JP6516238B2 - Austenitic stainless steel and method for producing the same - Google Patents
Austenitic stainless steel and method for producing the same Download PDFInfo
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- 229910000963 austenitic stainless steel Inorganic materials 0.000 title claims description 28
- 238000004519 manufacturing process Methods 0.000 title claims description 9
- IJGRMHOSHXDMSA-UHFFFAOYSA-N nitrogen Substances N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 101
- 239000007789 gas Substances 0.000 claims description 57
- 229910052757 nitrogen Inorganic materials 0.000 claims description 50
- 238000005121 nitriding Methods 0.000 claims description 43
- 150000004767 nitrides Chemical class 0.000 claims description 31
- 238000000034 method Methods 0.000 claims description 29
- 230000018199 S phase Effects 0.000 claims description 22
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 21
- 229910052799 carbon Inorganic materials 0.000 claims description 21
- 229910001220 stainless steel Inorganic materials 0.000 claims description 19
- 239000010935 stainless steel Substances 0.000 claims description 17
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 claims description 11
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 claims description 11
- 238000001816 cooling Methods 0.000 claims description 10
- 229910000037 hydrogen sulfide Inorganic materials 0.000 claims description 10
- 239000004215 Carbon black (E152) Substances 0.000 claims description 9
- 229930195733 hydrocarbon Natural products 0.000 claims description 9
- 150000002430 hydrocarbons Chemical class 0.000 claims description 9
- 229910001873 dinitrogen Inorganic materials 0.000 claims description 6
- QJGQUHMNIGDVPM-UHFFFAOYSA-N nitrogen(.) Chemical compound [N] QJGQUHMNIGDVPM-UHFFFAOYSA-N 0.000 claims description 6
- 239000010410 layer Substances 0.000 description 97
- 230000007797 corrosion Effects 0.000 description 38
- 238000005260 corrosion Methods 0.000 description 38
- 239000011651 chromium Substances 0.000 description 22
- 229910001566 austenite Inorganic materials 0.000 description 19
- 239000000463 material Substances 0.000 description 14
- 238000005452 bending Methods 0.000 description 11
- 238000010521 absorption reaction Methods 0.000 description 10
- 239000011159 matrix material Substances 0.000 description 10
- 239000000047 product Substances 0.000 description 9
- 239000006104 solid solution Substances 0.000 description 8
- 229910000831 Steel Inorganic materials 0.000 description 6
- 239000010959 steel Substances 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
- 229910052804 chromium Inorganic materials 0.000 description 4
- MUBZPKHOEPUJKR-UHFFFAOYSA-N Oxalic acid Chemical compound OC(=O)C(O)=O MUBZPKHOEPUJKR-UHFFFAOYSA-N 0.000 description 3
- 229910052736 halogen Inorganic materials 0.000 description 3
- 150000002367 halogens Chemical class 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 150000002829 nitrogen Chemical class 0.000 description 3
- 238000002407 reforming Methods 0.000 description 3
- 239000002344 surface layer Substances 0.000 description 3
- CSCPPACGZOOCGX-UHFFFAOYSA-N Acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 description 2
- KRHYYFGTRYWZRS-UHFFFAOYSA-M Fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 description 2
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 2
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
- 238000002441 X-ray diffraction Methods 0.000 description 2
- 229910021529 ammonia Inorganic materials 0.000 description 2
- 238000000137 annealing Methods 0.000 description 2
- 239000011248 coating agent Substances 0.000 description 2
- 238000000576 coating method Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
- 239000011737 fluorine Substances 0.000 description 2
- 229910052731 fluorine Inorganic materials 0.000 description 2
- 150000004820 halides Chemical class 0.000 description 2
- 150000002500 ions Chemical class 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 230000035515 penetration Effects 0.000 description 2
- 238000005554 pickling Methods 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 231100000331 toxic Toxicity 0.000 description 2
- 230000002588 toxic effect Effects 0.000 description 2
- 229910052721 tungsten Inorganic materials 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- KRHYYFGTRYWZRS-UHFFFAOYSA-N Fluorane Chemical compound F KRHYYFGTRYWZRS-UHFFFAOYSA-N 0.000 description 1
- 230000004913 activation Effects 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- HSFWRNGVRCDJHI-UHFFFAOYSA-N alpha-acetylene Natural products C#C HSFWRNGVRCDJHI-UHFFFAOYSA-N 0.000 description 1
- 239000004566 building material Substances 0.000 description 1
- 150000001805 chlorine compounds Chemical class 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 238000007872 degassing Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 125000002534 ethynyl group Chemical group [H]C#C* 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 229910000040 hydrogen fluoride Inorganic materials 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 235000006408 oxalic acid Nutrition 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
- 230000009291 secondary effect Effects 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
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- Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、腐食環境で使用される、窒素を表面に濃化させることによる高耐食性を有し、かつ曲げ性に優れたオーステナイトステンレス鋼板及びその製造方法に関する。 The present invention relates to an austenitic stainless steel sheet which is used in a corrosive environment, has high corrosion resistance by concentrating nitrogen on the surface, and is excellent in bendability, and a method of manufacturing the same.
通常ステンレス鋼の耐食性を向上させるためには、Ｃｒ、Ｍｏ、Ｗなどの高価な原料を多量に含有させた素材とする場合が多い。しかしながら、原料コストの増大に伴い製品価格が上昇する点が、経済的・技術的な問題である。 Usually, in order to improve the corrosion resistance of stainless steel, it is often used as a material containing a large amount of expensive raw materials such as Cr, Mo and W. However, it is an economic and technical problem that product prices rise with the increase of raw material costs.
