Novel nodular cast iron material and application thereof

Abstract

The invention relates to the technical field of nodular cast iron, and discloses a novel nodular cast iron material and application thereof, the novel nodular cast iron material comprises the following raw materials by mass: 1.8-2.3% of C, 1.3-2.2% of Si, 0.3-0.7% of Mn, 0.002-0.005% of Bi, 0.002-0.01% of Sb, less than 0.06% of P, less than 0.2% of S, and the balance Fe; the CE value is 2.2-2.8, and the Si / C ratio is 0.7-1. According to the scheme, the composition of all the raw materials is combined and optimized, and the use amount of C and Si is limited to a reasonable range, so that normal graphitization is guaranteed, a good spheroidal graphite structure is obtained, adverse effects on a matrix structure and performance due to the fact that the content of C and Si is too high are avoided, the elasticity modulus and compressive strength of the prepared novel spheroidal graphite cast iron material are effectively improved, and the service life of the spheroidal graphite cast iron material is prolonged. And meanwhile, certain toughness and processability can be guaranteed, and the application field of the novel nodular cast iron material is effectively widened.

Description

Technical Field

[0001] The present invention relates to the technical field of ductile iron, and particularly relates to a new type of ductile iron material and its application. Background Art

[0002] The mechanical properties (such as strength and toughness) and process characteristics (such as castability and feeding capacity) of ductile iron highly depend on its material composition. The typical composition of traditional ductile iron is: carbon 3.6 - 3.8 wt%, silicon 2.3 - 2.8 wt%, manganese 0.1 - 0.4 wt%, and 0.03 - 0.06 wt% of magnesium is added as a nodulizer. However, there are inherent contradictions in its composition design: (1) Carbon-silicon balance defect: A high carbon equivalent (CE = C + 1 / 3Si > 4.55%) promotes graphite nodulation but easily causes graphite floating, resulting in a decrease in the surface hardness of the casting; while a high silicon content (> 2.8 wt%) will trigger graphitization expansion in advance, weaken the self-feeding capacity, and increase the shrinkage porosity rate in the core of thick-walled parts. (2) Harm of residual trace elements: Residual aluminum and titanium catalyze the formation of subcutaneous pores, and residual sulfur reacts with magnesium to form magnesium sulfide inclusions, reducing the fluidity of the molten iron. (3) Mechanical property bottleneck: The tensile strength of conventional ductile iron (such as QT400-18, QT600-3) is mostly lower than 700 MPa, and the impact energy at low temperature (-40 °C) is generally < 12 J, making it difficult to meet the requirements of scenarios such as deep-sea pipelines and mechanical components in alpine regions. Even occasionally, the tensile strength of ductile iron (such as QT800-2, QT900-2) reaches 800 - 900 MPa, however, its elongation after fracture A5 ≤ 2%, and the risk of brittle fracture increases significantly. The above problems make the relatively low-cost ductile iron unable to replace the high-cost high-end alloy steel all the time, significantly restricting the application fields of ductile iron.

[0003] In view of the above problems, the prior art has achieved some performance breakthroughs through composition regulation: for example, Patent CN118441201A discloses a method for improving the performance of ductile iron and components obtained from the ductile iron. By combinatorially introducing trace elements copper 0.1-0.5%, tin 0.005-0.3% and vanadium 0.03-0.15%, and adopting a composite spheroidizing and inoculation technology during smelting, the performance is improved from the basic requirement of QT420-5 to the standards of QT500-7, QT500-10, QT550-5 (the tensile strength is increased to 550 MPa). Another patent CN112680648A discloses ductile iron and crankshafts with high modulus and high strength. By introducing multi-element alloying strengthening, adding copper, chromium, nickel to stabilize the pearlite matrix and promote graphitization, or compound adding molybdenum, cobalt, rare earth to refine the grains, the as-cast ultimate tensile strength is as high as 750-960 MPa. However, the prior art still has the following core problems: (1) Insufficient stability of composition control: Fluctuations of impurities such as sulfur and titanium in the original molten iron lead to an increase in the dispersion of the number of graphite balls and an increase in batch differences in tensile strength, reducing the performance stability of the material. (2) The contradiction between process and cost is prominent: The price of high-purity pig iron (Ti < 0.03 wt%) is 30-40% higher than that of ordinary pig iron, and the compound addition of multiple elements further increases the smelting cost. Moreover, the rapid cooling process relied on by Patent CN118441201A requires special equipment, significantly increasing energy consumption. (3) Performance ceiling and environmental constraints: It is difficult to balance high tensile strength and high elastic modulus under traditional composition design, and the risk of brittle fracture increases significantly, restricting its application in low-temperature extreme environments.

