Differences Between Hot Rolled Steel and Cold Rolled Steel

The World Steel Association lists over 3,500 different grades of steel, each with unique properties such as grades, specifications, shapes and finishes.

With so many diverse types, steel can by widely used in infrastructure, appliances, vehicles, wind turbines and many more applications. Some types of industrial steel products available for the manufacturers includes cold rolled steel coils, hot dipped galvanized steel coils, Galvalume steel coils, PPGI steel coils and steel plate.

Optimizing steel’s properties for each application goes beyond simply changing the chemical composition of the metal. The manufacturing processing of steel can also have a significant impact on steel products, even when grades and specifications of the products are the same. 

With pre-fabricated steel products, one key distinction is the difference between the process for hot rolled steel and cold rolled steel.

As the name implies, “hot rolling” refers to processing which is done with heat.  The process for “cold rolling” refers to the use of a lower temperature. done at or near room temperature. While each of these process techniques affect overall performance and application, not impacted are the formal specifications and grades of steel, which relate specifically to the metallurgical composition and performance ratings. Steels of different grades and specifications, including both basic carbon steels and other alloy steels, can be either hot rolled or cold rolled.

Some types of steel are better suited for certain applications, so knowing which steel type to use can help avoid over-spending on raw materials and also save time and money on additional processing.

What is the difference between hot rolled steel and cold rolled steel

Hot rolled steel

Hot rolled steel being processed in a steel mill 
Hot rolled steel is ideal for products where tight tolerances are not required.
Hot rolled steel is steel which has been roll-pressed at very high temperatures, usually over 1,700˚F, which is above the recrystallization temperature for most steels. This makes the steel easier to form, and the resulting products 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. The white-hot strands of steel are pushed through the rollers 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.

It should be noted that 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. This can result in slightly rounded edges and corners for bar and plate products and even slight distortions, like slightly trapezoidal forms during cooling.

Hot rolled steel is often used in applications where dimensional tolerances aren’t as important as overall material strength, and where surface finish isn’t a key concern. Where the scaled surface finish is a concern, scaling can be removed by grinding, sandblasting or acid-bath pickling. Once scaling has been removed, various brush or mirror finishes can also be applied. Descaled steel also offers a better surface for painting and other surface coatings. Railroad tracks and construction projects often use hot rolled steel.

Because hot rolled steel normally requires less processing than cold rolled steel, it is less expensive, and because hot rolled steel cools at room temperature, it is essentially called normalized which means the steel is free from internal stresses which can arise from quenching or work-hardening processes.

Cold rolled steel

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. Cold rolled steel is ideal for products demanding exact dimensions and improved surface characteristics.

The term cold “rolled” steel is often used to describe a range of finishing processes for steel, though correctly applied, “cold rolled” refers only to steel sheets which undergo compression between rollers. The steel forms which are pulled, such as bars or tubes, are identified as “drawn”, not rolled. Other cold finishing processes include turning, grinding and polishing, steps which are used to modify existing hot rolled steel stock into more refined products.

In terms of physical characteristics, cold rolled steels are typically harder and stronger than standard hot rolled steels, with closer tolerances on finished surfaces. As the metal is shaped at the lower temperatures, the steel’s hardness, resistance to tension breaking and resistance against deformation are all increased due to work hardening. One characteristic of cold rolled steel is the smooth surfaces of the metal often feel oily to the touch.

Cold rolled steel is often used for more technically precise applications, or where aesthetics are important, and due to the additional processing for cold finished products, they come at a higher price.

It should be noted that these additional treatment processes can also create internal stress within the material being worked, and can cause unpredictable warping if the steel is not stress relieved prior to cutting, grinding or welding.

SOURCE: http://www.reliance-foundry.com/blog/hot-vs-cold-rolled-steel#gref