Post on 07-Dec-2015
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Advances in Steelmaking and Secondary Steelmaking
Smarajit SarkarDepartment of Metallurgical and Materials Engineering
NIT Rourkela
In BOF steelmaking, oxygen of high purity (at least 99.9%
oxygen) is blown at supersonic speed onto the surface of the
bath using a vertical lance, inserted through the mouth of the
vessel. During the initial stages of development of the BOF
process, only single-hole lances were used, but with
increasing vessel size, multi-hole lances have come into vogue
so that large volumes of oxygen (typically 1000-1200 Nm3/min.
for 160-180 t converters) can be blown within the restricted
blowing time of 15-20 minutes.
The Lance
The use of multi-hole lances reduces the chances of any
individual oxygen jet penetrating anywhere near the vessel
bottom, since with a larger number of holes, the total jet
energy gets dispersed along the diameter of the vessel rather
than in the vertical direction. This has also resulted in higher
productivity, since more liquid metal is exposed to oxygen.
Further, the larger the number of holes in the lance, the
faster will be the slag-metal reactions like
dephosphorisation. Such reactions can then take place at a
greater number of reaction sites.
Next slide schematically shows the nature of jet-bath
interaction. The Mach Number can be as high as 2.5, when the
Supersonic Jet emerges from the nozzle. In the potential core
(length three to seven times nozzle diameter), the velocity is
constant. Then the jet starts entraining the surrounding fluid (in
this case, the gaseous converter atmosphere). This Jet
Entrainment causes lateral expansion of the jet and decreases
the jet velocity to make it finally subsonic. Beyond a distance
about 25-30 nozzle diameters, the supersonic jet becomes
fully subsonic.
Interaction of the Oxygen Jet with Surroundings and the bath
The jet ultimately impinges on the liquid metal bath surface to form a cavity. The impingement of the jet and the dissipation of the jet momentum causes circulation of the liquid bath in the upward direction at the vessel central axis. The intensity of jet-bath interaction is expressed in terms of the Jet Force Number (JFN) defined as:
Where L, the height of the lance tip above the bath surface,
is a key operating variable in the BOF process. With
changing JFN (say, by changing L), the following behaviour
of the liquid bath at the impact zone has been observed.
At low JFN, dimpling with a slight surface depression
At medium to high JFN, splashing with a shallow
depression
At high JFN, penetrating mode of cavity with reduction in
splashing.
The L.D. process was developed using a lance with a cylindrical nozzle. The
physics of a jet issuing from such a nozzle was hardly understood at that time.
The drawbacks of using a cylindrical nozzle for steelmaking were, therefore,
unknown. The successful development and commercial adoption of the L.D.
process later on led to the study of physics of the supersonic jets and thereby
develop a proper lance design.
It is now known that the supersonic jet issuing from the nozzle of a lance in a
L.D. process should penetrate the bath adequately and that the area of its
impact on the bath should be maximum. These conditions are essential
chiefly for efficient refining, i.e. for decarburisation as well as dephosphorisation.
.
The static pressure in a jet from a cylindrical
nozzle, as it emerges into the ambient atmosphere,
is more than the atmospheric pressure. It,
therefore, interacts with the atmosphere generating
shock waves and the velocity of the jet decreases
with damped fluctuations. This affects the bath
penetration as well as area of impact adversely
For a given size nozzle the length of the supersonic core depends on
the blowing pressure and the ratio of the densities of the jet-gas and the
ambient atmosphere. Although the density of the ambient atmosphere in
the L.D. process changes during the blow, an average value is assumed
to calculate the length of the supersonic core.
During the blow the jet should be expanded to obtain maximum impact
area at the bath surface. At the same time, it should also penetrate the
bath surface to a maximum extent. The depth of penetration of a jet
in a metal bath varies inversely with the impact area at the bath
surface. The requirements, therefore, can only be met at the optimum.
In the blowing position the lance height from the still bath level has to be
more than the length over which the supersonic core extends in the jet,
since the jet is not fully expanded until that point. In actual practice the
proper height would be around 40-50 times the diameter of the
nozzle.
It may be mentioned here that decarburisation is faster for greater
values of JFN and dephosphorisation is faster for reverse conditions.
The gas flow rate from a nozzle can be calculated by assuming a
frictionless and adiabatic flow through the nozzle. The jet behaviour does
not alter adversely even if the actual flow rate deviates by ± 20% from this
nominal value.
Much of these drawbacks are eliminated if a convergent
divergent laval shaped nozzle is used. The static
pressure in a jet from a laval shaped nozzle disappears
within a short distance from the nozzle tip and hence it
does not interact much with the ambient atmosphere.
The velocity of the jet decreases more uniformly with
much less of damped fluctuations, if the inside and the
outside diameters of the nozzle are properly designed..
The velocity at any point in the stream is more than
at the corresponding point of the stream from a
similar size cylindrical nozzle under similar
conditions of blowing. The resultant bath
penetration is more in the case of laval shaped
nozzle than that due to cylindrical nozzle. The laval
shaped nozzle is, therefore, universally adopted
Increase in total throughput oxygen without any adverse effect,
at the same pressure Improvement in jet spread on the metal bath.
These two lead to: less of slopping and spitting and thus less of mechanical losses, in turn
better yield improved mixing of slag and metal and thereby better mass transport and
hence better rate of refining less of danger of burning vessel bottom in spite of increased oxygen blowing
rate better gas recovery and improved lining life better thermal balance and hence more of coolant scrap or ore is required improved slag basicity from around 3 to 3·5 much improved turndown %P, from earlier 0·034 % to 0·017% high residual Mn in the bath so that less of Fe-Mn is subsequently required for
deoxidation.
The advantages of multi hole lance
Comparison of performance of single and multi-hole lance
OXYGEN BOTTOM MAXHUTTE PROCESS(OBM)
The OBM vessel is essentially a Bessemer-like converter fitted with a special bottom .
The tuyeres are inserted from the bottom in such a way that the oxygen would be surrounded by a protective hydrocarbon gas like propane.
On entry propane cracks down in an endothermic reaction and takes up some of the heat-gene rated by the entry of oxygen.
The relative feed rates of these two fluids are adjusted to obtain optimum temperatures at the tuyere tip and thereby ensure its reasonable life as well as speed of refining.
The deposition of carbon, which is a product of cracking, also helps to protect the bottom from heat generated due to the refining reactions at the tips of tuyeres.
In order to promote turbulence in the bath and thereby ensure good slag-metal contact, the tuyeres are arranged only on half the converter bottom.
Experience dictated that provision of a few bigger tuyeres is better than large number of fine tuyeres. Maintenance problems are minimised without loosing in terms of metallurgical requirements of turbulence. By this arrangement, it is ensured that the direction of metal circulation is upwards in the tuyere half of the vessel, and downwards in the other half.
This arrangement is also helpful in minimising the damage to tuyeres while charging scrap, since it can now be charged on that part where there are no tuyeres.
Sequence of elimination of impurities
Oxidation of carbon : Bottom blowing increases sharply the
intensity of bath stirring and increases the area of gas-metal
boundaries (10-20 times the values typical of top blowing) .
Since the hydrocarbons supplied into the bath together with
oxygen dissociate into H2, H2O and CO2 gas bubbles in the
bath have a lower partial pressure of carbon monoxide (Pco )
All these factors facilitate substantially the formation and
evolution of carbon monoxide, which leads to a higher rate of
decarburization in bottom blowing
Bottom blowing Vs Top blowing
The degree of oxidation of metal and slag
Removal of phosphorous: Since the slag of the bottom-blown converter process have a low degree of oxidation almost during the whole operation, the conditions existing during these periods are unfavorable for phosphorus removal
Cont..
Almost 98% oxygen being reacted with metal in OBM and
hence that much scrap rate is lower in the OBM. If scrap is
cheaper the top blowing can offer some cost advantage in
this respect.
The iron losses in top blown are nearly 5% more than
those in OBM. Very low carbon steers are achievable in
top blowing only at the expense of extra iron loss in slag.
But this is readily achievable in OBM.
OXYGEN BOTTOM MAXHUTTE PROCESS(OBM)
This also, leads to situation wherein higher carbon levels
can be obtained by 'catch carbon techniques' easily in
LD than in OBM, at low P contents.
The stirring intensity, which is estimated to be nearly ten
times more in OBM than in LD gives better partition of
phosphorus and sulphur, higher manganese and lower
oxygen at turndown result ing in better ferroalloy
recovery.
Cont..
