Silicon-molybdenum-chromium-nickel-niobium nodular cast iron material and preparation method thereof

Abstract

The invention relates to the technical field of nodular cast iron, in particular to a silicon-molybdenum-chromium-nickel-niobium nodular cast iron material and a preparation method thereof. The material comprises the following chemical components in percentage by mass: 3.0%-3.5% of carbon, 4.3%-4.7% of silicon, 0.1%-0.3% of manganese, less than or equal to 0.05% of phosphorus, less than or equal to 0.02% of sulfur, 0.7%-1.0% of molybdenum, 0.6%-0.8% of chromium, 0.4%-0.7% of nickel, 0.3%-0.6% of niobium, 0.03%-0.06% of magnesium, 0.05%-0.1% of rare earth, 0.006%-0.012% of antimony and the balance of iron. The preparation method comprises the steps of smelting, first-time spheroidizing and inoculation, second-time spheroidizing, second-time inoculation and heat treatment in sequence. According to the obtained nodular cast iron material, the form of graphite and the number of graphite nodules are improved, carbon elements are separated out as many as possible in the form of high-carbon phase-spheroidal graphite, generation of pearlite is reduced, and the mechanical strength and high-temperature resistance of nodular cast iron are improved.

Description

Technical Field

[0001] The invention relates to the technical field of ductile iron, and in particular to a silicon-molybdenum-chromium-nickel-niobium ductile iron material and a preparation method thereof. Background Art

[0002] Cast iron is one of the most widely used engineering structural materials. According to the different silicon content in the chemical composition, cast iron can be divided into three categories: white cast iron, gray cast iron and ductile cast iron. Among them, ductile cast iron has the advantages of high convexity and easy preparation into thin-walled complex-shaped parts. It is widely used in the manufacture of important parts such as automobile engine cylinder blocks, brake discs, and machine tool beds.

[0003] Ductile iron has advantages in manufacturing and economy, and has excellent comprehensive properties such as high strength, good toughness and good wear resistance. It is used to replace carbon steel, alloy steel and other materials as an excellent engineering material. It is widely used in important parts such as crankshafts, gears, connecting rods, housings, and cast pipes of internal combustion engines.

[0004] The Chinese invention patent with the publication (announcement) number CN117385268A discloses a cast ductile iron and a preparation method thereof. The composition of the cast ductile iron is: C3.75%-3.9%, Si2.65%-2.8%, Mn0.45%-0.6%, Cu0.65%-0.8%, Ni0.1%-0.2%, Cr0.2%-0.3%, Sn0.03%-0.04%, Mg0.035%-0.055%, Re0.025%-0.04%, Bi0.02%-0.03%, Ba0.01%-0.02%, P≤0.03%, S≤0.02%, and the balance is Fe; the tensile strength is ≥900MPa and the elongation is ≥6%. The invention achieves the effect of composite strengthening by adjusting the low alloying and trace alloying elements in the cast iron composition, and improves the strength and toughness of the ductile iron to a certain extent.

[0005] The Chinese invention patent with the publication (announcement) number CN117604371A discloses a ductile iron and a preparation method thereof. The ductile iron provided includes carbon, silicon, manganese, copper, vanadium, niobium, aluminum, zirconium and magnesium elements, adopts a low-carbon-low-silicon design concept, and achieves a composite strengthening effect by adding manganese, copper, vanadium, niobium, aluminum, zirconium and magnesium elements and adjusting the dosage of trace elements, thereby improving the strength, toughness and spheroidization effect of the ductile iron and improving the problem of poor comprehensive performance of the existing ductile iron.

[0006] With the rapid development of the automobile industry, users have increasingly stringent requirements for safety, comfort, intelligence, lightweight, energy saving, environmental protection, good quality and low price, especially in improving the loading capacity of the whole vehicle and the demand for more and faster running. In order to win the market, various vehicle manufacturers have put forward higher requirements on the material performance of supporting parts and components, requiring better mechanical properties of the materials. Especially for parts such as automobile exhaust manifolds and turbocharger housings, which need to work in high temperature environments, it is necessary to further improve the heat resistance of ductile iron so that it has good mechanical strength under high temperature working conditions. Among the current ductile iron materials, only some austenitic ductile iron materials with a nickel content of 18%-22% can meet this use requirement, but their high cost cannot meet market demand.

[0007] Therefore, there is an urgent need to develop a high-temperature resistant and low-cost ductile iron material to better meet market demand. Summary of the invention

[0008] In order to solve the above problems, the present invention provides a silicon-molybdenum-chromium-nickel-niobium ductile iron material and a preparation method thereof. The ductile iron material obtained by the present invention improves the morphology of graphite and the number of graphite nodules, allows as much carbon element as possible to precipitate in the form of high-carbon phase-spheroidal graphite, reduces the formation of pearlite, and improves the mechanical strength and high temperature resistance of the ductile iron.

[0009] The technical solution adopted by the present invention to achieve the above-mentioned purpose is: A silicon-molybdenum-chromium-nickel-niobium ductile iron material, comprising the following chemical components in percentage by mass: carbon: 3.0-3.5%, silicon: 4.3-4.7%, manganese 0.1-0.3%, phosphorus ≤0.05%, sulfur ≤0.02%, molybdenum 0.7-1.0%, chromium 0.6-0.8%, nickel 0.4-0.7%, niobium 0.3-0.6%, magnesium 0.03-0.06%, rare earth 0.05-0.1%, antimony 0.006-0.012%, and the balance is iron.

