Production method of thick and large-section as-cast nodular iron casting

Abstract

A production method of a thick and large-section as-cast nodular iron casting comprises the following steps: weighing raw materials: a carburant, silicon carbide, pig iron, waste steel and foundry returns, firstly putting the silicon carbide, the pig iron, the waste steel and the foundry returns into a smelting furnace, and adding the carburant according to a formula proportion in the charging process; then heating and melting, and after melting down, adding FeMn65 and FeSi75 to obtain raw iron liquid; continuously heating the original molten iron to 1440 to 1480 DEG C, wherein the obtained molten iron comprises the following components: 3.40 to 3.70 percent of C, 1.7 to 1.9 percent of Si, 0.50 to 0.60 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.025 percent of S and the balance of iron; the preparation method comprises the following steps of: performing spheroidization by adopting a pouring method, then adding an inoculant with the particle size of 3-8mm and a pretreating agent with the particle size of 1-5mm, compacting, and finally adding pure antimony which accounts for 0.009-0.011% of the mass of the raw iron liquid and electrolytic copper which accounts for 0.90-1.20% of the mass of the raw iron liquid, and performing spheroidization and inoculation treatment; the molten iron subjected to spheroidizing and inoculation treatment is subjected to slagging-off and standing, and when the temperature is reduced to 1275-1295 DEG C, the molten iron is poured into a casting cavity through a pouring structure to form a casting; the method has the advantages that formation of pearlite is promoted, and plasticity reduction is not obvious.

Description

Technical Field

[0001] The present application relates to the technical field of casting metals, and particularly to a production method for thick and large-section as-cast ductile iron castings. Background Art

[0002] In recent years, the rapid development of the major equipment manufacturing industry has promoted the production and application of thick and large-section ductile iron (ductile iron with a wall thickness greater than or equal to 100 mm). The market demand for thick and large-section ductile iron castings is becoming increasingly strong, especially for high-strength thick and large-section pearlitic ductile iron castings. When thick and large-section pearlitic ductile iron solidifies, it is affected by the wall thickness, and the cooling condition is poor. The growth of graphite is easily affected by composition segregation, growth time, etc., resulting in the distortion of graphite balls, forming abnormal graphite such as thick, blooming, fragmented, and sharp shapes. If the cooling condition of the thick and large-section pearlitic casting cannot be changed, simply adjusting the chemical composition cannot guarantee the graphite morphology in the core of the thick and large section. Even if the feeding riser is used to solve the problem of shrinkage porosity, the problems of graphite ball distortion and performance deterioration in the core of the thick and large section still cannot be solved.

[0003] The conventional method to increase the cooling rate of the mold is to use direct chill to rapidly cool the molten iron, accelerate the cooling and solidification of the molten iron, increase the number of graphite balls in the core of the casting, refine the graphite balls, and improve the mechanical properties of the core. Since the direct chill is in direct contact with the casting liquid, the surface of the chill is required to be smooth, free of oxide layer, oil stain, pores or shrinkage depressions. During use, the surface of the chill also needs to be protected from rust. If the above requirements are not met, on the one hand, it will lead to defects such as surface wrinkles and cracks in the casting; on the other hand, the direct external chill placed in the mold for a long time will absorb moisture, resulting in porosity defects on the surface of the casting; in addition, the gases generated by the oxide layer and oil stain on the surface of the direct chill will react with the molten iron, directly affecting the quality of the molten iron and deteriorating the overall performance of the casting.

[0004] The strength and hardness of ductile iron increase with the increase of pearlite content, while the plasticity and toughness tend to decrease. As-cast high-strength ductile iron castings eliminate the heat treatment process, save intermediate transportation costs, shorten the supply cycle, can effectively reduce the production cost of enterprises, and improve product competitiveness. Although raw materials such as pig iron and alloys contain alloying elements that strengthen the matrix structure, the elongation rate of as-cast ductile iron is often low, so it has a high casting difficulty. Summary of the Invention

[0005] Aiming at the above deficiencies of the prior art, the present application provides a production method for thick and large-section as-cast ductile iron castings that can promote the formation of pearlite, with insignificant decrease in plasticity, and can solve the influence of direct chill on the quality of molten iron.

[0006] To solve the above technical problems, the technical solution adopted in this application is as follows: A production method for thick and large-section as-cast ductile iron castings, and the steps of this method include:

[0007] (1) Weigh the following main raw materials by mass percentage: pig iron 40% - 60%; scrap steel 40% - 50%; return scrap 0% - 20%; silicon carbide, accounting for 0.5% - 0.8% of the total weight of the above pig iron, scrap steel, and return scrap; recarburizer, accounting for 0.7% - 1.2% of the total weight of the above pig iron, scrap steel, and return scrap;

[0008] (2) First, put all the silicon carbide, pig iron, scrap steel, and return scrap weighed in step (1) into the melting furnace, and add the recarburizer in the formula ratio during the middle of feeding; then heat to make the furnace charge melt, and add FeMn65 (65 manganese iron) and FeSi75 (75 silicon iron) after the furnace charge is completely melted. The addition amount of FeMn65 (65 manganese iron) is 0.5% - 0.7% of the total mass of pig iron, scrap steel, and return scrap, and the addition amount of FeSi75 (75 silicon iron) is 0.6% - 1.0% of the total mass of pig iron, scrap steel, and return scrap to obtain the original molten iron;

