Low-temperature-resistant offshore wind power flange steel, heat treatment method and production method thereof

Abstract

The application discloses a kind of low-temperature-resistant offshore wind power flange steel and its heat treatment method and production method, the low-temperature-resistant offshore wind power flange steel contains by weight percentage: C 0.05%~0.10%, Si 0.20%~0.40%, Mn 1.70%~2.00%, Cr 0.30%~0.60%, Mo 0.10%~0.30%, Ni 0.40%~0.60%, Cu 0.030%~0.050%, V 0.10%~0.20%, Ti 0.015%~0.035%, B 0.0020%~0.0040%, Al 0.015%~0.025%, N 0.0050%~0.0090%, the yield strength level of the steel is Q420NE grade, with good low-temperature toughness, rotary bending fatigue strength, excellent corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of steel technology, specifically relating to a low-temperature resistant steel for offshore wind turbine flanges and its heat treatment and production methods. Background Technology

[0002] Offshore wind power, characterized by its cleanliness and high efficiency, represents the future trend of wind power development. my country's installed wind power capacity and power output have been increasing year by year, while onshore wind power capacity is gradually approaching saturation, leading to a period of rapid growth for offshore wind power. Offshore wind power is developing towards higher power output and deeper waters, which places higher demands on the performance of key supporting components. Wind turbine flanges are crucial supporting and connecting parts of wind turbine towers, and their performance directly impacts the safety of wind power generation.

[0003] With the increasing size and depth of offshore wind power, the diameter and height of wind turbine towers have both increased. To reduce the weight of the towers, the strength of the steel used in the towers has been increased, with S355NL grade currently being commonly used. In the future, the strength grade of steel used in wind turbine towers will be increased to Q390 and Q420 grades, and the strength grade of flanges, as an important component of the tower, will also be correspondingly increased. Currently, in China, increasing the tower wall thickness is often used to increase support strength in order to solve the problem of offshore wind power support. Research on high-strength steel for wind turbine flanges is relatively limited, and the need to develop high-strength offshore wind turbine flanges to reduce tower weight is becoming increasingly urgent.

[0004] Patent CN 111893394A specifies the manufacturing process of flanges for offshore wind turbine foundation piles. This patent emphasizes the flange forging, ring rolling, and heat treatment processes, while the strength grade of the flange steel is 355. It does not provide a clear explanation of improvements in steel strength or low-temperature toughness, and the product's strength and toughness are not mentioned in the embodiments.

[0005] Patent CN 111286668A discloses a low-cost, high- and low-temperature toughness rare-earth wind turbine flange steel and its production process. The patent focuses on using rare-earth treatment to improve the steel's low-temperature toughness at -60℃, and the steel's strength grade is Q345. However, the strength grade of this patent is still insufficient.

[0006] Patent CN 110773692A discloses a forging method for low-temperature, high-strength offshore wind turbine flanges. The flange steel listed in this patent has a carbon content as high as 3.7%, which exceeds the carbon content of ordinary steel. This system should be a cast iron system, which has low low-temperature toughness. Furthermore, the patent does not provide any examples, lacks final product performance data, and the product's strength level and toughness are unknown.

[0007] Patent CN 1115058645A discloses a continuous casting round flange for low-temperature resistant tower flanges with large wall thickness in wind power applications, and its manufacturing method. The steel grade designed in this patent has a yield strength of 285MPa and a maximum impact energy of 168J at -50℃, neither of which meets the strength and toughness requirements for offshore wind power flanges. Furthermore, the patent title mentions "large wall thickness," but the patent does not specify a thickness level.

[0008] Patent CN 113913690A discloses a steel for offshore wind turbine flanges and a method for its preparation. The patent proposes a method for manufacturing steel for wind turbine flanges with a yield strength of 460MPa, but the steel has insufficient low-temperature toughness and does not address the offshore fatigue performance of the flanges.

