Electro-zinc coated bake-hardening steel HC180B at Salzgitter Flachstahl

Bake-hardening steel products are used in the automotive industry to increase the strength of bodywork parts, in particular for parts of the outer body shell. In this process, the manufacturing processes of the automotive industry are utilized in a highly targeted, specific manner. The component forming process increases dislocation density and hardens the material. Following their joining and painting, the body shell passes a heat treatment process, which is also known as paint baking, at a temperature of approx. 170 °C for the duration of approx. 20 minutes. The dissolved carbon atoms that are embedded in the bake-hardening steels diffuse at the dislocations and restrict their further motion, forming a distinct yield strength and increase of material strength (bake hardening). The precise adjustment of the content of interstitially dissolved carbon is prerequisite for the appropriate bake hardening (BH) effect and in order to avoid aging at room temperature.

Previously BH steel was only produced on the two hot dip galvanizing plants of Salzgitter Flachstahl, which are equipped with continuous annealing furnaces. Now it is also possible to produce cold-rolled sheets and electro-zinc coated sheets at Salzgitter Flachstahl.
These products are batch annealed at Salzgitter Flachstahl. By comparison with batch annealing, it is much less complicated to adapt the dissolved carbon content in the continuous annealing process, as the higher cooling rate retains an appropriate content of dissolved carbon. Now Salzgitter Flachstahl achieved this also successfully in the batch annealing process. The composition was adapted (reduction of the carbon content) for batch annealed BH steel to the effect that the carbon remains in a dissolved state even at the very low cooling rate of the batch annealing process. Batch annealed BH steel features good forming properties with high r-values, excellent aging resistance, least zinc abrasion and bake-hardening parameters (BH2) to standards. Moreover, the product is available both in the form of steel sheet and electroplated steel sheet with ultimate finish quality.

In the meantime for example, the electroplated variant of grade HC180B has been highly successful in series deliveries for door outer panels and replaces the previous grade HC220I for this application. The use of BH steel has made it possible to realize applications using steel of a lower strength class, while meeting demanded component properties thanks to the BH effect.

Once the grade HC180B has been integrated in the product portfolio of Salzgitter Flachstahl, it is intended to develop the next strength class with grade HC220B as electro-zinc coated material.