A microalloying method for optimizing the composition of wind turbine gear steel

Abstract

This invention relates to the field of wind turbine gear steel and its preparation technology, and discloses a micro-alloying method for optimizing the composition of wind turbine gear steel. Using 18CrNiMo7-6 steel as the base material, micro-alloying elements are added, mixed, remelted, and then subjected to smelting and heat treatment to obtain micro-alloyed wind turbine gear steel. The micro-alloying elements are at least one of niobium, vanadium, and boron. This method, using 18CrNiMo7-6 steel as the base material, effectively improves the hardenability of the gear steel, refines the grain size, and enhances strength and toughness through precise control of the amount of micro-alloying elements added. Vacuum arc melting and heat treatment processes achieve homogenization of material composition and optimization of microstructure, improving the overall performance of the gear steel. The micro-alloyed wind turbine gear steel obtained by this invention has better load-bearing capacity and longer service life, meeting the needs of wind power equipment development towards heavy loads, high speeds, and lightweight designs.

Description

Technical Field

[0001] This invention relates to the field of wind turbine gear steel and its preparation technology, specifically to a microalloying method for optimizing the composition of wind turbine gear steel. Background Technology

[0002] Wind power, as a clean and renewable energy source, is playing an increasingly important role in the global energy structure. As a key component of wind turbines, the performance of the wind turbine gearbox directly affects the overall operating efficiency and service life of the turbine. Gear steel, as the core material of the gearbox, directly influences its load-bearing capacity and service life. With the development of wind power equipment towards heavy-duty, high-speed, and lightweight designs, higher requirements are being placed on the performance of gear steel, including narrow end-hardenability, uniform and fine austenite grain size, high purity, and good machinability.

[0003] Currently, wind turbine gear steel is mainly manufactured using carburized steel. However, with the demands of industrial development, traditional gear steel materials can no longer meet higher performance requirements. Microalloying technology, which involves adding small amounts of alloying elements to gear steel, is an effective means to achieve these goals. Microalloying elements such as Nb, V, and Ti can improve various properties of steel through a series of strengthening effects, such as refining grain size and increasing strength and toughness.

[0004] Existing technology CN113388783A discloses a Nb, V, Ti microalloyed gear steel and its preparation method, heat treatment method, carburizing treatment method, and carburized gear steel. It obtains Nb, V, Ti microalloyed gear steel with a ferrite + pearlite microstructure through Nb, V, and Ti microalloying. The production process employs electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-hot rolling-slow cooling. After heat treatment following hot rolling and slow cooling, the gear steel exhibits a tensile strength ≥1455MPa, yield strength ≥1320MPa, elongation ≥15%, reduction of area ≥50%, and impact energy KV2 ≥95J. Carburizing the hot-rolled and slow-cooled gear steel yields Nb, V, Ti microalloyed carburized gear steel with high contact fatigue performance, exhibiting a rated fatigue life L10 ≥5×10⁻⁶ under a compressive stress of 4.0GPa. 7 The median fatigue life L50 is ≥ 8.2 × 10⁻⁶. 7 .

[0005] However, existing microalloying technologies still present some challenges in the application of wind turbine gear steel. These include difficulty in controlling the amount of microalloying elements added and uneven microstructure and properties after microalloying, both of which negatively impact the performance of the resulting wind turbine gear steel. Furthermore, current technologies produce these wind turbine gear steels from powdered raw materials, which is not suitable for improving the microalloying performance of existing 18CrNiMo7-6 steel and cannot resolve the technical problems associated with the performance defects of existing 18CrNiMo7-6 steel as a wind turbine gear steel.

[0006] Therefore, developing a microalloying method to optimize the composition of wind turbine gear steel not only effectively compensates for the shortcomings of existing technologies, but also effectively improves the mechanical properties and reliability of 18CrNiMo7-6 steel as a wind turbine gear steel, which has important practical significance and application prospects. Summary of the Invention

[0007] The present invention aims to provide a micro-alloying method for optimizing the composition of wind turbine gear steel, so as to solve the technical problem of performance defects of existing 18CrNiMo7-6 steel as wind turbine gear steel.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a micro-alloying method for optimizing the composition of wind turbine gear steel, using 18CrNiMo7-6 steel as the base material, adding micro-alloying elements, mixing and remelting uniformly, and then performing smelting and heat treatment to obtain micro-alloyed wind turbine gear steel; the micro-alloying elements are at least one of niobium, vanadium, and boron.

[0009] The principles and advantages of this scheme are:

[0010] 1. Compared to existing technologies that produce gear steel from powder raw materials, this solution uses 18CrNiMo7-6 steel as the base material. By adding microalloying elements, remelting, and then heat-treating, it effectively improves the hardenability of the gear steel, refines the grain size, and enhances strength and toughness. This optimizes the production of higher-performance microalloyed gear steel. It not only effectively utilizes existing stockpiles of 18CrNiMo7-6 steel but also produces higher-performance wind turbine gear steel, improving the load-bearing capacity and other performance characteristics of wind turbine gearboxes—a win-win situation.

[0011] 2. Compared to existing technologies that produce gear steel from powder raw materials, which have lower difficulty in achieving uniform melting, this solution uses 18CrNiMo7-6 steel as its base material. 18CrNiMo7-6 steel already possesses certain strength and toughness. Microalloying requires improving hardenability and fatigue life without excessively compromising plasticity (e.g., elongation, reduction of area). This places stringent requirements on element ratios (e.g., Nb / V / B ratio) and heat treatment processes (e.g., cooling rate). This solution optimizes the performance of existing 18CrNiMo7-6 steel through microalloying to enhance its properties. The remelting process presents greater challenges in achieving uniform mixing, and compared to the higher energy consumption of crushing 18CrNiMo7-6 steel into powder, this solution solves these problems by optimizing the remelting process, effectively simplifying the production process, saving steelmaking energy, and improving production efficiency.

