This document summarizes steel melt processing and refinement techniques. It discusses primary steelmaking processes like electric arc furnaces and basic oxygen furnaces. It also covers secondary refining using various furnaces and vessels. Some key secondary processes mentioned are argon oxygen decarburization (AOD), vacuum induction melting, and ladle metallurgy techniques. The document provides detailed information on the equipment, processes, reactions, and purposes involved in steel melt processing and refinement.

2. Steel Melt Processing
• MELT PROCESSING of steels classified - primary steelmaking or secondary
steel making.
• The two most important primary steel making processes are the electric arc
furnace (EAF) process and the basic oxygen furnace (BOF) process
• EAF process has been more used than the BOF as the primary steelmaking
furnace
• Some other processes- open-hearth furnace still practiced in a few
countries to produce special steels.
• Primary melt refinement also includes the uses of converters, which are
furnaces in which oxygen is blown through a bath of molten metal, oxidizing
the impurities and maintaining the temperature through the heat produced
by the oxidation reaction.
• A converter in steel refining is the argon oxygen decarburization (AOD)
vessel.
• In most modern steel operations, the steel is melted and only roughly
refined in one furnace. Then, it goes to a secondary operation where it is
further processed to yield the final desired chemistry.
• This secondary operation can be carried out while the steel is in the
furnace, ladle, the degasser unit, or, in the case of stainless steel, the liquid
metal can be poured into an AOD vessel.
• After secondary refining, the molten metal is transported to a tundish or a
bottom pure ingot making facility for teeming.

3. • Electric arc and induction melting are common methods of melting steels.
• After furnace melting, the molten steel may be refined in a converter
(outside the melting unit) or in a ladle.
• steel melt refinements are classified as treatments by either converter
metallurgy or ladle metallurgy.
• Converter metallurgy includes melt refinement in AOD vessels and vacuum
oxygen decarburization in a converter vessel.
• Ladle metallurgy refers to melt treatments in a ladle refining station after
melting in the furnace/converter unit.
• Ladle metallurgy is used for deoxidation, decarburization, and adjustment
of chemical composition, including gases.
• Methods of ladle metallurgy include:
• Vacuum induction degassing
• Vacuum oxygen decarburization
• Vacuum ladle degassing
• Coreless induction furnaces are also used for melting carbon, low-alloy
steels, stainless steels, high-alloy steels, or nickel-base heat-resistant alloys

4. Direct Arc Melting
• The direct arc furnace consists essentially of a metal shell lined with
refractories
• This lining forms a melting chamber, the hearth of which is bowl shaped.
• Three carbon or graphite electrodes carry the current into the furnace.
• Steel, either solid or molten, is the common conductor for the current
flowing between the electrodes
• The metal is melted by arcs from the electrodes to the metal charge—both
by direct impingement of the arcs and by radiation from the roof and walls
• The electrodes are controlled automatically so that an arc of proper height
can be maintained
For basic electric furnace melting,
• the furnace lining is a basic refractory such as magnesite or dolomite
• During the melting period, small quantities of lime are occasionally added
to form a protective slag over the molten metal
• Iron ore is added to the bath just as melting is complete.
• The slag is then highly oxidizing and in the correct condition to take up
phosphorus from the metal.
• Shortly after all of the steel has melted, this first slag is taken off (if a two-
slag process is to be used), and a new slag composed of lime, fluorspar, and
sometimes a little sand is added.

5. • As soon as the second slag is melted, the current is reduced, and pulverized
coke, carbon, or ferrosilicon, or a combination of these, is spread at
intervals over the surface of the bath.
• This period of furnace operation is known as the refining period, and its
purposes are to reduce the oxides of iron and manganese in the slag and to
form a calcium carbide slag, which is essential to the removal of sulfur from
the metal
• The refining slag has approximately the following composition:
• Adjustments are made in the carbon content of the bath by the addition of
a low-phosphorus pig iron
• After the proper bath temperature is obtained, ferromanganese and
ferrosilicon are added and the furnace is tapped.
• Aluminum is generally added in the ladle as a final deoxidizer.

