The metallurgical characterisation of ZnFe alloy coatings on galvannealed steel sheet

Galvannealed steel sheets used for the production of car bodies in the automobile industry are manufactured by the process of continuous hot dip galvanising of wide steel strips followed by a subsequent heat treatment of the zinc coating. As a result of the reaction of the zinc with the iron, an alloy coating is produced containing ca. 8 - 11 % Fe. The nature of the ZnFe phases produced is of considerable importance to the customer in respect to the further working of the material. The subsequent annealing process of the galvanised steel sheet is required to produce a defined phase structure in the ZnFe coating. Due to the demanding manufacturing processes of the automobile industry, the structure of the coating is usually checked by metallographical examination. The comprehensive physical metallurgical characterisation of such ZnFe coatings has been shown to be particularly effective using a combination of optical light microscopy, scanning electron microscopy and X-ray diffraction.