Non-magnetic austenitic steel and magnetic field confinement type nuclear fusion device comprising same

Abstract

A magnetic field confinement type nuclear fusion device (1) includes a first member (e.g., a cladding (20)) containing a non-magnetic austenitic steel. The nonmagnetic austenitic steel contains 28% by mass or more and 31% by mass or less of Mn, 5% by mass or more and 7% by mass or less of Al, 0.5% by mass or more and 2% by mass or less of Si, 0% by mass or more and 0.3% by mass or less of C, 0% by mass or more and 1.5% by mass or less of Y2O3, and the balance Fe and unavoidable impurities.

Description

TECHNICAL FIELD

[0001] The present application relates to a non-magnetic austenitic steel and a magnetic field confinement type nuclear fusion device containing the steel. BACKGROUND

[0002] Japanese Patent Application Publication No. 2021-133804 (Patent Literature 1) discloses a low-radioactivity ferritic steel (for example, F82H) as a cladding material for a blanket for nuclear fusion.

[0003] Prior Art Documents Patent Literature Patent Literature 1: Japanese Patent Application Publication No. 2021-133804 SUMMARY Problems to be Solved by the Invention A magnetic field confinement type nuclear fusion device is a device that utilizes magnetic lines of force to confine plasma, thereby continuously causing a nuclear fusion reaction. Therefore, a material constituting a component or member incorporated in a magnetic field confinement type nuclear fusion device is preferably a non-magnetic material. However, a material for a nuclear fusion device needs to improve properties such as oxidation resistance, neutron radiation resistance, or mechanical strength in addition to being non-magnetic.

[0004] An object of the present application is to provide a non-magnetic austenitic steel suitable as a material contained in a magnetic field confinement type nuclear fusion device, and a magnetic field confinement type nuclear fusion device using the non-magnetic austenitic steel.

[0005] Means for Solving the Problems The non-magnetic austenitic steel according to one embodiment contains 28 mass% or more and 32 mass% or less of Mn, 5 mass% or more and 7 mass% or less of Al, 0.5 mass% or more and 2 mass% or less of Si, 0 mass% or more and 0.3 mass% or less of C, 0 mass% or more and 1.5 mass% or less of Y2O3 (yttria), and the balance of Fe and inevitable impurities.

[0006] The magnetic field confinement type nuclear fusion device according to another embodiment includes a first component containing a non-magnetic austenitic steel. The non-magnetic austenitic steel contains 28 mass% or more and 32 mass% or less of Mn, 5 mass% or more and 7 mass% or less of Al, 0.5 mass% or more and 2 mass% or less of Si, 0 mass% or more and 0.3 mass% or less of C, 0 mass% or more and 1.5 mass% or less of Y2O3, and the balance of Fe and inevitable impurities.

[0007] Effects of the Invention According to one representative embodiment of the present application, an austenitic steel suitable as a material contained in a magnetic field confinement type nuclear fusion device can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 is a cross-sectional perspective view showing a structure example of a magnetic field confinement type nuclear fusion device according to an embodiment.

[0009] Figure 2 is a graph showing a mass increase curve of Example 1 and Comparative Example 1.

[0010] Figure 3 is a graph showing an observation image obtained by using a scanning transmission electron microscope (STEM) for a sample of Example 1.

[0011] Figure 4 is a graph showing an observation image obtained by using an energy dispersive X-ray analysis device (EDS) for a sample of Example 1.

[0012] Figure 5 is an explanatory view showing a forged material of an austenitic steel prepared as Example 2.

[0013] Figure 6 is an explanatory view showing results of X-ray crystal structure analysis of each test piece after a thermal aging test of the austenitic steel of Example 2 at 700°C for 10 hours, 100 hours, and 1000 hours.

[0014] Figure 7 is an explanatory view showing results of X-ray crystal structure analysis of a test piece before a thermal aging test of the austenitic steel of Example 2.

[0015] Figure 8 is an explanatory view showing results of analysis by an electron microprobe analyzer (EMPA) of each test piece after a thermal aging test of the austenitic steel of Example 2 at 700°C for 10 hours, 100 hours, and 1000 hours.

[0016] Figure 9 is an explanatory view showing evaluation results of nanoindentation hardness in the austenitic steels of Example 2 and Comparative Example 2.

[0017] Figure 10 is an explanatory view showing evaluation results of elastic modulus in the austenitic steels of Example 2 and Comparative Example 2.

[0018] Figure 11 is an explanatory view showing results of X-ray crystal structure analysis of each of six regions from the circumference toward the center in the forged material of the austenitic steel of Example 2.

[0019] Figure 12 is an explanatory view showing results of a tensile strength test of the austenitic steels of Example 3 and Comparative Example 3. DETAILED DESCRIPTION

[0020] The present inventors have researched and developed a nuclear fusion device that performs a nuclear fusion reaction by confining a high-temperature, high-density plasma in a furnace. As one aspect thereof, a suitable non-magnetic austenitic steel for a structural material of a nuclear fusion device or a material of a component such as a blanket or a diverter disposed in a nuclear fusion device has been researched.

