Rolling mill
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Introduction to Rolling
Rolling is a bulk deformation process in
which the thickness of the work is reduced
by compressive forces exerted by two
opposing rolls. The rolls rotate to pull and
simultaneously squeeze the work between
them.
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Introduction to Rolling
The basic process shown in the previous
figure is “Flat Rolling”, used to reduce the
thickness of a rectangular cross section. A
closely related process is “shape rolling”,
in which a square cross section is formed
into a shape such as an I-beam.
Shape Rolling
Flat Rolling
Shape Rolling
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Introduction to Rolling
After casting, ingots are rolled into one of three intermediate
shapes called blooms, billets, and slabs:
Blooms have square cross section 6” x 6” or larger. They
are rolled into structural shapes.
Billets have square cross section 1.5” x 1.5” or larger.
they are rolled into bars and rods.
Slabs have rectangular cross section 10” x 1.5” or larger.
They are rolled into plates, sheets and strips.
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Introduction to Rolling
•As any other metal forming process, rolling can
be performed hot (hot rolling) or cold (cold
rolling).
•Most rolling is carried out by hot rolling, owing to
the large amount of deformation required.
•Hot-rolled metal is generally free of residual
stresses, and has isotropic properties. On the
other hand, it does not have close dimensional
tolerances, and the surface has a characteristic
oxide scale. Moreover, cold rolled metals are
stronger.
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Types of Rolling
Based on work piece geometry :
Flat rolling - used to reduce thickness of a
rectangular cross section
Shape rolling - square cross section is formed
into a shape such as an I-beam
Based on work temperature :
Hot Rolling – most common due to the large
amount of deformation required
Cold rolling – produces finished sheet and
plate stock
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The Rolls
Rotating rolls perform two main functions:
•Pull the work into the gap between them by
friction between work part and rolls.
•Simultaneously squeeze the work to reduce its
cross section.
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Roll configurations in rolling mills
Two High Rolling Mill.
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Roll configurations in rolling mills
Three High Rolling Mill.
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Roll configurations in rolling mills
Four High Rolling Mill.
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Roll configurations in rolling mills
Multiple backing rolls allow even smaller roll diameters
Cluster Rolling Mill.
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Roll configurations in rolling mills
A series of rolling stands in sequence
Tandem Rolling Mill.
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Production steps in rolling
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Change in grains structure in rolling
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Rolling analysis
• In flat rolling, the work is squeezed between two
rolls so that its thickness is reduced by an amount
called the draft:
d = to - t f
where
d: draft
to: starting thickness
tf : final thickness
As a fraction of the starting thickness:
% reduction = % r = (d/ to) * 100%
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Rolling analysis
Rolling increases work width. This is called
“spreading”.
Spreading is expected because of the volume
constancy in plastic deformation. Since the
material is compressed in the thickness direction,
both the length and width will increase provided
that the material is not constrained in the width
direction.
Spreading is more pronounced with low widthto-thickness ratios and low coefficients of friction,
since there is small resistance to flow in the width
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direction.
Rolling analysis
The width-to-thickness ratio can be calculated as
follows:
w/t Ratio = initial width/ initial thickness
After rolling, percentage spread can be calculated as
follows:
% Spread = (Final width-initial width)/ (initial
width) *100%
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Rolling analysis
To calculate the roll force required to maintain
separation between the two rolls:
F = 1.15 * Yavg, i * Li * wi
where:
F
: roll force
Yavg, i : the average flow stress in the ith pass
Li
: the approximate contact length in the ith pass
wi
: the width of the sheet in the ith pass
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Rolling analysis
The torque in rolling can be estimated by:
T = 0.5 * F * L
Where:
T: Torque (lb.in or N.m)
F: Roll Force
L: Contact length
The Power required to drive the two rolls
is calculated as follows:
P = 2π*N*F*L
Where:
P: Power (in J/s =Watt or in-lb/min)
N: Rolls rotational speed (RPM)
F: Roll Force
Two High Rolling Mill
L: Contact length
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Rolling Defects
Defects in rolling may be either surface or structural
defects:
• Surface defects include scale and roll marks.
• Structural defects (see next figure) include:
1. Wavy edges: bending of the rolls causes the sheet
to be thinner at the edges, which tend to elongate
more. Since the edges are restricted by the
material at the center, they tend to wrinkle and
form wavy edges.
2. Center and edge cracks: caused by low material
ductility and barreling of the edges.
3. Alligatoring: results from inhomogeneous
deformation or defects in the original cast ingots.
• Other defects may includes residual stresses (in
some casesTwo
residual
stresses are desirable).
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Rolling Defects
Structural defects in sheet rolling:
Wavy Edges
Center cracking
Edge cracking
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Alligatoring
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A rolling mill for hot
flat rolling. The
steel plate is seen
as the glowing
strip in lower left
corner (photo
courtesy of
Bethlehem Steel).
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