1. Smelting process:
Basic principles of stainless steel smelting

Unlike carbon steel production, stainless steel contains at least 10.5% Cr, so during decarburization reactions, reactions occur:

Cr3O4+4[C]=3[Cr]+4CO

reaction equilibrium constant

Because in high Cr molten steel, Cr is preferentially oxidized than C. At normal smelting temperature, when C is below 0.03%, the equilibrium Cr is only about 4%; increasing the temperature can increase the equilibrium Cr content, but the refractory material is unbearable. For example, the temperature of molten steel with 18% Cr should reach above 1900 ℃, and a smelting method to reduce the partial pressure of CO was developed for this purpose. Stainless steel commonly used refining methods:

Electric arc furnace (EF) direct smelting method, AOD (argon oxygen decarburization method), VOD (vacuum oxygen decarburization method), converter top low re-blowing method (K-OBM-S, K-BOP method, etc.), RH- The OB method is dominated by the AOD method and the VOD method. The two-step and three-step processes are used to smelt stainless steel, which can produce low-carbon and ultra-low-carbon stainless steel in large quantities, and the recovery rate of Cr also reaches a high level.

1) Several stainless steel smelting methods:

u One-step method:

EAF original electric furnace direct production process

u Two-step method:

EAF+AOD EAF+VOD

EAF+CLU LD-OB+VOD

u Three-step method:

EAF+K-OBM-S+VOD or AOD+(LF)

EAF+AOD+VOD

EAF+LD-OB+VOD

2) Advantages and disadvantages of AOD and VOD:

AOD smelting has the advantages of low investment cost, fast decarburization speed, and high production efficiency; high thermal efficiency and low smelting cost; good stirring effect of molten steel, which is conducive to the removal of S and O; relatively simple equipment, easy to master the process, etc., currently about 70% of the world’s The left and right stainless steel is produced by AOD. The disadvantage is that it is difficult to produce low C and N steel grades.

VOD smelting has good effect of removing C, retaining Cr, and deoxidizing effect; it is suitable for the production of low C and N steel grades. Disadvantages: The investment cost is high, and the equipment maintenance is difficult.

2. Heat treatment process:
The purpose of stainless steel heat treatment is to obtain the best performance of stainless steel, or to create conditions for subsequent processing of stainless steel. Generally, stainless steel needs to be heat treated before leaving the factory.

u Martensitic stainless steel: softening (martensite → ferrite + carbide); carbide diffusion; adjustment of grain size.

u Ferritic stainless steel: improve plasticity; adjust grain size.

u Austenitic stainless steel: carbide solid solution; adjust grain size; soften; reduce delta ferrite.

Duplex stainless steel: carbide solid solution; improve plasticity; reduce brittle phase precipitation.