Thick and large-section nodular cast iron inoculation method based on laminated slow-release inoculation block

Abstract

The invention relates to a thick and large-section nodular cast iron inoculation method based on a laminated slow-release inoculation block. The method comprises the following treatment steps that S1, raw materials are prepared; s2, the high-purity pig iron, the waste steel and the nodular cast iron foundry returns in the step S1 are put into an electric induction furnace to be smelted; s3, performing carbon and silicon testing on the molten iron subjected to slag removal in the step S2 by using a carbon and silicon analyzer, and adjusting components; s4, carrying out primary spheroidizing inoculation by adopting a pouring method; and S5, after spheroidizing inoculation is completed, molten iron is poured into a sand mold, a laminated slow-release inoculation block is placed in the sand mold for slow-release inoculation, and a thick and large-section spheroidal graphite cast iron casting is formed. The method has the advantages that slow-release inoculation is implemented in the cross gate, and molten iron continuously and slowly releases an inoculant in the pouring process through the laminated slow-release inoculation blocks. By means of the method, inoculation recession can be effectively inhibited, the graphite form is improved, and finally the density and the spheroidization rate of graphite nodules in the center of the thick and large-section nodular iron casting are increased.

Description

Technical Field

[0001] This invention belongs to the technical field of large-module ductile iron inoculation methods, specifically relating to a method for inoculating thick-section ductile iron based on layered slow-release inoculation blocks. Background Technology

[0002] In current large-module ductile iron casting technology, controlling the morphology of graphite nodules is one of the biggest technical challenges. As the casting wall thickness increases, the cooling rate slows down significantly, creating conditions for unstable growth and metamorphism of graphite nodules, posing a severe challenge to their formation and growth. This results in thick-section ductile iron failing to meet the requirements of high-strength service environments and limiting its application range. Therefore, it is necessary to improve the graphite nodule problem in thick-section ductile iron while simultaneously enhancing its strength and toughness. In particular, the problem of graphite nodule inoculation degradation at the center of thick-section ductile iron is particularly difficult to control. Therefore, it is urgent to propose an inoculation method for thick-section ductile iron based on layered slow-release inoculation blocks. Summary of the Invention

[0003] Purpose of the invention The purpose of this invention is to solve the problems of graphite spheroid count and graphite morphology in thick-section ductile iron. Therefore, it provides a method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks. By using layered slow-release inoculating blocks, the purpose of slow-release inoculation is achieved, thereby inhibiting inoculation decline and optimizing graphite morphology.

[0004] To achieve the above objectives, the present invention provides the following technical solution: A method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks includes the following processing steps: S1: Prepare raw materials. The raw material composition by weight percentage includes: 40-70% high-purity pig iron, 30-65% scrap steel, 5-20% ductile iron remelting material, and the remainder is 75# ferrosilicon, carbon raiser, inoculant and spheroidizing agent. S2: The high-purity pig iron, scrap steel and ductile iron recycled material from step S1 are placed in an induction furnace for heating to form molten iron, and then slag removal is performed. S3: The molten iron after slag removal in step S2 is tested for carbon and silicon content using a carbon and silicon analyzer. When the carbon content is less than 3.4% and the silicon content is less than 1.1%, a carbon raiser and ferrosilicon are used for adjustment. After adjustment, the composition is controlled at 3.4%≤C≤3.9%, 1.1%≤Si≤2.3%, Mn≤0.2%, S≤0.03%, P≤0.02%, Ti≤0.03%, and Cr≤0.03%. After the composition is accurate, the temperature of the molten iron is raised to maintain it at 1520±30℃ and then kept at that temperature. S4: Use the pouring method to perform one spheroidizing inoculation. Place a layer of rare earth magnesium silicon iron alloy spheroidizing agent on the bottom side away from the spheroidizing ladle pouring port, then place a layer of inoculant on the rare earth magnesium silicon iron alloy spheroidizing agent, and then place a layer of iron filings on the inoculant. Pour the pure molten iron after slag removal in step 3 from the medium frequency induction furnace into the spheroidizing ladle for spheroidizing inoculation. S5: After the spheroidization inoculation is completed, pour the molten iron into the sand mold, place the stacked slow-release inoculation block containing inoculant in the horizontal runner for slow-release inoculation, set the ceramic filter screen at the ingate, complete the filling, and form a thick cross-section ductile iron casting.

