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

Abstract

This flux used for an electroslag remelting method is in granular form at 25 °C, wherein when passed through a sieve having a nominal opening of 500 μm defined in accordance with JIS Z 8801-1:2019, the flux has a passing rate of 18% or less, said passing rate being the ratio of the mass of the flux that has fallen under the sieve to the mass of the flux remaining on the sieve.

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 has low hygroscopicity and is capable of producing high-purity metals with reduced hydrogen embrittlement at 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: It is granular at 25°C, When the flux is passed through a sieve with a nominal opening 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 below the sieve to the mass of the flux that has been sieved, is 18% or less. is provided.

[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 has low hygroscopicity and is capable of producing high-purity metals with reduced hydrogen embrittlement at 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 is granular at 25°C, and when the flux is passed through a sieve having 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 18% or less, preferably 15% or less, more preferably 12% or less, even more preferably 10% or less, and even more preferably 8% or less. Here, the lower limit of the passing rate is ideally 0%, but in reality it is, for example, 0.00001% or more.

[0019] The inventors focused on the particle size of the flux in particular in order to improve the yield of ingots obtained by the ESR method. Specifically, in order to suppress the hygroscopicity of the mixed flux, the inventors removed low-particle fine powder, which tends to be highly hygroscopic, before mixing, and designed the flux so that the passing rate when the mixed flux was passed through a sieve with a nominal mesh size of 500 μm specified in JIS Z 8801-1:2019 was 18% or less. Although the reason for this tendency is unclear, it is believed that the surface area of ​​low-particle-size fine powders is larger than that of high-particle-size particles, which contributes to their high hygroscopicity. By removing these highly hygroscopic fine powders before blending and increasing the particle size of the flux, the surface area of ​​the flux is reduced. This effectively suppresses the hygroscopicity of the flux, enabling the production of high-purity metals with reduced hydrogen embrittlement and high yields. Furthermore, suppressing the hygroscopicity of the flux eliminates the need for a flux drying process during the production of metal materials, allowing the omission of the drying process, resulting in cost reductions during the production of metal materials.

[0020] To achieve the above pass rate, 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.

[0021] As described in (1) above, the flux of this embodiment preferably contains a mixture of particles containing CaF2 and particles containing a metal oxide. In other words, it is preferable to separately produce particles containing CaF2 and particles containing a metal oxide and then finally mix them to form a mixture. This can more effectively suppress the hygroscopicity of the flux, allowing for the production of high-purity metal with reduced hydrogen embrittlement at a high yield.

[0022] 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 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, allowing for the production of high-purity metal with reduced hydrogen embrittlement at a high yield.

[0023] In this case, it is preferable that the metal oxide contains an Al2O3 component, which allows the flux to have an appropriate melting temperature.

[0024] Furthermore, it is preferable that the metal oxide further contains CaO. As mentioned above, the CaO crystalline phase present as a single phase in the flux is preferably suppressed from the viewpoint of deterioration of the hygroscopicity of the flux. However, by incorporating CaO into the metal oxide and reacting it with the Al2O3 in the metal oxide to form a calcium aluminate crystalline phase, for example, it is possible to appropriately increase 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. In other words, by including a mixture of particles containing CaF2 and particles containing a metal oxide and including CaO as the metal oxide, it is possible to highly control the hygroscopicity, melting point, desulfurization performance of the flux, and the basicity of the slag obtained by melting the flux.

[0025] 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.

[0026] Incidentally, Patent Document 1 does not specifically mention the effect of flux particle size on moisture absorption. Therefore, it is thought that the flux contains small particles, which does not sufficiently suppress moisture absorption.

[0027] 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.

[0028] The flux of this embodiment will now be described in more detail.

[0029] (Flux penetration rate) 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.

[0030] 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.

[0031] (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 Al2O component 3 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.

[0032] 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.

[0033] 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.

[0034] ·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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] -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.

[0039] 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.

[0040] 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.

[0041] (Flux properties) The flux of this embodiment may have various properties. From the viewpoint of controlling the flux penetration rate 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.

[0042] <Flux manufacturing method> Next, a method for producing the flux according to this embodiment will be described. The method for producing the flux of this embodiment includes a firing step of firing at least a compound containing a CaF component to obtain particles containing the CaF component, and a melt-mixing step of melt-mixing at least a compound containing a CaO component and a compound containing an AlO component to obtain a molten mixture containing the CaO component and the AlO component. The flux can be obtained by a manufacturing method including: a crushing step of crushing the molten mixture containing the CaO component and the Al2O3 component to obtain a crushed material containing the CaO component and the Al2O3 component; a sorting step of passing the crushed material containing the CaO component and the Al2O3 component through a sieve with a nominal mesh size of 500 μm specified in JIS Z 8801-1:2019 to obtain particles containing the CaO component and the Al2O3 component; and a mixing step of mixing at least the particles containing the CaF2 component with the particles containing the CaO component and the Al2O3 component to obtain a flux.

[0043] Each step will be described in detail below.

[0044] (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.

[0045] (melt 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.

[0046] (Crushing process) Next, 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 penetration rate of 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 passing rate of 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.

[0047] (Sorting process) Next, as described in (2) above, the pulverized material containing the CaO and Al2O3 components obtained in the 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 a small particle size and high hygroscopicity generated in the pulverization step, thereby more effectively suppressing the hygroscopicity of the flux of this embodiment.

