03 rolling of metals

RRoolllliinngg ooff mmeettaallss
Chapter 3
• Introduction/objectives
• Rolling mills
• Classification of rolling processes
• Hot rolling
• Cold rolling
• Forces and geometry relationships in rolling
• Simplified analysis of rolling load: Rolling variables
• Problems and defects in rolled products
• Rolling-mill control
• Theories of cold rolling
• Theories of hot rolling
• Torque and power
Subjects of interest
Suranaree University of Technology Tapany Udomphol
Jan-Mar 2007

OObbjjeeccttiivveess
• This chapter provides information on different types of metal
rolling processes which can also be divided in to hot and cold
rolling process.
• Mathematical approaches are introduced for the
understanding of load calculation in rolling processes.
• Finally identification of defects occurring during and its
solutions are included.
Jan-Mar 2007

IInnttrroodduuccttiioonn-- DDeeffiinniittiioonn ooff rroolllliinngg pprroocceessss
• Definition of Rolling : The process
of plastically deforming metal by
passing it between rolls.
• Rolling is the most widely used
forming process, which provides high
production and close control of final
product.
• The metal is subjected to high
compressive stresses as a result of
the friction between the rolls and the
www.world-aluminium.org
Rolling process metal surface.
Note: rolling processes can be mainly divided
into 1) hot rolling and 2) cold rolling.
Jan-Mar 2007

IInnttrroodduuccttiioonn-- HHoott aanndd ccoolldd rroolllliinngg pprroocceesssseess
Hot rolling
• The initial breakdown of
ingots into blooms and billets
is generally done by hot-rolling.
This is followed by further hot-rolling
into plate, sheet, rod, bar,
pipe, rail.
Cold rolling
• The cold-rolling of metals has
played a major role in industry by
providing sheet, strip, foil with
good surface finishes and
increased mechanical strength
with close control of product
dimensions.
www.vatech.at
Jan-Mar 2007

Rolled strips
Rollforming machine
Sheet rolling machines
Jan-Mar 2007

Terminology
• Bloom is the product of first breakdown of ingot
(cross sectional area > 230 cm2).
• Billet is the product obtained from a further reduction by hot rolling
(cross sectional area > 40x40 mm2).
• Slab is the hot rolled ingot
(cross sectional area > 100 cm2 and with a width ≥ 2 x thickness).
Suranaree University of Technology Jan-Mar 2007
Semi-finished
products
• Plate is the product with a thickness > 6 mm.
• Sheet is the product with a thickness < 6 mm and width > 600 mm.
• Strip is the product with a thickness < 6 mm and width < 600 mm.
Further
rolling
steps
Mill
products
Bloom Billet Slap
Plate Sheet Strip
Tapany Udomphol

Mill rolls
Mill rolls
Rolls
Suranaree University of Technology Tapany Udomphol Jan-Mar 2007

Ring rolls
• Ring rolls are used for tube rolling,
ring rolling.
• Ring rolls are made of spheroidized
graphite bainitic and pearlitic matrix or
alloy cast steel base.
Cantilever mill roll Tube mill roll Universal roll
Jan-Mar 2007

Typical arrangement of rollers for rolling mills
Two-high mill, pullover
The stock is
returned to the
entrance for
further reduction.
Two-high mill, reversing
The work can be
passed back and forth
through the rolls by
reversing their
direction of rotation.
Three-high mill
Four-high mill
Small-diameter rolls
(less strength &
rigidity) are
supported by
larger-diameter
backup rolls
Cluster mill or
Sendzimir mill
Consist of upper and
lower driven rolls and
a middle roll, which
rotates by friction.
Each of the work
rolls is supported
by two backing
rolls.
Jan-Mar 2007

Continuous rolling
• Use a series of rolling mill and
each set is called a stand.
• The strip will be moving at
different velocities at each
stage in the mill.
σb
A four stand continuous mill or tandem mil.
σf
• The speed of each set of rolls is synchronised so that the input speed of
each stand is equal to the output speed of preceding stand.
• The uncoiler and windup reel not only feed the stock into the rolls and coiling up
the final product but also provide back tension and front tension to the strip.
Jan-Mar 2007

