Steel making

Smarajit Sarkar
Department of Metallurgical and Materials Engineering
NIT Rourkela

 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
 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.

 There are as many as two thousand odd
varieties of steels in use. These specifically
differ in their chemical composition. However, a
couple of hundred varieties are predominantly in
use. The chemical composition of steels broadly
divide them into two major groups, viz. (i) plain
carbon steels and (ii) alloy steels.

 The plain carbon steels are essentially alloys of
iron and carbon only whereas, if one or more of
elements other than carbon are added to steel in
significant amounts to ensure specific better
properties such as better mechanical strength,
ductility, electrical and magnetic properties,
corrosion resistance and so on it is known as an
alloy steel. These specifically added elements
are known as alloying additions in steels.

 Steels may contain many other elements such as AI, Si,
Mn, S, P, etc. which are not added specifically for any
specific purpose but are inevitably present because of
their association in the process of iron and steelmaking
and can not be totally eliminated during the known
process of iron and steelmaking. These are known as
impurities in steel.

 Every attempt is made to minimise them during the
process of steelmaking but such efforts are costly and
special tech-niques are required for decreasing their
contents below a certain level in the case of each
element.

 For cheaper variety of steels therefore their contents at
high levels are tolerated. These high. levels are however
such that the properties of steels are not signifi-cantly
adversely affected. These tolerable limits of impurities
are considered as 'safe limits' and the impurity levels are
maintained below these safe limits.

 For example, for ordinary steels sulphur contents up to
0.05% are tolerable ,whereas for several special steels
the limit goes on decreasing to as low as 0.005% or
even lower. For most high quality steels now the total
impurity level acceptable is below 100 ppm and the aim
is 45 ppm.

Plain carbon steels are broadly sub-divided into four
major types based on their carbon contents. These are
not strict divisions based on carbon contents but are
generally broad divisions as a basis of classification.
This division is definitely useful. These are:

(i) Soft or low carbon steels up to 0·15% C
(ii) Mild steels in the range 0·15-0·35% C
(iii) Medium carbon steels in the range 0·35-0·65% C
(iv) High carbon steels in the range 0·65-1·75% C

The alloy steels are broadly sub-divided into three groups
on the basis of the total alloying elements present. This
division is also only a broad division and not a rigid one.
This is :

(i) Low alloy steels up to 5% total alloying contents
(ii) Medium alloy steels 5-10% total alloying
(iii) High alloy steels above 10% total alloying

The products in the above reactions are only those which are
stable at steelmaking temperatures. The oxides which are not
thermodynamically stable at steelmaking temperatures need not
be considered here.
 Except the sulphur reaction all the rest are oxidation processes

and are favoured under the oxidizing condition of steelmaking .
In the case of oxidation of carbon the product, being a gas,
passes off into the atmosphere but the rest of the oxide products
shall remain in contact with the iron melt in the form of a slag
phase.

I n steelmaking the reactions should move to the right in

preference to the oxidation of iron and that the danger of
reversion of an impurity to the metal phase is as remote as
possible.
 From the point of view of law of mass action the required
conditions can be achieved by increasing the activities of the
reactants and decreasing those of the products.
For a given composition of iron melt the activity of the impurity
is fixed and hence can not be increased.
The oxidising potential of an oxidising agent can be increased.

The oxidising potential of an oxidising agent can be increased by using
atmospheric air (ao = 0·21) in place of iron oxide in slag phase and pure

oxygen (ao = 1) in place of air. But once the nature of the oxidizing agent is
chosen it cannot be increased.
The activity of the product can however be decreased by combining it with
oxide of opposite chemical character, i.e. an acid oxide product is mixed
with basic oxide and vice-versa.
As far as the physical requirement of the oxide product is concerned it
should be readily separable from the iron melt.
This is achieved by keeping the slag and the metal both as thin liquids so
that the metal being heavier settles down and the slag floats on top in the
form of two immiscible liquids which can be separated readily.

 If the oxide products of iron refining reactions are examined, silicon and
phosphorus form acid oxides and hence a basic flux is needed to form a
suitable slag for their effective removal.
 The higher the proportion of base available the lesser will be the danger of
backward reaction. For manganese elimination, since manganese oxide is
basic, an acid flux will be required. The nature of the process itself has
made the task little simpler.
 During refining, being the largest bulk, iron itself gets oxidised to some
extent as (FeO) which is basic in nature. It is possible to adjust the
contents of silicon and manganese in pig iron such that the amounts of
(FeO + MnO) formed during refining would be able to form a slag
essentially of the type FeOMnOSi02 and fix up silica in it.

 In such a slag P20S is not stable because (FeO + MnO) together
are not strong enough bases to fix it up in slag.
 In order to oxidise phosphorus in preference to iron, a strong
external base like CaO and/or MgO is needed in sufficient
proportion to form a basic slag to hold P20S without any danger
of its reversion.
 Phosphorus is best eliminated by a slag of the type CaOFeO
P205 It is quite interesting to note that such a slag is also
capable·of removing sulphur from iron melt to a certain extent.



The steelmaking processes can now be divided into two broad
categories :

(i) when silicon is the chief impurity to be eliminated from iron and that
phosphorus and sulphur need not be eliminated and,

(ii) when phosphorus and, to some extent, ,sulphur are the chief impurities
to be eliminated along with even silicon.

The elimination of manganese will take place under both the categories.

In the finished steel, except a few exceptions, phosphorus and sulphur
each must be below 0·05%. If phosphorus is above this limit, steel
becomes coldshort and if sulphur is more it becomes hotshort. Higher
sulphur contents are recommended for freecutting variety of steels and
a slightly high phosphorus level is desirable for efficient pack rolling of
steel sheets.

 If the pig iron composition is such that phosphorus and sulphur both
are below 0·05% and, therefore, need not be eliminated it is possible to
remove silicon along with manganese in such a way that slag of the type
MnOFeOSi02 is formed without the necessity of addition of an external
flux. Such a process of steelmaking is called acid steelmaking process
which is carried out in an acid brick lined furnace.
 On the other hand, to eliminate phosphorus and sulphur, the reverse
reaction rate can only be suppressed if the slag contains a good amount
of stronger base than as is internally available in the form of FeO and
MnO. External CaO (and also MgO) is used as a flux and slag of the type
CaOFeOP205 is made. Such a process is called basic steelmaking
process. The furnace lining in this case has to be basic in nature.


 In brief the composition of pig iron is the only factor that
determines the acid or the basic character of the process to be
adopted for steelmaking. In an acid process slag is acidic and
the furnace lining has to be acidic to withstand the slag.
Similarly in a basic process the slag contains excess basic
oxide and the furnace lining should be basic in nature. If the
lining is of opposite chemical character slag will readily react
with the lining and cause damage to the furnace. Besides the
acid or the basic nature, the slag needs to possess many other
physical and chemical properties to carry out refining
efficiently.

By

Dr. Smarajit Sarkar
Associate Professor
Dept. of Metallurgical and Materials
Engg.
National institute of Technology,
Rourkela

 Introduction to metallurgical slag
 Structure of pure oxide
◦ Role of ionic radii
◦ Metal – oxygen bond
 Structure of slag
 Properties of slag
◦ Basicity
◦ Oxidising power
◦ Sulphide capacity
◦ Electrical and thermal conductivity
◦ Viscosity
◦ Surface tension
 Constitution of slag

 The slag comprising of simple and/or complex compounds
consists of solutions of oxides from gangue minerals,
sulphides from the charge or fuel and in some cases
halides added as flux.

 Slag cover protects the metal and from oxidation and
prevents heat losses due to its poor thermal conductivity.

 It protects the melt from contamination from the furnace
atmosphere and from the combustion products of the fuel

 In primary extraction, slags accept gangue and unreduced
oxides, whereas in refining they act as reservoir of
chemical reactant(s) and absorber of extracted impurities.

