A ductile cast iron, its preparation method and application

Abstract

The present invention discloses a ductile cast iron and its preparation method and application, belonging to the technical field of cast iron. The element composition of the ductile cast iron in the present invention includes C, Si, Mn, Mg, Re, Mo, W, Al, Sn, Ti, etc. When preparing the ductile cast iron in the present invention, Mo, W and Ti are added in the form of powder, avoiding the composition segregation caused by the lag of dissolution. When heat-treating the ingot in the present invention, a two-stage austenitization treatment process is adopted. The first-stage treatment can fully dissolve the (Mo, W)C composite carbide and eliminate the casting stress; the second-stage treatment can inhibit the formation of pearlite and promote the precipitation of nano-scale (Mo, W)C. The ductile cast iron in the present invention successfully solves the problems of insufficient high-temperature strength, poor tissue stability, and weak environmental resistance of traditional materials through the multi-stage synergistic and antagonistic effects between elements, enabling it to have excellent comprehensive properties in high-temperature environments as well.

Description

Technical Field

[0001] The present invention belongs to the technical field of cast iron, and particularly relates to a ductile iron and its preparation method and application. Background Art

[0002] Due to its excellent mechanical properties and castability, ductile iron is widely used in fields such as internal combustion engines, hydraulic components, and heavy machinery. Traditional ductile iron (such as QT400-18, QT600-3) has Fe-C-Si-Mn-Mg as its main components, and its room-temperature tensile strength is generally lower than 700 MPa. Moreover, when it serves at temperatures above 400 °C, creep deformation and tissue deterioration are likely to occur, severely restricting its application in high-temperature working conditions (such as gas turbine casings, high-temperature valves, etc.).

[0003] In the prior art, there are significant defects in the solutions for improving high-temperature performance by adding alloying elements. For example, a Mo-containing ductile iron (Mo 0.2-0.4%) disclosed in the prior art can improve the short-term high-temperature strength, but excessive Mo will cause carbide coarsening, and the creep rate at 600 °C is still as high as 5×10 -8 / s. And after long-term thermal exposure, the graphite balls are distorted (the spheroidization rate drops below 75%); the prior art also discloses a nodular cast iron that uses Sb (0.02-0.05%) to inhibit the pearlite transformation, but it exacerbates the grain boundary segregation of low-melting-point elements such as Sn and Sb, resulting in a tensile strength attenuation rate of more than 40% at 600 °C; there are also reports in the prior art on improving the oxidation resistance of ductile iron by adding Al (0.5-1.0%), but the interaction between Al and Si is not coordinated, resulting in the eutectic cell size increasing to more than 200 μm, significantly reducing the fatigue life. In addition, the rare earth treatment of conventional ductile iron mostly uses Ce-based alloys (such as Fe-Si-Mg-Ce). When its Re content exceeds 0.1%, it is easy to form coarse Ce2O3 inclusions (size > 10 μm), causing microdefects and accelerating the propagation of high-temperature cracks. And the addition of trace Sn (<0.03%) can improve the ferrite morphology, but due to the non-equilibrium segregation of Sn at the grain boundaries, the material generates stress corrosion sensitivity under thermal cycling conditions.

[0004] The above technical bottlenecks indicate that there is an urgent need to develop a new type of ductile iron material. On the premise of ensuring process feasibility, through multi-component synergistic alloying and precise control of the microstructure, the comprehensive improvement of high-temperature strength, creep resistance, and oxidation resistance can be achieved to meet the stringent requirements of advanced energy equipment for high-temperature resistant structural materials. Summary of the Invention

[0005] Aiming at the above prior art, the present invention provides a ductile iron and its preparation method and application to solve the technical problems such as poor high-temperature strength and poor creep resistance of the existing ductile iron.

[0006] To achieve the above object, the technical solution adopted by the present invention is to provide a ductile iron, which comprises the following components by mass percentage:

[0007] C 3.4 - 3.6%, Si 2.8 - 3.2%, Mn 0.6 - 0.8%, Mg 0.04 - 0.07%, Re 0.03 - 0.05%, Mo 0.3 - 0.5%, W 0.2 - 0.4%, Al 0.8 - 1.2%, Sn 0.05 - 0.08%, Ti 0.15 - 0.25%, S ≤ 0.012%, P ≤ 0.025%, and the balance is iron.

