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Hot rolled vs cold rolled steel

08-08-17
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Hot rolled vs cold rolled steel

 

Hot rolling steel is a process which involves rolling steel at a high temperature (typically over 1700° F), which is higher than the steel’s recrystallization temperature. When steel is heated above the recrystallization temperature, it can be formed more easily. It can also be made in much larger sizes. Hot rolled steel is generally cheaper than cold rolled steel as it is often manufactured without any delays in the process. Cold rolled steel on the other hand needs to be re-heated.

Hot rolled steel reconfigures during the cooling stage. This results in the finished product having looser tolerances than the original material, compared to cold rolled steel products. When steel is hot rolled it is more malleable than when cold rolled. This allows it to be forced into a wider variety of shapes. It is preferable for the manufacturing of structural components, such as beams and rail tracks. It is also used to produce sheet metal.

The term ‘cold rolled’ is mistakenly used on all products, when it actually refers to the rolling of flat rolled sheet and coil products. Cold rolled steel is essentially hot rolled steel that has gone through further processing. It is processed further in cold reduction mills, where the material is cooled to room temperature and then annealed and/or rolled. This results in steel with tighter dimensional tolerances and a wider range of surface finishes. As it adds an extra step to the manufacturing process, costs are increased.

Due to these properties, cold rolled steel is limited to forming fewer shapes, such as round, square, flat and variations of those. Further shapes can be cold rolled if the transverse dimension is small and the cross section is uniform. A series of shaping operations, known as breakdown, sizing, semi-finishing, semi-roughing, finishing and roughing and are required to create cold rolled shapes.

Finished products created using the cold rolled steel process include rods, bars, strips and sheets which are generally smaller than the same products available using hot rolled methods. Smaller products have much tighter tolerances than the larger hot rolled versions.

Cold rolled steel is available in different conditions, including: full hard, half hard, quarter hard and skin rolled. The full hard condition reduces the overall thickness of the steel up to 50 percent, while the other conditions can create less of a reduction. Skin rolling is also referred to as a ‘skin pass’ in the metal industry as it involves the least amount of reduction in thickness; usually between a half to one percent.

The bottom line is that larger sized materials that need to be in more distinct shapes are better created using the the hot rolled steel process. Smaller products that are required to be more durable and tolerant, are better created using the cold rolled steel process.

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