JP2019073783A - Nb-CONTAINING FERRITIC STAINLESS STEEL MATERIAL EXCELLENT IN CORROSION RESISTANCE, AND MANUFACTURING METHOD - Google Patents
Nb-CONTAINING FERRITIC STAINLESS STEEL MATERIAL EXCELLENT IN CORROSION RESISTANCE, AND MANUFACTURING METHOD Download PDFInfo
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- 239000010959 steel Substances 0.000 claims abstract description 51
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 24
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 23
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 18
- 239000000203 mixture Substances 0.000 claims abstract description 17
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- 229910052720 vanadium Inorganic materials 0.000 claims abstract description 4
- 229910052726 zirconium Inorganic materials 0.000 claims abstract description 4
- 229910052796 boron Inorganic materials 0.000 claims abstract description 3
- 238000010438 heat treatment Methods 0.000 claims description 39
- 238000004833 X-ray photoelectron spectroscopy Methods 0.000 claims description 31
- 239000012298 atmosphere Substances 0.000 claims description 27
- 239000002344 surface layer Substances 0.000 claims description 23
- 238000000034 method Methods 0.000 claims description 20
- 229910004298 SiO 2 Inorganic materials 0.000 claims description 19
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 14
- 239000007789 gas Substances 0.000 claims description 13
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- 239000012535 impurity Substances 0.000 claims description 2
- 229910052715 tantalum Inorganic materials 0.000 claims description 2
- 125000004435 hydrogen atom Chemical class [H]* 0.000 claims 1
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 abstract description 3
- 230000006872 improvement Effects 0.000 description 16
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- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 3
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- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
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- 241000694440 Colpidium aqueous Species 0.000 description 1
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 1
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- 235000014113 dietary fatty acids Nutrition 0.000 description 1
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- 229930195729 fatty acid Natural products 0.000 description 1
- 239000000194 fatty acid Substances 0.000 description 1
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
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Abstract
Description
本発明は、表面に耐食性の良好な酸化皮膜を形成したＮｂ含有フェライト系ステンレス鋼材、およびその製造方法に関する。 The present invention relates to a Nb-containing ferritic stainless steel material having an oxide film with good corrosion resistance formed on the surface, and a method of manufacturing the same.
Ｎｂを含有するフェライト系ステンレス鋼は従来から種々の用途で使用されている。例えば、ＪＩＳ Ｇ４３０５：２０１２に規定されているＮｂ含有フェライト系ステンレス鋼種として、ＳＵＳ４３０ＬＸ、ＳＵＳ４３０Ｊ１Ｌ，ＳＵＳ４３６Ｌ、ＳＵＳ４３６Ｊ１Ｌ、ＳＵＳ４４３Ｊ１、ＳＵＳ４４４などがあり、広く普及するに至っている。 Nb-containing ferritic stainless steels are conventionally used in various applications. For example, there are SUS430LX, SUS430J1L, SUS436L, SUS436J1L, SUS443J1, and SUS444 as Nb-containing ferritic stainless steels specified in JIS G4305: 2012, and have come into widespread use.
フェライト系ステンレス鋼に添加されるＮｂの作用として、従来一般的に、Ｃ、Ｎを固定・安定化させることによる粒界腐食の抑制作用や、Ｎｂの固溶強化、析出強化による常温強度あるいは高温強度の向上作用が知られている。また、本出願人は最近の研究により、Ｎｂ含有析出物を利用して結晶配向を適正化することにより、ｒ値および耐リジング性を同時に向上させることが可能になることを知見し、その具体的手法を特願２０１７−０６３８８２にて開示した。 As the action of Nb added to a ferritic stainless steel, the action of suppressing intergranular corrosion by fixing and stabilizing C and N, the solid solution strengthening of Nb, normal temperature strength by precipitation strengthening, and high temperature have been generally performed. It is known to improve the strength. In addition, the applicant has found from recent research that by optimizing the crystal orientation by using Nb-containing precipitates, it is possible to simultaneously improve the r value and the ridging resistance. Japanese Patent Application No. 2017-063882 discloses a technical method.
一方、ステンレス鋼の耐食性は、よく知られているようにＣｒが濃化した薄い酸化皮膜（不動態被膜）によって発揮される。単体の金属として比べた場合、Ｎｂは本来、Ｃｒよりも不動態となる電位−ｐＨ領域が広く、かつ耐食性に優れた不動態皮膜を形成する金属である。このようなＮｂの特性をうまく引き出すことができれば、ステンレス鋼表面の酸化皮膜の耐食性機能を更に強化することが可能になるのではないかと期待される。 On the other hand, the corrosion resistance of stainless steel is exerted by a thin oxide film (passive film) enriched with Cr as is well known. When compared as a single metal, Nb is a metal that forms a passive film having a wider potential-pH range, which is intrinsically passive than Cr, and excellent corrosion resistance. It is expected that it will be possible to further enhance the corrosion resistance function of the oxide film on the surface of stainless steel if it is possible to bring out such properties of Nb well.
Ｎｂを含有するフェライト系ステンレス鋼に適用可能な表面皮膜の改善技術として、特定の雰囲気中で熱処理する手法が知られている。
例えば、特許文献１、２には、Ｎ2を含有する減圧雰囲気、あるいはＨ2雰囲気を採用することにより、表面にＣｒ、Ｓｉ、Ｎｂ、Ａｌ、Ｔｉに富む酸化皮膜を形成し、これにより排ガス凝縮水環境（特許文献１）、あるいは高温酸性脂肪酸環境（特許文献２）での耐食性を改善する技術が記載されている（特許文献１段落００２７〜００３２、特許文献２段落００２１〜００２８）。具体例として、いずれの文献にも１０-1〜１０-2ｔｏｒｒのＮ2雰囲気中で１１００℃１０分加熱する処理や、露点−６５℃の１００％Ｈ2中で１１００℃１０分加熱する処理が示されている（特許文献１段落００６３、特許文献２段落００５８）。これらの文献ではＸＰＳ（Ｘ線光電子分光分析法）により表面皮膜の分析を行っているが（特許文献１段落００６６、特許文献２段落００６５）、深さ方向の元素濃度プロファイルに関する開示はない。
As a technique for improving the surface film applicable to Nb-containing ferritic stainless steel, a method of heat treatment in a specific atmosphere is known.
For example, in Patent Documents 1 and 2 , an oxide film rich in Cr, Si, Nb, Al, and Ti is formed on the surface by adopting a reduced pressure atmosphere containing N 2 or an H 2 atmosphere, and thereby an exhaust gas Techniques for improving the corrosion resistance in a condensed water environment (Patent Document 1) or a high temperature acidic fatty acid environment (Patent Document 2) have been described (Patent Document 1 paragraphs 0027-0032, Patent Document 2 paragraphs 0021-0028). As a specific example, in any document, a treatment of heating at 1100 ° C. for 10 minutes in a N 2 atmosphere of 10 −1 to 10 −2 torr or a treatment of heating at 1100 ° C. for 10 minutes in 100% H 2 of dew point −65 ° C. (Patent Document 1 paragraph 0063, Patent Document 2 paragraph 0058). In these documents, the surface film is analyzed by XPS (X-ray photoelectron spectroscopy) (Patent Document 1 paragraph 0066, Patent Document 2 paragraph 0065), but there is no disclosure about the element concentration profile in the depth direction.
特許文献３には、Ｓｎを添加したフェライト系ステンレス鋼板に、水素ガスあるいは水素と窒素の混合ガス中８００〜１０００℃で光輝焼鈍を施すことにより、ＣｒやＳｎの濃度が高い表面皮膜を形成し、これにより耐銹性を向上させる技術が記載されている（段落００５６〜００５８）。その際、Ｎｂを含有する鋼を用いた場合には表面皮膜にＮｂが濃化され、耐銹性を高めるうえで効果的であることが教示されている（段落００５９）。この文献でもＸ線光電子分光分析法により表面皮膜を調べているが、「非破壊分析」（段落００６７）とあることから、この分析は最表面を測定するものであり、スパッタリングによって深さ方向に掘り進んで元素濃度プロファイルを調べるものではない。 According to Patent Document 3, a surface film having a high concentration of Cr and Sn is formed by subjecting a ferritic stainless steel plate to which Sn is added to bright annealing at 800 to 1000 ° C. in hydrogen gas or a mixed gas of hydrogen and nitrogen. Thus, techniques for improving weather resistance have been described (paragraphs 0056 to 0058). At that time, it is taught that when a Nb-containing steel is used, Nb is enriched in the surface film, which is effective in enhancing the weather resistance (paragraph 0059). Although this document also examines the surface film by X-ray photoelectron spectroscopy, this analysis is to measure the outermost surface because it is “nondestructive analysis” (paragraph 0067), and it is in the depth direction by sputtering. It does not dig in and investigate the element concentration profile.
