Smelting equipment and method for high-purity cast molten iron for 100-ton ductile iron cask

Abstract

This invention relates to a smelting apparatus and method for producing high-purity cast iron for 100-ton ductile iron casks. It solves at least one of the problems inherent in conventional smelting, such as the inability of induction furnaces alone to simultaneously achieve the desired molten iron volume, chemical composition, process temperature, purity, homogenization, and refinement, as well as the difficulty of controlling the graphite shape of the finished metallurgical structure. The apparatus includes an induction furnace, a first ladle refining furnace, a second ladle refining furnace, a distribution system, and a spheroidizing system. This invention employs two ladle refining furnaces, with the induction furnace for primary rough smelting and the ladle refining furnace for secondary refining, thereby achieving precise control of the major alloying elements, such as C, P, and S, in the molten iron. The secondary refined molten iron is distributed by the distribution system and then transferred to the spheroidizing system for spheroidizing, ensuring the overall compositional uniformity and homogenization level of the 100-ton cask castings.

Description

[Technical Field]

[0001] The present invention relates to the technical field of the smelting industry, and more particularly to an apparatus and method for smelting high-purity cast molten iron for 100-ton ductile iron casks. [Background technology]

[0002] Spent nuclear fuel, also known as irradiated nuclear fuel, generally refers to nuclear fuel that has lost its uranium content to a certain level in a nuclear power plant reactor and is no longer able to sustain a nuclear reaction. Spent nuclear fuel contains large amounts of radioactive elements, making it highly harmful to the environment. Its long half-life also necessitates proper disposal. The disposal process involves storage, transportation, reprocessing, and deep geological disposal, with spent nuclear fuel storage and transportation equipment serving as the storage and transportation container for these processes. As of 2021, China had 22 nuclear power plants in operation or under construction, creating an increasing need for spent nuclear fuel disposal. Due to the unique characteristics of spent nuclear fuel, storage and transportation equipment requires extremely high technical standards, high technical requirements, and high costs. For the time being, storage and transportation equipment is primarily imported, with no domestically produced alternatives yet available. These factors make this a high-value-added "strangulation" project.

[0003] In the prior art, ductile iron castings are mainly produced by using a cupola or induction furnace to produce raw molten iron, which is then subjected to a spheroidizing and inoculation process, followed by casting and molding, followed by a series of processes including demolding, cleaning, and heat treatment. The amount of molten iron that can be produced at one time is limited by the nominal capacity of the smelting furnace body, and is usually several tons to several tens of tons. The composition of the molten iron varies greatly due to limitations on the raw materials and the smelting process. Induction heating alone makes it difficult to control the process temperature during the process and lacks an inclusion removal process, making it difficult to control the overall metallurgical quality of the raw molten iron.

[0004] Storage and transportation cask equipment has a unit weight of over 100 tons, strict technical standards, and a special casting process, so induction furnace smelting equipment alone cannot simultaneously meet metallurgical quality requirements such as molten iron volume, chemical composition, process temperature, purity, and uniformity. Summary of the Invention [Problem to be solved by the invention]

[0005] Based on the above description, an embodiment of the present invention aims to provide an apparatus and method for smelting high-purity cast molten iron for 100-ton ductile cast iron casks, in order to solve at least one of the problems of the prior art, such as the inability of an induction furnace smelting apparatus alone to simultaneously satisfy the molten iron volume, chemical composition, process temperature, molten iron purity, homogenization, and precision requirements for storage and transportation cask equipment with a unit weight of 100 tons or more, and the difficulty in controlling the graphite shape of the metal structure of the finished product. [Means for solving the problem]

[0006] The object of the present invention is mainly achieved by the following technical solutions: The present invention provides a smelting apparatus for high-purity cast molten iron for a 100-ton ductile iron cask, the apparatus comprising: an induction furnace including a first station and a second station, used for crude smelting molten iron to obtain first-station crude smelted molten iron and second-station crude smelted molten iron; a first ladle refining furnace, in which the first station crude smelted molten iron enters the first ladle refining furnace and undergoes primary refining to obtain primarily refined molten iron; a second ladle refining furnace in which the primary refined molten iron and the second station crude smelted molten iron are sequentially introduced to perform secondary refining to obtain secondary refined molten iron; a distribution facility in which secondary refined molten iron is distributed and transported within said distribution facility; and a spheroidizing facility for performing a spheroidizing treatment on the distributed and transported molten iron in the spheroidizing facility.

[0007] The present invention also provides a method for smelting molten iron using the apparatus, the method comprising: Step (1) of crude smelting using an induction furnace, comprising adding cold steel, cast iron, nickel alloy plate, and graphite powder to a first station to crude smelt the cold steel, cast iron, nickel alloy plate, and graphite powder to obtain first-station crude smelted molten iron, and adding cast iron and graphite powder to a second station to crude smelt the cold steel, cast iron, and graphite powder to obtain second-station crude smelted molten iron; (2) a step of secondary refining using a ladle refining furnace, in which the crude smelted molten iron from the first station is added to a first ladle refining furnace for primary refining to obtain primary refined molten iron, and the crude smelted molten iron from the first station and the crude smelted molten iron from the second station are added to a second ladle refining furnace in that order for secondary refining to obtain secondary refined molten iron; and (3) transferring and distributing the secondary refined molten iron in the distribution facility and subjecting it to spheroidizing treatment in the spheroidizing treatment facility.

[0008] Furthermore, the mass of the cold steel material in the raw materials for producing molten iron is strictly limited to 15 to 20% of the total mass of the cold steel material, pig iron, and nickel alloy plate.

[0009] Furthermore, the steel material is obtained by dephosphorization and decarburization through crude smelting in an electric arc furnace, carbon adjustment and desulfurization through refining in a ladle furnace, and billet casting in a mold in an atmospheric environment.

[0010] Furthermore, the cold-rolled steel material described in step (1) contains, on a mass percent basis, 0.25 to 0.45% C, 0.01% or less Si, 0.05% or less Mn, 0.005% or less P, 0.005% or less S, 0.05% or less Cr, 0.05% or less Mo, and trace amounts of Sb, W, V, Pb, As, Sn, and Zr.

[0011] Further, in step (1), the foundry pig iron contains, by mass percent, 4.50 to 4.70% C, 0.40 to 0.60% Si, 0.100% or less Mn, 0.030% or less P, 0.025% or less S, 0.010% or less Cr, 0.10% or less Ni, 0.010% or less Mo, and 0.050% or less Ti.

[0012] Furthermore, in step (1), the mass fraction of Ni in the nickel alloy plate exceeds 99.5%.

