improvement of cleanness in melt of tool steel by control of...

IN DEGREE PROJECT MATERIALS SCIENCE AND ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2017

Improvement of Cleanness in Melt of Tool Steel by Control of Gas Bubbles And Electromagnetic stirring in the ladle during vacuum treatment

MOHAMMED ALI AYOUB

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

ABSTRACT The first part of the project is mainly about changing of slag compositions and see it affect on the inclusions in the ladle furnace. The second part of the project is to study the influence of stirring (EMGAS) on the large inclusions in the steel by changing stirring conditions.

The project has done at Uddeholm AB, Hagfors. First, the scrap is melted in ARC furnace then moved to the ladle de-slagging station here removing the slag. After de-slagging, a synthetics slag is added to the melt. During that time, the melt reheating and alloying until getting required compositions and temperature. Slag mainly consists of CaO- SiO2-Al2O3-MgO (quaternary system).

The project has divided into four periods; The first period was to evaluate the current slag compositions that used in Uddeholm AB for both types of steel, steel A and steel B. The samples of slag and steel were taken before and after vacuum degassing treatment. In this period we can see also the effect of EMGAS on the inclusion removal and Sulfur refining. During vacuum, the melt exposes to two kinds of stirring, electromagnetic stirring and argon as stirring (EMGAS). Electromagnetic stirring applies with very high-intensity stirring reach to 900 A, the direction of stirring is upward. High flow rate of gas argon applies at the same time from two plugs at the bottom of the ladle. Strong stirring aims to remove the sulphur, hydrogen and nitrogen during 50 minutes for steel A and 30 minutes for steel B of vacuum. After vacuum, the cover has been opened and apply only induction stirring with soft stirring, 650A for 20 minutes for both steel A and B, without gas stirring, to remove the large inclusion.

The second period is to add l-granules manually after synthetic slag addition stage to kill the slag. Both slag and steel samples have been taken before adding Al, before and after vacuum.

The third period is to change slag amount and add pure alumina Al2O3 (260 kg) to ensure the slug was wholly melted at the process temperature. Both slag and steel samples were taken before adding Al2O3, after adding Al2O3 (before vacuum) and after vacuum.

The fourth period is related about how the changing of the stirring condition during vacuum and soft stirring after vacuum affects the large inclusions removal in steel melt. The samples for the steel and slag have taken before and after vacuum and after 20 min of soft stirring (before casting).

ACKNOWLEDGEMENT

First, I would like to thank my thesis adviser Dr Andrey Karasev of the Materials Science and Engineering department at Royal Institute of Technology, KTH. He consistently allowed this paper to be my work but steered me in the right the direction whenever he thought I needed it.

I would also like to thank the ABB AB and Uddeholm AB for giving me this opportunity to work on this project. I would like to thank my supervisors, at Uddeholm AB Dr Mselly Nzotta and ABB AB Dr Lidong Teng and the other experts at Uddeholm AB. Without their passionate participation and input, the validation survey could not have been successfully conducted.

Finally, I must express my very deep appreciation to my parents for supporting me and continuous encouragement during my study and through the process of researching and writing this thesis. This achievement would not have been possible without them. Thank you.

Mohammed Ali Ayoub

Stockholm, 2017

CONTENTS

1 INTRODUCTION ........................................................................................................................1

1.1 The effect of slag compositions on steel melt ........................................................................1

1.2 The effect of EMGAS on sulfur removal and nonmetallic inclusion. ...................................2

2 THEORY ......................................................................................................................................3

2.1 Basicity of the slag .................................................................................................................3

2.2 Sulfide capacity ......................................................................................................................3

2.3 Sulphur Removal ...................................................................................................................5

2.4 De-Oxidation ..........................................................................................................................5

2.5 Argon Gas Stirring .................................................................................................................6

2.6 Electromagnetic Stirring (EMS) ............................................................................................6

2.7 EMGAS Stirring ....................................................................................................................6

2.7.1 Flow Field ......................................................................................................................7

2.7.2 Stirring Energy ...............................................................................................................8

2.7.3 Mixing Time ..................................................................................................................8

3 EXPERIMENTAL .......................................................................................................................9

3.1 Plant Description ....................................................................................................................9

3.2 Plant Trails .............................................................................................................................9

3.3 Overall Work Table .............................................................................................................10

3.4 Procedure of Sampling .........................................................................................................11

3.5 Analysis the Samples ...........................................................................................................11

4 RESULT AND DISCUSSION...................................................................................................12

4.1 Sulphur Removal .................................................................................................................12

4.2 The basicity and sulphide capacity ......................................................................................13

4.2.1 Effect of Al2O3 in the slag on sulfur removal ..............................................................14

4.3 The total number of inclusions in AISI H13 Tools Steel A. ................................................16

4.3.1 Reference heats ............................................................................................................16

4.3.2 Effects of adding pure Al on the nonmetallic inclusion ...............................................19

4.3.3 Effects of changing slag composition on nonmetallic inclusion ..................................21

4.3.4 Effects of changing stirring conditions during vacuum degassing treatment on nonmetallic inclusion ...................................................................................................................24

4.4 The total number of inclusions in AISI H13 Tools Steel B. ................................................31

4.4.1 Reference heats ............................................................................................................32

4.4.2 Effects of adding pure Al on the nonmetallic inclusion ...............................................35

4.4.3 Effects of changing stirring conditions during vacuum degassing treatment on nonmetallic inclusion ...................................................................................................................39

5 CONCLUSION ..........................................................................................................................45

6 FUTURE WORK .......................................................................................................................46

7 REFERENCES ...........................................................................................................................47

1

1 INTRODUCTION

One of the main demands of the steel market is clean steel. To approach this goal many studies and experiments have been occurred to reduce as much as possible the inclusions in steel and other impurities like sulphur and nitrogen. [1-5]

1.1 The effect of slag compositions on steel melt

High sulphide capacity and low viscosity of the slag are the keys to obtaining preferable reactions between slag and steel to get high refining of sulphur. The kinetics reaction between the slag and the melt are critical for desulphurization. Furthermore, kinetics reaction is improved when the slag is liquid at the steelmaking temperature. The presence of solids grains in the slag leads to increase the viscosity which in term gives severe kinetic reaction between slag and steel melt. By changing slag compositions, we can get a slag as a liquid phase at steelmaking temperature. The process of getting liquid slag is very critical where any change in slag composition changes melting temperature of the slag. It can be seen from the quaternary phase diagram, CaO- Al2O3- SiO2 - MgO, fig (1), it should keep the top slag in certain composition to keep it in the liquid area.

Fig. 1 quaternary phase diagram (CaO- Al2O3- SiO2-MgO) in constant amount of Alumina, 35% Al2O3 [27]

Homogeneos liquid

2

The deoxidation processes and refractory wearing directly affect on changing of slag composition which in terms effects on melting temperature of slag. The high amount of CaO in the slag gives high sulphide capacity because of the presence of the ions in the CaO leads to reacting with the sulfur gas to get the sulphur in the slag as CaS and release the O2 gas.

From that, a slag has to be in a liquid state to gives ion and react with melt as is shown in equation (1):

CaO (Ca²+) + (O²-) (1)

That ion plus 2 for Ca react with dissolved sulphur in the steel to gives slag CaS:

(Ca²+) +S CaS (2)

It can be concluded that ore basicity Ca²+ we have, more reaction with dissolved sulphur would be done. As well as, a high SiO2 and Al2O3 content decreases the viscosity of the slag and decreases the sulphur capacity [7]. The stirring of the melt during vacuum degassing treatment is one of the most important parameters which have a high influence on the cleanliness of steel. As mentioned above, for sulphur refining, we need good reaction between steel and slag, and by controlling the intensity of stirring, we can get the required reaction between steel and slag. To control the oxygen and nitrogen content, we need to minimise the open eye zone .the higher stirring intensity the bigger open eye zone, more steel exposes to the surrounding. The stirring method that used in Uddeholm is combined of Electromagnetic stirring (EMS) and the stirring by Argon gas flowing from two plugs from the bottom; this combined method is named as MGAS [1].

1.2 The effect of EMGAS on sulphur removal and nonmetallic inclusion.

The importance of induction stirring and the direction of stirring to reduce the number of inclusion has been studied. The most reduction of inclusion occurred during vacuum degassing treatment with upward stirring direction, while the alternating stirring leads to increasing the number of inclusions [2]. Some studies show that gas stirring has a good effect on decreasing the number of inclusions by creating large inclusions from the collision of small inclusions and rise to the top slag by buoyancy force [3][4]. Other studies showed that it could entrap some slag into the melt if we supply high flow rate of argon as, contrary, the soft stirring has more effect on the removal of inclusion [5]. A study showed that the stirring by gas has more effect on melt mix than the induction stirring, while the induction stirring has more effect on the removal of inclusion. This study showed that the combined between these two stirring methods would result in a high effect on melt mixing and inclusions removal at the same time [6].

