Hot rolled and cold rolled steel differ in three major aspects: rolling temperature, thickness and physical properties. Coil or sheet steel is initially hot reduced from slabs, either to a finished product or to produce strip steel for further processing. Cold rolled steel begins with a hot rolled strip product that, through the rolling process, is further reduced in thickness, a process that also changes the physical properties (primarily the crystalline structure) of the product.
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Hot-rolling requires temperatures of 1,652 degrees to 2,012 degrees Fahrenheit (900 to 1,100 degrees Celsius), while cold-rolled steel is rolled at room temperature.
Steel coming off the finishing stand at the hot mill is cooled by water spray and coiled at temperatures of 932 degrees Fahrenheit (500 degrees Celsius) and 1,400 degrees Fahrenheit (760 degrees Celsius). The hot-rolled steel is then cooled to room temperature. Once at ambient temperature, the steel can be cold reduced or cold rolled.
Typically, hot rolling begins with a standard 10-inch (250 mm) thick slab, either directly from a continuous caster or reheated to the desired temperature. The steel moves through a series of mill stands that reduce the slab to a hot roll strip with thicknesses ranging from 0.071 inches (1.80 mm) to 0.350 inches (8.89 mm).
Cold rolling through a second series of mills further reduces the hot rolled steel to a cold rolled product with a thickness range from .0126 inches (.32 mm) to .157 inches (3.99 mm).
Steel's physical structure varies by product; heating and cooling change the molecular structure of the metal. Steel coming off the hot mill will be softer and more ductile than material further processed through a cold mill.
Ductility, measured by elongation testing, determines the amount of material deformation that occurs before the material fractures. Elongation (in 2 inches) for hot rolled is from 39 to 43 per cent, while cold rolled ranges from 40 to 46 per cent.
Because it is softer, hot rolled steel forms easily, while cold rolled has superior drawing quality. Cold rolled's tighter tolerances make it a better choice for highly engineered end uses. The blue-grey oxide that forms the finish on hot rolled is unsuitable for painting without further processing and surface preparation, while cold rolled's smooth grey finish is ready to paint.
Each product can be produced with a variety of chemical and physical characteristics to meet specific applications.
Hot-rolled steel is used in construction, industrial machinery, and shipbuilding as well as for unexposed automotive and appliance parts such as frames, brackets, wheels and clutch plates. Hot-rolled steel is a suitable product where thickness, strength and formability are important.
Cold-rolled applications include refrigerators, stoves, washers, dryers and small appliances as well as a wide variety of exposed automotive parts. Cold-rolled applications often require tight tolerances and a smooth, paintable surface.
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