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Fundamentals of Stainless steel
melting In EAF-AOD route
By
Amartya Talukdar
E-mail: amartya.talukdar@gmail.com
Stainless steel demand is growing
continuously but prices are stagnant
To be successful it is necessary to
• Optimise Qual...
Presentation Road map
CARBON-CHROMIUM-TEMPERATURE
RELATIONSHIPS
Slag Characteristics in Stainless steel
making
Kinetics...
• 1960: Laboratory investigations on CARBON-
CHROMIUM-TEMPERATURE RELATIONSHIPS
Isothermal chemical reaction process of st...
Equilibrium of Chromium and Carbon
with variation in Temperature
• Line 1: Equilibrium
C-Content is
decrasing at higher
te...
Process
Triplex
Duplex
Prime Reactions in Stainless steel making
When oxygen is injected into the liquid metal reactions for Chromium & carbon
ox...
ConCept of oxide ellingham /
oxygen potential diagram
A reaction proceeds forward spontaneously when ∆Go
the
free energy o...
Oxide Ellingham / oxygen Potential Diagram
Control of Temperature
• Ellingham diagram : At Higher temperature C
oxidation is favorable than Cr oxidation
• The critic...
Reaction involved
3CO+2Cr = Cr2O3+3C
a
Cr2O3 X
a
C
3
Equilibrium constant K = --------------------- where
a
Cr
2
&
a
Cr2O3...
Oxygen potential
diagram with
conditions like
reduced partial
pressure & C, Cr
concentrations etc.
imposed on it for
C CO►...
Carbon – Chromium equilibrium in AOD/VOD
Decarburisation is effected
 high Temperature
 Reduced pCO
• Cr2O3 level in slag can vary from 5-
20 %
• High Cr oxide in slag make the slag
stiff viscous & non workable
SLAG CHARAC...
Viscosity of Cr2O3 slags at 1550o
C
High Cr oxide in slag make the slag stiff , viscous
& non workable
Area of Low viscosi...
Role of Slag basicity on Cr2O3 Reduction
Cr2O3
Cr2O3
Cr2O3
Cr2O3
Cr2O3Cr2O3
SiO2 SiO2
SiO2
CaO
Typical SiO2 –Cr2O3 Tetrahe...
Resultant Slag
• Viscous Stiff slag in Acid regime
• Until CaO breaks structure to individual Cr2O3 - CaO
units Cr2O3 can ...
Typical Evolution of Cr2O3 In slag During EAF
0
2
4
6
8
10
12
14
16
End
Melting
End oxy
blow
Start
Reduction
Finish
Reduct...
ISO-Activity contours of CrOx for CaO-SiO2- CrOx
system at 1873 K with Po2=6.95x10 -11
Combination of Slag CharaCteriStiCS & melt
CompoSition
• Optimum Slag basicity
• Optimum Silicon level in melt
RelativeCr2...
Combination of Slag Characteristics & Melt composition
RelativeCr2O3%inSlag
• Preferable Transfer Carbon to AOD should be ...
Basicity
• B = ( CaO+MgO ) / SiO2
• Start with Si from hot metal : B = 5
• Reduction: B = 1,4
• Desulfurisation: B = 2,0
•...
Cr oxidation reaction and Cr protection during
melting in EAF
• Cr to Cr2O3 oxidation& its Reduction to Cr is highly
compl...
• Cr2O3 solubility in slag varies with Cr in metal
• At 14-20% Cr in metal Cr2O3 solubility in slag
is~9-10% at 1690o
C
• ...
RelativeCr2O3%inSlag
• Preferable Al2O3 level in slag ~6 - 8%
• Al2O3 in slag increases Cr2O3 activity
• Al2O3 in slag inc...
Control points to restrict Cr oxidation &
enhance Cr recovery
• High Temperature
• Chemistry of melt
• Slag composition
• ...
