Forming method of nodular iron casting, nodular iron casting and wind generating set

Abstract

The invention provides a nodular iron casting forming method, a nodular iron casting and a wind generating set, the nodular iron casting forming method comprises the steps that a forming mold is prepared, the forming mold comprises an outer mold box and an inner mold core located in the outer mold box, and the outer mold box comprises a metal casting mold and a sand covering layer covering the surface of an inner cavity of the metal casting mold; a cavity is formed between the outer mold box and the inner mold core; and casting molding is conducted, specifically, molten iron is poured into the cavity, the nodular iron casting is formed after cooling and curing are conducted, and the nodular iron casting comprises, by mass, 3.6-3.8 wt% of C, 2.3-2.6 wt% of Si, smaller than or equal to 0.14 wt% of Mn and 0.20-0.55 wt% of Ni. The nodular iron casting obtained according to the embodiment of the invention has excellent comprehensive performance, especially can obviously improve the low-temperature impact energy, ensures the low-temperature strength of the casting, and reduces the fracture risk of the casting.

Description

Technical Field

[0001] The present invention relates to the technical field of casting molding, and particularly relates to a molding method for ductile iron castings, ductile iron castings, and a wind power generation unit. Background Art

[0002] With the continuous development of ductile iron technology, ductile iron has gradually replaced cast steel and become a new type of metal material. As a kind of ductile iron, as-cast ferrite ductile iron has been widely used in construction machinery, basic components of injection molding machines, basic components of die-casting machines, and large mechanical components or key components of wind power generation units, etc.

[0003] Currently, the research on ductile iron for wind power mainly focuses on solving problems such as how to meet the performance requirements of impact energy at low temperatures on the basis of maintaining the existing strength, and preventing the castings from cracking. However, the existing chemical composition formulations and production processes cannot ensure that while significantly improving the strength, the low-temperature impact energy meets the performance indicators. From the research results of the existing technology, improving the strength while ensuring the low-temperature impact energy is a contradiction. Increasing the strength will inevitably affect the low-temperature impact energy. How to solve this difficult problem has become the current research direction. Summary of the Invention

[0004] The purpose of the present invention is to provide a molding method for ductile iron castings, ductile iron castings, and a wind power generation unit.

[0005] According to an embodiment of the present invention, there is provided a molding method for ductile iron castings. The molding method includes: preparing a molding die, the molding die including an outer mold box and an inner core located in the outer mold box. The outer mold box includes a metal casting mold and a sand covering layer covering the inner cavity surface of the metal casting mold, and a cavity is formed between the outer mold box and the inner core; performing casting molding, pouring molten iron into the cavity, and forming the ductile iron casting after cooling and solidification. Among them, the ductile iron casting includes, by mass percentage: C 3.6 - 3.8 wt%, Si 2.3 - 2.6 wt%, Mn ≤ 0.14 wt%, Ni 0.20 - 0.55 wt%.

[0006] According to an aspect of the embodiment of the present invention, the ductile iron casting further includes, by mass percentage: V 0.001 - 0.01 wt%, Ti 0.02 - 0.03 wt%, P ≤ 0.035 wt%, S ≤ 0.02 wt%, Cr ≤ 0.025 wt%, Mg 残 0.035 - 0.060 wt%.

[0007] According to one aspect of an embodiment of the present invention, the molten iron is prepared by the following method: (1) Melting: Carrying out carbon addition treatment on the materials including pig iron and melting them into molten iron; (2) Nodulizing and inoculating: Nodulizing and inoculating the molten iron obtained from step (1); and (3) Pouring: Pouring the molten iron obtained from step (2) into the cavity, wherein, in step (1) and / or step (2), the preparation method further includes microalloying treatment, and the microalloying treatment includes adding Ni, adding a nodulizer containing Si and an inoculant containing Si in step (2), and adding a stream inoculant containing Si in step (3).

[0008] According to one aspect of an embodiment of the present invention, in step (1), the content of Si included in the materials is in the range of 1.4 - 1.6 wt%, in step (2), a nodulizer containing Si and an inoculant containing Si are added such that the content of Si is in the range of 2.25 - 2.45 wt%, and in step (3), a stream inoculant containing Si is added such that the content of Si is in the range of 2.3 - 2.6 wt%.

[0009] According to one aspect of an embodiment of the present invention, based on the total mass of the nodulizer, the nodulizer includes 40 - 50 wt% of Si by mass percentage, based on the total mass of the inoculant, the inoculant includes 72 - 78 wt% of Si by mass percentage, and based on the total mass of the stream inoculant, the stream inoculant includes 70 - 80 wt% of Si by mass percentage.

[0010] According to one aspect of an embodiment of the present invention, based on the total mass of the inoculant, the inoculant includes by mass percentage: 72 - 78 wt% of Si, 1.0 - 2.0 wt% of Ca, 2.0 - 3.0 wt% of Ba, < 1.5 wt% of Al, and the balance being iron; based on the total mass of the nodulizer, the nodulizer includes by mass percentage: 4.5 - 6 wt% of Mg, 0.15 - 0.3 wt% of RE, 40 - 50 wt% of Si, and the balance being iron; based on the total mass of the stream inoculant, the stream inoculant includes by mass percentage: 70 - 80 wt% of Si, 0.5 - 2.5 wt% of Bi, ≤ 2.0 wt% of Ca, ≤ 2.0 wt% of Al, and the balance being Fe.

[0011] According to one aspect of an embodiment of the present invention, in step (2), the addition amount of the inoculant is 0.35 - 0.67 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.0 - 1.3 wt% of the total mass of the molten iron, and in step (3), the addition amount of the stream inoculant is 0.08 - 0.2 wt% of the total mass of the molten iron.

[0012] According to one aspect of an embodiment of the present invention, the inoculant in step (2) includes a primary inoculant and a covering inoculant. Based on the total mass of the primary inoculant, the primary inoculant includes, by mass percentage: Si 72 - 78 wt%, Ca 1.0 - 2.0 wt%, Ba 2.0 - 3.0 wt%, Al < 1.5 wt%, and the balance is iron. Based on the total mass of the covering inoculant, the covering inoculant includes, by mass percentage: Si 72 - 78 wt%, Ca 1.0 - 2.0 wt%, Ba 2.0 - 3.0 wt%, Al < 1.5 wt%, and the balance is iron.

[0013] According to one aspect of an embodiment of the present invention, in step (2), the addition amount of the primary inoculant is 0.30 - 0.55 wt% of the total mass of the molten iron, and the addition amount of the covering inoculant is 0.05 - 0.12 wt% of the total mass of the molten iron.

[0014] According to one aspect of an embodiment of the present invention, Mg is added in step (2) such that, based on the total mass of the molten iron, the content of Mg is in the range of 0.045 - 0.078 wt%. Based on the total mass of the spheroidizing agent, the spheroidizing agent includes, by mass percentage, 4.5 - 6 wt% of Mg. The addition amount of the spheroidizing agent in step (2) is 1.0 - 1.3 wt% of the total mass of the molten iron. In step (1) and / or step (2), the addition amount of Ni is 0.2 - 0.4 wt% of the total mass of the molten iron. The microalloying treatment in step (1) and / or step (2) includes adding Ni such that the content of Ni is in the range of 0.20 - 0.55 wt% of the total mass of the molten iron. In step (1) and / or step (2), the microalloying treatment further includes adding TiC to the molten iron, and the addition amount of TiC is 0.01 - 0.03 wt% of the total mass of the molten iron.

[0015] According to one aspect of an embodiment of the present invention, the pig iron in step (1) is pig iron of grade Q10 or above, and the content of Mn is adjusted in step (1) such that the content of Mn in step (1) is in the range of 0.10 - 0.12 wt% of the total mass of the molten iron.

[0016] According to one aspect of an embodiment of the present invention, before pouring the molten iron into the cavity (130), the temperature of the cavity (130) is preheated to 50 - 200 °C. In step (4), the pouring temperature of the molten iron is 1330 - 1360 °C, and the pouring speed of the molten iron is 100 - 150 kg / s.

[0017] According to one aspect of an embodiment of the present invention, the metal mold is an iron mold, the thickness of the iron mold is 50 mm - 200 mm, and the thickness of the sand - covered layer is 6 mm - 20 mm.

[0018] According to one aspect of an embodiment of the present invention, the outer mold box includes a bottom box, a top box, and a middle box disposed between the bottom box and the top box. The middle box is a shell mold casting box, and the top box and the bottom box are sand boxes. The inner core is a sand core.

[0019] According to one aspect of an embodiment of the present invention, the molding die further includes a bottom gating system, and a runner for transporting molten iron into the cavity is disposed in the bottom box.

[0020] According to one aspect of an embodiment of the present invention, a riser is disposed in the top box, and an exhaust passage is disposed in the inner core. The exhaust passage is communicated with the riser.

[0021] According to one aspect of an embodiment of the present invention, the matrix of the ductile cast iron is ferrite.

[0022] According to one aspect of an embodiment of the present invention, the ductile cast iron part is a main shaft of a wind power generating set. In the axial direction of the main shaft, the outer mold box and / or the inner core are divided into multiple sections.

[0023] According to another aspect of an embodiment of the present invention, a ductile cast iron part is provided, and the ductile cast iron part is made by the molding method described above.

[0024] According to another aspect of an embodiment of the present invention, the wall thickness of the ductile cast iron part is greater than or equal to 60 mm.

[0025] According to another aspect of an embodiment of the present invention, the ductile cast iron part is a main shaft of a wind power generating set, and the outer diameter of the shaft body of the main shaft is greater than or equal to 1 m.

[0026] According to another aspect of an embodiment of the present invention, a wind power generating set is provided, and the wind power generating set includes the ductile cast iron part described above.

[0027] The core structure of the ductile cast iron part obtained by the ductile cast iron molding method according to the embodiment of the present invention is good, and there is basically no abnormal graphite such as fragmented graphite, and the comprehensive performance is excellent. In particular, the tensile strength of the ductile cast iron test block is greater than 400 MPa, the yield strength is greater than 280 MPa, and the low-temperature impact energy at -20 °C is greater than 7 J, which significantly improves the mechanical properties of large-section ductile cast iron. Therefore, the high-toughness and ultra-high-strength ferrite ductile cast iron provided by the present invention is particularly suitable for mass-producing key components such as large-section castings in wind power generating sets. In particular, it can significantly improve the low-temperature impact energy, ensure the low-temperature strength of the components, and reduce the risk of component fracture. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Through the following description in conjunction with the drawings shown by way of example, the above and other objects and features of the present invention will become clearer, wherein:

[0029] Figure 1 is a perspective view of a fan main shaft according to an embodiment of the present invention;

[0030] Figure 2 is a cross-sectional view of a fan main shaft according to an embodiment of the present invention;

[0031] Figure 3 is a cross-sectional view of a molding die according to an embodiment of the present invention;

[0032] Figure 4 is a cross-sectional view of another example of a molding die according to an embodiment of the present invention;

[0033] Figure 5 is a schematic diagram of the sand injection process of a molding die according to an embodiment of the present invention;

[0034] Figure 6A and Figure 6B respectively show the graphite morphology of ductile iron according to the comparative example and the invention example;

[0035] Figure 7A and Figure 7B respectively show the tensile fracture morphology of ductile iron according to the comparative example and the invention example;

[0036] Figure 8A and Figure 8B respectively show the three-dimensional tensile fracture morphology of ductile iron according to the comparative example and the invention example; and

[0037] Figure 9A and Figure 9B respectively show the impact fracture morphology of ductile iron according to the comparative example and the invention example. Detailed Embodiments

[0038] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention.

