Effects of Cooling Conditions on Tensile and Charpy Impact Properties of API X80 Linepipe Steels
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LINEPIPE steels used to transport crude oil or natural gas over a long distance require essentially high strength and toughness, and have become thicker and larger to improve transportation efficiency under highpressure conditions.[1–4] Because oil drilling activities in extremely cold regions such as Siberia and Alaska have been actively attempted due to serious resource depletion, there are increasing demands for linepipe steels having excellent low-temperature toughness. As the finish cooling temperature during the rolling of linepipe steels decreases, the yield strength tends to increase because of the grain refinement effect and the increased volume fraction of low-temperature transformation phases. The yield strength is rather reduced while the tensile strength increases, however, when the finish cooling temperature decreases further below a certain point. This phenomenon is closely related to the SEUNG YOUB HAN, Research Assistant, and SANG YONG SHIN, Postdoctoral Research Associate, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is also with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: [email protected] NACK J. KIM, Professor, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology. JIN-HO BAE, Principal Researcher, and KISOO KIM, Group Leader, are with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Manuscript submitted June 21, 2009. Article published online December 19, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
volume fraction of secondary phases such as martensiteaustenite (MA) constituents.[5,6] When the cooling rate increases, the grain size decreases overall, but the volume fraction of MA increases, which leads to higher strength and lower ductility and toughness.[5–7] Recent efforts have focused on the fabrication of linepipe steels with high strength and high toughness at reduced costs, and methods to optimize their microstructures have been suggested by controlling various cooling conditions.[2–6] Linepipe steels used in low-temperature environments should have ductile-brittle transition temperatures low enough to prevent abrupt brittle fracture, and high absorbed energy to prevent unstable ductile fracture propagation.[1–4] The absorbed energy and strength are affected by microstructural factors such as the kind, volume fraction, and shape of the secondary phases, grain size, and matrix structure. The transition temperature is largely affected by the unit crack path, which is the distance between cleavage fracture facets and which is closely related to the effective grain size.[6–8] Because API X80-grade linepipe steels, which are widely used, have various micro
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