A spheroidizing inoculation method for ductile iron with high spheroidizing rate

Abstract

The invention discloses a kind of spheroidizing inoculation method of high spheroidizing rate ductile iron, belong to ductile iron technical field, described method includes: prepare composite inoculant, prepare spheroidizing agent, fix, spheroidizing inoculation, heat treatment;Described preparation composite inoculant, by silicon powder and carbon black after mixing, carry out microwave sintering, microwave sintering 30 35min after, grind, remove carbon;Described preparation spheroidizing agent, rare earth magnesium silicon type spheroidizing agent is crushed, obtain millimeter grade spheroidizing agent;Then take out one-third from millimeter grade spheroidizing agent, continue to crush, ball milling, then after mixing with remaining millimeter grade spheroidizing agent, obtain mixed rare earth magnesium silicon type spheroidizing agent;Mixed rare earth magnesium silicon type spheroidizing agent is mixed with iron powder, obtain compound, use tinplate to wrap compound.The method of the present invention can improve the spheroidizing rate, plasticity and toughness of ductile iron, and improves the utilization rate of the spheroidizing agent used in ball-milled cast iron production.

Description

Technical Field

[0001] The invention relates to the technical field of ductile iron, and in particular to a spheroidizing inoculation method for ductile iron with a high spheroidizing rate. Background Art

[0002] Ductile iron (DPI) is a cast iron with a spheroidal graphite structure created by adding a certain amount of nodulizer and inoculant to the molten iron during the casting process. This process, combined with spheroidizing and inoculating treatments, creates a structure known as ductile iron. The emergence of ductile iron has dispelled the traditional perception that cast iron suffers from low mechanical strength, poor plasticity, and toughness. While retaining the original composition and excellent properties of ordinary cast iron, it significantly improves mechanical strength, toughness, and plasticity. This allows it to offer superior overall performance compared to gray cast iron and malleable cast iron. Compared to carbon steel, ductile iron can reduce costs while meeting performance requirements. Precisely because of these excellent properties, ductile iron is increasingly being used in a wide range of technical fields, including automotive, shipbuilding, wind power, pressure piping, and industrial production.

[0003] For ductile iron, the spheroidization rate of spheroidal graphite directly affects its overall performance. This is because spheroidal graphite expands in volume during growth, effectively reducing shrinkage and porosity during solidification, thereby ensuring internal quality. During the production of ductile iron, adding appropriate amounts of spheroidizing agents and inoculants to the molten iron for spheroidization and inoculation can induce the formation of spheroidal graphite in the molten iron and promote its growth, thereby achieving control over the spheroidization rate of spheroidal graphite.

[0004] The microstructure of ductile iron consists of spheroidal graphite and the surrounding matrix. In addition to the spheroidization rate of the spheroidal graphite, other structural factors also significantly influence the performance of ductile iron, such as the size of the spheroidal graphite, the uniformity of its distribution within the matrix, and the composition of the matrix. Based on the composition of the matrix, ductile iron is primarily classified as ferritic, pearlitic, and ferrite-pearlite mixed. Ferrite improves the plasticity and toughness of ductile iron, while pearlite enhances its strength. Ferrite-pearlite mixed ductile iron offers the best overall mechanical properties and the widest range of applications. The matrix structure of ductile iron can be altered by controlling the austenite phase transformation. Currently, alloying and heat treatment are the primary methods for influencing the austenite phase transformation. Spheroidizing and inoculation also have a certain impact on the austenite phase transformation.

[0005] In summary, spheroidizing and inoculation are core operations in ductile iron production, especially for ferrite-pearlite mixed ductile iron. Spheroidizing and inoculation can affect the microstructure of ductile iron, thereby controlling the spheroidization rate and size of spheroidal graphite in ductile iron, the uniformity of spheroidal graphite distribution in the matrix structure, the ratio of ferrite to pearlite, and further controlling the properties of ferrite-pearlite mixed ductile iron. The spheroidizing agent in the spheroidizing treatment can promote the spherical growth of graphite in the molten iron, and the inoculant in the inoculation treatment can establish more crystallization nuclei and also play a role in dehydrogenation, deoxidation, desulfurization and eliminating white cast. The use of spheroidizing agents and inoculation agents has a great influence on the treatment results of spheroidizing and inoculation.

