Rolling

R. V. COLLEGE OF ENGINEERING,

BANGALORE.

PRODUCTION TECHNOLOGY ASSIGNMENT

ROLLING

By

Avinash Kumar (1RV11ME033)

Basavaraj Devakki (1RV11ME034)

Biswajyothi Dutta (1RV11ME035)

Brian Nikil Quadras (1RV11ME036)

3rd Sem A Section

R o l l i n g P a g e 2

Contents:

Introduction

Types of rolling Two high rolling mill Three high rolling mill Four high rolling mill Tandem rolling mill Cluster rolling mill Continuous rolling mill Planetary rolling mill

Defects in rolling Surface defects Wavy defects Edge cracking Center split Zipper crack

Flatness and shape Profile Draft


Introduction

Rolling is a process of reduction of the cross-sectional area or shaping a metal

piece through the deformation caused by a pair of rotating in opposite directions

metal rolls.

A scheme of rolling process is shown in the picture:

The gap between the rotating rolls is less than the thickness of the entering

bar H0 therefore a friction force is necessary in order to bite the bar and to pull it

through the rolls.

A metal bar passing through the rotating rolls is squeezed, and it elongates

while its cross section area decreases. The amount of deformation R achieved in a flat rolling operation (thickness reduction) is determined by the relationship:

R = 100% * (H H0)/H0

A machine used for rolling metal is called rolling mill.

A typical rolling mill consists of a pair of rolls driven by an electric motor

transmitting a torque through a gear and pair of cardans. The rolls are equipped

with bearings and mounted in a stand with a screw-down mechanism.

A force applied to the rolls in vertical direction is called roll separating force.

A rolling mill is characterized by the maximum values of its roll separating force

and the torque.

The maximum amount of deformation (thickness reduction) which may be

achieved in a single rolling pass is determined by the maximum roll separating


force, maximum torque, work roll diameter, friction coefficient and mechanical

strength of the rolled material and its width.

Low roll diameter results in low roll contact area and consequently in low

absolute value of the roll separating force and the torque required for achieving a

certain thickness reduction.

However such rolls are susceptible to bending and causing non-uniform

widthwise strip thickness distribution (convex crown).

Complex rolling mill designs employing back-up rolls are used to diminish the

bending effect

Hot rolling is a rolling operation carried out at a temperature exceeding

the recrystallization temperature and permitting large amount of deformation.

Cold rolling is a rolling operation carried out at room temperature. Cold rolling

is commonly conducted after hot rolling when good surface quality and low

thickness tolerance are needed. Cold rolling causes material strengthening and may

be followed by annealing.

Types of Rolling mills

Rolling mills may be classified according to the number and arrangement of

the rolls.

1) Two high rolling mills

2) Three high rolling mills

3) Four high rolling mills

4) Tandem rolling mills

5) Cluster rolling mills

1) Two high rolling mills

Two high rolling mills may further classified as Reversing mill

Non reversing mill

A two high rolling mill has two rolls only.


Two high reversing mill

In two high reversing rolling mills the rolls rotate it in one direction and then

in the other, so that rolled metal may pass back and forth through the rolls several

times. This type is used in pluming and slabing mills and for roughing work in

plate, rail, structural and other mills.

These are more expensive compared to the non-reversing rolling mills.

Because of the reversible drive needed.

Two high non reversing mill

In two high non reversing mills as two rolls which revolve continuously in

same direction therefore smaller and less costly motive power can be used.

However every time material is to be carried back over the top of the mill for again

passing in through the rolls. Such an arrangement is used in mills through which

the bar

passes once

and in open

train plate

mill.


2) Three high rolling mill:

It consists of a roll stand with three parallel rolls one above the other. Adjacent

rolls rotates in opposite direction. So that the material may be passed between the

top and the middle roll in one direction and the bottom and middle rolls in opposite

one.

In three high rolling mills the work piece is rolled on both the forward and

return passes. First of all the work piece passes through the bottom and middle

rolls and the returning between the middle and the top rolls.

So that thickness is reduced at each pass. Mechanically operated lifted tables

are used which move vertically or either side of the stand. So that the work piece

fed automatically into the roll gap.

Since the rolls run in one direction only a much less powerful motor and

transmission system is required. The rolls of a three high rolling mills may be

either plain or grooved to produce plate or sections respectively.

3) Four high rolling mill:

It has a roll stand with four parallel rolls one above the other. The top and the

bottom rolls rotate in opposite direction as do the two middle rolls. The two middle

are smaller in size than the top and bottom rolls which are called backup rolls for

providing the necessary rigidity to the smaller rolls.


