Classification of Rolling mills
Classification of Rolling mills
Rolling is the process of plastically deforming metal by passing it between the rolls. It is the most widely used forming process, which provides high production and close control of the final product. It is a process used to shape metal into a thin long layer by passing it through a gap of two rolls rotating in different directions (clockwise and anti-clockwise). The gap between the two rolls is supposed to be smaller than the thickness of the work piece of the material to be formed. When the metal piece is put between the rolls, it experiences forces of friction and compression from the rolls compressing it to become thin and elongated, or longer than its original length. When the piece completes its way through the gap between the rolls, it has lesser thickness than the original one with an increased length and width. This decrease in thickness is referred to as draft and the increase in length and width is called an absolute elongation and spread respectively.
Hot and cold metalworking between two or more rolls has been in existence since long before the industrial revolution. The first known design of a rolling mill dates back no less than to Leonardo who in one of his drawings, dated 1480 (Fig 1), describes for the first time the possibility of ‘making a material pass’ between two cylindrical rollers with parallel axes to modify its thickness. Also, in this case, it was provided for the cold machining of ductile materials, especially for lead, tin etc but it is not certain that it was ever built. The first rolls were small, hand driven and they were used to flatten gold and silver in the manufacture of jewelry and art. By the 1600s, rolling machines, rather than small, hand-driven rolls, were known to have been in operation and iron was just being introduced as a metal capable of rolling.
There are reports of two rolling mills in the sixteenth century: one used to obtain gold sheets with uniform thickness from which to draw coins, the second to cut already formed sheets into strips. Both were more used as “finishing devices” rather than to reduce the thickness.
The first industrial plant for which there exists certain news was used in 1615, to obtain lead and tin plates. By the late 1700s, the first hot rolling mills appeared, allowing iron to become a more popularly rolled material. Modern rolling practice is attributed to Henry Cort, who got a patent for use of grooved rolls for rolling iron bars. Henry Cort is also called ‘father of modern rolling’. The first rail mill was established in 1820, while the first plate mill was exhibited in 1851. Three high mills for rolling heavy sections were introduced in 1853. Hot strip mill were developed in America in the first half of the twentieth century. Fig 1 gives some of the earlier rolling mills.
Fig 1 Some of the earlier rolling mills
In its simplest form, a rolling mill (Fig 2) 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. 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.
Fig 2 A simple rolling mill
The classification of mills can be done in several ways as 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. They are namely (i) hot rolling mill (ii) cold rolling mill.
In the hot rolling mill, 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.
In case of cold rolling mill, rolling is done below the recrystallization temperature of the metal. Rolling in this mill is normally done at the room temperature. Rolling increases the strength of the metal because of the strain hardening. It improves surface finish and the rolled metal and te metal has tighter tolerance when compared with the hot rolling.
Classification based on the rolling mill product
This classification of the rolling mills is done normally in three ways. In the first classification based on the rolling mill product, the rolling mills are of two types namely (i) flat mills, and (ii) long product mills. The flat mills are for the rolling of steel plates, sheets and strips, while the long product mills are for the rolling of rounds, rods, reinforcement bars, and shapes.
In the second way of classifying rolling mills based on products is done on the nature of the product. Under this classification rolling mills are of two types namely (i) semi-finishing mill, and (ii) finishing mill. The semi finishing mill produces semi finished products such as slabs, blooms, and billets etc. which need further rolling in the finishing mill. On the other hand, the finishing mill is that mill which produces saleable products.
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.
- Blooming, cogging, and slabbing mills – These mills are the preparatory mills to roll blooms or slabs from ingots. With the wide spread acceptance of continuous castings of slabs and blooms, these mills are no more required.
- Billet mills – These mills produce billets from the blooms.
- Beam mills – These mills are used for the production of heavys beam and large channel sections from the blooms.
- Rail mills – As the name suggest, rails mills are used for rolling of rail sections from the blooms.
- Shape or structure mills – In these mills medium and smaller sizes of beam and channel sections and other structural shapes are rolled normally from billets. These rolling mills are also called section mills.
- Merchant bar mills – These mills are for the rolling of merchant grades of bar products and reinforcement bars. Merchant bar products include rounds, flats, squares, angles, tees, channels, and beams etc.
- Special bar quality mills – These mills are used for rolling special bar quality steel grade products. Special bar quality mill products include rounds, flats, squares, hexagons, and tubes.
- Wire rod mills – These mills produces wire rods from billets. These mills are provided with no twist rolling in the blocks and laying head to form rings for the controlled cooling of the rods on the cooling conveyor after rolling, The product of the wire rod mill is the wire rod in coil form.
