steel making

1. Ladle Metallurgy (Secondary) steel Making
• In steel making, ladles are employed to transfer molten steel from BOF/ EAF to
ingot casting or continuous casting. Holding and transportation of molten steel
Contained in ladle can lead to a substantial drop in the temperature of steel.
• Therefore, for casting steel at desired superheat, it is necessary to compensate
such heat loss through appropriate melt reheating arrangements following furnace
tapping operations.
• These are collectively referred to as ladle metallurgy steel making and help bring
versatility and cleanliness to enhance mechanical properties to the final product.
• The total duration of secondary steel making operations (include deoxidation,
alloying, reheating, degassing and so on) is long and often exceeds that of
primary steel making.
• Ladle metallurgy steel making techniques are generally concerned about:
– Composition control: These include alloy additions for adjustment of melt
chemistry, powder injection for desulphurization and vacuum treatment for
removal of dissolved gases and production of ultra low carbon steel.
– Cleanliness control: This is concerned with the production of clean steel and
involves synthetic slag preparation for better inclusion absorption engineering
of flow in tundish and molds to help inclusion floatation as well as powder.

2. injection for modifying morphology and composition of oxide and sulphide
inclusions.
Temperature control: Electrical energy is used to increase heat content of molten
steel. Typically heat is produced through arcing in a LRF and convection and
radiation help to transport heat increasing melt temperature.
Inert gas stirring in Ladles:
• All secondary steel making operations in one way or other utilize gas injected
into the melt through one or more porous plug.
• The primary objective is to stir the bath resulting in homogenization of
temperature and composition of the melt. It offers additional advantages of faster
deoxidation and floatation of inclusions.
• The gas raising through the liquid create a central two phase gas liquid region,
known as plume. The plume, during its rise gets progressively wider due to
entrainment of the surrounding liquid steel. Ultimately, the gas bubbles escape to
the ambient through the free surface while the entrained liquid flow radially to
create turbulent re-circulatory motion which provides the necessary bath agitation
for increasing the rates of various heat and mass transfer controlled processes (
such as melting of deoxidizer and alloying additions as well as their dissolution
and dispersion).

3. • As the injected gas escapes to the surroundings, the bulk flow from the plume
eye pushes the slag layer radially outwards, exposing the melt surface to the
ambient.
• The uncovered area of the melt thus created is typically referred to as the slag
eye. The slag eye is a potential site for reoxidation, nitrogen pickup and slag
entrapment phenomena and hence can influence quality of steel profoundly.
• Therefore, during the final stage of ladle refining and immediately prior to
continuous casting, it is customary to practice gentle stirring (commonly termed
in the industry as argon rising) to ensure a small eye area.
• Depending on the end requirements a wide range of argon flow rates is used in

4. the industry. Correct stirring is of at most importance. Vigorous stirring would be
required for slag/metal reaction such as desulphurization, a relatively high argon
flow rate is used (1 Nm3/h/ton). Whereas, for inclusion removal, thermal and bath
composition homogenization relatively low flow rates are needed ( 0.1 Nm3/h/ton).
• More than the required amount of gas injection increases the possibility of
atmospheric reoxidation and erosion of refractory.
• Synthetic slag practice for secondary steel making : (for desulphurization)
• Secondary steel making is a critical quality control step between the primary steel
making and the continuous casting of the liquid steel.
• Synthetic slag practice is normally used to obtain clean steel and also for the
desulphurization of the liquid steel.
• The desirable properties of synthetic slag include:
– Slag is to have high sulphide capacity
– It is to be basic in nature
– It is to be fluid to obtain faster reaction rates
– It is not to cause excessive refractory wear

5. • Synthetic slag practice is adopted to meet the following objectives:
– To cover the liquid steel with an insulating layer to reduce heat losses.
– To remove the possibility of reoxidation of steel from atmospheric oxygen.
– To absorb inclusions present in the liquid steel.
– To desulphurize liquid steel. using synthetic slag of desired basicity and
sulphide capacity, deoxidized steel can be desulphurized to as low as 0.005
% of Sulphur.
– It helps in absorbing inclusions and impurities, thus producing cleaner steels.
• Design of synthetic slag:
• Main components of synthetic slag are CaO, Al2O3 and SiO2. synthetic slag
having these component is also known as calcium aluminate (CA) flux.
• when the ladles are lined with magnesia carbon or dolomite refractories then
MgO forms an important component of the synthetic slag. This synthetic slag is
also called calcium magnesium aluminate (CMA) flux.
• CMA slag allows a quick formation of a homogeneous and liquid slag with a
high capacity to absorb Sulphur and oxide inclusions from the steel bath.
• Earlier CaF2 was also used to be a component of the synthetic slag. CaF2 helps
in increasing the slag fluidity as well as sulphide capacity of slag but it attacks

