A method for smelting low-nitrogen steel using an electric arc furnace
By using an AC electric arc furnace for smelting, combined with iron addition at the rear side wall of the furnace, a high iron-to-water ratio, electromagnetic stirring at the furnace bottom, and optimized oxygen supply, the problem of controlling nitrogen content in electric arc furnace steelmaking has been solved, achieving low-cost and high-efficiency production of low-nitrogen steel.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- INST OF RES OF IRON & STEEL JIANGSU PROVINCE
- Filing Date
- 2023-09-01
- Publication Date
- 2026-05-12
AI Technical Summary
In the existing electric arc furnace steelmaking process, it is difficult to control the nitrogen content, which leads to a reduction in the plasticity and toughness of the steel. In addition, the existing methods have problems such as low production stability and high cost.
The process employs an AC electric arc furnace for smelting, using scrap steel and molten iron as raw materials. It combines iron addition to the rear side wall of the furnace, a high iron-to-molten iron ratio, electromagnetic stirring at the furnace bottom, optimized oxygen supply system, and high-temperature tapping. The process involves heating through carbon-oxygen reaction, reducing the energizing time, preventing the molten steel from being exposed, controlling the temperature and compositional uniformity of the molten steel, and avoiding contact with air.
This technology achieves a nitrogen content of ≤25ppm in molten steel from electric arc furnaces, meeting the requirements of most steel grades, reducing refining processes, and lowering production costs and time.
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Figure CN117230273B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a method for smelting low-nitrogen steel using an electric arc furnace, belonging to the technical field of steelmaking. Background Technology
[0002] In most cases, nitrogen (N) is a harmful element in steel. Excessive N content reduces the plasticity and toughness of steel, increases its aging tendency and brittleness, and impairs its weldability and cold bending performance.
[0003] With the continued advancement of the "dual carbon" target, electric arc furnace steelmaking will see greater development. However, based on past experience, one of the disadvantages of electric arc furnaces compared to converters, which are also primary smelting furnaces, is the greater difficulty in controlling nitrogen content. Generally speaking, the nitrogen content of liquid steel at the end of a converter run is in the range of 15-30 ppm, while that of an electric arc furnace is 50-100 ppm. The nitrogen content in the primary smelting furnace basically determines the nitrogen content level of the final product; therefore, how to effectively control the nitrogen content in electric arc furnace steel is an urgent problem to be solved.
[0004] A search of existing patents revealed Chinese patent CN107502702A, which discloses a "Clean and Rapid Smelting Method for All-Scrap Steel Electric Arc Furnace." This method uses all-scrap steel for smelting and employs a spray gun embedded in the refractory material on the side of the furnace bottom to inject different types of media at different smelting stages, promoting denitrification of the molten steel. Its limitations include: insufficient molten steel to undergo a vigorous carbon-oxygen reaction for denitrification in the later stages of smelting; and the embedded spray gun is difficult to maintain, resulting in low production stability.
[0005] The method disclosed in Chinese patent CN111996334A uses an electric arc furnace with all scrap steel for smelting, which optimizes the formation of foamy slag in the furnace and reduces the nitrogen absorption of the molten steel. Its limitations are: the electric arc furnace uses all scrap steel for charging, resulting in large fluctuations in the nitrogen content of the raw materials fed into the furnace; the carbon content in the raw materials is not high, and the carbon-oxygen reaction is not intense, resulting in unsatisfactory denitrification effect and low stability.
[0006] Chinese patent CN101899548A discloses a new process for preheating scrap steel and high-efficiency electric arc furnace steelmaking. It mentions first melting the scrap steel in an induction furnace and then adding it to the electric arc furnace for smelting, which can shorten the electric arc furnace smelting cycle. Its limitations are: the induction furnace has high energy consumption and a long smelting cycle, which is not compatible with the production rhythm of the electric arc furnace.
[0007] The method disclosed in Chinese patent CN108715912A involves controlling the oxygen content of molten steel in an electric arc furnace to achieve a slightly over-oxidized state, thereby reducing nitrogen absorption during the tapping process. Its limitations are that over-oxidized molten steel can affect the quality of subsequent products, and the deoxidation process in refining can also increase the nitrogen content of the molten steel.
