Electric furnace steelmaking mainly uses arc heat, and in the arc action area, the temperature is as high as 4000 °C. The smelting process is generally divided into melting period, oxidation period and reduction period. In the furnace, not only an oxidizing atmosphere but also a reducing atmosphere can be formed, so the efficiency of dephosphorization and desulfurization is very high.
Keyword: electric arc furnace steel
Operation method of electric furnace smelting
The operation methods are generally divided according to the characteristics of the slagging process. There are single slag oxidation method, single slag reduction method, double slag reduction method and double slag oxidation method, and the latter two are generally used at present.
Double slag reduction method
Also known as the return oxygen blowing method, it is characterized in that there is a short oxidation period (≤10min) in the smelting process, which produces oxidized slag and reduced slag, which can decarburize by blowing oxygen, and remove gas and inclusions. However, because this method is difficult to dephosphorize, it is required that the furnace charge should be composed of returned scrap steel containing low phosphorus.
Due to its small decarburization amount and short oxidation period, it can not only remove harmful elements, but also recover a large amount of alloy elements in scrap steel. Therefore, this method is suitable for smelting stainless steel, high-speed steel and other steels with high Cr and W content.
Double slag oxidation method
Also known as the oxidation method, its characteristic is that the smelting process has a normal oxidation period, which can decarburize, dephosphorize, degas, and inclusions, and has no special requirements for the furnace charge; there is also a reduction period, which can smelt high-quality steel.
At present, almost all kinds of steel can be smelted by oxidation method. The following mainly introduces the oxidation method smelting process.
Electric furnace smelting process
The traditional oxidation smelting process is the basis of the electric arc furnace steel
steelmaking method.
Its operation process is divided into six stages: furnace repair, charging, melting, oxidation, reduction and tapping. Because it is mainly composed of melting, oxidation, and reduction stages, it is commonly known as the third stage.
electric arc furnace Repair
1) “Three elements” that affect the lining life
The type, nature and quality of the lining;
High temperature arc radiation and chemical etching of slag;
Oxygen blowing operation and impact of slag, steel and other mechanical scours and charging.
2) electric arc furnace repair part
The working conditions of each part of the furnace lining are different, and the damage is also different. The main parts of the lining damage are as follows:
Furnace wall slag line: It is seriously damaged by high temperature arc radiation, chemical erosion and mechanical erosion of slag and steel, and oxygen blowing operation;
Slag line hot spot: especially the 2# hot spot is severely eroded by the influence of high arc power and partial arc, etc. The degree of damage at this point often becomes the basis for furnace replacement;
Near the tap hole: it is also easy to be thinned due to the erosion of slag steel;
Both sides of the furnace door: often affected by rapid cooling and rapid heating, slag washing and collision between operation and tools, etc., are also seriously damaged.
3) Furnace repair method )
Furnace repairing methods are divided into manual repairing and mechanical gunning repairing, and can be divided into dry repairing and wet repairing according to the different mixing methods of selected materials.
At present, mechanical gunning is mostly used on large-scale electric furnaces. Mechanical gunning equipment includes furnace door gunning machines and furnace rotary furnace repairing machines. Mechanical gunning repairs furnaces with fast speed and good effect.
The principle of repairing the furnace is: high temperature, quick repair and thin repair.
4) Furnace repair material
Charge
At present, furnace top material tanks (or material baskets, material baskets) are widely used for charging, and the material of each furnace of steel is added in 1 to 3 times. The quality of the charge affects the lining life, smelting time, power consumption, electrode consumption and burning loss of alloying elements. Therefore, reasonable charging is required, which mainly depends on whether the distribution of the charge in the material tank is reasonable or not.
On-site material distribution (charging) experience: the bottom is dense, the top is loose, the middle is high, the surrounding is low, there is no large material at the furnace door, the well is passed quickly, no bridge is built, the melting is fast, and the efficiency is high.
Melting period
The melting period of the traditional smelting process accounts for 50% to 70% of the entire smelting time, and the power consumption accounts for 70% to 80%. Therefore, the length of the melting period affects productivity and power consumption, and the operation of the melting period affects the smoothness of the oxidation and reduction periods.
The main tasks of the melting period
Melt the blocky solid charge quickly and heat it to the oxidation temperature; make slagging in advance, remove phosphorus early, and reduce the gas absorption and volatilization of molten steel.
