Effects of Rolling and Cooling Conditions on Microstructure and Tensile and Charpy Impact Properties of Ultra-Low-Carbon
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IN order to safely construct and maintain structures, many studies have been made for improved strength and toughness of structural steels. Particularly, demands for new advanced steels development, which can meet with requirements of environmental friendliness such as prevention of earth warming effect and recycling issue and with functional requirements for more excellent properties, have been emphasized. To satisfy all the requirements, the development of structural steels, which are more environmentally friendly and more economical by low alloying, are essentially needed.[1–4] Recent efforts on achieving the nice combination of high
HYO KYUNG SUNG, Research Assistant, Center for Advanced Aerospace Materials, and SANG YONG SHIN, Research Professor, Department of Materials Science and Engineering, are with the Pohang University of Science and Technology, Pohang, 790-784, Korea. BYOUNGCHUL HWANG and CHANG GIL LEE, Senior Researchers, are with the Ferrous Alloys Group, Korea Institute of Materials Science, Changwon, 641-831, Korea. NACK J. KIM, Professor, is with the Graduate Institute of Ferrous Technology and the Center for Advanced Aerospace Materials, Pohang University of Science and Technology. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials and the Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted June 7, 2010. Article published online December 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
strength and toughness by promoting fine low-temperature transformation microstructures such as bainite at rapid cooling rates are good examples.[5–10] For better formation of bainitic microstructures, it is necessary to optimize the rolling and cooling processes, because various microstructures can be formed and their volume fractions are varied with these process parameters. One of the widely used conventional rolling and cooling processes is a thermomechanical control process (TMCP). The grain refinement effect is sufficiently achieved by the controlled rolling process at high temperatures, and low-temperature transformation microstructures are formed by the accelerated cooling process.[11–13] It was recently reported in the Nippon Steel Corporation that the formation of fine acicular ferrite (AF) instead of bainite was promoted by the slow cooling after the controlled rolling process using a mild accelerated cooling process.[14] The JFE (Japan Fe Engineering) Steel Company (Chiba) developed a heattreat online process to obtain dual-phase microstructures composed of bainite and martensite-austenite (MA) constituents.[15] Research on steels used by dualphase microstructures of ferrite and martensite together with a small amount of hard MA and AF is also actively performed.[4,11] According to this recent steel research, it is likely that dual- or multiple-phase microstructures mostly composed of ductile ferrite and strong lowtemperature transformation microstructures occupy the main strea
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