Test and analysis of plating leakage defects on surface of hot galvanized BH180 sheet
The issue of surface plating leakage on bake-hardened steel BH180 products produced by a hot-dip galvanizing production line in a domestic steel plant was analyzed and studied.The formation mechanism of plating leakage was ana-lyzed by observing the macroscopic and microscopic morphology of defects as well as detecting the composition of defects.It was found that there were numerous pits on the surface of substrate where the defects appeared as orange peel shapes.Even after cleaning,the grease mixture residue remained in these pits.The organic residues were carbonized and deposited with-in the pits during annealing,which reduced the wettability of zinc solution and inhibited the formation of Fe2Al5 inhibition layer on the substrate surface.As a result,the plating leakage defects were generated.By comparing the composition of the defective coil substrate with that of the normal coil substrate,it was found that the boron content in the defective substrate exceeded the standard.The trace boron elements were segregated at the grain boundaries,which led to the existence of mixed structure of coarse block and long grain and fine grain in the substrate.The hot-rolled substrate with mixed grain structure would form orange peel pits after pickling and cold rolling.After strictly controlling the boron content in BH180 substrate and improving the process efficiency of galvanized cleaning section,the problem of plating leakage of BH180 could be effectively solved.
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