1. ROLLING1.1 PROCESSPractically almost all the metals that are not cast are reduced to desired shapes before subsequent processing. Metals are produced by the manufacturing companies in the form of slabs, billets, blooms which are obtained by casting liquid metal into a square or circular cross section; continuous casting techniques are also employed to achieve the same. These shapes are further processed through Forging, Rolling, Extrusion, Drawing, and Sheet metal forming to produce materials in standard form such as plates, sheets, rods, tubes and structural sections.The desired shapes may be obtained in two basic ways:a. By plastic deformation processes: volume and mass of the material are conserved, material just gets displaced from one location to another.b. By metal removal or machining processes: Material is removed to obtain the desired shape.

Fig 1.1: Schematic Diagram of rolling process [1]In metalworking process, rolling is defined as the process of reduction of thickness or cross sectional area of a bar by passing it through a set of rolls; it belongs to the first category. It may also be used to obtain a uniform thickness of the bar. A solid piece of the material in the form of cast ingots are broken down and converted into shapes such as slabs, plates and billets by rolling a primary metal working operation. Rolling is a bulk deformation process: the aspect ratio i.e. the surface area to volume ratio of the workpieces are small. It is the thickness (cross sectional area) that changes during this process. The invention of rolling processes dates back to 1500s and today, more than ninety percentage of the metal working processes comprises of rolling. Rolling processes can be grouped based on the temperature, size of the workpiece. Based on temperature, it is classified into hot rolling and cold rolling. Recrystallization temperature of a materials differentiates the two processes. If the working temperature is above recrystallization temperature, then it is hot rolling; if below, then cold rolling. In terms of size, it classified into plates and sheets. Plates have thickness greater than 6mm, sheets have less than 6mm. Flat rolling or Strip rolling is a rudimentary process where the end products are flat plates, sheets and foil in long lengths, the main aim being reducing thickness. The process is fast and products have a good surface finish, especially in the case of cold rolling, albeit requiring high capital investment and operational cost. Shape rolling is another routes which allows us to produce various shapes; not limited to plates and sheets. Specially designed rolls produce long and straight structural shapes such as channels, I beam, and railroad rails with ease. Shape rolling is relatively a new process and it is extensively used in the manufacturing industry. Contrary to the conventional process of producing ingots and rolling, today continuous casting and rolling is employed which is more efficient compared to the conventional process.

Fig 1.2: Schematic outline of various flat and shape rolling processes [1]

The material to be rolled is drawn by means of friction into the two revolving roll gap. Rolling materials should be of high strength and have good wear resistance. Hence, the choice of roll materials are usually cast iron, cast steel and forged steel. Hot rolls are usually rough so that they can bite the work, and cold rolls are ground and polished for good surface finish. In rolling process, crystal usually undergo elongation in the rolling direction which is somewhat retained in the cold rolling; in hot rolling, crystal start reforming after coming out of the deformation zone. When a strip of metal enters a set of rolls, velocity of the strip is less than that of the rolls; when it exists, velocity of strip is greater than that of the rolls. In the deformation zone, thickness of the metal decreases and it elongates. This increases the linear speed at the exit. As the velocity of strip increases, there is a neutral point where strip velocity is equal to the roll speed. At this point, friction reverses its direction. Power and torque of the rolls increase with increase in contact length and roll radius.

1.2 FRICTION IN ROLLING

Friction during rolling depends on the following factors:a. Lubrication: Friction reduces with lubrication.b. Work material: Depending on the type of material rolled, coefficient of static friction will vary.c. Temperature: In cold rolling, the coefficient of friction is around 0.1, in warm rolling 0.2, in hot rolling 0.4. Since sticking friction has to be included in the case of hot rolling, it may go up to 0.7. Hot workpiece gets stuck to the rolls and undergoes severe deformation.

Fig 1.3: Pressure during rolling [2]Typical pressure variation along the contact length in flat rolling is shown in fig 1.3. The peak pressure is located at the neutral point. The area beneath the curve represents roll force.

1.3 POWDER ROLLINGA thick sheet may be directly produced directly from rolling by the process of powder rolling. In powder rolling, a green strip is produced by introducing metal powder between the rolls and compacting. Further, it is sintered and subjected to subsequent hot or working processes. A major advantage of this process is the elimination of hot-ingot breakdown step. It also reduces the capital investment for the rolling process. Contamination in hot-rolling is minimized and the production of sheet with very fine grain size with desired orientation is possible with powder rolling.

