The difference between hot and cold rolled steel
One of the most apparent differences between hot and cold rolled steel is the process itself. While hot rolling is done with heat, cold rolled steel is processed at low temperatures. Both techniques have an impact on the type of application and the performance of the steel grade. However, steels of different types and properties can undergo both treatments, although it must be noted that some grades are more suitable for a specific process. In order to avoid wasting material and money, it is crucial to understand the difference between both techniques.
How is hot rolled steel processed?
The process occurs at very high temperatures above the recrystallization temperature of the metal, which is higher than 930° C. At these temperatures, the steel is more formable and the workpieces can be processed more easily.
Hot rolling starts with semi-finished steel. Usually it is a large piece of metal, such as a bloom, slab or billet. In the first preprocessing step, the workpiece is heated and formed into a large roll. To achieve the final dimensions, the steel is kept at high temperatures and run through several rollers at rapid processing speeds.
Typically, large workpieces are heated by oil- or gas-fired soaking pits, while smaller workpieces undergo induction heating. The temperature must be kept above the recrystallization point. Therefore, it is crucial to monitor the operating temperature at all times.
Like all materials, steel shrinks when it cools. This makes it difficult to control the final shape of the workpiece. Therefore, this process is rarely applied on steel parts that require high precision. Typical applications for hot rolled steel are railroad tracks, truck frames, water heaters or metal buildings. Further it used for doors, compressor shells and discs.
Hot rolled steel shows the following properties:
- The cooling from extreme temperatures leads to a scaled surface
- Due to shrinkage, hot rolled steels have slightly rounded edges and shapes
- The workpieces fall short at dimensional tolerances
Such types of steel are cheaper due to the fact that they do not require a lot of processing. Further, hot rolled steel is normalized because it is not quenched and cools at room temperature. This makes it free of internal stress.
As the dimensions of hot rolled steel parts are not very precise, they are used in applications where material strength and not precision or surface quality is of high importance. To improve the surface quality, the workpieces can undergo a pickling procedure. Furthermore, the scaling can be removed by grinding or sand blasting.
How is cold rolled steel processed?
First, cold rolled steel undergoes the same procedure as hot rolled steels. However, cold rolled steel requires additional processing. Once the workpieces have cooled, they get rerolled at room temperature. Thus, cold rolling achieves more precise dimensions and improved surface qualities.
Technically, cold rolled steel only applies to sheets that are compressed between rollers. However, the term describes a variety of finishing processes. Bars and tubes and other steel forms that are pulled, for example, are “drawn” and therefore not cold “rolled”.
Grinding, turning and polishing are other cold finishing processes which modify hot rolled steels into more refined workpieces.
Cold rolled steel show the following properties:
- Surface is far more refined
- Cold rolled steel show precise shapes and well-defined edges and corners
- Cold rolled tubes are straighter and have exact dimensions
Cold rolled steel shows better surface and dimension characteristics. Therefore, it is used when precision and aesthetics of the finished product are important. However, due to the further processing, such steel is more expensive.
Furthermore, it displays improved physical properties. Generally, cold rolled steel is harder and stronger. The resistance against deformation and tension breaking are improved, as well. However, the better physical characteristics come with a price. The additional treatments can create internal stresses within cold rolled steel.