The effect of B on microstructure and toughness at the heat affected zone after welding thermal cycle simulations has been investigated in steels containing titanium oxide particles. The titanium oxide is identified as Ti₂O₃ with cation vacancies, which contribute to preferential nucleation of MnS and TiN precipitates on Ti₂O₃. The Mn-depleted zone is formed around Ti₂O₃ after the precipitation of MnS. The Ti₂O₃, particles with TiN and Mn-depleted zone act as preferential nulceation sites for intra-granular ferrite. The segregation of B at austenite grain boundaries suppresses effectively the nucleation of grain boundary ferrite. Ferrite nucleation at the interface between Ti₂O₃ and austenite matrix is not affected by B addition because of B-depleted zone arisen from the diffusion of segregated B into Ti₂O₃ via cation vacancies. Consequently the B addition to steels with Ti₂O₃ promotes the formation of fine intra-granular ferrite grains so that the toughness in heat affected zone is improved even after large heat input welding.