Numerical Simulation Research On Die Wear During The Thermal Process Of High Strength Steel

With the wide application of high strength steel(HSS) in the automobile body structure and covering parts, the lightweight of automobile has developed rapidly. As a key technique in the thermal forming process of HSS, hot stamping technology has become a hotspot for all the researchers. Research on forming die of HSS is also an important aspect of it. The service life of the mold has become a vital economic indicator to measure the final cost because of the expensive materials, complex structure, large machining difficulty and high precision during the whole process. Accurate prediction of the wear volume which is one of the major factors influencing the die life during the service process is so significant to optimize the mold structure and technology, to reduce the mold manufacturing costs and to improve the productivity.In this paper, boron steel B1500 HS produced by cold rolling was used as the sheet metal, and the typical thermal die steel H13 was used as the punch and the die. The wear between the sheet and the die was deeply researched during the hot stamping process of HSS based on a lot of experiments exploration and literature summary which included many important parameters. How the parameters such as temperature influenced the wear process was explained. The main results were shown as following:(1) The friction coefficient between the boron steel B1500 HS and the die steel H13 and the wear rate in different conditions were measured based on the pin-on-disc wear test. With the increase of contact pressure, the friction coefficient decreased approximately linearly. While the wear rate whose average value was 2.541×10-5 mm/(MPa·m) had little variation and it can be applied in the following wear model to calculate the wear volume.(2) The relationship between stress and strain of B1500 HS was obtained through the tensile experiments under different temperature at different tensile rates, and its constitutive formula was fitted out in mathematical methods combined with the Johnson-cook model. The formula would be applied in the following simulation as an important parameter.(3) A 2D thermo-mechanical coupled model of the U shaped parts was established during the continuous stamping process. According to the temperature field distribution of the die from the simulation results, the die temperature fluctuated regularly in the area of 50 oC to 150 oC during the whole process. Within this temperature section, the thermal physical parameters and stress-strain relationship of H13 steel at room temperature can be applied to simplify the model reasonably.(4) A die wear model of HSS during the thermal process was established with the help of the FEM software called Abaqus and the modified Archard model. The wear volumes of each node at the corner of the die were estimated based on the its displacement, contact stress and the wear rate. The common rules and characteristics that the severe wear zone at the corner was within the range of 10o~20o and 65o~75o were pointed out. At the same time, the relationships between stamping times, initial sheet temperature, mold clearance, sheet thickness, stamping velocity, blank holder force and the wear volume were qualitatively explained.