Nodular cast iron casting and preparation method and application thereof

Abstract

The invention discloses a nodular cast iron casting and a preparation method and application thereof, and relates to the technical field of nodular cast iron, and the nodular cast iron casting comprises the following raw material components in percentage by mass: 3.5-3.8% of C, 2.5-2.75% of Si, 0.1-0.3% of Mn, less than or equal to 0.04% of P, less than or equal to 0.02% of S, 0.03-0.06% of Mg, 0.35-0.55% of Cu, 0.85-1.15% of Ni, and the balance of Fe and inevitable impurities. The ductile iron casting provided by the invention has relatively high strength and good toughness.

Description

Technical Field

[0001] This application relates to the field of automobile manufacturing technology, and in particular to a ductile iron casting, its preparation method, and its application. Background Technology

[0002] Ductile iron, due to its good strength, excellent casting processability, and relatively low cost, is widely used in the automotive industry for various structural components subjected to complex stresses. With the increasing demands for electrification and lightweighting in the automotive industry, higher requirements are being placed on the comprehensive mechanical properties of ductile iron materials. Developing high-performance ductile iron that combines high strength and good ductility and toughness while maintaining excellent casting performance and cost-effectiveness is of significant engineering importance for achieving lightweight components and reducing overall vehicle manufacturing costs.

[0003] As a critical load-bearing component of commercial vehicles, the drive axle housing's performance directly impacts the vehicle's safety and reliability. With the trend towards integrated and lightweight electric drive axles, cast axle housings exhibit significant advantages due to their greater design freedom and component integration capabilities. Compared to cast steel, ductile iron excels in casting performance, vibration damping, and cost control; however, its mechanical properties, particularly the balance between strength and ductility, remain insufficient, hindering further lightweighting of the axle housing. Therefore, improving the material properties of ductile iron axle housings to achieve a good balance between high strength and high toughness has become a key focus of industry technological research. Summary of the Invention

[0004] This application provides a ductile iron casting, its preparation method, and its application. The ductile iron casting has high strength and good toughness.

[0005] In a first aspect, this application provides a ductile iron casting comprising the following raw material components by mass percentage: C: 3.5-3.8%, Si: 2.5-2.75%, Mn: 0.1-0.3%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.35-0.55%, Ni: 0.85-1.15%, with the balance being Fe and unavoidable impurities.

[0006] This application significantly improves the casting fluidity of molten iron by controlling the carbon (C) content at a relatively high level of 3.5%-3.8%, increasing the number of graphite nodules during solidification and improving their roundness, thereby effectively suppressing shrinkage porosity and improving the density of the casting. Simultaneously, a suitable magnesium (Mg) content (0.03-0.06%) ensures good graphite spheroidization, providing an excellent strength and toughness foundation for ductile iron castings. Increasing the silicon (Si) content to 2.5-2.75% not only enhances graphitization ability and promotes ferrite formation but also effectively improves the material's plasticity and processing properties. Furthermore, by limiting the manganese (Mn) content to a low level of 0.1%-0.3%, the formation of excessive pearlite is suppressed, avoiding adverse effects on toughness. Finally, a "high-nickel, low-copper" alloying approach was adopted, with the nickel (Ni) content controlled at 0.85-1.15%. This significantly improved the matrix strength through grain refinement and solid solution strengthening mechanisms, while also improving low-temperature toughness without compromising plasticity. Simultaneously, the copper (Cu) content was limited to 0.35-0.55% to prevent it from drastically promoting pearlite at high cooling rates, thus avoiding a decrease in elongation. Phosphorus (P) and sulfur (S), as harmful elements, were strictly limited (P≤0.04%, S≤0.02%), effectively avoiding phosphorus eutectic brittleness and spheroidization interference, further ensuring the toughness and stability of the ductile iron castings. Therefore, the composition design of this ductile iron casting, through an alloying route of "high C, high Si, low Mn, low Cu, controlled sulfur and phosphorus, and added nickel," enables the ductile iron castings to maintain excellent casting performance and graphite morphology while possessing high strength and good toughness.

[0007] In some embodiments, the ductile iron casting comprises the following raw material components by mass percentage: C: 3.55-3.76%, Si: 2.56-2.73%, Mn: 0.21-0.26%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.36-0.52%, Ni: 0.88-1.10%, with the balance being Fe and unavoidable impurities.

[0008] Secondly, this application provides a method for preparing the ductile iron casting described in the first aspect, comprising: The raw materials are melted into molten iron, and the tapping temperature is controlled within the range of 1480℃-1540℃. Before tapping out of the furnace, spheroidizing agent and inoculant are placed in sequence at the bottom of the casting ladle, and then the tapped molten iron is added for spheroidizing and inoculation treatment. Molten iron that has undergone spheroidization inoculation is poured into a casting mold, and in-flow inoculation is carried out during the pouring process. After cooling, the ductile iron casting is obtained.

[0009] The process method provided in this application achieves the performance targets of tensile strength ≥800MPa and elongation ≥8% for ductile iron bridge shell castings through systematic control and optimization of raw material chemical element design, molding process, and spheroidizing inoculation process, and ensures the stability and consistency of microstructure and properties of bridge shell castings under mass production conditions.

