Survey of Rolling Processes

MEE 6420

Four Basic Bulk Deformation Processes

1. Rolling – slab or plate is squeezed between opposing rolls

2. Forging – work is squeezed and shaped between opposing dies

3. Extrusion – work is squeezed through a die opening, thereby taking the shape of the opening

4. Wire and bar drawing – diameter of wire or bar is reduced by pulling it through a die opening1. Is the exception in it that uses tensile stress to

accomplish deformation

Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls

Rolling

flat rolling

Shape Rolling

Types of Rolling

• Based on workpiece geometry :• Flat rolling - used to reduce thickness

of a rectangular cross section • Shape rolling - square cross section is

formed into a shape such as an I‑beam

• Ring rolling• Thread Rolling

• Based on work temperature :• Hot Rolling – most common due to

the large amount of deformation required to go from cast product to finished sizes without fracture

• Cold rolling – produces finished sheet and plate stock

Note appearance of surfaces

Shape Rolling is Progressive Shape Change

Hot rolling is used to reduce forces and promote ductility

Shape Rolling Equipment

Shape RollingWork is deformed into a

contoured cross section rather than flat (rectangular)

• Accomplished by passing work through rolls that have the reverse of desired shape

• Products include: • Construction shapes such as

I‑beams, L‑beams, and U‑channels

• Rails for railroad tracks• Round and square bars and

rods

Ring Rolling – Seamless Rings

Ring Rolling

Ring RollingDeformation process in which a

thick‑walled ring of smaller diameter is rolled into a thin‑walled ring of larger diameter

• As thick‑walled ring is compressed, deformed metal elongates, causing diameter of ring to be enlarged

• Hot working process for large rings and cold working process for smaller rings

• Applications: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery

• Advantages: material savings, ideal grain orientation, strengthening through cold working, fatigue resistance

Ring Rolling

Ring rolling is used to reduce the wall thickness and increase the diameter of a ring.

Making a Seamless Ring

Hot Ring Rolling – Radial Axial Mill

Threaded Products

Thread Rolling (Chipless Machining)Bulk deformation process used to form

threads on cylindrical parts by rolling them between two dies

• Important commercial process for mass producing bolts and screws

• Performed by cold working in thread rolling machines

• Advantages over thread cutting (machining):

• Higher production rates• Better material utilization• Stronger threads and better fatigue resistance

due to work hardening

Thread rolling with flat dies: 1) start of cycle and 2) end of cycle.

Thread Rolling

Thread Rolling-Flat Die

Thread Rolling-Circle Die Process

Flat Rolling

Rolling in the Steelmaking Process

Flat rolled products

Some of the steel products made in a rolling mill.

Rolled Products Made of Steel

Rolling Mills• Equipment is massive and expensive• Multiple stands/mills are usually needed• Rolling mill configurations:

• Two-high – two opposing rolls• Three-high – work passes through rolls in both directions• Four-high – backing rolls support smaller work rolls• Cluster mill – multiple backing rolls on smaller rolls • Tandem rolling mill – sequence of two-high mills

Rolling Mills

A rolling mill for hot flat rolling. (Bethlehem Steel).

Frame, drive, jacks, and rolls comprise a mill or stand which are connected together via conveyors

Flat Rolling Technology – Mill Design

The Rolls Rotating rolls perform two main functions:• Friction from Rolls pull the work into the gap• Simultaneously squeeze the work to reduce its cross

section and increase its length

CompressiveStress

2‑high mill

Rolling Mill Designs3‑high reversing mill.

4‑high mill 6‑high or cluster

mill

Flat Rolling TerminologyDraft = amount of thickness

reduction fo ttd

where d = draft; to = starting thickness; and tf = final thickness

Reduction = draft expressed as a fraction of starting stock thickness:

otd

r

where r = reduction

Workpiece may not allow for rolling to be done using a single mill. A series of rolling stands in sequence must be used.

