The main difference between the two processes is the temperature at which the raw material are processed.
Hot rolled sheets: There is a temperature above the distorted grain structure of a metal is replaced by a strain-free grain structure. This temperature is known as the temperature of recrystallization.
Hot Roll sheets are processed above the recrystallization temperature. Hot rolled steel is typically cheaper as it is often manufactured in one go, the re-heating of the steel is not required, therefore, the cost of reheating is saved. The main problem in using hot steel is that when it cools, the shrinks, thus, you have lesser control over the final size of what you will get. Hence, it can’t be used where high precision is required in the shape and tolerance.
Cold rolled sheets: These sheets are processed below the crystallisation temperature. The steel is processed further where the material is annealed (heated and allow it to cool slowly at room temperature) in order to remove internal stresses and toughen it. Then the sheet undergoes a process called turning and the absence of crystals makes the surface is quite smooth and dimensions are precise. The sheet is then grinded and polished which further increases the smoothness of the surface and precision in the dimension. The cold sheet is typically has a low carbon content and is annealed which makes it softer (in comparison to hot rolled) and easier to work with.
Firstly, one needs to know what is the main application of the sheet and the factors that are a must before deciding to buy the product. The factors one need to consider before deciding on a cold rolled or a hot rolled sheets are-
- Strength– If the sheet is to withstand high stresses the use of hot rolled sheets is advisable as they are stronger as compared to cold rolled sheets. There is a misconception regarding this which arises due to the fact that cold rolled bars are stronger(as they have more carbon content) but for sheets and coils, the opposite is true.
- Precision– If precision in dimension is more important than the strength, cold rolled sheets are the better choice.
- Surface finish- If smoothness of the surface is must, then cold rolled sheet is the favourable choice
- Price- If high precision and good surface finish are not very important, then it is advisable to buy Hot rolled sheets as they come at a cheaper price than Cold rolled sheets.
Being ignorant about the processes and the advantages of one process over the other can make you pay more for a product that doesn’t even satisfy your needs.