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2 THEORY

2.1 Basicity of the slag Increasing of basicity in slag means increasing of basic oxide in slags like increasing CaO in the slag to give Ca2+ and O2-. This increasing of basicity gives a rise the number of free ions of oxygen. Therefore, we can measure the basicity of slag by finding the value of oxygen activity.

The presence of impurities in the steel melt during steelmaking leads to having defects in the steel after casting. One of the critical process during steelmaking is to remove these impurities by oxygen reduction process followed by adding de-oxidation agents like Al, FeSi to reduce the oxygen in the steel melt forming nonmetallic inclusion in terms is very harmful and affect negatively on mechanical properties of the product. Therefore controlling the amount of oxygen in the steel is a very important process. [9]

2.2 Sulfide capacity One of the most harmful impurities on the properties of the product is high sulphur content in the steel for some steel grades. The success of the process depends on the ability of slag for removing the sulphur from the steel melt. That ability called sulphide capacity of the slag. Every slag has different composition and various properties as well as different sulphide capacity. [7]

The kinetics of desulphurization is by adding CaO to the melt as a slag, at high temperature, CaO releases Ca2+ and O2- in the slag phase. The steel phase already has dissolved sulphur S. dissolved sulphur transfer from the steel melt to the slag/metal interface and as well as the O2- moved in the slag towards slag/melt interface. At the interface, occurring the following chemical reaction:

[S]metal + (O2–) slag = (S2–) slag + [O]metal (3)

Where, [S]metal is dissolved sulphur in steel, (O2–) slag is oxygen ion in the slag. The result is S2–, which transfer to the slag and get [O] which moves to the steel melt [9]

The sulphur refining during ladle treatment obey to following reacting: 12 S2 (gas) + (O2-)slag = (S2–)slag + 1

2 O2 (gas) (4)

[7], [13]

4

The sulphide capacity can be calculated. According to the definition by Finchman and Richardson12

for sulphide capacity is given by the following equation:

𝐶𝐶𝑠𝑠 =𝐾𝐾1 𝑎𝑎𝑜𝑜2−𝑓𝑓𝑠𝑠2−

= (% 𝑆𝑆)𝑠𝑠𝑙𝑙𝑎𝑎𝑎𝑎 �𝑝𝑝𝑜𝑜2𝑝𝑝𝑠𝑠2�12� (5)

ao2-: activity of oxygen in the top slag fs2-: activity coefficient of sulphur Ps2: partial pressures of S2 Po2: partial pressures of O2 (%S) Slag: sulphur content in the slag K1: equilibrium constant of reaction 1 [13] the relationship between the sulphide capacity and equilibrium sulphur distribution (slag/metal) can be found from the following equation:

𝑆𝑆 Metal +12 O2 (gas) =O metal+

12 S2 (gas) (6)

From Eq. (6) we can find constant equilibrium K3

𝐾𝐾3 = 𝑎𝑎𝑜𝑜𝑎𝑎𝑠𝑠

�𝑝𝑝𝑠𝑠2𝑝𝑝𝑜𝑜2�1 2⁄

=(%𝑆𝑆)𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

[%𝑆𝑆]𝑚𝑚𝑚𝑚𝑚𝑚𝑠𝑠𝑠𝑠 𝑎𝑎𝑜𝑜𝑓𝑓𝑠𝑠 𝐶𝐶𝑠𝑠

(7)

ao and as are activities of Oxygen and Sulphur in the steel respectively

fs is the activity coefficient of sulphur in the steel

(%S) slag is sulphur content in the slag

[%S] metal is sulphur content in the steel

Now we can find the equilibrium sulphur distribution LS by combined the two equations (5) and (7) to get eq. (8)…

𝐿𝐿𝑠𝑠 =(%𝑆𝑆)𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

[%𝑆𝑆]𝑚𝑚𝑚𝑚𝑚𝑚𝑠𝑠𝑠𝑠 = 𝐶𝐶𝑠𝑠

𝑓𝑓𝑠𝑠𝑎𝑎𝑜𝑜𝐾𝐾 (8)

Since the equilibrium constant K3 described as

Log K3 = −935𝑇𝑇

+ 1.375 (9)

𝑙𝑙𝑙𝑙𝑎𝑎𝐿𝐿𝑠𝑠 = log(%𝑆𝑆)𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

[%𝑆𝑆]𝑚𝑚𝑚𝑚𝑚𝑚𝑠𝑠𝑠𝑠 − 935

𝑇𝑇 + 1.375 + 𝑙𝑙𝑙𝑙𝑎𝑎𝐶𝐶𝑠𝑠 + log𝑓𝑓𝑠𝑠 − 𝑙𝑙𝑙𝑙𝑎𝑎𝑎𝑎𝑜𝑜 (10)

[7],[13]

5

We can see from the eq. (8) that sulphur refining increases by decreasing in oxygen activity in steel, high sulphide capacity and high activity constant of sulphur.

By using Wagner’s equation, the activity coefficient can calculate

log𝑓𝑓𝑗𝑗 = ∑�𝑒𝑒𝑗𝑗𝑖𝑖[%𝑖𝑖]� (11)

𝑓𝑓𝑗𝑗 ∶ activity coefficient for element j

𝑖𝑖 ∶ desolved element in the steel

𝑒𝑒𝑗𝑗𝑖𝑖: 𝑖𝑖𝑖𝑖𝑖𝑖𝑒𝑒𝑖𝑖𝑎𝑎𝑖𝑖𝑖𝑖𝑖𝑖𝑙𝑙𝑖𝑖 𝑝𝑝𝑎𝑎𝑖𝑖𝑎𝑎𝑝𝑝𝑒𝑒𝑖𝑖𝑒𝑒𝑖𝑖 𝑓𝑓𝑙𝑙𝑖𝑖 𝑒𝑒𝑙𝑙𝑒𝑒𝑝𝑝𝑒𝑒𝑖𝑖𝑖𝑖 𝑗𝑗

The reaction between steel and slag can give by general eq.

MexO=XMe+O (12)

The oxygen activity in the steel can be calculated. [13]

2.3 Sulphur Removal One of the most critical processes removes the sulphur, the same for the following parts during the refining process, not for all type of steel, adding CaO to the top slag helps for Sulphur removal, this process illustrated in the equation (13):

[𝑆𝑆 ] + (𝐶𝐶𝑎𝑎𝐶𝐶 ) + 23

[𝐴𝐴𝑙𝑙 ] ↔ (𝐶𝐶𝑎𝑎𝑆𝑆 ) + 13

( 𝐴𝐴𝑙𝑙2𝐶𝐶3 ) (13)

We can see the Sulphur react with Ca to form CaS as slag, with the presence of Al, the O reacts with Al and form Al2O3 in top slag.

To find the Sulphur distribution ratio between the slag and the steel melt [10]

2.4 De-Oxidation

Adding an element to the steel melt like Ca, Si or Al leads to reduce the oxygen amount in the steel melt. This element called deoxidizer, which has high efficiency for reacting with solved oxygen in steel melt. The oxidizer is usually adding to the steel melt as a wire. The first step of refining is removed or decreased the oxygen amount from the steel. The result of this process is oxides formation of the oxidizer. Al is one of the most important element to remove or decrease the oxygen amount from the steel melt where it is powerful tendency to interact with the oxygen to form alumina. The following reaction explains the process of de-oxidation by adding Ca, Si or Al. [10]

[𝑠𝑠𝑖𝑖] + 2[𝐶𝐶] ↔ 𝑆𝑆𝑖𝑖𝐶𝐶2(𝑠𝑠)… [20] (14)

2[𝐴𝐴𝑙𝑙] + 3[𝐶𝐶] ↔ 𝐴𝐴𝑙𝑙2𝐶𝐶3(𝑠𝑠)…. [20] (15)

[𝐶𝐶𝑎𝑎] + [𝐶𝐶] ↔ 𝐶𝐶𝑎𝑎𝐶𝐶(𝑠𝑠)…. [20] (16)

6

Each one of this deoxidiser reduces a certain amount of oxygen inside the steel melt [10]. The deoxidation of steel with ferromanganese give about 100-200 ppm dissolved oxygen while deoxidation of steel with aluminium gives about 2-4 ppm dissolved oxygen and that called killing the steel by aluminium. [21].

2.5 Argon Gas Stirring

During vacuum, argon gas applies from two porous plugs at the bottom of the ladle. It helps for removal of nonmetallic inclusions from the steel to the top slag.

The inclusion removal efficiency for argon gas stirring is high but decreased with increasing of flow of the gas, so we can say that the removal efficiency for inclusion by argon gas is inversely proportional to flow rate of the gas. Removal efficiency for gas stirring is low for the large inclusions ≥ 10 µm. Applying of gas stirring during vacuum gives rise to desulfurization rate and gas removals like H and N. [22]

2.6 Electromagnetic Stirring (EMS) Induction stirring applies at the same time with the argon gas stirring during vacuum. The purpose of the EMS is to get more homogeneous melt and increase the reaction on the interface between slag and the steel melt, and that leads to rising in sulphur, nitrogen and hydrogen removal.

