Smelting apparatus and method for high-purity cast molten iron for 100-ton class ductile cast iron casks

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 particularly to a smelting apparatus and method for high-purity casting molten iron for a hundred-ton ductile iron casting.

Background Art

[0002] Spent nuclear fuel, also called irradiated nuclear fuel, generally refers to nuclear fuel whose uranium content has decreased to a predetermined level in an atomic power plant reactor and thus cannot maintain a nuclear reaction. Since spent nuclear fuel contains a large amount of radioactive elements, it is extremely harmful to the environment, and due to its long half-life, it must be properly processed. Its treatment process includes processes such as storage, transportation, reprocessing, and deep geological disposal. The spent nuclear fuel storage and transportation device is the storage and transportation container for this treatment process. By 2021, there are 22 operating or under-construction nuclear power plants in China, and the treatment of spent nuclear fuel is becoming increasingly necessary. Due to the particularity of spent nuclear fuel, very high technical standards are required for its storage and transportation devices. The technical requirements are high and the price is very high. Therefore, for the time being, the storage and transportation devices mainly rely on imports from foreign countries, and there are still no domestic substitutes. From these circumstances, it can be said that this is a high-value-added "strangling" project.

[0003] In the prior art, ductile iron castings are mainly produced by using a cupola or an induction furnace to produce raw molten iron. After spheroidizing inoculation treatment, casting forming is carried out, and then it is obtained through a series of processes such as mold breaking, cleaning, and heat treatment. The amount of molten iron that can be provided at one time is limited by the nominal capacity of the smelting furnace body, and most of them are several tons to dozens of tons. Its composition fluctuates greatly due to the limitations of raw materials and the smelting process. With only the induction heating method, the controllability of the process temperature during the process is poor, and the inclusion removal process is also lacking. Therefore, it is difficult to conduct quality control on the overall metallurgical quality of the raw molten iron.

[0004] Storage and transport cask equipment weighs over 100 tons per unit weight, has stringent technical standards, and involves a specialized casting process; therefore, induction furnace smelting equipment alone cannot simultaneously meet metallurgical quality requirements such as molten iron volume, chemical composition, process temperature, purity, and uniformity. [Overview of the project] [Problems that the invention aims to solve]

[0005] Based on the above description, the embodiment of the present invention aims to provide a smelting apparatus and method for high-purity cast molten iron for 100-ton class ductile cast iron casks in order to solve at least one of the problems of the prior art, such as the inability of induction furnace smelting apparatus alone to simultaneously satisfy the requirements for molten iron quantity, chemical composition, process temperature, molten iron purity, homogenization, and refinement for storage and transport cask apparatus of 100 tons or more per unit weight, and the difficulty in controlling the graphite shape of the metallic 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 100-ton class ductile cast iron casks, and the apparatus is An induction furnace, comprising a first station and a second station, is used to crudely smelt molten iron and obtain crudely smelted molten iron from the first station and crudely smelted molten iron from the second station. A first ladle smelting furnace, wherein crude molten iron from a first station enters the first ladle smelting furnace and undergoes primary smelting to obtain primary refined molten iron, A second ladle smelting furnace, wherein the primary smelted molten iron and the crude smelted molten iron of the second station are sequentially fed into the second ladle smelting furnace to perform secondary smelting and obtain secondary smelted molten iron, A distribution facility, wherein secondary refined molten iron is distributed and transferred within the distribution facility, A spheroidizing apparatus, comprising a spheroidizing apparatus that performs spheroidizing treatment on molten iron that has been distributed and transported.

[0007] The present invention also provides a method for smelting molten iron using the apparatus, and the method is Step (1) of crude smelting using an induction furnace, comprising adding cold-worked steel, cast iron, nickel alloy sheet, and graphite powder to a first station and crude smelting to obtain crude molten iron from the first station, and adding cast iron and graphite powder to a second station and crude smelting to obtain crude molten iron from the second station, (2) A secondary refining method using a ladle smelting furnace, comprising the steps of: (1) adding the crude molten iron from the first station to the first ladle smelting furnace for primary refining to obtain primary refined molten iron; (2) sequentially adding the primary refined molten iron and the crude molten iron from the second station to the second ladle smelting furnace for secondary refining to obtain secondary refined molten iron; The process includes step (3) sequentially transferring and distributing the secondary refined molten iron in a distribution facility and then performing a spheroidization treatment in a spheroidization treatment facility.

[0008] Furthermore, the mass of cold-worked steel in the molten iron production raw materials is strictly limited to 15-20% of the total mass of cold-worked steel, pig iron, and nickel alloy plates.

[0009] Furthermore, the 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.

[0010] Furthermore, the cold-worked steel described in step (1) contains, on a mass percentage 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] Furthermore, in step (1), the cast iron contains, on a mass percentage basis, 4.50-4.70% C, 0.40-0.60% 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 crude smelting of molten iron at the first and second stations is carried out at a process temperature of 1550°C or lower, and after all the charge has been cleaned, the iron is kept warm at 1500-1520°C for 5-20 minutes before being tapped.

[0014] Furthermore, in step (2), a method is employed in which argon gas is blown in from the bottom of the ladle and stirred throughout the entire process in both the primary and secondary refining stages.

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

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

[0017] Furthermore, in step (2), cold steel and cast iron are 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 in from the bottom of the ladle to equalize the temperature of the molten iron, thereby raising the spheroidizing treatment temperature to 1350-1450°C.