Since the slag of the bottom-blown converter process have a low
degree of oxidation almost during the whole operation, the
conditions existing during these periods are unfavorable for
phosphorus removal. Only at the end of blowing, when the bath
is low in carbon, the oxidation degree of the slag increases
sharply, thus favouring dephosphorization. At that moment,
phosphorus passes intensively to slag. When using lumpy lime in
the charge, it is difficult to make medium or high carbon steels
with a low content of phosphorus. The metal must be blown to a
low carbon content, so as to form an oxidizing slag at the end of
heat, and then carburized in the ladle.
Cont..
Problems arise when the layer of foaming slag created on the surface of the molten metal exceeds the height of the vessel and overflows, causing metal loss, process disruption and environmental pollution. This phenomenon is commonly referred to as slopping.
Slopping
Better mixing and homogeneity in the bath offer the following
advantages:
Less slopping, since non-homogeneity causes formation of
regions with high supersaturation and consequent violent
reactions and ejections.
Better mixing and mass transfer in the metal bath with closer
approach to equilibrium for [C]-[O]-CO reaction, and consequently,
lower bath oxygen content at the same carbon content.
Metallurgical features of Bath Agitated Process:
Better slag-metal mixing and mass transfer and consequently, closer approach to slag - metal equilibrium, leading to: o lower FeO in slag and hence higher Fe yield o transfer of more phosphorus from the metal to the slag (i.e.
better bath dephosphorisation) o transfer of more Mn from the slag to the metal, and thus
better Mn recovery o lower nitrogen and hydrogen contents of the bath.
More reliable temperature measurement and sampling of metal and slag, and thus better process control
Faster dissolution of the scrap added into the metal bath
•A small amount of inert gas, about 3% of the volume of oxygen
blown from top, introduced from bottom, agitates the bath so
effectively that slopping is almost eliminated.
•However for obtaining near equilibrium state of the system inside
the vessel a substantial amount of gas has to be introduced from
the bottom.
•If 20-30% of the total oxygen, if blown from bottom, can cause
adequate stirring for the system to achieve near equilibrium
conditions. The increase beyond 30% therefore contributes
negligible addition of benefits.
Hybrid Blowing
• The more the oxygen fraction blown from bottom the
less is the post combustion of CO gas and
consequently less is the scrap consumption in the
charge under identical conditions of processing.
• Blowing of inert gas from bottom has a chilling effect on
bath and hence should be minimum. On the contrary
the more is the gas blown the more is the stirring effect
and resultant better metallurgical results. A optimum
choice therefore has to be made judiciously.
Cont..
As compared to top blowing, the hybrid blowing
eliminates the temperature and concentration gradients
and effects improved blowing control, less slopping and
higher blowing rates. It also reduces over oxidation and
improves the yield. It leads the process to near
equilibrium with resultant effective dephosphorisation
and desulphurisation and ability to make very low
carbon steels.
Cont..
What is blown from the bottom, inert gas or oxygen? How much inert gas is blown from the bottom? At what stage of the blow the inert gas is blown,
although the blow, at the end of the blow, after the blow ends and so on?
What inert gas is blown, argon, nitrogen or their combination?
How the inert gas is blown, permeable plug, tuyere, etc.?
What oxidising media is blown from bottom, oxygen or air?
If oxygen is blown from bottom as well then how much of the total oxygen is blown from bottom ?
The variety of hybrid processes along with amount of basal gas injected
The processes have been developed to obtain the combined ad vantages of
both LD and OBM to the extent possible. Therefore the metallurgical
performance of a hybrid process has to be evaluated in relation to these two
extremes, namely the LD and the OBM. The parameters on which this can be
done are :
Iron content of the slag as a function of carbon content of bath
Oxidation levels in slag and metal
Manganese content of the bath at the turndown
Desulphurisation efficiency in terms of partition coefficient
Dephosphorisation efficiency in terms of partition coefficient
Hydrogen and nitrogen contents of the bath at turndown
Yield of liquid steel
Metallurgical Superiority of Hybrid Blowing
The oxidizing conditions of a heat in a steelmaking plant, the
presence of oxidizing slag, and the interaction of the metal with the
surrounding atmosphere at tapping and teeming - all these factors
are responsible for the fact that the dissolved oxygen in steel has a
definite, often elevated, activity at the moment of steel tapping. The
procedure by which the activity of oxygen can be lowered to the
required limit is called deoxidation. Steel subjected to deoxidation is
termed 'deoxidized'. If deoxidized steel is 'quiet during solidification
in moulds, with almost no gases evolving from it, it is called 'killed
steel'.
Deoxidation of steel
If the metal is tapped and teemed without being deoxidized, the reaction
[O] + [C] = COg will take place between the dissolved oxygen and
carbon as the metal is cooled slowly in the mould. Bubbles of carbon
monoxide evolve from the solidifying metal, agitate the metal in the
mould vigorously, and the metal surface is seen to 'boil'. Such steel is
called 'wild'; when solidified, it will be termed 'rimming steel' .
In some cases, only partial deoxidation is carried out, i.e. oxygen is only
partially removed from the metal. The remaining dissolved oxygen
causes the metal to boil for a short time. This type of steel is termed
'semi-killed'.
Thus, practically all steels are deoxidized to some or other extent so
as to lower the activity of dissolved oxygen to the specified limit.
The activity of oxygen in the metal can be lowered by two methods: (I)
by lowering the oxygen concentration, or
(2) by combining oxygen into stable compounds.
There are the following main practical methods for deoxidation of steel:
(a) precipitation deoxidation, or deoxidation in the bulk;
(b) diffusion deoxidation;
(c) treatment with synthetic slags; and
(d) vacuum treatment.
The advantages of continuous casting (over ingot casting) are:
It is directly possible to cast blooms, slabs and billets, thus eliminating blooming, slabbing mills completely, and billet mills to a large extent.
Better quality of the cast product. Higher crude-to-finished steel yield (about 10 to
20% more than ingot casting). Higher extent of automation and process control.
Continuous casting
Continuous casting may be defined as teeming of liquid metal in a short
mould with a false bottom through which partially solidified ingot is
continuously withdrawn at the same rate at which metal is poured in the
mould. The equipment for continuous casting of steel consists of :
The ladle to hold steel for teeming.
The tundish to closely regulate the flow of steel in the mould.
The mould to allow adequate solidification of the product.
The withdrawal rolls to pullout the ingot continuously from the
mould.
The cooling sprays to solidify the ingot completely.
The bending and/or cutting devices to obtain hand able lengths of
the product.
The auxiliary electrical and/or mechanical gears to help run the
machine smoothly.
Vertical type continuous casting machine
High rate of flow of cooling water on the mould surface
continuously removes heat, which is known as primary cooling.
The metal is only partially solidified at the mould exit; the remainder
of the cooling and solidification occurs below the mould by:
Secondary cooling by water sprays
Tertiary cooling by radiation below the secondary cooling zones.
Next slide gives a schematic representation of the steps involved in
continuous casting. The length of the secondary cooling zone is
normally 8 to 10 times that of the primary cooling zone.
HEAT TRANSFER AND SOLIDIFICATION IN CONTINUOUS CASTING
Simplified sketch of continuous casting
Solidification must be completed before the withdrawal rolls.
The liquid core should be bowl-shaped as shown in the
Figure and not pointed at the bottom (as indicated by the dotted lines), since the latter increases the tendency for undesirable centerline (i.e. axial) macro-segregation and porosity
The solidified shell of metal should be strong enough at
the exit region of the mould so that it does not crack or breakout under pressure of the liquid.
The major requirements of continuous casting
All the above requirements can be achieved only if the heat
extraction from the metal, both in the mould region and in the
secondary cooling zone, is carried out satisfactorily. The higher the
casting speed, the lesser is the time available for heat extraction in
the mould. By convention, casting speed (vc) is expressed as the
rate of linear movement of the ingot in meters per minute.
Therefore, the longer the length of the liquid core as well as the
mushy zone, the lesser would be the thickness of the shell when
the ingot emerges from the mould. Hence, there is a maximum
permissible (i.e. limiting) casting speed (vc,max)
qav is not the same for all the strands in CC machines. qav exhibits an
overall range of 800 to 2000 kW/m2 of ingot surface area.
It is clear that vc,max can be increased by increasing qav for a certain
strand (i.e. for R = constant).
At the same value of qav , P max increases proportionately with R (i.e.
strand size).
