Abstract:Experiments were designed to manufacture square-box-shaped part, and the effects of hot stamping process parameters including blank holder force (BHF), forming temperature and tool temperature on the hot formation quality were investigated. Since the hot formation quality is highly sensitive to BHF, a BHF controlling system was developed using six hydraulic cylinders to improve the accuracy of applied BHF to ±10 N. The experimental results showed that a mixture microstructure of martensite and bainite with large fraction of martensite at forming temperature of 850 °C was obtained in the hot stamped part, while the microstructure was dominated by the softened phase of pearlite as the forming temperature decreased to 550 °C. The tensile strength was raised from 1550 MPa to 1750 MPa as the tool temperature decreased from 200 °C to ambient temperature. The optimum BHF of 1.62 MPa was determined which can avoid the formation of drawbacks of wrinkle and crack.

Key words: hot stamping; high strength steel; martensite; microstructure; cooling system