[0010] In some embodiments, the casting model is formed using a sand-coated iron mold process. This sand-coated iron mold process significantly accelerates the cooling rate of the casting through the rapid cooling effect of the metal mold wall, thereby effectively promoting graphite spheroidization, increasing pearlite content, and refining the matrix structure. This rapid cooling process, synergistically with the adjusted Si, Ni, and Cu element ranges in the composition design, significantly improves the uniformity and density of graphite morphology and the matrix structure, which is beneficial for obtaining high-strength, high-toughness ductile iron bridge shell castings with a tensile strength ≥800MPa and an elongation ≥8%.

[0011] In some embodiments, the mass percentage of the spheroidizing agent is 1.0-1.3% based on the total mass of the spheroidizing agent, the inoculant, and the molten iron. When the amount of spheroidizing agent added is less than 1.0%, the Mg and RE content is insufficient to completely neutralize the sulfur, oxygen, and anti-spheroidizing elements in the molten iron, leading to poor spheroidization and the appearance of worm-like or flake-like graphite, which seriously affects mechanical properties. When the amount added exceeds 1.3%, excessive Mg will drastically increase the solidification shrinkage tendency and white cast iron tendency of the molten iron, which not only easily produces casting defects such as shrinkage porosity and shrinkage cavities, but may also lead to deterioration of graphite morphology and increased brittleness of castings due to excessive residual magnesium. This range ensures that there are sufficient effective elements (Mg, RE) to achieve stable spheroidization and resist fading, while minimizing the negative risks caused by excessive addition.

[0012] In some embodiments, the spheroidizing agent comprises the following raw material components by mass percentage: 7-8% Mg, 2-5% RE, 2-4% Ca, 35-44% Si, balance Fe, and unavoidable impurities. Mg, as the dominant spheroidizing element, effectively promotes graphite spheroidization, ensuring round graphite morphology. The addition of appropriate amounts of RE neutralizes the interference of anti-spheroidizing impurities such as sulfur and oxygen, enhancing spheroidization stability and helping to suppress fragmented or deformed graphite that is easily generated in thick sections of the casting due to slow cooling. Simultaneously, the combination of Ca and Si further improves the fluidity of the molten iron, enhances the inoculation effect, and increases the number and uniformity of graphite spheroid precipitation. Therefore, this spheroidizing agent composition ensures that the casting as a whole achieves a high level of spheroidization and a uniform, fine graphite structure, providing a stable and reliable foundation for high strength and toughness.

[0013] In some embodiments, based on the total mass of the spheroidizing agent, the inoculant, and the molten iron being tapped, the mass ratio of the inoculant at the bottom of the ladle is 0.2-0.4%. If the amount of inoculant added at the bottom of the ladle is less than 0.2%, insufficient nucleation may occur, potentially leading to deterioration of graphite morphology or the formation of cementite in the matrix. Conversely, if the amount of inoculant added exceeds 0.4%, it is not only economically unfeasible but may also introduce defects due to excessive inclusions, and may even lead to excessive silicon content in the molten iron, causing excessive ferritification of the matrix and sacrificing strength, while increasing the shrinkage tendency of the casting and causing shrinkage porosity defects. Therefore, controlling the inoculant within the above range can effectively control production costs and process risks while ensuring microstructure optimization and performance stability.

[0014] In some embodiments, the inoculant at the bottom of the ladle comprises the following raw material components by mass percentage: 60-70% Si, 1-3% Al, 0.5-2.5% Ca, and 4-6% Ba. This silicon-barium composite inoculant significantly improves the overall mechanical properties of the casting through the synergistic effect of its components. The high Si content, as a basic inoculant element, effectively promotes graphite nucleation, increases the number of graphite spheres, and makes their distribution more uniform. The introduction of Ba significantly enhances the resistance to fading of the inoculant effect and prolongs the effective inoculant time, thereby ensuring good graphite morphology even in thicker sections of the bridge shell. Simultaneously, the combination of Ca and Al further purifies the molten iron, improves fluidity, and helps improve the sphericity of the graphite spheres. Experiments show that castings treated with this specific silicon-barium inoculant exhibit significantly better elongation performance than castings without such an inoculant, which is beneficial for obtaining high-strength and high-ductility ductile iron bridge shell castings.

[0015] In some embodiments, based on the total mass of the molten iron undergoing the spheroidizing inoculation treatment and the inoculant used in the in-flow inoculation, the mass ratio of the inoculant used in the in-flow inoculation is 0.1-0.2%. If the amount of inoculant added is less than 0.1%, the effect is insufficient, failing to effectively compensate for the decline in inoculation at the bottom of the ladle, and making it difficult to suppress the formation of white iron at thin-walled areas or improve graphite morphology; while exceeding 0.2% results in over-inoculation, which may lead to the entrainment of gas and inclusions due to the violent reaction. Controlling the inoculant within this range ensures good inoculation results.

