The influence of preformed hole machining processes on the hole expansion limit of high-strength steel CP800

In this study, the effects of various preforming processes, including punching, WEDM (Wire Electrical Discharge Machining), laser cutting, and milling, as well as different processing speeds, on the stretch-flangeability of CP800 steel sheets were evaluated using the hole expansion test (HET) method. The cut edge characteristics of CP800 steel under different processing conditions were analyzed in terms of surface morphology, roughness, and burr formation. The influence mechanisms of various processes and cutting speeds on the stretch-flangeability of CP800 steel were examined in conjunction with the two-dimensional hardness distribution of the cut edges. A comparative analysis of crack propagation behavior along the hole walls in specimens subjected to different process conditions was also conducted. The experimental results demonstrate that laser cutting, WEDM, and milling significantly improve the stretch-flangeability of CP800 steel compared to traditional punching processes. Furthermore, adjusting the cutting speed leads to a more pronounced enhancement of stretch-flangeability. Among the four processing methods investigated, the impact of surface roughness on the hole expansion ratio (HER) was found to be less significant than that of surface hardening and burr formation. However, the influence of the shape and orientation of surface defects on the presence of crack initiation and extension behavior of the material cannot be ignored. Additionally, the punched specimens exhibited typical brittle fracture with multiple through-thickness cracks, while other processing methods better preserved material toughness, resulting in only a single through-thickness crack.