[0006] The main components of spheroidizers currently used in industry are magnesium, calcium, and rare earth elements. While magnesium offers the advantages of strong spheroidizing ability and relatively low cost, its low boiling point leads to violent vaporization during the spheroidization reaction, reducing safety at the production site and polluting air quality. Rare earth elements have a significantly higher boiling point than magnesium and boil relatively smoothly in molten iron. However, they are expensive, and when used alone, they have weak spheroidizing ability, resulting in poor roundness of the graphite in the produced ductile iron. Calcium presents similar problems to rare earth elements: while it boils relatively smoothly in molten iron, it has weak spheroidizing ability. Therefore, composite spheroidizers containing magnesium are often used in ductile iron production. Ferrosilicon inoculants are currently commonly used in industry. Depending on the performance requirements for the ductile iron, calcium, aluminum, barium, titanium, copper, zirconium, tin, molybdenum, and other additives are often added to the ferrosilicon inoculant.

[0007] In addition to the use of spheroidizing agents and inoculants, the methods used for spheroidizing and inoculating also have a significant impact on the results of these treatments. The main methods for spheroidizing and inoculating include the injection molding method, the cover molding method, the in-mold method, the transfer molding method, and the wire feeding method. The injection molding method requires the use of a magnesium-containing composite spheroidizing agent. When producing ductile iron using the injection molding method, the spheroidizing agent is first placed in the pit of the molten iron ladle, and the inoculant and covering agent are then covered on top. The molten iron is then slowly poured in, and during the pouring process, the molten iron is controlled so that it flows to the other side of the pit, i.e., the side without the spheroidizing agent. The flushing method has the advantages of simple and convenient operation and can achieve stable production. However, during the pouring process of molten iron, the following problems exist: First, since the temperature of the molten iron is not lower than 1400°C, the density and boiling point of the magnesium in the magnesium-containing composite spheroidizer are lower than those of the molten iron. The magnesium will float up and convert into a gaseous state, resulting in the problem of magnesium escape. Magnesium will also undergo oxidation and ablation, resulting in a low magnesium absorption rate. Further, the amount of magnesium-containing composite spheroidizer used is large; second, the magnesium-containing composite spheroidizer used is generally block-shaped, and the gaps between each composite spheroidizer are large, which makes it easy for molten iron to penetrate, resulting in a large amount of spheroidizer rapidly floating up. The internal spheroidizer has not yet been released and has already floated to the surface of the molten iron, causing a large amount of magnesium to escape, resulting in a further reduction in the magnesium absorption rate.

[0008] In view of the above problems, after searching, the existing solutions are as follows:

[0009] The first solution is to add a layer of iron filings after the spheroidizing agent, inoculant and covering agent are added in sequence to prevent the molten iron from quickly penetrating between the spheroidizing agents and to control the speed of adding the molten iron. Specifically, when the molten iron is just started to be added, the speed of adding the molten iron is increased to quickly form a certain height of molten iron, extend the floating route of the magnesium-containing composite spheroidizing agent, reduce the oxidation and burning loss of magnesium, and further improve the absorption rate of magnesium.

[0010] The second solution is described in "Development and Application of Powdered Nodulizers" by Geng Gang. Dalian Jiaotong University, Master of Engineering Thesis, 2009. This solution involves crushing and grinding the bulk magnesium-containing composite nodulizer into powdered magnesium-containing composite nodulizer, and then wrapping the powdered magnesium-containing composite nodulizer with tinplate to slow down the melting rate of the powdered magnesium-containing composite nodulizer. The gaps between the powdered magnesium-containing composite nodulizers are small, making it difficult for molten iron to penetrate, thus preventing a large amount of composite nodulizer from floating up at the same time. In addition, the powdered magnesium-containing composite nodulizer has a large specific surface area and high activity, and will quickly react with anti-spheroidizing factors in the molten iron, shortening the free growth time of graphite balls. The powdered magnesium-containing composite nodulizer will be quickly consumed during the floating process, thus avoiding the problem of magnesium oxidation and burning caused by the powdered magnesium-containing composite nodulizer floating to the surface of the molten iron, and further improving the magnesium absorption rate.

[0011] A third solution, described in Chinese patent CN113337774B, involves adding a layer of nodulizing agent and a layer of covering agent sequentially from bottom to top into the molten iron ladle's concavity. Ferrosilicon blocks are first added to the ladle above the covering agent, followed by the molten iron. Ferrosilicon blocks are then added as the molten iron is injected. The first and second ferrosilicon blocks gradually rise to the surface as they react with the molten iron, where they react with oxygen in the air to form silicates. As the molten iron tumbles, the silicates are drawn into the molten iron, accelerating the nodulization reaction and, consequently, improving magnesium absorption.

[0012] However, after testing and analysis, the above three methods have the following problems: For the first solution, since the composite spheroidizing agent containing magnesium has a fast floating speed, even if its floating route is extended, there is still a problem of oxidation and burning of a large amount of magnesium.

