Microstructure Prediction Of Hot Stamping And Research On S-rail Tailored Die Quenching

Hot stamping components are widely used in automobile industry because of high strength and hardness. Their applications are critical for lightweight of automobile. However, the elongation of hot stamping parts is lower, so they have problems in the energy absorption. Therefore, a new forming technology is derived from the origin hot stamping technology, named as tailored tempering process. In this technology, it is possible to reduce cooling rates in local regions in order to make the microstructure transforming to softer phase and manufacture components with tailored mechanical properties.In the process of hot stamping, phase transformation during quenching determines the final mechanical property, so it is important for tailored tempering process to correctly predict the phase transformation. In this paper, three models were programmed to predict the volume of each microstructure and the final hardness, and were compared to experimental results. Secondly, a numerical simulation of S-rail tailored die tempering quenching was carried out, and the influence of temperature on cooling rates and Vickers hardness distribution was analyzed. Furthermore, the hardness in inner fillet and outer fillet was compared so that the stress state’s influencing was obtained. Lastly, according to S-rail simulations, corresponding experiments were done by using the S-rail mold that was designed by our team. And the results from experiments were compared to simulation results.The conclusions prove that:(1) The phase fraction and hardness values of A-O model、Li model�'�K-V model for cooling rates from 1 ℃/s to 100℃/s were successfully predicted. Compared to experimental results, Li model shows the best prediction, K-V model provide maximum deviation, and A-O model is between them.(2) The higher the tool temperature is, the lower the cooling rates and hardness results become. The cooling rates at 400℃/s is declined to 16.7℃/s from 44.4℃/s at 20℃. When the tool temperature is up to 400℃, hardness is reduced about 40.6% from 480HV at 20℃to 285HV. And, with the increase of tool temperature, the forming of martensite is restrained while the forming of bainite is promoted.(3) The trends of hardness gradient of experiments and simulations are in quiet good agreement. The simulation model and related theories are certified to be practicable.(4) The stress state has great influence on hardness distribution in inner fillet and outer fillet. The tensile stress is contributed to martensite transformation, while the compressive stress forbids the forming of martensite. So the hardness value in inner fillet which is under tensile stress is higher than that in outer fillet which is compressed during forming.