Rolling Fundamentals

ROLLING 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.

Types of Rolling  Based on work piece geometry : reduce thickness of a rectangular rectangular cross section  Flat rolling - used to reduce  Shape rolling - square cross section is formed into a shape such as an I- beam  Based on work temperature :  Hot rolling – Temperature above recrystallization temperature of metal.  Cold rolling  – carried out at room temperature  Warm rolling  – carried out at elevated temperature but below recrystallization temperature. Hot rolling Coarse grain structure is broken up and elongated by the rolling action. Because of the high temperature, recrystaliization starts immediately and small grains begin to form. These grains grow rapidly until recrystallization is complete. Growth continues at high temperatures, if further work is not carried on, until the low temperature of the recrystalline range is reached.

Semi-finished products in rolling Ingot : Ingots are large rough castings designed for storage and transportation. The shape usually  resembles a rectangle or square. They are tapered, usually with the big-end-down. Billet : A billet is a length of metal that has a round or square cross-section, with an area less than 2 36 sq in (230 cm  ). Billets are created directly via continuous casting or extrusion or extrusion or indirectly  via rolling an ingot. Billets are further processed via profile rolling and drawing. and drawing. Final products include bar stock and  stock and wire. wire.

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Bloom: Blooms are similar to billets except the cross-sectional area is greater than 36 sq in 2 (230 cm  ). Blooms are usually further processed via rotary piercing, structural shape rolling and   profile rolling. Common Common final products include structural shapes, rails, rods, and seamless and seamless pipes. Slab: A slab is a length of metal that is rectangular in cross-section. It is created directly from continuous casting or indirectly by rolling an ingot. Slabs are usually further processed via flat  rolling and pipe rolling. Common final products include sheet metal, plates, strip metal, pipes, and tubes. and tubes. Plates: They have thickness greater than 6mm and are used for structural applications such as machine structures, ship hulls, boilers, bridges and nuclear vessels. Sheets: They are generally less than 6mm thick. They are used for automobile and aircraft bodies,  food and beverage beverage containers etc.

Hot rolling vs Cold rolling Hot Rolling

Cold Rolling

Metal is fed to the rolls after being heated above the recrystallization temperature. Rolled metal does not show work hardening effect. Co-efficient of friction between the rolls and the stock is higher. Heavy reductions in area of the work piece can be obtained. Very thin sections are not obtained by hot rolling.

Metal is fed to the rolls when it is below recrystallization temperature. The metal shows work hardening effect after being cold rolled. Co-efficient of friction between the rolls and the stock is comparatively lower. Heavy reduction is not possible.

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Very thin sections can be made.

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Hot Rolling Cold Rolling Mechanical properties are improved by Hardness increases. Excessive cold working breaking cast structure and refining grain generates cracks. Ductility of the metal reduces. size. Blow holes and other similar defects Cold rolling increases the tensile strength and yield present in the ingot are removed. The strength of steel. strength and toughness of the metal increases. Roll radius is generally larger. Roll radius is smaller The hot rolled surface has scale on it; the The cold rolled surface is smooth and oxide-free. surface finish is not good. Close tolerances on dimensions cannot be Close dimensional tolerances are possible. obtained. Yield stress varies with temperature, it varies Theoretical analysis can be easily carried out and with each pass, and therefore it is difficult to extensively developed theory is available. formulate a theory for hot rolling. Experimental measurements are difficult to Experimental measurements are easily carried out. make. Hot rolling is the father of cold rolling. Cold rolling follows hot rolling. Hot rolled objects are thoroughly cleaned of their surface scale by pickling in an acid solution and are then cold rolled. Pack Rolling It is a flat rolling operation in which two or more layers of metal are rolled together; this process improves productivity. Aluminium foil is pack rolled in two layers. The foil to foil side has a matt finish. The foil to roll side is shiny and bright because it has been in contact with the polished rolls. Temper rolling or skin pass Mild steel, when stretched during sheet forming operations, undergoes yield point elongation, a phenomenon that causes surface irregularities called stretcher strains. To correct this problem, the sheet metal is subjected to a final light pass of 0.5% to 1.5% reduction. This process is known as temper rolling or skin pass. Levelling rolls A rolled sheet may not be sufficiently flat as it leaves the roll gap, because of variations in the material or in the processing parameters during rolling. To improve flatness, the rolled strip is passed through a series of levelling rolls. Each roll is usually driven separately by an individual electric motor. The strip is flexed in opposite directions as it passes through a set of rollers.

