Investigation of the spring-back behaviour of high strength steels under various forming parameters

Bending operations are one of the most commonly used methods to form sheet metals to a desired shape. However, spring-back of the sheet metal after the completion of the bending process significantly impairs the productivity of the workshop. Thus, assessing the critical factors that influence the spring-back behaviour needs to be well investigated to control and limit the spring-back on the formed sheet metals. In this study, hat-shaped bending experiments have been conducted on MART1200 and MART1400 Ultra-High-Strength Steels (UHSS). To investigate the influence of different lubrication conditions, on the spring-back, three different lubricant types (Graphite, Teflon film and Teflon film + Graphite) have been used in the experiments. A constant 6.5 tons of bottoming force has been applied at the end of the bending stroke and the force has been held on the samples for 0, 10s and 60s to observe the effect of force holding time on the spring-back. In addition, the effect of blankholder force on the spring-back has also been investigated for MART1400 steel. It has been found that the decrease of blankholder force and the friction between the die and sheet interfaces has considerably reduced the spring-back in the sidewalls and flange regions and also lowered the side wall curl. However, force holding time has not resulted in a meaningful difference for the spring-back. It has been revealed through finite element analysis that the sheet bottom has suffered a larger deformation in lower friction conditions and blankholder forces and caused in the reduction of spring-back. Keywords: Spring-back, Mart steels, Hat-shaped bending, FEA, Stress analysis