ステンレス鋼では、以下の孔食指数に見られるように、Ｎの添加により耐孔食性が増すことが知られている。Ｎの添加により、Ｃｒ、Ｍｏ、Ｗなどの高価な元素の使用を極力抑えながら耐食性をあげることができる。すなわち、コストの低い材料に表面からＮを添加することにより、コストの高い材料と同等以上の耐食性を付与することができる。 In stainless steel, addition of N is known to increase the pitting resistance as seen in the pitting index below. By the addition of N, the corrosion resistance can be improved while minimizing the use of expensive elements such as Cr, Mo and W. That is, by adding N from the surface to a low cost material, corrosion resistance equal to or higher than that of a high cost material can be provided.
耐孔食指数（ＰＲＥＷ）：Ｃｒ＋３．３Ｍｏ＋１．６５Ｗ＋１６Ｎ … （１） Pitting resistance index (PREW): Cr + 3.3Mo + 1.65W + 16N (1)
特許文献１には、ステンレス鋼を、窒素ガスを含有する雰囲気中で、９５０〜１１５０℃の温度範囲で光輝焼鈍し、表面に窒素濃化層を形成させることにより、耐孔食性と耐時期割れ性に優れたオーステナイト系ステンレス鋼を提供する方法が記載されている。 In Patent Document 1, pitting corrosion resistance and time cracking are achieved by bright annealing stainless steel in a temperature range of 950 to 1150 ° C. in an atmosphere containing nitrogen gas to form a nitrogen-rich layer on the surface. A method of providing an austenitic stainless steel with excellent properties is described.
一方、４５０℃程度以下の低い処理温度でオーステナイト系ステンレス鋼に、塩化物系化合物やハロゲンガス、フッ化水素系ガスを表面活性化ガスとして用いるガス窒化法やイオン・プラズマ法などの方法により窒化処理を施すと、耐食性劣化の原因となるＣｒの窒化物の生成を抑え、最表面層に１０重量％以上の過飽和窒素を含むオーステナイト相、いわゆるＳ相が得られることが知られている。 On the other hand, nitriding by a method such as gas nitriding method or ion plasma method using chloride compound, halogen gas or hydrogen fluoride gas as surface activation gas to austenitic stainless steel at low processing temperature of about 450 ° C. or less It is known that the treatment suppresses the formation of nitrides of Cr which causes corrosion resistance deterioration, and obtains an austenite phase containing supersaturated nitrogen of 10% by weight or more, so-called S phase, in the outermost layer.
Ｓ相は高い硬度と耐食性を有している。Ｓ相の高耐食性は、（１）式から示唆されるように、過飽和に含まれる高濃度の窒素に由来すると考えられる。ステンレス鋼に対するこれらの窒化法は工業化され、硬度と耐摩耗性、耐食性が要求される装置部品などの窒化に広く適用されている。 The S phase has high hardness and corrosion resistance. The high corrosion resistance of the S phase is considered to be derived from the high concentration of nitrogen contained in supersaturation, as suggested by the equation (1). These nitriding methods for stainless steel are industrialized and widely applied to the nitriding of equipment parts that require hardness, wear resistance and corrosion resistance.
特許文献２、特許文献３、特許文献４では、４５０℃以下の窒化で、前処理ガスとして、フッ化物ガスやハロゲン化物ガス、炭化水素系ガスを用いて、表面に耐食性を著しく低下させるクロム系窒化物が生成しない方法が記されている。これらの特許文献での、窒化最表層はＳ層である。 According to Patent Document 2, Patent Document 3 and Patent Document 4, a chromium-based system which significantly reduces corrosion resistance on the surface by nitriding at 450 ° C. or less and using fluoride gas, halide gas and hydrocarbon gas as a pretreatment gas A method is described where nitrides do not form. The outermost layer of nitriding in these patent documents is an S layer.
前述したように、表面窒素量が多いほど耐孔食指数は増加し、耐孔食性は増加する。しかしながら、表面に固溶できる窒素量には、通常、温度や雰囲気中の窒素分圧、材料によってその限界がある。以下に、１０５０℃前後の高温における窒素吸収と４５０℃以下での窒化における問題点を個別に述べる。 As described above, the pitting resistance index increases and the pitting resistance increases as the amount of surface nitrogen increases. However, the amount of nitrogen that can form a solid solution on the surface usually has limits depending on the temperature, the nitrogen partial pressure in the atmosphere, and the material. In the following, nitrogen absorption at high temperatures around 1050 ° C. and problems in nitridation below 450 ° C. will be individually described.
１０５０℃前後での窒素吸収では、温度が高いのでほぼ熱平衡状態にあり、表面に添加できる窒素量は、温度、雰囲気ガス中の窒素分圧、材料により決まる。長時間の熱処理により、素材表面からの窒素の浸入距離は増加するものの、耐食性に直接影響する表面の窒素量を増加させることはできない。 The nitrogen absorption at around 1050 ° C. is almost in thermal equilibrium because the temperature is high, and the amount of nitrogen that can be added to the surface depends on the temperature, the nitrogen partial pressure in the atmosphere gas, and the material. Long-term heat treatment increases the penetration distance of nitrogen from the surface of the material, but can not increase the amount of nitrogen on the surface which directly affects the corrosion resistance.
温度に関しては、少し複雑である。１０５０℃近辺以下では窒化物とγ相への固溶窒素を合計した全窒素量は増加するが、Ｃｒ２Ｎを主体とする窒化物が多く表面近傍に析出する。耐孔食性向上に寄与するのは固溶窒素であり、窒化物は耐食性を劣化させる。１０５０℃近辺以上では、窒化物の生成は抑制されるものの、固溶窒素量も温度とともに減少する。 The temperature is a bit complicated. At around 1050 ° C. or less, the total amount of nitrogen, which is the sum of nitride and nitrogen dissolved in the γ phase, increases, but a large amount of nitride mainly composed of Cr 2 N precipitates in the vicinity of the surface. It is the solid solution nitrogen that contributes to the improvement of the pitting resistance, and the nitride deteriorates the corrosion resistance. Above about 1050 ° C., although the formation of nitride is suppressed, the amount of solid solution nitrogen also decreases with the temperature.
