The effect of surface morphology on friction during forming of electrogalvanized sheet steel

In the automotive industry, the surface morphology of sheet steel is controlled to improve both paint finish and formability. Formability, in turn, is influenced by the friction between the sheet and stamping tools, which the industry commonly measures with the ‘drawbead simulator’ (DBS) test. This investigation evaluated the use of various surface parameters as predictors of the frictional properties of electrogalvanized (EG) steel sheet as measured in the DBS test. The surfaces of a set of commercial EG steel sheets were measured using a profilometer and tested for friction in a DBS machine. The results were compared to various statistical measures of the sheet surfaces. The best of the simple measures proved to be a combination of two parameters: the arithmetic 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 friction can be obtained with lower values of the R a if the roughness is concentrated at the shorter wavelengths (below 500 μm). Surfaces with roughness distributed in the shorter wavelengths have the additional benefit of being easily painted to a high quality finish.