6. Induction Melting
• The high-frequency induction furnace is essentially an air transformer in which the
primary is a coil of water-cooled copper tubing and the secondary is the metal
charge.
• The circular winding of copper tubing is placed inside the shell
• Firebrick is placed on the bottom of the shell, and the space between that and the
coil is rammed with grain refractory.
• The furnace chamber may be a refractory crucible or a rammed and sintered lining.
• Basic linings are often preferred; a rammed lining of magnesia grain or a clay-
bonded magnesia crucible can be used.
• The process consists of charging the furnace with steel scrap and then passing a
high-frequency current through the primary coil, thus inducing a much heavier
secondary current in the charge, which heats furnace to the desired temperature.
• As soon as a pool of liquid metal is formed, a pronounced stirring action takes place
in the molten metal, which helps to accelerate melting.
• In this process, melting is rapid, and there is only a slight loss of the easily oxidized
elements
• If a capacity melt is required, steel scrap is continually added during the melting-
down period
• Once melting is complete, the desired superheat temperature is obtained, and the
metal is deoxidized and tapped.

7. • Because only 10 to 15 min elapse from the time the charge is melted down
until the heat is tapped, there is not sufficient time for chemical analysis.
• Composition can be closely controlled in this manner
• In most induction furnaces, no attempt is made to melt under a slag cover,
because the stirring action of the bath makes it difficult to maintain a slag
blanket on the metal.
• However, slag is not required, because oxidation is slight.
• The induction furnace is especially valuable because of its flexibility in
operation, particularly in the production of small lots of alloy steel castings.
• Induction melting is also well adapted to the melting of low-carbon steels,
because no carbon is picked up from electrodes, as may occur in an EAF.
Coreless Induction Furnaces
• Both channel-and coreless-type induction electric furnaces are used in the
steel industry.
• The coreless induction furnace melts by passing an alternating current
through a water-cooled coil that surrounds the refractory linings of the
furnace
• The current generates a magnetic field that induces currents in the charge
and molten metal
• Coreless induction furnaces provide operational flexibility when processing
different alloys, because the furnace can be easily emptied and recharged.

8. Vacuum Induction Melting (VIM).
• In the VIM remelting process, the charge material has basically the
same chemical analysis as the final melt
• No major refining or metallurgical work is performed other than
lowering the residuals and inclusions
• When remelting alloy steel, certain elements that readily vaporize
or oxidize must be added under controlled conditions.
• As a rule of thumb, alloying elements that oxidize should be added
as late as possible, and difficult-to-dissolve elements should be
added early and during high stirring periods.
• Oxygen, hydrogen, and, to some extent, nitrogen can be removed
during vacuum melting.
• Carbon can also be lowered to produce some types of stainless
steels
• Pressures as low as 5 mm of mercury are used in these furnaces
• The control of pressure and composition of gas over the melt
makes it possible to deoxidize the melt with carbon or hydrogen
• because they produce gaseous deoxidation products, preventing
the formation of nonmetallic inclusions.

9. • Use of low pressures also eliminates nitrogen pickup by steel.
• The volatility of certain alloying elements such as chromium, aluminum, and
manganese can result in high losses that can be minimized by replacing the
vacuum with an inert gas atmosphere over the melt during additions.
• Vacuum induction melting is often employed as a remelting operation for
very pure steels
• It is also used as a first stage in conjunction with vacuum arc remelting
(VAR) furnaces and ESR furnaces for duplex (VIM-VAR) or triplex (VIM-ESR-
VAR) melting
• Some modified types of VIM equipment use either a vacuum induction
melter under low pressure or as a stream degasser
• The system can be used for cold charging of steel in the induction furnace
and melting, refining, and teeming of steel under low pressure
• The steel could also be hot charged under low pressure in the vacuum
furnace during which stream degassing occurs, reheated to compensate for
the losses during degassing, adjusted for alloy additions to meet the
chemical composition specifications, and teemed, all under low pressure
• Superheating of steel is not required if hot charging is performed.
• The steel can be partly deoxidized before charging into the vacuum
induction melter.
• This equipment is mostly used as a vacuum induction unit to melt a cold
charge.