[0021] Definitions of terms First, the definitions of the terms used in the present application will be explained.

[0022] The "nuclear fusion device" refers to a device that takes out energy generated by a nuclear fusion reaction to the outside.

[0023] The "magnetic field confinement type nuclear fusion device" refers to a nuclear fusion device that adopts a method of confining a plasma for generating a nuclear fusion reaction by magnetic lines of force. The magnetic field confinement type has various methods such as a "Tokamak type" or a "screw type" according to the shape of the magnetic lines of force. In the present application, as a general term for the method of confining a plasma by magnetic lines of force, it is referred to as a "magnetic field confinement type".

[0024] The "non-magnetic" refers to a state that does not have the properties of a magnetic body and a state that is substantially considered not to be a magnetic body. The state that is substantially considered not to be a magnetic body refers to a case where, in a use environment, the characteristics as a magnetic body are sufficiently small, and thus the influence is small to the extent that it can be ignored. For example, in the case of a material for a magnetic field confinement type nuclear fusion device, it refers to a case where the magnetic properties of the material have an influence on the magnetic lines of force of the confined plasma to the extent that it can be ignored. In addition, when the "relative magnetic permeability" is used to express the non-magnetic state, as long as the relative magnetic permeability of the material is in a range of 1.00 or more and 1.02 or less, the material can be said to be a non-magnetic body.

[0025] The measurement of the relative magnetic permeability of the steel can be performed, for example, by a method using a vibrating sample magnetometer.

[0026] Magnetic field confinement type nuclear fusion device Figure 1 is a cross-sectional perspective view that shows a structure example of the magnetic field confinement type nuclear fusion device of one embodiment. Figure 1 The nuclear fusion device 1 shown is a magnetic field confinement type nuclear fusion device, specifically a screw type nuclear fusion device.

[0027] Figure 1 The X direction, the Y direction, and the Z direction are described in the middle. The X direction, the Y direction, and the Z direction intersect each other. In the example shown, the X direction, the Y direction, and the Z direction are orthogonal to each other. Figure 1 In the example shown, the X direction, the Y direction, and the Z direction are orthogonal to each other.

[0028] The nuclear fusion device 1 has a vacuum vessel 10, a blanket 20, a diverter 30, a magnet (superconducting magnet) 40, and a cryostat 50. The blanket 20, the diverter 30, the magnet 40, and the cryostat 50 are respectively arranged inside the vacuum vessel 10. The diverter 30 is arranged inside the blanket 20.

[0029] The vacuum vessel 10 is a vessel for housing components for continuously generating a nuclear fusion reaction. In the nuclear fusion reaction, an isotope of hydrogen such as deuterium or tritium is used as a raw material. Therefore, the inside of the vacuum vessel 10 needs to be maintained in an ultrahigh vacuum state (for example, 10 -5 Pa or less) before the start of the operation of the nuclear fusion plasma. The vacuum vessel 10 is formed of, for example, stainless steel.

[0030] In addition, as a modification example, the vacuum vessel 10 is sometimes composed of a non-magnetic austenitic steel described later. However, the vacuum vessel 10 is arranged at a position far from the nuclear fusion plasma 21, and there is the blanket 20 between the vacuum vessel 10 and the nuclear fusion plasma 21. Therefore, the characteristics of the oxidation resistance or the neutron radiation resistance required for the vacuum vessel 10 are relatively low compared to the characteristics of the oxidation resistance or the neutron radiation resistance required for the blanket 20.

[0031] The blanket 20 is a component that constitutes a flow path of the liquid metal 2. The nuclear fusion plasma 21 is formed in a ring shape (a toroidal shape) in an X-Y plane of an X direction and a Y direction. Figure 1 The blanket 20 is arranged so as to sandwich the nuclear fusion plasma 21 in the X direction or the Y direction. In other words, the nuclear fusion plasma 21 is sandwiched by the blanket 20 in the X direction or the Y direction at the time of the operation of the nuclear fusion device 1.

[0032] In the example shown in FIG. 1, the liquid metal 2 flowing inside the blanket 20 is irradiated with neutrons generated in the nuclear fusion plasma 21. In other words, the blanket 20 has a function of receiving and shielding the neutrons generated in the nuclear fusion plasma 21. Figure 1 The liquid metal 2 functions as a coolant that transfers heat generated by the irradiation of neutrons to the outside. In other words, the blanket 20 has a function of transferring heat energy generated by the irradiation of neutrons to the outside (for example, a power generation device such as a turbine generator) via the coolant (the liquid metal 2).

[0033] In addition, when the neutrons are irradiated, the liquid metal 2 functions as a fuel source that generates tritium as a fuel of the nuclear fusion reaction. The tritium is produced by a nuclear break-up reaction of neutrons irradiated to lithium contained in the liquid metal 2. In other words, the blanket 20 has a function as a raw material supply path for generating a fuel of the nuclear fusion reaction.

