Influence of microstructure and M/A island evolution on toughness of pipeline steel under controlled cooling process
NIU Yan-long1, LIU Qing-you1, JIA Shu-jun1, TONG Shuai1, WANG Bing1, REN Yi2
1. Institute of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China; 2. State Key Laboratory of Metal Materials and Applications for Marine Equipment, Anshan 114009, Liaoning, China
Abstract：In order to study the influence of microstructures,volume percent and length/width ratio of M/A islands on low temperature toughness of pipeline steels,the multiscale means including optical microscope(OM),scanning electron microscopy (SEM),electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) were used. The results showed that the microstructure of three different cooling process was the same and all contained of acicular ferrite (AF) +quasi-polygonal ferrite (QF) +M/A island+grain bainite(GB). However,with the increase of cooling rate,the microstructure was refined remarkably,and the effective grain size decreased from 3.21 to 2.88 μm,the volume percent of M/A islands decreased from 11.2% to 5.8%,and the length/width ratio of M/A islands also decreased from 3.5 to 1.2. When the temperature was above -40 ℃,the low temperature toughness of pipeline steel increased greatly with the decrease of effective grain size and decrease of M/A island size and length/width ratio. However,when the temperature decreased to -80--60 ℃,the circular M/A islands dominated by martensite tended to embrittle and fracture,resulting in the lowest impact toughness,when the volume percent and the length/width ratio of M/A islands and were the lowest. However,the low temperature toughness of pipeline steel was not directly related to the percentage of large angle grain boundaries. For the high strength pipeline steel in this experiment,when the cooling process was 20 ℃/s,the M/A volume percent was about 9.8%,and the length/width ratio is 2.5,the material had excellent comprehensive toughness.
牛延龙, 刘清友, 贾书君, 童帅, 汪兵, 任毅. 控冷工艺下组织及M/A岛对管线钢韧性的影响[J]. 钢铁, 2020, 55(6): 91-100.
NIU Yan-long, LIU Qing-you, JIA Shu-jun, TONG Shuai, WANG Bing, REN Yi. Influence of microstructure and M/A island evolution on toughness of pipeline steel under controlled cooling process. Iron and Steel, 2020, 55(6): 91-100.
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2020, Vol. 55 