The soft induction stirring (EMS) keep the melt surface covered by slag layer as well as natural controlled gives high predictability and operational flexibility. Also, the yield of alloyed addition and cleanness is greater with Electromagnetic Stirring. [22]

2.7 EMGAS Stirring The process of applying two kinds of stirring at the same time during vacuum called combined stirring or EMGAS. The efficiency of combined stirring is much higher than the efficiency of both of stirring alone. Where sulphur, nitrogen and hydrogen removal as well as nonmetallic inclusion removal taking place at same time.

The efficiency of stirring relies on the intensity. The strong flow of the argon gas gives a good desulphurization. While the calm flow gives better removal of inclusions (excellent inclusions floatation, avoiding re-oxidation and avoiding slag entrapment)

Some companies using both stirrings, induction stirring and gas stirring together to increase the efficiency of removal of inclusion as well as increasing in desulfurization.

Advantageous of EMGAS are:

a. This process takes advantage of two methods ( Gas & EMS stirring ) leads to improvement of

performance of ladle furnace

7

b. The idea of EMGAS is to control the gas bubbles in the melt spread the bubbles in the complete melt. c. The EMGAS shorten the refining time or/and improve the steel cleanness

2.7.1 Flow Field

Fig (2) shows the average melt speed that controlled by EMS after 25 sec of stirring is

Mean value = 0.5 m/sec upwards (stirring direction)

Mean value = 0.7 m/sec downwards (stirring direction)

While with gas stirring, the final average speed is much smaller

Mean value = 0.05 m/sec

Fig. 2 Time history of average melt speed. [23]

As shown in the figure above, the velocity of melt flow decreased a lot when the stirring is carried out by only gas to 0.05 m/sec. Therefore, we can see that the stirring by EMS has more efficiency than the gas stirring [23]

8

2.7.2 Stirring Energy Stirring energy is equivalent to energy input rate and can calculate from the equation; ɛᵒ = ∫𝛷𝛷

𝑀𝑀𝑑𝑑𝑑𝑑 (17)

ɛᵒ: stirring energy

Ω: unit volume

Φ: dissipation rate of mechanical energy

M: weight of liquid steel

The advantageous of this equation is - integration of whole liquid volume - how is the slag layer, free surface and stirring force affected by flow speed and turbulence

2.7.3 Mixing Time

Using only gas stirring that leads to the long mixing time of the steel melt. Using EMS leads to shortened mixing time while using both argon gas and electromagnetic stirring together EMGAS leads to the shortest mixing time. [23]

9

3 EXPERIMENTAL

3.1 Plant Description

The trails carried out at Uddeholm AB. Hagfors, the base materials are come from melting of scrap by using an electric arc furnace. Melting and tapping 65-ton steel melt into the ladle furnace, then moving to the ladle furnace station (heating station) where de-slagging and alloying was performed. After taking off the slag, synthetic slag added to the melt to protect the melt from the surrounding and prepare the steel melt for desulphurization. Reheating and alloying taking place until getting the desired temperature and composition, after that, the ladle transfers to the vacuum station. Before vacuum treatment, taking a sample for the steel, lollipop shape, and sample for the slag, then the steel melt is covering and degassing under low-pressure reach to 5 mbar. For steel A, vacuum treatment time is 50 min, while for steel B, vacuum treatment takes 30 min. In this stage applying two different methods of stirring at the same time (combine stirring), stirring by argon gas occurred by blowing argon gas through two plugs at the bottom of the ladle and induction stirring by applying electromagnetic field on the steel melt. The stirring under vacuum treatment usually is very strong, which creates open eye zone in the melt which in turns helps for removing of hydrogen, nitrogen and Sulphur from the melt. After vacuum treatment, the cover is open and apply only soft induction stirring for 20 minutes to remove the large inclusions, and that called floatation process .last stage, the steel cast into ingots.

The trails apply for two different types of steel grade with different vacuum treatment duration. Steel A, the vacuum’s time is 50 minutes. Steel B, the vacuum’s time is 30 minutes and the Argon gas flowing from two plugs at the bottom with flow rate 40-140 L/min as well as Induction stirring apply with 900 A, upward direction.

3.2 Plant Trails Fig. 3 shows the Trails performed at Uddeholm AB, Hagfors

Fig. 3 Trails performed at Uddeholm AB, Hagfors

Sampling steel and slag

Sampling steel and slag

10

3.3 Overall Work Table The table 1, demonstrates the procedure of the processes that done during thesis work

Table 1 overall work

Periods Number of heats (7 Heats)

Steel Grade

Sampling (18samples)

Analysis Slag Sampling (18 samples)

Analysis methods Analysis methods Measurement

conditions 1) Adding current slag (Reference)

3 heats Steel A (1 heat)

3 (BV) 3 (AV)

XRF and OES (Steel composition)for Period 1,2&3 PC-MIC (number

Done by operator

Period 1

3 (BV) 3 (AV)

XRF analysis

Steel B (2 heats)

of inclusion) for Period 1,2&3

100X magnification

2) Adding Aluminum (on the slag)

3 heats Steel A (1 heat)

3 (Before Al addition)

3 (BV) 3 (AV)

Period 2

3(Before Al addition)

3(BV) 3(AV)

XRF analysis Classificat

ion Size interval (μm)

Medium (DM)

11.2≤ DM <22.4

Heavy (DH)

22.4≤ DH <44.8

Steel B (2 heats)

Particular (DP)

DP >44.8

200X magnification Thin (DT) 2.8≤

DT <5.6

Medium (DM)

5.6≤ DM <11.2

3) Adding pure Alumina (Al2O3)

I heat Steel A (1 heat)

1(Before Alumina Addition) 1(BV) 1(AV)

Period 3

1(Before Alumina Addition) 1(BV) 1(AV)

XRF analysis

4) Changing of stirring conditions during vacuum and during soft stirring after vacuum.

6 heats Steel A (3 heats) Steel B (3heats)

6 BV 6 AV 6 before casting

Period 4

6 BV 6 AV 6 before casting

XRF analysis

SEM (composition of inclusion) for Period 1,2&3

Compositions of Three biggest inclusions

11

3.4 Procedure of Sampling

Sampling is carried out before and after vacuum for the first part of the project (period 1, 2 and 3). For the second part of the project (period 4), the samples of the steel and slag have been taken before and after vacuum and after 20 min of stirring (before casting). The steel sample as lollipop is taking automatically with argon protection(to avoid slag penetration into the steel sample) from the steel melt with different deeps of the melt 10%, 30% and 50% .the usual depth for taking samples is 10%. We obtain two samples with different thickness, one with 5 mm and another one is 12 mm thickness. Then quenched by water. The slag sample is taking manually by using a small scope then leave it on the floor to be cold.

3.5 Analysis For the steel sample, the 12mm thickness sample measured by XRF and OES to get total steel composition, from the pin we get carbon, nitrogen and sulphur content. Then the sample is cut off for total oxygen content measurement by LECO. The 5 mm thickness samples are polished to use later for determining of inclusions size by Light Optical Microscope (LOM). By PC-MIC software [19], the number of inclusions/mm2 can be calculated and the area percentage of inclusion. The inclusions are analysed and classified according to the Swedish standard SS111116. [3]

The table 2 and table 3 show that we using optical microscopic with 100X magnification, the inclusions classified into three groups and 200X magnification used and the inclusions classified into two groups respectively

Table 2 classification of inclusion by using LOM with 100 magnification

Classification Size interval (μm) Medium (DM) 11.2≤ DM <22.4 Heavy (DH) 22.4≤ DH <44.8 Particular (DP) DP >44.8

After that

Table 3 classification of inclusion by using LOM with 200 magnification

Classification Size interval (μm) Thin (DT) 2.8≤ DT <5.6 Medium (DM) 5.6≤ DM <11.2

Later on, the three biggest inclusion be marked from each sample to be analysed by SEM for inclusion composition analysis.

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4 RESULT AND DISCUSSION

In the first period, the study focuses on the current process at Uddeholm AB to know what we have for slag compositions and how that effect on the number of inclusions. This period took as a reference to be compared later with other periods. In the second period, the slag has killed by adding 10 kg of Al (granular) on the top slag in the ladle furnace during heating and alloying stage. The purpose of this change is to increase deoxidation in the ladle furnace and how that could be the effect on sulphur and non-metallic inclusion removal. In period 3, the primary focus was to study the changing of slag composition on the number of non-metallic inclusion in the melt. The slag composition has changed where the CaO has decreased to the half of its typical value. Al2O3 has been added manually as pure alumina to the melt instead of adding mix with the MgO while the amount of (CaMg (CO3)2) had increased or decreased In period 4, the study was a focus on the effect of change stirring conditions during vacuum degassing treatment on the Sulphur and nonmetallic inclusion removal.