Operation during the melting period
Reasonable power supply, timely oxygen blowing, and early slagging.
1) Furnace charge melting process and power supply
After charging, it can be energized and melted. Furnace charge melting process diagram can be basically divided into four stages (periods), namely arc ignition, well penetration, main melting and melting end heating.
Point (starting) arc period
The arcing period is from when the power is turned on to when the electrode end drops to a depth of d.
In this period, the current is unstable, and the electric arc burns and radiates near the furnace top. The higher the secondary voltage, the longer the arc, the stronger the radiation on the furnace top, and the more heat loss.
In order to protect the furnace roof, some light and thin materials are placed on the upper part of the furnace so that the electrodes can quickly enter the material and reduce the radiation of the arc to the furnace roof.
Lower voltage and lower current are used for power supply.
Well penetration period
The well-piercing period is from the end of the arc to the drop of the electrode end to the bottom of the furnace.
In this period, although the arc was covered by the charge, the arc combustion was unstable due to the continuous occurrence of material collapse.
Pay attention to protecting the bottom of the furnace. The method is: add lime as the bottom before feeding, and arrange large and heavy scrap steel and a reasonable furnace type in the middle of the furnace.
The power supply adopts a larger secondary voltage and a larger current to increase the diameter of the well and the speed of the well.
Main melting period
The main melting phase begins when the electrode starts to rise after falling to the bottom of the furnace. With the continuous melting of the charge, the electrode gradually rises until the charge is basically melted, only a small amount of charge exists near the furnace slope and slag line, and the main melting period ends when the arc begins to be exposed.
During the main melting period, because the arc is buried in the charge, the arc is stable, the thermal efficiency is high, and the heat transfer conditions are good, so the power supply should be powered with the maximum power, that is, the highest voltage and the highest current.
The main melting period accounts for more than 70% of the entire melting period.
Melting end heating period
The arc begins to be exposed to the furnace wall until the furnace charge is completely melted, which is the melting end heating period.
At this stage, due to the exposure of the furnace wall, especially the exposure of the hot spot area of the furnace wall, it is strongly radiated by the arc.
Care should be taken to protect the furnace wall, that is, foam slag should be made in advance for submerged arc operation, otherwise low voltage and high current should be used for power supply.
2) Timely oxygen blowing and element oxidation
During the melting period, oxygen is blown to help the melting, and the initial stage is mainly cutting. When the charge is basically melted to form a molten pool, oxygen is mainly blown into the molten steel.
Oxygen blowing is the use of elemental oxidation heat to accelerate the melting of the charge. It is most appropriate to start blowing oxygen when the solid material turns red (~900°C). Blowing oxygen is too early to waste oxygen, and too late to prolong the melting time.
In general, Si, Al, Ti, V, etc. in the steel during the melting period are almost all oxidized, and Mn, P are oxidized by 40% to 50%. This is related to the alkalinity and oxidation of slag; while C is oxidized by 10% to 30% and Fe is oxidized by 2% to 3% when oxygen is blown.
3) Early slagging
Use 2% to 3% lime to pad the bottom of the furnace or use the steel and slag left by the forehearth to realize slagging in advance. In this way, when the molten pool is formed, it is covered with slag, which makes the arc stable, which is beneficial to the melting and heating of the charge, and can reduce heat loss and prevent gas absorption and metal volatilization.
Since the initial slag has a certain oxidation and high alkalinity, part of the phosphorus can be removed; when the phosphorus is high, the operation of automatic slag flow and new slag replacement can be adopted, and the dephosphorization effect is better, which creates conditions for the oxidation period.
Why? Dephosphorization reaction and dephosphorization conditions:
Dephosphorization reaction:
2［P］＋5(FeO)＋4(CaO)＝(4CaO·P2O5)＋5［Fe］, △H＜0
Analysis: The reaction is carried out on the slag-steel interface, which is an exothermic reaction.
Conditions for the dephosphorylation reaction:
- lHigh alkalinity creates high alkalinity slag and increases calcium oxide in the slag;
- lHigh oxidizing property, making highly oxidizing slag, increasing iron oxide in slag;
- l Low temperature, pay close attention to the melting period;
- lLarge amount of slag (appropriately large), use flow slag to make new slag.