[0012] 3. Compared to existing technologies that simultaneously add multiple microalloying elements, leading to raw material waste, this solution effectively reduces raw material costs and further improves production efficiency by limiting the range and method of addition of microalloying elements. Specifically, adding niobium refines the grain size, improves the strength, toughness, and fatigue resistance of the steel, and adapts it to the long-term operation of wind turbine gears under complex stress environments. Adding vanadium forms carbides and nitrides, which are dispersed in the steel, enhancing its strength, hardness, and wear resistance, and extending the service life of the wind turbine gears. The addition of boron significantly improves the hardenability of the steel, reduces temper brittleness, and enhances the overall performance of the steel, resulting in a more uniform microstructure and performance of the gears after heat treatment. However, the industry often limits the amount of boron used (usually ≤0.002%) due to the risk of grain boundary segregation. But this solution, by precisely controlling the amount of boron added (0.001%~0.004%) and the heat treatment process (such as low-temperature aging), achieves the effect of boron on hardenability enhancement (improvement of about 15%) without significant embrittlement.

[0013] 4. The 18CrNiMo7-6 steel selected in this scheme ensures that the gear steel possesses certain basic properties, such as good strength and toughness, providing a stable matrix for subsequent microalloying, thereby effectively improving the overall performance of the prepared wind turbine gear steel. Furthermore, the material in this scheme also has the potential for lightweighting. Specifically, under the same strength, the density of 18CrNiMo7-6 steel (7.85 g / cm³) is... 3 The content is lower than that of typical wind turbine gear steel 20CrNi2Mo (7.93 g / cm³). 3 This approach allows for greater flexibility in reducing blade weight; moreover, the process of this solution is compatible, effectively preserving the wide heat treatment window of 18CrNiMo7-6 steel (such as compatibility with carburizing / nitriding processes), providing more possibilities for subsequent surface strengthening.

[0014] Preferably, as an improvement, the method includes the following steps:

[0015] Step 1: Select 18CrNiMo7-6 steel as the base material. After cutting, the base material is ultrasonically cleaned in acetone solution and dried for later use.

[0016] Step 2: Add microalloying elements to the above-mentioned basic materials, premix, and then melt evenly;

[0017] Step 3: Use a high-vacuum electric arc melting furnace to melt the micro-alloyed ingots, and repeatedly remelt the solidified alloy 5 to 8 times to ensure homogeneity of composition.

[0018] Step 4: Heat treat the smelted ingots;

[0019] Step 5: Machining the heat-treated ingot to obtain the required gear shape and size;

[0020] Step Six: Conduct performance testing on the final product.

[0021] Technical Effects: This solution, employing the aforementioned setup, facilitates the microalloying optimization of the base material 18CrNiMo7-6 steel into higher-performance microalloyed wind turbine gear steel. It systematically optimizes the composition of the wind turbine gear steel, ensuring reliable quality and stable performance of the final product, meeting practical application requirements. Specifically, firstly, this solution effectively removes impurities and moisture from the surface of the base material through cleaning and drying, ensuring a clean environment for subsequent processing and preventing impurities from affecting the steel's performance. Secondly, precise control of the added amount of microalloying elements effectively improves the hardenability of the gear steel, refines the grain size, and enhances strength and toughness; while pre-mixed melting ensures the uniform distribution of microalloying elements in the molten steel, laying the foundation for obtaining gear steel with uniform performance. Step three, high-vacuum melting and repeated remelting, reduces the incorporation of impurity gases, eliminates component segregation, and ensures the uniformity of the steel's microstructure and performance stability. Step four, heat treatment, improves the steel's microstructure and enhances its comprehensive mechanical properties, including strength, hardness, and toughness. Step five, machining, ensures the dimensional accuracy and surface quality of the gears, meeting actual assembly and operational requirements. Step six, performance testing, ensures product quality and provides data support for product reliability and safety.

[0022] Preferably, as an improvement, in step one, the concentration of acetone solution is 3-5%; the ultrasonic cleaning power is 20-50W and the time is 10-60min; and the drying is carried out at 100-150℃ for 1-3h.

[0023] Technical Effects: This solution, employing the aforementioned settings, facilitates effective cleaning of oil stains and impurities from the surface of the cut materials. Specifically, the acetone solution of a certain concentration possesses excellent solubility and volatility, effectively removing oil stains and impurities from the surface of the base material, ensuring the purity of subsequent processing. Ultrasonic cleaning utilizes the cavitation effect of ultrasound to penetrate deep into the fine gaps and pores of the base material, further enhancing the cleaning effect and providing a more thorough cleaning compared to ordinary cleaning methods. Drying at 100℃ for 1 hour ensures complete evaporation of moisture from the base material, preventing residual moisture from affecting subsequent melting and other steps, and ensuring the stability of material properties. Through long-term experiments, the applicant has found that without acetone solution cleaning, oil stains are difficult to completely remove; if the acetone solution concentration is too low, the cleaning effect is poor; if it is too high, there may be a risk of corrosion to the material; if the ultrasonic cleaning power is too low, the cleaning is incomplete; if the power is too high, it can damage the material surface; if the cleaning time is too short, impurities remain; if it is too long, production efficiency is affected; if the drying temperature is too low or the time is too short, residual moisture affects subsequent processes; if the temperature is too high or the time is too long, material properties may be altered.

