COMPARATIVE-STUDY OF SHOTBLASTED AND ELECTRICAL-DISCHARGE-TEXTURED ROLLS WITH REGARD TO FRICTIONAL BEHAVIOR OF THE ROLLED STEEL SHEET SURFACES

In tribology it has become common knowledge that surface roughness pla ys a part in boundary lubrication and mixed lubrication systems. There have been different ways of explaining friction and wear in the conta ct area. Some claim that the main factor is adhesion between the inter acting surfaces, while others maintain that wear particles entrapped a t the interface are the predominant factor. In both cases, changing th e surface characteristics can influence the factors mentioned and alle viate frictional problems. In order to investigate how surface topogra phy influences frictional behavior and how the topography changes duri ng deformation, a study on steel sheets of press-forming quality has b een carried out. Steel sheets are manufactured in a railing process wh ere the rolls are used to reduce the sheet thickness and to achieve th e desired surface characteristics. The steel sheets chosen for this st udy were manufactured using two different sets of rolls. The first set had been textured by shotblasting, the second by using electrical dis charge manufacturing, thus giving two different types of surface textu re. To be able to investigate how the wear of the rolls influences fri ctional 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 carri ed out: bending under tension (BUT) and a strip drawing test (modified Inland), using the same lubricant. In both cases, the pulling speed w as high in order to simulate the actual conditions during press-formin g. 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 ro ughness parameters and other methods characterizing the 3D content of the surface. In order to exclude other factors, such as different chem ical properties of the contact surfaces, two different chemical analys es have been made: ESCA (electrical spectroscopy for chemical analysis ) and GDOES (glow discharge optical emission spectroscopy). Correlatio n tests between the frictional behavior of the steel sheets and the mo re widely used surface topography parameters, such as S-a (3D equivale nt of R-a), have shown that other ways of characterizing the surface a re needed to explain how a change in topography changes frictional beh avior. The wear of the rolls does not seem to have any effect on the l evel of friction in these tests. The two test methods show different r esults as to how the surfaces change during the test. (C) 1997 Elsevie r Science S.A.