Silicon-molybdenum-vanadium nodular cast iron material as well as preparation method and application thereof

Abstract

The invention relates to a silicon-molybdenum-vanadium nodular cast iron material and a preparation method and application thereof. The silicon-molybdenum-vanadium nodular cast iron material comprises the following components in percentage by mass: 2.9-3.4% of C; 3.9% to 4.3% of Si; mn: 0.1%-0.3%; less than or equal to 0.05% of P; s: less than or equal to 0.02%; 0.03% to 0.055% of Mg; 0.9% to 1.15% of Mo; 0.25% to 0.45% of V; cr: 0.1% to 0.25%; 0.008%-0.02% of RE (rare earth); and the balance of Fe and inevitable impurities. The tensile strength of the material at the temperature of 800 DEG C is 60-65 MPa, the ductility of the material at the temperature of 800 DEG C is 30%-45%, the linear expansion coefficient of the material at the temperature of 800 DEG C is 16-19 micrometers / (m.K), the material is low in cost, and the risk that the exhaust manifold of the heavy-duty engine loses efficacy and cracks under extreme conditions can be reduced.

Description

Technical Field

[0001] The present invention relates to the field of high-temperature resistant alloys for automobiles, and in particular to a silicon-molybdenum-vanadium ductile iron material and a preparation method and application thereof. Background Art

[0002] The operating environment of the exhaust manifold is very harsh. It not only serves as a load-bearing component, bearing the impact loads caused by the supercharger system, intake and exhaust pipes, etc.; it also serves as a sealing pressure-bearing component, withstanding the high pressure of the gas to ensure the sealing characteristics of the entire system; and it also has to bear the risk of thermal engine fatigue failure caused by the high-temperature working environment.

[0003] High-explosion-pressure diesel engines, gas engines, and hydrogen-ammonia engines are becoming key technologies for commercial vehicle engines. The temperature of engine exhaust manifolds will gradually rise to around 800°C. Traditional exhaust manifold materials often struggle to withstand such high temperatures and complex operating conditions, and are prone to deformation, cracking, and fracture. Therefore, researching and developing exhaust manifold materials with excellent high-temperature resistance, high strength and toughness, and good thermal fatigue resistance has become crucial.

[0004] Currently, the main materials used for heavy-duty engine exhaust manifolds include silicon-molybdenum ductile iron, high-nickel austenitic ductile iron, and heat-resistant cast steel. Silicon-molybdenum heat-resistant ductile iron, due to its excellent high-temperature strength, thermal fatigue resistance, excellent oxidation resistance, growth resistance, and good high-temperature creep resistance, has become a preferred material for automotive exhaust manifolds. However, the heat-resistant temperature of silicon-molybdenum heat-resistant ductile iron is below 760°C, making it difficult to meet the high-temperature material requirements of future commercial vehicle engines. High-nickel austenitic ductile iron and heat-resistant cast steel have higher heat-resistant temperatures and can meet the performance requirements of future commercial vehicle engines, but their high cost makes them unsuitable for heavy-duty engine exhaust manifolds. Therefore, developing a lower-cost silicon-molybdenum ductile iron material with improved heat resistance, high-temperature strength, thermal fatigue resistance, and oxidation resistance, and increasing its heat resistance from the current 760°C to 800°C and above, is a current research priority. Summary of the Invention

[0005] The present invention provides a silicon-molybdenum-vanadium ductile iron material and a preparation method and application thereof, aiming to reduce the risk of deformation, cracking and fracture of heavy-duty engine exhaust manifolds at temperatures above 800°C at a low cost.

[0006] The technical solutions provided by the present invention are as follows: In a first aspect, the present invention provides a silicon molybdenum vanadium ductile iron material, which has the following composition and mass percentages: C: 2.9% - 3.4%; Si: 3.9% - 4.3%; Mn: 0.1% - 0.3%; P: ≤0.05%; S: ≤0.02%; Mg: 0.03% - 0.055%; Mo: 0.9% - 1.15%; V: 0.25% - 0.45%; Cr: 0.1% - 0.25%; RE: 0.008% - 0.02%; the balance is Fe and unavoidable impurities.

[0007] Through the combined action of adding vanadium (V), chromium (Cr) and rare earth elements (RE), the high-temperature strength of the matrix material is significantly improved, ensuring that its strength at 800°C is ≥60 MPa, and reducing the risk of deformation, cracking and fracture of the heavy-duty engine exhaust manifold under extreme conditions.

