Basic Oxygen Furnace (BOF)

Basic Oxygen Furnace (BOF) is a steel making furnace, in which molten pig iron and steel scrap convert into steel due to oxidizing action of oxygen blown into the melt under a basic slag.
The Basic Oxygen Process (Basic Oxygen Furnace, Basic Oxygen Steelmaking, Basic Oxygen Converter) is the most powerful and effective steel making method. About 67% of the crude steel in the world is made in the Basic Oxygen Furnaces (BOF).

• Furnace structure
• Refractory lining
• Chemical and physical processes
• Operation

Structure of a Basic Oxygen Furnace

The scheme of a Basic Oxygen Furnace (BOF) is presented in the picture.

Typical basic oxygen furnace has a vertical vessel lined with refractory lining.
Only 8-12% of the furnace volume is filled with the treated molten metal. The bath depth is about 4-6.5 ft (1.2-1.9 m). The ratio between the height and diameter of the furnace is 1.2-1.5. The typical capacity of the Basic Oxygen Furnace is 250-400 t.

The vessel consists of three parts: spherical bottom, cylindrical shell and upper cone. The vessel is attached to a supporting ring equipped with trunnions.
The supporting ring provides stable position of the vessel during oxygen blowing.
The converter is capable to rotate about its horizontal axis on trunnions driven by electric motors. This rotation (tilting) is necessary for charging raw materials, sampling the melt and pouring the steel out of the converter.

The top blown basic oxygen furnace is equipped with the water cooled oxygen for blowing oxygen into the melt through 4-6 nozzles. Oxygen flow commonly reaches 200-280 ft³/(min*t) (6-8 m³/(min*t)). The oxygen pressure is 150-220 psi (1-1.5 MPa). Service life of oxygen lance is about 400 heats.

The bottom blown basic oxygen furnace is equipped with 15-20 tuyeres for injection of oxygen (or oxygen with lime powder). The tuyeres are cooled by either hydrocarbon gas (propane, methane) or oil supplied to the outer jacket of the tube.
Refractory lining of a Basic Oxygen Furnace

The refractory lining of basic oxygen furnaces work in severe conditions of high temperature and oxidizing atmosphere. The lining wear is fastest in the zone of contact with the oxidizing slag (slag line).

Refractory bricks for lining basic oxygen furnaces are made of either resin bonded magnesite or tar bonded mixtures of magnesite (MgO) and burnt lime (CaO). The bonding material (resin, tar) is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from chemical attack of the molten metal.

The following measures allows to prolong the service life of the lining:

• Control of the content of aggressive oxidizing oxide FeO in the slags at low level.
• Addition of MgO to the slags.
• Performing “slag splashes” - projecting residual magnesia saturated slag to the lining walls by Nitrogen blown through the lance.
• Repair the damaged zones of the lining by gunning refractory materials.

Properly maintained lining may serve 20000 heats.

Chemical and physical processes in a Basic Oxygen Furnace

The basic oxygen furnace uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel. Iron and its impurities oxidize evolving heat necessary for the process.

Oxidation of the molten metal and the slag is complicated process proceeding in several stages and occurring simultaneously on the boundaries between different phases (gas-metal, gas-slag, slag-metal). Finally the reactions may be presented as follows:
(square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas)

1/2{O₂} = [O]

[Fe] + 1/2{O₂} = (FeO)

[Si] + {O₂} = (SiO₂)

[Mn] + 1/2{O₂} = (MnO)

2[P] + 5/2{O₂} = (P₂O₅)

[C] + 1/2{O₂} = {CO}

{CO} + 1/2{O₂} = {CO₂}

Most oxides are absorbed by the slag.

Gaseous products CO and CO₂ are transferred to the atmosphere and removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio: {CO₂}/({CO₂}+{CO}).

Basic Oxygen Process has limiting ability for desulfurization. The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. However the slag formed in the Basic Oxygen Furnace is oxidizing (not reducing) therefore maximum value of distribution coefficient of sulfur in the process is about 10, which may be achieved in the slags containing high concentrations of CaO.

Operation of a Basic Oxygen Furnace

• Charging steel scrap (25-30% of the total charge weight).
• Pouring molten pig iron from blast furnace.
• Charging fluxes.
• Starting oxygen blowing. Duration of the blowing is about 20 min.
• Sampling. Temperature measurements (by disposable thermocouple) and taking samples for chemical analysis are made through the upper cone in tilted position of the furnace.
• Tapping - pouring the steel to a ladle. Special devices (plugs, slag detectors) prevent penetration (carry-over) of the slag into the ladle.
• De-slagging - pouring the residual slag into the slag pot. The furnace is turned upside down in the direction opposite to the tapping hole.

The Basic Oxygen Furnace has a capacity up to 400 t and production cycle (tap-to-tap) of about 40 min.

Desulfurization of steel

• Sulfur in steel
• Desulfurization of steel by slags
• Desulfurization of steel by injection of active agents

Sulfur in steel

Sulfur (S) may dissolve in liquid iron (Fe) at any concentration. However solubility of sulfur in solid iron is limited: 0.002% in α-iron at room temperature and 0.013% in γ-iron at 1832°F (1000°C).

