Simulation of contact friction in the hot rolling of steel sheet

Abstract

Hot rolling creates complex deformation conditions at contact between the quenched strip and the working rollers and radically changes the boundary friction, which complicates simulation of this process. On the basis of many years’ experience with experimental lubricants in the hot rolling of steel strip in laboratory and industrial conditions, data regarding the influence of lubricant on the rolling parameters and the surface quality of the strip, and observations of the working surface of rollers in different periods, we derive conclusions regarding the formation of a lubricant contact layer at the rollers in accordance with the molecular-physics theory of contact friction and corrections to the theory. The process may be divided into three periods. First, in the brief period of mill operation without a load, a boundary layer of active lubricant is formed at the roller surface, in accordance with the molecularphysics theory of boundary friction [1]. In the absence of an external load, the thickness of the lubricant layer formed is half of the boundary phase determined by the force field of the solid roller surface, the temperature, and the properties of the atomic structure of the lubricant molecules. In the initial period, these factors determine the physical state of the material forming the boundary phase. When active lubricant is supplied to the rollers, a half layer of boundary phase is formed at the roller surface. In four-roller cells, if the lubricant is supplied to the working or supporting rollers, the boundary phase is transferred from one roller to another through the frictional contact between them. The thickness of the lubricant layer formed on subsequent rollers is also half the boundary phase without an external load. This transfer of the active-lubricant layers between the working and supporting rollers, confirmed in investigating the lubricants in industrial mills, permits the creation of rational methods and the design of oiling systems for thin-sheet hot rolling with lubricant, in production conditions. The second stage, which is the most prolonged, is associated with partial or complete combustion of the thin organic-lubricant layer in the deformation source. The combustion products, which have antifrictional properties, reduce the frictional contact forces. There has been virtually no study of the complex frictional mechanism in the deformation source during hot rolling. It is obvious, however, that occlusion occurs at the contact of the strip with the working rollers: specifically, the frictional forces are reduced, while the thickness, structure, and color of the scale are changed. Research shows that the working surface of the rollers is covered by a thin soot coating, which is retained throughout rolling and stabilizes the contact friction at a lower level; this determines the efficiency of the technological lubricant.