A kind of fine-grained steel for wind power flange and its manufacturing method

Abstract

The present application discloses a fine-grained steel for wind power flange and a manufacturing method thereof, belonging to the fields of steel metallurgy technology and mechanical manufacturing technology. In terms of weight percentage, the fine-grained steel for wind power flange includes: C: 0.13-0.15%, Si: 0.17-0.40%, Mn: 1.2-1.6%, P≤0.025%, S≤0.003%, Nb: 0.03-0.05%, Al<0.06%, Ce≤300ppm, La≤300ppm, H≤2ppm, O≤20ppm, N≤300ppm, and the remainder is Fe and unavoidable impurities. The present invention refines the solidification structure and grain size through rare earth sulfide oxides through microalloying, the addition of La rare earth elements and the content ratio of other elements; on the other hand, the precipitation behavior of AlN along the crystal is controlled during hot forming and heat treatment to achieve full-process organization refinement, while improving the strength and toughness of the flange.

Description

Technical Field

[0001] The present application relates to a fine-grained steel for wind power flange and a manufacturing method thereof, belonging to the technical fields of steel metallurgy and mechanical manufacturing. Background Art

[0002] Wind energy contains huge energy and is a clean renewable energy, so wind power generation is widely valued by various countries (including China). Wind power flange is an important component used to connect the tower, hub, hub, and blades, and is one of the key core structural parts of wind turbines.

[0003] With the rapid development of the wind power industry, the environment in the site selection area of ​​new wind farms is becoming increasingly harsh. Wind turbines often have to withstand extreme climate tests such as high cold, high humidity, wind and sand, and salt corrosion. In the face of extreme climates, the quality of flanges directly determines the safety of wind turbines. Therefore, people's requirements for wind power flanges are becoming more and more stringent. Not only must the raw materials and processing and production processes of wind power flanges be strictly controlled, but the material properties of flange steel must also be continuously improved.

[0004] Patent publication number CN107058877A discloses a method for manufacturing a wind power flange for low temperature environment, and its chemical composition is: C: 0.08-0.16%, Si: 0.15-0.30%, Mn: 1.00-1.60%, P: ≤0.020%, S: ≤0.015%, V: ≤0.12%, Nb: ≤0.05%, Ni: ≤0.50%, Cr: ≤0.20%, Cu: ≤0.25%, Als: ≥0.015%, Mo ≤0.10%, Ti: ≤0.010%. The invention optimizes the chemical composition, forging process and heat treatment process, and the flange performance meets the requirements of -60℃ low temperature impact performance. However, the invention lacks measures to control the solidification structure of the steel ingot, and the coarse solidification structure may be inherited to the final product, affecting the quality stability.

[0005] Patent publication number CN11128668A discloses a low-cost, high-low temperature toughness rare earth steel for wind power flanges and its production process, with chemical composition: C: 0.17-0.20%, Si: 0.15-0.35%, Mn: 1.20-1.40%, P≤0.015%, S≤0.005%, Nb: 0.020-0.040%, Ce≤0.0010%, H≤2ppm, 0≤15ppm, N≤60ppm. The invention reduces the amount of alloy added, reduces costs, improves the structure of the ingot and the morphology of inclusions, and improves the low-temperature toughness of the flange. The disadvantage of the invention is that the rare earth content is low, so there are fewer solid-dissolved rare earths in the steel, and the Ce rare earth element contained only cannot fully exert the rare earth grain boundary purification effect, and the rare earth composite inclusions obtained in the preparation cannot inhibit the grain growth behavior during thermal deformation and heat treatment. Summary of the invention

[0006] In order to solve the above problems, a fine-grained steel for wind power flange and a manufacturing method thereof are provided. Through micro-alloying, the addition of La rare earth element and the content ratio of other elements, on the one hand, the solidification structure is refined by rare earth sulfide oxides and the grain size is refined; on the other hand, the AlN precipitation behavior along the crystal is controlled during the hot forming and heat treatment process to achieve full-process structure refinement, while improving the strength and toughness of the flange.

