A ductile cast iron and a heat treatment method and application thereof

Abstract

The application discloses nodular cast iron and a heat treatment method and application thereof, and belongs to the technical field of nodular cast iron. The nodular cast iron comprises the following components in mass fraction: C: 3.4%-3.8%, Si: 2.0%-3.0%, Mn: 0.6% or less, Mg: 0.03%-0.06%, Mo: 0.15% or less, Cu+Ni: 0.2%-1%, rare earth Re: 0.02% or less, and the balance of Fe and inevitable impurities. The nodular cast iron has high pearlite content and small interlamellar spacing, can significantly improve fatigue strength, tensile strength and yield strength, and has the advantages of low cost; and a planet carrier prepared from the nodular cast iron material can significantly improve the service reliability of a wind power gear box and reduce the whole life cycle maintenance cost.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nodular cast iron, and particularly relates to a nodular cast iron and a heat treatment method and application thereof. 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 a wind power gearbox, a planet carrier is usually made of nodular cast iron material, which has good casting performance and cost advantage, but it is difficult to improve its performance.

[0003] At present, the performance improvement of the planet carrier mainly has the following ways: (1) increasing the amount of alloying elements such as copper, molybdenum, nickel, etc., still adopting normalizing treatment, which leads to high material cost and limited performance improvement, and a large amount of use also has a certain negative impact on the performance; (2) using isothermal quenching method to obtain isothermal quenching nodular cast iron (ADI material), but the ADI material also needs to add expensive metals such as molybdenum and nickel, and the ADI material also needs isothermal quenching treatment, which has high heat treatment cost, and it is difficult to quench the thick and large castings such as planet carriers, which easily leads to uneven organization and unqualified performance.

[0004] Therefore, it is an urgent technical problem to develop a nodular cast iron with excellent performance and a corresponding heat treatment method to coordinate the casting performance and cost. SUMMARY

[0005] In order to solve the above problems, a nodular cast iron is provided, which has high pearlite content and smaller interlamellar spacing, can significantly improve the fatigue strength, tensile strength and yield strength, and has the advantage of low cost, 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.

[0006] According to one aspect of the application, a nodular cast iron is provided, which comprises, in mass fraction, 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 is Fe and unavoidable impurities.

[0007] The nodular cast iron prepared by using the above components has high pearlite content and finer pearlite, which can improve the uniformity of the internal organization of the nodular cast iron, thereby improving its tensile strength, yield strength and fatigue strength, and also maintaining a certain elongation, which is convenient for relieving stress concentration and reducing the risk of sudden fracture of the casting.

[0008] In the spheroidal graphite cast iron of the present application, in addition to the above-mentioned elements, other metal elements can be added, and the balance is Fe and unavoidable impurities, such as Sb and Sn, which still belong to the protection scope of the present application and achieve the same technical effects, but the addition of new elements will increase the cost, which can be increased according to the production demand.

[0009] In the spheroidal graphite cast iron of the present application, Si: 2.0-3.0%, under the heat treatment method of the present application, Si in the material has the effect of inhibiting the formation of pearlite; Mn and Cu and Ni have a synergistic effect, which can promote the formation of pearlite and the refinement of the interlamellar spacing of pearlite, so under the limitation of the above-mentioned elements, the formation of pearlite can be promoted and the average interlamellar spacing of pearlite can be refined, thereby improving the performance of the material. Since the mechanism of Ni and Cu is similar, as long as the total content of Ni and Cu is within the above-mentioned range, it is acceptable.

[0010] If the content of Mn in the spheroidal graphite cast iron of the present application is greater than 0.6%, the castings are prone to have cementite, which reduces the product quality; for a planetary carrier of 0.5-30 tons, if the content of Mg in the spheroidal graphite cast iron is lower than 0.03%, the castings are prone to have vermicular graphite, which reduces the product performance, and if the content of Mg is higher than 0.06%, the castings are prone to have shrinkage; if the content of Mo in the spheroidal graphite cast iron is higher than 0.15%, since the water-based normalizing medium has a faster cooling speed, martensite is more likely to be formed, and it is difficult to obtain the spheroidal graphite cast iron material with a pearlite content of ≥95% according to the method of the present application.

[0011] The content of rare earth Re element in the spheroidal graphite cast iron of the present application is relatively low, which is suitable for preparing large pieces of more than 500 kg, such as large planetary carriers, and if the content of rare earth element is greater than 0.02%, it will promote the formation of chunky graphite in the preparation of thick and large pieces, which will reduce the performance of the spheroidal graphite cast iron.

[0012] Compared with the alloying scheme, the component ratio of the above-mentioned spheroidal graphite cast iron directly leads to the decrease of the plasticity of the castings, and the alloying elements also reduce the heat conduction performance of the castings, which makes it easy to form coarse pearlite in the core in the second cooling stage, thereby reducing the plasticity of the castings; and the alloying elements promote the formation of cementite and are more sensitive to cooling speed, which is prone to have problems such as stress concentration and cracking, and cannot meet the performance requirements of the spheroidal graphite cast iron. The component ratio of the spheroidal graphite cast iron of the present application can solve the above-mentioned problems caused by the alloying elements, so as to obtain the castings with improved performance.

[0013] Optionally, the spheroidal graphite cast iron contains, in mass fraction, C: 3.4%-3.8%, Si: 2.0%-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.

[0014] The content of Cu and Ni in the spheroidal graphite cast iron is further defined here. Generally, the higher the content of Ni in the product of thick pieces, the better the performance of the product. Under the limitation of the subsequent heat treatment method, the content of Ni can be reduced on the basis of preparing the spheroidal graphite cast iron material with the same structure and performance, thereby realizing the reduction of production cost and facilitating the industrialized popularization and use. If the content of Cu is greater than the maximum value, the graphite morphology will be poor, and the cost will also be increased.

[0015] Optionally, the spheroidal graphite cast iron comprises, in mass fraction, 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 of Fe and inevitable impurities.

[0016] When the content of Si in the spheroidal graphite cast iron is in the range of 2.5-3.0%, the fatigue strength of the spheroidal graphite cast iron is further improved, which can be improved to more than 300 MPa. Under the improvement of the fatigue strength, the deformation resistance of the spheroidal graphite cast iron is further improved, thereby further prolonging the service life of the planet carrier.