[0013] Regarding the second solution, although the particle size of the powdered magnesium-containing composite spheroidizer is small, the molten iron cannot penetrate too deeply into the spheroidizer, and the heated layer is thin, thereby prolonging the spheroidization reaction time, the powdered magnesium-containing composite spheroidizer has poor process stability. On the one hand, because the gaps in the powdered magnesium-containing composite spheroidizer are too small, the molten iron cannot penetrate, resulting in the inability to form a molten layer. Further, a large amount of the powdered magnesium-containing composite spheroidizer is violently splashed under the impact of the molten iron. Therefore, only when the amount of the powdered magnesium-containing composite spheroidizer is small can the magnesium absorption rate be improved; on the other hand, when the amount of the powdered magnesium-containing composite spheroidizer is large, due to the high activity of the powdered magnesium-containing composite spheroidizer, it is easy to quickly combine with sulfur and oxygen in the molten iron to form sulfides or sulfur oxides, resulting in too fast growth of graphite crystals and over-spheroidization. Further, the spheroidization rate decreases. Therefore, it is difficult to prepare ductile iron with high spheroidization rate by using the powdered magnesium-containing composite spheroidizer.

[0014] The third solution is mainly aimed at promoting the rapid oxidation of silicon in ferrosilicon blocks to further promote the spheroidization reaction. However, ferrosilicon blocks are used as inoculants, and the dispersibility of ferrosilicon blocks is poor, resulting in uneven dispersion in the molten iron. The spheroidal graphite and collective structure in the prepared ductile iron are unevenly distributed, further resulting in reduced plasticity and toughness of the ductile iron. Summary of the Invention

[0015] In view of the shortcomings of the existing technology, the present invention provides a spheroidizing inoculation method for ductile iron with high spheroidizing rate, which can improve the spheroidizing rate, plasticity and toughness of ductile iron and improve the utilization rate of spheroidizing agent used in the production of ductile iron.

[0016] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0017] A spheroidizing inoculation method for ductile iron with high spheroidization rate, comprising: preparing a composite inoculant, preparing a spheroidizing agent, fixing, spheroidizing inoculation, and heat treatment;

[0018] The composite inoculant is prepared by uniformly mixing silicon powder and carbon black, adding the mixture into a corundum crucible, and transferring the mixture into a microwave sintering furnace for microwave sintering. After microwave sintering for 30-35 minutes, the microwave sintered product is taken out from the corundum crucible, ground, and then burned in the presence of oxygen to remove carbon to obtain the composite inoculant.

[0019] In the preparation of the composite inoculant, the mass ratio of silicon powder to carbon black is 28:23-25;

[0020] The microwave sintering furnace has a frequency of 2.45 GHz, a temperature of 1000-1020°C, and a vacuum degree of 0.08-0.085 MPa.

[0021] The grinding process has a particle size of 5-6 μm after grinding;

[0022] The D50 particle size of the silicon powder is 100 μm;

[0023] The average particle size of the carbon black is 31 nm and the specific surface area is 80 m 2 / g;

[0024] The spheroidizer is prepared by crushing a rare earth magnesium silicon type spheroidizer to obtain a millimeter-sized spheroidizer; then taking out one third of the millimeter-sized spheroidizer, continuing to crush and ball mill, and then mixing it with the remaining millimeter-sized spheroidizer to obtain a mixed rare earth magnesium silicon type spheroidizer; the mixed rare earth magnesium silicon type spheroidizer is uniformly mixed with iron powder to obtain a mixture, and the mixture is wrapped with tinplate to obtain a spheroidizer;

[0025] In the preparation of the spheroidizing agent, the mass ratio of the mixed rare earth magnesium silicon type spheroidizing agent to the iron powder is 100:2.4-2.6;

[0026] The particle size of the millimeter-scale spheroidizing agent is 1 mm;

[0027] The particle size after the continued pulverization is 100-150 μm;

[0028] The ball milling process has a particle size of 4-5 μm after ball milling;

[0029] When using tinplate to wrap the mixture, ensure that the tinplate just completely covers all the mixture;

[0030] The mass content of magnesium in the rare earth magnesium silicon type nodularizer is 8%, the mass content of rare earth is 4.8%, the mass content of calcium is 2.5%, the mass content of silicon is 41.8%, the mass content of manganese is 1.8%, the mass content of aluminum is 0.2%, the mass content of titanium is 0.3%, and the rest is iron;

[0031] The particle size of the iron powder is 100-150 μm;

[0032] The thickness of the tinplate is 2 mm;

[0033] The fixing step includes adding a spheroidizing agent, a covering agent, and an inoculant into the pit of the molten iron ladle from bottom to top, and tamping the spheroidizing agent;

