A Reaction Between High Mn-High Al Steel and CaO-SiO 2 -Type Molten Mold Flux: Part I. Composition Evolution in Molten M
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NT advances in the development of highalloyed steel such as twin-induced plasticity (TWIP) steel have been led by noticeable product performance such as its extraordinary ductility and high tensile strength. Thanks to its high strain-hardening rate associated with dynamic strain aging and mechanical twins, the TWIP steel has attracted much attention as a next-generation automotive steel.[1] In the TWIP steel, the addition of Al is a usual practice because it is indispensable to suppress delayed fractures in the pressformed parts of the TWIP steel.[2,3] Moreover, the MIN-SU KIM, Graduate Student, HAE-GEON LEE, Professor Emeritus, and YOUN-BAE KANG, Assistant Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] SU-WAN LEE, formerly Graduate Student with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, is now Engineer with the Steelmaking Department, Pohang Works, POSCO Ltd, Pohang, Republic of Korea. JUNG-WOOK CHO, formerly Principal Researcher with the Technical Research Laboratories, POSCO Ltd, Gwangyang, Republic of Korea, is now Research Associate Professor with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology. MIN-SEOK PARK, Senior Researcher, is with the Technical Research Laboratories, POSCO Ltd. Manuscript submitted August 21, 2012. Article published online December 11, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
addition of Al also increases the stacking fault energy which is linked to the deformation mechanism[4] such that increasing the stacking fault energy changes the deformation mode from -martensite formation to mechanical twinning during the deformation of austenite. High Al content in the steel also additionally reduces weight of the steel, which is favorable in reducing automobile weight. However, contrary to those beneficial aspects on the product side, such high Al content in the steel induces severe process difficulties such as poor casting performances. In particular, among a number of issues responsible for the poor casting performances, strong chemical reactions taking place at the interface between molten steel and molten mold flux during the continuous casting process cause several operating problems. The problems stem from Al2O3 accumulation (coupled with SiO2 reduction) in the mold flux. This significantly affects the physico-chemical properties of the flux (viscosity, melting behavior, crystallization, heat transfer, etc.). A continuous casting process for a similar kind of high Al steel such as transformation-induced plasticity (TRIP) steel suffers problems such as breakout prediction (BOP) alarms, poor surface quality (transverse and longitudinal depressions), sticking on hot slabs, and deterioration of conditions in the mold.[5,6] A casting of a nonmagnetic high Mn-high Al steel (21 to 25 Mn-1.5 to 2.5 Al) also suffers similar problems.[7,8] In order to VOLUME 44B, APRIL 2013—299
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