Low-temperature nodular cast iron for wind power casting and preparation method thereof

Abstract

The invention discloses low-temperature nodular cast iron for wind power castings and a preparation method thereof, and belongs to the technical field of nodular cast iron materials. The nodular cast iron is prepared from, by mass, 3.2%-3.6% of C, 2.0%-2.5% of Si, 0.1%-0.2% of Cu, smaller than or equal to 0.3% of Mn, smaller than or equal to 0.03% of P, smaller than or equal to 0.015% of S, 0.03%-0.05% of Mg, 0.08%-0.15% of a nano additive and the balance Fe and inevitable impurities. The material matrix is mainly made of ferrite, the pearlite content is low, graphite nodules are uniformly distributed and regular in shape, and the comprehensive mechanical properties are as follows: the tensile strength is greater than or equal to 500 MPa (QT400-18AL is required to be greater than or equal to 360 MPa); the yield strength is greater than or equal to 350 MPa (the standard requirement is greater than or equal to 220 MPa); and the ductility is not less than 20% (the standard requirement is not less than 12%).

Description

Technical Field

[0001] The present invention belongs to the technical field of ductile iron materials, and particularly relates to a low-temperature ductile iron for wind power castings and a preparation method thereof. Background Art

[0002] Ductile iron has a high strength utilization rate due to the spherical dispersion of graphite particles in the matrix, and the unique self-lubricating and shock-absorbing effects endow ductile iron with excellent comprehensive mechanical properties and broad application prospects.

[0003] The service environment temperature range of wind turbines is relatively wide, within the range of -40 - 40°C. To avoid brittle fracture of ductile iron castings in wind turbines at low temperatures, compared with ordinary ductile iron castings, the production of ductile iron wind power castings has higher requirements and also has its uniqueness and complexity in aspects such as raw material selection, chemical composition control, spheroidizing treatment, and inoculation treatment processes. According to the national standard GB / T1348 (2009) or the European standard EN1563 (2012), when the wall thickness of the casting is > 60 - 200 mm, the tensile property requirements for QT400-18AL (EN-GJS-400-18-LT) material are a tensile strength ≥ 360 MPa, a yield strength ≥ 220 MPa, an elongation ≥ 12%, and the low-temperature impact property requirements are a -20°C low-temperature impact toughness of ≥ 7 J for each single value and ≥ 10 J for the average of three values.

[0004] Traditional low-temperature ductile iron (QT type) improves low-temperature toughness by adding alloying elements such as nickel and molybdenum, but its cost is high and excessive addition easily leads to a decline in casting performance.

[0005] For example, Chinese Patent Application CN202011050809.7 discloses a ductile iron material for a wind turbine, a preparation method and application of the ductile iron for a wind turbine, belonging to the field of wind turbines. The mass percentages of each element component of the ductile iron material for a wind turbine are: carbon 3.5% - 3.9%, manganese ≤ 0.2%, silicon ≤ 2.3%, phosphorus ≤ 0.003%, sulfur ≤ 0.025%, nickel ≤ 1.7%, antimony ≤ 0.005%, residual magnesium ≤ 0.05%, residual rare earth ≤ 0.03%, and the balance of iron and impurities generated during the preparation process. Compared with the prior art, the ductile iron obtained in the present invention accurately determines the proportion content of each component, so that the performance of QT400-18AL ductile iron can be improved in an ultra-low temperature environment.

[0006] In addition, the existing method of refining grains by rare earth elements has problems such as scarce rare earth resources and complex processes. In extremely low temperatures (such as below -40°C) and marine corrosion environments, traditional ductile iron is prone to brittle fracture and corrosion failure, which restricts its application in the field of offshore wind power.

[0007] Therefore, how to optimize the raw material selection and preparation process to obtain a new type of ductile iron material that is resistant to low temperature and corrosion to meet the application needs of various complex environments is a technical problem that needs to be solved urgently. Summary of the invention

[0008] In view of the problems existing in the prior art, the present invention provides a low-temperature ductile iron for wind power castings and a preparation method thereof. By optimizing the chemical composition and process flow, it is ensured that the material still has excellent mechanical properties and corrosion resistance at -40°C, meeting the harsh environmental requirements of offshore wind power.

