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