Sulphur Removal

The effect of sulfur content was same where decreasing of sulphur content is the result. When looking at the sulphur content in Figure 4, it can be seen that in period one which is the reference period, the decreasing percentage of the Sulphur is higher than in other periods. Also, it can be seen that this percentage decrease is lowest in period 3, where the slag composition has changed. Probably the reason is that the low amount of CaO which added as synthetic slag, where the CaO in period 3 added only half of its real value as mentioned earlier.

It can also be seen, in period four where the slag composition is same slag composition in period 1 (Reference), the removing of Sulphur content is higher than period two and three but lower than period 1. It seems like the changing of stirring condition does small effect on removing of Sulphur content since we reduce the stirring intensity from 900A to 800A as well as minimise the flow of argon gas from 40 to 10 L/min.

Fig. 4 Effect of four periods on sulphur content in the steel melt.

0.003

0.0004

0.00510.0044

0.0015

0.006 0.0058

0.00270.003333

0.000833

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Bv Av B addingAl

BV AV B addingAl2O3

BV AV BV AV

period 1, Ref. period 2 killing the slag byAl

period 3, changing of slag comp. period 4 , changing instirring condition

Sulp

hur c

onte

nt in

stee

l [%

S]

13

Since every heat has different conditions, for example, the slag carry-over that is coming from arc furnace varies from heat to heat that gives various conditions for each heat. Hence, the better way to compare the sulphur removal between the heats is to calculate the decreasing percentage of each heat by using the following equation:

[(BV-AV)/BV] *100%= percentage decreasing of Sulphur (18) The fig. 5, shows in period 1, the amount of Sulphur has decreased about %86 after vacuum. In period 2, killing the slag by added 10 kg Aluminium on the slag during ladle treatment, the decreasing of Sulphur was about %65 after vacuum. In period 3, changing of slag composition, added 260 kg pure Al2O3 on the slag during ladle treatment, the Sulphur deceased about %55. In period 4, has the same slag composition as reference period but decreasing in stirring conditions gives 74% of decreasing percentage of Sulphur.

Fig. 5 decreasing percentage of sulphur.

4.1 The basicity and sulphide capacity

Higher basicity gives higher sulphide capacity of the slag which in terms leads to higher removal of sulphur from the steel melt to the top slag. From the previous result, it can be seen that the basicity of slag has been changed from a period to another. in period 2, adding pure Al on the slag, probably formed alumina in the slag as well as in period 3, where alumina has added as pure to the slag. In both cases, the amount of Al2O3 has increased. Since Al2O3 and SiO2 are acidic oxides that lead to decrease the basicity of the slag, therefore, decreasing in sulphur removal. The decreasing of CaO and MgO content in slag also leads to decreasing the basicity of the slag, where CaO and MgO are basic oxides, thus, diminishing of sulphide capacity of the slag. In fig 6, it can be seen the relationship between basicity and sulphide capacity where the basicity has calculated in the following Eq. [14]

(CaO + MgO)/Al2O3 + SiO2) = basicity of the slag (19)

86%

65%55%

74%

0%10%20%30%40%50%60%70%80%90%

100%

period 1, Ref. period 2 killingthe slag

period 3changing slag

comp.

period 4, chingof stirringcondition

decr

easi

ng o

f sul

phur

cont

ent

afte

r vac

uum

14

Fig. 6 shows the relationship between basicity and sulphide capacity [14]

4.1.1 Effect of Al2O3 in the slag on Sulphur removal

Table 4, 5 and 6 show the slag composition before and after vacuum for period one, two and three respectively.

Table 4 Period 1, slag composition BV & AV

Table 5 Period 2 , slag composition BV & AV

%CaO % SiO2 %MgO %Al2O3 Total Charge

49.3 8.9 15.9 25.7 100 BV

44.2 11.3 16.8 27.5 100 AV Table 6 Period 3, slag composition BV & AV


46 1 11.9 32 100 BV

41.1 6.8 13.7 38.3 100 AV From the tables above shows that the amount of alumina has increased in the slag after vacuum. In the period 3, the decreasing percentage of sulphur removal is deficient. As is shown in the table, changing of the slag composition by adding pure alumina gives rise the alumina amount in the slag which in turn decreases the basicity of the slag.

0

2

4

6

8

10

12

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

C S*1

03

(CAO+MGO)/AL2O3+SIO2)

S U L F I D E C A P A C I T I E S A G A I N S T T H E E X T E N D E D B A S I C I T Y A T 1 8 7 3 K( 1 6 0 0 C ) .

CARL ALLERTZ and DU SICHEN [14] Nzotta [16]Al2O3-CaO-MgO Hino et al [17] Al2O3- CaO- SiO2 Hino et al [17]


53.8 11.1 11.8 23.1 100 BV

51.2 6.1 11.5 31.08 100 AV

15

It is possible to compare period 2 and 3 with the period 1. Al2O3 has also increased in period 1 but if the amount of SiO2 is also taking into account, sees that the SiO2 amount in period 1 after vacuum decreased to the half and that leads to increasing the basicity of slag while the amount of SiO2 in period 2 increased after vacuum. It would explain why in period one the slag has more sulphide capacity than period 2 and 3.

Table7 and fig 7, shows an overview of how the basicity of the slag has decreased before and after vacuum during period 1, 2, 3 and 4

Table 7 Basicity of the slag

Periods Basicity BV (%CaO+%MgO)/(%

SiO2+% Al2O3) Basicity AV (%CaO+%MgO)/(%

SiO2+% Al2O3) period 1 1.9 1.68 period 2 1.88 1.57 period 3 1.3 1.21 Period 4 1.9 1.75

Fig. 7 The amount of slug basicity for each period before and after vacuum.

1.9 1.88

1.3

1.901

1.681.57

1.21

1.75

00.20.40.60.8

11.21.41.61.8

2

period 1,Ref. period 2, killing the slagby Al

period 3 changing slagcomp.

period 4 chang stirringcondition

(%Ca

O+%

MgO

)/(%

SiO

2+%

Al2O

3)

BV AV

16

4.2 The total number of inclusions in AISI H13 Tools Steel A.

In table 8, can see the composition of AISI H13 Tools Steel A.

This grade of steel has a long vacuum’s time reach to 50 minutes while another steel grade spends only 30 minutes in the vacuum degassing treatment process.

Table 8 Composition of steel grade A

Typical Analysis %

C 0.35

Si 0.2

Mn 0.5

Cr 5.0

Mo 2.3

V 0.6

4.2.1 Reference heats

From the table 9 and the fig 8, it can be seen that the small inclusion < 11.3 increased after vacuum. The large inclusion ≥ 11.3 has decreased. The study focused on large inclusion since the small inclusion has no severe effect on properties of this steel grade..

Table 9 total number of nonmetallic inclusion before and after vacuum.

Number/mm² BV AV

DT 2.8 0.07958 0.08952

DM 5.7 0.05471 0.06466

DM 11.3 0.02997 0

DH 22.6 0.01776 0

DP 45.3 0.00777 0

total 0.1898 0.15418

17

Fig. 8 The total number of inclusion /mm2 before and after vacuum for period 1.

As can be seen in table 10. The slag composition before and after vacuum

Table 10 The slag composition BV & AV

The table 11 and fig 9, show the Type and compositions of inclusion that found before vacuum.

Table 11 Compositions of inclusions found before vacuum

%CaO %SiO2 %MgO % Al2O3 Total2

43.6 16.4 7.1 32.8 100

Fig. 9 Shape of inclusions found before vacuum by using Scanning Electron Microscope

The data in table 12 shows the compositions of inclusions that found after vacuum and the images in fig 10 show the shape of these inclusions

00.020.040.060.08

0.10.120.140.160.18

0.2

DT 2.8 DM 5.7 DM 11.3 DH 22.6 DP 45.3 total

Num

ber/

mm

²

BV AV

%CaO %SiO2 %MgO % Al2O3 Total Charge

53.8 11.1 11.8 23.2 100 BV

51.2 6.1 11.5 31.1 100 AV

18

Table 12 Compositions of inclusions found after vacuum

%CaO %SiO2 %MgO % Al2O3 Total2

28.9 4.3 46.1 20.6 100

Fig. 10 Shape of inclusions found after vacuum by using Scanning Electron Microscope

From fig. 11, before and after vacuum, shows the decrease in Al2O3, SiO2 and CaO amount in inclusions after vacuum while increases of MgO inclusion. Since the amount of MgO in the slag before and after the vacuum is almost same (as shown in tables above), therefore the MgO inclusions are probably coming from the refractory during stirring of the melt during vacuum degassing treatment.

Fig. 11 Inclusions composition BV & AV

0

5

10

15

20

25

30

35

40

45

BV AV BV AV BV AV BV AV

Al2O3 MgO CaO SiO2

incl

usio

n co

mp.

