Large megawatt wind power nodular cast iron and preparation method thereof

Abstract

The invention belongs to the field of wind power generation, and relates to large megawatt wind power nodular cast iron and a preparation method thereof. A traditional pouring basin mode is converted into a seat ladle casting mode, the slag content in the casting is effectively reduced, the quality of the surface of the casting is improved through the improvement, internal defects caused by slag inclusion are avoided, the purity and mechanical performance of the casting are ensured, and the casting quality is improved. And a large-particle secondary inoculant with the particle size of 10-50mm is adopted for ladle-to-ladle operation, so that the inoculation decline phenomenon can be effectively weakened, the number of graphite nodules is increased, and the graphite nodules are distributed more uniformly. Through the improvement measure, the uniformity of the internal structure of the casting is greatly improved, and the comprehensive performance of the material is enhanced.

Description

Technical Field

[0001] The present invention belongs to the field of wind power generation and relates to a large-megawatt wind power ductile iron and a preparation method thereof. Background Art

[0002] In recent years, with the rapid development of high-power wind power ductile iron units, the functional requirements for ductile iron castings have increased significantly. This is not only reflected in the requirements for mechanical properties such as material strength and toughness, but also includes aspects such as cost control, energy conservation and emission reduction, and resource conservation. Due to the limitations of their design and production capabilities, traditional small-power wind turbine units have been difficult to meet the current market demands.

[0003] In the production process of ductile iron castings, every link from raw material preparation, melting to spheroidizing treatment, pouring, inoculation, etc. directly affects the final performance of the cast iron. Especially when producing large-section ductile iron castings, the slow cooling rate becomes a prominent technical challenge. The slow cooling process easily leads to problems of graphite distortion in the thick-wall center or hot spot area of the casting. This phenomenon will cause a reduction in the number of spheroids, coarsening of the microstructure, and abnormal graphite distribution (graphite floating), which are all manifestations of spheroidizing inoculation decline and seriously affect the mechanical properties of the casting.

[0004] To solve these problems and improve the quality of ductile iron castings in high-power ductile iron units, the secondary in-stream inoculation gating cup pouring technology has been widely adopted. This method effectively improves the graphite morphology and distribution inside the casting by optimizing the inoculation treatment during the pouring process, reduces the spheroidizing inoculation decline phenomenon, and thus improves the comprehensive performance of the casting.

[0005] However, with the progress of technology and the change of market demands, the traditional gating cup pouring method has gradually revealed its deficiencies. Especially for high-power wind power ductile iron units, the excessive slag content in the casting has become a new challenge. This not only affects the surface quality of the casting but also may cause the MT (magnetic particle inspection) results to fail to meet the high standards of customers. Summary of the Invention

[0006] The purpose of the present invention is to address the above problems existing in the prior art and propose a large-megawatt wind power ductile iron. By adjusting the addition method, particle size of the secondary inoculant, and its combination with the ladle pouring process, the microstructure of the ductile iron can be effectively optimized and its mechanical properties can be improved.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] A large-megawatt wind power ductile iron, wherein the number of graphite balls in the ductile iron ≥ 185 pieces / mm 2, the spheroidization rate of graphite is ≥95%, and it is composed of components with the following mass fractions: 3.82 - 3.88% C, 2.0 - 2.15% Si, 0.1 - 0.15% Mn, 0.03 - 0.035% P, 0.008 - 0.01% S, 0.05 - 0.055% Mg, 0.002 - 0.005% RE, 0.006 - 0.008% Sb, and the balance is Fe.

[0009] The present invention also provides a method for preparing the above-mentioned large - megawatt wind power ductile iron, and the method comprises the following steps:

[0010] S1. Melting scrap steel, low - silicon pig iron, medium - silicon pig iron, ferrosilicon, metallic antimony, and carburant into molten iron for desulfurization treatment, and then performing high - temperature standing treatment;

[0011] S2. After the high - temperature standing ends, tapping the molten iron for spheroidizing inoculation treatment;

[0012] S3. Performing slag skimming treatment on the molten iron after spheroidizing inoculation treatment to obtain ladle molten iron, and sprinkling a heat - insulating covering agent on the surface of the molten iron and transporting it to the casting site;

[0013] S4. Then skimming off the heat - insulating covering agent, adding a silicon - aluminum secondary inoculant with a median particle size of 10 - 50 mm to the surface of the ladle molten iron, and then pouring the molten iron in the ladle into a bottom - pouring ladle;

[0014] S5. The molten iron in the bottom - pouring ladle undergoes standing, filling, and cooling in sequence to obtain large - megawatt wind power ductile iron.

