Flux used in electroslag remelting, method for manufacturing said flux, and method for manufacturing high-purity steel

Abstract

Provided is a flux that is used in an electro-slag remelting process, the flux comprising a mixture of particles containing a CaF2 component and particles containing an Al2O3 component and a CaO component, and the flux having a W value of 0.05% by mass or less and a ΔW value of 0.03% by mass or less wherein W represents an initial water content in the flux which is determined by a Karl Fischer's method and ΔW represents a change in water content in the flux when the flux is placed under an environment of 30°C and 85% RH for 14 days.

Description

[Technical Field]

[0001] The present invention relates to a flux used in electroslag remelting, a method for producing the flux, and a method for producing high-purity steel. [Background technology]

[0002] Electro-slag remelting is a special melting method that produces high-purity metals by remelting and refining metal materials (such as steel). This method is also called the "ESR method" or "ESR process," an abbreviation of the English term "Electro-Slag Remelting process" (these terms will be used hereinafter in this specification).

[0003] In the ESR method, (1) first, the electrode metal is melted by the resistance heat of the slag, (2) impurities (sulfur, oxygen, etc.) are removed as droplets of the molten metal pass through a slag pool, and (3) the molten metal solidifies at the bottom of the furnace to form an ingot, resulting in a highly purified metal material. The slag that is often used here is one whose main components are CaF2, Al2O3, and CaO. That is, a solid flux whose main components are CaF2, Al2O3, and CaO is often melted at high temperature to produce slag.

[0004] For example, Patent Document 1 describes slag for electroslag remelting, with an analyzed composition of 0.2-15 wt% SiO2, 12-40 wt% Al2O3, 15-40 wt% alkaline earth metal oxides, 12-75 wt% CaF2, 0.5 wt% or less FeO, 1 wt% or less MnO, and 10 wt% or less TiO2. The upper left column of page 2 of this document also states that the moisture content of the slag must be kept low to prevent the moisture from the flux from causing hydrogen embrittlement of the metal material being melted and refined. The lower left column of page 3 of this document further describes that granular slag was heated to 650°C, and the released moisture was absorbed by magnesium perchlorate, and the moisture content of the slag and its change over time were measured.

[0005] As another example, Patent Document 2 describes that the composition of the flux added in the ESR method is CaO: 20 to 60 mass%, Al2O3: 10 to 40 mass%, CaF2: 20 to 60 mass%, iron oxide: 1 to 10 mass%, and CaO / Al2O3: 1.0 to 6.0. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 57-060411 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-049908 Summary of the Invention [Problem to be solved by the invention]

[0007] It is known that the ESR method has a problem in that defects tend to occur in the resulting ingot, particularly at the bottom. Specifically, it is known that moisture from the flux causes hydrogen embrittlement of the metallic material being melted and refined, especially in the initial stage of the entire ESR process. This is also described in Patent Document 1, which was explained in the Background Art section above.

[0008] However, the inventors have found that conventional fluxes with reduced moisture content still have room for improvement in terms of the hydrogen embrittlement problem of metallic materials and the omission of processes during the production of metallic materials. For example, there is room for further improvement in the yield of ingots obtained by the ESR method and in the need for a drying process aimed at reducing the moisture content of the flux during the production of metallic materials.

[0009] The present invention has been made in view of the above circumstances, and one of its objects is to provide a flux for use in an electroslag remelting process that contains highly hygroscopic CaO and yet is capable of producing high-purity metals that are inhibited from hydrogen embrittlement with a high yield. [Means for solving the problem]

[0010] As a result of extensive research, the present inventors have completed the invention provided below and solved the above-mentioned problems.

[0011] According to the present invention, A flux for use in an electroslag remelting process, comprising: A mixture of particles containing a CaF2 component and particles containing an Al2O3 component and a CaO component, The initial moisture content of the flux determined by the Karl Fischer method is W, and the moisture content W after the flux is left in an environment of 30°C and 85% RH for 14 days is 14 When the change in moisture content, which is expressed as the difference between the initial moisture content W and the initial moisture content W, is ΔW, the flux is provided in which W is 0.05 mass % or less and ΔW is 0.03 mass % or less.

[0012] Further, according to the present invention, The method for producing the flux includes the steps of: a firing step of firing a compound containing at least a CaF2 component to obtain particles containing the CaF2 component; a melt-mixing step of melt-mixing at least a compound containing a CaO component and a compound containing an Al2O3 component to obtain a molten mixture containing the CaO component and the Al2O3 component; a pulverization step of pulverizing the molten mixture containing the CaO component and the Al2O3 component to obtain a pulverized material containing the CaO component and the Al2O3 component; a sorting step of passing the pulverized material containing the CaO component and the Al2O3 component through a sieve with a nominal opening of 500 μm specified in JIS Z 8801-1:2019 to obtain particles containing the CaO component and the Al2O3 component; a mixing step of mixing at least particles containing the CaF2 component with particles containing the CaO component and Al2O3 component to obtain a flux; A method for producing a flux, comprising: is provided.

