含Tiオキサイド鋼における粒内フェライト変態におよぼすBの効果

抄録

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 vacanies, which contribute to preferential nucleation of MnS and TiN precipitates on Ti₂O₃. The Mn-depleted zone is formed around Ti₂O₃ after the MnS precipitation. The Ti₂O₃ particles with TiN and Mn-depleted zone act as preferential nucleation sites for intra-granular ferrite. The segragation of B at austenite grain boundaries suppresses effectively the nucleation of grain boundary ferrite. While 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 intragranular ferrite grains so that the heat affected zone toughness is improved even after large heat input welding.