Analysis of the causes of linear surface defects on cold-rolled hot-dip galvanized steel sheets

In view of the surface linear defects in a hot-dip galvanizing production line, the microscopic morphology and chemical composition of the defects in hot-dip galvanized sheets, dezincification substrates and cold-hard plates were systematically studied by optical microscopy(OM), scanning electron microscope(SEM) and energy dispersive spectrometer(EDS), and the causes were clarified. The results show that the cause of linear defects can be attributed to the inherited oxide inclusions in the steelmaking process. During the rolling process, as the metal matrix undergoes plastic deformation, the inclusions gradually approach the surface of the strip. Under the combined action of strip tensile stress and rolling compressive stress, the matrix around the inclusion cracks, forming microcracks. These microcracks propagate and extend to the strip surface, resulting in some inclusions being exposed to the surface. On the one hand, the exposed inclusions and the rough surface of the fractured place affect the normal reflux of the molten zinc before solidification, resulting in the retention and local thickening of the molten zinc, and on the other hand, provide additional nucleation points for the solidification of the molten zinc, resulting in the formation of local fine zinc crystals. This inhomogeneity of the microstructure can alter the light reflection properties of the product surface, resulting in visible linear defects on the macroscopic surface. Through improvement measures such as reducing the oxygen content at the end of the converter and extending the refining soft blowing time, the purity of the molten steel was improved, and the grades of Class B and Class D coarse inclusions were controlled below 1.5 grade(the internal inclusions of thin cold-rolled products were controlled below grade 1.0), so that the incidence of inclusion defects in cold-rolled products was reduced from 2.05% to 0.6%.