Effects of Annealing Treatment Prior to Cold Rolling on Delayed Fracture Properties in Ferrite-Austenite Duplex Lightwei
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CENTLY, a large amount of Mn and Al has been added to automotive steels to achieve the lightweight effect as well as excellent strength and ductility because the reduction in specific gravity due to substitutional atoms beneficially works for reducing the weight.[1–7] This addition leads to about 10 pct of weight reduction in comparison with transformation-induced plasticity (TRIP) steels or twinning-induced plasticity (TWIP) steels.[8] Mn, an austenite stabilizer, raises the amount of austenite at high temperatures. Al, a ferrite stabilizer, helps to form a duplex structure of ferrite and austenite at high temperatures.[9,10] It also promotes the precipitation of j-carbides (composition; (Fe,Mn)3-Al-C, perovskite structure) during cooling, and the amount of j-carbides varies with contents of Mn and C as well as Al.[11–13] SEOK SU SOHN, Postdoctoral Research Associate, and HYEJIN SONG and JUNG GI KIM, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. JAI-HYUN KWAK, Senior Principal Researcher, is with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Kwangyang 545-090, 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 Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted November 4, 2014. Article published online November 18, 2015 706—VOLUME 47A, FEBRUARY 2016
Tensile properties of recently developed advanced high-strength steels (AHSSs) for automotive applications are superior to other conventional steels, but the active commercialization of these steels has been postponed because they are easily subjected to the cracking during formation or to the delayed fracture after formation.[14–19] The delayed fracture, a kind of hydrogen embrittlement, is one of the major issues in developments of AHSSs because resistance to hydrogen embrittlement decreases with increasing strengths.[20,21] Recently, high-Mn 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.[22,23] Chin et al.[15] reported that the cracking or delayed fracture did not occur during or after the cup formation of an Al-added TWIP steel, whereas it occurred in a high-Mn TWIP steel. These results were explained by the stress concentration at the cup side in the high-Mn steel and by the occurrence of homogeneous twinning in the Al-added steel. Berrahmoune et al.[24] found the delayed fracture after deep drawing of 301LN austenite steels, and explained it by the localization of residual stresses. However, exact mechanisms behind the delayed fracture are not sufficiently understood in relation to microstructures. This is because deformation mechanisms including TRIP and TWIP,[2
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