[0009] Continue to heat the original molten iron to 1440 - 1480 °C. At this time, the composition and mass percentage of the obtained molten iron are: C 3.40% - 3.70%, Si 1.7% - 1.9%, Mn 0.50% - 0.60%, P ≤ 0.025%, S ≤ 0.025%, and the rest is iron;

[0010] (3) Adopt the impouring method for spheroidizing. First, add the spheroidizing agent into the spheroidizing dam on one side of the spheroidizing ladle and compact it. The spheroidizing agent is a mixture of 70wt% light rare earth magnesium alloy and 30wt% yttrium-based heavy rare earth magnesium alloy. Then add the inoculant with a particle size of 3 - 8 mm and the pretreatment agent with a particle size of 1 - 5 mm and compact them. Finally, add 0.009% - 0.011% pure antimony based on the mass of the original molten iron and 0.90% - 1.20% electrolytic copper based on the mass of the original molten iron for spheroidizing and inoculation treatment;

[0011] (4) Skim the slag from the molten iron after spheroidizing and inoculation treatment and let it stand. When the temperature drops to 1275 °C - 1295 °C, pour the molten iron into the casting cavity through the pouring structure to form a casting.

[0012] Furthermore, the element mass percentage of the silicon carbide described in step (1) is: SiC ≥ 85%, Si ≥ 60%, C ≥ 25%, S 0.02% - 0.05%, and the silicon carbide with a particle size of 1 - 5 mm, such as the silicon carbide produced by Anhui Jiuhua Fukang Metallurgical Materials Co., Ltd.

[0013] Further, the element mass percentages of the recarburizer described in step (1) are as follows: C≥98%, S≤0.05%, N≤0.01%, ash content≤0.3%, volatile content≤0.3%. The recarburizer has a particle size of 0.5 - 3 mm, such as the DC series recarburizer (DC-(1 - 4) type recarburizer) produced by Dansheng Industry (Shanghai) Co., Ltd.

[0014] Further, the element mass percentages of the light rare earth magnesium alloy described in step (3) are as follows: Mg 5.5% - 6.0%, RE (rare earth) 0.4% - 0.6%, Si 42% - 46%, Ca 0.8% - 1.2%, Ba 2.3% - 2.7%, Al≤1.0%, MgO≤0.40%, and the balance is Fe.

[0015] Further, the element mass percentages of the yttrium-based heavy rare earth magnesium alloy described in step (3) are as follows: Mg 5.5% - 6.0%, RE (rare earth) 0.8% - 1.2%, Si 42% - 46%, Ca 2.0% - 2.5%, Al≤1.0%, MgO≤0.40%, and the balance is Fe.

[0016] Further, the inoculant described in step (3) is a silicon-barium inoculant, and its element mass percentages are as follows: Si 71% - 73%, Ca 0.7% - 1.3%, Ba 1.6% - 2.4%, Al≤1.2%, S≤0.02%, and the balance is iron.

[0017] Further, the pretreatment agent described in step (3) is a high-barium treatment agent, and its element mass percentages are as follows: Si 60% - 70%, Ca 0.5% - 1.5%, Ba 8.0% - 11.0%, Al≤1.5%, and the balance is iron, such as the YFYY-2 pretreatment agent produced by Jiangsu Yafeng Alloy Materials Co., Ltd.

[0018] Further, the spheroidizing reaction time described in step (3) is completed within 90 s - 120 s. Controlling this spheroidizing time can improve the absorption rates of magnesium and rare earth, enhance the desulfurization effect, and correspondingly reduce the addition amount of the spheroidizing agent.

[0019] Further, in step (3), control the initiation time of the spheroidizing reaction and the duration of the magnesium explosion reaction. When the amount of molten iron discharged reaches 70% - 80% of the total amount of molten iron for spheroidizing treatment, start the initiation reaction, and the duration of the magnesium explosion reaction is 90 s - 120 s; this operation improves the absorption rates of magnesium and rare earth, enhances the desulfurization effect, and correspondingly reduces the addition amount of the spheroidizing agent.

[0020] Further, in step (3), the addition amount of the spheroidizing agent is 0.9% - 1.15% of the total amount of the original molten iron, the addition amount of the inoculant is 0.6% - 0.8% of the mass of the original molten iron, and the addition amount of the pretreatment agent is 0.15% - 0.25% of the mass of the original molten iron.

[0021] Further, the composition and mass percentage of the molten iron obtained after spheroidizing and inoculating in step (3) are: C 3.30% - 3.55%, Si 2.65% - 2.85%, Mn 0.50% - 0.60%, P ≤ 0.025%, S 0.008 - 0.012%, Cu 0.85% - 1.20%, Mg 0.025 - 0.038%, RE 0.003 - 0.009%, Sb 0.008% - 0.012%, CE = 4.20 - 4.50, and the rest is iron.

[0022] Further, in step (4), during the pouring of the molten iron, in-stream inoculation is carried out with inoculant powder, and the addition amount is 0.10% - 0.12% of the total amount of the original molten iron. The inoculant powder is a silicon-barium inoculant, and its elemental mass percentage is: Si 71% - 73%, Ca 0.7% - 1.3%, Ba 1.6% - 2.4%, Al ≤ 1.2%, S ≤ 0.02%, and the rest is iron.

