Friction at strip-roll interface in cold rolling

Friction and lubrication are important in the roll bite of cold rolling process as they influence the rolling force and other rolling process parameters. The rolling force determines the reduction of strip being rolled, the rolling mill stretch and final strip thickness and shape accuracy. The ability to control more accurately the rolling force in industry is becoming increasingly important. Inaccurate understanding of friction in the roll bite has affected the accuracy of rolling force prediction, and hence the accuracy the final strip thickness. Much research work has been done in modeling the pressure distribution for the rolling process under fully hydrodynamic lubrication and mixed film lubrication. At the same time, some experimental works have also been done in determining the value of friction coefficient in cold rolling by using embedded pin transducer method in the work roll. This study proposes to determine the friction coefficient and temperature in the roll bite in cold rolling by experimental methods. A sensor roll embedded with loadcells and strain gauged pins was designed and manufactured to determine the friction coefficient along the roll bite in cold rolling. The loadcells and strain gauges were calibrated insitu. The average friction coefficient was also derived from the forward slip which was determined by Laser Doppler method and a strip marking method. The temperature over the roll bite was measured by the embedded thermocouple under different rolling conditions. A large number of experiments were performed to determine the friction coefficient under different reduction, rolling speed, surface roughness, material property and lubricants. The relationships between the rolling parameters such as rolling force, rolling torque, temperature, lubricant, friction coefficient and surface roughness etc. have been discussed. Empirical formulae of friction coefficient and rolling force were given. The measured friction coefficient was used in the calculation of rolling force and torque which were compared with the measured values to validate the measured friction coefficient. A theoretical model was developed by to consider the hydrodynamic inlet zone, plastic work zone and hydrodynamic outlet zone. The effects of lubricant and friction coefficient etc. in the mixed film model were discussed. The effect of the modification to the hydrodynamic film thickness at the inlet caused by the strip elastic recovery at entry was also considered. The temperatures in the lubricant and at the asperity contacts were calculated, respectively. The lubricant temperature at the roll and strip interface and lubricant mean temperature was obtained by using the energy equation. The temperature at asperity contact was calculated by using the moving heat source theory. The temperature calculation considers not only the plastic deformation of the bulk material, but also the frictional heat at the interface. The effects of different friction coefficient, reduction and fraction of plastic work converted to heat have been discussed. The final strip and roll surface calculated temperature was verified by the experimental work and other author\u27s work