One of the many questions I had when joining this industry was the title of this post: “what’s the difference between hot and cold rolled steel?” and I figured there might be a few people out there who are wondering the same! So let’s get into the meat of it.
To make sense of all the lingo, the first thing you should probably understand is what the rolling process is and why they do it. Rolling is a metal forming process, in which steel is passed through one or more pairs of rolls to reduce the thickness and make the steel uniform.
It’s pretty similar to rolling dough, although the result is much worse for your teeth. The type of rolled steel that you create depends on temperature of the steel when it is rolled. If the temperature of the metal is above its re-crystallization temperature, then the process is known as hot rolling. If the temperature of the metal is below its re-crystallization temperature, it’s known as cold rolling. Basically, if you use heat to process the steel, it’ll be hot rolled. Cold rolled is usually processed at or near room temp.
Hot Rolled Steel
The minimum temp for processing hot rolled steel is 926°C (1700°F), which is pretty hot. This is above the re-crystallization temperature for most steels. This makes the steel easier to form, resulting in products that are easier to work with.
Steel shrinks slightly as it cools, and since hot rolled steel is cooled after processing, there is less control over its final form. This makes it less suitable for precision applications. Hot rolled steel is often used for projects where the minute, specific dimensions aren’t crucial, such as construction, road barriers or railroad tracks.
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 trapezoid forms, as opposed to perfectly squared angles.
So why use hot rolled steel?
Hot rolled steel usually requires much less processing than cold rolled. And because of this it needs less time to manufacture and is therefore a lot cheaper. It’s also almost always allowed to cool at room temperature, meaning it is normalized. This makes it free from internal stresses that can arise from quenching or work-hardening processes.
Hot rolled 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 priority, 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 or other surface coatings.
Cold rolled steel
So you’ve heard about hot rolled steel, now what about cold? This is the part that surprised me; cold rolled steel goes through the exact same processing as hot rolled, including the temperature, but then once the steel has cooled it is re-rolled at room temperature to achieve the desired dimensions.
Cold rolled is often used to describe a range of finished processes, though technically cold rolled applies to sheets that undergo compression between rollers. Steel forms that are pulled, such as bars or tubes, are drawn not rolled.
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
So why use cold rolled?
With all of the descaling necessary to get a clean finish on hot rolled, it’s no surprise that one of the main benefits of cold rolled is that it’s got a better surface. For technically precise applications or projects where aesthetics are important, cold rolled steel is your best friend. But this comes with a heftier price tag.
As a comparison to hot rolled, cold rolled steels are typically harder and stronger. As the metal is shaped at lower temperatures, the steel’s hardness, resistance against tension breaking, and resistance against deformation are all increased due to work hardening.
These additional treatments, however, can also create internal stress within the material. This can cause unpredictable warping if the steel is not stress relieved prior to cutting, grinding, or welding.
What about Neil’s Steels?
With over 30 years of working in the steel industry we can give you the advice you need, as well as a competitive quote.
That’s all for now, catch you in the next one!