Springback behaviour in bending of ultra-high-strength steel sheets using CNC servo press

Abstract

The springback behaviour of ultra-high-strength steel sheets in bending was investigated under controlled conditions using a CNC servo press. Although the ultra-high-strength steel sheets are attractive in reducing weight of cars, the amount of springback of the ultra-high-strength steel sheets in the forming is very large due to high strength. The CNC servo press has the function of accurately controlling the motion by two servo motors. The effects of the material, the finishing reduction in thickness, the forming speed and the holding time at the bottom dead centre on the amount of springback in V-shaped bending were examined. The scatter of the springback for the ultra-high-strength steel sheets was improved by using real thickness and not nominal thickness of each individual sheet in the control of the punch stroke. The amount of springback for the ultra-high-strength steel sheet in the V-shaped bending was much larger than that for the mild steel sheet, and the amount was decreased by the finishing reduction in thickness direction because of uniform stress distribution. The effects of the forming speed and the holding time at the bottom dead centre were small. The amount of springback for the steel sheets was almost proportional to the ratio of the tensile strength to the elastic modulus.