Section 1: Overview of Basic Electric Arc Furnace Steelmaking
The basic arc furnace oxidation process mainly uses electric arc heating to remove impurities in the molten steel through oxidation reactions. During the oxidation period, the main oxidation reactions in the molten pool are the oxidation of elements such as carbon, silicon, and manganese. These reactions release a large amount of heat, which helps to maintain the furnace temperature. At the same time, the CO gas generated by the oxidation reaction can stir the molten pool to make the composition and temperature of the molten steel uniform.
The main process is as follows:
1. Batching and charging: According to the steelmaking target, scrap steel, pig iron and other raw materials are reasonably allocated, and an appropriate amount of slag-making materials (such as lime, fluorite, etc.) are added.
2. Melting period: The charge is quickly melted by arc heating to form molten steel. In this process, it is necessary to pay attention to controlling the stability of the arc and the melting rate of the charge.
3. Oxidation period: After the molten steel is melted, the oxidation of carbon, silicon, manganese and other elements in the molten steel is promoted by adding oxidants (such as oxygen, iron ore, etc.) or blowing oxygen. At the same time, the intensity and duration of the oxidation reaction need to be controlled to avoid excessive oxidation and lead to a decrease in the quality of the molten steel.
4. Reduction period: After the oxidation period, a reducing agent (such as ferrosilicon, ferromanganese, etc.) is added to carry out deoxidation and desulfurization reactions to further purify the molten steel. At the same time, the composition and temperature of the molten steel need to be adjusted to meet the requirements of the steelmaking target.
The basic arc furnace oxidation steelmaking process is suitable for making basic slag in the steelmaking process of various raw materials, which can effectively remove phosphorus and sulfur in the molten steel; during the oxidation period, the molten steel is boiled by the oxidation reaction of carbon, which can effectively remove the gas and inclusions in the molten steel, making the molten steel relatively pure. This method can be used to smelt most types of cast steel (including carbon steel and alloy steel).
Section 2: Repair of electric arc furnace
The arc furnace lining is constantly mechanically eroded and chemically eroded by the high-temperature molten steel during the steelmaking process. After each batch of steel is smelted, it will be damaged to varying degrees, especially in the high-temperature area at the slag line.
In order to ensure the normal smelting of the next furnace and extend the service life of the furnace lining, the furnace must be repaired after each steel tapping. The furnace repair material should be the same material as the furnace lining itself. If the furnace lining is knotted with brine magnesia, brine magnesia should be used for repair. The principle of furnace repair is “high temperature, fast, and thin repair”, that is, after steel tapping, the furnace should be repaired quickly while the temperature in the furnace is high.
The repaired magnesia layer should be thin, which is conducive to the sintering of the repair layer and avoids the phenomenon of peeling of the repair layer. If the furnace is seriously damaged, it needs to be repaired in batches. After each batch of repairs, the electrode should be put down for suffocation and sintering. The furnace is repaired with a large shovel and thrown with a shovel. The key parts of the furnace repair are the slag line at point 2 and nearby, both sides of the steel tapping place, and both sides of the furnace door.
Section 3: Electric Arc Furnace Batching and Charging
The basic charge for electric arc furnace steelmaking is scrap steel, return material, pig iron, and ferroalloy. Before batching, we must first understand the chemical composition and planned consumption quota of various raw materials, and then batch according to the technical standards and process requirements of the steel to be made.
The basic requirement for batching is to make the average carbon content of the charge reach a certain value to ensure that the carbon content of the molten steel after melting meets the process requirements, so as to ensure that there is enough carbon in the oxidation period for carbon-oxygen reaction to achieve the purpose of degassing and removing inclusions. Usually, considering the decarbonization amount during the oxidation period and the carbon burnout during the melting period, the carbon amount should be 0.05%~0.70% higher than the specified lower limit.
After the furnace is replenished, the charge can be loaded. At present, most electric arc furnaces use top loading. The prepared materials are placed in the material tank in a reasonable order of distribution in advance and lifted by a crane. Add them all at once from the top of the furnace. In this way, the charge can still be kept in the position of the charge tank after loading the furnace. In order to smelt more molten steel, after most of the solid materials in the furnace are melted, a secondary material turning operation can be performed, that is, the solid materials in the appropriate small cans (or small buckets) are put into the furnace for melting.
