Carburized steel for wind power gearbox bearings and production method thereof

Abstract

The present invention relates to carburizing steel for wind turbine gearbox bearings and a production method thereof, belonging to the technical field of metallurgy. The steel comprises, by mass percentage, C: 0.23-0.27%, Si: 0.3-0.5%, Mn: 0.8-1.0%, Cr: 1.0-1.2%, Ni: 0.6-0.8%, Mo: 0.10-0.15%, S≤0.005%, P≤0.020%, Cu≤0.20%, Al≤0.05%, Ti≤0.002%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the remainder being Fe and unavoidable impurities. A low-cost, high-efficiency vacuum degassing, continuous casting, and forging process is employed to produce a steel with high cleanliness, high structural uniformity, excellent mechanical properties, and a long service life. This is a new steel that meets the requirements of wind turbine gearbox bearings.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and more particularly relates to bearing steel and a production method thereof. Background Art

[0002] Wind energy is currently the most promising renewable energy source, a clean, pollution-free, green energy source. Faced with the dual pressures of a shortage of conventional energy and a deteriorating global ecological environment, the development of wind energy is imperative. Wind energy is particularly advantageous in coastal islands and areas with limited access. As a highly efficient and clean new energy source, it is gaining increasing attention worldwide, with its enormous potential.

[0003] Wind turbine bearings are the core equipment of wind turbine generator systems and primarily include: yaw bearing assemblies, blade main shaft bearings, gearbox bearings, generator bearings, etc. The gearbox is the transmission component that connects the unit's main shaft to the generator. Its primary function is to convert the low-speed input of the main shaft into the high-speed output required for the generator, making it a crucial component of wind turbines. Wind turbine gearbox bearings are subject to powerful, multiple alternating loads and impact torques, and the transmission load varies greatly, placing high demands on the raw materials used for the bearings. Furthermore, and most critically, due to the inconvenience and high cost of hoisting and replacing bearings, installation and removal costs can range from hundreds of thousands to millions of yuan per installation. This makes wind turbine gearbox bearings highly technically complex and is recognized as one of the most challenging technologies to domestically produce.

[0004] Due to the high stability and long life of wind turbine gearbox bearings, their raw materials are required to have high purity, high structural uniformity, and high mechanical properties. Currently, the steel used in domestic wind turbine gearbox bearings is mainly produced using the electroslag remelting process, and the steel used is mainly traditional high-carbon bearing steel GCr15 or GCr15SiMn. The high-carbon bearing steel GCr15 or GCr15SiMn produced by the electroslag remelting process has relatively mature smelting processes and heat treatment processes, and has many quality advantages such as fine non-metallic inclusion particles, high structural uniformity, and high density. However, the electroslag remelting process has obvious disadvantages such as very low production efficiency and production capacity, high energy consumption and production costs. In addition, the comprehensive mechanical properties of this type of high-carbon steel are not ideal, mainly due to the poor plasticity of the core of the material. In the final use, the poor core performance leads to premature failure of the wind turbine gearbox bearings, affecting the actual service life of the bearings.

[0005] In view of the current defects of poor core plasticity of steel used in wind turbine gearbox bearings, low production efficiency and high energy consumption of electroslag remelting process, technical personnel in this field are working hard to develop steel for wind turbine gearbox bearings with high comprehensive performance and reduce production costs. Summary of the Invention

[0006] According to the use conditions of wind turbine gearbox bearings, a steel for wind turbine gearbox bearings in this application is produced through a unique composition design and a low-cost and high-efficiency vacuum degassing + continuous casting + forging process to produce steel with high cleanliness, high structural uniformity, high comprehensive mechanical properties and long life. It is a new steel that meets the requirements of wind turbine gearbox bearings.

[0007] The present application is a carburizing steel, which not only has the characteristics of high purity and high structural uniformity of high carbon bearing steel GCr15 or GCr15SiMn, but also has high mechanical properties and good core plasticity. The production method designed by the present invention has the advantages of high production efficiency, low energy consumption and production cost compared with the electroslag remelting process.