For example, a sample calculation shows that vc.max for a slab caster
of size 2 m x 0.3 m, it is about 3.5 to 4 times lower than in the case of
a 0.15 m x 0.15 m billet caster. On the other hand, Pmax is 3.5 to 4
times higher for the slab casting case
Sustained efforts are being made by steel plants to the increase
casting speed without sacrificing quality. Next slide shows the
increase in casting speeds of slab casters in recent years in the case
of some steel companies in Japan.
It will be seen that after 1990, there has not been any substantial
increase. This is because at very high casting speeds, problems are
encountered in terms of product quality. Hence, it is not possible to
increase Vc arbitrarily.
In recent years, the principal emphasis has been on increasing the
heat flux in the mould region to increase productivity.
Increase in casting speed in recent years in the main slab casters in Japan.
The surface area-to-volume ratio per unit length of continuously cast ingot is larger than that for ingot casting. As a consequence, the linear rate of solidification (dx/dt) is an order of magnitude higher than that in ingot casting.
The dendrite arm spacing in continuously cast
products is smaller compared with that in ingot casting.
METALLURGICAL COMPARISON OF CONTINUOUS CASTING WITH INGOT CASTING
Macro-segregation is less, and is restricted to the
centreline zone only.
Endogenous inclusions are smaller in size, since they
get less time to grow. For the same reason, the blow
holes are, on an average, smaller in size.
Inclusions get less time to float-up. Therefore, any non-
metallic particle coming into the melt at the later stages
tends to remain entrapped in the cast product.
Cont…
In addition to more rapid freezing, continuous casting differs from ingot casting in several ways. These are noted below.
Mathematically speaking, continuously cast ingot is infinitely long.
Hence, the heat flow is essentially in the transverse direction, and
there is no end-effect as is the case in ingot casting (e.g. bottom
cone of negative segregation, pipe at the top, etc.).
The depth of the liquid metal pool is several metres long. Hence,
the ferrostatic pressure of the liquid is high during the latter
stages of solidification, resulting in significant difficulties of blow-
hole formation.
Since the ingot is withdrawn continuously from the mould, the frozen layer
of steel is subjected to stresses. This is aggravated by the stresses
arising out of thermal expansion/ contraction and phase transformations.
Such stresses are the highest at the surface. Moreover, when the ingot
comes out of the mould, the thickness of the frozen steel shell is not very
appreciable. Furthermore, it is at around 1100-1200°C, and is therefore,
weak. All these factors tend to cause cracks at the surface of the ingot
leading to rejections.
Use of a tundish between the ladle and the mould results in extra
temperature loss. Therefore, better refractory lining in the ladles, tundish,
etc. are required in order to minimise corrosion and erosion by molten
metal.
Segregation
Segregation means departure from the average composition. If the concentration
is greater than the average it is called positive and if it is less than the average, it
is called negative segregation.
It is often estimated as percentage departure from the average composition.
Segregation is the result of differential solidification, a characteristic of all liquid
solutions.
Steel is a liquid solution of S, Si, C, P, Mn, etc. in iron and hence is prone to
segregation during solidification. The initial chill layer of the ingot has practically
the same composition as that of the steel poured in the mould, i.e. there is no
segregation in the chill layer because of very rapid rate of solidification. The
progressive solidification thereafter results in solidifications of purer phase (rich in
iron) while the remaining liquid gets richer in impurity contents.
A killed ingot cast in wide-end-up mould shows two types of seg
regation as shown in slide. The impurity segregation at the top follows
the shape of the pipe and is known as V segregation. Side by side
inversed V or A-shaped segregation is also observed at the top.
It may be due to the sinking of purer crystals down and rising up of
the impure liquid in the upper part. The impurities get entrapped in
impure part at the end of solidification. This is the positive
segregation.
The negative segregation is confined to the lower central portion of
the ingot. In the actual ingot these zones are not as sharp as are
shown in slide; these are quite diffused.
Killed steel ingot showing segregation
When an ingot of wide freezing range is poured against a chill
mould, a solute-rich region (instead of the usual pure, solute-poor
region) may be obtained in the vicinity of the chill. This
phenomenon is called inverse segregation. The shrinkage during
solidification causes the solute-rich liquid to flow through the inter-
dendritic channels in a direction opposite to the interface motion.
Segregation increases with increasing time of solidification
required for an ingot, so that large ingots tend to segregate more
than small ingots.
During solidification of an alloy, the solute atom partitions itself in different
proportions in the liquid and solid. Under nonequilibrium conditions of
cooling, coring manifests itself and the solute gets segregated in the
volume of the liquid that solidifies last. During dendritic growth, the liquid to
solidify last is in the spaces between the dendritic arms. This segregation
of the solute in the solid that forms last is known as microsegregation.
The chill zone which solidifies first is usually purer. The central part of the
ingot has a concentration of solutes higher than the average.
Macrosegregation is caused by the physical movement of the liquid and
the solid in the semi-solidified '"mushy" region.
Microsegregation and Macrosegregation
Homogenization is the process of heating the casting for a
prolonged time at a high temperature. This allows diffusion to
occur in the solid state and tends to wipe out or reduce micro-
segregation.
The distance over which diffusion is to occur and the time of
annealing during homogenization are determined by the dendritic
arm spacing. Interstitial elements such as carbon in steel become
fully homogeneous, whereas substitutional elements, which
diffuse much more slowly, may be only partly homogenized.
Homogenization does not remove macrosegregation, where the
diffusion distances are much larger.
AS the fraction of the solid in the "mushy" region increases, the liquid is not
able to flow freely and compensate for shrinkage. This results in
microporosity. The strains generated by shrinkage can fracture the weak
solid. This phenomenon is known as hot tearing.
When a deoxidizer is added to a melt, the deoxidatIon product is often a
solid. When aluminium or silicon is used to deoxidize molten steel, Al2O3 or
SiO2 particles form in the melt. These are called pri mary inclusions, as
they form before solidification starts. Secondary inclusions form during or
after solidification, e.g., MnS in steels.
Porosity and inclusions
Secondary inclusions are usually present in interdendritic
regions. Primary inclusions are present within the dendrites,
but sometimes found in interdendritic regions, if they have
been pushed by the thickening dendrites.
A troublesome class of impurities in cast metals are the
dissolved gases. The decrease in solubility of oxygen in steel
results in the reaction between oxygen and carbon in the steel
to produce bubbles of CO. These are examples of gas porosity.
The following are some of the characteristics of different steel ingots.
The upper part containing the exposed pipe in killed steels has to be rejected
and this decreases the yield to about 80%. The yield from a rimmed ingot is
higher.
Only a killed steel can be continuously cast. In contrast to ingot steel, the yield
in continuous casting is more than 90 %. A rimmed steel cannot be
continuously cast, as the rimming action can puncture holes through the thin
solidified layer of the cast slab and the liquid steel may pour out
uncontrollably.
The turbulence during gas evolution in a rimmed ingot physically transports
the metal to different parts, causing macro-segregation to a greater extent.
Secondary Steelmaking
Smarajit SarkarDepartment of Metallurgical and Materials Engineering
NIT Rourkela
Primary steelmaking is aimed at fast melting and rapid refining. It is capable of refining at a macro level to arrive at broad steel specifications, but is not designed to meet the stringent demands on steel quality, and consistency of composition and temperature that is required for very sophisticated grades of steel. In order to achieve such requirements, liquid steel from primary steelmaking units has to be further refined in the ladle after tapping. This is known as Secondary Steelmaking.
Secondary steelmaking
improvement in quality improvement in production rate decrease in energy consumption use of relatively cheaper grade or
alternative raw materials use of alternate sources of energy higher recovery of alloying elements.
Secondary steelmaking is resorted to achieve one or more of the following requirements :
Lower impurity contents . Better cleanliness. (i.e. lower inclusion
contents) Stringent quality control. (i.e. less variation
from heat-to-heat) Microalloying to impart superior properties. Better surface quality and homogeneity in
the cast product.
Quality of Steel
The term clean steel should mean a steel free of inclusions. However, no steel can be free from all inclusions.
Macro-inclusions are the primary harmful ones. Hence, a clean steel means a cleaner steel, i.e., one containing a much lower level of harmful macro-inclusions.)
Clean Steel
In practice, it is customary to divide inclusions by size into macro inclusions and micro inclusions. Macro inclusions ought to be eliminated because of their harmful effects. However, the presence of micro inclusions can be tolerated, since they do not necessarily have a harmful effect on the properties of steel and can even be beneficial. They can, for example, restrict grain growth, increase yield strength and hardness, and act as nuclei for the precipitation of carbides, nitrides, etc.