[0013] Further, according to the present invention, A method for producing high purity steel, comprising a step of refining raw steel into high purity steel by remelting using an electroslag remelting method, A method for producing high purity steel using a melt of the above flux as the molten slag is provided. [Effects of the Invention]

[0014] According to the present invention, there is provided a flux for use in an electroslag remelting process, which is capable of producing high-purity metals that are suppressed from being embrittled by hydrogen with a high yield. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram for explaining a method for producing high-purity steel by electroslag remelting. [Figure 2] FIG. 1 is a diagram for explaining a method for producing high-purity steel by electroslag remelting. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, similar components are denoted by similar reference numerals and descriptions thereof will be omitted where appropriate. To avoid complexity, when there are multiple identical components in the same drawing, only one of them may be labeled with a symbol, and not all of them. The drawings are for illustrative purposes only, and the shapes and dimensional ratios of the components in the drawings do not necessarily correspond to the actual products.

[0017] In this specification, the term "slag" basically refers to a state in which flux that is solid at room temperature (25°C) is heated to a high temperature and melted. However, this is not always the case depending on the context (for example, in Patent Document 1, flux that is solid at room temperature appears to be expressed as "slag").

[0018] <Flux> The flux of this embodiment is used in the electroslag remelting method. This flux contains a mixture of particles containing CaF2 components and particles containing Al2O3 components and CaO components. The initial moisture content of the flux (moisture content at the time of storage for 0 days) determined by the Karl Fischer method is W, and the moisture content W after storing the flux in an environment of 30°C and 85% RH for 14 days is 14 The moisture content change (moisture content after 14 days W) is expressed as the difference between the initial moisture content W and the 14 When the initial moisture content (W) is ΔW, the flux has W of 0.05% by mass or less and ΔW of 0.03% by mass or less. In this case, W is 0.05% by mass or less, preferably 0.03% by mass or less, more preferably 0.02% by mass or less, and even more preferably 0.015% by mass or less. Also, ΔW is 0.03% by mass or less, preferably 0.01% by mass or less, more preferably 0.007% by mass or less, and even more preferably 0.005% by mass or less. Here, the lower limit of W is ideally 0, but in reality it is, for example, 0.00001 mass % or more, and the lower limit of ΔW is, for example, −0.005% or more. The initial moisture content W and the moisture content change ΔW at this time refer to the initial moisture content W and the moisture content change ΔW for all particle sizes of the flux.

[0019] The properties required for ESR flux include low viscosity, high desulfurization, and high electrical resistance. To achieve these properties, the flux contains CaF2, Al2O3, and CaO components. However, the CaO component used here is highly hygroscopic when present as a single CaO crystalline phase, so if the CaO crystalline phase content is high, the hygroscopicity of the flux deteriorates, making the metal more susceptible to hydrogen embrittlement.

[0020] The present inventors have focused on the moisture content of the flux in order to improve the yield of ingots obtained by the ESR method. Specifically, in order to precisely control and optimize the moisture content of the flux, the present inventors have measured the moisture content by the Karl Fischer method and designed the flux so that the initial moisture content W determined by the measurement is 0.05 mass% or less.

[0021] Furthermore, the inventors of the present invention have considered that not only the moisture content of the flux but also its change over time is closely related to the yield of ingots, and have designed a flux so that the moisture content change ΔW is 0.03 mass% or less when the flux is left in an environment of 30°C and 85% RH for 14 days.

[0022] By using a flux that satisfies these conditions of "W being 0.05 mass% or less" and "ΔW being 0.03 mass% or less," it is possible to produce high-purity metal with reduced hydrogen embrittlement at a good yield, even though it contains the highly hygroscopic CaO component. In addition, by suppressing the hygroscopicity of the flux, the flux drying process becomes unnecessary during the production of metal materials, making it possible to omit the drying process, and as a result, reducing the cost of producing metal materials.

[0023] In order to set W and ΔW to the above values, for example, (1) A fired material containing a CaF2 component, which is a raw material for the flux, and a metal oxide containing a CaO component and an Al2O3 component are separately produced and then mixed together; and / or (2) It is preferable to sieve the ground material containing the CaO component and the Al2O3 component to remove fine powder with a low particle size and high hygroscopicity before mixing it with the fired material containing the CaF2 component. (1) will be explained below, and (2) will be explained in the section <Flux Manufacturing Method> below.

