Synergistic enhancement of strength, ductility, and toughness in 2.3 GPa medium-Mn steel via low-temperature forging-induced lamellar nanostructures

Overcoming the strength–ductility–toughness trade-off in ultra-high-strength steels remains a major challenge for aerospace applications, especially for aircraft landing gear steels. In this work, a V–Nb–Mo microalloyed medium-Mn steel was designed, and a lamellar martensitic microstructure with an excellent strength–toughness synergy was obtained through low-temperature forging followed by quenching, deep cryogenic treatment, and tempering. Lowering the forging temperature promoted prior austenite grain (PAG) refinement, which favored a close-packed (CP) group-dominated martensite variant selection mode and significantly increased the density of high-angle grain boundaries (HAGBs). In addition, owing to phase transformation inheritance, martensite laths were arranged preferentially along specific crystallographic orientations, resulting in a highly oriented lamellar structure. This unique microstructure enhanced strength and crack resistance through grain refinement and interfacial strengthening. More importantly, it promoted coordinated plastic deformation by facilitating long-range dislocation glide and a more homogeneous dislocation distribution, thereby alleviating the ductility loss induced by excessive dislocation accumulation. Consequently, the low-temperature forged specimen (L) achieved a yield strength of 1934 MPa, an ultimate tensile strength of 2292 MPa, a total elongation of 17%, and a room-temperature impact toughness of 46.46 J/cm², outperforming the widely used 300M steel. These findings clarify the strengthening and toughening mechanisms of the lamellar martensitic structure and provide guidance for the design of next-generation high-performance landing gear steels.