[0008] In order to meet the performance requirements of high cleanliness, high structural uniformity, high mechanical properties and other performance requirements for wind turbine gearbox bearing steel, the present invention has carried out a unique and reasonable chemical composition design, and its chemical composition design is as follows:

[0009] The mass percentage is C: 0.23~0.27%, Si: 0.3~0.5%, Mn: 0.8~1.0%, Cr: 1.0~1.2%, Ni: 0.6~0.8%, Mo: 0.10~0.15%, S≤0.005%, P≤0.020%, Cu≤0.20%, Al≤0.05%, Ti≤0.002%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, and the balance is Fe and unavoidable impurities.

[0010] The design principles of the elemental composition of the present invention are as follows:

[0011] 1) Determination of C content

[0012] Carbon is one of the most economical and fundamental elements affecting steel strength. It also promotes austenite formation during heating, imparting high hardness to the steel after carburizing heat treatment. Increasing the carbon content significantly improves the yield strength and tensile strength of steel, but also reduces its plasticity and impact toughness. The carbon content in this invention is specified to be within a range of 0.23% to 0.27%.

[0013] 2) Determination of Si content

[0014] Si is dissolved in the ferrite phase and has a strong solid solution strengthening effect, which can improve the strength, elastic limit and hardenability of the ferrite, but at the same time reduce the plasticity and toughness of the ferrite. The Si content of the steel of the present invention is determined to be within the range of 0.3-0.5%.

[0015] 3) Determination of Mn content

[0016] Manganese, as a deoxidizing element in the steelmaking process, improves the hardenability of steel. It also fixes the form of sulfur in steel, forming MnS and (Fe, Mn)S, which are less detrimental to steel properties, reducing or inhibiting the formation of FeS. Therefore, the inclusion of manganese in steel improves its purity and performance. Manganese also acts as a solid solution strengthening and grain refinement in steel, increasing strength and significantly improving hardenability. However, excessive manganese content can promote austenitized grain growth in steel. The Mn content in the present invention is determined to be within the range of 0.8 to 1.0%.

[0017] 4) Determination of Cr content

[0018] Cr is an element that improves the hardenability of steel and contributes to increased strength. When the carbon content is low, adding an appropriate amount of Cr can ensure the steel achieves the required hardenability and strength. However, excessive Cr content combines with carbon in the steel to form large carbides. These insoluble carbides reduce the toughness of the steel and shorten the bearing life. Therefore, the Cr content in this invention is set within a range of 1.0-1.2%.

[0019] 5) Determination of Ni content

[0020] Ni is an element that improves the hardenability of steel and is also the most commonly used element to effectively improve the wear resistance of steel. Ni, in combination with Cr and residual P in steel, will help improve the corrosion resistance and wear resistance of steel. The Ni content of the present invention is determined to be within the range of 0.6% to 0.8%.

[0021] 6) Determination of Mo content

[0022] Molybdenum refines steel grains, improves hardenability and thermal strength, and maintains sufficient strength and creep resistance at high temperatures. However, molybdenum is a ferrite-forming element. Excessive molybdenum content can lead to the formation of ferrite delta phase or other brittle phases, reducing toughness. The Mo content in this invention is specified to be within a range of 0.10% to 0.15%.

[0023] 7) Determination of Al content

[0024] Al, added as a deoxidizing element to steel, not only reduces dissolved oxygen in the molten steel but also forms finely dispersed aluminum nitride with nitrogen, which refines the grain size. However, excessive Al content can easily lead to the formation of large, brittle inclusions such as Al₂O₃ during the smelting process, reducing the purity of the molten steel and shortening the service life of the steel. The Al content in this invention is specified to be ≤0.05%.

[0025] 8) Determination of Ti content

[0026] During the solidification process of molten steel, Ti easily combines with nitrogen or carbon to form non-metallic inclusions such as TiN or Ti(C,N). These inclusions typically appear as angular, massive inclusions. These inclusions have high hardness, severely affecting microstructure uniformity. During operation, they can easily cause stress concentration at these corners, significantly reducing bearing fatigue life. The Ti content in this invention is specified to be ≤0.002%.

