Search Results

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.

Application of phosphate treatments to zinc-coated steel strip prior to stamping operations is termed prephosphating. Prephosphate treatments have been shown to improve zinc-coated steel strip formability in automotive stamping processes and its corrosion resistance during storage and transport prior to painting. A program was undertaken to establish the range of prephosphate coating weights which improve formability without significantly altering spot weldability, adhesive bonding characteristics and final phosphating immediately prior to painting. The effects of coating untreated and prephosphated galvannealed steel with mill oil were also examined. When galvannealed steel with predominantly a delta-phase surface was prephosphated with a trication phosphate coating weight of ∼1.5 g/m2, an 11% reduction in forming load was measured with cup tests. Limiting dome height analysis showed a 27% improvement in performance by prephosphating.