Application of Advanced High Strength Stainless Steel for Mass Reduction in Automotive Structures - A Front Bumper Beam Case Study

The front bumper of a current production vehicle, which is made of hot-stamped 15B21 aluminized steel, was studied for mass and cost reductions using the Advanced High Strength Stainless Steel product NITRONIC® 30 (UNS Designation S20400) manufactured by AK Steel Corporation. This grade of stainless steel offers a combination of high ductility and strength, which was utilized to significantly modify the design of the bumper beam to incorporate geometry changes that improved its stiffness and strength. The structural performance of the bumper assembly was evaluated using LS-Dyna-based CAE simulations of the IIHS 40% Offset Full-Vehicle Impact at 40 mph with a deformable barrier, and the IIHS Full Width Centerline 6 mph Low-Speed Impact. Optimization of the bumper beam shape and gauge was performed using a combination of manual design iterations and a multi-objective optimization methodology using LS-Opt. The objective was to minimize the mass and cost of the bumper assembly while maintaining an impact performance that was comparable to the baseline bumper beam. An optimized bumper design with a much deeper "W"-shaped section was developed, which allowed for down-gauging of the beam and removal of the attachment and bolster brackets through part consolidation. The resulting final design offered a mass savings of 3.75 lbs. A cost analysis was done taking into account the material, tooling and process costs, and a piece price cost reduction was demonstrated.