Effect of Heating Rate on the Austenite Formation in Low-Carbon High-Strength Steels Annealed in the Intercritical Regio
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VANCED high-strength steel (AHSS) grades including dual-phase (DP) steels are in high and growing demand in the automotive industry. This demand was driven by the need to improve fuel efficiency, while simultaneously meeting high environmental and safety standards. Final mechanical properties and structure of DP steels are dependent on the volume fraction, morphology, and distribution of microstructure constituents, which in turn are determined by the composition and state of the parent austenite phase. In other words, the evolution of the austenite condition during annealing has a direct impact on the final microstructure and, thereby, on the final mechanical properties of the steel. One of the typical processing routes used to produce dual-phase steels is through continuous annealing at galvanizing lines (CGLs). The processing parameters such as heating rate, intercritical annealing temperature, holding time, and cooling rate determine the type, volume fraction, and morphology of phases present in the microstructure of coated DP steels (which often contain not only ferrite and martensite, but also some fraction of bainite), and thereby the final properties of the steel. Therefore, optimization of critical processing parameters is necessary for the production of DP steels R.R. MOHANTY, Research Engineer, O.A. GIRINA, Staff Research Engineer, and N.M. FONSTEIN, Scientific Advisor, are with ArcelorMittal Global R&D, East Chicago, IN 46312. Contact e-mail: [email protected] Manuscript submitted November 22, 2010. Article published online June 9, 2011 3680—VOLUME 42A, DECEMBER 2011
with desired properties, while maintaining the required process robustness and yield. Formation of austenite in the intercritical region was widely studied and reported in the literature[1–6] for both hot-rolled and cold-rolled steels. Speich and his collaborators[1] divided the process of intercritical austenitization into three stages: (1) a rapid growth of austenite into pearlite until complete pearlite dissolution, (2) a slow growth of austenite into ferrite controlled by carbon diffusion and Mn partitioning between ferrite and austenite, and (3) slow equilibration of austenite and ferrite. Another study by Garcia and DeArdo[7] was related to the effect of cementite morphology and carbon and manganese contents on the kinetics of austenite formation. Austenite was found to form preferentially on ferrite/ferrite grain boundaries for all initial structures. Several authors investigated the effect of prior cold rolling on the formation of austenite.[8–10] Tokizane et al.[8] showed that austenite formation in deformed low-carbon steel differs from that without deformation. Yang et al.[9] established the fundamental role of the degree of ferrite recrystallization in the formation of austenite by observing austenite nucleation on grain boundaries of nonrecrystallized ferrite and on interphase boundaries between carbides and ferrite after completion of recrystallization of ferrite. Furthermore, Beswick[11] reported that cold deformation coul
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