Rolling
(Bulk Deformation Process)
4th Semester IED
Terminology
•Bloom is the product of first breakdown of ingot
Semi Finished Products
(cross sectional area > 230 sq cm).
• Billet is the product obtained from a further
reduction by hot rolling
(cross sectional area > 40x40 sq mm).
• Slab is the hot rolled ingot
(cross sectional area > 100 sq cm and with a
width ≥ 2 x thickness)
• Plate is the product with a thickness > 6 mm.
Mill products
• Sheet is the product with a thickness < 6 mm
and width > 600 mm.
• Strip is the product with a thickness < 6 mm
and width < 600 mm.
Rolling
• It is used to reduce the cross-section of large
ingots or plate
• These include structural steel sections,
automotive body sheet, food/beverage container
sheet, building siding etc
• The simplest arrangement is the two-high mill,
consisting of upper and lower driven rolls
between which the work piece passes, shown in
figure a below
• The four-high mill consists of two driven work
rolls, with large back-up rolls that provide
increased stiffness
• The back-up rolls prevent the work rolls from
bowing due to the rolling pressure which, if not
controlled, results in rolled products, thicker at
the centre than at the edges
For this reason four-high mills are used when
the sheet thickness must be controlled
accurately
Sendzimir mill
• In a Sendzimir mill each work roll is supported
along its entire length by two back-up rolls,
which in turn are supported by successive
layers of larger intermediate rolls.
• The advantage of this arrangement is that the
mill is very stiff and resists deflection
• Sendizmir mill can produce very flat, thin
sheet in a single pass, with minimal power
requirements
Analysis of Flat Rolling
In flat rolling, the work is squeezed between two rolls so
that its thickness is reduced by an amount called the
draft:
draft, mm (in); starting thickness, mm (in); final thickness, mm
(in)
Draft is sometimes expressed as a fraction of the
starting stock thickness, called the reduction
In addition to thickness reduction, rolling usually increases
work width. This is called SPREADING, and it tends to be
most pronounced with low width-to-thickness ratios and
low coefficients of friction.
Similarly, before and after volume rates of material flow
must be the same, so the before and after velocities
can be related:
Each roll has radius, R, its rotational speed gives it a
surface velocity vr. This velocity is greater than the
entering speed of the work vo and less than its exiting
speed vf.
There is one point along the arc where work velocity
equals roll velocity. This is called the no-slip point,
also known as the neutral point.
On either side of this point, slipping and friction
occur between roll and work. The amount of slip
between the rolls and the work can be measured by
means of the forward slip,
The true strain experienced by the work in rolling is
based on before and after stock thicknesses.
The average flow stress is used to compute estimates
of force and power in rolling.
Certain coefficient of friction is required to accomplish
process, and the compression force of the rolls,
multiplied by this coefficient of friction, results in a
friction force between the rolls and the work
The friction force on the entrance side is greater, so
that the net force pulls the work through the rolls.
There is a limit to the maximum possible draft that
can be accomplished in flat rolling with a given
coefficient of friction, defined by:
Given a coefficient of friction sufficient to perform rolling,
roll force F required to maintain separation between the
two rolls can be computed by integrating the unit roll
pressure
The integration requires two separate terms, one for
either side of the neutral point.
Variation in roll pressure along the contact length is
significant.
A sense of this
variation can be
obtained from the
plot
Pressure reaches a maximum at the neutral point, and
trails off on either side to the entrance and exit
points.
As friction increases, maximum pressure increases
relative to entrance and exit values.
As friction decreases, the neutral point shifts away
from the entrance and toward the exit in order to
maintain a net pull force in the direction of rolling.
Otherwise, with low friction, the work would slip rather
than pass between the rolls.
The product wL is the roll-work contact area, Contact
LENGTH can be approximated by
The torque in rolling can be estimated by assuming
that the roll force is centered on the work as it
passes between the rolls, and that it acts with a
moment arm of one-half the contact length L,
Torque for each roll is
The power required to drive each roll is the product of
torque and angular velocity.
Angular velocity is 2πN, where N = rotational speed of
the roll.
The power for each roll is 2πNT, doubling the value
to account for the fact that a rolling mill consists of
two powered rolls
Problem
A 300-mm-wide strip 25-mm thick is fed through a
rolling mill with two powered rolls each of radius =
250 mm. The work thickness is to be reduced to 22
mm in one pass at a roll speed of 50 rev/min. The
work material has a flow curve defined by K = 275
MPa and n = 0.15, and the coefficient of friction
between the rolls and the work is assumed to be
0.12. Determine if the friction is sufficient to permit
the rolling operation to be accomplished. If so,
calculate the roll force, torque, and horsepower.
The draft attempted in this rolling operation is
Maximum possible draft for the given coefficient of
friction is
Since the maximum allowable draft exceeds the
attempted reduction, the rolling operation is FEASIBLE.
Configurations of rolling mills
(a) 2-high, (b) 3-high, (c) 4-high, (d) cluster mill, and (e) tandem rolling
mill.
Thread Rolling
Thread rolling with flat dies: (1) start of cycle and (2) end of cycle.
Ring Rolling
Ring rolling used to reduce the wall thickness and increase the diameter
of a ring: (1) start and (2) completion of process.
Roll Piercing
setup of Mannesmann roll mill for producing seamless tubing.
Summary
• Rolling, an important bulk deformation
process
• Problem sheet assigned, solve problems to
grasp concepts
