Rolling

Rolling is a bulk deformation process in which the thickness of the metal is reduced by compressive forces
exerted by set of rolls rotate in opposite directions.

Rotating rolls perform two main functions:

1. Pull the work into the gap between them by friction
between work-part and rolls.
2. Simultaneously squeeze the work to reduce its cross
section.
Flat rolling

Important terminologies:
Bloom: It has a square cross section 150 mm x 150 mm or more.
Slab: It is rolled from an ingot or a bloom and has a rectangular cross section of 250 mm width or more and
thickness 40 mm or more.
Billet: It is rolled from a bloom and is square in cross-section with dimensions 40 mm on a side or more.
Blooms are rolled into structural shapes like rails for railroad tracks.

Billets are rolled into bars, rods. They become raw materials for machining, wire drawing, forging, extrusion
etc.

Slabs are rolled into plates, sheets, and strips. Hot rolled plates are generally used in shipbuilding, bridges, boilers,
welded structures for various heavy machines, and many other products.
Rolling is classified according to the temperature of work piece rolled as in the following comparison:

Hot Rolling Cold Rolling

Hot rolled steel (also known as HR Steel) is a steel Cold rolled steel (also known as CR steel) is a steel
which is heated above its recrystallization
temperature (above 780°C) and then it is roll- which is roll-pressed at (or near) the room
temperature to get the desired shape.
pressed to a desired shape.

Amount of mechanical force required for hot rolling Amount of force required for cold rolling is more
is less (because the metal is soft and ductile). (because the metal is hard and brittle).

Hot rolling operation is comparatively faster than cold Cold rolling operation is slower than hot rolling.
rolling.

Surface finish obtained by hot rolling is not good. Surface finish obtained by cold rolling is better than
that obtained with hot rolling.

Hot rolled steels show less dimensional accuracy. Cold rolled steels show higher
dimensional accuracy.
Large sized steels can be hot rolled Cold rolling is suitable for small sized steels.
The plates and sheets are further reduced in thickness by cold rolling to strengthen the metal and
permits a tighter tolerance on thickness.

Important advantage is that the surface of the cold-rolled sheet does not contain scales and generally
superior to the corresponding hot rolled product.

Later the cold-rolled sheets are used for stampings, exterior panels, and other parts used in automobile,
aerospace and house hold appliance industries.
 Types of Rolling mills Important

Two high rolling mill: This type of rolling mill consists of two rolls rotating in opposite directions.

Roll diameters: 0.6 to 1.4 m

Types: either reversing or non-reversing.

Non-reversing mill: rolls rotate only in one direction, and the slab always
move from entry to exit side.

Reversing mill: direction of roll rotation is reversed, after each pass, so that the slab can be passed
through in both the directions. This permits a continuous reductions to be made through the same
pairs of rolls.
Used for hot rolling and initial
breakdown of ingots and large
billets.

Two high rolling

mill
Advantages:
•Simple design – Easy to construct and maintain
•High durability – Suitable for rough breakdown of large workpieces
•Reversible type – Increases productivity with fewer handling steps
•Cost-effective – Good for small-scale or primary rolling operations
Limitations:
•Non-reversible mills require extra handling for multiple passes
•High friction between rolls and material → increased wear and energy consumption
•Not suitable for very thin sheet production or precision rolling
•Limited flexibility compared to multi-roll systems (e.g., four-high or cluster mills)
Applications:
•Hot rolling of steel and non-ferrous metals
•Initial breakdown of ingots into slabs, blooms, or billets
Three high rolling mill: In this case, there are three rolls one above the other. At
a time, for single pass, two rolls will be used. The roll direction will not be
changed in this case.
The top two rolls will be used for first reduction and the sheet is shifted to the
bottom two rolls and further reduction is done. This cycle is continued till actual
reduction is attained.

