Transient Behavior of Inclusion Chemistry, Shape, and Structure in Fe-Al-Ti-O Melts: Effect of Titanium/Aluminum Ratio

Abstract

During ladle processing of interstitial-free (IF) steel melts, it is possible for transient titanium-containing oxides to be formed if the local titanium/aluminum (Ti/Al) ratio is locally and temporarily increased after aluminum killing. The phase stability diagrams suggest that if the Ti/Al ratio is increased, then Al2TiO5 and/or a liquid Al-Ti-O region can become stable, and eventually at even higher Ti/Al ratios, Ti3O5 becomes stable. In this study, the Ti/Al ratio was successively altered to investigate (1) how the inclusions evolved after titanium addition to aluminum-killed iron melts and (2) whether the inclusions present after sufficient time were those predicted by thermodynamics. When the Ti/Al ratio was maintained at 1/4, such that Al2O3 is the only thermodynamically stable oxide, the results show that transient titanium-containing oxides exist temporarily after titanium addition, but with time, the predominant inclusion was Al2O3, which would generate little shape change and produce transient stage inclusions with less titanium contents. When the Ti/Al ratio was increased to 1/1 (Al2O3 still being the only thermodynamically stable oxide), the results show a more distinct increase in the titanium content of the transient inclusions. The transient reaction was, in this case, accompanied by an irreversible shape change from spherical to irregular inclusions. When the Ti/Al ratio in the melt was increased to 15/1 within the Al2TiO5 stable phase region, the inclusion population evolved from spherical-dominant ones to irregular ones. It was found that the final inclusion chemistry has more titanium but less aluminum content compared with the expected from the Al2TiO5 chemistry. Besides, the transmission electron microscopy (TEM) results showed the existence of Ti2O. When the Ti/Al ratio in the melt was increased such that Ti3O5 is the thermodynamically stable inclusion (Ti/Al ratio of 75/1 or ∞), the inclusions evolved after titanium addition toward TiOx inclusions, which is accompanied by a shape change from spherical to irregular. The TEM results revealed and confirmed the existence of metastable Ti2O besides the thermodynamically stable Ti3O5, and it was consistent with the results based on oxidation studies of thin layers of titanium with Al2O3 substrate. It was discovered that Ti2O has the tendency of transforming into the thermodynamically stable phase Ti3O5 under certain conditions.