上述のようにＮｂ含有フェライト系ステンレス鋼は、Ｎｂの有用な作用を利用して種々の用途に使用されている。また、Ｎｂ等の元素が濃化した酸化皮膜は、フェライト系ステンレス鋼の耐食性改善に有効であることが知られている（特許文献１〜３）。しかしながら発明者らの研究によれば、特許文献１〜３に示されるようなＮｂ等が最表面付近に濃化していると考えられる皮膜では、Ｎｂ自体が持つ本来の耐食性向上作用が十分に発揮できていないことがわかってきた。すなわち、Ｎｂ添加により耐食性を向上させる手法には、更なる改善の余地があると考えられる。 As mentioned above, Nb-containing ferritic stainless steels are used in various applications utilizing the useful action of Nb. Moreover, it is known that the oxide film which elements, such as Nb, concentrated, is effective in corrosion-resistant improvement of ferritic stainless steel (patent documents 1-3). However, according to the inventors' research, in a film in which Nb or the like shown in Patent Documents 1 to 3 is considered to be concentrated near the outermost surface, the inherent corrosion resistance improvement function possessed by Nb itself is sufficiently exhibited. It turned out that I was not able to do it. That is, it is considered that there is room for further improvement in the method of improving the corrosion resistance by the addition of Nb.
本発明は、Ｎｂ含有フェライト系ステンレス鋼材の耐食性、特に塩化物に対する耐孔食性を安定して向上させる技術を開示しようというものである。 The present invention is intended to disclose a technique for stably improving the corrosion resistance of a Nb-containing ferritic stainless steel material, in particular the pitting corrosion resistance to chlorides.
上記課題は、フェライト系ステンレス鋼表面の酸化皮膜において、最表面ではなく、一定の深さ領域にＮｂとＣｒが濃化した皮膜構造とすることによって達成される。また、そのような皮膜構造は、Ｃｒが還元され、かつＮｂが酸化される雰囲気中で熱処理を行って最表面近傍にＮｂを濃化させ、その後、低温の酸化性雰囲気中で酸化皮膜を安定化させることにより実現することができる。 The above object is achieved by forming a film structure in which Nb and Cr are concentrated not in the outermost surface but in a certain depth region in the oxide film on the surface of a ferritic stainless steel. In addition, such a film structure performs heat treatment in an atmosphere in which Cr is reduced and Nb is oxidized to concentrate Nb near the outermost surface, and thereafter the oxide film is stabilized in a low temperature oxidizing atmosphere. It can be realized by
本明細書では以下の発明を開示する。
［１］質量％で、Ｃ：０.００１〜０.０８０％、Ｓｉ：０.０５〜１.５０％、Ｍｎ：０.０５〜１.５０％、Ｐ：０.００１〜０.０８０％、Ｓ：０.０００１〜０.０１０％、Ｎｉ：０.０１〜０.６０％、Ｃｒ：１１.０〜２５.０％、Ｃｕ：０〜２.００％、Ｍｏ：０〜３.００％、Ｎ：０.００１〜０.０８０％、Ｎｂ：０.１０〜０.８０％、Ｔｉ：０〜０.５０％、Ｚｒ：０〜０.５０％、Ｖ：０〜０.５０％、Ｔａ：０〜０.５０％、Ｓｎ：０〜０.５０％、Ｂ：０〜０.０１０％、ＲＥＭ（希土類元素）：合計０〜０.１０％、Ａｌ：０〜０.２０％、残部Ｆｅおよび不可避的不純物である化学組成を有し、表面に酸化皮膜を有する鋼材であって、ＸＰＳ（Ｘ線光電子分光分析法）による深さ方向の元素濃度プロファイルにおいて、前記酸化皮膜の最表面からのＳｉＯ2換算深さが５〜３０ｎｍの領域にＮｂおよびＣｒのピークがあり、前記Ｎｂのピーク位置でのＮｂ濃度が、Ｆｅ、Ｃｒ、Ｍｏ、Ｎｉ、Ｃｕ、Ｍｎ、Ｓｉ、Ｔｉ、Ｎｂ、Ａｌの合計に占めるモル分率で１０ｍｏｌ％以上であるフェライト系ステンレス鋼材。
［２］前記酸化皮膜厚さが１０〜４０ｎｍである上記［１］に記載のフェライト系ステンレス鋼材。
ここで、酸化皮膜厚さは、前記最表面からのＳｉＯ2換算深さにおいてＯ濃度が最表面の１／２となる深さとする。
［３］酸洗表面または酸洗後に冷間圧延を施した表面を有する鋼材に、Ｃｒが還元されＮｂが酸化される雰囲気中で熱処理を施すことにより、少なくとも最表面から５ｎｍ未満までの深さ領域にＮｂが濃化しているＮｂ濃化表層を形成させる工程（Ｎｂ濃化表層形成工程）、
前記Ｎｂ濃化表層を有する鋼材に、４５０℃以下の酸化性雰囲気中で熱処理を施すことにより最表面から５〜３０ｎｍの深さ領域にＣｒ濃化層およびＮｂ濃化層を形成させる工程（酸化皮膜安定化工程）、
を有する上記［１］または［２］に記載のフェライト系ステンレス鋼材の製造方法。
ここで、最表面からの深さは真空中でのイオンスパッタリング時間により定まるＳｉＯ2換算深さである。
［４］Ｎｂ濃化表層形成工程において、ガス組成：水素９５〜１００体積％、窒素５〜０体積％、温度：１０５０〜１２００℃の熱処理雰囲気を採用する上記［２］または［３］に記載のフェライト系ステンレス鋼材の製造方法。
［５］酸化皮膜安定化工程において、大気中、２００〜４５０℃の熱処理雰囲気を採用する上記［３］または［４］に記載のフェライト系ステンレス鋼材の製造方法。
The following invention is disclosed in the present specification.
[1] By mass%, C: 0.001 to 0.080%, Si: 0.05 to 1.50%, Mn: 0.05 to 1.50%, P: 0.001 to 0.080% S: 0.0001 to 0.010%, Ni: 0.01 to 0.60%, Cr: 11.0 to 25.0%, Cu: 0 to 2.00%, Mo: 0 to 3.00 %, N: 0.001 to 0.080%, Nb: 0.10 to 0.80%, Ti: 0 to 0.50%, Zr: 0 to 0.50%, V: 0 to 0.50% , Ta: 0 to 0.50%, Sn: 0 to 0.50%, B: 0 to 0.010%, REM (rare earth element): 0 to 0.10% in total, Al: 0 to 0.20% A steel material having a chemical composition which is the balance Fe and an unavoidable impurity, and having an oxide film on the surface, in the element concentration profile in the depth direction by XPS (X-ray photoelectron spectroscopy), From the surface There are Nb and Cr peaks in the region of 5 to 30 nm in depth equivalent to SiO 2 , and the Nb concentration at the peak position of Nb is Fe, Cr, Mo, Ni, Cu, Mn, Si, Ti, Nb, Al A ferritic stainless steel material having a mole fraction of at least 10 mol% in the total of.
[2] The ferritic stainless steel material according to the above [1], wherein the thickness of the oxide film is 10 to 40 nm.
Here, the oxide film thickness is such a depth that the O concentration is 1/2 of the outermost surface in the SiO 2 converted depth from the outermost surface.
[3] A steel material having a pickled surface or a surface subjected to cold rolling after pickling is heat-treated in an atmosphere in which Cr is reduced and Nb is oxidized to a depth of at least 5 nm from the outermost surface Forming an Nb-concentrated surface layer enriched in Nb in the region (Nb-concentrated surface layer formation step),
A step of forming a Cr-concentrated layer and a Nb-concentrated layer in a depth region of 5 to 30 nm from the outermost surface by heat treating the steel material having the Nb-concentrated surface layer in an oxidizing atmosphere of 450 ° C. or less Film stabilization process),
The manufacturing method of the ferritic stainless steel materials as described in said [1] or [2] which has these.
Here, the depth from the outermost surface is the equivalent SiO 2 depth determined by the ion sputtering time in vacuum.
[4] In the Nb-concentrated surface layer forming step, the heat treatment atmosphere of gas composition: hydrogen 95 to 100% by volume, nitrogen 5 to 0% by volume, temperature: 1050 to 1200 ° C. is employed, the above [2] or [3] Manufacturing method of ferritic stainless steel material.
[5] The method for producing a ferritic stainless steel material according to the above [3] or [4], wherein a heat treatment atmosphere at 200 to 450 ° C. in air is employed in the oxide film stabilization step.
ここで、ＲＥＭ（希土類元素）は、Ｓｃ、Ｙ、ランタノイド系元素およびアクチノイド系元素を意味する。 Here, REM (rare earth element) means Sc, Y, a lanthanoid element and an actinide element.
本発明によれば、Ｎｂを含有するフェライト系ステンレス鋼材の耐食性を顕著に向上させることが可能となる。ステンレス鋼の耐食性レベルはＣｒをはじめとする合金成分の配合組成によって相違するが、それぞれのＮｂ含有フェライト系ステンレス鋼種において、その耐食性を引き上げることができる。 According to the present invention, the corrosion resistance of a ferritic stainless steel material containing Nb can be significantly improved. Although the corrosion resistance level of stainless steel differs depending on the composition of alloy components including Cr, the corrosion resistance of each Nb-containing ferritic stainless steel can be raised.
