A new method for analysing the mixed lubrication in metal rolling.

Abstract

Mixed lubrication is an important lubrication mode in metal rolling, and if controlled properly, can improve the performance of a metal rolling process. However, the choice of appropriate processing parameters for realising proper mixed lubrication is a big challenge due to the complex solid-fluid interaction, surface asperity deformation, and the random nature of the asperity distributions through the rolling gap. A complete solution to mixed lubrication with full resolution down to the microscopic asperity level is formidable, although the fundamental fluid and solid mechanics have been established. A practical approach is to solve the problem in macroscopic scale but with sufficient ingredients of microscopic contact mechanics. A great deal of research efforts in the past decades has led to some implicit solutions, which include slab method in coupling with some asperity contact models. In these studies, film thickness and asperity contact are solved when an external pressure is obtained from the deformation of the strip. The load sharing by the liquid and the asperities are then solved in an iterative procedure. Such a process, however, often leads to divergence.

The aim of this thesis is to establish an explicit numerical approach to investigate mixed lubrication in metal rolling. The asperities on a slab surface were treated as contact springs with non-linear stiffness based on the Greenwood-Williamson's formulation, which relates the dry contact stress to the separation between the roll and slab's reference surfaces. The lubricant film thickness was assumed to be a function of the separation and the actual dry contact area. This approach was successfully implemented as a user-interface subroutine in the ABAQUS explicit FE code. The developed of user interaction code was verified by Hertz contact problem. The results of a case study on strip rolling analysis were compared with the experimental results and a good agreement was found. A detailed parametric study was explored for the effect of the viscosity and rolling velocity on contact pressure, frictional force, friction reduction, hydrodynamic pressure in mixed lubrication.