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