[0015] In the present invention, when the molten iron is poured from the ladle into the bottom - pouring ladle, the silicon - aluminum secondary inoculant with a median particle size of 10 - 50 mm can be more evenly dispersed throughout the volume of the molten iron instead of concentrating in a certain part. This not only helps to increase the quantity and quality of graphite balls, but also provides sufficient time for the slag to float, thereby significantly reducing the amount of slag inclusions in the casting. In addition, this method can also optimize the fluidity of the molten iron to a certain extent, making the filling process smoother and reducing defects caused by insufficient filling.

[0016] However, directly using this large - particle - size silicon - aluminum secondary inoculant in the bottom - pouring ladle will cause the phenomenon of floating silicon, resulting in a decrease in the uniformity of the mechanical properties of the casting. To solve this problem, the improved method of the present invention is to add the large - particle - size silicon - aluminum secondary inoculant to the surface of the ladle molten iron and achieve more uniform dissolution through the pouring method of pouring the molten iron from the ladle into the bottom - pouring ladle.

[0017] In the above method for preparing large-megawatt wind power ductile iron, the scrap steel in step S1 consists of the following components by mass fraction: carbon 0.01 - 0.04%, silicon 0.02 - 0.05%, manganese 0.05 - 0.30%, phosphorus 0.015 - 0.025%, sulfur 0.008 - 0.010%, chromium 0.015 - 0.025%, and the balance is iron and unavoidable impurities;

[0018] The low-silicon pig iron consists of the following components by mass fraction: carbon 3.8 - 4.3%, silicon 0.3 - 0.5%, manganese 0.02 - 0.07%, phosphorus 0.02 - 0.05%, sulfur 0.01 - 0.03%, chromium 0.004 - 0.005%, titanium 0.01 - 0.03%, copper 0.001 - 0.008%, vanadium 0.002 - 0.006%, and the balance is iron and unavoidable impurities;

[0019] The medium-silicon pig iron consists of the following components by mass fraction: carbon 4.0 - 4.5%, silicon 0.5 - 0.75%, manganese 0.02 - 0.07%, phosphorus 0.02 - 0.05%, sulfur 0.01 - 0.03%, chromium 0.004 - 0.005%, titanium 0.01 - 0.03%, copper 0.001 - 0.008%, vanadium 0.002 - 0.006%, and the balance is iron and unavoidable impurities;

[0020] The ferrosilicon consists of the following components by mass fraction: silicon 72.0 - 80.0%, manganese 0.15 - 0.30%, chromium 0.005 - 0.02%, aluminum 0.60 - 1.0%, and the balance is iron and unavoidable impurities;

[0021] The carburizer uses a micro-sulfur carburizer with a carbon content of not less than 99.7%.

[0022] Low-silicon pig iron usually contains a relatively low silicon content, which helps to more easily add other alloy elements or adjust the overall formula when needed to meet specific mechanical property requirements. In contrast, medium-silicon pig iron has a higher silicon content. As one of the important elements in ductile iron, silicon can not only promote graphite spheroidization but also improve the tensile strength and wear resistance of the material. However, too high a silicon content may also cause problems such as increased brittleness. Therefore, in the present invention, by reasonably matching these two kinds of pig iron, the proportion of silicon elements can be adjusted more flexibly according to actual needs, and at the same time, the content of other elements is optimized to achieve an ideal chemical composition, so as to more precisely control the performance indexes of ductile iron while maintaining cost-effectiveness.

[0023] Preferably, the melting temperature in step S1 is 1450 - 1500 °C.

