Nodular cast iron, heat treatment method and application

Abstract

The invention discloses nodular cast iron and a heat treatment method and application, and belongs to the technical field of nodular cast iron. The nodular cast iron is prepared from, by mass, 3.4%-3.8% of C, 2.0%-3.0% of Si, smaller than or equal to 0.6% of Mn, 0.03%-0.06% of Mg, smaller than or equal to 0.15% of Mo, larger than or equal to 0.2% and smaller than or equal to 1% of Cu and Ni, smaller than or equal to 0.02% of rare earth Re and the balance Fe and inevitable impurities. The nodular cast iron is high in pearlite content and smaller in lamellar spacing, the fatigue strength, the tensile strength and the yield strength can be remarkably improved, meanwhile, the nodular cast iron has the advantage of being low in cost, and a planet carrier prepared from the nodular cast iron material can remarkably improve the service reliability of a wind power gear box and reduce the full-life-cycle maintenance cost.

Description

Technical Field

[0001] The present application belongs to the technical field of ductile iron, and specifically relates to ductile iron, a heat treatment method and an application thereof. Background Art

[0002] With the accelerated advancement of global clean energy strategies, the increasing size and power density of wind turbine equipment are placing higher demands on the mechanical properties of core components. Planet carriers, as key load-bearing components in wind turbine gearboxes, are typically made of ductile iron. While this material offers excellent casting properties and cost advantages, improving its performance is difficult.

[0003] Currently, there are mainly the following ways to improve the performance of planetary carriers: (1) increasing the amount of alloying elements, such as copper, molybdenum, nickel, etc., while still using normalizing treatment. This method results in higher material costs and limited performance improvement, and large-scale use also has a certain negative impact on performance; (2) using austempering methods to obtain austempered ductile iron (ADI material), but ADI materials also require the addition of more expensive metals such as molybdenum and nickel, and ADI materials also require austempering treatment. The heat treatment cost is high, and this treatment method is difficult to quench thick and large castings such as planetary carriers, which can easily lead to uneven structure and unqualified performance.

[0004] Therefore, developing a ductile iron with excellent performance and a corresponding heat treatment method to coordinate casting performance and cost has become a technical problem that needs to be solved urgently. Summary of the Invention

[0005] In order to solve the above problems, a ductile iron is provided, which has a high pearlite content and smaller lamellar spacing, can significantly improve fatigue strength, tensile strength and yield strength, and has the advantage of low cost. The planetary carrier prepared using this ductile iron material can significantly improve the service reliability of the wind turbine gearbox and reduce the maintenance cost throughout its life cycle.

[0006] According to one aspect of the present application, a ductile iron is provided, which comprises, by mass, C: 3.4%-3.8%, Si: 2.0%-3.0%, Mn: ≤0.6%, Mg: 0.03%-0.06%, Mo≤0.15%, 0.2%≤Cu+Ni≤1%, rare earth Re≤0.02%, and the balance being Fe and unavoidable impurities.

[0007] The ductile iron prepared using the above components has a high pearlite content and finer pearlite, which can improve the uniformity of the internal structure of the ductile iron, thereby improving its tensile strength, yield strength and fatigue strength. At the same time, it can also maintain a certain elongation, which is convenient for alleviating stress concentration and reducing the risk of sudden fracture of the casting.

[0008] In the ductile iron of the present application, in addition to the above-mentioned elements, other metal elements may be added, with the remainder being Fe and unavoidable impurities, such as Sb and Sn, which still fall within the scope of protection of this solution and achieve the same technical effect. However, the addition of new elements will lead to increased costs, which can be increased according to production needs.

[0009] The Si content in the ductile iron of this application is 2.0-3.0%. Under the heat treatment method of this application, the Si in this material inhibits the formation of pearlite. Mn, Cu, and Ni act synergistically to promote the formation of pearlite and refine the spacing between pearlite lamellae. Therefore, within the limits of these elements, pearlite formation can be promoted and the average spacing between pearlite lamellae can be refined, thereby improving various material properties. Since Ni and Cu have similar mechanisms of action, it is sufficient to ensure that the total content of Ni and Cu is within the above range.

[0010] If the Mn content of the ductile iron in this application is greater than 0.6%, cementite is likely to appear in the casting, reducing product quality. For planetary carriers weighing 0.5-30 tons, if the Mg content in the ductile iron is less than 0.03%, vermicular graphite is likely to appear in the casting, reducing product performance. If the Mg content is higher than 0.06%, shrinkage is likely to occur in the casting. If the Mo content in the ductile iron is higher than 0.15%, martensite is more likely to form due to the faster cooling rate of the water-based normalizing medium in this method, making it difficult to obtain the ductile iron material with a pearlite content ≥ 95% as described in this application.

[0011] The ductile iron of the present application has a low content of rare earth Re elements and is suitable for the preparation of large parts weighing more than 500 kg, such as large planetary carriers. If the rare earth element is greater than 0.02%, it will promote the formation of fragmented graphite during the preparation of thick and large parts, resulting in a decrease in various properties of the ductile iron.

[0012] Compared to alloying solutions, the aforementioned ductile iron composition ratio: alloying elements directly reduce the plasticity of the casting. They also reduce the thermal conductivity of the casting, leading to the formation of coarse pearlite in the core during the second cooling stage, reducing the plasticity of the casting. Furthermore, alloying elements promote cementite formation, making it more sensitive to cooling rates and prone to stress concentration and cracking, thus failing to meet the performance requirements of this ductile iron. The ductile iron composition ratio of this application can address these issues caused by alloying elements, resulting in castings with improved performance.

[0013] Optionally, the ductile iron contains, by mass, 3.4%-3.8% C, 2.0%-3.0% Si, ≤0.6% Mn, 0.03%-0.06% Mg, 0.15%-0.15% Mo, 0.2%-0.8% Cu, ≤0.2% Ni, ≤0.02% rare earth Re, and the remainder is Fe and unavoidable impurities.

[0014] The content of Cu and Ni in ductile iron is further limited here. Generally speaking, in thick and large products, a higher Ni content indicates better product performance. This application, under the limitations of the subsequent heat treatment method, can reduce the Ni content while preparing ductile iron materials with the same structure and performance, thereby reducing production costs and facilitating industrial promotion and use. If the Cu content exceeds this maximum value, the graphite morphology will deteriorate and the cost will increase.

