stirring and shrouding gases

8/2/2019 Stirring and Shrouding Gases

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Selection of Stirring and Shrouding Gases for SteelmakingApplications

Dr. Ronald J. Selines

Manager - Process Metallurgy, Linde Division, Union Carbide Corporation

Tarrytown Technical Center Tarrytown, New York

Copyright 1988, Union Carbide Corporation

ABSTRACT

Argon. nitrogen, carbon dioxide, and carbon

monoxide are the gases used to provide stirring or

shrouding in steelmaking applications. The behaviorof these gases when in contact with molten steel is

reviewed, and the criteria that can influence the

selection of a particular gas are discussed in

general. The specific issues associated with three

representative applications; BOF stirring, billet

caster shrouding, and ladle stirring are discussed in

detail.

INTRODUCTION

The use of nitrogen and argon to provide stirring

and protection from atmospheric contamination is

widespread in steel meltshops, and resultant

benefits are well documented. Recently, experience

using carbon dioxide or carbon monoxide for such

applications has been reported. The selection of the

most appropriate gas for a specific application may

not be straightforward and can involve

consideration of a number of factors including steel

chemistry and quality, injection device life, gas and

overall process cost, and safety. This paper will

review the relative merits of each gas with respect

to each of the factors which can influence the

selection process. Included is a description of the

fundamental behavior of each gas in contact with

molten steel and resultant consequences for specific

steel melting, refining, and casting operations.

GAS CHARACTERISTICS

Argon

Argon is completely inert to molten steel. Itprovides stirring and a protective atmosphere with

no potential for undesired reactions and no

measurable solubility. Its only effect on steel

chemistry is to remove dissolved hydrogen, oxygen,

and nitrogen via a sparging mechanism. Figure 1

shows theoretical argon degassing requirements for

nitrogen and hydrogen removal. Argon is also used

as an inert diluting gas to promote carbon removal

in Union Carbide's proprietary AOD process.

Nitrogen

Nitrogen has a solubility of 380 ppm in molten iron

at 1530C, and its solubility increases with

temperature. The presence of elements such as

aluminum, titanium, vanadium, etc., further increases

nitrogen solubility. Consequently, its use for stirring

or shrouding can result in higher final nitrogen

contents. The kinetics of nitrogen absorption via the

reaction:

N2(g) 2N (1)

are strongly influenced by the oxygen and sulfur

content of the steel. These surface active elements

retard nitrogen dissolution kinetics by preferentially

occupying surface sites where reaction (1) would

otherwise occur. Thus, highly desulfurized steels


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and aluminum or calcium deoxidized steels are

particularly prone to nitrogen pick-up. Figure 2

shows the effect of sulfur content on the kinetics of

nitrogen absorption by iron droplets at 1600C.(1)

Carbon Dioxide

A thermodynamic analysis shows that carbon

dioxide can react with iron, carbon or any

deoxidizers which are present in the steel melt.

Reactions which are thermodynamically favored are

summarized in Table I. For applications where non-

deoxidized baths are present, such as AOD and

BOF converters, carbon dioxide can react with

either carbon or iron to form FeO and/or carbon

monoxide depending on carbon content. In

deoxidized steels, carbon dioxide can react with thedeoxidant to form the corresponding oxide and

either dissolved carbon or carbon monoxide.

The extent to which these reactions proceed is

controlled by kinetic considerations. D. R. Sain et

al(2) have studied the interfacial reaction kinetics of

carbon dioxide with carbon in liquid iron as a

function of temperature and pressure. The

experimentally measured reaction rates are

relatively high and suggest that bubbles of carbondioxide will be rapidly consumed by steel melts via

reaction with dissolved carbon assuming that mass

transfer is not limiting (see Appendix I).

T. Bruce et al(3) have considered carbon dioxide

stirring of aluminum killed steels and conclude that

aluminum diffusion in the bath is the rate controlling

step. They report the absence of visible bubbles at

low flow rates and up to a 50% decrease in injector

life time as evidence that reactions to form A1203,

FeO, and dissolved C do proceed to a significant

extent.

