Experimental and Numerical Evaluation of the Heat Transfer Coefficient in Press Hardening

When producing thin ultra high strength steel components with the press hardening process, it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components where it is of great importance to accurately predict the final component properties early in the product development process. The transfer of heat is a key process that affects the evolution of the mechanical properties in the product and it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulations. In this study an experimental setup is developed combined with an elementary inverse simulation approach to predict the interfacial heat transfer coefficient (IHTC) when the hot blank and cold tool are in mechanical contact. Different process conditions such as contact pressure and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation, a thermo-mechanical coupled simulation model is used with a thermo-elastic-plastic constitutive model including effects from changes in the microstructure during quenching. The results from simulations give the variations of the heat transfer coefficient in time for best match to experimental results. It is found that the pressure dependence for the two materials is different and the heat transfer coefficient is varying during quenching. This information together with further testing will be used as a base in a future model of the heat transfer coefficient influence at different conditions in press hardening process.