Classification of Rolling mills


Classification of Rolling mills
Rolling is the process of plastically deforming metal by passing it between rolls. It is the most widely used forming process, which provides high production and close control of final product. Leonardo da Vinci invented the first rolling mills but only after a few centuries rolling mills became important for the steel industry. In its simplest form, a rolling mill (Fig 1) consists of two driven rolls in a mill stand with a screw down. The work piece to be rolled is passed through the rotating rolls to get the desired shape.

a simple rolling mill

Fig 1 A simple rolling mill

But in practice rolling mills are not that simple. They are quite complicated and they are of several types. Though all the rolling mills utilize the same principle of rolling but they differ in many ways. The classification of mills can be done in many ways. These are described below.

Classification based on temperature of rolling

Under this classification, rolling mills are classified according to the temperature of the metal to be rolled. Based on this classification rolling mills are basically of two types: (i) hot rolling mill (ii) cold rolling mill.

  • Hot rolling mills – In these mills, rolling is done above the recrystallization temperature of the metal. During rolling in these mills the grains, which deform during the process of rolling, recrystallize, maintain an equiaxed microstructure and prevent the metal from work hardening. In this type of rolling hot rolled metals have very little directionality in the mechanical properties and deformation induced residual stresses.
  • Cold rolling mills – In these mills, rolling is done below the recrystallization temperature of the metal. Rolling in these mills which is normally done at the room temperature, increases the strength of the metal through strain hardening. It improves surface finish and holds tighter tolerance when compared with hot rolling.

Classification based on the mill product

This classification is usually done in three ways. The first way of classification is by the type of the product. These are:

  • Flat mills – These mills rolls plates, sheets and strips.
  • Long product mills – These mills rolls rounds, rods and shapes.

The second way of classifying mills on products is based on the nature of the product. These are

  • Finishing mills – These mills produced saleable products.
  • Semi finishing mills – These mills produce semi finished products which need further rolling in the finishing mills.

The third way classifying rolling mills based on products is the historical way of classifying the rolling mills. In this classification rolling mills are known by the product they produce after rolling. Under this classification mills are of the following types

  1. Blooming, cogging and slabbing mills – These are the preparatory mills to roll blooms and slabs from ingots. With the wide spread acceptance of slab and bloom continuous castings these mills are no more required.
  2. Billet mills – These mills produce billets from the blooms.
  3. Beam mills – These mills are used for the production of heavy beams and large channels.
  4. Rail mills – As the name suggest rails mills are used for rolling of rails from the blooms.
  5. Shape or structure mills – In these mills medium and smaller sizes of beams and channels and other structural shapes are rolled usually from billets.
  6. Merchant bar mills – These mills rolls merchant grades of rounds and reinforcement bars.
  7.  SBQ mills – These mills are used for rolling special bar quality rounds.
  8. Wire rod mills – These mills produces wire rods from billets. Usually these mills are provided with no twist rolling in the blocks and controlled cooling of the rods after rolling.
  9. Plate mills – These are flat mills to produce heavy plates.
  10. Hot strip mill – These are also flat mills and rolls hot strips from slabs.
  11. Cold strip mills – These mills rolls cold strips from hot strips by cold rolling.
  12. Universal mills – These mills are for the production of various wide flanged shapes by a system of vertical and horizontal rolls.

Classification based on rolling process

Under this classification, the rolling mills can be classified as follows:

  • Reversing mills – In this type of mills the rolling direction changes after each pass. In these mills the rolls are stopped, reversed, and then brought back up to rolling speed after each pass. In these materials the material being rolled moves in to and fro directions.
  • Continuous mills – In this type of mills the material to be rolled moves only in one direction and all the mill rolls rotates only in single direction. There are number of stands provided in the mill for giving total reduction to the material being rolled and for giving final shape to the rolled product.
  • Semi continuous mills – In this type of mills some roll stands (usually roughing stands) are reversing type while other rolling stands (usually finishing stands) constitutes continuous rolling.
  • Tandem mills – A tandem mill is a type of rolling mill where rolling is done in one pass. In a traditional rolling mill rolling is done in several passes, but in tandem mill there are several stands (>/=2 stands) and reductions take place successively. The number of stands ranges from 2 to 18. Tandem mills can be either of hot or cold rolling mill types.

Classification based on stand arrangements

Under this classification there are two types of rolling mills as given below.

  • Cross country mills – In these types of the mills the centre lines of initial rolling stands are parallel to each other and the material being rolled is shifted perpendicular to the rolling directions. Most of the cross country mills are reversing mills.
  • Straight line mills – In these mills all the roll stands have a common centre line and material being rolled moves only in forward or forward/backward direction.

Classification based on roll configuration

Rolling mills are also classified based on the roll configurations. The types of mills based on roll configurations are given below.

  • Two high mills – In this type of mills the rolling stands have two rolls (top and bottom).
  • Three high mills – In these types of mills the rolling stands have three rolls which rotate in one direction. The metal is fed in one direction through two of the rolls and then reversed through other pair. The middle roll is common in each feeding. The upper and the lower rolls are driven while the middle roll rotates by friction.
  • Four high mills – In these types of the mills two smaller diameter rolls (lesser strength and rigidity) are supported by two back up rolls with larger diameters.
  • Cluster mills – In this type of mills each set of the work rolls is supported by two back up rolls. These back up rolls have a further set of backing rolls in the third tier. Sendzimir mill is having this type of mill.
  • Planetary mill – This mill consists of a pair of heavy back up rolls surrounded by a large number of planetary rolls. Each planetary roll gives an almost constant reduction to the feed material as it sweeps out of a circular path between the backup roll and the feed material. As each pair of planetary rolls ceases to have contact with the work piece, another pair of rolls makes contact and repeat the reduction.

Specialized rolling mills

These rolling mills produce specialized products by combining rolling process with other processes. Under this classification the following are the major types of mills.

  • Pipe mills – Under this category, mills for producing different type of pipes (such as ERW, SAW, Seamless etc.) comes.
  • Ring mills – These mills are used for rolling rings
  • Thread rolling mills – These mills are used for threading of rods or pipes.
  • Transverse rolling mills – These mills are used for rolling in transverse direction.