Synergistic Refinement Mechanism of Carbides and Martensite in D2 Steel via Postforging Hot Rolling: Balancing High Hardness With Enhanced Toughness

Postforging hot rolling improves mechanical properties of D2 steel by optimizing M 7 C 3 carbides morphology and refining martensite laths. This study examines microstructural evolution under 15%, 25%, and 35% hot rolling reductions followed by heat treatments. The investigation traces M 7 C 3 carbides transformation from as‐forged to final heat‐treated states, revealing dual refinement mechanisms of both carbides and matrix microstructure. The result shows that postforging hot rolling significantly fragments M 7 C 3 carbides in D2 steel compared to conventional forging. After spheroidized annealing, the homogeneity grade of M 7 C 3 carbides improves from grade 3 in the as‐forged state to grade 1 for both 25% and 35% hot‐rolled specimens. The refinement mechanism is attributed to thermomechanical processing, wherein thermal activation enhances elemental diffusion and applied stress promotes carbides dissolution, reprecipitation, and fragmentation, resulting in simultaneous refinement and homogenization. This process refines prior austenite grains, reducing the tempered martensite lath width from 174.2 to 106.3 nm. With 35% reduction, the impact energy increases by 68% while maintaining a hardness of 60 HRC. The toughening mechanism combines M 7 C 3 network fragmentation, nanoscale dispersed M 23 C 6 precipitation, and multifaceted crack suppression by refined martensite laths. This study provides a reliable hot working process for balancing D2 steel's high hardness and toughness.