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:
(i) Blooms: 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.
(ii) Billet: A billet is the next product rolled from a bloom. Billets vary from 50 mm square to 125 mm square.
(iii) Slab: Slab is of rectangular cross-section with thickness ranging from 50 to 150 mm and is available in lengths up to 112 metres.
(iv) Plate: A plate is generally 5 mm or thicker and is 1.0 or 1.25 metres in width and 2.5 metres in length.
(v) Sheet: A sheet is up to 4 mm thick and is available in same width and length as a plate.
(vi) Flat: Flats are available in various thickness and widths and are long strips of material of specified cross-section.
(vii) Foil: It is a very thin sheet.
(viii) Bar: Bars are usually of circular cross-section and of several metres length. They are common stock (raw material) for capstan and turret lathes.
(ix) Wire: 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
Refer to Fig. 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,
between the two rollers at the narrowest point and r is the radius of rollers. For a given diameter of rollers and gap between them the value of h0 is limited by the value of μ which in turn depends upon the material of rolls and job being rolled, the roughness of their surfaces and the rolling temperature and speed.
In case of hot rolling when maximum reduction is cross-section per pass is aimed at, it may be necessary to artificially increase the value of μ by ‘‘ragging’’ the surface of rolls. Ragging means making the surface of rolls rough by making fine grooves on the roll-surface. However, in cold rolling which is a finishing operation and cross-section reduction is limited, ragging of rolls is neither required nor desirable. In fact, in that case, some lubrication is resorted to in addition to giving a fine finish to the rolls. Another reason for making do with a lower coefficient of friction in cold rolling is that in this process, very high pressures are used and even with a low value of μ, adequate frictional force becomes available.
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 Fig. called neutral or no slip section, the velocity of metal equals the velocity of rolls. As material is squeezed further, its speeds exceeds 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
TYPES OF ROLLING MILLS
Different types of rolling mills are described below in brief:
(i) 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.
|A two high mill AND A three high rolling mill
(ii) 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 are 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.
(iii) 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.
|Four high mill
(iv) Cluster mills: 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 themselves may offer deflection. So the backup rolls need support or backing up by further rolls. In the world famous SENDZIMIR MILL, as many as 20 backup rolls are used in the cluster. This mill is used for rolling stainless steel and other high strength steel sheets of thin gauge.
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 Fig. for conversion of a steel billet into a 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.
Seamless (i.e., without a joint) rings find wide application in industry. The inner and outer races of ball and roller bearings, steel tyres for railway wheel are some such applications. These rings are made by a special rolling process called ring rolling. The starting work piece is a thick walled circular piece of metal in whose centre a hole has been made by drifting and piercing. The work piece is heated until it becomes red hot and then placed between two rolls A and B which rotate in opposite directions. The arrangement of rolls and the ring is shown in Fig.
The pressure roll B exerts pressure on the material from inside. Caught between rolls A and B, the ring rotates. At the sametime, the inside and outside dia of ring progressively increase and the wall thickness keeps on reducing. In order to ensure that the ring is circular, two guide rolls are suitably placed on the outer surface of the ring. When the outer and inner dia of the ring increase to the size required, the rolling is stopped.
Hot rolled steel products look an unattractive greyish-black in colour. Non-ferrous materials also develop a tarnished colouring due to oxidation of outer surface. The surface finish is rough and the finished sizes of hot rolled products are far from satisfactory. In case of steel, the oxidation of carbon present in the surface leads to decarburisation. However hot rolling is very economical as due to increased plasticity, large reductions in cross-section are quickly achieved with low energy consumption. A great deal of hot rolled “black” bars and sheets/plates of steel are used in construction industry for fabrication of structures.
Thinner gauges, better surface finish, tighter size control and “bright” surfaces are obtained in cold rolling process. These products also develop greater strength and wear resistance due to strain hardening.
The effect of mechanical work done (i.e., strain hardening) is automatically nullified in hot rolling process, as recrystallisation in the hot worked material keeps on taking place simultaneously. This is shown schematically in Fig.
Effect of both cold working and hot working
|on the microstructure of metals
Thus the actual process employed in the industry for production of small gauge material is hot rolling to slightly above finished size required, cleaning/removing the oxidised surface by machining pickling or some other suitable process and finally using cold-rolling of work-material to finished sizes.
To understand the causes and remedies of rolling defects, we shall divide them in two classes:
1. Surface defects, and
2. Structural defects.
Surface defects include rusting and scaling, surface scratches, surface cracks, pits left on the surface of due to subsequent detachment or removal of scales which may have been pressed into the surface.
Structural defects are more important rolling defects some of which are difficult to remedy.
These defects include the following:
(i) Wavy edges
(ii) Zipper cracks
(iii) Edge cracks
(iv) Centre split
Wavy edges and zipper cracks: These defects are caused due to bending of rolls under the rolling pressure. A roll can be considered as a beam supported in its stands. Under rolling pressure, the rolls deflect in the manner shown in Fig. 3.9. Consequently the work material becomes thinner at the two edges and thicker in the central portion. In other words, it means that material becomes longer as measured along the edges than in centre. This causes tensile stress in the centre and compressive stress in the edges. The former causes zipper cracks in the centre and the latter causes wavy edges.
Remedy for zipper cracks and wavy edges lies in provide a “camber” to the rolls. They are made slightly convex in the central portion to offset the effect of deflection under rolling loads.
Edgecracks and centre split: These defects are caused due to non homogeneous plastic deformation of metal across the width. As the work piece passes through the rolls, under the rolling pressure its height decreases while its length increases. Some lateral spread i.e., increase in width also takes place. However the lateral spread is more towards the edges than in the centre as there is little resistance to lateral spread towards the edges. In the centre lateral spread is resisted by friction and the adjacent layer of material. Hence decrease in lateral spread in the central portion of work material results in greater increase in length in this region as compared to the edges. This effect is shown in Fig.
It can be realised that under such non homogeneous deformation of work material, the edges experience a tension (as the central portion tries to pull it due to continuity of material) and the central portion experience a compressive stress. Such a distribution of stress may result in edge crack or in severe cases, it may even lead to a split along the central portion.
Alligatoring: As pointed out earlier, rolling entails a reduction in the height with consequent increase in length. But due to friction present at the interface of the rolls and upper and lower surfaces of the work material, the elongation on the top and bottom surfaces is less than the material located at the centre of thickness of the work piece. If conditions become severe, it may cause a defect called “alligatoring” i.e., rupture of material along the length into an upper half and a lower half resembling the open mouth of an alligator. The defect is illustrated in Fig.
|Rolling defect (alligatoring)
Folds: This defect is encountered when the reduction per pass is very low.
Laminations: Laminations mean layers. If the ingot is not sound and has a piping or blow holes and during rolling they donot get completely welded (e.g., if the piping has got oxidised material or non-metallic inclusions it will not get welded), it will cause a defect called laminations. Very often in the ingot, there are non metallic inclusions; during rolling they will get lengthened along with sound material. This may also cause laminations.
These defects can only be remedied by discarding the portion of the ingot where piping and other defects are present and selecting only good metal portion for rolling.