[0015] Optionally, the ductile iron comprises, by mass, C: 3.4-3.8%, Si: 2.5-3.0%, Mn: ≤0.6%, Mg: 0.03%-0.06%, Mo≤0.15%, Cu: 0.2-0.8%, Ni≤0.2%, rare earth Re≤0.02%, and the balance is Fe and unavoidable impurities.

[0016] In the above-mentioned ductile iron, when the Si content is in the range of 2.5-3.0%, the fatigue strength of the ductile iron is further improved, which can be increased to more than 300 MPa. With this improvement in fatigue strength, the deformation resistance of the ductile iron is further improved, thereby further extending the service life of the planetary carrier.

[0017] Optionally, the ductile iron comprises, by mass, 3.5-3.7% C, 2.5-3.0% Si, 0.3-0.5% Mn, 0.04-0.06% Mg, 0.4-0.8% Cu, ≤0.1% Mo, ≤0.1% Ni, ≤0.1% rare earth Re, ≤0.01%, and the balance being Fe and unavoidable impurities.

[0018] Under further limitations, the silicon used in the ductile iron mentioned above plays a role in solid solution strengthening, forming a Si-Fe phase, which can replace the solid solution and strengthen the matrix. It also works synergistically with the cooling and refinement of pearlite by the normalizing medium, which can further improve fatigue performance. Compared with the scheme with a Si content of less than 2.5%, the scheme with a Si content greater than 2.5% can further improve the fatigue strength performance of the material. When the Si content is less than 2.5%, the solid solution strengthening effect is not obvious, and the improvement in fatigue performance is small. When the Si content is greater than 3.0%, the formation of pearlite can be significantly inhibited, and the ferrite content in the matrix increases.

[0019] Optionally, the ductile iron satisfies at least one of the following conditions: (C+Si / 3)=4.23-4.8%; Mn+Cu≥Si / 10.

[0020] In the aforementioned ductile iron, maintaining this ratio of C and Si approaches the eutectic composition, narrowing the solidification temperature range of the ductile iron casting and concentrating the transition from liquid to solid. This allows for longer liquid-phase fluidity during crystallization, enabling more timely shrinkage compensation in the final solidifying region and promoting the precipitation of spherical graphite. During the later stages of ductile iron solidification, the precipitation of spherical graphite produces a significant volume expansion, which effectively compensates for solidification and solid-state shrinkage in the casting, reducing shrinkage-related porosity. Therefore, within the aforementioned limits, the planetary carrier exhibits minimal shrinkage defects and more stable performance. If the C and Si contents exceed these ranges, shrinkage is more likely to occur, making normal use difficult.

[0021] In the matrix formation of this ductile iron material, Si, Mn, and Cu have opposing effects on pearlite formation. To ensure a pearlite content of ≥95%, the relationship Mn+Cu≥Si / 10 must be satisfied. As the Si content increases, its ability to inhibit pearlite increases, requiring the addition of pearlite-promoting elements Mn and Cu to offset Si's inhibition. This relationship is particularly beneficial for achieving the required pearlite content, strength, and hardness of the ductile iron.

[0022] Optionally, the ductile iron comprises, by mass, C: 3.4%-3.8%, Si: 2.0%-2.49%, Mn: ≤0.6%, Mg: 0.03%-0.06%, Mo≤0.15%, 0.2%≤Cu+Ni≤1%, rare earth Re≤0.02%, and the balance is Fe and unavoidable impurities.

[0023] In the above-mentioned ductile iron, the Si content is reduced, so that the tensile strength, yield strength and hardness properties of the material do not change much, but the fatigue performance will be reduced. For some products with low fatigue performance requirements, a low Si content solution can be used to reduce production costs while meeting the use requirements.

[0024] Optionally, the ductile iron meets the following conditions: Hardness is 270-320HB; The pearlite content in the matrix is ​​≥95%.

[0025] Optionally, the ductile iron meets the following conditions: Tensile strength above 900MPa; Tensile-compression fatigue with a stress ratio of R=-1, with an average fatigue strength of more than 290MPa; Yield strength above 600MPa; The pearlite interlamellar spacing within any circular area with a diameter of 2 mm is 30-450 nm.

[0026] The ductile iron of this application is based on pearlite, with a pearlite content of no less than 95%. The pearlite interlamellar spacing is small, demonstrating finer pearlite and ensuring a more uniform structure within the ductile iron material. This significantly improves the tensile strength, yield strength, and fatigue strength of the ductile iron compared to QT700-2. The ductile iron of this application has high hardness and strength, capable of withstanding higher static loads and alternating stresses. The material surface also exhibits strong wear resistance, reducing dimensional failure caused by wear. These properties ensure that the ductile iron meets the requirements for planetary carrier use.

[0027] The limiting condition of ductile iron in this application, "the content of pearlite in the matrix", refers to the content of pearlite in the entire matrix, which is obtained by referring to the GB / T 9441-2021 "Metallographic Examination of Ductile Iron" standard.

[0028] The lamellar spacing in the limiting condition of ductile iron in this application, "the pearlite lamellar spacing in a circular area with an arbitrary diameter of 2 mm", refers to the distance between the centers of two adjacent ferrite or cementite sheets in the lamellar pearlite structure. The method for determining the lamellar spacing in a circular area with an arbitrary diameter of 2 mm is as follows: using an optical microscope or a scanning electron microscope, at a magnification of 1000 times or more, randomly select N (N≥10) fields of view within a circular area with a diameter of 10 mm, and search for the suspected widest pearlite lamellae in the field of view. 10-20 lamellae are measured in each field of view, and the average value of the lamellar spacing measured in each field of view is taken as the pearlite lamellar spacing in that field of view. The final pearlite lamellar spacing of these N fields of view is added and averaged to obtain the pearlite lamellar spacing of the circular area.

[0029] In addition, the proportion of troostite and troostite in the pearlite of the ductile iron is significantly increased compared with the QT700-2 material. The spacing between troostite and troostite is smaller than that of pearlite. The greater their proportion, the higher the strength and hardness of the ductile iron.

[0030] Referring to the above test method for lamellar spacing, this method can be used to test that in the ductile iron of this application, for any two different circular areas with a diameter of 2 mm, |AB| is less than or equal to 400 nm, A is the pearlite lamellar spacing in one of the circular areas with a diameter of 2 mm, and B is the pearlite lamellar spacing in the other circular area with a diameter of 2 mm. |AB| is less than or equal to 400 nm, which means that the material can both reduce the pearlite lamellar spacing and improve the uniformity of the lamellar spacing, thereby making the performance more uniform macroscopically.

