The effects of phosphate treatments on surface friction and coating delamination of galvannealed sheet steels

Application of a prephosphate treatment to the surface of a galvannealed steel sheet prior to a forming operation has been commercially adapted in the steel industry to improve its formability. The issue of increased amounts of coating delamination, called powdering, of the prephosphate treated galvannealed steel during forming has also been a concern in industrial application of such material. The main focus on this study was to define the origins of the improved forming performance of prephosphate treated galvannealed steel and also the reason for increased powdering during forming.; The assessment of formability of bare and prephosphate treated galvannealed steels were performed using a cup drawing test at hold down loads that ranged from 5 to 45 kN and punch velocities that ranged from 4 to 300 mm/sec. Prephosphate treatment, eventually, reduced the maximum forming loads about 5 to 10 percent compared to the galvannealed steels without phosphate treatment. At a higher forming rate of 300 mm/sec the maximum forming loads of both bare and prephosphate treated galvannealed steels were significantly reduced compared to relatively low forming rates. The measured values of the coefficient of surface friction of the prephosphate treated galvannealed steels remained lower than those of untreated galvannealed steels as punch velocities increased from 4 to 300 mm/sec. The coefficient of surface friction was found to increase as punch displacement increased. The role of the prephosphate treatment in reducing surface friction was concluded to be due to prephosphate making asperities on the galvanneal surface more rigid and causing a weaker interface to be present at actual asperity contact points.; Prephosphate treatment increased the amount of powdering of galvannealed steels as measured using fully formed cup tests. Partially formed cup tests, Marciniak biaxial tension/plane strain tests were performed to understand the relationship between powdering and various strain states. From the results of powdering measurements and SEM analyses, coating delamination was found to be a strong function of the true minor compressive strain developed during a forming operation. Tensile strains alone did not produce a significant amount of coating delamination when the galvannealed steel sheets are deformed.; Based upon the experimental observations the mechanism of coating delamination of the galvannealed steel sheets were developed using fracture mechanics fundamentals. A wedging mechanism was much more relevant for galvanneal coating delamination rather than a buckling mechanism. From the model and the experimental observations, it can be expected that prephosphate treatments fill in the spaces caused by preexisting cracks in galvanneal coatings. The role of zinc phosphate is to reduce the coefficient of surface friction along the wedging interface which allows the generation of larger tensile stresses normal to the galvanneal coating/substrate interface which results in increased amount of powdering.