Nodular cast iron

Abstract

The invention provides nodular cast iron which comprises the following components in percentage by mass based on the total mass of the nodular cast iron: 3.6-3.8 wt% of C, 2.3-2.6 wt% of Si, less than or equal to 0.14 wt% of Mn and 0.20-0.55 wt% of Ni. According to the embodiment of the invention, the tensile strength of the nodular cast iron is greater than 400Mpa, the yield strength is greater than 280Mpa, the low-temperature impact energy at-20 DEG C is greater than 7J, and the nodular cast iron can be widely applied to castings in wind generating sets.

Description

Technical Field

[0001] The present invention relates to the field of metal materials, and particularly to a ductile iron and a preparation method thereof. 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 ferritic ductile iron has been widely used in construction machinery, basic components of injection molding machines, basic components of die-casting machines, and foundation or key components of wind power generation, 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 casting from cracking. However, the existing chemical composition formulations and production process methods cannot ensure that while significantly improving the strength, the low-temperature impact energy meets the indicators. From the research results of the existing technology, improving the strength while ensuring the low-temperature impact energy is a contradictory pair. Improving 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 present invention is proposed to overcome the above-mentioned disadvantages of the existing technology. The present invention provides a ductile iron and a preparation method thereof. Among them, 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.

[0005] According to an embodiment of the present invention, a ductile iron is provided. Based on the total mass of the ductile iron, the ductile iron 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%.

[0006] In the embodiment, the ductile iron may further include, by mass percentage: V 0.001 - 0.01 wt%, Ti 0.02 - 0.03 wt%.

[0007] In the embodiment, the ductile iron 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%.

[0008] In the embodiment, the matrix of the ductile iron may be ferritic.

[0009] In the embodiment, the content of Si may be in the range of 2.30 - 2.49 wt%.

[0010] In an embodiment, the content of Si may be in the range of 2.35 - 2.49 wt%.

[0011] In an embodiment, the content of Si may be in the range of 2.39 - 2.45 wt%.

[0012] In an embodiment, the content of Si may be in the range of 2.30 - 2.40 wt%.

[0013] In an embodiment, the content of Mn may be in the range of 0.10 - 0.12 wt%.

[0014] In an embodiment, the content of S may be less than 0.015 wt%.

[0015] In an embodiment, the content of Ni may be in the range of 0.35 - 0.55 wt%.

[0016] In an embodiment, the content of Ni may be in the range of 0.35 - 0.45 wt%.

[0017] In an embodiment, Mg 残 content may be in the range of 0.040 - 0.054 wt%.

[0018] The tensile strength of the ductile iron according to the embodiment of the present invention may be greater than 400 MPa, the yield strength may be greater than 280 MPa, and the low - temperature impact energy at - 20 °C may be greater than 7 J.

[0019] According to an embodiment of the present invention, a casting is provided, and the casting may include ductile iron.

[0020] In an embodiment, the wall thickness of the casting may be greater than or equal to 60 mm.

[0021] According to an embodiment of the present invention, a wind turbine generator is provided, and the wind turbine generator may include a casting.

[0022] According to an embodiment of the present invention, a method for preparing ductile iron is provided, and the preparation method may include the following steps: (1) Melting: Carrying out carbon - increasing treatment on the material including pig iron and melting it 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), Among them, in step (1) and / or step (2), the preparation method further includes microalloying treatment, and the microalloying treatment includes adding Ni. A spheroidizing agent containing Si and an inoculant containing Si are added in step (2), and a stream inoculant containing Si is added in step (3). Among them, based on the total mass of the molten iron, the content of Ni is 0.20 - 0.55 wt%, the content of Si is 2.3 - 2.6 wt%, and the content of Mn is less than or equal to 0.14 wt%.

[0023] In the embodiment, based on the total mass of the molten iron, in step (1), the content of Si in the material is in the range of 1.4 - 1.6 wt%. In step (2), a spheroidizing agent containing Si and an inoculant containing Si are added, so 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, so that the content of Si is in the range of 2.3 - 2.6 wt%.

[0024] In the embodiment, based on the total mass of the spheroidizing agent, the spheroidizing agent includes 40 - 50 wt% of Si by mass percentage. Based on the total mass of the inoculant, the inoculant includes 75 ± 3 wt% of Si by mass percentage. Based on the total mass of the stream inoculant, the stream inoculant includes 70 - 80 wt% of Si by mass percentage.

