decarburization of ul tra-low carbon …digitool.library.mcgill.ca/thesisfile61306.pdf ·...

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DECARBURIZATION

OF UL TRA-LOW CARBON STEEL

DY VACUUM LEVITATION

Jin Liu

A Thesis Submitted to The Faculty of Graduate Studies

and Research in Partial Fulfillment of the Requirements for The Degree of

Master of Engineering

Minin. ct Metalluraical Enaineerina Depanment McGi" University

Montreal September, 1991

To my dear father, mother and brother .

..,.,

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A BSTRACTS 1

Abstract

Improvement of the formability of ultra low carbon steel requires industry to reduce

the carbon conlent of the sleel to a level below 10 ppm. Process economics dictate that

it be done within a Iimited time. So far many methods have been triecl, but below 10

ppm is still very diffieult to reach. The carbon content of ultra '?W carbon steel made

by cllrrent commercial production techniques, generally ranges from IS to 30 ppm. It

has been observed that the rate of decarburization drops rapidly when the carbon is about

30 pp,". Thus the main reason for the laek of reduclion of carbon below these levels is

the slow rate of decarburization.

Vacuum levitation experiments have been eondueted to study the decarburization

kinf!lics of levitated steel droplets in order to determine the factors and relationships

whictt control the rate of decarburization especially at C levels below 30 ppm. The

beh:.lviour of sliifur and its intluence on the rate of decarburization were al50 studied al

the same time. It WBS found from the experiments that (1) vacuum cham ber pressure had

a significant effect on the rate of decarburization when the carbon content was below 3S

ppm; (2) sulfur did not show any signifieant effect on the rate of decarburization due to

the strong stirring inside the droplet caused by magnetie levitation field; (3) the rate of

decarburizalion of levitaled droplets was 3 ppm/sec al (C] = 30 ppm which was 40 limes

higher than the overall rate of decarburization in the RH process at [C] = 30 ppm; (4)

high initial oxygen contents improved the rate of decarburization al high carbon contents.

Il is believed Ihallhe rapid drop of the rate of decarburization at 30 ppm carbon is

due to the fact Ihat Ihe order of kinetics of decarburization reaction transferred from first

order to same higher order as a result of the increasing amount of COz gas in the

producls of decarburization. Lowering the vacuum ehamber pressure can reduce the

amount of COz gas produced in the gas phase 50 lhat reducing the chamber pressure can

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ABSTRACTS il ------------------.---------------------have a positive effect on accelerating the rate of decarburization.

The following suggestions are made for the acceleration of the rate of

decarburization on an industrial seale: (1) increase the alnOlint of liquid steel droplets

without increasing the size of the droplets; (2) increase the fraction of the amount of

decarburization reaction inside the molten steel by gas and powder injeclion; (3) further

rcduce the partial pressure of CO and CO2 gas in the gas phase. especially when carbon

content is ultra low, Le., lowcr the chamber pressure.

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A BSTRACTS III

Résumé

L'amélioration de la formabilité de l'acier. bas taux de carbone présuppose la

réduction du taux de cabone à un niveau audessous de 10 ppm. Les économiques du

traitement exige que cela soit fait dans un temps limité. Jusqu'. maintenant beaucoup

de méthodes ont été essayées, mais un taux au-dessous de 10 ppm est encore trb

difficile à atteindre. Le taux de carbone dans l'acier l ultra-bas niveau de carbone,

produit par les techniques comerciales courantes, se situe entre IS ppm et 3Oppm. Des

études ont montré que La vitesse de la dkarburation se ralentit brusquement lorsque le

taux de carbune s'approche de 30 ppm. Donc le ralentissement de décarburation semble

être la principale raison pour la difticulte à réduire le taux de carbone l un niveau

audessous de 30 ppm.

Des expériences sur la lévitation à vide ont été effectuœs pour étudier les cinétiques

dans les gouttelettes lévitées de l'acier, afin de déterminer les facteurs qui affectent la

vitesse de décarburation, surtout lorsque le taux de carbone se situe audessous de 30

ppm. Les comportements du soufre et son influence sur la vites.se de décarburation ont

imssi été étudiés. Les résultats montrent que: 1) La pression dans la chambre à vide joue

un rôle significatif dans la vitesse de décarburation lorsque le taux de carbone se situe

.m-dessous de 35 ppm; 2) Le soufre n'a pas un effet significatif sur la d6carburation, l

cause du remuement à l'intérieur des gouttelettes, remuement causé par le champs

magnétique de la lévitation; 3) La vitesse de décarburation des gouttelettes lévitées est

3 Pl,m/sec à C =30 ppm, laquelle est 40 fois plus vite que la vitesse globale de

décarburation dans le traitement RH l C=30 ppm; 4) La quantité initiale de l'oxYI_

accélère la vitesse de décarburation de l'acier contenant un taux élevé de carbone.

Le ralentissement accéléré de la décarburation, ce qui se produit lorsque le taux de

carbone s'approche de 30 ppm, semble être attribuable au fait que l'ordre des cinétiques

de la réaction de décarburation se trans~re du premier ordre l un certain ordre plus

haut, à cause de l'augmentation de la quantité de COl las dans les produits de

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l ABSTRACTS Îv

dbrburation. Réduire la pression de la chambre à vide peut réduire la quantité de COz

183 qui se produit à la phase de gas, donc réduire la pression de la chambre à vide lleut

avoir un effet positif sur l'accélération de la vitesse de décarburation.

Pour acc~lérer la décarburation au rlan industriel, les suggestions suivantes sont

proposœs: 1) augmenter la quantité de gouttelettes liquides de l'acier Solns augmenter la

taille des gouttelettes; 2) accroitre la fraction de la quantité de la réaction de

dbrburation • l'intérieur de l'acier fondu au moyen de l'injection de gas et de poudre;

3) r61uire davantage la pression partielle de CO et COl gas pendant la phase de gas,

surtout lorsque le taux de carbone devient très bas. En d'autres mots, réduire la pression

de la chambre à vide.

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ACKNOWLEDGEMENTS

ACKNOWLEDGEMENTS

1 would like to express my sincere gratitude to my supervisor, Professor R. Harris

for his excellent guidance, constant encouragement and continuous assistance.

My sincere thanks to Mr. Martin Knoepfel and R. Selby for their friendship and

support in building the vacuum levitation apparatus. My sincere appreciation to those

fcllow students who helped me in many ways in my work as weil u to Z. Wang. E.

Mast, R. Li, J. Jara, J. (.anglais and O. Shen.

The support of Dofasco Research in supplying some of the materials and performing

the chemical assay, and especially the assistance of Mr. R. Webber is gratefully

acknow l&!dged.

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,1 TABLE OF CONTENTS vi

TABLE OF CONTENTS

ABSTRACT

RESUME

ACKNOWLEDGEMENTS ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

TABLE OF CONTENTS ............................... VI

NOMENCLATURE .................................. XI

LIST OF FIGURES. . . . . . . . . . . . . . . . . . .. . ............. xv

LIST OF TABLES ..................... . . . . . . . . . . . xviii

CHAPTER 1 INTRODUCTION

1.1 Purposes of Producing Ultra-Low Carbon Steel ............ .

1. 1.1 Introduction of ultra-Iow carbon steel ............. 1

1.1.2 Formability improvement . . . . . . . . . . . . . . . . . . . .. 2

1.1.3 Nonaging property improvement . . . . . . . . . . . . . . .. 7

1. 1.4 Scavenging effect ......................... 8

1. I.S Annealing process . . . . . . . . . . . . . . . . ......... II

1. 1.6 Spot weldability ............. ............. 12

1.2 Processes for Producing Ultra-Low Carbon Steel . . . . . . . . . . . . 13

1.2.1 OH process ............................. 14

1.2.2 RH process ............................. 18

1.2.3 RH-OB, RH-OB·FD and RH-PB process ........... 22

1.2.4 YOD process ............................ 23

1.3 The Rate Behaviour of Decarburization .................. 25

1.3.1 The tirst period of decarburization . . . . . . . . . . . . ... 25 , t

1.3.2 The second period of decarburization ............. 27

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TA liLE or CONTENTS \'ii

1.3.3 The lhird period of decarburizalion .............. 28

CIIAI".,ER 2 LITt;RATURE SURVEY

2.1 The Commercial Production of Ultra-Low Carbon Steel. . . . • .. 29

2. 1. 1 The carbon content before vacuum treatment . . . . . • . . . 33

2. 1.2 The vacuum treatment duration . . . . . . • . . • . . • • . . • ~4

2.1.3 The vacuum minimum level, the cross section of the snorkels

and Ar flow rate . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.2 Current Problems in The Manufacture of Ultra-Low Carbon Steel . 37

2.2.1 Sources of carbon contamination 1 pick-up •.•...•... 38

2.2.2 Acceleration of the rate of decarburization .....•.... 40

2.3 Previolls Laboratory Research on The Kinetics of Decarburization 44

2.3.1 The mechanism of decarburization with oxidizing gas ... 44

2.3.2 Mathematical modelling of the kinetics of decarburization

............................................ " ...... .. 47

2.3.3 The influence of some elements on the rate of decarburization

.•••••...•..••••••••••••••.•••••••.• S 1

2.3.4 The circulati"g characteristics and stirring efticiency of molten

sleel in RH ...... 1 •••••••••••••••••••••• S5

2.3.5 The nuclealion of CO bubbles in molten steel ....•... 57

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CIIAI"., .. :R 3 IlECARBURIZATION MECHANISM

.ll Dccarburiz&lrion Mechnism When Carbon Content is Ultra Low ... 61

3.1.1 A hypothesis on the rate behaviour of decarburization when

carbon conlent is ultra low . . . . . . . . . . . . . . . . . ... 62

3.1.2 The feasibilily of carbon elimination (rom liquid steel ..• 65

TABLE OF CONTENTS ,'iii

3.1.3 Decarburization within the molten steel, at the free surfilee of

the molten steel and at the suspended droplet ........ 65

3.2 The Effect of Specitic Surface Area on The Rate of Decarburizatiol1

......................................... , , ....... .. 70

3.3 The Overall Rate of Decarburization in Practice ............. 73

3.3.1 The overall rate of decarburization under vacuum without imy

gas or powder injection ...................... 73

3.3.2 The overall rate of decarburization by adding powdered oxidi7cr

under vacuum ................ ........... 74

3.3.3 The overall rate of decarburization under vacuum with Gas

'n' t' 74 1 ~ec Ion .............................. .

CHAPTER 4 EXPERIMENTAL

4.1 Introduction ................................... 76

4.2 Levita1 ion Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.3 Experimental Variables . . . . . . . . . . . . . . . . . ........... 78

4.4 Experimental Setup ............. . . . . . . ........... 80

4.4.1 High frequency generator ......... ........... 80

4.4.2 Vacuum system .......................... 80

4.4.3 Vacuum levitation apparatus ................... 81

4.4.4 Temperature measurement . . . . . . . . . ........... 84

4.S Experimental Procedure ............... ............ 86

4.S.1 Experimental design ........................ 86

4.S.2 Vacuum Jevitation experiment procedure ........... 87

CRAFfER 5 RESULTS

S.l Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93

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TABLE OF CONTENTS lx

S.2 The Carbon Content Change .....................•.. 94

S.2. J The carbon content change for the tint group of specimens

(Initial [C] == 1870 ppm) .•...............•... 94

S.2.2 The carbon content chanae for the second group of specimens

(Initial [C] == 1230 ppm, [S] = 180 ppm) •.••.•••.. 96

S. 2.3 The carbon content chanae for the third group of specimens

(Initial [C] = 880 ppm, [5] == 3300 ppm) •..••..••. 97

S.2.4 The carbon content chanae for the fourth group of specimens

(Initial [C] == 3S ppm, [5] - 350 ppm) • • • . • • . . • • • . 98

S.3 The Evaporation of The Specimens During Vacuum Levitation •• 101

~.4 Levitation Temperature . . . . • . . . . . . . . . . .......•.••• 104

CHAPTER 6 DISCUSSIONS OF EXPERIMENTAL RESULTS

6.1 The Rate Behaviour of The Fint and Second Group of Specimens 106

6. I.i Effeet of carbon content on the rate of decarburization of the

fint and second group of specimens •.......•.••• 106

6.1.2 Effeet of oxygen from the vacuum chamber •.....•• 107

6.2 Decarburization mechanism of the fourth group of specimens .•. 1 \0

6.2.1 Mathematical model for the decarburization kinetics of the

fourth group of specimens •......•.•..•.••.•• 110

6.2.2 Discussion of the model .•••••••..••••.••.•• 116

6.3 Effeet of Suif ur on nte Rate of Decarburization of The Third Group of

SJJeCilllens ......•.........•.•••••..••••.••••• 121

6.4 Behaviour of Sulfur Content . . . . . . . . . . . . . . . . . . . . . . • . 122

CHAPTER 8 CONCLUSIONS

TABLE OF CONTENTS x ."'.

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

NOMENCLATURE xl

NOMENCLATURE

A effective interface area A' pre·exponential factor (frequency factor) a% fraclion of sample weight reduclion by evaporation a~ activity of carbon

30 activily of oxygen

il. activity of sulfur C .. carbon content inside the molten steel

CI! carbon conlent in equilibrium to CO partial pressure Co initial carbon content %C. sulfur concentration in molten steel C. carbon content al Ihe reaction surface

( d grain size D diffusivity D' the diameler of the bath in Ihe ladle

DA" binary diffusivity of the transfened species in the melt d .. diameler of the bubble Di Ihe inner diameler of Ihe snorkel of RH E activation energy f frequency of bubble nucleation ft frequency t cycles/sec F surface area of molten steel AF free energy

fIl'! activity coefficient of carbon Fa. levilation force f lOI activily coefficient of oxygen G Ar flow raie in RH process H depth of the molten steel bath 1 coil current, amps 1(1) inlensity of radioactivity (cps) al lime t lm flux of CO gas removal (rom reaction surface

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-~ NOMENCLATURE xii a

J,'Oa flux of COz gas removal from reaction surface J. flux of mass transfer of CO in the gas phase mole/cmJsec JL flux of mass transfer of carbon inside molten steel J. rate of bubble nucleation, bubbleslsec JIOI flux of mass transfer of oxygen in Iiquid steel, molr.icmlsec Jr number of moles of carbon being eliminated per unit area of reaction

surface K equilibrium constant k a constant of proportionality for the change of tensile stress with stmin k' a constant for a given position in a given coit K' isothermal rate constant K. mass transfer coefficient of the decarburization reaction occurring in the Ka the Bolzmann constant vacuum vessel and uptake snorkel of RH

Je. specific equilibrium constant

Kco mass transfer coefficient of CO gas in the gas phase

Kco. mass transfer coefficient of COz gas in the gas phase Ka. mass transfer coefficient of carbon in the liquid steel Ko mass transfer coefficient in Soejima's model KIOI mus transfer coefficient of oxygen in liquid steel, mole/sec Kr rate constant of decarburization reaction K., rate constant of decarburization in reverse direction, atm-amole/sec K, suJfur absorption coefficient m number of atoms CO atoms inside a bubble M charge mass in levitation, gram n the order of decarburization reaction n' work hardening exponent N total number of atoms in Iiquid steel Ne number of carbon atoms in liquid steel No number of oxygen atoms in liquid steel [0], oxygen content inside the droplet, mole/cm' [0], oxygen content at the surface of the droplet, mole/cm' P pressure in the vacuum vessel PL static pressure of liquid steel Po initial pressure in the vacuum vessel

.,.. .. p·co CO pressure in equilibrium with the melt p·eoa COz pressure in equilibrium with the melt

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NOMENCLATURE xiii

P('() Partial pressure of CO gas Peu, Partial pressure of COz gas Pc pressure in the gas phase 1". pressure of CO gas within the CO bubble Il, pressure of CO gas at the surface of the droplet, atm 1'. static pressure of slag due lO gravit y Q molten steel recirculating rate in RH QA argon injection rate, m'/min r radius of charge in levitation, cm r' radius of curvature of the CO gas trapped in the small cavities on the

refractory surface R gas constant R • exhausting rate constant RI overall rate of decarburization under vacuum without any gas or powder

injection ove rail rate of decarburization with powder injection overall rate of decarburization under vacuum with gas injection rate of decarburization in the bulle

rb radius of charge in levitation, cm Roi rate of decarburization at the suspended droplet RIII rate of decarburization at the melt surface r() radius of oxygen ions R(h rate of mass transfer of oxygen in gas phase, mole/sec T temperature. K

t' t' u

Vr

V.

w w W ..

decarburization reaction time, second thickness of specimen after tensile test thickness of specimen before tensile test temperature of the steel bath, K temperature of the environment, K liquid steel volume, m' tlow rate of gas, litre/sec volume of liquid steel being directly involved in decarburization reaction at the surface of droplet, cm' weight of molten steel treated, tonne width of specimen after tensile test value of the specifie surface area of droplet over the specifie surface area

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NOMENCLATURE xiv

of molten steel in ladle or crucible Wo width of sptCimen before tensile test W fi'" value of the specifie surface area of bubble or powdcr ovcr the spt.ocitic

surface area of molten steel in ladle or crucible Wy weight of molten steel in vacuum vessel of RH unit

Greek Symbols

CI constant fJ volume constant r. excess sulfur surface concentration, aloms/cm' -r plastic strain ratio cS thickness of diffusion layer

" constant , fraction of surface occupied by sul fur ). constant P4 chemical potential of CO dissolved in molten steel p. chemical potential of CO gas p density p' resistivjty (1 surface tension, dynes/cm (1.. tensile strength UI a constant depend on the chemical composition of steel .,. mixing time 0. molar volume of gas B. true strain in tensile test B stirring power

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LIST OF FlOURES .u

LIST OF FIGURES

Figure 1. 1 1 mprovement in the value of n (formability) by reducing alloying and

Figure 1.2 Figure '.3

Figure 1.4 Figure 1.5 Figure 1.6 Figure 1.7

Figure 1.8 Figure 1.9 Figure 1.10 Figure 1.11 Figure 1.12 Figure 1. 13 Figure 1. 14 Figure 1.15

Figure 11.1

Figure Il.2 Figure Il.3 Figure Il.4 Figure 11.5

Figure Il.6 Figure Il.8

impurity elements ......•..........••..••...•• 4 1 mprovement in elongation to fracture by reduction in carbon • 5 hnprovement in r value and elongation to fracture by reduction in carbc:Jn .•• • . • • • • . . • • • • • • • .. .. .. .. • • .. .. .. .. .. .. .. .. .. .. .. 6 The influence of aging on tensile test .........•....•• 8 Prevention of aging by reduction in carbon and nitrogen ...• 9 Improvement of r value by scavenging effeet •....•....• 10 Effeet of carbon + nitrogen content on fracture appearance transition temJlerature . . . . . . . . . . .. .. . . .. .. . . .. .. .. .. .. . .. .. .. .. .. .. .. .. 14 Principle of DH process .................•....•• 15 Decent of carbon in DH with AR injection . . . . . . . . . . . • . 16 Relation between Kc and injection gas flow rate . . . • . • . . . • 17 l'rinciple of RH process .................••....• 19 Effeet of height of circulation on the degassing rate in RH .. • 21 Principle of RH-OB and RH-OB-FD process . • . . . . . . . . . • 22 Principle of VOD process .•....•.........•....•• 24 The rate of decarburization versus carbon content in BOF process

.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . .26 Equilibrium between activity of carbon and olygen before and alter vacuum deearburization . . . . . . . . . . . . . . . . . . • . . • . • • 33 The reduction of RH treatment time during the last seven years 34 The reduction of tap temperature du ring the last five years • . • 35 The influence of slag line bricks on the rate of decarburization 38 The principle of pour;ng stream equipment for vacuum decarburization ........•....•....•....••...•• 41 The rate behaviour of decarburization at different temperatures 45 Carbon content change versus time in ultra-Iow carbon steel proouction . . . . . . . . . . . . . . . . .. .. . . . .. .. .. . . . . . . . . . 46

Figure Il.9 Carbon content versus lime in ultra-Iow carbon steel production 47

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1 LIST OF FlOURES xvi

Figure Il.10 Effeet of small concentration of sulfur on the rate constant fllr the decarburization of carbon-saturaled iron . . . . . . . . . . . . . . . !'i4

Figure 111.1 The behaviour of the rate of decarburization with the change of carbon content ............................ . . 63

Figure 111.2 The behaviour of COl gas as a decarburizalion product with the change of carbon content . . . . . . . . . . . . . . . . . . . . . . . . 64

Figure 111.3 c-o equilibrium at different partial CO gas pressure . . . . . . . 66 Figure 111.4 Three locations where decarburization reaction Decures ..... 67 Figure 111.3 CO bubble forming at the refractory surface ............ 68 Figure IV.l High frequency generator ....................... 79 Figure IV.2 Vacuum levitation apparatus . . . . . . . . . . . . . . . . . . . . . . 82 Figure IV.3 Levitation coils .............................. 83 Figure IV.4 Vacuum levitation apparatus . . . . . . . . . . . . . . . . . . . . . . 84 Figure IV.S The inner look of the vacuum levitation appara!'ls ........ 8!'i Figure IV.6 The specimen was held on top of the alumina crucible bcfore

experiment ................................ 88 Figure IV. 7 The specimen was being pushed up to the levitation coils . . . . 89 Figure IV.8 The specimen was being levitated .................. 90 Figure IV.9 The temperature of the specimen was increasing ......... 90 Figure IV.I 0 The speci men was being melted ................... 91 Figure IV. Il The spe-cimen was entirely melted .................. 91 Figure IV.12 The specimen was held in copper crucible after the experhnent 92 Figure IV. Il The glass tubes which have been used in the expcriment .... 92 Figure V.I Results of decarburization of the tirsl group of specimens .... 94 Figure V.2 Results of levitation experiment of the second group of specimens

•••••••••••••••••••••••••••••••••••• fi • • 96 Figure V.3 Results of vacuum levitation experiment of the third group of

sJJCCi men s • • • • . . • . • • . • . . . . • • . • . • . . . . . . . . . . . 98 Figure V.4 Results of vacuum levitation experiment of the fourth group of

specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9')

Figure V.S Weight losses of the specimens by evaporation during vacuum

Figure V.6

Figure V.7 Figure VI.I

levitation experiment ......................... 102 The adjusted value of carbon conlent versus lime for some of the Dofasco samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Temperature change during vacuum levitation experiment ... 104 The rate of decarburization versus carbon content of the four groups

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LIST OF fiGURES xvII

of specilnens ...............•........••.•.. 107 Figul'e VI.2 Effeet of gas flow rate and oxygen partial pressure on the rate of

oxygen absorption of liquid iron at 1600°C ••••••••••• 109

Figure VI.3 The change of COl(COz+CQ) in the aas phase versus the chanae of pressure (P'-'OaH,'O) in the aas phase venus the ('-.hanle of pressure (P(..'OI+~~) •••• , ••••••••••••••••••••••••••• • 121

Figure VI.4 Sulfur content of the first and second group of specimens ••• 123

Figure Vf.5 Sulfur content of the third grou~ of specimens •••••••••• 125

Figure VI.6 Sulfur content of the fourth group of specimens • • • • • • • •• 126

LIST OF TABLES

Table 1.1 TabIeI.1I Table Il.1 Table Il.11 Table Il.111 Table 111.1 Table IV.I Table V.I Table V.II Table V.III Table V.IV Table VI.I

•• 111

LIST OF TABLES

Mechanical properties of ferrite, cementite and pearlite ..... 2 Advan_es of RH-OB and RH-OB-FD methods ......... 23 Methods in various plants for ultra-Iow carbon steel production JI Experimental conditions for pouring stream decarburi7.ation .. 42 Kinetic coefficients of decarburization . • • . . . . . . . . . . . . . ~ Increue of C~ gu with a decrease of carbon content . . . . . . 64 Conditions of the four group of specimens ............. 86 Results of experiment of the fint group of specimens ...... 9S Results of experiment of the second group of specimens . . . . . 97 Results of experiment of the third group of specimens . . . . . . 97 Results of experiment of the fourth group of specimens .. .. 100 Carbon, sulfur content and S~ pressure of the four groups of specimens •••.••...••.....•.............. . 101

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CHAPTER 1 INTRODUCTION 1

CHAPTER OllE

INTRODUCTI()N

1.1. PURPOSE OF PRODUCING ULTRA-LOW CARBON STEEL

1.1.1. Ultra-Low Carbon Steel

The term ultra-Iow carbon steelgenerally refers to steel containing 30ppm or less

carbon which can meet the demand for economical manufacture of extra-deep drawinl

cold rolled steel sheet or high-tensile strength cold roUed s,~eel sheet with excellent deep

drawing properties. These cold rolled sheets are mostly \Jsed for the manufacture of

automobile and consumer goods.

