Effect of Nitrogen on Microstructure and Toughness of Coarse Grain Heat Affected Zone in Vanadium Microalloyed Steel

Microstructures and toughness of coarse grain heat affected zones(CGHAZ) in experimental steels with different nitrogen contents are studied by welding thermal simulation.Resultsindicate that with the increase of nitrogen content, CGHAZ microstructure changes from grain boundary ferrite, bainite and side lath ferrite to acicular ferrite, polygonal ferrite and a small amount of bainite, with the ferrite grains refined. The ductile-brittle transition temperature(FATT₅₀) of CGHAZ steel decreases first and then increases, meanwhile the yield strength rises. When the nitrogen content increases from 0.004 4% to 0.009 4%, the effective grain size decreases and the FATT₅₀ of CGHAZ decreases. When the nitrogen content increases from 0.009 4% to 0.019 0%, the combined influence of solid solution nitrogen and yield strength increment on FATT₅₀ is greater than that of grain refinement, leading to the increase of FATT₅₀.