Description of microstructures and mechanical properties of boron alloy steel in hot stamping process

Hot stamping process has been increasingly applied for producing automotive parts with ultra-high strength property. Hereby, reduction of car body weight and increasing crashworthiness of new generation vehicles could be simultaneously achieved. In this work, a direct hot stamping of the boron alloy steel was investigated by experiment and FE simulation. Temperature evolutions of blank and dies were gathered during the experiments and verified with FE results. Metallographic analysis, hardness measurement and tensile test were performed for different locations of formed samples. Based on time-temperature-transformation (TTT) diagram, local emerged microstructure constituents and corresponding distributed hardness values of the stamped parts were predicted. Additionally, quenching tests under various media were carried out for the examined boron steel. Determined tensile stress-strain curves from each cooling condition were described using Voce hardening equation. Relationships between hardness and Voce model parameters were established. Then, developed constitutive equations were used in combination with the obtained FE results to evaluate local mechanical properties of hot stamped samples. It was found that calculated stress-strain responses for different areas fairly agreed with those from the experiments.