With few carbide precipitates, such as tertiary cementite in the ferrite structure,

the mechanical properties of ultra-Iow carbon steel are influenced by the properties of

ferrite. In order to have a better undentanding, the mechanical propenies of ferrite,

cementite and pearlite are listed in Table 1.1.

Previous research(I, preclicted that lowerinl the carbon in steel will areatly

contribute to the following propenies and process improvemenlS if carbon content cu

be reduced to belcw 10 ppm:

( 1) Formability improvement.

(2) Nonaging propeny improvement.

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,- CHAPl'ER 1 INTRODUCTION

(3) ScavCftging effec:t.

(4) Annealing process.

(5) Spot weldability improvement.

(6) Ferritic stainless steel properties improvement.

Table 1.1 Mechanical properties of Ferrite, Pearlite and Cementite

Fenite Pearlite

Tensile strenath (f7t,) 26 • 33 (kpsi) 140 (kpsi)

Hardness (H8) SO·80 240

Elongation (3) 30 - SO~ 10~

Maximum carbon 218 ppm (727°C) If[C]< 218 ppm, solubility in iron 80 ppm (600°C) no pearlite formed.

1.1.Z. Fonnabliity Improwement

Formability generally refers to the following properties:

(i) Elongation to fracture;

(ii) Stretchability;

(iii) Deep drawability.

Cementite

4 (kpsi)

800

0

The areater is the elonaation to fracture or uniform elongalion (elonaation to a

maximum load) of a sheet in a tensile test, the better the formability of the sheet. The

relationship between the true strain e., and true stress, a, of the sheet in the tensile te:;t

CIO be approximated by:

a - Iœ Il' ~

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CHAPTER 1 INTRODUCTION 3 ---------------------------------------------

k is a constant of proportionality and n' is the work hardening exponent. Sinc:e it

corresponds to the true strain of uniform elongation, the value of n' is frequently used

to evaluate the formabUity of shect metal. Il is empirically known that the value of n'

is given by:

10 ft· - ----~ 1 . .,

10+a,+I.3d (1-2)

where d is the grain size and al is a constant which depends on the ehemical composition

of the sheet steel. The presence of alloying or impurity elements generally increases cr.

and decreases d, thereby decreasing the value of n. Two examples are shown in Fiaure

1.1 and Figure 1.2 III.

The stretchability of sheets is evaluated by the Erichsen test. In this test, the

specimen of sheet is stretched into the form of cup until a crack occurs and the height,

H, of the cup at cracking is measured. This heiaht, H, is called the Erichsen value. The

stretchability of shcets is greatty affected by nonlnetallic inclusions in the steel. When

the sulfur content of the steel increases to such a level as to farm larae sulphide

inclusions, cracks are apt to originate at such sulphide inclusions, lowering the Erichsen

value. The same is true for other inclusions, such as oxides.

Deep drawability is evaluated by the conical cup test. The specimen of shect is

strctched into a conical cup until. crack occurs and the diameter, D, of the conical cup

at cracking is measured. This diameter, D, is called the conical cup value (CCV). The

better the deep drawability of the shect, the smaller the CCV. The plastic strain ratio,

'Y, or Lankford value is also used widely to evaluate deep drawability. This value is

defined as the ratio of the strain in the transverse direction of the specimen aner the

specimen has been strained to a certain level (for example 20") in the tensile test, and

is liven by:

In(~/w) 'Y - -..;......-

In(I·.,II·) (1-3)


Where,

Wo' t'o = width and thickness of specimen before tension.

w, t' = width and thickness of specimen after being strained to specified level.

0.29 r----. ...,.. ',""'.-.. -----------.

0.28 oo', : ..

0.27 ... '..~ ",

" oo~. .;. .'. . : ' . ~,- ~, . ' l . ..," .. 4 .,., ...

0.26 ~' l-i .. 1 . . " ". '~.,.. - , (. ,,' ~ .... -, '"" , " " ' .. ~ ,." , , \ . .'

'<, ' '''" ~ l1li, " • .. -, '~' .. \. -" ~-. ,', ' . , '" . .. " ' .. .... .•... .. -

",,*.'

c

0.25

0.24

0.23 ... Ba.e,+·C· .' Ba •• + S . . .... . .. . .

''-~/::', 'a ••. +. 0: ,', ~' Ba •• + p'

0.22 o· 'Ô~Ô2·· 0.04 0.08 0.08 0.1 "

0.12

C, N, S, 0, P (wt. %)

Flaure 1.1 Improvement in the value of n' (formability) by reducing alloying and impurity elements.

( CHAPTER 1 INTRODUCTION 5

Sheets of excellent deep drawability have a high value of "'(.

52

ii -• 41 ... ::1 -U

• ... -0 -c 0

: 44 • ( lia C

0

au

40 o 0.01 0.02 0.03 0.04

C (wt. 1)

1 mp r 0 y • m. nt 1 n .1 0 n 1 Il Ion t 0 f rie t ur •

" r.ductlon ln carbon

Fiaure 1.2 Improvement in etongation to fracture by reduction in carbon.

(

...

~-


2

1.8

• :1

• > .. 1.1

1.4

o 0.01 0.02 0.03

C (wt. 1)

, •• , ...... , 'ft , U, .. III' "11111"" ,. "lClu. " ' ..... 1 .. Il CU,"

fi

Flaun 1.3 Improvement in r value and elongation to fracture by reduction in cubon.

The CCV or ~ value which is an index of deep drawability is related to the crystal

texture of the sheet steel. The more and larger the slip plane components, such as {Ill}

planes in 'Y-iron and {llO}, {l23} and {1l2} planes in a-iron, the better the deep

drawability. It is already known that texture varies greatly with the chemical

composition of the steel and the body-centrecl cubic a-iron lattice allows small impurity

" r ~

c

(

(


atoms (carbon, nitrogen, oxygen, hydrogen) to enter the lattice in interstitial positions.

Since ultra low carbon steel is in the form of a-iron, the possibility of forming interstitial

carbon can be reduced by reducing the carbon content. The interstitial carbon can reduce

the formability of ultra low carbon steel due to the following two possible reasons:

(1) The interstitial impurities can raise the deformation energy.

(2) The interstitial impurities can resist dislocation movement under certain

conditions.

An example for aluminum-killed steel is shown in Figure 1.3 III. Lowering the

carbon content of each grade increases the -r value and elonaation to fracture, and

improves the overall formability.

1.1.3. Nonallnl Property Improvement

Immediately arter temper rollina, the sh~t provides a smooth stress-strain curve

in the tensile test as shown in Figure 1.4(a) and no stretcher strains appear in formed

parts. If the sheet contains a certain amount of carbon and nitroaen in solid solution, it

develops yield-point elongation as shown in Figure 1.4(b) during a tensile test, perfonned

after storage of the sheet at room temperature for a long periocls of time (for example

one month). This change in sheet properties with time is called agina. With sheets that

produce yield-point elongation due to aging, a drop of elongation to fracture occun at

the same time. This behaviour can be understoocl in terms of dislocations locked by the

impuri'ies, which are most probably carbon and nitroaen atoms precipitated onto the

dislocations during aging, so that a larae stress is required to generate impurity-free

dislocations which can move under a smaller stress. To avoid such problems, sheets that

do not develop aging or nonaaina sheets are required. As shown in Fiaure I.S,

decreasing the amount of carbon and nitrogen in solid solution is effective in meetina this

nonaging requirement. On the other hand, lowerina the total carbon content and fixina

nitrogen by aluminum (aluminum killed steel) are used u mans for achievina this

purpose.

., ~ "

~ , l, ~

, 1 i' -[ , ~ "'" ~ Ir


• • • .. -en

• • • .. -en

( .,

St r al n

( .,

St r al n

Thil nf 1 uinci of Igi ng on t In.111 t I.t

Flpre 1.4 The influence of aging on tensile test

1.1.4. Selvenllnl Erreel

8

Il is known that the minimum solubility of carbon in ferrite (a) is below 80ppm

al room temperature (2). The carbon content of steel cao bt lowered by the present

(

(-

(


vac.uunl decarburization processes to a minimum of 15 to 30 ppm in consideration of

•

-lit -4 c: 0

-• -c: 0

• - 2 c:

CIl .. •

"'ID

• >-

0

D

, , "(...... ..'

D. DO 1 0.002 0.003 0.004

C+ Nin 101 id. 0 1 ut u Ion (wt.~)

Pr.v.nt Ion of •• 1 nI b, r .duct Ion

fn clrlton and nltro,.n

FlIUI'f 1.5 Pievention of a,ing by reduction in carbon and nitro,en.

r

l


2.2

-'" • s

• • ,. .. 0 -.-c • ~ -..

2.0

1 2 s 5

(M. X)

1 mp r 0 v • m. n lof r v • 1 u. b , • c • , • ft gin 1 • f f • c t

Flpre 1.6 Improvement of r value by scavenging effect.

economics and other factors involved such as equipment limitations and the Iimitcd

kinetics of the decarburization processes. However t sorne of the carbon in ultra low

carbon steel is in the form of intentitial carbon and precipitated tertiaty cementite,

which are harmful to the formability of steel. In order to prevent the forming of harmful

carbide and interstitial carbon by the residual carbon and to obtain a mataix iron of

(

(

(

CHAPTER 1 INTRODUCTION Il

substantially improved purity, vacuum decarburization was used at the same time as

adding titanium and other carbon scavengers which (orm harmfess precipitates, such as

TiC. This method is calleeS "scavenging". The scavenging effeet is frequently applied

because of the fact that it is technologically or economically difficult to decrease the

contents of impurity elements to extremely low levels. When alloying element M that

readily combines with an impurity element X in a steel is added to the steel, the

following reaction takes place to form a precipitate MIIIX.:

m[M] ... n[X] ut M".X,. ",

(1-4)

That is, X is fixed by M and the purity of the matrix iron itself is substMtially increased.

Most of carbon scavenging elements, including titanium, are relatively expensive.

'fthe carbon content can he lowered efficientlyand directly to below 10 ppm by vacuum

treatments, such expensive alloying elements will not have to he Used. Figure 1.6 shows

the influence of scavenging elements on the value of 'Y.

1.1.5. AnneaUn. Process

The degree of purity attained by current methods, including scavenging, has

permitted conversion from batch annealing to continuous annealing. That means the time

of annealing has becn shonened from about one week to about five minutes. The long

duration of batch annealing was suited for increasing the grain size and aligning the

texture in the [111] orientation, 50 that the batch annea1ing process was mostly used for

annealing sheets of excellent formability. In contrast, continuous annea1ing takes a total

trealment time of about five minutes, and is more efficient. However, because of the

shorter annealing lime, it was claimed that sheet products of lood formability cannot he

produced by the continuous annealing process. It was subsequently clarified that short

timf; annealing of about one minutt can increase the lrain size and provide z hiah 'Y

value for those steels whose contents of impurity elements (including carbon) are

extremely low (J'.

1

"".

CHAPTER 1 INTRODUCTION Il

ln view of the fact that the reerystallization temperature is about 550°C for

present commercial steels, a temperature about 150°C higher, i.e., 700"C, hils been

adopted as the annealing temperature. On the other hand, the recrystallilation

temperature of recent superlaigh-purity iron has been reported to be as low as 300"C.

This suggests that increasing the purity of steel can significantly decrease the anncaling

temperature and hence, the consumption of energy in sheet production.

"

1.1.6. Spot Weldabillty

Automobile parts press formed from cold-rolled steel sheets are chietly assembled

by spot welding. Previous research shows that decreasing the carbon, phosphorus and

sulfur contents will considerably improve the spot weldability of the steel. Various

methods are available for testing the strength of the spot weld, one of them is called the

peel test which is designed in such a way that one of the two sheets' spots welded

together is peeled to see if nuuet "efeets accur or not.

The condition under which no nugget defects accur in the peel test is given in

relation to the chemica1 composition of the steel by the following equation [2]:

As Equation 1-5 shows, deereasing the carbon, phosphorus and sulfur contents

improves the spot weldability of the steel.

1.1.7. Fenitle Stalnlcss Steel

Conventional ferritic stainless steels, such as Fe-1395er and Fe-17f1Cr grades,

have far hilher resistance to chloride stress corrosion cracking than their austenitic

counterparts. However, because of their poor resistance lo corrosion by acids and

alkalies as weil as low toughness, formability and weldability, their uses in chemical

(

(

(


plants and other fields have been limited, although they contain no nickel and are less

expensive. In rccent years, however, ferrilic stainless steels have becn expanding their

applications thanks to the development of high-purity molybdenum-bearing ferritic

stainless steels. The basic research findings that have encounged this development are

as follows:

(i) When the chromium content is increasecl and molybdenum is added,

ferritic stainless steels can be provided with acid and alkaU corrosion

(ii)

(iii)

... resistance equal to or better than that of austenitic stainless steels, while

maintaining good resistance to stress corrosion cracking in chloride

environments (41.

Toughness and other mechanical properties can he materially improved by

lowering the carbon and nitrogen contenls al the same lime lSJl61.

The simultaneous reduction of carbon and nitrogen contents and the

addition of powerful carbonitride formers, such as titanium and niobium,

can suppress the precipitation of chromium carbonitrides at lrain

baundaries, stabilize the mechanical propenies and increase the resistance

to intergranular corrosion PI.

The mechanism of sharply increasing toughness with decreasing carbon and

nitrogen contents may be explaintd by the fact that the chromium - carbonitrides

precipitate al the grain boundaries in decreasing amounts as the carbon and nitmaen

contents decrease (1191. Since the crystal structure of ferritic stainless steel is a body

centred cubic structure, the diffusion rate of chromium, carbon and nitrogeft is fu hilher

than that in austenitic stainless steels of face centred cubic structure. In addition, sinee

the solid solubility of carbon and nitrogen is low, the chromium carbonitrides precipitate

al the grain boundaries at a fast rate, embrittling the steel. The carbon + nitroaen

content must be lowered to about 100 ppm to obtain the ductile-to-briWe transition

temperature of under ooe, as shown in Figure 1.7, which is determinecl by charpy impact

tests or drop weight tcar tests (DWIT).

P li , ..

,. ~ f'

i [ f

., ! .. J ~ ~ .' f' r • 1 : , ,

l' •


-u

• zoo • .. ca • ... -• 150

-• .. • l' 100 • -c o

-• c • .. -•

50

u a c • .. • • la. la.

• ·50

• 0 .. :1 -• .. ~

0.01 0.02 0.03 0.04

C + N (")

Ef f Ict of car bon + ,,1 t r ogln conl.nt on

f r 1 c tu, 1 • pp •• r 1 no. 1 r 1 n lit Ion t. mp , l' U, •

Flpre 1.7 Effeet of C + N on the fracture appearance transition temperature

1.2. PROCFSSFS FOR PRODUCING ULTRA-LOW CARBON STEEL

14

The four major vacuum refininl processes. which an be used for commercial

mass production of ultra-low carbon content steel, are as follows:

('

'.,

(~

• CHAPTER 1 INTRODUCTION 15

I.Z.I. OH Process

The DH process, aJso called the vacuum lifter process, was mentioned for the

tirsl lime in 1956. Il was developed to full operational reliability by Dortmund-Harder

HüttenuÎon of Germany 1101,

Va cu u m

Pr 1 nel pl. of DH pr oc ••• Flpre 1.1 Schematic of DH procas

r


.00 .. • • - soo

• • 100 -• • • -; 100 c

10 • • -c 10 • 41

• 40 • .-.. • Co)

20

tG 0 10 40 80 10 100

Llttlnl n 11Mb Ir 1 1

o 10 1 S 1120

TrI,tI n. ,,"'. ("'1 n' DIU'''' of carlin ln DM wll" Ar l"ltel'ol

Flpre 1.9 Decrease of carbon in DH with Ar injection

A schematic of the DH process is shown in Figure I. 8. A ponion of steel in a

conventional castin, 1ac1le is sucked into a vacuum vessel and is returned to this ladle

after a shon deaassina period. The steel is sucked into a refractory-lined vacuum vessel

thoulh a reftaetory-linecl pipe which dips into the molten steel. Immediately afler the

suetion pipe is dipped into the molten steel the vacuum pumps are switched on 50 that

the vacuum vessel is evacuated. Owing to the difference in pressure, the steel riscs in

the suedon pipe until the difference in height between the steel in the ladle and that in

(

(

(


the vacuum vessel is about l.4m. On funher lowering the vesse! it will be filled with

steel, the difference in height of 1.4m being still maintained.

O. 1. " ... "' .....

, : • .>s

, '

O. 14 -c:

E --" ~

O. 12 , v

, ~ »

'v,

O. 1

o 400 800 1200 1.00

1 n J • c t Ion g 1 • f 1 0 W rit. (N 1 1 ml n)

Relation b.t ••• n K ,and InJ.ctlon '1' flow rit.

Flpre 1.10 Relation between Xc and injection gas tlow rate.

,-,

~.'


The atmospheric pressure therefore forces a cenain quantity of steel into the

vacuum vessel, this quantity depends upon the diR1~nsion of the vacuum vessel. As soon

as the steel enters the vacuum vessel, the degassing reaction starts causing vigorous

bubble formation and spattering of the steel Ill). When the reaction becomes more

calm and the degassing pracess is almost completed, the vacuum vessel is lifted so that

the treated steel flows baek into the ladle. Lowering and lifting are repeatcd as often as

required until a large enough quantity of steel has passed through the vacuum vessel and ... the desired degree of degassing has been obtained. Ali OH vacuum-degassing plants are

equipped with means of heating the vacuum vessel before flowing liquid steel into it 112) .

ln order to improve the rate of decarburization in DH process, the spced of

vertical movement of the vacuum vessel was incre.ased to 15 m/min. l'hus, the

recirculation rate was increased Il'). To ensure a large enough molten steel bath

surface area for a suffieiently fast degassing, the vacuum vessel of a OH has been

designed large, 50 much 50 that the amount of molten steel flowed can bt: about 33 tons

per lift, and the vessel is Iifted 5 times per minute by four hydraulic cylinders. Owing

to this, a large stirring energy, 8.4 watts per ton steel, is obtained and time required for

uniform mixing is as short as 72 seconds. Argon gas is also injected through the snorkcl

into the molten steel altemately with the lifting of vessel to improve the decarburization.

[C]-= 12ppm has been aehieved in a 19 minute treatment with an Ar ftow rate of 1500

NI/min 114), and for mass production, [e] =3Oppm has becn aehieved as shown in Figure

1.9 1141. The apparent decarburization rate constant, Ke, is increased proportionally to

the amount of Ar gas injected as shown in Figure 1.10 1151•

1.2.2. RH Process

The RH process or circulation degassing process, as shown in Figure 1.11, was

developecl for application on an industrial scale by Ruhrstahl A. G. Hattingen of

Germany. It is the most widely used and most effective pracess in ultra-Iow carbon steel

(

(


production 115111611171,

c o

-o • '-

Prlnclple of RH proc ••• FllUre 1.11 Schematic of RH process

19

tf."",.,

""'. CHAPTER 1 INTRODUCTION 20

ln contrast with the DH process, the refractory-lined vacuum chamber of the RH

process has two tubes attacheeS to the bottom; one of these is used fer the inlet and the

other for the outlet of the steel. After both tubes have been dipped into tht; molten steel

the vacuum vessel is evacuateeS 50 that the molten steel rises to the barometric hcight of

1.40m above the level of the steel surface. In the lower third of the inlet tube, close 10

the surface of the steel in the ladle, a conveyer Ils is injected after evacuation. Argon

is usually employed for this purpose. The upward movement of the metal in the intet

tube is further helped by the gases liberated from the steel in Ihe upper part of Ihe cntry

tube as a result of the lowerina of the pressure. In agreement with results on model

tests, it wu found that a mixture consisting of one part of steel and ten parts of gas by

volume flows at such a rate through th,: entry tube that the liquid steel reaches r, height

of about 1 m above the barometric height as shown in Figure 1.12. In some instances,

splashes of steel were thrown on to the refractory baffle provided at the upper part the

vacuum vessel. The delasseeS steel callects at the bottom of the vacuum vessel and

retums throuah the outlet tube into the ladle. The circulation speed of the steel is

lovemeeS by the elevating capa city of the inlet tube, which acts as a pump, and by the

bath in the delusina vessel.

The most conspicuous progress in the operation technology of the RH process has

been the suceessful production of extremely low carbon content steel with le] < 3Oppm.

To promote the decarburization by the RH rimming treatment, il is necessary 10

accelerate the transfer of carbon from the ladle to the vacuum vessel by increasing the

molten steel recirculatinl rate (Q) and to increase the volume mass transfer coefficient

(Ka) of the decarburization reaction occurring in the vacuum vessel and the upaake

snorkel. Accordinl to recent meuurements, the recirculatinl rate (Q) is proportional 10

inner diameter (DJ of the snorkel and Ar flow rate (0) to the power of 1.8 and 0.1

respectively, i.e. Q oc Dl.ljGA,o,l. An increase in Q is more advantageously attained by

increasina D, 50 that snorkels with a diameter as large as 60 cm are being used. On the

other band, the most effective way to increase Ka is to increase 0, whereby adoption of

a flow rate of 5000 IImin bas been reponed, and lC] < 30ppm is attained with 20 minutes

i , ..

('

(


of treatment.

• • ;; .. • • --• • • -• .. ---• • • 0

> '

1.11

• 71

•••

'.11

• ·11. • U. ·11 • Il

• • .. .. • .. • .. -• .. • .. • • ai

'" 'U

", •• 1111 ., "'"'"',, •• " .... , •• 111.,11' C .. ,

...

