Today, there are two main types of carbon alloys with iron – cold-rolled and hot-rolled steel. When smelting both types of mill products, there are no technological differences. Dissimilarity comes at the stage of final processing and rolling of metal into steel sheets.
Well, let’s have a look at the differences in the manufacturing processes of hot-rolled and cold-rolled sheets, and accordingly, in the metal properties that each type of rolling-mill products produces.
Hot-rolled steel features
The hot-rolling process occurs at temperatures above 900°C. Since the metal heats up, it is much easier to give it the necessary shape. Hot-rolled mill products are usually cheaper than the cold-rolled due to the fact that hot-rolled steel does not require re-heating (as in the case of cold rolling). Metal shrinks when cooling, so it is more difficult to predict the size and shape of the finished product than in the case of the cold-rolled coil.
As a result of heat treatment, crystal grains of small size and regular shape are formed in the structure of hot-rolled sheets. And it provides:
- increase in metal ductility and tenacity;
- a decrease in metal strength and hardness;
- improvement of the machining process.
Applicable scopes of hot-rolled steel products:
- construction activities, where it is required to strengthen foundations, erect metal structures, and loadbearing constructions;
- manufacturing of pipes;
- vehicle manufacturing, shipbuilding, and aircraft industry;
- production of electrical appliances;
- fabricated and bolted constructions;
- wherever a steel product is required, but the cost savings and structural strength are more important than an aesthetic appearance of the product.
Cold-rolled steel features
The cold-rolled steel, despite its name, does not get along without temperature treatment: any cold-rolled sheet is made from a hot-rolled steel workpiece. First of all, the hot-rolled workpiece undergoes treatment to remove the scale and oxide film. There are two treatment methods: pickling and shot blasting.
Pickling involves the immersion of a steel sheet in a 25% solution of hydrochloric or sulfuric acid. This method removes excess elements from the metal surface. Shot blasting implies the percussive action on the steel sheet, which allows knocking off the excess particles from the metal surface. Some manufactures, apply the method of combined treatment, which is the most effective.
And only after that, the workpiece is transferred to the cold-rolling mills. That is why the thickness of the cold-rolled steel sheet does not exceed 5 mm, and the surface of the sheet is much smoother than that of the hot-rolled steel.
This is how the direct manufacturing stage of cold-rolling sheets ends, and the stage of steel hardening begins. First of all, the steel undergoes furnace annealing at a temperature of about 700°C. It leads to the recrystallization of the molecular networks of the alloy so that the steel receives the necessary flexibility and elasticity. After annealing, the sheets are rolled in the mill again, which prevents the appearance of stretcher strain markings.
The final stage of the cold-rolled steel processing is called pinch-pass rolling. It implies the compression of the obtained steel sheet by up to 3%. This procedure increases the hardness and strength of steel sheets so that in the future, they can easily perform the functions assigned to them.
The features of cold-rolled steel are:
- high accuracy of geometric characteristics,
- evenness and flatness of the surface,
- homogeneity of the internal structure.
Applicable scopes of cold-rolled steel products:
- vehicle manufacturing and shipbuilding;
- instrument making industry;
- chemical and metallurgical industry;
- construction activities;
- production of cans and enamelled ware;
- production of pipes.
- wherever minimal metal thickness is required, and the attractive appearance of the steel sheet plays an important role.
Which type of rolling-mill products is better to choose?
Many people wonder which steel sheet is better: hot-rolled or cold-rolled. The answer is: neither of the two is better in relation to one another. Hot-rolled or cold-rolled steel sheets are different in their properties, and so, each of them is good for its own use. Therefore, when choosing a rolled-mill product, it is important to take into account the sphere of its application. For example, for the construction of large structures, it is advisable to use hot-rolled steel sheets. And for the finishing operations, it is better to use the cold-rolled steel, which differs by its smoothness and aesthetic appearance.