High-pearlite nodular cast iron and application

Abstract

The invention discloses high-pearlite nodular cast iron and application, and belongs to the technical field of nodular cast iron. The nodular cast iron comprises, by mass, 3.4%-3.8% of C, 2.0%-3.0% of Si, 0.1%-0.6% of Mn, 0.03%-0.06% of Mg, less than or equal to 0.1% of Mo, less than 0.2% of Cu + Ni, less than 0.1% of Sn, less than 0.1% of Sb, less than or equal to 0.02% of rare earth Re and the balance Fe and inevitable impurities. The preparation method of the nodular cast iron comprises the following steps: (1) carrying out normalizing heat treatment on a casting cast by the components, and cooling by adopting a water-based normalizing medium; and (2) the casting obtained in the step (1) is subjected to stress relief annealing treatment. According to the nodular cast iron, the content of pearlite in a matrix can be increased, the use requirement of QT700-2 can be met, the preparation cost of the nodular cast iron is remarkably reduced, the production mode of the QT700-2 nodular cast iron is widened, the service reliability of a wind power gear box is improved, and the maintenance cost of the whole life cycle is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nodular cast iron, and particularly relates to a high pearlite nodular cast iron and application. BACKGROUND

[0002] With the accelerated promotion of the global clean energy strategy, the large-scale and high-power density of wind power equipment put forward higher requirements on the mechanical properties of core components. As a key load-bearing component of the wind power gearbox, the planet carrier usually needs to achieve the performance of QT700-2. The preparation of the existing QT700-2 nodular cast iron usually has a low pearlite content, although it meets the performance requirements of QT700-2, but under the condition of periodic changes of wind power load, deformation accumulation easily occurs during long-term use, which affects the service reliability of the wind power gearbox and increases the whole life cycle maintenance cost. SUMMARY

[0003] In order to solve the above problems, a high pearlite nodular cast iron is provided, which reduces the addition amount of alloying elements and cooperates with the process of water-based normalizing medium cooling, improves the content of pearlite in the matrix, and significantly reduces the preparation cost of the nodular cast iron, widens the production mode of QT700-2 nodular cast iron, and the planet carrier prepared by using the nodular cast iron material can significantly improve the service reliability of the wind power gearbox and reduce the whole life cycle maintenance cost.

[0004] According to one aspect of the application, a high pearlite nodular cast iron is provided, which comprises, in mass fraction: C: 3.4%-3.8%, Si: 2.0%-3.0%, Mn: 0.1%-0.6%, Mg: 0.03%-0.06%, Mo≤0.1%, Cu+Ni<0.2%, Sn<0.1%, Sb<0.1%, rare earth Re≤0.02%, and the balance is Fe and unavoidable impurities. The preparation method of the nodular cast iron comprises the following steps: (1) normalizing heat treatment is performed on the castings cast according to the above components, and water-based normalizing medium cooling is adopted; (2) stress relief annealing treatment is performed on the castings obtained in step (1).

[0005] However, the spherulitic cast iron prepared by the components and the preparation method can overcome the traditional technical prejudice, can form more than 90% of pearlite in the matrix under the addition of low alloy content, obtain high pearlite spherulitic cast iron, and the tensile strength of the spherulitic cast iron is greater than 700 MPa, which can meet the use requirements of QT700-2. Meanwhile, the preparation method can promote the uniform dispersion of each component in the spherulitic cast iron, improve the microstructure uniformity, significantly improve the tensile strength and yield strength, and maintain a certain elongation, which is convenient for relieving stress concentration and reducing the risk of sudden fracture of the casting.

[0006] In the components of the spherulitic cast iron, the contents of Mo, Cu, Ni, Sn, Sb and rare earth Re can all be 0, that is, the above elements can not be added in the spherulitic cast iron, which can further reduce the production cost of the spherulitic cast iron. Meanwhile, when the above elements are not added, the element types in the spherulitic cast iron are less, and the elements are more easily uniformly dispersed in the spherulitic cast iron, which is beneficial to improve the performance uniformity of the spherulitic cast iron. In addition, too many element types in the spherulitic cast iron will cause the impurity content in the spherulitic cast iron to increase due to the addition of too many elements, so when the addition amount of the above elements is 0, the impurity content in the spherulitic cast iron can be reduced, and the mechanical property stability of the material can be improved.

[0007] In the spherulitic cast iron, if the content of C is less than 3.4%, the tendency of shrinkage porosity increases, if the content of C is greater than 3.8%, the graphite will float on the top surface of the casting; if the content of Si is less than 2.0%, the strengthening effect of Si on the matrix is insufficient, which reduces the mechanical properties of the material, and too low Si content will increase the shrinkage porosity, if the content of Si is greater than 3.0%, the amount of pearlite will be insufficient, which reduces the mechanical properties of the material; if the content of Mn is greater than 0.6%, the casting is prone to have cementite, which reduces the product quality, and if the content of Mn is less than 0.1%, the amount of pearlite will be insufficient, which reduces the mechanical properties of the material. For the planet carrier of 0.5-30 tons, if the content of Mg in the spherulitic cast iron is less than 0.03%, the casting is prone to have vermicular graphite, which reduces the spheroidizing ability of the spherulitic cast iron, reduces the spheroidizing rate, and reduces the tensile strength and elongation of the material, and if the content of Mg is higher than 0.06%, the casting is prone to have shrinkage porosity.

