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1. MY: 301 Steel Making Processes
AOD (Argon-Oxygen Decarburization) Vessel:
Why AOD?
• By using only EAF for the production of stainless steel the following
difficulties were arise.
1. Needs very high temperatures (about 19000
C) for
decarburization (requiring high cost refractories).
2. It was only possible low charge chromium… this meant that only
small amount of stainless steel scrap were used.
3. Low carbon FeCr was used for alloying… (requiring high cost)
4. One or more treatment (double slag practice)… (Time consuming)
AOD Vessel:
Introduction:
• It is used in the production of stainless steel or other high–alloy
grades that contain highly oxidizable elements such as Cr and low
carbon.
• Decarburization by regular oxygen lancing has the undesirable
consequence of oxidizing the alloying element as well.
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2. MY: 301 Steel Making Processes
• The heart of this process is to control the partial pressure of CO to
control the rate of oxidation of C and Cr so that Cr is retained in the
bath while C is removed to low levels.
• The AOD process alleviates this problem by diluting the injected
oxygen with Argon or Nitrogen.
• So that, based on the pressure-dependent equilibrium relationship
%C X %O = 0.0025 X CO pressure
• The oxygen prefers to combine with carbon and oxidize only small
amount of alloying element.
• The unit cost arc furnace alone would be higher than AOD and it is
difficult and expensive to achieve lower carbon less than 0.01%, this is
achieved in AOD unit.
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3. MY: 301 Steel Making Processes
Stainless Steel Production from the AOD Process:
Process flow = EAF  AOD  IC or CC
Aim (in %): Cr = 18.0, C = 0.025, Ni = 10.0, Si = 0.5, S = 0.015
Tapping Temp= 15500
C
Steps
In EAF
1. Charging (Stain less steel scrap high carbon Ferro alloys).
2. Melting
3. Sampling (If “C”,“P” and “S” does not with in the limit) and
(at melt out Cr 0.5% above the Minimum level)
Oxidizing Condition
4. Decarborization and Dephosphorization (Pure Oxygen
Blowing/Lancing + CaO)
5. Deslagging of Salg (Oxidizing slag)
6. Tapping (Temp: 1500 - 15500
C) in the ladle and transfer to the
AOD station.
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4. MY: 301 Steel Making Processes
In AOD
7. Blow 1 (Ar/O2) = 1:3 = 25%Ar and 75% O2, (25min)
Stopped when C = 0.4% and Cr = 17.8% and a Temp: = 17000
C)
• Such a mixture (Ar/O2) = 1:3 promotes rapid oxidation of carbon at
the low temperature.
• Towards the end of the blow as the C falls blow 0.5% then the
dilution effect of Ar on the partial pressure of CO is beginning to
have an effect.
• Blow 1 is stopped when C = 0.4% and Cr = 17.8% and a Temp: =
17000
C) which is approximately at the equilibrium point for 1atm
(pressure of CO 1atm).
• If the temperature rises higher than 17000
C the scrap coolant would
be added to reduce the temperature to about this level prior to
beginning blow2.
8. Blow 2 (Ar/O2) = 1:2= 34%Ar : 66% O2 (15min)
Stopped when C = 0.15% and Cr = 17% and a Temp: = 17200
C)
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5. MY: 301 Steel Making Processes
• Such a mixture (Ar/O2) = 1:2 reduces the partial pressure of CO to
promote C removal without significant temp increases or Cr loss.
9. Blow 3 (Ar/O2) = 2:1= 66%Ar : 34% O2 (15min)
• The final blow is (Ar/O2) = 2:1 so that partial pressure of the CO is
approaching 0.1 atm. Carbon removal continues with a small temp:
rise to give the bath conditions C = 0.018% and Cr = 16.5% and a
Temp: = 17400
C.
• 2% Cr has been oxidized into the slag.
10. Reduce Ar (blow) = 100%
• Addition of FeSi and lime and gently stirried with argon alone to
Recover the choromium which has been oxidized (about 1.7%Cr is
reduced back from the slag).
• Temperature reduced (down to 16500
C at this stage).
Reducing Condition
11. Desulphurization (CaO + FeMn) Ar (blow) = 100%
• Temperature reduced (down to 16250
C at this stage).
12. Deslagging of Salg (Reducing Slag)
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6. MY: 301 Steel Making Processes
13. Chemical Adjustment (Trim) Ar (blow) = 100%
Cr = 18.0, C = 0.025, Ni = 10.0, Si = 0.5, S = 0.015
14. Temperature Adjustment (15500
C)
15. Tapping
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7. MY: 301 Steel Making Processes
13. Chemical Adjustment (Trim) Ar (blow) = 100%
Cr = 18.0, C = 0.025, Ni = 10.0, Si = 0.5, S = 0.015
14. Temperature Adjustment (15500
C)
15. Tapping
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