%

19

4.2.2 Effects of adding pure Al on the nonmetallic inclusion In this heats, 10 kg of pure Aluminium, as granular, added on the top slag manually to kill the slag during heating and alloying in ladle furnace. The number of inclusion had been found in this process before and after vacuum illustrate in table 13 and fig. 12

Table 13 The total number of nonmetallic inclusion before and after vacuum

Fig. 12 total number of inclusion /mm2 before and after vacuum.

The result from the comparison above shows that adding of 10 kg of pure Al on the top slag in ladle furnace during heating and alloying gives rise the number of inclusions ≤ 22.6 µm after vacuum. The reason could be due to the forming of alumina in the slag (increasing of Al2O3 content in the slag) by adding 10 kg of pure Al to the slag. High Alumina content in the slag which not saturated with the CaO makes the slag easy to react with the refractory which made of dolomite to dissolve the calcium oxide from the wall to get saturated value with CaO. [24]. That explains the increase of CaO and MgO inclusion after vacuum. It is worth mentioning that the amount of alumina which formed in the slag during period 2 was higher than aluminium content in the slag during period one but was enough to give the slag a good Sulphide capacity. At the same time, this amount of alumina was also high enough to formed inclusions more than the inclusions which formed in period one after vacuum.

As in table 14 can be seen the slag compositions before and after vacuum

0

0.05

0.1

0.15

0.2

0.25


Num

ber/

mm

²

before Al addition BV AV

Number/mm² before Al addition BV AV

DT 2.8 0.06498 0.005 0.07998

DM 5.7 0.04999 0.02999 0.12496

DM 11.3 0.00444 0.00222 0.0222

DH 22.6 0 0.00111 0.00222

DP 45.3 0.00111 0.00111 0.00111

total 0.12052 0.03943 0.23047

20

Table 14 Slag composition BV & AV

%CaO %SiO2 %MgO % Al2O3 Total Charge

49.3 8.9 15.9 25.7 100 BV

44.2 11.3 16.8 27.5 100 AV

Table 15 and fig. 13 demonstrate the compositions and the type of inclusion before vacuum respectively

Table 15 Compositions of inclusion found before vacuum

%CaO %SiO2 %MgO %Al2O3 Total2

17.2 25.2 1.05 56.4 100


Table 16 and fig 14 demonstrate the compositions and the type of inclusion after vacuum respectively

Table 16 Compositions of inclusion found after vacuum

%CaO % SiO2 %MgO %Al2O3 Total2

24.2 10.6 5.4 59.7 100


21

From the fig. 15, it can be seen that small amount of Al2O3 as inclusion has increased after vacuum as well as MgO and CaO.


4.2.3 Effects of changing slag composition on nonmetallic inclusion

In these heats, the compositions of top slag have been modified to study the effect of top slag on nonmetallic inclusion in the steel melt. The CaO reduced to the half and added about 260 kg of pure AL2O3 manually on the top slag. In table 17 and figure 15 can see the effect of this change on the total number of inclusion after vacuum.

Table 17 total number of nonmetallic inclusion before and after vacuum

Number/mm² before Al2O3 addition BV AV

DT 2.8 0.02984 0.10497 0.04999

DM 5.7 0.02984 0.11997 0.07998

DM 11.3 0.00444 0.00555 0.01443

DH 22.6 0.00111 0.00111 0.00555

DP 45.3 0 0.00111 0.00222

total 0.06523 0.23271 0.15217

From Fig. 16 it was clear that the changing of slag composition gives rise the number of inclusions after vacuum. It can be observed from the same fig also that the inclusions smaller than 11.3 decreased after vacuum while inclusions ≥ 11.3 are increased after vacuum.

0

5

10

15

20

25

30

35

40

45

50


Al2O3 MgO CaO SiO2

incl

usio

n co

mp.

%

22

Fig.16 Shows the total number of inclusion /mm2 before and after vacuum in period 3

From the table 18 can see the slag composition before and after vacuum for the heats

Table 18 Slag composition BV & AV


46 10 11.9 32 100 BV

41.1 6.8 13.7 38.3 100 AV

Type and composition of inclusions

From the table 19 and fig. 17 it can be seen the compositions, and the shapes of nonmetallic inclusion found before vacuum degassing treatment

Table 19 composition of inclusion found before vacuum


0

0.05

0.1

0.15

0.2

0.25


Num

ber/

mm

²

before Al2O3 addition BV AV


25.4 51.5 6.3 16.7 100

23

From the table 20 and fig 17, it can be seen the compositions, and the shapes of nonmetallic inclusion found after vacuum degassing treatment

Table 20 composition of inclusion found after vacuum


43.9 0 10.4 45.6 100


In figure 19 illustrate the composition of inclusion and comparison for each oxide before and after vacuum.


In period 3, 260 kg of pure alumina has added to the slag. In the diagram above, increasing of different types of inclusion is shown. However, the increasing of alumina content in inclusion after vacuum expected. It could be due to add pure alumina to the slag. The purpose of changing the slag composition was to get liquid slag at steel process temperature. As mentioned in period 2, the increasing of alumina in a slag which not saturated with CaO makes the slag very easy to penetrate to the ladle wall and dissolve the CaO and MgO to the melt. In period 3, adding 260 kg of alumina to the slag result in high alumina content in the slag, higher than alumina content in the slag during

0

10

20

30

40

50

60


Al2O3 MgO CaO SiO2

incl

usio

n co

mp.

%

24

both period 1 and 2. That explained the lowest sulphide capacity of the slag for period 3 (The low CaO content in the slag could be the main reason for decreasing of sulphide capacity of the slag) and explained the high increase of Al2O3, CaO and MgO as nonmetallic inclusion after vacuum. In the fig below can show the comparison of number of inclusion ≥11.3 µm between period 1, 2 and 3 in steel grade A

4.2.4 Effects of changing stirring conditions during vacuum degassing treatment on nonmetallic inclusion

In this heats, as mentioned before, the changing occurred in stirring condition. As steel grade, A, the vacuum’s time is 50 min during this heats the strong induction stirring and argon gas applied for 50 min. The changing is after 10 min of vigorous stirring; the induction stirring reduces from 900A to 800 A for the rest of the time (40 min) while the flowing rate of the argon gas also reduced after 10 minutes from 40 to 10 L/min. After vacuum degassing treatment, the melt usually exposes to the soft induction stirring reach to 650 A. However, the induction stirring turn off and replaced by only soft argon gas stirring for 20 minutes. The samples in this periods had been taken before vacuum, after vacuum and after soft stirring (before casting)

The total number of nonmetallic inclusion before vacuum and after vacuum and before casting for three heats shown in the tables 21, 22 and 23, and figures 20,21 and 22 below.

Heat 1

Table 21 total number of nonmetallic inclusion before after vacuum and before casting.

Number/mm² BV AV BC

DT 2.8 0.015 0.015 0.02499

DM 5.7 0.05998 0.04499 0.01999

DM 11.3 0.0111 0.02442 0.01443

DH 22.6 0.00222 0.00555 0.00999

DP 45.3 0 0.00111 0.00333

total 0.0883 0.09107 0.07273

25

Fig. 20 Shows the total number of inclusion /mm2 before, after vacuum and before casting in period 4

Heat 2



DT 2.8 0.02499 0.01999 0.04999

DM 5.7 0.02999 0.03999 0.07498

DM 11.3 0.00555 0.00777 0.00888

DH 22.6 0.00222 0.00111 0.00555

DP 45.3 0 0 0

total 0.06275 0.06886 0.1394

Fig. 21 Shows the total number of inclusion /mm2 before, after vacuum and before casting in period

0

0.02

0.04

0.06

0.08

0.1


Num

ber/

mm

²heat 1, comparison BV ,AV & BC

Number/mm²

BV AV BC

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16


Num

ber/

mm

²

heat 2, comparison BV ,AV & BC Number/mm²

BV AV BC

26

Heat 3


Fig. 22 Shows the total number of inclusion /mm2 before, after vacuum and before casting in period 4

The comparison between 3 heats. The total numbers of all three heats 1,2 and 3 BV, AV and BC shown in the in fig. 23.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14


Num

ber/

mm

²


BV AV BC


DT 2.8 0.05498 0.03999 0.03999

DM 5.7 0.02499 0.06998 0.06998

DM 11.3 0.00222 0.01776 0.0111

DH 22.6 0 0.00333 0.00777

DP 45.3 0 0.00222 0.00111

total 0.08219 0.13328 0.12995

27

Fig. 23 Comparison of a total number of inclusions between heat 1,2 and 3 before, after and before casting.

From the chart above, it is evident that the number of inclusions after vacuum degassing increased. Properly the stirring condition was not enough to remove the inclusions. The result almost the same for larger inclusions ≥ 11.3 µm as illustrated in figure 24.

Fig. 24 Comparison of some inclusions ≥ 11.3 µm between 3 heats 1,2 and 3 before, after and before casting.