[0024] Preferably, as an improvement, in step two, the amount of the microalloying elements niobium, vanadium, and boron added accounts for the following percentages of the mass of the base material: niobium (Nb) 0.01%–0.04%, vanadium (V) 0.01%–0.04%, and boron (B) 0.001%–0.004%; the total amount of the alloying elements added is ≤0.04%.

[0025] Technical Effects: This solution, employing the aforementioned settings, maximizes the performance enhancement effect of microalloying elements on wind turbine gear steel while controlling costs, thereby improving the overall performance of the wind turbine gear steel. Specifically, with a total microalloying element addition of ≤0.04%, the cost per ton of steel increases by <500 yuan, but the tensile strength increases from 1295MPa to 1462MPa (+12.9%), and the yield strength increases from 1180MPa to 1320MPa (+11.9%), resulting in a significantly better cost-performance ratio than existing technologies. Furthermore, by limiting the specific addition amounts of different elements, niobium effectively refines the grains, improving strength and toughness; vanadium, added in proportion, forms carbides and nitrides, enhancing hardness and wear resistance; and trace amounts of boron significantly improve hardenability, enhancing overall performance. Through long-term experiments, the applicant has found that without the addition of microalloying elements or with amounts exceeding the specified range, the steel's performance cannot be effectively improved; if the addition amount is too low, the strengthening effect of the elements cannot be fully utilized; and if the addition amount is too high, it may lead to element aggregation and segregation, reducing the steel's performance and increasing costs.

[0026] Preferably, as an improvement, in step three, the vacuum level is controlled to be 10 during the melting process. -2 mbar, melting current 800~1800A, voltage 280~320V.

[0027] Technical Effects: This solution, employing the above-mentioned settings, facilitates stable smelting processes and improves the purity and quality consistency of the steel. Specifically, high vacuum reduces the incorporation of impurity gases, increasing steel purity and thus enhancing its quality and performance. Precise control of smelting current and voltage ensures the stability of the smelting process, allowing for the full dissolution and uniform distribution of microalloying elements. Stable smelting parameters contribute to improved production efficiency and product quality consistency. Through long-term experiments, the applicant has found that without vacuum control, the incorporation of impurity gases reduces the purity and performance of the steel; insufficient vacuum leads to incomplete impurity removal; excessively low smelting current and voltage result in incomplete smelting and uneven dissolution and distribution of microalloying elements; and excessively high current and voltage may damage equipment, affecting production safety and product quality.

[0028] Preferably, as an improvement, in step four, the heat treatment includes adding an anti-seepage coating to the surface, treating at 850–880°C, rapidly immersing in a medium for cooling, and finally holding at 680–740°C for 2–3 hours followed by slow cooling.

[0029] Technical Effects: This solution, employing the aforementioned setup, effectively improves the microstructure of wind turbine gear steel, enhancing its overall mechanical properties. Specifically, the surface anti-seepage coating prevents surface decarburization and nitriding during heat treatment of the ingot, ensuring surface performance and quality; rapid cooling after treatment at 850–880℃ forms a strengthened microstructure, increasing strength and hardness; slow cooling at 680–740℃ for 2–3 hours eliminates residual stress, improving toughness and plasticity. Through long-term experiments, the applicant discovered that without the anti-seepage coating, surface decarburization and nitriding affect surface performance; if the cooling rate after treatment at 850–880℃ is too slow, an effective strengthened microstructure cannot be formed, resulting in insufficient improvement in strength and hardness; if the holding time at 680–740℃ is too short or the cooling rate is too fast, residual stress cannot be effectively eliminated, reducing toughness and plasticity.

[0030] Preferably, as an improvement, this solution also provides a microalloyed wind turbine gear steel, which is obtained by adding microalloying elements, mixing and remelting to achieve uniformity, and then heat-treating; wherein the microalloying elements are at least one of niobium, vanadium, and boron.

[0031] Technical Effects: This solution, employing the aforementioned settings, facilitates the production of microalloyed wind turbine gear steel with uniform microstructure, stable performance, and compliance with stringent requirements for wind turbine gear applications. Specifically, based on 18CrNiMo7-6 steel, it possesses excellent initial properties, providing a foundation for microalloying modification; the addition of niobium, vanadium, and boron improves the microstructure, enhancing strength, toughness, wear resistance, hardenability, and other properties; mixed remelting and heat treatment ensure uniform steel microstructure and stable performance. Attached Figure Description

[0032] Figure 1The microhardness of the microalloyed 18CrNiMo7-6 gear steel obtained under different doping elements and different doping amounts in Experimental Example 1 of this invention is shown.

[0033] Figure 2 The stress-strain curves of microalloyed 18CrNiMo7-6 gear steel obtained under different doping elements and different doping amounts in Experimental Example 1 of this invention are shown (a represents V doping; b represents B doping; c represents Nb doping).

[0034] Figure 3 The ultimate tensile strength of the microalloyed 18CrNiMo7-6 gear steel obtained under different doping elements and different doping amounts in Experimental Example 1 of this invention.

[0035] Figure 4 The fracture strain of the microalloyed 18CrNiMo7-6 gear steel obtained under different doping elements and different doping amounts in Experimental Example 1 of this invention is shown.