Planetary mill • Consist of a pair of heavy backing rolls
surrounded by a large number of planetary rolls.
• Each planetary roll gives an almost constant
reduction to the slab as it sweeps out a circular
path between the backing rolls and the slab.
• As each pair of planetary rolls ceases to have
contact with the work piece, another pair of rolls
makes contact and repeat that reduction.
• The overall reduction is the summation of a
series of small reductions by each pair of rolls.
Therefore, the planetary mill can hot reduces a
slab directly to strip in one pass through the mill.
• The operation requires feed rolls to introduce the
slab into the mill, and a pair of planishing rolls on
the exit to improve the surface finish.
Jan-Mar 2007

RRoolllliinngg mmiillllss
Rolling mill is a machine or a factory for
shaping metal by passing it through rollers
A rolling mill basically consists of
• rolls
• bearings
• a housing for containing these parts
• a drive (motor) for applying power to the rolls and controlling the speed
• Requires very rigid
construction, large motors
to supply enough
power (MN).
Successive stands of a
large continuous mill
Modern rolling mill
+ Huge capital investment
• skills
• engineering design
• construction
Jan-Mar 2007

Different types of rolling processes
There are different types of rolling processes as listed below;
• Continuous rolling
• Transverse rolling
• Shaped rolling or section rolling
• Ring rolling
• Powder rolling
• Continuous casting and hot rolling
• Thread rolling
Jan-Mar 2007

Conventional hot or cold-rolling
The objective is to decrease the thickness of the metal with an
increase in length and with little increase in width.
• The material in the centre of the sheet
is constrained in the z direction (across
the width of the sheet) and the
constraints of undeformed
shoulders of material on each side of
the rolls prevent extension of the sheet
in the width direction.
• This condition is known as plane
strain. The material therefore gets
longer and not wider.
• Otherwise we would need the width
of a football pitch to roll down a steel
ingot to make tin plate!
σy
σz
σx
Jan-Mar 2007

Transverse rolling
• Using circular wedge rolls.
• Heated bar is cropped to length and
fed in transversely between rolls.
• Rolls are revolved in one direction.

Shaped rolling or section rolling
• A special type of cold rolling in
which flat slap is progressively bent
into complex shapes by passing it
through a series of driven rolls.
• No appreciable change in the
thickness of the metal during this
process.
• Suitable for producing moulded
sections such as irregular shaped
channels and trim.
Jan-Mar 2007

Shaped rolling or section rolling
A variety of sections can be produced by roll forming process using a
series of forming rollers in a continuous method to roll the metal sheet to
a specific shape
Applications:
- construction materials,
- partition beam
- ceiling panel
- roofing panels.
- steel pipe
- automotive parts
- household appliances
- metal furniture,
- door and window frames
- other metal products.
www.formtak.com
A variety of rolled sections
Jan-Mar 2007

Ring rolling
Seamless rings
Jan-Mar 2007

Simulation of ring rolling
www.qcforge.com
www.rz.rwth-aachen.de
Simulation
of ring
rolling
• The donut shape preform is placed
between a free turning inside roll and a
driven outside roll.
• The ring mills make the section thinner
while increasing the ring diameter.
Relative strain www.shape.co.kr in ring rolling
Jan-Mar 2007

Seamless ring rolling
Jan-Mar 2007

Powder rolling
Metal powder is introduced between the rolls and compacted into a
‘green strip’, which is subsequently sintered and subjected to further
hot-working and/or cold working and annealing cycles.
Advantage :
- Cut down the initial hot-ingot breakdown step (reduced capital investment).
- Economical - metal powder is cheaply produced during the extraction process.
- Minimise contamination in hot-rolling.
- Provide fine grain size with a minimum of preferred orientation.
Jan-Mar 2007

Continuous casting and hot rolling
• Metal is melted, cast and hot rolled continuously through a series of
rolling mills within the same process.
• Usually for steel sheet production.
Jan-Mar 2007

