Abstract
Role of microstructures on the improved fracture toughness has been analyzed for the heat affected zone of weldment in Ti-killed steel. A microstructural features of Ti-killed steel is decrease of coarse bainitic ferrite, αB°, and increase of the mixed area of αB°and quasipolygonal ferrite, αq, besides the presence of intragranular ferrite. The brittle fracture initiation is not from large particles or inclusions, but at the intersections of small αB° grains with different orientations in a mixed area of αB° and αq in the proximity of boundary between coarse αB°. Ductile crack growth resistance represented by the slop of R-curve is also higher in Ti-killed steel compared with Al-killed steel. Together with observation of deformed microstructures, it is discussed that the decrease of coarse αB° grains in Ti-killed steel suppresses brittle fracture initiation by reducing plastic constraint and the increase in the mixed area of αB° and αq enhances ductile crack growth resistance by accommodating large strain.
