What’s the Difference between Hot Rolling and Cold Rolling

Both hot rolling and cold rolling are forming processes of steel plate or profile, which have a great influence on the microstructure and properties of steel.

The rolling of steel is mainly hot rolling, and cold rolling is usually only used to produce small section steel and sheet steel.

Cold and hot rolling conditions of common steel:

Wire rod: diameter 5.5-40 mm, coil shape, all hot rolled. After cold drawing, it belongs to cold drawn material.  

Round steel: in addition to the size of precision bright materials are generally hot-rolled, there are also forging materials (surface forging traces).  

Strip steel: hot rolled and cold rolled, cold-rolled steel is generally thin.  

Steel plate: the cold-rolled sheet is generally thin, such as an automobile plate; there are many hot-rolled medium and heavy plates with similar thickness as cold rolling, but their appearance is obviously different.  

Angle steel: all hot rolled.  

Steel pipe: welding, hot rolling and cold drawing.  

Channel steel and H-beam: hot rolled.  

Steel bar: hot rolled material.

Hot-rolling

By definition, ingots or billets are difficult to deform and process at room temperature. Generally, they are heated to 1100-1250 ℃ for rolling. This rolling process is called hot rolling.

The end temperature of hot rolling is generally 800-900 ℃, and then it is cooled in air, so the hot rolling state is equivalent to normalizing treatment.

Most of the steel is rolled by hot rolling. Due to high temperature, a layer of iron oxide scale is formed on the surface of the steel delivered in a hot rolling state, so it has certain corrosion resistance and can be stored in the open air.

However, the oxide scale also makes the surface of hot-rolled steel rough and the size fluctuates greatly. Therefore, the steel with a smooth surface, accurate size and good mechanical properties should be produced by cold rolling with hot-rolled semi-finished products or finished products as raw materials.

Cold rolling

Cold rolling refers to the rolling method in which the steel is extruded by the pressure of the roller at room temperature and the shape of the steel is changed. Although the process will also raise the temperature of the steel plate, it is still called cold rolling. To be specific, the hot-rolled coil is used as raw material for cold rolling, and the finished product is hard rolled coil after acid pickling to remove the oxide scale.

Generally, cold-rolled steel such as galvanized steel and color steel plate must be annealed, so it has good plasticity and elongation, and is widely used in automobile, household appliances, hardware, and other industries. The surface of the cold-rolled sheet has a certain degree of smoothness, which is smooth by hand, mainly due to acid pickling. Generally, the surface finish of the hot-rolled strip can not meet the requirements, so the hot-rolled strip needs to be cold-rolled, and the thickness of the hot-rolled strip is generally 1.0 mm, and the cold-rolled strip can reach 0.1 mm. Hot rolling is rolling above crystallization temperature, cold rolling is rolling below crystallization temperature.

The change of steel shape by cold rolling belongs to continuous cold deformation. The cold work hardening caused by this process makes the strength and hardness of the rolled coil increase and the toughness and plasticity index decrease.

For the end-use, cold rolling makes the stamping performance worse, and the product is suitable for simple deformation parts.