[0004] In summary, the defects existing in the prior art result in the fact that existing ductile iron still cannot replace high-end alloy steel or new composite materials. There is an urgent need to innovate the material composition design, break through the performance upper limit of ductile iron, and achieve the unity of low cost and high performance. Summary of the Invention

[0005] The present invention aims to provide a new type of ductile iron material and its application to solve the technical problem that the performance upper limit of existing graphite cast iron materials is restricted by the material composition, resulting in a significant limitation in its application fields.

[0006] To achieve the above object, the present invention adopts the following technical solution: A new type of ductile iron material, comprising raw materials in the following mass percentages: C 1.8-2.3%, Si 1.3-2.2%, Mn 0.3-0.7%, P < 0.06%, S < 0.2%, and the balance is Fe.

[0007] The principle and advantages of this solution are:

[0008] 1. Compared with the existing graphite cast iron materials, whose performance ceiling is limited by their material composition, resulting in a significant limitation in their application fields, this solution optimizes the composition of each raw material through combination, limits the usage of C and Si to a reasonable range, not only ensures the normal progress of graphitization and obtains a good spherical graphite structure, but also avoids the adverse effects on the matrix structure and performance caused by excessive C and Si contents. Thus, it effectively improves the elastic modulus and compressive strength of the newly prepared ductile iron material, while also ensuring a certain toughness and machining performance, and further effectively broadens the application fields of the new ductile iron material. Specifically, when the new ductile iron material produced by this solution is applied to fields such as engine crankshafts, steering knuckles, cutting tools, and machine tool beds, it has performance advantages equivalent to or similar to those of alloy steels, significantly broadening the application fields of ductile iron materials.

[0009] 2. Compared with the use of high-cost alloy steels required due to high performance requirements in existing special fields, the new ductile iron material produced by this solution can not only meet the performance requirements, but also has a lower cost compared to alloy steels, effectively saving energy and improving production efficiency, achieving the unity of economic benefits and environmental benefits.

[0010] 3. This solution rationally matches various elements, and the raw materials interact with each other to synergistically improve the comprehensive performance of the newly produced ductile iron. Specifically as follows:

[0011] (1) Promote graphitization and optimize the structure: When the C content is 1.8 - 2.3%, it can moderately promote graphitization. The formed spherical graphite can serve as an internal support structure, effectively dispersing stress and reducing stress concentration. When the Si content is 1.3 - 2.2%, it cooperates with C to further promote graphitization, making the graphite balls more round, fine and evenly distributed. This evenly distributed spherical graphite in the cast iron matrix acts like "ball bearings", which can buffer and transfer stress when subjected to external forces, helping to improve the elastic modulus and compressive strength of the material.

[0012] (2) Strengthen the matrix and improve strength: When the Mn content is 0.3 - 0.7%, it can dissolve into ferrite, enhancing the strength and hardness of ferrite. At the same time, Mn forms manganese sulfide with S, reducing the harmful effects of S and improving the purity of the cast iron. The combined action of C, Si, and Mn optimizes the matrix structure of the cast iron, making the matrix more dense and uniform, thereby improving the overall strength and elastic modulus of the material and helping to improve the compressive strength.

[0013] (3) Control harmful elements to ensure performance: The P content is controlled at <0.06% and the S content is <0.2%. This avoids the formation of hard and brittle phosphide eutectics and excessive sulfide inclusions due to excessive P and S contents, preventing these substances from becoming crack sources, ensuring the toughness and strength of the material, and being of great significance for maintaining the stability of the elastic modulus and compressive strength of the material.

[0014] Preferably, as an improvement, the CE value is 2.2 - 2.8, and the Si / C ratio is 0.7 - 1.

[0015] Beneficial effects: With the above settings in this solution, it can ensure an appropriate degree of graphitization during the solidification of cast iron, which helps to form a uniform and dense matrix structure. Specifically, it can promote the formation of an appropriate amount of pearlite and ferrite during the cooling of cast iron. Pearlite has relatively high strength and hardness, while ferrite has a certain toughness. When the proportion of the two is appropriate, the material can have good strength and toughness, thereby improving the elastic modulus and compressive strength. It will neither produce excessive cementite due to insufficient graphitization, resulting in excessive hardness, decreased toughness and strength of the material, nor cause the graphite balls to be too large and the ferrite content in the matrix to be too high due to excessive graphitization, thus reducing the strength and elastic modulus of the material.

[0016] Preferably, as an improvement, it also includes raw materials with the following mass percentages: Bi 0.002 - 0.005%, Sb 0.002 - 0.01%.

