Changing Behavior of Non-metallic Inclusions in Solid Iron Deoxidized by Al-Ti Addition during Heating at 1 473 K

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 1 473 K, and observing inclusions by SEM-EDS. In the case of the specimen in which thermodynamically stable inclusion at 1 873 K is Al(2)O(3), mostly pure Al(2)O(3) 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 1 873 K is Ti(3)O(5), TiO 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 mu m. In the case of the specimen which composition is located at the boundary area of Al(2)O(3), Al(2)TiO(5), and Ti(3)O(5) stable regions at 1 873 K, various inclusions from pure Al(2)O(3) to TiO(x) were observed in an as cast sample. By heating, three types of inclusions, namely, Al(2)O(3), 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 1 473 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.