6. the refractories and has environmental issues because of formation of gaseous
compound SiF4 due to interaction of CaF2 with SiO2 in the slag.
2CaF2 + SiO2 = SiF4 + 2CaO
• Use of fluorite in the preparation of synthetic slag is generally no more done
these days.
• Some times Al is added in the synthetic slag to deoxidize the liquid steel, since
transfer of S from liquid steel to slag is followed by transfer of oxygen from slag
to the steel. Therefore deoxidation of steel is essential for efficient
desulphurization.
[S] + (O2-) = (S2-) + [O]
• Synthetic slag has CaO-25-55 %, Al2O3-30-55 %. The slag is normally low in
SiO2, Fe2O3 and TiO2. MgO in CMA slag is from 3-6 %. In case Al is added in
synthetic slag then usually it is in the range of 5-16 %.
• Generally synthetic slag is basic in nature. However, special synthetic slag can be
designed for a special purpose. For removal of oxide inclusions, a neutral slag
with C/S = 1 can be used.

7. • Desulphurization mechanism: (by powder injection method for desulphurization)
• Desulphurization can be carried out by injecting lime based powder. The
injection rate varies between 2-4 kg/ton of melt. When slag forming materials
are injected into melt, they melt and the molten slag particles begin to rise and
accumulate at the top surface of the melt.
 The desulphurization reaction occurs in two ways:
• During contact between rising molten solid powders and the melt. In this
mechanism of desulphurization it is important that the powder becomes molten
on injection. Residence time of the rising particles in the melt is also important.
Powder melts and the rising gas imparts mixing in the melt. This mechanism is
known as transitory contact.
• Contact between top slag and the melt. As the molten slag particles rise they
accumulate at the top surface of the melt and after a while top slag also takes
part in the desulphurization. In this mechanism slag/metal interface area is
important. Gas injection rate may be suitably selected to produce and entrain
slag droplets into the melt for the faster rate of reaction. Once all the powder is
injected, reaction between top slag and sulphur of the melt governs the final
sulphur content of the steel. This mechanism is known as permanent contact.

8. • Methods for injection of powder:
• The slag forming materials are lighter than steel and deep injection would be
required for the efficiency of the reaction. Powder can be injected either through
cored wire or pneumatic transport along with a stream of Ar gas.
• In both, argon is bubbled through a porous plug fitted at the bottom of the ladle
for speeding up mixing and mass transfer.

9. • It was found that at high gas flow rates there is extensive slag-metal emulsion
formation, resulting in a large slag-metal interfacial area, which speeds up
Sulphur transfer from the metal to slag.
• Additions are also made partly during tapping of the metal from the steel making
furnace in to the ladle. The tapping stream causes violent stirring, and during this
process some slag-metal reaction and desulphurization will occur.
• The addition of calcium metal into the melt led to deep deoxidation, deep
desulphurization. Calcium is a gas at steel making temperatures. At 1600ºC , Pca
= 1.81 atm. This is quite high and is likely to lead to instant , violent vapour
formation. Very little Ca would get chance to react with the melt if it were added
as such.
• Even in Ca-Si alloy also the solubility of Ca is very low 0.025 wt % at 1600ºC.
Therefore Si is expected to dissolve into the melt much faster than Ca. so, shortly
after addition, the liquid Ca-Si alloy would get depleted in Si. Consequently
raising Pca and leading to instant vaporization and loss of Ca.
• The problem was solved by injecting CaSi alloy at a depth of at least 1 to 1.5m
inside the melt so as to avoid vapour formation due to Ferro-static pressure. This
allowed the calcium to react with the oxygen and Sulphur of steel.

