Cold-rolled steel is steel that has been worked below its recrystallization temperature by passing it between a pair of rollers. Recrystallization temperature is the temperature at which grains in the lattice structure of the metal have been rearranged, leaving it free of strain and deformations. In this way, cold-rolling differs from hot-rolling, which is used to work metal above its recrystallization temperature. Cold-rolled steel stock is available in a variety of sizes and shapes with characteristics useful in a wide range of applications.
Steel is pre-treated before being cold rolled with a process known as pickling, which uses strong acids to remove scale and other impurities. The metal is then passed through rollers to reduce its thickness. Most cold rolling takes place in multiple passes and usually incorporates two stations to work the metal. One station is used for a larger initial reduction in size while the other produces a smaller finishing reduction to produce a quality surface. Due to increased hardness created by cold rolling, this process is more limited than hot rolling in the size reduction achieved with each pass.
There are several reasons for cold rolling steel. This process reduces the thickness of the metal worked and achieves tighter thickness tolerances than can be produced with hot rolling. Cold rolling can also produce a high quality surface finish and prepare the metal to receive a surface coating.
A variety of grades of cold-rolled steel stock may be produced depending on the reduction in size of the original stock. Skin rolled steel undergoes a reduction of 0.5–1.0% and is noted for its smooth surface finish and good ductility. Other grades include quarter hard, half hard, and full hard, which can reach up to 50% reduction in size from the original stock.
As the size of the cold-rolled steel is further reduced, its strength and hardness both increase, but its ductility decreases. Quarter Hard steel retains sufficient ductility that it can be bent back on itself without fracturing. Half hard steel can only be bent to 90 degrees and full hard steel can only be bent to 45 degrees without fracturing. After cold rolling, heating the metal up in a process known as annealing can restore some of its ductility. The final cold-rolled steel stock may be manufactured in the form of sheets, strips, bars, or other forms.
Cold-rolled steel products feature a variety of attributes suitable in a wide range of applications. In addition to its high strength, cold-rolled steel is highly resistant to denting and exhibits useful magnetic properties. It also readily accepts surface coatings such as enamel and paint. Manufactured items made with cold-rolled steel range from commercial products such as appliances and bathtubs to automobiles to industrial products including motors, generators, and transformers.