EAF & secondary steel making processes

Monday, April 28, 2025 10:05 PM

Secondary Steelmaking
Primary steelmaking is aimed at fast scrap 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 various 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 has become an integral feature of virtually all modern steel plants. The advent of the continuous casting
process, which requires stringent quality control is one of the main reasons for the growth of secondary steelmaking. The
harmful impurities in steel include: sulphur, phosphorus, oxygen, nitrogen and hydrogen. The latter three elements occupy
interstitial sites in the iron lattice and hence, are known as interstitials. The principal effect of these impurities in steel is loss of
ductility, lower impact strength and poorer corrosion resistance.
Oxygen and sulphur are also constituents of non-metallic particles in steel, known as inclusions. These particles are also harmful
for steel properties and should be removed to as low levels as possible. Carbon is also present as an interstitial in the iron lattice
as well as in the form of Cementite (Fe3C). Unlike the other interstitials, some carbon is always required in steel and hence, the
content of carbon forms a part of steel specifications. However, in recent times, in some special sophisticated grades like
Interstitial Free (IF) steels, carbon is considered as an impurity and has to be removed to very low levels (Ultra-low Carbon
steels).

Why the secondary processing is done on the laddle itself?

Normally the temperature drop of molten steel during tapping from the primary steelmaking furnace is around 20– 40°C. An
additional temperature drop of about 30–50°C occurs during secondary steelmaking. Continuous casting involving pouring into
a tundish causes additional drop of 10–15°C in liquid steel temperature. Therefore, provisions for heating and temperature
adjustment are very desirable, which has led to the development of liquid steel treatment in ladles in special units, such as Ladle
Furnace (LF), Vacuum Arc Degasser (VAD), CAS-OB unit, etc.
Degassing refers to the removal of nitrogen and hydrogen from liquid steel. In common terminology, the abbreviations used
and their full forms are:

With the passage of time, customers are demanding higher quality steels, which requires:

INERT GAS PURGING (IGP)

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INERT GAS PURGING (IGP)
This consists of purging molten steel by argon introduced from the bottom through porous bricks or slit plugs, fitted at the ladle
bottom. Purging by argon through a top lance, which is immersed into the melt in an open ladle, is also practised. The primary
objective is to stir the bath resulting in homogenisation of temperature and composition of the melt. It offers additional
advantages of faster deoxidation and floatation of inclusions (i.e. superior cleanliness). All secondary steelmaking ladles,
nowadays, have provision for gas purging.

DEOXIDATION OF LIQUID STEEL

Solubility of oxygen in solid steel is negligibly small. Therefore, during solidification of liquid steel, the excess oxygen is rejected
by the solidifying metal. This excess oxygen causes defects by reacting with C, Mn, Si, etc. resulting in the formation of
blowholes (primarily CO) and oxide inclusions (FeO–MnO, SiO2, Al2O3, etc.). Evolution of CO has a significant influence on the
structure and homogeneity of the cast metal as well. Therefore, dissolved oxygen levels in molten steel have to be lowered by
the addition of strong oxide formers, such as Mn, Si, Al, Ca (as ferromanganese, ferrosilicon, silico-manganese, aluminium,
calcium silicide) in the ladle. This is known as deoxidation.

If more than one deoxidiser is added to molten steel simultaneously, it is known as Complex Deoxidation. Some important
complex deoxidisers are Si + Mn, Si + Mn + Al, Ca + Si, Ca + Si + Al. For this, the deoxidation product is a slag consisting of more
than one oxide.

The Ladle Furnace (LF)

The ladle with liquid steel is brought to the LF station, where a top cover
is placed on the ladle and graphite electrodes are introduced. It is the
most commonly used unit in secondary steelmaking. In the LF,
deoxidation and composition adjustments are carried out by additions,
and the temperature of the melt is adjusted by arc heating. Bath stirring
is achieved by means of argon purging from the bottom. The top cover
gives significant protection from atmospheric oxidation, but is not
completely sealed and some ingress of air is inevitable.

