Research On Hot Stamping Of High Strength Boron Steel With Different Cooling Zones

The general development trend of automotive industry is energy-saving, environmental protection, safety and comfort. Weight lightening is one of the most effective ways to achieve the goal of energy conservation. Design and manufacturing of mixed strength parts is one of the most efficient methods for weight reduction. This kind of parts can adapt complex load profiles in the event of a collision by a defined distribution of locally varying properties.In the present work, in order to produce a hot stamped part with tailored mechanical properties, hot stamping of high strength boron steel with different cooling zones are studied by the method of numerical simulation, microstructure-properties prediction and experiment. The study includes the following aspects and meaningful results are obtained.The hot stamping process of a U-channel with22MnB5boron steels is simulated by using a FE program, and a program based on an existing model is developed to predict the final microstructure and mechanical properties. Through the analysis, the temperature, microstructure and hardness distribution of the blank is obtained. The numerical simulation is evaluated by comparing simulation results with the U-channel hot stamping experiment. The numerically obtained temperature history is basically in agreement with corresponding experimental observation. Then on this basis, the hot stamping process of the U-channel with different cooling zones is simulated. The influence law of the thermal conductivity of the die material, the tool-blank heat transfer coefficient and the initial die temperature on the component’s cooling rate, microstructure and hardness distribution is obtained.Based on the above research, using the multipurpose tool developed by the author’s team, the method of elevating die temperature is used to perform the hot stamping with different cooling zones experiments and mixed strength samples are obtained. The influence law of the die temperature on the component’s hardness and the relation between the air gap width and the transition zone width are also studied. The research indicates that die temperatures of300℃and greater are necessary before any reasonable amount of softening occurs. In the400℃and500℃heated die trials, hardness measurements as low as300HV-240HV are recorded, which represents a reduction in hardness of35%～50%compared to the fully cooled trials.