[0009] In order to achieve the above technical objectives, the technical solution adopted by the present invention is: A low-temperature ductile iron for wind power castings, wherein the mass percentages of the various element components in the ductile iron are C 3.2%-3.6%, Si 2.0%-2.5%, Cu 0.1%-0.2%, Mn≤0.3%, P≤0.03%, S≤0.015%, Mg0.03%-0.05%, nano additives 0.08%-0.15%, and the balance is Fe and unavoidable impurities.

[0010] Furthermore, the nano additive is obtained by mixing the nano SiC-TiCN composite and nano cerium oxide in a mass ratio of 1:1.

[0011] Furthermore, the preparation method of the nano-SiC-TiCN composite is: (1) Weigh 2 g of titanyl sulfate and place it in a container, add 20-30 mL of deionized water, place it in a constant temperature water bath at 28-30° C. and stir for 1-2 h until it becomes clear, and filter to obtain a titanyl sulfate solution; (2) 2.5 g of silicon carbide powder with an average particle size of 100 nm was placed in a container, 20 mL of (CH2OH)2 and 100 mL of deionized water were added, and the mixed solution was ultrasonically stirred for 60 min; 5 g of urea was added to the mixed solution, and the stirring was continued for 30 min, and then 60 mL of the titanyl sulfate solution obtained in step (1) was added, and the obtained mixed solution was placed in a 65°C oil bath and stirred for 24 h, and then allowed to stand for 12 h and then centrifuged and washed to obtain a nanocomposite precursor; (3) In a N2 atmosphere, the temperature was increased to 800°C at a rate of 10°C / min and kept at that temperature for 2 h. After naturally cooling to room temperature, the product was collected and fully ground. The ground sample was then placed in a tube furnace and kept at 300°C for 30 min in a N2 atmosphere. The temperature was then increased to 1350°C and kept at that temperature for 1 h. The sample was naturally cooled to room temperature to obtain a nano-SiC-TiCN composite.

[0012] The average particle size of the nano cerium oxide is ≤200 μm.

[0013] A preparation method of low-temperature ductile iron for wind power castings, comprising the following steps: (1) Melting: Use an intermediate frequency induction furnace to melt the raw materials. The melting temperature is 1450 - 1530 °C. During the heating process, high-purity pig iron, ferrosilicon, scrap steel, and nano-additives are sequentially added. Detect the content of each element in the molten iron and adjust the content of each element to meet the formula requirements to obtain qualified molten iron in terms of chemical composition. Then, carry out static slag skimming treatment to obtain molten iron; (2) Spheroidizing treatment: Add a rare earth magnesium alloy spheroidizing agent accounting for 1.1% - 1.3% of the weight of the molten iron into the spheroidizing ladle, and evenly cover it with an inoculant accounting for 0.4% - 0.6% of the total weight of the molten iron. Pour in one-half of the total amount of molten iron. After completing the spheroidizing treatment and the first inoculation treatment, pour in the remaining molten iron, and pour in the inoculant accounting for 0.2% - 0.4% of the total weight of the molten iron with the flow to complete the second inoculation treatment, and then cast to obtain castings; (3) Heat treatment: Place the casting in an electric resistance furnace, heat it up to 550 °C, keep it warm for 1 hour, then heat it up to 880 - 900 °C, keep it warm for 2.5 - 3.5 h, and then naturally cool it to room temperature with the furnace to obtain the ductile iron.

[0014] Further, in step (1), the nano-additive is added with the raw materials during the melting stage and is uniformly dispersed by electromagnetic stirring.

[0015] Further, the rare earth magnesium alloy spheroidizing agent in step (2) includes by mass percentage: 5% - 6% of Mg, 45% - 50% of Si, 0.5% - 0.7% of Re, and the balance of Fe.

[0016] Further, the composition of the inoculant in step (2) includes by mass percentage: 72% - 80% of Si, 0.5% - 1.0% of Ca, 0.5 - 1.5% of Al, 2% - 3% of Ba, and the balance of Fe.