 In order to achieve these objectives, slag must possess
certain optimum level of physical properties:
◦ Low melting point,
◦ Low viscosity,
◦ Low surface tension,
◦ High diffusivity
and chemical Properties:
◦ Basicity,
◦ Oxidation potential and
◦ Thermodynamic properties
 The required properties of slags are controlled by the
composition and structure.

 There are two principal types of bonds found in
crystals: electrovalent and covalent.
 Electrovalent bond strength is lower than the

covalent bond. High temperature is required to
destroy the covalent bond.
 However, oxides exhibit varying proportion of both

ionic and covalent bonding in slag.
 Ionic bond fraction indicates the tendency to

dissociate in liquid state.

Relative dimensions of cations and anions and type of bonds
between them are important factors in controlling the structure of
pure oxides

 TiO2, SiO2 and P2O5, bonding is mainly covalent and the
electrovalent proportion is strong due to small cations
carrying higher charge with a coordination number of 4.
 These simple ions combine to form complex anions

such as SiO4-4 and PO3-4 leading to the formation of stable
hexagonal network in slag systems.
 Hence they are classified as ‘network formers’ or “acidic

oxides”. For example
 SiO2 + 2O2- = SiO4-4
 P2O5 + 3O2- = 2(PO3-4)

 The oxides with high ionic fraction form simple ions
on heating beyond the melting point or when
incorporated into a liquid silicate slag. For example :
CaO→Ca2+ + O2-
Na2O → 2Na+ + O2-

 As they destroy the hexagonal network of silica by
breaking the bond they are called ‘network
breakers’or‘basic oxides.

Oxide z/(Rc+Ra) Ionic Coordination Nature of the

fraction number Oxide

of bond
Solid- -Liquid
Na2O 0.18 0.65 6 6 to 8

BaO 0.27 0.65 8 8 to 12
SrO 0.32 0.61 8 Network breakers

CaO 0.35 0.61 6 or
MnO 0.42 0.47 6 6 to 8 Basic oxides
FeO 0.44 0.38 6 6 Oxides like Fe2O3, Cr2O3 and
ZnO 0.44 0.44 6
Al2O3 are known to be
Mgo 0.48 0.54 6
amphoteric due to their dual
BeO 0.69 0.44 4
characteristics because they
…………. ……………... ……………... …… ……... …………………...
behave like acids in basic slag
Cr2O3 0.72 0.41 4
and as bases in acidic slag.

Fe2O3 0.75 0.36 4 Amphoteric oxides

Al2O3 0.83 0.44 6 4 to 6

…………. ……………... …………….. …….. ………. …………………...
TiO2 0.93 0.41 4 Network formers

SiO2 1.22 0.36 4 4 or

P2O5 1.66 0.28 4 4 Acidic oxides

 It is well known that most of the slags are silicates. When a basic
oxide is incorporated in to the hexagonal network of silica it forms
two simple ions.
 The fraction of basic oxide, expressed as O/Si ratio plays an
important role in destroying the number of Si-O joints.

O/Si Formula Structure
2/1 Si O2 Silica tetrahedra form a perfect three
dimensional hexagonal network
5/2 MO.2 One vertex joint in each tetrahedron
SiO2 breaks to produce two-dimensional
lamellar structure.
3/1 MO. Si O2 Two vertex joints in each tetrahedron
break to produce a fibrous structure

7/2 3MO. Three vertex joints in each tetrahedron
2SiO2 break
4/1 2MO.SiO2 All the four joints break

A knowledge of various chemical and physical
properties of slag is essential in order to adjust
them according to the need of extraction and
refining processes.
1. Basicity of Slags
 In slag systems, a basic oxide generates O2- anion

while an acidic oxide forms a complex by
accepting one or more O2 anions:
Base ↔ acid + O2-

 For example, SiO2, P2O5, CO2, SO3 etc are acidic oxides because
they accept O2- anions as per the reaction:
(SiO2) + 2 (O2-) = SiO44-
 On the other hand basic oxides like CaO, Na2O, MnO etc. generate
O2- anions:
(CaO) ↔ Ca2+ +O2-
 The amphoteric oxides like Al2O3, Cr2O3 Fe2O3 behave as bases in the
presence of acid (s) or as acids in presence of base (s):
(Al2O3) + (O2-) = 2 (Al O2-) or (Al2 O4 2- )
 (Al2O3) = 2(Al3+) + 3(O2-)

 In a binary slag viz. CaO-SiO2 the basicity index (I)
is given as:
I = wt % CaO / wt % SiO2
 For example a complex slag consisting of CaO,
MgO, SiO2 and P2O5 employed in
dephosphorisation of steel, basicity index 2 is
estimated as %CaO + 2 3 wt%MgO
wt follows:
I=
wt%SiO 2 + wt%P2 O 5

 Oxidizing power means the ability of the slag to
take part in smooth transfer of oxygen from and to
the metallic bath.
 The oxidizing power of the slag depends on the

activity of the iron oxide present in the slag.
 The equilibrium between iron oxide in slag and

oxygen dissolved in metal is represented as:
 (FeO) = [ Fe ] + [ O ]

K=
[ a ][ a ]
Fe O Thus [ a O ] ∝ ( a FeO )
(a ) FeO

 Since slags are employed to remove sulphur from
metal, chemistry of sulphur in silicate slags
becomes interesting.
 Sulphide is soluble in silicate melts but elemental

sulphur does not dissolve to any appreciable
extent.
1 1
S 2 ( g ) + (O 2 − ) = O2 ( g ) + ( S 2 − ) (18)
2 2

(a )  p
S 2−


1
2 x
S 2−
.γ
S 2−
 p O2



1
2

(a )  p
O2
K= = (19)
 x  pS 
O 2− H2  O 2−  2 

 The sulphur affinity of a slag, presented as molar
sulphide capacity is defined by the equation:
1
 pO  2  x 2− 
′ = x 2−  2
CS  = K O  (20)
S  pS   γ 2− 
 2   S 
 or a more useful term wt % sulphide capacity5 for
technologist is defined as
1
 p O2  2

C S = (wt% S)   (21)
 pS 
 2 
 Thus under similar conditions a slag with a high Cs will
definitely hold sulphur more strongly than the other with
a low Cs and hence will prove to be a better desulphuriser
in a metallurgical process.

 Molten silica is a poor electrical conductor3. However its
conductivity increases to a great extent by addition of basic
oxides e.g. CaO, FeO or MnO as flux.
 This increase is due to the formation of ions.
 The conductivity values serve as a measure of degree of
ionization of the slag. The electrical conductivity of slags
depends on the number of ions present and the viscosity of
liquid slag in which they are present.
 Thus conductivity will be greater in liquid state and further
increases with the temperature.
 In general thermal conductivity of slag is very low but heat
losses are much higher due to convection.

 Viscosity of slags are controlled by composition and
temperature. The viscosity , of a slag of a given
composition decreases exponentially with increase of
temperature according to the Arrhenius equation:

η = A exp (E η/ RT)

 Basic oxides or halides with large ionic bond fraction
are more effective in reducing viscosity than those
with smaller bond fraction by breaking bonds between
the silica tetrahedra.

Effect of addition of flux on activation
energy

 Viscosity decreases rapidly with temperature for both basic as well
as acid slags.
 But basic slags with higher melting points are more sensitive to
temperature.
 This indicates that activation energy for viscous flow of basic slags
is much lower than for acid slags.

 Use of CaF2 as flux is more effective in reducing viscosity of basic
slags than that of acidic slags.
 This may be due to ability of F- ions to break the hexagonal network
of silica and the low melting point of undissociated CaF 2.

 Figure shows that addition
of Al2O3 to a basic slag
increases viscosity by
acting as network former.
 Addition of Al2O3 to an
acidic slag reduces viscosity
because it now acts as
network breaker.

 The high rates of reaction in basic oxygen converters is
due to the physical conditions of the metal, slag and
gaseous phases in the converter.