When a liquid steel cools down and solidifies the solubility of sulfur drops and it is liberated from the solution in form of iron sulfide (FeS) forming an eutectic with the surrounding iron. The eutectic is segregated at the iron grain boundaries. The eutectic temperature is relatively low - about 1810°F (988°C).
Fe-FeS eutectic weakens the bonding between the grains and causes sharp drop of the steel properties (brittleness) at the temperatures of hot deformation (Rolling, Forging etc.).

Brittleness of steel at hot metal forming operations due to the presence of low-melting iron sulfides segregated at grain boundaries is called hot shortness.
In order to prevent formation of low-melting iron sulfide manganese (Mn) is added to steel to a content not less than 0.2%.
Manganese actively reacts with iron sulfides during solidification of steel transforming FeS to MnS according to the reaction:

(FeS) + [Mn] = (MnS) + Fe

(square brackets [ ] - signify concentration in steel, round brackets ( ) signify concentration in slag)

The melting temperature of manganese sulfide is relatively high - about 2930°F (1610°C) therefore the steels containing manganese may be deformed in hot state (no hot shortness).
Unfortunately MnS inclusions are:

• Brittle (less ductile than steel);
• They may have sharp edges;
• They are located between the steel grains.

All these factors determine negative influence of sulfide inclusions on the mechanical properties. Cracks may be initiated at brittle sharp edge inclusions. Sulfide inclusions especially arranged in a chain form also make easier the cracks propagation along the grain boundaries.
The negative effect of sulfur on the steel properties becomes more significant in large ingots and castings, some zones of which are enriched by sulfur (macrosegregation of sulfur).

The properties negatively affected by sulfur:

• Ductility;
• Impact toughness;
• corrosion resistance;
• Weldability.

 

Desulfurization of steel by slags

The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. Basic slag is a slag containing mainly basic oxides: CaO, MgO, MnO, FeO.
A typical basic slag consists of 35-60% CaO + MgO, 10-25% FeO, 15-30% SiO₂, 5-20% MnO.

Transition of sulfur from steel to slag may be presented by the chemical equation:

[S] + (CaO) = (CaS) + [O]

The equilibrium constant K_S1 of the reaction is:

K_S1 = a_[O]*a_(CaS)/a_[S]*a_(CaO)

Where:
a_[O], a_[S] - activities of oxygen and sulfur in the liquid steel;
a_(CaS), a_(CaO) - activities of CaS and CaO in the slag.

The same reaction in ionic form:

[S] + (O^2-) = (S^2-) + [O]

The equilibrium constant K_S2 of the reaction is:

K_S2 = a_[O]*a_(S2-)/a_[S]*a_(O2-)

Where:
a_(S2-), a_(O2-) - activities of S^2- and O^2- in the slag.

Capability of a slag to remove sulfur from steel is characterized by the distribution coefficient of sulfur:

L_S = (S)/[S]

Where:
(S) - concentration of sulfur in slag;
[S] - concentration of sulfur in steel;

As appears from the above equations desulfurization is effective in deoxidized (low (O)) basic (high (CaO)) slags. Therefore ability of Basic Oxygen Process (BOP) to remove sulfur is low due to its highly oxidized slag.
Desulfurization may be effectively conducted in the reducing slag stage of the steel making process in Electric-arc furnace. At this stage the oxidizing slag is removed and then lime flux is added to form basic slag with high CaO content.

Deep desulfurization by slags may be achieved in ladle:

• The refining (desulfurizing) slag with high content of CaO and no FeO is prepared and placed in an empty ladle.
• The molten steel is poured into the ladle filled with the refining slag.
• Energy of the falling steel stream causes mixing the slag with the steel, during which sulfur is removed from the steel to slag phase.

Effect of desulfurization may be enhanced by additional stirring, for which electromagnetic (induction) stirrers or argon bubbling are used.
Desulfurization of steel by injection of active agents

Injection of desulfurizing agents to a molten steel is the most effective method of sulfur removal.Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing.

Deep desulfurization by injection of active agents are achieved due to the following factors:

• High chemical activity of the desulfurization agents (Ca, Mg);
• High contact area between the steel and slag phases;
• Stirring providing good kinetic conditions of desulfurization;
• Presence of basic non-oxidized slag capable to absorb the products of the desulfurization reaction (CaS, MgS).

The following materials are used as desulfurizing agents:

• Slag mixtures CaO (50-90%) + CaF₂ (10-20%) + A₂lO₃ (0-30%);
• CaSi;
• CaC₂;
• CaC₂ + Mg;
• Lime (CaO) + Mg;
• Ca + Al;
• Ca;
• Mg.

The desulfurizing agents are injected into molten steel either in form of powder transported by an argon blown to the steel through a lance or in form of a cored wire containing powder of desulfurizing agent. In the latter method stirring by argon bubbling from the porous plug mounted in the ladle bottom is used.



Chemical reactions between desulfurizing agents and sulfur dissolved in steel may be presented by the following equations:

Ca + [S] = (CaS)

Mg + [S] = (MgS)

Injection of desulfurizing agents allows to achieve ultra-low concentrations of sulfur in steel (0.0002%).

Structure of killed steel ingot

 Typical ingot structure consists of five zones:

• Zone of small equiaxed grains

The thin layer of small crysrtals forms when a melt comes to a contact with a wall of a cold metallic mold. The crystals (grains) have no favorable direction (equiaxed) and their chemical composition is close to that of the liquid steel. Heat liberated as a result of Crystallization depresses the nucleation and crystal growth.

