What is Rolling Mills and Their Types?

What is Rolling?

In the industry of metalworking, rolling is a metal forming process where metal stock is passed through one or more pairs of rolls to reduce the thickness; to make the thickness uniform; and/or to impart an intended mechanical property.

Rolling is analogous to rolling out dough. Rolling differs based upon the temperature of the metal rolled. If the temperature of the metal is above the recrystallization temperature, the process is called hot rolling.

If the temperature of the metal is below the recrystallization temperature, it is called cold rolling.

In terms of tonnage, hot rolling has more tonnage than any other manufacturing process and cold rolling has the most tonnage of all cold working processes .

Roll stands holding pairs of rolls are assembled in a rolling mills to process metal, usually steel, quickly into products such as structural steel (I-beams, angle, channel), bar stock, and rail.

Rolling mills are typically part of steel mills, where they are part of a division that converts semi-finished casting products to finished products.

There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.

History of Rolling Mills

The invention of the rolling mill in Europe can be attributed to Leonardo da Vinci with his drawings. However early forms of rolling mills (while it is in crude form but working on the similar fundamental principles) are present in the Middle East and South Asia as early as 600 BC.

The slitting mills were the first rolling mills developed and were brought to England from parts of present Belgium in 1590.

These will pass flat bars between rolls form an iron plate, which will pass between grooves in rolls (the slitters) to make iron rods. The first experiments at rolling iron for tinplate occurred about 1670.

Major John Hanbury built a mill at Pontypool in 1697 for rolling ‘Pontypool plates’ – black plate. Eventually this was to be re-rolled and tinned to make tinplate. The earliest production of plate iron on the continent of Europe had been in forges, not rolling mills.

The slitting mill was adapted to make hoops (for barrels) and iron with half-round or other sections by means that was subject to two patents c.1679.

Some of the earliest writings concerning rolling mills can be traced to the Swedish engineer Christopher Polhem in his Patriotista Testamente of 1761 when he speaks of rolling mills for plates and bar iron.

He also writes of how rolling mills save time and labor because a rolling mill allows the operator to roll out 10 to 20 or more bars at one and the same time.

A patent granted to Thomas Blockley of England in 1759 for the polishing and rolling of metals. A patent granted to Richard Ford of England in 1766 was for the first tandem mill.

A tandem mill, is a mill where the metal is rolled in successive stands; Ford’s tandem mill was for the hot rolling of wire rods.

Components of a Rolling Mill

Despite differences in design, most rolling mills have the same basic components.

The components of most rolling mills include:

• Rolls (or Rollers): These are the principal working components that apply force to the metal in a rolling mill. Rolls can take on a number of different materials, such as forged steel or cast iron, and the rolls are machined to exact specifications to provide the shape and surface finish of the final product.
• Mill Stands: Supports and aligns the rolls, and provides a sturdy structure to hold the rolls in place during rolling.
• Bearings: Necessary to support the rotating shafts of the rolls and creates a low friction surface when in operation.
• Screwdown Mechanism: Supplemental to the rolls, the screwdown mechanism adjusts the gaps between the rolls to allow for distinct levels of thickness reductions- screwdowns can be mechanical, hydraulic or electromechanical.
• Drive System: Usually includes motors, gearboxes and spindles that provide the torque and power to rotate the rolls.
• Guides and Manipulators: Provide direction to the workpiece as it goes through the mill from entry to exit of the roll gaps.
• Cooling Systems: Used in hot rolling, such as water sprays, to help dissipate heat on the rolls and on the workpiece to prevent overheating and degradation of properties of the material.
• Shears/Cutters: For cutting the rolled product to lengths or trimming edges.
• Control Systems: Modern rolling mills usually have sophisticated feedback control systems typically using a computer for automated rolling operations, monitoring trends during the process and quality control.

How do Rolling Mills Work?

Even though these may vary, rolling mills utilize at least two rollers in a set. The two rollers will rotate and grip the material and pull the material through.

The material is then pulled through an opening that is smaller than the thickness of the original material. The rolls reduce the thickness of the material and produce a finished thickness that is less than the original or starting thickness.

The rolls decrease the materials thickness in a similar way as a draw die or swager dies decrease the area of a material without any loss of material or scrap.