[0031] Preferably, |AB| is less than or equal to 300 nm, more preferably, |AB| is less than or equal to 200 nm.

[0032] Optionally, the spheroidization rate of the ductile iron is ≥90%.

[0033] In the production of the above-mentioned ductile iron, a spheroidization rate of more than 90% can still be met to ensure the improvement of various properties of ductile iron. The higher the spheroidization rate, the smaller the proportion of irregular ductile iron, and the better the tensile strength, yield strength, elongation and fatigue strength of the ductile iron.

[0034] Optionally, the graphite size in the ductile iron is 5-8.

[0035] Under the conditions of meeting the above-mentioned pearlite formation and spheroidization rate restrictions, the size of the graphite in the ductile iron of the present application is 5-8 levels, which can not only ensure a certain strength and avoid the matrix being split due to excessive graphite, but also provide better toughness, avoid insufficient expansion due to excessively small graphite, and improve the fatigue strength of the ductile iron material.

[0036] In the ductile iron matrix of the present application, the remainder is ferrite or cementite except pearlite.

[0037] Optionally, the ferrite content of the ductile iron is ≤4%, and the ferrite is distributed around the graphite nodules or dispersed in the matrix; The cementite content of the ductile iron is ≤1%, and the cementite is dispersed on the pearlite grain boundaries.

[0038] In the present application, the ductile iron contains ferrite or cementite in addition to pearlite. The lower the ferrite content, the higher the strength and hardness of the ductile iron. The ferrite is distributed around the graphite nodules or dispersed in the matrix, which can improve the dispersion uniformity of the ferrite and avoid ferrite agglomeration. Cementite is a high-hardness, high-brittle structure. The cementite content in the ductile iron of the present application is controlled to be lower than the ferrite content, which can improve the elongation of the ductile iron. The cementite is dispersed on the pearlite grain boundaries, which can further reduce the impact of cementite on performance compared to massive cementite, thereby achieving a synergistic improvement in the strength, hardness and elongation of the ductile iron.

[0039] Optionally, the ductile iron meets the following conditions: The elongation is more than 2%.

[0040] The elongation of the ductile iron of the present application is above 2%, which enables the material to produce a certain amount of plastic deformation when overloaded, relieves stress concentration, and absorbs energy through slight deformation when subjected to impact or vibration loads, thereby avoiding planetary carrier failure due to complete brittle failure and reducing the risk of sudden breakage of the planetary carrier. The above-mentioned properties make the ductile iron of the present application a combination of "high strength + medium-high hardness + moderate toughness", achieving the coordinated optimization of load-bearing capacity, wear resistance and anti-failure ability, and is suitable for heavy-load wear-resistant working conditions, and can maintain reliability in complex stress environments. At the same time, the composition and heat treatment method of the ductile iron have the advantages of controllable method and low cost.

[0041] Optionally, the ductile iron satisfies: elongation ≥ (tensile strength / 350)%.

[0042] Under the above definition, the unit of tensile strength is MPa. Only the numerical value of tensile strength is used here, and the unit of MPa is not introduced into elongation. Generally speaking, the higher the tensile strength of ductile iron material, the lower the elongation. Based on the use requirements of the planetary carrier, the tensile strength of ductile iron material is usually increased to the higher the better. However, when the tensile strength is too high, the elongation is too low, and there will be a problem of brittle fracture of the casting due to poor plasticity during the stress process. The ductile iron pearlite sheet spacing of the present application is better uniform, which can reduce the internal defects and unevenness of ductile iron. These defects and unevenness are often the starting point of crack initiation. At the same time, the uniform microstructure of the ductile iron of the present application can disperse and withstand external loads more effectively. Therefore, the ductile iron can ensure that the elongation does not decrease excessively while the strength is improved, and the dual effect of tensile strength and elongation is maintained.

[0043] The ductile iron satisfies the following conditions: 0.6≤yield strength / tensile strength≤0.75.

[0044] The ratio of yield strength to tensile strength in the ductile iron of this application affects the material's elongation. If the ratio is too large, the elongation decreases, the material becomes brittle, and brittle failure of the planetary carrier occurs. If the ratio is too small, the elongation increases. This application improves the yield strength and tensile strength of ductile iron so that the yield strength and tensile strength satisfy the above relationship, thereby coordinating the elongation of the ductile iron and significantly improving the deformation resistance of the ductile iron.

[0045] According to another aspect of the present application, there is provided a heat treatment method for obtaining any of the above-mentioned ductile irons, comprising the following steps: (1) normalizing heat treatment of a casting made of any of the above ductile iron compositions, followed by cooling with a water-based normalizing medium; (2) The casting obtained in step (1) is subjected to stress relief annealing treatment and cooled to obtain the casting.

[0046] When the Mo content in the components of ductile iron is less than 0.15%, normalizing heat treatment of the casting using the water-based normalizing medium of the present application can increase the proportion of pearlite in the matrix, refine the spacing between pearlite sheets, and synergistically improve the various properties of ductile iron.

[0047] Optionally, the normalizing heat treatment is performed at a temperature of 870-940° C. and for a time of 2-10 hours.

[0048] The normalizing heat treatment temperature and duration promote the diffusion of various components in the casting, laying the foundation for the formation of pearlite. If the normalizing heat treatment temperature is too high, the cost will increase, the strength of the casting will decrease, and the casting will be more susceptible to deformation. If the normalizing heat treatment temperature is too low, the amount of pearlite formed in the matrix will decrease, making it difficult to improve the strength, hardness and elongation. If the normalizing heat treatment time is too short, the temperature and pearlite content will be uneven across the casting. If the normalizing heat treatment time is too long, the cost will increase and the casting will be more susceptible to deformation.

[0049] When using a water-based normalizing medium, the casting typically only needs to be immersed in the medium to cool it. Mist cooling, on the other hand, requires specialized, complex spray equipment and precise control of spray parameters such as pressure and flow, making it relatively difficult to control. Furthermore, mist cooling struggles to fully cover complex or large castings, resulting in poor cooling uniformity and the tendency to cause deformation. This method can further exacerbate the uneven cooling of castings with widely varying wall thicknesses, making them more susceptible to deformation and making it difficult to obtain ideal casting dimensions.

[0050] Optionally, the heating rate in the normalizing stage is 30-100°C / h.

[0051] The heating rate in the normalizing stage is 30-100°C, which is a relatively slow heating rate, which can ensure that no new residual stress is added to the casting during the heating process.

