A method for producing high-toughness nodular cast iron

Abstract

The application discloses a preparation method of high-toughness nodular cast iron and belongs to the technical field of cast iron manufacturing. The method selects low-phosphorus and low-sulfur cast pig iron and low-carbon carbon scrap steel, obtains pure molten iron through vacuum nitrogen protection smelting and slag removal and desulfurization, adopts silicon-barium inoculant to compoundly coat, screen and ultrasonic disperse rare earth magnesium spheroidizing agent, and prepares the compound spheroidizing inoculant with uniform dispersion; the bismuth alloy and zirconium alloy are subjected to vacuum non-oxidation pretreatment, the compound spheroidizing inoculant, bismuth-zirconium alloy and covering agent are placed in a spheroidizing ladle, and the pretreated molten iron is poured to complete spheroidizing inoculation, and the high-toughness nodular cast iron is prepared through pouring forming under high-purity nitrogen protection and secondary isothermal annealing. The application can make the nodular cast iron have high spheroidizing rate and excellent low-temperature toughness through the silicon-barium inoculation and bismuth-zirconium synergistic effect to inhibit the grain boundary segregation of bismuth elements, and the stable structure can be obtained through the pure smelting and annealing process.

Description

Technical Field

[0001] This invention relates to the field of cast iron manufacturing technology, specifically to a method for preparing high-toughness ductile iron. Background Technology

[0002] Ductile iron has the combined advantages of high strength, good plasticity and low cost, making it a core structural material in high-end manufacturing fields such as machinery and equipment, rail transportation and wind power components. As application scenarios expand towards low temperature, heavy load and high reliability, the industry has put forward extremely high requirements for the low temperature toughness of ductile iron, and the graphite spheroidization rate and microstructure uniformity are the key indicators that determine its toughness level.

[0003] Existing technologies typically improve graphite spheroidization and roundness by adding bismuth. However, bismuth, as a surface-active element, readily agglomerates at grain boundaries, forming brittle phases that directly reduce the material's low-temperature toughness. Simultaneously, conventional rare-earth magnesium spheroidizing agents lack composite modification treatment, resulting in insufficient graphite nucleation sites and poor microstructure refinement. Furthermore, the lack of inert protection during molten iron smelting makes them prone to oxidation inclusions and harmful gases, failing to simultaneously address the core contradiction between improving spheroidization and deteriorating toughness. Current mainstream preparation processes can only optimize either spheroidization or toughness individually, failing to achieve a synergistic balance between bismuth distribution control, graphite microstructure refinement, improved molten iron purity, and residual stress elimination. This makes it difficult to produce ductile iron with both high spheroidization and excellent low-temperature toughness.

[0004] Therefore, it is necessary to provide a method for preparing high-toughness ductile iron to solve the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-toughness ductile iron, so as to solve the technical problem of poor low-temperature toughness in traditional high-sphericity ductile iron due to bismuth grain boundary segregation, insufficient synergy between inoculation and alloy control.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing high-toughness ductile iron, comprising the following steps: (1) Pretreatment of furnace charge: Pig iron and scrap steel are added to the medium frequency induction furnace, and after vacuuming, nitrogen is introduced for protection. After smelting, molten iron is obtained. The molten iron is deslagified and desulfurized to obtain pretreated molten iron. (2) Preparation of spheroidizing inoculant: The rare earth magnesium spheroidizing agent was subjected to drying pretreatment, silicon-barium composite coating modification, particle size sieving and ultrasonic dispersion in sequence to obtain composite spheroidizing inoculant; (3) Bismuth-zirconium pretreatment: Bismuth alloy particles and zirconium alloy particles are subjected to non-oxidative pretreatment; (4) Spheroidizing inoculation: The composite spheroidizing inoculator, bismuth alloy particles, zirconium alloy particles and covering agent are placed in the spheroidizing ladle, and then the pretreated molten iron is poured in to carry out spheroidizing inoculation treatment to obtain spheroidized inoculated composite molten iron. (5) Casting heat treatment: The spheroidized inoculated composite molten iron is cast into shape under the protection of inert gas, and the casting is subjected to secondary isothermal annealing to obtain high toughness ductile iron.

[0007] Preferably, in step (2), the drying pretreatment specifically includes the following steps: 1a) Place the rare earth magnesium spheroidizing agent in a vacuum drying oven and dry it at 80~100℃ for 1~2 hours; 2a) The dried rare earth magnesium spheroidizing agent is screened to remove lumps and impurities; 3a) The screened spheroidizing agent is sealed and stored in a nitrogen atmosphere with a purity of ≥99.99% for later use, thus obtaining a pretreated rare earth magnesium spheroidizing agent.