The principle of laying is to load as much as possible and melt the charge quickly. Generally, medium-sized charge is laid on the bottom, large blocks are placed in the middle, the largest blocks and refractory materials are placed under the electrode, and small blocks and steel scraps are placed on the top. The charge should be packed tightly to facilitate electrical and thermal conduction. It should be noted that non-conductive debris such as refractory bricks and sand blocks are not allowed to be placed under the electrode to prevent non-conductivity or even break the electrode. The charge must not be mixed with closed containers and flammable and explosive items, nor can it be mixed with mud, oil, hydrated snow balls and other inclusions.
Before loading the furnace, a layer of lime should be laid on the bottom of the furnace, with a weight of at least 1% of the weight of the charge. Its function is to reduce the impact of the charge on the furnace bottom and protect the furnace bottom when adding charge; secondly, to slag and remove phosphorus in advance during the melting of charge.
“Steelmaking is slag making”, slag plays an extremely important role in the steelmaking process. The role of slag is mainly manifested in 5 aspects.
1. Collect and remove impurities in molten steel. The sources of impurities in molten steel include impurities in the charge, oxides generated by oxidation of metal elements, and eroded or knocked-off lining refractory materials. These impurities are called non-metallic inclusions in molten steel. When various inclusions are combined with slag-making materials to form slag, the melting point is relatively low, floating on the molten steel and separating from it.
2. Remove harmful elements phosphorus and sulfur from molten steel.
3. Control the oxidation and reduction of molten steel. By adjusting the slag, control the chemical reaction process during the oxidation and reduction periods.
4. Protect molten steel from being oxidized in large quantities and from gas infiltration. The slag covers the surface of the molten steel, isolating the molten steel from the furnace gas and preventing oxidation and air inhalation.
5. Avoid a sharp drop in the temperature of the molten steel. Because the slag has poor conductivity, it can play a role in heat preservation.
The physical and chemical properties of slag are determined by its composition. Slag mainly contains oxides such as CaO, SiO₂, Al₂O₃, MgO, FeO, FeS, MnS and CaF₂. The oxides are acidic (such as SiO₂, P₂O₅), alkaline (such as CaO, MnO, MgO, FeO, CaO) and neutral (such as Al₂O₃).
Slag is divided into acidic slag and alkaline slag according to the relative content of acidic oxides and alkaline oxides. The acidity and alkalinity of slag are generally expressed by alkalinity, which is usually calculated by the ratio of the amount of strong alkaline oxide CaO to the amount of strong acidic oxide SiO₂ in the slag: R = ωCaO / ωSiO₂
For slags with high phosphorus content, P2O2 should be included in the calculation: R = ωCaO / ωSiO2 + ωP2O2
When R>1, it is alkaline slag, and when R<1, it is acidic slag. Generally, alkaline slag is made in alkaline furnace, and acidic slag is made in acidic furnace.
Another property of slag is viscosity. The viscosity of slag is largely determined by its melting point. Under certain temperature conditions, the lower the melting point, the lower its viscosity. For alkaline slag, adding lime to the slag increases the melting point of the slag and increases the viscosity. Conversely, adding quartz sand can reduce the viscosity. In order to reduce the viscosity without reducing the alkalinity of the slag and make the slag thinner, fluorite (CaF2) can be added.
Section 4: Melting Period of Electric Arc Furnace
The melting period refers to the entire process from the start of melting to the melting of the charge. The task of the melting period is to melt the solid charge into molten steel and dephosphorize it. The melting period accounts for about half of the entire smelting time, and the power consumption accounts for about two-thirds of the total power consumption. Accelerating the melting of the charge and shortening the melting period are important ways to increase production and reduce power consumption.
In the arcing stage, the medium voltage is generally selected, and the input transformer rated power is about two-thirds. The arcing stage is short, about 5~10 minutes. Sometimes non-conductivity occurs during operation, and the arc cannot be induced. This is because there are non-conductive debris under the electrode or cold slag at the lower end of the electrode. The solution is to raise the electrode, open the furnace cover, remove the non-conductive debris, and knock off the cold slag at the end of the electrode. You can also put a small amount of broken electrode blocks or coke blocks under the electrode.