[0052] Optionally, the temperature of the water-based normalizing medium is 0-80°C.

[0053] In the present application, the temperature of the water-based normalizing medium in the cooling stage is lower than 80°C. This setting can effectively promote the contact between the water-based normalizing medium and the casting, improve the cooling efficiency of the casting, reduce stress generation, and save production costs.

[0054] Optionally, in the cooling of the water-based normalizing medium, the maximum cooling rate in the first stage at 500-850°C is 10-30°C / s, and the maximum cooling rate in the second stage below 500°C is ≤100°C / s.

[0055] Preferably, the maximum cooling rate below 500°C is not less than 10°C / s.

[0056] The cooling rate of the water-based normalizing medium is measured using a cooling characteristic tester. The specific test method is: heat a temperature probe (made of Inconel 600) to above 850°C, quickly place it in at least 600ml of water-based normalizing medium (ensuring the probe is completely immersed), and record the cooling curve of the temperature probe. Alternatively, a computer can be used to generate a cooling characteristic curve using simultaneous data processing. This test method refers to the continuous cooling process of the same medium.

[0057] The applicant discovered that the cooling rate of the core of a ductile iron casting is not directly controlled by the cooling medium, but rather by heat conduction between the core and the casting surface. Conventional thinking involves controlling the cooling rate in the temperature range where austenite transforms to pearlite to form a pearlite structure. However, large planetary carrier castings have thicker walls. When the casting surface temperature is below the pearlite transformation temperature, the core may still be above the pearlite transformation temperature. At this point, the temperature difference between the surface and the surrounding medium decreases, making it difficult to dissipate heat from the casting. This situation results in delayed heat conduction from the core. Furthermore, to prevent cracking in the casting, the cooling rate in the second stage is typically slowed. This setting results in coarser interlamellar spacing in the pearlite core of the casting, resulting in poorer overall performance.

[0058] In the present application, under the cooling rate of the above-mentioned water-based normalizing medium cooling, a steam film is formed at the contact interface between the water-based normalizing medium and the casting in the first stage. The water-based normalizing medium basically does not directly contact the casting, and pure convection heat transfer is used for cooling. The maximum cooling rate is maintained in the range of 10-30°C, which can promote the formation of pearlite and reduce the interlamellar spacing of pearlite. This cooling rate range promotes the transformation of thinner wall parts and surfaces to pearlite, while avoiding the formation of bainite and martensite, obtaining a fine pearlite structure, improving strength performance, and avoiding a decrease in plasticity (elongation). During the second stage, below 500°C, the cooling rate can also be maintained at a high level. Preferably, the cooling rate between 400-500°C can be controlled at 10-50°C / s. This maintains a large temperature difference between the casting surface temperature and the casting core temperature, promoting the core to pearlite transformation and avoiding slow core cooling. This allows for a plateau period where the heat transfer from the core to the surface and the heat transfer from the surface to the surrounding medium are consistent, promoting the core to pearlite transformation, resulting in a fine structure and improved performance. If the cooling rate in the second stage is reduced, the core structure will become coarse and uneven, leading to a decrease in various properties of ductile iron, especially plasticity. In addition, the thin-walled parts and surfaces of the casting in the second stage, because they have already transformed into pearlite, will not transform to bainite or martensite.

[0059] Therefore, for planetary carrier products with large differences in thickness (the minimum wall thickness may be 10mm, the maximum is over 200mm, and may even reach 300mm), the above-mentioned water-based normalizing medium cooling conditions can ensure that a dense pearlite structure can be obtained both inside and outside the casting, improving performance while avoiding excessive reduction in plasticity.

[0060] Compared to air or air cooling, the first stage cooling rate is faster. This reduces the spacing between pearlite lamellae because, during the pearlite transformation, carbon atoms need to diffuse to form cementite and ferrite. Therefore, the first stage cooling rate is fast, and the carbon atoms have a short diffusion time. To complete the transformation within this limited time, pearlite lamellae can only form with a smaller spacing to meet the diffusion distance and transformation kinetic requirements of carbon atoms. However, the second stage cooling rate should not be too fast, otherwise it will cause large residual stresses during casting, leading to cracking and difficulty in continued use of the casting.

[0061] Optionally, the water-based normalizing medium comprises 70 to 99 parts of water and 0.5 to 30 parts of thickener, calculated by mass.

[0062] The addition of a thickener to the water-based normalizing medium can increase its viscosity, thereby affecting its cooling process. The amount of thickener required can ensure the cooling effect of the water-based normalizing medium. If the amount of thickener used decreases while the amount of water remains constant, the cooling rate of the casting will not meet specified requirements, and various properties of the casting will deteriorate. The greater the amount of thickener added, the higher the concentration of the water-based normalizing medium, and the slower the cooling rates in the first and second stages. Excessive thickener addition increases costs and slows the cooling rate, resulting in a decrease in various properties of the ductile iron.

[0063] Optionally, the thickener is selected from at least one of polyacrylamide, sodium alginate, polyvinyl alcohol, sodium polyacrylate, and sodium carboxymethyl cellulose.

[0064] Optionally, the water-based normalizing medium further comprises 0.1 to 1 parts of a corrosion inhibitor and 0.1 to 1 parts of a rust inhibitor. The addition of the corrosion inhibitor can reduce the corruption and deterioration of the water-based normalizing medium. However, if the amount of corrosion inhibitor added is too high, the cost will increase, while if the amount of corrosion inhibitor added is too low, the corrosion protection effect will be poor and the service life of the water-based normalizing medium will be reduced. The addition of the rust inhibitor can prevent rust on the surface of the casting after heat treatment. However, if the amount of corrosion inhibitor added is too high, the cost will increase, while if the amount of corrosion inhibitor added is too low, the rust protection effect of the casting will be poor.

[0065] Optionally, the water-based normalizing medium includes 75-85 parts of water, 5-8 parts of polyacrylamide, 3-7 parts of sodium alginate, 7-9 parts of polyvinyl alcohol, 0.5-0.9 parts of preservative, and 0.6-0.9 parts of rust inhibitor.

[0066] Optionally, the molecular weight of the polyacrylamide is 10 million to 50 million, the molecular weight of the sodium alginate is 50,000 to 250,000, and the molecular weight of the polyvinyl alcohol is 100,000 to 300,000.

[0067] The lack of preservatives and rust inhibitors in the water-based normalizing medium of this application does not affect the cooling rate of the medium, and thus does not affect the performance of the product. If preservatives and rust inhibitors are not added, it may only cause the castings to rust easily in the later stage and reduce the service life of the water-based normalizing medium.