[0008] In the spherulitic cast iron, if the content of Mo is greater than 0.1%, the cost will greatly increase, and when the content of Mo is greater than 0.15%, martensite is prone to appear, which makes the content of pearlite in the matrix of the spherulitic cast iron less than 90%, if the content of Cu+Ni is higher than 0.2%, the cost will greatly increase, if the content of Sn is greater than 0.1%, the cost will greatly increase, and if the content of Sb is greater than 0.1%, the cost will also greatly increase.

[0009] The rare earth Re element in the spheroidal graphite cast iron of the present application is low, which is suitable for preparing large pieces of more than 500 kg, such as large planetary gear, if the rare earth element is greater than 0.02%, it will promote the formation of chunky graphite in the preparation of thick pieces, resulting in the decline of various properties of spheroidal graphite cast iron.

[0010] Alternatively, the spheroidal graphite cast iron is prepared from the following components: C: 3.4-3.8%, Si: 2.0-2.5%, Mn: 0.3%-0.6%, Mg: 0.03%-0.06%, rare earth Re≤0.02%, the balance being Fe and unavoidable impurities. This scheme is a better scheme based on cost consideration, which can greatly reduce the production cost of spheroidal graphite cast iron.

[0011] Alternatively, the spheroidal graphite cast iron is prepared from the following components: C: 3.8%, Si: 2.0%, Mn: 0.6%, Mg: 0.06%, Mo: 0.1%, Cu: 0.19%, Sn: 0.09%, Sb: 0.09%, rare earth Re: 0.02%, the balance being Fe and unavoidable impurities. This scheme is a better scheme based on performance consideration, which can improve the performance of the material while ensuring the cost as low as possible.

[0012] Alternatively, the pearlite content in the spheroidal graphite cast iron matrix is ≥90%.

[0013] Alternatively, the spheroidal graphite cast iron at least meets one of the following conditions: (C+Si / 3)=4.23-4.8%.

[0014] In the above spheroidal graphite cast iron, C and Si are maintained at this ratio, which is close to the eutectic composition, so that the solidification temperature range of the spheroidal graphite cast iron casting is narrowed, the transition from liquid phase to solid phase is more concentrated, the liquid phase flowability is maintained for a longer time during the crystallization process, and the last solidification area can be more timely supplemented and spheroidal graphite precipitation is promoted. At the late stage of spheroidal graphite cast iron solidification, spheroidal graphite precipitation will produce a large volume expansion, which can effectively compensate for the solidification shrinkage and solid state shrinkage of the casting, and reduce the shrinkage porosity caused by shrinkage. Therefore, under the above limitation, the shrinkage porosity of the planetary gear is small, and the performance is more stable, if the content of C and Si exceeds the above range, shrinkage porosity will easily occur, which is difficult to meet normal use.

[0015] Alternatively, the spheroidal graphite cast iron meets the following conditions: hardness of 230-300 HB; pearlite content in the matrix is ≥90%.

[0016] Preferably, the hardness is 230-270 HB.

[0017] Alternatively, the spheroidal graphite cast iron meets the following conditions: The tensile strength is greater than 700 MPa. The yield strength is greater than 420 MPa.

[0018] The pearlite content in the matrix of the spheroidal graphite cast iron in the application is not less than 90% on the basis of low alloy content, so that the tensile strength and yield strength of the spheroidal graphite cast iron can meet the use requirements of QT700-2. At the same time, on the basis of low alloy content, the spheroidal graphite cast iron in the application can achieve the same hardness and strength as high-alloy-content spheroidal graphite cast iron, can withstand higher static load and alternating stress, and has strong anti-wear ability on the surface, which can reduce the size failure caused by wear, so the above-mentioned performance can ensure that the spheroidal graphite cast iron meets the use requirements of the planet carrier.

[0019] The limited condition "the pearlite content in the matrix" of the spheroidal graphite cast iron in the application refers to the content of pearlite in the whole matrix, which is obtained by referring to the standard detection of GB / T 9441-2021 "Metallographic examination of spheroidal graphite cast iron".

[0020] Optionally, the spheroidization rate of the spheroidal graphite cast iron is greater than or equal to 90%.

[0021] In the generation of the above-mentioned spheroidal graphite cast iron, the spheroidization rate of more than 90% can still be met under the condition of low alloy content, so as to ensure the improvement of the performance of the spheroidal graphite cast iron. The higher the spheroidization rate is, the less the irregular spheroidal graphite is, and the better the tensile strength, yield strength and elongation of the spheroidal graphite cast iron are.

[0022] Optionally, the graphite size of the spheroidal graphite cast iron is 5-8 grade.

[0023] Under the condition of meeting the formation of the above-mentioned high-content pearlite and the limitation of spheroidization rate, the graphite size of 5-8 grade in the spheroidal graphite cast iron in the application can ensure a certain strength, avoid the splitting of the matrix caused by too large graphite, and also provide better toughness to avoid insufficient expansion caused by too small graphite. The above-mentioned comprehensive effects improve the mechanical strength of the spheroidal graphite cast iron.

[0024] Optionally, the spheroidal graphite cast iron meets the following conditions: The elongation is greater than or equal to 2%.