10. • Kinetics of desulphurization reaction:
• The overall desulphurization reaction consists of the following kinetic steps:
• Transfer of sulphur dissolved in liquid iron to slag-metal interface.
• Transfer of O2- from the bulk of the slag to the slag-metal interface.
• Chemical reaction at the interface i.e.
[S] + (O2-) = (S2-) + [O]
• Transfer of S2- from the interface into the bulk slag.
• Transfer of [O] from the interface into the bulk metal phase.
• Mixing in slag phase
• Mixing in metal phase.
Deoxidation
• Steel making is a process of selective oxidation of impurities in molten iron.
During this however, the molten steel also dissolves some oxygen. Solubility of
oxygen in solid steel is negligibly small. Therefore during solidification of steel
in ingot or continuous casting, the excess oxygen is rejected by the solidifying
melt. This excess oxygen causes defects such as blow holes and non metallic
inclusions in casting.

11. • Therefore it is necessary to control the oxygen content in molten steel and
bring it down by carrying out deoxidation after primary steel making and
before teeming the molten metal into an ingot or continuous casting mold.
• Source of oxygen in steel:
• Oxygen blowing
• Steel making slag
• Atmospheric oxygen dissolved in steel during teeming
• Oxidizing refractories
• At 1600ºC solubility of oxygen in liquid steel is 0.23 % which decreases to
0.003 % in solid steel during solidification. According to the degree of
deoxidation, carbon steels are subdivided into three groups.
– Killed steels: oxygen is removed completely. Solidification of such steel
does not give gas porosity.
– Semi-killed steel: incompletely deoxidized steels containing some amount
of oxygen which forms CO during solidification.
– Rimming steel: partially or non deoxidized low carbon steels evolving
sufficient CO during solidification. These steels have good surface finish.

12. • Deoxidation can be carried out either by single element such as Si, Mn Al etc, which
have high affinity towards oxygen than iron or by mixture of elements such as Si
+Mn , Ca+ Si+ Al etc.
• Deoxidation by single element is known as simple deoxidation, whereas, deoxidation
by a mixture of element is known as complex deoxidation.
• Deoxidation is also carried out by carbon under vacuum; which is called vacuum
deoxidation. Elements are added in the form of ferro alloys Fe-Si, Fe-Mn or FeSi +
FeMn etc.
• Typically, lump additions of de-oxidizer elements are made to the bath during
tapping. Alternative modes of addition are also used at times for better utilization of
such additives.---- example: Aluminum wire, injected into the bath at high speed,
ensures subsurface melting and dissolution, increasing the efficiency of aluminum
utilization.
• The requirements of a deoxidizer are: high reactivity with dissolved oxygen and
formation of a stable deoxidation product that is easily separable from molten steel.
• Deoxidation product such as SiO2 and Al2O3 are non metallic in nature and if they
remain entrapped in molten steel, then they are referred to as endogenous inclusions.
Whereas, worn-out refractory pieces remaining entrapped in steel on the other hand
form exogenous inclusions.
• During deoxidizer addition, some silica and lime are also added such that an
adequate basic slag (calcium aluminosilicate) is formed. Such a slag helps absorb
nonmetallic inclusions as these float up due to buoyancy.

13. • Ferroalloys , at room temperature, is projected into a ladle containing molten
steel, heat flows from the melt to the cold solid, raising the temperature
progressively to its melting point. Subsequently, the molten alloy dissolves into
steel to take part in various chemical reactions.
• As soon as a cold solid addition such as ferroalloy or Al is made, a layer of steel
freezes around it and forms a solid crust.
• From then on, the mechanism of dissolution would depend on the melting point
of the addition. If it lower than steel, it may become molten, with the crust of
solid steel.
• If the melting point of the addition is higher than that of steel, such as
ferrotungsten, then the crust of steel will remelt, exposing the alloy to the melt
and leading to its dissolution by simultaneous heat and mas transfer.
• Factors that govern the rate of dissolution are density, melting point, thermal
conductivity and size of the additions.

14. • De-Oxidation kinetics:
• Deoxidation process can be visualized interms of several kinetic steps.
– melting and dissolution of the deoxidizing element.
– Dispersion of the dissolved deoxidizer in the melt.
– Chemical reaction between dissolved oxygen and the deoxidizer element
– Heterogeneous nucleation of the new oxide phase
– Growth of the product phase.
• Deoxidation involves the formation of a new phase (i.e. deoxidation product) as
a result of reaction.
x [M] + y [O] = (MxOy)
• New phases form by the processes of nucleation and growth.
• Nucleation refers to formation of a small embryo of the new phase that is
capable of growth. Such an embryo consists of a small number of molecules and
has a dimension on the order of 10Aº.
• Growth of the oxide phase occurs via diffusion and is also reasonably fast due to
high temperature. Since the rate of chemical reaction is generally fast at elevated
temperature, it is reasonable to assume that rate of deoxidation reaction is
appreciable.