10. Converter Metallurgy
• Converters are used in secondary metallurgy to refine melts outside the
primary metallurgical melting unit
• Various converters are available that apply the bottom blowing of
oxygen/inert gas mixtures.
• These bottom-blowing converters use different types, numbers, and
arrangements of injection nozzles and the following gases:
Argon as a cooling inert gas with a purity ranging from 85 to 99.99%
Nitrogen as a cooling inert gas with a purity of 99 to 99.9%
Argon-oxygen and nitrogen-oxygen mixtures in the case of the AOD
converter
Dry air as a diluted reactive gas
Oxygen as a reactive gas with a purity of 80 to 99.5%
Steam as a cooling reactive gas
Carbon dioxide as a diluted reactive gas
• Bubbling methods allow quick and uniform mixing of alloys, temperature
homogenization, adjustment of chemical composition, and partial removal
of nonmetallic inclusions
• These functions are accomplished by lowering a refractory- protected lance
close to the ladle bottom or by blowing through porous refractory plugs at
the bottom or sidewall of the ladle or vessel

11. • converter designs for degassing:
Argon oxygen decarburization
Oxygen top and bottom blowing converter, which is an extension of AOD
technology
Vacuum oxygen decarburization converter
• In addition to AOD in a converter unit, argon bubbling processes also include
capped argon bubbling and composition adjustment by sealed argon bubbling
• These bubbling methods are described with other ladle injection methods in the
section “Ladle Metallurgy” of this article.
Argon Oxygen Decarburization
• These units look very much like Bessemer converters with tuyeres in the lower
sidewalls for the injection of argon or nitrogen and oxygen
• Up to approximately 20% cold charge can be added to an AOD unit; however, the
cold charge is usually less than 20% and consists of solid ferroalloys
• The continuous injection of gases causes a violent stirring action and intimate
mixing of slag and metal, which can lower sulfur values to below 0.005%.
• The gas contents approach or may be even lower than those of vacuum induction
melted steel
• The dilution of oxygen with inert gas, argon, or nitrogen causes the carbon-oxygen
reaction to go to completion in favor of the oxidation reaction of iron and the
oxidizable elements, notably chromium in stainless steel.
• Therefore, superior chromium recoveries from less expensive high-carbon
ferrochromium are obtained compared to those of electric arc melting practices.

13. • Argon oxygen decarburization is a secondary refining process that was originally
developed to reduce material and operating costs and to increase the productivity
in production of chromium- bearing stainless steels
• A premelt is prepared in an electric arc furnace by charging high-carbon
ferrochrome, ferrosilicon, stainless steel scrap, burned lime, and fluorspar and
melting the charge to the desired temperature.
• The heat is then tapped, deslagged, weighed, and transferred into an AOD vessel,
which consists of a refractory-lined steel shell mounted on a tiltable trunnion ring,
process gases (oxygen, argon, and nitrogen) are injected through submerged, side
mounted tuyeres
• The primary aspect of the AOD process is the shift in the decarburization
thermodynamics that is afforded by blowing with mixtures of oxygen and inert gas
as opposed to pure oxygen.
• The heat is decarburized in the AOD vessel to 0.03% C in stages, during which the
inert gas-to-oxygen ratio of the blown gas increases from 1-to-3 to 3-to-1
• During the blowing, fluxes are added to the furnace and a slag is prepared
• Following the decarburization blow, ferrosilicon is added, and the heat is argon
stirred for a short period
• The furnace is then turned down, a chemistry sample is taken, and the heat is
deslagged
• Additional alloying elements are added if adjustments are necessary, and the heat
is tapped into a ladle and poured into ingot molds or a continuous casting machine.

