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Modern electric arc furnace steelmaking process

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Modern electric arc furnace steelmaking process only retains steel-chemical equipment for melting, heating and necessary refining functions (dephosphorization, decarburization). And those process operations that require only lower power are transferred to the ladle refining furnace. Try to advance dephosphorization and even partial decarburization to the melting period as much as possible. In the oxidation refining and heating period after melting, only carbon control is carried out and the melting of iron alloys that are easier to oxidize and have larger amounts of additions that are not suitable for addition during the feeding period is particularly beneficial to shortening the smelting cycle, reducing consumption, and improving productivity.

The electric furnace adopts the operation of retaining steel and slag. There is a ready-made molten pool at the beginning of melting, supplemented by enhanced oxygen blowing and bottom blowing stirring, which provides good conditions for metallurgical reactions in advance.

The electric furnace is required to have the shortest melting time, the fastest heating rate and the least auxiliary time (such as repairing the furnace, feeding, replacing electrodes, tapping, etc.) in order to achieve the best economic benefits.

electric arc furnace steel making process

Rapid melting and heating operation

Rapid melting and heating are the most important functions of today’s electric arc furnaces. Generally, the following operations are used to complete the process: power supply at the maximum possible power, oxygen-burning nozzle for melting, oxygen blowing for melting and stirring, bottom blowing for stirring, foaming slag and other enhanced smelting and heating technologies. These are all based on the premise of providing primary molten steel with composition and temperature that meet the requirements for refining outside the furnace.

Dephosphorization operation

Three elements of dephosphorization operation: oxidizing properties of slag, lime content and temperature. As the FeO and CaO in the slag increase and the temperature decreases, the distribution coefficient of phosphorus between the slag and the steel increases significantly.

Main processes adopted in dephosphorization operation

Strengthen oxygen blowing and oxygen-combustion fluxing to improve the oxidation of the primary slag;

Create foamy slag with strong oxidation and high alkalinity in advance, and make full use of the favorable conditions of lower temperature during the melting period to improve the dephosphorization ability of the slag;

Timely release the primary slag with high phosphorus content and replenish new slag to prevent the slag from returning to phosphorus after the temperature rises and during tapping;

Injection operation is used to enhance dephosphorization, that is, oxygen is used to blow lime and fluorite powder directly into the molten pool. The dephosphorization rate can generally reach 80%, and desulfurization can be carried out at the same time, with the desulfurization rate approaching 50%;

Slag-free tapping technology is adopted to strictly control the amount of slag and reduce phosphorus to a minimum after tapping. Generally, the amount of slag can be controlled at 2kg/t. For slag with a P2O5 content of 1%, the amount of phosphorization should not exceed 0.001%.

Decarbonization operation

The electric furnace ingredients adopt a high carbon content, and its main purposes are:

When oxygen is blown to assist during the melting period, carbon oxidizes before iron, thereby reducing iron burning loss;

Carburization can lower the melting point of scrap steel and accelerate melting;

The carbon-oxygen reaction causes molten pool agitation, promotes the slag-steel reaction, and is conducive to early dephosphorization;

During the refining heating period, the active carbon-oxygen reaction expands the slag-steel interface, which is conducive to further dephosphorization, the homogenization of the composition and temperature of the molten steel, and the floating of gases and inclusions;

The active carbon-oxygen reaction contributes to the formation of foam slag, improves heat transfer efficiency, and accelerates the heating process.

The amount of carbon is closely related to the form of carbon addition, oxygen blowing method, oxygen supply intensity and the power of the furnace, and needs to be determined according to the actual situation.

Alloying

Modern electric furnace alloying is generally completed in the ladle during the tapping process. Alloys that are not easily oxidized and have higher melting points, such as Ni, W, Mo and other ferrous alloys, can be added to the furnace after being melted. However, when using the steel retention operation, the influence of the steel retention in the previous furnace on the composition of the molten steel in the next furnace should be fully considered. When tapping, the tapping temperature should be adjusted appropriately according to the amount of alloy added, coupled with good ladle baking and heat compensation in the ladle, which can increase the alloy yield without causing low temperatures.

