JP2016183412A - High strength austenitic stainless steel excellent in hydrogen embrittlement resistance, manufacturing method therefor, and device for hydrogen used in high pressure hydrogen gas and liquid hydrogen environment - Google Patents
High strength austenitic stainless steel excellent in hydrogen embrittlement resistance, manufacturing method therefor, and device for hydrogen used in high pressure hydrogen gas and liquid hydrogen environment Download PDFInfo
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- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 title claims abstract description 124
- 229910052739 hydrogen Inorganic materials 0.000 title claims abstract description 86
- 239000001257 hydrogen Substances 0.000 title claims abstract description 86
- 229910000963 austenitic stainless steel Inorganic materials 0.000 title claims abstract description 40
- 239000007788 liquid Substances 0.000 title claims abstract description 12
- 238000004519 manufacturing process Methods 0.000 title claims description 22
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 7
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 6
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 6
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 6
- 229910052802 copper Inorganic materials 0.000 claims abstract description 5
- 239000012535 impurity Substances 0.000 claims abstract description 5
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 4
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 4
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 4
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 3
- 239000002244 precipitate Substances 0.000 claims description 74
- 229910000831 Steel Inorganic materials 0.000 claims description 34
- 239000010959 steel Substances 0.000 claims description 34
- 238000010438 heat treatment Methods 0.000 claims description 26
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- 239000000203 mixture Substances 0.000 claims description 7
- 238000012360 testing method Methods 0.000 description 22
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- 239000000463 material Substances 0.000 description 16
- 230000000694 effects Effects 0.000 description 15
- 238000001556 precipitation Methods 0.000 description 15
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- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 3
- 238000011161 development Methods 0.000 description 3
- 238000000605 extraction Methods 0.000 description 3
- 150000004767 nitrides Chemical class 0.000 description 3
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- 238000009628 steelmaking Methods 0.000 description 3
- 230000002159 abnormal effect Effects 0.000 description 2
- 238000000151 deposition Methods 0.000 description 2
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- 238000000227 grinding Methods 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- 230000003647 oxidation Effects 0.000 description 2
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- 239000002994 raw material Substances 0.000 description 2
- 239000002436 steel type Substances 0.000 description 2
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- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- 229910052765 Lutetium Inorganic materials 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 239000010962 carbon steel Substances 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
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- 239000005431 greenhouse gas Substances 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- FZLIPJUXYLNCLC-UHFFFAOYSA-N lanthanum atom Chemical compound [La] FZLIPJUXYLNCLC-UHFFFAOYSA-N 0.000 description 1
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- 238000002844 melting Methods 0.000 description 1
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- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
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- 239000000243 solution Substances 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
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- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
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Abstract
Description
本発明は、耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼およびその製造方法、ならびに高圧水素ガスおよび液体水素環境中で用いる水素用機器に関し、特に、高圧水素ガスおよび液体水素環境下で使用され、高い強度を有する耐水素脆化特性に優れたオーステナイト系ステンレス鋼およびその製造方法に関する。
本願は、２０１５年３月２６日に、日本に出願された特願２０１５−６４９５１号に基づき優先権を主張し、その内容をここに援用する。
The present invention relates to a high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance, a method for producing the same, and a hydrogen device used in a high-pressure hydrogen gas and a liquid hydrogen environment, and particularly in a high-pressure hydrogen gas and a liquid hydrogen environment. The present invention relates to an austenitic stainless steel which is used and has high strength and excellent hydrogen embrittlement resistance and a method for producing the same.
This application claims priority on March 26, 2015 based on Japanese Patent Application No. 2015-64951 for which it applied to Japan, and uses the content here.
近年、地球温暖化防止の観点から、温室効果ガス（ＣＯ２、ＮＯｘ、ＳＯｘ）の排出を抑制するため、水素をエネルギーの輸送・貯蔵媒体として利用する技術開発が進んでいる。このため、水素の貯蔵・輸送用の機器で使用される金属材料の開発が期待されている。 In recent years, from the viewpoint of preventing global warming, technological development using hydrogen as an energy transport / storage medium is progressing in order to suppress the emission of greenhouse gases (CO 2 , NO x , SO x ). For this reason, the development of metal materials used in equipment for storing and transporting hydrogen is expected.
従来、圧力４０ＭＰａ程度までの水素ガスは、厚肉（厚さの厚い）のＣｒ−Ｍｏ鋼製ボンベに高圧ガスとして充填・貯蔵されている。また、配管用材料あるいは燃料電池自動車の高圧水素ガスタンクライナーとしては、ＪＩＳ規格のＳＵＳ３１６系オーステナイト系ステンレス鋼（以下、「ＳＵＳ３１６鋼」と記載）が使用されている。ＳＵＳ３１６鋼は、高圧水素ガスの環境下での耐水素脆化特性が、例えば上記のＣｒ−Ｍｏ鋼を含む炭素鋼や、ＪＩＳ規格のＳＵＳ３０４系オーステナイト系ステンレス鋼（以下、「ＳＵＳ３０４鋼」と記載）と比較して良好である。 Conventionally, hydrogen gas up to a pressure of about 40 MPa is filled and stored as a high-pressure gas in a thick (thick) Cr-Mo steel cylinder. Further, JIS standard SUS316 austenitic stainless steel (hereinafter referred to as “SUS316 steel”) is used as a piping material or a high-pressure hydrogen gas tank liner of a fuel cell vehicle. SUS316 steel has a resistance to hydrogen embrittlement in an environment of high-pressure hydrogen gas, for example, a carbon steel containing the above-mentioned Cr-Mo steel or a JIS standard SUS304 austenitic stainless steel (hereinafter referred to as "SUS304 steel"). ).
近年、燃料電池自動車の一般販売に先駆けて、水素ステーションの公的な試作・実証実験が進行している。例えば、大量の水素を液体水素として貯蔵でき、かつ液体水素を昇圧して７０ＭＰａ以上の高圧水素ガスとして供給可能な水素ステーションが実証段階にある。また、水素ステーションにおいて、燃料電池自動車のタンクに充填する水素を−４０℃程度の低温に予冷するプレクールと呼ばれる技術が実用化されている。
これらのことから、水素ステーションのディスペンサーに付随する液体水素用の貯蔵容器や水素ガス配管などに用いられる金属材料は、７０ＭＰａの高圧かつ低温の水素ガスに曝されることが想定される。
In recent years, prior to the general sale of fuel cell vehicles, public trials and demonstration experiments of hydrogen stations are in progress. For example, a hydrogen station that can store a large amount of hydrogen as liquid hydrogen and that can be pressurized and supplied as high-pressure hydrogen gas of 70 MPa or more is in the demonstration stage. Also, a technology called precooling has been put into practical use in which hydrogen filled in a fuel cell vehicle tank is precooled to a low temperature of about −40 ° C. in a hydrogen station.
From these facts, it is assumed that the metal material used for the storage container for liquid hydrogen and the hydrogen gas piping associated with the dispenser of the hydrogen station is exposed to high-pressure and low-temperature hydrogen gas of 70 MPa.
より過酷な水素脆化環境下で水素脆化しない金属材料として、Ｎｉを１３％程度含有したＳＵＳ３１６鋼およびＳＵＳ３１６Ｌ鋼が挙げられるが、これら２鋼種を国内の７０ＭＰa級水素ステーションで使用することが高圧ガス保安協会の定める例示基準にて認められている。 SUS316 steel and SUS316L steel containing about 13% of Ni are examples of metal materials that do not become hydrogen embrittled in a more severe hydrogen embrittlement environment. However, it is high pressure to use these two steel types at domestic 70 MPa class hydrogen stations. Recognized by the example standards established by the Gas Safety Association.
一方、将来の燃料電池自動車を中心とした水素エネルギー社会の普及および自律的発展のためには、燃料電池自動車や水素ステーションのコスト削減が必要不可欠である。つまり、水素脆化環境下で用いられる金属材料に対しては、各種の機器の小型化・薄肉化により鋼材の使用量を削減するため、より一層高い強度が求められている。特に、高圧水素配管用途では、用いられる金属材料に対して６５０ＭＰａ程度の引張強さが要求される。 On the other hand, cost reduction of fuel cell vehicles and hydrogen stations is indispensable for the dissemination and autonomous development of hydrogen energy society centering on future fuel cell vehicles. In other words, metal materials used in a hydrogen embrittlement environment are required to have higher strength in order to reduce the amount of steel used by reducing the size and thickness of various devices. In particular, in high-pressure hydrogen piping applications, a tensile strength of about 650 MPa is required for the metal material used.
