Rolling is a forming process in which the metal and alloys are plastically deformed into semifinished or finished condition, by passing these between circular or contoured rotating cylinders (rolls). The metal is drawn into the opening between the rolls by friction forces between the metal and the roll surface. In deforming metal between rolls, the work piece is subjected to high compressive force the squeezing action. A rolling mill is a complex of machines for deforming metal in rotary rolls and performing auxiliary operations such as transportation of stock to rolls, disposal after rolling, cutting, melting, piling or coiling etc., A set of rolls in their massive housing is called a ‘stand’. Rolling mills are classified according to the number of rolls are in the working stand, the product rolled arrangement of stand etc.,
Classification based on number of Rolls in the stand
- Two-high pull over mill
- Two-high reversing mill
- Three-high rolling mill
- Four- high rolling mill
- Cluster mill
- Continuous mill
- Planetary mill
Two-High pull over mill
As the name implies, it has two rolls with a constant direction of rotation about horizontal axes, It is the simplest and most common type of rolling mill. For successive reductions, the stock is returned to the entrance of the rolls by hand carrying or by means of a platform which can be raised pass the stock over the rolls.The rolling is accomplished in one direction only by drawing the work through friction. The upper roll may be raised or lowered to change the distance between the rolls. And regulated opening between rolls the work piece is taken over the rolls and put into original position for the rolling and thus the process is repeated.
Two-High reversing mill
An improvement in productivity results from the use of TWO-HIGH reversing mill. In which the work can be passed back and front through the rolls by reversing their direction of rotation. The enables the work to travel through in one direction the back through in the other direction.This enables the work to travel through in one direction the back through in the other direction. A series of reductions can be made using the same set of rolls, by passing the work back and front
Three-High rolling mill
The disadvantages of a two-high reversing mill are over come by using a three-high rolling mill in which three rolls with a constant direction of rotation are arranged in a single vertical plane. Lifting tables are provided on one or both sides of the stand to raise and lower the bar after each pass. In these mills, the top and bottom rolls are drive rolls and the middle roll rotates by friction. These mills are employed as blooming mills, for billet rolling and finish rolling.
A large decrease in the power required for rolling can be achieved by the use of small diameter rolls. However, small diameter rolls have less strength and rigidity than large rolls. In four-high rolling mill small diameter rolls (less strength & rigidity) are supported by large-diameter back up rolls. Here, only two horizontal rolls of smaller diameter are used and these rolls are supported by another two horizontal rolls of large diameter. In this arrangement,the pressure during rolling is counteracted by back rolls. This also proves greater reduction, without any intermediate annealing.
Cluster-Rolling mil
By using cluster mill, very thin sheets can be rolled to very close on a mill with small diameter work rolls. Here each of the work rolls is supported by two backing rolls. It is very typical mill. The sendzimir mill is a modification of the cluster mill with 20 rolls. Which is very well adapted to rolling thin sheets or foil from high-strength alloys. Hard materials, including stainless steels,titanium alloys and nickel alloys may be rolled in these mills.
Continuous mill
In 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 mill and each set is called a stand. If the cross-section is reduced, the speed must be increased proportionately. Thus the rolls of each successive stand must turn faster than those of preceding one. The strip will be moving at different velocities at each stage in mill. The speed of each set of rolls is synchronised so that the input speed of each stand is equal to the output speed of preceding stand. The uncoiler and wind up reel not only feed the stock into the rolls and coiling up the final product but also provides back tension and front tension to the strip.
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.
