Thermodynamic conditions of MgO and MgO∙Al₂O₃ formation and variation of inclusions formed in Fe-17mass%Cr steel at 1 873 K

Demands for cleanliness of high chromium steel have been increasing. In steel refining process, aluminum is usually added in molten steel as a deoxidizing agent. As a result, such inclusions as alumina (Al₂O₃) and spinel (MgO∙Al₂O₃) are formed, which cause fatigue failures and surface defects. Therefore, it is important to understand the conditions of the inclusions which form in high chromium steel, and to reduce their harmful effects on steel qualities. In this work, to begin with, thermodynamic conditions of MgO and MgO∙Al₂O₃ formation in Fe-17mass%Cr molten steel at 1 873 K were investigated. The results showed that MgO is more stable in high chromium steel than in plain steel. The boundary of the stable condition of MgO and MgO∙Al₂O₃ shifts toward higher Al and lower Mg contents in high Cr steel. This cause is judged to be the effect of thermodynamic interaction between Cr and Mg. The interaction parameter of Cr on Mg was estimated to be 0.040 so that the boundary of stable condition of MgO and MgO∙Al₂O₃ can be explained. Moreover, phase stability diagram of Fe-Cr-Al-Ca-Mg-O system at 1 873 K was developed to estimate the effect of chromium on the stable condition of MgO, MgO∙Al₂O₃ and CaO-MgO-Al₂O₃(l). Subsequently, the variations of inclusions which formed in Fe-17mass%Cr molten steel were also investigated at 1 873 K. The variations of inclusions in molten Fe-Cr steel were reasonably explained by considering the stable conditions of MgO and MgO∙Al₂O₃ investigated in this work.