Method for cold start of alternating current electric arc furnace with pure scrap steel

Abstract

The application discloses an alternating current electric arc furnace pure scrap steel cold-state start-up method, which comprises the following steps: laying carbonizer on the furnace bottom and adding light and thin scrap steel, carrying out the first furnace drying after an initial molten pool is formed by power-on; continuously adding scrap steel and melting by power-on until the molten slag covers the furnace slope, and then carrying out the second furnace drying; adding scrap steel again and smelting by power-on, continuing to heat after the feeding is stopped, and carrying out the third furnace drying after a preset temperature is reached; then, heating again and tapping after the tapping conditions are met. The three-stage progressive furnace drying corresponding to the molten pool forming and expanding process is adopted, so that the furnace bottom, the furnace slope and the slag line area of the furnace wall are sequentially dried and sintered, which is favorable for improving the uniformity of the heating and sintering of each area of the new furnace lining, and reducing the risk of local thermal shock and insufficient sintering in the cold-state start-up process.

Description

Technical Field

[0001] This invention belongs to the field of electric arc furnace steelmaking technology, specifically a method for cold start-up of pure scrap steel in an AC electric arc furnace. Background Technology

[0002] Electric arc furnace steelmaking is a short-process steelmaking technology that uses scrap steel as the main raw material. It features flexible production, short construction period, and high resource recycling rate. With the development of horizontal continuous feeding technology, electric arc furnaces can achieve continuous addition of scrap steel and can utilize the high-temperature flue gas in the furnace to preheat the scrap steel, which is beneficial to improving energy efficiency and smelting continuity.

[0003] For newly built electric arc furnaces or electric arc furnaces that have undergone major lining overhauls, cold start-up is usually required before they can be put into normal production. Since the refractory materials in the furnace bottom, furnace slope and furnace wall of the new furnace lining still contain a certain amount of physical water and bound water, they need to be heated gradually and fully sintered during the start-up process. If the temperature rises too quickly or the local heating inside the furnace is too concentrated, it is easy to cause uneven heating of the furnace lining, which will affect the sintering quality of the refractory materials. In severe cases, problems such as furnace lining cracking and peeling may occur, which will affect the subsequent furnace life and production stability.

[0004] Current methods for cold start-up of electric arc furnaces often involve batch charging via a charging basket, or starting the furnace with molten iron or pig iron. When charging in batches via a charging basket, the amount of scrap steel charged at one time is large, and the concentrated entry of cold material into the furnace can easily cause significant thermal shock to the new furnace lining. Using molten iron or pig iron as auxiliary start-up requires additional reliance on these resources, and the direct entry of high-temperature molten iron into the new furnace lining can also cause significant localized thermal stress. These methods also fail to fully utilize the characteristics of horizontally continuously charged electric arc furnaces. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies and solve at least one of the problems mentioned in the background art, a cold start-up method for pure scrap steel in an AC electric arc furnace is proposed.

[0006] The technical solution adopted by this invention to solve its technical problem is: a method for cold start-up of pure scrap steel in an AC electric arc furnace, comprising the following steps:

[0007] S1. Initial molten pool formation: A carbon raiser is laid at the bottom of the AC electric arc furnace and light scrap steel is added. The light scrap steel is melted by electricity to form an initial molten pool in the furnace. S2. First furnace drying: After the initial molten pool is formed, the power is cut off to perform the first furnace drying. S3. Second furnace drying: After the first furnace drying is completed, scrap steel is added into the furnace and electricity is turned on to melt it, so that the molten pool in the furnace expands until the molten slag covers the furnace slope, and then the power is turned off to carry out the second furnace drying. S4. Third furnace drying: After the second furnace drying is completed, continue to add scrap steel into the furnace and carry out smelting with electricity. After stopping the feeding, continue to heat the molten steel until the molten steel reaches the preset third stage furnace drying temperature. Then, stop the power and carry out the third furnace drying. S5. Heating and tapping: After the third furnace drying is completed, power is turned on again to raise the temperature, and the steel is tapped after it reaches the tapping conditions.

[0008] Furthermore, in step S1, the carbon raiser is laid on the bottom of the AC electric arc furnace in a gradient pattern with a thicker central area and a thinner peripheral area. The total amount of carbon raiser is 1.0~2.0t, the thickness of the carbon raiser in the central area of ​​the furnace bottom is 30~100mm, and the thickness of the carbon raiser in the peripheral area of ​​the furnace bottom is 10~35mm.

[0009] Furthermore, the diameter of the central area of ​​the furnace bottom is 40% to 60% of the furnace bottom diameter, and 60% to 70% of the total amount of recarburizing agent is laid in the central area of ​​the furnace bottom, while the remaining recarburizing agent is laid in the surrounding area of ​​the furnace bottom. In step S1, the bulk density of the light and thin scrap steel is 0.5t / m³, the weight of a single piece is no more than 3kg, and after the light and thin scrap steel is laid, the distance between its upper surface and the lower end of the electrode is 150~300mm.

