Method for preparing secondary spheroidal graphite precipitation in a spheroidal graphite cast iron material

Abstract

The application discloses a preparation method of secondary spherulitic graphite precipitation in nodular cast iron material, and specifically comprises the following steps: firstly, preparing nodular cast iron material; secondly, carrying out high-temperature annealing treatment on the prepared nodular cast iron material, so that pearlite in the nodular cast iron material is decomposed, and carbon atoms are solid-solved in the matrix; and then, carrying out annealing below eutectoid temperature on the nodular cast iron after high-temperature annealing, so that secondary spherulitic graphite is precipitated in the structure; the nodular cast iron material prepared by the application has a large number of uniformly distributed fine secondary spherulitic graphites, solves the problem of less graphite in the current nodular cast iron material, solves the problem of poor aluminum liquid corrosion resistance of the current aluminum alloy die casting mold, and greatly improves the service life of the aluminum alloy die casting mold.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal materials, and particularly relates to a preparation method of secondary spherulitic graphite precipitation in nodular cast iron material. BACKGROUND

[0002] In recent years, the development of light manufacturing with aluminum alloy as the main consumable is rapid and promising, the application field is continuously expanding, the proportion of aluminum materials is continuously increasing, and die casting has become the main forming method. The aluminum profile die casting mold is mainly developed in the service environment of 4Cr5Mo2V1 series hot work die steel by hot forging of steel parts. Although the mechanical properties meet the thermal strength requirements under the die casting working condition, the short service life and high consumption caused by the accelerated melt corrosion of aluminum thermal reaction have become one of the main bottlenecks for the improvement of production efficiency of aluminum industry. Therefore, how to obtain a die casting mold material with good aluminum melt corrosion resistance has important engineering significance.

[0003] Reducing the aluminum / iron contact area can effectively inhibit the aluminum thermal reaction. By using the physical property that graphite does not wet aluminum below 800 DEG C, graphite is generated in the prepared material organization to block the aluminum melt corrosion of the material matrix. The nodular cast iron material meets this property, but the number of graphite balls in the traditional nodular cast iron material is limited, that is, the effect of blocking the aluminum melt is limited. Therefore, obtaining more number, smaller size and smaller spacing of graphite balls has important significance for the aluminum alloy die casting mold resistance to aluminum melt corrosion. Therefore, how to greatly improve the number of graphite balls in the nodular cast iron organization has become a difficult problem to be solved.

[0004] At present, the traditional nodular cast iron cannot obtain more number, smaller size and smaller spacing of graphite balls. According to the material theory, the number of graphite obtained by liquid-solid phase change during casting is difficult to break through this order of magnitude, because at high temperature, the diffusion and precipitation growth speed of carbon atoms in the liquid phase is very fast, and it is easy to form relatively thick eutectic graphite balls. The solid phase change in the traditional heat treatment process cannot change the number of graphite balls, cannot generate new graphite crystal nucleus, and can only make the original eutectic graphite balls grow, so a heat treatment method for generating new graphite grains is urgently needed to obtain more number, smaller size and smaller spacing of graphite balls. SUMMARY

[0005] In view of the problems of poor melt corrosion resistance of the aluminum alloy die casting mold and the small number of graphite in the traditional nodular cast iron material, the application provides a preparation method of secondary spherulitic graphite precipitation in nodular cast iron material.

[0006] To achieve the above purpose, the technical scheme adopted by the application is as follows:

[0007] A preparation method of secondary spherulitic graphite precipitation in nodular cast iron material is specifically implemented according to the following steps:

[0008] Step 1, the scrap steel, pig iron raw material, pretreatment agent, recarburizer, inoculant, spherulitic agent are weighed and dosed, the scrap steel is Q235 steel, the mass percentage is: 25%~90%; the pig iron raw material is Q10 bread iron, the mass percentage is: 10%~75%; the pretreatment agent is SiC, the mass percentage is: 0.3%~0.9%; the recarburizer is high-purity graphite, the mass percentage is: 0.2%~3.8%;

