Microstructure and properties of coarse grain region for high-strain pipeline X80 steel

The microstructure, microhardness and toughness of coarse grain zone for high-strain pipeline steel X80 were studied by means of welding thermal simulation.Influence of cooling rates after welding on the microstructure and properties of coarse grain zone were investigated.The results illustrated that there were mainly four types of microstructure in coarse grain zone of high-strain pipeline steel X80, which includes polygonal ferrite(PF) and a small quantity of pearlite, granular bainite, bath bainite and bath martensite.The softening in coarse grain zone is an ubiquitous problem for high-strain pipeline steel X80.While the cooling rate after welding was between 15℃/s and 30℃/s, the coarse grain zone was mainly composed of bath bainite, which has the better microhardness, charpy toughness at room temperature and-10℃.