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

[0020] Compared to the prior art, the present invention can achieve at least one of the following beneficial effects. (1) Compared to conventional ductile cast iron molten iron manufacturing equipment, the apparatus of the present invention solves the problems that conventional equipment and molten iron manufacturing processes that use only an induction furnace for smelting cannot precisely control the composition and temperature of the molten iron, nor can they homogenize or unify the composition temperature of the molten iron raw material for spheroidization. The present invention employs two ladle smelting furnaces and employs crude smelting by an induction furnace and secondary smelting by a ladle smelting furnace to achieve precise control of major alloy components such as C, P, and S in the molten iron. The secondary smelted molten iron is distributed by a distribution facility and further transferred to a spheroidization treatment facility for spheroidization treatment, thereby ensuring the overall composition identity and homogenization level of the cask casting. The apparatus of the present invention can be used with conventional smelting equipment even when large-scale specialized equipment is unavailable. Through process and technological innovation, it can provide highly homogenized 100-ton class spheroidizing raw material molten iron in a single batch. Furthermore, the product quality meets stringent technical standards, solving the challenge of single-furnace smelting of molten iron for 100-ton class ductile cast iron cask equipment for the storage and transport of spent nuclear fuel. In physicochemical property tests of the finished product, all technical indicator items are superior to the design requirements of the product specifications. (2) In this invention, in addition to cold-worked steel, cast iron, and nickel alloy sheets, alloys are not added during the process to adjust the target composition of the molten iron. Adding alloys increases residual elements, which affects the purity of the molten iron and lowers the final evaluation of the graphite shape of the metal structure of the product. In particular, a dedicated alloy design and 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 purifying the molten iron, significantly reducing the content of endogenous and exogenous inclusions in the molten iron. By employing a secondary refining process using a large-tonnage ladle smelting furnace, the molten iron can simultaneously meet special technical standards for precise control of the design composition, high purity with small amounts of inclusions, high uniformity of composition and temperature, and single-furnace smelting of 100-ton class products. This represents a new technological path and concept, possessing clear technological progress, thereby obtaining ductile cast iron products with superior performance indicators. (4) Molten iron with a high phosphorus content undergoes phosphorus eutectic phenomena at grain boundaries, leading to deterioration of the matrix structure and a decrease in mechanical properties of the finished casting. Therefore, molten iron used as raw material for spheroidization is required to have a phosphorus content of 0.020% or less. Conventional molten iron smelting processes (induction furnace melting + off-furnace treatment) cannot meet this technical standard. The main reason for this is that, for the time being, the phosphorus content of domestically produced pig iron for castings is basically 0.025% or more, and when melting in an induction furnace alone, it is not possible to limit the phosphorus element to the target component range. Therefore, the cold-worked steel used in the present invention is obtained through primary rough smelting in an arc furnace, secondary smelting in a ladle smelting furnace, and finally billet casting in a mold under atmospheric conditions. By limiting the phosphorus (P) content of the cold-worked steel to 0.001% or less through a special process, the weight of P in the final molten iron raw material for spheroidization is reduced to 0.020% or less, thus eliminating the process limitation that dephosphorization (P) is not possible when molten iron is smelted in an induction furnace. (5) In the present invention, the carbon content in the molten iron raw material for spheroidization is 3.50-4.00%, and in the induction furnace smelting, tapping, transfer, mixing, ladle furnace tapping, distribution, and spheroidization processes, a certain amount of carbon combustion loss or carbon leakage occurs in the high-temperature, high-carbon molten iron. Therefore, in order to guarantee the target carbon (C) of the product, a recarburization operation with graphite powder is necessary in the induction furnace smelting process, and the carbon (C) component value in the tapped iron from the induction furnace must satisfy the final carbon weight value of the refining process. The present invention achieves precise control of carbon in the smelting process by limiting the factors influencing carbon fluctuations to the minimum range through precise measurement of raw materials, design of auxiliary materials, and process carbon loss based on experience. (6) In the present invention, the process temperatures for crude smelting and refining of molten iron are limited, and the process design already takes into account compensating for the temperature drop of molten iron during subsequent transfer, distribution, and waiting processes with the tapping temperature of secondary refining, thereby precisely realizing the optimal process temperature requirements necessary for spheroidizing molten iron. (7) The method of the present invention makes the temperature and composition of molten iron highly uniform by blowing argon gas from the bottom of the ladle during refining for stirring, and combines slag formation to promote the floating of endogenous and exogenous inclusions in molten iron and their adsorption and removal by metallurgical slag, realizing high purity of molten iron, high uniformity of composition and temperature, and precise control of the optimum process temperature for spheroidization treatment, which cannot be achieved in the conventional molten iron manufacturing process. Since the high-pressure argon gas is imparted with a very large kinetic energy by stirring, the mass transfer process of molten iron from the bottom to the surface of the molten metal is promoted. The fine high-purity argon gas bubbles generated from the porous brick at the bottom of the ladle correspond to innumerable tiny vacuum chambers. As the floating argon gas bubbles increase in volume as the hydrostatic pressure of the molten iron decreases, hydrogen gas and nitrogen gas in the molten iron enter the argon gas bubbles, and various inclusions are also adsorbed during the collision with the surface of the argon gas bubbles and come to the iron-slag interface, where they are adsorbed and removed by basic slag. By limiting S in the molten iron for spheroidization to 0.004 - 0.009%, the method of the present invention can significantly improve the effect of spheroidization inoculation and improve the graphite shape of the metal structure of the casting.

[0021] In the present invention, each of 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 described in the following description of the specification, and some advantages will be obvious from the specification or will become known through the implementation of the present invention. The object and other advantages of the present invention are realized and considered to be obtained by the content specifically described in the specification and drawings.

Brief Description of the Drawings

[0022] The drawings are only for showing the purpose of specific embodiments and are not regarded as limitations to the present invention. In all the drawings, the same reference numerals represent the same parts.

