Nodular cast iron material with thick and large section and preparation method of nodular cast iron material

Abstract

The invention discloses a nodular cast iron material with a thick and large section and a preparation method thereof. The nodular cast iron material is mainly prepared from the following raw materials: 35-50% of pig iron, 30-50% of scrap steel and 0-30% of foundry returns. And the carburant accounts for 0.45%-0.9% of the total amount of the pig iron, the scrap steel and the foundry returns. According to the technical scheme, on one hand, the silicon content is increased, and on the other hand, all alloy elements in the molten iron are subjected to specific proportion regulation and control, such as carbon (C), manganese (Mn) and other main elements and magnesium (Mg) auxiliary elements; through the adjustment, the nodular cast iron material with the thick and large section has good basic mechanical properties, the prepared as-cast nodular cast iron with the thick and large section is ferrite nodular cast iron, the tensile strength Rm of the as-cast nodular cast iron is larger than or equal to 650 MPa, the Rp yield strength is larger than or equal to 500 MPa, and the elongation A is larger than or equal to 7%.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of cast metal, in particular to a high-strength and high-toughness thick-section nodular cast iron material and a preparation method thereof. BACKGROUND

[0002] Thick-section nodular cast iron refers to large nodular cast iron parts with a wall thickness exceeding 100 mm. In actual industrial applications, the wall thickness of key components can even reach more than 300 mm. Such castings have an irreplaceable position in the fields of nuclear power, wind power, heavy machinery and large mining equipment, because they can withstand extremely high static and dynamic loads while maintaining excellent wear resistance and structural integrity. With the development of the national major equipment manufacturing industry towards large-scale and integration, the demand for thick-section nodular cast iron parts is becoming increasingly strong.

[0003] However, the production of thick-section nodular cast iron faces many technical difficulties. For example, the iron liquid of such products is heavy, and the solidification speed of the iron liquid is slow during pouring into the cavity, and the solidification time can last for several hours (for example, the solidification time of a 250mm x 250mm x 300mm sample can be more than 95 minutes). During this long solidification process, the physical and chemical state inside the iron liquid will change complexly, leading to a series of quality control difficulties; the main manifestations are: the number of graphite balls decreases, the graphite morphology is distorted (including the formation of chunky graphite), the matrix structure is abnormal (such as abnormal increase of ferrite content), and the element segregation is serious, etc. These organizational defects directly lead to the decrease of the mechanical properties of the castings, especially the tensile strength and impact toughness of the core, which are difficult to meet the design requirements. Therefore, how to prepare a thick-section as-cast nodular cast iron with high strength and high toughness is crucial. SUMMARY

[0004] In view of the above shortcomings of the prior art, the application provides a thick-section nodular cast iron material with ferrite as the matrix structure, so that the thick-section castings have good strength and toughness.

[0005] In order to solve the above technical problems, the technical scheme adopted by the application is as follows: a thick-section nodular cast iron material, the preparation raw materials of the material mainly include: pig iron 35% to 50%, scrap steel 30% to 50%, and recycled material 0% to 30%; carbon additive, accounting for 0.45% to 0.9% of the total mass of pig iron, scrap steel and recycled material.

[0006] Further, the preparation raw materials of the material mainly include: pig iron 35% to 40%, scrap steel 30% to 40%, and recycled material 10% to 30%; carbon additive, accounting for 0.45% to 0.8% of the total mass of pig iron, scrap steel and recycled material.

[0007] Further, the preparation raw material of the material mainly comprises: pig iron 40%-50%, scrap steel 35%-40%, and recycled material 25%-30%; carbon additive, accounting for 0.5%-0.8% of the total amount of pig iron, scrap steel and recycled material.

[0008] Further, the preparation raw material of the material further comprises FeSi75 ferrosilicon, and the FeSi75 ferrosilicon accounts for 3%-5% of the total mass of pig iron, scrap steel and scrap steel.