[0016] In some embodiments, the inoculant used for in-flow inoculation comprises the following raw material components by mass percentage: 70-80% Si, 3-6% Sb, with the balance being iron and unavoidable impurities. Through the synergistic effect of silicon (Si) and antimony (Sb), the inoculation effect and microstructure uniformity of the casting can be improved. The high Si content provides sufficient graphite nucleation sites, effectively increasing the number of graphite spheroids and refining their size. The addition of Sb significantly enhances the anti-fading ability of the inoculation, especially in molten iron containing RE, effectively eliminating abnormal structures such as broken graphite caused by slow cooling in thick sections of the casting, ensuring the stability and consistency of the spheroidization effect. Experiments show that using this Sb-containing in-flow inoculant in combination with a silicon-barium inoculant not only ensures excellent anti-fading performance and spheroidization grade in the casting but also significantly improves the elongation of the casting, thus contributing to the production of high-strength and high-toughness ductile iron castings.

[0017] In some embodiments, the casting temperature is 1380°C–1420°C. This temperature range ensures sufficient fluidity of the molten iron for complete filling, avoiding cold shuts or incomplete casting due to excessively low temperatures. More importantly, this temperature matches the higher cooling rate of the iron mold sand coating process, promoting rapid solidification, thereby refining graphite spheroids and eutectic clusters, and obtaining a dense matrix structure. Excessively high temperatures exacerbate the degradation of inoculation effects, leading to a tendency for shrinkage porosity, and also intensify thermal shock to the metal mold, reducing its lifespan. Lower temperatures are prone to incomplete casting defects.

[0018] Thirdly, this application provides an electric drive bridge housing, which includes the ductile iron casting described in the first aspect or the ductile iron casting obtained by the preparation method of ductile iron described in the second aspect.

[0019] Fourthly, this application provides an automobile that includes the electric drive axle housing described in the third aspect. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application are clearly and completely described. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0021] As a core load-bearing component of medium and heavy-duty commercial vehicles, the drive axle housing's performance directly affects the vehicle's safety, service life, and lightweighting level. With the increasing demands for vehicle electrification and lightweighting, electric drive axles, due to their high integration and efficient transmission, have become an important development direction in the commercial vehicle sector. Compared to stamped and welded axle housings, integral cast axle housings offer significant advantages in structural integration, vibration damping performance, and cost control, and are easier to integrate with the motor and axle body. Currently, cast axle housings mainly use ductile iron or cast steel, with ductile iron becoming the mainstream choice due to its excellent casting performance, low density, and superior cost-effectiveness. However, existing ordinary ductile iron materials often have low strength and insufficient toughness, making it difficult to simultaneously meet the stringent requirements of high-load conditions and lightweight design for the comprehensive mechanical properties of materials.

[0022] To improve the mechanical properties of ductile iron bridge housings, existing technologies often employ a combination of alloying and process control. For example, Chinese patent CN202111252041.6 achieves tensile strength ≥700MPa and elongation ≥10% by adding boron (B) and strictly controlling the C / B and Si / B ratios. However, its composition control range is extremely narrow, especially the stability of trace amounts of B, which is difficult to guarantee, leading to poor consistency and high scrap rates during large-scale production. Another Chinese patent CN200910066633.1 uses Cu alloying to prepare QT600-5 grade ductile iron. Although the process performance is good, the strength and plasticity levels cannot meet the current requirements of electric drive bridges for higher mechanical properties and further weight reduction. Generally speaking, in existing technologies, the strength and toughness of ductile iron often come at opposite ends, making it difficult to achieve both simultaneously. Moreover, improving strength is often accompanied by a decrease in toughness and uncertain fatigue performance, which restricts its application in the field of high-performance electric drive bridges.

[0023] Therefore, developing a ductile iron bridge housing material that can stably achieve both high strength and high toughness, as well as good casting performance and fatigue life, without significantly increasing production costs and process complexity, has become a technical problem that urgently needs to be solved in this field.

[0024] In view of this, this application provides a ductile iron casting, its preparation method and application, wherein the ductile iron casting has high strength and good toughness.

[0025] In a first aspect, this application provides a ductile iron casting comprising the following raw material components by mass percentage: C: 3.5-3.8%, Si: 2.5-2.75%, Mn: 0.1-0.3%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.35-0.55%, Ni: 0.85-1.15%, with the balance being Fe and unavoidable impurities.

[0026] The ductile iron castings provided in this application achieve a tensile strength of ≥800 MPa and an elongation of ≥8% through the precise proportioning and synergistic effect of each element component.

[0027] By precisely controlling the carbon (C) content within the relatively high range of 3.5-3.8%, the casting fluidity of the molten iron is effectively improved, the number of graphite spheres during solidification is significantly increased and their roundness is improved, thereby reducing the tendency of shrinkage porosity in the casting and improving the overall density. This composition range fully utilizes the beneficial effects of carbon while effectively avoiding graphite morphology deterioration and graphite floating defects caused by excessive carbon content, providing a stable and reliable graphite structure basis for the material.