[0024] In the above-mentioned preparation method of large-megawatt wind power ductile iron, the high-temperature standing temperature in step S1 is 1515 - 1525 °C, and the time is 15 - 25 min. Through high-temperature standing in the present invention, the impurities in the molten iron can float to the surface more fully, facilitating removal, thereby reducing their negative impact on the quality of the casting. Under such temperature conditions, the atomic activity of the molten iron increases, promoting the interaction between atoms and making it easier for crystal nuclei to form. This not only helps to refine the grain structure but also improves the morphology and distribution of graphite, making it more uniform and finer, which is crucial for improving the mechanical properties of ductile iron. Moreover, it can further promote the dissolution and uniform distribution of alloying elements in the molten iron.

[0025] In the above-mentioned preparation method of large-megawatt wind power ductile iron, in the desulfurization and slag skimming treatment in step S1, the desulfurizer includes the following components by mass fraction: 45 - 55% calcium oxide, 5 - 10% calcium fluoride, and 40 - 45% calcium carbide.

[0026] In the above-mentioned preparation method of large-megawatt wind power ductile iron, the tapping temperature in step S2 is 1420 °C - 1480 °C. Controlling the tapping temperature in the present invention helps to maintain an appropriate cooling rate, which is particularly important for the formation of graphite balls. An ideal cooling rate can ensure that graphite is evenly distributed in the cast iron matrix in the form of small and round balls, which not only improves the strength and toughness of the material but also enhances its wear resistance. If the molten iron temperature is too high, the cooling rate will be significantly reduced. A slower cooling process may lead to abnormal graphite morphology, increased size, or reduced quantity, thereby weakening the spheroidizing effect and affecting the mechanical properties of ductile iron. If the molten iron temperature is too low, the cooling rate will be too fast. Rapid cooling may increase the risk of filling, that is, the molten metal fails to completely fill the mold cavity, resulting in cold shut defects in the casting. Such defects are manifested as discontinuous seams or cracks on the surface or inside of the casting, seriously affecting the structural integrity and appearance quality of the casting, and even possibly leading to product rejection.

[0027] In the above-mentioned preparation method of large-megawatt wind power ductile iron, the process of spheroidizing and inoculating treatment in step S2 is as follows: Add a spheroidizing agent accounting for 0.9 - 1.2 wt% of the weight of the molten iron into the spheroidizing pit, and sequentially cover it with a high-calcium barium inoculant accounting for 0.2 - 0.5 wt% of the weight of the molten iron and scrap steel sheets accounting for 0.6 - 0.8 wt% of the weight of the molten iron.

[0028] During the casting process, the molten iron has a very high kinetic energy when flowing out. Directly impacting the spheroidizing agent will cause premature local reaction of the spheroidizing agent or spheroidization decay caused by uneven distribution, which will affect the fluidity of the molten iron. The invention covers 0.6 - 0.8wt% of scrap steel sheets, which can effectively buffer the impact force of the molten iron without affecting the fluidity of the molten iron, thereby reducing the risk of spheroidization decay. This not only helps to maintain the integrity of the spheroidizing agent but also ensures its uniform dispersion throughout the molten iron. Moreover, an appropriate amount of scrap steel can adjust the cooling rate and solidification process of the molten iron, which also has a positive effect on refining grains and improving the internal quality of the casting. At the same time, the presence of scrap steel sheets will not have a significant impact on the final composition of the casting because they mainly play a role in physical isolation and buffering.

[0029] In the preparation method of the above large - megawatt wind power ductile iron, the spheroidizing agent is composed of the following components by mass fraction: 42 - 46% silicon, 5.5 - 6.5% magnesium, 0.3 - 0.38% RE, 0.6 - 0.8% aluminum, 0.45 - 0.55% magnesium oxide, 0.9 - 1.0% calcium, and the balance is iron and inevitable impurities. The spheroidizing agent of the present invention helps to ensure that the graphite spheroidizes in the best state, which is particularly important for preventing the appearance of fragmented, nail - shaped or other distorted graphite morphologies in large ductile iron castings. Without proper spheroidizing treatment, the graphite may exist in irregular forms such as flakes or needles, which will significantly reduce the strength and toughness of the material. On the contrary, when the graphite is evenly distributed in the matrix in a spherical form, its cutting effect on the matrix is the smallest, so that the casting has higher tensile strength, ductility and impact toughness.

