ROLLING ROLLING
It is the process of reducing the thickness or changing the cross-section of a long work-piece by 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.
compressive forces applied through a set of rolls.
Types of Rolling Types of Rolling
Based on work piece geometry :Based on work piece geometry :
Flat rolling - used to reducFlat rolling - used to reduce thickness e thickness of a rectangof a rectangular cross ular cross sectionsection
Shape rolling - square cross section is formed into a Shape rolling - square cross section is formed into a shape such as an I- beamshape such as an I- beam
Based on work temperature :Based on work temperature :
Hot rollingHot rolling – – Temperature above recrystallization temperature of metal.Temperature above recrystallization temperature of metal.
Cold rollingCold rolling – – carried out at room temperaturecarried out at room temperature
Warm rollingWarm rolling – – carried out at elevated temperature but below recrystallizationcarried out at elevated temperature but below recrystallization temperature.
temperature. Hot rolling
Hot rolling
Coarse grain structure is broken up and elongated by the rolling action. Because of the high 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 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 grow rapidly until recrystallization is complete. Growth continues at high temperatures, if further work is not carried on, until the low temperature of the
work is not carried on, until the low temperature of the recrystalline range is reached.recrystalline range is reached.
Semi-finished products in rolling Semi-finished products in rolling Ingot
Ingot : Ingots are large rough castings designed for storage and transportation. The shape usually : Ingots are large rough castings designed for storage and transportation. The shape usually resembles a rectangle or square. They are tapered, usually with the
Bloom
Bloom:: BloomsBlooms are similar are similar to billets to billets except the except the cross-sectional area cross-sectional area is greater is greater than 36 than 36 sq sq inin (230 cm
(230 cm22 ). Blooms ). Blooms are are usually usually further further processed processed viavia rotary piercing,rotary piercing, structural shape rollingstructural shape rolling and and profile rolling. Com
profile rolling. Common final products mon final products includeinclude structural shapes,structural shapes, rails,rails, rods, and rods, and seamless pipes.seamless pipes. Slab
Slab: A slab is a length of metal that is rectangular in cross-section. It is created directly from: 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 continuous casting or indirectly by rolling an ingot. Slabs are usually further processed via flat flat rolling
rolling and pipe rolling. Common final products includeand pipe rolling. Common final products include sheet metal,sheet metal, plates, strip metal,plates, strip metal, pipes,pipes, and
and tubes.tubes. Plates
Plates: They have thickness greater than 6mm and are used for structural applications such as: They have thickness greater than 6mm and are used for structural applications such as machine structures, ship hulls, boilers, bridges and nuclear vessels.
machine structures, ship hulls, boilers, bridges and nuclear vessels. Sheets:
Sheets: They are generally less than 6mm thick. They are used for automobile and aircraft bodies,They are generally less than 6mm thick. They are used for automobile and aircraft bodies, food and beve
food and beverage containers etc.rage containers etc.
Hot rolling vs
Hot rolling vs Cold rollingCold rolling Hot Rolling
Hot Rolling Cold RollingCold Rolling
Metal is fed to the rolls after being heated Metal is fed to the rolls after being heated above the
above the recrystallization temperature.recrystallization temperature.
Metal is fed to the rolls when it is below Metal is fed to the rolls when it is below recrystallization temperature.
recrystallization temperature. Rolled metal does not show work hardening
Rolled metal does not show work hardening effect.
effect.
The metal shows work hardening effect after being The metal shows work hardening effect after being cold rolled.
cold rolled. Co-efficient of friction between the rolls and
Co-efficient of friction between the rolls and the stock is higher.
the stock is higher.
Co-efficient of friction between the rolls and the Co-efficient of friction between the rolls and the stock is comparatively lower.
stock is comparatively lower. Heavy reductions in area of the work piece
Heavy reductions in area of the work piece can be obtained.
can be obtained.
Heavy reduction is not possible. Heavy reduction is not possible. Very thin sections are not obtained by hot
Very thin sections are not obtained by hot rolling.
rolling.
Very thin sections can be made. Very thin sections can be made.
