Partitioning Temperature Dependence of Microstructure and Mechanical Property Balance in Q-P-Treated 300M Steel

300M ultra-high-strength steel for critical load-bearing components such as aircraft landing gear requires a better balance of strength, ductility, and toughness. However, the effect of partitioning temperature on the microstructural evolution and mechanical property balance of Q-P-treated 300M steel under a fixed interrupted quenching condition remains unclear. In this work, 300M steel was subjected to quenching–partitioning treatment with interrupted quenching at 220 °C for 300 s, followed by partitioning at 250–400 °C for 1 h. As the partitioning temperature increased, the yield strength and ultimate tensile strength decreased from 1599 MPa to 1499 MPa and from 1987 MPa to 1801 MPa, respectively, whereas the elongation to failure and impact toughness increased from 11.52% to 16.50% and from 240 kJ·m−2 to 271 kJ·m−2. The microstructure remained lath-martensitic throughout, while higher partitioning temperature promoted martensite recovery, reduced dislocation density, and caused precipitate coarsening. Retained austenite remained mainly between martensite laths and exhibited both morphology variation and a non-monotonic diffraction response. Within the investigated window, partitioning at 350 °C gave the most favorable combination of strength, ductility, and impact toughness. These results establish the partitioning temperature dependence of microstructural evolution and mechanical property balance in Q-P-treated 300M steel, and provide guidance for heat treatment optimization.