Transient Behavior of Inclusion Chemistry, Shape, and Structure in Fe-Al-Ti-O Melts: Effect of Gradual Increase in Ti
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THE ladle processing of interstitial-free (IF) steel melts involves deoxidation by Al followed by Ti addition for binding the interstitial elements C and N. The deoxidation products are often found to contain Ti, which is not predicted by equilibrium conditions. The Ti-containing oxide inclusions have been suggested to cause clogging of the submerged entry nozzle, which decreases continuous casting productivity and is a source of surface defects on the steel products.[1–3] A systematic effort has been applied to simulate the transient conditions after the Ti addition under laboratory conditions, and the resulting inclusions have been identified.[4–7] Specifically, the effect of Ti sources and the influence of Ti/Al ratio on inclusion evolution CONG WANG, formerly Ph.D. Candidate, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, is now Senior Research Engineer, Alcoa Technical Center, Alcoa Center, PA 15069. NOEL THOMAS NUHFER, Director, Electron Microscopy and Materials Characterization, and SEETHARAMAN SRIDHAR, POSCO Professor and NETL Resident Faculty Fellow, are with the Department of Materials Science and Engineering, Carnegie Mellon University. Contact e-mail: [email protected]. Manuscript submitted March 22, 2010. Article published online July 1, 2010. 1084—VOLUME 41B, OCTOBER 2010
behaviors, primarily in terms of composition and morphology, have been described. Particularly, it was demonstrated that when no Ti was added after Al killing, the results showed that Al killing could result in rapid deoxidation and removal of a significant portion of the solid oxide products through clustering and flotation. The morphology of the inclusions remaining inside the melt did not change appreciably with time. However, when Ti was incurred after Al deoxidation, Ti-containing inclusions were produced, which was accompanied by changes in morphology. It was suggested that these results are caused by a local and temporary super-saturation of soluble Ti, which generates either a metallothermic reduction of Al2O3 and/or a reaction between the added Ti and the dissolved oxygen (residual oxygen or impurities in the Ti source). The inclusion chemistry reverted with prolonged time toward the thermodynamically stable Al2O3, but the morphology change remained. A question arises accordingly: whether the temporary and permanent changes caused by Ti can be alleviated and/or eliminated by lowering any local Ti supersaturation. This result could be achieved by adding the Ti in multiple smaller batches such that the final desired melt chemistry is maintained. The aim of this work was to investigate whether dividing the Ti addition into several batches would have an influence on lowering the METALLURGICAL AND MATERIALS TRANSACTIONS B
temporary and/or permanent changes that the inclusions undergo. The approach was first to carry out experiments where the Ti/Al ratio was lowered to 1/2 and 1/4 to establish how small each Ti batch needed to be to have an effect. This process was followed by ex
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