１０５０℃以下であっても、４５０℃程度以下の低い処理温度での窒化では１０５０℃前後の窒素吸収過程とは異なり、熱平衡状態にはなく、耐食性劣化の原因となるＣｒの窒化物の生成を抑え、最表面層に１０重量％以上の過飽和窒素を固溶させることができる（Ｓ相）。このＳ相は、高い固溶窒素を含有しているため高耐食性を示すが、非常に硬質なので、鋼板や板状部品に曲げのような加工をすることができない。 Even at 1050 ° C. or lower, nitriding at a low processing temperature of 450 ° C. or lower differs from the nitrogen absorption process at around 1050 ° C. and is not in a thermal equilibrium state, resulting in the formation of Cr nitrides that cause corrosion resistance deterioration. In the outermost surface layer, 10% by weight or more of supersaturated nitrogen can be dissolved (S phase). The S phase exhibits high corrosion resistance because it contains high solid solution nitrogen, but because it is very hard, it can not be processed like a bend in a steel plate or plate-like part.
すなわち、１０５０℃前後での窒素吸収では、十分な耐食性を得るために十分な窒素を吸収させることが難しく、４５０℃程度での窒化では、耐食性のよい過飽和に窒素含んだ表面層は得られるが、硬質のＳ相のために、鋼板や板状部品に曲げのような加工をすることが困難である。 That is, it is difficult to absorb sufficient nitrogen to obtain sufficient corrosion resistance by nitrogen absorption at around 1050 ° C., and by nitriding at about 450 ° C., a highly corrosion-resistant supersaturated nitrogen-containing surface layer can be obtained. Because of the hard S phase, it is difficult to process steel plates and plate-like parts like bending.
本発明の目的は、窒素添加による耐食性の向上であり、硬質の皮膜を得ることではない。窒素添加は、加工を施した最終形状品に施すことが考えられる。さらに、窒素耐食性確保に十分な固溶窒素を含みつつ、曲げ性に優れた鋼板、あるいは板状部品に窒化処理を施し、その後曲げなどの加工を付与することも想定される（図１）。このように、窒化後の板、あるいは板状部品に曲げなどの加工が行えれば、高耐食性窒化ステンレス鋼の適用範囲は大きく広がる。 The object of the present invention is to improve the corrosion resistance by the addition of nitrogen, not to obtain a hard coating. Nitrogen addition can be considered to be applied to the processed final shaped product. Furthermore, it is also conceivable to subject a steel plate or plate-like part having excellent bendability to a nitriding treatment and then apply a process such as bending while containing solid solution nitrogen sufficient to secure nitrogen corrosion resistance (FIG. 1). As described above, if the plate or plate-like part after nitriding can be processed by bending or the like, the application range of the high corrosion resistance nitrided stainless steel is greatly expanded.
本発明では、従来技術では達成できないレベルの窒素を、オーステナイト系ステンレス鋼の表面に濃化し、耐食性を向上したオーステナイト系ステンレス鋼及びその製造方法を提供することを課題とする。本発明のオーステナイト系ステンレス鋼には、鋼板やオーステナイト系ステンレス鋼からなる製品が含まれる。 An object of the present invention is to provide an austenitic stainless steel having a corrosion resistance improved by concentrating nitrogen of a level which can not be achieved by the prior art on the surface of an austenitic stainless steel and a method of manufacturing the same. The austenitic stainless steels of the present invention include products made of steel plates and austenitic stainless steels.
本発明者らは、前記課題を解決するために鋭意検討を重ねた。その結果、最表層をＳ相に比べて軟質であるが耐食性は良好な窒素を含む層、その直下にＳ相からなる層、その直下に炭素濃化層の三層を形成することにより、耐食性が向上できることを知見した。 The present inventors diligently studied to solve the above-mentioned problems. As a result, the outermost layer is softer than the S phase but has a good corrosion resistance by forming a layer containing nitrogen, a layer consisting of the S phase immediately below it, and three layers of a carbon enriched layer directly below it. Found that it could improve.
本発明は、上記の知見に基きなされたものであって、その要旨は以下のとおりである。 The present invention has been made based on the above findings, and the summary thereof is as follows.
（１）表面から内層へ順番に、γ相中にＦｅの窒化物とＣｒの窒化物を含んだ最表層と、Ｎを１０ｍａｓｓ％以上過飽和に固溶したＳ相からなる層と、炭素の濃化層の三層を、表面に有することを特徴とするオーステナイト系ステンレス鋼。 (1) In order from the surface to the inner layer, the outermost layer containing Fe nitride and Cr nitride in the γ phase, the layer consisting of the S phase in which N is dissolved in a supersaturation of 10 mass% or more, and carbon concentration An austenitic stainless steel characterized by having three layers of an oxide layer on its surface.
（２）前記最表層のγ相中に含まれるＦｅの窒化物がＦｅ４Ｎを主体とし、かつ、γ相中のＦｅの窒化物に含まれるＮの総量がγ相中のＣｒの窒化物に含まれるＮの総量よりも多いことを特徴とする前記（１）のオーステナイト系ステンレス鋼。 (2) The nitride of Fe contained in the γ phase of the outermost layer is mainly composed of Fe 4 N, and the total amount of N contained in the nitride of Fe in the γ phase is a nitride of Cr in the γ phase The austenitic stainless steel according to (1) above, characterized in that it is larger than the total amount of N contained in.