[0068] Optionally, the preservative includes at least one of sodium benzoate, potassium sorbate, and paraben.

[0069] Optionally, the rust inhibitor includes at least one of sodium nitrite and triethanolamine borate.

[0070] Optionally, the stress relief annealing treatment is performed at a temperature of 530-590° C. and a holding time of 2-20 hours.

[0071] The second stage of cooling with the water-based normalizing medium in this application has a faster cooling rate. Compared to conventional air cooling, this cooling method introduces higher residual stresses. Therefore, the temperature and time of the stress relief annealing treatment in this application are set within the above ranges to eliminate the stress introduced in the second stage of cooling with the water-based normalizing medium and maintain the structure of the pearlite structure. If the stress relief annealing temperature is lower than 530°C, the residual stress of the ductile iron will not be completely eliminated. If the stress relief annealing temperature is higher than 590°C, the pearlite will decompose. Therefore, if the stress relief annealing temperature is too high or too low, the various properties of the ductile iron will be reduced.

[0072] Optionally, after the stress relief annealing, the temperature in the furnace is lowered to ≤300° C. at a cooling rate of ≤60° C. / h, and the steel is taken out of the furnace for air cooling.

[0073] Setting the cooling rate after stress relief annealing treatment to be slower can ensure that no residual stress is generated during the cooling process, and further eliminate the stress generated in the water-based normalizing cooling stage, so as to improve the various properties of ductile iron.

[0074] According to another aspect of the present application, there is provided a use of the ductile iron described in any one of the above items or the ductile iron prepared by the heat treatment method described in any one of the above items in a planetary carrier.

[0075] The beneficial effects of this application include but are not limited to: 1. The ductile iron according to this application, through optimization of composition and heat treatment methods, can have improved tensile strength and yield strength compared to the original QT700-2 ductile iron, meeting the higher use standards of planetary carriers.

[0076] 2. According to the ductile iron of the present application, the content of pearlite in the matrix is ​​high, and the pearlite formed is finer, the microstructure of the ductile iron is more uniform, and both mechanical properties and uniformity of mechanical properties can be improved.

[0077] 3. The ductile iron of this application can be used to manufacture planetary carriers with a tonnage of 0.5-30 tons. When the planetary carrier has a wall thickness of 10 mm or greater, it can still meet a pearlite content of 95% or greater in the matrix. This reduces the performance difference between the ductile iron test piece and the planetary carrier, and increases the service life of the planetary carrier made from the ductile iron.

[0078] 4. According to the heat treatment method of ductile iron of the present application, by using a water-based normalizing medium for cooling and controlling the cooling rate, it is possible to promote the formation of pearlite in the ductile iron and reduce the lamellar spacing, thereby refining the ductile iron structure and improving various properties of the ductile iron. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 This is a metallographic photograph of the ductile iron involved in Example 3 of the present application.

[0080] Figure 2 This is a scanning electron microscope image of the ductile iron involved in Example 3 of the present application.

[0081] Figure 3 This is a metallographic photograph of the ductile iron involved in Comparative Example 10 of this application.

[0082] Figure 4 This is a scanning electron microscope image of the ductile iron involved in Comparative Example 10 of this application.

[0083] Figure 5 This is a cooling characteristic curve diagram of the water-based normalizing medium in Example 1 of the present application.

[0084] Figure 6 This is a cooling characteristic curve diagram of the water-based normalizing medium in Example 3 of the present application.

[0085] Figure 7 This is a cooling characteristic curve diagram of the water-based normalizing medium in Example 10 of the present application.

[0086] Figure 8 This is a cooling characteristic curve diagram of the water-based normalizing medium in Example 11 of the present application.

[0087] Figure 9 This is the cooling characteristic curve of water in comparative example 9 of this application.

[0088] Figure 10 This is a schematic diagram of the cooling characteristics tester involved in this application.

[0089] List of parts and reference numerals: 1. Water-based normalizing medium; 2. Tubular heating furnace; 3. Temperature probe; 4. Starting point setter; 5. Test recording system. DETAILED DESCRIPTION

[0090] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0091] Unless otherwise specified, the raw materials in the examples and comparative examples of the present application were purchased through commercial channels.

[0092] Unless otherwise specified, the methods used in the examples and comparative examples of this application are conventional methods in the prior art. The specific casting operations include shaping, assembling, melting, and pouring, followed by solidification to obtain the desired casting. The operating conditions of the above methods can be those commonly used in the art and do not constitute a limitation to this application.

[0093] Figure 10 The figure is a schematic diagram of a cooling characteristics tester. The test method using this tester is as follows: The starting point setter 4 fixes the temperature probe 3, and the tubular heating furnace 2 heats the temperature probe 3 (made of Inconel600) to above 850°C. The probe is then quickly placed in more than 600ml of water-based normalizing medium 1, and the temperature probe 3 is completely immersed in the water-based normalizing medium 1. Then, the test recording system 5 records the cooling process curve of the temperature probe 3 and simultaneously generates a cooling characteristic curve through computer data processing.

[0094] Example 1 The present embodiment relates to a ductile iron and a heat treatment method. The ductile iron comprises, by mass, 3.8% C, 2.0% Si, 0.6% Mn, 0.06% Mg, 0.15% Mo, 0.8% Cu, 0.2% Ni, 0.02% rare earth Re, and the balance Fe and unavoidable impurities.

[0095] A heat treatment method for ductile iron comprises the following steps: (1) Castings are cast according to the component ratio, and the heating rate is controlled to be 100℃ / h to heat the castings to 940℃. Normalizing heat treatment is performed for 2h, and then cooled with a water-based normalizing medium. The temperature of the water-based normalizing medium is 80℃. The maximum cooling rate of the first stage of 500-850℃ in the water-based normalizing medium cooling is 27℃ / s, and the maximum cooling rate of the second stage below 500℃ is 77℃ / s. The cooling characteristic curve is as follows: Figure 5As shown; the water-based normalizing medium includes 75 parts of water, 0.8 parts of polyacrylamide, 0.5 parts of preservative sodium benzoate and 0.6 parts of sodium nitrite; (2) The casting obtained in step (1) is subjected to stress relief annealing treatment at a temperature of 530°C and a holding time of 20h. Thereafter, the temperature in the furnace is reduced to ≤300°C at a cooling rate of 60°C / h, and the casting is taken out of the furnace and air-cooled.