[0036] Figure 5 The impact properties of microalloyed 18CrNiMo7-6 gear steel obtained under different doping elements and different doping amounts in Experimental Example 1 of this invention are shown.

[0037] Figure 6 This represents the hardness gradient from the surface to the core of the undoped microalloyed 18CrNiMo7-6 gear steel in Experimental Example 2 of this invention.

[0038] Figure 7 This represents the hardness gradient from the surface to the core of the 18CrNiMo7-6 gear steel microalloyed with 0.02% V doping in Experimental Example 2 of this invention.

[0039] Figure 8 This represents the hardness gradient from the surface to the core of the 18CrNiMo7-6 gear steel microalloyed with 0.003% B doping in Experimental Example 2 of this invention.

[0040] Figure 9 This represents the hardness gradient from the surface to the core of the 18CrNiMo7-6 gear steel microalloyed with 0.02% Nb in Experimental Example 2 of this invention.

[0041] Figure 10 The tensile strength of gear steels with different microalloying processes in Experimental Example 2 of this invention is shown. Detailed Implementation

[0042] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the technical means used in the following embodiments and experimental examples are conventional means well known to those skilled in the art, and the materials and reagents used can all be obtained commercially.

[0043] Overview of the Plan

[0044] This solution provides a micro-alloying method for optimizing the composition of wind turbine gear steel, comprising the following steps:

[0045] Step 1: Select 18CrNiMo7-6 steel as the base material. After cutting, the base material is ultrasonically cleaned in acetone solution for 10 minutes; then dried at 100℃ for 1 hour.

[0046] Step 2: Add at least one of the following microalloying elements to the above base material: 0.01%–0.04% niobium (Nb), 0.01%–0.04% vanadium (V), and 0.001%–0.004% boron (B). Premix and then melt evenly.

[0047] Step 3: Melt the micro-alloyed ingot using a high-vacuum electric arc melting furnace, with the vacuum level controlled at 10. -2 mbar, melting current 800~1800A, voltage 280~320V; the solidified alloy is repeatedly remelted 5~8 times to ensure homogeneity of composition;

[0048] Step 4: Heat treat the smelted ingot. This includes adding an anti-seepage coating to the surface, treating at 850–880℃, rapidly immersing it in a cooling medium, and finally holding it at 680–740℃ for 2–3 hours before slow cooling.

[0049] Step 5: Machining the heat-treated ingot to obtain the required gear shape and size;

[0050] Step Six: Conduct performance tests on the final product, including hardness testing, tensile testing, and impact testing, to verify the material's performance.

[0051] Specifically, the hardness test is conducted in accordance with standard GB / T 4340.1-2024 "Metallic materials - Vickers hardness test - Part 1: Test method", and the steps are as follows:

[0052] (1) Select a suitable specimen and ensure that the specimen surface is flat, clean, and free of oxide scale, dirt, etc. The thickness of the specimen should meet the standard requirements. For materials such as cemented carbide, the specimen thickness should be at least 1 mm.

[0053] (2) Select appropriate test force (such as HV0.001, HV0.002, etc.) according to the material and hardness range of the sample; set the application time and holding time of the test force. Usually, the application time is 10-15 seconds and the holding time is 10-15 seconds.

[0054] (3) Place the sample on the stage of the hardness tester, ensuring that the sample is perpendicular to the indenter. Start the hardness tester and press the diamond indenter into the sample surface with the set test force. After maintaining the test force for a period of time, automatically measure the diagonal length of the indentation.

[0055] (4) The hardness tester automatically calculates and displays the Vickers hardness value and records the value. Multiple points can be tested simultaneously to obtain more accurate results.

[0056] Tensile testing was conducted in accordance with standard GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test at room temperature," and the steps are as follows:

[0057] (1) Select a suitable specimen shape (such as a circular or rectangular cross-section) according to the standard requirements. Use a vernier caliper to measure the diameter or width and thickness of the specimen and record the data.

[0058] (2) Turn on the electronic universal testing machine and change to the appropriate clamps according to the shape and size of the sample. Adjust the position of the crossbeam of the testing machine to ensure that the upper and lower clamps are in the correct positions.

[0059] (3) Clamp the sample in the upper chuck of the testing machine, then move the lower chuck to the appropriate position and clamp the sample.

[0060] (4) Apply a small preload (the stress corresponding to the load should not exceed the proportional limit of the material), then unload to zero and check whether the testing machine is working properly.

[0061] (5) Start the testing machine and slowly and evenly apply the load until the specimen breaks. Record the maximum load value at which the specimen breaks.

[0062] (6) Calculate the tensile strength based on the recorded maximum load and the original cross-sectional area of ​​the specimen. Calculate the elongation after fracture based on the gauge length after fracture and the original gauge length.

[0063] The impact test was conducted in accordance with GB / T 229-2020 "Metallic Materials - Charpy Pendulum Impact Test Method", and the steps are as follows:

[0064] (1) Prepare a standard Charpy V-notch specimen according to the standard requirements. The specimen size is 10mm×10mm×55mm and the notch depth is 2mm. Use a vernier caliper to measure the size of the specimen to ensure that it meets the standard requirements.

[0065] (2) Ensure that the impact testing machine is calibrated and meets the requirements of GB / T 229-2020. Check that the pendulum, anvil and support of the testing machine are installed correctly and that there is no looseness or damage.

[0066] (3) Position the notch of the specimen directly opposite the impact direction of the pendulum, and ensure that the plane of symmetry of the notch is within 0.5 mm of the pendulum's swing plane. Use clamps to fix the specimen to the support of the testing machine, ensuring accurate specimen positioning.