[0013] Furthermore, in step (1), the process temperatures of the first station and the second station for crude smelting of molten iron are both 1550°C or less, and in both cases, after all the charged materials have been purified, the temperature is maintained at 1500 to 1520°C for 5 to 20 minutes before tapping.

[0014] Furthermore, in step (2), a method of stirring by blowing argon gas into the bottom of the ladle is adopted throughout the entire process of primary refining and secondary refining.

[0015] Furthermore, in step (2), a slag formation process is performed in both the primary refining and secondary refining. The slag forming materials in the primary refining are metallurgical lime and fluorite, with the mass ratio of metallurgical lime to fluorite being 4:1, and the slag forming material in the secondary refining is fluorite.

[0016] Furthermore, the process temperature of the primary refining and secondary refining of the molten iron in step (2) is 1550°C or less.

[0017] Furthermore, in step (2), cold steel and foundry pig iron are further added to adjust the mass and weight of the components of the molten iron during the primary refining process.

[0018] Furthermore, in step (3), before the spheroidizing treatment, argon gas is blown into the bottom of the ladle to make the temperature of the molten iron uniform, thereby setting the spheroidizing treatment temperature at 1350 to 1450°C.

[0019] Furthermore, step (3) further includes transferring the spheroidized molten iron to a casting process for final inoculation, deslag and pouring operations. [Effects of the Invention]

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: (1) Compared with conventional ductile iron smelting equipment, the equipment of the present invention solves the difficult problem of conventional equipment and smelting processes that use only an induction furnace for smelting, which make it impossible to precisely control the composition and temperature of the molten iron or to uniformize and synchronize the composition and temperature of the molten iron raw material for spheroidization.The present invention employs two ladle refining furnaces, with the induction furnace for primary smelting and the ladle refining furnace for secondary refining, thereby achieving precise control of the main alloying components in the molten iron, such as C, P, and S.The secondary refined molten iron is distributed by a distribution facility and then transferred to the spheroidizing facility for spheroidizing treatment, ensuring the overall composition uniformity and homogenization level of the cask castings. The equipment of the present invention can utilize ordinary smelting equipment when large-scale specialized equipment is not available. Through process and technological route innovation, it is possible to provide highly homogenized molten iron as raw material for spheroidization in a single operation, and the product quality meets strict technical standards. This solves the difficult problem of single-furnace smelting of molten iron for ductile iron cask equipment for storing and transporting 100-ton spent nuclear fuel. In the physical and chemical property tests of the finished product, all technical index items exceed the design requirements of the product specifications. (2) In the present invention, in addition to cold steel, cast pig iron, and nickel alloy plate, alloys are not added during the process to adjust the target composition of the molten iron. Adding alloys will increase the residual elements, affect the purity of the molten iron, and reduce the final evaluation of the graphite shape of the metal structure of the product. In particular, a dedicated alloy design composition control process must be strictly implemented in the secondary refining process of molten iron production. (3) The molten iron smelting method of the present invention controls the slag composition through a scientific slag formation process, thereby regulating the sulfur content and realizing the purification treatment of the molten iron, and significantly reducing the content of endogenous and exogenous inclusions in the molten iron. By adopting a secondary refining process using a large-tonnage ladle refining furnace, the molten iron can simultaneously meet the special technical requirements for precise control of the designed composition, high purity and small amount of inclusions, high uniformity of composition and temperature, and one-time single-furnace smelting of 100-ton class products. This is a new technological path and technical concept, with obvious technological progress, and thereby produces ductile iron products with excellent performance indexes. (4) Molten iron with a high phosphorus content causes the phosphorus eutectic phenomenon at the grain boundaries, which leads to deterioration of the parent phase structure and reduced mechanical properties of the finished casting. Therefore, the P content of the raw molten iron used for spheroidizing must be 0.020% or less. The conventional molten iron smelting process (induction furnace melting + out-of-furnace treatment) cannot meet this technical standard. The main reason for this is that the phosphorus content of domestically produced pig iron for casting is currently basically 0.025% or more, and when melting using only an induction furnace, it is not possible to limit the P element to the target component range. Therefore, the cold-formed steel employed in the method of the present invention is obtained by undergoing primary crude smelting in an electric arc furnace, secondary refining in a ladle refining furnace, and finally billet casting in a mold in an atmospheric environment. By limiting the P content of the cold-formed steel to 0.001% or less through a special process, it is possible to keep the P weight of the final molten iron raw material for spheroidizing to 0.020% or less, thereby eliminating the process limitation that dephosphorization (P) is not possible when smelting molten iron in an induction furnace. (5) In the present invention, the carbon content of the raw material molten iron used for spheroidizing is 3.50-4.00%. During the induction furnace smelting, tapping, transferring, mixing, ladle furnace tapping, distribution, and spheroidizing processes, high-temperature, high-carbon molten iron will experience some degree of carbon burnout or carbon washout. Therefore, to ensure the target carbon (C) content of the product, graphite powder recarburization is required during the induction furnace smelting process. Furthermore, the carbon (C) content of the molten iron tapped from the induction furnace must meet the final carbon weight value of the refining process. The present invention achieves precise control of carbon during the smelting process by minimizing the influence of carbon fluctuations through precise measurement of raw materials, design of auxiliary materials, and empirical process carbon loss. (6) In the present invention, the process temperatures for the rough smelting and refining of molten iron are limited, and the process design already takes into account that the tapping temperature of secondary refining compensates for the temperature drop of molten iron during the subsequent transport, distribution, and waiting processes, thereby precisely achieving the optimal process temperature requirements for the spheroidizing treatment of molten iron. (7) The method of the present invention achieves high purity, uniformity of composition and temperature, and precise control of the optimal process temperature for spheroidization by injecting argon gas into the bottom of the ladle during refining. This method, combined with slag formation, promotes the rising of endogenous and exogenous inclusions in the molten iron, which are then adsorbed and removed by the metallurgical slag. This achieves high purity, uniform composition and temperature, and precise control of the optimal process temperature for spheroidization, which are unattainable with conventional molten iron production processes. The high kinetic energy imparted by the high-pressure argon gas during stirring accelerates the mass transfer process of the molten iron from the bottom to the surface. The fine, high-purity argon gas bubbles generated from the porous bricks at the bottom of the ladle act as countless microscopic vacuum chambers. As the hydrostatic pressure of the molten iron decreases, the rising argon gas bubbles increase in volume, transporting hydrogen and nitrogen gases from the molten iron into the argon gas bubbles. Various inclusions are also adsorbed during collisions with the argon gas bubbles and reach the iron-slag interface, where they are adsorbed and removed by the basic slag. The method of the present invention clearly improves the effect of inoculation for spheroidization by limiting the S content in the spheroidizing molten iron to 0.004 to 0.009%, and can improve the graphite shape of the metal structure of the casting.

