Thermo-mechanical Processing of TRIP-Aided Steels
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THE increasing demand for better safety standards, fuel efficiency, and competition from light weight metals has led to the development of low-alloyed TRIP-aided steels as one of the most exciting material for automotive body structures. TRIP-aided steels generally consist of ferrite (a), bainitic ferrite (ab), and retained austenite (cret).[1] Ferrite, being a soft phase, begins to deform as soon as the strain is applied, along with the transformation of a part of the relatively less mechanically stableretained austenite, which in turn helps maintain the initial work hardening of the material. Once ferrite strain hardens, metastable-retained austenite starts to progressively transform to martensite (a¢), which maintains the strain hardening to higher strains. This effect, identified first by Zackay et al.,[2] is known as the TRIP (transformation-induced plasticity) effect. The TRIP effect, when allowed to trigger at the appropriate strain level by optimal stabilization of retained austenite, leads to a very good balance between strength and ductility.[3] The presence of retained austenite also leads to an improvement in the fatigue properties and crashworthiness due to the crack tip blunting effect.[4–6] Hence, the amount of retained austenite and its thermal and mechanical stability need to be optimized to obtain the best results. Successful retention of austenite of the desired composition requires a careful design of the alloy composition and processing parameters. The carbon enrichment RAVI RANJAN, Ph.D. Scholar, and SHIV BRAT SINGH, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. Contact e-mail: [email protected] HOSSEIN BELADI, Senior Research Fellow, and PETER D. HODGSON, Professor, are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Manuscript submitted October 7, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
of austenite is of critical importance in this regard, and this necessitates addition of the carbide inhibitor elements such as Si and/or Al to the steel. However, the presence of Si in high amounts results in the formation of a very strong oxide layer, which easily gets rolled into the surface during hot rolling resulting in poor surface finish, coatability, and other surface-related properties.[7,8] Hence, it has been proposed to use Al to partially or completely replace Si to address this issue.[9] However, due to the stronger solid solution strengthening effect of Si and its greater ability to inhibit carbide precipitation compared with Al, complete replacement of Si with Al has not been recommended.[9] In the present work, Si has been partly replaced with Al. Phosphorous (P) when used in small amounts (
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