Oxide dispersion strengthened iron alloy material

Abstract

Provided is an oxide dispersion strengthened iron alloy material which, even in high-temperature environments, has higher long-term stability than conventional ones. This oxide dispersion strengthened iron alloy material has a chemical composition which contains 10.5-14 mass% Cr, 3-8 mass% Al, 0.35-0.45 mass% Y, 0.1-1 mass% Zr and / or Hf, 0.2-0.5 mass% O, up to 0.08 mass% Ti, and less than 0.1 mass% N, the remainder comprising Fe and unavoidable impurities.

Description

Oxide Dispersion Strengthened Iron Alloy Material

[0001] The present invention relates to an oxide dispersion strengthened iron alloy material used in nuclear power systems such as boiling water reactors and fast reactors.

[0002] Conventionally, zirconium alloy materials (also called zircaloy, for example, Zr-Sn-Fe-Cr-Ni alloy materials) have been widely used as components (for example, fuel cladding tubes) of fuel assemblies in nuclear power systems. Zircaloy has excellent characteristics such as a small neutron absorption cross-section, and thus has been used in light water reactor nuclear power systems including boiling water reactors.

[0003] On the other hand, when zircaloy is exposed to steam in a high-temperature environment, a chemical reaction occurs to generate hydrogen. If this hydrogen leaks from the reactor pressure vessel or the containment vessel and accumulates in the building of the nuclear power plant, it may lead to serious events such as a hydrogen explosion. Using a material with high corrosion resistance and suppressed hydrogen generation instead of zircaloy as the material for components of the fuel assembly can be a countermeasure against such events.

[0004] From the perspective of the confinement property of radioactive substances, high mechanical strength, high corrosion resistance, and high stability over time are required as material characteristics for the fuel cladding tube. Oxide dispersion strengthened alloy materials (ODS alloy materials) are expected as one of the candidates to replace zircaloy because of their high mechanical strength and corrosion resistance.

[0005] For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2010-065302) teaches an oxide dispersion strengthened alloy steel containing, by weight ratio, Cr: 13.0 to 23.0%, Al: 3.5 to 5.0%, Y: 0.18 to 0.38%, C: 0.02 to 0.05%, and O: 0.15 to 0.25%, and at least one of Hf: 0.2 to 0.7% and Zr: 0.4 to 1.0%, with the balance being composed of Fe and inevitable impurities. It also teaches an oxide dispersion strengthened alloy steel further containing at least one of Ti: 0.1 to 0.25% and W: 1.0 to 3.0%.

[0006] According to Patent Document 1, Hf and / or Zr inhibit the aggregation of oxides by Al, resulting in a fine and dense oxide distribution similar to that of 9Cr ODS steel, thus improving high-temperature strength. Furthermore, Hf and / or Zr form carbides and oxides at the grain boundaries, suppressing grain boundary sliding and improving high-temperature strength.

[0007] Patent Document 2 (JP 2018-070897) teaches an iron-chromium-aluminum oxide dispersion-strengthened steel in which an aluminum oxide film is formed on the surface and a composite oxide of yttrium and zirconium is dispersed inside, wherein the excess oxygen, which is present in greater quantities than that forming yttrium oxide in the oxide dispersion-strengthened steel, is contained in a molar ratio of 2 or more relative to the zirconium. Specifically, an iron-chromium-aluminum oxide dispersion-strengthened steel is taught in which, by mass%, Cr is 12.0 to 23.0%, Al is 4.0 to 8.0%, Ti is 0.1 to 1.0%, Zr is 0.2 to 1.0%, Y2O3 is 0.3 to 1.0%, and the remainder is Fe and unavoidable impurities.

[0008] According to Patent Document 2, it is possible to provide an iron-chromium-aluminum oxide dispersion-strengthened steel that has excellent high-temperature strength even at high temperatures of 1000°C or higher, and that can prevent a decrease in oxidation resistance.

