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Vancron 40, a nitrided powder metallurgical tool Steel, is used in cold work applications where the predominant failure mechanisms are adhesive wear or galling. Typical applications of Vancron 40 are among others fine blanking, cold extrusion, deep drawing and cold work rolls for cluster mills. Vancron 40 positive results for cold work rolls for cluster mills and as a tool for some severe metal forming process makes it competitive compared to other type of work rolls that require higher precision, among others in cold rolling of thin stainless steel, which required high surface finish quality. In this project, three roll materials for cold rolling of stainless steel strip was examined, Vancron 40, Narva 12B (a high-carbon, high-chromium tool steel alloyed with tungsten) and Supra 3 (a Chromium-molybdenum tungsten-vanadium alloyed high speed steel). The purpose of this project was to study the depth profiles of the ironed stainless steel strips, emergence of galling and to study the lubrication performance used by steel industries. Laboratory experiments were conducted to examine scratch of the strip, galling and surface roughness of the roll materials under severe tribological conditions. The critical sliding length for onset of galling was estimated for stainless steel with four different lubricants. Laboratory experiments result of performance evaluation of resistance capability of rolls toward adhesive wear under severe conditions for low and high reductions. Vancron 40 in combination with cold rolling lubricant gave good surface quality, prevents galling of metal surfaces and good bearing capacity., {"references":["M. Tahir, N.G. Jonsson, J. Lagergren, L. Wikström, H. Hedenlund,\n\"Evaluation of tool material Vancron 40 with regard to wear, surface\nquality and galling\" in Proc. JSI 2012 - 30th International Steel Industry\nConference, Paris, 2012.","S. Hatami, A. Nafari, L. Nyborg, U. Jelvestam, \"Galling related surface\nproperties of powder metallurgical tool steels alloyed with and without\nnitrogen\" Wear vol. 269. 3-4, Jun. 2010, pp.229-240.","O.N. Cora, M. Koc \"Experimental investigations on wear resistance\ncharacteristics of alternative die materials for stamping of advanced\nhigh-strength steels (AHSS)\" International Journal of Machine Tools\nand Manufacture vol. 49.12-13, Oct. 2009, pp. 897-905.","M. Nilsson, M. Olsson, \"Microstructural, mechanical and tribological\ncharacterisation of roll materials for the finishing stands of the hot strip\nmill for steel rolling\", Wear vol. 307.1-2, Sep 2013, pp. 209-217.","A.S. Galakhar, J. D. Gates, JT. William, A. Paul; \"Adhesive tool wear in\nthe cold roll forming process\", Wear vol. 271.11-12, Sep 2011, pp.\n2728-2745.","D. Bacci, G. Ius, \"Wear Behavior of AISI D3 and AISI M3:2 Tool\nSteels in a Mass Production Shearing Operation\", Wear vol. 91.2, Nov.\n1983, pp.209-217.","J. L. Andreasen, N. Bay, \"A strip reduction test for measurement of\nlubricity in ironing\", in Proc. 19th Biennial IDDRG Congress, Advanced\nSheet Metal Forming; Eger, 1996.","J, L. Andreasen, N. Bay, M. Andersen, E. Christensen, N. Bjerrum,\n\"Screening the performance of lubricants for the ironing of stainless\nsteel with a strip reduction test\" Wear vol. 207.1-2, (June 1997, pp. 1-5)."]}

The only way to establish the true rolling pressure and the true friction condition in cold rolling is to conduct measurements in the roll bite. A new transducer design is therefore proposed, in order to overcome problems in previous measurements in the past 70 years. The new idea is to increase the contact surface of the transducer, to be larger than the arc of contact. This is in contrast to the smaller and smaller contact pin design that has been prevailing. The measurements were conducted during cold dry rolling of both copper strips and stainless steel strips in a pilot mill. The recordings were selected from a steady state with no disturbance from the material flow. The transducer was able to simultaneously measure both the normal pressure and the friction stress. An estimation of the coefficient of friction was accordingly performed. The new transducer works very well, it was seen to be robust and able to avoid signal disturbance. The pressure and friction stress distribution results was as expected by the authors and showed good reproducibility, together with a proven agreement between recorded and simulated signals.