[0007] According to one aspect of the present application, a fine-grained steel for wind turbine flanges is provided, which includes, by weight percentage: C: 0.13-0.15%, Si: 0.17-0.40%, Mn: 1.2-1.6%, P≤0.025%, S≤0.003%, Nb: 0.03-0.05%, Al<0.06%, Ce≤300ppm, La≤300ppm, H≤2ppm, O≤20ppm, N≤300ppm, and the balance is Fe and unavoidable impurities.

[0008] Two rare earth elements Ce and La are added to the fine-grained steel for wind turbine flanges. On the one hand, the two elements can react with S and O in the steel to form rare earth sulfides and rare earth oxides, which can reduce the content of S and O impurity elements in the steel and prevent the impurity elements from destroying the steel matrix structure. The rare earth sulfides and rare earth oxides can also serve as the basis for the growth of the steel structure, so that the steel structure can grow rapidly into a stable structure with the rare earth sulfides and rare earth oxides as nuclei. In addition, since the content of rare earth sulfides and rare earth oxides is moderate and evenly distributed, the particle uniformity and density of the growing steel structure can be improved, and the fine grain steel structure can be smooth and uniform. On the other hand, Ce and La elements are dispersed in the steel in the form of pure elements, which can increase the content of rare earth elements dissolved in the steel. The sizes of the above two rare earth elements are larger than other elements. When mixed with other elements in the steel, they can confine impurity elements such as P, Sn, Sb, etc. within the grain boundaries, effectively inhibiting the diffusion of the above elements outside the grain boundaries and avoiding damage to the steel structure. The elements confined within the grain boundaries can support the grain boundaries, thereby strengthening the grain boundaries and significantly improving the low-temperature impact toughness of the flange.

[0009] Optionally, the Al×N in the fine-grained wind turbine flange steel is ≤1.2×10 -7 By limiting the content of the product of Al and N elements, the precipitation behavior of AlN along the crystal can be strictly controlled, so that the content of AlN precipitation is moderate, the purpose of inhibiting the growth of austenite and ferrite is achieved, and the whole process organization is refined. Al can also be used as a deoxidizing element, thereby improving the cleanliness of the steel matrix.

[0010] Optionally, the Ce content is greater than 80 ppm, and the La content is greater than 20 ppm.

[0011] Optionally, the weight ratio of Ce to La is (1.69-3.8):1.

[0012] In the present application, when the Ce and La elements are set in the above-mentioned content and weight ratio, firstly, the number and precipitation form of rare earth sulfides and rare earth oxides can be optimized, and the impurity content can be reduced while the steel structure can be refined and homogenized to the greatest extent; secondly, the solid solubility of the two elements can be increased, and the purification effect of rare earth grain boundaries can be more fully exerted, thereby effectively inhibiting the grain boundary segregation of impurity elements such as P, and significantly improving the low-temperature toughness of steel; thirdly, the above-mentioned weight ratio of the two elements La and Ce can more effectively exert the alloying effect, further improve the distribution uniformity of ferrite and pearlite and the gaps between them, and improve the various mechanical properties of steel.

[0013] Optionally, the weight ratio of P to La is (1.2-3.35): 1. Under this ratio, the La element can play an optimal limiting role on the P element and can control the content of the P element in the grain boundary, so that the P element can be evenly dispersed in the grain boundary, while avoiding the agglomeration of the P element in the grain boundary, and on the basis of grain refinement, the morphological uniformity of the steel organization can be improved, thereby improving the moisture resistance and corrosion resistance of the steel.

[0014] Optionally, the purity of Ce and La is greater than 95%. The purity is set to reduce the amount of impurities introduced during the addition of Ce and La to prevent impurities from affecting the steel structure; secondly, it can ensure that Ce and La fully exert the above-mentioned effects and further improve the grain refinement effect.

[0015] Optionally, the fine-grained wind turbine flange steel has a low-temperature impact energy of -60°C ≥177J and an elongation at break ≥35%.

[0016] Optionally, the tensile strength of the fine-grained wind turbine flange steel is 620-670 MPa, and the yield strength is 420-480 MPa.

[0017] Preferably, the tensile strength of the fine-grained wind turbine flange steel is 640-670 MPa, and the yield strength is 440-480 MPa.

