JP3588826B2 - Heat treatment method for high nitrogen containing stainless steel - Google Patents
Heat treatment method for high nitrogen containing stainless steel Download PDFInfo
- Publication number
- JP3588826B2 JP3588826B2 JP22436294A JP22436294A JP3588826B2 JP 3588826 B2 JP3588826 B2 JP 3588826B2 JP 22436294 A JP22436294 A JP 22436294A JP 22436294 A JP22436294 A JP 22436294A JP 3588826 B2 JP3588826 B2 JP 3588826B2
- Authority
- JP
- Japan
- Prior art keywords
- stainless steel
- heat treatment
- gas
- nitrogen
- high nitrogen
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Landscapes
- Heat Treatment Of Articles (AREA)
- Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
- Heat Treatment Of Steel (AREA)
Description
【０００１】
【産業上の利用分野】
本発明は、優れた耐食性が要求される高窒素含有ステンレス鋼の鋼管、鋼板、棒鋼等に適用される熱処理方法に関するものである。
【０００２】
【従来の技術】
ステンレス鋼の用途として、オーステナイト系ステンレス鋼（ＳＵＳ ３０４ 、３１６ 等）、フェライト系ステンレス鋼（ＳＵＳ ４３０ 等）が、それぞれ特徴ある材料として広く用いられており、さらに、この両者の特徴を兼ね備え、弱点を相補完しうるフェライト・オーステナイト二相ステンレス鋼（ＳＵＳ ３２９ 等）が注目されている。
【０００３】
最近、耐食性金属材料一般について、海水環境での使用など使用環境が過酷になるに従い、要求される耐食性能が高度になってきており、この要求に対応した耐食性ステンレス鋼の開発が多くなされている。
【０００４】
ステンレス鋼の耐食性、特に局部腐食である孔食や隙間腐食に対する抵抗力を高める方法として、合金成分であるＣｒ、Ｍｏ、Ｗ、さらにＮを高めることが有効であり、これらの孔食に対する抵抗力を、Ｃｒ１％と等価な値に換算して定量的に表現するパラメータとして、下記 （１）式で示すＰＲＥＷ（Ｐｉｔｔｉｎｇ Ｒｅｓｉｓｔａｎｃｅ Ｅｑｕｉｖａｌｅｎｔ ｗｉｔｈ Ｔｕｎｇｓｔｅｎ ）が知られている。
【０００５】
ＰＲＥＷ＝Ｃｒ＋ ３．３（Ｍｏ＋ ０．５Ｗ）＋１６Ｎ ・・・ （１）
一般に、海水環境での使用などを想定する場合には、このＰＲＥＷが３５以上になるようにＣｒ、Ｍｏ、Ｗ、さらにＮの含有量が調整される。
【０００６】
（１）式から明らかなように、これらの合金成分のうちのＮは優れた耐食性改善元素であるとともに、強力なオーステナイト生成元素である。また、フェライト生成元素であるＣｒ、Ｍｏも有効な耐食性改善成分であるから、通常、耐食性ステンレス鋼にはＣｒ、Ｍｏが多量に添加されることになる。この場合には、Ｎはオーステナイトの熱的安定性を図るため、比較的高濃度に添加されるので、高耐食性ステンレス鋼として高窒素含有ステンレス鋼になるのが特徴である。
【０００７】
上記の高窒素含有ステンレス鋼を、耐食性材料として鋼管、鋼板、棒鋼等に熱間加工または冷間加工する場合、加工により硬化した材料を軟化するため、および最終製品としての要求に合致した組織、強度を得るために熱処理を施す必要がある。この熱処理として、燃焼排ガス雰囲気中で行う方法と制御された雰囲気中で行う光輝熱処理法とがある。
【０００８】
燃焼排ガス雰囲気中で行う方法は、ステンレス鋼の熱処理として汎用されている方法ではあるが、表面に酸化スケールが発生するため、熱処理後に表面の脱スケールが必要になる。脱スケール法には、例えば、ショットブラスト法、砥石研削法等のメカニカル脱スケール法があるが、これらは極めて非能率（特に、小径鋼管の内面脱スケールは困難）であることから、酸洗による脱スケールが多用される。しかし、酸洗脱スケールを行う場合であっても、対象が高窒素含有ステンレス鋼であると、鋼自体が酸に対する高い耐食性をもつため、脱スケールが困難となって、多大な工数および費用が掛かることになる。
【０００９】
制御雰囲気中で行う方法を採用すれば、酸化スケールの発生が防止できて脱スケールが省略できる、または薄い酸化スケールの生成に留まって脱スケールが容易になることが期待できる。このような観点から、特公昭６１−８１３０号公報で開示されているように、雰囲気ガスとしてＨ２単味のガスとＮ２単味のガスとを最適割合に混合したガスを用いることを特徴とするステンレス鋼の光輝焼鈍法の提案がある。
【００１０】
しかしながら、この光輝熱処理法が対象とする鋼種は、ＳＵＳ ３０４ 、ＳＵＳ ２０２ およびＳＵＳ ４３０ に規定される窒素含有量が低いオーステナイト系およびフェライト系のステンレス鋼 （Ｎ＜０．１ ％） に限定されるものであり、高窒素含有ステンレス鋼の熱処理に関するものではない。さらに高窒素含有ステンレス鋼に光輝熱処理を適用すると、鋼表面に窒素吸収や脱窒素の現象が生じ、極端に耐食性が劣化する事態も発生した。
【００１１】
【発明が解決しようとする課題】
本発明の目的は、耐食性金属材料として優れる高窒素含有ステンレス鋼の実用化を一層進めるために、上記した従来技術の問題点を克服して、光輝熱処理法を確立することにある。
【００１２】
【課題を解決するための手段】
本発明は、次の （１）、 （２）の高窒素含有ステンレス鋼の熱処理方法を要旨としている。
【００１３】
（１）質量％で、Ｎ：０．２７２〜０．９％を含有する高窒素含有ステンレス鋼を、Ｎ2ガスが１〜１５容量％、残部がＨ2ガスからなる混合ガスであって、かつその露点が−１０℃未満である混合ガスの雰囲気で、１０３０〜１２５０℃の温度範囲に加熱したのち、５００℃までの平均冷却速度を１００℃／分以上として冷却することを特徴とする高窒素含有ステンレス鋼の熱処理方法。