Comparison between reference heats and new heats after changing stirring conditions. A better way to see how the number of inclusions after the vacuum has been affected by changing of stirring conditions during vacuum is to make a comparison between reference heats that exposed the normal stirring condition during vacuum and the heats that exposed to the new stirring conditions during vacuum. That comparison is evident in fig 25.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

heat 1 heat 2 heat 3

Num

ber/

mm

²heat 1,2 &3 comparison of total number of inclusions BV ,AV & BC

Number/mm²

BV AV BC

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

heat 1 heat 2 heat 3Tota

l no.

of i

ncl-

≥ 11

.3µm

heat 1,2 &3 comparison of number of inclusions ≥ 11.3 µm BV ,AV & BC Number/mm²

BV AV BC

28

Fig.25 comparison between reference heats and new heats with changing of stirring conditions

It was seen that in only one heat the inclusion is reduced after vacuum, the reference heat gave almost the same result that the new heat gave. The changing in conditions does not change too much.

Percentage increase in a total number of inclusion ≥ 11.3 µm in 6 Heats can see in fig 26. It is clear to see the effect of changing stirring conditions on the number of inclusions ≥ 11.3 µm after vacuum. The average of percentage increase of inclusion in reference heats is lower than average of percentage increase of inclusion in new heats. Thus, the changing of stirring condition gives rise the number of inclusions after vacuum.

Fig. 26 Percentage increase of the number of inclusion ≥ 11.3 µm in 6 Heats after vacuum

Table 24. Demonstrates the Slag composition of heat 1, 2 and 3 Before Vacuum, After Vacuum and before Casting

0

0.01

0.02

0.03

0.04

0.05

0.06

heat 1 heat 2 heat 3 heat 1 heat 2 heat 3

reference heats new (changing stirring condition)

Tota

l no.

of i

ncl-

≥ 11

.3µm

BV AV

-200%

0%

200%

400%

600%

800%

1000%

1200%

heat 1 heat 2 heat 3 Average heat 4 heat 5 heat 6 Average

ref new

Perc

enta

ge In

crea

se n

o. O

f inc

l-≥1

1.3

29

Table 24 Slag composition of heat 1, 2 and 3 Before Vacuum, After Vacuum and before Casting

Heats %CaO % SiO2 %MgO %Al2O3 Total2

BV1 53.6 8.9 10.7 26.5 100

AV1 49.1 10.1 13.1 27.4 100

BC1 49.4 10.2 12.8 27.5 100

BV2 51 8.4 17.6 22.8 100

AV2 48.8 7.6 18.3 25.1 100

BC2 49.2 7.6 17.7 25.3 100

BV3 53 10.3 9.9 26.7 100

AV3 48.8 8.6 11.8 30.6 100

BC3 49.1 8.6 11.6 30.6 100

Table 25. Demonstrates the Inclusions compositions found in samples from heat 1,2 and 3 Before Vacuum, After Vacuum and Before Casting

Table 25 Inclusions compositions found in samples from heat 1,2 and 3 Before Vacuum, After Vacuum and Before Casting

Heats %CaO % SiO2 %MgO %Al2O3 Total BV1 25.2 37.1 11.4 26 100 AV1 42.4 8.5 10.9 38 100 BC1 38.6 15.4 8.1 37.7 100 BV2 11.6 0.5 17.2 70.6 100 AV2 43.7 0.8 13.8 41.5 100 BC2 44.2 0.8 16.1 38.8 100 BV3 14.7 1.5 11.2 72.4 100 AV3 39.3 7.04 9.7 43.8 100 BC3 37.6 5.7 11.6 44.9 100

Three graphs show the position of the average composition of inclusions that were found and analysed by SEM in three heats as well as demonstrate the position of the slag composition for these three heats before vacuum, after vacuum and before casting. In figures 27, 28 and 29 show the Phase diagrams of Al2O3-CaO-MgO- SiO2 with 10 mass % MgO and phase diagram Al2O3-CaO-MgO- SiO2 with 15 mass % MgO have been used to demonstrate the position of slag and nonmetallic inclusion in the steel.

30

Fig. 27 Liquidus surface in the system Al2O3-CaO-MgO- SiO2 with 10 mass % MgO after Cavalier, Sandreo-dendon. [25] For the base system Al2O3-CaO- SiO2. [26]


Heat 1

Heat 2

31


It was also possible to observe the inclusion before vacuum for three heat is located far away from the slag composition, probably these inclusions came from carry-over slag from ARC furnace. Later on, it can see also the inclusion after the vacuum is closer to the slag composition. The 50 minutes vacuum may give the inclusions enough time to develop and react chemically to be closer to slag composition. Also, could be a physical effect like move some pieces from the top slag into the steel melt during vacuum due to the high stirring for 50 minutes.

4.3 The total number of inclusions in AISI H13 Tools Steel B.

Another steel grade has studied in this project is steel grade B. as can be seen from table 26 the compositions of this grade.

Table 26 Compositions of steel grade B

Typical Analysis %

C 0,39

Si 0.1

Mn 0.4

Cr 5.3

Mo 1.3

V 0.9

This kind of steel spends 30 minutes in the vacuum degassing treatment exposed to very strong electromagnetic stirring (900A) and argon gas stirring (40-140 L/min) just like the same in steel grade A but for a shorter time.

Steel grade B has been examined for only two periods, period one which is the reference one and period two were killing the slag by adding 10 kg of pure Al on the slag during heating and alloying in ladle furnace.

Heat 3

32

4.3.1 Reference heats

The total number and composition of nonmetallic inclusion before and after vacuum can be seen in table 27.

Table 27 A total number of inclusion before and after vacuum.

Number/mm² BV AV

DT 2.8 0.04974 0.05968

DM 5.7 0.02984 0.03482

DM 11.3 0.00555 0.01332

DH 22.6 0.00111 0.00777

DP 45.3 0 0.00222

total 0.08624 0.11781

In Figure 30 and Table 27 above, a total number of nonmetallic inclusion shows before and after vacuum for steel grade B. it is evident from the fig. that all size of nonmetallic inclusion for steel grade B has increased after vacuum.

Fig. 30 shows the numbers of the inclusion before and after vacuum

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

DT 2.8 DM 5.7 DM 11.3 DH 22.6 DP 45.3 totalBV AV

33

The slag compositions in this period have obtained before, and after vacuum, it shows in table 28.

Table 28 slag composition BV & AV (AISI H13 Tools Steel B)

%CaO % SiO2 %MgO %Al2O3 Total2 Charges

51.5 9.2 9.2 29.9 100 BV

48.6 9.4 9.8 32 100 AV

As table 29 and fig 31 demonstrate the compositions and type of inclusion respectively

Table 29 Compositions of inclusion found before vacuum


32.1 7.3 9.2 51.3 100


As table 30 and fig 32 both demonstrate the compositions and type of inclusion found after vacuum

Table 30 compositions of inclusion


40.6 12 7.1 40.1 100


34

In figure 33 illustrate the composition of inclusion and comparison for each oxide before and after vacuum.


From the diagram above, it can be seen the comparison between inclusions composition before and after vacuum. In this period, reference slag and reference heat condition applied. The result from all heats of steel grade B leads to increase in inclusion after vacuum.

The phase diagram Al2O3-CaO- SiO2 with 10% of MgO shows the average compositions of inclusions before and after vacuum which represented by red and blue circle respectively. The composition of the slag before and after the vacuum is represented by red and blue square respectively. It is possible to observe the composition of inclusions after vacuum became closer to the top slag composition compared to the composition of inclusion before vacuum.

It also be can be observed as in fig 33 that the composition of inclusion before vacuum (red circle in the phase diagram) within Spinel region. That shown in fig of inclusion before vacuum where the inclusion structure consists of spinel phase of MgO-Al2O3, which is a lighter area in figure 34.

05

101520253035404550


Al2O3 MgO CaO SiO2

35

Fig. 34 Liquidus surface in the system Al2O3-CaO-MgO- SiO2 with 10 mass % MgO after Cavalier, Sandreo-dendon.[25] For the base system Al2O3-CaO- SiO2. [26] 4.3.2 Effects of adding pure Al on the nonmetallic inclusion Figure 35 and the table 31, show the number of inclusions for all size before adding Al, before vacuum and after vacuum. It can be seen from the fig that the result is increasing the number of inclusion after vacuum compared with the number of inclusions before vacuum. It also shows that two sizes DH 22.6 µm and DP 45.3 µm was zero before vacuum and didn’t change after vacuum. The high number of inclusion before adding Al could be due to the way that the sample was taken from the steel melt, the equipment using for sampling is not protected by argon gas during sampling, and that makes the oxygen pickup of steel during sampling.