[0018] According to another aspect of the present application, a method for manufacturing the fine-grained steel for wind turbine flange as described in any one of the above items is provided, comprising the following steps:

[0019] (1) Smelting:

[0020] The scrap steel is melted in an electric arc furnace, pig iron is added, and the tapping temperature is less than 1650°C to obtain molten steel;

[0021] LF refining and VD vacuum treatment ensure the cleanliness of molten steel. LF refining uses a large amount of slag to make slag to ensure that the S content is ≤0.003%. Niobium iron and Al are added during the refining process to control the Al content to <0.06%;

[0022] The VD vacuum treatment time is greater than 25 minutes, and the N content is controlled to be ≤300ppm. Ce and La elements are added when the vacuum treatment lasts for 20 minutes. At this time, the added amount per ton of steel is ≤200×([S%]+[O%])kg; VD vacuum treatment ensures the deep vacuum cycle time of molten steel. Slag surface protective agent is added immediately after vacuum treatment to prevent slag surface oxidation. The argon soft blowing time is greater than 20 minutes;

[0023] (2) Casting:

[0024] During continuous casting, the casting is protected throughout the whole process, the superheat and casting speed are controlled, and electromagnetic stirring technology is used to reduce the center segregation of the continuous casting billet;

[0025] During die casting, the ladle water inlet is protected by argon gas, the superheat and pouring speed are controlled, and protective slag and heating agent are added to the riser in time;

[0026] (3) Processing and molding

[0027] The blank is cut by sawing to meet the weight requirement of the flange; the flange is formed by free forging + ring rolling. After each forging, the forging surface is cooled to less than 950℃ before it can be heated in the furnace. The heat treatment adopts normalizing + tempering process. After heat treatment, it is processed by lathe to obtain the product.

[0028] Since the addition of Ce and La rare earth elements reacts in the molten steel or exists in the form of pure elements, the amount of steel added in step (1) is controlled by the content of S and O elements. Under this setting, the content of rare earth sulfides and rare earth oxides can be controlled, further improving the effect of rare earth sulfide oxides in refining the solidification structure.

[0029] Optionally, in step (3), the normalizing temperature is 1100-1300° C., and the normalizing time T1=9th, wherein T1 is in min, and th is the effective thickness of the workpiece in mm, with a value range of 200-400;

[0030] The tempering temperature is 700-750°C, and the tempering time is T2=120+th, where T2 is in min, and th is the effective thickness of the workpiece in mm, with a value range of 200-400.

[0031] The above-mentioned normalizing and tempering temperature and time can make rare earth oxides and sulfides quickly and effectively induce austenite nucleation to form a uniform fine-grained structure. During the cooling process, the solid-dissolved Ce and La atoms quickly occupy the grain boundary vacancies, which can not only inhibit the grain boundary segregation of elements such as P and S, but also induce pearlite nucleation to further refine the structure. And because of the two rare earth elements Ce and La added in this application, one normalizing or tempering can meet the needs of steel for wind power flanges, saving processing steps and reducing manufacturing costs. If the number of heat treatments is increased, the performance of the steel in this application will be reduced.

[0032] Optionally, during continuous casting in step (2), the superheat is controlled at 25-30° C. and the casting speed is ≤0.90 m / min;

[0033] During the step (2) molding, the superheat is controlled at 50-55° C. and the pouring speed is ≤2.5 tons / min.

[0034] During continuous casting, the superheat is controlled at 25-30°C, which can make the rare earth oxides and sulfides precipitate evenly and dispersed in the molten steel. The pulling speed is ≤0.90m / min, and the best AlN precipitation morphology and precipitation amount can be obtained. During mold casting, the superheat is controlled at 50-55°C, which can further make the rare earth oxides and sulfides precipitate evenly and improve the uniformity of their dispersion distribution in the molten steel. The casting speed is ≤2.5 tons / min, which can cooperate with the pulling speed to ensure that AlN is uniformly precipitated and the precipitation morphology is highly consistent, and the precipitation amount of AlN is further increased.

[0035] The beneficial effects of this application include but are not limited to:

[0036] 1. According to the fine-grained steel for wind power flanges of the present application, the genetic and nucleation characteristics of the steel structure are utilized, the solidification structure is refined by inducing the solidification nucleation of the molten steel through rare earth sulfur oxides, and the growth of austenite and ferrite is inhibited by inducing AlN precipitation in the steel, and finally the refinement of the flange steel matrix is ​​achieved.

