Hierarchical B2 Architecture Enabling Exceptional Strength–Ductility Synergy in Fe–Mn–Al–C–Ni Lightweight Steel

Fe–Mn–Al–C lightweight steels are promising structural materials, but their performance is often limited by insufficient work hardening and premature failure associated with κ‐carbides. Here, a hierarchical B2 design strategy is developed in a Ni‐alloyed Fe–Mn–Al–C steel by increasing Al content and tailoring annealing conditions. Three B2 morphologies, i.e., banded B2, grain‐boundary B2 (GB‐B2), and intragranular B2 (IG‐B2), are stabilized within a fully recrystallized austenitic (Rex‐γ) matrix. The hierarchical structure arises from the controlled sequence of γ → B2 transformation and γ recrystallization, leading to increasing Ni/Al content and hardness from IG‐B2 to GB‐B2 to banded B2. During deformation, IG‐B2 particles deform compatibly with the matrix while impeding dislocation motion, sustaining work hardening, and delaying strain localization. In contrast, GB‐B2 particles act as crack initiation sites, whereas banded B2 mainly contributes to strengthening. The optimized microstructure achieves a specific ultimate tensile strength–total elongation product exceeding 8 GPa·g −1 ·cm 3 ·%, among the highest reported for Fe–Mn–Al–C–Ni steels. This work establishes a formation–deformation coupling mechanism and suggests a microstructural design strategy for improving the strength–ductility–density synergy of B2‐strengthened Fe–Mn–Al–C austenitic steels.