Comparative study of shotblasted and electrical-discharge-textured rolls with regard to frictional behavior of the rolled steel sheet surfaces

Abstract

In tribology it has become common knowledge that surface roughness plays a part in boundary lubrication and mixed lubrication systems. There have been different ways of explaining friction and wear in the contact area. Some claim that the main factor is adhesion between the interacting surfaces, while others maintain that wear particles entrapped at the interface are the predominant factor. In both cases, changing the surface characteristics can influence the factors mentioned and alleviate frictional problems. In order to investigate how surface topography influences frictional behavior and how the topography changes during deformation, a study on steel sheets of press-forming quality has been carried out. Steel sheets are manufactured in a rolling process where the rolls are used to reduce the sheet thickness and to achieve the desired surface characteristics. The steel sheets chosen for this study were manufactured using two different sets of rolls. The first set had been textured by shotblasting, the second by using electrical discharge manufacturing, thus giving two different types of surface texture. To be able to investigate how the wear of the rolls influences frictional behavior, sheets were chosen which had been made at different stages of roll lifetime: newly textured, half-way spent, and ready to be taken out of production. Two types of friction test have been carried out: bending under tension (BUT) and a strip drawing test (modified Inland), using the same lubricant. In both cases, the pulling speed was high in order to simulate the actual conditions during press-forming. Vickers indentations were made on the sheets in both friction tests, thus allowing relocation and comparative studies on the same area of the sheet, before and after testing. The corresponding surfaces have been measured with a stylus instrument and analysed with 3D surface roughness parameters and other methods characterizing the 3D content of the surface. In order to exclude other factors, such as different chemical properties of the contact surfaces, two different chemical analyses have been made: ESCA (electrical spectroscopy for chemical analysis) and GDOES (glow discharge optical emission spectroscopy). Correlation tests between the frictional behavior of the steel sheets and the more widely used surface topography parameters, such as S a (3D equivalent of R a), have shown that other ways of characterizing the surface are needed to explain how a change in topography changes frictional behavior. The wear of the rolls does not seem to have any effect on the level of friction in these tests. The two test methods show different results as to how the surfaces change during the test.