After the arc is struck, the charge at the lower end of the electrode is melted first, forming 3 holes, which is called “penetration”. At this time, high-power power can be used, because the arc is completely surrounded by the charge and will not burn the land sink. Generally, the “penetration” time is about 20 minutes.
When the electrode drops to the bottom of the furnace, the electrode is raised to continue melting the charge. At this time, the charge next to it will collapse, which is called “collapse”. In this way, the charge is gradually melted and the liquid level of the molten pool continues to rise. When three-quarters of the charge is melted, the remaining solid charge is far from the electrode. You can use a rake to push it under the electrode to accelerate melting. This operation is called “pushing charge to assist melting”. You can also use oxygen blowing to accelerate the melting of the charge, which is called “oxygen blowing to assist melting”.
During the melting period, the charge often hits the electrode and causes a short circuit. Therefore, a plug current coil (reactance) must be brought into the circuit, which is called “with reactance”. After the arc is stable, the plug current coil (reactance) is cut off, which is called “throwing reactance”.
During the melting process of the charge, the Fe, Si, Mn, P and other elements in the charge are oxidized by O₂ in the furnace gas to generate FeO, SiO₂, MnO, P₂O₅, etc., which are combined with the lime (CaO) added before charging to form slag, covering the surface of the molten steel.
In order to dephosphorize, small pieces of ore are added in batches at the end of melting. The total amount varies according to the phosphorus content, generally about 1% – 2% of the charge. During the melting period, the oxidation loss of carbon is related to the carbon content of the charge and the oxygen blowing flux, generally about 10% – 30% of the carbon content. When using carbon blocks and waste electrode blocks as carbon increasers, they should be placed at the bottom of the charge, and not placed directly under the electrode. Do not operate towards the carbon increaser when blowing oxygen to assist fluxing.
After the charge is melted, the melting period ends. Samples should be taken for analysis of C and P. If the carbon equivalent (melted carbon) of the molten steel does not meet the process requirements at this time, carbon increase operation should be performed. At this time, the phosphorus content in the slag is high, most of the slag should be discharged, and lime, fluorite, etc. should be added to make new slag.
Section 5: Oxidation Period of Electric Arc Furnace
Usually the oxidation period refers to the process stage from the melting and sampling of the charge to the removal of the oxidized slag. The task of the oxidation period is to dephosphorize, remove gas and inclusions in the molten steel, and increase the temperature of the molten steel. In the previous stage of the oxidation period, the temperature of the molten steel is relatively low, and the main purpose is slag making and dephosphorization.
Phosphorus exists in the molten steel in the form of iron phosphide (Fe₂P). During the steelmaking process, the iron phosphide in the molten steel reacts with the ferrous oxide and calcium oxide in the slag to form calcium phosphate.
2[Fe₂P]+5(FeO) + 4(CaO) → (CaO)₄·P₂O₅ + 9[Fe] is an exothermic reaction (in the reaction formula, square brackets [] indicate dissolution in the molten steel, and round brackets () indicate dissolution in the slag.) The favorable conditions for dephosphorization are high alkalinity and strong oxidizing properties, low viscosity slag, large slag volume and low temperature.
When the temperature of the molten steel rises to a certain level (generally the thermocouple temperature is required to be above 1560°C), oxidation decarburization boiling refining is carried out to remove gas and inclusions in the molten steel.
There are three methods of decarburization: ore decarburization, oxygen blowing decarburization and oxygen blowing-ore combined decarburization.
The main component of the ore is FeO. The added ore is first reduced by the iron in the steel and dissolved in the molten steel:
FeO + [Fe] = [FeO] + 3[Fe] (endothermic reaction)
The ferrous oxide dissolved in the molten steel reacts with the [C] in the steel to decarbonize:
[FeO] + [C] = [Fe] + CO↑ (endothermic reaction)
The reaction generates a large number of CO bubbles, which make the molten steel boil. The bubbles stir the molten pool strongly during the process of floating up, making the temperature and chemical composition of the molten steel uniform; when these bubbles encounter inclusions in the molten steel during the process of floating up, these inclusions adhere to the surface of the bubbles and float up with them and enter the slag; these CO bubbles also capture gases such as hydrogen and nitrogen dissolved in the steel during the process of floating up, and take them out to achieve the purpose of purifying the molten steel.
