Influence of the Initial Microstructure on the Heat Treatment Response and Tensile Properties of TRIP-Assisted Steel
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oncern on the CO2 emission and gradually tightened safety regulations make the high-strength steels a more attractive option as a car-body material. However, the strength is only one of many engineering parameters considered for the automotive applications, and indeed, other properties, for instance formability, should be acceptable as well. It is known that the strength and formability are mutually exclusive properties, which has been an obstacle for the application of high-strength steels. Nevertheless, a persistent effort has been given to make the high-strength steels a viable material, leading to the development of specific classes of high-strength steels such as transformation-induced plasticity (TRIP) steels and dual-phase (DP) steels.
KYOOYOUNG LEE, Senior Principal Researcher, and JOO HYUN RYU, SEA WOONG LEE, and WON HWI LEE, Senior Researchers, are with the Technical Research Laboratories, POSCO, Kwangyang, 545-090 Korea. JEONG IN KIM, Graduate Student, and DONG-WOO SUH, Associate Professor, are with the Graduate Institute of Ferrous Technology, POSTECH, Pohang, 790-784 Korea. Contact e-mail: [email protected] Manuscript submitted April 21, 2016. Article published online August 23, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
In the TRIP steels, an exceptional combination of strength and uniform elongation is achieved by using the transformation of retained austenite into martensite during deformation, which generates a remarkable work hardening contributing to the homogeneous deformation by delaying the occurrence of local necking.[1–5] Developments of advanced TRIP steels containing a larger amount of alloying elements are also attracting interest.[6–8] However, it has been known that TRIP steels are likely to suffer from a poor stretch-flangeability that is a measure of specific formability involving a shear, bending, and stretch-drawing operation.[9,10] It is reported that a fine lath microstructure is more beneficial for improving the stretch-flangeability compared with the polygonal one.[10–12] A mixture of ferrite and austenite with lath-type morphology can be produced by using martensite as an initial microstructure for the intercritical annealing or by annealing at the temperature above Ac3 followed by austempering. The evolution of a lath-type microstructure has been considered to be attributed to the nucleation of austenite at carbide particles in the lath boundaries of martensite[13] or the growth of interlath austenite retained in the initial martensite[14] even though it is not clear which one has the more dominant effect. Recently, Kim et al.[15] revealed that the retention of interlath austenite in the initial microstructure is critical to obtain the lath-type microstructure after intercritical annealing. However, the work was not linked to the influence of the initial microstructure on the evolution of final microstructure and tensile properties of TRIP steels subjected to the intercritical annealing and austempering, which might be the most important issue on the role of the initial microst
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