Study on the prediction of tensile strength and phase transition for ultra-high strength hot stamping steel

Abstract

The accurate prediction of mechanical properties and phase microstructures is of great significance to the design and development of the ultra-high strength hot stamping steel (UHSHSS). Based on the prediction of JMatPro and the experimental measurement, an accurate modified prediction model of JMatPro for the tensile strength of UHSHSS was proposed in the study, considering the influence of austenitizing temperature. At the same time, the prediction model of phase transition in quenching process of UHSHSS was established. The tensile strength and phase transition models of UHSHSS under different conditions were studied using the prediction models. Results showed that the tensile strength of UHSHSS increased with the increase of C content and decreased with the increase of Si content. For every 0.01 wt% increase in C content, the tensile strength increased by around 30 MPa. For the hot stamping process used in this study, the carbon content of hot stamping steel should be more than 0.35 wt% to achieve 2000 MPa in tensile strength. The volume fraction of the residual austenite in UHSHSS increased with the increase of C and Si content. During the quenching process, the phase transition in UHSHSS was the fastest between 4 s and 8 s, and the average decomposition rate of the austenite reached 17.8% per second. The average volume fraction of residual austenite and martensite in UHSHSS was 3.3% and 96.7%, respectively, when the quenching die temperature was 150 °C. With the increase of the die temperature, the volume fraction of residual austenite in steel increased.