[0009] Step 2, the scrap steel, pig iron raw material, pretreatment agent and recarburizer weighed in step 1 are placed in a medium-frequency induction furnace to melt into molten iron;

[0010] Step 3, the molten iron is poured into a ladle with spherulitic agent, inoculant and covering agent laid at the bottom;

[0011] Step 4, after slagging, the molten iron in step 3 is quickly poured into a prefabricated sand mold to form, and after solidification, a nodular cast iron material is obtained;

[0012] Step 5, the nodular cast iron material obtained in step 4 is heated from room temperature to 900℃~1100℃ at a heating rate of 4℃ / min~7℃ / min for high-temperature annealing treatment, and then is kept for 10h~15h, and is air-cooled to room temperature after discharging, so that the pearlite in the nodular cast iron material is decomposed, and carbon atoms are dissolved in the matrix;

[0013] Step 6, the nodular cast iron material after high-temperature annealing in step 5 is annealed below the eutectoid temperature, so that secondary spheroidal graphite is precipitated in the structure, and the specific method of annealing below the eutectoid temperature is: the nodular cast iron material after high-temperature annealing is heated from room temperature to 720℃~750℃ at a heating rate of 4℃ / min~7℃ / min, and then is kept for 10h~15h, and is air-cooled to room temperature after discharging.

[0014] Further, the melting temperature of the medium-frequency induction furnace in step 2 is 1530℃~1570℃, and the holding time is 2~5min.

[0015] Further, the spherulitic agent in step 3 is Si-Mg-Ca-RE-Al or NiMg30 alloy, the Si-Mg-Ca-RE-Al spherulitic agent includes the following components according to mass fraction: Si: 44%~48%, Mg: 5.8%~6.5%, Ca: 2%~3%, Re: 1%~1.5%, Al: 0.4%~0.6%, and the rest is Fe; the mass of the Si-Mg-Ca-RE-Al spherulitic agent accounts for 1.0%~1.2% of the total mass of the molten iron, and the residual Mg content in the nodular cast iron material is 0.03%~0.05%; the NiMg30 alloy spherulitic agent includes the following components according to mass fraction: Ni: 70%~75%, Mg: 25%~30%, and the mass of the NiMg30 alloy spherulitic agent accounts for 1.0%~1.2% of the total mass of the molten iron.

[0016] The inoculant is Si-Ba-Ca, and includes the following components by mass fraction: Si: 66%~69%, Ba: 3.9%~4.0%, Ca: 1.3%~1.7%, and the rest is Fe, and the inoculant preparation amount accounts for 0.6%~0.8% of the total mass of the molten iron;

[0017] The covering agent is rust-free iron filings.

[0018] Further, the interval time of pouring the molten iron into the sand mold in step 4 is not more than 20s.

[0019] Further, the spherulitic cast iron material in step 4 includes the following components by mass percentage: C: 3.4%~4.0%, Si: 1.0%~2.6%, Mn: ≤0.6%, S: ≤0.02%, P: ≤0.1%, Mg: 0.03%~0.05%, Re: 0%~0.04%, and the rest is Fe and inevitable impurities.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] The present application adopts the traditional sand mold casting method, the molding material used in the method is cheap and easy to obtain, and the casting mold is simple to manufacture, so the production cost is low. The spherulitic cast iron material prepared by the method has the following microstructure: pearlite, ferrite and eutectic graphite spheres. After high-temperature annealing and annealing below the eutectoid temperature, the pearlite is completely decomposed, the supersaturated carbon atoms in the iron matrix are precipitated in the form of fine and dispersed secondary spherical graphite, and finally the microstructure of ferrite as the matrix, eutectic graphite spheres and secondary spherical graphite dispersed on the matrix is obtained. Below 800 DEG C, graphite does not wet the aluminum liquid, that is, the graphite sphere array can form a capillary barrier of the aluminum liquid corrosion matrix, and the surface tension of the combined surface of the graphite sphere array and the aluminum liquid. Therefore, the present application can precipitate secondary graphite spheres in the spherulitic cast iron microstructure, increase the number of graphite spheres, reduce the size of the graphite spheres, and shorten the distance between adjacent graphite spheres, so as to achieve the graphite distribution state of "increasing the number and shortening the distance", which can significantly improve the anti-corrosion effect of the material on the aluminum melt, solve the problem of poor anti-aluminum liquid corrosion performance of the current aluminum alloy die casting mold, and greatly improve the service life of the aluminum alloy die casting mold. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a metallographic structure photo of a spherulitic cast iron material prepared by example 2 of the present application: figure (a) is the metallographic structure without corrosion, and figure (b) is the metallographic structure after corrosion;

