Rolling is a deformation process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls.

1. Lecture No.6
Rolling
By
Ass. Lect. Ali H. Almaily
enginerali1989@gmail.com

2. 6.1 Rolling definition:
Rolling is a deformation process in which the thickness of the work is reduced by
compressive forces exerted by two opposing rolls.
6.2 Types of Rolling:

3. 6.3 Type of rolling Based on work temperature:
6.3.1 Hot rolling: is a rolling operation carried out at a
temperature just below the metal melting point, permitting
large amount of deformation. Hot rolling is a mill process
which involves rolling the steel at a high temperature
(typically at a temperature over 1700° F), which is above the steel’s recrystallization
temperature. When steel is above the recrystallization temperature, it can be shaped and
formed easily, and the steel can be made in much larger sizes.
Uses: Hot rolled products like hot rolled steel bars are used in the welding and construction
trades to make railroad tracks and I-beams, for example. Hot rolled steel is used in situations
where precise shapes and tolerances are not required.

4. 6.3.2 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. Cold rolled steel is
essentially hot rolled steel that has had further processing. This process results in higher
yield points and has four main advantages:
 Cold rolling increases the yield and tensile strengths, often eliminating further costly
thermal treatments.
 Turning gets rid of surface imperfections.
 Grinding narrows the original size tolerance range.
 Polishing improves surface finish.
Uses: Any project where tolerances, surface condition, concentricity, and straightness are
the major factors. These characteristics make cold-rolled sheets, strips, and coils ideal for
stampings, exterior panels, and other parts of products ranging from automobiles to
appliances and office furniture.

5. 6.4 Type of rolling Based on work piece geometry
6.4.1 Flat rolling:
The rolls rotate as illustrated in Figure 6.1 to pull and simultaneously squeeze the work
between them. The basic process shown in our figure is flat rolling, used to reduce the
thickness of a rectangular cross section.
Figure 6.1 The rolling process (specifically, flat rolling).

6. 6.4.1.1 Flat Rolling and Its Analysis:
Flat rolling is illustrated in Figures 6.1 .In flat rolling, the work is squeezed between two
rolls so that its thickness is reduced by an amount called the draft:
where d = draft (mm) (in); to = starting thickness, mm (in); and tf = final thickness, mm (in).
Draft is sometimes expressed as a fraction of the starting stock thickness, called the
reduction:
where r = reduction. Conservation of matter is preserved, so the volume of metal exiting the
rolls equals the volume entering:
where wo and wf are the before and after work widths, mm; and Lo and Lf are the before
and after work lengths, mm . Similarly, before and after volume rates of material flow must
be the same, so the before and after velocities can be related:
where vo and vf are the entering and exiting velocities of the work.

7. Along an arc defined by the angle u. Each roll has radius R, and 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. The amount of slip between the rolls and the work can be
measured by means of the forward slip, a term used in rolling that is defined:
where s = forward slip; vf = final (exiting) work velocity, m/s ; and vr = roll speed, m/s
.The true strain experienced by the work in rolling is based on before and after stock
thicknesses. In equation form,
The true strain can be used to determine the average flow stress Yf:

8. There is a limit to the maximum possible draft that can be accomplished in flat rolling with a
given coefficient of friction, defined by:
where dmax= maximum draft, mm(in); µ=coefficient of friction; and R= roll radius mm. the
roll force F required for rolling operation is equal to the flow stress multiplied by the area of
the contact between roll and work piece ,given as flowing:
Where average flow stress MPa ; and the product wL is the roll-work contact area, mm2 .
Contact length can be approximated by:

9. Figure 6.2: Side view of flat rolling, indicating before and after thicknesses, work
velocities, angle of contact with rolls, and other features.

10. 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. Thus, torque for each roll is
T = 0.5 FL
The power required to drive each roll is the product of torque and angular velocity. we get
the following expression:
P = 2 NFL
where P = power, J/s or W ; N = rotational speed, 1/s (rev/min); F = rolling force, N ; and L
= contact length, m (in).
Example 6.1: Flat peed 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 Rolling: 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 s the rolling
operation to be accomplished. If so, calculate the roll force, torque, and horsepower.

11. Solution: The draft attempted in this rolling operation is
d = 25 - 22 = 3mm
the maximum possible draft for the given coefficient of friction is
dmax = (0.12)2(250) = 3.6mm
To compute rolling force, we need the contact length L and the average
flow stress ̅. The contact length is given

12. Rolling force is determined
F = 175.7(300)(27.4) = 1, 444, 786 N
Torque required to drive each roll is
T = 0.5(1, 444,786) (27, 4)(10-3) =19, 786 N-m
and the power is obtained
P = 2 (50)(1, 444,786)(27.4)(10-3) = 12,432, 086 N-m/min = 207,201 N-m/s(W)
For comparison, let us convert this to horsepower (we note that one horsepower = 745.7 W):
6.4.2 Shape Rolling:
In shape rolling, the work is deformed into a contoured cross section. Products made by
shape rolling include construction shapes such as I-beams, L-beams, and U-channels; rails
for railroad tracks; and round and square bars and rods (see Figure6.2). The process is
accomplished by passing the work through rolls that have the reverse of the desired shape.
Shaping rolls are more complicated; and the work, usually starting as a square shape,
requires a gradual transformation through several rolls in order to achieve the final cross
section.

13. 6.5 Rolling Mills:
Various rolling mill configurations are available to deal with the variety of applications and
technical problems in the rolling process.
1. two-high rolling mill The basic rolling mill consists of two opposing rolls and is
referred to as a two-high rolling mill, shown in Figure 6.4(a). The two-high
configuration can be either:
 Reversing. The reversing mill allows the direction of roll rotation to be reversed, so that
the work can be passed through in either direction.
 Non-reversing. In the non-reversing mill, the rolls always rotate in the same direction,
and the work always passes through from the same side.
2. In the three-high configuration, Figure 6.4(b), there are three rolls in a vertical column,
and the direction of rotation of each roll remains unchanged.
3. The four-high rolling mill uses two smaller-diameter rolls to contact the work and two
backing rolls behind them, as in Figure 6.4 (c).
4. The cluster rolling mill: Each of the work rolls is supported by two backing
rolls. (Figure 6.4(d)).

14. 5. Tandem rolling mill is often used. To achieve higher throughput rates in standard
products, This configuration consists of a series of rolling stands, as represented in Figure
6.4(e).
Figure 6.4:Various configurations of rolling mills: (a) 2-high, (b) 3-high, (c) 4-high, (d)
cluster mill, and (e) tandem rolling mill.