When it came time to reengineer the most popular model in the Mercedes-Benz lineup, engineers focused on making the body-in-white stronger, safer, yet lighter. These seeming contradictions (i.e., don’t stronger and safer mean heavier?) forced the team to reevaluate its material choices to meet this challenge, and resulted in a surprising outcome: Approximately 70% of all the steel panels on the body shell of the 2008 C-Class use high-strength steel alloys; a rate Mercedes says is unprecedented for a passenger car. Engineers also used ultra high-strength steel in 20% of the body shell to enhance durability and safety. Together, their expanded use provides unexpected weight savings and improved body rigidity. “We have managed to put an end to the weight spiral,” said Thomas Weber, head of Mercedes research and development. “In the body shell alone, intelligent use of high-strength steels has produced a weight savings of 17.6 pounds.” Torsional resistance also improved by 13% when compared to the outgoing model.
Beyond the weight and stiffness benefits, Mercedes also touts the safety benefits of using ultra- and high-strength alloys. Development engineers were able to enlarge the crumple zones through the use of the materials, resulting in improved energy flow and occupant protection. The structure on which the engine, steering, front axle and transmission are mounted, is constructed from high-strength steel and is extended forward, forming an additional impact plane at the lower section of the vehicle, allowing it to act as part of the front deformation zone. The use of high-strength steel also improved deformation and energy absorption characteristics. Ultra high-strength steels and panels of varied thickness are used in the construction of the occupant cell. The main floor structure is comprised of three steel sheets laser-welded together with the thickest sections forming the center tunnel backbone. Continuous floor side members, constructed from steel with reinforced inner sections, provide added protection. Hot-formed, ultra high-strength steel in portions of the B-pillars absorbs large forces and transfers them to the main body side structure in a side impact. The pillars are made up of three formed steel layers with a large, reinforced area that extends to the upper belt deflector point. High-strength steel plates with reinforcements are placed in the frame, waistline and bumper levels of the door. At the rear, the side members are comprised of continuous, closed box sections with defined material thickness at various points that enables the structure to absorb large energy forces. An ultra high-strength bolt-on steel cross member is manufactured via a flexible rolling process that allows the thickness to vary—the outer edge thickness is greater than the inner section—for better impact load management. As a final touch, Mercedes engineers made the steering rack from high-strength steel, which cut 1.7 lb from the steering system.
Plenty of automotive engineers are taking advantages of the comparatively new steels. The 2008 Mitsubishi Lancer utilizes high- and ultra-high-strength steel in its body-in-white, improving torsional rigidity by 56% and bending rigidity by 50% over the previous model. Engineers use the ultra-high-strength 980 MPa alloy for constructing the rocker panel between the A- and C-pillars, resulting in improved side-crash performance. High-strength 590 MPa alloy makes up the B-pillar, roof joints and cross-car structure, adding to the safety credentials.