Surface and interior residual stress analysis of a deep-drawn 1180-MPa class ultra-high strength steel sheet with scratch marks

This study examined the residual stresses on the surface and in the interior of a deep-drawn cup made from a 1180-MPa class ultra-high strength steel sheet. A high residual stress causes a delayed material fracture, and an accurate evaluation of residual stress reduces failure risks. Nevertheless, the residual stress in the vicinity of the scratch marks, induced by galling in the stamping process, remains unclear. This study accordingly presented X-ray diffraction measurements of the considerable residual compressive stress associated with the scratch marks. Measurements obtained using the hole drilling method revealed a previously unknown high residual tensile stress (exceeding 1000 MPa) in the interior at the scratch mark locations, corresponding to a 0.1-mm-thick compressive zone. These measurements were then compared with the results of a finite element (FE) simulation of the deep drawing process performed without the formation of the scratch marks; the simulated residual stress in the scratch mark region considerably deviated from the measurements. A compressive stress of approximately 1000 MPa was therefore imposed on the local surface area to account for the formation of the scratch marks. The prediction of the actual residual stress on the surface and in the interior of the scratch mark region was then discussed. A simple surface stress modification technique for scratch mark prediction was realized from the comparison between the measured and FE simulation results. The stored friction work, modified by the equivalent plastic strain induced by deep drawing, could be adopted to accurately predict the location of the scratch mark region.