[0009] Further, the preparation raw material of the material further comprises pure antimony, and the addition amount of the pure antimony is 0.003%-0.005% of the mass of the original iron liquid obtained after the mixed melting of the pig iron, scrap steel, recycled material, carbon additive and FeSi75 ferrosilicon.

[0010] Further, the preparation raw material of the material further comprises electrolytic copper, and the addition amount of the electrolytic copper is 0.30%-0.60% of the mass of the original iron liquid obtained after the mixed melting of the pig iron, scrap steel, recycled material, carbon additive and FeSi75 ferrosilicon.

[0011] The application also provides a preparation method of the thick-section nodular cast iron material, and the steps of the method comprise: (1) sand casting: a small sample wooden mold with consistent entity structure size is designed, the size of the wooden mold is 200mm*200mm*200mm, then a heating riser with an inner diameter of 80mm is used to communicate with the casting cavity, bottom pouring is adopted, and the casting cavity is obtained after the sand mold is filled and the pouring structure is designed; (2) pouring production: the following raw materials are weighed: pig iron 35%-50%, scrap steel 30%-50%, recycled material 0%-30%, and carbon additive, accounting for 0.45%-0.9% of the total mass of pig iron, scrap steel and recycled material; (3) all the pig iron, scrap steel and recycled material are put into a medium-frequency induction furnace, 0.45%-0.9% of carbon additive is added in the middle of the feeding, the furnace charge is heated to be melted, FeSi75 ferrosilicon is added after the furnace charge is completely melted, the addition amount of the ferrosilicon is 3%-5% of the total mass of pig iron and scrap steel and recycled material, and the original iron liquid is obtained; the original iron liquid is continuously heated to 1450-1490℃, and the composition and mass percentage of the original iron liquid obtained are as follows: C 3.00%-3.30%, Si 3.0%-3.6%, Mn <0.30%, P≤0.03%, S≤0.020%, and the rest is iron; (4) using the method of pouring into the spheroidization, spheroidization package one side of the spheroidization dam in the first plus spheroidizing agent and compact, spheroidizing agent for light rare earth magnesium alloy, the quality of the original iron liquid obtained in step (3) is 1.2%~1.3%, adding 0.003%~0.005% of pure antimony to the original iron liquid obtained in step (3), then adding the inoculant with a particle size of 3-8mm and compacting, the mass of the inoculant is 0.3%~0.6% of the mass of the original iron liquid obtained in step (3); finally, adding 0.30%~0.60% of the electrolytic copper to the original iron liquid obtained in step (3) to carry out spheroidization and inoculation; the composition and mass percentage of the iron liquid obtained after the above spheroidization and inoculation are: C 2.9%~3.2%, Si 3.8~4.40%, Mn <0.30%, P≤0.03%, S≤0.020%, Cu 0.30%~0.60%, RE (rare earth) ≤0.01%, Mg 0.035%~0.06%, Sb 0.003%~0.005%, the rest is iron; (5) the iron liquid after spheroidization and inoculation treatment in step (4) is slagged, and is allowed to stand, when the temperature of the iron liquid is reduced to 1300℃~1320℃, the iron liquid is poured into the casting cavity in the pouring structure to form a casting, and in-process stream inoculation is carried out during pouring; the in-process stream inoculation is to set an in-process inoculation structure in the pouring structure, and the in-process inoculation structure is pre-embedded with inoculation blocks; after the casting is cooled, the casting is obtained.

[0012] Further, the recarburizer in step (2) is a recarburizer with an element mass percentage of C≥98%, S≤0.05%, N≤0.01%, ash content (ash) ≤0.3%, volatile content (volatile) ≤0.3%, and a particle size of 0.5-3mm, such as the DC series recarburizer (DC-(1-4) type recarburizer) produced by Dan Sheng Industry (Shanghai) Co., Ltd.