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Rolling Mill A rolling mill consists of one or more roll stands, motor drive, reduction gears, and flywheel and coupling gears between units. The roll stand is the main part of the mill, where the rolling process is performed. It basically consists of housings in which bearings are fitted, which are used for mounting the rolls. Depending upon the profile of the rolled product, the body of the roll may be either flat for rolling sheets (plates or strips) or grooved for making structural members (channel, I-beam, rail). Rolling mills are classified according to the number and arrangement of rolls in a stand. They are classified as: For hot rolling of metals (Two-high rolling mill, Three-high rolling mill) For cold rolling of metals (Four high rolling mill, Cluster rolling mill)

(1) Two-high rolling mill: It is basically of two types i.e., non-reversing and reversing rolling mill. The two high non-reversing rolling stand arrangements is the most common arrangement. In this the rolls always move in only one direction, while in a two-high reversing rolling stand the direction of roll rotation can be reversed. This type of stand is particularly useful in reducing the handling of the hot metal in between the rolling passes. About 30 passes are required to reduce a large ingot into a bloom. This type is used in blooming and slabbing mills. (2) Three-high rolling mill : It is used for rolling of two continuous passes in a rolling sequence without reversing the drives. After all the metal has passed through the bottom roll set, the end of  the metal is entered into the other set of the rolls for the next pass. For this purpose, a tabletilting arrangement is required to bring the metal to the level with the rolls. Such type of  arrangement is used for making plates or sections. (3) Four-high rolling mill: It is generally a two-high rolling mill, but with small sized rolls. The other two rolls are the backup rolls for providing the necessary rigidity to the small rolls. It is used for both hot and cold rolling of wide plates and sheets. (4) Cluster rolling mill: It uses backup rolls to support the smaller work rolls. In this type of mill, the roll in contact with the work can be as small as 1/4 in. in diameter. Foil is always rolled on cluster mills since the small thickness requires small-diameter rolls. (5) Tandem rolling mill: In tandem rolling, the strip is rolled continuously, through a number of  stands, to smaller gaps with each pass. Each stand consists of a set of rolls with its own housing JO/VJCET

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and controls. The control of the gap and of the speed at which the sheet travels through each roll gap is critical. Electronic and computer controls, along with extensive hydraulic controls are used in tandem rolling operations.

(6) Planetary rolling mill: In the planetary mill, a great number of small rolls, which in turn serve as work rolls, are mounted on the surface of two large backup rolls. Since multiple sets of rolls work on the strip simultaneously, the pass reduction can be very high. Due to the high complicity, a pair of feed rolls is installed in some cases.

Roll Passes The final rolled products such as plates, flats, sheets, rounds and sections are obtained in a number of passes starting from billet or slabs. For rolling the flat product, plain cylindrical rolls are used but for sections, grooved rolls are used. The type of grooving done is decided by the final section desired. The roll pass sequence can be broadly classified into three types: 1. Breakdown passes: These are used for reducing the cross-sectional area nearer to what is desired. These would be the first to be present in the sequence. 2. Roughing passes: In these passes also, the cross-section gets reduced, but along with it, the shape of the rolled material comes nearer to the fi nal shape. 3. Finishing passes: These are the final passes which give the required shape of pass follows a leader pass. The principal breakdown pass sequence is: (i) box pass series (ii) diamond square series (iii) oval square series Rolling of square sections Figure below shows the pass sequence for the rolling of squares. Diamond-diamond and diamond-square roughing passes, diamond leader passes and square finishing passes are employed for rolling.