[0025] The elongation of the spheroidal graphite cast iron of the present application is more than 2%, so that the material can produce certain plastic deformation when overloaded, relieve stress concentration, and absorb energy through micro deformation when subjected to impact or vibration load, thereby avoiding the failure of the planet carrier caused by complete brittle failure and reducing the risk of sudden breakage of the planet carrier. The above performance makes the spheroidal graphite cast iron of the present application a combination of "medium-high strength + medium-high hardness + moderate toughness", achieving the synergistic optimization of load-carrying capacity, wear resistance and failure resistance, which is suitable for heavy load and wear conditions and can maintain reliability in complex stress environment. At the same time, the component combination and preparation method of the spheroidal graphite cast iron have the advantages of controllable method and low cost. Under the combination, the spheroidal graphite cast iron with high pearlite content and tensile strength not less than 700 MPa can be obtained by low alloy content components, which solves the long-standing problem in the industry, reduces the production cost of spheroidal graphite cast iron, and opens up a new formula and production process for QT700-2 spheroidal graphite cast iron with high pearlite content, and points out a new direction for the research and development of low-cost, high-pearlite-content and high-performance spheroidal graphite cast iron.

[0026] Optionally, the temperature of the normalizing heat treatment is 870-940℃, and the time is 2-10h.

[0027] The normalizing heat treatment temperature and time promote the diffusion of each component in the casting, laying a foundation for the formation of pearlite, so as to obtain spheroidal graphite cast iron with pearlite content not less than 90% on the basis of low alloy content. If the normalizing heat treatment temperature is too high, the cost will increase, and the strength of the casting will decrease, making the casting more prone to deformation; if the normalizing heat treatment temperature is too low, the pearlite content formed in the matrix will decrease, making it difficult to improve the strength, hardness and elongation of the spheroidal graphite cast iron; if the normalizing heat treatment time is too short, the temperature of the casting at each position is not uniform, and the pearlite content is not uniform; if the normalizing heat treatment time is too long, the cost is high, and the casting is more prone to deformation.

[0028] The water-based normalizing medium is usually used by immersing the casting into the water-based normalizing medium for cooling. The mist cooling needs a special spraying device, which is relatively complex, and needs to precisely control the parameters such as the pressure and flow of the spraying, and the control difficulty is relatively large. Meanwhile, for the casting with complex structure or large size, the mist cooling is difficult to realize the full coverage of the casting, and the cooling uniformity is poor, which is easy to cause the deformation of the casting. When the mist cooling is used for cooling the casting with large wall thickness difference, the cooling non-uniformity of the casting is further increased, and the casting is more likely to be deformed, and it is difficult to obtain the casting with ideal size. Therefore, the nodular cast iron is cooled by the water-based normalizing medium on the basis of low alloy content, which can not only reduce the production cost, but also can promote the formation of more than 90% of pearlite in the matrix under the condition of low alloy content, so as to ensure the mechanical strength of the casting, and reduce the deformation of the casting. The water-based normalizing medium cooling of the application refers to that the casting is moved into the water-based normalizing medium within 20 minutes after being taken out of the normalizing heat treatment. The total time of the whole casting entering the water-based normalizing medium for cooling is not more than 2 hours.

[0029] Optionally, the heating rate of the normalizing stage is 30-100℃ / h.

[0030] The heating rate of the normalizing stage is 30-100℃, which is a relatively slow heating rate, and can ensure that the casting does not increase new residual stress during the heating process.

[0031] Optionally, the temperature of the water-based normalizing medium is 0-80℃.

[0032] The temperature of the water-based normalizing medium in the cooling stage of the application is lower than 80℃, which can effectively promote the contact between the water-based normalizing medium and the casting, improve the cooling efficiency of the casting, reduce the stress generation, and save the production cost.

[0033] Optionally, in the water-based normalizing medium cooling, the maximum cooling rate of the first stage at 500-850℃ is 10-30℃ / s, and the maximum cooling rate of the second stage below 500℃ is ≤100℃ / s.

[0034] Preferably, the maximum cooling rate below 500℃ is not less than 10℃ / s.

[0035] The cooling rate of the above water-based normalizing medium is measured by using a cooling characteristic tester. The specific test method is as follows: the temperature measuring probe (material Inconel 600) is heated to above 850℃, and then quickly immersed into the water-based normalizing medium (more than 600ml, to ensure that the temperature measuring probe is completely immersed) above, and the cooling process curve of the temperature measuring probe is recorded, or the cooling characteristic curve is drawn by computer at the same time in the form of data processing. Under the above test method, it refers to the test result obtained in the same medium and continuous cooling process.

[0036] The applicant finds that the cooling rate of the nodular cast iron casting core is not directly controlled by the cooling medium, but is controlled by the heat conduction between the core and the surface of the casting; under the conventional idea, the cooling rate of the temperature range of the austenite transformation pearlite is controlled to form the pearlite structure. However, the wall thickness of the large planetary gear casting is large, when the surface temperature of the casting is lower than the pearlite transformation temperature, the actual core of the casting can still be higher than the pearlite transformation temperature, at this time, due to the temperature difference between the surface and the surrounding medium, the heat dissipation of the casting is difficult, which makes the heat conduction of the core not timely; and in the conventional case, to avoid the cracking of the casting, the second stage cooling rate is usually slowed down, which makes the overall performance worse, and the performance uniformity also decreases.

