THE HISTORY OF COLD ROLLING
Q – Cold rolling is the mechanical working of the Steel below Recrystallization Temperature to achieve close thickness tolerances, better surface finish.
One of the unique selling points of the Corus steel mills at IJmuiden is the quality of the steel they produce.
The lubrication used during the last production stage is one of the factors that determine how clean and smooth the steel will emerge from the mill.
The usual lubricant consists of an emulsion of oil in water. Until recently, little was known about the physical principles underlying the action of lubricant emulsions in steel mills.
The only way to test new lubricants was by applying them during the actual production process. Physicist Marjolijn Trijssenaar of the subfaculty of Applied Physics Delft University of Technology has changed all this.
She managed to unravel the complex chemical and physical processes that take place in the roll gap during cold rolling, and to translate this knowledge into a model that can be used to describe and optimise the lubrication process.
Cold rolling is one of the last stages in the production process of strip steel. Slabs of steel are rolled, first hot, and then cold if necessary, into strips several kilometres long and a few tenths of a millimetre thick. At the end of the process the speed at which the material travels through the mill is over 70 km per hour. Yet cold rolling is a relative term, for the rolling process causes the temperature of the steel to increase so much that cooling water has to be continuously sprayed onto the steel strip and the rolls.
THE NECESSITY OF ROLL COOLANT
Coolant is an emulsion of water, rolling oil, and an emulsifier. The emulsifier molecules, which consists of hydrophobic and hydrophilic parts, ensures that the oil can float in the water as finely distributed particles.
The very fine oil droplets, just about 3.5 micrometres in diameter, are enveloped in a single layer of emulsifier molecules, which prevents them from sticking together, and thus helps to keep them suspended in the water.
Using an emulsion as coolant ensures that both the rolls and the steel strip are lubricated at the same time.
The lubricating oil reduces the friction between the roll and the steel strip, which are pressed together at high pressure. The result is that wear and tear on the rolls is reduced, and the rolled steel strip comes out nice and smooth.
The composition and the action of the rolling emulsion are essential to the production process and for the quality of the steel. Too much lubrication, and the steel cannot be pulled through the mill, because the rolls will slip on its surface.
Too little lubrication, and the resulting friction may cause the steel surface to become severely polluted and damaged.
On top of that, the roll power may be insufficient to achieve the desired reduction rate.As a result of these quality requirements, and more recently, environmental and health requirements, manufacturers of rolling oil are continuously introducing new, sometimes better, products.
Pure oil cannot be used in the cold rolling process, for this would soon render the steel strip too greasy. Oil in a highly diluted form is thus applied as an emulsion.
The oil film thickness on the rolled strip is a measure of the performance of the lubricant. The oil film is invisible to the naked eye. The thickness of the film is of the order of magnitude of 0.1 micrometre, which is nothing compared with the irregularities of the material surface, which measure between 0.5 and 0.3 micrometre.