[0040] In addition, unless there are conflicting descriptions, the various embodiments described below can also be combined with each other. Furthermore, unless otherwise specified, the devices with the same reference numerals in the various embodiments have the same structure, and redundant descriptions will not be provided again.

[0041] Mechanical components of wind turbines in the prior art, especially components such as the main shaft and base of wind turbines, are usually made by forging steel. However, with the development of the enlargement of wind turbines, the sizes of the mechanical components in wind turbines are getting larger and larger. Conventional forging methods can no longer meet the production requirements, and for large-sized mechanical components, casting methods have begun to be used. For example, molten iron is poured into a sand mold for component forming. However, sand casting molds use a large amount of sand, and the sand treatment system is huge, complex, energy-consuming, and costly. In addition, sand molds cannot be reused, and new molds need to be made for each casting, resulting in low efficiency in manufacturing sand molds. Moreover, due to the low hardness and poor rigidity of the sand mold, it cannot resist the graphitization expansion force generated during the solidification of molten iron. Therefore, shrinkage cavities and porosity defects are likely to occur in the castings, affecting the quality of the castings and resulting in a low yield rate.

[0042] Therefore, on the one hand, according to an embodiment of the present invention, a forming method for manufacturing a wind turbine component using an improved mold is provided. Different from traditional sand molds, the embodiment of the present invention uses a metal mold with sand coating. The forming mold includes a metal mold made of metal and a sand coating layer laid on the inner surface of the metal mold, combining the advantages of metal molds and sand molds. As an example, the metal mold can be an iron mold.

[0043] The forming method of the wind turbine component according to the embodiment of the present invention includes the following steps: preparing a forming mold, the forming mold including an outer mold box and an inner core located in the outer mold box, the outer mold box including a metal mold and a sand coating layer laid on the inner cavity surface of the metal mold, and a cavity is formed between the core and the sand coating layer; casting and forming, pouring molten iron into the cavity, and forming the ductile iron casting after cooling and solidification.

[0044] Hereinafter, with reference to the attached Figures 1 to 5 Describe the forming method of the wind turbine component, the main shaft of the wind turbine, and the wind turbine according to the embodiment of the present invention. In the following description, for convenience, the casting of the main shaft of the wind turbine is taken as an example for description. However, the forming method of the present invention is also applicable to manufacturing other mechanical components of the wind turbine. Accordingly, the shape and structure of the forming mold, especially the cavity, are changed accordingly according to different mechanical components.

[0045] Figure 1 is a perspective view of the main shaft 100 of the wind turbine according to the embodiment of the present invention, Figure 2It is a cross-sectional view of the main shaft of a wind turbine according to an embodiment of the present invention. The main shaft 100 includes a shaft body 110, a large-end flange 120 provided at one end of the shaft body 110, and a small-end flange 130 provided at the other end of the shaft body 110. The large-end flange 120 is used to connect with the hub of the wind turbine, and the small-end flange 130 is used to connect with the gearbox of the wind turbine. Since there is a large size difference between the shaft body 110 and the large-end flange 120, a trumpet-shaped connecting portion 140 is also used for transitional connection between the shaft body 110 and the large-end flange 120.

[0046] Figure 3 It shows a cross-sectional view of a molding die according to an embodiment of the present invention. As Figure 3 shown, the molding die 200 according to an embodiment of the present invention includes an outer mold box. The outer mold box 210 has a hollow inner cavity, and the inner surface of the outer mold box 210 is consistent with the contour of the outer surface of the main shaft 100, and is used for forming the outer contour of the main shaft 100.

[0047] In order to reduce the weight of the main shaft 100 and save manufacturing costs, as Figure 2 shown, the main shaft 100 is usually formed as a hollow structure. Therefore, the molding die according to an embodiment of the present invention further includes an inner core 220. The outer surface of the inner core 220 is consistent with the contour of the inner cavity surface of the main shaft 100, and is used for forming the inner surface of the main shaft 100. The inner core 220 can be inserted into the outer mold box 210 and has a gap with the outer mold box 210, so as to form a cavity 300 consistent with the shape of the main shaft 100 between the two.

[0048] The molding die 200 according to an embodiment of the present invention is a metal mold with sand coating. As Figure 3 shown, the outer mold box 210 includes a metal casting mold 212 and a sand coating layer 213 covered on the inner surface of the metal casting mold 212. The metal casting mold 212 can be an iron casting mold. Compared with the traditional sand mold, for the molding die according to an embodiment of the present invention, due to the thinner sand coating layer, the iron casting mold has an obvious chilling effect during the crystallization process of ductile iron molten iron, which can refine the grain size of the casting and improve the nodularity rate, and can significantly improve the dimensional accuracy and density of the casting. At the same time, due to the existence of the sand coating layer 213, the graphite expansion of ductile iron can be used for self-feeding and contraction of the casting to obtain high-quality castings.

[0049] According to an embodiment of the present invention, the weight of the main shaft 100 is approximately 10 tons to 20 tons. In the axial direction of the main shaft 100, the thickness of the main shaft varies, and the wall thickness range of the main shaft is 30 mm to 200 mm. Correspondingly, the thickness of the iron mold is also variable, with different thicknesses at different positions, approximately in the range of 50 mm to 200 mm. In addition, the thickness of the sand-covered layer 113 is 6 mm to 20 mm. The thickness of the sand covering affects the quality of the casting and the production cost. If the thickness of the sand covering is too thick, it not only affects the chilling effect but also increases the production cost. In addition, a too-thick sand layer emits a large amount of gas during the casting process, and the casting is prone to porosity defects and is not easily cured uniformly by heat. If the thickness of the sand covering is too thin, the chilling is too severe, the hardness of the casting is high, and it is not convenient for finish machining.

[0050] According to an embodiment of the present invention, the iron mold with sand covering forming die is adopted, reducing the amount of sand used, lowering the production cost. The iron mold 212 can be reused, with high production efficiency, and the obtained casting has high dimensional accuracy, small machining allowance, fine graphite, dense crystal structure, good internal quality of the casting, and high mechanical properties.

[0051] As Figure 3 shown, for the forming die 200 according to an embodiment of the present invention, the outer mold box 210 is divided into three sections, namely the bottom box 214, the middle box 211, and the top box 218. The bottom box 214 corresponds to the large-end flange of the main shaft, and the top box 218 corresponds to the small-end flange of the main shaft.

[0052] Furthermore, as Figure 4 shown, the middle box can also be divided into multiple sections. For example, the middle box is divided into three sections, namely the first middle box section 215, the second middle box section 216, and the third middle box section 217. The number of sections of the outer mold box 210 can be determined according to factors such as the lifting capacity of the lifting mechanism and the height of the sand blaster, and no specific limitation is made here.

[0053] According to an embodiment of the present invention, the gating system can adopt the bottom gating type, that is, the molten iron enters from the lower part of the cavity 300. A part of the gating pipeline can be arranged in the bottom box 214. To facilitate the arrangement of the gating system in the bottom box 214, the bottom box 214 can adopt a sand mold box. In addition, the gating system also includes a riser arranged in the top box 218, etc. Similarly, to facilitate the formation of a riser, etc. in the top box 218, the top box 218 can also be a sand mold box. In this case, only the middle box 211 is an iron mold with sand covering box.

[0054] To obtain an iron mold with sand covering box, the iron mold 212 can be prepared first, and then a sand blaster is used to sandblast the inner surface of the iron mold to form a sand-covered layer. Figure 5 The schematic diagram of sandblasting the segmented iron mold by a sand blaster is shown.

[0055] As Figure 5As shown, the iron mold 212 is placed on the workbench 310, and the iron die mold 330 is placed inside the iron mold 212, thereby forming a sand injection cavity 340 between the iron mold 212 and the iron die mold 330. Here, the outer contour of the iron die mold 330 is consistent with the outer contour of the main shaft 100, so that the inner surface of the sand-covered layer formed is consistent with the shape of the main shaft 100. In the case where the iron mold 212 is segmented, the iron die mold 330 also has corresponding segments and corresponding shapes, which will not be elaborated here.

[0056] After arranging the iron mold 212, a sand injection and covering operation is carried out. Specifically, the iron mold 212 and the iron die mold 330 are heated, and the sand material is injected into the sand injection cavity 340 by the sand injection machine 400 to fully cure the sand material between the sand material and the iron mold 212. Finally, the mold is removed to obtain the iron mold covered with sand. By separately performing sand injection and film covering on the segmented iron molds, and then stacking multiple segmented iron molds with the bottom box 214 and the top box 218, the outer box 210 of the molding die according to the embodiment of the present invention can be obtained.

[0057] According to an embodiment of the present invention, the inner core 220 can be made of a sand core to facilitate the demolding of the casting. The inner core 220 can be directly cast on the bottom box 214. As Figure 3 shown, an exhaust passage 229 can be formed in the middle of the inner core 220. The exhaust passage extends along the height direction of the inner core 220 and communicates with the outside of the molding die, so as to timely discharge the gas generated by the inner core 220 during the process of casting and molding with molten iron. A plurality of risers are provided on the top box 218 for slag discharge, gas exhaust, etc. The exhaust passage 229 in the inner core 220 can communicate with the risers for gas exhaust on the top box 218.

[0058] According to an embodiment of the present invention, when manufacturing the main shaft 100 of a wind power generation unit with the molding die 200, the outer box 210 and / or the inner core 220 can be divided into multiple segments in the axial length direction of the main shaft 100 to facilitate manufacturing. However, the molding die 200 according to the embodiment of the present invention is not limited to manufacturing the main shaft of a wind power generation unit and can be used to manufacture other mechanical components.

[0059] In addition, in order to reduce the manufacturing cost, the embodiment of the present invention uses low-cost ductile iron to manufacture large castings.

[0060] With the continuous development of ductile iron technology, ductile iron has gradually replaced cast steel and become a new type of metal material. As a kind of ductile iron, as-cast ferritic ductile iron has been widely used in construction machinery, basic components of injection molding machines, basic components of die-casting machines, and foundations or key components of wind power generation, etc. Currently, the research on ductile iron for wind power mainly focuses on solving how to meet the performance requirements of impact energy at low temperatures while maintaining the existing strength. However, improving strength while ensuring low-temperature impact energy is a contradictory issue. Increasing strength will inevitably affect low-temperature impact energy. To solve this problem, the present invention provides a ductile iron casting for wind turbine generators.

[0061] For large components of wind turbine generators, during the manufacturing and testing processes, usually the performance of the casting specimens needs to reach certain indicators. For example, the tensile strength is greater than 400 MPa, the yield strength is greater than 280 MPa, and at the same time, the low-temperature impact energy at -20 °C is greater than 7 J. Therefore, in the following embodiments of the present invention, taking the main shaft of the wind turbine generator made of ductile iron casting as an example, the performance of the casting specimens reaching these requirements is used as the performance index. However, the ductile iron castings of the present invention are not limited to the mechanical components of wind turbine generators, and can also be other large-section large castings, for example, large-section castings with a wall thickness greater than 60 mm.