[0067] (4) Raise the pendulum to its initial position to ensure the testing machine is at zero. Release the pendulum to allow the specimen to break under impact force. Record the final position of the pendulum and read the impact absorbed energy.

[0068] (5) Record the impact absorbed energy of the specimen (in joules, J). Test multiple specimens with the same alloy composition, calculate the average impact absorbed energy, and evaluate the impact toughness of the material.

[0069] Example 1:

[0070] A microalloying method for optimizing the composition of wind turbine gear steel includes the following steps:

[0071] Step 1: Select 18CrNiMo7-6 steel as the base material and wire cut the base material into 20×20×20mm pieces. 3 The small cubes, after being cut, were ultrasonically cleaned in an acetone solution for 10 minutes to remove residual oil from the wire cutting process. They were then dried in a drying oven at 100°C for 1 hour.

[0072] Step Two: To improve the hardness and strength of the gear steel, nitrogen (Nb) was selected as the microalloying element, with a doping mass fraction of 0.02%. Based on the mass of the gear steel, the actual mass required for Nb smelting was calculated and precisely weighed using an electronic balance. To compensate for element volatilization during the smelting process, the actual weighed mass was 1.25 times the calculated mass.

[0073] Step 3: Place the base materials and microalloying elements into a vacuum arc melting furnace, controlling the vacuum level at 10. - 2 mbar is used to reduce contamination from harmful gases such as oxygen and nitrogen, ensuring the purity and uniformity of the alloy composition. The melting process is controlled with a melting current of 1400A and a voltage of 320V to ensure complete melting and uniform temperature, while avoiding overheating that could lead to component segregation or microstructure coarsening. The solidified alloy is repeatedly remelted six times to ensure compositional homogenization.

[0074] Step 4: Heat treatment of the smelted ingot. First, an anti-seepage coating is added to the surface of the gear steel. After holding at 850℃ for 1 hour, it is quickly immersed in water to cool, obtaining a martensitic structure and improving its hardness, strength, and toughness. Second, after holding at 700℃ for 2 hours, it is slowly cooled in air to reduce internal stress and improve plasticity and toughness.

[0075] Step 5: Machining the heat-treated ingot to obtain the required gear shape and size;

[0076] Step Six: Conduct performance tests on the final product, including hardness testing, tensile testing, and impact testing, to verify the material's performance.

[0077] Example 2:

[0078] A microalloying method for optimizing the composition of wind turbine gear steel includes the following steps:

[0079] Step 1: Select 18CrNiMo7-6 steel as the base material and wire cut the base material into 20×20×20mm pieces. 3 The small cubes, after being cut, were ultrasonically cleaned in an acetone solution for 10 minutes to remove residual oil from the wire cutting process. They were then dried in a drying oven at 100°C for 1 hour.

[0080] Step Two: To improve the hardness and impact toughness of the gear steel, boron (B) was selected as a microalloying element, with a doping mass fraction of 0.003%. Based on the mass of the gear steel, the actual mass of boron required for smelting was calculated and precisely weighed using an electronic balance. To compensate for element volatilization during the smelting process, the actual weighed mass was 1.25 times the calculated mass.

[0081] Step 3: Place the base materials and microalloying elements into a vacuum arc melting furnace, controlling the vacuum level at 10. - 2 mbar is used to reduce contamination from harmful gases such as oxygen and nitrogen, ensuring the purity and uniformity of the alloy composition. The melting process is controlled with a melting current of 1200A and a voltage of 320V to ensure complete melting and uniform temperature, while avoiding overheating that could lead to component segregation or microstructure coarsening. The solidified alloy is repeatedly remelted six times to ensure compositional homogenization.

[0082] Step 4: Heat treatment of the smelted ingot. First, an anti-seepage coating is added to the surface of the gear steel. After holding at 850℃ for 1 hour, it is quickly immersed in water to cool, obtaining a martensitic structure and improving its hardness, strength, and toughness. Second, after holding at 700℃ for 3 hours, it is slowly cooled in air to reduce internal stress and improve plasticity and toughness.

[0083] Step 5: Machining the heat-treated ingot to obtain the required gear shape and size;

[0084] Step Six: Conduct performance tests on the final product, including hardness testing, tensile testing, and impact testing, to verify the material's performance.

[0085] Example 3:

[0086] A microalloying method for optimizing the composition of wind turbine gear steel includes the following steps:

[0087] Step 1: Select 18CrNiMo7-6 steel as the base material and wire cut the base material into 20×20×20mm pieces. 3 The small cubes, after being cut, were ultrasonically cleaned in an acetone solution for 10 minutes to remove residual oil from the wire cutting process. They were then dried in a drying oven at 100°C for 1 hour.

[0088] Step Two: To improve the hardness and impact toughness of the gear steel, vanadium (V) was selected as a microalloying element, with a doping mass fraction of 0.02%. Based on the mass of the gear steel, the actual mass of V required for smelting was calculated and precisely weighed using an electronic balance. To compensate for element volatilization during the smelting process, the actual weighed mass was 1.25 times the calculated mass.

[0089] Step 3: Place the base materials and microalloying elements into a vacuum arc melting furnace, controlling the vacuum level at 10. - 2 mbar is used to reduce contamination from harmful gases such as oxygen and nitrogen, ensuring the purity and uniformity of the alloy composition. The melting process is controlled with a melting current of 1200A and a voltage of 320V to ensure complete melting and uniform temperature, while avoiding overheating that could lead to component segregation or microstructure coarsening. The solidified alloy is repeatedly remelted six times to ensure compositional homogenization.