[0008] In addition, electric arc furnaces can reduce the exposure of molten steel to air and decrease nitrogen absorption by operating with high current, low voltage, and a short arc. However, this method not only prolongs the smelting process but also increases costs such as electrode consumption and power consumption. Therefore, this method is not suitable for practical production. Summary of the Invention
[0009] To address the aforementioned problems, this invention discloses a method for smelting low-nitrogen steel using an electric arc furnace, the specific technical solution of which is as follows:
[0010] A method for smelting low-nitrogen steel using an electric arc furnace includes the following steps:
[0011] Raw material charging operation: In the AC electric arc furnace, scrap steel and molten iron are used as the main raw materials. The scrap steel is charged into a circular ladle. The top-opening furnace cover is opened, and the scrap steel is charged from the top of the furnace. After the furnace cover is closed, the molten iron ladle is placed on the ladle rotary table and added into the furnace through the rear chute. The charging rate is controlled at 5-10 tons / min. The CO generated by the decarburization of the electric arc furnace removes some of the nitrogen element from the raw materials. The strong oxidizing atmosphere in the furnace and the foamy slag covering the molten steel reduce the absorption of nitrogen element by the molten steel.
[0012] Power supply operation: After the furnace cover is closed, the scrap steel inside the furnace is melted by conductive heating through electrodes;
[0013] Oxygen supply operation: Oxygen supply from the furnace wall oxygen lance is divided into burner, low oxygen, medium oxygen, high oxygen, and hold modes; oxygen supply from the furnace door oxygen lance is divided into low oxygen and high oxygen modes.
[0014] Electromagnetic stirring at the bottom of the furnace: The electromagnetic stirring technology at the bottom of the furnace is adopted. A magnetic field that penetrates the bottom of the furnace is generated by the stirring coil, thereby strengthening the stirring during the scrap steel smelting process, forcing convection to reduce stratification, and making the temperature and chemical composition in the molten pool reach uniformity more quickly, reducing the time to melt large pieces of scrap steel.
[0015] Slag-making process: Lime is added in small amounts multiple times. The fluidity of the steel slag and the temperature of the molten steel must be controlled throughout the smelting process.
[0016] Steel tapping operation: Power off when taking temperature samples before tapping, and switch the oxygen lance on the furnace wall to hold mode; try to remove slag before tapping to ensure that no slag flows out of the tapping port when the furnace returns to the slag tapping position after tapping; during the tapping process, the longer the contact time between the molten steel and the air and the larger the contact surface area, the more nitrogen is added to the molten steel. Avoid scattering at the tapping port, and do not open the bottom argon blowing of the ladle during tapping to avoid the molten steel inside the ladle churning and coming into contact with the air.
[0017] Furthermore, the electric arc furnace is selected from the electric arc furnaces with AC electric arc power supply systems, and the transformer unit power is above 650kVA / t.
[0018] Furthermore, the raw material feeding operation is carried out by adding molten iron to the rear wall of the furnace.
[0019] Furthermore, the molten iron added during the raw material feeding process has a temperature ≥1250℃ and a composition requirement of C content ≥4.0%, Si content ≤0.7%, P content ≤0.13%, and S content ≤0.03%.
[0020] Furthermore, the electric arc furnace has three furnace wall oxygen lances and one furnace door carbon oxygen lance with carbon powder injection function. The furnace wall oxygen lances are named No. 1, No. 2 and No. 3 in a clockwise direction.
[0021] The oxygen supply operation is as follows: When the smelting begins, the furnace wall oxygen lance is in burner mode. After running for 4 minutes, it is switched to low oxygen mode. Then, lances #1 and #2 use medium oxygen mode, and lance #3 maintains low oxygen mode. High oxygen mode is avoided throughout the process. After the molten steel temperature reaches 1600℃, it is switched to furnace wall lance maintenance mode.