[0062] The present invention provides a ductile iron casting. Based on the total mass of the ductile iron casting, the ductile iron casting may include by mass percentage: C 3.6 - 3.8 wt%, Si 2.3 - 2.6 wt%, Mn ≤ 0.14 wt%, Ni 0.20 - 0.55 wt%. In addition, the matrix of the ductile iron casting according to the embodiments of the present invention may be ferritic.

[0063] The present invention controls the content of each element in the ductile iron casting, especially the content of Si, Mn, and Ni, so as to obtain the technical effects that the tensile strength is greater than 400 MPa, the yield strength is greater than 280 MPa, and at the same time, the low-temperature impact energy at -20 °C is greater than 7 J.

[0064] In the embodiments, the ductile iron casting according to the exemplary embodiments of the present invention may further include by mass percentage: V 0.001 - 0.01 wt%, Ti 0.02 - 0.03 wt%. In another embodiment, the ductile iron casting according to the exemplary embodiments of the present invention may further include by mass percentage: P ≤ 0.035 wt%, S ≤ 0.02 wt%, Cr ≤ 0.025 wt%, Mg 残 0.035 - 0.060 wt%.

[0065] Specifically, Si can play a role in solid solution strengthening. The principle is to form a solid solution by incorporating a certain solute element into the molten iron, thereby strengthening the metal. The solute atoms incorporated into the solid solution cause lattice distortion, which increases the resistance to dislocation movement and makes slip difficult to occur, thus increasing the strength and hardness of the alloy solid solution. When the concentration of solute atoms in the melt is appropriate, the strength and hardness of the material are significantly improved, but its toughness and plasticity decrease. In the embodiments of the present invention, the Si content can be controlled within the range of 2.3 - 2.6 wt%, preferably within the range of 2.30 - 2.49 wt%, within the range of 2.35 - 2.49 wt%, or within the range of 2.30 - 2.40 wt%, more preferably within the range of 2.39 - 2.45 wt%. If the Si content is outside the range defined in the present invention, the toughness and plasticity of the ductile iron will decrease, so the Si content must be strictly controlled.

[0066] Mn is a positive segregation element, which is likely to form alloy carbides at grain boundaries, stabilize pearlite, and inhibit the formation of ferrite. Mn will significantly increase the ductile-brittle transition temperature and reduce the plasticity and toughness of ductile iron. For every 0.1% increase in manganese, the low-temperature brittle transition temperature of ductile iron will increase by 12 °C. To ensure the low-temperature impact toughness of ductile iron, the Mn content must be strictly controlled. In the embodiments of the present invention, the Mn content can be controlled within the range of less than or equal to 0.14 wt%, preferably within the range of 0.10 - 0.12 wt%. If the Mn content is outside the range defined in the present invention, the ductile-brittle transition temperature will increase, which will lead to a decrease in low-temperature impact performance.

[0067] Ni can be infinitely dissolved in ductile iron and has no effect on the graphite morphology and the number of eutectic clusters. Nickel in the as-cast state can promote the formation of pearlite and refine pearlite, and has a certain solid solution strengthening effect on ferrite, which is beneficial to improving the tensile strength of ductile iron, but will reduce the elongation and low-temperature impact toughness of ductile iron. Adding nickel to low-temperature high-toughness ductile iron cannot effectively reduce the ductile-brittle transition temperature, and the improvement effect of nickel on the low-temperature impact toughness of the material is not obvious. However, it can increase its tensile strength on the premise of not reducing the low-temperature impact toughness of the material, thereby making up for the problem of insufficient strength caused by the reduction of the silicon content. In the embodiments of the present invention, the Ni content can be controlled within the range of 0.20 - 0.55 wt%, preferably within the range of 0.35 - 0.55 wt%, more preferably within the range of 0.35 - 0.45 wt%. If the Ni content is outside the range defined in the present invention, the strength and hardness of the ductile iron will deteriorate, so the Ni content must be strictly controlled.

[0068] Excessive contents of Ti and Cr will have adverse effects on the microstructure and properties of ductile iron. Both Ti and Cr are very active elements, which are prone to form particles with very high microhardness with C and N. Moreover, Ti and Cr have the characteristic of selective crystallization and tend to enrich at grain boundaries and the last solidified part of the casting during the crystallization process. Excessive contents of Ti and Cr will affect the spheroidization quality, interfere with spheroidization, and cause distortion of graphite shape. Therefore, preferably, the contents of Ti and Cr can be controlled to further improve the properties of ductile iron. In the embodiments of the present invention, preferably, the content of Ti can be in the range of 0.02 - 0.03 wt%, and the content of Cr can be less than or equal to 0.025 wt%.

[0069] S and Mg 残 are elements that form grain boundary inclusions, reduce impact properties, and are one of the reasons for low-temperature impact properties. Therefore, in order to further improve the properties of ductile iron, their contents can be controlled within a relatively low range. In addition, Mg can also promote the spheroidization of graphite, making the spheroidization rate of graphite higher and the distribution more uniform. In the embodiments of the present invention, preferably, the content of S can be controlled within the range of less than or equal to 0.02 wt%, and more preferably, less than 0.015 wt%. Preferably, the content of Mg 残 can be controlled within the range of 0.035 - 0.060 wt%, and more preferably, within the range of 0.040 - 0.054 wt%.

[0070] P is one of the elements that affect the ductile-brittle transition temperature of ductile iron. Therefore, in order to further improve the properties of ductile iron, the content of P can be controlled. In the embodiments of the present invention, preferably, the content of P can be controlled within the range of less than or equal to 0.035 wt%.

[0071] In the embodiments of the present invention, the solid solution strengthening of V in ductile iron, the precipitated phases and formed compounds during the solidification process, and the strengthening effect on the matrix can be utilized to further improve the mechanical properties of the ductile iron of the present invention. Therefore, preferably, the content of V can be controlled within the range of 0.001 - 0.01 wt%.

[0072] In the ductile iron of the embodiments of the present invention, the content of Si is controlled within the range of 2.3 - 2.6 wt%, the content of Mn is controlled within the range of less than or equal to 0.14 wt%, and the content of Ni is controlled within the range of 0.20 - 0.55 wt%. More preferably, the content of V is also controlled within the range of 0.001 - 0.01 wt%, and the content of Ti is also controlled within the range of 0.02 - 0.03 wt%. Therefore, the ductile iron of the embodiments of the present invention can achieve a technical effect that the tensile strength is greater than 400 MPa, the yield strength is greater than 280 MPa, and the low-temperature impact energy at -20 °C is greater than 7 J. The relevant technical effect can refer to that the performance of the casting reaches the relevant indicators, or the effect of the test block reaches the relevant indicators. The test block can be a test block prepared according to GB_T 1348-2019 Ductile Iron Castings, or a test block that can reflect the performance of the overall casting. In other words, the ductile iron of the present invention can greatly improve the low-temperature impact energy of the casting on the basis of maintaining the existing strength. For example, the impact energy meets the requirement of being greater than 7 J at -20 °C. Therefore, the ductile iron of the present invention can be widely used in castings, especially in the castings of wind turbine generators.

[0073] The preparation method of the ductile iron according to the embodiments of the present invention will be described in detail below.

[0074] The preparation method of the ductile iron according to the embodiments of the present invention may include the following steps:

[0075] (1) Melting: Carrying out carbon addition treatment on the materials including pig iron and melting them into molten iron;

[0076] (2) Spheroidizing and inoculating: Spheroidizing and inoculating the molten iron obtained from step (1); and

[0077] (3) Pouring: Pouring the molten iron obtained from step (2),

[0078] Among them, in step (1) and / or step (2), the preparation method of the ductile iron according to the embodiments of the present invention may further include microalloying treatment.

[0079] Specifically, in step (1) and / or step (2), the microalloying treatment may include adding Ni so that the content of Ni is 0.20 - 0.55 wt% based on the total mass of the molten iron. A spheroidizing agent containing Si and an inoculant containing Si may be added in step (2), and a stream inoculant containing Si may be added in step (3) so that the content of Si is 2.3 - 2.6 wt% based on the total mass of the molten iron. The materials added to the furnace in step (1) may be adjusted so that the content of Mn is less than or equal to 0.14 wt% based on the total mass of the molten iron.

[0080] In the present invention, the addition amount of additives (such as carburizer, nodulizer, inoculant, etc.) in each step is much smaller than the total mass of the molten iron in each step. Therefore, for the total mass of the molten iron, the addition amount of additives in each step can be negligible. That is, the total mass of the molten iron in each step in the present invention is substantially equal. In other words, the "total mass of the molten iron" in the present invention may refer to the mass of the materials including pig iron melted into molten iron in step (1).

[0081] In addition, the method for preparing ductile iron according to the embodiments of the present invention can control the contents of Si, Mn, and / or Ni in different steps to further improve the comprehensive properties of the ductile iron (such as improving tensile strength, yield strength, and low-temperature impact energy).

[0082] The present invention adds Ni through microalloying treatment in step (1) and / or step (2) to control the content of Ni based on the total mass of the molten iron within the range of 0.20 - 0.55 wt%, preferably within the range of 0.35 - 0.55 wt%, and more preferably within the range of 0.35 - 0.45 wt%. Additionally, the present invention controls the content of Si based on the total mass of the molten iron within the range of 1.4 - 1.6 wt% in step (1), adds a nodulizer containing Si and an inoculant containing Si in step (2) to control Si within the range of 2.25 - 2.45 wt% based on the total mass of the molten iron, and adds a stream inoculant containing Si in step (4) to control the content of Si based on the total mass of the molten iron within the range of 2.3 - 2.6 wt%, preferably within the range of 2.35 - 2.49 wt%, and more preferably within the range of 2.39 - 2.45 wt%. Furthermore, the present invention adjusts the content of Mn in step (1) to control the content of Mn based on the total mass of the molten iron to be less than or equal to 0.14 wt%, preferably within the range of 0.10 - 0.12 wt%.

[0083] Specifically, in step (1), the materials including pig iron can be added into a melting furnace and heated to melt, so that the above materials are melted into molten iron. Pig iron, return scrap, and scrap steel commonly used in the art can be used as long as the content of Si based on the total mass of the molten iron in step (1) is within the range of 1.4 - 1.6 wt% and the content of Mn based on the total mass of the molten iron is less than or equal to 0.14 wt%. For example, the pig iron in the present invention can include pig iron of grade Q10 or above. However, the embodiments of the present invention are not limited thereto.

[0084] In addition, in step (1), if the Si content is less than 1.4 wt%, ferrosilicon with a higher Si content needs to be added. If the Si content is higher than 1.6 wt%, other pig irons with a lower Si content (such as plain carbon steel, but the present invention is not limited thereto) need to be added. In addition, in step (1), the Mn content is preferably controlled within the range of 0.10 - 0.12 wt%. If the Mn content of the hot metal in step (1) is low, ferromanganese can be added to increase the Mn content in the hot metal. However, the embodiments of the present invention are not limited thereto.

