Basics of Rolling and Rolling Mills
Basics of Rolling and Rolling Mills
Metal rolling is an important production process. It has industrial importance among various production processes because of its several advantages such as cost effectiveness, enhanced mechanical properties, flexible operations, higher productivity, and considerable material saving. Rolling is the most widely used forming process, since it provides high production rates and close control of the final product.
Rolling is the process of plastically deforming metal by passing it between rolls. It is the process of reducing the thickness or changing the cross section of a long work-piece by compressive forces applied through a set of rolls rotating in the opposite direction. The rolling operation takes a solid piece of metal and breaks it down successively in several steps into different shapes such as flats, rounds, and sections etc. During rolling, metal is subjected to high compressive stresses as a result of the friction between the roll and the metal surface.
Rolling has existed for hundreds of years. The first rolls were small, hand driven and they were used to flatten gold and silver in the manufacture of jewelry and art. In 1480, Leonardo da Vinci sketched a machine for the rolling of lead for stained glass windows. This was the world’s first available record of a rolling mill, but no evidence exists which shows that the machine was built. 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. 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.
Basic concept of the rolling
Rolling is a process used to shape metal into a thin long layer by passing it through a gap of two rollers rotating in different directions (clockwise and anti-clockwise). The gap between the two rollers 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 rollers, it experiences forces of friction and compression from the rollers compressing it to become thin and elongated, or longer than its original length (Fig 1). When the piece completes its way through the gap between rollers, it has lesser thickness than the original one with an increased length and width. This decrease in thickness is referred to draft and the increase in length and width is called an absolute elongation and spread respectively.
The initial materials for the primary rolling mills are ingots or continuous cast slabs, blooms or billets. In addition to producing a useful shape for further processing, the hot rolling process converts the cast grain structure into a wrought grain structure. The initial cast material possesses a non-uniform grain structure, typically consisting of large columnar grains which grow in the direction of solidification. These structures are normally brittle with weak grain boundaries. Cast structure characteristically contains many defects such as porosity caused by gases, shrinkage cavities, and solid inclusions of foreign materials which are trapped in the metal. Rolling a metal above its recrystallization temperature breaks apart the old grain structure and forms a new one. Grain boundaries are destroyed and new tougher ones are formed, along with a more uniform grain structure. The rolling process also closes the vacancies and cavities within the metal. In addition, hot rolling process breaks up the inclusions and distributes the material throughout the work piece.
The process of rolling is a specialized form of metal forming for shaping large bulk material into thin sheets, plates, or different types of cross-sections such as rounds, flats, squares, angles, channels, T-bars, rails, and beams etc. of large lengths. The rolling operation is to ensure the final shape geometry of the work piece being rolled, the uniformity of the material, and the change in property due to the deformation process. Fig 1 shows the concept of rolling of metals.
Fig 1 Concept of rolling of metals
Most metal rolling operations are similar in that the work piece is plastically deformed by compressive forces between two constantly spinning rolls. These forces act to reduce the thickness of the metal and affect its grain structure. The reduction in thickness can be measured by the difference in thickness before and after the reduction, this value is called the draft. In addition to reducing the thickness of the work, the rolls also act to feed the material as they spin in opposite directions to each other. Friction is hence a necessary part of the rolling operation, but too much friction can be detrimental for a variety of reasons. It is essential that in a metal rolling process the level of friction between the rolls and work material is controlled, use of lubricants can help with this.
During a metal rolling operation, the geometric shape of the work is changed but its volume remains essentially the same. The roll zone is the area over which the rolls act on the material. It is the place where the plastic deformation of the work occurs. An important factor in metal rolling is that due to the conservation of the volume of the material with the reduction in thickness, the metal exiting the roll zone is moving faster than the metal entering the roll zone. The rolls themselves rotate at a constant speed, hence at some point in the roll zone the surface velocity of the rolls and that of the material are exactly the same. This is point is known as the no slip point. Before this point the rolls are moving faster than the material, after this point the material is moving faster than the rolls.
Fig 2 Metal rolling concepts
Sometimes in metal rolling practice, tension, (force), is applied to a work piece as it is being rolled (Fig 2). This tension can be applied to the front, (front tension), the back, (back tension), or both sides. This technique assists the forces necessary to form the work piece, and is normally used on hard to roll materials.
At only one point along the surface of contact between the roll and the work piece, two forces act on the work piece. These are (i) a radial force, and (ii) a tangential frictional force. Between the entrance plane and the neutral point the work piece is moving slower than the roll surface, and the tangential frictional force act in the direction to draw the metal into the roll (Fig 3). On the exit side of the neutral point, the work piece moves faster than the roll surface. The direction of the frictional force is then reversed and opposes the delivery of the work piece from the rolls.
Fig 3 Rolling process concepts
In metal 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 can decrease a little bit due to the closing up of the pores. The velocity of the rolls is considered to be constant during the rolling process. The metal only extends in the rolling direction and there is practically no extension in the width of the material. The cross sectional area normal to the rolling direction is not distorted.
