1. Overview In our company's steering series, there are U-shaped curved parts as shown in Figure 1. When bending is formed on the steel structure mold originally designed and used, the mouth size of the workpiece is always unstable due to the springback after the workpiece is bent and formed, and the surface of the workpiece is always scratched as the number of times of punching increases. From light development to obvious, then to severe scratching, until the workpiece is scrapped after severe strain.