【００１４】
（２） 高窒素含有ステンレス鋼を、Ｎ２ガスが ０．５〜５０容量％、残部がＨ２ガスからなる混合ガスであって、かつその露点が−１０℃以上である混合ガスの雰囲気で、１０３０〜１２５０℃の温度範囲に加熱したのち、 ５００℃までの平均冷却速度を １００℃／分以上として冷却することを特徴とする高窒素含有ステンレス鋼の熱処理方法。
【００１５】
上記の高窒素含有ステンレス鋼として、質量％で、Ｃｒ：１６〜３０％、Ｍｏ＋０．５Ｗ：２〜８％、Ｎ：０．２７２〜０．９％を含有する二相ステンレス鋼、さらにＮｉ：１０％以上を含有するオーステナイト系ステンレス鋼であって、かつ下記（１）式で表されるＰＲＥＷが３５以上であることが望ましい。
【００１６】
ＰＲＥＷ＝Ｃｒ＋ ３．３（Ｍｏ＋ ０．５Ｗ）＋１６Ｎ ・・・ （１）
ただし、 （１）式中の元素記号は各元素の含有量（重量％）を示す。
【００１７】
【作用】
本発明は、高窒素含有を特徴とする耐食性ステンレス鋼の合理的な熱処理を実現するために、光輝熱処理を前提として、鋼管、鋼板、棒鋼等の鋼表面に生じる窒素吸収層や脱窒素層の形成による耐食性の劣化を防止するため、熱処理雰囲気の露点とＮ２ガス混合比率を規定すること、さらに、炭窒化物あるいは金属間化合物の析出による耐食性劣化を防止するため、加熱温度および冷却速度を規定することを特徴としている。
【００１８】
以下に、光輝熱処理の条件限定の理由と対象となる高窒素含有ステンレス鋼の望ましい成分範囲を説明する。
【００１９】
１．光輝熱処理の条件限定の理由
熱処理温度：高窒素含有ステンレス鋼の加熱温度は、１０３０〜１２５０℃の範囲とする。加熱温度が１０３０℃未満では、昇温中に析出する金属間化合物（σ相等）が未固溶になるため、熱処理後も十分な機械的性質および耐食性が得られない。一方、加熱温度が１２５０℃を超えると、二相ステンレス鋼の場合には、フェライト量が増加し、耐食性および靱性が劣化し、オーステナイト系ステンレス鋼の場合には、著しい粒成長を起こし、同様に耐食性が劣化し、強度も低下する。ただし、昇温速度については、十分に加熱温度が確保される限りにおいて、問題とならない。
【００２０】
熱処理雰囲気：高窒素含有ステンレス鋼の光輝熱処理においては、鋼表面の窒素吸収、脱窒素の現象が重要である。前述の特公昭６１−８１３０号公報では、Ｎ２ガスの多い雰囲気でＳＵＳ ３０４ 、ＳＵＳ ２０２ の鋼種を熱処理すると、窒素吸収の現象が生じるが、吸収された窒素は窒化物（ＣｒＮ、 Ｃｒ２Ｎ）としては存在せずに、すべて固溶状態で存在すると開示している。しかし、発明者らの実験結果によれば、光輝熱処理時に鋼表面に生じる窒素吸収、脱窒素の現象は熱処理雰囲気の条件（露点）によって左右され、吸収された窒素の挙動も異なったものになる。
【００２１】
熱処理雰囲気の露点が低い （−１０℃未満） 場合には、Ｎ２ガスとＨ２ガスとの混合からなる雰囲気ガスの割合がＮ２ガスが３０容量％を超えると、鋼表面では窒素吸収の現象が生じる。このとき吸収された窒素の一部は、ステンレス鋼中の金属元素と反応して窒化物を生成し、析出する。このため、表面に脆化層が形成され、耐食性が劣化する。また熱処理対象が二相ステンレス鋼であれば、その表層のみにオーステナイト化が起こり、二相ステンレス鋼の基本的な性質が失われる。一方、Ｎ２ガスの混合割合が１容量％未満になると、鋼表面では脱窒素の現象が生じ、このため鋼表面の耐食性が劣化する。さらに表面近傍では、フェライト単相組織となり、本来の特徴がなくなり耐食性および靱性の劣化を招く。したがって、高窒素含有ステンレス鋼の光輝熱処理において、雰囲気ガスはＮ２ガスが１〜３０容量％である場合には、その露点が−１０℃未満である混合ガスを使用しなければならない。
【００２２】
熱処理雰囲気の露点が比較的高くなると （−１０℃以上） 、鋼表面に薄い酸化膜が生成する。この酸化膜が鋼表面での窒素吸収、脱窒素の現象を防止するので、Ｎ２ガスの割合が５０容量％を超えるまで窒素吸収が発生せず、Ｎ２ガスの割合が ０．５容量％未満になるまで脱窒素が生じない。したがって、高窒素含有ステンレス鋼の光輝熱処理で使用する雰囲気ガスは、Ｎ２ガスが ０．５〜５０容量％である場合には、その露点が−１０℃以上の条件を具備する必要がある。
【００２３】
平均冷却速度：加熱後の冷却速度が １００℃／分未満では、冷却中に炭窒化物および金属間化合物（σ相等）が析出し、耐食性および靱性の劣化を招くことになる。冷却速度が平均して １００℃／分以上であれば、耐食性劣化の問題が生じない。
【００２４】
２．高窒素含有ステンレス鋼の成分範囲（含有量の％は全て質量％を意味する）
Ｃｒ：Ｃｒは耐食性を維持するために有効な基本成分である。その含有量が１６％未満では、耐食性を確保するために、Ｍｏ、ＷおよびＮを高濃度に添加しなければならず、熱間加工性が困難となり、また、窒化物、金属間化合物の析出によって耐食性の劣化が顕著になる。一方、Ｃｒ含有量が３０％を超えると、金属間化合物（特にσ相）の析出が顕著となって、靱性および耐食性の劣化を招く。
【００２５】
ＭｏおよびＷ：
Ｍｏは耐食性を向上させるのに非常に有効な成分である。特に、Ｎとの複合作用による耐食性を高めるために、２％以上の含有が必須である。一方、含有量が８％を超えると、Ｃｒと同様に、金属間化合物の析出が顕著となるため、素材の靱性および耐食性が劣化する。
【００２６】
Ｗは、Ｍｏと同様に、耐食性を向上させる効果を有し、また低温側での時効熱処理に際し、ＣｒあるいはＭｏに比べ金属間化合物の析出を抑制することができるので、添加するのが有効である。しかし、高価な元素であるため経済性から含有量を５％以下とする。さらに、上記の効果を得るために積極的に添加する場合は、 ０．１％以上含有させるのが望ましい。
【００２７】
一般にＷは合金成分としてＭｏと同等の作用効果を有するものとされ、Ｍｏとその１／２ 量のＷが均等物として取り扱われることが多い。このため、本発明ではＭｏ＋ ０．５Ｗの含有量を、２〜８％とする。
【００２８】