[0085] In step (1), the hot metal obtained by smelting can be subjected to carbon addition treatment. Specifically, a carbon additive can be added to the hot metal to perform carbon addition treatment on the hot metal. For example, the temperature of the hot metal can be controlled within the range of 1400 °C - 1450 °C, and a carbon additive can be added to the hot metal to perform carbon addition treatment. In the embodiments of the present invention, in order to reduce the influence of trace elements on ductile iron, the carbon content in the carbon additive can be greater than 90 wt% and the sulfur content can be less than 0.05 wt%. Under the guidance of the concept of the present invention, those skilled in the art can select a suitable carbon additive as long as it meets the foregoing conditions.

[0086] In step (2), the hot metal obtained from step (1) can be subjected to spheroidizing, inoculating, and microalloying treatments. Specifically, as an example, first, the inoculant can be laid on one side of the dam in the spheroidizing ladle, second, the spheroidizing agent and the microalloy are laid on the other side of the dam in the spheroidizing ladle and covered with iron filings, and then the carbon addition-treated hot metal is added to the spheroidizing ladle, thereby performing the spheroidizing, inoculating, and microalloying treatment process. Alternatively, the hot metal can also be subjected to microalloying treatment in step (1).

[0087] In step (3), a stream inoculant is added during the pouring of the hot metal obtained from step (2) to perform stream inoculation.

[0088] In an embodiment of the present invention, based on the total mass of the spheroidizing agent, the spheroidizing agent may include 40 - 50 wt% of Si by mass percentage. Preferably, the spheroidizing agent may include, by mass percentage: 4.5 - 6 wt% of Mg, 0.15 - 0.3 wt% of RE, 40 - 50 wt% of Si, and the balance is iron. However, the embodiments of the present invention are not limited thereto. Based on the total mass of the inoculant, the inoculant may include 75 ± 3 wt% of Si by mass percentage. Preferably, the inoculant may include, by mass percentage: 75 ± 3 wt% of Si, 1.0 - 2.0 wt% of Ca, 2.0 - 3.0 wt% of Ba, Al < 1.5 wt%, and the balance is iron. However, the embodiments of the present invention are not limited thereto. In addition, in step (2), the addition amount of the inoculant may be 0.35 - 0.67 wt% of the total mass of the molten iron, and the addition amount of the spheroidizing agent may be 1.0 - 1.3 wt% of the total mass of the molten iron, so as to control Si within the range of 2.25 - 2.45 wt% based on the total mass of the molten iron in step (2).

[0089] In an embodiment of the present invention, based on the total mass of the in - stream inoculant, the in - stream inoculant may include 70 - 80 wt% of Si by mass percentage. Preferably, the in - stream inoculant may include, by mass percentage: 70 - 80 wt% of Si, 0.5 - 2.5 wt% of Bi, Ca ≤ 2.0 wt%, Al ≤ 2.0 wt%, and the balance is Fe. However, the embodiments of the present invention are not limited thereto. In step (3), the addition amount of the in - stream inoculant may be 0.08 - 0.2 wt% of the total mass of the molten iron, so as to control the content of Si based on the total mass of the molten iron within the range of 2.3 - 2.6 wt%.

[0090] In a preferred embodiment, the inoculant in step (2) may include a primary inoculant and a covering inoculant. In an embodiment, based on the total mass of the primary inoculant, the primary inoculant may include 75 ± 3 wt% of Si by mass percentage. Preferably, the primary inoculant may include, by mass percentage: 75 ± 3 wt% of Si, 1.0 - 2.0 wt% of Ca, 2.0 - 3.0 wt% of Ba, Al < 1.5 wt%, and the balance is iron. In an embodiment, based on the total mass of the covering inoculant, the covering inoculant may include 75 ± 3 wt% of Si by mass percentage. Preferably, the covering inoculant may include, by mass percentage: 75 ± 3 wt% of Si, 1.0 - 2.0 wt% of Ca, 2.0 - 3.0 wt% of Ba, Al < 1.5 wt%, and the balance is iron.

[0091] Specifically, as an example, a primary inoculant can be laid on one side of the dam in the spheroidizing ladle, and a spheroidizing agent, a microalloy, and a covering inoculant can be laid on the other side, and the spheroidizing agent and the microalloy can be covered with iron filings, and then the molten iron after carbon addition treatment is added into the spheroidizing ladle. In the embodiment, the addition amount of the primary inoculant can be 0.30-0.55 wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent can be 1.0-1.3 wt% of the total mass of the molten iron, the addition amount of the covering inoculant can be 0.05-0.12 wt% of the total mass of the molten iron, and the addition amount of the in-stream inoculant can be 0.08-0.20 wt% of the total mass of the molten iron. However, the embodiments of the present invention are not limited thereto, as long as the content of Si based on the total mass of the ductile iron is within the range of 2.3-2.6 wt%. More preferably, as long as the content of Si in step (1) is within the range of 1.4-1.6 wt%, the content of Si in step (2) is within the range of 2.25-2.45 wt%, and the content of Si in step (3) is within the range of 2.3-2.6 wt%.

[0092] In the embodiment, Mg can be added in step (2) such that the content of Mg based on the total mass of the molten iron is within the range of 0.045-0.078 wt%. In the embodiment, Mg can be included in the spheroidizing agent. Based on the total mass of the spheroidizing agent, the spheroidizing agent can include 4.5-6.5 wt% of Mg by mass percentage, and the addition amount of the spheroidizing agent in step (2) can be 1.0-1.3 wt% of the total mass of the molten iron. However, the embodiments of the present invention are not limited thereto.

[0093] In step (1) and / or step (2), the microalloy treatment can include adding Ni to control the content of Ni based on the total mass of the molten iron within the range of 0.20-0.55 wt%. Specifically, the addition amount of Ni can be 0.20-0.40 wt% of the total mass of the molten iron.

[0094] In another embodiment, in step (1) and / or step (2), the microalloy treatment can further include adding TiC to the molten iron, wherein the addition amount of TiC can be 0.01-0.03 wt% of the total mass of the molten iron. In some preferred embodiments, a carrier can be added to TiC, and the carrier is 75 ferrosilicon, and the mixing of TiC and 75 ferrosilicon can make the alloy components uniformly distributed at the grain boundaries, playing a role of solid solution strengthening. Preferably, the mass ratio of TiC to 75 ferrosilicon can be 1:2.

[0095] In step (3), an in-stream inoculant is added to the molten iron obtained from step (2) and pouring is carried out, and the addition amount of the in-stream inoculant can be 0.1-0.2 wt% of the total mass of the molten iron.

[0096] The pouring temperature in step (3) can be 1330 - 1360 °C. In the embodiment, step (3) further includes slowly cooling the castings after pouring to below 400 °C, where the slow cooling rate can be 10 - 20 °C / min.

[0097] In the present invention, by controlling the contents of Si, Mn, and Ni, more preferably, controlling the contents of Si and Mn in step (1), controlling the content of Ni in step (1) and / or (2), and controlling the content of Si in step (2) and step (3) respectively, ductile iron with a tensile strength greater than 400 MPa, a yield strength greater than 280 MPa, and a low-temperature impact energy greater than 7 J at -20 °C is obtained. Specifically, in step (1), the content of Si is controlled to be 1.4 - 1.6 wt%, and the content of Mn is controlled to be less than or equal to 0.14 wt%. In step (1) and / or step (2), the content of Ni is controlled to be 0.20 - 0.55 wt% by microalloy treatment. In step (2), a spheroidizing agent and an inoculant are added to control the content of Si to be 2.25 - 2.45 wt%. In step (3), a stream inoculant is added to control the content of Si to be 2.3 - 2.6 wt%.

[0098] In the prior art, for small parts (such as wall thickness less than 60 mm), due to good heat dissipation conditions and fast solidification speed, the time for graphite growth is relatively short, the tendency of graphite distortion is small, its structure is easy to control, and the material properties are good. However, for large-section castings, its cooling speed is slow, the time for graphite ball growth is long, and this material relies on solid solution strengthening of silicon. With a high silicon content, large-section castings often appear deformed graphite such as fragmented graphite, and the performance also deteriorates sharply. The ductile iron prepared by the present invention solves the above problems, has good core structure, and basically no abnormal graphite such as fragmented graphite appears, and is especially suitable for large-section castings.

[0099] Hereinafter, the beneficial effects of the present invention will be more clearly elaborated in combination with the examples and comparative examples of the present invention. The preparation methods in the following examples are all conventional methods unless otherwise specified; the reagents and materials used are all conventional reagents and materials in the art unless otherwise specified, and can be obtained through commercial purchase.

[0100] Example 1

[0101] (1) Using a 20T intermediate frequency furnace, loading high-temperature graphitizing carburizer and pig iron of Q10 grade, carrying out melting and carburizing treatment at 1430 °C to obtain molten iron, adding 75 ferrosilicon to adjust the silicon content in the molten iron to 1.48 wt%; adding ferromanganese to make the manganese content in the molten iron 0.125 wt%; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0102] The element composition of the recarburizer is: carbon > 98.5 wt%, sulfur < 0.05 wt%, ash < 0.5 wt%, moisture < 0.5 wt%.

[0103] (2) Carry out nodulizing, inoculating and microalloying treatments on the molten iron after the carburizing treatment in step (1):

[0104] Lay the inoculant on one side of the dam in the 20T nodulizing ladle. On the other side of the dam, lay the nodulizer, microalloy Ni from bottom to top in sequence, and then cover the microalloy with cast iron chips with a covering thickness of 6 mm. Pour the molten iron into the nodulizing ladle from the side where the inoculant is laid in the nodulizing ladle;

[0105] Among them, the addition amount of the inoculant is 0.54 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloy Ni is 0.46 wt% of the total mass of the molten iron;

[0106] Among them, the element composition of the inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron;

[0107] The element composition of the nodulizer is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron;

[0108] (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.14 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0109] The element composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0110] The elements of the obtained ductile iron casting include: C 3.71 wt%, Si 2.49 wt%, Mn 0.125 wt%, P 0.025 wt%, S 0.02 wt%, Ni 0.46 wt%, Cr 0.011 wt%, Mg 残 0.04 wt%, and the rest is iron and unavoidable impurities.

[0111] Example 2

[0112] (1) Use a 20T intermediate frequency furnace, charge high-temperature graphitized carburizer and pig iron of grade Q10, carry out melting and carburizing treatment at 1430 °C to obtain molten iron, add 75 ferrosilicon, and adjust the silicon content in the molten iron to 1.47 wt%; add ferromanganese to make the manganese content in the molten iron 0.122 wt%; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0113] The elemental composition of the carburizer is: carbon greater than 98.5 wt%, sulfur less than 0.05 wt%, ash less than 0.5 wt%, and moisture less than 0.5 wt%.

[0114] (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after carburizing treatment in step (1):

[0115] Lay the primary inoculant on one side of the dam in a 20T spheroidizing ladle. On the other side of the dam, lay the spheroidizing agent, covering inoculant, and microalloying Ni from bottom to top in sequence, and then cover the microalloying with cast iron chips, with a covering thickness of 6 mm. Pour the molten iron into the spheroidizing ladle from the side where the primary inoculant is laid.

[0116] Among them, the addition amount of the primary inoculant is 0.45 wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1 wt% of the total mass of the molten iron, the addition amount of the covering inoculant is 0.09 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.46 wt% of the total mass of the molten iron.

