Preparation method and application of nodular cast iron material with high elastic modulus

Abstract

The invention relates to the technical field of nodular cast iron and a preparation method thereof, and discloses a preparation method and application of a nodular cast iron material with high elastic modulus, and the material comprises the following components in percentage 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 of Fe. The preparation method comprises the following steps: step 1, smelting and casting to obtain a casting blank; secondly, the casting blank is rapidly discharged out of the furnace and cooled in air after being subjected to first-stage annealing and second-stage annealing in sequence; and thirdly, oxide skin is removed through shot blasting, and the nodular cast iron material with the high elastic modulus is obtained. According to the scheme, by optimizing the component proportion and performing two-stage annealing after smelting and casting, a matrix structure with high graphite nodule roundness, uniform components and good continuity can be formed, the elasticity modulus of the nodular cast iron material is effectively improved, the nodular cast iron material can conveniently and effectively bear the long-time continuous operation load when replacing alloy steel to manufacture a crankshaft, and the service life of the crankshaft is prolonged. And the alternating stress tolerance performance is improved, so that the crankshaft cost is reduced, and light weight is realized.

Description

Technical Field

[0001] The invention relates to the technical field of ductile iron and a preparation method thereof, and in particular to a preparation method and application of a high elastic modulus ductile iron material. Background Art

[0002] In today's world industrial field, ductile iron and alloy steel materials are widely used. The application fields and core differences of the two materials are: the economy of ductile iron products is much higher than the cost of alloy steel forgings, and has obvious cost advantages; and ductile iron has good flow formability and casting mold, which can meet the casting of complex shapes and has obvious weight reduction potential. Alloy steel can meet extreme performance requirements (such as high temperature, corrosion, and ultra-high strength) due to its higher strength and elastic modulus, and is commonly used in precision or high-load scenarios. The two complement each other and play a key role in different industrial scenarios. However, if the elastic modulus and strength of ductile iron can be effectively improved, it can have the advantages of both, and its market prospects will be very broad.

[0003] Taking the crankshaft of an automobile engine as an example, with the development of new energy vehicles, the market demand for extended-range vehicles has risen rapidly. Traditional fuel engines cannot replace extended-range engines (range extenders). The reason is that the only function of the extended-range engine (range extender) is to generate electricity for the battery or drive motor, and the vehicle is completely driven by the electric motor. The extended-range engine (range extender) must usually run at a constant speed under the highest thermal efficiency conditions (such as 30-40% thermal efficiency range) to achieve higher overall energy efficiency. As the core component of the engine, the crankshaft has the following core requirements: high durability and fatigue resistance. The extended-range engine usually works in a constant high-efficiency speed range (such as medium and high speed). The crankshaft needs to withstand the load of long-term continuous operation, alternating stress tolerance, and higher requirements for the fatigue life of the material. In addition, the extended-range engine often runs at a fixed speed, which is easy to cause resonance, and the vibration needs to be suppressed by optimizing the dynamic balance of the crankshaft. At the same time, a vibration-reducing pulley or damper may be integrated to reduce the interference of the crankshaft torsional vibration on the generator and ensure the stability of power generation. At present, in order to meet the above core requirements, OEMs mostly use high-strength alloy steel (such as 42CrMo, 38MnVS6) in material selection, supplemented by surface treatment to meet the requirements. However, although alloy steel can meet the stiffness requirements, its production process is expensive and has low lightweight potential, which makes the existing crankshaft materials still have certain defects and deficiencies. It is urgent to optimize the elastic modulus of ductile iron materials to meet the high performance, low cost and lightweight requirements including crankshafts.

[0004] In view of this, the development of a preparation method for high elastic modulus ductile iron material not only effectively compensates for the defects of the existing technology, but also improves the elastic modulus and strength of ductile iron material so that it can replace alloy steel, which is of great significance for optimizing material properties, reducing costs and lightweight equipment. Summary of the Invention

[0005] The present invention aims to provide a preparation method and application of ductile iron materials with high elastic modulus, so as to solve the technical problems that existing ductile iron materials cannot replace alloy steel due to their low elastic modulus, resulting in high equipment costs and low potential for lightweight of alloy steel materials.