[0030] Preferably, the average diameter of the scrap steel sheets is 3 - 8mm, and it is composed of the following components by mass fraction: 0.01 - 0.1% carbon, 0.01 - 0.05% silicon, 0.10 - 0.30% manganese, 0.01 - 0.035% phosphorus, 0.003 - 0.010% sulfur, 0.001 - 0.020% copper, 0.01 - 0.04% chromium, 0.001 - 0.004% molybdenum, 0.001 - 0.002% titanium, and the balance is iron and inevitable impurities.

[0031] In the preparation method of the above large - megawatt wind power ductile iron, the high - silicon calcium barium inoculant is composed of the following components by mass fraction: 70 - 75% silicon, 0.8 - 1.0% aluminum, 0.8 - 0.85% calcium, 2 - 2.5% barium, and the balance is iron and inevitable impurities.

[0032] In the above-mentioned preparation method of large megawatt wind power ductile iron, the pretreatment agent is composed of the following components by mass fraction: 62.00 - 69.00% silicon, 3.00 - 5.00% aluminum, 0.60 - 0.90% calcium, 3.0 - 5.0% zirconium, and the balance is iron and inevitable impurities. The pretreatment agent with this specific composition in the present invention can effectively reduce the sulfur content in the molten iron, thereby improving the spheroidization effect; at the same time, it can also purify the molten iron, remove impurities and oxides therein, reduce the presence of inclusions, and further improve the purity of the molten iron. In addition, by adjusting the chemical composition of the molten iron to improve the spheroidization reaction environment, better reaction conditions are created for the spheroidizing agent (such as magnesium), further improving the spheroidization efficiency. The pretreatment agent can also optimize the temperature and chemical composition of the molten iron, ensure the stability of the molten iron properties, and reduce the fluctuations during the spheroidization treatment. Finally, these measures work together to reduce casting defects by improving the quality of the molten iron, significantly enhancing the mechanical properties and surface quality of the castings.

[0033] In the above-mentioned preparation method of large megawatt wind power ductile iron, the heat preservation covering agent in step S3 includes the following components by mass fraction: 65 - 76% SiO2, 8 - 14% Al2O3.

[0034] In the above-mentioned preparation method of large megawatt wind power ductile iron, the silicon-aluminum secondary inoculant in step S4 is added to the molten iron surface of the ladle after slag skimming is completed, and the addition amount is 0.1 - 0.2% of the mass of the molten iron. It is composed of the following components by mass fraction: 70 - 75% silicon, 3.5 - 4.5% aluminum, 0.50 - 1.50% calcium, and the balance is iron and inevitable impurities.

[0035] In the above-mentioned preparation method of large megawatt wind power ductile iron, the pouring temperature in step S5 is 1320 - 1360 °C, and the filling speed is controlled below 0.5 m / s according to the design of the gating system to prevent turbulence.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. By changing the traditional sprue basin method to the ladle casting method, the present invention effectively reduces the slag content in the casting. This improvement not only improves the surface quality of the casting but also avoids internal defects caused by slag inclusions, ensuring the purity and mechanical properties of the casting.

[0038] 2. The number of graphite balls in the ductile iron castings prepared by the present invention through the improvement of the overall preparation process increases by an average of 41 per mm 2 , and more graphite balls mean better material toughness and higher strength, which is crucial for improving the overall mechanical properties of ductile iron.

[0039] 3. By subjecting the invention to high-temperature static treatment at 1515 - 1525 °C, partial impurities in the molten iron can be effectively removed, and the nucleation process can be promoted. This step is crucial for obtaining the desired quantity and morphology of graphite, thereby further enhancing the mechanical properties of ductile iron.

[0040] 4. The invention uses a large-particle secondary inoculant with a particle size of 10 - 50 mm for the pouring bag operation, which can effectively weaken the inoculation decay phenomenon, increase the number of graphite balls and make their distribution more uniform. This improvement measure greatly improves the uniformity of the internal structure of the casting and enhances the comprehensive properties of the material.

[0041] 5. By precisely controlling the content of various raw materials and additives, the invention obtains an excellent chemical element ratio, thereby producing ductile iron with excellent mechanical properties such as high tensile strength, yield strength, and elongation rate. This precise regulation ensures the high-performance performance of the material in different application scenarios.