The alloying in the ladle is pre-alloyed during tapping, and the precise alloy composition adjustment is ultimately completed in the refining furnace. In order to smoothly adjust the components during the refining process, it is required that the components to be adjusted during pre-alloying do not exceed the mid-range limit of the specification.

Temperature control

Dephosphorization requires high oxidation and high alkalinity slag, good temperature coordination, which is why it is emphasized that dephosphorization should be done at an early stage. During the oxidative refining period, active carbon-oxygen boiling occurs. A higher temperature (>1550°C) is required; the initial steelmaking liquid in the electric furnace has a certain degree of superheat to compensate for the temperature loss during the tapping process, refining outside the furnace, and transportation of molten steel.

If the tapping temperature is too low, the fluidity of the molten steel will be poor, resulting in short lengths or solidified steel in the ladle after casting; if the tapping temperature is too high, the cleanliness of the steel will deteriorate, the defects of the cast slab (or ingot) will increase, and the consumption will increase. In short, the tapping temperature should be controlled as low as possible on the premise that casting can be successfully completed.

The EBT electric furnace has a low tapping temperature (small tapping temperature drop), which saves energy and reduces phosphorization.

Foam slag operation

Advantages of foam slag operation

Foam slag control

Factors affecting foam slag

Advantages of foam slag operation

With the emergence of electric furnace foamed slag technology, the foamed slag thickness can reach 300~500mm, which is more than twice the arc length, allowing the electric furnace to achieve submerged arc operation. It can solve two problems: on the one hand, it truly plays the role of water-cooled furnace wall and improves the life of the furnace body; on the other important aspect, it makes long arc power supply possible, that is, high voltage and low current. Its superiority lies in making up for the shortcomings of the early “ultra-high power power supply” and bringing the following advantages: 1) Increase the life of the furnace lining and reduce the consumption of refractory materials; 2) Reduce the power loss and power consumption; 3) Reduce the consumption of electrodes ;4) Improvement of three-phase arc power balance; 5) Improvement of power factor.

The molten pool of modern electric furnaces is formed early, so appropriate high carbon content and early oxygen blowing can be used to foam the slag. The foamed slag operation of the electric furnace is mainly carried out during the period between the arc exposure of the molten powder and the end of oxidation. It uses carbon monoxide bubbles generated by spraying carbon powder and blown oxygen into the slag to foam the slag through the slag layer. Good foam slag requires the arc to be buried for a long time, which requires bubbles to be generated in the slag and also requires the bubbles to have a certain lifespan.

Factors affecting foam slag

Amount of oxygen blown: Foaming slag is mainly caused by the carbon-oxygen reaction generating a large amount of CO. Therefore, increasing the oxygen supply intensity not only increases the oxygen content but also increases the stirring intensity.

Carbon content in the molten pool: a necessary condition for the generation of CO bubbles. If the carbon is insufficient, the carbon-oxygen reaction will be weak, and carbon should be replenished in time to promote the generation of CO bubbles.

Physical properties of slag: increasing the viscosity of the slag, reducing the surface tension and increasing the number of suspended particles in the slag will improve the foaming performance of the slag and the stability of the foamed slag.

Chemical composition of slag: In alkaline steelmaking slag, FeO content and alkalinity have a great influence on the height of foamed slag. Generally speaking, as the FeO content increases, the foaming performance of the slag becomes worse. This may be due to FeO dissolving suspended particles in the slag and reducing the viscosity of the slag. The alkalinity has a peak value near index 2, when the foam value reaches its maximum height.

Temperature: Within the steelmaking temperature range, as the temperature increases, the viscosity of the slag decreases. The higher the temperature of the molten pool, the worse the conditions for generating foamed slag.