[0005] As a further description of the above scheme, the modulus of the thick-section ductile iron casting is ≥2.5cm, the ferrite content is ≥85%, the spheroidization rate is ≥85%, and the density of graphite nodules per unit area is ≥100 nodules / mm². 2 The casting has a tensile strength ≥ 410 MPa and an elongation ≥ 17%; in step S2, the melting temperature is controlled at 1310℃~1450℃, and light stony tuff is used as a slag remover, which is sprinkled on the molten iron; in step S5, the casting time is controlled at 20~30 seconds.

[0006] As a further description of the above scheme, the high-purity pig iron comprises, by weight percentage: 4.20–4.45% C, 0.32–0.46% Si, 0.061–0.069% Mn, 0.023–0.026% P, 0.014–0.019% S, 0.0032–0.0036% Cr, 0.0012–0.0016% Ni, 0.0012–0.0016% Cu, and trace elements with a content of less than 0.003%, the remainder being iron; the trace elements are one or more combinations of Mo, V, Ti, Nb, Al, As, B, Bi, Ca, Cd, Ce, Co, La, Mg, N, Pb, Sb, Se, Sn, Te, W, Zn, or Zr.

[0007] As a further description of the above scheme, the composition of the scrap steel by mass percentage is as follows: Mn < 0.24%, S < 0.02%, P < 0.03%, Ti < 0.03%, Cr < 0.03%, Cu < 0.03%, Mo < 0.01%, and the total of other trace alloying elements is less than 0.05%, with the remainder being iron. The total of the other trace alloying elements is less than 0.05%. The ductile iron remelting material is the waste, scrap, or leftover molten iron generated during the production of ductile iron, which is recycled and remelted in the furnace for reuse. The carbon raiser is an additive used to increase the carbon content in molten metal, and its main component is elemental carbon.

[0008] As a further description of the above scheme, the spheroidizing agent is a rare earth magnesium-silicon-iron alloy spheroidizing agent, the content of which accounts for 0.8-1.1% of the final formed casting mass percentage, the particle size of which is 5-20 mm, and the composition of which by mass percentage includes: 46-48% Si, 6.0-6.2% Mg, 2.1-2.2% Ca, and 0.7-0.9% Al. The content of Re is less than 1%, and the remainder is iron; the inoculant is a composite of barium silicon inoculant and sulfur-oxygen inoculant, with the barium silicon inoculant accounting for 0.6-1.2% of the mass of the molded casting and having a particle size of 0.5-2 mm; the sulfur-oxygen inoculant accounting for 0.3-0.5% of the mass of the molded casting and having a particle size of 0.5-1.5 mm; the barium inoculant and the sulfur-oxygen inoculant are evenly spread on the top surface of the rare earth magnesium silicon iron alloy spheroidizing agent.

[0009] As a further description of the above scheme, the silicon barium inoculant composition by mass percentage is: 44-47% Si, 0.8-1.2% Ca, 0.4-1.0% Al, 0.4-0.6% RE, 5.5-6.5% Mg, MgO < 0.6%, with the remainder being iron; the SO inoculant composition by mass percentage is: 68-70% Si, 0.03-0.05% Ba, 0.96-1.02% Ca, 1.10-1.16% Al, and 2.32-2.37% RE, with the remainder being iron.