Whereas dies have only the process setting to create a fixed precision, rolling mills have the ability to adjust the roll gap to reduce various thicknesses with one set of tooling.

For this reason, rolling mills are the most common type of machine for controlling material thickness in various strips.

The rolling mill operator will typically control the entire running process with one integrated HMI station.

As stated previously, there are a multitude of rolling mill types. However, the function of rolling mills is fundamentally the same, to process various metals with one or more rolls to reduce thickness of, uniform thickness of, imprint an identify or design into, or compact loose materials.

The basic steps in the rolling process are:

1. Feeding the Workpiece: The raw material, often pre-warmed initially (in hot rolling) so as to create more ductility, is fed into the rolling mill.
2. Reduction: After the metal passes through the rolls, the thickness of the material has reduced. Each pass through a set of rolls (or stands), produces a total reduction.
3. Elongation: With a conservation of volume in mind, as the thickness is reduced the length of the workpiece increases.
4. Shaping: Depending on the roll design, the metal can take on different shapes as opposed to a only flat sheet.
5. Cooling and Finishing: Generally the material is cooled after the rolling process, and some additional milling processes may occur such as cutting, coiling, or surface treatment.

How Are Rolling Mills Primarily Classified by Temperature?

One of the simplest methods for classification of a rolling mill is the temperature range at which the metal is rolled:

Hot Rolling Mills

Hot rolling is where the metal is heated above its recrystallization temperature (typically above 900°C for steel). This means the metal is very ductile and can be easily deform due to the higher temperature.

This enables a large reduction in thickness from a single pass and is often used for the initial breakdown of large ingots or slabs into intermediate forms called blooms, billets, or thicker plates.

Hot rolling can help refine grain structure, remove cast defects, and allow for considerable shape changes, however dimensional accuracy is not great, and the surface may be scaled due to oxidation.

Cold Rolling Mills

Cold rolling is where the metal is below the recrystallization temperature, typically at room temperature. The advantage from cold rolling are very nice surfaces finishes, tolerances are much tighter, and any strength/hardness properties are increased due to “formed” or “work” hardened.

However, at low temperatures the metal is less ductile, requires more deformation force, and has less reduction potential per pass.

Cold rolling is typically applied in the production of sheets, strips, and foils for the precision and aesthetics required by consumers, such as automotive body panels, appliance casings, or aluminum foil.

Warm Rolling Mills

This is an intermediate form of processing generally performed at a temperature that is between the hot and the cold rolling process (i.e. Steel at a temperature between 600°C to 900°C.).

However, it still has the potential of giving some operational benefits associated with hot rolling, and gives better dimensional control and surface finish than can be obtained through hot rolling, although it requires less force than cold rolling.

Warm rolling is typically applied to resulting properties that need to be described as not necessarily hot-rolled properties, nor cold-rolled properties also, such as in the case of aluminum foil.

Types of Rolling Mills

Depending on the special function or arrangement rolls dependent on the special process, the designs of rolling mills can be classified into the following categories:

#1. Two High Rolling Mills.

A two high rolling mill consists of two rollers which rotate in opposite directions for the desired movement of the workpiece. The workpiece is fed between the rollers, and the rollers apply a full force, to deform a workpiece and form it into the desired shape.

If you are looking for a strong and high-quality two high rolling mill, then you could check with mill rolls manufacturers to find which would suit your purpose.

The two high rolling mills can be further divided into two more categories namely high non-reversible machines to rotate in only one direction by changing workpiece direction and high reversible which have potential to rotate in either direction.

Non-reversing two-high mills:

The rolls’ direction of rotation can only go in one direction so the work-piece can be passed through only in one direction.

The two-high rolling mill will be employed in on-going rolling operations where the material progresses through multiple stands in one direction.

For instance, you will be operating straight-cut shop-type saws when cutting small laying parts.

Reversing two-high mills:

The rolls’ direction of rotation can be reversed so the work-piece can be passed back and forth multiple times through the rolls in the same direction. The back and forth passing allows greater thickness reductions.

The reversing two high mill is a two-high mill used for breakdown rolling of ingots to blooms/slabs and as well for rolling rails, beams, and heavy plates.