Linde's experience with carbon dioxide in AOD

steelmaking confirms these conclusions.

Specifically, complete reaction of carbon dioxide

with carbon must be inferred to close an oxygen

balance for the decarburization step. In addition,

stoichiometric increases in bath carbon content

when blowing a mixture of oxygen and carbon

dioxide into a bath containing aluminum are

observed.

Carbon Monoxide

The use of carbon monoxide in Q-BOP and BOP

furnaces has been recently reported.(4,5) However,

there is little experience to date, and the behavior of

carbon monoxide in the melt is not well understood.

Effects attributable to low reactivity and a resultant

high partial pressure of carbon monoxide as well as

effects attributed to significant conversion to

dissolved carbon and carbon dioxide via the

reaction:

2CO C + CO2 (2)

are described. In general, the use of carbon

monoxide for steelmaking applications is of reduced

interest due to availability and safety considerations,

and only its use for BOF stirring will be considered.

SELECTION CRITERIA

Having reviewed the fundamental behavior ofargon, nitrogen, and carbon dioxide in molten iron

and steel, this section will consider the major factors

which can influence the selection of a gas for

steelmaking applications.

Criteria related to steel chemistry, quality, refractory

plug wear, economics and safety are examined. A

relative ranking of these gases for each of these

categories is given in Table II, and a more detailed

discussion follows.

Steel Chemistry

Since argon is totally inert, it is the gas of choice

when ladle stirring or shrouding is required and

changes in nitrogen, carbon, or deoxidant levels

must be minimized. It is also used in AOD, vacuum

processing, and ladle degassing when substantial


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reductions in nitrogen, hydrogen, or oxygen content

are desired, and when producing ultra-low carbon

content grades (C


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effective choice of gas usually requires

consideration of several factors, more detailed

analyses of these economic issues will be presented

in the section of this paper which discusses specific

applications.

Safety

The use of any of these gases introduces potential

hazards, and equipment design, operating practices,

and maintenance procedures must be established to

reduce the liklihood and seriousness of potential

accidents to acceptable levels. The most serious

hazard associated with argon, nitrogen or carbon

dioxide is asphyxiation in confined spaces due to

lack of oxygen. In many situations, the relative

severity of the hazard increases with increasingspecific gravity (sg). Consequently. a ranking in

order of increasing hazard would be; nitrogen (sg-

0.97), argon (sg-1.38), and carbon dioxide (sg-

1.52). It should also be noted that the reactivity of

carbon dioxide in the blood stream results in a

maximum eight hour exposure limit of 0.5% in air as

recommended by the American Conference of

Governmental Industrial Hygienists.

An additional potential hazard associated withcarbon dioxide use is the formation of carbon

monoxide due to dissociation or reaction with iron,

carbon or silicon. Carbon monoxide is flammable

and toxic with a recommended maximum exposure

limit of 50 ppm (ACGIH 1984-85). Consequently,

the environment should be checked to assure safe

levels for operations that require relatively high flow

rates such as billet shrouding . Obviously, the use of

carbon monoxide for BOF stirring poses a

significantly greater hazard due to the large quantity

of gas required and risk of explosion.

APPLICATIONS

This section will consider the issues which can

impact gas selection for specific steelmaking

applications. Since it is not possible to cover all

uses, three applications have been selected which

are in widespread use and represent a significant

portion of total inert gas use by the industry.

BOF Stirring

Pneumatic stirring provides a variety of benefits to

BOF steelmaking and has been widely adopted

throughout the world. Most common is a practicewhich uses nitrogen and argon, with final nitrogen

and carbon levels dictating the relative amounts of

each gas used. A BOF stirring practice with either

carbon dioxide or carbon monoxide is used in

several Japanese and European mills and in one US

location. The gases are injected through tuyeres

located in the bottom of the vessel. Tuyere design is

usually either: concentric tubes to provide an

annulus for gas injection; or multiple small diameter

tubes or channels incorporated in a high qualitycarbon-magnesite block.

