JP2004083965A - Method for producing austenitic stainless steel pipe and austenitic stainless steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an austenitic stainless steel pipe with which an oxidizing passive film showing good corrosion resistance when used to a heat-exchange pipe in which water, steam or mixture thereof is a main fluid, can be formed only with a finish solid-solution heat treatment, and to provide this steel pipe. <P>SOLUTION: In the method for producing the austenitic stainless steel pipe, the finish solid-solution heat treatment is performed under hydrogen atmosphere controlling the dew point in the range of -25°C to 0°C, or under mixed gas atmosphere having ≥ 50 vol% hydrogen gas and the balance inert gas. In the inner surface of the austenitic stainless steel pipe obtained with this method, the oxidizing passive film having 2 - 10 mm thickness and ≥ 0.01 ratio (Cr/Fe) of Cr concentration (at%) and Fe concentration (at%) at the most surface part is present. <P>COPYRIGHT: (C)2004,JPO

Description

【００１２】
上記の工程７（固溶化熱処理）は、通常、大気雰囲気中で行われる。しかし、表面が特に美麗なことが要求される場合には、工程７の固溶化熱処理を光輝熱処理とすることもある。この光輝熱処理は、雰囲気ガスとして水素ガスを用いた還元性雰囲気の連続炉を使用して行われる。その際、雰囲気ガスの水素ガスとしては、光輝熱処理が目的であるので、できるだけ高純度のものを使用し、露点は概ね−３０℃以下とされる。また、加熱温度は通常の固溶化熱処理温度である９５０〜１２００℃とされる。
【００１３】
しかし、最終の熱処理である固溶化熱処理を光輝熱処理とした製品鋼管は、これを例えば水や水蒸気、あるいはこれらの混合物を主要な流体とする熱交換用に使用した場合、使用条件によっては耐食性が不十分な場合があり、その解決が望まれていた。
【００１４】
【発明が解決しようとする課題】
本発明は、上記の実状に鑑みてなされたもので、最終熱処理の固溶化熱処理を光輝熱処理とした場合でも、使用条件によらず、水や水蒸気、あるいはこれらの混合物に対して十分な耐食性を発揮する熱交換管等として用いて好適なオーステナイト系ステンレス鋼管が確実に得られるだけでなく、この鋼管を低コストで製造することができるオーステナイト系ステンレス鋼管の製造方法を提供することを目的とする。
【００１５】
【課題を解決するための手段】
本発明の要旨は、下記（１）のオーステナイト系ステンレス鋼管の製造方法、および下記（２）のオーステナイト系ステンレス鋼管にある。
（１）固溶化熱処理を行う際の炉内の雰囲気を、露点が−２５℃から０℃までの範囲内に制御された水素ガス雰囲気、または露点が−２５℃から０℃までの範囲内に制御された５０体積％以上の水素ガスと残部が不活性ガスである混合ガス雰囲気とすることを特徴とするオーステナイト系ステンレス鋼管の製造方法。
（２）上記（１）に記載の方法により製造されたオーステナイト系ステンレス鋼管であって、少なくとも内表面に、厚さが２〜１００ｎｍ、最表面のＣｒ濃度（原子％）とＦｅ濃度（原子％）との比（Ｃｒ／Ｆｅ）が０．０１以上の酸化不動態皮膜を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管。
