Correlation of Prephosphate Treatment, Surface Friction and Powdering of Galvanneal During Forming

Prephosphating of galvanneal steel sheet has been shown to improve its forming performance. This improved formability is due to a reduced coefficient of surface friction caused by the prephosphate treatment. Also, increased levels of powdering have been observed with prephosphated galvanneal sheet during cup-drawing operations. It has also been shown that powdering is strongly dependent on the strain state, especially the compressive strain component, during the forming process. Combining these two observations, a possible explanation for the increased powderability of prephosphated galvanneal may be that the prephosphate treatment alters the strain state during the forming of galvanneal sheet, which in turn impacts the powderability. In order to examine this hypothesis, cups were drawn from delta-phase galvanneal sheet samples which were subjected to various prephosphate treatments. The changes in strain state along the walls of the cups were measured using circle grid analysis. Numerical simulations using finite element analysis were also performed to assess the changes in local strain state with variations in the coefficient of friction. Correlations among prephosphate treatment, surface friction, strain state and powdering will be discussed.