", •• , ., Il.'.111' ., ." •• ,., •••••• 111. ' ••••• , •• '1" ,. RN

Figure 1.12 Effeet of height of circulation on the delusina rate in RH

21

Further, increasing Q is effective in erasina the uneven distribution of carbon

concentration in the ladle which arises durin, the decarburizaûon treatment and which

causes a reduction in decarburizaûon rate. Another effecûve way to alleviate the uneven

carbon distribution is to have the pre-treatment [e] to be lowered to approximate1y 100

ppm. An average value of final carbon content, which is 20 ppm, can be obtained in an

18 minute treatment.

,

i

L __


1.2.3. RH-OB, RH-OB-FD and RH·PB Processes

• , ,

o." ..

o." .. Ar AI

RH • OB m. t ho d RH· OB • F D III' t h 0 d

FllUre 1.13 Schemalie of RH-OB and RH-OB-FD process.

The RH-OB is a new process relardin. the addition of new funetions to the RH

process (either for healing or decarburization), by injecting Oz to the vacuum vessel

throulh a tube situatecl above melt. The more aclvaneed RH-OB-FD involves injecting

~ throulh the inner tube of a double annular tube nozzle whieh is embedded above the

uplel, as shown in Filure 1.13, while supplyinl eoolant Nz or Ar gas to the outer

annulus. The advantalcs of the IWo processes are listecl in Table 1.11 "1'. In the

{

(

(


RH-OB-FD process, by oxidizing the AI in molten steel, bath heatina of about 3°C/min

is attained. The RH-PB process directly injects flux powder together with carrier gas

through the oxygen blowing (OB) nozzles provided in the lower vessel for the purposes

of heating and acceleration of dephosphorization and desulfurization (l9).

Table 1.11 The advantages of RH-OB and RH-OB-FD methods

RH • OB method RH - OB - FD method

Oxygen efficiency 70~ 80_

Splash violent medium violent

Dissolved oxygen content high low (after OB)

Construction cost inexpensive expensive at tuyere

1.2.4. YOD Process

The schematic of the YOD process is shown in Filure 1.14. Alter the lacIle has

becn placed in the vacuum vessel, the dome is moved into the closed position and the

vessel is then evacuated. OXYlen is injected into the laclle by top blowinl throulh a tube

as shown in Fig.1.14.

The YOD (vacuum oxygen decarburization) process was fint developed by the

Edelstahlwerke Witten Company of west Oermany in 1967. The spread of this pracess

and the progress of peripheral techniques enabled the mass production of ferritic stainless

steels as commercial steels.

The process is said to be capable of reducing the carbon + nitfOSen content of

18-30%Cr and 1-2~Mo steels to between ISO and 200 ppn'. It is reported that when

cou pied with forced stirring by argon las injection, the YOD process can reduce the

carbon + nitrogen content to under 140 to 100 ppm I20Il211. The important point to

he considered for YOD is to prevent the pick-up of cuban, nitroaen and oXYlen durinl

ingot casting.

1 •

T r k ~ , <' ~ (

~

t ~ l,-f î ;t u' ,. f

~ ~

~<

~ , ~ , \ ,

~"

y-


Ox y D • n D' • 1 n J • c t Ion

v. cu u m

Pr 1 ncl pl. of YOD pr oc ••• Flpre 1.14 Schematic of YOD process

24

ln lapan, Kawasaki Steel improved the YOD process by creating a stronger Ar .U stirrin. by providinl more than one porous plulS and securing elimination of more

than 1. of carbon, and they have been able to realize [C] < 2Oppm, [N] < 30ppm for

18" Cr steel. Subsequendy, they have improved the porous plugs funher to increase

{

(

(

ft


the Ar gas flow r.te and by developina a new process usina this new plug which has

made stronger stirring possible they have succeeded in producing extremely low carbon,

low nïtrogen 301 Cr steel.

There are some other processes for decarburization such as: ASEA-SKF and AOD

processes but their application for producing ultra-Iow carbon steel is limited comparecl

to the above four processes.

... 1.3. mE RATE BEHAVIOUR OF DECARBURIZATION

The kinetics of decarburization of liquid steel have becn extensively studied on

both laboratory and commercial scale. Nevenheless, the previous studies mainly

concentrate on the carbon content range above 20 to 30 ppm. Investigation on ultra low

carbon content range (below 20-30 ppm) under reduced pressure is rare.

Decarburization in BOF (Top Blowing Oxygen) accurs with amuIt that carbon

can be reduced from 4~wt to O.02~wt. As shown in Fiaure I.IS 1221, the rate of

decarburization in BOF can be divided into three periods according to their carbon

content.

1.3.1. Decarburlzatlon in The Fini Period

The first period refers to the beginning of steelmalcing, taking the BOF process

fOf example, the tirst period has a carbon content chanaina from 4 to 2.S" and the

temperature ranaing from 12S0 to 1400·C. Since the injected oxygen (mt reacts with

silicon instead of carbon, the rate of decarburization increases with a decrease of the

carbon content accompanied by a increasing melt temperature within this periode In the

BOF process, the silicon content decreases from 0.3" to O.IS-I, then remains constant

for the rest of the processll2l• For the production of ultra low carbon steel, silicon and

carbon are oxidized simultaneously, for instance, silicon cao be reduced to below 0.01"

in the extremely low carbon ranael22J.

. b Il


0.25 -c: -E ---c: 0.2 0

-• N

-=-0.15 ~ -• c.t

• ~-~ "a

- O. 1 0

• -• a:

0.05

o o 1 2 3

C. r bon con t • nt (%)

Flpre 1.15 The rate of decarburization versus carbon content in the BOF process

(

(

(


At temperatures below 1370-1 380°C, the oXYlen concentration in the metal is low

and the rate of the carbon elimination reaction is very slow. At 1380-1400°C, the

necessary energy for the formation of CO 115 an be obtained by increasina oxygen

concentrations to a value higher than the equilibrium value at these temperatures.

However, because of the hilh viscosity of the metal al these temperatures, conditions for

the diffusion of oXYlen into the interior of the bath are still not created and the reaction

of carbon oxidation proceeds only in the direct vicinity of the metal surface so that the ' ..

observed rate of decarburization is low. The surface boil of he bath causes the

Iiberation of very minute carbon monoxide bubbles which eject very minute droplets of

metal into the gas atmosphere. These minute droplets are rapidly oxidized by the oXYlen

of the gas atmosphere because of their large specific surface ara. On the other hand,

these minute metal droplets also result in an abundant formation of brown smoke.

An increase in temperature to over 1380-1400°c creatcs conditions for the

diffusion of oxygen within the interior of the metal. The carbon monoxide bubbles that

are formed are able to increase in sile durinl the process of floatinl out from the

reaction zone to the surface of the bath. The increase in the sile and the number of the

bubbles Iiberated from the bath is accompanied by an increase in the sile and the number

of metal droplets that are ejected from the bath by these bubbles. Their specific surface

area is greatly reduced, and the rate at which brown smoke is formed diminishes to a low

value. Most of the ejected metal droplets will fan back into bath. This large number

of metal droplets provides new sites for the decarburization reaction. There is sufficient

supply of carbon to the reaction sites in this fint period, and therefore, the rate of

decarburization increases as the decarburization re&Ction proceeds.

1.3.1. Decarburlzatlon ln The Second Perlod

ln the second period, as shown in Figure 111.1, the rate of decuburization venus

carbon content stays constant when the carbon content ranles approximately from 2.'. to 0.1 ~ in the BOF process. An increase in temperature and sdmn. rnay have a

r ,

,/

-

1


favourable effect on the rate of diffusion of oxygen into the dceper layers of metal and

for the enlargement of the carbon monoxide bubbles liberated from the metal. Since

there is a sufficient supply of carbon, the overall rate of decarburization is controlled by

the supply of oxygen, and moreover, if the flow rate of oxygen injection stays constant.

the observed rate of decarburization stays constant a'5O which, in other words, means the

observed rate of decarburization obeys zeroth order kinetics in this period.

1.3.3. Decarburizatlon in 'Ibe 'Ibird Period

ln the third period, the observed rate of decarburization is proportional to the

carbon content which ranges approximately from 0.1" to 0.003". The critieal value

of carbon content, which is the carbon content from which the second period transfers

to the third period, lies between 0.1 ~ and 0.2~ or O.07~ and 0.1" depending on the

conditions 1»1 mentioned before. In commercial production, il lies between 0.1" and

0.2'1 or 0.2'1 and 0.3'1, but sometimes mayeven reach 1.2" to 1.0". The critical

carbon content is controlled by: (a> the flow rate and the means of oxygen injection; (b)

the stirring in the molten steel and (c) the mass transfer coefficient. The critical carbon

content increases as the tlow rate of oxygen and stirring increase.

Since the carbon content i5 greatly reduced after the tirst and second period,

which means that there is not sufficient sapply of carbon to the reaction sites any more,

and the oXYlen content is much higher than its equilibrium value for the corresponding

carbon activity of the molten 5tetl, most of the decarburization tends to occur at the melt

surface instead of the interior of the liquid metal. Therefore, the rate of decarburization

is most probably controlled by the rate of mass transfer of carbon in the liquid phase

with the result that the rate of decarburization is proportional to the carbon content.

(

(

(

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CHAPTER 2 LlTERATURE SURVEY 29

CHAPTBIlI'WO

LITERATURE SURVEY

2.1. niE COMMERCIAL PRODUCTION OF ULTRA-LOW CARBON STEEL

Recently, demands have arisen for a new. more effective method for removina

carbon and nitroaen from molten steel. This is because it bas become eviclent that

carbon and nitroaen need to be held to as low a level as possible, especially lower than

10 ppm, in order to improve deep drawability of thin sheetf241.

The initial carbon content lies within the ranae of loo-4OOppm in producinl ultra

low carbon steel by RH and OH. The final carbon content by RH tratment in 9

Japanese companies lies within the range of 100JOppm for a treatment time of 10 to 25

minutes. The final carbon content by DH treatment lies within the ranle of 3O-100ppm

for an operatina time of 20-30 minutes. Table Il.1lists the reponed methocIs in mous plants for ultra low carbon steel production.

Low nitl'OJen contents in steel are promoted by hiah araon flow rate and

decarburization. a aood tiahtness of the vessel and, as far as killed steels are concemed,

low sulfur content in the liquid metal. However, it seems impossible to reduce the

nitroaen content when the nitfOlen content is 20 or lOppm before depssinl of low

alloyed steels 1:!51.

" CHAPTER 2 LITERATURE SURVEY 30

Stainless ultra low carbon grades, with carbon content Iying between 10 and

30ppm at the end of decarburization, are produced by the AOD or YOD route. The

VOD route seems to be more favourable than the AOD route because a high nitrogen

pick-up can be observed during the AOD-Iadle tapping. The nilrogen conlent at the end

of ladle metallurgy in VOD lies within the range of 10-40ppm. Low-alloy, ultra low

carbon grades with carbon contents between ISppm and 30ppm are produced by vacuum

ladle metallurgy treatments such as RH and DH for higher productions and ladle furnace ,~

or ladle degassing tank and YOD for smaller productions. The efticiency of thcsc

vacuum decarburizing treatments depends on:

(1) the carbon content before vacuum treatment;

(2) the vacuum treatment duration;

(3) the ultimate vacuum level and the pumping unit capability;

(4) the total volume of argon injected into the vessel;

(5) the cross section of snorkels.

National Steel Corp (U.S.A) has launched a 10 year program by investing $1.8

billion for the construction and modemization of its RH vacuum degasser. The new RH

vacuum degasser at Great Lakes Division near Detroit will be able to produce a 240-ton

heat of ultra low carbon steel every 35 minutes total operating time. Moreovcr, il will

improve the productivity by enable the steelr.'laking operations to work more smoothly

and efficiently and increase refractory life of the BOF's by reducing the need to lowcr

the carbon content of steel in the BOF's 1261.

CHAPTER 2 LlTERATURE SURVEY 31 (

Table Il.1 Reported methods in various plants for ULC steel production

References Equipment [C]. (ppm) [C], (ppm) Treatment Initial C Final C Time (min.)

KSC Chibal27J RH (240 t) ISO 20 - 30 2S '.,

IS - 20 2S

20 10 10 15 - 20

KSC Mizushimal281 RH (275 t) 300 - 400 SO 20

15 20

10 13

Sumitomo Kashimal291 RH (2S0 t) 240 - 300 20 IS

NKK KehinllOllll1 RH (2S0 t) 200 ., 300 20 20 - 25

100 - 120 10 IS

NSC RH (100 t) 300 - 400 60 IS Hirohatal32l1JJIIJ"1

200 - 300 25 IS

200 - 3S0 16 20

9 20

NSC Nagoya 21351 RH (2S0 t) 400 < 30 20

NSC Oital361 RH (3S0 t) 300 - 400 30 20

NKK Fukuyamal)71 RH (260 t) 200 - 300 15 IS

10

Kobe Steel RH (240 t) 120 - 300 20 KakogawalJl1

NSC YawaralJ91 OH ISO - 370 S5 19

( 29 19

CHAPTER 2 LITERATURE SURVEY 32

References Equipment [C]o (ppm) [C]f(ppm) Treatment Initial C Final C time (min)

Krupp Reheinhaussen f40J OH (300 t) 400 90 30

60 30

OH (120 t) 400 50 30

100 20

... 75 20

50 20

OH (400 t) 400 45 20

Nisshin Steel Kure141) VAD (90 t) 270 - 340 20 - 40 25

Daido Steel LF (35 t) 100 - 180 30 - 50 10 Hoshizakil421

Sumitomo Centre de YOD (1.5 t) 150 10 s 5 30 - 40 Recherchesl4'1

NICK Keihin12IJ VOD-VAD 20 (SO t)

Fried. Krupp Gas injection 150 - 300 20 - 75 Hüttenwerke AG Degasser Bochuml4411451 (120 t)

KSC Nishinomiyal461 YOD (SO t) 0.4 - 2% 1) 30 - 150 30 - 140 2) 3 - 10 70 - 120

Sumitomo Centre de VOD(l.St) 300 10 160 Recherches(4?J

Nihon Kinzokul41' AOD (SS t) 3-S" 10 - 30 80 - 130

(

(


2.1.1. The Carbon Content Berore Vacuum Treatment.

Natural decarburization, that is, decarburization without vacuum and oxidizinggu

injection is possible if the steel carbon content at tapping from the BOF or EAF (electric

arc furnace) is higher than 400 ppm. If the carbon content is less than 400 ppm, it is

necessary to suppl Y oxygen to promote decarburization, via an oxidized slag or throulh

oxygen blowing by lance or OB tuyere.

a. ae , ....... ".1 2, 000 2,000

, 9 1 , 000 :"'1 , 0 0 0

d f 500 5 a a

b

200 200

100 (. 100

-

d

•• ., .. Il

. ~" hltl ll ... '.I.'IIl . ..I.1 ' ......... .. , " / ............ c . '" " b

•. , 1'" ....... 50 50 ~--------~------~----------~----------~ 5 10 20 50 100 200 500 1,000

1 •

• ,uilibliu. betveen Ict!yity of cllbon lad oay,e. belore lad afcel y.cuu. decarburllicloi.

Filure Il.1 Equilibrium between activity of carbon and oxygen before and alter vacuum decarburization.

The carbon and oxygen equilibrium, before and alter vacuum treatment, is shown

in Figure Il.1 (011' for the production of ultra low carbon steels in a 30 ton LF vacuum

refining unit. Since no deoxidizers such as silicon or aluminum were aclded durinl

degassing, carbon and oxygen activities sealter around the line of Pco ID 1 atm before

.. ..

l,

,,-,

CHAPTER 2 LITERATURE SURVEY

depssinl. The equilibrium values for ollYlen and carbon .ner vacuum tratment differ

very much dependinl on the initial carbon content and ollYleII content. For 600 ppm

carbon initialty, the decarburization l'aCtion comes to a halt at 300 ppm due to the lack

of ollYlen, which indicates that the addition of ollYlen is required for the reaction to

further lower the carbon to below 1 S ppm 1491.

2.1.2. 1be Duratlon or The Vacuum Treatment

Il __ ----------------------------------------~ • t ' < '"'' --• • .. -•

"i -• • --• • • -• • ~ -:1: •

1'" 11.. 1 •• 7 1 •• 7 1 ••• 1,.' t ,.,

" .• , '7. U' ".., (7. U, " .• , (7. Il' Flpre 0.2 The reeluetion of RH treatment time durinl the last seven yan.

Currcnt typical vacuum treatments last bctween 15 to 25 minutes in the RH

process. USually, a lORaer vacuum tratment improves the reduction of carbon 10 ultra

low level. However 1 such a lonl vacuum treatment willlead 10 an increued temperatule

drop in the RH unit and pose problems with respect to production efficicncy and cost

(

(

(


no heating of molten steel occurs in the RH unit. In the past, the production of ultra low

carbon steel in the RH process took about 40 minutes, and the temperature of steel

tapped from the con vener was higher than 1680°C. With research kl1d development, the

gas injection conditions improved and the decarburization time was shonened ta 10-15

minutes with the result that the RH treatmcnt time and temperature drop were reduced

extensiveJy. Changes in the entire treatment time for deoxidatior and chemistry

trimming are shown in Figure n.2. Figure Il.3 shows how the tap temperature5 have

been reduced over the past five years 1501.

" .70

-Co) 1, •• 0

• • .. ca • 'a

• .. ::1 -• .. • IL

1, '50

E ','40 • -IL

• ... 1, '30 '4 • 5 •• 17

VI., •

Filure Il.3 The reduction of tap temperature during the last five yeuse

Il

, r

-.. ,.

CHAPl'ER 2 LITERA TURE SURVEY

Owinl to the addition of a1uminum at the time of tappina, the tappina temperature

drop can be œduc:ed by about lOoe by addinl 0.4 (ka/tonne steel) Al. Tappina

temperatures which had had to be 1680°C or hiaher. were reduced to 1640°C, remllina

in an extension of the service life of refractory materials in the convener, IlCIle and RH.

ln Kawasaki Chiba Works, an over all avenge tap temperature of 1615°C for the Q-BOP

bas been achieved 1531.

Z.1.3. The Minimum VlCuum Lewel, ne C .... Seetlon 01 The S ..... and

The Araon Flow Rate.

A vacuum of 0.3 torr can be attained in S minutes aRer the start of the RH

treatment by raisinl the speed of evacuation 15. J • To promote the decarburization by

the RH rimmina treatment, it is necessary to accelerate the transfer of carbon from the

ladle to the vacuum vessel by increasinl the molten steel recirculation rate (Q) and to

increase the volume mus transfer coefficient (ka) of the decarburization reaction

occurrin, in the vacuum vessel and the uptake snorkel. Accordina to a recent

measurement, the recirculation rate (Q) is proponional to inner diameter (Di) of the

snorlœl and Ar flow rate (0) raised to the powen of 1.8 and 0.1, respectively. Thus,

an increase in Q wu more ICIvantaaeously attained by increasina Di' 50 that snorkels

with a diameter u larae u 60 cm have been ua. On the other hand, the most effective

way to increase Je. wu to increase 0, whereby adoption of a flow rate of SOOO IImin wu

reported. At'ter meuuleS were taken to increase Q and k., lC) < 15 ppm was attained

with a 20 minute RH treatment. Furthermore, the increase in Q effectively erucd the

uneven distribution of carbon concentration in the Iaclle which arose durin. the

decarburization treatment, and causecl a reeluetion in the decarburization rate.

Moreover, il wu found that the dqrœ of vacuum in the chamber had no effect on the

ciJeulation rate, 50 lonl u the pressure wu between 0.3 mmHa and 20 mmHa 1521•

OH unilS are as efficient as RH units if arion las is injec:ted into the DH vessel

with fIow Tates equivalent to those injected into RH. If the decarburizina treatment is

(

(

CHAFTER 2 L!TERATURE SURVEY 37

carried out in a vacuum tank, the ladle mllst be equipped with several porous plugs and

have a large freeboard to allow high argon f10w rates to be UsedISlI•

2.2. CURRENT PROBLEMS IN mE MANUFACTURE OF ULTRA LOW

CARBON STEEL

The most suitable devites for a mass production of ultra low carbon steel are RH

" and OH degassers for ordinary steels and YOD units for stainless ultra low carbon steels.

It is very difficult to produce ultra low carbon steels with carbon lower than 10 ppm

using present RH technology. Even if it was possible to produce such extra low cubon

steels by using a longer RH treatment, it would be very diffieult to provide uitable

starting temperature needed for continuous casting because of the temperature drop due

to the increase in treatment timel2"1•

The following aspects should he considered for the funher improvement and the

general adoption of vacuum refining processes for ultra low carbon steel production.

( 1 ) Reduction of tatment costs:

• shonening of treatment time;

• proper selection of refractories;

• simplification of the refining system.

(2) pursujl of the eçonomy of the refininl system as a total system:

• optimum combination of hot metal treatment;

• BOF operation and ladle trcatment;

• making steel produet manufacturing processes continuous and direct.

(3) ImprQyemcOl of the refinjnl dClra:

• prevention of contamination .fter vacuum refininl;

• measures to effectively reduce the nitrolen content of steel;

• addition of molten steel heatinl and si .. refininl funetions.

Vacuum retining processes which overcome these technical problems are expected

to bccome widespread. For the funher reduction of carbon content durinl the production

r


of ultra low carbon steel, the rate of decarburization reaction is crucially needed to be

accelerated.

2.2.1. Sourees or Carbon Contamination 1 Pick-Up

• .. .. -Co)

'10

, , ' , , Il .,,, Be ,ft ck

., , "+ Mt 0 • DI ••• l ••

5 ~~--~~--------~----~----~----~--~ o 2 4 • • 10 12 14

TI m. u ft d IF ta c U U III (III ft. ,

Flpre Il.4 The influence of slag line bricks on the rate of decarburization.

Sources of carbon contamination are numerous and can lead to considerable pick

up of carbon. Most of the sources of contamination are as follows:

(1) RcfracIQriC$;

Dolomitic ladle refractories can contain up to S - 9% graphite, MgO-C

bricks for slll Iines up to 17~ graphite. Cl' ...... content of powder for

lad1e sUde late lies between 0 and fi". Carbon pick-up can also accur in

the tundish due to the contact of steel with glue or phenyl resins from the

(

(

(

CHAPTER 2 LlTERATURE SUR VEY 39

refractory layer, especially at the beginning of castina. Casting and

shrouding refractories, composed of AI20 3-C.,.,..." can also be a source of

contamination of the metal. Figure Il.4 shows the decarburization lcinetics

for two different kinds of slag line bricks.

(2) ~

(3)

(4)

Risks due to ladle metallurgy slag are generally negligible, bùt it is not the

case for the tundish slaa. The usual rice ash powder contains lS lO 20"

graphite, but also special ones containing about 6" C ...... caii cause non

reproducible C pick-up, up to 20 ppm, even during steady state, 50 that

a C-free tundish powder must be recommended. Mould sial usecl al the

beginning of casting must also he free of graphite.

FeuoallO,ys and sçrap:

Feuoalloys and also uncontrolled scrap an cause large carbon pick-up.

For example, 1 kg/t low carbon ferromangnese with 1.S" C bring about

15 ppm C to the steel.