Electric furnace dephosphorization operation:
The actual dephosphorization operation of electric furnace is to make high alkalinity and high oxidizing slag in advance, and adopt the operation of flowing slag and making new slag, so as to basically complete the dephosphorization task during the melting period.
4) Measures to shorten the melting period
- Reduce thermal shutdown time, such as improving mechanization and automation, reducing the number and time of loading materials, etc.;
- Strengthening the use of oxygen, such as blowing oxygen to boost melting, oxygen-combustion boosting, to realize synchronous melting of scrap steel and increase the melting speed of scrap steel;
- Increase the input power of the transformer to speed up the melting speed of scrap steel;
- Scrap steel preheating, use the high temperature waste gas produced in the electric furnace smelting process to preheat scrap steel, etc.
Oxidation period
The oxidation period is the main process of oxidation smelting, which can remove phosphorus, gas and inclusions in steel.
When the scrap steel is completely melted and reaches the oxidation temperature, more than 70% to 80% of phosphorus is removed and enters the oxidation period. In order to ensure the progress of the metallurgical reaction, the oxidation start temperature is 50-80°C higher than the melting point of molten steel.
The main task of the oxidation phase
Continue to dephosphorize to the requirement – dephosphorization;
Decarburization to the lower specification limit – decarburization;
Remove gas, remove inclusions – two to go;
Raise the temperature of molten steel—heating.
Oxidation period operation
1) Slagging and dephosphorization
In the traditional smelting method, dephosphorization continues during the oxidation period, as can be seen from the dephosphorization reaction formula. In the early stage of oxidation (low temperature), it is feasible to make slag with high oxidation, high alkalinity and good fluidity, and to flow slag in time and replace it with new slag to achieve rapid dephosphorization.
2［P］＋5(FeO)＋4(CaO)＝(4CaO·P2O5)＋5［Fe］ △H＜0
2) Oxidation and decarburization
In recent years, the practice of strengthening oxygen use has shown that. Unless the phosphorus content in the steel is particularly high and crushed ore (or iron oxide scale) is used to make high-oxidizing slag, oxygen blowing oxidation is used, especially when the dephosphorization task is not heavy, the carbon content in steel is reduced by strengthening oxygen blowing oxidation steel .
Carbon reduction (de)carbonization is one of the important tasks of electric furnace steelmaking, but the role of decarburization reaction is not only for carbon reduction, but also the role of decarburization reaction?
- Reduce the carbon in the steel and use the method of carbon-oxygen reaction (C+O2→CO) to achieve the following goals;
- Stirring the molten pool, accelerating the reaction, uniform composition and temperature;
- Remove gas and inclusions in steel.
In fact, the electric furnace achieves the purpose of accelerating the reaction, uniform composition and temperature, and removing gas and inclusions through high carbon distribution and oxygen blowing decarburization.
Decarburization reaction and decarburization conditions:
[C]＋[O] =CO↑ , △HCO＝－0.24kcal=－22kJ＜0
Analysis: The reaction is carried out in steel and is an exothermic reaction.
- High oxidative property, strengthen oxygen supply, make [%O] actual > [%O] balance.
- High temperature, accelerates the diffusion between C-O (because the decarburization reaction is a “weak” exothermic reaction, the temperature has little effect (thermodynamic temperature), but from a kinetic point of view, the increase in temperature improves the kinetic conditions and accelerates the C-O Diffusion between them, so high temperature is beneficial to decarburization).
- Reduce PCO, such as filling inert gas (AOD), pumping and vacuum treatment (VD, VOD), etc. are all beneficial to the decarburization reaction.
3) Removal of gas and inclusions
At what stage and how is the removal of gas and inclusions in the electric furnace steelmaking process?
Degassing and de-inclusions are carried out in the decarburization stage of the electric furnace oxidation period. It uses the carbon-oxygen reaction and the rising of carbon monoxide bubbles to make the molten pool boil violently, promote the removal of gas and inclusions, and uniform composition and temperature.
The mechanism of removing gas and impurities?
- C-O reaction produces CO to make the molten pool boil;
- CO bubbles for N2, H2, etc., PN2, PH2 partial pressure is zero, N2, H2 are very easy to merge into CO bubbles, grow up and eliminate;
- C-O reaction, easy to make oxides such as 2FeO·SiO2, 2FeO·Al2O3 and 2FeO·TiO2 grow up and float;
- The process of rising CO2 adheres oxide inclusions and floats up to remove them.
