Forming-Induced Residual Stress and Material Properties of Roll-Formed High-Strength Steels

Martensitic steels are widely used in the automotive lightweight application but less understood in aspects of post-forming material properties. The steels show good ductility in roll forming but occasionally experience delayed (hydrogen) fracture issues, which are believed to be due to the formation of localized residual stress and a reduced product of strength and elongation. To characterize the effect of roll forming process on the formation of residual stress and material properties variation of martensitic steel components, this paper investigates the forming-induced longitudinal residual stress and material property variation in a roll-formed high-strength MS1180 automotive rocker panel. The finite element analysis results for residual stress are validated by neutron diffraction measurements. The numerical model is used to analyze the full evolution of residual stress during the roll forming process and the effect on material properties with major focus on the product of strength and elongation. It is found that the flower design, in particular the overbending stages, play a significant role in the formation of residual stress and the change in material properties. The product of strength and elongation is significantly reduced across the profile, in particular in the corners. The achieved understanding will assist researchers comprehend the material properties of roll-formed component and therefore assist future studies aimed at preventing the occurrence of hydrogen fracture.