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Rolling of rounds Figure below shows the pass sequ ence for the rolling of a rod. A ½’’ diameter rod is rolled from 4’’ x 4’’ billet in 10 passes. Oval -square passes have been employed for shaping the bar from the billet.

Rolling of flats, angle sections and channels

Spreading in rolling In the rolling of work pieces with smaller width to thickness ratios, such as with a square cross-section, the width increases considerably in the roll gap. This increase in width is called spreading. In the calculation of roll force, the average width is considered. Spreading increases with a) A decrease in width to thickness ratio of entering material b) An increase in the friction c) A decrease in the ratio of roll radius to strip thickness Spreading can be prevented by the use of vertical rolls in contact with the edges of the rolled product.

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Absolute draught / draft: It is the difference between the initial and final thickness of material being rolled. It is expressed as ∆h = h 0 - hf  , where h0 is initial thickness and h f  is final thickness. 2 The maximum possible draft is given by ∆h max = µ R where µ is co-efficient of friction between roll and work piece and R is roll radius. Relative draught/ draft: It is the ratio of absolute draught to initial thickness of the work piece as expressed as percentage. Absolute elongation: It is the difference between the final and initial length of the work piece being rolled. Absolute spread: It is the difference between the final and initial width of the work piece being rolled. Thread rolling Thread rolling is used to form threads on cylindrical parts by rolling them between two dies. It is the most important commercial process for mass producing external threaded components (eg: bolts and screws). Most thread rolling operations are performed by cold working in thread rolling machines. These machines are equipped with special dies that determine the size and form of the thread. The dies are of two types: (1) flat dies, which reciprocate relative to each other and (2) round dies which rotate relative to each other. Production rates in thread rolling can be high, ranging up to eight parts per second for small bolts and screws. Advantages of thread rolling include (a) better material utilization (b) stronger threads due to work hardening (c) smoother surface (d) better fatigue resistance due to compressive stresses introduced by rolling.

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Gear rolling Gear rolling is a cold working process to produce certain gears. The automotive industry is an important user of these products. The deformed features of the cylindrical blank are oriented parallel to its axis or at an angle depending on the type of gear. Compared to machining, gear rolling process has the advantages of higher production rates, better strength, better fatigue resistance and less material wastage. Gear rolling is also used for surface densification of gears manufactured by powder metallurgy process.

Ring rolling It is a deformation process in which a thick walled ring of smaller diameter is rolled into a thin walled ring of larger diameter. The process is illustrated in the figure below. As the thick walled ring is compressed, the deformed material elongates, causing the diameter of the ring to be enlarged. Ring rolling is usually performed as a hot-working process for large rings and as cold working process for small rings.

The ring walls are not limited to rectangular cross-sections; the process permits rolling of more complex shapes as shown in the figure below.

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Applications Ball and roller bearing races Steel tires for rail road wheels Rings for pipes, pressure vessels and rotating machinery. Advantages Raw material savings Ideal grain orientation for the application Strengthening through cold working Roll piercing / Rotary tube piercing / Mannesmann process It is a specialized hot working process for making seamless thick walled tubes. It utilizes two opposing rolls. The process is based on the principle that when a solid cylindrical part is compressed on its circumference, high tensile stresses are developed at its centre. If compression is high enough, an internal crack is formed. Compressive stresses on a cylindrical solid billet are applied by two rolls, whose axes are oriented at slight angles (about 6⁰) from the axis of the billet, so that their rotatio n tends to pull the billet through the rolls. A mandrel is used to control the size and finish of the hole created by the action.

Tube Rolling Tube rolling is a process employed for reducing the diameter and/or thickness of tubes and pipes. The process can be done with or without the help of mandrels.