[0006] To achieve the above object, the present invention adopts the following technical scheme: A preparation method of ductile iron materials with high elastic modulus, the ductile iron materials with high elastic modulus include the following components in mass percentages: C 1.8 - 2.3%, Si 1.3 - 2.2%, Mn 0.3 - 0.7%, Bi 0.002 - 0.005%, Sb 0.002 - 0.01%, P < 0.06%, S < 0.2%, and the balance is Fe; the preparation method includes the following steps:

[0007] Step 1, billet stage; melting the raw materials and casting them into billets, the billets containing the above components in mass percentages;

[0008] Step 2, annealing stage: successively undergoing first-stage annealing and second-stage annealing, and then quickly taking out of the furnace and air-cooling;

[0009] Step 3, surface treatment stage: shot blasting to remove the oxide scale to obtain ductile iron materials with high elastic modulus.

[0010] The principle and advantages of this solution are as follows:

[0011] 1. Compared with existing ductile iron whose highest elastic modulus is only 150 - 180 GPa of QT800 - 3 and cannot replace alloy steel as a crankshaft material, this solution can form a matrix structure with high roundness, uniform composition and good continuity of graphite balls by optimizing the component ratio and through two-stage annealing after melting and casting. Thus, it can effectively improve the stability of the material to maintain its shape and size when subjected to external forces, and further improve the elastic modulus of the ductile iron material. When it is used to replace alloy steel to make a crankshaft, it can effectively withstand the load of long-term continuous operation, improve its alternating stress tolerance performance, thereby reducing the cost of the crankshaft and achieving lightweight.

[0012] 2. Carbon (C) is the core element in cast iron, and silicon (Si) is a strong graphitizing element. By limiting the amount of the two, the composition of the material matrix can be effectively limited, which plays a decisive role in improving the elastic modulus of the material. Specifically, this scheme limits C1.8~2.3%, which is convenient for forming graphite balls with an appropriate number, uniform size and high roundness. The graphite balls are evenly distributed in the matrix, which plays a role in strengthening and toughening. Limiting S1wei to 1.3~2.2% can effectively inhibit the formation of cementite and promote the transformation of carbon atoms into graphite balls, thereby increasing the number of graphite balls and improving their roundness. The applicant has found through long-term experiments that if the C content is too much, it is easy to cause an excessive number of graphite balls or graphite coarsening. Both situations will weaken the continuity of the matrix, reduce the bearing capacity of the matrix, and reduce the strength and toughness of ductile iron, thereby reducing the compressive strength and elastic modulus. When the silicon content exceeds 2.2%, too much ferrite will appear in the matrix of ductile iron, reducing the proportion of pearlite and reducing the strength and hardness of the material, resulting in a decrease in compressive strength and elastic modulus. If the silicon content is less than 1.3%, the graphitization effect is weakened, cementite is easily formed, resulting in a decrease in the number of graphite balls and uneven size, which reduces the toughness of the material and becomes a stress concentration source when subjected to force, reducing the compressive strength and elastic modulus.

[0013] 3. This scheme can further remove impurities and refine grains by limiting manganese (Mn), antimony (Sb) and bismuth (Bi), and further improve the elastic modulus of the material. Specifically, manganese can effectively remove harmful impurities (deoxidation and desulfurization) in molten iron and improve the purity of molten iron. And an appropriate amount of manganese can also dissolve in ferrite and pearlite, play a role in solid solution strengthening, and improve the strength and hardness of the matrix. Antimony has the functions of promoting graphitization, refining grains, and increasing pearlite content in ductile iron, effectively improving the morphology and distribution of graphite balls, making the matrix structure more uniform and dense, which is conducive to uniform deformation of the material when subjected to force and improving the elastic modulus. Bismuth can promote the stability of pearlite matrix structure, improve the wear resistance and corrosion resistance of materials, and has positive significance for some ductile iron products that need to be used in wear or corrosive environments. The applicant found through long-term experiments that if the antimony content is too high (> 0.03%), it may cause the graphite morphology to deteriorate, fragmented graphite to appear, and defects in the casting process. If the antimony content is too low, its effects of refining grains and promoting graphitization will be weakened. If the bismuth content is too high, it may cause too many hard and brittle phases to appear in the matrix 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, its effects of promoting pearlite formation and improving elastic modulus will be weakened.

[0014] 4. By limiting the component ratio in the casting blank, this solution facilitates sampling and testing after melting on the one hand, and also enables precise control of the material composition and properties on the other hand. Specifically, this solution samples and tests the components after melting to check if they meet the requirements. If not, the corresponding raw materials can be added during the melting stage to adjust the components, and the ingot is cast only after ensuring that the components meet the requirements, effectively guaranteeing that the components in the casting blank meet the requirements. At the same time, the matrix structure of the casting blank can also be detected during the testing process, laying a foundation for the preparation of ductile iron materials with high elastic modulus.

