Preparation method of high-strength high-toughness as-cast ultralow-temperature nodular cast iron

Abstract

The invention belongs to the field of wind power generation, and relates to a preparation method of high-strength high-toughness as-cast ultralow-temperature nodular cast iron. The proportion of all alloy elements in molten iron is strictly regulated and controlled, such as carbon (C), silicon (Si), manganese (Mn) and other main elements and magnesium (Mg), nickel (Ni), titanium (Ti) and other auxiliary elements. Through the adjustments, the basic mechanical properties of the material are improved, formation of bad tissues (such as fragment-shaped or aquatic plant-shaped graphite) is effectively inhibited, good roundness and uniform distribution of graphite nodules are ensured, and stream inoculation treatment is carried out by adopting a unique nodulizer combination and cooperatively using a silicon-calcium-barium inoculant and sulfur-oxygen inoculation powder. According to the multi-stage and multi-layer inoculation strategy, the nucleation condition in molten iron can be remarkably improved, generation of finer and more graphite nodules is promoted, and therefore the toughness and low-temperature impact performance of the material are further improved.

Description

Technical Field

[0001] The invention belongs to the field of wind power generation and relates to a method for preparing high-strength and high-toughness cast ultra-low-temperature ductile iron. Background Art

[0002] In recent years, with the increasing global environmental awareness and the pursuit of sustainable development, the new energy industry has ushered in unprecedented development opportunities, especially in the field of wind power generation. Against this background, the construction of wind farms not only covers traditional land and plain areas, but also extends to some extreme environments, such as the Arctic Circle, showing a trend of rapid expansion of the wind power industry. As one of the key components of wind turbines, the role of wind power gearboxes is crucial. It converts wind energy into mechanical kinetic energy and transmits it to the generator set to generate electricity, thereby promoting the application and development of clean energy.

[0003] However, with the growth of market demand and the advancement of technology, the wind power industry is facing more complex and harsh working conditions. Traditionally, the working environment temperature of wind turbines is about -20℃, but now, in order to adapt to the application in extremely cold areas, wind turbines need to be able to operate stably in ultra-low temperature environments as low as -40℃ or even -60℃. This not only poses a challenge to the overall design of wind turbines, but also prompts researchers in the field of materials science to re-examine the materials used to manufacture these devices - especially ductile iron, an important material widely used in wind power gearboxes.

[0004] For wind power manufacturers, it is particularly important to develop a ductile iron that can maintain high strength and toughness under such extreme conditions. Unlike the sequential solidification process of cast steel, ductile iron is prone to uneven distribution of wall thickness inside and outside during the casting process due to its unique paste-like solidification method. Specifically, the thicker parts of the casting cool slowly, resulting in a longer solidification time, causing certain elements that are not conducive to spheroidization to gather in the center of the casting, resulting in inconsistency between the internal and external structures, and the graphite morphology is not round enough, sometimes forming fragmented or waterweed-like graphite structures. Such defects will significantly weaken the impact performance of ductile iron at low temperatures, thereby affecting the safety and reliability of the entire wind power gearbox and even the wind turbine. Summary of the invention

[0005] The purpose of the present invention is to solve the above problems in the prior art and to propose a method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron to meet the performance requirements of tensile strength and -60°C low temperature impact.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron, the method comprising the following steps:

[0008] S1, smelting scrap steel, pig iron, ferrosilicon, carburizer, ferrous sulfide and nickel into molten iron, and then adding a pretreatment agent;

[0009] S2, adjusting the composition of the molten iron, and then raising the temperature to perform spheroidization inoculation treatment on the molten iron;

[0010] S3. After spheroidizing and inoculation treatment, remove the slag in the furnace, cover the surface of the molten iron with a heat-insulating covering agent and transport it to the casting site;

[0011] S4. After the iron is tapped, the molten iron is transferred to a casting mold for pouring, and sulfur-oxygen inoculant powder is added with the molten iron for inoculation to obtain the as-cast ultra-low temperature ductile iron.