Contamination due ta the tXRC of proce.s jnvolved:

For the RH or OH process, the skull which forms in the vesse! due to

splashing under vacuum is a source of contamination of the decarburized

steel, especially after killing. The graphite resistor which is usecl for

keeping the refractory walls at a higher temperature can represent a supply

of about ISO ppm of carbon to the metal. In RH-OBN or OH-OB vessels,

C pick-up occurs during oxygen blowing due to cooling fuel crackina. In

the ladte fumace, C pick-up can also he caused by arc healinl, and il may

be necessary to modify the electrical operating conditions to avoid it .

--

CHAPl'ER 2 LITERA TURE SURVEY 40

2.2.2. Acceleration or The Rate or Detarburlzation

ln the production of ultra low carbon steel, vacuum decarburization to the desircd

level of carbon content takes 50 lonl a time as to cause a drop in steel temperature and

effons to compensate for the temperature drop by an increased tapping temperaturc of

the convener results in a higher steelmaking costs including those for refractory

materials. On the other hand, reducing the carbon content in the steel just before tapping

50 as ta shonen the time required for the vacuum decarburization increases the oxidation

loss of iron, thus lowerinl the steel yield. Moreover, a considerably longer processing

line after tapping causes difticulty in performing \.ontinuous-continuous casting

(continuous casting and annealing). Therefore, it is necessary to accelerate the rate of

decarburization. The methods for accelerating the rate of decarburization which have

becn used in commercial seale are as follows:

(1) Production of ultra low carbon steel by a combined process of bottom-blown

convener and RH delasser. In the bottom blown converter ail the retining oxygen gas

is injected from the bottom to agitate molten steel intensively, allowing decarburization

to an adequately low level of carbon content while suppressing the oxidation 1055 of steel

bath, in compari5On with the conventional lop-blown converter. This is a highly efficient

and economicaJ technique in makinl ultra low carbon steel via RH vacuum

decarburization of molten steel containinll00-200 ppm carbon derived from the bottom

blown convetter. The reduction of carbon concentration to 20ppm or less within 10-15

minutes of the decarburization treatment in RH was established at Chiba Works of

Kawasaki Steel 1521. The ultra low carbon steel can be consistently produced in a total

treatment time of 24-25 minutes, allowing continuous casting. The tap temperature can

be lowered from about 1680 to 1630°C due to the reduction of vacuum treatment time

in the RH process 1'1).

(2) As mentioned in ~tion 2.1.3, enlargement of the inner diameter of the

snorkel is effective in enhancinl the rate of decarburization reaction by promoting

increased circulation of steel. Unfonunately, it al50 caused the reduction of effective

i ____ ~ ~ __ ~ _

(

(

(


refractory thickness and resulted in lowering the vessellife. Moreover, it wu found that

the degree of vacuum in the chamber had no effeet on the circulation rate, 50 lonl as the

pressure was between 0.3 mmHg and 20 mmHI I52I•

(3) Prevention of argon leakage and reduction of [C] analysis lime. Ar injection

was practised through Ar brick, but sometimes Ar leaked to the lower vessel throulh the

backup lining of the refractory or through the joint instead of flowing through the porous

inserl. This Ar leakale not only caused a reduction of the circulation rate of steel and

a reduction oflhe interfacial faction area, but also deterioration of decarburization

rea('tion rate. To solve the above shortcomings, Kawasaki Steel Corporation adopted the

method of Ar injection through stainless steel pipes 1501.

Ar

Ty p. 1 Ty P' 2

Filure Il.5 The principle of pourin. stream equipment for vacuum decarburization.

'"

CHAPTER 2 LITERA TURE SURVEY 42

Table Il.0 Pourinl stream experiment conditionsl~1

Molten steel IS kg/heat. 16S0 - 1700°C

Ar o - 200 NI/min.

Noule diameter 10 mm

Pressure in chamber 20 - ISO torr

Experimental time about 15 seconds

(4) The decarburization treatment time is made up of the decarbul'izabOn reaction

time, the carbon analysis time, and the deoxidation time. Reduction of the time for [Cl

analysis can lead to the reduction of the decarburization treatment time. Kawasaki Steel

Cooperation has established the method of on-line [C] estimation by measuring the fr~'C

oXYlen content in steel and has reduced the [Cl analysis time by 2 minutes.

(S) One of the lat est developments was to accelerate the rate of decarburization

and nitrolen removal by powdered oxidizer injection under reduced pressure I·HH\1I.

The powdered oxidizers were mainly iron-ore (F~O,) and manganese-ore or Mn-oxide

wbich was scale from producinl thick plates. The penetration distance of oxidizer

particles below the free surface of the molten steel was calculated to be more than 70 mm

in the relion immediately under the lance by K. Shinme et al14JI•

ln the powdered oxidizer injection method, it was thoulht that the decarburization

rate lreatly increased because the blown and penetrated oxidizing panic les acted as

sources of oxyaen, (F~O,) - 2[Fe] + 3[0], and provided fine nuclei for the formation

of CO bubbles in the molten steel. In spite of low oXYlen content in the molten steel,

the oXYlen content just around the oxidizer particles in the molten steel was extremely

hiah. Thus, it was considered that the decarburization rcaction was accelerated at the

oxidizer panicles where the resistance to the formation of CO gas bubbles was relatively

low because the particles acted as nuclei.

(6) ln order to accelerate the rate of decarburization, vacuum refining of a stream

of non-cleoxidizecl steel formed durina pourinl was studied under laboratory conditions.

Takahashi et allMI reponed tbat in their pourinl stream experiment about 40-75"

(

(

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CHAPTER 2 L1TERATURE SURVEY

carbon wu removed by CO from steel in a reaction time of around 0.1 second.

Schematic rcpraentations of the experimental apparatus and experimental conditions are

shown in Filure Il.5 and Table Il.11 respectively. In their experiment, 50 ka of moIten

steel wu poured into a tundish located on the top of the vacuum charnber. As shown

in Fiaure Il.5 (Type 1,2), aluminum sheet wu used 10 sai the bole of the nozzie .. al

the boltom of the stream by two lnethodl durina pusaae throuah the nozzle 10 the

camber. Due to the rapid expansion of Illon bubbles injected into the liquid .... as il enteRd the vacuum chamber, the liquid steel wu broken into fine droplets. The

droplcts were collected in a container placed at the base of the chamber. SImples for

chcmical analysis were withdnw from the tundish by silica tubes and later by machinin.

cast blocks. Hieh speed photography found that the stream of liquid steel wu dispened

wilh man droplet diameter of 0.5 mm and that ilS fallina velocity lay in the ranle 6-7

m/sec. The drop in temperature of liquid steel durin. ils fall wu eslimated Ilaround

50°C, because radialion could completely dominate the heat loss. Assumin. that

decarburization rate wu determined by the transport step of carbon in liquid steel, the

mus transfer coefficient of carbon Kt = 0.04 cm/sec wu obtained.

Sokoloyl551 investieated the process of vacuum decarburization and cIeoxidation

of a stream of 10w-carbon(O.04~-o.07~C) steel. The experiment provided for the

vacuum clecantine of small portions of steel in such 1 muner chat the refinina proœslel

wen: completed while the stram wu fallin. and that solidified drvplets of metal enteNd

the reœptacle. In orcier to obtain a non-oxidizin,ltmosphere, the chamber wu tlushed

with hi,h-purily araon before the metaI wu decanted and the workinl vlCuum wu

created. The main mus of the dropletl fell within the sile ranle of 0.5-1.0 mm. There

wen: almost no droplets biller than 1.5 mm. For the total fin time of 0.2. seconds, the

stream passed throuah the tirst half of its path in 0.15 seconds and the second put in

0.09 seconds. The calculated rates of oxidation of carbon were 8.6 and 2. "'C/min

respectively. The coefficient of mus transfer (01&) with vacuum decarbUriZltion wu

6.5 x 10.3 cm/sec. The average clearee of carbon elimination wu 40" for 25 mmH.

vacuum, 57~ for 4 mmHg vacuum. Sokolov furtller explained that the exllemely hilh

CHAPTER 2 llTERATURE SURVEY

rate was due to the following reason: "A aggregate of bubbles should have incidentally

developed into a twin-phase bubble-film structure having continuousty renewabte reaction

surfaces when the Jases are liberated in the cavities of the bubbles which are rapidly

growing from the gas nuclei within the interior of the freely t'alling metal. As distcnded

droplets, the coefficient of mass transfer KL =D/<\ in such a structure must have rcachcd

a very high value since the thickness of the diffusion layer 8 was close to the value of

zero. "

2.3. PREVIOUS LABORATORY RESEARCH ON THE KINETICS OF

DECARBURIZATION

Previous research on the kinetics of decarburization concentrated on five arcas:

(1) The mechanism of decarburization with oxidizing gas.

(2) Mathematical modelling of the kinetics of decarburization.

(3) The influences of some elements on the rate of decarburization.

(4) The circulating characteristics and stirring efticiency.

(S) The nucleation of CO bubbles.

2.3.1. The Mechanlsm or DecarburizaUon Wlth Oxidlzlnl Gas

The behaviour of the rate of decarburization changes with the change of carbon

content. Previous research on the kinetics of decarburization can he divided into a high

carbon content region ([C] > 300 ppm) and a low carbon content region ([e] < 300

ppm).

As mentioned in section 1.3, the kinetics of decarburization at high carbon content

have been extensively studied on both laboratory and plant seale and the resuns are

similar 15tIS7JlSl,DII01I61 1162J(6',.

As shown in Figure Il.6, in high carbon content, the temperature dependence of

the rate of decarburization is more sianiticant than that in low carbon content in the

(

(

(


experiment done by T. Watanabe et a1(2',.

Ct '4 Ct .. Ct ..

, . ... , .... "'~',' v .. <H.$ ..... !-~~ .. _'

" " ., ",',O"{ 4'.f .'~ -~ , 2

'Ë -Co) ~ , 0 -~

0 8 -•

N .. :1 ~ .. • u

• 4 ~

-0 2 • -• a: o

o 0.2 0.4 o .• 0.8

1 C 'ft) Figure Il.6 The rate behaviour of decarburization al different temperatures.

It was observed that the rate of decarburization followed first order kinetics down

to about 30 ppm in RH systems, then, il transfered to some order hiaher than the tint

order at carbon levels below 30 ppm'6I. to 20 ppm.

A crucible decarburization experiment with CO-C02-Ar gas mixture blowinC onto

the liquid iron surface has been reponed by Harshima et al '65'. As shown in Filure

Il.7. the behaviour of decarburization reaction was divided into three. Ac:cordinlly, in

the range of carbon content hiaher than 200 ppm, il wu described as the zeroth order

reaction with respect to carbon content. In the ranle of carbon content


200

100

• .. .. 50 . ~ -g

20

10

:e TI",. (",1 ft)

t f r ,

Fllure D.7 Carbon content change versus time in ultra low carbon steel production.

between 20 ppm and 200 ppm t it was described as the first order reaction with respect

to the carbon content and in the case of carbon content less than 20 ppm, the rate of

decarburization was tnnsferred into a higher order of kinetics and ils mechanism in this

region was reponed as unknown.

Norio SumidalS2J discovered a similar phenomenom by indicating that the rate of

decarburization drops abruptly in the vicinity of a 30 ppm. The rate equation is

uansfered into higher order as shown in Figure Il.8. As Norio Sumida explained, it was

difficult to attribute this drop in rate to the approach to the equilibrium. This

phenomenon seemed to be due to a decrease in evolution of CO gas owing to the

reduction of carbon concentration, apparently caused by the lowered agitation of molten

steel in the vacuum vessel through the suppression of CO boiling and the reduccd

interfacial area for reaction.

(

(

(

---------------


2 0 0 ........................ '" ...... ~n. ••• • n. '"_._.-._-_ ........... -....,!-t:" ......... ~

\,' .......... ~ ..... , .. .... .. .. 100

50

-u 20

.' '

5 10 15 u so TI Il' (III ft)

Figure Il.8 Carbon content change versus time in ultra low carbon steel production.

The behaviour of the carbon and oxygen contents with powder oxidizer blown bas

been studied by Kaoru Shinmel"'I. The constant value of the rate of the decarburization

obtained was around 0.00226 sec'· at the relion of [C] > 30 ppm, and it decreased with

the decarburization reaction, when the carbon content was lower than 30 ppm, with final

carbon content reaching 2-3 ppm after 80 minutes vacuum treatment time. He funher

explained that the reason why the vacuum decarburization treatment was not 50 effective

for the acceleration of the decarburization rate was due to the lower oXYlen content

during the treatment. An experiment by Kaoru Shinme (661 on stainless steels with CO

nuclei formation in the molten steel by blowinl powdered oxidizer (Cr20,) in a 2.5t VIF

(vacuum induction fumace) has shown that the decarburization reaction rate equation

changed from tirst order into hilher order when the carbon content was around 30 ppm.

1.3.2. Mathematical Modelllni of the Klnetics 01 DecarburizatloD

CHAFfER 2 LITERATURE SURVEY 41

Most of the previous studies believcd that the rate of decarburization is controlled

by one of the followinl three steps or a mixture of them:

(1) The rate of mus transfer in the au phase.

(2) The rate of interfacial chemical reaction on the reaction surface.

(3) The rate of mus transfer in the liquid phase.

1be mathematical modellinl of the kinetics of decarburization has bcen the subject

of many investiptions. Mou of the investiption results show that the mechanism of

decalburization chanles accnrdina to the chanle of carbon content. When the rate of

decalburization wu zeroth order with respect to the carbon content which, in most eues,

is in hilh carbon content the rate of decarburization, it wu either COtltrollcd by the mus

traDsfer in the Ils phase or by the interfacial raction rate, or more likely a mixture of

the rate of mus transfer in the au phase and interfacial reaction rate. When the carbon

content wu reduced to a level lower than 0.1 5, the rate of decarburization wu chanled

to the fint orcier with respect to the carbon content and the rate of decarburization wu

mainly controlled by the rate of mus transfer in the Iiquid phase.

(1) Gas phase mus transfer

For the decarburizatÎon with C~/CO or other oxidizinl las blown onto the

reaction surface, the flux of CO2 or other oxidizinllas to the surface is liven by:

(11-1)

where the terms are clefined as:

p. cm the C~ pressure in equilibrium with the melt,

Kc02 the mass transfer coefficient of C~ Ils in the las phase.

This equation bas been used by Fruehan, Swisher, Baker 1561, 0010, Nomura, Lee 16'11

and Distin (57) in their studies.

(

(

(

CHAPTER 2 LITERA TURE SURVEY 4' rcaction surface is given by a simplified form if the panial pressure CO las is far below

1 atm:

(0 .. 2)

Similar equations have been used by Harris l'l', !to 169) and many other investigaton

in their vacuum refining studies.

. ...

(2) Mass transfer in the liguid phase

The simplified rate expression is given as follows by assuming that the rate of

mas:, transfer of carbon in the Iiquid phase is the rate controlling factor:

de" AD - - ---(C.-C) dl V 6 1

(D-3)

where the terms are defined as:

A: the area of the reaction interface (cm'),

V: the volume of melt (cm2),

D: the diffusion coefficient of [Cl in the melt (cm1/sec),

0: the thickness of the diffusion layer in the metal,

Cb: the carbon content inside the molten steel (mol/cm'),

CI: the carbon content at the reaction sites (mol/crn').

This equation has been used by Watanabe 12') and many other investi.ators

POII\211J1JIiIJln l. Table Il.111 lists the coefficients of decarburization of liquid phase rrom

expcriments by different investigators.

According to Winklerml the mass transfer coefficient of CO in the lU phase

could range from 0.3 cmls to 0.03 cmls for boiling steel melt and non-boilinl steel melt.

(3) Gas-metal surface reaction

Previous research on decarburization by blowing CC>,/CO ,as showed that at hi,h

p ..

r , t , f • ,. ; ;. {

p

1 1, .. i • ! 1 ,

CHAPTER 2 LITERA TURE SURVEY

carbon content the rate of decarburization is tirst ,,'rder with respect to C~ gas pressure

and zeroth order with respect to the carbon content. Swisher and Turkdogan 1&\1 proposcd

that their observed deearburization rates are attributable to a slow gas-metal surface

Table Il.DI The coefficients of decarburization obtained by different investigators

(D) (a) (Kt.> (K,) Diffusivity Thickness mass reaction rate

of boundary transfer constant layer coefficient

... in liquid

Watanabel231 4.7xIO·' O.OO3cm 0.015cm/s cm'/s

(16S0 C)

NadifP1J O.ll/min

Sumidal'ZI 0.09-0. lS/min

Kazuumi 0.02cm/s Harashimal6SJ

H. Konda et allSOI 0.1 - O.2/min

KaON Shinme et O.136/min all"l (powder

injection) O.~~/min (oxygen injection)

O. Winkler (731 6x 10.5 2.Sx 10·lem cm'/s

Kenji TakahashilS41 O.04cm/s (pouring stream)

reaction with the retardinl cffeet of chcmisorbed oxygen on the reaction surface. They

reported that, for the decarburization of Iiquid steel by CO2-CO mixture, the dissociation

of adsorbed carbon dioxide via an activated state (C~· would appear to be the rate

controllinl stcp:

(

(

(


(0-4)

(0-5)

The rate equation becomes as follows:

(0-6)

The units of dnc/dt (g-mole Co, per sq cm sec) may be converted to dlC/dt, living:

dffoC __ 1200 K'(l-6>P dl pH co

where the terms are defined as follows:

p

H

density of liquid iron,

depth of liquid iron,

K' isothermal rate constant,

( )-8) the fraction of vacant surface sites,

P cm the panial pressure of CO2 in the reactant gas.

(11-7)

Similar equations were derived or used by Nomura Poil, El-Kaddah (751,

Sain(76l , Lee167), and Ooto(77).

At low carbon concentrations ([C] < 30 ppm), carbon and oXYlen in molten iron

react with each other at the gas-melt interface. It is expected from thermodynamic

calculation that decarburization reaction shO'Jld produce a CO/C02 mixture.

2.3.3. lbe Innuence or Some Elements on the Rate of DecarburizatloD

Sain et al 1761 have investigated the influence of some elements on the rate of

decarburization and concluded that the elements Si, P, Pb, Cu, Cr, B and AI were found

r i

'f


to have no measurable effeet on the rate. An experiment with 0.5 wt% Si resulted in a

definite film on the iron surface but. otherwise. il is c1ear that Ihese solules did not result

in a sianificant depression in the rate. Sn did result in a depression of the

decarburization rate. For example, at 1330°C, 0.2 wt% Sn reduced the mie of

decarburization by about 30 pct and 0.5 wt pct Sn reduced il by about 65 pet. Clearly.

however. at the low Sn concentrations usually encountered. it is unlikely that Sn would

have a measurable effect.

Halden and Kinaeryf71l, and many others, found that sliifur markcdly lowers the

surface tension of iron. For the Fe-S binary solution. the Gibbs adsorption equation is:

"C, a" r, - - RT a9LC

1


r.

" excess sulfur surface concentration in atoms per sq cm,

surface tension in dynes per cm,

"CI sulfur concentration in wt pct in iron.

(11-1)

Since the surface tension" can be reduced by increasing sulfur content, which

means a",a"C. is less than zero, thus, r. is areater than zero which means a positive

surface absorption reaction could oc:cur. Funhermore, by invoking the Gibbs adsorption

equation, the excess sulfur concentration is calculated to be about 7.5 X 1014 sulfur atoms

per sq cm for the liquid with more than 0.01 wt"S which presumably corresponds to

monolayer coverqe.

Sain et al. also found that trace amounts of sulfur depressed the rate of

,decarburization and it was shown that this depression was in good accord with a simple

surface bloctaae mec:hanism with ideal adsorption of sulfur. They have donc a few

experiments with traces of suif ur added to pure iron (0.5 ma FeS for 10 gm Fe) which

showecl a reduction in the rate by a factor of about two. Figure Il.9 shows the effeet of

suif ur on the raIe constant at two different temperatures: 13S0°C and 1450°C. They

1 1

,

1

(

(

(

CHAPTER 2 LlTERATURE SURVEY

experiments with traces of sulfur added to pure iron (0.5 ml FoS for 10 lm Fe) which

showed a reeluetion in the rate by a factor of about two. Filure Il.9 shows the effect of

sulfur on the rate constant at two different temperatures: 135O°C and 1450°C. ne, have further explained the residual rate of decarburizatiOll in the presence of sulfur by

proposinl that there are sorne surface sites which cannot be blocked by the chemilOlplion

of sulfur. Such a phenomenon milht be a result of the stirrinl effect of induction fcnes

in the melt, or the penetration of the chemisorbed sulfur layer by C~ molecules which

arrive at the surface with a parlicular orientation, or residual disorder, or misfit in the

chemi!l'lbed surface layer itself. If the dissociation of CO:! is assumed to occur only on

the fraction of surface which is IlOt covered by sulfur, absorption of oIher COIIltituenlS

is assumed wak, and the ideal Lanlmuir absorption isotherm for sinlle site occupancy

holds, wc may write:

(0-9)

where the terml are defined as follows:

, the fraction of surface which is covered by sulfur,

K. the absorption coefficient for sulfur,

a.: the activity of sulfur.

More decarburization experiments with varyinl carbon concentration al 1400 and

15OO°C have been done by Sain et at 1761• The results show considerable experimental

saauer at the lonler times but are consistent with an increasinl rate as carbon content

decreases. sUllestin, that the interference by sulfur decreues as the carbon content

clecreases. Values of the apparent idealldsorption coeffICient for sulfur in the carbon

saturated alloys were estimated to be about 450 and 600 (wt petri at 1450 and 1350°C,

feSl*tively.

Harashima et al 1651 found in their experiment that the adsorption coefficient of

sul fur • at 1600°C was liven as follows: ln the range of carbon content hilher than 200

-t'/

...


have funher explained the residual rate of decarburization in the presence of sul fur by

Proposinl that there are some surface sites which cannot be blocked by the chemisorption

of sulfur. Such a phenomenon might be a result of the stirring effect of induction forces

in the melt, or the penetration of the chemisorbed sulfur layer by CO2 molecules which

anive at the surface with a particular orientation, or residual disorder, or mistit in the

chemisorbed surface layer itself. If the dissociation of COl is assumed to occur only on

the fraction of surface which is not covered by sul fur , absorption of other constituents

is assumed weak, and the ideal lanamuir absorption isotherm for single site occupancy

holds, we rnay write:

9 1-9 - K,a, (11·9)

where the terms are detined as follows:

9 the fraction of surface which is covered by sulfur,

Je. the absorption coefticient for sulfur,

a,: the activity of sulfur.

More decarburization experiments with varying carbon concentration at 1400 and

1500°C have been done by Sain et al 1761. The results show considerable experimental

alter at the longer times but are consistent with an increasing rate as carbon content

decreases, suuesûng that the interference by sulfur decreases as the carbon content

decreases. Values of the apparent ideal adsorption coefticient for sulfur in the carbon

saturated a110ys were estimated to be about 450 and 600 (wt pet)"1 at 1450 and 1350°C,

respectively.

