Optimizing the processing parameters of the 4Cr5Mo2VNb hot-work die steel: Constitutive analysis, hot-processing map, and an anomaly at 950℃

This study investigated the unusual temperature-dependent dynamic recrystallization (DRX) behavior of the 4Cr5Mo2VNb hot-work die steel to determine its optimal hot-processing parameters. Increasing the strain rate delayed the DRX onset at 1050℃–1150℃ but promoted earlier DRX at 950℃. Based on the flow-stress curves obtained from isothermal compression at 950℃–1150℃ and deformation at strain rates of 0.01–1 s −1 to a true strain of 0.693, a strain-compensated constitutive model was established, which accurately reproduced the rheological behavior and enabled the quantification of DRX kinetics. According to optical microscopy and X-ray diffraction results, the early DRX onset at low temperatures was caused by the augmented nucleation process of DRX at high strain rates. Electron backscatter diffraction analysis indicated that high strain rates and low temperatures increased local strain but did not facilitate the low-angle grain boundary–high-angle grain boundary transformation indicative of DRX. Transmission electron microscopy revealed that this behavior is linked to the microstructure. Continuous DRX (CDRX) at high temperatures transitioned into discontinuous DRX at low temperatures. At 1150℃, fully developed fine DRXed grains along with minimal NbC precipitates were observed, which explained the limited precipitation strengthening effect. Meanwhile, a high dislocation density was observed alongside NbC and fine VC precipitates at 950℃. A hot-processing map based on the constitutive model indicated optimal processing temperature and strain rate ranges of 1020℃–1150℃ and 0.01–0.1 s −1 , respectively. Under these conditions, a high power dissipation efficiency ( ∼ 0.4) and fully recrystallized microstructures were achieved