[0010] Existing ductile iron materials are mostly strengthened by a single or a few alloying elements, and it is difficult to balance strength and toughness. In the present invention, multiple alloying elements such as molybdenum, chromium, and nickel work synergistically. Molybdenum and chromium form carbides to improve hardness, wear resistance, and high-temperature strength. Nickel refines grains, improves toughness and fatigue resistance, and reduces the brittle transition temperature, so that the material has good mechanical properties at low temperatures, achieving an effective balance between strength and toughness, and can be applied to a wider range of engineering fields. Chromium can improve oxidation resistance and corrosion resistance, and work together with molybdenum to enable the material to maintain good performance in corrosive environments such as moisture, acid and alkali, reduce wear and corrosion, and extend service life. Rare earth elements can react with impurities such as sulfur and oxygen in molten iron, purify molten iron, create favorable conditions for spheroidization, and promote the growth of graphite balls, making them more round, small and evenly distributed, reducing stress concentration, and further improving the mechanical properties, toughness, and anti-degradation ability of ductile iron. As a strong carbide-forming element, niobium can be combined with nickel and other elements to effectively refine the matrix structure and graphite nodules of ductile iron, making the microstructure more uniform and dense, thereby improving comprehensive properties such as strength, hardness and toughness. The modification of antimony cooperates with other elements to further optimize the morphology and distribution of graphite nodules and improve the strength and hardness of ductile iron. The solid solution strengthening effect of silicon works together with other strengthening elements to comprehensively improve the comprehensive properties of ductile iron, such as strength, toughness, wear resistance, etc., and better meet the strict requirements of different engineering applications for material properties. The present invention also reduces the content of impurity elements such as phosphorus and sulfur, reducing their adverse effects on performance, such as phosphorus segregation at grain boundaries will reduce toughness and increase cold brittleness, sulfur will consume spheroidizing agent and affect spheroidizing effect, etc. At the same time, manganese and sulfur form manganese sulfide inclusions, reducing the influence of sulfur on spheroidization, and rare earth elements will also react with sulfur and oxygen to purify molten iron, further ensuring the quality and performance of ductile iron.

[0011] The preparation method of the above-mentioned silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following steps: S1, smelting raw materials to obtain molten iron, the smelting temperature is 1620°C-1650°C; the raw materials include pig iron, low-carbon steel, ferromolybdenum, ferroniobium, electrolytic manganese, ferrosilicon, low-carbon ferrochrome, and electrolytic nickel; S2, first spheroidization and inoculation: the molten iron in step S1 is spheroidized for the first time by using a pressing ladle injection method, and then the first inoculation is performed using an inoculant. During the first inoculation, metal antimony is added to increase the degree of supercooling; S3, second spheroidization: the second spheroidization is carried out by using the wire feeding spheroidization method; S4, casting the molten iron after the second spheroidization to obtain a casting, and performing a second inoculation during the casting process, wherein the second inoculation uses an inoculant for inoculant flow inoculation; S5, heat treatment: after the obtained casting is kept at 900℃-940℃ for 1.5-2 hours, the temperature is reduced to 820℃-850℃, and kept at this temperature for 3-5 hours.

[0012] Preferably, the silicon content in the molten iron in step S1 is 3.4-3.6%.

[0013] Preferably, in step S2, the dosage of the spheroidizing agent for the first spheroidization and the inoculant for the first inoculation are 0.7-1.2% and 0.5-1.0% of the weight of the molten iron, respectively; in step S4, the dosage of the inoculant for the second inoculation is 0.05-0.2% of the weight of the molten iron.

[0014] Preferably, in step S3, a spheroidizing cored wire is used for wire feeding and spheroidizing, wherein the spheroidizing cored wire has a spheroidizing agent of nickel-magnesium spheroidizing agent with a magnesium content of 14-18%, a specification of the spheroidizing cored wire is 0.4-0.5 kg / m, a content of nickel-magnesium spheroidizing agent is 0.24-0.4 kg / m, a wire feeding speed of the spheroidizing cored wire is 12-15 m / min, and an amount of the spheroidizing cored wire added to the molten iron is 35-45 m / t.

[0015] Preferably, the inoculants used in the first inoculation and the second inoculation are both silicon strontium inoculants, the particle size of the inoculant used in the first inoculation is 3-8 mm, and the particle size of the inoculant used in the second inoculation is 0.1-0.5 mm.

[0016] Preferably, in step S2, the spheroidizing agent for the first spheroidization comprises the following chemical components in terms of mass percentage: Mn: 6.5-6.8%, Si: 2.2-2.5%, Cr: 0.1-0.2%, Mg: 3.6-3.7%, S: ≤0.01%, Bi: 0.05-0.08%, Te: 0.02-0.03%, rare earth: 0.01-0.02%, and the balance Fe.

[0017] Preferably, the preparation method of the spheroidizing agent for the first spheroidization is as follows: Step 1, weighing raw materials such as ferromanganese alloy, ferrosilicon alloy, ferrochrome alloy, magnesium ingot, rare earth ferrosilicon alloy, bismuth alloy, tellurium alloy and scrap steel, and crushing them respectively; Step 2, mixing and melting the crushed ferrosilicon alloy, rare earth ferrosilicon alloy and scrap steel, then adding tellurium alloy, ferromanganese alloy, ferrochrome alloy and bismuth alloy, stirring and melting, removing slag, then adding magnesium ingot, melting and mixing, and obtaining alloy melt; Step 3, after casting the alloy melt and cooling it, the molded material is taken and divided into four parts, which are crushed to 15-20mm, 25-30mm, 35-40mm and 45-50mm respectively to obtain four particles of different sizes, and then the above four particles of equal mass are mixed to obtain.

