Effect of element Nb on microstructures and impact toughness of CGHAZ in TiNbV micro-alloyed steels

The microstructure evolution and impact toughness of the coarse grain heat-affected-zone (CGHAZ) of TiNbV micro-alloyed steel were investigated by the simulated welding thermal cycle. The microstructure and grain size of the simulated CGHAZ between low-niobium steel and high-niobium steel is significantly different. When the Nb content is low (0.005%), the CGHAZ consists of ferrite, acicular ferrite, and pearlite. The ratio of the large-angle grain boundary and the small-angle grain boundary is equal, and the grain size of the CGHAZ is coarse and uneven. With the increase of Nb content, the number of grains in the high-angle grain boundary increases, and the grains are refined. However, the formation of acicular ferrite is suppressed, and the bainite content of CGHAZ increases. The formation of bainite in the micro-alloyed steel plays a leading role in the decrease of the impact toughness of CGHAZ. The content of element Nb should be controlled within an appropriate range (~ 0.02%) to obtain good impact toughness.