Influence of Heating Rate on Ferrite Recrystallization and Austenite Formation in Cold-Rolled Microalloyed Dual-Phase St
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INTRODUCTION
THE development of advanced high-strength steels (AHSS) is of prime importance with regard to current challenges facing the automotive industry.[1] Consequently, the demand for dual-phase (DP) steels has gradually increased during past decades. Such steels have become one of the most important contributions with regard to the weight reduction of the global motor vehicle fleet.[2] The metallurgical concept of a mixture of ferrite and martensite (and sometimes bainite) gives access to a wide range of high-strength steels with appropriate formability[3,4] and adequate in-use properties (IUP). Low-carbon microalloyed DP steels appear to be suitable candidates for such industrial
C. PHILIPPOT, J. DRILLET, and V. HEBERT are with the ArcelorMittal Research SA, Voie Romaine, BP 30320, 57283 Maizie´res les Metz, France. M. BELLAVOINE is with the ArcelorMittal Research SA, and also with the Aix Marseille Univ, Univ Toulon, CNRS, IM2NP, 13 397 Marseille, France. M. DUMONT, K. HOUMMADA, and P. MAUGIS are with the Aix Marseille Univ, Univ Toulon, CNRS, IM2NP. Contact e-mail: [email protected] Manuscript submitted January 19, 2017. Article published online November 20, 2017 66—VOLUME 49A, JANUARY 2018
applications.[5,6] However, establishing a link between the properties of such steels and their microstructures is problematic because the microstructural evolution of these steels has not been elucidated. The processing routes used to manufacture cold-rolled products involve hot rolling, followed by cold rolling and intercritical annealing; this leads to a mixture of austenite and ferrite.[7] Subsequent cooling results in the formation of a ferrite and martensite mixture, which is typical of DP steels. In particular, the chemical composition of the steel, including alloying elements such as Mn, Mo, or Cr, is optimized to avoid the transformation of austenite into ferrite during cooling.[1] The final martensite microstructure (volume fraction, composition, and spatial distribution) is inherited from the intercritical austenite phase.[8] Therefore, the austenite evolution is of prime importance with regard to understanding the final microstructure and properties of the material. The austenite development depends on the nominal chemical composition, the initial hot-rolled microstructure, cold rolling reduction, heating rate, soaking temperature, and time. Regarding the effect of the initial microstructure, previous studies have primarily focused on
METALLURGICAL AND MATERIALS TRANSACTIONS A
ferrite-pearlite microstructures.[9–11] Austenite formation is primarily associated with cementite; coarse-grained ferrite-pearlite microstructures (including cementite clusters localized in pearlite areas) result in the formation of coarse and heterogeneous austenite microstructures, which are detrimental to the mechanical properties of the material due to microstructural banding.[12,13] Such observations led to research into alternative initial microstructures. Alternatives could be based on fully martensitic microstructures
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