Inclusions
The critical inclusion size is not fixed but depends on many factors, including service requirements.
Broadly speaking, it is in the range of 5 to 500 µm (5 X 10-3 to 0.5 mm). It decreases with an increase in yield stress. In high-strength steels, its size will be very small.
Scientists advocated the use of fracture mechanics concepts for theoretical estimation of the critical size for a specific situation.
Macro and Micro Inclusions
Precipitation due to reaction from molten steel or during freezing because of reaction between dissolved oxygen and the deoxidisers, with consequent formation of oxides (also reaction with dissolved sulphur as well). These are known as endogenous inclusions.
Mechanical and chemical erosion of the refractory lining Entrapment of slag particles in steel Oxygen pick up from the atmosphere, especially during
teeming, and consequent oxide formation. Inclusions originating from contact with external sources
as listed in items 2 to 4 above, are called exogenous inclusions.
Sources of Inclusions
With a lower wettability (higher value of σMe – inc ),
an inclusion can be retained in contact with the
metal by lower forces, and therefore, can break
off more easily and float up in the metal. On the
contrary, inclusion which are wetted readily by the
metal, cannot break off from it as easily.
Removal of Inclusions
Carryover slag from the furnace into the ladle should be minimised, since it contains high percentage of FeO + MnO and makes efficient deoxidation fairly difficult.
Deoxidation products should be chemically stable.
Otherwise, they would tend to decompose and transfer oxygen back into liquid steel. Si02 and Al203 are preferred to MnO. Moreover the products should preferably be liquid for faster growth by agglomeration and hence faster removal by floatation. Complex deoxidation gives this advantage.
Cleanliness control during deoxidation
Stirring of the melt in the ladle by argon flowing through bottom tuyeres is a must for mixing and homogenisation, faster growth, and floatation of the deoxidation products. However, very high gas flow rates are not desirable from the cleanliness point of view, since it has the following adverse effects:
o Too vigorous stirring of the metal can cause disintegration
of earlier formed inclusion conglomerates.o Re-entrainment of slag particles into molten steel. o Increased erosion of refractories and consequent
generation of exogenous inclusions. o More ejection of metal droplets into the atmosphere with
consequent oxide formation.
Cont…
The speed of floating of large inclusion can be found by Stoke’s formula
The varieties of secondary steelmaking processes that have proved to be of commercial value can broadly be categorised as under:
Stirring treatments Synthetic slag refining with stirring Vacuum treatments Decarburisation techniques Injection metallurgy Plunging techniques Post-solidification treatments.
Process Varieties
Various secondary process and their capabilities
VOD(vacuum oxygen
decarburization)
Submitted byABHISEK PANDA
108MM003
What is VOD???
A modification of the tank degassers is the vacuum oxygen decarburizer (VOD), which has an oxygen lance in the centre of the tank lid to enhance carbon removal under vacuum. The VOD is often used to lower the carbon content of high-alloy steels without also overoxidizing such oxidizable alloying elements as chromium.
This process is charecterized by:slag-free tapping at the melting furnace, application of ladles with sufficient freeboard, inert gas stirring through the ladle’s bottom by means of porous plugs, oxygen lance with high efficiency and minimised splashing.
Here the vital player is the vacuum treatment which reduces carbon without reducing the alloying elements to a greater extent.
Why vacuum treatment needed??? Vacuum treatment of molten steel descreases
the partial pressure of CO, bubbles of CO are formed in liquid state,float up and then they are removed.At this pressure oxidation of chromium is not feasible.hence low carbon high alloy status is maintained.
It also helps in removing hydrogen dissolved in liquid steel.gaseous nitrogen and nitrogen inclusions are also removed.
Movement of molten steel caused by CO bubble also helps in refining steel from non-metallic inclusions.
Steels refined in vacuum are characterized by homogenous structure,low-content of non-metallic inclusion and low gas porosity.
VOD design
VOD design contd.
The VOD system essentially consists of a vacuum tank,a ladle furnace with or without argon stirring,a lid with oxygen lancing facility.
The ladle has a free board of about a metre to contain violent agitation of the bath during lancing.the charge is molten metal from arc furnace.the percentage of carbon in molten metal in VOD process is about 0.7%-0.8%.
Argon stirring is required to faster the kinetics.
VOD design contd.
At the end of refining,the vacuum is broken and the bath is deoxidized with Al and Fe-Si.
Desulphurization can be carried out by putting synthetic slag .
Argon purging would also result in sulphur removal around 80%.
Since many of the stainless steels are required to be vacuum treated to decrease the gas content,the vacuum system could easily be modified to incorporate oxygen lancing facility and there by VOD can be brought about for producing low carbon steels,without much xtra investment.
Final Composition
THE total VOD cycle lasts for 2 -2.5 hours. Final sulphur content-0.01% Final carbon content-0.02% Final chromium content-15-18%(recovery
~97%) The final composition shows that for
producing low carbon high alloy steel,it’s a very good method.
Benefits of VOD
Deep carbon removal Low loses of chromium in treatment of
stainless steel Sulphur removal Precise alloying Temperature and chemical uniformity. Non-metallic inclusions removal
Application of VOD
Stainless steel production Homogenization of ladle content Manufacturing large steel ingots Manufacturing rails,bail-bearings,other high
quality steels Here the initial carbon percentage in molten
metal before treated in VOD is 0.7%-0.8%.that is a limitation ,where as in other ladle degassing routes,it could be allowed up to 2%.
Recent improvement on VOD
To improve the operation and control of the vacuum oxygen decarburization process,the treatment of stainless steel will be optimized in terms of major metallurgical operations and resource consumption.New VOD operation practices like injection of scale FeO and EAF slag;and the control of vacuum pressure will be investigated with respect to their influence on temperature and decarburization.further an increase of energy efficiency of EAF-VOD mode is required
REFERENCE Steel Making by A. Ghosh, A. Chaterjee An introduction to Modern Steel Making by
R.H.Ttupkarey, V.R. Tupkarey
Composition Adjustment by Sealed argon bubbling
It is a simple ladle like furnace provided with bottom plug for argon
purging and lid with electrodes to become an arc furnace for heating
the bath.
Another lid may be provided to connect it to vacuum line, if required.
Chutes are provided for additions and an opening even for injection.
In short it is capable of carrying out stirring, vacuum treatment,
synthetic slag refining, plunging, injection etc. all in one unit
without restraint of temperature loss, since it is capable of being
heated independently.
Ladle Furnace
Every ladle furnace need not be equipped with all these arrangements. As per the requirements of refining the ladle furnace may be provided with the necessary facilities. For example if gas content is no consideration, vacuum attachment may be eliminated. The principal component of the facilities are shown in next slide schematically.
Cont..
Principle component of a ladle furnace facility
The ASEA-SKF furnace is a special variety of LF furnace only.
The SKF furnace is essentially a teeming ladle for which additional
fittings are provided.
The metal in the ladle is stirred by an electromagnetic stirrer provided
from outside.
The ladle shell is made of austenitic stainless steel for this reason.
Two ladle covers are employed. One of these fits tightly on to the ladle
forming a vacuum seal, and is connected to a steam ejector unit for
evacuation of the ladle chamber.
For vacuum decarburisation oxygen lance is introduced through a
vacuum sealed port located in the cover.
ASEA-SKF Furnace
When the decarburisation and vacuum degassing is over the first cover is replaced by the second cover which contains three electrodes. Final alloying and temperature adjustments are then made.
Steel can also be desulphurised by preparing a reducing
basic slag under the electrode cover.
The process is schematically shown in next slide. The nearly
re fined steel in only one of the primary steelmaking processes
can be treated in this furnace by carrying out the following
operations :
Tapping primary furnace into the SKF ladle directly .
Controlled stirring during the entire secondary processing
Vacuum treatment including minor decarburisation
Extensive decarburisation for stainless steelmaking.
Deoxidation.
Desulphurisation and deslagging. Alloying to desired extent.
Temperature adjustment.
Teeming from the same SKF ladle.
The scheme of operation of SKF Furnace
Quality improvement of steel can also be brought about after steel
is refined and cast into ingots from the primary refining furnace,
by remelting and casting once again. Typical examples of this
type is zone refining which is adopted to produce purer metals.
The other two techniques that have been developed are meant
for the production of, not pure metals, but alloy steels of better
cleanliness and low sulphur contents. The vacuum arc remelting,
VAR(750kWh/ton) for short and the electro slag refining, ESR
(900-1300kWh/ton) for short, are commercially used for further
refining of steels after these are cast into ingots.