[0023] Further, in step (4), the pouring system described includes a casting cavity and a pouring structure. Indirect chill blocks are arranged at intervals on the corresponding sides of each outer surface of the casting cavity. The pouring structure is connected to the casting cavity, and a choke plate is arranged in the pouring structure.

[0024] With the above structure, since the present application adopts the structure of the choke plate, the pouring medium or the molten iron coming out of the cross gate does not directly enter the casting cavity, but is blocked by the choke plate, which slows down the flow rate of the molten iron. Moreover, the molten iron between the choke plate and the sprue can be collected and buffered, and then enters the casting cavity more smoothly. The pouring speed of the molten iron can be effectively controlled to maintain a steady inflow of the molten iron, avoiding defects caused by too fast or too slow pouring speeds, thereby improving the purity of the molten iron and reducing shrinkage porosity and slag inclusion defects in the casting. By using indirect chill blocks, these chill blocks do not directly contact the casting surface, overcoming the casting defects of direct chill blocks.

[0025] Furthermore, the size of the indirect chill placed on the upper top surface of the casting cavity is 100 mm (length) × 100 mm (width) × 80 mm (height), and the sizes of the indirect chills placed on the lower bottom surface and the four side surfaces of the casting cavity are a combination of 200 mm (length) × 100 mm (width) × 80 mm (height) and 100 mm (length) × 100 mm (width) × 80 mm (height); the thickness of the molding sand layer between the indirect chill and the outer surface of the corresponding casting cavity is 30 - 40 mm, and the spacing between the indirect chills on the same outer surface is 35 - 40 mm; the setting of the above-mentioned indirect chills can not only avoid the casting defects caused by direct chills, but also ensure the uniform temperature of the molten iron during the pouring process and avoid the defects caused by excessive temperature difference; by controlling the thickness of the molding sand layer, the cooling rate of the molten iron can be adjusted. If the thickness of the molding sand layer is too thick, the cooling rate of the molten iron is too slow. If the thickness of the molding sand layer is too thin, the sand layer is likely to separate from the indirect chill, and the separated sand blocks or broken sand directly enter the molten iron, resulting in the scrapping of the casting. The present application effectively solves the above technical problems through the control of the thickness of the above-mentioned molding sand layer.

[0026] Furthermore, the pouring structure includes a sprue, a runner, a choke plate, and an ingate. The sprue is located at one end of the runner and is perpendicularly connected to the runner. One end of the ingate is horizontally connected to the runner, and the other end of the ingate is connected to the casting cavity. The choke plate is located at the end of the runner close to the sprue.

[0027] Furthermore, the depth (h) of the blockage of the choke plate is 0.2 - 0.3 times the height (H) of the runner. The thickness of the choke plate is 20 mm - 30 mm (the thickness in the extending direction of the length of the runner). The length and width of the choke plate extend 40 mm - 60 mm outward along the periphery of the runner respectively; adopting the above scheme is convenient for firmly fixing the choke plate at a specific position in the molding sand.

[0028] Furthermore, the lower bottom surface of the ingate is flush with the lower bottom surface of the runner. Adopting this structure can make the molten iron enter the casting cavity more smoothly and reduce casting defects.

[0029] Furthermore, four small risers are also provided on the upper surface of the casting cavity. The small risers are respectively arranged at the positions near the four corners of the top surface of the casting cavity; adopting this structure can feed the casting and prevent the defects caused by the shrinkage of the casting. In addition, the bubbles, dross, etc. of the molten iron can be led out from the position of the riser, further reducing the casting defects of the casting.

[0030] Furthermore, the total cross-section of the ingate is smaller than that of the runner. With this structure, the molten iron in the runner can be kept full, and the molten iron flowing into the cavity is purer and more stable, thereby reducing the casting defects of the casting.

[0031] The method of the present application has the following advantages and beneficial effects:

[0032] 1. Since the strength and hardness of ductile iron increase with the increase of pearlite content, while the plasticity and toughness tend to decrease. Although alloying elements for strengthening the matrix structure are included in raw materials such as pig iron and alloys, the elongation of as-cast ductile iron is often low, so it has a high casting difficulty. The present application uses a resin sand casting process to prepare as-cast thick-section pearlitic ductile iron castings. The effective addition of copper elements promotes the formation of pearlite while ensuring that the plasticity does not decrease significantly. The addition of an appropriate amount of silicon elements promotes the formation of ferrite, and at the same time has a significant strengthening effect on ferrite within a certain range, and the decrease in plasticity is not significant. In addition, by using the chilling effect of indirect chill blocks, the influence of direct chill blocks on the quality of molten iron is solved, and good matrix structure is obtained by rapid cooling. Also, small safety risers are used to achieve simultaneous solidification of the casting and solve the technical problem of shrinkage porosity of the casting. Therefore, the method of the present application realizes the low-cost production of thick-section high-strength as-cast QT700-2A ductile iron castings under as-cast conditions.

