Rolling

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Rolling
FLAT ROLLING
Def: Rolling is the process of reducing the thickness or
changing the cross section of a long workpiece by
compressive forces applied through a set of rolls.
It is the first process in converting a cast material (ingot)
into a finished wrought product.
90% of products produced by metal working processes
The initial breaking down of an ingot or of a continuously
cast slab is done by hot rolling, which converts the cast
structure to a wrought structure.
Wrought structure has finer grains and enhanced
ductility, both resulting from the breaking up of brittle
grain boundaries and the closing up of defects, especially
porosity
The work is subjected to high compressive stresses from
squeezing action of rolls

The frictional force between metal and rolls is
responsible for drawing the metal into the rolls.
The initial material form for
rolling is an ingot
Slabs, Billets and Bloom are
known as semi-finished products
because they are subsequently
formed into other mill products.
slab
ROLLED PRODUCTS
i) Bloom:
 First breakdown product of ingots
 Generally, square in cross-section.
 Ex: I-beams & railroad rails
ii) Billet:
 Further reduction of bloom by rolling results in a billet.
 Cross sectional area smaller than blooms
 Ex: round rods and bars (wire drawing-wire rods)
iii) Slab:
 A hot rolled ingot with a width to thickness ratio of 2:1 at
least.
iv) Plate:
 It has thickness greater than 6mm
Ex: Ship hulls,boilers,bridges

v) Sheet and Strip:
 Generally have thickness lesser than 6mm
Ex:Automobile & aircraft bodies, appliances

vi) Foil:
 Thickness is very small around 0.008mm
Ex: Aluminium foil to wrap candy & cigarettes, thickness
0.008mm
 Plates: > 6 mm
 Sheets: < 6 mm
 Foil: ~ 0.008 mm
Ingot
Flat-Rolling and Shape-Rolling Processes
Flat-Rolling and Shape-Rolling Processes
Flat-Rolling and Shape-Rolling Processes
Typical Products by Rolling
Classification of Rolling Process

i) Hot Rolling
ii) Cold Rolling
iii) Powder Rolling
Effects of Hot Rolling
Hot rolling is an effective way to reduce grain size in metals for improved
strength and ductility.
Cast structures of ingots or continuous castings are converted to a wrought
structure by hot working.
 Hot rolling temp of Al-450
0
C, alloy steels -1250
0
C
The product of the first hot-rolling operation is called
Bloom or Slab.
Cold rolling is carried out at room temp. and compared
to hot rolling, produces sheet and strip with much better
surface finish, dimensional tolerances, and mechanical
properties because of strain hardening
Hot Rolling:

Hot working - above the re-crystallization temperature
It is employed where large reduction in cross-sectional area is
required.

Used for bars, rods, rails etc.
Cold Rolling:

Cold working - below the re-crystallization temperature.

Employed for finishing the metal to given specification of
sizes and surface quality.

Produces sheets, strips and foils with good surface finish and
increased mechanical strength.
Powder Rolling:
Metal powder is introduced between the rolls and
turned into a “green strip” which is subsequently sintered
to high density.

This produces a tough sheet with very fine grain size
Rolling Process:
 In the process of rolling the billet is fed from one side into the
rolls and reduces in thickness according to the section and gap
between the rolls.
 The reduction ratios have maximum allowable values for
different metals beyond which reduction is not practicable in a
single process.
 Rolling sets in series may be employed.

There are fundamentally two types of rolls:

i) Supporting rolls
ii) Driving rolls

ROLLING MILLS:
 A rolling mill consists basically of rolls, bearings, a
housing for containing these parts, and a drive for
applying power to the rolls and controlling there speeds.

 Rolling mills can be conventionally classified with
respect to the number and arrangement of the rolls.
Classification of rolling mills:
i) Two-high mills
ii) Two-high reversing mills
iii) Three-high mills
iv) Four-high mills
v) Cluster mills(Sendzimir mills)
vi) Continuous mills(Tandem mill)
vii) Planetary mills
Two-high mill: Two rolls revolving at same speed but in
opposite direction

The rolls are supported on bearings housed in sturdy
upright side frames called stands

The space between the rolls can be adjusted by raising or
lowering the upper roll.