• Zone of columnar grains

Columnar grains start to grow when a stable and directed heat flow is formed as a result of heat transfer through the zone of small equiaxed grains. Direction of the columnar grains growth is oppsite to the direction of heat flow. Columnar grains continue to grow untill the heat flow decreases due to the following causes:

• Large width of the solidified metal;
• Heating the mold wall;
• Formation of an air gap between the ingot and the mold wall. The air gap is a result of shrinkage caused by solidification.

When the temperature of the melt, adjacent to the solidification front, increases due to the liberation of the latent heat, constitutional undercooling will end and the columnar grains growth will stop.

• Zone of large equiaxed grains

Low temperature gradient (low heat flow) and low cooling rate of the solute-enriched liquid in the cenral zone of the ingot result in formation of equiaxed grains. This process is slow due to slow heat extraction therefore the number of nuclei (seed crystals) is low and the grains size is large.
Zone of large equiaxed grains is enriched by the impurities (sulfur, phosphorous, carbon).

• Bottom cone

This cone-shaped zone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.
Bottom cone is characterized by negative segregation of the impurities.

• Shinkage cavity zone

Shrinkage cavity is located in the top part of the ingot (which is later discarded) where last portion of liquid solidifies. The mold design should provide upwards direction of solidification at its last stage. Below the shrinkage cavity the zone of shrinkage porosity is located. This zone forms when the feeding of solidifying metal by the residual liquid is insufficient. Isolated pockets of liquid metal separated from the liquid pool by “bridges” form their own shrinkage cavities (shrinkage pores).
The mold shape, which is wider in upper levels and thermal isolation of the “hot top” favor to diminish the shrinkage porosity.

Fabrication of large steel ingots

Large steel ingots are required for manufacturing electric power plant turbine shafts, generator rotor shafts, nuclear pressure vessels, chemical pressure vessels, ship parts and other heavy machinery parts.
Metalforming technology used for final shaping of large ingots is Forging.

The largest ingot (570 metric tons) was produced in 1980 by “Kawasaki Steel” (Japan).
Solidification of a large mass of steel is characterized by significant development of micro and macro-defects of the ingot structure:

• Non-metallic inclusions
• Macrosegregation
• Hydrogen in steel

The structure defects decrease the reliability of the part manufactured from the ingot. Since such parts work in equipment, failure of which is potentially catastrophic (nuclear equipment, electric power plants, chemical equipment, large scale machinery), Technology of large ingots fabrication should provide minimum degree of steel structure defects.

Non-metallic inclusions

Non-metallic inclusions in steel are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, oxysulfides, carbonitrides).

Despite small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:

• Tensile strength
• Deformability (ductility)
• Toughness
• Fatigue strength
• corrosion resistance
• Weldability
• Polishability
• Machinability

The following parameters of non-metallic inclusions influence on the properties of parts made of large steel ingots:

• Chemical composition (oxides, sulfides etc.)
  • Oxides
  • Sulfides
  • Oxysulfides
  • Carbides
  • Nitrides
  • Carbonitrides
  • Phosphides
• Inclusions size

Size of non-metallic inclusions is determined by the processes of nucleation, growth and coalescence/agglomeration. High surface energy causes the nucleation at higher supersaturation of the solutes (oxygen, sulfur, nitrogen, aluminum, silicon, titanium, vanadium, etc.) and favors coalescence and agglomeration of the inclusions.

• Shape (morphology) of non-metallic inclusions
  • Globular inclusion form in liquid state at low concentration of aluminum. Globular shape inclusions exert moderate influence on the steel properties therefore globular morphology is preferable.
  • Platelet shaped inclusions form at the grain boundaries as a result of eutectic transformation during solidification. Platelet shaped inclusion exert adverse effect on the steel properties and therefore this morphology is undesirable.
  • Dendrite shaped inclusions form at high concentration of aluminum. Their shape is characterized by sharp edges, which may cause concentration of stresses during the ingot forging and decrease of ductility, toughness and fatigue strength of the steel part fabricated from the ingot.
  • Polyhedral inclusions are result of modification of the dendrite shaped inclusions by addition of strong deoxidizers and rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. The effect of polyhedral inclusions on the steel properties is less than that of the dendrite shaped inclusions.
• Distribution (homogeneous, clusters, chains)

Homogeneous distribution of non-metallic inclusions is most desirable. Clusters of inclusions are unfavorable since they may result in local drop of mechanical properties such as toughness and fatigue strength.

• Physical and mechanical properties (hardness, ductility, melting point)

Microscopic hard inclusions (carbides, nitrides) strengthen the metal however larger hard inclusions may cause drop of the steel ductility without increase of the strength and hardness. Ductile and brittle inclusions behave different during plastic deformation (steel Forging). Ductile inclusions elongate in the direction of deformation. Brittle inclusions break to fragments and form chains.