【００１６】
上記（１）の方法においては、加熱温度を９５０〜１２００℃とするのが好ましい。
【００１７】
上記（２）の酸化不動態皮膜の厚さとは、次のようにして求められる厚さである。例えば、後述の図２に示すように、オージェ電子分光分析器（ＡＥＳ）によって皮膜厚さ方向の各元素の濃度（原子％）分布を測定する。そして、酸素濃度の分布曲線からその濃度が最表面濃度の１／２になる深さを求める。その深さ（図２のＬ）が上記の皮膜厚さである。
【００１８】
本発明者は、最終熱処理の固溶化熱処理を光輝熱処理とした製品鋼管の表面を詳細に調べた結果、以下のことを知見し、本発明を完成させた。
【００１９】
高純度で、かつ露点が−３０℃以下の水素雰囲気中で光輝熱処理された製品鋼管の内表面には、前述のように定義される厚さで１ｎｍ程度と極めて薄い酸化皮膜がある。この皮膜は損傷しやすく、しかもＣｒ酸化物が最表面にほとんど存在しないために保護皮膜としての性能が不十分である。
【００２０】
そこで、水や水蒸気、あるいはこれらの混合物に対して十分な耐食性を発揮する酸化不動態皮膜とこの酸化不動態皮膜を形成させるための条件を見出すための実験を行った結果、次のことが判明した。
【００２１】
（１）　水や水蒸気、あるいはこれらの混合物に対する耐食性は、前述のように定義される厚さが２〜１００ｎｍで、かつ最表面におけるＣｒ濃度（原子％）がＦｅ濃度（原子％）の０．０１倍以上の酸化不動態皮膜であれば確保される。
【００２２】
（２）　上記（１）　の酸化不動態皮膜は、炉内の雰囲気を、露点が−２５℃から０℃までの範囲内に制御された水素ガス雰囲気、または露点が−２５℃から０℃までの範囲内に制御された５０体積％以上の水素ガスと残部が不活性ガスである混合ガス雰囲気、にすれば形成される。
【００２３】
【発明の実施の形態】
以下、本発明の方法における雰囲気ガスの条件および熱交換用オーステナイト系ステンレス鋼管の酸化不動態皮膜の条件を上記のように定めた理由について詳しく説明する。
（１）雰囲気ガスの露点と混合ガスの水素ガス量と残部ガスについて
水素ガスまたは混合ガスの露点が−２５℃未満では最表面のＣｒ濃度がＦｅ濃度の０．０１倍以上の酸化不動態皮膜が形成されず、所望の耐食性が確保されない。一方、露点が０℃を超えると、炉内の構造物やヒーター等の酸化を速め、炉の寿命が著しく短くなる。よって、雰囲気ガスの露点は−２５〜０℃とした。好ましいのは−２０〜−１０℃である。
【００２４】
混合ガスの場合における水素ガスの割合が５０体積％未満では、酸化不動態皮膜の厚さが厚くなりすぎ、かえって耐食性を損う。また、残部のガスをＣＯガスやＣＯ_２ガスとしたのでは浸炭が生じ、母材の耐食性が損なわれる。よって、混合ガスの水素ガス量は５０体積％以上、残部は不活性ガスとした。好ましい水素ガス量は８０体積％以上である。なお、不活性ガスとしてはアルゴンガスやヘリウムガス、および窒素ガスを挙げることができる。
（２）鋼管内面の酸化不動態皮膜の厚さと最表面のＣｒ濃度について
酸化不動態皮膜の厚さが２ｎｍ未満では、酸化が不十分で、最表面にＣｒ酸化物が存在する被膜とならず、所望の耐食性が確保されない。一方、１００ｎｍを超えると、皮膜を構成する酸化物の粒径が粗大になりすぎ、かえって耐食性が損なわれる。よって、酸化不動態皮膜の厚さは２〜１００ｎｍとした。好ましいのは２〜５ｎｍである。
【００２５】
酸化不動態皮膜の最表面のＣｒ濃度（原子％）がＦｅ濃度（原子％）の０．０１倍未満では、所望の耐食性が確保されないので、０．０１倍以上とした。好ましいのは０．１倍以上である。なお、最表面のＣｒ濃度は、高ければ高いほどよいので、その上限は特に制限しない。
【００２６】
上記の酸化不動態皮膜は、被処理鋼管を上記の雰囲気中でオーステナイト系ステンレス鋼の固溶化熱処理温度である９５０〜１２００℃に加熱することにより形成させることができる。従って、本発明の方法によれば、前述した従来技術のように固溶化熱処理とは別の酸化不動態皮膜形成のための特別な熱処理が不要なので、その分だけ製造コストが安くなる。
【００２７】
なお、形成させるべき酸化不動態皮膜の厚さは、被処理鋼管の化学組成、加熱温度および炉内雰囲気に応じて保持時間を適宜調整する（例えば、後述する実施例に示す条件を採用する）ことにより所望の厚さとすればよいので、保持時間については特に制限しない。
【００２８】
