What Is The Difference Between Cold Rolled And Hot Rolled?
Modern metalworking widely uses two types of steel sheets in production—cold-rolled and hot-rolled together with their modifications. In this case, when buying a hot rolled steel coil, the customer often wants to know the characteristics of both types of steel and their differences.
Melting steel for these materials is the same, but the final rolling technology has its peculiarities. If you want to make the right choice, you have to go over the following information.
Usage and Production of Cold-Rolled Steel
The procedure of cold-rolled metal at most stages doesn’t involve the use of heat treatment. The drawing process is carried out with the help of a rolling mill, turning the billet into a sheet. This is followed by cleaning the oxide film and scale, for which etching or shot blasting technology is used.
Pickling means immersing a steel plate in a bath of 25% sulfuric or hydrochloric acid. Shot blasting is the impact cleaning of the surface of the steel sheet by applying high pressure through the use of small particles.
The next step is hardening in a special furnace at 700°C. Heating can lead to the emergence of shear lines, which additional rolling helps to remove.
Dressing the sheet is the last processing step, in which the workpiece is compressed by up to 3% to increase its strength and hardness. The finished sheet can then be subjected to additional processing depending on the purpose of further use, such as galvanizing.
Such rolled metal products are mainly used in the manufacture of precision parts in terms of their physical and mechanical parameters:
- aircraft and space industry;
- machine tool and electrical engineering industry;
- for construction purposes.
Usage and Production of Hot Rolled Steel
The principle of drawing is also used to obtain this material, but already at a high temperature—over 1700°F. Heated steel is easier to shape, which reduces the cost of such rolled steel. However, after cooling, such a sheet can change its size and shape, so it’s used in those industries that don’t require exact compliance with the physical and mechanical parameters of the parts.
Heating imparts other properties to the structure of the metal, which makes it more plastic and harder, increases its toughness, and improves machining. Such products are of low quality in shape and thickness, but their attractiveness is the product’s final price.
That’s extremely important for use in large construction projects, where a huge number of different metal structures with low requirements for the perfection of several parameters are required.
Cold-Rolled vs. Hot-Rolled Sheets
- Thickness
In cold-rolled metal rolling, this parameter does not exceed 5 mm; in hot-rolled sheets can reach 200 mm. But the main difference is that x / k steel has the same thickness over the entire sheet area, while h / k steel dimensions may vary.
- Accuracy
A clear match of lengths and angles is obtained only by cold drawing. Thermal heating leads to shrinkage of the billet, so h/c sheets always have rounded corners.
- Surface
Cold-rolled sheets have a perfectly smooth and clean surface, without scale or structural shear lines.
- Cost
The complexity of processing, the number of additional operations, and the price of the equipment used significantly increase the cost of cold-rolled steel compared to its hot-rolled counterpart.
