Study on morphological characteristics and formation mechanism of chrysanthemum pattern defect on hot-dip galvanized steel strip surface

To address Chrysanthemum Pattern defects on galvanized sheet surfaces, this study starts from the microscopic morphological characteristics of the defects and combines microstructural observation with process analysis to reveal their formation mechanism. Through corrosion treatment, argon-ion etching stripping, and EBSD analysis of defect samples from DX56D+Z galvanized sheets, it was found that coke deposits on the rollers in the annealing furnace create micro-indentations approximately 0.5–3 μm deep on the surfaces of red-hot steel strips. These indentations are filled by molten zinc during galvanizing, leading to localized zinc coating thickening up to 12 μm (compared to 6.5 μm in normal areas). The thickened zinc layer, due to insufficient substrate support, exhibits poor plastic deformation coordination during the skin-pass process, resulting in a loose structure. After oil-wax polishing, optical dark patterns emerge due to roughness variations. EBSD analysis confirmed that the substrate consists of equiaxed ferrite without distortion, ruling out the influence of cold working. Acid pickling corrosion tests verified that the defects are unrelated to substrate oxidation or the inhibition layer. Inspection of furnace rollers identified coke deposits as the direct cause. The essence of the defect lies in the non-uniform deformation of the zinc layer structure induced by micro-damage from roller–strip contact during the annealing stage. Optimizing the cleaning and maintenance of furnace rollers can mitigate or even eliminate this defect, providing the industry with a clearly defined mechanistic solution.