Fig 1.4: Powder rolling [3]The main objective of rolling process is to decrease the thickness. But usually, a little increase in width associated with the corresponding decrease in the thickness. Roll forming and thread rolling are specialized processes of rolling used to produce long, molded sections and to form thread respectively.

2. HOT ROLLINGPrimary roughening mill blooming, slabbing or cogging mills is the initial hot-working process for most metal products. Mills are two-high reversing mills with 0.6 to 1.4m diameter rolls. The main aim of this process is to convert ingots to blooms or slabs for further processing into intermediate products (bars, plate, and sheets). The initial process involve only small breakdowns. Heavy scale is removed initially by rolling the ingot while lying on edge, while the thickness is reduced by rolling after the ingot has been turned 90 so as to be lying flat. There is appreciable spreading of the ingot width in hot-rolling of ingots. To maintain the desired width and preserve the edges, the ingot is turned 90 on intermediate passes and passed through edging grooves in the rolls. The production rate of the reversing mills is often low since the workpiece has to be passed back and forth and turned many times. This makes the process slower and production lesser. Universal mills are installed to mitigate the problem of production rates. Universal mills employ two different rolling mills one with two large diameter rolls and the other with vertical rolls. This allows the control of width and reduction of thickness at the same time. As said earlier, cast ingots can be eliminated from the process tree by the use of continuous casting technique in which the materials are rolled directly from their molten state. Another option is to use bottom pressure casting for producing slabs. Plates are produced in two ways by rolling: from reheated slabs or directly from cast ingots. Sheared plate is produced by rolling between straight horizontal rolls and trimming all the edges. An edge is produced in hot rolling between horizontal finishing rolls known as Mill edge. Mill edge plates have two mill edges and two trimming edges. Universal mill produces Universal mill plates with edges trimmed.The strip is differentiated from sheet according to their width. Sheet is usually more than 600m wide. Irrespective of the width, continuous hot-strip mill is used to produce sheets and strips. The flow chart of the mill is shown below:

If the width of the sheet produced is less than the width required, broadside mill in the roughening train can be used to broaden the sheet to its required specifications by cross rolling. Scales are removed by spraying high pressure water jets. Depending upon whether the final product required is pieces or long bars, flying shear or coiler is used. The operating temperature during hot rolling is around 12000C. The final temperature of the last finishing strand is around 800oC. For non-ferrous metals, the equipment used is not as specialized as steel products. Advantages of non-ferrous rolling are smaller ingot sizes and flow stresses are lower. All these factor makes the usage of smaller rolling mills feasible for non-ferrous metals.

3. COLD ROLLING Cold rolling allows the production of sheets and plates with good surface finish and higher dimensional tolerances. Since the cold work causes strain hardening, it increases the strength of the finished product. It also makes the product brittle. Hence, a compromise has to be made between strength and brittleness. Between ferrous and non-ferrous metals, a higher percentage of non-ferrous metals are cold rolled. In case of steels and ferrous metals, the starting material for cold rolling is hot rolled strip from hot strip mill. In the case of copper, cold rolling is done directly from cast state. No hot rolling is involved.Cold rolling of steel, aluminum and copper alloys employs a high speed four-high tandem mills with 3 to 5 strands. This is done so as to provide both front and back tension. It is imperative to apply front and back tension to achieve greater reduction ratio. Continuous tandem mills require large capital investment and they are not versatile. But they can achieve far greater speeds compared to single-stand reversing mills. Delivery speed can go upto 30 ms-1. The reduction percentage that can be achieved by cold rolling varies from 50 to 90 %. It depends on various factors such as lubrication, application of front and back tension, diameter of the rolls etc. The work is distributed uniformly over several passes to get a good finish and a higher dimensional tolerance. Rolling load has to be maintained a content value by adjusting reduction in each pass. The rolling stress on the material should not exceed the yield stress as it will result in inhomogeneous deformation. Temper rolling or skin pass may be used to reduce yield point elongation. Roller leveling and stretcher leveling are some techniques which are used to obtain flat surface with good finish. Cold rolling is done below the recrystallization temperature of the material. For steel, it is around 17000C. Hot rolling temperature of a certain metal like steel may be a cold rolling temperature for a metal like Titanium. Cold rolling increases the strength and eliminates the need of costly heat treatments. Turning gets rid of size imperfections; grinding and polishing take care of size tolerance and surface finish. Cold rolling introduces defects into the crystal structure of the metal creating a hardened microstructure which prevents further slip. Grain size is also reduced by Hall-Petch Hardening. y = 0 + ky/D0.5Hall Petch Equation

