What is cold rolling and hot rolling?

Cold Rolling

Rolling below recrystallization temperature is called cold rolling. Hot-rolled coils are usually used as raw materials for cold rolling. After removing oxide scales by pickling, the products are rolled coils. The cold work hardening caused by continuous cold deformation makes the strength, hardness and toughness index of rolled coils increase, so the stamping performance will deteriorate and can only be used for parts with simple deformation. The rolled coil can be used as raw material for hot galvanizing plant because annealing lines are set up in hot galvanizing units. The weight of the rolled coil is generally 20-40 tons. The hot-rolled pickling coil is rolled continuously at room temperature.

Features of cold-rolled products: Because it has not been annealed, its hardness is very high (HRB is greater than 90), and its machinability is very poor, so it can only carry out simple bending with directivity less than 90 degrees.

Simply speaking, cold rolling is processed and rolled on the basis of hot-rolled coils. Generally speaking, it is a process of hot rolling – pickling – phosphating – saponification – cold rolling.

Cold rolling is made of Hot-rolled sheets at room temperature. Although cold rolling also warms up the steel plate during the process, it is still called cold rolling. Because of the poor mechanical properties and high hardness of continuous cold rolling, the mechanical properties of hot rolling can only be restored by annealing. The cold rolling without annealing is called rolled coil. Rolled coils are generally used for products that do not need bending or stretching. Rolled coils of less than 1.0 thickness have good luck in bending both sides or four sides. Cold rolling is usually annealed.

Cold rolling oil must be used in cold rolling process. The advantages of using cold rolling oil are as follows: 1. Reducing friction coefficient effectively, providing corresponding rolling force, reducing rolling energy consumption, and obtaining satisfactory rolling parameters; 2. Making surface brightness high and rolling thickness uniform; 3. The good cooling effect, which can quickly take away rolling heat and protect rolls and rolls. Good annealing performance, no oil burning; 4. Short-term anti-rust performance can provide temporary anti-rust protection for rolling parts.

Hot Rolling

Hot rolling has the advantages of high production efficiency, large scale, low energy consumption, low cost, high mechanization and automation. It is suitable for mass continuous production and is the most important metal pressure processing method. Hot rolling is widely used because of its low deformation resistance and good plasticity. However, due to the uneven rolling temperature and the existence of an oxide scale on the surface, the dimensional accuracy and surface roughness of the products are not as good as those of the cold-rolled products. The main equipment of hot rolling is rolling mill.

There are many kinds of rolling mills for hot rolling, such as two-roll mill, three-roll mill, three-roll Lauter mill, four-roll mill, Y-type mill, two-roll skew mill, three-roll skew mill, cross mill, universal mill and various special mills. According to the product variety, product quality requirements, output, site conditions and other factors, there are many forms of rolling mill layout in hot rolling workshop, such as single-machine layout, single-row or multi-row horizontal layout, tracking layout, continuous rolling layout, chessboard layout, etc. The process flow of hot rolling generally includes raw material preparation, heating, rolling, finishing and other major processes. Hot rolling can be used to roll large ingots or billets to produce large-size products. For example, large ingots of 70 t_and billets of 50 t can be rolled, thick plates of 500 mm thickness, seamless steel tubes of 1070 mm diameter, 1200 mm 530 mm H section steel and 78 kg/m rail can be produced. In the process of hot rolling, the temperature of the rolled piece decreases gradually, and the deformation resistance of the material increases gradually, which eventually makes hot rolling impossible. Therefore, the size of hot-rolled products can not be too small, only rolling 1 mm thick sheet, 20 mm diameter seamless steel pipe, 5.0 mm diameter wire and so on.

Hot rolling can have large production capacity, such as a blooming mill with an annual output of 6 million tons and a hot strip mill with an annual output of 6 million tons. Hot-rolled products can be divided into the hot-rolled strip, hot-rolled seamless steel pipe, hot-rolled section steel and hot-rolled special steel according to their cross-section shape characteristics. Hot rolling can be divided into longitudinal rolling, cross rolling and cross rolling.

The Difference between Cold Rolling and Hot Rolling

1. Local buckling of the cold-rolled section is allowed, which can make full use of the post-buckling bearing capacity of members, while local buckling of the hot-rolled section is not allowed.
2. The causes of residual stresses in hot-rolled and cold-rolled sections are different, so the distribution of residual stresses in sections is also very different. The distribution of residual stress on the section of cold-formed thin-walled steel is bending, while that on the section of hot-bonded steel or welded steel is thin film.
3. The free torsional stiffness of hot-rolled section steel is higher than that of cold-rolled section steel, so the torsional resistance of hot-rolled section steel is better than that of cold-rolled section steel.