Prediction of Continuous Cooling Transformation Diagrams for Dual-Phase Steels from the Intercritical Region
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INTRODUCTION
THE microstructure of dual-phase steels consists of high-strength hard phase (mostly martensite sometimes with small amounts of bainite or retained austenite) embedded in a relatively soft and ductile matrix of ferrite. Due to this composite microstructure, dualphase steels exhibit excellent mechanical properties such as continuous yielding behavior, low yield stress to ultimate tensile strength (UTS) ratio, superior strengthductility combination, better formability, high energy absorption, very high initial work hardening rate with good elongation values, and excellent surface finish compared with other high-strength low-alloy steels of similar grade. As a consequence, they are attracting more and more interest and attention from automotive industries to reduce vehicle weight, achieve higher crash resistance, and improve fuel efficiency.[1–4] Dual-phase steels are generally produced by continuous annealing after cold rolling. The continuous annealing includes heating, soaking, slow cooling, rapid cooling, and low-temperature tempering. Although the number of relevant factors is not so large, the determination of the optimal values of the related process parameters (such as, for instance, intercritical temperature and cooling rate) needs a large number of V. COLLA, Technical Research Manager, and A. DIMATTEO, Young Researcher, are with PERCRO-CEIICP, Scuola Superiore Sant’Anna, Polo Sant’Anna Valdera, 56125 Pontedera, PI, Italy. Contact e-mail: [email protected] M. DESANCTIS, Associate Professor, G. LOVICU, Research Assistant, and R. VALENTINI, Assistant Professor, are with the Department of Chemical Engineering, Industrial Chemistry and Materials Science, Pisa University, 56100 Pisa, Italy. Manuscript submitted April 27, 2010. Article published online April 23, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
preliminary experiments before being transferred to the industrial production. Furthermore, for commercial exploitation of these steels, it is necessary to thoroughly understand their phase transformation behavior with emphasis on their chemical composition and response to intercritical annealing and continuous cooling treatments. In fact, from the industrial point of view, it would be very useful to know more precisely the quantitative distribution of the different phases that are present in these steels as a function of the aforementioned process parameters.[5,6] The experimental elaboration of the continuous cooling transformation (CCT) diagram is time consuming and requires expensive testing equipment. A model able to calculate CCT diagrams directly from the chemical composition of the steel and its austenitizing temperature would surely allow considerable time and economical savings. Therefore, many attempts at modeling austenite transformations in the steel during cooling are being undertaken for completely austenitized steels. The basis of temperature and time prediction of transformations of supercooled austenite and volume fraction calculations of the microstructural components, a
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