Role of retained austenite on the deformation of an Fe-0.07 C-1.8 Mn-1.4 Si dual-phase steel
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INTRODUCTION
RETAINED austenite is inevitably present in ferritemartensite dual-phase steels. In some cases, quite a large portion of the second phase has been found to be retained austenite. Thus, many investigators have been interested in the role of the retained austenite on the strength-ductility combination of dual-phase steel. Dual-phase steel slow-cooled from the alpha-gamma region has a higher content of retained austenite and a better strength-ductility combination than water-quenched type, ~ even though the presence of retained austenite is not a prerequisite for the achievement of 'dual-phase' properties. It has also been reported that the ductility of dual-phase steel depends on the retained austenite content.2'3'4 In these cases, some fraction of the retained austenite did not transform to martensite even at strains over 10 pct. In contrast to these observations, Eldis 5 reported that the retained austenite did not have a measurable effect on mechanical properties partly because the austenite had been eliminated from the microstructure by the first few percent plastic strain. Unfortunately, there have been few attempts to clarify these contradictory views on the effect of retained austenite on the mechanical properties of dual-phase steels. It is the purpose of this work to understand the strain induced transformation of retained austenite during tensile deformation and the relevant effects of the austenite to martensite transformation on the deformation behavior of dualphase steels. JOON JEONG YI is Graduate Student, Department of Materials Science and Engineering, The Korea Advanced Institute of Science and Technology, Seoul, and Senior Researcher, The Technical Research Laboratory, Pohang Iron and Steel Company Ltd., Pohang, Korea. KI JOON YU, formerly Graduate Student, Department of Metallurgical Engineering, Seoul National University, Seoul, Korea, is now Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. IN SUP KIM is Principal Research Scientist, Material and Metallurgical Research Division, KAIST, P.O. Box 131, Dongdaemun, Seoul 131, Korea. SANG JOO KIM is Professor, Department of Metallurgical Engineering, Seoul National University, Seoul, Korea. Manuscript submitted July 8, 1982.
METALLURGICALTRANSACTIONS A
The chemical composition in wt pct of the steel for this investigation was 0.07 C, 1.78 Mn, 1.36 Si, 0.04 A1, 0.003 P, and 0.005 S. The steel was induction melted to the form of a 1 x 10-tm square ingot. The ingot was hotforgedat 1000to 1200 ~ • 10-2mby 1.2 x 10-~m slab. The slab was soaked at 1250 ~ for two hours, then hot-rolled to 3 x 10-3m in thickness with a finishing temperature of 870 ~ and air-cooled. The hot-rolled strips were pickled and cold-rolled to 1.5 x 10-3m in thickness. Tensile specimens were cut from the cold-rolled sheet with the tensile axis parallel to the rolling direction. The specimens were intercritically annealed in a radiation type furnace which was designed to heat a sample by fo
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