In this paper, the springback characteristics are quantified and analyzed in the stamping process of the hat-type standard product with ultra high strength steel (UHSS) sheet. The standard shape of the hat-type product is proposed in order to simplify the geometry of auto-body members such as a side sill and its stamping method is proposed in order to verify the effect of the stamping procedure on the springback amount.
A hat type product and press tools are designed and fabricated in order to analyze the springback characteristics. Stamping experiments and the finite element analyses are conducted with two kinds of UHSS such as SGAFC980DP and SGAFC1180DP. Experiments and analyses with four kinds of the forming sequences are conducted so as to investigate the effect of embossing and lower padding on the deformation mechanism and the springback tendency after stamping. From the base bending-type method, this paper uses the embossing operation before and after forming and also considers the form-draw type procedure. Three dimensional shapes of the deformed blank are extracted. The springback amounts are measured and compared according to the blank material and the forming procedure with the embossing shape. This paper measures three kinds of springback modes for the comparison such as the angle change at the punch shoulder, the angle change at the die shoulder and the radius of wall curl.
Experimental result in company with analysis one illustrate that the springback amount is reduced with embossing in the padding operation. When embossing is applied before forming, the elastic recovery of embossment induces springback in the reverse direction while the additional tensile stress is applied to the punch shoulder region when embossing is applied after forming. The lower padding force before the bottom dead point applies the tensile stress to the flange and die shoulder region, which remarkably reduces the amount of springback.
This paper inquires into the effect of the process parameters such as the embossing radius and the padding pressure on the springback amount with the finite element analysis. The analysis result explains that the springback amount decreases as the embossing radius increases and as the padding pressure increases to a certain extent.
With the process parameters obtained in the parameter study for the hat-type standard product, the process design is carried out with the stamping process of a side sill made of UHSS in order to reduce the excessive springback. The study investigates the change of the springback amount according to the embossing procedure, which demonstrates that the springback amount is remarkably reduced when embossing is introduced after the second stage of forming.
The results in this paper show that the springback phenomena can be improved by modifying the stress distribution at the round part of the product with the appropriate forming method. They also fully demonstrates the proposed forming procedure and the analysis method can be effectively applied to the process design for producing parts with ultra high strength steel.