A production process for accurately controlling the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel in an ecological electric furnace

Abstract

The present invention discloses a production process for accurately controlling the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel smelted in an ecological electric furnace. The process adopts an ecological electric furnace smelting, LF refining, RH vacuum degassing, and a large square billet continuous casting production process. A series of measures such as steel tapping endpoint control, slag control, optimization of LF white slag effect and time, bottom blowing flow control, and vacuum time control are used to control the key control points of each production link throughout the entire process. Without the need for additional investment in funds and production equipment, the aluminum yield and nitrogen content in the smelting process can be stabilized, and the aluminum-nitrogen ratio in the steel can be accurately controlled to be 2.5-2.8, thereby preventing grain growth. This greatly solves the possibility of scrapping workpieces in the forging process due to grain reasons, reduces user risks, and helps to enhance the company's economic benefits and brand image. At the same time, the use of ECOARC ecological electric furnace smelting reduces power consumption and reduces carbon dioxide and dioxin emissions.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a production process for accurately controlling the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel in an ecological electric furnace. Background Art

[0002] 18CrNiMo7-6, a case-hardening steel grade specified in the European standard EN10084, is primarily used in China for wind turbine reducer gears. This product offers high added value and substantial profit margins, attracting a surge in production among specialty steel companies. However, since finished 18CrNiMo7-6 gears are often installed in critical locations within wind turbine systems and experience long service lives, customers demand not only stable chemical composition and high molten steel purity, but also a narrow hardenability band, high contact fatigue strength, and good wear resistance, hardness, and toughness. To achieve these excellent properties, 18CrNiMo7-6 requires carburizing, but this process often results in the formation of network or angular carbides, which reduces the gear's fatigue strength. Furthermore, the carburizing process exhibits a significant tendency for austenite grain growth, and mixed crystal formation can occur. This increases the gear's tendency to deform and crack during quenching, making cracks more likely to form during quenching or machining, resulting in component failure. Studies have shown that when the ratio of aluminum to nitrogen in carburizing steel ([Al] / [N]) is between 2 and 4, AlN is primarily distributed at grain boundaries, pinning them and preventing grain growth. Therefore, controlling [Al] / [N] is an effective means of reducing the tendency of carburized steel to crack. 18CrNiMo7-6 gear steel requires Al ≤ 0.030% and N ≤ 100 ppm. Simply controlling the [Al] and [N] contents is not difficult in actual production; the key is to accurately control the [Al] / [N] ratio within a reasonable range. This requires innovation and refinement of the entire smelting process, thereby reducing the tendency of grain growth in 18CrNiMo7-6 gear steel during carburization, preventing the occurrence of mixed crystals, and alleviating the burden of the heat treatment process.

[0003] Currently, existing technologies for refining the grain size of 18CrNiMo7-6 primarily focus on the forging and heat treatment stages, with limited methods for controlling grain growth from a smelting perspective. Furthermore, precisely controlling the content of certain elements in steel can facilitate stable and smooth production, reducing the risk of entire furnaces being scrapped due to substandard chemical composition, thereby achieving the goals of refined management and lean operations. For these reasons, there is an urgent need to develop a production process that precisely controls the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel smelted in an ecological electric furnace. This process can meet user requirements for wind turbine gear steel and has significant practical significance for improving corporate profitability. Summary of the Invention

[0004] In order to solve the above-mentioned technical problems in electric furnace smelting of 18CrNiMo7-6 gear steel, the present invention stabilizes the aluminum yield and nitrogen content during the smelting process through endpoint control, slag discharge control, optimization of LF white slag effect and time, bottom blowing flow control and vacuum time control, thereby accurately controlling the aluminum-nitrogen ratio in the steel.

