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Traditional Electric arc furnace steelmaking smelting process

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Electric furnace smelting operation method

Double slag reduction method

The characteristic of the double slag reduction method (return oxygen blowing method) is that there is a short oxidation period (≤15min) during the smelting process, which creates oxidized slag and reduced slag, and can blow oxygen to remove carbon, gas, and inclusions. However, since it is difficult to remove phosphorus by this method, it is required that the charge should be composed of returned scrap steel containing low phosphorus.

Due to the small decarburization amount and short oxidation period, the double slag reduction method can not only remove harmful elements, but also recover a large amount of alloying elements. This method is suitable for smelting stainless steel, high-speed steel and other steel types containing high Cr and W.

Double slag oxidation method

The characteristic of the double slag oxidation method (oxidation method) is that the smelting process has an oxidation period, which can remove carbon, phosphorus, gas, inclusions and other impurities. There are no special requirements for the charge. The smelting process has both an oxidation period and a reduction period, which is conducive to the improvement of steel quality. At present, almost all steel types can be smelted by oxidation method.

Traditional electric arc furnace steelmaking smelting process

1. Repair furnace

1) Main factors affecting furnace lining life:

  • The type, nature and quality of the furnace lining (including production, knotting and masonry quality);
  • High temperature arc radiation and chemical erosion of slag;
  • Oxygen blowing, mechanical flushing of molten steel, slag, etc. and impact of charging.

In order to improve the life of furnace lining. In addition to selecting high-quality refractory materials and advanced furnace construction techniques, maintenance must also be strengthened, that is, after each furnace of steel is produced, the furnace must be repaired. In special circumstances, special methods must be used for repairs and padding.

2) Furnace repair parts:

The working conditions of various parts of the furnace lining are different, and the damage conditions are also different. The main part of the furnace lining damage is the furnace wall slag line. The slag line is seriously damaged by radiation from high-temperature arcs, chemical erosion and mechanical erosion of slag steel, and smelting operations.

The area near the tap hole is easily thinned by the erosion of slag steel, and both sides of the furnace door are often subject to rapid cooling and rapid heating, erosion by flowing slag, and collisions between operations and tools. The damage is also serious.

Therefore, after tapping, the electric furnace generally needs to repair the slag line, the tapping port and the vicinity of the furnace door. Regardless of whether gunning or pouring is carried out, these parts should be repaired.

3) Principles of furnace repair

The principles of furnace repairing are: high temperature, fast repairing, and thin repairing. Furnace repairing is to spray the repairing material into the damaged area of the furnace lining, and use the residual heat in the furnace to sinter the newly repaired refractory material and the original furnace lining into a whole at high temperature. This sintering requires very high temperatures to complete. . After the electric furnace taps steel, the surface temperature of the furnace lining drops quickly, so you should seize the time to repair it while it is hot. The purpose of thin patching is to ensure good sintering of refractory materials. Experience tells us that the thickness of the new patching should not be greater than 30mm at a time. When thicker patching is needed, it should be done in layers and multiple times.

4) Furnace repair method

Furnace repairing methods can be divided into manual repairing and mechanical gunning repairing. According to the mixing method of the selected materials, it can be divided into dry repairing and wet repairing. Manual mending has poor quality, high labor intensity, long operation time, and high consumption of refractory materials, so it is only suitable for small furnaces. At present, mechanical gunning is mostly used in large electric furnaces. There are many types of mechanical gunning equipment, including furnace door gunning machines, such as Fushun Iron and Steel Co., Ltd.’s 50t furnace, and in-furnace rotary gunning machines, such as Shanghai Wuhan Iron and Steel Co., Ltd. Mechanical gunning and repairing of furnaces is fast and effective.

5) Furnace repair materials

The filling materials for manual repairing of alkaline electric furnaces are magnesia, dolomite or partially recycled magnesia. The binders used are: brine or water glass is used for wet patching; asphalt powder is generally mixed for dry patching.

Mechanical gunning materials mainly use magnesia, dolomite or a mixture of the two, and can also be mixed with binders such as phosphate or silicate.

2. Loading

The furnace top basket is used for charging, and the charge for each furnace of steel is added in 1 to 3 times. The quality of the charging affects the life of the furnace lining, smelting time, power consumption, electrode consumption and burning loss of alloy elements. Therefore, reasonable charging is required, and reasonable charging mainly depends on whether the distribution of the furnace charge in the basket is reasonable or not.

