The process of metalworking consists of running metal stock through pairs of rolls to lessen the thickness or improve uniformity of thickness. It’s like rolling dough. Rolling is classified based on the temperature of the metal. The starting material is usually large pieces of metal such as slabs, blooms, and billets.
These pieces of metal are fed into rolling mills at a prescribed temperature. If the temperature is above recrystallization temperature, the process is known as hot rolling. However, if the temperature of the metal is below its recrystallization temperature, it is considered cold rolling.
Hot rolling produces more tonnage than cold working processes. Hot rolled milled steel is also considered to have wider tolerances for the amount of the included carbon than cold rolling processes. For example, a blacksmith uses hot rolled steel since it is more easily manipulated. Hot rolled products are also significantly less expensive than cold rolled metals.
Hot rolling is most often used to create sheet metal or simple cross sections like rail tracks. Other common uses for hot rolled metal includes truck frames, varieties of pipes, water heaters, mechanical equipment, stampings, railcars and components, wheel rims, metal structures like buildings, hopper cars, doors, shelving, and highway guard rails to name a few.
Cold rolling improves surface finish and holds tighter tolerances. Cold-rolled products include metal sheets, bars, strips, and certain types of metal rods. Cold rolled products are usually smaller than hot rolled metals. Because of their smaller size and greater strength, four-high or cluster mills are used. A limitation of cold rolling is less ability to reduce the thickness of a workpiece as compared to hot rolling.
Cold-rolled sheets come in a variety of conditions: skin-rolled, full-hard, half-hard, and quarter-hard. Full-hard rolling reduces thickness by half, while the other conditions bring much less reduction. Skin-rolling involves the least amount of thickness reduction at just 1/2 – 1%. This process is most often used to produce a very smooth surface with uniform thickness. Skin-rolled stock is mostly used in cold-working processes requiring good ductility.
Cold rolled steel is typically used for items like metal furniture like filing cabinets, desks, tables, and chairs. Other uses include vehicle exhaust pipes, certain home appliance components, lighting fixtures, door handles or hinges, lawn mower parts, cookery like fying pans, and other construction related products.