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Chapter 13: Rolling of Metals
Rolled strips
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Introduction
Rolling is the process of reducing the thickness of a
long work-piece by compressive forces appliedthrough a set of rolls.
Plates, which are generally regarded as having a
thickness greater than 6 mm are used for structuralapplications such as machines structures, boilers.
Sheets are generally less than 6 mm thick; they areprovided to manufacturing facilities as flat pieces.
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Flat-Rolling
and Shape-Rolling Processes
Figure 13.1 Schematic outline of various flat-rolling and shape-rolling processes.Source: After the American Iron and Steel Institute.
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Flat-Rolling Process
Figure 13.2 (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on
strip surfaces. (c) Roll force, F, and the torque, T, acting on the rolls. The width of the strip,
w, usually increases during rolling, as shown later in Fig. 13.5.
A metal strip of thickness enters the roll gap and is reduced to thickness by a
pair of rotating rollseach roll being powered individually by electric motors.
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Flat Rolling
Frictional Forces : The rolls pull the material into the roll
gap through a net frictional force of the material. The
draft, defined as the difference between the initial and
final thickness which is the function of the coefficient of
friction and the roll radius R.
In figure (13.2b) the frictional force is shown.
Roll Force : Because the rolls apply pressure on the
material in order to reduce its thickness, a force
perpendicular to the arc of contact is needed. In figure
(13.2c) the roll force is shown.
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Reducing Roll Force: Roll forces can cause deflection and flattening
of the rolls, which adversely affect the rolling operation.
Roll forces can be reduced by any of the following means:
1. Reducing friction2. Using smaller-diameter rolls, to reduce the contact area
3. Taking smaller reduction per pass, to reduce the contact area
4. Rolling at elevated temperatures, to lower the strength of the
material
5. Another effective method of reducing roll forces is to apply
longitudinal tension to the strip during rolling. Because they
require high roll forces, tensions are important particularly in
rolling high-strength metals. Tensions can be applied to the strip
either at the entry zone (back tension), at the exit zone (front
tension), or both.
Flat Rolling
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Figure 13.3 Schematic illustration of a four-high rolling-mill stand, showing
its various features. The stiffness of the housing, the rolls, and the roll
bearings are all important in controlling and maintaining the thickness of
Flat Rolling
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Flat Rolling
Geometric considerations: Just as a straight beam
deflects under a transverse load, roll forces tend to
bend the rolls elastically during rolling. As expected,
the higher the elastic modulus of the roll material, thesmaller the roll deflection.
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Bending of Rolls:
Figure 13.4 (a) Bending
of straight cylindrical rollscaused by roll forces. (b)
Bending of rolls ground
with camber, producing a
strip with uniform
thickness through the strip
width. Deflections have
been exaggerated forclarity.
Flat Rolling
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Spreading: In rolling plates and sheets with high width-to-thickness ratios, the width of the strip remains effectively constant
during rolling. However, with smaller ratios (such as a strip with a
square cross-section), its width increases significantly as itpasses through the rolls (an effect commonly observed in the
rolling of dough with a rolling pin). This increase in width is called
spreading.
It can be shown that spreading increases with (a) decreasing
width-to-thickness ratio of the entering strip (because of reduction
in the width constraint), (b) increasing friction, and (c) decreasing
ratio of the roll radius to the strip thickness.
Spreading can be prevented by the use of vertical rolls in contact
with the edges of the rolled product.
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Spreading in Flat Rolling
Figure 13.5 Increase in strip width (spreading) in flat rolling. Note that
similar spreading can be observed when dough is rolled with a rolling
pin.
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Flat-Rolling Practice: Hotrolling
The initial rolling steps (breaking down) of the material typically isdone by hot rolling.
Hot rolling converts the cast structure to a wrought structure with
finer grains and enhanced ductility, both of which result from the
breaking up of brittle grain boundaries and the closing up of internal
defects (especially porosity).
Fig: Hot rolling
The product of the first hot-rolling
operation is called a bloom or slab.
A bloom usually has a square cross-
section, at least 150 mm on the side; aslab usually is rectangular in cross-
section.
Billets usually are square (with a
cross-sectional area smaller than
blooms) and later are rolled into
various shapes, such as round rods
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Effects of Hot Rolling:
Figure 13.6 Changes in the grain structure of cast or of large-grain
wrought metals during 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.