[0117] Among them, the elemental composition of the primary inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron.

[0118] The elemental composition of the spheroidizing agent is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron.

[0119] The elemental composition of the covering inoculant is: Si 73.3 wt%, C 8.0 wt%, Ca 0.45 wt%, Ba 1.94 wt%, Al 0.52 wt%, and the rest is iron.

[0120] (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.14 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0121] The elemental composition of the in - stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0122] The elements of the obtained ductile iron parts include C 3.70 wt%, Si 2.47 wt%, Mn 0.122 wt%, P 0.027 wt%, S 0.017 wt%, Ni 0.46 wt%, Cr 0.009 wt%, Mg 残 0.05 wt%, and the rest is iron and inevitable impurities.

[0123] Example 3

[0124] (1) Use a 20T intermediate - frequency furnace, load high - temperature graphitizing carburizer and pig iron of grade Q10, carry out melting and carburizing treatment at 1430 °C to obtain molten iron. Add 75 ferrosilicon to adjust the silicon content in the molten iron to 1.45 wt%; add ferromanganese to make the manganese content in the molten iron 0.121 wt%; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0125] The elemental composition of the carburizer is: carbon greater than 98.5 wt%, sulfur less than 0.05 wt%, ash less than 0.5 wt%, moisture less than 0.5 wt%.

[0126] (2) Carry out spheroidizing, inoculating and micro - alloying treatment on the molten iron after the carburizing treatment in step (1):

[0127] Lay the primary inoculant on one side of the dam in a 20T spheroidizing ladle, and lay the spheroidizing agent, micro - alloying Ni, micro - alloying TiC (the mass ratio of TiC to 75 ferrosilicon is 1:2) and the covering inoculant from bottom to top on the other side of the dam in sequence. Then cover the micro - alloying and the covering inoculant with cast iron chips, with a covering thickness of 6 mm. Add the molten iron into the spheroidizing ladle from the side where the primary inoculant is laid.

[0128] Among them, the addition amount of the primary inoculant is 0.45 wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1 wt% of the total mass of the molten iron, the addition amount of the covering inoculant is 0.09 wt% of the total mass of the molten iron; the addition amount of micro - alloying Ni is 0.45 wt% of the total mass of the molten iron; the addition amount of micro - alloying TiC is 0.025 wt% of the total mass of the molten iron.

[0129] Among them, the elemental composition of the primary inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron.

[0130] The elemental composition of the spheroidizing agent is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron;

[0131] The elemental composition of the covering inoculant is: Si 73.3 wt%, C 8.0 wt%, V 0.16 wt%, Ca 0.45 wt%, Ba 1.94 wt%, Al 0.52 wt%, and the rest is iron;

[0132] (3) Pouring: During the pouring process, in-stream inoculation is carried out by adding an in-stream inoculant, and the addition amount of the in-stream inoculant is 0.14 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is then a ductile iron casting.

[0133] The elemental composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0134] The elements of the obtained ductile iron casting include C 3.75 wt%, Si 2.45 wt%, Mn 0.121 wt%, P 0.024 wt%, S 0.01 wt%, V 0.001 wt%, Ni 0.45 wt%, Ti 0.021 wt%, Cr 0.006 wt%, Mg 残 0.04 wt%, and the rest is iron and unavoidable impurities.

[0135] Example 4

[0136] (1) Use a 20T intermediate frequency furnace, load high-temperature graphitizing carburizer and pig iron of Q10 grade, carry out melting and carburizing treatment at 1430 °C to obtain molten iron, add 75 ferrosilicon, and adjust the silicon content in the molten iron to 1.15 wt%; add ferromanganese to make the manganese content in the molten iron 0.125 wt%; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0137] The elemental composition of the carburizer is: carbon greater than 98.5%, sulfur less than 0.05%, ash less than 0.5%, moisture less than 0.5%;

[0138] (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1):

[0139] The inoculant is laid on one side of the dam inside the 20T nodulizing ladle. On the other side of the dam, the spheroidizing agent, microalloying Ni are laid successively from bottom to top, and then covered with cast iron filings on the microalloying, with a covering thickness of 6 mm. The molten iron is added into the nodulizing ladle from the side where the inoculant is laid in the nodulizing ladle;

[0140] Among them, the addition amount of the inoculant is 0.75 wt% of the total mass of the molten iron, and the addition amount of the spheroidizing agent is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.46 wt% of the total mass of the molten iron;

[0141] Among them, the element composition of the inoculant is: Si 75±3%; Ca 1.0 - 2.0%; Ba 2.0 - 3.0%; Al < 1.5%, and the rest is iron;

[0142] The element composition of the spheroidizing agent is: Si 45.5%, Mg 5.1%, RE 0.243%, Ba 1.31%, Ca 0.94%, and the rest is iron;

[0143] (3) Pouring: In the pouring process, in-stream inoculant is added for in-stream inoculation. The addition amount of the in-stream inoculant is 0.14 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling speed is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0144] The element composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0145] The elements of the obtained ductile iron casting include C 3.71 wt%, Si 2.31 wt%, Mn 0.125 wt%, P 0.025 wt%, S 0.02 wt%, Ni 0.46 wt%, Cr 0.011 wt%, Mg 残 0.05 wt%, and the rest is iron and inevitable impurities.

[0146] Example 5

[0147] (1) Use a 20T intermediate frequency furnace, load high-temperature graphitizing carburant and pig iron of Q10 grade, carry out melting and carburizing treatment at 1430 °C to obtain molten iron, add 75 ferrosilicon, and adjust the silicon content in the molten iron to 1.23 wt%; add ferromanganese to make the manganese content in the molten iron 0.13 wt%; the addition amount of the carburant is 0.2 wt% of the total mass of the molten iron,

[0148] The element composition of the carburant is: carbon is greater than 98.5%, sulfur is less than 0.05%, ash content is less than 0.5%, and moisture content is less than 0.5%;

[0149] (2) Carry out nodulizing, inoculating, and microalloying treatments on the molten iron after the carbon addition treatment in step (1):

[0150] Lay the inoculant on one side of the dam in a 20T nodulizing ladle. On the other side of the dam, lay the nodulizer, microalloying Ni from bottom to top in sequence, and then cover the microalloying with cast iron chips with a covering thickness of 6 mm. Pour the molten iron into the nodulizing ladle from the side where the inoculant is laid;

[0151] Among them, the addition amount of the inoculant is 0.54 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.46 wt% of the total mass of the molten iron; the addition amount of the recarburizer is 0.2 wt% of the total mass of the molten iron;

[0152] Among them, the element composition of the inoculant is: Si 75 ± 3%; Ca 1.0 - 2.0%; Ba 2.0 - 3.0%; Al < 1.5%, and the rest is iron;

[0153] The element composition of the nodulizer is: Si 45.5%, Mg 5.1%, RE 0.243%, Ba 1.31%, Ca 0.94%, and the rest is iron;

[0154] (3) Pouring: Add in-stream inoculant during the pouring process for in-stream inoculation. The addition amount of the in-stream inoculant is 0.47 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0155] The element composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0156] The elements of the obtained ductile iron casting include C 3.71 wt%, Si 2.47 wt%, Mn 0.13 wt%, P 0.024 wt%, S 0.017 wt%, Ni 0.46 wt%, Cr 0.015 wt%, Mg 残 0.048 wt%, and the rest is iron and inevitable impurities.

[0157] Example 6

[0158] (1) Use a 20T intermediate frequency furnace, charge high-temperature graphitized carburizer and pig iron of grade Q10, carry out melting and carburizing treatment at 1430 °C to obtain molten iron, add 75 ferrosilicon, and adjust the silicon content in the molten iron to 1.44 wt%; add ferromanganese to make the manganese content in the molten iron 0.125 wt%; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0159] The elemental composition of the carburizer is: carbon greater than 98.5 wt%, sulfur less than 0.05 wt%, ash less than 0.5 wt%, moisture less than 0.5 wt%.

[0160] (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after carburizing treatment in step (1):

[0161] Lay the inoculant on one side of the dam in a 20T spheroidizing ladle. On the other side of the dam, lay the spheroidizing agent, microalloying Ni from bottom to top in sequence, and then cover the microalloying with cast iron filings with a covering thickness of 6 mm. Add the molten iron into the spheroidizing ladle from the side where the inoculant is laid in the spheroidizing ladle.

[0162] Among them, the addition amount of the inoculant is 0.75 wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.46 wt% of the total mass of the molten iron.

[0163] Among them, the elemental composition of the inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron.

[0164] The elemental composition of the spheroidizing agent is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron.

[0165] (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.14 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0166] The elemental composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0167] The elements of the obtained ductile iron casting include: C 3.71 wt%, Si 2.60 wt%, Mn 0.125 wt%, P 0.025 wt%, S 0.02 wt%, Ni 0.46 wt%, Cr 0.011 wt%, Mg残 0.048 wt%, with the balance being iron and unavoidable impurities.

[0168] Example 7

[0169] (1) Use a 20T intermediate frequency furnace, charge high-temperature graphitized carburizer and pig iron of grade Q10, carry out melting and carburizing treatment at 1430 °C to obtain molten iron, add 75 ferrosilicon, and adjust the silicon content in the molten iron to 1.23 wt%; add ferromanganese to make the manganese content in the molten iron 0.14 wt%; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0170] The element composition of the carburizer is: carbon greater than 98.5 wt%, sulfur less than 0.05 wt%, ash less than 0.5 wt%, and moisture less than 0.5 wt%.

[0171] (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1):

[0172] Lay the inoculant on one side of the dam in a 20T spheroidizing ladle. On the other side of the dam, lay the spheroidizing agent, microalloying Ni from bottom to top in sequence, and then cover the microalloying with cast iron chips with a covering thickness of 6 mm. Add the molten iron into the spheroidizing ladle from the side where the inoculant is laid.

[0173] Among them, the addition amount of the inoculant is 0.54 wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.46 wt% of the total mass of the molten iron.

[0174] Among them, the element composition of the inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the balance is iron.

[0175] The element composition of the spheroidizing agent is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the balance is iron.

[0176] (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.47 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0177] The element composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the balance is iron.

[0178] The elements of the obtained ductile iron parts include: C 3.71 wt%, Si 2.47 wt%, Mn 0.14 wt%, P 0.025 wt%, S 0.02 wt%, Ni 0.46 wt%, Cr 0.011 wt%, Mg 残 0.048 wt%, and the rest is iron and inevitable impurities.

[0179] Example 8

[0180] Except that the addition amount of the microalloy Ni in Example 2 is adjusted to 0.2 wt% of the total mass of the molten iron, the ductile iron parts are prepared in the same manner as in Example 2.

[0181] The elements of the obtained ductile iron parts include C 3.70 wt%, Si 2.47 wt%, Mn 0.122 wt%, P 0.027 wt%, S 0.017 wt%, Ni 0.20 wt%, Cr 0.009 wt%, Mg 残 0.05 wt%, and the rest is iron and inevitable impurities.

[0182] Example 9

[0183] Except that the addition amount of the microalloy Ni in Example 2 is adjusted to 0.55 wt% of the total mass of the molten iron, the ductile iron parts are prepared in the same manner as in Example 2.

