There are so many types of steel out there, making up 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.
Why the difference?
It may seem obvious, but some types of steel are better suited for certain applications. Knowing which to use can help avoid spending more than you have to on raw materials. It can also save you time and money on additional processing. In other words, this guide will help you avoid complications and achieve better results—at the best price possible.
It's important to note that the difference between hot rolled and cold rolled 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 both basic carbon steels and other alloy steels.
With that, let's get into some specifics.
Hot rolled steel
Hot rolled steel is steel that has been roll-pressed at very 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 first start with a large, rectangular 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. Picture glowing, white-hot strands of steel running through compression rollers at high speed. 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.
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. Hot rolled steel is often used in applications where minutely specific dimensions aren't crucial. Railroad tracks and construction projects often use hot rolled steel.
Hot rolled steel can often be identified by the following characteristics:
- A scaled surface—a remnant 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 result in slightly trapezoidal forms—as opposed to perfectly squared angles
What are the benefits of hot rolled steel? It typically requires much less processing than cold rolled steel, which makes it a lot cheaper. Because hot rolled steel is allowed to cool at room temperature, it's essentially normalized—meaning it's free from internal stresses that can arise from 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. Where surface finish is a concern, scaling can be removed by grinding, sand blasting 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.
Cold rolled steel
Cold rolled steel is essentially hot rolled steel that has been processed further. Once hot rolled steel has cooled, it is then rolled to achieve more exact dimensions and better surface qualities.
Cold "rolled" steel is often used to describe a range of finishing processes—though, technically, "cold rolled" applies only to sheets that undergo compression between rollers. Things like bars or tubes are "drawn," not rolled. Other cold finishing processes include turning, grinding and polishing—each of which is used to modify existing hot rolled stock into more refined products.
Cold rolled steel can often be identified by the following characteristics:
- Better, 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 steels are typically harder and stronger than standard hot rolled steels. This is because cold finishing essentially creates a work-hardened product. It's worth noting that these additional treatments can also create internal stresses within the material. In other words, when fabricating cold-worked steel—whether cutting, grinding or welding it—this can release tensions and lead to unpredictable warping.
Manufacturing and product design
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.
Whatever your project, know that Reliance Foundry houses many metal fabricating operations. To learn more about our offerings, contact our Sales Department.
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- Rolled steel wound into coils: Jean-Etienne Minh-Duy Poirrier, CC BY-SA 2.0, via Flickr