〔鋼組成〕
本発明では、Ｎｂを含有するフェライト系ステンレス鋼を対象とする。以下、鋼の成分元素に関する「％」は特に断らない限り「質量％」を意味する。
[Steel composition]
The present invention is directed to a ferritic stainless steel containing Nb. Hereinafter, “%” relating to the component elements of steel means “mass%” unless otherwise specified.
Ｎｂは、上述のように、Ｃ、Ｎを固定・安定化させることによる粒界腐食の抑制作用や、Ｎｂの固溶強化、析出強化による常温強度あるいは高温強度の向上作用を発揮する元素であり、フェライト系ステンレス鋼には従来から必要に応じて添加されている。また、Ｎｂ含有析出物を利用して結晶配向を適正化することにより、ｒ値および耐リジング性を同時に向上させることが可能になる。本発明ではこれらの作用に加えて、Ｎｂを耐食性向上元素として活用する。後述するように、Ｎｂが所定深さに濃縮した構造の表面酸化皮膜を形成させたとき、特に塩化物に対する耐食性を顕著に向上させることができるのである。これらの作用を十分に発揮させるため、ここではＮｂ含有量が０.２０％以上のフェライト系ステンレス鋼種を対象とする。ただし、過剰のＮｂ含有は靭性を低下させる要因となる。種々検討の結果、Ｎｂ含有量は０.８０％以下に制限される。０.７０％以下に管理してもよい。 Nb is an element that exerts the action of suppressing intergranular corrosion by fixing and stabilizing C and N, solid solution strengthening of Nb, and improving normal temperature strength or high temperature strength by precipitation strengthening as described above. The ferritic stainless steel is conventionally added as needed. In addition, by optimizing the crystal orientation using the Nb-containing precipitate, it is possible to simultaneously improve the r value and the ridging resistance. In the present invention, in addition to these effects, Nb is utilized as a corrosion resistance improving element. As described later, when a surface oxide film having a structure in which Nb is concentrated to a predetermined depth is formed, the corrosion resistance to, in particular, chloride can be remarkably improved. In order to exert these effects sufficiently, here, ferritic stainless steels having an Nb content of 0.20% or more are targeted. However, excessive Nb content causes the toughness to be reduced. As a result of various studies, the Nb content is limited to 0.80% or less. You may manage to 0.70% or less.
Ｃ、Ｎは、Ｎｂとの複合添加において、Ｎｂ炭窒化物を形成する。Ｎｂ炭窒化物はｒ値の向上に有利な集合組織の形成に寄与する。また、鋼板製造過程でコロニーの形成を抑制し、耐リジング性の向上にも寄与する。一方、粒界腐食を抑制する観点からは、Ｃ、Ｎ含有量は低いことが好ましいが、極度の低Ｃ化、低Ｎ化には製鋼でのコスト上昇を伴う。また、多量のＣ、Ｎ含有は鋼を硬質化させ、熱延鋼板の靱性を低下させる要因となる。固溶Ｎｂによる高温強度の向上を狙う場合には、十分な固溶Ｎｂ量を確保するために、鋼中のＮｂが炭窒化物として過剰に消費されないよう、Ｎｂ含有量に応じてＣ、Ｎ含有量を調整する必要がある。このように、Ｃ、Ｎ含有量は目的に応じて種々調整される。ここでは、Ｃ含有量０.００１〜０.０８０％、Ｎ含有量０.００１〜０.０８０％の鋼を対象とする。Ｃ含有量は０.０５０％以下、あるいは０.０３０％以下に制限してもよい。Ｎ含有量についても０.０５０％以下、あるいは０.０３０％以下に制限してもよい。 C and N form Nb carbonitride in complex addition with Nb. Nb carbonitrides contribute to the formation of texture which is advantageous for the improvement of the r value. Moreover, formation of a colony is suppressed in a steel plate manufacturing process, and it contributes also to the improvement of ridging resistance. On the other hand, from the viewpoint of suppressing intergranular corrosion, the C and N contents are preferably low, but the extreme reduction of C and reduction of N involve cost increase in steel making. In addition, a large amount of C and N contained hardens the steel and causes the toughness of the hot-rolled steel sheet to decrease. When aiming at improvement of high temperature strength by solid solution Nb, C, N according to Nb content so that Nb in steel is not consumed excessively as carbonitride in order to secure sufficient solid solution Nb amount. It is necessary to adjust the content. Thus, the C and N contents are variously adjusted according to the purpose. Here, steels having a C content of 0.001 to 0.080% and an N content of 0.001 to 0.080% are targeted. The C content may be limited to 0.050% or less, or to 0.030% or less. The N content may also be limited to 0.050% or less, or to 0.030% or less.
ＳｉおよびＭｎは、脱酸剤として有効である他、耐高温酸化性を向上させる作用を有する。Ｓｉについては０.０５％以上、Ｍｎについては０.０５％以上の含有量を確保することがより効果的である。これらの元素の含有量が多くなると鋼が硬質化し、加工性および靭性が低下する場合がある。Ｓｉ含有量は１.５０％以下に制限され、１.００％以下とすることがより好ましい。Ｍｎ含有量も１.５０％以下に制限される。 Si and Mn are effective as a deoxidizer and also have an effect of improving high-temperature oxidation resistance. It is more effective to secure a content of 0.05% or more for Si and 0.05% or more for Mn. When the content of these elements is increased, the steel may be hardened and the workability and toughness may be reduced. The Si content is limited to 1.50% or less, more preferably 1.00% or less. The Mn content is also limited to 1.50% or less.
ＰおよびＳは、多量に含有すると耐食性低下などの要因となる。Ｐ含有量は０.０８０％まで許容でき、Ｓ含有量は０.０１０％まで許容できる。過剰な低Ｐ化、低Ｓ化は製鋼への負荷を増大させ不経済となる。Ｐ含有量は、例えば０.００１％以上、Ｓ含有量は例えば０.０００１％以上の範囲で調整すればよい。 When P and S are contained in a large amount, they cause factors such as deterioration in corrosion resistance. P content is acceptable up to 0.080% and S content is acceptable up to 0.010%. Excessive reduction of P and reduction of S increase the load on steelmaking and become uneconomical. The P content may be adjusted, for example, in the range of 0.001% or more, and the S content, for example, in the range of 0.0001% or more.
Ｎｉは、腐食の進行を抑制する作用があり、必要に応じて添加することができる。その場合、０.０１質量％以上のＮｉ含有量を確保することがより効果的である。ただし、Ｎｉはオーステナイト相安定化元素であるため、フェライト系ステンレス鋼に過剰に含有させるとマルテンサイト相を生成し、加工性が低下する。また、過剰のＮｉ含有はコスト上昇を招く。ここではＮｉ含有量を０.６％以下に制限する。 Ni has the effect of suppressing the progress of corrosion, and can be added as needed. In that case, it is more effective to secure the Ni content of 0.01% by mass or more. However, since Ni is an austenite phase stabilizing element, if it is excessively contained in a ferritic stainless steel, a martensitic phase is generated and the workability is reduced. In addition, excessive Ni content causes cost increase. Here, the Ni content is limited to 0.6% or less.
Ｃｒは、ステンレス鋼としての耐食性を確保するために重要である。耐高温酸化性の向上にも有効である。これらの作用を発揮させるために、１１.０％以上のＣｒ含有量が必要である。多量にＣｒを含有すると加工性が低下して問題となる場合がある。ここではＣｒ含有量が２５.０％以下の鋼を対象とする。Ｃｒ含有量は２０.０％以下に管理してもよい。 Cr is important to ensure corrosion resistance as stainless steel. It is also effective in the improvement of high temperature oxidation resistance. In order to exert these effects, a Cr content of 11.0% or more is required. If a large amount of Cr is contained, the processability may be reduced to cause a problem. Here, steels with a Cr content of 25.0% or less are targeted. The Cr content may be controlled to 20.0% or less.
Ｃｕは、低温靱性の向上に有効であると共に、高温強度の向上にも有効な元素であり、必要に応じて添加することができる。上記作用を得るためには０.０１％以上のＣｕ含有量を確保することがより効果的である。ただし、多量にＣｕを添加すると加工性がむしろ低下するようになる。Ｃｕ含有量は２.００％以下に制限される。 Cu is an element effective for improving the low temperature toughness and also for improving the high temperature strength, and can be added as needed. In order to obtain the above-mentioned action, it is more effective to secure a Cu content of 0.01% or more. However, if Cu is added in a large amount, the processability is rather lowered. The Cu content is limited to 2.00% or less.
Ｍｏは、耐食性の向上に有効な元素であり、必要に応じて添加することができる。Ｍｏを添加する場合は、０.０１％以上の含有量を確保することがより効果的である。ただし、多量の添加は、鋼の熱間加工性、加工性や靭性を低下させるとともに、製造コストの上昇につながる。Ｍｏ含有量は３.００％以下とする。 Mo is an element effective for improving the corrosion resistance, and can be added as needed. When adding Mo, it is more effective to secure a content of 0.01% or more. However, a large amount of addition lowers the hot workability, workability and toughness of the steel and leads to an increase in manufacturing cost. The Mo content is 3.00% or less.