Advantages:
•No need to reverse roll rotation, reducing mechanical complexity Three high rolling
•Increased production speed due to continuous rolling mill
•Lower downtime since roll reversal is not required
•Useful for rolling large sections or billets in successive passes
•Better utilization of rolling energy compared to two-high non-reversing mills
Limitations:
•Automated mechanism is required to shift the slab
•Not ideal for very thin or precision rolling
•Higher initial setup cost than two-high mills
•Roll wear is not uniform due to alternate rolling directions
Applications:
•Hot rolling of slabs and billets
•Used in rail and structural beam production
Four high rolling mill: Small-diameter rolls (less strength & rigidity) are supported by
larger-diameter backup rolls. Using small rolls reduces power consumption but increases
the roll deflection. In this configuration, two small rolls, called working rolls, are used to
reduce the power and another two, called backing rolls, are used to provide support to the
working rolls. Two backup rolls, generally much larger than the operating rolls, is placed
against the two operating rolls to prevent their distortion. These are called four-high
stands.
• It is primarily used for rolling thin sheets with high accuracy and surface finish.
• The metal is passed between the two small working rolls. Four high rolling
• The backup rolls apply pressure to prevent deflection of working rolls under load. mill
• This allows for higher rolling forces, enabling the production of thinner sheets.
• Ensures uniform thickness and better control of mechanical properties
Advantages:
•Enables rolling of very thin sheets without roll deflection
•Improved surface finish and dimensional accuracy Applications:
•Can handle higher rolling loads than two-high or three-high mills •Cold rolling of stainless steel, aluminum,
•Reduces material wastage and increases production efficiency copper, and high-carbon steels
•Suitable for cold rolling of high-strength materials •Production of thin sheets, strips, and foils
Limitations: •Commonly used in automotive, aerospace,
•More complex and expensive setup compared to simpler mills electrical, and packaging industries
•Requires precise alignment and maintenance of backup rolls •Ideal for high-precision rolling in modern steel
•Not suitable for initial breakdown of large ingots (used after plants
roughing)
 Cluster rolling mill: It is a type of multi-roll rolling mill that consists of two small-
diameter working rolls supported by two or more backup rolls arranged in a cluster
formation. This design minimizes deflection and allows precise rolling of very thin
materials.
•Metal is passed between the small working rolls
•Backup rolls provide support, reducing deflection under high rolling loads Cluster rolling mill
•Smaller working rolls allow rolling of very thin sheets with great precision
•This configuration permits higher rolling forces while maintaining roll rigidity
•Cluster mills are specialized for applications requiring extreme precision and thin material rolling

Tandem rolling mill:

This consists of series of rolling stations of the order of 8 to 10. In each station, thickness reduction is given to the
sheet. With each rolling station, the work velocity increases.
This is fully used in industry practice, along with continuous casting operation. This results in reduction in floor space,
shorter manufacturing lead time.

Tandem rolling mill:

Thread
rolling

Thread rolling is used to create threads on cylindrical parts by rolling them between two
dies as shown in figure.
It is used for mass production of external threaded parts like bolts and screws.

Ring rolling
Ring rolling is a forming process in which a thick walled ring part of smaller diameter is
rolled into a thin walled ring of larger diameter.
As the thick walled ring is compressed, the deformed material elongates, making the
diameter of the ring to be enlarged.

Application: ball and roller bearing races, steel tires for railroad wheels, rings for pipes,
pressure vessels, and rotating machinery
Start of process Completion of process
(thick (thin walled, large
walled, small diameter)
diameter)
Ring
rolling
 Simple analysis of flat
strip rolling
The schematic of flat rolling is shown in previous slides. It involves rolling of
sheets, plates having rectangular cross section in which the width is greater
than the thickness.

In flat rolling, the plate thickness is reduced by squeezing between two rolls.
The thickness reduction is quantified by draft which is given by,
d = t0 – tf
here t0 and tf are initial thickness and final thickness of the sheet used for
rolling.

Draft is also defined as, r = d / t0 . Here r is reduction.

During rolling, the workpiece width increases which is termed as spreading.

It will be large when we have low width to thickness ratio and low friction
coefficient.

In strip rolling, t0 w0l0  t f w f l f and hence t0 w0v0  t f w f v f

In strip rolling, the width will not change much after rolling.
From the previous equation, it is observed that the exit
velocity vf is greater than entry velocity v0. In fact, the
velocity of the rolled sheet continuously increases from entry
to exit.
The rolls contact the rolling sheet along
an arc defined by angle θ. Each roll has
radius R, and its has surface velocity vr.
This velocity is in between entry and exit
velocity.

However, there is one point or zone along

the contact arc where work velocity equals
roll velocity. This is called the no-slip point,
or neutral point.