[0021] In the present invention, the above technical solutions can be combined with each other to realize more preferred combined solutions. Other features and advantages of the present invention will be set forth in the following specification, and some advantages will be apparent from the specification or may be learned by practice of the invention. It is believed that the objectives and other advantages of the present invention will be realized and obtained by the particular points pointed out in the specification and drawings. [Brief explanation of the drawings]

[0022] The drawings are only for purposes of illustrating specific embodiments and are not to be considered as limitations on the invention, and like reference numerals represent like parts throughout the drawings.

[0023] [Figure 1] 1 is a schematic diagram of the smelting equipment and process flow of high-purity cast molten iron for 100-ton ductile cast iron casks of the present invention. [Figure 2]1 is a metallographic diagram (scale 50 μm) of a cask for storing and transporting spent nuclear fuel made from the ductile cast iron material prepared in Example 2 of the present invention after cask corrosion. [Figure 3] 1 is a metallographic diagram (scale 200 μm) of a cask for storing and transporting spent nuclear fuel made from the ductile cast iron material prepared in Example 2 of the present invention before cask corrosion. [Figure 4] This is an actual on-site view of a cask for storing and transporting spent nuclear fuel made from ductile cast iron material according to Example 2 of the present invention after semi-finishing processing. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The drawings, which form part of this application, serve to explain the principles of the present invention together with the embodiments of the present invention, but are not intended to limit the scope of the present invention.

[0025] As a specific embodiment of the present invention, as shown in FIG. 1, the present invention provides a smelting apparatus for high-purity cast molten iron for a 100-ton ductile iron cask, the apparatus comprising: an induction furnace including a first station and a second station, used for crude smelting molten iron to obtain first-station crude smelted molten iron and second-station crude smelted molten iron; a first ladle refining furnace, in which the first station crude smelted molten iron enters the first ladle refining furnace and undergoes primary refining to obtain primarily refined molten iron; a second ladle refining furnace in which the primary refined molten iron and the second station crude smelted molten iron are sequentially introduced to perform secondary refining to obtain secondary refined molten iron; a distribution facility in which secondary refined molten iron is distributed and transported within said distribution facility; and a spheroidizing facility for performing a spheroidizing treatment on the distributed and transported molten iron in the spheroidizing facility.

[0026] In operation, the operating principle of the device is as follows: raw materials are crudely smelted in the first and second stations of the induction furnace, respectively; the crude smelted molten iron from the first station enters the first ladle refining furnace for primary refining; the molten iron after primary refining enters the second ladle refining furnace; the crude smelted molten iron from the second station is further added to the second ladle refining furnace for secondary refining; the secondary refined molten iron is distributed by the distribution equipment and then enters the spheroidizing equipment for spheroidizing treatment.

[0027] Unlike conventional processes that use only an induction furnace for smelting, this system employs two ladle refining furnaces. This allows for precise control of the major alloying elements, such as C, P, and S, in the molten iron. The secondary refined molten iron is distributed through a distribution system and then transferred to a spheroidizing system for spheroidizing, ensuring the overall compositional uniformity and homogenization of the cask castings. In the absence of large-scale specialized equipment, this system utilizes conventional smelting equipment and innovative processes and technological pathways to produce highly homogenized 100-ton molten iron for spheroidizing in one go. The product quality meets strict technical standards, solving the difficult task of single-furnace smelting of molten iron for 100-ton ductile iron casks for storing and transporting spent nuclear fuel. The physicochemical property tests of the finished product demonstrated that all technical indicators exceeded the design requirements of the product specifications.

[0028] The present invention also provides a method for smelting molten iron using the apparatus, the method comprising: Step (1) of crude smelting using an induction furnace, comprising adding cold steel, cast iron, nickel alloy plate, and graphite powder to a first station to crude smelt the cold steel, cast iron, nickel alloy plate, and graphite powder to obtain first-station crude smelted molten iron, and adding cast iron and graphite powder to a second station to crude smelt the cold steel, cast iron, and graphite powder to obtain second-station crude smelted molten iron; (2) a step of secondary refining using a ladle refining furnace, in which the crude smelted molten iron from the first station is added to a first ladle refining furnace for primary refining to obtain primary refined molten iron, and the crude smelted molten iron from the first station and the crude smelted molten iron from the second station are added to a second ladle refining furnace in that order for secondary refining to obtain secondary refined molten iron; and (3) transferring and distributing the secondary refined molten iron in the distribution facility and subjecting it to spheroidizing treatment in the spheroidizing treatment facility.

[0029] The high-purity cast molten iron for ductile cast iron casks of this embodiment can be used not only for the manufacture of spent nuclear fuel storage casks but also for the manufacture of other storage and transportation cask equipment, and its technological route and process flow are suitable for molten iron manufacturing processes that have the same or similar quality, composition, and temperature requirements.

[0030] In addition, a certain amount of carbon element loss occurs in molten iron during the processes of crude smelting, tapping, and pouring. This loss is corrected using empirical parameters in the process, but the specific values ​​depend on the smelting time, molten iron composition, tapping method, and tapping temperature.

[0031] In this example, apart from the cold-rolled steel, cast iron, and nickel alloy plate, no alloys are added during the process to adjust the target composition of the molten iron, as adding alloys would increase residual elements, affect the purity of the molten iron, and reduce the final evaluation of the graphite shape of the metallographic structure of the product. Therefore, a dedicated alloy design and composition control process must be strictly implemented, especially in the secondary refining process of molten iron production. The auxiliary materials are graphite powder, metallurgical lime, and fluorite. In the induction furnace crude refining process, graphite powder is used for recarburization, and nickel plate is used to adjust the nickel content. In the refining process, cold-rolled steel and cast iron are used to adjust the amount and composition of the molten iron, and metallurgical lime and fluorite are added as needed to form slag. Other raw materials and auxiliary materials are not required.

[0032] In addition, except for cold steel and pig iron for casting, elemental carbon in the form of graphite electrode powder or carbon powder should not be added to the molten iron when adjusting the chemical component C of the molten iron in the secondary refining process using a ladle furnace. This is to avoid errors in the measurement and judgment of the chemical component of the final finished product due to the influence of errors in the yield of elemental carbon and measurement and inspection equipment on the main component C of high-temperature molten iron.