#2. Three High Rolling Mills.

The Three High Rolling Mills use a roll stand with three parallel rolls one above another.

The adjacent rolls are rotating in the opposite direction so the material is passed between the top and middle rolls in one direction and bottom and central roll in the opposite direction.

This means the work-piece is rolled on all of both a forward and a return passes. The work-piece passes between the bottom and intermediate rolls and returns between top and middle rolls.

There are various steel roll suppliers that build high quality roll mills for so many of the every type of industrial applications.

#3. Four High Rolling Mills.

Four high rolling mills are comprised of four rolls and arranged in a vertical stack. A four high rolling mill has two smaller sized work rolls that directly contact the metal, and two larger backup rolls that support the work rolls.

The work rolls are smaller to limit the contact area and thereby limit the rolling force required for the deformation, while the larger backup rolls react to the bending of the work rolls suffered under immense loads to obtain a more constant thickness across the width of the rolled product.

Used mainly for finish rolling narrow gauge stock, plates, sheets and strips especially in cold rolling where precise thickness is required. Also used for hot rolling of armor and other plates.

#4. Tandem Rolling Mills.

Tandem Rolling Mills consist of two or three strands of rolls set in parallel arrangement. A continuous pass is possible through each one, utilizing the direction change of the material. Many mills roll manufacturers provide quality tandem rolling mills for use in a range of industries.

A continuous mill or a tandem mill is a series of two or more rolling stands set out in line. The workpiece of metal is passed through the series of loops one right after the other without interruption, providing for each roll to reduce the material upon each pass. 

The rolls speed of operation in each passes of stand is progressively increased to accommodate the elongation of the metal as it reduces in thickness, therefore allowing for the void to be compensated and flow.

High volume sheets, strips, and bars hot or cold rolled. Mobilized widely in steel mills to provide hot rolled coils and cold rolled strips.

#5. Cluster Rolling Mills.

Cluster Rolling Mills consist of a four high rolling mill where each working roll has two or more rolls backing up each roll so that you can roll hard material. At times it is necessary to use some minimum diameter work rolls.

Cluster Mills are simply a higher technology version of four-high mills; which means where each of the two working rolls are backed up by two or more (we could have 6, 8, 12, 20 rolls if desired) bigger rolls.

The greater the number of backing up rolls; the better the support to the smaller diameter work rolls, and the effect of roll deflection is virtually eliminated.

Cluster mills possess the ability to roll extremely hard material – such as stainless steel, titanium alloys, nickel alloys; also, they can produce very thin and wide foils (as in copper foil) while adhering to extremely small dimensions tolerances during cold rolling.

#6. Continuous Mill.

A continuous roll mill is the same concept but the same amount of material must pass by each stand in a specific period of time using one of a series of rolling mills.

If the Cross section is reduced, the speed must increase proportionally.  Each set of rolls must be adjusted to keep the input speed of each stand the same as the output speed of the previous stand.

The coiler and wind up reel not only feed the stock into the rolls but provide back force from front force to the strip.

#7. Planetary Rolling Mill.

The planetary rolling mill has a pair of heavy backing rolls with a large number of small planetary rolls around those rolls.

The key feature of this mill is that it hot reduces a slab to coiled strip in one pass. This process requires the use of feed rolls to bring the slab into the mill as well as a pair of planishing rolls at the exit to further improve the surface finish.

This type of mill is mainly used only for the hot reduction of slabs into coiled strips. Hot reduction offers substantial thickness reductions utilizing only one pass.

#8. Universal Rolling Mills.

Universal mills have horizontally mounted rolls and vertical mounted rolls. This allows for thickness and width reduction at the same time, or forming a complex cross-sectional shape.

Mainly used to produce structural shapes such as H-beams, I-beams, channels, and angles from rolled forming process where accurate dimensions and sharp corners are essential.

#9. Specialized Rolling Mills.

