Nano grains-induced high tensile strength-plastic strain synergy in martensitic steel

The plasticity is always detrimentally affected by the presence of the delta-ferrite (δ) phase in martensitic steels. This work presents a novel processing strategy employing successive warm-rolling, cold-rolling, and low-temperature annealing to achieve almost complete dissolution of the δ phase in a martensitic steel containing Fe-12Cr-11Ni-1.1Mo-1.7Ti-0.3Al-0.01C (in wt.%). This novel δ-free martensitic steel is primarily composed of nanoscale α′ martensite accompanied with high-density dislocations, demonstrating a ∼4-fold increase in plastic strain (4.82±0.19%) while maintaining an enhancement of ∼36.4% in ultimate tensile strength (1855±18 MPa) compared with its δ-containing counterpart fabricated via the conventional method. Nano-grains deformation induces a substantial dislocation accumulation at delayed strains (i.e., >2%), contributing to a sustained work-hardening capacity and thus preventing early necking. This mechanism allows for greater dislocation-mediated deformation, enabling the high-density dislocations to exhibit an outstanding plasticizing capability while maintaining significant dislocation strengthening.