[0033] 2. The present application realizes the effect of increasing the cooling rate of the mold through the control of specific element compositions and preparation processes. The cooling rate of the mold has an important influence on the increase in the number of graphite nuclei, the refinement of the matrix structure, and the improvement of the properties of ductile iron. Therefore, controlling its cooling rate can effectively control casting defects. In addition, in order to neutralize the fast cooling rate of the molten iron, the carbon equivalent is controlled to be higher than that set in conventional sand mold casting. A high carbon equivalent not only does not cause graphite floating, but also increases the number of graphite balls, which is beneficial to preventing shrinkage porosity. A mold with high stiffness can also achieve small riser casting and ensure that the properties of different parts of the casting are basically the same.

[0034] 3. During the casting process of thick-section ductile iron, due to its large cross-sectional size, defects such as shrinkage cavity, shrinkage porosity, and slag inclusion are likely to occur. Therefore, there are special requirements for the gating system. The gating system of the present application can ensure the high fluidity, high filling ability and temperature uniformity of the molten iron, ensure the temperature uniformity of the molten iron during pouring, and avoid defects caused by excessive temperature difference. This gating system can control the smooth pouring speed of the molten iron to keep the molten iron flowing smoothly and avoid defects caused by too fast or too slow pouring speed.

[0035] 4. In the steps of the production method, the spheroidizing agent is a mixture of two spheroidizing agents. This is because the morphology of graphite will change during the spheroidizing process, and nail-shaped graphite or waterweed-like graphite structure is likely to appear. In order to eliminate these non-spheroidized graphite states, the present application uses a mixture of two spheroidizing agents for spheroidization, especially the addition of yttrium-based heavy rare earth magnesium alloy, thereby eliminating the non-spheroidized state of graphite and improving the spheroidization rate.

[0036] 5. In the production process of the present application, a pretreatment agent is added. The addition of the pretreatment agent can increase the ability to form a graphite core to ensure that the obtained casting has a greater spheroidization rate, and ensure the uniformity and small particle performance of the spheroidized graphite, thereby improving the performance of the overall casting.

[0037] 6. The production process of the present application adopts two inoculation steps. This is because the effect of the first inoculation will decay within a dozen minutes of inoculation, which will cause the graphite morphology to change and produce irregular graphite. The present application remedies this by performing a secondary inoculation to improve the graphite spheroidization efficiency of the casting.

[0038] 7. The composition of the molten iron after inoculation and spheroidization treatment and before pouring in the casting prepared in the present application effectively controls the dosage ratio of copper, manganese and silicon (Si 2.65% to 2.85%, Mn 0.50% to 0.60%, Cu 0.85% to 1.20%). Through the effective control of the dosage ratio of the three, the synergistic effect of the three can be brought into play to form a synergistic effect, thereby improving the overall plasticity of the casting and promoting the formation of pearlite. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 A schematic structural diagram of a pouring system (including indirect chiller) for thick and large-section as-cast ductile iron parts of the present application.

[0040] Figure 2 Structural schematic diagram of the first view of the pouring system of the present application (without indirect chiller).

[0041] Figure 3 Structural schematic diagram of the second view of the pouring system of the present application (without indirect chiller).

[0042] Figure 4 Structural schematic diagram of the first view of the casting structure of this application.

[0043] Figure 5 Structural schematic diagram of the second view of the casting structure of the present application.

[0044] Figure 6 A structural schematic diagram of the first view of the combination of the casting cavity and the indirect chiller of the present application.

[0045] Figure 7Schematic diagram of the second view of the combination of the casting cavity and the indirect chill in this application.

[0046] Figure 8 Schematic diagram of the third view of the combination of the casting cavity and the indirect chill in this application.

[0047] Figure 9 Schematic diagram of the cross-sectional view after filling the mold sand for the combination of the casting cavity and the indirect chill in this application.

[0048] Figure 10 Flow chart for obtaining specimens in this application.

[0049] Figure 11 Metallographic structure of the specimen prepared in Example 1 before corrosion.

[0050] Figure 12 Metallographic structure of the specimen prepared in Example 1 after corrosion.

[0051] Figure 13 Metallographic structure of the specimen prepared in Example 1 before corrosion.

[0052] Figure 14 Metallographic structure of the specimen prepared in Example 1 after corrosion.

[0053] As shown in the attached drawings: a. casting, a1. test bar, a2. specimen, 1. casting cavity, 2. gating system, 201. sprue, 202. runner, 203. ingate, 3. indirect chill, 4. choke, 5. mold sand layer, 6. small riser. Detailed implementation manners

[0054] Next, the technical solutions in the embodiments of this application will be clearly and completely described in conjunction with the embodiments and the attached drawings. Obviously, the described embodiments are only preferred embodiments, rather than all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention;

[0055] In addition, it should be noted that when a component is referred to as "fixed to" another component, it can be directly on the other component or there may be another intermediate component through which it is fixed. When a component is considered to be "connected to" another component, it can be directly connected to the other component or there may be another intermediate component at the same time. When a component is considered to be "disposed on" another component, it can be directly disposed on the other component or there may be another intermediate component at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are only for illustrative purposes. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used herein in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0056] For the castings and casting cavities mentioned in this application, their structures are basically the same. During the casting process, molten iron enters the casting cavity and finally cools and solidifies to form a casting. Therefore, the description of the corresponding positions of the subsequent castings is equivalent to the description of the corresponding positions of the casting cavities.