Ｎ：前述の通り、Ｎは強力なオーステナイト生成元素であるから、Ｃｒ、Ｍｏが多量に添加される場合には、０．１％以上の含有が必要となる。一方、Ｎの含有量が０．９％を超えると、窒化物の生成にともなう靱性、耐食性の劣化を招くとともに、凝固時または溶接時にブローホールが生成し易くなる。そのため、Ｎの含有量は０．２７２〜０．９％とする。
【００２９】
Ｎｉ：Ｎｉはオーステナイトを安定化するために必須の成分であり、熱処理対象がオーステナイト系ステンレス鋼の場合には、オーステナイト単相組織とするために、含有量は１０％以上とする。Ｎｉ含有量の上限は、経済性を考慮して４０％とするのが望ましい。
【００３０】
対象が二相ステンレス鋼である場合には、その含有量が１０％を超えるとフェライト量の減少によって二相ステンレス鋼の基本的な性質が確保しにくくなり、一方、含有量が４％より少ないとフェライト量が多くなりすぎて二相ステンレス鋼の特徴が失われるので、Ｎｉの含有量は４〜１０％とするのが望ましい。
【００３１】
Ｍｎ：Ｍｎも高窒素含有オーステナイト系ステンレス鋼に多量に添加されるが、その含有量は特に限定されない。
【００３２】
ＰＲＥＷ（Ｃｒ＋ ３．３（Ｍｏ＋ ０．５Ｗ）＋１６Ｎ）：
ＰＲＥＷ値の増大にともなって、耐食性、特に耐孔食性が向上するので、海水環境での使用であっても耐孔食性を確保するため３５以上とする。現在、使用環境の過酷化に対応して、ＰＲＥＷ値が４６を超える耐食性ステンレス鋼の開発がなされているが、勿論これらも本発明の対象とされる。
【００３３】
【実施例】
２０ｋｇの真空溶解炉で表１に示す高窒素ステンレス鋼（二相ステンレス鋼としてＡ、ＤおよびＥ鋼の３鋼種、オーステナイト系ステンレス鋼としてＣ鋼の１鋼種）を溶製してインゴットとし、これを１２００℃に加熱して厚さ１５ｍｍの板材に鍛造した。
【００３４】
【表１】
これらの板材を下記の条件で光輝熱処理した後、幅１０ｍｍ×厚さ３ｍｍ×長さ４０ｍｍの試験片を切り出して、耐孔食性の調査を行った。
【００３６】
１．熱処理温度：加熱温度１１４０℃で、均熱時間１０分。
【００３７】
２．熱処理雰囲気：露点は−４５℃、−１５℃および−１０℃の３条件。
【００３８】
Ｎ２ガスの割合を０から５５容量％に変化させる（残部はＨ２ガスを使用）。
【００３９】
３．平均冷却速度： １５０℃／分、 １００℃／分および９０℃／分の３条件。
【００４０】
耐孔食性の調査は、各温度（５０℃、５５℃、６０℃、６５℃、７０℃、７５℃および８０℃）の６％ＦｅＣｌ3溶液に試験片をそれぞれ２４時間浸漬した後、孔食が発生しない限界の温度（ＣＰＴ）を求めた。孔食発生の判定は、２４時間浸漬後１０ｍｇ以上の減量がある場合を孔食発生と判定した。各鋼種の判定結果を表２および表４に示す。表中の耐孔食性の判定は、ＣＰＴが７０℃以上の場合を○とし、８０℃以上の場合を◎とした。
【００４１】
【表２】
【表４】
表２および表４から明らかなように、Ｎ2ガスの割合や冷却速度が本発明で規定する条件から外れる条件で光輝熱処理を行った比較例では、ＣＰＴは７０℃未満となっているのに対して、本発明例ではいずれもＣＰＴが７０℃以上となって、優れた耐孔食性を示している。
【００４５】
【発明の効果】
本発明方法によれば、最近使用が増加している高窒素含有ステンレス鋼の鋼管、鋼板、棒鋼製品における表面の窒素吸収、脱窒素による耐食性の劣化が防止でき、さらに不完全な酸洗脱スケールに起因する表面付着物を防いで、清浄な耐食性ステンレス鋼製品の提供することが可能となる。しかも、酸洗脱スケール工程を省略あるいは簡略化できるので製造コストの低減、ならびに弗酸、硝酸等の薬品の使用量の削減が可能となり環境改善にも貢献することができる。
【００４６】[0001]
[Industrial applications]
The present invention relates to a heat treatment method applied to a steel pipe, a steel plate, a steel bar, or the like of a high nitrogen-containing stainless steel requiring excellent corrosion resistance.
[0002]
[Prior art]
Austenitic stainless steels (SUS 304, 316, etc.) and ferritic stainless steels (SUS 430, etc.) are widely used as distinctive materials for stainless steel applications. Ferrite-austenite duplex stainless steels (such as SUS 329), which can complement each other, have attracted attention.
[0003]
Recently, as for corrosion-resistant metal materials in general, the required corrosion resistance performance has become higher as the use environment becomes severer such as use in a seawater environment, and corrosion-resistant stainless steel corresponding to this requirement has been developed much. .
[0004]
As a method of increasing the corrosion resistance of stainless steel, especially the resistance to pitting and crevice corrosion, which are local corrosion, it is effective to increase the alloy components Cr, Mo, W, and N, and the resistance to these pitting corrosion is effective. Is known as a parameter equivalent to Cr 1% and quantitatively expressed by the following formula (1): PREW (Pitting Resistance Equivalent with Tungsten).