[0042] 6. During the preparation process of ductile iron, the invention precisely controls the temperature of the molten iron, ensuring the uniformity during the cooling process of the casting and reducing the occurrence of cold lap defects. This method not only improves the overall quality of the casting but also extends its service life.

[0043] 7. The invention also optimizes the filling speed to ensure the structural stability during the cooling process of the casting, which avoids the generation of a large amount of secondary slag caused by too fast filling speed, ensuring the high quality and reliability of the casting. Description of the Drawings

[0044] Figure 1 For Example 1, the inoculant is poured into the bag + placed in the ladle for casting.

[0045] Figure 2 For Comparative Example 1, the inoculant is placed in the ladle + inoculated during the flowing process for casting.

[0046] Figure 3 For the metallographic diagram of the ductile iron prepared in Example 1.

[0047] Figure 4 For the metallographic diagram of the ductile iron prepared in Comparative Example 1. Detailed Description of the Invention

[0048] The following are specific examples of the invention, which further describe the technical solutions of the invention, but the invention is not limited to these examples.

[0049] The raw materials for the examples are as follows:

[0050] The molten iron is selected according to the following mass parts: 35 parts of low-silicon pig iron, 35 parts of medium-silicon pig iron, 28 parts of scrap steel, 1.1 parts of ferrosilicon, 0.7 parts of recarburizer, and 0.003 parts of antimony metal;

[0051] The high-silicon pig iron consists of the following components by mass fraction: carbon 4.55%, silicon 0.52%, manganese 0.037%, phosphorus 0.029%, sulfur 0.011%, chromium 0.009%, titanium 0.021%, copper 0.0035%, vanadium 0.0033%, and the balance is iron and unavoidable impurities.

[0052] The low-silicon pig iron consists of the following components by mass fraction: carbon 4.58%, silicon 0.31%, manganese 0.57%, phosphorus 0.018%, sulfur 0.01%, chromium 0.009%, titanium 0.02%, copper 0.0032%, vanadium 0.0056%, and the balance is iron and unavoidable impurities;

[0053] The scrap steel consists of the following components by mass fraction: carbon 0.05%, silicon 0.03%, manganese 0.17%, phosphorus 0.013%, sulfur 0.007%, chromium 0.030%, and the balance is iron and unavoidable impurities;

[0054] The ferrosilicon consists of the following components by mass fraction: silicon 74.3%, manganese 0.28%, chromium 0.045%, aluminum 0.93%, and the balance is iron and unavoidable impurities;

[0055] The recarburizer is a micro-sulfur recarburizer with a carbon content of 99.7%;

[0056] The desulfurizer consists of the following components by mass fraction: 50.0% calcium oxide, 7.0% calcium fluoride, and 43.0% calcium carbide;

[0057] The average diameter of the high-calcium barium inoculant is 5 mm, and it consists of the following components by mass fraction: silicon 72.45%, aluminum 0.94%, calcium 0.83%, barium 2.18%, and the balance is iron and unavoidable impurities;

[0058] The average diameter of the scrap steel sheet is 5 mm, and it consists of the following components by mass fraction: carbon 0.01%, silicon 0.01%, manganese 0.22%, phosphorus 0.015%, sulfur 0.006%, copper 0.009%, chromium 0.037%, molybdenum 0.003%, titanium 0.001%, and the balance is iron and unavoidable impurities;

[0059] The average diameter of the spheroidizing agent is 20 mm, and it consists of the following components by mass fraction: silicon 45.41%, magnesium 5.86%, rare earth elements 0.34%, aluminum 0.76%, magnesium oxide 0.50%, calcium 0.98%, and the balance is iron and unavoidable impurities;

[0060] The pretreatment agent consists of the following components by mass fraction: 68% silicon, 3.5% aluminum, 0.8% calcium, 3.5% zirconium, and the balance is iron.

[0061] The median particle size of the silicon-aluminum inoculant is 30 mm, and it is composed of the following components by mass fraction: 71.58% silicon, 4.32% aluminum, 0.85% calcium, and the balance is iron and inevitable impurities.

[0062] The median particle size of the silicon-aluminum inoculant is 0.2 - 0.8, and it is composed of the following components by mass fraction: 71.88% silicon, 4.24% aluminum, 0.72% calcium, and the balance is iron and inevitable impurities.

