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 1,700˚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.
Source: machinedesign.com