[0012] As a preferred method, electric furnace smelting is selected to avoid high temperature burning of molten iron nuclei during the melting process, so as to form more stable nucleation particles, provide good conditions for subsequent spheroidization reactions, obtain qualified metallographic structures, and lay a good foundation for ultra-low temperature performance testing.

[0013] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the amount of pretreatment agent added in step S1 is 0.08-0.10wt% of the weight of molten iron, and the component contents by mass percentage are: 66.5-68.5% Si, 3.5-4.5% Al, 3-5% Zr, and the rest is Fe.

[0014] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the molten iron after the composition adjustment in step S2 is composed of the following components by mass fraction: 3.76-3.80% C, 1.7-1.90% Si, 0-0.15% Mn, 0-0.025% P, 0.006-0.012% S, 0.040-0.055% Mg, 0.50-0.80% Ni, 0-0.0008% B, 0-0.015% Ti, 0.002-0.006% RE, 0.002-0.005% Sb, and the remainder is Fe.

[0015] In the present invention, C is as high as possible while ensuring that no graphite floats, so as to reduce the tendency of shrinkage and affect the size and number of graphite balls, thereby affecting the ultra-low temperature impact performance. The carbon content of molten iron is set to 3.75-3.80%; Si is an element that promotes graphitization in the present invention, which can improve the tensile strength, yield strength and elongation, etc., improve the tough-brittle transition temperature, and improve the eutectoid transition temperature. When the content is too high, the ultra-low temperature impact value will be greatly reduced, and it is also very easy to form fragmented graphite in the core of the casting and blooming graphite on the surface of the casting. The content of Mn is controlled to be 1.70-1.90%; Mn is an alloy element that promotes the production of pearlite and improves the tensile strength. It is easy to cause positive segregation and is formed at the core grain boundary. Pearlite is produced at the place where the low-temperature impact toughness is significantly reduced. Its content is determined by the pig iron scrap and should be controlled below 0.15%; P of the present invention can improve the fluidity of molten iron. When the content is too high, phosphorus eutectic will be formed at the grain boundary, significantly reducing the impact value. Its content is determined by the raw and auxiliary materials and should be controlled below 0.025%; S of the present invention is to ensure good spheroidization and good nucleation. The sulfur content in the original iron liquid should be between 0.008-0.015%, and the sulfur content after spheroidization should be between 0.006-0.010%; Mg of the present invention is an element that strongly stabilizes carbides and promotes the formation of spheroidal graphite. To ensure the spheroidization effect, the residual amount of magnesium should be between 0.040-0.053%. The ability of Ni of the present invention to promote graphitization is about one-fourth of Si, which reduces the tough-brittle transition temperature and still has good impact toughness at extremely low temperatures. Its content is controlled at 0.50-0.80%. Ti of the present invention is an anti-spheroidizing element for ductile iron with thick wall, which promotes the formation of vermicular graphite and is one of the elements that strongly promotes intergranular carbides. Its content is controlled below 0.015%; B of the present invention is one of the positive segregation elements. For ductile iron with thick wall, it is one of the elements that strongly promotes intergranular carbides and seriously affects the low-temperature impact performance. Its content is controlled below 0.0008%; RE of the present invention has favorable effects such as deoxidation and desulfurization, and neutralizes the anti-spheroidizing elements in the molten iron. However, when the content is too high, the graphite morphology will be deteriorated and the mechanical properties will be reduced. The residual amount should be between 0.002-0.006%. Sb of the present invention is an element that strongly promotes pearlite. When the content is too high, it will promote the formation of waterweed-like graphite and reduce the ultra-low temperature impact performance. In order to improve the roundness of graphite and eliminate the influence of RE on the deterioration of graphite, the process sets the Sb content of the molten iron to be 0.002-0.005%.

[0016] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the process of spheroidizing inoculation treatment is to place a spheroidizing agent accounting for 1.15-1.23wt% of the weight of the molten iron on one side of the bottom dam of the spheroidizing ladle, and cover it with 0.35-0.45wt% of silicon-calcium-barium inoculant, and evenly cover the silicon-barium inoculant with 0.65-0.75wt% of silicon steel sheets accounting for 0.65-0.75wt% of the weight of the molten iron.