[0022] The oxygen lance at the furnace door is not opened in the early stage of smelting. After the raw materials in the furnace are melted and cleared, the low oxygen mode is turned on. The high oxygen mode is not used throughout the process. After the molten steel temperature reaches 1600℃, the oxygen lance at the furnace door is closed. Depending on the situation in the furnace, carbon powder is sprayed using the oxygen lance at the furnace door, with a total amount of 2-5 kg / t. The purpose is to thicken the slag and avoid slag discharge when tapping steel.
[0023] Furthermore, in the oxygen supply operation, the flow rate ranges for various modes of the furnace wall oxygen lance and the furnace door oxygen lance are shown in the table below:
[0024] .
[0025] Furthermore, during the entire slag-making process, 3-4 batches of lime are added. When slag flows out of the furnace door, the first batch of lime is added in 2-3 batches, with an addition amount of 1.0-1.5 tons. When the carbon-oxygen reaction occurs after the furnace is cleared, the second batch of lime is added in 2-3 batches, with an addition amount of about 1.0-1.5 tons. When the raw materials are completely melted and the temperature reaches 1510℃-1530℃, the third batch of lime is added, with an addition amount of 0.5-0.8 tons in 2-3 batches. If the carbon-oxygen reaction is intense, resulting in a large amount of slag runoff, an additional 0.2-0.3 tons of lime are added.
[0026] Furthermore, the coil current intensity and stirring frequency of the electromagnetic stirring system at the bottom of the electric arc furnace are controlled and adjusted in stages, namely the scrap melting period and the temperature uniformity period. The electromagnetic stirring current intensity ranges from 100-1000A, and the electromagnetic stirring frequency ranges from 0.5-10Hz.
[0027] The working principle and beneficial effects of this invention are as follows:
[0028] This invention provides a method for smelting low-nitrogen steel using an electric arc furnace. In the electric arc furnace process, iron is added to the rear side wall of the furnace and smelted with a high iron-to-water ratio (≥45%). The oxygen supply system is optimized, and electromagnetic stirring technology is used at the bottom of the furnace. Steel is tapped at a relatively high temperature (1620-1650℃) to reduce the refining energizing time. This results in a nitrogen content of ≤25ppm in the steel produced by the electric arc furnace process, avoiding the need for additional vacuum treatment steps in refining and reducing production costs.
[0029] This invention achieves stable control of nitrogen (N) content in steel through electric arc furnace (EAF) smelting. The EAF uses a hot metal ratio of over 45% to achieve high carbon content and replace some electrical heating with carbon oxidation reactions, thus reducing the energizing time. The EAF does not use a high-oxygen mode throughout the process, instead supplying oxygen in a medium-low oxygen mode. This is to avoid the reduced oxidation reaction of elements in the furnace later, which would make heating difficult and necessitate electrical heating, but at this point, the slag foam height is insufficient, easily causing the molten steel to absorb nitrogen. The EAF employs bottom electromagnetic stirring technology to reduce cold zones in the furnace, accelerate the homogenization of steel composition and temperature, and prevent the phenomenon of large pieces of unmelted scrap steel in the later stages of smelting, thereby shortening the smelting time. The EAF tapping temperature requirement is designed to ensure that the molten steel reaches a sufficiently high temperature before refining, reducing refining heating time. The EAF tapping port condition is designed to prevent the molten steel from increasing its contact area with air without timely correction. Using the method of this invention, the nitrogen content of the molten steel produced by electric arc furnace smelting is ≤25ppm, which meets the requirements of most steel grades and reduces the vacuum treatment process, thereby saving production costs.
[0030] This invention provides a method for smelting low-nitrogen steel using an electric arc furnace, thereby meeting the future market demand for short-process production of low-nitrogen steel grades. Attached Figure Description
[0031] Figure 1 This is a diagram showing the distribution of oxygen lances on the furnace wall and at the furnace door of the electric arc furnace of the present invention.
[0032] Figure 2 This is a schematic diagram of the flow of molten steel driven by the electromagnetic stirring at the bottom of the furnace according to the present invention. Implementation
[0033] The smelting process used in this invention is: electric arc furnace (EAF) smelting and eccentric bottom (EBT) tapping. The invention will be further described below with reference to specific embodiments.
