Phase-Field Modeling for Intercritical Annealing of a Dual-Phase Steel
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TRODUCTION
DUAL-PHASE steels are advanced high-strength steels that are now widely used in the automotive industry. Compared with conventional high-strength steels, e.g., high-strength low-alloy (HSLA) steels, dualphase steels have a better combination of strength and formability as well as higher crashworthiness. There are two principal processing routes to produce DP steel sheets: (i) controlled cooling from the austenite phase (hot-rolled products); (ii) intercritical annealing of coldrolled steels in continuous annealing or hot-dip galvanizing lines. In particular, the intercritical annealing route is of primary interest to produce sheets with the required surface quality and thickness of 1 mm and below. A number of microstructure process models were developed for controlled run-out table cooling of hotrolled DP steels.[1–3] There is, however, a remarkable lack of such models for intercritical annealing. Bos et al. developed a 3D cellular automaton model describing the microstructure evolution during intercritical annealing of a cold-rolled Fe-0.1 wt pctC-1.5 wt pctMn steel with an initial pearlite/ferrite microstructure.[4] The model consists of sub-models for ferrite recrystallization, austenite formation, and the austenite-to-ferrite transformation. These microstructure phenomena are assumed to occur in sequence rather than concurrently, i.e., the potential interaction of ferrite recrystallization and austenite formation is not taken into account. Moreover, it is assumed that the ferrite-to-austenite transformation is interface controlled even though the BENQIANG ZHU, Ph.D. Student, and MATTHIAS MILITZER, Professor, are with The Center for Metallurgical Process Engineering, The University of British Columbia, 309-6350 Stores Road, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: zhubenqiang@ gmail.com Manuscript submitted August 14, 2014. Article published online December 10, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation is typically of mixed-mode character in a low-carbon steel, i.e., both interface reaction and longrange diffusion would have to be considered. Nevertheless, the model provides a framework for meso-scale simulation of microstructure evolution during intercritical annealing. In contrast to Bos et al., Rudnizki et al. simulated austenite formation as a transformation controlled by long-range diffusion of carbon.[5] Using the commercial phase-field package MICRESS, they described austenite formation in a Fe-0.1 wt pctC-1.65 wt pctMn steel from an annealed pearlite/ferrite microstructure. Austenite nucleation was assumed to take place within pearlite, and nucleation at ferrite grain boundaries was not considered. The modeling of the pearlite-to-austenite transformation and the ferrite-to-austenite transformation was performed separately in two steps, i.e., quick growth of austenite into pearlite which is assumed to be an effective phase with eutectoid carbon composition followed by growth into ferrite. A sufficiently high interface mobility was adopted such that the ferrite-toaustenite
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