しかしながら、上記の例示基準に記載されたＳＵＳ３１６系オーステナイト系ステンレス鋼は、レアメタルであるＮｉとＭｏを多量に含んでいるため高価である。しかし、ＳＵＳ３１６系オーステナイト系ステンレス鋼に溶体化処理を施しても、このような引張強度さを満足しないため、ＳＵＳ３１６系オーステナイト系ステンレス鋼に冷間加工を施して強度を補強して使用される。 However, the SUS316 austenitic stainless steel described in the above exemplary criteria is expensive because it contains a large amount of rare metals Ni and Mo. However, even if a solution treatment is performed on SUS316 austenitic stainless steel, such tensile strength is not satisfied. Therefore, SUS316 austenitic stainless steel is cold-worked to reinforce the strength.
ＳＵＳ３１６鋼およびＳＵＳ３１６Ｌ鋼の引張強さを高めた鋼材としては、Ｎの固溶強化を活用したＪＩＳ規格鋼種のＳＵＳ３１６ＮおよびＳＵＳ３１６ＬＮが知られている。しかしながら、例えば、非特許文献１で報告されている通り、ＳＵＳ３１６ＮおよびＳＵＳ３１６ＬＮは、低温の高圧水素ガス中では延性が低下してしまう。 As steel materials with increased tensile strength of SUS316 steel and SUS316L steel, JIS standard steel types SUS316N and SUS316LN utilizing the solid solution strengthening of N are known. However, for example, as reported in Non-Patent Document 1, the ductility of SUS316N and SUS316LN decreases in a low-temperature high-pressure hydrogen gas.
特許文献１（特開２００２−１７３７４２号公報）で開示されたステンレス鋼は、Ｎｉ含有量を４〜１２％としつつ、加工熱処理により金属組織をオーステナイト相とマルテンサイト相の二相組織に制御している。これによりビッカース硬さが５００程度と非常に硬質なステンレス鋼を達成している。 The stainless steel disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2002-173742) controls the metal structure to a two-phase structure of an austenite phase and a martensite phase by thermomechanical treatment while the Ni content is 4 to 12%. ing. This achieves a very hard stainless steel having a Vickers hardness of about 500.
特許文献２（特開２００９−１３３００１号公報）で開示されたステンレス鋼は、１μｍ以上の大きさのＴｉおよびＮｂの炭窒化物の活用により耐水素脆化特性を向上させており、ＳＵＳ３１６鋼に対してＭｏ添加を省略しているため、経済性に優れている。 The stainless steel disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2009-133001) has improved resistance to hydrogen embrittlement by utilizing carbonitrides of Ti and Nb having a size of 1 μm or more. On the other hand, since the addition of Mo is omitted, the economy is excellent.
しかしながら、特許文献１に記載のステンレス鋼は、水素脆化しやすいマルテンサイト相を含んでいるため、水素環境下での適用は困難である。
また、特許文献２に記載のステンレス鋼は、強度に関してはＳＵＳ３１６鋼と同程度であり、さらなる強度の向上が望まれる。
However, since the stainless steel described in Patent Document 1 contains a martensite phase that is easily hydrogen embrittled, it is difficult to apply it in a hydrogen environment.
Further, the stainless steel described in Patent Document 2 has the same degree of strength as SUS316 steel, and further improvement in strength is desired.
このように、低温かつ４０ＭＰａ超の高圧水素ガス環境下での耐水素脆化特性を有する高強度オーステナイト系ステンレス鋼は、未だ出現していないのが現状である。
本発明は、前述の現状に鑑みてなされたもので、低温かつ４０ＭＰａ超の高圧水素ガス環境下で好適に使用できる耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼を提供することを課題とする。
Thus, the present condition is that the high-strength austenitic stainless steel which has the hydrogen embrittlement resistance in the low temperature and high pressure hydrogen gas environment of more than 40 Mpa has not yet appeared.
The present invention has been made in view of the above-mentioned present situation, and it is an object to provide a high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance that can be suitably used in a low-temperature and high-pressure hydrogen gas environment exceeding 40 MPa. And
析出強化による高強度化を指向した高圧水素用ステンレス鋼としては、例えば特許文献３（特開２０１４−４７４０９号公報）に開示されている。
特許文献３記載のステンレス鋼では、η相金属間化合物を活用している。しかしながら、２０％以上のＮｉの添加が必要となり合金コストの増加を招く。
そこで、本発明者らは、主要元素を活用して得られる析出物として、Ｃｒ系炭窒化物に着目した。
As stainless steel for high pressure hydrogen aimed at increasing strength by precipitation strengthening, for example, it is disclosed in Patent Document 3 (Japanese Patent Laid-Open No. 2014-47409).
In the stainless steel described in Patent Document 3, an η phase intermetallic compound is utilized. However, addition of 20% or more of Ni is required, resulting in an increase in alloy costs.
Therefore, the present inventors have focused on Cr-based carbonitrides as precipitates obtained by utilizing main elements.
一方で、一般的にステンレス鋼の各種特性は、Ｃｒ系炭窒化物の影響により低下する。例えば、特許文献４（特開２０１４−１４２２号公報）で開示されている通り、Ｃｒ系炭窒化物が析出すると、Ｃｒ系炭窒化物と母相との界面が破壊の起点となり、成型性の低下を招く。 On the other hand, various characteristics of stainless steel generally deteriorate due to the influence of Cr-based carbonitride. For example, as disclosed in Patent Document 4 (Japanese Patent Application Laid-Open No. 2014-1422), when Cr-based carbonitride is precipitated, the interface between the Cr-based carbonitride and the parent phase becomes a starting point of fracture, and the moldability is improved. Incurs a decline.
さらに、ステンレス鋼の耐水素ガス脆化特性に及ぼすＣｒ系炭窒化物の影響も例外ではない。非特許文献１によれば、金属組織中にＣｒ系炭窒化物を析出させた場合、この析出物の周囲には、Ｃｒ濃度が著しく低下したＣｒ欠乏層が形成される。このＣｒ欠乏層の付近ではオーステナイト相の安定度が低下するため、変形時に加工誘起マルテンサイト相が優先的に生成してしまい、高圧水素ガス中での延性の低下を招く。Ｃｒ欠乏層は追加で熱処理を行い、Ｃｒを拡散させることで消失させることができるが、製造コストが増加してしまう。 Furthermore, the influence of Cr-based carbonitrides on the hydrogen gas embrittlement resistance of stainless steel is no exception. According to Non-Patent Document 1, when Cr-based carbonitride is deposited in a metal structure, a Cr-deficient layer having a significantly reduced Cr concentration is formed around the precipitate. In the vicinity of the Cr-deficient layer, the stability of the austenite phase is lowered, so that a work-induced martensite phase is preferentially generated during deformation, leading to a reduction in ductility in high-pressure hydrogen gas. The Cr-deficient layer can be eliminated by additional heat treatment and diffusing Cr, but the manufacturing cost increases.
ここで、発明者らは、主要元素であるＣｒ、Ｎｉ、Ｍｏと微量元素で構成されているオーステナイト系ステンレス鋼の合金成分組成と、金属組織、析出物の平均サイズ、高圧水素ガス環境下における耐水素脆化特性および強度特性の関係について鋭意研究を行った。その結果、以下の（ａ）〜（ｆ）の新しい知見を得た。 Here, the inventors, alloy components composition of the austenitic stainless steel composed of the main elements Cr, Ni, Mo and trace elements, the metal structure, the average size of the precipitates, under high pressure hydrogen gas environment We have conducted extensive research on the relationship between hydrogen embrittlement resistance and strength properties. As a result, the following new findings (a) to (f) were obtained.