[0024] As described in (1) above, the flux of this embodiment contains a mixture of particles containing CaF2 and particles containing Al2O3 and CaO. In other words, the particles containing CaF2 and the particles containing Al2O3 and CaO are produced separately and then mixed together to form a mixture. This effectively suppresses the hygroscopicity of the flux, allowing for the production of high-purity metal with reduced hydrogen embrittlement at a high yield.

[0025] Although the reason for this is unclear, it is thought that the generation of CaO crystalline phase in the flux is suppressed by separately producing particles containing CaF2 and particles containing metal oxides such as Al2O3 and CaO, and then finally mixing them. Because the CaO crystalline phase is highly hygroscopic as mentioned above, suppressing the generation of the CaO crystalline phase in the flux is thought to more effectively suppress the hygroscopicity of the flux, making it possible to produce high-purity metal with reduced hydrogen embrittlement at a good yield.

[0026] As mentioned above, it is preferable to suppress the occurrence of the CaO crystalline phase present as a single phase in the flux from the viewpoint of the hygroscopicity of the flux. On the other hand, from a viewpoint other than the suppression of hygroscopicity, for example, from the viewpoint of maintaining a low melting point of the flux while appropriately increasing the desulfurization performance of the flux and the basicity of the slag obtained by melting the flux, it is preferable for the flux to contain CaO. In other words, by including a mixture of particles containing CaF2 and particles containing Al2O3 and CaO, the amount of CaO crystalline phase in the flux can be appropriately controlled, and the hygroscopicity, melting point, desulfurization performance, and basicity of the slag obtained by melting the flux can be highly controlled.

[0027] Here, it is preferable that the particles containing the CaF2 component do not contain a CaO crystalline phase, which makes it easier to control the amount of CaO crystalline phase in the flux, and also makes it easier to control the hygroscopicity, melting point, and desulfurization performance of the flux, as well as the basicity of the slag obtained by melting the flux.

[0028] Incidentally, Patent Document 1 describes a moisture content measurement method in which granular slag is heated to 650°C and the released moisture is absorbed by magnesium perchlorate. However, this method is less accurate in determining moisture content than the Karl Fischer method because it is not possible to absorb all of the moisture with magnesium perchlorate, and there is a possibility that substances other than moisture may be absorbed by magnesium perchlorate.

[0029] Furthermore, in Patent Document 1, the food is placed in a "paper bag with polyethylene lining," i.e., a moisture-blocking bag, and the absolute humidity of the indoor air is "4 to 17 g / m 3 The moisture absorption amount of the flux stored under the environment where the temperature fluctuates greatly between " and " is measured. It can be said that under such evaluation conditions, the moisture absorption of the "flux itself" is not evaluated properly. In this embodiment, the moisture absorption of the flux itself can be properly measured by evaluating the moisture absorption of the flux in a bare state without being placed in a bag under the constant conditions of 30°C and 85% RH. The ΔW obtained by the measurement of the flux of this embodiment falls within a certain range.

[0030] The flux of this embodiment will now be described in more detail.

[0031] (Flux penetration rate) When the flux of this embodiment is passed through a sieve with a nominal mesh size of 500 μm as specified in JIS Z 8801-1:2019, the passing rate, which is the ratio of the mass of the flux that falls through the sieve to the mass of the flux that is passed through the sieve, is preferably 18% or less, more preferably 15% or less, even more preferably 12% or less, even more preferably 10% or less, and even more preferably 8% or less. This makes it possible to more suitably suppress the hygroscopicity of the flux, and to produce high-purity metal with suppressed hydrogen embrittlement at a high yield. The lower limit of the passing rate is ideally 0%, but in reality it is, for example, 0.00001% or more.

[0032] When the flux of this embodiment is passed through a sieve with a nominal mesh size of 1.0 mm as specified in JIS Z 8801-1:2019, the passing rate, which is the ratio of the mass of the flux that falls through the sieve to the mass of the flux that is passed through the sieve, is preferably 40% or less, more preferably 37% or less, even more preferably 35% or less, even more preferably 33% or less, even more preferably 31% or less, even more preferably 29% or less, and even more preferably 28% or less. This makes it possible to more effectively suppress the hygroscopicity of the flux, and to produce high-purity metal with reduced hydrogen embrittlement at a high yield. The lower limit of the passing rate is preferably 18% or more, more preferably 20% or more, and even more preferably 21% or more, which reduces the bulk of the flux when stored and makes it easier to melt the flux during refining.