Macrosegregation

The following factors favors macrosegregation in large steel ingots:

• Large absolute amount of solutes (sulfur, phosphorous, carbon). Steel is being enriched with the solutes rejected by the moving solidification front therefore at the final solidification stage the residual liquid contain large solute content.
• Low cooling rate of the metal in the central ingot zone. Low cooling rate results in larger spaces between the dendrite arms filled with the liquid metal enriched with the solutes. Partition of solutes between the dendrite arms and interdendritic liquid is called Microsegregation, which is the main cause of macrosegregation. Additionally solidification conditions at low cooling rate are closer to equilibrium resulting in decrease of Equilibrium Partition Coefficient and higher microsegregation.
• Large distances, which liquid metal and separate dendrite crystals and their fragments may be transferred. Transfer of liquid and solid phases in a solidifying ingot is the result of solidification shrinkage, buoyancy forces acting on the liquid metal, sedimentation of solid crystals homogeneously nucleated in liquid phase and fragments of broken or melted off dendrites.

 Development of macrosegregation zones in a steel ingot and their locations are associated with the ingot grain structure.

• Bottom negative segregation

Bottom negative segregation is a result of low solute concentration in the crystals formed in the early stage of solidification and comprising bottom cone. The bottom cone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.

• V-segregation

The central zone of ingot is enriched with solute rejected by the solidification front progressing from the mold wall to its center. The central zone consists of large equiaxed grains, which settle down to the V-shaped solidification front. The residual liquid surrounding the large equiaxed grains is solute-rich and it forms V-segregates when solidifies.

• A-segregation

A-segregates (freckles) form in the Zone of columnar grains at the regions with structure characterized by the transition from the columnar grains to large equiaxed grains. A-segrgates present channels enriched by sulfur, carbon, phosphorus and other impurities.

• Hot top segregation

Hot top segregation zone is located in the top central ingot region below the shrinkage cavity. Hot top segregation is formed at the final solidification stage from the residual liquid enriched by the solutes as a result of microsegregation (rejection by solidifying dendrites) followed by penetration of the liquid through the dendrite skeleton.

Factors allowing to diminish macrosegregation in large steel ingots:

• Lowering contents of impurities. Steel for large ingots is treated in the melting furnace and in Ladle refining stands in order to remove undesirable impurities such as sulfur (desulfurization]]), phosphorous (dephosphorization), hydrogen (degassing). Steel for large steel ingots commonly contains not more than 0.005% of sulfur, 0.005 of phosphorous and up to 2 ppm of hydrogen.
• Alloying of steel by alkaline (Ca, Mg) or rare earth (Ce,La) elements in amount of about 2*[S].
• Low concentration of silicon in steel (about 0.1%).
• Using steel grades with lower carbon content.
• Modification of the ingot dimensions. If low level of A-segregation is required the ratio of the ingot height to its diameter should be as low as possible (about 1.0-1.2).
• Decrease of the pouring temperature. Lower pouring temperature results in increase of the cooling rate, which is favorable for depressing macrosegregation.

Hydrogen in steel

Sources of hydrogen in liquid steel:

• Damp scrap;
• Fluxes and alloying additives;
• Furnace and ladle refractories;
• Atmospheric humidity;
• Fuel (if used).

Hydrogen is easily dissolved in liquid steel in dissociated (atomic) state. Solubility of hydrogen in steel drops sharply during solidification resulting in formation of gaseous hydrogen form H₂. In solid steel hydrogen is dissolved in form of interstitial solution.

Carbon, nickel, chromium (up to 10%), vanadium, titanium, zirconium, columbium, tantalum increase the solubility of hydrogen in solid steel.
Silicon, aluminum, tungsten, chromium (10% and higher) decrease the solubility of hydrogen in solid steel.
Solubility of hydrogen in austenite is much higher than in ferrite.

Both gaseous and dissolved forms of hydrogen exert adverse effect on mechanical properties of steels:

• Hydrogen flakes

Solubility of hydrogen decrease during solidification and cooling down of steel ingot. Hydrogen atoms possessing high mobility are collected at internal voids such as non-metallic inclusions (sulfides, oxides) and their clusters, shrinkage pores, cracks caused by internal stresses.
Hydrogen atoms collected at internal voids combine and form gaseous hydrogen H₂, which may cause formation of cracks (flakes) when the gas pressure exceeds the steel strength.
Hydrogen flakes is particularly dangerous for parts fabricated from large ingots. Vacuum ladle degassing methods allow to decrease the content of hydrogen to 2 ppm, which does not cause flaking formation.

• Hydrogen embrittlement

Hydrogen in dissolved form also decreases steel properties such as ductility, Fracture Toughness and fatigue strength.

Technology of large ingots fabrication

The following tasks are accomplished by the technology of large ingots fabrication:

• Dephosphorization (to about 0.005%) in a steel making furnace (melting furnace).
• Desulfurization of steel (to about 0.005%) in the melting furnace and then by a ladle refining desulfurization method (eg. Ladle desulfurization by injection of active agents, Ladle Furnace (LF)).
• Deoxidation of steel by metallic deoxidizers to the required concentrations of aluminum, silicon and other deoxidizing elements.
• Removal of non-metallic inclusions from the liquid steel in both melting furnace and Ladle refining stand.
• Vacuum treatment of steel in ladle by one of the degassing methods: Recirculation Degassing (RH), Ladle Degassing (VD).
• *Achievement of the required temperature of steel before pouring to the mold by using Ladle Furnace (LF).
• Final vacuum treatment of the steel in the Ladle-to-mold degassing operation.
• Providing non-intermittent supply of liquid steel to the pouring-to-mold operation.