[0017] Beneficial effects: With the above settings in this solution, it is convenient to further improve the comprehensive performance of the material. Specifically, antimony (Sb) has various beneficial effects in ductile iron, including refining grains, making the matrix structure more uniform and dense, and improving the strength and toughness of the material; antimony can also promote graphitization, improve the morphology and distribution of graphite balls, and further enhance the performance of the material. For example, in terms of compressive strength, the refined grains and good graphite ball state enable the material to better withstand pressure and improve the compressive strength. In terms of elastic modulus, the uniform and dense structure is conducive to the uniform deformation of the material when stressed, improving the elastic modulus. In terms of microstructure, antimony can stabilize pearlite and inhibit the formation of ferrite, thereby increasing the pearlite content and contributing to the improvement of the hardness and strength of the material. The applicant found through long-term experiments that if the antimony content is too high (>0.03%), it may lead to the deterioration of graphite morphology, the appearance of fragmented graphite, and defects during the casting process. If the antimony content is too low, its functions such as refining grains and promoting graphitization cannot be fully exerted. Coarse grains and poor graphite ball morphology will reduce the comprehensive performance of the material and are prone to crack propagation when stressed, reducing the compressive strength.

[0018] Bismuth (Bi) can improve the strength and hardness of the material in ductile iron, especially in promoting the stability of the pearlite matrix structure. Bismuth can also improve the wear resistance and corrosion resistance of the material, which is of positive significance for some ductile iron products that need to be used in wear or corrosive environments. The applicant has found through long-term experiments that if the bismuth content is too high, it may cause too many hard and brittle phases to appear in the matrix structure of ductile iron. These hard and brittle phases increase the brittleness of the material, reduce toughness, and easily cause crack propagation when under pressure, reducing compressive strength. If the bismuth content is too low, it cannot fully play its role in promoting pearlite formation and improving strength and hardness. The proportion of pearlite in the matrix structure is insufficient, the strength and hardness are reduced, and the compressive strength decreases accordingly.

[0019] Preferably, as an improvement, the ductile iron material has a pearlite matrix structure, the volume fraction of pearlite is 98-99%, and the remainder is cementite and ferrite.

[0020] Technical effect: This scheme adopts the above-mentioned setting to make the structure of ductile iron more uniform and dense, and effectively improve the elastic modulus and compressive strength of the material. Specifically, pearlite is a mixture of ferrite and cementite, which has high hardness and rigidity. A large amount of pearlite forms a skeleton-like structure, which effectively limits the elastic deformation of the material when it is stressed, allowing the material to withstand greater stress during the elastic deformation stage, thereby improving the elastic modulus of the material. The cementite in pearlite has a high hardness and can hinder dislocation movement. The material is less likely to undergo plastic deformation when under pressure, thereby improving the compressive strength.

[0021] Preferably, as an improvement, the ductile iron material has a spheroidal graphite size grade of 6 to 7 and a spheroidization grade of 2.

[0022] Beneficial effects: This solution effectively improves the comprehensive performance of the material by limiting the spheroidization grade and spheroidal graphite size grade of the prepared ductile iron material. Specifically, graphite size grades 6 to 7 correspond to fine graphite balls (diameter 0.02 to 0.05 mm), which are uniform in size and densely distributed, effectively reducing stress concentration and significantly improving tensile strength. Graphite spheroidization grade 2 indicates that the graphite morphology is mainly spherical with high roundness, reducing the brittle effect of non-spherical graphite and ensuring the elongation of the material. This solution comprehensively optimizes the size and morphology of the graphite balls in the material, and the two work together to further improve the strength, toughness and fatigue life of the material.

[0023] Preferably, as an improvement, the ductile iron material has a tensile strength of 800-950 MPa, an elongation of 3-4%, and a yield strength of 500-650 MPa.

[0024] Technical effects: With the above settings, the new ductile iron material in this solution can replace high-performance alloy steel in some fields, significantly reducing the production cost of products and enhancing the market competitiveness of products. Specifically, the high tensile strength and yield strength of the new ductile iron material in this solution enable it to withstand large external forces and loads, and it can be used to manufacture large building structural components, bridge support components, etc., effectively broadening the application scope of ductile iron in the field of engineering structures. The tensile strength and yield strength of the ductile iron in this solution can meet the strength requirements of mechanical parts (such as crankshafts, gears, etc.) under high-speed and high-load operating conditions, and the elongation rate enables it to withstand a certain degree of impact and vibration, reducing the possibility of part failure due to fatigue, which helps to broaden the application of ductile iron in the high-end field of mechanical manufacturing. Moreover, the high strength of the ductile iron in this solution can ensure the reliability of key components such as the engine block and chassis of automobiles. The appropriate elongation rate makes it not easily break when subjected to impact. At the same time, its density is smaller than that of alloy steel, which is conducive to the lightweight of automotive parts, thus being more widely used in the automotive industry and broadening the material selection range of automotive parts.

[0025] In the specific replacement and use process, the new ductile iron material in this solution also has the following advantages:

[0026] (1) Cost advantage: The production process of alloy steel is usually relatively complex, requiring the addition of various precious metal elements, resulting in a high production cost. However, the main elements of ductile iron, such as iron, carbon, and silicon, are abundant in nature, with a wide source and low cost; its production process is relatively simple. Under the condition of achieving the same performance, the cost of ductile iron is usually lower than that of alloy steel. Replacing alloy steel with ductile iron can significantly reduce the production cost of products and enhance the market competitiveness of products.