[0025] In the embodiment, based on the total mass of the inoculant, the inoculant includes by mass percentage: 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. Based on the total mass of the spheroidizing agent, the spheroidizing agent includes by mass percentage: Mg 4.5 - 6 wt%, RE 0.15 - 0.3 wt%, Si 40 - 50 wt%, and the balance is iron. Based on the total mass of the stream inoculant, the stream inoculant includes by mass percentage: Si 70 - 80 wt%, Bi 0.5 - 2.5 wt%, Ca ≤ 2.0 wt%, Al ≤ 2.0 wt%, and the balance is Fe.

[0026] In the embodiment, 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 spheroidizing agent 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.

[0027] In the embodiment, the inoculant in step (2) includes a primary inoculant and a covering inoculant. Among them, based on the total mass of the primary inoculant, the primary inoculant includes 75 ± 3 wt% of Si by mass percentage. Among them, based on the total mass of the covering inoculant, the covering inoculant includes 75 ± 3 wt% of Si by mass percentage.

[0028] In the embodiment, based on the total mass of the primary inoculant, the primary inoculant comprises by mass percentage: 75±3 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 is iron. Based on the total mass of the covering inoculant, the covering inoculant comprises by mass percentage: 75±3 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 is iron.

[0029] In the embodiment, 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.

[0030] In the embodiment, 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.

[0031] In the embodiment, based on the total mass of the spheroidizing agent, the spheroidizing agent comprises 4.5 - 6.5 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.

[0032] In the embodiment, 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.

[0033] In the embodiment, 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.

[0034] In the embodiment, in step (1) and / or step (2), the microalloying treatment further includes adding TiC to the molten iron, wherein the addition amount of TiC is 0.01 - 0.03 wt% of the total mass of the molten iron.

[0035] In the embodiment, the pig iron in step (1) is pig iron of grade Q10 or above.

[0036] In the embodiment, based on the total mass of the molten iron, the content of Mn is in the range of 0.10 - 0.12 wt%.

[0037] In the embodiment, the content of Mn is adjusted in step (1) so 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.

[0038] In the embodiment, based on the total mass of the molten iron, the content of Ni is 0.35 - 0.45 wt%.

[0039] In the embodiment, based on the total mass of the hot metal, the content of V is 0.001-0.01 wt%.

[0040] In the embodiment, based on the total mass of the hot metal, the content of Si is 2.39-2.45 wt%.

[0041] In the embodiment, based on the total mass of the hot metal, the content of Si is 2.30-2.40 wt%.

[0042] The core structure of the ductile iron according to the embodiment of the present invention is good, and there is basically no abnormal graphite such as fragmented graphite. 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, significantly improving the mechanical properties of the large-section ductile iron. Therefore, the high-toughness and ultra-high-strength ferritic ductile iron provided by the present invention is particularly suitable for mass-producing key components such as large-section castings in wind turbine generators. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Through the following description with reference to the drawings that exemplarily show embodiments of the present invention, the above and other objects and features of the present invention will become clearer. In the drawings: Figure 1A and Figure 1B respectively show the graphite morphologies of ductile iron according to the comparative example and the inventive example; Figure 2A and Figure 2B respectively show the tensile fracture morphologies of ductile iron according to the comparative example and the inventive example; Figure 3A and Figure 3B respectively show the three-dimensional tensile fracture morphologies of ductile iron according to the comparative example and the inventive example; and Figure 4A and Figure 4B respectively show the impact fracture morphologies of ductile iron according to the comparative example and the inventive example. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] In the following description, for the purpose of explanation, many specific details are set forth to provide a thorough understanding of various embodiments or implementations of the invention. As used herein, "embodiment" and "implementation" are interchangeable terms and are non-limiting examples of a device or method that employs one or more of the inventive concepts disclosed herein. However, it is obvious that various embodiments can be practiced without these specific details or with one or more equivalent arrangements. In addition, various embodiments can be different but not necessarily exclusive.

[0045] It should be understood that when the terms "comprising" and / or "including" are used in the specification, the recited materials and / or components are present, but one or more other materials and / or components are not excluded from being present or added.

[0046] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Terms (such as those defined in a general dictionary) should be interpreted as having a meaning consistent with their meaning in the context of the relevant art, and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0047] The present invention provides a ductile iron. Based on the total mass of the ductile iron, the ductile iron 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 according to an embodiment of the present invention may be ferrite.

[0048] The present invention controls the content of each element in the ductile iron, in particular, 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 the low-temperature impact energy at -20 °C is greater than 7 J.

[0049] In an embodiment, the ductile iron according to an exemplary embodiment 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 according to an exemplary embodiment 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%.

[0050] 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 should be strictly controlled.

[0051] Mn is a positive segregation element, which is easy 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. In order 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.

