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

Abstract

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.