Heat transfer in hot stamping of high-strength steel sheets

This paper focuses on the determination of the heat transfer coefficient (HTC) between blanks and dies in the hot stamping of high-strength steel sheets. Experiments to record the temperature evolution inside the dies were carried out on custom-built test equipment, where process conditions typical of hot stamping could be reproduced, but avoiding any deformation of the metal sheet. The HTC was determined by using an inverse analysis procedure, based on the comparison between temperature measured inside the dies and temperatures calculated by means of a numerical model of the test. HTCs between the steel blank and dies made of different materials were identified, showing that the use of ceramic inserts can modify the cooling rate inside the blank in order to avoid a complete martensitic transformation. Moreover, the influence of the contact pressure was investigated, proving that HTC dependence on this factor is not negligible.