[0052] Ni can be infinitely dissolved in ductile iron, and has no effect on the graphite morphology and the number of eutectic cells. Nickel in the as-cast state can promote the formation of pearlite, 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 rate 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 improve its tensile strength on the premise of not reducing the low-temperature impact toughness of the material, thus making up for the problem of insufficient strength caused by the reduction of 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 should be strictly controlled.

[0053] Excessive contents of Ti and Cr will have adverse effects on the structure and properties of ductile iron. Both Ti and Cr are very active elements, which are prone to generating particles with very high microhardness with C and N. Moreover, Ti and Cr have the characteristics of selective crystallization and are prone to enrichment at grain boundaries and the last solidified parts of castings 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%.

[0054] S and Mg 残 is one of the reasons for forming grain boundary inclusions, reducing impact properties, and being the cause of low-temperature impact properties. Therefore, in order to further improve the properties of ductile iron, its content 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%.

[0055] 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%.

[0056] 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%.

[0057] 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 the technical effects 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 effects 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, on the basis of maintaining the existing strength, make the impact energy meet the requirement of being greater than 7 J at -20 °C.

[0058] Therefore, the ductile iron of the present invention can be widely applied to castings, especially castings in wind turbine generators.

[0059] Next, the preparation method of the ductile iron according to the embodiments of the present invention will be described in detail.

[0060] The preparation method of the ductile iron according to the embodiments of the present invention may include the following steps: (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), wherein, 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.

[0061] Specifically, in step (1) and / or step (2), the microalloying treatment may include adding Ni such that, based on the total mass of the molten iron, the content of Ni is 0.20 - 0.55 wt%. A nodulizer containing Si and an inoculant containing Si may be added in step (2), and a in-mold inoculant containing Si may be added in step (3) such that, based on the total mass of the molten iron, the content of Si is 2.3 - 2.6 wt%. The materials added to the furnace in step (1) may be adjusted such that, based on the total mass of the molten iron, the content of Mn is less than or equal to 0.14 wt%.

[0062] 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 ignored. That is, the total mass of the molten iron in each step in the present invention is basically 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).

[0063] 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 ductile iron (such as improving tensile strength, yield strength, and low-temperature impact energy).

[0064] 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%, within the range of 2.30 - 2.10 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%.

[0065] Specifically, in step (1), 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.

[0066] 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), preferably, the Mn content is controlled within the range of 0.10 - 0.12 wt%. If the Mn content of the molten iron in step (1) is low, ferromanganese can be added to increase the Mn content in the molten iron. However, the embodiments of the present invention are not limited thereto.

[0067] In step (1), the molten iron obtained by smelting can be subjected to carbon increasing treatment. Specifically, a carbon increasing agent can be added to the molten iron to perform carbon increasing treatment on the molten iron. For example, the temperature of the molten iron can be controlled within the range of 1400 °C - 1450 °C, and a carbon increasing agent can be added to the molten iron to perform carbon increasing 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 increasing agent can be greater than 90 wt% and the sulfur content can be less than 0.05 wt%. Under the teaching of the inventive concept, those skilled in the art can select a suitable carbon increasing agent as long as it meets the foregoing conditions.

[0068] In step (2), the molten iron 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, secondly, the spheroidizing agent and the microalloy are laid on the other side of the dam in the spheroidizing ladle and the spheroidizing agent and the microalloy are covered with iron filings, and then the carbon-increased molten iron is added to the spheroidizing ladle, thereby performing the spheroidizing, inoculating, and microalloying treatment process. Alternatively, the molten iron can also be subjected to microalloying treatment in step (1).

[0069] In step (3), a stream inoculant is added during the pouring of the molten iron obtained from step (2) to perform stream inoculation.

[0070] 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 to control Si within the range of 2.25-2.45 wt% based on the total mass of the molten iron in step (2).

[0071] 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 to control the content of Si based on the total mass of the molten iron within the range of 2.3-2.6 wt%. In a preferred embodiment, the inoculant in step (2) may include a primary inoculant and a covering inoculant. In the 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 the 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.

[0072] Specifically, as an example, a primary inoculant can be laid on one side of the dam in the spheroidizing ladle, 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 carburizing 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%.

[0073] 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.

[0074] 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.

[0075] 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 also 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 solution strengthening. Preferably, the mass ratio of TiC to 75 ferrosilicon can be 1:2.

[0076] 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.

[0077] 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.

[0078] 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 respectively controlling the content of Si in step (2) and step (3), 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 2.25 - 2.45 wt%. In step (3), a stream inoculant is added to control the content of Si to 2.3 - 2.6 wt%.

