Formability of 800 MPa Grade Hot‐Dip Galvanized Steel Sheet

Abstract

Hot-dip galvanized steel sheets are widely used for their better corrosion resistance. With the development of steel materials and the need for higher performance of galvanized steel, it is urgent to carry out research on the formability of 800 MPa or even higher grade hot-dip galvanized steel sheets. In this paper, forming limit diagram (FLD) is assessed in laboratory by stretch-forming galvanized sheets with a hemispherical punch. The intersection of forming limit curve (FLC) and the major strain axis is around 0.28. By comparison to zinc layer removed samples, galvanized samples have higher yield strength and lower strain hardening coefficient. Formability of experimental galvanized sheets declines significantly attributed to the existence of brittle intermetallic Fe-Zn phase. The original positions of cracking in zinc layer are microcracks and voids around the zinc grain boundaries. During punch forming, the crack networks are formed by the microcracks from the different layers of the zinc coating and may accumulate and propagate to the surface. Forming failure including cracks and flaking makes the zinc layer thinner and increases the tendency of matrix to be exposed.