[0037] Under the cooling rate of the above water-based normalizing medium cooling, the water-based normalizing medium and the casting contact interface form a vapor film in the first stage, the water-based normalizing medium is basically not in direct contact with the casting, the pure convection heat transfer cooling, the maximum cooling rate is maintained in the range of 10-30℃, which can promote the formation of pearlite, so as to ensure that the content of pearlite in the matrix can also be increased under low alloy content; the cooling rate range promotes the transformation of the thin wall part and the surface to pearlite, while avoiding the formation of bainite and martensite, improving the strength performance, and avoiding the decrease of plasticity (elongation). The cooling rate below 500℃ in the second stage can also be maintained at a high level, preferably the cooling speed in the temperature range of 400-500℃ is controlled in the range of 10-50℃ / S, so that the surface temperature of the casting and the core temperature of the casting can maintain a large temperature difference, promote the core to transform to pearlite, avoid the slow cooling of the core, and further avoid the internal heat transfer consistent with the external heat transfer, so as to break the formation of the platform period, promote the core to transform to pearlite, obtain fine structure, so that the content of the core pearlite is also above 90%, and the mechanical properties of the material are improved; if the second stage cooling rate is reduced within the above limited cooling rate range, the core structure is coarse and uneven, which leads to the decrease of various properties of nodular cast iron, especially the decrease of plasticity. In addition, the thin wall part and the surface of the casting in the second stage will not transform to bainite or martensite because they have already transformed to pearlite.

[0038] The application comprehensively considers the influence of components and structure on the thermal conductivity of the casting, coordinates the cooling speed of the outer wall and the core of the casting, so that the internal and external cooling rates are maintained in a high range, the heat conduction is not hindered, the pearlite content is increased, the uniformity of the structure of the large wall thickness casting is realized, the pearlite content of the outer wall and the core is consistent or close, and the difference between the internal and external pearlite content is not more than 3%. Therefore, for the planetary gear product with large thickness difference (the minimum wall thickness can be 10mm, the maximum wall thickness is more than 200mm, and even can reach 300mm), the above water-based normalizing medium cooling condition can ensure that the pearlite content of 90% or more can be obtained in the internal and external of the casting, the performance of the nodular cast iron is improved, and the nodular cast iron can meet the use requirements of QT700-2 under low alloy content.

[0039] The first stage cooling speed has the characteristic of fast cooling speed compared with air cooling and air blast cooling, and the reason why it can increase the pearlite content in the matrix to more than 90% under low alloy content is that carbon atoms need to diffuse to form cementite and ferrite during the pearlite transformation process, so the first stage cooling speed is fast, the carbon atom diffusion time is short, and there is no time to form graphite, more of which remains in austenite, directly combines with iron atoms to generate cementite crystal nucleus, resulting in higher pearlite content in the matrix under low alloy content.

[0040] Optionally, the water-based normalizing medium comprises 70-99 parts of water and 0.5-30 parts of thickening agent by mass fraction.

[0041] The addition of the thickening agent in the water-based normalizing medium can increase the viscosity of the water-based normalizing medium, and then affect the cooling process of the water-based normalizing medium. The fraction of the thickening agent can ensure the cooling effect of the water-based normalizing medium. If the amount of the thickening agent is small under the condition that the fraction of water is unchanged, the cooling rate of the casting does not meet the defined requirements, and the performance of the casting decreases, especially the pearlite content decreases significantly. The more the fraction of the thickening agent, the higher the concentration of the water-based normalizing medium, and the slower the cooling rate of the first stage and the second stage. If the amount of the thickening agent is too high, it will lead to high cost and slow cooling speed, and then the performance of the ductile cast iron decreases. However, if the cooling rate is still within the defined range, it can ensure that the pearlite content in the matrix is not less than 90%.

[0042] Optionally, the thickening agent is at least one selected from polyacrylamide, sodium alginate, polyvinyl alcohol, sodium polyacrylate and sodium carboxymethyl cellulose.

[0043] Optionally, the water-based normalizing medium further comprises 0.1-1 part of preservative and 0.1-1 part of rust inhibitor. The addition of the preservative can reduce the corruption and deterioration of the water-based normalizing medium. If the amount of the preservative is too high, it will lead to high cost, and if the amount of the preservative is too low, the preservative effect is not good and the service life of the water-based normalizing medium decreases. The addition of the rust inhibitor can prevent the surface of the casting from rusting after heat treatment. If the amount of the rust inhibitor is too high, it will lead to high cost, and if the amount of the rust inhibitor is too low, the rust prevention effect of the casting will be poor.

[0044] Optionally, the water-based normalizing medium comprises 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.

[0045] Optionally, the molecular weight of the polyacrylamide is 10-50 million, the molecular weight of the sodium alginate is 50-250 thousand, and the molecular weight of the polyvinyl alcohol is 100-300 thousand.

[0046] The absence of the preservative and the rust inhibitor in the water-based normalizing medium of the application does not affect the cooling rate of the medium, so as not to affect the performance of the product, and if the preservative and the rust inhibitor are not added, only the castings may be prone to rust in the later stage and reduce the service life of the water-based normalizing medium.

[0047] Optionally, the preservative comprises at least one of sodium benzoate, potassium sorbate, and nipagin.

[0048] Optionally, the rust inhibitor comprises at least one of sodium nitrite and triethanolamine borate.

[0049] Optionally, the temperature of the stress relief annealing treatment is 530-590℃, and the holding time is 2-20h.

[0050] The second stage cooling rate of the water-based normalizing medium of the application is relatively fast, and compared with ordinary air cooling, the introduced residual stress is high, so the temperature and time of the stress relief annealing treatment of the application are set in the above range, so as to eliminate the stress introduced in the second stage of the water-based normalizing medium cooling and maintain the structure of the pearlite structure, so that the pearlite content is maintained at more than 90%. If the temperature of the stress relief annealing treatment is lower than 530℃, the residual stress of the ductile cast iron will not be completely eliminated, and if the temperature of the stress relief annealing treatment is higher than 590℃, the pearlite will be decomposed, so it is difficult to ensure that the pearlite content of the ductile cast iron is not less than 90% under low alloy content, so the temperature of the stress relief annealing treatment is too high or too low, which will reduce the performance of the ductile cast iron.