[0017] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the spheroidizer is spheroidizer one and spheroidizer two at a ratio of 1: (0.4-0.45).

[0018] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the spheroidizer 1 is composed of the following components in mass fraction: 70-72% Si, 5-6% Mg, 0.3-0.5% RE, 0.55-0.65% Al, 0.35-0.4% MgO, 0.5-1.0% Ca, and the balance is Fe.

[0019] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the second spheroidizer is composed of the following components by mass fraction: 40-50% Si, 5-6% Mg, 0.40-0.55% Al, 0.25-0.35% MgO, 0.8-0.9% Ca, and the balance is Fe.

[0020] In the above-mentioned method for preparing high-strength and high-toughness cast ultra-low temperature ductile iron, the silicon-calcium-barium inoculant consists of the following components by mass fraction: 65-75% Si, 0.8-1.2% Al, 0.65-0.75% Ca, 1.5-2.5% Ba, and the balance is Fe.

[0021] Preferably, the spheroidization inoculation time is 4-6 minutes. The present invention strictly controls the inoculation time, and the silicon calcium barium inoculant has good anti-fading performance, and a good inoculation effect can be obtained within 4-6 minutes of inoculation.

[0022] In the above method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron, the particle size of the sulfur-oxygen inoculated powder in the flow is 0.2-0.7 mm, and the addition amount is 0.25-3.5% of the mass of the molten iron.

[0023] The invention adopts sulfur-oxygen inoculant powder in the pouring flow, the purpose is to improve the instantaneous inoculation effect, can effectively increase the elements such as S and O in the molten iron, add a small amount of nucleation mass center in the molten iron, increase the number of graphite balls, reduce the diameter of graphite balls, and improve the ultra-low temperature impact performance. In addition, the particle size of the inoculant powder in the pouring flow of the invention must be guaranteed to be between 0.2-0.7mm, the purpose is that when pouring the flow, the inoculant powder contacts the molten iron and can melt quickly, increase the uniformity of the molten iron inoculation, and provide a sufficient number of nuclei. The flow rate of the inoculant powder must be consistent with the filling speed of the molten iron, and the inoculant powder is never allowed to flow out early and lag behind, otherwise it will affect the inoculation effect. When selecting the best inoculation amount, it should be noted that the inoculation amount should not be increased blindly, otherwise the excessive inoculation amount will increase the viscosity of the molten iron and the tendency of slag inclusion, shrinkage cavity and shrinkage. The inoculant amount during pouring is controlled at 0.25-3.5%.

[0024] As a preference, during pouring, the inoculant powder must be directly poured onto the molten iron being filled at the ladle spout, and the inoculant powder should be in full contact with the molten iron to achieve the best inoculation effect, so as to improve the roundness and number of graphite nodules in the core of the casting and improve the -60°C ultra-low temperature impact performance of the casting.

[0025] Preferably, the time difference between the filling speed of the molten iron and the flow rate of the pouring inoculant powder must be controlled within ±10 seconds.

[0026] Preferably, the tapping temperature in step S4 is 1450-1500°C, and the pouring temperature is 1300-1400°C. The tapping temperature has an indirect effect on the low-temperature impact properties of the material. When the tapping temperature of the molten iron is high, the absorption rate of the Mg content in the spheroidizer will be low, and the low Mg content will easily cause the spheroidization of the molten iron to decline, affecting the mechanical properties; when the tapping temperature is set too low, the temperature of the molten iron drops too fast, there is a risk of filling the mold, and the casting is prone to cold shut defects, affecting the low-temperature impact properties.

