Rolling is a process where metal is passed through a pair of heavy rollers, which compress and shape the metal into desired shapes and sizes. Rolled steel is classified into two types- hot rolled and cold rolled, which differ in their manufacturing process and end-use.
In this blog post, we will look at the manufacturing process of these two types of steel, as well as some of their pros and cons, to help you decide which one is right for your project.
The hot rolling process begins with a big hunk of metal, called a billet or slab, that’s heated up until it is red hot.
Then, it gets passed through a series of rollers (which help to shape it) and then cooled.
The result of this manufacturing process is a grain structure that is more relaxed, which also makes it more of a malleable piece of steel. Thus, the term “Hot Rolled” is because it is made into the final shape while the material is in a heated state.
This steel is less expensive compared to cold rolled because it does not require as many processes to make the final product.
This steel is pliable and can be easily formed into different shapes, making it a good choice for projects that require intricate shapes. Examples include sheets, plates, angles, beams, and channels.
Because the steel is cooled after the rolling process, it can end up with more variation of the final dimension.
This is caused by the scale that’s left behind from the hot rolling process. The cooling process causes oxides to form on the surface of the steel which can protect it from oxidation but can be a nuisance for stamping or painting.
With this rolled steel, the manufacturing process begins with a hot rolled product that is processed further to get to a final thickness.
Hot Rolled steel is de-scaled (typically by shot blasting or pickling) to remove the mill scale.
A cold reduction (without adding heat) process follows, which reduces the dimensions of the steel giving it a smoother finish with tighter dimensional tolerances. At this stage, the material is in a “Full Hard” condition. In this condition, the grains are stressed from the reduction process causing the surface to be stiff and very brittle. Many round, flat and square bars remain in this condition in the market.
Annealing occurs next in most sheet products (and some bar stock) where the material is gradually heated in a controlled atmosphere, “soaking” the material for a period and then gradually cooling it to room temperature. This relieves the grain stresses to a “Dead Soft” condition.
Finally, the material is processed further by tempering material which returns some stress to the grain structure. This process gives the Cold Rolled material the ability to hold its shape when formed and be able to further process into a final product
The removal of the mill scale allows for a cleaner surface which is great for painting. However, when oil is removed from the surface, it can begin oxidizing quickly, so painting soon thereafter is highly recommended.
Due to the absence of added heat in the process, cold rolled material can be held to very tight tolerances on all dimensions.
Bar stock that is Cold Rolled typically have higher mechanical properties and are much stiffer than Hot Rolled products.
The process of creating this steel requires more time and resources, making it more expensive compared to other steel.
The reason why this steel is limited to certain shapes is because of its brittle nature. When the material is processed, it becomes very hard and difficult to work within the full hard condition. To process structural shapes such as angles, channels, and beams, the annealing and tempering process is not practical. This means that it can only be formed into certain shapes economically. Full hard material in these shapes would be too brittle for practical use.
It depends on what you need it for. If you need affordable, durable steel that is easy to manipulate, then hot rolled is a good choice.
If you need steel that has a smoother, scale-free surface, then cold rolled is the better option.
No matter what your project entails, make sure to do your research and pick the right rolled steel for the job!
Contact New Mexico Metals LLC today if you have any queries!