For this reason, the decarburization reaction speed must be well controlled to ensure that the molten pool has a certain intense boiling time.
4) Temperature control during the oxidation period
The temperature control in the oxidation period should take into account the needs of both dephosphorization and decarburization, and dephosphorization should be given priority. In the early stage of oxidation, the heating rate should be properly controlled, and the temperature should be raised after the phosphorus reaches the requirement.
Generally, the temperature at the end of oxidation is required to be slightly higher than the tapping temperature by 20-30°C to compensate for the cooling of molten steel caused by slag removal, new slag formation and alloy addition, as shown in the figure.
When the temperature, phosphorus, carbon, etc. of the molten steel meet the requirements, the oxidation slag is removed and thin slag is made to enter the reduction period.
Reduction period
In the traditional electric arc furnace smelting process, the existence of reduction period shows the characteristics of electric furnace steelmaking. The main difference of the modern electric furnace smelting process is that the reduction period is moved outside the furnace.
Main tasks in the recovery period
Deoxidation to the requirement – deoxidation;
Desulfurization to a certain value – desulfurization;
Adjusting composition – alloying;
Adjust the temperature – adjust the temperature.
Among them: deoxidation is the core, temperature is the condition, and slagging is the guarantee.
1) Deoxidation method
There are precipitation deoxidation, diffusion deoxidation and comprehensive deoxidation.
Electric furnace steelmaking adopts a comprehensive deoxidation method that is alternately carried out by precipitation deoxidation and diffusion deoxidation, that is, at the end of oxidation, precipitation deoxidation is used before reduction-pre-deoxidation, diffusion deoxidation is used during the reduction period, and precipitation deoxidation-final deoxidation is used before tapping.
Among them, the precipitation deoxidation reaction formula is:
x[M] block +y[O]=(MxOy) ↑
Precipitation deoxidation is to add block deoxidizer into molten steel to deoxidize molten steel directly.
Commonly used deoxidizers are: Fe-Mn, Fe-Si, Al, V and composite deoxidizers Mn-Si, Ca-Si, etc., and the deoxidation capacity increases in turn.
The characteristics of this method: simple operation, rapid deoxidation; deoxidation products are easy to stay in the steel (when the floating time is short).
Diffusion deoxygenation reaction formula
x(M) powder+y(FeO)=(MxOy)+y[Fe]
[FeO] → (FeO)
- Diffusion deoxidation is to add powdery deoxidizer to slag to deoxidize the slag, and the oxygen in the steel diffuses into the slag to indirectly remove the oxygen in the steel.
- Powder deoxidizers include: C, Fe-Si, Ca-Si, CaC, Al powder, etc.
- Compared with the precipitation deoxidation method, the characteristics of the diffusion deoxidation method: the reaction is carried out in the slag, the product does not enter the steel, and the steel quality is good; the deoxidation speed is slow and the time is long. This method is often used after the thin slag is formed during the reduction period of the electric furnace.
2) Desulfurization reaction and desulfurization conditions
[FeS]+(CaO)=(CaS)+(FeO), △H＞0
Analysis: This reaction is carried out on the slag-steel interface, which is an endothermic reaction.
- High alkalinity creates high alkalinity slag and increases calcium oxide in the slag;
- Strong reducing gas (or low oxidation), creating reducing slag and reducing iron oxide in slag;
- High temperature, at the same time high temperature improves the fluidity of slag;
- Large amount of slag (properly large), fully stir to increase slag-steel contact.
Due to the strong reducing gas in the reduction period of the electric furnace or the refining period of the refining furnace, (FeO) < 0.5% ~ 1.0%, which is particularly beneficial to desulfurization.
Reduction operation – deoxygenation operation
The comprehensive deoxidation method is commonly used in electric furnaces, and the reduction operation takes deoxidation as the core.
