Effect of Change of Surface Morphology on Press-formability of Steel Sheets

To clarify the influence of plastic deformation of asperities under contact with forming tool on press-forming behavior, the relationship between drawing force and bead holding force was evaluated for various types of steel strips. The result showed that the friction coefficient between the steel strip and the forming-tool decreased as the bead holding force was increased. However it was found that, particularly for lowstrength steel strips, the friction coefficient decreased considerably as contact pressure became near to the yield-strength. A number of microholes were observed after the drawing tests of low strength steels, which were formed by the plastic deformation during the drawing process. We conclude that these holes acted as micro oil reservoirs during the drawing and were responsible for drop of the friction coefficient. The above-mentioned dependency of friction coefficient on bead holding force was not observed for galvannealed steels (GA) and organic lublicant film coated GA. To evaluate the effect of contact pressure dependent friction coefficient on the drawing force, a finite element method analysis, which incorporated the effect of contact pressure dependent friction coefficient was developed. Using this finite element analysis, we confirmed that the increment of the drawing force could be suppressed by choosing a suitable range of the bead holding force.