Stirring and Shrouding Gases
Transcript of 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%).