[0008] Based on the above technical solution, the present invention can be further improved as follows.

[0009] Further, the ductile iron comprises the following components by mass percentage:

[0010] C 3.5%, Si 3.0%, Mn 0.7%, Mg 0.05%, Re 0.04%, Mo 0.4%, W 0.3%, Al 1.0%, Sn 0.06%, Ti 0.2%, S ≤ 0.012%, P ≤ 0.025%, and the balance is iron.

[0011] Further, Re includes Y and La, the mass percentage of Y in the ductile iron is 0.02 - 0.04%, and the mass percentage of La in the ductile iron is 0.01 - 0.02%.

[0012] The present invention also discloses a preparation method of the above ductile iron, which comprises the following steps:

[0013] (1) Crush Q12 pig iron into iron blocks with a particle size of 30 - 50 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50 × 50 mm; perform magnetic separation and impurity removal treatment on the return material; make Mo-Fe master alloy, W-Fe master alloy and Fe-Ti master alloy into alloy powders with a particle size of 80 - 120 μm; crush Al blocks into particles of 3 - 5 mm; make Sn grains and Fe-Mn alloy into cored wire;

[0014] (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel and return material into the intermediate frequency induction furnace from bottom to top in sequence, heat up to 1540 - 1560 °C with step power, then add Mo-Fe master alloy powder, W-Fe master alloy powder, Fe-Ti master alloy powder and Al grains, and perform electromagnetic stirring; after all are melted, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements;

[0015] (3) Load the rare earth magnesium alloy into the preheated nodulizing ladle, then pour in molten iron, and simultaneously pour in the liquid modifier and silicon carbide particles during the flowing process, and inject the cored wire prepared in step (1) by wire feeding method; the liquid modifier is Fe-Ti alloy powder;

[0016] (3) Cast to obtain the casting;

[0017] (4) Heat the casting to 900 - 950 °C, hold for 2 - 4 h, then cool down to 850 - 880 °C, hold for 1 - 3 h; then cool down at a cooling rate of 30 - 50 °C / min to 300 °C, and then air-cool to room temperature;

[0018] (5) Heat the ingot treated in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product.

[0019] Further, in step (2), the mass ratio of Q12 pig iron, low sulfur and phosphorus scrap steel, and return scrap is 55:30:15.

[0020] Further, in the stepwise power heating mode in step (2), first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW, heat for 20 min, and then adjust the power to 2000 kW to heat to the final heating temperature.

[0021] Further, the preheating temperature of the nodulizing ladle in step (3) is 800 °C.

[0022] Further, the rare earth magnesium alloy is Y-Mg alloy and La-Mg alloy.

[0023] Further, in step (3), the charging rates of the liquid modifier and the silicon carbide particles are both 0.1 - 0.3 kg / min; the particle size of the silicon carbide particles is 50 - 100 nm.

[0024] The present invention also discloses the application of the above ductile iron in the preparation of high-temperature resistant structural parts.

[0025] The beneficial effects of the present invention are:

[0026] 1. The constituent elements of the ductile cast iron in the present invention include C, Si, Mn, Mg, Re, Mo, W, Al, Sn, Ti, etc. Among them, Ti preferentially forms the primary TiC phase, which can serve as the substrate for the heterogeneous nucleation of (Mo,W)C, promoting the formation of nano-carbides in the ductile cast iron. The presence of nano-carbides can pin dislocations, thereby enhancing the high-temperature strength of the ductile cast iron; Mo and W form a composite solid solution in austenite, which can effectively reduce the diffusion coefficient of carbon and synergistically promote the uniform precipitation of (Mo,W)C carbides, thus forming fine grains with uniform distribution in the ductile cast iron, and further reducing the high-temperature creep rate of the ductile cast iron. Al and Si form an Al2O3-SiO2 interpenetrating network structure during high-temperature oxidation, filling the micro-cracks of the single oxide film, and Re segregates at the oxide film / matrix interface, inhibiting the inward diffusion of oxygen along the grain boundaries, so that the oxidation resistance of the ductile cast iron is improved. Sn and Re co-segregate at the austenite grain boundaries to form Sn-Re atomic clusters, occupying the segregation sites of harmful elements such as P and S, thereby reducing the impurity concentration; moreover, Mg and Re form Mg-Re-O composite inclusions, which can effectively adsorb free S impurities and effectively reduce the generation of brittle MgS phases, thus enhancing the strength of the ductile cast iron. In addition, by increasing the contents of C and Si in this application, the eutectic reaction range can be expanded, effectively promoting the formation of a fine and uniform ferrite matrix. And Al is dissolved in the ferrite finally, compensating for the strength loss caused by high Si, not only ensuring the strength of the cast iron but also improving the ductility of the cast iron.

