The effect of surface deformation on lubrication and oxide-scale fracture in cold metal rolling
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INTRODUCTION
LUBRICATION is vital to the success of cold metal rolling processes. The requirements of low friction and good strip-surface finish dictate that most rolling processes are operated in the “mixed” lubrication regime, where there is asperity-to-asperity contact between the roll and strip as well as oil drawn in due to the wedge-entraining action at the inlet to the bite. Previous studies show that friction is strongly dependent on the ratio of the true asperity contact area to the nominal contact area between the roll and the strip (the “contact ratio”). It also depends on the effectiveness of any boundary lubrication mechanisms present.[1–6] The contact ratio depends on an inlet lubrication parameter (s), the ratio of a theoretical oil-film thickness (hw) to the combined roll, and the strip-surface roughness (t). The film thickness is given by Wilson and Walowit’s formula[7] for hydrodynamic entrainment at the inlet to the bite with smooth rolls and strip, as illustrated in Figure 1(a): hw
6h0 au f0 (1 eaYs)
[1]
where 0 is the oil viscosity at ambient pressure, is the pressure-viscosity index of the oil, u is the entraining speed (i.e., the mean of the roll and strip speeds at entry), 0 is the roll slope at entry, and Ys is the plane-strain yield stress of the strip. Equation [1] shows that the wedge angle (0) between the roll and the strip at the entry to the bite is an important parameter affecting lubrication in the bite. It is usually assumed that the strip is undeformed ahead of the bite, so that this wedge angle equals the roll slope at the entry to the bite. However, distortion of the strip surface ahead of the bite will tend to reduce the wedge angle, as illustrated in Figure 1(b). The effect of a roll’s elastic deformation on the elastohydrodynamic lubrication of elastic Hertzian contacts has been addressed by Grubin and later by Greenwood,[8] based on half-space solutions. For the plastic rolling process, the elastic deformation of the rolls, which is particularly important for foil rolling, has been considered by Johnson[9]
H.R. LE, Senior Research Associate, and M.P.F. SUTCLIFFE, Senior Lecturer, are with the Department of Engineering, University of Cambridge, Cambridge C82 1PZ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted March 17, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
and Fleck et al.,[10] although the effect of strip-surface deformation ahead of the entry was not accounted for. Deformation of the strip surface is also responsible for the fracture of oxide films and the subsequent extrusion of metal through the microcracks in the metal rolling process. This has a significant impact on the interface between the tool and workpiece. Milner and Rowe[11] reported that aluminum oxide breaks up into small particles during rolling. Ball et al.[12] showed that the oxide scale formed on aluminum ingots during preheating is broken into arrays and that metal is extruded through the cracks under compressive stress during hot rolling. Similar pheno
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