Problem of Slag Carryover

• The primary steelmaking slag has a high percentage of iron oxide. A portion of this slag comes into the ladle during
tapping because of slag carryover. Owing to its high iron oxide content, a significant quantity of deoxidiser is consumed by
it.
• Another serious problem is what is known as Phosphorus Reversion in ladles from slag to metal. High concentration of
iron oxide in slag is a major factor for the retention of phosphorus in slag. Deoxidation of slag, therefore, tends to transf er
phosphorus back from slag into metal.
Therefore, modern steel melting shops aim at slag-free tapping; however, complete prevention of slag carryover is seldom
achieved and minimisation becomes the objective.

The CAS-OB Process

• CAS stands for Composition
Adjustments by Sealed argon
bubbling. Following this treatment,
the losses by oxidation of deoxidisers
and alloys is low.
• The dissolution becomes faster
because of high turbulence and

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 the losses by oxidation of deoxidisers
and alloys is low.
• The dissolution becomes faster
because of high turbulence and
stirring at the plume eye.
• With additional facilities for oxygen
blowing, the temperature can be
raised because of exothermic heat
supplied by aluminium oxidation
• As a result, arc heating is not
required.
• The process has the advantage of low
capital cost and several steel plants
around the world have installed it.

DEGASSING AND DECARBURISATION OF LIQUID STEEL

The gases, i.e. hydrogen, nitrogen and oxygen, dissolve as atomic H, N, O respectively in molten steel. However, their solubilities
in solid steel are very low. Removal of oxygen from steel is carried out by adding deoxidisers. When liquid steel solidifies, excess
nitrogen forms stable nitrides of Al, Si, Cr, etc. The dissolved nitrogen affects the toughness and ageing characteristics of steel
as well as enhancing the tendency towards stress corrosion cracking. Nitrogen is, by and large, considered to be harmful for
properties of steel.
Its strain hardening effect does not allow extensive cold working without intermittent annealing and hence low nitrogen is
essential for deep drawing steels. However, in some applications, nitrogen has a beneficial effect, such as the grain refinement
by fine AlN precipitates.

hydrogen is picked-up from the moisture in solid charges. Hydrides are thermodynamically unstable. Therefore, the excess
hydrogen in solid steel tends to form H2 gas in the pores and also diffuses out to the atmosphere, since H has very high
diffusivity even in solid steel because of its low atomic mass. In relatively thin sections, such as those produced by rolling,
diffusion is fairly rapid. Hence, excess hydrogen is less, reducing the tendency towards the development of high gas pressure
and the formation of pinholes.
However, diffusion is not that efficient in forgings because of their large sizes. So H rejected by the solidifying steel accumulates
in blowholes and pinholes, where high gas pressure is developed. During forging, the combination of hot working stresses and
high gas pressure in the pinholes near the surface tend to cause fine cracks in the surface region. Efforts to avoid these cracks
have led to the commercial development of vacuum degassing processes. Dissolved hydrogen also causes a loss of ductility of
steel; hence, low H content is a necessity for superior grades of steel with high strength and impact resistance

Vacuum Degassing Processes

Vacuum degassing processes have been traditionally classified into the following categories:

Amongst ladle degassing and circulation degassing RH are popular.

Since, an additional temperature drop of 20–40°C occurs during secondary steelmaking and temperature control of the steel
melt is important for proper casting, provisions for heating and temperature adjustment have been made in RH, as well as in
ladle degassing [vacuum arc degassing (VAD)]. Vacuum-oxygen decarburisation (VOD) where oxygen lancing is done under
vacuum was originally developed for stainless steel refining, but is now used for the production of ultra-low carbon steels (ULC)
as well.
RH process:
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. 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 downleg snorkel, causing melt circulation.
VAD process:
Heating is done by the arc with graphite electrodes, as in a ladle furnace. Heating, degassing, slag treatment and alloy
adjustment are carried out without interrupting the vacuum.
In industrial vacuum degassing, the treatment time should be short enough to match logistically with converter steelmaking on
the one hand and continuous casting on the other. To achieve this, in addition to proper choice and design of the process, the

