Welding CCT Curve and Microstructure of the Magnesium Treated X100 Pipeline Steel

Microstructure in a continuously cool of magnesium treated X100 high strength pipeline steel was investigated by means of heat simulation technique with a DIL805A simulator,optical microscopy(OM) and scanning electron microscopy(SEM).The results show that comparing the welding simulation CCT with 950℃ insulated CCT process,the AC1 and AC3 temperatures of the former are 25℃ higher than those of the latter,and the austenite grain size is also significantly larger than that of the latter too,but the temperature of cooling phase is 30~50℃ lower than that of the latter.Microstructure of X100 high strength pipeline steel is mainly composed of quasi-polygonal ferrite(QF) and granular bainite(GB) when the cooling rate is in the range of 0.1~3℃/s.Acicular ferrite and granular ferrite(GB+BF) prevail in the steel when the cooling rate is in the range of 3~30℃/s.Martensite(M) forms in the steel when the cooling rate is higher than 50℃/s.