[0033] When the flux of this embodiment is passed through a sieve with a nominal mesh size of 2.8 mm as specified in JIS Z 8801-1:2019, the passing rate, which is the ratio of the mass of the flux that falls through the sieve to the mass of the flux that is passed through the sieve, is preferably 90% or less, more preferably 87% or less, even more preferably 84% or less, and even more preferably 82% or less. This makes it possible to more suitably suppress the hygroscopicity of the flux, and to produce high-purity metal with suppressed hydrogen embrittlement at a high yield. The lower limit of the passing rate is preferably 45% or more, more preferably 55% or more, even more preferably 60% or more, even more preferably 65% ​​or more, even more preferably 70% or more, even more preferably 74% or more, and even more preferably 77% or more, which reduces the bulk of the flux when stored and makes it easier to melt the flux during refining.

[0034] (Chemical composition of flux) CaF2 and Al2O3 components The flux of the present embodiment preferably contains a CaF2 component and an Al2O3 component. Here, when the flux further contains CaO component, the Al2O3 component exists as a calcium aluminate crystalline phase (3CaO-Al2O3, 12CaO-7Al2O3, CaO-Al2O3, CaO-2Al2O3, CaO-6Al2O3). Therefore, in this embodiment, the "Al2O3 component" includes not only the Al2O3 component itself but also the Al2O3 component in the calcium aluminate crystalline phase. More preferably, in the flux of this embodiment, part or all of the CaF2 component is present as a CaF2 crystalline phase, and part or all of the Al2O3 component is present as a calcium aluminate crystalline phase. The flux containing CaF2 and Al2O3 components has the advantage of being able to obtain an appropriate melting temperature of the flux, etc. Furthermore, the flux further contains CaO and the Al2O3 component exists as a calcium aluminate crystalline phase, which makes it possible to make the melting temperature of the flux more suitable.

[0035] When the flux of this embodiment contains a CaF2 component, the content of the CaF2 crystalline phase in the entire crystalline phase in the flux is preferably 20 mass % or more and 80 mass % or less, more preferably 35 mass % or more and 65 mass % or less. When the flux of this embodiment contains an Al2O3 component, the total content of the Al2O3 crystalline phase and the calcium aluminate crystalline phase in the entire crystalline phase in the flux is preferably 20 mass% or more and 80 mass% or less, more preferably 35 mass% or more and 65 mass% or less.

[0036] In order to achieve the above-mentioned crystalline phase content in the flux of this embodiment, the upper limit of the amount of CaF2 component in the entire flux when the raw materials are mixed is preferably 80 mass% or less, more preferably 75 mass% or less, even more preferably 65 mass% or less, and even more preferably 63 mass% or less. Also, the lower limit of the amount of CaF2 component in the entire flux is preferably 20 mass% or more, more preferably 30 mass% or more, even more preferably 35 mass% or more, and even more preferably 38 mass% or more. Furthermore, the upper limit of the amount of Al2O3 component in the entire flux at the time of mixing the raw materials is preferably 40 mass% or less, more preferably 37 mass% or less, even more preferably 35 mass% or less, and even more preferably 32 mass% or less. The lower limit of the amount of Al2O3 component in the entire flux is preferably 5 mass% or more, more preferably 10 mass% or more, even more preferably 15 mass% or more, and even more preferably 16 mass% or more.

[0037] ·CaO component In the flux of this embodiment, the upper limit of the amount of CaO in the entire flux when the raw materials are mixed is preferably 40 mass% or less, more preferably 37 mass% or less, even more preferably 35 mass% or less, even more preferably 30 mass% or less, and even more preferably 29 mass% or less. CaO is a highly hygroscopic substance. Therefore, by reducing the amount of CaO in the entire flux during raw material mixing and by reducing the amount of CaO crystalline phase in the flux, the hygroscopicity of the flux can be more effectively suppressed, and high-purity metal with reduced hydrogen embrittlement can be produced with a good yield.

[0038] Furthermore, in the flux of this embodiment, from a viewpoint other than suppression of hygroscopicity, for example, from the viewpoint of appropriately increasing the desulfurization performance of the flux and the basicity of the slag obtained by melting the flux while maintaining a low melting point of the flux, the lower limit of the amount of CaO in the entire flux at the time of mixing the raw materials is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, still more preferably 20% by mass or more, and even more preferably 21% by mass or more.

[0039] Other components: MgO, SiO2, etc. The flux of this embodiment may contain one or more components other than those mentioned above, such as an MgO component and an SiO2 component. It is believed that the inclusion of an appropriate amount of MgO in the flux allows the melting point of the flux to be maintained low while the basicity of the slag obtained by melting the flux can be appropriately increased, which is preferable in terms of the ability to remove sulfur components during ingot production. By including an appropriate amount of SiO2 in the flux, it becomes easier to design a flux that increases the electrical resistance of the slag obtained by melting the flux while maintaining its performance, such as its ability to remove sulfur components. This is desirable because it makes it easier to dissolve electrode metals. In addition, by including an appropriate amount of SiO2 in the flux, the ferrite phase of the steel is strengthened, which tends to increase the strength of the steel after refining.