[0027] (2) Machining performance advantage: The machining performance of ductile iron is better than that of some alloy steels. It has good cutting performance and is not prone to problems such as excessive tool wear and poor machining surface quality during the machining process, which can reduce the machining process and machining time, improving production efficiency. At the same time, ductile iron has good casting performance and can manufacture parts with complex shapes, reducing the subsequent machining amount, further reducing the production cost and machining difficulty.

[0028] (3) Corrosion resistance advantage: In some specific working environments, such as humid environments with corrosive media, the corrosion resistance of ductile iron is often better than that of some alloy steels. Through appropriate surface treatment, ductile iron can maintain good performance in these environments, extending its service life. Therefore, in these fields, it can replace alloy steels with poor corrosion resistance, reducing the equipment maintenance cost and replacement frequency.

[0029] Preferably, as an improvement, the elastic modulus of the ductile iron material is 210 - 240 GPa.

[0030] Technical effect: With the above settings in this solution, the elastic modulus reflects the ability of a material to resist elastic deformation. The relatively high elastic modulus of ductile iron means that when subjected to external forces, it produces less elastic deformation. In building structures and bridge engineering, components made of this ductile iron can maintain good shape and dimensional stability under long-term loads, provide reliable stiffness support for the structure, and ensure the safety and normal service functions of the structure. For some engineering structures prone to vibration, such as the foundations of large machinery and equipment, high-rise buildings, etc., the high elastic modulus of ductile iron can quickly absorb and dissipate vibration energy, reduce fatigue damage caused by vibration in the structure, and improve the vibration resistance performance and service life of the structure. In precision machining equipment, such as components like the bed and workbench of a machine tool, the high elastic modulus of ductile iron enables it to withstand the cutting force of the tool and the weight of the workpiece during the machining process without excessive deformation, thereby ensuring machining accuracy, improving the manufacturing quality and consistency of mechanical parts. In a mechanical transmission system, such as components like gears and transmission shafts, ductile iron with a high elastic modulus can maintain good rigidity when transmitting power, reduce transmission errors caused by elastic deformation, improve transmission efficiency and stability, and ensure the reliable operation of the entire mechanical system.

[0031] Preferably, as an improvement, the hardness of the ductile iron material is 280 - 300 HB.

[0032] Technical effect: With the above settings in this solution, the relatively high hardness enables ductile iron to better resist wear during the friction process, be able to withstand larger loads when bearing weight, and also have good fatigue resistance under cyclic loads. For example, in some power transmission components, it can withstand long-term alternating stress without easily showing fatigue cracks and fractures, thereby improving the reliability and stability of the entire system. And this hardness range enables ductile iron to exhibit good cutting performance during the machining process. It will neither cause the tool to wear too quickly and the machining difficulty to be too high due to being too hard, nor have problems with difficult-to-guarantee machining accuracy due to being too soft. Therefore, during machining, operations such as turning, milling, and drilling can be carried out more efficiently, improving machining efficiency and product quality, and reducing machining costs. And this hardness range combined with other characteristics of ductile iron gives it good shock absorption performance. In applications such as equipment foundations and machine tool bases with shock absorption requirements, it can effectively absorb vibration energy, reduce the impact of vibration on the equipment and the surrounding environment, improve the working accuracy and stability of the equipment, and also help reduce noise.

[0033] Preferably, as an improvement, the density of the ductile iron material is 7.23 - 7.26 g / cm 3 .

[0034] Technical effect: With the above settings, this solution facilitates improving the comprehensive properties of the material while reducing its mass. Compared with materials with higher density such as steel, ductile iron has a lighter mass for the same volume. In some applications with weight requirements, such as automotive manufacturing and aerospace component manufacturing, this helps reduce the overall weight, improve transportation efficiency, and enhance energy utilization efficiency. The molten ductile iron with relatively low density has good fluidity and can more easily fill complex mold cavities, enabling the manufacture of components with complex shapes, reducing casting defects, and increasing the yield rate of castings.

[0035] Preferably, as an improvement, this solution also provides an application of a new type of ductile iron material, including the application of ductile iron material in the preparation of engine crankshafts, steering knuckles, cutting tools, and machine tool beds.

[0036] The principle and advantages of this solution are as follows:

[0037] 1. Compared with the prior art where the dispersion of graphite balls in the prepared ductile iron material increases and the batch difference in tensile strength is significant, the ductile iron material prepared by this solution has higher performance stability, that is, the differences in properties such as tensile strength, elastic modulus, and elongation rate among products of the same batch are small, effectively ensuring product quality and enhancing the product performance stability during actual application.