[0008] Combined with the first aspect of the present invention, in some embodiments, the pearlite content of the silicon molybdenum vanadium ductile iron material is not higher than 20%.

[0009] Although increasing the pearlite content can improve the high-temperature strength of the silicon molybdenum ductile iron material, pearlite will melt at 700°C. When the pearlite content is higher than 20%, the material size becomes larger, resulting in greater thermal stress and increasing the probability of material failure and cracking. Therefore, by controlling the pearlite content below 20%, the present invention aims to maintain good thermal fatigue performance of the material, ensuring that the exhaust manifold has a longer service life and higher safety under high temperature and dynamic stress.

[0010] Combined with the first aspect of the present invention, in some embodiments, the tensile strength of the silicon molybdenum vanadium ductile iron exhaust manifold material at 800°C is 60 - 65 MPa, the elongation at 800°C is 30% - 45%, and the linear expansion coefficient at 800°C is 16 - 19 μm / (m·K).

[0011] The high strength and low linear expansion coefficient of this material at 800°C ensure that the exhaust manifold can withstand greater stress without cracking when stressed at 800°C. At the same time, when the temperature changes, the thermal expansion or contraction is small, and the thermal stress is low. Therefore, the risk of deformation, cracking and fracture is low.

[0012] In combination with the first aspect of the present invention, in some embodiments, the silicon molybdenum vanadium ductile iron material has the following composition and mass percentages: C: 2.9% - 3.4%; Si: 3.9% - 4.3%; Mn: 0.15% - 0.25%; P: ≤0.03%; S: ≤0.015%; Mg: 0.04% - 0.05%; Mo: 0.9% - 1.10%; V: 0.25% - 0.40%; Cr: 0.1% - 0.25%; RE: 0.010% - 0.012%; the balance is Fe and unavoidable impurities.

[0013] In a second aspect, the present invention provides a method for manufacturing a silicon molybdenum vanadium ductile iron part, including: Melting the raw materials into molten iron; Pouring the molten iron into a nodulizing ladle for nodulizing and inoculating, and then pouring it into a part mold, while performing in-stream inoculation synchronously; Cooling the part mold to obtain a part made of the silicon molybdenum vanadium ductile iron material of the first aspect.

[0014] In combination with the second aspect of the present invention, in some embodiments, the tapping temperature of the molten iron is 1510°C - 1540°C.

[0015] In combination with the second aspect of the present invention, in some embodiments, an Fe-Si-Mg-Ca nodulizer is placed at the bottom of the nodulizing ladle, and a Si-Ba inoculant and metallic bismuth (Bi) are covered above the Fe-Si-Mg-Ca nodulizer. Compared with common inoculants, the inoculation treatment with the Si-Ba inoculant and a trace amount of Bi element can increase the high-temperature strength of the material and reduce the linear expansion coefficient.

[0016] In combination with the second aspect of the present invention, in some embodiments, the temperature of the molten iron poured into the part mold is 1370°C - 1430°C.

[0017] In combination with the second aspect of the present invention, in some embodiments, the in-stream inoculant used for in-stream inoculation is a ferrosilicon inoculant.

[0018] In a third aspect, the present invention provides an exhaust manifold made of the above-mentioned silicon molybdenum vanadium ductile iron material.

[0019] In a fourth aspect, the present invention provides a vehicle including the above-mentioned exhaust manifold.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects: The silicon molybdenum vanadium ductile iron material provided by the present invention has high tensile strength, high elongation rate and low linear expansion coefficient at 800°C, which can reduce the failure risk of parts at 800°C, improve their stability and durability during high-temperature operation, and is especially suitable for manufacturing exhaust manifolds. Description of the Drawings

[0021] Figure 1 : SEM observation diagram of the matrix structure after the engine bench test of the exhaust manifold material of Comparative Example 1 (currently produced QTRSi4Mo1).

[0022] Figure 2 : EDS surface scanning distribution diagram of Mo element in the matrix structure after the engine bench test of the exhaust manifold material of Comparative Example 1 (currently produced QTRSi4Mo1).

[0023] Figure 3 : SEM observation diagram of the matrix structure after the engine bench test of the exhaust manifold material of Example 1 (the material of the present invention).

[0024] Figure 4 : EDS surface scanning distribution diagram of Mo element in the matrix structure after the engine bench test of the exhaust manifold material of Example 1 (the material of the present invention).

[0025] Figure 5 : EDS surface scanning distribution diagram of V element in the matrix structure after the engine bench test of the exhaust manifold material of Example 1 (the material of the present invention).