[0017] Optionally, the spheroidal graphite cast iron comprises, in mass fraction, C: 3.5-3.7%, Si: 2.5-3.0%, Mn: 0.3-0.5%, Mg: 0.04-0.06%, Cu: 0.4-0.8%, Mo ≤0.1%, Ni ≤0.1%, rare earth Re ≤0.01%, and the balance of Fe and inevitable impurities.

[0018] Further limitation, the silicon used in the above spheroidal graphite cast iron plays a solid solution strengthening role to form Si-Fe phase, which can play a role of replacing solid solution and strengthening the matrix, and has a synergistic effect with the normalizing medium cooling and refining pearlite, and can further improve the fatigue performance. Compared with the scheme in which the content of Si is less than 2.5%, the scheme in which the content of Si is greater than 2.5% can further improve the fatigue strength performance of the material. When Si is less than 2.5%, the solid solution strengthening effect is not obvious, and the fatigue performance improvement is small. When Si is higher than 3.0%, the formation of pearlite can be obviously inhibited, and the content of ferrite in the matrix is increased.

[0019] Optionally, the spheroidal graphite cast iron at least meets one of the following conditions:

[0020] (C+Si / 3)=4.23-4.8%;

[0021] Mn+Cu ≥Si / 10.

[0022] In the above spheroidal graphite cast iron, when C and Si are kept in this ratio, close to eutectic composition, 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 keeps for a longer time during the crystallization process, the last solidification area can be fed more timely, and the precipitation of spheroidal graphite is promoted. In the late stage of spheroidal graphite cast iron solidification, a larger volume expansion is generated when spheroidal graphite is precipitated, and this expansion can effectively compensate the solidification shrinkage and solid state shrinkage of the casting, and reduce the shrinkage porosity generated by shrinkage. Therefore, under the above limitation, the shrinkage porosity of the planet carrier is small, and the performance is more stable. If the contents of C and Si exceed the above range, shrinkage porosity is prone to occur, and normal use is difficult to meet.

[0023] In the formation of the spheroidal graphite cast iron material matrix, Si and Mn and Cu have opposite effects on the formation of pearlite, and in order to ensure that the amount of pearlite is greater than or equal to 95%, the relationship Mn+Cu≥Si / 10 needs to be met. With the increase of Si content, the inhibition ability to pearlite is enhanced, and the elements Mn and Cu promoting pearlite need to be increased to offset the inhibition of Si to pearlite. The limitation of the relationship is more conducive to making the pearlite content, strength and hardness of the spheroidal graphite cast iron meet the requirements.

[0024] Alternatively, the spheroidal graphite cast iron comprises, in mass fraction, 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.

[0025] In the above spheroidal graphite cast iron, the decrease of Si content makes the tensile strength, yield strength and hardness performance of the material change little, but the fatigue performance is reduced. For some products with low fatigue performance requirements, the low Si content scheme can be used to reduce the production cost under the condition of meeting the use requirements.

[0026] Alternatively, the spheroidal graphite cast iron satisfies the following conditions:

[0027] The hardness is 270-320HB;

[0028] The pearlite content in the matrix is greater than or equal to 95%.

[0029] Alternatively, the spheroidal graphite cast iron satisfies the following conditions:

[0030] The tensile strength is greater than or equal to 900MPa;

[0031] The tensile-compression fatigue with stress ratio R=-1 is greater than or equal to 290MPa;

[0032] The yield strength is greater than or equal to 600MPa;

[0033] The pearlite lamellar spacing in a circular area with a diameter of 2 mm is 30-450 nm.

[0034] The pearlite in the spheroidal graphite cast iron of the present application is not less than 95%, and the pearlite lamellar spacing is small, which proves that the pearlite is finer, and can ensure that the structure of the spheroidal graphite cast iron is more uniform, so that the tensile strength, yield strength and fatigue strength of the spheroidal graphite cast iron are obviously improved compared with QT700-2. The spheroidal graphite cast iron of the present application has high hardness and high strength, can bear higher static load and alternating stress, and has strong anti-wear ability on the surface of the material, which can reduce the size failure caused by wear, so the above-mentioned performances can ensure that the spheroidal graphite cast iron meets the use requirements of the planet carrier.

[0035] The content of the pearlite in the matrix of the spheroidal graphite cast iron of the present application refers to the content of the 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".

[0036] The pearlite lamellar spacing in the spheroidal graphite cast iron of the present application refers to the distance between the centers of adjacent two ferrite or cementite in the lamellar pearlite structure. The measuring method of the pearlite lamellar spacing in a circular area with a diameter of 2 mm is as follows: using an optical microscope or a scanning electron microscope, randomly selecting N (N≥10) fields of view in a circular area with a diameter of 10 mm under a magnification of 1000 times or more, finding the widest pearlite lamellar in each field of view, measuring 10-20 lamellars in each field of view, taking the average value of the measured lamellar spacing in each field of view as the pearlite lamellar spacing of the field of view, and then summing up and averaging the pearlite lamellar spacings of the N fields of view to obtain the pearlite lamellar spacing of the circular area.

[0037] In addition, the proportions of sorbite and troostite in the pearlite of the spheroidal graphite cast iron are also obviously increased compared with the QT700-2 material. The spacing between sorbite and troostite is smaller than that between pearlite, and the higher the proportion, the higher the strength and hardness of the spheroidal graphite cast iron.

[0038] According to the above test method for the lamellar spacing, the pearlite lamellar spacing of the spheroidal graphite cast iron of the present application in any two different circular areas with a diameter of 2 mm is less than or equal to 400 nm, A is the pearlite lamellar spacing of one of the circular areas with a diameter of 2 mm, B is the pearlite lamellar spacing of the other circular area with a diameter of 2 mm, and |A-B| less than or equal to 400 nm represents that the material can not only reduce the pearlite lamellar spacing, but also improve the uniformity of the lamellar spacing, so that the performance is more uniform in macroscopic.

[0039] Preferably, |A-B| is less than or equal to 300 nm, more preferably |A-B| is less than or equal to 200 nm.

[0040] Optionally, the spherulitic ratio of the spheroidal graphite cast iron is greater than or equal to 90%.

