The Difficulties of Predicting the Coefficient of Friction in Cold Flat Rolling

Abstract

The flat rolling process is initiated when the frictional forces draw the strip to be rolled into the roll gap. These forces depend on the coefficient of friction, knowledge of which is essential to understand, describe and analyze the process. Several predictive formulae for the coefficient have been presented in the technical literature. Contradictions are observed, however, when their predictions are compared to each other. The data obtained while cold rolling aluminum and steel strips are used in the analyses. A model of the rolling process – accounting for strain hardening, frictional events and varying speeds - is then used to determine the coefficients of friction. Use of statistical analyses is found to yield more reliable results than the use of the predictive relations.

Models for symmetric cold rolling of an aluminum sheet

“…where ε is a thickness reduction, z is a coordinate along the thickness of the sheet, x is a displacement of the sheet in the rolling direction, γ is a derivative of displacement of the sheet in the rolling direction, ε S is the shear deformation, and ε vM is a von Mises strain. An important parameter of the rolling process is the friction coefficient µ, which is the result of a complex mechanism, effected by various physical parameters, as is summarised by Szűcs [3,14] and Sidor [15]. The most important parameters allowing estimation of µ are the temperature, the pressure that acts on the surface, and the slip velocity between the sheet and the rolls [3].…”

“…An important parameter of the rolling process is the friction coefficient µ, which is the result of a complex mechanism, effected by various physical parameters, as is summarised by Szűcs [3,14] and Sidor [15]. The most important parameters allowing estimation of µ are the temperature, the pressure that acts on the surface, and the slip velocity between the sheet and the rolls [3]. According to a commonly used approximation, these dependencies can be neglected, and one can estimate a representative value of µ for the entire process, i.e.…”

“…The µ min calculated by using the parameters presented in Table 2 is 0.049. Alternative models dealing with the friction are described in [3,14,[16][17][18][19][20][21][22]. The studies can be extended with further parameters such as microstructural properties as described by Sidor [23].…”

“…This technological parameter expresses the rate of shear and normal stresses in the contacts of the sheet and the rolls, as it is expressed among others by Avitzur [2]. There are many models for describing the deformation rate as a function of various technological and physical parameters such as temperature, slip velocity in the contact, normal pressure, the shear strength of the sheets material, or the roughness of the surface [3,4], however, in this study, a simple approximation is used with a constant value.…”

Investigation of materials flow during the manufacturing process by experimental evidence and numerical approaches

“…The other important parameters, which can be used during the description of the deformation processes, are the "coefficient of friction (COF)", and the "roll gap parameter". The friction coefficient means the rate of the maximal slip stress and the normal pressure [30], there are many other, similar parameters for describing this phenomenon [31][32][33]. On other hand, there are more parameters that can be applied during this calculation [34,35], but in our study, we have used only the geometrical parameters for calculating a minimal value of the coefficient of friction [36].…”