[0034]

[0035] ​From the viewpoint of efficiently converting neutrons generated from the nuclear fusion plasma 21 into thermal energy, it is preferable that the distance between the blanket 20 and the nuclear fusion plasma 21 be short. Therefore, as the material constituting the blanket 20, a non-magnetic austenitic steel described later is preferable.

[0036] The diverter 30 is a component (heat receiving device) provided in a portion of the blanket 20 that contacts the diverted plasma 31. A portion of the nuclear fusion plasma 21 is connected to a portion of the surface of the blanket 20 by magnetic lines of force. This is called diverted plasma. Charged particles in the plasma move along the magnetic lines of force. The diverter 30 suppresses the intrusion of impurities as charged particles into the nuclear fusion plasma 21 by making use of this characteristic of charged particles.

[0037] The diverter 30 is disposed between the blanket 20 and the nuclear fusion plasma 21. Therefore, the distance between the diverter 30 and the nuclear fusion plasma 21 is short. Therefore, as the material constituting the diverter 30, a non-magnetic austenitic steel described later is preferable.

[0038] The magnet 40 is a component for generating magnetic lines of force that maintain the nuclear fusion plasma 21 in a pre-designed shape. The magnet 40 is a superconducting electromagnet formed by laminating superconducting wires (omitted from the drawing) composed of a superconducting material. The shape of the magnet 40 is variously modified depending on the shape of the nuclear fusion plasma 21.

[0039] The cryostat 50 is a container that houses the magnet 40. In addition, the cryostat 50 is a heat shield for maintaining the superconducting state of the magnet 40.

[0040] Figure 1 Among the plurality of components (the blanket 20, the diverter 30, the magnet 40, and the cryostat 50) possessed by the nuclear fusion device 1 shown in the drawing, the blanket 20 and the diverter 30 preferably contain a non-magnetic austenitic steel described later.

[0041] However, in the nuclear fusion device 1, there are various modification examples of the components containing the non-magnetic austenitic steel described later. For example, there is a case where only either one of the blanket 20 and the diverter 30 contains the non-magnetic austenitic steel described later. Alternatively, there is a case where a component other than the blanket 20 and the diverter 30 contains the non-magnetic austenitic steel described later.

[0042] <Non-magnetic Austenitic Steel> Hereinafter, the austenitic steel of the present embodiment will be described.

[0043] The austenitic steel of the present embodiment is an alloy containing iron (Fe), Mn (manganese), Al (aluminum), and Si (silicon) as essential components.

[0044] Mn and Al described later are necessary components for achieving single-phase stabilization (hereinafter, also simply referred to as "stabilization") of austenitic steel (FCC phase) at high temperatures. In order to stabilize the austenitic steel, Mn and Al need to have a prescribed relationship with each other in terms of content, and these elements have a positive correlation in terms of content. That is, if Mn is increased, Al also needs to be increased, and if Mn is decreased, Al also needs to be decreased.

[0045] Mn is an element used instead of Ni (nickel) conventionally used for stabilizing the austenitic steel, but further in the present embodiment, by increasing the content of Mn and making the contents of other components fall within prescribed ranges, the obtained austenitic steel can be a non-magnetic alloy.

[0046] Making the austenitic steel non-magnetic is of great importance in the case of using it as a material for the nuclear fusion device 1. In particular, in a magnetic field confinement type nuclear fusion device, as described above, magnetic lines of force formed by the magnets 40 are used for confining the plasma. Therefore, in the case where a magnetic body is disposed in the vicinity of the nuclear fusion plasma 21 and the divertor plasma 31, the magnetic body affects the shape of the magnetic lines of force used for confining the plasma, and an undesirable situation can occur. The austenitic steel of the present embodiment suppresses the occurrence of such an undesirable situation, and enables stable operation of the nuclear fusion device.

[0047] In addition, when a non-magnetic austenitic steel is used as a material for the nuclear fusion device 1, it is preferable to have the following characteristics. Since the components disposed in the vicinity of the nuclear fusion plasma 21 (particularly, the blanket 20 and the divertor 30) are in a high-temperature environment, it is preferable that they have high oxidation resistance (corrosion resistance). Details will be described later, but the non-magnetic austenitic steel of the present embodiment can achieve high oxidation resistance.

[0048] Since more neutrons are irradiated to the components disposed in the vicinity of the nuclear fusion plasma 21 (particularly, the blanket 20 and the divertor 30), it is preferable to have high neutron irradiation resistance.

[0049] In addition, as a common requirement for any component, it is preferable to achieve a degree of mechanical strength that does not damage the component.

[0050] The content of Mn is, for example, preferably 28% by mass or more and 31% by mass or less, more preferably 29% by mass or more and 30.5% by mass or less. If the content is less than 28% by mass, it is also necessary to relatively reduce the content of Al, and it can be impossible to sufficiently obtain the oxidation resistance (corrosion resistance) of the austenitic steel. On the other hand, if the content exceeds 31% by mass, the stability of the austenitic steel can be reduced due to the balance with other components (particularly, β-Mn phase is generated at high temperatures), and the like. Furthermore, as described above, the austenitic steel containing a high proportion of Mn of 28% by mass or more exhibits high mechanical strength as compared with a chromium-nickel-based stainless steel containing Ni and Cr as elements for stabilizing austenite.