[0096] Example 2 The present embodiment relates to a ductile iron and a heat treatment method. The ductile iron comprises, by mass, 3.4% C, 3.0% Si, 0.4% Mn, 0.03% Mg, 0.1% Mo, 0.2% Ni, 0.01% rare earth Re, and the balance Fe and unavoidable impurities.

[0097] A heat treatment method for ductile iron comprises the following steps: (1) Castings were cast according to the component ratio, and the temperature was raised to 870°C at a controlled rate of 30°C / h. The castings were subjected to normalizing heat treatment for 10 hours, and then cooled using a water-based normalizing medium. The temperature of the water-based normalizing medium was 0°C. The maximum cooling rate of the first stage of 500-850°C in the water-based normalizing medium cooling was 18°C / s, and the maximum cooling rate of the second stage below 500°C was 33°C / s. The water-based normalizing medium included 80 parts of water, 6 parts of polyacrylamide, 5 parts of sodium alginate, 5 parts of polyvinyl alcohol, 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite. (2) The casting obtained in step (1) is subjected to stress relief annealing treatment at a temperature of 590°C and a holding time of 2h. Thereafter, the temperature in the furnace is reduced to ≤300°C at a cooling rate of 50°C / h, and the casting is taken out of the furnace and air-cooled.

[0098] Example 3 The present embodiment relates to a ductile iron and a heat treatment method. The ductile iron comprises, by mass, 3.7% C, 2.7% Si, 0.4% Mn, 0.05% Mg, 0.04% Mo, 0.5% Cu, 0.03% Ni, 0.005% rare earth Re, and the balance Fe and unavoidable impurities.

[0099] A heat treatment method for ductile iron comprises the following steps: (1) Castings are cast according to the component ratio, and the heating rate is controlled at 50℃ / h to heat the castings to 900℃. Normalizing heat treatment is performed for 5h, and then cooled with a water-based normalizing medium. The temperature of the water-based normalizing medium is 60℃. The maximum cooling rate of the first stage of 500-850℃ in the water-based normalizing medium cooling is 18℃ / s, and the maximum cooling rate of the second stage below 500℃ is 33℃ / s. The cooling characteristic curve is as follows: Figure 6As shown; the water-based normalizing medium includes 80 parts of water, 6 parts of polyacrylamide, 5 parts of sodium alginate, 5 parts of polyvinyl alcohol, 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite; (2) The casting obtained in step (1) is subjected to stress relief annealing treatment at a temperature of 570°C and a holding time of 10 hours. Thereafter, the temperature in the furnace is reduced to ≤300°C at a cooling rate of 40°C / h, and the casting is taken out of the furnace and air-cooled.

[0100] Example 4 The difference between this embodiment and embodiment 3 is that the Si content is 2.2%, and the rest is the same as embodiment 3.

[0101] Example 5 The difference between this embodiment and embodiment 3 is that the C content is 3.4%, the Si content is 2.2%, and the rest is the same as embodiment 3.

[0102] Example 6 The difference between this embodiment and embodiment 3 is that there is no Mn and the Cu content is 0.2%. The rest is the same as embodiment 3.

[0103] Example 7 The difference between this embodiment and embodiment 3 is that 0.01% of Sb is further included, and the rest is the same as embodiment 3.

[0104] Example 8 The difference between this embodiment and embodiment 3 is that it further includes 0.04% Sn, and the rest is the same as embodiment 3.

[0105] Example 9 The difference between this embodiment and embodiment 3 is that in step (1), the heating rate is controlled to be 150°C / h to heat the casting to 900°C, and the rest is the same as embodiment 3.

[0106] Example 10 The difference between this embodiment and embodiment 3 is that in the water-based normalizing medium cooling, the water-based normalizing medium includes 99 parts of water, 0.3 parts of polyacrylamide, 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite. The maximum cooling rate of 500-850°C in the first stage is 140°C / s, and the maximum cooling rate of below 500°C in the second stage is 150°C / s. The rest is the same as in embodiment 3. The cooling characteristic curve is shown in FIG. Figure 7 As shown, the rest is the same as Example 3.

[0107] Example 11 The difference between this embodiment and embodiment 3 is that in the water-based normalizing medium cooling, the water-based normalizing medium includes 90 parts of water, 28 parts of sodium polyacrylate, 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite. The maximum cooling rate of 500-850°C in the first stage is 13.5°C / s, which is lower than 20°C / s. The maximum cooling rate of below 500°C in the second stage is 8.5°C / s, which is lower than 10°C / s. The rest is the same as embodiment 3. The cooling characteristic curve is shown in FIG. Figure 8 As shown, the rest is the same as Example 3.

[0108] Example 12 The difference between this embodiment and embodiment 3 is that the stress relief annealing treatment temperature in step (2) is 500° C., and the rest is the same as embodiment 3.

[0109] Example 13 The difference between this embodiment and embodiment 3 is that after the stress relief annealing treatment in step (2), the temperature in the furnace is lowered to ≤300°C at a cooling rate of 80°C / h, and the rest is the same as embodiment 3.

[0110] Comparative Example 1 The difference between this embodiment and embodiment 3 is that the Mo content is 0.2%, and the rest is the same as embodiment 3.

[0111] Comparative Example 2 The difference between this embodiment and embodiment 3 is that the Si content is 3.2%, and the rest is the same as embodiment 3.

[0112] Comparative Example 3 The difference between this embodiment and embodiment 3 is that the Mn content is 0.8%, and the rest is the same as embodiment 3.

[0113] Comparative Example 4 The difference between this embodiment and embodiment 3 is that the Mg content is 0.02%, and the rest is the same as embodiment 3.

[0114] Comparative Example 5 The difference between this embodiment and embodiment 3 is that Cu and Ni are not added, and the rest is the same as embodiment 3.

[0115] Comparative Example 6 The difference between this embodiment and embodiment 3 is that the Re content is 0.04%, and the rest is the same as embodiment 3.

[0116] Comparative Example 7 The difference between this embodiment and embodiment 3 is that the C content is 3.9%, and the rest is the same as embodiment 3.

[0117] Comparative Example 8 The difference between this embodiment and embodiment 3 is that the C content is 4.2%, and the rest is the same as embodiment 3.

[0118] Comparative Example 9 The difference between this comparative example and Example 3 is that the water-based normalizing medium cooling in step (1) is cooled by pure water, the water temperature is 60°C, and the cooling characteristic curve is as follows: Figure 9 As shown, the rest is the same as Example 3.

