Liquid steel is usually cast in continuous casting machines in the shape of billets, blooms, or slabs. In some plants, it is also being cast in continuous casting machines in the shape of thin slabs or dog bone sections. These shapes are processed by hot rolling by passing them through plain or grooved cylindrical rotating rolls to produce plates, sheets, rods, structural sections, and tubes etc.
Rolling process is one of the most important and widely used industrial metal forming operations. It provides high production and close control of the final product. It was developed in late 1500s. It accounts for 90 % of all metals produced by metal working processes.
Rolling steel is a metal forming process in which steel is passed through a pair of rotating rolls for plastic deformation of the steel. Plastic deformation is caused by the compressive forces applied through the rotating rolls. High compressive stresses are as a result of the friction between the rolls and the steel stock surface. The steel material gets squeezed between the pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Rolling is classified according to the temperature of the steel rolled. If the temperature of the steel is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the steel is below its recrystallization temperature, the process is termed as cold rolling.
The rolls run on massive neck bearings mounted in housings of enormous strength and driven by powerful electric motors. These are known as mill stands. There are several types of rolling mill stands as given below.
- Two- high stand – It is the basic type of rolling mill stand in which two opposing rolls are used.
- Two-high pullover stand – The rolling steel stock is returned to the entrance for further reduction.
- Two-high stand, reversing – The work can be passed back and forth through the rolls by reversing their direction of rotation.
- Three-high stand – It consists of three rolls. The upper and lower rolls are driven while the middle roll rotates by friction.This improves the productivity.
- Four-high stand – It has small diameter rolls (less strength and rigidity) are supported by larger-diameter backup rolls. Rolling power is directly proportional to roll diameter.
- Universal mill stand – This stand has two horizontal rolls and two vertical rolls. Vertical rolls are used to control the width of the work simultaneously.
- Cluster mill or Sendzimir mill stand – In this rolling mill stand, each of the work rolls is supported by two backing rolls.
- Planetary mill – It consists of a pair of heavy backing rolls surrounded by a large number of planetary rolls. Each planetary roll gives an almost constant reduction to the slab as it sweeps out a circular path between the backing rolls and the slab.
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| Two high Rolling mill |