Since this is an oxygen based decarburization

process, the reactivty of carbon dioxide is not

detrimental as long as decarburization to low levels

is not required. In fact, the reaction of carbon

dioxide with carbon to form carbon monoxide (See

Table I) should decrease oxygen blowing times and

consumptions slightly (less than 3%). However, as

shown in Figure 3

(8)

, dissolved oxygen contents atend of blow are significantly higher when using

carbon dioxide rather than argon or nitrogen for

carbon levels below 0.1%. In addition, as carbon

content decreases in this range, the reaction of

carbon dioxide with iron to form dissolved carbon

as well as carbon monoxide (See Table I) will be

increasingly favored, further hindering

decarburization to low levels. Consequently, the

amount of low carbon steel grades produced can

significantly impact potential savings associated with

the substitution of carbon dioxide for argon in this

process. Its use can reduce the ability to achieve

low carbon levels, and decreases in yield and

refractory life associated with high oxygen and FeO

contents will lead to higher refining costs.

The other consideration affecting the use of carbon

dioxide is its effect on tuyere wear rates. Figure 4


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shows that tuyere wear rates are significantly

accelerated when using carbon dioxide rather than

argon or carbon monoxide. Examination of worn

tuyeres indicates that this acceleration of wear rate

is due to the formation of FeO at the tuyere which

results in local 4ttack of the carbon-magnesite

refractory as shown schematically in Figure 08)Such an increase in tuyere wear rate can result in

premature loss of stirring and associated benefits,

reduced vessel life and productivity, or both.

An attempt to put some perspective on the trade-

off between gas cost and overall BOF operating

costs is presented in Figure 6. The case considers a

230 ton BOF with a 1500 heat campaign life and

treats the effect of carbon dioxide on tuyere wear

rate as a variable. The basic assumption made isthat any increase in tuyere wear rate will result in a

corresponding decrease in the number of stirred

heats but will not shorten overall vessel life. In other

words, the consequences of premature loss of

stirring translate into lost savings due to a lack of

stirring on remaining production. The vessel remains

in service for the full 1500 heats with no loss in

productivity or refractories. The analysis shows that

for the stated assumptions for relative gas costs and

stirring cost benefits, a 25% increase in tuyere wearrate is the break even point. This break even point

will shift depending on the relative magnitudes of

cost premium for argon vs. carbon dioxide and

overall cost savings associated with stirring, and an

analysis based on actual costs and operating data is

recommended. However, such an analysis does

point out that the economic consequences of

accelerated tuyere wear can offset the cost savings

associated with substituting carbon dioxide for

argon.

A discussion of BOF stirring would not be

complete without mentioning carbon monoxide.

Published results indicate that, compared with argon

stirring, slag FeO contents are unchanged, dissolved

ox en contents are slightly higher, and tuyere wear

rates are about equal.(4,5,8) Consequently, carbon

monoxide appears to be an acceptable alternative

to argon on the basis of metallurgical and

operational considerations. The troublesome

aspects of carbon monoxide use are increased

safety risk due to its toxicity and flammability and a

requirement for additional capital equipment to

reclaim the carbon monoxide from the BOF off-gas

stream.

Billet Casting

Gas purging of stovepipe type shrouds is the usual

method for protecting tundish to mold streams from

atmospheric contamination. Elimination of large

reoxidation type inclusions is the most common

objective. A reduction in nitrogen pick-up may also

be important. Due to visibility and accessibility

requirements, stovepipe shrouds may havesignificant gaps or openings and typically require

gas consumptions in the range of 50-200 ft3 (1.4-

5.7m3) per ton.

If prevention of reoxidation is the only need,

nitrogen is the best gas choice. For best results, the

shroud design and gas flow rates used should be

capable of achieving oxygen levels below 1%, and

preferably below 0.5% as measured within the

stovepipe shroud during casting. The magnitude ofnitrogen pick-up depends on steel chemistry and

casting conditions and is usually in the 5 to 10 ppm

range.