[0015] 5. By setting two annealing processes for the ductile iron casting blank of this solution, it is convenient to adjust the annealing temperature according to the material strength requirements while meeting the high elastic modulus of the material, obtaining ductile iron materials with high elastic modulus required in various fields and further expanding the application range of the materials. For example, when producing high-strength parts, high annealing temperature treatment is required. For products such as crankshafts that require pearlite, the annealing temperature in the second stage is controlled higher; while when producing low-strength parts, low annealing temperature treatment can be selected. For example, for some chassis parts that require ferrite matrix, the annealing temperature in the second stage can be selected for relatively low-temperature treatment.

[0016] Preferably, as an improvement, the raw materials include low-carbon and low-manganese scrap steel, carburizer, ferrosilicon, ferromanganese alloy, antimony-iron alloy, bismuth-iron alloy and spheroidizing agent.

[0017] Technical effect: With the above settings in this solution, it is convenient to obtain a casting blank matrix mainly composed of ledeburite at low temperature. Among them, by using low-carbon and low-manganese scrap steel, not only the processing waste is effectively recycled, but also it is convenient to add other additives to ensure the material properties. Moreover, compared with other mixed raw materials, the composition of pure scrap steel is relatively simple, which can reduce the introduction of harmful elements such as sulfur and phosphorus, ensuring that the phosphorus content is as low as the excellent steel standard.

[0018] Preferably, as an improvement, in step one, the casting blank stage includes two-stage melting, and the steps are as follows:

[0019] S1-1. Primary melting stage: Melting with pure scrap steel, adding carburizer, ferrosilicon, alloy and spheroidizing agent to form spherical graphite;

[0020] S1-2. Intermediate frequency furnace melting stage, the melting temperature is 1530 - 1600 °C, and the casting temperature > 1420 °C to obtain the casting blank.

[0021] Technical effect: With the above settings in this solution, it is convenient to obtain a matrix structure material mainly composed of ledeburite at low temperature. At this time, the material has high strength and high hardness (measured above 400 HB), but insufficient toughness, so annealing heat treatment is required.

[0022] Preferably, as an improvement, the addition amount of the alloy is 2-2.2% of the total mass of the raw materials, and the spheroidizing agent is any one of rare earth ferrosilicon magnesium alloy spheroidizing agent and pearlite spheroidizing agent; the addition amount of the spheroidizing agent is 1.0-1.2% of the total mass of the raw materials.

[0023] Technical effect: With the above settings in this solution, it is convenient to effectively improve the properties of ductile iron. Specifically, ferromanganese alloy can improve the strength and hardness of cast iron and enhance its wear resistance; antimony iron alloy and bismuth iron alloy can refine grains, improve the morphology and distribution of graphite balls, improve the toughness and density of cast iron, and make ductile iron have better comprehensive mechanical properties.

[0024] Preferably, as an improvement, the billet comprises a mixed matrix tissue material of hypoeutectic ledeburite, spherical graphite, pearlite and cementite, and the proportion of ledeburite in the matrix tissue is 15-20%.

[0025] Technical effect: The billet in this solution is a mixed matrix tissue material mainly composed of hypoeutectic ledeburite. The combination of hypoeutectic ledeburite matrix and spherical graphite can provide high strength and hardness, thereby improving the compressive strength and elastic modulus of ductile iron. The applicant found through long-term experiments that if there is too much pearlite in the billet, it will increase the brittleness of the material, reduce the toughness, easily cause crack propagation when under pressure, and reduce the compressive strength. The change of the structure will also affect the uniformity of the material during elastic deformation, resulting in fluctuations in the elastic modulus and unable to meet the performance requirements of the product.

[0026] Preferably, as an improvement, in step one, the first-stage annealing is that the billet is heated in the furnace to 880-980 °C and kept warm for 15-36 h.

