Abstract: Spheroidized microstructure of modified 4Cr5Mo2MnV1Si die-casting module steel didn't meet the technical requirements by the traditional spheroidizing treatment of “residual heat annealing+normalizing+isothermal spheroidizing annealing”, then the traditional spheroidizing process was improved, and the microstructure and hardness of the specimens treated by the improved process were tested. The results show that after residual heat annealing+normalizing+isothermal spheroidizing annealing, and then heated at 1010 ℃ for 0.5 h by furnace cooling to different temperatures (820, 790 and 760 ℃) held for 1 h by air cooling, the microstructure of the steel is lath martensite and carbide particles uniformly dispersed on the matrix, but the hardness is higher than 400 HBW, which can not meet the requirement of spheroidizing structure with hardness less than 240 HBW. However, when heated at 1010 ℃ for 0.5 h air-cooled to room temperature and then tempered at 820, 790 and 760 ℃ for 1 h, the microstructure is equiaxed ferrite with particle-like carbides uniformly distributed, the hardness is 321, 235 and 245 HBW respectively. And the tempering effect at 790 ℃ is the best, the spheroidization grade reaches GB3, the hardness is less than 240 HBW. In conclusion, the spheroidizing treatment after forging of the modified 4Cr5Mo2MnV1Si die-casting module can be realized by using residual heat annealing+normalizing+high temperature tempering (790 ℃) instead of residual heat annealing+normalizing+isothermal spheroidizing annealing.

Key words: die-casting module steel, spheroidizing treatment, spheroidized structure, brinell hardness