Thread rolling
• Dies are pressed against the surface of cylindrical
blank. As the blank rolls against the in-feeding die
faces, the material is displaced to form the roots of
the thread, and the displaced material flows radially
outward to form the thread's crest.
• A blank is fed between two grooved die plates to form
the threads.
• The thread is formed by the axial flow of material in
the work piece. The grain structure of the material is
not cut, but is distorted to follow the thread form.
• Rolled threads are produced in a single pass at
speeds far in excess of those used to cut threads.
• The resultant thread is very much stronger than
a cut thread. It has a greater resistance to
mechanical stress and an increase in fatigue
strength. Also the surface is burnished and work
hardened.
Cut thread and rolled
thread
Jan-Mar 2007

HHoott--rroolllliinngg
• The first hot-working operation for
most steel products is done on the
primary roughing mill (blooming,
slabbing or cogging mills).
• These mills are normally two-high
reversing mills with 0.6-1.4 m diameter
rolls (designated by size).
Plate rolling
www.msm.cam.ac.uk
• The objective is to breakdown the cast ingot into blooms or slabs for
subsequent finishing into bars, plate or sheet.
• In hot-rolling steel, the slabs are heated initially at 1100 -1300 oC. The
temperature in the last finishing stand varies from 700 - 900 oC, but should
be above the upper critical temperature to produce uniform equiaxed
ferrite grains.
Jan-Mar 2007

Example for hot strip mill process
www.nzsteel.co.nz
Oxidation scale is
removed
Mill reverses after each pass (5 or 7)
and the roll gap is reduced each time
Slabs are organised
according to rolling
schedule
Red hot slab 210 mm
thick is ready for
rolling
Slab is reduced to a long strip
approx 25 mm thick
The strip is coiled and uncoiled
to make the tail end lead
Leading edge and tail
The strip is progressively reduced end are removed
to the required thicknesses
Strip is coiled and up ended or
passed through if heavy plate
Plate 12 to 30 mm thick
Coiled steel 1.8 to 12 mm thk
910 mm to 1550 mm wide
Jan-Mar 2007

• Flat plate of large thickness (10-50 mm) is
passed through different set of working
rolls, while each set consecutively reduces
thickness.
www.uksteel.org.uk
Hot rolled coil produced
on strip mill
www.reverecopper.com
Plate rolling
• Hot strip is coiled to reduce its
increasing length due to a reduction of
thickness.
• Reducing the complication of controlling
strips of different speeds due to
different thicknesses. (thinner section
moves faster)
Jan-Mar 2007

CCoolldd--rroolllliinngg
• Cold rolling is carried out under
recrystallisation temperature and
introduces work hardening.
• The starting material for cold-rolled steel
sheet is pickled hot-rolled breakdown coil
from the continuous hot-strip mill.
www.williamsonir.com
• The total reduction achieved by cold-rolling generally will vary from about
50 to 90%.
• The reduction in each stand should be distributed uniformly without falling
much below the maximum reduction for each pass.
• Generally the lowest percentage reduction is taken place in the last pass
to permit better control of flatness, gage, and surface finish.
Jan-Mar 2007

Example for cold strip mill process. www.nzsteel.co.nz
In batches of 9 coils, cold
rolled steel is annealed to
reduce work hardening
The 6 roll configuration enables this
high speed mill to produce steel of
high quality with consistent shape
and flatness
Cold rolling mill
Coils are transferred to
and from the annealing
furnace
The combination mill has a dual
function, cold rolling and single pass
temper rolling
Temper rolling improves the shape
of the strip after its workability has
been improved by annealing
Cold rolled, annealed and
tempered coils are transferred
to the Cold Finishing Section
Cold rolling reduces the thickness
and increases the strength of hot
rolled steel. The surface finish and
shape improve and work hardening
results
Coils up to 40 tones enter on a
conveyor from the pickle line
20 tonne coils of cold rolled
steel are dispatched to
Metal Coating Line
Jan-Mar 2007

CCoolldd--rroolllliinngg
• Cold rolling provide products with
superior surface finish (due to low
temperature no oxide scales)
• Better dimensional tolerances
compared with hot-rolled products due
to less thermal expansion.
• Cold-rolled nonferrous sheet may be produced from
hot-rolled strip, or in the case of certain copper alloys
it is cold-rolled directly from the cast state.
Cold rolled strips
Cold rolled metals are rated as ‘temper’
• Skin rolled : Metal undergoes the least rolling ~ 0.5-1%
harden, still more workable.
• Quarter hard : Higher amount of deformation. Can be bent
normal to rolling direction without fracturing
• Half hard : Can be bent up to 90o.
• Full hard : Metal is compressed by 50% with no cracking.
Can be bent up to 45o.
Jan-Mar 2007