[0010] Furthermore, in step S1, when melting the thin scrap steel by energizing it, the voltage is 40%~42% of the rated voltage and the current is 45%~48% of the rated current; In step S2, the first oven baking time is 15~25 minutes; In step S3, the second oven drying time is 20~30 minutes; In step S4, the oven temperature in the third stage is not lower than 1630℃, and the oven time for the third stage is 25~35 minutes.

[0011] Furthermore, during the first furnace drying, the AC electric arc furnace is tilted towards the tapping side; during the second furnace drying, the AC electric arc furnace is tilted towards the slag tapping side; during the third furnace drying, the AC electric arc furnace is tilted towards the tapping side, with each tilt angle being 1~3°.

[0012] Furthermore, in steps S3 and S4, scrap steel is added to the AC electric arc furnace in a horizontal continuous feeding manner. In step S4, before the furnace body reaches 90% of the rated loading capacity, scrap steel is added into the furnace at a rate of 1.2~1.8t / min.

[0013] Furthermore, the furnace wall of the AC electric arc furnace is equipped with four oxygen lances along the circumference. The two oxygen lances located on both sides of the horizontal feeding port constitute the feeding side oxygen lances, and the other two oxygen lances constitute the opposite side oxygen lances. In step S4, when the depth of the molten pool is no more than 500 mm, only the oxygen lance on the feeding side is turned on; when the depth of the molten pool is greater than 500 mm and the foam slag has been stably formed, the oxygen lance on the opposite side is turned on, so that all four oxygen lances are put into operation. After all four oxygen lances are in operation, the oxygen flow rate of the oxygen lance on the feeding side is 10% to 20% higher than that of the oxygen lance on the opposite side.

[0014] Furthermore, the furnace wall is also equipped with four carbon lances arranged alternately with the oxygen lances; In step S4, when only the oxygen lance on the feeding side is turned on, the carbon lance corresponding to the oxygen lance on the feeding side is turned on; after the oxygen lance on the other side is turned on, all carbon lances are turned on. Each carbon gun independently adjusts the amount of carbon sprayed according to the thickness of the foam slag in the corresponding area, so that the thickness of the foam slag is maintained at 400-600mm.

[0015] Furthermore, in step S4, the feeding rate is adjusted in stages as the feed rate approaches the rated loading capacity: When the furnace body reaches 90% of the rated loading capacity, the feeding rate is reduced to 0.8t / min; When the furnace body weighs up to 95% of the rated loading capacity and the liquid level inside the furnace reaches the preset near-full level, the feeding rate is reduced to 0.3t / min. When the furnace body reaches 98% to 100% of the rated loading capacity and the liquid level inside the furnace reaches the full level, feeding should be stopped.

[0016] Furthermore, in step S4, before stopping the feeding, each oxygen lance is kept in low-fire mode, and the oxygen flow rate of a single oxygen lance is 1000~1300 Nm³ / h; after stopping the feeding, the molten steel continues to be decarburized and dephosphorized and heated, and the oxygen flow rate of each oxygen lance is increased so that the oxygen supply intensity is 0.8~1.2 Nm³ / (min·t). The oxygen flow rate of a single oxygen lance on the feeding side is 1800~2200 Nm³ / h, and the oxygen flow rate of a single oxygen lance on the opposite side is 1600~2000 Nm³ / h, maintaining the oxygen flow rate of the oxygen lance on the feeding side 10%~20% higher than that of the oxygen lance on the opposite side. In step S5, when the temperature of the molten steel is not lower than 1650℃, the temperature difference between two consecutive temperature measurements is not greater than 20℃, and the chemical composition of the molten steel meets the preset requirements for molten steel composition, the steel is tapped. After tapping, 15% to 25% of the rated charge amount of molten steel is retained.

[0017] The beneficial effects of this invention are as follows: 1. The AC electric arc furnace cold start-up method for pure scrap steel of the present invention, by sequentially setting the first, second and third furnace drying stages during the furnace start-up process, and taking the initial molten pool formation, slag covering the furnace slope and the steel reaching the third stage furnace drying temperature as the corresponding process nodes, allows the range of action of the high-temperature melt in the furnace to extend from the furnace bottom to the furnace slope and slag line area of ​​the furnace wall as the molten pool gradually expands, thereby forming a progressive furnace lining sintering process from bottom to top. This is beneficial to improving the uniformity of heating and sintering in different areas of the new furnace lining and reducing the adverse effects on the furnace lining quality caused by excessively rapid local heating or insufficient sintering.

[0018] 2. The method for cold start-up of pure scrap steel in an AC electric arc furnace of the present invention involves laying a carbon raiser in a gradient manner with a thicker central area and a thinner peripheral area at the bottom of the furnace, and adding thin scrap steel on top for low voltage and low current arc ignition. This allows the bottom of the furnace to form more suitable electrical conductivity and arc ignition conditions, which is conducive to the stable formation of the initial molten pool under cold conditions of pure scrap steel. At the same time, it avoids the large concentrated thermal shock to the new furnace lining caused by directly using a high smelting intensity in the early stage of furnace start-up.