[0013] Further, the spheroidizing agent in step (4) is a rare earth magnesium alloy: Mg 5.5%~6.5%, RE 0.8%~1.2%, Si 38%~42%, Ca 1.5%~2.5%, Ba 2.0%~3.0%, Al≤1.2%, MgO≤0.70%, and the balance is Fe.

[0014] Further, the spheroidization reaction time in step (4) is controlled to be completed within 100s~120s, which can improve the absorption rate of magnesium and rare earth, enhance the desulfurization effect, and correspondingly reduce the amount of spheroidizing agent added.

[0015] Further, the inoculant in step (4) and step (5) is a silicon-barium inoculant, and the element mass percentage is Si 71%~73%, Ca 0.7%~1.3%, Ba 1.6%~2.4%, Al≤1.2%, S≤0.02%, and the balance is iron.

[0016] Further, the added amount of the inoculant in the mold inoculation structure in step (5) is 0.015%-0.03% of the total weight of the poured molten iron.

[0017] The advantages and beneficial effects of the present application are as follows:

[0018] 1. In the spheroidal graphite cast iron, silicon significantly improves the strength, hardness and yield strength of the ferrite matrix through strong solid solution strengthening effect; it is the core means to regulate the mechanical properties of spheroidal graphite cast iron, especially for ferrite spheroidal graphite cast iron (such as QT400-18, QT450-10); this makes it possible to have "high strength and high toughness" ferrite spheroidal graphite cast iron; therefore, in the raw material of the molten iron of the present application, thick section high strength and high toughness spheroidal graphite cast iron material is prepared by adding silicon element solid solution strengthening (high silicon content Si 3.8~4.40%).

[0019] 2. On the one hand, the present application improves the silicon content, and on the other hand, the specific proportion of each alloying element in the molten iron is regulated, such as carbon (C), manganese (Mn) and other main elements and magnesium (Mg) auxiliary elements; these adjustments make the thick section spheroidal graphite cast iron material have good basic mechanical properties, and the as-cast thick section spheroidal graphite cast iron prepared is ferrite spheroidal graphite cast iron, the as-cast spheroidal graphite cast iron has a tensile strength R m ≥650MPa, a yield strength R p ≥500MPa, and an elongation A≥7%; adding electrolytic copper in the raw material of the present application and controlling the reasonable addition amount can improve the strength and hardness of the final casting, and can also promote the formation and stability of pearlite in spheroidal graphite cast iron, can realize the formation of spheroidal graphite cast iron, avoid the occurrence of flaky defects, and also can improve the corrosion resistance of the casting; adding trace amount of pure antimony in the raw material of the present application and controlling the precise addition amount can promote and stabilize the pearlite structure, so as to improve the strength, hardness and wear resistance of the casting, and can make the structure more fine and uniform, and also can prevent the generation of ferrite and distorted graphite; and the reasonable content of rare earth and other elements are controlled to cooperate with each other, and the anti-spheroidization effect is reduced.

[0020] 3. The manufacturing process of the present application can make the graphite spheroidization rate reach more than 90% through different stages and different levels of inoculation means, the graphite size is 6 levels, the hardness is >230HB, and the as-cast sample prepared has good performance index and relatively low production cost. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 Sampling locations for the cast obtained in Example 1.

[0022] Figure 2 First metallographic picture of the cast obtained in Example 1.

[0023] Figure 3 Second metallographic picture of the cast obtained in Example 1.

[0024] Figure 4 Sampling locations for the cast obtained in Example 2.

[0025] Figure 5 First metallographic picture of the cast obtained in Example 2.

[0026] Figure 6 Second metallographic picture of the cast obtained in Example 2.

[0027] Figure 7 Sampling locations for the cast obtained in Example 3.

[0028] Figure 8 First metallographic picture of the cast obtained in Example 3.