[0033] The second ladle refining furnace will be explained using a 160-ton example. After the 160-ton ladle furnace refines and taps off a total of 145 tons of molten iron, it must be transferred to a distribution station for distribution and transfer. The distribution order is to first pour 60 tons into a tundish, then pour 80 tons into another tundish, and the transfer order is to transfer the 60-ton tundish first, then the 80-ton tundish. After distribution, the temperature of the molten iron in the tundish is measured and found to be 1440-1460°C. To avoid the need for secondary heating due to a significant drop in the temperature of the molten iron, the time interval between the transfer and distribution of the molten iron after tapping and the spheroidizing treatment must be strictly limited to 30 to 60 minutes. When the tundish reaches the spheroidizing treatment station, the temperature measuring contact point of the disposable thermocouple is inserted at least 200 mm below the surface of the molten iron. If the temperature is too low, the molten iron is returned to the refining station for heating; if the temperature is too high, argon gas is blown in from the bottom to cool the molten iron.

[0034] This casting process involves splitting 145 tons of molten iron into two separate 60-ton and 80-ton molten iron portions, which undergo spheroidizing, slag removal, and silicon floating inoculation. The molten iron is then poured into the casting cavity at regular intervals from separate dedicated pouring basins. To ensure the molten iron surface rises quickly, both pouring steps must be completed simultaneously. The spheroidizing temperature for this process is 1350-1450°C, with the actual spheroidizing temperature being 1405°C. To ensure the molten iron in the cavity meets the purity standards, the remaining molten iron in the refining ladle must be at least 3 tons after splitting; in fact, it was measured to be 5 tons.

[0035] Specifically, the mass of cold-rolled steel in the raw materials for molten iron production must account for 15 to 20% of the total mass of cold-rolled steel, pig iron, and nickel alloy plate. The cold-rolled steel is obtained by dephosphorization and decarburization in an electric arc furnace, carbon adjustment and desulfurization in a ladle furnace, and billet casting in a mold in an atmospheric environment.

[0036] In a specific embodiment, the cold-rolled steel product described in step (1) contains, by mass percent, 0.25 to 0.45% C, 0.01% or less Si, 0.05% or less Mn, 0.005% or less P, 0.005% or less S, 0.05% or less Cr, 0.05% or less Mo, and trace amounts of Sb, W, V, Pb, As, Sn, and Zr.

[0037] In this example, the cold-rolled steel is prepared by the above-mentioned method, and the weight of each ingot is 250 to 6000 kg. The composition ratio of the steel is limited, and preferably, the main components according to chemical analysis are C 0.25%, Si 0.01% or less, P 0.001% or less, and S 0.002% or less. The reason for setting these values ​​is to satisfy a series of technical standard restrictions such as the subsequent smelting quality of the molten iron and the weight of the components.

[0038] Specifically, in step (1), the foundry pig iron contains, by mass percent, 4.50 to 4.70% C, 0.40 to 0.60% Si, 0.100% or less Mn, 0.030% or less P, 0.025% or less S, 0.010% or less Cr, 0.10% or less Ni, 0.010% or less Mo, and 0.050% or less Ti, and the weight of each lump of foundry pig iron is 5 kg.

[0039] Specifically, in step (1), the mass fraction of Ni in the nickel alloy plate is greater than 99.5%, and the grade of the nickel alloy plate in this embodiment is Ni9950.

[0040] In this example, the graphite powder has a carbon content of more than 99% and a particle size of less than 1 mm.

[0041] Specifically, in step (1), the temperature of the first station crude smelting and the second station crude smelting is 1550°C or less, and in both cases, after all the charged materials have been purified, they are kept at 1500 to 1520°C for 5 to 20 minutes before being tapped.

[0042] Specifically, in step (2), a method of stirring by blowing argon gas into the bottom of the ladle is adopted throughout the entire process of primary refining and secondary refining.

[0043] In addition, the temperature and composition of the molten iron are made highly uniform by blowing argon gas into the bottom of the ladle to stir it, and by using slag formation in combination, endogenous and exogenous inclusions in the molten iron are promoted to float and be adsorbed and removed by the metallurgical slag. This achieves high purity of molten iron, high uniformity of composition and temperature, and precise control of the optimum process temperature for spheroidizing treatment, which cannot be achieved with conventional molten iron manufacturing processes.

[0044] Specifically, in step (2), a slag formation process is performed in both the primary refining and secondary refining. In the primary refining, the slag forming materials are metallurgical lime and fluorite, with the mass ratio of metallurgical lime to fluorite being 4:1. In the secondary refining, fluorite is used as the slag forming material.

[0045] In this example, Class 1 metallurgical lime was selected, with the following technical parameters (by mass): CaO 90% or more, MgO 5.0% or less, SiO2 2.0% or less, S 0.03% or less, caustic soda 4% or less, activity 320 or more (activity 4 mol / L, 40°C ± 1°C for 10 minutes), and lumpiness 20-100 mm. The fluorite ore was FL-85 grade, with the following technical parameters: CaF2 85% or more, SiO2 14.3% or less, P 0.06% or less, S 0.10% or less, and lumpiness 5-100 mm. All raw materials and auxiliary materials used must be clean, dry, and clearly labeled. Large metal cutting materials must be specifically labeled by unit weight.

[0046] In this example, the first and second ladle refining furnaces are both newly constructed cylindrical ladle furnaces, and the refractory bricks in the working layer are made of magnesia-carbon material. These refractory bricks are obtained by cold compression molding of magnesia clinker, flake graphite, organic binder, and antioxidant. When used for the first time, a special heating curve must be followed to heat for 24 hours, and the energy medium is natural gas or industrial gas. After heating is stopped, infrared radiation temperature measurement is performed at 1 / 2 depth of the inner wall. The temperature exceeded 1000°C, and after heating was stopped for 30 minutes, the inner bottom was visually observed to be red-hot, the infrared radiation temperature measured at half the depth of the inner wall was above 750°C, and the infrared radiation temperature measured at half the depth of the outer wall was above 150°C, which ensured that the newly installed ladle furnace was thoroughly heated and nearly thermally saturated, and no wet gas or crystallized water remained in the refractory bricks, ensuring the safety of the smelting process and ensuring that the [H] of the molten iron would not increase due to environmental factors during the smelting process.

