What's the Difference Between Hot Rolled Steel and Cold Rolled Steel?

In the production of steel products, steel is molded and reshaped with different machinery at different temperatures. One process is steel rolling, which involves metal stock passing through a pair of rolls. Rolling produces flat steel sheets of a specific thickness, and the process is classified according to the temperature at which the metal is rolled. If the temperature of the metal is above its recrystallization temperature, or the temperature at which the grain structure of the metal can be altered, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling.

Hot Rolled Steel

Hot rolled steel has been roll-pressed at high temperatures (over 1,700˚F), which is above the re-crystallization temperature for most steels. This makes the steel easier to form, and also results in products that are easier to work with.

To process hot rolled steel, manufacturers start with a large, rectangular billet. The billet gets heated and sent for pre-processing, where it is flattened into a large roll. From there, it is kept at a high temperature, and the glowing white-hot steel is run through a series of compression rollers to achieve its finished dimensions. For sheet metal, manufacturers spin the rolled steel into coils and leave it to cool. For other forms, such as bars and plates, materials are sectioned and packaged.

Steel shrinks slightly as it cools. Because 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 when minutely specific dimensions aren’t crucial—in railroad tracks and construction projects, for example.

Hot rolled steel can often be identified by the following characteristics:
• Scaled surfaces, the remnants of cooling from extreme temperatures.
• Slightly rounded edges and corners for bar and plate products
(due to shrinkage and less precise finishing).
• Slight distortions, where cooling may leave slightly trapezoidal forms rather than perfectly squared angles.

Hot rolled steel typically requires much less processing than cold rolled steel, which makes it a lot less expensive. Hot rolled steel is also allowed to cool at room temperature, so it’s essentially normalized, meaning it’s free from internal stresses that can arise during quenching or work-hardening processes.
Hot rolled steel is ideal where dimensional tolerances aren’t as important as overall material strength, and where surface finish isn’t a key concern. If surface finish is a concern, scaling can be removed by grinding, sand blasting, or acid-bath pickling. Once scaling is removed, various brush or mirror finishes can be applied. Descaled steel also offers a better surface for painting and other surface coatings.

Uses: Hot rolled products like hot rolled stainless steel pipe hot rolled steel bars are used in the welding and construction trades to make railroad tracks and I-beams, for example. Hot rolled steel is used in situations where precise shapes and tolerances are not required.

Cold Rolled Steel

Cold rolled steel is essentially hot rolled steel that has gone through more processing. To get cold rolled steel, manufacturers generally take cooled-down hot rolled steel and roll it more to get more exact dimensions and better surface qualities.

But the term “rolled” is often used to describe a range of finishing processes such as turning, grinding, and polishing, each of which modifies existing hot rolled stock into more refined products. Technically, “cold rolled” applies only to sheets that undergo compression between rollers. But forms like bars or tubes are “drawn,” not rolled. So hot rolled bars and tubes, once cooled, are processed into what are called “cold finished” tubes and bars.

Cold rolled steel can often be identified by the following characteristics:
• More finished surfaces with closer tolerances.
• Smooth surfaces that are often oily to the touch.
• Bars are true and square, and often have well-defined edges and corners.
• Tubes have better concentric uniformity and straightness.

With better surface characteristics than hot rolled steel, it’s no surprise that cold rolled steel is often used for more technically precise applications or where aesthetics are important. But, due to the additional processing for cold finished products, they come at a higher price.
In terms of their physical characteristics, cold worked treatments can also create internal stresses within the material. In other words, fabricating cold worked steel—whether by cutting, grinding, or welding it—can release tensions and lead to unpredictable warping.

Depending on what you’re looking to build, different types of materials each have their own benefits and drawbacks. For unique projects or one-off productions, prefabricated steel materials can provide the building blocks for any structural configuration imaginable.

Uses: Any project where tolerances, surface condition, concentricity, and straightness are the major factors.

Conclusion

These are the fundamental differences between hot and cold rolled steel. Each type of steel is better suited to some jobs than others, so it is important to know the advantages and weaknesses of each variation. By staying knowledgeable, any contractor will have the ability to ensure that the job is completed as efficiently and safely as possible.

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