[0051] Al is a component that improves the oxidation resistance (corrosion resistance) of the austenitic steel and stabilizes the austenitic steel, and is a necessary component. As described above, the content thereof has a positive correlation with Mn in terms of maintaining the stability of the austenitic steel.

[0052] In the case where the austenitic steel is used under high-temperature conditions like the nuclear fusion device 1, Al can form an oxide film containing alumina (Al203or the like) on the surface layer of the austenitic steel to improve the oxidation resistance. According to the research of the present inventors, in the surface layer of the non-magnetic austenitic steel, an oxide scale of magnesium oxide, iron oxide, or the like is formed on the outermost surface layer, and an oxide film containing alumina is densely formed along the interface between the oxide scale and the base material. It is considered that, by forming the dense oxide film containing the alumina, the invasion of oxygen into the base material and the movement of elements constituting the base material to the surface are suppressed. In the present embodiment, it is considered that, by coexisting with Si described later, a denser film is formed, and the oxidation resistance (corrosion resistance) is improved.

[0053] The content of Al is preferably 5% by mass or more and 7% by mass or less, more preferably 5.5% by mass or more and 6.5% by mass or less. If the content is less than 5% by mass, it can be impossible to sufficiently obtain the oxidation resistance (corrosion resistance) of the austenitic steel. On the other hand, if the content exceeds 7% by mass, the stability of the austenitic steel can be reduced due to the balance with other components (particularly, β-Mn phase is generated at high temperatures due to an increase in the content of Mn), and the like. Furthermore, as described later as Embodiment 3, the austenitic steel containing Al of 5% by mass or more and 7% by mass or less, particularly preferably 5.5% by mass or more and 6.5% by mass or less, exhibits high mechanical strength.

[0054] Si is a component that improves the oxidation resistance (corrosion resistance) of the austenitic steel and stabilizes the austenitic steel, and is a necessary component. As described above, Si is considered to function as a component that more densely forms an oxide film containing alumina and improves the corrosion resistance of the austenitic steel when used under high-temperature conditions like a nuclear fusion device.

[0055] The content of Si is, for example, preferably 0.5 mass% or more and 2 mass% or less, more preferably 0.8 mass% or more and 1.5 mass% or less. If the content is less than 0.5 mass%, it can be difficult to sufficiently obtain the oxidation resistance (corrosion resistance) of the austenitic steel. On the other hand, if the content exceeds 2 mass%, the strength characteristics can be degraded. In addition, the austenitic steel containing Si in the range of 0.5 mass% or more and 2 mass% or less, preferably 0.8 mass% or more and 1.5 mass% or less, as a necessary component exhibits high mechanical strength. C (carbon) is a component that improves the strength of the austenitic steel, and is an arbitrary component. C can be contained or not contained.

[0056] When C is contained, the content thereof is preferably 0.3 mass% or less. If C is contained in an amount exceeding 0.3 mass%, the austenite-forming component decreases, a BCC phase (ferrite phase) is easily generated, the stability of the austenitic steel decreases, and the austenitic steel can have magnetic properties.

[0057] However, in the case where the content of C is a small amount, that is, 0.3 mass% or less, C functions as a component for stabilizing the austenitic steel. In the present embodiment, the stabilization of the austenitic steel is due to the large amount of Mn contained, and thus by containing C in an amount of 0.3 mass% or less, the austenitic steel can be stabilized.

[0058] Y2O3 (yttrium oxide) is a component that improves the strength characteristics of the austenitic steel, and is an arbitrary component. By incorporating Y2O3, improvement of the oxidation resistance (corrosion resistance) and improvement of the neutron irradiation resistance can also be achieved. Here, the neutron irradiation resistance refers to the property of generating a high-level radioactive substance after neutron emission of a nuclear fusion device, and by incorporating Y2O3, the neutron irradiation resistance can be reduced by the synergistic effect against irradiation defects.

[0059] In addition, as described above, C functions as a component that improves the strength of the austenitic steel, but in the present embodiment, the content of C is made low in consideration of the stability of the austenitic steel at high temperatures. Therefore, in the present embodiment, from the viewpoint of improving the strength characteristics of the austenitic steel, it is preferable to incorporate Y2O3. In the case where Y2O3 is incorporated, the content thereof is, for example, preferably 0.1 mass% or more and 1.5 mass% or less, more preferably 0.15 mass% or more and 1 mass% or less.

[0060] Ni is a component for achieving the stabilization of the austenitic single phase, and is an arbitrary component. As described above, in the present embodiment, since Mn is contained in a large amount as a necessary component, a small amount of Ni can be contained, but from the viewpoint of the stability of the austenitic steel, it is not problematic even if Ni is not contained. In addition, from the viewpoint of suppressing the magnetic properties of the austenitic steel, it is preferable to substantially not contain Ni. Here, substantially not containing Ni means that the content of Ni is 1000 ppm or less.