[0009] Japanese Patent Publication No. 2010-65302 Japanese Patent Publication No. 2018-70897

[0010] C. Capdevila, MM Aranda, R. Rementeria, J. Chao, E. Urones-Garrote, J. Aldazabal, MK Miller: Strengthening by intermetallic nanoprecipitation in Fe-Cr-Al-Ti alloy, Acta Materialia 107 (2016), pp. 27-37.Peng Dou, Zong-Xi Xin, Wei Sang, Akihiko Kimura: Age-hardening mechanisms of 15Cr ODS ferritic steels with 5, 7 and 9 wt.% Al at 475 ℃ for 9000 h, Journal of Nuclear Materials 540 (2020), 152368.Yasuhide Yano, Takashi Tanno, Satoshi Ohtsuka, Takeji Kaito and Shigeharu Ukai: Effects of Thermal Aging on the Mechanical Properties of FeCrAl-ODS Alloy Claddings, Materials Transactions, Vol. 62, No. 8 (2021) pp. 1239 to 1246.

[0011] In ODS alloy materials, fine dispersion of oxide particles (essentially yttrium oxide particles) is important from the viewpoint of mechanical strength. Patent documents 1 and 2 teach that the addition of a Ti component is effective in finely dispersing oxide particles.

[0012] On the other hand, Non-Patent Documents 1, 2, and 3 report on age hardening in ODS steel containing Ti components. According to Non-Patent Documents 1, 2, and 3, when age heat treatment is performed at 435 to 475°C, a TiAl-enriched intermetallic compound phase (β' phase) precipitates in the matrix phase of the ODS steel, increasing its Vickers hardness. Furthermore, Non-Patent Document 3 reports that age hardening is observed with age heat treatment, resulting in a significant increase in tensile strength, but this is accompanied by a significant decrease in ductility.

[0013] In metallic materials, an increase in Vickers hardness has the advantage of leading to an increase in tensile strength, but it also has the disadvantage of leading to a decrease in ductility and toughness. When considering use as fuel cladding, cracking and failure of the fuel cladding due to a decrease in ductility and toughness are undesirable outcomes. In other words, high long-term stability is one of the important material properties for fuel cladding materials.

[0014] Therefore, the object of the present invention is to provide an ODS iron alloy material that has higher temporal stability than conventional materials, even in high-temperature environments.

[0015] (I) One aspect of the present invention provides an oxide dispersion strengthened iron alloy material having a chemical composition comprising 10.5% to 14% by mass of chromium (Cr), 3% to 8% by mass of aluminum (Al), 0.35% to 0.45% by mass of yttrium (Y), 0.1% to 1% by mass of at least one of zirconium (Zr) and hafnium (Hf), 0.2% to 0.5% by mass of oxygen (O), 0.08% by mass or less of titanium (Ti), and less than 0.1% by mass of nitrogen (N), with the remainder being iron (Fe) and unavoidable impurities.

[0016] The present invention allows for the following improvements and modifications to be freely combined in the oxide dispersion-strengthened iron alloy material (I) according to the present invention: (i) The material contains 0.05% by mass or less of carbon as an unavoidable impurity.

[0017] According to the present invention, it is possible to provide an ODS iron alloy material that has higher temporal stability than conventional materials, even in high-temperature environments. Other issues, configurations, and effects will be clarified by the description of the embodiments below.

[0018] [Basic Concept of the Invention] In developing an ODS iron alloy material with higher temporal stability than conventional materials, the inventors considered it crucial to suppress the increase in Vickers hardness due to β' phase precipitation (and the resulting decrease in ductility and toughness). Therefore, in order to suppress β' phase precipitation under high-temperature conditions, they considered reducing the Ti content compared to conventional materials.

[0019] On the other hand, as taught in Patent Documents 1 and 2, the addition of a Ti component has been considered effective for the fine dispersion of oxide particles. Therefore, the present inventors investigated and examined in detail the balance of other components while reducing the Ti content compared to conventional methods. As a result, they found a chemical composition range that achieves fine dispersion of oxide particles while suppressing β' phase precipitation. The present invention was completed based on this finding.

[0020] Embodiments of the present invention will be described below. However, the present invention is not limited to the specific embodiments described, and it is possible to combine it with or improve upon prior art as appropriate, without departing from the technical spirit of the invention.

