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 :-
Reduce the thickness
Improve the surface finish
Improve the thickness tolerances
To offer a range of "tempers"
As a preparation for surface coating
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.
