Korean Journal of Metals and Materials (대한금속재료학회지)

The Korean Institute of Metals and Materials (대한금속재료학회)
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Effect of Cooling Conditions on Microstructures and Mechanical Properties in API X80 Linepipe Steels

API X80 라인파이프강의 미세조직과 기계적 특성에 미치는 냉각조건의 영향

  • 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)
  • 한승엽 (포항공과대학교 항공재료연구센터) ;
  • 신상용 (포항공과대학교 항공재료연구센터) ;
  • 이성학 (포항공과대학교 항공재료연구센터) ;
  • 배진호 (POSCO 기술연구소 박판연구그룹) ;
  • 김기수 (POSCO 기술연구소 박판연구그룹)
  • Received : 2009.04.27
  • Published : 2009.09.25
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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

Acknowledgement

Supported by : POSCO, 한국과학재단

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