[0027] 2. When preparing the ductile cast iron in the present invention, intermediate alloys such as Mo-Fe intermediate alloy, W-Fe intermediate alloy, and Fe-Ti intermediate alloy are first made into alloy powders, which can shorten the melting time and avoid composition segregation caused by dissolution lag. The Sn granules and Fe-Mn alloy are made into cored wires, and the Fe-Mn / Sn cored wires are injected with the flowing metal, which can effectively avoid the volatilization of Sn and the oxidation of Mn, ensuring the effective play of the synergistic effect of Sn / Mn elements; what is also injected with the flowing metal includes a liquid modifier and silicon carbide particles. Ti in the liquid modifier forms the primary TiC phase, increasing the number of graphite balls in the cast iron, and the silicon carbide particles, as the silicon-based nucleation substrate, can increase the number and regularity of graphite balls. When heat-treating the ingot in the present invention, a two-stage austenitization treatment process is adopted. The first-stage treatment (900 - ^{\circ}C / 2 - 4h) can fully dissolve the (Mo,W)C composite carbide and eliminate the casting stress; the second-stage treatment (850 - ^{\circ}C / 1 - 3h) can inhibit the formation of pearlite and promote the precipitation of nano-scale (Mo,W)C; after the two-stage treatment, uniformly dispersed graphite balls are formed inside the ductile cast iron, ensuring the performance of the ductile cast iron.

[0028] 3. In the ductile iron of the present invention, through the multi-level synergistic and antagonistic effects among elements, the problems of insufficient high-temperature strength, poor tissue stability, and weak environmental resistance of traditional materials are successfully solved, enabling it to also have excellent comprehensive properties in high-temperature environments. Detailed Embodiments

[0029] The following is a detailed description of the specific embodiments of the present invention in conjunction with the embodiments.

[0030] Embodiment 1

[0031] A kind of ductile iron, comprising the following components by mass percentage:

[0032] C 3.5%, Si 3.0%, Mn 0.7%, Mg 0.05%, Y 0.02%, La 0.02%, Mo 0.4%, W 0.3%, Al 1.0%, Sn 0.06%, Ti 0.2%, S≤0.012%, P≤0.025%, and the balance is iron.

[0033] The ductile iron in this embodiment is prepared through the following steps:

[0034] (1) Crush Q12 pig iron into iron blocks with a particle size of about 40 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50×50 mm; perform magnetic separation and impurity removal on the return material; make Mo-Fe master alloy, W-Fe master alloy, and Fe-Ti master alloy into alloy powders of about 100 μm; crush Al blocks into particles of about 4 mm; make Sn grains and Fe-Mn alloy into cored wire;

[0035] (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel, and return material into the intermediate frequency induction furnace in sequence from bottom to top, and the mass ratio of the three is 55:30:15; first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW and heat for 20 min, and then adjust the power to 2000 kW and heat to 1550 °C; then add Mo-Fe master alloy powder, W-Fe master alloy powder, Fe-Ti master alloy powder, and Al particles, and perform electromagnetic stirring; after all are melted, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements;

[0036] (3) Load the formulated Y-Mg alloy and La-Mg alloy into a spheroidizing ladle preheated to 800 °C, then pour in the molten iron, and simultaneously pour in Fe-Ti alloy powder and silicon carbide particles with a particle size of about 80 nm, and the pouring rate of both is 0.2 kg / min; at the same time, inject the cored wire prepared in step (1) by the wire feeding method;

[0037] (3) Pour to obtain a casting;

[0038] (4) Heat the casting to 920 °C, hold for 3 h, then cool down to 860 °C and hold for 2 h; then cool down to 300 °C at a cooling rate of 40 °C / min, and then air-cool to room temperature;

[0039] (5) Heat the ingot treated in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product.