[0017] Beneficial effects: Through the unique nano-additive system and process optimization of the present invention, the low-temperature toughness, corrosion resistance, and comprehensive mechanical properties of the ductile iron are significantly improved. The specific positive effects are as follows: (1) The present invention prepares SiC-TiCN biphasic nanoparticles. The high hardness of nano-SiC and the chemical stability (high temperature resistance) of TiCN together serve as heterogeneous nucleation cores, promoting the spheroidization rate of graphite to ≥90%. CeO2 nanoparticles preferentially adsorb impurity elements such as sulfur and phosphorus through surface oxygen vacancies, reducing the content of brittle phases at grain boundaries (such as FeS and Fe3P) and enhancing the grain boundary bonding strength. The two are mixed in equal proportions to produce a synergistic effect, significantly improving the mechanical properties of the material. Secondly, the TiCN phase forms a passivation layer at the crack tip to inhibit crack propagation. Nano-SiC particles increase the impact energy absorption efficiency to ≥15 J (-40 °C) through pinning dislocations (Orowan mechanism) and inducing crack deflection (the tortuosity of the path increases by more than 50%), far exceeding 7-10 J of traditional materials, and significantly enhancing the low-temperature toughness.

[0018] (2) The TiCN phase forms a passivation layer in the matrix, reducing the electrochemical corrosion rate. The redox characteristics of cerium oxide can repair the surface oxide film and inhibit pitting corrosion and stress corrosion cracking in the marine environment. The two act together, and in the 5% NaCl salt spray test (GB / T 10125-2021), the corrosion current density of the material is reduced to 0.12 μA / cm² (0.35 μA / cm² for traditional materials), and the potential range of the passivation zone is broadened to -0.2 - +0.6 V ( -0.1 - +0.3 V for traditional materials), significantly enhancing the ability to resist Cl⁻ erosion.

[0019] (3) After subsequent isothermal heat treatment process, the material matrix is mainly composed of ferrite, with a low pearlite content. The graphite balls are evenly distributed and regular in shape. The comprehensive mechanical properties are as follows: tensile strength ≥500 MPa (the requirement for QT400-18AL is ≥360 MPa); yield strength ≥350 MPa (the standard requirement is ≥220 MPa); elongation ≥20% (the standard requirement is ≥12%). Description of the Drawings

[0020] Figure 1 It is a schematic diagram of the dimensions of a ductile iron tensile specimen; Figure 2 It is a schematic diagram of the dimensions of a ductile iron impact specimen; Figure 3 Metallographic structure diagrams of different specimens, where A-E are specimens of Comparative Example 1-5, and F is the specimen of Example 1; Figure 4 It is a SEM photo of the impact fracture surface of the specimen, where A-E are specimens of Comparative Example 1-5, and F is the specimen of Example 1. Detailed Embodiments

[0021] The technical solutions of the present invention will be further described below in conjunction with specific embodiments, but not limited thereto.

[0022] Example 1 A low-temperature ductile iron for wind power castings, wherein the mass percentages of the various element components in the ductile iron are C3.6%, Si 2.5%, Cu0.2%, Mn≤0.3%, P≤0.03%, S≤0.015%, Mg0.03%, nano additives 0.08%, and the balance Fe and unavoidable impurities.

[0023] The nano additive is obtained by mixing the nano SiC-TiCN composite and nano cerium oxide in a mass ratio of 1:1.

[0024] The preparation method of the nano-SiC-TiCN composite is: (1) Weigh 2 g of titanyl sulfate and place it in a container, add 20 mL of deionized water, place it in a constant temperature water bath at 28-30° C. and stir for 1-2 h until it becomes clear, and filter to obtain a titanyl sulfate solution; (2) 2.5 g of silicon carbide powder with an average particle size of 100 nm was placed in a container, 20 mL of (CH2OH)2 and 100 mL of deionized water were added, and the mixed solution was ultrasonically stirred for 60 min; 5 g of urea was added to the mixed solution, and the stirring was continued for 30 min, and then 60 mL of the titanyl sulfate solution obtained in step (1) was added, and the obtained mixed solution was placed in a 65°C oil bath and stirred for 24 h, and then allowed to stand for 12 h and then centrifuged and washed to obtain a nanocomposite precursor; (3) In a N2 atmosphere, the temperature was increased to 800°C at a rate of 10°C / min and kept at that temperature for 2 h. After naturally cooling to room temperature, the product was collected and fully ground. The ground sample was then placed in a tube furnace and kept at 300°C for 30 min in a N2 atmosphere. The temperature was then increased to 1350°C and kept at that temperature for 1 h. The sample was naturally cooled to room temperature to obtain a nano-SiC-TiCN composite.