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(a) Tube rolling with fixed mandrel, (b) Tube rolling with moving mandrel (c) Tube rolling without mandrel (d) Pilger rolling over a mandrel and shaped rolls

Shape rolling of an H-section/I section

Skew rolling of steel balls

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Roll materials The basic requirements for roll material are strength and resistance to wear. Common roll materials are cast iron, cast steel and forged steel. Tungsten carbides are also used for small diameter rolls such as the working roll in a cluster mill. Forged steel rolls although more costly have greater strength, stiffness and toughness than cast iron rolls. Rolls for cold rolling are ground to a fine finish. Elastic modulus of the roll material influences roll deflection and flattening. Rolls made for cold rolling should not be used for hot rolling, because they may crack from thermal cycling and spalling. Lubricants in rolling Hot rolling of ferrous alloys is usually carried out without lubricants, although graphite may be used. Nonferrous alloys are hot rolled with a variety of compounded oils, emulsions and fatty acids. Cold rolling is carried out with water-soluble oils, emulsions and fatty acids. Cold rolling is carried out with water soluble oils or low viscosity lubricants such as mineral oils, emulsions, paraffin and fatty oils. Residual salts from molten salt baths employed for heat treating billets and slabs also offer effective lubrication during rolling. Roll deflections and roll flattening Roll forces tend to bend the rolls elastically during rolling. The higher the elastic modulus of the roll material, the smaller is the roll deflection. As a result of roll bending, the rolled strip tends to be thicker at its centre rather than at its edges. The usual method of avoiding this problem is to grind the rolls so that their diameter at the centre is slightly larger than at their edges (give them camber). Thus when the roll bends, its contact along the width of the strip becomes straight and the strip being rolled has a constant thickness along its width. For rolling sheet metals, the radius of the maximum camber point is generally 0.25mm greater than that at the edges of the roll. When properly designed, cambered rolls produce flat strips. However a particular camber is correct only for a certain load and a certain strip width. To reduce the effects of deflection, the rolls can be subjected to bending, by the application of moments at their bearings; this manipulation simulates camber.

Roll forces also tend to flatten the rolls elastically. This flattening of the rolls is undesirable. It produces a large roll radius and hence a larger contact area for the same draft. The roll force in turn increases with increased flattening. New (distorted) roll radius is given by the expression

 R'  R 1

CF ' h0

h f  

where C  2.3 10

2

mm 2 / kN for steels and 4.57 10

2

mm2 / kN for cast - iron rolls

 F ' roll force per unit width

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Defects in rolled products Defects may be surface defects (scale, rust, scratches, gouges, pits and cracks) or internal structural defects. Some typical defects generally found in rolled plates and sheets are Wavy edges: They are the result of roll bending. The strip is thinner along its edges than at its centre because the edges elongate more than the centre.

Zipper cracks: They are caused by poor material ductility at rolling temperature.

Edge cracks: They are also caused by poor material ductility.

Alligatoring: It is a complex phenomenon and may be caused by non-uniform deformation during rolling or by the presence of defects in the original cast billet.

Residual stresses in rolled products Because of non uniform deformation of the material in the roll gap, residual stresses can develop in rolled plates and sheets, especially during cold rolling. Small diameter rolls or small reductions per pass tend to deform the metal plastically at its surfaces. This produces compressive residual stresses on the surfaces and tensile stresses in the middle. On the other hand, large diameter rolls and high reductions tend to deform the bulk more than the surfaces. This produces residual stresses that are opposite of those in the case of small diameter rolls.

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Rolling of wheels It’s a long way from sheet metal to the finished wheel. Initially, the wheel disk and wheel rim are produced in separate production processes and subsequently welded together in an assembly line. The wheel disk is produced from coil stock in a deep-drawing process using a progressive press (shaping and punching). The associated wheel rim is produced in the wheel rim line. Here too, the basic product is a sheet metal coil. The coil is unwound and cut to length, and subsequently pre-bent (round bending) and welded into a circular blank. This circular blank is then profiled via roll stands and is shaped into the typical wheel rim profile. In the assembly line the disk is pressed into the wheel r im, welded, checked and subsequently dip-primed.

