By definition, steel ingots or billets are challenging to deform at room temperature and difficult to process. Generally, they are heated to 1100-1250 °C for rolling. This rolling process is called hot rolling.
The termination temperature of hot rolling is generally 800 to 900 ° C, and then it is generally cooled in air, so the hot rolling state is equivalent to normalizing treatment.
Most steels are rolled by the hot rolling method. Due to the high temperature, the steel delivered in the hot-rolled state has a layer of iron oxide scale on the surface, so it has certain corrosion resistance and can be stored in the open air.
However, this layer of iron oxide scale also makes the surface of the hot-rolled steel rough and the size fluctuates greatly. Therefore, steel with a smooth surface, accurate size, and good mechanical properties are required, and hot-rolled semi-finished products or finished products are used as raw materials for cold rolling production.
Advantages of hot rolled:
The forming speed is fast, the output is high, the coating is not damaged, and it can be made into a variety of cross-sectional forms to meet the needs of use conditions; cold rolling can cause a large plastic deformation of the steel, thereby increasing the yield point of the steel.
The shortcoming of hot rolled:
1. Although there is no hot plastic compression during the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The style of cold-rolled steel is generally an open section, which makes the free torsional stiffness of the section low. It is prone to torsion when under bending, prone to bending-torsional buckling under compression, and has poor torsional performance;
3. The wall thickness of cold-rolled forming steel is small, and it is not thickened at the corners where the plates are connected, and the ability to withstand concentrated local loads is weak.
The hot coil is uncoiled, the continuous welding starts and the cold rolling process is officially entered: after pickling, it is rolled by the rolling mill to produce a hard-rolled coil, and the cleaned hard-rolled coil enters the heat treatment stage.