Transient Behavior of Inclusion Chemistry, Shape, and Structure in Fe-Al-Ti-O Melts: Effect of Titanium Source and Labor
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INTRODUCTION
NONMETALLIC inclusions in steels form as a result of the interaction between reactive elements in the steel melt and its environment (atmosphere, refractory, or slag). In general, the population of inclusions needs to be minimized, because inclusions can cause problems with process control. They can also have a deleterious effect on the toughness of the final products and on their ductility and weldability.[1,2] This project aims to study the evolution of oxide inclusions in the ladle during the processing of interstitial-free (IF) steels. Generally, 400 to 700 ppm of dissolved oxygen will remain in steel after the converter process,[3–5] and steel melts with such a high dissolved oxygen content need to be deoxidized to prevent blister formation during continuous casting. Deoxidation and adjustment of the final steel composition is carried out during ladle treatment. CONG WANG, Graduate Student, and NOEL THOMAS NUHFER, Director of Electron Microscopy and Materials Characterization, are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15253. SEETHARAMAN SRIDHAR, POSCO Professor of Steelmaking, Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, is with the National Energy Technology Laboratory, Pittsburgh, PA 15236. Contact e-mail: sridhars@andrew. cmu.edu Manuscript submitted April 24, 2009. Article published online July 21, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
Aluminum is a common deoxidizer because it is inexpensive compared to other reactive elements and its high oxygen affinity results in a fairly low content of soluble oxygen. During ladle processing of IF steels, titanium in the form of Fe-Ti alloys is added after deoxidation to bind the soluble nitrogen or soluble carbon remaining after the secondary refining process into TiC or TiN compounds during solidification. Binding of the interstitials is necessary to ensure the required mechanical properties of IF steel during hot rolling, because interstitial atoms prohibit dislocation rearrangement and reduce the formability of IF steel. Because IF steels are used for structural parts such as doors in automotive applications, it is vital that formability is ensured. While titanium is not added for the purpose of deoxidation, it can form relatively stable oxides if oxygen is available. Therefore, titanium oxide inclusions can precipitate during solidification because of solute rejection and the enrichment of titanium and oxygen in the interdendritic liquid.[6] Furthermore, titanium oxides have good ability to assist ferrite nucleation within austenite grains and dramatically promote the formation of acicular ferrite microstructure, which is interlocking in nature and deflects the propagation of cleavage cracks, causing an increase in toughness.[7,8] Titanium-containing inclusions are, however, generally not wanted, because titanium bound as oxides decreases the amount of titanium available
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