Hot Rolling
Hot Rolling Cold RollingCold Rolling
Mechanical properties are improved by Mechanical properties are improved by breaking cast structure and refining grain breaking cast structure and refining grain size. Blow holes and other similar defects size. Blow holes and other similar defects present in the ingot are removed. The present in the ingot are removed. The strength and toughness of the metal strength and toughness of the metal increases.
increases.
Hardness increases. Excessive cold working Hardness increases. Excessive cold working generates cracks. Ductility of the metal reduces. generates cracks. Ductility of the metal reduces. Cold rolling increases the tensile strength and yield Cold rolling increases the tensile strength and yield strength of steel.
strength of steel.
Roll
Roll radius radius is is generally generally larger. larger. Roll Roll radius radius is is smallersmaller The hot rolled surface has scale on it; the
The hot rolled surface has scale on it; the surface finish is not good.
surface finish is not good.
The cold rolled surface is smooth and oxide-free. The cold rolled surface is smooth and oxide-free. Close tolerances on dimensions cannot be
Close tolerances on dimensions cannot be obtained.
obtained.
Close dimensional tolerances are possible. Close dimensional tolerances are possible. Yield stress varies with temperature, it varies
Yield stress varies with temperature, it varies with each pass, and therefore it is difficult to with each pass, and therefore it is difficult to formulate a theory for hot rolling.
formulate a theory for hot rolling.
Theoretical analysis can be easily carried out and Theoretical analysis can be easily carried out and extensively developed theory is available.
extensively developed theory is available. Experimental measurements are difficult to
Experimental measurements are difficult to make.
make.
Experimental measurements are easily carried out. Experimental measurements are easily carried out. Hot
Hot rolling rolling is is the the father father of of cold cold rolling. rolling. Cold Cold rolling rolling follows follows hot hot rolling. rolling. Hot Hot rolled rolled objectsobjects are thoroughly cleaned of their surface scale by are thoroughly cleaned of their surface scale by pickling in an acid solution and are then cold rolled. pickling in an acid solution and are then cold rolled. Pack Rolling
Pack Rolling
It is a flat rolling operation in which two or more layers of metal are rolled together; this 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
process improves productivity. Aluminium foil is pack rolled in two layers. The foil to foil two layers. The foil to foil side has aside has a matt finish. The foil to roll side is shiny and bright because it has been
matt finish. The foil to roll side is shiny and bright because it has been in contact with the polishedin contact with the polished rolls.
rolls.
Temper rolling or skin pass Temper rolling or skin pass
Mild steel, when stretched during sheet forming operations, undergoes yield point Mild steel, when stretched during sheet forming operations, undergoes yield point elongation, a phenomenon that causes surface irregularities called stretcher strains. To correct 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 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.
process is known as temper rolling or skin pass. Levelling rolls
Levelling rolls
A rolled sheet may not be sufficiently flat as it leaves the roll gap, because of variations in 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 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 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.
Rolling Mill Rolling Mill
A rolling mill consists of one or more roll stands, motor drive, reduction gears, and flywheel and 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 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 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 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, 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 I-beam, rail). Rolling mills are classified according to the number and arrangement of rolls in a stand. They are classified as:
stand. They are classified as:
For hot rolling of metals (Two-high rolling mill, Three-high rolling mill) 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) For cold rolling of metals (Four high rolling mill, Cluster rolling mill)
(1) Two-high rolling mill:
(1) Two-high rolling mill: It is basically of two types i.e., non-reversing and reversing 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 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 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 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 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
large ingot into a bloom. This type is used in blooming and slabbing mills.blooming and slabbing mills. (2) Three-high rolling mill
(2) Three-high rolling mill: It is used for rolling of two continuous passes in a rolling sequence: 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
without reversing the drives. After all the metal has passed through the bottom roll set, the bottom roll set, the end of end of the metal is entered into the other set of the rolls for the next pass. For this purpose, a the metal is entered into the other set of the rolls for the next pass. For this purpose, a table-tilting arrangement is required to bring the metal to the level with the rolls. Such type of tilting arrangement is required to bring the metal to the level with the rolls. Such type of arrangement is used for making
arrangement is used for making plates or sections.plates or sections. (3) Four-high rolling mill:
(3) Four-high rolling mill: It is generally a two-high rolling mill, but with small sized rolls. The otherIt 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 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.
