Heat transfer coefficient calculations between tool and boron steel blanks for HPF

Abstract

Hot press forming technology based on austenite to martensite transformation has been used to manufacture automobile parts. To predict final microstructure and mechanical property of the hot press formed part accurately, determining heat transfer coefficient is important. Hot pressing experiments with 22MnB5 blank sheets and flat-shaped die were conducted to calculate proper heat transfer coefficients. Initial die temperature, pressure, and coating material affected the heat transfer coefficients. Temperature histories, measured with thermocouples inside the blank and the die, were used for heat transfer calculations. Because the thermocouples did not measure the surface temperature, the determination of heat transfer coefficients required solution of an ill-posed problem. One of the inverse heat conduction method, named Beck’s function specification, was used for this ill-posed problem. The peak of heat transfer coefficients (HTCs) decreased with increasing initial blank temperature below 700 ℃ due to the low temperature difference between the blank and the die. However, the HTC peak increased for blank temperature over 700 ℃ because of the increased austenite fraction, which is a dominant factor. At higher holding pressure, higher peak HTC and lower time interval to reach the maximum value were obtained. Coarser surfaces lead to lower HTC because of the larger air gap distance between blank and die surfaces.