[0010] As a further description of the above scheme, in step S5, the slow-release inoculation uses SO inoculant, the weight of which accounts for 0.1-0.2% of the final molded casting mass, and the particle size of which is 0.1-0.5mm; the slow-release inoculation SO inoculant is mixed with iron powder and then hydraulically injected to form inoculated iron sheets, the iron powder accounting for 20-40% of the total weight of the inoculant and iron powder during mixing, the hydraulically inoculated iron sheets are 20-28mm wide, 20-28mm long, and 3-4mm high, the inoculated iron sheets are mixed with iron powder to increase weight and prevent them from floating during casting.

[0011] As a further description of the above scheme, in step S5, the stacked slow-release inoculum block is made of 2 to 4 layers of iron foil, each layer being a cuboid of 30 to 35 mm × 30 to 35 mm × 5 mm, with an iron foil thickness of 0.2 to 0.4 mm. Slow-release inoculum is achieved by melting layer by layer. The stacked slow-release inoculum block is made by cutting the iron foil into rectangles and then placing them into an inoculum sheet and pressing them together.

[0012] As a further description of the above scheme, each of the ductile iron laminated slow-release inoculant blocks contains SO inoculant accounting for 0.07 to 0.18% of the casting mass; the laminated slow-release inoculant blocks are fixed to the runner by steel nail structure, and a 0.3 to 0.5 mm thick iron sheet baffle is provided at the ingate in the direction of molten iron flow in the runner.

[0013] As a further description of the above scheme, the ceramic filter screen is a zirconia-based composite material with a pore size of 1.5 to 2.0 mm; the zirconia-based composite material includes zirconia and the stabilizer yttrium oxide.

[0014] Advantages and effects of the present invention: 1. This invention implements slow-release inoculation within the horizontal runner, using layered slow-release inoculation blocks to continuously and slowly release the inoculator during the pouring process. This method effectively inhibits inoculation decline and improves graphite morphology, ultimately achieving a graphite spheroid density of over 100 spheroids / mm² in the center of large-modulus ductile iron castings, and increasing the spheroidization rate to over 85%.

[0015] The layered slow-release inoculant block of this invention adopts a layered iron foil encapsulation structure (2-4 layers), achieving slow release of the inoculant through layer-by-layer melting. Combined with steel nail fixing and iron sheet baffle guiding design, it effectively prevents the inoculant block from being dispersed or floating by the molten iron, ensuring a stable and controllable slow-release process. This design is highly robust, easy to operate, and directly compatible with existing casting production lines.

[0016] This invention utilizes a composite inoculation process (in-package barium silicon + sulfur-oxygen inoculator + slow-release SO inoculator) to synergistically optimize the matrix microstructure. While improving the density and sphericity of graphite spheres, it promotes ferrite formation rate ≥85%, ultimately achieving a balance between high strength and high toughness—tensile strength ≥410MPa and elongation ≥17%, meeting the mechanical performance requirements of heavy-duty components. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the stacked sustained-release incubation block used in an embodiment of the present invention; Figure 2 This is a schematic diagram of the sustained-release incubation method used in an embodiment of the present invention; Figure 3 This is a schematic diagram of the sustained-release incubation planar projection used in an embodiment of the present invention; Figure 4 This image shows the microstructure of the thick-section ductile iron core according to an embodiment of the present invention.