[0028] By controlling the silicon (Si) content within the range of 2.5-2.75%, its role in promoting graphitization and ferrite formation is effectively utilized, significantly improving the casting fluidity, machinability, and plasticity of ductile iron castings. If the silicon content is below this range, its effect on improving toughness is not fully realized, leading to insufficient elongation; while exceeding this range, the brittleness of ductile iron castings increases significantly, making it difficult to guarantee strength. This composition design, while ensuring good strength and toughness, overcomes the problem of insufficient plasticity caused by the generally low silicon content in large castings such as commercial vehicle axle housings, which is often used to maintain strength.

[0029] By limiting the manganese (Mn) content to a low range of 0.1-0.3%, it is possible to stabilize pearlite, moderately improve the strength and hardness of castings, and effectively avoid the adverse effects of excessively promoting pearlite formation and inhibiting ferrite formation caused by excessive content, thereby significantly ensuring and improving the toughness and elongation of castings.

[0030] By controlling the sulfur (S) content to below 0.02%, the chemical reaction consumption with spheroidizing elements magnesium (Mg) and rare earth elements (RE) is effectively reduced, thereby significantly improving the processing efficiency of the spheroidizing agent and ensuring good spheroidization and round shape of the graphite. At the same time, the low sulfur content reduces the generation of slag during the smelting process, improves the purity of the molten iron, and ultimately ensures the reliable internal quality and mechanical properties of the castings.

[0031] By controlling the phosphorus (P) content to no more than 0.04%, the formation of brittle phosphorus eutectic structure can be effectively avoided, significantly improving the toughness and impact resistance of the material, while reducing the tendency of castings to produce shrinkage porosity and cold cracks, thereby ensuring the uniformity of the overall structure and the reliability of ductile iron castings.

[0032] By controlling the Mg content within the range of 0.03-0.06%, it is possible to effectively neutralize anti-spheroidizing elements such as S and O in the molten iron, forming sufficient spheroidal graphite nucleation cores to ensure good spheroidization of graphite and provide the necessary strength and toughness foundation for the casting. It also avoids the defects of poor spheroidization caused by too low Mg content, as well as the problems of increased white iron tendency, increased shrinkage porosity risk and increased production costs caused by too high Mg content. Thus, while ensuring excellent spheroidization effect and mechanical properties, it also takes into account the stability and economy of the casting process.

[0033] By controlling the copper (Cu) content within a low range of 0.35-0.50%, this invention effectively avoids the significant decrease in elongation caused by excessive Cu content while ensuring the strength of ductile iron. Cu promotes graphitization and reduces the tendency for white iron formation during the eutectic stage, and stabilizes pearlite during the eutectoid stage, contributing to increased strength and reduced cross-sectional sensitivity. However, excessive Cu can over-promote pearlite formation, severely impairing the material's plasticity. This invention, through precise control of the Cu content, utilizes its strengthening effect while successfully stabilizing the elongation of the casting above 8%, achieving a good balance between strength and toughness.

[0034] By controlling the Ni content within the range of 0.85-1.15% and employing a high-Ni, low-Cu alloy strengthening scheme, Ni effectively improves the strength of the casting by promoting graphitization, refining the pearlite and ferrite structure, and generating significant grain refinement and solid solution strengthening effects, achieving a tensile strength of ≥800MPa. On the other hand, Ni can maintain or even improve the toughness of the casting while strengthening it, lowering the brittle-ductile transition temperature, and has no negative impact on the elongation. Thus, while ensuring high strength, the elongation is kept stable at ≥8%, achieving a good balance between high strength and high plasticity.

[0035] It should be noted that RE here represents the general term for rare earth elements, not a specific element.

[0036] Furthermore, the preferred mass percentage of each raw material component in the ductile iron casting is: C: 3.55-3.76%, Si: 2.56-2.73%, Mn: 0.21-0.26%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.36-0.52%, Ni: 0.88-1.10%, with the balance being Fe and unavoidable impurities.

[0037] Secondly, this application provides a method for preparing the ductile iron casting described in the first aspect, comprising: S100: Melt the raw materials into molten iron and control the tapping temperature within the range of 1480℃-1540℃.

[0038] In this step, the furnace temperature is strictly controlled within the high-temperature range of 1480℃-1540℃. On the one hand, this ensures that the molten iron has good fluidity, allowing the alloying elements to fully dissolve and homogenize, thus avoiding compositional segregation. On the other hand, it provides the necessary overheating temperature for the subsequent spheroidizing process, ensuring that the spheroidizing reaction can proceed fully and stably. This provides a core guarantee for finally obtaining high-quality castings with round graphite morphology and dense matrix structure.

[0039] It should be noted that there are no special restrictions on the types of raw materials. The raw materials provided in this application include, but are not limited to, recycled materials, pig iron and scrap steel.

[0040] It should be explained that "carbon enhancer" is a substance that can increase the carbon content in molten iron, and can be one or more of the following: carbon-enhancing pig iron, graphite, electrode powder, petroleum coke powder, charcoal powder, and coke powder.

[0041] S200. Before tapping out of the furnace, spheroidizing agent and inoculant are placed in sequence at the bottom of the casting ladle, and then the molten iron is added for spheroidizing and inoculation treatment.