[0061] Cr is a component that improves the oxidation resistance (corrosion resistance) of the austenitic steel, and is an optional component. However, if the content of Cr is increased, there is a possibility that a σ phase will be generated, which can be imagined to adversely affect the oxidation resistance (corrosion resistance), mechanical properties, and sometimes the magnetic properties. In the present embodiment, it is preferable that Cr be substantially not contained. Here, by substantially not contained is meant that the content of Cr is 0.5 mass% or less.

[0062] The austenitic steel having the above composition is composed of a single phase of an FCC phase, and becomes a non-magnetic austenitic steel. By using a non-magnetic material, as described above, the nuclear fusion device can be stably operated. Here, the definition of "non-magnetic" is the same as that already described.

[0063] The austenitic steel of the present embodiment having the above composition has very good oxidation resistance (corrosion resistance) at high temperatures, and is a non-magnetic material. Furthermore, by incorporating Y2O3, a material having improved strength properties and neutron irradiation resistance can also be produced. This austenitic steel is a material that is very suitable as a member forming a nuclear fusion device.

[0064] In particular, the austenitic steel of the present embodiment, as described above, has the following characteristic: by having a high content of Mn and incorporating Al and Si as essential components, the oxidation resistance (corrosion resistance) at high temperatures can be effectively improved. This can be understood as being due to the fact that the oxide film of aluminum that is formed at high temperatures is denser than that of the alloys known in the past, as shown in Example 1 described later, although there is a temporary increase in mass at the initial stage of the oxidation treatment, the increase in mass thereafter is greatly suppressed, and thus can be understood. That is, in the austenitic steel of the present embodiment, because the oxide film that is formed is dense, once the oxide film is formed, the intrusion of oxides from the outside and the movement of Fe and Mn from the inside of the austenitic steel to the surface side can be effectively suppressed.

[0065] In addition, the austenitic steel of the present embodiment, as described above, by having a high content of Mn and incorporating Al and Si as essential components, a high mechanical strength can be obtained. As explained in Example 2 described later, for example, in the case of an austenitic steel that does not contain C, Y2O3, a high mechanical strength can also be obtained.

[0066] The austenitic steel of the present embodiment can be produced without particular limitation using a method of producing such an alloy, that is, by mixing the above-described metals as raw materials to the above-described contents, heating to a high temperature and dissolving, and then performing forging, casting, or the like, whereby the austenitic steel can be produced. Furthermore, by performing rolling or heat treatment or the like, the desired shape, form, and the like can be processed, and thus an austenitic steel member can be produced. Each of the processes at this time can be performed under conditions that are normally employed.

[0067] In addition, the austenitic steel can also be obtained by powder metallurgy. In the case of powder metallurgy, the metal powder described above as a raw material is put into a mold to be compression-molded by pressurization, and then heated to a high temperature to sinter, or the metal powder as a raw material is mechanically mixed (mechanical alloying) using a ball mill device and sintered, whereby an austenitic steel can be produced.

[0068] In the case of using Y2O3 as a raw material, it is preferable to produce the austenitic steel as a dispersion type strengthening alloy by powder metallurgy such as mechanical alloying. The obtained austenitic steel is further heated and processed into various shapes by hot forging to be shaped into a desired shape.

[0069] With Figure 1 The nuclear fusion device 1 described is a helical type nuclear fusion device. The helical type nuclear fusion device 1 has a complex shape of the nuclear fusion plasma 21 compared to the tokamak type nuclear fusion device. Therefore, the divertor 30 and the blanket 20 of the nuclear fusion device 1 have a complex shape. As a method of producing such a component having a complex shape, the AM (Additive Manufacturing) technique of layering a raw material three-dimensionally is effective.

[0070] In the case of the production method using powder metallurgy, the forming method using the AM (Additive Manufacturing) technique is particularly effective because a powder can be used as a raw material.

[0071] Example Hereinafter, the austenitic steel of the present embodiment will be described in more detail with reference to examples, comparative examples, and research examples, but the present application is not limited to these examples.

[0072] (Example 1) A raw material powder was prepared in an argon atmosphere glove box. Next, the prepared raw material powder was subjected to ball mill pulverization for 48 hours at 300 rpm using a planetary ball mill at a ball-to-powder ratio of 10:1. The pulverized powder was solidified by a spark plasma sintering (SPS) method at 1050°C under a certain axial pressure of 50 MPa. Finally, the sintered cylinder was subjected to hot rolling at 1050°C at a forging ratio of 3:1, annealed at 1050°C for 2 hours, and then quenched with water to obtain an austenitic steel having a composition of Fe-30Mn-6Al-0.3C-1Si-1Y2O3 in mass%.

[0073] (Comparative Example 1) An austenitic steel having a composition of Fe-30Mn-6Al-0.3C-1Cr-1Y2O3 in mass% was obtained by the same method as Example 1.

