Springback and side-wall curl of galvanized and galvalume steel sheet

The introduction of high tensile steel sheet to permit the use of thinner gauge in large pressings has increased the occurrence of the forms of elastic distortion known as springback and side-wall curl, and has increased, likewise, the need to control these by appropriate changes to the selection of process parameters when they lead to an unsightly product, or, more significantly, to a product in which loss of dimensional accuracy causes loss of function. They may also affect manufacture by making the stripping of the tooling more difficult. The use of thin sheet of high unit cost has increased the need for protection by zinc or aluminium/zinc (galvalume) coatings, and their effect on springback and side-wall curl under conditions appropriate to the pressing of large shallow panels is the subject of the present investigation. It is concluded that the resulting modification of friction does not significantly change the behaviour of steel sheet. Generally, any change in friction which increases the transverse stress gradient increases springback and side-wall curl. It appears that differences in the reported effect of tool geometry on springback and side-wall curl can be attributed largely to differences in the rate of work hardening of the sheet tested. It is confirmed that springback and side-wall curl increase with yield strength and work-hardening rate and with tool radius and tool clearance. © 1989