Ｔｉは、ＣおよびＮを固定する作用があり、鋼の耐食性および耐高温酸化性を高く維持する上で有効な元素である。そのため必要に応じてＴｉを添加することができる。上記作用を十分に発揮させるためには０.０５％以上のＴｉ含有量を確保することがより効果的である。ただし、過剰のＴｉ含有は鋼の靭性や加工性を低下させるとともに、製品の表面性状に悪影響を及ぼす。Ｔｉを添加する場合は０.５０％以下の範囲で行う。 Ti functions to fix C and N, and is an element effective to maintain high corrosion resistance and high temperature oxidation resistance of the steel. Therefore, Ti can be added as needed. It is more effective to secure 0.05% or more of Ti content in order to fully exhibit the above-mentioned effect. However, excessive Ti content reduces the toughness and workability of the steel and adversely affects the surface properties of the product. When adding Ti, it carries out in 0.50% or less of range.
Ｚｒ、Ｖ、Ｔａ、Ｓｎは、耐食性向上に有効であることから、必要に応じてこれらの１種以上を添加することができる。その場合、Ｚｒ：０.０５％以上、Ｖ：０.０５％以上、Ｔａ：０.０５％以上、Ｓｎ：０.０５％以上の含有量を確保して、これらの１種以上を添加することがより効果的である。ただし、これらの元素を過剰に添加すると、靭性低下やコスト上昇を招く。これらの元素の１種以上を添加する場合は、いずれも０.５０％以下の含有量範囲とすることが望ましい。 Since Zr, V, Ta, and Sn are effective in improving the corrosion resistance, one or more of these can be added as needed. In that case, the content of Zr: 0.05% or more, V: 0.05% or more, Ta: 0.05% or more, Sn: 0.05% or more is secured, and one or more of these are added. Is more effective. However, if these elements are added in excess, the toughness will decrease and the cost will increase. In the case of adding one or more of these elements, it is preferable that the content range of each is 0.50% or less.
Ｂは、２次加工性向上に有効であり、必要に応じて添加することができる。その場合、０.０００２％以上の含有量を確保することがより効果的である。ただし、Ｂ含有量が多くなるとＣｒ2Ｂの生成により金属組織の均一性が損なわれ、加工性が低下する場合がある。Ｂを添加する場合は０.０１０％以下の含有量範囲で行う。 B is effective for improving the secondary processability, and can be added as needed. In that case, it is more effective to secure the content of 0.0002% or more. However, when the B content is increased, the uniformity of the metal structure may be impaired due to the formation of Cr 2 B, and the processability may be reduced. When adding B, it carries out in 0.010% or less of content range.
ＲＥＭ（希土類元素）は、溶接性および耐酸化性の改善に有効であり、必要に応じて添加することができる。その場合、ＲＥＭの合計含有量を０.０１０％以上確保することがより効果的である。過剰のＲＥＭ添加は連続鋳造時のノズル詰まりの原因になるなど、製造性を損なう要因となる。ＲＥＭを添加する場合は、その合計含有量が０.１０％以下となる範囲で行う。 REM (rare earth element) is effective for improving weldability and oxidation resistance, and can be added as needed. In that case, it is more effective to secure 0.010% or more of the total content of REM. Excessive addition of REM causes clogging of the nozzle during continuous casting and causes a loss of manufacturability. When adding REM, it carries out in the range which the total content becomes 0.10% or less.
Ａｌは、脱酸剤として有効である。その作用を十分に得るために、０.００１％以上のＡｌ含有量となるようにＡｌを添加することがより効果的である。過剰にＡｌを含有させると硬さが上昇し、加工性および靭性が低下する。Ａｌ含有量は０.２０％以下に制限され、０.１０％以下とすることがより好ましい。 Al is effective as a deoxidizer. It is more effective to add Al so as to have an Al content of 0.001% or more in order to sufficiently obtain the action. If the Al content is excessive, the hardness increases and the processability and toughness decrease. The Al content is limited to 0.20% or less, more preferably 0.10% or less.
〔酸化皮膜〕
ステンレス鋼の表面は通常、Ｃｒが濃化した薄い酸化皮膜（不動態皮膜）で覆われており、これによって耐食性が維持される。Ｎｂ含有フェライト系ステンレス鋼の場合、酸洗仕上げ（いわゆるＮｏ.２Ｄ仕上げ）ではＮｂはＣｒと異なり表面皮膜中に濃化しないが、水素ガス雰囲気中あるいは水素と窒素の混合ガス雰囲気中で熱処理すると最表面付近のごく浅い部分にＮｂの濃化が生じる。このような最表面付近にＮｂが濃化した酸化皮膜を形成させると、酸洗仕上げ（Ｎｂの濃化が認められない酸化皮膜を有する表面状態）と比べ、耐食性の改善が認められる。しかし、その改善効果は小さい。これに対し、後述のＡ処理によって、最表面付近のごく浅い部分ではなく、それより少し深い部分にＮｂが濃化し、かつＣｒも濃化している特異な構造を有する比較的厚い酸化皮膜を形成させたとき、特に塩化物に対する耐食性が大幅に向上することがわかった。
[Oxide film]
The surface of stainless steel is usually covered with a thin Cr-rich oxide film (passivation film), which maintains corrosion resistance. In the case of Nb-containing ferritic stainless steel, Nb does not concentrate in the surface film unlike Cr in pickling finish (so-called No. 2D finish), but heat treatment in hydrogen gas atmosphere or mixed gas atmosphere of hydrogen and nitrogen Nb enrichment occurs in a very shallow portion near the outermost surface. When an oxide film enriched with Nb is formed in the vicinity of such an outermost surface, the corrosion resistance is improved compared to pickling (a surface state having an oxide film in which no concentration of Nb is observed). However, the improvement effect is small. On the other hand, by the treatment A described later, a relatively thick oxide film having a unique structure in which Nb is enriched and Cr is also enriched in a portion slightly deeper than the outermost portion, not a very shallow portion near the outermost surface It was found that the corrosion resistance to chloride, in particular, is significantly improved.
上記特異な構造の酸化皮膜は、具体的には、ＸＰＳ（Ｘ線光電子分光分析法）による深さ方向の元素濃度プロファイルにおいて、前記酸化皮膜の最表面からのＳｉＯ2換算深さが５〜３０ｎｍの領域にＮｂおよびＣｒのピークがあり、前記Ｎｂのピーク位置でのＮｂ濃度が、Ｆｅ、Ｃｒ、Ｍｏ、Ｎｉ、Ｃｕ、Ｍｎ、Ｓｉ、Ｔｉ、Ｎｂ、Ａｌの合計に占めるモル分率で１０ｍｏｌ％以上である皮膜として特定される。また、ＸＰＳによる深さ方向の元素濃度プロファイルにおいて、最表面からのＳｉＯ2換算深さでＯ濃度が最表面の１／２となる深さまでを酸化皮膜と定義したとき、上記酸化皮膜厚さは例えば１０〜４０ｎｍである。前記プロファイルにおけるＮｂのピーク位置は、Ｃｒのピーク位置より浅い位置（最表面側）にある。ここで、「Ｎｂのピーク位置」は、上記深さ方向の元素濃度プロファイルにおいて、Ｓｉ換算深さ５〜３０ｎｍの範囲でＮｂ濃度が最も高いＳｉＯ2換算深さ位置を意味する。同様に「Ｃｒのピーク位置」は、上記深さ方向の元素濃度プロファイルにおいて、Ｓｉ換算深さ５〜３０ｎｍの範囲でＣｒ濃度が最も高いＳｉＯ2換算深さ位置を意味する。 Specifically, in the element concentration profile in the depth direction by XPS (X-ray photoelectron spectroscopy), the oxide film having the above-described unique structure has a SiO 2 equivalent depth of 5 to 30 nm from the outermost surface of the oxide film. There is a peak of Nb and Cr in the region of Nb, and the Nb concentration at the peak position of Nb is 10 mol as a mole fraction of the total of Fe, Cr, Mo, Ni, Cu, Mn, Si, Ti, Nb, and Al. It is specified as a film which is% or more. In addition, in the element concentration profile in the depth direction by XPS, when the O concentration reaches a depth at which the O concentration becomes half of the outermost surface in terms of SiO 2 conversion depth from the outermost surface, the above oxide film thickness is For example, it is 10-40 nm. The peak position of Nb in the profile is at a position shallower (uppermost surface side) than the peak position of Cr. Here, the “peak position of Nb” means the SiO 2 converted depth position at which the Nb concentration is highest in the range of 5 to 30 nm in terms of Si in the element concentration profile in the depth direction. Similarly, “Cr peak position” means a SiO 2 converted depth position at which the Cr concentration is highest in the range of 5 to 30 nm in terms of Si conversion in the element concentration profile in the depth direction.