[0040] Example 2

[0041] A ductile iron includes the following components by mass percentage:

[0042] C 3.4%, Si 3.2%, Mn 0.6%, Mg 0.07%, Y 0.02%, La 0.01%, Mo 0.5%, W 0.2%, Al 1.2%, Sn 0.05%, Ti 0.25%, S ≤ 0.012%, P ≤ 0.025%, and the balance is iron.

[0043] The ductile iron in this example is prepared through the following steps:

[0044] (1) Crush Q12 pig iron into iron blocks with a particle size of about 30 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50×50 mm; perform magnetic separation and impurity removal on the return material; make Mo-Fe master alloy, W-Fe master alloy and Fe-Ti master alloy into alloy powders with a particle size of about 80 μm; crush Al blocks into particles of about 3 mm; make Sn grains and Fe-Mn alloy into cored wire;

[0045] (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel and return material into the intermediate frequency induction furnace in sequence from bottom to top, and the mass ratio of the three is 55:30:15; first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW and heat for 20 min, and then adjust the power to 2000 kW and heat to 1540 °C; then add Mo-Fe master alloy powder, W-Fe master alloy powder, Fe-Ti master alloy powder and Al grains, and perform electromagnetic stirring; after all are melted, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements;

[0046] (3) Load the formulated Y-Mg alloy and La-Mg alloy into a spheroidizing ladle preheated to 800 °C, then pour in the molten iron, and simultaneously pour in Fe-Ti alloy powder and silicon carbide particles with a particle size of about 50 nm during the pouring process, and the pouring rates of both are 0.1 kg / min; at the same time, inject the cored wire prepared in step (1) by the wire feeding method;

[0047] (3) Cast to obtain a casting;

[0048] (4) Heat the casting to 900 °C, hold for 4 h, then cool down to 850 °C and hold for 3 h; then cool down to 300 °C at a cooling rate of 30 °C / min, and then air-cool to room temperature;

[0049] (5) Heat the ingot treated in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product.

[0050] Example 3

[0051] A ductile iron includes the following components by mass percentage:

[0052] C 3.6%, Si 2.8%, Mn 0.8%, Mg 0.04%, Y 0.04%, La 0.01%, Mo 0.3%, W 0.4%, Al 0.8%, Sn 0.08%, Ti 0.15%, S ≤ 0.012%, P ≤ 0.025%, and the balance is iron.

[0053] The ductile iron in this example is prepared through the following steps:

[0054] (1) Crush Q12 pig iron into iron blocks with a particle size of about 50 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50×50 mm; perform magnetic separation and impurity removal on the return materials; make Mo-Fe master alloy, W-Fe master alloy and Fe-Ti master alloy into alloy powders of about 120 μm; crush Al blocks into particles of about 5 mm; make Sn grains and Fe-Mn alloy into cored wires;

[0055] (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel and return materials into the intermediate frequency induction furnace from bottom to top in a mass ratio of 55:30:15; first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW and heat for 20 min, and then adjust the power to 2000 kW and heat to 1560 °C; then add Mo-Fe master alloy powder, W-Fe master alloy powder, Fe-Ti master alloy powder and Al particles, and perform electromagnetic stirring; after complete melting, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements;

[0056] (3) Load the formulated Y-Mg alloy and La-Mg alloy into a spheroidizing ladle preheated to 800 °C, then pour in the molten iron, and simultaneously pour in Fe-Ti alloy powder and silicon carbide particles with a particle size of about 100 nm at a pouring rate of 0.3 kg / min for both; at the same time, inject the cored wire prepared in step (1) by the wire feeding method;

[0057] (3) Cast to obtain the casting;

[0058] (4) Heat the casting to 950 °C, hold for 2 h, then cool down to 880 °C and hold for 1 h; then cool down to 300 °C at a cooling rate of 50 °C / min, and then air-cool to room temperature;

[0059] (5) Heat the ingot treated in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product.