[0023] Figure 2 is a corrosion metallographic structure photo of a spherulitic cast iron material prepared by example 2 of the present application after heat treatment;

[0024] Figure 3 The aluminum melting loss weight percentage of the spherulitic cast iron material prepared in Embodiment 2 of the present application. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions of the embodiments of the present application will be described clearly and completely below in combination with the drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the described embodiments of the present application, all other embodiments obtained by those skilled in the art without any creative work also belong to the protection scope of the present application.

[0026] The present application will be described in detail below with reference to the drawings and specific embodiments as follows:

[0027] The present application is a preparation method of secondary spherulitic graphite precipitation in spherulitic cast iron material, comprising the following steps:

[0028] Step 1, batching and melting of raw materials. According to the component requirements of spherulitic cast iron material, according to the component composition of raw materials, each raw material is calculated and prepared according to the percentage, Q235 scrap steel: 25%~90%, Q10 bread iron: 10%~75%, SiC pretreatment agent: 0.3%~0.9%, high-purity graphite carburizer: 0.2%~3.8%, silicon barium calcium inoculant: 0.6%~0.8%, rare earth magnesium or nickel magnesium spheroidizing agent: 1.0~1.2%.

[0029] Step 2, the weighed scrap steel, pig iron and carburizer raw materials are put into a medium frequency induction furnace to heat and melt into molten iron, and the melting temperature of the medium frequency induction furnace is 1530℃~1570℃, and the holding time is 2~5min.

[0030] Step 3, the molten iron obtained in step 2 is poured into a ladle which is pre-laid with spheroidizing agent, inoculant and covering agent at the bottom to perform spheroidizing and inoculating treatment, and the temperature of the molten iron after spheroidizing and inoculating treatment is controlled to be 1480℃~1520℃.

[0031] The spherulizing agent is Si-Mg-Ca-RE-Al or NiMg30 alloy. The Si-Mg-Ca-RE-Al spherulizing agent comprises the following components in mass fraction: Si: 44% to 48%, Mg: 5.8% to 6.5%, Ca: 2% to 3%, Re: 1% to 1.5%, Al: 0.4% to 0.6%, and the rest is Fe. The mass of the Si-Mg-Ca-RE-Al spherulizing agent accounts for 1.0% to 1.2% of the total mass of the molten iron. The residual Mg content in the spheroidal graphite cast iron material is 0.03% to 0.05%. The NiMg30 alloy spherulizing agent comprises the following components in mass fraction: Ni: 70% to 75%, Mg: 25% to 30%. The mass of the NiMg30 alloy spherulizing agent accounts for 1.0% to 1.2% of the total mass of the molten iron. The inoculant is Si-Ba-Ca, which comprises the following components in mass fraction: Si: 66% to 69%, Ba: 3.9% to 4.0%, Ca: 1.3% to 1.7%, and the rest is Fe. The preparation amount of the inoculant accounts for 0.6% to 0.8% of the total mass of the molten iron. The covering agent is rust-free iron filings.

