2019 Volume 60 Issue 697 Pages 45-50
Automobile body parts produced by the hot stamping process show excellent shape fixability with an ultra high tensile strength of 1.5 GPa. We investigated the effect of flow stress during forming and transformation after forming on shape fixability in the hot stamping process. Referring to the experimental and FEM coupled simulation results, we discussed the mechanism behind the excellent shape fixability in the hot stamping process. Steel of 0.2% C was used for hot stamping in this study. IF steel and SUS 304, which have different transformation behaviors from that of 0.2% C steel, were used for comparison. The temperature of forming start was varied from 400 to 800 °C. After hot stamping, the shape fixability of the parts was evaluated. The result showed that the excellent fixability in hot stamping was caused by not only low flow stress during stamping, but also martensitic transformation. When martensitic transformation occurs after stamping, excellent shape fixability is obtained without any relation to the flow stress during forming. Accordingly, it was concluded that the stress introduced by hot stamping is relaxed and becomes uniform throughout the thickness during martensitic transformation. In addition, the application of tensile stress due to thermal contraction also contributes to the decrease in spring back.