[0029] Figure 9 Second metallographic picture of the cast obtained in Example 3. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the embodiments and drawings. Obviously, the described embodiments are only preferred embodiments, but not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application. It is further noted that when a member is referred to as being "on" another member, it can be directly on the other member or intervening members can also be present. When a member is referred to as being "connected" to another member, it can be directly connected to the other member or intervening members can also be present. When a member is referred to as being "disposed on" another member, it can be directly disposed on the other member or intervening members can also be present. The terms "vertical", "horizontal", "left", "right", and similar expressions used herein are for illustrative purposes only. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0031] The embodiment takes a casting cavity formed by sand casting as an example: a small sample wooden mold with the same size as the entity size of the casting is designed and the size of the wooden mold is 200mmx200mmx200mm, which is used to form a cavity structure consistent with the casting in the sand (the cavity of the casting is formed by using a conventional wooden mold sand casting process in the art, which does not need to be described in detail here), then a Ф80 (inner diameter 80mm) is used as a hot riser (the hot riser is communicated with the cavity for supplementing the iron liquid or for the role of iron liquid outgas during pouring and cooling), then the bottom pouring method is used, and the pouring structure of the final casting is designed to obtain the casting cavity after the box is filled.

[0032] Example 1 (1) The following mass percentages of raw materials are weighed: pig iron 35%, scrap steel 40%, return material 25%, and carbon additive: 0.8% of the total amount of pig iron, scrap steel, and return material; (2) Put all the pig iron, scrap steel, return material, and carbon additive weighed in step (1) into a smelting furnace, heat to melt the charge, and after the charge is melted, add FeSi75 ferrosilicon, the amount of which is 4% of the total mass of pig iron, scrap steel, and return material, to obtain a primary iron liquid, and continue to heat the primary iron liquid to 1450℃; the composition and mass percentage of the primary iron liquid are measured as follows: C 3.10%, Si 3.40%, Mn 0.27%, P 0.028%, S 0.020%, and the rest is iron; (3) Spheroidization: spheroidization is carried out by the impingement method, spheroidizing agent is first added to one side of the spheroidizing dam and compacted, 0.0035% of the mass of the primary iron liquid is added as pure antimony, the spheroidizing agent is light rare earth magnesium alloy with a mass of 1.2% of the mass of the primary iron liquid, 3-8mm particle size inoculant is then added and compacted, the mass of the inoculant added is 0.3% of the mass of the primary iron liquid, and finally 0.30% of the mass of the primary iron liquid is added as electrolytic copper for spheroidization and inoculation; after spheroidization and inoculation, the composition and mass percentage of the iron liquid are measured as follows: C 3.0%, Si 4.05%, Mn 0.28%, P 0.028%, S 0.011%, Cu 0.30%, RE ≤0.01%, Mg 0.045%, Sb 0.0035%, and the rest is iron; (4) the iron liquid after spheroidizing and inoculating treatment is slagged, and is left to stand, and when the temperature drops to 1300℃, the iron liquid is poured into the casting cavity in the pouring structure to form a casting, and in the pouring process, in-mold inoculation is carried out; the pouring structure contains an in-mold inoculation structure (the in-mold inoculation structure can be an isosceles trapezoidal structure or a square structure, is fixed in the casting cavity, and an inoculation block is stacked in the in-mold inoculation structure, and the inoculation block can be fully contacted and melted with the poured iron liquid), and the in-mold inoculation structure is pre-embedded with an inoculation block (the inoculation iron block accounts for 0.015% of the weight of the liquid iron); after the casting is cooled, the casting is obtained, and the sleeve material rod is obtained after machining.

[0033] The physical properties of the casting test rod obtained in Example 1 are shown in Tables 1 and 2; The sampling position is shown in Figure 1 The metallographic picture is shown in Figure 3 , Figure 4 The microstructure of the sample before etching is shown in Figure 3 , and the microstructure of the sample after etching is shown in Figure 4 , and the same is true in Examples 2-3; Table 1 Mechanical properties of the casting test rod obtained in Example 1 Item Tensile strength (Mpa) Yield strength (Mpa) Elongation (%) Hardness (HB) Measured value 656 514 10 232 Table 2 Metallographic structure of the casting test block obtained in Example 1 Item Spheroidizing rate Graphite size Measured value 94.95% 6 levels Example 2