 

Deoxidation of steel

The main sources of Oxygen in steel are as follows:

• Oxygen blowing (example: Basic Oxygen Furnace (BOF));
• Oxidizing slags used in steel making processes(example: Electric-arc furnace;
• Atmospheric oxygen dissolving in liquid steel during pouring operation;
• Oxidizing refractories (lining of furnaces and ladles);
• Rusted and wet scrap.

Solubility of oxygen in molten steel is 0.23% at 3090°F (1700°C). However it decreases during cooling down and then drops sharply in Solidification reaching 0.003% in solid steel.

Oxygen liberated from the solid solution oxidizes the steel components (C, Fe, alloying elements) forming gas pores (blowholes) and non-metallic inclusions entrapped within the ingot structure. Both blowholes and inclusions adversely affect the steel quality.

In order to prevent oxidizing of steel components during solidification the oxygen content should be reduced.

Deoxidation of steel is a steel making technological operation, in which concentration (activity) of oxygen dissolved in molten steel is reduced to a required level.

There are three principal deoxidation methods:

• Deoxidation by metallic deoxidizers
• Deoxidation by vacuum
• Diffusion deoxidation.

Deoxidation by metallic deoxidizers

This is the most popular deoxidation method. It uses elements forming strong and stable oxides. Manganese (Mn), silicone (Si), aluminum (Al), cerium (Ce), calcium (Ca) are commonly used as deoxidizers.
Deoxidation by an element (D) may be presented by the reaction:

n[D] + k[O] = (D_nO_k)

The equilibrium constant K_D-O of the reaction is:

K_D-O = a_ox/(a_Dⁿ x a_O^k)
or
log K_D-O = log a_ox - n*log a_D - k*log a_O

where:
a_ox - activity of the oxide (D_nO_k) in the resulted non-metallic inclusion;
a_D - activity of the deoxidizer in liquid steel;
a_O - activity of oxygen in liquid steel.

Thermodynamic activity of a solute in a solution is a parameter related to the solute concentration. Activity substitutes concentration in thermodynamic equations describing chemical reactions in non-ideal solutions (activities of solutes in a diluted solution are close to their concentrations).

The equilibrium constant of a deoxidation reaction is determined by the steel temperature:

log K_D-O = A_D/T - B_D

where:
A_D, B_D - characteristic parameters determined for the particular deoxidizer D;
T - steel temperature, °K

The table presents parameters of the deoxidation reactions for some metallic oxidizers:

Deoxidizer	Reaction	A	B	Equilibrium constant at 1873 °K (2912°F, 1600°C)
Manganese	[Mn] + [O] = (MnO)	12440	5.33	1.318
Silicone	[Si] + 2[O] = (SiO2)	30000	11.5	4.518
Aluminum	2[Al] + 3[O] = (Al2O3)	62780	20.5	13.018

Values of the equilibrium constant parameters are used for calculation of equilibrium concentrations of oxygen and the deoxidizer by the equation:

A_D/T - B_D = log a_ox - n*log a_D - k*log a_O

In the simplest case a_ox=1, a_D=[D], a_O=[O], therefore:

A_D/T - B_D = n*log [D] - k*log [O]

According to the degree of deoxidation Carbon steels may be subdivided into three groups:

• Killed steels - completely deoxidized steels, solidification of which does not cause formation of carbon monoxide (CO). Ingots and castings of killed steel have homogeneous structure and no gas porosity (blowholes).
• Semi-killed steels - incompletely deoxidized steels containing some amount of excess oxygen, which forms carbon monoxide during last stages of solidification.
• Rimmed steels - partially deoxidized or non-deoxidized low carbon steels evolving sufficient amount of carbon monoxide during solidification. Ingots of rimmed steels are characterized by good surface quality and considerable quantity of blowholes.

Deoxidation in vacuum

Method of deoxidation in vacuum utilizes carbon dissolved in steel as the deoxidizer according to the equation:

[C] + [O] = {CO}

where:
[C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide.

The equilibrium constant of this chemical reaction is expressed as follows:

K_CO = p_CO/(a_C x a_O)

where:
p_CO - partial pressure of carbon monoxide in the atmosphere; a_C and a_O - activities of carbon and oxygen in liquid steel.

Temperature dependence of K_CO is insufficient.
For approximate calculations the following equation may be used:

[C]*[O] = 0.0025*p_CO at 2948°F (1620°C)
According to the above expressions the oxygen activity (concentration) is proportional to the partial pressure of carbon monoxide therefore decrease of the latter will cause reduction of the oxygen activity.Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation. Bubbles of carbon monoxide form in the liquid steel, float up and then they are removed by the vacuum system.

In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel. Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump.
Vacuum deoxidation is used mainly in Ladle refining.

Steels deoxidized in vacuum are characterized by homogeneous structure, low content of non-metallic inclusions and low gas porosity.
Vacuum treatment is used for manufacturing large steel ingots, rails, ball bearings and other high quality steels.