[0023] [Figure 1] It is a schematic diagram of the smelting device and process flow of high-purity casting molten iron for a 100-ton-class ductile iron casting. [Figure 2]This is a metallographic diagram (scale 50 μm) of a cask for storing and transporting spent nuclear fuel, made from ductile cast iron material prepared in Example 2 of the present invention, after cask corrosion. [Figure 3] This is a metallographic diagram (scale 200 μm) of a cask for storing and transporting spent nuclear fuel, made from ductile cast iron material prepared in Example 2 of the present invention, before cask corrosion. [Figure 4] This is a field drawing of a workpiece after semi-finishing of a spent nuclear fuel storage and transport cask made from ductile cast iron material according to Embodiment 2 of the present invention. [Modes for carrying out the invention]

[0024] Preferred embodiments of the present invention will be specifically described below with reference to the drawings. Hereinafter, the drawings, as parts constituting the present invention, are used together with the embodiments to explain the principles 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 Figure 1, the present invention provides a smelting apparatus for high-purity cast molten iron for 100-ton class ductile cast iron casks, and the apparatus is An induction furnace, comprising a first station and a second station, is used to crudely smelt molten iron and obtain crudely smelted molten iron from the first station and crudely smelted molten iron from the second station. A first ladle smelting furnace, wherein crude molten iron from a first station enters the first ladle smelting furnace and undergoes primary smelting to obtain primary refined molten iron, A second ladle smelting furnace, wherein the primary smelted molten iron and the crude smelted molten iron of the second station are sequentially fed into the second ladle smelting furnace to perform secondary smelting and obtain secondary smelted molten iron, A distribution facility, wherein secondary refined molten iron is distributed and transferred within the distribution facility, A spheroidizing apparatus, comprising a spheroidizing apparatus that performs spheroidizing treatment on molten iron that has been distributed and transported.

[0026] In carrying out the process, the operating principle of the apparatus is as follows: The raw materials are crudely smelted in the first and second stations of the induction furnace. The crudely smelted molten iron from the first station enters the first ladle smelting furnace for primary smelting. The molten iron after primary smelting enters the second ladle smelting furnace. The crudely smelted molten iron from the second station is then added to the second ladle smelting furnace for secondary smelting. After the secondary smelted molten iron is distributed by the distribution equipment, it enters the spheroidizing treatment equipment for spheroidizing treatment.

[0027] Compared to conventional technologies, the apparatus of this embodiment differs from conventional processes that use only an induction furnace for smelting. The present invention employs two ladle smelting furnaces, employing crude smelting by the induction furnace and secondary smelting by the ladle smelting furnace to achieve precise control of major alloy components such as C, P, and S in the molten iron. The secondary smelted molten iron is distributed by a distribution facility and further transferred to a spheroidizing treatment facility for spheroidizing treatment, thereby ensuring the overall compositional identity and homogenization level of the cask casting. The apparatus of the present invention, even when large-scale specialized equipment is unavailable, utilizes ordinary smelting equipment and, through process and technological path innovation, can provide highly homogenized 100-ton class spheroidizing raw material molten iron in one go. Furthermore, the product quality meets stringent technical standards, solving the difficult problem of single-furnace smelting of molten iron for 100-ton class ductile cast iron cask equipment for spent nuclear fuel storage and transport. In physicochemical property tests of the finished product, all technical indicator items are superior to the design requirements of the product specifications.

[0028] The present invention also provides a method for smelting molten iron using the apparatus, and the method is Step (1) of crude smelting using an induction furnace, comprising adding cold-worked steel, cast iron, nickel alloy sheet, and graphite powder to a first station and crude smelting to obtain crude molten iron from the first station, and adding cast iron and graphite powder to a second station and crude smelting to obtain crude molten iron from the second station, (2) A secondary refining method using a ladle smelting furnace, comprising the steps of: (1) adding the crude molten iron from the first station to the first ladle smelting furnace for primary refining to obtain primary refined molten iron; (2) sequentially adding the primary refined molten iron and the crude molten iron from the second station to the second ladle smelting furnace for secondary refining to obtain secondary refined molten iron; The process includes step (3) sequentially transferring and distributing the secondary refined molten iron in a distribution facility and then performing a spheroidization treatment in a spheroidization treatment facility.

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

[0030] It should be noted that a certain amount of carbon element loss occurs in molten iron during the crude smelting, tapping, and injection processes. While these are corrected using empirical parameters during the process, the specific values ​​are related to the smelting time, molten iron composition, tapping method, and tapping temperature.

[0031] In this embodiment, in addition to cold-worked steel, cast iron, and nickel alloy sheets, no alloys are added during the process to adjust the target composition of the molten iron. Adding alloys increases residual elements, which affects the purity of the molten iron and lowers the final evaluation of the graphite shape of the product's metal structure. In particular, a dedicated alloy design and composition control process must be strictly executed in the secondary refining process of molten iron production. The auxiliary materials are graphite powder, metallurgical lime, and fluorite. In the crude smelting process using an induction furnace, graphite powder is used for recarburization and nickel is adjusted with nickel sheets. In the refining process, cold-worked 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. No other raw materials or auxiliary materials are required.

[0032] Furthermore, in the secondary refining process using a ladle furnace, with the exception of cold-worked steel and cast iron, elemental carbon must not be added to the molten iron in the form of graphite electrode powder or carbon powder when adjusting the chemical composition C of the molten iron in this process. This is to avoid inaccuracies in the measurement and judgment of the chemical composition of the final product, as the main component C of high-temperature molten iron is affected by the yield of elemental carbon and the errors of the measuring and inspection equipment.

[0033] Furthermore, the second ladle smelting furnace will be interpreted and explained using a 160-ton furnace as an example. After a total of 145 tons of molten iron are tapped from the 160-ton ladle furnace, it is necessary to transfer the molten iron to the distribution station for distribution and transfer processing. The distribution order is to first pour 60 tons into the tundish, and then pour 80 tons into the tundish. The transfer order is to first transfer the 60-ton tundish, and then transfer the 80-ton tundish. After distribution, the temperature is measured, and the temperature of the molten iron in the tundish is 1440-1460°C. To avoid the need for secondary heating due to the significant temperature drop 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 30-60 minutes. When the tundish reaches the spheroidizing treatment station, the temperature sensing junction of a disposable thermocouple is inserted at a depth of at least 200 mm below the liquid surface of the molten iron. If the temperature is very low, the molten iron is returned to the smelting station for heating, and if the temperature is very high, the molten iron is cooled by blowing argon gas from the bottom.