 The theories regarding rapid reaction rates rely heavily
on the formation of slag – metal emulsion and slag foams
leading to creation of the large required reaction surface.

 The most important feature of emulsion and foam is the
considerable increase of the interfacial area between the
two phases leading to the high rate of reaction.

 As surface tension is the work required to create unit
area of the new surface, the necessary energy for
emulsifying a liquid or a gas in another liquid increases
with increasing surface tension value.

 In a similar manner energy is liberated when interfacial
area decreases.

 Hence a low interfacial tension favors both formation and
retention of emulsion.

 On this basis slag / metal and slag /gas systems are not
suitable for emulsification because of the high equilibrium
slag/metal interfacial tension.

 However the slag/metal interfacial tension is considerably
lowered to 1/100 of the equilibrium value due to mass
transfer.

 Addition of SiO2 or P2O5 to a basic oxide lowers3 the
surface tension due to the absorption of a thin layer of
anions, viz. SiO44- , PO43- on the surface.

 It has been reported that lowering of surface tension of
FeO by excess oxygen.

 The major constituents of
iron blast furnace slags can
be represented by a ternary
system: SiO2 – CaO – Al2O3.

 On the other hand all the
steelmaking and many
nonferrous slags are
represented by the ternary
system: SiO2- CaO – FeO.

1.Basic open hearth steel furnace
2.Acid open hearth steel furnace
3.Basic oxygen converter
4.Copper reverberatory
5.Copper oxide blast furnace
6.Lead blast furnace
7.Tin smelting

 Introduction
 Changing Pattern of Steel Making
 Modern steel making – BOF / LD steel making
 Silicon Reaction
 Manganese reaction
 Phosphorous Reaction
 Carbon Reaction
 Vacuum Degassing

 Steelmaking is conversion of pig iron containing about
10 wt weight of carbon , silicon, manganese,
phosphorus, sulphur etc to steel with a controlled
amount of impurities to the extent of about 1 weight
percent.
 With the exception of sulphur removal of all other
impurities is favored under oxidizing conditions.
 In the case of oxidation of carbon the product, being a
gas, passes off into the atmosphere but rest of the oxide
products shall remain in contact with the iron melt in the
form of a slag phase.

 SiO2, MnO and P2O5 formed by oxidation of Si, Mn and P,
respectively will join the slag phase.

 The formation of these oxides can be facilitated by
decreasing their activities which is possible by providing
oxides of opposite chemical character serving as flux.

 As SiO2 and P2O5 are acid oxides a basic flux is required
for formation and easy removal of the slag.

 A strong basic slag is formed by addition of CaO and / or
MgO to absorb P2O5 and SiO2.

 The removal of carbon will take place in the form of
gaseous products (CO).

 During refining, controlled oxidation of the impurities in hot metal
(with the exception of sulphur) takes place once oxygen is blown at
supersonic speeds onto the liquid bath.
 The interaction of the oxygen jet(s) with the bath produces crater(s)
on the surface, from the outer lip(s) of which, a large number of tiny
metal droplets get splashed.
 These droplets reside for a short time in the slag above the bath.
Therefore, the existence of a metal-slag-gas emulsion within the
vessel, virtually during the entire blowing/refining period is an integral
part of BOF steelmaking.

 This is the reason why the slag-metal reactions like
dephosphorisation and gas--metal reactions like decarburisation
proceed so rapidly in the BOF process
 The droplets ultimately return to the metal bath. The extent of
emulsification varies at different stages of the blowing period, as
depicted schematically .
 A minimum amount of slag, with the desired characteristics, is
necessary for ensuring that the emulsion is stable, i.e. the slag
should not be too viscous, or too 'watery'. Only in this way can the
kinetics of the removal of the impurities be enhanced.

 For encouraging quick formation of the appropriate type of slag,
lime/dolomite/other fluxing agents with adequate reactivity are added right from
the beginning of the blow. The reactivity of the fluxing agents, primarily lime
(consumption 60-100 kg/tls), determines how quickly slag is formed (typically
within 4-5 minutes after the commencement of the blow).
 The rate at which oxygen is blown through the lance, the number of openings
(holes) on the lance tip, the distance between the lance tip and the bath surface
(lance height), the characteristics of the oxygen jets as they impinge on the bath
surface, the volume, basicity and fluidity of the slag, the temperature conditions
in the bath and many other operational variables influence the refining.

There are two distinct zones of refining in a LD vessel viz. the
reactions in the emulsion and in the bulk phase. The
contribution of bulk refining, i.e. refining in impact zone and at
the bulk slag-metal interface, is dominant in the beginning
since emulsion is yet to form properly. It has also been
believed that substantial decarburisation of droplets can occur
because of its free exposure to an oxidising gas, particularly
in the beginning. As the emulsion builds up the emulsion
refining attains a dominant role. The bulk phase refining
dominates again towards the end when the emulsion
collapses.

 Conditions for dephosphorisation are that the slag should be basic, thin and
oxidising and, that the temperature should be low.
 Dephosphorisation, therefore, does not take place efficiently until such a slag is
formed. Such a slag is formed in LD process only after the initial 4-6 minutes of
blowing.
 The rate of dephosphorisation picks up concurrently with the rate of
decarburisation.
 For efficient decarburisation as well as dephosphorisation the slag should,
therefore, form as early as possible in the process. If a preformed slag is
present as in a double slag practice wherein the second, slag is retained in the
vessel in part or full, the decarburisation rate curve rises more steeply in the
beginning

Dephosphorisation is very rapid in the emulsion because of
the increased interfacial area and efficient mass transport.
Phosphorus should, therefore, be fully eliminated before the
emulsion collapses. If this is not achieved the heat will have
to be kept waiting for dephosphorisation to take place and, in
the bulk phase, it is extremely slow as compared to that in the
emulsion. In general dephosphorisation should be over by the
time carbon is down to 0·7-1·0%, i.e. well ahead of the
collapse of emulsion which begins at around 0·3%C.


 The relative rates of dephosphorisation and decarburisation can be
controlled by adjusting the lance height or by adjusting the flow rate of
oxygen.
 Raising the height of the lance or decreasing the oxygen pressure
decreases the gas-metal reactions in the emulsion (i.e. decarburisation)
and vice versa.
 The dephosphorisation reaction is thus relatively increased by the above
change and vice versa. Towards the end when temperature is high the
danger of phosphorus reversion does exist but it can be prevented by
maintaining a high basicity of the slag.

The process of decarburization includes at least three stages:
 supply of reagents - carbon and oxygen - to the reaction site;
 the reaction [c] + [0] proper; and
 evolution of reaction products - CO bubbles into the gaseous
phase .
. The apparent activation energy of the reaction [C] + [0] = CO is
relatively small (according to various researchers, E = 80000-
120000 J/mol), which suggests that the reaction occurs practically
instantaneously. The solubility of CO in molten steel is also
negligible. Accordingly, the process can be limited by either the first
or the third stage.

 The nature of kinetic curves of carbon burning-off at its various
concentrations is different: on attaining a certain 'critical’ level of
concentration of carbon (0.15-0.35%), the rate of carbon oxidation is
observed to drop noticeably.
 It has also been established in experiments that the critical carbon
concentration is determined by the intensity of supply of oxidant to the bath
(it increases with increasing intensity of oxygen supply and decreases
during bath boil or metal stirring).

 Thus, at carbon concentrations above the ‘critical value’, the intensity of
decarburisation reaction is determined by the supply of the oxidant and at
those below the critical value, by carbon diffusion to the reaction place .
 This means practically that, if the carbon content of the metal is sufficiently
high, the rate of carbon oxidation will be higher at a higher intensity of oxygen
supply. At low concentrations of carbon, however, a higher level of intensity of
oxygen supply will not produce the desired effect and the bath should be
agitated forcedly (in order to intensify the supply of carbon to the reaction
place) so as to increase the rate of carbon oxidation.