[0009] Patent CN 114921720A discloses a steel ingot for flanges of high-power offshore wind turbines of 6 MW and above, and its production method. The flange designed in this patent has a strength of Q355 grade, but its low-temperature toughness is insufficient. The raw material is steel ingot, which results in low material utilization and high cost.

[0010] Patent CN 112342459A discloses a steel for low-temperature resistant wind turbine flanges and its rolling method. The flange designed in this patent has a strength of Q355 grade, but its low-temperature toughness is insufficient.

[0011] Therefore, based on the requirements of offshore wind turbine flanges, it is urgent to develop a flange steel that is resistant to low temperatures, high strength, high fatigue, and corrosion, and to design a targeted heat treatment process for the flanges to solve the safety problems of large-scale, deep-sea offshore turbine equipment. Summary of the Invention

[0012] To solve the above-mentioned technical problems, the present invention provides a steel for low-temperature resistant offshore wind turbine flanges, its heat treatment method and production method, wherein the steel has a yield strength level of Q420NE, good low-temperature toughness, rotational bending fatigue strength and excellent corrosion resistance.

[0013] The technical solution adopted in this invention is as follows:

[0014] A type of steel for low-temperature resistant offshore wind turbine flanges contains, by weight percentage: C 0.05%–0.10%, Si 0.20%–0.40%, Mn 1.70%–2.00%, Cr 0.30%–0.60%, Mo 0.10%–0.30%, Ni 0.40%–0.60%, Cu 0.030%–0.050%, V 0.10%–0.20%, Ti 0.015%–0.035%, B 0.0020%–0.0040%, Al 0.015%–0.025%, P≤0.015%, S≤0.010%, N 0.0050%–0.0090%, O≤0.0040%, with the remainder being Fe and other unavoidable impurities.

[0015] The composition of the steel used for the low-temperature resistant offshore wind turbine flanges meets the following requirements:

[0016] A=(4.5×%C)×(1+3.4×%Mn)×(1+0.7×%Si)×(1.2+2.6×%Cu)×(1+2.7×%Ni)×(1+3.

[0017] 1×%Cr)×(1+2.3×%Mo)×(1+1.6×%V+4.6×%N+1.3×%Ti+1.7×%Cu);

[0018] 40.0≤A≤55.0.

[0019] The composition of the steel used for the low-temperature resistant offshore wind turbine flanges satisfies: Y = 2.5%Cr + 3.8%Mo + 16.5%Ni + 2.5%Cu + 1.2%V + 1.4%Ti - 1%C - 4%Mn ≥ 2.0.

[0020] The metallographic structure of the steel used for the low-temperature resistant offshore wind turbine flange is tempered sorbite.

[0021] The wall thickness of the low-temperature resistant offshore wind turbine flange is ≥240mm.

[0022] The steel used for the low-temperature resistant offshore wind turbine flange has a yield strength grade of Q420NE; low-temperature toughness at 1 / 2 wall thickness at -50℃ KV2 ≥ 210J; rotational bending fatigue strength ≥ 300MPa; and room temperature corrosion rate ≤ 0.09mm / a.

[0023] The present invention also provides a heat treatment method for the steel used in the low-temperature resistant offshore wind turbine flange, the heat treatment method comprising quenching and tempering steps.

[0024] The quenching conditions are as follows: the flange semi-finished product is heated to T1 = 800~900℃, held at that temperature for t1 min, and then water-cooled, wherein S-T1 / 10≤t1≤S-T1 / 50, and S is the flange wall thickness in mm. The furnace entry temperature of the flange semi-finished product is ≤400℃.

[0025] The tempering conditions are as follows: heat the flange semi-finished product to T2 = 600~700℃, hold for t2 min, and then water cool. Wherein, 1.5×S-T2 / 10≤t2≤1.5×S-T2 / 50, and S is the flange wall thickness in mm.

