On Surface Topography of Steel Sheet for Car Bodies

Doktorsavhandling, 1997

This thesis is a contribution to the scientific area of surface topography on sheet metal surfaces. The studies are restricted to exploring the influence of the steel sheet,s surface topography on forming properties and on appearance after painting. They also deal with problems associated with the measuring of surface topography in three dimensions (3D). In six papers the results from experimental studies concerning measuring, friction, and painting of sheet metal surfaces are presented. The results can contribute, on the one hand to the development of new surface geometries for the steel producer, while on the other hand increasing the possibilities of the steel users to specify the sheet material,s surface topography more linked to the desired functions of the surface. The 3D approach for assessment of surface topography has been adopted in order to understand the relations between the functional demands on the sheet metal surface and its surface topography. Compared to profile measurements, a 3D measurement offers completely new possibilities to understand the surface microgeometry from a functional point of view. The lack of standards for 3D analysis of surface topography prompted investigations of different measurement and evaluation strategies. The sampling area size was identified as important in order to obtain single measurements that are representative of the whole sheet surface. The filtering of surface measurements is a critical part of the topographical analysis. A ball filtering technique was adopted and implemented. The result of such an envelope filtering is determined by the topmost part of the surface, similar to the actual contact between sheet and tool. The influence of surface topography on paint appearance was investigated by fourier analysis of both surface profiles and 3D measurements. The results revealed that the possibility to produce a superior paint finish was increased when no waviness existed in the range 0.8-3 mm in the unpainted surface. Analysis of the areal information in the surface topography, such as contact and lubrication morphologies, shows relations to friction properties. For a heavy oil it was shown that the presence of relatively large isolated oil pockets decreases the coefficient of friction. The results point at a minor influence from the amplitude of the surface topography while the lateral surface features are of major importance for obtaining a desirable surface topography in the contact interface between sheet and tool.