[0005] To achieve the above-mentioned purpose, the present invention provides a production process for accurately controlling the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel in an ecological electric furnace, which adopts the following technical solutions:

[0006] Ecological Electric Furnace Smelting: The ECOARC ecological electric furnace utilizes continuous scrap feeding and a molten pool retention mode. Scrap is preheated to above 600°C, with a loading of 110-120 tons. The nitrogen content of the incoming scrap and auxiliary materials is kept below 60 ppm, minimizing nitrogen carryover from the incoming raw materials. As smelting progresses, lime (39-41 kg / t), dolomite (19-21 kg / t), and coke pellets (26-42 kg / t) are added in batches to create foamed slag. The slag basicity (CaO / SiO2) is controlled between 2.0 and 2.5, with FeO ≤ 25%. The tapping point C is 0.04-0.10%, P ≤ 0.010%, and the tapping temperature is 1620-1640°C, ensuring the desired carbon content and temperature are achieved immediately. Before tapping, pre-purge argon for 2-3 minutes. During tapping, the flow rate is 200-400 NL / min. The alloy is baked to 400-500°C. Slag is blocked during tapping, and slag and molten steel dispersion are strictly prohibited. The tapping time is 3-5 minutes. After 10-15 tons of steel have been tapped, 800 kg of ladle lime per furnace, 1.8-2.1 kg / t of aluminum wire segments, 21-22 kg / t of low-carbon ferrochrome, and 4.5-6.0 kg / t of medium-carbon ferromanganese are added to the ladle. The ladle clearance is 450-500 mm.

[0007] LF refining: The ladle is fed into the LF refining furnace. The arrival temperature of the molten steel is 1520-1540°C, and the carbon content is above 0.075%. Excessive carbon addition is required to prevent molten steel from inhalation. Close contact between the LF furnace cover and the ladle is ensured. A slight positive pressure of 10-20 Pa is maintained inside the furnace. Submerged arc heating is used to prevent arc light leakage. The argon flow rate is continuously adjusted during the refining process, and excessive argon blowing is prohibited to prevent nitrogen absorption. Diffusion deoxidation is performed at 200 NL / min, carburization, desulfurization, and alloying at 350 NL / min, and the final refining stage at 120 NL / min. Temperature measurement, sampling, and wire feeding are performed at 90 NL / min. Lime is added at 4.0-6.5 kg / t to the LF refining furnace. After the initial 2-3 minutes of power supply to form liquid slag, carbon powder is added to initiate carburization. Silicon carbide (1-2 kg / t) is then added in batches for diffusion deoxidation. White slag is produced throughout the entire process. Refining slag is added during the desulfurization process, and its composition is controlled as shown in Table 1.

[0008] Table 1 Chemical composition mass content of LF refined slag

[0009] CaO <![CDATA[Al2O3]]> <![CDATA[SiO2]]> MgO FeO 50～58％ 27～32％ 5～10％ ≤8％ ≤0.65％

[0010] When the temperature reaches 1560-1600℃, take a full analysis sample. After the composition is reported, add 0.5-1.5kg / t of aluminum wire, 1.0-2.5kg / t of ferrosilicon, 1.0-2.0kg / t of high carbon ferromanganese, 1.5-2.0kg / t of low carbon ferrochrome, 7.5-8.5kg / t of high carbon ferrochrome, 4.5-5.5kg / t of ferromolybdenum, and 16.0-18.0kg / t of electrolytic nickel to adjust the composition. The aluminum content in the steel is adjusted to 0.020-0.040%. After refining, the liquid surface is strictly prohibited from being exposed during the transfer of the ladle. The chemical composition before leaving the station must meet the requirements of C: 0.16-0.19%, Al: 0.025-0.035%. The temperature of the first furnace is 1676-1686℃, the second furnace is 1656-1666℃, the middle furnace is 1636-1646℃, the tail furnace is 1546-1556℃, and then enters the RH process.