The amount of ore should be determined according to the oxidation decarburization. Generally, adding 1~1.2kg of ore per ton of molten steel can decarburize 0.01%. Since the overall reaction process between ore and carbon is endothermic, increasing the temperature is beneficial to the reaction. Generally, the minimum ore addition temperature should be greater than 1560℃. In order to prevent the molten steel from cooling down significantly, the method of adding ore in batches is adopted. Equivalent to 10~15kg of ore per ton of molten steel, each batch adds 10~15kg of ore. Low-temperature ore addition or too much ore at one time should be avoided, otherwise the temperature of the molten steel will be too low for a long time after the ore is added, and it will not boil. When the temperature of the molten steel rises, it is easy to “boil” again, that is, due to the excessive carbon-oxygen reaction, the amount of gas produced is too large, causing the molten steel to roll violently. “Big boiling” causes the molten steel to come into direct contact with the furnace gas, causing it to absorb a large amount of air and drop in temperature drastically. It may also cause the molten steel to gush out of the furnace door and cause an accident of “steel running out of the furnace door”. This not only causes loss of molten steel, but can also easily lead to safety accidents, causing damage to people and equipment.
When oxygen is blown into the molten steel, carbon is oxidized in two ways. One is direct oxidation:
2[C] + O₂ = 2CO↑ (exothermic reaction)
The other is indirect oxidation:
2[Fe] + O₂ = 2[FeO] (exothermic reaction)
[C] + [FeO] = [Fe] + CO↑ (endothermic reaction)
Under the same conditions, the oxygen decarburization speed is much faster than the ore decarburization speed, and a large amount of heat can be released. Therefore, oxygen decarburization has many advantages such as short oxidation time, low power consumption, and fast temperature rise. Generally, the oxygen blowing pressure is required to be 0.6~0.8MPa.
Decarburization with ore also has its advantages. Due to the high FeO content in the slag, the dephosphorization conditions are good, the iron loss is small, the environmental pollution is not as serious as oxygen blowing, and the endothermic characteristics of the reaction can be used to control the temperature in the furnace. The decarburization method combining ore and oxygen takes into account the advantages of both aspects. The ore can be added in 2 to 3 batches, and oxygen is blown between the two batches of ore to increase the temperature of the molten steel and promote the decarburization reaction.
In order to purify the molten steel, a certain amount of oxidation decarburization is required. The amount of decarburization should be determined according to the charge conditions and the quality requirements of the molten steel.
If the charge is relatively damp, or the furnace is newly built or overhauled, the amount of oxidation decarburization should be appropriately increased. In order to effectively remove the gas and inclusions in the molten steel, the boiling intensity of the molten steel should be appropriate, so a certain decarburization speed must be achieved. The appropriate boiling condition is both intense and stable, and the decarburization speed at this time is equivalent to 0.01%~0.03% per minute. If the decarburization speed is too slow, the degassing effect will be very poor; on the contrary, if the decarburization speed is too fast, there is a danger of “big boiling”.
When the temperature and chemical composition are appropriate, the addition of ore or oxygen blowing should be stopped, and the molten pool should be allowed to enter the natural boiling of net boiling, which is called net boiling. The purpose of net boiling is to reduce the residual oxygen content in the molten steel and allow the gas and inclusions to float fully. The net boiling time is about 5~15min. The net boiling time of a small furnace can be shorter, and the net boiling time of a large furnace can be longer; the net boiling time of oxygen blowing decarburization can be shorter, and the net boiling time of adding ore decarburization can be longer. During the net boiling, ferromanganese can be added at a ratio of 0.20% for pre-deoxidation to prevent carbon from being overoxidized, which is called “manganese boiling”.
The slag in the oxidation period contains high oxygen and phosphorus. In order to successfully deoxidize and prevent phosphorus reversion during the reduction period, slag must be removed. The slag removal should be thorough and the slag removal speed should be fast to prevent excessive air intake and cooling of the molten steel. Slag removal conditions: the slag removal temperature is 10~20℃ higher than the steel tapping temperature; the chemical composition meets the process requirements, among which C is generally 0.03%~0.10% lower than the lower limit of the finished product specification, and P≤0.015%.