Fundamental concept of metal rolling
1) The arc of contact between the rolls and the
metal is a part of a circle.
2) The coefficient of friction, μ, is constant in
theory, but in reality μ varies along the arc of
contact.
3) The metal is considered to deform
plastically during rolling.
4) The volume of metal is constant before and
after rolling. In practical the volume might
decrease a little bit due to close-up of pores.
5) The velocity of the rolls is assumed to be
constant.
6) The metal only extends in the rolling direction
and no extension in the width of the
material.
7) The cross sectional area normal to the
rolling direction is not distorted.
Assumptions
o R
α
y
vo x
vf
hf ho
Lp
o
x
y
Jan-Mar 2007

Forces and geometrical rreellaattiioonnsshhiippss
iinn rroolllliinngg
o R
α
x
y
vo vf
hf ho
Lp
o
x
y
• A metal sheet with a thickness ho enters
the rolls at the entrance plane xx with a
velocity vo.
• It passes through the roll gap and leaves
the exit plane yy with a reduced thickness
hf and at a velocity vf.
• Given that there is no increase in
width, the vertical compression of the
metal is translated into an elongation in
the rolling direction.
• Since there is no change in metal
volume at a given point per unit time
o o f f throughout the process, therefore bh v = bhv = bh v …Eq.1
Where b is the width of the sheet
v is the velocity at any thickness h intermediate between ho and hf.
Jan-Mar 2007

vo
R
x y
vf
hf ho
x’
y’
From Eq.1
o o f f bh v = bh v
Given that bo = bf
L
o f h = h
t
L
t f
o
Then we have
o o f f v h = v h
v f
= …Eq.2
h
h
o
o
f
v
vo vf
When ho hf , we then have vo vf
The velocity of the sheet must steadily increase
from entrance to exit such that a vertical element
in the sheet remain undistorted.
Jan-Mar 2007

• At only one point along the surface of contact between the roll and the
sheet, two forces act on the metal: 1) a radial force Pr and 2) a tangential
frictional force F.
• If the surface velocity of the roll vr equal to the velocity of the sheet, this
point is called neutral point or no-slip point. For example, point N.
• Between the entrance plane (xx)
and the neutral point the sheet is
moving slower than the roll surface,
and the tangential frictional force,
F, act in the direction (see Fig) to
draw the metal into the roll.
• On the exit side (yy) of the neutral
point, the sheet moves faster than
the roll surface. The direction of the
frictional fore is then reversed and
oppose the delivery of the sheet
from the rolls.
x
Pr
N point : vroll = vsheet
F
α
θ
β Friction acts in
y
N
x y
opposite directions
Tapany Udomphol

Pr is the radial force, with a vertical
component P (rolling load - the load with
which the rolls press against the metal).
The specific roll pressure, p, is the rolling
load divided by the contact area.
P
p = …Eq.3
p bL
Where b is the width of the sheet.
Lp is the projected length of the arc of contact.
( ) ( ) [ ( )]
 −
= − − 1 2
L R h
R h h
h h
L R h h
p
o f
o f
p o f
≈

≈ −

 

 

1 2
2
4
…Eq.4

• The distribution of roll pressure
along the arc of contact shows that the
pressure rises to a maximum at the
neutral point and then falls off.
• The pressure distribution does not
come to a sharp peak at the neutral
point, which indicates that the neutral
point is not really a line on the roll
surface but an area.
vo
N
R
hf ho
p
A B
Friction hill in rolling
• The area under the curve is
proportional to the rolling load.
• The area in shade represents the
force required to overcome
frictional forces between the roll
and the sheet.
• The area under the dashed line
AB represents the force required to
deform the metal in plane
homogeneous compression.
Jan-Mar 2007