[0025] A method for preparing low-temperature ductile iron for wind power castings comprises the following steps: (4) Smelting: Use a medium frequency induction furnace to melt the raw materials at a smelting temperature of 1450°C. During the heating process, high-purity pig iron, ferrosilicon, scrap steel and nano-additives are added in sequence. The content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. Then, the molten iron is subjected to static slag treatment to obtain molten iron; (5) Spheroidizing treatment: 1.1% of the weight of the molten iron is added to the spheroidizing bag. The spheroidizing agent of rare earth magnesium alloy is evenly covered on the spheroidizing bag. One-half of the total weight of the molten iron is poured in. After the spheroidizing treatment and the first inoculation treatment are completed, the remaining molten iron is poured in. The inoculant of 0.2% of the total weight of the molten iron is poured in along with the flow. The second inoculation treatment is completed, and casting is performed to obtain a casting; (6) Heat treatment: the casting is placed in a resistance furnace, heated to 550° C., kept at this temperature for 1 hour, then heated to 880° C., kept at this temperature for 2.5 hours, and then naturally cooled to room temperature in the furnace to obtain the ductile iron.

[0026] The nano additives in step (1) are added with the raw materials during the smelting stage and are evenly dispersed by electromagnetic stirring.

[0027] Step (2) The rare earth magnesium alloy spheroidizing agent comprises, by mass percentage, 5% Mg, 45% Si, 0.5% Re and the balance Fe.

[0028] The composition of the inoculant in step (2) includes, by mass percentage, 72% Si, 0.5% Ca, 0.5% Al, 2% Ba, and the balance Fe.

[0029] Example 2 A low-temperature ductile iron for wind power castings, wherein the mass percentages of the various element components in the ductile iron are C3.2%, Si 2.0%, Cu0.1%, Mn≤0.3%, P≤0.03%, S≤0.015%, Mg0.05%, nano additives 0.1%, and the balance Fe and unavoidable impurities.

[0030] The nano additive is obtained by mixing the nano SiC-TiCN composite and nano cerium oxide in a mass ratio of 1:1.

[0031] Furthermore, the preparation method of the nano-SiC-TiCN composite is: (1) Weigh 2 g of titanyl sulfate and place it in a container, add 25 mL of deionized water, place it in a constant temperature water bath at 28-30° C. and stir for 1-2 h until it becomes clear, and filter to obtain a titanyl sulfate solution; (2) 2.5 g of silicon carbide powder with an average particle size of 100 nm was placed in a container, 20 mL of (CH2OH)2 and 100 mL of deionized water were added, and the mixed solution was ultrasonically stirred for 60 min; 5 g of urea was added to the mixed solution, and the stirring was continued for 30 min, and then 60 mL of the titanyl sulfate solution obtained in step (1) was added, and the obtained mixed solution was placed in a 65°C oil bath and stirred for 24 h, and then allowed to stand for 12 h and then centrifuged and washed to obtain a nanocomposite precursor; (3) In a N2 atmosphere, the temperature was increased to 800°C at a rate of 10°C / min and kept at that temperature for 2 h. After naturally cooling to room temperature, the product was collected and fully ground. The ground sample was then placed in a tube furnace and kept at 300°C for 30 min in a N2 atmosphere. The temperature was then increased to 1350°C and kept at that temperature for 1 h. The sample was naturally cooled to room temperature to obtain a nano-SiC-TiCN composite.