In addition to the main classifications, there are also some specialized mills for producing specific products or processing:

• Tube Piercing Mills (Mannesmann Mills): These mills are used to make seamless hollow tube from solid cylindrical billets. The tube is made through the use of two conical rolls (that are angled) and with a stationary internal mandrel that pierces the solid progressive length and transforms it into a hollow tube.
• Ring Rolling Mills: Ring rolling mills roll three rollers (drive roller, idler roller, and axial roller) to create seamless rings and tubes by compressing a workpiece that looks like a donut.
• Shape Rolling Mills: Shape rolling mills are designed with special grooves in their rolls, in order to produce rolled products in various profiles (I-sections, H-sections, angles, and railway rail).
• Skew Rolling Mills: These mills use a special type of roller to create spherical shapes, most famously ball bearings.
• Thread and Gear Rolling Mills: Equipment that employs thread dies on rollers to produce threads and gears on metal workpieces with consistency of quality and high rates of production.
• Transverse Rolling Mills: Where tapered surfaces can be produced by passing between rollers with a tapered portion.
• Roll Bending Mills: Form sheets or plates of metal into cylindrical, conical, and other curved shapes.
• Laboratory Rolling Mills: Smaller rolling plants usually with adjustable features are used for research and development, material testing, and pilot line production.

Applications of Rolling Mills

Rolling mills are vital to a wide variety of metals processing industries. For example, rolling mills are employed in:

• Steel Industry: Production of steel sheets, plates, structural beams, rails, wire rod, rebar.
• Aluminum Industry: Production of aluminum sheets, foils, plates, and extrusions for aerospace, automotive, and packaging.
• Copper and Brass Industry: Production of copper and brass sheets, strips, and tubes used in electrical, plumbing, and decorative applications.
• Automotive Industry: Production of body panels, chassis components and other rolled parts.
• Construction: Structural steel and reinforcing bars and roofing materials.
• Aerospace: High-strength aluminum and titanium alloys for aircraft structures.
• Electronics: Very thin foils for batteries and circuit boards.
• Coinage: Strips of metal rolled to exact thickness for coin blanks.
• Research and Development: Laboratory-scale rolling mills for metallurgical research and testing new alloys and processes.

Advantages of Rolling Mills

• Productivity: Rolling is a production process that is continuous and produces large quantities of production.
• Cost: With continuous, high volume production – rolling can be more cost-effective than other forming processes.
• Versatility: Rolling mills can process many metals and alloys into many shapes and sizes.
• Properties: Rolling improves material properties – rolling improves grain structure leading to improved mechanical properties like strength and ductility, hot rolling will increase properties through recrystallization as well.
• Dimensional Accuracy: Cold rolling equipment is capable of achieving precision tolerances in dimensions and or thickness.
• Waste: The rolling process produces little waste relative to some other processes.

Disadvantages of Rolling Mills

• Initial cost: Rolling mills are large, complex and expensive pieces of machinery with high capital costs.
• Energy: The process of rolling (especially hot rolling) consumes a lot of energy.
• Maintenance: Rolls and other parts of the mill will wear down and need regular maintenance, often costly maintenance.
• Material Limitations: Some materials are inherently difficult to roll (like very brittle materials, materials that stick, etc).
• Residual Stresses: If cold rolled, residual stresses form in the material, requiring an annealing process to relieve.
• Edge Elongation: In a 4-high mill, the center can stay the same and the edges elongate resulting in a quality concern.

References:

PPRM. “What is rolling mill and Its Process?” Accessed June 7, 2025. https://pprm.in/blogs/rolling-mill-everything-you-need-to-know/

Fenn Torin. “What is a Rolling Mill Used for.” Accessed June 7, 2025. https://www.fenn-torin.com/blog/what-is-a-rolling-mill-used-for

Ulbrich Stainless Steels & Special Metals, Inc. “Types of Rolling Mills.” Accessed June 7, 2025. https://www.ulbrich.com/blog/types-of-rolling-mills/

Maco Corporation. “The Impact of Rolling Mills and Rolling Machines Across Industries.” Accessed June 7, 2025. https://www.macocorporation.com/blog/rolling-mills/

The Steefo Group. “5 Ways Rolling Mills Prove Beneficial to Small Manufacturers.” Accessed June 7, 2025. https://www.thesteefogroup.com/5-benefits-of-rolling-mills-for-small-manufacturers/

Bellcliffe Engineering Ltd. “Rolling Mill Components.” Accessed June 7, 2025. https://www.bellcliffe.com/rolling-mill-components/

FAQs