[0018] Preferably, in step 2, the melting temperature of ferrosilicon alloy, rare earth ferrosilicon alloy and scrap steel is 1400-1450°C, then the temperature is raised to 1450-1520°C, tellurium alloy, ferromanganese alloy, ferrochrome alloy and bismuth alloy are added for melting, and finally, the temperature is lowered to 700-800°C, magnesium ingot is added for melting.

[0019] The spheroidizing agent used in the first spheroidizing contains 0.1-0.2% chromium, which is a carbide-forming element. In ductile iron, chromium combines with carbon to form carbides. These carbides are dispersed in the matrix structure and can effectively hinder dislocation movement, improve the high-temperature strength and hardness of the material, thereby enhancing the casting's ability to resist deformation at high temperatures and thus improving its temperature resistance. The rare earth element content is 0.02-0.03%. Its boiling point is relatively high. When added to the molten iron, it will not cause the molten iron to churn and splash. It also has stronger desulfurization and deoxidation capabilities than magnesium. Rare earth elements turn flake graphite into spherical graphite. This graphite spheroidization can reduce stress concentration, refine the cast structure, improve the shape and distribution of non-metallic inclusions, and thus improve the density of ductile iron, reduce defects such as pores and shrinkage at high temperatures, thereby improving the material properties, improving the strength, toughness, plasticity and temperature resistance of castings, and improving the anti-degradation performance of ductile iron, so that spherical graphite remains unchanged for a long time, and reduces casting defects such as segregation and thermal cracking during the casting process. During the solidification stage of molten iron, magnesium can change the growth morphology of graphite, transforming the originally flake graphite into spherical graphite. Spherical graphite can effectively reduce stress concentration, improve the mechanical properties of ductile iron, and reduce the possibility of defects such as cracks in castings due to stress concentration at high temperatures, thereby improving temperature resistance. The various elements in the spheroidizing agent used in the first spheroidizing work together, such as rare earth elements and magnesium elements, which can effectively remove impurities such as sulfur and oxygen in molten iron, purify the molten iron, reduce the segregation of impurity elements at grain boundaries, avoid the weakening effect of these impurities on grain boundaries at high temperatures, improve the bonding strength of grain boundaries, and enable castings to maintain good mechanical properties at high temperatures.

[0020] Furthermore, the smaller particles (15-20 mm) of the spheroidizing agent used for the first spheroidization in the present invention dissolve quickly in the molten iron. At the beginning of the spheroidization process, these small particles can quickly release spheroidizing elements, such as magnesium, rare earth, etc. This enables the spheroidization reaction to start quickly, which is conducive to the formation of a good spherical shape of graphite balls at the beginning of solidification. The large particles (35-40 mm, 45-50 mm) of the spheroidizing agent dissolve slowly, and they continue to release spheroidizing elements in the middle and late stages of the spheroidization process. When the elements released by the small particle spheroidizing agent are partially consumed in the early stage of the reaction, the large particle spheroidizing agent continues to provide the elements required for spheroidization, ensuring that the spheroidization reaction is stable for a long time and avoiding the spheroidization decline phenomenon in which the spheroidization effect gradually deteriorates over time. By mixing spheroidizing agents of different particle sizes, spheroidization decline can be effectively prevented. Particles of different sizes release spheroidizing elements at different stages, maintaining the effective concentration of spheroidizing elements in the molten iron, ensuring that the growth of graphite balls is always well controlled during the entire spheroidization process, and making the quality of ductile iron more stable. When spheroidizers of different particle sizes are dissolved in molten iron, they will release spheroidizing elements at different positions and times due to the difference in their dissolution speed. This dispersed release method can make the spheroidizing elements more evenly distributed in the molten iron, avoid the local concentration of spheroidizing elements being too high or too low, and ensure a uniform spheroidizing effect, which helps to uniformly spheroidize all parts of the entire casting, avoid the formation of pearlite, reduce the performance differences of castings caused by uneven spheroidization, and improve the overall quality of ductile iron. For ductile iron parts used in complex working conditions, such as high temperature, high pressure, high friction and other environments, uniform spheroidizing effect and good graphite ball morphology are essential. Spheroidizers with mixed particle sizes can provide stable spheroidizing quality, ensuring that ductile iron parts have excellent mechanical properties, wear resistance and fatigue resistance under these harsh working conditions. The second spheroidizing process uses a nickel-magnesium spheroidizer with a magnesium content of 14-18% to be added through cored wire feeding to accurately control the amount and speed of magnesium added during the spheroidizing process. The wire feeding spheroidizing method allows the spheroidizer to enter the molten iron evenly, reducing the situation where local reactions are too intense, and further strengthening the spheroidizing effect. The two spheroidizers complement each other to ensure that there are enough spheroidizing elements in the molten iron, making the graphite spheroidizing more fully, improving the spheroidizing rate, and ensuring the quality stability of ductile iron. Combined with the stable spheroidizing characteristics of the first spheroidizing agent, it can effectively reduce spheroidizing decay and casting defects, improve production efficiency and product quality stability. In the large-scale production of ductile iron, this combination can better meet the requirements of continuous production and ensure the consistency of casting performance.

