Novel Laboratory Lubrication Tests for Cold Rolling Emulsions

research scientist, Quaker Chemical Corp., Uithoorn, The Netherlands [email protected] Cold rolling involves intensive frictional contact between two highly loaded metallic surfaces in sliding contact. Together with the deformation-related parameters, such as hardness and reduction, the frictional conditions in the roll bite determine the roll forces and torques required for rolling (Figure 1). Steel manufacturers are often looking for ways to reduce these roll forces and torques, thereby allowing them to speed up the process. The alloy hardness and reductions are process requirements and cannot be altered, but the friction in the roll bite can be influenced by considering the lubricant’s physico-chemical properties. Just prior to the roll bite, one finds the inlet, or entry zone, filled with lubricant. This zone is characterized by two fast-moving, converging surfaces. According to the Reynolds equation,1 under such conditions, pressures in the lubricant will rise, causing a lubricant film to form between the mating surfaces. In cold rolling, due to the surface roughness, this film generally only partially separates the surfaces. So, just beyond the inlet zone, the surfaces will start to touch, and one can define the beginning of the roll bite, or contact zone, where the frictional conditions are determined. Cold rolling occurs in the socalled “mixed lubrication (ML) regime.” This means that in the roll bite, there are regions of some surface separation, where friction is determined by the rheological properties of the lubricant under high pressures, temperatures and shear rates, and there are also regions of close surface contact, where friction is determined by the surface chemistry of strip and roll. The friction in the roll bite is strongly influenced by the properties of the lubricant, either through its bulk rheological properties, or by its ability to react with the mating surfaces (see Figure 2 for a schematic drawing). Figure 2a shows the entry zone, where the film thickness is determined, and the contact zone, where friction occurs due to the sliding of the work roll over the accelerating strip. Figure 2b illustrates that there are several lubrication