EFFECT OF SCRATCH ON A SUBSTRATE ON A PRO TRUSIVE 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 o n a steel substrate, during the galvanizing and galvannealing processe s. As a result of galvanizing, the concavity was reduced because the s cratch was filled up with molten zinc. After galvannealing, however, a protrusive surface defect appeared on the sheet, higher than the orig inal scratch. Moreover, during galvanizing, the alloy layer formed fas ter over the concavity than over the unscratched area. During galvanne aling, the alloy layer over the concavity grew even more because it re acted with the molten zinc over the unscratched area. It was therefore concluded that the presence of more Zn-Fe alloy leads to increased de velopment of protrusive surface defect.