A continuous casting large round billet production wind power tower drum flange steel and a manufacturing method thereof

Abstract

The present application relates to a kind of continuous casting big round billet production wind power tower flange steel and its manufacturing method, the chemical composition of adoption is according to mass percentage C:0.09~0.14%, Si:≤0.15%, Mn:1.45~1.60%, Cr≤0.30%, P≤0.010%, S≤0.005%, Mo≤0.05%, V:0.0.030~0.090%, Ni≤0.05%, Cu≤0.05%, Ti≤0.003%, O≤0.0015%, Al:0.025~0.050%, N≤0.015%, CEV≤0.43%, the balance is Fe and inevitable impurities.CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.The preparation method includes that molten steel is refined in converter, then is refined outside LF furnace and is treated by RH furnace high vacuum degassing, and molten steel is poured into continuous casting big round billet, and after heating, it is rolled into φ500mm intermediate billet, and φ500mm intermediate billet is cut and forged into wind power tower flange.The present application improves the fatigue strength, full cross-section uniformity, tensile strength, yield strength and low temperature impact performance of wind power tower flange steel, and welding performance is excellent, and the service life of wind power tower flange is increased.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a method for producing steel for wind turbine tower flanges from continuously cast large round billets. Background Technology

[0002] During the operation of wind power equipment, wind turbine tower flanges are structural components connecting different sections of the tower or between the tower and the hub, and between the hub and the blades. Although flanges are seemingly insignificant parts of a wind turbine, their role is crucial. Wind turbines cannot operate stably in natural environments for more than 20 years without the role of flanges. Wind turbines are installed in windy locations such as islands, mountains, wilderness, and beaches, subjected to unpredictable wind forces and strong gusts, and constantly experiencing extreme heat and temperature differences. Coupled with the inconvenient transportation in these environments, repairs are extremely difficult once a malfunction occurs. Furthermore, wind turbine flanges are subjected to various stresses such as impact, contact stress, and friction during operation, and are also affected by factors such as machining precision, assembly precision, and grinding from external hard particles. This places higher demands on wind turbine flanges. In addition to meeting the requirements of low-temperature resistance, high weldability, high strength, and good crack sensitivity, wind turbine flanges also need to meet the requirements of good fatigue resistance and good flange uniformity to ensure a service life of more than 20 years for wind power equipment.

[0003] Patent publication number CN102517494 B discloses a type of steel for wind turbine flanges and its manufacturing method. The chemical composition is as follows: C: 0.15–0.19%, Si: 0.17–0.35%, Mn: 1.45–1.55%, V: 0.060–0.090%, Nb: 0.02–0.04%, Al: 0.015–0.040%, Ti≤0.005%, [O]≤0.0015%, [N]≤0.0120%, [H]≤0.00015%, P≤0.012%, S≤0.005%, As≤0.04%, with the balance being iron and other unavoidable impurities. It possesses low-temperature resistance, high weldability, high strength, and good crack resistance. However, the patent does not mention the fatigue performance of the steel or the uniformity of the flange.

[0004] Patent publication number CN 113969372 B discloses a low-carbon, fatigue-resistant steel plate for wind power and its preparation method. The chemical composition is as follows: C: 0.08–0.12%, Si: 0.25–0.35%, Mn: 1.45–1.60%, Nb: 0.02–0.03%, Al: 0.03–0.05%, Ti: 0.01–0.02%, Ni: 0.10–0.20%, V: 0.02–0.03%, Cr≤0.10%, N≤0.008%, P≤0.012%, S≤0.005%, O≤0.002%, with the remainder being Fe and unavoidable impurities. The preparation method includes billet heating, rough rolling, finish rolling, cooling, and tempering heat treatment. This invention improves the yield strength and fatigue strength of the wind power steel plate, provides excellent weldability, and increases the service life of the wind power steel. However, this steel material is a steel plate, which cannot be used to manufacture wind turbine tower flanges, and the uniformity of the flanges is not mentioned in this patent.

[0005] Patent publication number CN 112342459 B discloses a low-temperature resistant steel for wind turbine flanges and its rolling method. The chemical composition is as follows: C: 0.15–0.20%, Si: 0.15–0.35%, Mn: 1.25–1.50%, P≤0.020%, S≤0.015%, Al≤0.020–0.050%, Cr≤0.30%, Cu≤0.30%, Ni≤0.30%, [H]≤2.0ppm, [O]≤20ppm, [N]≤120ppm, Al / N≥2, CE≤0.43%, with the remainder being Fe and unavoidable impurities. This low-temperature resistant steel for wind turbine flanges exhibits excellent low-temperature impact toughness, but the patent does not mention the fatigue performance of the steel or the uniformity of the flange.

