Cold rolling

A coil of cold-rolled steel

Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used.[2] Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.

Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Quarter-hard is defined by its ability to be bent back onto itself along the grain boundary without breaking. Half-hard can be bent 90, while full-hard can only be bent 45, with the bend radius approximately equal to the material thickness. Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield-point phenomenon (by preventing Luder bands from forming in later processing).[6] It is also used to breakup the spangles in galvanized steel.[citation needed] Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.[2]

Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small; approximately less than 50 mm (2.0 in). This may be a cost-effective alternative to extruding or machining the profile if the volume is in the several tons or more. Cold rolling shapes requires a series of shaping operations, usually along the lines of: sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.