Chapter 13
Rolling of Metals
13.1 Introduction
• Rolling: is the process of reducing the thickness or changing the
cross section of a long work-piece by compressive forces applied
through a set of rolls (F13.1)
• Rolling is 1st carried out at elevated temperatures (hot rolling):
the coarse-grained, brittle, and porous structure of the ingot or
the cont. cast metal is broken down into a wrought structure
having finer grain size and enhanced properties.
• Subsequently, rolling is carried out at room temperature (cold
rolling), whereby the rolled product has higher strength and
hardness and a better surface finish. But more energy required?
• Plates: thickness more than 6mm and up to 300mm like bridges
• Sheets: thickness less than 6mm and can reach 0.28mm as in
Aluminum beverage cans.
Flat-Rolling and
Shape-Rolling
Processes
Figure 13.1 Schematic
outline of various flatrolling and shape-rolling
processes. Source: After
the American Iron and
Steel Institute.
Flat-Rolling Process
• A schematic illustration of the flat rolling process is shown in
F13.2a.
• A strip of thickness ho enters the roll gap and is reduced to thickness
hf by a pair of rotating rolls.
• The surface speed of the rolls is Vr
• The velocity of the strip increases from its entry value Vo as it enters
the roll gap and it id maximum at the end Vf
Flat rolling
• Because surface speed of the roll is constant, there is
relative sliding between the roll and the strip along the arc
of contact in the roll gap, L
• At the neutral point or no-slip point, the velocity of the
strip equals that of the roll.
• To the left of this point, the roll moves faster than the
strip, to the right, the strip moves faster than the roll.
• Therefore, the frictional forces, act on the strip as shown
in F13.2b
Flat rolling – Frictional Forces
• The rolls pull the material into the roll gap through a
net frictional force on the material which must be to
the right.
• The max possible draft: difference between the initial
and final thickness, (ho – hf), is a function of the
coefficient of friction, m, and the roll radius, R:
ho – hf = m2R
• The higher the friction and the larger the roll radius,
the greater the max draft.
Flat rolling – Roll Force and Power Requirement
• The roll force, F = LwYav
• Yav = average true stress of the strip in the roll gap.
L  R (ho  hf )
• This equation is used for a frictionless situation.
• The higher the coefficient of friction is between the rolls and the
strip, the greater the divergence, and the formula predicts
lower roll force than the actual force.
• The power required per roll can be estimated by assuming that
the force F acts in the middle of the arc of contact: a = L/2
2 FLN
power 
kW
60000
EXAMPLE |3.l Calculation of Roll Force and
Torque in Flat-rolling
•
An annealed copper strip 250 mm wide and 25 mm thick is rolled to a thickness of 20
mm in one pass. The roll radius is 300 mm, and the rolls rotate at 100 rpm. Calculate
the roll force and the power required in this operation.
Solution:
•
The average true stress, Yavg, for annealed copper is determined as follows: First note
that the absolute value of the true strain that the strip undergoes in this operation is
EXAMPLE |3.l Calculation of Roll Force and
Torque in Flat-rolling
•
Referring to figure 2.6
the annealed copper
has true unstrained
stress 80 MPa and at
0.223 true strain, the
true stress is 280 MPa,
then the average true
stress is 180 MPa.
Flat rolling – Reducing Roll Force
•
•
•
•
Roll force can cause flattening of the rolls.
Also, the roll stand (F13.3) may stretch under the roll force to
such an extent that the roll gap can open up significantly.
Therefore, the rolls have to be set closer than was calculated,
to compensate for this deflection and to obtain the desired final
thickness.
Roll forces can be reduced by any of the following means:
1. Reducing friction.
2. Using smaller diameter roll, to reduce contact area
3. Smaller reductions per pass
4. Rolling at elevated temperatures
5. Apply longitudinal tension to the strip during rolling, thus
reducing the compressive stresses required to deform the
material plastically.
Roll Arrangements
Figure 13.3 Schematic illustration of various roll arrangements: (a) four-high
rolling mill showing various features. The stiffness of the housing, the rolls, and the
roll bearings are all important in controlling and maintaining the thickness of the
rolled strip; (b) two-high mill; (c) three-high mill; and (d) cluster (or Sendzimir) mill.
Flat rolling – Reducing Roll Force
• Back tension is applied to the sheet by applying a braking
action to the supply reel.