[0032] Step 4: The spherulizing and inoculating molten metal in step 3 is rapidly poured into a sand mold to form a spheroidal graphite cast iron material. The interval time for pouring the molten iron into the sand mold is not more than 20 s. The spheroidal graphite cast iron material comprises the following components: C: 3.4% to 4.0%, Si: 1.0% to 2.6%, Mn: ≤0.6%, S: ≤0.02%, P: ≤0.1%, Mg: 0.03% to 0.05%, Re: 0% to 0.04%, and the rest is Fe and inevitable impurities.

[0033] Step 5: The spheroidal graphite cast iron material is subjected to high-temperature annealing treatment to decompose the pearlite in the spheroidal graphite cast iron material and make carbon atoms dissolve in the iron matrix. The high-temperature annealing treatment is as follows: the spheroidal graphite cast iron material is heated to 900°C to 1100°C at a heating rate of 4°C / min to 7°C / min from room temperature and then is kept for 10 h to 15 h, and is quenched or air-cooled to room temperature to complete the high-temperature annealing treatment. The high-temperature annealing treatment decomposes the pearlite in the spheroidal graphite cast iron material and makes carbon atoms dissolve in the iron matrix.

[0034] Step 6: The castings after high-temperature annealing are subjected to annealing below the eutectoid temperature to make fine secondary spheroidal graphite precipitate in the structure. The annealing below the eutectoid temperature is as follows: the spheroidal graphite cast iron material after high-temperature annealing is heated to 720°C to 750°C at a heating rate of 4°C / min to 7°C / min from room temperature and then is kept for 10 h to 15 h, and is air-cooled to room temperature to complete the annealing below the eutectoid temperature. After the annealing below the eutectoid temperature, fine and uniformly distributed secondary spheroidal graphite precipitates in the structure.

[0035] Example 1

[0036] Step 1, batching and melting of raw materials. Calculate and prepare each raw material according to percentage: Q235 scrap steel: 90%, Q10 bread iron: 10%, SiC pretreatment agent: 0.3%, high-purity graphite carburizer: 3.8%, silicon-barium-calcium inoculant: 0.8%, nickel-magnesium spheroidizing agent: 1.2%.

[0037] Step 2, place the weighed scrap steel, pig iron and carburizer raw materials into the medium-frequency induction furnace to heat and melt into molten iron, the melting temperature of the medium-frequency induction furnace is 1530℃, and the holding time is 3min.

[0038] Step 3, pour the 1530℃ molten iron obtained in step 1 into the ladle pre-laid with spheroidizing agent, inoculant and covering agent at the bottom to perform spheroidizing and inoculating treatment, and the temperature of the molten iron after spheroidizing and inoculating treatment is controlled at 1480℃.

[0039] Step 4, quickly pour the spheroidized and inoculated melt in step 2 into a sand mold to form, and obtain nodular cast iron material after solidification, the composition of the nodular cast iron material is C: 4.0%, Si: 1.0%, Mn: 0.6%, S: ≤0.02%, P: ≤0.1%, Mg: 0.05%, and the balance is Fe.

[0040] Step 5, heat the nodular cast iron material from room temperature to 1100℃ at a heating rate of 7℃ / min, hold for 15h, and air cool to room temperature after discharging from the furnace to complete high-temperature annealing treatment.

[0041] Step 6, heat the nodular cast iron material after high-temperature annealing from room temperature to 720℃ at a heating rate of 6℃ / min, hold for 15h, and air cool to room temperature after discharging from the furnace to complete annealing below the eutectoid temperature.

[0042] Example 2

[0043] Step 1, batching and melting of raw materials. Calculate and prepare each raw material according to percentage: Q235 scrap steel: 75%, Q10 bread iron: 25%, SiC pretreatment agent: 0.73%, high-purity graphite carburizer: 2.9%, silicon-barium-calcium inoculant: 0.8%, nickel-magnesium spheroidizing agent: 1.2%.

[0044] Step 2, place the weighed scrap steel, pig iron and carburizer raw materials into the medium-frequency induction furnace to heat and melt into molten iron, the melting temperature of the medium-frequency induction furnace is 1540℃, and the holding time is 3min.

