Effect of nitrogen content on the second phase particles in V–Ti microalloyed shipbuilding steel during weld thermal cycling

Highlights: • Increasing the nitrogen can improve toughness of simulated CGHAZ and weld joint. • The nitrogen-enhanced V–Ti steels exhibit superior tolerance for high inputs. • The chemical stability of particles during the welding cycle was investigated. • Increasing N decreased soluble Ti resulting in low coarsening rate of particles. • Fine Ti- (Ti, V) (C, N) particles in process of welding cycle refined austenite grain. The microstructures, mechanical properties, and second-phase particles in the base plate and simulated coarse grain heat affected zone (CGHAZ) of V–Ti microalloyed steels with differing amounts of nitrogen were investigated. A Gleeble-3800, submerged arc welding, and transmission electron microscope were used during this investigation. The results showed that austenite grain size was refined and microstructure and toughness of CGHAZ improved with increasing nitrogen. Furthermore, in the case of low- nitrogen steel, base plate consisted mainly of Ti-rich (Ti, V) (C, N), which were coarsened after welding thermal cycling, thereby leading to coarsening of the austenite grains. In the case of high-nitrogen steels, base plate consisted mainly of Ti-rich (Ti, V) (C, N), V-rich (V, Ti) (C, N), and V (C, N) particles. During welding heating, Ti-rich (Ti, V) (C, N) dissolved only partially, whereas V-rich (V, Ti) (C, N) and V (C, N) particles dissolved completely. Fine Ti-rich (Ti, V) (C, N) and V (C, N) re-precipitated during cooling stage of welding. The sub-50-nm Ti-rich (Ti, V) (C, N) particles present at peak temperature and during cooling, were effective in refining the austenite grain size, by preventing migration of austenite grain boundaries. The nitrogen-enhanced V–Ti steels exhibit excellent toughness for high heat inputs.