（ａ）水素脆化を示した試験片では、Ｃｒ系炭窒化物やＮｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物の周辺で、き裂（ｃｒａｃｋｓ）が生成する。これら析出物の周辺で生成したき裂が連結・伝ぱすることで延性が低下する。
（ｂ）しかしながら、析出物のサイズを１００ｎｍ以下かつ質量％を１．０％以下に制御することで水素脆化により生成するき裂の生成・進展が著しく抑制され、その結果、耐水素脆化特性が向上する。
（ｃ）このような析出物の平均サイズを１００ｎｍ以下に制御し、かつ析出物の量を質量％で０．００１％〜１．０％に制御することで、Ｃｒ系炭窒化物やＮｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物、あるいはＴｉ、Ｎｂ、Ｖ系炭窒化物などの析出物はオーステナイト系ステンレス鋼の高強度化にも有効に作用する。さらにＮの固溶強化を活用しつつ、析出強化を複合的に作用させることで、ＳＵＳ３１６鋼の冷間加工材と同等以上の６５０ＭＰａ程度の引張強さを得ることができる。
（ｄ）析出物のサイズは熱処理条件の影響を強く受ける。Ｃｒ系炭窒化物やＮｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物の析出ノーズは８００℃程度であり、これより高い温度で保持すると短時間で析出物が析出するが、粗大化が速やかに進行する。このため、析出物の平均サイズを１００ｎｍ以下に制御するのは困難である。８００℃以下で鋼材を保持すると、析出物の粗大化は抑制できるが、析出開始に時間がかかる。
（ｅ）最終熱処理後の冷却時、７５０℃までの平均冷却速度を２．０℃／ｓ未満に制御することで、ステンレスの高強度化と耐水素脆化特性向上を両立させる析出物のサイズ・質量％を確保することができる。
（ｆ）また、炭窒化物を形成しやすいＴｉ、Ｎｂ、Ｖを微量かつ単独あるいは複合的に添加することでＴｉ、Ｎｂ、Ｖ系の炭窒化物を析出させる、あるいはＣｕ添加によりＣｕを析出させることで、耐水素脆化特性を損なうことなく、さらに強度を高めることができる。
(A) In the test piece which showed hydrogen embrittlement, cracks generate | occur | produce around Cr type | system | group carbonitride and Ni * Fe * Cr * Mo * Si intermetallic compound. Ductility is reduced by the connection and propagation of cracks generated around these precipitates.
(B) However, by controlling the size of the precipitates to 100 nm or less and the mass% to 1.0% or less, the generation and propagation of cracks generated by hydrogen embrittlement are remarkably suppressed, and as a result, hydrogen embrittlement resistance Improved characteristics.
(C) By controlling the average size of such precipitates to 100 nm or less and controlling the amount of precipitates to 0.001% to 1.0% by mass%, Cr-based carbonitrides and Ni · Precipitates such as Fe / Cr / Mo / Si intermetallic compounds or Ti, Nb, and V-based carbonitrides also effectively act to increase the strength of austenitic stainless steel. Further, by making the precipitation strengthening work in combination while utilizing the solid solution strengthening of N, a tensile strength of about 650 MPa, which is equal to or higher than that of the cold-worked material of SUS316 steel, can be obtained.
(D) The size of the precipitate is strongly influenced by the heat treatment conditions. The precipitation nose of Cr-based carbonitrides and Ni / Fe / Cr / Mo / Si intermetallic compounds is about 800 ° C., and if kept at a temperature higher than this, precipitates precipitate in a short time, but the coarsening rapidly proceed. For this reason, it is difficult to control the average size of the precipitates to 100 nm or less. If the steel material is held at 800 ° C. or lower, the coarsening of the precipitate can be suppressed, but it takes time to start the precipitation.
(E) At the time of cooling after the final heat treatment, by controlling the average cooling rate up to 750 ° C. to less than 2.0 ° C./s, the size of the precipitate that achieves both high strength of stainless steel and improved resistance to hydrogen embrittlement -Mass% can be secured.
(F) Also, Ti, Nb, V, which easily forms carbonitrides, is added in a small amount, alone or in combination to precipitate Ti, Nb, V-based carbonitrides, or Cu is added to precipitate Cu. By doing so, the strength can be further increased without impairing the hydrogen embrittlement resistance.
本発明は、上記（ａ）〜（ｆ）の新たな知見に基づいてなされたものであり、その要旨は以下の通りである。 The present invention has been made based on the new findings (a) to (f) described above, and the gist thereof is as follows.
（１）質量％で、Ｃ：０．２％以下、Ｓｉ：０．２〜１．５％、Ｍｎ：０．５〜２．５％、Ｐ：０．０６％以下、Ｓ：０．００８％以下、Ｎｉ：１０．０〜２０．０％、Ｃｒ：１６．０〜２５．０％、Ｍｏ：３．５％以下、Ｃｕ：３．５％以下、Ｎ：０．０１〜０．５０％、Ｏ：０．０１５％以下を含有し、残部がＦｅおよび不可避的不純物であり、
さらに、析出物の平均サイズが１００ｎｍ以下であり、かつ析出物の量が質量％で０．００１％以上１．０％以下であることを特徴とする耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼。
(1) By mass%, C: 0.2% or less, Si: 0.2-1.5%, Mn: 0.5-2.5%, P: 0.06% or less, S: 0.008 %: Ni: 10.0-20.0%, Cr: 16.0-25.0%, Mo: 3.5% or less, Cu: 3.5% or less, N: 0.01-0.50 %, O: 0.015% or less, the balance being Fe and inevitable impurities,
Further, the high-strength austenite having excellent hydrogen embrittlement resistance, wherein the average size of the precipitate is 100 nm or less, and the amount of the precipitate is 0.001% to 1.0% by mass. Stainless steel.
（２）更に、質量％で、Ａｌ：０．３％以下、Ｍｇ：０．０１％以下、Ｃａ：０．０１％以下、ＲＥＭ：０．１０％以下、Ｂ：０．００８％以下から選択される１種または２種以上含むことを特徴とする（１）に記載の耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼。 (2) Further, by mass%, Al: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, REM: 0.10% or less, B: 0.008% or less The high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to (1), characterized by comprising one or more of the above.
（３）質量％で、Ｔｉ：０．５％以下、Ｎｂ：０．５％以下、Ｖ：０．５％以下から選択される１種または２種以上を含むことを特徴とする（１）または（２）に記載の耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼。 (3) By mass%, Ti: 0.5% or less, Nb: 0.5% or less, V: One or more selected from 0.5% or less (1) Or the high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance as described in (2).
（４）高圧水素ガスおよび液体水素環境中で用いられることを特徴とする（１）〜（３）のいずれか一項に記載の耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼。 (4) The high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to any one of (1) to (3), which is used in a high-pressure hydrogen gas and liquid hydrogen environment.
（５）（１）〜（３）のいずれか一項に記載の成分組成を有する鋼片を熱間加工する工程と、１０００℃〜１２００℃で最終熱処理する工程と、前記最終熱処理の工程後に冷却する工程と、を有し、前記冷却の工程では、７５０℃までの平均冷却速度を２．０℃／ｓ未満に制御することを特徴とする耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼の製造方法。 (5) After the step of hot working the steel slab having the component composition according to any one of (1) to (3), the step of final heat treatment at 1000 ° C. to 1200 ° C., and the step of the final heat treatment A high-strength austenitic system excellent in hydrogen embrittlement resistance, characterized in that in the cooling step, an average cooling rate up to 750 ° C. is controlled to less than 2.0 ° C./s. Stainless steel manufacturing method.
（６）（１）〜（４）のいずれか一項に記載の耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼を使用した、高圧水素ガスおよび液体水素環境中で用いる水素用機器。 (6) An apparatus for hydrogen used in a high-pressure hydrogen gas and liquid hydrogen environment using the high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to any one of (1) to (4).
本発明の一態様によれば、高圧水素ガスおよび液体水素環境下で好適に使用され、高い強度を有する耐水素脆化特性に優れたオーステナイト系ステンレス鋼およびその製造方法を提供することができる。 According to one embodiment of the present invention, it is possible to provide an austenitic stainless steel that is suitably used in a high-pressure hydrogen gas and liquid hydrogen environment and has high strength and excellent hydrogen embrittlement resistance and a method for producing the same.
以下、本実施形態に係るオーステナイト系ステンレス鋼およびその製造方法について詳細に説明する。
まず、本実施形態のオーステナイト系ステンレス鋼の成分組成について説明する。なお、以下の説明において、各元素の含有量の「％」表示は「質量％」を意味する。
Hereinafter, the austenitic stainless steel and the manufacturing method thereof according to the present embodiment will be described in detail.
First, the component composition of the austenitic stainless steel of this embodiment will be described. In the following description, “%” display of the content of each element means “mass%”.
本実施形態に係るオーステナイト系ステンレス鋼は、質量％で、Ｃ：０．２％以下、Ｓｉ：０．２〜１．５％、Ｍｎ：０．５〜２．５％、Ｐ：０．０６％以下、Ｓ：０．００８％以下、Ｎｉ：１０．０〜２０．０％、Ｃｒ：１６．０〜２５．０％、Ｍｏ：３．５％以下、Ｃｕ：３．５％以下、Ｎ：０．０１〜０．５０％、Ｏ：０．０１５％以下を含有する。さらに、析出物の平均サイズが１００ｎｍ以下であり、かつ析出物の量が質量％で０．００１％以上、１．０％以下含むことを特徴とする。 The austenitic stainless steel according to the present embodiment is mass%, C: 0.2% or less, Si: 0.2 to 1.5%, Mn: 0.5 to 2.5%, P: 0.06. %: S: 0.008% or less, Ni: 10.0-20.0%, Cr: 16.0-25.0%, Mo: 3.5% or less, Cu: 3.5% or less, N : 0.01 to 0.50%, O: 0.015% or less. Furthermore, the average size of the precipitate is 100 nm or less, and the amount of the precipitate is 0.001% or more and 1.0% or less in mass%.