[0051] Optionally, after the stress relief annealing, the temperature in the furnace is reduced to ≤300℃ at a cooling rate ≤60℃ / h, and the furnace is discharged for air cooling.

[0052] The cooling rate after the stress relief annealing treatment is set to be relatively slow, which 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 performance of the ductile cast iron.

[0053] According to still another aspect of the application, the application provides the use of the high pearlite ductile cast iron of any one of the above in a planet carrier.

[0054] The beneficial effects of the application include but are not limited to: 1. The high pearlite ductile cast iron according to the application, by the cooperation of components and preparation methods, can improve the tensile strength and yield strength compared with air cooling or air cooling heat treatment scheme, so as to meet the higher use standard of the planet carrier.

[0055] 2. The high pearlite nodular cast iron according to the present application solves the problem that it is difficult to prepare a nodular cast iron with a pearlite content of more than 90% by using a low alloy content, and makes the pearlite content in the matrix more than 90% under a low alloy content, and has good uniformity, so that the mechanical properties and the uniformity of the mechanical properties are improved on the basis of reducing the production cost.

[0056] 3. The high pearlite nodular cast iron according to the present application can be used to prepare a planet carrier with a tonnage of 0.5-30 tons, and when the wall thickness of the planet carrier is greater than or equal to 10 mm, the pearlite content in the matrix is still more than 90% under a low alloy content, the performance difference between the nodular cast iron test block and the planet carrier is reduced, and the service life of the planet carrier prepared from the nodular cast iron is improved.

[0057] 4. The high pearlite nodular cast iron according to the present application can promote the formation of pearlite in the nodular cast iron by using a water-based normalizing medium for cooling and controlling the cooling rate, so as to improve the pearlite content under a low alloy content and improve the performance of the nodular cast iron.

[0058] 5. The high pearlite nodular cast iron according to the present application comprehensively considers the influence of components and structures on the thermal conductivity of the castings, coordinates the cooling speed of the outer wall and the core of the castings, keeps the internal and external cooling speeds in a higher range, does not hinder the heat conduction, improves the pearlite content, and realizes the uniformity of the structure of the large-wall-thickness castings. BRIEF DESCRIPTION OF DRAWINGS

[0059] The drawings described herein are used to provide further understanding of the present application, constitute a part of the present application, the schematic embodiments of the present application and the description thereof are used to explain the present application, and do not constitute an improper limitation on the present application. In the drawings: Figure 1 It is a schematic diagram of a cooling characteristic tester related to the present application.

[0060] Figure 2 It is a cooling characteristic curve diagram of a water-based normalizing medium in Example 1 of the present application.

[0061] Figure 3 It is a cooling characteristic curve diagram of a water-based normalizing medium in Example 3 of the present application.

[0062] Figure 4 It is a cooling characteristic curve diagram of a water-based normalizing medium in Example 8 of the present application.

[0063] Figure 5 It is a cooling characteristic curve diagram of a water-based normalizing medium in Example 9 of the present application.

[0064] Figure 6 It is a cooling characteristic curve diagram of water in Comparative Example 10 of the present application.

[0065] Figure 7The microstructure photograph of the spheroidal graphite cast iron after corrosion of Example 3 of the present application.

[0066] Figure 8 The microstructure photograph of the spheroidal graphite cast iron after corrosion of Comparative Example 11 of the present application.

[0067] Figure 9 The microstructure photograph of the spheroidal graphite cast iron after corrosion of Comparative Example 12 of the present application.

[0068] Parts and list of reference numerals: 1, water-based normalizing medium; 2, tubular heating furnace, 3, temperature measuring probe, 4, starting point setter; 5, test recording system. DETAILED DESCRIPTION

[0069] The present application will be described in detail below with reference to examples, but the present application is not limited to these examples.

[0070] The raw materials in the examples and comparative examples of the present application are all purchased through commercial channels unless otherwise specified.

[0071] The methods used in the examples and comparative examples of the present application are conventional methods in the prior art unless otherwise specified. The specific operation of casting: after the processes of molding, clamping, smelting and pouring, the desired castings are obtained after solidification, and the operating conditions of the above methods can use the operating conditions commonly used in the art, which do not constitute a limitation on the present application.

[0072] Figure 1 The schematic diagram of the cooling characteristic tester is shown in the figure, and the test method using the tester is as follows: The temperature measuring probe 3 is fixed by the starting point setter 4, the tubular heating furnace 2 heats the temperature measuring probe 3 (material: Inconel 600) to above 850℃, and then it is quickly put into the water-based normalizing medium 1 with a volume of more than 600ml, the temperature measuring probe 3 is completely immersed in the water-based normalizing medium 1, and then the cooling process curve of the temperature measuring probe 3 is recorded by the test recording system 5, and the cooling characteristic curve is drawn by computer at the same time in the form of data processing.

[0073] Example 1 This example relates to a spheroidal graphite cast iron and a heat treatment method, which comprises, by mass fraction, C: 3.8%, Si: 2.0%, Mn: 0.6%, Mg: 0.06%, Mo: 0.01%, Cu: 0.19%, Sn: 0.09%, Sb: 0.09%, rare earth Re: 0.02%, and the balance is Fe and unavoidable impurities.