4. FLAT ROLLINGA schematic diagram of the flat rolling processes is shown in Fig 4.1. Initial thickness of the strip is h0 and the thickness when the strip leaves is hf. A pair of rotating rolls are used that are being powered through its own shaft by electric motors. Roll surface is Vr. Strip velocity increases as it passes through the rolls from Vo at the entry to Vf at the exit, the highest being at the exit. This is analogous to a fluid passing through a convergent nozzle with subsonic velocity. Surface speed of the roll is constant and there is a sliding between the roll and strip in the roll gap. Velocity of strip at the entry less than that of roll surface velocity (V0 < Vr), and at the exit, velocity of strip is greater than roll surface velocity (Vf > Vr). Therefore, in the roll gap there must exist a point where both the velocities are equal. That point is called no-slip point. To the left of this point, roll moves faster than the strip and vice versa. Also, the direction of friction changes at this point.

Fig 4.1a: Flat rolling process [1]Fig 4.1c: Roll forces and torque acting on the strip [1]

L Roll gap lengthh0 initial thickness of the workpiece.hf final thickness of the workpiece.wo initial width of the workpiecewf final width of the workpieceR Radius of the roll.

Fig 4.1b: Friction forces acting on the roll gap [1]

The strip is pulled into the roll gap due to the frictional force that exists between the roll and the metal strip. Since the friction opposes the apparent movement of the body, it acts towards right. The magnitude of frictional force to the left of the neutral point should be greater than the right for obvious reasons. Friction is a required for rolling materials albeit it consumes energy. Increase in friction leads to higher energy dissipation and the efficiency of the process reduces. High friction also leads to the damage of the surface of the product. Friction causes local temperature to rise which can have negative effects on the product. An effective use of lubricants can help in reducing the friction.The maximum possible draft, h, is defined as the difference in thickness before and after rolling. h = (ho hf)where, ho = initial thicknesshf = final thicknessh = the maximum possible drafth is a function of the coefficient of friction, , and the roll radius, R:h = (ho hf) = 2RTherefore, to achieve greater draft and hence more reduction, radius of the rolls and coefficient of friction have to increased. A balance between energy dissipation and reduction has to be maintained for effective process. 4.1 ROLL FORCE AND POWER REQUIREMENTRolls apply pressure in the direction perpendicular to the workpiece to reduce the thickness. As we can see in the fig 4.1c, force is perpendicular to the plane of the strip and it is a valid approximation. Roll gap is significantly small compared to the roll radius. So it is reasonable to assume that the force acts only in the perpendicular direction.Roll force is given byF = LW YavgWhere L = roll strip contact length (roll gap)W = width of the stripYavg = average true stressF = roll forceThis formula does not take friction into to account. Stark deviation is observed from this formula when the coefficient of static friction between rolls and strip is high. The roll force is higher than that predicted by the above formula POWER REQUIREMENT:To calculate power, torque must be calculated. Torque = Force x Length.In this case, T = F x L/2Where T Torque;F Roll force;L Roll gap;

Therefore, Power= fLN/ 60,000 (KW) orPower= fLN/ 33,000 (hp)This is the formula for one roll. This is multiplied with the number of rolls to obtain the total power.

EFFECT OF ROLL FORCE:Roll forces often cause flattening of rolls and deflections in the final product. In the long run, it will affect the rolling process drastically. Several parts of the roll stand such as housing, chocks and bearing also stretch under the roll force and increase the roll gap significantly. This would mean the rolls have to be set more closely than was calculated to mitigate these defects and ensure smooth operation. The following are some remedies to counter the deflection and roll flattening:a. Lubrication (reduces friction)b. Using smaller roll radius (reduces contact area)c. Smaller reductions per pass (reduces contact area)d. High temperature rolling (reduces the strength of the material)e. Using carbide rolls (higher )f. Applying front and back tensiong. Using backup rolls

Fig 4.2: Illustration of four-high rolling-mill stand with its features [1]

4.2 GEOMETRIC CONSIDERATIONSThe force acting on the rolls leads to a stress on them as a result of which they undergo geometric changes. Elastic deformation occurs during rolling which causes a bending effect. Using a material with high elastic modulus for the rolls, this can be controlled. The result of roll bending is that the metal strip is thicker at its center than at its edges. One way to curb this problem is by making the roll with a greater radius at its center than at its edges. This way, even when roll bending occurs, the strip will be of same thickness. For rolling sheet metals, the diameter at the center of the rolls are 0.25mm greater than at its edges. However, this remedy is applicable only for specific a certain load and strip width.