[0008] Preferably, in step (2), the silicon-barium composite coating modification specifically includes the following steps: 1b) Add the pretreated rare earth magnesium spheroidizing agent to anhydrous ethanol and disperse it evenly by ultrasonication to obtain a spheroidizing agent dispersion; 2b) Add a barium silicon inoculant to the spheroidizing agent dispersion. The barium silicon inoculant is a barium silicon iron-based inoculant alloy, with a Si content of 60%–75% and a Ba content of 3%–8% by mass, and the balance being Fe. The amount of the barium silicon inoculant is 5%–10% of the mass of the rare earth magnesium spheroidizing agent. The reaction is carried out at a constant temperature of 40–50°C for 20–30 min. 3b) After the reaction is complete, anhydrous ethanol is removed by vacuum distillation to obtain the barium silicon coated modified spheroidizing agent.

[0009] Preferably, in step (2), the particle size sieving specifically includes the following steps: 1c) Crushing the barium silicon-coated modified spheroidizing agent; 2c) Screen and collect modified spheroidizing agent particles with a particle size of 3~8mm; 3c) The sieved modified spheroidizing agent is sealed in an inert gas atmosphere to obtain a sieved composite spheroidizing agent.

[0010] As a preferred embodiment, in step (2): the qualified composite spheroidizing agent is subjected to ultrasonic dispersion treatment with an ultrasonic power of 80~120W and a treatment time of 5~10min to obtain the composite spheroidizing inoculant.

[0011] Preferably, in step (1): the pig iron is low-phosphorus and low-sulfur foundry pig iron with a carbon content of 3.5wt% to 4.2wt%, P ≤ 0.03wt%, and S ≤ 0.02wt%; the scrap steel is low-carbon scrap steel with a carbon content ≤ 0.25wt%, P ≤ 0.03wt%, and S ≤ 0.02wt%; when adding pig iron and scrap steel to the medium-frequency induction furnace, ductile iron remelting material is added, and the mass ratio of pig iron, scrap steel, and ductile iron remelting material is 70:20:10 to 80:10:10; vacuum is applied during smelting to a vacuum degree ≤ -0.08MPa; the slag removal and desulfurization temperature is 1420 to 1450℃, and the holding time is 0.5 to 1h.

[0012] Preferably, in step (3): the bismuth alloy particles have a particle size of 1~3mm and are added at 0.002%~0.0035% of the mass of molten iron; the zirconium alloy particles have a particle size of 1~3mm and are added at 0.02%~0.05% of the mass of molten iron; the non-oxidative pretreatment of the bismuth alloy particles and zirconium alloy particles is to dry them at 80~100℃ in a vacuum environment for 1~2h.

[0013] Preferably, in step (4), the covering agent is one or more of perlite and silicon carbide, and the amount of the covering agent is 0.5% to 1.5% of the total mass of molten iron.

[0014] As a preferred embodiment, in step (4): the spheroidizing inoculation temperature is 1430~1460℃; the amount of composite spheroidizing inoculator added is 1.0%~1.3% of the mass of molten iron; the spheroidizing reaction time is 1~2 min, and the reaction is allowed to stand for 3~5 min.

[0015] Preferably, in step (5): the inert gas is high-purity nitrogen; the secondary isothermal annealing temperature is 720~760℃, the holding time is 2~3h, the furnace is cooled to below 500℃, and then the furnace is taken out and air-cooled to room temperature.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention utilizes low-phosphorus and low-sulfur cast iron and low-carbon scrap steel, combined with vacuum nitrogen-protected smelting and high-temperature slag removal and desulfurization processes to form a basic synergistic effect. At the raw material stage, the content of impurities such as phosphorus and sulfur, which are prone to causing grain boundary embrittlement, is strictly controlled. At the same time, the vacuum environment is used to reduce the oxidation and gas absorption of molten iron, and the nitrogen atmosphere is used to isolate the entry of harmful external gases. Then, the high-temperature slag removal and desulfurization process thoroughly removes the oxide inclusions and sulfide impurities in the melt. This effectively avoids the superposition of impurities such as phosphorus and sulfur with bismuth elements, which can cause more serious grain boundary embrittlement problems. At the same time, a pure and uniform molten iron matrix is ​​formed, providing a stable and reliable basis for the uniform dispersion and distribution of bismuth and zirconium elements in the melt.