[0021] [ODS Iron Alloy Material of the Present Invention] As described above, the ODS iron alloy material of the present invention has a chemical composition comprising 10.5% to 14% by mass of Cr, 3% to 8% by mass of Al, 0.35% to 0.45% by mass of Y, 0.1% to 1% by mass of Zr and / or Hf, 0.2% to 0.5% by mass of O, 0.08% or less by mass of Ti, and less than 0.1% by mass of N, with the remainder being Fe and unavoidable impurities. Each component will be described in detail.

[0022] (Cr: 10.5–14% by mass) The Cr component is essential for ensuring corrosion resistance, and its content is preferably between 10.5% by mass and 14% by mass. If the Cr content is less than 10.5% by mass, it becomes difficult to ensure the expected corrosion resistance. On the other hand, if the Cr content exceeds 14% by mass, the Cr-enriched ferrite phase (α' phase) tends to precipitate under high-temperature environments, which can lead to a decrease in ductility and toughness.

[0023] (Al: 3-8% by mass) The Al component is also effective in ensuring corrosion resistance, and its content is preferably between 3% by mass and 8% by mass. If the Al content is less than 3% by mass, the effects of the Al component will not be fully exhibited. On the other hand, if the Al content exceeds 8% by mass, coarse Al oxide grains may precipitate or undesirable intermetallic compound phases may be formed, which can lead to a decrease in ductility and toughness.

[0024] (Y: 0.35–0.45% by mass) The Y component contributes to securing mechanical strength by forming Y2O3 particles, and its content is preferably between 0.35% by mass and 0.45% by mass. If the Y content is less than 0.35% by mass, it becomes difficult to secure the expected mechanical strength. On the other hand, if the Y content exceeds 0.45% by mass, the Y2O3 particles tend to aggregate, making it difficult to secure the expected mechanical strength.

[0025] (Zr and / or Hf: 0.1 to 1% by mass) The Zr and Hf components suppress the aggregation of Y2O3 particles and contribute to the fine dispersion of Y2O3 particles. The respective content is preferably 0.1% to 1% by mass, and more preferably 0.2% to 0.6% by mass. When both components are included, the total content is preferably 0.2% to 1% by mass. If the total content of Zr and Hf is less than 0.1% by mass, the effects will not be fully exhibited. On the other hand, if the total content of Zr and Hf exceeds 1% by mass, it will cause a decrease in ductility and toughness.

[0026] (O: 0.2–0.5 mass%) The oxygen component is essential for forming Y2O3 particles. In addition, some of the oxygen component contributes to improved corrosion resistance by combining with the al and / or chromium components. If the oxygen content is less than 0.2 mass%, it becomes difficult to secure the expected mechanical strength. On the other hand, if the oxygen content exceeds 0.5 mass%, excess oxide is produced, leading to a decrease in the ductility and toughness of the ODS iron alloy material.

[0027] (Ti: 0.08 mass% or less) While the Ti component has the advantage of contributing to the fine dispersion of oxide particles, it also has the disadvantage of reducing the stability over time in high-temperature environments, and is therefore not a component that should be actively included in this invention. If the Ti component is included, its content is preferably 0.08 mass% or less, as this suppresses the disadvantages. A Ti content of 0.07 mass% or less is more preferable, and 0.06 mass% or less is even more preferable, with the most preferable being to intentionally omit it (for example, an unavoidable impurity level of 0.01 mass% or less).

[0028] (N: less than 0.1 mass%) The N component contributes to improving the mechanical strength of the iron alloy material by solid-solubilizing it in the matrix as an interstitial element, and also contributes to improving toughness by refining the crystal grains of the matrix. Although it is not an essential component in this invention, if it is included, it is preferable to have less than 0.1 mass%. If the N content is 0.1 mass% or more, it may promote low-temperature brittleness of the iron alloy material.

[0029] (C: 0.05 mass% or less) Although C is an essential component in conventional ODS alloy steel materials, it is not a component that is actively included in the present invention. If C is included, its content is preferably 0.05 mass% or less, and no particular problems will occur if it is 0.05 mass% or less. It is even more preferable that C is not included intentionally (for example, an unavoidable impurity level of 0.01 mass% or less).