[0063] Example 1:

[0064] S1. Melting scrap steel, low-silicon pig iron, medium-silicon pig iron, ferrosilicon, metallic antimony, and carburant in a medium-frequency induction furnace at 1480 °C;

[0065] S2. Adding a desulfurizer of 0.2 wt% of the molten iron mass to the ladle for slag skimming treatment, and then standing at a high temperature of 1520 °C for 20 min;

[0066] S3. Adding a nodulizer of 1.0 wt% of the molten iron mass to the desulfurized ladle and tamping it. The surface of the nodulizer is successively covered with a calcium-silicon-barium inoculant of 0.3 wt% of the molten iron weight, scrap steel sheets of 0.65 wt%, and a pretreatment agent of 0.35 wt%. When tapping at 1480 °C, it is poured into the ladle for nodulization;

[0067] S4. Conducting slag skimming treatment on the nodulized and inoculated molten iron to obtain ladle molten iron, and sprinkling a heat preservation covering agent on the surface of the molten iron and transporting it to the casting site;

[0068] S5. When the ladle is transported to the casting site, remove the heat preservation covering agent. Add a silicon-aluminum secondary inoculant with a median particle size of 30 mm of 0.15 wt% of the molten iron mass to the surface of the ladle molten iron, and then Figure 1 By the method of pouring from the ladle to the seat ladle as shown, the inoculant is evenly diffused. After standing, filling starts at 1330 °C, and finally, ductile iron is obtained after cooling;

[0069] The components of the ductile iron are 3.86% carbon, 2.12% silicon, 0.031% phosphorus, 0.12% manganese, 0.053% magnesium, 0.004% rare earth elements, 0.011% sulfur, 0.0062% antimony, and the balance is iron.

[0070] Example 2:

[0071] The difference from Example 1 is only that in step S5, a silicon-aluminum secondary inoculant with a median particle size of 10 mm of 0.15 wt% of the molten iron mass is added to the surface of the ladle molten iron.

[0072] Example 3:

[0073] The difference from Example 1 is only that in step S5, a silicon-aluminum secondary inoculant with a median particle size of 20 mm of 0.15 wt% of the molten iron mass is added to the surface of the ladle molten iron.

[0074] Embodiment 4:

[0075] The only difference from Example 1 is that in step S5, 0.15 wt% of silicon-alumina secondary inoculant with a median particle size of 40 mm is added to the surface of the molten iron in the ladle.

[0076] Embodiment 5:

[0077] The only difference from Example 1 is that in step S5, 0.15 wt% of silicon-alumina secondary inoculant with a median particle size of 50 mm is added to the surface of the molten iron in the ladle.

[0078] Comparative Example 1:

[0079] The only difference from Example 1 is that according to Figure 2 In the step S5, the secondary inoculation method is to select the in-stream inoculation of 0.15wt% silicon-aluminum inoculant with a median particle size of 0.5mm, and to pre-embed 0.05wt% silicon-aluminum inoculant with a median particle size of 10mm at the bottom of the seat package.

[0080] Comparative Example 2:

[0081] The only difference from Example 1 is that in step S5, the secondary inoculation method selects in-stream inoculation of 0.10wt% silicon-aluminum inoculant with a median particle size of 0.5mm, and 0.05wt% silicon-aluminum inoculant with a median particle size of 20mm is pre-embedded at the bottom of the seat package.

[0082] Comparative Example 3:

[0083] The only difference from Example 1 is that in step S5, the secondary inoculation method selects the in-flow inoculation of 0.10wt% silicon-aluminum inoculant with a median particle size of 0.5mm, and 0.05wt% silicon-aluminum secondary inoculant with a median particle size of 30mm is pre-embedded in the bottom of the seat package.

[0084] Comparative Example 4:

[0085] The only difference from Example 1 is that in step S5, the secondary inoculation method selects the in-stream inoculation of 0.10wt% silicon-aluminum inoculant with a median particle size of 0.5mm, and 0.05wt% silicon-aluminum secondary inoculant with a median particle size of 40mm is pre-embedded in the bottom of the seat package.