[0006] In summary, while the composition of existing wind turbine tower flange steel must meet the requirements of high strength, low-temperature toughness, and good weldability, the fatigue performance of the steel and the uniformity of the final wind turbine flange remain unknown. Given the current state of wind turbine tower flange material production, the urgent technical problem to be solved by those skilled in the art is to develop a high-strength, low-temperature resistant, high-fatigue-performance, and highly homogeneous wind turbine tower flange steel with high production efficiency and high yield. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for producing steel for wind turbine tower flanges by continuous casting of large round billets and the method for manufacturing the steel by continuous casting of large round billets. The steel is rolled and forged to form the wind turbine tower flanges. The method can meet the requirements of high strength and low temperature toughness of wind turbine tower flanges, as well as high tensile strength ≥500Mpa, fatigue performance and high homogeneity.

[0008] The technical solution adopted by this invention to solve the above problems is as follows: C: 0.09-0.14%, Si: ≤0.15%, Mn: 1.45-1.60%, Cr ≤0.30%, P ≤0.010%, S ≤0.005%, Mo ≤0.05%, V: 0.030-0.090%, Ni ≤0.05%, Cu ≤0.05%, Ti ≤0.003%, O ≤0.0015%, Al: 0.025-0.050%, N ≤0.015%, CEV ≤0.43%, with the balance being Fe and unavoidable impurities. Wherein, CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

[0009] The design basis for the chemical composition is as follows:

[0010] Carbon (C) is the most economical and basic strengthening element in steel. Through solid solution strengthening and precipitation strengthening, it can significantly improve the strength of steel. However, excessively high C content can adversely affect the toughness and ductility of steel. To ensure that the carbon equivalent does not exceed 0.43% and guarantee weldability, the C content range in this invention is determined to be 0.09–0.14%.

[0011] Si is a deoxidizing element in steel. It increases the hardness and strength of steel through solid solution strengthening. However, Si increases the degree of work hardening, which reduces the toughness and plasticity of steel. In order to improve toughness, this invention controls the Si content to be ≤0.15%.

[0012] Mn (Mn) is a deoxidizing element in the steelmaking process, improves the hardenability of steel, and plays a role in solid solution strengthening. Mn can also fix the form of sulfur in steel and form MnS and (Fe,Mn)S, which are less detrimental to steel properties, reducing or inhibiting the production of FeS and improving the purity and performance of steel. Excessive Mn content, however, can lead to grain coarsening and increase the tendency for temper brittleness. Furthermore, higher Mn content can easily cause center segregation and cracks in continuously cast round billets, reducing the toughness of wind turbine tower flanges. In this invention, the Mn content is controlled at 1.45–1.60%.

[0013] Cr: A carbide-forming element. In steel, some Cr replaces iron to form alloy cementite, improving the tempering stability of the steel. This steel grade is delivered after normalizing, so no additional Cr is needed. This invention controls its content to ≤0.30%.

[0014] Mo, Ni, and Cu are precious metals, and to control production costs, they are not added as additional elements. In this invention, the content of Mo, Ni, and Cu is ≤0.05%.

[0015] V: It can refine the grain structure of steel, improve its strength, toughness, and wear resistance, and enhance its thermal stability. Its impact performance and fatigue strength are also improved. The V content in this invention is determined to be in the range of 0.030–0.090%.

[0016] Al has a strong affinity for oxygen. As a deoxidizing element in steel, it is added not only to reduce dissolved oxygen in molten steel, but also to form fine, dispersed aluminum nitride inclusions with nitrogen, which can refine the grain size. However, a high Al content can easily lead to the formation of brittle inclusions such as Al₂O₃ during steelmaking, reducing the purity of the molten steel. In this invention, the Al content is determined to be in the range of 0.025% to 0.050%.

[0017] Nitrogen (N) can improve the strength, low-temperature toughness, and weldability of steel, but also increase its aging sensitivity. In this invention, the N content is defined as N ≤ 0.015%.

[0018] Ti: In steel, Ti readily combines with N and C to form titanium nitride inclusions, which remain in the steel. These inclusions are hard and angular, severely affecting the fatigue life of gears, especially when the purity is significantly improved and the amount of other oxide inclusions is very small. To improve the fatigue strength and purity of this steel, the Ti content in this invention is determined to be ≤0.003%.