[0027] Technical effect: With the above settings in this solution, it is convenient to promote the full decomposition of cementite in ductile iron. Through long-term experiments, the applicant found that if the annealing temperature in the first stage exceeds 980 °C, the austenite grains in the ductile iron structure will grow rapidly, and coarse ferrite or pearlite matrix structures will be formed during the subsequent cooling process. When stressed, cracks are likely to propagate, reducing the material's ability to resist deformation and resulting in a decrease in both compressive strength and elastic modulus. If the annealing temperature in the first stage is lower than 880 °C, the cementite decomposition is insufficient. It is not only difficult to form a sufficient number of graphite balls with uniform sizes, but the undecomposed cementite will exist as a hard and brittle phase in the matrix, reducing the material's toughness. At the same time, it becomes a stress concentration source when stressed, which is not conducive to the improvement of elastic modulus and compressive strength. If the holding time exceeds 36 h, although the cementite decomposition may be more complete, other problems will arise, such as excessive growth of graphite balls, occupying a large space, weakening the matrix's load-bearing capacity and deformation coordination ability, and instead reducing the material's elastic modulus, strength, and toughness. In addition, high-temperature holding for too long may also lead to energy waste and reduced production efficiency, increasing production costs. If the holding time is less than 15 h, the cementite fails to be fully transformed into graphite balls but exists as a hard and brittle phase, which is prone to crack initiation when stressed, reducing the material's compressive strength. At the same time, due to the non-uniformity of the structure, the elastic modulus is also difficult to reach the ideal value.

[0028] Preferably, as an improvement, in step one, the second-stage annealing is to cool down to 650 - 860 °C and hold for 3 - 5 h; the cooling rate is 40 - 60 °C / h.

[0029] Technical effect: With the above settings, this solution facilitates improving the elastic modulus of the material while obtaining components with different strength requirements by adjusting the annealing temperature. Specifically, when producing high-strength components, a high annealing temperature is required for treatment. For example, for crankshaft products that require pearlite, the annealing temperature in the second stage is selected as 860°C. When producing low-strength components, a low annealing temperature can be selected for treatment. For example, for some chassis parts that require a ferrite matrix, the annealing temperature in the second stage can be selected as 650°C. Through long-term experiments, the applicant found that if the annealing temperature in the second stage is higher than 880°C, the transformation rate of austenite to ferrite will be too fast, resulting in uneven formation of ferrite, and massive or coarse ferrite structures may appear, thereby reducing the strength, toughness, and elastic modulus of the material. If the temperature is cooled to below 650°C, the graphitization process will slow down significantly, 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 brittle or unstable phases will reduce the toughness and elastic modulus of the material and easily cause crack propagation when stressed, reducing the compressive strength of ductile iron. If the holding time exceeds 5h, although the graphite balls may continue to grow, there may be a phenomenon of excessive growth and agglomeration of graphite balls, which will instead reduce the compressive strength and elastic modulus of ductile iron. If the holding time is less than 3h, the tissue transformation is insufficient, the growth and roundness of graphite balls are insufficient, and the transformation of austenite to ferrite is incomplete, which will also reduce the compressive strength and elastic modulus.

[0030] Preferably, as an improvement, the elastic modulus of the high-elastic-modulus ductile iron material is 210 - 240 GPa; the tensile strength is 800 - 950 MPa, the elongation is 3 - 4%, and the yield strength is 500 - 650 MPa.

[0031] Technical effect: The elastic modulus reflects the ability of the material to resist elastic deformation. The high elastic modulus ductile iron material of this scheme means that when subjected to external force, the elastic deformation produced is small, and it can maintain good shape and dimensional stability under long-term load or vibration conditions, provide reliable rigidity support for the structure, ensure the safety, seismic performance, processing accuracy and service life of the structure, and ensure the reliable operation of the equipment. The higher tensile strength, yield strength and appropriate elongation can enable the material of this scheme to replace high-performance alloy steel in some fields, significantly reduce the production cost of the product, and improve the market competitiveness of the product. Specifically, the high elastic modulus ductile iron material of this scheme has both high tensile strength and yield strength, so that it can withstand large external forces and loads, and can be used to manufacture large building structural parts, bridge support components, etc., effectively broadening the application scope of ductile iron in the field of engineering structures. This scheme can also meet the strength requirements of mechanical parts (such as crankshafts, gears, etc.) under high-speed and high-load operation conditions, and the elongation enables it to withstand a certain degree of impact and vibration, reducing the possibility of parts failure due to fatigue, which helps to broaden the application of ductile iron in the high-end field of mechanical manufacturing.

[0032] Preferably, as an improvement, the density of the high elastic modulus ductile iron material is 7.23-7.26 g / cm 3 Ductile iron material has a pearlite matrix structure, the volume fraction of pearlite is 98-99%, and the remainder is cementite and trace ferrite.

[0033] 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.

[0034] Preferably, as an improvement, the present solution also provides an application of a high elastic modulus ductile iron material, including application of the above-mentioned high elastic modulus ductile iron material in the preparation of engine crankshafts, steering knuckles, cutting tools, and machine tool beds.

