Rolling

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1. ROLLING
1.1 PROCESS
Practically 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 ROLLING
A 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 ROLLING
Primary roughening mill – blooming, slabbing or cogging mills – is the initial hotworking 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:

Scalebrea
ker mill

Roughing
train

• 4 four Highmills

Finishing
train

• 6 four high
finishing mills

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.

σy = σ0 + ky/D0.5
Hall Petch Equation

4. FLAT ROLLING
A 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 length
h0 – initial thickness of the workpiece.
hf – final thickness of the workpiece.
wo – initial width of the workpiece
wf – final width of the workpiece
R – 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, Δh, is defined as the difference in thickness before and after
rolling.

Δh = (ho – hf)
where, ho = initial thickness
hf = final thickness
Δh = the maximum possible draft
Δh is a function of the coefficient of friction, μ, and the roll radius, R:

Δh = (ho – hf) = μ2R
Therefore, 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 REQUIREMENT
Rolls 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 by

F = LW Yavg
Where L = roll strip contact length (roll gap)
W = width of the strip
Yavg = average true stress
F = roll force
This 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/2
Where T – Torque;
F – Roll force;
L – Roll gap;

Therefore,

Power= πfLN/ 60,000 (KW) or
Power= πfLN/ 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.
b.
c.
d.
e.
f.
g.

Lubrication (reduces friction)
Using smaller roll radius (reduces contact area)
Smaller reductions per pass (reduces contact area)
High temperature rolling (reduces the strength of the material)
Using carbide rolls (higher σ)
Applying front and back tension
Using backup rolls

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

4.2 GEOMETRIC CONSIDERATIONS
The 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.
SPREADING
In 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.

Metals

Temperature range of hot
working

Aluminum

4500C

Alloy steels

12500C

Refractory metals

16500C

Copper

5000C

5. EQUIPMENT – ROLLING MILLS
Till 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 MILLS
The 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 REMEDIES

Several 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 LOAD
Variables that are considered for the calculation:






The roll diameter
The deformation resistance
Friction between rolls and workpiece
Presence of front and back tension

Three conditions are considered

a)
b)
c)
a)

No friction condition
Normal friction condition
Sticking friction condition
NO FRICTION CONDITION

When friction is ignored, the rolling load (P) is roll pressure times the area between the
workpiece and the rolls (bL0)

P = pbLp = σ0’b(RΔh)0.5 …… Eqn (1)
Where P – rolling load
p – roll pressure
σ0 – yield stress
b – width
R – radius
2) NORMAL FRICTION SITUATION
In the normal case of friction situation in plane strain, the average pressure P is.

p/σ0 = (eq – 1)/Q
Where Q = µL/h
h – the mean thickness between entry and exit.
From eqn 1 we have,

P = pbLp
Therefore,

P = 1.155σ0[(eq -1)b(RΔh)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) - µ is 0.05 – 0.10.
For hot rolling - µ is 0.2

3) STICKY FRICTION SITUATION
Continuing with analogy with compression in plane strain,

p = σ0’(Lp + Δh)/Δh
P = pbLp
P = σ0’b(RΔh)0.5 [(RΔh)0.5/4h) + 1]

REFERENCES
1.
2.
3.
4.
5.
6.

Alexandria University - Faculty of Engineering, Report: Metal Rolling.
Metal Forming Processes, Dr. Pulak M. Pandey.
Rolling of metals, Suranaree University of Technology, 2007
“Manufacturing Engineering and Technology”, Kalpakjian & Schmid, 2010
Metal Rolling, library of manufacturing.
Wikipedia

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