Post-Solidification Treatments
In both of these processes the steel ingot produced by the primary refining forms the electrode to be drip-melted into a water cooled copper mould.
In VAR melting is carried out under vacuum and in ESR it is in open atmosphere.
In VAR arc is struck between the electrode and the mould and it generates the heat required for melting the electrode.
In ESR a slag layer is used to act as a resistor between the electrode and the mould and which is responsible for melting the electrode. The slag also acts as a refining agent.
VAR and ESR Processes
In both of these processes the electrode melts progressively and is resolidified on the mould, nearly unidirectionally.
Because of the high temperature, small pool of molten metal and almost unidirectional solidification, both of these processes can produce sound ingots of high density. The composition of the product is nearly the same as that of the original material but with improved cleanliness, decreased segregation and with practically no cavities. The ingot size ranges from about 200 to 1500mm on industrial level
The product of both of these processes is exceptionally suited for the production of forgings of high alloy steels. But because of high cost of such a process, applications are limited to specialty products like turbo rotor shafts and so on.
In VAR the hydrogen and oxygen contents are very low but in ESR they are like ordinary steels. In ESR the choice of the slag composition is fairly critical since it has to act as a resistor as well as a refin ing agent. These are essentially oxy-fluoride type reducing slag like CaO-CaF2·
The ESR however has some advantages over VAR and these are given below:
Multiple electrode can be melted into a single electrode.
Spacing between the mould wall and the electrode is not critical.
Surface quality is superior requiring little or no conditioning.
Steel can be desulphurised to as low as 0·002% sulphur.
Round, square, hollow and rectangular shapes of ingots can be produced.
Ingots of much larger weight can be produced.
Vacuum Arc Remelting Process
Electro Slag Remelting Process
Ladle degassing processes (VD, VOD, VAD)
Stream degassing processes
Circulation degassing processes (DH and RH).
Vacuum Degassing Processes
Sketch of a RH degasser
Molten steel is contained in the ladle. The two legs of the vacuum
chamber (known as Snorkels) are immersed into the melt. Argon is
injected into the up leg.
Rising and expanding argon bubbles provide pumping action and lift
the liquid into the vacuum chamber, where it disintegrates into fine
droplets, gets degassed and comes down through the down leg
snorkel, causing melt circulation.
The entire vacuum chamber is refractory lined. There is provision for
argon injection from the bottom, heating, alloy additions, sampling and
sighting as well as video display of the interior of the vacuum chamber.
RH DEGASSER
RH-OB Process
Why RH-OB Process?
To meet increasing demand for cold-rolled steel sheets with improved
mechanical properties, and to cope with the change from batch-type to
continuous annealing, the production of ULC steel (C < 20 ppm) is
increasing.
A major problem in the conventional RH process is that the time
required to achieve such low carbon is so long that carbon content at
BOF tapping should be lowered. However, this is accompanied by
excessive oxidation of molten steel and loss of iron oxide in the slag.
It adversely affects surface the quality of sheet as well.
Hence, decarburization in RH degasser is to be speeded up. This is achieved by some oxygen blowing (OB) during degassing.
The RH-OB process, which uses an oxygen blowing facility during degassing, was originally developed for decarburization of stainless steel by Nippon Steel Corp., Japan, in 1972.
Subsequently, it was employed for the manufacture of ULC steels.
The present thrust is to decrease carbon content from something like 300 ppm to 10 or 20 ppm within 10 min.
SS MakingFerrochrome, which contains about 55 to 70% chromium is the principal source of Chromium. This ferroalloy can be classified into various grades, based primarily on their carbon :ontent, such as:
Low carbon ferrochrome (about 0.1 % C). Intermediate carbon ferrochrome (about 2% C). High carbon ferrochrome (around 7% C).
Amongst these grades, the high carbon variety has the drawback that though it is the least expensive, it raises the carbon content of the melt. This is undesirable, since all SS grades demand carbon contents less than 0.03%.
Chromium forms stable oxides. Hence, the removal of carbon from the bath by oxidation to CO is associated with the problem of simultaneous oxidation of chromium in molten steel.
The higher the temperature, the greater is the tendency for preferential
oxidation of carbon rather than chromium. From this point of view, higher bath
temperatures are desirable; however, too high a temperature in the bath gives rise to
other process problems.
The dilution of oxygen with argon lowers the partial pressure of CO, which
helps in preferential removal of CO without oxidising bath chromium. Attempts
were made to use this in the EAF, but the efforts did not succeed. Hence, as is the
case with the production of plain carbon steels, the EAF is now basically a melting
unit for stainless steel production as well. Decarburisation is carried out partially in
the EAF, and the rest of the carbon is removed in a separate refining vessel. In this
context, the development of the AOD process was a major breakthrough in stainless
steelmaking.
AOD is the acronym for Argon-Oxygen Decarburisation. The process was patented by the Industrial Gases Division of the Union Carbide Corporation In an AOD converter, argon is used to dilute the other gaseous species (02, CO, etc.). Hence, in some literature, it is designated as Dilution Refining Process. After AOD, some other dilution refining processes have been developed. Lowering of the partial pressures, such as the partial pressure of carbon monoxide, is achieved either by argon or by employing vacuum
The combination of EAF and AOD is sufficient for producing ordinary grades
of stainless steels and this combination is referred to as a Duplex Process.
Subsequent minor refining, temperature and composition adjustments, if
required, can be undertaken in a ladle furnace. Triplex refining, where electric
arc furnace melting and converter refining are followed by refining in a vacuum
system, is often desirable when the final product requires very low carbon and
nitrogen levels.
About 65-70% of the world's total production of stainless steel is in the
austenitic variety, made by the duplex EAF-AOD route. If the use of AOD
converters even in the triplex route is included, the share of AOD in world
production would become as high as 75-80%.
AOD PROCESS
Conventional AOD, no top blowing is involved. Only a
mixture of argon and oxygen is blown through the
immersed side tuyeres. However, the present AOD
converters are mostly fitted with concurrent facilities for
top blowing of either only oxygen, or oxygen plus inert
gas mixtures using a supersonic lance as in BOF
steelmaking.
AOD PROCESS
Initially, when the carbon content of the melt is high, blowing
through the top lance is predominant though the gas mixture
introduced through the side tuyeres also contains a high
percentage of oxygen.
However, as decarburisation proceeds, oxygen blowing from
the top is reduced in stages and argon blowing increased. As
stated earlier, some stainless steel grades contain nitrogen as
a part of the specifications, in which case, nitrogen is
employed in place of argon in the final stages.
Cont..
Use of a supersonic top lance as in the case of BOFs
allows post-combustion of the evolved CO gas with
consequent minimisation of toxic carbon monoxide in the
exit gas as well as utilisation of the fuel value of CO to
raise the bath temperature.
Towards the end of the blow, when the carbon content is
very low and is close to the final specification, only argon
is blown to effect mixing and promote slag-metal reaction.
At this stage, ferrosilicon and other additions are made.
Silicon reduces chromium oxide from the slag.
If extra-low sulphur is required, the first slag is removed and
a fresh reducing slag is made along with argon stirring.
The purpose of the other additions is to perform both alloying
as well as cooling of the bath, since the bath temperature
goes beyond 1700°C following the oxidation reactions.
Simplified by Hiltey and Kaveney
Thermodynamics of reactions in the AOD Process
Influence of pressure and temperature on the retention of Cr by oxygen saturated steel melts at 0.05%C
ARGON OXYGEN DECARBURIZATION
(AOD)
Over 75% of the world’s stainless steel is made using the Argon Oxygen Decarburization (AOD) process
Invented by Praxair. It provides an economical way to produce stainless steels with minimal losses of precious elements.
AOD is widely used for the production of stainless steels and specialty alloys such as silicon steels, tool steels, nickel-base alloys and cobalt-base alloys.
After initial melting the metal is then transferred to an AOD vessel where it will be subjected to three steps of refining
Decarburization Reduction Desulphurization
Schematic diagram of AOD
How It works AOD is part of a duplex process in which scrap or
virgin raw materials are first melted in an electric arc furnace (EAF) or induction furnace.
Molten steel containing most of the chromium and nickel needed to meet the final composition of SS is tapped from electric arc furnace into a transfer ladle
AOD vessel is rotated into a horizontal position during charging of liquid steel so that the side mounted tuyuers are above the bath level.
The molten metal is then decarburized and refined in a special AOD vessel to less than 0.05% carbon.
The key feature in the AOD vessel is that oxygen for decarburization is mixed with argon or nitrogen inert gases and injected through submerged tuyeres.
