One of the most common questions we get about materials is: Whats the difference between hot and cold rolled steel? The simple answer is how the material is processed and what the end product looks like. The more detailed answer requires taking a look into the steel manufacturing process.
Rolled Steel
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| Billets |
Similar to using a rolling pin to roll out dough, steel billets (large blocks of metal) are passed through machines that use heavy cylinders to flatten the metal into sheets. There are many different kinds of rolling machines to achieve the various shapes steel mills produce.
So how do the terms hot and cold tie into the rolling process?
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| Hot Rolled Sheet Steel |
Hot Rolled means that the billet has been worked through the rolling process at temperatures over 1700 degrees. At those temperatures the superheated metal is worked down the line of rollers into its final thickness and shape. Hot rolled steel has a mill scale which is the scaring/marking pattern that is left on the metal after processing. These marks form due to the temperature being so hot that it prevents the steels natural recrystallization process. Recrystallization is a process in which the grains of the steel that had ben damaged through the rolling process are replaced with a new set of grains. Recrystallization can only happen when the metal is cool, so any metal that is processed in temperatures above the recrystallization point are considered hot rolled.
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| Cold Rolled Sheet Steel |
Cold Rolled is a process that allows the metal to form new grains to replace the ones that were lost or deformed by the hot rolling process. Mills take cooled hot rolled steel and work it to its final shape by either rolling or extruding it. When you cold work the steel you can reduce its strength, so the mill adds one final processing step, annealing. Annealing works by heating the metal just to the upper limit of the recrystallization point. The metal is then cooled very slowly to room temperature, this allows the metal to create a uniform microstructure and resets its grain. It also increases the strength of the metal.
So which is right for your project?
Cold rolled steel overall will be stronger than hot rolled, but will also be more expensive due to the extra milling process. Since cold rolled steel has a grain you can only work it with its grain. Working opposite of the grain will make it significantly weaker, and cutting it wrong can cause warpage and change the integrity of the metal. Because hot rolled steel does not have a grain there is no difference in strength regardless of how its worked, and no matter how its cut it will not experience any warping. Additionally hot rolled steel contains a visual variation allowing a random pattern to occur when matched up. There is also a small tactile variation. Conversely cold rolled steel has a very smooth almost oily feeling finish to it, as well as a consistent grain pattern that is often covered with a patina like blackening to control a more regular appearance.
Whether hot rolled or cold rolled steel is a great material and can be used to achieve many fantastic looks.
Here are some projects we have completed using hot and cold rolled steel:
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| Hot Rolled Steel |
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| Hot Rolled Steel |
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| Cold Rolled Steel |
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| Cold Rolled Steel |
Would you like help utilizing steel in your project? Give us a call at 206.922.8007 or email us at info@creoworks.com.
