Emerging secondary steel making processes
Introduction
Secondary steel making is the term applied to the processes for treatment
of liquid steel between the primary steel making unit and the casting.
These processes involve in refining operation to provide metallurgical
benefits which include better alloy recovery, homogenous molten bath
composition, accurate control of temperature for casting, better de-oxidation,
excellent de-sulphurisation, low levels of hydrogen oxygen & nitrogen
contents, achievement of low inclusion rating through promoting inclusion
separation coupled with inclusion modification and so on. It is the
increasing demand of high quality steel that prompted development of
secondary steel making technology. Also, secondary steel making enhance
the productivity of primary steel making units and improves the overall
economics.
Methods of secondary steel
making
There are varied categories of secondary steel making facilities that
are available in the world today. Broadly, secondary steel making units
categorized are based on (a) Stirring Systems (b) Ladle Heating Systems
(c) Vacuum Degassing Systems and (d) Injection Systems. The application
of a particular unit for the melt shop depends upon the specific needs
of the plant and the product made. However, it is the final product
that determines the choice of the process.
Stirring
systems
These systems involve in stirring the molten steel bath for obtaining
homogenous temperature, composition, inclusion floatation and promotion
of slag-metal refining reaction. Stirring systems are further classified
as Ladle Stirring and Vessel Stirring.
Ladle stirring
Here, stirring is carried out either by gas or by electro-magnetic methods.
Gas Stirring process is a method where stirring is done through injection
of inert gas into the steel bath. Stirring results from the expansion
of gas due to heating and decrease in pressure as the gas rises. One
of the methods is injection of inert gas through deeply inserted refractory
lance from the top in to the molten steel bath. These lances may be
of T, Y or straight bore type. Initially, nitrogen was used as medium
for purging that resulted in increased nitrogen pick up in steels. This
led to application of argon gas for stirring to produce steels with
low nitrogen. Gas stirring by purging argon through the porous plug
located at the bottom of the ladle has evolved as the most effective
method of gas stirring. From the simple argon purging from porous plug,
further developments took place in the form of using snorkel over the
steel bath by sealed argon bubbling and provision of composition adjustment
through the process known as CAS method. Here, the slag remains undisturbed
and limits the detrimental effects of primary furnace slag contamination
like poor recovery of aluminium, increased phosphorous reversion etc.
The best advantages of gas stirring method can be accrued through mixing
a basic reducing slag with steel in the inert environment to simultaneously
achieve de-oxidation & de-suphurisation. Also, the argon stirring helps
in reducing the hydrogen content and improves the cleanliness of the
steel by floatation of oxide inclusions. Electro-Magnetic Stirring process
is a stirring method involving induction stirring through electro-magnetic
coils positioned close to the ladle. Here, the supply of turbulent currents
through the coils diametrically at 1/3rd and 2/3rd of the ladle depth
below the surface of the molten steel induces stirring action. This
method provides lower stirring energy than the gas stirring system with
better stirring energy distribution with controlled stirring action.
The stirring system is reported to be an excellent process for floatation
and separation of non-metallic inclusions.
Vessel stirring
One of the most popular secondary steel making process for stainless
steel production is through Argon Oxygen De-carburization (AOD) unit.
It is a low cost stainless steel production method that can absorb large
amounts of scrap and high carbon ferrochrome. The initial carbon content
of the melt is about 3% and the process possesses the capability to
achieve carbon levels of maximum 0.015%.The steel melted in Electric
Arc Furnace is transferred to AOD where oxygen and argon are injected
into the molten bath through the tuyeres located at the lower side wall
of the converter. Chromium oxidation increases as the carbon content
is reduced. In this process, to ensure rapid de-carburization but low
chromium losses while conserving argon, a low ratio of argon : oxygen
is injected initially. As the carbon content of the bath decreases,
the ratio is increased. After de-carburization, FeSi is used as reductant
to recover chromium lost to the slag. Basic slag is produced through
addition of sufficient amount of lime for decreasing the activity of
silica and followed by vigorous stirring that enables to offset the
detrimental effect of chromium on bath oxygen content for production
of low oxide inclusions coupled with high degree of de-sulphurization
of the stainless steel. Further developments took place through application
of top and bottom blowing leading to improved production rates.
