Crystallographic study on deformed bainite structure of ultra-high strength steel and its relationship with strength and ductile-brittle transition temperature

Abstract

• Variation of strength and DBTT in thickness direction of medium plate is elaborated. • The surface can obtain higher strength and lower DBTT than the core. • Surface can weaken variant selection and refine crystallographic packet and block. • Dominant Bain group forms in the core, resulting in low density of grain boundaries. • Driving force of bainite transformation is the guarantee to weaken variant selection. The significant contribution of deformed bainitic structure to the strength and ductile-brittle transition temperature (DBTT) of ultra-high strength steel was investigated from the aspect of crystallography in order to elucidate the mechanism responsible for the variation in strength and toughness between surface and core. The surface has yield strength of 860 MPa, tensile strength of 913 MPa and DBTT of −110 °C. However, lower strength (yield and tensile strength: 721 and 835 MP) and higher DBTT (−84 °C) appeared in the core, although the ductility changed little. Visualization and quantification of crystallographic structures show that weakening of variant selection occurs on the surface, inducing the staggered configuration of different Bain and CP (closed-packed plane) groups, and then promoting the formation of high density of various grain boundaries conducive to both of toughness and strength. The insufficient transformation degree inferred from the retained austenite leads to the strengthening of variant selection in the core, resulting in the formation of dominant Bain group and reduction of various grain boundary density.