[0079] 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 growth time of graphite 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 growth time of graphite balls is long, and this material relies on solid solution strengthening of silicon. With a high silicon content, large-section castings often have abnormal graphite such as fragmented graphite, and the performance also deteriorates sharply. The ductile iron prepared by the present invention solves the above problems, has a good core structure, and basically no abnormal graphite such as fragmented graphite appears, and is especially suitable for large-section castings.

[0080] 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 all be obtained through commercial purchase.

[0081] Example 1 (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. The element composition of the recarburizer is: carbon > 98.5 wt%, sulfur < 0.05 wt%, ash < 0.5 wt%, moisture < 0.5 wt%. (2)Perform nodulizing, inoculating and microalloying treatments on the molten iron after the recarburizing treatment in step (1): 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; 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; 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; 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; (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.

[0082] 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.

[0083] 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 inevitable impurities.

[0084] Example 2 (1)Use a 20T intermediate - frequency furnace, load high - temperature graphitizing recarburizer and pig iron of grade Q10, carry out melting and recarburizing 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 recarburizer is 0.2 wt% of the total mass of the molten iron, The element composition of the recarburizer is: carbon > 98.5 wt%, sulfur < 0.05 wt%, ash < 0.5 wt%, moisture < 0.5 wt%. (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, covering inoculant, and microalloying Ni from bottom to top in sequence. 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 primary inoculant is laid.

[0085] 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 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 0.46 wt% of the total mass of the molten iron; 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; 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; The element 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; (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 nodular iron casting.

[0086] 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.

[0087] The elements of the obtained nodular iron casting 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 unavoidable impurities.

[0088] Example 3 (1)Use a 20T intermediate frequency furnace, charge high-temperature graphitized carburizer and pig iron of grade Q10, carry out smelting and carburizing treatment at 1430°C to obtain molten iron, add 75 ferrosilicon, and adjust the silicon content in the molten iron to 1.45wt%; add ferromanganese to make the manganese content in the molten iron 0.121wt%; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron. The element composition of the carburizer is: carbon greater than 98.5wt%, sulfur less than 0.05wt%, ash less than 0.5wt%, and moisture less than 0.5wt%. (2)Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1): 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, microalloying Ni, microalloying TiC (the mass ratio of TiC to 75 ferrosilicon is 1:2) and the covering inoculant from bottom to top in sequence, and then cover the microalloying and the covering inoculant with cast iron chips, with a covering thickness of 6mm. Add the molten iron into the spheroidizing ladle from the side where the primary inoculant is laid. Among them, the addition amount of the primary inoculant is 0.45wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1wt% of the total mass of the molten iron, the addition amount of the covering inoculant is 0.09wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.45wt% of the total mass of the molten iron; the addition amount of microalloying TiC is 0.025wt% of the total mass of the molten iron. Among them, the element composition of the primary inoculant is: Si 75±3wt%; Ca 1.0 - 2.0wt%; Ba 2.0 - 3.0wt%; Al<1.5wt%, and the rest is iron. The element composition of the spheroidizing agent is: Si 45.5wt%, Mg 5.1wt%, RE 0.243wt%, Ba 1.31wt%, Ca 0.94wt%, and the rest is iron. The element composition of the covering inoculant is: Si 73.3wt%, C 8.0wt%, V 0.16wt%, Ca 0.45wt%, Ba 1.94wt%, Al 0.52wt%, and the rest is iron. (3)Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.14wt% 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.

[0089] The element composition of the in-stream inoculant is: Si 72.09wt%, Bi 1.23wt%, Ca 1.32wt%, Al 1.09wt%, and the rest is iron.

[0090] The elements of the obtained ductile iron parts include C 3.75wt%, Si 2.45wt%, Mn 0.121wt%, P 0.024wt%, S 0.01wt%, V 0.001wt%, Ni 0.45wt%, Ti 0.021wt%, Cr 0.006wt%, Mg 残 0.04wt%, and the rest is iron and unavoidable impurities.

[0091] Example 4 (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, and adjust the silicon content in the molten iron to 1.15wt%; add ferromanganese to make the manganese content in the molten iron 0.125wt%; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron, The element 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%; (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1): Lay the inoculant on one side of the dam in the 20T spheroidizing ladle. On the other side of the dam, lay the spheroidizing agent, microalloy Ni from bottom to top in sequence, and then cover the microalloy with cast iron chips, with a covering thickness of 6mm. Pour the molten iron into the spheroidizing ladle from the side where the inoculant is laid; Among them, the addition amount of the inoculant is 0.75wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1wt% of the total mass of the molten iron; the addition amount of microalloy Ni is 0.46wt% of the total mass of the molten iron; 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; 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; (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.14wt% 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 part.