[0074] [Antioxidant Test] For the austenitic steels obtained in Example 1 and Comparative Example 1, the rolled samples were cut by wire electrical discharge machining and ground with 1 μm diamond spray to achieve a final size of 9 mm × 9 mm × 0.5 mm. A 1.5 mm diameter hole was drilled, and the sample was hooked using an Al₂O₃ rod. An oxidation test was conducted in an air atmosphere at 600 °C using a muffle furnace. The mass gain caused by oxidation at multiple exposure times (1, 4, 9, 16, 25, 49, 100, 225 hours) was determined at a resolution of 1 μg using a precision electronic balance (trade name: MC 5, manufactured by Sartorius).

[0075] The results are shown in Figure 2 . Figure 2 This is a graph showing the mass increase curves of Example 1 and Comparative Example 1. In Comparative Example 1, since the evaluation of mass increase was sufficiently large, the oxidation test was completed after 100 hours. As shown in the graph, in Comparative Example 1, as time passed, oxidation proceeded and the mass increased significantly. In contrast, in Example 1, although there was a temporary mass increase initially, no significant mass increase was subsequently observed.

[0076] exist Figure 2 In the results shown, when plotting the horizontal axis as the square root of time, the graph lies on a straight line, indicating that the increase in mass is proportional to the square root of time. This means that the movement of oxygen is limited by diffusion, implying that the growth rate of the oxide film is inversely proportional to its thickness. This proportionality coefficient is typically called the oxidation rate, measured in mg. 2 cm - 4 h -1 .

[0077] Calculate using the method described above. Figure 2 The oxidation rate of the austenitic steel in Example 1 shown is 5.80 × 10⁻⁶. -6 mg 2 cm -4 h -1 On the other hand, for Figure 2 The comparative example 1 shown has a 6.97 × 10 -2 mg 2 cm -4 h -1 .

[0078] In addition, cross-sectional EPMA observations were performed on the samples of Example 1 and Comparative Example 1 using an electron probe microanalysis device (JEOL JXA-8530F). The results showed that in Comparative Example 1, the oxide film thickened over time, while in Example 1, the oxide scale was very thin and changed little over time, which also confirmed this.

[0079] In addition, a scanning transmission electron microscope (STEM) observation image of the sample of Example 1 is shown in Figure 3 . Figure 4 is a view showing an observation image obtained by an energy dispersive X-ray analysis device (EDS) for the sample of Example 1. As shown in Figure 3 , the austenitic steel (non-magnetic austenitic steel) 60 has a scale layer 63 composed of an oxide film 61 having magnesium oxide as a main component and an oxide film 62 having iron oxide as a main component. In addition, the austenitic steel 60 has an oxide coating film 65 continuously extending along the interface between the base material 64 and the scale layer 63. In addition, as shown in Figure 4 , since a large amount of Al was detected at a position corresponding to the oxide coating film 65 of Figure 3 , it is considered that the oxide coating film 65 is a dense film of aluminum oxide.

[0080] That is, from Figure 3 and Figure 4 it is confirmed that the austenitic steel 60 has the oxide coating film 65 containing aluminum oxide in the surface layer. The improvement in oxidation resistance is strongly presumed to be due to this dense aluminum oxide film.

[0081] As described above, the austenitic steel of the present embodiment is very excellent in oxidation resistance at high temperatures.

[0082] (Example 2) After vacuum melting of the raw material metal, hot forging into a columnar shape was performed to obtain a forged material 70 shown in Figure 5 . Figure 5 is a view showing a forged material as an austenitic steel prepared in Example 2. Figure 5 The forged material 70 shown in Figure 5 is an austenitic steel having a composition of Fe-30Mn-5Al-1Si. The forged material 70 shown in

[0083] [Evaluation of the effect of thermal aging] After electric discharge machining of the forged material 70 shown in Figure 5 , it was punched into a columnar shape, and used as a test piece to perform thermal aging evaluation by keeping it at a temperature of 700°C for 10 hours, 100 hours, and 1000 hours. Then, the results of evaluation of each sample by an X-ray diffractometer are shown in Figure 6 . In addition, the results of evaluation of the test piece of the forged material 70 before the thermal aging evaluation by the X-ray diffractometer (i.e., the evaluation results at a keeping time of 0 hours) are shown inFigure 7 As shown in Figure 6 and Figure 7 , the diffraction peaks of the metal surface of each test piece having a changed holding time did not change. From this result, it was known that the austenite stably existed even in the case of being held at a high temperature (for example, 700 degrees Celsius) for a long time (for example, 1000 hours).

[0084] Further, the surface of the test piece punched into a column shape from the forged material 70 (refer to Figure 5 ) was analyzed with an electron beam micro analyzer (EMPA), and the result is shown in Figure 8 . As shown in Figure 8 , it was known that aluminum was uniformly dispersed on the surface of the test piece. From this, it was considered that, as in Example 1, the surface layer of the forged material 70 of Example 2 after being held in a high temperature environment was densely formed with the alumina-containing film. In other words, it was presumed that the austenitic steel of Example 2 was also formed with the dense alumina-containing oxide film on the surface layer, like the austenitic steel of Example 1, and thus exhibited high oxidation resistance.

