Changing Behavior of Non-metallic Inclusions in Solid Iron Deoxidized by Al–Ti Addition during Heating at 1473 K

Abstract

Changing behavior of non-metallic inclusions in composition and size distribution at high temperature was investigated by preparing three kinds of Fe–Al–Ti steels, heating at 1473 K, and observing inclusions by SEM-EDS. In the case of the specimen in which thermodynamically stable inclusion at 1873 K is Al₂O₃, mostly pure Al₂O₃ inclusions were observed in an as cast sample, while many dual phase inclusions were observed after heating. Iron content in inclusions increased and many Al–Ti–Fe–O inclusions in narrow composition range were observed. In the case of the specimen in which stable inclusion at 1873 K is Ti₃O₅, TiO_x inclusions were observed in an as cast sample. Inclusion compositions changed to Fe–Ti–O by heating and inclusion size became smaller to 2 to 3 μm. In the case of the specimen which composition is located at the boundary area of Al₂O₃, Al₂TiO₅, and Ti₃O₅ stable regions at 1873 K, various inclusions from pure Al₂O₃ to TiO_x were observed in an as cast sample. By heating, three types of inclusions, namely, Al₂O₃, Al–Fe–O, and Al–Ti–Fe–O inclusions were observed and average inclusion size decreased. Although the stable phase of oxide equilibrated with Fe–Al–Ti–O system at 1473 K are not clarified, inclusions equilibrated with molten metal are no longer stable in solid state steel and thus inclusion composition and size change by reacting with solid steel.