図１〜図３に、本発明に従うフェライト系ステンレス鋼材表面の酸化皮膜について、ＸＰＳ（Ｘ線光電子分光分析法）による深さ方向の元素濃度プロファイルを例示する。図１は後述実施例の試料Ｎｏ.１０、図２は同試料Ｎｏ.１３、図３は同試料Ｎｏ.１４である。それぞれの図において、Ｏ、Ｃｒの濃度プロファイルは上段のグラフに、Ｎｂの濃度プロファイルは中段のグラフに示してある（後述図４〜６において同じ）。グラフの横軸はＳｉＯ2換算深さ（ｎｍ）、縦軸は元素濃度（ｍｏｌ％）である。Ｎｂ、Ｃｒなどの金属元素Ｍの濃度は下記（１）式、酸素Ｏの濃度は下記（２）式によって定めることができる。
金属元素Ｍの濃度（ｍｏｌ％）＝Ｍ／（Ｆｅ＋Ｃｒ＋Ｍｏ＋Ｎｉ＋Ｃｕ＋Ｍｎ＋Ｓｉ＋Ｔｉ＋Ｎｂ＋Ａｌ）×１００ …（１）
酸素Ｏの濃度（ｍｏｌ％）＝Ｏ／（Ｏ＋Ｆｅ＋Ｃｒ＋Ｍｏ＋Ｎｉ＋Ｃｕ＋Ｍｎ＋Ｓｉ＋Ｔｉ＋Ｎｂ＋Ａｌ）×１００ …（２）
ここで、（１）式および（２）式の右辺の元素記号の箇所には、その元素の相対強度の値が代入される。Ｍは、Ｆｅ、Ｃｒ、Ｍｏ、Ｎｉ、Ｃｕ、Ｍｎ、Ｓｉ、Ｔｉ、Ｎｂ、Ａｌのいずれかである。
The element concentration profile of the depth direction by XPS (X-ray photoelectron spectroscopy) is illustrated about the oxide film on the surface of the ferritic stainless steel material according to this invention to FIGS. FIG. 1 shows Sample No. 10 in the Example described later, FIG. 2 shows the same Sample No. 13, and FIG. 3 shows the same Sample No. 14. In each figure, the concentration profiles of O and Cr are shown in the upper graph, and the concentration profiles of Nb are shown in the middle graph (the same in FIGS. 4 to 6 described later). The horizontal axis of the graph is the SiO 2 conversion depth (nm), and the vertical axis is the element concentration (mol%). The concentration of the metal element M such as Nb or Cr can be determined by the following equation (1), and the concentration of oxygen O can be determined by the following equation (2).
Concentration of metal element M (mol%) = M / (Fe + Cr + Mo + Ni + Cu + Mn + Si + Ti + Nb + Al) × 100 (1)
Concentration of oxygen O (mol%) = O / (O + Fe + Cr + Mo + Ni + Cu + Mn + Si + Ti + Nb + Al) × 100 (2)
Here, the value of the relative intensity of the element is substituted in the place of the element symbol on the right side of the equations (1) and (2). M is any of Fe, Cr, Mo, Ni, Cu, Mn, Si, Ti, Nb, and Al.
（ＸＰＳ分析方法）
ＸＰＳによる分析は、Ｐｈｙｓｉｃａｌ Ｅｌｅｃｔｒｏｎｉｃｓ社製、Ｑｕａｎｔｅｒａ ＳＸＭを用いて、最表面からＳｉＯ2換算深さ１００ｎｍまで行った。Ａｒイオンスパッタ速度は約１.１５ｎｍ／ｍｉｎである。分析条件は以下の通りである。
分析エリア：φ２００μｍ
Ｘ線：単色化ＡｌＫα
Ｘ線源の出力：４３.７Ｗ
分析角度：４５°
スペクトル線：Ｏ１ｓ、Ｆｅ２ｐ3/2、Ｃｒ２ｐ3/2、Ｍｏ３ｄ5/2、Ｎｉ２ｐ3/2、Ｃｕ２ｐ3/2、Ｍｎ２ｐ3/2、Ｓｉ２ｐ、Ｔｉ２ｐ3/2、Ｎｂ３ｄ5/2、Ａｌ２ｓ
バックグラウンド処理：ｓｈｉｒｌｅｙ法
(XPS analysis method)
The analysis by XPS was performed from the outermost surface to a SiO 2 conversion depth of 100 nm using Quantera SXM manufactured by Physical Electronics. The Ar ion sputtering rate is about 1.15 nm / min. The analysis conditions are as follows.
Analysis area: φ 200 μm
X-ray: monochromized AlKα
Output of X-ray source: 43.7 W
Analysis angle: 45 °
Spectral lines: O1s, Fe2p 3/2, Cr2p 3/2 , Mo3d 5/2, Ni2p 3/2, Cu2p 3/2, Mn2p 3/2, Si2p, Ti2p 3/2, Nb3d 5/2, Al2s
Background processing: Shirley method
本発明に従う酸化皮膜は、図１〜図３に見られるように、最表面付近のごく浅い部分ではなく、それより少し深い部分にＮｂとＣｒが濃化した構造を有している。具体的には、深さ方向プロファイルにおいて、前記酸化皮膜の最表面からのＳｉＯ2換算深さが５〜３０ｎｍの領域にＮｂおよびＣｒのピークがある。そして、そのＮｂのピーク位置での上記（１）式により定まるＮｂ濃度は１０ｍｏｌ％以上に調整されている。上記ＸＰＳ分析およびＥＤＸ分析（エネルギー分散型Ｘ線分析）の結果、このＮｂの濃化はＮｂ酸化物の形成によるものであり、上記ＣｒのピークはＣｒ酸化物の形成によるものである。また、最表面からのＳｉＯ2換算深さが５〜３０ｎｍの範囲に、Ｃｒの濃化領域（Ｃｒ濃度が鋼中の平均Ｃｒ濃度よりも高い領域）とＮｂ濃化領域（Ｎｂ濃度が鋼中の平均Ｎｂ濃度よりも高い領域）には重なり合う部分があるが、Ｎｂのピーク位置は、Ｃｒのピーク位置より浅い位置（最表面側）にある。このような構造の皮膜を形成させたときに耐食性が向上するメカニズムについては現時点で未解明であるが、通常の不動態皮膜よりも皮膜厚さが厚いこと、およびＮｂ酸化物層の言わば「後ろ盾」としてＣｒ酸化物層が存在することにより、Ｃｌイオンの侵入に対する保護作用が強化されたものと推察される。 The oxide film according to the present invention, as seen in FIGS. 1 to 3, has a structure in which Nb and Cr are concentrated at a portion slightly deeper than the very shallow portion near the outermost surface. Specifically, in the depth direction profile, there are peaks of Nb and Cr in a region where the SiO 2 equivalent depth from the outermost surface of the oxide film is 5 to 30 nm. Then, the Nb concentration determined by the above equation (1) at the peak position of Nb is adjusted to 10 mol% or more. As a result of the XPS analysis and EDX analysis (energy dispersive X-ray analysis), the enrichment of Nb is due to the formation of Nb oxide, and the peak of Cr is due to the formation of Cr oxide. In addition, the concentration area of Cr (area where the Cr concentration is higher than the average Cr concentration in the steel) and the Nb concentration area (Nb concentration in the steel) in the range of 5 to 30 nm in SiO 2 conversion depth from the outermost surface Although there is an overlapping portion in the region higher than the average Nb concentration of Nb, the peak position of Nb is at a position shallower (uppermost surface side) than the peak position of Cr. Although the mechanism by which the corrosion resistance is improved when forming a film having such a structure is unknown at present, the thickness of the film is thicker than that of the ordinary passive film, and the so-called “back of Nb oxide layer” It is inferred that the presence of the Cr oxide layer as a "shield" enhances the protective action against the penetration of Cl ions.
なお、発明者らの調査によれば、上記のような構造の皮膜において、Ｎｂ濃化領域は、浅い部分（最表面側）にＮｂ2Ｏ5が多く、深い部分（鋼材の肉厚中心側）にＮｂＯが多い傾斜組成を有することがわかった。そして、ＮｂＯが比較的多い部分（Ｎｂ濃化領域の比較的深い部分）に、Ｃｒ酸化物が多く存在する。 According to the investigation of the inventors, in the film having the above-described structure, the Nb-concentrated region is rich in Nb 2 O 5 in the shallow portion (the outermost surface side) and the deep portion (the thickness center side of the steel) ) Was found to have a gradient composition rich in NbO. Then, a large amount of Cr oxide is present in the relatively large portion of NbO (the relatively deep portion of the Nb-rich region).
〔製造方法〕
従来一般的なフェライト系ステンレス鋼板の製造プロセスを利用して、所定の板厚に調整された焼鈍酸洗鋼板（酸洗後に調質圧延が施されたものである場合を含む。）、または酸洗後に冷間圧延が施された冷延鋼板を、中間製品として用意する。継目なし鋼管の場合は通常の鋼管製造プロセスで得られた酸洗済みの鋼管を、中間製品とすればよい。また、上記の鋼板または鋼管からなる中間製品に加工を施し、所定の部材形状に加工された段階の中間製品を作製してもよい。このような中間製品を出発材料として、以下に示す「Ｎｂ濃化表層形成工程」と「酸化皮膜安定化工程」を順に有する熱処理を施すことによって、上述の皮膜を有する鋼材を得ることができる。この熱処理の手法を本明細書では「Ａ処理」と呼んでいる。
〔Production method〕
Annealed and pickled steel plate (including the case where temper rolling is applied after pickling) or acid adjusted to a predetermined thickness using a conventional general ferritic stainless steel plate manufacturing process A cold rolled steel sheet subjected to cold rolling after washing is prepared as an intermediate product. In the case of a seamless steel pipe, the pickled steel pipe obtained in the normal steel pipe manufacturing process may be used as an intermediate product. Further, the intermediate product made of the above-described steel plate or steel pipe may be processed to produce an intermediate product of a stage processed into a predetermined member shape. By subjecting such an intermediate product as a starting material to a heat treatment having the following “Nb-concentrated surface layer forming step” and “oxide film stabilization step” in order, a steel material having the above-mentioned film can be obtained. The heat treatment method is referred to as "A treatment" in the present specification.