Fig 4.3a: Non uniform strip width due to roll bending [1]Fig 4.3b: Uniform strip width after correction is made [1]

Plastic deformation causes a barreling effect in the strip as a result of heat generated due to the deformation. Roll forces tend to flatten the rolls which is analogous to automobile tires getting punctured. This flattening effect increases the roll radius which results in the increased roll force. SPREADINGIn rolling of strips having larger width to thickness ratio, the increase in width during rolling is insignificant. However, when the ratio is small, it becomes important. Width increases when the strip is rolled. This increase in width is termed as Spreading. This effect can be remedied with the decrease in friction and increase in the ratio of roll radius to the thickness.

Fig 4.4: Spreading [1]4.3 CHANGE IN THE GRAIN STRUCTURE DURING ROLLING

Fig 4.5: Change in the grain structure [2]

Microstructural change that the strip undergoes during hot rolling is depicted in the picture above. Cast metal has larger non-uniform grains; when the cast metal is hot rolled, it results in the formation of uniformed equiaxed smaller grains as seen in the figure. Dynamic recrystallization and grain growth play an important role in determining the final microstructure. The transformation of cast structure to wrought structure takes place during hot rolling. The final structure has finer grains, better ductility as result of breaking up brittle grain boundaries and closing up of internal defects. Hot rolling is an effective way of reducing the brittleness and grain size, at the same time improving strength by considerable amount. The hot working temperature of a few metals are tabulated.MetalsTemperature range of hot working

Aluminum 4500C

Alloy steels12500C

Refractory metals16500C

Copper5000C

5. EQUIPMENT ROLLING MILLSTill date, various types of rolling mills and equipment are developed depending upon the starting material and the product required. The equipment for hot and cold rolling are essentially the same; however, they differ in the operating parameters like temperature, roll speed. For hot rolling, cooling systems have to be used in a proper way. Lubricants used for hot and cold rolling are very different. The construction of a rolling mill involves a huge capital investment. The cost varies depending the type of mill installed. Highly automated rolling mills that produce high quality strips in faster rates involve high investment cost and maintenance cost. Hot rolling mills is installed with the cooling systems and their costs are more than cold rolling mills. Most often, continuous casting is integrated with the rolling mills. Though it requires comparatively more investment, in the long run it may prove cost effective. Fig 5.1: Rolling mills [6]Fig 5.2: Cold rolling mill brass sheet [6]Operational parameters Width of rolled products up to 5m. Thickness 0.0025mm (minimum) Rolling speed up to 25m/s (cold rolling) Roll diameters 0.6m to 1.4m Lubricants oils, grease, way lube oil etc.

TWO-HIGH OR THREE-HIGH ROLLING MILLS These are used mainly for roughening or cogging mills in the initial breakdown during hot rolling. The roll diameters range from 20 to 45 inches. In a three-high mill or a reversing mill, the direction of the metal strip being rolled is changed after each pass; the metal strip is raised and lowered using elevators and manipulators. It is rolled repeatedly to give a smooth surface finish.

Fig 5.3: Three high rolling mill and its operation [5]

FOUR-HIGH MILLS AND CLUSTER MILLS (SENDZIMIR OR Z MILL) Fg 5.4: Sendzimir Mill [1]Fig 5.5: Four-high rolling mill [1]In four-high cluster mills, cascaded supporting rolls are used in the place of single pair of rolls. The supporting rolls are smaller in diameter compared to the conventional rolls. This reduces the roll force and power required to run the mill. However, if there are many such supporting rolls, power requirement will be more. A balabce has to be maintained between the number of rolls and their sizes. This arrangement allows higher roll pressure and reduces spreading of rolls by a significant amount. Maintenance costs are less than the conventional mills: if a roll fails (worn or broken), it is easier and cheaper to replace small rolls. The investment cost of Sendzimir or Z mills may be crores. But in the long run, it will prove effective.

PLANETARY ROLLING MILLSThe planetary mills consist of a pair of backing rolls, with greater radius, surrounded by smaller rolls. There are several advantages of using this arrangement. It reduces the number of process steps from cathode to coiled mother tube. It is optimized and has a compact layout, low noise generation. Charge weights can be three to four times those of conventional plants. It is possible to get a high reduction up to 98% in one pass. There is no speed reduction during rolling-over the shell junction. Cooling devices can be used effectively to get an improved microstructure. It can be operated at high speed and provides extended tool lifetime. Planetary rolling is mainly used for copper tubes where the thickness reduction of more than 90% is required in one single pass.