[0034] (1) the following mass percentages of raw materials are weighed: pig iron 40%, scrap steel 30%, recycled material 30%, and carbon additive: 0.45% of the total amount of pig iron, scrap steel and recycled material; (2) put all the pig iron, scrap steel and carbon additive into a smelting furnace, heat to melt the charge, and after the charge is melted, add FeSi75 ferrosilicon, the amount of ferrosilicon added is 3.7% of the total mass of pig iron, scrap steel and recycled material, obtain the original iron liquid, and continue to heat the original iron liquid to 1450℃; the measured composition and mass percentage of the original iron liquid are C 3.20%, Si 3.50%, Mn 0.25%, P 0.025%, S 0.018%, and the rest is iron; (3) Spheroidization: Spheroidization is carried out by the injection method. Spheroidizing agent is first added and compacted in the spheroidizing dam on one side of the spheroidizing package. 0.004% pure antimony of the original molten iron mass is added. The spheroidizing agent is light rare earth magnesium alloy, with a mass of 1.25% of the original molten iron mass. Then, an inoculant with a particle size of 3-8 mm is added and compacted. The mass of the inoculant is 0.35% of the original molten iron mass. Finally, 0.55% electrolytic copper of the original molten iron mass is added before spheroidization and inoculation. The composition and mass percentage of the molten iron after spheroidization and inoculation are as follows: C 3.1%, Si 4.25%, Mn 0.25%, P 0.025%, S 0.010%, Cu 0.55%, RE ≤0.01%, Mg 0.051%, Sb 0.0039%, and the remainder is iron. (4) After spheroidizing and inoculation treatment, the molten iron is slag removed and allowed to stand. When the temperature drops to 1320℃, the molten iron is poured into the casting cavity in the casting structure to form a casting. In-mold inoculation is carried out during the pouring process. The casting structure contains an in-mold inoculation structure, and an inoculation block (0.025% of the weight of the molten iron) is pre-embedded in the in-mold inoculation structure. After the casting cools, the casting is obtained and machined to obtain the bushing.

[0035] The physical properties of the casting test bars were measured and are shown in Tables 3 and 4. Sampling location such as Figure 4 As shown, the metallographic image is as follows Figure 5 , Figure 6 As shown; Table 3 Mechanical properties of casting test bars from Example 2 Item Tensile strength (Mpa) Yield strength (Mpa) Elongation (%) Hardness (HB) Measured value 675 534 7 251 Table 4. Metallographic structure of the cast test blocks from Example 2 Item Spheroidizing rate Graphite size Measured value 92.37% 6 levels Example 3

[0036] (1) Weigh the following raw materials by mass percentage: 50% pig iron, 40% scrap steel, 10% recycled material, and 0.5% carbon raiser of the total amount of pig iron, scrap steel, and recycled material; (2) Put all the pig iron, scrap steel and carbon raiser into the smelting furnace and heat it to melt the furnace charge. After the furnace charge is melted, add FeSi75 ferrosilicon. The amount of ferrosilicon added is 4.8% of the total mass of pig iron, scrap steel and recycled material to obtain the original molten iron. Continue to heat the original molten iron to 1450℃. The composition and mass percentage of the original molten iron were measured to be C 3.00%, Si 3.50%, Mn 0.24%, P 0.028%, S 0.021%, and the remainder was iron. (3) Spheroidization: Spheroidization is carried out by the injection method. Spheroidizing agent is first added and compacted in the spheroidizing dam on one side of the spheroidizing package. 0.0045% pure antimony of the original molten iron mass is added. The spheroidizing agent is light rare earth magnesium alloy, with a mass of 1.3% of the original molten iron mass. Then, an inoculant with a particle size of 3-8 mm is added and compacted. The mass of the inoculant is 0.4% of the original molten iron mass. Finally, 0.45% electrolytic copper of the original molten iron mass is added before spheroidization and inoculation. The composition and mass percentage of the molten iron after spheroidization and inoculation are as follows: C 2.95%, Si 4.35%, Mn 0.24%, P 0.028%, S 0.0098%, Cu 0.45%, RE≤0.01%, Mg 0.055%, Sb 0.0044%, and the remainder is iron. (4) After spheroidizing and inoculation treatment, the molten iron is slag removed and allowed to stand. When the temperature drops to 1310℃, the molten iron is poured into the casting cavity in the casting structure to form a casting. In-mold inoculation is carried out during the pouring process. The casting structure contains an in-mold inoculation structure, and an inoculation block (0.03% of the weight of the molten iron) is pre-embedded in the in-mold inoculation structure. After the casting is cooled, the casting is obtained and machined to obtain the sleeve bar.

