12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

With the increasing use of galvanized high-strength steel sheets in automobiles, the surface quality requirements of galvanized sheets are also increasing. When hot-dip galvanized C-Si-Mn dual-phase steel (such as DP600~DP1000) is produced, a serious orange peel-like rough surface defect appears on the surface of the steel sheet, which is mainly found in the middle area of the middle section of the coil. The orange peel-like defect samples are investigated and analyzed by Keynes microscope, scanning electron microscope (SEM) and glow discharge spectrometer (GDS). It was found that the zinc grains on the surface of orange peel defect area was abnormally large, about 300 microns, while it was only about 50 microns in normal area. After the zinc coating was etched by hydrochloric acid, the morphology of the inhibition layer was irregular, and there were transverse microcracks perpendicular to the rolling direction on the surface of the substrate and the full hard coil. The Al content at the interface between zinc layer and matrix is lower than that in the normal region. The analysis shows that the microcracks on the surface of the full hard coil are closely related to the type and composition of the scale of the hot rolled steel sheet. The hot-rolling coiling temperature is lowered to 580 °C or less, avoiding the eutectoid transformation of FeO, changing the surface oxide scale and composition, and eliminating orange peel defects. The analysis shows that the microcracks on the surface of the full hard coil are closely related to the type and composition of the scale of the hot rolled steel sheet. When the coiling temperature is lower than 580 °C, the orange peel-like defect is eliminated. Due to the avoidance of the eutectoid transformation of FeO, the type of scale is changed, and the surface enrichment of Si during hot rolling is suppressed.