[0092] The element composition of the in-stream inoculant is: Si 72.09wt%, Bi 1.23wt%, Ca 1.32wt%, Al 1.09wt%, and the rest is iron.

[0093] The elements of the obtained ductile iron parts include C 3.71wt%, Si 2.31wt%, Mn 0.125wt%, P 0.025wt%, S 0.02wt%, Ni 0.46wt%, Cr 0.011wt%, Mg 残 0.05wt%, and the rest are iron and inevitable impurities.

[0094] Example 5 (1) Use a 20T intermediate frequency furnace, charge 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, and adjust the silicon content in the molten iron to 1.23wt%; add ferromanganese to make the manganese content in the molten iron 0.13wt%; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron, The element 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%; (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1): 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, microalloy Ni from bottom to top, and then cover the microalloy with cast iron chips with a covering thickness of 6mm. Pour the molten iron into the spheroidizing ladle from the side where the inoculant is laid; Among them, the addition amount of the inoculant is 0.54wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1wt% of the total mass of the molten iron; the addition amount of microalloy Ni is 0.46wt% of the total mass of the molten iron; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron; 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; 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; (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.47wt% 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 part.

[0095] The element composition of the in-stream inoculant is: Si 72.09wt%, Bi 1.23wt%, Ca 1.32wt%, Al 1.09wt%, and the rest is iron.

[0096] The elements of the obtained ductile iron parts include C 3.71wt%, Si 2.47wt%, Mn 0.13wt%, P 0.024wt%, S 0.017wt%, Ni 0.46wt%, Cr 0.015wt%, Mg 残 0.048wt%, and the rest is iron and inevitable impurities.

[0097] Example 6 (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 1.44wt%; add ferromanganese to make the manganese content in the molten iron 0.125wt%; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron. The element composition of the carburizer is: carbon greater than 98.5wt%, sulfur less than 0.05wt%, ash less than 0.5wt%, moisture less than 0.5wt%. (2) Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after carburizing treatment in step (1): 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, microalloy Ni from bottom to top in sequence, and then cover the microalloy with cast iron chips, with a covering thickness of 6mm. Pour the molten iron into the spheroidizing ladle from the side where the inoculant is laid. Among them, the addition amount of the inoculant is 0.75wt% of the total mass of the molten iron, the addition amount of the spheroidizing agent is 1.1wt% of the total mass of the molten iron; the addition amount of microalloy Ni is 0.46wt% of the total mass of the molten iron. Among them, the element composition of the inoculant is: Si 75±3wt%; Ca 1.0 - 2.0wt%; Ba 2.0 - 3.0wt%; Al<1.5wt%, and the rest is iron. The element composition of the spheroidizing agent is: Si 45.5wt%, Mg 5.1wt%, RE 0.243wt%, Ba 1.31wt%, Ca 0.94wt%, and the rest is iron. (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.14wt% 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 part.

[0098] The element composition of the in-stream inoculant is: Si 72.09wt%, Bi 1.23wt%, Ca 1.32wt%, Al 1.09wt%, and the rest is iron.

[0099] The elements of the obtained ductile iron parts include: C 3.71wt%, Si 2.60wt%, Mn 0.125wt%, P 0.025wt%, S 0.02wt%, Ni 0.46wt%, Cr 0.011wt%, Mg 残 0.048wt%, and the rest is iron and unavoidable impurities.

[0100] Example 7 (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, and adjust the silicon content in the molten iron to 1.23wt%; add ferromanganese to make the manganese content in the molten iron 0.14wt%; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron. The element composition of the carburizer is: carbon greater than 98.5wt%, sulfur less than 0.05wt%, ash less than 0.5wt%, moisture less than 0.5wt%. (2) Carry out nodulizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1): 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 6mm. Add the molten iron into the nodulizing ladle from the side where the inoculant is laid in the nodulizing ladle. Among them, the addition amount of the inoculant is 0.54wt% of the total mass of the molten iron, the addition amount of the nodulizer is 1.1wt% of the total mass of the molten iron; the addition amount of microalloying Ni is 0.46wt% of the total mass of the molten iron. Among them, the element composition of the inoculant is: Si 75±3wt%; Ca 1.0 - 2.0wt%; Ba 2.0 - 3.0wt%; Al<1.5wt%, and the rest is iron. The element composition of the nodulizer is: Si 45.5wt%, Mg 5.1wt%, RE 0.243wt%, Ba 1.31wt%, Ca 0.94wt%, and the rest is iron. (3) Pouring: Add in-stream inoculant for in-stream inoculation during the pouring process. The addition amount of the in-stream inoculant is 0.47wt% 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 part.

