Introduction:

For most applications the quality of steel produced by
conventional processes is satisfactory. But with time the
quality requirement of steel is increased continuously and
for some special applications secondary steelmaking or
Ladle Metallurgy is developed.

In early days, this was done by modifying or prolonging
the furnace refining process.

However these methods were not practical and if they
proved to be feasible, it was at the cost of loss of
production & under utilization of the equipments.

So this led to development of a series of new and
supplementary steel processing techniques as per
requirements.
Introduction(Contd.)

So major portion of total refining including melting is carried
out in the steelmaking process, so it is called as “Primary
Steelmaking” processes.

The final refining and finishing is carried out in subsequent
processes, so they are called as “Secondary Steelmaking”
processes.

This can also be understood as duplex practice of
steelmaking.

As major refining is done in secondary process, primary
process can be more effectively utilized for melting & primary
refining, thus productivity increases considerably.

As most of these processes are carried out in Ladle itself, it
is referred as Ladle Metallurgy.
Process Varieties:
The varieties are as follows:

Stirring treatments

Synthetic slag refining with stirring

Vacuum treatments

Decarburization techniques

Injection metallurgy

Plunging techniques

Post-solidification treatments

Tundish metallurgy
Stirring Techniques:

This technique was introduced to float deoxidation
products in form of non-metallic inclusions and to obtain
homogeneous chemistry of the bath.

It can be used to help to mix slag & metal to have better
equilibrium conditions in primary steelmaking also.

Stirring of steel bath is achieved by bubbling inert gas like
nitrogen or argon.

It can be done through immersed pipe from top, from side
tuyere, from bottom using a single eccentric plug and
from bottom through multiple plugs as shown in fig.
Stirring Techniques Figure:
Stirring Techniques(Contd.)

The efficiency of stirring is measured in terms of energy
associated with bubble formation & its size.

Homogenization of bath temperature & composition is
achieved because of buoyant energy of injected gas which is
calculated as:
E=14.23[VT/M]*log[(1+H)/1.48 Po]
Where E=Stirring power
V=gas flow rate,Nm3 /min
T=bath temperature in K
M=bath weight, ton
H=depth of gas injection, m
P=gas pressure at bath surface, atm
Stirring Techniques(Contd.)

The rising bubbles tend to lift up non-metallic inclusions, due to its
surface tension effect.

These lifted-up particles are supposed to be absorbed in slag
present at top.

As bubbles rise their size increase due to reduction of ferrostatic
head and helps to lift up the inclusions along with it.

But we have to avoid excessive blowing rate, because it leads to
mixing of slag with metal & increases inclusions in metal.

So the gas is blown at 3.5-4.5 bar pressure & at rate of 100500Ndm3/min.

Since gas is bubbled in cold condition it has a cooling effect on steel
bath temperature.

Temperature drop is given by,
(Mass of bath)x(Sp. Heat)x(Temp drop)=
(Mass of Gas)x(Sp. Heat)x(Bath Temp.)
Stirring Techniques(Contd.)

Argon purging may reduce temperature nearly 1.5° C/min
in bath temperature.

Gas can also be bubbled through porous refractory plugs
made of magnesia or alumina and fitted at the bottom of
ladle.

But the porosity of plug must be maintained at around 3035%.

Also the pores need to be fine enough so that metal does
not enter them due to surface tension effect.

The amount of gas purged is very small as compared to
tuyeres. It is in terms of few m3 per hour.
Synthetic Slag Refining with
Purging:

This involves use of specially prepared pre-melted slags to act as a
sink for non-metallic inclusions or to help refining of some specific
impurities like sulphur, phosphorous etc.

But unstirred slag-metal system is most ineffective in carrying out
interaction because apparent area of contact is too small, which can
be increased by stirring.

These are prepared by melting required oxides in desired
proportions in separate furnace.

Synthetic slags for desulphurization are almost almost free from
FeO & MnO & for dephosphorisation are highly oxidizing & highly
basic in nature.
Synthetic Slag Refining with
Purging(Contd.)