• Front tension is applied by increasing the rotational speed of
the take up reel.
• Rolling can also be carried out by front tension only, with no
power supplied to the rolls (Steckel rolling)
Flat rolling – Geometric Considerations
(roll bending)
• Roll forces tend to bend the rolls
elastically (F13.4a).
• Therefore, the rolled strip tends to be
thicker at its center than at its edges.
• To avoid this problem, grind the rolls so
that the their diameter at the center is
slightly larger than at their edges.
• For rolling sheet metals, the radius of
max camber is generally 0.25mm
greater than that at the edges of the roll.
• a particular camber is correct only for a
certain load and a certain strip width.
• To reduce the effects of deflection, the
rolls can be subjected to bending
moments at their bearings, to simulate
camber.
Flat rolling – Geometric Considerations
(thermal camber)
• Because of heat generated by plastic deformation, rolls can
become slightly barrel-shaped (thermal camber)
• To avoid the strip being thinner in the center, there should be
some way to compensate this camber.
• Therefore, the net camber can be controlled by varying the
location of the coolant on the HR
• Roll forces also tend to flatten the rolls elastically.
• This flattening produces large roll radius, hence, larger contact
area for the same draft.
• Therefore, the roll force increases.
Flat rolling – Geometric Considerations
(spreading)
• In rolling plates and sheets having high
width-to-thickness ratios, width of the
material remains constant.
• With smaller ratios (square x-section),
the width increases in the roll gap
(spreading), F13.5
• Spreading can be prevented by use of
vertical rolls in contact with edges of the
rolled product.
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.
Flat rolling practice - Effects of Hot Rolling
• A cast structure is typically dendritic, and it includes coarse and
non-uniform grains, is usually brittle and may contain porosities.
• HR converts the cast structure to a wrought structure (F13.6),
which has finer grains and enhanced ductility.
• Temperature ranges for HR from 450oC for Al alloys to 1250oC for
alloy steels, and up to 1650oC for refractory alloys.
Flat rolling practice
• The product of 1st HR operation is called a bloom or slab.
• Bloom: square x-section, at least 150mm side, Slab:
rectangular x-section.
• Billets: square x-section smaller than blooms.
• In HR blooms, slabs, and billets, the surface is usually
conditioned prior to rolling by any of the following means:
1. Use of a torch to remove heavy scale or by rough grinding
2. In CR scale is removed by etching with acids.
Flat rolling practice
• Pack rolling: two or more layers of metal are rolled together to improve
productivity (Al foil)
• To improve flatness, the rolled strip is passed through a series of
leveling rolls (F13.7)
Figure 13.7 (a) A method of roller leveling to flatten rolled sheets. (b)
Roller leveling to straighten drawn bars.
Flat rolling practice - Defects
• Scale, rust, scratches, and cracks may
be caused by inclusions and impurities in
the original cast material, or due to
surface and material conditions
• Wavy edges are the results of roll
bending: the strip is thinner along its
edges than at its centers (F13.3a).
Because the edges elongate more than
the center, they buckle because they are
restrained from expanding freely in the
long. direction.
• cracks shown in F13.8b and c are due to
poor material ductility at the rolling temp.
• Alligatoring (F13.8d): caused by nonuniform deformation or by the presence
of defects in the original cast billet.
• Edge defects are removed by shearing
and slitting operations.
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.
Flat rolling practice – residual stresses
• Because of non-uniform
deformation of the material in
the roll gap, residual stresses
may develop.
• Small diameter rolls or small
reductions per pass tend to
deform the metal plastically at
its surfaces (F13.9a)
• Large diam rolls & high
reductions tend to deform the
bulk more than the surfaces
(F13.9b). This is due to the
frictional constraint at the
surfaces along the arc of
contact
Figure 13.9
(a) Residual stresses developed in rolling
with small-diameter rolls or at small reductions in
thickness per pass. (b) Residual stresses developed in
rolling with large-diameter rolls or at high reductions per
pass. Note the reversal of the residual stress patterns.
Rolling Mills
• Width of rolled products: up to
5m and be as thin as
0.0025mm.
• Rolling speeds: up to 25m/s for
CR.
• 2-high or 3-high mills are used
for HR in initial breakdown
passes on cast ingots or in
continuous casting, with roll
diam range from 0.6m to 1.4m.
