Influence of Al on Microstructure and Mechanical Behavior of Cr-Containing Transformation-Induced Plasticity Steel
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en using scrap during steelmaking, impurities identified as tramp elements usually remain in the steel products.[1] These impurities can have unexpected effects on the performance of advanced high strength steels, the microstructures of which have to be precisely controlled to achieve the desired properties.[2–5] There have been reports that Cr in transformation-induced plasticity (TRIP) steels suppresses the formation of ferrite during heat treatment after intercritical annealing, which degrades the thermal stability of austenite.[6,7] We recently reported that ferrite formation during fast cooling after intercritical annealing was stimulated by the addition of Al in TRIP steel.[8] The original purpose of the Al addition was to improve the galvanizability,[9,10] but an additional beneficial effect was to accelerate the ferrite transformation.[11,12] Thermodynamically, both Cr and Al enhance the stability of ferrite because they expand the ferrite region in phase diagram.[13] This is not in conflict with the effect of Cr in TRIP steels, because the suppression of ferrite formation possibly originates from the increase of DONG-WOO SUH, Assistant Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: dongwoo1@ postech.ac.kr SEONG-JUN PARK, Senior Researcher, and SUNG-JOON KIM, Principal Researcher, are with the Department of Advanced Metallic Materials, Korea Institute of Materials Science, Kyungnam 641-010, Korea. HEUNG NAM HAN, Associate Professor, is with the School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea. Manuscript submitted May 7, 2010. Article published online October 28, 2010 3276—VOLUME 41A, DECEMBER 2010
hardenability, which is associated with kinetic phenomena.[6] Meanwhile, in Cr-containing TRIP steel, the acceleration of ferrite formation by Al may benefit the stability of austenite by promoting carbon enrichment in austenite. This study aims to investigate the influence of Al on the microstructure and mechanical behavior of low carbon TRIP steel containing Cr. The alloys investigated are designated CMnSiCr (C0.15, Mn1.52, Si1.49, Cr0.39 in wt pct) and CMnSiAlCr (C0.15, Mn1.49, Si0.53, Al1.0, Cr0.39) steels. CMnSi steel (C0.15, Mn1.49, Si1.51) having typical composition of TRIP steel is also prepared for comparative purpose. The CMnSiCr and CMnSi steels also contain Cu (0.51 wt pct). The vacuum-melted ingots were hot rolled to 4-mm thickness with a finishing temperature above 1223 K (950 C). The hot-rolled sheets were pickled in 10 pct HCl solution and then cold rolled to 1-mm thickness. The heat treatment was performed using an infrared heating furnace. The cold-rolled CMnSiCr and CMnSi sheets were annealed at 1053 K (780 C) for 300 seconds, and the CMnSiAlCr steel was annealed at 1103 K (830 C) to obtain similar austenite fractions at the end of intercritical annealing. The specimens were then cooled to 723 K (450 C) at 20 K/s and austempered for 10 to 300 seconds follow
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