[0047] In this embodiment, the mass of the molten iron and the weight of the components are adjusted using cold steel and foundry pig iron, and slag is formed using metallurgical lime and fluorite, with a slag ratio of CaO:CaF2=4:1. Graphite electrodes are heated to perform desulfurization treatment, and after the smelting of the crude molten iron in the second station is completed, it is added to the second ladle refining furnace. After mixing is completed, fluorite is added to the second ladle refining furnace to form slag, which is then arc-heated. Argon gas is blown in from the bottom to stir the components, and the component temperatures are made uniform. Samples are taken, and if the component temperatures are appropriate, the molten iron is tapped, which can then be transferred to the subsequent molten iron distribution and spheroidization inoculation treatment.

[0048] Specifically, in step (2), the temperature of the primary refining and secondary refining is 1550°C or less.

[0049] Specifically, in step (2), cold steel and cast pig iron are further added to adjust the mass and weight of the components of the molten iron in the primary refining process.

[0050] In ladle furnace refining, (1) a slag-forming material must be added simultaneously with the addition of cold steel and cast pig iron. This arc-heats a graphite electrode, desulfurizing the slag and adsorbing and removing various endogenous and exogenous inclusions from the molten iron. In conventional induction furnace smelting, the slag floating on the molten pool surface is primarily acidic SiO2 derived from the original gangue in the cast pig iron raw material. Compared to alkaline slag, which is primarily CaO, this acidic slag cannot be desulfurized and has a lower ability to adsorb inclusions. (2) When recarburizing high-carbon molten iron with carbon powder, undissolved graphite particles tend to get trapped in the casting cavity. Furthermore, the yield of carbon powder varies greatly due to various factors. Therefore, the carbon (C) content of the molten iron is adjusted to the lower limit of the standard during the induction furnace crude smelting stage. Only cast pig iron and cold steel are used for carbon adjustment during the refining process. (3) The method of stirring molten iron by injecting argon gas from the bottom of the ladle significantly shortens the process of equalizing the temperature, homogenizing, and removing inclusions, thereby improving the purity and uniformity of the molten iron, offering technological advantages unparalleled compared to conventional processes. The high-pressure argon gas imparts significant kinetic energy during stirring, accelerating the mass transfer process from the bottom to the surface. The fine, high-purity argon gas bubbles generated from the porous bricks at the bottom of the ladle act as countless microscopic vacuum chambers. As the rising argon gas bubbles increase in volume as the hydrostatic pressure of the molten iron decreases, hydrogen (H) and nitrogen (N) gases in the molten iron enter the argon gas bubbles. Various inclusions are also adsorbed during collisions with the argon gas bubble surfaces and reach the iron-slag interface, where they are adsorbed and removed by the basic slag. The process parameters are as follows: The ladle bottom is fitted with an argon gas injection type rectangular brick, with two gas supply holes. At half radius, the flow rate is 150-300NL / min, and the argon gas purity is above 99.99%. In actual operation, the slag surface should be no more than 200mm x 200mm high. The complete temperature equalization and homogenization cycle takes 10-15 minutes depending on the height of the molten iron in the ladle.(4) During the refining process, not only must the composition of the molten iron meet the design requirements, but its temperature must also take into account compensation for the temperature drop during subsequent transfer, distribution, and waiting processes, thereby achieving the optimal process temperature for the final spheroidizing treatment of the molten iron. In actual operation, the empirical formula for the temperature drop rate during transfer is as follows: If the transfer time is t = 20-30 minutes, the temperature drop rate is ΔT / t ≒ 1.5°C / min; if the transfer time is t = 30-45 minutes, the temperature drop rate is ΔT / t ≒ 1.0°C / min. In the process design, the tapping temperature for secondary refining is T = 1480-1520°C.

[0051] Specifically, in step (3), the molten iron for spheroidizing treatment contains, on a mass percent basis, C 3.50 to 4.00%, Si 0.30 to 0.40%, P 0.020% or less, S 0.004 to 0.009%, and Ni 0.55 to 0.65%.

[0052] Specifically, before the spheroidizing treatment in step (3), argon gas is blown into the bottom of the ladle to make the temperature of the molten iron uniform, thereby setting the spheroidizing treatment temperature at 1350 to 1450°C.

[0053] The spheroidizing temperature in the process is determined by using the tapping temperature of the second ladle refining furnace as the reference temperature, and by determining the cooling rate during the molten iron transfer, distribution, and standby processes using an empirical formula. The optimum process temperature range for the spheroidizing treatment is precisely controlled by injecting argon gas into the bottom of the ladle.

[0054] Specifically, step (3) further includes transferring the spheroidized molten iron to a casting process for final inoculation, deslag, and pouring operations.

[0055] Specifically, the molten iron ladle and the molten iron ladle are both heated according to a dedicated heating curve before use, so that the refractory material of the molten iron transfer equipment is almost thermally saturated, and the temperature reduction rate during the molten iron transfer process is stable, thereby enabling precise control of the spheroidizing treatment temperature of the molten iron.

[0056] In the present invention, the carbon content of the molten iron for spheroidizing treatment is 3.50-4.00%. Since high-temperature, high-carbon molten iron will experience some carbon burnout or carbon washout during the induction furnace crude smelting, tapping, transfer, mixing, ladle furnace tapping, distribution, and spheroidizing treatment processes, recarburization with graphite powder is required in the induction furnace smelting process to ensure the target carbon (C) content of the product, and the carbon (C) content of the tapped iron from the induction furnace must meet the final carbon weight value of the refining process.

[0057] Furthermore, precise in-process control of the carbon element in molten iron is the most important technical challenge in the entire molten iron production process, and since carbon element has the greatest impact on the metal structure and mechanical properties of the finished cask, the process design of the technological route adopted by the present invention is based almost entirely on precise in-process control of carbon element. Because carbon loss occurs in many parts of the entire molten iron production process and the range of variation is large, it is difficult to precisely control the carbon element in the final molten iron using the graphite powder recarburization method. Through repeated testing, the inventors further optimized the technological route of the process, and by precisely measuring raw materials, designing auxiliary materials, and learning from experience in-process carbon loss, they were able to minimize the influence factors of carbon variation, thereby achieving precise control of the smelting process.

[0058] Secondly, carbon adjustment using graphite powder often results in inaccurate carbon yields, leading to process defects such as graphite floating and slag entrapment in the cask metal structure after spheroidization, and carbon loss from high-carbon molten iron throughout the entire production process. Specific empirical carbon loss values ​​are as follows: if the blended carbon (C) is 2.50-3.50%, the carbon loss ΔC is 0.15%, and if the blended carbon (C) is 4.50-5.50%, the carbon loss ΔC is 0.20%.