- Plate mills – These are flat mills for the rolling of heavy plates. These mills normally include on-line heat treatment of the rolled plates.
- Hot strip mills – These are also flat mills and roll hot steel strips from slabs. Hot strips are produced in the form of coils.
- Skelp mills – These mills hot roll narrow strips in the form of coils. Narrow strips are used for the production of welded pipes.
- Cold rolling mills – Cold rolling mills roll cold rolled strips from hot rolled strips by cold rolling. Cold rolling is done after the removal of mill scale from the hot rolled strip by the acid pickling. Cold rolled strips are rolled in the form of coils. Cold rolled strips after rolling are annealed and skin passed before their dispatch from the mill.
- Universal mills – These mills are for the production of various universal sections such as I beam, H or wide flanged beam and other universal shapes by a system of vertical and horizontal rolls.
Classification based on stand arrangements
Based on the rolling stands configurations, rolling mills are classified either cross-country mills or the straight line mills. The cross country mills are those rolling mills where the centre lines of rolling stands are parallel to each other and the material being rolled is shifted perpendicular to the rolling direction from one stand to other stand. Most of the cross country mills are reversing mills. In the straight line 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 rolling process
Based upon the rolling process, rolling mills are classified as (i) reversing mill, (ii) semi continuous mills, (iii) continuous mills, and (iv) tandem mill.
Reversing mills are those rolling mills in which 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 mills the material is being rolled moves in to and fro directions.
Semi continuous mills are those rolling mills in which some roll stands (normally the roughing stands) are reversing type while other rolling stands (normally finishing stands) constitutes continuous rolling.
Continuous mills are those rolling mills in which the material to be rolled moves only in one direction and all the mill rolls rotates only in single direction. The required numbers of stands are provided in these rolling mills for giving total reduction to the material being rolled and for giving final shape to the rolled product.
Tandem mills are the type of rolling mills where rolling is done in one pass (Fig 3). In a traditional rolling mill rolling is done in several passes, but in tandem mill there are several stands (equal to or more than 2 stands) and reductions take place successively. The number of stands can range from 2 to 18. Tandem mills can be either of hot rolling or cold rolling mill types.
Fig 3 Types of rolling mills
Classification based on roll configuration
Rolling mills are designed with different types of roll configurations, and hence the rolling mills are also classified based on the roll configurations. The types of rolling mills based on roll configurations are described below and are shown in Fig 3.
- Two-high mills – These mills have the most commonly used rolling mill configuration. In this configuration, there are two horizontally mounted rolls. The rolling mill motor drives either both rolls (top and bottom) or only one roll (normally the bottom roll) with the top roll rotating due to the friction between the roll and the work piece. As per the rolls rotation direction, the mill can be either non-reversing (unidirectional) mill or reversing mill.
- Three-high mills – These mills have the roll configuration in which there are three horizontally mounted rolls. Rolls in the mills with this configuration rotate permanently only in one direction. These mills make it possible rolling with increased number of grooves than in case of two-high mill stands. The work piece is rolled in one direction between the bottom and intermediate roll and then in the opposite direction between the intermediate and top roll. The fix-fitted intermediate roll is directly driven. The bottom and top roll are driven via the gearbox and they are normally adjustable. This roll configuration is used for the rolling of the shaped rolled products in grooved rolls. The drive of these mills normally include a fly wheel.
- Four-high mills – In the four high rolling mills, the roll configuration consists of four horizontal rolls, mounted in a single vertical plane. Two rolls (inner) are work rolls and two rolls (outer) are back-up rolls. Significance of the back-up rolls consists in a chance of using higher roll forces and decrease in bending (deflection) of the work rolls. Small diameters of work rolls also permit (except for greater elongation of the rolling stock) a possibility of achieving of more favourable dimensional thickness deviations. The work rolls of the four-high mills are driven while the back-up rolls are normally friction driven. The four-high rolling mills are used for rolling of plates and strips in case of hot rolling and strips in case of cold rolling. These mills are both non-reversing and reversing types of the rolling mills.
- Six -high mills – In six high rolling mills, there are six horizontal rolls, mounted in a single vertical plane. Two rolls (inner) are work rolls and four rolls are back-up rolls. These types of rolling mills are normally used in cold rolling of steel strip.
