To explore the effect of inclusions in the coarse-grain heat-affected zone (CGHAZ), four groups were designed using the ‘Al/Ti deoxidised + Nb/V microalloyed’ technical route. The toughness of the CGHAZ of Al–Ti–V–N steel was greatly promoted by the presence of polygonal ferrite (PF) after high heat input rather than the traditionally used acicular ferrite (AF). Furthermore, micron-sized inclusions in AF or PF were analysed, and the results indicated that the TiN and V (CN) particles attached on the Ti/Al oxide inclusions induced the nucleation of AF and PF. That can be concluded that the nucleation mechanism of PF is similar to that of AF. In addition, smaller micron- and nanoscale-size inclusions observed by transmission electron microscopy could not induce the nucleation of AF and PF because of the low heterogeneous nucleation energy. Nevertheless, these inclusions can have a drag effect on grain boundary migration.
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