Ladle heating systems
Ladle furnace Ladle Furnace has come out as a great relief to the primary
steel making. Here, a refractory or a water cooled lid sits on a seal
along the rim of the ladle. Three phase electric power is introduced
through the graphite electrodes for heating the molten steel as a means
to increase temperature with heating rate of about 3ºC – 4 ºC/min. With
the hoppers provided for alloying addition, chemistry adjustment can
be carried out effectively. This furnace thus, acts as an excellent
buffer between the primary melting unit and the continuous caster giving
precise temperature and compositional control. This provides an option
to the primary melting unit to tap at low temperatures leading to saving
in time and energy. Through appropriate slag composition control, de-oxidation
practice and argon stirring, it is possible to produce clean steels
through Ladle furnace.
CAS- OB Process
This is a development to the earlier CAS method of gas stirring wherein
there is provision for oxygen lancing and feeding aluminium through
the snorkel to enable chemical heating of steel for increasing temperature.
AC Plasma method
This is a heating system employing three plasma torches using argon
as the carrier gas. The advantage of this method is avoiding carbon
pick up which is evident in arc heating systems. It is expected that
this technology would come out in a big way for production of ultra
low carbon steels.
Vaccum degassing
systems
The concept of degassing started primarily to control the hydrogen content
in steels but sooner it served many purposes for production of clean
steels. The degassing systems can be further classified as Circulation
Degassers, Tank Degassers and Stream Degassers.
Circulation
degassers
In this process, a vacuum chamber is positioned above the ladle possessing
a snorkel or snorkels which are dipped into the molten steel bath. There
are two types of Circulation Degassers namely Dortmund – Hörder (DH)
and Ruhrstahl – Heraeus (RH) units. DH unit has a single snorkel and
operates by repeatedly sucking the metal into the vacuum chamber and
then releasing it back into the ladle. RH unit has two snorkels dipped
into the ladle. Similar to the DH degasser, the snorkels are covered
with a sheet metal cone at the start of the operation to act as slag
breaker preventing slag from entering the vacuum chamber. Metal is circulated
into the chamber by injecting argon gas into the bottom of one leg.
This induces an up flow; and down flow occurs in the other leg creating
a circulating movement. Here, the slag remains undisturbed leading to
poor de-sulphurisation. New developments in DH and RH degassing units
took place in the form of increased vessel size, stirring energy for
faster & efficient operation coupled with changes in design & refractories
to limit temperature losses about 12ºC to 15ºC through fast and repeated
use. RH-OB is a process which incorporates an oxygen injection facility
near to the bottom of the vacuum chamber to enable production of low
carbon steels. Also, temperature recovery is achieved through use of
aluminium in combination with oxygen and normal degassing practice is
carried out for production of clean steels. Considering suppression
of slag-metal mixing in circulation degassers with no de-sulphurisation,
new techniques have been developed which involve injecting refining
slag into the up leg of RH vessel and is reported to achieve de-sulphurisation
to the tune of 80%.
Tank degassers
Here, the ladle is placed in a vacuum tank and stirred with an inert
gas while the tank is evacuated. Alternatively, the ladle may have a
sealing arrangement on its periphery for a lid to be fitted which forms
the vacuum chamber.
Vacuum
degassing (VD) unit
This is a simple ladle degassing unit with provisions for alloying additions.
Here, vacuum is created through steam ejectors. Pressures as low as
0.5 mm Hg are created and the process is capable in homogenization of
molten steel bath with regard to both temperature & composition, fine
adjustment of chemistry, improved de-oxidation and reduction in hydrogen,
oxygen & nitrogen contents. De-sulphurisation is a big problem for heats
directly processed through VD unit from primary steel melter. However,
the problem can be sorted out through ensuring reduced slag in the ladle
before sending the heat to VD unit and enhanced de-sulphurisation is
caused by slag-metal mixing.
Vacuum arc degassing (VAD)
unit
This is a single station unit in which the ladle sits in a vacuum tank
and is stirred 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. In this system, 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%. VAD is now a widely used method
of producing clean steels in the world.
ASEA – SKF unit
It is a process which possesses integrated group of treatment stations
usually consisting of separate de-slagging, arc heating and vcuum treatment
stations. Here, slag is removed by re-ladling to prevent re-phosphorisation
after which ferro-alloy addition is carried out. Arc heating is done
to raise temperature for compensating the cooling effect of the alloying
additions followed by degassing in a vacuum atmosphere for reducing
the oxygen content and de-hydrogenation for achieving hydrogen contents
as low as 1.5 ppm. The method involves application of electro-magnetic
stirring which helps in floating inclusions and result in production
of clean steels. Presently, ASEA-SKF units have incorporated basal inert
gas stirring to enable de-sulphurisation.
