Prediction of the Stress Decreasing Behavior in the Early Stage of Stress Relaxation in Steel Sheet

Stress relaxation, which occurs during holding at the bottom dead center in press forming, affects press formability. Because holding at the bottom dead center is completed within several seconds, it is important to predict the stress relaxation behavior of the first few seconds after the start of holding. The purpose of this study is to propose a model, the material parameters of which are obtained from tensile tests, to predict the stress decreasing behavior in the early stage of stress relaxation in steel sheets. We constructed the model by modifying the Kocks-Mecking model based on the following assumptions: Stress relaxation at room temperature is due to the slip motion of dislocations, which indicates that the plastic strain rates at the start of stress relaxation are same as those in the previous tensile process. In this study, it was assumed that the change in microstructure during stress relaxation is negligible; hence material parameters remain constant during stress relaxation. The measured and predicted stress relaxation behaviors for various strain rates during the tensile process were compared. It was observed that an increase in the plastic strain rate during the tensile process accelerated stress relaxation. The proposed model was able to predict this phenomenon. However, the relative error between experimental results and the model increased as the stress relaxation progressed mainly owing to dynamic strain aging. Therefore, the proposed model is applicable in the condition when effect of dynamic strain aging is negligible.