Effect of Cooling Rate on Microstructure Evolution of Hot Forming High Strength Steel Based on Non-Isothermal Constitutive Model

Constitutive equations based on isothermal tensile test were applied when traditional researchers performed numerical analysis for mechanism of high strength hot stamping. However, dramatic thermal exchange occurs because of large contact area between temperature-elevated hot blank and cold die tools, causing it virtually a complicated non-isothermal process. Since the martensitic transformation strongly depends on minimum feasible cooling rate, influence of temperature change needs to be considered and new non-isothermal constitutive equation is necessary for numerical analysis of desired phase transformation. Firstly, using Al–Si coating high strength steel boron steel BR1500HS, non-isothermal uniaxial tensile tests were carried out on a hydraulic servo-simulator Gleeble1500. The quenchable boron steel sample had plastic deformation with different cooling rates beginning from 800 °C and ending at 700, 650, 550, 500, 450 and 400 °C, respectively. Optical microscope was used to study microstructural evolution. Different from conventional isothermal deformation, work-hardening types of high-temperature rheological curve were obtained, and new non-isothermal constitutive relationships were regressed to take into account the thermal–mechanical phase coupling effect. Secondly, numerical modelling of box-shaped parts hot forming was constructed based on the obtained new constitutive equations. The die tool’s cooling water flow rates varied at 0.1, 0.2 and 0.3 m/s. Their influences on the microstructure evolution and mechanical properties were analysed. Finally, experiments of typical box-shaped sheet metal hot stamping were conducted. Deep drawings of these sheets were developed with different cooling rates in order to simulate the actual industrial non-isothermal thermal–mechanical environment. Through comparison of experimental data and numerical results, the non-isothermal constitutive model of high strength steel has been verified. It provides new support for numerical optimization for quality improvement of hot stamping automotive components.