Hot stamping components with 1500 MPa ultra-high strength are obtained by press hardening during hot stamping, and the properties depend on the microstructures. It is very important that the microstructure evolution rule is found out during hot stamping process. To characterize the microstructure evolution during hot stamping, a method combining finite element and experiment is carried out. Samples were heated to 950˚C and held for 300 second at a induction heating furnace, then taken out from the furnace and stayed in the air at different time (7 s, 11 s, 13 s, 22 s), respectively, finally the specimens were formed and quenched at a die. Microstructural observation as well as surface hardness profiling of formed specimens was performed. And the numerical simulation to predict the austenite transformation into ferrite, pearlite, bainite, and martensite and the volume fraction of each phase during the hot stamping process was made with ABAQUS software. The results show that the ferrite is observed when the specimen stays in the air for 22 s, and the temperature drops to 325˚C when the dwell time increases from 7 s to 22 s. The results of numerical simulation and experimental results are in good agreement. So the method finite element can be used to guide the optimization of hot stamping process parameters.

In this paper, we present the effect of austenitizing temperature on the work hardening behavior of air-hardening steel LH800 by evaluating the influence of austenitizing temperature on microstructure evolution and mechanical properties, using Hollomon, Differential Crussard–Jaoul (DC-J), and Modified C-J (MC-J) work hardening models. The results reveal that with an increase in austenitizing temperature, there is an increase in the percentage of martensite, along with an increase in the strength and hardness of the LH800 steel; on the other hand, there is a decrease in the plasticity. Austenitized at 825 °C, LH800 steel exhibits its highest strength and good plasticity, with a tensile strength of 897 MPa and an elongation of 13.6%. The comparison between the three strain hardening models revealed that the Hollomon model was the finest fit for the experimental data utilized and could illustrate the work hardening behavior of LH800 steel most suitably. This model manifests a two-stage work hardening mechanism; the first stage is related to the plastic deformation of ferrite phase, while the second stage deals with the co-deformation of ferrite and martensite/bainite phase. As austenitizing temperature increases, the work hardening ability of LH800 steel diminishes at each stage, the transition strain decreases, and the plastic deformation of martensite starts earlier.