（Ｎｂ濃化表層形成工程）
Ｎｂ濃化表層形成工程は、酸洗表面または酸洗後に冷間圧延を施した表面を有する鋼材に、Ｃｒが還元されＮｂが酸化される雰囲気中で熱処理を施すことにより、少なくとも最表面から５ｎｍ未満までの深さ領域にＮｂが濃化しているＮｂ濃化表層を形成させる工程である。「Ｃｒが還元されＮｂが酸化される雰囲気」は、低酸素分圧の高温状態において実現できる。具体的には例えば、ガス組成：水素９５〜１００体積％、窒素５〜０体積％、温度：１０５０〜１２００℃の熱処理雰囲気を挙げることができる。この場合、露点は−３０〜−６０℃とすることが好ましい。この範囲の熱処理により少なくとも最表面から５ｎｍ未満までの深さ領域にＮｂが濃化しているＮｂ濃化表層を形成させることができる。Ｎｂが酸化される条件のなかでも、できるだけＮｂの酸化領域と還元領域の境界に近い条件を採用することがより好ましい。そのようなより好ましい条件として、例えば、水素ガス、１０７０〜１１５０℃、露点−４５〜−５５℃の熱処理雰囲気を採用することができる。
(Nb enrichment surface formation process)
In the Nb-concentrated surface layer forming step, a steel material having a pickled surface or a surface subjected to cold rolling after pickling is heat-treated in an atmosphere in which Cr is reduced and Nb is oxidized to at least 5 nm from the outermost surface This is a step of forming an Nb-concentrated surface layer in which Nb is concentrated in a depth region of less than 10 nm. The “atmosphere in which Cr is reduced and Nb is oxidized” can be realized at a high temperature under a low oxygen partial pressure. Specifically, for example, a heat treatment atmosphere having a gas composition of 95 to 100% by volume of hydrogen, 5 to 0% by volume of nitrogen and a temperature of 1050 to 1200 ° C. can be mentioned. In this case, the dew point is preferably −30 to −60 ° C. By heat treatment in this range, it is possible to form an Nb-concentrated surface layer in which Nb is concentrated at least in a depth region from the outermost surface to less than 5 nm. Among the conditions under which Nb is oxidized, it is more preferable to adopt conditions that are as close as possible to the boundary between the oxidized region and the reduced region of Nb. As such more preferable conditions, for example, a heat treatment atmosphere of hydrogen gas, 1070 to 1150 ° C., dew point −45 to −55 ° C. can be adopted.
上記のような「Ｃｒが還元されＮｂが酸化される雰囲気」での熱処理によって、少なくとも最表面から５ｎｍ未満までの深さ領域にＮｂが濃化しているＮｂ濃化表層を形成させることができる。熱処理時間（鋼板表面が上記温度域の雰囲気に曝される時間）は、３〜１２０分とすることが好ましい。熱処理時間が短すぎるとＮｂ濃化表層の形成が不十分となる。熱処理時間が長すぎると不経済となる。上記Ｎｂ濃化表層が形成される条件として、例えば、最表面からＳｉＯ2換算深さ５ｎｍまでの深さ領域において、平均Ｎｂ濃度が平均Ｃｒ濃度の０.３倍以上となる条件を採用することが好ましい。上記の平均Ｎｂ濃度および平均Ｃｒ濃度は、ＸＰＳ（Ｘ線光電子分光分析法）による深さ方向の元素濃度プロファイルにおいて、それぞれＮｂ濃度およびＣｒ濃度を最表面からＳｉＯ2換算深さ５ｎｍまでの深さ領域について積分することによって求めることができる。 By the heat treatment in the above-described “atmosphere in which Cr is reduced and Nb is oxidized”, it is possible to form an Nb-concentrated surface layer in which Nb is concentrated at least in a depth region from the outermost surface to less than 5 nm. The heat treatment time (the time when the steel sheet surface is exposed to the atmosphere in the above temperature range) is preferably 3 to 120 minutes. If the heat treatment time is too short, formation of the Nb-concentrated surface layer will be insufficient. If the heat treatment time is too long, it becomes uneconomical. As the conditions under which the Nb-concentrated surface layer is formed, for example, a condition in which the average Nb concentration is 0.3 times or more the average Cr concentration in the depth region from the outermost surface to the depth of 5 nm converted to SiO 2 is adopted. Is preferred. The above average Nb concentration and average Cr concentration are respectively the Nb concentration and the Cr concentration in the element concentration profile in the depth direction by XPS (X-ray photoelectron spectroscopy), from the outermost surface to the depth of 5 nm converted to SiO 2. It can be determined by integrating over the domain.
Ｎｂ濃化表層形成工程は、鋼材の形態がコイル状に巻かれた鋼板である場合にはベル焼鈍炉などによるバッチ式熱処理にて実施できる他、光輝焼鈍炉を用いた連続熱処理にて実施することも可能である。なお、ＢＡ仕上げステンレス鋼を製造する際の光輝焼鈍条件（例えば、ガス組成：水素８０体積％、窒素２０体積％、温度：１０００℃、露点：−５５℃、熱処理時間：１分）をＮｂ濃化表層形成工程にそのまま適用すると、Ｎｂ濃化表層の形成が不十分となる場合があり、最終的に十分な耐食性向上効果を安定して得ることは難しい。 The Nb-concentrated surface layer forming step can be carried out by batch heat treatment using a bell annealing furnace or the like if the form of the steel material is a steel plate wound into a coil, or continuous heat treatment using a bright annealing furnace. It is also possible. In addition, bright annealing conditions (for example, gas composition: hydrogen: 80% by volume, nitrogen: 20% by volume, temperature: 1000 ° C., dew point: −55 ° C., heat treatment time: 1 minute) at the time of producing BA finish stainless steel If it is applied as it is to the surface layer forming step, the formation of the Nb-concentrated surface layer may be insufficient, and it is difficult to finally obtain a sufficient effect of improving the corrosion resistance stably.
Ｎｂ濃化表層形成工程を終えた鋼材を、そのままの形状で酸化皮膜安定化工程に供することもできるし、この段階で所定の部材形状に加工を施した後に酸化皮膜安定化工程に供することもできる。 The steel material that has finished the Nb-concentrated surface layer forming step can be used as it is for the oxide film stabilization step in the same shape, or it can be processed to a predetermined member shape at this stage before being subjected to the oxide film stabilization step. it can.
（酸化皮膜安定化工程）
酸化皮膜安定化工程は、上述のＮｂ濃化表層形成工程を終えてＮｂ濃化表層を形成させた鋼材に、４５０℃以下の酸化性雰囲気中で熱処理を施すことにより最表面から５〜３０ｎｍの深さ領域にＣｒ濃化層およびＮｂ濃化層を形成させる工程である。熱処理温度が４５０℃より高いとテンパーカラーが生じる恐れがあり、好ましくない。４００℃以下に管理してもよい。酸化性雰囲気ガスとしては大気を利用することができる。この熱処理では、最表面に近い領域に新たにＦｅ酸化物を形成させることにより酸化皮膜のトータル厚さを増大させ、結果的にＮｂ濃化領域を最表面近傍から内部へとシフトさせる。その際、Ｎｂ濃化領域を構成するＮｂ酸化物は、外側がＮｂ2Ｏ5リッチ、内側がＮｂＯリッチである傾斜組成を呈するようになり、より安定な酸化物層構造が構築される。このような酸化皮膜の構造変化を効果的に生じさせるためには、熱処理温度を２００℃以上とすることが望ましく、２５０℃以上とすることがより好ましい。
(Oxide film stabilization process)
In the oxide film stabilization step, the steel material on which the Nb-concentrated surface layer is formed after the above-described Nb-concentrated surface layer formation step is subjected to heat treatment in an oxidizing atmosphere at 450 ° C. or less in the oxidizing atmosphere of 5 to 30 nm from the outermost surface. In this step, a Cr-rich layer and an Nb-rich layer are formed in the depth region. If the heat treatment temperature is higher than 450 ° C., temper color may be generated, which is not preferable. You may manage at 400 degrees C or less. The atmosphere can be used as the oxidizing atmosphere gas. In this heat treatment, the total thickness of the oxide film is increased by newly forming Fe oxide in a region near the outermost surface, and as a result, the Nb-rich region is shifted from the vicinity of the outermost surface to the inside. At this time, the Nb oxide constituting the Nb-rich region has a gradient composition in which the outer side is Nb 2 O 5 rich and the inner side is NbO rich, and a more stable oxide layer structure is constructed. In order to effectively cause such structural change of the oxide film, the heat treatment temperature is preferably 200 ° C. or higher, and more preferably 250 ° C. or higher.