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the one hand and continuous casting on the other. To achieve this, in addition to proper choice and design of the process, the
principal variables are as follows:

Increasing the argon flow rate increases the rate of degassing and gas evolution. This tends to raise the chamber pressure and
requires a higher exhausting rate. Hence, optimisation of the two is required. It shows that there is no advantage in increasing
the pumping rate if the argon flow rate is not adequate. Also, there is no advantage of having large argon flow rates until there
is a certain minimum exhausting rate.
Manufacture of Ultra-Low Carbon (ULC) Steel by RH-OB Process:
AOD was not commercially viable till the price of argon became reasonably low. To meet the increasing demand for cold-rolled
sheets with improved mechanical properties and to cope with the changeover from batch to continuous annealing, the demand
for ULC (C < 20 ppm) is increasing. The RH process was modified by oxygen blowing under vacuum. It is known as RH-OB and
was first developed by Nippon Steel Corp. in Japan for producing stainless steel in 1972 and subsequently, employed for the
production of ULC steels.
The present thrust is to bring down the carbon content of the melt from 300 ppm to 10–20 ppm in 10 minutes. RH-OB was
subsequently made more versatile by incorporating provisions for chemical heating by aluminium addition. Oxidation of
aluminium generates heat rapidly and counters temperature drop during degassing Powder injection for desulphurisation, alloy
additions, etc. were later incorporated. This versatile and flexible process is known as the RH-injection process.

DESULPHURISATION IN SECONDARY STEELMAKING

Except in free-cutting steels ( have good machinability due to MnS ),
sulphur is considered to be a harmful impurity, since it causes hot
shortness (brittleness of steels at elevated temperatures, particularly
when heated above 460 °C).
Therefore, it is necessary to limit it to 0.02% for general carbon steels.
In special steel plates, the normal specification for sulphur is at present
0.005%, but there is demand for ultra-low sulphur (ULS) steels with as
low as 10 ppm (0.001%) S in grades such as line-pipe steel, HIC
resistant steels (Hydrogen induced cracking) and alloyed steel forgings.

Sulphur comes into iron principally through coke ash, and is effectively removed first during ironmaking and further during hot
metal pre-treatment in ladles. However, levels below 0.01% have to be achieved by further desulphurisation during secondary
steelmaking. There are now processes such as the MPE process of Mannesman and the EXOSLAG process of U.S. Steel, where
desulphurisation is done, to some extent, by adding a synthetic slag. Desulphurisation by use of synthetic slags is also carried in
IGP, ladle furnace and vacuum degasser; but deep desulphurisation can only be achieved by the injection of powders like
calcium silicide into the melt.

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calcium silicide into the melt.

Nowadays it is a standard practice to carry out some desulphurisation of hot metal in the transfer ladle after tapping from the
blast furnace and before charging it into the primary steelmaking furnace. This is known as external desulphurisation and is a
part of hot metal ladle pre treatment. The major difference between the two is that normally the hot metal ladles do not have
bottom stirring through porous plugs, while in secondary steelmaking, ladles are fitted with bottom porous plugs for argon
purging. Therefore, in hot metal desulphurisation, a lance has to be immersed into the melt from the top for gas stirring and
injection of desulphurising agents. This comes under the broad area of Injection Metallurgy (IM).

IM is practiced in secondary steelmaking only for deep desulphurisation for the production of ultra-low sulphur (ULS) steels.
Otherwise, desulphurisation by treatment with synthetic slag on top of molten steel in the ladle, ladle furnace or during vacuum
treatment is sufficient. The principal additions are CaO and Al, though some CaF2, SiO2 and Al2O3 are also required for slag
formation.