[0045] Step 3, pour the 1540℃ molten iron obtained in step 1 into the ladle pre-laid with spheroidizing agent, inoculant and covering agent at the bottom to perform spheroidizing and inoculating treatment, and the temperature of the molten iron after spheroidizing and inoculating treatment is controlled at 1490℃.

[0046] Step 4, the spheroidized inoculated melt in step 2 is quickly poured into a sand mold to form, and a nodular cast iron material is obtained after solidification, and the composition of the nodular cast iron material is C: 3.9%, Si: 1.4%, Mn: 0.3%, S: ≤0.02%, P: ≤0.1%, Mg: 0.05%, and the balance is Fe.

[0047] Step 5, the nodular cast iron material is heated from room temperature to 1050℃ at a heating rate of 6℃ / min, and then held for 12h, and then discharged and air-cooled to room temperature to complete the high-temperature annealing treatment.

[0048] Step 6, the nodular cast iron material after high-temperature annealing is heated from room temperature to 725℃ at a heating rate of 4℃ / min, and then held for 15h, and then discharged and air-cooled to room temperature to complete the annealing treatment below the eutectoid temperature.

[0049] Figure 1 Fig. (a) is a metallographic structure of the nodular cast iron material prepared in Example 2 without etching, and Fig. (b) is a metallographic structure after etching; Figure 2 Fig. is an etched metallographic structure of the nodular cast iron material prepared in Example 2 after heat treatment.

[0050] Fig. is a schematic diagram of the graphite ball array formed by the graphite balls in the nodular cast iron material prepared in Example 2. Figure 1 Fig. is a schematic diagram of the graphite ball array formed by the graphite balls in the nodular cast iron material prepared in Example 2. Figure 2 As can be seen, the number of graphite balls increases, the size of the graphite balls decreases, and the distance between adjacent graphite balls decreases, the capillary block of the graphite ball array to the aluminum liquid corrosion matrix is formed, and the surface tension of the combined surface of the graphite ball array to the matrix is isolated from the aluminum liquid, which solves the problem of poor aluminum liquid corrosion resistance of the current aluminum alloy die casting mold.

[0051] Figure 3 Fig. is the aluminum melting loss rate of the nodular cast iron material prepared in Example 2. Figure 3 The results show that the aluminum melting loss rate of the nodular cast iron material prepared in this embodiment is reduced by about one time compared with the traditional H13 steel aluminum die casting mold, and therefore the service life of the aluminum alloy die casting mold can be greatly improved.

[0052] Example 3

[0053] Step 1, batching and melting of raw materials. The raw materials are calculated and prepared according to the percentage: Q235 scrap steel: 60%, Q10 bread iron: 40%, SiC pretreatment agent: 0.43%, high-purity graphite carbonizer: 2.1%, silicon barium calcium inoculant: 0.8%, and rare earth magnesium spheroidizing agent: 1.1%.

[0054] Step 2, the weighed scrap steel, pig iron and carbonizer raw materials are placed in a medium-frequency induction furnace to heat and melt into molten iron, and the melting temperature of the medium-frequency induction furnace is 1550℃, and the holding time is 3min.

[0055] Step 3, the 1550℃ molten iron obtained in step 1 is poured into a ladle with balling agent, inoculant and covering agent pre-laid at the bottom for balling and inoculation treatment, and the temperature of the molten iron after balling and inoculation treatment is controlled at 1500℃.

[0056] Step 4, the melt after balling and inoculation in step 2 is quickly poured into a sand mold for molding, and a ductile cast iron material is obtained after solidification, the composition of the ductile cast iron material being C: 3.7%, Si: 1.8%, Mn: 0.3%, S: ≤0.02%, P: ≤0.1%, Mg: 0.04%, Re: 0.03%, and the balance being Fe.

[0057] Step 5, the ductile cast iron material is heated from room temperature to 1000℃ at a heating rate of 6℃ / min, and then held for 12h, and then discharged and air-cooled to room temperature to complete high-temperature annealing treatment.

