THE PROCESS OF COLD ROLLING
Cold rolling is a process by which the sheet metal or strip stock is introduced between rollers and then compressed and squeezed. The amount of strain introduced determines the hardness and other material properties of the finished product.
The advantages of cold rolling are good dimensional accuracy and surface finish
The mill roll itself has a certain roughness, which it needs to get a proper grip on the steel strip. At the same time, the roll surface acts as a stamp, so the roughness of the rolled strip does not decrease as the strip becomes thinner and longer.
As a result of the rolling process, the strip is stretched. Unless the quantity of oil is replenished, the stretching process would cause the oil film to become much thinner as it has to cover a much larger surface.
To keep the oil film on the steel strip at the required level, it has to be replenished with oil from the emulsion that is sprayed onto the material. In principle there is no shortage of oil. During the rolling process, an excess of emulsion, which contains about 2.5 percent lubricating oil, is sprayed onto the steel strip and the rolls from a series of nozzles.
To determine the thickness of the film of oil after rolling, sections of steel were cut from the rolled strip, and rinsed in Freon, which dissolves the oil (due to environmental considerations, alternative methods are now being used). The oil concentration in the wash was determined using infrared spectrometer. The quantity of oil particles that actually ended up from the emulsion spray in the oil film was calculated by taking differential measurements between consecutive roll passes.
The results indicated that the preset reduction rate (thickness per pass) was the determining factor. As the reduction rate increased, more oil particles passed from the emulsion to end up in the oil film during rolling, which is exactly what makes a good cold rolling process.
This does not mean that we can recommend simply increasing the reduction rate in the production plant, for the capabilities of the system are limited by the pressure of the roll and the resulting heat development.’
During the rolling process, wear and tear of the roll, or damage to the strip can cause steel debris to land on the surface. Good lubrication can prevent wear. In addition, cold rolling will always remain a kind of balancing act between strip tension, equipment power, and slip.
RELATIONSHIP OF ROLL BITE AND EMULSION
A rough surface can pick up quite a lot of emulsion so the oil film had to be formed in the entry zone. The emulsion on the steel strip moves with the strip at a constant speed in the direction of the roll contact point. The emulsion on the roll also moves in that direction. There is an accumulation of excess emulsion in the contact angle between the roll and the strip.
Just before the roll and the strip meet, the two liquid flows collide, and the excess emulsion is pushed back through the middle of the contact angle. A very small quantity of the emulsion ends up between the roll and the strip.
Model-based calculations of these flows showed that the shape of the flow profiles, like the oil film thickness readings, depended on the reduction rate.
The speed at which the rolled strip leaves the machine is slightly higher than the turning speed of the roll. Since the strip becomes thinner and stretched by the rolling process, the speed of the steel strip is much lower before it passes through the rolls.
As the reduction rate increases, the difference in speed between the roll and the strip as it enters the rolls increases, which results in a highly asymmetrical distribution in the flow profile of the emulsion in the entry zone.
The particles moved towards a position of equilibrium, which in this case is either towards the strip or the roll. The segregation of the oil particles from the emulsion reaches its maximum in the angle where the strip and the roll almost come into contact with each other. The oil concentration increases at that point of the entry zone. In the contact zone I have measured increases to as much as 30% oil. So, the entry zone contains a highly enriched emulsion.
SCHEMATIC DIAGRAM OF COLD ROLLING SET
Schematic diagram of a cold rolling set containing two work rolls and two, larger, backup rolls.
Cold rolling does involve high temperatures, up to about 400°C. Since the process involves high pressures with steel facing steel, proper lubrication is essential, as is cooling to cope with the high temperatures. If the oil/water mixture can no longer cope, the rolling process is affected, and the results can be bizarre.
Schematic Diagram of Work Rolls and Oil Distribution over the strip
The films varied in thickness between 50 and 190 nanometres for varying reduction rates. The reduction rate is expressed as:(thickness in – thickness out) : (thickness in) x 100%
The reduction rate was the determining factor for the precipitation of oil on the rolled steel. Remarkably, this proved to be independent of the type of oil and the type of emulsifier.
Important parameters include the radius of the work roll, the turning speed of the roll, the speed of the steel strip, the thickness of steel strip as it enters and leaves the rolls, the angle between the incoming strip and the roll, and finally, the height and length of the entry zone (i.e. the area between the roll and the strip just before they make contact).
Schematic representation of the flow in the entry zone. Lubricant is carried along by the surfaces of the roll and the steel strip, and is pushed back through the centre of the entry zone. Only a small fraction of the lubricant ends up in the contact zone between the roll and the strip.
The figures above show the direction and the speed of the lubricant in the entry zone. The entry zone is limited by the surface of the steel strip (the horizontal line at the bottom) and the surface of the roll (the curved line at the top). The three figures show the effect of the reduction rate on the speed profiles. In figure A, the reduction rate is nil, and the speed profile is fairly symmetrical. Increasing the reduction rate changes the shape of the speed profile. In figure B, the reduction rate is 0.166, and the speed profile has become more asymmetrical. As the reduction increases, the backflow moves closer to the steel strip. In figure C, with a reduction rate of 0.457, the speed profile is even more asymmetrical.
Segregation occurs in the entry zone, i.e. the oil phase and the water phase separate more or less. This is caused by the migration of the oil droplets in the emulsion to specific locations in the entry zone. As a result of this, certain locations in the entry zone become enriched with oil. The figure shows the segregation number. A segregation number of zero indicates a homogeneous distribution of particles, higher numbers indicate increasing separation in the entry zone. At a point about three quarters into the entry zone, segregation clearly occurs. Increasing the speed during rolling results in the segregation shifting to a point earlier on in the entry zone.