A Study of Reduction of Sidewall Curl in Sheet Metal Forming of Advanced High Strength Steels via Blank Holder Force

Due to the goal of reducing emission of CO2, lightweight design of automobiles has become a major goal for vehicle manufacturers in recent years. It results in the tendency towards the study of sheet metal forming using advanced high strength steels (AHSS). This research aims at reducing of sidewall curl effect after the sheet metal forming process of AHSS via blank holder force. Taking U-hat drawing as an example, this thesis investigates the basic relation between blank holder force and sidewall curl. So far, there is no reference of maximum blank holder force for AHSS in sheet metal forming handbook. The results of this study provide maximum blank holder force of U-hat drawing with AHSS DP590、DP780、DP980 and DP1180 for the future mold design. In order to eliminate drawbacks by using constant blank holder force, variable blank holder force is proposed. This thesis aims at researching the mechanism of four basic variable blank holder force curve based on CAE simulations. Then the effects of using each variable blank holder force on sidewall curl are compared. Another method of reducing sidewall curl effect is using step mold. The CAE simulation results of stamping with step mold are compared with stamping using variable blank holder force. Side sill is a common automobiles structure. The traditional way of forming side sill is bending with two operation processes. The simulation results of forming side sill using drawing with variable blank holder force are presented. With the use of variable blank holder force, bending operation process can be replaced by drawing with variable blank holder force.