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

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. This revised version was published online in November 2006 with corrections to the Cover Date.