（３）前記（１）又は（２）のオーステナイト系ステンレス鋼の製造方法であって、
ステンレス鋼板、又は製品形状に加工されたステンレス鋼を硫化水素、窒素、及びアンモニアガスを導入した雰囲気下で４００〜４５０℃の範囲まで昇温する酸化皮膜除去工程と、上記ステンレス鋼を、硫化水素ガスの導入を停止し、窒素、及びアンモニアガスを導入した雰囲気下で４００〜４５０℃で、２０〜１００時間保持する窒化工程と、上記ステンレス鋼を、アンモニアガスの導入を停止し、窒素ガスを導入した雰囲気下で冷却する冷却工程を順に備えることを特徴とするオーステナイト系ステンレス鋼の製造方法。
(3) A method for producing the austenitic stainless steel according to (1) or (2) above,
An oxide film removing step of raising the temperature of a stainless steel plate or stainless steel processed into a product shape to a range of 400 to 450 ° C. in an atmosphere containing hydrogen sulfide, nitrogen and ammonia gas; Stop the introduction of gas, hold nitriding for 20 to 100 hours at 400 to 450 ° C. in an atmosphere of nitrogen and ammonia gas introduced, stop the introduction of ammonia gas for the stainless steel and stop nitrogen gas A method for producing an austenitic stainless steel, comprising a cooling step of cooling in an introduced atmosphere in order.
（４）前記酸化皮膜除去工程において、さらに、炭化水素系ガスを導入することを特徴とする前記（３）のオーステナイト系ステンレス鋼の製造方法。 (4) The method for producing an austenitic stainless steel according to (3), further including introducing a hydrocarbon-based gas in the oxide film removing step.
本発明によれば、窒素を利用することで高価な合金元素の添加を極力少なくし、従来の窒化層より薄化した三層構造からなる窒化層を有する耐食性に優れ、表面曲げ性にも優れたオーステナイト系ステンレス鋼が得られる。 According to the present invention, the use of nitrogen reduces the addition of expensive alloying elements as much as possible, and it is excellent in corrosion resistance having a nitrided layer consisting of a three-layer structure thinner than the conventional nitrided layer, and also excellent in surface bendability. An austenitic stainless steel is obtained.
また、コスト面でも優れたオーステナイト系ステンレス鋼を提供できる。さらに、鋼板を窒化して、その後曲げなどの加工を施すことができるので、適用製品範囲が大幅に拡がる。本発明によるオーステナイト系ステンレス鋼は、自動車部品のみならず、厨房関連用品、食器、浴槽、便器、家電製品、建材製品、車両等に幅広く適用できる。 Further, it is possible to provide an austenitic stainless steel excellent in cost. Furthermore, since the steel plate can be nitrided and then subjected to processing such as bending, the range of applied products is greatly expanded. The austenitic stainless steel according to the present invention can be widely applied not only to automobile parts but also to kitchen related products, dishes, baths, toilet bowls, home appliances, building materials products, vehicles and the like.
本発明のオーステナイト系ステンレス鋼は、三層構造の窒化層を有することを特徴とする。母材となるステンレス鋼は、オーステナイト系ステンレスであれば、特に限定はされない。たとえば、ＳＵＳ３０４Ｌ、ＳＵＳ３１６Ｌ等のステンレス鋼を使用することができる。 The austenitic stainless steel of the present invention is characterized by having a nitride layer of a three-layer structure. The stainless steel as the base material is not particularly limited as long as it is austenitic stainless steel. For example, stainless steel such as SUS304L or SUS316L can be used.
窒化層は、表面から順番に、最表層は、オーステナイト（γ）相中に多量のＦｅとＣｒの窒化物を含む層（以下「層Ｉ」という）、その直下の層は、窒素を１０重量％以上の窒素を過飽和に含むオーステナイト相、すなわちＳ相からなる層（以下「層II」という）、さらにその直下の層は、炭素が濃化したＦＣＣ相（γ相）からなる層（以下「層III」という）から構成される。以下、本発明における、これら三層について詳述する。 The nitrided layer is, in order from the surface, the outermost layer is a layer containing a large amount of Fe and Cr nitride in the austenite (γ) phase (hereinafter referred to as “layer I”, and the layer immediately below it is nitrogen 10 wt. Austenite phase containing supersaturated nitrogen in a supersaturation state, ie, a layer consisting of S phase (hereinafter referred to as “layer II”), and a layer directly under it is a layer consisting of a carbon-enriched FCC phase (γ phase) Layer III)). Hereinafter, these three layers in the present invention will be described in detail.
層Ｉは、Ｆｅ４ＮとＣｒＮを主体とする、多量の窒化物が析出したオーステナイト相である。Ｆｅ４ＮとＣｒＮの量比はＦｅ４Ｎが多いことが好ましい。通常、窒化処理で表面に生成するＣｒ窒化物は、耐食性を著しく低下させるとされる。表面のＣｒ窒化物が耐食性を低下させる原因は、表面近傍のオーステナイト相にＣｒ窒化物であるＣｒＮが多量に析出生成することにより、オーステナイト中の固溶Ｃｒ濃度が大幅に低下し、ステンレス鋼本来の不動態皮膜生成能力が著しく低下することにある。 The layer I is an austenite phase in which a large amount of nitride is precipitated, which is mainly composed of Fe 4 N and CrN. The quantitative ratio of Fe 4 N to CrN is preferably high in Fe 4 N. Usually, Cr nitride formed on the surface by nitriding treatment is considered to significantly reduce the corrosion resistance. The reason why the Cr nitride on the surface lowers the corrosion resistance is that a large amount of CrN, which is Cr nitride, is precipitated and formed in the austenite phase near the surface, so that the concentration of solid solution Cr in austenite is significantly reduced. The ability to form a passive film is significantly reduced.
