Effect of Thickness Reduction in BIW Weight Reduction: Transition from 600DU to 800DU on Splitting Behavior

Material selection for creating a rigid and high-energy absorption lightweight BIW is critical in terms of fuel economy and emissions. In this context, material forming limits and mechanical behavior during stamping processes are the most important factors in the creation of lightweight BIW’s. Therefore, enhancing strength and reducing material thickness stand out as the primary ways to comply with industry standards and reduce the body weight. In weight reduction efforts with dual-phase steels, thinning may lead to splitting due to the increased martensitic island structure in regions with the highest Mn and C content in dual-phase materials. In hot- and cold-rolled dual-phase 800DU material with a thickness of 1mm, splitting may occur during forming simulation due to the martensitic structure to distribute inhomogeneously, surpassing the critical threshold of the forming simulation curve when compared to cold-rolled 600DU material with a thickness of 1.2mm. This study addresses the thinning-induced splitting issue encountered in weight reduction applications, determining the optimal choice between different rolling types under a die produced for 600DU material with a thickness of 1.2mm, using both hot and cold-rolled materials. In line with the purpose of the study, trials were conducted with two different 800DU materials under forming dies. Formability simulations and mechanical tests were performed, and analyses were carried out in terms of microstructure, thinning, and edge cracking performance. The characterization of the materials was conducted using a scanning electron microscope (SEM) equipped with an EDX apparatus. Following the characterization, tensile tests were performed in three different rolling directions to determine the mechanical properties of the materials. To evaluate the resistance of the materials to edge cracking, Hole Expansion (HE) tests were conducted. As a result of the improvements and analyses made by replacing the 600DU material with a thickness of 1.2mm with cold-rolled 800DU material with a thickness of 1.0mm, the proposed application appears feasible for the part geometry under consideration.