Effect of Different Conical Punch Angle Geometries and the Initial Hole Diameters on the Hole Expansion Ratio of DP steels

Abstract

In the recent decade, advanced high strength steels (AHSS) have gained a great popularity in the automotive manufacturing industries due to their high strength to weight ratio, which significantly improves the safety of the manufactured automobiles while reducing the weight and thus, enabling to improve the fuel efficiency. However, it is known that some types of AHSSs, especially DP steels, are highly susceptible to edge cracking behaviour during the forming operations. Edge cracking behaviour is generally investigated with a 600 conical punch as suggested by the ISO 16630 standard. However, in this study, to observe the behaviour of edge cracking ability of DP steels under different conical punch angles for different initial hole diameters, hole expansion tests have been performed with conical punches with three different angles (300 ,600, 900) for three different initial hole diameters (14, 16, 18 mm). The results have shown that the hole expansion ratio (HER) does not differ considerably with the variation of the conical punch angle and the initial hole diameter due to low fracture strain of DP steels observed after hole expansion tests. The major factor for the edge stretching ability of DP steels have been observed to be microstructure rather than geometrical factors such as conical punch angles.