Classification of rolling mill types

Universal rolling mills can be classified according to the arrangement and number of rolls, as well as by the configuration of the mill stands.

Two-roll

It features a simple structure and wide-ranging applications. It is classified into reversible and irreversible types. The former includes equipment such as roughing mills, rail‑beam mills, and medium‑thick plate mills; the latter comprises billet continuous rolling mills, tandem cold‑rolling mills for thin sheets, and cold‑straightening machines for sheet or strip steel. In the early 1980s, two‑high rolling mills had roll diameters of 1,500 mm, roll lengths of 3,500 mm, and rolling speeds ranging from 3 to 7 m/s.

Three-roll

It replaces the conventional method of rolling left or right relative to the gap between the upper and lower rolls, typically used for structural steel and rail beams. This process has largely been superseded by the more efficient two-roll mill.

Lauter triple-roll system

The upper and lower rolls are driven, while the center roll is floating; the workpiece passes either above or below the center roll. Because the center roll has a smaller diameter, it helps reduce rolling force. This type of mill is commonly used for rolling rails, structural steel, and medium‑thick plates, and can also be employed for billet opening. However, this configuration has gradually been replaced by four‑high mills.

Four-roll

The work rolls have a smaller diameter and transmit the rolling torque, while the rolling pressure is borne by the larger-diameter backup rolls. This arrangement offers advantages such as higher stiffness, greater reduction per pass, and lower rolling forces, enabling the production of thinner sheet metal. It is available in both reversible and continuous‑mill configurations and is widely used for hot or cold rolling of medium‑ and thick‑gauge plates and strips, as well as in skin‑pass mills.

Five-roll

There are two types: one is the C‑B‑S configuration (Contact–Bend–Straighten), which employs a four‑roll mill equipped with small-diameter idler rolls—about 1/20 the diameter of the work rolls—that induce bending in the strip. This setup delivers a reduction per pass many times greater than that of conventional four‑roll mills. The strip undergoes plastic bending around these small idler rolls, enabling the rolling of metals and alloys that are difficult to deform into thin strips. The other type features an adjustable center idler roll whose position can be fine-tuned along either the entry or exit side, thereby maintaining the desired strip thickness. This arrangement is used to produce stainless steel, carbon steel, and non‑ferrous metal strips with extremely tight thickness tolerances.

HC

It features high performance and controllable roll crown. Specifically, a pair of axially movable center rolls is added between the work rolls and backup rolls of a four-high mill, with the corresponding ends of the two center rolls precisely aligned to positions corresponding to the edges of the strip. This arrangement improves the uniformity of the pressure distribution and the elastic flattening of the work rolls, thereby ensuring dimensional accuracy of the strip while reducing edge thinning and defects such as cracking. The HC mill is well suited for cold rolling wide strip steel.

Offset eight-roll

It is a variant of the four-high mill. The work rolls have a diameter equal to one-sixth that of the backup rolls and are offset relative to each other to prevent lateral buckling of the work rolls; the rolling force is about half that of a conventional four-high mill. With excellent roll stability and high lateral stiffness, it can be used to produce certain products that typically require a twenty-high mill. Its structure and adjustment mechanisms, however, are considerably simpler than those of a twenty-high mill. This type of mill can be readily reconfigured into two-, four-, eight-, or sixteen-high configurations, making it well suited to a wide range of applications; hence it is also known as a multipurpose mill. It is available in both reversing and continuous‑rolling versions and is employed for cold rolling difficult-to-deform steels, silicon steels, and nonferrous metal strips.

Six-roll

It consists of a pair of work rolls and two pairs of backup rolls, providing a relatively stable roll system. However, its rigidity is comparable to that of a four-roll mill, and it is inconvenient to operate, so it is not widely used; it is typically employed for rolling copper strips for high-precision submarine cables.

Multi-roll

There are three configurations: twelve-roll, twenty-roll, and thirty-six-roll. In the middle, the pair of rolls with the smallest diameter serves as the working rolls, while the remaining rolls function as backup rolls. This design offers advantages such as high relative stiffness, large reduction per pass, low rolling forces, and the ability to produce extremely thin strip. Early multi-roll mills were driven by the working rolls; later, drive was transferred to the center backup rolls. They can process foils with widths ranging from 200 to 1,000 millimeters and thicknesses from 0.02 to 0.0015 millimeters.

Planet

Two large-diameter backup rolls drive two sets of planetary roll stands, enabling slab or bloom billets to be hot‑rolled into thin strip in a single pass, with a reduction ratio of 90–95%. When paired with a continuous caster in the upstream process and a skin‑pass mill in the downstream process, this configuration can streamline the production of hot‑rolled strip steel. The roll bodies of large planetary rolling mills can reach lengths of up to 1,450 mm.

Almighty

It consists of a pair of horizontal rolls and a pair of vertical rolls, with all rolls driven by power. It is used for rolling slab billets or flat steel.

All-purpose steel beam

Its structure is similar to that of a universal rolling mill, except that the axes of the vertical rolls and the horizontal rolls lie in the same straight plane, and the vertical rolls are driven. It is suitable for rolling wide-flange steel beams with heights ranging from 300 to 1200 millimeters.

Disc-type perforating machine

The axis of the workpiece lies below the axis of the rolls; as the rolls rotate, they advance the workpiece along a helical path against the mandrel, extruding solid round steel into a hollow blank tube. This method is suitable for producing seamless steel tubes with diameters ranging from 60 to 150 mm. Another type is the skew‑rolling piercing mill, in which drum‑shaped rolls intersect at an angle and rotate; its operating principle is similar to that of the disc‑type piercing mill and it is used for manufacturing seamless steel tubes with diameters from 60 to 650 mm. The skew‑rolling piercing mill, when combined with the automatic tube‑rolling mill, sizing and diameter‑reducing mill, and the skew‑roll straightening machine shown in Figure 7, forms a complete hot‑working line for seamless steel tubes.

Wheel tire

It consists of an inner and outer circle that oppose the rolling and expanding of wheel rims, as well as a pair of obliquely mounted conical rollers that engage the end faces of the wheel rim. It is used for rolling wheels or the inner and outer rings of rolling bearings.

Rollers with helical semicircular grooves are used; bar stock spirals through the rotating rollers and is rolled into spherical shapes, enabling the production of spheres with diameters ranging from 60 to 650 millimeters.