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Products and solutions, 2001-04-23, 02:00 AM

Energy-absorbing body structures made of high-strength steel

Design proposal for lightweight safety components

In serving the automobile industry, Thyssen Krupp Stahl AG not only successfully improves the strength and processing properties of its materials but also investigates which forming processes, joining techniques and design solutions are best able to exploit the lightweight potential of modern high-strength steels. A new car side beam developed by Thyssen Krupp Stahl AG is a recent example of the company`s integrated approach to design.

The component is crucially important to the passive safety of a vehicle. In the event of a crash a side beam has to absorb as much of the impact energy as possible. To do so it needs to deform in a defined, uniform way under high loads. Conventionally side beams are made of deep-drawn steel shells which are subsequently welded together. But to exploit the weight advantage of high-strength steels new design solutions are needed. If the wall thickness of the shells were reduced to the extent the high strength of the new steels would actually allow, the deformation behavior of the component would change. The beam would be subject to non-uniform deformation even under low strain loads and in extreme cases would completely collapse.

To continue to meet crash requirements and at the same time use the weight advantages of modern steels, especially for future ultralight steel bodies, Thyssen Krupp Stahl AG has developed a new design proposal for side beams in which two tubes are placed on top of each other. An apron connects the tubes and fixes them in the correct position the correct distance from each other. The tubes are made of steel RA/K4070 and are formed by hydroforming (IHU). The side beam meets all energy absorption requirements in the event of a crash and is approx. 25 percent lighter than conventional designs.

RA/K4070 is a so-called TRIP steel which attains its ultimate strength during forming. Its initial tensile strength of 700 megapascals (MPa) increases during hydroforming to up to 850 Mpa, its yield point rises from 400 to 650 Mpa. This TRIP effect (Transformation Induced Plasticity) is produced by residual austenite in the material`s microstructure which is converted into hard martensite during forming.

Thin-walled tubular sections for hydroforming are another innovation from Thyssen Krupp Stahl AG: while hydroformed components are already used as standard in chassis structures, the challenge in body structures so far has been to produce tubular blanks with significantly higher diameters (60 to 150 millimeters) and at the same time lower wall thicknesses (0.6 to 1.5 millimeters). For thin-walled lightweight structures of this kind Thyssen Krupp Stahl AG developed a manufacturing technique in which the sections are produced without a roll former using a new forming technique and subsequently laser welded. Even though the complex shape of the side beam means it has to pass through several forming stages during production, the TRIP steel still retains enough residual forming capacity to absorb energy in the event of a crash.

The same principle can also be used to produce Thyssen Tailored Tubes®, in which Thyssen Krupp Stahl AG has adapted the idea behind its very successful tailored blanks for use with tubes. Like tailored blanks, Thyssen Tailored Tubes® can be produced using steels of different grades and different wall thicknesses to make lightweight components tailored to the demands of specific loads and applications.

For further reference: ThyssenKrupp AG, Trade Press, Bernd Overmaat Tel: +49-211-824-36012, Fax: +49-211-824-36035 e-mail:

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