Hot stamped parts are becoming increasingly popular in the automotive industry because of the demand for vehicle weight reduction and improved crashworthiness. Hot stamped parts are manufactured by austenized boron steel blanks in a furnace and then simultaneously forming and quenching them within a tool. As the blanks is quenched at cooling rate above 30℃/s, austenitic structure transforms into the martensitic structure which has very high tensile strength of approximately 1.5GPa. However, a fully martensitic hot stamped part exhibits low ductility which leads to insufficient energy absorption characteristics. Therefore, recent technology of hot stamping has been tried to use the tailored blanks such as tailor welded blanks (TWB), tailor rolled blank (TRB) and partial quenching (PQ) in order to manufacture multi-strength hot stamped part.
In concept of tailored blank for multi-strength hot stamped part, TWBs are semi-finished parts that consist of at least two single sheets which are welded together prior to the forming process. The sheets can exhibit different mechanical properties, thickness or coating of which characteristics lead to vehicle weight reduction and improved crashworthiness.
This paper aims to improve crashworthiness and reduce weight of center pillar with TWB in hot stamping process of which blanks are consisted of USIBOR 1500P and DUCTIBOR 500P. First of all, microstructure and hardness of welded zone was measured to verify reliable mechanical property of welded zone because instability of weld material leads to low formability and crashworthiness. Mechanical properties of USIBOR 1500P and DUCTIBOR 500P was also measured to apply FE-simulation by tensile test according to forming temperature and strain rate. Then, FE-simulation of forming stage was conducted to confirm formability and cooling rate, and impact analysis was performed to evaluated crashworthiness and displacement depending on the thickness combination of the blanks.
Center pillar with TWB was manufactured in optimum thickness combination based on the results of FE-simulation and impact analysis. Finally, mechanical properties of the part were measured to verify satisfaction of demand property in welded zone, high strength region and low strength region. The crashworthiness of center pillar with TWB was evaluated by drop test and compared with it of the conventional hot stamped part. Based on the analytical and experimental studies, improved crashworthiness and reduced weight of center pillar with TWB was achieved by hot stamping process using USIBOR 1500P and DUCTIBOR 500P.