There are some fundamental differences between hot rolled and cold rolled steel. The differences are related to the methods of processing and are not relevant to the specification of the metal or the grade.
The Hot Rolled steel
In the hot rolling process, the steel is rolled at a temperature higher than the recrystallization point, typically higher than 1700° F. When heated to such high temperatures, steel can be manipulated easily. The hot rolling process can produce great amounts of steel without the delays of cooling and reheating.
However, when hot rolled steel cools the final side is less predictable than it is with cold rolled steel.
The most common uses for hot rolled steel are found in construction and welding industries. They are also used to create railroad tracks and I-beams. Hot rolled steel can be applied anywhere that precision shapes and dimensions are not required.
Cold Rolled Steel
The cold rolling process is much like the hot rolling process only with a bit more processing. The hot rolled steel is taken to a cold reduction mill where it is cooled at room temperatures. This is followed by temper rolling. When completed, this type of steel can be produced in precisions shapes and specific sizes.
Not all steel is cold rolled, when referring to bar products for example, the correct term is cold finishing. This is a similar process but includes grinding, polishing, turning or cold drawing.
There are four main advantages that cold rolling has over hot rolling. These are:
-the cold drawing process improves the production process by producing higher yields with greater tensile strength. This often eliminates the need for further thermal treatments.
-Surface imperfections can be eliminated with the process of turning. The surface can be polished to perfection and ground to the original size tolerance range.
-Cold rolled products feature a far superior surface finish and this makes them better in concentricity and form and therefore suitable to precise measurements. This increases their applicability in precision designs.
Of course, cold rolled bars are not as easy to work with as hot rolled bars and this is due to the increased carbon that also makes them more durable. This is not the same with hot and cold rolled sheets. In the cold rolled sheet there is typically less carbon than the hot rolled sheet making the cold sheet softer.