In conventional AOD no top blowing is involved. Only a mixture of argon and oxygen is blown through the immersed side tuyeres.
Present AOD convertors are mostly fitted with concurrent facilities for top blowing of either only oxygen or oxygen + inert gas mixture using a supersonic lance as in BOF steel making
This argon dilution minimizes unwanted oxidation of precious elements contained in special steels, such as chromium.
ELECTRIC ARC FURNACE
TRANSFER LADLE O2
N2
ARGON AOD ARGON OXYGEN DECARBURIZATION
BOTTOM POURINGLADLE
INGOT PROCESS
PACKAGE CONTINUOUS CASTING MACHINE
CUT OFF
Decarburization Prior to the decarburization step, one more step should be
taken into consideration: de-siliconization, which is very important factor for refractory lining and further processing.
The decarburization step is controlled by ratios of oxygen to argon or nitrogen to remove the carbon from the metal bath. The ratios can be done in any number of phases to facilitate the reaction. The gases are usually blown through a top lance (oxygen only) and tuyeres in the sides/bottom (oxygen with an inert gas shroud). The stages of blowing remove carbon by the combination of oxygen and carbon forming CO gas.
To drive the reaction to the forming of CO the partial pressure of CO is lowered using argon or nitrogen. Since the AOD vessel isn't externally heated, the blowing stages are also used for temperature control. The burning of oxygen increases the bath temperature.
Reduction After a desired carbon and temp level have been
reached the process moves to reduction
Reduction recovers the oxidized elements such as Cr from the slag
To achieve this, alloy additions are made with elements that have a higher affinity for oxygen than Cr, using either Si alloy or Al
The reduction mix also includes CaO and fluorspar CaF2.
The addition of lime and fluorspar help with driving the reduction of Cr2O3 and managing the slag, keeping the slag fluid and volume small
Desulphurization Desulphurization is achieved by having a high lime
concentration in the slag and a low oxygen activity in the bath
S(bath) + CaO (slag)→ CaS (slag) +O(bath) So, additions of lime are added to dilute sulfur in the metal
bath. Also, Al or Si may be added to remove oxygen. Other trimming alloy additions might be added at the end
of the step. After sulfur levels have been achieved the slag is removed
from the AOD vessel and the metal bath is ready for tapping. The tapped bath is then either sent to a stir station for further chemistry trimming or to a caster for casting.
References
http://www.praxair.com/praxair.nsf/0/48740DF62F17EB22852569DE007457CC/$file/P-10018.pdf
http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=220
IRON MAKING AND STEEL MAKING By:Ahindra Ghosh and Amit Chatterjee
Inert Gas Purging
COREX smelting reduction process
This process produces molten iron in a two-step reduction melting
operation. One reactor is melter-gasifier and the other is pre-
reducer. In the pre-reducer, iron oxide is reduced in counter-flow
principle. The hot sponge is discharged by screw conveyors into the
melting reactor.
Coal is introduced in the melting-gassifying zone along with
oxygen gas at the rate of 500-600 Nm3/thm. The flow velocity is
chosen such that temperature in the range of 1500-1800°C is main
tained. The reducing gas containing nearly 85% CO is hot dedusted
and cooled to 800-900°C before leading it into the pre-reducer
Finex process
In the FINEX Process fine ore is preheated and reduced to DRI in a
train of four or three stage fluidized bed reactors.
The fine DRI is compacted and then charged in the form of Hot
Compacted Iron (HCI) into the melter gasifier. So, before charging
to the melter- gasifier unit of the FINEX unit, this material is
compacted in a hot briquetting press to give hot compacted iron
(HCI)
since the melter- gasifier can not use fine material (to ensure
permeability in the bed).
Non-coking coal is briquetted and is fed to the melter gasifier where
it is gasified with oxygen
FINEX PROCESS
As a standard guide the temperature rise attainable by oxidation of 0·01 % of each of the element dissolved in liquid iron at 1400°C by oxygen at 25°C is calculated assuming that no heat is lost to the surroundings and such data are shown below.
Ahindra Ghosh and Amit Chatterjee: Ironmaking and Steelmaking Theory and Practice, Prentice-Hall of India Private Limited, 2008
Anil K. Biswas: Principles of Blast Furnace Ironmaking, SBA Publication,1999 R.H.Tupkary and V.R.Tupkary: An Introduction to Modern Iron Making, Khanna Publishers. R.H.Tupkary and V.R.Tupkary: An Introduction to Modern Steel Making, Khanna Publishers. David H. Wakelin (ed.): The Making, Shaping and Treating of Steel (Ironmaking Volume), The
AISE Steel Foundation, 2004. Richard J.Fruehan (ed.): The Making, Shaping and Treating of Steel (Steeelmaking Volume), The
AISE Steel Foundation, 2004. A.Ghosh, Secondary Steel Making – Principle & Applications, CRC Press – 2001. R.G.Ward: Physical Chemistry of iron & steel making, ELBS and Edward Arnold, 1962. F.P.Edneral: Electrometallurgy of Steel and Ferro-Alloys, Vol.1 Mir Publishers,1979 B. Ozturk and R. J. Fruehan,: "Kinetics of the Reaction of SiO(g) with Carbon Saturated Iron":
Metall. Trans. B, Vol. 16B, 1985, p. 121. B. Ozturk and R. J. Fruehan: "The Reaction of SiO(g) with Liquid Slags,” Metall. Trans.B,
Volume 17B, 1986, p. 397. B. Ozturk and R. J. Fruehan:”.Transfer of Silicon in Blast Furnace": , Proceedings of the fifth
International Iron and Steel Congress, Washington D.C., 1986, p. 959. P. F. Nogueira and R. J. Fruehan:” Blast Furnace Softening and Melting Phenomena - Melting
Onset in Acid and Basic Pellets", , ISS-AIME lronmaking Conference, 2002, pp. 585.
Paulo Nogueira, Richard Fruehan: "Blast Furnace Burden Softening and Melting Phenomena-Part I Pellet Bulk Interaction Observation", , Metallurgical and Materials Transactions B, Volume 35B, 2004, pp. 829.
P.F. Nogueira, Richard J. Fruehan: 'Fundamental Studies on Blast Furnace Burden Softening and Melting", Proceedings of 2nd International Meeting on lronmaking, September 2004, Vitoria, Brazil.
Paulo F. Nogueira, Richard J. Fruehan, "Blast Furnace Softening and Melting Phenomena - Part III: Melt Onset and Initial Microstructal Transformation in Pellets", submitted to Materials and Metallurgical Transactions B.
Paulo F. Nogueira, Richard J. Fruehan :Blast Furnace Burden Softening and Melting Phenomena-Part II Evolution of the Structure of the Pellets", Metallurgical and Materials Transactions, Volume 36B, 2005, pp. 583
MA Jitang: “Injecuion of flux into Blast Furnace via Tuyeres for optimizing slag formation” ISIJ International, Volume 39, No7 1999,pp697
Y.S.Lee, J.R.Kim, S.H.Yi and D.J.Min: “Viscous behavior of CaO-SiO2-Al2O3-MgO-FeO Slag”, Proceedings of VIIInternational Conferenceon -Molten slags,fluxes and salts, The South African Institute of Minig and Metallurgy, 2004,pp225
Electric Steelmaking
Smarajit SarkarDepartment of Metallurgical and Materials Engineering
NIT Rourkela
The furnace proper looks more like a saucepan covered from top with an
inverted saucer as shown in next slide. The electrodes are inserted through
the cover from top. Arc furnaces are of two different designs:
The roof along with the electrodes swing clearly off the body to facilitate
charging from top.
The roof is lifted a little and the furnace body moves to one side clearly off
the roof to facilitate charging.
For smaller furnaces both of these alternatives are equally well suited but
for bigger sizes the body becomes too heavy to move and hence the
swing-aside roof design is favoured. It is quite popular even with small
furnaces.
Cross section of an electric arc furnace
Vertical section of an electric arc furnace shop
The furnace unit consists of following parts:
Furnace body i.e. the shell, the hearth, the walls,
the spout, the doors, etc.
Gears for furnace body movements.
Roof and roof-lift arrangements.
Electrodes, their holders and supports.
Electrical equipments i.e. the transformer, the
cables, the electrode control mechanism, etc.
Acid Process: If the raw materials are very low in P and S acid lined furnace
can be used for refining, using an acid slag as in an acid open hearth practices.
It is generally restricted to foundries.