＜Ｃ：０．２％以下＞
Ｃは、オーステナイト相の安定化に有効な元素であり、耐水素脆化特性の向上に寄与する。また、固溶強化およびＣｒ系炭化物による析出強化のため、強度増加にも寄与する。これら効果を得るため、Ｃ含有量を０．０１％以上とすることが好ましい。一方、過剰な量のＣの添加は、Ｃｒ系炭化物の過剰な析出を招き、耐水素脆化特性の低下に繋がる。このため、Ｃ含有量の上限を０．２％とする必要がある。より好ましいＣ含有量の上限は０．１５％である。
<C: 0.2% or less>
C is an element effective for stabilizing the austenite phase and contributes to the improvement of hydrogen embrittlement resistance. Moreover, it contributes to an increase in strength due to solid solution strengthening and precipitation strengthening by Cr-based carbides. In order to obtain these effects, the C content is preferably 0.01% or more. On the other hand, addition of an excessive amount of C leads to excessive precipitation of Cr-based carbides, leading to a decrease in hydrogen embrittlement resistance. For this reason, it is necessary to make the upper limit of C content 0.2%. A more preferable upper limit of the C content is 0.15%.
＜Ｓｉ：０．２〜１．５％＞
Ｓｉは、オーステナイト相の安定化に有効な元素である。オーステナイト相の安定化により耐水素脆化特性を向上させるため、Ｓｉ含有量を０．２％以上とする必要がある。Ｓｉ含有量は０．４％以上であることが好ましい。一方、過剰な量のＳｉの添加は、シグマ相などの金属間化合物の生成を促進させ、熱間加工性や靭性低下を招く。このため、Ｓｉ含有量の上限を１．５％とする必要がある。Ｓｉ含有量は、より好ましくは１．１％以下である。
<Si: 0.2 to 1.5%>
Si is an element effective for stabilizing the austenite phase. In order to improve the hydrogen embrittlement resistance by stabilizing the austenite phase, the Si content needs to be 0.2% or more. The Si content is preferably 0.4% or more. On the other hand, the addition of an excessive amount of Si promotes the formation of intermetallic compounds such as a sigma phase, and causes hot workability and toughness deterioration. For this reason, the upper limit of Si content needs to be 1.5%. The Si content is more preferably 1.1% or less.
＜Ｍｎ：０．５〜２．５％＞
Ｍｎは、オーステナイト相の安定化に有効な元素である。オーステナイト相の安定化により加工誘起マルテンサイト相の生成が抑制され、これにより耐水素脆化特性が向上する。このため、Ｍｎ含有量を０．５％以上とする必要がある。Ｍｎ含有量は０．８％以上であることが好ましい。一方、過剰な量のＭｎの添加は、粗大なＭｎＳ介在物の生成が促進させ、オーステナイト相の延性低下を招くほか、窒化物の生成を促進する作用も有するため、上限を２．５％とする必要がある。Ｍｎ含有量は、より好ましくは２．０％以下である。
<Mn: 0.5 to 2.5%>
Mn is an element effective for stabilizing the austenite phase. Stabilization of the austenite phase suppresses the formation of a work-induced martensite phase, thereby improving the hydrogen embrittlement resistance. For this reason, it is necessary to make Mn content 0.5% or more. The Mn content is preferably 0.8% or more. On the other hand, the addition of an excessive amount of Mn promotes the formation of coarse MnS inclusions and causes a reduction in the ductility of the austenite phase, and also has the effect of promoting the formation of nitrides, so the upper limit is 2.5%. There is a need to. The Mn content is more preferably 2.0% or less.
＜Ｐ：０．０６％以下＞
Ｐは、本実施形態のオーステナイト系ステンレス鋼中に不純物として含まれる。Ｐは、熱間加工性を低下させる元素であるため、Ｐ含有量を極力低減させることが好ましい。具体的には、Ｐ含有量は０．０６％以下と制限することが好ましく、０．０５％以下と制限することがより好ましい。しかし、Ｐ含有量の極度の低減は製鋼コストの増大に繋がるため、Ｐ含有量は０．００８％以上であることが好ましい。
<P: 0.06% or less>
P is contained as an impurity in the austenitic stainless steel of the present embodiment. Since P is an element that reduces hot workability, it is preferable to reduce the P content as much as possible. Specifically, the P content is preferably limited to 0.06% or less, and more preferably 0.05% or less. However, since extreme reduction of the P content leads to an increase in steelmaking costs, the P content is preferably 0.008% or more.
＜Ｓ：０．００８％以下＞
Ｓは、熱間加工時にオーステナイト粒界に偏析し、粒界の結合力を弱めることで熱間加工時の割れを誘発する元素である。そのため、Ｓ含有量の上限を０．００８％に制限する必要がある。Ｓ含有量の好ましい上限は０．００５％である。Ｓ含有量は、極力低減させることが好ましいため、特に下限は設けないが、極度の低減は製鋼コストの増大に繋がる。このためＳ含有量は０．０００１％以上であることが好ましい。
<S: 0.008% or less>
S is an element that segregates at the austenite grain boundary during hot working and induces cracking during hot working by weakening the bonding force of the grain boundary. Therefore, it is necessary to limit the upper limit of the S content to 0.008%. The upper limit with preferable S content is 0.005%. Since it is preferable to reduce the S content as much as possible, there is no particular lower limit. However, extreme reduction leads to an increase in steelmaking costs. For this reason, it is preferable that S content is 0.0001% or more.
＜Ｎｉ：１０．０〜２０．０％＞
Ｎｉは、オーステナイト系ステンレス鋼の耐水素脆化特性を向上させる効果が大きい元素である。また、Ｎｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物の生成を促進させ、高強度化にも寄与する。これらの効果を十分に得るため、Ｎｉ含有量を１０．０％以上とする必要がある。成分偏析を解消することにより、これらの効果は更に向上するため、Ｎｉ含有量は１１．５％以上であることが好ましい。一方、過剰な量のＮｉの添加は材料コストの上昇を招くため、Ｎｉ含有量の上限を２０．０％とする。Ｎｉ含有量は、好ましくは１４．０％以下である。
<Ni: 10.0-20.0%>
Ni is an element having a large effect of improving the hydrogen embrittlement resistance of austenitic stainless steel. Moreover, the production | generation of a Ni * Fe * Cr * Mo * Si intermetallic compound is accelerated | stimulated and it contributes also to high intensity | strength. In order to sufficiently obtain these effects, the Ni content needs to be 10.0% or more. Since these effects are further improved by eliminating component segregation, the Ni content is preferably 11.5% or more. On the other hand, the addition of an excessive amount of Ni causes an increase in material cost, so the upper limit of the Ni content is 20.0%. The Ni content is preferably 14.0% or less.
＜Ｃｒ：１６．０〜２５．０％＞
Ｃｒは、ステンレス鋼に要求される耐食性を得るために欠くことのできない元素である。加えて、Ｃｒは、オーステナイト系ステンレス鋼の強度上昇にも寄与する元素である。この効果を十分に得るため、Ｃｒ含有量は１６．０％以上とする必要がある。Ｃｒ含有量は、好ましくは１６．５％以上である。一方、過剰な量のＣｒの添加は、Ｃｒ系炭窒化物の過剰な析出を招き、耐水素脆化特性を低下させる。このため、Ｃｒ含有量の上限を２５．０％とする必要がある。Ｃｒ含有量は、好ましくは２２．５％以下である。
<Cr: 16.0 to 25.0%>
Cr is an element indispensable for obtaining the corrosion resistance required for stainless steel. In addition, Cr is an element that contributes to an increase in strength of austenitic stainless steel. In order to sufficiently obtain this effect, the Cr content needs to be 16.0% or more. The Cr content is preferably 16.5% or more. On the other hand, the addition of an excessive amount of Cr causes excessive precipitation of Cr-based carbonitrides, and deteriorates the hydrogen embrittlement resistance. For this reason, it is necessary to make the upper limit of Cr content 25.0%. The Cr content is preferably 22.5% or less.