both hot and cold rolling of wide plates and sheets. (4) Cluster rolling mill:
(4) Cluster rolling mill: It uses backup rolls to support the smaller work rolls. In this type of 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 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
cluster mills since the small thickness requires small-diameter rolls.small thickness requires small-diameter rolls. (5) Tandem rolling mill:
(5) Tandem rolling mill: In tandem rolling, the strip is rolled continuously, through a number of 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 stands, to smaller gaps with each pass. Each stand consists of a set of rolls with its own housing
and controls. The control of the gap and of the speed at which the sheet travels through each roll 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 gap is critical. Electronic and computer controls, along with extensive hydraulic controls are used in tandem rolling operations.
in tandem rolling operations.
(6) Planetary rolling mill:
(6) Planetary rolling mill: In the planetary mill, a great number of small rolls, which in turn serveIn 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 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, work on the strip simultaneously, the pass reduction can be very high. Due to the high complicity, a pair of feed rolls is installed
a pair of feed rolls is installed in some cases.in some cases.
Roll Passes Roll Passes
The final rolled products such as plates, flats, sheets, rounds and sections are obtained in a 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
number of passes starting from billet or slabs. For rolling the flat product, plain cylindrical rolls arecylindrical rolls are used but for sections, grooved rolls are used. The type of grooving done is decided by the final 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:
section desired. The roll pass sequence can be broadly classified into three types: 1. Breakdown passes:
1. Breakdown passes: These are used for reducing the cross-sectional area nearer to what isThese are used for reducing the cross-sectional area nearer to what is desired. These would be the first to be
desired. These would be the first to be present in the sequence.present in the sequence. 2. Roughing passes:
2. Roughing passes: In these passes also, the cross-section gets reduced, but along with it, theIn these passes also, the cross-section gets reduced, but along with it, the shape of the rolled material comes nearer to the fi
shape of the rolled material comes nearer to the final shape.nal shape. 3. Finishing passes:
3. Finishing passes: These are the final passes which give the required shape of pass follows aThese are the final passes which give the required shape of pass follows a leader pass.
leader pass.
The principal breakdown pass sequence is: The principal breakdown pass sequence is:
(i) box pass series (i) box pass series
(ii) diamond square series (ii) diamond square series (iii) oval square series (iii) oval square series Rolling of square sections Rolling of square sections
Figure below shows the pass sequence for the rolling of squares. Diamond-diamond and 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 diamond-square roughing passes, diamond leader passes and square finishing passes are employed for rolling.
Rolling of rounds Rolling of rounds
Figure below shows the pass sequ
Figure below shows the pass sequence for the rolling of a rod. A ½’’ diameter rod is rolledence for the rolling of a rod. A ½’’ diameter rod is rolled from 4’’ x 4’’ billet in 10 passes. Oval
from 4’’ x 4’’ billet in 10 passes. Oval-square passes have been employed for shaping the bar from-square passes have been employed for shaping the bar from the billet.
the billet.
Rolling of flats, angle sections and channels Rolling of flats, angle sections and channels
Spreading in rolling Spreading in rolling
In the rolling of work pieces with smaller width to thickness ratios, such as with a square 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 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 spreading. In the calculation of roll force, the average width is considered. Spreading increases with
with a)
a) A decrease in width to thickness ratio of entering materialA decrease in width to thickness ratio of entering material b)
b) An increase in the frictionAn increase in the friction c)
c) A decrease in the ratio of roll A decrease in the ratio of roll radius to strip thicknessradius to strip thickness
Spreading can be prevented by the use of vertical rolls in contact with the edges of the rolled Spreading can be prevented by the use of vertical rolls in contact with the edges of the rolled product.
Absolute draught / draft:
Absolute draught / draft: It is the difference between the initial and final thickness of materialIt is the difference between the initial and final thickness of material bei
being rolled. It is expressed as ∆h = hng rolled. It is expressed as ∆h = h00 - - hhf f , where h, where h00 is initial thickness and his initial thickness and hf f is final thickness.is final thickness.