Roll bite condition For the workpiece to enter the throat
of the roll, the component of the
friction force must be equal to or
greater than the horizontal
component of the normal force.
cosα sinα r F ≥ P
α
α
sin ≥ ≥
α
tan
cos
F
r P
But we know
r F = μP
Therefore μ = tanα
Prsineα
α
Pr
α
Fcosα
F
α
The angle of
bite or the angle
of contact
F is a tangential friction force
Pr is radial force
…Eq.5
• If tan α μ, the workpiece cannot be drawn.
• If μ = 0, rolling cannot occur.
Jan-Mar 2007

Therefore Free engagement will occur when μ tan α
Increase the effective values of μ
, for example grooving the rolls
parallel to the roll axis.
Using big rolls to reduce tan α or
if the roll diameter is fixed, reduce
the ho
+
α1
α1αααα α2 2
Jan-Mar 2007

From triangle ABC, we have
= + −
2 2 2
R L R a
( )
p
= − − +
2 2 2 2
L R R Ra a
( 2 )
p
= −
2 2
L 2
Ra a
p
As a is much smaller than R, we
can then ignore a2.
L Ra R h p ≈ 2 ≈

h = ho – hf = 2a
…Eq.6
A
α
a
h
R
R-a
B C
Lp
v D o
Lp

R h
ho
The critical variables are
Lp and h
A large diameter roll will permit a
thicker slab to enter the rolls than will
a small-diameter roll.
R
R h
R h
≈
−

= μ
The maximum reduction
Tapany Udomphol

Problem with roll flattening
When high forces generated in rolling are transmitted to the workpiece through
the rolls, there are two major types of elastic distortions:
1) The rolls tends to bend along their length because the workpiece tends to
separate them while they are restrained at their ends. thickness
variation.
2) The rolls flatten in the region where they contact the workpiece. The radius
of the curvature is increased R R’. (roll flattening)
According to analysis by Hitchcock,

 

 
CP
−
= +
( )
1
'
'
o f b h h
R R
Where C = 16(1-ν2)/πE = 2.16 x 10-11 Pa-1 for steel rolls.
Rollling
R R ‘
P’ = rolling load based on the deformed roll radius.
Roll flattening
Jan-Mar 2007

Example: Determine the maximum possible reduction for cold-rolling
a 300 mm-thick slab when μ = 0.08 and the roll diameter is 600
mm. What is the maximum reduction on the same mill for hot rolling
when μ = 0.5?
From Eq.7, ( h) R 2
max

= μ
( h) (0.08) (300) 1.92mm 2
max For cold-rolling

= =
For hot-rolling ( h) (0.5) (300) 75mm 2
max

= =
Alternatively, we can use the relationship below
( )
Lp
α = = α =
− μ

h =
mm
R h
R
1.92
Jan-Mar 2007

Simplified analysis ooff rroolllliinngg llooaadd
The main variables in rolling are:
• The roll diameter.
• The deformation resistance of the metal as influenced by metallurgy,
temperature and strain rate.
• The friction between the rolls and the workpiece.
• The presence of the front tension and/or back tension in the plane of the
sheet.
We consider in three conditions:
1) No friction condition
2) Normal friction condition
3) Sticky friction condition
Jan-Mar 2007

1) No friction situation
In the case of no friction situation, the rolling load (P)
is given by the roll pressure (p) times the area of contact
between the metal and the rolls (bLp).
P pbL b R h p o = =σ '

Where the roll pressure (p) is the yield stress in plane strain
when there is no change in the width (b) of the sheet.
…Eq.8
Jan-Mar 2007

2) Normal friction situation
In the normal case of friction situation in plane strain, the average
pressure p can be calculated as.
_
= Q −
( 1)
1
_ '
o
e
Q
p
σ
…Eq.9
Where Q = μLp/h
h = the mean thickness between entry and exit from the rolls.
From Eq.8,
_
=
p P pbL

( ) 

1
2 _σ
We have …Eq.10
= e − b R

h
P Q
o 1

Q
3
Roll diameter Rolling load
Jan-Mar 2007

•Therefore the rolling load P increases with the roll radius R1/2,
depending on the contribution from the friction hill.
• The rolling load also increases as the sheet entering the rolls
becomes thinner (due to the term eQ).
• At one point, no further reduction in thickness can be achieved if
the deformation resistance of the sheet is greater than the roll
pressure. The rolls in contact with the sheet are both severely
elastically deformed.
• 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 of the sheet is possible.
Backup rolls
Example: the rolling of aluminium cooking foil.
Roll diameter 10 mm with as many as 18
backing rolls.
Jan-Mar 2007