Injection Metallurgy (IM)

IM is practised in secondary steelmaking not only for the removal of sulphur from metal, but also for inclusion modification.
The processes may be broadly classified into the following two categories:

The addition of calcium metal into the melt in the form of Ca-Si alloy causes deep deoxidation, deep desulphurisation and
modification of inclusions to yield desirable properties. The line-pipe steel for transporting gas over long distances has to
withstand high pressure, corrosion from H2S in gas, and sub-zero temperatures without any tendency towards brittle fracture.
The steel for this purpose required treatment by calcium.
• In powder injection processes, therefore, the powders should be injected as deep as possible into the melt. Powder
injection gives better desulphurisation owing to the increased surface area.
• In wire feeding, Ca–Si powder is encased in a hollow steel tube and swaged before the tube plus powder is continuously
fed into the melt by a machine. Wire feeding is particularly suitable for inclusion modification. It is worth mentioning here
that wire feeding is practised for other purposes—such as Al wire feeding for deoxidation and alloy powder feeding for
precision alloying. Many steel plants, therefore, have provision for both powder injection and cored wire feeding.
CLEAN STEEL TECHNOLOGY
non-metallic inclusions in steel have been found to be harmful for desirable mechanical properties and corrosion resistance of
steel. The inclusion particles are mostly oxides and to some extent, sulphides. This is more so in the case of high-strength steels
for critical applications. As a result, there is a move to produce what is known as Clean Steel, i.e. steel free from inclusions.

Inclusions originating from contact with external sources as listed in items 2 to 4 above, are called exogenous inclusions. These
are formed during the entire process of secondary steelmaking
Cleanliness Control during Deoxidation:

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Cleanliness Control during Teeming:
Subsequent to processing in LF/CAS-OB/Vacuum degasser and/or injection treatment, the liquid steel is held in the ladle for 20
to 40 minutes before and during teeming. Larger non-metallic particles get plenty of time to float up. The presence of a well-
deoxidised top slag does not allow much atmospheric re-oxidation.The presence of a well-deoxidised top slag does not allow
much atmospheric re-oxidation. Refractory erosion is also low, since there is no gas stirring. However, during teeming, if no
precautions are taken, the melt will become dirty because of:
1. Oxygen absorption by the teeming stream
2. Slag entrainment consequent to funnel formation
3. Erosion of the nozzle refractory
The falling stream of liquid steel absorbs oxygen from the surrounding air by the jet entrainment effect. The oxygen content of
steel has been found to increase by as much as 40 to 400 ppm depending on the nature of the stream. This leads to the
formation of large inclusions, rich in FeO and MnO. Moreover, it increases the dissolved oxygen content and causes further
generation of inclusions by reaction during solidification in the mould. Atmospheric oxidation has been eliminated by the use of
Submerged Entry Nozzles (SEN) in the continuous casting route, where the nozzle tip is submerged into the melt. Additional
protection can also be provided by shrouding the stream further with a gentle flow of argon.
Formation of a funnel-shaped cavity towards the end of tapping causes the slag to flow out along with the metal. This
phenomenon also leads to entrainment of slag by metal during teeming from the ladle and tundish. This can be minimised by:

For producing clean steels, the refractory used in the teeming nozzles is of considerable importance, since there is a high
probability that inclusions and impurities introduced at the stage of teeming will not be eliminated. Here, both erosion and
corrosion are severe owing to the high flow velocity of liquid steel through the nozzle.
Tundish Metallurgy for Clean Steel:

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Tundish Metallurgy for Clean Steel:
The tundish is a shallow, refractory-lined vessel that is located in between the ladle and the continuous casting mould. Liquid
steel flows from the ladle into the tundish and from the tundish into the mould. A tundish is a must in continuous casting for
proper regulation of the rate of flow into the mould.
Inclusion Modification:
One of the defects in continuous casting is formation of subsurface pinholes owing to the presence of dissolved gases.
Therefore, the oxygen content of the melt should be kept very low by use of aluminiun also as deoxidiser (Al-killed steel). A
certain minimum level of dissolved Al is required in the melt. This gives rise to the problem of nozzle clogging in continuous
casting owing to sticking of alumina inclusions to the inner wall of the casting nozzle. Calcium treatment at the final stage in a
ladle or a tundish has been found to eliminate this, because the deoxidation product is a liquid consisting of CaO and Al2O3,
occasionally with some amount of SiO2.
Tellurium (Te) or Selenium (Se) is also added as a reagent for inclusion modification to improve the machinability of sulphur
containing steels. Their basic effect is to make the inclusions globular, thus leading to better deformation characteristics during
hot working as well.
Temperature Changes during Secondary Steelmaking:
For obtaining the desired cast structure as well as for elimination of some casting defects, the temperature of liquid steel should
be controlled within a desired range before it is teemed into the mould. Continuous casting demands more stringent
temperature control than ingot casting. In secondary steelmaking, the temperature may drop by as much as 100°C from the
furnace to the mould. Therefore, some secondary steelmaking units, such as LF, VAD and RH have provisions for heating the
melt. Pre-heating of the lining of empty ladle is also standard practice so that the interior hot face lining temperature is above
800°C.
Arc heating is most common, followed by chemical heating, plasma arc or induction heating for reducing the temperature loss
during secondary treatment. Chemical heating requires aluminium addition as well as some oxygen lancing, as is the case in RH–
OB. Exothermic oxidation of aluminium provides the heat. The overall temperature change of liquid steel from the furnace to
the mould is a sum total of the following:

Stainless Steelmaking
Amongst high alloy steels, stainless steels (SS) are the most important. The world production of SS is about 20 Mtpa. Stainless
steels contain 10 to 30% chromium. Varying amounts of nickel, molybdenum, copper, sulphur, titanium, niobium, nitrogen, etc.
may be added to obtain the desired properties. SS is used wherever superior resistance to corrosion is desired. Stainless steels
are primarily classified as austenitic, ferritic, martensitic, duplex or precipitation hardening grades. Most grades contain 17–18%
chromium. Addition of nickel enhances the corrosion resistance further and tends to stabilise the austenitic structure—the most
popular SS is the 18% Cr and 8% Ni variety.

Production of ferritic SS requires the removal of carbon and nitrogen in steel to very low levels. Nickel, which is an integral part
of stainless steels is expensive and efforts have been made from the 1950s to replace it, partially or completely, by other
elements, such as manganese, which tend to stabilise the austenite phase. However, this can be achieved more easily, if the
steel contains high percentages of nitrogen (0.06–0.08%); high nitrogen SS grades are being produced today.

. The quantities of various charge materials that are now used are as follows:

Ferrochrome, 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 content, such as:

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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%.
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 (Argon–Oxygen Decarburisation) was a major breakthrough in stainless steelmaking.

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%.
MELTING AND REFINING OF STAINLESS STEELS FOR SCRAP AND FERROALLOY-BASED PROCESSES:
Melting:
the primary melting unit used for producing stainless steel is the electric arc furnace. Melting in induction furnaces is popular in
the case of smaller scale operation and in foundries. there are some salient differences which are:

Stainless steel melting in some plants is also carried out in a converter. It requires the injection of carbon and oxygen and post-
combustion of CO to CO2 inside the converter to generate heat.

The AOD Converter Process:

Molten steel containing most of the chromium and nickel needed to meet the final composition of SS steel, is tapped from the
electric arc furnace into a transfer ladle. The AOD vessel is rotated into a horizontal position during charging of liquid steel, so
that the side-mounted tuyeres are above the bath level. Then, the vessel is made vertical for gas blowing. Charging of solids
during the blow, temperature measurement and sampling are done in a similar manner to that in BOF steelmaking.
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. 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.

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.

BASIC OPEN HEARTH STEELMAKING:

the open hearth process of steelmaking was invented in 1861. Initially, acid open hearth furnaces were used, but they were
soon overtaken by basic open hearth (BOH) steelmaking. BOH was the primary method of steelmaking for about 100 years till
the advent of the basic oxygen process. With the gradual evolution of basic oxygen steelmaking, the number of open hearth
furnaces decreased steadily. By the mid 1990s, all recently installed steel plants as well as older plants that had been
modernised did not have any BOH furnaces.

