Effect of Strain-Induced Age Hardening on Yield Strength Improvement in Ferrite-Austenite Duplex Lightweight Steels
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TRODUCTION
RECENTLY, highly deformable steels such as TRansformation-induced plasticity (TRIP) and TWinning-induced plasticity (TWIP) steels have been developed worldwide in automotive industries for mass-efficient vehicle structures, improved vehicle impact performance, and greater formability for complex stampings.[1–9] The demanding requirements for high strength and ductility, environmental friendliness, low-cost production, and reduction in automotive vehicle weight are key issues governing application of automotive steels.[1–11] Lowerdensity Al-containing (ferrite + austenite) duplex lightweight steels have also been actively developed,[12–17] where the ranging from 4 to 6 wt pct of Al addition provides excellent tensile properties such as strengths
HYEJIN SONG and SEOK GYU LEE, Research Assistants, and SEOK SU SOHN, Research Assistant Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. Contact e-mail: bbosil7@ postech.ac.kr JAI-HYUN KWAK, Senior Principal Researcher, is with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Kwangyang, 545-090, Korea. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea. Manuscript submitted March 16, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
greater than 830 MPa and elongation above 40 pct, with savings in terms of weight ranging from 6 to 9 pct compared with conventional steels.[1,18–20] Structural components in automotive bodies are classified into energy absorption or reinforcement, and these two require different properties. The steels for energy absorption mostly require a high tensile strengthxelongation product, high strain hardening exponent, and low yield ratio for reduced spring-back. Deformation mechanisms such as TRIP, TWIP, and microband-induced plasticity (MBIP) that utilize austenite have been used for these components.[21] Reinforcement components, on the other hand, preferentially require a high yield-to-tensile strength ratio, rather than a high tensile strengthxelongation product. Passenger compartments enclosed in a rigid ‘safety cage’ are designed to protect passengers in cases of low- to high-speed crashes, and compartment structures should prevent any deformation or intrusion for serving spaces around passengers. The most important material property to meet the needs of passenger compartment is the high yield strength because achieving high geometric stiffness to prevent or minimize deformation is more important than absorbing impact energy by deformation of the structure itself.[22] Recently developed duplex lightweight steels have lower C and Mn contents than those of austenitic lightweight steels in view of environmental friendliness and low-cost production. The TRIP phenomenon is the main deformation mechanism that confers the improved
properties
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