Diffusion deoxidation

Oxygen dissolves in both steel and slag. Equilibrium between the two systems may be presented by the equation:

[O] = (O)

The equilibrium constant of the reaction:

K_FeO = a_[O]/a_(O)
or
a_[O] = K_FeO*a_(O)

Thus reduction of the oxygen activity (concentration) in steel may be achieved by decreasing the oxygen activity in the slag.
When the oxygen activity in the slag is reduced oxygen ions dissolved in steel begin to diffuse from the steel into the slag, and the equilibrium conditions are restored. In other words, deoxidation of slag results in deoxidation of the steel.
Carbon (coke), silicone, aluminum and other elements are used for slag deoxidation.
Since deoxidizers in the difusion method are not introduced directly into the steel melt, oxide non-metallic inclusions do not form.
Diffusion deoxidation allows to produce steel less contaminated by non-metallic inclusions.

Ladle refining

• Vacuum ladle degassing
  • Recirculation Degassing (RH)
  • Recirculation Degassing with oxygen top lance (RH-OB)
  • Ladle Degassing (VD, Tank Degassing)
  • Vacuum Oxygen Decarburization (VOD)
• Ladle Furnace (LF)
• Ladle desulfurization by injection of active agents
  • Powder injection
  • Cored wire injection
• Ladle-to-mold degassing

Vacuum ladle degassing

Methods of vacuum ladle degassing utilize the reaction of deoxidation by carbon dissolved in steel according to the equation:

[C] + [O] = {CO}

where:
[C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide.

Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation. Bubbles of carbon monoxide form in the liquid steel, float up and then they are removed by the vacuum system.

In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel. Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump.

Movement of the molten steel caused by CO bubbles also results in refining the steel from non-metallic inclusions, which agglomerate, float up and are absorbed by the slag.
CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous Nitrogen.

Steels refined in vacuum are characterized by homogeneous structure, low content of non-metallic inclusions and low gas porosity.

Vacuum degassing methods are used for manufacturing large steel ingots, rails, ball bearings and other high quality steels.

Vacuum ladle degassing methods:

• Recirculation Degassing (RH)

Recirculation degassing unit uses a vacuum chamber having two snorkels connected to the chamber bottom. One of the snorkels is equipped with pipes supplying Argon through its refractory lining.

The snorkels of the vacuum chamber are immersed into the ladle with molten steel. Liquid metal fills the chamber to a level determined by the atmospheric pressure (4.2ft/1.3m). Argon bubbles floating up in one of the snorkels (up-leg) force the melt to rise in the snorkel. Through the second snorkel (down-leg) the molten steel flows down back to the ladle producing circulation. The circulation rate may reach 150-200 t/min.
The recirculation degassing vacuum chambers are usually equipped with addition hoppers, through which alloying elements or/and desulfurization slag may be added.

Benefits of Recirculation Degassing (RH):
-Hydrogen removal (degassing);
-Oxygen removal (deoxidation);
-Carbon removal (decarburization);
-Sulfur removal (desulfurization);
-Precise alloying;
-Non-metallic inclusions removal;
-Temperature and chemical homogenizing.

• Recirculation Degassing with oxygen top lance (RH-OB)

In this method a conventional Recirculation degassing (RH) vessel (chamber) is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface.
Oxygen intensifies the reaction [C] + [O] = {CO} resulting in fast and effective decarburization. Oxygen also oxidizes phosphorus like in Basic Oxygen Process (BOP) or in oxidizing slag stage in Electric-arc furnace.
Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source.

Benefits of Recirculation degassing with oxygen top lance (RH-OB):
-Hydrogen removal (degassing);
-Fast carbon removal (decarburization);
-Phosphorus removal (dephosporization);
-Sulfur removal (desulfurization);
-Reheating; -Precise alloying;
-Non-metallic inclusions removal;
-Temperature and chemical homogenizing.

• Ladle Degassing (VD, Tank Degassing)

In the Tank Degassing method the ladle with molten steel is placed into a vacuum chamber. The ladle is equipped with a porous refractory plug mounted in the ladle bottom. Through the plug argon is supplied during vacuum treatment. There is an addition hopper with vacuum lock on the chamber cover. The hopper is used for adding alloying elements and/or slag components.
The reaction [C] + [O] = {CO} starting in the steel under vacuum conditions causes stirring, which is additionally intensified by argon blown through the bottom porous plug.
Intensive stirring of the melt and the slag results in deep desulfurization of the steel. Desulfurizing slags possessing high sulfur solubility are used in this process.
Argon and CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous nitrogen.

Benefits of Ladle Degassing (VD, Tank Degassing):
-Hydrogen removal (degassing);
-Oxygen removal (deoxidation);
-Deep sulfur removal (desulfurization);
-Carbon removal (decarburization);
-Precise alloying;
-Non-metallic inclusions (oxides and nitrides) removal;
-Temperature and chemical homogenizing.

• Vacuum Oxygen Decarburization (VOD)

In this method a conventional Ladle Degassing (VD, Tank Degassing) chamber is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface.
Vacuum Oxygen Decarburization (VOD) method is used for manufacturing Stainless steels. Oxidation of liquid steel components under vacuum differs from that at normal pressure: oxygen is consumed mainly by the reaction [C] + [O] = {CO} rather than by oxidation of chromium, which is the main constituent of stainless steels.
VOD process allows to decarburize the steel with minor chromium losses.
Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source.
After having the decarburization (oxidation) stage completed deoxidizers are added to the steel in order to remove excessive oxygen. Then a Desulfurizing slag is added to the molten steel surface. Stirring of the melt and the slag caused by argon blown through the porous bottom plug results in deep desulfurization of the steel.

