In this article we will explore the difference between hot rolled, cold rolled and cold drawn steel. Knowing the differences between these three fabrications can help you avoid wasting resources on raw materials, save time and additional processing costs.
Hot rolled steel- as the name suggests, is steel which is pressed at extreme temperatures, usually exceeding 1000. This causes the steel to rise above re-crystallisation. This makes the steel easier to form and results in products which are easier to work with.
Manufactures begin with a large length of metal called a billet. The billet is heated and then flattened into a large roll. Then, whilst kept at a high temperature the billet is then run through a series of rollers to achieve its finished dimensions- this can occur at high speeds. For sheet metal, rolled steel is spun into coils and left to cool. For other forms, such as bars or plates, materials are sectioned and packaged.
Due to steel shrinking when it cools, there is less control over the final shape, making hot rolled steel not suitable for precise applications. Railroad tracks and construction projects often use hot rolled steel.
Hot rolled steel is a lot cheaper than cold drawn and cold rolled, this I due to it not running through as many processes and being allowed to cool at room temperature. The scaling can also be removed, by sanding or grinding; this then results in a surface finish which is easy to paint and put other surface coatings on.
How to identify hot rolled steel?
• A scaled surface- caused by cooling after being at extreme temperatures
• Slightly rounded edges on bar and plate products- due to shrinkage when cooling
• Slight distortions and not perfectly shaped angles
Cold rolled steel – as the name suggest is not completed at high temperatures. It is essentially hot rolled steel but put through more processes. Once the hot rolled steel has been left to cool, it is then re-rolled but at room temperature to achieve a better surface finish and more precise dimensions.
Cold rolled steel involves the process of rolling out sheets- this causes the steel to harden due to the increased pressure used to form its new shape- rolling also causes the steel to have improved mechanical properties.
Cold rolled steel is less malleable than hot rolled, making it limited to only a few simple shapes. However recent methods of uniform cross sections and transverse dimension had resulted in forming other cold rolled shapes.
The surface characteristics of cold rolled steel are more improved than that of hot, and is often used in applications where appearance is important, such as parts on automobiles, however due to the cold rolled steel going through more processes than hot, it does result in a higher price. In terms of physical characteristics, cold rolled steels are typically harder and stronger than standard hot rolled steels.
Cold drawn steel
Cold drawn steel is also processed at room temperature- alike cold rolled. Hot rolled bars or hot rolled coils are bought down to room temperature, the steel is then hammered and rolled to ensure it fits through a die. A die sis essentially a large cookie cutter, which will cut steel down to specific shapes. Once the steel has been pushed through the die- to reshape it into a thinner shape without altering the volume.
Compared to hot rolled steel and cold rolled steel, cold drawn steel products can be manufactured to give more precise measurements and sharper corners. This is because hot rolled steel products changes in shape during the cooling process.
Cold drawn steel products include long products used in the production of machined parts, automotive motor parts, shafting and fabricated structural units.