 The rate of decarburization can also be limited by the third stage, the evolution
of CO. For a bubble of CO to form in metal, It must overcome the pressure of
the column of metal (pm), slag (psl), and of the atmosphere (pat) above the bubble
and also the forces of the cohesion with the liquid, 2σ/r i.e.

pCOev ≥ pm + psl + pat + 2σ/r
 The value of 2σ/r becomes practically sensible at low values of bubble radius: at
r > I mm it can be neglected. Formation of bubbles in the bulk of liquid metal is
practically impossible.. They can only form on interfaces between. phases, such
as slag - metal, non-metallic inclusion - metal, gas bubble - metal or lining -
metal. The most favorable conditions for the nucleation of CO bubbles exist on
boundaries between the metal and refractory lining which has a rough surface
and is poorly wettable by the metal


 High silicon pig iron is required in the acid steelmaking
processes to make relatively acid slag to ensure longer life of
the refractory lining.

 Oxidation of silicon also generates sufficient heat required in
case of the Bessemer process.

 However basic steelmaking processes need low silicon iron
because the entire amount of acid silica due to the oxidation of
silicon has to be neutralized by lime to produce slag with
basicity (CaO / SiO2 ratio) between 2 and 4 needed for
effective desulphurisation and dephosphorisation.

 Due to the strong attraction between iron and silicon, the
Fe-Si system exhibits large negative deviation from the
Raoults low. The activity coefficient of silicon in iron in
presence of other elements is given by :
 log fSi = 0.18×%C + 0.11×% Si + 0.058×% Al
-0.058 × %S + 0.025 × % V + 0.014 × % Cu
+ 0.005 × % Ni
+ 0.002 × % Mn – 0.0023 × % Co – 0. 23 × %O
 Oxidation of silicon is an exothermic reaction and provides
some of the heat necessary for rise of temperature of the
bath during blowing.

 Si –O reaction is governed by ∆G0 vs T equation:
[Si ]+ 2 [O] = (SiO2 ), Go = -14200 + 55.0 T cals.
 The activity coefficient of oxygen decreases and that of
silicon increases with increasing silicon content in iron.
 Silica is a very stable oxide, hence once silicon is
oxidised to SiO2 the danger of its reversion does not
arise.

a SiO 2 a SiO 2
K= = ( 20 )
[ a Si ][ aO ] 2
[ f Si .% Si ][ f O .% O ] 2

a SiO 2  a SiO 2 
∴ [ % Si ][ % O ] =
2
= 2.8 × 10 −5   ( 21)
f Si f O2 . K  f Si f O2 

 The extremely low activity of silica in basic steelmaking slag poses
no danger of preferential reduction of silica like that of phosphorus
removal.
 In basic steelmaking process the silicon content of pig iron should
be kept as low as possible to decrease the lime consumption with
the prime objective of controlling the required basicity for
phosphorus removal at a minimum slag volume.
 In case of high silicon entering the basic steelmaking furnace
double slag practice has to be adopted.
 Alternatively, external desiliconisation of the hot metal has to be
done outside the blast furnace before charging it in a basic
steelmaking furnace.

 About 50 to 75% of the manganese in the burden gets
reduced along with the pig iron resulting its
manganese content between 0.5 to 2.5%.
 During steelmaking major amount of manganese is
lost into the slag and very little is utilized to meet the
specifications.
 Some manganese is required to control the
deleterious effects of sulphur and oxygen and also for
improvement of mechanical properties of the steel.

 Hence conditions for maximum recovery of manganese
can be derived by considering the equilibria:
(Fe2+) + [Mn] = (Mn2+) +[Fe]
(FeO) +[ Mn] = (MnO) + [Fe]
( a Mn 2 + ) [ a Fe ] ( χ Mn 2 + ) f Fe [ % Fe ]
K= = ( 23)
( a Fe 2 + ) [ a Mn ] ( χ Fe 2 + ) f Mn [ % Mn ]
( χ Mn 2 + ) [ % Fe ]
or K ′ = ( 24 )
( χ Fe 2 + ) [ % Mn ]

 At equilibrium the Mn slag-metal distribution relation is
( χ given by( χ Fe 2 + )
)
Mn 2 +
= K′ ( 25)
[ % Mn ] [ % Fe ]

 From the equation it is apparent that the conditions for
the highest possible recovery of Mn i.e. minimum slag-
metal distribution ratio are
 i) min (χFe2+), requiring a low FeO content in the slag.
 ii) min K’ requires a low SiO2 content and a high
temperature as evident from the relation showing effect
various anions in the slag.

log K ′ = 3.1 χ SiO 4 − + 2.5 χ PO 3 − + 2.4 χ O 2 − + 1.5 χ F − ( 26 )
4 4

From the figure it is evident
that for slags containing
about 20% MnO, a
maximum of 0.1% Mn is
found in metal.

The slag containing 50%
SiO 2 (the rest being FeO
and MnO), with increasing
Mn content of the metal the
(MnO) content of the slag
increases whereas the
oxygen content of the metal
decreases and silicon
content increases.

 Despite its very low boiling point significant
amount of P gets dissolved in pig iron due to
strong attraction for iron.
 Making use of the interaction coefficients for the
effect of various elements on the activity
coefficient of phosphorus in iron, the activity of P
can be estimated by the expression:
 logfP = 0.13×%C + 0.13×%O + 0.12×%Si + 0.062×
%P + 0.024×%Cu + 0.028×%S + 0.006×%Mn –
0.0002×%Ni – 0.03×%Cr

 A very close stability of FeO, Fe2O3 and P2O5 is evident
from the iron and phosphorus lines in the Ellingham
diagram.
 Hence practically all the phosphorus present in the ore
gets reduced along with iron in the blast furnace and
joins pig iron.
 During steelmaking the activity of P2O5 in the slag of
basicity 2.4 is reduced drastically to 10-15-10-20.
 Activity of P2O5 in steelmakig slag is very low even if it
contains 25% P2O5.

•i.e. 2[P] + 5[O] + 3(O2-) = 2 (PO43-) (12)

 Thus for effective removal of phosphorus basic steelmaking
processes have to employ slags of high basicity.
 The distribution of phosphorus between slag and metal can be
dessribed as
2[P] + 5(FeO) + 3 (CaO) = (3 CaO.P2O5) + 5[Fe]
i.e. 2[P] + 5[O] + 3(O2-) = 2 (PO43-)
2
a PO 3−
K= 4
Applying Temkin rule : (13)
a[2P ] .a[5O ] .a (3O 2 − )
χ PO 3−
2

= 4
(14 )
[ f P % P] [ fO % O] 5 χ O
2 3
2−

The dephosphorising index, D P which is the ratio of phosphorus in slag to that in metal, is given as
( χ PO 3− )
[ % O]5/ 2 ( χ O
1/ 2
∴ DP = 4
= K′ )3 / 2 (15)
[ % P] 2−

From the figure it is clear that D P
increases with increase in the (FeO)
content upto 15% due to the high
oxidizing power.

Beyond this D P decreases due to
decrease in the lime proportion.

Dephosphorisation is more effective
at lower temperature because D P
increases with decrease of
temperature.

 The soda ash is 100 times more effective compared to lime
on molar basis but it is avoided in practice due to its severe
corrosive action on furnace lining.
 The magnesia content of a basic steelmaking slag reaches
equilibrium with the lining hence not under control and MnO
depends on charge and hence not much adjustable.
 The steel maker has the option of controlling lime, silica and
FeO.
 For charges containing high % P more than one slags are
made to dephosphorise metal bath to the desired level.
 In brief ,high basicity, low temperature, and FeO content
around 15% favour dephosphorisation of metal by basic
slags.