The reduction period refers to the process stage from slag removal to steel tapping. The tasks of the reduction period are deoxidation, desulfurization, and adjustment of the temperature and chemical composition of the molten steel.
After slagging, the molten steel is exposed to the atmosphere and needs to absorb air and cool down. Therefore, lime, fluorite and crushed ferrosilicon blocks should be added immediately in a ratio of (3-4):1:1 to make thin slag, and ferromanganese (sometimes silicon-manganese alloy or aluminum insert) should be added according to the lower limit of the specification for pre-deoxidation. The total amount of thin slag is about 2%-3% of the molten steel. After the thin slag is formed, reducing slag is immediately made for reduction deoxidation. Deoxidation is to reduce the residual ferrous oxide in the molten steel with a deoxidizer, so that oxygen reacts with the deoxidizer to generate deoxidation products that enter the slag.
There are two methods of deoxidation: precipitation deoxidation and diffusion deoxidation. Precipitation deoxidation is to add deoxidizer directly into the molten steel to react with ferrous oxide in the steel for deoxidation. Precipitation deoxidation generally selects elements with strong deoxidation ability and the generated deoxidation products that are easy to discharge from the molten steel as deoxidizers, such as Mn, Si, Al, etc. The advantage of this method is that the deoxidation process is fast, and the disadvantage is that the deoxidation products are easy to remain in the molten steel and form inclusions.
The reaction formula for deoxidation with ferromanganese is: Mn + [FeO] = MnO + [Fe]
The reaction formula for deoxidation with ferrosilicon is: Si + 2[FeO] = SiO₂ + 2[Fe]
The diffusion deoxidation method is to add deoxidizer to the steel slag, so that the deoxidizing element reacts with the ferrous oxide in the slag, reducing the ferrous oxide content in the slag and promoting the diffusion of ferrous oxide in the molten steel into the slag, indirectly removing oxygen from the molten steel. The advantage of this method is that the deoxidation product remains in the slag and the quality of the molten steel is higher. The disadvantage is that the diffusion process is slow and the deoxidation time is long.
At present, electric arc furnace steelmaking generally adopts a comprehensive deoxidation method, that is, a method combining precipitation deoxidation and diffusion deoxidation. Its deoxidation system is to use ferromanganese, ferrosilicon or aluminum blocks (aluminum needs to be inserted) for precipitation deoxidation at the beginning of the reduction period, which is called pre-deoxidation; after the thin slag is formed, powdered deoxidizer is used for diffusion deoxidation; and then aluminum is used for precipitation deoxidation, which is called final deoxidation. This deoxidation method can ensure the deoxidation effect and the deoxidation time will not be too long.
The slag-making materials of the reducing agent include lime, fluorite and powdered diffusion deoxidizer. When initially making reducing slag, the amount of lime added per ton of molten steel can be 8-12kg, and the amount of fluorite added can be 1-2kg. Later, as the reaction proceeds and the slag-making materials are consumed, slag-making and new slag must be continuously added to ensure the progress of the reaction process.
Diffusion deoxidizers usually use carbon powder and ferrosilicon powder. During the reduction period, the first stage is deoxidation with carbon powder, and the deoxidation reaction is:
C + (FeO) = CO↑ + [Fe]
The second stage is deoxidation with ferrosilicon powder, and the deoxidation reaction is:
Si + 2[FeO] = (SiO₂) + 2[Fe]
Carbon powder is used for diffusion deoxidation. Its deoxidation product is CO gas, which will not contaminate the molten steel. At the same time, it can create a reducing atmosphere in the furnace. However, due to the incomplete reaction and slow speed, it can only deoxidize the molten steel to a certain extent. Moreover, adding too much carbon powder in the later stage of reduction can easily increase the carbon content of the molten steel. Therefore, ferrosilicon powder or other strong deoxidizers must be used for further diffusion deoxidation in the later stage of reduction.
Since the density of ferrosilicon powder is between slag and steel, part of the ferrosilicon powder may sink to the slag-steel interface for reaction, and the (SiO₂) produced may contaminate the molten steel. In addition, the high SiO₂ content in the slag will also seriously corrode the furnace lining. Therefore, ferrosilicon powder should not be used too much for deoxidation. After 15-20 minutes of deoxidation with carbon powder, the oxygen content in the molten steel and slag is relatively low when ferrosilicon powder is used for deoxidation, and its negative impact becomes very light.