[0184] The elements of the obtained ductile iron parts include C 3.70 wt%, Si 2.47 wt%, Mn 0.122 wt%, P 0.027 wt%, S 0.017 wt%, Ni 0.55 wt%, Cr 0.009 wt%, Mg 残 0.05 wt%, and the rest is iron and inevitable impurities.

[0185] Comparative Example 1

[0186] (1) A 20T intermediate frequency furnace is used to load high-temperature graphitizing carburizer and pig iron of Q10 grade, and smelting and carburizing treatment are carried out at 1430 °C to obtain molten iron; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0187] The element composition of the carburizer is: carbon greater than 98.5 wt%, sulfur less than 0.05 wt%, ash less than 0.5 wt%, and moisture less than 0.5 wt%.

[0188] (2) The molten iron after carburizing treatment in step (1) is subjected to spheroidizing, inoculating and microalloying treatment:

[0189] On one side of the dam in the 20T nodulizing ladle, inoculant is laid. On the other side of the dam, nodulizer and micro-alloy Ni are laid successively from bottom to top. Then, cast iron filings are used to cover the micro-alloy, with a covering thickness of 6 mm. Molten iron is added into the nodulizing ladle from the side where the inoculant is laid.

[0190] Among them, the addition amount of the inoculant is 0.5 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1 wt% of the total mass of the molten iron; the addition amount of micro-alloy Ni is 0.45 wt% of the total mass of the molten iron;

[0191] Among them, the element composition of the inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron;

[0192] The element composition of the nodulizer is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron;

[0193] (3) Pouring: During the pouring process, in-stream inoculant is added for in-stream inoculation. The addition amount of the in-stream inoculant is 0.15 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0194] The element composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0195] The elements of the obtained ductile iron casting include: C 3.75 wt%, Si 2.65 wt%, Mn 0.12 wt%, P 0.024 wt%, S 0.01 wt%, Ni 0.45 wt%, Mg 残 0.04 wt%, and the rest is iron and unavoidable impurities.

[0196] Comparative Example 2

[0197] (1) Use a 20T intermediate frequency furnace, load high-temperature graphitizing carburizer and pig iron of Q10 grade, and carry out melting and carburizing treatment at 1430 °C to obtain molten iron; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron,

[0198] The element composition of the carburizer is: carbon > 98.5%, sulfur < 0.05%, ash < 0.5%, moisture < 0.5%;

[0199] (2) Carry out nodulizing, inoculating and micro-alloying treatment on the molten iron after the carburizing treatment in step (1):

[0200] On one side of the dam in the 20T nodulizing ladle, inoculant is laid, and on the other side of the dam, nodulizer and microalloy Ni are laid in sequence from bottom to top. Then, cast iron filings are used to cover the microalloy, with a covering thickness of 6 mm. The molten iron is added into the nodulizing ladle from the side where the inoculant is laid.

[0201] Among them, the addition amount of the inoculant is 0.54 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloy Ni is 0.46 wt% of the total mass of the molten iron;

[0202] Among them, the element composition of the inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron;

[0203] The element composition of the nodulizer is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron;

[0204] (3) Pouring: During the pouring process, in - stream inoculant is added for in - stream inoculation. The addition amount of the in - stream inoculant is 0.14 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling speed is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0205] The element composition of the in - stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0206] The elements of the obtained ductile iron casting include: C 3.75 wt%, Si 2.49 wt%, Mn 0.16 wt%, P 0.024 wt%, S 0.01 wt%, Ni 0.46 wt%, Mg 残 0.04 wt%, and the rest is iron and inevitable impurities.

[0207] Comparative Example 3

[0208] (1) Use a 20T intermediate - frequency furnace to load high - temperature graphitizing carburant and pig iron of Q10 grade, and carry out melting and carburizing treatment at 1430 °C to obtain molten iron; the addition amount of the carburant is 0.2 wt% of the total mass of the molten iron,

[0209] The element composition of the carburant is: carbon greater than 98.5%, sulfur less than 0.05%, ash less than 0.5%, moisture less than 0.5%;

[0210] (2) Perform nodulizing, inoculating, and microalloying treatments on the molten iron after the carburizing treatment in step (1): Lay the primary inoculant on one side of the dam in a 20T nodulizing ladle. On the other side of the dam, lay the nodulizer, microalloying Ni, microalloying TiC (the mass ratio of TiC to 75% ferrosilicon is 1:2), and the covering inoculant in sequence from bottom to top. Then cover the microalloying and the covering inoculant with cast iron filings, with a covering thickness of 6 mm. Pour the molten iron into the nodulizing ladle from the side where the primary inoculant is laid.

[0211] Among them, the addition amount of the primary inoculant is 0.35 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1 wt% of the total mass of the molten iron, the addition amount of the covering inoculant is 0.09 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.3 wt% of the total mass of the molten iron; the addition amount of microalloying TiC is 0.025 wt% of the total mass of the molten iron;

[0212] Among them, the elemental composition of the primary inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron;

[0213] The elemental composition of the nodulizer is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron;

[0214] The elemental composition of the covering inoculant is: Si 73.3 wt%, C 8.0 wt%, V 0.16 wt%, Ca 0.45 wt%, Ba 1.94 wt%, Al 0.52 wt%, and the rest is iron;

[0215] (3) Pouring: Add in-stream inoculant during the pouring process for in-stream inoculation. The addition amount of the in-stream inoculant is 0.13 wt% of the total mass of the molten iron; after the pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0216] The elemental composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0217] The elements of the obtained ductile iron casting include C 3.75 wt%, Si 2.23 wt%, Mn 0.13 wt%, P 0.024 wt%, S 0.01 wt%, V 0.001 wt%, Ni 0.46 wt%, Ti 0.021 wt%, Cr 0.006 wt%, Mg 残 0.04 wt%, and the rest is iron and unavoidable impurities.

[0218] Comparative Example 4

[0219] (1) Charge the high-temperature graphitized recarburizer and pig iron of grade Q10 into a 20T intermediate frequency furnace, and carry out melting and recarburization treatment at 1430 °C to obtain molten iron; the addition amount of the recarburizer is 0.2 wt% of the total mass of the molten iron.

[0220] The element composition of the recarburizer is: carbon greater than 98.5%, sulfur less than 0.05%, ash less than 0.5%, and moisture less than 0.5%.

[0221] (2) Carry out nodulizing, inoculating and microalloying treatment on the molten iron after the recarburization treatment in step (1): Lay the primary inoculant on one side of the dam in a 20T nodulizing ladle. On the other side of the dam, lay the nodulizer, microalloying Ni, microalloying TiC (the mass ratio of TiC to 75 ferrosilicon is 1:2) and covering inoculant in sequence from bottom to top. Then cover the microalloying and covering inoculant with cast iron chips, and the covering thickness is 6 mm. Add the molten iron into the nodulizing ladle from the side where the primary inoculant is laid.

[0222] Among them, the addition amount of the primary inoculant is 0.41 wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1 wt% of the total mass of the molten iron, the addition amount of the covering inoculant is 0.09 wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.6 wt% of the total mass of the molten iron; the addition amount of microalloying TiC is 0.025 wt% of the total mass of the molten iron.

[0223] Among them, the element composition of the primary inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron.

[0224] The element composition of the nodulizer is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron.

[0225] The element composition of the covering inoculant is: Si 73.3 wt%, C 8.0 wt%, V 0.16 wt%, Ca 0.45 wt%, Ba 1.94 wt%, Al 0.52 wt%, and the rest is iron.

[0226] (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process, and the addition amount of the in-stream inoculant is 0.13 wt% of the total mass of the molten iron; after pouring is completed, the casting is slowly cooled to below 400 °C in the mold (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, which is the ductile iron casting.

[0227] The elemental composition of the in-stream inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0228] The elements of the obtained ductile iron parts include C 3.75 wt%, Si 2.33 wt%, Mn 0.125 wt%, P 0.024 wt%, S 0.01 wt%, V 0.001 wt%, Ni 0.6 wt%, Ti 0.021 wt%, Cr 0.006 wt%, Mg 残 0.04 wt%, and the rest is iron and unavoidable impurities.

[0229] Comparative Example 5

[0230] (1) Use a 20T intermediate frequency furnace, load high-temperature graphitizing carburizer and pig iron of grade Q10, carry out melting and carburizing treatment at 1430 °C to obtain molten iron; the addition amount of the carburizer is 0.2 wt% of the total mass of the molten iron.

[0231] The elemental composition of the carburizer is: carbon greater than 98.5 wt%, sulfur less than 0.05 wt%, ash less than 0.5 wt%, and moisture less than 0.5 wt%.

[0232] (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1):

[0233] Lay the primary inoculant on one side of the dam in a 20T spheroidizing ladle. On the other side of the dam, lay the spheroidizing agent, microalloy Ni, microalloy TiC (the mass ratio of TiC to 75 ferrosilicon is 1:2) and the covering inoculant from bottom to top in sequence. Then cover the microalloy and the covering inoculant with cast iron filings, and the covering thickness is 6 mm. Pour the molten iron into the spheroidizing ladle from the side where the primary inoculant is laid.

[0234] Among them, the addition amount of the primary inoculant is 0.45 wt% of the total mass of the molten iron, and the addition amount of the spheroidizing agent is 1.1 wt% of the total mass of the molten iron; the addition amount of microalloy Ni is 0.15 wt% of the total mass of the molten iron.

[0235] Among them, the elemental composition of the inoculant is: Si 75 ± 3 wt%; Ca 1.0 - 2.0 wt%; Ba 2.0 - 3.0 wt%; Al < 1.5 wt%, and the rest is iron.

[0236] The elemental composition of the spheroidizing agent is: Si 45.5 wt%, Mg 5.1 wt%, RE 0.243 wt%, Ba 1.31 wt%, Ca 0.94 wt%, and the rest is iron.

[0237] (3) Pouring: During the pouring process, in-mold inoculant is added for in-mold inoculation, and the addition amount of the in-mold inoculant is 0.14 wt% of the total mass of the molten iron. After pouring is completed, the casting is slowly cooled in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold to obtain a ductile iron casting.

[0238] The elemental composition of the in-mold inoculant is: Si 72.09 wt%, Bi 1.23 wt%, Ca 1.32 wt%, Al 1.09 wt%, and the rest is iron.

[0239] The elements of the obtained ductile iron casting include: C 3.75 wt%, Si 2.45 wt%, Mn 0.121 wt%, P 0.024 wt%, S 0.01 wt%, Ni 0.15 wt%, Mg 残 0.04 wt%, and the rest is iron and unavoidable impurities.

[0240] Pour the molten iron prepared according to the above-described examples and comparative examples into test blocks of ductile iron castings. Design the dimensions of the ductile iron test blocks according to the GB / T 1348-2019 standard. The thickness of the ductile iron test blocks is 70 mm to compare the effects of different molten iron compositions on the properties of ductile iron castings.

[0241] 1. Performance Evaluation of Ductile Iron Test Blocks

[0242] 1.1 Tensile strength, yield strength and low-temperature impact energy

[0243] Test the tensile strength, yield strength and low-temperature impact energy of the cast ductile iron castings according to the following test methods.