The optimum conditions for dephosphorisation can be
derived from the equation defining the index:
( χ PO 3− )
[ %O]5/ 2 (χO
1/ 2
DP = 4
= K′ )3 / 2
[ % P] 2−

1. Basic slag gives a high value of χO2-
2. High lime content – lime is the divalent oxide making the
largest contribution to K’ (log K' = 21N'Ca++ + 18 N'Mg++ +
13N'Mn++ + 12 N'Fe++
3. Ferrous oxide close to 15% .
4. Low temperature gives a high value of K‘.

 In refining of steel oxidation of Si, Mn and P takes place
at the slag-metal interface.
 The oxidation of carbon practically does not take place
at the slag-metal interface because of the difficulty of
nucleation of CO bubbles there.
 C-O reaction takes place at the gas –metal interface
since it eliminates the necessity of nucleating gas
bubbles.
 During refining of steel oxygen has to dissolve first in the
bath before it reacts with the dissolved impurities.
 In the absence of other slag forming constituents at
1600oC liquid iron can dissolve oxygen up to at 0.23 wt.
%

 In steel making the reaction between carbon and
dissolved oxygen is of utmost importance.
 Generally pig iron contains about 4 wt% carbon.
 The solubility of carbon in steel is effected by the
presence of impurities and alloying elements.
 Presence of Nb, V, Cr, Mn and W increase
solubility of carbon in iron where as presence of
Co, Ni, Sn and Cu decrease it.

•Thus solubility of carbon in steel can be calculated by combining the binary data from the following equation:

 Oxidation of Carbon can be discussed according to
the reaction:
C + O = CO, ΔG0= -5350 – 9.48T cals.
pCO pCO
K= =
a c aO [ fc%C ][ fo%O ]

pCO pCO
∴[%C ][%O ] = =
K fc fo K

 At any chosen pressure of CO, % C vs % O
indicates inverse hyperbolic relationship

During oxidation period oxygen is continuously transferred from
the slag to the bath, where it continuously reacts with carbon to
give CO.
The main resistance to the oxygen flow is the slag–metal and the
metal–gas interfaces, whereas inside the steel bath the transfer
of dissolved oxygen is very fast.

 The activity coefficient
of carbon in iron
increases with
increasing carbon
content and that of
oxygen decreases with
increasing carbon
content.
 The net result is that
the product [% C] [% O]
for a given pCO
decreases slightly with
increasing carbon
content as shown in
Figure

 Since steel making is a dynamic process, the
concentration of carbon and oxygen in the bulk metal
phase is not in equilibrium with the prevailing CO-
pressure in the bubbles.

 At the gas bubble–metal interface the reaction is close to
equilibrium.

 The experimentally observed excess oxygen and carbon
in the bulk metal phase is thus helpful in transfer of the
reactants by diffusion to the gas-metal interface in the
violently stirred metal bath.

 As [% O] increases with (aFeO) in slag and [% O]
decreases with [% C] in the bath.

 it follows that the iron oxide contents of the slag
increases with decreasing carbon in steel during refining.

 Hence there is a general trend in the variation of slag
composition with the carbon content of the metal.

 For a given total iron oxide in a slag, a lower carbon in
the steel corresponds to a higher sum of (% SiO2 + %
P2O5) in the slag.

 Within the range of basic slags, for a given sum of % CaO + %
MgO + % MnO the carbon content of steel does not vary much
with the FeO content of the slag.

 During steelmaking i.e. refining of pig iron where impurities like
carbon, silicon, manganese and phosphorus are eliminated to
the required level oxygen, nitrogen and hydrogen get
dissolved as harmful impurities.

 As solubility decreases with decrease of temperature excess
gases dissolved in steel are liberated during solidification.

 The evolution of the gas gives rise to the formation of skin or
pin holes, blow holes, pipes etc.

 The unsoundness caused by these cavities affect the
mechanical properties of steel

 Nitrogen pick up during steel making:
◦ open atmosphere
◦ raw material charged
◦ during melting and/or refining
 Effect of nitrogen in steel:
◦ yield-point phenomena
◦ AlN causes intergranular fracture
◦ nitrogen stabilizes the austenitic structure
 Factors affecting the nitrogen solubility in steel.
◦ partial pressure of nitrogen in the blast
◦ time of contact
◦ length of after blow and
◦ the bath temperature

[wt.%H] =

 Since nitrogen dissolves atomically in liquid iron and steel
in very small proportion its solubility can be discussed in
terms of Sievert’s and Henry’s laws

 There is slow rise in solubility in solid state with increasing
temperature but at the melting point it increases very
rapidly. It also rises in liquid steel but at a slow rate.

 Presence of vanadium, niobium, tantalum, chromium,
manganese, molybdenum, and tungsten increases
nitrogen solubility in iron whereas it decreases in presence
of nickel, cobalt, silicon and carbon

 Hydrogen pick up steel making:
◦ wet solid and rusty charge materials
◦ atmospheric humidity
◦ wet refractory channels, runners and containers
 Effect of hydrogen in steel
◦ Decreases ductility
◦ Appearance of hairline cracks seriously affect the
mechanical properties
◦ Formation of blow holes and pin holes.

 Water vapour coming in contact
with steel or slag leads to the
formation of hydrogen which gets
dissolved in steel melt as per
reaction:
H2O (g) = 2[H]+ [0]
 At the melting point of iron
solubility in delta iron is
approximately 10 mL/ 100g.
 Beyond this hydrogen will be
rejected during solidification to
produce unsound porous ingots
due to gas evolution.

 Thus partial pressure of hydrogen, and composition of
steel and its temperature control the hydrogen content of
steel. According to Sievert’s law solubility of hydrogen
in pure iron is expressed as:

 Presence of niobium, tantalum, titanium and nickel
increases the solubility of hydrogen in iron whereas it
decreases in pressure of carbon, silicon, chromium and
cobalt.

 The objectives of vacuum degassing include removal of
hydrogen from steel to avoid long annealing treatment,
removal of oxygen as carbon monoxide and production
of steels with very low carbon content (< 0.03%).

 The principle is based on the usefulness of the Sievert’s
law relationship.

 The equation demonstrates that subjecting the molten
steel to vacuum will decrease the hydrogen, nitrogen as
well as the oxygen content of the steel according to the
following reasons:

2[H] = H2 (g)

2[N] = N2 (g)

[C] + [O] = CO (g)

 The effectiveness of vacuum treatment increases with increase in
the surface area of liquid steel exposed to vacuum.
 For this purpose metal is allowed to flow in the form of thin stream
or even fall as droplets to accelerate the degassing process.

A number of methods available on commercial scale for
vacuum treatment of steel may categorized into three groups :
1. Ladle Degassing
 The teeming ladle filled with steel to one fourth of its height is

placed inside a vacuum chamber.
 the melt is stirred either by bubbling argon or by

electromagnetic induction
 Introduction of gas for stirring provides interface which

facilitates degassing.
 In general pumping is carried out to attain the ultimate

vacuum of 1-10 mm Hg. which is supposed to be adequate
for degassing.

2. Stream Degassing
 In this case molten steel is allowed to flow down
under vacuum as a stream from the furnace to ladle
to another ladle or a mould.
 A very high rate of degassing is achieved due to
large increase in surface area of molten steel in the
form of falling droplets.
 Thus choice of proper vacuum pump and vacuum
chamber is important to achieve the adequate level of
degassing.

3. Circuilation Degassing

R-H degassing process
The average rate of circulation is
12 tons/min.
Twenty minutes are required to
treat 100 tons of steel to bring
down 90% reduction of
hydrogen content.

D-H Vessel.
The chamber is moved through
50-60 cm with a cycle time of
20 sec. 10-15% steel is
exposed at a time.

7-10 cycles are required to
expose the entire steel once.

Adequate degassing is
obtained
in 20-30 cycles in 15-20
minutes.