[0059] To improve the effectiveness of spheroidizing inoculation and the graphite morphology of the casting's metallurgical structure, the S content in the raw molten iron for spheroidization must be limited to 0.004-0.009%, with a target S content of 0.006%. These process parameter values ​​were determined based on analysis of test data and metallurgical structure measurements from multiple pilot tests. Specifically, they relate to the evaluation of the process effectiveness of spheroidizing inoculation of the molten iron and the graphite morphology of the casting's metallurgical structure. The main difficulty in controlling this element in the process is that the amount of S removed becomes uncontrollable as the refining process progresses in a ladle refining furnace. Therefore, the optimal process parameter values ​​for S in the raw molten iron can be obtained by designing the process route, selecting a slag system with various compositions, and rationally limiting the refining time.

[0060] Because high-phosphorus content molten iron can cause phosphorus eutectic formation at grain boundaries, leading to deterioration of the matrix structure and reduced mechanical properties of the finished casting, the P content of molten iron used for spheroidization must be 0.020% or less. Conventional molten iron smelting processes (induction furnace melting + ex-furnace treatment) cannot meet this technical standard. The main reason for this is that the P content of domestically produced pig iron for casting is currently generally 0.025% or more, and melting using only an induction furnace cannot limit the P element to the target range. Therefore, the cold-rolled steel used in the method of the present invention is obtained by arc furnace smelting, ladle furnace refining, and finally billet casting in a mold under atmospheric conditions. By limiting the P content to 0.001% or less, the P weight is kept to 0.020% or less, eliminating the process limitation of not being able to dephosphorize molten iron when smelting using an induction furnace.

[0061] Example 1 As shown in FIG. 1, this embodiment is a smelting apparatus for high-purity cast molten iron for a 100-ton ductile cast iron cask, and the apparatus includes: an induction furnace including a first station and a second station, used for crude smelting molten iron to obtain first-station crude smelted molten iron and second-station crude smelted molten iron; a first ladle refining furnace, in which the first station crude smelted molten iron enters the first ladle refining furnace and undergoes primary refining to obtain primarily refined molten iron; a second ladle refining furnace in which the primary refined molten iron and the second station crude smelted molten iron are sequentially introduced to perform secondary refining to obtain secondary refined molten iron; a distribution facility in which secondary refined molten iron is distributed and transported within said distribution facility; and a spheroidizing facility for performing a spheroidizing treatment on the distributed and transported molten iron in the spheroidizing facility.

[0062] Example 2 In this embodiment, the ductile cast iron material is QT400-18AL, and the total amount of spheroidized molten iron is 140 tons.

[0063] 1.Technical standards See Table 1 for technical standards for evaluation of cask materials (smelted and finished) (wt.%). See Table 2 for technical standards (wt.%) for steel materials specifically for ductile cast iron. Refer to Table 3 for technical standards (wt.%) for ultra-pure pig iron exclusively for ductile cast iron. Table 1: Technical criteria for evaluation of cask materials (smelted and finished) (wt.%) [Table 1] Table 2: Technical standards for ductile cast iron steel materials (wt.%) [Table 2] Table 3: Technical standards for ultra-pure pig iron for ductile cast iron (wt.%) [Table 3]

[0064] 2. Main facilities One 40-ton nominal EBT (eccentric bottom tapping, which allows for more effective control of slag outflow during tapping than trough tapping) electric arc furnace, used in this example to crudely smelt cold steel. One 60-ton nominal double-station medium-frequency induction furnace, used for pre-melting molten iron. Four cylindrical ladles constructed of magnesia-carbon bricks, with nominal capacities of 160 tons, 130 tons, 90 tons, and 40 tons, respectively, used as metallurgical vessels for refining and transferring molten iron. One 160-ton and one 130-ton refining station, capable of performing metallurgical functions such as power supply heating, argon gas injection and stirring, slag formation and desulfurization, and warm-keeping standby. One 80-ton and one 60-ton cylindrical molten iron ladle, used as metallurgical vessels for spheroidizing and pouring molten iron.

[0065] 3. Equipment checks and condition requirements (1) Regarding the charging basket of the electric arc furnace, the basket should be checked before charging and no residual waste steel from the previous furnace should be attached to it to prevent contamination. (2) For the furnace body of an electric arc furnace, it is required to completely discharge the steelmaking slag from the previous furnace, and smelting is not possible if it is at the end of its life, and the Mo content of the residual components of the steel grade of the previous furnace is less than 0.20%. (3) For induction furnace bodies, thoroughly discharge the heated steelmaking slag from the previous furnace, and no viscous steel or slag is visible in the steel-tapping trough, furnace wall, or furnace bottom. (4) Regarding the refining ladle, it is a new ladle, and the thermal condition is good after heating. Argon gas injection is tried to check the permeability of the porous brick at the bottom. (5) For Perrin process ladles, the thermal condition of the ladle is good after heating, no steel or slag remains on the bottom or rim, and the slag of the steel grade smelted in the previous furnace is a powdered slag system mainly consisting of CaO and SiO2. (6) Regarding the tundish, it is a new ladle. After brickwork is completed, the condition of the rim on the tapping side is checked. The heating process for a new ladle is carried out, and the heating time exceeds 24 hours. The temperature is measured before pouring molten iron, and the center of the wall lining is required to exceed 750°C. After heating, the refractory peeling inside is removed by suction. (7) For the molten iron ladle, the heating temperature should be 500-800°C. For a new ladle, the heating time should exceed 24 hours. For an old ladle, the heating time should exceed 12 hours. In humid weather, the heating time should be extended accordingly. Before the spheroidizing process, the temperature of the molten iron ladle should be measured. The central temperature of the lining should be 150-300°C, and the central temperature of the outer shell of the ladle should be above 100°C.

[0066] 4. Preparation of raw materials and auxiliary materials The materials included 32 tons of steel, 125 tons of foundry pig iron, 2,000 kg of Class 1 metallurgical lime, 1,000 kg of fluorite, 1,000 kg of nickel alloy plate, and 300 kg of graphite powder. All raw and auxiliary materials must be provided with chemical analysis of their alloying elements and tested and verified on-site before use. They must be precisely weighed, clearly labeled, clean, and dry. Mixing is prohibited. Large steel blocks must also have their masses specifically labeled.