[0041] In the production of the spheroidal graphite cast iron described above, a spherulitic ratio of more than 90% can still be achieved to ensure the improvement of various properties of the spheroidal graphite cast iron. The higher the spherulitic ratio, the smaller the proportion of irregular spheroidal graphite, and the better the tensile strength, yield strength, elongation, and fatigue strength of the spheroidal graphite cast iron.

[0042] Optionally, the graphite size in the spheroidal graphite cast iron is grade 5-8.

[0043] Under the above-mentioned limitations of pearlite formation and spherulitic ratio, the graphite size of grade 5-8 in the spheroidal graphite cast iron of the present application can ensure a certain strength, avoid the splitting of the matrix caused by excessively large graphite, provide better toughness, avoid insufficient expansion caused by excessively small graphite, and improve the fatigue strength of the spheroidal graphite cast iron material.

[0044] In addition to the pearlite, the balance of the spheroidal graphite cast iron matrix is ferrite or cementite.

[0045] Optionally, the ferrite content of the spheroidal graphite cast iron is less than or equal to 4%, and the ferrite is distributed around the graphite balls or dispersed in the matrix.

[0046] The cementite content of the spheroidal graphite cast iron is less than or equal to 1%, and the cementite is dispersedly distributed on the pearlite grain boundaries.

[0047] In addition to the pearlite, the spheroidal graphite cast iron of the present application contains ferrite or cementite. The smaller the ferrite content, the higher the strength and hardness of the spheroidal graphite cast iron. The ferrite is distributed around the graphite balls or dispersedly distributed in the matrix, which can improve the dispersion uniformity of the ferrite and avoid the agglomeration of the ferrite. Cementite is a high-hardness and high-brittle structure. The content of cementite in the spheroidal graphite cast iron of the present application is lower than that of ferrite, which can improve the elongation of the spheroidal graphite cast iron. The cementite is dispersedly distributed on the pearlite grain boundaries, which can further reduce the influence of cementite on performance compared to blocky cementite, to achieve the synergistic improvement of the strength, hardness, and elongation of the spheroidal graphite cast iron.

[0048] Optionally, the spheroidal graphite cast iron satisfies the following conditions:

[0049] The elongation is greater than or equal to 2%.

[0050] The elongation of the spheroidal graphite cast iron of the present application is above 2%, so that the material can produce certain plastic deformation when overloaded, and relieve stress concentration. When subjected to impact or vibration load, the material can absorb energy through micro-deformation to avoid the failure of the planet carrier caused by complete brittle failure, and reduce 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 "high strength + medium-high hardness + moderate toughness", achieving the synergistic optimization of load-carrying capacity, wear resistance and anti-failure ability, which is suitable for heavy load and wear conditions, and can maintain reliability in complex stress environment. At the same time, the composition and heat treatment method of the spheroidal graphite cast iron have the advantages of controllable method and low cost.

[0051] Optionally, the spheroidal graphite cast iron satisfies: elongation ≥ (tensile strength / 350)%.

[0052] Under the above limitation, the unit of tensile strength is MPa, and only the numerical value of tensile strength is used here, and the unit of MPa is not introduced into the elongation. Generally speaking, the higher the tensile strength of the spheroidal graphite cast iron material, the lower the elongation. Based on the use requirements of the planet carrier, the tensile strength of the spheroidal graphite cast iron material is usually increased as high as possible. However, when the tensile strength is too high, the elongation is too low, and the problem of brittle fracture of the casting during the stress process due to poor plasticity may occur. The pearlite interlamellar spacing of the spheroidal graphite cast iron of the present application is more uniform, which can reduce the internal defects and non-uniformity of the spheroidal graphite cast iron, which are often the starting point of crack initiation. At the same time, the uniform microstructure of the spheroidal graphite cast iron of the present application can more effectively disperse and withstand external load. Therefore, the spheroidal graphite cast iron can ensure that the elongation does not decrease too much while the strength is improved, realizing the dual effect of maintaining the tensile strength and the elongation.

[0053] The spheroidal graphite cast iron satisfies: 0.6 ≤ yield strength / tensile strength ≤ 0.75.

[0054] The ratio of yield strength and tensile strength of the spheroidal graphite cast iron of the present application will affect the elongation of the material. If the ratio is too large, the elongation will decrease, the material will be high in brittleness, and the planet carrier will fail due to brittle failure. If the ratio is too small, the elongation of the material will increase. The present application improves the yield strength and tensile strength of the spheroidal graphite cast iron, and makes the yield strength and tensile strength satisfy the above relationship, so as to coordinate the elongation of the spheroidal graphite cast iron, thereby significantly improving the anti-deformation ability of the spheroidal graphite cast iron.

[0055] According to another aspect of the present application, a heat treatment method for obtaining any one of the above spheroidal graphite cast irons is provided, comprising the following steps:

[0056] (1) subjecting the castings cast with the composition of any one of the above spheroidal graphite cast irons to normalizing heat treatment, and then cooling with water-based normalizing medium;

[0057] (2) the castings obtained in step (1) are subjected to stress relief annealing treatment and cooling, and the castings are obtained.

[0058] In the composition of the nodular cast iron, when the content of Mo is less than 0.15%, the castings are subjected to normalizing heat treatment by using the water-based normalizing medium, so that the proportion of pearlite in the matrix is increased, the pearlite interlamellar spacing is refined, and the performance of the nodular cast iron is improved.

[0059] Optionally, the temperature of the normalizing heat treatment is 870-940℃, and the time is 2-10h.

[0060] The normalizing heat treatment temperature and time promote the diffusion of each component in the castings and lay the foundation for the formation of pearlite. If the normalizing heat treatment temperature is too high, the cost will increase, and the strength of the castings will decrease, so that the castings are more prone to deformation; if the normalizing heat treatment temperature is too low, the content of the pearlite formed in the matrix decreases, so that it is difficult to improve the strength, hardness and elongation; if the normalizing heat treatment time is too short, the temperature of the castings at each position is not uniform, and the amount of pearlite is not uniform; if the normalizing heat treatment time is too long, the cost is high, and the castings are more prone to deformation.

