High-strength high-elongation qt400-18 nodular cast iron and preparation method thereof

Abstract

The application relates to the technical field of casting, in particular to a high-strength high-elongation QT400-18 nodular cast iron and a preparation method thereof. The chemical components of the nodular cast iron are as follows in percentage by mass: C: 3.6%-3.9%, Si: 2.2%-2.6%, Mn: <=0.15%, P: <=0.02%, S: <=0.015%, Cr: <=0.04%, Mo: <=0.02%, Ni: 0.031%, Cu: <=0.035%, Ti: <=0.01%, Mg: 0.03%-0.05%, RE: 0.015%-0.03%, 0.06%<=∑(Cr+Mo+Ni+Cu)<0.1%, and the mass percentage of ∑(Cr+Mo) is not greater than the mass percentage of ∑(Ni+Cu), and the balance is Fe and inevitable impurities. The high-strength high-elongation QT400-18 nodular cast iron is prepared through accurate control of chemical component adjustment and optimization of a heat treatment process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of casting, in particular to a high-strength and high-elongation QT400-18 nodular cast iron and a preparation method thereof. BACKGROUND

[0002] The suspension swing arm is a key safety component and load-bearing component in the automobile suspension system, and its core role is to transmit various forces and moments between the wheels and the vehicle body, and to ensure that the wheels move along a specific trajectory. Therefore, the comprehensive performance of the suspension swing arm is directly related to the handling stability, safety and ride comfort of the whole vehicle.

[0003] Nodular cast iron, especially QT400-18 grade nodular cast iron, is widely used to manufacture suspension arms with high impact toughness and fatigue performance requirements due to its excellent casting performance, good shock absorption, and good toughness and plasticity (its performance standard requires: tensile strength Rm≥400 MPa, elongation after fracture A≥18%). The excellent plasticity and toughness of QT400-18 enable it to absorb energy through plastic deformation when subjected to overload or accidental impact, thereby avoiding instantaneous brittle fracture.

[0004] However, with the development of the automobile industry, especially the continuous improvement of lightweight, high performance and safety requirements, the traditional QT400-18 material manufactured suspension swing arm also has certain limitations in performance. The performance indicators of this material can only meet the minimum standard of industry access, and cannot meet the high standard requirements of existing lightweight and high performance. The specific performance is that the tensile strength (≥430 MPa) and elongation ≥22% are high standard requirements, which have higher strength and higher elongation, so that the suspension swing arm can absorb more impact energy, produce slower and more controllable deformation, and provide higher quality and safety protection.

[0005] In the field of materials science, the strength and plasticity of nodular cast iron are usually a pair of contradictory contradictions. For example, replacing the material with QT500-7 (Rm≥500 MPa, A≥7%) or QT700-2 (Rm≥700 MPa, A≥2%) with higher strength, although it meets the strength requirement, but the significant decrease of elongation makes the suspension arm lose the valuable energy absorption capacity when facing extreme impact, greatly reducing the safety performance. SUMMARY

[0006] The present application aims at the deficiencies of the prior art, and provides a high-strength and high-elongation QT400-18 nodular cast iron and a preparation method thereof. The present application combines the precise control of key ingredients with the production process, so that the elongation (≥22%) and tensile strength (≥430 MPa) of the casting are simultaneously improved, and are significantly higher than the standard requirements.

[0007] The technical solution adopted by this invention to solve its technical problem is: a high-strength, high-elongation QT400-18 ductile iron, the chemical composition of which, by mass percentage, is:

[0008] C: 3.6%~3.9%, Si: 2.2%~2.6%, Mn: ≤0.15%, P: ≤0.02%, S: ≤0.015%, Cr: ≤0.04%, Mo: ≤0.02%, Ni: 0.031%, Cu: ≤0.035%, Ti: ≤0.01%, Mg: 0.03%~0.05%, RE: 0.015%~0.03%, with the balance being Fe and unavoidable impurities; 0.06% < ∑(Cr+Mo+Ni+Cu) < 0.1%, and the mass percentage of ∑(Cr+Mo) is not greater than the mass percentage of ∑(Ni+Cu), preferably the mass percentage of ∑(Cr+Mo) is 0.5~0.8 times the mass percentage of ∑(Ni+Cu).

[0009] The basis for the design of the components of this invention is described below.