本発明では、最表層である層Ｉに析出する窒化物は、ＣｒＮのほかに、Ｆｅ４Ｎが多く析出している。そのため、オーステナイト相中の固溶Ｃｒ濃度の低下はわずかであり、層Ｉの不動態皮膜生成能力はほとんど低下しない。かつＣｒＮ自体は良好な耐食性を示すので、オーステナイト母相以上の耐食性は保持している。硬度は層IIより低いが、母相オーステナイト相よりも高い。 In the present invention, in the nitride deposited in the outermost layer I, a large amount of Fe 4 N is precipitated in addition to CrN. Therefore, the decrease in solid solution Cr concentration in the austenite phase is slight, and the passive film forming ability of layer I is hardly reduced. And, since CrN itself exhibits good corrosion resistance, the corrosion resistance above the austenite matrix is maintained. The hardness is lower than in layer II but higher than the parent austenite phase.
層IIは、窒素を過飽和に含むオーステナイト相であるＳ相である。窒素を過飽和に高濃度で含むため、前記（１）式の孔食指数計算式からもわかるように、母相オーステナイト相より高い耐食性を有している。本発明のオーステナイト系ステンレス鋼の高耐食性は本層の存在に拠っている。層IIの硬度は非常に高く、８００〜１０００Ｈｖほどの硬度を持っている。 The layer II is an S phase which is an austenite phase containing nitrogen in supersaturation. Since nitrogen is contained at a high concentration in supersaturation, it has higher corrosion resistance than the parent phase austenite phase, as can be understood from the pitting index calculation formula of the above-mentioned formula (1). The high corrosion resistance of the austenitic stainless steel of the present invention is due to the presence of this layer. The hardness of layer II is very high and has a hardness of about 800 to 1000 Hv.
層IIIは炭素が層Ｉ、層II、及び層III直下のオーステナイト母相の炭素濃度より高い、炭素が濃化したオーステナイト相である。層IIIのオーステナイト結晶粒は、層IIのS相及び内部のオーステナイト母相に連続して続いている。層IIIは、層II以上の高い耐食性を有している。本発明のオーステナイト系ステンレス鋼の高耐食性には、層Ｉ、層IIとともに、層IIIも寄与している。層IIIの炭素濃度は、オーステナイト母相の約２〜５倍程度である。 Layer III is a carbon-rich austenite phase in which carbon is higher than the carbon concentration of layer I, layer II, and the austenite matrix immediately below layer III. The austenite grains of layer III continue to the S phase of layer II and the austenite matrix inside. Layer III has higher corrosion resistance than Layer II. In addition to the layers I and II, the layer III also contributes to the high corrosion resistance of the austenitic stainless steel of the present invention. The carbon concentration of layer III is about 2 to 5 times that of the austenite matrix.
炭素の濃化は、もともと母相オーステナイトに含有されていた炭素が、表面から進入してきた窒素による炭素ポテンシャルの変化により、再分布して生じる現象である。これは、本発明での窒化ガス及び前処理ガス（硫化水素ガス）において、炭素を含んだガスを使用しない場合においても炭素濃化層が観察されることから明らかである。 Carbon enrichment is a phenomenon that occurs due to redistribution of carbon originally contained in matrix austenite due to a change in carbon potential due to nitrogen entering from the surface. This is apparent from the fact that a carbon-rich layer is observed even in the case where the gas containing carbon is not used in the nitriding gas and the pretreatment gas (hydrogen sulfide gas) in the present invention.
炭素濃化層が、もともとオーステナイト母相が含有していた炭素が窒素吸収によって再分布し形成されることから、ステンレス母相の炭素含有量によって炭素濃化層が制御できることが示唆される。実際、ステンレス母相の炭素濃度が０．０１重量％でも炭素濃化層が観察され、０．０３重量％に母相炭素濃度が増加すると、窒化条件が同じならば、ピーク高さ、と濃化層幅は増加する。すなわち、耐食性等を制御できる。 The carbon enriched layer is formed by redistribution and formation of carbon originally contained in the austenite matrix by nitrogen absorption, which suggests that the carbon enriched layer can be controlled by the carbon content of the stainless matrix. In fact, a carbon-rich layer is observed even when the carbon concentration of the stainless matrix is 0.01% by weight, and when the matrix carbon concentration increases to 0.03% by weight, the peak height, if the nitriding conditions are the same, Layer width increases. That is, corrosion resistance etc. can be controlled.
本発明のオーステナイト系ステンレス鋼は、以上に述べた三層の存在により、母相オーステナイト以上の耐食性を保持している。一般に、ステンレス鋼の耐食性保持のためには、所定の窒化層厚みが必要である。本発明のオーステナイト系ステンレス鋼における三層構造の場合には、従来の最表層がS相の場合に比較して、三層合計の厚みが、従来の窒化層厚みより薄い場合でも同等以上の耐食性を示す。 The austenitic stainless steel of the present invention maintains corrosion resistance higher than that of the parent phase austenite by the presence of the three layers described above. Generally, in order to maintain the corrosion resistance of stainless steel, a predetermined nitrided layer thickness is required. In the case of the three-layer structure in the austenitic stainless steel of the present invention, compared to the conventional outermost layer having S phase, even if the total thickness of the three layers is thinner than the conventional nitrided layer thickness, Indicates
また、曲げ性に関しては、最表層の層ＩがS相である層IIよりも軟質なため、最表層がS相である場合に比較して、良好である。さらに、耐食性を従来の窒化層並みに維持しつつ、窒化層厚みを薄くできるので、良好な曲げ性を確保できる。すなわち、従来の最表面S相の窒化層に比較して同等以上の耐食性を維持しながら、窒化層厚みを薄くできるので曲げ性を確保できる。これにより、窒化処理後であっても、ある程度の曲げ加工が可能となる。このため、鋼板、あるいは板状部品に窒化処理を施し、その後曲げなどの加工を付与することが可能である。 Further, the bendability is better than the case where the outermost layer is the S phase because the outermost layer I is softer than the layer II which is the S phase. Furthermore, since the thickness of the nitrided layer can be reduced while maintaining the corrosion resistance to the level of the conventional nitrided layer, good bendability can be secured. That is, since the thickness of the nitrided layer can be reduced while maintaining the corrosion resistance equal to or more than the conventional outermost surface S phase nitrided layer, the bendability can be secured. As a result, even after the nitriding treatment, bending can be performed to some extent. Therefore, it is possible to subject a steel plate or plate-like part to a nitriding treatment and then to apply a process such as bending.