[0019] 3. The AC electric arc furnace cold start-up method for pure scrap steel of the present invention, through horizontal continuous feeding and phased coordinated control of oxygen lance and carbon lance, enables the oxygen supply and carbon injection process to adapt to the regional heat load changes formed by the continuous addition of cold scrap steel, which helps to reduce the temperature difference in the furnace, making the scrap steel addition and melting process more stable, and reducing the thermal shock and scouring of the new furnace lining during the start-up stage while improving the scrap steel melting efficiency. Attached Figure Description

[0020] Figure 1 This is a schematic flowchart of the method for cold start-up of pure scrap steel in an AC electric arc furnace according to the present invention. Detailed Implementation

[0021] To illustrate in detail the technical solutions adopted by the present invention to achieve the intended technical objectives, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Furthermore, the technical means or technical features in the embodiments of the present invention can be replaced without creative effort. The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0022] This invention is applicable to the cold start-up of an AC electric arc furnace using pure scrap steel after the new furnace lining has been constructed and no steel or slag remains in the furnace. The AC electric arc furnace can be equipped with a horizontal continuous feeding device, a wall-mounted oxygen lance, a carbon lance, a furnace weighing system, and conventional power supply, hydraulic, water cooling, and dust removal equipment. The horizontal continuous feeding device can be an existing Consteel-type horizontal continuous feeding device for conveying scrap steel into the furnace.

[0023] Before formally starting the furnace in a cold state, the water cooling system, power supply system, dust removal system, pneumatic system, hydraulic system, furnace weighing system, and the pipelines and cooling status of the oxygen lance and carbon lance of the AC electric arc furnace should be checked first. The furnace weighing system should be zero-point calibrated, and the following steps should be carried out after confirming that each system is in normal working condition.

[0024] S1, Initial molten pool formation.

[0025] First, a carbon raiser is laid at the bottom of the AC electric arc furnace. The carbon raiser is laid in a gradient pattern, with a thicker layer in the center of the furnace bottom and a thinner layer in the periphery. The total amount of carbon raiser is 1.0~2.0t, the thickness of the carbon raiser in the center of the furnace bottom is 30~100mm, and the thickness of the carbon raiser in the periphery is 10~35mm.

[0026] Furthermore, the diameter of the central area of ​​the furnace bottom can be 40% to 60% of the furnace bottom diameter. 60% to 70% of the total amount of recarburizing agent is laid in the central area, and the remaining 30% to 40% of the recarburizing agent is laid in the surrounding area of ​​the furnace bottom, thereby forming a gently sloping recarburizing agent layer with a high center and a low periphery.

[0027] In this embodiment, the carbon raiser can be a low-sulfur calcined petroleum coke carbon raiser with a fixed carbon content of not less than 95%, a sulfur content of not more than 0.05%, and a particle size of 5-20 mm, of which the proportion of carbon raiser with a particle size of 10-15 mm can be more than 60%. Using the above particle size range can, on the one hand, reduce the removal of excessively fine carbon raiser particles by the dust removal system, and on the other hand, avoid excessively large particles that melt slowly.

[0028] After the carbon raiser is laid, add light and thin scrap steel on top of the carbon raiser. The bulk density of the light and thin scrap steel is about 0.5t / m³, and the weight of a single piece is no more than 3kg. After adding the light and thin scrap steel, the surface of the light and thin scrap steel is located near the lower part of the electrode, and the distance between the upper surface of the light and thin scrap steel and the lower end of the electrode is maintained at 150~300mm, so that the electrode can more easily establish arc conditions with the scrap steel layer after it is lowered.

[0029] Subsequently, a lower voltage and lower current are used for the initial power-on. The voltage can be controlled at 40% to 42% of the rated voltage of the AC electric arc furnace, and the current can be controlled at 45% to 48% of the rated current. In principle, no oxygen is blown during the initial arc ignition stage, or only a small flow rate is used for auxiliary oxygen supply to reduce the scouring of the new furnace lining caused by the high temperature and high speed oxygen jet.

[0030] After an alternating electric arc is formed by the electrodes, the thin scrap steel gradually melts and forms an initial molten pool together with the carburizing agent at the bottom of the furnace. The combination of carburizing agent and thin scrap steel is beneficial to improving the electrical conductivity and arc-starting environment in the furnace under cold conditions, so that the alternating electric arc can be established more stably. At the same time, it avoids the large local thermal shock to the new furnace lining caused by directly charging a large amount of ordinary scrap steel and then using high-power arc starting.

[0031] S2, First oven baking.

[0032] Once the thin scrap steel in the furnace has been basically melted and an initial molten pool has been formed, the power supply is stopped, and the furnace is heated for the first time. In one specific implementation, the depth of the initial molten pool can be about 300 mm, and the temperature of the molten steel can reach about 1500 ℃.

[0033] The first furnace drying time is 15-25 minutes. During the first furnace drying process, the AC electric arc furnace is slightly tilted towards the tapping side, with an inclination angle of 1-3°, preferably 2° in this embodiment. By moderately shifting the initial molten pool towards the tapping side, the heat carried by the initial molten pool can be further applied to the corresponding area of ​​the furnace bottom, causing the newly built furnace bottom refractory material to gradually heat up and sinter.