- Cluster mills – These mills are also known as ‘Z’ mills. In the cluster rolling mills, there are six, seven, twelve, or twenty horizontally mounted rolls. The popular Sendzimir mill is a type of cluster mill with having 20 rolls. In all the mills having this configuration, there are only two rolls which are work rolls while all the other rolls are back-up rolls. Normally work rolls are driven and back-up rolls are friction driven. The cluster mills are used for cold rolling of very thin sheets, strips and foils.
- Universal mills – Universal mills have roll configuration, where there are two horizontally mounted rolls and two vertically mounted rolls which are driven through transmission of bevel gear wheels. The vertical rolls act by edging effect on lateral sides of the work piece, which leads to creating its lateral ‘walls’, precision angles and sharp edges. The edging rolls are used to be mounted from the front of the mill stand, less frequently from the rear side, but sometimes also from both sides of the mill. Universal mills are used for rolling of slabs, universal plates, and universal steel sections such as beams etc. To enable rolling of wide-flange beams, the vertical rolls are mounted in the same plane with axes of rolls placed horizontally. Only the horizontal rolls are driven.
- Planetary mills – In planetary mills, there are a pair of heavy backup 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. This configuration is used for giving high reduction in a single pass.
Pipe rolling mills
Pipe rolling mills are used for the production of different types of pipes. Pipe mills can be for the production of welded pipes or for the production of seamless pipes.
Welded steel pipes are produced with either a longitudinal seam or a spiral (helical) seam. The starting material for the production of the welded pipes is rolled flat product which depending on the pipe production process, pipe dimensions and application, can be hot rolled (HR) or cold rolled (CR) steel strip/skelp, and HR wide strip or plate. This starting material can be formed into pipe shape in either hot or cold condition. The forming process can be either a continuous process or a single pipe forming process. In continuous pipe forming process, uncoiled strip material is taken from an accumulator, with the leading end and the trailing end of the consecutive coils being welded together. In single pipe forming process, the pipe forming and welding operation is carried out in single pipe length. There are two types of welding processes which are mainly used for the welded pipe production. These are (i) pressure welding processes, and (ii) fusion welding processes. The commonly used pressure welding processes are (i) pressure welding process e.g. Fretz-Moon process, (ii) direct current electric resistance welding, (iii) low frequency electric resistance welding, (iv) high frequency induction welding, and (v) high frequency conduction welding. The commonly used fusion welding processes are (i) submerged arc welding, and (ii) gas shielded welding.
Seamless pipes are made from round billet, which is pierced through the centre to make it a hollow shell and then rolled or extruded and drawn to size. The seamless pipe manufacturing process (Fig 4) consists of three stages namely (i) making of a hollow pipe shell in the piercing or extrusion operation, (ii) elongating the hollow pipe shell by reducing its diameter and wall thickness, and (iii) making of a final pipe in the hot or cold rolling process. As the manufacturing process does not include any welding, seamless pipe is perceived to be stronger and more reliable. Seamless pipe is regarded as withstanding pressure better than other types of pipes.
Fig 4 Seamless pipe rolling processes
Specialized rolling mills
Specialized rolling mills produce specialized products by combining rolling process with other processes. Under this classification the following are the major types of mills.
Skew rolling mills – Skew rolling mills (Fig 5) are used for the production steel balls for ball bearing. The semi-finished balls made by the skew rolling process are subsequently ground and polished for use in ball bearings.
Transverse rolling mills – These types of rolling mills are also called cross rolling or roll forging mills (Fig 5). These mills use circular wedge rolls. Heated bar is cropped to length and fed in transversely between rolls. Rolls are revolved in one direction.
Fig 5 Skew rolling and transverse rolling processes
Ring rolling mills – In the ring rolling mills (Fig 6), the donut shape pre-formed work piece is placed between a free turning inside roll and a driven outside roll. The ring rolling mills make the section thinner while increasing the ring diameter. Several cross-sections can be formed by the ring rolling.
Fig 6 Ring rolling and thread rolling processes
Thread rolling mills – These mills are used for threading of rods or pipes (Fig 6). Rolled threads are produced in a single pass at speeds far in excess of those used to cut threads. Dies are pressed against the surface of cylindrical blank. As the blank rolls against the in-feeding die faces, the material is displaced to form the roots of the thread, and the displaced material flows radially outward to form the thread’s crest. A blank is fed between two grooved die plates to form the threads. The thread is formed by the axial flow of material in the work piece. The grain structure of the material is not cut, but is distorted to follow the thread form. The resultant thread is very much stronger than a cut thread. It has a greater resistance to mechanical stress and an increase in fatigue strength. Also the surface is burnished and work hardened.