[0101] The element composition of the in-stream inoculant is: Si 72.09wt%, Bi 1.23wt%, Ca 1.32wt%, Al 1.09wt%, and the rest is iron.

[0102] The elements of the obtained ductile iron parts include: C 3.71wt%, Si 2.47wt%, Mn 0.14wt%, P 0.025wt%, S 0.02wt%, Ni 0.46wt%, Cr 0.011wt%, Mg 残 0.048wt%, and the rest is iron and inevitable impurities.

[0103] Example 8 Except that the addition amount of the microalloy Ni in Example 2 is adjusted to 0.2wt% of the total mass of the molten iron, the ductile iron parts are prepared in the same manner as in Example 2.

[0104] The elements of the obtained ductile iron parts include C 3.70wt%, Si 2.47wt%, Mn 0.122wt%, P 0.027wt%, S 0.017wt%, Ni 0.20wt%, Cr 0.009wt%, Mg 残 0.05wt%, and the rest is iron and inevitable impurities.

[0105] Example 9 Except that the addition amount of the microalloy Ni in Example 2 is adjusted to 0.55wt% of the total mass of the molten iron, the ductile iron parts are prepared in the same manner as in Example 2.

[0106] The elements of the obtained ductile iron parts include C 3.70wt%, Si 2.47wt%, Mn 0.122wt%, P 0.027wt%, S 0.017wt%, Ni 0.55wt%, Cr 0.009wt%, Mg 残 0.05wt%, and the rest is iron and inevitable impurities.

[0107] Comparative Example 1 (1) A 20T intermediate frequency furnace is used to load high-temperature graphitizing carburizer and pig iron of grade Q10, and smelting and carburizing treatment are carried out at 1430°C to obtain molten iron; the addition amount of the carburizer is 0.2wt% of the total mass of the molten iron. The element composition of the carburizer is: carbon greater than 98.5wt%, sulfur less than 0.05wt%, ash less than 0.5wt%, and moisture less than 0.5wt%. (2) The molten iron after carburizing treatment in step (1) is subjected to spheroidizing, inoculating and microalloying treatment: An inoculant is laid on one side of the dam in a 20T spheroidizing ladle. On the other side of the dam, a spheroidizing agent and microalloy Ni are laid in sequence from bottom to top, and then cast iron chips are used to cover the microalloy, with a covering thickness of 6mm. The molten iron is added into the spheroidizing ladle from the side where the inoculant is laid. Among them, the addition amount of the inoculant is 0.5 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 the microalloy Ni is 0.45 wt% of the total mass of the molten iron; 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; 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 rest is iron; (3)Pouring: During the pouring process, in-stream inoculant is added for in-stream inoculation, and 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, and it is a ductile iron casting.

[0108] 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.

[0109] 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 inevitable impurities.

[0110] Comparative Example 2 (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, The element composition of the carburizer is: carbon > 98.5%, sulfur < 0.05%, ash < 0.5%, moisture < 0.5%; (2)Carry out spheroidizing, inoculating and microalloying treatment on the molten iron after the carburizing treatment in step (1): Lay the inoculant on one side of the dam in a 20T spheroidizing ladle, and lay the spheroidizing agent and microalloy Ni on the other side of the dam from bottom to top in sequence, and then cover the microalloy with cast iron chips, and the covering thickness is 6 mm. Pour the molten iron into the spheroidizing ladle from the side where the inoculant is laid in the spheroidizing ladle; Among them, the addition amount of the inoculant is 0.54 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 the microalloy Ni is 0.46 wt% of the total mass of the molten iron; 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; 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; (3)Pouring: Inoculation with in-stream inoculant is carried out during the pouring process, 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 the ductile iron casting.

[0111] 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.

[0112] 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.

[0113] Comparative Example 3 (1)Charge a high-temperature graphitizing carburizer and pig iron of grade Q10 into a 20T intermediate-frequency furnace, 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, The element composition of the carburizer is: carbon > 98.5%, sulfur < 0.05%, ash < 0.5%, moisture < 0.5%; (2)Carry out nodulizing, inoculating and microalloying treatment on the molten iron after carburizing treatment in step (1): Lay the primary inoculant on one side of the dam in a 20T nodulizing ladle, and lay the nodulizer, microalloying Ni, microalloying TiC (the mass ratio of TiC to 75 ferrosilicon is 1:2) and covering inoculant from bottom to top on the other side of the dam in turn, then cover the microalloying and covering inoculant with cast iron chips, and the covering thickness is 6 mm, and add the molten iron into the nodulizing ladle from the side where the primary inoculant is laid; 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; 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; 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; 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 0.5'2 wt%, and the rest is iron; (3) Pouring: During the pouring process, in - stream inoculant is added for in - stream inoculation, 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 in the mold to below 400 °C (the slow cooling rate is 10 °C / min), and the casting is cleaned out of the mold, and thus the ductile iron casting is obtained.