[0027] 9) Determination of O content

[0028] Oxygen naturally enters steel during the steelmaking process and remains in the steel later. Oxygen primarily exists in the form of non-metallic inclusions such as SiO2 and Al2O3. Al2O3 inclusions, in particular, significantly impact bearing fatigue life. Numerous fatigue life tests have shown that reducing oxygen content significantly improves steel purity, particularly by reducing the content and size of brittle oxide inclusions. The oxygen content in this invention is specified to be ≤ 0.0009%.

[0029] 10) Determination of P and S content

[0030] Phosphorus (P) causes severe segregation during solidification in steel. P dissolves in ferrite, causing grain distortion and coarsening, and increasing cold brittleness. The P content in this invention is set to ≤ 0.020%. Sulphur (S) causes hot brittleness in steel, reducing its ductility and toughness. The S content in this invention is set to ≤ 0.005%.

[0031] 11) Determination of As, Sn, Sb, and Pb content

[0032] Trace elements such as As, Sn, Sb, and Pb are all low-melting-point non-ferrous metals. Their presence in steel causes soft spots on the surface of parts and uneven hardness. Therefore, they are considered harmful elements in steel. The content range of these elements in the present invention is determined to be As≤0.04%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.

[0033] In order to obtain a wind turbine gearbox bearing steel with high cleanliness, high structural uniformity, high mechanical properties and long life, the present invention also clearly defines the steel to have the following main technical indicators:

[0034] The non-metallic inclusions in the steel of the present invention are tested according to the GB / T 10561A method, and the levels of various inclusions do not exceed the requirements of Table 1.

[0035] Table 1

[0036]

[0037] The macro defects of the steel of the present invention are inspected by the water immersion high frequency flaw detection method according to SEP 1927 (water immersion ultrasonic determination method for purity of forged steel bars). The length of a single inclusion does not exceed 1mm, and the defect index does not exceed 10mm / dm. 3 .

[0038] The macrostructure of the steel of the present invention is inspected and graded according to ASTM E381, requiring that S, R, and C all do not exceed level 1.0, and shrinkage cavities, cracks, and subcutaneous bubbles are not allowed to appear.

[0039] The grain size of the steel material of the present invention is tested and graded by the carburizing method according to GB / T6394, and the grain size is required to be ≥ grade 7.

[0040] The end hardenability of the steel of the present invention is tested according to GB / T225, and the heat treatment system is: normalizing at 920±(0-10)℃, holding for 1 hour, air cooling; end quenching at 900±(0-5)℃, water cooling, and the requirements are J3: 50-55HRC, J7: 45-50HRC.

[0041] The mechanical properties of the steel of the present invention are tested in accordance with GB / T228, and the specific requirements are shown in Table 2 below.

[0042] Table 2

[0043]

[0044] The present invention discloses a method for producing carburizing steel for wind turbine gearbox bearings, and the process flow is as follows: primary refining in a converter or electric furnace → refining in a ladle refining furnace (LF furnace) → vacuum degassing in a vacuum circulating degassing furnace RH or a VD furnace → continuous casting CCM (large cross-section) → slow cooling of the continuously cast billet → heating and forging into a finished product → finishing → surface and internal flaw detection → packaging.

[0045] The main steps are as follows:

[0046] (1) Molten steel smelting:

[0047] Primary smelting in a converter or electric furnace: It is preferred to use high-quality molten iron and high-nickel scrap steel (Ni accounts for more than 70% of the mass of the scrap steel) and add them into a converter or electric furnace for primary smelting. Oxygen is blown into the top of the furnace mouth for oxidation reaction, and argon is blown into the bottom for stirring. Preferably, 30 to 35 cubic meters of oxygen and 2 to 3 cubic meters of argon are blown into one ton of steel, and active composite lime (CaO-FeO-CaF2) is added. Under the combined action of oxygen and argon, harmful elements phosphorus (P≤0.020%) and titanium (Ti≤0.002%) are removed. The end point carbon when tapping from the primary smelting furnace is 0.12% to 0.15%, the tapping temperature is ≥1630℃, slag blocking is used for tapping, and some alloys are added during tapping (initial adjustment of composition). Slag removal is carried out immediately after tapping. After slag removal, it is quickly lifted to the refining LF furnace for smelting.