Reasonable order of fabrics

    a. First, place some small pieces of material at the bottom of the material basket to protect the chain plate or clam leaf plate of the material basket, slow down the impact of heavy materials on the furnace bottom, protect the furnace bottom, and form a molten pool as early as possible;

    b On top of the small pieces of material, put a block of aniseed or refractory material in the center of the basket. And fill small pieces of material to make it smooth, dense and without large gaps. It is not only conducive to conduction, but also eliminates material bridges and prevents the electrode from breaking when the material collapses, that is, protecting the electrode;

    c. The remaining medium and small pieces of materials are placed on and around the aniseed or refractory materials;

d. Finally, load a small piece of light material into the upper part of the material basket to facilitate arc starting, stabilize the current and reduce the radiation damage of the arc to the furnace cover, that is, to protect the furnace top.

Furnace materials that are not easily conductive such as slag steel and soup steel should be installed away from the electrodes to avoid affecting conductivity;

Pig iron should not be installed near the furnace door or on the furnace slope, but should be installed around aniseed or refractory materials to use its carburizing effect to lower the melting point of aniseed or refractory materials, thus accelerating melting.

All ferroalloys loaded with the charge should be installed in different locations according to their different physical and chemical properties in order to ensure the yield of elements. For example, ferrotungsten and ferromolybdenum have high melting points and are not easily oxidized. They can be installed in high-temperature areas, but do not install them under the electrodes;

Ferrochrome, nickel plate, ferromanganese, etc. should be installed away from high temperature areas to reduce their volatilization losses.

Fabric (loading) experience

Dense at the bottom, loose at the top, high in the middle, low on all sides, fast to penetrate the well, no bridges, no aniseed at the furnace door, and good advance fluxing effect.

3. Melting period

The melting period of the traditional process accounts for 50% to 70% of the entire smelting time, and the power consumption accounts for 60% to 80%. Therefore, the length of the melting period affects productivity and power consumption, and the operation of the melting period affects the success of the oxidation and reduction periods.

Main tasks during the melting period

Rapidly melt the massive solid charge and heat it to the oxidation temperature;

Make slag in advance and remove phosphorus in advance;

Reduce gas inhalation and volatilization of molten steel.

Operations during the melting period

1) Charge melting process and power supply

2) Timely oxygen blowing and elemental oxidation

Oxygen blowing uses elemental oxidation heat to heat and melt the charge. It is most appropriate to start blowing oxygen when the solid material turns red (~900°C). Blowing oxygen too early wastes oxygen, and blowing too late increases the melting time. During the melting period, oxygen is blown to assist, with cutting being the main focus in the initial stage. When the charge is basically melted to form a molten pool, oxygen is blown into the molten steel.

Under normal circumstances, Si, Al, Ti, V, etc. in the steel during the melting period are almost completely oxidized. Mn is oxidized by 40% to 60%, and P is oxidized by 40% to 50%, which is related to the alkalinity and oxidizing properties of the slag; while during oxygen blowing, C is oxidized by 10% to 30%, and Fe is oxidized by 2% to 3%.

3) Make slag in advance

In order to make slag in advance, use 2% to 3% lime to pad the furnace bottom (except for steel retention operations, conductive furnace bottom, etc.), so that when the molten pool is formed, it is covered with slag, which stabilizes the arc and is conducive to the melting and heating of the charge. , and can reduce heat loss and prevent suction and metal volatilization. Since the initial slag has certain oxidizing properties and high alkalinity, part of the phosphorus can be removed; when the phosphorus is high, automatic slag flow and replacement of new slag can be adopted to achieve better dephosphorization effects and create conditions for the oxidation period.

The dephosphorization task is mainly completed during the melting period. By adding about 2% to 3% to the furnace bottom before feeding, the slag with high alkalinity and high oxidation is made in advance, and the operation of flowing slag to create new slag is basically completed during the melting period.

Measures to shorten the melting period

Reduce thermal downtime

Increase transformer input power

Strengthen the use of oxygen, such as blowing oxygen fluxing, oxygen-burning fluxing

Yu Gang, slag return

Scrap steel preheating, etc.

electric arc furnace steel making process

4. Oxidation period

To remove phosphorus, gases and inclusions from steel, oxidation must be used. The oxidation period is the main process of oxidation smelting. In the traditional smelting process, when the scrap charge is completely melted and reaches the oxidation temperature, more than 70% of the phosphorus is removed and the oxidation period begins. In order to ensure the progress of the metallurgical reaction, the oxidation starting temperature should be 50 to 80°C higher than the melting point of the molten steel.

The main tasks of the oxidation period

When the dephosphorization task is heavy, continue dephosphorization to the requirement (<0.02%)

Decarburize to lower specification limit

Degassing and inclusion removal (using C-O reaction)

Increase molten steel temperature

Slagging and dephosphorization

During the oxidation period, slag with high oxidation, high alkalinity and good fluidity must be produced. And promptly remove the slag, replace it with new slag, and seize the opportunity to quickly dephosphorize in the early stage of oxidation (low temperature).