また、鋼管母材のオーステナイト系ステンレス鋼は、オーステナイト系のステンレス鋼でありさえすればよく、その化学組成に特別な制約はないが、代表的なものを例示すれば、ＪＩＳ規格に規定されるＳＵＳ３０４、ＳＵＳ３０４Ｌ、ＳＵＳ３４７、ＳＵＳ３４７Ｈ、ＳＵＳ３１６、ＳＵＳ３１６Ｌおよびこれらの相当鋼等を挙げることができる。
【００２９】
【実施例】
表２に示す化学組成を有するＳＵＳ３０４Ｌ相当鋼からなり、冷間仕上げ後、脱脂、洗浄した外径１６ｍｍ、肉厚１ｍｍ、長さ１５ｍのオーステナイト系ステンレス鋼管を準備した。
【００３０】
【表２】

【００３１】
準備した鋼管は、図１に示すような連続炉を用い、表２に示す種々の条件で固溶化熱処理した。その際、鋼管内面の空気を雰囲気ガスで置換するために炉内の圧力を０．９８〜９８Ｐａのプラス圧とした。
【００３２】
固溶化熱処理後の各鋼管は、内面に形成された酸化不動態皮膜の厚さと最表面のＣｒ濃度を調べた後、硫酸濃度が１０体積％の沸騰水溶液中に６時間浸漬する加速腐食試験に供し、その耐食性を調べた。
【００３３】
酸化不動態皮膜の厚さと最表面のＣｒ濃度は、ＡＥＳを用いて皮膜厚さ方向の各元素の濃度（原子％）分布を測定し、前述したとおりの方法によりそれぞれ求めた。また、耐食性は、試験による腐食減量を試験時間で除し、平均腐食速度を求めることにより評価した。
【００３４】
以上の結果を、固溶化熱処理条件と併せて表３に示すとともに、試番１と４のＡＥＳによる分析結果を図２と図３に示した。
【００３５】
【表３】

【００３６】
表３および図３に示すように、本発明で規定する条件で固溶化熱処理した試番２〜５および６の鋼管の内面には、本発明で規定する条件を満たす酸化不動態皮膜が形成されており、加速腐食試験における平均腐食速度が０．１７〜０．３５ｇ／ｍ^２／ｈと遅く、耐食性が良好である。また、これらの鋼管の表面の光輝状態は、本来の光輝熱処理品には及ばないものの、大気雰囲気中品に比べればはるかに美麗であった。
【００３７】
これに対し、雰囲気ガスの露点が低すぎる試番１と６の鋼管の内面には、厚さは本発明で規定する条件を満たすものの、最表面にＣｒ酸化物が存在しない酸化不動態皮膜皮膜（図２参照）が形成されており、平均腐食速度が０．５５〜０．６３ｇ／ｍ^２／ｈと速く、耐食性が不十分である。
【００３８】
また、露点は本発明で規定する条件を満たすものの、混合ガス中の水素濃度が低すぎる試番８の鋼管の内面には、最表面にＣｒ酸化物は存在するが、厚さが１２０ｎｍと厚すぎる酸化不動態皮膜皮膜が形成されており、平均腐食速度が１．５ｇ／ｍ^２／ｈと極めて速く、耐食性が悪い。
【００３９】
【発明の効果】
本発明の方法によれば、熱処理工程を増やすことなく、水や水蒸気、あるいはその混合物を主要な流体とする熱交換に使用して良好な耐食性を発揮する安価なオーステナイト系ステンレス鋼管を確実に製造することができる。また、本発明のオーステナイト系ステンレス鋼管は耐食性に優れるので、これを熱交換管に用いた機器装置は安全性が向上するだけでなく、寿命も長くなる。
【図面の簡単な説明】
【図１】固溶化熱処理の実施の形態を示す模式図である。
【図２】オージェ電子分光分析器による分析結果の一例を示す図である。
【図３】オージェ電子分光分析器による分析結果の他の一例を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing an austenitic stainless steel pipe, and more particularly, to form a passivation film having better corrosion resistance than a passivation film originally provided on steel at the same time as solution heat treatment on a steel surface. The present invention relates to a method for producing an austenitic stainless steel pipe and an austenitic stainless steel pipe suitable for use in heat exchange using water or steam or a mixture thereof as a main fluid.