[0074] The heat treatment method of the spheroidal graphite cast iron comprises the following steps: (1) Casting the castings according to the component allocation ratio, controlling the heating rate to be 100℃ / h to heat the castings to 940℃, and then carrying out normalizing heat treatment for 2h, and then cooling with a water-based normalizing medium, the temperature of the water-based normalizing medium being 80℃, the maximum cooling rate in the first stage being 27℃ / s at 500-850℃, and the maximum cooling rate in the second stage being 77℃ / s below 500℃, and the cooling characteristic curve being as shown in Figure 2 The water-based normalizing medium comprises 75 parts of water, 0.8 parts of polyacrylamide (molecular weight being 30 million), 0.5 parts of preservative sodium benzoate, and 0.6 parts of sodium nitrite. (2) Carrying out stress relief annealing treatment on the castings obtained in step (1), the temperature of the stress relief annealing treatment being 530℃, and the holding time being 20h, and then reducing the temperature in the furnace to ≤300℃ at a cooling rate of 60℃ / h, and then taking out the castings to carry out air cooling, and then obtaining the castings.

[0075] Example 2 The present example relates to a nodular cast iron and a heat treatment method, the nodular cast iron comprising, in mass fraction, C: 3.4%, Si: 3.0%, Mn: 0.1%, Mg: 0.03%, Mo: 0.05%, Ni: 0.19%, Sb: 0.09%, rare earth Re: 0.01%, and the balance being Fe and unavoidable impurities.

[0076] The heat treatment method of the nodular cast iron comprises the following steps: (1) Casting the castings according to the component allocation ratio, controlling the heating rate to be 30℃ / h to heat the castings to 870℃, and then carrying out normalizing heat treatment for 10h, and then cooling with a water-based normalizing medium, the temperature of the water-based normalizing medium being 0℃, the maximum cooling rate in the first stage being 18℃ / s at 500-850℃, and the maximum cooling rate in the second stage being 33℃ / s below 500℃; the water-based normalizing medium comprises 80 parts of water, 6 parts of polyacrylamide (molecular weight being 20 million), 5 parts of sodium alginate (molecular weight being 100 thousand), 5 parts of polyvinyl alcohol (molecular weight being 200 thousand), 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite. (2) Carrying out stress relief annealing treatment on the castings obtained in step (1), the temperature of the stress relief annealing treatment being 590℃, and the holding time being 2h, and then reducing the temperature in the furnace to ≤300℃ at a cooling rate of 50℃ / h, and then taking out the castings to carry out air cooling, and then obtaining the castings.

[0077] Example 3 The present example relates to a nodular cast iron and a heat treatment method, the nodular cast iron comprising, in mass fraction, C: 3.7%, Si: 2.2%, Mn: 0.4%, Mg: 0.04%, rare earth Re: 0.005%, and the balance being Fe and unavoidable impurities.

[0078] A heat treatment method of nodular cast iron, comprising the following steps: (1) Casting the castings according to the component ratio, controlling the heating speed to be 50℃ / h to heat the castings to 900℃, carrying out normalizing heat treatment for 5h, and then cooling with a water-based normalizing medium, the temperature of the water-based normalizing medium being 60℃, the maximum cooling speed in the first stage being 18℃ / s at 500-850℃, and the maximum cooling speed in the second stage being 33℃ / s below 500℃, and the cooling characteristic curve being as shown in Figure 3 ; the water-based normalizing medium comprising 80 parts of water, 6 parts of polyacrylamide (molecular weight being 20 million), 5 parts of sodium alginate (molecular weight being 100 thousand), 5 parts of polyvinyl alcohol (molecular weight being 200 thousand), 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite; (2) Carrying out stress relief annealing treatment on the castings obtained in step (1), the temperature of the stress relief annealing treatment being 570℃, the holding time being 10h, and then reducing the temperature in the furnace to ≤300℃ at a reducing speed of 40℃ / h, and taking out the castings to carry out air cooling, and the nodular cast iron is obtained.

[0079] Example 4 The difference between this example and Example 3 is that the content of C is 3.4%, and the rest is the same as Example 3.

[0080] Example 5 The difference between this example and Example 3 is that it further comprises 0.05% of Sb, and the rest is the same as Example 3.

[0081] Example 6 The difference between this example and Example 3 is that it further comprises 0.05% of Sn, and the rest is the same as Example 3.

[0082] Example 7 The difference between this example and Example 3 is that the heating speed in step (1) is controlled to be 120℃ / h to heat the castings to 900℃, and the rest is the same as Example 3.

[0083] Example 8 The difference between this example and Example 3 is that in the cooling of the water-based normalizing medium, the water-based normalizing medium comprises 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 speed in the first stage being 140℃ / s at 500-850℃, and the maximum cooling speed in the second stage being 150℃ / s below 500℃, and the rest is the same as Example 3, and the cooling characteristic curve is as shown in Figure 4 , and the rest is the same as Example 3.

[0084] Example 9 The difference between the present example and Example 3 is that the water-based normalizing medium cooling comprises 90 parts of water, 28 parts of sodium polyacrylate, 0.7 parts of potassium sorbate, 0.8 parts of sodium nitrite, the maximum cooling rate of the first stage of 500-850℃ is 13.5℃ / s, lower than 20℃ / s, the maximum cooling rate of the second stage below 500℃ is 8.5℃ / s, lower than 10℃ / s, and the rest is the same as Example 3. The cooling characteristic curve is shown in Figure 5 , and the rest is the same as Example 3.