Harashima et al (651 found in their experiment that the adsorption coefficient of

sulfur, at 1600°C was liven as follows: ln the range of carbon content higher than 200

ppm, the value equals 65; ln the range of carbon content between 10 ppm and 200 ppm,

the value equals 50-60. The value of the adsorption coefficient of sulfur decreases with

a decrease of carbon content. It also proved that the interference of sulfur on the rate

(

(

(


of decarburization decreases as the carbon content decreases.

7 .-------------~----------------~~ , '

., ......... \' ~ .~.::.~ y

r ':!.N'> -.:(' .. .. '*"y .......... ....: .. .. ..... ~,,'

1 ...... ,.... '", ......... ..

, "

0 0 0 0 - 1 le -E -• CI

• • N • E CI -• 0 E

2 -c • .... > , ...... -• " , c 0 CI

• • a:

o ~----~------------~----~----~ o 0.002 0.004 0.001 0.001 0.01

Sul , ure 0 nt. ft t (wt 1)

Filure Il.9 Effect of small concentrations of sulfur on the rate constant for the decarburization of carbon - saturated iron.

ln Goto's experiment l'm, the addition of sulfur to the liqUld Fe-C system did not

show signifieant retarding effect on the rate of the decarburization.

Elanskii et al. 1'791 reponed that in their experiment a sharp rise in the rate of

carbon oxidation with nickel and copper contents of around 1 wt" wu observed. The

rise in the rate was associated with a change in the close-order structure of iron melts.

,

1

-

CHAPrER 2 LITERATURE SURVEY

ln GoIo's experiment 1771, the addition of sulfur to the liquid Fe-C system did ROI

show silnificant retardinl effeet on the rate of the decarburization.

Elanskii et aI.I791 reportecl that in their experiment a sharp rise in the rate of

carbon oxidation with nickel and copper contents of around 1 wt~ wu ob.crved. The

rise in the rate wu associated with a chanae in the close-order structure of iron melts.

It is Imown that nickel and c:opper raises the thermodynamic activity of carbon in iron

melts. AllO a rise in the carbon diffusion coefficient at a temperature of 1600°C in the

presence of nickel ha been observed.

2.3.4. The Clma ... I. Chande ..... 1a and SllrrI .. EfI1clenry 01 Mol. Steel

InRH

The early work in vacuum treatment discovered the nccessity for stirrina and

mixinl in the steel ladies for adcquate raults to œme from the proœss. In

decarburization, strona stirrina rneans a preferential oxidation of C, or fast

decarburization, and weak Slirrina implies a delay in C supply to the reaction sites,

meaninl the oxidation of Fe is apt to occurilOl• In the LVP (llCIle vacuum proccss)

systems, a mixinl time and stirrinl power clensity have been adopted as carrelatinl

variables. In the vep (vacuum cirtulatina proc:ess refcm:d as RH) systems, which are

bued on the principle of creatinl flow by injectina au to chanae the density of the llecl

in one les of a siphon, the steel is c:ontinuously cirtulated throulh the vacuum chambcr.

The reaclion surface of the vessel is essentially 5111 free beause the lower ends of the

ICIS of the vessel are below the steel surface in the Iaclle. The circulation rate throulh

the vacuum vessel is the important factor since it supports both the deçarburization rate

and the mixina in the ladle. The foUowina three most important factors have to be

consiclered: slimnl power dcnsity, mixina time and relation bctwecn mixinl time and

stirrina power dcnsity.

(1) Stlrrl .. power denslty

(

(

(


There an: two contributions to the ener&Y input to the bath by an araon bubble.

The fint of these contributions, '., comes from the expansion of the au of bubbles. The

second ,~, comes from the expansion of bubbles due to pressure drop durinl its ri. in

the bath. The followina equation lives the stirrinl power density for PI bubblinl

'111:

1 • e +, • 832 T [I-..!!] +In [1 + 9.8IPH! Q (11-11) 1 2 22.4xl()3x60xW. T. P A


W: weiaht of the steel bath (ton);

T.: temperature of the steel bath (K);

Tu: temperature of the environment (K);

p: steel density (kJlm');

H: depth of the steel bath (m);

P: residual vacuum pressure in Pa (1 bar - 10' Pa);

QA ar,OII injection rate (m3/min).

The value of '2 depends upon the residual vacuum pressure. Assume ~ is the

stirrina eneray rate of bubblinl under atmospheric conditions acc:ordinl to Equation Il

if Pis small:

-=. • ln -1., [ II ,", P

(II-Il)

therefore. under 1 millibar residual pressure. this stirrina enerlY of bubblinl is about 6

or 9 times ,reater than under atmospheric pressure.

(2) Mbdna'lIne

The mixing time is defined as the time required to obtain a concentration of the

element within. usuaUy. 9S~ of the steady state concentration. In the circulation model

.. ..

"""


f' • Ot(D/2/Hr't-Z (II-Il)

f' the mixing time,

t the stirring po\\'~r density,

D' the diameter of the bath in the ladle,

H the height of the bath in the ladle,

Ot the constant.

Many equations have been developed to describe the circulation in the RH. In

ail of these, snorkel diameter and lift las rate are key variables. Other variables included

in the equations are the pressure in the vacuum vessel, the up and down snorkel

diameters and the height of the lift gas bubble column. Kuwabara1w1 has studied the

circulation rate by usinl isotope measurement. The key conclusions from these analyses

are that the circulation can be increased mainly by increasing the lift gas rate and by

increasing the diameter of the up leg snorkel. In practical terms, the effect of pressure

is not important because of the low pressures already achieved. Some of developments

in changine the way the lift gas is injected in the snorkel are continuing in practice. In

several plants, lift las has becn increased to 1000 NI/min and more for normal

operations. Trials have becn run with lift gas rates as high as SOOO NI/min and with

snorkels enlarged to 600, 750, and 780 mm diameter IMI. Repons from KuwabaralMI,

and Barros ''', have mentioned that the up lei snorkel was enlarged by changing the

section to oval. The larler snorkels have an advantage that the vertical velocity of the

steel in the up leg is reduced, thus reducinl splash and tendency to skulling, at a given

circulation rate. The steel stream from the down leg carries considerable momentum and

provides excellent mixinl in the steelladle because it disperses mainly at the bottom of

the laclle IMI.

2.3.5. The Nueleatlon 01 CO lubbles ln l\lolten Steel

~

(

(

(


CO gas has been considered as the most important reaction product in most of the

decarburization processes. CO gas bubbles may form either in melts through

spontaneous nucleation from supersaturated solutions or at the surfaces of ,as bubbles

which are the direct result of injection of a gas stream through nozzles or orifices.

Richardson 1871 has mentioned that a very high degree of supersaturation <say of the

order of 50-100 atm) must exist in order that homogeneous nucleation of the bubbles may

occur. For this reason, il is generally thought that gas bubble nucleation in melts of

metallurgical interest is heterogeneous; thal' is, bubbles are formed either al solid surfaces

in contact with the melt or on the surface of soUd particles (impurities) present in the

melt 111111. The pressure inside the small, initialgas bubble P"o is liven by Kelvin's

equationl90l:

20' PO-PL+-r. r


P,. the static pressure in the liquid at the level of the bubble,

CI the surface tension,

(8-13)

r' the radius of curvature of the CO gas trapped in the small cavities on the

refractory surface.

Most refractories tend to be porous 50 that the surfaces in contact with melts

usually contain an adequate number of nucleation sites lltl. If the supersaturation

pressure, with respect to the transferred component, exceeds P"o, the bubble will,row.

The lwO most important facton in the nuc1eation of CO gas are: (1) The critical

size of CO bubble, (2) the rate of CO bubble nucleation.

So far, the total rate al which gas bubbles would evolve from a given surface still

cannot be predicted. Because the number of active nucleation sites that may exist on a

liven surface in a bath cannOl be predicted at any time. A very rough estimation of the

bubble frequency, f. at a given nucleation sile has been ,iven by Szekely lll):

"

, CHAPTER 2 L1TERATURE SURVEY

f. 16~ A' 1 C. - C" ]2 d. PG

(11-.4)

where the tenns are defined as follows:

' ..

(C.-CeJ

Po

d.

the binary diffusivity of the transferred spt-cies in the melt;

the concentration of the transferred species in the bulk of the melt

minus the concentration of the transferrcd species at the bubblc

surface;

the density of the gas in the bubble.

diameter of the bubble.

As mentioned by Szekely, the above equation is inapplicable when the growth rate

is very fast, e.g., in vacuum degassing; moreover, in the initial stages of growth, when

surface tension and viscous effeets could al50 become important, this simple expression

would not be appropriate.

Kaplan et al (191 calculated the rate of CO bubble nucleation at oxide metal

interfaces within liquid iron alloys by using von Bogdandy's equation:

(11-15)

The value of f is estimated by Von Bogdandy to be .()21 nuclei per second,

however, according to Hinh, the value of f should be between 1019 and IOn nuclei per

second. KI is the Boltzamnn's constant. 4F· is the free energy change which includs the

following three factors: (1) the changes in the chemical potential when the bubble is

formed, (2) the PV work done when the bubble is formed, (3) the changes in the surface

free enerlY when bubble is formed.

The quantity of ~F· was computed by determining the free energy of formation

of a CO embryo from carbon and oxygen dissolved in iron. For homogeneous

nucleation, an embryo was considered to be a sphere; for heterogeneous nucleation at an

iron-iron oxide interface, an embryo was considered to consist of two spherical caps base

1

CHAPTER 2 LITERATURE SURVEY • to base. During the formation of this embryo, surface is ereatcd between liquid iron and

gas and between liquid iron oxide and las while some surface previously existinl

bctween iron and iron oxide is destroyed. The area of the iron-iron oxide interface

destroyed was assumed equal to the ara of the base of the spherical caps.

Assuming that the temperature wu 1990 K, ["CJ wu 0.8, ["OJ wu 0.23, the

cquilibrium constant Kwas 486, the pressure of CO las in equilibrium with the bulk

steel, Pet was 90.5 X 10' (dyne/cm2) and the radius of • eritical-size nucleus f· wu

1.74 x 10"' (cm), the caleulated free encray of formation fOf critical-size embryos and

nucleation rates of CO bubbles arel"':

(1) Homogeneous nueleation:

4F· = 10"' erl which lives:

J. = 1 OZl/exp(3. 8 x 10') bubbles per second at 1900 Je

(2) Heterogeneous nucleation:

4F· == 3.9 x 10-7 erg whieh gives:

J. = IOZ'/exp(1.4xIO') bubbles per second at 1900 K

The results of this calculation show that the rate of nueleation is essentially zero

for bath homogeneous and heterogeneous. Furthermore, even when both carbon and •

ollygen are at their saturation limit in molten iront the pressure of CO in equilibrium

with the bulk alloy is insufficient to produce heteroaeneous bubble nucleation. 50 lhal

Kaplan concluded that CO bubble nucleation muId not oceur al an iron-iron oxide

interface, and CO bubble nucleation must occur by another mechanitm.

,1

l 1

CHAPTER 3 DECARBURIZATION MECHANISM 61

CHAPTER THREE

DECARBURIZATION MECHANISM

3.1 DECARURIZATION MECHANISM WHEN CARBON CONTENT IS ULTRA

LOW

The study of decarburization mechanism consists of the following considerations:

(1) The pot~ibility of carbon elimination from liquid steel which refers to a

thermodynamic study of the decarburization reaction.

(2) The kinetics of decarburization reaction which include:

a) The rate of nucleation of CO gas.

b) The rate of mass transfer of carbon and oxygen in the liquid steel.

c) The rate of mass transfer of reaction products in the gas phase.

d) The rate of chemical reaction of decarburization.

(

(

(

CHAPTER 3 DECARBURIZATION MECHANISM

3.1.1 A lIypothesis on The Rate Behavlour or Decarburlzatlon When 'lbe Carbon

Content is Ultra Low

The cquatlon for the decarburization reaction with one mole of CO and i moles of COz las as

the reaction products can be described as:

(1 +;)[CJ + (1 +2;)[0] • CO + ; CO2 (m.1)

The ra~c of chcmical reaction of decarburization at ultra low carbon levels an be

approximatcly describcd as:

des _ KC" dl ' S

whcre the tCrlns are defined as follows:

Kr the rate constant of decarburization reaction;

C, the carbon content at the reaction sites;

n the order of the reaction.

(m-2)

The order of reaction, n, with respect to reagent carbon must he found experimenta1ly and

cannot he prcdicted or deduced from the equation for reactionl1l•

The v,lIue of rate constant of decarburization reaction K, can be obtained direc:dy from

experimcnl. For some simple reactions, an empirical equation proposed by Arrhenius in 1889

shown as f(lllows can also be used:

00·3)

r ,

." .j. CHAPrER 3 DECARBURIZATION MECHANISM

where the tenns are defined as follows:

A • the pre-exponential factor;

E the activation enel1Y.

It is known that the observed rate of decarburization versus carbon content no Ionler

follows tint orcier kinetics at carbon content less than lOppm. and that the rate of

decarburization drops abruptly as shown in Fi,ure 111.1. It is also known that more and more

C~ au is produced with a decrase in the carbon c:onten~221 as shown in Fi,ure 111.2 and Table

111.1. Based on these two facts the followin, hypothesis is put rorward to explain the chanae of

rate behaviour when carbon content is ultra low: With the decrease of carbon content. the orcier

of decarburization reaction increases from 1 to some value hi,her than 1 as a result of incRUe

of Co, due to the fact that the CO., increases with the decrease of carbon content. - -

-c

i

'.11

~ '.1 -c • -• N

-• • -• c

'.n

.. ,

....

•

t,. ......

. , .....

~ .. N~

/ ' ........ _.&......~_---' ___ .~ . ..&._.~~._~L ....... _ .... t __ .. _ .... : •• .J ............... : ... _~ .. : .... ,._ .

• • 1 •

Car b 0 ft con t ." t (-.)

Flpre IU.l The behaviour of the rate of decarburization with the change of carbon content

(

(

(


Table 111.1 l'he chanac of dccarburization product composition u carbon content clecreuedlDl•

C~(CO+COz)(") under equilibrium condition

(CI (~) Temperature (OC)

ISOO ISSO 1600 1650 1700

0.01 20.1 16.7 13.8 Il.S 9.S

0.05 S.6 4.3 3.3 2.7 2.1

0.10 2.8 2.2 J.7 1.3 I.J

O.S 0.44 0.34 0.26 0.21 0.16

1.00 0.16 0.12 0.034 0.07 0.06

21 __ ------------------------------------------__ T'. pr ••• ur. of CO+COIII t Il ..

20

- 15 - ................................................................................................................................................................................................. ..

--~ o U tO + o U

••••••••••••••• '" t ... I ......... ~ .... ..H ••••••••• ~ ...... , .................... 'It •• IIU ......... h ••••• hl.hI>tU .................... " ••••••••••• 1 ................. ., ........... ,. ...... t •

--~ o U

" .. C""" C,

5 -

o LS~ ________ ----I o 0.2 0.4 O., O •• t 1.2

Carbon cont.nt ( •• 1)

Filure 111.2 The behaviour of COz gas as a decarburization product with the change of carbon contentrHl

4" CHÂPrER 3 DECARBURIZATION MECHANISM

3.1.2 1'IIe Felllblilly 01 Carbon Ellml_11on r ..... Llquld Sleel

Fiaure 111.31901 shows the equilibrium oXYlen activity versus carbon activity at different

partial CO au pressures, Pco. As it shows, the equilibrium carbon activity declaleS with a

decrase in Pco at a constant oXYlen activity. For instance. the carbon activity CID be u Iow

as 3 ppm when a activity of 50 ppm oXYlen, a pressure of 0.5 torr and a temperature of 1873

K are maintained. This leads to the conclusion that the feasibility of carbon elimination from

liquid steel to below 10 ppm exists bascd on thermodynamic calculations.

3.1.3. Decarburlzatlon .lIhln Ihe moita lleel

Since calculations by previous reserchers show that the rate of nucleation is essentially zero

for both homoaeneous and heteroaeneous nucleation, the following hypothesis arises for

decarburization reaction irlside the molten steel: Decarburization reaction can only take place

at existina ps-liquid interfaces within the molten steel. It has been observed durinl vacuum

decarburization processes, such as YOD, that the relationship between the panial pressure of CO

whieh was assumed to be the pressure in the vacuum ehamber. and the carbon content, (C), and

wu described by the following equation:

(111-4)

did not hold when the partial pressure of CO gas decreased to a certain value. In other words,

the carbon content in the liquid steel does not decrease with a decrease of vacuum chamber

pressure if the pressure in the ehamber is lower than a certain level.

Takinl hiah ehromium content stainless steel for example, when the panial pressure of CO

gBS ranles from 0.01 to 0.1 atm, Equation 111-4 describes the decrease of carbon with pressure,

however, at the partial pressure of 0.001-0.0001 atm, such a relationship as indicated by

Equation 111-4 did not hold even thoulh all the other conditions were the same. This eritial

~ value of pressure can be lowered by increasing the stirring and temperature, nevertheless a lower

(

(

{


limit of pressure was still observed. Such a lower limit is due to the fact that the

uecarburization reaction Wilh gas injection occurs in the following three different locations: (1)

in the bulk. (2) at the free surface of the melt. (3) at temporarily suspended droplets. The rate

of dccarburil.ation is greatly affected by the proportions of the amount of reaction occurrina at

the three different locations because of the different conditions for the formation of CO las.

1. 000

500

~.,u"

300 'co • 1 •• or' -E a- 200 D-

e

• ca 100 .. te 0

-0 50 .. -.-:. 30

- tOrl u 20 ~

'00 -O. • 1 or r

, 0

2 3 5 10 20 ao 10

1 CI ( Pli m)

Figure 111.3 C-Q equilibrium al low partial CO gas pressures

'.

i,

l


~---------~--~-----,...-,------~---._- -------, -~ .. _-- -, - ~,

Th. bul k

FllUre 111.4 Three locations where decarburization reaction occurs

67

SI ag

~

tS

The capillary pressure (2(7/r) acting on a CO gas bubble can be as high as 29600 atm if the

radius of the bubble is )()"7cm, thus, decarburization is almost impossible within the molten steel

by homoaeneous nuclea.tion of CO gas.

ln most cases, decarburization reaction occurs on existing gas-Iiquid phase interfaces arising

from cracks at the rough surface of refractory, or the surface of injected gas bubbles and

powdered oxidizer. Those cracks are filled by gas since molten steel can not flow into them duc

to the resistance of the capillary pressure. Assuming that the radius of a crack is about 0.05 cm,

the minimum capillary pressure will he 46 mmHg, which is an acceptable value for

decarburization. Figure III.S shows the process of nucleation of CO gas in slims or cracks at

(

(


the roulh surface of the refl'lCtory.

( .)

LI,uld "0.1

.... r.ctor'

( b)

1 CI + 1 0) • co

( d,

• co

( .)

CO bu b b 1 0 • 0 r •• ft. 0 ft t". , •• , 1 ct 0 r ,

lur' .e.

( .)

Flpft 111.5 The process of CO gas bubble forming at the refractory surface

• • co

Sinc:e every 100 mm increase in the depth of the molten steelleads to a SO mmHI ine ...

in the llatie pressure, the effeet of statie pressure, PL' on decarburization cu not be ilnored.

If mo!t of the decarburization reaction occurs within the molten steel, it would be observed that

the carbon content could not be further reduced by increasinl the de&ree of vacuum when

chamber pressure and carbon and oxygen content were lower than a cenain value since vacuum

only reduces the pressure of the gas phase, p.. Vacuum becomes less and less effective with

decreasinl carbon content if most of the decarburization reaction takes place within the molten

steel. One of the best methods to accelerate the decarburization reaction within the molten steel

(' is to provide more sites for the nudeation of CO las bubbles by injecting oxidizinl cas and

1 CHAPTER 3 DECARBURIZATION MECHANISM 69

powdered oxidizer.

The formation of CO gas at the free melt surface does not change the arca of the gas-liquid

interface and the radius of CO gas bubbles, r, can be considered as the radius of the free surface

of the melt which is infinite. !hus, the capillary pressure, 2alr, approaches zero. In other

words, the effeet of surface tension can be ignored. Moreover, the static pressure of slag and

liquid steel due to gravit y can al50 be ignored. Therefore, the minimum pressure necdcd for the

~' formation of CO gas is as follows: '.

1 . .:1

1

(111·5)

As shown by Equation III-S, the lower the pressure in the gas phase. p •• the better the

condition for deearburization to occur. Since CO gas can be directly transferred into the gas

phase, il is not necessary to form any kind of gas bubbles al the free surface of the molten stccl.

Therefore, it is believed that the condition for decarburization at the meh surface is much bener

than that inside the molten steel. If most of the decarburization occurs at the melt surface, it

should be observed that the equilibrium carbon content decreases with the partial pressure in the

las phase. On the other hand, the rate of mass transfer of le] in the liquid phase plays an

imponant role in the overall rate of decarburization since the proportion of the amount of

decarburization oc:curring at the free surface of the melt increases with decreasing carbon

content.

The minimum pressure needed for the formation of CO gas against the surrounding

pressures at the surface of temporarily suspended droplets is:

2a Pc;o > P,-r

(lJI.6)

It is obvious that the suspended droplets have the best condition for the formation of CO

gas.

Since most of the decarburization occurs at the free melt surface when carbon content is

ultra low, increasina the proponion of the decarburization occuring within the molten steel and

al the suspended droplets can lead to an acceleration of the overall rate of decarburization.

...

(

(

(

CHAPTEK 3 DECARBURIZATION MECHANISM 70

3.1. TUE Et1 FECT OF SPECIFIC SURFACE AREA ON THE RATE OF

DECARBURIZATION

The flux of carbon (J .. mole/cm2sec) in the liquid steel can be approximated as:

wherc the terms are detined as follows:

Ch the carbon content inside the bulk (mole/cm');

Ct the carbon content at the reaction sites which, in most cases, refers to the free surface

of molten steel (mole/cm);

KI mass transfer coefficient (cm/sec).

The rate of decarburization (dCJdt, mole/sec) can be described as:

dC.. F -- • -KL(C .. -C) dl V .. 1

As Equation 111-9 shows. the rate of decarburization could be improved by improvina the

following two factors:

( 1 ) the mass transfer coefficient KL;

(2) the value of specitic surface F/V, F is the reaction surface area (cm2), V is the total

volume of the liquid steel (cm').

KI. can be increased by improving the stirring. Levich established the followina relationship

for the mass transfer coefficient of carbon in levitated liquid steel droplets in a magnenc fiel,I):

K • DI 'V l'pO.,,,., L ,

1 CHAPI'ER 3 DECARBURIZATION MECHANISM

where the terniS are defined as follows:

D the diffusivity,

V c eddy vclocity,

P the density,

" the surface tension.

71

On the OIhcr hand. the specifie surface area. F/V. tan bc inereased by the following ways:

(1) Produc:c more suspcnded liquid steel droplets.

(2) Gas injection into the molten steel.

(3) Adelina powcler into molten steel.

Assuming that the suspended droplets are spheres. the average value of the specifie surface

FIV of the suspended droplets is:

[FI 4-r,2 3 V J'OP/II'· ~-rr3 • r

3

whcre r is the average radius of the suspended steel droplets.

The value of FIV of molten steel in a eylindrica~ erucible can be described as:

(111-10)

(III-II)

whcre H is depth of the molten steel (height of the erucible) and R is the radius of the erucible.