[0085] [Evaluation of mechanical properties of a micro region] Evaluation of mechanical properties of a micro region was performed using the forged material 70 (refer to Figure 5 ) obtained in Example 2. More specifically, the evaluation was performed by pressing a diamond indenter or the like against the surface of the forged material to draw a displacement curve of the indentation load. The result is shown in Figure 9 and Figure 10 . Figure 9 is the evaluation result of nanoindentation hardness, Figure 10 is the evaluation result of elastic modulus.

[0086] (Comparative Example 2) Here, as a comparative example for evaluating the mechanical strength of the austenitic steel of Example 2, a single crystal of an austenitic steel having a composition of Fe-15Cr-20Ni (corresponding to chromium-nickel-based stainless steel, SUS316) was prepared, and, like Example 2, a displacement curve of the indentation load was drawn by pressing a diamond indenter or the like against the surface of the single crystal of Comparative Example 2. Figure 9 and Figure 10 The result of Comparative Example 2 is shown together with the result of Example 2 in Figure 9 and Figure 10 . The austenitic steel of Comparative Example 2 shown in

[0087] As shown in Figure 9 and Figure 10As shown, Example 2 exhibits greater nanoindentation hardness and lower elastic modulus compared to Comparative Example 2, indicating that the austenitic steel of Example 2 has relatively higher mechanical strength than the austenitic steel of Comparative Example 2. Furthermore, Comparative Example 2 is a single crystal, which differs from the crystal structure of the austenitic steel of Example 2, which is not a single crystal. Additionally, the composition of Comparative Example 2 differs from that of Example 2 in the metals contained other than Fe. Therefore, a pure comparison is difficult. However, Comparative Example 2 is identical to Example 2 in that it does not contain C or Y₂O₃. Therefore, it is believed that a high Mn content (not Cr and Ni) and the presence of Al and Si as essential components contribute to mechanical strength. From the above results, it can be inferred that even without C and Y₂O₃, austenitic steel with a high Mn content and the presence of Al and Si as essential components can achieve higher mechanical strength than Comparative Example 2.

[0088] [Evaluation of the use of X-ray diffraction equipment] Next, Figure 5 The circumferential portion 71 and the central portion 72 of the forged material 70 shown are each divided into three regions. Regions 70F, 70E, 70D, 70C, 70B, and 70A were evaluated using an X-ray diffraction apparatus. The evaluation results are shown below. Figure 11 .like Figure 11 As shown, peak P1 was identified in region 70F, the outermost surface of the circumferential portion 71. No peak corresponding to peak P1 was identified in other regions. Peak P1 is an aluminum nitride (AlN) peak. That is, in the case of Example 2, aluminum nitride was confirmed to be present on the outermost surface of the circumferential portion 71, and it was considered that aluminum nitride was not distributed in regions other than the outermost surface.

[0089] Since aluminum nitride can potentially cause embrittlement, when machining the austenitic steel of this embodiment (i.e., austenitic steel with a high Mn content and containing Al and Si as essential components), it is preferable to avoid machining the outermost surface containing aluminum nitride on the periphery. This allows for the production of components with high toughness. Furthermore, as already explained, aluminum is distributed in the forging material 70 (see...). Figure 5 Therefore, even with the outermost surface of the circumferential side removed, as long as the high-temperature environment is maintained, an oxide film mainly composed of aluminum oxide can be formed on the surface of the forging material 70. Therefore, it is believed that the described oxidation resistance (corrosion resistance) is maintained even after the surface of the forging material is removed.

[0090] Next, the results of the tensile strength test will be explained as another indicator of the mechanical strength of austenitic steel. Furthermore, Example 3 and Comparative Example 3 will be described below.

[0091] (Example 3) By the same method as in Example 1, an austenitic steel of the composition of Fe-30Mn-6Al-1Si-1.5Y2O3 was obtained.

[0092] (Comparative Example 3) By the same method as in Example 1, an austenitic steel of the composition of Fe-30Mn-5Al-1Si-1.5Y2O3 was obtained.

[0093] [Quality analysis] The austenitic steels of Example 3 and Comparative Example 3 were subjected to quality analysis, and the results are shown in Table 1. As shown in Table 1, Example 3 and Comparative Example 3 are identical except for the mass proportion of aluminum and the mass proportion of Fe in the remaining portion accompanying this. The mass proportion of Al of the austenitic steel of Comparative Example 3 is less than 5 mass%. That is, the mass proportion of aluminum, which is a necessary component, of the austenitic steel of Comparative Example 3 is less than the lower limit value already described, and therefore Comparative Example 3 is described as a comparative example.

[0094] In addition, in Table 1, the austenitic steels of Example 3 and Comparative Example 3 are not austenitic steels in which C (carbon) is actively added. The component of C (carbon) shown in Table 1 is the mass proportion of carbon that is accidentally mixed due to contaminants and the like in the preparation process. The mass proportion of carbon described in Table 1 is sufficiently small compared to the mass proportion of Si (silicon) and the mass proportion of Y (yttrium), and therefore the influence of the carbon component that contributes to mechanical strength is considered to be small to the extent that it is substantially negligible. Therefore, even if the mass proportion of carbon shown in Table 1 is zero, the influence on the results of the tensile strength test described below is considered to be small to the extent that it is negligible. In other words, both Example 3 and Comparative Example 3 can be regarded as examples (and comparative examples) in which C (carbon) is not contained.