[0058] Step 6, the ductile cast iron material after high-temperature annealing is heated from room temperature to 735℃ at a heating rate of 4℃ / min, and then held for 15h, and then discharged and air-cooled to room temperature to complete sub-eutectic annealing treatment.

[0059] Example 4

[0060] Step 1, batching and melting of raw materials. The raw materials are calculated and prepared according to the percentages: Q235 scrap steel: 40, Q10 bread iron: 60%, SiC pretreatment agent: 0.78%, high-purity graphite carbonizer: 1.0%, silicon-barium-calcium inoculant: 0.8%, and rare earth magnesium spheroidizing agent: 1.1%.

[0061] Step 2, the weighed scrap steel, pig iron and carbonizer raw materials are placed in a medium-frequency induction furnace for heating and melting into molten iron, and the melting temperature of the medium-frequency induction furnace is 1560℃, and the holding time is 3min.

[0062] Step 3, the 1560℃ molten iron obtained in step 1 is poured into a ladle with balling agent, inoculant and covering agent pre-laid at the bottom for balling and inoculation treatment, and the temperature of the molten iron after balling and inoculation treatment is controlled at 1510℃.

[0063] Step 4, the melt after balling and inoculation in step 2 is quickly poured into a sand mold for molding, and a ductile cast iron material is obtained after solidification, the composition of the ductile cast iron material being C: 3.6%, Si: 2.2%, Mn: 0.3%, S: ≤0.02%, P: ≤0.1%, Mg: 0.03%, Re: 0.03%, and the balance being Fe.

[0064] Step 5, the ductile cast iron material is heated from room temperature to 950℃ at a heating rate of 5℃ / min, and then held for 10h, and then discharged and air-cooled to room temperature to complete high-temperature annealing treatment.

[0065] Step 6, after the high-temperature annealed ductile cast iron material is heated from room temperature to 740℃ at a heating rate of 5℃ / min and kept for 15h, it is discharged and air-cooled to room temperature to complete the annealing treatment below the eutectoid temperature.

[0066] Example 5

[0067] Step 1, batching and melting of raw materials. The raw materials are calculated and prepared according to the percentage: Q235 scrap steel: 25%, Q10 bread iron: 75%, SiC pretreatment agent: 1.26%, high-purity graphite carburizer: 0.18%, silicon-barium-calcium inoculant: 0.8%, rare earth magnesium spheroidizing agent: 1.0%

[0068] Step 2, the weighed scrap steel, pig iron and carburizer raw materials are put into a medium-frequency induction furnace to be heated and melted into molten iron, and the melting temperature of the medium-frequency induction furnace is 1570℃, and the holding time is 3min.

[0069] Step 3, the 1570℃ molten iron obtained in step 1 is poured into a ladle pre-laid with spheroidizing agent, inoculant and covering agent at the bottom for spheroidizing and inoculating treatment, and the temperature of the molten iron after spheroidizing and inoculating treatment is controlled at 1520℃.

[0070] Step 4, the spheroidized and inoculated melt in step 2 is quickly poured into a sand mold to form a ductile cast iron material, and the composition of the ductile cast iron material is C: 3.4%, Si: 2.6%, Mn: 0.4%, S: ≤0.02%, P: ≤0.1%, Mg: 0.04%, Re: 0.02%, and the balance is Fe.

[0071] Step 5, the ductile cast iron material is heated from room temperature to 900℃ at a heating rate of 4℃ / min and kept for 10h, and then discharged and air-cooled to room temperature to complete the high-temperature annealing treatment.

[0072] Step 6, after the high-temperature annealed ductile cast iron material is heated from room temperature to 750℃ at a heating rate of 4℃ / min and kept for 10h, it is discharged and air-cooled to room temperature to complete the annealing treatment below the eutectoid temperature.

[0073] Obviously, the above examples are only examples for clarity and do not limit the embodiments. For those skilled in the art, without departing from the concept of the present application, some simple deductions or substitutions can be made, which should be considered as falling within the scope of the present application.

