Effect of Tempering Time on Microstructure, Tensile Properties, and Deformation Behavior of a Ferritic Light-Weight Stee
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LIGHT-WEIGHT steels have been developed for automotive steels sheets in order to increase fuel efficiency and to decrease CO2 emission. They are classified into austenite- and ferrite-based light-weight steels according to their matrix microstructure. Austenitic light-weight steels typically include transformationinduced plasticity (TRIP) steels or twinning-induced plasticity (TWIP) steels, in which the austenite is stabilized at room temperature. Ferritic light-weight steels are named as such since their Al content is generally higher than their Mn content. These steels only have ferrite as the matrix at room temperature, while the austenite and ferrite exist simultaneously at high temperatures. SEUNG YOUB HAN, Research Assistant, SANG YONG SHIN, Research Professor, BYEONG-JOO LEE, Professor, and SUNGHAK LEE, Professor, are with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea Contact e-mail: [email protected] BYEONG-JOO LEE and SUNGHAK LEE, are jointly appointed with Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea. NACK J. KIM, Professor, is with Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, 790-784, Korea and jointly appointed with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. JAI-HYUN KWAK, Senior Principal Researcher, Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Gwangyang, 545-711, Korea. Manuscript submitted July 17, 2011. Article published online August 16, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Light-weight steels have been actively developed since Fe-Al-C alloying systems were proposed in late 1990s.[1,2] As the Al and C are controlled within 8.5 to 11 and 0.5 to 1.0 wt pct, respectively, in these steels, large amounts of j-carbides with a composition of (Fe,Mn)3-Al-C and a perovskite structure are homogeneously distributed within the ferrite matrix, leading to enhancement in both strength and ductility.[1–10] Recently, j-carbides have gained attention for increasing mechanical properties by controlling their size, fraction, and distribution. Quenching and tempering treatments of these ferritic light-weight steels fabricated by varying their C content has shown that a number of j-carbides were precipitated in the tempered band-shaped martensite and ferrite matrix.[3–8] Tensile test of these steels tempered at 973 K (700 °C) showed that deformation bands formed widely, and j-carbides were sufficiently deformed inside bands leading to improved ductility and strength. However, the formation of j-carbides has not yet been fully understood, and their deformation and fracture mechanisms have not been sufficiently investigated with regard to the microstructures that containing the j-carbides and the band-shaped tempered martensites. In addition, the microstructure modification that occurs as tempering proceeds towards equilibrium has not been an
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