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Rolling mill designs come in different types of configurations, the most basic being the twin roll non-reversible, meaning that there are two rolls that can only turn in one direction.The rolls of a two-roll reversing mill can rotate in both directions, but the disadvantage is that the rolls must be stopped, reversed, and then returned to rolling speed between each pass.To solve this problem, the three-roll mill was invented, which uses three rolls that rotate in one direction; the metal is fed through two rolls and sent back through the other pair. The disadvantage of this system is that a lift must be used to raise and lower the workpiece.All of these mills are typically used for primary rolling with roll diameters ranging from 60 to 140 cm (24 to 55 inches).To minimize roll diameters, four-high mills or cluster mills are used.Smaller roller diameters are advantageous because fewer rollers are in contact with the material, which results in lower force and power requirements.The problem with small rolls is the reduced stiffness, which can be overcome by using backup rolls.These backup rollers are larger and touch the back of the smaller rollers. A four-high mill has four rolls, two small and two large.Cluster mills have more than four rolls, usually in three tiers.These types of rolling mills are commonly used for hot rolling wide plate, most cold rolling applications and rolling foil.
Historically,mills were classified by the product produced:
Blooming,slotting and slab mills, which are preparation mills for rolling finished rails, sections or plates, respectively.They range in diameter from 34 to 48 inches if reversed, or 28 to 42 inches if triple high.
3-high billet mill with roll diameters from 24” to 32” for further rolling of blooms to 1.5x1.5” billets as pre-rolling mill for bars.
Beam mills,three-high, roll diameters from 28 to 36 inches, for the production of heavy beams and channels 12 inches and larger.
Rail mills with roll diameters from 26 to 40 inches.
Formers with roll diameters from 20 to 26 inches for smaller beam and channel sizes and other structural shapes.
Commercial bar mills with roll diameters of 16 to 20 inches.
Small commercial bar mills with finish roll diameters of 8 to 16 inches, usually with larger roughing stands.
Rod and wire mills with finishing roll diameters of 8 to 12 inches, always with larger size roughing stands.
Hoop and cotton tie mills, similar to small commercial bar mills.
Armor plate mills with 44 to 50 inch diameter rolls and 140 to 180 inch main bodies.
Plate mills with roll diameters from 28 to 44 inches.
Plate mills with roll diameters from 20 to 32 inches.
Universal mills for the production of square-edge or so-called universal plates and various wide-edge plates by means of vertical and horizontal roll systems.
Continuous rolling mill
A tandem rolling mill is a special type of modern rolling mill in which rolling is done in one pass.In a conventional rolling mill, rolling is done in several passes,but in a tandem rolling mill, there are multiple stands (>=2 stands) and the reduction is done continuously.The number of booths varies from 2 to 18.
Tandem rolling mills can be of the hot or cold rolling mill type:Cold rolling mills can be further classified as continuous or batch processing.A continuous mill has an annular tower which allows the mill to continue slowly rolling strip in the tower while a strip welder joins the tail of the current coil to the head of the next coil.At the exit end of the mill there is usually a flying shear (used to cut the strip at or near the weld) followed by two coilers; one is unloaded while the other is wound on a current coil.Ring towers are also used in other places; such as continuous annealing lines and continuous electrolytic tinning lines and continuous galvanizing lines.