Hot rolled steel is steel that has been roll-pressed at very high temperatures—over 1,700, which is above the re-crystallization temperature for most steels. This makes the steel easier to form, and resulting in products that are easier to work with. To process hot rolled steel, manufacturers first start with a large, rectangular length of metal, called a billet. The billet is heated and then sent for pre-processing, where it is flattened into a large roll. From there, it is kept at a high temperature and run through a series of rollers to achieve its finished dimensions. Steel shrinks slightly as it cools. Since hot rolled steel is cooled after processing, there is less control over its final shape, making it less suitable for precision applications. Hot rolled steel is often used in applications where minutely specific dimensions aren't crucial.
 

Cold rolled steel is essentially hot rolled steel that has been through further processing. Once hot rolled steel has cooled, it is then re-rolled at room temperature to achieve more exact dimensions and better surface qualities.