[0011] RH vacuum degassing: After the ladle is hoisted onto the ladle car, open the argon gas blow, the flow rate is 300 ~ 450NL / min, after entering the working position, start the jacking operation, the immersion tube insertion depth is greater than 400mm, the vacuum degree is <67Pa, the duration is 10 ~ 12min, restore the argon gas in time after breaking the air, feed 150m / furnace of silicon calcium wire, 60kg / furnace of covering agent, static argon blowing time >10min, the temperature of the first furnace meets 1601 ~ 1611℃, the second furnace 1586 ~ 1596℃, the middle furnace 1571 ~ 1581℃, the tail furnace 1581 ~ 1591℃, and then carry out the casting operation.

[0012] Continuous Casting: Vacuum-degassed molten steel is hoisted to the casting position, with the superheat controlled at 25-40°C. A continuous slag detection device is used in the continuous casting ladle to prevent slag from falling. Electromagnetic stirring parameters are shown in Table 2. Low-carbon mold slag is used.

[0013] Table 2 Electromagnetic stirring parameters

[0014]

[0015] Different casting speeds are set according to the different superheat of molten steel. The control targets are shown in Table 3.

[0016] Table 3 Casting speed control targets

[0017] Superheat ℃ Blank <20 20～35 36～45 >45 Casting speed m / min 390×480 0.48 0.45 0.42 0.39

[0018] Heating: The heating temperature should be based on the lower limit of Cr-Ni-Mo steel at 1180-1200℃. The total heating time should not be less than 330min. Ensure that the heating furnace is in good condition and production is running smoothly to avoid excessive extension of the heating time.

[0019] Rolling: High-pressure water descaling before rolling, descaling pressure 28 ~ 30MPa, starting rolling temperature ≥ 1100℃, finishing rolling temperature ≥ 850℃.

[0020] An 18CrNiMo7-6 gear steel prepared by the above production process, wherein the chemical composition of the 18CrNiMo7-6 gear steel is, by mass percentage, C: 0.180-0.195%, Si: 0.22-0.30%, Mn: 0.53-0.57%, P≤0.015%, S≤0.010%, Cr: 1.60-1.64%, Ni: 1.44-1.48%, Mo: 0.27-0.29%, Al≤0.028%, Ca≤0.0015%, and Cu≤0.12%; and the aluminum-nitrogen ratio is controlled to be 2.5-2.8.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] This invention utilizes an ECOARC ecological electric furnace smelting process, including LF refining, RH vacuum degassing, and bloom continuous casting. Through a series of measures, including tapping endpoint control, slag discharge control, optimization of LF white slag effect and timing, bottom blowing flow control, and vacuum time control, key control points in each production link are fully controlled throughout the entire process. Without the need for additional investment in capital and production equipment, the aluminum yield and nitrogen content in the smelting process can be stabilized, and the aluminum-nitrogen ratio in the steel can be precisely controlled to 2.5-2.8, preventing grain growth. This greatly reduces the possibility of workpiece scrapping during the forging process due to grain problems, reduces user risks, and helps enhance the company's economic benefits and brand image. Furthermore, the use of the ECOARC ecological electric furnace smelting reduces electricity consumption and reduces carbon dioxide and dioxin emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Metallographic photograph of inclusions in finished steel prepared in Example;

[0024] Figure 2 This is a metallographic photograph of the grain size of the finished steel prepared in the example. DETAILED DESCRIPTION

[0025] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way. To avoid redundancy, the raw materials in the following examples are all commercially available products unless otherwise specified, and the processes used are all conventional processes unless otherwise specified.

[0026] Example

[0027] This invention proposes a production process for precisely controlling the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel smelted in an ecological electric furnace. The specific process flow is: ecological electric furnace smelting → LF → RH → continuous casting of 390×480 large square blooms → heating → rolling. The controlled chemical composition of 18CrNiMo7-6 gear steel is, by mass percentage, C: 0.180-0.195%, Si: 0.22-0.30%, Mn: 0.53-0.57%, P ≤ 0.015%, S ≤ 0.010%, Cr: 1.60-1.64%, Ni: 1.44-1.48%, Mo: 0.27-0.29%, Al ≤ 0.028%, Ca ≤ 0.0015%, and Cu ≤ 0.12%.