[0026] Figure 6 : EDS surface scanning distribution diagram of Cr element in the matrix structure after the engine bench test of the exhaust manifold material of Example 1 (the material of the present invention). Detailed implementation manners

[0027] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0028] The term "silicon molybdenum ductile iron material" of the present invention represents a ductile iron material containing silicon (Si), molybdenum (Mo), and multi-element alloy elements, with its graphite morphology being spherically and regularly distributed and the matrix structure being a composite structure of pearlite and ferrite.

[0029] The silicon molybdenum vanadium ductile iron material provided by the present invention has the following composition and mass percentage: C: 2.9% - 3.4%; Si: 3.9% - 4.3%; Mn: 0.1% - 0.3%; P: ≤0.05%; S: ≤0.02%; Mg: 0.03% - 0.055%; Mo: 0.9% - 1.15%; V: 0.25% - 0.45%; Cr: 0.1% - 0.25%; RE: 0.008% - 0.02%; the balance is Fe and inevitable impurities.

[0030] On the basis of the silicon molybdenum ductile iron material, elements such as V, Cr, and RE are added to transform the fishbone-shaped carbide containing Mo in the traditional silicon molybdenum ductile iron material (see Figure 1 ), into a dispersed massive composite carbide containing Mo, V, and Cr (see Figure 2 ), thereby significantly enhancing the high-temperature strength of the matrix material. Its strength at 800 °C is ≥60 MPa, and it ensures a reduced linear expansion coefficient, reducing the risk of deformation, cracking, and fracture of the heavy-duty engine exhaust manifold under extreme conditions.

[0031] Element V can refine the grain structure of the silicon molybdenum ductile iron material, forming fine and dispersed carbides, which can effectively improve the mechanical properties of the silicon molybdenum ductile iron material at room temperature and high temperature. The addition of V reduces the content of pearlite, inhibits the risk of pearlite increase brought by the addition of Cr, and also improves the thermal fatigue performance of the material at high temperature, enabling it to better maintain its stability and durability under high-temperature cyclic loads.

[0032] The addition amount of V must be strictly controlled. Too little V may not fully exert its strengthening effect, while too much V may cause the material to be too brittle, reducing its toughness and impact resistance. In the present invention, the content of V is controlled within the range of 0.25% - 0.45%. Beyond this content range, it will be difficult to further improve the high-temperature mechanical properties of the material and increase the material cost; below this content range, the pearlite content of the material will exceed the requirement of 20%. The volume expansion occurs during the decomposition of pearlite at high temperature, generating phase transformation stress and affecting the reliability of the exhaust manifold.

[0033] The addition of RE further optimizes the organizational structure of the material, helps promote the formation of graphite balls, eliminates inclusions in the material, and improves the purity of the material. Especially the microalloying effect of the rare earth element RE can not only promote the uniformity inside the material but also help improve its corrosion resistance and thermal shock resistance. In addition, RE can form a stable oxide film at high temperature, effectively preventing the oxidation loss of the material, thereby extending the service life of the exhaust manifold. In the present invention, the content of RE is controlled within the range of 0.008% - 0.02%. By using RE and Mg synergistically and combining appropriate ingredient ratios, pouring, inoculation processes, etc., the purposes of graphite spheroidization, molten iron purification, microalloy strengthening, and improvement of tissue properties can be fully achieved.

[0034] Cr promotes the formation of carbides and pearlite, improves the strength of the material, and enhances its mechanical properties under high-temperature conditions. Cr can also form a dense oxide film on the material surface, effectively resisting high-temperature oxidation erosion, enhancing the oxidation resistance, and extending the service life of the exhaust manifold. In addition, Cr improves the thermal fatigue performance of the material, increases the eutectoid transformation temperature of the material, slows down the crack propagation under high-temperature cycles, and improves its long-term use reliability. Excessive Cr will cause problems such as an increase in pearlite content, a decrease in material toughness, an increase in cold cracking and shrinkage tendencies. In the actual application process, the various effects of Cr should be comprehensively considered to meet the use requirements of the exhaust manifold under high-temperature and complex working conditions.

[0035] Cr can improve the high-temperature strength, and its strengthening effect is higher than that of the Nb element. However, its effect of promoting pearlite formation is also higher than that of the Nb element. When the content of Cr exceeds 0.25%, the elongation of the material will be reduced, and it is difficult to ensure that the pearlite content in the as-cast material is controlled within 20%. In this way, the stress generated by the decomposition of pearlite in the high-temperature environment will offset the increase in high-temperature strength. Therefore, the content range of Cr in the present invention is 0.1% to 0.25%.