For desulphurization binary type CaO-Al 2 O 3 with
liquidus temperature around 1450. C.
However due to SiO 2 present in system this slag
makes ternary system CaO-Al2O3-SiO2 .
Usual composition is 42-48% Al2O3, 52-58% CaO &
balance is SiO2.
Slag should not contain carbon, free undissolved
lime or fluorine & should have stable composition.
These slags are made by melting them fully then
solidified and supplied in powder form for use when
melted again in steelmaking furnace.
Vacuum Treatments:

Depending on the steel grades to be produced, various after treatment methods
and process combinations can be applied to steel.

Vacuum degassing is practiced in the steel industry for several purposes. They
are : -

to remove hydrogen

to improve cleanliness by removing part of the oxygen

to produce steel of low carbon content ( < 0.03%)

to produce steels to close chemical composition ranges (including deoxidizers),

to control pouring temperatures, especially for continuous casting operations.

To control composition and temperature control; decarburization; micro
cleanliness; and inclusion morphology.

Hydrogen removal is a diffusion and partial pressure phenomenon. Oxygen
removal is a function of chemical reaction of oxygen with carbon and the partial
pressure of carbon monoxide.

Vacuum degassing treatments are divided into 3 categories:
1. Ladle degassing processes(VD, VOD, VAD)
2. Stream degassing processes
3. Circulation degassing processes(DH & RH)
Vacuum Degassing(VD):

Vacuum degassing of steel takes place after the molten steel has left the
furnace and before the steel is poured into ingots or processed through a
caster. The main objectives of steel degassing are:

Reduction/elimination of dissolved gases, especially hydrogen and
nitrogen

Reduction of dissolved carbon for more ductile steel

Preferential oxidation of dissolved carbon over chromium when refining
stainless steel grades.

After leaving the furnace, molten steel is moved in a ladle to the
degassing area and positioned inside the degasser. The ladle is covered
with a layer of slag that is penetrated approximately 18” deep by the
snorkels. As the snorkels are inserted, the steam ejectors create a
vacuum of 0.5 mm HgA in the vacuum chamber to draw the steel into the
chamber. The lower partial pressure within the vacuum chamber removes
both hydrogen and nitrogen gases from the liquid steel, which are both
vented as the steel is continuously circulated. The evacuation time is
usually five minutes or less.
Figure of Vacuum Degassing
Unit:

Vacuum Arc
This is a single station unit in which the ladle sits in a vacuum tank and is stirred
Degassing(VAD):
by inert gas through porous plug at the bottom with provision for heating through
electrodes and alloying additions.

After addition of lime in the molten steel ladle, arcing is carried out at 250 Torr –
300 Torr to raise the temperature & fuse the lime followed by short duration
degassing, additions for chemistry adjustment and deep degassing to pressures
as low as 1 Torr.

Argon stirring is continued in all the operational steps and the adjustment of flow
rate is done for varied operations carried out during processing. The heating rate
is about 3ºC – 4 ºC/min and during heating, argon flow rate is kept on the lower
side.

Under vacuum, carbon-oxygen reaction and carbon-Al2O3 reaction under the
high temperature arc are of great help in achieving low oxygen content without
any solid reaction product.

Hydrogen levels as low as 1.5 ppm are achieved caused by intense mass
transfer by argon and low partial pressure of hydrogen because of dilution of
liberated carbon monoxide.

The greatest advantage of this process is the high degree of de-sulphurisation
as high as 80% for production of steels with sulphur levels as low as 0.005%.
Vacuum Arc
Degassing(VAD) Figure:
Stream Degassing:

In stream degassing technology, molten steel is teemed
into another vessel which is under vacuum. Sudden
exposure of molten stream in vacuum leads to very rapid
degassing due to increased surface area created by
breakup of stream into droplets. The major amount of
degassing occurs during the fall of molten stream. Height
of the pouring stream is an important design parameter.