In AOD extent /degree of chromium oxidation depends of
blowing procedures
AOD fundamentals
Fundamentals converted into AOD Actions
• High input metal temperature
• Optimum C & Silicon content in AOD input metal
• ...
Gas Phase
Diatomic
Liquid/Gas Inter-phaseLiquid Phase
N2
[N][N]
N2
[N]
Schematic representation for nitrogen adsorption
Su...
[N] = k* √ pN2
[N]
[N]
[N]
[N]
Ar/N2
Ar/N2
Ar/N2
Ar/N2
Ar
{Ar}
DepthofBathSchematic representation of nitrogen removal by ...
Increase in nitrogen content of stainless steel
 Use of High Nitrogen containing inputs
 CO-Boil in AOD reduces the nitr...
Inclusions in 304 type stainless steel
Change in constituent percentage in Inclusions
with temperature during steel making...
Inclusions in 304 type Ti treated stainless steel
 Inclusions in AOD• Inclusions in Mold
Inclusion compositions In 304 ty...
Inclusions in 304 type stainless steel
• Inclusions are scattered in
boundary of Sphene (CaO-SiO2-
TiO2) and Perovskite (C...
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Fundamentals of Stainless steel melting

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Fundamentals of Stainless steel melting

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Fundamentals of Stainless steel melting

  1. 1. Fundamentals of Stainless steel melting In EAF-AOD route By Amartya Talukdar E-mail: amartya.talukdar@gmail.com
  2. 2. Stainless steel demand is growing continuously but prices are stagnant To be successful it is necessary to • Optimise Quality • Maximise Productivity • Reduce Operating cost
  3. 3. Presentation Road map CARBON-CHROMIUM-TEMPERATURE RELATIONSHIPS Slag Characteristics in Stainless steel making Kinetics of gas adsorption/ de-sorption in Stainless steel-w. r. t nitrogen
  4. 4. • 1960: Laboratory investigations on CARBON- CHROMIUM-TEMPERATURE RELATIONSHIPS Isothermal chemical reaction process of stainless steel making • ARGON was introduced in the REACTION ZONE to control TEMPERATURE • REDUCED CARBON level can be achieved WITHOUT ANY MAJOR LOSS OF Cr • RESULTS COULD NOT BE REPEATED IN EAF – HENCE AOD Preamble
  5. 5. Equilibrium of Chromium and Carbon with variation in Temperature • Line 1: Equilibrium C-Content is decrasing at higher temperature. • Line 2 : Equilibrium C-Content is incrasing at higher Cr-content.old standard
  6. 6. Process Triplex Duplex
  7. 7. Prime Reactions in Stainless steel making When oxygen is injected into the liquid metal reactions for Chromium & carbon oxidation • 4[Cr] + 3{O}= 2(Cr2O3) not preferred • [C]+{O}={CO} • 3CO+Cr =Cr2O3+3C A major loss of valuable chromium to the slag in the form of oxides is not acceptable Thus preferential oxidation of Carbon is desired over Cr oxidation reaction  (Cr2O3)+3[C] =2[Cr ]+3{CO} preferred 
  8. 8. ConCept of oxide ellingham / oxygen potential diagram A reaction proceeds forward spontaneously when ∆Go the free energy of the reaction is negative and those are reactions are more favorable compared to other where ∆Go is more negative -∆Go = -∆ Ho +T ∆So If this equation is plotted - ∆Go as y and T as x- axis for any (in y= mx+C format) particular reaction we get Ellingham Diagram Where ∆ Ho formulate intercept & ∆So decide the slope
  9. 9. Oxide Ellingham / oxygen Potential Diagram