[0017] 2. This invention employs a rare-earth magnesium spheroidizing agent modified with a silicon-barium composite coating. This agent, along with bismuth and zirconium, forms a core synergistic effect. The silicon-barium inoculant significantly increases the non-spontaneous nucleation sites of graphite, refines graphite grains, and improves the uniformity of graphite distribution. Combined with bismuth, it enhances the graphite spheroidization effect and improves the roundness of graphite spheroids, achieving simultaneous optimization of graphite structure and spheroidization rate. Simultaneously, the numerous graphite-matrix interfaces formed by silicon-barium inoculation can preferentially adsorb bismuth. Combined with the binding effect of the dispersed precipitation of zirconium relative to the solid-solid bismuth, this constitutes a dual synergistic regulatory mechanism. This fundamentally prevents bismuth from migrating and agglomerating to grain boundaries, eliminates grain boundary embrittlement defects caused by bismuth, and synergistically solves the technical contradiction of achieving both high spheroidization rate and high toughness in ductile iron.

[0018] 3. This invention utilizes vacuum oxidation-free pretreatment of bismuth-zirconium alloys, combined with the protective effect of a covering agent and a precise spheroidizing inoculation process. The vacuum oxidation-free pretreatment avoids oxidation and burn-off of bismuth and zirconium alloys at high temperatures, and the covering agent effectively inhibits the volatilization and burn-off of magnesium in the spheroidizing agent. At the same time, it stabilizes the spheroidizing reaction atmosphere and ensures that the composite spheroidizing inoculator, bismuth, and zirconium are uniformly dispersed and fully reacted in the molten iron. This avoids local element enrichment and spheroidization decay, and achieves simultaneous and efficient completion of spheroidizing reaction, nucleation, and alloying control. It ensures the stable and stable synergistic effect of each functional component and guarantees the uniformity of the casting structure and the consistency of mechanical properties. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 This embodiment provides a method for preparing high-toughness ductile iron, including the following steps: (1) Pretreatment of furnace charge: Low phosphorus and low sulfur foundry pig iron, low carbon scrap steel and ductile iron recycled material are added to the medium frequency induction furnace in a mass ratio of 70:20:10. After vacuuming to a vacuum degree ≤-0.08MPa, nitrogen gas is introduced for protection and molten iron is obtained. The molten iron is deslagred and desulfurized at 1420℃ and kept at the temperature for 1h to obtain pretreated molten iron.

[0021] (2) Preparation of spheroidizing inoculant: The rare earth magnesium spheroidizing agent was placed in a vacuum drying oven and dried at 80°C for 2 hours. The agglomerates and impurities were removed by screening. The agent was then sealed and stored in a nitrogen atmosphere with a purity of ≥99.99% for later use to obtain a pretreated rare earth magnesium spheroidizing agent. The pretreated rare earth magnesium spheroidizing agent was added to anhydrous ethanol and ultrasonically dispersed evenly. 5% by weight of silicon barium iron-based inoculant was added. The silicon barium inoculant contained 60% Si and 3% Ba, with the remainder being Fe. The mixture was reacted at a constant temperature of 40°C for 30 minutes. Anhydrous ethanol was removed by vacuum distillation to obtain a silicon barium coated modified spheroidizing agent. The silicon barium coated modified spheroidizing agent was crushed and sieved to collect modified spheroidizing agent particles with a particle size of 3 mm. The mixture was sealed in an inert gas atmosphere to obtain a sieved composite spheroidizing agent. The sieved composite spheroidizing agent was ultrasonically dispersed at an ultrasonic power of 80 W for 10 minutes to obtain a composite spheroidizing inoculant.

[0022] (3) Bismuth-zirconium pretreatment: Bismuth alloy particles with a particle size of 1 mm and zirconium alloy particles with a particle size of 1 mm are dried at 80°C for 2 hours in a vacuum environment to complete the oxidation-free pretreatment.

[0023] (4) Spheroidizing inoculation: The composite spheroidizing inoculator, bismuth alloy particles, zirconium alloy particles and perlite covering agent are placed in the spheroidizing pot. The amount of covering agent is 0.5% of the total mass of molten iron. Then, the pretreated molten iron is poured in and spheroidizing inoculation is carried out at 1430℃. The amount of composite spheroidizing inoculator added is 1.0% of the mass of molten iron, the amount of bismuth alloy added is 0.002% of the mass of molten iron, the amount of zirconium alloy added is 0.02% of the mass of molten iron, the spheroidizing reaction time is 1 min, and the reaction is allowed to stand for 5 min to obtain spheroidized inoculated composite molten iron.

