Thermo-mechanical Processing of TRIP-Aided Steels RAVI RANJAN, HOSSEIN BELADI, SHIV BRAT SINGH, and PETER D. HODGSON The effects of the partial replacement of Si with Al and the addition of P on the microstructure and mechanical properties of experimental TRIP-aided steels subjected to different thermo- mechanical cycles were studied. Based on the available literature and thermodynamics-based calculations, three steels with different compositions were designed to obtain optimum results from a relatively low number of experiments. Different combinations of microstructure were developed through three different kinds of thermo-mechanical-controlled processing (TMCP) routes, and the corresponding tensile properties were evaluated. The results indicated that partial replacement of Si with Al improved the strength-ductility balance along with providing an improved variation in the incremental change in the strain-hardening exponent. However, the impact of the P addition was found to depend more on the final microstructure obtained by the different TMCP cycles. It has also been shown that an increase in the volume fraction of the retained austenite (V c ret ) or its carbon content (C c ret ) resulted in an improved strength-ductility balance, which can be attributed to better exploitation of the TRIP effect. DOI: 10.1007/s11661-015-2885-5 Ó The Minerals, Metals & Materials Society and ASM International 2015 I. INTRODUCTION 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 (a b ), and retained austenite (c ret ). [1] Ferrite, being a soft phase, begins to deform as soon as the strain is applied, along with the transforma- tion of a part of the relatively less mechanically stable- retained austenite, which in turn helps maintain the initial work hardening of the material. Once ferrite strain hardens, metastable-retained austenite starts to progres- sively 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 (transfor- mation-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 improve- ment 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 com- position requires a careful design of the alloy composi- tion and processing parameters. The carbon enrichment of austenite is of critical importance in this regard, and this necessitates addition of the carbide inhibitor ele- ments 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 ( < 0.1 wt pct) has been reported to increase the volume fraction of retained austenite without significantly dete- riorating other formability-related properties, such as strength, ductility, etc. [10] Hence, in the current work, about 0.07 wt pct P has been added to one of the steels to study its influence on the retained austenite stability and subsequent effect on the mechanical properties. Moreover, 0.04 wt pct Nb and 0.25 wt pct Mo were also added in all the steels studied in this work. Nb raises the non-recrystallization (T nr ) temperature of the steel, and therefore if a critical amount of deformation is applied, the phases formed during subsequent thermo- mechanical processing will be refined. [11–13] This in turn reduces the M s temperature of the retained austenite due to decrease in its size and thus increases its stability at the room temperature. [14,15] The effect of Nb is further enhanced when it is used with Mo. [16] Nb in the presence of Mo produces a synergetic effect on the transforma- tion behavior, so that the austenite to pearlite transfor- mation is suppressed. [16] The presence of Mo in turn delays the carbide precipitation [16,17] and therefore increases the stability of metastable-retained austenite at the room temperature. 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: ravimetalbitjsr@gmail.com 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