[0035] Technical effects: The high elastic modulus ductile iron in this solution can ensure the reliability of the key components of the equipment. Its appropriate elongation rate makes it not easy to break when impacted. At the same time, its density is smaller than that of alloy steel, which is conducive to the lightweight of the equipment, thus being more widely used in related industries and broadening the material selection range of equipment parts. In the specific replacement and use process, the high elastic modulus ductile iron material in this solution also has the following advantages:

[0036] (1) Cost advantage: The production process of alloy steel is usually more complex, requiring the addition of various precious metal elements, resulting in higher production costs. While the main elements of ductile iron, such as iron, carbon, and silicon, are abundant in nature, with wide sources and low costs; 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 improve the market competitiveness of products.

[0037] (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 procedures and machining time and improve production efficiency. At the same time, ductile iron has good casting performance and can manufacture parts with complex shapes, reducing the subsequent machining amount and further reducing the production cost and machining difficulty.

[0038] (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 and extend its service life. Therefore, in these fields, it can replace alloy steels with poor corrosion resistance, reducing the maintenance cost and replacement frequency of equipment. 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 from experimental group 2 in Example 1 of the present invention.

[0040] Figure 2 It is the partial metallographic structure diagram of the ductile iron material obtained from experimental group 2 in Example 1 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 from experimental group 6 in Example 1 of the present invention under 100 - fold magnification (showing the influence of too high annealing temperature in the first stage).

[0042] Figure 4 This is a partial metallographic structure diagram of the matrix of the ductile iron material obtained in Experimental Group 7 in Example 1 of the present invention under a 100-fold microscope (showing the influence of too low annealing temperature in the first stage. The bright network in the figure is ferrite).

[0043] Figure 5 This is a partial metallographic structure diagram of the matrix structure of the ductile iron material obtained in Experimental Group 11 in Example 1 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. 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] Example 1

[0046] The present solution provides a preparation method for a high elastic modulus ductile iron material, including the following steps:

[0047] Step 1: Melting and casting the raw material components into a billet, including the following contents:

[0048] S1-1, Primary melting stage: Using pure scrap steel for melting, adding graphitized petroleum coke for carbon addition, silicon carbide for silicon addition, ferromanganese alloy, antimony iron alloy, bismuth iron alloy and spheroidizing agent to form spherical graphite; during the process, 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 ferrosilicon 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 high elastic modulus ductile iron material includes the following components in mass percentages: C 1.8-2.3%, Si 1.3-2.2%, Mn 0.3-0.7%, Bi 0.002-0.005%, Sb 0.002-0.01%, P < 0.06%, S < 0.2%, and the balance is Fe. Specifically, after melting in this example, the high elastic modulus ductile iron material includes the following components in mass percentages: C 1.8%, Si 1.4%, Mn 0.35%, Bi 0.004%, Sb 0.004%, P < 0.06%, S < 0.2%, and the balance is Fe.

[0049] As a reference, this embodiment specifically is: The pure scrap steel melting adopted uses low-carbon and low-manganese scrap steel (cold-rolled thin plates pressed into blocks). The composition of this material is relatively single, which can reduce the introduction of harmful elements such as sulfur and phosphorus. It has been widely used in medium-frequency furnace casting at present and will not be elaborated here; then use graphitized petroleum coke (carburizer) with a carbon content of about 99% and an absorption rate of 92% - 94% for carburization. After melting, the carbon content is 1.8%; use silicon carbide with a silicon content of about 58% and an absorption rate of 90% for silicon addition. After melting, the silicon content is 1.4%; through calculation, the carbon equivalent CE value is 2.3, and the Si / C ratio is 0.83 (the optional range of the carbon equivalent CE value is 2.2 - 2.8, and the optional range of Si / C is 0.7 - 1); use ferromanganese with a manganese content of about 65% for manganese addition, and the total manganese content is 0.35%; use antimony iron and bismuth iron with an alloy content of 99%, and the final content is controlled at 0.004%.

[0050] S1-2. In the medium-frequency furnace melting stage, the melting temperature is 1530 - 1600 °C, and the casting temperature > 1420 °C to obtain a casting blank. The casting blank includes a mixed matrix structure material of ledeburite, spherical graphite, pearlite, and cementite. The proportion of ledeburite in the matrix structure is 15 - 20%. The metallographic structure of the obtained casting blank is as Figure 1 shown, showing a mixed matrix structure of ledeburite, graphite, pearlite, and ferrite as the matrix.