本発明の三層構造からなる窒化層を有するオーステナイト系ステンレス鋼の製造方法について説明する。 A method of producing an austenitic stainless steel having a nitrided layer having a three-layer structure according to the present invention will be described.
本発明のオーステナイト系ステンレス鋼は、冷間加工後のコイルあるいは板を焼鈍し、その後酸洗でスケールを除去した２Ｂ仕上げの板に窒素吸収工程で窒素を吸収させることにより製造する。その後、窒素吸収させた板を所定の製品形状に加工し最終製品とする。あるいは、２Ｂ仕上げの板を所定の製品形状に加工し、窒素吸収工程で窒素を吸収させることもできる。 The austenitic stainless steel of the present invention is manufactured by annealing a cold-worked coil or plate and then absorbing nitrogen in a 2B-finished plate whose scale has been removed by pickling in a nitrogen absorption step. Thereafter, the plate absorbed with nitrogen is processed into a predetermined product shape to obtain a final product. Alternatively, the 2B-finished board can be processed into a predetermined product shape and nitrogen can be absorbed in the nitrogen absorption step.
本発明の窒素吸収工程は、（１）硫化水素ガスを主体とするガスによるＣｒ酸化皮膜を改質する、酸化皮膜除去工程、（２）窒素とアンモニアの混合ガスによる窒化工程、（３）冷却工程の三工程に分けられる。本発明においては、１０５０℃以上の窒素吸収処理では、十分な固溶窒素量を確保できないことから、４５０℃以下の窒化法をベースに、Ｓ相を含む複層窒化層を形成する。 The nitrogen absorption process of the present invention comprises (1) reforming the Cr oxide film with a gas mainly composed of hydrogen sulfide gas, removing the oxide film, (2) nitriding process with a mixed gas of nitrogen and ammonia, (3) cooling It is divided into three steps of steps. In the present invention, since a sufficient amount of solid solution nitrogen can not be secured by nitrogen absorption treatment at 1050 ° C. or higher, a multilayer nitride layer containing S phase is formed based on the nitriding method at 450 ° C. or lower.
一般に、ステンレス鋼に窒化処理を施す場合、窒素を効率的に表面から内部に侵入させるために、窒化過程に先立ち、Ｃｒの酸化皮膜を改質あるいは破壊・除去する必要がある。イオン・プラズマ窒化では、プラズマ放電により酸化皮膜を改質する。ガス窒化ではフッ化物ガスやハロゲン化物ガス、炭化水素系ガスを用い皮膜を改質する。 In general, when stainless steel is subjected to nitriding treatment, it is necessary to reform or destroy or remove a Cr oxide film prior to the nitriding process in order to allow nitrogen to efficiently infiltrate from the surface to the inside. In ion plasma nitriding, the oxide film is reformed by plasma discharge. In gas nitriding, a film is reformed using a fluoride gas, a halide gas, or a hydrocarbon gas.
本発明の酸化皮膜除去工程では、窒化温度に到達する前の昇温過程で、Ｃｒ酸化皮膜改質ガスとして硫化水素（Ｈ２Ｓ）ガス、あるいはそれをベースにした炭化水素系ガス（たとえば、アセチレン）との混合ガスを用いてＣｒ酸化皮膜を除去する。硫化水素ガスを用いて皮膜除去を行うことによって、Ｆｅ系の窒化物の形成と、窒素の過飽和での浸入が可能となり、続く窒化工程、冷却工程を適切な条件で行うことにより、最表面に軟質な窒化層（層Ｉ）が形成される。 In the oxide film removing step of the present invention, hydrogen sulfide (H 2 S) gas as a Cr oxide film reforming gas or a hydrocarbon-based gas (for example, a hydrocarbon gas based thereon) is used in the temperature raising process before reaching the nitriding temperature. The Cr oxide film is removed using a mixed gas with acetylene). By removing the film using hydrogen sulfide gas, formation of Fe-based nitride and penetration of nitrogen with supersaturation become possible, and by performing the subsequent nitriding process and cooling process under appropriate conditions, the outermost surface can be obtained. A soft nitrided layer (layer I) is formed.
炭化水素ガスは、通常の窒化処理でも、表面を活性化するために用いられる。本発明でも同様の目的で皮膜除去用のガスに混合させてもよいが、多量に含有させると、硫化水素ガスによる皮膜除去の効果が薄れ、本発明の目的である層Ｉが形成されなくなる。したがって、炭化水素ガスの量は、硫化水素ガスの１／２以下とすることが好ましい。 Hydrocarbon gas is also used to activate the surface, even in conventional nitriding processes. Even in the present invention, the gas may be mixed with the film removal gas for the same purpose, but if it is contained in a large amount, the effect of the film removal by hydrogen sulfide gas will diminish, and the layer I which is the object of the present invention will not be formed. Therefore, the amount of hydrocarbon gas is preferably 1/2 or less of hydrogen sulfide gas.