[0028] Ecological Electric Furnace Smelting: The ECOARC ecological electric furnace utilizes continuous scrap feeding and a molten pool retention mode. Scrap is preheated to above 600°C, with a loading of 110-120 tons. The nitrogen content of the incoming scrap and auxiliary materials is kept below 60 ppm, minimizing nitrogen carryover from the raw materials. As smelting progresses, active lime and dolomite are added to the furnace in batches, and coke particles are injected to create foamed slag, as shown in Table 4. The slag basicity (CaO / SiO2) is controlled between 2.0 and 2.5, with FeO ≤ 25%. The slag composition is shown in Table 5. The tapping endpoint C is 0.04-0.10%, P ≤ 0.010%, and the tapping temperature is 1620-1640°C. The carbon content and temperature are guaranteed to be achieved immediately, as shown in Table 6. Before tapping, pre-purge argon for 3 minutes at a flow rate of 300 L / min. The alloy is baked to 400-500°C. Slag is blocked during tapping, and slag and molten steel dispersion are strictly prohibited. The tapping time is 3-5 minutes. After 10-15 tons of tapping, add ladle lime, aluminum wire segments, low-carbon ferrochrome, medium-carbon ferromanganese, and other alloys into the ladle, as shown in Table 4. Ensure the ladle clearance is 450-500 mm.

[0029] Table 4 Amount of materials added to the electric furnace

[0030]

[0031] Table 5 Composition of electric furnace slag

[0032] Example CaO <![CDATA[SiO2]]> FeO R 1 38.92 17.98 24.74 2.16 2 31.85 14.55 22.84 2.19

[0033] Table 6 Composition and temperature of molten steel

[0034] Example C / ％ P / ％ Temperature / ℃ 1 0.057 0.007 1630 2 0.043 0.006 1631

[0035] LF refining: The ladle is fed into the LF refining station. The arrival temperature of the molten steel is 1520-1540°C, and the carbon content is above 0.075%. Excessive carbon addition is required to prevent molten steel from inhalation. Close contact between the LF furnace cover and the ladle is ensured. A slight positive pressure of 10-20 Pa is maintained inside the furnace. Submerged arc heating is used to prevent arc light leakage. The argon flow rate is continuously adjusted during the refining process, and excessive argon blowing is prohibited to prevent nitrogen absorption. Diffusion deoxidation is performed at 200 NL / min, carburization, desulfurization, and alloying at 350 NL / min, and the final refining stage at 120 NL / min. Temperature measurement, sampling, and wire feeding are performed at 90 NL / min. Lime is added to the LF refining station. After the initial 2-3 minutes of power supply to form liquid slag, carbon powder is added to initiate carburization. Silicon carbide (1.0-2.5 kg / t) is then added in batches for diffusion deoxidation. White slag is produced throughout the entire process, and the refining slag composition is within the target range. The refining slag composition is shown in Table 7, with the remainder being unavoidable impurities. When the temperature reaches 1560-1600°C, a full analysis sample is taken. After the composition is reported, aluminum wire segments, ferrosilicon, high-carbon ferromanganese, low-carbon ferrochrome, high-carbon ferrochrome, ferromolybdenum, and electrolytic nickel are added to adjust the composition. The aluminum content in the steel is adjusted to 0.020-0.040% in one step. The alloy addition amounts are shown in Table 8. After refining, the liquid surface is strictly prohibited during the transfer of the ladle. The chemical composition of the steel outgoing from the refining station is guaranteed to meet C: 0.16-0.19%, Al: 0.025-0.035%. The temperature of the first refining furnace is 1676-1686°C, the second refining furnace is 1656-1666°C, the intermediate refining furnace is 1636-1646°C, and the tail refining furnace is 1546-1556°C (the outgoing temperature and composition of LF refining are shown in Table 9). The steel then enters the RH process.

