Effect of Carbon Distribution During the Microstructure Evolution of Dual-Phase Steels Studied Using Cellular Automata,
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NTINUOUS drive from the automotive sector for improving the fuel efficiency by reducing the vehicle weight, without compromising passenger safety, has led CHANDAN HALDER, ANISH KARMAKAR, and SK. MD. HASAN, Ph.D. Students, DEBALAY CHAKRABARTI, Associate Professor, and NIRUPAM CHAKRABORTI, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India. Contact e-mail: [email protected] MACIEJ PIETRZYK, Professor, is with the Department of Applied Computer Science and Modelling, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krakow, Poland. Manuscript submitted January 4, 2016. Article published online September 16, 2016 5890—VOLUME 47A, DECEMBER 2016
to the development of different grades of high-strength steel. Dual-phase (DP) steels consist of fine ferrite grains and hard martensite regions (less than 30 pct volume) with a low-carbon equivalent. DP steels are suitable for automotive applications due to their high strength with satisfactory ductility, strain-hardening ability, formability, and continuous yielding behavior.[1–10] To develop ferrite-martensite dual-phase structures in a hot-rolling operation, the steel is rolled in the austenite region, isothermally held at an intercritical temperature (in austenite-ferrite two-phase region), and water quenched. This treatment is known as step-quenching treatment (SQ).[11,12] On the other hand, in a cold-rolling mill, the cold-rolled sheet is annealed at an intercritical temperature and rapidly cooled to room temperature for developing a
METALLURGICAL AND MATERIALS TRANSACTIONS A
dual-phase structure. This treatment is known as intercritical annealing treatment.[9,13] Irrespective of the processing route, the fraction, size, and distribution of martensite in the final product, which determine the mechanical properties, depend on the composition and microstructural parameters of austenite, which are present at the intercritical temperature.[14] Several studies have been carried out to understand the austenite formation during intercritical annealing of cold-rolled steels having a ferrite-pearlite starting structure.[13–22] The effect of different processing parameters such as the amount of cold-rolling, heating rate, annealing temperature, annealing time, and cooling rate on the microstructural evolution in dual-phase steels has been investigated.[9,22,23] Among the different processing parameters associated with the formation of a dual-phase structure, the effect of heating rate during annealing on the microstructure and mechanical properties has attracted considerable attention in recent research.[13,23,24] This is due to the fact that the increase in heating rate has been found to significantly refine the ferrite grain size. The size, morphology, and distribution of martensite islands in the annealed structure also depends on the heating rate.[25] Now, the heating rate and annealing temperature determine the kinetics of isothermal and non-isothermal austenite formation du
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