[0034] This casting process involves spheroidization inoculation and pouring. The process involves dividing 145 tons of molten iron into two 60-ton and 80-ton portions. After spheroidization, slag removal, and silicon floating inoculation, the molten iron is poured into the casting cavity from dedicated pools at different locations at regular time intervals. Both pouring processes must be completed simultaneously to ensure the molten iron surface rises. The spheroidization temperature is 1350-1450°C, with the actual spheroidization temperature being 1405°C. To meet the purity standards for the molten iron in the cavity, the remaining molten iron in the ladle after distribution must be at least 3 tons; in actual measurements, this amount was 5 tons.

[0035] Specifically, the mass of cold-worked steel in the molten iron production raw materials is required to account for 15-20% of the total mass of cold-worked steel, pig iron, and nickel alloy sheets. The cold-worked steel is obtained by dephosphorization and decarburization through arc furnace crude smelting, carbon adjustment and desulfurization through ladle furnace refining, and billet casting using molds in an atmospheric environment.

[0036] In specific embodiments, the cold-worked steel described in step (1) contains, on a mass percentage 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.

[0037] In this embodiment, the cold-worked steel was prepared using the method described above, with a weight of 250 to 6000 kg per ingot. The component ratios in the steel were limited, preferably, the main components determined by chemical analysis were C 0.25%, Si 0.01% or less, P 0.001% or less, and S 0.002% or less. This setting was made to satisfy a series of technical standard limitations, such as the smelting quality of the molten iron and the weight of the components.

[0038] Specifically, in step (1), the cast iron contains, on a mass percentage basis, 4.50-4.70% C, 0.40-0.60% 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, with a weight of 5 kg per lump of cast iron.

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

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

[0041] Specifically, in step (1), both the first station crude smelting and the second station crude smelting are performed at a temperature of 1550°C or lower, and after all the charge is cleaned, the material is kept warm at 1500-1520°C for 5-20 minutes before tapping.

[0042] Specifically, in step (2), a method is employed in which argon gas is blown in from the bottom of the ladle and stirred throughout the entire process during both primary and secondary refining.

[0043] Furthermore, by injecting argon gas from the bottom of the ladle and stirring, the temperature and composition of the molten iron are made highly uniform. Combined with slag formation, this promotes the floating of endogenous and exogenous inclusions in the molten iron, which are then 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 optimal process temperature for spheroidization, which are unattainable with conventional molten iron manufacturing processes.

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

[0045] In this example, Class 1 metallurgical lime was selected, and the main technical indicators, based on mass percentage, were 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℃±1℃ for 10 minutes), and lumpiness 20~100 mm. The fluorite ore was grade FL-85, and the main technical indicators were CaF 285% 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 are clean and dry, clearly labeled, and the unit weight of large metal cut materials must be specifically indicated.

[0046] In this embodiment, both the first and second ladle smelting furnaces are newly constructed cylindrical ladle furnaces. The refractory bricks in the working layer are made of magnesia carbon, and these refractory bricks are obtained by room-temperature compression molding with magnesia clinker, flake graphite, an organic binder, and an antioxidant. When used for the first time, it is necessary to heat them for 24 hours using a special heating curve. The energy medium is natural gas or industrial gas. After heating is stopped, infrared radiation thermometers are used to measure the temperature at half the depth of the inner wall. The temperature exceeded 1000°C, and after heating was stopped for 30 minutes, the inner bottom was visible as red-hot. Infrared radiation temperature measurements at half the depth of the inner wall were above 750°C, and infrared radiation temperature measurements at half the depth of the outer wall were above 150°C. This ensures that the newly installed ladle furnace is thoroughly heated and nearly heat-saturated, guaranteeing the safety of the smelting process by preventing the retention of wet gases or crystal water in the refractory bricks, and ensuring that the [H] content of the molten iron does not increase due to environmental factors during the smelting process.

[0047] In this embodiment, the mass and weight of the molten iron are adjusted using cold-worked steel and cast iron, and slag is formed using metallurgical lime and fluorite. The slag mixture ratio is CaO:CaF2 = 4:1. Desulfurization is performed by heating a graphite electrode, and after the smelting of the crude molten iron at the second station is completed, it is added to the second ladle smelting furnace. After mixing is complete, fluorite is added to the second ladle smelting furnace to form slag, which is then arc-heated, and argon gas is blown in from the bottom to stir and equalize the component temperature. If the component temperature is appropriate after sampling, the molten iron can be tapped, and the process can proceed to the subsequent molten iron distribution and spheroidization inoculation treatments.

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

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

[0050] Furthermore, in ladle furnace smelting, (1) it is necessary to add a slag-forming agent at the same time as adding cold steel and cast iron. This causes the graphite electrode to arc-heat, desulfurizing the slag and adsorbing and removing various endogenous and exogenous inclusions from the molten iron. In the conventional induction furnace smelting process, the slag floating on the surface of the molten pool is mainly acidic SiO2 derived from the raw gangue in the cast iron raw material in the molten iron. Compared to alkaline slag mainly composed of CaO, this acidic slag cannot be desulfurized and has a low inclusion adsorption capacity. (2) When carbonizing high-carbon molten iron with carbon powder, undissolved graphite particles easily enter the casting cavity, and the yield of carbon powder fluctuates greatly due to various factors. Therefore, in the crude smelting stage using an induction furnace, the weight of the carbon (C) component of the molten iron is adjusted to the lower limit of the standard, and in the refining process, only cast iron and cold steel are used for carbon adjustment. (3) The method of blowing argon gas from the bottom of the ladle to stir the molten iron significantly shortens the process of equalizing the temperature of the molten iron, homogenizing it, and removing inclusions, improving the purity and uniformity of the molten iron, and has a technological advantage that is incomparable to conventional processes. Because the high-pressure argon gas is given very large kinetic energy by stirring, the mass transfer process from the bottom to the surface of the molten iron is promoted, and the fine, high-purity argon gas bubbles generated from the porous brick at the bottom of the ladle correspond to countless tiny vacuum chambers, and as the volume of the rising argon gas bubbles increases as the hydrostatic pressure of the molten iron decreases, hydrogen (H) gas and nitrogen (N) gas in the molten iron enter the argon gas bubbles, and various inclusions are adsorbed as they collide with the surface of the argon gas bubbles and come to the iron-slag interface, where they are adsorbed and removed by the basic slag. The process parameters are as follows. The ladle bottom is an argon gas-injected rectangular brick with two gas supply holes. At a 1 / 2 radius, the flow rate is 150-300 NL / min, and the argon gas purity exceeds 99.99%. In actual operation, a slag top surface elevation of 200 mm x 200 mm or less is appropriate, and the complete temperature equalization and homogenization cycle takes 10-15 minutes depending on the height of the molten iron in the ladle.(4) In the refining process, the molten iron must not only meet the design requirements in terms of its composition, but its temperature must also take into account compensation for the temperature drop of the molten iron during subsequent transfer, distribution, and waiting processes, thereby achieving the optimal process temperature for the final molten iron spheroidizing treatment. In actual work, the empirical formula for the rate of temperature drop during transfer is as follows: If the transfer time is t=20~30 minutes, the rate of temperature drop is ΔT / t ≈ 1.5℃ / min, and if the transfer time is t=30~45 minutes, the rate of temperature drop is ΔT / t ≈ 1.0℃ / min. In the process design, the tapping temperature for secondary refining is T=1480~1520℃.