[0037] The physical properties of the casting test bars were measured and are shown in Tables 5 and 6. Sampling location such as Figure 7 As shown, the metallographic image is as follows Figure 8 , Figure 9 As shown; Table 5 Mechanical properties of casting test bars from Example 3 Item Tensile strength (Mpa) Yield strength (Mpa) Elongation (%) Hardness (HB) Measured value 677 505 8.5 237 Table 6. Metallographic results of the cast test blocks in Example 3 Item Spheroidizing rate Graphite size Measured value 91.93% 6 levels The mechanical properties and metallographic structure of the test blocks obtained through the above embodiments show that the ductile iron material obtained by the technical solution of this application has "high strength and high toughness"; the prepared as-cast thick-section ductile iron is ferritic ductile iron, with a tensile strength Rm≥650MPa, a yield strength Rp≥500MPa, and an elongation A≥7%; through different stages and different levels of inoculation methods, the graphite spheroidization rate can reach more than 90%, the graphite size is grade 6, the hardness is >230HB, and the prepared as-cast sample has good performance indicators and relatively low production cost.

Claims

1. A thick-section ductile iron material, characterized in that: The main raw materials for preparing this material include: 35%–50% pig iron, 30%–50% scrap steel, and 0%–30% recycled material; and a carbon raiser, accounting for 0.45%–0.9% of the total mass of pig iron, scrap steel, and recycled material.

2. The thick-section ductile iron material according to claim 1, characterized in that: The raw materials for preparing the material mainly include: 35% to 40% pig iron, 30% to 40% scrap steel, and 10% to 30% recycled material; and a carbon raiser, accounting for 0.45% to 0.8% of the total amount of pig iron, scrap steel, and recycled material.

3. The thick-section ductile iron material according to claim 1, characterized in that: The raw materials for preparing the material mainly include: 40% to 50% pig iron, 35% to 40% scrap steel, and 25% to 30% recycled material; and a carbon raiser, accounting for 0.5% to 0.8% of the total amount of pig iron, scrap steel, and recycled material.

4. The thick-section ductile iron material according to claim 1, characterized in that: The raw materials for preparing the material also include FeSi75 ferrosilicon, which accounts for 3% to 5% of the total mass of pig iron, scrap steel and recycled materials.

5. The thick-section ductile iron material according to claim 1, characterized in that: The raw materials for preparing the material also include pure antimony, and the amount of pure antimony added is 0.003% to 0.005% of the mass of the original molten iron obtained after mixing and melting pig iron, scrap steel, recycled material, carbon raiser and FeSi75 ferrosilicon.

6. The thick-section ductile iron material according to claim 1, characterized in that: The raw materials for preparing the material also include electrolytic copper, and the amount of electrolytic copper added is 0.30% to 0.60% of the mass of the original molten iron obtained after mixing and melting pig iron, scrap steel, recycled material, carbon raiser and FeSi75 ferrosilicon.