[0061] When the water-based normalizing medium is used, the castings are usually only immersed in the water-based normalizing medium for cooling. The mist cooling needs special spraying equipment, and the equipment is relatively complex. In addition, the pressure and flow of the spraying need to be accurately controlled, and the control difficulty is relatively large. At the same time, for complex structure or large castings, the mist cooling is difficult to cover the castings completely, and the cooling uniformity is poor, which easily leads to deformation of the castings. When the mist cooling is used for cooling the castings with large wall thickness difference, the cooling non-uniformity of the castings will be further increased, so that the castings are more prone to deformation, and it is difficult to obtain castings with ideal size.

[0062] Optionally, the heating speed of the normalizing stage is 30-100℃ / h.

[0063] The heating speed of the normalizing stage is 30-100℃, which is a relatively slow heating speed, so that it can ensure that the castings do not increase new residual stress during the heating process.

[0064] Optionally, the temperature of the water-based normalizing medium is 0-80℃.

[0065] The temperature of the water-based normalizing medium in the cooling stage is less than 80℃, which can effectively promote the contact between the water-based normalizing medium and the castings, improve the cooling efficiency of the castings, reduce the stress generation, and save the production cost.

[0066] Optionally, in the cooling of the water-based normalizing medium, the maximum cooling speed of the first stage is 10-30℃ / s at 500-850℃, and the maximum cooling speed of the second stage is ≤100℃ / s below 500℃.

[0067] Preferably, the maximum cooling rate below 500℃ is not less than 10℃ / s.

[0068] The cooling rate of the above-mentioned water-based normalizing medium is measured by using a cooling characteristic tester. The specific test method is as follows: a temperature measuring probe (material: Inconel 600) is heated to above 850℃, and then it is quickly immersed in the water-based normalizing medium (to ensure that the temperature measuring probe is completely immersed) with a volume of more than 600ml. The cooling process curve of the temperature measuring probe is recorded, or the cooling characteristic curve is drawn by a computer in a data processing manner. Under the above-mentioned test method, it refers to the cooling rate obtained in the same medium and continuous cooling process.

[0069] The applicant finds that the cooling rate of the core of the ductile iron casting 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 for controlling the temperature range of the austenite-to-pearlite transformation 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 decrease of the temperature difference between the surface and the surrounding medium, the heat dissipation of the casting is difficult, which causes the heat of the core to be not timely conducted outward. In addition, in order to avoid the cracking of the casting, the cooling rate in the second stage is usually reduced under the conventional condition, which leads to the coarse lamellar spacing of the pearlite in the core of the casting, and the overall performance is poor.

[0070] Under the cooling rate of the above-mentioned water-based normalizing medium, a vapor film is formed at the interface between the water-based normalizing medium and the casting in the first stage, the water-based normalizing medium is basically not in direct contact with the casting, the cooling is maintained by pure convection heat transfer, the maximum cooling rate is in the range of 10-30℃, which can promote the formation of pearlite and reduce the lamellar spacing of pearlite. The cooling rate in this range promotes the transformation of the thin wall part and the surface to pearlite, and at the same time avoids the formation of bainite and martensite, so that fine pearlite structure is obtained, the strength performance is improved, and the plasticity (elongation) is avoided to decrease. The cooling rate below 500℃ in the second stage can also be maintained at a high level, and preferably the cooling rate at 400-500℃ is controlled in the range of 10-50℃ / s, so that the temperature difference between the surface temperature of the casting and the core temperature of the casting is maintained, the core is promoted to transform to pearlite, and the core is avoided to cool slowly, so that a platform period is formed when the heat transfer amount of the core to the surface is consistent with the heat dissipation amount of the surface to the surrounding medium, the core is promoted to transform to pearlite, fine structure is obtained, and the performance is improved. If the cooling rate in the second stage is reduced, the core structure is coarse and uneven, which leads to the decrease of various performances of the ductile iron, especially the decrease of plasticity. In addition, the thin wall part and the surface of the casting in the second stage have been transformed to pearlite, and will not be transformed to bainite or martensite.

[0071] Thus, for the planetary gear product thickness difference is large (the minimum wall thickness can be up to 10mm, the maximum is more than 200mm, and even up to 300mm), the above water-based normalizing medium cooling condition can ensure that dense pearlite structure can be obtained inside and outside the casting, and the performance is improved while avoiding excessive reduction of plasticity.

[0072] The first stage cooling speed has the characteristics of fast cooling speed compared with air cooling and air cooling, and the reason why it can reduce the pearlite lamella spacing is that in the process of pearlite transformation, carbon atoms need to diffuse to form cementite and ferrite. Therefore, the first stage cooling speed is fast, the carbon atom diffusion time is short, and in order to complete the transformation in a limited time, the pearlite lamella can only be formed with a small spacing to meet the diffusion distance and transformation kinetics requirements of carbon atoms. However, the second stage cooling speed cannot be too fast, otherwise there will be a large residual stress in the product casting, which will cause the casting to crack and be difficult to continue to use.

[0073] Optionally, the water-based normalizing medium comprises 70-99 parts of water, 0.5-30 parts of thickening agent by mass fraction.

[0074] The addition of the thickening agent in the above water-based normalizing medium can improve the viscosity of the water-based normalizing medium, and then affect the cooling process of the water-based normalizing medium. The fraction of the above 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 amount of water is constant, the cooling rate of the casting does not meet the defined requirements, and the performance of the casting is reduced. 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, the cost will be increased and the cooling speed will be too slow, which will reduce the performance of the nodular cast iron.

[0075] Optionally, the thickening agent is at least one selected from polyacrylamide, sodium alginate, polyvinyl alcohol, sodium polyacrylate, and sodium carboxymethyl cellulose.

[0076] Optionally, the water-based normalizing medium further comprises 0.1-1 parts of preservative and 0.1-1 parts 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, the cost will be increased. If the amount of the preservative is too low, the preservative 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 the surface of the casting from rusting after heat treatment. If the amount of the rust inhibitor is too high, the cost will be increased. If the amount of the rust inhibitor is too low, the rust prevention effect of the casting will be poor.

[0077] 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.

[0078] Optionally, the polyacrylamide has a molecular weight of 10-50 million, the sodium alginate has a molecular weight of 0.5-2.5 million, and the polyvinyl alcohol has a molecular weight of 1-3 million.

