In this process, metals and alloys are plastically deformed into semifinished or finished products by being pressed between two rolls which are rotating.
The metal is initially pushed into the space between two rolls, thereafter once the roll takes a ‘‘bite’’ into the edge of the material, the material gets pulled in by the friction between the surfaces of the rolls and the material.
The material is subjected to high compressive force as it is squeezed (and pulled along) by the rolls. This is a process to deal with material in bulk in which the cross-section of material is reduced and its length increased.
The final cross-section is determined by the impression cut in the roll surface through which the material passes and into which it is compressed. The essentials of the rolling process can be understood from the Fig
Rolling is done both hot and cold. In a rolling mill attached to a steel plant, the starting point is a cast ingot of steel which is broken down progressively into blooms, billets and slabs.
The slabs are further hot rolled into plate, sheet, rod, bar, rails and other structural shapes like angles, channels etc. Conversion of steel into such commercially important sections is usually done in another rolling mill called merchant mill.
Rolling is a very convenient and economical way of producing commercially important sections. In the case of steel, about three-fourth’s of all steel produced in the country is ultimately sold as a rolled product and remaining is used as forgings, extruded products and in cast form. This shows the importance of rolling process.
NOMENCLATURE OF ROLLED PRODUCTS
The following nomenclature is in common usage:
It is the first product obtained from the breakdown of Ingots. A bloom has a crosssection ranging in size from 150 mm square to 250 mm square or sometimes 250 × 300 mm rectangle.
A billet is the next product rolled from a bloom. Billets vary from 50 mm square to 125 mm square.
Slab is of rectangular cross-section with thickness ranging from 50 to 150 mm and is available in lengths up to 112 metres.
A plate is generally 5 mm or thicker and is 1.0 or 1.25 metres in width and 2.5 metres in length.
A sheet is up to 4 mm thick and is available in same width and length as a plate.
Flats are available in various thickness and widths and are long strips of material of specified cross-section.
It is a very thin sheet.
Bars are usually of circular cross-section and of several metres length. They are common stock (raw material) for capstan and turret lathes.
A wire is a length (usually in coil form) of a small round section; the diameter of which specifies the size of the wire.
MECHANISM OF ROLLING
Each of the two rolls contact the metal surface along the arc AB, which is called arc of contact. Arc AB divided by the radius of rolls will gives angle of contact (α). The rollers pull the material forwards only due to the friction existing between roll surface and the metal. At the moment of the bite, the reaction at the contact point A will be R acting along radial line O1A and frictional force will be acting along tangent at A at right angles to O1A. In the limiting case,
R sin α = μR cos α
∴ μ = tan α or α = tan–1 μ
If α is greater than tan–1 μ, the material would not enter the rolls unaided.
|Mechanism of Rolling
The usual values of biting angles employed in industry are:
2–10° … for cold rolling of sheets and strips;
15–20° … for hot rolling of sheets and strips;
24–30° … for hot rolling of heavy billets and blooms.
In the rolling process, although the material is being squeezed between two rolls, the width (b0) of the material does not increase or increases only very slightly. Since volume of material entering the rolls is equal to the volume of material leaving the rolls, and the thickness of material reduces from h0 to h1, the velocity of material leaving the rolls must be higher than the velocity of material entering the rolls. The rolls are moving at a uniform r.p.m. and their surface speed remains constant. The rolls are trying to carry the material into the rolls with the help of friction alone, there is no positive grip between rolls and the material. On one side, therefore, i.e., point A where contact between the rolls and work material starts, the rolls are moving at faster surface speed than the work material. As the material gets squeezed and passes through the rollers, its speed gradually increases and at a certain section CC called neutral or no slip section, the velocity of metal equals the velocity of rolls. As material is squeezed further, its speeds exceed the speed of the rolls. The angle subtended at the centre of the roll at the neutral section is called angle of no slip or critical angle (angle BO1C).
The deformation zone to the left of the neutral section is called lagging zone and the deformation zone to the right of the neutral section is termed leading zone. If Vr is the velocity of roll surface, V0 the velocity of material at the entrance to the deformation zone and V1 at the exit of the rolls, we have
Forward slip= V1 – Vr/Vr x 100 %
Backward slip= Vr – V0/Vr x 100 %
The value of forward slip normally is 3–10% and increases with increase in roll diameter and coefficient of friction and also with reduction in thickness of material being rolled. Some other useful terms associated with rolling are explained below:
Absolute draught: h1-h2
Relative draught: h1-h2/h1 x 100
Absolute elongation, Δl = Final length – Original length of work material
Coefficient of elongation = Final length/Original length
Absolute spread = Final width of work material – Original width of material
TYPES OF ROLLING MILLS
Different types of rolling mills are described below in brief:
Two high mills:
It comprises of two heavy rolls placed one over the other. The rolls are supported in bearings housed in sturdy upright frames (called stands) which are grouted to the rolling mill floor. The vertical gap between the rolls is adjustable. The rolls rotate in opposite directions and are driven by powerful electrical motors. Usually the direction of rotation of rolls cannot be altered, thus the work has to be fed into rolls from one direction only. If rolling entails more than one ‘pass’ in the same set of rolls, the material will have to be brought back to the same side after the first pass is over. Since transporting material (which is in red hot condition) from one side to another is difficult and time consuming (material may cool in the meantime), a ‘‘two high reversing mill’’ has been developed in which the direction of rotation of rolls can be changed. This facilitates rolling of material by passing it through back and forth passes. A two high rolling mill arrangement is shown in Fig.
|Two high mills and Three high mills
Three high mills:
A three high rolling mill arrangement is shown in Fig. It consists of three rolls positioned directly over one another as shown. The direction of rotation of the first and second rolls are opposite as in the case of two high mill. The direction of rotation of second and third rolls is again opposite to each other. All three rolls always rotate in their bearings in the same direction. The advantage of this mill is that the work material can be fed in one direction between the first and second roll and the return pass can be provided in between the second and third rolls. This obviates the transport of material from one side of rolls to the other after one pass is over.
