Consideration on the Equilibrium between Silicon and Oxygen in the Liquid 18-8 Stainless Steel

Abstract

At several steps of stainless-steelmaking, the most important thing is the relation between chemical compositions of various elements, which are determined by the thermodynamic data. Practically, many elements dissolve from raw materials, fuel, and refractory, into liquid steel and the actual composition is different from the composition in the melt prepared in laboratory.
It needs complicated calculations to know the equilibrium constants. Presence of too many elements in the liquid steel and the uncertainty of their activity coefficients make the results of the calculations not always satisfactory.
Silicon is one of the important elements for the deoxidation in 18-8 stainless-steelmaking practice. Concerning the equilibrium relation between silicon and oxygen, the present authors obtained the following results.
1) The effect of chromium concentration on the activity of dissolved oxygen, f(^Cr)₀, the temperature dependence of which has not yet been measured, is calculated by the solubilities of oxygen and silicon in the system Fe-Cr solution equilibrated with acidic slag saturated with silica.
The value of f(^Cr)₀ obtained is in good agreement at 1600°C with that reported by SAKAO and SANO.
The effects of both the concentration of chromium and the temperature on f(^Cr)₀ are expressed by the following empirical formula;
log f^(Cr)₀=-(369/T-0.137)[%Cr]+0.0024[%Cr]^1.75
2) The equilibrium constant between silicon and oxygen in the liquid F_e-C_r system is calculated at 1600°C, 1650°C and 1700°C.The relationship between the equilibrium constant Ksi and the temperature is as follows:
log K_Si=-33210/T+13.01
3) By using f(^Cr)₀ given above, the equilibrium constant Ksi is calculated for the liquid 8-8 stainless steel, which is practically produced in the steelmaking.
The result agrees relatively well with that of Ksi in the F_e-C_r system.