[0037] 2. According to the fine-grained wind turbine flange steel of the present application, by controlling the content of solid-dissolved rare earth in the steel, the grain boundary segregation of P and other impurity elements is effectively suppressed, and the low-temperature toughness of the steel is significantly improved, and finally a high-strength wind turbine flange with refined grains and high and low-temperature impact toughness is obtained.

[0038] 3. According to the manufacturing method of fine-grained steel for wind power flanges of the present application, the actual components in the steel are matched, the content of rare earth sulfur oxides is controlled by the amount of steel added per ton, and the process conditions of continuous casting, die casting and heat treatment are controlled, so that the steel can achieve full-process organizational refinement, further improving the strength and toughness of the flange. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0040] Figure 1 This is a schematic diagram of the metallographic structure of the test steel 2# involved in Example 1 of the present application. DETAILED DESCRIPTION

[0041] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0042] Unless otherwise specified, the raw materials in the examples of this application were purchased from commercial sources.

[0043] Example 1

[0044] Test steels 1#-6# and comparative test steels D1#-D4# were prepared according to the following process. The specific weight percentages of the components are shown in Table 1, and neodymium was used to replace the La element in test steel 2# to obtain comparative test steel D5#.

[0045] (1) Smelting:

[0046] The scrap steel is melted in an electric arc furnace, pig iron is added, and the tapping temperature is less than 1650°C to obtain molten steel;

[0047] LF refining and VD vacuum treatment ensure the cleanliness of molten steel. LF refining uses a large amount of slag to make slag to ensure that the S content is ≤0.002%. Niobium iron and Al are added during the refining process to control the Al content to <0.035%;

[0048] The VD vacuum treatment time is greater than 25 minutes, and the N content is controlled to be ≤300ppm. Ce and La elements are added when the vacuum treatment lasts for 20 minutes. At this time, the added amount per ton of steel is ≤200×([S%]+[O%])kg; VD vacuum treatment ensures the deep vacuum cycle time of molten steel. Slag surface protective agent is added immediately after vacuum treatment to prevent slag surface oxidation. The argon soft blowing time is greater than 20 minutes;

[0049] (2) Casting:

[0050] During continuous casting, the casting is protected throughout the whole process, the superheat is controlled at 30℃, the casting speed is 0.90m / min, and electromagnetic stirring technology is used to reduce the center segregation of the continuous casting billet;

[0051] During die casting, the ladle water inlet is protected by argon gas, the superheat is controlled at 50℃, the pouring speed is 2.5 tons / min, and protective slag and heating agent are added to the riser in time;

[0052] (3) Processing and molding

[0053] The blank is cut by sawing to meet the weight requirement of the flange; the flange is formed by free forging + ring rolling. After each forging, the surface of the forging is cooled to less than 950℃ before it can be heated in the furnace. The heat treatment adopts normalizing + tempering process. The effective thickness of the workpiece is 300mm, the normalizing temperature is 1200℃, the normalizing time is 45h, the tempering temperature is 750℃, and the tempering time is 7h. After heat treatment, it is processed on a lathe to obtain the product.

[0054] Table 1

[0055]

[0056] The mechanical properties and grain size of the test steel obtained above were tested, wherein the grain size test was carried out according to the GB-T6394-2002 metal average grain size determination method, as shown in Table 2 below:

[0057] Table 2

[0058]

[0059] According to the data in Table 2, the Ce and La elements added in this application can refine the grain size and the microscopic solidification structure of the steel ingot, improve the performance of the flange in various aspects, and the Ce and La elements as well as the P and La elements in a specific proportion can further improve the density and morphological uniformity of the steel structure, ensuring that each component fully works synergistically.

[0060] Example 2

[0061] The composition of the test steel 2# was used to prepare the test steels 7#-14# according to the manufacturing method in Table 3 below. Except for the process conditions listed in Table 3 below, the other process conditions were the same as those in Example 1.

