Strength-toughness enhancement in sub-micron heterogeneous lamellar ferrite-martensite dual-phase steel at −84 °C extreme low-temperature conditions

• A novel quenching–intercritical quenching process produced sub-micron heterogeneous lamellar ferrite–martensite (HL) dual-phasel. • At –84 °C, HL steel shows 1007 MPa UTS, 24 % elongation, and 187 J impact energy. • Sub-micron HL steel improves toughness via uniform HAGBs/voids and crack resistance. This study proposes a novel quenching- intercritical quenching (Q-IQ) process to fabricate a ferrite-martensite dual-phase steel with sub-micron heterogeneous lamellar (HL) structures. The final HL microstructure exhibited a uniform lamellar distribution of ferrite (45.3 %, average width 0.39 μm) and martensite (54.7 %, average width 0.37 μm), along with a relatively high fraction of high-angle grain boundaries (HAGBs, 42.8 %). Mechanical testing at –84 °C demonstrates superior performance, with an ultimate tensile strength (UTS) of 1007 MPa, total elongation (El) of 24 %, and impact energy (Akv) of 187 J. Microstructural analysis indicates that the sub-micron heterogeneous lamellar architecture structure promotes a more uniform stress distribution and promotes cooperative deformation between soft and hard phases. The nucleation of voids is not limited to the hard martensitic phase, and the interfaces remain intact, which significantly improves the cooperative deformation between the soft and hard phases. Sustained work hardening at –84 °C was enabled by cooperative deformation and heterogeneous deformation-induced (HDI) hardening. Additionally, the sub-micron heterogeneous lamellar structure promotes a high-volume fraction of HAGBs and facilitates the uniform distribution of both HAGBs and voids, thereby enhancing the impact toughness at − 84 °C.