[0051] Step two: Anneal the casting blank to obtain ductile iron material, including the following content:

[0052] S2-1: The first-stage annealing and high-temperature decomposition: Heat up in the furnace to 880 - 980 °C and keep warm for 15 - 36 h. The holding time is specifically determined according to the thickness of the product and the added alloying elements.

[0053] S2-2: The second-stage annealing and low-temperature graphitization: Cool down to 650 - 860 °C, and the cooling rate is 40 - 60 °C / h. After keeping warm for 3 - 5 h, quickly take out of the furnace and air-cool.

[0054] Step three: Surface treatment: Shot blast to remove the oxide scale to obtain ductile iron material.

[0055] This solution also includes a high elastic modulus ductile iron material prepared by the above method. After testing, the high elastic modulus ductile iron material of this solution has a pearlite matrix structure. The volume fraction of pearlite is 98 - 99%, and the balance is cementite and ferrite. The elastic modulus is 210 - 240 GPa. The nodule size grade of the material is 6 - 7 levels, the spheroidization grade is 2 levels, and the density is 7.23 - 7.26 g / cm 3 . Other properties are as follows: The tensile strength is 800 - 950 MPa, the elongation is 3 - 4%, the yield strength is 500 - 650 MPa, and the hardness is 280 - 300 HB.

[0056] Experimental Example 1: Influence of Parameters in Two Annealing Stages of the Preparation Method on Material Properties

[0057] In this solution, multiple experimental groups were set for parameters such as temperature and time in the two annealing stages. The specific parameter combinations in each experimental group for the annealing stage are shown in Table 1.

[0058] Table 1 Design of Process Parameters in the Annealing Method

[0059]

[0060] The raw materials of each experimental group were 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. The metallographic structure of the ductile iron material prepared in Experimental Group 2 is as Figure 2 shown, and the metallographic structure of the ductile iron material prepared in Experimental Group 6 (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 Experimental Group 7 (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 Experimental Group 11 (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.

[0061] Table 2 Properties of Ductile Iron Materials

[0062]

[0063]

[0064] Experimental data show that this solution significantly improves the comprehensive properties of the prepared ductile iron materials, especially the elastic modulus and tensile strength, by optimizing the parameters in the annealing process.

[0065] Specifically, the excessively high melting temperature (such as 1620 °C) and casting temperature (such as 1532 °C) in experimental group 4 will disrupt the graphitization process, resulting in thick and distorted graphite, inhibiting graphite precipitation, and causing coarse grains, thereby reducing the performance. In experimental group 5, the excessively low melting temperature (such as 1514 °C) and casting temperature (such as 1371 °C) lead to poor fluidity of the molten iron, making it difficult to form products. The alloying elements in the molten iron cannot be fully melted and diffused insufficiently, resulting in uneven graphite distribution and unstable matrix structure. In experimental group 6, the excessively high first-stage annealing temperature (such as 1000 °C) causes the graphite in the casting to dissolve into austenite and precipitate as carbides during cooling, increasing the brittleness of the material. At the same time, the surface oxidation is obvious. As Figure 3 shown, the graphite at the material edge is oxidized and decomposed due to high temperature, and the undecomposed graphite is also too thick and unevenly distributed, seriously affecting its performance. In experimental group 7, 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 carbides in the casting cannot be completely decomposed (as Figure 4 shown), and the residual carbides increase the brittleness of the casting. In experimental group 8, the excessively fast cooling rate (such as 100 °C / h) leads to the formation of abnormal phases, such as retained tempered martensite, increasing brittleness. At the same time, it will inhibit the normal phase transformation, resulting in abnormal grain structure. In experimental group 9, 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 the toughness. In experimental group 10, the excessively high second-stage annealing temperature (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 experimental group 11, the excessively low second-stage annealing temperature (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 incompletely transformed austenite remaining in the matrix. The presence of these hard and brittle phases or unstable phases will reduce the toughness of the material and easily cause crack propagation when stressed, 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, and the elastic modulus cannot reach the optimal value.

[0066] Experimental Example 2: Influence of Component Contents in Cast Blanks on the Properties of High-Elasticity-Modulus Ductile Iron Materials

[0067] This solution sets up multiple experimental groups according to the content of each component in the cast billet to explore the influence of the dosage of each component on the properties of high elastic modulus ductile iron materials. The component formulations in each experimental group are shown in Table 3.