[0019] O: Numerous experiments have shown that reducing oxygen content is significantly beneficial for improving the fatigue strength of wind turbine tower flanges. The oxygen content range determined in this invention is ≤0.0015%.

[0020] P and S: In steel, P significantly causes segregation during solidification. P dissolves in ferrite, leading to grain distortion and coarsening, and increasing cold brittleness. To improve the impact toughness of wind turbine gearbox gears, P content is strictly controlled. In this invention, the P content range is determined to be ≤0.010%. S mostly exists in steel in the form of FeS, causing hot brittleness and reducing ductility and toughness. In this invention, the S content range is determined to be ≤0.005%.

[0021] CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and ≤0.43%. In order to ensure the welding performance of wind turbine tower flanges, the carbon equivalent requirement of no more than 0.43% is also a conventional requirement, and this steel grade also needs to be specially controlled.

[0022] The steel used for the wind turbine tower flange of this invention is a continuously cast round billet with a diameter of φ1000-φ1320mm. The round billet is rolled into an intermediate billet with a diameter of φ500mm. The intermediate billet is forged into a tower flange after secondary heating. After normalizing heat treatment, the tensile strength is ≥500Mpa, the yield strength is ≥325Mpa, and the impact KV2 (-60℃) is ≥60J. Non-metallic inclusions are rated according to Method A in GB / T 10561, satisfying the following conditions: Class A fine series ≤1.5; Class A coarse series ≤1.0; Class B fine series ≤1.0; Class B coarse series ≤0.5; Class C fine series = 0; Class C coarse series = 0; Class D fine series ≤1.0; Class D coarse series ≤0.5; Class Ds ≤1.0. The steel has good fatigue strength. The tower flange has good uniformity across its entire cross-section, with a carbon content difference of ≤0.03%.

[0023] The present invention also provides a method for manufacturing wind turbine tower flanges based on the above chemical composition. The method mainly involves the following steps: molten steel is initially refined in a converter, then refined in an LF ladle and degassed in a RH furnace under high vacuum. The molten steel is then cast into a large continuously cast round billet, heated and rolled into a φ500mm intermediate billet, and the φ500mm intermediate billet is cut and forged into a wind turbine tower flange.

[0024] Specifically, the manufacturing method of the steel for wind turbine towers in this application includes:

[0025] Step 1, Primary Refining: A top-and-bottom blowing converter is used. Oxygen is introduced at the top of the converter and argon is introduced at the bottom to stir the molten steel. The carbon content at the end of the primary refining is controlled above 0.05% and the phosphorus content at the end is controlled below 0.008%. Slag is blocked and the steel is tapped. During the tapping process, aluminum iron is added with the steel stream at a rate of 2.5 kg / t for precipitation and deoxidation.

[0026] Step 2, Refining: The ladle after tapping from the converter is transferred to the LF station. Argon gas is blown from the bottom to stir the molten steel. Lime and fluorite are added to the furnace during the smelting process. SiC is added to the surface of the refining slag in batches at a rate of 1.5 kg / t for diffusion deoxidation. The refining slag adopts a quaternary slag system of CaO-Al2O3-SiO2-MgO (CaO≥48%, SiO2<15%, Al2O3>25%, MgO>3.5%). The entire white slag refining time is ≥30 min. All alloys are adjusted to the design requirements in the refining furnace. After refining, the ladle temperature is ≥1620℃.

[0027] Step 3, Vacuum Degassing: The ladle is hoisted into the vacuum degassing furnace, and argon is used as the lifting gas. The high vacuum with a vacuum degree below 100 Pa is maintained for more than 10 minutes. After the vacuum is broken, the hydrogen concentration is ≤1.0 ppm.

[0028] Step 4: Continuous Casting Round Billet: The ladle containing molten steel is hoisted to the continuous casting station via the continuous casting ladle turret. The superheat of continuous casting is controlled at 15-30℃ to obtain a continuous casting round billet with a diameter of φ1200-φ1320mm. The continuous casting process includes ladle slag detection, crystallizer electromagnetic stirring, and solidification end electromagnetic stirring. To ensure the surface and core quality of the continuous casting round billet, a new casting flow heating technology combined with solidification end heavy pressure technology is used during the continuous casting process. Alkane combustible gas is burned with air and applied to the surface of the continuous casting round billet. The flame temperature is above 1000℃ to compensate for the surface temperature loss of the continuous casting round billet, reduce the temperature difference between the surface and the core, and control the temperature difference between the surface and the core of the φ1200-φ1320mm continuous casting round billet to within 400℃. This promotes the flow of molten steel in the core, promotes central molten steel feeding, and reduces central porosity and central crack defects caused by insufficient feeding during continuous casting. Simultaneously, the heavy pressing technology at the end of solidification is combined, with the heavy pressing carried out within the range of central solid fraction fs < 5.0, single roll reduction < 25 mm, and total reduction controlled at < 100 mm. The round billet is slowly cooled after leaving the line.