Rolling of I beams

I-beams have a large geometrical moment of inertia per unit weight and resist bending and twisting, and are therefore used as columns, beams, and bridge girders in architectural and civil construction. Products such as I-beams, whose cross-sectional shape is not rectangular, can also be produced by rolling. The figure shows the rolling equipment, forming process and JO/VJCET

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names of the parts of an I-beam. Here, caliber rolling is conducted in the roughing stage. The materials are rolled by caliber rolls in order to obtain the same cross-sectional shape as that of the rolls. After producing a near I shape by caliber rolling, the product is finished by a universal mill and an edging mill. An I-shaped cross-section is formed when the material passes through four rolls, making the universal mill, which is equipped with a pair of vertical rolls and a pair of  horizontal rolls, suitable for rolling I-beams. The edging mill is equipped with caliber rolls as shown in the figure, and has the function of adjusting the flange widths of products. In the universal mill, variations of flange- and web- thickness can be made easily by adjusting the roll gap. However, when products with different web heights and flange widths are to be rolled, it is necessary to employ exclusive-use rolls for these sizes, necessitating roll changes. In particular, since the web heights are determined by sum of the width of the horizontal rolls and flange thickness, it has to date been necessary to have the same number of horizontal roll sizes as product web heights. Development to overcome this problem has resulted in recent rolling mills and rolling techniques capable of adjusting the web heights by one roll with changeable width without changing rolls.

Rolling of axles Axles, especially tapered ones can be manufactured by roll forging. Roll forging (also known as hot forge rolling) is a process for reducing the cross-sectional area of heated bars or billets by passing them between two driven rolls that rotate in opposite directions and have one or more matching grooves in each roll. Roll dies designed for forging the required shape are bolted to the roll shafts, which rotate in opposite directions during operation. Roll dies (or their effective forging portion) usually occupy about one-half the total circumference; therefore, at least some forging action takes place during half of the revolution. Machines can be operated continuously or stopped between passes, as required. In the roll forging of long tapered work pieces, the more common practice is to operate the machine intermittently The operator lays the heated stock on the table of the machine, grasps the stock with tongs, and starts the machine (commonly controlled by a foot treadle) During the portion of the revolution when the roll dies are in the open position, the operator places the stock between them against a stock gage and in line with the first roll groove, retaining his tong hold on the work piece. The tables are usually grooved to assist in aligning the stock. As the roll dies rotate to the closed position, forging begins. The work piece is forced back toward the operator, who moves it to the position of the next roll-die groove and again pushes it against the stop during the open position of the roll dies. This is repeated until the work piece has been forged through the entire series of grooves In a few mass-production applications, the roll-forging procedure described above has been automated, but manual operation is by far the most common practice.

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Forging of an axle shaft in ten passes through eight-groove eight -groove semi-cylindrical semi-cylindrical roll dies

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Analysis of flat rolling

A metal sheet with a thickness h 0 enters the rolls at the entrance plane XX with a velocity v 0. It passes through the roll gap and leaves at an exit plane YY with a reduced thickness h f  and at a velocity vf .  Assumptions The arc of contact between the rolls and the metal is a part of the circle. The co-efficient of friction µ is constant. (In reality, µ varies along the arc of contact.) The metal deforms plastically during rolling. The volume of metal remains constant before and after rolling. The velocity of rolls is assumed to be constant The metal extends only in rolling direction and there is no extension widthwise. Given that there is no increase in width, the vertical compression of the metal is translated into an elongation in the rolling direction. Since there is no change in metal volume at a given point per unit time throughout the process, b0hoνo = bhν = bf hf νf  ……………………….(1) where b is the width of the sheet, v is the velocity at any thickness h between h 0 and hf . We know that b 0 = bf  = b. Therefore , hoνo = hν = hf νf 

………………….(2) Since h0>hf , vf >v0 The velocity of the sheet must steadily increase from entry to exit. Rolling load and roll pressure At any point along the surface of the contact between the roll and the sheet, two forces act on the metal 1) a radial force Pr 2) a tangential frictional force F.