[0034] The electric arc furnace has three furnace wall oxygen lances and one furnace door carbon-oxygen lance with carbon powder injection function. The furnace wall oxygen lances are named No. 1, No. 2, and No. 3 in a clockwise direction; see [link to specific locations] for details. Figure 1 The electric arc furnace tapping process incorporates a furnace bottom electromagnetic stirring system with a current intensity of 100-1000A and an electromagnetic stirring frequency of 2-50Hz. The achieved flow effect of molten steel within the furnace is illustrated in the diagram. Figure 2 .
[0035] Example 1:
[0036] The furnace lid of the 100-ton electric arc furnace was opened, and 34 tons of scrap steel were loaded from the top. The furnace lid was then closed. 41 tons of molten iron were added to the rear chute for the first time. Oxygen lances #1, #2, and #3 on the furnace wall were activated with burner mode. After 15 minutes, a second batch of 40 tons of molten iron was added. Oxygen lances #1, #2, and #3 were adjusted to low-oxygen mode, and smelting was initiated. Simultaneously, electromagnetic stirring was activated at the furnace bottom with a current intensity of 300A and a stirring frequency of 2Hz. After 5 minutes, power was cut off, oxygen lances #1 and #2 were adjusted to medium-oxygen mode, and the oxygen lance at the furnace door was opened and set to low-oxygen mode. After 30 minutes of smelting, the power consumption was 3430 kWh, and the oxygen consumption was 2456 Nm³. 3 The temperature was measured at 1617℃, and the nitrogen content was measured to be 16.7 ppm. Oxygen lances #1 and #2 were adjusted to low-oxygen mode, and the electromagnetic stirring current at the furnace bottom was set to 400A, with a stirring frequency of 3Hz. After 34 minutes of smelting, the power consumption was 3430 kWh, and the oxygen consumption was 2587 Nm³. 3 The temperature was measured at 1630℃, and the nitrogen content was found to be 16.3 ppm. After 42 minutes of smelting, the electromagnetic stirring at the bottom of the furnace was turned off, the oxygen lance at the furnace door was closed, and oxygen lances 1, 2, and 3 on the furnace wall were set to holding mode. The final power consumption of this furnace was 3430 kWh and the oxygen consumption was 3046 Nm³. 3 The temperature was measured at 1640℃. The iron-to-liquid ratio in this furnace was 70.4%. During the smelting process, lime was added in multiple batches from the top hopper, with each batch containing 300 kg, consuming a total of 3362 kg of lime. The final nitrogen content at the electric arc furnace endpoint was 18 ppm.
[0037] The steel grade smelted in this furnace is cold heading steel 35K. When the output of the electric arc furnace reaches 70 tons, 500 kg of lime and 210 kg of aluminum blocks are added to the ladle. The N content was measured to be 23.9 ppm after refining.
[0038] Example 2:
[0039] The furnace lid of the 100-ton electric arc furnace was opened, and 40 tons of scrap steel were loaded from the top. The furnace lid was then closed. 40 tons of molten iron were added to the rear chute for the first time. Oxygen lances #1, #2, and #3 on the furnace wall were activated with burner mode. After 15 minutes, a second batch of 35 tons of molten iron was added. Oxygen lances #1, #2, and #3 were adjusted to low-oxygen mode, and smelting was initiated. Simultaneously, electromagnetic stirring was activated at the furnace bottom with a current intensity of 300A and a stirring frequency of 2Hz. After 6 minutes, power was cut off, oxygen lances #1 and #2 were adjusted to medium-oxygen mode, and the oxygen lance at the furnace door was opened and set to low-oxygen mode. After 31 minutes of smelting, the power consumption was 3960 kWh, and the oxygen consumption was 2260 Nm³. 3The temperature was measured at 1590℃, and the nitrogen content was found to be 17.5 ppm. Oxygen lances #1 and #2 were adjusted to low-oxygen mode, and the electromagnetic stirring current at the furnace bottom was set to 400A and the stirring frequency to 3Hz. After 32 minutes of smelting, the power consumption was 3960 kWh and the oxygen consumption was 2423 Nm³. 3 The temperature was measured at 1612℃, and the nitrogen content was found to be 16.8 ppm. After 43 minutes of smelting, the electromagnetic stirring at the bottom of the furnace was turned off, the oxygen lance at the furnace door was closed, and oxygen lances 1, 2, and 3 on the furnace wall were set to holding mode. The final power consumption of this furnace was 3960 kWh and the oxygen consumption was 3204 Nm³. 3 The measured temperature was 1643℃. The iron-to-liquid ratio in this furnace was 65.2%. During the smelting process, lime was added in multiple batches from the top hopper, 300 kg per batch, consuming a total of 2983 kg of lime. The final nitrogen content of the electric arc furnace was 17.6 ppm.