[0029] Step 5, Heating and Rolling: The continuously cast round billet is slowly heated to a preheating temperature of 650-750℃, a heating section temperature of 1050-1090℃, a heating section temperature of 1150-1200℃, and a soaking section temperature of 1250-1280℃ for 7-9 hours. The initial rolling temperature is 1100-1150℃, and the rolling speed is ≤1.0m / s. Large reduction rolling is used, with a reduction of more than 10% in two passes. During rough rolling, water is sprayed onto the surface of the continuously cast round billet to appropriately lower the surface temperature, allowing the rolling force to penetrate to the core of the billet and improve defects such as shrinkage cavities in the core. To avoid excessively rapid temperature drop, the cooling water is shut off in the last four passes of rough rolling to ensure the billet's temperature when entering subsequent rolling mills. The final rolling temperature is ≥850℃. The rolled size is φ500mm. After rolling, the billet is placed in a cooling pit for slow cooling for more than 48 hours.

[0030] Step 6: Wind turbine tower flange forming: After the φ500mm rolled billet is cut, it is reheated to a temperature of 1250℃ and held for more than 5 hours. Using a hydraulic press of 2000t or more, it is upset to a height of 600mm and punched with a center hole of φ450mm. After the billet is formed, it is put on a ring rolling mill for diameter expansion to the required flange size and then air cooled.

[0031] Step 7, Heat Treatment: The wind turbine tower flange is subjected to normalizing heat treatment at a temperature of 920℃ and a holding time of more than 6 hours.

[0032] Compared with the prior art, the advantages of the present invention are as follows:

[0033] 1. The refining process adopts a special CaO-Al2O3-SiO2-MgO quaternary slag system (CaO≥48%, SiO2<15%, Al2O3>25%, MgO>3.5%), and the entire refining white slag time is ≥30min, which improves the adsorption capacity of the refining slag for inclusions and ensures the purity of the steel. At the same time, by controlling the element Ti≤0.003%, the fatigue strength of the steel used for wind turbine tower flanges is improved.

[0034] 2. The continuous casting round billets with diameters of φ1200-φ1320mm are produced by combining a new casting flow heating technology with a solidification end pressure technology during the continuous casting process. This ensures the surface quality and core quality of the continuous casting round billets, and further guarantees the good uniformity of the wind turbine tower flanges.

[0035] 3. Large-sized φ1200mm-φ1320mm billet rolling is carried out using large reduction rolling, with the reduction in two passes exceeding 10%. During the rough rolling process, water is sprayed on the surface of the continuously cast round billet to appropriately reduce the surface temperature of the billet, allowing the rolling force to penetrate into the core of the round billet, improving defects such as shrinkage cavities in the core of the continuously cast round billet, and ensuring good and uniform internal quality of the wind turbine tower flange.

[0036] 4. Wind turbine tower flange forming: A hydraulic press with a capacity of 2000t or more is used to punch a φ450mm center hole, which ensures the removal of the loose area in the center of the continuously cast round billet and guarantees high fatigue strength and uniform cross-sectional composition of the wind turbine tower flange. Attached image description:

[0037] Figure 1 This is an SN curve diagram of the steel used for the wind turbine tower flange in Embodiment 1 of the present invention;

[0038] Figure 2 This is a carbon content distribution diagram from the inner ring to the outer ring of the wind turbine tower flange in Embodiments 1, 2, and 3 of the present invention and the comparative example. Detailed Implementation

[0039] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0040] Examples 1-3

[0041] The manufacturing process of wind turbine tower flange steel is as follows: converter primary refining—refining—continuous casting—cooling of continuously cast round billets—heating of continuously cast round billets—rolling—reheating—flange forming—heat treatment—warehousing. Details are as follows:

[0042] 1) Smelting: After 100 tons of patented smelting, the steel is refined outside the furnace and then degassed under vacuum. The mass percentage of each chemical element is strictly controlled according to requirements. Specific smelting parameters are shown in Table 1.