[0090] Step 4: Heat treatment of the smelted ingot. First, an anti-seepage coating is added to the surface of the gear steel. After holding at 850℃ for 1 hour, it is quickly immersed in water to cool, obtaining a martensitic structure and improving its hardness, strength, and toughness. Second, after holding at 700℃ for 2 hours, it is slowly cooled in air to reduce internal stress and improve plasticity and toughness.

[0091] Step 5: Machining the heat-treated ingot to obtain the required gear shape and size;

[0092] Step Six: Conduct performance tests on the final product, including hardness testing, tensile testing, and impact testing, to verify the material's performance.

[0093] Experimental Example 1: Performance Comparison of Microalloyed 18CrNiMo7-6 Gear Steels Obtained with Different Doping Elements and Doping Amounts

[0094] As-cast 18CrNiMo7-6 gear steel was used, without any subsequent machining or heat treatment. Microalloying elements were selected from elemental V, elemental B, and elemental Nb with a purity of ≥99.9%. The elements were combined in a gradient manner according to their content, with the addition amounts of V ranging from 0.01% to 0.04%, B from 0.001% to 0.004%, and Nb from 0.01% to 0.04%. Based on the different amounts of added elements, the ingots are named V1 to V4 (where V1 represents 0.01% V, V2 represents 0.02% V, V3 represents 0.03% V, and V4 represents 0.04% V), B1 to B4 (where B1 represents 0.001% B, B2 represents 0.002% B, B3 represents 0.003% B, and B4 represents 0.040% B), and Nb1 to Nb4 (where Nb1 represents 0.01% Nb, Nb2 represents 0.02% Nb, Nb3 represents 0.03% Nb, and Nb4 represents 0.04% Nb).

[0095] The performance test results of the wind turbine gear steels obtained by microalloying optimization of V1~V4, B1~B4, and Nb1~Nb4 using this scheme are as follows:

[0096] Figure 1 This paper presents the microhardness of microalloyed 18CrNiMo7-6 gear steel obtained with different doping elements and doping amounts. Experimental data show that for gear steels doped with different amounts of V, the overall microhardness increased by approximately 15%, with an average value of 370±7 HV. With the gradual increase of V content, the microhardness showed a significant upward trend, increasing by approximately 1.3%, reaching a peak hardness of 388±8.96 HV at a V doping amount of 0.03%. However, further addition of V to 0.04% resulted in a slight decrease in hardness. For gear steels doped with different amounts of B, the overall microhardness increased by approximately 12%, with an average value of 360±6 HV. However, with the gradual increase of B content, the microhardness showed a slight downward trend, with a change of approximately 1%, which can be considered within the error range. For gear steels doped with different amounts of Nb, the overall microhardness increased by approximately 9%, with an average value of 350±8 HV. With the gradual increase of Nb content, the microhardness showed a significant upward trend, increasing by approximately 1.7%, reaching a peak hardness of 361.67 ± 8.58 HV when the Nb doping content was 0.03%. However, when Nb was further added to 0.04%, the hardness decreased slightly.

[0097] Figure 2The stress-strain curves of microalloyed 18CrNiMo7-6 gear steel with different doping elements and different doping amounts are shown: (a) doped with V; (b) doped with B; (c) doped with Nb.

[0098] Figure 3 This paper presents the ultimate tensile strength of microalloyed 18CrNiMo7-6 gear steel obtained with different doping elements and doping amounts. Experimental data show that for gear steels doped with different amounts of V, the overall tensile strength increases by about 1%, with an average value of 1028±10 MPa. From the perspective of doping amount, as the V content gradually increases, the tensile strength increases to some extent, but the increase is small, about 1%. The tensile strength reaches a peak of 1036.39±9.72 MPa when the V doping amount is 0.02%. However, when V is further added to 0.03% and 0.04%, the tensile strength decreases slightly. For gear steels doped with different amounts of B, the overall tensile strength increases by about 10%, with an average value of 1120±10 MPa. From the perspective of doping amount, as the B content gradually increases, the tensile strength decreases, by about 2%. The tensile strength reaches a peak of 1138.65±10.39 MPa when the B doping amount is 0.001%. This indicates that excessive boron doping has a negative impact on tensile strength. For gear steels doped with different amounts of nitrogen (Nb), the overall tensile strength increased by approximately 4%, with an average value of 1040 ± 10 MPa. From the perspective of doping amount, the tensile strength gradually increased with increasing Nb content, by about 2%. The tensile strength reached a peak of 1042.67 ± 3.96 MPa when the Nb doping amount was 0.04%. This demonstrates that Nb doping can significantly improve the tensile strength of gear steel.

[0099] Figure 4 The fracture strain of microalloyed 18CrMo7-6 gear steel obtained with different doping elements and doping amounts is shown. Experimental data indicate that the fracture strain of gear steel doped with vanadium (V) is significantly increased. When the doping amount is 0.02% and 0.03%, the fracture strain exceeds 50%, indicating a certain improvement in the plasticity of the gear steel. The fracture strain of gear steel doped with boron (B) shows no significant change, remaining around 51%, indicating that B doping has a relatively small impact on the plasticity of the gear steel. The fracture strain of gear steel doped with nitrogen (Nb) gradually decreases. When the doping amount is 0.01%, the fracture strain is around 51%. When the doping amount is 0.04%, the fracture strain is around 46%. This indicates that Nb doping reduces the plasticity of the gear steel.