[0085] Example 10 The difference between the present example and Example 3 is that the stress relief annealing temperature in step (2) is 600℃, and the rest is the same as Example 3.

[0086] Example 11 The difference between the present example and Example 3 is that the furnace temperature is lowered to ≤300℃ at a cooling rate of 70℃ / h after the stress relief annealing in step (2), and the rest is the same as Example 3.

[0087] Comparative Example 1 The difference between the present example and Example 3 is that the Mo content is 0.2%, and the rest is the same as Example 3.

[0088] Comparative Example 2 The difference between the present example and Example 3 is that the Si content is 1.5%, and the rest is the same as Example 3.

[0089] Comparative Example 3 The difference between the present example and Example 3 is that no Mn is added, and the rest is the same as Example 3.

[0090] Comparative Example 4 The difference between the present example and Example 3 is that the Mg content is 0.08%, and the rest is the same as Example 3.

[0091] Comparative Example 5 The difference between the present example and Example 3 is that it also includes 0.8% of Cu, and the rest is the same as Example 3.

[0092] Comparative Example 6 The difference between the present example and Example 3 is that it also includes 0.6% of Ni, and the rest is the same as Example 3.

[0093] Comparative Example 7 The difference between the present example and Example 3 is that the Re content is 0.04%, and the rest is the same as Example 3.

[0094] Comparative Example 8 The difference between the present example and Example 3 is that the C content is 3.9%, and the rest is the same as Example 3.

[0095] Comparative Example 9 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.

[0096] Comparative Example 10 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 6 As shown, the rest is the same as Example 3.

[0097] Comparative Example 11 The difference between this comparative example and Example 3 is that the water-based normalizing medium cooling in step (1) is replaced by air cooling, and the rest is the same as Example 3.

[0098] Comparative Example 12 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 15 m / s. The rest is the same as Example 3.

[0099] 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 QT700-2 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.

[0100] Test Example 1 The metallographic structure of the ductile iron materials prepared in the above examples and comparative examples was tested, and the test results are shown in Table 1.

[0101] Table 1

[0102] Figure 7 This is a metallographic photograph of the ductile iron after corrosion according to Example 3 of the present application. Figure 8 This is a metallographic photograph of the ductile iron after corrosion in Comparative Example 11 of this application. Figure 9 This is a metallographic photograph of the ductile iron after corrosion in comparative example 12 of the present application.Figures 7-9 It can be seen that the same composition castings, different cooling processes can obtain different pearlite content in the matrix. The pearlite content in the matrix of the comparative example 11 using air cooling or the comparative example 12 using air cooling will be insufficient, and it is difficult to obtain high pearlite content nodular cast iron by low alloy content.

[0103] Test Example 2 The mechanical test and estimated cost test of the nodular cast iron material prepared by the above examples and comparative examples are shown in Table 2. The estimated cost in Table 2 is calculated based on the cost of the components used in the comparative example 5. From Table 2, it can be seen that the material cost rises or falls compared to the comparative example 5.

[0104] Table 2

[0105] According to the above data, the nodular cast iron of the present application mainly has pearlite as the matrix at low alloy content, has higher overall mechanical properties, and has lower cost.

[0106] When the components and heat treatment methods of the above examples and comparative examples are used to prepare a planet carrier with a wall thickness of ≥10 mm and a weight of 0.5-30 tons, the materials of examples 7, 8 and 11 will crack, resulting in the planet carrier cannot be used normally.

[0107] According to the comparison of example 4 and example 3, when the C content is reduced to less than 4.23% of the sum of C+Si / 3, the graphitization expansion during solidification will be reduced, the pearlite content will not be affected, but the performance of the nodular cast iron will be reduced, and the tendency of shrinkage will be increased, but the overall cost will not change much.

[0108] According to the comparison of example 5, 6 and example 3, the increase of Sb and Sn elements will increase the pearlite content and mechanical properties of the matrix, but the cost will be increased accordingly.

[0109] According to the comparison of example 7 and example 3, the fast heating temperature of the normalizing treatment has little effect on the pearlite content and mechanical properties of the matrix, but it will increase the residual stress in the normalizing process, thereby causing the casting to crack.

[0110] According to the comparison of example 8 and example 3, the fast cooling rate of the water-based normalizing medium at the first stage of 500-850℃ will reduce the pearlite content, increase the martensite content, and also increase the residual stress, thereby causing the casting to crack.

[0111] From the comparison of Example 9 and Example 3, it can be seen that the increase of the polyacrylamide content in the water-based normalizing medium reduces the cooling speed of the water-based normalizing medium, slightly reduces the pearlite content, and reduces the strength and hardness of the ductile iron material. However, the increase of the content increases the cost of the water-based normalizing medium. The estimated cost in Table 2 refers only to the cost of the material components, and therefore the estimated cost in Table 2 is the same for Example 3 and Example 9.

[0112] From the comparison of Example 10 and Example 3, it can be seen that the stress relief annealing temperature that is too high causes the decomposition of pearlite, thereby reducing the tensile strength, yield strength, and hardness of the casting. From the comparison of Example 11 and Example 3, it can be seen that the cooling rate after the stress relief annealing is too fast, which has little effect on the pearlite content, but increases the residual stress in the ductile iron, thereby reducing the performance and causing cracks in the casting.

[0113] From the comparison of Comparative Example 1 and Example 3, it can be seen that the Mo content greater than 0.1% causes the generation of martensite in the ductile iron matrix, thereby causing abnormal structure and significantly reducing the performance.

