After hot rolling, many steel products undergo a further processing in the cold state.
This stage of processing may not necessarily alter the shape of the steel product, but it does reduce it in thickness and significantly improve its performance characteristics.
Hot rolled coil
Hot rolled coil is commonly cold rolled (also known as cold reduced).
The strip is first de-coiled (uncoiled) and then passes through a series of rolling mill stands which apply pressure to the strip and progressively reduce its thickness - down to as low as 0.15 mm. The strip is then recoiled.
Cold rolling processes are also used to improve the surface quality of the steel. Cold rolling also has the effect of hardening steel, so cold reduced strip is subsequently annealed: a process of very carefully controlled heating and cooling to soften it.
Cold reduced strip and sheet is able to withstand subsequent forming and pressing operations without the steel cracking.
The elaborate shapes used to make car bodies are pressed out of cold reduced sheets. Very thin cold reduced sheet (known also as blackplate), after coating with a thin layer of tin, is used to make food and drink cans.
So sophisticated have modern steelmaking and rolling techniques become that it is now possible to press the shape of the complete can (bottom and sides) from sheet steel, leaving only the lid to be sealed on after filling. Many drinks cans are formed in this way.
Another form of cold processing is cold drawing. Steel rod is dragged at pressure (drawn) through a series of dies which progressively reduce the rod's circumference to produce wire.
The drawing process substantially increases the steel's tensile strength - steel wires can be spun into huge ropes strong enough to support the world's largest suspension bridges.