Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Stre
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RECENTLY, demands for developing new advanced steels, which can meet with the requirements of environmental-friendliness and excellent mechanical properties simultaneously, have been emphasized. To achieve these requirements, studies on improving functionality by optimizing many properties obtained from various microstructural combinations have been actively conducted.[1–3] For example, thick plates of high-strength high-toughness steels having bainitic and martensitic microstructures are utilized in large-scale constructions of linepipes, buildings, bridges, industrial plants, and ships.[3–5] The addition of alloying elements into these steel plates is a simple method to produce high-strength high-toughness steels, but alloying poses problems of deteriorating ductility and toughness as strength is improved. Recently, efforts have been made to realize HYO KYUNG SUNG, Research Assistant, is with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SANG YONG SHIN, Research Professor, is with Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. BYOUNGCHUL HWANG and CHANG GIL LEE, Senior Researchers, are with Ferrous Alloys Department, Korea Institute of Materials Science, Changwon 641-831, Korea. SUNGHAK LEE, Professor, is with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted February 24, 2012. Article published online August 21, 2012 294—VOLUME 44A, JANUARY 2013
a good combination of high strength and toughness by promoting fine low-temperature transformation microstructures such as bainite at rapid cooling rates, while the addition of alloying elements is minimized.[6–9] Among alloying elements, carbon is the most effective and economical element for strengthening,[10] and chromium homogeneously distributes bainitic microstructures, while refining grains and preventing the formation of martensite.[11,12] Niobium plays a role in improving strength and toughness by refining austenite grains and forming carbonitrides.[12,13] In addition to alloying methods, various complex and mixed microstructures can be formed by varying cooling conditions, and bainite or martensite is readily formed with increasing cooling rates.[7,14] As the finish cooling temperature (FCT) during rolling of bainitic steels decreases, the yield strength tends to increase because of the grain refinement effect and the increased volume fraction of low-temperature transformation microstructures. The yield strength is rather reduced, while the tensile strength increases, however, when the FCT decreases further below a certain temperature. This is closely related to the volume fraction of secondary phases such as martensite-austenite constituents.[15,16] When the cooling rate increases, the grain size decreases overall, and the volume frac
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