What is Rolling Mills and Their Types?

What is Rolling?

In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property.

The concept is similar to the rolling of dough. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is known as hot rolling.

If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling.

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

Roll stands holding pairs of rolls are grouped together into rolling mills that can quickly process metal, typically steel, into products such as structural steel (I-beams, angle stock, channel stock), bar stock, and rails.

Most steel mills have rolling mill divisions that convert semi-finished casting products into finished products.

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

What is Rolling Mills?

The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. The earliest rolling mills in crude form but the same basic principles were found in the Middle East and South Asia as early as 600 BCE.

The earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590.

These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron. The first experiments at rolling iron for tinplate took place about 1670.

In 1697, Major John Hanbury erected a mill at Pontypool to roll ‘Pontypool plates’ – black plate. Later this began to be rerolled and tinned to make tinplate. The earlier production of plate iron in Europe had been in forges, not rolling mills.

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

Some of the earliest literature on rolling mills can be traced back to the Swedish engineer Christopher Polhem in his Patriotista Testamente of 1761, where he mentions rolling mills for both plates and bar iron.

He also explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 or more bars at the same time.

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

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

Types of Rolling Mills

As per the requirement of the process and arrangement of the rolls, the rolling mill rolls can be divided into the following categories:

#1. Two High Rolling Mills.

It consists of two rollers, which rotate in the opposite direction for the desired movement of the workpiece. The workpiece is fed between the rollers, which apply a full force, and tends to deform a workpiece and convert it into the desired shape.

If you want a robust and quality two high rolling mill, you can look for mill rolls manufacturers to know which one suits your purpose.

The two high rolling mills are further divided into two more categories namely high non-reversible machines in which rollers rotate in only one direction, and the workpiece can be fed in only one direction.

On the other hand, the second one high reversible machine, in which both rollers rotate in both directions.

#2. Three High Rolling Mills.

The Three High Rolling Mills comprises a roll stand with three parallel rolls one above another. The adjacent rolls rotate in the opposite direction to pass the material between the top and middle roll in a single direction, and the bottom and central role in the opposite direction.

The workpiece is rolled on both forward and return passes. The workpiece passes through the bottom and intermediate rolls, and returns between the middle and top rolls.

Various steel roll manufacturers provide top-quality roll mills to meet every type of industrial requirement.

#3. Four High Rolling Mills.

The Four High Rolling Mills has a roll stand with four parallel rolls placed one above another. The top and bottom rolls work in the opposite direction. The two in the middle are smaller than the top and bottom rolls, which are also known as backup rolls.

#4. Tandem Rolling Mills.

The Tandem Rolling Mills comprises of a set of two or three strands of roll set in parallel alignment. A continuous pass might be possible through each one with the change in the direction of the material. Many mills roll manufacturers provide quality tandem rolling mills to various industries.

#5. Cluster Rolling Mills.

Cluster Rolling Mills is a first four high rolling mills, where each of the working rolls is backed up by two or more rolls for rolling hard material. At times, one might need to employ work rolls of minimum diameter.

#6. Continuous Mill.

With a continuous rolling mill, the same amount of material must pass through each stand in a given period of time using a series of rolling mills. 

If the cross-section is reduced, the speed must be increased proportionately.  The speed of each set of rolls is synchronized so that the input speed of each stand is equal to the output speed of the preceding stand. 

The uni-coiler and wind up reel not only feed the stock into the rolls, but also provide back tension from front tension to the strip.

#7. Planetary Rolling Mill.

This mill consists of a pair of heavy backing rolls surrounded by a large number of small planetary rolls. The main feature of this mill is that it hot-reduces a slab to coiled strip in a single pass.

The operation requires feed rolls to introduce the slab into the mill, and a pair of planishing rolls on the exit to improve the surface finish.

How Do Rolling Mills Work?

Although quantities can vary, rolling mills use a set of at least two rollers. The set of rotating rollers grip the material and drive it forward.

The material is then driven through an opening that is smaller than its starting thickness. The rolls themselves reduce the materials thickness and in the process produce a finished thickness less than that of the starting thickness.

The rolls themselves function similarity to a draw die or swager dies in terms of reducing the area of the material without any loss of material or scrap.

However, the adjustability in the roll gap allows the operator to run various thickness reductions using one set of tooling.

It is for this reason that a rolling mill is the common use of machine for controlling the thickness in various strips.

The operator is able to control the process and make adjustments during production by using an integrated HMI station.

As mentioned, there are many different types of rolling mills. However, the operation of rolling mills is to process various metals through one or more sets of rolls to reduce thickness, create uniform thickness, imprint a design or compact loose material.

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