Stream degassing technology has following variants in
the practice
i. Ladle to mold degassing
ii. Ladle to ladle degassing
Ladle to Mold Degassing:

F i g u r e s h o w s
arrangement of vessels .

Preheated mold with hot
top is placed in vacuum
c h a m b e r. A b o v e t h e
chamber a tundish is
placed.

Steel tapped in the ladle at
superheat equivalent to
30℃ is placed above the
tundish.

Steel is bottom poured in
the tundi sh. One i ngot
could weigh around as
high as 400 tons.
Ladle to Ladle Degassing:

In ladle to ladle degassing, a ladle with the stopper rod is placed in a
vacuum chamber.

Ladle containing molten steel from BOF or EAF is placed on top of
the vacuum chamber and the gap is vacuum sealed.

Alloy additions are made under vacuum.

Stream is allowed to fall in the ladle where molten steel is degassed.

In some plants degassing is done during tapping.

In this arrangement molten steel from EAF is tapped into tundish or
pony ladle.

From the pony ladle molten stream is allowed to fall into a ladle
which is evacuated.

Ladle is closed from top with a special cover which contains exhaust
opening.

Steel with 25℃ to 30℃ superheat is tapped into ladle.
Recirculation Degassing:

In the recirculation degassing technology, molten steel is
allowed to circulate in the vacuum chamber continuously
by special arrangement.

It is done by two techniques:
1. RH degassing(Rheinstahl Heinrich – Shutte developed
at Hattingen, Germany)
2. DH degassing
RH Degassing:

In this technique, cylindrical refractory
lined shell with two legs (also called
snorkel) is designed such that steel is
raised in one leg and falls back into the
ladle after degassing through the second
leg.

Top side of the cylindrical shell is
provided with exhaust, alloy additions,
observation and control window.

Cylindrical shell is lined with fire bricks in
the upper portion, and alumina bricks in
the lower portion in order to sustain high
temperature.

The legs are lined with alumina refractory.

A lifter gas argon is injected at the inlet
snorkel in order to increase the molten
steel velocity entering into inlet snorkel.

Advantages:

Heat losses are relatively low.

Alloy additions can be adjusted more
closely

Small vacuum pumping capacity is
adequate since smaller volume is to be
evacuated as compared with ladle to
ladle or stream degassing.
Steps of Operation:
i) Cylindrical chamber is heated to the desired temperature (varies in between
900℃ to 1500℃ ).
ii) The chamber is lowered into molten steel up to a desired level.
iii)The chamber is evacuated so that molten steel begins to rise in the
chamber. Lifter gas is introduced. This gas expands and creates a
buoyant force to increase the speed of molten steel rising into the inlet
snorkel.
iv)Molten steel in the chamber is degassed and flows back through the other
snorkel into the ladle. This degassed steel is slightly cooler than steel in
the ladle. Buoyancy force created by density difference ( density of cooler
liquid steel is > hot steel) stirs the bath
v)Rate of circulation of molten steel in cylindrical chamber controls the
degassing. Circulation rate depends upon amount of lifter gas and the
degree of vacuum. A 110 T steel can be degassed in 20 minutes by
circulating molten steel at 12 tons/min., amount of argon is around 0.075 �
o 0.075 m3/ton.
vi)Alloy additions can be made at the end of degassing depending on the
superheat.
DH Degassing:

A small amount 10-15% of the
total mass of steel is degassed
at a time. The process is
repeated until required level of
degassing is achieved.

In DH unit, the cylindrical vessel
has one snorkel .

Cylindrical vessel has heating
facility.

The length of the snorkel is
sufficiently large to realize the
effect of atmospheric pressure
on rise of steel in the snorkel.

It can operate with lower
superheats compared with RH
since DH unit has heating facility.
Steps of Operation:
i) DH vessel is preheated and lowered in the ladle so that
snorkel tip dips below the molten steel surface
ii) The evacuated chamber is moved up and down so that
steel enters the chamber
iii) The chamber is moved for 50-60 times with a cycle time
of 20 seconds.
iv) Adequate degassing is possible in 20 -30 cycles.
v) A layer of slag is kept in the ladle to minimize heat losses.
Decarburization Techniques:

Extra low-carbon soft steels can be produced by effective
decarburization in a primary steelmaking furnace itself.