[0100] Figure 5This study demonstrates the impact properties of microalloyed 18CrMo7-6 gear steel obtained with different doping elements and doping amounts. Experimental data show that for V-doped gear steel, the overall impact toughness is improved by about 60%, with an average value of 8.4 J. From the perspective of doping amount, the impact toughness first increases and then decreases with increasing V content. At a V doping amount of 0.01%, the impact energy reaches a peak of 12.54 ± 2.34 J. However, further addition of V to 0.02% and 0.03% results in a slight decrease in impact energy. For Bo-doped gear steel, the overall impact toughness is improved by about 25%, with an average value of 6.4 J. From the perspective of doping amount, the impact toughness gradually increases with increasing Bo content. At a Bo doping amount of 0.003%, the impact energy reaches a peak of 9.17 ± 1.33 J. However, further addition of Bo to 0.004% results in a decrease in impact toughness. For Nb-doped gear steel, the overall impact toughness only increased by about 6%, with an average of only 6.4 J. From the perspective of doping amount, the impact toughness first increased and then decreased with increasing Nb content. At an Nb doping level of 0.01%, the impact energy was only 3.45 ± 0.62 J, which did not exceed the performance of un-microalloyed gear steel. Adding Nb to 0.02% increased the impact energy to 7.21 ± 1.03 J. However, further additions of Nb to 0.03% and 0.04% resulted in a decrease in impact toughness.

[0101] Based on the above, 0.02% V, 0.003% B, and 0.02% Nb were selected to correspond to Examples 1, 2, and 3, respectively, and subsequent heat treatment was performed to compare the performance after adding the anti-seepage coating.

[0102] Experimental Example 1: Performance Comparison of Microalloyed 18CrNiMo7-6 Gear Steels Obtained in Examples 1-3

[0103] The performance of the microalloyed 18CrNiMo7-6 gear steel prepared in Examples 1-3 before and after applying the anti-seepage layer is compared with that of the 18CrNiMo7-6 steel without microalloying elements. Table 1 shows the numbering method of the microalloyed gear steel after heat treatment.

[0104] Table 1. Numbering method for heat-treated microalloyed gear steel

[0105] Example Gear steel No seepage barrier With seepage prevention layer - Undoped 1-1 1-2 Example 3 Doped with 0.02% V 2-1 2-2 Example 2 Doped with 0.003% B 3-1 3-2 Example 1 Doped with 0.02% Nb 4-1 4-2

[0106] Figure 6This study demonstrates the hardness gradient from the surface to the core of undoped, microalloyed 18CrNiMo7-6 gear steel. Experimental data shows that the microhardness value generally decreases gradually with increasing distance from the surface. Without the anti-seepage coating, the edge hardness reaches a maximum of 727 HV, while the core hardness is around 500 HV. After adding the anti-seepage coating, the hardness gradient decreases, with the edge hardness reaching a maximum of approximately 542 HV and the core hardness around 420 HV. This indicates that adding the anti-seepage coating helps reduce the hardenability of the gear steel. Regarding hardness values, the overall hardness of the uncoated gear steel is higher than that of the coated gear steel. This is because the uncoated gear steel produces a higher volume fraction of martensite during the carburizing heat treatment process, and martensite has a significantly higher hardness than austenite.

[0107] Figure 7 This study demonstrates the hardness gradient from the surface to the core of 18CrNiMo7-6 gear steel microalloyed with 0.02% V. Experimental data show that for ingots without an anti-seepage coating, the V-doped gear steel achieves a maximum edge hardness of 866 HV, while the core hardness is approximately 450 HV. For microalloyed ingots with an anti-seepage coating, the V-doped gear steel achieves a maximum edge hardness of 497 HV, while the core hardness is approximately 380 HV. The trend of the hardness gradient after adding the microalloying element is the same as that without it. That is, without an anti-seepage coating, the edge hardness is high, and the core hardness is low, resulting in a large hardness gradient; with an anti-seepage coating, the difference between the edge and core hardness is smaller, resulting in a lower hardness gradient.

[0108] Figure 8 This study demonstrates the hardness gradient from the surface to the core of 18CrNiMo7-6 gear steel microalloyed with 0.003% boron. Experimental data show that for ingots without an anti-seepage coating, the edge hardness of the boron-doped gear steel is the highest at 847 HV, while the core hardness is approximately 470 HV. For microalloyed ingots with an anti-seepage coating, the edge hardness is the highest at 510 HV, while the core hardness is approximately 420 HV. The trend of the hardness gradient after adding the microalloying element is the same as that without it. That is, without an anti-seepage coating, the edge hardness is high, and the core hardness is low, resulting in a large hardness gradient; with an anti-seepage coating, the difference between the edge and core hardness is smaller, resulting in a lower hardness gradient.

[0109] Figure 9This study demonstrates the hardness gradient from the surface to the core of 18CrNiMo7-6 gear steel microalloyed with 0.02% Nb. Experimental data show that for ingots without an anti-seepage coating, the Nb-doped gear steel has a maximum edge hardness of 782 HV, while the core hardness is approximately 470 HV. For microalloyed ingots with an anti-seepage coating, the maximum edge hardness is 450 HV, while the core hardness is approximately 400 HV. The trend of the hardness gradient after adding the microalloying element is the same as that without it. That is, without the anti-seepage coating, the edge hardness is high, and the core hardness is low, resulting in a large hardness gradient; with the anti-seepage coating, the difference between the edge and core hardness is smaller, resulting in a lower hardness gradient.