Nitrogen sensitive grades, most notably boron

containing steels or wire grades, are sometimes

shrouded with argon to reduce the nitrogen pick-up

which would be associated with gaseous nitrogen

shrouding. Due to the high gas consumption

required, the use of argon rather than nitrogen

involves a significant cost increase, and carbon

dioxide shrouding of nitrogen sensitive grades has

been suggested as a more economic alternative.

However, while carbon dioxide is as effective as

argon in preventing nitrogen pick-up, there is a

question regarding its ability to also prevent re-

oxidation.


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Reoxidation occurs due to contact of the metal

stream with the surrounding atmosphere and the

entrainment and subsequent reaction (if any) of this

atmosphere in the stream and, subsequently, the

continuous casting mold itself as the stream

impinges on the molten metal surface. Levitated

drop experiments have been used to evaluate thekinetics of oxygen absorption in atmospheres

containing

1-20% oxygen, nitrogen, and carbon dioxide, and

the results are summarized in Figure 7. The data

clearly show that the rate of oxygen absorption in

pure carbon dioxide is significantly lower than in

atmospheres containing one or more percent

oxygen, and that nitrogen does indeed completely

eliminate reoxidation. This data suggests that carbon

dioxide shrouding should offer an intermediate levelof protection from reoxidation compared to argon

or nitrogen.

Results from commercial trials comparing the

relative performance of nitrogen, argon, and carbon

dioxide are not consistent and controversy

concerning the degree of protection afforded by

carbon dioxide remains. Figure 8 shows typical

results from tests at Auburn Steel using an enclosed

shroud with residual oxygen contents less than0.2%.(6) The data show that carbon dioxide

shrouded billets contain significantly more large

reoxidation inclusions compared to nitrogen or

argon shrouded material, and, surprisingly,

compared to unshrouded material as well. This last

result may be due to the protective atmosphere

formed by partial combustion of the mold lubricant

by air which is lost when shrouding with carbon

dioxide.

Figure 9 shows typical results from tests at CF&I

Steel using a shroud design which usulted in residual

oxygen contents of 1-11% (2.5% average)(7).

These results show comparable levels of cleanliness

in argon compared to carbon dioxide shrouded

material. However, in view of the data presented in

Figure 7, one may speculate that it was the residual

oxygen levels present with both gases that was

controlling the overall extent of reoxidation. In view

of the importance of preventing reoxidation, the use

of carbon dioxide is not recommended due to its

questionable performance in this regard.

While argon does offer the best protection against

both reoxidation and nitrogen pick-up, it appears

that it can result in an increase in pin-hole content.Both of the tests referred to above report such an

effect. One may speculate that such an effect is due

to the physical entrapment of insoluble and non-

reactive argon bubbles, and that solidification

conditions control their occurence. In any event, it is

recommended that argon shrouded billets be

carefully evaluated to assess whether pin-hole

content remains at acceptable levels.

Ladle Stirring

Ladle stirring is commonly practiced to aid

desulfurization, homogenize temperature and

composition, remove inclusions, assist vacuum

degassing, etc. Gas is usually injected through a

porous refractory stirring element located in the

bottom of the ladle or through a refractory coated

lance. Gas injection rates are usually less than 10

scfm (0.29 m3/min). and gas consumptions are

usually about 1 ft

3

/ton (0.03 m

3

/ton).

Since most ladle stirring is performed to further

improve the quality of deoxidized steel, argon is the

gas most often selected for this application. The

required stirring is provided while minimizing

possible adverse reactions. Argon also provides

good stirring element life. Nitrogen is used when the

associated increase in nitrogen content can be

tolerated. The increase in nitrogen content de ends

on the steel chemistry and amount of nitrogen used.