For general steel grades, carbon powder and ferrosilicon powder can also be mixed and added. For some low-carbon high-alloy steels, ferrosilicon powder, calcium silicon powder, aluminum powder, etc. can be used for deoxidation.
The use of powdered silicon carbide as a diffusion deoxidizer also achieves good results. The following issues should be noted: First, the particle size of silicon carbide should not be too large, and should be between 0 and 3 mm; second, the amount added at one time should not be too much, and the powder should be spread out when added; third, do not stir or add other furnace materials immediately after adding silicon carbide.
The reaction formula for silicon carbide deoxidation is:
SiC + 3[FeO] = CO↑ + (SiO₂) + 3[Fe]
During the deoxidation process during the reduction period, diffusion deoxidation is the key link, and the quality of slag making determines the effect of diffusion deoxidation.
There are two types of reduction slag: white slag and carbide slag. White slag is suitable for smelting steel with low carbon content (C≤0.35%), while carbide slag is suitable for smelting steel with high carbon content (C＞0.35%). When making white slag, do not add too much deoxidizer per batch. When reducing with carbon powder and ferrosilicon powder, the first batch of carbon powder can be added at 1.5-2kg per ton of molten steel, and then the furnace door is closed immediately for reduction. As the reaction proceeds, the reducing property of the slag weakens, and 1-3kg/t of carbon powder or ferrosilicon powder can be added every 5 minutes or so.
When using silicon carbide as a diffusion deoxidizer, the amount added per batch can be 1-2kg/t. It must be added in small amounts and multiple times.
During the reduction process, attention should be paid to forming a good reducing atmosphere in the furnace, and the slag should be adjusted continuously until white slag is formed; frequent stirring and temperature measurement should be carried out to make the temperature and composition uniform. Refining should generally be continued for 15-30 minutes under a good reducing atmosphere and white slag with good fluidity.
The characteristics of good white slag are: observe the slag surface in the furnace to show uniform small bubbles; use a steel rod to dip the slag, the slag layer is uniform and the thickness is about 3-5mm, the surface is white fish roe after cooling, the cross section is white with gray, and it will automatically powderize soon after cooling; sample and analyze the FeO content in the slag, which should be below 1.0%. As the deoxidation reaction proceeds, the (FeO) content in the slag decreases, and the [FeO] in the steel diffuses into the slag, thereby reducing the [FeO] content in the steel and achieving the purpose of deoxidation. [FeO]→(FeO)
When making carbide slag, the amount of deoxidizer added should be larger to enhance the reducibility of the slag. First, add 4-5kg of carbon powder per ton of molten steel, then close the furnace door and increase the current for reduction for 15-20 minutes. Under the high temperature and reducing atmosphere of the arc, a part of CaO is reduced to carbide (CaC₂) by C: (CaO) + 3C → (CaC₂) + CO↑ There are also people who directly add crushed carbide to the slag to make carbide slag.
In addition to the deoxidation effect of C in carbide slag, calcium carbide also plays a deoxidation role: (CaC₂) + (FeO) + CaO) = 2 (CaO) + 2CO↑ + [Fe]
As the reaction proceeds, a batch of slag-forming materials such as lime and carbon powder should be added every 5 to 10 minutes. The amount added depends on the condition of the slag. Generally, 4 to 6 kg/t of lime and 1 to 2 kg/t of carbon powder should be added. The slag is adjusted to form good carbide slag and reduced under carbide slag for 15 to 25 minutes as needed.
After cooling, carbide slag turns gray (weakly conductive carbide slag) or gray-black. It has strong deoxidation ability, but it is easy to adhere to the molten steel and difficult to separate and float. Therefore, the carbide slag must be turned into white slag before tapping. Basic arc furnaces mainly use white slag for refining. White slag is very easy to separate from the molten steel and float, and it rarely contaminates the molten steel, so it is usually necessary to tap steel under the white slag.
The favorable conditions for achieving diffusion deoxidation are reducing furnace gas and slag, high furnace temperature and low slag viscosity. Deoxidation and desulfurization of molten steel are carried out simultaneously, and the principle of desulfurization is similar to that of diffusion deoxidation.