[0119] Comparative Example 10 The difference between this comparative example and Example 3 is that in step (1), the water-based normalizing medium cooling is replaced by air cooling, and the rest is the same as Example 3.

[0120] Comparative Example 11 The difference between this comparative example and Example 3 is that in step (1), the water-based normalizing medium cooling is replaced by air cooling, and the blowing rate is 10-20 m / s. The rest is the same as Example 3.

[0121] The ductile iron materials obtained in the above embodiments and comparative examples were tested in the following Test Example 1 and Test Example 2. Both test examples were conducted on samples removed from test blocks prepared in accordance with the GB / T 1348-2019 "Ductile Iron Castings" standard. It is a common practice in this field to define the grade of casting materials by the test performance of samples removed from the test blocks according to the standard. According to the test results, the ductile iron obtained by this scheme can meet the mechanical property requirements of QT900-2 grade in the GB / T 1348-2019 "Ductile Iron Castings" standard; at the same time, the GB / T 1348-2019 "Ductile Iron Castings" standard indicates that the main body sample is affected by multiple factors and may not be representative. In addition, in this industry, in order to avoid damaging the product body, planetary carrier products usually use test block performance as the product acceptance standard, so the test block performance should be used as the judgment standard first.

[0122] Test Example 1 The ductile iron materials prepared in the above-described embodiments and comparative examples were subjected to metallographic testing, and the test results are shown in Table 1. When testing the pearlite lamellar spacing of the ductile iron materials in Table 1, at least ten circular areas with a diameter of 2 mm were selected for testing. Within each 2 mm diameter circular area, ten fields of view (magnified 1000x or more) were randomly selected. The suspected widest pearlite lamellae within the field of view were searched for, and 10-20 lamellae were measured in each field of view. The average of the lamellae spacing measured in each field of view was used as the pearlite lamellar spacing for that field of view. The lamellae spacings of these ten fields of view were then summed and averaged to obtain the value obtained as the lamellae spacing for the 2 mm diameter circular area.

[0123] Since the test was carried out in at least 10 circular areas with a diameter of 2 mm, each area has a lamellar spacing in the area. Therefore, the data in this column of Table 1 are range values, which means that the pearlite lamellar spacing obtained from at least 10 circular areas with a diameter of 2 mm are all within this range. The maximum and minimum values ​​of the range can also represent the maximum value (|AB|) of the absolute value of the difference between the pearlite lamellar spacing obtained from any two circular areas with a diameter of 2 mm. A is the pearlite lamellar spacing of one of the circular areas with a diameter of 2 mm, and B is the pearlite lamellar spacing of the other circular area with a diameter of 2 mm.

[0124] Table 1

[0125] In the above table, since Example 10, Comparative Example 1 and Comparative Example 9 formed a martensitic matrix, which does not meet the matrix requirements of ductile iron in this application, the data of the average spacing of the lamellar layers of Comparative Example 1 and Comparative Example 9 are given.

[0126] Figure 1 This is a metallographic photograph of the ductile iron of Example 3. Figure 2 This is a scanning electron microscope test image of the ductile iron material of Example 3. It can be seen intuitively that the average spacing between the pearlite lamellae of this material is mainly in the range of 30-450nm, and the spacing difference between adjacent lamellae is small, which proves that the pearlite structure has better uniformity.

[0127] Figure 3 This is a metallographic photograph of the ductile iron of Comparative Example 10. Figure 4 This is a scanning electron microscope test image of the ductile iron material of Comparative Example 10. It can be seen that the distance between the pearlite sheets of this material is 500-1000nm, and the distance between adjacent sheets is quite different.

[0128] Test Example 2 The ductile iron materials prepared in the above examples and comparative examples were subjected to mechanical tests, and the test results are shown in Table 2.

[0129] Table 2

[0130] Based on the above data, the ductile iron of the present application has a pearlite matrix and a low average lamellar spacing, resulting in higher overall mechanical properties. Since Example 10, Comparative Examples 1, and 9 formed a martensitic matrix, which does not meet the matrix requirements for the ductile iron of the present application, fatigue strength testing was not performed on Comparative Examples 1 and 9.

[0131] When the components and heat treatment methods of the above embodiments and comparative examples are used to prepare planetary carriers with a wall thickness of ≥10 mm and a weight of 0.5-30 tons, cracks will appear in the materials of Examples 9, 10, and 12, causing the planetary carriers to be unable to function normally.

[0132] A comparison of Example 4 and Example 3 shows that a reduced Si content weakens the solid solution strengthening effect of silicon, thereby reducing the fatigue performance of ductile iron. A comparison of Comparative Example 2 and Example 3 shows that a Si content greater than 3.0% inhibits the formation of pearlite in the matrix, thereby reducing the strength and hardness of the ductile iron.

[0133] According to the comparison among Example 5, Example 4 and Example 3, when the C content is reduced to a sum of C+Si / 3 below 4.23%, the graphitization expansion during the solidification process is reduced, thereby causing the various properties of the ductile iron to deteriorate and the tendency to produce shrinkage to increase.

[0134] According to the comparison between Example 6 and Example 3, when the Mn and Cu contents are reduced to the point where (Mn + Cu) ≥ Si / 10 cannot be satisfied, the ability of Mn and Cu to promote pearlite is insufficient, the amount of pearlite in the matrix is ​​reduced, and the spacing between pearlite lamellae in the circular area with a diameter of 2 mm increases, thereby reducing the strength and hardness of the ductile iron material.

[0135] According to the comparison between Examples 7 and 8 and Example 3, adding Sb and Sn elements can promote the formation of pearlite and improve the strength and hardness of the ductile iron material.

[0136] According to the comparison between Example 9 and Example 3, it can be seen that if the normalizing temperature is raised too quickly, the residual stress in the normalizing process will increase, thereby causing cracks in the casting.

[0137] According to the comparison between Example 10 and Example 3, the cooling rate of the water-based normalizing medium in the first stage of 500-850° C. is too fast, which will increase the residual stress and thus cause cracks in the casting.

[0138] According to the comparison between Example 11 and Example 3, an increase in the amount of polyacrylamide in the water-based normalizing medium will reduce the cooling rate of the water-based normalizing medium, thereby reducing the strength and hardness of the ductile iron material and increasing the production cost.