[0024] (5) Casting heat treatment: The spheroidized inoculated composite molten iron is cast into shape under the protection of high-purity nitrogen. The casting is subjected to secondary isothermal annealing at a temperature of 720°C and a holding time of 3 hours. It is then cooled to below 500°C in the furnace and then taken out and air-cooled to room temperature to obtain high-toughness ductile iron.

[0025] Example 2 This embodiment provides a method for preparing high-toughness ductile iron, including the following steps: (1) Pretreatment of furnace charge: Low phosphorus and low sulfur foundry pig iron, low carbon scrap steel and ductile iron recycled material are added to the medium frequency induction furnace in a mass ratio of 75:15:10. After vacuuming to a vacuum degree of ≤-0.08MPa, nitrogen gas is introduced for protection and molten iron is obtained. The molten iron is deslagred and desulfurized at 1435℃ and kept at the temperature for 0.8h to obtain pretreated molten iron.

[0026] (2) Preparation of spheroidizing inoculant: The rare earth magnesium spheroidizing agent was placed in a vacuum drying oven and dried at 90°C for 1.5 h. The agglomerates and impurities were removed by screening. The agent was then sealed and stored in a nitrogen atmosphere with a purity of ≥99.99% for later use to obtain a pretreated rare earth magnesium spheroidizing agent. The pretreated rare earth magnesium spheroidizing agent was added to anhydrous ethanol and ultrasonically dispersed evenly. 7.5% of the rare earth magnesium spheroidizing agent by mass of silicon barium iron-based inoculant was added. The silicon barium inoculant contained 67.5% Si and 5.5% Ba, with the remainder being Fe. The reaction was carried out at a constant temperature of 45°C for 25 min. The anhydrous ethanol was removed by vacuum distillation to obtain a silicon barium coated modified spheroidizing agent. The silicon barium coated modified spheroidizing agent was crushed and sieved to collect modified spheroidizing agent particles with a particle size of 5.5 mm. The particles were sealed in an inert gas atmosphere to obtain a sieved composite spheroidizing agent. The sieved composite spheroidizing agent was ultrasonically dispersed at an ultrasonic power of 100 W for 7.5 min to obtain a composite spheroidizing inoculant.

[0027] (3) Bismuth-zirconium pretreatment: Bismuth alloy particles with a particle size of 2 mm and zirconium alloy particles with a particle size of 2 mm are dried at 90°C for 1.5 h in a vacuum environment to complete the non-oxidation pretreatment.

[0028] (4) Spheroidizing inoculation: The composite spheroidizing inoculator, bismuth alloy particles, zirconium alloy particles and silicon carbide covering agent are placed in the spheroidizing ladle. The amount of covering agent is 1.0% of the total mass of molten iron. Then, the pretreated molten iron is poured in and spheroidizing inoculation is carried out at 1445℃. The amount of composite spheroidizing inoculator added is 1.15% of the mass of molten iron, the amount of bismuth alloy added is 0.00275% of the mass of molten iron, the amount of zirconium alloy added is 0.035% of the mass of molten iron, the spheroidizing reaction time is 1.5 min, and after the reaction, it is allowed to stand for 4 min to obtain spheroidized inoculated composite molten iron.

[0029] (5) Casting heat treatment: The spheroidized inoculated composite molten iron is cast into shape under the protection of high-purity nitrogen. The casting is subjected to secondary isothermal annealing at a temperature of 740℃ and a holding time of 2.5h. It is then cooled to below 500℃ in the furnace and then taken out and air-cooled to room temperature to obtain high-toughness ductile iron.

[0030] Example 3 This embodiment provides a method for preparing high-toughness ductile iron, including the following steps: (1) Pretreatment of furnace charge: Low phosphorus and low sulfur foundry pig iron, low carbon scrap steel and ductile iron recycled material are added to the medium frequency induction furnace in a mass ratio of 80:10:10. After vacuuming to a vacuum degree ≤ -0.08MPa, nitrogen gas is introduced for protection and smelting to obtain molten iron. The molten iron is deslagified and desulfurized at 1450℃ and kept at the temperature for 0.5h to obtain pretreated molten iron.