• 4-high mills and cluster mills are
based on the principle that small
diameter rolls lower roll forces &
power req. and reduce
spreading.
• Also, when worn or broken,
small rolls can be replaced at
less cost than large rolls.
Figure 13.10 A general view of a rolling mill. Source:
Courtesy of Ispat Inland.
Rolling Mills
• Tandem rolling (F13.12): the strip is rolled continuously, through a
number of stands to smaller gages with each pass.
• Rolls have to have strength and resistance to wear.
• Common roll materials: cast iron, forged steel, Tungsten Carbide.
• HR of ferrous alloys usually done without lubricants, (may use graphite)
• Water based solutions are used to cool the rolls and to break up scale.
• Non-ferrous alloys are HR with a variety of compounded oils,
emulsions, and fatty acids.
• CR is done with water soluble oils or low viscosity lubricants such as
mineral oils, emulsions, paraffin and fatty oils.
Shape Rolling Operations –Ring Rolling
• Shape rolling (F13.13)
• A thick ring is expanded into a larger diam ring with a
reduced x-section.
• The ring is placed between two rolls, one of which is
driven (F13.14a), and its thickness is reduced by
bringing the rolls closer together as they rotate.
• The process can be done at room temp or at high
temp, depending on size, strength, and ductility of the
work piece material.
• Advantages: short production times, material saving,
close dimensional tolerances, favorable grain flow in
the product.
Shape Rolling of an H-section part
Figure 13.12 Steps in the
shape rolling of an Hsection part. Various other
structural sections, such as
channels and I-beams, also
are rolled by this kind of
process.
Ring-Rolling
Figure 13.15 (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.
Roll-Forging
Figure 13.13 Two examples of the roll-forging operation, also known as cross-rolling.
Tapered leaf springs and knives can be made by this process. Source: After J. Holub.
Production of Steel Balls
Figure 13.14 (a) Production of steel balls by the skew-rolling process. (b) Production of
steel balls by upsetting a cylindrical blank. Note the formation of flash. The balls made by
these processes subsequently are ground and polished for use in ball bearings.
Shape Rolling Operations – Thread Rolling
• Threads are formed
on rod or wire with
each stroke of a pair
of flat reciprocating
dies (F13.15a)
• Another method
uses rotary dies
(F13.15b) at prod
rates as high as 80
pieces per sec.
Figure 13.16 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.
Shape Rolling Operations – Thread Rolling
• Advantages: no scrap, good strength, very
smooth surface finish, induced compression
residual stresses, improving fatigue life.
• Machining threads cuts through the grain flow
lines of material, while rolling leaves a grain-flow
pattern that improves the strength of the thread
(F13.16)
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.
Production of Seamless Pipe and Tubing
• Rotary tube piercing is a HW process for making long,
thick-walled seamless pipe and tubing (F13.17).
• 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’s subsequently subjected to cyclic compressive
stresses (F13.17b), a cavity begins to form at the center
of the bar.
Cavity Formation in Bar
Figure 13.18 Cavity formation in a solid, round bar and its utilization in the rotary tubepiercing process for making seamless pipe and tubing. (see also Fig. 2.9.)
Production of Seamless Pipe and Tubing
• Rotary tube piercing is
carried out using an
arrangement of rotating
rolls (F13.17c).
• Axis of rolls are skewed,
in order to pull the round
bar through the rolls by
axial comp of the rotary
motion.
• An internal mandrel
assists the operation by
expanding the hole and
sizing the inside
diameter of the tube.
• The diameter &
thickness of tubes and
pipes can be reduced
by tube rolling (F13.18)
Continuous casting and rolling: integrated
mills and minimills
• Integrated mills: large facilities that involve complete activities
from the production of hot metal in a blast furnace to the casting
and the rolling of finished products.
• Minimills: scrap metal is melted in electric furnaces, cast
continuously, and rolled directly into specific lines of products.
• Each minimill produces one kind of rolled product.
• The scrap metal is obtained locally to reduce transportation
costs (old machinery, cars, and farm equipment)
• Minimills have the economic advantages of low investment.
Forming of Solid Rocket Casings
Figure 13.20 The Space Shuttle U.S.S.
Atlantis is launched by two strapped-on solidrocket boosters. Source: Courtesy of NASA.
Figure 13.21 The forming
processes involved in the
manufacture of solid rocket
casings for the Space
Shuttles.