Influence of the Initial Microstructure on the Reverse Transformation Kinetics and Microstructural Evolution in Transfor
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RETAINED austenite plays a critical role in obtaining the characteristic properties in transformation-induced plasticity (TRIP)–assisted steels. By gradual transformation into martensite during straining, the retained austenite contributes to the homogeneous deformation as well as significant work hardening, which effectively delays the onset of plastic instability.[1–5] The deformation-induced martensite transformation renders an impressive combination of strength and elongation, making the application of TRIP-assisted steels to automotive parts successful with respect to weight reduction as well crashworthiness.[6–13] However, relatively poor stretch flangeability has been an obstacle for more active application of TRIP-assisted steel.[14–16] The manufacturing processes of automobile components such as wheels, suspensions, and structural components involve primarily shearing, bending, and stretch drawing operations. In that circumstance, an ability to resist a failure from localized deformation, so-called stretch flangeability, is important. A hole expansion ratio (HER) is a representative index to
JEONG IN KIM, Graduate Student, YOON-UK HEO, Research Assistant Professor, and DONG-WOO SUH, Associate Professor, are with the Graduate Institute of Ferrous Technology, POSTECH, Pohang, 790-784 Korea. Contact email: [email protected] JOO HYUN RYU and SEA WOONG LEE, Senior Researchers, and KYOOYOUNG LEE, Senior Principal Researcher, are with the Technical Research Laboratories, POSCO, Kwangyang, 545-090 Korea. Manuscript submitted February 23, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
evaluate that kind of formability of sheet steels. Generally, the materials having higher HER value show better stretch flangeability. The stretch flangeability is strongly affected by microstructure. For instance, dual-phase steels have inferior stretch flangeability compared to bainitic steels, while they exhibit better elongation.[2,6,17] This finding is due to the fact that the interface between hard martensite and soft ferrite often provides a crack initiation site. Fine, homogenous microstructure is generally thought to be beneficial to obtain enhanced stretch flangeability. This microstructure led to the introduction of alternative TRIP-assisted steels in which coarse polygonal ferrite and retained austenite were replaced by fine lath-type morphology.[18,19] Even though it was reported that the fine lath-type morphology could be generated by starting from the martensite as the initial microstructure for heat treatment, the mechanism that governs the evolution of the lath-type reverse-transformed microstructure upon heating to an intercritical temperature is not fully understood. Yi et al.[20] reported that the austenite nucleated at carbide particles in the lath boundaries of martensite during heating to the intercritical temperature, which grew along the lath boundaries to form the lath-type morphology. On the other hand, Wei et al.[21] reported that lath-type reverse-transformed microstructure originated from the interlat
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