THE EFFECT OF SURFACE-MORPHOLOGY ON FRICTION DURING FORMING OF ELECTROGALVANIZED SHEET STEEL

In the automotive industry, the surface morphology of sheet steel is c ontrolled to improve both paint finish and formability. Formability, i n turn, is influenced by the friction between the sheet and stamping t ools, which the industry commonly measures with the 'drawbead simulato r' (DBS) test. This investigation evaluated the use of various surface parameters as predictors of the frictional properties of electrogalva nized (EG) steel sheet as measured in the DBS test. The surfaces of a set of commercial EG steel sheets were measured using a profilometer a nd tested for friction in a DBS machine. The results were compared to various statistical measures of the sheet surfaces. The best of the si mple measures proved to be a combination of two parameters: the arithm etic average roughness (R-a) and the mean wavelength of the roughness, as determined from the Fourier power spectrum. Surfaces with higher R -a were more likely to have lower friction in the DBS tests. But low f riction can be obtained with lower values of the R-a if the roughness is concentrated at the shorter wavelengths (below 500 mu m) Surfaces w ith roughness distributed in the shorter wavelengths have the addition al benefit of being easily painted to a high quality finish. (C) 1997 Elsevier Science S.A.