Table 31 total and compare number of nonmetallic inclusion before and after vacuum

Number/mm² before Al addition BV AV

DT 2.8 0.08997 0.01999 0.04999

DM 5.7 0.08498 0.01999 0.03999

DM 11.3 0.00666 0.00222 0.01221

DH 22.6 0.00444 0 0

DP 45.3 0.00444 0 0

total 0.19049 0.0422 0.10219

36

Fig. 35 Number Of inclusion before al addition, before and after vacuum

• Slag composition

table 32 shows the slag composition before adding Aluminum, before vacuum and after vacuum.

Table 32 slag composition before adding Aluminum, before vacuum and after vacuum.

%CaO % SiO2 %MgO %Al2O3 Total2 Charge

43.5 7.5 15.5 33.2 100 before adding Al

44 7.3 15.5 32.9 100 BV

47.1 9.3 13.8 29.6 100 AV

• Type and composition of inclusion

Before vacuum, in table 33 and fig 36 can be seen the composition and shapes of inclusion found before vacuum respectively

Table 33 compositions of inclusion BV


13.7 30.3 14.8 41 100

0

0.05

0.1

0.15

0.2

0.25


No. Of inclusion befoer al addition ,before and after vacuum

before Al addition BV AV

37


After vacuum, in table 34 and fig 37 can be seen the composition and shapes of inclusion found after vacuum respectively

Table 34 Composition of inclusion found after vacuum


41.6 3.7 13.1 41.4 100


In this case, it can be seen that adding of Al to kill the slag leads to increase the alumina in the slag which became not saturated with CaO and make the slag more aggressive to the refractory and dissolved CaO. From the fig 38, it is obvious that the content of CaO has increased more than other oxides. By plotting the composition of the inclusion and slag on the phase diagram of Al2O3-CaO-MgO- SiO2 with 15 mass % MgO, it can predict the source of inclusion after vacuum.

38


As same as in period 1, the plot of the phase diagram Al2O3-CaO-SiO2 with 15% of MgO, fig 39 shows the average compositions of inclusions before and after vacuum which represented by red and blue circle respectively. While the composition of the slag before and after the vacuum is represented by a square, red and blue respectively. It is possible to observe the composition of inclusions after vacuum became closer to the top slag composition compared to the composition of inclusion before vacuum


0

5

10

15

20

25

30

35

40

45

50


Al2O3 MgO CaO SiO2

39

4.3.3 Effects of changing stirring conditions during vacuum degassing treatment on nonmetallic inclusion

In this period, applied the same changing of stirring condition in steel grade A. used lower induction stirring after 10 minutes from 900A to 800A and the flowing of argon gas reduced from 40 to 10 L/min after 10 minutes. The difference with steel grade B is the time of vacuum is 30 minutes.

The total number of nonmetallic inclusion before vacuum, after vacuum and before casting for heats 1shown in the tables 35 and figure 40 and for heat 2 shown in table 36 and figure 41

Heat 1

Table 35 total number of nonmetallic inclusion for heat 1, before after vacuum and before casting.


DT 2.8 0.02999 0.03999 0.04499

DM 5.7 0.02499 0.02499 0.03499

DM 11.3 0.00444 0.00666 0.01554

DH 22.6 0.00111 0.00333 0

DP 45.3 0 0.00111 0.00222

total 0.06053 0.07608 0.09774

Fig. 40 The total number of nonmetallic inclusion for heat 1 before after vacuum and before casting.

0

0.02

0.04

0.06

0.08

0.1

0.12


Num

ber/m

m²


BV AV BC

40

Heat 2

Table 36 total number of nonmetallic inclusion for heat 2 before after vacuum and before casting.


DT 2.8 0.04999 0.01999 0.02999

DM 5.7 0.06498 0.03499 0.04499

DM 11.3 0.00222 0.00777 0.00444

DH 22.6 0 0.00444 0.00666

DP 45.3 0 0.00111 0.00222

total 0.11719 0.0683 0.0883

Fig. 41 The total number of nonmetallic inclusion for heat 2 before after vacuum and before casting.

Comparison between Heat 1 and 2, total numbers of two heats 1 and 2 BV, AV and BC

as can be seen from fig 42 The comparison between two heats of steel grade B shown in the diagram above. It is evident from the diagram the two heats have a different result. However, this result related to all size of inclusion. Figure 42 shows the result of only large inclusions, larger than 11.3 µm.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14


Num

ber/

mm

²


BV AV BC

41

Fig. 42 A total number of inclusion for heat 1 and heat 2 before and after vacuum and before casting.

The figure 43 shows that the number of large inclusions is increasing inclusions after vacuum for heat 1 and 2. Comparing with the fig c, in heat two the inclusion decreased after vacuum while in fig above the inclusion increased after vacuum. It is clear that the increase took place only on inclusion larger than 11.3 µm.

Fig. 43 Heat 1 &2 comparison of number of inclusions ≥ 11.3 µm BV, AV & BC Comparison between reference heats and new heats after changing stirring conditions.

The effect of the changing of stirring conditions during vacuum degassing on the large inclusions larger 11.3 μm is evident in figure 44. Comparing the New heat1 and 2 with reference heat 1, 2 and 3, it is evident to see that the changing of stirring condition did not affect too much on the number of inclusion smaller than 11.3 μm after vacuum. In both cases, the result was increasing in the number of inclusions after vacuum.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

heat 1 heat 2

Num

ber/m

m²

heat 1&2 comparison of total number of inclusions BV ,AV & BC Number/mm²

BV AV BC

0

0.005

0.01

0.015

0.02

heat 1 heat 2

Num

ber/m

m²

heat 1 &2 comparison of number of inclusions ≥ 11.3 µm BV ,AV & BC Number/mm²

BV AV BC

42

Fig. 44 Comparison between reference heats and new heats after changing stirring conditions.

Percentage increase in a total number of inclusion ≥ 11.3 µm in 5 Heats.

From the average values for reference heats and new heats in figure 45, it was seen that they have an almost same value that supports the conclusion which took from the previous figure that changing of stirring condition did not affect inclusion removal during vacuum degassing treatment.

Fig. 45 Percentage increase in a total number of inclusion ≥ 11.3 µm in 5 Heats.

Table 37. Shows the slag composition of heat 1and 2 Before Vacuum, After Vacuum and before Casting

0

0.005

0.01

0.015

0.02

0.025

0.03

heat 1 heat 2 heat 3 heat 1 heat 2

Reference New (changing stirring condition)

Num

ber/m

m²

BV AV

0

100

200

300

400

500

600

heat 1 heat 2 heat 3 Average heat 1 heat 2 Average

Reference New (changing stirring condition)

Perc

enta

ge In

crea

se n

o. O

f inc

l-

≥11.

3

43

Table 37 slag composition of heat 1and 2 Before Vacuum, After Vacuum and before Casting

Heats %CaO % SiO2 %MgO % Al2O3 Total2

BV1 50.7 7.2 11.9 30.1 100

AV1 48.4 8.9 12.5 30 100

BC1 48.7 8.9 12 30.3 100

BV2 50.5 10.5 12.3 26.5 100

AV2 48.4 11.2 12.8 27.4 100

BC2 48.7 11.2 12.3 27.5 100

Table 38. Shows the Inclusions compositions have found in samples from heat 1 and 2 Before Vacuum, After Vacuum and Before Casting

Table 38 Inclusions compositions have found in samples from heat 1 and 2 Before Vacuum, After Vacuum and Before Casting

Heats %CaO % SiO2 %MgO % Al2O3 Total

BV1 10.8 3.9 11 74.1 100

AV1 41.9 6.9 9.2 41.9 100

BC1 43.8 5.5 9.3 41.2 100

BV2 14.9 2.1 11.2 71.6 100

AV2 43.2 5 10.2 41.3 100

BC2 41.4 5.2 8.1 45.1 100

The compositions of inclusions found in the sample taken before and after vacuum treatment and before casting for steel grade B plotted on the phase diagram, fig 45 and fig 46, with slag compositions of same heats before and after vacuum and before casting. Figure 46 and 47 for both heat1 and 2 respectively. The average compositions of inclusion found before vacuum located far away from its slag and after vacuum degassing treatment the inclusion move closer to the slag composition and that what happen in steel grade A

.

44

Heat 1

Fig. 46 Liquidus surface in the system Al2O3-CaO-MgO- SiO2 with 10 mass % MgO after Cavalier, Sandreo-dendon. [25] For the base system Al2O3-CaO- SiO2. [26] Heat 2

Fig. 47 Liquidus surface in the system Al2O3-CaO-MgO- SiO2 with 10 mass % MgO after Cavalier, Sandreo-dendon. [25] For the base system Al2O3-CaO- SiO2. [26].