The specifie surface area, F/V. of suspended liquid steel droplets can be compared with that

of molten steel in a erucible:

[~] d<opM ! 3N

[F] · ~ · r V c,,,c;II/,

(111-12)

(

(


Sinœ the specific surface area for suspended steel droplets is 3H/r times grealer than that of the

molten steel in a crucible, as Equation 111-12 shows, the rate of decarburization of suspended

droplets would be 3H/r times arater than that of molten steel in cnac:ible.

Assuming that the height of cruc:ible H equals 1000m, radius of droplet r equals O.5cm and

KI. is the same in both cases, the value of 3H/r will be 600, whieh mans the rate of

decarburization for the suspended droplet (mol sec-lem") would be 600 times hilber than that

of the molten steel in crucible if most of the reaction oceurs at the free surface of the melt.

ln gas or powder injection, assumina that the averaae radius of las bubbles or powder is

r .. , the number of gas bubbles or powder partieles per unit weight of the metal is m, the radius

of the free surface of the molten steel is R, the total weight of the molten metal is w and the

height "f the crucible is H, the specifie surface area for gu and powder injection (F/V),."...

is:

1 FI 4mwrr,,l + ftR' 4mwr,,2 1 V [ffI.""''''.' = --"R~2~H-- = R'H + H

(111-13)

The specifie surface area of molten steel with aas or powder (F/V).ulpowlicr œmpand to the

specific surface area of molten steel in crucible (F/V)cnd,1r is:

I~L_ = 4mr.' +1

1 FI R' V c',..,.."",

(10-14)

Assuming that the radius of agas bubble rll is S cm, the radius of the free surface of molten steel R is SO cm, the number of gas bubbles per unit weight of metal mis 500 tonne' and the

weight of molten metal. w, is 1 tonne, then, the value of Equation 111-14 is 21, whieh means \hat

the decarburization rate can be increased by 21 times with gas injection. For powder injection,

the value of m could be as much as 10' per metric tonne of metal and the radius could be

O.OSmm I~'I, the calculated value of the ratio. Equation 111-14, for adding powder is 21 whieh

is the same value with that in gas injection.

( ln most cases. the value of Equation 111-12 is larger than the value of Equation 111-14, thus,

-~. '


the rate of decarburization of suspended steel droplers should be (aster than that of molten metal

with las or powder injection.

3.3. mE OVERALL RATE OF DECARBURIZATION IN PRACTICE

Vacuum decarburization processes can be divided into three types:

(1) Vacuum decarburization without any gas or powder injection. "

(2) Vacuum decarburization with gas injection.

(3) Vacuum decarburization with powdered oxidizer injection.

3.3.1. The Overall Rate of Decarburizatioll Under VaclIum Withoua ally Gas or

Powder Injection.

Assuming that RI is the overall rate of decarburization u'.Jer vacuum without any gas or

powder injection, il can be divided into the following two parts:

(1) The rate of decarburization at the (ree surface of molten steel: Rm.

(2) The rate of decarburization in the bulk of molten steel: Rb'

cv represents the value of the ratio of the amount of decarburization occurring at the free

surface of molten steel over the total amount of decarburization in the whole process. The

overall rate of decarburization, RI, is as follows, based on a mass balance:

RI - cvR", + (1- a)R. (111-15)

Since the decarburization conditions al the free surface of molten steel are much better than

that of the decarburization within the molten steel, especially under vacuum, the rate of

decarburization at the free surface (Rm) is greater than that in the bulk (~). 1t is known that the

value of Ct increases with a increase of the specifie surface area (FI V) and a decrease of carbon

content. On the other hand, Ra. is controlled by the rate of mass transfer which indicates that

a strong sûrring and a very high degree of vacuum should have a favourable effect on R... Since

Rb is controlled by the conditions for the nucleation of CO gas inside the molten steel, an

...


increase of the number of nucleation sites inside the molten steel should Icad to the increase of

R.. Moreover, a very high local oxygen or carbon concentration inside the molten steel may

also increase the value of Rb' however, vacuum is less effective to increase the value of Rh

3.3.2. The OveraU Rate of Decarburlzatlon by \Vith Powdered Oxidizer Injettion

Under Vacuum

Similar to the overaU rate of decarburization without gas or powder injection. the overall

rate of decarburization with powder injection can be expressed as follows under the assumption

that R.' represents the overaU rate of decarburization with the addition of powdered oxidizcr:

(111 .. 16)

An the symbols on the right hand side of the Equation 111-17 have the same meaning as

tho.e useeS in Equation 111-16. The only difference between the rate of decarburization with and

without powder injection is that the value of Rb for the situation with powder injection is much

larger than that without powder injection which can be explained by the following reasons:

(1) The injected powder oxidizer provides a large number of ~ites for the nucleation of CO

gas bubb'''s.

(2) The local oxygen content around the oxidizer particles in the molten steel is high.

(3) A good dispersion of the oxidizer particles increase the specifie surface area of molten

steel and shonens the th~e needed for the mass transfer of carbon to the surface of the

oxidizer particles.

3.3.3. The OvenU Rate of Decarburizatlon Wlth Gas Injection Under Vacuum

Assuming that R2 represents the overaU rate of decarburization in the whole process with

lU injection, ~ represents the rate of decarburization inside the molten steel, Rd represents the

average rate of decarburization for suspended droplets, " represents the ratio of the total mass

of droplets over the total mass of liquid steel and that the size of the droplets produced is

(

(

(


approximately uniform, the expression for the overall rate of decarburization with gas injection

is as follow~:

(rn-11)

With powder injection, RI has the same meaning as defined in Equation III-IS. Without

powder injection, it has the same meaning as that of RI' in Equation 111-16. Equation 111-17 cao

also be expressed as:

(rn-11)

Here, the value of Rb has been increased by the injection of gas due to the following

rcasons:

( 1 ) The injected gas bubbles provide a large number of decarburization reaction sites.

(2) The injected gas bubble increase the specifie surface area for decarburization

reaction.

(3) The injected gas bubbles provide a very high local oxygen concentration if

oxygen gas was injected.

Moreover, the value of Rn. has also been increased due to the increase of the rate of mass

transfer by gas injection. Previous studies on levitated droplets showed that the rate of

decarburiziltion for a single levitated droplet was nearly 200 times faster than the overall rate

of decarburization in RH process and was nearly 600 limes faster than that in crucible without

gas injection, so that it is believed that R.. is much greater than RI or R2• However, since the

value of" is small, in other words, the mass of droplets is much less than the total mass Iiquid

steel, the contribution made by Rd to R2 is not as big as it could he. One of the ways to increase

the valuc of R2 is to increase the value of." which means to increase the mass and number of

droplets without increasing the average size of droplets, especially, when the r.arbon content is

below .10ppm.

, ,

CHAPTER 4 EXPERIMENTAL 76

CHAPTER FOUR

'..,

EXPERIMENTAL

4.1 INTRODUCTION

Vacuum levitation experiments were conducted to measure the kinetics of

decarburization of levitated steel droplets. Il was assumed that the levitated droplct

simulated the temporarily suspended droplet in industrial vacuum degassing processes

with gas injection like RH. It has been shown that the rate of decarburi7.éllion is very fast

al the temporarily suspended droplets due to lheir large specifie surface area and their

good condition for the formation of CO gas at the reaction surface. A relatively large ,

amount of these temporarily suspended droplets in RH or similar vacuum refining

processes could play an important role in the reduction of carbon to below lOppm within

a limited time.

If the mass transfer in the gas phase con trois the rate of decarburization, the

levitated droplet could be used to simulate the droplet without any significant error.

However, if liquid phase mass transfer is the rate controlling factor, e'Toneous

conclusions oould be made when levltated droplet is used to simulate the suspended

droplet becau5e, due to the magnetic field of levitation, the stirring inside the levitated

(

(

c


droplet is st ronger than that inside the same droplet in free fall.

The three main objectives of the experiment were:

(1) to study the elimination of carbon to below IOppm within a limited time;

(2) to study the decarburization mechanism of a levitated droplet under

vacuum;

(3) to study the influence of oxygen and sulfur on the rate of decarburization

of levitated steel droplets under vacuum.

4.2. LEVITATION MELTING

Il is known that a conductor placed in a high frequency electromagnetic field will

tend to move from the stronger to the weaker part of the field. The force on the

conduclor due to the field will be associated with both the magnitude and gradient of the

field strength, the steeper the gradient, the greater the force. Thus for levitation, a field

is required which decreases in strength in a vertical direction to obtain an upward force

equal to the weight of the conductor. To obtain stable levitation, a field is required

which also increases in strength radially from the field axis so that a restoring force

operates towards the axis. The methods so far used to obtain such a field involve the use

of two coils, one above the other, the current in cach being out of phase. In this way,

it is possible to obtain a position of minimum field strength between the two coils 50 that

the forces on a condllctor between them are always towards this point, thus producing

the conditions necessary for a stable levittotion. Because the levitatio!l force is associated

with the divergence (field strength gradient) of the field as weil as the field strength, and

because the heating of the condurtc. is llssociated with field strength, it is possible to

decrease the heating rate and therefore the equilibrium temperature by increasing the field

d:, !rgence and decreasing the field strength. In sorne coil systems, this can be achieved

shnply by increasing the coil current so that the conductor rises to a more diveraent part

of the tield closer to the point of minimum field strengthl92J, in other words, the

temperature of specimen might decrease with the increase of lifting force by increasing

, ,.

'.


the power in such coil system.

The factors which are of importance in determining the total levitalion force FI.

have been expressed in the following relationship, suggested by Okress el al I~]II'NI:

(lV-1)


k' constant for a given position in a given coil,

1 coil current (amps),

M charge mass (grams),

p density of charge (gram/cc),

G(x) = 1 - 3(sinh2x-sin2x)/[4x(sinh2x+sin2x»).

x = 2">..r,.Jr/lOOp',

rc radius of charge, assumed spherical (cm),

r frequency (cycles/sec),

p' resistivity of charge (l' ohm cm).

This relationship was derived for a sphere between two co-axial loops, wilh

current flowing in reverse directions. The levitating fOI'f!e increases with increasing

frequency, but this effect ceases to be significanl al certain frequency, depending on the

actual values of rc and p'. The value of this frequency decreases as R increases and p

decreases. Previous experience showed that coils wound in a series of horizontal layers

in preference to conica1layers appear to he more effective, although the reasan for this

suceess is not yet understood.

4.3. EXPERIMENTAL VARIABLES

There were three experimental variables which may have a significant influence

on the results of the experiment:

(

(

(


(1) Chamber pressure, which was controlled within the range of 0.2 to C.4

mbar. The mininlum value which could be obtained was 0.1 mbar.

(2)

(3)

Droplet surface temperature, which varied from lSS0°C to 17S0°C due

10 the irregular shape and weight of the specimens.

Input power of the high frequency generator, which was controlled to be

11.5 kVA with very little change from one run to another.

Figure IV.I High frequency generator

..f',

~.-

CHAPTER 4 EXPERIMENTAL 80 ------------------------------------------------------4.4. EXPERIMENTAL SETUP

The experimental apparatus can be divided into four parts:

(1) high frequency generator;

(2) vacuum system including a dual stage mechanical pump;

(3) the vacuum levitation apparatus;

(4) temperature measurement system.

4.4.1. Ulah Frequency Generator:

As ~hown in Figure IV. l, a Toccotron radio frequency generator type No.

4EG202-14 was used with the following two major sections:

(a) The rectifier (or power) section, which converts line frequency (3 phase.

60 cycle) to high voltage OC;

(b) The oscillator section, which con verts the high voltage DC to radio

frequency output.

The equipment operated at approximately 440 kHz and 20 kW with an external

7.S: 1 step-down transformer.

4.4.2. Vacuum System:

A E2M80 rotary vacuum pump, made by E,d,vards Hig" Vacuum, was used for

the vacuum system. The pump was a direct drive, sliding vane type two-stage version

incorporating 5epar8te high vacuum (HV) and low vacuum (LV) stages with inter

connecting pons. The pump was air-cooled and incorporated gas ballast facilities. The

drive from the HV rotor to the LV rotor of the E2M80 wa; via steel drive plates strcsscd

ta prevent bacldash; the HV rotur is supported in roller bearings housed in the front and

interstage housings and the l V rotor in journal bearings.

During operation, the rotor blades sweep the crescent shaped volume formed by

(

(

(

CHAnER 4 EXPERIMHNT AL Il

the ecœntrically mounted rotor and the stator; as each blade passes the inlet, a quantity

of Ils is induœd and subsequently trapped and compressed by the followinl blade until

finally ejected via the discharge valve. The model E2MBO incorporates a au ballal

facility to enable it to pump most condensable vapours directly without silnificant

contamination of the pump oil. Technical data and specifications of the E2M80 pump

are as follows:

Ultimate VKuum:

Displacement (swept volume):

Speed (PNEUROP):

Maximum H20 inlet pressure:

Pump rotational speed (rev/min):

Normal operating temperature:

Oil capacity:

2)( 10-4 mbar (without lU ballut); S)( Ut' mbar

(with gas ballast).

80 m3h-1 (SO Hz motor); 64 m'h-I (60 Hz motor).

74 m3h-1 (SO Hz motor); 61 m'h-I (60 Hz motor).

S mbar.

1420 (50Hz motor); 1140 (60 Hz motor).

36°C (without gas ballast).

7 litres.

Vacuum was measured by a Edwards tilting type method mercury vacuum pUle

with a maximum measurement value of 0.001 mbar.

4.4.3. Vacuum Levitation Appantus:

As shown in Figure IV.2, the apparatus consisted of an aluminum base plate, A,

over which a plastic bell. B, was placecl, the junction beinl sealeci by an O-rinl recesled

in the base plate. The levitation coil, C, with coaling water flowinl inside, wu

connected to the power with O-ring seals and rubber insulation at the junction with the

plate. A carriage. D, containing specimens held on top of alumina crucibles, 0, and

copper crucibles, E. used to hold specimens after experiments, could be rotated by a

shaft brought through an O-ring scal in the base plate. With two o-rinl seals at the

junction with base plate, a pushing rod, F, was used to push the specimen up to the

levitation coil. In order to prevent the ionization of gas in vacuum chamber, levitation

l

, .


coils were insulated by silicone rubber tape. Five specimens could be held on top of the

alumina crucibles at the one time inside the vacuum chamber. The volume of the

vacuum chamber was about 20 litres.

Power

Vacuum FilUre 1V.2 The vacuum levitation apparatus.

~G " , ,

r

(

(

(

CHAPTER 4 EXPERIMENTAL 1.'

The levitation time was limited to 60 seconds in consideration that a longer

levitation time may damage the insulation of levitation coil. A Ilass tube wu placecl

inside the levitation coil to prevent vaporized iron from condensinl on the levitation

coils. The tube was replaced every experiment due to the loss of observations caused

by the black condensate forrned on the inner wall of the tube. The weilht increue of

the glass tube was used to measure the weight loss of the specimen by evaporation durinl

the vacuum levitation ex peri ment.

"

Figure 1V.3 Levitation coils

f

t

CHAPTER 4 EX PERIMENT AL

The coils used in the vacuum levitation experiment were wound in a series of

horizontal layers with 3 tums in the lower part and 1 turn in the upper part as shown III

Figure IV. 3. The dimensions of the coits were determined by trial and crror. the COli

was made of 0.32 x 10'2 m (l/8 an) copper tubing with 0.05 x 10 ~ III (lUl2 in) wall

which was almost the smallest that can be adequately cooled. Figure IV A iUld Figure

IV.S are the of the vacuum levitation apparatus.

4.4.4. Temperature Measurement:

Il

li Flpre IV.4 Vacuum levitation apparatus

The droplet surface temperature was measured by an optical pyrometcr. Usually,

the use of an optical thermometer will be more accu rate than that of contact

thermometers if one or several of the following conditions are prc~ent:

(1)

(2)

Measurements of relatively high temperatures, espcciallyabovc 170(Y'K

Temperature measurements on materials having a very low thermal

conductivity.

r (

{

CHAPTER 4 EXPERIMENTAL 8S

(3) Measurements of the tempe ratures of sm ail or inaccessible objects.

(4) Temperature measurements of a moving object.

ln vacuum levitation, conditions J and 3 were present, and it would have been

very difficult to probe the levitated droplet with a thermocouple since there might have

becn ~ome kind of droplet contamination by contact with the thermocouple wire. For all

thcsc reasons, an optical pyromeler was used for temperalure measurement in vacuum

Icvitation experiment.

.... -;-.-.~ .

Figure IV.S The inlerior look of the vacuum levitalion apparatus.

The Mikron model 78 two-colour tibre optics infrared thermometer, as shown in

FIgure IV.4, was sent to Mikron Instrument Co. lnc. by the previous user to calibrate

and adjust in order to meet the following conditions:

(1 ) Droplet temperature 1300 - 2300 0 K.

(2) Distance between droplet and lens assembly range from 15 to 20 cm.

(3) Droplet diameter 0.-. x 10.2 - 1.0 X 10.2 m.

.,..,


(4) Window transmittance near to l.

Installed in a glass tube and sealed at the bottoln by a crystal glass window, the

lens assembly aimed at the light source was placed vertically facing down at the centre

the plastic bell roof, at a distance of 20 cm to the centre of the levitation coils where the

levitated ~pecimen resided. The lens assembly was connected to a digital rcading target

pyrometer by a fibre optic cable with flexible stainless steel protective sheeting. The ,~

minimum measurable temperature was 1073°C.

4.5. EXPERIMENTAL PROCEDURE

4.5.1 Experimental Design

As shown in Table IV.I, four groups of specimens with different carbon, oxygcn

and sulfur content were prepared. With a relatively high carbon content, the tirst thrcc

groups of specimens were made from steel bars and plates by cutting them into small

pellets and cubes. On the other hand, the fourth group of specimens were ultra-Iow

carbon steelobtained from Dofasco; 35 ppm initial carbon and 350 ppm initial oxygen.

Figure IV.6 shows the specimens before the levitation experiments.

Table IV.I Conditions of the four groups of specimens

Group Average Initial Initial Jnitial Droplet Vacuum Timc number weight carbon oxygen sulfur surface (mbar) (sec)

(g) (ppm) (ppm) (ppm) temperature (OC)

1 1.1 1870 800 220 1550 - 1750 0.4 0-40

2 LI 1230 300 170 1550 - 1750 0.4 0-50

3 1.1 880 200 3300 1550 - 1750 0.4 0-40

4 LI 35 350 40 1550 - 1750 0.2 or 0-40 0.4

( ..


The objectives of the experiments on the first three groups specimens were:

(1) to study the rate of decarburization of levitated steel droplet under vacuum

when the initial carbon content is higher than 800 ppm;

(2) to study the influence of oxygen on decarburization of levitated steel

droplet under vacuum when the carbon content was relatively high;

(3) to study the influence of sulfur on the rate of decarburization of levitated

steel droplets under vacuum when the carbon content was relatively high.

As shown in Table IV. 1, a vacuum of 0.4 mbat and droplet surface temperatures

of 1550°C to 1750°C were used for the first three groups of specimens.

The objectives of the experiments for the fourth group of specimens were:

(1) to study the mechanism of decarburization of levitated ultra low carbon

steel droplets under vacuum;

(2) to study the influence of pressure on the rate of decarburization of

levitated droplets under vacuum when the carbon content wa., ultra low;

(3) to calculate the rate of decarburization of levitated steel droplets under

vacuum when the carbon content was ultra low.

As shown ira Table IV.I, two vacuums (0.2 or 0.4 mbar) were applied for the

fourth groups of samples and ihe droplet surface temperature varied from lSSO°C to

1750°C. The weight loss of the specimen by evaporation was evaluated by measuring

the Y. eight increase of the glass tube which was installed inside the coils. Over 300

experiments were conducted, however, only SO~ of them were successful. Ali

specimens were sent to Dofaseo for carbon and sulfur analysis.

4 • .5.2. Vacuum levitation Experlment Procedure

The vacuum levitation experiments proceeded via the followinl steps:

(1) Stan the water coaling system.

(2) Start the vacuum pump.

(3) Start the levitation coil power when the vacuum reached a requirecl value,

..


and increase the power to the desired value.

(4) Use the push rod to place a specimen in the levitation coU. (When the

specimen was levitated, place the alumina crucible back on the carriage

and tum the caniage to put the copper crucible under the levitation coil).

(5) Use a stop watch to measure the levitation time from the moment the

specimen was totally melted.

(6) Tum off the power and let the levitated specimen fall into the copper

crucible. (Stop the watch).

(7) Break the vacuum, colleet the specimen after levitation, replace the gla"s

tube inside the levitation coil with a new one.

(8) Measure the weight oC the specimen and the weight of the glass tube.

(9) Send samples to DoCaseo for chemical analysis of carbon and sulfur.

Flpre IV.' The specimen wu held on top of the alumina crucible before experiment

(

(

CHAPTER 4 EXPERIMENTAL l'

Figure IV.6 to Figure IV. Il show the melting process of levitated specimens

during the levitation experiment.

Figure IV.12 shows one of the specimens in the coppel crucible after levitation

experimt~nt.

Figure IV.13 shows two glass tubes which have been usee! in levitation

experiment. The one on the left sicle has been used under vacuum, the one on the rilhl

side has bœn used withoul vacuum. Black condensates from the evaporadon of the

specimen were formed al the innel' wall of the tube.

Fllure IV. 7 The specimen wu beinl pushed up 10 the levitadon co ils

., CHAPTER 4 EXPERIMENTAL 90

"l'In IV.I The specimen wu beinglevitated

. ~ , ,r, 1 \

~, ).

f ~ "&un 1V.9 The temperature of the specimen was increasing

t

" "'.r-i,

(. CHAPTER 4 EXPERIMENTAL '1

Flpre IV.IO The specimen wu beinl melted

( FllUre IV .11 The specimen was entirely melted

CHAPTER 4 EXPERlMENTAL

Figure IV.12 The specimen was held in copper crucible after the experiment

.~~,

1 4 : H 1 . ~ l, ~ ,1 1 ,t . ! j

Filure IV.13 The glass tubes which have been used in the experiment

(

(

(

CHAPTER 5 RESULTS 93

CRAPTER FIVE

RESULTS

5.1. INTRODUCTION

More than 300 vacuum decarburization levitation experiments were conducted and

about 50" of them were successful. It wa::; observed that the rates of decarburization

were fast for each of the four groups of specimens, and particularly fast for the fourth

group of specimens, for which the rate of decarburization was 40 times hilher than that

in RH at 30 ppm carbon. A fast rate of evaporation of iron in the droplet wu also

observed.

The failed experiments were the result of the following factors:

(1) A specimen could not be levitated, which may due to incorrect weight,

shape and dimension of the specimen.

(2) The levitated specimen feU down before or jusl after it was melted. This

may have been due to the same reasons as (1).

(3) The levitated droplet exploded and attached to the inner wall of the Ilus

tubes. Reasons could be either the high H20 content in atmosphere,

especially the high humidity in Montreal's summer, which increased

hydrogen content in the molten steel giving rise to H2 gas evolution or the

decarburization reaction taking place inside the droplet at an inclusion with

the result that CO gas bubbles were formed in the droplet.