酸化皮膜安定化工程は、バッチ式熱処理にて実施できる他、連続熱処理炉をゆっくり通過させることにより連続熱処理にて実施することも可能である。 The oxide film stabilization step can be carried out not only by batch heat treatment but also by continuous heat treatment by slowly passing it through a continuous heat treatment furnace.
表１に示す化学組成のフェライト系ステンレス鋼を溶製し、熱間圧延、熱延板焼鈍、酸洗、冷間圧延の工程により、板厚１.５ｍｍの冷延鋼板を得た。鋼Ｎｏ.１２以外の材料については、更に仕上げ焼鈍、仕上げ酸洗を施し、焼鈍酸洗鋼板（ＪＩＳ Ｇ０２０３：２００９の番号４２２１「Ｎｏ.２Ｄ仕上げ」に該当する鋼板）とした。上記仕上げ酸洗は、各鋼種の耐酸洗性に応じて硝酸濃度が５０〜１００ｇ／Ｌ、弗酸濃度が５〜１５ｇ／Ｌの範囲で調整された６０℃の弗硝酸水溶液を用い、酸化スケールが完全に除去される浸漬時間で行った。 A ferritic stainless steel having a chemical composition shown in Table 1 was melted, and a cold-rolled steel plate having a thickness of 1.5 mm was obtained by the steps of hot rolling, hot-rolled sheet annealing, pickling and cold rolling. The materials other than steel No. 12 were further subjected to finish annealing and finish pickling to obtain annealed pickled steel plates (steel plates corresponding to No. 4221 “No. 2D finish” of JIS G0203: 2009). The above-mentioned finish pickling is carried out using a 60 ° C. aqueous hydrofluoric acid solution adjusted to have a nitric acid concentration of 50 to 100 g / L and a hydrofluoric acid concentration of 5 to 15 g / L according to the acid washing resistance of each steel type. The immersion time was to be completely removed.
鋼Ｎｏ.１２については上記の冷間圧延を終えた段階の材料を、また鋼Ｎｏ.１２以外については上記の仕上げ酸洗を終えた段階の材料を、それぞれ出発材料として、以下に示すＡ処理、Ｂ処理、Ｃ処理、Ｄ処理のいずれかを施した。鋼Ｎｏ.１２の出発材料は「酸洗後に冷間圧延を施した表面を有する鋼材」であり、以下、この表面に仕上げられた材料を「ＨＴ仕上げ材」と呼ぶ。また、鋼Ｎｏ.１２以外の出発材料は「酸洗表面を有する鋼材」であり、以下、この表面に仕上げられた材料を「２Ｄ仕上げ材」と呼ぶ。 The material of the stage after cold rolling for steel No. 12 and the material of the stage after finishing finish pickling for steel No. 12 are treated as shown below as starting materials. Process B, process C, or process D was applied. The starting material of steel No. 12 is “Steel material having a surface subjected to cold rolling after pickling”, and hereinafter, the material finished on this surface is called “HT finish material”. Moreover, starting materials other than steel No. 12 are "Steel materials which have a pickled surface", and the material finished on this surface is called "2D finish material" hereafter.
（Ａ処理）出発材料に対して、ガス組成：水素１００体積％、露点：−５０℃、温度：１１００℃のガス中で１０分保持した後、炉内で冷却する第１の熱処理を施し、次いで、大気中３００℃で１時間保持する第２の熱処理を施した。
（Ｂ処理）出発材料に対して、大気中３００℃で１時間保持する熱処理を施した。
（Ｃ処理）出発材料に対して、ガス組成：水素１００体積％、露点：−５０℃、温度：１１００℃のガス中で１０分保持した後、炉内で冷却する熱処理を施した。
（Ｄ処理）出発材料に対して、ガス組成：水素８０体積％、窒素２０体積％、露点：−５５℃、温度：１０００℃のガス中で１分保持した後、炉内で冷却する第１の熱処理を施し、次いで、大気中３００℃で１時間保持する第２の熱処理を施した。
(A treatment) The starting material is subjected to a first heat treatment of cooling in a furnace after being held for 10 minutes in a gas of gas composition: 100% by volume of hydrogen, dew point: −50 ° C., temperature: 1100 ° C. Then, a second heat treatment was performed, which was maintained at 300 ° C. in the atmosphere for 1 hour.
(Process B) The starting material was heat-treated at 300 ° C. in air for 1 hour.
(C treatment) The starting material was heat treated for cooling in a furnace after being held for 10 minutes in a gas of gas composition: 100% by volume of hydrogen, dew point: -50 ° C, temperature: 1100 ° C.
(D treatment) Gas composition: 80% by volume of hydrogen, 20% by volume of nitrogen, dew point: −55 ° C., temperature: 1000 ° C. for 1 minute with respect to the starting material, and then cooled in a furnace C., and then subjected to a second heat treatment maintained at 300.degree. C. in the atmosphere for 1 hour.
本発明に従う化学組成の鋼板について適用するＡ処理では、第１の熱処理が前記「Ｎｂ濃化表層形成工程」に該当し、第２の熱処理が前記「酸化皮膜安定化工程」に該当する。Ｂ処理は、Ａ処理における第２の熱処理のみを施す処理である。Ｃ処理は、Ａ処理における第１の熱処理のみを施す処理である。Ｄ処理は、Ａ処理での第１の熱処理に代えて一般的なＢＡ仕上げステンレス鋼の製造に適用可能な光輝焼鈍条件を採用した処理である。 In the treatment A applied to the steel plate of the chemical composition according to the present invention, the first heat treatment corresponds to the “Nb-concentrated surface layer forming step”, and the second heat treatment corresponds to the “oxide film stabilization step”. The B treatment is a treatment to which only the second heat treatment in the A treatment is applied. The C treatment is a treatment to which only the first heat treatment in the A treatment is applied. The D treatment is a treatment employing bright annealing conditions applicable to the production of a general BA finish stainless steel in place of the first heat treatment in the A treatment.
上記のＡ処理、Ｂ処理、Ｃ処理、Ｄ処理のいずれかを施して得られた材料を以下において「処理材」と呼ぶ。各出発材料（２Ｄ仕上げ材またはＨＴ仕上げ材）および各処理材の表面について、上掲の「ＸＰＳ分析方法」に従い、ＸＰＳ（Ｘ線光電子分光分析法）による深さ方向の元素濃度プロファイルを測定した。その元素濃度プロファイルにおいて、Ｎｂのピーク位置とＣｒのピーク位置を調べ、最表面からのＳｉＯ2換算深さが５〜３０ｎｍの領域にＮｂおよびＣｒのピークがあるものを、Ｎｂ、Ｃｒのピーク位置評価：○、それ以外のものをＮｂ、Ｃｒのピーク位置評価：×とした。また、Ｎｂのピーク位置におけるＮｂ濃度（Ｎｂピーク濃度）を、Ｆｅ、Ｃｒ、Ｍｏ、Ｎｉ、Ｃｕ、Ｍｎ、Ｓｉ、Ｔｉ、Ｎｂ、Ａｌの合計に占めるＮｂのモル分率として、前記（１）式に基づいて求めた。さらに、最表面からのＳｉＯ2換算深さでＯ濃度が最表面の１／２となる深さまでを酸化皮膜と定義した場合の酸化皮膜厚さを、前記（２）式に基づいて求めた。 The material obtained by performing any of the above-mentioned treatment A, treatment B, treatment C, and treatment D is hereinafter referred to as "treatment material". The elemental concentration profile in the depth direction was measured by XPS (X-ray photoelectron spectroscopy) according to the above-mentioned “XPS analysis method” for each starting material (2D finish or HT finish) and the surface of each treatment. . In the element concentration profile, check the peak position of Nb and the peak position of Cr, and for peaks with Nb and Cr peaks in the region of 5 to 30 nm in SiO 2 equivalent depth from the outermost surface, the peak positions of Nb and Cr Evaluation: ○, other than that, the peak position evaluation of Nb and Cr: x. Also, the Nb concentration (Nb peak concentration) at the peak position of Nb is defined as the molar fraction of Nb in the total of Fe, Cr, Mo, Ni, Cu, Mn, Si, Ti, Nb, and Al. It calculated based on the formula. Further, an oxide film thickness in the case of O concentration in terms of SiO 2 depth from the outermost surface is defined as an oxide film to a depth which is a half of the outermost surface was calculated based on the equation (2).
各出発材料（２Ｄ仕上げ材またはＨＴ仕上げ材）および各処理材から５０ｍｍ×５０ｍｍの試験片を切り出し、以下に示す条件の複合サイクル腐食試験（ＣＣＴ）に供し、ＣＣＴ３００サイクル後の試験片表面を光学顕微鏡で観察し、焦点深度法により最大浸食深さを測定した。
［ＣＣＴ条件］
・塩水噴霧（１５ｍｉｎ、３５℃、５％ＮａＣｌ）、
・乾燥（１ｈ、６０℃、３０％ＲＨ）、
・湿潤（３ｈ、５０℃、９５％ＲＨ）、
を１サイクルとする。
Specimens of 50 mm x 50 mm are cut out from each starting material (2D finish or HT finish) and each treatment, subjected to combined cycle corrosion test (CCT) under the conditions shown below, and the surface of the specimen after CCT 300 cycles is optically It observed with the microscope and measured the maximum erosion depth by the depth of focus method.