Fig 5.5: Planetary Rolling Mill [1]

6. ROLLING DEFECTS AND REMEDIESSeveral types of defects are possible in the manufacturing of metals. It is no wonder that a wide variety of them occur during rolling. Surface must be cleaned and prepared diligently to avoid impurities, scale, rust or dirt. Improper material distribution leads to the internal defects: cracks, wavy edges etc.In hot rolling, defects occur mainly due to temperature gradient. When the temperature is high, the metal flow is more and vice versa. Non-uniform distribution thus occurring will lead to cracks and tears.

FLATNESS

Fig 6.1: Roll deflection [5]The gap between the rolls increases because of the deflection of the rolls that occurs due to the load required for rolling the workpiece. It causes thicker middle portions and thinner edges. Crowned roller can be used to mitigate the effect; but it is specific for the material, temperature and deformation.Continual varying crown (CVC), pair cross rolling, and work roll bending are some alternatives that are currently used to overcome this defect. Applying longitudinal loads to decrease the load on the rolls will also work against the deflection. Other remedies include, using roll material of higher elastic modulus and adding front and back tension to the rolls.

Various types of flatness defects are: Symmetrical edge wave Asymmetrical edge wave Center buckle Quarter buckle It is important that the material is subjected to uniform stresses across the width so that the strain remains the same across the width. This will ensure uniform mass flow of the material and elongation is in the same manner across the width.Defects are undesirable, not only because they degrade surface appearance but also because they may adversely affect the strength, the formability, and other manufacturing characteristics.WAVY EDGES This is a result of roll bending. The thickness varies across the width: thinner along its edges than at its center. As the workpiece is compressed, it expands laterally in a non-uniform manner due to which the edges become wavy.

Fig 6.2: Wavy Edges [1]CRACKS

Fig 6.3: Cracks [1]

Cracks are the effect of high rolling temperature, without proper use of coolant and poor material ductility.

ALLIGATORING

Fig 6.4: Alligatoring[5]It is a complex rolling defect which arises due to friction between rolls and workpiece. Due to the friction acting between rolls and workpiece, the top and bottom elongate more than the middle portion. In extreme cases, this may lead to opening up of the sheet as shown in the figure.

7. ANALYSIS OF ROLLING LOADVariables that are considered for the calculation: The roll diameter The deformation resistance Friction between rolls and workpiece Presence of front and back tensionThree conditions are considereda) No friction conditionb) Normal friction condition c) Sticking friction conditiona) NO FRICTION CONDITIONWhen friction is ignored, the rolling load (P) is roll pressure times the area between the workpiece and the rolls (bL0)P = pbLp = 0b(Rh)0.5 Eqn (1)Where P rolling loadp roll pressure0 yield stressb widthR radius2) NORMAL FRICTION SITUATIONIn the normal case of friction situation in plane strain, the average pressure P is.p/0 = (eq 1)/Q

Where Q = L/hh the mean thickness between entry and exit.From eqn 1 we have,P = pbLp

Therefore,P = 1.1550[(eq -1)b(Rh)0.5)]

This relation suggests that rolling load P increases with the radius depending upon the frictional force. The rolling load also increases when the thickness of the sheet gets thinner. There exists a certain point where no further reduction in thickness is possible. This happens when the deformation resistance of the sheet is greater than the roll pressure. Severe elastic deformation of the sheet takes place. Small-diameter rolls which are properly stiffened against deflection by backup rolls can produce a greater reduction before roll flattening become significant and no further reduction is possible.Friction does useful work too. It is needed to pull the workpiece into rolls and contributes a large portion to the rolling load. But high friction leads to high rolling load and a steep friction hill and great tendency for edge cracking.

Fig 7.1: Roll pressure vs Length of contact [1] For cold rolling (with lubricants) - is 0.05 0.10. For hot rolling - is 0.2

3) STICKY FRICTION SITUATIONContinuing with analogy with compression in plane strain,p = 0(Lp + h)/hP = pbLpP = 0b(Rh)0.5 [(Rh)0.5/4h) + 1]

REFERENCES1. Alexandria University - Faculty of Engineering, Report: Metal Rolling.2. Metal Forming Processes, Dr. Pulak M. Pandey.3. Rolling of metals, Suranaree University of Technology, 20074. Manufacturing Engineering and Technology, Kalpakjian & Schmid, 20105. Metal Rolling, library of manufacturing.6. Wikipedia