[0010] C: It is the basis for the formation of graphite spheres, ensuring that the casting has good mechanical and casting properties; the content is 3.6%~3.9%, preferably 3.7%~3.85%.

[0011] Si: a strong graphitizing element and solid solution strengthening element, with a content of 2.2%~2.6%, preferably 2.3%~2.5%, providing an optimal balance between strength and toughness.

[0012] Mn: ≤0.15%: Increasing the Mn content increases the strength of ductile iron, but it also leads to a decrease in its plasticity and elongation. QT400-18 is a ferritic matrix ductile iron widely used in high-toughness critical components in shipbuilding, automobiles, and mechanical engineering due to its excellent ductility (elongation ≥18%), impact toughness, and certain strength (tensile strength ≥400MPa). QT400-18 produced by traditional methods typically has strength at the critical level required by the standard (400-450MPa), making further improvement difficult. Increasing the strength would severely compromise its ductility, making it difficult to consistently achieve an elongation of over 18%. This invention strictly limits the Mn content to ≤0.15%, preferably controlled at 0.1%~0.15%. This control successfully breaks the traditional trade-off between tensile strength and ductility in ductile iron, resulting in castings with high tensile strength (stable at 430 MPa ~ 480 MPa), significantly higher than the standard requirement of 400 MPa in QT400-18. At the same time, the castings have high ductility (elongation stable at 22% ~ 28%), far exceeding the standard of 18% in QT400-18.

[0013] In this invention, the Mn content is ≤0.15%, which significantly reduces the microscopic segregation of Mn at grain boundaries, fundamentally eliminating the brittle phase at grain boundaries and ensuring a high-purity ferrite matrix. Ferrite itself has excellent plasticity; under tension, the entire matrix can undergo sufficient and uniform plastic deformation, rather than prematurely fracturing at the brittle boundary. This is the fundamental guarantee for achieving ultra-high elongation (≥22%). Simultaneously, pure grain boundaries avoid stress concentration, significantly improving impact toughness, especially low-temperature toughness. Furthermore, by eliminating the brittle phase at grain boundaries, the continuity and uniformity of force transmission within the matrix are ensured. This allows the ferrite matrix to more effectively bear loads through its own deformation, avoiding strength loss caused by premature cracking of the brittle phase.

[0014] P and S are harmful elements, and their contents must be strictly controlled in this invention: P ≤ 0.02%, S ≤ 0.015%. P tends to segregate at grain boundaries, reducing grain boundary bonding strength, increasing the brittleness of the material, and severely impairing the toughness and elongation of ductile iron. S can form sulfide inclusions with elements such as Mg, which can become crack initiation sites, reducing the mechanical properties of the material. Strictly controlling the P and S contents can reduce these harmful effects and ensure that ductile iron has excellent comprehensive properties.

[0015] The addition and content control of alloying elements such as Cr, Mo, Ni, and Cu (0.06% < ∑(Cr + Mo + Ni + Cu) < 0.1%, and the mass percentage of ∑(Cr + Mo) is not greater than the mass percentage of ∑(Ni + Cu), preferably 0.5-0.8 times the mass percentage of ∑(Ni + Cu)) are to further optimize the performance of ductile iron and achieve a synergistic improvement in tensile strength and elongation. Cr and Mo can improve the strength and hardness of the material, but excessive addition will lead to a decrease in toughness. By reasonably controlling their content and the ratio with Ni and Cu, the strength can be improved while avoiding a significant decrease in toughness caused by adding too many strengthening elements. Ni can stabilize the austenitic structure and improve the toughness and low-temperature performance of the material; Cu can play a role in solid solution strengthening and graphitization, which helps to improve the strength and hardness of the material, and also has a certain effect on improving toughness.

[0016] The content of Ti is controlled at ≤0.01%. Ti is a strong carbide-forming element, which can combine with C to form titanium carbide, thus refining the grain size. Fine grains can improve the strength and toughness of the material, but excessive addition of Ti will lead to too many carbides, forming a hard and brittle phase, which will reduce the performance of the material. Therefore, strictly controlling the Ti content is crucial to obtaining good overall performance.

[0017] Mg and RE are key elements in ductile iron that promote graphite spheroidization. The Mg content is 0.03% - 0.05%, and the RE content is 0.015% - 0.03%. Mg enables graphite to precipitate in a spherical shape, significantly improving the mechanical properties of ductile iron, especially toughness and elongation. RE has functions such as purifying molten iron, desulfurizing, and degassing, and can also improve the morphology and distribution of graphite spheroids, further enhancing the performance of ductile iron. By precisely controlling the content of Mg and RE, a moderate number of graphite spheroids with rounded shapes and uniform distribution can be obtained, thus ensuring that ductile iron has high tensile strength and high elongation.