Variables influencing rolling
There are several variables which have influence on the process of rolling. These variables are (i) diameters of the rolls, (ii) amount of reduction in a single pass, (iii) initial thickness of the work piece, (iv) speed of rolling since it decides the strian rate, (v) front and back tension, (vi) nature of friction between the rolls and the material rolled, (vii) physical properties of the material being rolled, (viii) temperature field in the material and the rolls, (ix) shape of the roll contour or roll pass in which the material is getting deformed, (x) behaviour of the rolling mill under load, (xi) effect of previous treatment of the material resulting in work hardening or other effects, (xii) elastic deformation of rolls under load, (xiii) state of the anisotropy of the material, and (xiv) aspect ratio, or the ratio of the width of the stock to the initial thickness.
These 14 variables can singly or jointly, in combinations of two or more, create secondary parameters anf phenomena more directly related to and normally associated with the rolling process. These secondary parameters are given below.
- Coefficient of draught, absolute draught and relative draught which are established by the initial and final mean thickness of the rolling stock.
- Slip which is characterized by the difference of the linear speed of the rolling stock and the periphral speed of the roll.
- Neutral angle which determines the point of no slip.
- Spread which is the difference in the width of the exit material in comparison to the width of the in going material.
- Coefficient of elongation which is dependent on the relative value of draught and spread.
- Bite which is a function of draught, and roll diameter.
- Coefficient of friction and in going thickness which is important for the workpiece to enter the throat of the roll. The component of the friction force is to be equal to or greater than the horizontal component of the normal force.
- Rolling pressure which is a useful quantity for characterizing the mechanics of the rolling process. It is the average normal stress, pressure, acting between the work piece and roll. The pressure is not constant since the stress acting to deform the work piece is the stress needed to overcome material strength, frictional forces, and any constraints placed on the deformation by process characteristics.
- Specific roll pressure which is the rolling load divided by the contact area.
- Rolling load which is the load with which the rolls press against the metal.
- Torque which is the measure of the force applied to a member to produce rotational motion.
- Power which is applied to the rolling mill by applying a torque to the rolls and by means of work piece tension. The power is spent principally in four ways namely (i) energy needed to deform the metal, (ii) energy needed to overcome the frictional force, (iii) 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 (i) rolling stand or roll housing which needs a very rigid construction, (ii) roller table conveying system for the movement of the work piece in the rolling mill, (iii) descaling device which removes the scale on the work piece before rolling, (iv) shears which are needed to cut the head end and tail end of the work piece, to cut the rolled material in cooling bed lengths or saleable lengths, and to cut the work piece being rolled to small lengths in case of cobble, (v) guides which are normally used in bar and section mills for guiding the feed material to the roll groove, (vi) work rolls which carry out the function of rolling and which are in contact with the work piece being rolled, (vii) back up rolls which are used in flat hot and cold rolling mills and which are intended to provide rigid support needed by the work rolls to prevent bending under the rolling loads, (viii) roll bearings and roll chocks which support the rolls at their two ends, (ix) roll balance system which ensures that upper rolls are maintained in proper position relative to the lower rolls, (x) roll changing device which is a special device designed to attach to the neck of the roll for the removal or insertion of the rolls into the rolling mill stands, (xi) mill protection devices which ensure that the forces applied to the roll chocks are not of such a magnitude to fracture the roll necks or damage the housing, (xii) screw down mechanism which controls the gap between the top and bottom rolls, (xiii) roll cooling and lubrication systems, (xiv) hydraulic systems, (xv) pinions which are gears to divide power between the two spindles connected to the rolls, rotating them at the same speed but in the different direction, (xvi) gearing assembly which establishes the desired speed of rolling, (xvii) drive motors which provide power to the rolls and roller conveyors and control the speed and which are to be large enough to supply enough power, (xviii) electric control system which control the quality of the power to the drive motors, (xix) automation system which is normally used in large capacity mills for the elimination of the human errors, (xx) cooling of the rolled products which is done by water cooling, air mist cooling or cooling by air on the cooling beds or cooling conveyor (in case of wire rod mills), (xxi) coilers and uncoilers which are used for coiling and uncoiling of rolled steel, and (xxii) packing and bundling devices for the rolled products.
Roll configuration in rolling mills
Rolling mills are designed with different types of roll configurations. 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 direction of rolling, the rolls are to be stopped, reversed and then brought up back to the rolling speed. Various types of roll configurations used in the rolling mills are given in Fig 4 and described below.
Fig 4 Roll configurations in rolling mills
Two-high mill roll configuration – This is 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 mill roll configuration – In this type of roll configuration, 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 rolled stock 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.
Four-high mill roll configuration – In this type of roll configuration, there are 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 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 mill are driven while the back-up rolls are normally friction driven. The four-high roll configuration is used for rolling of plates and for hot rolling and cold rolling of steel strip. It is used both in the non-reversing and reversing rolling mills.
Six -high mill roll configuration – In this type of roll configuration, there are six horizontal rolls, mounted in a single vertical plane. Two rolls (inner) are work rolls and four rolls are back-up rolls. This configuration is normally used in cold rolling of steel strip.
Cluster mill roll configuration – In this type of roll configuration, there are six, seven, twelve, or twenty horizontally mounted 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 multi-roll mill configuration is used for cold rolling of very thin sheets, strips and foils.
Universal mill roll configuration – In this type of roll configuration, 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 rolling stock, 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 mill configuration is 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 mill roll configuration – In this type of roll configuration, there are 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. This configuration is used for giving high reduction in a single pass.