[0043] Table 1

[0044]

[0045] 2) Continuous Casting: Large round billets of φ1200-φ1320mm are continuously cast. A sealed protective pouring method is used to isolate the molten steel from air, controlling secondary oxidation. The tundish superheat is 15-30℃. The continuous casting process includes slag detection in the ladle, electromagnetic stirring in the crystallizer, and electromagnetic stirring at the end of solidification. To ensure the surface and core quality of the continuously cast billets, advanced equipment and processes combining a new casting flow heating technology and a heavy pressure reduction technology at the end of solidification are used during the casting process. Specific continuous casting parameters are shown in Table 2.

[0046] Table 2

[0047]

[0048] 3) Billet preparation and forming: The φ1200-φ1320mm large round billet is slowly heated, descaled by high pressure water, and rolled by a continuous rolling mill. It is hot rolled into φ500mm, cooled slowly, and then discharged. It is heated again, upset, punched in the center, and rolled into a ring. Specific parameters are shown in Table 3.

[0049] Table 3

[0050]

[0051] The chemical composition of the steel involved in the above embodiments is shown in Table 4.

[0052] Table 4

[0053] Example 1 0.12 0.12 1.53 0.007 0.002 0.10 0.0019 0.01 0.03 0.05 0.0075 0.0014 0.01 0.01 0.41 Example 2 0.11 0.11 1.55 0.007 0.002 0.10 0.0018 0.01 0.03 0.05 0.0075 0.0013 0.01 0.01 0.40 Example 3 0.13 0.11 1.5 0.007 0.002 0.08 0.0022 0.01 0.03 0.04 0.0070 0.0014 0.01 0.01 0.41 Comparative Example 0.15 0.25 1.35 0.015 0.01 0.09 0.0069 0.01 0.02 0.04 0.0075 0.0021 0.01 0.01 0.40

[0054] In the above embodiments, after the wind turbine tower flange is normalized, tensile and impact tests are performed. Tensile tests are performed according to GB / T228.1 standard, and impact tests are performed according to GB / T229 standard, as shown in Table 5.

[0055] Table 5

[0056] Example 1 510 330 115 / 120 / 118 Example 2 515 330 117 / 120 / 119 Example 3 518 335 122 / 119 / 125 Comparative Example 485 300 40 / 55 / 30

[0057] Inclusions: Non-metallic inclusions shall be inspected according to Method A in GB / T 10561; the range of carbon round blank cross-section of tower flange is as shown in Table 6.

[0058] Table 6

[0059]