Benefits of Vacuum Oxygen Decarburization (VOD):
-Deep carbon removal (decarburization);
-Low losses of chromium in treatment of stainless steels; -Hydrogen removal (degassing);
-Sulfur removal (desulfurization);
-Precise alloying;
-Reheating; -Non-metallic inclusions (oxides and nitrides) removal;
-Temperature and chemical homogenizing.

Ladle Furnace (LF)

 Molten steel in a ladle may be treated (refined) in a device called Ladle Furnace (LF).

The ladle is transported to the Ladle Furnace stand where it is placed under a cover equipped with three graphite electrodes connected to a three-phase arc transformer. The ladle bottom has a porous refractory plug, which is connected to the argon supply pipe at the Ladle Furnace stand. The LF stand is also equipped with an addition hopper mounted on the cover and a lance for injection of desulfurizing agents. Fumes formed during the operation are extracted through the cover.

Molten steel treated in Ladle Furnace is covered by a layer of desulfurizing slag. The graphite electrodes are submerged into the slag, which protects the ladle lining from overheating produced by the electric arcs. The arcs are capable to heat the steel at the rate about 5°F/min (3°C/min).

During the treatment process argon is blown through the bottom porous plug providing continuous metal stirring. Stirring results in distribution of heat produced by the arcs, chemical homogenization and desulfurization of the steel by the slag.
Alloying elements and/or slag components may be added through the addition hopper.

If deep desulfurization is required active desulfurizing agents are injected into the melt through the injection lance or in form of cored wire.
Besides refining operations Ladle Furnace (LF) may serve as a buffer station before Continuous casting.

Benefits of Ladle Furnace (LF):

• Deep sulfur removal (desulfurization);
• Controllable reheating by electric power;
• Alloying;
• Temperature and chemical homogenizing;
• Non-metallic inclusions removal.

Ladle Furnace process is used for refining a wide variety of steels, in which degassing (hydrogen removal) is not required.

Ladle desulfurization by injection of active agents

Injection of desulfurizing agents (Ca, Mg, CaSi, CaC₂, CaF₂+CaO) to a molten steel is the most effective method of sulfur removal.
Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing.

A ladle with deoxidized (killed) molten steel is transported to the injection stand where it is placed under a cover, through which the injection lance may lower and immerse into the melt.
Steel treated in the stand is covered by a layer of desulfurizing slag having high solubility of sulfur and capable to absorb sulfides formed as a result of active agents injection.
Desulfurization agents are injected in argon stream. Argon bubbles produce stirring of the molten steel and the slag promoting desulfurization. Stirring also provides thermal and chemical homogenization of the melt.

When the desulfurizing agents are injected into molten steel in form of a cored wire containing powder of desulfurizing agent stirring by argon bubbling from the porous plug mounted in the ladle bottom is used.

Fumes formed during the operation are extracted through the cover.

Injection of desulfurizing agents allows to achieve ultra-low concentrations of sulfur in steel (0.0002%).

Benefits of Ladle desulfurization by injection of active agents:

• Deep sulfur removal (desulfurization);
• Temperature and chemical homogenizing;
• Non-metallic inclusions removal.
  
  Ladle-to-mold degassing

 Ladle-to-mold degassing is a vacuum degassing method, in which the mold is placed in a vacuum chamber.

The molten steel is poured from a tundish attached to the cover of the chamber.
The tundish is continuously filled with the melt poured from the ladle.

The steel stream “boils” when it is falling to the mold cavity in vacuum due to the deoxidation reaction [C] + [O] = {CO}.
Hydrogen dissolved in steel diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump.

Intensity of the deoxidation and degassing during Ladle-to-mold pouring is indicated by the angle, at which the melt stream “opens” as a result of CO bubbles formation.

Steel strip processing

Steel strips are commonly manufactured of continuously cast slabs.

Coiled steel strips have the thickness (gauge) up to 0.2” (5 mm).

The strips are supplied in different conditions:

• Hot rolled
• Pickled
• Cold rolled
• Coated

Hot rolling

Continuously cast steel slabs are processed in a hot Rolling line:

• Heating in a gas/oil fired reheating furnace to the temperature 2200-2280ºF (1200-1250ºC).
• Removing oxide scale in a scale breaker. Iron oxides form on the slab surface as a result of high temperature oxidation in the reheating furnace. Scale breaker utilizes high pressure water jets directed to the slab surface.
• Rough rolling in either reversing or tandem roughing rolling mill. Rough rolling mills may have either two-high or four-high configuration. The strip temperature decreases in the rough rolling operation to about 1830ºF (1000ºC).
• Finishing rolling in a tandem four-high rolling mill (5-7 stands). The strip dimensions (thickness, width) are strictly controlled in this operation.
• Cooling by water jets. The temperature is reduced to max. 1110ºF (600ºC).
• Coiling the finished strip. Hot rolled strips have a thin scale of iron oxides on their surface. Part of the strips is used in as-rolled condition. Other part is processed in a pickling line where the oxide scale is removed.