[0031] (2) Preparation of spheroidizing inoculant: The rare earth magnesium spheroidizing agent was placed in a vacuum drying oven and dried at 100℃ for 1h. The agglomerates and impurities were removed by screening. The agent was then sealed and stored in a nitrogen atmosphere with a purity of ≥99.99% for later use to obtain a pretreated rare earth magnesium spheroidizing agent. The pretreated rare earth magnesium spheroidizing agent was added to anhydrous ethanol and ultrasonically dispersed evenly. 10% of the mass of rare earth magnesium spheroidizing agent was added to a silicon barium iron-based inoculant alloy. The silicon barium inoculant contained 75% Si and 8% Ba, with the remainder being Fe. The mixture was reacted at a constant temperature of 50℃ for 20min. Anhydrous ethanol was removed by vacuum distillation to obtain a silicon barium coated modified spheroidizing agent. The silicon barium coated modified spheroidizing agent was crushed and sieved to collect modified spheroidizing agent particles with a particle size of 8mm. The mixture was sealed in an inert gas atmosphere to obtain a sieved composite spheroidizing agent. The sieved composite spheroidizing agent was ultrasonically dispersed with an ultrasonic power of 120W for 5min to obtain a composite spheroidizing inoculant.

[0032] (3) Bismuth-zirconium pretreatment: Bismuth alloy particles with a particle size of 3 mm and zirconium alloy particles with a particle size of 3 mm are dried at 100°C for 1 hour in a vacuum environment to complete the non-oxidation pretreatment.

[0033] (4) Spheroidizing inoculation: The composite spheroidizing inoculator, bismuth alloy particles, zirconium alloy particles and perlite and silicon carbide mixed covering agent are placed in the spheroidizing pot. The amount of covering agent is 1.5% of the total mass of molten iron. Then, the pretreated molten iron is poured in and spheroidizing inoculation is carried out at 1460℃. The amount of composite spheroidizing inoculator added is 1.3% of the mass of molten iron, the amount of bismuth alloy added is 0.0035% of the mass of molten iron, the amount of zirconium alloy added is 0.05% of the mass of molten iron, the spheroidizing reaction time is 2 min, and the reaction is allowed to stand for 3 min to obtain spheroidized inoculated composite molten iron.

[0034] (5) Casting heat treatment: The spheroidized inoculated composite molten iron is cast into shape under the protection of high-purity nitrogen. The casting is subjected to secondary isothermal annealing at a temperature of 760°C and a holding time of 2 hours. It is then cooled to below 500°C in the furnace and then taken out and air-cooled to room temperature to obtain high-toughness ductile iron.

[0035] Comparative Example 1 The only difference between this comparative example and Example 1 is that the rare earth magnesium spheroidizing agent is not modified by silicon-barium composite coating in step (2), and the spheroidizing inoculant is directly prepared.

[0036] Expected performance: Insufficient number of graphite spheroidal nuclei, easy segregation of bismuth at grain boundaries, decreased spheroidization rate of castings, significantly reduced toughness, and failure to meet the requirements for high-toughness ductile iron.

[0037] Comparative Example 2 The only difference between this comparative example and Example 1 is that bismuth alloy particles are not added in step (4).

[0038] Expected performance: Poor graphite spheroidization, insufficient spheroidization rate and graphite roundness, poor matching between the strength and toughness of the casting matrix, and mechanical properties that do not meet the high toughness standard.

[0039] Comparative Example 3 The only difference between this comparative example and Example 1 is that zirconium alloy particles are not added in step (4).

[0040] Expected performance: It cannot suppress the segregation of bismuth at grain boundaries, resulting in grain boundary embrittlement in castings, a significant decrease in toughness, and a tendency to fracture.

[0041] Comparative Example 4 The only difference between this comparative example and Example 1 is that: in step (1), no vacuuming and nitrogen protection treatment is performed during the melting process.

[0042] Expected performance: Molten iron is prone to oxidation, resulting in more inclusions, higher levels of harmful gases, increased internal defects in castings, and significant deterioration in mechanical properties and toughness.

[0043] Comparative Example 5 The only difference between this comparative example and Example 1 is that the casting is not subjected to secondary isothermal annealing in step (5).

[0044] Expected performance: The residual stress inside the casting cannot be eliminated, the microstructure is uneven, the casting is brittle, the toughness is poor, and the dimensional stability is poor.

[0045] Comparative Example 6 The only difference between this comparative example and Example 1 is that the bismuth alloy particles and zirconium alloy particles in step (3) are not subjected to non-oxidative pretreatment.

[0046] Expected performance: Bismuth and zirconium alloys are prone to oxidation and failure. Their synergistic effect cannot be achieved after addition, resulting in a decrease in spheroidization effect and toughness control ability, and unstable casting performance.