The maximum possible draft is given by ∆h
The maximum possible draft is given by ∆hmaxmax= µ= µ22R where µ is co-efficient of friction between rollR where µ is co-efficient of friction between roll
and work piece and R is roll radius. and work piece and R is roll radius. Relative draught/ draft:
Relative draught/ draft: It is the ratio of absolute draught to initial thickness of the work piece asIt is the ratio of absolute draught to initial thickness of the work piece as expressed as percentage.
expressed as percentage. Absolute elongation:
Absolute elongation: It is the difference between the final and initial length of the work pieceIt is the difference between the final and initial length of the work piece being rolled.
being rolled. Absolute spread:
Absolute spread: It is the difference between the final and initial width of the work piece beingIt is the difference between the final and initial width of the work piece being rolled.
rolled.
Thread rolling Thread rolling
Thread rolling is used to form threads on cylindrical parts by rolling them between two 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 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
components (eg: bolts and screws). Most thread rolling operations are performed by cold workingcold working in thread rolling machines. These machines are equipped with
in thread rolling machines. These machines are equipped with special dies that determine the sizespecial dies that determine the size and form of the thread. The dies are of two types: (1) flat dies, which reciprocate relative to each 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 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 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) rolling include (a) better material utilization (b) stronger threads due to work hardening (c) smoother surface (d) better fatigue resistance due
Gear rolling Gear rolling
Gear rolling is a cold working process to produce certain gears. The automotive industry is 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 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 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 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 resistance and less material wastage. Gear rolling is also used for surface densification of gears manufactured by powder metallurgy process.
manufactured by powder metallurgy process.
Ring rolling Ring rolling
It is a deformation process in which a thick walled ring of smaller diameter is rolled into a 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 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 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 be enlarged. Ring rolling is usually performed as a hot-working process for large rings and as cold working process for small rings.
working process for small rings.
The ring walls are not limited to rectangular cross-sections; the process permits rolling of more The ring walls are not limited to rectangular cross-sections; the process permits rolling of more complex shapes as shown in the figure below.
Applications Applications
Ball and roller bearing races Ball and roller bearing races Steel tires for rail road wheels Steel tires for rail road wheels Rings for pipes, pressure vessels
Rings for pipes, pressure vessels and rotating machinery.and rotating machinery. Advantages
Advantages
Raw material savings Raw material savings
Ideal grain orientation for the
Ideal grain orientation for the applicationapplication Strengthening through cold working Strengthening through cold working Roll piercing / Rotary tube piercing /
Roll piercing / Rotary tube piercing / Mannesmann processMannesmann process
It is a specialized hot working process for making seamless thick walled tubes. It utilizes 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 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
compressed on its circumference, high tensile stresses are developed at its centre. If compressionat its centre. If compression is high enough, an internal crack is formed.
is high enough, an internal crack is formed.
Compressive stresses on a cylindrical solid billet are applied by two rolls, whose axes are 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
oriented at slight angles (about 6⁰) from the axis of the billet, so that their rotatio n tends to pulln tends to pull the billet through the rolls. A mandrel is used to control the size and finish of the hole created by the billet through the rolls. A mandrel is used to control the size and finish of the hole created by the action.
(a)
(a) Tube rolling with fixed mandrel, (b) Tube rolling with moving mandrelTube rolling with fixed mandrel, (b) Tube rolling with moving mandrel (c)
(c) Tube rolling without mandrel (d) Pilger rolling over a Tube rolling without mandrel (d) Pilger rolling over a mandrel and shaped rollsmandrel and shaped rolls Shape rolling of an
Shape rolling of an H-section/I sectionH-section/I section
Skew rolling of steel balls Skew rolling of steel balls
Roll materials Roll materials
The basic requirements for roll material are strength and resistance to wear. Common roll 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 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 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
have greater strength, stiffness and toughness than cast iron rolls. Rolls for cold rolling are groundare ground to a fine finish. Elastic modulus of the roll material influences roll deflection and flattening. Rolls 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 made for cold rolling should not be used for hot rolling, because they may crack from thermal cycling and spalling.
