Selection and use of coated advanced high-strength steels for automotive applications

The hot-dip coating process faces a particular challenge with the production of high-strength steels. The backdrop to this challenge in the case of the high-strength steels is the enrichment of alloying elements necessary to control the microstructure at the steel's surface. These elements diffuse to the strip surface during the annealing treatment in the furnace and affect the wetting action of the molten zinc. The hot-dip galvanising of high-strength steels has been tested in numerous laboratory and field trials. New methods for improving zinc coatability have been developed around the world. A focal point of the work at the Division Auto of ThyssenKrupp Stahl AG (TKS-CS) has for this reason lain in the development of a new generation of multiphase steels that achieve an optimum surface quality also in conventional hot-dip coating lines. It is, as a result, possible to save between 25 and 35% in auto body weight by using high-strength steels and optimised forming and joining techniques [1... 3].