Metal Production- Stainless Steel Production

Question 1

Problem of conventional routes when making stainless steel

Answer 1

To achieved desired C levels the conditions need to be oxidising and this leads to loss of Cr to the slag. To rescue the Cr back from the slag requires additions of ferrosilicon which is expensive. Any shortfall in Cr means low C ferrochrome needs to be added which is also a large additional cost

Question 2

What is different about the AOD process?

Answer 2

The electric furnace is used to melt a charge.
The hot metal is then transferred to a converter.

Question 3

What does AOD stand for?

Answer 3

Argon oxygen decarburisation.

Question 4

How does C removal generally work in AOD process?

Answer 4

C removal can be encouraged by lowering the effective pressure of CO gas. This is achieved by dilution with an inert gas, argon (blown together with oxygen into the vessel). Carbon removal encouraged rather than chromium oxidation

Question 5

Problems with making stainless steel using electric steelmaking techniques (e.g EAF)

Answer 5

Possible but problems. Need to use very high temperatures (about 2000°C). At these very high temperatures there is a greater % of Cr that remains unoxidised in the melt for a given level of carbon. Very high T also has implications for energy footprint and refractory wear.

Question 6

Reactions that occur in the AOD vessel

Answer 6

C (in Fe) + 1/2O2 -> CO
2Cr (in Fe) + 3/2O2 -> Cr2O3 (solid or liquid)

Question 7

Difference with VOD and RH:OB processes

Answer 7

Reducing effective pressure of CO done by introducing oxygen into a vacuum chamber

Question 8

Percentages of Cr and carbon that are good for AOD

Answer 8

Has been shown that 11% Cr metal with C over 0.3% means very little Cr loss and all oxygen is used for decarburisation.

Question 9

How is the oxygen/argon ratio varied over the course of the blow in AOD?

Answer 9

In the early stages use a high oxygen/argon ratio mixture. Amount of oxygen in mixture changes the value of the oxygen partial pressure pO2. When C falls below 0.3% the Cr oxidation increases and we need to decrease the oxygen/argon ratio in the gas.
At start oxygen/argon is 3:1. 2c+O2->CO
At finish ratio is 1:2. Cr2O3+3C->2Cr+3/2O2

Question 10

Problems with AOD

Answer 10

1. When a Cr containing steel is decarburised to C under 0.05% the slag contained 20-25% Cr2O3. This increases slag viscosity and this must be counteracted by adding Fe/Si or Fe/Cr/Si.
2. When the steel has been decarburised to the desired level still about 3% Cr is oxidised. This can be recovered from the slag by reducing with Si.
2CrO+Si->2Cr+SiO2

Question 11

Other disadvantages of AOD

Answer 11

Main weakness is high refractory wear. The vessels are lined with dolomite or magnesium/chrome refractories. Lining life is low, only a few hundred heats. Due to:
Refractory erosion caused by efficient mixing,
Acidic slags in the melt as the Si oxidises,
High T (1750-2000°C).