[0042] In this step, the molten iron is modified by undergoing spheroidizing inoculation treatment after being brought out of the furnace at high temperature. The spheroidizing agent promotes the transformation of graphite into spheroids, thereby significantly improving the strength and toughness of the casting. The inoculant effectively refines the graphite spheroids, increases the number of graphite spheroids, eliminates the tendency for white iron, and improves the matrix structure. The synergistic effect of the two ensures that ductile iron obtains round and uniform spheroidal graphite and an ideal metallic matrix, which is conducive to obtaining high-strength and high-toughness ductile iron castings.

[0043] In conjunction with the second aspect, in some embodiments provided in this application, the mass percentage of the spheroidizing agent is 1.0-1.3% based on the total mass of the spheroidizing agent, the inoculant, and the molten iron. When the amount of spheroidizing agent added is less than 1.0%, the Mg and RE content is insufficient to completely neutralize the sulfur, oxygen, and anti-spheroidizing elements in the molten iron, leading to poor spheroidization and the appearance of worm-like or flake-like graphite, severely affecting mechanical properties. When the amount added exceeds 1.3%, excessive Mg will drastically increase the solidification shrinkage tendency and white cast iron tendency of the molten iron, not only easily producing casting defects such as shrinkage porosity and shrinkage cavities, but also potentially causing graphite morphology deterioration and increased casting brittleness due to excessive residual magnesium. This range ensures sufficient effective elements (Mg, RE) to achieve stable spheroidization and resist fading, while minimizing the negative risks caused by excessive addition.

[0044] In conjunction with the second aspect, in some embodiments provided in this application, the spheroidizing agent comprises the following raw material components by mass percentage: 7-8% Mg, 2-5% RE, 2-4% Ca, 35-44% Si, balance Fe, and unavoidable impurities. Mg, as the dominant spheroidizing element, effectively promotes graphite spheroidization by adding an appropriate amount, ensuring the spheroids are round and well-formed. The addition of an appropriate amount of RE neutralizes the interference of anti-spheroidizing impurities such as sulfur and oxygen, enhancing spheroidization stability and helping to suppress fragmented or deformed graphite that is easily generated in thick parts of the casting due to slow cooling. Simultaneously, the combination of Ca and Si elements further improves the fluidity of the molten iron, enhances the inoculation effect, and increases the number and uniformity of graphite spheroid precipitation. Therefore, this spheroidizing agent composition can ensure that the casting as a whole obtains a high level of spheroidization and a uniform and fine graphite structure, providing a stable and reliable foundation for high strength and toughness.

[0045] In conjunction with the second aspect, in some embodiments provided in this application, based on the total mass of the spheroidizing agent, the inoculant, and the molten iron being tapped, the mass ratio of the inoculant at the bottom of the ladle is 0.2-0.4%. If the amount of inoculant added at the bottom of the ladle is less than 0.2%, insufficient nucleation may occur, potentially leading to deterioration of graphite morphology or the appearance of cementite in the matrix. Conversely, if the amount of inoculant added exceeds 0.4%, it is not only economically unfeasible but may also introduce defects due to excessive inclusions, and may even lead to excessive silicon content in the molten iron, causing excessive ferritification of the matrix and sacrificing strength, while increasing the shrinkage tendency of the casting and causing shrinkage porosity defects. Therefore, controlling the inoculant within the above range can effectively control production costs and process risks while ensuring microstructure optimization and performance stability.

[0046] In conjunction with the second aspect, in some embodiments provided in this application, the inoculant at the bottom of the ladle comprises the following raw material components by mass percentage: 60-70% Si, 1-3% Al, 0.5-2.5% Ca, and 4-6% Ba. This silicon-barium composite inoculant, through the synergistic effect of its components, significantly improves the overall mechanical properties of the casting. The high Si content, as the basic inoculant element, effectively promotes graphite nucleation, increases the number of graphite spheres, and makes their distribution more uniform. The introduction of Ba significantly enhances the resistance to fading of the inoculant effect and prolongs the effective inoculant time, thereby ensuring good graphite morphology even in thicker sections of the bridge shell. Simultaneously, the combination of Ca and Al further purifies the molten iron, improves fluidity, and helps improve the sphericity of the graphite spheres. Experiments show that castings treated with this specific silicon-barium inoculant exhibit significantly better elongation performance than castings without such an inoculant, which is beneficial for obtaining high-strength and high-ductility ductile iron castings.

[0047] S300: The molten iron treated with spheroidization and inoculation is poured into the casting mold. During the pouring process, in-flow inoculation is carried out, and after cooling, the ductile iron casting is obtained.

[0048] In this step, by adding an inoculant during the pouring process, the inoculation decline phenomenon after spheroidization treatment can be overcome, significantly increasing the graphite nucleation core, refining the graphite spheroids and improving their distribution uniformity. On the other hand, it can further purify the molten iron, improve its fluidity, and reduce the tendency of the casting to shrink and porosity, thereby ensuring that the final ductile iron casting has a high spheroidization rate, fine and uniform graphite structure and excellent mechanical properties.