[0034] The first furnace drying mainly corresponds to the initial sintering in the furnace bottom area. Since the depth of the molten pool in the furnace is relatively small at this time, the high-temperature molten steel is mainly concentrated in the furnace bottom. Therefore, the first furnace drying by shutting down the power using the initial molten pool can make full use of the residual heat of the molten steel without excessively increasing the input power, so that the physical water and bound water in the refractory material at the furnace bottom can gradually escape.

[0035] During the first power outage for furnace drying, the mechanical, electrical, and hydraulic systems of the AC electric arc furnace can also be checked simultaneously. For example, the operating status of the furnace tilting mechanism, electrode lifting mechanism, and horizontal feeding equipment can be checked; the transformer, short-circuit connection, and insulation status can be checked; and the pressure, cylinders, and accumulator status of the hydraulic system can be checked. After confirming that there are no abnormalities in the equipment, proceed to the next step.

[0036] S3, Second oven drying.

[0037] After the first furnace drying is completed, the furnace body is restored to the corresponding feeding position, and scrap steel is continuously added to the AC electric arc furnace in a horizontal feeding manner.

[0038] The scrap steel added in this stage can be a mixture of light scrap steel, medium scrap steel and crushed material, with light scrap steel accounting for 35% to 40%, medium scrap steel accounting for 30% to 35%, and crushed material accounting for 30%. Using a mixture of different material types is beneficial to taking into account the furnace filling state, permeability and melting efficiency of the scrap steel.

[0039] After adding scrap steel, the furnace continues to melt using a lower power source, while lime and dolomite are added as needed for slag formation. As the scrap steel continues to melt, the molten pool inside the furnace gradually expands and forms high-temperature molten slag.

[0040] In actual production, the stage of feeding can be determined based on the feeding status of the horizontal continuous feeding equipment. For example, in a horizontal continuous feeding equipment using a hydraulic pusher mechanism, as the material pile in the furnace gradually fills, the pushing resistance increases accordingly. When the hydraulic system pressure reaches the rated working pressure of the equipment or the opening pressure of the overflow valve, it can be considered that the scrap steel in the furnace has reached the corresponding filling state. The rated working pressure of some equipment is 16~20MPa.

[0041] Continue to melt the scrap steel by applying electricity, and add more scrap steel and slag as needed. As the scrap steel gradually melts and high-temperature slag gradually forms, the slag will eventually cover the furnace slope area.

[0042] Power supply was stopped after the molten slag covered the furnace slope, and a second furnace drying process was carried out, which lasted for 20 to 30 minutes.

[0043] During the second furnace drying process, the AC electric arc furnace is slightly tilted towards the slag discharge side, with an inclination angle of 1~3°, preferably 2° in this embodiment. Since the high-temperature molten slag formed in the second stage has already covered the furnace slope area, by slightly tilting the furnace body in a direction different from the first furnace drying, the high-temperature molten steel and slag further act on the furnace slope area on the slag discharge side.

[0044] Therefore, the first furnace drying mainly targets the furnace bottom area for sintering, while the second furnace drying further targets the furnace slope area as the molten pool expands and the slag layer rises, thus forming a progressive sintering process that gradually moves upward from the furnace bottom to the furnace slope.

[0045] During the second furnace drying process, the equipment status checks of the mechanical, electrical, and hydraulic systems can also be carried out simultaneously.

[0046] S4, Third oven drying.

[0047] After the second furnace drying was completed and the equipment inspection showed no abnormalities, scrap steel was added to the AC electric arc furnace again using a horizontal continuous feeding method, and smelting continued with electricity.

[0048] During the continued electrified smelting process, the horizontal continuous feeding rate can be maintained at 1.2~1.8t / min to add scrap steel into the furnace until it approaches the predetermined charging amount.

[0049] In this embodiment, four oxygen lances are arranged circumferentially on the furnace wall of the AC electric arc furnace. The two oxygen lances located on both sides of the horizontal charging port are designated as charging-side oxygen lances, and the two oxygen lances located on the other side of the furnace body are designated as opposing-side oxygen lances. The four carbon lances and four oxygen lances are arranged alternately along the circumferential direction of the furnace wall.

[0050] For example, with the center of the horizontal feed port as 0°, the two feed-side oxygen lances can be arranged at approximately +45° and -45° on both sides of the horizontal feed port, respectively, and the two opposite-side oxygen lances can be arranged at approximately +135° and -135°, respectively. The four carbon lances are respectively placed between adjacent oxygen lances.

[0051] The oxygen lance is a wall-mounted type, and the height of the lance head from the molten slag surface can be maintained at 800~1200mm, preferably 1000mm in this embodiment. Using a relatively high lance position during the start-up stage helps to reduce the direct impact of the oxygen jet on the new furnace lining.

[0052] In the initial stage of S4, when the depth of the molten pool is no more than 500mm, only the two oxygen lances on the charging side on both sides of the horizontal charging port are turned on, and the two oxygen lances on the opposite side are not put into operation for the time being. The oxygen lances on the charging side are kept at a low flame to supply oxygen in order to assist in the melting of scrap steel on the charging side. At the same time, it is to avoid turning on all the oxygen lances directly when the depth of the molten pool is small, so as to avoid the high-speed jet from scouring the furnace bottom and furnace slope.