[0027] In the above-mentioned method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron, the number of graphite nodules in the as-cast ultra-low temperature ductile iron prepared in step S4 is ≥180 / mm 2 , composed of the following components in mass fractions: 3.76-3.80% C, 1.70-1.90% Si, 0-0.15% Mn, 0-0.020% P, 0.006-0.010% S, 0.040-0.053% Mg, 0.50-0.80% Ni, 0-0.015% Ti, 0-0.0008% B, 0.002-0.006% Re, and 0.002-0.005% Sb.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention strictly regulates the proportion of various alloy elements in molten iron, such as carbon (C), silicon (Si), manganese (Mn) and other main elements and magnesium (Mg), nickel (Ni), titanium (Ti) and other auxiliary elements. These adjustments not only improve the basic mechanical properties of the material, but also effectively inhibit the formation of undesirable structures (such as fragmented or waterweed-like graphite), ensuring the good roundness and uniform distribution of graphite balls.

[0030] 2. The present invention adopts a unique spheroidizing agent combination (spheroidizing agent 1 and spheroidizing agent 2), and uses silicon calcium barium inoculant and sulfur oxygen inoculant powder for in-stream inoculation. This multi-stage and multi-level inoculation strategy can significantly improve the nucleation conditions inside the molten iron, promote the formation of smaller and more numerous graphite nodules, and further improve the toughness and low-temperature impact performance of the material.

[0031] 3. The present invention ensures that the tensile strength and -60°C low-temperature impact performance of the final product are significantly enhanced by optimizing the component design and process control, solves the problem of a sharp drop in mechanical properties of traditional ductile iron under extremely low temperature conditions, and meets the requirements of the wind power industry for the stability and reliability of materials in extreme environments.

[0032] 4. From the temperature management in the electric furnace smelting process to the precise matching between the molten iron filling speed and the inoculant addition rate during pouring, each step is carefully designed to ensure the best metallurgical effect. In particular, the selection of the tapping temperature and the pouring temperature takes into account both the risk of preventing spheroidization decay and the occurrence of cold shut defects, thus ensuring the overall quality of the casting.

[0033] 5. The present invention can make full use of recycled resources without sacrificing material properties through the selection of raw materials (such as scrap steel, pig iron, etc.) and pretreatment measures (such as the application of pretreatment agents), which is in line with the concept of modern industry pursuing sustainable development. At the same time, its excellent low-temperature performance makes this kind of ductile iron very suitable for use in extremely cold regions, such as high-strength and high-toughness components required in Arctic wind power projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is the molten iron curve after smelting the ductile iron prepared in Example 1.

[0035] Figure 2 This is the molten iron curve of the ductile iron prepared in Example 1 after spheroidization.

[0036] Figure 3 This is a metallographic diagram of the ductile iron mold prepared in Example 1, a is the ductile iron in the original state after preparation, and b is the ductile iron corroded by the corrosive liquid. DETAILED DESCRIPTION

[0037] The following are specific embodiments of the present invention, which further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0038] The raw materials for molten iron are selected according to the mass proportions: 40 parts of high-quality pig iron, 40 parts of ultra-high purity pig iron, 20 parts of scrap steel, 0.5 parts of carburizer, 0.9 parts of ferrosilicon, 0.02 parts of ferrous sulfide, and 0.65 parts of nickel;

[0039] Table 1: Chemical composition of molten iron raw materials

[0040]

[0041] The pretreatment agent also contains 4wt% Zr.

[0042] Table 2: Chemical composition of spheroidizer and inoculant

[0043]

[0044] The average particle size of spheroidizer 1 is 15 mm;

[0045] The average particle size of spheroidizer 2 is 10 mm;

[0046] The average particle size of the silicon-calcium-barium inoculant is 5 mm;

[0047] Sulfur-oxygen inoculant powder was purchased from Elkem Ce inoculant.