[0062] Table 3

[0063]

[0064] The mechanical properties of the test steel obtained above were tested, as shown in Table 4 below:

[0065] Table 4

[0066]

[0067] According to the data in Table 4, the manufacturing method of the present application matches the components of the flange. When the superheat of continuous casting and die casting is too low or too high, the uniformity of precipitation of rare earth oxides and sulfides is reduced, thereby reducing the mechanical properties of the product. If the pulling speed and casting speed are too fast, the uniformity of AlN precipitation morphology and the amount of precipitation will decrease, which will also reduce the mechanical properties of the product, especially the tensile strength. The decrease is most obvious; if the normalizing temperature and annealing temperature are too low, and the normalizing time or tempering time is too short, the uniformity of the fine-grained structure will be reduced, which is not conducive to grain refinement, thereby reducing the performance of the product in all aspects.

[0068] The above is only the embodiment of the present application, and the protection scope of the present application is not limited by these specific embodiments, but is determined by the claims of the present application. For those skilled in the art, the present application can have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the technical ideas and principles of the present application should be included in the protection scope of the present application.

Claims

1. A fine-grained steel for wind turbine flange, characterized in that: In terms of weight percentage, it comprises: C: 0.13-0.15%, Si: 0.17-0.40%, Mn: 1.2-1.6%, P≤0.025%, S≤0.003%, Nb: 0.03-0.05%, Al<0.06%, Ce≤300ppm, La≤300ppm, H≤2ppm, O≤20ppm, N≤300ppm, and the balance is Fe and unavoidable impurities; The Al×N content of the fine-grained steel for wind turbine flange is ≤1.2×10 -7 ; The Ce content is greater than 80 ppm, and the La content is greater than 20 ppm; The weight ratio of Ce to La is (1.69-3.8):1; The weight ratio of P to La is (1.2-3.35):1; The fine-grained wind power flange steel has a low-temperature impact energy of ≥177J at -60°C and an elongation at break of ≥35%.

2. The fine-grained steel for wind turbine flange according to claim 1 is characterized in that: The purity of Ce and La is greater than 95%.

3. The method for manufacturing fine-grained steel for wind power flange according to any one of claims 1 to 2, characterized in that: The steps include: (1) Smelting: The scrap steel is melted in an electric arc furnace, pig iron is added, and the tapping temperature is less than 1650°C to obtain molten steel; LF refining and VD vacuum treatment ensure the cleanliness of molten steel. LF refining uses a large amount of slag to make slag to ensure that the S content is ≤0.003%. Niobium iron and Al are added during the refining process to control the Al content to <0.06%; The VD vacuum treatment time is greater than 25 minutes, and the N content is controlled to be ≤300ppm. Ce and La elements are added when the vacuum treatment lasts for 20 minutes. At this time, the added amount per ton of steel is ≤200×([S%]+[O%])kg; VD vacuum treatment ensures the deep vacuum cycle time of molten steel. Slag surface protective agent is added immediately after vacuum treatment to prevent slag surface oxidation. The argon soft blowing time is greater than 20 minutes; (2) Casting: During continuous casting, the casting is protected throughout the whole process, the superheat and casting speed are controlled, and electromagnetic stirring technology is used to reduce the center segregation of the continuous casting billet; During die casting, the ladle water inlet is protected by argon gas, the superheat and pouring speed are controlled, and protective slag and heating agent are added to the riser in time; (3) Processing and molding The blank is cut by sawing to meet the weight requirement of the flange; the flange is formed by free forging + ring rolling. After each forging, the forging surface is cooled to less than 950℃ before it can be heated in the furnace. The heat treatment adopts normalizing + tempering process. After heat treatment, it is processed by lathe to obtain the product.

4. The manufacturing method according to claim 3, characterized in that: In step (3), the normalizing temperature is 1100-1300° C., and the normalizing time T1=9th, where T1 is in min, and th is the effective thickness of the workpiece in mm, ranging from 200 to 400; The tempering temperature is 700-750°C, and the tempering time is T2=120+th, where T2 is in min, and th is the effective thickness of the workpiece in mm, with a value range of 200-400.

5. The manufacturing method according to claim 3, characterized in that: Step (2) During continuous casting, the superheat is controlled at 25-30° C. and the casting speed is ≤0.90 m / min; During the step (2) molding, the superheat is controlled at 50-55° C. and the pouring speed is ≤2.5 tons / min.

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