Performance Evaluation of Prephosphated Galvannealed Steel Sheet for Automotive Applications

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. Draw bead simulator friction was found to decrease by 11% for ∼0.5 g/m² and by 14% for ∼1.5 g/m² prephosphate treatments. Also, a correlation between decreasing surface friction and increasing amounts of powdering was observed. Prephosphate treatments did not adversely affect the spot weldability of galvannealed sheet. Electrode tip life was unaffected by prephosphating. Rephosphating studies showed that final phosphating coating weight decreased with increasing prephosphate coating weight. Prephosphating caused no change in adhesive bond strength or adhesive bond durability of galvannealed steel sheet.