[0049] It needs to be explained that "in-flow inoculation" refers to breaking the inoculant into fine particles and feeding it into the molten iron along with the flow of the molten iron during the pouring process of the molten iron, so as to play an inoculation role.

[0050] In conjunction with the second aspect, in some embodiments provided in this application, based on the total mass of the molten iron undergoing the spheroidizing inoculation treatment and the inoculant used in the in-flow inoculation, the mass ratio of the inoculant used in the in-flow inoculation is 0.1-0.2%. If the amount of inoculant added is less than 0.1%, the effect is insufficient, failing to effectively compensate for the decline in inoculation at the bottom of the ladle, and making it difficult to suppress the formation of white iron at thin-walled areas or improve graphite morphology; while exceeding 0.2% results in over-inoculation, which may not only lead to the entrainment of gas and inclusions due to the violent reaction. Controlling the inoculant within this range ensures a good inoculation effect.

[0051] In conjunction with the second aspect, in some embodiments provided in this application, the inoculant used for in-flow inoculation comprises the following raw material components by mass percentage: 70-80% Si, 3-6% Sb, with the balance being iron and unavoidable impurities. Through the synergistic effect of silicon (Si) and antimony (Sb), the inoculation effect and microstructure uniformity of the casting can be improved. The high Si content provides sufficient graphite nucleation sites, effectively increasing the number of graphite spheroids and refining their size. The addition of Sb significantly enhances the anti-fading ability of the inoculation, especially in molten iron containing RE, effectively eliminating abnormal structures such as broken graphite caused by slow cooling in thick parts of the casting, ensuring the stability and consistency of the spheroidization effect. Experiments show that using this Sb-containing in-flow inoculator in combination with a silicon-barium inoculator not only ensures excellent anti-fading performance and spheroidization grade in the casting, but also significantly improves the elongation of the casting, thus contributing to the production of high-strength and high-toughness ductile iron castings.

[0052] In conjunction with the second aspect, in some embodiments provided in this application, the casting temperature is 1380℃-1420℃. This temperature range ensures that the molten iron has sufficient fluidity to completely fill the mold, avoiding cold shuts or incomplete casting due to excessively low temperatures. More importantly, this temperature matches the higher cooling rate of the iron mold sand coating process, promoting rapid solidification, thereby refining graphite spheroids and eutectic clusters, and obtaining a dense matrix structure. Excessively high temperatures will exacerbate the decline in inoculation effect, leading to a tendency for shrinkage porosity, and will also intensify the thermal shock of the metal mold, reducing its lifespan. Lower temperatures are prone to incomplete casting defects.

[0053] In conjunction with the second aspect, in some embodiments provided in this application, the casting model is molded using a sand-coated iron mold process. This sand-coated iron mold process significantly accelerates the cooling rate of the casting through the rapid cooling effect of the metal mold wall, thereby effectively promoting graphite spheroidization, increasing pearlite content, and refining the matrix structure. This rapid cooling process, synergistically with the adjusted Si, Ni, and Cu element ranges in the composition design, significantly improves the uniformity and density of graphite morphology and the matrix structure, which is beneficial for obtaining high-strength, high-toughness ductile iron bridge shell castings with a tensile strength ≥800MPa and an elongation ≥8%.

[0054] In this application, through systematic material design and process optimization, the tensile strength of the bridge housing casting is ≥800 MPa, the elongation is ≥8%, and the stability and consistency of the microstructure and properties of the casting are good in mass production.

[0055] To achieve the performance targets of tensile strength ≥800MPa and elongation ≥8% for ductile iron bridge shell castings, and to ensure the stability and consistency of microstructure and properties under mass production conditions, this invention systematically controls and optimizes aspects such as the chemical element design of raw materials, molding process, and spheroidizing inoculation process.

[0056] Firstly, in terms of composition design, the carbon content is increased (3.5-3.8%) to improve graphite morphology and density, while the silicon content is controlled (2.5-2.75%) to balance strength and elongation. The contents of elements such as Mn, S, and P are strictly limited to ensure toughness. Furthermore, a "high Ni, low Cu" alloying scheme (Ni: 0.85-1.15%, Cu: 0.35-0.5%) is introduced. This utilizes the grain refinement and solid solution strengthening effects of Ni to significantly improve strength while avoiding elongation loss.

[0057] Secondly, in terms of process, an iron mold sand-coated molding process is adopted, utilizing its high cooling rate to refine the matrix structure, increase the pearlite content, and improve the uniformity of graphite distribution, providing a structural basis for toughening. Combined with dual inoculation treatment of silicon-barium inoculation and Sb-containing in-flow inoculation, inoculation fading is effectively suppressed, and broken graphite in thick areas is eliminated, ensuring the stability and consistency of the spheroidization effect. Mg is the main component in the spheroidization treatment, supplemented with a small amount of RE, effectively achieving spheroidization while neutralizing the interference of impurity elements.