＜Ｍｏ：３．５％以下＞
Ｍｏは、オーステナイト系ステンレス鋼の強度の上昇と耐食性の向上に寄与する元素である。しかしながら、Ｍｏの添加は合金コストの増加を招く。したがって、Ｍｏ含有量は３．５％以下とする。一方、Ｍｏはスクラップ原料から不可避に混入する元素である。Ｍｏ含有量の過度な低減は溶解原料の制約を招き、製造コストの増加に繋がる。したがって、上記効果と製造性を両立させるため、Ｍｏ含有量の下限は０．０５％とすることが好ましい。
<Mo: 3.5% or less>
Mo is an element contributing to an increase in strength and corrosion resistance of austenitic stainless steel. However, the addition of Mo causes an increase in alloy cost. Therefore, the Mo content is 3.5% or less. On the other hand, Mo is an element inevitably mixed from scrap raw materials. Excessive reduction of the Mo content invites restrictions on the melting raw material, leading to an increase in manufacturing cost. Therefore, in order to achieve both the above effects and manufacturability, the lower limit of the Mo content is preferably 0.05%.
＜Ｃｕ：３．５％以下＞
Ｃｕは、オーステナイト相の安定化に有効な元素である。オーステナイト相の安定化により耐水素脆化特性を向上させるため、Ｃｕ含有量は０．１５％以上であることが好ましい。一方、ＣｕはＣｕ析出強化による強度上昇にも寄与するが、過剰な量のＣｕの添加は、オーステナイト相の強度低下につながり、熱間加工性も損なわれるため、Ｃｕ含有量の上限を３．５％とする必要がある。Ｃｕ含有量は、より好ましくは３．０％以下である。
<Cu: 3.5% or less>
Cu is an element effective for stabilizing the austenite phase. In order to improve the hydrogen embrittlement resistance by stabilizing the austenite phase, the Cu content is preferably 0.15% or more. On the other hand, although Cu contributes to an increase in strength due to Cu precipitation strengthening, addition of an excessive amount of Cu leads to a decrease in strength of the austenite phase and also impairs hot workability, so the upper limit of Cu content is 3. Need to be 5%. The Cu content is more preferably 3.0% or less.
＜Ｎ：０．０１〜０．５０％＞
Ｎは、オーステナイト相の安定化と耐食性の向上に有効な元素である。また、固溶強化およびＣｒ系窒化物の析出強化により、強度上昇に寄与する。これら効果を得るため、Ｎ含有量は０．０１％以上とする。Ｎ含有量は、好ましくは０．０４％以上である。一方、過剰な量のＮの添加はＣｒ系窒化物の過剰な生成を促進し、オーステナイト相の耐水素脆化特性や耐食性、靭性を低下させる。このため、Ｎ含有量の上限を０．５０％とする必要がある。Ｎ含有量は、より好ましくは０．３５％以下である。
<N: 0.01 to 0.50%>
N is an element effective for stabilizing the austenite phase and improving the corrosion resistance. In addition, solid solution strengthening and precipitation strengthening of Cr-based nitride contribute to an increase in strength. In order to obtain these effects, the N content is set to 0.01% or more. The N content is preferably 0.04% or more. On the other hand, the addition of an excessive amount of N promotes excessive formation of Cr-based nitrides, and deteriorates the hydrogen embrittlement resistance, corrosion resistance, and toughness of the austenite phase. For this reason, the upper limit of N content needs to be 0.50%. The N content is more preferably 0.35% or less.
＜Ｏ：０．０１５％以下＞
Ｏは、鋼中で酸化物を形成することで、オーステナイト相の熱間加工性および靭性を低下させる。このため、Ｏ（酸素）含有量の上限を０．０１５％以下と制限する必要がある。Ｏ含有量は、好ましくは、０．０１０％以下である。Ｏ（酸素）含有量は、極力低減させることが好ましいが、極度の低減は製鋼コストの増大に繋がる。このためＯ（酸素）含有量は０．００１％以上であることが好ましい。
<O: 0.015% or less>
O reduces the hot workability and toughness of the austenite phase by forming oxides in the steel. For this reason, it is necessary to limit the upper limit of O (oxygen) content to 0.015% or less. The O content is preferably 0.010% or less. The O (oxygen) content is preferably reduced as much as possible, but the extreme reduction leads to an increase in steelmaking costs. Therefore, the O (oxygen) content is preferably 0.001% or more.
本実施形態に係るオーステナイト系ステンレス鋼は、上述してきた元素以外は、Ｆｅ及び不可避的不純物からなるが、後述する任意に添加される元素を含有してもよい。 Although the austenitic stainless steel which concerns on this embodiment consists of Fe and an unavoidable impurity other than the element mentioned above, you may contain the element added arbitrarily mentioned later.
＜Ａｌ：０．３％以下、Ｍｇ、Ｃａ：０．０１％以下、ＲＥＭ：０．１０％以下、Ｂ：０．００８％以下＞
Ａｌ、Ｍｇ、Ｃａ、ＲＥＭ、Ｂは、脱酸および熱間加工性、耐食性の向上に有効な元素である。必要に応じて、これらから選択される１種または２種以上の元素を添加してもよい。ただし、これら元素の過剰な量の添加は、製造コストの著しい増加を招く。このため、これら元素の含有量の上限をＡｌ：０．３％以下、Ｍｇ、Ｃａ：０．０１％以下、ＲＥＭ：０．１０％以下、Ｂ：０．００８％以下とする必要がある。これら元素の含有量の下限は特に設ける必要はないが、脱酸効果を十分に得るため、Ａｌ：０．０１％、Ｍｇ、Ｃａ：０．０００２％、ＲＥＭ：０．００１％、Ｂ：０．０００２％とすることが好ましい。ここで、ＲＥＭ（希土類元素）は一般的な定義に従い、スカンジウム（Ｓｃ）、イットリウム（Ｙ）の２元素と、ランタン（Ｌａ）からルテチウム（Ｌｕ）までの１５元素（ランタノイド）の総称を指す。単独の元素を添加してもよいし、２種以上の元素を添加してもよい。ＲＥＭの含有量は、これら元素の合計量である。
<Al: 0.3% or less, Mg, Ca: 0.01% or less, REM: 0.10% or less, B: 0.008% or less>
Al, Mg, Ca, REM, and B are elements that are effective for improving deoxidation, hot workability, and corrosion resistance. If necessary, one or more elements selected from these may be added. However, the addition of an excessive amount of these elements causes a significant increase in manufacturing cost. For this reason, the upper limit of the content of these elements needs to be Al: 0.3% or less, Mg, Ca: 0.01% or less, REM: 0.10% or less, B: 0.008% or less. The lower limit of the content of these elements is not particularly required, but in order to obtain a sufficient deoxidation effect, Al: 0.01%, Mg, Ca: 0.0002%, REM: 0.001%, B: 0 .0002% is preferable. Here, REM (rare earth element) refers to a generic name of two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu) according to a general definition. A single element may be added, or two or more elements may be added. The content of REM is the total amount of these elements.
＜Ｔｉ、Ｎｂ、Ｖ：０．５０％以下＞
Ｔｉ、Ｎｂ、Ｖは鋼中に固溶するか、または炭窒化物として析出し、強度を増加させるために有効な元素である。必要に応じてこれらから選択される１種または２種以上の元素を添加してもよい。この場合、Ｔｉ、Ｎｂ、Ｖのそれぞれの含有量は、０．０１％以上が好ましい。ただし、Ｔｉ、Ｎｂ、Ｖのそれぞれの含有量が０．５０％より多くなるとＣｒ系炭窒化物の生成を抑制してしまい、Ｃｒ系炭窒化物による析出強化の効果を十分に得ることができない。したがって、Ｔｉ、Ｎｂ、Ｖのそれぞれの含有量の上限を０．５０％以下とする必要がある。好ましいＴｉ、Ｎｂ、Ｖのそれぞれの含有量の上限は０．４０％である。
<Ti, Nb, V: 0.50% or less>
Ti, Nb, and V are effective elements for increasing the strength by dissolving in steel or precipitating as carbonitride. If necessary, one or more elements selected from these may be added. In this case, the content of each of Ti, Nb, and V is preferably 0.01% or more. However, when the content of each of Ti, Nb, and V exceeds 0.50%, the formation of Cr-based carbonitrides is suppressed, and the effect of precipitation strengthening by Cr-based carbonitrides cannot be sufficiently obtained. . Therefore, the upper limit of each content of Ti, Nb, and V needs to be 0.50% or less. The upper limit of each content of preferable Ti, Nb, and V is 0.40%.
以上説明した元素以外の他の元素も、本実施形態の効果を損なわない範囲で含有させることが出来る。 Other elements than the elements described above can also be contained within a range not impairing the effects of the present embodiment.
「析出物に係る限定の理由」
次に、鋼中の析出物のサイズおよび生成量について説明する。
水素脆化を示した試験片では、Ｃｒ系炭窒化物あるいはＮｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物の周辺で、き裂が生成する。これは、各析出物の周辺に形成されるＣｒ欠乏層に起因して、各析出物の周辺において局所的に耐水素ガス脆化特性が低下するためである。析出物の周辺を起点として生成したき裂が連結・伝ぱすることで延性が低下する。
"Reason for limitation related to precipitates"
Next, the size and amount of precipitates in the steel will be described.