cycling and spalling. Lubricants in rolling Lubricants in rolling
Hot rolling of ferrous alloys is usually carried out without lubricants, although graphite may 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 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 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, 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
paraffin and fatty oils. Residual salts from molten salt baths employed for heat treating billets andfor heat treating billets and slabs also offer effective lubrication during
slabs also offer effective lubrication during rolling.rolling. Roll deflections and roll flattening
Roll deflections and roll flattening
Roll forces tend to bend the rolls elastically during rolling. The higher the elastic modulus 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 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 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 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 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 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 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 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 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 of deflection, the rolls can be subjected to bending, by the application of moments at their bearings; this manipulation simulates camber.
bearings; this manipulation simulates camber.
Roll forces also tend to flatten the rolls elastically. This flattening of the rolls is undesirable. It 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 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 turn increases with increased flattening. New (distorted) roll radius is given by the expression
Defects in rolled products Defects in rolled products
Defects may be surface defects (scale, rust, scratches, gouges, pits and cracks) or internal 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
structural defects. Some typical defects generally found in rolled plates and sheets are Wavy edges:
Wavy edges: They are the result of roll bending. The strip is thinner along its edges than at itsThey 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.
centre because the edges elongate more than the centre.
Zipper cracks:
Zipper cracks: They are caused by They are caused by poor material ductility at rolling temperature.poor material ductility at rolling temperature.
Edge cracks:
Edge cracks: They are also caused by poor material ductility.They are also caused by poor material ductility.
Alligatoring:
Alligatoring: It is a complex phenomenon and may be caused by non-uniform deformation duringIt is a complex phenomenon and may be caused by non-uniform deformation during rolling or by the presence of
rolling or by the presence of defects in the original cast billet.defects in the original cast billet.
Residual stresses in rolled products Residual stresses in rolled products
Because of non uniform deformation of the material in the roll gap, residual stresses can 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 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.
reductions per pass tend to deform the metal plastically at its surfaces. This produces compressiveThis produces compressive residual stresses on the surfaces and tensile stresses in the middle. On the other hand, large 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 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
Rolling of wheels Rolling of wheels
It’s a long way from sheet metal to the finished wheel. Initially, the wheel disk
It’s a long way from sheet metal to the finished wheel. Initially, the wheel disk and wheeland wheel rim are produced in separate production processes and subsequently welded together in an 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 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 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 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.
subsequently pre-bent (round bending) and welded into a circular blank. This circular blank isThis circular blank is then profiled via roll stands and is shaped into the
then profiled via roll stands and is shaped into the typical wheel rim profile.typical wheel rim profile. In the assembly lineIn the assembly line the disk is pressed into the wheel r
the disk is pressed into the wheel r im, welded, checked and subsequently dip-primed.im, welded, checked and subsequently dip-primed.
Rolling of I beams Rolling of I beams
names of the parts of an I-beam. Here, caliber rolling is conducted in the roughing stage. The 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
materials are rolled by caliber rolls in order to obtain the same cross-sectional shape as that of order to obtain the same cross-sectional shape as that of thethe rolls. After producing a near I shape by caliber rolling, the product is finished by a universal mill 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 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 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
horizontal rolls, suitable for rolling I-beams. The edging mill is equipped with caliber rolls is equipped with caliber rolls as shownas shown in the figure, and has the function of adjusting the flange widths of products.
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 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 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, 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 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 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 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 has resulted in recent rolling mills and rolling techniques capable of adjusting the web heights by one roll with changeable width without changing rolls.
one roll with changeable width without changing rolls. Rolling of axles
Rolling of axles
Axles, especially tapered ones can be manufactured by roll forging. Roll forging (also known as hot 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 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 them between two driven rolls that rotate in opposite directions and have one or more matching grooves in each roll.
grooves in each roll.
