Cold Rolled Steel Strip is produced, as the term suggests by the further rolling of strip produced by the hot strip mills. Prior to cold rolling, the mill scale has to be removed, normally by the Pickling Process which uses mechanical manipulation (around small diameter rolls) and acid to dissolve the surface scale. The surface is then washed to remove the acid and a light oil added to prevent rusting.
Cold Rolling is undertaken to :-
Most Cold Rollers, incorporate two rolling stations. The in initial cold rolling is undertaken on a more powerful “break down” mill that is capable of heavy reductions. After annealing to remove the stress and work hardening introduced in the process, the final cold rolling or “skin-pass” takes place. This is a light reduction (typically 3%) to improve the surface finish.
Cold rolling mills are typically reversing mills. A reversing mill is where the steel enters the rolling mill from one side, passes through the other side and then comes back through the mill again. Normally it will go left to right through the mill a number of times being rolled a little thinner each time it goes through.
Cold rolled strip can be produced in various conditions such as skin-rolled, quarter hard, half hard, full hard depending on how much cold work has been performed. This cold working (hardness) is often called temper, although this has nothing to do with heat treatment temper.
In skin rolling, the metal is reduced by 0.5 to 1% and results in a surface that is smooth and the yield point phenomenon–excessive stretching and wrinkling in subsequent operations, is eliminated. This makes the metal more ductile for further forming and stretching operations.
Quarter Hard, Half Hard, Full Hard stock have higher amounts of reduction, up to 50%. This increases the yield point; grain orientation and material properties assume different properties along the grain orientation. However, while the yield point increases, ductility decreases.
Quarter Hard material can be bent (perpendicular to the direction of rolling) on itself without fracturing. Half hard material can be bent 90º; full hard can be bent 45º. Thus, these materials can be used for in applications involving great amounts of bending and deformation, without fracturing.
A Z mill or Sendzimir mill operates with a very small diameter work roll, normally about 2 inches, backed up by a number of rolls in a pyramid-shaped stack. This roll set up allows you to exert extremely high forces through the work roll and yet keep the work roll from extreme flexing. The take-up roll on the Z-mill also exerts a tension on the coil as it comes through the mill. The combination of high pressure and tension makes the mill capable of rolling material thin and flat.
Classical Cold Rolled is produced on narrow strip mills typically offering widths up to 500mm with “close” gauge control and a wide range of sizes, qualities, hardness’s and surface finishes.
Cold Reduced is an alternative product in a more limited range of sizes and grades. The strip is produced on “wide” mills. Usually the qualities offered are restricted to general purpose pressing and blanking qualities.