[0040] The steel grade smelted in this furnace is high-carbon steel 70#. When the output of the electric arc furnace reaches 68 tons, 500 kg of lime is added, and the nitrogen content is measured to be 19.1 ppm after refining.
[0041] Example 3:
[0042] The furnace lid of the 100-ton electric arc furnace was opened, and 60 tons of scrap steel were loaded from the top. The furnace lid was then closed. 35 tons of molten iron were added to the rear chute. Oxygen lances #1, #2, and #3 on the furnace wall were activated (burner mode). After 15 minutes, a second batch of 20 tons of molten iron was added. Oxygen lances #1, #2, and #3 were adjusted to low-oxygen mode, and smelting was initiated. Simultaneously, electromagnetic stirring was activated at the furnace bottom, with a current intensity of 300A and a stirring frequency of 2Hz. After 18 minutes, power was cut off, oxygen lances #1 and #2 were adjusted to medium-oxygen mode, and the furnace door oxygen lance was opened and set to low-oxygen mode. After 25 minutes of smelting, the power consumption was 10700 kWh, and the oxygen consumption was 2260 Nm³. 3 The temperature was measured at 1578℃, and the nitrogen content was found to be 20.5 ppm. Smelting was then resumed with the power supply restored. Oxygen lances #1 and #2 were adjusted to low-oxygen mode, and the electromagnetic stirring current at the furnace bottom was set to 400A, with a stirring frequency of 3Hz. After 32 minutes of smelting, the power consumption was 13300 kWh, and the oxygen consumption was 2510 Nm³. 3 The temperature was measured at 1612℃, and the nitrogen content was found to be 22.2 ppm. After 41 minutes of smelting, the electromagnetic stirring at the bottom of the furnace was turned off, the oxygen lance at the furnace door was closed, and oxygen lances 1, 2, and 3 on the furnace wall were set to hold mode. The final power consumption of this furnace was 13300 kWh, the oxygen consumption was 2820 Nm3, and the temperature was measured at 1630℃. The iron-to-metal ratio of this furnace was 47.8%. During the smelting process, lime was added in multiple batches from the top hopper, 300 kg per batch, consuming a total of 3250 kg of lime. The final nitrogen content of the electric arc furnace was 22.1 ppm.
[0043] The steel grade smelted in this furnace is medium carbon steel 45#. When the output of the electric arc furnace reaches 65 tons, 500 kg of lime is added, and the nitrogen content is measured to be 23.5 ppm after refining.
[0044] The technical means disclosed in this invention are not limited to those disclosed above, but also include technical solutions composed of any combination of the above technical features.
[0045] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.