[0048] Embodiment 1:

[0049] S1, smelting scrap steel, pig iron, ferrosilicon, carburizer, ferrous sulfide and nickel into molten iron, and then adding 1% pretreatment agent by mass of the molten iron;

[0050] S2. The composition of the molten iron is adjusted to obtain the following composition by mass fraction: 3.72% C, 1.74% Si, 0.07% Mn, 0.012% P, 0.010% S, 0.050% Mg, 0.60% Ni, 0.0006% B, 0.012% Ti, 0.003% RE, 0.003% Sb, and the balance is iron;

[0051] S3, then heating to 1480°C to perform spheroidizing inoculation treatment on the molten iron for 5 minutes; placing a spheroidizing agent accounting for 1.20wt% of the weight of the molten iron on one side of the dam at the bottom of the spheroidizing ladle, covering it with 0.40% of silicon-calcium-barium inoculant, and evenly covering the silicon-barium inoculant with 0.70wt% of silicon steel sheets accounting for 0.70wt% of the weight of the molten iron; wherein the spheroidizing agents are spheroidizing agent 1 and spheroidizing agent 2 in a mass ratio of 70:30;

[0052] S3. After spheroidizing and inoculation treatment, remove the slag in the furnace, cover the molten iron surface with Ishikawa 4S slag remover and Jinnaiyuan JF-1 insulation covering agent, and transport it to the casting site;

[0053] S4, tapping at 1480°C, then lifting and transferring the molten iron to a casting mold for pouring at 1360°C, and adding 0.3% sulfur-oxygen inoculant powder by mass of the molten iron with the molten iron for inoculation to obtain as-cast ultra-low temperature ductile iron.

[0054] Embodiment 2:

[0055] S1, smelting scrap steel, pig iron, ferrosilicon, carburizer, ferrous sulfide and nickel into molten iron, and then adding 1% pretreatment agent by mass of the molten iron;

[0056] S2. The composition of the molten iron is adjusted to obtain the following composition by mass fraction: 3.74% C, 1.74% Si, 0.07% Mn, 0.012% P, 0.010% S, 0.051% Mg, 0.60% Ni, 0.0006% B, 0.011% Ti, 0.003% RE, 0.003% Sb, and the balance is iron;

[0057] S3, then raise the temperature to 1480°C to perform spheroidizing inoculation treatment on the molten iron for 5 minutes; place a spheroidizing agent accounting for 1.20wt% of the weight of the molten iron on one side of the dam at the bottom of the spheroidizing ladle, cover it with 0.40% of silicon calcium barium inoculant, and evenly cover the silicon barium inoculant with 0.70wt% of silicon steel sheet accounting for 0.70wt% of the weight of the molten iron; wherein the spheroidizing agent is spheroidizing agent 1;

[0058] S3. After spheroidizing and inoculation treatment, remove the slag in the furnace, cover the molten iron surface with Ishikawa 4S slag remover and Jinnaiyuan JF-1 insulation covering agent, and transport it to the casting site;

[0059] S4, tapping at 1480°C, then lifting and transferring the molten iron to a casting mold for pouring at 1360°C, and adding 0.3% sulfur-oxygen inoculant powder by mass of the molten iron with the molten iron for inoculation to obtain as-cast ultra-low temperature ductile iron.

[0060] Embodiment 3:

[0061] S1, smelting scrap steel, pig iron, ferrosilicon, carburizer, ferrous sulfide and nickel into molten iron, and then adding 1% pretreatment agent by mass of the molten iron;

[0062] S2. The composition of the molten iron is adjusted to obtain the following composition by mass fraction: 3.73% C, 1.75% Si, 0.07% Mn, 0.012% P, 0.010% S, 0.056% Mg, 0.60% Ni, 0.0006% B, 0.012% Ti, 0.003% RE, 0.003% Sb, and the balance is iron;

[0063] S3, then heating to 1480°C to perform spheroidizing inoculation treatment on the molten iron for 5 minutes; placing a spheroidizing agent accounting for 1.20wt% of the weight of the molten iron on one side of the dam at the bottom of the spheroidizing ladle, covering it with 0.40% of silicon-calcium-barium inoculant, and evenly covering the silicon-barium inoculant with 0.70wt% of silicon steel sheets accounting for 0.70wt% of the weight of the molten iron; wherein the spheroidizing agent is spheroidizing agent II;

[0064] S3. After spheroidizing and inoculation treatment, remove the slag in the furnace, cover the molten iron surface with Ishikawa 4S slag remover and Jinnaiyuan JF-1 insulation covering agent, and transport it to the casting site;

[0065] S4, tapping at 1480°C, then lifting and transferring the molten iron to a casting mold for pouring at 1360°C, and adding 0.3% sulfur-oxygen inoculant powder by mass of the molten iron with the molten iron for inoculation to obtain as-cast ultra-low temperature ductile iron.

