Rolling Process

Rolling
Rolling is a metal forming process in which the thickness of the work is reduced by
compressive forces exerted by two rolls rotating in opposite direction. Flat rolling is
shown in figure

Flat rolling

Rolling is the plastic deformation of materials caused by compressive force applied

through a set of rolls. The cross section of the work piece is reduced by the process.
The material gets squeezed between a pair of rolls, as a result of which the thickness
gets reduced and the length gets increased.
Important terminologies:
Bloom: It has a square cross section 150 mm x 150 mm or more. Blooms are rolled
into structural shapes like rails for railroad tracks.

Slab: It is rolled from an ingot or a bloom and has a rectangular cross section of 250
mm width or more and thickness 40 mm or more. Slabs are rolled into plates,
sheets, and strips. Hot rolled plates are generally used in shipbuilding, bridges,
boilers, welded structures for various heavy machines, and many other products.

Billet: It is rolled from a bloom and is square in cross-section with dimensions

40mm on a side or more. Billets are rolled into bars, rods. They become raw
materials for machining, wire drawing, forging, extrusion etc.
Plates have thickness greater than 6 mm where as strips and sheets have less than 6
mm thickness. Sheets have greater width and strip has lower width – less than 600
mm.
➢Mostly, rolling is done at high temperature, called hot rolling because of
requirement of large deformations.
➢Hot rolling results in residual stress-free product. However, scaling is a major
problem, due to which dimensional accuracy is not maintained.
➢Cold rolling of sheets, foils etc is gaining importance, due to high accuracy and
lack of oxide scaling.
➢Cold rolling also strengthens the product due to work hardening.
 Rolling mills
Two high rolling mill:

This type of rolling mill consists of two rolls rotating in opposite directions.
Roll diameters: 0.6 to 1.4 m
Types: Either reversing or non-reversing.
Non-reversing mill: Rolls rotate only in one direction, and the slab always move
from entry to exit side.
Reversing mill: Direction of roll rotation is reversed, after each pass, so that the
slab can be passed through in both the directions. This permits a continuous
reductions to be made through the same pairs of rolls.
Three high rolling mill:

➢In this case, there are three rolls one above the
other.
➢At a time, for single pass, two rolls will be
used.
➢The roll direction will not be changed in this
case.
➢The top two rolls will be used for first
reduction and the sheet is shifted to the bottom
two rolls and further reduction is done.
➢This cycle is continued till actual reduction is
attained.
➢Disadvantage: automated mechanism is
required to shift the slab
Four high rolling mill:
➢This consists of two small rolls for
thickness reduction and two large backing
rolls to support the small rolls.
➢The small rolls will reduce the roll force
required as the roll-sheet contact area will
be reduced.
➢The large backing rolls are required to
reduce the elastic deflection of small rolls
when sheet passes between them
Cluster rolling mill:

For very thin sheets, small diameter work rolls need to be supported by
more back up rolls. The cluster mill may have 6 or 12 rolls.
Tandem rolling mill (Continuous Rolling Mill)

➢For high production rates, a series of rolling mills are installed in tandem form.
Each set of rolls is called a stand.
➢This consists of series of rolling stations of the order of 8 to 10.
➢In each station, thickness reduction is given to the sheet.
➢With each rolling station, the work velocity increases.
➢This is fully used in industry practice, along with continuous casting operation.
➢This results in reduction in floor space, shorter manufacturing lead time.
This technique was recently adopted for thermo-mechanical rolling, which
involves control of both thermal and mechanical treatment through the rolling
stand for the purpose of producing thin gauge steel sheets with high strength
and high toughness/ductility (HSLA steels)
Thread Rolling

➢Thread rolling is used to create threads on cylindrical parts by rolling

them between two dies as shown in figure.
➢It is used for mass production of external threaded parts like bolts
and screws.
➢ Thread rolling is a high productivity process involving no loss of
materials. Due to grain flow in thread rolling strength is increased.
➢Surface finish of rolled threads is very good.
➢Gears can also be produced by the thread rolling process.
➢Compressive stresses introduced during the process is favorable for fatigue
applications.
➢Auto power transmission gears are made by thread rolling.
Ring Rolling

➢Ring rolling is a forming process in which a thick walled ring part of smaller
diameter is rolled into a thin walled ring of larger diameter.
➢As the thick walled ring is compressed, the deformed material elongates,
making the diameter of the ring to be enlarged.
➢In this process, two circular rolls, one of which is idler roll and the other is driven
roll are used.
➢A pair of edging rollers are used for maintaining the height constant.
➢The ring is rotated and the rings are moved closer to each other, thereby
reducing the thickness of ring and increasing its diameter.
➢Rings of different cross-sections can be produced.
➢The major merits of this process are high productivity, material saving,
dimensional accuracy and grain flow which is advantageous.

