Effects of Annealing Temperature on Microstructure and Tensile Properties in Ferritic Lightweight Steels
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RESEARCH on how to reduce greenhouse gases and to control global warming has been conducted by governments, businesses, and research institutes throughout the world. The reduction in a vehicle’s weight has been focused on in order to increase fuel efficiency and to decrease CO2 emissions, which could have economic advantages and be eco-friendly in the research area of automotive steel sheets.[1–9] Recently, ferritic lightweight steels, in which Mn content decreases (to below 10 wt pct) and Al content increases, have been noted. This increase in Al content to 6 wt pct, for example, could possibly lead to a 10 wt pct reduction in automotive components. In addition, it could have excellent properties such as a strength of over 780 MPa and an elongation of over 30 pct.[4] Lightweight steels can be divided into austenite-based and ferrite-based according to their matrix microstructures. Typical austenitic lightweight steels are transforSEUNG YOUB HAN and HYUK-JOONG LEE, Research Assistants, and SANG YONG SHIN, Research Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. BYEONG-JOO LEE and SUNGHAK LEE, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, are jointly appointed with the Materials Science and Engineering Department, Pohang 790-784, Korea. Contact e-mail: [email protected] NACK J. KIM, Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, and is jointly appointed with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology. JAI-HYUN KWAK, Senior Principal Researcher, is with the Sheet Products and Process Research Group, Technical Research Laboratories, POSCO, Gwangyang 545-711, Korea. Manuscript submitted January 10, 2011. Article published online November 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
mation-induced plasticity steels or twinning-induced plasticity steels, in which the austenite is stabilized at room temperature. There have been many research studies intended to increase formability by adding Al, which raises stacking fault energy and stabilizes the ferrite, and by controlling deformation mechanisms. Ferritic lightweight steels have the advantages of lightweight effects because of the higher content of Al rather than Mn. They have only ferrite as a matrix at room temperature, although both austenite and ferrite exist at high temperatures. The development of ferritic lightweight steels started in the late 1990s, as Baligidad and co-workers[10,11] suggested Fe-Al-C alloying systems. Controlling Al and C contents in 8.5 to 11 wt pct and 0.5 to 1.0 wt pct, respectively, both strength and ductility were improved by homogeneously distributing large amounts of j-carbides, which have the composition of (Fe,Mn)3-Al-C and perovskite structure. In the 1980s, when research on j-carbides was in its infancy, it was thought that j-carbides had a harmful effect on ductility
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