[0048] Ladle refining: The entire refining process uses bottom-blown argon to stir the molten steel and slag formation on the steel surface. Al granules and a high-performance CaO-SiC-MnO composite slag-forming agent are used for deoxidation and removal of harmful non-metallic inclusions. Al granules are added to the molten steel for precipitation deoxidation, while the CaO-SiC-MnO composite slag-forming agent diffuses and deoxidizes the steel surface and removes harmful non-metallic inclusions by adsorption. During the smelting process, Al granules (e.g., 100-150 kg) are first added to the molten steel, followed by the CaO-SiC-MnO composite slag-forming agent (e.g., 400-450 kg) added all at once. Argon is then connected to the bottom of the ladle, and electrodes are inserted into the slag for submerged arc current. Power is then turned off at intervals (e.g., every 15 minutes) to measure the molten steel temperature and sample for analysis. Required major elements are added according to target requirements. Temperature measurements and sampling in the refining furnace are limited to 3-4 times until the composition meets product requirements. Preferably, the refining time is controlled to be more than 45 minutes, and the molten steel soft blowing time is 10 to 15 minutes.

[0049] Vacuum degassing: The maximum vacuum degree in the vacuum furnace is ≤1.33mbar, and the vacuum circulation treatment time of the molten steel is maintained at ≥20min to ensure that harmful gases in the steel are effectively removed. After the vacuum treatment is completed, argon is blown into the bottom of the ladle. The argon flow rate is controlled so that the molten steel is not exposed to the air. Preferably, the soft blowing time of the molten steel is 15-20min. At the same time, 50-80m of silicon calcium wire is fed. The silicon calcium wire can convert Al2O3 or MgO·A12O3 in the molten steel into calcium aluminate and composite inclusions with lower melting points, further removing harmful inclusions.

[0050] (2) Continuous casting: Argon protection is used throughout the casting process to prevent secondary contamination and oxidation of the molten steel; preferably, large cross-sections of 300mm×300mm and above are used for continuous casting, and low superheat casting (superheat △T≤20℃) is adopted; the amount of molten steel in the tundish is controlled at 20-25 tons, and light pressure control technology is adopted (pressure reduction 20mm-25mm), and the casting speed is 0.55-0.75m / min; an appropriate steel flow to water ratio (0.5-0.7L / kg) is adopted; electromagnetic stirring is used in the tundish to interrupt the bridging of the columnar crystal zone during the solidification of the continuous casting billet; through the above various control technologies, the center segregation of the steel is further improved and the uniformity of the steel structure is improved.

[0051] (3) Slow cooling of continuous casting billets: The continuous casting billets are quickly lifted off the production line and slow cooled in the slow cooling pit. The slow cooling temperature is preferably ≥550℃, covered with an insulation cover, the slow cooling time is ≥36 hours, and the pit temperature is ≤150℃.

[0052] (4) Heating and forging: The continuous casting billet is sawn into a length of 1.4 to 1.6 m and sent to a heating furnace in a neutral or weak oxidizing atmosphere through a sling for heating (temperature control: 1000 to 1100 ° C, time ≥ 5 h). After being taken out of the furnace, it is subjected to the first pier under the forging machine (large reduction ≥ 200 mm), and then enters the drawing machine for drawing to an intermediate billet with a length of 3 to 3.5 m and a cross-section of 200 mm × 200 mm to 220 mm × 220 mm. The intermediate billet is again heated in a heating furnace in a neutral or weak oxidizing atmosphere (temperature control: 1220 to 1260 ° C, time ≥ 8 h). After being taken out of the furnace, it is forged into round bars with specifications of φ80 mm to φ130 mm.

[0053] (4) Finishing: including straightening, chamfering and other finishing processes to ensure that indicators such as size and curvature meet the requirements.