[0066] Embodiment 4:

[0067] The only difference from Example 1 is that the mass ratio of spheroidizing agent 1 to spheroidizing agent 2 is 1:0.1.

[0068] Embodiment 5:

[0069] The only difference from Example 1 is that the mass ratio of spheroidizing agent 1 to spheroidizing agent 2 is 1:1.

[0070] Embodiment 6:

[0071] The only difference from Example 1 is that in step S3, a spheroidizing agent accounting for 1.5 wt% of the weight of the molten iron is placed on one side of the dam at the bottom of the spheroidizing ladle.

[0072] Embodiment 7:

[0073] The only difference from Example 1 is that no nickel is added to the raw materials.

[0074] Embodiment 8:

[0075] The only difference from Example 1 is that no ferrous sulfide is added to the raw materials.

[0076] Comparative Example 1:

[0077] The only difference from Example 1 is that the inoculant in step S4 is silicon strontium zirconium.

[0078] The weight of the casting products prepared for the embodiments and comparative examples is 3 tons and above, and the wall thickness range of the casting is 120mm. The entire production process is planned from the preparation of raw and auxiliary materials in the early stage, the melting method, the spheroidization inoculation process design, and the pouring, so that the product can meet the ultra-low temperature impact performance of -60℃. Material technical index requirements: The tensile strength of the product cast test block is ≥370MPa, the yield strength is ≥220MPa, the elongation is ≥18, the three average values ​​of -60℃ low temperature impact are ≥10J, and the single value is >7J. The size of the cast test block is 70mm*70mm*180mm.

[0079] Table 3: Mechanical properties test results of ductile iron prepared in Examples 1-5 and Comparative Example 1

[0080]

[0081] Table 4: Test results of -60°C ultra-low temperature impact properties of ductile iron prepared in Examples 1-5 and Comparative Example 1

[0082]

[0083]

[0084] Figure 1 This is the molten iron curve after smelting of the ductile iron prepared in Example 1. As can be seen from the figure, the LET value of the original molten iron in Example 1 is 1145°C, indicating that the nucleation value of the original molten iron is relatively good, and the G1 interval of 27% indicates that the original molten iron is a hypoeutectic component.

[0085] Figure 2 This is the molten iron curve after spheroidization of ductile iron prepared in Example 1. The LET value after spheroidization treatment is 1140°C (without adding flow), indicating that the molten iron has excellent nucleation ability; the R value after spheroidization is 2.6°C, indicating that the molten iron supercooling is significantly reduced after spheroidization inoculation, reflecting the optimal solution for spheroidization inoculation treatment.

[0086] Figure 3 This is the metallographic diagram of the ductile iron mold prepared in Example 1. Figure 3 It can be seen from a that the ductile iron prepared in Example 1 of the present invention has a spheroidization rate of 95%, a graphite size of 7 levels, and a graphite ball number of 195 / mm 2 After the ductile iron was corroded by 4wt% nitric acid alcohol, Figure 3 As can be seen from b, the matrix structure is mainly ferrite, and its content is >95%.

[0087] In summary, the present invention ensures that the tensile strength and -60°C low-temperature impact performance of the final product are significantly enhanced by optimizing the component design and process control, solves the problem of a sharp drop in mechanical properties of traditional ductile iron under extremely low temperature conditions, and meets the requirements of the wind power industry for the stability and reliability of materials in extreme environments.

[0088] The parts of the embodiments herein that are not exhaustive of the midpoint values ​​of the technical scope claimed for protection by the present invention and the new technical solutions formed by equivalent replacement of single or multiple technical features in the technical solutions of the embodiments are also within the scope claimed for protection by the present invention; at the same time, in all the listed or unlisted embodiments of the scheme of the present invention, each parameter in the same embodiment merely represents an example of its technical solution (i.e., a feasible solution), and there is no strict coordination and limitation relationship between the parameters, wherein the parameters can be replaced with each other without violating the axioms and the claims of the present invention, unless otherwise stated.