[0021] The present invention has the following beneficial effects: The silicon-molybdenum-chromium-nickel-niobium ductile iron material of the present invention contains a variety of alloy elements, and its strength and toughness are greatly improved. Elements such as molybdenum and chromium can improve the hardness and wear resistance of the material, and the addition of nickel and chromium can improve the corrosion resistance of ductile iron and slow down the corrosion rate. The addition of chromium and nickel also improves the oxidation resistance of the material and improves its high temperature resistance.

[0022] The preparation method of the present invention realizes uniform spheroidization in all parts of the entire casting through two spheroidizations and two inoculations, as well as annealing treatment, thereby avoiding the formation of pearlite, reducing the difference in casting performance caused by uneven spheroidization, and improving the overall quality of ductile iron. The two inoculations use silicon-strontium inoculants of different particle sizes, the silicon element promotes graphitization, increases the number of graphite balls, and the strontium element refines the graphite balls and improves their morphology. The combination of the two inoculations makes the graphite balls smaller, rounder and more evenly distributed, reduces the size difference and local aggregation of the graphite balls, and significantly improves the quality of ductile iron. Uniform and fine graphite balls can effectively reduce stress concentration and improve the mechanical properties of ductile iron such as tensile strength, yield strength and elongation. The small-particle inoculant of the second inoculation can quickly merge with molten iron during pouring, improving the fluidity and surface tension of the molten iron. This helps the molten iron to be better filled in the mold, making the surface of the casting smoother and reducing surface defects such as slag inclusions and pores. The two inoculations process the molten iron at different stages, which is conducive to reducing the internal structural defects of the casting. The first inoculation provides good initial conditions for the solidification of the molten iron, and the second inoculation optimizes the state of the molten iron during the pouring process, making the internal structure of the casting more compact, reducing internal stress concentration, improving its mechanical properties at high temperatures, and improving the fatigue resistance and service life of the casting. Then, through the heat treatment process, a part of the strengthening phase is retained, and the strengthening phase can also be diffused into granular and dispersed in the metal base surface, which not only reduces the difference in the expansion and contraction coefficients of the strengthening phase and the matrix during the hot and cold cycle, but also plays a role in dispersion strengthening. The ductile iron material obtained by the preparation method of the present invention improves the morphology and graphite ball number of graphite, makes the carbon element precipitate as much as possible in the form of high carbon phase-spheroidal graphite, reduces the generation of pearlite, and has only about 8-10% of granular pearlite and a small amount of carbide, thereby improving the mechanical strength and high temperature resistance of ductile iron. DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0024] The raw materials used in the following examples are all common commercially available products. Among them, the pig iron is Q10 pig iron, the low carbon steel is Q215 low carbon steel, the ferroniobium is 60 ferroniobium, the low carbon ferrochrome is C0.25 low carbon ferrochrome, the electrolytic manganese: the manganese content is 99.7%, the electrolytic nickel: the nickel content is 99.99%, the ferromolybdenum is 60 ferromolybdenum, the ferrosilicon is FeSi75, the ferromanganese alloy is FeMn84C0.4, the ferrochrome alloy is high carbon ferrochrome: the carbon content is 4%-10%, and the chromium content is between 52%-60%; the purity of the magnesium ingot is above 99.9%, and the rare earth ferrosilicon alloy is It is FeSiRe32-A, the bismuth alloy is a manganese-bismuth alloy with the molecular formula of BiMn and the molecular weight of 263.92; the tellurium alloy is a tellurium-bismuth alloy with the model of Bi2Te39; the main chemical components in the silicon-strontium inoculant are as follows by weight: Si: 73-78%, Ca≤0.1, Al≤0.3, Sr: 1.1-1.7%; in the nickel-magnesium spheroidizer, Ni80-85%, Mg14-18%, particle size: 10-30mm, Nanjing Pujiang Special Alloy Co., Ltd.

[0025] Example 1 A silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following chemical components by mass percentage: carbon: 3.2%, silicon: 4.5%, manganese 0.2%, phosphorus 0.03%, sulfur 0.01%, molybdenum 0.8%, chromium 0.7%, nickel 0.5%, niobium 0.5%, magnesium 0.05%, rare earth 0.08%, antimony 0.01%, and the balance is iron.

[0026] The preparation method of the above silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following steps: S1, sequentially putting pig iron, low-carbon steel, ferroniobium, low-carbon ferrochrome, electrolytic manganese, electrolytic nickel, ferromolybdenum and ferrosilicon into a medium-frequency induction furnace for smelting to obtain molten iron, the smelting temperature is 1640°C, and the silicon content of the obtained molten iron is 3.5%; S2, first spheroidization and inoculation: the molten iron in step S1 is spheroidized for the first time by a pressing ladle punching method, and then a silicon strontium inoculant is added to the bottom of the spheroidization ladle for the first inoculation. During the first inoculation, metal antimony is added to increase the supercooling degree to avoid the precipitation of fragmented graphite. The dosage of the spheroidizing agent for the first spheroidization and the inoculant for the first inoculation are 0.9% and 0.7% of the weight of the molten iron, respectively. The inoculant used for the first inoculation is a silicon strontium inoculant with a particle size of 3-8 mm; S3, second spheroidization: using spheroidizing cored wire for wire feeding spheroidization, wherein the spheroidizing cored wire has a spheroidizing agent of nickel-magnesium spheroidizing agent with a magnesium content of 16%, a specification of the spheroidizing cored wire of 0.44 kg / m, a content of nickel-magnesium spheroidizing agent of 0.32 kg / m, a wire feeding speed of the spheroidizing cored wire of 14 m / min, and an amount of the spheroidizing cored wire added to the molten iron of 40 m / t; S4, casting the molten iron after the second spheroidization to obtain a casting, and performing a second inoculation during the casting process, wherein the second inoculation is performed using a silicon strontium inoculant with a particle size of 0.1-0.5 mm, and the amount used is 0.10% of the weight of the molten iron; S5, heat treatment: after the obtained casting is kept at 920°C for 1.8 hours, the temperature is reduced to 830°C and kept at this temperature for 4 hours.