Vaccum oxygen de-carburization
(VOD) unit
This is considered to be an important vacuum process for production
of stainless steel. It is particularly suitable for special stainless
steels that require the lowest carbon, nitrogen and hydrogen levels.
In this process, the ladle is placed in vacuum chamber and there is
a provision for oxygen lancing through vacuum tight gland and alloying
additions. Basically, the method involves preferential oxidation of
carbon over chromium leading to minimum chromium losses. Due to reduced
freeboard available in the ladle, the initial carbon content of the
melt should be as low as 1%. Here, oxygen injection is carried out at
100 torr – 250 torr. Silicon is oxidized followed by carbon. De-carburisation
occurs through start of co bubbling determined by initial temperature
and silicon content of the liquid bath. Constant rate of de-carburization
occurs depending on the oxygen flow rate. The CO:CO2 ratio is monitored
and a bath carbon content of 0.08%, it increases rapidly. So, beyond
this limiting carbon percentage, de-carburization rate falls independent
of oxygen flow rate with simultaneous chromium oxidation. Oxygen lancing
is ceased and the vessel pressure is reduced and argon stirring is carried
out further to the reaction between the dissolved oxygen and the remaining
carbon. It has been reported that through vigorous stirring carbon can
be reduced to levels of 0.005% and total (Carbon + Nitrogen) less than
0.015% are achieved. The refining sequence in general is controlled
by combination of variation in oxygen flow rate, the lance tip – bath
surface distance, control of vacuum pressure and the argon flow rate.
Addition of sufficient amount of lime and aluminum helps in excellent
de-sulphurisation of the melt. Stream degassers One of the prominent
stream degassing techniques is Tap Degassing where in the metal from
the furnace is tapped into a small ladle which is seated on a vacuum
tight seal on a vacuum tank containing a second ladle. Here, the stream
is broken up in the vacuum chamber giving fine dispersion of droplets
which provide highly efficient degassing. Another type of stream degassing
is pouring the liquid steel through refractory stream limiter device
in ingot moulds located in a vacuum tank. This is applied for heavy
steel forgings. Injection systems The injection systems are broadly
classified as Gas injection with synthetic slag treatment system, Powder
injection system and Cored Wire injection system. Gas injection with
synthetic slag treatment In this process, a lime based slag mixture
is added to the de-oxidised carry over slag to produce suitable basic
top slag possessing high sulphide capacity. Argon or Nitrogen is injected
into the melt to impart requisite stirring energy for better slag-metal
reaction. De-sulphurisation to the tune of 70% is achieved through addition
of aluminum. The difficulty with this system is the prolonged treatment
time to the tune of 20 min – 25 min for effective de-sulphurisation
resulting in temperature drop coupled with chances of hydrogen pick
up from the lime based slag. Powder Injection systems In this system,
T type refractory coated lances are used which are inserted in ladle
with provision for argon purging. Here, powders like lime based reagents
(CaO-CaF2-Al2O3), calcium carbide, lime based flux with calcium silicide
etc. are injected through these lances by powder dispenser. Low argon
flow rates are applied during injection to result in less splashing
and higher retention time of the reactant in steel due to decreased
plume velocity. This process helps in excellent de-sulphurisation, de-oxidation
and inclusion control for production of clean steels. Cored wire injection
systems This injection system has evolved as a widely acceptable one
in the world today. Cored wire is used for injecting aluminium, calcium,
calcium silicide, sulphur etc. It is made up of a low carbon steel sheath
into which the powdered element or alloy to be injected is encased.
It is generally injected by means of an uncoiler, a feeder and a guide
tube vertically into the ladle containing the molten steel. This system
do not require a carrier gas and hence the turbulence is lower and nitrogen
pick up is lesser. The important parameters that control the wire feeding
is the wire diameter and the feed rate. Commonly, wire diameter of 9
mm to 13 mm are applied with wire feed rate ranging from 100 – 350 mts/min.
The advantages of this system is better alloy recovery as well as effective
inclusion shape control. Highly oxidisable elements like calcium and
aluminium are fed through cored wire leading to improved alloy recovery.
For high sulphur bearing steels, sulphur cored wire injection provides
better recovery of sulphur and ease in heat making. Calcium silicide
cored wire injection helps in better recovery of calcium helping in
modification of sulphide inclusions coupled with calcium aluminate formation
avoiding nozzle clogging in caster caused by high aluminum in steels.
Conclusions The growth of secondary steel making led to the development
of new steel products which enhanced the competitive position of steel.
Continuous developments in the area of secondary steel making are taking
place for improving the productivity, quality of the steel products
and improving the overall economics of steel production in the world.