Hagerty et al (6) reported an increase in the 10 to 20

ppm range for 10 to 15 minutes of nitrogen stirring

for AISI 1016 and 1035M grades containing

0.04% sulfur. In view of the potential for significant

increases in nitrogen contents, the small added cost

associated with argon can often be justified.


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The use of carbon dioxide for ladle stirring has also

been evaluated.(3) However, there are a number of

considerations which make it a poor choice for this

application as well. Since ladle stirring operations

often involve deoxidized melts, the potential

reactions of carbon dioxide with aluminum or silicon

to form the corresponding oxide can compromisefinal product quality. While most of the oxide

inclusion reaction products appear to be eliminated

due to the stirring action, the possiblility of some

amount carrying through to the final product will

always exist. The reported decrease in porous

refractory element life is of course another negative

aspect associated with its use. Finally, it is not at all

clear that the use of carbon dioxide rather than

argon results in any overall cost savings. The

reported observation that, Under certainconditions of injection, no bubbles seem to reach

the surface. suggests that reaction with aluminum is

proceeding to near completion. The analysis in

Table III shows that the value of the deoxidant

consumed far exceeds the potential savings in gas

costs for aluminum killed grades. If it is likewise

assumed that the reaction of carbon dioxide

proceeds to near completion in silicon killed grades,

then the value of the lost silicon is comparable to the

potential savings in gas costs. Such an analysis canalso be applied to the billet casting application

which would also involve deoxidized steel.

However, in this case, it is difficult to estimate how

much of the total carbon dioxide introduced into the

shroud reacts to consume deoxidant.

CONCLUSIONS

A review of the behavior of the gases used to

provide stirring and atmosphere protection in

steelmaking has shown that argon alone is

completely non-reactive. Consequently, it is the gas

of choice for applications that require the best

possible quality and the least possible change in

steel chemistry. The only exception to this

conclusion is its use for shrouding on continuous

casters where a possibility of increased pinhole

content exists. Since argon is denser than air, it is

particularly effective for purging molds and is widely

used to improve quality in ingot teeming operations.

However, this property also increases the risk of

asphyxiation, and special precautions must be taken

when entering confined spaces. Argon is the most

expensive of the gases normally used for such

applications, and nitrogen, carbon dioxide, andcarbon monoxide are substituted when possible.

Nitrogen is used whenever the associated increase

in nitrogen content can be tolerated. BOF stirring,

AOD refining, billet caster shrouding, and ladle

stirring are typical applications. However, the

relatively small cost savings associated with ladle

stirring may not justify the resultant increase in

nitrogen content. Nitrogen is less dense than air and

consequently is not an efficient gas for mold purgingand is less likely to introduce asphyxiation hazards.

Carbon dioxide can be substituted for nitrogen or

argon to reduce nitrogen pick-up or gas cost

respectively. However, there are several

undesirable characteristics associated with its use

which should be considered in order to fully assess

overall suitability. Carbon dioxide can undergo

appreciable reaction when in contact with molten

steel, thereby changing steel chemistry and possiblyincreasing inclusion content. Its use can also

increase the rate of wear of gas injection tuyeres.

The reactivity of carbon dioxide is of less concern in

oxygen based applications such as BOF and AOD

refining unless ultra-low carbon contents are

required, and these are the processes in which its

use may be justified. The potential for reaction with

deoxidant in killed steels and associated formation

of oxide inclusions make its use for shrouding or

ladle stirring applications questionable from both

quality and overall economic viewpoints. Carbon

dioxide is also denser than air and poses increased

risk of asphyxiation in confined spaces. There may

also be a potential for appreciable levels of carbon

monoxide associated with its use.

Carbon monoxide is being used for BOF stirring by

a few Japanese steelmakers. It appears to be an


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excellent substitute for nitrogen and argon from both

metallurgical and operational viewpoints. However,

since bulk quantities of carbon monoxide are not

available commercially, it must be recovered from

the BOF off-gas stream, and overall economics can

be strongly influenced by capital requirements and

the value of any remaining carbon monoxide as afuel gas. In addition, there are obvious safety

considerations associated with the storage, handling

and use of such large quantities of a gas which is

both toxic and flammable.