[0057] As shown in the attached Figures 1 - 9 figure, a gating system for a thick and large section as-cast ductile iron casting of this application is shown. The gating system includes a casting cavity 1 and a gating structure 2. Indirect chillers 3 are arranged at intervals on the corresponding sides of each outer surface of the casting cavity 1. The gating structure 2 is communicated with the casting cavity 1, and a choke plate 4 is arranged in the gating structure 2.

[0058] With the above structure, since the choke plate structure is adopted in this application, the pouring medium, that is, molten iron, coming out of the cross-riser does not directly enter the casting cavity, but is blocked by the choke plate, which slows down the flow rate of the molten iron. Moreover, the molten iron between the choke plate and the sprue can be collected and buffered, and then enters the casting cavity more smoothly. The pouring speed of the molten iron can be effectively controlled to maintain the smooth inflow of the molten iron and avoid defects caused by too fast or too slow pouring speed, thereby improving the purity of the molten iron and reducing the shrinkage porosity and slag inclusion defects of the casting. By using indirect chillers, these chillers do not directly contact the casting surface, overcoming the casting defects of direct chillers.

[0059] As an example, the size of the indirect chill 3 provided on the upper top surface of the casting cavity 1 described in this application is 100 mm (length) × 100 mm (width) × 80 mm (height), and the sizes of the indirect chills 3 provided on the corresponding sides of the lower bottom surface and the four side surfaces of the casting cavity 1 are a combination of 200 mm (length) × 100 mm (width) × 80 mm (height) and 100 mm (length) × 100 mm (width) × 80 mm (height); the thickness of the sand layer 5 between the indirect chill 3 and the outer surface of the corresponding casting cavity 1 is 30 - 40 mm (as shown by h2 in Figure 9 ), and the spacing between multiple indirect chills 3 corresponding to the same outer surface is 35 - 40 mm (as shown by h1 in Figure 9 ); the setting of the above-mentioned indirect chill 3 can not only avoid the casting defects caused by direct chills, but also ensure the uniform temperature of the molten iron during the pouring process and avoid the defects caused by excessive temperature difference; by controlling the thickness of the sand layer to adjust the cooling rate of the molten iron, if the thickness of the sand layer is too thick, the cooling rate of the molten iron is too slow, and if the thickness of the sand layer is too thin, the sand layer is easily separated from the indirect chill, and the separated sand blocks or broken sand directly enter the molten iron, resulting in the scrapping of the casting. This application effectively solves the above technical problems through the control of the thickness of the above-mentioned sand layer.

[0060] As shown in the attached Figures 1 - 5 figures, the pouring structure 2 described in this application includes a sprue 201, a runner 202, a choke 4, and an ingate 203. The sprue 201 is located at one end of the runner 202 and is perpendicularly connected to the runner 202. One end of the ingate 203 is horizontally connected to the runner 202, and the other end of the ingate 203 is connected to the casting cavity 1. The choke 4 is located at one end of the runner 202 close to the sprue 201; specifically, the sprue 201 of this application is a cylindrical structure with a circular cross-section, the cross-section of the runner 202 of this application is an isosceles trapezoidal structure, and the choke 4 of this application is a ceramic refractory flat brick (such as the manufacturer: Changxing Litang Refractory Materials Company can be used).

[0061] As an example, the blocking depth (h) of the choke 4 described in this application is 0.2 - 0.3 times the height (H) of the runner, the thickness of the choke is 20 mm - 30 mm (the thickness extending in the extending direction of the runner length), and the length and width of the choke extend 40 mm - 60 mm outward along the periphery of the runner; adopting the above scheme facilitates the firm fixation of the choke in a specific position in the sand.

[0062] As shown in the attached Figure 3As shown, the lower bottom surface of the ingates 203 described in the present application is flush with the lower bottom surface of the cross runner 202. Specifically, the ingates 203 feeds from the side wall near the bottom of the entire casting cavity 1. With this structure, the molten iron can enter the casting cavity more smoothly, reducing casting defects.

[0063] As attached Figures 1 - 3 As shown, four small safety risers 6 are also arranged on the upper surface of the casting cavity 1 described in the present application, and the small safety risers 6 are respectively arranged at the four corners of the top surface of the casting cavity 1; adopting this structure, the shrinkage of the casting can be compensated to prevent defects of the casting caused by shrinkage. In addition, bubbles, slag, etc. in the molten iron can be led out from the position of the safety risers to further reduce the casting defects of the casting.

[0064] As an example, the total cross-section of the inner runner 203 described in the present application is smaller than the total cross-section of the cross runner 202; adopting this structure, the molten iron in the cross runner can be filled, and the molten iron flowing into the mold cavity can be purer and smoother, thereby reducing casting defects of the casting.

