Modeling of the Recrystallization and Austenite Formation Overlapping in Cold-Rolled Dual-Phase Steels During Intercriti
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TRODUCTION
DUAL-PHASE Steels (DP steels) are one of the most used Advanced High Strength Steels. This is due to the fact that their microstructures combining a ferritic matrix and martensite islands leads to a very good compromise between strength and ductility. In the case of cold-rolled steels, this type of microstructure is obtained by continuous annealing: an intercritical annealing (performed between Ac1 and Ac3) is followed by a rapid cooling, so that austenite transforms into martensite. The parameters (heating rate (RH), annealing time and temperature) of the heating and holding stages play a major role on the final properties of DP steels, as they control the austenite fraction, the M. OLLAT is with Univ. Lyon INSA Lyon - MATEIS UMR CNRS 5510, 69621 Villeurbanne, France and also with the Fives Keods, 108-112 Avenue de la liberte´, 94700 Maisons-Alfort, France. Contact e-mail: [email protected] V. MASSARDIER, D. FABREGUE, and M. PEREZ are with the Univ. Lyon INSA Lyon - MATEIS UMR CNRS 5510. E. BUSCARLET and F. KEOVILAY are with the Fives Keods, 108-112 Avenue de la liberte´, 94700 Maisons-Alfort, France. Manuscript submitted March 15, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite grain size, and the carbon content of austenite at the end of the annealing treatment, before the quench. During the first part of the treatment of DP steels, from a cold-rolled ferrite–pearlite microstructure, several microstructural evolutions are expected to occur more or less simultaneously depending on the applied parameters for the heating and soaking stages. This makes the understanding and modeling of microstructural evolutions during these stages rather difficult. First, the cold-rolled ferrite matrix starts to recrystallize during the heating stage. A full recrystallization before reaching the austenite start temperature is assumed to be obtained only at very slow heating rates (
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