7. A method for preparing thick-section ductile iron material according to any one of claims 1-6, characterized in that: The steps of this method include: (1) Sand casting: Design a small wooden mold with the same dimensions as the solid structure of the casting. The size of the wooden mold is 200mm×200mm×200mm. Then, use a Ф80 heating riser to connect with the casting cavity. Bottom pouring is adopted. After designing the pouring structure, the casting cavity is obtained by filling the box sand casting. (2) Casting production: Weigh the following raw materials by mass percentage: 35% to 50% pig iron, 30% to 50% scrap steel, 0% to 30% recycled material; carbon raiser, accounting for 0.45% to 0.9% of the total mass of pig iron, scrap steel, and recycled material; (3) Put all the pig iron, scrap steel and recycled materials into the medium frequency induction furnace, and add 0.45% to 0.9% of carbon raiser during the charging process; heat the furnace charge to melt it, and after the furnace charge is melted, add FeSi75 ferrosilicon. The amount of ferrosilicon added is 3% to 5% of the total mass of pig iron, scrap steel and recycled materials to obtain the original molten iron; continue to heat the original molten iron to 1450-1490℃. The composition and mass percentage of the obtained original molten iron are: C 3.00% to 3.30%, Si 3.0% to 3.6%, Mn < 0.30%, P ≤ 0.03%, S ≤ 0.020%, and the remainder is iron; (4) Spheroidization is carried out by the spheroidizing method. First, a spheroidizing agent is added and compacted inside the spheroidizing dam on one side of the spheroidizing package. The spheroidizing agent is a light rare earth magnesium alloy, and its mass is 1.2% to 1.3% of the mass of the original molten iron obtained in step (3). Then, 0.003% to 0.005% of pure antimony is added, and then an inoculant with a particle size of 3-8 mm is added and compacted. The mass of the inoculant is 0.3% to 0.6% of the mass of the original molten iron obtained in step (3). Finally, the inoculant from step (3) is added. The obtained molten iron, after being mixed with 0.30%–0.60% electrolytic copper, is then spheroidized and inoculated. The composition and mass percentage of the molten iron obtained after spheroidization and inoculation are as follows: C 2.9%–3.2%, Si 3.8–4.40%, Mn < 0.30%, P ≤ 0.03%, S ≤ 0.020%, Cu 0.30%–0.60%, RE ≤ 0.01%, Mg 0.035%–0.06%, Sb 0.003%–0.005%, with the remainder being iron. (5) Remove the slag from the molten iron after spheroidization and inoculation treatment in step (4) and let it stand. When the temperature of the molten iron drops to 1300℃~1320℃, pour the molten iron into the casting cavity in the casting structure to form a casting. In the pouring process, in-mold inoculation is carried out. The in-mold inoculation is to set an in-mold inoculation structure in the casting structure. The inoculation block is pre-embedded in the inoculation structure. After the casting cools down, the casting is obtained.

8. The method for preparing thick-section ductile iron material according to claim 7, characterized in that: The carbon raiser mentioned in step (2) is a carbon raiser with an elemental mass percentage of C≥98%, S≤0.05%, N≤0.01%, ash content≤0.3%, volatile matter≤0.3%, and particle size of 0.5-3mm.

9. The method for preparing thick-section ductile iron material according to claim 7, characterized in that: The spheroidizing agent mentioned in step (4) is a rare earth magnesium alloy: Mg 5.5%~6.5%, RE 0.8%~1.2%, Si 38%~42%, Ca 1.5%~2.5%, Ba 2.0%~3.0%, Al≤1.2%, MgO≤0.70%, with the balance being Fe.

10. The method for preparing thick-section ductile iron material according to claim 7, characterized in that: In step (4), the spheroidizing reaction time is controlled to be completed within 100s to 120s, which can improve the absorption rate of magnesium and rare earth, enhance the desulfurization effect, and correspondingly reduce the amount of spheroidizing agent added. The inoculant in step (4) is a barium silicon inoculant, whose elemental mass percentage is 71% to 73% Si, 0.7% to 1.3% Ca, 1.6% to 2.4% Ba, Al≤1.2%, S≤0.02%, and the balance is iron.

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