Combined Effect of Heating Rate and Microalloying Elements on Recrystallization During Annealing of Dual-Phase Steels
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DUAL-PHASE (DP) steels are the most developed and widely used advanced high-strength steels. Their final microstructure is made of a hard martensitic phase dispersed in a soft bainitic/ferritic matrix and offers a good compromise between high strength and high formability.[1–3] The processing route for cold-rolled DP products consists of hot-rolling followed by cold-rolling and continuous annealing. Continuous annealing of cold-rolled sheets consists of heating and soaking in the intercritical temperature range, leading to the formation of a microstructure composed of ferrite and austenite. Subsequent cooling leads to the final microstructure made of martensite, bainite, and ferrite. As the microstructure obtained at the end of the intercritical annealing has a major influence on the final mechanical properties of DP steels, understanding microstructural changes during this stage is of
MARION BELLAVOINE is with the IM2NP – UMR CNRS 7334 – Aix-Marseille Universite´, Univ. Toulon, Faculte´ des Sciences et Techniques de St-Je´roˆme – Service 251, 13 397, Marseille Cedex 20, France and also with ArcelorMittal Research SA, Maizieres-les-Metz, France. MYRIAM DUMONT and PHILIPPE MAUGIS are with the IM2NP – UMR CNRS 7334 – Aix-Marseille Universite´, Univ. Toulon, Faculte´ des Sciences et Techniques de St-Je´roˆme – Service 251. Contact e-mail: [email protected] JOSE´E DRILLET and VE´RONIQUE He`BERT are with ArcelorMittal Research SA. Manuscript submitted September 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
considerable interest. These microstructural changes are the results of three major mechanisms: precipitation of microalloying elements, ferrite recrystallization, and austenite formation. These mechanisms may occur simultaneously or successively and may interact with each other. Their kinetics and interactions essentially depend on three key factors: the nominal composition of the steel, the microstructure after cold-rolling, and the processing parameters of the annealing cycle (heating rate, soaking temperature, and soaking time). Monitoring the microstructure obtained at the end of annealing thus implies to know and to take benefit of the separate role of these factors on the different mechanisms. Recrystallization is one of the mechanisms that are essential to control. Recrystallization is the formation of new dislocation-free grains within the deformed structure by the migration of high-angle grain boundaries.[4] Recrystallization kinetics are important to control for two main reasons. The first reason is that recrystallization and subsequent grain growth determine the final ferrite grain size. The second reason is that recrystallization strongly affects austenite formation and distribution, and thus the morphology of the microstructure.[6,7] In particular, it was shown that the overlap of ferrite recrystallization and austenite formation results in a finer, more heterogeneous and anisotropic microstructure.[7] The cold-rolled microstructure influences recrystallization kinetics during annealing through th
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