[0110] Figure 10 This study demonstrates the tensile strength of gear steels with different microalloying elements. Experimental data shows that the gear steels without microalloying elements (1-1 and 1-2) have a tensile strength of 1295 MPa without an impermeable layer, which increases to 1307 MPa with an impermeable layer. For gear steels doped with V (2-1 and 2-2), the tensile strength is 1462 MPa without an impermeable layer, but decreases to 1230 MPa with an impermeable layer, a reduction of 15.8%. For gear steels doped with B (3-1 and 3-2), the tensile strength is 1370 MPa without an impermeable layer, but decreases to 1296 MPa with an impermeable layer, a reduction of 5.4%. For gear steels doped with Nb (4-1 and 4-2), the tensile strength is 1327 MPa without an impermeable layer, but decreases to 1237 MPa with an impermeable layer, a reduction of 6.7%.

[0111] In summary, this solution optimizes the performance of 18CrNiMo7-6 steel by adding microalloying elements to the base material, significantly improving its service performance. This not only effectively solves the problem of utilizing existing 18CrNiMo7-6 steel inventory, but also enables the production of high-performance wind turbine gear steel, thereby enhancing the overall performance of wind turbine gearboxes.

[0112] Specifically, the applicant explored the effects of different doping elements, different addition amounts, different element combinations, different melting conditions, and different heat treatment conditions on the properties of the obtained microalloyed 18CrNiMo7-6 gear steel. They found that the best performance of the microalloyed 18CrNiMo7-6 gear steel was achieved when the combination of doping with 0.02% Nb, melting at 1680℃ under vacuum, remelting six times, and heat treatment conditions were as follows. Doping with 0.04% Nb reduced fracture toughness, thus decreasing the performance of the obtained microalloyed 18CrNiMo7-6 gear steel. Melting temperatures exceeding 1700℃ resulted in coarse grains, reducing material strength and toughness; and too few remelting cycles (less than three times) led to excessive inclusion residues, affecting the fatigue life of the steel.

[0113] This solution also provides a microalloyed wind turbine gear steel, which uses 18CrNiMo7-6 steel as the base material, and adds any one or a combination of microalloying elements niobium, vanadium, and boron to the base material. After remelting and homogenizing the base material, it is then heat-treated. The mechanical properties of the microalloyed optimized wind turbine gear steel are as follows: tensile strength up to 1462MPa, yield strength ≥1050MPa, elongation ≥50%, and impact energy KV2 ≥80J.

[0114] In summary, this solution has the following technical advantages:

[0115] 1. This solution, through precise control of the amount of microalloying elements added, can effectively improve the hardenability of gear steel, refine the grains, and improve strength and toughness.

[0116] 2. Through vacuum arc melting and heat treatment processes, the material composition was homogenized and the microstructure was optimized, thereby improving the overall performance of gear steel;

[0117] 3. The micro-alloyed wind turbine gear steel of the present invention has better load-bearing capacity and longer service life, which can meet the needs of wind power equipment to develop towards heavy load, high speed and lightweight.

[0118] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method of optimizing the microalloying of wind turbine gear steel compositions, characterized in that: The micro-alloying wind power gear steel is obtained by adding micro-alloying elements to 18CrNiMo7-6 steel as a base material, remelting the mixture uniformly, smelting treatment and heat treatment; the micro-alloying elements are at least one of niobium, vanadium and boron; the adding amount of the micro-alloying elements niobium, vanadium and boron accounts for the mass ratio of the base material as follows: niobium (Nb) 0.01%~0.04%, vanadium (V) 0.01%~0.04%, boron (B) 0.001~0.004%; the total adding amount of the alloying elements is ≤0.04%; the heat treatment includes adding a permeation-preventing coating on the surface, cooling in a medium after treatment at 850~880℃, and finally slow cooling after heat preservation at 680~740℃ for 2~3h.

2. A method of microalloying of wind turbine gear steel composition as claimed in claim 1, wherein: The method comprises the following steps: Step one: 18CrNiMo7-6 steel is selected as a base material, and the cut base material is cleaned in an acetone solution by ultrasonic wave, dried and reserved; Step two: micro-alloying elements are added to the above base material, and the mixture is smelted uniformly after premixing; Step three: the micro-alloyed ingot is smelted by using an electric arc smelting furnace with high vacuum degree, and the solidified alloy is repeatedly remelted for 5~8 times to ensure homogenization of the composition; Step four: the smelted ingot is heat treated; Step five: the heat treated ingot is mechanically processed to obtain the required gear shape and size; Step six: the final product is tested for performance.

3. A method of microalloying of wind turbine gear steel composition as claimed in claim 2, wherein: In step one, the concentration of the acetone solution is 3~5%(v / v); the ultrasonic cleaning power is 20~50W, and the time is 10~60min; the drying is heat preservation at 100~150℃ for 1~3h.

4. A method of microalloying of wind turbine gear steel composition as claimed in claim 3, wherein: In step three, the vacuum degree is controlled to be 10 -2 mbar, the smelting current is 800~1800 A, and the voltage is 280~320 V.

5. A microalloyed wind turbine gear steel, characterized in that: The micro-alloying method for optimizing the composition of wind power gear steel according to any one of claims 1~4 is prepared. The micro-alloying method for optimizing the composition of wind power gear steel according to any one of claims 1~4 is prepared.

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