Thermodynamic Conditions of MgO and MgO·Al₂O₃ Formation and Variation of Inclusions Formed in Fe-17 mass%Cr Steel at 1873 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-17 mass%Cr molten steel at 1873 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 1873 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-17 mass%Cr molten steel were also investigated at 1873 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.