L13-Rolling of metals.ppt

8/10/2019 L13-Rolling of metals.ppt

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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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Three High Rolling Mill.


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Four High Rolling Mill.


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Multiple backing rolls allow even smaller roll diameters

Cluster Rolling Mill.


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A series of rolling stands in sequence

Tandem Rolling Mill.

Documents

L13-Rolling of metals.ppt