In the test piece showing hydrogen embrittlement, a crack is generated around the Cr-based carbonitride or the Ni / Fe / Cr / Mo / Si intermetallic compound. This is because the hydrogen gas embrittlement resistance locally deteriorates around each precipitate due to the Cr-deficient layer formed around each precipitate. Ductility decreases when cracks generated from the periphery of precipitates are connected and propagated.
しかしながら、析出物の平均サイズを１００ｎｍ以下に制御し、かつ、析出物の生成量を質量％で１．０％以下に制御することで、水素ガス脆化により生成するき裂の生成・進展が著しく抑制される。その結果、耐水素ガス脆化特性が向上する。 However, by controlling the average size of precipitates to 100 nm or less and controlling the amount of precipitates generated to 1.0% or less by mass%, the generation and propagation of cracks generated by hydrogen gas embrittlement can be prevented. Remarkably suppressed. As a result, the hydrogen gas embrittlement resistance is improved.
さらに、析出物の析出強化により強度を増加させるとともに、Ｎの固溶強化を複合的に作用させることで、ＳＵＳ３１６鋼の冷間加工材と同等以上の６５０ＭＰａ程度の引張強さを得ることができる。当該効果を享受するために、析出物の生成量の下限値は０．００１％以上とする。析出物の生成量の下限値は、好ましくは０．００５％以上である。 Furthermore, by increasing the strength by precipitation strengthening of precipitates and by combining N solid solution strengthening, it is possible to obtain a tensile strength of about 650 MPa equivalent to or higher than that of a cold-worked material of SUS316 steel. . In order to enjoy the effect, the lower limit of the amount of precipitate generated is set to 0.001% or more. The lower limit of the amount of precipitate generated is preferably 0.005% or more.
析出物の平均サイズおよび析出物の生成量については、後述する最終熱処理後の平均冷却速度を制御することで制御できるが、この平均冷却速度が遅いほど析出物は徐々に粗大化する。そのため、析出物の存在を透過型顕微鏡（ＴＥＭ）にて確認することが可能となる。好ましい析出物の平均サイズは７０ｎｍ以下である。
一方、平均冷却速度が速い場合（上限に近い場合）、析出物は非常に微細であることから、析出物の平均サイズの下限については特に設けないが、５ｎｍ以上であることが好ましい。
The average size of precipitates and the amount of precipitates generated can be controlled by controlling the average cooling rate after the final heat treatment, which will be described later. The slower the average cooling rate, the larger the precipitates become. Therefore, it is possible to confirm the presence of the precipitate with a transmission microscope (TEM). The average size of the preferred precipitate is 70 nm or less.
On the other hand, when the average cooling rate is fast (close to the upper limit), the precipitates are very fine. Therefore, the lower limit of the average size of the precipitates is not particularly provided, but is preferably 5 nm or more.
炭窒化物および金属間化合物（析出物）の生成量は、例えば電解抽出残渣法により測定できる。
過剰な量の析出物が生成すると、析出物の周辺を起点として生成したき裂の連結・伝ぱが助長されるため、析出物の生成量を質量％で１．０％以下とする必要がある。好ましくは、析出物の生成量は質量％で０．９０％以下である。一方、冷却速度が速い場合（上限に近い場合）、析出物は非常に微細であることから、析出物の平均サイズの下限については特に設けない。しかし、Ｃｒ系炭窒化物およびＮｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物による強度上昇効果を得るためには、質量％で０．０２％以上であることが好ましい。
The amount of carbonitride and intermetallic compound (precipitate) produced can be measured, for example, by the electrolytic extraction residue method.
When an excessive amount of precipitates is generated, the connection and propagation of cracks generated from the periphery of the precipitates is promoted, so the amount of precipitates generated must be 1.0% or less by mass%. . Preferably, the amount of precipitates produced is 0.90% or less by mass. On the other hand, when the cooling rate is fast (close to the upper limit), the precipitates are very fine, so there is no particular lower limit for the average size of the precipitates. However, in order to obtain the effect of increasing the strength by the Cr-based carbonitride and the Ni / Fe / Cr / Mo / Si intermetallic compound, it is preferably 0.02% or more by mass%.
また、析出物の平均サイズは、例えば、以下の方法により測定される。ＴＥＭにより析出物を観察し、ＥＤＸにより析出物を同定して析出物を特定する。次いで、１個の析出物の長径と短径をＴＥＭ写真により測定する。そして、長径と短径の平均値（（長径＋短径）／２）を求め、その析出物のサイズとする。同様にして、複数個の析出物のサイズを求める。複数個の析出物のサイズの平均値を算出し、その平均値を、ステンレス鋼における析出物の平均サイズとすることができる。なお、本実施形態においては、１個の析出物に対して、面積が最小になるように外接長方形を描く。そして、この外接長方形の長辺を析出物の長径とし、外接長方形の短辺を析出物の短径とする。
また、本発明における「析出物」とは、鋼中に析出した全ての析出物を意味し、Ｃｒ系炭窒化物、Ｎｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物のほか、Ｔｉ、Ｎｂ、Ｖ系の炭窒化物や析出したＣｕなども含まれる。
Moreover, the average size of the precipitate is measured by the following method, for example. The precipitate is observed by TEM, and the precipitate is identified by EDX to identify the precipitate. Next, the major axis and the minor axis of one precipitate are measured with a TEM photograph. Then, an average value of the major axis and the minor axis ((major axis + minor axis) / 2) is obtained and set as the size of the precipitate. Similarly, the size of a plurality of precipitates is obtained. An average value of the sizes of the plurality of precipitates can be calculated, and the average value can be set as the average size of the precipitates in the stainless steel. In the present embodiment, a circumscribed rectangle is drawn so as to minimize the area for one precipitate. The long side of the circumscribed rectangle is the major axis of the precipitate, and the short side of the circumscribed rectangle is the minor axis of the precipitate.
Further, the “precipitate” in the present invention means all precipitates precipitated in the steel, in addition to Cr-based carbonitrides, Ni / Fe / Cr / Mo / Si intermetallic compounds, Ti, Nb, V-based carbonitrides and precipitated Cu are also included.
「製造方法」
次に、本実施形態に係るオーステナイト系ステンレス鋼の製造方法の一例について説明する。
本実施形態のオーステナイト系ステンレス鋼を製造するには、まず、上記の成分組成からなるステンレス鋼を溶製し、スラブなどの鋼片を製造する。次に、鋼片を所定の温度に加熱して熱間圧延等の熱間加工を行う（熱間加工工程）。
なお、本実施形態のオーステナイト系ステンレス鋼は、鋼板に限定されるものではない。したがって、鋼片は、スラブに限定されるものではなく、目的の製品（棒、管等）の形状に対して、好ましい形状の鋼片（ビレット、ブルーム等）を選択しても達成可能であることは言うまでもない。
"Production method"
Next, an example of the manufacturing method of the austenitic stainless steel which concerns on this embodiment is demonstrated.
In order to manufacture the austenitic stainless steel of this embodiment, first, a stainless steel having the above component composition is melted to manufacture a steel slab such as a slab. Next, the steel slab is heated to a predetermined temperature to perform hot working such as hot rolling (hot working step).
In addition, the austenitic stainless steel of this embodiment is not limited to a steel plate. Therefore, the steel slab is not limited to the slab, and it can be achieved by selecting a steel slab (billette, bloom, etc.) having a preferable shape with respect to the shape of the target product (bar, pipe, etc.). Needless to say.
以下、熱間加工後の最終熱処理の条件について詳細な説明を行う。
熱間加工後の最終熱処理は温度が高すぎると、過剰な粒成長により鋼材の強度が低下する場合や、異常酸化により研削工程を追加することになり生産コストの増加を招く場合がある。このため、最終熱処理温度の上限を１２００℃とする。一方、最終熱処理の温度が低すぎると、熱間加工時の変形組織が残存し、鋼製品の延性が低下するため、下限を１０００℃とする。好ましい最終熱処理の温度範囲は１０５０℃〜１１８０℃である。
上記温度範囲での熱処理の保持時間は、１秒〜１時間とする。これより保持時間が短すぎると、鋼中に加工組織が残存し、延性低下を招く。好ましい保持時間の下限は３０秒である。また、熱処理の保持時間が長すぎると、過剰な粒成長により強度が低下する場合や、異常酸化により研削工程を追加することになり生産コストの増加を招く場合がある。好ましい保持上限時間は４０分である。
Hereinafter, the conditions for the final heat treatment after hot working will be described in detail.
