Characterization of galvanized/galvannealed sheet steel defects towards enabling defect free zinc coatings

Abstract

As more emphasis is being placed on reducing the weight of vehicles while increasing safety ratings through advanced high strength steels allowing use of thinner metal gauge corrosion properties become more important. To protect these steels against corrosion, a hot-dip galvanizing process is used. An additional annealing step may follow to produce galvannealed (GA) steels. The present thesis characterized microstructure and chemistry of defects in four industrially produced steels, 3 GA steels and one galvanized (GI) steel. Material A was shown to have a streaking type defect in a GA coating. Light optical microscopy (LOM) showed macroscopic periodicity of 1.5-2 mm between streaks. Time of flight secondary ion mass spectrometry (TOF-SIMS) analysis showed the presence of Mn rich oxides at the steel-coating interface. Two mechanisms were attributed to defect formation, the Zn pot sink roll micro-grooved surface and Mn rich oxides were shown to affect coating thickness. Material B was received as having acne type defects in a GI coating. Scanning electron microscopy (SEM) showed a uniform coating, and the steel-coating interface was planar. TOF-SIMS showed that the areas associated with the defects were Zn rich, and no evidence of chemical contribution to coating formation. The defect formation mechanism was unclear, however it appeared to be a Zn splash and a function of processing parameters e.g.. air knife operating conditions. Two ends of an identical coil were received for Material C: one showing a flame pattern defect, and one end exhibiting a defect free coating. SEM showed the coating surface exhibited a temper roll surface finish, and that the coating thickness for the end without defects was thinner than the end with defects. Defect formation may relate to the difference in temper roll response from the coil and/or Zn solidification growth due to difference in substrate microstructure. Material D was received as having a striped defect, denoted “tiger stripes”. The stripes could be observed in the hot rolled, as-pickled, cold rolled, and GA condition. SEM analysis showed that the coating in areas associated with the stripe was approximately half as thick as areas without stripes. Chemical analysis showed that high levels of Cr, Mn and Si were found within area associated with the stripe in the cold rolled condition. Defect formation may relate to insufficient oxide removal by pickling.