[0054] (5) The surface and interior are subjected to 100% non-destructive testing, and only products that pass the inspection can be qualified.

[0055] The advantages of the present invention are:

[0056] 1. The present invention is a high-mechanical-performance carburizing steel with a new chemical composition. Compared with the high-carbon bearing steel GCr15 or GCr15SiMn produced by the traditional electroslag remelting process, the present invention has high production efficiency and low production cost. It not only has high cleanliness and high structural uniformity, but also has high mechanical properties that traditional wind turbine gearbox bearing steels do not have, thereby obtaining long-life wind turbine gearbox bearings.

[0057] 2. Special smelting process:

[0058] (1) Converter or electric furnace: Add high-quality molten iron and high-nickel scrap steel (Ni accounts for 60% of the scrap steel) for primary smelting. Oxygen and argon are blown into the steel, and active composite lime (CaO-FeO-CaF2) is added. Under the combined action of oxygen and argon, harmful elements such as phosphorus and titanium are removed.

[0059] (2) Refining furnace and vacuum degassing furnace: In the LF refining furnace, Al particles and CaO-SiC-MnO high-performance composite slag-making agent are used for deoxidation and removal of harmful non-metallic inclusions. In the RH or VD vacuum degassing furnace, non-metallic inclusions and harmful gases in the steel are removed in a vacuum environment. Silicon calcium wire fed after soft blowing converts Al2O3 or MgO·Al12O3 in the molten steel into calcium aluminates and composite inclusions with lower melting points, further removing harmful inclusions. Ultimately, the non-metallic inclusions and O content in the steel are reduced to extremely low levels, and the number and size of inclusions reach the level of electroslag remelting.

[0060] (3) Continuous casting adopts argon protection throughout the casting process to protect the molten steel from being contaminated by secondary oxidation; large cross-sections of 300mm×300mm and above are used for continuous casting, and low superheat casting (superheat △T≤20℃) is adopted; the amount of molten steel in the tundish is controlled at 20-25 tons, and light pressure control technology is adopted (pressure reduction 20mm-25mm), and the casting speed is 0.55-0.75m / min; an appropriate steel flow water ratio (0.5-0.7L / kg) is adopted to further improve the center segregation of steel and improve the uniformity of steel structure.

[0061] 3. Special rolling process:

[0062] The high-reduction forging process uses a sawn, spooled, and then transported via a sling to a heating furnace in a neutral or slightly oxidizing atmosphere for heating. After exiting the furnace, the billet undergoes a first forging pass under a forging machine, then enters a drawing machine for drawing to length, and then enters a heating furnace in a neutral or slightly oxidizing atmosphere for heating again before being forged into round bars. This forming process offers the following advantages over rolling: 1. More uniform microstructure; 2. Finer grains; and 3. Improved segregation reduction.

[0063] The comprehensive mechanical performance level of the product of this application is: tensile strength Rm ≥ 1400MPa, elongation A% ≥ 19; shrinkage Z% ≥ 68, and impact absorption energy KU2 ≥ 140J at room temperature. DETAILED DESCRIPTION

[0064] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0065] The chemical composition (wt%) of the steel for the wind turbine gearbox bearings of various embodiments of the present invention is shown in Tables 3 and 4.

[0066] Table 3

[0067] C Si Mn P S Cr Cu Ni Al Embodiment 1 of the present invention 0.25 0.40 0.90 0.009 0.002 1.10 0.02 0.70 0.015 Embodiment 2 of the present invention 0.25 0.41 0.90 0.009 0.002 1.11 0.02 0.71 0.014 Embodiment 3 of the present invention 0.26 0.41 0.89 0.008 0.003 1.10 0.02 0.71 0.016

[0068] Table 4

[0069] Mo As Sn Sb Pb Ti O Embodiment 1 of the present invention 0.12 0.0055 0.0023 0.0013 0.001 0.0009 0.00057 Embodiment 2 of the present invention 0.12 0.0053 0.0019 0.0015 0.001 0.0010 0.00061 Embodiment 3 of the present invention 0.13 0.0052 0.0020 0.0016 0.001 0.0009 0.00056