14. • With the AOD process, steels with low hydrogen (<2 ppm) and nitrogen
(<0.005%) can be produced with complete recovery of chromium.
• In addition to its economic merits, AOD offers improved metal cleanliness,
which is measured by low unwanted residual-element contents and gas
contents
• The AOD process is duplexed, with molten metal supplied from a separate
melting source to the AOD refining unit (vessel)
• The source of the molten metal is usually an EAF or a coreless induction
furnace.
• Foundries and integrated steel mills use vessels ranging in nominal capacity
from 1 to 175 tons
• Although the process was initially targeted for stainless steel production,
AOD is used in refining a wide range of alloys, including:
Stainless steels, Tool steels, Silicon (electrical) steels, Carbon steels,
low-alloy steels, and high strength low-alloy steels, High-temperature alloys
and superalloys
• Argon oxygen decarburization units are used in the production of high-alloy
castings, particularly of grades that are prone to defects due to high gas
contents
• Carbon and low- alloy steels for castings with heavy wall sections may be
subject to hydrogen embrittlement and are also processed in these units
with good results.

15. Fundamentals
• In the AOD process, oxygen, argon, and nitrogen are injected into a molten metal
bath through submerged, side-mounted tuyeres
• The primary aspect of the AOD process is the shift in the decarburization
thermodynamics that is afforded by blowing with mixtures of oxygen and inert gas
as opposed to pure oxygen.
• To understand the AOD process, it is necessary to examine the thermodynamics
governing the reactions that occur in the refining of stainless steel, that is, the
relationship among carbon, chromium, chromium oxide (Cr3O4), and carbon
monoxide (CO)
• The overall reaction in the decarburization of chromium-containing steel can be
written as:
• 1/4 Cr3O4 + C = 3/4 Cr + CO(g)
• At a given temperature, there is a fixed, limited amount of chromium that can exist
in the molten bath that is in equilibrium with carbon.
• By examining Eq , one can see that by reducing the partial pressure of CO, the
quantity of chromium that can exist in the molten bath in equilibrium with carbon
increases
• The partial pressure of CO can be reduced by injecting mixtures of oxygen and inert
gas during the decarburization of stainless steel
• the relationship among carbon, chromium, and temperature for a partial pressure
of CO equal to 1 and 0.10 atm
• The data shown in Fig. 4 indicate that diluting the partial pressure of CO allows
lower carbon levels to be obtained at higher chromium contents with lower
temperatures.

17. • In refining stainless steel, it is generally necessary to decarburize
the molten bath to less than 0.05% C.
• Chromium is quite susceptible to oxidation; therefore, prior to the
introduction of the AOD process, decarburization was
accomplished by withholding most of the chromium until the bath
had been decarburized by oxygen lancing
• After the bath was fully decarburized, low-carbon ferrochromium
and other low-carbon ferroalloys were added to the melt to meet
chemical specifications.
• Dilution of the partial pressure of CO allows the removal of carbon
to low levels without excessive chromium oxidation
• This practice enables the use of high-carbon ferroalloys in the
charge mix, avoiding the substantially more expensive low-carbon
ferroalloys
• Figure 5 compares the refining steps in the two processes. are
equipped with a computer to assist in process control by
calculating the required amount of oxygen as well as alloying
additions
• Some installations have computer control systems capable of
sending set points and flow rates to the gas control systems.

18. AOD Processing of Stainless Steels.
• Charge materials (scrap and ferroalloys) arevmelted in the melting furnace
• The charge is usually melted with the chromium, nickel, and manganese
concentrations at midrange specifications.
• The carbon content at meltdown can vary from 0.50 to 3.0%, depending on
the scrap content of the charge
• Once the charge is melted down, the heat is tapped, and the slag is
removed and weighed prior to charging the AOD vessel.
• In the refining of stainless steel grades, oxygen and inert gas are injected
into the bath in a stepwise manner
• The ratio of oxygen to inert gas injected decreases (3:1, 1:1, 1:3) as the
carbon level decreases
• Once the aim carbon level is obtained, a reduction mix (silicon, aluminum,
and lime) is added
• If extra low sulfur levels are desired, a second desulfurization can be added.
• Both of these steps are followed by an argon stir
• After reduction, a complete chemistry sample is usually taken and trim
additions made following analysis.
• Figure 6 illustrates the relationships among carbon, chromium,
temperature, and the various processing steps for refining a typical type
304 stainless steel using AOD

20. • Another critical aspect of AOD refining is the ability
to predict when to change from nitrogen to argon
to obtain the aim nitrogen specification.
• The point during refining when the oxygen-to-inert-
gas ratio is lowered is based on carbon content and
temperature
• The ratios and carbon switch points are designed to
provide optimal carbon removal efficiency without
exceeding a bath temperature of 1730 C (3150 F).

