Sep 13 2005
The Auto Division of ThyssenKrupp Stahl AG has investigated the possibility of using advanced high-strength steels for exposed automotive body panels. Large outer skin panels offer great potential for weight reduction. Using higher-strength steels makes it possible to reduce sheet thicknesses. However, it also reduces component stiffness, which means that additional measures are necessary to prevent engine hoods, roofs or door outers from fluttering or denting too readily.
ThyssenKrupp Stahl AG has developed a concept study for ultra-thin exposed panels that both reduces weight and at the same time compensates for the loss of stiffness. For example, the company designed a door outer panel weighing twelve percent less than a production benchmark yet offering 20 percent higher stiffness.
The material used for this panel was the dual-phase steel DPK 30/50 with a strength of 500 megapascals, significantly higher than the 300 to 400 megapascals of today's commonly used outer skin steels. At 0.48 millimeters, the outer panel from ThyssenKrupp Stahl is considerably thinner than the benchmark panel (0.7 millimeters). The company produces such materials on its 'TAKO' mill in Duisburg-Beeckerwerth – the world's most advanced cold rolling mill of its kind, capable of reducing advanced high-strength steels to cold strip in thicknesses of 0.3 to four millimeters and widths of 1,000 to 2,040 millimeters while delivering the excellent surface quality which is essential for outer skin parts.
To compensate for the loss of stiffness associated with such low thicknesses, the ThyssenKrupp Stahl concept provides for a latticed backing of narrow steel and foamed polymer strips. The strips are some 2.5 millimeters thick and 30 millimeters wide. The 0.48 millimeter thick steel strips serve as a support for the polymer foam. The structure is attached to the inside of the formed panel by adhesive bonding.
Experts will know that many of the adhesives used in modern automotive bodymaking shrink during curing and thus create stresses in the component. On the smooth surfaces of outer skin parts this would result in the backing structure showing through. ThyssenKrupp Stahl investigated this problem using a new process in which a 3D scanner is used to digitize entire components. This allowed the visual impression of the backing to be converted into precisely quantifiable and reproducible data. The process allowed developers from the Auto Division to establish which adhesives provide good results even for sensitive outer skin applications. The concept study also showed that thin outer skin panels can be formed and joined with minor tooling adjustments.
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