[0079] 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, and thus does not affect the performance of the product. If the preservative and the rust inhibitor are not added, only the castings may be prone to rust in the later stage and the service life of the water-based normalizing medium may be reduced.

[0080] Optionally, the preservative includes at least one of sodium benzoate, potassium sorbate, and propyl gallate.

[0081] Optionally, the rust inhibitor includes at least one of sodium nitrite and triethanolamine borate.

[0082] Optionally, the temperature of the stress relief annealing treatment is 530-590°C, and the holding time is 2-20h.

[0083] The second stage of the cooling of the water-based normalizing medium of the application has a faster cooling rate. Compared with ordinary air cooling, the introduced residual stress is higher. Therefore, the temperature and the 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 cooling of the water-based normalizing medium and maintain the structure of the pearlite organization. If the temperature of the stress relief annealing treatment is lower than 530°C, the residual stress of the ductile cast iron may not be completely eliminated. If the temperature of the stress relief annealing treatment is higher than 590°C, the pearlite may be decomposed. Therefore, the performance of the ductile cast iron may be reduced if the temperature of the stress relief annealing treatment is too high or too low.

[0084] Optionally, after the stress relief annealing, the temperature in the furnace is reduced to ≤300°C at a cooling rate of ≤60°C / h, and the furnace is discharged for air cooling.

[0085] The cooling rate after the stress relief annealing treatment is set to be slow, so as to ensure that no residual stress is generated in the cooling process and the stress generated in the cooling stage of the water-based normalizing is further eliminated, thereby improving the performance of the ductile cast iron.

[0086] According to still another aspect of the application, the application provides the use of the ductile cast iron prepared by the above-mentioned heat treatment method in a planet carrier.

[0087] The beneficial effects of the application include but are not limited to:

[0088] 1. The ductile cast iron according to the application, by optimizing the components and the heat treatment method, the tensile strength and the yield strength are improved compared with the original QT700-2 ductile cast iron, and the higher use standard of the planet carrier is met.

[0089] 2. The spheroidal graphite cast iron according to the present application, the pearlite content in the matrix is high, and the pearlite formed is finer, and the spheroidal graphite cast iron has better uniformity of the structure, and the mechanical properties and the uniformity of the mechanical properties can be improved.

[0090] 3. The spheroidal graphite cast iron according to the present application, the material can be used to prepare a planet carrier with a tonnage of 0.5-30 tons, and the planet carrier with a wall thickness of ≥10 mm can still meet the requirement that the pearlite content in the matrix is ≥95%, the performance difference between the spheroidal graphite cast iron test block and the planet carrier is reduced, and the service life of the planet carrier prepared from the spheroidal graphite cast iron is improved.

[0091] 4. The heat treatment method of the spheroidal graphite cast iron according to the present application, by using water-based normalizing medium cooling and controlling the cooling rate, the formation of the pearlite in the spheroidal graphite cast iron and the reduction of the interlamellar spacing can be promoted, the structure of the spheroidal graphite cast iron is refined, and the performance of the spheroidal graphite cast iron is improved. BRIEF DESCRIPTION OF DRAWINGS

[0092] The drawings described herein are used to provide further understanding of the present application, and constitute a part of the present application. The schematic embodiments of the present application and the descriptions thereof are used to explain the present application, and do not constitute an improper limitation on the present application. In the drawings:

[0093] Figure 1 The metallographic photograph of the spheroidal graphite cast iron related to Example 3 of the present application.

[0094] Figure 2 The scanning electron microscope photograph of the spheroidal graphite cast iron related to Example 3 of the present application.

[0095] Figure 3 The metallographic photograph of the spheroidal graphite cast iron related to Comparative Example 10 of the present application.

[0096] Figure 4 The scanning electron microscope photograph of the spheroidal graphite cast iron related to Comparative Example 10 of the present application.

[0097] Figure 5 The cooling characteristic curve diagram of the water-based normalizing medium in Example 1 of the present application.

[0098] Figure 6 The cooling characteristic curve diagram of the water-based normalizing medium in Example 3 of the present application.

[0099] Figure 7 The cooling characteristic curve diagram of the water-based normalizing medium in Example 10 of the present application.

[0100] Figure 8 The cooling characteristic curve diagram of the water-based normalizing medium in Example 11 of the present application.

[0101] Figure 9 The cooling characteristic curve diagram of water in Comparative Example 9 of the present application.

[0102] Figure 10 A schematic diagram of a cooling characteristic tester.

[0103] Parts and reference numeral list:

[0104] 1. Water-based normalizing medium; 2. Tubular heating furnace; 3. Temperature measuring probe; 4. Initial point setting device; 5. Test recording system. DETAILED DESCRIPTION

[0105] The present application will be described in detail below with reference to examples, but the present application is not limited to these examples.

[0106] The raw materials in the examples and comparative examples of the present application are all purchased through commercial channels unless otherwise specified.

[0107] 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 molding, clamping, melting, pouring process, the required casting is obtained after solidification, and the operating conditions of the above method can use the operating conditions commonly used in the art, which does not constitute a limitation on the present application.

[0108] Figure 10 A schematic diagram of a cooling characteristic tester, and the test method using the tester is as follows:

[0109] The initial point setting device 4 fixes the temperature measuring probe 3, and the tubular heating furnace 2 heats the temperature measuring probe 3 (material Inconel 600) to above 850℃, and then quickly puts it into the water-based normalizing medium 1 of more than 600ml, and the temperature measuring probe 3 is completely immersed in the water-based normalizing medium 1, and then the test recording system 5 records the cooling process curve of the temperature measuring probe 3, and the cooling characteristic curve is drawn by computer at the same time in the form of data processing.

[0110] Example 1

[0111] The present embodiment relates to a nodular cast iron and a heat treatment method, which comprises C: 3.8%, Si: 2.0%, Mn: 0.6%, Mg: 0.06%, Mo: 0.15%, Cu: 0.8%, Ni: 0.2%, rare earth Re: 0.02%, and the balance is Fe and unavoidable impurities, by mass fraction.