Oxidation and decarburization

According to the different sources of oxygen in the molten pool, the oxidation period operations are divided into three types: ore oxidation, oxygen blowing oxidation and ore oxygen comprehensive oxidation. In recent years, the practice of intensifying the use of oxygen has shown that in addition to the extremely high phosphorus content in steel, comprehensive oxidation of mineral oxygen is used. Oxygen blowing is used for oxidation. Especially when the dephosphorization task is not heavy, the carbon content in the steel is reduced by strengthening oxygen blowing to oxidize the steel liquid.

Removal of gases and inclusions

Degassing and inclusion removal of molten steel in electric furnace steelmaking are carried out during the oxidation period. It relies on the carbon-oxygen reaction and the floating of carbon monoxide bubbles to cause intense boiling in the molten pool, promote the removal of gases and inclusions, and even out the composition and temperature. For this reason, the decarburization reaction speed must be controlled to ensure that the molten pool has a certain period of intense boiling.

Temperature control during oxidation period

The temperature control during the oxidation period should take into account the needs of both P removal and C removal, and give priority to P removal. In the early stage of oxidation, the temperature rise rate should be appropriately controlled, and the temperature should be raised after P reaches the requirement. Generally, the temperature at the end of oxidation is required to be slightly higher than the tapping temperature by 20~30℃.

Consider two points: 1) Slag removal, making new slag and adding alloy will cool down the molten steel; 2) The molten steel is not allowed to heat up during the reduction period. Otherwise, the molten steel under the arc will be overheated, and the high-current arc light reflection will damage the furnace lining and the molten steel will absorb air (according to data, the temperature at the end of the oxidation stage should be 120 to 130°C higher than the liquidus line).

When the temperature, phosphorus, carbon, etc. of the molten steel meet the requirements, the oxidized slag is removed and the thin slag is created to enter the reduction period.

5. Recovery period

In the traditional electric furnace steelmaking process, the existence of the reduction period shows the characteristics of electric furnace steelmaking.

Main tasks during the recovery period

Deoxidize to requirements (deoxidize to 30~80×10-4%);

Desulfurization to a certain value;

Adjust the composition of molten steel and perform alloying;

Adjust the temperature of molten steel.

Among them: deoxidation is the core, temperature is the condition, and slagging is the guarantee.

Deoxygenation operation

When the T, P, and C of the molten steel meet the requirements, the slag removal is >95%

Add Fe-Mn, Fe-Si blocks, etc. to pre-deoxidize (precipitation deoxidation)

Add lime, fluorite and fire bricks to create thin slag

After the thin slag is formed, it is reduced, and C powder, Fe-Si powder, etc. are added for deoxidation (diffusion deoxidation), divided into 3 to 5 batches, 7 to 10 minutes/batch (the reason for the long reduction period of the third phase of steelmaking)

Stirring, sampling, temperature measurement;

Adjust the composition—alloying (alloying calculation will be discussed separately later);

Add Al or Ca-Si block for final deoxidation (precipitation deoxidation);

Tapping steel.

temperature control

Considering the temperature loss from tapping to casting, the tapping temperature should be 100 to 140°C higher than the melting point of steel.

Since the temperature of the molten steel is not controlled to be 20-30°C higher than the tapping temperature during the oxidation period, the temperature control during the reduction period after slag removal is actually a heat preservation process. If the temperature rises significantly during the reduction period, firstly, the molten steel will seriously absorb air; secondly, the high-temperature arc will aggravate the erosion of the furnace lining; thirdly, the molten steel will be overheated locally. For this reason, heating operations during the reduction period should be avoided.

Tapping steel

In the traditional electric furnace steelmaking smelting process, after the molten steel has been oxidized and reduced, 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 appropriate. Since the steel slag contact during the tapping process can further deoxidize and desulfurize, the tapping method of “large mouth, deep drawing, and steel slag mixing” is required.

The third phase of the smelting process of traditional electric furnaces 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, dephosphorization, decarburization, degassing, etc. of the molten steel must be completed. Remove inclusions and raise temperature, deoxidize, desulfurize, alloy and adjust temperature and composition of molten steel.

EAF magnesia carbon brick, EAF refractory

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As professional one-stop solution provider, LIAONING MINERAL & METALLURGY GROUP CO., LTD(LMM GROUP) Established in 2007, and focus on engineering research & design, production & delivery, technology transfer, installation & commissioning, construction & building, operation & management for iron, steel & metallurgical industries globally. 

Our product  have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time.

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