Some Basics of Rolling and Rolling Mills
Some Basics of Rolling and 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. During rolling metal is subjected to high compressive stresses as a result of the friction between the roll and the metal surface.
Leonardo da Vinci invented the first rolling mills but only after a few centuries rolling mills became important for the steel industry. 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.
Basic concept of the rolling
- The arc of contact between the rolls and the metal is a part of a circle.
- The coefficient of friction is constant in theory, but in reality it varies along the arc of contact.
- The metal is considered to deform plastically during rolling.
- The volume of metal is constant before and after rolling. In practice the volume might decrease a little bit due to close up of pores.
- The velocity of the rolls is assumed to be constant.
- The metal only extends in the rolling direction and there is no extension in the width of the material.
- The cross sectional area normal to the rolling direction is not distorted.
Factors influencing rolling
The major factors which influences the rolling process are given below
- The diameters of the rolls
- The amount of reduction in a single pass
- The initial thickness of the rolling stock
- The speed of rolling since it decides the strian rate
- The front and back tension
- The nature of friction between the rolls and the steel material rolled
- The physical properties of the steel material being rolled
- The temperature field in the steel material and the rolls
- The shape of the roll contour or roll pass in which the steel material is getting deformed
- The behaviour of the rolling mill under load
- The effect of previous treatment of the steel material resulting in work hardening or other effects.
- The elastic deformation of rolls under load.
- The state of the anisotropy of the material
- The aspect ratio, or the ratio of the width of the stock to the iniial thickness
The above 14 parameters may singly or jointly, in combinations of two or more, create secondary parameters anf phenomena more directly related to and commonly associated with the rolling process. These are given below
- Coefficient of draught, absolutr draught and relative draught – these are established by the initial and final mean thickness of the rolling stock
- Slip – It is characterized by the difference of the linear speed of the rolling stock and the periphral speed of the roll
- Neutral angle – It determines the point of no slip
- Spread – It is the difference in the width of the exit material in comparison to the width of the in going stock
- Coefficient of elongation – It is dependent on the relative vaue of draught and spread
- Bite – It is a function of draught, roll diameter, coefficient of friction and in going thickness. For the workpiece to enter the throat of the roll, the component of the friction force must be equal to or greater than the horizontal component of the normal force.
- Rolling pressure – A useful quantity for characterizing the mechanics of the rolling process is the average normal stress, pressure, acting between the work and roll. The pressure is not constant since the stress acting to deform the work material is the stress needed to overcome material strength, overcome frictional forces and overcome any constraints placed on the deformation by process characteristics.
- Specific roll pressure – It is the rolling load divided by the contact area.
- Rolling load – It is the load with which the rolls press against the metal.
- Torque – It is the measure of the force applied to a member to produce rotational motion
- Power – It is applied to a rolling mill by applying a torque to the rolls and by means of strip tension. The power is spent principally in four ways (i) the energy needed to deform the metal (ii) the energy needed to overcome the frictional force (iii) the power lost in the pinions and power transmission system and (iv) Electrical losses in the various motors and generators
Major components of a rolling mill
The major components of a rolling mill consist of the following:
- Rolling stand or roll housing – Requires very rigid construction
- Roller table – For material movement in the rolling mill
- Descaling device – For removal of scale before rolling
- Shears – to cut the steel materials at different stages of rolling
- Guides – They are usually used in long mills for guiding the feed material to the roll groove.
- Work rolls – The rolls which are in contact with the work piece being rolled.
- Back up rolls – they are intended to provide rigid support required by the working rolls to prevent bending under the rolling loads. They are normally used in flat mills.
- Roll bearings and roll chocks
- Roll balance system – This is to ensure that upper rolls are maintained in proper position relative to lower rolls.
- Roll changing device – This is a special device designed to attach to the neck of the roll to be removed or inserted into the rolling mills.
- Mill protection devices – These are to ensure that the forces applied to the roll chocks are not of such a magnitude to fracture the roll necks or damage the housing
- Roll cooling and lubrication systems
- Hydraulic system
- Pinions – These are gears to divide power between the two spindles, rotating them at the same speed but in the different direction
- Gearing- This is to establish desired rolling speed
- Drive motors – They provide power to the rolls and roller conveyors. They should be large enough to supply enough power. The drive motors also controls the speed.
- Electric controls – To control the quality of power to the drive motors and the automation system for the mill
- Cooling of the rolled products – It is done either by water cooling, air mist cooling or cooling by air on the cooling beds.
- Coilers and uncoilers – These are used for coiling and uncoiling of rolled steel.
- Packing and bundling devices
Roll configuration in a rolling mill
Rolling mills are designed with different types of roll configurations (Fig 1). Rolls configuration can be reversing (rolls can rotate in forward and backward direction) or non reversing (Rotation of rolls is in a single direction). In the reversing types of rolls for reversing the rolls must be stopped, reversed and then brought back up to the rolling speed. Various types of roll configurations are given below.
- Two high configurations – This means rolling stand has two rolls (top and bottom). This configuration is normally used for rolling long products
- Three high configurations – The rolling stand has 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. In this configuration work piece is lifted or lowered using an elevator. This configuration is normally used in cross country rolling mills.
- Four high configurations – In this configuration two smaller diameter rolls (lesser strength and rigidity) are supported by two back up rolls with larger diameters. This configuration is used for flat rolling.
- Cluster mill configurations – In this type of configuration 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 configuration
- Planetary mill configuration – It consist 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 back up 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.
Fig 1 Types of roll configurations