 High carbon steels like rail steels (0.65%-0.74% C, 0.6%-1.0%
Mn, 0.27-0.30% Si), ball-bearing steel (1.0% C, 1.2% Cr), etc. are
also manufactured in the LD converter by the catch carbon
technique. In this technique, dephosphorization is accelerated
and completed before decarburization. Extra lime and fluorspar
are charged and the lance is raised to a higher position for
maintaining a soft blow condition till phosphorus removal is
completed. Thereafter, decarburization is continued by a
harder blow till the bath carbon content drops to the desired
level.

 Alternatively, blowing may be continued to complete both
dephosphorization and decarburization. Required amount of
carburizer is then added to the low carbon steel bath to raise
the carbon content to the desired level. However, this method
involves a risk of increasing the inclusion and nitrogen
contents in the steel. These are picked up from the carburizer
(e.g., petroleum coke or graphite). For production of low alloy
steel, the alloying elements are usually added in the ladle
during tapping the steel.

 As will be evident from the discussion [Mn] from the bath is
lost in the slag. (MnO) thus formed quickly combines with (SiO 2
to form (2MnO· Si02). Thus, there is a reduction in the Mn
content in the bath in the initial period of the blow. As the slag
basicity increases due to lime dissolution, (MnO) is gradually
released and is reduced by carbon during intensive carbon
oxidation according to the following reactions:
 (MnO) +[C] → [Mn]+{CO}
 [Mn] content in the bath increases again. As the intensity of the
carbon-oxygen reaction decreases towards the end of the
blow,. manganese is reoxidized from the bath. As a result, the
bath manganese content drops again. This accounts for the
characteristic 'manganese hump' in the LD converter reaction
diagram.


 A basic and highly reactive slag is necessary to attain
desulphurization and dephosphorization in LD steel making at
the turndown stage. Hence the physical and chemical
characteristics of the lime used are of utmost importance.
Some common quality criteria for steel making lime are listed
below:
 Chemical composition
 Size distribution
 Reactivity
 Loss on ignition
 Moisture content
 Si02 in the lime reduces the CaO activity due to the formation of
larger amount of slag by fixing up about two times its mass of
CaO. This is detrimental both from "yield" and "cost" points of
view.

 The sulphur content in lime should be as low as possible. An
MgO content of approximately 3.5% in lime is thought to be
beneficial because an MgO content of around 5% in the slag
has been found to hinder the formation of dicalcium silicate,
thereby ensuring a faster lime dissolution in the slag. However,
lowering of melting point and the viscosity of slag due to
increased proportion of MgO can result in early slopping. An
adequate level of MgO in slag also ensures less corrosion of
the vessel refractories because of its known properties of
neutralizing the FeO level of the bath.





 Formation of slag as early as possible during the blow requires a
uniform and rapid dissolution of lime. A size range of +8 to -40 mm,
minimum proportion of fines in the lime charge and soft burnt lime
promote early slag formation. A soft burnt lime is highly porous,
having a large specific area. This results in its favorable reactivity.
 Thermal dissociation reaction of unreacted CaC03 is endothermic.
It adversely affects the heat balance of the converter and leads to
operating problems. Similarly, a moisture content in lime directly
affects the heat balance of the vessel because of temperature
losses during its disintegration. It also acts as a potent source of
hydrogen in steel. Hence both loss on ignition (LOI) and moisture
content of lime should be low.


The lining of oxygen converters is usually made up of three
layers of bricks. First an inner layer of magnesite or burnt
dolomite brick is made. Gaps between the brick and the shell
are filled with tardolomite ramming mass. The same ramming
composition is used for making up the second intermediate
layer. The upper working layer is made of magnesia carbon
brick.
The performance of refractories is generally evaluated by the
life of the lining or by the consumption of refractories per ton
of steel produced. However, this is greatly influenced by the
severity of service conditions that prevail during operation.
In brief, these are:
 Furnace atmosphere, Composition of slag, Mechanical
stresses ,Thermal shock, Effect of high temperature, Geometry
of the vessels, Operational procedure or the blowing technique
Quality of hot metal, Quality of refractories.

 A rapid sequence of blows, without pause, increases the lining life. A
high silicon hot metal produces a silica rich slag which increases the
wear of basic lining. At high temperature, the corrosive attack of the
slag is enhanced. Combustion of the CO generated inside the vessel
also raises the temperature in the upper zone of the furnace. This
enhances lining wear in the region. The distance of the oxygen lance
from the bath has a considerable effect on the refractory wear. Usually,
a high position of the lance leads to a reduced wear of the furnace
bottom, but it increases wear at the top and upper part of converter.
However, with the introduction of the multi-hole lance nozzles, the
oxygen is evenly distributed on the bath surface. This has solved the
problem of preferential bottom or top lining wear.

The early refractory lining for LD vessel was based generally
on doloma, magnesia or magnesia-chrome of the same quality
as used in the earlier steel making processes, e.g., Bessermer,
open hearth, etc. However, the high basicity of the LD
converter slag and the high temperature of the bath promoted
rapid wear of the refractories. Modern LD converters are,
therefore, lined with magnesia-carbon refractories. The total
Fe20s, Si02 and Al20s content in the magnesia refractory should
be low-definitely less than 4.0%-to improve its resistance to
slag attack. Sea water magnesia is usually added along with
natural magnesia to enrich the MgO content in the brick

Care would be taken to lower the B20s content in sea
water magnesia to a level at which it does not affect
the high temperature properties. The presence of
submicroscopic carbon particles in magnesia
carbon refractories inhibits penetration of slag into
the refractory.

The capacity of graphite to reduce wear is based upon its
large wetting angle for oxide melts. The melt can
penetrate the bricks only when the graphite is burnt
away. near the hot face owing to diffusion of oxygen in
between blows during a campaign. Thus, the infiltration
zone progressively advances, resulting in a continuous
wear of the lining. The slag resistance of magnesia
particles is improved by its high bulk density, low
impurity content and large crystal size of MgO particles.

LD refractory lining life has been greatly enhanced in recent
years by adopting the slag splashing technology. In this
technology, a portion of the slag is retained in the vessel after
tapping. A low FeO and a high MgO slag is desirable for slag
splashing. Such improvement in slag condition is achieved
through addition of dolomite lime after tapping. Slag splashing
is accomplished by injecting nitrogen into a conditioned slag at
a given flow-rate and lance height. The existing oxygen-lancing
equipment is used.

 By varying lance height and nitrogen flow-rates, slag can be
selectively targeted and blown into particular areas of the
furnace. This is schematically illustrated in Figure given below.
The process time for slag splashing is between 1 and 4
minutes. A well-designed nitrogen slag splashing programme
can extend furnace lining life to 8,000 heats. Once slag
splashing is started, it would be done on a regular basis. Slag
splashing presents some operating challenges like lance shell

DEOXIDATION METHODS
AND PRACTICES

By

Dr. S.Sarkar
Associate Professor
Dept. of Metallurgical and
Materials Engg.
National institute of Technology,
Rourkela

PLAN OF PRESENTATION

 Introduction
 Deoxidation methods
 Choice of deoxidisers
 Removal of deoxidation product
 Deoxidation equilibria
 Silicon – manganese deoxidation
 Complex deoxidisers
 Deoxidation practices

INTRODUCTION
 Contrary to iron making steelmaking is practiced in
oxidizing conditions.
 In all the steelmaking processes either air or oxygen is
blown or surplus air/oxygen is provided to facilitate
quick oxidation of impurities.
 Under these conditions oxygen easily gets dissolved in
the steel melt.
 During solidification of steel castings excess oxygen is
evolved because of very low solid solubility and is one
of causes of defective casting.
 This excess oxygen has to be eliminated for production
of sound casting. The process of removal of residual
oxygen of the refined steel called deoxidation

DEOXIDATION METHODS
1. Diffusion deoxidation
 When dissolved oxygen is lowered down by
diffusion of oxygen from the steel melt to the
slag in the steelmaking furnace, the method is
called Diffusion deoxidation.
 This can also be done outside the furnace under
vacuum according to the reaction:
2[O] → O2 (g)
 But the method can be used effectively to a
limited extent.