REFERENCES

(1) L.A.Greenberg and A.McLean, Nitrogen

Pick-up in Low Sulfur Steel, Ironmaking and

Steelmaking, 9, 2, 1982, pp.58.

(2) D.R.Sain and G.R.Belton, Interfacial Reaction

Kinetics in the Decarburization of Liquid Iron

by Carbon Dioxide, Met.Trans. B, Vol. 7B,

June 1976, p. 235.

(3) T.Bruce et al, Effects Of CO2 Stirring in a

Ladle, Electric Furnace Conference

Proceedings, Vol. 45, Chicago, IL, 1987, pp.

293-297.

(4) T.Sakuraya et al, Protection of OxygenBottom Blown Tuyeres by CO Gas,

Steelmaking Conference Proceedings,

Washington, DC, Vol. 69, 1986, pp. 639-646.

(5) H.Yamana et al, CO Gas Bottom Blowing in

the Top and Bottom Blowing Converter,

Ironmaking and Steelmaking, Steelmaking

Conference Proceedings, Pittsburgh, PA, Vol.

70, 1987, pp. 339-346.

(6) L.J.Hagerty and J.A.Rossi, Shrouding ofContinuous Billet Castings at Auburn Steel with

Argon, Nitrogen and Carbon Dioxide, Electric

Furnace Conference Proceedings, Vol. 44,

Dallas, TX, 1986, pp. 153-159.

(7) C.T.Jensen et al, Atmospheric Protection of

Billet Streams Using Carbon Dioxide, Electric

Furnace Conference Proceedings, Vol. 45,

Chicago, IL, 1987, pp. 57-63.

(8) Rinsing Effect of LD-KGC Process,

Kawasaki Steel Corporation, private

communication.

(9) M.Nishi et al, Development of the MultipleHole Plug for Top and Bottom Blown

Converter, Nippon Kokan KK, private

communication.


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TABLE I - CO2 REACTIONS

Non-Deoxidized Carbon Steels

CO2 + C 2CO

CO2 + Fe CO + FeO

Deoxidized Steels

3CO2 + 4Al 2Al203 + 3C

2CO2 + Si SiO2 + 2CO

TABLE II - QUALITATIVE RANKING OF GAS TYPES FOR SEVERAL SELECTION CRITERIA

SELECTION CRITERIA

Steel Chemistry Steel Quality Cost Safety#

Gas

Type

Low

N

High

N

Low

O

Low

Inclusions

Low

Pinholes

Gas

Cost

Plug

Wear

Deox.

Use

Ar + - + + - - + + 0

N2 - + + + + + + + +

CO2 + - - - + + - - 0

CO + - 0 NA NA SD 0 NA -

- Poor

0 Acceptable

+ Recommended

# See text for safety information for all gases

NA Not applicable - used for BOF stirring only

ND Not determinedSD Site dependent - recovered from BOF off-gas


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TABLE III - ECONOMICS OF CARBON DIOXIDE STIRRING DEOXIDIZED STEELS

STEEL TYPE REACTION

DEOXIDANT CONSUMPTION

lbs/ft3 of CO2 (kg/m3 Of CO2)

BREAK EVEN GAS

COST DIFFERENTIAL*

$/100 ft3 ($/m3)

Al Deoxidized 3CO2 + 4Al 2Al2O3 + 3C 0.093 (1.49) 7.00 (2.48)

Si Deoxidized 2CO2 + Si SiO2 + 2CO 0.036 (0.58) 2.40 (0.85)

*Assumes $0.75/lb Aluminum, $0.65/lb Silicon and complete reaction of carbon dioxide.

