The Effects of Die Temperature and Cooling Condition on Friction Behavior of Bare 22MnB5 Boron Steel in Hot Stamping

Hot stamping is an effective method to improve both the elongation and plasticity of ultra-high-strength steels and thus a higher specific strength can be achieved, making weight reduction in motor vehicles possible. However, due to the die temperature rise and the cooling liquid within hot stamping applications, the friction in its forming stage is always a nonisothermal mode. Therefore, a high-temperature slider-on-sheet strip (SOSS) tribotester to simulate sliding friction in hot stamping of 22MnB5 hot stamping steel sheet was designed and custom-made. Then, it was employed to exploit the high-temperature friction behavior and mechanism of bare 22MnB5 hot-stamped boron steel sheets under different die temperatures and extra cooling conditions. Results show that the friction coefficient obtained under the pure die cooling conditions is lower than that obtained under extra cooling conditions, which is attributed to the formation of thicker oxide layers on the surface of specimen during the cooling process. In addition, the friction coefficient is obviously decreased when the die temperature is increased to 200 degrees C, which is ascribed to the co-action of the destruction of the oxide layers and the formation of bainite.