[0032] A preparation method of low-temperature ductile iron for wind power castings, comprising the following steps: (1) Melting: Use an intermediate frequency induction furnace to melt the raw materials. The melting temperature is 1530 °C. During the heating-up process, high-purity pig iron, ferrosilicon, scrap steel, and nano-additives are sequentially added. Detect the content of each element in the molten iron and adjust the content of each element to meet the formula requirements to obtain molten iron with qualified chemical composition. Then, carry out static slag skimming treatment to obtain molten iron; (2) Spheroidizing treatment: Add a rare earth magnesium alloy spheroidizing agent accounting for 1.3% of the weight of the molten iron into the spheroidizing ladle, and evenly cover it with an inoculant accounting for 0.6% of the total weight of the molten iron. Pour in one-half of the total amount of molten iron. After completing the spheroidizing treatment and the first inoculation treatment, pour in the remaining molten iron, and pour in the inoculant accounting for 0.4% of the total weight of the molten iron with the flow to complete the second inoculation treatment, and then cast to obtain castings; (3) Heat treatment: Place the casting in an electric resistance furnace, heat it up to 550 °C, keep it warm for 1 hour, then heat it up to 900 °C, keep it warm for 3.5 h, and then naturally cool it to room temperature with the furnace to obtain the ductile iron.

[0033] In step (1), the nano-additive is added with the raw materials during the melting stage and is uniformly dispersed by electromagnetic stirring.

[0034] The rare earth magnesium alloy spheroidizing agent in step (2) includes, by mass percentage: 6% of Mg, 50% of Si, 0.7% of Re, and the balance of Fe.

[0035] The composition of the inoculant in step (2) includes, by mass percentage: 76% of Si, 0.5% of Ca, 1% of Al, 2% of Ba, and the balance of Fe.

[0036] Example 3 A low-temperature ductile iron for wind power castings, wherein the mass percentages of the components of each element in the ductile iron are C 3.2%, Si 2.1%, Cu 0.2%, Mn ≤ 0.3%, P ≤ 0.03%, S ≤ 0.015%, Mg 0.04%, nano-additive 0.15%, and the balance is Fe and unavoidable impurities.

[0037] The nano-additive is obtained by mixing nano SiC-TiCN composite and nano cerium oxide in a mass ratio of 1:1.

[0038] The preparation method of the nano SiC-TiCN composite is as follows: (1) Weigh 2 g of titanium oxysulfate and place it in a container. Add 30 mL of deionized water, place it in a constant temperature water bath at 28 - 30 °C and stir for 1 - 2 h until it becomes clear, and filter to obtain a titanium oxysulfate solution; (2) Put 2.5 g of silicon carbide powder with an average particle size of 100 nm into a container, add 20 mL of (CH2OH)2 and 100 mL of deionized water, and ultrasonically treat the mixture. Stir the mixed solution ultrasonically for 60 min; add 5 g of urea to the mixed solution, continue stirring for 30 min, then add 60 mL of the titanium oxysulfate solution obtained in step (1). Place the obtained mixed solution in an oil bath at 65 °C and stir for 24 h, then let it stand for 12 h and centrifuge and wash to obtain a precursor of the nanocomposite; (3) Under a N2 atmosphere, heat up to 800 °C at a rate of 10 °C / min, hold for 2 h, naturally cool to room temperature, collect the product and grind it thoroughly. Then place the ground sample in a tubular furnace. Under a N2 atmosphere, hold at 300 °C for 30 min, then heat up to 1350 °C and hold for 1 h; naturally cool to room temperature to obtain the nano SiC-TiCN composite.

[0039] A preparation method of ductile iron for wind power castings includes the following steps: (7) Melting: Use an intermediate frequency induction furnace to melt the raw materials. The melting temperature is 1450 °C. During the heating process, put high-purity pig iron, ferrosilicon, scrap steel, and nano additives in sequence, detect the content of each element in the molten iron, and adjust the content of each element to meet the formula requirements to obtain molten iron with qualified chemical composition. Then perform static slag skimming treatment to obtain molten iron; (8) Spheroidizing treatment: Add a rare earth magnesium alloy spheroidizing agent accounting for 1.1% of the weight of the molten iron to the spheroidizing ladle, evenly cover it with an inoculant accounting for 0.6% of the total weight of the molten iron, pour in half of the total amount of molten iron. After completing the spheroidizing treatment and the first inoculation treatment, pour in the remaining molten iron, and pour in the inoculant accounting for 0.2% of the total weight of the molten iron with the flow to complete the second inoculation treatment, and then cast to obtain the casting; (9) Heat treatment: Place the casting in an electric resistance furnace, heat up to 550 °C, hold for 1 hour, then heat up to 880 °C, hold for 3 h, and then naturally cool to room temperature with the furnace to obtain the ductile iron.

