EFFECT OF SCRATCH ON A SUBSTRATE ON A PROTRUSIVE SURFACE-DEFECTS OF GALVANNEALED STEEL SHEET

The mechanism by which scratch leads to protrusive surface defect was investigated by observing a scratched concavity artificially created on a steel substrate, during the galvanizing and galvannealing processes. As a result of galvanizing, the concavity was reduced because the scratch was filled up with molten zinc. After galvannealing, however, a protrusive surface defect appeared on the sheet, higher than the original scratch. Moreover, during galvanizing, the alloy layer formed faster over the concavity than over the unscratched area. During galvannealing, the alloy layer over the concavity grew even more because it reacted with the molten zinc over the unscratched area. It was therefore concluded that the presence of more Zn-Fe alloy leads to increased development of protrusive surface defect.