硫化水素ガスによる皮膜処理は、副次的な効果ももたらす。すなわち、ハロゲンガスやフッ素系ガスは腐食性・毒性が強く、装置や作業に特別な配慮が必要となり、必然的に製造コストの増加をもたらす。硫化水素ガスも毒性を持つ危険なガスであるが、腐食性はこれらガスほどではなく、窒化炉装置の気密が十分に保たれていれば問題ない。また、排ガス装置も簡便である。炭化水素系ガスも、可燃性であり、装置や作業に特別な配慮が必要であることでは、ハロゲンガスやフッ素系ガスと同じである。 Coating treatment with hydrogen sulfide gas also has secondary effects. That is, halogen gas and fluorine-based gas are highly corrosive and toxic, and special consideration is required for equipment and operations, which inevitably leads to an increase in manufacturing cost. Hydrogen sulfide gas is also a toxic and dangerous gas, but it is not as corrosive as these gases and there is no problem if the air tightness of the nitriding furnace is sufficiently maintained. In addition, the exhaust gas device is also simple. The hydrocarbon-based gas is also flammable, and is the same as the halogen gas and the fluorine-based gas in that special consideration is required for the apparatus and operation.
窒化工程は、窒化温度到達後から開始される。窒化温度到達後、定温保持し、アンモニアガスと窒素ガスの所定の比率からなる混合ガス中で窒化を施す。ガス導入は、酸化皮膜除去工程が始まる、昇温開始後から開始する。窒化温度到達後は、窒化ガスのみ所定の流量で炉へ導入し続ける。この過程でＳ相（層II）が形成される。同時に炭素濃化層（層III）がＳ相と母相の界面に形成される。 The nitriding process is started after reaching the nitriding temperature. After reaching the nitriding temperature, the temperature is maintained and nitriding is performed in a mixed gas consisting of a predetermined ratio of ammonia gas and nitrogen gas. The gas introduction is started after the start of the temperature rise, at which the oxide film removing process starts. After reaching the nitriding temperature, only the nitriding gas is continuously introduced into the furnace at a predetermined flow rate. The S phase (layer II) is formed in this process. At the same time, a carbon-rich layer (layer III) is formed at the interface between the S phase and the parent phase.
窒化工程における加熱温度は４００〜４５０℃、加熱時間は２０〜１００ｈｒとする必要がある。これにより、続く冷却工程を適切な条件で行うことにより、最表面に軟質な窒化層（層Ｉ）が形成される。 The heating temperature in the nitriding step needs to be 400 to 450 ° C., and the heating time needs to be 20 to 100 hr. Thereby, a soft nitrided layer (layer I) is formed on the outermost surface by performing the subsequent cooling process under appropriate conditions.
窒化終了後、アンモニアの導入を停止する。冷却工程における高温中には、窒素ガスのみを導入し続ける。これにより、最表層にＳ相ではない軟質の窒化膜（層Ｉ）が形成される。冷却工程における窒素ガスの導入が十分でないと、窒素が過飽和で浸入できず、適切な層Ｉが形成されない。適切な処理により形成された層ＩはＣｒ系の表面皮膜が除去され、Ｆｅ系の窒化物が形成され、窒素が過飽和で浸入するので、好ましくは、層Ｉ中のγ相中のＦｅ窒化物に含まれるＮの総量が、γ相中のＣｒ窒化物に含まれるＮの総量よりも多くなる。 After the end of the nitriding, the introduction of ammonia is stopped. During the high temperature in the cooling step, only nitrogen gas is continuously introduced. Thereby, the soft nitride film (layer I) which is not the S phase is formed in the outermost layer. If the introduction of nitrogen gas in the cooling step is not sufficient, nitrogen can not enter at supersaturation and a proper layer I can not be formed. The layer I formed by appropriate treatment has a Cr-based surface film removed, an Fe-based nitride is formed, and nitrogen penetrates at supersaturation. The total amount of N contained in is larger than the total amount of N contained in Cr nitride in the γ phase.
このようにして、オーステナイト系ステンレス鋼の表層に耐食性にすぐれ、曲げ試験によっても表面に顕著な割れが観察されない三層からなる窒化層を形成させる。 In this manner, the surface layer of austenitic stainless steel is formed into a three-layer nitrided layer which is excellent in corrosion resistance and in which no noticeable cracks are observed on the surface even by a bending test.
（１）供試材
表１に供試材の化学組成を示す。供試材Ａ、ＢはそれぞれＳＵＳ３０４Ｌ、ＳＵＳ３１６Ｌに相当する。熱延材を供試材は厚さ１ｍｍの板で表面仕上げは２Ｂである。すなわち厚み１ｍｍの冷間圧延材を焼鈍しその後、スケールを酸洗に除去した板を用いた。
(1) Test material Table 1 shows the chemical composition of the test material. Test materials A and B correspond to SUS304L and SUS316L, respectively. The hot-rolled material is a 1 mm thick plate with a surface finish of 2B. That is, a cold-rolled material having a thickness of 1 mm was annealed and then a plate from which scale was removed by pickling was used.