[0040] The nano additive described in step (1) is added with the raw materials during the melting stage and is uniformly dispersed by electromagnetic stirring.

[0041] The rare earth magnesium alloy spheroidizing agent in step (2) includes by mass percentage: 5% of Mg, 50% of Si, 0.5% of Re, and the balance of Fe.

[0042] The composition of the inoculant in step (2) includes by mass percentage: 80% of Si, 1.0% of Ca, 1.5% of Al, 3% of Ba, and the balance of Fe.

[0043] Comparative Example 1 This comparative example is the same as Example 1 in terms of the remaining raw materials and process steps except that nano SiC-TiCN composite is not used in the nano additive. That is: A low-temperature ductile iron for wind power castings, wherein the mass percentages of the elements in the ductile iron are: C 3.2%, Si 2.0%, Cu 0.1%, Mn ≤ 0.3%, P ≤ 0.03%, S ≤ 0.015%, Mg 0.05%, nano additive 0.1%, and the balance is Fe and inevitable impurities.

[0044] The nano additive is nano cerium oxide.

[0045] Comparative Example 2 This comparative example is the same as Example 1 in terms of the remaining raw materials and process steps except that nano cerium oxide is not used in the nano additive. That is: A low-temperature ductile iron for wind power castings, wherein the mass percentages of the elements in the ductile iron are: C 3.2%, Si 2.0%, Cu 0.1%, Mn ≤ 0.3%, P ≤ 0.03%, S ≤ 0.015%, Mg 0.05%, nano additive 0.1%, and the balance is Fe and inevitable impurities.

[0046] The nano additive is nano SiC-TiCN composite, and the preparation method is the same as that in Example 1.

[0047] Comparative Example 3 This comparative example is the same as Example 1 in terms of the remaining raw materials and process steps except that the mass ratio of nano SiC-TiCN composite and nano cerium oxide in the nano additive is changed. That is: A low-temperature ductile iron for wind power castings, wherein the mass percentages of the elements in the ductile iron are: C 3.6%, Si 2.5%, Cu 0.2%, Mn ≤ 0.3%, P ≤ 0.03%, S ≤ 0.015%, Mg 0.03%, nano additive 0.08%, and the balance is Fe and inevitable impurities.

[0048] The nano additive is obtained by mixing nano SiC-TiCN composite and nano cerium oxide in a mass ratio of 2:1.

[0049] Comparative Example 4 This comparative example is the same as Example 1 in terms of the remaining raw materials and process steps except that the mass ratio of nano SiC-TiCN composite and nano cerium oxide in the nano additive is changed. That is: A low-temperature ductile iron for wind power castings, wherein the mass percentages of the elements in the ductile iron are: C 3.6%, Si 2.5%, Cu 0.2%, Mn ≤ 0.3%, P ≤ 0.03%, S ≤ 0.015%, Mg 0.03%, nano additive 0.08%, and the balance is Fe and inevitable impurities.

[0050] The nano-additive is obtained by mixing nano-SiC-TiCN composite and nano-cerium oxide in a mass ratio of 1:2.

[0051] Comparative Example 5 In this comparative example, except for not using the nano-additive, the other raw materials and process steps are the same as those in Example 1. That is: A low-temperature ductile iron for wind power castings, wherein the mass percentages of the respective elements in the ductile iron are C 3.2%, Si 2.0%, Cu 0.1%, Mn ≤ 0.3%, P ≤ 0.03%, S ≤ 0.015%, Mg 0.05%, and the balance is Fe and unavoidable impurities.

