自己焼戻しを考慮したホットスタンピングの硬さ予測のシミュレーション

抄録

The application of ultrahigh-strength steel plates in automobile bodies is aimed at reducing weight and enhancing collision safety, with hot stamping (HS) technology being increasingly used for components with a tensile strength of 1.5 GPa or higher. HS simulations that consider phase transformation are conducted to predict formability, hardenability, and shape accuracy. However, the hardness of hot-stamped components obtained via die quenching is lower than that of martensitic structures obtained via water quenching owing to self-tempering beyond the martensite-transformation temperature (Ms point). Few HS simulations consider self-tempering. We developed a material model to accurately reproduce self-tempering behavior. By measuring the hardness of water-quenched martensite after tempering and calculating the activation energy of the tempering reaction, we determined the tempering parameter. The hardness of the self-tempering martensite is predicted by accumulating the tempering parameter during continuous cooling. An HS test and numerical analysis were conducted using a die with partially clearance sections, and the results were compared to verify the accuracy of the hardness prediction. The model developed improves the accuracy of hardness prediction in HS simulations.