Abstract: Microstructure and mechanical properties of different hot formed parts based on a body-in-white were systematically studied. The results show that the microstructure of the typical hot formed parts is mainly lath martensite, and some parts have residual austenite structure in local areas. The microstructure of the soft zone in the mold is mainly composed of ferrite, martensite and a small amount of bainite, where the martensite has no obvious lath characteristics and relatively low proportion. The decarburization layer thickness of the left front/rear door collision beam is distributed between 38.55-45.57 μm. The average force value of the three-point bending of the left front door anti-collision beam is 6962 N, and the maximum force value is 9709 N; while the average force value of the left rear door anti-collision beam is 8740 N, and the maximum force value is 11 392 N. The surface of the parts is in good condition and there are no cracks generate after three-point bending. The performance of the anti-collision beams meets the requirements of vehicle safety collision testing. For the typical hot formed parts, the average hardness distribution is 456-507 HV10, the yield strength is greater than 1000 MPa, the tensile strength is greater than 1400 MPa, and the elongation is ≥5.5%. While in the soft zone, the hardness distribution is between 225-243 HV10, the yield strength is greater than 470 MPa, the tensile strength is greater than 650 MPa, and the elongation is ≥17%. The performance of the parts meets the standard requirements.

Key words: body-in-white, hot forming, microstructure, mechanical properties