• Frictional force is needed to
pull the metal into the rolls and
responsible for a large portion
of the rolling load.
• High friction results in high rolling load, a steep friction hill and great
tendency for edge cracking.
• The friction varies from point to point along the contact arc of the roll.
However it is very difficult to measure this variation in μ, all theory of
rolling are forced to assume a constant coefficient of friction.
• For cold-rolling with lubricants, μ ~ 0.05 – 0.10.
• For hot-rolling , μ ~ 0.2 up to sticky condition.
Jan-Mar 2007

Example: Calculate the rolling load if steel sheet is hot rolled 30% from
a 40 mm-thick slab using a 900 mm-diameter roll. The slab is 760 mm wide.
Assume μ = 0.30. The plane-strain flow stress is 140 MPa at entrance and
200 MPa at the exit from the roll gap due to the increasing velocity.
x
−
−
(40) ( h
)
h h
o f
=
h mm
x
=
=
h h h mm
(40) (28)
(0.30) 450 12
R h
μ L
p μ
Q _ _ = =
140 200
' '
' =

=σ −

P Q
e b R h
Q
o
1


= −

h
o f
f
f
o
(40) (28) 12
28
100 30
(40)
100 30%

= − = − =
mm
h h
h o f 34
2
2
_
=
+
=
+
=
0.65
(34)

= = x
h
h
MPa
entrance exit
o 170
2
2
+
=
+
=
σ σ
σ
P (e 1 ) (0.76) 0.45 x 0.012 13.4
MN
(0.65)
170
( 1)
1
0.65
'
= 



From Eq.10
Tapany Udomphol

3) Sticky friction situation
What would be the rolling load if sticky friction occurs?
Continuing the analogy with compression in plane strain
L
=  + 
1
p p
+ = 
σ o 1
σ
'
0
a
2 _
_
=
h
P pbLp
From Eq.8,

 


 


4
'
_
h
From example;
=σ
'
P o
P

R h
_
h

 
1
4

+
0.45 0.012


 

= +
4 0.034

 
170
=

P MN

 
1 (0.76) 0.45 x
0.012
x
x
b R h
14.6

Jan-Mar 2007

Example: The previous example neglected the influence of roll flattening
under very high rolling loads. If the deformed radius R’ of a roll under load is given
in Eq.11, using C = 2.16x10-11 Pa-1, P’ =13.4 MPa from previous example.

CP
'
' 1 …Eq.11
( ) 
  
−
= +
o f b h h
R R
Where C = 16(1-ν2)/πE ,P’ = Rolling load
based on the deformed roll radius.
( )
m
−
11 6
2.16 x 10 13.4 x
10


' = +
= 
R 0.464
0.76 x
0.012
0.45 1

We now use R’ to calculate a
new value of P’ and in turn
another value of R’
0.30 464 12
= =
34
( )
x

R h
_
h
Q
=
μ
1
=  −
0.66
'' 0.66

= 
P e x MN
1 0.76 0.464 0.012 13.7
m
−
11 6
x x
0.66
2.16 10 (13.7 10 )
x
R
0.465
0.76 0.012
170


= +
0.45 1
''

= 

The difference between
the two estimations of R’
is not large, so we stop
the calculation at this
point.
Jan-Mar 2007

Relationship of μ, rolling load and torque
• We have known that the location of the neutral
point N is where the direction of the friction force
changes.
• If back tension is applied gradually to the sheet,
the neutral point N shifts toward the exit plane.
• The total rolling load P and torque MT (per unit
of width b) is given by
x
N point : vroll = vsheet
Pr
F N
α
β
θ
y
Friction acts in
opposite
directions
Lp
x y
pdx
p p
( )
L
p
∫ ∫
∫
0
= = =
M
L
o
T
PR
P
b
M
=
T
b
thus
P
b
L
μ μ μ
pdx R R pdx R
o
=
μ
Where μ is obtained by
measuring the torque and the
rolling load at constant roll speed
and reduction with the proper
back tension.
Jan-Mar 2007