[0036] The influence of alloy strengthening on the microstructure and properties of the Si-Mo ductile iron material is mainly reflected in the addition of alloying elements and the control of their contents. These alloying elements include silicon (Si), molybdenum (Mo), vanadium (V), nickel (Ni), niobium (Nb), etc. They affect the microstructure and properties of the Si-Mo ductile iron material through different mechanisms. The functions of other elements in the Si-Mo-V ductile iron exhaust manifold material are described as follows: Si is the main element of the Si-Mo ductile iron exhaust manifold material and has a significant influence on the microstructure and properties. Adding an appropriate amount of Si can form a single ferrite matrix, avoiding the size increase caused by the decomposition of pearlite and resulting in a failure risk. Si can increase the eutectoid transformation temperature, which can be raised to 880 °C, reducing the risk of phase transformation of the material due to excessive temperature in the exhaust manifold and ensuring the dimensional stability of the material microstructure and properties. Si can generate a dense oxide film mainly composed of silicon oxide on the surface of the casting at high temperature. This oxide film can prevent the diffusion of oxygen ions into the casting interior, thereby improving the high-temperature oxidation resistance of the casting. In addition, an appropriate amount of Si can refine graphite and produce a solid solution strengthening effect, improving the hardness and strength of the Si-Mo ductile iron material to a certain extent. However, too high Si content will lead to risks such as silicon embrittlement, graphite fragmentation, and an increase in cold cracking tendency, and it should be controlled within an appropriate range.

[0037] Higher Si can improve the high-temperature strength of the material and reduce the pearlite content. However, when the Si content exceeds a certain range, the toughness and high-temperature strength of the material will be reduced, deteriorating the material properties. In the present invention, the Si content is controlled between 3.9% and 4.3%, ensuring the oxidation resistance of the material and meeting the performance requirements of the material with a high-temperature strength of ≥60 Mpa and an elongation rate of ≥5% at 800 °C.

[0038] Mo can dissolve in ferrite and cementite to form stable special carbides (such as Fe3Mo3C), thereby strengthening the matrix and effectively improving the strength of cast iron, especially the strength at high temperatures. Mo has a high melting point and high thermal stability, and Mo-containing heat-resistant ductile iron has excellent oxidation resistance and creep resistance at high temperatures. In addition, Mo also has a low coefficient of thermal expansion, increasing the thermal conductivity, enabling the heat-resistant ductile iron to maintain good dimensional stability during high-temperature operation and reducing deformation or cracks caused by thermal stress. The addition of Mo has no significant adverse effect on the machining performance of heat-resistant ductile iron. Instead, due to the refinement of the structure and the improvement of performance, the material is more stable during the machining process. The addition amount of Mo should be controlled within a certain range to avoid excessive molybdenum leading to a decline in material performance or an increase in cost. At the same time, the influence of other alloying elements also needs to be comprehensively considered during the production process to ensure that the heat-resistant ductile iron has excellent comprehensive performance.

[0039] Although Mo can increase the high-temperature strength of the material and reduce the linear expansion coefficient of the material, it also reduces the elongation rate of the material. In the present invention, when the Mo content exceeds 1.15%, the carbide shape will have obvious fishbone network characteristics, making it difficult to continue effectively improving the high-temperature strength of the material and improving the linear expansion coefficient. To obtain relatively ideal comprehensive performance, the Mo content in the present invention is controlled within the range of 0.9% to 1.15%.

[0040] The Mg element is a spheroidizing element, which promotes the formation of graphite balls and makes the graphite balls smaller, rounder and more regular. In the present invention, Mg is used in combination with RE and inoculant, and combined with the corresponding casting process, the Mg content is controlled within the range of 0.03% to 0.055%, which can effectively ensure the graphite structure and high-temperature performance.

[0041] The present invention provides a method for manufacturing silicon-molybdenum-vanadium ductile iron parts, including: Melting the raw materials into molten iron; Pouring the molten iron into a spheroidizing ladle for spheroidizing and inoculating, and then pouring it into a part mold, while simultaneously performing in-stream inoculation; Cooling the part mold to obtain parts made of the above-mentioned silicon-molybdenum-vanadium ductile iron material.

