Influence of prephosphating on the forming behavior of galvanneal.

摘要

Prephosphating of galvanneal steel sheet has been shown to improve its forming performance. This improvement is due to a reduced coefficient of surface friction caused by the prephosphate treatment. Increased levels of powdering have also been observed with prephosphated galvanneal during cup-drawing operations. A major aim of the current study was to gain an increased understanding of the reasons why prephosphating causes the coefficient of surface friction of galvanneal to decrease and the amount of powdering to increase.;The bare galvanneal steel substrate was treated with two different types of prephosphate treatments, a conventional spray process designed to deposit micron-sized phosphate crystals and a dried-in-place treatment designed to deposit a featureless phosphate coating on the galvanneal surface. The spray and dried-in-place treatments were each applied at two different coating weights so that different area fractions of the galvanneal surface were covered by the phosphate. Besides the four prephosphate treatments, a fluoride-etchant treatment designed to remove alumina from the galvanneal surface was also applied. In all cases, the effect of applying mill oil to the blanks was examined by forming cups in the oiled as well as in the non-oiled condition.;The different surfaces were characterized by SEM, ESCA and surface resistance measurements. Cup testing was used to evaluate forming behavior. Cups were drawn from blanks of each material and the maximum forming loads were measured. The changes in strain state along the walls of the cups as they were formed were measured using circle grid analysis. Numerical simulations using finite element analysis also were performed to assess the changes in local strain state with variations in the coefficient of surface friction. Powdering was also measured in conjunction with the cup tests by loss of weight and XRF methods. Both total and localized powdering amounts were measured.;Both spray and dried-in-place prephosphate treatments were found to reduce the maximum forming load required to form galvanneal cups. The maximum forming load was reduced by up to 12% when ;The prephosphate treatments increased the amount of galvanneal powdering from about 5 g/m