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During the process of hot rolling, seam defects often appear on the edges of the interstitial‐free (IF) steel, which seriously affects product quality and yield. To this end, the macro‐ and micromorphology of edge seam defects are analyzed, their specific characteristics are summarized, and the defect cast slab rolling experiment is conducted. The experiment shows that the pore, crack, and slag inclusion are not the causes of the seam defects. Then, through a combination of theoretical analysis, actual parameters' detection, and experiments, the formation mechanisms of the seam defects are evidently side folding of the intermediate slab and flipping of the corner metal. The mechanisms of the two types of folds are also described in detail. Finally, in view of the mechanisms, the advantages and disadvantages of control measures are analyzed, and the concave anvil of the slab sizing press is selected at last. To determine the effectiveness of the scheme, a pass experiment is conducted on a slab sizing press at the production site. Results show that the steel coils without slab sizing press generally have seam defects, whereas the coils with a concave anvil slab sizing press do not. The defects are almost totally eliminated. [ABSTRACT FROM AUTHOR]