[0027] The spheroidizing agent used for the first spheroidization contains the following chemical components in terms of mass percentage: Mn: 6.7%, Si: 2.4%, Cr: 0.15%, Mg: 3.65%, S: 0.007%, Bi: 0.06%, Te: 0.025%, rare earth: 0.015%, and the balance Fe.

[0028] The preparation method of the spheroidizing agent for the first spheroidization is as follows: Step 1, weighing raw materials such as ferromanganese alloy, ferrosilicon alloy, ferrochrome alloy, magnesium ingot, rare earth ferrosilicon alloy, bismuth alloy, tellurium alloy and scrap steel, and crushing them respectively; Step 2, at a temperature of 1430° C., the crushed ferrosilicon alloy, rare earth ferrosilicon alloy, and scrap steel are mixed and melted, and then tellurium alloy, ferromanganese alloy, ferrochrome alloy, and bismuth alloy are added, stirred and heated to 1500° C. for melting, slag is removed, and then the temperature is lowered to 750° C., a magnesium ingot is added, melted, and mixed to obtain an alloy melt; Step 3, after casting the alloy melt and cooling it, the molded material is taken and divided into four parts, which are crushed to 15-20mm, 25-30mm, 35-40mm and 45-50mm respectively to obtain four particles of different sizes, and then the above four particles of equal mass are mixed to obtain.

[0029] Example 2 A silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following chemical components by mass percentage: carbon: 3.0%, silicon: 4.7%, manganese 0.1%, phosphorus 0.05%, sulfur 0.02%, molybdenum 0.9%, chromium 0.6%, nickel 0.4%, niobium 0.4%, magnesium 0.04%, rare earth 0.05%, antimony 0.008%, and the balance is iron.

[0030] The preparation method of the above silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following steps: S1, sequentially putting pig iron, low-carbon steel, ferroniobium, low-carbon ferrochrome, electrolytic manganese, electrolytic nickel, ferromolybdenum and ferrosilicon into a medium-frequency induction furnace for smelting to obtain molten iron, the smelting temperature is 1620°C, and the silicon content of the obtained molten iron is 3.4%; S2, first spheroidization and inoculation: the molten iron in step S1 is spheroidized for the first time by a pressing ladle punching method, and then a silicon strontium inoculant is added to the bottom of the spheroidization ladle for the first inoculation. During the first inoculation, metal antimony is added to increase the supercooling degree to avoid the precipitation of fragmented graphite. The dosage of the spheroidizing agent for the first spheroidization and the inoculant for the first inoculation are 1.2% and 1.0% of the weight of the molten iron, respectively. The inoculant used for the first inoculation is a silicon strontium inoculant with a particle size of 3-8 mm; S3, second spheroidization: using spheroidizing cored wire for wire feeding spheroidization, wherein the spheroidizing cored wire has a spheroidizing agent of nickel-magnesium spheroidizing agent with a magnesium content of 18%, a specification of the spheroidizing cored wire of 0.45 kg / m, a content of nickel-magnesium spheroidizing agent of 0.24 kg / m, a wire feeding speed of the spheroidizing cored wire of 15 m / min, and an amount of the spheroidizing cored wire added to the molten iron of 35 m / t; S4, casting the molten iron after the second spheroidization to obtain a casting, and performing a second inoculation during the casting process, wherein the second inoculation is performed using a silicon strontium inoculant with a particle size of 0.1-0.5 mm, and the amount used is 0.15% of the weight of the molten iron; S5, heat treatment: After the obtained casting is kept at 900°C for 2 hours, the temperature is reduced to 850°C and kept at this temperature for 3 hours.

[0031] The spheroidizing agent used for the first spheroidization contains the following chemical components in terms of mass percentage: Mn: 6.5%, Si: 2.5%, Cr: 0.1%, Mg: 3.6%, S: 0.008%, Bi: 0.08%, Te: 0.023%, rare earth: 0.01%, and the balance Fe.

[0032] The preparation method of the spheroidizing agent for the first spheroidization is as follows: Step 1, weighing raw materials such as ferromanganese alloy, ferrosilicon alloy, ferrochrome alloy, magnesium ingot, rare earth ferrosilicon alloy, bismuth alloy, tellurium alloy and scrap steel, and crushing them respectively; Step 2, at a temperature of 1400° C., the crushed ferrosilicon alloy, rare earth ferrosilicon alloy, and scrap steel are mixed and melted, and then tellurium alloy, ferromanganese alloy, ferrochrome alloy, and bismuth alloy are added, stirred and heated to 1450° C. for melting, slag is removed, and then the temperature is lowered to 700° C., a magnesium ingot is added, melted, and mixed to obtain an alloy melt; Step 3, after casting the alloy melt and cooling it, the molded material is taken and divided into four parts, which are crushed to 15-20mm, 25-30mm, 35-40mm and 45-50mm respectively to obtain four particles of different sizes, and then the above four particles of equal mass are mixed to obtain.

