Microstructure and Precipitation Behavior in Heat Affected Zone of C-Mn Microalloyed Steel Containing Nb, V and Ti

Effects of total N content on HAZ toughness, microstructure and precipitate behavior in Nb-V-Ti bearing C-Mn microalloyed steels has been investigated under non-equilibrium state such as welding thermal cycle.The HAZ toughness is improved with increasing total N content and the lowest 100 J transition temperaturre concerning HAZ toughness was obtained at a steel with high total N content about 80 ppm. Improbement of HAZ toughness with increasing total N content is caused by refinement of microstructure and decreasing souluble Nb and Ti with increasing total N content. Complex precipitates that contain both Nb and Ti are observed in Nb-V-Ti bearing steels after a weld thermal cycle, and they are refined and their density tend to increase with increasing total N content. The amount of Nb and Ti as precipitates increase with increasing total N content. And then the complex precipitation behavior in HAZ could be quantitatively predicted by a thermodynamic model.In Nb-V-Ti steels refinement of prior austenite grains occurs at higher total N content than in Ti bearing steels. It is caused by increasing the stability of precipitates through the formation of complex precipitates in Nb-V-Ti bearing steels.