Hot-rolled strip is rolled even thinner by means of cold rolling at room temperature. Adjustments made during this process result in the desired processing properties of the strip.
Cold-rolled thin sheet is mostly produced with thicknesses of from 0.4 to 3.0 mm and widths of up to 2,010 mm. Ultra-thin sheet is 0.12 to 0.49 mm thick and is largely used as an input material for the packaging industry.
Cold rolling consists of the following process steps:
The layer of scale on the hot strip is removed from endless strip in continuous pickling lines or from individual strips in push-pull pickling lines.
The pickled strip is cold rolled as endless strip in a tandem mill or as individual strips in a reversing stand. Tandem mills consist of 4 – 6 quarto stands arranged in a row and the strip is located in all the stands simultaneously. The pickling line and tandem mill can be run coupled with one another by means of an intermediate storage system for the strip (looper).
20-roll stands, so-called Sendzimir planetary rolling mills, have become established for cold rolling stainless steels. They can best meet the demands for high forming levels, tight tolerances and good surface quality..
The material hardened during cold rolling must undergo recrystallising annealing to restore the forming properties for further processing. This takes place in batch annealing or in continuous annealing lines. In order to ensure that the surface quality achieved during cold rolling is retained, annealing must be carried out under a protective gas atmosphere or under oxygen exclusion in order to suppress oxidation.
In the case of batch annealing, several coils are stacked on a base and annealed below a hood for about of 2 – 3 days.
Continuous annealing lines permit flexible annealing cycles with higher heating and cooling rates, as necessary for heat treating high-strength steel grades, among others.
A yield-point elongation is overcome with this process step; the sheet surface is smoothed or deliberately roughened and compacted.
In this final work step the material is inspected for surface defects in coil preparation lines and provided with corrosion-protection. Depending on requirements, the strip is slit into narrower strips or cut-to-length to make sheets. Then the material is packaged and prepared for despatch. There may, however, be a final surface-finishing process.