Causes of white dot imprint defects on the surface of alloyed hot dip galvanized interstitial free atomic steel sheet

The white dot imprint defects on the surface of alloyed hot dip galvanized interstitial free atomic steel sheet were analyzed by using scanning electron microscope, energy dispersive spectrometer and optical microscope, and the cause of the defects was found out. The results show that the surface of the defect was not fully alloyed, there were many flat prints, the zinc layer was partially thicker, and iron oxide scale was found after partial zinc dissolution. The iron oxide scale on the steel plate surface was partially thick, and the iron oxide scale could not be completely dissolved during pickling, and it was pressed on the steel plate surface. After galvanizing and alloying, it showed a white dot impression. By reducing the coiling temperature, the thickness of iron oxide scale on the steel plate surface was reduced effectively, and no such defect was found on the subsequent steel plate.