[0038] 2. By using the ductile iron material with optimized properties to prepare products such as engine crankshafts, steering knuckles, cutting tools, and machine tool beds, this solution not only effectively meets the high-performance requirements of the products, but also compared with the prior art where alloy steel is used with higher production costs and energy consumption, the ductile iron material in this solution can achieve the same performance level as alloy steel while effectively reducing the production cost and energy consumption of the material, thus realizing the unity of economic benefits and environmental benefits. Brief Description of the Drawings

[0039] Figure 1 It is the metallographic structure diagram (100 - fold magnification) of the matrix structure in the cast billet obtained in Example 1 of the present invention.

[0040] Figure 2 It is the partial metallographic structure diagram of the ductile iron material obtained in Example 7 of the present invention (a shows the state of graphite balls under 100 - fold magnification, the size of graphite balls is 0.5 - 0.8 mm; b shows the state of graphite balls under 500 - fold magnification; c shows the matrix structure under 100 - fold magnification; d shows the matrix structure under 500 - fold magnification).

[0041] Figure 3 It is the partial metallographic structure diagram of graphite balls in the ductile iron material obtained in Comparative Example 14 of the present invention under 100 - fold magnification (showing the influence of too high annealing temperature in the first stage).

[0042] Figure 4This is a partial metallographic structure diagram of the matrix of the ductile iron material obtained in Comparative Example 15 of the present invention under a 100-fold microscope (showing the influence of too low annealing temperature in the first stage, and the bright network in the figure is ferrite).

[0043] Figure 5 This is a partial metallographic structure diagram of the matrix of the ductile iron material obtained in Comparative Example 19 of the present invention (showing the influence of too low annealing temperature in the second stage; the upper figure is a 100-fold microscope, and the lower figure is a 500-fold microscope, and the bright network in the figure is ferrite). Detailed implementation manners

[0044] The present invention will be further described in detail below in conjunction with embodiments, but the implementation manners of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following embodiments and experimental examples are conventional means well-known to those skilled in the art, and the materials, reagents, etc. used can all be obtained from commercial channels.

[0045] Embodiment 1

[0046] This solution provides a new type of ductile iron material, including raw materials with the following mass percentages: C 1.8 - 2.3%, Si 1.3 - 2.2%, Mn 0.3 - 0.7%, P < 0.06%, S < 0.2%, and the balance is Fe. The CE value is 2.2 - 2.8, and the Si / C ratio is 0.7 - 1.

[0047] In order to further improve the performance of the material, the ductile iron material also includes raw materials with the following mass percentages: Bi 0.002 - 0.005%, Sb 0.002 - 0.01%.

[0048] The ductile iron material prepared by this solution has a pearlite matrix structure, and the volume fraction of pearlite is 98 - 99%, and the balance is cementite and ferrite. The nodule size grade of the material is 6 - 7, the spheroidization grade is 2, and the density is 7.23 - 7.26 g / cm 3 . Its performance is as follows: tensile strength is 800 - 950 MPa, elongation is 3 - 4%, yield strength is 500 - 650 MPa, elastic modulus is 210 - 240 GPa, and hardness is 280 - 300 HB.

[0049] The raw material compositions of the ductile iron materials in Embodiments 2 - 3 and Comparative Examples 1 - 13 are basically the same as those in Embodiment 1, and the differences are shown in Table 1.

[0050] Table 1 Differences in raw material compositions of Embodiments 1 - 3 and Comparative Examples 1 - 13

[0051]

[0052] In order to fully explore the influence of the raw material dosage on the performance of the ductile iron material, the method for preparing the ductile iron material in this solution includes the following steps:

[0053] Step 1: Melting and casting the raw material components into a billet, the steps are as follows:

[0054] S1-1. Primary melting stage: Melting with pure scrap steel, adding graphitized petroleum coke for carbon increment, silicon carbide for silicon increment, alloys and spheroidizing agents to form spherical graphite. Among them, the pure scrap steel melting uses low-carbon and low-manganese scrap steel (cold-rolled thin plates pressed into blocks), the graphitized petroleum coke has a carbon content of about 99% and an absorption rate of 92% - 94%, the silicon carbide has a silicon content of about 58% and an absorption rate of 90%; the alloys include ferromanganese alloy (ferromanganese alloy with a manganese content of 65% can be selected), antimony-iron alloy and bismuth-iron alloy (both with a purity of 99%), and the addition amount of the alloy is 2 - 2.2% of the total mass of the raw materials; the spheroidizing agent is any one of rare earth-silicon-magnesium alloy spheroidizing agent and pearlite spheroidizing agent, and the addition amount of the spheroidizing agent is 1 - 1.2% of the total mass of the raw materials. After melting in this stage, the composition of each raw material in the ductile iron material meets the requirements described above.