[0060] Comparative Example 1

[0061] A ductile iron includes the following components by mass percentage:

[0062] C 3.5%, Si 3.0%, Mn 0.7%, Mg 0.05%, Y 0.02%, La 0.02%, Mo 0.4%, W 0.3%, Sn 0.06%, S ≤ 0.012%, P ≤ 0.025%, and the balance is iron.

[0063] The ductile iron in this comparative example is prepared through the following steps:

[0064] (1) Crush Q12 pig iron into iron blocks with a particle size of about 40 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50×50 mm; conduct magnetic separation and impurity removal treatment on the return material; make Mo-Fe master alloy and W-Fe master alloy into alloy powders of about 100 μm; make Sn pellets and Fe-Mn alloy into cored wires;

[0065] (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel and return material into the intermediate frequency induction furnace in sequence from bottom to top, and the mass ratio of the three is 55:30:15; first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW and heat for 20 min, and then adjust the power to 2000 kW and heat to 1550 °C; then add Mo-Fe master alloy powder and W-Fe master alloy powder, and conduct electromagnetic stirring; after complete melting, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements;

[0066] (3) Load the formulated amount of Y-Mg alloy and La-Mg alloy into a spheroidizing ladle preheated to 800 °C, then pour in the molten iron, and simultaneously pour in silicon carbide particles with a particle size of about 80 nm at a pouring rate of 0.2 kg / min; at the same time, inject the cored wire prepared in step (1) by the wire feeding method;

[0067] (3) Pour to obtain the casting;

[0068] (4) Heat the casting to 920 °C, hold for 3 h, then cool down to 860 °C and hold for 2 h; then cool down to 300 °C at a cooling rate of 40 °C / min, and then air-cool to room temperature;

[0069] (5) Heat the ingot processed in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product.

[0070] Comparative Example 2

[0071] A ductile iron includes the following components by mass percentage:

[0072] C 3.5%, Si 3.0%, Mn 0.7%, Mg 0.05%, Y 0.02%, La 0.02%, Al 1.0%, Sn 0.06%, Ti 0.2%, S ≤ 0.012%, P ≤ 0.025%, and the balance is iron.

[0073] The ductile iron in this comparative example is prepared through the following steps:

[0074] (1) Crush Q12 pig iron into iron blocks with a particle size of about 40 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50×50 mm; conduct magnetic separation and impurity removal treatment on the return material; make Fe-Ti master alloy into alloy powder with a particle size of about 100 μm; crush Al blocks into particles of about 4 mm; make Sn grains and Fe-Mn alloy into cored wire;

[0075] (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel, and return material into the intermediate frequency induction furnace in sequence from bottom to top, and the mass ratio of the three is 55:30:15; first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW and heat for 20 min, and then adjust the power to 2000 kW and heat to 1550 °C; then add Fe-Ti master alloy powder and Al particles and conduct electromagnetic stirring; after all are melted, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements;

[0076] (3) Load the formulated Y-Mg alloy and La-Mg alloy into a nodulizing ladle preheated to 800 °C, then pour in the molten iron, and simultaneously pour in Fe-Ti alloy powder and silicon carbide particles with a particle size of about 80 nm along with the flow, and the pouring rate of both is 0.2 kg / min; at the same time, inject the cored wire prepared in step (1) by the wire feeding method;

[0077] (3) Pour to obtain a casting;

[0078] (4) Heat the casting to 920 °C, hold for 3 h, then cool down to 860 °C and hold for 2 h; then cool down at a cooling rate of 40 °C / min to 300 °C, and then air-cool to room temperature;

[0079] (5) Heat the ingot processed in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product.

[0080] Experimental Example

[0081] The high-temperature strength of the ductile iron prepared in each of the above examples and comparative examples was tested by the method described in ASTM E21, and the graphite morphology of the ductile iron prepared in each of the above examples and comparative examples was studied by the method described in ASTM A247. The results are shown in Table 1.