[0052] Performance Test The ductile iron prepared from the examples and comparative examples was subjected to performance tests. The test methods are as follows: According to the national standard GB / T 231.1-2018 "Metallic materials - Brinell hardness test - Part 1: Test method", the hardness of the specimens was tested using a 310HBS-3000 type Brinell hardness tester. The hardness test was carried out at 5 points evenly distributed on the surface of the specimens, and the average value of each group of data was taken to obtain the Brinell hardness value. Mechanical properties such as yield strength and tensile strength were tested 3 times for each specimen using a universal tensile machine, and the average value was taken. Refer to GB / T228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature". The dimensions of the tensile specimens are as Figure 1 , unit mm.

[0053] According to the standard of GB / T 9441—2009 "Metallographic examination of ductile iron", indexes such as nodularity and graphite ball diameter were statistically analyzed. The size of the metallographic sample was 10 mm×10 mm×10 mm. It was pre-ground successively with 400, 800, 1200, and 2000 mesh sandpapers, then polished with diamond polishing agent, and then the surface of the sample was corroded with 4% nitric acid alcohol for 7-10 s. Then it was rinsed with anhydrous ethanol and quickly dried. Finally, the as-cast structure of the ductile iron sample was observed using a VM3000I inverted metallographic microscope. The nodularity, size, number per unit area, and relative content of ferrite and pearlite of the graphite balls in the structure were the statistical average values of multiple pictures.

[0054] Low-temperature impact test: The impact specimens were processed according to the regulations of the national standard GB / T 229-2011 "Metallic materials - Charpy pendulum impact test method". The impact test used Charpy V-notch specimens, and the NI300F impact testing machine was used for the impact test. Before the test, the specimens were cooled in absolute ethanol at -20 °C for 20 min, then quickly removed from the absolute ethanol and placed on the impact testing machine, and the impact test was completed within 2 s. Three specimens were tested in each group, and the average value was taken as the result. After the test, the SEM was used to observe the morphology of the impact fracture surface. The impact specimens are as Figure 2 shown (unit: mm).

[0055] Corrosion performance test: Electrochemical test: Using a PARSTAT 4000 electrochemical workstation, a three-electrode system (working electrode, platinum sheet counter electrode, saturated calomel reference electrode), scanning rate 1 mV / s, potential range -1.0~+1.0 V, in an environment containing Cl⁻ (such as 5% NaCl solution).

[0056] Table 1 Performance test results As can be seen from the data in Table 1, the tensile strength, yield strength, elongation and hardness of the example group are significantly better than those of the comparative example group and the QT400-18L standard, indicating that the optimized ductile iron samples have significantly improved mechanical properties. In addition, the impact test results show that the specimens in the example group still maintain high toughness in a low-temperature environment, and the fracture surface morphology analysis further verifies their excellent impact resistance. The results of the electrochemical test and the salt spray test also show that the optimized ductile iron samples are significantly superior to the control group in terms of corrosion resistance and have a wider application prospect. Through up to 2000 hours of corrosive brine immersion and electrochemical reaction testing, Table 2 Performance test results Table 3 Electrochemical test results Through comprehensive analysis in the embodiments of the present invention, the optimized ductile iron samples significantly exceed the existing standards in terms of mechanical, impact resistance and corrosion resistance, showing excellent comprehensive performance and providing a reliable material guarantee for high-end industrial applications. Compared with Comparative Examples 1-5 with changed nano-additive compositions, the example group is particularly outstanding in terms of low-temperature impact and electrochemical stability, verifying the scientificity and practicability of the optimized formula. In Comparative Examples 1-5 with changed nano-material compositions, the synergistic balance effect of the nano SiC-TiCN composite and nano-ceria is broken, resulting in performance degradation. From Figure 4It can also be seen from the SEM photos of the impact fracture surface of the specimens that river pattern fracture morphologies appeared in Comparative Examples 1-5, which are brittle fracture characteristics. While the examples showed obvious dimpled morphologies, which are ductile fracture characteristics.

[0057] It should be noted that the above-mentioned embodiments are only partial embodiments of the preferred ways to implement the present invention, rather than all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

Claims

1. A low-temperature ductile iron for wind power castings, characterized in that, The mass percentage of each element component in the ductile iron is C 3.2%-3.6%, Si 2.0%-2.5%, Cu0.1%-0.2%, Mn≤0.3%, P≤0.03%, S≤0.015%, Mg0.03%-0.05%, nano additive 0.08%-0.15%, and the balance is Fe and unavoidable impurities.