[0034] In the fixing, the covering agent is rust-free iron filings with a particle size of 1-2 mm;

[0035] The inoculant is FeSi75 with a particle size of 5-6 mm;

[0036] The pit is cylindrical, the ratio of pit depth to diameter is 1.8, and the diameter of the pit accounts for one third of the diameter of the lower part of the molten iron ladle;

[0037] After tamping, the remaining depth of the pit is 25-30 mm;

[0038] The spheroidizing inoculation is performed by pouring molten iron into the other side of the pit in the ladle, first quickly pouring 62-67% of the total mass of the molten iron, and the pouring time accounts for one tenth of the total pouring time, then adding the composite inoculant, and slowly pouring the remaining molten iron to obtain a casting;

[0039] During the spheroidization inoculation, the temperature of the molten iron is 1420-1460° C., the mass content of carbon in the molten iron is 3.8%, the mass content of silicon is 2.5%, the mass content of copper is 0.48%, the mass content of molybdenum is 0.42%, the mass content of manganese is 0.26%, the mass content of tin is 0.07%, the mass content of magnesium is 0.04%, the mass content of phosphorus is 0.05%, the mass content of sulfur is 0.032%, and the remainder is iron;

[0040] The mass fraction of the mixed rare earth magnesium silicon type spheroidizer in the spheroidizer is 1.4-1.5% of the total mass of the molten iron;

[0041] The mass fraction of inoculant used in the total mass of molten iron is 0.7-0.73%;

[0042] The mass fraction of the composite inoculant used in the total mass of the molten iron is 0.2-0.22%;

[0043] The heat treatment comprises heating the casting to 900-920°C at a heating rate of 10-15°C / min, keeping the temperature at 900-920°C for 4.5-5h, cooling the casting to 700-720°C at a cooling rate of 4-6°C / min, keeping the temperature at 700-720°C for 2-2.5h, air-cooling the casting to room temperature, then heating the casting to 900-920°C at a heating rate of 10-15°C / min, cooling the casting to 400-450°C at a cooling rate of 10-15°C / min, keeping the temperature at 400-450°C for 1-1.5h, and air-cooling the casting to room temperature to obtain ductile cast iron.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) The spheroidization inoculation method of ductile iron with high spheroidization rate of the present invention uses a composite inoculant prepared by mixing silicon powder and carbon black, performing microwave sintering, and controlling the particle size of the silicon powder, the temperature and time of microwave sintering to ensure that only part of the silicon powder reacts with the carbon black to generate a mixture of silicon powder, carbon black and nano-silicon carbide, which is then ground and burned in an aerobic atmosphere to remove carbon to obtain a composite inoculant. The composite inoculant is a mixture of small-particle silicon powder and nano-silicon carbide. After the composite inoculant is added, the composite inoculant can be quickly dispersed as the iron liquid is poured in. The silicon therein generates silicate under the oxidation of air, which promotes the spheroidization reaction. The nano-silicon carbide can play a role in grain refinement, thereby improving the spheroidization rate and the uniformity of the distribution of spheroidal graphite and collective organization. The preparation method of the spheroidizer is as follows: after the rare earth magnesium silicon type spheroidizer is crushed into millimeter-sized spheroidizer, part of the millimeter-sized spheroidizer is taken and continued to be ground and ball-milled to a particle size of micron-sized spheroidizer, and then the millimeter-sized spheroidizer, micron-sized spheroidizer and iron powder are mixed, and the particle size distribution of the millimeter-sized spheroidizer, micron-sized spheroidizer and iron powder is controlled to ensure that there are appropriate gaps in the spheroidizer 1 to promote the inflow of molten iron. The iron powder can further promote the inflow of molten iron, avoiding the problem that the molten iron cannot penetrate. At the same time, the spheroidizer is released step by step, that is, the micron-sized spheroidizer reacts quickly first, and quickly reacts with the anti-spheroidizing factor in the molten iron, shortening the free growth time of the graphite balls. Then the millimeter-sized spheroidizer is released and spheroidization continues, thereby avoiding the problem that the activity of the spheroidizer is too high and the growth rate of the graphite crystals is too fast.

[0046] (2) The spheroidizing inoculation method for high-spheroidizing ductile iron of the present invention can improve the spheroidizing rate, plasticity and toughness of ductile iron, and improve the utilization rate of the spheroidizing agent used in the production of ductile iron. The absorption rate of magnesium in the spheroidizing inoculation method of the present invention is 54.2-55.5%, and the spheroidizing rate of the obtained ductile iron is 92.9-93.4%, the tensile strength is 926.7-945.2MPa, the yield strength is 653.0-668.1MPa, and the elongation is 9.0-9.2%. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram of the spheroidizing inoculation method of ductile iron in Example 1 and Example 2;

[0048] In the figure: 1-spheroidizing agent, 2-covering agent, 3-inoculant, 4-molten iron bag, 5-composite inoculant;

[0049] Figure 2 This is the metallographic structure diagram of the ductile iron obtained in Example 1. DETAILED DESCRIPTION

[0050] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific embodiments of the present invention are now described.