[0040] When the flux of this embodiment further contains an MgO component, the content thereof is preferably 0.005 mass % or more and 5.0 mass % or less, and more preferably 0.01 mass % or more and 3.0 mass % or less, of the entire flux. When the flux of this embodiment further contains an SiO2 component, the content thereof is preferably 0.3 mass % or more and 3.0 mass % or less, more preferably 0.5 mass % or more and 2.5 mass % or less, based on the total mass of the flux.

[0041] -Method for quantifying the amount of each component The content of each crystalline phase of CaF2, Al2O3, CaO, and calcium aluminate formed by reaction of Al2O3 with CaO can be quantitatively analyzed, for example, by analyzing powder X-ray diffraction (XRD) patterns by the Rietveld method. More specifically, the content of each material can be determined by comparing the diffraction intensity-incident angle chart obtained by XRD analysis with the XRD pattern simulated from a crystal structure model, and optimizing the mass fraction using the least squares method so as to minimize the residual between the experimental and calculated XRD patterns.

[0042] In addition, the amounts of chemical components such as CaF2, Al2O3, CaO, MgO, and SiO2 can be quantified by X-ray fluorescence analysis (XRF), ion electrode method, a combination of these methods, or the like.

[0043] It is preferable that the flux of this embodiment basically contains as few components as possible other than those described above. In other words, it is preferable that the flux of this embodiment contains as few impurities as possible. The low content of components other than those described above can suppress fluctuations in the properties of the slag obtained by dissolving the flux. Specifically, the content of impurities in the entire flux is preferably 5 mass % or less, more preferably 3 mass % or less, and even more preferably 1 mass % or less.

[0044] (Flux properties) The flux of this embodiment may have various properties. From the viewpoint of controlling the change in moisture content of the flux to a desired value (and thereby producing high-purity metal with reduced hydrogen embrittlement at a high yield), it is preferable that the flux be granular at room temperature. Room temperature is, for example, 25°C.

[0045] <Flux manufacturing method> Next, a method for producing the flux according to this embodiment will be described. The method for producing the flux of the present embodiment includes a firing step of firing at least a compound containing CaF to obtain particles containing CaF, and a melt-mixing step of melt-mixing at least a compound containing CaO and a compound containing AlO to obtain a molten mixture containing CaO and AlO. The flux can be obtained by a production method including: a crushing step of crushing the molten mixture containing CaO and Al2O3 to obtain a crushed material containing CaO and Al2O3; a sorting step of passing the crushed material containing CaO and Al2O3 through a sieve with a nominal mesh size of 500 μm specified in JIS Z 8801-1:2019 to obtain particles containing CaO and Al2O3; and a mixing step of mixing at least the particles containing CaF2 and the particles containing CaO and Al2O3 to obtain a flux.

[0046] Each step will be described in detail below.

[0047] (Firing process) First, a compound containing the CaF2 component is fired to obtain particles containing the CaF2 component. This process evaporates the moisture in the particles containing the CaF2 component, thereby more effectively suppressing the hygroscopicity of the flux. The firing temperature and firing time in this step may be any temperature and time that can evaporate the moisture in the compound containing the CaF2 component, for example, firing at 1200°C for 1 hour.

[0048] (Melting and mixing process) Next, a compound containing CaO and a compound containing AlO are melt-mixed to obtain a molten mixture containing CaO and AlO, separately from the particles containing CaF obtained in the firing step. This step allows the flux of this embodiment to have a suitable melting temperature while controlling the CaO content in the flux. At this time, the melting temperature for melt-mixing the compound containing the CaO component and the compound containing the Al2O3 component is preferably 1800° C. or higher, and the melting time for melt-mixing the compound containing the CaO component and the compound containing the Al2O3 component is preferably 3 hours or longer.

[0049] (Crushing process) Next, as described in (2) above, the molten mixture containing the CaO and Al2O3 components obtained in the melt mixing step is pulverized to obtain a pulverized product containing the CaO and Al2O3 components. This step controls the change in moisture content in the flux of this embodiment, and more effectively suppresses the hygroscopicity of the flux of this embodiment. As a method for pulverizing the molten mixture, any conventionally known pulverization method may be used as long as it can achieve the change in moisture content in the flux of this embodiment, and examples thereof include pulverizers and dispersers such as a ball mill, a bead mill, a vibration mill, a turbo mill, a mechanofusion, a roll crusher, a cone crusher, a jaw crusher, a hammer mill, a disc mill, and a roll mill; a rock drill; a vibration drill; an impact driver; a high-pressure gliding roll; and vertical mills such as a roller-type vertical mill and a ball-type vertical mill.

