Cold-rolling mills for coiled steel

• Two-high mills,
• Three-high mills,
• Four-high mills,
• Twenty-high mills,
• Strip gage: 0.3–4.0 mm,
• Coil weight: up to 30 t,
• Annual output: up to 800,000 t.

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:

• billet; for rolling of billets - slabs and blooms;
• sheet; for rolling coils and bands with width of up to 600 mm;
• rolled steel; for high-quality rolled steel with various cross-section;
• pipe rolling;
• special design mills: for profiles with varying sections, wheels, bands, etc.

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:

• Two-high rolling mills. The stand has two rolls revolving towards each other. Often such mills have a reverse for the billets to pass back and forth through the rolls several times. Reversing models are used for three-dimensional profiles and thick sheets; non-reversing models are usually a part of multi-stand mills.
• Three-high roll mills. The rolls are one above the other and revolve without changing its direction. Revolving from the drive is transmitted only to the top and bottom rolls. Operation reversibility is created by the billet passing in one direction under the middle roll and in the other direction – passing above it, for which it is necessary to equip the mill with additional equipment - a special sheet elevator.
• Cluster mills (four-, six-, twelve- high roll). Such mills have two drive rolls and two, four or more backup rolls. The billet is placed between the working rolls. They have a smaller diameter than the drive ones, which reduces deflection of the working rolls and increases rigidity of the entire structure. This allows to adjust the product thickness much more accurately, and with twelve-high roll structures a very thin band is produced. They may have a reverse. Four-high roll stands are widely used in single and multi-stand units.

By number of stands:

• Single-stand. Consists of one stand with rolls driven by a single, often reversing, drive. Usually single-stand mills produce billets (slabs and blooms) and mills for various applications.
• Linear multi-stand. A mill consists of stands arranged in one or more lines. All stands are driven by a single drive. Billets and mills for large profiles often have such design. Because of the drive simplicity of such mills they have a major drawback – impossibility of speed adjustment for individual sections to accelerate the total rolling.
• Tandem. A mill consists of several stands arranged successively. A strip in a stand is rolled only in one pass that is why the mill has pretty many stands, consequently, large sizes. It avoids arranging the stands in several rows.
• Continuous. A mill consists of stands arranged one by one. However, unlike tandem mills the stands here are spaced very closely to each other and a strip is rolled simultaneously in several stands. These are the most productive mills.

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.