Kinetics of Solute Partitioning During Intercritical Annealing of a Medium-Mn Steel
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generation, medium-Mn, and advanced high-strength steels are under intense investigation as a substitute to 1st (low alloy) and 2nd generation (high-Mn) steels. These steels aim at improved combinations of strength and ductility.[1–4] In medium-Mn steels, the Mn content is reduced, relative to the high-Mn steels, in the range between 3 and 12 pct and the microstructure consists of an ultrafine ferrite-austenite mixture. The transformation-induced plasticity (TRIP) of the retained austenite is responsible for the enhanced formability in these steels. Several processing routes have been developed in order to stabilize the austenite phase for optimum TRIP interactions. For steels containing between 3 and 8 weight pct Mn, the quench and partitioning (Q&P) process has been H. KAMOUTSI, Researcher, E. GIOTI, Student, and GREGORY N. HAIDEMENOPOULOS, Professor, are with the Department of Mechanical Engineering, University of Thessaly, Pedion Areos, 38500 Volos, Greece. Contact e-mail: [email protected] Z. CAI and H. DING, Professors, are with the School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China. Manuscript submitted March 4, 2015. Article published online September 1, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
proposed.[5,6] In this process, austenite is stabilized by carbon partitioning from martensite to austenite, the partitioning taking place between the Ms and Mf temperatures. For steels containing 5 to 12 pct Mn, intercritical annealing, following the cold rolling of the martensitic microstructure, is investigated as a means of stabilizing the austenite by carbon and Mn partitioning.[7–12] The retained austenite fraction and stability depend, therefore, on the intercritical annealing temperature and time. Solute partitioning during intercritical annealing in medium-Mn steels has been investigated recently. Most of the research work is concerned with experimental determination of austenite volume fraction and composition.[13–16] Mn partitioning during intercritical annealing has been studied by atom probe tomography[13,17] and experimental results have been compared with simulation predictions. Mn partitioning has also been studied by TEM[14,18–20] indicating that partitioning of Mn from ferrite to the austenite is slow. Thorough thermodynamic analyses of solute partitioning during intercritical annealing have been performed[21,22] indicating the role of alloying elements. However, kinetic analysis of the partitioning process is limited to relatively few alloy systems, 3 pct Mn,[23] 5 pct Mn[24,25], and 12.2 pct Mn.[17] These works considered a specific intercritical annealing temperature and examined the partition of alloying elements at specific stages of annealing. Furthermore, the evolution of composition spikes of the substitutional solutes with annealing temperature and time is not discussed. It is the aim of the present paper to investigate the evolution of the austenite volume fraction, solute partitioning, and composition spikes during intercritical annealing of as-quenched
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