The origin of striations on the surface of iron–zinc coated steel sheet produced by hot-dip galvannealing


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Abstract

Dark striations are often observed on the surface of iron–zinc coated steel sheet annealed immediately after hot-dip galvanizing (galvanneal). The striations can be explained on the basis of the differential formation of an iron–aluminium barrier layer at the steel-bath interface during galvanizing. The contact of a submerged, grooved sink roll in the galvanizing bath with the steel sheet causes variations in the iron–aluminium barrier layer at the interface. A more coherent layer is formed in the areas where there is no contact i.e., the grooved areas on the sink roll. The growth of the iron–zinc coating under a higher local aluminium concentration during subsequent annealing leads to a pitted surface in those groove areas, and creates the appearance of dark striations on the surface of the coated sheet. The aluminium content of the galvanizing bath is a key factor in determining the extent of the non-uniformity imposed by the contact with the sink roll. Consequently the striations can be reduced by lowering the aluminium content of the galvanizing bath.

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