CHAPTER S RESULTS 94

5.2. THE CARBON CONTENT Cil ANGE

5.2.1. The Carbon Content Chanle ror The Fini Group of SpedlIIelLIii

(Initiai le] = 1870 ppm)

1,100

Ê : 1,eoo --• • -c o o

• o .A .. • u

-II.

1, .. 0 0

, , 200

1, 000

100 o 2 •

.. '. < ~ -:(0" ....

........ ," ....

.. ~:. ' .......... ,' .. ->'; .. , .-", , ,

1

, .

1 0

Ti mt (. te)

" :

12

Filure V.l Results of decarburization for the tirst group of spL>cimens

, .

,',

With 1870 ppm initial carbon and 800 ppm initial oxygen in the samples, the

experiments were conducted under a vacuum of 0.4 mbar at droplet surface temperatures

ranging from 1550 - 1750°C. Over 50 experiments were conductcd, 20 of them werc

successful.

· , ..

(

( ...

( ,

CHAPTER 5 RESULTS 9S

Table V.I Results of levitation experiments for the first group of specimens

Specimen number [C] (ppm) [S] (ppm) Time t (sec)

153 991 216 10.5

155 1441 228 1.74

156 1428 236 3

157 1461 223 O.S

161 1563 214 1.42

194 1567 227 3.13

196 911 212 13.76

197 1630 217 2.31

198 1541 234 0.5

199 1482 219 3.6

200 1578 214 0.5

202 1457 211 S.O

203 1512 222 2.66

207 1186 212 10

208 1429 221 2.4S

209 1405 211 3

210 1403 217 3.41

211 1415 222 2

214 1482 21S 2.S

215 1390 226 2.73

The experimental results which include the final carbon and sulfur content al

different levitation times are listed in Table V.1. The final carbon content versus time

is Illotted in Figure V. 1. As indicated from the Iinear regression through the data, the

rate of decarburization was constant, about 47 ppm/sec, over the range 1600 ppm to 800

ppm carbon (R2=0.84, [C]=-47.4t+ 1590 for Figure V.l). The constant rate of

~ -------------------------------------------

CHAPTER 5 RESUL TS

Table V.I Results of levitation experiments for the first group of specimens

Specimen number [C]( x l()lppm) [S)( x l()2ppm) Time t (sec)

153 0.99 2.2 10.5

15S 1.44 2.8 1.7

156 1.43 2.4 3

l57 1.46 2.2 0.5

161 1.56 2.1 1.4

194 1.57 2.3 3.1

196 0.91 2.1 13.8

197 1.63 2.2 2.3

198 1.54 2.3 0.5

199 1.48 2.2 3.6

200 1.58 2.1 0.5

202 1.46 2.2 5.0

203 1.51 2.2 2.6

207 1.19 2.1 10

208 1.43 2.2 2.5

209 1.41 2.2 3

210 1.40 2.2 3.4

211 1.42 2.2 2

214 1.48 2.2 2.5

215 1.39 2.3 2.7

The experimental results which include the final carbon and sulfur content at

different levitation times are listai in Table V.I. The tinal carbon content versus lime

is plotted in Figure V.l. As indicated from the linear felfession through the data, the

rate of decarburization was constant, about 47 ppm/sec:, over the range 1600 ppm to 800

ppm carbon (R2=O.84, [C]=-47.4t+1590 for Figure V.I). The constant rate of

r. .. i

(

(

(

CHAPTER S RESULTS

decarburi7.8tion indicated that the farst group of specimens were in the region where

carbon content had no signiflcant effect on the rate of decarburization.

Ê ft. A

.. c • .. c o u

c: o .G ... .. Co)

-.. u.

5.Z.2. 1be Carbon Content Chanle for The Second Group of Specimens

(Initiai (C) '..; 1230 ppm, Initial (S] = 110 ppm)

.00

700

.; ., " , '

~/;. /~;~ ~ .. :".~'".t ::~' .... " .. :·~~l~"; .. ",," n~ .. -;/

" ,. , , ....... ,

....... 500 ....... ~ ........ _._ .... , ... t.ooJ __ ...... , ... ~ __ ... ____ ....... _ .................... _

o 10 20 30 40

TI m. ( •• c)

10 10

.~igure V.Z Results of levitation experiment of the second group of specimens

Under the same experimental conditions as those for the tirst group of specimens,

the second group of specimens, having 1230 ppm initial carbon, 300 ppm initial oxygen

and 180 ppm initial sulfur concentration, the levitation experiments resulted in a constant

raie of decarburization which was 6.9 ppm/sec for linear regression through the data

(R1=O.84, (C]=-6.9t+ 1010.9 for Figure V.2). The measurcd final carbon content versus

lime is plotted in Figure V.2 which displays that carbon has bœn reduced (rom 1000

ppm 10 800 ppm within 30 seconds. The experimental results are listed in Table V.II.

Bolh Ihe tirst and second groups of specimens were in the region that mass transfer of

CHAPTER 5 RESULTS 97

oxygcn to the reaction sites rather than chemical reaction controls the rate of

decarburization.

Table V.U Rcsults of levitation experiments for the second group of specimens

Samples number [C]( x l()lppm) [S]( x l()1ppm) Time t (sec)

114 0.81 1.3 23.7

115 0.98 1.7 3.7

116 0.84 1.2 28.0

118 1.00 1.6 0.0

120 0.58 1.2 50.0

122 0.95 1.6 1.5

123 0.98 1.2 0.5

125 0.91 1.0 25.8

126 0.90 1.3 25.3 . 142 0.90 1.2 18.7

143 0.87 1.2 22.5

218 0.85 1.3 25.5

219 0.89 1.4 21.7

5.2.3. The Carbon Content Chanle 01 The Third Group 01 Specimens (Initial

IC) = 110 ppm, (S) = 3300 ppm)

Table V.UI Results of levitation experiments for the third group of specimens

Sample number (Cl( x l()1ppm) (S)(x IOWm) Time t (sec)

221 0.56 27.8 10.1 ~-

223 0.55 21.7 21.9

224 0.64 32.8 1.0

253 0.49 20.8 30.9

254 0.60 31.4 0.5

CHAPTER 5 RES U LTS (

&50

• ï. ~,

èi. [) Ij bOIl

t,J " C

" 1\) t-' \ '.. . 0 V

550 -

.~ .' L 0 n , "1 V

11..1 ',1\11 ('

Il

4'i0 '--__ -1-__ --1 ________ ....... ___ ....... _______ _

D lU 1S 20 2S 30 35

( lime (sec)

(

Fllure V.3 Results of vacuum levitation experiment of the third group of specimens

With 880 ppm initial carbon, 200 ppm initial oxygen and 3300 ppm initial

sulfur, the carbon content of the third group of specimens was reduced from 650 to 500

l'Pm within 30 seconds as plotted in Figure V.3. The experimental results are listed in

Table V.III. Under the same experimental conditions as that of the fint and second

groups of specimens, the average rate of decarhurization was about 4.1 ppm/sec by Iinear

regression through the data even though the initial sulfur content was about 18 limes

higher than that of the second group specimens (R2=O.87, (C]=-4.lt+619.9 for Figure

V.3). The nonlinear curve in Figure V.3 is not indicative due to the Jack of data.

5.1.4. The Carbon Content Chanle of The Fourth Group of Specimens

(Initiai (C] = 35 ppm, (S] = 350 ppm)

ln order to understand the influence of chamber pressure on the rate of

1 1

,1.'

CHAPTER 5 RESULTS

decarburization, two

vacuums, 0.2 mbar and 0.4

mbar, were employed for

experiments on the fourth

group of specimens which had

35 ppm initial carbon, 350

ppm initial oxygen. Droplet

surface temperature was 1550

to 1750°C. The experimental

results are listed in Table

V.IV and the carbon content

change versus time is plotted

in Figure V.4. Carbon

content was reduced (rom 35

ppm to 5 ppm within 35

seconds. Moreover, As

shown in Figure V.4, carbon

contents of specimens under

the vacuum of 0.2 mbar were

concentrated in the lower

50 r------:------~--.----~.---... ~

ê .. Q. --c

40

• 30 -c o u

c o .. .. • 20 u

-• c

t 0

• •

.... ~.CUUIII: 0:.2 ",bu R· .... ar.: o. u.! y-u. n .. ;,(. O. OUXI

1 :

..... ~v. C U U III: ~. 4 IIIb Ir : i i

R· .~.u.: O. ,~, Y-31. S4 ••• C· o. ou

o ~----~,----- -~-----_._--·---,--,", ____ I o

Figure V.4

t 0 20 40

TI III' (Ife)

Results of vacuum levitation expcriment for the fourth group of specimens

shaded areaand exhibited an exponential relationship ([C] =32. 76cxp(-O.052t), R2 =O.63}.

Furthermore, the rate of decarburization can be derived as a result of the above relation

as: d[C)/dt=-O.052[C). Those under the vacuum of 0.4 mbar were concentratcd in the

upper shaded area and exhibited a similar relationship ([C] =31.34exp(-O.03t), R2 =0.63).

The rate equation can be derived as: d[C)/dt=-O.03[C). The instantancous

decarburization rate at 30ppm carbon was calculated by the rate cquations as 1.56

ppm/sec for 0.2mbar and 0.9 ppm/sec for O.4mbar.

Statistical analysis on the results has bœn conductcd by using th:: sttpwist

re,rtssion function in a program called Stat,raphics. It was confirmed that temperature

CHAPTER S RESULTS 1. hld no significant effeet on the rate of decarburization, however, pressure wu found

havc a significant effeet. Such a result su_csts that the rate of decarburization miaht

be controlled by the rate of mass transfer of the reaction product in the pl phase, and

that the rate constant (1(, in Equation 111-2) for the chemica1 re&Ction of decarburiration

is probably not be the rate controlling factor for a Icvitated ultra low carbon steel droplet

under vacuum.

Table V.IV Results of Icvitation experiment for the forth group of specimens

0.2 mbar 0.4 mbar

No [e] ppm [8] ppm Time No [e] ppm [8] ppm Time (sec) (sec)

S2 S 2S 27.7 1 20 36 IS.0

S3 5 18 33.8 2 20 31 14.0

S4 4 20 34.0 3 26 36 16.0

66 S 29 16.7 4 21 31 18.0

S6 7 2S 31.4 S 19 32 IS.0

S7 12 24 27.S 6 24 26 20.0

S8 19 29 20.2 7 20 3S 18.0

S9 22 33 7.0 8 30 38 3.0

60 21 33 17.3 9 10 27 26.0

61 21 33 lS.1 10 21 36 14.0

62 17 32 16.6 11 27 33 14.0

63 21 34 10.9 14 26 37 3.0

64 12 36 15.2 1.5 29 38 3.S

65 18 3S 10.0 16 28 34 11.0

66 S 29 16.6 18 27 38 4.2

67 25 3S 8.9 19 32 34 1.0

68 29 33 10.6 22 41 34 4.0

, l

l ' , '

CHAPl'ER 5 RESUL TS 101

69 19 33 17.8 23 25 40 0.2

70 13 29 13.3 24 24 40 0.7

71 38 29 2.6 25 33 38 0.0

72 16 16 25.8 26 32 29 0.5

73 23 34 8.4 27 30 38 0.0

74 21 37 6.8 28 28 38 0.5

75 21 38 6.6 29 31 39 0.0

76 23 38 7.4 32 19 32 18.2

77 21 36 9.3 33 18 24 27.2

78 23 38 5.8 34 13 33 20.6

79 21 37 4.5 35 12 25 25.1

80 19 37 12.7 37 23 25 28.8

81 22 37 8.0 38 18 27 22.9

82 21 38 4.6 39 9 27 37

83 6 22 28.0

85 22 34 4.0

86 27 39 2.8

87 23 28 7.0

99 19 25 18.0

5.3. DIE EVAPORATION OF THE SPECIMEN DURING VACUUM

LEVITATION

Evaporation of the specimens was easily observed when they had been levitated

for about 3 to 5 seconds under a vacuum of 0.2 or 0.4 mbar. Furthermore, the glass

tube, beinl placed inside the levitation coil, was completely covered by black condensate

on its inner wall after 10 to 15 seconds of vacuum levitation. As liven by the

~ -A ..

-(

.

CHAPTER S RESULTS 102

evaporation curve in Figure V.S, almost 15" to 25" of the levitated specimen wa!

evapCJratcd aCter 25 to 3S seconds of vacuum levitation. Assumina that ail the evaporated

matcrials were condensed at the inner wall of the glass tube, the fraction of wei,ht loss

of the specimen due to evaporation was calculated from the measured the weight increase

of the glass tube.

-• 25 • • E

-• 20 -'-c

-0 1 5 'lit -• -c 1 0 0

-• .. 0 CL

• 5 ~

• r 0 .. o o 10 20 30 40

TI m. ( •• c) .'igufC V.S Weight 1055 of the specimens by evaporation durina the vacuum

levitation experiments.

Sucn a reduction of sample weight by evaporation increased the measurecl final

carbon content because carbon was not evaporated. Thus, the measured value of final

carbon content was adjusted as follows to remove the error introduced by evaporation of

the iron:

whcre the terms are defined as follows:

CHAPTER S RESULTS

SI -----------------------

'i' .. .. --Il • -• o

30

u 21 II: o .. .. • .. 20

• •

-o

• •

'al

• -• :1 .. c

10

5

• 1 1 1

• v ..... :~ 1_1 .Ur ! • v ..... :. 1 ••• Ur ,

o ~ ______ ~ _______________ ~ ______ _J

o 10 30 40

Tt Il' ( .. c)

10l

Figure V.6 The adjusted value of carbon content versus time for sorne group 4 samples which have been accurately measurcd for the weight losses.

(V-I)

the adjusted value of carbon content (ppm);

the measured value of carbon content by chemical analysis (ppm);

a" : the percentage of weight reduction by evaporation;

The adjusted values of carbon contents versus lime for the forth group of

specimens are plotted in Figure V.6. Similar to Figure V.4, carbon content under a

vacuum of 0.2 mbar were concentrated in the lower shaded area.

(

(

(

CHAPTER S RESULTS 1 ..

5.4 LEVITATION TEMPERATURE

'. . '. '.

'f?: ., ......... ! ...... ',' . . . . . .. . .,',' ',' ....... ';":' . ','" . , ... ',' .. ;

--Col 1700 - 11." Ui ... '1 .••• : . i : . .' .• 6.. '11'" •• "., ••••••• ,.41 ..... ..," ~ .

• • ... .,. 1545 •

~

• 1110 ... :li ~ .. ... 1210 • • • • 1071 ~

Flaure V. 7 Temperature change during vacuum levitation experiment.

Figure V. 7 shows the measured surface temperature of five Dofasco specimens

recorcled by computer. As given by the curve in Figure V. 7, the surface temperature of

Sample 1 was 1700°C, however the surtace temperature of the remaining four samples

were about 1 SSO°C. Reasons for 5uch a difference in temperature between samples werc

not clear ever. though, in principle, it was known that the heating rate, or in sorne cases,

the equilibrium temperature could be increased by increasing the electrical resistivity, the

density of the specimen and the strength of the levitating field. However, in practic:e,

the surface temperature may al50 be controlled by physical factors which are mOlt likely

to be the weight and the shape of the specimen. Since all the specimens were cut by

hand. the weights ranged from 0.9 to 1.3 grams and the shapes were also different from

specimen to specimen. Thus, such an irregularity in the physical condition of the

specimens increased the difficulty of temperature control.

r

CHAPTER S RESULTS lM

5.5 LEVITATION TlME AND ERROR AN.~LVSIS

Levitation time was measured manuaUy by a digital stop watch with an accuracy

of 0.01 second. Since the time measurement was conducted by human vision, the errer

of the stop watch operation should be the normal human reaction time (from visual to

teaetion) which is considered to be 0.01 to 0.03 sec. The time measurement staned from

the moment that the levitated specimen was observed as being completely melted (see

Filure IV.6 te Filure IV. 1 1). The time measurement stopped during that fall of the

specimen from the levitated state to the copper mould undemeath (Figure IV. 12). The

falUnl lOOk about 0.1 second. The total error of the levitation time measurement should

be the stan and stop errer (0.02 to 0.06 second) plus the specimen free falling time (0.1

second) which is 0.12 to 0.16 second Levitation time (t) can be expressed as t±0.16

(second), taking 0.16 second as the maximum error.

The relative error of carbon content ([e)) caused by the levitation time

measurement can be expressed as d[C)/dt xdt/[C). If the carbon content is expressed by

the relression equations of the four group of specimens (see section 5.2.1 to 5.2.4), the

relative error can be calculated as follows:

hllrouP 2nd IrouP 3rd IrouP 4th IrouP

d[C]/dt (ppm/sec)

-47.4 -6.9 -4.1

-O.052[C]

dt (sec)

0.16 0.16 0.16 0.16

d[C]/dtxdl (ppm) -7.58 -1.1 -0.656

-0.OO83[C]

Relative error d[C]/dt xdt/[C] x 100

(~)

0.48 - 0.8 0.11 - 0.17 0.1 - 0.13

0.83

(

(

(

CHAlyrER 6 DISCUSSION OF EXPERIMENTAL RESULTS 106

CHAPTERSIX

DISCUSSION OF EXPERIMENTAL RESULTS

6.1. TUE RATE DEIIAVIOUR OF THE FIRST AND SECOND GROUP OF

SI'ECIMENS

6.1.1. Erreet or Carbon Cuntent on The Rate of Detarburlzatlon of The Flrst

and Second Group of Specimens

As shown in Figure VI.I, the rate of decarburization of the tirst and second group

of specimens was constant, that is, the carbon content had no effect on the rate of

dccarburization when carbon was above 800 ppm. Similar behaviour has been observed

by JJrevious researchers12Zl•

When carbon was higher than 800 to 1000 ppm the rate behaviour, as shown in

Figure VI. l, was due 10 the fact that there was sufticient supply of carbon to the

dccarbuiÏzation reaction sites, and the rate of decarburization was controll~ by the rate

of sUI>ply 01 oxygen. In steelmaking processes with oxygen injection, the rate of supply

of oxygen can be increascd by increasing the flow rate of oxygen injection. Here,

oxygen was supplied by the following two sources in the vacuum levitation experiments:

(1) Oxygen which was dissolved in the droplets.

(2) Oxygen which was absorbed from the gas phase du ring the levitation

CHAPTER 6 DISCUSSION OF EXPERIMENTAL RESULTS 107

experimen\.

tlO

-CI

• .. • -E A A If -c 0 te

-• 1 lOI

~

~ .. 1 .. • ct

• " - '.1 0

• -• ••• II:

• " 10 1 .. ... , .. , . , ... Car bon cont Int ( p pm)

Filure Vl.t The rate of decarburization versus carbon content of the four groups of specimens

6.1.2. Erreet of Oxy&en Pickup rrom The Vacuum Chamber

The initial oxygen concentrations of the firsl and second group of specimens were

800 ppm and 300 ppm, respectively. However, the eliminatcd amount of carbon was

about 970 ppm for the first group of specimens and 430 ppm for the second group of

specimens. In order to eliminate such an amount of carbon, 1300 ppm and 570 ppm of

· a

(

(

(


oxygcn had to be consul11cd, thus, over 500 ppm and 270 ppm extra oXYlen had to be

addcll for the lirst and second groups of specimens, respectively. This extra oXYlen

must have come from the absorption of oxygen from the las phase durin. the levitation

expcriment ,even though the chamber pressure was about 0.4 mbar.

During an experiment, a steady state was rcached between the rate of las leavinl

the vacuum chamber (pumping) and the rate of air enterinl the chamber from lealcage

of the seals. Assuming the flow rate of lcakage was equalto the vacuum pumping speed

which was 74 m1h'I (20 litre/sec), and the partial pressure of oxygen was 0.08 mbar since

the chamber pressure was 0.4 mbar and 21 ~ of the air leakage was oxygen, the rate of

transrer of Oz into the \'8cuum chamber, Rou is:

PV, R --0, RT

whcrc the tcrms were defined as follows:

V,:

ROI:

P:

R:

T:

the flow rate of gas (litre/sec);

the rate of mass transfer of oxygen in the las phase (mole/sec);

the vacuum chamber pressure (atm);

gas constant (0.082 litre atm/K mol);

temperature (K).

(VI-l)

Il was round that the rate of transfer of oxygen was 7 x Ut' mol/sec. Under the

cnrrent cxperimental conditions, il was unnecessary to consider the formation of oxide

at the interface because in most cases of low oxygen potential or low concentration of

dcoxidizer in Iiquid iron, a visible oxide film is not formed at the interface until the

oxygen concentration in the melt approaches saturation l']).

Previous study found that the rate of oxygen absorption of liquid iron was a

funclion of oxygen gas content and flow rate of gas. As shoWJi in Figure VI.2, the rates

of oxygcn absorption are plotted against the partial pressure of oXYlen on a lolarithmic

seale by Takao Choh et a11911• Il is clearly observed that the rates of oxygen absorption

1

,. 1


are proportional lO the oXYIen partial pressures and increasc with increasing gas flow

rate.

-• • • -• 1

• • -

. ... , ..... ,

lE· ..

.. tI· .. ... • • • • • • • ... • • -• • -• •

n ... ",~~",~""",......N_""'''' __ ···'·''';''''''''''·''''···''''''''· .. ·I

n· 17 "'--"-_~-"" __ -...I ___ ..a.--"" __ __

•• 1 '.1 •.• , t 1 Il JI Il U,

o." .. Il Ih ,U (II'

Fllure VI.2 Effeet of las flow rate and ox YIen partial pressure on the rate of oxygen absorption of liquid iron at 1 600°C.

According to the experimental results obtained in Chapter S on the raie of

evaporation of Iiquid iron during vacuum levitation, the average rate of evaporation of

Iiquid iron within the tirst 30 seconds was 6x 10-5 mol/sec. From above, the rate of

transfer of oxygen into the vacuum chamber during the experiment was 7 x 10~ mol/sec,

that is, 14 x 10.5 alom/sec. Assuming that all of the evaporated iron atoms reacted with

oxygen molecules to form FeO in the gas phase, it was estimated that 40% of oxygen

molecules supplied from the gas phase were consumed by the iron vapour. The

remaining rate of transfer of oxygen was 4 x 10' mol/sec, that is, 8 x IO,J atom/sec. The

,"'.

(

(

n

CHAfJTER 6 DISCUSSION OF EXPERIMENTAL RESULTS 110

very fast flow rate (1200 litre/min) had a positive effeet on increasing the rate of oxygen

adsorption, but the very low partial pressure of oxygen in the vacuum chamber had a

negative effccl.

The average rates of decarburization of the first and second groups of specimens

were 47 ppm/sec (3.8)( 10-6 mol/sec) and 6.9 ppm/sec (S.8x 10-7 mol/sec), respectively.

Such values were slJ1aller than the rate of mass transfer of uxygen in the gIS phase, and

th us oxygen pickup from the gas phase may have contributed to the decarburization

proccss.

