In order to ascertain the reason for the formation of softening zone in heat affected zone of high-strain pipeline steel in this paper, the welding thermal cycle process of high-strain pipeline steel of X80 grade was studied using welding thermal simulation. The microstructure characteristic, crystallology and impact toughness of the softening zone in heat affected zone was examined by optical microscope, scanning electron microscope, electronbackscattered diffraction ( EBSD) , transmission electron microscopy and impact testing. The results indicated that the softening zonein heat affected zone formed at high temperature tempering zone with a peak temperature of 600～700 ℃, when the microstructure of base materials consisted of ferrite and bainite. At this moment, the microstructure transformed to the coarse ferrite plus tempering bainite with low hardness and the recovery process was accelerated. Then the substructure decreased significantly and the dislocation density reduced dramatically, causing the formation of softening zone. The softening zone has excellent impact toughness. However, the M/A component assembled and coarsened at the intercritical zone of 800 ℃. The percentage of high angle grain boundary was also decreased, resulting in the lower toughness.%为了研究抗大变形管线钢热影响区出现软化区的具体原因,本文采用焊接热模拟实验研究了X80级抗大变形管线钢的焊接热循环过程,结合金相显微镜、扫描电镜、EBSD、透射电镜和冲击实验,分析了热影响区软化区的组织变化、晶体学特征和冲击韧性.结果表明:当母材组织为多边形铁素体+贝氏体时,焊接热影响区的软化区出现在峰值温度600～700℃的高温回火区,此时组织转变成硬度较低的粗大铁素体+回火贝氏体,并且回复过程加快,组织中亚结构的大幅度减少和位错密度的显著降低是产生软化区的主要原因;软化区的韧性较好,但是在800℃的临界区,M/A组元发生了聚集和粗化,并且大角度晶界比例降低,导致了韧性低谷的出现.
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