[0244] (1) Tensile strength is detected according to the method specified in GB / T 228.1;

[0245] (2) Yield strength is detected according to the method specified in GB / T 228.1;

[0246] (3) -20 °C low-temperature impact toughness is detected according to the method specified in GB / T 229.

[0247] The main element compositions and specific test results of the ductile iron in the examples and comparative examples are shown in Table 1 below. The main element compositions of the ductile iron are detected by EDS. The instrument used in this test is the German Zeiss SUPRA 55 field emission scanning electron microscope used in conjunction with an energy spectrometer.

[0248] Table 1

[0249]

[0250] From the experimental data in Table 1 above, it can be seen that the contents of Si, Mn, and Ni in the test blocks of ductile iron in Examples 1-9 are in the ranges of 2.3-2.6 wt%, ≤0.14 wt%, and 0.20-0.55 wt% respectively. Therefore, the tensile strength of the test blocks of ductile iron in Examples 1-9 is greater than 400 MPa, the yield strength is greater than 280 MPa, and the low-temperature impact energy at -20°C is greater than 7 J.

[0251] The Si content of the ductile iron in Comparative Example 1 exceeds 2.6 wt%. Therefore, the low-temperature impact energy of the ductile iron in Comparative Example 1 at -20°C is less than 7 J.

[0252] The Mn content of the ductile iron in Comparative Example 2 exceeds 0.14 wt%. Therefore, the low-temperature impact energy of the ductile iron in Comparative Example 2 at -20°C is less than 7 J.

[0253] The Si content of the ductile iron in Comparative Example 3 is less than 2.3 wt%. Therefore, the yield strength of the ductile iron in Comparative Example 3 is less than 280 MPa.

[0254] The Ni content of the ductile iron in Comparative Example 4 is greater than 0.55 wt%. Therefore, the low-temperature impact energy of the ductile iron in Comparative Example 4 at -20°C is less than 7 J.

[0255] The Ni content of the ductile iron in Comparative Example 5 is less than 0.20 wt%. Therefore, the tensile strength of the ductile iron in Comparative Example 5 is less than 410 MPa, and the yield strength is less than 280 MPa.

[0256] In the preparation process of Examples 1 to 3, in step (1), the content of Si is controlled to be 1.4-1.6 wt%, and the content of Mn is controlled to be less than or equal to 0.14 wt%. In step (2), the content of Ni is controlled to be 0.20-0.55 wt% by microalloy treatment, and a spheroidizing agent and an inoculant are added to control the content of Si to be 2.25-2.45 wt%. In step (3), a stream inoculant is added to control the content of Si to be 2.3-2.6 wt%.

[0257] In the preparation process of Example 4, the Si content in step (1) is 1.15 wt%, and the content of Si is not controlled within the range of 1.4-1.6 wt%. The Si content in step (2) is 2.21 wt%, and the content of Si is not controlled within the range of 2.25-2.45 wt%.

[0258] During the preparation process of Example 5, the Si content in step (1) was 1.23 wt%, and the Si content was not controlled within the range of 1.4 - 1.6 wt%. The Si content in step (2) was 2.13 wt%, and the Si content was not controlled within the range of 2.25 - 2.45 wt%.

[0259] As can be seen from Table 1 above, compared with Examples 4 and 5, the ductile iron of Examples 1 to 3 can achieve more excellent technical effects in terms of tensile strength, yield strength, and / or low-temperature impact energy at -20°C.

[0260] 1.2 Metallography of Ductile Iron Test Blocks

[0261] 1.2.1 Scanning Electron Microscope Test

[0262] The scanning electron microscope used in this test is the Zeiss SUPRA 55 field emission scanning electron microscope from Germany. Its principle is to use secondary electron signals to form images to observe the surface morphology of the sample. The adjustment range is 20V - 30KV, the adjustment step is continuously adjustable every 10V, and the magnification is 12X - 1,000,000X. The scanning electron microscope is mainly used to observe the tensile fracture morphology, the surface morphology of wear specimens, and the thermal fatigue crack morphology.

[0263] 1.2.2 Three-dimensional Video Electron Microscope Test

[0264] In this test, a Keyence-1000E three-dimensional video electron microscope was used to detect and analyze the tensile and wear specimens. According to the taken photos, the concavity and convexity of the specimen surface were observed, and the average scratch depth of the wear surface was calculated.

[0265] 1.2.3 Test Results

[0266] (1) Graphite Morphology

[0267] Figure 6A To compare the graphite morphology of the ductile iron in Example 1, Figure 6B is the graphite morphology of the ductile iron in Example 1. It can be seen that compared with Figure 6A compared, Figure 6B the spheroidization rate of the graphite is higher and the distribution is more uniform.

[0268] (2) Tensile Fracture Morphology

[0269] Figure 7A To compare the tensile fracture morphology of the ductile iron in Example 1, Figure 7B is the tensile fracture morphology of the ductile iron in Example 1. From Figure 7BIt can be seen that macroscopically, the undulation degree of the entire cross-section is not large. The surface morphology of the tensile fracture of the original specimen shows an uneven state, and some graphite balls fall off or break during the tensile test, resulting in holes on the cross-section surface, indicating that the bonding ability between the graphite balls and the matrix is relatively weak. From Figure 7B It can be seen that compared with the original specimen, macroscopically, from the overall flatness of the cross-section, the roughness and undulation degree of the specimen cross-section increase, making it more three-dimensional and having a stronger sense of hierarchy; microscopically, in the tensile fracture morphology of the specimen, the separation of graphite balls from the matrix is greatly reduced, the number of intact graphite balls in the specimen increases, and the dispersion is more uniform. In addition, Figure 7A In the original specimen, the cleavage steps with a "river-like" pattern on the fracture morphology are wide and steep, and the formation of tear ridges in the form of ductile fracture is not obvious, and the number of dimples on the few remaining tear ridges is small, scattered between the ridges. In Figure 7B , by comparing the fracture morphology of the specimen, the number of tear ridges on the cross-section is significantly increased, the distribution of dimples on the ridges is more uniform and dense, the tear ridges show a bright white color, and each tear ridge is interconnected to form a net-like closed structure, enclosing the graphite balls in it.

[0270] (3) Three-dimensional morphology of tensile fracture

[0271] Figure 8A is the three-dimensional morphology of the tensile fracture of the ductile iron in Comparative Example 1, Figure 8B is the three-dimensional morphology of the tensile fracture of the ductile iron in Example 1. From Figure 8A and Figure 8B , it can be seen that the concavity and convexity of the three-dimensional morphology of the fracture of the two specimens are quite different. Among them, Figure 8A the maximum protrusion height of the tensile fracture of the specimen is 596.4 μm, Figure 8B the maximum protrusion height of the tensile fracture is 687.3 μm, increasing the value of the maximum height difference of the tensile fracture by 15.24%; from the test results, the toughness of the tensile fracture of the ductile iron in Example 1 has been improved.

[0272] (4) Impact fracture morphology

[0273] Figure 9A is the impact fracture morphology of the ductile iron in Comparative Example 1, Figure 9B is the impact fracture morphology of the ductile iron in Example 1. It can be known from Figure 9A that the number of tear ridges on the impact specimen fracture is not large, but the size is large. A certain number of thick dimples are formed around the tear ridges, and a dimple band is formed in some parts. At the same time, there are cleavage planes in some parts of the cross-section, which indicates that during the fracture process of the impact specimen, quasi-cleavage fracture occurs, belonging to mixed fracture. It can be known from Figure 9B that the number of dimples in its fracture is the same as that in Figure 8ACompared with the specimens, there is an obvious increasing trend, the length of the tearing ridge is significantly shorter and thinner, and the degree of curvature increases. Under the action of impact stress, the adjacent dimples can be connected to form a fracture with a certain directionality.

[0274] According to the embodiments of the present invention, the tensile strength of the ductile iron is greater than 400 MPa, the yield strength is greater than 280 MPa, and the low-temperature impact energy at -20 °C is greater than 7 J, and it can be applied to large castings in wind turbine generators.

[0275] 2. Performance Testing of Large Ductile Iron Castings

[0276] According to the embodiments of the present invention, by using a metal mold coated with sand mold, the tissue compactness of ductile iron castings can be further improved by the strong chilling of the metal mold. Therefore, by improving the cooling rate of the molten iron and the composition of the molten iron in two aspects, the comprehensive mechanical properties of large castings can be improved. Next, taking the main shaft of a wind turbine generator formed by pouring the molten iron prepared according to Example 1 above through the iron mold coated with sand mold according to the embodiments of the present invention and a common sand mold as an example, it is tested whether the molten iron according to the embodiments of the present invention can also meet the performance requirements of the castings when casting large castings. The outer diameter of the shaft body 110 of the main shaft is 1500 mm. The pouring temperature is 1330 - 1360 °C, and the pouring speed of the molten iron is 100 - 150 kg / s.

[0277] Example 10

[0278] The same materials as in Example 1 are used, and the main shaft of the wind turbine generator is cast and formed by using the iron mold coated with sand mold according to the embodiments of the present invention. That is to say, compared with Example 1, the materials and process control processes used in Example 10 are exactly the same. The difference is that in Example 1, a traditional sand mold is used to cast ductile iron specimens, and in Example 10, the main shaft of the wind turbine generator is cast by using the iron mold coated with sand mold according to the embodiments of the present invention.

[0279] Comparative Example 6

[0280] Comparing Comparative Example 6 with Example 10, the materials and process control processes used are exactly the same. The difference is that in Example 10, the main shaft of the wind turbine generator is manufactured by using the iron mold coated with sand mold according to the embodiments of the present invention, and in Comparative Example 6, a traditional sand mold is used to cast the main shaft of the wind turbine generator.

[0281] Comparative Example 7

[0282] Comparing Comparative Example 7 with Example 10, the materials and process control procedures used are exactly the same. The difference is that in Example 10, a sand-coated iron mold according to the embodiment of the present invention is used to manufacture the main shaft of a wind turbine generator, while in Comparative Example 7, a traditional iron mold is used to cast the main shaft of a wind turbine generator.

[0283] Samples are taken from the shaft body, large-end flange, and small-end flange of the cast main shaft according to the following testing methods to test the tensile strength, yield strength, and low-temperature impact energy:

[0284] (1) Tensile strength is detected according to the method specified in GB / T 228.1;

[0285] (2) Yield strength is detected according to the method specified in GB / T 228.1;

[0286] (3) The low-temperature impact toughness at -20°C is detected according to the method specified in GB / T 229.

[0287] The specific test results of the main shafts in Example 10, Comparative Example 6, and Comparative Example 7 are shown in Table 2 below.

[0288] Table 2

[0289]

[0290] As shown in Table 2, when using a sand-coated iron mold to cast the molten iron obtained according to Example 1 of the present invention, the low-temperature impact energy of the main shaft can be significantly improved, meeting the comprehensive performance requirements of the main shaft. When casting the molten iron prepared according to Example 1 of the invention into a main shaft through the traditional sand casting process, the shaft body, large-end flange, and small-end flange of the main shaft cannot simultaneously meet the comprehensive performance requirements of the main shaft. When using an iron mold to prepare the main shaft, the comprehensive performance requirements of the main shaft can generally be met, but the improvement effect of the low-temperature impact energy is not as good as that of the main shaft obtained by the sand-coated iron forming mold. Therefore, when manufacturing large fan components with high performance requirements for each part, the molten iron composition provided according to the embodiment of the present invention and the sand-coated iron process provided according to the embodiment of the present invention can be used for casting, thereby significantly improving the low-temperature impact energy and enabling each part of the large ductile iron casting to meet the comprehensive performance requirements.

