Effects of Refining Slag on Transformation and Removal of Inclusions in Type 430 Stainless Steel

Abstract

Laboratory “steel–slag” equilibrium experiments at 1873 K (1600 °C) were conducted to study the effects of basicity (CaO/SiO₂) and the C/A ratio (CaO/Al₂O₃) in CaO–SiO₂–Al₂O₃–MgO slag on the inclusions in Si–Mn-deoxidized 430 stainless steels. A thermodynamic analysis was performed to predict the transformation of the chemical composition of the inclusions. A mathematical model was introduced to describe the inclusion behavior at the liquid “steel–slag” interface and predict the removal of oxide inclusions during slag refining. The results showed that increasing slag basicity (R) from 1.0 to 5.0 lowered the total oxygen content (T.O.) from 0.0070 to 0.0038 mass pct and dissolved [O] content in steel from 0.0013 to 0.0008 mass pct, and led to the formation of inclusions with high Al₂O₃ content, while increasing the C/A ratio from 0.7 to 2.5 had little effect on the steel cleanliness. The values of both ${\text{log}}{{X}}_{{\text{Al}}_{2}{{\text{O}}}_{3}}^{\text{i}}$ and ${\text{log}}\frac{{X}_{{\text{Cr}}_{2}{{\text{O}}}_{3}}^{\text{i}}\cdot{X}_{{\text{Si}}{\text{O}}_{2}}^{\text{i}}}{{X}_{{\text{Al}}_{2}{{\text{O}}}_{3}}^{\text{i}}\cdot{X}_{\text{MnO}}^{\text{i}}}$ were positively correlated with that of log $\frac{{a}_{{\text{Al}}_{2}{{\text{O}}}_{3}}^{\text{s}}\cdot{a}_{\text{MnO}}^{\text{s}}}{{a}_{{\text{Si}}{\text{O}}_{2}}^{\text{s}}\cdot{{a}}_{{\text{Cr}}_{2}{{\text{O}}}_{3}}^{\text{s}}}$, indicating that the activity ratio of the slag components directly affected the concentration of the inclusions. The separation was less than 90 pct for all inclusions with a diameter of 5 μm for different refining slags. The experimental results and prediction of the mathematical model for inclusion removal after slag refining were in reasonable agreement.