FIGURE CAPTIONS

1. Theoretical argon requirement for removal of hydrogen or nitrogen from molten steel at 2912F.

2. Effect of sulfur content on nitrogen pick-up: (a) increase in nitrogen content vs. time for levitated droplets

with varying sulfur contents; (b) effect of sulfur content on the rate of nitrogen pick-up by levitated

droplets.(1)

3. Effect of carbon dioxide compared to argon or nitrogen stirring on bath oxygen content at the end of oxygen

blowing in the BOF.(8)

4. Effect of carbon dioxide compared to argon or nitrogen stirring on the wear rate of BOF stirring elements.(8)

5. Schematic illustration of the mechanism of increased element wear in the BOF due to carbon dioxide stirring

and associated formation of FeO which locally attacks the magnesite-carbon refractory element.(9)

6. Relationship between increased refractory wear due to carbon dioxide use and overall savings due to BOF

stirring assuming an inverse linear relation between increased tuyere wear rate and percentage of heats

stirred for a campaign.

7. Effect of oxygen-nitrogen mixtures, pure nitrogen, and carbon dioxide on the variation of the n9en content of

levitated steel droplets weighing approximately one gram.(7)

8. Effect of nitrogen, argon, or carbon dioxide shrouding on the macro-inclusion content of grade 1016 billet.(6)

(Shroud oxygen content less than 0.2%.)

9. Effect of argon or carbon dioxide shrouding on the macro-inclusion content of grade SAE J422a billet.(7)

(Average shroud oxygen content about 2.5%.)


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APPENDIX I - REACTION OF A CARBON DIOXIDE BUBBLE IN A STEEL MELT

REACTION: CO2 + C 2CO

(Rate Constant K = 3.16 x 10-4 mole/cm2/sec/atm2)

ASSUMPTIONS: Average bubble radius (R) = 5cm

Initial CO2 pressure (PCO2i) = 1.75 atm

Average CO2 pressure ( )P atmCO2 = 05.

Temperature (T) = 1900K

Initial CO2 content of bubble (niCO2) = ( )

P

RT4 3 RCO2

i3

=1.75 atm 522 cm

cm atm

mol k K

3

3

8205 1900.

= 6 10 3 mole

Rate Of CO2 depletion = K bubble surface area PCO2

= K R P2 CO2 4

= 5 10 2 mole/ sec

Complete bubble reaction time (tR) =6 10

5 10

3

2

mole

mole / sec

= 0.12 seconds


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FIGURE 1: Theoretical argon requirement for

removal of hydrogen or nitrogen from molten

steel at 2912F.


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FIGURE 2: Effect of sulfur content on nitrogen

pick-up: (a) increase in nitrogen content vs. time

for levitated droplets with varying sulfur contents;

(b) effect of sulfur content on the rate of nitrogen

pick-up by levitated droplets.(1)


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FIGURE 3: Effect of carbon dioxide compared

to argon or nitrogen, stirring on bath oxygen

content at the end of oxygen blowing in the

BOF.(8)


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FIGURE 4: Effect of carbon dioxide compared

to argon or nitrogen stirring on the wear rate of

BOF stirring elements.(8)


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FIGURE 5: Schematic illustration of the

mechanism of increased element wear in the

BOF due to carbon dioxide stirring and

associated formation of FeO which locally

attacks the magnesite-carbon refractory

element.(9)


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FIGURE 6: Relationship between increased

refractory wear due to carbon dioxide use and

overall savings due to BOF stirring assuming an

inverse linear relation between increased tuyere

wear rate and percentage of heats stirred for a

campaign.


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FIGURE 7: Effect of oxygen-nitrogen mixtures,

pure nitrogen, and carbon dioxide on the

variation of the oxygen content of levitated steel

droplets weighing approximately one gram. (7)


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FIGURE 8: Effect of nitrogen, argon or carbon

dioxide shrouding on the macro-inclusion content

of grade 1016 billet.(6) (Shroud oxygen content

less than 0.2%).


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FIGURE 9: Effect of argon or carbon dioxide

shrouding on the macro-inclusion content of

grade SAE J422a billet.(7) (Average shroud

oxygen content about 2.5%).

stirring and shrouding gases

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