15. • These indicate that the deoxidation process, accompanied by heat and mass transfer as
well as nucleation and growth, should be complete and attain equilibrium with in a
few seconds.
• According to the classical theory, the work required to form a spherical nucleus
homogeneously is given by :
W = 4𝜋r𝜎 + 4/3 𝜋 r3 (
∆𝐺
𝛾
)
R = radius of the nucleus
𝜎 = interfacial tension between liquid steel and deoxidation product
∆𝐺 = change in free energy per mole
𝛾 = 𝑚𝑜𝑙𝑎𝑟 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑑𝑒𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡
Removal of deoxidation product:
• The deoxidized products are nonmetallic inclusions, they are lighter than steel,
therefore, tend to float naturally.
• The rising velocity (terminal velocity) for a spherical solid inclusion can be
obtained from the stokes law.
Vt
=
g𝑑2
𝑝∆𝑒
18μ
, ∆𝑒 = (𝜌𝑠 − 𝜌𝑓), 𝜌𝑠 = density of solid
• 𝜌𝑓 = density of fluid, g = acceleration due to gravity, dp= dia of particle, µ =
viscosity of fluid.

16. • In the above expression vt α d2
p , other factors constant. Therefore, particles of
different sizes would move at different speeds. Larger sizes more faster.
• During their movement many of them are likely to collide with one another and
forms one particle and this is the mechanism of growth.
• Fluid flow and turbulence play important role in inclusion agglomeration and
flotation of deoxidation product.
• Vigorous stirring may not be of much help since deoxidation product may be
circulated in the liquid.--- (optimum 0.1Nm3/hr/tonne)
• Degassing
• During steel making gases like oxygen, hydrogen and nitrogen dissolve in steel
in atomic form. Nitrogen finds its way into molten steel during furnace tapping
when large scale interaction takes place between the falling stream of liquid
steel and the ambient.
• Whereas, hydrogen finds its way via moisture present in charge material as well
as in the atmosphere.
• Both nitrogen and hydrogen impair the mechanical properties of steel. These
gases have an extremely low solubility in solid steel. During solidification
excess nitrogen is rejected which may form blow holes.

17. • Excess nitrogen causes embrittlement of heat affected zone of welded steels.
• Whereas, hydrogen is soluble in liquid steel but does not combine with the iron and
its alloying element. During solidification it is released, entrapped forming porosity
or promotes cracking.
• Therefore the term degassing is employed to remove nitrogen and hydrogen from
steel.
• Principle:
• Chemical reactions involved in the removal of dissolved gases such as nitrogen and
hydrogen from molten steel are :
[N] = ½ {N2}
[H] = ½ {H2}
• Sieverts law: It states that, at a constant temperature, the amount of a given gas that
dissolves in a liquid is directly proportional to the partial pressure of that gas in
equilibrium with that liquid.
• If partial pressure of gas decreases, solubility of gas decreases and vice versa.
• If a steel bath is exposed to a very low pressure environment, degassing will be
facilitated leading to a smaller level of dissolved hydrogen in the melt. Similar for
nitrogen removal also.
• This principle is exploited in vacuum degassing, in which the molten steel bath is
subjected to an extremely low pressure environment ( 1 m bar or lower)

18. • Degassing reactions take place at the melt-vacuum boundary. It is essential that a large
area of exposure of the melt to the vacuum is maintained throughout the process.
• Rate of degassing depends on the rate of transport of dissolved gases from the bulk to
the melt- vacuum interface. Ar injection helps achieve adequate bath stirring, facilitating
transport of dissolved species in the melt phase.
• Criteria for efficient degassing:
– A low pressure environment
– large melt-vacuum contact area
– Adequate stirring of the bath are essential to achieve reasonable degassing of the
steel melt.
• Several types of degassing techniques have been employed to remove dissolved gases
from steel. Of these, tank degassing and circulation degassing are mentioned below.
• Tank degassing Circulation degassing
Ladle degassing stream degassing DH degassing RH degassing
Ladle-mold Ladle-Ladle

19. • Ladle degassing:
• Ladle degassing is typically employed when the heat size is relatively small (100 tonnes)
• In this, a ladle containing molten steel is placed inside a separate vessel equipped with a
removable lid, suction and alloying addition chute.
• Once the chamber is covered and sealed, suction is applied and continued till a stable
low pressure regime (1 m bar) is established. This period is called the pressurizing
period.
• The ladle containing molten steel is held in the evacuated chamber at such low pressure
for a period of time called holding period essential to achieve the desired level of
dissolved hydrogen/nitrogen in the melt.
• During vacuum treatment, argon purging from the bottom is continuously used.