1) When the T, P, and C of the molten steel meet the requirements, the slag removal is >95%;
2) Add Fe-Mn, Fe-Si blocks, etc. for pre-deoxidation (precipitation deoxidation);
3) Add lime, fluorite, and fire bricks to make thin slag;
4) Reduction, add C powder, Fe-Si powder and other deoxidation (diffusion deoxidation), divide into 3-5 batches, 7-10min/batch;
5) Stirring, sampling and temperature measurement;
6) Adjust composition – alloying;
7) Add Al or Ca-Si blocks for final deoxidation (precipitation deoxidation);
8) Tapping
Temperature control
Considering the temperature loss from tapping to pouring, the tapping temperature should be 100-140°C higher than the melting point of steel. Since the molten steel temperature is controlled to be 20-30°C higher than the tapping temperature at the end of the oxidation period, the temperature control in the reduction period after slag removal is generally a heat preservation process. If the temperature rises sharply during the reduction period, it will cause severe gas absorption of molten steel, aggravated erosion of the furnace lining by high-temperature arcs, and local overheating of molten steel. For this reason, the “post-heating” operation during the reduction period should be avoided.
Tapping
In the traditional electric furnace smelting process, after the molten steel is oxidized and reduced, the steel can be tapped when the chemical composition is qualified, the temperature meets the requirements, the molten steel is well deoxidized, and the alkalinity and fluidity of the slag are suitable.
Because the slag-steel contact in the tapping process can further deoxidize and desulfurize, it is required to adopt the tapping method of “big mouth, deep drawing, and slag-steel mixing”.
The operation of the third phase of the traditional electric furnace smelting process integrates melting, refining and alloying in one furnace, including the melting period, oxidation period and reduction period. In the furnace, the melting of scrap steel, the heating of molten steel, the dephosphorization of molten steel Decarburization, degassing, removal of inclusions, deoxidation, desulfurization of molten steel, and adjustment of temperature and composition are also required, so the smelting cycle is very long.
This is not only difficult to ensure the increasingly stringent quality requirements for steel, but also limits the increase in productivity of electric furnaces.
Alloying of molten steel
The operation of adjusting the alloy composition of molten steel in the steelmaking process is called alloying, which includes the alloying of molten steel in the electric furnace process and the fine-tuning of the alloy composition of molten steel in the later stage of the refining process.
The alloying of the traditional electric furnace smelting process is generally pre-alloyed at the end of oxidation and at the beginning of reduction, and fine-tuned the alloy composition at the end of reduction, before tapping or during tapping.
In modern electric furnace steelmaking, the alloying is generally completed in the ladle during the tapping process, and the alloying in the ladle is called pre-alloying during tapping, and the precise alloy composition adjustment is finally completed in the refining furnace.
Alloying operation
It mainly refers to the time, quantity and method of adding the alloy.
Alloy addition time
The general principle is: if the melting point is high, elements that are not easily oxidized can be added early; if the melting point is low, elements that are easily oxidized can be added later.
Specific principles of alloying operation:
A) The principle of post-addition of easily oxidizable elements:
- Elements that are not easily oxidized can be added during charging, oxidation or reduction, such as Ni, Co, Mo, W, etc.;
- Elements that are more easily oxidized are generally added at the initial stage of reduction, such as P, Cr, Mn, etc.; elements that are easily oxidized are generally added at the end of reduction, that is, when molten steel and slag are well deoxidized, such as V, Nb, Si , Ti, Al, B, rare earth elements (La, Ce, etc.).
In order to increase the yield of easily oxidized elements, many factories add rare earth elements, ferro-titanium, etc. during the tapping process, and sometimes rare earth elements are added during the pouring process.
B) The principle of strengthening stirring for large proportion:
For iron alloys with high melting point and high specific gravity, stirring should be strengthened after adding. For example, ferro-tungsten has a high density and a high melting point, and if it sinks to the bottom of the furnace, its lumpiness should be smaller.
C) Cheap first add principle:
Under permitting conditions, use cheap high-carbon iron alloys first, and then consider using medium-carbon iron alloys or low-carbon iron alloys.
D) Precious lower control limit principle:
Precious iron alloys should be controlled in the middle and lower limits as far as possible to reduce the cost of steel. For example, when smelting W18Cr4V (W 17%~19%), every 1% less W can save 15kg/t ferro-tungsten.
In addition, the deoxidation operation and the alloying operation cannot be completely separated. Generally speaking, the deoxidizing elements are added first, and the alloying elements are added later; the deoxidizing ability is relatively strong, and the more expensive alloy elements should be added when the molten steel is well deoxidized.
