Steel mills smelt raw materials with limestone or coke in a blast furnace. This produces molten ore, which is converted to such products as sheets, ingots and slabs via hot or cold rolling.
The hot rolling process takes place above a material's recrystallization temperature. The grains contained in the molten steel deform during heating, then recrystallize. The starting material is fed through the rolling mill and processed at a temperature that must be maintained above the recrystallization temperature to prevent hardening during the process. Hot rolling is often used to produce steel sheets or simple cross-section materials such as rail tracks.
Cold rolling occurs when the steel is at a temperature below its recrystallization temperature. This increases the steel's strength via strain hardening, or hardening that is produced via dislocation movements within a material's crystal structure. Cold rolled materials are often smaller because it is not as effective as hot rolling at decreasing the thickness of a workpiece in a single pass. Cold rolled products often come in the form of rods, strips and bars.