[0139] A comparison of Example 12 with Example 3 shows that a stress relief annealing temperature that is too low can lead to poor stress relief at this stage, resulting in cracks in the casting. A comparison of Example 13 with Example 3 shows that a cooling rate that is too fast after the stress relief annealing can also increase residual stress in the ductile iron at this stage, leading to a decrease in various properties and, consequently, cracks in the casting.

[0140] According to the comparison between Comparative Example 1 and Example 3, when the Mo content is greater than 0.15%, martensite will be generated in the ductile iron matrix, resulting in abnormal structure and significant degradation of various properties.

[0141] According to the comparison between Comparative Example 3 and Example 3, an increase in the Mn content promotes the formation of cementite, thereby reducing the tensile strength, fatigue strength and elongation of the ductile iron.

[0142] According to the comparison between Comparative Example 4 and Example 3, a decrease in Mg content will lead to insufficient spheroidization ability of ductile iron, a decrease in spheroidization rate, and a decrease in tensile strength, fatigue strength and elongation of the material.

[0143] According to the comparison between Comparative Example 5 and Example 3, if Cu and Ni are not added, the ability to form pearlite in the matrix will decrease, thereby forming partial ferrite, resulting in a decrease in the strength and hardness of the ductile iron material.

[0144] According to the comparison between Comparative Example 6 and Example 3, an increase in Re content promotes the formation of fragmented graphite and reduces the spheroidization rate, thereby reducing the tensile strength, fatigue strength and elongation of ductile iron.

[0145] According to the comparison between Comparative Examples 7 and 8 and Example 3, an increase in C content will cause the graphite nodules to become larger, resulting in graphite blooming and a decrease in the spheroidization rate, thereby reducing the tensile strength, fatigue strength and elongation of the ductile iron material.

[0146] According to the comparison between Comparative Example 9 and Example 3, water cooling is used to cool the casting after normalizing. Figure 6 and Figure 9 It can be seen that the maximum cooling rate of water is 10-20 times the maximum cooling rate of water-based normalizing medium, which will cause martensite to form in the matrix and make it impossible to obtain ductile iron material with pearlite content ≥95%, resulting in abnormal structure and reduced performance of ductile iron.

[0147] According to the comparison between Comparative Example 10 and Example 3, it can be seen that when air cooling is used to cool the casting after normalizing, the maximum cooling rate of air cooling is about 5°C / s, which will increase the spacing between pearlite sheets and also produce more ferrite, thereby reducing the strength and hardness of ductile iron.

[0148] According to the comparison between Comparative Example 11 and Example 3, it can be seen that using air cooling to cool the casting after normalizing will increase the pearlite plate spacing and increase the ferrite content, thereby reducing the strength and hardness of the ductile iron material.

[0149] The foregoing is merely an embodiment of the present application, and the scope of protection of the present application is not limited by these specific embodiments, but is determined by the claims of the present application. For those skilled in the art, the present application may have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc. made within the technical ideas and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A ductile iron, characterized in that: The ductile iron comprises, by mass, 3.4%-3.8% C, 2.0%-3.0% Si, ≤0.6% Mn, and Mg. 0.03%-0.06%, Mo≤0.15%, 0.2%≤Cu+Ni≤1%, rare earth Re≤0.02%, the balance is Fe and unavoidable impurities.

2. The ductile iron according to claim 1, characterized in that The ductile iron comprises, by mass, 3.4%-3.8% C, 2.0%-3.0% Si, ≤0.6% Mn, and Mg. 0.03%-0.06%, Mo≤0.15%, Cu: 0.2-0.8%, Ni≤0.2%, rare earth Re≤0.02%, the balance is Fe and unavoidable impurities.

3. The ductile iron according to claim 2, characterized in that The ductile iron comprises, by mass, 3.4-3.8% C, 2.5-3.0% Si, ≤0.6% Mn, 0.03%-0.06% Mg, ≤0.15% Mo, 0.2-0.8% Cu, ≤0.2% Ni, ≤0.02% rare earth Re, and the balance Fe and unavoidable impurities.

4. The ductile iron according to claim 3, characterized in that The ductile iron comprises, by mass, 3.5-3.7% C, 2.5-3.0% Si, 0.3-0.5% Mn, 0.04-0.06% Mg, 0.4-0.8% Cu, ≤0.1% Mo, ≤0.1% Ni, ≤0.1% rare earth Re, ≤0.01%, and the balance being Fe and unavoidable impurities.

5. The ductile iron according to claim 1, characterized in that The ductile iron meets at least one of the following conditions: C+(Si / 3)=4.23-4.8%; Mn+Cu≥Si / 10.

6. The ductile iron according to any one of claims 1 to 5, characterized in that: The ductile iron meets the following conditions: Hardness is 270-320HB; The content of pearlite in the matrix is ​​≥95%.

7. A heat treatment method for obtaining the ductile iron according to any one of claims 1 to 6, characterized in that: The steps include: (1) subjecting a casting made of the ductile iron composition according to any one of claims 1 to 6 to normalizing heat treatment, and cooling the casting using a water-based normalizing medium; (2) The casting obtained in step (1) is subjected to stress relief annealing treatment.

8. The heat treatment method according to claim 7, characterized in that The temperature of normalizing heat treatment is 870-940℃ and the time is 2-10h.

9. The heat treatment method according to claim 7, characterized in that The heating rate in the normalizing stage is 30-100℃ / h.

10. The heat treatment method according to claim 7, characterized in that The temperature of the water-based normalizing medium is 0-80°C.

11. The heat treatment method according to claim 7, characterized in that The maximum cooling rate of the water-based normalizing medium at 500-850°C in the first stage is 10-30°C / s, and the maximum cooling rate below 500°C in the second stage is ≤100°C / s.

12. The heat treatment method according to claim 7, characterized in that Calculated by mass, the water-based normalizing medium includes 70-99 parts of water and 0.5-30 parts of thickener.

13. The heat treatment method according to claim 12, characterized in that The thickener is selected from at least one of polyacrylamide, sodium alginate, polyvinyl alcohol, sodium polyacrylate, and sodium carboxymethyl cellulose.

14. The heat treatment method according to claim 7, characterized in that The temperature of the stress relief annealing treatment is 530-590°C, and the holding time is 2-20h.

15. The heat treatment method according to claim 7, characterized in that: After stress relief annealing, the temperature in the furnace is lowered to ≤300°C at a cooling rate of ≤60°C / h, and the steel is taken out of the furnace for air cooling.

16. Use of the ductile iron according to any one of claims 1 to 6 or the ductile iron prepared by the heat treatment method according to any one of claims 7 to 15 in a planetary carrier.

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