[0058] Thirdly, this application provides an electric drive bridge housing, which includes the ductile iron casting described in the first aspect or the ductile iron casting obtained by the preparation method of ductile iron described in the second aspect.

[0059] The electric drive bridge housing is based on the above-mentioned ductile iron casting. The specific technical features of the ductile iron casting can be referred to in the above embodiments. Since the electric drive bridge housing adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0060] Fourthly, this application provides an automobile that includes the electric drive axle housing described in the third aspect.

[0061] The vehicle is based on the aforementioned electric drive axle housing. The specific technical features of the electric drive axle housing can be referred to in the above embodiments. Since the vehicle adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0062] The technical solutions provided in this application will be described in detail below with reference to the embodiments.

[0063] Example 1 Examples 1-3 and Comparative Examples 1-6 of this application provide a ductile iron casting comprising the following raw material composition by mass percentage: C: 3.5-3.8%, Si: 2.5-2.75%, Mn: 0.1-0.3%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.35-0.55%, Ni: 0.85-1.15%, with the balance being Fe and unavoidable impurities. The parameters are shown in Table 1. Table 1

[0064] The ductile iron castings provided in Examples 1-3 and Comparative Examples 1-6 can be prepared by the following methods. Taking Example 1 as an example, the specific preparation steps of the ductile iron casting are as follows: 1) Shaping: The shaping process is carried out using iron molds covered with sand.

[0065] 2) Smelting: Use an induction furnace to smelt molten iron, melt recycled materials, pig iron and scrap steel to a molten state, add carburizing agent and alloy according to the chemical composition test results to adjust the chemical composition of molten iron, control the tapping temperature at about 1500℃, and then tap it out of the furnace; 3) Spheroidizing treatment: Before tapping, add 1.1% spheroidizing agent and 0.3% inoculant to the bottom of the ladle in sequence, and pour the molten iron into the ladle for spheroidizing and inoculation treatment.

[0066] The spheroidizing agent comprises the following raw material components by mass percentage: 7-8% Mg, 2-5% RE, 2-4% Ca, 35-44% Si, balance Fe, and unavoidable impurities.

[0067] The inoculant for the bottom of the package comprises the following raw material components by mass percentage: 60-70% Si, 1-3% Al, 0.5-2.5% Ca and 4-6% Ba.

[0068] 4) Casting: The molten iron after spheroidizing and inoculation treatment is cast at a temperature of 1410℃. At the same time as casting, 0.15% in-flow inoculation is carried out. After cooling, ductile iron castings are obtained.

[0069] The inoculant used for in-flow inoculation comprises the following raw material components by mass percentage: 70-80% Si, 3-6% Sb, with the balance being iron and unavoidable impurities.

[0070] Comparative Example 7 Comparative Example 7 of this application provides a ductile iron casting, similar to Example 1, except that no Sb-containing inoculant is used in the preparation of the ductile iron casting.

[0071] Performance testing (1) The performance of the ductile iron castings of Examples 1-3 and Comparative Examples 1-7 was tested. The tensile specimens were prepared in accordance with the requirements of GB / T 1348-2019 "Ductile Iron Castings", with a specimen diameter of 7 mm, a gauge length of 35 mm, and a parallel section length of 42 mm. The tensile strength, yield strength, and elongation were tested in accordance with the requirements of GB / T 228.1-2021 "Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature". The specific test results are shown in Table 2: Table 2 Mechanical properties of the examples and comparative examples

[0072] As shown in Table 2, compared with Comparative Examples 1-6, the content of elements such as Si, Cu, and Ni in the raw material composition of ductile iron castings exceeds the range provided in this application, and the strength or elongation of ductile iron castings cannot meet the requirements.

[0073] Comparative Example 7, due to the absence of a Sb-containing inoculant during the preparation of ductile iron castings, exhibited fragmented graphite defects, which deteriorated the mechanical properties of the material.

[0074] (2) Mechanical properties of Example 1 and cast steel ZG650-830 were tested, including tensile strength, yield strength, elongation, and fatigue strength. The testing methods for tensile strength, yield strength, and elongation were the same as those described above. The requirements and testing methods for the material fatigue strength specimens were in accordance with GB / T 3075-2021 "Metallic Materials Fatigue Test Axial Force Control Method". The specimen diameter was 8 mm, the parallel section length was 18 mm, and the transition fillet radius was 48 mm. The stress ratio R was 0, the number of fatigue cycles was 107, and the fatigue strength value with a 50% survival rate was taken.

[0075] The specific test results are shown in Table 3: Table 3

[0076] As shown in Table 3, the ductile iron castings provided in this application, while possessing excellent tensile properties (tensile strength ≥ 800 MPa, elongation ≥ 8%), exhibit superior fatigue performance, with fatigue strength significantly exceeding that of ZGD650-830 grade cast steel. This is mainly due to the unique microstructure of ductile iron: although the microstructure contains a large number of graphite spheroids, which act as natural "pores" in the matrix, they reduce the sensitivity of the casting to external defects to a certain extent, thereby effectively inhibiting the initiation and propagation of fatigue cracks. In contrast, ZGD650-830 cast steel lacks this graphite structure. Although its overall microstructure is dense, it is more sensitive to surface or internal defects, and under high stress, stress concentration easily leads to a decrease in fatigue performance.