Claims

1. A method of producing secondary spheroidal graphite precipitation in a ductile cast iron material, characterized by, The method is implemented according to the following steps: Step 1, the scrap steel, pig iron raw material, pretreatment agent, recarburizer, inoculant and spheroidizing agent are weighed and mixed, the scrap steel is Q235 steel, the mass percentage is 25%-90%; the pig iron raw material is Q10 bread iron, the mass percentage is 10%-75%; the pretreatment agent is SiC, the mass percentage is 0.3%-0.9%; the recarburizer is high-purity graphite, the mass percentage is 0.2%-3.8%; Step 2, the scrap steel, pig iron raw material, pretreatment agent and recarburizer weighed in step 1 are placed in a medium-frequency induction furnace to melt into molten iron; Step 3, the molten iron is poured into a ladle in which the spheroidizing agent, inoculant and covering agent are laid at the bottom; Step 4, after slagging, the molten iron in step 3 is quickly poured into a prefabricated sand mold to form, and the spheroidal graphite cast iron material is obtained after solidification; Step 5, the spheroidal graphite cast iron material obtained in step 4 is heated to 900-1100 DEG C at a heating rate of 4-7 DEG C / min from room temperature, then is kept for 10-15 hours, and is air-cooled to room temperature after discharging, so that the pearlite in the spheroidal graphite cast iron material is decomposed, and carbon atoms are dissolved in the matrix; Step 6, the spheroidal graphite cast iron material after high-temperature annealing in step 5 is annealed below the eutectoid temperature, so that secondary spheroidal graphite is precipitated in the structure, and the specific method of annealing below the eutectoid temperature is as follows: the spheroidal graphite cast iron material after high-temperature annealing is heated to 720-750 DEG C at a heating rate of 4-7 DEG C / min from room temperature, then is kept for 10-15 hours, and is air-cooled to room temperature after discharging.

2. The method of claim 1, wherein the method is characterized by: The melting temperature of the medium-frequency induction furnace in step 2 is 1530-1570 DEG C, and the holding time is 2-5 min.

3. The method of producing secondary spheroidal graphite precipitation in a ductile cast iron material according to claim 1, characterized by, The spheroidizing agent in step 3 is Si-Mg-Ca-RE-Al or NiMg30 alloy, the Si-Mg-Ca-RE-Al spheroidizing agent comprises the following components in mass fraction: Si: 44%-48%, Mg: 5.8%-6.5%, Ca: 2%-3%, Re: 1%-1.5%, Al: 0.4%-0.6%, and the rest is Fe; the mass of the Si-Mg-Ca-RE-Al spheroidizing agent accounts for 1.0%-1.2% of the total mass of the molten iron, and the residual Mg content in the spheroidal graphite cast iron material is 0.03%-0.05%; the NiMg30 alloy spheroidizing agent comprises the following components in mass fraction: Ni: 70%-75%, Mg: 25%-30%, and the mass of the NiMg30 alloy spheroidizing agent accounts for 1.0%-1.2% of the total mass of the molten iron; The inoculant is Si-Ba-Ca, which comprises the following components in mass fraction: Si: 66%-69%, Ba: 3.9%-4.0%, Ca: 1.3%-1.7%, and the rest is Fe, and the preparation amount of the inoculant accounts for 0.6%-0.8% of the total mass of the molten iron; The covering agent is rust-free iron filings.

4. The method of claim 1, wherein the method is characterized by: The interval time of pouring the molten iron into the sand mold in step 4 is not more than 20 s.

5. The method of claim 1, wherein the method is characterized by: The nodular cast iron material of step 4 comprises the following components in percentage by mass: C: 3.4%-4.0%, Si: 1.0%-2.6%, Mn: ≤0.6%, S: ≤0.02%, P: ≤0.1%, Mg: 0.03%-0.05%, Re: 0%-0.04%, the balance being Fe and unavoidable impurities.

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