Effective grain size and charpy impact properties of high-toughness X70 pipeline steels |
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Authors: | Byoungchul Hwang Yang Gon Kim Sunghak Lee Young Min Kim Nack J. Kim Jang Yong Yoo |
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Affiliation: | (1) Center for Advanced Aerospace Steels, Pohang University of Science and Technology, 790-784 Pohang, Korea;(2) Plate Research Group, Technical Research Laboratories, POSCO, 790-785 Pohang, Korea |
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Abstract: | The correlation of microstructure and Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel was investigated in this study. Six kinds of steel were fabricated by varying the hot-rolling conditions, and their microstructures, effective grain sizes, and CVN impact properties were analyzed. The CVN impact test results indicated that the steels rolled in the single-phase region had higher upper-shelf energies (USEs) and lower energy-transition temperatures (ETTs) than the steels rolled in the two-phase region because their microstructures were composed of acicular ferrite (AF) and fine polygonal ferrite (PF). The decreased ETT in the steels rolled in the single-phase region could be explained by the decrease in the overall effective grain size due to the presence of AF having a smaller effective grain size. On the other hand, the absorbed energy of the steels rolled in the two-phase region was considerably lower because a large amount of dislocations were generated inside PFs during rolling. It was further decreased when coarse martensite or cementite was formed during the cooling process. |
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