[0030] As described above, the ODS iron alloy material of the present invention suppresses the formation of the β' phase by reducing the Ti content compared to conventional materials, achieves fine dispersion of Y2O3 particles by controlling the Zr and / or Hf content, controls the amount of oxide formation by controlling the O content, suppresses the formation of the α' phase while ensuring corrosion resistance by controlling the Cr content, and suppresses the formation of undesirable phases while ensuring corrosion resistance by controlling the Al content. The ODS iron alloy material of the present invention can be suitably used for fuel assemblies in nuclear power systems and for fuel cladding tubes constituting said fuel assemblies.

[0031] [Method for Manufacturing the ODS Iron Alloy Material of the Present Invention] The method for manufacturing the ODS iron alloy material of the present invention is not particularly limited as long as a desirable microstructure (a structure in which Y2O3 particles are finely dispersed in an α-phase matrix) can be obtained, and conventional manufacturing methods can be used as appropriate. For example, one method involves weighing and mixing Fe powder, Fe2O3 powder, Cr powder, Al powder, Y2O3 powder, and at least one of Zr powder and Hf powder to obtain the aforementioned chemical composition, then performing a mechanical alloying treatment on the mixed powder to prepare a bulk ODS iron alloy material, and then performing plastic deformation or machining on the bulk body to obtain a desired shape.

[0032] The present invention will be described in more detail below through various experiments. However, the present invention is not limited to the configurations and structures described in these experiments.

[0033] [Experiment 1] (Preparation of Example 1) Fe powder, Fe2O3 powder, Cr powder, Al powder, Zr powder, and Y2O3 powder (each commercially available reagents) were weighed and mixed to prepare a mixed starting powder. Next, the mixed starting powder was subjected to mechanical alloying to obtain a bulk ODS iron alloy material (Example 1).

[0034] The obtained Example 1 was subjected to compositional analysis using an electron probe microanalyzer (EPMA, JEOL Ltd., JXA-8530F). The quantitative analysis results are shown in Table 1. Since commercially available reagents were used as raw material powders, the obtained samples contain a certain amount of unavoidable impurities. The amount of unavoidable impurities is assumed to be contained in "Fe".

[0035]

[0036] [Experiment 2] (Test and evaluation of the time-dependent stability of Example 1 1) Each of the samples prepared for Example 1 was divided into several parts to be used as test samples. One of the divided samples was left untreated with aging heat treatment, while the other samples were treated with aging heat treatment at 475°C in air.

[0037] For each sample, the Vickers hardness HV of the surface was measured using a micro-Vickers hardness tester (Akashi Corporation, model MVK-E). The measurement results are shown in Table 2. The Vickers hardness HV was taken as the average value of 10 measurements.

[0038]

[0039] As shown in Table 2, it was confirmed that in Example 1, there was almost no change in the Vickers hardness (no age hardening) even after the aging heat treatment at 475°C. The fact that there is no age hardening means that there is almost no change in the microstructure (for example, β' phase precipitation) even in the heat treatment environment, which means high stability over time in a high-temperature environment.

[0040] It seems that the Vickers hardness of the sample subjected to the aging heat treatment of "475°C × 750 h" decreased slightly compared to the sample of "0 h (without aging heat treatment)". As an example of the factor for the slight decrease in the Vickers hardness, the following events can be considered. The sample of "0 h (without aging heat treatment)" is in the state after mechanical alloying treatment and is considered to be work-hardened to some extent. It is thought that because the aging heat treatment was performed on such a sample, a part of the work hardening was relaxed and the Vickers hardness decreased slightly.

[0041] (Age hardening of the sample taught in Non-Patent Document 2) As described above, Non-Patent Document 2 reports research on age hardening in ODS steels containing Ti components, and experiments were conducted on 4 samples having the chemical compositions shown in Table 3. Also, when reading the plot in Fig. 1(a) of Non-Patent Document 2 showing the relationship between the heat treatment time by the 475°C aging heat treatment and the Vickers hardness, it becomes as shown in Table 4. <000008​​​​​​As shown in Table 3, all four samples in Non-Patent Document 2 contain 0.2 mass% or more of Ti. As shown in Table 4, the Vickers hardness clearly increases with the increase in the aging heat treatment time. In the relationship between the aging heat treatment time and the Vickers hardness, it is confirmed that there are significant differences between the technology of Non-Patent Document 2 and the present invention.