[0042] This manufacturing method uses a punching method for spheroidization and inoculation. By strictly controlling the casting process, the Mg content can be controlled within the range of 0.03% to 0.055% and the RE content within the range of 0.008% to 0.020%. This ensures a fine, uniform, and rounded graphite structure, and a uniform distribution of pearlite and carbides. This also reduces the material's linear expansion coefficient, improves its thermal conductivity, and ensures high-temperature performance. Experiments have shown that, under the same chemical composition and spheroidization conditions, the use of a silicon-barium inoculant and a trace amount of Bi metal inoculation improves the material's high-temperature strength, elongation, and thermal conductivity, reduces its linear expansion coefficient, and thus improves its reliability, compared to traditional ferrosilicon inoculants.

[0043] Spheroidizing and inoculating treatments: Selecting the appropriate type and amount of spheroidizing agent and inoculant can significantly improve the morphology, size, and distribution of graphite nodules, promote ferrite formation, and refine grain size. These changes not only enhance the mechanical properties of silicon-molybdenum ductile iron, such as tensile strength and elongation, but also improve the material's linear expansion coefficient and thermal conductivity, enhancing its thermal stability. In some embodiments of the present invention, the spheroidizing agent is an iron-silicon-magnesium-calcium spheroidizer, specifically FeSiMg7RE3Ca2.5, and the bottom inoculant is a mixture of a silicon-barium inoculant and a trace amount of Bi metal. The core component of the silicon-barium inoculant is an alloy of Si and Ba, with a typical ratio of 60% to 75% Si and 2% to 8% Ba, with the remainder being iron (Fe) and trace amounts of other elements (such as Al and Ca).

[0044] In some embodiments of the present invention, raw materials are smelted into molten iron, and the raw materials used include recycled materials, pig iron, scrap steel, etc., and recarburizers, alloys, etc. are used to control the C, Mn, P, S, Mo, V, and Cr elements in the molten iron to meet the target composition requirements. The Si element content is met by the subsequent addition of spheroidizers and inoculants to meet the target requirements. The rare element RE and the spheroidizing element Mg are added through spheroidizers, inoculants, etc.

[0045] In some embodiments of the present invention, the temperature of the molten iron out of the furnace is controlled at 1510°C ~ 1540°C. 1510°C ~ 1540°C is the optimal reaction temperature window of magnesium (Mg) and rare earth (RE). By controlling the temperature of the molten iron out of the furnace within this temperature range, the residual Mg can be controlled at 0.03% ~ 0.06%.

[0046] In some embodiments of the present invention, an Fe-Si-Mg-Ca spheroidizing agent is placed at the bottom of the spheroidizing ladle, and a Si-Ba inoculant is covered above the Fe-Si-Mg-Ca spheroidizing agent. This setting of placing the spheroidizing ladle and the inoculant in layers can release spheroidizing and inoculating elements in stages, reducing the oxidation and burning loss of magnesium. The first step (dissolution of the Si-Ba inoculant): When the molten iron is poured in, it first contacts the upper Si-Ba inoculant, and Si and Ba quickly melt. Si reacts with the oxygen in the molten iron, reducing the oxidizability of the molten pool and creating a low-oxygen environment for the subsequent spheroidizing reaction; Ba promotes heterogeneous nucleation and forms fine eutectic clusters (≤150 / mm²), providing a nucleation substrate for subsequent graphite spheroidization. The second step (reaction of the Fe-Si-Mg-Ca spheroidizing agent): After the molten iron penetrates the Si-Ba layer, it contacts the spheroidizing agent at the bottom, and at this time, magnesium and calcium elements are released. Mg performs spheroidizing strengthening to ensure the roundness of graphite balls; Ca combines with residual impurities (such as Pb and Bi) to form high-melting-point compounds, reducing the brittle phase at grain boundaries and improving toughness.

[0047] In some embodiments of the present invention, the addition amount of the Fe-Si-Mg-Ca spheroidizing agent is 1.2% - 1.5% of the total weight of the molten iron, the addition amount of the Si-Ba inoculant is 0.6% - 0.8% of the total weight of the molten iron, and the addition amount of Bi metal is 0.002% - 0.005% of the total weight of the molten iron.

[0048] In some embodiments of the present invention, the temperature of the molten iron poured into the part mold is 1370°C - 1430°C. 1370°C - 1430°C is the best superheat range for ductile iron (about 150 - 200°C higher than the liquidus), which reduces the viscosity of the molten iron to 2.0 - 3.5 mPa·S, ensuring the complete filling of complex thin-walled structures (such as manifold branches) and avoiding cold shuts and misruns (defect rate < 1%).