[0002]
[Prior art]
Austenitic stainless steels are used for various applications because of their excellent corrosion resistance. For example, steel tubes made of austenitic stainless steel are widely used for heat transfer tubes such as boilers and heat exchangers, piping for chemical plants, and clean gas piping for semiconductor manufacturing equipment. Austenitic stainless steel pipes used for these applications are required to have higher corrosion resistance and less outgassing than austenitic stainless steel originally has.
[0003]
As is well known, the corrosion resistance of austenitic stainless steel depends on a passive film mainly composed of Cr oxide on the steel surface. For example, when the amount of Cr in the base material is increased, a passive film is easily formed. Further, when the amount of impurities such as S and O (oxygen) present in the steel is reduced, the amount of outgassing decreases. However, an increase in alloying elements such as Cr and a reduction in the amount of impurities cause an increase in the cost of the base material.
[0004]
For this reason, various measures have been made in the past to form a passivation film having better corrosion resistance and gas release suppression effect on the steel surface than the austenitic stainless steel original passivation film, such as the following: A method has been proposed.
[0005]
(A) The steel surface is subjected to electrolytic polishing or electrolytic combined polishing, and then baked to remove water from the steel surface, and then a hydrogen gas or a hydrogen gas having an O ₂ or H ₂ O concentration of about 100 ppb is mixed with an inert gas. By heating to 300 to 600 ° C. in a mixed gas atmosphere, the ratio of Cr to Fe on the outermost surface (Cr / Fe) is 1 or more and the maximum thickness is about 2 nm. A method of forming a dynamic film (a conventional method described in paragraph 0008 of JP-A-6-116632).
[0006]
(B) The surface of the steel is electrolytically polished to impart processing strain to the surface layer, and then baked to remove the water on the surface of the steel, and then subjected to 450 ppm in an inert gas atmosphere having an H ₂ O concentration of 500 ppb to ₂ %. By heating to -600 ° C., a two-layered structure composed of an outer layer mainly composed of a Cr oxide having a thickness of 2 nm or more in which no Fe oxide is present on the outermost surface and an inner layer mainly composed of a Cr oxide containing Fe oxide is formed. A method of forming a dynamic film (JP-A-6-116632).
[0007]
(C) A steel surface is polished and then heated to 300 to 500 ° C. in an air atmosphere to form an inner layer of “Cr-containing oxide content> Cr-free oxide content” having a thickness of 5 to 100 nm and “Cr-free oxide content”. A method of forming an oxidation passivation film having a two-layer structure consisting of an outer layer having a thickness of 5 nm or less with "amount of oxide> amount of Cr-containing oxide" or removing only the outer layer after forming the film (Japanese Patent Laid-Open No. 3-153858). .
[0008]
(D) A high Cr austenitic stainless steel having a Cr content of less than 20 to 30% and an austenite grain size number of 7 or less is subjected to 700 to 1000 in steam or a low oxygen partial pressure atmosphere in which Fe ₂ O ₃ is not generated. A method in which an oxidation passivation film made of Cr oxide is formed on a steel surface by heating to a temperature of 0 ° C. (JP-A-3-176824).
[0009]
However, all of the above methods (a) to (d) are directed to austenitic stainless steel after solution heat treatment, and these methods are applied to the production of austenitic stainless steel pipes. Then, there is a disadvantage that the heat treatment cost increases and the production cost of the product increases. This is for the following reason.
[0010]
Austenitic stainless steel pipes are generally manufactured through the basic steps shown in Table 1. For this reason, the above methods (a) to (d) are performed after the final heat treatment, that is, between the steps 7 and 8.
