Making low-alloyed steel strong and tough by designing a dual-phase layered structure

As a challenge to avoid the strength-toughness trade-off for high-strength steel, strength and toughness are contradictory, especially at low temperatures. This study proposed a new strategy to evade this trade-off dilemma by designing an ultrafine-grained (UFG) ferrite/martensite (F/M) layered structure in low-alloyed steel. Compared to the quenched and tempered steel with a martensitic microstructure, the present UFG F/M layered microstructure offers a remarkable increase by 9.5 times and 30 times of room temperature (RT) and cryogenic impact energy (401 J at RT and 245 J at 77 K), respectively, while the strength is not sacrificed (tensile strength of 1.67 GPa). The UFG microstructures, high fraction of martensite and strong strain hardening render the steel high strength. The excellent cryogenic toughness of the current steel mainly can be ascribed to plastic deformation and steady-state crack propagation absorbing energy, by which the UFG F/M microstructure suppresses strain localization and rapid shear-band propagation during impact loading and thus renders the steel good plastic deformation ability and high toughness. Furthermore, the grain refinement also contributes to the cryogenic toughness enhancement. Particularly, it was experimentally confirmed that the martensite layer should be refined to a critical value (∼620 nm) to make the dual-phase layered steel tough at LNT (liquid nitrogen temperature); such thinner martensite layer favours larger bending fracture strain and better effect of suppressing strain localization. This strategy may provide a new economical route to produce high-strength engineering materials for cryogenic application.