Evolution of microstructure and impact property in welding HAZ of TiNb steel

The influence of welding thermal cycle parameters on the microstructure and impact property of the coarse grain zone in the HAZ of TiNb steel for large-line energy welding was studied by thermal simulation technology. The results show that the coarse-grained microstructure of the heat-affected zone of TiNb steel is mainly composed of bainite, ferrite and pearlite. When the peak temperature of thermal cycle increases, the content of pearlite and ferrite decreases, and the content of bainite increases, the slab structure is obviously roughened, resulting in the decrease of impact toughness, the prolonging of high temperature residence time, the reduction of bainite and pearlite content, and the increasing of polygonal ferrite content. The high-temperature ferrite structure is thick when the high temperature residence time is above 10 s. Then the impact toughness is drastically reduced. Under the appropriate cooling time conditions, the main microstructure is fine acicular ferrite structure, and the impact toughness reaches the maximum. With a lower thermal cycle peak temperature, a shorter high temperature residence time and a suitable cooling time, a fine grained ferrite structure can be obtained, which can significantly improve the impact toughness of the heat affected coarse grain zone.