  10. 10. Control of Temperature • Ellingham diagram : At Higher temperature C oxidation is favorable than Cr oxidation • The critical temperature for kinetic reasons 1680-1700o C • Not feasible and detrimental to vessel lining • Oxygen lancing on low temperature bath: lots of Cr oxidised & lands in slag as Cr2O3 • Late O2 injection & High Oxygen lancing rate is preferred in EAF
  11. 11. Reaction involved 3CO+2Cr = Cr2O3+3C a Cr2O3 X a C 3 Equilibrium constant K = --------------------- where a Cr 2 & a Cr2O3 =1 a Cr 2 X p CO 3 aC is directly proportional to CO partial pressure i.e. lower equilibrium C level at reduced p CO ai - activities of reaction constituents At reduced CO partial pressure pCO : 2C+O2= 2CO will preferentially proceed over Cr ► Cr2O3 reaction At reduced CO partial pressure pCO high Cr should be Explanation with Sivert’s law
  12. 12. Oxygen potential diagram with conditions like reduced partial pressure & C, Cr concentrations etc. imposed on it for C CO► & Cr ► Cr2O3 reactions -120 -60 -70 -90 -110 -80 -100 pco= 0.01 atm. pco= 1.0 atm. 1200 1600 200018001400 RTlnPo2 Temperature o C
  13. 13. Carbon – Chromium equilibrium in AOD/VOD Decarburisation is effected  high Temperature  Reduced pCO
  14. 14. • Cr2O3 level in slag can vary from 5- 20 % • High Cr oxide in slag make the slag stiff viscous & non workable SLAG CHARACTERISTICS
  15. 15. Viscosity of Cr2O3 slags at 1550o C High Cr oxide in slag make the slag stiff , viscous & non workable Area of Low viscosity Cr2O3 scale CaO scale
  16. 16. Role of Slag basicity on Cr2O3 Reduction Cr2O3 Cr2O3 Cr2O3 Cr2O3 Cr2O3Cr2O3 SiO2 SiO2 SiO2 CaO Typical SiO2 –Cr2O3 Tetrahedral Slag Structure Process of breaking tetrahedron by CaO start at basicity 1.05
  17. 17. Resultant Slag • Viscous Stiff slag in Acid regime • Until CaO breaks structure to individual Cr2O3 - CaO units Cr2O3 can not be released for reduction to Cr • High Basicity : more slag volume and more Cr2O3 entrapment • Thus Slag basicity must be optimum Between 1.3 to 1.8 • This is also true for AOD slags under reduction
  18. 18. Typical Evolution of Cr2O3 In slag During EAF 0 2 4 6 8 10 12 14 16 End Melting End oxy blow Start Reduction Finish Reduction Tap ladle Cr2O3 in Slag min Cr2O3 in Slag max Preferred Slag Basicity After Melt 2.0 Melting & Oxy Injection 1.9 Reduction start 1.8 Reduction finish 1.7 Tapping 1.5 EAF SLAG CHARACTERISTICS
  19. 19. ISO-Activity contours of CrOx for CaO-SiO2- CrOx system at 1873 K with Po2=6.95x10 -11
  20. 20. Combination of Slag CharaCteriStiCS & melt CompoSition • Optimum Slag basicity • Optimum Silicon level in melt RelativeCr2O3%inSlag RelativeCr2O3%inSlag
  21. 21. Combination of Slag Characteristics & Melt composition RelativeCr2O3%inSlag • Preferable Transfer Carbon to AOD should be ~1.25 - 1.50% • Decarburisation in AOD is more efficient compared to EAF • Thus optimum Transfer/ Tap Carbon preferred
  22. 22. Basicity • B = ( CaO+MgO ) / SiO2 • Start with Si from hot metal : B = 5 • Reduction: B = 1,4 • Desulfurisation: B = 2,0 • Combined Red. + Desulf. : B = 2,2
  23. 23. Cr oxidation reaction and Cr protection during melting in EAF • Cr to Cr2O3 oxidation& its Reduction to Cr is highly complicated process • Dissolved Silicon and carbon should be high to protect Cr and to secure sound reduction of Cr2O3 • At least 0.2% Silicon should be maintained melting to tapping to protect Cr from being oxidised • Substantial improvement in Cr recovery may obtained when Silicon level in melt is improved from 0.1 to 0.40 %