[0086] Comparative Example 5:

[0087] The only difference from Example 1 is that in step S5, the secondary inoculation method selects the in-stream inoculation of 0.05wt% silicon-aluminum inoculant with a median particle size of 0.5mm, and 0.10wt% silicon-aluminum secondary inoculant with a median particle size of 50mm is pre-embedded in the bottom of the seat package.

[0088] For the ductile iron products prepared in the above examples and comparative examples, the tensile strength, yield strength, and elongation of their attached test blocks need to be measured to verify whether they meet the technical index requirements that the tensile strength of the attached test block of the product ≥ 370 MPa, the yield strength ≥ 220 MPa, and the elongation ≥ 12%. The size of the attached test block is 70 mm × 70 mm × 180 mm.

[0089] Table 1: Performance data of ductile iron attached test blocks obtained in examples and comparative examples

[0090] Example Tensile strength Yield strength Elongation Hardness Graphite grade Number of graphite nodules Example 1 377 MPa 237 MPa 26.5％ 138 HB 6+5 <![CDATA[183 · mm -2 <!-- 6 -->]]> Example 2 378 MPa 236 MPa 25.5％ 141 HB 6 <![CDATA[174 · mm -2 > Example 3 379 MPa 237 MPa 25.0％ 141 HB 6 <![CDATA[190 pieces·mm -2 > Example 4 380 MPa 237 MPa 27.0％ 139 HB 6+5 <![CDATA[188 pieces·mm -2 > Example 5 382 MPa 241 MPa 26.5％ 143 HB 6 <![CDATA[175个·mm -2 ]]> Comparative example 1 377 MPa 235 MPa 26.5％ 143 HB 6 <![CDATA[145 pieces·mm -2 > Comparative example 2 368 MPa 226 MPa 26.5％ 135 HB 6 <![CDATA[140个·mm -2 ]]> Comparative example 3 377 MPa 235 MPa 26.0％ 140 HB 6 <![CDATA[137 · mm -2 > Comparative example 4 374 MPa 233 MPa 26.0％ 137 HB 6 <![CDATA[155个·mm -2 ]]> Comparative example 5 374 MPa 233 MPa 26.5％ 136 HB 6+5 <![CDATA[126个·mm -2 ]]>

[0091] From the analysis of the above data, it can be seen that the tensile strength of the ductile iron attached test blocks prepared in Examples 1 to 5 all reached 377 MPa and above, the yield strength was all 236 MPa and above, and the elongation was all 25.0% and above, showing high mechanical properties. In contrast, the tensile strength of Comparative Example 2 was 368 MPa and the yield strength was 226 MPa, slightly lower than the performance indexes of the examples of the present invention.

[0092] In terms of graphite grade and the number of graphite balls, the average number of graphite balls in the ductile iron prepared in the examples of the present invention reached 182 per mm². Compared with the number of graphite balls in the ductile iron prepared in the comparative examples (with an average of about 140 per mm²), the average increased by 41 per mm². Combining Figure 3 and Figure 4 with the metallographic diagrams of the attached test blocks of Example 1 and Comparative Example 1, it can be seen that the spheroidization rate of graphite in both reached 95%, but the number of graphite balls in Example 1 was significantly increased. More graphite balls mean better material toughness and higher strength, which is crucial for improving the overall mechanical properties of ductile iron.

[0093] For the parts of the technical scope claimed in the present invention where the midpoint values are not exhausted, as well as the new technical solutions formed by the equivalent replacement of single or multiple technical features in the technical solutions of the examples, they are also within the scope claimed in the present invention; at the same time, in all the listed or unlisted examples of the present invention, each parameter in the same example only represents an example (i.e., a feasible solution) of its technical solution, and there is no strict coordination and limitation relationship between the parameters. Among them, the parameters can be mutually replaced when not violating the axioms and the claims of the present invention, except as otherwise specifically stated.

[0094] The technical means disclosed in the solution of the present invention are not limited to those disclosed by the above technical means, but also include technical solutions composed of any combination of the above technical features. The above is the specific implementation manner of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present invention.