[0082] Table 1 Performance test results of ductile iron

[0083] High-temperature strength (600 °C, MPa) VI nodularity rate (%) Example 1 478 98.3 Example 2 471 97.7 Example 3 474 97.9 Comparative Example 1 407 93.2 Comparative Example 2 402 88.7

[0084] Although the specific embodiments of the present invention have been described in detail in conjunction with the embodiments, it should not be construed as a limitation on the protection scope of this patent. Within the scope described in the claims, various modifications and deformations that can be made by those skilled in the art without creative efforts still fall within the protection scope of this patent.

Claims

1. A preparation method of ductile iron, the ductile iron being used for preparing high-temperature resistant structural parts, characterized in that, It includes the following steps: (1) Crush Q12 pig iron into iron blocks with a particle size of 30 - 50 mm; cut low-sulfur and low-phosphorus scrap steel into flakes of 50×50 mm; perform magnetic separation and impurity removal treatment on the return materials; make Mo-Fe master alloy, W-Fe master alloy, and Fe-Ti master alloy into alloy powders with a particle size of 80 - 120 μm; crush Al blocks into particles of 3 - 5 mm; make Sn grains and Fe-Mn alloy into cored wire; (2) Load Q12 pig iron, low-sulfur and low-phosphorus scrap steel, and return materials into the intermediate frequency induction furnace from bottom to top, heat up to 1540 - 1560 °C by stepped power, then add Mo-Fe master alloy powder, W-Fe master alloy powder, Fe-Ti master alloy powder, and Al grains, and perform electromagnetic stirring; after complete melting, detect the content of each element in the molten iron and adjust the content of each element to the formula requirements; (3) Load the rare earth magnesium alloy into the preheated nodulizing ladle, then pour in the molten iron, and pour in the liquid modifier and silicon carbide particles along with the flow, and at the same time inject the cored wire prepared in step (1) by the wire feeding method; the liquid modifier is Fe-Ti alloy powder; (3) Cast to obtain the casting; (4) Heat the casting to 900 - 950 °C, hold for 2 - 4 h, then cool down to 850 - 880 °C, hold for 1 - 3 h; then cool down at a cooling rate of 30 - 50 °C / min to 300 °C, and then air-cool to room temperature; (5) Heat the ingot treated in step (4) to 450 °C, hold for 4 h, and then air-cool to room temperature to obtain the product; The ductile iron includes the following components by mass percentage: C 3.4 - 3.6%, Si 2.8 - 3.2%, Mn 0.6 - 0.8%, Mg 0.04 - 0.07%, Re 0.03 - 0.05%, Mo 0.3 - 0.5%, W 0.2 - 0.4%, Al 0.8 - 1.2%, Sn 0.05 - 0.08%, Ti 0.15 - 0.25%, S≤0.012%, P≤0.025%, and the balance is iron.

2. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of Q12 pig iron, low-sulfur and low-phosphorus scrap steel, and return materials is 55:30:

15.

3. The preparation method according to claim 1, wherein: In step (2), the stepped power heating method is to first heat at a power of 800 kW for 20 min, then adjust the power to 1500 kW, heat for 20 min, and then adjust the power to 2000 kW and heat to the final heating temperature.

4. The preparation method according to claim 1, characterized in that: In step (3), the preheating temperature of the nodulizing ladle is 800 °C.

5. The preparation method according to claim 1, characterized in that: The rare earth magnesium alloy is Y-Mg alloy and La-Mg alloy.

6. The preparation method according to claim 1, characterized in that: In step (3), the charging rates of the liquid modifier and the silicon carbide particles are both 0.1 - 0.3 kg / min; the particle size of the silicon carbide particles is 50 - 100 nm.

7. The preparation method according to claim 1, characterized in that, The ductile iron includes the following components by mass percentage: C 3.5%, Si 3.0%, Mn 0.7%, Mg 0.05%, Re 0.04%, Mo 0.4%, W 8. The preparation method according to claim 1 or 7, characterized in that, The Re includes Y and La. The mass percentage of Y in the ductile iron is 0.02 - 0.04%, and the mass percentage of La in the ductile iron is 0.01 - 0.02%.