[0005]
PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)
Generally, when use in a seawater environment is assumed, the contents of Cr, Mo, W, and N are adjusted so that the PREW is 35 or more.
[0006]
As is apparent from the equation (1), N among these alloy components is an excellent corrosion resistance improving element and a powerful austenite forming element. Further, since Cr and Mo, which are ferrite forming elements, are also effective corrosion resistance improving components, a large amount of Cr and Mo is usually added to corrosion resistant stainless steel. In this case, since N is added at a relatively high concentration in order to achieve the thermal stability of austenite, it is characterized in that it becomes a high-nitrogen-containing stainless steel as a high corrosion-resistant stainless steel.
[0007]
When hot-working or cold-working the above high-nitrogen-containing stainless steel into a steel pipe, a steel plate, a steel bar, or the like as a corrosion-resistant material, in order to soften the material hardened by the working, and a structure that meets the requirements as a final product, Heat treatment must be performed to obtain strength. As this heat treatment, there are a method performed in a combustion exhaust gas atmosphere and a bright heat treatment method performed in a controlled atmosphere.
[0008]
The method performed in a combustion exhaust gas atmosphere is a method widely used for heat treatment of stainless steel. However, since scale is generated on the surface, descaling of the surface is required after the heat treatment. As the descaling method, for example, there are mechanical descaling methods such as a shot blast method and a grindstone grinding method. However, these methods are extremely inefficient (especially, it is difficult to descaling the inner surface of a small diameter steel pipe). Descaling is frequently used. However, even when pickling and descaling, if the target is a high-nitrogen-containing stainless steel, the steel itself has high corrosion resistance to acids, making descaling difficult, resulting in large man-hours and costs. Will hang.
[0009]
If a method performed in a controlled atmosphere is adopted, it can be expected that the generation of oxide scale can be prevented and descaling can be omitted, or that descaling can be facilitated by forming only a thin oxide scale. From such a point of view, as disclosed in Japanese Patent Publication No. 61-8130, a gas obtained by mixing a gas of plain H 2 and a gas of plain N 2 in an optimal ratio is used as an atmosphere gas. There is a proposal of a bright annealing method for stainless steel.