DEOXIDATION METHODS
2. Precipitation deoxidation
 The residual oxygen is allowed to react with
elements having higher affinity for oxygen
(compared to what iron has for oxygen) to form
oxide products.
 The product being lighter than steel rises to the
top surface and can be easily removed.
 Precipitation deoxidation is practiced
extensively because it is very effective in
decreasing oxygen content of steel.

PRECIPITAION DEOXIDATION -
CHOICES OF DEOXIDISER
 Thermodynamically best
deoxidinsing element
(deoxidiser) should have the
least amount of dissolved
oxygen [O] left in equilibrium
with its own lowest
concentration in the steel
melt.
 Al and Si are very effective in
deoxidation of steel and
hence they are used
extensively.
 Al, Si and Mn are reasonably
cheap and hence used as
common deoxidizers.

CHOICE OF DEOXIDISER
 Some times Zr, Ti, V, Nb etc. are used in
deoxidation of steel but they are costlier than
common deoxidisers.
 The residual content of the deoxidiser in steel
after deoxodation should not adversely affect the
mechanical properties of steel.
 The rate of deoxidation i.e. formation of oxide
products must be fast.
 Since kinetic data on deoxidation are very limited
thermodynamic consideration play major role in
selection of deoxidisers and estimation of
residual content of the deoxidisers in steel at the
end of deoxidation.

REMOVAL OF DEOXIDATION
PRODUCTS
 The mechanically entrapped oxide products in steel
are called nonmetallic inclusions which deteriorate the
mechanical properties.

 Size, shape, distribution and chemical composition of
inclusions make effective contribution in controlling
the properties of steel.

 This makes it essential to remove the deoxidation
products from the steel melt to get clean steel.

 Thus from cleanliness point of view a gaseous product
of deoxidation would be most appropriate.

PRODUCTS
 Only carbon produces gaseous deoxidation
product under reduced pressure according to the
reaction:
[ C ] + [ O ] = CO ( g )
 Though the reaction is favoured under reduced
pressure but economics do not permit for vacuum
treatment.
 Hence carbon cannot be used as a deoxidiser for
production of clean steel.
 Deoxidisers other than carbon form liquid or
solid products.

PRODUCTS
 Formation of a solid deoxidation product will give rise to a
new phase which will grow during the course of deoxidation
and has to rise to surface of the melt for elimination.

 Otherwise it will disperse in the melt and on solification
may be entrapped in steel as nonmetallic inclusions.

 For nucleation and growth of deoxidation products required
interface may be provided by inhomogenities, for example
formation of Al2O3/steel interface while deoxidising steel
with aluminium at the beginning.

PRODUCTS of the decoxidation product (v) in a quiet
 The rate of rise

bath may be estimated from Stoke’s law:

 Where g, r, ρliq, ρdp and η stand respectively for acceleration
due to gravity, radius of the deoxidation product, densities
of the liquid metal and the deoxidation product and the
viscosity of the liquid metal.

 that r2 factor plays an important role in controlling the time
required for the particles to rise to the surface of the
metallic bath.

PRODUCTS
 On the basis of Stoke’s law it can be
demonstrated that particles of deoxidation
product less than 0.001cm radius will not move
to the surface of the metallic bath in a usual
ladle within the normal holding time of 20
minutes, whereas larger particles ( radius
greater than 0.01cm) should be completely
eliminated.

 These figures emphasise the significance of
coalescence of deoxidation products in formation
of particles of larger radius to facilitate rapid rise
to the surface of the steel melt

PRODUCTS
 Since coalescence of the deoxidation product is
more likely in liquid state, deoxidation is often
carried out to obtain liquid products.

 The rate of removal is also affected by the
interfacial energy between the liquid metal and
the deoxidation product.

 High interfacial energy will enhance the rate of
removal of the product by lowering the dragging
affect.

PRODUCTS
 The rate of rise of the decoxidation product (v) in a
quiet bath may be estimated from Stoke’s law:

 Where g, r, ρliq, ρdp and η stand respectively for
acceleration due to gravity, radius of the deoxidation
product, densities of the liquid metal and the
deoxidation product and the viscosity of the liquid
metal.

 that r2 factor plays an important role in controlling the
time required for the particles to rise to the surface of
the metallic bath.

for which the equilibrium constant is given as :

DEOXIDATION EQUILIBRIA
 A generalised form of chemical equilibrium dealing
with the deoxidation product in contact with steel melt
may be represented as:
x [M] + y [O] = MxOy (s, l )

 By and large all the solid deoxidation products except
Fe(Mn)O have stoichiometric compositions.
 Since we are dealing with infinitely dilute solutions of
deoxidisers in the melt according to Henry’s law we
can write

DEOXIDATION EQUILIBRIA

 The activity coefficient of oxygen decreases and
that of alloying element increase, with increases
in concentration of the alloying element.
 However the minimum oxygen content decreases
with the increasing stability of the deoxidation
product.

SILICON MANGANESE
DEOXIDATION widely carried out by common
 Deoxidation is most
deoxidisers like silicon and manganese.
 The deoxidation with manganese giving rise to the
formation of liquid or solid solution of FeO and MnO
may be represented as:
[Mn] + (FeO) ( s, l ) = [Fe] ( l ) + (MnO) ( s, l )
 Deoxidation by silica is given by

[Si] + 2 [O] = (SiO2)
 Deoxidation with silicon is much more effective as
compared to manganese but simultaneous deoxidation
by both the elements leaves much lower residual
oxygen in the melt due to reduced activity of SiO2 in
FeO – MnO – SiO2 slag.

SILICON MANGANESE
DEOXIDATION
 Assuming that the deoxidation product is pure
manganese silicate and the sum of the deoxidation
reactions by silicon and manganese are represented as:
[Si] + 2 (MnO) = 2 [Mn] + ( SiO2 )
 The figure highlights the role of manganese in
boosting5 the deoxidising power of silicon with
increasing silicon content.
 For example at 0.05% Si in solution, the residual
oxygen is lowered from 0.023% to 0.016% when the
manganese content is increased from zero to 0.8% ;
while at 0.2% Si, a similar increase in manganese
lowers the residual oxygen from 0.0104% to 0.0094%”6.

SILICON MANGANESE
DEOXIDATION

Simultanious deoxidation by silicon and
manganese at 1600oC.

SILICON MANGANESE
DEOXIDATION

Residual oxygen and
silicon contents of iron
after deoxidation of 0.10
% oxygen steel at 1650oC
at various residual
manganese contents
from 0.2 to 0.6 % Mn.

SILICON MANGANESE
DEOXIDATION
 From the figure it is evident that at all temperatures
for the metal compositions lying above the curve,
manganese does not take part in deoxidation reaction
and solid silica is formed.
 On the other hand metal composition lying below the
curve the deoxidation product is liquid manganese
silicate whose composition is controlled by the ratio [%
Si]/[% Mn]2 in the metal.
 From the above discussion it is clear that silicon alone
is a very effective deoxidiser but it produces solid
product which poses problems in separation from the
steel melt.

SILICON MANGANESE
DEOXIDATION
 Though manganese is not effective it produces
liquid deoxidation product. Both silicon and
manganese used together give better result.

SILICON MANGANESE
DEOXIDATION
 Deoxidation first carried out by addition of
ferromanganese in steel melt produces FeO –MnO
liquid slag which dissolves SiO2 when ferrosilicon
deoxidises the melt in second step.

 In the resulting slag FeO – MnO – SiO2 the activities of
SiO2 and MnO are much lower than when Fe–Mn and
Fe–Si are used separately for deoxidation.

 Lowering of activity improves their effectiveness in
reducing the residual oxygen in steel when Mn and Si
are added in correct proportion.

SILICON MANGANESE
DEOXIDATION (Mn/Si) is normally maintained
 In practice the ratio

between 7 and 4 to obtain a thin liquid slag as the
deoxidation product.