[0048] (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, the penetration rate of the flux of this embodiment can be controlled, and as a result, the hygroscopicity of the flux can be more suitably suppressed, and high-purity metal with suppressed hydrogen embrittlement can be produced with a good yield.

[0049] <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.

[0050] 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.

[0051] 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.

[0052] 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]

[0053] The present invention will be described in detail based on examples and comparative examples, but the present invention is not limited to these examples.

[0054] (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). The passage rates of these particles were measured according to the method described below in (Passage Rate). The results are shown in Table 1.

[0055] [Table 1]

[0056] 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.

[0057] (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.

[0058] (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.

[0059] 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. This procedure was repeated until all of the molten mixture passed through the sieve (fell below the sieve). The sieve 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. The particle passing rate is shown in Table 1 above.

[0060] <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%.

[0061] <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.

[0062] <Evaluation> The obtained fluxes of the Examples and Comparative Examples were evaluated by the following methods.

[0063] (passage rate) The obtained fluxes of each of the examples and comparative examples were sieved according to the following (1) to (5), and the passing rate was measured. The results are shown in Table 2.

[0064] (1) The flux was passed through a sieve with a nominal mesh size of 4.75 mm (corresponding to the ASTM E11 No. 4 sieve) specified in JIS Z8801-1:2019 at 25°C, and it was confirmed that no residue remained on the sieve (i.e., the passing rate was 100%) (in this case, no residue was confirmed 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 (equivalent to the ASTM E11 No. 7 sieve) specified in JIS Z8801-1:2019. This allowed the selection of flux with particle sizes of 2.8 mm or larger. The mass of the components that passed through the sieve was then measured. (3) The flux was passed through a sieve with a nominal mesh size of 1 mm (equivalent to the ASTM E11 No. 18 sieve) specified in JIS Z8801-1:2019. This allowed the selection of flux with particle sizes of 1 mm or more. The mass of the components that passed through the sieve was then measured. (4) The flux was passed through a sieve with a nominal mesh size of 500 μm (equivalent to the ASTM E11 No. 35 sieve) specified in JIS Z8801-1:2019. This separated the flux into particles with a particle size of 0.5 mm or more and those with a particle size of less than 0.5 mm. The mass of the flux with a particle size of 0.5 mm or more (sieve residue) and the mass of the flux with a particle size of less than 0.5 mm (components that passed through the sieve) were then measured. (5) The ratio of the mass of the flux of each particle size measured in (2) to (4) above to the mass of the flux before sieving was calculated and used as the passing rate.

[0065] (Measurement of initial moisture content 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).

[0066] (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 allowed for the selection of flux with particle sizes of 2.8 mm or more and less than 4.75 mm. The mass of the 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 1.0 mm specified in JIS Z8801-1:2019 (equivalent to the No. 18 sieve of ASTM E11). This allowed for the selection of flux with a particle size of 1.0 mm or more and less than 2.8 mm. The mass of the sieve residue that could 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 out flux with a particle size of 0.5 mm or more and less than 1 mm from flux with a particle size of less than 0.5 mm. The mass of the flux with a particle size of 0.5 mm or more and less than 1 mm (sieve residue) and the mass of the flux with a particle size of less than 0.5 mm (component that passed through the sieve) were then measured.

[0067] (5) The fluxes obtained above, each having a particle size of 2.8 mm or more and less than 4.75 mm, the fluxes having a particle size of 1.0 mm or more and less than 2.8 mm, the fluxes having a particle size of 0.5 mm or more and less than 1 mm, and the fluxes having a particle size less than 0.5 mm, were separately subjected to an apparatus (Nitto Seiko Analytech Co., Ltd. / Model CA-310) to determine the moisture content by the Karl Fischer method.

[0068] (6) Each of the above flux 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 particles in total), and aged at 30°C and 85% RH. After 7, 14, and 28 days, the aluminum dish containing the flux particle size was removed and thoroughly mixed, and the moisture content was determined in the same manner as in (5) above. The results are shown in Table 2.

[0069] [Table 2]

[0070] 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.

[0071] <Manufacturing of high-purity steel (refining)> The fluxes of Examples 1 and 2, which had been produced for 28 days or more, were used as they were without 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 drying, hydrogen embrittlement could not be suppressed compared to Examples 1 and 2, and the ingot yield was poor.

[0072] From the above, it was found that by refining raw steel using the ESR method with a flux that has a passage rate of 18% or less through a 500 μm sieve, it is possible to produce high-purity metal with low moisture absorption and suppressed hydrogen embrittlement with a good yield.

[0073] This application claims priority based on Japanese Patent Application No. 2021-182350, filed on November 9, 2021, the disclosure of which is incorporated herein in its entirety. [Explanation of symbols]

[0074] 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: It is granular at 25°C, when the flux is passed through a sieve having a nominal mesh size of 500 μm as specified in JIS Z 8801-1:2019, a 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 12% or less; The composition contains a CaF 2 component, an Al 2 O 3 component, and a CaO component, 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.

2. 2. The flux according to claim 1, 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 40% or less.

3. The flux according to claim 1 or 2, A flux in which, when the flux is passed through a sieve having 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 90% or less.

4. A method for producing the flux according to claim 1 or 2, At least, CaF ２ The compound containing the component is fired to form 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 crushed 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:

5. 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, 3. A method for producing high purity steel, comprising using a melt of the flux according to claim 1 or 2 as the molten slag.

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