Chengxi Lei

Hot formation quality of high strength steel BR1500HS for hot stamping without cooling system

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.

Predictions of the Mechanical Properties and Microstructure Evolution of High Strength Steel in Hot Stamping

Hot stamping is an innovative operation in metal-forming processes which virtually avoids the cracking and wrinkling of high strength steel (HSS) sheets. Examining the phase transformation and mechanical properties of HSS by means of experiments is challenging. In this article, a numerical model of the hot stamping process including forming, quenching, and air cooling was developed to reveal the microstructure evolution and to predict the final mechanical properties of hot-stamped components after multi-process cycles. The effects of the number of process cycles and the holding times on the temperature of HSS were examined using the model. The microstructure evolution of HSS under variable holding times is illustrated. The mechanical properties, particularly hardness and tensile strength, were predicted. It was found that the martensitic content increased with increasing holding time, and the martensitic content of the formed component at the flange and end was higher than for the sidewall, and lowest for the bottom. The hardness trend was consistent with the martensitic content. After six process cycles, the predictive errors of the model for hardness and tensile strength were acceptable for practical applications in engineering. Comparison between the predicted results and the experiment results showed that the developed model was reliable.