[0070] Table 5 Inclusions in the steel materials of each example

[0071]

[0072] Table 6 Macroscopic data of steel materials in various examples

[0073]

[0074] Table 7 Grain size data of steel materials in each example

[0075]

[0076] Table 8 Water immersion high frequency flaw detection data of steel materials in various embodiments

[0077]

[0078] Table 9 Mechanical properties data of steel materials in various embodiments

[0079]

[0080] Table 10 End hardenability data of steel materials in various examples

[0081]

[0082] The present invention discloses a method for producing carburizing steel for wind turbine gearbox bearings, the process flow of which comprises the following steps: primary refining in a converter or electric furnace → refining in a ladle refining furnace (LF furnace) → vacuum degassing in a vacuum circulating degassing furnace RH or a VD furnace → continuous casting CCM (large cross-section) → slow cooling of the continuously cast billet → heating and forging into a finished product → finishing → surface and internal flaw detection → packaging.

[0083] Specifically, high-quality molten iron and 1 ton of high-nickel scrap steel (Ni accounts for 70% of the scrap steel) are added to a converter or an electric furnace for primary smelting. Oxygen is blown into the top of the furnace mouth for oxidation reaction, and argon is blown into the bottom for stirring. 30 to 35 cubic meters of oxygen and 2 to 3 cubic meters of argon are blown into each ton of steel, and 3 to 4 tons of active composite lime (CaO-FeO-CaF2) are added for smelting. The tapping end point C is controlled at 0.12% to 0.15%, the end point P is controlled at ≤0.020%, the end point Ti is ≤0.002%, the tapping temperature is controlled at 1632°C to 1643°C, and a slag blocking system is used to block the slag. Part of the alloy (initial adjustment composition) is added during tapping. ), after all the molten steel is discharged, the slag is removed and then quickly hoisted to the LF refining furnace; when arriving at the LF refining furnace, first cover the molten steel with a ladle cover, and pass argon into the ladle cover to protect the molten steel. During the smelting process, first add Al particles (100-150 kg) to the molten steel, and then add a CaO-SiC-MnO high-performance composite slag-making agent (400-450 kg) at one time. The bottom of the ladle is connected to argon, and then the electrode is inserted into the slag for submerged arc powering. The power is cut off every 15 minutes to measure the temperature of the molten steel and take samples for analysis. The required main elements are added according to the target requirements. The number of temperature measurements and sampling of the refining furnace is controlled at 3-4 times until the composition meets the product requirements. The refining time is controlled at more than 45 minutes, and the soft blowing time of molten steel is 10 to 15 minutes; during RH or VD vacuum degassing, the maximum vacuum degree in the vacuum furnace is ≤1.33mbar, the vacuum circulation treatment time of molten steel is 20 to 25 minutes, the soft blowing time of molten steel is 15 to 20 minutes, and 50 to 80m of silicon calcium wire is fed at the same time; the continuous casting adopts large sections of 300mm×300mm and above, and adopts low superheat pouring (superheat △T≤20℃); the amount of molten steel in the tundish is controlled at 20 to 25 tons, and the light pressure control technology is adopted (pressure reduction of 20mm to 25mm), and the pouring speed is 0.55 to 0.75m / min; an appropriate steel flow ratio to water is adopted (0.5 to 0.7L / kg); the continuous casting billet is quickly hoisted off the line and slowly cooled in the slow cooling pit, and the slow cooling temperature of the continuous casting billet off the line is ≥5 50℃, covered with insulation cover, slow cooling time ≥36 hours, pitting temperature ≤150℃; after pitting, the continuous casting billet is sawn into a length of 1.4~1.6m, and sent to a heating furnace in a neutral or weak oxidizing atmosphere through a sling for heating (temperature control: 1000~1100℃, time ≥5h). After being taken out of the furnace, it is subjected to the first pier under the forging machine (large pressing reduction ≥200mm), and then enters the drawing machine for drawing to a length of 200mm×200mm~220mm×220mm with a length of 3~3.5m, and then enters the heating furnace in a neutral or weak oxidizing atmosphere for heating again (temperature control: 1220~1260℃, time ≥8h). After being taken out of the furnace, it is forged into round bars with a specification of φ80mm~φ130mm, and slowly cooled to room temperature off the line, and then the bars are subjected to subsequent flaw detection and finishing.