21. AOD Processing of Carbon and Low-Alloy Steels
• The refining of carbon and low-alloy steels involves a two-step practice: a
carbon removal step, followed by a reduction/heating step
• The lower alloy content of these steels eliminates the need for injecting less
than a 3:1 ratio of oxygen to inert gas
• Once the aim carbon level is obtained, carbon steels are processed similarly
to stainless steels
• Figure 7 illustrates the carbon content and temperature relationship for the
AOD refining of carbon and low-alloy steels
• Because the alloy content of these grades of steel is substantially lower
than that of stainless steel and because the final carbon levels are generally
higher, there is no thermodynamic or practical reason for using an
oxygen/inert gas ratio of less than 3:1.
• Oxidation measurements indicate that all of the oxygen reacts with the bath
and that none leaves the vessel unreacted
• By monitoring and recording the oxygen consumption during refining, very
close control of end-point carbon is achieved
• Because the oxygen and inert gases are introduced below the bath and at
sonic velocities,
• there is excellent bath mixing and intimate slag/metal contact
• As a result, the reaction kinetics of all chemical processes that take place
within the vessel are greatly improved.

22. Composition and Property Improvement with AOD
• It has been well documented that AOD-refined steels exhibit significantly
improved ductility and toughness, along with impact energy increases of
over 50%
• These improved properties result from a decrease in the number and size of
inclusions
• The capability to produce low-gas-content steel with exceptional micro
cleanliness, along with alloy savings, is the primary factor for the growth of
the AOD process for refining stainless, carbon, and low-alloy steels.
Decarburization
• In both stainless and low alloy steels, the dilution of oxygen with inert gas
results in increased carbon removal efficiencies without excessive metallic
oxidation.
• In stainless grades, carbon levels of 0.01% are readily obtained.
AOD Chemistry Control
• The injection of a known quantity of oxygen with a predetermined bath
weight enables the steelmaker to obtain very tight chemical specifications.
Desulfurization with AOD
• Sulfur levels of 0.01% or less are routinely achieved, and levels less than
0.005% can be achieved with single slag practice
• When extra low sulfur levels are required, a separate slag treatment for
3min is sufficient.

23. Slag Reduction
• During oxygen injection for carbon removal, there is some metallic
oxidation.
• Efficient slag reduction with stoichiometric amounts of silicon or aluminum
permits overall recoveries of 97 to 100% for most metallic elements.
• Chromium recovery averages approximately 97.5%, and the recovery of
nickel and molybdenum are approximately 100%.
Nitrogen Control
• Degassing in AOD is achieved by inert gas sparying
• Each argon and CO bubble leaving the bath removes a small amount of
dissolved nitrogen and hydrogen.
• Final nitrogen content can be accurately controlled by substituting nitrogen
for argon during refining
• Nitrogen levels as low as 25 to 30 ppm can be obtained in carbon and low-
alloy steels, and 100 to 150 ppm N can be obtained in stainless steels
• The ability to obtain aim nitrogen levels substantially reduces the need to
use nitrided ferroalloys for alloy specification, and this also minimizes the
use of argon
• Hydrogen levels as low as 1.5 ppm can be obtained

24. Oxygen top and bottom blowing (OTB)
• also referred to as combined blowing, is an extension of AOD technology for
refining steel.
• During OTB, oxygen is injected into the molten steel through a top lance during
carbon removal, and inert gases, such as argon, nitrogen, or carbon dioxide, are
injected with oxygen through submerged tuyeres or alternate forms of gas injectors
such as canned bricks, porous bricks, or thin pipes set in the refractory brick
• The top-blown oxygen can react with the bath, reducing refining times, or with CO
• The combustion of CO above the surface of the bath increases the thermal
efficiency of the refining process and decreases the quantity of silicon and
aluminum required for reduction of metallic oxides.
• If the top-blown oxygen system is designed so that more than 65% of the oxygen
reacts with the bath, the system is referred to as hard blown; if less than 65% of
the oxygen reacts with the bath, it is referred to as soft blown.
• As a general guideline, a top-blown oxygen system installed in AOD vessels with a
nominal capacity greater than 50 tons will be hard blown; smaller vessels will be
soft blown.
• In hard-blown top oxygen systems, the flow rate of oxygen through the top lance
will be between 50 and 150% of the oxygen injected through tuyeres or injectors.
• In soft-blown systems, the top oxygen flow rate is between 50 and 100% of the
oxygen flow through the tuyeres or injectors.