[0036] Table 7 LF refined slag composition

[0037] Example CaO <![CDATA[Al2O3]]> <![CDATA[SiO2]]> MgO FeO 1 50.56 28.30 5.63 7.92 0.62 2 51.02 27.28 8.36 7.90 0.64

[0038] Table 8 LF alloy addition amount

[0039]

[0040] Table 9 LF refining outlet temperature and composition

[0041] Example Outgoing station temperature / ℃ Outbound C / % Al％ 1 1662 0.185 0.030 2 1637 0.160 0.031

[0042] RH vacuum degassing: As shown in Table 10, after the ladle seat is on the ladle car, open the argon blow, the flow rate is 300 ~ 450NL / min, after entering the working position, start the jacking operation, the immersion tube insertion depth is greater than 400mm, the vacuum degree is <67Pa, the duration is ≥12min, and the argon is restored in time after breaking the air, 150m / furnace of silicon calcium wire and 60kg / furnace of covering agent are fed, the static argon blowing time is ≥10min, the temperature of the first furnace meets 1601 ~ 1611℃, the second furnace 1586 ~ 1596℃, the middle furnace 1571 ~ 1581℃, the tail furnace 1581 ~ 1591℃, and then the casting operation is carried out.

[0043] Table 10 RH treatment parameters

[0044] Example Vacuum time / min Static blowing time / min Outgoing station temperature / ℃ 1 20 19 1610 2 22 27 1589

[0045] Continuous Casting: The vacuum-degassed molten steel was hoisted to the casting position. The superheat of the molten steel was controlled at 25-40°C. The casting speed met the target requirements, as shown in Table 12. A continuous slag detection device was used in the continuous casting ladle to prevent slag from falling. The electromagnetic stirring parameters are shown in Table 11. Low-carbon mold slag was used. The chemical composition of the tundish met the internal control requirements: C: 0.180-0.195%, Si: 0.22-0.30%, Mn: 0.53-0.57%, P ≤ 0.015%, S ≤ 0.010%, Cr: 1.60-1.64%, Ni: 1.44-1.48%, Mo: 0.27-0.29%, Al ≤ 0.028%, Ca ≤ 0.0015%, and Cu ≤ 0.12%.

[0046] Table 11 Electromagnetic stirring parameters

[0047]

[0048] Table 12 Continuous casting process parameters

[0049] Example Superheat / ℃ Casting speed m / min 1 35 0.45 2 35 0.45

[0050] Heating: The heating temperature should be based on the lower limit of Cr-Ni-Mo steel at 1180-1200℃. The total heating time should not be less than 330min. Ensure that the heating furnace is in good condition and production is running smoothly to avoid excessive extension of the heating time.

[0051] Rolling: High-pressure water descaling before rolling, descaling pressure 28 ~ 30MPa, starting rolling temperature ≥ 1100 ℃, finishing rolling temperature ≥ 850 ℃. The chemical composition of the finished steel is shown in Table 13, its inclusions and grain size are shown in Table 14, and the metallographic photos of the inclusions are shown in Table 15. Figure 1 As shown in the metallographic photograph of the grain size Figure 2 The mechanical properties of the finished steel are shown in Table 15, and the terminal hardenability of the finished steel is shown in Table 16. The above test results show that the present invention can accurately control the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel to between 2.5 and 2.8, effectively control the grain size to above level 7, and control the level of Class A inclusions to below level 1.0. The mechanical properties and hardenability of the gear steel meet customer requirements.

[0052] Table 13 Chemical composition of finished steel

[0053]

[0054] Table 14 Inclusions and grain size of finished steel

[0055]

[0056] Table 15 Mechanical properties of finished steel

[0057] Example Yield strength / Mpa Tensile strength / Mpa Sectional shrinkage / % Elongation after break / % 1 1093 1383 60 12 1 1088 1330 62 11 2 1006 1309 63 13.5 2 1062 1360 61 12.5