[0095] [Table 1]

[0096] [Tensile strength test] From the austenitic steels of Example 3 and Comparative Example 3, SS-J3 (parallel portion thickness 0.50 mm, parallel portion length 5.0 mm, parallel portion width 1.2 mm) micro tensile strength test pieces were collected. Then, the test pieces were set on a testing machine, and the pure plastic deformation was evaluated at a temperature of 25°C and a deformation rate of 6.67 x 10 -4 / s. The results are shown in Figure 12 .

[0097] In addition, the 0.2% yield strength (σ 0.2% ) of the austenitic steels of Example 3 and Comparative Example 3 was 1378 MPa and 1218 MPa, respectively. In addition, the ultimate tensile strength (σ UTSThe pressures at break were 1435 MPa and 1268 MPa, respectively. Additionally, the elongation at break (ε0.05) of the austenitic steels in Example 3 and Comparative Example 3 were... u The figures were 9.0% and 10%, respectively.

[0098] from Figure 12 The test results shown, along with the aforementioned 0.2% yield strength and ultimate tensile strength results, demonstrate that, with a fixed mass ratio of Mn and Si in the austenitic steel, increasing the mass ratio of aluminum can improve the mechanical strength of the austenitic steel. In particular, the ultimate tensile strength of Example 3 shows a value approximately 13% higher than that of Comparative Example 3.

[0099] From the perspective of improving the oxidation resistance (corrosion resistance) of austenitic steel, it has been stated that the preferred mass ratio of aluminum is 5% or more. However, from... Figure 9 The results show that, from the viewpoint of improving mechanical strength, the mass ratio of aluminum is preferably 5% by mass or more, and particularly preferably 5.5% by mass or more.

[0100] In Example 3, the mass ratio of Mn was approximately 29.8 to 30.1% by mass. Therefore, if the mass ratio of aluminum is too high while the mass ratio of Mn is fixed, it may impair the stability of austenite. However, by further increasing the mass ratio of Mn within a range not exceeding 31% by mass, the mass ratio of aluminum can be increased to 7% by mass.

[0101] This invention is not limited to the above-described embodiments and examples; various modifications can be made without departing from its spirit. For example, in Figure 1 In the example of a magnetic field confinement nuclear fusion device, a spiral-type nuclear fusion device was used as an example. However, the non-magnetic austenitic steel described above can be applied to magnetic field confinement nuclear fusion devices such as tokamak or spherical mak.

[0102] Additionally, for example, when using Figure 1 The example described uses a nuclear fusion device 1 having a blanket 20 and a shunt 30. As a variation of nuclear fusion device 1, it can be applied to nuclear fusion devices without a shunt 30. In this case, the blanket 20 also functions as the shunt 30.

[0103] Furthermore, for example, various modifications have been described above, but it is possible to combine a portion of the embodiments with other embodiments.

[0104] Industrial applicability This invention can be used, for example, in magnetic field confinement nuclear fusion devices.

[0105] Explanation of symbols 1 nuclear fusion device 10 vacuum vessel 20 cladding 21 nuclear fusion plasma 30 diverter 31 divertor plasma 40 magnet 50 cryostat 60 austenitic steel (non-magnetic austenitic steel) 61 oxide film 62 oxide film 63 oxide skin layer 64 base material 65 oxide coating 70 forged material 70A, 70B, 70C, 70D, 70E, 70F regions 71 circumferential portion 72 central portion

Claims

1. A non-magnetic austenitic steel containing 28% by mass and 31% by mass of Mn, 5% by mass and 7% by mass of Al, 0.5% by mass and 2% by mass of Si, 0% by mass and 0.3% by mass of C, 0% by mass and 1.5% by mass of Y2O3, and the balance of Fe and unavoidable impurities.

2. The non-magnetic austenitic steel according to claim 1, wherein, The non-magnetic austenitic steel has a magnetic permeability of less than 1.

02.

3. The non-magnetic austenitic steel according to claim 1, wherein, The surface has a coating containing aluminum oxide.

4. A magnetic field confinement nuclear fusion device, comprising a first component, the first component being composed of non-magnetic austenitic steel. The non-magnetic austenitic steel contains 28% to 31% by mass of Mn, 5% to 7% by mass of Al, 0.5% to 2% by mass of Si, 0% to 0.3% by mass of C, 0% to 1.5% by mass of Y2O3, and the balance of Fe and unavoidable impurities.

5. The magnetic field confinement nuclear fusion device according to claim 4, wherein, The non-magnetic austenitic steel has a magnetic permeability of less than 1.

02.

6. The magnetic field confinement nuclear fusion device according to claim 4, wherein, The magnetic field confinement nuclear fusion device has an aluminum oxide-containing coating on its surface.

7. The magnetic field confinement fusion device according to claim 4, wherein, The first component is a cladding.

8. The magnetic field confinement fusion device according to claim 4, wherein, The first component is a splitter.

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