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Flat-Rolling Practice
After casting, ingots are rolled into one of three intermediate shapes
called blooms, billets, and slabs. In the hot rolling of blooms, billets,
and slabs, the surface of the material usually is conditioned
(prepared for a subsequent operation) prior to rolling them.
Blooms have square cross section 6x 6or larger. They are
rolled into structural shapes.
Billets have square cross section 1.5x 1.5or larger. they are
rolled into bars and rods.
Slabs have rectangular cross section 10x 1.5or larger. They
are rolled into plates, sheets and strips.
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Cold rolling is carried out at room temperature and, compared withhot rolling, produces sheets and strips with a much better surface
finish (because of lack of scale), dimensional tolerances, and
mechanical properties (because of strain hardening).
Pack rolling is a flat-rolling operation in which two or more layers of
metal are rolled together; the process improves productivity.Aluminum foil, for example, is pack rolled in two layers.
Fig: Cold rolling
Flat-Rolling Practice: ColdRolling
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Defects in rolled plates and
sheets Defects may be present on the surfaces of rolled plates and
sheets, or there may be internal structural defects.
Defects are undesirable not only because they degrade
surface appearance but also because they may adverselyaffect the strength, formability, and other manufacturing
characteristics.
Wavy edges on sheets are the result of roll bending.
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Defects in Flat Rolling
Figure 13.8 Schematic
illustration of typical
defects in flat rolling:
(a) wavy edges; (b)
zipper cracks in the
center of the strip; (c)
edge cracks; and (d)
alligatoring.
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Various rolling processes and
millsShape rolling
Straight and long structural shapes (such as
channels, I-beams, railroad rails, and solid bars) are
formed at elevated temperatures by shape rolling
(profile rolling), in which the stock goes through aset of specially designed rolls.
Cold shape rolling also can be done with the
starting materials in the shape of wire with various
cross-sections. Fig 13.12 shows the Steps in the shape rolling of an
I-beam part. Various other structural sections, such
as channels and rails, also are rolled by this kind of
process.
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Shape Rolling of an H-section part
Figure 13.13 Stages in
the shape rolling of an
H-section part. Various
other structuralsections, such as
channels and I-beams,
are also rolled by this
kind of process.
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Ring-Rolling
Figure 13.14 (a) Schematic illustration of a ring-rolling operation. Thickness
reduction results in an increase in the part diameter. (b-d) Examples of cross-
sections that can be formed by ring-rolling.
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Thread Rolling Thread rolling is a cold-forming process by which
straight or tapered threads are formed on round
rods or wire by passing them between dies.
Threads are formed on the rod or wire with eachstroke of a pair of flat reciprocating dies.
Fig 13.16 shows Thread rolling processes: (a) and
(b) reciprocating flat dies; (c) two-roller dies; (d) A
collection of thread-rolled parts madeeconomically at high production rates.
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Thread-Rolling Processes
Figure 13.15 Thread-rolling processes: (a) and (c) reciprocating flat dies; (b) two-
roller dies. (d) Threaded fasteners, such as bolts, are made economically by these
processes at high rates of production. Source: Courtesy of Central Rolled Thread
Die Co.
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Machined and Rolled Threads
Figure 13.17 (a) Features of a machined or rolled thread. Grain flow in (b)
machined and (c) rolled threads. Unlike machining, which cuts through the
grains of the metal, the rolling of threads imparts improved strength because of
cold working and favorable grain flow.
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Production of Seamless Pipe
Rotary tube piercing is a hot-working process for makinglong, thick-wall seamless pipe and tubing. It is based
on the principle that when a round bar is subjected to
radial compressive forces, tensile stresses develop at
the center of the bar. When it is subsequentlysubjected to cyclic compressive stresses, a cavitybegins to form at the center of the bar.
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Cavity Formation in Bar
Figure 13.17 Cavity formation in a solid, round bar and its utilization in the rotary
tube-piercing process for making seamless pipe and tubing.
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Tube Rolling
The diameter and thickness of pipes and tubing
can be reduced by tube rolling, which utilizes
shaped rolls.
Fig 13.18 shows the schematic illustration of
various tube-rolling processes: (a) with a fixed
mandrel; (b) with a floating mandrel; (c) without a
mandrel; and (d) pilger rolling over a mandrel and
a pair of shaped rolls. Tube diameters andthicknesses also can be changed by other
processes, such as drawing, extrusion, and
spinning.
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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.
The Rolls
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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.