[0055] S1-2. Intermediate frequency furnace melting stage, the melting temperature is 1530 - 1600 °C (specifically 1565 °C in this embodiment), the casting temperature > 1420 °C (specifically 1483 °C in this embodiment), obtaining a billet, the billet is a matrix structure material mainly composed of low-temperature ledeburite, and the proportion of ledeburite in the matrix structure is 15 - 20%, and the rest of the matrix is spherical graphite, pearlite, and cementite. The metallographic structure diagram is as Figure 1 shown.

[0056] Step 2: Annealing the billet to obtain ductile iron material, including the following steps:

[0057] S2-1: First-stage annealing, high-temperature decomposition: Heating in the furnace to 880 - 980 °C (specifically 950 °C in this embodiment), holding for 15 - 36 h (specifically 28 h in this embodiment), and specifically determining the holding time according to the thickness of the product and the added alloying elements.

[0058] S2-2: Second-stage annealing, low-temperature graphitization: Cooling to 650 - 860 °C (specifically 780 °C in this embodiment), the cooling rate is 40 - 60 °C / h (specifically 50 °C / h in this embodiment), holding for 3 - 5 h and then quickly taking out of the furnace and air-cooling (specifically 3 h in this embodiment);

[0059] In this stage, low-temperature graphitization can also adopt a two-stage method, that is, after heating the billet in the furnace to 880 - 980 °C and holding for 15 - 36 h in the first-stage annealing, taking it out of the furnace and air-cooling to room temperature, then loading it into the furnace again and heating to 650 - 860 °C, holding for 3 - 6 h according to the product thickness and taking it out of the furnace and air-cooling to obtain various properties with a high elastic modulus as required.

[0060] S2-3: Surface treatment: Shot blasting to remove the oxide scale to obtain ductile iron material.

[0061] The raw materials in each example in Table 1 were all prepared into ductile iron materials according to the above steps, and the properties of the obtained ductile iron materials were detected with reference to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", GB / T 22315-2008 "Test method for elastic modulus and Poisson's ratio of metallic materials", and GB / T 9441-2021 "Metallographic examination of ductile iron". The results were recorded in Table 2.

[0062] Table 2 Properties of ductile iron materials

[0063]

[0064]

[0065] Experimental data show that by limiting the composition of each element in the ductile iron material, the overall performance of the material is effectively improved. Specifically, too high carbon equivalent (such as 3.5 or 3) in Comparative Example 1 and Comparative Example 3 will cause abnormal graphite morphology, such as graphite blooming and floating, and at the same time inhibit the improvement of the product strength; too low carbon equivalent (such as 2.1) in Comparative Example 2 will inhibit graphitization, resulting in part of the carbon existing in the form of cementite, increasing hardness and brittleness; the appropriate Si / C value in Examples 1 to 3 can just avoid the occurrence of the above problems. And too high P content (such as 0.1%) in Comparative Example 7 will form hard and brittle phosphide eutectic in ductile iron, distributed at the grain boundaries, forming crack sources. Although it can be decomposed by subsequent heat treatment, there will still be residues, which has a great impact on the product performance. And too high sulfur content (such as 0.25%) in Comparative Example 8 will consume the spheroidizing agent, reduce the spheroidizing effect, seriously reduce the mechanical properties of the product, and at the same time increase casting defects. The appropriate addition of antimony and bismuth in Examples 1 to 3 can significantly stabilize the morphology of graphite balls, making them refined and round. However, the addition of antimony and bismuth beyond the specified amount (such as Bi content of 0.0055% in Comparative Example 9 and Sb content of 0.012% in Comparative Example 10) will excessively increase the pearlite content, making the material brittle and the impact toughness decrease. And if there is no addition of antimony and bismuth (such as not adding Bi in Example 4 and not adding Sb in Example 5), the comprehensive performance of the material will also be appropriately reduced, but the overall performance is still significantly improved compared with the existing ductile iron QT800-3.

[0066] Particularly for Comparative Example 11, if the raw material composition in the prior art "CN115074609A A Ductile Iron with Low Residual Stress and High Elastic Modulus and Its Application" is adopted, even if the annealing process in this solution is used, the comprehensive performance of the obtained ductile iron material is significantly higher than the highest performance of the materials obtained by the prior art (for example, in "CN115074609A A Ductile Iron with Low Residual Stress and High Elastic Modulus and Its Application", the highest elastic modulus is 213 GPa and the highest strength is 728 MPa). This fully shows that in addition to the influence of the raw material composition on the product performance, the process parameters in each stage of the optimized annealing method in this solution can also significantly improve the elastic modulus and strength of the material. And by combining the optimized annealing method and the raw material composition of the material in this solution, the two work synergistically to further improve the elastic modulus and compressive strength of the prepared ductile iron material, thereby improving the comprehensive performance of the material.