[0045] An ODS ferrous alloy material having the same chemical composition as that of Example 1 except that the Ti content was adjusted to 0.21 mass% (adjusted by the Fe content) was prepared, and the same tests and evaluations as in Experiment 2 were carried out. As a result, it was separately confirmed that the Vickers hardness increases with the increase in the aging heat treatment time, similar to Non-Patent Document 2.

[0046] [Experiment 3] (Test and Evaluation 2 of Long-Term Stability for Example 1) The bulk body of Example 1 prepared in Experiment 1 was subjected to plastic processing (such as extrusion and drawing) to be processed into a tube shape (outer diameter 10.26 mm × wall thickness 0.33 mm × length 120 mm), and a plurality of test samples were prepared. Next, as in Experiment 2, samples that underwent aging heat treatment (in air, 475°C × 750 h) and samples that did not undergo aging heat treatment were prepared.

[0047] Thereafter, each sample was subjected to machining to produce a No. 14B test piece of JIS Z 2241:2022 (Test Method for Tensile Test of Metallic Materials). A tensile test was performed on the produced test piece at a test temperature of 300°C using a general-purpose universal testing machine (Shimadzu Corporation, model AGS-10kNX), and mechanical properties (0.2% proof stress, maximum tensile stress, uniform elongation, and total elongation) were measured. The measurement results are shown in Table 5. The measurement results were all taken as the average values of three-point measurements.

[0048]

[0049] As shown in Table 5, it is confirmed that for Example 1, even after aging heat treatment at 475°C × 750 h, the 0.2% proof stress, maximum tensile stress, uniform elongation, and total elongation hardly change compared to the case without aging heat treatment. This means that there is no decrease in ductility and toughness due to aging heat treatment, and it means that the long-term stability in a high-temperature environment is high.

[0050] (Age hardening of samples as taught in Non-Patent Literature 3) As mentioned above, Non-Patent Literature 3 reports on age hardening of ODS steel containing Ti components, and experiments were conducted on nine samples having the chemical compositions shown in Table 1. For example, the relationship between the heat treatment time and mechanical properties (maximum tensile stress and total elongation at room temperature and 450°C) for age heat treatment at 450°C for these samples is reported in Figure 7.

[0051] Of the nine samples shown in Table 1 of Non-Patent Document 3, the chemical composition of sample "SP9," which is thought to have a chemical composition close to that of Example 1 of the present invention, is shown in Table 6, and its mechanical properties are shown in Table 7 by reading the plot of "SP9" shown in Figure 7 of Non-Patent Document 3.

[0052]

[0053]

[0054] As shown in Tables 6 and 7 above, the sample "SP9" taught in Non-Patent Document 3 contains 0.5 mass% Ti, and undergoing aging heat treatment clearly increases the maximum tensile stress and significantly decreases the total elongation. It can be confirmed that the technology in Non-Patent Document 3 and the present invention differ significantly in the relationship between aging heat treatment and mechanical properties.

[0055] The embodiments and experiments described above are explained to aid in understanding the present invention, and the present invention is not limited to the specific configurations described. For example, it is possible to replace some of the configurations of the embodiments with configurations that are common knowledge to those skilled in the art, and it is also possible to add configurations that are common knowledge to those skilled in the art to the configurations of the embodiments. In other words, the present invention allows for the deletion, substitution, and addition of some of the configurations of the embodiments and experiments specified herein, as long as it does not depart from the technical spirit of the invention.

Claims

1. An oxide dispersion-strengthened iron alloy material having a chemical composition comprising 10.5% to 14% by mass of chromium, 3% to 8% by mass of aluminum, 0.35% to 0.45% by mass of yttrium, at least one of zirconium and hafnium in 0.1% to 1% by mass, 0.2% to 0.5% by mass of oxygen, 0.08% or less of titanium, and less than 0.1% by mass of nitrogen, with the remainder being iron and unavoidable impurities.

2. The oxide dispersion-strengthened iron alloy material according to claim 1, characterized in that the unavoidable impurity contains 0.05% by mass or less of carbon.

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