[0114] 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.

[0115] 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 inevitable impurities.

[0116] Comparative Example 4 (1) Charge a high - temperature graphitizing carburizer and pig iron of grade Q10 into a 20T intermediate - frequency furnace, 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, The elemental composition of the carburizer is: carbon > 98.5%, sulfur < 0.05%, ash content < 0.5%, moisture content < 0.5%; (2) Carry out spheroidizing, inoculating, and micro - alloying treatment on the molten iron after the carburizing treatment in step (1): 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, micro - alloying Ni, micro - alloying 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 micro - alloying and the covering inoculant with cast iron chips, and the covering thickness is 6 mm. Pour the molten iron into the spheroidizing ladle from the side where the primary inoculant is laid; 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 spheroidizing agent is 1.1 wt% of the total mass of the molten iron, and the addition amount of the covering inoculant is 0.09 wt% of the total mass of the molten iron; the addition amount of the micro-alloy Ni is 0.6 wt% of the total mass of the molten iron; the addition amount of the micro-alloy TiC is 0.025 wt% of the total mass of the molten iron; 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; 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; 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; (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 0.13 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.

[0117] 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.

[0118] The elements of the obtained ductile iron casting 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.

[0119] Comparative Example 5 (1) A 20T intermediate frequency furnace is used to load high-temperature graphitizing carburizer and pig iron of Q10 grade, and melting 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, The elemental composition of the carburizer is: carbon > 98.5 wt%, sulfur < 0.05 wt%, ash content < 0.5 wt%, moisture content < 0.5 wt%; (2) Carry out spheroidizing, 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 spheroidizing ladle. On the other side of the dam, lay the spheroidizing agent, 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 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. 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 microalloying Ni is 0.15 wt% of the total mass of the molten iron; 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; 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 rest is iron; (3) Pouring: Add in-mold inoculant during the pouring process for in-mold inoculation. 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, which is the ductile iron casting.

[0120] The element 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.

[0121] 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 inevitable impurities. 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 influence of different molten iron compositions on the properties of ductile iron castings.

[0122] 1. Effect Evaluation of Ductile Iron (1) Tensile strength, detected by the method specified in GB / T 228.1; (2) Yield strength, detected by the method specified in GB / T 228.1; (3)The low-temperature impact toughness at -20°C was detected by the method specified in GB / T 229.

[0123] The main element compositions and specific test results of the ductile iron in the examples and comparative examples are shown in Table 1 below.

[0124] The main element compositions of the ductile iron were detected using EDS. The instrument used in this experiment was the combination of the Zeiss SUPRA 55 field emission scanning electron microscope and energy spectrometer from Germany.

[0125] Table 1

[0126] It can be seen from the experimental data in Table 1 above that the contents of Si, Mn, and Ni in the test blocks of the ductile iron in Examples 1-9 are respectively in the ranges of 2.3-2.6 wt%, ≤0.14 wt%, and 0.20-0.55 wt%. Therefore, the tensile strength of the test blocks of the 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] During the preparation processes of Examples 1 to 3, in step (1), the Si content was controlled to be 1.4 - 1.6 wt%, and the Mn content was controlled to be less than or equal to 0.14 wt%. In step (2), through microalloying treatment, the Ni content was controlled to be 0.20 - 0.55 wt%, and a spheroidizing agent and an inoculant were added to control the Si content to be 2.25 - 2.45 wt%. In step (3), a stream inoculant was added to control the Si content to be 2.3 - 2.6 wt%.

[0133] During the preparation process of Example 4, the Si content in step (1) was 1.15 wt%, and the Si content was not controlled within the range of 1.4 - 1.6 wt%. The Si content in step (2) was 2.21 wt%, and the Si content was not controlled within the range of 2.25 - 2.45 wt%.

[0134] 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%.

[0135] 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.

[0136] 2. Metallographic Structure of Ductile Iron 2.1 Scanning Electron Microscope Test The scanning electron microscope used in this test was 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 samples. 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.

[0137] 2.2 Three-Dimensional Video Electron Microscope Test 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.

[0138] 2.3 Test Results (1) Graphite Morphology Figure 1A To compare the graphite morphology of the ductile iron in Example 1 Figure 1BThe graphite morphology of the ductile iron in Example 1. It can be seen that compared with Figure 1A compared with Figure 1B , the spheroidization rate of the graphite is higher and the distribution is more uniform.