[0053] Once the molten pool depth exceeds 500mm and a relatively stable foamy slag has formed inside the furnace, two more oxygen lances on opposite sides are turned on, so that all four oxygen lances are in operation.

[0054] Before continuous feeding is stopped, each oxygen lance maintains a low-fire oxygen supply, with an oxygen flow rate of 1000~1300 Nm³ / h for a single lance. After all four oxygen lances are turned on, the oxygen flow rate of the lance on the feeding side is increased by 10%~20% compared to the lance on the opposite side. This allows the higher oxygen supply on the feeding side to compensate for the large amount of heat absorbed by the continuously added cold scrap steel, thus reducing the temperature difference between the feeding side and the opposite side of the furnace.

[0055] In conjunction with the oxygen lance, the carbon lance corresponding to the oxygen lance on the feeding side can be activated first in the early stage of S4; after the molten pool has expanded further and all oxygen lances have been activated, all carbon lances can then be activated.

[0056] Each carbon lance independently adjusts its carbon injection rate based on the foamy slag condition of its corresponding furnace zone, maintaining the foamy slag thickness within the range of 400-600 mm. As a specific adjustment method, for independently adjustable carbon lances, when the foamy slag thickness is below 400 mm, the carbon injection rate can be increased, for example, to approximately 100 kg / min; when the foamy slag thickness is above 600 mm, the carbon injection rate can be decreased, for example, to approximately 50 kg / min; and when the foamy slag thickness is within the 400-600 mm range, the current carbon injection rate can be maintained.

[0057] When the horizontal continuous feeding speed is increased, the carbon injection amount of the carbon gun on the feeding side can be increased accordingly to adapt to the increase in the amount of cold scrap steel melted on the feeding side, and to help maintain the stability of the foam slag state on the feeding side.

[0058] When the furnace approaches its rated charge capacity, the feeding rate is adjusted in stages. Specifically, when the furnace weighing reaches 90% of the rated charge capacity, the feeding rate is reduced to 0.8 t / min, and close monitoring of the liquid level inside the furnace is maintained; when the furnace weighing reaches 95% of the rated charge capacity and the liquid level inside the furnace is close to full, the feeding rate is further reduced to 0.3 t / min; when the furnace weighing reaches 98%~100% of the rated charge capacity and the liquid level inside the furnace reaches full, continuous feeding is stopped.

[0059] The above method uses both the furnace body weight and the liquid level in the furnace to determine the end point of continuous feeding. Compared with judging solely by the weight of the added scrap steel, it can reduce the risk of underloading or overflow caused by the actual furnace filling state not completely corresponding to the weighing weight due to the different bulk densities of pure scrap steel in different batches.

[0060] After stopping the feeding, continue to decarburize, dephosphorize and heat the molten steel; at this stage, based on the formation of a relatively deep molten pool in the furnace, appropriately increase the oxygen supply of the oxygen lance.

[0061] Specifically, the oxygen flow rate of a single oxygen lance on the feeding side can be increased to 1800~2200 Nm. 3 / h, the oxygen flow rate of a single oxygen lance on the opposite side can be increased to 1600~2000 Nm³ / h. 3 / h, while maintaining a higher oxygen lance flow rate on the feeding side than on the opposite side; the overall oxygen supply intensity during this stage is controlled at 0.8~1.2 Nm. 3 / (min·t).

[0062] Although the oxygen supply is higher than that of the low-fire oxygen supply before the feeding is stopped, it is still lower than that of the high-flow blowing condition in normal production. This allows the high-speed oxygen jet to reduce the scouring of the new furnace lining during the cold start-up stage while meeting the needs of decarburization, dephosphorization and heating.

[0063] Power and oxygen supply are stopped once the molten steel reaches the preset third-stage furnace drying temperature. The third-stage furnace drying temperature is no lower than 1630℃, followed by a third furnace drying process, which lasts for 25-35 minutes.

[0064] During the third furnace drying process, the AC electric arc furnace is tilted slightly towards the tapping side again, with an inclination angle of 1~3°, preferably 2° in this embodiment.

[0065] By tilting the furnace towards the tapping side during the first drying, towards the slag tapping side during the second drying, and towards the tapping side again during the third drying, the high-temperature molten steel and slag alternately act on different areas inside the furnace. As the depth and temperature of the molten pool gradually increase, the sintering area expands sequentially from the furnace bottom to the furnace slope, and then to the furnace wall and slag line area, thus forming a three-stage progressive furnace lining sintering process from bottom to top.

[0066] The three-stage furnace drying process, which involves alternating slight tilting in different directions, can reduce localized overheating caused by the high-temperature melt being concentrated on the same side of the furnace for a long time, and also reduce the possibility of insufficient sintering on the other side due to insufficient contact of the high-temperature melt.

[0067] During the third furnace drying process, the mechanical, electrical, and hydraulic systems can be checked again to assess their equipment status.

[0068] S5, heating up and tapping steel.