Roll dies designed for forging the required shape are bolted to the roll shafts, which rotate Roll dies designed for forging the required shape are bolted to the roll shafts, which rotate in opposite directions during operation. Roll dies
in opposite directions during operation. Roll dies (or their effective forging portion) usually (or their effective forging portion) usually occupyoccupy about one-half the total circumference; therefore, at least some forging action takes place during 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 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 required. In the roll forging of long tapered work pieces, the more common practice is to operate the machine intermittently
the machine intermittently
The operator lays the heated stock on the table of the machine, grasps the stock with The operator lays the heated stock on the table of the machine, grasps the stock with tongs, and starts the machine (commonly controlled by
tongs, and starts the machine (commonly controlled by a foot treadle)a foot treadle)
During the portion of the revolution when the roll dies are in the open position, the 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 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 groove, retaining his tong hold on the work piece. The tables are usually grooved to assist in aligning the stock.
in aligning the stock.
As the roll dies rotate to the closed position, forging begins. The work piece is forced back 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 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 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
the work piece has been forged through the entire series of groovesseries of grooves
In a few mass-production applications, the roll-forging procedure described above has 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.
Forging of an
Analysis of flat rolling Analysis of flat rolling
A metal sheet with a thickness h
A metal sheet with a thickness h00 enters the rolls at the entrance plane XX with a velocity venters the rolls at the entrance plane XX with a velocity v00. . ItIt
passes through the roll gap and leaves at an exit plane YY with a reduced thickness h
passes through the roll gap and leaves at an exit plane YY with a reduced thickness hf f and at aand at a
velocity v velocity vf f ..
Assumptions Assumptions
The arc of contact between the rolls and the metal is
The arc of contact between the rolls and the metal is a part of the circle.a part of the circle. The co-efficient of friction µ is constant. (In reality, µ varies along the arc of
The co-efficient of friction µ is constant. (In reality, µ varies along the arc of contact.)contact.) The metal deforms plastically during rolling.
The metal deforms plastically during rolling.
The volume of metal remains constant before and after rolling. The volume of metal remains constant before and after rolling. The velocity of rolls is assumed to be
The velocity of rolls is assumed to be constantconstant
The metal extends only in rolling direction and there is no extension widthwise. The metal extends only in rolling direction and there is no extension widthwise. Given that there is no
Given that there is no increase in width, the vertical compression of the metal is increase in width, the vertical compression of the metal is translated into antranslated into an elongation in the rolling direction.
elongation in the rolling direction.
Since there is no change in metal volume at
Since there is no change in metal volume at a given point per unit time throughout the process,a given point per unit time throughout the process, b
b00hhooννoo==bhνbhν= b= bf f hhf f ννf f ……….(1)……….(1)
where b is the width of
where b is the width of the sheet, v is the the sheet, v is the velocity at any thickness h between hvelocity at any thickness h between h00and hand hf f ..
We know that b We know that b00= b= bf f = b.= b. Therefore , h Therefore , hooννoo == hνhν = h= hf f ννf f ……….(2) ……….(2) Since h Since h00>h>hf f ,,
v
v
f f>v
>v
00The velocity of the sheet must steadily increase from entry
The velocity of the sheet must steadily increase from entry to exit.to exit. Rolling load and roll
Rolling load and roll pressurepressure
At any point along the surface of the contact between the roll and the sheet, two forces act on the At any point along the surface of the contact between the roll and the sheet, two forces act on the metal
metal 1)
1) aa radial force Prradial force Pr 2)
The vertical component of the radial force,
The vertical component of the radial force, P is the rolling loadP is the rolling load – – the load with which rolls pressthe load with which rolls press against the metal.
against the metal. The
The specific roll pressure pspecific roll pressure p is the rolling load per unit contact area.is the rolling load per unit contact area. ... (3) ... (3) Where b is the width of
Where b is the width of sheet and Lsheet and Lppis the projected length of arc of contact.is the projected length of arc of contact.
