Machine Design
What’s the Difference Between Hot and Cold Rolled Steel?

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

There are many types of steel out there, with various shapes, grades, specifications, finishes, and other processing considerations. One key distinction among prefabricated steel products is the difference between hot rolled versus cold rolled steel.

It may seem obvious, but some types of steel are better suited for certain applications than others. Knowing which to use can help avoid spending more than necessary on raw materials. It can also save time and money on additional processing. In other words, understanding the differences between hot and cold rolled steel will help designers and engineers achieve better results—and at the best price possible.

The basic difference between these two types of steel is one of process. As you can imagine, “hot rolling” refers to processing done with heat. “Cold rolling” refers to processes done at or near room temperature. Although these techniques affect overall performance and application, they should not be confused with formal specifications and grades of steel, which have to do with metallurgical composition and performance ratings. Steels of different grades and specifications can be either hot rolled or cold rolled—including basic carbon and other alloy steels.

Hot Rolled Steel

Hot rolled steel has been roll-pressed at high temperatures (over 1,700ËšF), 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.

Fig. 1
Hot rolled steel gets stored at a production yard.

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.

Fig. 2
Stacks of hot rolled steel sheets.

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.

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.

Fig. 3
Cold rolled steel pipe is bundled for shipment.

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.

For projects where you'll be manufacturing many units, casting is another option that can save time in machining and assembly. Cast parts can be made to almost any form in a range of quality materials.

Brad Done is vice president of Reliance Foundry Co. Ltd., British Columbia, Canada

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