 At 16000C the equilibrium oxygen level is
approximately 0.1% with 0.5% Mn but addition of 0.1%
Si reduces residual oxygen to 0.015%.

OTHER DEOXIDISERS deoxidiser as it has more
 Aluminum is even more effective

affinity for oxygen compared to silicon and manganese. But
it cannot be used alone to deoxidise steel completely
because the deoxidation product, Al2O3 is solid at the
steelmaking temperature.

 While using along with manganese and silicon alumina will
dissolve in the liquid slag product of deoxidation. Boron,
titanium and zirconium are also very effective deoxidisers.

 The extent of deoxidation achieved by 8% Si can be easily
obtained by 0.7% B, or 0.1% Ti or 0.002% Al or 0.00003%
Zr.

COMPLEX DEOXIDISERS
 The rare earth elements or alloys based on them are
employed in conjunction with common deoxidisers for
bringing down sulphur and oxygen to a low desired level.

 A commercial rare earth mixture, known as “REM”
containing 48-50% Ce, 32-34% La, 13-14% Nd, 4-5% Ps,
and 0.6-1.6% higher lanthanides has been reported.

 For achieving low residual oxygen in steel the complex
deoxidisers must exhibit
 low vapour pressure
 Liquid deoxidation products

COMPLEXin steel calcium silicide reacts with oxygen
 Dissolution DEOXIDISERS
to form molten calcium silicate slag which can flux
alumina inclusion.
 Possessing similar characteristics an alloy of Ca, Si, Al
and Ba is a good deoxidiser to produce clean steel.
 Occasionally the deoxidation products are beneficial if
they remain entrapped in a very finely dispersed form.
 For example, very fine dispersion of Al2O3 particles
without coagulation provides the possible nucleation
sites during solidification of steel resulting in a very
fine grain structure of steel.

DEOXIDATION PRACTICE
 On industrial scale there are three methods of
deoxidation.

 After refining, molten steel can be deoxidized either
inside the furnace, called furnace deoxidation or
during tapping in a ladle, called ladle deoxidation.

 For production of fine grained steel or in case of
inadequate deoxidation a small portron of total
deoxidation may be done in the ingot moulds.

 As deoxidation lowers the oxidizing potential of
the bath there is a fair chance of reversion of the
refining reactions if oxidised refining slag is
present in contact with the metal.
 Stable oxides like SiO and MnO are not prone to
2
reversion in acid steelmaking processes.
 However P O in basic steelmaking is very easily
2 5
reduced from the slag to the metal phase on drop
of oxygen potential.


 In general the refining slag is flushed off in basic
process and deoxidation may be carried out
partly in the furnace and major part in the ladle.

 As products of deoxidation in a furnace get more
time to reach the surface of the bath furnace
deoxidation is useful in production of clean steel.

CONTROL OF INGOT STRUCTURE
 The final structure of an ingot is entirely determined
by the degree of deoxidation carried out prior to
solidification of steel in a mould.

 The residual oxygen in the steel at the end of refining
is determined by the steel making practice and the
type of steel produced.

 For a given type of steel the steel making and
deoxidation practices have to properly adjusted to
finally obtain the desired ingot structure.

RIMMING STEEL
 Rimming steel require a lot of gas evolution during
solidification. The steel, therefore, must contain
enough dissolved oxygen and which is possible only in
low carbon steel (<0.15%).

 The heat must be finished in the furnace in such a way
that the bath contains desired level of oxygen having
carbon level < 0.15%.

 In general, no deoxidation is carried out inside the
furnace. Only a small amount of deoxidation is carried
out in the ladle using Fe-Mn and Al.

RIMMING STEEL
 The zone between the
primary and secondary
blow holes is called rim
which is characteristics of
rimming ingots.
 Rimming ingot is
relatively cleaned due to
less inclusions and brisk
evolution of gas in the
beginning of
solidification.

SEMI-KILLED STEEL
 These are partially deoxidised steel such that
only small amount of gas is evolved during
solidification.
 The carbon content has been in the range of 0.15-
0.30%.
 Partial deoxidation is carried out in the furnace
itself using Fe-Mn and Al.
 The gas is evolved towards the end of the
solidification. The blow holes are therefore,
present in the middle and top of the ingot.

SEMI-KILLED STEEL
 Aluminium is put into
the ladle toward the
end of pouring to
completely deoxidises
the top of the ingot to
compensate the pipe
formation.

KILLED STEEL
 No gas evolution take place in killed steel during
solidification.
 All steels containing 0.3% C are killed.

 The heat is worked in such a way that by the
time carbon level drops close to specification level
the refining should be over.
 In general the heat is then blocked by adding Fe-
Si, Fe-Mn and high silicon pig iron.
 Blocking stops the carbon oxygen reaction by
lowering oxygen content of the bath

KILLED STEEL
 Deoxidation product should
be given sufficient time to
rise to the surface
otherwise it will form
nonmetallic inclusions in
steel.
 Solidification of Killed
steel is accompanied by V
or A type seggregation

ADVANCES IN STEELMAKING
AND SECONDARY
STEELMAKING

Smarajit Sarkar
Department of Metallurgical and Materials
Engineering
NIT Rourkela

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 .

OXYGEN BOTTOM MAXHUTTE PROCESS(OBM)

BOTTOM BLOWING VS TOP BLOWING
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

CONT..
 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

SEQUENCE OF ELIMINATION OF
IMPURITIES IN OBM PROCESS

SLOPPING
 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.

METALLURGICAL FEATURES OF BATH
AGITATED PROCESS:

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

 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

HYBRID BLOWING
•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.

CONT..

• 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.

 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

HYBRID BLOWING
 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

DEOXIDATION OF STEEL

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'.

 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 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 macrosegregation to a greater
extent.


CONTINUOUS CASTING
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.

SIMPLIFIED SKETCH OF
CONTINUOUS CASTING

THE MAJOR REQUIREMENTS 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.

METALLURGICAL COMPARISON OF
CONTINUOUS CASTING WITH INGOT
CASTING

 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.

CONT…

 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.

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.

SECONDARY STEELMAKING

Smarajit Sarkar
Engineering
NIT Rourkela

SECONDARY STEELMAKING
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.

RESORTED TO ACHIEVE ONE OR MORE
OF THE FOLLOWING REQUIREMENTS :

 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.

QUALITY OF STEEL
 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.

CLEAN 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.)

INCLUSIONS

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.

MACRO AND MICRO
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.

SOURCES OF 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.

REMOVAL 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.

CLEANLINESS CONTROL DURING
DEOXIDATION
 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.


CONT…
 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.

THE SPEED OF FLOATING OF LARGE
INCLUSION CAN BE FOUND BY STOKE’S
FORMULA

PROCESS VARIETIES
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.

VARIOUS SECONDARY PROCESS
AND THEIR CAPABILITIES

VACUUM DEGASSING PROCESSES

 Ladle degassing processes (VD, VOD,
VAD)
 Stream degassing processes
 Circulation degassing processes (DH and
RH).

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.


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. Cont…

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.

CONT..

 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.


THERMODYNAMICS OF REACTIONS IN THE
AOD PROCESS

 Simplified by Hiltey and Kaveney

INFLUENCE OF PRESSURE AND TEMPERATURE
ON THE RETENTION OF CR BY OXYGEN
SATURATED STEEL MELTS AT 0.05%C

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

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 Paulo Nogueira, Richard Fruehan: "Blast Furnace Burden Softening and Melting
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Phenomena-Part II Evolution of the Structure of the Pellets", Metallurgical and
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formation” ISIJ International, Volume 39, No7 1999,pp697
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FeO Slag”, Proceedings of VIIInternational Conferenceon -Molten slags,fluxes and
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

DESIGN OF BLAST
FURNACE

Smarajit Sarkar
Engineering
NIT Rourkela

Steel making

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