[0018] Al (Al): With a content of 0.03~0.01%, aluminum strongly promotes the graphitization of carbon during the solidification process of molten cast iron, which is beneficial for the precipitation of carbon in cast iron in the form of graphite, thus making it more conducive to obtaining cast iron with a ferritic matrix. At the same time, aluminum helps to improve the mechanical properties of cast iron at low temperatures, enabling it to maintain good toughness and strength at room temperature or even lower temperatures. However, the Al content should not be too high, as excessive addition may lead to an increase in inclusions, which will have an adverse effect on performance. Therefore, its content should be controlled within the range of 0.03~0.01% to obtain the best overall performance.

[0019] This invention also provides a method for preparing the high-strength, high-ductility QT400-18 ductile iron, characterized by comprising the following steps:

[0020] a. Smelting and Composition Control: 20-40% high-purity pig iron and 60-80% pure scrap steel are smelted in a medium-frequency induction furnace. After melting, the temperature is raised to 1500-1520℃ for refining for 3-5 minutes. High-purity, low-sulfur crystalline graphite recarburizing agent is used to adjust the carbon content to prepare molten iron that meets the above chemical composition requirements. The preferred high-purity, low-sulfur crystalline graphite recarburizing agent has a sulfur content ≤0.05% and a fixed carbon content >99%. Using this agent effectively ensures that 0.06% < ∑(Cr+Mo+Ni+Cu) < 0.1%, thus ensuring both tensile strength and plasticity, achieving a balance between high tensile strength and high ductility. Simultaneously, it ensures that the spheroidizing agent Mg is fully utilized for graphite spheroidization, rather than being ineffectively consumed in the reaction with S, thereby helping to ensure that the produced castings consistently meet the high standards of tensile strength ≥430MPa and elongation ≥22%.

[0021] b. Spheroidizing and Inoculation Treatment: Spheroidizing is performed using the pouring method. The spheroidizing agent is placed in a dam-type ladle preheated to above 600℃, and a 75SiFe buffer layer is placed on top of the spheroidizing agent and compacted. The mass ratio of 75SiFe to the spheroidizing agent is 1:4-1:8. Molten iron is poured into the ladle. When the liquid level submerges the dam and the ladle is 1 / 2 to 2 / 3 full, 0.3%-0.5% of a silicon-barium inoculant (by mass of the total molten iron) is added in the flow for the first inoculation. After the spheroidizing reaction is complete, the slag is removed, and a second inoculation is performed before casting, adding 0.1%-0.2% of a silicon-strontium inoculant (by mass of the total molten iron).

[0022] The spheroidizing agent consists of: Mg: 5% ~ 6%, Si: 40~45%, Ca: 1.0~2.2%, RE: 0.5~1%, with the balance being Fe. 75SiFe has a lower melting point than the spheroidizing agent; when molten iron is poured in, it melts first, forming a molten barrier that delays the reaction of the spheroidizing agent after a short period. This avoids the large amount of magnesium vapor splashing and combustion caused by the instantaneous and violent reaction between the molten iron and the spheroidizing agent, resulting in a stable and complete reaction and significantly improving the magnesium absorption rate.

[0023] c. Pouring and cooling: The treated molten iron is poured within 10 minutes at 1380℃-1420℃; the casting is opened when it is cooled in the sand mold to 750℃-800℃, and immediately transferred to a heat preservation and slow cooling pit at about 250~320℃, and slowly cooled to below 250℃ before being taken out of the furnace and air-cooled.

[0024] d. Heat treatment: The castings undergo two-stage heat treatment: the first stage is to hold at 920℃-940℃ for 1-2 hours, followed by programmed cooling at a rate of ≤60℃ / hour to slowly cool with the furnace to 720℃-740℃; the second stage is to hold at 720℃-740℃ for 2-4 hours, followed by programmed cooling again at a rate of ≤50℃ / hour to slowly cool with the furnace to below 500℃ before air cooling.

