Transient Behavior of Inclusion Chemistry, Shape, and Structure in Fe-Al-Ti-O Melts: Effect of Titanium/Aluminum Ratio
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NTRODUCTION
INCLUSIONS obtained during industrial sampling of interstitial-free (IF) steel melts are often found to contain titanium oxides, although these are not thermodynamically predicted.[1–3] The inclusion stability diagrams, proposed in literature by Ruby-Meyer et al.[4,5] and Jung et al.[6] suggested that Al2O3 is the stable inclusion phase under conditions prevalent during ladle processing of IF steel melts. If, however, the Ti/Al CONG WANG, Graduate Student, NOEL THOMAS NUHFER, Director of Electron Microscopy and Materials Characterization, and SEETHARAMAN SRIDHAR, POSCO Professor, are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. SEETHARAMAN SRIDHAR, POSCO Professor, is also with the National Energy Technology Laboratory, Pittsburgh, PA 152360940. Contact e-mail: [email protected]. Manuscript submitted July 21, 2009. Article published online September 1, 2009. 1022—VOLUME 40B, DECEMBER 2009
ratio is raised because of, e.g., transient conditions, then Al2TiO5 and/or a liquid Al-Ti-O phase could become temporarily stable, and if the titanium content is even higher, i.e., at even higher Ti/Al ratios, then Ti3O5 becomes stable. It should be noted that the thermodynamic inclusion stability diagram is only of limited use because literature on titanium oxides and titaniumoxidation studies suggests that the oxides that can form can be different, such as TiO and Ti2O,[7–16] where some metastable phases are eventually proceeded by others, and solid-state transport limitations could prevent the appearance of the thermodynamic stable phase. However, a consensus has not been reached on the formation of certain types of titanium oxides even under the same experimental conditions. In a preceding paper,[17] it was shown that when the Ti/Al ratio in the melt was in the Al2O3 stable region, then after titanium addition, titanium-containing inclusions, such as Al2TiO5, did form. With time, the inclusion chemistry reverted to the thermodynamically METALLURGICAL AND MATERIALS TRANSACTIONS B
stable Al2O3. It was suggested that these are caused by a local and temporary supersaturation of soluble titanium, which causes either a metallothermic reduction of Al2O3 and/or a between titanium and the residual dissolved oxygen. This temporary process was accompanied by an irreversible shape change in the inclusions from spherical to nonspherical. Because of the temporary and local nature of this reaction, a variety of inclusions could have been produced that continuously change with time. In this study, experiments were undertaken to simulate different conditions of titanium supersaturation in the melt by successively varying the Ti/Al ratio. The objectives are to identify the inclusion-reaction products that result from varying the titanium content in aluminum-killed steels. It is also intended that the transient stage is identified for these various cases. Transient conditions, where Ti/Al ratios different from those are expec
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