Control of foam slag

Good foamed slag is achieved by controlling the amount of CO gas generated, the FeO content in the slag and the alkalinity of the slag. The gases that form foamy slag can be generated not only in the metal bath but also in the slag. The bubbles generated in the molten pool mainly come from the reaction of dissolved carbon, gaseous oxygen, and dissolved oxygen. The premise is that there is sufficient carbon content in the molten pool. CO in the slag is mainly produced by a series of reactions between carbon and gaseous oxygen, iron oxide, etc. The carbon can be added in the form of particles or directly injected in the form of powder. The fine dispersed bubbles produced by the molten pool are not only beneficial to the flow of molten pool metal, promote metallurgical reactions, but also conducive to the formation of foamy slag, while the gas generated in the slag will not cause the molten pool metal to flow. Studies have shown that increasing the viscosity of the slag, reducing the surface tension, making the basicity of the slag 2.0 to 2.5, (FeO) = 15% to 20%, etc. are beneficial to the foaming of the slag.

The water-cooled carbon-oxygen lance developed by the United States and Germany is specially used to create foamed slag by operating the electric furnace door, and the effect is particularly good. It has been widely adopted in China. Recently, Germany and Italy have developed carbon-oxygen-combustion composite furnace wall spray guns. According to different stages in the furnace, enhanced oxygen operations such as oxygen-combustion fluxing, carbon-oxygen slagging, oxygen blowing to remove carbon and secondary combustion can be carried out. This composite furnace wall spray gun realizes the furnace door closing operation, and its effects are: eliminating cold spots, slagging and submerged arc, accelerating reaction and recovering energy.

Alloying of molten steel

Alloying in traditional electric furnace steelmaking generally involves pre-alloying at the end of oxidation and early reduction, and fine-tuning of the alloy composition at the end of reduction, before tapping or during the tapping process.

Alloying in modern electric furnace steelmaking is generally completed in the ladle during the tapping process. During tapping, the alloying in the ladle is pre-alloyed. The precise alloy composition adjustment is ultimately completed in the refining furnace. Alloying operation mainly refers to the time and amount of alloy addition.

Alloy adding time

Elements that are not easily oxidized can be added during charging, oxidation period or reduction period, such as Cu, Ni, Co, Mo, and W. Elements that are more easily oxidized are generally added in the early stages of reduction, such as P, Cr, and Mn. Elements that are easily oxidized are generally added at the end of reduction, such as V, Nb, Si, Ti, Al, B, and rare earth elements (La, Ce, etc.).

Iron alloys with high melting points and heavy specific gravity should be stirred after addition. For example, ferrotungsten has high density and high melting point, and it sinks to the bottom of the furnace, so its block size should be smaller.

Elements that are added in large amounts and are easily oxidized should be baked and heated in order to melt quickly.

Prioritize the use of cheap high-carbon iron alloys (such as high-carbon ferromanganese, high-carbon ferrochrome, etc.), and then consider using medium-carbon iron alloys or low-carbon iron alloys.

Precious ferrous alloys should be controlled at the mid-to-lower limit as much as possible to reduce the cost of steel.

Deoxidation operations and alloying operations cannot be completely separated. Generally speaking, deoxidizing elements are added first, and alloying elements are added later; alloying elements with relatively strong deoxidizing ability and relatively valuable elements should be added when the molten steel is well deoxidized. For example, the order and purpose of adding easily oxidizable elements such as Al, Ti, and B: insert aluminum for deoxidation 2 to 3 minutes before tapping, add titanium to fix nitrogen, and add boron during the tapping process to increase the recovery rate of boron.

Add quantity

The chemical composition has a great influence on the quality and performance of steel. The amount of alloy added can be calculated quickly and accurately on-site based on the type of steel smelted, the amount of molten steel in the furnace, the composition in the furnace, the alloy composition and the alloy yield.

Modern electric arc furnace steelmaking process

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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. 

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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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