[0011]
[Table 1]

[0012]
The above step 7 (solution heat treatment) is usually performed in an air atmosphere. However, if the surface is required to be particularly beautiful, the solution heat treatment in step 7 may be a bright heat treatment. This bright heat treatment is performed using a continuous furnace in a reducing atmosphere using hydrogen gas as an atmosphere gas. At this time, the hydrogen gas as the atmosphere gas is used for the purpose of bright heat treatment, and therefore, a gas having a purity as high as possible is used, and the dew point is set to approximately −30 ° C. or less. The heating temperature is 950 to 1200 ° C., which is a normal solution heat treatment temperature.
[0013]
However, when the product steel tube is used as the final heat treatment, the solution heat treatment is a bright heat treatment, for example, when it is used for heat exchange using water, steam, or a mixture thereof as a main fluid, the corrosion resistance depends on the use conditions. In some cases, it was not enough, and a solution was desired.
[0014]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and even when the solution heat treatment of the final heat treatment is a bright heat treatment, regardless of the use conditions, sufficient corrosion resistance to water, steam, or a mixture thereof. An object of the present invention is to provide a method for manufacturing an austenitic stainless steel pipe that not only reliably obtains an austenitic stainless steel pipe suitable for use as a heat exchange pipe and the like but also can manufacture this steel pipe at low cost. .
[0015]
[Means for Solving the Problems]
The gist of the present invention resides in a method for producing an austenitic stainless steel pipe of the following (1) and an austenitic stainless steel pipe of the following (2).
(1) The atmosphere in the furnace at the time of performing the solution heat treatment is set to a hydrogen gas atmosphere in which the dew point is controlled in a range of −25 ° C. to 0 ° C. or a dew point in a range of −25 ° C. to 0 ° C. A method for manufacturing an austenitic stainless steel pipe, characterized in that the mixed gas atmosphere is a controlled 50% by volume or more of hydrogen gas and the balance is an inert gas.
(2) An austenitic stainless steel pipe manufactured by the method described in (1) above, wherein at least the inner surface has a thickness of 2 to 100 nm, and the Cr concentration (at%) and the Fe concentration (at%) of the outermost surface. An austenitic stainless steel pipe characterized by having an oxidation passivation film having a ratio (Cr / Fe) of 0.01 or more to (a).
[0016]
In the method (1), the heating temperature is preferably set to 950 to 1200 ° C.
[0017]
The thickness of the oxidation passivation film of the above (2) is a thickness determined as follows. For example, as shown in FIG. 2 described later, the concentration (atomic%) distribution of each element in the film thickness direction is measured by an Auger electron spectrometer (AES). Then, a depth at which the concentration becomes 1/2 of the outermost surface concentration is obtained from the distribution curve of the oxygen concentration. The depth (L in FIG. 2) is the above film thickness.
[0018]
The present inventor has conducted detailed studies on the surface of a product steel pipe in which the solution heat treatment of the final heat treatment is performed as a bright heat treatment, and as a result, has found the following, and has completed the present invention.
[0019]
The inner surface of the product steel pipe which has been subjected to bright heat treatment in a hydrogen atmosphere having a high purity and a dew point of −30 ° C. or less has an oxide film as extremely thin as about 1 nm as defined above. This film is easily damaged, and its performance as a protective film is insufficient because Cr oxide hardly exists on the outermost surface.
[0020]
Therefore, we conducted an experiment to find an oxidation passivation film that exhibits sufficient corrosion resistance to water, water vapor, or a mixture of these, and conditions for forming this oxidation passivation film. did.
[0021]
(1) The corrosion resistance to water, water vapor, or a mixture thereof is such that the thickness as defined above is 2 to 100 nm, and the Cr concentration (atomic%) at the outermost surface is 0.1% of the Fe concentration (atomic%). An oxidation passivation film of 01 times or more is ensured.