[0291] In addition, compared with the existing sand casting process, manufacturing ductile iron castings according to the embodiments of the invention using the iron mold with sand coating process also has the following technical effects: 1) High dimensional accuracy. The dimensional accuracy of the castings obtained by the iron mold with sand coating process can reach CT10 level, while the accuracy of the castings obtained by the sand casting process is CT12 level; 2) Denser structure and high UT grade. The UT grade of the main shaft obtained by the iron mold with sand coating process can reach UT1 level, while the UT grade of the main shaft obtained by the sand casting process is UT2 - 3 level; 3) Fast cooling time, which can save 3 days of mold opening time compared with the sand casting process; 4) Improve the utilization rate of sand boxes and sites, and increase the production capacity per unit area.

[0292] Comparative Example 8

[0293] In Comparative Example 8, the molten iron obtained in Comparative Example 1 was used to manufacture the main shaft of a wind power generation unit by the sand casting process.

[0294] Comparative Example 9

[0295] In Comparative Example 9, the molten iron obtained in Comparative Example 1 was used to manufacture the main shaft of a wind power generation unit by an iron mold.

[0296] Comparative Example 10

[0297] In Comparative Example 10, the molten iron obtained in Comparative Example 1 was used to manufacture the main shaft of a wind power generation unit by the iron mold with sand coating mold provided in the embodiments of the present invention.

[0298] In the above Comparative Examples 8, 9, and 10, by using the molten iron obtained in Comparative Example 1, the main shafts of wind power generation units were respectively manufactured by traditional sand molds, traditional iron molds, and iron mold with sand coating molds according to the embodiments of the present invention. After sampling various parts of the main shafts for performance testing, the comparison of performance test data is shown in Table 3 below.

[0299] Table 3

[0300]

[0301] It can be seen from Table 3 that for molten iron with ordinary material components, whether using ordinary sand molds, ordinary iron molds, or iron mold with sand coating molds, the obtained main shafts are difficult to meet the comprehensive performance requirements.

[0302] In summary, the comprehensive performance of the ductile iron casting test blocks according to the embodiments of the present invention is excellent. Especially for the casting test blocks with large cross-sections, excellent performance with a tensile strength greater than 400 MPa, a yield strength greater than 280 MPa, and a low-temperature impact energy greater than 7 J at -20°C can be obtained. Therefore, it can be widely applied to the manufacture of ductile iron castings with high comprehensive performance requirements.

[0303] In addition, the comprehensive properties of the molten iron obtained according to the embodiments of the present invention and the ductile iron castings obtained by the iron mold compound sand process according to the embodiments of the present invention are excellent. In particular, the low-temperature impact energy of the main shaft of the wind turbine generator can be greatly improved, reducing the fracture risk in low-temperature environments. Therefore, the iron mold compound sand molds for the ductile iron castings according to the present invention can be widely applied to large castings in wind turbine generators, and can also be applied to castings with high comprehensive property requirements in other fields, especially large-section castings, and can also meet the comprehensive property requirements.

[0304] Although the exemplary embodiments of the present invention have been specifically described with reference to its exemplary embodiments, those skilled in the art should understand that various changes in form and details can be made without departing from the spirit and scope of the present invention defined by the claims.

Claims

1. A forming method of ductile iron castings, characterized in that, The molding method includes: Preparing a molding die, which includes an outer mold box and an inner core located in the outer mold box. The outer mold box includes a metal casting mold and a sand coating layer covering the inner cavity surface of the metal casting mold. A cavity is formed between the outer mold box and the inner core; Pouring and molding, pouring molten iron into the cavity, and forming the ductile iron casting after cooling and solidification, wherein, the ductile iron casting includes by mass percentage: C 3.6 - 3.8wt%, Si 2.3 - 2.6wt%, Mn ≤ 0.14wt%, Ni 0.20 - 0.55wt%.

2. The forming method of the ductile iron casting according to claim 1, wherein, The ductile iron casting further includes, by mass percentage: V 0.001-0.01 wt%, Ti 0.02-0.03 wt%, P ≤ 0.035 wt%, S ≤ 0.02 wt%, Cr ≤ 0.025 wt%, Mg 残 0.035-0.060 wt%.

3. The forming method of the ductile iron casting according to claim 1, characterized in that, The molten iron is prepared by the following preparation method: (1) Melting: Carrying out carbon addition treatment on the materials including pig iron and melting them into molten iron; (2) Nodulizing and inoculating: Nodulizing and inoculating the molten iron obtained from step (1); and (3) Pouring: Pouring the molten iron obtained from step (2) into the cavity, wherein, in step (1) and / or step (2), the preparation method further includes microalloying treatment, and the microalloying treatment includes adding Ni, adding a nodulizer containing Si and an inoculant containing Si in step (2) and adding a stream inoculant containing Si in step (3).

4. The molding method of the ductile iron casting according to claim 3, characterized in that in step (1), the content of Si in the materials is in the range of 1.4 - 1.6wt%, in step (2), adding a nodulizer containing Si and an inoculant containing Si, such that the content of Si is in the range of 2.25 - 2.45wt%, and in step (3), adding a stream inoculant containing Si, such that the content of Si is in the range of 2.3 - 2.6wt%.

5. The molding method of the ductile iron casting according to claim 3, characterized in that based on the total mass of the nodulizer, the nodulizer includes 40 - 50wt% of Si by mass percentage, based on the total mass of the inoculant, the inoculant includes 72 - 78wt% of Si by mass percentage, based on the total mass of the stream inoculant, the stream inoculant includes 70 - 80wt% of Si by mass percentage.

6. The molding method of the ductile iron casting according to claim 5, characterized in that based on the total mass of the inoculant, the inoculant includes by mass percentage: Si 72 - 78wt%, Ca 1.0 - 2.0wt%, Ba 2.0 - 3.0wt%, Al < 1.5wt%, and the rest is iron, based on the total mass of the nodulizer, the nodulizer includes by mass percentage: Mg 4.5 - 6wt%, RE 0.15 - 0.3wt%, Si 40 - 50wt%, and the rest is iron, based on the total mass of the stream inoculant, the stream inoculant includes by mass percentage: Si 70 - 80wt%, Bi 0.5 - 2.5wt%, Ca ≤ 2.0wt%, Al ≤ 2.0wt%, and the rest is Fe.

7. The molding method of the ductile iron casting according to claim 5, characterized in that In step (2), the addition amount of the inoculant is 0.35 - 0.67 wt% of the total mass of the molten iron, and the addition amount of the spheroidizing agent is 1.0 - 1.3 wt% of the total mass of the molten iron. In step (3), the addition amount of the in-stream inoculant is 0.08 - 0.2 wt% of the total mass of the molten iron.

8. The forming method of the ductile iron casting according to claim 7, characterized in that, The inoculant in step (2) includes a primary inoculant and a covering inoculant. Based on the total mass of the primary inoculant, the primary inoculant includes, by mass percentage: Si 72 - 78 wt%, Ca 1.0 - 2.0 wt%, Ba 2.0 - 3.0 wt%, Al < 1.5 wt%, and the balance is iron. Based on the total mass of the covering inoculant, the covering inoculant includes, by mass percentage: Si 72 - 78 wt%, Ca 1.0 - 2.0 wt%, Ba 2.0 - 3.0 wt%, Al < 1.5 wt%, and the balance is iron.

9. The forming method of the ductile iron casting according to claim 8, characterized in that, In step (2), the addition amount of the primary inoculant is 0.30 - 0.55 wt% of the total mass of the molten iron, and the addition amount of the covering inoculant is 0.05 - 0.12 wt% of the total mass of the molten iron.

10. The method for forming a ductile iron casting according to claim 3, wherein In step (2), Mg is added such that the content of Mg is in the range of 0.045 - 0.078 wt% based on the total mass of the molten iron. Based on the total mass of the spheroidizing agent, the spheroidizing agent includes 4.5 - 6 wt% of Mg by mass percentage, and the addition amount of the spheroidizing agent in step (2) is 1.0 - 1.3 wt% of the total mass of the molten iron. In step (1) and / or step (2), the addition amount of Ni is 0.2 - 0.4 wt% of the total mass of the molten iron. The microalloying treatment in step (1) and / or step (2) includes adding Ni such that the content of Ni is in the range of 0.20 - 0.55 wt% of the total mass of the molten iron. In step (1) and / or step (2), the microalloying treatment further includes adding TiC to the molten iron, and the addition amount of TiC is 0.01 - 0.03 wt% of the total mass of the molten iron.

11. The forming method of the ductile iron casting according to claim 3, characterized in that, The pig iron in step (1) is pig iron of grade Q10 or above, and the content of Mn is adjusted in step (1) such that the content of Mn in step (1) is in the range of 0.10 - 0.12 wt% of the total mass of the molten iron.

12. The forming method of the ductile iron casting according to claim 3, characterized in that, Before pouring the molten iron into the cavity (130), the temperature of the cavity (130) is preheated to 50 - 200 °C. In step (4), the pouring temperature of the molten iron is 1330 - 1360 °C, and the pouring speed of the molten iron is 100 - 150 kg / s.

13. The forming method of the ductile iron casting according to claim 1, characterized in that, The metal mold is an iron mold, the thickness of the iron mold is 50 mm - 200 mm, and the thickness of the sand covering layer is 6 mm - 20 mm.

14. The forming method of the ductile iron casting according to claim 1, characterized in that, The outer mold box includes a bottom box, a top box, and a middle box disposed between the bottom box and the top box. The middle box is an iron mold sand-covered box, the top box and the bottom box are sand boxes, and the inner core is a sand core.

15. The forming method of the ductile iron casting according to claim 14, characterized in that, The forming mold further includes a bottom-gating gating system, and a runner for transporting the molten iron into the cavity is provided in the bottom box.

16. The forming method of the ductile iron casting according to claim 14, characterized in that, A riser is provided in the top box, and an exhaust passage is provided in the inner core, and the exhaust passage communicates with the riser.

17. The forming method of the ductile iron casting according to claim 1, characterized in that The matrix of the ductile iron is ferrite.

18. The forming method of the ductile iron casting according to any one of claims 1-17, characterized in that, The ductile iron casting is the main shaft of a wind turbine generator. In the axial direction of the main shaft, the outer mold box and / or the inner core are divided into multiple sections.

19. A ductile iron casting, characterized in that, The ductile iron casting is made by the molding method according to any one of claims 1 to 18.

20. The ductile iron casting according to claim 19, wherein The wall thickness of the ductile iron casting is greater than or equal to 60 mm.

21. The ductile iron casting according to claim 19, characterized in that, The ductile iron casting is the main shaft of a wind turbine generator, and the outer diameter of the shaft body of the main shaft is greater than or equal to 1 meter.

22. A wind turbine generator, characterized in that, The wind turbine generator includes the ductile iron casting according to any one of claims 19-21.

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