Study on Microstructure and Properties of Continuous Cooling Phase Transformation for 1800 MPa Hot Stamping Steel

On the basis of the traditional 1500 MPa hot stamping steel 22MnB5, the research and development of 1800MPa hot stamping steel and tissue performance evaluation research of the sample were carried out through the composition optimization. Thermal expansion instrument was used to measure the curve of temperature-expansion at different cooling rates of 1800 MPa experimental steel. Combining SEM microstructure and microhardness, the cooling phase transition law of the experimental steel under different cooling rates was obtained, and the CCT curve was plotted. The results show that when the cooling rate is less than 1℃/s, the microstructure contains ferrite and pearlite. When the cooling rate is increased to 2.0 ℃/s,partial bainite appears in the microstructure. When the cooling rate is greater than 10 ℃/s, the microstructure is all converted to martensitic, and the hardness of the experimental steel substrate increases to more than 500 HV, and no significant changes occur later. According to the above results, the hot forming trial production and evaluation of bumper bar samples were carried out. The tensile strength of the hot forming part of the experimental steel reaches 1851 MPa, which is 21.3% higher than that of 22MnB5（1525 MPa）, and its elongation and bending performance are good. Compared with that of the 22MnB5 parts, the maximum deformation of the hot-formed bumper is reduced by 32%, and its maximum load is increased by 20%.