Rolling Process: Definition, Working, Application, Defects, Type of Rolling Mills [PDF]

Saswata Baksi
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Rolling is the deformation process of a metal that is widely used in the metal forming process.

It is done by passing the strip of the metal between the rollers. This paper discussion will be made about the rolling process, the working principle of the rolling process along with that the working principle of the rolling mills.

Then the types of rolling mills will be discussed. Apart from that, in this assessment rolling and its defects are included briefly.

rolling operation

Definition of Rolling process:

Rolling is defined as a process to form metals where the metal strip is pressed by two or multiple rollers, thus the uniform thickness is formed. To do this, the temperature is essential.

There are two types of processes. One is Hot rolled and another is Cold Rolled.

If the strip is rolled after heating the strip above the re-crystallization temperature then it is termed as Hot rolled and if that done in room temperature then it is termed as the Cold rolled. Rolling is a process that is widely used and has very high production.

Working Principle of Rolling Process:

The rolling process is a metal forming process, in which stock of the material is passed between one or more pairs of rollers in order to reduce and to maintain the uniform thickness.

This process is mainly focused on the cross-section of the ingot or the metal which is forming. Mainly by this process, we reduce the thickness of the metal workpiece.

Now, the rolling processes are mainly focused on the increasing length and the decreasing thickness without changing the width of the workpiece.

There are certain types of the rolling process, whereas, in the hot rolling process, the metal is heated at its desirable temperature, when the metal is properly heated then the metal should be passed between the one or more rolling mills to gain the proper desirable shape.

This process is vastly used in respect of any other rolling process. In this process, the metal is heated above the recrystallization temperature. In the hot working process, the metal is changing its grain structure because of the heat, now there were a new set of strain-free grains in the metal and this process needs less amount of force which correspondingly reduces the quality of the surface finish, of that metal.

Now there is another rolling process, which is a cold rolling process. This rolling process is done below the recrystallization temperature of the metal it varies upon the metal, room temperature can also be a below recrystallization temperature.

In this process, the force is much more required than the hot working process to pass the metal from the rollers and this process offers good surface finish.

Types of Rolling Mills:

There are five rolling mills which are commonly used for rolling metals:

  • Two-High Rolling Mills
  • Three-High Rolling Mills
  • Four High Rolling Mills
  • Tandem Rolling Mills
  • Cluster Rolling Mills

Description of these mills are as follows:

1. Two-High Rolling Mills:

This type of mill has two types. Those are:

  • Reversing Mills
  • In reversing Mills

In this rolling mill, there are two rolls used.

Two High Reversing Mill:

In this type of mill, the rollers are both adjustable. In these mills, rotation of that two rolls is made in two different directions. In this operation, the metal is passed between two rollers that rotate at the same speed but it is in the opposite direction.

It is used in slabbing, plumbing, rail, plate roughing work and many other areas. As there is the need for a reversible drive, this mill is cheaper compared to the others.

Two High Non-Reversing Mills:

In these mills, two rolls continuously revolve in the same direction and we can't reverse the direction of the rollers. In this operation, the motive power is less costly.

two high rolling mills

2. Three-High Rolling Mills:

In this mill, the three rolls stand in parallel one by others. The rolls are rotating in opposite directions. In this mill, between the first and the second rolls, the material passes. If the second roll rotates in a direction then the bottom roll rotates in another direction. The material is rolled both in forward and return in three high rolling mills.

At first, it passes forward through the last and second roller and then comes back through the first and second roller. In that mill, the thickness of the material is reduced and being uniform by each pass. Here transition system and a motor are needed which is less powerful.

three high rolling mills

3. Four High Rolling Mills:

In this type of mill, there are four parallel rolls one by another. In this operation, the rotation of the first and the fourth rolls take place in the opposite direction of the second and the third rolls. The second and third rolls are smaller to provide rigidity in necessity. So those are known as back up rolls. It is used in the hot rolling process of the armor and in the cold rolling process of sheets, strips, and plates.


4. Tandem Rolling Mills:

In this type of rolling mill, there are two or more sets of rolls in the parallel alignment which make the continuous passes and successively decrease the thickness and make that uniform.


