FRICTION IN SHEET FORMING WITH SOFT AND HARD TOOLING

Comparative studies were conducted with the drawbead-simulation (DBS) test on a bare steel, a galvannealed steel, three electrogalvanized st eels, and a hot-dip galvanized steel as representatives of the major c lasses of steel sheets used in the automotive industry. Drawbeads of b are. plasma-nitrided and hard chromium-coated tool steels were prepare d with various surface finishes. The zinc alloy Kirksite represented t he soft tooling used in die tryout. A low-viscosity mineral oil, neat and compounded with oleic acid or a borate compound, served as lubrica nt. Correlation between friction on hard and soft surfaces was governe d primarily by the extent and stability of a zinc transfer layer on th e hard beads. Kirksite may be regarded as the equivalent of a very sta ble, more hard and less reactive transfer film. Correlation was genera lly better when the sheet surface was capable of entrapping the lubric ant, when oleic acid (a boundary additive) was absent, berate (a parti ng compound) was present, and the plowing component of friction was lo w.