Effect of nitrogen on the microstructures and mechanical properties in simulated CGHAZ of vanadium microalloyed steel varied with different heat inputs

The microstructures and mechanical properties of simulated weld coarse heat affected zone (CGHAZ) of two kinds of vanadium microalloyed steel, i.e. V-Ti and V-N-Ti steel, had been studied using the welding thermal simulation method on a Gleeble-3800 thermal simulator, under different welding heat input. The yield strength of simulated CGHAZ decreases with heat input increasing. The 50% fraction appearance transition temperature (50% FATT) of simulated CGHAZ in V-N-Ti steel decreased at first, and then increased. While for the V-Ti steel, 50% FATT increased with t8/5 increasing. The V-N-Ti steel had a better combination of strength and toughness of simulated CGHAZ compared with V-Ti steel. Increasing nitrogen cannot only refine the austenite grain size, but also simulate V-rich cap part forming on the TiN core part which can be as intragranular ferrite nucleation positions, resulting in refining the effective grain size and raising cleavage fracture stress of simulated CGHAZ. The detrimental effect of the free N on toughness can be remedied by austenite grain and effective grain size refinement.