[0047] To compare the performance differences between the high-toughness ductile iron preparation methods provided in Examples 1-3 and Comparative Examples 1-6, the present invention provides the following test methods: I. Room Temperature Tensile Properties Test Method The test was conducted according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test at room temperature," and the specific procedures are as follows: 1. Sample preparation Standard circular cross-section tensile specimens were prepared from the standard sampling locations of the ductile iron castings prepared in the examples and comparative examples using wire cutting and machining. The parallel length diameter of the specimens was 10 mm, the gauge length was 50 mm, and the surface of the specimens was finely polished with a roughness Ra≤1.6 μm, free from tool marks, burrs, cracks and mechanical damage.

[0048] 2. Test equipment An electronic universal testing machine with a precision class of 1 was used. The machine has been calibrated and is qualified. The test force measurement error is no greater than ±1%.

[0049] 3. Test conditions The test environment was room temperature 23℃±5℃ and relative humidity 50%±10%. There was no significant vibration or airflow interference during the test.

[0050] 4. Test Procedure The specimen is concentrically clamped in the testing machine fixture, ensuring that the specimen axis coincides with the force-applying axis of the testing machine; an electronic extensometer is installed and the gauge length is accurately calibrated; an axial tensile force is applied uniformly at a strain rate of 2 mm / min and continuously loaded until the specimen breaks; the load-deformation curve during the test is automatically recorded, the maximum force at specimen breakage is read, and the tensile strength is calculated; the gauge length after specimen breakage is measured, and the elongation after fracture is calculated.

[0051] 5. Results Processing Three samples were tested in parallel for each group, and the arithmetic mean was taken after removing outliers as the tensile property result of the group.

[0052] II. Low-Temperature Charpy Impact Performance Test Method The test was conducted according to GB / T 229-2020 "Metallic Materials Charpy Impact Test Method", with a focus on testing the impact energy at -20℃. The specific operation is as follows: 1. Sample preparation Samples were taken from the casting body and machined into standard V-notch Charpy impact test specimens with dimensions of 10mm×10mm×55mm, V-notch depth of 2mm, and notch root radius of 0.25mm. The notch was precision machined without notch skew or burrs, and the surface roughness Ra of the specimen was ≤0.8μm.

[0053] 2. Test equipment The low-temperature fully automatic impact testing machine is used, with a pendulum rated impact energy of 300J and a low-temperature control system accuracy of ±1℃. The equipment has been metrologically calibrated and qualified.

[0054] 3. Low-temperature treatment of samples The sample was placed in a low-temperature constant temperature bath, and anhydrous ethanol was injected as the cooling medium. The temperature was set to -20℃ and kept at that temperature for 30 minutes to ensure that the overall temperature of the sample was uniform and stable, without local temperature differences.

[0055] 4. Test Procedure After the heat preservation is completed, a special clamp is used to quickly transfer the sample to the support of the testing machine, ensuring that the notch of the sample faces away from the pendulum blade and the transfer time does not exceed 5 seconds; the testing machine is started to release the pendulum, impacting the center position of the notch of the sample, and the impact energy absorbed by the sample fracture is automatically recorded.

[0056] 5. Result Processing Three parallel tests are performed for each group of samples. After eliminating outliers, the arithmetic average is taken as the low-temperature impact energy result of the group at -20°C.

[0057] III. Brinell Hardness Test Method The test is carried out in accordance with GB / T 231.1-2018 *Metallic materials - Brinell hardness test - Part 1: Test method*, and the specific operations are as follows: 1. Sample Preparation Samples are taken from the casting body. After the test surface is processed by a milling machine, it is gradually polished to 1000 mesh with sandpaper, and then polished. The test surface is flat and smooth, free of oxide scale, scratches, corrosion and inclusions; the thickness of the test surface of the sample is not less than 8 times the indentation depth, ensuring no deformation or warping during the test.

[0058] 2. Test Equipment A digital-display Brinell hardness tester equipped with a φ5mm cemented carbide ball indenter is used, with a test force of 750kgf (7355N), and the equipment has passed metrological calibration and is qualified.

[0059] 3. Test Conditions The test is carried out at room temperature of 23°C±5°C, and the test table is stable without vibration interference.

[0060] 4. Test Procedure Place the sample stably on the worktable of the hardness tester, adjust the position of the sample so that the indenter is vertically aligned with the test surface; start the equipment to apply the test force, unload stably after holding the load for 15s; use the microscope attached to the hardness tester to measure the diameters of the indentation in two perpendicular directions, and take the average to calculate the Brinell hardness value.