25. Vacuum Oxygen Decarburization (VODC)
• it can be carried out in a converter vessel (VODC) or in the ladle (VOD).
• The parameters that favor the choice of ladles as reaction vessels include
the following:
• Tapping, treatment, and teeming are done in the same reaction vessel.
• Thus, there are no temperature losses due to any final transfer of the melt,
and the high level of cleanliness achieved during the treatment can be
preserved up to teeming.
• The use of electric power as an inexpensive energy source permits the
highest flexibility in the melt shop
• The ladle unit can act as a time buffer between the melting unit and the
casting stand.
• The primary reasons for selecting the converter as the VOD reaction vessel
are as follows:
• Initial carbon contents are as high as possible, together with high oxygen
blow rates.
• Ease of deslagging and strong stirring action
• Shop restrictions such as a limited number of ladles, prohibited use of slide
gates, and restricted crane capacity in the casting bay make converter
technology more attractive.

26. Equipment.
• Vacuum oxygen decarburization converters are similar to AOD
and OTB converters in terms of design and the tilting device
used
• Bottom blowing is, however, restricted to the introduction of
small amounts of inert gas through simple pipes, thus
avoiding the special erosion-resistant refractory material used
around tuyeres
• Flue gas handling is easier and is incorporated into the
vacuum system
• In terms of vessel design, the conical converter top is closed
by a vacuum hood with an oxygen lance feed through and
vacuum addition lock .
• Because the VODC system is closed and no air enters the
vessel, permanent control of the decarburization rate and the
carbon level in the bath can be maintained and monitored
with a flue gas analyzing device
• Pollution control for carbon monoxide (CO) and dust is also
incorporated into the system.

27. Ladle Metallurgy
• Ladle metallurgy follows furnace melting or refining in a converter
• A primary reactor is emptied into a tapping ladle, and then the steel can be
transferred into other ladles, furnaces, or vessels for treatment
• Alternatively, the steel can be treated in the tapping ladle before being
poured into molds.
• Ladle metallurgy is used for deoxidation, decarburization, and adjustment
of chemical composition, including gases
• Refining times in the furnace can often be shortened and production rates
increased with an efficient secondary steel making practice
• Ladle metallurgy also allows the control of teeming temperature, especially
required for continuous casting.
• Steels with as low as 0.002 wt% S can be produced free of oxide or sulfide
inclusions
• Inclusion shape control is also performed in ladles to improve mechanical
properties.
• There are various types of ladle treatment.
• These methods can be categorized under four major groups : stirring,
injection, vacuum, and heating processes

28. Vacuum treatments can be broadly classified into vacuum
stream degassing, ladle degassing, and recirculation degassing
• Vacuum processes are designed to reduce the partial pressure
of hydrogen, nitrogen, and carbonmonoxide gas in the
ambient atmosphere to enhance the kinetics of degassing,
decarburization, and deoxidation.
• Alloy additions can also be made during the vacuum
treatment
• Alloy additions susceptible to oxidation, for example, iron-
niobium, are added after deoxidation
• The recovery of expensive alloys is improved by ensuring
complete dissolution.
• Injection treatment in ladles with inert gas, inert gas/oxygen
mixtures, or pure oxygen is practiced
• Injection of powdered agents of CaSi, CaC2, CaAl, and
magnesium with an inert gas carrier, or of submerged cored
wire with a prior synthetic slag treatment, allows for
simultaneous deoxidation, desulfurization, and sulfide shape
control.