[0112] The heat treatment method of the nodular cast iron comprises the following steps:

[0113] (1) Casting the castings according to the component allocation ratio, controlling the heating rate to be 100℃ / h to heat the castings to 940℃, carrying out normalizing heat treatment for 2h, and then cooling by using 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℃, the cooling characteristic curve being as shown in Figure 5 The water-based normalizing medium comprises 75 parts of water, 0.8 parts of polyacrylamide, 0.5 parts of preservative sodium benzoate and 0.6 parts of sodium nitrite.

[0114] (2) Carrying out stress relief annealing treatment on the castings obtained in step (1), the temperature of the stress relief annealing treatment being 530℃, the holding time being 20h, and then reducing the temperature in the furnace to ≤300℃ at a cooling rate of 60℃ / h, and taking out the castings to carry out air cooling, thereby obtaining the castings.

[0115] Example 2

[0116] The present embodiment 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: 0.4%, Mg: 0.03%, Mo: 0.1%, Ni: 0.2%, rare earth Re: 0.01%, and the balance being Fe and inevitable impurities.

[0117] The heat treatment method of the nodular cast iron comprises the following steps:

[0118] (1) Casting the castings according to the component allocation ratio, controlling the heating rate to be 30℃ / h to heat the castings to 870℃, carrying out normalizing heat treatment for 10h, and then cooling by using 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, 5 parts of sodium alginate, 5 parts of polyvinyl alcohol, 0.7 parts of potassium sorbate and 0.8 parts of sodium nitrite.

[0119] (2) Carrying out stress relief annealing treatment on the castings obtained in step (1), the temperature of the stress relief annealing treatment being 590℃, the holding time being 2h, and then reducing the temperature in the furnace to ≤300℃ at a cooling rate of 50℃ / h, and taking out the castings to carry out air cooling, thereby obtaining the castings.

[0120] Example 3

[0121] The embodiment relates to a nodular cast iron and a heat treatment method, the nodular cast iron comprises the following components in mass fraction: C: 3.7%, Si: 2.7%, Mn: 0.4%, Mg: 0.05%, Mo: 0.04%, Cu: 0.5%, Ni: 0.03%, rare earth Re: 0.005%, and the rest is Fe and inevitable impurities.

[0122] The heat treatment method of the nodular cast iron comprises the following steps:

[0123] (1) casting the castings according to the component proportion, controlling the heating speed to be 50 DEG C / h, heating the castings to 900 DEG C, carrying out normalizing heat treatment for 5h, and then cooling by using a water-based normalizing medium, the temperature of the water-based normalizing medium is 60 DEG C, the maximum cooling speed of the water-based normalizing medium in the first stage is 18 DEG C / s at 500-850 DEG C, the maximum cooling speed of the water-based normalizing medium in the second stage is 33 DEG C / s below 500 DEG C, and the cooling characteristic curve is as shown in the figure Figure 6 The water-based normalizing medium comprises 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.

[0124] (2) carrying out stress relief annealing treatment on the castings obtained in the step (1), the temperature of the stress relief annealing treatment is 570 DEG C, the holding time is 10h, then the temperature in the furnace is reduced to <=300 DEG C at the reducing speed of 40 DEG C / h, and the castings are taken out to carry out air cooling, and the nodular cast iron is obtained.

[0125] Embodiment 4

[0126] The embodiment is different from the embodiment 3 in that the Si content is 2.2%, and the rest is the same as the embodiment 3.

[0127] Embodiment 5

[0128] The embodiment is different from the embodiment 3 in that the C content is 3.4% and the Si content is 2.2%, and the rest is the same as the embodiment 3.

[0129] Embodiment 6

[0130] The embodiment is different from the embodiment 3 in that the Mn is not contained, and the Cu content is 0.2%, and the rest is the same as the embodiment 3.

[0131] Embodiment 7

[0132] The embodiment is different from the embodiment 3 in that the Sb of 0.01% is further contained, and the rest is the same as the embodiment 3.

[0133] Embodiment 8

[0134] The embodiment is different from the embodiment 3 in that the Sn of 0.04% is further contained, and the rest is the same as the embodiment 3.

[0135] Example 9

[0136] The difference between this example and Example 3 is that the temperature increasing rate in step (1) is controlled to be 150°C / h to heat the casting to 900°C, and the rest is the same as Example 3.

[0137] Example 10

[0138] The difference between this example and Example 3 is that the water-based normalizing medium in the cooling process 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 rate in the first stage of 500-850°C is 140°C / s, the maximum cooling rate in the second stage below 500°C is 150°C / s, and the rest is the same as Example 3. The cooling characteristic curve is shown in Figure 7 , and the rest is the same as Example 3.

[0139] Example 11

[0140] The difference between this example and Example 3 is that the water-based normalizing medium in the cooling process comprises 90 parts of water, 28 parts of polyacrylamide, 0.7 parts of potassium sorbate, and 0.8 parts of sodium nitrite, the maximum cooling rate in the first stage of 500-850°C is 13.5°C / s, which is lower than 20°C / s, the maximum cooling rate in the second stage below 500°C is 8.5°C / s, which is lower than 10°C / s, and the rest is the same as Example 3. The cooling characteristic curve is shown in Figure 8 , and the rest is the same as Example 3.

[0141] Example 12

[0142] The difference between this example and Example 3 is that the stress relief annealing temperature in step (2) is 500°C, and the rest is the same as Example 3.

[0143] Example 13

[0144] The difference between this example and Example 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 Example 3.

[0145] Comparative Example 1

[0146] The difference between this example and Example 3 is that the Mo content is 0.2%, and the rest is the same as Example 3.

[0147] Comparative Example 2

[0148] The difference between this example and Example 3 is that the Si content is 3.2%, and the rest is the same as Example 3.

[0149] Comparative Example 3

[0150] The difference between this example and Example 3 is that the Mn content is 0.8%, and the rest is the same as Example 3.

[0151] Comparative Example 4

[0152] The difference between this example and Example 3 is that the Mg content is 0.02%, and the rest is the same as Example 3.

[0153] Comparative Example 5

[0154] The difference between this example and Example 3 is that Cu and Ni are not added, and the rest is the same as Example 3.

[0155] Comparative Example 6

[0156] The difference between this example and Example 3 is that the Re content is 0.04%, and the rest is the same as Example 3.

[0157] Comparative Example 7

[0158] The difference between this example and Example 3 is that the C content is 3.9%, and the rest is the same as Example 3.

