The Significance of Nitrogen Addition on the Impact Toughness of CGHAZ in V‐Bearing FH36 Grade Shipbuilding Steel

The influence of nitrogen (N) addition on the evolution mechanism of microstructure and the variation law of impact toughness in the coarse‐grained heat‐affected zone (CGHAZ) of vanadium (V)‐bearing FH36 grade shipbuilding steel with different N contents was investigated via microstructure characterization and simulation calculations. In this investigation, the chemical compositions and rolling processes of the steels with different N contents were designed, and multiscale characterization techniques (OM, SEM‐EBSD, HT‐CLSM, TEM‐EDS) as well as Thermo‐Calc calculations were employed for the analysis. Key findings revealed that N addition refined the CGHAZ microstructure and shifted precipitation from VC to a higher density of V(C, N). The enhanced strength and toughness of the N‐containing base metal (BM) steel, despite a marginal ductility reduction (∼1%), were attributed to solid solution and precipitation strengthening. For the CGHAZ, enhanced impact toughness was mediated by an increased fraction of ductile phases and regulated precipitate features. Moreover, the impact toughness of the simulated CGHAZ exhibited an initial increase followed by a decrease with the increasing of welding heat input, and the intrinsic mechanism was further discussed in detail.