[0049] In some embodiments of the present invention, the in-stream inoculant used for in-stream inoculation is an Fe-Si inoculant. The Si content in the Fe-Si inoculant is usually 72% - 75%, which can effectively promote the nucleation and growth of graphite in the molten iron. The Fe-Si inoculant can quickly dissolve in the molten iron at 1370°C - 1430°C and can complete effective inoculation in a short time. In some embodiments of the present invention, the addition amount of the in-stream inoculant is 0.05% - 0.15% of the total weight of the molten iron.

[0050] In some embodiments of the present invention, the method for manufacturing a Si-Mo-V ductile iron part specifically includes the following steps: 1. Melting: Use a melting furnace to melt the molten iron, melt raw materials such as return materials, pig iron, and scrap steel to a molten state, add a carbon increasing agent and alloys according to the chemical composition test results to adjust the chemical composition of the molten iron, and control the tapping temperature within the range of 1510°C - 1540°C.

[0051] 2. Spheroidizing and inoculating treatment: Pour the above molten iron into a spheroidizing ladle. Before pouring, place FeSiMg7RE3Ca2.5 spheroidizing agent accounting for 1.2% - 1.5% of the total weight of the molten iron (where the Mg content is 6% - 8%, the RE content is 2% - 4%, and the Ca content is 1.5% - 3.5%) at the bottom of the spheroidizing ladle. Cover the spheroidizing agent with silicon-barium inoculant accounting for 0.6% - 0.8% of the total weight of the molten iron (where the Ba content is 3.5% - 6.5%) and 0.002% - 0.005% of Bi metal. Then pour the above molten iron into the spheroidizing ladle for spheroidizing and inoculating treatment.

[0052] 3. Pouring: Pour the spheroidized and inoculated molten iron into the exhaust manifold mold. Control the pouring temperature within the range of 1370°C - 1430°C. Conduct in-stream inoculation treatment while pouring, and the in-stream inoculant is ferrosilicon inoculant.

[0053] The technical solution of the present invention will be described in detail through specific embodiments as follows: Example 1 A kind of silicon-molybdenum-vanadium ductile iron material, including the following chemical components by mass percentage: C: 2.98%; Si: 4.26%; Mn: 0.25%; P: 0.023%; S: 0.011%; Mg: 0.043%; Mo: 1.08%; V: 0.39%; Cr: 0.24%; RE: 0.011%; the balance is Fe and unavoidable impurities.

[0054] A preparation method of silicon-molybdenum-vanadium ductile iron includes the following preparation steps: 1. Material preparation: Prepare raw materials such as return scrap, pig iron, and scrap steel to ensure that their chemical components meet the requirements.

[0055] 2. Melting: Add the raw materials into a melting furnace, heat to 1520°C, and add a carbon increasing agent and alloys according to the chemical composition test results to adjust the chemical composition of the molten iron.

[0056] 3. Spheroidizing and inoculating treatment: Pour the molten iron out of the furnace and into a spheroidizing ladle. Lay silicon-barium inoculant (0.70% of the total weight of the molten iron), Bi metal (0.003% of the total weight of the molten iron), and FeSiMg7RE3Ca2.5 spheroidizing agent (1.3% of the total weight of the molten iron) from top to bottom in the spheroidizing ladle for spheroidizing and inoculating treatment.

[0057] 4. Pouring: Pour the treated molten iron into the exhaust manifold mold. Control the pouring temperature at 1400°C. Conduct in-stream inoculation treatment while pouring, and the in-stream inoculant is ferrosilicon inoculant, and the addition amount is 0.10% of the total weight of the molten iron.

[0058] 5. Naturally cool the exhaust manifold mold to room temperature to obtain a silicon-molybdenum-vanadium ductile iron exhaust manifold.

[0059] Example 2 A kind of Si-Mo-V ductile cast iron material, including the following chemical components by mass percentage: C: 3.35%; Si: 3.93%; Mn: 0.18%; P: 0.026%; S: 0.012%; Mg: 0.045%; Mo: 0.95%; V: 0.27%; Cr: 0.11%; RE: 0.010%; the balance is Fe and unavoidable impurities.

[0060] A preparation method of Si-Mo-V ductile cast iron, including the following preparation steps: 1. Material preparation: Prepare raw materials such as return scrap, pig iron and scrap steel to ensure that the elements of C, Mn, P, S, Mo, V, and Cr meet the above mass percentage requirements.