In pull over type, direction of rotation is fixed and can’t
be reversed.
Three-high mill: The direction of rotation of the upper
and lower rolls are the same but the intermediate roll
rotates in a direction opposite to both of these.
Four high mill: The smaller size rolls are known as
working rolls which concentrate the total rolling pressure
over the work piece.

The larger diameter rolls are called back-up rolls and
their main function is to prevent the deflection of the
smaller rolls, which otherwise would result in thickening
of rolled plates or sheets at the centre.

Common products are hot or cold rolled plates and
sheets.
Cluster or Sendzimir roll mill: Each of the working
roll is backup by 2 or more larger backup rolls for
rolling hard materials.

It may be necessary to employ work rolls of very
small diameter but of considerable length.
Or Tandem rolling
Tandem rolling mill or continuous rolling mill
A tandem mill is a special type of modern rolling mill
where rolling is done in one pass. In a traditional
rolling mill rolling is done in several passes, but in
tandem mill there are several stands (>=2 stands) and
reductions take place successively. The number of
stands ranges from 2 to 18. Tandem mills can be either
of hot or cold rolling mill types.
Tandem rolling
Planetary Rolling Mill
Schematic illustration of various roll arrangements:
(a) two-high; (b) three- high; (c) fourhigh;
(d) cluster (Sendzimir) mill
Type of Rolling
Shape Rolling Flat Rolling
Roll Forging
Ring Rolling
Shape Rolling
Stages in the
shape rolling of an
H-section part.
Various other
structural
sections, such as
channels and
I-beams, are also
rolled by this kind
of process.
Roll gap, Roll bite condition, Force, Torque, Maximum reduction
Neutral point: At a certain point along the arc of contact strip velocity and roll
velocity are the same. It is known as neutral point, or no slip point.

To the left of the neutral point, roll moves faster than the work piece, and to right
the work piece moves faster than the roll
Schematic illustration of the flat-rolling process
(b) Friction forces acting on strip surfaces in opposite direction

 Neutral Point:
The location where
To the left,
To the right,
workpiece roll
V V =
workpiece roll
V V >
workpiece roll
V V <
Mechanics of Rolling cont..
The width b of the strip usually increases during rolling
a
R-a
R
L
R
2
= L
2
+(R-a)
2
a is much smaller than R, So a
2
is ignored
 Force:

 Torque:
 Power:


A
B
2
L
F
f
Lw F o =
roll per
L
F T
2
=
( )
) (
500 , 16
) (
000 , 30
2
2 2
rpm ft lb hp
FLN
P or
rpm m N KW
FLN
P or
NFL
N T P
÷ ÷ =
÷ ÷ =
=
- =
t
t
t
t
w
L
σ
Mechanics of Rolling cont..
 True Strain:

 Average Flow Stress:



n
K
n
f
+
=
1
c
o
f
h
h
0
ln = c
R
2
0
h
2
f
h
O
A
B
Mechanics of Rolling cont..

n
K
n
K
u u
u energy specif ic Euivalent
n
K
d K d u energy Specif ic
n
f
f
n
f
n
n
+
=
=
+
¬ =
=
+
= = =
+
+
} }
1
1
1
1
0 0
1
c
o
c o
c
c o
c
c c c o
c c
o
c
f
o
f
o
c
Mechanics of Rolling cont..
 Contact Length:

) (
0 f
h h R L ÷ =
R
L
2
0
h
2
f
h
O
A
B
C
) (
) (
2
)
2
(
0
0
2
0
2
0
2
2 2 2
f
f
f f
h h R L
h h R
h h h h
R R
BC AC L
÷ =
÷ =
|
|
.
|


\
|
÷
+
÷
÷ ÷ =
+ =
Mechanics of Rolling cont..
 Frictional Force:
Friction direction
 Draft:

R d
h h d
f
2
max
0
µ =
÷ =
Workpiece
f
h
f
h
0
h
Mechanics of Rolling cont..

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