Influence of the Duration of Transfer From the Furnace to the Press During the Hot Stamping Process

During the hot stamping process a blank is first heated up 900 °C for 6 min 30 s. Then, the blank is transferred to the press where it is hot stamped. During the transfer, the blank is slowly cooled down by convection and radiation with the ambient air and metallurgical transformations can occur and modify the mechanical properties after hot stamping. Using samples with welded thermocouples, a method is presented to estimate the kinetic of the metallurgical transformations. The results are compared with the metallurgical analysis of samples that are quenched in the water after different transfer times. The influence of the thickness of the blank is also investigated. It is highlighted that using different thicknesses leads to different cooling rates and different resulting microstructures. The influence of the transfer time on the mechanical properties after hot stamping is highlighted with hardness measurements and tensile tests. Finally, a coupled heat transfer/metallurgical model is built to simulate the kinetic of the transformations and a diagram can be plotted that presents the evolution of the metallurgy as a function of the thickness and the transfer time. These results show the high importance of the transfer time from the furnace to the press and a tool is proposed to help the engineers to define a process window.