Back and front tteennssiioonnss iinn sshheeeett
• The presence of back and front
tensions in the plane of the sheet
reduces the rolling load.
• Back tension may be produced
by controlling the speed of the
uncoiler relative to the roll speed.
Back tension, σb Front tension, σf
Uncoiler Coiler
p
σ’
o σ’
o
σ’
o- σb
σ’
o- σf
• Front tension may be created
by controlling the coiler.
• Back tension is ~ twice as
effective in reducing the rolling
load P as front tension.
• The effect of sheet tension on
reducing rolling pressure p can be
shown simply by
_
2
p =σ '
o −σ h = σ o −σ
h 3
…Eq.11
• If a high enough back tension is applied,
the neutral point moves toward the roll exit
– rolls are moving faster than the metal.
• If the front tension is used, the neutral
point will move toward the roll entrance.
Where σh = horizontal sheet tension.
Jan-Mar 2007

Problems and ddeeffeeccttss iinn rroolllleedd
pprroodduuccttss
Defects from cast ingot before rolling
Defects other than cracks can result from defects introduced during the
ingot stage of production.
• Porosity, cavity, blow hole occurred in the cast ingot will be closed up
during the rolling process.
• Longitudinal stringers of non-metallic inclusions or pearlite banding
are related to melting and solidification practices. In severe cases, these
defects can lead to laminations which drastically reduce the strength in the
thickness direction.
Jan-Mar 2007

Defects during rolling
There are two aspects to the problem of the shape of a sheet.
1) Uniform thickness over the width and thickness – can be precisely
controlled with modern gage control system.
h h
2) Flatness – difficult to measure accurately.
Jan-Mar 2007

Uniform thickness
• Under high rolling forces, the rolls flatten
and bend, and the entire mill is elastically
distorted.
• Mill spring causes the thickness of the
sheet exiting from the rolling mill to be
greater than the roll gap set under no-load
conditions.
• Precise thickness rolling requires the
elastic constant of the mill. Calibration
curves are needed, see Fig.
(1–3 GNm-1 for screw-loaded rolling mills, 4
GNm-1 for hydraulically loaded mills).

• Roll flattening increases the roll pressure and eventually causes
the rolls to deform more easily than the metal.
•The limiting thickness is nearly proportional to μ, R, σ’
o but
inversely proportional to E.
For example in steel rolls the limiting thickness is given by
_ '
μ R σ
o
= …Eq.12
12.8
min
h
In general, problems with limiting gauge can be expected when the
sheet thickness is below 1/400 to 1/600 of the roll diameter.
Jan-Mar 2007

Flatness
• The roll gap must be perfectly parallel to produce sheets/plates
with equal thickness at both ends.
• The rolling speed is very sensitive to flatness. A difference in
elongation of one part in 10,000 between different locations in the
sheet can cause waviness.
Rolling
direction
Perfectly flat
More elongated
along edges
More elongated in
the centre
Tapany Udomphol

Solutions to flatness problems
• Camber and crown can be used to correct the roll deflection (at only one
value of the roll force). Or use rolling mill equipped with hydraulic jacks to
permit the elastic distortion of the rolls to correct deflection.
(a) (b)
(a) The use of cambered rolls to compensate for roll bending.
(b) Uncambered rolls give variation of thickness.
Jan-Mar 2007

• Hot mill can be provided with
facilities for crown control to
improve the control of the
profile of hot strip mill.
• For example work roll
bending with continuous
variable crown and pair cross
mills.
•The roll cross angle of rolls incorporated
in a stand of each rolling mill is set at a
predetermined value beforehand.
• If there is a roll cross angle that will
enable a target sheet crown to be applied
to each sheet and the roll bender load of
each stand is adjusted on-line, thereby
effecting sheet crown control.

Possible effects when rolling with insufficient camber
(a) (b)
(c)
(d)
(e)
• Thicker centre means the edges would be plastically elongated more
than the centre, normally called long edges.
• This induces the residual stress pattern of compression at the edges
and tension along the centreline.
• This can cause centreline cracking (c), warping (d) or edge
wrinkling or crepe-paper effect or wavy edge (e).
Jan-Mar 2007

03 rolling of metals

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