[0067] The steel materials in this example were all self-produced, specifically by the following process: they were obtained by arc furnace smelting, ladle furnace refining, and finally billet casting in a mold under atmospheric conditions, and the steel materials contained, in mass percent, 0.25% C, 0.01% or less Si, 0.001% or less P, and 0.002% or less S. The foundry pig iron contains 4.50% C, 0.40% Si, 0.100% or less Mn, 0.030% or less P, 0.025% or less S, 0.010% or less Cr, 0.10% or less Ni, 0.010% or less Mo, and 0.050% or less Ti. The nickel content in the nickel alloy plate is more than 99.50%, and the grade is Ni9950. The carbon content of the graphite powder is more than 99%, and the particle size of the graphite powder is less than 1 mm. The main technical indicators of Class 1 metallurgical lime are CaO 90% or more, MgO 5.0% or less, SiO2 2.0% or less, S 0.03% or less, caustic soda 4% or less, activity 320 or more (activity 4 mol / L, 40°C ± 1°C for 10 minutes), and lumpiness 20 to 100 mm. The fluorite is of grade FL-85, with the main technical indices being CaF 285% or more, SiO2 14.3% or less, P 0.06% or less, S 0.10% or less, and lumpiness 5-100mm.

[0068] This example is a method for smelting molten iron using the apparatus described in Example 1, and includes the following steps. (1) In the case of crude smelting using an induction furnace, 25 tons of steel, 35 tons of pig iron for casting, 800 kg of nickel alloy plate, 75 kg of graphite powder, and 2.75% carbon blend are crudely smelted in the first station of the induction furnace, and after cleaning, sampling is performed and a full analysis is carried out. The temperature of the molten iron throughout the entire smelting process is below 1550°C. After all the charged materials are cleaned, the first temperature retention treatment is carried out by keeping the temperature at 1500-1520°C for 5-20 minutes, and then the molten iron crudely smelted in the first station can be obtained by tapping. If the amount of slag during the smelting process is abnormally large (visually exceeding 200 kg), it is necessary to transfer the slag and strictly measure the amount of slag outflow, or to carry out slag removal work. 61 tons of foundry pig iron, 125 kg of graphite powder, and 4.85% carbon content are added to the second station of the induction furnace for crude smelting. After cleaning, samples are taken and fully analyzed. The temperature of the molten iron throughout the smelting process is below 1550°C. After all the charged materials have been cleaned, the second temperature retention treatment is carried out at 1500-1520°C for 5-20 minutes, at which point the molten iron can be tapped off to obtain the second station crude smelted molten iron. (2) The capacity of the first ladle refining furnace is selected as 130 tons, and the process control points are as follows: the ladle nozzle hole diameter Φ is 100mm, the heating temperature is above 1000°C, the molten iron temperature is above 900°C, the thermal condition is good, and the temperature drop of the molten iron is less than 50°C. After the first station crude smelting molten iron is poured into the first ladle refining furnace, it is sampled and slag is formed. The slag forming materials are 1000kg of metallurgical lime and 250kg of fluorite. The appropriate amount of fluorite is added during the slag slag fluidity process. The temperature of the molten iron must not exceed 1550°C throughout the entire refining process. A medium-level voltage is supplied to uniformly heat the molten iron to 1500-1520°C, and the temperature is maintained for 5-20 minutes. Argon gas is injected from the bottom throughout the entire process. During the heat retention process, the flow rate of argon gas is adjusted to prevent a slag layer from being exposed on the surface of the molten iron. When heating up and adding foundry pig iron and steel, the flow rate of argon gas can be increased appropriately to promote the heat transfer process of the molten iron associated with mass transfer. The composition and amount of molten iron are adjusted by adding steel and foundry pig iron. The tapping temperature is 1500-1520°C, the molten iron contains 3.05-3.15% C, 0.40% or less Si, 0.020% or less P, 0.002% or less S, and 1.00-1.05% Ni. The amount of molten iron is limited to 81-85t. Recarburization with carbon powder is not permitted. Primary refined molten iron is obtained. The capacity of the second ladle refining furnace is selected as 160 tons, and the process control points are as follows: the ladle nozzle diameter is 100mm, the heating temperature is over 1000°C, the molten iron temperature is over 900°C, the thermal condition is good, and the temperature drop of the molten iron is less than 50°C. At this point, the primary refined molten iron is poured into the second ladle refining furnace, and then the second station crude smelting molten iron is poured into the second ladle refining furnace. After mixing, the mixture is sampled and slag is formed. The slag-forming material is 750 kg of fluorite from the first batch. After the chemical composition results are obtained, metallurgical lime is added according to the S content to continue desulfurization to maintain standard control. The temperature of the molten iron when metallurgical lime is added is 1500-1520°C. The temperature of the molten iron throughout the refining process must not exceed 1550°C. Medium-level voltage is used to uniformly heat the molten iron to 1500-1520°C and then maintain the temperature for 5-20 minutes. Argon gas is injected from the bottom throughout the process. During the maintenance process, the flow rate of argon gas is adjusted to prevent the slag layer from being exposed on the surface of the molten iron. When heating the molten iron, the flow rate of argon gas can be increased appropriately to promote the heat transfer process of the molten iron associated with mass transfer. The smelting time is 60 to 120 minutes, the tapping temperature T is 1480 to 1520°C, the optimum composition ranges of the molten iron are C 3.70 to 3.80%, Si 0.30 to 0.40%, P 0.020% or less, S 0.004 to 0.009%, and Ni 0.55 to 0.65%, the amount of molten iron is limited to 140 to 145 t, and recarburization with carbon powder is not permitted. Secondary refined molten iron is obtained. (3) Key points for controlling the molten iron transfer and distribution process are as follows: Check all machinery and equipment in advance, including weighing equipment, overhead crane equipment, hydraulic systems, and motor systems, to ensure they are all in working order. Measure the temperature of the center of the tundish lining 30 minutes before distributing the secondary refining molten iron. Ensure it is visually glowing and exceeds 900°C. This indicates that the tundish is nearly thermally saturated, and apply the empirical formula for the temperature drop of molten iron during the transfer process. The amounts of molten iron distributed are 80t and 60t, respectively. After distribution, measure and sample the temperature of the molten iron in the tundish. The spheroidizing temperature is 1395-1405°C. The spheroidizing temperature is precisely controlled by injecting argon gas from the bottom of the ladle. The argon gas flow rate per gas supply hole is 30-50 nL / min, and the temperature reduction rate of the molten iron is 1.5-2.0°C / min. Cooling the molten iron with a large flow of argon gas is prohibited. The entire process time from the completion of distribution to the start of spheroidizing is limited to 30-45 minutes. In this example, the 140 ton spheroidized molten iron is subjected to secondary refining in a 160-ton ladle furnace to adjust the composition and temperature, and achieve temperature uniformity and homogenization before distribution and spheroidizing. Each process is assigned a responsible person, on-site safety procedures are strictly followed, and safety measures and emergency response measures are in place in advance. After spheroidizing is completed, the molten iron ladle is transferred to the casting process for final inoculation, slag removal, and pouring.