[0084] It can be seen from Tables 3, 4, 5, 6, 7, 8, 9, and 10 that the carburizing steel for wind turbine gearbox bearings in each embodiment of the present invention has a control level of harmful elements such as phosphorus, sulfur, oxygen, titanium, and non-metallic inclusions that has reached the international advanced level. From the results of low magnification, mechanical properties, terminal hardenability, and microstructure grain size, the low magnification quality, hardenability, microstructure density, and mechanical properties of the present invention all meet the requirements for wind turbine gearbox bearing steel.

[0085] At the same time, each embodiment was subjected to water immersion high-frequency flaw detection according to the SEP 1927 method, and the macroscopic inclusions were zero defects.

[0086] In summary, a carburizing steel for wind turbine gearbox bearings in each embodiment of the present invention adopts a high-efficiency, high-capacity, low-cost process route of vacuum degassing + continuous casting + forging. Through a unique chemical composition design, special smelting process and forging process, a steel with high purity, high structural uniformity and high mechanical properties is obtained, producing a new steel with a high-efficiency, high-capacity, low-cost and high-quality production model.

[0087] In addition to the above embodiments, the present invention also includes other implementation methods. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of protection of the claims of the present invention.

Claims

1. A method for producing carburized steel for wind turbine gearbox bearings, characterized in that: The chemical composition of the steel is as follows by mass percentage: C: 0.23-0.27%, Si: 0.3-0.5%, Mn: 0.8-1.0%, Cr: 1.0-1.2%, Ni: 0.6-0.8%, Mo: 0.10-0.15%, S≤0.005%, P≤0.020%, Cu≤0.20%, Al≤0.05%, Ti≤0.002%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, and the balance is Fe and unavoidable impurities; Production methods include:

1. Molten steel smelting: 1.1 Primary refining: High-quality molten iron and scrap steel are put into the furnace for primary refining. Oxygen and argon are blown into the furnace, and active limestone is added. Under the combined action of oxygen and argon, harmful elements phosphorus and titanium are removed until P ≤ 0.020% and Ti ≤ 0.002%. The final carbon content of the primary refining furnace is 0.12% to 0.15% when tapping. The tapping temperature is ≥ 1630℃. Slag blocking is used for tapping. Some alloys are added during tapping to adjust the initial composition. Slag removal is carried out immediately after tapping, and the steel is then quickly hoisted to the refining furnace. 1.2 Refining: The entire refining process uses bottom-blown argon to stir the molten steel and slag on the surface of the molten steel. During the process, Al particles and CaO-SiC-MnO high-performance composite slag-forming agent are used for deoxidation and removal of harmful non-metallic inclusions. Al particles are added to the molten steel for precipitation deoxidation, and the CaO-SiC-MnO high-performance composite slag-forming agent diffuses and deoxidizes on the surface of the molten steel and removes harmful non-metallic inclusions by adsorption. During the smelting process, argon gas is connected to the bottom of the ladle, and then electrodes are inserted into the slag for submerged arc power. The power is turned off at intervals to measure the temperature of the molten steel and sample and analyze. The required main alloying elements are added according to the target requirements until the composition reaches the product design range. 1.3 Vacuum degassing: The maximum vacuum degree in the vacuum furnace is ≤1.33mbar, and the vacuum circulation treatment time of the molten steel is maintained for ≥20min. After the vacuum treatment is completed, argon is blown into the bottom of the ladle. The argon flow rate is controlled so that the molten steel is not exposed to the air. At the same time, silicon calcium wire is fed to convert Al2O3 or MgO·Al2O3 in the molten steel into calcium aluminate and composite inclusions with lower melting points, thereby removing harmful inclusions; 2. Continuous Casting: 2.1 The whole process of pouring is protected by argon to prevent secondary contamination and oxidation of molten steel; 2.2 The continuous casting billet is quickly hoisted off the production line and slowly cooled in the slow cooling pit; 3. Heating and forging: The continuous casting billet after the pit is sawn into a length of 1.4 to 1.6 m, and sent to a heating furnace in a neutral or weak oxidizing atmosphere for heating through a sling. After it comes out of the furnace, it is subjected to the first upsetting under the forging machine: the reduction amount is ≥ 200 mm, and then enters the drawing machine to draw an intermediate billet with a length of 3 to 3.5 m and a cross-section of 200 mm × 200 mm to 220 mm × 220 mm. The intermediate billet is again heated in a heating furnace in a neutral or weak oxidizing atmosphere. After it comes out of the furnace, it is forged into round bars with a specification of φ80 mm to φ130 mm and finished.