[0060] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing steel for wind turbine tower flanges from continuously cast large round billets, characterized in that: The steel is smelted from the following components by mass percentage: C: 0.09–0.14%, Si: ≤0.15%, Mn: 1.45–1.60%, Cr ≤0.30%, P ≤0.010%, S ≤0.005%, Mo ≤0.05%, V: 0.030–0.090%, Ni ≤0.05%, Cu ≤0.05%, Ti ≤0.003%, O ≤0.0015%, Al: 0.025–0.050%, N ≤0.015%, CEV ≤0.43%, with the balance being Fe and unavoidable impurities, wherein CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and CEV ≤0.43%; the method specifically includes the following steps: Step 1, Primary Refining: A top-and-bottom combined blowing converter is used. Oxygen is introduced at the top of the converter and argon is introduced at the bottom to stir the molten steel. The carbon content at the end of the primary refining is controlled above 0.05% and the phosphorus content at the end is controlled below 0.008%. Slag is blocked and the steel is tapped. During the tapping process, aluminum iron is added with the steel stream at a rate of 2.5 kg / t for precipitation and deoxidation. Step 2, Refining: The ladle after the converter tapping is transferred to the LF station, the molten steel is stirred by bottom blowing argon gas, lime and fluorite are added into the furnace during the smelting process, SiC is added to the surface of the refining slag in batches at 1.5kg / t for diffusion deoxidation, all alloys are adjusted to the design requirements in the refining furnace, and the ladle temperature is ≥1620℃ after refining. Step 3, Vacuum Degassing: The ladle is hoisted into the vacuum degassing furnace, and argon is used as the lifting gas. The high vacuum with a vacuum degree below 100 Pa is maintained for more than 10 minutes. After the vacuum is broken, the hydrogen concentration is ≤1.0 ppm. Step 4: Continuous Casting of Round Billets: The ladle containing molten steel is hoisted to the continuous casting station via the continuous casting ladle turret. The superheating temperature is controlled at 15-30℃ to obtain continuously cast round billets with a diameter of ϕ1200-ϕ1320mm. During the continuous casting process, a combination of casting flow heating technology and solidification end-pressure technology is used. Alkane combustible gas is burned with air, acting on the surface of the continuously cast round billet. The flame temperature is above 1000℃ to compensate for the surface temperature loss of the continuously cast round billet, reduce the temperature difference between the surface and the core, and ensure that the diameter of the round billet is ϕ1200-ϕ1320mm. The temperature difference between the surface and core of the ϕ1320mm continuous casting round billet is controlled within 400℃ to promote the flow of molten steel in the core, promote central molten steel feeding, and reduce central porosity and central crack defects caused by insufficient feeding during continuous casting; the tower flange has good uniformity across its entire cross section, with a carbon content difference of ≤0.03%; heavy pressing technology is used at the end of solidification, and heavy pressing is carried out within the range of central solid fraction fs <5.0, with a single roll reduction of <25mm and a total reduction controlled at <100mm, and the round billet is slowly cooled after leaving the line; Step 5, Heating and Rolling: The continuous casting round billet is slowly heated to a preheating temperature of 650-750℃, a heating section temperature of 1050-1090℃, a heating section temperature of 2150-1200℃, and a soaking section temperature of 1250-1280℃ for 7-9 hours. The initial rolling temperature is 1100-1150℃, the rolling speed is ≤1.0m / s, the final rolling temperature is ≥850℃, and the rolled size is ϕ500mm. After rolling, the billet is placed in a pit for slow cooling for more than 48 hours. Step 6: Wind turbine tower flange forming: After the ϕ500mm rolled billet is cut, it is reheated to a temperature of 1250℃ and held for more than 5 hours. Using a hydraulic press of 2000t or more, it is upset to a height of 600mm and the center hole is punched to ϕ450mm. After the billet is formed, it is put on a ring rolling mill for diameter expansion to the required flange size and then air-cooled. Step 7, Heat Treatment: The wind turbine tower flange is subjected to normalizing heat treatment at a temperature of 920℃ and a holding time of more than 6 hours.

2. The method for manufacturing steel for wind turbine tower flanges from continuously cast large round billets according to claim 1, characterized in that: The steel used for the wind turbine tower flange is a continuously cast round billet with a diameter of ϕ1000-ϕ1320mm. The round billet is rolled into an intermediate billet with a diameter of ϕ500mm. The intermediate billet is forged into a tower flange after secondary heating. After normalizing heat treatment, the tensile strength is ≥500MPa, the yield strength is ≥325MPa, and the impact KV2 (-60℃) is ≥60J. Non-metallic inclusions are rated according to Method A in GB / T 10561, satisfying the following conditions: Class A fine series ≤1.5; Class A coarse series ≤1.0; Class B fine series ≤1.0; Class B coarse series ≤0.5; Class C fine series = 0; Class C coarse series = 0; Class D fine series ≤1.0; Class D coarse series ≤0.5; Class Ds ≤1.0; The steel has good fatigue strength.

3. The method for manufacturing steel for wind turbine tower flanges from continuously cast large round billets according to claim 1, characterized in that: In step two, the refining slag adopts a quaternary slag system of CaO-Al2O3-SiO2-MgO, wherein CaO≥48%, SiO2<15%, Al2O3>25%, MgO>3.5%, and the entire refining time for white slag is ≥30min.

4. The method for manufacturing steel for wind turbine tower flanges from continuously cast large round billets according to claim 1, characterized in that: In step four, the continuous casting process includes slag detection in the ladle, electromagnetic stirring in the crystallizer, and electromagnetic stirring at the end of solidification to ensure the surface and core quality of the continuously cast billet.

5. The method for manufacturing steel for wind turbine tower flanges from continuously cast large round billets according to claim 1, characterized in that: In step five, roughing is performed with a large reduction, with a reduction of more than 10% in two passes. During the roughing process, water is sprayed on the surface of the continuously cast billet to appropriately reduce the surface temperature of the billet, so that the rolling force can penetrate into the core of the billet and improve defects such as shrinkage cavities in the core of the continuously cast billet. To avoid the temperature dropping too quickly, the cooling water is turned off in the last four passes of roughing to ensure the temperature of the billet when it enters the subsequent rolling mill.

Images