To improve the accuracy of rolling force prediction, some important force models were evaluated through applied computation for cold rolling of low carbon steel and aluminum alloy according to measured data on lab mill. The effects of model structure and three important variables - flow stress, contact length and friction coefficient - on the precision of computed force were quantitatively studied. Flow stress was measured with plane-strain compression test, contact length was based on elastic flattening of work-roll by Hitchcock, and friction-coefficient was determined by rolling strain and numerical iteration. In steel rolling Bland & Ford integration model and Bryant & Osborn algebraic equation are better in accuracy than Ekelund and Parkins. In aluminum rolling all the models produce large deviations ΔF R = 10-20% if flow stress, contact length and friction coefficient are determined with the same method as steel rolling. The elastic deformation of aluminum strip is now taken into account for its low elastic modulus. An effective method to determine plastic and elastic contact has been developed in this investigation. The accuracy of force computation is obviously improved for aluminum rolling.

Using a model duo mill, and model material and lubricants, the hot strip rolling process has been simulated. The normal roll pressure, the shear stress and the shear-stress angle in the roll gap of the mill was measured by load tranducers, the purpose being to evaluate assumed frictional influencing factors, such as: strip reduction, rolling velocity, lubrication and different materials rolled. In order to improve the present simple-friction model for force calculation in hot strip rolling that uses just an average friction value, three friction models were evaluated: the friction laws by Coulomb, Tresca and Wanheim-Bay. The best model was then chosen on the basis of the measurements made in this study. Finally, suggestions for the improvement of the model of strip rolling is made.

During normal pressure measurements in flat rolling, conducted in the roll gap of laboratory mills, unexpected multiple pressure peaks appeared. These multiple pressure peaks, such as a double peak, have not only been measured during rolling of billets, but also when rolling slabs or strips. Not explained by the established rolling theory stating just a single maximum point. In order to investigate its physical mechanisms a number of rolling experiments are presented. A model duo mill, load transducers, powder lubrication or by vaseline and wax as model material were used. The normal pressure, the shear stress and its angular movement were measured simultaneously. By first measuring a double normal pressure peak during rolling of a wax specimen and then by consciously making changes in draft, specimen thickness, rolling velocity, specimen width or the friction conditions, respectively, the double peak phenomena were investigated by means of the responding normal pressure distribution. A number of direct measurements in the roll gap by other researchers using different techniques, have also been investigated. The conclusion from the wax rolling experiments and the rewiew of measurements by others is that the peak at entry of contact is caused by the presence of compressive stresses from rigid zones outside the deformation zone, raising the hydrostatic pressure and the normal pressure. The effect is observed by inhomogeneous deformation. The peak value at the entry of contact can then be very high, by analogy to plane indentation with elastic regions present. The resistance of the wax material to the height deformation of the specimen was seen to increase at small reductions or by a specimen with a greater nominal thickness, supporting the rigid end theory. The result of this study is also that the exit peak in the double peak is caused by frictional surface stresses at the roll/specimen interface. The effect was seen by changes in frictional influencing factors, such as the reduction or the surface friction condition. Lateral spread of the material rolled was at the same time seen to make the double peak more distinct, decreasing the normal pressure in the valley between the two peaks and at exit of contact.

In the strip rolling process friction is needed at first to pull the strip into the roll gap, otherwise slipping occurs. After the neutral plane however, where the velocity of the work roll equals the velocity of the strip, the friction stress alters direction to the opposite. If the friction stress after the neutral plane then is lowered, the total roll force will decrease. This can be done by a properly applicated oil- and water dispersion close to the work roll gap and between the roll cooling wipers. The technique and the key to a successful lubrication is a work roll surface which is as dry as possible. If the roll surface is not dry, neither large changes in the oil concentration, nor changes in the oil composition can make the lubricant stick to the roll surface. The lubricate was partly washed away by the water and only a small roll force decrease could therefore be measured. Only when the roll cooling water on the entry side of the work roll was set off on purpose, the roll force decrease was significant. Due to this result, further test were performed in an upstream stand in the hot strip mill. Here, the work roll wipers were modified and the roll was substituted by one with a larger radius and lubricated. The result was very slight or no water leakage at all. The roll force decrease was then much greater than before and close to those measured for the same reduction in the case of no entry water for the later stand. The electrical current in main drives and the roll wear in the lubricated stand could also be lowered significantly. In addition, the strip surface was greatly improved, under conditions with a high possibility of oxide residual contamination from the roll surface.