[0068] Table 3 Component formulations in each experimental group

[0069]

[0070] The above experimental groups and Comparative Example 1 both selected the parameter conditions of each annealing stage in Experimental Group 2 of Example 1 to prepare ductile iron materials, and referred to the standards GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", GB / T 22315-2008 "Test methods for elastic modulus and Poisson's ratio of metallic materials", and GB / T 9441-2021 "Metallographic examination of ductile iron" to detect the properties of the obtained ductile iron materials. The results are recorded in Table 4.

[0071] Table 4 Properties of high elastic modulus ductile iron materials

[0072]

[0073]

[0074] Experimental data show that this solution effectively improves the overall performance of the material by limiting the composition of each element in the ductile iron material. Specifically, excessive carbon-silicon equivalent (such as 3.5 or 3) in Experimental Group 15 and Experimental Group 17 will cause abnormal graphite morphology, such as graphite blooming and floating, and at the same time inhibit the improvement of product strength; too low carbon-silicon equivalent (such as 2.1) in Experimental Group 16 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 Experimental Groups 12-14 can just avoid the occurrence of the above problems. And too high P content (such as 0.1%) in Experimental Group 21 will form hard and brittle phosphide eutectic in ductile iron, distributing at the grain boundaries to form crack sources. Although it can be decomposed through subsequent heat treatment, there will still be residues, which have a great impact on product performance. And too high sulfur content (such as 0.25%) in Experimental Group 22 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 Experimental Groups 12-14 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 0.0055% in Experimental Group 23 and Sb content 0.012% in Experimental Group 25) 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 no addition of Bi in Experimental Group 24 and no addition of Sb in Experimental Group 26), it will also significantly reduce the comprehensive performance of the material.

[0075] In particular, for Comparative Example 1, if the raw material composition in the prior art "CN115074609A Ductile Iron with Low Residual Stress and High Elastic Modulus and Its Application" is used, even if the annealing process in this solution is adopted, 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 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 of each stage in the optimized annealing method of 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.

[0076] This solution also provides an application of a ductile iron material with high elastic modulus in the preparation of engine crankshafts, steering knuckles, cutting tools, and machine tool beds.

[0077] Experimental Example 3: Application of Ductile Iron Material with High Elastic Modulus in the Preparation of Engine Crankshafts

[0078] This embodiment provides an application of a ductile iron material with high elastic modulus in the preparation of engine crankshafts, that is, after melting, casting, heat treatment, and surface treatment of the ductile iron material in Experimental Group 3, an engine crankshaft is obtained. Specifically, taking the production of Chery F4J20 crankshaft (2.0T new energy engine) and Changan HE15 crankshaft (1.5T new energy engine) as examples for illustration, the performance test results are shown in Table 5.

[0079] Table 5 Crankshaft Performance

[0080] Test Items Performance Elastic Modulus ≥200 GPa Tensile Strength ≥850 MPa Yield Strength ≥520 MPa Elongation ≥3.5％ Hardness 260 - 300 HB Impact Energy Absorbed 50J

[0081] And it has passed the 1150 N·m fatigue test, which can greatly improve the durability and anti-fatigue performance of parts, can better suppress the noise problem caused by high vibration, and at the same time has a greater expansion space in terms of lightweight.

[0082] Experimental Example 4: Application of Ductile Iron Material with High Elastic Modulus in the Preparation of Steering Knuckles

[0083] This embodiment provides an application of a ductile iron material with high elastic modulus in the preparation of steering knuckles, that is, after melting, casting, heat treatment, and surface treatment of the ductile iron material in Experimental Group 1, a steering knuckle is obtained. Specifically, taking the production of a steering knuckle part of a well-known main engine factory as an example for illustration, the performance test results are shown in Table 6.

[0084] Table 6 Steering Knuckle Performance

[0085] Test Items Performance Elastic Modulus ≥200 GPa Tensile Strength ≥850 MPa Yield Strength ≥520 MPa Elongation ≥3.5％ Hardness 260 - 300 HB Impact Energy Absorbed 50J

[0086] Experimental data show that the significant synchronous improvement of the tensile strength and elastic modulus of the material has far-reaching effects 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 can be selected in the second stage to meet the requirements of its strength and elastic modulus.