[0010]
However, the steel types targeted by the bright heat treatment method are limited to austenitic and ferritic stainless steels (N <0.1%) having a low nitrogen content specified in SUS 304, SUS 202 and SUS 430. And does not relate to heat treatment of stainless steel containing high nitrogen. Further, when the bright heat treatment is applied to the high nitrogen content stainless steel, a phenomenon of nitrogen absorption and denitrification occurs on the steel surface, and a situation in which the corrosion resistance is extremely deteriorated also occurred.
[0011]
[Problems to be solved by the invention]
An object of the present invention is to overcome the above-mentioned problems of the prior art and to establish a bright heat treatment method in order to further promote the practical use of a high-nitrogen-containing stainless steel excellent as a corrosion-resistant metal material.
[0012]
[Means for Solving the Problems]
The gist of the present invention is the following (1) and (2) heat treatment methods for high nitrogen-containing stainless steel.
[0013]
(1) A high nitrogen-containing stainless steel containing 0.272 to 0.9% by mass of N: 0.22 to 0.9%, a mixed gas comprising 1 to 15 % by volume of N 2 gas and H 2 gas as a balance, After heating to a temperature range of 1030 to 1250 ° C. in an atmosphere of a mixed gas having a dew point of less than −10 ° C., cooling is performed with an average cooling rate up to 500 ° C. being 100 ° C./min or more. Heat treatment method for nitrogen-containing stainless steel.
[0014]
(2) High-nitrogen-containing stainless steel was mixed in an atmosphere of a mixed gas containing 0.5 to 50% by volume of N 2 gas and the balance being H 2 gas, and having a dew point of −10 ° C. or more. And heating at a temperature in the range of 1030 to 1250 ° C., and then cooling at an average cooling rate up to 500 ° C. of 100 ° C./min or more.
[0015]
As the above high nitrogen-containing stainless steel, a duplex stainless steel containing, by mass% , Cr: 16 to 30%, Mo + 0.5W: 2 to 8%, and N: 0.272 to 0.9%, and Ni: It is preferable that the austenitic stainless steel contains 10% or more and PREW represented by the following formula (1) is 35 or more.
[0016]
PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)
Here, the element symbols in the formula (1) indicate the content (% by weight) of each element.
[0017]
[Action]
The present invention is based on the premise of bright heat treatment, in order to realize a rational heat treatment of corrosion-resistant stainless steel characterized by high nitrogen content, a steel pipe, a steel sheet, a nitrogen absorption layer or a denitrification layer generated on the steel surface of a steel bar or the like. In order to prevent the deterioration of the corrosion resistance due to the formation, the dew point of the heat treatment atmosphere and the N 2 gas mixture ratio are specified. Further, in order to prevent the deterioration of the corrosion resistance due to the precipitation of carbonitrides or intermetallic compounds, the heating temperature and the cooling rate are reduced. It is characterized by stipulating.
[0018]
The reason for limiting the conditions of the bright heat treatment and the desirable component range of the high-nitrogen-containing stainless steel as an object will be described below.
[0019]
1. Reasons for limitation of bright heat treatment conditions Heat treatment temperature: The heating temperature of the high nitrogen-containing stainless steel is in the range of 1030 to 1250 ° C. If the heating temperature is less than 1030 ° C., the intermetallic compound (eg, σ phase) that precipitates during the temperature rise becomes insoluble, so that sufficient mechanical properties and corrosion resistance cannot be obtained even after the heat treatment. On the other hand, when the heating temperature exceeds 1250 ° C., in the case of a duplex stainless steel, the amount of ferrite increases, the corrosion resistance and toughness deteriorate, and in the case of an austenitic stainless steel, significant grain growth occurs. Corrosion resistance deteriorates and strength also decreases. However, the heating rate does not matter as long as the heating temperature is sufficiently secured.
[0020]
Heat treatment atmosphere: In bright heat treatment of stainless steel containing high nitrogen, the phenomenon of nitrogen absorption and denitrification on the steel surface is important. According to the above-mentioned Japanese Patent Publication No. 61-8130, when SUS 304 and SUS 202 steels are heat-treated in an atmosphere containing a large amount of N 2 gas, a phenomenon of nitrogen absorption occurs, but the absorbed nitrogen is converted into nitride (CrN, Cr 2 N). ) Is disclosed as not being present but being all present in a solid solution state. However, according to the experimental results of the inventors, the phenomenon of nitrogen absorption and denitrification occurring on the steel surface during the bright heat treatment is affected by the conditions (dew point) of the heat treatment atmosphere, and the behavior of the absorbed nitrogen is different. .
[0021]
When the dew point of the heat treatment atmosphere is low (less than −10 ° C.), if the ratio of the atmosphere gas composed of a mixture of N 2 gas and H 2 gas exceeds 30% by volume of N 2 gas, the nitrogen absorption on the steel surface is reduced. A phenomenon occurs. Part of the nitrogen absorbed at this time reacts with the metal elements in the stainless steel to form nitrides and precipitates. For this reason, an embrittlement layer is formed on the surface, and the corrosion resistance deteriorates. If the object of heat treatment is a duplex stainless steel, austenitization occurs only in the surface layer, and the basic properties of the duplex stainless steel are lost. On the other hand, when the mixing ratio of the N 2 gas is less than 1% by volume, a phenomenon of denitrification occurs on the steel surface, and the corrosion resistance of the steel surface is deteriorated. Further, in the vicinity of the surface, a ferrite single phase structure is formed, and the original characteristics are lost, resulting in deterioration of corrosion resistance and toughness. Therefore, in the bright heat treatment of the high-nitrogen-containing stainless steel, when the N 2 gas is 1 to 30% by volume, a mixed gas having a dew point of less than −10 ° C. must be used as the atmosphere gas.