[0058] Table 16 End hardenability of finished steel

[0059] Example J9 / HRC J15 / HRC J40 / HRC 1 44.0 41.5 36.0 2 44.5 43.5 37.0

[0060] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A production process for accurately controlling the aluminum-nitrogen ratio of 18CrNiMo7-6 gear steel in an ecological electric furnace, characterized in that: The chemical composition of the 18CrNiMo7-6 gear steel is as follows: C: 0.180% to 0.195%, Si: 0.22% to 0.30%, Mn: 0.53% to 0.57%, P≤0.015%, S≤0.010%, Cr: 1.60% to 1.64%, Ni: 1.44% to 1.48%, Mo: 0.27% to 0.29%, Al≤0.028%, Ca≤0.0015%, Cu≤0.12% by mass; the aluminum-nitrogen ratio is controlled to be 2.75 to 2.80; The production process adopts the following technical solutions: Ecological electric furnace smelting: The ECOARC ecological electric furnace adopts continuous charging of all scrap steel and smelting mode of steel retention in the molten pool; the nitrogen content of the scrap steel and auxiliary materials entering the furnace is controlled below 60ppm; the tapping endpoint is controlled to C: 0.04% to 0.10%, P ≤ 0.010%, and the temperature is 1620℃ to 1640℃; argon is pre-blown for 2 to 3 minutes before tapping, and the flow rate during tapping is 200 to 400NL / min. The alloy is baked to 400℃ to 500℃, and the steel is tapped with slag blocking. Slag discharge and molten steel dispersion are strictly prohibited. The tapping time is 3 to 5 minutes. LF refining: During the refining process, the argon flow rate is continuously adjusted. Large argon blowing is prohibited to prevent nitrogen absorption. The diffusion deoxidation rate is 200NL / min, the carbonization, desulfurization, and alloying rates are 350NL / min, the late refining rate is 120NL / min, and the temperature measurement, sampling, and wire feeding rates are 90NL / min. LF is supplemented with lime at 4.0-6.5kg / t. After the initial power supply for 2-3 minutes to form liquid slag, carbon powder is added to begin carbonization, and 1-2kg / t of silicon carbide is added in batches for diffusion deoxidation. The entire process is a white slag operation. The refined slag is added during the desulfurization treatment, and the chemical composition of the refined slag is CaO: 50% to 58%, Al2O3: 27% to 32%, SiO2: 5% to 10%, MgO≤8%, and FeO≤0.65% by weight; After LF refining, the chemical composition of the product before delivery must meet the requirements of C: 0.16% to 0.19%, Al: 0.025% to 0.035%; RH vacuum degassing: After the ladle is hoisted onto the ladle car, open the argon gas with a flow rate of 300-450NL / min. After entering the working position, start the jacking operation, the immersion tube insertion depth is greater than 400mm, the vacuum degree is less than 67Pa, and the duration is 10-12min; Continuous casting: The vacuum degassed molten steel is hoisted to the casting position and the superheat of the molten steel is controlled at 25℃~40℃. A continuous slag detection device is used for the continuous casting ladle to prevent slag from falling. Different casting speeds are set according to the superheat of the molten steel: when the superheat is 20℃~35℃, the casting speed is controlled at 0.45m / min; when the superheat is 36℃~40℃, the casting speed is controlled at 0.42m / min. Heating: The heating temperature should be between 1180℃ and 1200℃, and the total heating time should not be less than 330min; Rolling: High-pressure water descaling before rolling, descaling pressure 28 ~ 30MPa, starting rolling temperature ≥ 1100℃, finishing rolling temperature ≥ 850℃.

2. The production process according to claim 1, characterized in that In the RH vacuum degassing process, the temperature is controlled to meet the requirements of 1601℃~1611℃ for the first furnace, 1586℃~1596℃ for the second furnace, 1571℃~1581℃ for the middle furnace, and 1581℃~1591℃ for the tail furnace.

3. The production process according to claim 1, characterized in that In the continuous casting process, the crystallizer electromagnetic stirring parameters are controlled as follows: current 290A, frequency 2Hz; the end electromagnetic stirring parameters are controlled as follows: current 500A, frequency 7Hz, and the stirring mode is alternating 20s-5s-20s.

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