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For 4Cr2Mo2V hot stamping die steel, the isothermal fatigue test was suspended at different cycles, and the stress-relief annealing was carried out in situ. After annealing, the fatigue tests were continued. The stress-relief annealing was carried out at 30%, 50% and 70% of the cycle life when the total strain amplitude was 0.5%, and the fatigue life was increased by 30.27%, 23.84% and 10.24% respectively. On the basis of Basquin-Coffin-Manson life prediction model, by adding the influence of stress-relief annealing on life, obtained the stress-relief annealing life prediction model. The dislocation density, microstructure and pole figure were analyzed by TEM and XRD to explore the mechanism of stress-relief annealing on prolonging fatigue life. After stress-relief annealing, the full width at half maximum (FWHM) decreased, and the dislocations originally piled up at the grain boundary moved into the grain, delayed the time of dislocation cell or wall formation. At the same time, it could also eliminate the trend of consistent orientation of most grains in the fatigue process. This is the main reason why stress-relief annealing prolongs fatigue life.

In this paper, the alternate immersion corrosion test of Cr-Mo-V series SDCM steel for hot stamping was carried out, and different stresses were loaded with self-made fixture. The results shown that regardless of hardness and stress, the corrosion mode of the material is uniform corrosion. Stress could significantly increase the corrosion rate, with lower hardness and higher corrosion rate. Because of the existence of Corrosion Removal Layer (CRL), the maximum corrosion pit depth would be reduced. The maximum corrosion pit depth and Corrosion Pit density (CPD, ρv) were used to describe the degree of corrosion damage. From low to high hardness, the CPD ρv and corrosion resistance increased gradually. With the increased of tempering temperature, the hardness decreased, and the percentage of carbide area in the field of view increased from 16.36% to 24.32%. The irregular spherical carbide M23(C, N)6 rich in Cr coarsens and consumes Cr element in the material, which lead to the decrease of corrosion resistance. Through the polarization curve of the dynamic potential, we known that the current density was increased with the hardness decreased, from 28.53 μA/mm2 to 40.93 μA/mm2.

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