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授課教師：楊宏智教授
1
楊宏智（台大機械系教授）
2
PART III: Forming and Shaping Processes and
Equipment

“Forming” indicates changing the shape of an
existing solid body
PART III: Forming and Shaping Processes and
Equipment
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For forming processes, the starting material may be in the
shape of a plate, sheet, bar, rod, wire, or tubing of various
cross sections
Shaping processes involve the molding and casting of
molten materials and the finished product is near the final
desired shape
Molten metalis cast into individual ingots or continuously cast
into slabs, rods, or pipes
Cast structures are converted to wrought structures by
plastic-deformation processes
Cast Structure and Wrought Structure

cast structure
- solidification front moves thr the molten metal from the mold walls
towards the center
- at the mold walls the metal cools rapidly and produces solidified skin,
or shell
- brittle grain boundaries and internal defects are formed

wrought structure
- converted from cast structure by hot rolling
- gives finer grains and enhanced ductility, improved strength resulting
from breaking up brittle grain boundaries and closing up internal
defects
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Chapter 13: Metal-Rolling Processes and Equipment
Chapter Outline
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Introduction
The Flat-rolling Process
Flat-rolling Practice
Rolling Mills
Various Rolling Processes and Mills
Introduction
Rolling is the process of reducing the cross section
of a long workpiece by compressive forces
applied through a set of rolls
(本圖表請參考Manufacturing Engineering Technology in SI
Units, 6th P.317 Figure 13.1)
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Slab: rectangular shape
Billet: (< 6”) square or circular
Bloom: (> 6”)
Plates and Sheets
Plates: 300mm>t>6mm
reactor vessels(150mm)
tanks (125mm) boiler
supports (300mm)
 Sheets: t<6mm
B747 (1.8mm)
sedan (1.2-0.7mm)
can (0.1mm)
Al foil (0.008mm)
 Slab, Billet and Bloom

The Flat-rolling Process
Flat-rolling process is shown
 Friction forces act on strip surfaces
 Roll force, F, and torque, T, acts on the rolls
(本圖表請參考Manufacturing Engineering Technology in SI
Units, 6th P.319 Figure 13.2)

The Flat-rolling Process
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As the surface speed of the rigid roll is constant, there is relative
sliding between the roll and the strip along the arc of contact in the
roll gap, L
At neutral point or no-slip point, the velocity of the strip is the
same as that of the roll
The maximum possible draft is defined as the difference between
the initial and final strip thicknesses
From the relationship, higher the friction and the larger the roll
radius, the greater the maximum possible draft becomes
The Flat-rolling Process:
Roll Force, Torque, and Power Requirements


Rolls apply pressure on the flat strip to reduce its thickness,
resulting in a roll force, F
Roll force in flat rolling can be estimated from
L = roll-strip contact length
w = width of the strip
Yavg = average true stress of the strip

Total power (for two rolls) is
Types of Rolling
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Based on workpiece 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 – can achieve significant deformation
Cold rolling – produces sheet and plate stock
Hot Rolling
process of reducing the thickness or changing the
cross-section of a long workpiece by compressive
forces thr a set of rolls
 Performed above recrystallization temp
 converted from cast structure to wrought structure
 gives finer grains and enhanced ductility, improved
strength resulting from breaking up brittle grain
boundaries and closing up internal defects

Grain Structure During Hot Rolling
(本圖表請參考Manufacturing Engineering Technology in SI Units, 6th P.323
Figure 13.6)
The Flat-rolling Process:
Roll Force, Torque, and Power Requirements
Reducing Roll Force

Roll forces can cause deflection and flattening of the rolls

The columns of the roll stand may deflect under high roll forces
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Roll forces can be reduced by:
1.
Reducing friction at the roll–workpiece interface
2.
Using smaller diameter rolls
3.
Reduce the contact area
4.
Rolling at elevated temperatures
5.
Applying front and/or back tensions to the strip
The Flat-rolling Process:
Roll Force, Torque, and Power Requirements
(本圖表請參考Manufacturing Engineering Technology in SI Units, 6th P.321
Figure 13.3)
The Flat-rolling Process:
Geometric Considerations

Roll forces will bend the rolls elastically during rolling
crown: strip thicker at center
- wavy edges or fractures in center

Use of a “crowned” roll for compensation, When the roll
bends, the strip has a constant thickness along its
width
The Flat-rolling Process:
Geometric Considerations
Equipment is massive and expensive
 Rolling mill configurations:
 Two-high – two opposing rolls
 Three-high – work passes through rolls in both
directions
 Four-high – backing rolls support smaller rolls
 Cluster mill – multiple backing rolls on smaller rolls
 Tandem rolling mill – sequence of two-high mills

Rolling Mill Configurations
two-high rolling mill
 two-high reversing
- used for hot rolling in initial breakdown passes on cast
ingots or in continuous casting, with the direction of
material movement is reversed after each pass.
- drawbacks are the energy waste in reversing the roll
rotation and time consuming in adjusting the roll gap

Rolling Mill Configurations
four-high rolling mill
- smaller rolls require lower force (fig 18-4) to reduce
spread and roll deflection
- the smaller cross-section however provides reduced
stiffness
- when worn or broken smaller rolls replaced at lower cost
- large back-up rolls are used to provide the necessary
support for the smaller work rolls (reduce crown problem)