[0061] 5. Result Processing Five test points are evenly selected on the test surface of each sample, and the spacing between test points is not less than 3 times the indentation diameter. After eliminating the maximum and minimum values, the arithmetic average of the remaining three points is taken as the Brinell hardness (HBW) result of the group. The experimental data are as follows: Table 1 Performance test results of examples and comparative examples

[0062] Based on the experimental data in Table 1, it can be seen that the tensile strength of Examples 1-3 is in the range of 475-490 MPa, with stable and excellent strength levels. The tensile strength of Comparative Examples 1-6 is significantly lower than that of the Examples, with Comparative Example 2, which does not contain bismuth alloy, having the lowest strength at only 365 MPa. The tensile strength of the other comparative examples is between 372 and 436 MPa. This invention achieves a dense matrix structure and uniform graphite distribution in the casting by rare earth magnesium spheroidization, silicon barium inoculation to refine the microstructure, and synergistic regulation by bismuth zirconium, thus ensuring the tensile properties of the material. However, the comparative examples, due to the lack of silicon barium coating, absence of bismuth element, iron oxidation, or lack of heat treatment, exhibit coarse microstructure, internal defects, and poor spheroidization effect, directly leading to a decrease in tensile strength.

[0063] The elongation after fracture of Examples 1-3 was 20.1-21.8%, showing outstanding plasticity and good deformation capacity. The elongation after fracture of Comparative Examples 1-6 was only 7.5-12.3%, far lower than the level of the Examples. This invention uses low-phosphorus and low-sulfur pure furnace charge, combined with vacuum nitrogen protection melting and secondary isothermal annealing process, which eliminates grain boundary embrittlement impurities and casting residual stress. Combined with the refining effect of silicon barium inoculation on graphite, the plasticity of the material is greatly improved. The comparative examples, due to problems such as bismuth grain boundary segregation, uneven structure, more internal defects or residual stress not being eliminated, have drastically reduced plasticity and the material is prone to brittle fracture.

[0064] The -20℃ low-temperature impact energy of Examples 1-3 is 38-42J, exhibiting excellent low-temperature toughness and meeting the core requirements for high-toughness ductile iron. In contrast, the low-temperature impact energy of Comparative Examples 1-6 is only 10-22J, with a significant decrease in toughness. Among them, Comparative Example 2 without bismuth alloy and Comparative Example 3 without zirconium alloy have the worst toughness, with impact energy less than one-third of that of the Examples. This invention increases graphite nucleation sites through silicon-barium inoculation and effectively suppresses bismuth grain boundary segregation by combining zirconium elements, fundamentally solving the technical contradiction that bismuth elements increase spheroidization rate while causing embrittlement. Furthermore, the invention employs a full-process clean treatment and annealing to stabilize the microstructure, endowing the material with excellent low-temperature impact resistance. The comparative examples, lacking any core control link, exhibit problems such as bismuth segregation embrittlement, insufficient spheroidization, microstructural defects, or excessive residual stress, resulting in a significant loss of low-temperature toughness.

[0065] Examples 1-3 have Brinell hardness values ​​of 185-192 HBW, which are moderate and achieve a balanced match of strength, plasticity and toughness. Comparative Examples 1-6 generally have higher hardness values, ranging from 215-242 HBW, exhibiting hard and brittle characteristics. The process of this invention can enable the casting to form a ferrite matrix with excellent toughness and fine and dispersed graphite distribution, ensuring both hardness and toughness. The comparative examples, due to problems such as coarse structure, increased tendency of white iron, and precipitation of embrittled phases at grain boundaries, have abnormally high hardness and significantly reduced toughness, making them unsuitable for high-toughness applications.

[0066] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing high-toughness ductile iron, characterized in that, Includes the following steps: (1) Pretreatment of furnace charge: Pig iron and scrap steel are added to the medium frequency induction furnace, and after vacuuming, nitrogen is introduced for protection. After smelting, molten iron is obtained. The molten iron is deslagified and desulfurized to obtain pretreated molten iron. (2) Preparation of spheroidizing inoculant: The rare earth magnesium spheroidizing agent was subjected to drying pretreatment, silicon-barium composite coating modification, particle size sieving and ultrasonic dispersion in sequence to obtain composite spheroidizing inoculant; (3) Bismuth-zirconium pretreatment: Bismuth alloy particles and zirconium alloy particles are subjected to non-oxidative pretreatment; (4) Spheroidizing inoculation: The composite spheroidizing inoculator, bismuth alloy particles, zirconium alloy particles and covering agent are placed in the spheroidizing ladle, and then the pretreated molten iron is poured in to carry out spheroidizing inoculation treatment to obtain spheroidized inoculated composite molten iron. (5) Casting heat treatment: The spheroidized inoculated composite molten iron is cast into shape under the protection of inert gas, and the casting is subjected to secondary isothermal annealing to obtain high toughness ductile iron.