[0033] Example 3 A silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following chemical components by mass percentage: carbon: 3.5%, silicon: 4.3%, manganese: 0.3%, phosphorus 0.04%, sulfur 0.01%, molybdenum 0.7%, chromium 0.8%, nickel 0.7%, niobium 0.6%, magnesium 0.03%, rare earth 0.1%, antimony 0.006%, and the balance is iron.

[0034] The preparation method of the above silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following steps: S1, sequentially putting pig iron, low-carbon steel, ferroniobium, low-carbon ferrochrome, electrolytic manganese, electrolytic nickel, ferromolybdenum and ferrosilicon into a medium-frequency induction furnace for smelting to obtain molten iron, the smelting temperature is 1650°C, and the silicon content of the obtained molten iron is 3.6%; S2, first spheroidization and inoculation: the molten iron in step S1 is spheroidized for the first time by a pressing ladle punching method, and then a silicon strontium inoculant is added to the bottom of the spheroidization ladle for the first inoculation. During the first inoculation, metal antimony is added to increase the supercooling degree to avoid the precipitation of fragmented graphite. The dosage of the spheroidizing agent for the first spheroidization and the inoculant for the first inoculation are 0.7% and 0.5% of the weight of the molten iron, respectively. The inoculant used for the first inoculation is a silicon strontium inoculant with a particle size of 3-8 mm; S3, second spheroidization: using spheroidizing cored wire for wire feeding spheroidization, wherein the spheroidizing cored wire has a spheroidizing agent of nickel-magnesium spheroidizing agent with a magnesium content of 14%, a specification of the spheroidizing cored wire of 0.5 kg / m, a content of nickel-magnesium spheroidizing agent of 0.4 kg / m, a wire feeding speed of the spheroidizing cored wire of 12 m / min, and an amount of the spheroidizing cored wire added to the molten iron of 45 m / t; S4, casting the molten iron after the second spheroidization to obtain a casting, and performing a second inoculation during the casting process, wherein the second inoculation is performed using a silicon strontium inoculant with a particle size of 0.1-0.5 mm, and the amount used is 0.05% of the weight of the molten iron; S5, heat treatment: after the obtained casting is kept at 940°C for 1.5 hours, the temperature is reduced to 820°C and kept at this temperature for 5 hours.

[0035] The spheroidizing agent used for the first spheroidization contains the following chemical components in terms of mass percentage: Mn: 6.8%, Si: 2.3%, Cr: 0.2%, Mg: 3.7%, S: 0.005%, Bi: 0.05%, Te: 0.03%, rare earth: 0.02%, and the balance Fe.

[0036] The preparation method of the spheroidizing agent for the first spheroidization is as follows: Step 1, weighing raw materials such as ferromanganese alloy, ferrosilicon alloy, ferrochrome alloy, magnesium ingot, rare earth ferrosilicon alloy, bismuth alloy, tellurium alloy and scrap steel, and crushing them respectively; Step 2, at a temperature of 1450° C., the crushed ferrosilicon alloy, rare earth ferrosilicon alloy, and scrap steel are mixed and melted, and then tellurium alloy, ferromanganese alloy, ferrochrome alloy, and bismuth alloy are added, stirred and heated to 1520° C. for melting, slag is removed, and then the temperature is lowered to 800° C., a magnesium ingot is added, melted, and mixed to obtain an alloy melt; Step 3, after casting the alloy melt and cooling it, the molded material is taken and divided into four parts, which are crushed to 15-20mm, 25-30mm, 35-40mm and 45-50mm respectively to obtain four particles of different sizes, and then the above four particles of equal mass are mixed to obtain.

[0037] Example 4 A silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following chemical components by mass percentage: carbon: 3.4%, silicon: 4.6%, manganese 0.3%, phosphorus 0.03%, sulfur 0.015%, molybdenum 1.0%, chromium 0.7%, nickel 0.6%, niobium 0.3%, magnesium 0.06%, rare earth 0.08%, antimony 0.012%, and the balance is iron.

[0038] The preparation method of the above silicon-molybdenum-chromium-nickel-niobium ductile iron material comprises the following steps: S1, sequentially putting pig iron, low-carbon steel, ferroniobium, low-carbon ferrochrome, electrolytic manganese, electrolytic nickel, ferromolybdenum and ferrosilicon into a medium-frequency induction furnace for smelting to obtain molten iron, the smelting temperature is 1630°C, and the silicon content of the obtained molten iron is 3.5%; S2, first spheroidization and inoculation: the molten iron in step S1 is spheroidized for the first time by a pressing ladle punching method, and then a silicon strontium inoculant is added to the bottom of the spheroidization ladle for the first inoculation. During the first inoculation, metal antimony is added to increase the supercooling degree to avoid the precipitation of fragmented graphite. The dosage of the spheroidizing agent for the first spheroidization and the inoculant for the first inoculation are 1.0% and 0.8% of the weight of the molten iron, respectively. The inoculant used for the first inoculation is a silicon strontium inoculant with a particle size of 3-8 mm; S3, second spheroidization: using spheroidizing cored wire for wire feeding spheroidization, wherein the spheroidizing cored wire has a spheroidizing agent of nickel-magnesium spheroidizing agent with a magnesium content of 17%, a specification of the spheroidizing cored wire of 0.4 kg / m, a content of nickel-magnesium spheroidizing agent of 0.36 kg / m, a wire feeding speed of the spheroidizing cored wire of 13 m / min, and an amount of the spheroidizing cored wire added to the molten iron of 38 m / t; S4, casting the molten iron after the second spheroidization to obtain a casting, and performing a second inoculation during the casting process, wherein the second inoculation is performed using a silicon strontium inoculant with a particle size of 0.1-0.5 mm, and the amount used is 0.2% of the weight of the molten iron; S5, heat treatment: after the obtained casting is kept at 930°C for 1.6 hours, the temperature is reduced to 840°C and kept at this temperature for 4.5 hours.