[0022]
When the dew point of the heat treatment atmosphere is relatively high (-10 ° C. or higher), a thin oxide film is formed on the steel surface. Since this oxide film prevents nitrogen absorption and denitrification on the steel surface, nitrogen absorption does not occur until the ratio of N 2 gas exceeds 50% by volume, and the ratio of N 2 gas is 0.5% by volume. No denitrification occurs until less than. Therefore, the atmosphere gas used in bright heat treatment of high nitrogen containing stainless steel, when N 2 gas is 0.5 to 50% by volume, it is necessary to its dew point to mount an -10 ° C. or more.
[0023]
Average cooling rate: If the cooling rate after heating is less than 100 ° C./min, carbonitrides and intermetallic compounds (such as σ phase) precipitate during cooling, resulting in deterioration of corrosion resistance and toughness. If the cooling rate is 100 ° C./min or more on average, the problem of deterioration of corrosion resistance does not occur.
[0024]
2. Composition range of stainless steel containing high nitrogen (% of content means mass% )
Cr: Cr is a basic component effective for maintaining corrosion resistance. If the content is less than 16%, Mo, W and N must be added at a high concentration in order to secure corrosion resistance, so that hot workability becomes difficult, and precipitation of nitrides and intermetallic compounds occurs. As a result, the deterioration of corrosion resistance becomes remarkable. On the other hand, if the Cr content exceeds 30%, precipitation of intermetallic compounds (especially σ phase) becomes remarkable, leading to deterioration in toughness and corrosion resistance.
[0025]
Mo and W:
Mo is a very effective component for improving corrosion resistance. In particular, in order to increase the corrosion resistance due to the combined action with N, the content of 2% or more is essential. On the other hand, if the content exceeds 8%, as in the case of Cr, precipitation of the intermetallic compound becomes remarkable, so that the toughness and corrosion resistance of the material deteriorate.
[0026]
W has an effect of improving corrosion resistance similarly to Mo, and can suppress precipitation of an intermetallic compound as compared with Cr or Mo at the time of aging heat treatment at a low temperature side. Therefore, it is effective to add W. is there. However, since it is an expensive element, the content is set to 5% or less from the viewpoint of economy. Further, in the case where it is positively added in order to obtain the above effects, it is desirable that the content be 0.1% or more.
[0027]
Generally, W is assumed to have the same function and effect as Mo as an alloy component, and Mo and a half amount of W are often treated as equivalents. Therefore, in the present invention, the content of Mo + 0.5W is set to 2 to 8%.
[0028]
N: As described above, N because is a strong austenite forming element, Cr, when Mo is added in large amounts is that Do required content of 0.1% or more. Hand, if the content of N exceeds 0.9%, toughness due to formation of nitrides, with leads to corrosion degradation, blowholes are easily generated during the solidification or during welding. Therefore, the content of N is set to 0.272 to 0.9%.
[0029]
Ni: Ni is an essential component for stabilizing austenite. When the object of heat treatment is austenitic stainless steel, the content is set to 10% or more in order to form an austenitic single phase structure. It is desirable that the upper limit of the Ni content be 40% in consideration of economy.
[0030]
When the target is a duplex stainless steel, if the content exceeds 10%, it becomes difficult to secure the basic properties of the duplex stainless steel due to a decrease in the amount of ferrite, while the content is less than 4%. And the amount of ferrite becomes too large and the characteristics of the duplex stainless steel are lost, so the Ni content is desirably 4 to 10%.
[0031]
Mn: Mn is also added in large amounts to the high nitrogen-containing austenitic stainless steel, but the content is not particularly limited.
[0032]
PREW (Cr + 3.3 (Mo + 0.5W) + 16N):
Corrosion resistance, especially pitting corrosion resistance, is improved with an increase in the PREW value. Therefore, even when used in a seawater environment, the pitting resistance is set to 35 or more to ensure pitting corrosion resistance. At present, corrosion-resistant stainless steels having a PREW value of more than 46 have been developed in response to severer use environments, but these are, of course, also objects of the present invention.
[0033]
【Example】
20kg high nitrogen stainless steels shown in Table 1 in a vacuum melting furnace (A as duplex stainless steel, 3 steels D and E steel, 1 steels C steel and austenitic stainless steel) was melted and an ingot, This was heated to 1200 ° C. and forged into a plate having a thickness of 15 mm.
[0034]
[Table 1]
After these plate materials were subjected to bright heat treatment under the following conditions, test pieces having a width of 10 mm, a thickness of 3 mm and a length of 40 mm were cut out, and the pitting corrosion resistance was investigated.
