Friction and formability of galvannealed interstitial free sheet steel

Galvannealing is a process by which zinc-coated steel sheet is heat-treated. During heat treatment, the zinc and the iron diffuse into each other. The coating of a galvannealed steel sheet is composed of a mixture of Fe–Zn intermetallic compounds. The coefficient of friction and powdering of the coating are two important properties that need to be controlled in order to effectively use these coated sheet steels in stamping operations. The distribution of the intermetallic phases on the surface of the coating is a major factor affecting the friction and powdering behavior. A single interstitial-free steel with four different distributions of intermetallic phases in the coating as well as steels from commercial producers were examined. A flat-die tribometer and a bending-under-tension tribometer were used to investigate the frictional behavior of these sheet steels. The double-Olsen powdering test was used to investigate the powdering behavior of these sheet steels. As the amount of the zinc-rich (iron lean) ζ-phase (zeta phase) increased on the surface of the coating, the friction coefficient increased. The coefficient of friction also increased as the amount of iron content in the coating decreased. In contrast, with increased iron content in the coating the amount of powdering increased. In order to use a galvannealed sheet steel in a metalforming operation, there needs to be a compromised choice between frictional resistance and powdering behavior.