Microstructural Evolution of a Low-Carbon Steel during Application of Quenching and Partitioning Heat Treatments after P
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RANSFORMATION-INDUCED-PLASTICITY (TRIP) steels are usually produced via a thermomechanical process of intercritical annealing followed by rapid cooling to a bainitic transformation regime, to obtain a microstructure of ferrite, bainite, and retained austenite. During the bainitic transformation, the formation of carbides is suppressed, due to the effect of alloying elements such as silicon and aluminum; the austenite is thus enriched with carbon and retained at room temperature. Carbon-enriched metastable retained austenite is considered beneficial, because the TRIP phenomenon during the deformation can significantly contribute to the formability and energy absorption of the material. Recently, Speer et al.[1,2] proposed a novel heattreatment concept, the so-called ‘‘quenching and partitioning’’ (Q&P) process, for the development of multiphase steels with considerable retention of austenite in the microstructure. The Q&P process consists of a first quench (quenching step) to a temperature below the martensite-start (Ms) temperature but above the martensite-finish (Mf) temperature, to form a mixture of martensite and austenite, and a subsequent isothermal treatment (partitioning step) at the same temperature M.J. SANTOFIMIA, Postdoctoral Researcher, and L. ZHAO, Research Fellow, Department of Materials Science and Engineering, Delft University of Technology, are with the Materials Innovation Institute (M2i), 2628 CD Delft, The Netherlands. J. SIETSMA, Associate Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands. Contact e-mail: m.santofi[email protected] Manuscript submitted March 26, 2008. Article published online November 14, 2008 46—VOLUME 40A, JANUARY 2009
(one-step treatment) or at a higher temperature (twostep treatment), in order to transfer the carbon from the supersaturated martensite into the austenite. In this heat treatment, alloying elements such as silicon and aluminum are also used, to avoid the carbide precipitation during the partitioning step, because carbide precipitation acts as a sink of carbon that is no longer available for the stabilization of the austenite. Combining this heat treatment with a previous partial austenitization, a microstructure consisting of ferrite, carbon-depleted martensite, and carbon-enriched retained austenite is obtained. This microstructure can lead to an interesting combination of mechanical properties,[3,4] from a good formability, as a result of the TRIP effect from the retained austenite, to a strength higher than that of conventional TRIP steels, due to the presence of martensite instead of bainite. The design of adequate Q&P heat treatments requires an understanding of all the phenomena that can affect the final microstructure of the material during this processing route. Most of the research on the Q&P process has been focused on treatments that start with full austenitization;[5,6] the use of partial austenitization has been studied less.[1] Therefore, there is insufficient experimental ev
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