5. Cluster Rolling Mills:

In this type of rolling mill, there are two basic roles that are backed up by two or more rolls which are bigger than those two basic rolls. These backed up rolls give more pressure to the basic rolls to heavily press the strip.


Application of Rolling:

The rolling operation used in various industries such as:

  • Rods, seamless hollow tubes are made by rolling.
  • Rolling is used to producing cross-section of large sections.
  • Rolling is used to cutting the gears on the gear blank.
  • The threaded parts, bolts, screws, etc. which have mass production is made by the rolling process.
  • In automotive industries, various parts are manufactured by the rolling process.
  • The rolling process is used to made plates, steel sheets, etc.
  • Bearing, Turbines rings are rolling products.

Apart from that, in the rolling applications, there are two types of rolling products. Those are as follows:

Hot Rolled Products:

Hot rolled products are made by a hot rolling process. In that process, the process is done at a very high temperature like over 1,700 Degree Fahrenheit. For most of the metals, this temperature is re-crystallization temperature. Such Hot Rolled products are as follows:

  • Rods
  • Rails
  • Plates
  • Sheets and strip,
  • Structural
  • Bars, etc.

Hot Rolled products also used to make body panel, piping and tubes, and construction materials.

Those Hot Rolled products have such characteristics. Those are as follows:

  • As the product shrinks from very high temperature, the edges of the product and the corners of the products such that plate or bars becomes rounded slightly.
  • As the product cooled from very high temperature there might be a chance of distortion is present. In many cases, the product becomes slightly trapezoidal in shape instead of the perfect square.
Cold Rolled Product: 

Cold rolled product is made from the hot-rolled product. When the hot-rolled product is cooled, then that is re-rolled in the room temperature. It is known as Cold Rolled Product. It is used to do to get more exact or perfect dimensions and to make the surface quality better. There are such Cold rolled products. Those are as follows:

  • wire sheet etc.
  • Cold-rolled products, such that the wire is used to make screws and bolts, cold-rolled sheets are used to make strip and sheets for external applications of the automotive industries like doors, electrical motors, etc.

Cold Rolled products have such characteristics. Those are as follows:

  • Glossy like an oily and smooth surface.
  • The surface is more finished and better with closer tolerances.
  • Tubes have straightness and the concentric uniformity is better.
  • The edges or the corners of the bars are well defined and the shape of the bars are more perfect than hot rolled products.

Defects of Rolling:

As the deflection of rolls occurred by the high forces of rolling,

There are two types of rolling defects. Those are as follows:

  • Surface rolling defects
  • Internal structural rolling defects.

Surface Defects:

Surface defects in rolling can be categorized into following types, and those are:

Internal Structural Defects:

There are some types of internal structure defects. Those are as follows:

Wavy Edges Crack: 

The result is thicker as the middle portion of the rolling part is bent or deflected by the compressive load. There are some different cases. Those are as follows.

  • For the imperfection of the roll gaps, variation occurs on the rolling sheets.
  • If the thickness varies and along with that volume and width are constant then the center is shortened than the edges. But the body is continuous.
  • Then the edges portions are in the compression and the center portion is the tension.
  • The result of the edge is wavy.

Zipper Cracks in Centre of Strip: 

Zipper Crack is a type of Wavy Crack. If there is an uneven stress distribution on the strip, then the crack occurs in the centreline of the strip. It is called Zipper Cracks in the Center of Strip. This crack looks like a zip so that it called Zipper Cracks.

Edge Crack: 

Edge cracks occur when the hot rolls are cooled. It happens as excessive quenching effects on the strip.

If excess water is used to cool the edges.

The use of excess water might give the result of unflattens in the strips.

The edges of the metal got rounded off as the friction force prevents the corners and increases the length of the center portion.

Alligator Crack:

Alligator Crack is one type of cracking where the metal has any inclusions or weakness of metallurgy. That causes factor in the strip. As this crack separates the layers and increases the slabs openings, it looks like the mouth of an alligator. So that name of this crack is Alligator Crack.


This study of the rolling process and its mills drew every minute aspect. In terms of manufacturing engineering, every engineer should consider the rolling process as a core metal forming process. This process is mainly used when a cross-sectional area of a material required to be changed by an engineer.


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