[0018] Explanation of reference numerals in the attached figures: The attached diagram lists the components represented by each number as follows: 101. Pour cup, 102. Sprue, 103. Runner, 104. Layered slow-release inoculum block, 105. Ingate, 106. Ceramic filter screen, 107. Casting, 108. Inoculum sheet, 109. Riser, 110. Iron sheet baffle, 111. Sand box, 112. Steel nail structure. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] A method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks includes the following processing steps: S1: Prepare raw materials. The raw material composition by weight percentage includes: 40-70% high-purity pig iron, 30-65% scrap steel, 5-20% ductile iron remelting material, and the remainder is 75# ferrosilicon, carbon raiser, inoculant and spheroidizing agent. S2: The high-purity pig iron, scrap steel and ductile iron recycled material from step S1 are placed in an induction furnace and heated to form molten iron. The smelting temperature is controlled at 1310℃~1450℃. Light stone tuff is used as a slag remover. The slag remover is sprinkled on the molten iron and slag is removed. S3: Use a carbon and silicon analyzer to test the carbon and silicon content of the molten iron after slag removal in step S2. When the carbon content is less than 3.4% and the silicon content is less than 1.1%, use a carbon raiser and ferrosilicon to adjust the composition. After adjustment, the composition should be controlled at 3.4%≤C≤3.9%, 1.1%≤Si≤2.3%, Mn≤0.2%, S≤0.03%, P≤0.02%, Ti≤0.03%, and Cr≤0.03%. After the composition is accurate, raise the temperature of the molten iron to 1520±30℃ and hold it for 6 to 8 minutes. S4: Use the pouring method for one spheroidizing inoculation. Place a layer of rare earth magnesium silicon iron alloy spheroidizing agent at the bottom on the side away from the spheroidizing ladle pouring port, then place a layer of inoculant on top of the rare earth magnesium silicon iron alloy spheroidizing agent, and finally place a layer of iron filings on top of the inoculant. Pour the pure molten iron after slag removal in step 3 from the medium frequency induction furnace into the spheroidizing ladle for spheroidizing inoculation. The spheroidizing treatment temperature is 1430-1480℃, and the spheroidizing inoculation to casting time is 5-15 minutes. S5: After spheroidization and inoculation are complete, the molten iron is poured into the sand mold. A layered slow-release inoculating block 104 containing inoculant is placed in the runner 103 for slow-release inoculation. A ceramic filter screen 106 is placed at the ingate 105. The pouring time is controlled at 20-30 seconds, and the mold is finally filled to form a thick-section ductile iron casting 107. This application implements slow-release inoculation in the runner 103, using the layered slow-release inoculating block 104 to continuously and slowly release the inoculant during pouring. This method effectively inhibits inoculation decline and improves graphite morphology, ultimately achieving a graphite spheroid density of over 100 spheroids / mm² in the center of the large-modulus ductile iron casting, and increasing the spheroidization rate to over 85%.

[0021] The thick-section ductile iron casting 107 of the present invention has a modulus ≥ 2.5 cm, a ferrite content ≥ 85%, a spheroidization rate ≥ 85%, and a unit graphite nodule density ≥ 100 nodules / mm². 2 The tensile strength of casting 107 is ≥410MPa, and the elongation of casting 107 is ≥17%.

[0022] The high-purity pig iron of this invention comprises, by weight percentage: 4.20–4.45% C, 0.32–0.46% Si, 0.061–0.069% Mn, 0.023–0.026% P, 0.014–0.019% S, 0.0032–0.0036% Cr, 0.0012–0.0016% Ni, 0.0012–0.0016% Cu, and trace elements with a content of less than 0.003%, the remainder being iron; wherein the trace elements are one or more combinations of Mo, V, Ti, Nb, Al, As, B, Bi, Ca, Cd, Ce, Co, La, Mg, N, Pb, Sb, Se, Sn, Te, W, Zn, or Zr. The use of high-purity pig iron in this application can reduce the influence of impurity elements on the spheroidizing effect. The scrap steel composition of this invention, by mass percentage, is as follows: Mn < 0.24%, S < 0.02%, P < 0.03%, Ti < 0.03%, Cr < 0.03%, Cu < 0.03%, Mo < 0.01%, and the total of other trace alloying elements is less than 0.05%, with the remainder being iron, wherein the total of other trace alloying elements is less than 0.05%; the ductile iron remelting material is the waste, scrap, or leftover molten iron generated during the production process of ductile iron, which is recycled and remelted in the furnace for reuse; the carbon raiser is an additive used to increase the carbon content in molten metal, and its main component is elemental carbon.