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The Open Hearth Furnace:
Scrap, cold pig, iron ore, manganese ore, and hot metal are used as charge in open hearth furnaces. The proportion of the solid
charge consisting especially of scrap (in various forms) is seldom less than 70%, and a lot of time (at least 3–4 hours) is
consumed in the process of charging and melting these solids. In fact, in any open hearth shop, extensive stock yard facilities
have to be provided for scrap storage in large heaps. The scrap is then loaded into relatively small charging boxes which can be
inserted into the furnaces through the front doors by using especially designed charging cars that operate on the furnace floor.

Different layers of refractories are laid on the steel shell to form the hearth bottom—first fireclay bricks, then magnesite bricks,
and finally a layer of fritted burnt magnesite (10–30 mm). The main roof of BOH furnaces is made of basic bricks like chrome-
magnesite or magnesite-chromite, while in acid furnaces, silica bricks are used.
Steelmaking in Basic Open Hearth Furnaces:

The charging sequence of solids is: iron ore at the bottom, then limestone, and finally scrap on top of limestone. Once heating
begins and scrap reaches the point of melting, molten hot metal is poured. Because considerable oxidation of the scrap takes
place during heating, the impurities in the hot metal, especially silicon and carbon, vigorously react with iron oxide. Limestone
also dissociates releasing CO2, which reacts with carbon and silicon in hot metal to form CO and SiO2 respectively. Evolution of
CO results in foaming of the slag, which continues throughout the melting period.

As the impurities get oxidised, the amount of slag on the top of the horizontal bath keeps on increasing and the chemical
composition of the metal bath changes continuously. Slag and metal samples are withdrawn throughout the refining period
(lasting 90–120 minutes) to monitor the oxidation of carbon, manganese and phosphorus. The SiO2, MnO and FeO contents in
the slag samples provide an idea of the extent of oxidation of the respective impurities.

Attempts to increase the productivity resulted in extensive usage of oxygen, once pure oxygen became available in larger
quantities and at a relatively low price. Oxygen was used to either enrich the burner flame or was introduced directly into the
molten bath using water-cooled top lances/submerged tuyeres. This helped increase the productivity of BOH furnaces by
around 20% and reduced the tap-to-tap time to 8–12 hours from 16–20 hours earlier. It also resulted in reduction in fuel
consumption. Since silicon is oxidised early and not much CaO is available from the decomposition of limestone, the foamy slag
that is initially formed is rich in silica and is acidic in nature. Most of this slag comes out through the doors. After foaming
subsides, fresh lime is added and a slag of high basicity (V-ratio 3 to 4) is made to begin the refining of phosphorus and carbon.

Evolution of carbon monoxide bubbles provides adequate stirring to ensure homogenisation of the bath as well as helping in
heat transfer. When the bath samples taken indicate that carbon in the metal bath has reached the specified level, the heat is
ready for tapping. However, before tapping, some deoxidisers are added into the bath in order to lower the bath oxygen
content as well as for minor alloying. Tapping is carried out by physically opening the tap hole at the centre of the back wall of
the furnace by oxygen lancing.
Advantages and disadvantages of Acid open hearth process:

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ELECTRIC ARC FURNACE STEELMAKING:
There are basically two types of electrically-heated melting furnaces:
∑ Electric arc furnace (EAF)
∑ Induction furnace (IF).
Induction furnaces are employed for steelmaking only on a relatively very small scale since the furnace size varies from 0.5 t to
maximum of 20/25 t per heat, with most furnaces below 5 t. Hence, IFs are used to a limited extent. On the other hand, arc
furnace steelmaking is widely used and about 35% of world’s steel is produced by EAFs at present (the rest by basic oxygen
steelmaking), which is expected to become 50% in the next 10–15 years. While earlier EAFs were mostly between 10 t and 50 t,
today 200–250 t EAFs are common and 400–500 t EAFs are available.

