The material to be rolled is fed between two rotating steel rolls whose separation is somewhat smaller than the thickness of the material to be rolled. As a result of friction, the rolls draw in the material and compress it until it has the same thickness as the spacing between the rolls. Deformation takes place mainly in the longitudinal direction so that the material being rolled becomes elongated. In order to achieve the final thickness, several so-called "passes" are usually needed. The thickness reduction per pass with aluminium is between ten and 50 per cent depending on the alloy.
Roll stands and mill trains
The rolls are installed in a roll stand and are driven by an electric motor. In two-high stands the lower of the two rolls is fixed and the upper one can be adjusted. In four-high stands there are two back-up rolls to ensure there is no deflection of the working rolls under the high forces (up to 1000 tonnes). In order to be able to roll down the material in several passes on a single roll stand, reversing roll stands are used which can roll the material in both the forward and reverse direction.
Where the passes are carried out on different roll stands, the stands and ancillary equipment are combined to form "mill trains". These are up to 350 metres long; they are computer controlled and are capable of rolling strip and foil at speeds of up to 2000 metres a minute. The product is edge trimmed and wound to coils, which can contain several thousand metres. The edge trimmings are recycled as scrap.
Conventional and new rolling processes
The feedstock for rolling are rolling ingots up to 2.25 metres wide and nine metres long that are produced by continuous casting and can weigh up to 30 tonnes. One differentiates between rolling to semi-finished products, such as strip and sheet, and rolling to foil. Depending on the rolling temperature, rolling is divided into:
- hot rolling: large billets are preheated to 500-600 °C and rolled down to strip three millimetres thick, for example using a two-high reversing mill stand
- cold rolling (with four-high stands): final rolling is carried out in several passes, for example to foil from hot-rolled strip. As a result of cold working, the material becomes increasingly stronger so that it may be necessary to perform full or intermediate annealing at 350-400 °C between passes. If the required final condition (temper) is "soft", one has to carry out annealing (see heat treatment) at these temperatures after final rolling.
Special rolled products and rolling processes:
- To produce patterned aluminium sheet, a patterned steel sheet is placed between two aluminium sheets. The pattern from the steel sheet is embossed on the aluminium sheet in a single pass (a surface treatment). Pattern rolling is also used for embossing.
- Profiling or roll forming, see drawing.
- In roll bonding, a method of joining, one places for example unalloyed aluminium sheet on one or both sides of a core sheet. After a single hot-rolling pass, the surface layers are already bonded to the core sheet. The aim is to improve the corrosion resistance of the core material (a high-strength aluminium alloy or an unalloyed or low-alloy steel).
- In continuous casting and rolling, molten metal is directed through a nozzle into the gap between the rolls, which are water-cooled on the inside and depending on the alloy rotate at a speed of up to 1.5 metres a minute. The molten metal solidifies just ahead of the narrowest point. The short solidification time produces a very fine-grained structure, which is beneficial for subsequent rolling out.