45

5 CONCLUSION

This project focuses on the cleanness of the steel melt of tools steel in ladle furnace during vacuum degassing treatment, In particular, this project concentrates on the effect of top slag and stirring condition on sulphur removal and non-metallic inclusion during vacuum degassing treatment. The samples have been taken before and after vacuum degassing and have been analysed in the lab by OES to determine the steel compositions, LOM with PC-MIC software to determine the number of inclusion according to its size and at the end use SEM to determine the composition of inclusions. According to the result, leading findings be summarised as follows:

• For sulphur removal, the sulphur content decreased with current slag and also decreased after

changing of the slag composition, but the percentage decrease of the sulphur was different. The highest percentage decrease of sulphur obtained during heats with current slag, which called reference heats, and the lowest percentage decrease of sulphur was in period three where the slag has changed. Hence, reducing the amount of CaO in the synthetic slag led to decreased sulphur removal from the steel melt. Also in period 4, when the stirring conditions during vacuum degassing were changed after 10 minutes from the begging of the vacuum process. The slag composition was similar to that one in period 1. The result was the removal of sulphur was high and almost equal to the percentage decrease in reference heats. Hence, it concludes that the removal of sulphur based mainly on the top slag composition especially the amount of CaO in the slag. Moreover, with 10 minutes of intense stirring of combine stirring (EMGAS) during vacuum can reduce the sulphur to the required value.

• In both steel grade which was examined in this project, the non-metallic inclusions have been increased after vacuum degassing treatment. However, killing the slag by Al or changing of slag composition lead to increase of the number of the inclusion ≥ 11.3 µm in both AISI H13 Tools Steel A and B. In both cases, the amount of Al2O3 in slag was increasing. It was concluded that the amount of Al2O3 in the top slag has a significant influence on non-metallic inclusions presence in the steel melt.

• Result from changing stirring conditions during vacuum gives rise in some inclusions after vacuum for all 3 heats in both steel grade AISI H13 Tools Steel A and B. The induction stirring and argon gas stirring has reduced during vacuum after 10 minutes. Reduction of EMGAS intensity increases the number of nonmetallic inclusions in the melted steel.

• The inclusion in the steel melt before vacuum has different compositions from the inclusion which

was found after vacuum. Probably came from the arc furnace. The composition of inclusion obtained after the vacuum is very close to the slag composition before and after vacuum. Therefore, it can be concluded that the source of inclusion which found after vacuum degassing was the top slag.

46

6 FUTURE WORK In this project, the effect of top slag on the sulphur and inclusion removal has been investigated. Also, the effect of stirring intensity on the inclusions removal during vacuum has also studied. Though much work has been done in the project, still there are more questions need to be answered. For the further understanding, some ideas have been suggested and will be investigated in future work.

• Optimise the slag composition, to suggest another composition of top slag with keeping the CaO and Al2O3 content within a safe range to avoid sulphur removal problem.

• Studying how much the top slag effect on inclusion during vacuum degassing by stopping the vacuum process and taking samples at different times.

• Taking more heats and more samples could help to get enough result to compare. • Changes stirring’s conditions during vacuum degassing treatment by increasing intensity of EMS with

decreasing the time of the vacuum to see the effect of stirring intensity on inclusion. • Study the effect of stirring on the hydrogen, nitrogen and sulphur removal by stopping of vacuum

process at a different time.

47

7 REFERENCES

1) Charlotte Médioni, Influence of Stirring on the Inclusion Characteristics during Vacuum Degassing in a Ladle, Licentiate Thesis Division of Applied Process Metallurgy, Department of Materials Science and Engineering, KTH Royal Institute of Technology SE-100 44 Stockholm, Sweden, P.30.).

2) P. Jönsson, L. Karlsson, F. Reinholdsson. The Effect of Final Induction Stirring Practice on Inclusion Characteristics in Bearing Steel Production. In proceedings: 5th International Conference on Clean Steel, 1997, Balatonfured, Hungary. )

3) Svensk standard SS11116, Steel – Method for Assessment of the content of non-metallic inclusions, Swedish Institute for standards, 1987.).

4) Mats Söder, Doctoral thesis, Growth and removal of inclusions during ladle refining, Stockholm 2004, Royal Institute of Technology, Department of material science and engineering, Division of Metallurgy. ISBN 91-7283-886-8, ISRN KTH/MSE-04/63/SE+APRMETU/AVH, P 18, 31.

5) L. Jonsson and P. Jo¨nsson: “Modelling of fluid flow conditions around the slag/metal interface in a gas-stirred ladle, ISIJ Int., 1996, 36, 1127–1134.

6) S. Chung, Y. Shin, J. Yoon. Flow Characteristics by Induction and Gas Stirring in ASEA-SKF Ladle. ISIJ. 1992, 32, 12.

7) Fredrik Dahl , master thesis , characterization of some slag used in ladle treatment, department of material science and engineering Royal Institute of technology SE-100 44, Sweden, Stockholm 2004, ISRN KTH/MSE—04/19- SE+MICROMODMETU/EX, P-16.

8) Estimation of Liquidus Temperatures for Multicomponent Silicates from Activation Energies for Viscous Flow S. SEETHARAMAN, S. SRIDHAR, DU SICHEN, and K.C. MILLS, VOLUME 31B, FEBRUARY 2000—111

9) Ghosh, Ahindra, Secondary Steelmaking: Principles and Applications, p. cm.Includes bibliographical references and index.ISBN 0-8493-0264-1, 1. Steel. I. Title.TN730 .G48 2000, 672—dc21.p44-193

10) Kamrooz Riyahimalayeri, Slag, Steel, Ladle and Non-metallic Inclusions Equilibria in an ASEA-SKF Ladle Furnace, Department of Materials Science and Engineering, School of Industrial Engineering and Management, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden, ISRN KTH/MSE--12/24--SE+ THERM/AVH, ISBN 978-91-7501-489-0, P.9

11) K Steneholm, M Andersson, A Tilliander & P G Jönsson (2013) Removal, of hydrogen, nitrogen and sulphur from tool steel during vacuum degassing, Ironmaking, &Steelmaking, 40:3, 199-205, DOI: 10.1179/1743281212Y.0000000029

12) c. j. b. fincham and f. d. richardson: Proc. R. Soc. A, 1954, 223, 40–62 13) M. Andersson, M. Hallberg, L. Jonsson & P. Jönsson (2002) Slag-metal reactions during ladle

treatment with a focus on desulphurisation, Ironmaking & Steelmaking, 29:3, 224-232, DOI: 10.1179/030192302225004106.p. 224 Ironmaking and Steelmaking 2002 Vol. 29 No. 3.

14) Carl Allertz, Du Sichen, Sulfide Capacity in Ladle Slag at Steelmaking Temperatures, Metallurgical and Materials Transactions B, 2015, Vol.46(6), pp.2609-2615

15) M. Ohta, T. Kubo, and K. Morita: Tetsu-to-Hagane´, 2003, vol. 89, pp. 742–49. 16) M. M. Nzotta: PhD thesis, Royal Institute of Technology, Stockholm, Sweden, 1999. 17) M. Hino, S. Kitagawa, and S. Ban ya: ISIJ Int., 1993, vol. 33, pp. 36 42. 18) M. Gornerup and O. Wijk: Scand. J. Metall., 1996, vol. 25, pp. 103 107. 19) PCMIC, Jernkontoret, TO23, 111 87 Stockholm, Sweden. 20) S.W. Cho, H. Suito, “Assessment of calcium-oxygen equilibrium in liquid iron’’, ISIJ Int., Vol.34

(1994), No.3, pp.265-269. 21) G.J.W.KOR, SCIENTIST, THE TIMKEN CO., P.C.GLAWS, SENIOR RESEARCH

SPECIALIST, THE TIMKEN CO. LADLE REFINING AND VACUUM DEGASSING. CHAPTER 11, P677.

48

22) Charlotte Médioni, Pär G. Jönsson, and Du Sichen Effect of Stirring Practice on Sulphur and Nitrogen Refining as well as Inclusion Removal in Ladle Treatment, steel research int. 85 (2015) No. 9999

23) Ulf Sand (top left), design analysis, Hongliang Yang (top right), senior metallurgist, and Jan-Erik Eriksson (bottom left), R&D manager, ABB AB Metallurgy, Västerås, Sweden ([email protected], [email protected], [email protected]); and Rebei Bel Fdhila (bottom right), senior principal scientist, ABB AB Corporate Research, Västerås, Sweden ([email protected]), Control of Gas Bubbles and Slag Layer in a Ladle Furnace by Electromagnetic Stirring.

24) K.C Ahlborg, T.H. Bieniosek and J.H. Tucci LTV Steel Company, Slagmaking practising at LTVsteels Cleveland Works. P[74]

25) Armaki, Sh.: Roy, R.: J. Am. Ceram. Soc., 45(1962),229 26) Slag atlas, verein Deutscher Eisenhuttenleute, ISBN 3-514-00457-9, p 180. 27) Hikarru Hayakawa, Masakatsu Hasegawa, Kazue Oh-nuki, Takashi Sawai and Masanori Iwase,

Sulphide capacities of CaO-SiO2-Al2O3-MgO Slags,Steel research int.77(2006)No.1

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