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The vertical component of the radial force, P is the rolling load  – the load with which rolls press against the metal. The specific roll pressure p is the rolling load per unit contact area. ........................... (3) Where b is the width of sheet and L p is the projected length of arc of contact. 2

2

2

From the figure, R = Lp + (R - ∆h/2) where change in thickness ∆h = h 0 - hf  2

1/2

On simplification, L p = *R∆h - ∆h /4] Neglecting the higher order term,

........................... (4)

Roll bite condition

For the work piece to enter the roll, the component of friction force must be equal to or greater than the horizontal component of the normal force.

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i.e.,

................................. (5)

On simplifying, But we know, F = µ Pr, where µ is the co-efficient of friction. Therefore,

, for the work piece to enter the roll.

• If tanα > μ,the work piece cannot be drawn. • If μ=0, rolling cannot occur.

Roll bite can be improved by increasing the value of µ and/or by reducing the value of tan α. The effective value of µ can be increased by grooving the rolls. The value of tan α can be reduced by using big rolls. If the roll diameter is fixed, tan α can be reduced by reducing the value of h 0. Maximum possible reduction / draft

Maximum possible reduction takes place when µ = tan α

For maximum possible reduction , On simplification, maximum possible reduction,

...................... (6)

Torque and power in rolling Torque is the measure of the force applied to a member to produce rotational motion. Power 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: a) The energy needed to deform the metal. b) The energy needed to overcome the frictional force. c) The power lost in the pinions and power -transmission system. d) Electrical losses in the various v arious motors and generators.

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The total rolling load is distributed over the arc of contact in the typical friction-hill pressure distribution. However the total rolling load can be assumed to be concentrated at a point along the arc of the contact at a distance ‘a’ from the line of centres of the rolls. The torque-MT is equal to the total rolling load P multiplied by the effective moment arm a. Since there are two rolls, the torque is given by MT = 2Pa During one revolution of the top roll the resultant rolling load P moves along the circumference of  a circle equal to 2πa.Since there are two work rolls, the work done W is given by Work = 2(2πa)P Since power is defined as the rate of doing work, the power (in watts) needed to operated a pair of rolls revolving at N rps, Power = 4πaPN where P is in Newton and a is in meters For hot rolling, a = 0.5 L p For hot rolling, a = 0.5 L p Therefore, for hot rolling, Torque, MT = P.Lp Work, W = 2π LpP Power = 2πLpPN

Neutral point or No slip point

Neutral point is the point at which the velocity of sheet becomes equal to the velocity of roll. Between the entrance plane and the neutral point, the sheet is moving slower than the roll velocity and the tangential frictional force act in the direction to draw the metal into the roll. On the exit side of the neutral point, the sheet moves faster than the roll surface. The direction of the frictional force is then reversed and it opposes the delivery of sheet from the rolls.

Roll pressure distribution / Friction hill in rolling

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The distribution of roll pressure along the arc of contact shows that the pressure rises to a maximum at the neutral point and then falls off. The pressure distribution does not come to a sharp peak at the neutral point which indicates that neutral point not really a line but an area. The area under the curve is proportional to the rolling load. The shaded area represents the force required to overcome frictional forces between the roll and sheet. The area under u nder the dashed line AB represents the force required to deform the metal in plane homogeneous compression.

Back and front tensions in sheet rolling The presence of back and front tensions in sheet rolling reduces the rolling load.

Back tension may be created by controlling the speed of uncoiler relative to the roll speed. Front tension may be created by controlling the coiler. Back tension is twice as effective in reducing the rolling load as compared to front tension. If a high enough back tension is applied, the neutral point moves towards roll exit. If the front tension is used, neutral point will move toward the roll entrance.

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