[0026] This invention also provides a method for producing the steel for low-temperature resistant offshore wind turbine flanges, the method comprising the following steps: smelting in an electric arc furnace or converter → refining in an LF furnace → RH or VD vacuum degassing → continuous casting of round billets → slow cooling of round billets, blanking of round billets → heating of round billets → upsetting → punching → ring rolling → heat treatment → machining → flaw detection → grinding → packaging and warehousing; the heat treatment is performed using the heat treatment method described in this invention.

[0027] The functions and controls of each component in the low-temperature resistant offshore wind power flange steel provided by this invention are as follows:

[0028] Carbon (C): Carbon is the cheapest strengthening element in steel. Each 0.01% increase in dissolved C can increase strength by approximately 45 MPa. C forms precipitates with alloying elements in steel, resulting in precipitation strengthening. C significantly improves hardenability, enabling thick-walled steel pipes to acquire a martensitic structure in the center. However, as its content increases, plasticity and toughness decrease; therefore, the C content is controlled between 0.05% and 0.10%.

[0029] Si: Si is an effective solid solution strengthening element in steel, increasing its strength and hardness. Si also acts as a deoxidizer during steelmaking. However, Si tends to segregate at austenite grain boundaries, reducing grain boundary bonding and causing brittleness. Furthermore, Si easily causes elemental segregation in steel. Therefore, the Si content is controlled between 0.20% and 0.40%.

[0030] Mn: Mn can play a solid solution strengthening role, but its solid solution strengthening ability is weaker than that of Si. Mn is an austenite stabilizing element that can significantly improve the hardenability of steel and reduce decarburization. Mn combined with S can prevent hot brittleness caused by S. However, excessive Mn will reduce the plasticity of steel. Therefore, the Mn content should be controlled between 1.70% and 2.00%.

[0031] Cr: Cr is a carbide-forming element. Cr can improve the hardenability and strength of steel, but it easily causes temper brittleness. Cr can improve the oxidation resistance and corrosion resistance of steel, but excessive Cr content will increase crack susceptibility. The Cr content should be controlled between 0.30% and 0.60%.

[0032] Mo: Mo primarily improves the hardenability of steel. Mo dissolved in the matrix helps maintain high stability of the steel's microstructure during tempering and effectively reduces the segregation of impurity elements such as P, S, and As at grain boundaries, thereby improving the steel's toughness and reducing temper brittleness. Mo reduces the stability of M7C3; when the Mo content is high, acicular Mo2C will form, leading to a reduction in the Mo content in the matrix. Mo can improve the strength of steel through the combined effects of solid solution strengthening and precipitation strengthening, and it can also alter the steel's toughness by changing the precipitation of carbides. Therefore, the Mo content should be controlled between 0.10% and 0.30%.

[0033] Ni: Ni can form an infinitely miscible solid solution with Fe. It is an austenite stabilizing element, expanding the phase region, increasing the stability of supercooled austenite, shifting the C-curve to the right, and improving the hardenability of steel. Ni can refine the width of martensite laths, increasing strength. Ni can significantly lower the ductile-brittle transition temperature of steel, improving low-temperature toughness. Ni is a precious metal element; excessive addition leads to excessively high costs. The Ni content should be controlled between 0.40% and 0.60%.

[0034] V: V is a strong C and N compound-forming element. V (C, N) is finely dispersed and maintains a coherent relationship with the matrix, thus playing a role in strengthening and refining the microstructure. The V content is controlled at 0.10% to 0.20%.

[0035] Ti: Ti is a strong C and N compound-forming element. Ti (C and N) is finely dispersed and maintains a coherent relationship with the matrix, thus strengthening and refining the microstructure. Strengthening the matrix increases resistance to fatigue crack initiation and propagation, thereby improving fatigue strength. The Ti content should be controlled between 0.015% and 0.035%.