2. The method according to claim 1, wherein: Non-metallic inclusions in the structure are tested according to GB / T 10561 A method, and the levels of various inclusions shall not exceed the requirements in the following table; ； Macro defects in the structure are inspected according to the water immersion high-frequency flaw detection method in SEP 1927 Water immersion ultrasonic determination method for purity of forged steel bars. The length of a single inclusion shall not exceed 1mm and the defect index shall not exceed 10mm / dm. 3 ; Macrostructure is inspected and graded according to ASTM E381, and S, R, and C levels are all no more than 1.0, with no shrinkage cavities, cracks, or subcutaneous bubbles. The grain size of steel is tested and rated using the carburizing method in accordance with GB / T6394, and the grain size is required to be ≥ grade 7.

3. The method according to claim 1, wherein: The mechanical properties of steel are tested in accordance with GB / T228 and meet the requirements in the following table; ； The end hardenability of steel is tested in accordance with GB / T225, and the heat treatment system is: normalizing at 920±(0~10)℃, holding for 1 hour, air cooling; end quenching temperature is 900±(0~5)℃, water cooling, meeting J3: 50~55HRC, J7: 45~50HRC.

4. The method according to claim 1, wherein: In step 1.1, high-nickel scrap steel is used, in which Ni accounts for more than 70% of the mass of the scrap steel; 30 to 35 cubic meters of oxygen and 2 to 3 cubic meters of argon are blown into one ton of steel during smelting; and active composite lime CaO-FeO-CaF2 is added to remove P and Ti from the steel.

5. The method according to claim 1, wherein: In step 1.2, during refining in the ladle refining furnace, first add Al particles: 100~150 kg to the molten steel, and then add CaO-SiC-MnO high-performance composite slag-making agent: 400~450 kg at one time. The power is turned off every 15 minutes to measure the temperature of the molten steel and take samples for analysis. The number of temperature measurement and sampling is controlled at 3~4 times.

6. The method according to claim 1, wherein: In step 1.3, the soft blowing time of argon blowing into the molten steel is 15 to 20 minutes, and 50 to 80 meters of silicon calcium wire is fed at the same time.

7. The method according to claim 1, wherein: In step 2.1, continuous casting uses large-section billets with a cross-section of 300 mm × 300 mm or larger, and low superheat pouring is adopted: superheat ΔT ≤ 20°C; the amount of molten steel in the tundish is controlled at 20 to 25 tons, and a soft reduction control technology is adopted: the reduction is 20 mm to 25 mm, and the casting speed is 0.55 to 0.75 m / min; Steel flow water content: 0.5~0.7L / kg; electromagnetic stirring is used in the tundish to interrupt the bridging of columnar crystal areas during the solidification of the continuous casting billet.

8. The method according to claim 1, wherein: In step 2.2, the slow cooling temperature of the continuous casting billet off the line is ≥550℃, covered with an insulation cover, the slow cooling time is ≥36 hours, and the pit starting temperature is ≤150℃.

9. The method according to claim 1, wherein: In step 3, the sawn steel billet is first heated to 1000-1100°C for a heating time of ≥5h; the heating temperature of the intermediate billet is 1220-1260°C for a heating time of ≥8h.

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