The relationships of the compounds in the Mg-Ti-Al-O system in equilibrium with molten iron are investigated at temperatures ranging from 1873 to 1973 K, and the thermodynamic calculations are conducted in avoiding Al 2 O 3 or spinel MgAl 2 O 4 formation and for inclusion control. The equilibrium relations between the compounds (Mg 2 TiO 4 , MgTi 2 O 4 , or MgAl 2 O 4 ) and the composition of solutes in steel are clarified. The conditions are shown that the transformation of the stable compound from Mg-Ti spinel to Mg-Al spinel occurs at Mg contents ranging from 1 to 10 ppm by mass. It is also found, on the Mg-Ti spinel, the stable compound is transformed from inverse-spinel Mg 2 TiO 4 to normal-spinel MgTi 2 O 4 at Ti ¼ about 60 ppm by mass. In addition, the stable compound is transformed from MgTi 2 O 4 to Ti 2 O 3 at Ti ¼ about 1000 ppm by mass.

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It is important to determine the equilibrium relationships between the oxide compounds in MgO-Ti2O3-Al2O3 and iron to avoid Al2O3 or MgO·Al2O3 formation and for inclusion control. In this study, the equilibrium relationships between the oxide compounds in MgO-Ti2O3-Al2O3 and molten iron are investigated at 1 873 K. The phase-stability diagrams, which show the equilibrium relationships between the oxide compounds in MgO-Ti2O3-Al2O3 and molten iron, are described. It is also important to know the oxides that newly form after solidification. Accordingly, the temperature-dependences of the stable compounds that newly form after solidification were investigated using the thermodynamic calculation software FactSage 6.1. Although the compounds formed after solidification are not necessarily the same as the equilibrium oxide compounds at 1 873 K, the correlation between them can be confirmed. In particular, under the unstable condition of MgAl2O4 at 1 873 K, it is found that the formation of MgAl2O4 could also be suppressed after solidification.