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Sung, H.K.,Shin, S.Y.,Cha, W.,Oh, K.,Lee, S.,Kim, N.J. Elsevier Sequoia 2011 Materials science & engineering. properties, micro Vol.528 No.9
This study was concerned with effects of acicular ferrite on Charpy impact properties in heat affected zones (HAZs) of two API X80 linepipe steels containing oxides. In the one steel, Mg and O<SUB>2</SUB> were additionally added to form a larger amount of oxides than the other steel, which was a conventional X80 steel containing a considerable amount of Al and Ti. Various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs of 35kJcm<SUP>-1</SUP> and 60kJcm<SUP>-1</SUP>. Oxides present in the API X80 linepipe steels were complex oxides whose average size was 1-2μm, and the number of oxides increased with increasing amount of Mg and O<SUB>2</SUB>. The volume fraction of acicular ferrite present in the steel HAZs increased with increasing number of oxides, and decreased with increasing heat input. When the volume fraction of acicular in the HAZ was higher than 20%, Charpy impact energy at -20<SUP>o</SUP>C was higher than 100J as the ductile fracture mode was dominant. Particularly in the steel HAZs having a larger amount of oxides, Charpy impact properties were excellent because oxides worked as nucleation sites of acicular ferrite during welding. Charpy impact properties of the HAZs could be well correlated with the volume fraction of acicular ferrite and number of oxides under different heat input conditions.
논문 : 단상영역에서 압연된 API X80 라인파이프강의 인장 및 샤르피 충격 특성에 미치는 Mo, Cr, V의 영향
한승엽 ( Seung Youb Han ),신상용 ( Sang Yong Shin ),서창효 ( Chang Hyo Seo ),이학철 ( Hak Cheol Lee ),배진호 ( Jin Ho Bae ),김기수 ( Ki Soo Kim ),이성학 ( Sung Hak Lee ),김낙준 ( Nack J. Kim ) 대한금속재료학회 ( 구 대한금속학회 ) 2008 대한금속·재료학회지 Vol.46 No.12
This study is concerned with the effects of Mo, Cr, and V addition on tensile and Charpy impact properties of API X80 linepipe steels. Four kinds of steels were processed by varying Mo, Cr, and V additions, and their microstructures and tensile and Charpy impact properties were investigated. Since the addition of Mo and V promoted to form fine acicular ferrite and granular bainite, while prohibiting the coarsening of granular bainite, it increased the strength and upper shelf energy, and decreased the energy transition temperature. The Cr addition promoted the formation of coarse granular bainite and secondary phases such as martensite-austenite constituents, thereby leading to the increased effective grain size, energy transition temperature, and strength and to the decreased upper shelf energy. The steel containing 0.3wt.% Mo and 0.06wt.% V without Cr had the highest upper shelf energy and the lowest energy transition temperature because its microstructure was composed of fine acicular ferrite and granular bainite, together with a small amount of hard secondary phases, while its tensile properties maintained excellent.
San Yong Shin,Kyungshik Oh,이성학,김낙준 대한금속·재료학회 2011 METALS AND MATERIALS International Vol.17 No.1
This study investigated the correlation between the microstructure, hardness, and Charpy impact properties in heat affected zones (HAZs) of API X80 linepipe steels containing complex oxides. Three types of steels were fabricated by adding Ti, Al, and Mg to form complex oxides; their microstructures, hardness, and Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al, and Mg was included in the steels. The simulated HAZs containing a number of oxides showed a high volume fraction of intra-granular transformation microstructure (IGT) region because the oxides acted as nucleation sites for the ferrites. According to the correlation study between the heat input, volume fraction of the IGT region, and Charpy impact properties, it was found that ductile fractures predominantly occurred when the volume fraction of the IGT region was 65 % or higher, and the Charpy absorbed energy was excellent over 200 J. When the volume fraction of the IGT region was 45 % or lower, the Charpy absorbed energy was poor below 50 J as brittle cleavage fractures prevailed. These findings suggest that the active nucleation of the ferrites in the oxide-containing steel HAZs is related to the improvement of the Charpy impact properties of the HAZs.
API X80 라인파이프강의 미세조직과 기계적 특성에 미치는 냉각조건의 영향
한승엽 ( Seung Youb Han ),신상용 ( Sang Yong Shin ),이성학 ( Sung Hak Lee ),배진호 ( Jin Ho Bae ),김기수 ( Ki Soo Kim ) 대한금속·재료학회 2009 대한금속·재료학회지 Vol.47 No.9
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.
API X80 라인파이프 강의 인장 및 샤르피 충격 특성에 미치는 Mo 및 Nb의 영향
민경준 ( Kyung Zoon Min ),배진호 ( Jin Ho Bae ),김기수 ( Ki Soo Kim ),이도재 ( Doh Jae Lee ) 대한금속재료학회(구 대한금속학회) 2011 대한금속·재료학회지 Vol.49 No.10
In this study, three kinds of linepipe steels were processed by changing the amount of Mo and Nb to investigate the effects on microstructures, tensile and Charpy impact properties. All the specimens consisted of acicular ferrite, granular bainite and secondary phases such as martensite and austenite constituents (MA). The increase in Mo raised the volume fractions of the granular bainite and MA, and raised the number of fine precipitates, which increased the yield and tensile strengths and decreased the upper self energy and energy transition temperatures. In the steel having less Mo and more Nb, the volume fractions of the granular bainite and MA decreased, and a finer microstructure was observed. This microstructure suppressed the formation of separation during Charpy impact testing and led to a higher upper shelf energy and lower energy transition temperature, while the yield and tensile strengths were lower than those of the steels with more Mo and less Nb.