[0114] From the comparison of Comparative Example 2 and Example 3, it can be seen that the Si content lower than 2.0% is insufficient to strengthen the matrix, thereby reducing the strength and hardness of the ductile iron. The decrease of the Si content corresponds to an increase of the Fe content, and the cost of Si is not much different from that of Fe, so the total cost is the same as that of Example 3.

[0115] From the comparison of Comparative Example 3 and Example 3, it can be seen that the absence of Mn causes a small amount of pearlite, thereby reducing the tensile strength and yield strength of the ductile iron. The absence of Mn corresponds to an increase of the Fe content, and the cost of Mn is not much different from that of Fe, so the total cost is the same as that of Example 3.

[0116] From the comparison of Comparative Example 4 and Example 3, it can be seen that the increase of the Mg content has little effect on the pearlite content in the matrix, but causes the casting to be prone to shrinkage, thereby reducing the performance uniformity and the mechanical strength of the material. The increase of the Mg content depends on the increase of the nodulizer, and has little effect on the cost of the entire material.

[0117] From the comparison of Comparative Examples 5 and 6 and Example 3, it can be seen that the Cu and Ni contents greater than 0.2% increase the cost, but do not significantly increase the pearlite content, which proves that the ductile iron prepared by the low-alloy content according to the application can achieve the same pearlite content as the ductile iron prepared by the high-alloy content.

[0118] According to the comparison between Comparative Example 7 and Example 3, the increase in Re content has little effect on the pearlite content, but will promote the formation of fragmented graphite, resulting in a decrease in the spheroidization rate, thereby reducing the yield strength, tensile strength and elongation of the ductile iron. The increase in Re content depends on the increase in the spheroidizing agent and has little effect on the cost of the entire material.

[0119] According to the comparison between Comparative Examples 8 and 9 and Example 3, an increase in the C content will cause the graphite nodules to become larger, resulting in graphite blooming and a decrease in the spheroidization rate. It will also cause a slight decrease in the pearlite content in the matrix, thereby reducing the yield strength, tensile strength and elongation of the ductile iron material, but has little overall impact on the cost of ductile iron.

[0120] According to the comparison between Comparative Example 10 and Example 3, water cooling is used to cool the casting after normalizing. Figure 3 and Figure 6 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 it is impossible to obtain ductile iron material with a pearlite content ≥90% under low alloy content components, resulting in abnormal structure and a decrease in various properties of ductile iron.

[0121] According to the comparison between Comparative Example 11 and Example 3, 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 produce more ferrite, thereby reducing the strength and hardness of the ductile iron.

[0122] A comparison of Comparative Example 12 and Example 3 shows that using air cooling to cool normalized castings also increases the ferrite content, thereby reducing the strength and hardness of the ductile iron. Since the components in Comparative Examples 10-12 are identical, only the cooling method is changed. The costs in Table 2 refer to the component costs, so the costs for Comparative Examples 10-12 are the same as those for Example 3.

[0123] When the planetary carrier body product is prepared from ductile iron using the components and preparation method of the present application, the body can ensure a hardness of 230-300HB, a yield strength of more than 420MPa, and a pearlite content of not less than 90%.

[0124] 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 high pearlite ductile iron, characterized in that: The ductile iron comprises, by mass, 3.4%-3.8% C, 2.0%-3.0% Si, 0.1%-0.6% Mn, 0.03%-0.06% Mg, ≤0.1% Mo, <0.2% Cu+Ni, <0.1% Sn, <0.1% Sb, <0.1% rare earth Re, <0.02%, and the balance Fe and unavoidable impurities. The preparation method of the ductile iron comprises the following steps: (1) subjecting the castings made of the above-mentioned composition to normalizing heat treatment and cooling with a water-based normalizing medium; (2) The casting obtained in step (1) is subjected to stress relief annealing treatment.

2. The high pearlite ductile iron according to claim 1, characterized in that: The ductile iron is prepared from the following components: C: 3.4-3.8%, Si: 2.0-2.5%, Mn: 0.3%-0.6%, Mg: 0.03%-0.06%, rare earth Re≤0.02%, and the balance Fe and inevitable impurities.

3. The high pearlite ductile iron according to claim 1 or 2, characterized in that: The pearlite content in the ductile iron matrix is ​​≥90%.

4. The high pearlite ductile iron according to claim 1, characterized in that: The temperature of normalizing heat treatment is 870-940℃ and the time is 2-10h.

5. The high pearlite ductile iron according to claim 1, characterized in that: The heating rate in the normalizing stage is 30-100℃ / h.

6. The high pearlite ductile iron according to claim 1, characterized in that: The temperature of the water-based normalizing medium is 0-80°C.

7. The high pearlite ductile iron according to claim 1, 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.

8. The high pearlite ductile iron according to claim 1, characterized in that: Calculated by mass, the water-based normalizing medium includes 70-99 parts of water and 0.5-30 parts of thickener.

9. The high pearlite ductile iron according to claim 8, characterized in that: The thickener is selected from at least one of polyacrylamide, sodium alginate, polyvinyl alcohol, sodium polyacrylate, and sodium carboxymethyl cellulose.

10. The high pearlite ductile iron according to claim 1, characterized in that: The temperature of the stress relief annealing treatment is 530-590°C, and the holding time is 2-20h.

11. The high pearlite ductile iron according to claim 1, 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.

12. Use of the high pearlite ductile iron according to any one of claims 1 to 11 in a planetary carrier.

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