Four high mills:
As shown in Fig, this mill consists of four horizontal rolls, two of smaller diameter and two much larger ones. The larger rolls are called backup rolls. The smaller rolls are the working rolls, but if the backup rolls were not there, due to deflection of rolls between stands, the rolled material would be thicker in the centre and thinner at either end. Backup rolls keep the working rolls pressed and restrict the deflection, when the material is being rolled. The usual products of these mills are hot and cold rolled plates and sheets.
It consists of two working rolls of small diameter and four or more backing rolls. The large number of backup rolls provided becomes necessary as the backup rolls cannot exceed the diameter of working rolls by more than 2–3 times. To accommodate processes requiring high rolling loads (e.g., cold rolling of high strength steels sheets), the size of working rolls becomes small. So does the size of backup rolls and a stage may be reached that backup rolls themselvesmay offer deflection. So the backup rolls need support or backing up by further rolls.
Geometric Considerations of Rolling
Because of the forces acting on them, rolls undergo changes in shape during rolling. just as a straight beam deflects under a transverse load, roll forces tend to bend the rolls elastically during rolling As expected, the higher the elastic modulus of the roll material, the smaller the roll deflection. As a result of roll bending, the rolled strip tends to be thicker at its center than at its edges (crown). The usual method of avoiding this problem is to grind the rolls in such way that their diameter at the center is slightly larger than at their edges (camber). Thus, when the roll bends, the strip being rolled now has a constant thickness along its width For rolling sheet metals, the radius of the maximum camber point is generally 0.25 mm greater than that at the edges of the roll. However, as expected, a particular camber is correct only for a certain load and strip width. To reduce the effects of deflection, the rolls also can be subjected to external bending by applying moments at their bearings
(a) Bending of straight cylindrical rolls caused by roll forces. (b) Bending of rolls ground with camber, producing a strip with uniform thickness through the strip width.
ROLLS AND ROLL PASS DESIGN
Two types of rolls—Plain and Grooved are shown in Fig. Rolls used for rolling consists of three parts viz., body, neck and wabbler. The necks rest in the bearings provided in the stands and the starshaped wabblers are connected to the driving shaft through a hollow cylinder. Wabbler acts like a safety device and saves the main body of the roll from damage if too heavy a load causes severe stresses. The actual rolling operation is performed by the body of the roll. The rolls are generally made from a special variety of cast iron, cast steel or forged steel. Plain rolls have a highly finished hard surface and are used for rolling flats, plates and sheets. Grooved rolls
have grooves of various shapes cut on their periphery. One-half of the required shape of rolled product is sometimes cut in the lower roll and one-half in the upper roll, so that when the rolls are assembled into its stands, the required shape in full will be produced on the work material, once it passes (i.e., rolled) through the groove in question. However it should be understood that the desired shape of the rolled section is not achieved in a single pass. The work material has to be rolled again and again through several passes and each pass brings the cross-section of the material closer to the final shape required. These passes are carefully designed to avoid any rolling defect from creeping in. Rolling is a painstaking process as would be noticed from the scheme of passes shown in Figure for conversion of a steel billet into a round bar.
Various stages of rolling and the number of passes for converting a steel billet into round bar
Various passes fall into the following groups:
(i) Breakdown or roughing passes,
(ii) Leader passes, and
(iii) Finishing passes.
Breakdown passes are meant to reduce the cross-sectional area. The leader passes gradually bring the cross-section of the material near the final shape. The final shape and size is achieved in finishing passes. Allowance for shrinkage on cooling is given while cutting the finishing pass grooves.
Defects in Rolling
Defects may be present on the surfaces of rolled plates and sheets, or there may be internal structural defects. Defects are undesirable not only because they compromise surface appearance, but also because they may adversely affect strength, formability, and other manufacturing characteristics. Several surface defects (such as scale, rust, scratches, gouges, pits, and cracks) have been identified in sheet metals. These defects may be caused by inclusions and impurities in the original cast material or by various other conditions related to material preparation and to the rolling operation. Wavy edges on sheets are the result of roll bending. The strip is thinner along its edges than at its center); thus, the edges elongate more than the center. Consequently, the edges buckle because they are constrained by the central region from expanding freely in the longitudinal (rolling) direction. The cracks are usually the result of poor material ductility at the rolling temperature. Because the quality of the edges of the sheet may affect sheet-metal-forming operations, edge defects in rolled sheets often are removed by shearing and slitting operations. Alligatoring is the phenomenon and typically is caused by non uniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material.
|(a) wavy edges; (b) zipper cracks in the center of the strip; (c) edge cracks; and (d) alligatoring