（２）窒化
供試材を図２に示す手順で、昇温、定温保持、降温し、所定のガスを各タイミングと流量で炉に導入し窒化処理を施した。詳細な条件は、表２に示す。窒化処理を施す炉には、日本テクノ製のガス窒化炉（型番ＮＰＭ−５５１２０）を用いた。炉内容量は２００Ｌである。供試材は、アセトンで洗浄後、ガス窒化炉に装入しガス窒化に供した。
(2) Nitriding According to the procedure shown in FIG. 2, the temperature rising, constant temperature holding and temperature lowering were performed, and a predetermined gas was introduced into the furnace at each timing and flow rate to perform nitriding treatment. Detailed conditions are shown in Table 2. The gas nitriding furnace (model number NPM-55120) made from Nippon Techno was used for the furnace which gives nitriding treatment. The furnace capacity is 200 liters. The test material was charged with a gas nitriding furnace after being washed with acetone and subjected to gas nitriding.
（３）各種評価試験 (3) Various evaluation tests
［１］耐食性評価（孔食電位）
孔食電位はＪＩＳ Ｇ０５７７（１９９５年、ステンレス鋼の孔食電位測定方法）に準拠する方法で測定した。溶液濃度３．５％ＮａＣｌ、温度は３０℃、脱気をし、掃引速度は２０ｍＶ／ｍｉｎとした。
[1] Corrosion resistance evaluation (pitting potential)
The pitting potential was measured by a method in accordance with JIS G0577 (1995, method of measuring pitting potential of stainless steel). The solution concentration was 3.5% NaCl, the temperature was 30 ° C., degassing was performed, and the sweep rate was 20 mV / min.
［２］耐曲げ性試験
曲げ半径０ｍｍで１８０度（密着）曲げ後、実体顕微鏡を用いて、曲げ加工部表面を４００倍で観察し、き裂有りを×、き裂無しを○とした。
[2] Bending Resistance Test After bending at 180 degrees (close contact) with a bending radius of 0 mm, the surface of the bent portion was observed at a magnification of 400 using a stereomicroscope.
［３］表層窒化物層の観察
窒化後の供試材の断面を研磨後、（１μｍダイヤバフ）、シュウ酸電解腐食を施し、光学顕微鏡で、２００〜１０００倍で観察し、窒化層の構造を観察し、層の厚みを測定した。
[3] Observation of surface nitride layer After polishing the cross section of the test material after nitriding, (1 μm dia buff), electrolytic oxidation is applied with oxalic acid, and observed with an optical microscope at 200 to 1000 times, the structure of the nitrided layer It observed and measured the thickness of the layer.
（４）試験結果
窒化条件と耐食性、耐曲げ性の試験結果を表２に示す。
(4) Test results Table 2 shows the test results of the nitriding conditions, corrosion resistance, and bending resistance.
試験番号１〜５は本発明例である。Ｃｒ酸化皮膜改質ガスにはＨ２Ｓを用いている。試験番号５はＨ２Ｓガスに加えて０．５Ｌ／ｈｒのＣ２Ｈ２ガスを流している。いずれも、窒化層は、前述の層Ｉ、層II、層IIIの三層から成り立っている。 The test numbers 1 to 5 are inventive examples. H 2 S is used as the Cr oxide film reforming gas. In Test No. 5, in addition to H 2 S gas, 0.5 L / hr of C 2 H 2 gas is flowed. In any case, the nitrided layer is composed of three layers of the aforementioned layer I, layer II and layer III.
図３に、試験番号１の組織写真を示す。図４に、試験番号１のＸ線回折図を示す。最表層にＦｅＮとＣｒＮが母相γ相中に析出している。層IIはＳ相のみで構成されている。 FIG. 3 shows a tissue photograph of Test No. 1. An X-ray diffraction diagram of Test No. 1 is shown in FIG. FeN and CrN are precipitated in the matrix γ phase in the outermost layer. Layer II is composed of only the S phase.
比較例である試験番号は、酸化皮膜除去工程、又は冷却工程のガス条件が適切で無いため、層Ｉが形成されず、曲げ性に劣る結果となった。 In the test number which is a comparative example, the layer I was not formed because the gas conditions in the oxide film removing step or the cooling step were not appropriate, resulting in poor bendability.
Claims (4)
γ相中にＦｅの窒化物とＣｒの窒化物を含んだ最表層と、
Ｎを１０ｍａｓｓ％以上過飽和に固溶したＳ相からなる層と、
炭素の濃化層
の三層を、表面に有することを特徴とするオーステナイト系ステンレス鋼。 In order from the surface to the inner layer,
The outermost layer containing Fe nitride and Cr nitride in the γ phase,
A layer consisting of an S phase in which N is dissolved in a supersaturation of 10 mass% or more,
An austenitic stainless steel characterized by having three layers of carbon enriched layers on the surface.
ステンレス鋼板、又は製品形状に加工されたステンレス鋼を硫化水素、窒素、及びアンモニアガスを導入した雰囲気下で４００〜４５０℃の範囲まで昇温する酸化皮膜除去工程と、
上記ステンレス鋼を、硫化水素ガスの導入を停止し、窒素、及びアンモニアガスを導入した雰囲気下で４００〜４５０℃で、２０〜１００時間保持する窒化工程と、
上記ステンレス鋼を、アンモニアガスの導入を停止し、窒素ガスを導入した雰囲気下で冷却する冷却工程
を順に備えることを特徴とするオーステナイト系ステンレス鋼の製造方法。 A method of producing an austenitic stainless steel according to claim 1 or 2, wherein
An oxide film removing step of raising the temperature of a stainless steel plate or stainless steel processed into a product shape to a range of 400 to 450 ° C. in an atmosphere containing hydrogen sulfide, nitrogen and ammonia gas;
A nitriding step of stopping the introduction of hydrogen sulfide gas and holding the above stainless steel at 400 to 450 ° C. for 20 to 100 hours in an atmosphere in which nitrogen and ammonia gas are introduced;
A method for producing an austenitic stainless steel, comprising a cooling step of sequentially cooling the stainless steel in an atmosphere in which introduction of ammonia gas is stopped and nitrogen gas is introduced.
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