Tandem Rolling Mill

Rolling Mechanics

Note how workpiece velocity must increase while roll velocity will be constant

Note how workpiece thickness decreases as the material passes through the roll gap- We need to use average flow stress

We need to calculate force and power needed to turn the rolls

Flat Rolling Mechanics

Roll speeds (ust account for increased work velocityAssume no deformation perpendicular to rolling direction (plane strain)

in in out out

in outin out

in in out out

in in out

out out in

A l A l

l lA At t

A v A v

A h w v

A h w v

∴𝑣𝑜𝑢𝑡>𝑣 𝑖𝑛Volume in = Volume Out

Rolling Mechanics

At point where friction changes direction, this is termed the no-slip point

At the no-slip point, the roll pressure distribution is at a maximum

We can consider that the average roll pressure is the same as the average flow stress for rolling

Rolling MechanicsWe can develop an estimate of the force and power for a single stand in cold rolling as follows:

For an annealed material:

1

n

fK

Yn

We can also calculate the average flow stress at the i th stand by:

1

1

i

i

n

f

i i

K d

Y

In cold rolling, the average flow stress will increase due to work hardening . In hot rolling we often assume that flow stress is constant.

Note average yield stress will increase as the next stand in tandem cold rolling.

Rolling MechanicsMill LoadForce = Average Yield Stress * areaChange in Width of workpiece = 0 (plane strain)Assume rolling is compression between 2 inclined plates

Roll radius is large compared to arc contact lengthWe can reasonably approximate the length, L, as being a straight line due to roll flattening where

Force F, on each roll, will be

L

Rolling MechanicsF

F

Based on the force equilibrium, we can see that a torque is generated on the roll

When no front or back pull is applied, F is midway along the contact length, L

L

𝑇=𝐹𝐿2

Power is more useful for the drive motor

(inches)Hp = *hp would need to double hp for 2 rolls

Rolling MechanicsWe would like to roll a workpiece using the minimum number of stands due to the equipment and operations costsi.e. maximize the reduction taken at each standClearly, mill load is one factor

Rolling requires that we have sufficient friction to pull the workpiece into the roll gap, this represents a second limiting factor as to the maximum reduction that can be taken

Consider an element at the entry to the roll gap- Roll force (normal)- Friction force (parallel)

Rolling Mechanics

q

q

FN

Ff

Minimum condition for feasible rolling

Fnsin q = Ffcosq= m tanq

R-d/2

d/2

R

Successive Application of Pythagoreans Theorem Yieldsdmax=m2R

Suggests max roughness and roll sizes

Rolling Friction Example• What is the maximum draft that can be

taken for a 300 mm thick slab for cold rolling where m = 0.08 and for hot rolling where m = 0.8 assuming 600 mm diameter rolls?

• m = 0.08 • m = 0.5

We can easily see why fewer mills are needed in hot rolling due to the quadratic function

Cold Rolling Load Example0.2510 ksi

D = 40” diameterW = 24”What is the rolling load at each stand?

.045”.050” .041”

Force F, on each roll, will be

𝐿=√𝑅 (𝑡𝑜−𝑡𝑓 )

1

n

fK

Yn

1

1

i

i

n

f

i i

K d

Y

Annealed

First Stand

Second Stand

Cold Rolling Load Example

0.25.05

10 ln.045

4.61 1.25

n

fK

Y ksin

First Stand

1

1.25 1.251

1 2 1

10 0.2 10 0.16

1 1.25 0.2 0.1

i

i

n

n

f

i i

K dK

Y ksin

Second Stand

.045”.050” .041”

Annealed

1.2510 0.2

Note work hardening from first stand

Cold Rolling Load Example

Area, A = 0.316*24 = 7.58

Now find contact areas, w is constant in flat rolling

First Stand Second Stand

Area, A = 0.288*24 = 6.91in2

Load = 4.6*7.58 = 34.9 kips Load = 6*6.91 = 41.46 kips

Problems in Rolling

No-load

Loaded

Minimum thickness achievable due to roll flatteningnote aluminum foil is 0.2 mm thick

Uneven thickness across workpiece width (crowning)

CrowningSolutions• Cambering the

rolls• Using a higher E

material• Backup rolls• Larger roll radius

(undesirable)

12 High Sendzimir Mill StandUsed for rolling very thin sheet material

Rolling Thin Sheet

Work Roll

Backer Rolls