[0036] Cu: Cu expands the austenite phase region. Elemental Cu can act as a second phase, significantly improving strength and enhancing the tempering stability and strength of the microstructure. However, excessive Cu content will lead to Cu brittleness. Therefore, the Cu content should be controlled between 0.030% and 0.050%.

[0037] Al: Al is the main deoxidizer in steelmaking. Al combines with N to form finely dispersed AlN, which maintains a coherent relationship with the matrix. This strengthens and refines the microstructure, increases resistance to fatigue crack initiation and propagation, and thus improves the endurance strength of steel. The Al content is controlled between 0.015% and 0.025%.

[0038] O and N: TO forms oxide inclusions in steel, so TO should be controlled at ≤0.0040%; N can form fine precipitates with nitride-forming elements in steel to refine the microstructure, and can also precipitate Fe4N, which has a slow diffusion rate, leading to aging of the steel and reduced processing performance. Therefore, N should be controlled at 0.0050% to 0.0090%.

[0039] The low-temperature resistant offshore wind power flange steel provided by this invention can improve the strength of the steel by adding beneficial alloying elements, improve the toughness of the steel by effectively balancing the elements, and improve the fracture performance by forming effective toughening precipitates. This study of the alloy system shows that, under this composition, Mn is the most effective alloying element in improving hardenability and strength, hence the coefficient is 3.4; Mo also contributes significantly to hardenability and strength by improving tempering stability and its interaction with Mn, with a coefficient of 2.3; Cr is a major substitutional solid solution element and carbide-forming element, contributing 3.1 to strength; Ni and Cu do not form carbides in steel, but improve hardenability and strength by altering the crystal morphology through solid solution strengthening, with coefficients of 2.7 and 2.6, respectively; C is a non-metallic element and the most important interstitial solid solution strengthening element in steel, affecting both strength and toughness, hence the coefficient is 4.5; Si is a non-metallic element and also a major solid solution strengthening element in steel, contributing 0.7 to the steel's performance; V, N, and Ti are microalloying elements that improve steel strength through interaction and the formation of a second phase. In addition, N can improve steel strength by altering the crystal lattice of C, hence the coefficients are 1.6, 4.6, and 1.3, respectively. Since the strength, plasticity, and toughness of steel are inversely proportional, and high strength leads to a decrease in plasticity and toughness, the strength cannot be increased indiscriminately to ensure the overall performance of steel. Let the strengthening factor in steel be represented by A, then 40.0≤A≤55.0, A=(4.5×%C)×(1+3.4×%Mn)×(1+0.7×%Si)×(1.2+2.6×%Cu)×(1+2.7×%Ni)×(1+3.1×%Cr)×(1+2.3×%Mo)×(1+1.6×%V+4.6×%N+1.3×%Ti+1.7×%Cu).

[0040] Flanges require good fatigue resistance during service, thus the proportions of C, Mn, Cr, Mo, Ni, and Cu need to be carefully controlled. While C and Mn significantly improve steel strength, their tendency to deviate can lead to microstructure inhomogeneity, increasing entropy and causing localized weakening of the matrix, thereby exacerbating crack formation. Cr, Mo, and V can form second phases with C and N in steel. These second phases act as fixed defect sources, improving fatigue resistance. Ni increases stacking fault energy, dislocation density, and dislocation slip rate, thus enhancing fatigue resistance. Cu exhibits good nanoscale bonding with steel, forming a semi-coherent relationship and effectively fixing defects, thus hindering fatigue cracking. Let Y represent the fatigue resistance factor in steel; then Y ≥ 2.0.

[0041] Y=2.5×%Cr+3.8×%Mo+16.5×%Ni+2.5×%Cu+1.2×%V+1.4×%Ti-1×%C-4×%Mn.