[0025] The beneficial effects of this invention are:

[0026] 1. This invention fundamentally eliminates the brittle grain boundary phase by strictly limiting the Mn content to ≤0.15%, providing a foundation for achieving extremely high ductility and successfully resolving the contradiction of difficulty in synergistically improving the strength and plasticity of ductile iron. Simultaneously, by controlling ∑(Cr+Mo+Ni+Cu) < 0.1% and ∑(Cr + Mo) / ∑(Ni + Cu) to 0.5-0.8, this invention utilizes the synergistic strengthening effect of elements at extremely low contents to further consolidate the strength improvement, breaking through the traditional technical bias that QT400-18 material cannot simultaneously achieve both strength and plasticity. Through the control of Mn content ≤0.15%, and the precise control of the composition and content of the four microalloying elements Cr, Mo, Cu, and Ni, the synergistic improvement of the material's tensile strength and ductility is achieved.

[0027] 2. The QT400-18 ductile iron prepared by this invention has a tensile strength that is stable at 430-480MPa, a yield strength ≥260MPa, an elongation that is stable at 22%-28%, and a low-temperature impact energy of -40℃ ≥12J. Its performance exceeds the national standard of QT400-18 and can meet the stringent performance requirements of high-end components. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to specific embodiments, but this is not intended to limit the scope of the invention.

[0029] A method for preparing high-strength, high-elongation QT400-18 ductile iron includes the following steps:

[0030] a. Smelting and Composition Control: 20-40% high-purity pig iron and 60-80% pure scrap steel are smelted in a medium-frequency induction furnace. After melting, the temperature is raised to 1500-1520℃ for refining for 3-5 minutes. High-purity, low-sulfur crystalline graphite recarburizing agent is used to adjust the carbon content to prepare molten iron that meets the above chemical composition requirements. The preferred high-purity, low-sulfur crystalline graphite recarburizing agent has a sulfur content ≤0.05% and a fixed carbon content >99%. Using this agent effectively ensures that 0.06% < ∑(Cr+Mo+Ni+Cu) < 0.1%, thus ensuring both tensile strength and plasticity, achieving a balance between high tensile strength and high ductility. Simultaneously, it ensures that the spheroidizing agent Mg is fully utilized for graphite spheroidization, rather than being ineffectively consumed in the reaction with S, thereby helping to ensure that the produced castings consistently meet the high standards of tensile strength ≥430MPa and elongation ≥22%.

[0031] b. Spheroidizing and Inoculation Treatment: Spheroidizing is performed using the pouring method. The spheroidizing agent is placed in a dam-type ladle preheated to above 600°C, and a 75SiFe buffer layer is placed on top of the spheroidizing agent and compacted. The mass ratio of 75SiFe to the spheroidizing agent is 1:4-1:8, preferably 1.5-1.6. Molten iron is poured into the ladle. When the liquid level submerges the dam and the ladle is 1 / 2 to 2 / 3 full, 0.3%-0.5% of a silicon-barium inoculant (by mass of the total molten iron) is added in the flow for the first inoculation. After the spheroidizing reaction is complete, slag is removed, and a second inoculation is performed before casting, adding 0.1%-0.2% of a silicon-strontium inoculant (by mass of the total molten iron).

[0032] c. Pouring and cooling: The treated molten iron is poured within 10 minutes at 1380℃-1420℃; the casting is opened when it is cooled in the sand mold to 750℃-800℃, and immediately transferred to a heat preservation and slow cooling pit at about 250~320℃, and slowly cooled to below 250℃ before being taken out of the furnace and air-cooled.

[0033] d. Heat treatment: The castings undergo two-stage heat treatment: the first stage is to hold at 920℃-940℃ for 1-2 hours, followed by programmed cooling at a rate of ≤60℃ / hour to slowly cool with the furnace to 720℃-740℃; the second stage is to hold at 720℃-740℃ for 2-4 hours, followed by programmed cooling again at a rate of ≤50℃ / hour to slowly cool with the furnace to below 500℃ before air cooling.

[0034] Chemical composition analysis, tensile strength, elongation (plasticity), yield strength, and minimum impact energy absorbed (room temperature) were performed on the cast ductile iron. The analysis of chemical composition, tensile strength, yield strength, elongation (plasticity), and minimum impact energy absorbed (room temperature) were all performed in accordance with the national standard GB 1348-2019. The chemical composition analysis results of ductile iron samples 1-18 are recorded in Table 1, and the results of the corresponding mechanical property tests are shown in Table 2.