[0023] The spheroidizing agent of this invention is a rare earth magnesium-silicon-iron alloy spheroidizing agent, wherein the content of the rare earth magnesium-silicon-iron alloy spheroidizing agent accounts for 0.8-1.1% of the mass percentage of the final formed casting, the particle size of the rare earth magnesium-silicon-iron alloy spheroidizing agent is 5-20 mm, and the composition of the rare earth magnesium-silicon-iron alloy spheroidizing agent by mass percentage includes: 46-48% Si, 6.0-6.2% Mg, 2.1-2.2% Ca, 0.7-0.9% Al, and Re < 1%. The remainder is iron; the inoculant is a composite of silicon-barium inoculant and sulfur-oxygen inoculant. The silicon-barium inoculant accounts for 0.6-1.2% of the mass of the 107-tonnage of the shaped casting, and the particle size of the silicon-barium inoculant is 0.5-2mm. The sulfur-oxygen inoculant accounts for 0.3-0.5% of the mass of the 107-tonnage of the shaped casting, and the particle size of the sulfur-oxygen inoculant is 0.5-1.5mm. The barium inoculant and the sulfur-oxygen inoculant are evenly spread on the top surface of the rare earth magnesium-silicon-iron alloy spheroidizing agent.

[0024] The silicon-barium inoculant of the present invention comprises, by mass percentage: 44-47% Si, 0.8-1.2% Ca, 0.4-1.0% Al, 0.4-0.6% RE, 5.5-6.5% Mg, MgO < 0.6%, with the remainder being iron; the SO inoculant comprises, by mass percentage: 68-70% Si, 0.03-0.05% Ba, 0.96-1.02% Ca, 1.10-1.16% Al, and 2.32-2.37% RE, with the remainder being iron.

[0025] In step S5 of this invention, slow-release inoculation uses SO inoculant, wherein the weight of SO inoculant accounts for 0.1 to 0.2% of the mass percentage of the final molded casting 107, and the particle size of SO inoculant is 0.1 to 0.5 mm; wherein the slow-release inoculation SO inoculant is mixed with iron powder and then hydraulically injected to form inoculated iron sheet 108, wherein the proportion of iron powder during mixing is 20 to 40% of the total weight of inoculant and iron powder, and the hydraulically inoculated iron sheet 108 is 20 to 28 mm wide, 20 to 28 mm long, and 3 to 4 mm high. The mixing of inoculated iron sheet 108 with iron powder increases its weight and prevents it from floating during casting.

[0026] In step S5 of this invention, the multilayered slow-release inoculant block 104 is made of 2 to 4 layers of iron foil, each layer being a cuboid of 30 to 35 mm × 30 to 35 mm × 5 mm, with an iron foil thickness of 0.2 to 0.4 mm. Slow-release inoculant is achieved by melting each layer sequentially. Specifically, the multilayered slow-release inoculant block 104 is made by cutting the iron foil into a rectangle and then placing it into an inoculant sheet 108 and pressing it down. The preparation method is as follows: the iron foil is cut into a rectangle and then folded into a U-shaped groove. The groove depth is adapted to the height of the inoculant sheet 108. The inoculant sheet 108 is placed in the groove and pressed down using a hydraulic tool to make the iron foil and the inoculant sheet 108 fit tightly together. Then, the multilayered iron foil grooves are folded and stacked, and fixed at the four corners with stainless steel rivets to form an integral structure. In this design, the U-shaped groove provides positioning guidance for the inoculum sheet 108, increasing the contact area between the iron foil and the inoculum sheet 108 to accelerate heat conduction. The rivets ensure that the multiple layers of iron foil do not loosen under the influence of high-temperature molten iron. Through the gradual melting of the iron foil from the outside to the inside, the SO inoculant in the inoculum sheet 108 is continuously released during the pouring process. This, combined with the guidance of the iron sheet baffle 110 in the ingate 103, ensures that the molten iron enters the casting cavity after the first layer of the laminated inoculum block 104 melts, achieving slow-release inoculuming of thick-section ductile iron, inhibiting inoculum decay, and improving graphite morphology. The laminated slow-release inoculum block 104 of this application adopts a 2-4 layered iron foil encapsulation structure, achieving slow release of the inoculant through layer-by-layer melting. The combination of steel nail structure 112 for fixing and iron sheet baffle 110 for guiding design effectively prevents the stacked slow-release inoculant block 104 from being dispersed or floating by the molten iron, ensuring the stability of the inoculant block in the flow of molten iron and guaranteeing a stable and controllable slow-release process. This design is highly robust, easy to operate, and directly compatible with existing casting production lines.