[0036]
1. Heat treatment temperature: heating temperature 1140 ° C, soaking time 10 minutes.
[0037]
2. Heat treatment atmosphere: Dew point is -45 ° C, -15 ° C and -10 ° C.
[0038]
The ratio of N 2 gas is changed from 0 to 55% by volume (the rest uses H 2 gas).
[0039]
3. Average cooling rate: 150 ° C./min, 100 ° C./min and 90 ° C./min.
[0040]
The pitting resistance was investigated by immersing the test pieces in 6% FeCl 3 solution at each temperature (50 ° C., 55 ° C., 60 ° C., 65 ° C., 70 ° C., 75 ° C. and 80 ° C.) for 24 hours, and then pitting. The limit temperature (CPT) at which no cracks occurred was determined. Pitting was determined to occur when there was a weight loss of 10 mg or more after immersion for 24 hours. Tables 2 and 4 show the determination results for each steel type. The pitting corrosion resistance in the table was evaluated as ○ when the CPT was 70 ° C or higher, and as ◎ when the CPT was 80 ° C or higher.
[0041]
[Table 2]
[Table 4]
As is clear from Tables 2 and 4, in the comparative example in which the bright heat treatment was performed under the condition that the ratio of the N 2 gas and the cooling rate were out of the conditions specified in the present invention, the CPT was less than 70 ° C. On the other hand, in each of the examples of the present invention, the CPT is 70 ° C. or higher, and the pitting corrosion resistance is excellent.
[0045]
【The invention's effect】
According to the method of the present invention, the use of high-nitrogen-containing stainless steel pipes, steel sheets, and bar steel products, which have recently been increasing in use, can prevent surface nitrogen absorption and deterioration of corrosion resistance due to denitrification, and further imperfect pickling descaling. Thus, it becomes possible to provide a clean corrosion-resistant stainless steel product while preventing surface deposits caused by the corrosion. In addition, since the pickling and descaling step can be omitted or simplified, the production cost can be reduced and the amount of chemicals such as hydrofluoric acid and nitric acid can be reduced, which can contribute to environmental improvement.
[0046]
Claims (4)
ＰＲＥＷ＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎ ・・・ （１）
ただし、（１）式中の元素記号は各元素の含有量（質量％）を示す。The high nitrogen-containing stainless steel contains, by mass%, Cr: 16 to 30%, Mo + 0.5W: 2 to 8%, and N: 0.272 to 0.9%, and is represented by the following formula (1). 3. The method for heat treating high nitrogen content stainless steel according to claim 1, wherein the stainless steel is a duplex stainless steel having a PREW of 35 or more.
PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)
Here, the element symbols in the formula (1) indicate the contents (% by mass) of each element.
ＰＲＥＷ＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎ ・・・ （１）
ただし、（１）式中の元素記号は各元素の含有量（質量％）を示す。The high nitrogen-containing stainless steel contains, by mass%, Cr: 16 to 30%, Mo + 0.5W: 2 to 8%, N: 0.272 to 0.9%, Ni: 10% or more, and the following ( The heat treatment method for a high nitrogen-containing stainless steel according to claim 1 or 2, wherein the PREW represented by the formula (1) is an austenitic stainless steel having 35 or more.
PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)
Here, the element symbols in the formula (1) indicate the contents (% by mass) of each element.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP22436294A JP3588826B2 (en) | 1994-09-20 | 1994-09-20 | Heat treatment method for high nitrogen containing stainless steel |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP22436294A JP3588826B2 (en) | 1994-09-20 | 1994-09-20 | Heat treatment method for high nitrogen containing stainless steel |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH0885820A JPH0885820A (en) | 1996-04-02 |
| JP3588826B2 true JP3588826B2 (en) | 2004-11-17 |
Family
ID=16812576
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP22436294A Expired - Fee Related JP3588826B2 (en) | 1994-09-20 | 1994-09-20 | Heat treatment method for high nitrogen containing stainless steel |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP3588826B2 (en) |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE527178C2 (en) * | 2003-03-02 | 2006-01-17 | Sandvik Intellectual Property | Use of a duplex stainless steel alloy |
| SE527175C2 (en) * | 2003-03-02 | 2006-01-17 | Sandvik Intellectual Property | Duplex stainless steel alloy and its use |
| SE528008C2 (en) * | 2004-12-28 | 2006-08-01 | Outokumpu Stainless Ab | Austenitic stainless steel and steel product |
| CN101878319B (en) | 2007-11-29 | 2013-11-13 | Ati资产公司 | Lean austenitic stainless steel |
| WO2009082501A1 (en) | 2007-12-20 | 2009-07-02 | Ati Properties, Inc. | Corrosion resistant lean austenitic stainless steel |
| US8337749B2 (en) | 2007-12-20 | 2012-12-25 | Ati Properties, Inc. | Lean austenitic stainless steel |
| CN103060718B (en) | 2007-12-20 | 2016-08-31 | 冶联科技地产有限责任公司 | Low-nickel austenitic stainless steel containing stabilizing elements |
| KR101323041B1 (en) * | 2009-12-21 | 2013-10-29 | 주식회사 포스코 | Austenitic Stainless Steel for Gas Nitriding and gas nitriding method of the same |
| CN103789624A (en) * | 2014-01-16 | 2014-05-14 | 安徽省杨氏恒泰钢管扣件加工有限公司 | High-toughness steel pipe material and preparation method thereof |
| CN104131229A (en) * | 2014-06-19 | 2014-11-05 | 宝钢不锈钢有限公司 | Dual-phase stainless steel having high plasticity and high corrosion resistance and manufacturing method thereof |
| JP2016006232A (en) * | 2015-07-28 | 2016-01-14 | 日本冶金工業株式会社 | Surface modification method of stainless steel plate |