Initially, for the bulk production of plain carbon steels, open hearth furnaces were less expensive because of their large size as
well as the higher cost of electricity. All these started to change from around 1970, because of the following.

Therefore, EAFs started gradually replacing BOHs for producing plain carbon steels as well. With EAFs ranging in size from 100 t
to 250 t (and even 500 t), there are some plants around the world which are producing 1.5 to 2 Mtpa of steel from 2–3 large
electric arc furnaces. This type of growth in EAF steelmaking has resulted in scrap shortage. This has been taken care of by
partial substitution of scrap by sponge iron (DRI/HBI) and, in some plants with additional charging of cold pig and hot metal.
The Furnace and the Auxiliaries:
EAF is a direct arc furnace, where the arc is struck between the graphite electrodes and the metallic charge/metal bath. The arc
temperature is above 4000°C and is used to heat the bath by radiative heat transfer. The traditional power supply is three-
phase AC, requiring three electrodes; the modern trend is to go for DC arc.

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Cross section of an electric arc furnace
(both acid and basic lining shown for illustration)

Large transformers are required for supplying power to any EAF. The primary voltage may be 33 KV or more, and the secondary
voltage anywhere from 200 V to 1000 V depending on the furnace size and power supply.

Conventional EAF Steelmaking Practice

EAF steelmaking involves most of those stages, with the exception of the following:
∑ Hot metal is not used in conventional practice
∑ The refining and finishing practices differ widely.
The basic purpose of refining in EAFs is two-fold:
∑ Removal of undesirable impurities (C, Si, P, S, N, H, etc.)
∑ Finishing the bath so as to ensure maximum alloy recovery

depending on the type of the solid metallic charge and the grade of steel to be produced, the refining practice differs
considerably. The type of refining employed may be broadly classified into the following.

Oxidising single slag practice: It is employed to produce tonnage grades of carbon and low alloy steels, as well as non-
deoxidised, semi-deoxidised and deoxidised steel. The physical and chemical specifications are not very stringent, i.e. the quality
of steel is similar to what is attainable in open hearths using a basic oxidising slag as the medium of refining.
Double slag practice: In this case, after refining using an oxidising slag, further refining is carried out under a reducing slag. The
reducing slag allows attainment of lower sulphur levels and also assists in higher alloy recovery after tapping. 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 slags 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.

1. 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.
2. As a process, EAF is far more versatile than BOH and can make a wide range of steel grades.
3. 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:

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4. In BOH steelmaking, refining begins with the bath containing about 1% 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.

Operating Costs

Steel Quality Control in EAF:

Of all the residual elements, nitrogen is the most difficult to control in EAFs, and is a major limitation in the production of high-
quality steels using the arc furnace route. Some nitrogen comes through the scrap, but the major amount absorbed by the
molten steel bath comes from nitrogen in the furnace atmosphere. This situation is aggravated by the very high temperature in
the arc zone which prompts dissociation of N2 molecules.
The principal method for the prevention of nitrogen absorption is to maintain a foamy slag right through the heat, thereby
shielding the metal bath from the arc zone. Nitrogen is also removed from the bath by the flushing action of the rising CO
bubbles. Therefore, all steps that promote these two factors will tend to lower nitrogen in steel. Features like continuous
charging of high percentages of DRI, extensive bottom argon injection, etc. are beneficial. However, even after incorporating
them, the minimum nitrogen in EAF steel is about 40 ppm
Hydrogen dissolved in steel also gets flushed out by the rising CO bubbles and typical tap hydrogen contents vary from 2 to 7
ppm. To arrive at lower hydrogen levels in the final product, it is necessary to take recourse to vacuum degassing during
secondary steelmaking.
It is not difficult to bring down phosphorus in steel to below 0.015% in EAFs, which is acceptable for most steel grades.
However, sulphur cannot be reduced to very low levels directly in EAF steelmaking without sacrificing productivity and incurring
high cost. Therefore, desulphurisation is often carried out during subsequent secondary steelmaking

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