[0050] (Sorting process) Next, the pulverized material containing the CaO and Al2O3 components obtained in the above-mentioned pulverization step is passed through a sieve with a nominal mesh size of 500 μm as specified in JIS Z 8801-1:2019 to obtain particles containing the CaO and Al2O3 components on the sieve. This step removes the fine powder with small particle size and high hygroscopicity generated in the pulverization step, thereby more effectively suppressing the hygroscopicity of the flux of this embodiment.

[0051] (Mixing process) Finally, the particles containing the CaF2 component obtained in the aforementioned firing step are mixed with the particles containing the CaO component and the Al2O3 component obtained in the aforementioned sorting step. By removing fine powder with a small particle size and high hygroscopicity in advance, it is possible to control the change in moisture content in the flux of this embodiment, and as a result, it is possible to more effectively suppress the hygroscopicity of the flux, and it is possible to produce high-purity metal with reduced hydrogen embrittlement with a good yield.

[0052] <Manufacturing method for high purity steel> 1 and 2 are schematic diagrams illustrating an example of a method for producing high-purity steel by refining raw steel by the ESR method. Specifically, Fig. 1 is a diagram that schematically shows the state at the start of production of high-purity steel by the ESR method, and Fig. 2 is a diagram that schematically shows the state after some time has passed since the start of Fig. 1.

[0053] In Fig. 1, raw steel (electrode 10) is electrically connected to one end of a power source 2. The composition of the raw steel (electrode 10) is not particularly limited. The composition can be determined to correspond to the high purity steel to be obtained. The raw steel (electrode 10) is installed in the ESR furnace 1 so that it can move up and down. At the start of production, solid (granular at room temperature) flux 11A is spread over the sides and bottom of the raw steel (electrode 10). The flux 11A has the above-mentioned composition. The other end of the power supply 2 is electrically connected to the conductive hearth of the ESR furnace 1 . The furnace wall of the ESR furnace 1 may be provided with a cooling means such as a water cooling means (not shown in FIG. 1). Although the ESR furnace 1 is shown in an open configuration in FIG. 1, the smelting of raw steel may be carried out in a closed, atmosphere-controlled ESR furnace.

[0054] To produce high-purity steel, a current is applied from a power source 2 to an electrode 10 and a flux 11A. This causes resistance heat to melt the flux 11A and the tip of the electrode 10. The flux 11A melts and becomes slag 11. The molten metal from the tip of the electrode 10 descends through the slag 11. As this occurs, impurities (such as sulfur atoms) in the metal are absorbed into the slag 11, refining the raw steel. The descending metal forms a molten pool 12 below the slag 11. The metal then gradually cools to produce high-purity steel (ingot 13) (see FIG. 2). 2, the liquid surface of the slag 11 gradually moves upward as the ingot 13 and the molten pool 12 are produced. Therefore, the electrode 10 is moved appropriately in accordance with this movement, and the electrode 10 is continuously remelted. In this way, high purity steel can be obtained.

[0055] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0056] The present invention will be described in detail based on examples and comparative examples, but the present invention is not limited to these examples.

[0057] (Flux raw material) First, the following compounds containing a CaF2 component, a compound containing an Al2O3 component, and a compound containing a CaO component were prepared as raw materials. Compounds containing CaF2: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name: calcium fluoride, purity 98% by mass or more Compound containing Al2O3: Nippon Light Metal Co., Ltd., product name = SMM-23A, purity 99% by mass or more Compounds containing CaO: Yoshizawa Lime Industry Co., Ltd., product name = quicklime, purity 99% by mass or more Here, the compound containing the CaF2 component contained small amounts of MgO component, SiO2 component, etc. These raw materials were mixed in the raw material ratios shown in Table 1 and processed according to the (method for manufacturing a fired product) or (method for manufacturing a molten product) described below to obtain particles 1 (particles containing CaF2 component), particles 2 and 3 (particles containing metal oxides).

[0058] [Table 1]

[0059] In Table 1, "baked product" and "melted product" refer to the methods for producing the particles. The manufacturing methods for the "fired product" and "molten product" are shown below.

[0060] (Method for manufacturing baked products) The above raw materials were placed in a rotary kiln according to the raw material ratios shown in Table 1 and fired at 1200°C for 1 hour to obtain particles.

[0061] (Melt manufacturing method) The above raw materials were mixed in a gravity mixer according to the raw material ratios shown in Table 1 to obtain a substantially uniform mixture. The mixture was then placed in a three-phase arc furnace and heated to 1800°C or higher under conditions of a current value of 3.0 to 3.5 kA and a voltage value of 150 to 200 V to obtain a molten mixture. The melting time was 3 hours.