[0069] After the third furnace drying was completed and the equipment inspection showed no abnormalities, the AC electric arc furnace was powered on and oxygen was supplied again to reheat the molten steel.

[0070] Continuous temperature measurement and sampling are performed on the molten steel; the steel is tapped when the temperature of the molten steel is not lower than 1650℃, the temperature difference between two consecutive temperature measurements is not greater than 20℃, and the chemical composition of the molten steel meets the tapping requirements.

[0071] The steel is tapped using an eccentric hearth tapping method; during the tapping process, 15% to 25% of the rated charge weight of molten steel is retained, for example, the retained steel weight can be 18 to 35 tons. The retained molten steel can be used for the next heat, allowing the AC electric arc furnace to enter a normal continuous production state after the cold start-up of pure scrap steel.

[0072] Through the above steps S1~S5, the new furnace lining first forms a relatively stable initial molten pool under cold conditions using carbon raisers and thin scrap steel. Then, the furnace is shut down for baking at three progressive nodes: the formation of the initial molten pool, the slag covering the furnace slope, and the steel temperature reaching the third stage baking temperature. This is combined with slight tilting of the furnace body in different directions, so that the furnace bottom, furnace slope, and slag line area of ​​the furnace wall are successively baked and sintered more thoroughly. At the same time, in the subsequent continuous charging process, the oxygen lance is opened in stages, oxygen is supplied asymmetrically on the charging side and the opposite side, foam slag is adjusted, and the combined stopping of weighing and liquid level is used to gradually improve the scrap steel melting efficiency while protecting the new furnace lining.

[0073] Example 1 A 100t-class ultra-high power AC electric arc furnace is used for cold start-up of pure scrap steel. The AC electric arc furnace is equipped with a horizontal continuous feeding device, a transformer with a rated capacity of about 90MVA, and a rated charging capacity of about 125t. Four oxygen lances and four carbon lances are alternately arranged along the circumference of the furnace wall.

[0074] First, the equipment was inspected and the zero-point calibration of the furnace weighing system was performed. Then, 1.5t of carbon raiser was laid at the bottom of the furnace. The diameter of the central area of ​​the furnace bottom was about 50% of the furnace bottom diameter. About 1.0t of carbon raiser was laid in the central area, accounting for about 67% of the total carbon raiser, with a thickness of about 75mm. About 0.5t of carbon raiser was laid in the surrounding area of ​​the furnace bottom, with a thickness of about 35mm.

[0075] Add thin, light scrap steel with a bulk density of about 0.5 t / m³ and a single piece weight of no more than 3 kg, so that the distance between its upper surface and the lower end of the electrode is about 220 mm.

[0076] The arc is initiated by applying a voltage of approximately 42% of the rated voltage and a current of approximately 48% of the rated current, without oxygen blowing. After about 30 minutes of energization, the thin scrap steel is basically melted and an initial molten pool with a depth of about 300 mm is formed.

[0077] Power is cut off and the furnace is first dried for about 20 minutes. At the same time, the furnace body is tilted about 2° toward the tapping side.

[0078] After the first furnace drying is completed, a mixture of light scrap steel, medium scrap steel and crushed material is added in a horizontal continuous feeding manner, and lime and dolomite are added to form slag. After the scrap steel continues to melt and the slag surface forms to cover the furnace slope, the power supply is stopped to carry out the second furnace drying, which takes about 25 minutes. At the same time, the furnace body is tilted about 2° towards the slag discharge side.

[0079] After the second drying process is completed, horizontal continuous feeding continues, with an initial feeding speed of approximately 1.5 t / min.

[0080] When the molten pool depth is no more than 500 mm, only the two oxygen lances on the feeding side located on both sides of the horizontal feed port are turned on, with a flow rate of about 1200 Nm³ / h for each lance; after the molten pool depth is greater than 500 mm and the foam slag has stabilized, the other two oxygen lances on the opposite side are turned on, with a flow rate of about 1050 Nm³ / h for each lance on the opposite side.

[0081] The four carbon guns are independently adjusted according to the foam slag state of the corresponding area to maintain the foam slag thickness at 400~600mm.

[0082] As feeding continues, when the furnace body weighs approximately 113t, which is about 90% of the rated charge, the feeding rate is reduced to 0.8t / min; when the furnace body weighs approximately 119t, which is about 95%, and the liquid level inside the furnace is close to full, the feeding rate is reduced to 0.3t / min; when the furnace body weighs approximately 125t and the liquid level inside the furnace reaches full, feeding is stopped.

[0083] After stopping the feeding, decarbonization, dephosphorization and heating operations are carried out; at this time, the flow rate of a single oxygen lance on the feeding side is increased to about 2000 Nm³ / h, and the flow rate of a single oxygen lance on the opposite side is increased to about 1800 Nm³ / h, corresponding to a total oxygen supply intensity of about 1.0 Nm³ / (min·t).

[0084] When the temperature of the molten steel reaches about 1635℃, the power and oxygen are turned off for the third furnace drying, which takes about 30 minutes, and the furnace body is tilted about 2° towards the steel tapping side.