[0022]
(2) The oxidation passivation film of the above (1) is prepared by changing the atmosphere in the furnace to a hydrogen gas atmosphere whose dew point is controlled within a range of -25 ° C to 0 ° C, or a dew point of -25 ° C to 0 ° C. Is formed in a mixed gas atmosphere in which hydrogen gas of 50% by volume or more and the balance are inert gas are controlled within the range described above.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the reason why the conditions of the atmosphere gas and the conditions of the oxidation passivation film of the austenitic stainless steel tube for heat exchange in the method of the present invention are determined as described above will be described in detail.
(1) Regarding the dew point of the atmosphere gas, the amount of hydrogen gas in the mixed gas, and the remaining gas When the dew point of the hydrogen gas or the mixed gas is lower than −25 ° C., the oxidation passivation film in which the Cr concentration on the outermost surface is 0.01 times or more the Fe concentration Are not formed, and desired corrosion resistance is not ensured. On the other hand, when the dew point exceeds 0 ° C., the oxidation of the structures and heaters in the furnace is accelerated, and the life of the furnace is significantly shortened. Therefore, the dew point of the atmosphere gas was set to -25 to 0 ° C. Preferred is -20 to -10C.
[0024]
If the ratio of the hydrogen gas in the case of the mixed gas is less than 50% by volume, the thickness of the oxidation passivation film becomes too large, and the corrosion resistance is rather deteriorated. Further, if the remaining gas is CO gas or CO ₂ gas, carburization occurs and the corrosion resistance of the base material is impaired. Therefore, the hydrogen gas amount of the mixed gas was 50% by volume or more, and the balance was an inert gas. The preferred amount of hydrogen gas is 80% by volume or more. Note that examples of the inert gas include an argon gas, a helium gas, and a nitrogen gas.
(2) Regarding the thickness of the oxidation passivation film on the inner surface of the steel pipe and the concentration of Cr on the outermost surface If the thickness of the oxidation passivation film is less than 2 nm, the oxidation is insufficient and the film does not have a Cr oxide on the outermost surface. , The desired corrosion resistance is not ensured. On the other hand, if it exceeds 100 nm, the particle size of the oxide constituting the film becomes too large, and the corrosion resistance is rather deteriorated. Therefore, the thickness of the oxidation passivation film is set to 2 to 100 nm. Preferred is 2-5 nm.
[0025]
If the Cr concentration (atomic%) on the outermost surface of the oxidation passivation film is less than 0.01 times the Fe concentration (atomic%), the desired corrosion resistance is not ensured, so that it is set to 0.01 times or more. It is preferably at least 0.1 times. Since the higher the Cr concentration on the outermost surface, the better, the upper limit is not particularly limited.
[0026]
The above-mentioned oxidation passivation film can be formed by heating the steel pipe to be treated to 950 to 1200 ° C., which is the solution heat treatment temperature for austenitic stainless steel in the above atmosphere. Therefore, according to the method of the present invention, a special heat treatment for forming an oxide passivation film, which is different from the solution heat treatment as in the prior art described above, is not required, and the manufacturing cost is reduced accordingly.
[0027]
The thickness of the oxide passivation film to be formed is appropriately adjusted for the holding time in accordance with the chemical composition of the steel pipe to be treated, the heating temperature, and the atmosphere in the furnace (for example, the conditions described in Examples described later are employed). The holding time is not particularly limited, since the desired thickness may be obtained.
[0028]
Further, the austenitic stainless steel of the steel pipe base material may be any austenitic stainless steel, and there is no particular restriction on the chemical composition thereof, but typical examples are specified in JIS standards. SUS304, SUS304L, SUS347, SUS347H, SUS316, SUS316L, and their equivalent steels can be mentioned.
[0029]
【Example】
An austenitic stainless steel pipe made of SUS304L equivalent steel having a chemical composition shown in Table 2, cold-finished, degreased and washed, and having an outer diameter of 16 mm, a wall thickness of 1 mm, and a length of 15 m was prepared.
[0030]
[Table 2]

[0031]
The prepared steel pipe was subjected to solution heat treatment under various conditions shown in Table 2 using a continuous furnace as shown in FIG. At that time, the pressure in the furnace was set to a positive pressure of 0.98 to 98 Pa in order to replace the air on the inner surface of the steel pipe with the atmospheric gas.
