Effect of punch taper angle and first hole diameter on hole expansion ratio of high strength dual phase and martenzite steels

Advanced high-strength steels (AHSS) both increase the safety of the manufactured vehicles due to their high strength and allow them to be produced in lower thicknesses. The decrease in thickness played an important role in the reduction of the vehicle's weight and thus the improvement of fuel economy. Due to its high strength-to-weight ratios, it has attracted a lot of attention in the automotive industry in recent years. However, the high strength of these sheets has revealed various forming problems. It is known that especially DP and MART type AHSS steels have a serious tendency to edge cracking errors during forming processes. The edge cracking behavior of sheet materials is often tested by expanding a fixed inner hole diameter of 10 mm with a 60° conical punch as specified in the ISO 16630 standard. However, in this study, in order to observe the edge cracking behavior of DP and MART steels, DP and MART sheet materials with three different initial hole diameters (14, 16, 18 mm) were drilled with conical punches at three different angles (30°, 60° and 90°) hole expansion tests (HET) were performed. The effects of conical punch angle and initial hole diameter changes on hole expansion rates of DP and MART sheets were investigated. According to the results obtained, it was observed that the hole expansion ratios (HER) of DP and MART steels did not change significantly with the conical punch angle and initial hole diameters. This situation was thought to be due to the low fracture strain of DP and MART steels. It was concluded that the most important factor in the edge cracking behavior of DP and MART steels is the microstructure composition rather than the cone punch angles and the initial hole diameter.