[0159] Comparative Example 8

[0160] The difference between this example and Example 3 is that the C content is 4.2%, and the rest is the same as Example 3.

[0161] Comparative Example 9

[0162] The difference between this example and Example 3 is that the water-based normalizing medium cooling in step (1) is replaced by pure water cooling, the water temperature is 60°C, and the cooling characteristic curve is as shown in Figure 9 , and the rest is the same as Example 3.

[0163] Comparative Example 10

[0164] The difference between this 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.

[0165] Comparative Example 11

[0166] The difference between this 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.

[0167] The nodular cast iron materials obtained in the above examples and comparative examples were tested in Test Example 1 and Test Example 2 described below. Both Test Example 1 and Test Example 2 were tests performed on samples taken from test blocks prepared according to GB / T 1348-2019 “Nodular Cast Iron Pieces”. It is a common practice in the art to define the grade of a cast iron material based on the test performance of samples taken from test blocks according to the standard. According to the test results, the nodular cast iron obtained in the present scheme can meet the mechanical property requirements of the QT900-2 grade in GB / T 1348-2019 “Nodular Cast Iron Pieces”. Meanwhile, GB / T 1348-2019 “Nodular Cast Iron Pieces” indicates that the bulk samples may not be representative due to the influence of multiple factors, and in the industry, the performance of test blocks is usually used as the acceptance standard for planet carrier products to avoid damaging the product bulk. Therefore, the performance of test blocks should be used as the judgment standard.

[0168] Test Example 1

[0169] The nodular cast iron materials prepared in the above examples and comparative examples were tested for metallographic structure, and the test results are shown in Table 1. In Table 1, when testing the pearlite interlamellar spacing of the nodular cast iron material, at least 10 circular regions with a diameter of 2 mm were selected for testing. In each circular region with a diameter of 2 mm, 10 random fields of view (magnification of 1000 times or more) were selected, and the widest suspected pearlite interlamellar spacing was found in the field of view. 10-20 interlamellar spacings were measured in each field of view, and the average value of the interlamellar spacings measured in each field of view was taken as the pearlite interlamellar spacing of the field of view. The sum of the interlamellar spacings of the 10 fields of view was averaged to obtain the interlamellar spacing of the circular region with a diameter of 2 mm.

[0170] Since the test was performed on at least 10 circular regions with a diameter of 2 mm, each region had an interlamellar spacing of the region. Therefore, the data in this column in Table 1 is a range value, which means that the pearlite interlamellar spacings of the at least 10 circular regions with a diameter of 2 mm selected are all within this range. The maximum and minimum values of the range can also indicate that the maximum absolute value of the difference between the pearlite interlamellar spacings of any two circular regions with a diameter of 2 mm (|A-B|), A being the pearlite interlamellar spacing of one circular region with a diameter of 2 mm and B being the pearlite interlamellar spacing of the other circular region with a diameter of 2 mm.

[0171] Table 1

[0172]

[0173] In the above table, since Example 10, Comparative Example 1 and Comparative Example 9 form a martensite matrix, they do not meet the matrix requirements of the nodular cast iron of the present application, and therefore the data of the interlamellar spacing of Comparative Example 1 and Comparative Example 9 are not included.

[0174] Figure 1 The metallographic photo of the nodular cast iron of Example 3, Figure 2 The scanning electron microscope test photo of the nodular cast iron material of Example 3 can be directly observed that the average spacing of the pearlite lamellas of the material is mainly in the range of 30-450 nm, and the spacing difference of adjacent lamellas is small, which proves that the pearlite structure uniformity is better.

[0175] Figure 3 The metallographic photo of the nodular cast iron of Comparative Example 10, Figure 4 The scanning electron microscope test photo of the nodular cast iron material of Comparative Example 10 can be observed that the pearlite lamellar spacing of the material is in the range of 500-1000 nm, and the spacing difference of adjacent lamellas is large.

[0176] Test Example 2

[0177] The nodular cast iron materials prepared in the above examples and comparative examples are subjected to mechanical tests, and the test results are shown in Table 2.

[0178] Table 2

[0179]

[0180] According to the above data, the nodular cast iron of the application has a pearlite matrix, and the average spacing of the lamellas is low, so the overall mechanical properties are higher. Since Examples 10, Comparative Example 1 and Comparative Example 9 form a martensite matrix, they do not meet the matrix requirements of the nodular cast iron of the application, so the fatigue strength of Comparative Example 1 and Comparative Example 9 is not detected.

[0181] 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 9, 10 and 12 will crack, resulting in the planet carrier cannot be used normally.

[0182] According to the comparison of Example 4 and Example 3, the reduction of Si content will weaken the solid solution strengthening effect of silicon, thereby reducing the fatigue performance of the nodular cast iron. According to the comparison of Comparative Example 2 and Example 3, when the Si content is greater than 3.0%, the formation of pearlite in the matrix will be inhibited, thereby reducing the strength and hardness of the nodular cast iron.

[0183] According to the comparison of Example 5, 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, thereby reducing the performance of the nodular cast iron, and the tendency of shrinkage porosity will be larger.

[0184] According to the comparison between Example 6 and Example 3, when the content of Mn and Cu is reduced to be unable to meet (Mn+Cu)≥Si / 10, the ability of Mn and Cu to promote pearlite is insufficient, the amount of pearlite in the matrix is reduced, the interlamellar spacing of pearlite in the circular area with a diameter of 2 mm is increased, and thus the strength and hardness of the ductile cast iron material are reduced.

[0185] According to the comparison between Example 7, 8 and Example 3, increasing the content of Sb and Sn can promote the formation of pearlite and improve the strength and hardness of the ductile cast iron material.

[0186] According to the comparison between Example 9 and Example 3, if the heating temperature of the normalizing treatment is too fast, the residual stress in the normalizing process is increased, and thus the castings are cracked.

[0187] According to the comparison between Example 10 and Example 3, if the cooling rate of the water-based normalizing medium in the first stage of 500-850℃ is too fast, the residual stress is increased, and thus the castings are cracked.

[0188] According to the comparison between Example 11 and Example 3, if the content of polyacrylamide in the water-based normalizing medium is increased, the cooling speed of the water-based normalizing medium is reduced, the strength and hardness of the ductile cast iron material are reduced, and the production cost is also increased.

