Wear 257 (2004) 471–480 Asperity deformation, lubricant trapping and iron fines formation mechanism in cold rolling processes S. Huart, M. Dubar ∗ , R. Deltombe, A. Dubois, L. Dubar Laboratoire d’Automatique, de Mécanique et d’informatique Industrielles et Humaines, UMR 8530, Université de Valenciennes et du Hainaut Cambrésis, F-59313 Valenciennes Cedex 9, France Received 25 June 2003; received in revised form 14 January 2004; accepted 26 January 2004 Abstract Contact conditions in cold rolling are decisive to improve quality and final aspect of rolled strip. The control of contact conditions is obtained by a good combination of process parameters, such as normal and tensile force, forward slip and lubrication regime. To understand and to optimise contact at roll–strip interface, a simulation test was developed in our laboratory. The upsetting rolling test (URT) permits reproduction of the main contact conditions for cold rolling. The URT is able to simulate and to define the influence of rolling parameters on the iron residue quantities, and on the quality of the final surface. In accordance with finite element simulations, we can estimate a mean Coulomb’s friction coefficient. A very good correlation was found between experimental and numerical results. A complementary finite element model has been developed to predict the local behaviour of surface asperities during cold rolling. In this simulation at local scale, the roughness and trapping of lubricant are introduced, in addition to the pressure force and forward slip. It has been found that forward slip and reduction ratio modify the plastic strain behaviour of asperity through pressure and shear force. Thus, we are able to define the quantity of iron residues, as a function of the local plastic strain and the plastic energy solutions. © 2004 Elsevier B.V. All rights reserved. Keywords: Cold rolling; Experimental simulation; Finite element method; Asperity; Trapping; Iron fines 1. Introduction Tribology in forming processes is a parameter often un- known by manufacturers. An optimum contact condition is required to improve rheology and quality of the produced parts. In cold rolling, productivity and surface quality re- quirements of rolled strip increase constantly. The produc- tivity can be obtained by the rise of reduction rate and rolling speed, but the final surface quality is more difficult to obtain and the control of contact conditions in roll-bite is necessary. The rolling mill studied here is based on the Sendzimir tech- nology. It is a 20 high cluster mill. This reversible rolling mill is used for reduction of low carbon steel strips, which have been previously pickled in sulphuric acid. The pickling gives a specific strip roughness before cold rolling. The de- gree of initial strip roughness, associated to bad parameter adjustments, favours surface degradation. A poor strip sur- face quality after cold rolling may have a large impact on the downstream process, like annealing, galvanisation and painting. A phenomenon caused by a direct contact between roll and strip, is the wrenching of asperity from the strip ∗ Corresponding author. Tel.: +33-3-27511381; fax: +33-3-27511317. E-mail address: mirentxu.dubar@univ-valenciennes.fr (M. Dubar). surface. These iron fines are removed in large quantities by emulsion, nevertheless, a residual quantity of fine can remain on the strip surface. At the next stage, an annealing section between cold rolling and galvanisation ensures degreasing of strip surface. The organic elements such as oil and im- purities will be reduced into carbon whereas the iron fines remain in iron residues. In contact with the galvanisation bath, the completely wrenched asperities create a reaction called dross, while the residues are dragged by the strip. In this case, fines remaining on the strip can cause adherence defects on the galvanising layer and a rapid oxidation [1–3]. Optimisation of friction and contact conditions between work-roll and strip goes through lubrication and process parameter adjustments. The lubricant (emulsion of oil in water) leads mainly to a reduction of torque and separating force on roll. An adapted emulsion is necessary to improve the contact conditions at roll–strip interface. Furthermore, the lubricant is used to evacuate heat produced by the plastic strain and friction energy in the strip. The process parameters have also a decisive effect on the wrenching of asperities and the surface quality. As ex- plained by Deltombe et al. [4,5], several parameters such as reduction rate or forward slip act on the surface degra- dation. To define the mechanical contact in cold rolling, an 0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.01.012