Low-temperature toughening mechanism of layered ferrite-martensite steel based on grain boundary synergy and crack deflection model

Traditional marine steel suffers from a ductile-brittle transition temperature, compromising its performance in extremely low temperatures. To enhance low-temperature toughness, research has focused on material heterostructures. A novel marine steel featuring a ferrite-martensite lamellar structure through critical quenching in the dual-phase region and an 85% reduction warm rolling process on quenched and tempered steel was successfully developed. The formation mechanism of this layered structure and its impact on low-temperature toughness were systematically analyzed using multi-scale characterization and mechanical property testing. Findings reveal that the layered heterostructure markedly improves the low-temperature toughness of the steel while preserving strength and plasticity, evidenced by an increase in Charpy impact energy at -60 °C by 81.35 J. The enhancement in low-temperature toughness of the tested steel is primarily attributed to grain refinement: Warm rolling markedly refines the grain structure, increasing the high-angle grain boundary density (> 15°) from 0.4 to 3.1 μm^-1. This dense grain boundary network effectively impedes crack propagation, enhancing fracture resistance. Additionally, the ferrite-martensite lamellar structure imparts significant anisotropic characteristics, resulting in a layered structure effect. A distinct orientation difference distribution exists between directions perpendicular and parallel to the rolling direction. This unique microstructure increases the tortuosity of the crack path, significantly boosting low-temperature impact toughness. The lamellar heterostructure notably improves the toughness of the steel with minimal plasticity loss, offering a potential design strategy for optimizing the mechanical properties.