[0095] The specific embodiments described herein are only illustrative of the spirit of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A large-megawatt wind power ductile iron, characterized in that, The average number of nodular graphite in the ductile iron is ≥182 pieces / mm 2 , the nodularity of graphite is ≥95%, and it is composed of components with the following mass fractions: 3.82 - 3.88% C, 2.0 - 2.15% Si, 0.1 - 0.15% Mn, 0.03 - 0.035% P, 0.008 - 0.01% S, 0.05 - 0.055% Mg, 0.002 - 0.005% RE, 0.006 - 0.008% Sb, and the balance is Fe and inevitable impurities.

2. A preparation method of ductile iron for large megawatt wind power as described in claim 1, characterized in that, The method includes the following steps: S1. Melting scrap steel, low-silicon pig iron, medium-silicon pig iron, ferrosilicon, metallic antimony, and carburizer into molten iron, conducting desulfurization and slag skimming treatment, and then conducting high-temperature standing treatment; S2. After the high-temperature standing ends, tapping the molten iron for nodulizing and inoculating treatment; S3. Conducting slag skimming treatment on the molten iron after nodulizing and inoculating treatment to obtain ladle molten iron, sprinkling a heat-insulating covering agent on the surface of the molten iron, and transporting it to the casting site; S4. Then skimming off the heat-insulating covering agent, adding a silicon-aluminum secondary inoculant with a median particle size of 10 - 50 mm to the surface of the ladle molten iron, and then pouring the molten iron in the ladle into a seat ladle; S5. The molten iron in the seat ladle undergoes standing, filling, and cooling in sequence to obtain large-megawatt wind power ductile iron.

3. The preparation method of a large megawatt wind power ductile iron according to claim 2, characterized in that, In step S1, the high-temperature standing temperature is 1515 - 1525 °C, and the time is 15 - 25 min.

4. The preparation method of a large megawatt wind power ductile iron according to claim 2, characterized in that, In the desulfurization and slag skimming treatment in step S1, the desulfurizer includes components with the following mass fractions: 45 - 55% calcium oxide, 5 - 10% calcium fluoride, and 40 - 45% calcium carbide.

5. The preparation method of a large megawatt wind power ductile iron according to claim 2, characterized in that, In step S2, the tapping temperature is 1420 °C - 1480 °C.

6. The preparation method of a large megawatt wind power ductile iron according to claim 2, characterized in that, The process of nodulizing and inoculating treatment in step S2 is as follows: adding a nodulizer accounting for 0.9 - 1.2 wt% of the weight of the molten iron to the nodulizing pit, and successively covering a high-silicon calcium barium inoculant accounting for 0.2 - 0.5 wt% of the weight of the molten iron, scrap steel sheets accounting for 0.6 - 0.8 wt%, and a pretreatment agent accounting for 0.3 - 0.4%.

7. The preparation method of a large-megawatt wind power ductile iron according to claim 6, characterized in that, The nodulizer is composed of components with the following mass fractions: 42 - 46% silicon, 5.5 - 6.5% magnesium, 0.3 - 0.38% RE, 0.6 - 0.8% aluminum, 0.45 - 0.55% magnesium oxide, 0.9 - 1.0% calcium, and the balance is iron and inevitable impurities.

8. The preparation method of ductile iron for large megawatt wind power according to claim 6, characterized in that, The high-silicon calcium barium inoculant is composed of components with the following mass fractions: 70 - 75% silicon, 0.8 - 1.0% aluminum, 0.8 - 0.85% calcium, 2 - 2.5% barium, and the balance is iron and inevitable impurities.

9. The preparation method of ductile iron for large megawatt wind power according to claim 6, characterized in that, The pretreatment agent is composed of components with the following mass fractions: 62.00 - 69.00% silicon, 3.00 - 5.00% aluminum, 0.60 - 0.90% calcium, 3.0 - 5.0% zirconium, and the balance is iron and inevitable impurities.

10. The preparation method of a large megawatt wind power ductile iron according to claim 2, characterized in that, In step S4, the silicon-aluminum secondary inoculant is added to the surface of the ladle molten iron with clean slag skimming, and the addition amount is 0.1 - 0.2% of the mass of the molten iron. It is composed of components with the following mass fractions: 70 - 75% silicon, 3.5 - 4.5% aluminum, 0.50 - 1.50% calcium, and the balance is iron and inevitable impurities.

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