Applications: Large rings for turbines, roller bearing races,

flanges and rings for pipes, steel tires for railroad wheels.
 Tube Piercing
Rotary tube piercing is used for producing long thick walled tubes. Cavity forms at
the center due to tensile stress, in a round rod when subjected to external
compressive stress – especially cyclic compressive stress.

Mannesmann Mill

The Mannesmann process makes use of a tube piercing in rotary mode. A pair of
skewed rolls are used for drawing the work piece inside the rolls. The roll axes are
oriented at 6 degrees with reference to axis of work piece. A mandrel is used for
expanding the central hole, and sizing the inner diameter. Pilger mill uses
reciprocating motion of both work and mandrel to produce tubes. Work is
periodically rotated additionally.
 Analysis of flat strip rolling
Geometric Relations

Consider the rolling of a strip of initial thickness ho,

between a pair of rolls of radius R. The rolls are
rotating in same direction. The strip is reduced in
thickness to hf with width of the strip assumed to
remain constant during rolling – because width is
much larger than thickness. Flat rolling is a plane
strain compression process.

Draft refers to reduction in thickness.

Draft = ho-hf
Reduction (R) is the ratio of thickness reduction

R = (ho-hf)/ ho = 1 - hf/ ho
If the change in width of the strip is taken into consideration, we can find the final
width by applying the volume constancy principle.
Volume of material before rolling = volume after rolling.
 Velocity variation in rolling process
The velocity of the strip increases from Vo to Vf
as it passes through the rolls. This velocity
increase takes place in order to satisfy the
principle of volume constancy of the billet
during the deformation process.

w is width of the strip, which is

assumed to be constant during rolling.
➢The strip velocity increases during rolling, as it passes between the rolls. At
some section the velocity of rolls and strip velocity are equal. This point is called
neutral point. Ahead of neutral point, the strip is trailing behind the rolls. Beyond
the neutral point the strip leads the rolls.

➢Frictional shear stress τ acts tangential to the rolls at any section along the arc
of contact between rolls and strip. However, the direction of τ reverses at the
neutral point. Between the entry section of the roll gap and the neutral section,
the direction of friction is the same as the direction of motion of the strip – into
the roll gap. Therefore, the friction aids in pulling the strip into the rolls in this
part of the travel.
➢The direction of friction reverses after the neutral point, as the velocity of strip
is higher than the velocity of the rolls.
➢Friction force opposes the forward motion of the strip in sections beyond the
neutral section.
➢However, the magnitude of the friction acting ahead of neutral section is greater
than that beyond the neutral section.

➢Therefore, the net friction is acting along the direction of the strip movement,
thereby aiding the pulling of the strip into the roll gap.

The forward slip is defined as the difference in velocity between the strip at
exit and roll divided by roll velocity.

At roll exit the forward slip is positive, meaning that the work piece moves faster than roll
here.
The projected arc length [Lp], which is the
length of the straight line got by
projecting the arc of contact onto a
horizontal line or plane.
Limiting condition for friction between roll and work

The roll exerts a normal pressure p on the

work. This pressure may be imagined to be
the pressure exerted by the work piece on
the rolls to separating them. Due to the
roll pressure a tangential friction shear
stress is induced at the interface contact
between roll and work piece.

This friction stress can be written as:

Sliding friction is assumed between roll and work. At the entry section, if the forces acting
on the strip are balanced, we get:
If the work piece is to be pulled into the rolls at entry section, the following condition is to be
satisfied:

The minimum condition for work to be pulled into the rolls can be written as: μ = tanα

If the tangent of angle of bite exceeds the coefficient of friction, the work piece will not
be drawn into the roll gap
α = 0 indicates rolling

From geometry of the roll-strip contact, we can write

It can be inferred from the above equation that for the same angle of bite [same
friction condition], a larger roll will enable thicker slab to be drawn into the roll gap.
This is because for large radius roll the arc length is larger, and hence Lp is larger.
Decreasing the roll radius reduces the maximum achievable reduction in thickness
of strip.

It can also be concluded that higher coefficient of friction can allow larger
thickness of the strip to be drawn into the roll throat.
Longitudinal grooves are made on the roll surface in order to increase
friction. This enables the breakdown of large thickness ingots during hot
rolling.
The parameters which influence the rolling process are: roll diameter, friction,
material flow stress, temperature of working etc.

Without considering friction, we can get the rolling load, approximately, from
the material flow stress and the area of contact between roll and strip.

Rolling load F is now written as:

From the above equation, it is observed that the roll force increases with increase in
roll radius or increase in reduction of thickness of the strip

Rolling load
Alternatively, we can write the average flow stress based on true strain during rolling. For
a material which obeys power law relation between plastic stress and strain, in the form:

The true strain in rolling is given as:

Roll force:

The above equation is based on the assumption that the material work hardens.
In cold rolling, the work material gets work hardened considerably. Therefore,
the above equation is more appropriate for cold rolling.
The mean flow stress is determined from plane strain compression test. It is
assumed that the rolls do not undergo elastic deformation.