[0069] (I) The molten iron smelted by the method of this embodiment has the following main components after the spheroidizing inoculation treatment: The purity of the molten iron produced by the method of the present invention is higher than that produced by the conventional technical route.

[0070] (II) A cask for storing and transporting spent nuclear fuel was made using the ductile cast iron material QT400-18AL prepared in this example. Figure 2 shows a metallographic diagram (50 μm scale) of a sample taken from the cask after corrosion. As can be seen from Figure 2, the graphite structure of the casting is nearly circular, precipitates along the grain boundaries, and is uniformly distributed, measuring 30 to 50 μm in size. Subsequent performance testing revealed that the casting possesses qualities such as a high degree of uniformity and purity not found in conventional castings. Figure 3 shows a metallographic diagram (200 μm scale) of the cask before corrosion. It can be seen that the metallographic structure of the spent nuclear fuel before and after corrosion did not change significantly.

[0071] (III) The actual on-site drawing of the processed product after semi-finishing of the cask for storing and transporting spent nuclear fuel made from the ductile cast iron material of this example is shown in Figure 4. The results of the subsequent non-destructive testing revealed that the casting fully met the technical standards set out in the design guidelines.

[0072] Example 3 In this example, the same method as in Example 2 is used to smelt molten iron, except that the mass of the steel in the raw materials accounts for 15% of the total mass of the steel, pig iron, and nickel alloy plate, the C in the steel is 0.35%, and the C in the foundry pig iron is 4.60%, and the Si is 0.50%.

[0073] Example 4 In this example, the same method as in Example 2 is used to smelt molten iron, except that the mass of the steel in the raw materials accounts for 17.5% of the total mass of the steel, pig iron, and nickel alloy plate, the C in the steel is 0.45%, and the C in the foundry pig iron is 4.70%, and the Si is 0.60%.

[0074] The above are preferred specific embodiments of the present invention, and the protection scope of the present invention is not limited thereto. Any modifications or substitutions that are easily understood by those skilled in the art within the technical scope disclosed in the present invention shall be included in the protection scope of the present invention.

Claims

1. an induction furnace including a first station and a second station, used for crude smelting molten iron to obtain first-station crude smelted molten iron and second-station crude smelted molten iron; a first ladle refining furnace, in which the first station crude smelted molten iron enters the first ladle refining furnace and undergoes primary refining to obtain primarily refined molten iron; a second ladle refining furnace in which the primary refined molten iron and the second station crude smelted molten iron are sequentially introduced to perform secondary refining to obtain secondary refined molten iron; a distribution facility in which secondary refined molten iron is distributed and transported within said distribution facility; a spheroidizing facility for performing a spheroidizing treatment on the distributed and transferred molten iron in the spheroidizing facility, This is a smelting device for high-purity cast molten iron for 100-ton class ductile cast iron casks.

2. Step (1) of crude smelting using an induction furnace, comprising adding cold steel, foundry pig iron, nickel alloy plate, and graphite powder to a first station for crude smelting to obtain first-station crude smelted molten iron, and adding foundry pig iron and graphite powder to a second station for crude smelting to obtain second-station crude smelted molten iron; (2) a step of secondary refining using a ladle refining furnace, in which the crude smelted molten iron from the first station is added to a first ladle refining furnace for primary refining to obtain primary refined molten iron, and the crude smelted molten iron from the first station and the crude smelted molten iron from the second station are added to a second ladle refining furnace in sequence for secondary refining to obtain secondary refined molten iron; and (3) sequentially transferring and distributing the secondary refined molten iron in a distribution facility and subjecting the secondary refined molten iron to spheroidizing treatment in a spheroidizing treatment facility. A method for smelting molten iron using the apparatus according to claim 1.

3. The mass of cold steel in the raw materials for molten iron production accounts for 15-20% of the total mass of cold steel, pig iron and nickel alloy plate; 3. The method for smelting molten iron according to claim 2.

4. The cold steel material is obtained by dephosphorization and decarburization through arc furnace crude smelting, carbon adjustment and desulfurization through ladle furnace refining, and billet casting using a mold in an atmospheric environment.

3. The method for smelting molten iron according to claim 2.

5. The cold-worked steel material according to step (1) contains, on a mass percent basis, 0.25 to 0.45% C, 0.01% or less Si, 0.05% or less Mn, 0.005% or less P, 0.005% or less S, 0.05% or less Cr, and 0.05% or less Mo; 3. The method for smelting molten iron according to claim 2.

6. In step (1), the foundry pig iron contains, by mass percent, 4.50 to 4.70% C, 0.40 to 0.60% Si, 0.100% or less Mn, 0.030% or less P, 0.025% or less S, 0.010% or less Cr, 0.10% or less Ni, 0.010% or less Mo, and 0.050% or less Ti; 3. The method for smelting molten iron according to claim 2.

7. In step (1), the mass fraction of Ni in the nickel alloy plate is greater than 99.5%; 3. The method for smelting molten iron according to claim 2.

8. In step (1), the process temperatures of the first station and the second station are both 1550°C or less, and after all the charged materials are purified, the molten iron is tapped after being kept at 1500-1520°C for 5-20 minutes.

3. The method for smelting molten iron according to claim 2.

9. In step (2), argon gas is injected from the bottom of the ladle to stir the entire process during the primary and secondary refining processes.

3. The method for smelting molten iron according to claim 2.

10. In step (2), slag formation is carried out in both the primary refining and secondary refining. Specifically, the slag composition requirements are that the slag-forming materials in the primary refining are metallurgical lime and fluorite, with the mass ratio of metallurgical lime to fluorite being 4:1, and the slag-forming material in the secondary refining is fluorite.

3. The method for smelting molten iron according to claim 2.

11. In step (2), the process temperature of the primary refining and secondary refining of the molten iron is 1550°C or less; 3. The method for smelting molten iron according to claim 2.

12. In step (2), cold steel and foundry pig iron are further added to adjust the mass and weight of the molten iron during the primary refining process.

3. The method for smelting molten iron according to claim 2.

13. In step (3), before the spheroidizing treatment, argon gas is blown into the bottom of the ladle to make the temperature of the molten iron uniform, thereby setting the spheroidizing treatment temperature at 1350 to 1450°C. The method for smelting molten iron according to any one of claims 2 to 12.

14. Step (3) further comprises transferring the spheroidized molten iron to a casting process for final inoculation, deslag and pouring operations.

3. The method for smelting molten iron according to claim 2.

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