Influence of steel sheet surface-topography on friction in stamping

Steel sheet is the prevalent material for car body panels. In the manufacturing of steel sheets a certain surface roughness is achieved from the ground or textured rolls used in the three final reduction steps. When manufacturing car body panels the sheets first are formed and then painted. For these two processes, the roughness obtained from the reduction rolls will influence both friction and paint results. Furthermore, the processes have environmental effects through the choice of lubricant, amount of lubricant, paint, and amount of paint. From knowledge of pollution and cost, it is easy to understand that it is of great interest to decrease the amounts of both lubricant and paint. Therefore, it is important to understand how the surface topography influences frictional behaviour and paint results. The work presented in this thesis is focused on finding and describing features in the sheet surface topography that influence friction. In order to investigate frictional behaviour and the change in surface roughness of different sheet topographies during forming, a test method that simulates the die radius in the forming tool was used. The method is called Bending-Under-Tension, BUT. In the test, steel strips were bent and drawn over a cylindrical tool, under prescribed pressure, sliding velocity, and lubrication. Thereafter, the surface topography was measured in the contact zone, where the tool was in contact when the test was stopped. By ball-filtering the surface roughness measurements, thereby simulating realistic contact conditions, and then estimating the area fraction of contact, a so-called Wihlborg-Crafoord index (WC index) can be calculated. This WC index indicates whether the surface has a good lubricant supply or not. It is well correlated to the friction coefficient achieved from BUT friction tests in mixed lubrication. The WC index is designed with respect to both the Micro Plasto HydroDynamic Lubrication (MPHDL) and Micro Plasto HydroStatic Lubrication (MPHSL) mechanisms. From this WC index and previous work on prediction of paint appearance, it can be stated that in regard to the surface topography, there is no conflict between the requirements for good paint results and low friction. The paint results depend on amplitudes of waviness between 0.8-3 mm, while the frictional behaviour is dependent on features in an interval of much shorter wavelengths. For low friction, the surface should have many contact areas with many small pockets on them, and the ratio of the perimeter to the area of the pockets should be maximised. The peaks of these contact areas should be located at the same height, and the angle to the edge of a lubricant pocket should be small.