Role of nano-bainite laths and nanosized precipitates: Strengthening a low-alloy steel to 1870 MPa

Strength and ductility trade-off is a dilemma in the field of metallic materials. In this study, the ultrafine-grained low-alloy steel was prepared using hot rolling, warm rolling, and bainitic treatment processing. The obtained steel has an average grain size of 800 nm and bainitic lath thickness of ∼350 nm, accompanied with dense nanosized precipitates. Hence the steel exhibits ultrahigh yield strength and ultimate tensile strength of 1550 ± 30 MPa and 1870 ± 50 MPa, respectively. The heterostructure consisting of numerous nanoscale bainitic laths and dense nanosized carbides is conducive for storing geometrical necessary dislocations, thus enhancing the heterostructure-deformation-induced hardening. In the deformed steel, the dislocation density is as high as 4.68 × 10 15 m −2 . In addition, the thickness of bainitic laths is the dominant factor responsible for the ultrahigh strength of the steel. In situ tensile testing-digital image correlation images demonstrate that the higher dense dislocations like to pile up at the interfaces than the interior of grains. The alternative arrangement of hard bainite and soft ferrite is helpful for promoting uniform deformation and delaying premature fracture. Thus, the optimal strength-ductility combination is achieved.