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Effect of Cooling Conditions on Microstructures and Mechanical Properties in API X80 Linepipe Steels  

Han, Seung Youb (Center for Advanced Aerospace Materials Pohang University of Science and Technology)
Shin, Sang Yong (Center for Advanced Aerospace Materials Pohang University of Science and Technology)
Lee, Sunghak (Center for Advanced Aerospace Materials Pohang University of Science and Technology)
Bae, Jin-ho (Sheet Products & Process Research Group Technical Research Laboratories, POSCO)
Kim, Kisoo (Sheet Products & Process Research Group Technical Research Laboratories, POSCO)
Publication Information
Korean Journal of Metals and Materials / v.47, no.9, 2009 , pp. 523-532 More about this Journal
Abstract
In this study, four API X80 linepipe steel specimens were fabricated with varying cooling rates and finish cooling temperatures, and their microstructures and crystallographic orientations were analyzed to investigate the effects of cooling conditions on their tensile and Charpy impact properties. All the specimens consisted of acicular ferrite, granular bainite, and secondary phases such as martensite and martensiteaustenite constituent. The volume fraction of secondary phases increased with increasing cooling rate, and the higher finish cooling temperature resulted in the reduction in volume fraction and grain size of secondary phases. According to the crystallographic orientation analysis data, the effective grain size and unit crack path decreased as fine acicular ferrites having a large amount of high-angle grain boundaries were homogeneously formed, thereby leading to the improvement of Charpy impact properties. The specimen fabricated with the higher cooling rate and lower finish cooling temperature had the highest upper shelf energy and the lowest energy transition temperature because it contained a large amount of fine secondary phases homogeneously distributed inside fine acicular ferrites, while its tensile properties well maintained.
Keywords
API X80 linepipe steel; cooing condition; tensile properties; charpy impact properties; effective grain size;
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