Rolling Mill
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Automated mills produce close-tolerance, low cost and
high quality plates and sheets at high production rates
(本圖表請參考Manufacturing Engineering Technology in SI
Units, 6th P.326 Figure 13.10)
Rolling Mills
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Two-high rolling mills are used for hot rolling in initial
breakdown passes (cogging mills) on cast ingots or in
continuous casting
In tandem rolling, the strip is rolled continuously
through a number of stands to thinner gages with each
pass
Flat-rolling Practice:
Defects in Rolled Plates and Sheets
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Defects may be present on the surfaces or there may be internal
structural defects
They are undesirable as they compromise surface appearance
and adversely affect strength, formability, and other manufacturing
characteristics
Surface defects may be caused by inclusions and impurities in the
original cast material
Wavy edges on sheets are the result of roll bending
Cracks are due to poor material ductility at the
rolling temperature
Flat-rolling Practice:
Other Characteristics of Rolled Metals
Residual Stresses
 Residual stresses develop in rolled plates and sheets
due to nonuniform deformation of materials in roll gap
The Flat-rolling Process:
Geometric Considerations
Spreading
 Increase in width is called spreading
 Spreading increases with:
1. Decreasing width-to-thickness ratio of the entering strip
2. Increasing friction
3. Decreasing ratio of the roll radius to the strip thickness
The Flat-rolling Process:
Vibration and Chatter
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Vibration and chatter have effects on product quality and the
productivity of metalworking operations
Chatter defined as self-excited vibration
Occur in rolling and in extrusion, drawing, machining, and grinding
operations
Chatter results from interactions between the structural dynamics
of the mill stand and the dynamics of the rolling operation
Chatter can be reduced by increasing the roll radius, strip-roll
friction and incorporating dampers in the roll supports
Flat-rolling Practice:
Other Characteristics of Rolled Metals
Dimensional Tolerances
 Thickness tolerances for cold-rolled sheets range from
±0.1~0.35 mm
 Flatness tolerances are within ±15 mm/m for cold
rolling and ±55 mm/m for hot rolling
Surface Roughness
 Cold rolling can produce a very fine surface finish
 Cold-rolled sheets products may not require additional
finishing operations
Rolling Mills
Roll Materials

Basic requirements for roll materials are strength and resistance to
wear
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Forged-steel rolls have higher strength, stiffness, and toughness
than cast-iron rolls

Rolls made for cold rolling should not be used for hot rolling as
they may crack from thermal cycling (and spalling
Lubricants

Hot rolling of ferrous alloys do not need lubricants

Water-based solutions are used to cool the rolls
Various Rolling Processes and Mills
Shape Rolling
 Straight and long structural shapes are formed at
elevated temperatures by shape rolling
Various Rolling Processes and Mills
Tube Rolling
 Diameter and thickness of pipes and tubing can be
reduced by tube rolling, which utilizes shaped rolls
Shape Rolling
Work is deformed into a contoured cross section
rather than flat (rectangular)
 Accomplished by passing work through rolls that
have the reverse of desired shape
 Products
 Construction shapes such as I-beams, L-beams,
and U-channels
 Rails for railroad tracks
 Round and square bars and rods

Various Rolling Processes and Mills
Thread Rolling
 Thread rolling is a cold-forming process by which
straight or tapered threads are formed on round rods or
wire
 Threads are formed with rotary dies at high production
rates
Various Rolling Processes and Mills
Thread Rolling
 Thread-rolling process has the advantages of
generating threads with good strength without any loss
of material
 Internal thread rolling can be carried out with a
fluteless forming tap, produces accurate internal
threads with good strength
Thread Rolling
Bulk deformation process used to form threads on cylindrical
parts by rolling them between two dies

Important process for mass producing bolts and screws

Performed by cold working in thread rolling machines

Advantages over thread cutting (machining):
•
Higher production rates (80 pieces/sec)
•
Better material utilization
•
Surface finish is very smooth
•
Stronger threads and better fatigue resistance
Various Rolling Processes and Mills
Ring Rolling
 A thick ring is expanded into a large-diameter thinner
one
 Thickness is reduced by bringing the rolls closer
together as they rotate
 Short production times, material savings and close
dimensional tolerances
Ring Rolling
Deformation process in which a thick-walled ring of smaller
diameter is rolled into a thin-walled ring of larger diameter

As thick-walled ring is compressed, deformed metal
elongates, causing diameter of ring to enlarge

Hot working process for large rings and cold working
process for smaller rings

Products: ball and roller bearing races, steel tires for railroad
wheels, and rings for pipes, pressure vessels, and rotating
machinery
Various Rolling Processes and Mills
Rotary Tube Piercing
 Also known as the Mannesmann process
 It is a hot-working operation for making long, thickwalled seamless pipe and tubing
 The round bar is subjected to radial compressive
forces while tensile stresses develop at the center of
the bar
Mannesmann Mills
principle:
-when round bar subjected to radial compressive
forces tensile stresses developed at the center of
the bar
- radial compressive force + bar roll, then gives cyclic
compressive stresses cavity formed at the center
of the bar

Mannesmann Mills
arrangement:
- skew rolls
pull the round bar by the axial component of the rotary
motion
- internal mandrel assists the expanding of the hole
and sizing of the tube inside diameter

Various Rolling Processes and Mills
Roll Forging
 Cross section of a round bar is shaped by passing it
through a pair of rolls with profiled grooves
Various Rolling Processes and Mills
Skew Rolling
 Similar to roll forging and used for making ball bearings
 Another method is to shear pieces from a round bar
and then upset them in headers between two dies with
hemispherical cavities