[0065] The above-mentioned pouring system is used below to further describe the casting prepared in this application in detail through specific examples: Example 1

[0066] (1) Weigh the following raw materials in percentage by mass: 40% pig iron; 40% scrap steel; 20% recycled materials; silicon carbide, accounting for 0.6% of the total amount of pig iron, scrap steel and recycled materials; recarburizer, accounting for 0.7% of the total amount of pig iron, scrap steel and recycled materials;

[0067] (2) Put all the silicon carbide, pig iron, scrap steel and recycled materials into a smelting furnace, and add a carburizer all at once during the process. Heat the charge to melt it, and add FeMn65 ferromanganese and FeSi75 ferrosilicon after the charge is completely melted. The amount of ferromanganese added is 0.6% of the total mass of pig iron, scrap steel and recycled materials, and the amount of ferrosilicon added is 0.8% of the total mass of pig iron, scrap steel and recycled materials to obtain molten iron. Continue to heat the molten iron to 1470°C, and the composition and mass percentage of the obtained molten iron are C 3.45%, Si 1.75%, Mn 0.55%, P 0.021%, S 0.023%, and the rest is iron;

[0068] (3) Spheroidizing by flushing method, first add spheroidizing agent into the spheroidizing dam on one side of the spheroidizing bag and compact it, then add inoculant with a particle size of 3-8 mm and pretreatment agent with a particle size of 1-5 mm and compact it, and finally add 0.009% pure antimony by mass of the original iron liquid and 1.0% electrolytic copper by mass of the original iron liquid;

[0069] The addition amount of the spheroidizing agent is 1.1% of the mass of the original molten iron, and the spheroidizing agent is a mixture of 70% light rare earth magnesium alloy + 30% yttrium-based heavy rare earth magnesium alloy; the duration of the magnesium explosion reaction is 101 s; for the light rare earth magnesium alloy: Mg 5.9%, RE 0.49%, Si 44.8%, Ca 0.98%, Ba 2.5%, Al 0.49%, MgO 0.37%, and the balance is Fe; for the yttrium-based heavy rare earth magnesium alloy: Mg 6.0%, RE 0.9%, Si 43.5%, Ca 2.2%, Al 0.55%, MgO 0.35%, and the balance is Fe.

[0070] The addition amount of the inoculant is 0.65% of the mass of the original molten iron, and the inoculant is a silicon-barium inoculant, and the mass percentage of its elements is Si 72%, Ca 1.0%, Ba 2.0%, Al 0.70%, S 0.015%, and the balance is iron.

[0071] The addition amount of the pretreatment agent is 0.15% of the mass of the original molten iron, and the pretreatment agent is a high-barium treatment agent, and the mass percentage of its elements is Si 62%, Ca 0.8%, Ba 10.5%, Al 0.65%, and the balance is iron;

[0072] After spheroidizing inoculation, the composition and mass percentage of the molten iron obtained are: C 3.38%, Si 2.75%, Mn 0.56%, P 0.021%, S 0.0098%, Cu 0.95%, Mg 0.035%, RE 0.0085%, Sb 0.0088%, CE = 4.30, and the rest is iron;

[0073] (4) Skim the slag and let the molten iron obtained in step (3) stand still. When the temperature drops to 1295 °C, pour the molten iron into the casting cavity through the gating system to form a casting; during pouring, carry out in-stream inoculation with inoculation powder (the composition of the inoculation powder is the same as the above inoculant), and the addition amount is 0.12% of the total amount of the original molten iron; after the casting cools, the casting is obtained. The physical properties of the specimens taken from the center position of the casting obtained in the above embodiments of this application are shown in Tables 1 and 2:

[0074] Table 1 Mechanical properties of the test bars in Example 1

[0075]

[0076] Table 2 Microstructure composition of the test bars

[0077] Item Spheroidization rate Graphite size Spheroidite Standard value ≥90％ 5～8 ≥75％ Measured value 92.3％ 6 85％

[0078] This example uses resin sand casting process to prepare as-cast thick large-section pearlite ductile iron castings. The casting size is 400mm×400mm×4000mm. Test rod a1 is taken from the center of casting a and processed into sample a2 for performance testing. The casting body nesting position and the sample acquisition process are as follows: Figure 10 As shown (other examples are the same).

[0079] The metallographic structure of the sample obtained in this example is as follows: Figure 11 The figure shows the metallographic structure before corrosion. Figure 12 Shown is the metallographic structure after corrosion.

[0080] Example 2

[0081] Weigh the following raw materials in percentage by mass: 50% pig iron, 40% scrap steel, 10% recycled materials, silicon carbide: 0.7% of the total amount of pig iron, scrap steel and recycled materials, and recarburizer: 0.95% of the total amount of pig iron, scrap steel and recycled materials.

[0082] Put all silicon carbide, pig iron, scrap steel and recycled materials into a smelting furnace, add a carburizer at one time during the process; heat the charge to melt, add FeMn65 ferromanganese and FeSi75 ferrosilicon after the charge is completely melted, the amount of ferromanganese added is 0.68% of the total mass of pig iron, scrap steel and recycled materials, and the amount of ferrosilicon added is 0.7% of the total mass of pig iron, scrap steel and recycled materials, to obtain molten iron. Continue to heat the molten iron to 1475℃, and the composition and mass percentage of the obtained molten iron are C3.65%, Si 1.72%, Mn 0.53%, P 0.020%, S 0.022%, and the rest is iron;

[0083] The spheroidizing is carried out by flushing method. The spheroidizing agent is first added and compacted into the spheroidizing dam on one side of the spheroidizing bag, and then the inoculant with a particle size of 3-8mm and the pretreatment agent with a particle size of 1-5mm are added and compacted, and finally 0.010% pure antimony of the original iron liquid mass and 1.05% electrolytic copper of the original iron liquid mass are added.

