Hydroforming performance of DP590/DP780 high-strength steel tube

In recent years, the automotive industry has become increasingly demanding for the strength of hollow structural parts. To meet the strength and toughness requirements, major automakers have begun to use high-strength steel for the production of automotive hollow structural parts, and the hydroforming process is the most economical way to achieve this purpose. However, studies on the hydroforming process of high-strength steel in the industry are few. To guide the production of high-strength steel hydroformed parts, the deformation behavior of DP590/DP780 high-strength steel welded tube during hydroforming was investigated in this study. The cross section of the circumferential direction of the tube was observed by scanning electron microscopy to determine the microstructure of the base metal. The sizes of the weld and the heat-affected zone of the tube were determined by VMHT30M microhardness tester to study the hydroforming fracture behavior. The deformation behavior of DP590/DP780 high-strength steel welded tube during hydroforming was studied by a tube hydroforming test machine. The experimental results are as follows: the fracture pressure of the tube during the bulging process is larger than the fracture pressure obtained by the theoretical calculation formula, and the rupture position is located in the base metal area near the weld and heat-affected zone. With the increase of the tube diameter and the length-to-diameter ratio, the maximum expansion ratio of the tube exhibits a downward trend. In the process of free bulging, the weld area of the tube is basically not thinned. The position of the minimum thickness is located in the heat-affected zone of the tube and the transition zone of the base body; the wall thickness reduction rate is the largest at the highest point of the bulging region, and the closer to the tube clamping zone, the smaller the wall thickness reduction rate. Finally, the following conclusions can be drawn: the hydroforming experiment of the tube can accurately obtain the mechanical properties of the tube. Improving the welding quality could help to control the failure rupture position. A reasonable selection of the length-to-diameter ratio of the tube is beneficial to the tube overall performance. It is beneficial to reduce the risk of cracking of the hydroformed part by reasonably controlling the thickness reduction rate of each part.