Investigation of Indentation Type Surface Defect on Interstitial Free Galvannealed Steel Sheet During Zinc Coating

The market capitalization of the coated sheets is continuously increasing in the automotive exposed and non-exposed panels such as door inner panels, dashboard, and cowl. The production of defect-free coated steel sheets requires increasingly more stringent norms for defects are visible after painting to end customer. In the present study, we have analyzed one of the typical surface defects that appear as “Comma” shape and commonly located at the edges of galvannealed steel sheets. Using Optical microscope, Profilometer and Scanning electron microscope coupled with Energy dispersive X-ray spectroscopy we have characterized the defect. Correlation with plant process parameters has been investigated for its likely origin. Defects are observed to align at a certain angle with respect to the RD (rolling direction), and consisting of a head, a tail, and a connecting trench inside, with tail oriented toward the edge of the sheet. The formation of these defects was hypothesized to be associated with dead zone creation, splashing and consequential back-splashing of zinc droplets near edges of the sheet during air knife operation. Because a dead zone is created at the edges of sheet, these zinc droplets pass through the air knife without getting wiped off, consequentially get adhered at the strip surface. During skin pass rolling, these solidified zinc particles get rolled onto the sheet and form the observed defects on the final coated products. Usage of baffle plate in air knife assembly is recommended to reduce/annihilate formation of dead zones at the edges of the coated sheet.