Experimental Research On The Springback Characteristic Of High Strength Steel Sheet Metal Forming Processes

Springback is one of the key factors influencing the quality of stamping parts in sheet metal forming. Springback is an unloading process after forming. The elastic strain energy will be released and the stress will be redistributed after springback. And the shape of part will change after springback, which will influence the part’s final shape. When the springback is beyond the tolerance, the part is turned to be unqualified.Springback is not only a key factor effecting and restricting the parts quality, but also a key factor influencing the die design. It’s hard to control or compensate the springback after forming if it’s not considered thoughtfully in die design, which will easily result in forming defects.High strength steel has been used widely in automobile and aviation industry for energy saving, pollution reducing and more safety, which makes the problem of springback more severe. Even with the development of automobile and aviation industry, the precision requirement of the part’s forming shape is getting more strict, and this also makes springback problem more urgent to be solved.In order to get the springback characteristic of high strength steel, this paper designed U-shaped bending die and box-shaped drawing die to research the springback in the two typical sheet metal forming process, the bending and drawing, by the way of physical tests on DP steel produced by Baosteel,which has the tensile strength grade of 600MPa.The U-shaped die is designed with device to position the blank in order to assure the right position of the blank in forming. The punch is designed with cross outline and manufactured by wire-cut, and the die is designed to fitting the punch, so that the gap between the punch and the die can be assured when forming. All this can help to avoid the dissymmetry problem of U-shaped part after forming. Auto-pushing device is also designed to help to saving the time getting out the parts and can also help to avoid the distortion when getting out parts with hands.The box formed by the box-shaped drawing die is a dissymmetry part, which has typical features of car panel stamping parts with two-curved bottom and one arced wall. The box-shaped drawing die is also designed with blank positing device, the gap between the punch and the bind is rigid designed, then there are guild-pins between the bind and the die, which can assure the even die gap. All blanks are manufactured by wire-cut in order to assure the right dimension of the blanks. Auto-pushing device is also designed. All those tests, which need changing the die parameters, are realized by changing different punch and different die in their own mould-base.Two sets of measuring tools are also designed to measure the springback of the U-shaped parts and the box-shaped parts. The U-shaped part can be measured direct in the measuring tool, while the box-shaped part is sent to manufactured by wire-cut after forming, then the springback of the cutting piece can be measured with the measuring tool.Based on the two sets dies and the two measuring tools, mass of physical tests and measures are taken, and some springback characteristics of DP steel in bending and drawing are summarized as follows:1. The springback of TRIP is the largest, and followed is the springback of DP, while the springback of HSLA is the least.2. All the springback increase with the increase of die gap and drawing depth.3. All the springback decrease with the increase of binder.4. Springback differe with the lubrication. In the tests of this paper, the springback without any lubricant is the largest. And the springback with the lubricant of pressing oil is the least.