[0051] Example 1

[0052] A spheroidizing inoculation method for ductile iron with a high spheroidizing rate, specifically comprising:

[0053] 1. Preparation of Composite Inoculant 5: Silicon powder and carbon black were uniformly mixed in a mass ratio of 28:23, added to a corundum crucible, and transferred to a microwave sintering furnace for microwave sintering. The frequency of the microwave sintering furnace was controlled to 2.45 GHz, the temperature in the microwave sintering furnace was controlled to 1000°C, and the vacuum degree was controlled to 0.08 MPa. After microwave sintering for 30 minutes, the microwave sintered product was removed from the corundum crucible, ground to a particle size of 5 μm, and then carbon was removed by oxygen burning to obtain Composite Inoculant 5.

[0054] The D50 particle size of the silicon powder is 100 μm;

[0055] The average particle size of the carbon black is 31 nm and the specific surface area is 80 m 2 / g;

[0056] 2. Preparation of Nodulizer 1: Adding a rare earth magnesium silicon type nodulizer to a grinder and crushing it to a particle size of 1 mm to obtain a millimeter-grade nodulizer; then taking out one-third of the millimeter-grade nodulizer, continuing to crush it to a particle size of 100 μm, then transferring it to a ball mill, ball-milling it to a particle size of 4 μm, and then mixing it with the remaining millimeter-grade nodulizer to obtain a mixed rare earth magnesium silicon type nodulizer; the mixed rare earth magnesium silicon type nodulizer and iron powder are uniformly mixed in a mass ratio of 100:2.4 to obtain a mixture, and wrapping the mixture with tinplate, ensuring that the tinplate just completely covers all the mixture during wrapping, to obtain Nodulizer 1;

[0057] The mass content of magnesium in the rare earth magnesium silicon type nodularizer is 8%, the mass content of rare earth is 4.8%, the mass content of calcium is 2.5%, the mass content of silicon is 41.8%, the mass content of manganese is 1.8%, the mass content of aluminum is 0.2%, the mass content of titanium is 0.3%, and the rest is iron;

[0058] The particle size of the iron powder is 100 μm;

[0059] The thickness of the tinplate is 2 mm;

[0060] 3.Fix: According to Figure 1 Schematic diagram, spheroidizing agent 1, covering agent 2, inoculant 3 are added into the pit of the iron ladle 4 from bottom to top and rammed;

[0061] The covering agent 2 is rust-free iron filings with a particle size of 1 mm;

[0062] The inoculant 3 is FeSi75 with a particle size of 5 mm;

[0063] The pit is cylindrical, the ratio of pit depth to diameter is 1.8, and the diameter of the pit accounts for one third of the diameter of the lower part of the molten iron ladle;

[0064] After tamping, the remaining depth of the pit is 25 mm;

[0065] 4. Spheroidization inoculation: according to Figure 1 Schematic diagram, pouring molten iron into the other side of the pit in the iron ladle 4, first quickly pouring 62% of the total mass of molten iron, the pouring time accounts for one tenth of the total pouring time, and then adding the composite inoculant 5, slowly pouring the remaining molten iron to obtain a casting;

[0066] The temperature of the molten iron is 1420° C., and the mass content of the molten iron is 3.8% carbon, 2.5% silicon, 0.48% copper, 0.42% molybdenum, 0.26% manganese, 0.07% tin, 0.04% magnesium, 0.05% phosphorus, 0.032% sulfur, and the remainder is iron;

[0067] The mass fraction of the mixed rare earth magnesium silicon type spheroidizer in spheroidizer 1 to the total mass of the molten iron is 1.4-1.5%;

[0068] The mass fraction of inoculant 2 in the total mass of molten iron is 0.7%;

[0069] The mass fraction of the composite inoculant 5 in the total mass of the molten iron is 0.2%;

[0070] 5. Heat treatment: heat the casting to 900°C at a heating rate of 10°C / min, keep it at 900°C for 4.5 hours, cool it to 700°C at a cooling rate of 4°C / min, keep it at 700°C for 2 hours, air-cool it to room temperature, then heat it to 900°C at a heating rate of 10°C / min, cool it to 400°C at a cooling rate of 10°C / min, keep it at 400°C for 1 hour, and air-cool it to room temperature to obtain ductile iron.

