Kinetics of Ferrite Recrystallization and Austenite Formation During Intercritical Annealing of the Cold-Rolled Ferrite/
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NEW materials are constantly being developed to provide desirable properties compared with previous materials. This evolution is usually done by changing the microstructure of the materials. Past decades have been associated with the introduction of advanced high-strength steels (AHSS) in sheet-metal-forming industries. In this category, dual-phase (DP) steels are the result of global efforts of a broad group of researchers. Their investigations have focused on developing steels with high strength and formability.[1–5] The microstructure of DP steels mainly consists of a soft ferrite phase with hard martensite islands. Although ferrite shows good ductility properties, martensite as a hard phase increases the strength of the material. DP steels can be considered one of the most important steels among AHSS, which have been developed for different industries, especially the automotive industry.[6] To generate the required microstructures, a processing route can be used, which involves cold rolling followed by intercritical annealing. The processing routes for cold-rolled and annealed DP steels involve reheating the steel into the intercritical region, where an
Y. MAZAHERI, formerly Ph.D. Student with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran, is now Assistant Professor with the Department of Materials Engineering, Bu-Ali Sina University, Hamedan 6517838695, Iran. Contact e-mail: [email protected] A. KERMANPUR, Associate Professor, A. NAJAFIZADEH, Professor, and A. GHATEI KALASHAMI, Researcher, are with the Department of Materials Engineering, Isfahan University of Technology. Manuscript submitted February 4, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite/ferrite mixture is formed. Cooling to room temperature produces a microstructure consisting of ferrite and martensite. The mechanical properties of the DP steels are primarily dependent on the volume fraction and distribution of the final transformation products,[7,8] which inherit the composition and distribution of the parent austenite phase. Therefore, the kinetics of austenite formation during intercritical annealing is most critical for the evolution of the final microstructure of the steel and, consequently, its mechanical properties. Arnold and McWilliams[9] reported that austenite forms as a nucleation and growth process upon heating. Roberts and Mehl[10] confirmed the nucleation and growth nature of the process by discussing in detail the rate of austenite formation from ferrite–cementite aggregates. By considering that austenite formation in DP steels is a diffusion-controlled growth,[11] some researchers[12–14] proposed that the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model can be used for austenite transformation during intercritical annealing, i.e.: Vc ¼ 1 expðktn Þ
½1
where Vc is the austenite volume fraction, t is annealing time, n is the JMAK exponent, and k is the temperature-dependent rate constant. According to this model, by going time to infinity, Vc becomes 1.
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