[0062] After cooling, the resulting molten mixture was pulverized using a cone crusher and sorted by passing it through a sieve with a nominal mesh size of 4.75 mm as specified in JIS Z 8801-1:2019. The molten mixture that did not pass through the sieve (remained on the sieve) was pulverized again using a cone crusher and sorted by passing it through a sieve with a nominal mesh size of 4.75 mm as specified in JIS Z 8801-1:2019. These operations were repeated until all of the molten mixture passed through the sieve (fell below the sieve). The mixture was then sorted by passing it through a sieve with a nominal mesh size of 500 μm as specified in JIS Z 8801-1:2019 to obtain particles.

[0063] <Examples 1 and 2> The obtained particles 1 to 3 were mixed in a gravity mixer to obtain the fluxes of Examples 1 and 2. The mixing ratios in Example 1 were particle 1: 62.2 mass% and particle 2: 37.8 mass%, and in Example 2 were particle 1: 40.2 mass%, particle 2: 35.4 mass%, and particle 3: 24.4 mass%.

[0064] <Comparative Example 1> First, the raw materials, a compound containing CaF2, a compound containing Al2O3, and a compound containing CaO, were mixed in a gravity mixer to obtain a nearly uniform mixture. The mixture ratios were 42.3 mass% of the compound containing CaF2, 31.4 mass% of the compound containing Al2O3, and 27.3 mass% of the compound containing CaO. Next, this mixture was treated in the same manner as described above (Method for producing a molten product) except that it was not passed through a sieve with a nominal mesh size of 500 μm as specified in JIS Z 8801-1:2019, and the resulting particles were designated as Comparative Example 1.

[0065] <Evaluation> The obtained fluxes of the Examples and Comparative Examples were evaluated by the following methods.

[0066] (Measurement of initial moisture content W and moisture absorption) In order to investigate the moisture absorption of the flux as a whole as well as the difference in moisture absorption due to particle size, the flux was separated by particle size and exposed to an environment of 30°C and 85% RH. Specifically, the flux was sieved and its moisture content was measured according to the following steps (1) to (6).

[0067] (1) The flux was passed through a sieve with a nominal mesh size of 4.75 mm (corresponding to the No. 4 sieve of ASTM E11) specified in JIS Z8801-1:2019, and it was confirmed that no residue remained on the sieve (in this case, no residue was found on the sieve in any of Examples 1 and 2 and Comparative Example 1). (2) The flux was passed through a sieve with a nominal mesh size of 2.8 mm specified in JIS Z8801-1:2019 (equivalent to the No. 7 sieve of ASTM E11). This resulted in the selection of flux with particle sizes of 4.75 to 2.8 mm. The mass of the sieve residue that could not pass through the sieve was then measured. (3) The components that passed through the sieve in (2) above were passed through a sieve with a nominal mesh size of 1 mm specified in JIS Z8801-1:2019 (equivalent to the No. 18 sieve of ASTM E11). This resulted in the selection of flux with particle sizes of 1 to 2.8 mm. The mass of the sieve residue that did not pass through the sieve was then measured. (4) The components that passed through the sieve in (3) above were passed through a sieve with a nominal mesh size of 500 μm as specified in JIS Z8801-1:2019 (equivalent to the ASTM E11 No. 35 sieve). This separated the flux into particles with a particle size of 0.5 to 1 mm and particles with a particle size of 0.5 mm or less. The masses of the flux with a particle size of 0.5 to 1 mm (sieve residue) and the flux with a particle size of 0.5 mm or less (components that passed through the sieve) were then measured.

[0068] (5) The fluxes obtained above, each with a particle size of 2.8 to 4.75 mm, 1 to 2.8 mm, 0.5 to 1 mm, and 0.5 mm or less, were separately subjected to an apparatus (Nitto Seiko Analytech Co., Ltd. / Model CA-310) to determine the initial moisture content W (moisture content at the time of storage for 0 days) by the Karl Fischer method.

[0069] (6) Each of the fluxes with the above particle sizes was placed separately in an open-top cylindrical aluminum dish (top diameter: φ59 mm, bottom diameter: φ45 mm), with 10 g of each particle size (15 pieces in total), and left to stand at 30°C and 85% RH. Then, after 7, 14, and 28 days from the start of the standing, the aluminum dish containing the flux of each particle size was removed along with the flux inside, and after thorough mixing, the moisture content W7 and W8 for each particle size were measured in the same manner as in (5) above. 14 and W 28 asked for.

[0070] (7) In addition to measuring the moisture content of each of the above fluxes of each particle size, the fluxes (fluxes of all particle sizes) that were not subjected to the sorting steps (1) to (4) were also measured for their initial moisture content W and their moisture contents W7, W8 after 7 days, 14 days, and 28 days in the same manner as in (5) and (6). 14 and W 28 asked for. After that, for all particle sizes of flux, the moisture content W after 14 days 14 The difference between the initial moisture content W and the moisture content W was calculated as the moisture content change ΔW. The results are shown in Table 2.