[0061] 2. Melting: Add the raw materials into the melting furnace, heat to 1530 °C, and add carbon increasing agent and alloy according to the chemical composition test results to adjust the chemical composition of the molten iron.

[0062] 3. Spheroidizing and inoculating treatment: Pour the molten iron out of the furnace and into the spheroidizing ladle. Lay barium-silicon inoculant (0.75% of the total weight of the molten iron), Bi metal (0.004% of the total weight of the molten iron), and FeSiMg7RE3Ca2.5 spheroidizing agent (1.4% of the total weight of the molten iron) from top to bottom in the spheroidizing ladle for spheroidizing and inoculating treatment.

[0063] 4. Pouring: Pour the treated molten iron into the exhaust manifold mold, control the pouring temperature at 1420 °C, and carry out in-stream inoculation treatment at the same time. The in-stream inoculant is ferrosilicon inoculant, and the addition amount is 0.08% of the total weight of the molten iron.

[0064] 5. Naturally cool the exhaust manifold mold to room temperature to obtain an Si-Mo-V ductile cast iron exhaust manifold.

[0065] Example 3 A kind of Si-Mo-V ductile cast iron material, including the following chemical components by mass percentage: C: 2.97%; Si: 4.28%; Mn: 0.25%; P: 0.023%; S: 0.010%; Mg: 0.041%; Mo: 1.09%; V: 0.38%; Cr: 0.25%; RE: 0.010%; the balance is Fe and unavoidable impurities.

[0066] A preparation method of Si-Mo-V ductile cast iron, including the following preparation steps: 1. Spheroidizing and inoculating treatment: The molten iron of Example 1 is tapped (tapping temperature 1520 °C) and poured into a spheroidizing ladle. In the spheroidizing ladle, ferrosilicon inoculant (0.70% of the total weight of the molten iron) and FeSiMg7RE3Ca2.5 spheroidizing agent (1.3% of the total weight of the molten iron) are laid from top to bottom for spheroidizing and inoculating treatment.

[0067] 2. Pouring: The treated molten iron is poured into an exhaust manifold mold, and the pouring temperature is controlled at 1400 °C. At the same time, in-stream inoculation treatment is carried out, and the in-stream inoculant is ferrosilicon inoculant, and the addition amount is 0.10% of the total weight of the molten iron.

[0068] 3. The exhaust manifold mold is naturally cooled to room temperature to obtain a silicon-molybdenum-vanadium ductile iron exhaust manifold.

[0069] Comparative Examples 1-7 The preparation methods of Comparative Examples 1-7 are the same as those of Example 1, except for the chemical composition of the exhaust manifold.

[0070] Table 1 Chemical compositions of the exhaust manifolds of each example and comparative example

[0071] Table 2 Performance comparison of silicon-molybdenum-vanadium ductile iron exhaust manifolds of each example and comparative example

[0072] Table 3 Comparison of engine bench tests of silicon-molybdenum-vanadium ductile iron exhaust manifolds of Example 1 and Comparative Example 1

[0073] The chemical composition analysis in Table 1 and the performance comparison results in Table 2 show that the content ratios of key elements such as V, Nb, Ni, Cr, and Si in the comparative example group deviate from the core protection range of the present invention (see Claim 1 for details), directly resulting in a significant weakening of the high-temperature creep resistance and fatigue strength indexes of its exhaust manifold compared with those of Examples 1-3.

[0074] The bottom inoculant of Example 1 and Example 2 is barium-silicon inoculant + trace amount of Bi metal, and the bottom inoculant of Example 3 is traditional ferrosilicon inoculant. Compared with Example 1 and Example 2, the high-temperature strength and room-temperature elongation of the exhaust manifold of Example 3 are reduced, and the linear expansion coefficient increases, but it can still meet the material performance requirements of the exhaust manifold.

[0075] The exhaust manifold of Comparative Example 1 uses traditional QTRSi4Mo1 material, and the high-temperature strength of the material at 800 °C is insufficient, which can pass the rapid thermal fatigue test of the diesel engine. However, the exhaust gas temperature of the gas engine is higher, and failure cracking occurs in the bench test, and its performance is difficult to meet the development requirements of gas engines and other models.