[0189] According to the comparison between Example 12 and Example 3, if the temperature of the stress relief annealing treatment is too low, the stress relief effect in this stage is poor, and thus the castings are cracked. According to the comparison between Example 13 and Example 3, if the cooling rate after the stress relief annealing treatment is too fast, the residual stress in the ductile cast iron is increased in this stage, the performance is reduced, and thus the castings are cracked.

[0190] According to the comparison between Comparative Example 1 and Example 3, if the content of Mo is greater than 0.15%, martensite is generated in the matrix of the ductile cast iron, the structure is abnormal, and the performance is significantly reduced.

[0191] According to the comparison between Comparative Example 3 and Example 3, if the content of Mn is increased, the formation of cementite is promoted, and thus the tensile strength, fatigue strength and elongation of the ductile cast iron are reduced.

[0192] According to the comparison between Comparative Example 4 and Example 3, if the content of Mg is reduced, the spheroidizing ability of the ductile cast iron is insufficient, the spheroidizing rate is reduced, and the tensile strength, fatigue strength and elongation of the material are reduced.

[0193] 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 is reduced, part of ferrite is formed, and thus the strength and hardness of the ductile cast iron material are reduced.

[0194] According to the comparison between Comparative Example 6 and Example 3, it can be seen that the increase of Re content promotes the generation of graphite lumps, and the spheroidization rate decreases, thereby reducing the tensile strength, fatigue strength and elongation of the ductile cast iron.

[0195] According to the comparison between Comparative Examples 7 and 8 and Example 3, it can be seen that the increase of C content causes the graphite spheres to become large, the graphite to bloom, and the spheroidization rate to decrease, thereby reducing the tensile strength, fatigue strength and elongation of the ductile cast iron material.

[0196] According to the comparison between Comparative Example 9 and Example 3, it can be seen that the use of water cooling for the cooling of the normalized castings causes the maximum cooling rate of the water to be 10-20 times that of the water-based normalization medium, which causes martensite to be generated in the matrix, and the ductile cast iron material with a pearlite content of ≥95% cannot be obtained, thereby causing the structure to be abnormal, and the various properties of the ductile cast iron to decrease. Figure 6 Figure 9 According to the comparison between Comparative Example 10 and Example 3, it can be seen that the use of air cooling for the cooling of the normalized castings causes the maximum cooling rate of the air cooling to be about 5℃ / s, which increases the pearlite interlamellar spacing and also generates a large amount of ferrite, thereby reducing the strength and hardness of the ductile cast iron.

[0197] According to the comparison between Comparative Example 11 and Example 3, it can be seen that the use of air cooling for the cooling of the normalized castings increases the pearlite interlamellar spacing and the ferrite content, thereby reducing the strength and hardness of the ductile cast iron material.

[0198] According to the comparison between Comparative Example 11 and Example 3, it can be seen that the use of air cooling for the cooling of the normalized castings increases the pearlite interlamellar spacing and the ferrite content, thereby reducing the strength and hardness of the ductile cast iron material.

[0199] The above merely illustrates the embodiments of the present application, and the protection scope of the present application is not limited by these specific embodiments, but is determined by the claims of the present application. Any modification, equivalent replacement, improvement, etc. within the technical thought and principle of the present application shall be included in the protection scope of the present application.​

Claims

1. A spheroidal graphite cast iron for a planetary carrier, characterized by, The planet carrier mass is 0.5-30 tons, the spheroidal graphite cast iron comprises, by mass fraction, 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 rare earth Re addition is greater than 0%, the balance being Fe and unavoidable impurities, and C+ (Si / 3) =4.23-4.8%, Mn+Cu ≥Si / 10; (1) normalizing heat treatment is performed on the castings cast with the above spheroidal graphite cast iron composition, the castings are immersed in a water-based normalizing medium for cooling, the maximum cooling rate at the first stage of 500-850℃ is 10-30℃ / s, to promote the transformation of the wall thickness thinner part and the surface to pearlite; the maximum cooling rate at the second stage below 500℃ is ≤100℃ / s, and the maximum cooling rate below 500℃ is not less than 8.5℃ / s, to promote the core pearlite transformation; (2) stress relief annealing treatment is performed on the castings obtained in step (1); The content of the pearlite in the spheroidal graphite cast iron matrix is ≥95%, and the pearlite lamella spacing in any 2mm diameter circular area is 30-450nm; The spheroidal graphite cast iron has a hardness of 270-320HB, a tensile strength ≥900MPa, and an elongation ≥2%.

2. The planetary carrier cast iron according to claim 1, characterized in that, The spheroidal graphite cast iron comprises, by mass fraction, C: 3.4%-3.8%, Si: 2.0%-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%, the balance being Fe and unavoidable impurities.

3. The planetary carrier cast iron according to claim 2, characterized in that, The spheroidal graphite cast iron comprises, by mass fraction, 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%, the balance being Fe and unavoidable impurities.

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

5. The planetary carrier ductile iron of claim 1, wherein, The normalizing heat treatment temperature is 870-940℃, and the time is 2-10h.

6. The planetary carrier ductile iron of claim 1, wherein, The heating rate at the normalizing stage is 30-100℃ / h.

7. The planetary carrier ductile iron of claim 1 wherein, The water-based normalizing medium temperature is 0-80℃.

8. The planetary carrier ductile iron of claim 1, wherein, The water-based normalizing medium comprises, by mass fraction, 70~99 parts of water, 0.5~30 parts of thickening agent.

9. The planetary carrier cast iron according to claim 8, characterized in that, The thickening agent is selected from at least one of polyacrylamide, sodium alginate, polyvinyl alcohol, sodium polyacrylate, and sodium carboxymethyl cellulose.

10. The planetary carrier ductile iron of claim 1, wherein, The stress relief annealing treatment temperature is 530-590℃, and the holding time is 2-20h.

11. The planetary carrier ductile iron of claim 1 wherein, After the stress relief annealing, the temperature in the furnace is decreased to ≤300℃ at a cooling rate of ≤60℃ / h, and the furnace is discharged for air cooling.

12. Use of the spheroidal graphite cast iron according to any one of claims 1 to 11 in a planet carrier.

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