The Slab analysis of strip rolling with friction – another approximate method
for load calculation will be discussed separately in class.
Roll deflection and roll flattening

Due to roll force, the rolls are subjected to deflection and they bend resulting in
larger thickness at the centre of the rolled sheet and the edge being thinner.
This defect is known as crown and camber.
In order to avoid this rolls are given a slight curvature on surface by grinding so
that the centre of the rolls has higher diameter than the edges. This is called
cambering of rolls. The bulged rolls, when subjected to bending during rolling
will produce flat sheets. For sheet rolling, normally camber of 0.5 mm on roll
diameter is provided.

Roll bending
Also during hot rolling, rolls get heated up and bulge out at the center, causing
camber of the rolls. This is due to temperature variation between edges and the
center of rolls.
Roll camber has to be varied during rolling in order to take care of roll camber
due to both thermal effects and roll deflection. This also avoids uneven roll
wear – rolls wear more at edges than at center.

Roll camber can be varied by:

[1] bending the work rolls by applying external force.
[2] Shifting of work rolls laterally with respect to centerline of the strip,
[3] using shaped rolls – rolls with profiles,
[4] Rotation of the axis of the work roll with respect to axis of backup roll in
horizontal plane – results in deflection of work roll ends, producing camber.
Roll flattening:
There is increase in radius of curvature of rolls due to the roll pressure which
causes elastic deformation of rolls. This is known as roll flattening. Roll flattening
leads to increase in contact length and hence an increase in roll force.

The distorted roll radius is given by:

P’ is roll pressure with flattened roll. Higher the Young’s modulus of the roll
material, the lower is roll flattening.
 Spread

Spread refers to the increase in width of rolled strips of low width to thickness
ratios – square sectioned strips for example. Reducing the friction, increasing the
roll radius to strip thickness ratio and using wider strips can reduce the roll
spread.
The spread given by wo=wf is given as:

A pair of vertical rolls called edger rolls can be used to reduce spread.
Lubrication:

➢Oils, soap emulsions, fatty acids are used as lubricants during hot
rolling non-ferrous metals.
➢Mineral oils, paraffin, fatty acids are used for cold rolling.
➢Normally, for ferrous alloys no lubricant is used.
Rolling defects
Mill spring is a defect in which the rolled sheet is thicker than the required
thickness because, the rolls get deflected by high rolling forces. Elastic deformation
of the mill takes place. If we use stiffer rolls, namely roll material of high stiffness
or elastic constant, we could avoid mill spring. Normally elastic constant for mills
may range from 1 to 4 GN/m.

Roll elastic deformation may result in uneven sheet thickness across. Roll
material should have high elastic modulus for reducing the roll deformation. For
producing very thin gage sheets like foils, small diameter rolls are used. They are
supported with larger rolls. The minimum thickness of rolled sheets achieved is
directly proportional to roll radius, friction, flow stress
 Flatness of rolled sheets depends on the roll deflection. Sheets become wavy as
roll deflection occurs.
Wavy edge and zipper cracks

Wavy edge Zipper cracks

If rolls are elastically deflected, the rolled sheets become thin along the edge, whereas at
centre, the thickness is higher. Similarly, deflected rolls result in longer edges than the
centre. Edges of the sheet elongate more than the centre. Due to continuity of the sheet,
the centre is subjected to tension, while edges are subjected to compression. This leads
to waviness along edges.
Along the centre zipper cracks occur due to high tensile stress there. Cambering of rolls
can prevent such defects. However, one camber works out only for a particular roll force.
Centre crack and edge cracks

Centre crack Edge crack

During rolling, the sheet will have a tendency to deform in lateral direction.
Friction is high at the centre. Therefore, spread is the least at the centre. This
leads to rounding of ends of the sheet. The edges of the sheet are subjected to
tensile deformation . This leads to edge cracks. If the center of the sheet is
severely restrained and subjected to excess tensile stress, center split may
happen.
Non-homogeneous material deformation across the thickness leads to high
secondary tensile stress along edge. This leads to edge cracks. Secondary tensile
stresses is due to bulging of free surface.
Edge cracks can be avoided by using edge rolls.

Due to non homogeneous flow of material across the thickness of the sheet,
another defect called alligatoring occurs. This is due to the fact that the surface
is subjected to tensile deformation and centre to compressive deformation. This
is because greater spread of material occurs at center.

Bowing of the sheet:

This is due to nonparallelism of the roll gap. If one edge of the sheet is smaller
in thickness than other, the length at the smaller edge will be linger
Centre buckle:
Which results when the edges of the sheet are restricted relative to the central
parts. This effect is opposite to the out-of-flatness

Defects due to lateral spread:

High friction hill with maximum pressure at the sheet centre, which suggests
higher tendency of the edge parts to spread laterally, with less elongation in the
length direction. This may develop slight rounding at the ends of the sheet.
Alternatively, edge cracking may arise.