Study on the Effect of Co Microalloying on the High‐Temperature Tensile Behavior of 4Cr5Mo2V Hot Work Die Steel

The hot work die steel 4Cr5Mo2V has insufficient high‐temperature strength, making it difficult to meet advanced manufacturing requirements. Herein, a novel modified 4Cr5Mo2V‐Co steel with 0.5% Co addition is designed and subjected to tensile tests at room and high temperature, and the precipitation formation and microstructure evolution are characterized by optical microscopy, scanning electron microscopy, electron backscatter diffraction, X‐ray diffractometer, and high‐resolution transmission electron microscopy. The results show that Co microalloying increases the strength of 4Cr5Mo2V steel while slightly decreasing its plasticity with temperatures ranging from 25 to 700 °C. The increased high‐temperature strength of 4Cr5Mo2V‐Co steel is mainly attributed to dislocation and precipitation strengthening. This can be attributed to the addition of Co lowering the stacking fault energy of bcc grains, preventing dislocation climb and cross slip, thus increasing the dislocation density. In turn, the dislocations provide more nucleation sites for M 2 C phase precipitation, promoting more dispersed nanoscale M 2 C phase formation. During high‐temperature tensile deformation, the recrystallization softening phenomenon of 4Cr5Mo2V steel is more pronounced due to the presence of continuous dynamic recrystallization.