[0035] Table 1 (All components are mass percentages, the balance is Fe)

[0036]

[0037] Table 2: Results of mechanical property tests corresponding to the numbers in Table 1

[0038]

[0039] Examples 13 and 14 are comparative examples of Invention Example 2 of this invention, with adjustments made to the Mn content. The Mn content in Comparative Example 13 is 0.2%, and in Comparative Example 14 it is 0.08%, both failing to meet the requirements specified in this invention. Although the tensile strength and elongation of Comparative Examples 13 and 14 meet the QT400-18 standard, these mechanical properties are still insufficient for the high-specification requirements of suspension swing arms, which require a tensile strength ≥430 MPa and an elongation ≥22%. Comparative analysis shows that when the Mn content deviates from the range specified in this invention, the mechanical properties of ductile iron are significantly affected. Specifically, excessively high (e.g., 0.2% in Comparative Example 13) or excessively low (e.g., 0.08% in Comparative Example 14) Mn content leads to a decrease in tensile strength and elongation, failing to meet the stringent requirements for tensile strength and elongation in high-performance castings. This indicates that by precisely controlling the Mn content between 0.1% and 0.15%, the present invention can have a positive effect on solid solution strengthening and stabilizing the microstructure, which is conducive to optimizing the mechanical properties of ductile iron and enabling it to stably achieve the high standards of tensile strength ≥430MPa and elongation ≥22%.

[0040] Examples 15-19 are comparative examples of Invention Example 3 of this invention, in which the contents of Cr, Mo, Ni, and Cu were adjusted so that at least one of ∑(Cr + Mo + Ni + Cu) and ∑(Cr + Mo) / ∑(Ni + Cu) did not meet the requirements of this invention. In Comparative Example 15, both ∑(Cr + Mo + Ni + Cu) and ∑(Cr + Mo) / ∑(Ni + Cu) were higher than the requirements of this invention; in Comparative Document 16, ∑(Cr + Mo + Ni + Cu) met the requirements of this invention, but ∑(Cr + Mo) / ∑(Ni + Cu) was higher than the requirements of this invention. Although the tensile strength of Comparative Examples 15 and 16 was improved, the elongation improvement effect was not significant due to the excessively high Cr + Mo content, making it difficult to meet the high standards required for the suspension arm of this invention. In Comparative Example 17, ∑(Cr + Mo + Ni + Cu) meets the requirements of this invention, but ∑(Cr + Mo) / ∑(Ni + Cu) is lower than the requirements of this invention. Although the tensile strength and elongation meet the minimum standard of QT400-18, they are insufficient to meet the high standard requirements of the suspension arm of this invention. In Comparative Example 18, ∑(Cr + Mo + Ni + Cu) is lower than the requirements of this invention, but ∑(Cr + Mo) / ∑(Ni + Cu) meets the requirements of this invention. Its tensile strength is low (lower than the minimum standard of QT400-18), and its elongation is only 18.1%, making it difficult to meet the high standard requirements of the suspension arm of this invention. In Comparative Example 19, ∑(Cr + Mo + Ni + Cu) is lower than the requirements of this invention, and ∑(Cr + Mo) / ∑(Ni + Cu) is also lower than the requirements of this invention. Both its tensile strength and elongation are low (both are lower than the minimum requirement of QT400-18), making it even more difficult to meet the high standard requirements of the suspension arm of this invention. Comparative analysis demonstrates that this invention, by strictly limiting ∑(Cr + Mo + Ni + Cu) to 0.06% < ∑(Cr + Mo + Ni + Cu) < 0.1% and ∑(Cr + Mo) / ∑(Ni + Cu) to the range of 0.5~0.8, effectively ensures that ductile iron possesses both high tensile strength and high elongation, meeting the stringent mechanical property requirements of high-performance castings. If the total amount of alloying elements exceeds or falls below this range, it will adversely affect the mechanical properties of ductile iron, making it impossible to achieve the synergistic improvement of high tensile strength and high elongation, and also preventing the stable attainment of the high standards of tensile strength ≥430MPa and elongation ≥22%.