[0032]
After the solution heat treatment, each steel pipe was subjected to an accelerated corrosion test in which the thickness of the oxide passivation film formed on the inner surface and the Cr concentration of the outermost surface were examined, and the steel pipe was immersed in a boiling aqueous solution having a sulfuric acid concentration of 10% by volume for 6 hours. And tested for its corrosion resistance.
[0033]
The thickness of the oxidation passivation film and the Cr concentration on the outermost surface were obtained by measuring the concentration (atomic%) distribution of each element in the thickness direction of the film using AES, and by the methods described above. The corrosion resistance was evaluated by dividing the weight loss due to the test by the test time and calculating the average corrosion rate.
[0034]
The above results are shown in Table 3 together with the solution heat treatment conditions, and the analysis results by AES of Test Nos. 1 and 4 are shown in FIGS. 2 and 3.
[0035]
[Table 3]

[0036]
As shown in Table 3 and FIG. 3, an oxide passivation film satisfying the conditions specified in the present invention is formed on the inner surfaces of the steel pipes of Nos. 2 to 5 and 6 subjected to solution treatment under the conditions specified in the present invention. The average corrosion rate in the accelerated corrosion test is as low as 0.17 to 0.35 g / m ² / h, and the corrosion resistance is good. The bright state of the surface of these steel pipes was not as good as that of the original bright heat-treated product, but was much more beautiful than the product in the air atmosphere.
[0037]
On the other hand, on the inner surfaces of the test tubes 1 and 6 in which the dew point of the atmospheric gas is too low, the oxidation passivation film having no Cr oxide on the outermost surface although the thickness satisfies the conditions specified in the present invention. (See FIG. 2), the average corrosion rate is as fast as 0.55 to 0.63 g / m ² / h, and the corrosion resistance is insufficient.
[0038]
Further, although the dew point satisfies the condition specified in the present invention, the inner surface of the steel pipe of Test No. 8 in which the hydrogen concentration in the mixed gas is too low contains Cr oxide on the outermost surface, but has a thickness of 120 nm. Excessive oxidation passivation film is formed, the average corrosion rate is extremely fast as 1.5 g / m ² / h, and the corrosion resistance is poor.
[0039]
【The invention's effect】
According to the method of the present invention, an inexpensive austenitic stainless steel pipe that exhibits good corrosion resistance by using it for heat exchange with water, steam, or a mixture thereof as a main fluid can be reliably produced without increasing the number of heat treatment steps. can do. Further, since the austenitic stainless steel pipe of the present invention is excellent in corrosion resistance, the equipment using the same as a heat exchange pipe not only improves safety but also has a longer life.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an embodiment of a solution heat treatment.
FIG. 2 is a diagram illustrating an example of an analysis result obtained by an Auger electron spectrometer.
FIG. 3 is a diagram showing another example of an analysis result by an Auger electron spectrometer.

Claims (3)

The atmosphere in the furnace during the solution heat treatment was controlled such that the dew point was controlled in a range of -25 ° C to 0 ° C in a hydrogen gas atmosphere, or the dew point was controlled in a range of -25 ° C to 0 ° C. A method for producing an austenitic stainless steel pipe, wherein a mixed gas atmosphere in which 50% by volume or more of hydrogen gas and the balance is an inert gas is used. The method for producing an austenitic stainless steel pipe according to claim 1, wherein the heating temperature of the heat treatment is 950 to 1200C. An austenitic stainless steel pipe manufactured by the method according to claim 1, wherein at least an inner surface has a thickness of 2 to 100 nm, and a Cr concentration (at%) and a Fe concentration (at%) of the outermost surface. An austenitic stainless steel pipe characterized by having an oxidation passivation film having a ratio (Cr / Fe) of 0.01 or more.
Here, the thickness of the oxidation passivation film is the distance from the outermost surface of the film to a position where the O (oxygen) concentration (atomic%) in the thickness direction of the film becomes 1/2 of the O concentration at the outermost surface of the film. It is.

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