Basic Process: It is capable of refining any type of charge by maintaining basic
slag in a basic lined furnace. Unlike any other steelmaking process electric
furnace has practically no oxidising atmosphere of its own. Oxidising as well as
reducing conditions for refining can be maintained by making slags of suitable
compositions. Oxidising refining is carried out under a slag containing good
amount of iron oxide. Reducing conditions can be maintained by having the slag
highly basic but practically free of iron oxide. The following describes the ways
in which these slags are used for refining in an arc furnace:
Process Types Known by Their Slags
Oxidising single slag practice. It is used for making carbon or
low alloy steels of a quality attainable in an open hearth process.
The charge is melted and refined under a basic oxidising slag as
in an open hearth. The alloy additions may be made in the furnace
or in the ladle.
Oxidising double slag practice. It is a modification over the
single slag practice. The early slag is removed and a similar new
slag is made again to obtain effective desulsphurisation and
dephosphorisation during refining.
Reducing single slag practice. It is used for high alloy steelmaking to
effect maximum recovery of alloying elements from the scrap. Hardly any
refining take place. Carbon and phosphorus contents in the scrap must be
well below the specification levels since these will not be removed during
refining. Sulphur however could be readily removed in this practice since
the conditions are reducing.
Oxidising slag converted to reducing. It is meant to remove carbon but
recover most of the alloying contents like Cr, Mn, etc. in the scrap during
high alloy steelmaking. Phosphorus content of the charge needs to be
below the specification level, since it will otherwise revert back to the metal
during the reducing period.
Double slag practice. It means refining under oxidising as well as
reducing slags made separately. The first slag is oxidising and it
eliminates all impurities like P, Si, C, Mn, etc. This slag is removed and a
reducing slag is made by fresh additions of lime, coke and spar to
desulphurise the metal and to carry out alloying very effectively. The
practice is a must if effective desulphurisation and the large alloying
additions are to be made. It is costly but the yield of the alloying
additions is very high and the quality of the product is much
better. Amongst the above practices the (i), (ii) and (v) types of practices
are more widely adopted in practice. The (iii) and (iv) types of practices
are used in induction furnace processes.
Slag compositions used in two slag EAF steelmaking
In this practice, the original oxidising slag can be modified by the addition of reducing
agents; however, it gives rise to danger of reversion of phosphorus from the slag back
into the metal. To preclude this possibility, generally the oxidising slag is completely
removed and fresh reducing slag is made by charging lime, fluorspar and silica. The
reducing agent may be graphite or coke breeze. This type of slag is commonly referred
to as carbide slag, since the carbon added reacts with CaO to form some amount of
CaC2• Carbide slag do not allow very low carbon contents to be attained in the bath; in
such cases, ferrosilicon is used as the reducing agent instead of carbon. The typical
compositions of slag are shown in the Table.
Since EAF steelmaking is primarily scrap/DRI based and both these materials have
relatively low levels of residual impurities, the extent of refining is much less than in
BOH steelmaking.
As a process, EAF is much more versatile than BOH and can make a wide range of
steel grades.
Sorting out of scrap and choosing the proper scrap grade are important for EAF
steelmaking, since the extent of refining has to be managed accordingly. For this purpose,
scrap may be classified into the following categories:
scrap containing elements that cannot be removed by oxidation during refining, such as
Cu, Ni, Sn, Mo, W, etc.
scrap containing partially oxidisable elements, such as P, Mn , Cr, etc.
scrap containing completely oxidisable elements, such as AI, Si, Ti, V, Zr, etc.
scrap containing volatile elements, such as Zn, Cd, Pb, etc.
Scrap of type (b) and (c) can be tackled easily during refining. Type (d)
scrap would require some special attention. However, type (a) scrap
gives rise to problems like undesirable residuals in the final steel. This is
where DRI scores over scrap--it is totally. free from all the above
undesirable elements.
In BOH steelmaking, refining begins with the bath containing about I %
excess carbon (often referred to as the opening carbon) in order that
evolution of CO following the oxidation of carbon provides the
necessary agitation for homogenisation of the bath as well as for
enhancing the reaction rates.
In EAF steelmaking also, the initial bath carbon is maintained at about 0.3%
above the final carbon specification during oxidising refining. However, stirring
is absent during refining under a reducing slag, and some other stirring
technique (use of mechanical stirrers called rabbles) is required. Recent
developments in EAF steelmaking have taken place primarily in the context of
large scale production of plain carbon and low alloy steels. Of course, some of
these developments have also been implemented in smaller scale of operation
as well as for the production of high alloy steels, such as stainless steels.
Besides a distinct trend towards increase in furnace size, the important
developments may be summarised as follows:.
Ultra high power supply (UHP)
DC arc furnace
Oxygen lancing (in some cases along with carbon/coke breeze)
Use of water-cooled elements in the furnace shell, water-cooled electrodes, etc.
Foamy slag practice
Bath stirring by argon
Auxiliary secondary steelmaking facility .~
Use of sponge iron (DRI/HBI) to substitute scrap
Hot metal or cold pig iron as scrap substitute
Pre-heating of scrap and DRI
Eccentric bottom tapping
Emission and noise control
Process automation and control.
Transformers supplying power to electric arc furnaces have
been classified as given below.
(i) Regular power, i.e. for old furnaces
100-400 kV A per tonne steel
(ii) High power
400- 700 kV A per tonne steel
(iii) Ultra high power (UHP)
above 700 kV A per tonne steel
Use of UHP enables faster melting of the solid charge, thereby decreasing the
tap-to-tap time with consequent increase in the production of steel.
An EAF of 100 tonne capacity will require a transformer capacity of above 70
MVA for UHP operations. It has been possible to achieve such figures owing to
major advances in electrical engineering in the last few decades.
Another important development is the use of DC (direct current) in the furnaces.
This requires conversion of three-phase AC into single-phase AC supply after
the step-down transformer conversion of AC into DC.
A DC arc has one electrode and the circuit is completed through the conducting
electrodes embedded in the furnace bottom. It offers certain distinct advantages
over three-phase AC arc, such as smoother arc operation, less noise, etc.
Oxygen lancing through a top lance gives certain advantages
that include: oxidation of carbon and some iron from the bath
releasing chemical energy with consequent saving of electrical
energy; faster removal of carbon and other impurities following
faster slag formation and the generation of a foamy slag.
In large EAFs the top lance is supersonic, as in BOFs. For
greater saving of electrical energy, coke or carbon breeze is
also injected along with oxygen in some plants. Coherent jet
lance design makes these injections more efficient and has
been adopted in some EAF shops.
We have already introduced the concept of foams and emulsions in the
context of BOF steelmaking. These are applicable to the foaming of slags in
EAFs as well.
To summarise, a slag foam is transient and is basically sustained by
vigorous evolution of CO following the reaction of bath carbon with oxygen.
A foamy slag is actually an emulsion of metal droplets and gas bubbles in
slag.
Higher slag viscosity and the presence of undissolved solid particles assist
foaming, which speeds-up slag-metal reactions, such as dephosphorisation.
All modem EAF shops, therefore, adopt foamy slag practice.
The subject of mixing and homogenisation of the bath in BOFs has
been elaborately discussed.
To help bath mixing, concurrent top and bottom blowing has been
adopted by all modem BOF shops.
In large EAFs also the problem of mixing exists, to some extent.
Oxygen lancing and flow of current through the metal bath in DC arc
furnaces induce some amount of bath motion, which is sometimes
insufficient.
Better mixing in the bath is desirable for all the advantages described
earlier. Therefore, many modem EAFs are equipped with bottom
tuyeres for injection of argon, etc.
However, excess hot metal usage can prolong the refining time and give rise to
uncontrolled foaming. Therefore, it is recommended that hot metal charge is restricted
to a maximum of 40-45% of the total charge and the best method of usage is to charge
it continuously through a side launder.
DRI/HBI has very low impurity content (i.e. P, Si, S, and, of course, the tramp elements)
and hence does not require any additional refining time. However, it is a porous material
that tends to get severely oxidised in contact with moist air at high temperature. Up to
about 30% DR! (of the total charge) can be charged along with scrap in buckets, if bucket
charging is practiced. First a layer of scrap, then DR! and then another layer of scrap are
used in each bucket. If continuous charging facilities for charging DRI throughout the heat
in small amounts are available, the proportion can be increased to 50-60% and
sometimes, even more. In all cases, HBI is preferred since it is dense and does not
get oxidised very readily.