[0042] In the heat treatment method for steel for low-temperature offshore wind power flanges provided by the present invention, the holding time for quenching and tempering is determined by the flange wall thickness and heating temperature, and 100% tempered sorbite structure can be obtained after heat treatment.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1. In this invention, the chemical components that affect the strength of steel are limited in formula A, and the ratio of 40.0 to A is controlled to 55.0. The effective proportion of each chemical component can improve the toughness of steel, and the formation of effective toughness precipitates can improve the fracture performance of steel.

[0045] 2. To ensure that the flange has good fatigue resistance during service, the present invention limits the composition of C, Mn, Cr, Mo, Ni and Cu in formula Y, and controls Y≥2.0.

[0046] 3. The steel for low-temperature resistant offshore wind turbine flanges provided by this invention has a yield strength grade of Q420NE, a tensile strength at 1 / 2 wall thickness ≥610MPa, a yield strength ≥440MPa, a low-temperature toughness at 1 / 2 wall thickness of -50℃ KV2 ≥210J, a rotational bending fatigue strength ≥300MPa, and a room temperature corrosion rate ≤0.09mm / a. It has good low-temperature toughness, rotational bending fatigue strength, and excellent corrosion resistance. Attached Figure Description

[0047] Figure 1 The image shows the metallographic structure of the steel used for wind turbine flanges in Example 1.

[0048] Figure 2 The image shows the metallographic structure of the steel used for the wind turbine flange in Comparative Example 3. Detailed Implementation

[0049] This invention provides a steel for low-temperature resistant offshore wind turbine flanges, comprising, by weight percentage: C 0.05%–0.10%, Si 0.20%–0.40%, Mn 1.70%–2.00%, Cr 0.30%–0.60%, Mo 0.10%–0.30%, Ni 0.40%–0.60%, Cu 0.030%–0.050%, V 0.10%–0.20%, Ti 0.015%–0.035%, B 0.0020%–0.0040%, Al 0.015%–0.025%, P ≤ 0.015%, S ≤ 0.010%, N 0.0060%–0.0090%, O ≤ 0.0040%, with the remainder being Fe and other unavoidable impurities.

[0050] The composition of the steel used for the low-temperature resistant offshore wind turbine flanges meets the following requirements:

[0051] A=(4.5×%C)×(1+3.4×%Mn)×(1+0.7×%Si)×(1.2+2.6×%Cu)×(1+2.7×%Ni)×(1+3.

[0052] 1×%Cr)×(1+2.3×%Mo)×(1+1.6×%V+4.6×%N+1.3×%Ti+1.7×%Cu);

[0053] 40.0≤A≤55.0.

[0054] The composition of the steel used for the low-temperature resistant offshore wind turbine flanges satisfies: Y = 2.5%Cr + 3.8%Mo + 16.5%Ni + 2.5%Cu + 1.2%V + 1.4%Ti - 1%C - 4%Mn ≥ 2.0.

[0055] The heat treatment method for the steel used in low-temperature resistant offshore wind turbine flanges includes quenching and tempering steps.

[0056] The quenching conditions are as follows: heat the flange semi-finished product to T1 = 800~900℃, hold for t1 min, and then water cool, where S-T1 / 10≤t1≤S-T1 / 50, and S is the flange wall thickness in mm.

[0057] The tempering conditions are as follows: heat the flange semi-finished product to T2 = 600~700℃, hold for t2 min, and then water cool. Wherein, 1.5×S-T2 / 10≤t2≤1.5×S-T2 / 50, and S is the flange wall thickness in mm.

[0058] The production method of the steel for low-temperature resistant offshore wind turbine flanges includes the following steps: smelting in an electric arc furnace or converter → refining in an LF furnace → RH or VD vacuum degassing → continuous casting of round billets → slow cooling of round billets, blanking of round billets → heating of round billets → upsetting → punching → ring rolling → heat treatment → machining → flaw detection → grinding → packaging and warehousing; the heat treatment is carried out using the heat treatment method described in this invention.

[0059] The present invention will now be described in detail with reference to the embodiments.