[CCT condition]
Salt spray (15 min, 35 ° C, 5% NaCl),
Drying (1 h, 60 ° C., 30% RH),
Wet (3 h, 50 ° C., 95% RH),
Is one cycle.
ステンレス鋼の耐食性レベルは鋼種によって異なる。そこで、Ａ処理、Ｂ処理、Ｃ処理、Ｄ処理のいずれかを施して得られた「処理材」の耐食性と、処理前の出発材料（２Ｄ仕上げ材またはＨＴ仕上げ材）の耐食性を比較し、各処理による耐食性向上効果を調べた。具体的には、ＣＣＴ３００サイクル後の試験片について、出発材料に対する処理材の最大浸食深さの比Ｒを下記（３）式により求めた。
最大浸食深さ比Ｒ＝処理材の最大浸食深さ（μｍ）／出発材料の最大浸食深さ（μｍ） …（３）
The corrosion resistance level of stainless steel varies depending on the steel type. Therefore, we compare the corrosion resistance of the “treated material” obtained by applying either treatment A, treatment B, treatment C or D with the corrosion resistance of the starting material (2D finish or HT finish) before treatment, The corrosion resistance improvement effect by each treatment was investigated. Specifically, for the test piece after CCT 300 cycles, the ratio R of the maximum erosion depth of the treated material to the starting material was determined by the following equation (3).
Maximum Erosion Depth Ratio R = Maximum Erosion Depth of Treated Material (μm) / Maximum Erosion Depth of Starting Material (μm) ... (3)
上記の最大浸食深さ比Ｒが０.７０以下である場合には、施した処理によって改質された表面皮膜による耐食性改善効果が顕著であると評価される。従って、最大浸食深さ比Ｒが０.７０以下であるものを○評価（耐食性改善効果；顕著）、それ以外を×評価とし、○評価を合格と判定した。
以上の結果を表２に示す。
When the above maximum erosion depth ratio R is 0.70 or less, the corrosion resistance improvement effect by the surface film modified by the applied treatment is evaluated to be remarkable. Therefore, when the maximum erosion depth ratio R was 0.70 or less, the evaluation was evaluated as 評 価 (corrosion improvement effect; remarkable), the other was evaluated as x, and the 評 価 evaluation was judged as pass.
The above results are shown in Table 2.
Ａ処理を施した本発明例のものは、最表面からのＳｉＯ2換算深さが５〜３０ｎｍの領域にＮｂおよびＣｒのピークがあり、Ｎｂが前記所定の濃度で濃化した皮膜構造を有している。これらにおいては顕著な耐食性改善が認められた。
図１にＮｏ.１０、図２にＮｏ.１３、図３にＮｏ.１４のＸＰＳ（Ｘ線光電子分光分析法）による深さ方向の元素濃度プロファイルをそれぞれ例示してある（前述）。参考のため、図４に鋼Ｎｏ.１３の２Ｄ仕上げ材（Ａ処理を施す前の段階）についてＸＰＳによる深さ方向の元素濃度プロファイルを例示する。
In the example of the present invention subjected to the A treatment, peaks of Nb and Cr are present in the region of 5 to 30 nm in terms of SiO 2 equivalent depth from the outermost surface, and Nb has a film structure concentrated at the predetermined concentration. doing. In these cases, significant improvement in corrosion resistance was observed.
The element concentration profiles in the depth direction by XPS (X-ray photoelectron spectroscopy) by No. 10 and FIG. 2 and No. 13 and 14 respectively are exemplified in FIG. 1 and FIG. 3 (described above). For reference, FIG. 4 exemplifies an element concentration profile in the depth direction by XPS for a 2D finish of steel No. 13 (step before the treatment A).
Ｂ処理を施した比較例Ｎｏ.３−２、１０−２、１３−２では皮膜中にＣｒは濃化しているが、Ｎｂは濃化しなかった。すなわち、Ａ処理で第１の熱処理（Ｎｂ濃化表層形成工程）を省略すると、Ｎｂの濃化した皮膜構造が得られない。これらは、耐食性の改善効果に劣った。
図５にＮｏ.１３−２（Ｂ処理材）についてＸＰＳによる深さ方向の元素濃度プロファイルを例示する。
In Comparative Example Nos. 3-2, 10-2 and 13-2 subjected to the B treatment, Cr was concentrated in the film but Nb was not. That is, when the first heat treatment (Nb-concentrated surface layer forming step) is omitted in the A treatment, a film structure in which Nb is concentrated can not be obtained. These were inferior to the improvement effect of corrosion resistance.
The element concentration profile of the depth direction by XPS is illustrated about No. 13-2 (B process material) in FIG.
Ｃ処理を施した比較例Ｎｏ.１３−３では、Ｎｂが最表面近傍の浅い部分のみに濃化しており、酸化皮膜厚さも本発明例のものに比べ薄い。Ａ処理で第２の熱処理（酸化皮膜安定化工程）を省略すると、ＮｂとＣｒが所定深さ領域に濃化した特異な皮膜構造を実現することができない。この場合、十分な耐食性改善効果は得られない。
図６にＮｏ.１３−３（Ｃ処理材）についてＸＰＳによる深さ方向の元素濃度プロファイルを例示する。
In Comparative Example No. 13-3 subjected to the C treatment, Nb is concentrated only in the shallow portion near the outermost surface, and the thickness of the oxide film is also thinner than that of the inventive example. If the second heat treatment (oxide film stabilization step) is omitted in the A treatment, it is impossible to realize a unique film structure in which Nb and Cr are concentrated in a predetermined depth region. In this case, a sufficient corrosion resistance improvement effect can not be obtained.
The element concentration profile of the depth direction by XPS is illustrated about No. 13-3 (C process material) in FIG.
Ｄ処理を施した比較例Ｎｏ.３−３では、所定深さ領域でのＮｂの濃化が不十分であった。すなわち、Ａ処理での第１の熱処理に代えて一般的なＢＡ仕上げステンレス鋼の製造に適用可能な光輝焼鈍条件を採用した場合には、Ｎｂを安定して十分に濃化させることが困難である。Ｎｏ.３−３の例では、Ａ処理を施した本発明例のものと比べ、耐食性改善効果は小さかった。 In Comparative Example No. 3-3 subjected to the D treatment, the concentration of Nb in the predetermined depth region was insufficient. That is, when bright annealing conditions applicable to the production of general BA finish stainless steel are adopted instead of the first heat treatment in the A treatment, it is difficult to stably and sufficiently concentrate Nb. is there. In the example of No. 3-3, the corrosion resistance improvement effect was small compared with the thing of the example of this invention which gave A treatment.
比較例Ｎｏ.１７はＮｂ含有量が少ない鋼であるため、Ａ処理を施しても十分な耐食性改善効果は得られなかった。 Since Comparative Example No. 17 is a steel having a small Nb content, even when the A treatment is performed, a sufficient corrosion resistance improvement effect can not be obtained.
図７に、鋼Ｎｏ.１３を用いた２Ｄ仕上げ材、Ｂ処理材、Ａ処理材について、上記のＣＣＴ３００サイクル後における表面外観写真を例示する。各試験片のサイズは５０ｍｍ×５０ｍｍである。 The surface appearance photograph after the above-mentioned CCT300 cycle is illustrated about the 2D finishing material which used steel No. 13, B processing material, and A processing material in FIG. The size of each test piece is 50 mm × 50 mm.
Claims (5)
ここで、酸化皮膜厚さは、前記最表面からのＳｉＯ2換算深さにおいてＯ濃度が最表面の１／２となる深さとする。 The ferritic stainless steel material according to claim 1, wherein the oxide film thickness is 10 to 40 nm.
Here, the oxide film thickness is such a depth that the O concentration is 1/2 of the outermost surface in the SiO 2 converted depth from the outermost surface.
前記Ｎｂ濃化表層を有する鋼材に、４５０℃以下の酸化性雰囲気中で熱処理を施すことにより最表面から５〜３０ｎｍの深さ領域にＣｒ濃化層およびＮｂ濃化層を形成させる工程（酸化皮膜安定化工程）、
を有する請求項１または２に記載のフェライト系ステンレス鋼材の製造方法。
ここで、最表面からの深さは真空中でのイオンスパッタリング時間により定まるＳｉＯ2換算深さである。 A steel material having a pickled surface or a surface subjected to cold rolling after pickling is heat-treated in an atmosphere in which Cr is reduced and Nb is oxidized, so that Nb at least in the depth region from the outermost surface to less than 5 nm. Forming an Nb-concentrated surface layer enriched with Nb (Nb-concentrated surface layer formation step),
A step of forming a Cr-concentrated layer and a Nb-concentrated layer in a depth region of 5 to 30 nm from the outermost surface by heat treating the steel material having the Nb-concentrated surface layer in an oxidizing atmosphere of 450 ° C. or less Film stabilization process),
The manufacturing method of the ferritic stainless steel materials of Claim 1 or 2 which has these.
Here, the depth from the outermost surface is the equivalent SiO 2 depth determined by the ion sputtering time in vacuum.
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