29. Metallurgical Objectives
• The fundamental objective of melt processing is the removal or addition of specific
elements to achieve desired steel properties.
• Residual solid as well as gaseous elements in steel, such as sulfur, phosphorus,
oxygen, nitrogen, and hydrogen, are present due to thermodynamic and kinetic
limitations in the primary and secondary steelmaking
• Other elements, such as carbon, silicon, and manganese, can be controlled
(lowered or raised) to the desired level
• Other alloy additions, such as chromium, nickel, titanium, copper, and so on, may
be present as residuals from the scrap used in steelmaking or as intentional
additions to manufacture special alloyed steels.
Degassing. Liquid steel absorbs gases, which can cause embrittlement, voids,
inclusions, and so on in the steel when solidified
• Failure to control oxygen, hydrogen, and nitrogen contents may result in porosity or
a serious decrease in ductility or both
• Gas content is largely adjusted during the oxygen boil
• After the cold charge is melted and the bath is in the temperature range of 1510 to
1540 C ,oxygen is introduced into the molten metal, usually by means of a piping
arrangement
• The oxygen combines with the dissolved carbon in the steel to form bubbles of
carbon monoxide
• As the bubbles form, dissolved hydrogen and nitrogen are caught up in the bubbles
in much the same way that dissolved oxygen finds its way into bubbles of boiling
water.
• Thus, the bubbles of gas contaminants are boiled out.

30. Oxygen is the principal refining agent in steelmaking and plays a role in
determining the final composition and properties of steel and influences
the consumption of deoxidizers.
• If the oxygen content of molten steel is sufficiently high during vacuum
degassing, the oxygen will react with some of the carbon to produce carbon
monoxide.
• The evolved carbon monoxide is removed by the created vacuum, known as
vacuum-carbon deoxidation
• In undeoxidized steel, the carbon and oxygen contents approach
equilibrium at a given temperature and pressure according to:
• As the pressure is lowered by vacuum treatments, more and more carbon
reacts with oxygen to establish the new carbon monoxide partial pressure,
thus making the oxygen unavailable for inclusion formation with the added
deoxidizers.
• Strong deoxidizers, such as aluminum, titanium, and silicon, react with
oxygen with greater affinity than carbon at atmospheric pressure, so carbon
cannot react with oxygen when vacuum degassed
• If not floated properly, oxides of deoxidants can result in inclusions when
solidified.
• Carbon is a stronger deoxidizer than aluminum, titanium, or silicon below a
pressure of 0.01 atm.

31. Hydrogen causes bleeding ingots, embrittlement, low ductility, thermal flaking, and
blowholes.
• Hydrogen is usually picked up from the moisture in the charge and the
environment and can be lowered by an effective vacuum treatment,
• Hydrogen removal during vacuum degassing is affected by factors such as surface
area exposed to vacuum, degassing pressure, steel composition, extent of prior
deoxidation, and hydrogen pickup from alloy additions, slags, and refractories.
• The normal hydrogen content in acid-melted steel is in the range of 2 to 4ppm.
• Austenitizing and tempering treatments serve to lower the hydrogen content
further in sections of average thickness
• However, these treatments may not suffice when very heavy sections are involved
• The alternative is to degas the molten steel under vacuum, particularly when
pouring heavy castings that will be subjected to severe dynamic loading.
Nitrogen is particularly harmful for low-carbon steels intended for drawing
applications and should be lowered as much as possible.
• Its removal by argon flushing or vacuum degassing is limited due to the tendency of
nitrogen to form stable nitrides
• Primary control of nitrogen is attempted during steelmaking practices.
• The lowest nitrogen contents are obtained with either acid or basic open-hearth
practice
• Nitrogen contents in electric arc melted steels range from 3 to 10 ppm.

32. Deoxidation and Alloying.
• Steels are required to meet certain temperature and chemical requirements
for proper ladle metallurgy
• Toward the end of tapping, some amount of furnace slag is carried over into
the ladle.
• Deoxidation and alloying additions can be made during tapping.
• However, if ultra low carbon steel grades are to be produced, deoxidation is
usually not performed at the time of tapping, since dissolved oxygen is later
required to react with carbon.
• The primary role of deoxidation is to prevent pinholes due to carbon
monoxide formation during solidification
• Oxygen content should be less than 100ppm to prevent porosity
• Silicon and manganese are mild deoxidizers; they are added to stop the
carbon boil and to adjust the chemistry.
• Manganese (>0.6%) and silicon (0.3 to 0.8%) additions are limited by other
alloy effects and are normally inadequate alone to prevent pinholes.
• Aluminum is the most common supplemental deoxidizer used to prevent
pinholes
• As little as 0.01% Al will prevent pinholes.
• Aluminum is normally added at the tap—approximately 1 kg/Mg with a
recovery of 30 to 50% for a final content of approximately 0.03 to 0.05%.