[0087] Experimental Example 5: Application of Ductile Iron Material with High Elastic Modulus in the Preparation of Machine Tool Beds

[0088] This embodiment provides an application of a ductile iron material with high elastic modulus in the preparation of machine tool beds, that is, the ductile iron material in Experimental Group 2 is used to produce machine tool beds. Specifically, taking the production of the base components of a well-known domestic machine tool manufacturing company as an example, the performance test results are shown in Table 7.

[0089] Table 7 Performance of Machine Tool Beds

[0090] Test Items Performance Elastic Modulus ≥200 GPa Tensile Strength ≥850 MPa Yield Strength ≥520 MPa Elongation ≥3.5％ Hardness 260 - 300 HB Impact Energy Absorbed 50J

[0091] Experimental data show that materials with high elastic modulus deform less under the same stress, can effectively resist elastic deformation caused by cutting force, gravity, etc., ensure the stability of the geometric accuracy of the machine tool, and avoid machining errors caused by the deformation of the bed. The 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 stronger creep resistance, are not easy to relax under long-term load, and extend the precision life.

[0092] 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 deformations 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. Preparation method of ductile iron material with high elastic modulus, characterized in that: The high elastic modulus ductile iron material comprises the following components in mass percentage: C 1.8 - 2.3%, Si 1.3 - 2.2%, Mn 0.3 - 0.7%, Bi 0.002 - 0.005%, Sb 0.002 - 0.01%, P < 0.06%, S < 0.2%, and the balance is Fe; the preparation method comprises the following steps: Step 1, billet stage: melting the raw materials and casting them into a billet, and the billet contains the components in the above mass percentage; Step 2, annealing stage: annealing in the first stage and the second stage in sequence, and then quickly taking out of the furnace and air-cooling; Step 3, surface treatment stage: removing the oxide scale by shot blasting to obtain the high elastic modulus ductile iron material.

2. The preparation method of the ductile cast iron material with high elastic modulus according to claim 1, characterized in that: The raw materials include low-carbon and low-manganese scrap steel, carburizer, ferrosilicon, ferromanganese alloy, antimony-iron alloy, bismuth-iron alloy and spheroidizing agent.

3. The preparation method of the ductile iron material with high elastic modulus according to claim 2, characterized in that: In Step 1, the billet stage includes two-stage melting, and the steps are as follows: S1-1, primary melting stage: melting with pure scrap steel, adding carburizer, ferrosilicon, alloy and spheroidizing agent to form spherical graphite; S1-2, intermediate frequency furnace melting stage, the melting temperature is 1530 - 1600 °C, and the casting temperature > 1420 °C to obtain a billet.

4. The preparation method of the ductile cast iron material with high elastic modulus according to claim 3, characterized in that: The addition amount of the alloy is 2 - 2.2% of the total mass of the raw materials, and the spheroidizing agent is any one of rare earth ferrosilicon magnesium alloy spheroidizing agent and pearlite spheroidizing agent; the addition amount of the spheroidizing agent is 1.0 - 1.2% of the total mass of the raw materials.

5. The preparation method of the ductile iron material with high elastic modulus according to claim 3, characterized in that: The billet includes a mixed matrix tissue material of hypereutectic ledeburite, spherical graphite, pearlite and cementite, and the proportion of ledeburite in the matrix tissue is 15 - 20%.

6. The preparation method of the ductile iron material with high elastic modulus according to claim 1, characterized in that: In Step 1, the first stage annealing is that the billet is heated in the furnace to 880 - 980 °C and kept warm for 15 - 36 h.

7. The preparation method of the ductile iron material with high elastic modulus according to claim 1, characterized in that: In Step 1, the second stage annealing is to cool down to 650 - 860 °C and keep warm for 3 - 5 h; the cooling rate is 40 - 60 °C / h.

8. The preparation method of the ductile iron material with high elastic modulus according to claim 1, characterized in that: The elastic modulus of the high elastic modulus ductile iron material is 210 - 240 GPa; the tensile strength is 800 - 950 MPa, the elongation is 3 - 4%, and the yield strength is 500 - 650 MPa.

9. The preparation method of the ductile cast iron material with high elastic modulus according to claim 1, characterized in that: The density of the high elastic modulus ductile iron material is 7.23 to 7.26 g / cm 3 , and the ductile iron material has a pearlite matrix structure. The volume fraction of pearlite is 98 to 99%, and the balance is cementite and trace ferrite.

10. Application of ductile iron material with high elastic modulus, characterized in that: The application of the high elastic modulus ductile iron material prepared by the method according to any one of claims 1 - 9 in the preparation of engine crankshafts, steering knuckles, cutting tools and machine tool beds.

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