[0027] The iron baffle 110 is located upstream of the ingate 105, forming a blocking-guiding effect, so that the molten iron first flows through the area of ​​the layered slow-release inoculant block 104 in the horizontal runner 103, and then enters the casting 107 through the ingate 105; at the same time, after the molten iron is blocked by the iron baffle 110, the flow rate near the layered slow-release inoculant block 104 is slowed down, the contact time is increased, and the inoculant is slowly released through the melting of the iron foil.

[0028] Each ductile iron laminated slow-release inoculant block 104 of the present invention contains SO inoculant accounting for 0.07 to 0.18% of the mass percentage of the casting 107; wherein the laminated slow-release inoculant block 104 is welded and fixed to the runner 103 by steel nail structure 112, and a 0.3 to 0.5 mm thick iron sheet baffle 110 is provided at the ingate 105 in the direction of molten iron flow in the runner 103 to prevent it from being rolled by molten iron to the gate, resulting in slow-release inoculation failure; and guides the molten iron to preferentially flush the inoculant block.

[0029] The ceramic filter 106 of this invention is a zirconia-based composite material with a pore size of 1.5–2.0 mm; wherein the zirconia-based composite material includes zirconia and the stabilizer yttrium oxide. The ceramic filter 106 of this application can effectively filter inclusions in molten iron, reduce internal defects in castings such as slag inclusions and porosity, and further improve the density and mechanical property stability of the castings.

[0030] The forming process of molten iron in the sand box 111 in this application is as follows: After spheroidization and inoculation, the molten iron is poured from the induction furnace into the casting system in the sand box 111. It is first received by the pouring cup 101 and flows into the vertically arranged sprue 102, and then turns into the horizontal runner 103. A stacked slow-release inoculating block 104 is installed in the runner 103 and fixed by a steel nail structure 112. It is made by folding 2-4 layers of iron foil into a U-shaped groove and embedding it into the inoculating iron sheet 108 and riveting it together. When the molten iron flows through, the heat causes the iron foil to melt layer by layer. The SO inoculating agent in the inoculating iron sheet 108 is continuously released into the molten iron to achieve slow-release inoculation. An iron sheet baffle 110 is set near the ingate 105 in the runner 103 to guide the molten iron to preferentially flush the inoculating block 104 and prevent it from floating. When molten iron flows into the ingate 105 through the gating system 103, it passes through a ceramic filter 106 (zirconia-based composite material, pore size 1.5-2.0 mm) to filter out inclusions, and then enters the mold cavity in the sand box 111 to form the casting 107. During the pouring process, the riser 109 is located above the casting 107 to collect gas and slag and to compensate for shrinkage. The entire filling process is controlled within 20-30 seconds. After the molten iron cools and solidifies in the sand box 111, a thick-section ductile iron casting with a modulus ≥ 2.5 cm is obtained, and the density of graphite nodules in its central part is ≥ 100 nodules / mm², and the nodularity is ≥ 85%, meeting the requirements for high-strength mechanical properties.

[0031] like Figure 4 The image shown is of the microstructure of the thick-section ductile iron core of this invention. It can be seen that the diameter of the graphite spheres is 24.92 μm and the number of graphite spheres is 110 per mm. 2 The spheroidization rate is 85.3%, which is about 15% higher than that of traditional in-flow inoculated graphite spheres, about 8% higher in spheroidization rate, and about 10% lower in diameter of graphite spheres. This effectively overcomes the problem of inoculation decline, resulting in a finer, rounder, and more numerous graphite sphere structure in thick-section ductile iron. The optimized graphite structure will directly translate into better mechanical properties and more reliable quality of the castings, especially in the application of thick-section components with strict performance requirements.

