Serration Phenomena Occurring During Tensile Tests of Three High-Manganese Twinning-Induced Plasticity (TWIP) Steels
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TRODUCTION
IN automotive industries, many efforts for decreasing CO2 emission and increasing fuel efficiency have been conducted on highly deformable steel sheets such as transformation-induced plasticity (TRIP) steels and twinning-induced plasticity (TWIP) steels. Particularly in TWIP steels, mechanical twins are formed during deformation, and prevent the dislocation movement as they play a role in refining grains, which is known to be Hall–Petch effect.[1–6] Since the necking is also suppressed because of high work hardening rate, TWIP steels show high strength and ductility simultaneously.[6–9] In general, a considerable amount (about 20 wt pct) of Mn is contained in TWIP steels composed of austenite.[6] Their active commercialization has been postponed because they are easily subjected to the cracking during forming or to the delayed fracture after forming.[6,10–14] According to the stabilization of austenite at room temperature and the sufficient formation SEOKMIN HONG, Postdoctoral Research Associate, JUNGHOON LEE, and DONG-HYUN AHN, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. SANG YONG SHIN, Assistant Professor, is with the School of Materials Science and Engineering, University of Ulsan, Ulsan, 680719, Korea. HYOUNG SEOP KIM and SUNGHAK LEE, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with the Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea. Contact e-mail: [email protected] SUNG-KYU KIM, Senior Principal Researcher, and KWANGGEUN CHIN, Senior Vice President, are with the HIMASS Research Project Team, Technical Research Laboratories, POSCO, Gwangyang, 545-711, Korea. Manuscript submitted October 19, 2012. Article published online September 27, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
of twins, however, the reduction in Mn content is somewhat difficult. Recently, TWIP steels containing Al have been developed to improve the formability and to prevent the delayed fracture. Here, Al plays an important role in decreasing twin formation because it works for increasing stacking fault energy.[15–20] These Al-added TWIP steels have excellent formability due to the increase in slip and the decrease in twin formation. Chin et al.[10] reported that cracks were formed during cup forming of a high-Mn TWIP steel, whereas they were not found in an Al-added TWIP steel, and explained the cracking behavior by the stress concentration on a cup side of the high-Mn steel. Berrahmoune et al.[21] found the delayed fracture after deep drawing of 301LN austenite steels, and explained it by the localization of residual stresses. However, detailed mechanisms of cracking or delayed fracture are not sufficiently understood in relation to microstructures. In addition, TWIP steels show a peculiar characteristic of serration phenomenon occurring during tensile deformation.[6,15,22–25] This serration is generally ex
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