6.2. DECARBURIZATION MECHANISM OF THE roVRTH GROUP OF

SI'ECIMENS

6.2.1. MatiaemaUea. Model For The OecarburlzaUon Klnetla or The Fourth

Group or Specimens

A model was developed for the deearburization of a levitated steel droplet for the

fourth group of specimens. Since the initial carbon content of the fourth group of

specimens was about 35 ppm, three assumptions were made for the model:

(1) The oxygen content was much higher than that of carbon 50 that the

oxygen content was considered as a constant during the decarburization

process.

(2) The deearburization reaction only occurred at the surface of the droplet.

(3) Molten steel inside the droplet was always mixed perfectly because of the

strong stirring due the magnetic field, that is, the concentration difference

was only aeross a liquid boundary layer at the surface.

The model was derived by relating the three mass transfer steps described below:

(i) LitlUid pila. te mass Iransler. The instantaneous flux of carbon through the

boundary layer of liquid phase was presented by:

CHAPTER 6 DISCUSSION OF EXPERIMENTAL RESULTS III

(VI-l)

where the terms were defined as follows:

Cil: carbon content inside the molten steel (mol/cml );

CI: carbon content at the reaction surface (mol/cm');

KL: mass lIansfer coefficient of carbon in molten steel (cm/sec).

(ii) Chtmical reaCI;on QI Ilrt .mrfilce 01 drop/el. The renction cqmltion for

decarburization reaction of Jevitated steel droplet with 35 ppm or less

carbon was described as follows:

(l+i)[C] + (1+2;)[0] - {CO} ~ ;{C02} (V 1-3)

i was defined as COz/CO. which means lhat every one mole of CO

produced will be accompanied by i moles of COl' Assuming only CO2

and CO were present. i increased with a decrease of carbon content and

a increase of COl/(C02+CO) which is the partial pressure of COl (Sec

Section 6.2.2). The rate of decarburization rcaction was describcd

approximately as foJlows:

_ de. _ KC"-K P dl r, -~ r

(VI-4)


Kr: reaction constant for the forward reaction (mol·(II.I)cmltl/s);

K.r: reaction constant for reaction in the reverse direction (atm·1molls);

Pr: pressure of the reaction products (CO+COl ) at the reaction surface

(atm);

CI: carbon content at the reaction surface (mol/cm1);

n: the order of decarburization reaction. Ils value can increase with

,~ the increase of i. ...,.

r

(

(

CHAJTfER 6 DISCUSSION OF EXPERIMENTAL RESULTS 112

The flux of the decarburization reaction at the reaction surface was written

as follows:

J Il _ V, de, = Y.(K C" - K p) , A dl If' 1 -"

(VI-!)


V.: the volume of liquid steel at the surface of the droplet which was

directly involved in the decarburization reaction (cm');

A: the surface area of the droplet (cm2).

Here Jr was the number of moles of carbon per unit area of reaction

surface being eliminated by the reaction (mol cm-z s-').

(iii) Ga ... pha.ft' ma.fS trans/no The flux of the reaction products (CO+C~)

in gas phase was described as:

K J Il .2.(P -p ) , RT r ,

(VI-fi)


P,: pressure of the reaction product at the reaction surface (atm);

JI: the flux of mass transfer of gas in the phase boundary of the gas

phase (mol/cm2s);

KI: mass transfer coefficient in gas phase (cm/s);

R: gas constant (82.0594 cm'atm/g-mol);

T: temperature (K);

PI: pressure of in the gas phase (atm).

Assuming lhal the decarbllrizalion process was at steady state, the following

expression held:

JL =) -J r • (VI-7)

,-'-IL:'

L


Under conditions of decarburization chemical equilibriull1, the ratio of raies of the

forward and reverse reactions were proportional to the chemical equilibrium constant.

ie:

(VI-II)

this expression is only rigorously true when ail values are expressed as activities. In the

present work, a term Ke has been used for Equation VI-4 and is detined as follows:

Kr P(.'() K ----~ K Cft

-, S

(V 1-9)

As shown in Equation VI-9, Ke is not exactly the equilibrium constant due to the fact that

the oxygen content [0]. was considered to be a constant and is hidden in the reaction

constant, K" of Equation VI-4. Since the equilibrium constant, K, of decarburil.ation

reaction is pcoICIl.[O]. when carbon and oxygen content are low, Kt can be approximately

expressed as K[01 •.

Dividing Equations VI-2, VI-S and VI-6 by KI., K., and K. respcctively:

(VI-IO)

J, " V- - K~C, -Pr ....!K Il -,

(VI-II)

J.RT _ P_P K " , (VI-11)

,1_ l t . 3 i i

(

(

(


Adding Equation VI-Il and VI-12 to eliminate Pr:

(VI-13'

Dividing Equation VI-13 by K" to eliminate K.r:

J,. J,RT "P, --+-- - C,--V'K K,K, K, A ,.

(VI-I",

Adding Equations VI-IO and VI-14:

(VI-I$)

Assuming that the relationship J., = Jr = J. = J exists al sleady stale, Equation

VI·15 can he wriuen ilS:

Jlt 1 + RT 1 Kt + ;K. K)C,

- c.- [t.+c,-c:] (VI-16)

The flux, J, and the rate, dCJdt, have the following relationship under the

assumption that the levitated droplet can be lreated as a sphere:

vdC. ,dC. J. --- • A dt -3 dt

(VI-11)

whcrc tenns were defined as follows:

V: volume of the levitated droplet (cm');

r: the radius of the droplet (cm);

A: surface area of the droplet (cm2).

Substituting Equation VI-17 into Equation VI-16:

'_:111.1 1

! ~

CHAPTER 6 DISCUSSION OF EXPERIMENTAL RESULTS

C !J.+c-c," de ,,- K Il

b 3 e - - ---~~~--==---.... dl , 1 1 RT -+-+-KI. V, K,K,

-K A r

liS

(VI-II)

ln orcier to solve the equation above, it was necessary to tind a value for, CI' the

carbon content at the surface. The Gibbs energy change for Equation VI-4 is much less

than zero for T - 1873 K, Poo = 1 torr, le] = 35 ppm. [0] = 350 ppm and ail values

of i. Therefore, the partial pressure of the gaseous components were smaU and for

mathematical convenience, the amount of back reaction was assumed to be negligible.

Equation VI-4 can then be rewritlen:

dC, _ -K C" dl r 1

(VI-19)

Integrating Equation VI-19 assuming n > 1;

c. de ,-, r -; -l-·K,dt l~ C, -0

(V 1-20)

the result is:

(VI-21)

Substituting Equation VI-21 into Equation VI-18 and assuming V/V = (1 and

V J A = (Jr/3, yields:

. -C.- ~ +(Krl(n-l)+C~-II).J; - (Krl(n-I)+{'~-")';

. -~~------'------------~~~~~-----------------~~~~ r 1 1 RT -+-+-KI. Q'K K,K,

"'''3 r

l 1

(

(


The numerator of the above equation replesents the drivinl force of

decarburization, and the denominator represents the resistance of decarburization, of

which, I/Kt represents the resistance from liquid phase mass transfer; 3/~rKr)

represents the resistance from decarburization reaction at the surface and RT/<KeK.)

represents the resistance from gas phase mass transfer.

6.2.2. DIscussion of De Mode.

The discussion of Equation VI-22 can be divided into the foUowina two parts:

(1) The discussioll of the denominator which represents the resistances to

decarburization.

(2) The discussion of the numerator which represents the drivinl force for

decarburization.

(1) DUc",siDlI' 0/111' "slsttlIIC' to d,ctU6"ritJIIlo,,:

Jt was known that the decarburization equilibrium constant, K, is 419 (atm/wtr.2),

that is 1.3x 107 (atm/[mol/cm3]~ with only CO las as the product. Xe wu

approximated to have the same value as K[OJ.,quilibrium' that is, Kc = 1.3 X 107 x2.S X 10-'

= 32S (atm/mol/lcm3]), under the assumption that lO]cquilibrium = SO ppm, ie 2.S x Ut'

mol/cm). Kt and Kg were chosen to be 0.02 cm/sec and 2 cm/sec accordinl to previous

studiesl"112311541.

Kr was estimated by the following calculations:

(1) It was seen in Equation 111-3 of Section 3.2 that the specifie surface arA of

suspencled droplet is 3H/r, [that is, 3x(depth of molten steel)/(raclius of the

suspended droplet)], times lreater than that of the molten steel in a crucible or

ladle. Since the depth of the ladle for a ISO tonnes RH unit is about 300 cm, and

the radius of the levitated steel droplet is 0.3 cm, the calculated value is 3000

which means the rate of reaction can be accelerated by a maximum of 3000 times

by increasing the specifie surface area.

CHAPTER 6 DISCUSSION OF EXPERIMENTAL RESUl TS 117

(2) The reaction rate constant in the RH process obtained by H. Konda et al1501 is

0.1 - 0.2/min. that is 0.OO17/sec - 0.OO33/sec with an average value of

O.OO2S/sec. From the value above, the rate constant of a levitated droplet could

be as high as 3000xO.OO2S/sec, that is 7.S/sec, as an average value. For

mathematical convenience, it was assumed that the following relationship exists

in order to calculate the rate constant Kr in Equation VI-IO and VI-22:

de - d,' - K,C," = K; V,C, (V 1-13)

Where the terms were defined as follows:

K t. r •

Kr:

VI:

CI:

n:

reaction rate constant of levitated droplet which equals 7.S/sec;

reaction rate constant of levitated droplet (mol'(II'''cmltl/sec);

the volume of liquid steel at the surface of the droplet which is directly

involved in the decarburization reaction (cm'). fts value was chosen to be

0.08 cm' for a 0.3 cm radius droplet with a 0.1 cm thick boundary layer

(see Section S.4).

the carbon content at the surface of the droplet which was assumed

ranging from 3S ppm to 3 ppm;

the order of the reaction which was assumed lo be > 1 (say 1.2) based on

the previous study (Section 2.3.1).

The calculated result showed that the rate constant, K" can range from 1.9 to 4.9

mol·O.2cm'·'/sec. Kr = 2 mol'O 2cm' 6/sec was th us chosen to be the value for further

calculation.

The lower limit of (J, the fraction of the droplet volume involved in the

decarburization, was determined by assuming that the chemical reaction step was not rate

controlling, i.e.:

R7' .. KJ<, (VI-24)

(

(

(


Substituting Kr = 2, KL = 0.02 and r = 0.3 into Equation VI-24, reveals that Il > 0.1

is required for the reaction step not to be rate limitina.

The upper limit wu then taken u a 1 mm thick layer at the surface of a 6 mm

diameter droplet:

(VI-25)

It was found that the simulation (Section 6.2.1) was insensitive to the chanae in

fJ over the range specified above. Thus a value of IJ = 0.1 wu taken to be a

representative value for the fraction of the droplet volume involved in the chemica1

reaction.

The denominator of Equation VI-22 was calculatecl by substitutina the specified

values mentioned above and the results are listed as follows:

(VI-26)

1 - -SO (J..!:

Kr

(VI-11)

(VI-.)

The value of the gas phase resistance is S times greater than the resistance in the liquid

phase and the rea<:tion at the surface. This led to the conclusion that the total pressure

affccts Kov. the overall mass transfer coefficient.

(2) Dist" •• " ol'h, to"t,lIIlDIÙI" dU/,rr"t, ,,"" ," ,h, /Ira 'flMlllo,,:

",_.

".r


The flux equation shows that the numerator is a function of the order of reaction,

n, which is greater than unit y when COz gas is one of the products of the decarburization

reaction, Section 3.1.1.

The decarburization reaction can be represented by the following two equations

when the carbon content is ultra low:

[C]+2[O] - COz (V 1-19)

[C]+[O] - CO (VI-3D)

At high (or usual) carbon contents, Equation VI-30 dominates, but at ultra low carbon

content, the reaction as described by Equation VI-29 occurs.

Subtracting 2 X Equation VI-30 from Equation VI-29 yields:

COz + [C] • 2CO 4GO - 33260-30.34T (VI-JI)

Taking K as the equilibrium constant for Equation VI-31 (C\iual to 560 at a temperature

of 1873K), and a.: and Pco as the activity of carbon and the partial pressure of CO gas,

respectively, the equilibrium constant for Equation VI-31 i-:.

(VI-Jl)

A~suming that the total pressure is constant and that only CO and COz are in the gas

phase, Equation VI-32 can be rewritten as:

K _ (l-X)2p 1 x 6r

(VI-JJ)

(

(


where x is the CO2 partial pressure in the gIS phase, ie.:

x - (V1-34)

x cao be determined by solving Equation VI-33:

(V1-35)

x -

where P = Pco+ PC02' Figure VI.3 shows the calculated values of x when K = 560 at

T = 1873K and ae = 0.0035 (=35 ppm).

As shown in Figure V1.3, COl partial pressure decreases with decreasing

pressure. Equation VI.3S also shows that the value of x is a function of activity <aJ, ie.,

the lower is the activity of carbon, the higher the value of x.

The observations and analysis can be summarized as follows:

(1) Total pressure affects C~/CO in the proclucts of the decarburization.

(2) ~ affects COlCO in the proclucts of decarburization.

(3) It is believed that as CO2/CO decreases, the orcier of decarburization

reaction tends loward unit y , Section 3.1.1.

Thus:

(1) At low pressure and high ~, the order of reaction would be close to unity.

(2) At low pressure and low~, the order of reaction may be > 1.

(3) At high pressure (ie. industrial vacuum) and low le (i.e., ultra low carbon

contents), the order of reaction would probably be greater 1.

.... .....


o ••

O.U

0.1 -0 Co)

+ N O.U 0 Co)

-N 0 O. 1 Co)

0.05

• Pr ... ur. of C02 + CO (atm)

Flaure VI.3 The change of COzl(C02+CO) in the gas phase versus the change of pressure (P COHCO)

6.3. EFFECT OF SVLPHUR ON THE RATE OF DECARBURIZATION OF

THE THIRD GROUP OF SPECIMENS J

Since sulphur is a surface active element, the presence of sulphur in the droplet

may form a chemisorbed sulphur layer al the mett surface with a result that some of the

decarburization reaclion siles al lhe surface might be blocked which could lead to a

reduction of the rate of deearburization. The initial sulphur content of the third group

of specimens was 3300 ppm, almost 18 times higher than the initial sulphur content of

the second group of specimens which was 180 ppm.

As shown in Figure VI. l, the third grOt'p of specimens did not shown any

significant retarding effeet on the rate of decarburi7.ation even though the sulphur content

'12:.; , l •

(

(


was extremely high. This is believed to be due to the strong stirring inside the droplet

causing eddies in the liquid phase with a result that the surface of the droplet will be

continually renewed by the arrival of new tluid elements which may break the bloclca&e

by the chemisorption of sulphur. Therefore, the effeet of sulphur on the rate of

decarburization was greatly reduced by strong stirrin. in the Iiquid phase.

6.4. DEHA VIOUR OF SULPHUR CONTENT

240

220

-E CL CL 200 --c

180 • -c 0 u 110 .. , ::1 140

fi)

120

100 0 10 20 30 40 10 80

TI m. (Ife)

"~ilure VI.4 Sulphur content of the tirst and second group of specimens

Il was round that there was almost no change of sulphur content for the fint and

second groups of specimens during the levitation experiment, as shown in Filure VI.4.

However, a reduction of sulphur has been observed in the third and fourth lroups of

sllCcimens as shown in Figure Vl.S and VI.6.

ln industrial steelmaking, almost 90~ of the sulphur in steel is removed by sIal

CHAPTER 6 DISCUSSION OF EXPERIMENTAL RESUL TS 123

desulphurization following the reaction indicated in Equation VI-l6:

[FeS] + (CaO) • (CaS) + (FtO) (V 1-36)


[FeS]: the form of sulphur in molten steel;

(CaO): calcium oxide in slag;

(caS): calcium sulphide in slag;

(FeO): iron oxide in slag.

Il is clear that high basicity and low oxygen are favourable for improving slag

desulphurization. Only the remaining 10% of the sulphur could possibly be removed

through the las phase in industrial steelmaking. The possible reactions and thcir value

of 40° can be wrillen as Equation VI-l7 and Equation VI-l8:

[SJ + 2[0] - {SOl} AG" - 1660 + 13.09T (VI-37)

(V 1-31)

Since 40° is positive for Equation VI-37 at steelmaking temperature (187JoK),

reaction of sulphur with dissolved oxygen will not occur unless the partial pressure of

S~ is extremely low. Since oxygen has a stronger affinity for C, in other words,

oXYlen may not react with sulfur unless the carbon content in steel is low. the

oxidization of C has to be taken into consideration even IhoUlh 40° is negative al

steelmaking temperatures for Equation VI-l8. The reaction of decarburi7.ation by oxygen

gas is shown in Equation VI-39:

2lc]+{O]} - 2{CO} AG" - -64200-21.70T (VI-39)

(~

(

(

CHAPTER 6 DISCUSSION OF EXPERIMENTAL RBSULTS 124

Under the elperimental levitation conditions, the temperature of the melted specimens

was about 1873°K, and a relationship Po. =- Pco - 0.4 mbar =- 4 x 104 atm éxisted, thus

. the folloWinl expression can be derivecl:

loaP IQ - - 210lGc + 101tl, - 16.4 • (VI-40)

Assuminl that [C] < 1" (10000 ppm), fe - l, f, = 2.8 and [S] == 300 ppm -= 0.03",

Equation VI-40 can be rewritten 81 Equation VI-4I:

100Pso. - -210I[C] -17.5 t

(VI-4l)

The calculated equilibrium partial pressure of Sa, 181, PI02, Equation VI-41, for the four

groups of specimens is listecl in Table VI.II

Since the partial pressure of S~ must be extremely low for desulphurizalion to

occur by forming SOz IlS, especiaJly when the carbon content is hiah, Sa, may possibly

be formed under the following conditions:

(1) A very high vacuum.

(2) A very low carbon content.

(3) A very high sulphur content.

(4) A very hiah olYlen activity.

Since the partial pressure of SOz miaht be very low inside sorne of the lU

bubbles provideel by gas injection into the motten steel, desulphurization may occur

inside gas bubbles, especiatly for olYlen JU injection.

ln the vacuum levitation elperiment, there were mainly three reasons for the poor

conditions of desulphurization:

(1) There wu liatle slag formed at the droplet surface.

(2) There wu almost no decarburization reaction inside the droplet so that no

CO gas and any other bubbles could be formed inside die droplet.

(3) The experiment time wu very short, 30 to 40 seconds.

" .. .. "~

~ .. ,

l',

i 1


3,200

-I! 3,000 A A - 2,800 -c • -c 2, .00 0 u .. ::1 2,400 -::1 fi)

2,200

2.000 0 5 1 0 1 5 20 25 30

TI me (.ee)

FllUre VI.S Sulphur content of the third group of specimens.

Table VI.II The calculated equilibrium partial pressure of S02

by Bq VI-42 for the entire four groups of specimens

Groups No. [C] ppm [S] ppm Pso. atm

1 1870 240 8.4 x 10-17

2 1230 180 1.3x 10-16

3 880 3000 4.4x 10-15

4 35 40 3.5x 10-14

us

35

(

(

(


45

Ê ca. 40

-c: o

-• --c:

• u c: o u

-::a -

35

30

25

:::1 20 ri)

1 5 o 1 0 20 30 TI me ( •• c)

.·'igl.re VI.6 Sulphur content of the fourth group of specimens

126

40

The following conclusions on desulphurization can be made as a mult of vacuum

Icvitation cxperiment with a vacuum of 0.4 to 0.2 mbar:

(1) As shown in Figure VI.4, there was almost no desulphurization on levitated steel

droplct whcn the carbon is higher than 1200ppm and sulfur is less than l8Oppm.

(2) As shown in Figure V1.5, desulphurization oceurs at [S] > 3000ppm and

lC] < 880ppm with a rate of 40ppm/sec.

(3) As shown in Figure V1.6, desulphuriz.ation occurs al [S] >4Oppm and

(e] < 35ppm with a average rate of 0.48ppm/sec.

..,.

L

CHAPTER 7 CONCLUSIONS 127

CHAPTER SEVEN

CONCLUSIONS

(1) The rate of decarburizalion of levitated steel droplets under vacuum was round

to be 1.S ppm/sec al 30 ppm carbon, which was about 20 times higher th an the

rate of decarburization in the RH process under the same conditions.

(2) The rate of decarburization was found to be constant at le] > 800 ppm and

increased with initial ox ygen content. Il was concluded that the rate of

decarburization was controlled by the rate of mass transfer of oxygen to the

reaction sites al [C] > 800 ppm or this level of carbon.

(3) Reduction in chamber pressure not only improved the rate of mass transfcr in

the gas phase, but also reduced the order of kinetics of decarburi7.ation reaction

which could have a positive effect on accelerating the rate of decarburi7.ation

at ultra low carbon.

(4) The resistance 10 mass transfer in gas phase was 5 times greater than both the

resistance to mass transfer in Iiquid phase and the resistancc of the chemical

reaction at the surface of the droplet. Therefore, the rate of mass transfer in

the gas phase is considered to be the rate controlling step for the overall rate

of decarburization when carbon content is ultra low.

4

(

(

(

CHAPTER 7 CONCLUSIONS lZ1

(5) An oxygen adsorption process at the surface of the droplet was round durina

the experiment.

(6) After 30 seconds of vacuum levitation, almost lS5wt to 2S5wt of the

speci men material was evaporated.

(7) The surface temperature of specimen during the levitation experiments wu

relatively stable for each single test, but varied from ISS0 to 17S0oe for

differcnt specimens.

(8) Strong stirring inside the droplet due to the magnetic induction reduced the

antlcipated retarding effeet on the rate of decarburization by sulfur. Il wu possible to reduce the sulfur content by formin, 802 ,as when the carbon

content was lower than 300 ppm and under a vacuum hi,her than 0.4 mbar.

(') The following suggestions are made for the further reduction of carbon to

below 10 ppm within a Iimiled lime:

(i) Increase the nurnber of suspended droplets durin, vacuum retining without

increasing the size of droplets.

(ii) Increase the fraction of decarburization occurring inside the molten steel

by injecting gas and adding powdered oxidizer.

(iii) Further reduce the partial pressure of CO las especially when carbon

contenl is ultra low.

. "

REFERENCES Il9

Rtlennees

1. Tldashi Ohtake. -Effect of Vacuum Refininl on Steel Properties (Changes in Steelmakin. TechnolOlY with Vacuum Refininl)". Proe. 1'" ICVM, '912, Tokyo, p.821.

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4. R. F. Steilerwald, A. P. Bond, Bond, H. J. Dunclas and B. A. Uzlov: "The new Fe-Cr-Mo Ferritic Stainless Steel" Co",,11011 Vol.33 (1977), p.279.

S. J. Lowe, C. Hunt and H. Harker. "The Mechanical Properties of Iron-Base Alloys Refined in the Electron-Beam Cold-Hearth Fumace". Amltic"l1 Vac"",.. Soe"'" V."",.. M"""",.., eo"',,,"e, (Pittsburl) (1969).

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TowtUfll"",.,., Duellllllllll4 TOII,""'u (held on KyOlo) (1971).

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8. B. Pollard. "Effect of Titanium on The Ductility of 26" Chromium, Low Intentitial Ferritic Stainless Steel" M""" T,c""o"". Jan. 1974, p.JI-36.

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r ·f

(

( "

-

REFERENCES 130

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...

• 131 REFRENeES

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