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

[0052] Specifically, in step (3), before the spheroidizing treatment, argon gas is blown in from the bottom of the ladle to equalize the temperature of the molten iron, thereby raising the spheroidizing treatment temperature to 1350-1450°C.

[0053] Furthermore, the spheroidization temperature of the process is determined by using the tapping temperature of the second ladle smelting furnace as the reference temperature, determining the cooling rate during the transfer, distribution, and waiting processes of molten iron using empirical formulas, and precisely controlling the optimal process temperature range for the spheroidization treatment by blowing argon gas from the bottom of the ladle.

[0054] Specifically, step (3) further includes transferring the molten iron after spheroidization to the casting process for final inoculation, slag removal, and pouring operations.

[0055] Specifically, both the ladle for molten iron and the molten iron ladle are heated using a dedicated heating curve before use. This ensures that the refractory material of the molten iron transfer device is nearly heat-saturated, and that the temperature decrease rate during the molten iron transfer process is stable, thereby enabling precise control of the spheroidizing temperature of the molten iron.

[0056] In this invention, the carbon content in molten iron for spheroidization is 3.50-4.00%. In the induction furnace crude smelting, tapping, transfer, mixing, ladle tapping, distribution, and spheroidization processes, some degree of carbon combustion loss or carbon leaching occurs in high-temperature, high-carbon molten iron. Therefore, in order to guarantee the target carbon (C) content of the product, a carbon restoration process with graphite powder is necessary in the induction furnace smelting process, and the carbon (C) content value in the tapped iron from the induction furnace must satisfy the final carbon weight value of the refining process.

[0057] Furthermore, precise control of the carbon (C) element in molten iron is the most significant technical challenge in the entire molten iron manufacturing process. Since carbon is the element that most influences the metallic structure and mechanical properties of the finished cask, the process design in the technical pathway adopted in this invention is almost entirely based on its precise control during the process. In the entire molten iron manufacturing process, carbon loss occurs in many parts, and the fluctuation range is large. Therefore, precisely controlling the carbon element in the final finished molten iron is difficult with the recarburization method using graphite powder. After repeated testing, the inventors further optimized the process's technical pathway and achieved precise control of the smelting process by limiting the influencing factors of carbon fluctuation to the minimum range through precise measurement of raw materials, design of auxiliary materials, and empirical analysis of process carbon loss.

[0058] Next, in carbon adjustment using graphite powder, inaccurate carbon yields often lead to process defects such as graphite flotation in the cask's metal structure after spheroidization and slag inclusion, and carbon loss of high-carbon molten iron occurs throughout the entire production process. The specific carbon loss empirical 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] Furthermore, from the perspective of improving the effectiveness of spheroidization inoculation and improving the graphite shape of the casting's metal structure, it is required to limit the sulfur (S) content inside the molten iron raw material to 0.004-0.009%, with a target S content of 0.006%. The values ​​of these process parameters were obtained by analyzing test data and metal structure measurement results from multiple pilot tests conducted to date. Specifically, they relate to the process effect of spheroidization inoculation of molten iron and the evaluation of the graphite shape of the casting's metal structure. The difficulty in controlling this element in the process lies mainly in the fact that, in the refining process using a ladle smelting furnace, the amount of S element removed becomes uncontrollable as the refining process progresses. Therefore, optimal process parameter values ​​for the S element in the raw molten iron are obtained through process route design, selection of slag systems with various components, and rational limitation of refining time.

[0060] Molten iron with a high phosphorus content undergoes phosphorus eutectic phenomena at grain boundaries, leading to deterioration of the matrix structure and a decrease in mechanical properties of the finished casting. Therefore, molten iron used as raw material for spheroidization requires a phosphorus content of 0.020% or less. Conventional molten iron smelting processes (induction furnace melting + off-furnace processing) cannot meet this technical standard. The main reason for this is that the phosphorus content of domestically produced pig iron for castings is basically 0.025% or more, and when melted in an induction furnace alone, the phosphorus element cannot be limited to the target component range. Therefore, the cold-worked steel material used in the present invention is obtained by arc furnace smelting, ladle furnace refining, and finally billet casting using a mold in an atmospheric environment. By limiting the phosphorus content to 0.001% or less, the weight of phosphorus is reduced to 0.020% or less, eliminating the process limitation that dephosphorization (P) is not possible when molten iron is smelted in an induction furnace.

