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Both hot rolled steel and cold rolled steel start out in essentially the same way and both can have the same grades and specifications. But cold rolled steel undergoes additional processing steps, resulting in improved properties that can be exploited for different applications. Each type of steel has its advantages and disadvantages and costs for the two types of steel are also different.
How It’s MadeBoth cold rolled and hot rolled steel start out as large steel slabs or billets cast from hot liquid metal. The billets are then heated, eventually reaching over 1700°F. At this high temperature, they are easily flattened into a long sheet using a set of rollers, and then wound up into large coils. To make bars or plates, the heated billet is rolled to the desired thickness and cut into sections before cooling.
As the rolled or cut steel cools to room temperature, it shrinks slightly, making the final dimensions of each piece less exact and the edges somewhat rounded. The surface is slightly rough and covered in scale. At this point, hot rolled steel products are ready for shipment, and require no further treatment.
Cold Rolling ProcessesBut cold rolled steel products are destined for further processing after the steel has cooled. Cold rolling is most often used to decrease the thickness of plate and sheet metal in the manufacturing stage. This “cold forming” occurs either by re-rolling at around room temperature and then coiling into sheet, or else drawing into bars or tubes. Additional steps such as drawing, grinding or turning create the desired finished product.
Work hardening of the metal at room temperature increases its hardness and yield strength by introducing crystalline defects, but also may cause internal stresses that must be relieved by heating, or else the final product may warp.
Finishing TouchesCold rolled or cold formed steel has a smooth, shiny finish with an oily texture that is free of rust or scale so it can easily be painted or chromed. The dimensions of the final product are more precise and square, with a sharper edge, and cold rolled steel sheet can hold tighter tolerances than hot rolled when machined or otherwise fabricated.
In general, cold rolled and cold formed steel costs more than hot rolled steel because of the extra processing steps.