[0039] The spheroidizing agent used for the first spheroidization contains the following chemical compositions by mass percentage: Mn: 6.6%, Si: 2.2%, Cr: 0.13%, Mg: 3.62%, S: 0.01%, Bi: 0.07%, Te: 0.028%, rare earth: 0.01%, and the balance Fe.

[0040] The preparation method of the spheroidizing agent for the first spheroidization is as follows: Step 1, weighing raw materials such as ferromanganese alloy, ferrosilicon alloy, ferrochrome alloy, magnesium ingot, rare earth ferrosilicon alloy, bismuth alloy, tellurium alloy and scrap steel, and crushing them respectively; Step 2, at a temperature of 1420° C., the crushed ferrosilicon alloy, rare earth ferrosilicon alloy, and scrap steel are mixed and melted, and then tellurium alloy, ferromanganese alloy, ferrochrome alloy, and bismuth alloy are added, stirred and heated to 1490° C. for melting, slag is removed, and then the temperature is lowered to 780° C., magnesium ingot is added, melted, and mixed to obtain an alloy melt; Step 3, after casting the alloy melt and cooling it, the molded material is taken and divided into four parts, which are crushed to 15-20mm, 25-30mm, 35-40mm and 45-50mm respectively to obtain four particles of different sizes, and then the above four particles of equal mass are mixed to obtain.

[0041] In the embodiment of the present invention, in step S1, the silicon content of the obtained molten iron is controlled at 3.4%-3.6%. If it is insufficient, silicon is added along the flow at the furnace outlet, and ferrosilicon particles with a particle size of 10-20 mm are used for silicon addition. The remaining silicon is added through the process of spheroidization and inoculation, so as to reduce the amount of primary graphite precipitated, reduce silicon segregation, improve the morphology of graphite and the number of graphite nodules, so that as much carbon element as possible is precipitated in the form of high carbon phase-spheroidal graphite, and reduce the formation of pearlite.

[0042] Comparative Example 1 A silicon-molybdenum-chromium-nickel-niobium ductile iron material and a preparation method thereof, wherein a spheroidizing agent used for the first spheroidization comprises the following chemical components by mass percentage: Mg: 8%, rare earth: 1.2%, Si: 15%, Ca: 1.2%, Ba: 0.5%, Al: 0.3%, Ti: 0.3%, MgO: 0.4%, and the balance is iron, and the particle size is 5-10 mm.

[0043] The rest is the same as in Example 1.

[0044] Comparative Example 2 A silicon-molybdenum-chromium-nickel-niobium ductile iron material and a preparation method thereof, wherein a spheroidizing agent used for the first spheroidization comprises the following chemical components by mass percentage: Mg: 12%, rare earth: 0.2%, Si: 10%, Ca: 2%, Ba: 1%, Al: 1%, Ti: 0.5%, MgO: 0.5%, and the balance is iron, and the particle size is 30-35 mm.

[0045] The rest is the same as Example 2.

[0046] Comparative Example 3 A silicon-molybdenum-chromium-nickel-niobium ductile iron material and a preparation method thereof, wherein a spheroidizing agent used for the first spheroidization comprises the following chemical components by mass percentage: Mg: 3%, rare earth: 2%, Si: 7%, Ca: 0.6%, Ba: 0.2%, Al: 0.6%, Ti: 0.2%, MgO: 0.2%, and the balance is iron, and the particle size is 45-50 mm.

[0047] The rest is the same as in Example 3.

[0048] According to the test method of the national standard "GB / T 26653-2011 Exhaust manifold cast iron parts", the mechanical strength test of the ductile iron obtained in Examples 1-4 and Comparative Examples 1-3 was carried out, and the tensile strength and yield strength were tested at 700°C, 750°C, 800°C, 850°C, and 870°C, respectively. The tensile strength and yield strength of the existing ductile iron QTRSi4Mo1 and QTRSi4Mo were tested under the same temperature conditions for further comparison. Three samples of each product were taken for testing, and the results are shown in Table 1.

[0049] Table 1. High temperature static strength test of ductile iron As can be seen from Table 1, the ductile iron materials obtained in Examples 1-4 of the present invention have better tensile strength and yield strength than those of Comparative Examples 1-3 and the cast iron materials of models QTRSi4Mo1 and QTRSi4Mo in the range of 700-870°C. It can be seen that the preparation method of the present invention improves the morphology of graphite and the number of graphite nodules, so that as much carbon element as possible is precipitated in the form of high-carbon phase-spheroidal graphite, reduces the formation of pearlite, improves the mechanical strength and high temperature resistance of ductile iron, and enables it to replace part of the austenitic ductile iron material with a nickel content of 18%-22%, and the cost is lower than that of the austenitic material.