[0061] (Example 1) As shown in Figure 1, this embodiment is a smelting apparatus for high-purity cast molten iron for 100-ton class ductile cast iron casks, and the apparatus is An induction furnace, comprising a first station and a second station, is used to crudely smelt molten iron and obtain crudely smelted molten iron from the first station and crudely smelted molten iron from the second station. A first ladle smelting furnace, wherein crude molten iron from a first station enters the first ladle smelting furnace and undergoes primary smelting to obtain primary refined molten iron, A second ladle smelting furnace, wherein the primary smelted molten iron and the crude smelted molten iron of the second station are sequentially fed into the second ladle smelting furnace to perform secondary smelting and obtain secondary smelted molten iron, A distribution facility, wherein secondary refined molten iron is distributed and transferred within the distribution facility, A spheroidizing apparatus, comprising a spheroidizing apparatus that performs spheroidizing treatment on molten iron that has been distributed and transported.

[0062] (Example 2) In this example, the interpretation and explanation will be based on the case where the ductile cast iron material is QT400-18AL and the total amount of spheroidized molten iron is 140 tons.

[0063] 1.Technical standards Refer to Table 1 for technical standards regarding the evaluation of cask materials (smelting and finished product) (wt.%). For technical standards (wt.%) regarding steel materials specifically for ductile cast iron, please refer to Table 2. For technical standards (wt.%) regarding ultrapure pig iron specifically for ductile cast iron, please refer to Table 3. Table 1: Technical standards for evaluating cask materials (smelting and finished product) (wt.%) [Table 1] Table 2: Technical standards for ductile cast iron-specific steel materials (wt.%) [Table 2] Table 3: Technical standards (wt.%) for ultrapure pig iron specifically for ductile cast iron. [Table 3]

[0064] 2. Main Equipment One EBT (Eccentric Bottom Tapping) arc furnace with a nominal capacity of 40 tons (allowing for more effective control of slag outflow during tapping than the chute tapping method) is used in this embodiment for the crude smelting of cold steel. One double-station medium-frequency induction furnace with a nominal capacity of 60 tons is used for the preliminary smelting of molten iron. Four cylindrical ladles constructed of magnesia-carbon bricks, with nominal capacities of 160 tons, 130 tons, 90 tons, and 40 tons respectively, are used as metallurgical vessels for smelting and transferring molten iron. One 160-ton and one 130-ton smelting station each are available, capable of providing metallurgical functions such as power supply and heating, argon gas injection and stirring, slag formation and desulfurization, and temperature retention. One 80-ton and one 60-ton cylindrical molten iron ladle each are available, used as metallurgical vessels for molten iron spheroidization and pouring.

[0065] 3. Equipment check and condition requirements (1) Regarding the charging basket of the arc furnace, the basket shall be checked before charging, and no residual waste steel from the previous furnace shall be attached to it to prevent contamination. (2) With respect to the furnace body of the arc furnace, it is required that the steelmaking slag be completely discharged from the previous furnace, and if it is near the end of its lifespan, smelting is not possible, and the Mo content of the residual components of the steel type from the previous furnace is less than 0.20%. (3) With respect to the furnace body of the induction furnace, the steelmaking slag after heating is thoroughly discharged from the previous furnace, and the accumulation of viscous steel and slag in the tap trough, furnace wall, and furnace bottom is not visible. (4) Regarding the refining ladle, since it is a newly installed ladle, the thermal condition is good after heating, and the permeability of the porous brick at the bottom is checked by attempting to blow in argon gas. (5) Regarding the Perrin ladle, the ladle is in good thermal condition after heating, no steel or slag remains at the bottom and rim, and the slag of the steel type smelted in the previous furnace is a powdery slag system mainly consisting of CaO and SiO2. (6) For the tundish, which is a newly constructed ladle, the condition of the rim on the tapping side is checked after the brickwork is completed. A new ladle heating process is performed, the heating time exceeds 24 hours, the temperature is measured before pouring in molten iron, and the middle of the wall lining is required to exceed 750°C, and after heating, any refractory material that has peeled off from the inside after heating is removed by suction. (7) For the molten iron ladle, the heating temperature is 500-800°C, the heating time exceeds 24 hours for a new ladle and exceeds 12 hours for an old ladle, and the heating time may be extended as appropriate in humid weather. The temperature of the molten iron ladle is measured before the spheroidizing treatment, and the temperature of the center of the lining is required to be 150-300°C, and the temperature of the center of the outer shell of the ladle is required to be above 100°C.

[0066] 4. Preparation of raw materials and auxiliary materials The materials consist of 32 tons of steel, 125 tons of cast iron, 2000 kg of Class 1 metallurgical lime, 1000 kg of fluorite, 1000 kg of nickel alloy sheet, and 300 kg of graphite powder. All of the above raw materials and auxiliary materials are provided with chemical analysis results of their alloy components, and are inspected and confirmed again on-site before use. They must be precisely weighed, clearly labeled, clean, and dry. Mixing is prohibited, and in the case of large steel ingots, the mass must also be specifically indicated.

[0067] All steel materials used in this embodiment were manufactured in-house, and the specific method is as follows: After arc furnace smelting and ladle furnace refining, the steel materials were finally obtained by billet casting using a mold in an atmospheric environment. On a mass percentage basis, the steel materials contained 0.25% C, 0.01% or less Si, 0.001% or less P, and 0.002% or less S. The aforementioned pig iron for casting 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 aforementioned nickel alloy sheet exceeds 99.50%, and its grade is Ni9950. The C content in the aforementioned graphite powder exceeds 99%, and the particle size of the graphite powder is less than 1 mm. The main technical indicators for 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 of 20-100 mm. Fluorite is graded FL-85, and its main technical indicators are CaF 285% or higher, SiO2 14.3% or lower, P 0.06% or lower, S 0.10% or lower, and a lumpiness of 5-100 mm.