[0041] Therefore, through a series of precisely controlled adjustments to the chemical composition and optimization of the heat treatment process, this invention successfully prepared a high-tensile-strength, high-elongation QT400-18 ductile iron. This ductile iron not only achieves a significant synergistic improvement in tensile strength and elongation, stably meeting the high standards of tensile strength ≥430MPa and elongation ≥22%, but also exhibits excellent performance in mechanical properties such as yield strength and minimum impact absorption energy. It fully meets the stringent material performance requirements of high-performance castings such as suspension swing arms, breaking through the bottleneck of traditional technologies where strength and plasticity are difficult to balance.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing high-strength, high-elongation QT400-18 ductile iron, characterized in that: The chemical composition of the ductile iron, by mass percentage, is as follows: C: 3.6%~3.9%, Si: 2.2%~2.6%, Mn: 0.1%~0.15%, P: ≤0.02%, S: ≤0.015%, Cr: ≤0.04%, Mo: ≤0.02%, Ni: ≤0.031%, Cu: ≤0.035%, Ti: ≤0.01%, Mg: 0.03%~0.05%, RE: 0.015%~0.03%, 0.06% < ∑(Cr+Mo+Ni+Cu) < 0.1%, and the mass percentage of ∑(Cr+Mo) is not greater than the mass percentage of ∑(Ni+Cu), the mass percentage of ∑(Cr+Mo) is 0.5~0.8 times the mass percentage of ∑(Ni+Cu), and the balance is Fe and unavoidable impurities; The ductile iron has a tensile strength of 430MPa-480MPa, a yield strength of ≥260MPa, an elongation of 22%-28%, and a minimum impact absorption energy of ≥12J at low temperature. a. Smelting and composition control: 20%~40% high-purity pig iron and 60%~80% pure scrap steel are smelted in a medium-frequency induction furnace. After melting, the temperature is raised to 1500℃-1520℃ for refining for 3-5 minutes. High-purity low-sulfur crystalline graphite carbonizer is used to adjust the carbon content to prepare molten iron that meets the chemical composition requirements. b. Spheroidization and inoculation treatment: Spheroidization is carried out by the pouring method; the spheroidizing agent is placed in the dam of the preheated ladle and covered with a buffer layer; the first inoculation is carried out in the flow when the ladle is tapped, and a silicon barium inoculator is added; after the spheroidization reaction is completed, the slag is removed, and the second inoculation is carried out in the flow before casting, and a silicon strontium inoculator is added. c. Pouring and cooling: The treated molten iron is poured within 10 minutes at 1380℃-1420℃; the casting is opened when it is cooled in the sand mold to 750℃-800℃ and immediately transferred to a heat preservation and slow cooling pit at 250℃~320℃, and slowly cooled to below 250℃ before being taken out of the furnace and air-cooled. d. Heat treatment: The castings undergo two-stage heat treatment: the first stage is to hold at 920℃-940℃ for 1-2 hours, followed by programmed cooling at a rate of ≤60℃ / hour to slowly cool with the furnace to 720℃-740℃; the second stage is to hold at 720℃-740℃ for 2-4 hours, followed by programmed cooling again at a rate of ≤50℃ / hour to slowly cool with the furnace to below 500℃ before air cooling.

2. The method for preparing high-strength, high-elongation QT400-18 ductile iron according to claim 1, characterized in that: The chemical composition of QT400-18 ductile iron also includes Al, with an Al mass percentage of 0.01% to 0.03%.

3. The method for preparing high-strength, high-elongation QT400-18 ductile iron according to claim 1, characterized in that: The spheroidizing agent consists of the following components: Mg: 5% ~ 6%, Si: 40% ~ 45%, Ca: 1.0% ~ 2.2%, RE: 0.5% ~ 1%, with the balance being Fe.

4. The method for preparing high-strength, high-elongation QT400-18 ductile iron according to claim 1, characterized in that: The high-purity, low-sulfur crystalline graphite carbon raiser has a sulfur content of ≤0.05% and a fixed carbon content of >99%.

5. The method for preparing high-strength, high-elongation QT400-18 ductile iron according to claim 1, characterized in that: The spheroidizing and inoculation treatment includes placing the spheroidizing agent in a dam-type casting ladle preheated to above 600°C, covering the spheroidizing agent with a 75SiFe buffer layer and compacting it; pouring molten iron into the ladle, and when the liquid level submerges the dam and the ladle reaches 1 / 2 to 2 / 3 full, adding 0.3%-0.5% of a silicon-barium inoculant by mass of the molten iron in the flow for the first inoculation; after the spheroidizing reaction is completed, removing the slag, and performing a second inoculation by flow before casting, adding 0.1%-0.2% of a silicon-strontium inoculant by mass of the molten iron.

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