[0050] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0051] Although the embodiments of the present application have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present application, and that the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A silicon-molybdenum-chromium-nickel-niobium ductile iron material, characterized in that: In terms of mass percentage, it contains the following chemical components: carbon: 3.0-3.5%, silicon: 4.3-4.7%, manganese 0.1-0.3%, phosphorus ≤0.05%, sulfur ≤0.02%, molybdenum 0.7-1.0%, chromium 0.6-0.8%, nickel 0.4-0.7%, niobium 0.3-0.6%, magnesium 0.03-0.06%, rare earth 0.05-0.1%, antimony 0.006-0.012%, and the balance is iron.

2. The method for preparing silicon-molybdenum-chromium-nickel-niobium ductile iron material according to claim 1, characterized in that: The following steps are involved: S1, smelting raw materials to obtain molten iron, the smelting temperature is 1620°C-1650°C; the raw materials include pig iron, low-carbon steel, ferromolybdenum, ferroniobium, electrolytic manganese, ferrosilicon, low-carbon ferrochrome, and electrolytic nickel; S2, first spheroidization and inoculation: the molten iron in step S1 is spheroidized for the first time by using a pressing ladle injection method, and then the first inoculation is performed using an inoculant. During the first inoculation, metal antimony is added to increase the degree of supercooling; S3, second spheroidization: the second spheroidization is carried out by using the wire feeding spheroidization method; S4, casting the molten iron after the second spheroidization to obtain a casting, and performing a second inoculation during the casting process, wherein the second inoculation uses an inoculant for inoculant flow-based inoculation; S5, heat treatment: after the obtained casting is kept at 900℃-940℃ for 1.5-2 hours, the temperature is reduced to 820℃-850℃, and kept at this temperature for 3-5 hours.

3. The method for preparing silicon-molybdenum-chromium-nickel-niobium ductile iron material according to claim 2, characterized in that: In step S1, the silicon content in the molten iron is 3.4-3.6%.

4. The method for preparing silicon-molybdenum-chromium-nickel-niobium ductile iron material according to claim 2, characterized in that: In step S2, the dosage of the spheroidizing agent for the first spheroidization and the inoculant for the first inoculation are 0.7-1.2% and 0.5-1.0% of the weight of the molten iron, respectively. In step S4, the dosage of the inoculant for the second inoculation is 0.05-0.2% of the weight of the molten iron.

5. The method for preparing silicon, molybdenum, chromium, nickel, and niobium ductile iron material according to claim 2, characterized in that: In step S3, a spheroidizing cored wire is used for wire feeding and spheroidizing. The spheroidizing cored wire contains a nickel-magnesium spheroidizing agent with a magnesium content of 14-18%. The specification of the spheroidizing cored wire is 0.4-0.5 kg / m, the content of the nickel-magnesium spheroidizing agent is 0.24-0.4 kg / m, the wire feeding speed of the spheroidizing cored wire is 12-15 m / min, and the amount of the spheroidizing cored wire added to the molten iron is 35-45 m / t.

6. The method for preparing silicon, molybdenum, chromium, nickel, and niobium ductile iron material according to claim 4, characterized in that: The inoculants used in the first and second inoculations are both silicon strontium inoculants. The particle size of the inoculant used in the first inoculation is 3-8 mm, and the particle size of the inoculant used in the second inoculation is 0.1-0.5 mm.

7. The method for preparing silicon, molybdenum, chromium, nickel, and niobium ductile iron material according to claim 2, characterized in that: In step S2, the spheroidizing agent used for the first spheroidization contains the following chemical components in terms of mass percentage: Mn: 6.5-6.8%, Si: 2.2-2.5%, Cr: 0.1-0.2%, Mg: 3.6-3.7%, S: ≤0.01%, Bi: 0.05-0.08%, Te: 0.02-0.03%, rare earth: 0.01-0.02%, and the balance Fe.

8. The method for preparing silicon, molybdenum, chromium, nickel, and niobium ductile iron material according to claim 7, characterized in that: The preparation method of the spheroidizing agent for the first spheroidization is as follows: Step 1, weighing raw materials such as ferromanganese alloy, ferrosilicon alloy, ferrochrome alloy, magnesium ingot, rare earth ferrosilicon alloy, bismuth alloy, tellurium alloy and scrap steel, and crushing them respectively; Step 2, mixing and melting the crushed ferrosilicon alloy, rare earth ferrosilicon alloy and scrap steel, then adding tellurium alloy, ferromanganese alloy, ferrochrome alloy and bismuth alloy, stirring and melting, removing slag, then adding magnesium ingot, melting and mixing, and obtaining alloy melt; Step 3, after casting the alloy melt and cooling it, the molded material is taken and divided into four parts, which are crushed to 15-20mm, 25-30mm, 35-40mm and 45-50mm respectively to obtain four particles of different sizes, and then the above four particles of equal mass are mixed to obtain.

9. The method for preparing silicon, molybdenum, chromium, nickel, and niobium ductile iron material according to claim 7, characterized in that: In step 2, the melting temperature of ferrosilicon alloy, rare earth ferrosilicon alloy and scrap steel is 1400-1450°C, then the temperature is raised to 1450-1520°C, tellurium alloy, ferromanganese alloy, ferrochrome alloy and bismuth alloy are added for melting, and finally, the temperature is lowered to 700-800°C, magnesium ingot is added for melting.

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