[0076] The exhaust manifold material of Comparative Example 2 added Nb element, and the content of Cr element exceeded the required range. Although Nb and Cr can significantly increase the high-temperature strength of the material, they also significantly increase the pearlite content. The pearlite content of the material far exceeds the required range, and the linear expansion coefficient at 800 °C increases significantly, which will cause greater thermal stress due to the increase in size at high temperature of the exhaust manifold, increasing the risk of failure and cracking.

[0077] The exhaust manifold material of Comparative Example 3 added 0.35% Ni element. Ni is a commonly used high-temperature resistant element, but when the addition amount of Ni is small, it will not optimize the high-temperature strength, elongation, pearlite content and linear expansion coefficient of the material, but only increase the material cost additionally.

[0078] The exhaust manifold material of Comparative Example 4 did not add Cr element, the high-temperature strength of the material was insufficient, and the linear expansion coefficient of the material increased.

[0079] The exhaust manifold material of Comparative Example 5 did not add V element. The strengthening effect of Cr element on high-temperature strength is not as good as that of V element, and at the same time, it increases the pearlite content, exceeding the required range, and the linear expansion coefficient of the material increases significantly.

[0080] The Si element content in the exhaust manifold material of Comparative Example 6 was relatively high, exceeding the required range. High Si can significantly increase the room-temperature strength, oxidation resistance, etc. of the material, but when the content of Si exceeds a certain range, it will instead reduce the high-temperature strength of the material and increase the brittleness of the material. The Si content should not exceed the required range.

[0081] The Cr element in the exhaust manifold material of Comparative Example 7 exceeded the required range. Cr can effectively increase the high-temperature strength, but when it exceeds a certain range, it will cause an increase in the pearlite content and an increase in the linear expansion coefficient of the material, easily causing the risk of failure of the exhaust manifold due to large thermal stress.

[0082] In the description of this specification, the description with reference to terms such as "one embodiment / way", "some embodiments / ways", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment / way or example are included in at least one embodiment / way or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment / way or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments / ways or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments / ways or examples described in this specification and the features of different embodiments / ways or examples.

[0083] It should be noted that in the present invention, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element. In the present invention, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0084] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather will be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A ductile silicon molybdenum vanadium cast iron material, characterized in that, It has the following components and mass percentages: C: 2.9% - 3.4%; Si: 3.9% - 4.3%; Mn: 0.1% - 0.3%; P: ≤0.05%; S: ≤0.02%; Mg: 0.03% - 0.055%; Mo: 0.9% - 1.15%; V: 0.25% - 0.45%; Cr: 0.1% - 0.25%; RE: 0.008% - 0.020%; the balance is Fe and unavoidable impurities.

2. The ferritic ductile cast iron material according to claim 1, characterized in that: The pearlite content of the silicon-molybdenum-vanadium ductile iron material is not higher than 20%.

3. The ferritic ductile iron material according to claim 1, characterized in that: The tensile strength of the silicon-molybdenum-vanadium ductile iron material at 800°C is 60 - 65 MPa, the elongation at 800°C is 30% - 45%, and the linear expansion coefficient at 800°C is 16 - 19 μm / (m·K).

4. A method for manufacturing a nodular graphite cast iron part containing silicon, molybdenum and vanadium, characterized in that, It includes: Melting the raw materials into molten iron. Pouring the molten iron into a nodulizing ladle for nodulizing and inoculating, and then pouring it into a part mold while simultaneously performing in-stream inoculation. Cooling the part mold to obtain a part made of the silicon-molybdenum-vanadium ductile iron material described in claim 1.

5. The method for manufacturing a silicon molybdenum vanadium ductile iron part according to claim 4, characterized in that: The tapping temperature of the molten iron is 1510°C - 1540°C.

6. The method for manufacturing a silicon molybdenum vanadium ductile iron part according to claim 4, characterized in that: An Fe-Si-Mg-Ca nodulizer is placed at the bottom of the nodulizing ladle, and a Si-Ba inoculant or Si-Fe inoculant is covered above the Fe-Si-Mg-Ca nodulizer.

7. The method for manufacturing a silicon molybdenum vanadium ductile iron part according to claim 4, characterized in that: The temperature of the molten iron poured into the part mold is 1370°C - 1430°C.

8. The method for manufacturing a silicon molybdenum vanadium ductile iron part according to claim 4, characterized in that: The in-stream inoculant used for in-stream inoculation is a Si-Fe inoculant.

9. An exhaust manifold, characterized in that, Using the silicon-molybdenum-vanadium ductile iron material described in any one of claims 1 - 3.

10. A vehicle, characterized in that, Including the exhaust manifold described in claim 9.

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