Claims
1. A method for smelting low-nitrogen steel using an electric arc furnace, characterized in that, Includes the following steps: Raw material charging operation: In the AC electric arc furnace, scrap steel and molten iron are used as the main raw materials. The top-opening furnace cover is opened, and the scrap steel is charged from the top of the furnace. After the furnace cover is closed, the molten iron ladle is placed on the molten iron ladle rotary table and added into the furnace through the rear chute. The iron charging speed is controlled at 5-10 tons / min. The CO generated by the decarburization of the electric arc furnace removes some of the N element from the raw materials. The strong oxidizing atmosphere in the furnace and the foamy slag covering the molten steel reduce the absorption of N element by the molten steel. Power supply operation: After the furnace cover is closed, the scrap steel inside the furnace is melted by conductive heating through electrodes; Oxygen supply operation: Oxygen supply from the furnace wall oxygen lance is divided into burner, low oxygen, medium oxygen, high oxygen, and hold modes; oxygen supply from the furnace door oxygen lance is divided into low oxygen and high oxygen modes. Electromagnetic stirring at the bottom of the furnace: The electromagnetic stirring technology at the bottom of the furnace is adopted. A magnetic field that penetrates the bottom of the furnace is generated by the stirring coil, thereby strengthening the stirring during the scrap steel smelting process, forcing convection to reduce stratification, and making the temperature and chemical composition in the molten pool reach uniformity more quickly, reducing the time to melt large pieces of scrap steel. Slag-making process: Lime is added in small amounts multiple times. The fluidity of the steel slag and the temperature of the molten steel must be controlled throughout the smelting process. Steel tapping operation: Power off when taking temperature samples before tapping, and switch the oxygen lance on the furnace wall to hold mode; try to remove slag before tapping to ensure that no slag flows out of the tapping port when the furnace returns to the slag tapping position after tapping; during the tapping process, the longer the contact time between the molten steel and the air and the larger the contact surface area, the more nitrogen is added to the molten steel. Avoid scattering at the tapping port, and do not open the bottom argon blowing of the ladle during tapping to avoid the molten steel inside the ladle churning and coming into contact with the air.
2. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 1, characterized in that, The electric arc furnace is selected from those using an AC electric arc power supply system, and the transformer unit power is above 650kVA / t.
3. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 1, characterized in that, The raw material feeding process involves adding molten iron through the rear wall of the furnace.
4. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 3, characterized in that, The molten iron added during the raw material feeding process has a temperature ≥1250℃ and a composition requirement of C content ≥4.0%, Si content ≤0.7%, P content ≤0.13%, and S content ≤0.03%.
5. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 1, characterized in that, The electric arc furnace has three furnace wall oxygen lances and one furnace door carbon oxygen lance with carbon powder injection function. The furnace wall oxygen lances are named No. 1, No. 2 and No. 3 in clockwise order. The oxygen supply operation is as follows: When the furnace wall oxygen lance starts smelting, the furnace wall lance burner mode is used. After running for 4 minutes, it is switched to the furnace wall lance low oxygen mode. Then, lances #1 and #2 use medium oxygen mode, and lance #3 maintains low oxygen mode. High oxygen mode is avoided throughout the process. After the molten steel temperature reaches 1600℃, it is switched to the holding mode. The oxygen lance at the furnace door should not be opened in the early stage of smelting. After the raw materials in the furnace are melted and cleared, the low oxygen mode should be turned on. After the temperature of the molten steel reaches 1600℃, the oxygen lance at the furnace door should be closed. Depending on the situation in the furnace, carbon powder should be sprayed using the oxygen lance at the furnace door, with a total amount of 2-5 kg / t. The purpose is to thicken the slag and avoid slag discharge when tapping steel.
6. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 5, characterized in that, The flow rate ranges for various modes of the furnace wall oxygen lance and furnace door oxygen lance during the oxygen supply operation are shown in the table below: 。 7. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 1, characterized in that, Throughout the slag-making process, 3-4 batches of lime are added. When slag flows out of the furnace door, the first batch of lime is added in 2-3 batches, with an amount of 1.0-1.5 tons. When the furnace is cleared and a carbon-oxygen reaction occurs, the second batch of lime is added in 2-3 batches, with an amount of 1.0-1.5 tons. When the raw materials are completely melted and the temperature reaches 1510℃-1530℃, the third batch of lime is added, in 2-3 batches, with an amount of 0.5-0.8 tons. If the carbon-oxygen reaction is intense, resulting in a large amount of slag runoff, an additional 0.2-0.3 tons of lime are added.
8. The method for smelting low-nitrogen steel using an electric arc furnace according to claim 1, characterized in that, The current intensity and stirring frequency of the electromagnetic stirring system at the bottom of the electric arc furnace are controlled and adjusted in stages, namely the scrap melting period and the temperature uniformization period; during the scrap melting period, the current intensity ranges from 200 to 350A and the electromagnetic stirring frequency ranges from 1 to 3Hz; during the temperature uniformization period, the current intensity ranges from 300 to 500A and the electromagnetic stirring frequency ranges from 2 to 5Hz; before tapping the steel, the electromagnetic stirring is turned off.