[0071] The metallographic structure diagram of the ductile iron obtained in this embodiment is analyzed. Figure 2 .Depend on Figure 2 It can be seen that the ductile iron obtained in this embodiment is a ferrite-pearlite mixed ductile iron, and the spheroidal graphite is evenly distributed in the matrix structure.

[0072] Comparative Example 1

[0073] Based on the spheroidization inoculation method for ductile iron with high spheroidization rate of Example 1, the first step of preparing the composite inoculant 5 is omitted, and in the fourth step of spheroidization inoculation, an equal mass of a mixture of silicon carbide and silicon powder is used instead of the composite inoculant 5, the particle size of the mixture of silicon carbide and silicon powder is 5 μm, and the mass ratio of silicon carbide to silicon powder in the mixture of silicon carbide and silicon powder is 1:1.

[0074] The remaining operations are the same as those in Example 1.

[0075] Comparative Example 2

[0076] Based on the spheroidizing inoculation method of ductile iron with high spheroidizing rate in Example 1, the second step of preparing the spheroidizing agent 1 is changed to:

[0077] A rare earth magnesium silicon type nodularizer is added to a grinder and crushed to a particle size of 1 mm to obtain a millimeter-grade nodularizer; the millimeter-grade nodularizer is then further crushed to a particle size of 100 μm, and then transferred to a ball mill and ball-milled to a particle size of 4 μm to obtain a ball mill material, and the ball mill material is wrapped with a tinplate sheet, ensuring that the tinplate sheet completely covers all the ball mill material during the wrapping process to obtain a nodularizer 1;

[0078] The mass content of magnesium in the rare earth magnesium silicon type nodularizer is 8%, the mass content of rare earth is 4.8%, the mass content of calcium is 2.5%, the mass content of silicon is 41.8%, the mass content of manganese is 1.8%, the mass content of aluminum is 0.2%, the mass content of titanium is 0.3%, and the rest is iron;

[0079] The thickness of the tinplate is 2 mm.

[0080] The remaining operations are the same as those in Example 1.

[0081] Example 2

[0082] A spheroidizing inoculation method for ductile iron with a high spheroidizing rate, specifically comprising:

[0083] 1. Preparation of Composite Inoculant 5: Silicon powder and carbon black were uniformly mixed in a mass ratio of 28:25, added to a corundum crucible, and transferred to a microwave sintering furnace for microwave sintering. The frequency of the microwave sintering furnace was controlled to 2.45 GHz, the temperature in the microwave sintering furnace was controlled to 1020°C, and the vacuum degree was controlled to 0.085 MPa. After microwave sintering for 35 minutes, the microwave sintered product was removed from the corundum crucible, ground to a particle size of 6 μm, and then carbon was removed by oxygen burning to obtain Composite Inoculant 5.

[0084] The D50 particle size of the silicon powder is 100 μm;

[0085] The average particle size of the carbon black is 31 nm and the specific surface area is 80 m 2 / g;

[0086] 2. Preparation of Nodularizer 1: Adding a rare earth magnesium silicon type nodularizer to a grinder and crushing it to a particle size of 1 mm to obtain a millimeter-grade nodularizer; then taking out one-third of the millimeter-grade nodularizer, continuing to crush it to a particle size of 150 μm, transferring it to a ball mill, ball-milling it to a particle size of 5 μm, and then mixing it with the remaining millimeter-grade nodularizer to obtain a mixed rare earth magnesium silicon type nodularizer; the mixed rare earth magnesium silicon type nodularizer and iron powder are uniformly mixed in a mass ratio of 100:2.6 to obtain a mixture, and wrapping the mixture with tinplate, ensuring that the tinplate just completely covers all the mixture during wrapping, to obtain Nodularizer 1;

[0087] The mass content of magnesium in the rare earth magnesium silicon type nodularizer is 8%, the mass content of rare earth is 4.8%, the mass content of calcium is 2.5%, the mass content of silicon is 41.8%, the mass content of manganese is 1.8%, the mass content of aluminum is 0.2%, the mass content of titanium is 0.3%, and the rest is iron;

[0088] The particle size of the iron powder is 150 μm;

[0089] The thickness of the tinplate is 2 mm;

[0090] 3.Fix: According to Figure 1 Schematic diagram, spheroidizing agent 1, covering agent 2, inoculant 3 are added into the pit of the iron ladle 4 from bottom to top and rammed;

[0091] The covering agent 2 is rust-free iron filings with a particle size of 2 mm;

[0092] The inoculant 3 is FeSi75 with a particle size of 6 mm;