Pickling of steel strip

Hot rolled strip is treated in a pickling line:

• Uncoiling.
• Pickling in a solution of hydrochloric or sulfuric acid. The acid dissolutes the oxide scale from the strip surface according to the reaction: Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O.
• Rinsing and passivation of the strip. At this stage the acid residuals are washed by water from the strip. Passivation film is formed on the strip surface providing corrosion protection.
• Drying.
• Edge trimming.
• Oiling.
• Recoiling.

Cold rolling of steel strip

Cold rolled steel strips are used mainly in the Deep drawing process.

The purposes of cold rolling processing are:

• Further reduction of the strip thickness;
• Improvement of the surface finish;
• Improvement of the surface flatness;
• Achievement of a required level of work hardening.

Cold rolling processing includes the following stages:

• Cold rolling in a tandem rolling mill. The rolling mill consists of 5-7 stands having four-high configuration (two large back-up rolls and two small work rolls).
• Annealing at a temperature 1100-1300ºF (600-700ºC) in a controlled reducing atmosphere (commonly a mixture of Hydrogen and Nitrogen) preventing oxidation of the steel surface. Annealing results in recrystallization of the steel Grain structure and in stress relief. There are two alternatives of annealing process: continuous annealing line and batch annealing furnace. In the continuous annealing line (see the picture below) steel strip passes annealing furnace at a controlled speed. In the batch process steel strip coils are stacked on top of each other in the bell type furnace. Batch annealing allows to achieve lower hardness and higher ductility of the steel than in alternative continuous annealing.
• Temper rolling (skin pass rolling) - final cold rolling operation with low thickness reduction conducted in order to impart to the steel required levels of hardness, evenness and surface finish. Four-high rolling mill is used for tempering.

Most of annealed low carbon steel strips are tempered since they are too soft (HV<110) in annealed condition. Bending and deep drawing operations of soft annealed steel may cause formation of kinks (cross breaks) and stretcher strains, which are the result of localized stretching of the strip at low cold deformation beyond the yield point. Light tempering of annealed strip (non-kinking temper, pinch pass) produces strip surface conditions, which do not cause formation of cross breaks and stretcher strains. Hardness of pinch passed steel is about 115 HV. Other temper conditions of steel strip are: eighth hard (105<HV<125), quarter hard (115<HV<130), half hard (130<HV<160), three quarter hard (150<HV<185).




Surface finish conditions of cold rolled steel strip:

• Mirror. Superior luster mirror finish is produced by rolling between fine polished rolls. Mirror finish strips are used mainly for electroplating.
• Bright. Bright finish is produced by rolling between polished rolls. It is the common surface finish condition of cold rolled strip.
• Matt. Matt (dull) surface finish is produced by rolling between roughened rolls. Matt surface is suitable for enameling and painting. Matt finish strips are also used in deep drawing due to the ability of the rough surface to hold lubricant providing low friction between the strip and the drawing tools (punch, blank holder, die).
• Blued. Blued surface finish is produced by controlled heating and cooling of bright finish strip resulting in formation of thin blue oxide film on the steel surface.

Strip edges conditions:

• Natural mill edges, which are result of cold rolling without any edge cutting operation. Natural mill edges may have micro-cracks due to non-uniform work hardening.
• Slit edges (shared edges), which are produced by rotary slitter. Slit edges are square with slight sharp burr.
• Dressed edges - slit edges, from which burr has been removed.
• Rolled edges, which are formed by edge rolling.

Steel strip coating

The most popular techniques of continuous steel strip coating are hot dip galvanizing and Electroplating.

Hot dip galvanizing process

• Uncoiling.
• Cleaning. This operation commonly combines alkaline cleaning (degreasing) and mechanical cleaning by rotating cylindrical brushes. Oil and other contaminants are removed from the steel surface.
• Pickling. Degreased and rinsed strip enters a pickling bath where oxides and rust are dissolved by hydrochloric acid and removed according to the reactions:

Iron oxide reaction: Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O
Rust reaction: Fe₂O₃*nH₂O + 6HCl = 2FeCl₃ + (n+3)H₂O

• Zinc coating (hot galvanizing). Clean strip passes through a bath with molten zinc (Zn).
• Wiping excessive zinc by air knives.
• Cooling. The coated strip is cooled by air and water.
• Levelling. Leveller produces smooth and flat strip surface.
• Recoiling.

Continuous electroplating process

• Uncoiling.
• Cleaning. This operation commonly combines alkaline cleaning (degreasing) and mechanical cleaning by rotating cylindrical brushes. Oil and other contaminants are removed from the steel surface.
• Pickling. Degreased and rinsed strip enters a pickling bath where oxides and rust are dissolved by hydrochloric acid and removed according to the reactions:

Iron oxide reaction: Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O
Rust reaction: Fe₂O₃*nH₂O + 6HCl = 2FeCl₃ + (n+3)H₂O

• Electroplating. Clean strip passes through a series of vertical electroplating baths. The strip is connected to the DC power supply as a cathode (negative). The positively connected anodes are arranged in parallel to the strip opposite to its surface. The bath is filled with an electrolytic solution. Electrolyte and anodes compositions are determined by the the deposited material and the electroplating method (Tin alloy electroplating, Decorative chromium electroplating|chromium electroplating, zinc electroplating).
• Recoiling.