Effect of CaO–SiO 2 –MgO–CaF 2 slags on deep deoxidation and desulphurisation of low-Ni 201 stainless steel by simultaneous consideration of multiple slag–steel reactions

Abstract

Effects of the CaO–SiO₂–MgO–CaF₂ slag on the deep deoxidation and desulphurisation of low-nickel 201 stainless steel (SS) are investigated via laboratory-scale experiments and a developed thermodynamic model. The model was developed based on the slag–steel equilibrium theory, the law of mass conservation, as well as the ion and molecule coexistence theory. And the model involves the Si–Mn–Cr–Fe–S–O reaction system. By comparing the measured values and the model-predicted values, the model has been proved that can effectively predict the equilibrium oxygen and sulphur contents in 201 SS with CaO–SiO₂–MgO–CaF₂ slag. The average deviation between the model-predicted value and the measured value is <1.5×10⁻⁶. Both the model-predicted values and the measured values show that a high basicity is beneficial for the deoxidation and desulphurisation of steel. Moreover, the measured values of the minimum oxygen and sulphur contents are 8.69×10⁻⁶ and 8.10×10⁻⁶, respectively, which are obtained at B≥1.9. Increasing the MgO content at B=1.7 to within 5 wt-% slightly improves the deoxidation and desulphurisation of the steel, while it has no effect at (% MgO)≥7.0. Increasing the CaF₂ content in the range of 20–40 wt-% slightly decreases the equilibrium sulphur content (3.6×10⁻⁶) in the steel, but it does not affect the equilibrium oxygen content. As a result, the optimum slag for both the deoxidation and desulphurisation of 201 SS is to maintain a binary basicity range of 1.9–2.1, with MgO content at saturation state and CaF₂ content at 40 wt-%. In addition, the deoxidation and desulphurisation mechanisms between CaO–SiO₂–MgO–CaF₂ initial slag and 201 SS are discussed based on the developed model.