Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength
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NOWADAYS natural resources are depleted seriously by fast increasing industrialization like active oil drilling, even in extremely cold areas. Structural steels require improved mechanical properties such as strength, ductility, and toughness, as industries demanding structural steels have developed rapidly. Development demands for new advanced steels, which can meet with the requirements of environmental friendliness and excellent mechanical properties simultaneously, have been emphasized. In order to achieve these requirements, studies on improving functionality by materializing many properties obtained from various microstructural combinations have been actively conducted.[1,2] For example, thick plates of high-strength high-toughness steels having bainitic and martensitic microstructures are positively utilized in large-scale constructions of pipelines, buildings, bridges, industrial plants, and ships.[3] The addition of alloying elements into these steel plates is a simple way to obtain the bainitic and martensitic microstructures, but can easily lead to the HYO KYUNG SUNG, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with the Materials Science and Engineering, Pohang University of Science and Technology. SANG YONG SHIN, Assistant Professor, is with the School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea. Contact e-mail: [email protected] Manuscript submitted February 5, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
abrupt deterioration of ductility and toughness as well as weldability and corrosion resistance.[4,5] Conventional alloying methods usually pose the problem of deteriorating toughness while the strength is improved. However, efforts have been made to come up with a favorable combination of high strength and toughness by promoting fine bainitic microstructures at rapid cooling rates.[6,7] For better formation of these microstructures, it is necessary to control and stabilize the austenite during cooling. One of the widely used conventional rolling and cooling processes is a thermo-mechanical 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.[8,9] 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.[10] The Japan Fe Engineering (JFE) steel company developed a heat-treat online process to obtain dual-phase microstructures composed of bainite and martensite–austenite (MA) constituents.[11] Hu et al.[12] controlled the cooling conditions to generate various microstructures including precip
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