[0089] The technical means disclosed in the scheme of the present invention are not limited to the technical means disclosed in the above technical means, but also include technical schemes composed of any combination of the above technical features. The above is a specific implementation of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications are also regarded as the protection scope of the present invention.

[0090] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron, characterized in that: The method comprises the following steps: S1, smelting scrap steel, pig iron, ferrosilicon, carburizer, ferrous sulfide and nickel into molten iron, and then adding a pretreatment agent; S2, adjusting the composition of the molten iron, and then raising the temperature to perform spheroidization inoculation treatment on the molten iron; S3. After spheroidizing and inoculating treatment, remove the slag in the furnace, cover the surface of the molten iron with a heat-insulating covering agent and transport it to the casting site; S4. After the iron is tapped, the molten iron is transferred to a casting mold for pouring, and sulfur-oxygen inoculant powder is added with the molten iron for inoculation to obtain the as-cast ultra-low temperature ductile iron.

2. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 1, characterized in that: The amount of pretreatment agent added in step S1 is 0.08-0.10wt% of the weight of the molten iron, and the composition content by mass percentage is: 66.5-68.5% Si, 3.5-4.5% Al, 3-5% Zr, and the rest is Fe.

3. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 1, characterized in that: The molten iron after the composition adjustment in step S2 is composed of the following components by mass fraction: 3.76-3.80% C, 1.7-1.90% Si, 0-0.15% Mn, 0-0.025% P, 0.006-0.012% S, 0.040-0.055% Mg, 0.50-0.80% Ni, 0-0.0008% B, 0-0.015% Ti, 0.002-0.006% RE, 0.002-0.005% Sb, and the balance is Fe.

4. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 1, characterized in that: The process of spheroidizing inoculation treatment is to place a spheroidizing agent accounting for 1.15-1.23wt% of the weight of the molten iron on one side of the bottom dam of the spheroidizing ladle, and cover it with 0.35-0.45wt% of silicon calcium barium inoculant, and evenly cover the silicon barium inoculant with 0.65-0.75wt% of silicon steel sheets accounting for 0.65-0.75wt% of the weight of the molten iron.

5. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 4, characterized in that: The spheroidizer is a spheroidizer 1 and a spheroidizer 2 in a mass ratio of 1: (0.4-0.45).

6. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 5, characterized in that: The spheroidizing agent 1 is composed of the following components in mass fraction: 70-72% Si, 5-6% Mg, 0.3-0.5% RE, 0.55-0.65% Al, 0.35-0.4% MgO, 0.5-1.0% Ca, and the balance is Fe.

7. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 5, characterized in that: The second spheroidizer is composed of the following components by mass fraction: 40-50% Si, 5-6% Mg, 0.40-0.55% Al, 0.25-0.35% MgO, 0.8-0.9% Ca, and the balance is Fe.

8. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 4, characterized in that: The silicon-calcium-barium inoculant is composed of the following components in mass fraction: 65-75% Si, 0.8-1.2% Al, 0.65-0.75% Ca, 1.5-2.5% Ba, and the balance is Fe.

9. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 1, characterized in that: The particle size of the sulfur-oxygen inoculated powder in the flow is 0.2-0.7 mm, and the addition amount is 0.25-3.5% of the mass of the molten iron.

10. The method for preparing high-strength and high-toughness as-cast ultra-low temperature ductile iron according to claim 1, characterized in that: The number of graphite nodules of the as-cast ultra-low temperature ductile iron prepared in step S4 is ≥ 180 / mm 2 , composed of the following components in mass fractions: 3.76-3.80% C, 1.70-1.90% Si, 0-0.15% Mn, 0-0.020% P, 0.006-0.010% S, 0.040-0.053% Mg, 0.50-0.80% Ni, 0-0.015% Ti, 0-0.0008% B, 0.002-0.006% Re, and 0.002-0.005% Sb.

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