Influence of Heating Rate on Annealing and Reverse Transformation Behavior of TRIP Steels Having Martensite as Starting
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P steel is an advanced high strength steel that is currently used for automotive parts.[1–7] TRIP is an acronym, standing for ‘transformation-induced plasticity’; the transformation is the conversion of austenite into martensite. Therefore, TRIP steel refers to steel products that use deformation-induced martensite transformation from retained austenite to obtain an excellent combination of strength and elongation.[8–12] The response of retained austenite to strain, so-called mechanical stability, has been believed to play a critical role in controlling the tensile properties of TRIP steels and has led to a vast number of studies on the parameters affecting the characteristics of austenite. In most studies on TRIP steels, the major focus has been optimizing the austenite chemistry and grain size with an emphasis on improving the mechanical
JEONG IN KIM, YONG HOON CHOI, and DONG-WOO SUH are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Kyungbuk, 790-784, Korea. Contact e-mail: [email protected] JOO HYUN RYU, SEA WOONG LEE, and KYOOYOUNG LEE are with Technical Research Laboratories, POSCO, Kwangyang, 545-090, Korea. Manuscript submitted March 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
properties. However, it was recently reported that the morphology of austenite strongly affected the mechanical performance of TRIP steels as well.[1,9] Indeed, the manufacturing process of automobile components involves various kinds of forming; shearing, bending, and stretch drawing operations. Thus, apart from the tensile properties, an ability to resist failure from localized deformation, so-called stretch-flangeability, is an important design parameter. Generally, TRIP steels have been known to be inferior in terms of stretch-flangeability because the interface between ferrite and transformed martensite is vulnerable to cracking.[13,14] However, recent studies show that morphological control of retained austenite is a promising option, revealing that TRIP steels having a lath type morphology of retained austenite exhibit an improved stretch-flangeability.[15–17] A martensitic initial microstructure is an effective way to obtain lath-type austenite after the intercritical annealing. In addition, the authors suggest that the presence of interlath austenite along the lath boundary in the initial martensite is important for the morphological control of the microstructure.[15] The reason is that the interlath austenite in the initial microstructure, not decomposed upon heating, readily grows along the lath boundary of martensite at the intercritical temperature. Considering the importance of the retention of austenite upon heating to the intercritical temperature, the heating rate will have a significant influence on the morphological evolution of intercritical austenite. In the present study, therefore, we examined the annealing behavior of TRIP steel at various heating rates with respect to the microstructural evolution and transformation kinetics, with martensite as the initia
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