2020 Volume 61 Issue 11 Pages 2121-2127
Automobile body parts produced by the hot stamping process exhibit excellent shape fixability with an ultra-high tensile strength of 1.5 GPa. We investigated the effect of flow stress during forming and phase transformation in the hot stamping process. Referring to both 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, were used for comparison. The forming start temperature varied from 400°C to 800°C. After hot stamping, the springback of the parts was evaluated. The results showed that shape fixability in hot stamping is caused by low flow stress during stamping and martensitic transformation. When martensitic transformation occurs after stamping, excellent shape fixability is obtained regardless of the flow stress during forming. Accordingly, it was concluded that the stress introduced by hot stamping is relaxed and becomes uniform during martensitic transformation. The application of tensile stress due to thermal contraction also contributes to the decrease in springback.
This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 45–50.