[0093] The pit is cylindrical, the ratio of pit depth to diameter is 1.8, and the diameter of the pit accounts for one third of the diameter of the lower part of the molten iron ladle;

[0094] After tamping, the remaining depth of the pit is 30 mm;

[0095] 4. Spheroidization inoculation: according to Figure 1 Schematic diagram, pouring molten iron into the other side of the pit in the iron ladle 4, first quickly pouring 62-67% of the total mass of molten iron, the pouring time accounts for one tenth of the total pouring time, and then adding the composite inoculant 5, slowly pouring the remaining molten iron to obtain a casting;

[0096] The temperature of the molten iron is 1460° C., and the mass content of the molten iron is 3.8% carbon, 2.5% silicon, 0.48% copper, 0.42% molybdenum, 0.26% manganese, 0.07% tin, 0.04% magnesium, 0.05% phosphorus, 0.032% sulfur, and the remainder is iron;

[0097] The mass fraction of the mixed rare earth magnesium silicon type spheroidizer in spheroidizer 1 to the total mass of the molten iron is 1.5%;

[0098] The mass fraction of inoculant 2 in the total mass of molten iron is 0.73%;

[0099] The mass fraction of composite inoculant 5 in the total mass of molten iron is 0.22%;

[0100] 5. Heat treatment: heat the casting to 920°C at a heating rate of 15°C / min, keep it at 920°C for 5 hours, cool it to 720°C at a cooling rate of 6°C / min, keep it at 720°C for 2.5 hours, air-cool it to room temperature, then heat it to 920°C at a heating rate of 15°C / min, cool it to 450°C at a cooling rate of 15°C / min, keep it at 450°C for 1.5 hours, air-cool it to room temperature, and obtain ductile iron.

[0101] Comparative Example 3

[0102] Based on the spheroidization inoculation method for ductile iron with high spheroidization rate of Example 2, the first step of preparing the composite inoculant 5 is omitted, and in the fourth step of spheroidization inoculation, an equal mass of a mixture of silicon carbide and silicon powder is used instead of the composite inoculant 5, the particle size of the mixture of silicon carbide and silicon powder is 6 μm, and the mass ratio of silicon carbide to silicon powder in the mixture of silicon carbide and silicon powder is 1:1.

[0103] The remaining operations are the same as those in Example 2.

[0104] Comparative Example 4

[0105] Based on the spheroidizing inoculation method of ductile iron with high spheroidizing rate in Example 2, the second step of preparing the spheroidizing agent 1 is changed to:

[0106] A rare earth magnesium silicon type nodularizer is added to a grinder and crushed to a particle size of 1 mm to obtain a millimeter-grade nodularizer; the millimeter-grade nodularizer is then further crushed to a particle size of 150 μm, and then transferred to a ball mill and ball-milled to a particle size of 5 μm to obtain a ball mill material, and the ball mill material is wrapped with a tinplate sheet, ensuring that the tinplate sheet completely covers all the ball mill material during the wrapping process to obtain a nodularizer 1;

[0107] The mass content of magnesium in the rare earth magnesium silicon type nodularizer is 8%, the mass content of rare earth is 4.8%, the mass content of calcium is 2.5%, the mass content of silicon is 41.8%, the mass content of manganese is 1.8%, the mass content of aluminum is 0.2%, the mass content of titanium is 0.3%, and the rest is iron;

[0108] The thickness of the tinplate is 2 mm.

[0109] The remaining operations are the same as those in Example 2.

[0110] Test Example 1

[0111] The magnesium absorption rate in Examples 1-2 and Comparative Examples 1-4, and the spheroidization rate, tensile strength, yield strength, and elongation of the obtained ductile iron were tested. The test results are as follows:

[0112]

[0113] The above test results show that the test results of Example 1 are better than those of Comparative Examples 1 and 2, and the test results of Example 2 are better than those of Comparative Examples 3 and 4. This indicates that the composite inoculant 5 and spheroidizing agent 1 used in Examples 1 and 2 can both improve the magnesium absorption rate, as well as the spheroidization rate, tensile strength, yield strength, and elongation of the ductile cast iron.