[0067] Example 6

[0068] This solution also fully explores the influence of each parameter in the annealing stage on the performance of ductile iron materials. Taking the raw material ratio of ductile iron materials "C 1.8%, Si 1.5%, Mn 0.35%, Bi 0.004%, Sb 0.004%, P 0.02%, S 0.03%, the balance is Fe" as an example, the design of each process parameter is shown in Table 3.

[0069] Table 3 Design of Each Parameter in the Annealing Stage

[0070]

[0071]

[0072] The raw materials with the above ratio are prepared into ductile iron materials according to the processes of each example and comparative example in Table 3, and the performance of the obtained ductile iron materials is detected with reference to GB / T 228.1-2021 "Metallic Materials - Tensile Testing - Part 1: Method of Test at Room Temperature", GB / T 22315-2008 "Test Method for Elastic Modulus and Poisson's Ratio of Metallic Materials", and GB / T 9441-2021 "Metallographic Examination of Ductile Iron". The results are recorded in Table 2. The metallographic structure of the ductile iron material prepared in Example 7 is as Figure 2 shown, and the metallographic structure of the ductile iron material prepared in Comparative Example 14 (showing the influence of too high annealing temperature in the first stage) is as Figure 3 shown, and the metallographic structure of the ductile iron material prepared in Comparative Example 15 (showing the influence of too low annealing temperature in the first stage) is as Figure 4 shown, and the metallographic structure of the ductile iron material prepared in Comparative Example 19 (showing the influence of too low annealing temperature in the second stage) is as Figure 5 shown. Different small figures show the metallographic structures under different fields of view.

[0073] Performance of Ductile Iron Materials in Table 4

[0074]

[0075] Experimental data show that by optimizing the parameters in the annealing stage, this solution significantly improves the comprehensive performance of the prepared ductile iron materials, especially the elastic modulus and tensile strength. Specifically, as Figure 2 shown, the graphite balls of the materials prepared by this solution have good roundness, the size of the graphite balls is 0.5 - 0.8 mm, the matrix structure is uniform, which significantly ensures the overall performance of the prepared new ductile iron materials.

[0076] Specifically, in Comparative Example 12, the excessively high melting temperature (such as 1620 °C) and casting temperature (such as 1532 °C) will damage the graphitization process, resulting in thick and distorted graphite, inhibiting graphite precipitation, and will also make the grains thick, reducing the performance. In Comparative Example 13, the excessively low melting (such as 1514 °C) and casting temperature (such as 1371 °C) result in poor fluidity of the molten iron, making it difficult to form the product, the alloying elements in the molten iron cannot be melted sufficiently, the diffusion is insufficient, the graphite is unevenly distributed, and the collective organization is unstable. In Comparative Example 14, the excessively high first-stage annealing temperature (such as 1000 °C) dissolves the graphite in the casting into austenite, and when cooled, it precipitates as carbide, resulting in an increase in the brittleness of the material, and at the same time, the surface oxidation is obvious. As Figure 3 shown, the graphite at the edge of the material is oxidized and decomposed due to high temperature, and the undecomposed graphite is also too thick and unevenly distributed, seriously affecting its performance. In Comparative Example 15, the excessively low first-stage annealing temperature (such as 850 °C) makes the graphite in the casting unable to precipitate effectively. Since it is below the phase transformation temperature, the carbide in the casting cannot be completely decomposed (as Figure 4As shown in the figure, the residual carbides increase the brittleness of the casting. In Comparative Example 16, the excessively fast cooling rate (such as 100 °C / h) leads to the formation of abnormal phases, such as retained tempered martensite, increasing brittleness, and at the same time inhibiting the normal phase transformation, resulting in abnormal grain structure. In Comparative Example 17, the excessively slow cooling rate (such as 30 °C / h) causes the material to stay in the high-temperature zone for too long, and the grains have sufficient time to grow. The coarse grains will reduce the toughness, strength and other properties of the material. At the same time, it will promote the precipitation of carbides at the grain boundaries, forming a continuous network structure, reducing toughness. In Comparative Example 18, the excessively high annealing temperature in the second stage (such as 900 °C) will cause the transformation rate of austenite to ferrite to be too fast, resulting in uneven formation of ferrite, and massive or coarse ferrite structures may appear. This uneven ferrite structure will reduce the strength and toughness of the material. At the same time, due to the inhomogeneity of the structure, the elastic modulus will also be affected. In Comparative Example 19, the excessively low annealing temperature in the second stage (such as 600 °C) significantly slows down the graphitization process. The diffusion of carbon atoms is difficult, and it is difficult to further improve the morphology and distribution of graphite balls, resulting in poor roundness and uniformity of graphite balls. At the same time, due to insufficient graphitization, there may be more cementite or untransformed austenite remaining in the matrix. The presence of these hard and brittle phases or unstable phases will reduce the toughness of the material, and cracks are likely to propagate under stress, reducing the compressive strength of ductile iron. And a network of ferrite is also formed, which is not conducive to improving the strength of the material (as Figure 5 shown), comprehensively making the structure inhomogeneous and imperfect, reducing its elastic modulus.