2. The method for preparing high-toughness ductile iron according to claim 1, characterized in that, In step (2), the drying pretreatment specifically includes the following steps: 1a) Place the rare earth magnesium spheroidizing agent in a vacuum drying oven and dry it at 80~100℃ for 1~2 hours; 2a) The dried rare earth magnesium spheroidizing agent is screened to remove lumps and impurities; 3a) The screened spheroidizing agent is sealed and stored in a nitrogen atmosphere with a purity of ≥99.99% for later use, thus obtaining a pretreated rare earth magnesium spheroidizing agent.

3. The method for preparing high-toughness ductile iron according to claim 2, characterized in that, In step (2), the silicon-barium composite coating modification specifically includes the following steps: 1b) Add the pretreated rare earth magnesium spheroidizing agent to anhydrous ethanol and disperse it evenly by ultrasonication to obtain a spheroidizing agent dispersion; 2b) Add a barium silicon inoculant to the spheroidizing agent dispersion. The barium silicon inoculant is a barium silicon iron-based inoculant alloy, with a Si content of 60%–75% and a Ba content of 3%–8% by mass, and the balance being Fe. The amount of the barium silicon inoculant is 5%–10% of the mass of the rare earth magnesium spheroidizing agent. The reaction is carried out at a constant temperature of 40–50°C for 20–30 min. 3b) After the reaction is complete, anhydrous ethanol is removed by vacuum distillation to obtain the barium silicon coated modified spheroidizing agent.

4. The method for preparing high-toughness ductile iron according to claim 3, characterized in that, In step (2), the particle size sieving specifically includes the following steps: 1c) Crushing the barium silicon-coated modified spheroidizing agent; 2c) Screen and collect modified spheroidizing agent particles with a particle size of 3~8mm; 3c) The sieved modified spheroidizing agent is sealed in an inert gas atmosphere to obtain a sieved composite spheroidizing agent.

5. The method for preparing high-toughness ductile iron according to claim 4, characterized in that, In step (2): the qualified composite spheroidizing agent is subjected to ultrasonic dispersion treatment with an ultrasonic power of 80~120W and a treatment time of 5~10min to obtain the composite spheroidizing inoculant.

6. The method for preparing high-toughness ductile iron according to claim 1, characterized in that, In step (1): the pig iron is low-phosphorus and low-sulfur foundry pig iron with a carbon content of 3.5wt% to 4.2wt%, P ≤ 0.03wt%, and S ≤ 0.02wt%; the scrap steel is low-carbon scrap steel with a carbon content ≤ 0.25wt%, P ≤ 0.03wt%, and S ≤ 0.02wt%. When adding pig iron and scrap steel to the medium-frequency induction furnace, ductile iron recycled material is added. The mass ratio of pig iron, scrap steel, and ductile iron recycled material is 70:20:10 to 80:10:

10. Vacuum is drawn during smelting to a vacuum degree ≤ -0.08MPa. The slag removal and desulfurization temperature is 1420~1450℃, and the holding time is 0.5~1h.

7. The method for preparing high-toughness ductile iron according to claim 1, characterized in that, In step (3): the bismuth alloy particles have a particle size of 1~3mm and are added at 0.002%~0.0035% of the mass of molten iron; the zirconium alloy particles have a particle size of 1~3mm and are added at 0.02%~0.05% of the mass of molten iron; the non-oxidative pretreatment of the bismuth alloy particles and zirconium alloy particles is to dry them at 80~100℃ in a vacuum environment for 1~2h.

8. The method for preparing high-toughness ductile iron according to claim 1, characterized in that, In step (4): the covering agent is one or more of perlite and silicon carbide, and the amount of the covering agent is 0.5% to 1.5% of the total mass of molten iron.

9. The method for preparing high-toughness ductile iron according to claim 1, characterized in that, In step (4): the spheroidizing inoculation temperature is 1430~1460℃; the amount of composite spheroidizing inoculator added is 1.0%~1.3% of the mass of molten iron; the spheroidizing reaction time is 1~2 min, and the reaction is allowed to stand for 3~5 min.

10. The method for preparing high-toughness ductile iron according to claim 1, characterized in that, In step (5): the inert gas is high-purity nitrogen; the secondary isothermal annealing temperature is 720~760℃, the holding time is 2~3h, the furnace is cooled to below 500℃, and then it is taken out and air-cooled to room temperature.

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