Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity ste
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DUCTION
THE high strength and ductility of ferrite-bainite-austenite transformation-induced-plasticity TRIP sheet steels make them attractive materials for automotive applications such as door impact beams and bumpers. These steels owe their superior mechanical properties to the TRIP mechanism.[1] The transformation of retained austenite to martensite during forming operations enhances local strain hardening and delays the onset of necking. The successful operation of the TRIP mechanism depends on the amount and stability of retained austenite. Chemical composition, particularly C, Si, and Mn,[2] and thermal processing history[3,4,5] strongly influence austenite retention. The TRIP microstructures are produced in hot-rolled steels by cooling to the bainite transformation temperature after rolling, at which temperature the sheet is coiled. During cooling through the intercritical regime, ferrite may form on prior austenite grain boundaries. The subsequent hold at the coiling temperature converts much of the remaining austenite to a bainitic structure. The presence of silicon suppresses carbide formation,[6] creating a microstructure of retained austenite films between bainitic-ferrite laths.[7] Carbon enrichment of the austenite during the bainite transformation stabilizes the austenite and promotes its retention when the product is cooled to room temperature. Hot deformation might be expected to play an important role in ferrite and bainitic-ferrite nucleation and growth; however, there are few results relating to such effects.[8,9] H.-B. RYU, Assistant Manager, is with the Hot-Rolling Department, Pohang Iron and Steel Co., Ltd., Pohang, Korea. J.G. SPEER, Professor, is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] J.P. WISE, Research Engineer, is with the Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted November 28, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Therefore, the purpose of this study was to evaluate the influence of hot deformation on the microstructure and retained austenite content of a Si-Mn TRIP steel. In particular, hot deformation was evaluated in conjunction with variations in coiling temperature and cooling rate from the rolling temperature. II. PROCEDURE The chemical composition of the steel evaluated in this study is shown in Table I. The steel was vacuum melted, cast, and hot rolled to a thickness of 15 mm prior to further experimental processing. The Ac1, Ac3, BS , and MS temperatures of the steel are 765 ⬚C, 909 ⬚C, 640 ⬚C, and 414 ⬚C, respectively, based upon the empirical formulas of Andrews[10] and Steven and Hayes.[11] Heating and deformation were performed with a Gleeble 1500 thermomechanical simulator. Specimens were machined into cylinders with a diameter of 10 mm and a height of 20 mm. Tests were conducted in an inert atmosphere of nitrogen gas. Argon gas jets were applied to obtain rapid cooling rates. Figure 1 shows the processing
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