[0077] In summary, this application significantly improves the casting fluidity of molten iron by controlling the carbon (C) content at a relatively high level of 3.5%-3.8%, increases the number of graphite spheroids during solidification, and improves their roundness, thereby effectively suppressing shrinkage porosity and improving the density of the casting. Simultaneously, a suitable magnesium (Mg) content (0.03-0.06%) ensures good graphite spheroidization, providing an excellent foundation for the strength and toughness of ductile iron castings. Increasing the silicon (Si) content to 2.5-2.75% not only enhances graphitization ability and promotes ferrite formation but also effectively improves the material's plasticity and processing properties. Furthermore, limiting the manganese (Mn) content to a low level of 0.1%-0.3% suppresses the formation of excessive pearlite, avoiding adverse effects on toughness. Finally, a "high-nickel, low-copper" alloying approach was adopted, with the nickel (Ni) content controlled at 0.85-1.15%. This significantly improved the matrix strength through grain refinement and solid solution strengthening mechanisms, while also improving low-temperature toughness without compromising plasticity. Simultaneously, the copper (Cu) content was limited to 0.35-0.55% to prevent it from drastically promoting pearlite at high cooling rates, thus avoiding a decrease in elongation. Phosphorus (P) and sulfur (S), as harmful elements, were strictly limited (P≤0.04%, S≤0.02%), effectively avoiding phosphorus eutectic brittleness and spheroidization interference, further ensuring the toughness and stability of the ductile iron castings. Therefore, the composition design of this ductile iron casting, through an alloying route of "high C, high Si, low Mn, low Cu, controlled sulfur and phosphorus, and added nickel," enables the ductile iron castings to maintain excellent casting performance and graphite morphology while possessing high strength and good toughness.

[0078] In the description of this specification, the references to terms such as "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments / modes or examples described in this specification, as well as the features of different embodiments / modes or examples.

[0079] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this application, "a plurality of" means at least two, such as two, three, etc., unless otherwise expressly specified.

[0080] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A ductile cast iron casting, characterized by, The spherulitic cast iron casting comprises raw material components with the following mass percentages: C: 3.5-3.8%, Si: 2.5-2.75%, Mn: 0.1-0.3%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.35-0.55%, Ni: 0.85-1.15%, and the balance of Fe and inevitable impurities.

2. The ductile iron casting of claim 1 wherein, The spherulitic cast iron casting comprises raw material components with the following mass percentages: C: 3.55-3.76%, Si: 2.56-2.73%, Mn: 0.21-0.26%, P≤0.04%, S≤0.02%, Mg: 0.03-0.06%, Cu: 0.36-0.52%, Ni: 0.88-1.10%, and the balance of Fe and inevitable impurities.

3. A method of producing a spheroidal graphite cast iron casting as claimed in claim 1 or 2, characterized in that, Comprise: The raw materials are melted into molten iron, and the tapping temperature is controlled in the range of 1480-1540 ℃; Before tapping, the spherulitizing agent and the inoculant are sequentially placed in the ladle bottom of a casting ladle, and then the molten iron for tapping is added to perform spherulitizing and inoculating treatment; The spherulitizing and inoculating treated molten iron is poured into a casting mold, and the stream inoculating is performed during pouring, and the spherulitic cast iron casting is obtained after cooling.

4. The production method according to claim 3, wherein The casting mold is made by using the sand-coated iron mold process.

5. The production method according to claim 3, wherein Based on the total mass of the spherulitizing agent, the inoculant and the molten iron for tapping, the mass percentage of the spherulitizing agent is 1.0-1.3%, and the spherulitizing agent comprises raw material components with the following mass percentages: 7-8% Mg, 2-5% RE, 2-4% Ca, 35-44% Si, the balance of Fe and inevitable impurities.

6. The production method according to claim 3, wherein Based on the total mass of the spherulitizing agent, the inoculant and the molten iron for tapping, the mass percentage of the inoculant in the ladle bottom is 0.2-0.4%, and the inoculant in the ladle bottom comprises raw material components with the following mass percentages: 60-70% Si, 1-3% Al, 0.5-2.5% Ca and 4-6% Ba.

7. The production method according to claim 3, wherein Based on the total mass of the spherulitizing and inoculating treated molten iron and the stream inoculating inoculant, the mass percentage of the stream inoculating inoculant is 0.1-0.2%, and the stream inoculating inoculant comprises raw material components with the following mass percentages: 70-80% Si, 3-6% Sb, and the balance of Fe and inevitable impurities.

8. The production method according to claim 3, wherein The pouring temperature is 1380-1420 ℃.

9. An electric drive axle housing characterized in that The electric drive axle housing comprises the spherulitic cast iron casting of claim 1 or 2 or the spherulitic cast iron casting obtained by using the preparation method of the spherulitic cast iron of any one of claims 3-8.

10. An automobile characterized by comprising: The automobile comprises the electric drive axle housing of claim 9.

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