From the figure, R
From the figure, R22= L= Lpp22+ (R -+ (R - ∆h/2)∆h/2)22where change in thickness ∆h = hwhere change in thickness ∆h = h00- h- hf f
On simplification, L
On simplification, Lpp= *R∆h= *R∆h -- ∆h∆h22/4]/4]1/21/2
Neglecting the higher order term, Neglecting the higher order term,
... (4) ... (4) Roll bite condition
i.e.,
i.e.,
... (5)... (5) On simplifying,On simplifying, But we know, F = µ
But we know, F = µ Pr, where µ is the Pr, where µ is the co-efficient of friction.co-efficient of friction. Therefore,
Therefore,
,,
for the work piece to enter the for the work piece to enter the roll.roll. • If tanα > μ,the work piece cannot be drawn.• If tanα > μ,the work piece cannot be drawn. • If μ=0, rolling cannot occur.
• 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 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
effective value of µ can be increased by grooving the rolls.increased by grooving the rolls. The value of tan α can be reduced byThe 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
using big rolls. If the roll diameter is fixed, tan α can be reduced by reducing the value of h00..
Maximum possible reduction / draft Maximum possible reduction / draft
Maximum possible reduction takes place when µ =
Maximum possible reduction takes place when µ = tan αtan α
For maximum possible reduction For maximum possible reduction
,,
OnOn simplification, simplification, maximum maximum possible possible reduction, reduction, ... ... (6)(6) Torque and power in rolling
Torque and power in rolling
Torque is the measure of the force applied to a member to produce rotational motion. 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. 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:
The power is spent principally in four ways: a) The energy needed to deform the
a) The energy needed to deform the metal.metal. b) The energy needed to overcome
b) The energy needed to overcome the frictional force.the frictional force. c) The power lost in the pinions and power
c) The power lost in the pinions and power-transmission system.-transmission system. d) Electrical losses in the v
The total rolling load is distributed over the arc of contact in the typical friction-hill pressure 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 distribution. However the total rolling load can be assumed to be concentrated at a point along the arc of the contact at a distance
the arc of the contact at a distance ‘‘aa’’ from the line of centres of the rolls.from the line of centres of the rolls. The torque-M
The torque-MTT is equal to the total rolling load P multiplied by the effective moment arm a. Sinceis equal to the total rolling load P multiplied by the effective moment arm a. Since
there are two rolls, the torque is
there are two rolls, the torque is given bygiven by M
MTT = 2Pa= 2Pa
During one revolution of the top roll
During one revolution of the top roll the resultant rolling load P moves along the circumference of 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
a circle equal to 2πa.Since there are two work rolls, the work done W is given bygiven by Work =
Work = 2(2πa)P2(2πa)P
Since power is defined as the rate of doing work, the power (in watts) needed to operated a pair 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,
of rolls revolving at N rps,
Power =
Power = 4πaPN4πaPN
where P is in Newton
where P is in Newton and a is in metersand a is in meters For hot rolling, a = 0.5 L
For hot rolling, a = 0.5 L p p
For hot rolling, a = 0.5 L For hot rolling, a = 0.5 L p p
Therefore, for hot rolling, Therefore, for hot rolling,
Torque, M Torque, MTT= P.Lp= P.Lp Work, W = 2π Work, W = 2πLLppPP Power = 2πLpPN Power = 2πLpPN Neutral point or No slip point
Neutral point or No slip point
Neutral point is the point at which the velocity of sheet becomes equal to the velocity of roll. 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 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 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 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
frictional force is then reversed and it opposes the delivery of sheet from the rolls.delivery of sheet from the rolls. Roll pressure distribution / Friction hill
The distribution of roll pressure along the arc of contact shows that the pressure rises to a 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 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. 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 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
required to overcome frictional forces between the roll and sheet. The area uand sheet. The area under the dashed linender the dashed line AB represents the force required to deform the metal in plane
AB represents the force required to deform the metal in plane homogeneous compression.homogeneous compression. Back and front tensions in sheet rolling
Back and front tensions in sheet rolling
The presence of back and front tensions in sheet rolling reduces the rolling load. The presence of back and front tensions in sheet rolling reduces the rolling load.
Back tension may be created by controlling the speed of
Back tension may be created by controlling the speed of uncoiler relative to the roll speed.uncoiler relative to the roll speed. Front tension may be created by controlling the coiler.
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. 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 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.
