Yves Carretta

MetaLub – a slab method software for the numerical simulation of mixed lubrication regime in cold strip rolling

A cold-rolling model taking into account mixed lubrication regime has been developed and included into a simulation software named MetaLub. The main objective is to enhance the performances of rolling mills from a lubrication point of view. It means that lubricant rheology but also roll diameters and roughness, etc. can be optimized to improve stability and efficiency of the rolling tool. The main features of MetaLub are briefly presented in this article. Then, two studies of the influence of rolling speed and negative forward slip are discussed. The obtained numerical results are presented and compared to some experimental data from literature and from ArcelorMittal facilities in order to validate the model and to show its capacity to understand and help to improve industrial rolling conditions.

Numerical Simulations of Asperity Crushing—Application to cold rolling

Asperity flattening has a huge influence on friction and wear in metal forming processes. Nevertheless, phenomena that occur at the microscopic scale are still not well understood. Since no experiments can be easily performed in real forming conditions, numerical models are essential to achieve a better knowledge of what happens in these contact regions. In this paper, two finite elements models are presented. The first one represents the flattening of a serrated asperity field in plane strain conditions. The results are compared to the experiments conducted by Sutcliffe [1]. The second one is a tri‐dimensional asperity model flattened by a rigid plane. The boundary conditions applied to this model correspond to the ones encountered in a real cold rolling case. The results are compared to the relative contact area computed by a strip rolling model using the analytical laws proposed by Wilson & Sheu [2] and Marsault [3].

Ultrasonic roll bite measurements in cold rolling: Contact length and strip thickness

In cold rolling of thin metal strip, contact conditions between the work rolls and the strip are of great importance: roll deformations and their effect on strip thickness variation may lead to strip flatness defects and thickness inhomogeneity. To control the process, online process measurements are usually carried out; such as the rolling load, forward slip and strip tensions at each stand. Shape defects of the strip are usually evaluated after the last stand of a rolling mill thanks to a flatness measuring roll. However, none of these measurements is made within the roll bite itself due to the harsh conditions taking place in that area. This paper presents a sensor capable of monitoring strip thickness variations as well as roll bite length in situ and in real time. The sensor emits ultrasonic pulses that reflect from the interface between the roll and the strip. Both the time-of-flight of the pulses and the reflection coefficient (the ratio of the amplitude of the reflected signal to that of the incident signal) are recorded. The sensor system was incorporated into a work roll and tested on a pilot rolling mill. Measurements were taken as steel strips were rolled under several lubrication conditions. Strip thickness variation and roll-bite length obtained from the experimental data agree well with numerical results computed with a cold rolling model in the mixed lubrication regime.

Complementary Approaches for the Numerical Simulation of the Micro-Plasto-Hydrodynamic Lubrication Regime

This paper presents recent investigations in the field of lubricant escapes from asperities. This phenomenon, named Micro Plasto Hydrodynamic Lubrication (MPHL), induces friction variation during metal forming processes. A better understanding of MPH lubrication would lead to a better management of friction, which is a central element in most sheet metal forming processes. To fulfil that goal, experiments were conducted in plane strip drawing using a transparent upper tool in order to observe lubricant flow around macroscopic pyramidal cavities. These experiments were then numerically reproduced with two complementary Finite Element models. The numerical results are discussed in this paper and show good agreement with experimental measurements.

Numerical Simulations of Asperity Crushing Using Boundary Conditions Encountered in Cold-Rolling

The general framework of this paper is in the field of numerical simulation of asperity crushing. Different material forming processes, such as strip-rolling and deep drawing, imply mixed lubrication. In this lubrication regime, two types of contact are present at the same time: a direct contact between the two solids at the asperity level and also valleys filled with pressurized oil. Theses contact conditions have a large influence on friction and wear taking place during the upsetting process. As this mixed type of contact is not yet fully understood from the physics point of view, numerical models are essential to achieve a better understanding. For example, semi-analytical asperity crushing models have been developed by Wilson&Sheu [1] and Sutcliffe [2] to take into account the influence of bulk plastic deformations on asperity crushing. The finite element method has also been used to model asperity crushing. Ike&Makinouchi [3] studied the behavior of 2D triangular-shaped asperities under different boundary conditions. Krozekwa et al. [4] modeled 3D triangular asperities behavior, for various bulk strain directions. More recently, Lu et al. [5] compared experimental results of pyramid-shaped asperity and ridge-shaped asperity crushing with finite element simulation results. As in the three former references mentioned above, it has been decided, to study the interaction between a rigid plane and a simplified geometry asperity without lubricant. In this article, numerical asperity crushing results obtained with Metafor[6], a home made large strains software, will be presented. Those results will illustrate the influence of boundary conditions, contact pressure, large bulk strain and geometry of asperities on the evolution of the contact area. As the asperity crushing behaviour is known to be very sensitive to the boundary conditions, in this article, we will also present results using boundary conditions from a cold rolling model named MetaLub. MetaLub [7-8] is a software developed at the University of Liege in partnership with ArcelorMittal R&D center. It iteratively solves the equations resulting from the discretisation using the slab method of the strip coupled to a mixed lubrication model at the interface. This lubrication model takes into account the evolution of the oil film thickness as well as the asperity crushing along the roll bite. We will compare the evolution of the relative contact area obtained with MetaLub to the results obtained with finite elements simulations using the same boundary conditions. [1] Wilson, W.R.D and Sheu, S. Real area of contact and boundary friction in metal forming. Int. J. Mech. Sci. 1988, 30(7), 475-489. [2] Sutcliffe, M.P.F Surface asperity deformation in metal forming processes. Int. J. Mech. Sci., 1988, 30(11), 847-868. [3] Ike, H. and Makinouchi, A. Effect of lateral tension and compression on plane strain flattening processes of surface asperities lying over a plastically deformable bulk. Wear, 1990, 140, 17-38. [4] Korzekwa, D.A., Dawson, P.R. and Wilson W.R.D., Surface asperity deformation during sheet forming. Int. J. Mech. Sci., 1992, 34(7), 521-539. [5] Lu, C., Wei, D., Jiang, Z., and Tieu, K., Experimental and theoretical investigation of the asperity flattening process under large bulk strain, Proc. Inst. Mech. Eng. J. 222 (2008), 271–278. [6] LTAS-MN2L. ULg. http://metafor.ltas.ulg.ac.be/. [7] Stéphany, A., Contribution à l’étude numérique de la lubrification en régime mixte en laminage à froid. PhD dissertation (in French), Université de Liège (2008) [8] Carretta, Y., Stephany, A., Legrand, N., Laugier, M., and Ponthot, J.-P., MetaLub – A slab method software for the numerical simulation of mixed lubrication regime. Application to cold rolling. In Proceedings of the 4th International Conference on Tribology In Manufacturing Processes (ICTMP), 2010,799-808.