Difference Between Hot and Cold Rolled Carbon Steel

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Mild steel (low carbon steel) is an affordable, workable material that remains a go-to for welders, builders, and fabricators everywhere. We offer three surface finishes for mild steel here at SendCutSend: Cold Rolled (1008), Hot Rolled (A36), and Hot Rolled Pickled and Oiled (A36). Each one brings its own unique value to a project, and they have a few key differences that are important to know.

What is Mild Steel?

Mild steel is a low carbon steel that is more workable and weldable than its higher carbon counterparts. Mild steel is classified as being .05% to .25% carbon by weight, with little to no additional alloying elements. Higher carbon count in a material increases its hardness and decreases its workability, so being considered a “low carbon” steel lends mild steel to projects where machinability is necessary.

Since mild steel only contains traces of carbon and very little additional alloying elements, mild steel loses a little bit of tensile strength and corrosion resistance as compared to higher carbon steels and steel alloys. It’s for this reason that mild steel is not recommended for long term exposure to aggressive elements without additional coating or finishing processes to protect the material.

Although the simplicity of the chemical makeup of mild steel causes some of its weaknesses, it also makes mild steel one of the most affordable steels available. Mild steel can be up to four to five times more cost effective than other steels in raw material costs.

We have generalized many of mild steel’s applications, strengths, and weaknesses, but mild steel is heavily affected by the finishing process it goes through. Each process changes the properties of the mild steel drastically, also changing its recommended uses.

Hot Rolled Carbon Steel (A36)

A small part laser cut out of hot rolled carbon steel

Hot rolled carbon steel is heated to temperatures that exceed the material’s recrystallization temperature. It is vital that the steel reach a temperature greater than ~1700°F because only then is it able to be rolled, formed, and shaped without cracking or breaking. Once the steel has been formed, it is then allowed to slowly cool to room temperature. Since it’s cooled slowly, hot rolled steel is normalized, making it free from the internal stress that other materials experience during work hardening. This gives the steel high strength properties.

Because the cooling process is imprecise during its forming, hot rolled steel has somewhat inconsistent tolerances and may warp slightly during cooling. For this reason, it’s not recommended that hot rolled steel be used for applications where tight tolerances are of high concern. In addition to warping, hot rolled carbon steel will also have a rough, scaly finish post-cooling. However, this scaling can be removed with some finishing such as grinding or sand-blasting.

What hot rolled carbon steel lacks in precision, however, it makes up for in affordability. Of the three main methods for finishing mild steel, hot rolled is by far the most cost effective because it sustains no further processing post-cooling. This makes it ideal for high quantity applications because it’s budget-friendly. And for all its affordability, hot rolled carbon steel retains the core strength properties that make carbon steel inherently valuable to high strength projects.

Applications:

  • Structural components
  • Vehicle framing
  • Construction materials
  • Railroad equipment

Hot rolled pickled and oiled carbon steel (A36)

Two parts laser cut out of hot rolled pickled and oiled mild steel

Hot rolled pickled and oiled (HRP&O) carbon steel is simply hot rolled carbon steel that has been washed in an acid bath to remove the scaling. The material is then brushed with a dry oil to protect the surface from oxidation and corrosion, making the end result more corrosion resistant than unwashed hot rolled steel.

Although HRP&O is slightly more expensive than its less processed counterpart, it is worth it if your project is going to spend any significant amount of time in aggressive weathering conditions. 

Applications:

  • Guardrails
  • Welded structures
  • Long-term outdoor equipment

Cold Rolled Carbon Steel (1008)

A gear laser cut out of cold rolled carbon steel

Cold rolled carbon steel goes through all the same initial processes as hot rolled, but with a few additional steps to make it more workable and precise. Once the mild steel is cooled completely, it’s then drawn through rollers at room temperature. This process reduces the inconsistencies often found in just hot rolled carbon steel, making it more reliable to work with. It also reduces external scaling and overall warping of the material.

The biggest downside to cold rolled carbon steel is that it is more expensive than hot rolled because of the additional processing costs. Cold rolled steel is also subject to slightly more internal stress due to work hardening, but this process is necessary to achieve cold rolled steel’s characteristic hardness and tension resistance. When a project requires both strength and tight tolerances, cold rolled is a must. 

Thanks to its clean edges and higher hardness level, cold rolled carbon steel is also perfect for welded projects. It’s often the first choice for our welding customers who are looking for strength and durability alongside ductility. Cold rolled steel is also better suited for projects where aesthetics should be taken into account since it retains little to no scaling that’s characteristic of hot rolled carbon steel. 

Applications:

  • Aerospace structures
  • Furniture
  • School lockers
  • Floor decking

Use Mild Steel In Your Next Laser Cut Project

Each finish of mild steel has its own wide variety of uses and strengths, and hopefully we have helped you narrow down which one to use in your next project. If you have any further questions about picking the right material for your parts, check out our other material breakdown blog posts or reach out to our support team

If you’ve already decided on your material or we have convinced you to use mild steel on your next project, upload your files to our website and get an instant quote today!

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