Abstract
The effects of drawing speed were explored in drawbead simulation tests on galvannealed, electrogalvanized, and hot-dip galvanized sheets, with a cold-rolled steel serving as reference. Draw speeds ranging from 5 to 240 mm s−1 gave, in combination with base oils of 4.5 to 285 mm2 s−1 viscosity, a viscosity × velocity product (S∗) spanning four decades. The nominal coefficient of friction (COF) always declined with increasing S∗ according to a power law; the decline was steeper with sheets having a deformable (zinc) surface with a structure suitable for lubricant entrapment, such as is typical of electrogalvanized and non-temper-rolled hot-dip galvanized sheets. At low S∗, electrogalvanized sheets of greater macroroughness and higher basal component in the coating as well as temper-rolled hot-dip galvanized sheets gave widely fluctuating COF and high enough drawing forces to cause breakage of the sheet. Friction settled to lower values upon the development of a stable transfer layer (coating) on the draw beads. Addition of 2% stearic acid increased friction on zinc-coated sheets, most likely because of junction growth.
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