33. • This is normally supplemented at the pour ladle with additional
deoxidation, which could be more aluminum or calcium, barium, silicon,
manganese, titanium, or zirconium.
• More than the minimum amount of deoxidizer required for preventing
porosity is needed to maintain sulfide inclusion shape control
• but excessive amounts can cause intergranular failures or dirty metal
• The second addition at the pour ladle is normally 1 to 3 kg/Mg
• The commonly used elements for deoxidation are (in order of decreasing
power) zirconium, aluminum, titanium, silicon, carbon, and manganese.
• The primary role of aluminum as a deoxidant is to react with the dissolved
oxygen and float up as alumina.
• However, some of the finer alumina particles are retained in the steel as
inclusions.
• Making low-inclusion steels requires further treatment to float the fine
oxide particles.
• Slag carry-over is minimized to control the hydrogen and nitrogen pickup by
the steel as well as the phosphorus reversal due to reduction of P2O5 by
deoxidizing agents.
• Several types of devices are available as slag stoppers.

34. Desulfurization. Ladle processes allow for desulfurization by judicious selection
of an agent
• Oxygen is a deterrent in sulfur removal and therefore should be pre-
deoxidized to the lowest practicable oxygen content, preferably with
aluminum.
• Simultaneous deoxidation and desulfurization is also commonly performed.
• For specialty steels, the ladle furnace is used to desulfurize the melt after
vacuum degassing.
• Efficiency of desulfurization is lowered by the carry-over of oxidizing
furnace slags, since the oxides are unstable during the sulfur removal
treatment
• In addition, FeO lowers the desulfurization efficiency
• Similarly, oxides in the ladle lining can affect the efficacy of sulfur removal.
• Because desulfurization involves slag metal reactions, intimate mixing of
steel and desulfurizing agents is a prerequisite
• in addition to the previous requirements, the initial and final levels of sulfur
are critical according to the law of mass action
• Thus, steels with less than 0.006% S are difficult to make.

35. Decarburization.
• It is difficult to produce steel by conventional steel making with <0.03% C.
• Carbon can be lowered to 0.01% from 0.04 to 0.06% levels by simply exposing it to a
vacuum,
• Particular attention must be paid to the amount of carbon in alloying additions made after
decarburization of steel.
Sulfur and Phosphorus Control.
• It has been emphasized that the primary removal of sulfur is effected during iron making in
the blast furnace, where reducing conditions prevail
• However, in steelmaking, direct oxidation at the gas-metal interface in the jet impact zone
causes 15 to 25% of dissolved sulfur to directly oxidize into the gaseous phase due to the
turbulent and oxidizing conditions existing in the impact zone
Argon Stirring
• Clean steel melt processing can be carried out at atmospheric pressure without any
supplemental reheating by various methods that include ESR, ladle injection methods, and
various argon bubbling processes such as AOD, capped argon bubbling (CAB), and
composition adjustment by sealed argon bubbling (CAS).
• The CAB and CAS methods for argon bubbling were developed to make controlled additions
to the ladle as well as to improve the steel refining capability.
• Stirring treatment with argon gas, also known as argon rinsing, can be done to homogenize
the bath and to promote decarburization by lowering the carbon monoxide partial pressure
• Bubbles agitate the bath and help in alloy dissolution, deoxidation, quick and uniform mixing
of alloys, temperature homogenization, adjustment of chemical composition, and partial
removal of nonmetallic inclusions.
• These functions are accomplished by either blowing argon through a refractory-protected
lance lowered to within 300 mm of the ladle bottom or by blowing argon through porous
refractory plugs inserted in the bottom or sidewall of the ladle.