MY: 301 Steel Making Processes Lecture No: 18, 10 Feb 2011

What is Stainless Steel?

Stainless steel is the name given to a group of corrosion resistant

and high temperature steels.

The corrosion resistant property is due to a thin, adherent stable

chromium oxide or nickel oxide film that effectively protects the

surface from many corroding media.

When ordinary carbon steel is exposed to rain water, it corrodes,

forming a brown iron oxide, commonly called rust, on the surface.

This is not protective and eventually the entire piece of steel will

corrode and be converted to rust.

But when enough chromium (more than about 10%) is added to

ordinary steel, the oxide on the surface is transformed - it is very thin,

virtually invisible and protective in a wide range of corrosive media.

This is what we call stainless steel.

There are several different types, and many different grades.

1. Ferrite stainless steels.(14 – 27 %Cr)

2. Martensitic stainless steels. (11.5 – 18 % Cr)

3. Austenitic stainless steels. (Cr - Ni or Cr – Ni - Mn)

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MY: 301 Steel Making Processes Lecture No: 18, 10 Feb 2011

4. Duplex stainless steels.

5. Precipitation hardening stainless steels.(16.5-17%Cr)

Stainless Steel Production from the Arc Furnace alone:

Steps

1. Charging (Scrap not more than 4 % Cr)

2. Melting

3. Sampling (If “C”,“P” and “S” does not with in the limit than)

Oxidizing Condition

4. Decarborization and Dephosphorization (Pure Oxygen Blowing/Lancing + CaO)

5. Deslagging of Salg (Oxidizing slag)

Reducing Condition

6. Desulphurization (CaO + FeMn)

7. Deslagging (Reducing Slag)

Oxidizing Condition

8. Oxygen Blowing (Lancing) for more decarburization.

9. Addition of FeSi (to Recover the choromium which has been oxidized

(Cr2O3) + 2/3[Si] 2[Cr] + 3/2(SiO2)

10. Chemical AdjustmentPage 2 of 6

MY: 301 Steel Making Processes Lecture No: 18, 10 Feb 2011

11. Temperature Adjustment

12. Tapping to ladle

Problem:

“Most of the stainless steel contains equal or less than 0.03%C.It

is difficult to obtain low carbon with out excessive oxidation of

chromium”.

During Oxygen Blow

3/4 <Cr> + {O2} 2/3 <Cr2O3> Reaction #1

2<C> + {O2} 2{CO} Reaction #2

Solution:

From thermodynamics, the activity of the “C” is increased by raising

the temperature or by reducing the pressure (Vacuum Creation).

The possible way in EAF is only rising the temperature.

At high temperature the reaction # 2 takes place efficiently in

comparison to reaction # 1. (As Shown in Figure).

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MY: 301 Steel Making Processes Lecture No: 18, 10 Feb 2011

In figure, curves clearly shows that carbon is removed from the bath,

the equilibrium Cr content is also lowered and any Cr in excess of

equilibrium is very rapidly oxidised out.

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MY: 301 Steel Making Processes Lecture No: 18, 10 Feb 2011

For- example,

At 0.05%C and 1700 0 C (Point A in Figure)

The equilibrium Cr content is approximately 2%.

At 0.05%C and 1800 0 C (Point B in Figure)

The equilibrium Cr content is approximately 7.5%.

Most Stainless steel production requires carbon at or below 0.03%C at

which the equilibrium Cr level even at 18000C is only 4%Cr

Since C is present/contained in small amount in all Ferro alloys

For a specification of 0.03%C, it would be necessary to finish at

0.02%C, than low carbon FrCr is added to maintain Cr % up to the

required level

Production of 188 steel (18% Cr , 8% Ni and 0.04%C-Steel)

It is only possible to use a charge mix with 4 % Cr in it; the molten

charge would be blown down with pure oxygen to about 0.02%C giving

a temperature to about 18000C.

During the blow about 2% Cr of the charge would be oxidised into the

slag.

About 1%Cr would be reduced back into the metal by using FeSi.Page 5 of 6

MY: 301 Steel Making Processes Lecture No: 18, 10 Feb 2011

(The Ni content is largely unaffected by the decarburisation reaction)

By using only EAF for the production of stainless steel the following

difficulties were arise.

1. Needs very high temperatures (about 18000C or above) for

decarburization (require 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)

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