On either side of the neutral point,

slipping and friction occur between roll
and sheet. The amount of slip between
the rolls and the sheet can be quantified
v
by forwardv slip,r S, vf is the final velocity, vr is the roll
S v
f
r velocity
The true strain during rolling is  
given by, t0 tf
ln( )
The true strain is used to find the average flow stress (Yf) and
power,
further rolling K n
force.
Yf 
1 n

On the entry side of the neutral point, friction force is in one direction,
and on
the other side it is in the opposite direction, i.e., the friction force acts
towards the neutral point. But the two forces are unequal.

The friction force on the entry side is greater, so that the net force
pulls the sheet
through the rolls. Otherwise, rolling would not be possible.
d max   2
The limit to
R the maximum possible draft that can be accomplished in
flat rolling
The is given
equation by, that if friction were zero, draft is zero, and it is
indicates
not possible to accomplish the rolling operation.
The friction coefficient in rolling depends on lubrication, work
material, and
working temperature.

In cold rolling, the value is app. 0.1, in warm rolling, a typical value
is around 0.2; and in hot rolling, it is around 0.4.

Hot rolling is characterized by sticking friction condition, in which

the hot work surface adheres to the rolls over the contact region.
This condition often occurs in the rolling of steels and high-
temperature alloys.
When sticking occurs, the coefficient of friction can be as high as
0.7.
The contact length (projected) is approximated R(t0  t f )
by,  force (F) is calculated by, F  Y wL , wL is the contact area
Lroll
The rolling
The power required for two powered rolls
f is given by, P=
(2πN)FL (watts)
 L
Area under the
curve, rolling F  w
force, F,
pdL
0

Typical variation in roll pressure along the contact length in

flat rolling
Practice 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 0.12.
Determine if the friction is sufficient to permit the rolling operation to be
accomplished. If so, calculate the roll force, and horsepower (or rolling power).

Inference from equations: The strip rolling force and/or power of a given width and
work material can be reduced by the following methods: (1) using hot rolling
rather than cold rolling to reduce strength and strain hardening (K and n) of the
work material; (2) reducing the draft in each rolling pass; (3) using a smaller roll
radius ‘R’ to reduce force; and (4) using a lower rolling speed ‘N’ to reduce power.
 Important
Defects
1. Wavy Edges
Cause:
• Non-uniform elongation across the width of the strip
• Roll bending due to high rolling force
Prevention:
• Use backup rolls to prevent working roll deflection
Wavy Edges
• Proper roll alignment and load distribution
• Use cambered rolls for thickness uniformity

2. Zipper Cracks (Center Cracks)

Cause:
• Excessive tensile stress in the center during rolling
• Low ductility of material
Prevention:
• Reduce the amount of reduction per pass
• Use materials with better ductility Zipper Cracks (Center Cracks)
• Apply appropriate intermediate annealing
3. Edge Cracks
Cause:
• High tensile stress at the edges
• Material inhomogeneity or improper roll pass design
Prevention:
• Control reduction ratio
• Ensure uniform temperature distribution
• Use correct roll profiles Edge Cracks

4. Alligatoring (Splitting of Material)

Cause:
• Non-uniform flow of material across thickness
• Trapped scale or oxide layer
• Defective raw material
Prevention:
• Ensure proper heating before rolling
• Use high-quality billets or ingots Alligatoring
• Maintain good surface preparation to remove scale
5. Laminations
Cause: 7. Scale Pits
• Inclusions or gas porosity in the raw material Cause:
• Poor consolidation during previous processes • Oxide scale from hot rolling embeds into surface
Prevention: Prevention:
• Use clean, high-quality billets/ingots • Descale properly before rolling (e.g., water jet,
• Apply vacuum degassing or proper forging brushing)
before rolling • Control reheating temperature to minimize scale
formation

6. Thickness Variation
Cause: 8. Surface Scratches or Dents
• Uneven roll wear or misalignment Cause:
• Non-uniform temperature/lubrication during • Foreign particles or debris on rolls or workpiece
rolling • Damaged roll surfaces
Prevention: Prevention:
• Regular roll maintenance and alignment • Ensure clean rolling surfaces
checks • Regular inspection and polishing of rolls
• Maintain consistent temperature and
lubrication