  24. 24. • Cr2O3 solubility in slag varies with Cr in metal • At 14-20% Cr in metal Cr2O3 solubility in slag is~9-10% at 1690o C • Industrial slag shows much higher solubility this is primarily due to solvent effect of Al2O3 • Quantity or Percent of Cr2O3 in slag can be estimated through some approximate - rule (Cr) Slag / [Cr] Metal = 0.3(%FeO) Slag Cr2O3 in solubility slag and Cr2O3 partition
  25. 25. RelativeCr2O3%inSlag • Preferable Al2O3 level in slag ~6 - 8% • Al2O3 in slag increases Cr2O3 activity • Al2O3 in slag increases slag fluidity and kinetic conditions for Cr transfer rate at 1500-1600o C • Cr recovery rate is slow and improves with increase in Al2O3 % in slag • Al3+ replaces the Cr3+ ion in Cr2O3 spinel based particles Combination of Slag Characteristics & Melt composition
  26. 26. Control points to restrict Cr oxidation & enhance Cr recovery • High Temperature • Chemistry of melt • Slag composition • Extent of dilution of CO
  27. 27. In AOD extent /degree of chromium oxidation depends of blowing procedures AOD fundamentals
  28. 28. Fundamentals converted into AOD Actions • High input metal temperature • Optimum C & Silicon content in AOD input metal • Moderate to low slag volume in transfer • Deep Blow from top lance • Increased oxygen lancing rate at initial decarburisation period • Quick reduction of temperature after blow finish • Optimum basicity of slag during reduction
  29. 29. Gas Phase Diatomic Liquid/Gas Inter-phaseLiquid Phase N2 [N][N] N2 [N] Schematic representation for nitrogen adsorption Sulphur & Oxygen put hindrance at this surface Nitrogen at the end of the AOD varies between 150-500 PPM in metal depending on blow gas regimes
  30. 30. [N] = k* √ pN2 [N] [N] [N] [N] Ar/N2 Ar/N2 Ar/N2 Ar/N2 Ar {Ar} DepthofBathSchematic representation of nitrogen removal by argon rinsing
  31. 31. Increase in nitrogen content of stainless steel  Use of High Nitrogen containing inputs  CO-Boil in AOD reduces the nitrogen level  Oxygen purity of blowing should be reduced  After Sulphur & Oxygen removal during reduction step Nitrogen should be blown
  32. 32. Inclusions in 304 type stainless steel Change in constituent percentage in Inclusions with temperature during steel making  Al2O3 & TiO2 content increase as the process proceeds from AOD to Mold  Al2O3 & TiO2 form during deoxidation  MgO content : constant or decrease due to dilution  Inclusions nucleate heterogeneously on surface of AOD or slag particles
  33. 33. Inclusions in 304 type Ti treated stainless steel  Inclusions in AOD• Inclusions in Mold Inclusion compositions In 304 type Stainless on CaO- Al2O3-SiO2 – 10 MgO quasi-ternary Normally the nucleation site for Inclusion formation in AOD – suspended AOD slag droplets Mainly four types • Spinel MgO-Al2O3 • Perovskite CaO-TiO2 • Pure Rutile TiO2 • Spinel in MnO-Silicate matrix Reduction in Aluminium content of metal
  34. 34. Inclusions in 304 type stainless steel • Inclusions are scattered in boundary of Sphene (CaO-SiO2- TiO2) and Perovskite (CaO-TiO2) region • Inclusion Crystallization temperature Highly sensitive to basicity  Perovskite- high melting point  Sphene- lower melting point Reduction in AOD Basicity to avoid formation of high temperature phases  Inclusions in AOD • Inclusions in Mold Inclusion compositions In 304 type Stainless on CaO- TiO2-SiO2–10 MgO quasi-ternary Perovskite Sphene Rutile

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