[0139] (2) Tensile fracture morphology Figure 2A shows the tensile fracture morphology of the ductile iron in Comparative Example 1, Figure 2B shows the tensile fracture morphology of the ductile iron in Example 1. It can be seen from Figure 2B that macroscopically, the undulation degree of the whole 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 section surface, indicating that the bonding ability between the graphite balls and the matrix is relatively weak. It can be seen from Figure 2B that compared with the original specimen, macroscopically, from the overall flatness of the section, the surface roughness and undulation degree of the specimen 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 complete graphite balls in the specimen increases, and the dispersion is more uniform. In addition, Figure 2A in the original specimen of Figure 2B , the cleavage steps with a "river-like" pattern in the fracture morphology are wide and steep, the tear ridges generated in the form of ductile fracture are not obvious, and the number of dimples on the few remaining tear ridges is small, scattered between the ridges. In

[0140] (3) Three-dimensional morphology of the tensile fracture Figure 3A shows the three-dimensional morphology of the tensile fracture of the ductile iron in Comparative Example 1, Figure 3B shows the three-dimensional morphology of the tensile fracture of the ductile iron in Example 1. It can be seen from Figure 3A and Figure 3B that the concavity and convexity of the three-dimensional morphology of the fracture of the two specimens are quite different. Among them, Figure 3A the maximum protrusion height of the tensile fracture of the specimen is 596.4 μm, Figure 3B 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.

[0141] (4) Impact fracture morphology Figure 4A shows the impact fracture morphology of the ductile iron in Comparative Example 1, Figure 4BThe impact fracture morphology of the ductile iron in Example 1. From Figure 4A it can be seen that the number of tear ridges on the fracture of the impact specimen is not large, but the size is relatively large. A certain number of coarse dimples are formed around the tear ridges, and a dimple zone is formed in some parts. At the same time, cleavage planes exist in some parts of the cross-section. This phenomenon indicates that during the fracture process of the impact specimen, quasi-cleavage fracture occurs, belonging to mixed fracture. From Figure 4B it can be seen that the number of dimples in its fracture is Figure 3A compared with that of the specimen, there is an obvious tendency to increase, the length of the tear ridge becomes significantly shorter and thinner, and the degree of bending increases. Under the action of impact stress, adjacent dimples can be connected to form a fracture with a certain directionality.

[0142] The tensile strength of the ductile iron according to the embodiment of the present invention 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 widely used in the castings of wind turbine generators.

[0143] Although the exemplary embodiments of the present invention have been specifically described with reference to the 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 ductile iron, characterized in that, Based on the total mass of the ductile iron, the ductile iron comprises, 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%.

2. The ductile cast iron according to claim 1, characterized in that, The ductile iron further comprises, by mass percentage: V 0.001 - 0.01 wt%, Ti 0.02 - 0.03 wt%.

3. The ductile cast iron according to claim 1, characterized in that, The ductile iron further includes, by mass percentage: P ≤ 0.035 wt%, S ≤ 0.02 wt%, Cr ≤ 0.025 wt%, Mg 残 0.035 - 0.060 wt%.

4. The ductile cast iron according to claim 1, wherein, The matrix of the ductile iron is ferrite.

5. The ductile cast iron according to claim 1, characterized in that, The content of Si is in the range of 2.30 - 2.49 wt%.

6. The ductile cast iron according to claim 1, characterized in that, The content of Si is in the range of 2.35 - 2.49 wt%.

7. The ductile cast iron according to claim 6, characterized in that, The content of Si is in the range of 2.39 - 2.45 wt%.

8. The ductile cast iron according to claim 1, characterized in that, The content of Si is in the range of 2.30 - 2.40 wt%.

9. The ductile cast iron according to claim 1, wherein, The content of Mn is in the range of 0.10 - 0.12 wt%.

10. The ductile cast iron according to claim 3, characterized in that, The content of S is less than 0.015 wt%.

11. The ductile cast iron according to claim 1, characterized in that, The content of Ni is in the range of 0.35 - 0.55 wt%.

12. The ductile cast iron according to claim 11, characterized in that, The content of Ni is in the range of 0.35 - 0.45 wt%.

13. The ductile cast iron according to claim 3, characterized in that, Mg 残 The content of is in the range of 0.040 - 0.054 wt%.

14. The ductile cast iron according to any one of claims 1 to 13, characterized in that, 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.

15. A casting, characterized in that, The casting comprises the ductile iron according to any one of claims 1 to 13.

16. The casting according to claim 15, characterized in that, The wall thickness of the casting is greater than or equal to 60 mm.

17. A wind turbine generator, characterized in that, The wind turbine generator set comprises the casting according to claim 15.

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