[0077] This solution also provides an application of a new type of ductile iron material, including the application of ductile iron material in the preparation of engine crankshafts, steering knuckles, cutting tools, and machine tool beds.

[0078] Example 9: Application of the new type of ductile iron material in the preparation of engine crankshafts

[0079] In this example, specifically taking the production of engine crankshafts such as Chery F4J20 crankshaft (2.0T new energy engine), Changan HE15 crankshaft (1.5T new energy engine) as examples, that is, after melting, casting, heat treatment, and surface treatment of the new type of ductile iron material in Example 8, an engine crankshaft is obtained, and the performance test results are shown in Table 5.

[0080] Table 5 Crankshaft performance

[0081] Test items Performance Elastic modulus ≥200 GPa Tensile strength ≥850 MPa Yield strength ≥520 MPa Elongation ≥3.5％ Hardness 260 - 300 HB Impact energy absorption 50J

[0082] And it has passed the 1150 N·m fatigue test, which can greatly improve the durability and fatigue resistance of parts, better suppress the noise problem caused by high vibration, and at the same time has greater expansion space in terms of lightweight.

[0083] Example 10: Application of the new type of ductile iron material in the preparation of steering knuckles

[0084] In this embodiment, taking the production of steering knuckle parts for a well-known vehicle factory as an example, that is, after melting, casting, heat treatment, and surface treatment of the new nodular cast iron material in Embodiment 6, a steering knuckle is obtained, and the performance test results are shown in Table 6.

[0085] Table 6 Performance of Steering Knuckle

[0086] Test items Performance Elastic modulus ≥200 GPa Tensile strength ≥850 MPa Yield strength ≥520 MPa Elongation ≥3.5％ Hardness 260 - 300 HB Impact energy absorption 50J

[0087] Experimental data show that the significant synchronous improvement in the tensile strength and elastic modulus of the material has profound impacts in many aspects on the optimization of automotive chassis components, bringing significant advantages. For the requirement of its relatively low strength (i.e., a ferritic matrix is required), a low annealing temperature can be selected for treatment, and 650 °C in the second stage can meet the requirements for its strength and elastic modulus.

[0088] In summary, experimental data show that materials with a high elastic modulus deform less under the same stress, can effectively resist elastic deformation caused by cutting forces, gravity, etc., ensure the stability of the geometric accuracy of the workpiece, and avoid machining errors caused by workpiece deformation. A high-rigidity structure has a higher natural frequency, can reduce resonance with the vibration source during the machining process, reduce the amplitude, thereby reducing chatter and surface waviness, and improving the surface finish of the workpiece. High-rigidity materials can reduce thermal deformation caused by temperature changes, and at the same time have a stronger creep resistance, are not easily relaxed under long-term load, and extend the precision life.

[0089] The above are only embodiments of the present invention. Specific technical solutions and / or common knowledge such as characteristics well known in the art are not described in detail herein. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. A new type of ductile iron material, characterized in that: The raw materials include the following in mass percentages: C 1.8 - 2.3%, Si 1.3 - 2.2%, Mn 0.3 - 0.7%, P < 0.06%, S < 0.2%, and the balance is Fe.

2. A new ductile iron material according to claim 1, characterized in that: The CE value is 2.2 - 2.8, and the Si / C ratio is 0.7 - 1.

3. A novel ductile iron material according to claim 2, characterized in that: It also includes the following raw materials in mass percentages: Bi 0.002 - 0.005%, Sb 0.002 - 0.01%.

4. A novel ductile iron material according to claim 3, characterized in that: The ductile iron material has a pearlite matrix structure, the volume fraction of pearlite is 98 - 99%, and the balance is cementite and ferrite.

5. A novel ductile iron material according to claim 3, characterized in that: In the ductile iron material, the nodule size grade is 6 - 7, and the spheroidization grade is 2.

6. A novel ductile iron material according to claim 3, characterized in that: The tensile strength of the ductile iron material is 800 - 950 MPa, the elongation is 3 - 4%, and the yield strength is 500 - 650 MPa.

7. A novel ductile cast iron material according to claim 3, characterized in that: The elastic modulus of the ductile iron material is 210 - 240 GPa.

8. A new ductile iron material according to claim 3, characterized in that: The hardness of the ductile iron material is 280 - 300 HB.

9. A novel ductile cast iron material according to claim 3, characterized in that: The density of the ductile iron material is 7.23 to 7.26 g / cm 3 .

10. Application of a new ductile iron material according to any one of claims 1 to 9, characterized in that: It includes the application of the ductile iron material in the preparation of engine crankshafts, steering knuckles, cutting tools, and machine tool beds.

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