Experimental study on strain hardening of steel panels in the hemming process

In the production of automotive door panels and other types of panels, it is important to clarify the mechanism of the bending phenomena of the outer panel during the flanging, pre-hemming and hemming processes. This paper reports an experimental study of a planar straight-edge-shaped hot-dip-galvanised steel sheet panel that was set and pressed in prototype equipment. The pressing was performed under dry contact of the flange end on the punch surface, and the study included the processes of flanging to 90 degrees, pre-hemming at 15 degrees, and hemming. The load profiles vs. punch movement distance and time-lapse images captured from videos of the processes were used to analyse the panel shape changes. The Micro Vickers hardness distribution and scanning electron microscope images of the lateral surface of the panel were also measured. A detailed examination of the obtained results gives an industrially useful guideline for panel hemming drawn from new findings wherein both the microhardness and morphology changed during these processes.