Sulfur in steel exists in the form of iron sulfide (FeS). While deoxidation is in progress, lime (CaO) in the reducing slag plays a role in desulfurization:
(CaO) + (FeS) = (CaS) + (FeO)
CaC₂ in carbide slag also plays a role in desulfurization: CaC₂ + 3 (FeS) + 2 (CaO) → 3 (CaS) + 3 [Fe] + 2CO↑
As the reaction proceeds, the (FeS) content in the slag gradually decreases, and the [FeS] in the steel diffuses into the slag, achieving the purpose of desulfurization:
[FeS] → (FeS)
The favorable conditions for achieving desulfurization are high furnace temperature, reducible and high basicity slag and sufficient slag volume. For effective desulfurization, the slag volume during the reduction period should be around 4%. When the sulfur content in the charge is high, the slag volume can be appropriately increased, and slag flow operation can be performed.
Since the desulfurization reaction is far from reaching equilibrium in the furnace, the “steel slag mixed flushing” operation method during steel tapping can greatly increase the contact area between slag and steel, reducing the sulfur content by 30% – 50%, so the steel tapping process is a key link in desulfurization.
After the chemical composition is adjusted, aluminum can be used for final deoxidation. There are usually two methods for final deoxidation: aluminum insertion method and aluminum flushing method. The aluminum insertion method is to insert the aluminum block into the molten steel with a steel chisel for deoxidation before tapping. The aluminum flushing method is to place the aluminum block on the tapping trough during tapping, and use the molten steel to flush the aluminum into the ladle for deoxidation, or to put the aluminum block into the ladle along the steel flow during tapping. Some also add part of the aluminum into the ladle in advance (the aluminum is melted when the ladle is baked), and mix it with the molten steel for deoxidation during tapping. Of these two methods, the aluminum insertion method has the best effect. The aluminum flushing method is easy to operate, but sometimes the aluminum block is wrapped by the slag and floats to the top of the ladle, which does not play a deoxidation role.
The amount of aluminum added in these two methods is 1-1.5kg per ton of molten steel. The practice of adding aluminum for final deoxidation by wire feeding method has been widely used. This method is to use a wire feeder to feed aluminum wire directly into the ladle at a certain speed for deoxidation after tapping. After adjusting the wire feeding speed, the aluminum wire can just go down to the bottom of the ladle to melt and evenly carry out the deoxidation reaction in the ladle. Using this method, the amount of aluminum added can be reduced to about 0.5kg per ton of molten steel, and the residual aluminum content in the steel is relatively high, but care should be taken not to lower the temperature of the molten steel too much, otherwise the reaction will be inadequate.
Section 7: Alloying of Molten Steel
Electric arc furnaces can smelt many kinds of steel. The alloying and composition control of molten steel runs through all links from batching to steelmaking. When adjusting the composition, the recovery rate of alloying elements should be increased as much as possible, the burning loss should be reduced, and the chemical composition of the finished steel should all meet the requirements of the standards.
The chemical stability of the alloying elements plays a decisive role in the alloying process, followed by the melting point, density, and addition amount of the alloy.
Elements such as nickel, cobalt, and copper will not be oxidized during the steelmaking process, so they can be added during charging, or during the melting period or oxidation period. Tungsten and molybdenum have a lower affinity with oxygen, and have a high density and melting point, so they should be added earlier. When adding during the reduction period, the block size should be small, and it should be added to the high temperature zone and stirred more. Silicon has a stronger affinity with oxygen at high temperatures, so ferrosilicon is generally added 5-10 minutes before steelmaking. However, practice has shown that adding ferrosilicon in the early stage of reduction (after adding ferromanganese) can quickly produce white slag and make the reducibility of the slag more stable, which is also conducive to the control of silicon composition.
Aluminum, titanium, and boron are very easy to oxidize elements, so the molten steel must be well deoxidized before adding, the slag basicity is appropriate, and the reducing atmosphere in the furnace is strong. Before adding boron, you must first add an appropriate amount of aluminum and titanium to the molten steel to deoxidize and fix nitrogen to increase the recovery rate of boron. The method of adding is to wrap the ferroboron with aluminum foil, quickly insert it 2-3 minutes before tapping or add it to the bag during tapping.