[0068] This embodiment is a molten iron smelting method using the apparatus described in Example 1, and includes the following steps. (1) In the crude smelting process using an induction furnace, 25 tons of steel, 35 tons of cast iron, 800 kg of nickel alloy plate, 75 kg of graphite powder, and 2.75% blended carbon are crudely smelted in the first station of the induction furnace. After cleaning, samples are taken and the entire mixture is analyzed. Throughout the entire smelting process, the temperature of the molten iron is kept below 1550°C. After all the charges are cleaned, the first heat treatment is performed by holding the mixture at 1500-1520°C for 5-20 minutes. This allows for tapping and obtaining crude molten iron from the first station. If the amount of slag is abnormally large during the smelting process (exceeding 200 kg by visual inspection), it is necessary to transfer the mixture and strictly measure the amount of slag flowing out or to perform slag removal work. 61 tons of cast iron, 125 kg of graphite powder, and 4.85% blended carbon are added to the second station of the induction furnace for crude smelting. After cleaning, samples are taken and fully analyzed. Throughout the smelting process, the temperature of the molten iron remains below 1550°C. After all the charges are cleaned, the mixture is kept warm at 1500-1520°C for 5-20 minutes for a second warming treatment, after which pig iron is tapped to obtain crude molten iron from the second station. (2) The capacity of the first ladle smelting furnace is set to 130 tons, and the process control points are as follows: The nozzle hole diameter of the ladle is Φ100 mm, the heating temperature exceeds 1000°C, the molten iron temperature exceeds 900°C, the thermal condition is good, and the temperature drop of the molten iron is less than 50°C. After pouring the crude molten iron from the first station into the first ladle smelting furnace, it is sampled and slag is formed. The slag forming material is 1000 kg of metallurgical lime and 250 kg of fluorite. An appropriate amount of fluorite is added during the smelting process according to the fluidity of the slag. Throughout the smelting process, the temperature of the molten iron must not exceed 1550°C. Power is supplied at a medium-pressure level voltage to raise the temperature uniformly, and the temperature is raised to 1500-1520°C for a holding period of 5-20 minutes. Throughout the entire process, argon gas is blown in from the bottom, and when performing the heat retention process, the flow rate of argon gas is adjusted so that the slag layer is not exposed on the surface of the molten iron. When power heating is performed and casting pig iron and steel are added, the flow rate of argon gas may be increased as appropriate to promote the heat transport process of the molten iron due to mass transfer. The composition and amount of molten iron are adjusted by adding steel and casting pig iron, the tapping temperature is 1500-1520°C, the C content in the molten iron is 3.05-3.15%, Si is 0.40% or less, P is 0.020% or less, S is 0.002% or less, and Ni is 1.00-1.05%, the amount of molten iron is limited to 81-85 tons, recarburization with carbon powder is not performed, and primary refined molten iron is obtained. The capacity of the second ladle smelting furnace is set to 160 tons, and the process control points are as follows: The hole diameter of the ladle nozzle is Φ100 mm, the heating temperature exceeds 1000°C, the molten iron temperature exceeds 900°C, the thermal condition is good, and the temperature drop of the molten iron is less than 50°C. Here, the primary smelted molten iron is poured into the second ladle smelting furnace, and then the crude smelted molten iron from the second station of the induction furnace is poured in. After mixing, samples are taken to form slag. The slag-forming material is 750 kg of fluorite from the first lot. Once the chemical composition results are obtained, metallurgical lime is added in an appropriate amount according to the sulfur content, and desulfurization is continued to control the standards. The temperature of the molten iron to which metallurgical lime is added is 1500-1520°C. Throughout the entire refining process, the temperature of the molten iron must not exceed 1550°C. Power is supplied at a medium-pressure level voltage to raise the temperature uniformly, and after raising the temperature to 1500-1520°C, a heat retention process is carried out for 5-20 minutes. Argon gas is blown in from the bottom throughout the entire process. When carrying out the heat retention process, the flow rate of argon gas is adjusted so that the slag layer is not exposed on the surface of the molten iron. During power supply and heating, the flow rate of argon gas may be increased as appropriate to promote the heat transport process of the molten iron due to mass transfer. The smelting time is 60 to 120 minutes, the tapping temperature T is 1480 to 1520°C, the optimal composition range of the molten iron is 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 tons, recarburization with carbon powder is not permitted, and secondary refined molten iron is obtained. (3) The following are the points regarding the control of the molten iron transfer and distribution process: Each piece of machinery, such as the weighing equipment, overhead crane equipment, hydraulic system, and motor system, should be checked in advance to ensure that they are all in normal working order. 30 minutes before the distribution of the molten iron in the secondary refining process, the temperature of the middle of the tundish lining should be measured and it should be visible as red-hot, with the temperature exceeding 900°C. This can be considered as the tundish being nearly heat-saturated, and the empirical formula for the temperature drop of the molten iron during the transfer process should be applied. The amounts of molten iron to be distributed are 80t and 60t, respectively. After distribution, the temperature of the molten iron in the tundish should be measured and sampled. The spheroidizing temperature is 1395-1405°C. The spheroidizing temperature of the molten iron is precisely controlled by blowing argon gas from the bottom of the ladle. The flow rate of argon gas per gas supply hole is 30-50 NL / min, and the rate of temperature reduction of the molten iron is 1.5-2.0°C / min. Cooling of the molten iron with a large flow rate of argon gas is not permitted. The entire process, from distribution completion to the start of spheroidizing, is limited to 30-45 minutes. In this embodiment, 140 tons of molten iron for spheroidizing must be secondary refined in a 160-ton ladle furnace to adjust the composition and temperature, homogenize and equalize the temperature, and then distribute and perform the spheroidizing process. At the site, a person in charge is assigned to each process, on-site safety procedures are strictly followed, and safety measures and emergency measures in case of accidents are taken in advance. Once the spheroidizing process is complete, the molten iron ladle is transferred to the casting process for the final inoculation, slag removal, and pouring operations.

[0069] (I) The molten iron smelted using the method of this embodiment, after spheroidization inoculation, has the following main components, specifically: The composition is C 3.77%, Si 1.53%, Mn 0.11%, P 0.015%, S 0.005%, Ni 0.61%, Cr 0.04%, Mo 0.03%, H 1.6ppm, O 10ppm, and N 33ppm. This shows that the purity of molten iron smelted using the method of the present invention is higher than that produced by conventional technological routes.