[0060] The composition and weight percentage of the steel used for offshore wind turbine flanges in each embodiment and comparative example are shown in Table 1.

[0061] Table 1

[0062] steel grades C Si Mn Cr Ni Mo Cu V Ti Example 1 0.06 0.35 1.95 0.47 0.59 0.23 0.032 0.11 0.023 Example 2 0.08 0.32 1.84 0.37 0.56 0.29 0.038 0.12 0.027 Example 3 0.10 0.22 1.71 0.56 0.42 0.12 0.048 0.18 0.016 Comparative Example 1 0.08 0.38 1.94 0.45 0.41 0.28 0.042 0.17 0.025 Comparative Example 2 0.09 0.29 1.92 0.56 0.42 0.18 0.045 0.16 0.034 Comparative Example 3 0.07 0.34 1.71 0.38 0.48 0.23 0.031 0.13 0.018 steel grades B Al P S N O A value Y value Example 1 0.0025 0.019 0.009 0.009 0.0056 0.0036 41.23 4.17 Example 2 0.0035 0.020 0.013 0.007 0.0085 0.0031 49.68 4.10 Example 3 0.0022 0.023 0.010 0.009 0.0072 0.0029 49.71 2.2 Comparative Example 1 0.0025 0.023 0.012 0.002 0.0062 0.0032 52.81 <![CDATA[ 1.46 ]]> Comparative Example 2 0.0028 0.018 0.008 0.004 0.0073 0.0026 <![CDATA[ 56.24 ]]> <![CDATA[ 1.58 ]]> Comparative Example 3 0.0032 0.021 0.007 0.005 0.0064 0.0032 <![CDATA[ 34.18 ]]> 3.09

[0063] The manufacturing process for steel used in offshore wind turbine flanges is as follows:

[0064] Electric furnace smelting: oxygen is determined before tapping, and steel is left in place during tapping to avoid slag discharge;

[0065] LF furnace: C, Si, Mn, Cr, Ni, Mo, V, Ti, Cu and other elements are adjusted to the target values;

[0066] Vacuum degassing: Pure degassing time ≥ 15 minutes, ensuring that the [H] content after vacuum treatment is ≤ 1.5 ppm, avoiding white spots in the steel and causing hydrogen embrittlement;

[0067] Continuous casting: The target temperature of molten steel in the ladle is controlled at 10-40℃ above the liquidus temperature, and round billets with a diameter of ≥700mm are continuously cast.

[0068] Flange manufacturing route: Electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → round billet continuous casting → Slow cooling of round billet, blanking of round billet → heating of round billet → upsetting → punching → ring rolling → heat treatment → machining → flaw detection → grinding → packaging and warehousing.

[0069] The heat treatment methods for the steel used in offshore wind turbine flanges in each embodiment and comparative example are shown in Table 2.

[0070] Table 2 lists the process details of the embodiments and comparative examples of the present invention.

[0071]

[0072] The performance of the offshore wind turbine flange steel produced in each embodiment and comparative example was tested according to the following method:

[0073] Organization: Samples were taken from half the thickness of the flange for metallographic, grain size, and hardness difference analysis.

[0074] Performance: Samples were taken from half the thickness of the flange for tensile, impact, fatigue and corrosion tests. Tensile, impact and fatigue performance tests were performed according to GB / T228, GB / T229 and GB / T 8650 respectively.

[0075] Corrosion test: A corrosion sample was taken from half the thickness of the flange. The corrosion was reduced according to GB / T 4334 standard. The corrosion reagent was artificial seawater. After a 720-hour immersion test, the corrosion rate was measured.

[0076] The mechanical performance test results are shown in Table 3.