If the temperature of the final heat treatment after hot working is too high, the strength of the steel material may decrease due to excessive grain growth, or a grinding process may be added due to abnormal oxidation, which may increase production costs. For this reason, the upper limit of the final heat treatment temperature is set to 1200 ° C. On the other hand, if the temperature of the final heat treatment is too low, a deformed structure at the time of hot working remains and the ductility of the steel product decreases, so the lower limit is set to 1000 ° C. A preferable temperature range for the final heat treatment is 1050 ° C to 1180 ° C.
The heat treatment holding time in the above temperature range is 1 second to 1 hour. If the holding time is too short, the processed structure remains in the steel, resulting in a decrease in ductility. A preferable lower limit of the holding time is 30 seconds. In addition, if the holding time of the heat treatment is too long, the strength may decrease due to excessive grain growth, or a grinding process may be added due to abnormal oxidation, leading to an increase in production cost. A preferable holding upper limit time is 40 minutes.
Ｃｒ系炭窒化物およびＮｉ・Ｆｅ・Ｃｒ・Ｍｏ・Ｓｉ金属間化合物の析出ノーズ温度は８００℃程度である。これより高い温度で鋼材を保持すると、析出物の粗大化が速やかに進行するため、析出物の平均サイズを１００ｎｍ以下に制御するのは困難である。一方、８００℃以下で鋼材を保持すると、析出物の粗大化は抑制できるが、析出開始に時間がかかってしまう。このため、製造コストの増加に繋がる。 The precipitation nose temperature of the Cr-based carbonitride and the Ni / Fe / Cr / Mo / Si intermetallic compound is about 800.degree. If the steel material is held at a temperature higher than this, the coarsening of the precipitate proceeds rapidly, so that it is difficult to control the average size of the precipitate to 100 nm or less. On the other hand, when the steel material is held at 800 ° C. or lower, the coarsening of the precipitate can be suppressed, but it takes time to start the precipitation. For this reason, it leads to the increase in manufacturing cost.
しかしながら、１０００℃〜１２００℃で最終熱処理後、７５０℃までの平均冷却速度を２．０℃／ｓ未満に制御することで、ステンレスの高強度化と耐水素脆化特性の向上を両立させる析出物の平均サイズ・生成量を確保することができる。 However, after the final heat treatment at 1000 ° C. to 1200 ° C., by controlling the average cooling rate up to 750 ° C. to less than 2.0 ° C./s, precipitation that achieves both high strength of stainless steel and improvement of hydrogen embrittlement resistance The average size and production amount of objects can be secured.
以上のことから、最終熱処理後の冷却工程においては、７５０℃までの平均冷却速度を２．０℃／ｓ未満に制御する必要がある。平均冷却速度が２．０℃／ｓより速い場合、析出物が析出する時間を確保できないため、鋼製品の強度を高めることができない。一方、冷却速度が過剰に遅い場合、析出物の平均サイズが１００ｎｍよりも大きくなるおそれがあり、鋼製品の良好な耐水素脆化特性を確保することができないおそれがある。そのため、好ましい平均冷却速度の下限は０．３℃／ｓ以上である。さらに好ましい下限は０．４℃／ｓ以上である。 From the above, in the cooling step after the final heat treatment, it is necessary to control the average cooling rate up to 750 ° C. to less than 2.0 ° C./s. When the average cooling rate is faster than 2.0 ° C./s, it is not possible to secure the time for depositing, so the strength of the steel product cannot be increased. On the other hand, if the cooling rate is excessively slow, the average size of the precipitates may be larger than 100 nm, and the good hydrogen embrittlement resistance of the steel product may not be ensured. Therefore, the preferable lower limit of the average cooling rate is 0.3 ° C./s or more. A more preferred lower limit is 0.4 ° C./s or more.
なお、上記熱間加工、最終熱処理を行った後は、必要に応じて酸洗、冷間加工を施してもよい。 In addition, after performing the said hot processing and final heat processing, you may perform pickling and cold processing as needed.
また、本実施形態に係るオーステナイト系ステンレス鋼は、上述してきた製造方法に限らず、析出物の平均サイズと生成量を上記範囲内に制御できる方法であれば如何なる製造方法を採用してもよい。
また、本発明範囲の成分を満たすオーステナイト系ステンレス鋼を使用した水素用機器の製造工程における熱処理、あるいは水素用機器への熱処理によって析出物の平均サイズと生成量を上記範囲内に制御してもよい。
In addition, the austenitic stainless steel according to the present embodiment is not limited to the above-described manufacturing method, and any manufacturing method may be adopted as long as the average size and amount of precipitates can be controlled within the above range. .
In addition, even if the average size and amount of precipitates are controlled within the above range by the heat treatment in the manufacturing process of hydrogen equipment using austenitic stainless steel satisfying the components of the present invention range or the heat treatment to the hydrogen equipment. Good.
以下に本発明の実施例について説明するが、本発明は、以下の実施例で用いた条件に限定されるものではない。
なお、表中の下線は本実施形態の範囲から外れているものを示す。
Examples of the present invention will be described below, but the present invention is not limited to the conditions used in the following examples.
In addition, the underline in a table | surface shows what has remove | deviated from the range of this embodiment.
表１の成分組成を有するステンレス鋼供試材を溶製し、厚さ１２０ｍｍの鋳片を製造した。その後、鋳片を１２００℃で加熱して、熱間鍛造および熱間圧延を行うことにより、厚さ２０ｍｍの熱延板を作製した。その後、熱延板に対して、表２に記載の条件で最終熱処理および冷却を施し、熱延焼鈍板を得た。最終熱処理における保持時間は３分〜２０分の範囲内で行った。表２中の「熱処理温度（℃）」は最終熱処理の温度を示し、「冷却速度（℃／ｓ）」は７５０℃までの平均冷却速度を示す。 A stainless steel specimen having the composition shown in Table 1 was melted to produce a cast piece having a thickness of 120 mm. Thereafter, the slab was heated at 1200 ° C., and hot forging and hot rolling were performed to produce a hot-rolled sheet having a thickness of 20 mm. Thereafter, the hot-rolled sheet was subjected to final heat treatment and cooling under the conditions shown in Table 2 to obtain a hot-rolled annealed sheet. The holding time in the final heat treatment was performed within a range of 3 minutes to 20 minutes. In Table 2, “heat treatment temperature (° C.)” indicates the temperature of the final heat treatment, and “cooling rate (° C./s)” indicates an average cooling rate up to 750 ° C.
各供試材の析出物の平均サイズおよび析出物量を表２に示す。
得られた熱延焼鈍板から抽出レプリカ法により試料を作製し、次いでＴＥＭによる析出物の観察を行った。１個の析出物のサイズは、長径と短径の平均値（（長径＋短径）／２）として定義した。３０個の析出物に対してサイズの測定を行い、３０個の析出物のサイズの平均値を、その供試材における析出物の平均サイズと定めた。
析出物の量は、同様に供試材から分析用サンプルを採取し、電解抽出残渣法により測定した。残渣を濾すフィルターのメッシュサイズは０．２μｍのものを使用した。
Table 2 shows the average size and amount of precipitates of each test material.
A sample was prepared from the obtained hot-rolled annealed plate by the extraction replica method, and then the precipitate was observed by TEM. The size of one precipitate was defined as the average value of the major axis and the minor axis ((major axis + minor axis) / 2). The size of 30 precipitates was measured, and the average size of the 30 precipitates was determined as the average size of the precipitates in the test material.
Similarly, the amount of the precipitate was measured by the electrolytic extraction residue method after collecting a sample for analysis from the test material. The filter used for filtering the residue had a mesh size of 0.2 μm.
次に、各供試材の熱延焼鈍板について、以下に示す方法により、耐水素ガス脆化特性を評価した。
上記の厚さ２０ｍｍの熱延焼鈍板の長手方向かつ板厚の中心部から、外径３ｍｍ、長さ２０ｍｍの平行部を持つ丸棒引張試験片を採取した。この丸棒引張試験片を用いて、（１）大気中引張試験と、（２）高圧水素ガス中での引張試験を行った。
Next, the resistance to hydrogen gas embrittlement was evaluated for the hot-rolled annealed plates of each test material by the following method.
A round bar tensile test piece having a parallel portion with an outer diameter of 3 mm and a length of 20 mm was collected from the longitudinal direction of the hot-rolled annealing plate having a thickness of 20 mm and from the center of the plate thickness. Using this round bar tensile test piece, (1) an atmospheric tensile test and (2) a tensile test in high-pressure hydrogen gas were performed.