[0084] The amount of spheroidizer added is 1.05% of the mass of the original iron liquid, and the spheroidizer is a mixture of 70% light rare earth magnesium alloy + 30% yttrium-based heavy rare earth magnesium alloy; the duration of magnesium explosion reaction is 95s.

[0085] Light rare earth magnesium alloy: Mg 5.9%, RE 0.49%, Si 44.8%, Ca 0.98%, Ba 2.5%, Al0.49%, MgO 0.37%, and the balance is Fe.

[0086] Yttrium-based heavy rare earth magnesium alloy: Mg 6.0%, RE 0.9%, Si 43.5%, Ca 2.2%, Al 0.55%, MgO 0.35%, and the balance is Fe.

[0087] The addition amount of the inoculant is 0.61% of the mass of the original molten iron. The inoculant is a silicon-barium inoculant, and its elemental mass percentages are 72% for Si, 1.0% for Ca, 2.0% for Ba, 0.70% for Al, 0.015% for S, and the balance is iron.

[0088] The addition amount of the pretreatment agent is 0.18% of the mass of the original molten iron. The pretreatment agent is a high-barium treatment agent, and its elemental mass percentages are 62% for Si, 0.8% for Ca, 10.5% for Ba, 0.65% for Al, and the balance is iron.

[0089] The composition and mass percentages of the obtained molten iron are 3.55% for C, 2.72% for Si, 0.53% for Mn, 0.020% for P, 0.0096% for S, 1.08% for Cu, 0.034% for Mg, 0.009% for RE, 0.0091% for Sb, CE = 4.45, and the rest is iron.

[0090] Skim the slag from the molten iron and let it stand. When the temperature drops to 1278 °C, pour the molten iron into the mold to form a casting.

[0091] During pouring, in-mold inoculation is carried out with inoculation powder, and the addition amount is 0.10%. After the casting cools, the casting of the present invention is obtained.

[0092] The physical properties of the specimens taken from the center position of the casting obtained in the above embodiments of this application are shown in Tables 3 and 4:

[0093] Table 3 Mechanical properties of the test bars in Example 2

[0094]

[0095] Table 4 Microstructure composition of the test bars in Example 2

[0096] Item Spheroidization rate Graphite size Spheroidite Standard value ≥90％ 5～8 ≥75％ Measured value 92.8％ 6 87％

[0097] The obtained specimen was detected for the metallographic structure as Figure 13 shown, which is the metallographic structure diagram before corrosion, Figure 14 and the following is the metallographic structure diagram after corrosion.

[0098] It can be seen from the above embodiments that in this application, copper is used to promote the formation of pearlite when the plasticity does not decrease significantly; appropriate silicon is used to promote the formation of ferrite, and at the same time, it has a significant strengthening effect on ferrite within a certain range, and the plasticity does not decrease significantly; the chilling effect of indirect chill is utilized to solve the influence of direct chill on the quality of molten iron and obtain a good matrix structure by rapid cooling; the use of a small safety riser realizes the simultaneous solidification of the casting and solves the problem of shrinkage porosity in the casting; it realizes the low-cost production of thick and large-section high-strength as-cast QT700-2A ductile iron castings under as-cast conditions.

Claims

1. A production method of thick and large cross-section as-cast ductile iron castings, characterized in that: The method steps include: (1) Weigh the following main raw materials by mass percentage: pig iron 40% - 60%; scrap steel 40% - 50%; remelted material 0% - 20%; silicon carbide, accounting for 0.5% - 0.8% of the total weight of the above pig iron, scrap steel, and remelted material; carburizer, accounting for 0.7% - 1.2% of the total weight of the above pig iron, scrap steel, and remelted material; (2) First, put all the silicon carbide, pig iron, scrap steel, and remelted material weighed in step (1) into the melting furnace, and add the carburizer in the formula ratio during the feeding process; then heat to melt the furnace charge. After the furnace charge is melted and clarified, add FeMn65 and FeSi75. The addition amount of FeMn65 is 0.5% - 0.7% of the total mass of pig iron, scrap steel, and remelted material, and the addition amount of FeSi75 is 0.6% - 1.0% of the total mass of pig iron, scrap steel, and remelted material to obtain the primary molten iron; Continue to heat the primary molten iron to 1440 - 1480 °C. At this time, the composition and mass percentage of the obtained molten iron are: C 3.40% - 3.70%, Si 1.7% - 1.9%, Mn 0.50% - 0.60%, P ≤ 0.025%, S ≤ 0.025%, and the rest is iron; (3) Adopt the impouring method for spheroidizing. First, add the spheroidizing agent into the spheroidizing dam on one side of the spheroidizing ladle and compact it. The spheroidizing agent is a mixture of 70wt% light rare earth magnesium alloy and 30wt% yttrium-based heavy rare earth magnesium alloy. Then add the inoculant with a particle size of 3 - 8mm and the pretreatment agent with a particle size of 1 - 5mm and compact them. Finally, add 0.009% - 0.011% pure antimony based on the mass of the primary molten iron and 0.90% - 1.20% electrolytic copper based on the mass of the primary molten iron for spheroidizing and inoculation treatment; (4) Skim the slag from the molten iron after spheroidizing and inoculation treatment and let it stand. When the temperature drops to 1275 °C - 1295 °C, pour the molten iron into the casting cavity through the pouring structure to form a casting.

2. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 1, characterized in that: The element mass percentage of the silicon carbide described in step (1) is: SiC ≥ 85%, Si ≥ 60%, C ≥ 25%, S 0.02% - 0.05%, and the silicon carbide with a particle size of 1 - 5mm; the element mass percentage of the carburizer described in step (1) is: C ≥ 98%, S ≤ 0.05%, N ≤ 0.01%, ash content ≤ 0.3%, volatile content ≤ 0.3%, and the carburizer with a particle size of 0.5 - 3mm.

3. The production method of thick and large cross-section as-cast ductile iron castings according to claim 1, characterized in that: The element mass percentage of the light rare earth magnesium alloy described in step (3) is: Mg 5.5% - 6.0%, RE 0.4% - 0.6%, Si 42% - 46%, Ca 0.8% - 1.2%, Ba 2.3% - 2.7%, Al ≤ 1.0%, MgO ≤ 0.40%, and the rest is Fe; The element mass percentage of the yttrium-based heavy rare earth magnesium alloy described in step (3) is: Mg 5.5% - 6.0%, RE 0.8% - 1.2%, Si 42% - 46%, Ca 2.0% - 2.5%, Al ≤ 1.0%, MgO ≤ 0.40%, and the rest is Fe; The inoculant described in step (3) is a silicon-barium inoculant, and its elemental mass percentages are as follows: Si 71% - 73%, Ca 0.7% - 1.3%, Ba 1.6% - 2.4%, Al ≤ 1.2%, S ≤ 0.02%, and the balance is iron; The pretreatment agent described in step (3) is a high-barium treatment agent, and its elemental mass percentages are as follows: Si 60% - 70%, Ca 0.5% - 1.5%, Ba 8.0% - 11.0%, Al ≤ 1.5%, and the balance is iron; In step (3), the initiation time of the spheroidizing reaction and the duration of the magnesium explosion reaction are controlled. When the amount of tapped iron reaches 70% - 80% of the total amount of molten iron for spheroidizing treatment, the initiation reaction starts, and the duration of the magnesium explosion reaction is 90s - 120s; In step (3), the addition amount of the spheroidizing agent is 0.9% - 1.15% of the total amount of the original molten iron, the addition amount of the inoculant is 0.6% - 0.8% of the mass of the original molten iron, and the addition amount of the pretreatment agent is 0.15% - 0.25% of the mass of the original molten iron; After spheroidizing and inoculating in step (3), the composition and mass percentages of the obtained molten iron are as follows: C 3.30% - 3.55%, Si 2.65% - 2.85%, Mn 0.50% - 0.60%, P ≤ 0.025%, S 0.008 - 0.012%, Cu 0.85% - 1.20%, Mg 0.025 - 0.038%, RE 0.003 - 0.009%, Sb 0.008% - 0.012%, CE = 4.20 - 4.50, and the rest is iron.

4. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 1, characterized in that: In step (4), during the pouring of the molten iron, in-stream inoculation is carried out with inoculation powder, and the addition amount is 0.10% - 0.12% of the total amount of the original molten iron. The inoculation powder is a silicon-barium inoculant, and its elemental mass percentages are as follows: Si 71% - 73%, Ca 0.7% - 1.3%, Ba 1.6% - 2.4%, Al ≤ 1.2%, S ≤ 0.02%, and the balance is iron.

5. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 1, characterized in that: The pouring system described in step (4) includes a casting cavity and a pouring structure. Indirect chillers are arranged at intervals on the corresponding sides of each outer surface of the casting cavity. The pouring structure is connected to the casting cavity, and a choke plate is arranged in the pouring structure.

6. The production method of thick and large section as-cast ductile iron castings according to claim 5, characterized in that: The size of the indirect chiller arranged on the corresponding side of the upper top surface of the casting cavity is 100mm (length) × 100mm (width) × 80mm (height). The sizes of the indirect chillers arranged on the corresponding sides of the lower bottom surface and the four side surfaces of the casting cavity are a combination of 200mm (length) × 100mm (width) × 80mm (height) and 100mm (length) × 100mm (width) × 80mm (height).

7. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 6, characterized in that: The thickness of the molding sand layer between the indirect chiller and the outer surface of the corresponding casting cavity is 30 - 40mm, and the distance between the indirect chillers on the same outer surface is 35 - 40mm.

8. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 6, characterized in that: The described pouring structure includes a sprue, a runner, a choke plate, and an ingate. The sprue is located at one end of the runner and is perpendicularly connected to the runner. One end of the ingate is horizontally connected to the runner, and the other end of the ingate is connected to the casting cavity. The choke plate is located at the end of the runner close to the sprue.

9. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 8, characterized in that: The depth of the blockage of the choke plate is 0.2 - 0.3 times the height of the runner. The thickness of the choke plate is 20 mm - 30 mm, and the length and width of the choke plate extend outward by 40 mm - 60 mm along the periphery of the runner.

10. The production method of the thick and large cross-section as-cast ductile iron casting according to claim 8, characterized in that: The lower bottom surface of the ingate is flush with the lower bottom surface of the runner; four small risers are further provided on the upper surface of the casting cavity, and the small risers are respectively arranged at the positions near the four corners of the top surface of the casting cavity; the total cross-sectional area of the ingate is smaller than the cross-sectional area of the runner.

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