As mentioned earlier, some alloying elements are more difficult to oxidise
than Fe, such as Cu, Ni, Sn, Mo, W, etc. Hence, they cannot be
satisfactorily removed during steelmaking and are also known as tramp
elements. One way of getting around this problem is not to use scrap
containing these tramp elements, but this is not always economically
viable. Substitution of scrap, partly or fully, by alternative iron sources
(AIS) is a solution, since these inputs do not contain tramp elements.
Besides DRI, the other alternative iron sources are:
Hot briquetted iron (HBI), which is a dense, compacted form of DRI
Solid pig iron
Hot metal (i.e. molten pig iron).
Use of AIS is gaining popularity in EAF steelmaking. DRI/HBI is now the principal feed
stock next to scrap. In 2005, the worldwide DRI/HBI production was just over 56 million
tonnes, which was slightly more than 15% of the scrap consumption.
Solid pig iron and hot metal are also important AIS, constituting about 5-8% of the total
feed. In the case of EAF shops located inside an integrated steel plant, blast furnace hot
metal is available. Otherwise, hot metal can be produced either in a mini blast furnace or
in a smelting reduction unit. Both these have been used in EAF steelmaking,
since hot metal charging is advantageous from a thermal point of view being already
molten and the oxidation of its impurities provides chemical energy; 1 kg hot metal
charge per tonne of steel saves electricity by about 0.5 kWh/t
promotes foaming by the evolution of CO and gives all the advantages of a foamy slag.
With the use of DRI/HBI, melting and refining can proceed simultaneously. In some
EAF shop even up to 100% DRI is used by adopting what is known as the hot heel
practice. Here, molten steel from a previous heat is not tapped out completely and is
allowed to remain in the EAF to provide a liquid metal bath for DR! charging right
from the beginning of the next heat.
The quality of DRI is judged by its following characteristics:
Gangue content
Percentage metallisation
Carbon content
Levels of other impurities
The gangue in DRI consists principally of silica and alumina associated with
the iron oxide feedstock. For optimum usage in steelmaking, the gangue
content should be as low as possible; otherwise, large slag volumes and
hence more lime addition are required. This has an adverse effect on the
consumption of energy.
The percentage metallisation (i.e. the percentage of metallic iron in the DRI
as a percentage of total iron; the remaining iron is present as wustite) should
also be high to keep the energy consumption low.
Typically, steelmakers prefer metallisations between 92% and 96% (too
high metallisation lowers the turbulence that is induced in the bath when
FeO in DRI reacts with the bath carbon).
During the production of DRI (particularly gas-based DRI) carbon in the form of iron
carbide gets absorbed in the final product.
The carbon percentage in DRI depends on the process of sponge iron making-in
coal-based processes it is about 0.10-0.15%, while in gaseous reduction
processes it can be varied anywhere from 1.5 to 4% depending on the
customer demand.
Carbon in DRI lowers its melting point and when it reacts readily with any unreduced
iron oxide, CO is evolved, which contributes towards the formation of a foamy slag.
This is required for efficient steelmaking and hence, steel makers prefer higher
carbon containing DRI, say above1 %.
In case this amount of carbon is not available in DRI, additional carbon input by
injection of coke breeze along with oxygen becomes necessary. The addition of hot
metal can also provide a source of carbon
If the solid charge can be pre-heated, it can obviously reduce electricity
consumption. The economics would depend on the cost of pre-heating.
Under normal circumstances, scrap is charged into the furnace in cold condition
and during the progress of the EAF heat, vigorous evolution of CO and some
amount of hydrogen takes place.
This gas can be an additional heat source by post-combustion of CO and H2,
either in the furnace atmosphere or above the furnace in a separate pre-heating
chamber.
The oxygen required can be supplied by injecting pure oxygen at the appropriate
location. Several systems of pre-heating within the furnace chamber or in a
separate vessel have been used in EAF steelmaking.
Charge pre-heating
Separate pre-heating of DRI/HBI is difficult since it would oxidise.
At the same time, since it is at high temperature when it comes out of the
reduction reactor, it is a matter of retaining this temperature during the
transport of DRI/HBI to the electric furnace. Several systems have been
reported in literature. One of the latest that has been developed by Midrex
Corporation, USA consists of directly conveying hot DRI through an insulated
pipeline directly into the EAF shop and then charging it with the aid of gravity.
Essar Steel, India, has developed refractory lined containers for transport.
Using such techniques, it is possible to charge hot DRI at a temperature of
600-700°C, resulting in 10-15% power saving. As a result, use of pre-
heated DRI/HBI has become a standard practice in many EAF plants.
Performance Assessment of EAF Steelmaking
In these furnaces, electromagnetic induction is used
to heat the metal.
An alternating current supplied to a primary coil
(inductor) sets up a variable magnetic field around that
coil. The variable magnetic flux in turn induces an
electromotive force in the secondary circuit (metallic
charge), so that the metal is melted by the alternating
currents formed in it.
Induction furnaces
There are two basic laws of electricity which form the
foundation of induction melting theory.
The first is that a current flowing through a conductor will
produce a magnetic field around that conductor. If this wire
is wound into a cylindrical coil, the magnetic field of each
turn is added producing an intensified magnetic field. The
field is related to the amount and direction of the current.
The field is maximum when the current is maximum and will
reverse direction if the current reverses direction.
Principles of Induction Melting
The second fundamental is related to Faraday's Law, which says that
when a flux which links a coil is changing, there is an electro-motive
force (emf) induced in the coil. If these flux linkages change in a
closed electric circuit, the emf produced causes a current to flow. A
solid metallic block will produce currents swirling around in eddys in a
plane perpen dicular to the flux. These eddy currents produce the I2R
losses which generate the heat required. Proper selection of coil
frequency and power density allows for the practical application of
induction heating and melting .
The material to be heated or melted by induction must be
conductive, but does not have to be magnetic.
The two basic designs of induction furnaces, the core type or channel furnace and the
coreless.
Both types have advantages which make one or the other suitable to a particular operation.
The channel furnace is the most efficient type of induction furnace. The core-type
construction provides maximum power transfer into the metal. This design has a distinct
advantage of providing a large capacity of molten metal with low holding power level. It is
an excellent furnace for small foundries with special requirements for large cast ings,
especially if off-shift melting is practiced. It is widely used for duplexing operations and
installations where pro duction requirements demand a safe cushion of readily available
molten metal. Because of the requirement to keep the channel molten, core-type furnaces
are energized 24 hours a day. This limits its use to single alloys or similar base-alloy
applications. Power supplies are of line frequencies of 60 or 50 Hz.
The coreless induction furnace is used when a quick melt of one alloy is desirable, or it is
necessary to vary alloys frequently. The coreless furnace may be completely emptied and
restarted easily, which makes it perfect for one-shift operation
Induction Melting
Coreless induction furnaces possess certain advantages over other types of arc furnace:
since there are no electrodes, it is possible to melt steels very low in carbon;
the absence of arcs ensures that the metal made is very low in gases;
alloying additions are oxidized only insignificantly and the furnace productivity is high;
the temperature of the process can be controlled quite accura tely. The drawbacks of induction furnaces for melting steel are as fol lows:
low temperature of the slag, which is heated from the metal; and
low durability of the basic lining.
They can be either open-top or vacuumized. According to the frequency of the current supplied they may be classed into types as follows: high-frequency furnaces operating with valve generators (200 1000 kHz); medium-frequency furnaces (500-10,000 Hz) supplied from rotary or thyristor converters; and low-frequellcy furnaces (50 Hz) which are fed directly from the mains.
There are two concentric conductors in a coreless induction furnace, the
inductor being the external conductor and the molten metal, the internal
one.
Since currents flow in opposite directions through them, they repel each
other. The inductor, which is a rigid conductor, remains fixed, while the
molten metal is compressed from the walls towards the axis of the crucible.
Upon passage from the annular gap between the inductor and metal, the
magnetic flux extends horizontally over the metal surface. The horizontal
component of magnetic field strength produces electrodynamic forces
acting perpendicular to the metal surface, i.e. down wards at the open
surface and upwards at the crucible bottom, the forces being at their
maximum at the wall of the crucible. The·
Electrodynamic phenomena in coreless induction furnaces
The total action of these forces causes the metal to circulate
and to form a convex meniscus at its surface.
A positive effect of this phenomenon is that the metal is
stirred, which equalizes its temperature and composition
and speeds up melting, but the convex portion of the metal
is thus exposed, since the slag flows towards the walls. It is
possible to cover the meniscus by increasing the bulk of
slag, but this may have an adverse effect on the lining.
Electrodynamic circulation of metal in the crucible of an induction furnace