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.

• 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

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

maintainedFor 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.

• 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.

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.

• 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.

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.

• 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.

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 gasCarbon 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

• converter designs for degassing:Argon oxygen decarburizationOxygen top and bottom blowing converter, which is an extension of AOD technologyVacuum 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.

• 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.

• 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.

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.

• 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.

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

• 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).

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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%.

• 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.

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.

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.