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A rolling mill is a machine for shaping metal billets or metal alloys with two or more rolls.
There are two main types of rolling: hot and cold. Cold rolling is used mainly for products of small thickness (up to 5-6 mm) due to the danger and difficulty of heating thin metal. Besides, products of better quality are made with cold rolling: smoother surface, better physical and mechanical properties.
The most common rolling is a longitudinal one - between two parallel rolls, however, transverse, helical rolling or their combinations are typical.
By type of end product:
By number of rolls:
The number of rolls is selected depending on the final product, its quantity, thickness of the metal used.
Classification is as follows:
By number of stands:
All similar arrangements can be applied to both strip and bar-rolling mills.
Besides those listed above different combinations of stands and their drives arrangements are possible.
For cold rolling of rolled steel various mills are used depending on the required machine performance, product application, etc. Reversing (three-high roll) as well as cluster (up to 5-6) continuous rolling mills are used. Depending on the thickness of the end product (1-2.5 mm), the source material has the thickness of 1.5 to 5 mm. For thick rolls four-high roll mills or mills with successively arranged breaking-down and finishing stands.
Right before rolling the material, it is necessary to clean its surface from scale.
Cleaning is performed chemically - by etching: the roll is rolled, placing its part in a bath with a solution of sulfuric or hydrochloric acid. To ensure the continuity of the process, the end of one roll is pre-welded to the beginning of the next roll, and then, before rolling, it is cut off. After etching, the metal is washed in water and then dried with hot air and oiled. Oiling is essential to reduce friction between the rolls and the metal, which reduces the wear of the rolls and their deflection and in general contributes to a longer service life of entire structure. At the same time, the beginning of the strip is not oiled to make the capture of metal by the rolls easier and to exclude cobble. If the scale on the metal is very hard, then instead of etching, mechanical cleaning is used – shot-blasting or roller descalers.
After preparation, the roll goes to the rolling mill where the metal is rolled to a preset thickness. Currently continuous 4-6- high roll mills are the optimal and most used solution for the production of rolled steel due to their speed and full automation. Hot-rolled coils weighing up to 50 tons with a strip thickness of up to 6 mm are used as billets.
Then the roll is subjected to heat treatment in bell-type or chamber furnaces to have specified mechanical characteristics or eliminate deformations. In particular, surface hardening is removed by annealing - it is heated up to about 700 °C and then slowly cooled. Annealing is carried out between rolling processes in case it is necessary to achieve b metal deformation.
If necessary temper rolling is carried out. It is rolling with light draft (1-3%) to improve flatness and have a certain metal hardness.
Then the rolls are subjected to cutting – longitudinal cutting when there is a need for narrower rolls or cutting-to-length to produce sheets or smaller and less heavy rolls.
At the final stage, the metal undergoes a protection treatment - polishing, painting, galvanizing, etc.