Enhanced strength-ductility-toughness synergy in an HSLA steel with multi-gradient ultrafine grained structure by adopting a two-stage rolling coupling inter-pass ultra-fast cooling process

In this work, a multi-gradient ultrafine grain (GUFG) structured high strength low alloy (HSLA) steel was successfully prepared by adopting a two-stage rolling coupling ultra-fast cooling process. The heterogeneous microstructure with gradients in grain size, shape, and texture across the thickness direction of the GUFG HSLA steel was characterized by large area electron backscattered diffraction (LA-EBSD) technique, and the quasi-static tensile properties, dynamic Charpy impact toughness, and its anisotropy were evaluated at ambient temperature. Results show that optimal matching of strength and ductility can be attained in the GUFG steel compared to the ultrafine grain (UFG) and coarse grain (CG) uniformed grain-sized structures. Furthermore, the GUFG specimens have a significantly enhanced toughness under dynamic impact loading, which was, respectively, 1.6 and 5.9 times that presented in uniform UFG and CG structures when impacted along the rolling direction. The refined grains and synergistic deformation of heterogeneous microstructures can contribute to the high strength without compromising ductility and toughness in GUFG specimens. In addition, the GUFG, UFG, and CG structures all exhibit obvious anisotropy in mechanical properties, which originate from distinct microstructure, texture, and delamination characteristics. This work not only reveals that the quasi-static/dynamic fracture resistance can be markedly improved by heterogenous structure-design strategies but also provide a realistic preparation technology for the exploitation of superior steel for engineering applications.