[0032] This invention utilizes a composite inoculation process (in-cell barium silicate + sulfur-oxygen inoculator + slow-release SO inoculator) to synergistically optimize the matrix microstructure. While improving the density and sphericity of graphite spheres, it promotes ferrite formation at a rate ≥85%, ultimately achieving a balance between high strength and high toughness—tensile strength ≥410 MPa and elongation ≥17%, meeting the mechanical performance requirements of heavy-duty components. The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks, characterized in that, The following processing steps are included: S1: Prepare raw materials. The raw material composition by weight percentage includes: 40-70% high-purity pig iron, 30-65% scrap steel, 5-20% ductile iron remelting material, and the remainder is 75# ferrosilicon, carbon raiser, inoculant and spheroidizing agent. S2: The high-purity pig iron, scrap steel and ductile iron recycled material from step S1 are placed in an induction furnace for heating to form molten iron, and then slag removal is performed. S3: The molten iron after slag removal in step S2 is tested for carbon and silicon content using a carbon and silicon analyzer. When the carbon content is less than 3.4% and the silicon content is less than 1.1%, a carbon raiser and ferrosilicon are used for adjustment. After adjustment, the composition is controlled at 3.4%≤C≤3.9%, 1.1%≤Si≤2.3%, Mn≤0.2%, S≤0.03%, P≤0.02%, Ti≤0.03%, and Cr≤0.03%. After the composition is accurate, the temperature of the molten iron is raised to maintain it at 1520±30℃ and then kept at that temperature. S4: Use the pouring method to perform one spheroidizing inoculation. Place a layer of rare earth magnesium silicon iron alloy spheroidizing agent on the bottom side away from the spheroidizing ladle pouring port, then place a layer of inoculant on the rare earth magnesium silicon iron alloy spheroidizing agent, and then place a layer of iron filings on the inoculant. Pour the pure molten iron after slag removal in step 3 from the medium frequency induction furnace into the spheroidizing ladle for spheroidizing inoculation. S5: After the spheroidization inoculation is completed, pour the molten iron into the sand mold, place the stacked slow-release inoculation block (104) containing the inoculant in the horizontal runner (103) for slow-release inoculation, set the ceramic filter screen (106) at the ingate (105) to complete the filling and form a thick cross-section ductile iron casting (107).

2. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, The casting (107) of the thick-section ductile iron has a modulus ≥ 2.5 cm, a ferrite content ≥ 85%, a spheroidization rate ≥ 85%, and a unit graphite spheroid density ≥ 100 spheroids / mm². 2 The tensile strength of the casting (107) is ≥410MPa and the elongation of the casting (107) is ≥17%. In step S2, the melting temperature is controlled at 1310℃~1450℃, and light stone tuff is used as a slag remover, which is sprinkled on the molten iron. In step S5, the casting time of the test piece is controlled at 20~30 seconds.

3. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, The high-purity pig iron comprises, by weight percentage: 4.20–4.45% C, 0.32–0.46% Si, 0.061–0.069% Mn, 0.023–0.026% P, 0.014–0.019% S, 0.0032–0.0036% Cr, 0.0012–0.0016% Ni, 0.0012–0.0016% Cu, and trace elements with a content of less than 0.003%, the remainder being iron; the trace elements are one or more combinations of Mo, V, Ti, Nb, Al, As, B, Bi, Ca, Cd, Ce, Co, La, Mg, N, Pb, Sb, Se, Sn, Te, W, Zn, or Zr.

4. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, The scrap steel composition by mass percentage is as follows: Mn < 0.24%, S < 0.02%, P < 0.03%, Ti < 0.03%, Cr < 0.03%, Cu < 0.03%, Mo < 0.01%, and the total of other trace alloying elements is less than 0.05%, with the remainder being iron. The total of the other trace alloying elements is less than 0.05%. The ductile iron remelting material is the waste, scrap, or leftover molten iron generated during the production of ductile iron, which is recycled and remelted in the furnace for reuse. The carbon raiser is an additive used to increase the carbon content in molten metal, and its main component is elemental carbon.

5. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, The spheroidizing agent is a rare earth magnesium-silicon-iron alloy spheroidizing agent. The content of the rare earth magnesium-silicon-iron alloy spheroidizing agent accounts for 0.8-1.1% of the mass percentage of the final formed casting (107). The particle size of the rare earth magnesium-silicon-iron alloy spheroidizing agent is 5-20 mm. The composition of the rare earth magnesium-silicon-iron alloy spheroidizing agent by mass percentage includes: 46-48% Si, 6.0-6.2% Mg, 2.1-2.2% Ca, 0.7-0.9% Al, and Re < 1%, with the remainder being... The inoculant is a composite of barium silicon inoculant and sulfur-oxygen inoculant. The barium silicon inoculant accounts for 0.6-1.2% of the mass percentage of the molded casting (107), and the particle size of the barium silicon inoculant is 0.5-2 mm. The sulfur-oxygen inoculant accounts for 0.3-0.5% of the mass percentage of the molded casting (107), and the particle size of the sulfur-oxygen inoculant is 0.5-1.5 mm. The barium inoculant and the sulfur-oxygen inoculant are evenly spread on the top surface of the rare earth magnesium silicon iron alloy spheroidizing agent.

6. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 5, characterized in that, The silicon-barium inoculant composition by mass percentage is: 44-47% Si, 0.8-1.2% Ca, 0.4-1.0% Al, 0.4-0.6% RE, 5.5-6.5% Mg, MgO < 0.6%, with the remainder being iron; the SO inoculant composition by mass percentage is: 68-70% Si, 0.03-0.05% Ba, 0.96-1.02% Ca, 1.10-1.16% Al, and 2.32-2.37% RE, with the remainder being iron.

7. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, In step S5, slow-release inoculation uses SO inoculator, the weight of which accounts for 0.1-0.2% of the mass of the final molded casting (107), and the particle size of which is 0.1-0.5 mm. The slow-release inoculator is mixed with iron powder and then hydraulically injected to form an inoculated iron sheet (108). During mixing, the iron powder accounts for 20-40% of the total weight of the inoculator and iron powder. The hydraulically injected iron sheet (108) is 20-28 mm wide, 20-28 mm long, and 3-4 mm high. The mixing of the inoculated iron sheet (108) with the iron powder increases its weight to prevent it from floating during casting.

8. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, In step S5, the stacked slow-release inoculum block (104) is made of 2 to 4 layers of iron foil, each layer being a cuboid of 30 to 35 mm × 30 to 35 mm × 5 mm, with an iron foil thickness of 0.2 to 0.4 mm. Slow-release inoculum is achieved by melting layer by layer. The stacked slow-release inoculum block (104) is made by cutting the iron foil into a rectangle and then placing it into an inoculum sheet (108) and pressing it.

9. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 8, characterized in that, Each of the aforementioned ductile iron laminated slow-release inoculant blocks (104) contains SO inoculant accounting for 0.07 to 0.18% of the mass percentage of the casting (107); the laminated slow-release inoculant blocks (104) are welded and fixed to the runner (103) by steel nail structure (112), and a 0.3 to 0.5 mm thick iron sheet baffle (110) is provided at the ingate (105) in the direction of molten iron flow of the runner (103).

10. The method for inoculating thick-section ductile iron based on layered slow-release inoculating blocks according to claim 1, characterized in that, The ceramic filter screen (106) is a zirconia-based composite material with a pore size of 1.5 to 2.0 mm; the zirconia-based composite material includes zirconia and the stabilizer yttrium oxide.

Images