Analysis of the Causes of Stripe Black Spot Defects in Hot-Dip Galvanized Strip of Thin Slab Continuous Casting and Rolling Production Line

After the thin slab continuous casting and rolling production line of a domestic steel plant was put into operation, with the vigorous development of hot-dip galvanized products, the strip blackening defect on the surface became more prominent. After hot-dip galvanizing, the coil had a certain spacing of strip black spot defects on the surface, which could not meet the needs of high-end users and restricted the improvement of the manufacturing capacity of the headless production line.Therefore, this article uses scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), three-dimensional profilometer, and optical microscope (OM) to study the microscopic characteristics and causes of black spot defects on the surface of hot-dip galvanized steel. There are a large number of oxidized black spots on the surface of the stripe black area. The zinc layer thickness of the cross-section at the blackened area is smaller than that of the normal area, and the secondary dendrites inside the zinc layer at the blackened area are not well-developed in the normal area. The surface roughness Ra and Rt of the blackened position in the three-dimensional morphology display show significant differences from the normal position. Electron microscopy observation shows that the grain size of the substrate at the blackened area is different from that at the normal area after removing the zinc layer, which is the direct cause of the final surface showing stripe black spot defects. By optimizing the heating temperature uniformity of the IH on the thin slab continuous casting and rolling production line, the stripe black spot defects were ultimately eliminated.