Claims

1. A spheroidizing inoculation method for ductile iron with high spheroidizing rate, characterized in that: include: preparing a composite inoculant (5), preparing a spheroidizing agent (1), fixing, spheroidizing and inoculating, and heat treating; The composite inoculant (5) is prepared by uniformly mixing silicon powder and carbon black, performing microwave sintering, and then grinding to remove carbon after microwave sintering for 30-35 minutes to obtain the composite inoculant (5); In the composite inoculant (5), the mass ratio of silicon powder to carbon black is 28:23-25; The frequency of microwave sintering is 2.45 GHz, the temperature is 1000-1020°C, and the vacuum degree is 0.08-0.085 MPa; The composite inoculant (5) is a mixture of small-particle silicon powder and nano-silicon carbide; The preparation of the spheroidizer (1) comprises crushing a rare earth magnesium silicon type spheroidizer to obtain a millimeter-sized spheroidizer; then taking out one third of the millimeter-sized spheroidizer, continuing to crush and ball mill, and then mixing it with the remaining millimeter-sized spheroidizer to obtain a mixed rare earth magnesium silicon type spheroidizer; uniformly mixing the mixed rare earth magnesium silicon type spheroidizer with iron powder to obtain a mixture, and wrapping the mixture with tinplate to obtain the spheroidizer (1); In the preparation of the spheroidizing agent (1), the particle size of the millimeter-scale spheroidizing agent is 1 mm; The particle size after the continued pulverization is 100-150 μm; The ball milling process has a particle size of 4-5 μm after ball milling; The particle size of the iron powder is 100-150 μm.

2. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 1, characterized in that: In the preparation of the composite inoculant (5), the grinding process is performed so that the particle size after grinding is 5-6 μm; The D50 particle size of the silicon powder is 100 μm; The average particle size of the carbon black is 31 nm and the specific surface area is 80 m 2 / g.

3. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 1, characterized in that: In the preparation of the spheroidizing agent (1), the mass ratio of the mixed rare earth magnesium silicon type spheroidizing agent to the iron powder is 100:2.4-2.6; When using tinplate to wrap the mixture, ensure that the tinplate just completely wraps all the mixture.

4. The spheroidizing inoculation method for ductile iron with high spheroidizing rate according to claim 1, characterized in that: In the preparation of the spheroidizing agent (1), the mass content of magnesium in the rare earth magnesium silicon type spheroidizing agent is 8%, the mass content of rare earth is 4.8%, the mass content of calcium is 2.5%, the mass content of silicon is 41.8%, the mass content of manganese is 1.8%, the mass content of aluminum is 0.2%, the mass content of titanium is 0.3%, and the remainder is iron; The thickness of the tinplate is 2 mm.

5. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 1, characterized in that: The fixing step includes sequentially adding a spheroidizing agent (1), a covering agent (2), and an inoculant (3) into the pit of the molten iron ladle from bottom to top, and tamping the spheroidizing agent (1).

6. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 5, characterized in that: In the fixing, the covering agent (2) is rust-free iron filings with a particle size of 1-2 mm; The inoculant (3) is FeSi75 with a particle size of 5-6 mm; The pit is cylindrical, the ratio of pit depth to diameter is 1.8, and the diameter of the pit accounts for one third of the diameter of the lower part of the molten iron ladle; After compaction, the remaining depth of the pit is 25-30 mm.

7. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 1, characterized in that: The spheroidizing inoculation is performed by pouring molten iron into the other side of the pit in the molten iron ladle (4), first pouring 62-67% of the total mass of the molten iron, and the pouring time accounts for one tenth of the total pouring time, then adding the composite inoculant (5), and pouring the remaining molten iron to obtain a casting.

8. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 7, characterized in that: During the spheroidization inoculation, the temperature of the molten iron is 1420-1460° C., the mass content of carbon in the molten iron is 3.8%, the mass content of silicon is 2.5%, the mass content of copper is 0.48%, the mass content of molybdenum is 0.42%, the mass content of manganese is 0.26%, the mass content of tin is 0.07%, the mass content of magnesium is 0.04%, the mass content of phosphorus is 0.05%, the mass content of sulfur is 0.032%, and the remainder is iron; The mass fraction of the mixed rare earth magnesium silicon type spheroidizer in the spheroidizer (1) to the total mass of the molten iron is 1.4-1.5%; The mass fraction of the inoculant (3) used in the total mass of the molten iron is 0.7-0.73%; The mass fraction of the composite inoculant (5) used in the total mass of the molten iron is 0.2-0.22%.

9. The spheroidizing inoculation method for high spheroidizing rate ductile iron according to claim 1, characterized in that: The heat treatment comprises heating the casting to 900-920°C at a heating rate of 10-15°C / min, keeping the temperature at 900-920°C for 4.5-5h, cooling the casting to 700-720°C at a cooling rate of 4-6°C / min, keeping the temperature at 700-720°C for 2-2.5h, air-cooling the casting to room temperature, then heating the casting to 900-920°C at a heating rate of 10-15°C / min, cooling the casting to 400-450°C at a cooling rate of 10-15°C / min, keeping the temperature at 400-450°C for 1-1.5h, and air-cooling the casting to room temperature to obtain ductile cast iron.

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