Low alloy low carbon steels can be produced by ladle
additions of low carbon ferro-alloys.

But the production of low carbon high alloy steel is difficult.

Manufacture of stainless steel is a typical example of this.

So special decarburization techniques are developed such
as:
1. Argon Oxygen decarburization(AOD)
2. Vacuum Oxygen decarburization(VOD)
Injection Metallurgy:

It is mainly developed for decreasing sulphur levels.

In injection metallurgy, a strong desulphurizing agent in the
form of fine powder is injected in refined steel held in transfer
ladle, along with an inert gas as carrier.

Very high interfacial area of contact of particles with bath
leads to very efficient interaction.

This process takes about 8-10 minutes & the bath looses 30
to 35 °C temperature.

The reagents may be metallic like calcium, magnesium etc.
in forms like calcium silicide, magnesium coke, lime, calcium
carbide etc. to give fluid oxide melt product.

Strong desulphurizing agents along with strong deoxidizing
agents are injected to give excellent desulphurization &
deoxidation.
Injection Metallurgy(Contd.)

Thyssen Niederrhein(TN) system is most popular in this field.

In this, injecting lance is made up of ceramic material and
argon is used as a carrier gas.

Usually when sulphur specification levels are low, residual Si
and Mn contents are also very low. Even they are high, they
by themselves do not oxidize the bath to the extent required
fot effective desulphurisation.

So it have to be desulphurized by use of aluminium, because
it is stronger deoxidizer.

Al is added in form of big lumps or cubes and rest amount is
added in form of Al wire.

The rate at which it melts into bath is such that it travels in
solid state well below bath surface before it melts.
Plunging Techniques:

It is developed to desulphurize steel bath in transfer ladle or to alloy
it with micro-alloy additions.

In this a small crucible containing reagent is attached at end of
refractory protected rod.

The crucible is plunged in bath with upside down and held inside the
bath for required duration and during which reagent is expected to
interact with steel.

This technique is adopted when total amount of reagent to be added
is so small that it cannot be injected with a carrier gas or addition is
not a routine process required in plant.

Reagents like magnesium-coke briquettes or Rare Earth Metals are
plunged in refined steel bath for desulphurization on commercial
level.

Virgin metals or ferro alloys can similarly be added in steel bath for
achieving micro-alloying additions.
Post-Solidification
Treatments:

In this technique, quality improvement is obtained after
casting into molds primary refining furnace & then
remelting and casting once again.

Typical example: zone refining applied to produce pure
metals.

Techniques applied to steels are developed to produce
alloy steels of better cleanliness & low sulphur contents.

Two techniques are:
1. Vacuum Arc Remelting (VAR)
2. Electro Slag Refining(ESR)
Vacuum Arc Remelting (VAR):

In this, steel ingot from
primary refining acts as
electrode which is drip
melted into a water cooled
copper mold.

This is carried out under
vacuum.

The arc is struck between
the electrode and the mold
& generates heat required
to melt electrode.

The hydrogen & oxygen
contents are very low.
Electro Slag Refining(ESR):

In this a slag layer is used to act as a resistor between electrode and
the mold which gives heat required for melting.

It is carried in open atmosphere.

The choice of slag is highly critical since it has to act as a resistor as
well as refining agents. Mainly used slag is oxy-fluoride type
reducing slags like CaO-CaF2.

In both processes electrodes melts progressively and resolifies
unidirectionally.

Due to high temperature, small pool of molten metal and almost
unidirectional solification, these processes produce sound castings
of high density.

The composition of product is nearly same as that of original material
but with improved cleanliness, decreased segregation and with
practically no cavities.

Ingot size ranges from about 200 to 1500 mm on industrial level.
Electro Slag Refining(ESR)
Figure:
VAR & ESR(Contd.)

Product of both processes is exceptionally suited for production of
forgings of high alloy steels.