[0070] (II) A casks for storing and transporting spent nuclear fuel were made from the ductile cast iron material QT400-18AL prepared in this embodiment. The metallographic diagram (scale 50 μm) after sampling and corrosion of the casks is shown in Figure 2. As can be seen from Figure 2, the graphite structure of the casting is almost circular in shape, uniformly distributed after precipitation along the grain boundaries, and has a size of 30-50 μm. Subsequent performance tests revealed that the casting possesses a high level of uniformity and purity not found in conventional castings. The metallographic diagram (scale 200 μm) before cask corrosion is shown in Figure 3. It can be seen that the metallographic structure before and after corrosion of the spent nuclear fuel did not change significantly.

[0071] (III) Figure 4 shows the actual on-site drawing of the semi-finished workpiece of the spent nuclear fuel storage and transport cask made from ductile cast iron material in this embodiment. Subsequent non-destructive testing results clearly show that the casting fully meets the technical standards specified in the design specifications.

[0072] (Example 3) This embodiment uses the same method as in Example 2 to smelt molten iron, except that the mass of steel in the raw materials accounts for 15% of the total mass of steel, pig iron, and nickel alloy plate, the carbon content in the steel is 0.35%, the carbon content in the casting pig iron is 4.60%, and the silicon content is 0.50%.

[0073] (Example 4) This embodiment uses the same method as in Example 2 to smelt molten iron, except that the mass of steel in the raw materials accounts for 17.5% of the total mass of steel, pig iron, and nickel alloy plate, the carbon content in the steel is 0.45%, the carbon content in the casting pig iron is 4.70%, and the silicon content is 0.60%.

[0074] The foregoing describes preferred specific embodiments of the present invention, and the scope of protection of the present invention is not limited thereto. Any modification or substitution that is readily apparent to a person skilled in the art within the scope of the art disclosed herein is included within the scope of protection of the present invention.

Claims

1. A method for molten iron smelting using a smelting apparatus for high-purity cast molten iron for 100-ton class ductile cast iron casks, The aforementioned smelting apparatus is An induction furnace, including a first station and a second station, is used to crudely smelt molten iron and obtain crudely smelted molten iron from the first station and crudely smelted molten iron from the second station. A first ladle smelting furnace, wherein crude molten iron from a first station enters the first ladle smelting furnace and undergoes primary smelting to obtain primary refined molten iron, A second ladle smelting furnace, wherein the primary smelted molten iron and the crude smelted molten iron of the second station are sequentially fed into the second ladle smelting furnace to perform secondary smelting and obtain secondary smelted molten iron, A distribution facility, wherein secondary refined molten iron is distributed and transferred within the distribution facility, A spheroidizing apparatus, comprising a spheroidizing apparatus that performs spheroidizing treatment on molten iron that has been distributed and transported, The molten iron smelting method is, Step (1) of the induction furnace crude smelting, which includes adding cold steel material, cast iron, nickel alloy plate, and graphite powder to the first station and crude smelting to obtain crude molten iron from the first station, and adding cast iron and graphite powder to the second station and crude smelting to obtain crude molten iron from the second station, The secondary refining is performed using a ladle smelting furnace, and the steps include: (2) adding the crude molten iron from the first station to the first ladle smelting furnace for primary refining to obtain primary refined molten iron, and then sequentially adding the primary refined molten iron and the crude molten iron from the second station to the second ladle smelting furnace for secondary refining to obtain secondary refined molten iron; The process includes step (3) sequentially transferring and distributing the secondary refined molten iron in a distribution facility and performing a spheroidization treatment in a spheroidization treatment facility. A method for smelting molten iron characterized by the following features.

2. Cold-worked steel accounts for 15-20% of the total mass of cold-worked steel, pig iron, and nickel alloy sheets in the raw materials for molten iron production. The molten iron smelting method according to feature 1.

3. The aforementioned cold-worked steel is obtained by dephosphorization and decarburization by arc furnace crude smelting, carbon adjustment and desulfurization by ladle furnace refining, and billet casting in a mold under atmospheric conditions. The molten iron smelting method according to feature 1.

4. The cold-worked steel described in step (1) contains, by mass percentage, 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, the remainder Fe and unavoidable impurities. more The molten iron smelting method according to feature 1.

5. In step (1), the cast iron is composed of, by mass percentage, 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, 0.050% or less Ti, the remainder being Fe and unavoidable impurities. The molten iron smelting method according to feature 1.

6. In step (1), the mass fraction of Ni in the nickel alloy plate exceeds 99.5%. The molten iron smelting method according to feature 1.

7. In step (1), the crude smelting of molten iron at the first and second stations is carried out at a process temperature of 1550°C or lower, and after all the charge is cleaned, the iron is kept warm at 1500-1520°C for 5-20 minutes before tapping. The molten iron smelting method according to feature 1.

8. In step (2), a method is employed in which argon gas is blown in from the bottom of the ladle and stirred throughout the entire process in both the primary and secondary refining stages. The molten iron smelting method according to feature 1.

9. In step (2), slag formation treatment is performed in both primary and secondary refining. Specifically, the component requirements for the slag system are as follows: in primary refining, the slag-forming material is metallurgical lime and fluorite, with a mass ratio of metallurgical lime to fluorite of 4:1; and in secondary refining, fluorite is used as the slag-forming material. The molten iron smelting method according to feature 1.

10. In step (2), the process temperature for the primary and secondary refining of the molten iron is 1550°C or lower. The molten iron smelting method according to feature 1.

11. In step (2), cold steel and cast iron are added to adjust the mass and weight of the molten iron during the primary refining process. The molten iron smelting method according to feature 1.

12. In step (3), before the spheroidizing treatment, argon gas is blown in from the bottom of the ladle to equalize the temperature of the molten iron, thereby raising the spheroidizing treatment temperature to 1350-1450°C. A method for smelting molten iron according to any one of claims 1 to 11, characterized by the features described herein.

13. Step (3) further includes transferring the molten iron after spheroidization to the casting process for final inoculation, slag removal, and pouring operations. The molten iron smelting method according to feature 1.

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