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High-tensile steel grades are being used to an increasing extent in the construction of car bodies with a minimum weight. A special importance must be placed on a particularly high stretch forming ability, since stretch forming is that forming strain to which car body closures are most frequently exposed. In an effort to meet those requirements ThyssenKrupp Stahl AG (TKS) has developed a new generation of steel grades with moderately enhanced yield strengths of up to 350 MPa and tensile strengths of up to 450 MPa. These new grades which have an excellent stretch formability are mainly characterized by a high hardening ability which can be further enhanced by an additional bake-hardening effect. Moreover, they are excellently suited for surface treatment including hot-dip coating. The new steel grades close the gap between the high-strength IF and the multiphase steel, thus completing to the product range of high-strength grades at TKS.

In 1989 Daimler Benz AG, Stuttgart, introduced hot dip aluminized steel sheet for this application in passenger vehicles. By now, after 10 years' experience, most of this manufacturer's current models are equipped with fuel tanks made from aluminized steel sheet. This material incorporates a combination of a specially designed deep drawing IF steel with the excellent corrosion protection properties of Al coatings. The two methods for joining the upper and lower halves of the tank body are resistance roller seam welding with an additional wire or laser beam welding. The first method was applied to the fuel tank bodies illustrated in this paper. In laboratory corrosion test series, aluminized steel sheet performed very well with regard to fuel resistance, including resistance to fuels containing methanol and rape-methyl-ester (bio diesel). Attempts to improve external corrosion resistance by in-line chromate treatment were successful.

The benefits of bake-hardening steels for body applications in the automotive industry are well known since several years. These steels offer the potential of weigth savings without major loss of formability. Thus this group of high-strength steels is the first to find broad application for exposed panels. This paper sums up the different concepts of producing cold-rolled electrogalvanized and hot-dip galvanized bake-hardening steels. These concepts are critically discussed from the point of view of stable production. Important aspects for batch annealed or continuously annealed steels are the control of grain size and temper rolling. New hot-dip galvanized bake-hardening steels have been developed; as a result of degassing to ultra low carbon contents in modern vacuum treatment facilities. Beside bake-hardening effects special emphasis is given to the strain-hardening behaviour and to the ageing resistance of bake-hardening steels.