2. The ductile iron for wind power castings according to claim 1, wherein The mass percentages of the various element components in the ductile iron are C 3.2%, Si 2.0%, Cu 0.1%, Mn≤0.3%, P≤0.03%, S≤0.015%, Mg 0.05%, nano additive 0.1%, and the balance Fe and unavoidable impurities.

3. The ductile iron for wind power castings according to claim 1, wherein The nano additive is obtained by mixing a nano SiC-TiCN composite and nano cerium oxide in a mass ratio of 1:

1.

4. The ductile iron for wind power castings according to claim 3, characterized in that, The preparation method of the nano SiC-TiCN composite is: (1) Weigh 2 g of titanyl sulfate and place it in a container, add 20-30 mL of deionized water, place it in a constant temperature water bath at 28-30° C. and stir for 1-2 h until it becomes clear, and filter to obtain a titanyl sulfate solution; (2) 2.5 g of silicon carbide powder with an average particle size of 100 nm was placed in a container, 20 mL of (CH2OH)2 and 100 mL of deionized water were added, and the mixed solution was ultrasonically stirred for 60 min; 5 g of urea was added to the mixed solution, and the stirring was continued for 30 min, and then 60 mL of the titanyl sulfate solution obtained in step (1) was added, and the obtained mixed solution was placed in a 65°C oil bath and stirred for 24 h, and then allowed to stand for 12 h and then centrifuged and washed to obtain a nanocomposite precursor; (3) In a N2 atmosphere, the temperature was increased to 800°C at a rate of 10°C / min and kept at that temperature for 2 h. After naturally cooling to room temperature, the product was collected and fully ground. The ground sample was then placed in a tube furnace and kept at 300°C for 30 min in a N2 atmosphere. The temperature was then increased to 1350°C and kept at that temperature for 1 h. The sample was naturally cooled to room temperature to obtain a nano-SiC-TiCN composite.

5. The preparation method of the low-temperature ductile iron for wind power castings according to any one of claims 1-4, characterized in that, The following steps are involved: (1) Melting: Use a medium frequency induction furnace to melt the raw materials at a melting temperature of 1450-1530°C. During the heating process, high-purity pig iron, ferrosilicon, scrap steel and nano-additives are added in sequence. The content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. Then, the molten iron is subjected to static slag treatment to obtain molten iron; (2) Spheroidizing treatment: adding 1.1%-1.3% of the weight of the molten iron to the spheroidizing bag, a rare earth magnesium alloy spheroidizing agent is evenly covered on the spheroidizing bag with 0.4%-0.6% of the total weight of the molten iron inoculant, and half of the total amount of the molten iron is poured in. After the spheroidizing treatment and the first inoculation treatment are completed, the remaining molten iron is poured in, and 0.2%-0.4% of the total weight of the molten iron inoculant is poured in along with the flow, and the second inoculation treatment is completed, and casting is performed to obtain a casting; (3) Heat treatment: placing the casting in a resistance furnace, raising the temperature to 550° C., keeping the temperature for 1 hour, then raising the temperature to 880-900° C., keeping the temperature for 2.5-3.5 hours, and then naturally cooling to room temperature in the furnace to obtain the ductile iron.

6. The preparation method of the low-temperature ductile iron for wind power castings according to claim 5, characterized in that, The nano-additive described in step (1) is added with the raw materials during the smelting stage and is uniformly dispersed by electromagnetic stirring.

7. The preparation method of the low-temperature ductile iron for wind power castings according to claim 5, characterized in that, The rare earth magnesium alloy spheroidizing agent in step (2) includes, by mass percentage: 5%-6% of Mg, 45%-50% of Si, 0.5%-0.7% of Re, and the balance of Fe.

8. The preparation method of the low-temperature ductile iron for wind power castings according to claim 5, characterized in that, The composition of the inoculant in step (2) includes, by mass percentage: 72%-80% of Si, 0.5%-1.0% of Ca, 0.5-1.5% of Al, 2%-3% of Ba, and the balance is Fe.

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