[0085] After the third furnace drying, power and oxygen were supplied again to raise the temperature; the temperature was measured twice in succession, reaching approximately 1652℃ and 1658℃ respectively, with a temperature difference of about 6℃ between the two measurements. After the chemical composition of the molten steel met the requirements for tapping, the steel was tapped.

[0086] The eccentric furnace bottom tapping method is adopted, with a steel retention of approximately 25 tons.

[0087] In this embodiment, the total power-on time is approximately 122 minutes, and the total smelting time is approximately 232 minutes.

[0088] Example 2 A 120t ultra-high power AC electric arc furnace is used for cold start-up of pure scrap steel. The AC electric arc furnace is equipped with a horizontal continuous feeding device, a transformer with a rated capacity of about 110MVA, a rated charging capacity of about 150t, and four oxygen lances and four carbon lances are alternately arranged on the furnace wall.

[0089] Approximately 1.8 tons of carbon raiser were laid at the bottom of the furnace, with approximately 1.2 tons laid in the central area at a thickness of approximately 90 mm, and approximately 0.6 tons laid in the surrounding area at a thickness of approximately 30 mm.

[0090] After the thin scrap steel is laid, the distance between its upper surface and the lower end of the electrode is about 180mm.

[0091] When energized for the first time, the voltage is about 40% of the rated voltage and the current is about 45% of the rated current. The initial molten pool is formed after about 32 minutes of energization.

[0092] The furnace was then dried for the first time, which took about 22 minutes, with the furnace body tilted about 2° toward the tapping side.

[0093] Continue to add a mixture of 40% light scrap steel, 30% medium scrap steel, and 30% crushed material in a horizontal continuous feeding manner. After melting and covering the furnace slope with molten slag, carry out a second furnace drying process, which takes about 28 minutes. The furnace body is tilted about 2° towards the slag discharge side.

[0094] After the second furnace drying, continue horizontal continuous feeding; when the molten pool depth is no more than 500 mm, open two oxygen lances on the feeding side, with a flow rate of about 1300 Nm³ / h for each lance; after the molten pool depth is greater than 500 mm and stable foamy slag is formed, open all four oxygen lances, with a flow rate of about 1300 Nm³ / h for a single oxygen lance on the feeding side and about 1150 Nm³ / h for a single oxygen lance on the opposite side.

[0095] When the continuous feeding approaches the rated loading capacity, the feeding speed is gradually reduced according to the furnace body weight and the liquid level in the furnace. Feeding is stopped when the furnace body weight is about 148~150t and the liquid level in the furnace reaches the corresponding full level.

[0096] After feeding is stopped, decarbonization, dephosphorization and heating continue. The flow rate of a single oxygen lance on the feeding side is increased to about 2200 Nm³ / h, and the flow rate of a single oxygen lance on the opposite side is increased to about 1900 Nm³ / h. Based on a rated charge of 150 t, the total oxygen supply intensity at this stage is about 0.9 Nm³ / (min·t).

[0097] Once the temperature of the molten steel reaches no less than 1630℃, the power and oxygen supply are stopped for a third furnace drying process, which takes about 32 minutes. The furnace body is then tilted about 2° toward the tapping side.

[0098] After the third furnace drying, the temperature was raised again. When the temperature of the molten steel reached about 1655℃, the temperature was continuously measured to be stable and the composition of the molten steel met the requirements, and the steel was tapped, leaving about 28t of steel.

[0099] In this embodiment, the total power-on time is approximately 128 minutes, and the total smelting time is approximately 238 minutes.

[0100] As can be seen from the above two embodiments, the present invention does not simply increase the number of times the furnace is shut down for baking. Instead, based on the actual state of the molten pool expanding from shallow to deep and the slag's range of action gradually expanding from the furnace bottom to the furnace slope and slag line area of ​​the furnace wall during the cold start-up process, three baking nodes with different triggering conditions are set up respectively. Through three baking processes, the new furnace lining forms a progressive sintering process from bottom to top. At the same time, by combining horizontal continuous feeding, staged oxygen lance input, asymmetrical oxygen supply on the feeding side and the opposite side, and alternating micro-tilting, the smelting intensity can be gradually improved while reducing the concentrated thermal shock and oxygen jet scouring of the new furnace lining.

[0101] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the scope of the present invention, based on the technical essence of the present invention and within the spirit and principles of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A method for cold start-up of a pure scrap steel furnace in an AC electric arc furnace, characterized in that, Includes the following steps: S1. Initial molten pool formation: A carbon raiser is laid at the bottom of the AC electric arc furnace and light scrap steel is added. The light scrap steel is melted by electricity to form an initial molten pool in the furnace. S2. First furnace drying: After the initial molten pool is formed, the power is cut off to perform the first furnace drying. S3. Second furnace drying: After the first furnace drying is completed, scrap steel is added into the furnace and electricity is turned on to melt it, so that the molten pool in the furnace expands until the molten slag covers the furnace slope, and then the power is turned off to carry out the second furnace drying. S4. Third furnace drying: After the second furnace drying is completed, continue to add scrap steel into the furnace and carry out smelting with electricity. After stopping the feeding, continue to heat the molten steel until the molten steel reaches the preset third stage furnace drying temperature. Then, stop the power and carry out the third furnace drying. S5. Heating and tapping: After the third furnace drying is completed, power is turned on again to raise the temperature, and the steel is tapped after it reaches the tapping conditions.

2. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 1, characterized in that, In step S1, the carbon raiser is laid on the bottom of the AC electric arc furnace in a gradient pattern with a thicker central area and a thinner peripheral area. The total amount of carbon raiser is 1.0~2.0t, the thickness of the carbon raiser in the central area of ​​the furnace bottom is 30~100mm, and the thickness of the carbon raiser in the peripheral area of ​​the furnace bottom is 10~35mm.

3. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 2, characterized in that, The diameter of the central area of ​​the furnace bottom is 40% to 60% of the diameter of the furnace bottom. 60% to 70% of the total amount of the carbon raiser is laid in the central area of ​​the furnace bottom, and the remaining carbon raiser is laid in the surrounding area of ​​the furnace bottom. In step S1, the bulk density of the thin scrap steel is 0.5t / m³, the weight of a single piece is no more than 3kg, and after the thin scrap steel is laid, the distance between its upper surface and the lower end of the electrode is 150~300mm.

4. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 1, characterized in that, In step S1, when the thin scrap steel is energized to melt it, the voltage is 40% to 42% of the rated voltage and the current is 45% to 48% of the rated current. In step S2, the first oven baking time is 15~25 minutes; In step S3, the second oven drying time is 20~30 minutes; In step S4, the oven temperature in the third stage is not lower than 1630℃, and the oven time for the third stage is 25~35 minutes.

5. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 1, characterized in that, During the first furnace drying, the AC electric arc furnace is tilted towards the steel tapping side; during the second furnace drying, the AC electric arc furnace is tilted towards the slag tapping side; during the third furnace drying, the AC electric arc furnace is tilted towards the steel tapping side, with each tilt angle being 1~3°.

6. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 1, characterized in that, In both steps S3 and S4, scrap steel is added to the AC electric arc furnace using a horizontal continuous feeding method. In step S4, before the furnace body reaches 90% of the rated loading capacity, scrap steel is added into the furnace at a rate of 1.2~1.8t / min.

7. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 6, characterized in that, The furnace wall of the AC electric arc furnace is provided with four oxygen lances along the circumference. The two oxygen lances located on both sides of the horizontal feeding port constitute the feeding side oxygen lances, and the other two oxygen lances constitute the opposite side oxygen lances. In step S4, when the depth of the molten pool is no more than 500 mm, only the oxygen lance on the feeding side is turned on; when the depth of the molten pool is greater than 500 mm and the foam slag is stably formed, the oxygen lance on the opposite side is turned on, so that all four oxygen lances are put into operation. After all four oxygen lances are in operation, the oxygen flow rate of the feeding side oxygen lance is 10% to 20% higher than that of the opposite side oxygen lance.

8. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 7, characterized in that, The furnace wall is also provided with four carbon lances arranged alternately with the oxygen lance; In step S4, when only the oxygen lance on the feeding side is turned on, the carbon lance corresponding to the oxygen lance on the feeding side is turned on; after the oxygen lance on the opposite side is turned on, all carbon lances are turned on. Each carbon gun independently adjusts the amount of carbon sprayed according to the thickness of the foam slag in the corresponding area, so that the thickness of the foam slag is maintained at 400-600mm.

9. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 6, characterized in that, In step S4, the feeding rate is adjusted in stages as the feed rate approaches the rated loading capacity: When the furnace body reaches 90% of the rated loading capacity, the feeding rate is reduced to 0.8t / min; When the furnace body weighs up to 95% of the rated loading capacity and the liquid level inside the furnace reaches the preset near-full level, the feeding rate is reduced to 0.3t / min. When the furnace body reaches 98% to 100% of the rated loading capacity and the liquid level inside the furnace reaches the full level, feeding should be stopped.

10. The method for cold start-up of pure scrap steel in an AC electric arc furnace according to claim 7, characterized in that, In step S4, before stopping the feeding, each oxygen lance is kept in low fire mode, and the oxygen flow rate of a single oxygen lance is 1000~1300 Nm³ / h; after stopping the feeding, the molten steel continues to be decarburized and dephosphorized and heated, and the oxygen flow rate of each oxygen lance is increased so that the oxygen supply intensity is 0.8~1.2 Nm³ / (min·t). The oxygen flow rate of a single oxygen lance on the feeding side is 1800~2200 Nm³ / h, and the oxygen flow rate of a single oxygen lance on the opposite side is 1600~2000 Nm³ / h, maintaining the oxygen flow rate of the oxygen lance on the feeding side 10%~20% higher than that of the oxygen lance on the opposite side. In step S5, when the temperature of the molten steel is not lower than 1650℃, the temperature difference between two consecutive temperature measurements is not greater than 20℃, and the chemical composition of the molten steel meets the preset requirements for molten steel composition, the steel is tapped. After tapping, 15% to 25% of the rated charge amount of molten steel is retained.

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