A four high rolling mill is used for the hot rolling of armor and other plates as

well as cold rolling of plates, sheets and strips.

4) Tandem rolling mills:

It is a set of two or three stands of roll set in parallel alignment. So that a

continuous pass may be made through each one successively with change the

direction of material.

5) Cluster rolling mills:

It is a special type of rolling mill in which each of the two working rolls is backup

by two or more of larger backup rolls for rolling hard in materials .It may be

necessary to employ work rolls of very small diameter but considerable length. In

such cases adequate of the working rolls can be obtained using a cluster mill.


6) Continuous rolling mill

A continuous mill consists of several stands of rolls arranged in a straight line

(in tandem), with each succeeding stand operating with roll surface speed greater

than its predecessor. This type of mill is in very common usage for rolling strip,

sheet, billets, bars, rods, etc. Any part of the workpiece, after pass through the

roughing, intermediate and finish stands, is rolled from initial shape into the finish

one, and emerges from the last roll stand.

7) Planetary rolling mill

This consists of one pair of heavy back up rolls. Each back up roll is

surrounded by a series of small planetary rolls as shown in the figure. The

planetary rolls sweeps out a circular path between the slab and the backup roll as a

result it gives almost a constant reduction to the slab. When one pair of planetary

rolls make contact and the reduction continues. Thus the total reduction undergone

by the workpiece is the summation of the series of small reductions effected by

each pair of rolls


Defects in Rolling

As with any other manufacturing process, it is extremely difficult roll

products which are totally defect free. To produce quality roll product, rolling

practice requires balancing many factors, including material properties, process

variables and lubrication .The defects observed in rolled products can be broadly

considered as follows.

In hot rolling, if the temperature of the workpiece is not uniform the flow of

the material will occur more in the warmer parts and less in the cooler. If the

temperature difference is great enough cracking and tearing can occur.

1) Surface defects:

These result from inclusions and impurities in the material , scale, rust, dirt, roll

marks, and various other causes related to the prior treatment and working of the

material.


2) Wavy edges:

This is caused by bending of thee rolls which results in the edge of the strip

being thinner than at the center.

3) Edge cracking:

During rolling inhomogeneity in the deformation is a major problem. The

decrease in thickness at the center of the sheet results in increase in length. In

addition a part of the thickness decrease at the edges results in lateral spread.

Due to continuity between the edges and center the the edges of the sheet are

placed in tension which results in edge cracklings.

4) Center split:

Under severe conditions, the reduction in thickness of the sheet at the center

may result in center split.


5) Zipper cracks:

These cracks appear at the center of the strip and are usually accused by low

ductility and barreling.

Flatness and Shape:

In a flat metal workpiece, the flatness is a descriptive attribute characterizing the

extent of the geometric deviation from a reference plane. The deviation from

complete flatness is the direct result of the workpiece relaxation after hot or cold

rolling, due to the internal stress pattern caused by the non-uniform transversal

compressive action of the rolls and the uneven geometrical properties of the entry

material. The transverse distribution of differential strain/elongation-induced stress

with respect to the material's average applied stress is commonly referenced to as

shape. Due to the strict relationship between shape and flatness, these terms can be

used in an interchangeable manner. In the case of metal strips and sheets, the

flatness reflects the differential fiber elongation across the width of the workpiece.

This property must be subject to an accurate feedback-based control in order to

guarantee the machinability of the metal sheets in the final transformation

processes.


Profile:

Profile is made up of the measurements of crown and wedge. Crown is the

thickness in the centre as compared to the average thickness at the edges of the

workpiece. Wedge is a measure of the thickness at one edge as opposed to the

other edge. Both may be expressed as absolute measurements or as relative

measurements. For instance, one could have 2 mil of crown (the centre of the

workpiece is 2 mil thicker than the edges), or one could have 2% crown (the centre

of the workpiece is 2% thicker than the edges).

It is typically desirable to have some crown in the workpiece as this will cause

the workpiece to tend to pull to the centre of the mill, and thus will run with higher

stability.

Draft:

The difference between the thickness of initial and rolled metal piece is called Draft. Thus if is initial thickness and is final thickness, then the draft is

given by

The maximum draft that can be achieved via rollers of radius with

coefficient of static friction between the roller and the metal surface is given by

This is the case when the frictional force on the metal from inlet contact

matches the negative force from the exit contact.


Bibliography

Manufacturing technology by P N Rao http://engineeringhut.blogspot.in http://www.subtech.com Image Courtesy: http://www.google.co.in

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