[0077] Table 3

[0078]

[0079]

[0080] As can be seen from the above, the chemical composition and production methods of the steels in Examples 1-3 were appropriately controlled, ensuring that the chemical composition was 35.0≤A≤55.0 and Y≥2.0. The resulting steels exhibited good strength, plasticity, toughness, and corrosion resistance. While the chemical composition of Comparative Examples 1-3 was controlled within the range required by this invention, the chemical composition was not guaranteed to be 35.0≤A≤55.0 and Y≥2.0. Furthermore, the heat treatment processes in Comparative Examples 2 and 3 were not properly controlled, resulting in steels with excessively low strength, insufficient plasticity and toughness, and unsatisfactory overall performance.

[0081] The above detailed description of a low-temperature resistant offshore wind turbine flange steel and its heat treatment and production methods, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A low-temperature-resistant offshore wind power flange steel, characterized in that, By weight percentage: C 0.05%~0.10%, Si 0.20%~0.40%, Mn 1.70%~2.00%, Cr 0.30%~0.60%, Mo 0.10%~0.30%, Ni 0.40%~0.60%, Cu 0.030%~0.050%, V 0.10%~0.20%, Ti 0.015%~0.035%, B 0.0020%~0.0040%, Al 0.015%~0.025%, P≤0.015%, S≤0.010%, N 0.0050%~0.0090%, O≤0.0040%, the rest is Fe and other inevitable impurities; The composition of the low-temperature-resistant offshore wind power flange steel satisfies: A=(4.5*%C)*(1+3.4*%Mn)*(1+0.7*%Si)*(1.2+2.6*%Cu)*(1+2.7*%Ni)*(1+3.1*%Cr)*(1+2.3*%Mo)*(1+1.6*%V+4.6*%N+1.3*%Ti+1.7*%Cu); The composition of the low-temperature-resistant offshore wind power flange steel satisfies: Y=2.5*%Cr+3.8*%Mo+16.5*%Ni+2.5*%Cu+1.2*%V+1.4*%Ti-1*%C-4*%Mn≥2.0; 40.0≤A≤55.0； The yield strength grade of the low-temperature-resistant offshore wind power flange steel is Q420NE grade; the rotary bending fatigue strength is≥300MPa; the room temperature corrosion rate is≤0.09mm / a. The metallographic structure of the low-temperature-resistant offshore wind power flange steel is tempered sorbite.

2. Steel for low temperature offshore windmill flanges according to claim 1, characterized in that, The wall thickness of the low-temperature-resistant offshore wind power flange is≥240mm.

3. Steel for low temperature offshore windmill flanges according to claim 1, characterized in that, The low-temperature toughness at 1 / 2 wall thickness is KV2≥210J at-50℃.

4. Steel for low temperature offshore windmill flanges according to claim 1 characterized in that, The heat treatment method comprises quenching and tempering steps.

5. The heat treatment method of a low-temperature-resistant offshore wind power flange steel according to any one of claims 1-4, characterized in that, The quenching condition is: heating the flange semi-product to T1=800~900℃, keeping for t1min, and then water cooling, wherein S-T1 / 10≤t1≤S-T1 / 50, S is the wall thickness of the flange, unit: mm.

6. A heat treatment method of a low-temperature-resistant offshore wind power flange steel according to claim 5, characterized in that, The tempering condition is: heating the flange semi-product to T2=600~700℃, keeping for t2min, and then water cooling, wherein 1.5*S-T2 / 10≤t2≤1.5*S-T2 / 50, S is the wall thickness of the flange, unit: mm.

7. The heat treatment method of a low temperature resistant offshore wind turbine flange steel according to claim 5, characterized in that, The production method comprises the following steps: electric arc furnace or converter smelting→LF furnace refining→RH or VD vacuum degassing→round billet continuous casting→round billet slow cooling, round billet blanking→round billet heating→upsetting→punching→ring rolling→heat treatment→machining→flaw detection→grinding→packaging and warehousing; the heat treatment is carried out by the heat treatment method in any one of claims 5-7.

8. The method of producing a low-temperature-resistant offshore wind power flange steel according to any one of claims 1 to 4, characterized in that, ​

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