Controlling inclusion content in high chromium steel is very important to prevent submerged entry nozzle from clogging in continuous casting and avoid the negative impacts of inclusions on steel properties. Therefore, effects of temperature and content of elements on phase stability diagram should be clarified in chromium bearing steel. However, the effect of chromium content on boundaries of MgO, MgO•Al 2 O 3 and Al 2 O 3 in phase stability diagram are much different among the researchers. The direction of boundaries shift is affected by chromium content differently. Temperature dependencies of deoxidation equilibrium constants below 1 873 K are also scattered. Calcium, which is used to avoid the negative effect of MgO•Al 2 O 3 inclusion, enlarges liquid region in phase stability diagram. However, the region replaced by liquid oxide is understood differently in low alloyed steel and high chromium steel. In TiO X -Al 2 O 3 -MgO system inclusion, commercial thermochemical software predicts that boundaries of Ti 2 O 3 , Ti 3 O 5 , Al 2 O 3 and TiOx-Al 2 O 3 shift toward lower titanium content in high chromium steel. However, the calculated phase stability diagrams vary among studies even in liquid iron or low alloyed steel. Therefore, equilibrium experiments under various conditions and reliable technique of thermodynamic calculation with high accuracy are desired.

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 (Al 2 O 3 ) and spinel (MgO•Al 2 O 3 ) 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•Al 2 O 3 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•Al 2 O 3 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•Al 2 O 3 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•Al 2 O 3 and CaO-MgO-Al 2 O 3 (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•Al 2 O 3 investigated in this work.

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