But the cost is prohibitively high.

So applications is limited to specialty products such as turbo rotor
shafts etc.

ESR has several advantages:
1.
Multiple electrodes can be melted.
2.
Spacing between mold & electrode is not critical.
3.
Surface quality is superior.
4.
Can do desulphurisation upto 0.002% sulphur.
5.
Round, square, hollow & rectangular ingots can be produced.
6.
High weight ingots can be produced.
Tundish Metallurgy:

It is a simple device where liquid steel stays before entering mold.

It can be used for further removal of inclusions with or without additional
deoxidation.

So gentle stirring of bath and increasing residence time of steel are the
parameters to achieve this.

Tundish is designed to improve steel flow patterns to help coalescence and
floating of inclusions. Synthetic slag covers, ceramic traps are used to remove
them.

Final deoxidation adjustments, desulphurization, trimming alloy additions,
inclusion modifications and temperature homogenisation and contol can be done
in tundish metallurgy.

However tundish can also be a source of silicon pick up if proper precautions are
not taken.

It is necessary to have some insulating material layer to decrease heat losses.

Rice husk is used as insulating material which is rich is silica & also very active
so gives Si pick up.

So if rice husk is to be used, a thin liquid layer first on bath surface & then the
rice husk.
Furnace Developed for Ladle
Metallurgy:

Ladle metallurgy is carried out in transfer ladle fitted with
a porous plug for argon purging.

If plug is not there, a refractory lance can be used for
purging.

However bath tends to loose temperature in this
treatment so it cannot be carried out for so long time.

So Ladle Furnace is developed for carrying out most of
secondary refining, also proved to be economical.
Ladle Furnace:

A Ladle Furnace is used to relieve the primary melter of most
secondary refining operations, and its primary functions are:

Reheating of liquid steel through electric power conducted by
graphite Electrodes.

Homogenization of steel temperature and chemistry through inert
gas stirring.

Formation of a slag layer that protects refractory from arc damage,
concentrates and transfers heat to the liquid steel, trap inclusions
and metal oxides, and provide the means for desulphurization.

Alloy additions to provide bulk or trim chemical control.

Cored wire addition for trimming or morphology control.

Provide a means for deep desulphurization.

Provide a means for dephosphorization.

Act as a buffer for down stream steelmaking equipment.
Typical Sketch of ASEASKF Ladle Furnace:
Process Steps:
1.
Tapping primary furnace into the ladle directly.
2.
Controlled stirring during the entire secondary
processing.
3.
Vacuum treatment including minor decarburization.
4.
Extensive decarburization for stainless steelmaking.
5.
Deoxidation.
6.
Desulphurization and deslagging.
7.
Alloying to desired extent.
8.
Temperature adjustment.
9.
Teeming from the same ladle for further processing.
Slag fluidity requirements for
operations with a ladle furnace
(LF):
All the fluxing oxides should be added to ladle as
soon as possible, preferably during tap.
The amounts of lime, dolomite and fluxing oxides
should be sufficient to cover the arc in the ladle
furnace (3 to 4") Mixtures of pure components can be
used (lime + bauxite or lime + fluorspar + silica sand).
The slag will be superheated by the arc and the heat
will spend enough time at the LF station for all the
components to go into solution.
The slags should be fluid, but not watery - slags with
a "creamy" consistency.
The "creaminess" of the slag should be maintained
throughout the heat by adding lime to slag if it
becomes too watery.
Heating and arc control:




The electrode arcs must be electrically balanced
and proper aligned to prevent excessive arc
impingement on the ladle walls.
The temperatures of carbon arc plasmas are over
6000°F and well melt quickly trough any type of
refractory lining.
If the arcs are imbalanced electrically or are
physically misaligned, this can cause the arc flare
to overheat a small section of the ladle slag line
leading to early failure of the slagline.
Serious damage can also occur if the arc length is
to long and the arc is not covered by the slag layer
in the ladle. The maximum heating rate that is
typically used is 5-7°F/min.
Thank You…