| CN108368560B (en) * | 2016-08-17 | 2020-06-02 | 华为技术有限公司 | High-nitrogen stainless steel and heat treatment process thereof |
| JP7262176B2 (en) * | 2018-03-30 | 2023-04-21 | 日鉄ステンレス株式会社 | Ferritic and Austenitic Duplex Stainless Steel Sheets and Pipes |
| CN111519008B (en) * | 2020-06-11 | 2021-10-26 | 扬州大学 | Heat treatment method for controlling high-nitrogen low-nickel austenitic stainless steel structure and performance gradient |
-
1994
- 1994-09-20 JP JP22436294A patent/JP3588826B2/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| JPH0885820A (en) | 1996-04-02 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| TWI377257B (en) | Excellent heat-resistance ferrite stainless steel | |
| JP5335502B2 (en) | Martensitic stainless steel with excellent corrosion resistance | |
| EP2548988B1 (en) | Ferrite-based stainless steel for use in components of automobile exhaust system | |
| JP3588826B2 (en) | Heat treatment method for high nitrogen containing stainless steel | |
| CA2443545C (en) | Method of producing stainless steels having improved corrosion resistance | |
| EP3556880A1 (en) | Ferrite stainless hot-rolled steel sheet and production method therefor | |
| JP6112273B1 (en) | Ferritic stainless hot-rolled steel sheet, hot-rolled annealed sheet, and methods for producing them | |
| WO2011093516A1 (en) | Highly corrosion-resistant hot-rolled ferrite stainless steel sheet having excellent toughness | |
| JPH10503809A (en) | Martensitic stainless steel with sulfide stress cracking resistance with excellent hot workability | |
| RU2280707C2 (en) | Duplex stainless steel, method of making such steel and industrial article made from this steel (versions) | |
| JP4116134B2 (en) | Austenitic stainless steel excellent in high temperature sag resistance and method for producing the same | |
| JP2002030346A (en) | METHOD FOR PRODUCING Cr-CONTAINING HEAT AND CORROSION RESISTANT STEEL SHEET EXCELLENT IN FORMABILITY | |
| JP3514889B2 (en) | Austenitic stainless clad steel sheet with excellent corrosion resistance and method for producing the same | |
| JP2006219718A (en) | Steel for high strength bolt with excellent delayed fracture resistance, and high strength bolt | |
| JPS61119654A (en) | Steel for line pipe having superior corrosion resistance and weldability | |
| JPH0148345B2 (en) | ||
| TWI667357B (en) | Ferritic stainless steel and automotive exhaust path components | |
| WO2018116792A1 (en) | Ferritic stainless steel | |
| JP7271789B2 (en) | Highly corrosion-resistant austenitic stainless steel with excellent impact toughness and hot workability | |
| JP3276303B2 (en) | Ferritic stainless steel with excellent rust resistance that does not easily cause initial rust | |
| JPH0158249B2 (en) | ||
| JP7465955B2 (en) | Low Cr ferritic stainless steel sheet with improved pipe expansion workability and its manufacturing method | |
| JP2001003144A (en) | High purity ferritic stainless steel sheet excellent in secondary working brittleness after deep drawing | |
| JP2000160247A (en) | Manufacture of duplex stainless steel tube | |
| JP7022633B2 (en) | Ferritic stainless steel sheets with excellent high-temperature salt damage resistance and automobile exhaust system parts |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| A521 | Written amendment |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20040212 |
|
| A02 | Decision of refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A02 Effective date: 20040406 |
|
| A521 | Written amendment |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20040602 |
|
| A911 | Transfer of reconsideration by examiner before appeal (zenchi) |
Free format text: JAPANESE INTERMEDIATE CODE: A911 Effective date: 20040617 |
|
| TRDD | Decision of grant or rejection written | ||
| A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 Effective date: 20040727 |
|
| A61 | First payment of annual fees (during grant procedure) |
Free format text: JAPANESE INTERMEDIATE CODE: A61 Effective date: 20040809 |
|
| R150 | Certificate of patent or registration of utility model |
Free format text: JAPANESE INTERMEDIATE CODE: R150 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20070827 Year of fee payment: 3 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20080827 Year of fee payment: 4 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20080827 Year of fee payment: 4 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20090827 Year of fee payment: 5 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20090827 Year of fee payment: 5 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20100827 Year of fee payment: 6 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20110827 Year of fee payment: 7 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20110827 Year of fee payment: 7 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120827 Year of fee payment: 8 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120827 Year of fee payment: 8 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130827 Year of fee payment: 9 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130827 Year of fee payment: 9 |
|
| S111 | Request for change of ownership or part of ownership |
Free format text: JAPANESE INTERMEDIATE CODE: R313111 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130827 Year of fee payment: 9 |
|
| R350 | Written notification of registration of transfer |
Free format text: JAPANESE INTERMEDIATE CODE: R350 |
|
| LAPS | Cancellation because of no payment of annual fees |