20. • Stirring the bath enhances rate of gas removed. Vigorous removal of gases causes
metal splashing too. Therefore, ladle is not filled completely and about 25 % of
its height is kept as free board to accommodate the splashed metal droplets.
Pressure is maintained in between 1 mm Hg to 10 mm Hg for effective
degassing.
• To quantify the efficiency of tank degassing process, a parameter, commonly
termed as circulation number is used. This is estimated as a ratio of processing to
mixing times.
• The relation between processing time, mixing time and circulation number is
therefore:
Circulation number (CN) =
Process Time
Mixing Time
• The final content of gas in steel depends on degree of vacuum and time of
treatment. Hydrogen is generally reduced to below 2 ppm.

21. • Stream degassing:
• In stream degassing technology, molten steel is teemed into another vessel which is under
vacuum. Sudden exposure of molten stream in vacuum leads to very rapid degassing.
• The major amount of degassing occurs during the fall of molten stream. Height of the
pouring stream is an important design parameter.
• Ladle to mold degassing:
• Preheated mold with hot top is placed in vacuum chamber. Above the chamber a tundish is
placed.
• Steel tapped in the ladle at superheat equivalent to 30ºC is placed above the tundish. Steel
is bottom poured in the tundish.

22. • Ladle to ladle degassing:
• In this process, a ladle is placed in a vacuum chamber. Ladle containing molten
steel from BOF or EAF is placed on top of the vacuum chamber and the gap is
vacuum sealed. Stream is allowed to fall in the ladle where molten steel is
degassed.

23. • Circulation degassing:
• In circulation degassing, the liquid steel in a ladle is formed into an evacuated chamber
where it is exposed to low pressure and returned back into the ladle. The steel is
recirculated through the low pressure chamber 40-50 times to achieve the desired level
of degassing.
• DH (Dortmand-Hoerder):
• Degasser with one snorkel and ladle moving up and down to force the liquid metal in
and out of the vacuum chamber.
• The DH chamber is equipped with heating facility, alloying addition.
• DH vessel is preheated (900-1500ºC) and lowered in the ladle so that snorkel tip dips
below the molten steel surface.
• The chamber is moved up and down for 50-60 times with a cycle time of 20 sec, so that
steel enters the evacuated chamber and undergoes degassing.

24. • RH (Ruhrstahl-Heraeus) degasser with two snorkels and stationary ladle. Argon
is injected into one snorkel to force liquid steel into the vacuum chamber; steel
flows back into the ladle through the other snorkel.
• By reducing the system pressure through vacuum and by injecting inert gas into
the up-leg snorkel the melt raises into the evacuated chamber.
• The liquid thus degassed, flow back into the ladle through the down leg. This
degassed steel is slightly cooler than steel in the ladle. Buoyancy forces created
by density difference (density of cooler liquid steel > hot metal) stirs the bath.
Also, the inert gas injection in the up-leg help bring fresh steel (rich in dissolved
gas) inside the vacuum chamber.

25. • For small capacity ladles (50-100 tonnes size) the temperature can drop at a rate
of 2-2.5ºC/min, while for bigger size ladles, the drop is smaller about 0.6-
0.7ºC/min.
• Rate of circulation of molten steel in cylindrical chamber controls the degassing.
Circulation rate depends upon amount of inert gas and the degree of vacuum.
• The dia of the up leg is greater than that of the down leg; this gives a greater
depth of metal in the vessel and increased circulation rate.
• The speed of degassing increases with the increased rate of circulation (R) of
liquid steel through the vacuum chamber. Typically “R” ranges from 10 t/min to
100t/min. circulation velocity increases with an increase Ar gas flow rate.
• The circulation rate (R) can be determined by :
R = 7.42 x 103 Q1/3 d 1/3 {ln (
𝑃1
𝑃2
)}
R = circulation rate
Q = Ar injection rate Nm3/min.
P1 = pressure at the base of down leg
P2 = pressure in vacuum chamber
D = internal dia of leg (m)