（１）の大気中での引張試験は、試験温度：２５℃および−４０℃、歪速度：５×１０−５／ｓの条件で実施した。２５℃の引張試験で得られた引張強さが６５０ＭＰａを超えるものを合格と評価した。
（２）の高圧水素ガス中での引張試験は、試験温度：−４０℃、試験環境：７０ＭＰａ水素ガス、歪速度：５×１０−５／ｓの条件で実施した。なお、試験片Ｎｏ．Ａ３、Ａ４、Ａ６については、試験環境：１０３ＭＰａ水素ガス中でも同様の引張試験を行った。
そして−４０℃における「（高圧水素ガス中での絞り／大気中での絞り）×１００（％）」の値（相対絞り）を算出した。この値が８０％以上である共試材を、高圧水素ガス中での耐水素脆化特性が合格であると評価した。特に、２５℃の引張強さが６５０ＭＰａを超え、さらに絞りが８０％以上、８５％未満のものを「○」、８５％以上のものを「◎」として評価した。
その結果を表３および表４に示す。
The tensile test in the atmosphere of (1) was performed under the conditions of test temperature: 25 ° C. and −40 ° C., strain rate: 5 × 10 −5 / s. Those having a tensile strength exceeding 650 MPa obtained in a 25 ° C. tensile test were evaluated as acceptable.
The tensile test in high-pressure hydrogen gas (2) was performed under the conditions of test temperature: −40 ° C., test environment: 70 MPa hydrogen gas, strain rate: 5 × 10 −5 / s. The test piece No. For A3, A4, and A6, the same tensile test was performed in the test environment: 103 MPa hydrogen gas.
Then, a value (relative aperture) of “(throttle in high-pressure hydrogen gas / throttle in air) × 100 (%)” at −40 ° C. was calculated. A co-test material having this value of 80% or more was evaluated as having passed hydrogen embrittlement resistance in high-pressure hydrogen gas. In particular, when the tensile strength at 25 ° C. exceeded 650 MPa, the drawing was evaluated as “◯” when the drawing was 80% or more and less than 85%, and “◎” when 85% or more.
The results are shown in Tables 3 and 4.
試験片Ａ１ａ、Ａ１ｃ、Ａ２〜Ａ１８は、好ましい条件で最終熱処理および冷却を実施した供試材（発明例）である。
これらの相対絞り値は８０％以上でありながら、２５℃における大気中の引張強さは６５０ＭＰａ以上を示した。特に、耐水素脆化性の向上に大きな影響を及ぼすＮｉ、Ｃｕや平均冷却速度が本実施形態の好ましい範囲であるＡ１ａ、Ａ２〜Ａ６、Ａ８〜Ａ１７は、相対絞り値が８５％以上となり、耐水素脆化性が非常に優れる結果となった。
また、試験片Ａ３、Ａ４、Ａ６は１０３ＭＰａ水素中でも引張試験を実施したが、相対絞りは９０％以上であり、目標値の８０％を上回った。
Test pieces A1a, A1c, and A2 to A18 are specimens (invention examples) that were subjected to final heat treatment and cooling under preferable conditions.
Although these relative aperture values were 80% or more, the tensile strength in the atmosphere at 25 ° C. was 650 MPa or more. In particular, Ni, Cu and A1a, A2 to A6, and A8 to A17 whose average cooling rate has a great influence on the improvement of hydrogen embrittlement resistance are within a preferred range of this embodiment, and the relative aperture value is 85% or more, As a result, the hydrogen embrittlement resistance was extremely excellent.
The test pieces A3, A4, and A6 were subjected to a tensile test even in 103 MPa hydrogen, but the relative aperture was 90% or more, exceeding 80% of the target value.
試験片Ａ１ｂは、最終熱処理後の冷却速度が本発明の範囲を満たしていない。その結果、最終熱処理後の冷却時、供試材中に析出物が析出せず、析出強化の効果を得ることができなかったため、室温・大気中の引張強さが６５０ＭＰａを下回った。 In the test piece A1b, the cooling rate after the final heat treatment does not satisfy the scope of the present invention. As a result, at the time of cooling after the final heat treatment, precipitates did not precipitate in the test material, and the effect of precipitation strengthening could not be obtained, so the tensile strength at room temperature and in the air was below 650 MPa.
試験片Ｂ１は、Ｎｉ量が本発明の範囲を下回る。その結果、耐水素脆化特性が不足し、相対絞り値が５９％となった。 As for test piece B1, Ni amount is less than the range of this invention. As a result, the hydrogen embrittlement resistance was insufficient, and the relative aperture value was 59%.
試験片Ｂ２は、Ｃｕ量が本発明の範囲を上回る。その結果、オーステナイト相の強度が低下し、２５℃における大気中の引張強さが目標値である６５０ＭＰａを下回った。 In the test piece B2, the amount of Cu exceeds the range of the present invention. As a result, the strength of the austenite phase was lowered, and the tensile strength in the atmosphere at 25 ° C. was lower than the target value of 650 MPa.
試験片Ｂ３は、Ｓｉ量が本発明の範囲を上回る。その結果、耐水素脆化特性が不足し、相対絞り値が６８．８％となった。 In the test piece B3, the Si amount exceeds the range of the present invention. As a result, the hydrogen embrittlement resistance was insufficient, and the relative aperture value was 68.8%.
試験片Ｂ４は、Ｃｒ量が本発明の範囲を上回る。その結果、本発明の範囲を上回る量の析出物が析出した結果、水素ガス脆化感受性が高くなり、耐水素脆化特性が不足し、相対絞り値が６１．５％となった。 The specimen B4 has a Cr amount exceeding the range of the present invention. As a result, as a result of depositing an amount of precipitate exceeding the range of the present invention, the hydrogen gas embrittlement sensitivity was increased, the hydrogen embrittlement resistance was insufficient, and the relative aperture value was 61.5%.
試験片Ｂ５は、Ｍｎ量が本発明の範囲を上回る。その結果、耐水素脆化特性が不足し、相対絞り値が７１．３％となった。 Test piece B5 has an Mn content that exceeds the range of the present invention. As a result, the hydrogen embrittlement resistance was insufficient, and the relative aperture value was 71.3%.
試験片Ｂ６は、Ｃｒ量が本発明の範囲を下回る。その結果、オーステナイト相安定度が低下したことで耐水素脆化特性が不足し、相対絞り値が７７．５％となった。 As for test piece B6, Cr amount is less than the range of this invention. As a result, the stability of the austenite phase was lowered, resulting in insufficient hydrogen embrittlement resistance and a relative aperture value of 77.5%.
試験片Ｂ７は、Ｎ量が本発明の範囲を下回る。その結果、オーステナイト相の強度が低下し、２５℃における大気中の引張強さが目標値である６５０ＭＰａを下回った。 As for test piece B7, N amount is less than the range of this invention. As a result, the strength of the austenite phase was lowered, and the tensile strength in the atmosphere at 25 ° C. was lower than the target value of 650 MPa.
本発明のオーステナイト系ステンレス鋼は、４０ＭＰａ超の高圧の水素ガス中で極めて優れた耐水素脆化特性と６５０ＭＰａ超の引張強さが得られる。このため、本発明のオーステナイト系ステンレス鋼は、圧力が４０ＭＰａを超える水素ガスを貯蔵する高圧水素ガス用タンク、高圧水素用ガスタンクライナー、高圧水素ガス用熱交換器、高圧水素ガスおよび液体水素用配管の材料として適用可能である。 The austenitic stainless steel of the present invention has extremely excellent hydrogen embrittlement resistance and tensile strength of over 650 MPa in a high-pressure hydrogen gas of over 40 MPa. For this reason, the austenitic stainless steel of the present invention includes a high-pressure hydrogen gas tank that stores hydrogen gas having a pressure exceeding 40 MPa, a high-pressure hydrogen gas tank liner, a high-pressure hydrogen gas heat exchanger, a high-pressure hydrogen gas, and a liquid hydrogen pipe. It is applicable as a material.
Claims (6)
析出物の平均サイズが１００ｎｍ以下であり、かつ析出物の量が質量％で０．００１％以上１．０％以下であることを特徴とする耐水素脆化特性に優れた高強度オーステナイト系ステンレス鋼。 In mass%, C: 0.2% or less, Si: 0.2-1.5%, Mn: 0.5-2.5%, P: 0.06% or less, S: 0.008% or less, Ni: 10.0-20.0%, Cr: 16.0-25.0%, Mo: 3.5% or less, Cu: 3.5% or less, N: 0.01 to 0.50%, O : 0.015% or less, the balance being Fe and inevitable impurities,
High-strength austenitic stainless steel excellent in hydrogen embrittlement resistance, characterized in that the average size of precipitates is 100 nm or less and the amount of precipitates is 0.001% to 1.0% by mass steel.
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