[0071] [Table 2]

[0072] In Table 2, when looking at the change in moisture content over time by particle size, it can be seen that the smaller the particle size, the higher the moisture content tends to be.

[0073] <Manufacturing of high-purity steel (refining)> The fluxes of Examples 1 and 2, which had been produced for more than 28 days, were used as they were without any prior drying treatment to refine raw steel containing impurities such as sulfur in an ESR furnace as shown in Figures 1 and 2 to produce ingots. It was confirmed that ingots with sufficiently suppressed hydrogen embrittlement could be produced with a good yield. On the other hand, when the flux of Comparative Example 1, which had been produced for more than 28 days, was used instead of the flux of Examples 1 and 2 without prior drying treatment, hydrogen embrittlement could not be suppressed compared to Examples 1 and 2, and the ingot yield was poor.

[0074] From the above, it was found that by refining raw steel by the ESR method using a flux with a W content of 0.05 mass% or less and a ΔW of 0.03 mass% or less, it is possible to produce high-purity metal with suppressed hydrogen embrittlement, even though it contains the highly hygroscopic CaO component, with good yield.

[0075] This application claims priority based on Japanese Patent Application No. 2021-182353, filed on November 9, 2021, the disclosure of which is incorporated herein in its entirety. [Explanation of symbols]

[0076] 1 ESR furnace 2 power supply 10 electrodes 11A Flux 11 Slag 12 Melt Pool 13 Ingots

Claims

1. A flux for use in an electroslag remelting process, comprising: CaF ２ Particles containing Al ２ O ３ and particles containing a CaO component, The initial moisture content of the flux determined by the Karl Fischer method is W, and the moisture content W when the flux is left in an environment of 30°C and 85% RH for 14 days is １４ and the initial moisture content W, W is 0.02 mass% or less, and ΔW is 0.01 mass% or less, the amount of the CaF 2 component in the entire flux at the time of mixing the raw materials is 20% by mass or more and 80% by mass or less; the amount of the Al 2 O 3 component in the entire flux at the time of mixing the raw materials is 5 mass % or more and 40 mass % or less; The amount of the CaO component in the entire flux at the time of mixing the raw materials is 5% by mass or more and 40% by mass or less, A flux in which, when the flux is passed through a sieve having a nominal mesh size of 1.0 mm as specified in JIS Z 8801-1:2019, the passing rate, which is the ratio of the mass of the flux that falls through the sieve to the mass of the flux that is passed through the sieve, is 28% or less.

2. A method for producing a flux for use in an electroslag remelting process, comprising: the flux includes a mixture of particles containing a CaF 2 component and particles containing a CaO component and an Al 2 O 3 component; When the initial moisture content of the flux determined by the Karl Fischer method is W, and the change in moisture content expressed by the difference between the moisture content W14 obtained when the flux is placed in an environment of 30°C and 85% RH for 14 days and the initial moisture content W is ΔW, W is 0.02 mass% or less, and ΔW is 0.01 mass% or less, the amount of the CaF 2 component in the entire flux at the time of mixing the raw materials is 20% by mass or more and 80% by mass or less; the amount of the Al 2 O 3 component in the entire flux at the time of mixing the raw materials is 5 mass % or more and 40 mass % or less; A method for producing a flux, wherein the amount of the CaO component in the entire flux at the time of mixing raw materials is 5% by mass or more and 40% by mass or less, At least, CaF ２ The compound containing the component is fired, and the CaF ２ a calcination step to obtain particles containing the component; At least a compound containing CaO and Al ２ O ３ The compound containing the CaO component and the Al component is melt-mixed. ２ O ３ a melt mixing step to obtain a molten mixture containing the components; The CaO component and Al ２ O ３ The molten mixture containing the components is crushed to obtain the CaO component and the Al component. ２ O ３ a grinding step for obtaining a ground product containing the component; The CaO component and Al ２ O ３ The ground material containing the CaO component and Al component is passed through a sieve with a nominal opening of 500 μm as specified in JIS Z 8801-1:2019. ２ O ３ a sorting step to obtain particles containing the component; At least the CaF ２ particles containing the CaO component and Al component on the sieve obtained in the screening step. ２ O ３ a mixing step of mixing particles containing the component to obtain a flux; A method for producing a flux, comprising:

3. A method for producing high purity steel, comprising a step of refining raw steel into high purity steel by remelting using an electroslag remelting method, A method for producing high purity steel, which uses a melt of the flux according to claim 1 as the molten slag.

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