JP2018163148A - Method and apparatus for evaluating sulfide stress corrosion cracking of steel material - Google Patents
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本発明は、鋼材の硫化物応力腐食割れ性の評価方法および評価装置に関する。 The present invention relates to an evaluation method and an evaluation apparatus for sulfide stress corrosion cracking of steel materials.
金属材料の中でもステンレス鋼は、表面に自然に形成される酸化皮膜、いわゆる不動態皮膜の形成により優れた耐食性を示すことが知られている。ステンレス鋼の腐食形態は全面腐食と局部腐食に大別される。全面腐食は不動態皮膜が形成されにくい希硫酸や塩酸環境下などで発生する。一方、局部腐食は、介在物を除けば、局所的に不動態皮膜が破壊されることで発生する。不動態皮膜が破壊された部分がアノード反応、他の部分がカソード反応となり、その位置が固定され断続的に進行することで、点または孔状に深く侵食されて孔食が形成される。一旦形成された孔食の孔食内は低ｐＨ化し、より腐食が進みやすくなり、耐食性や外観性の劣化など様々な問題が表面化する。 Among metal materials, stainless steel is known to exhibit excellent corrosion resistance due to the formation of an oxide film naturally formed on the surface, a so-called passive film. The corrosion forms of stainless steel are broadly divided into general corrosion and local corrosion. Overall corrosion occurs in dilute sulfuric acid or hydrochloric acid environments where a passive film is difficult to form. On the other hand, local corrosion is caused by local destruction of the passive film except for inclusions. The portion where the passive film is destroyed becomes an anodic reaction, the other portion becomes a cathodic reaction, and the position is fixed and progresses intermittently. Once formed, the pH of the pitting corrosion is lowered, the corrosion is more likely to proceed, and various problems such as corrosion resistance and deterioration of appearance appear on the surface.
このような孔食の形成は、腐食のみならず、割れの問題を招く場合がある。特に、硫化水素Ｈ２Ｓを含む環境（例えば、油井管材料等）下での孔食の発生は、水素脆化という問題に発展する。水素脆化とは、原子状水素（Ｈ）が金属中に吸蔵されて材質が脆くなる現象である。腐食反応を通して金属表面に原子状水素が生成されるが、その多くはその後会合し水素ガス（Ｈ２）として金属表面から離脱する。しかし、硫化水素は水素原子の会合を阻害することで知られており、原子状水素として金属表面に滞在する時間が長くなり、金属に吸蔵される原子状水素の量が増え、水素脆化で割れに至る（硫化物応力腐食割れ：以下、硫化物応力腐食割れを「ＳＳＣ」ともいう）。 Such formation of pitting corrosion may cause not only corrosion but also cracking problems. In particular, the occurrence of pitting corrosion under an environment containing hydrogen sulfide H 2 S (for example, oil well pipe material) develops a problem of hydrogen embrittlement. Hydrogen embrittlement is a phenomenon in which atomic hydrogen (H) is occluded in a metal and the material becomes brittle. Atomic hydrogen is generated on the metal surface through the corrosion reaction, and many of them are then associated and separated from the metal surface as hydrogen gas (H 2 ). However, hydrogen sulfide is known to inhibit the association of hydrogen atoms, and it takes longer time to stay on the metal surface as atomic hydrogen, increasing the amount of atomic hydrogen occluded by the metal, resulting in hydrogen embrittlement. (Sulphide stress corrosion cracking: hereinafter referred to as “SSC”).
このような孔食起因から発生するＳＳＣの試験方法としては、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに規定されている定荷重試験が知られている。この試験は、地層水または凝縮水を模擬した試験溶液のｐＨおよびＨ２Ｓ濃度の各条件下で、試験片に負荷応力を与えた状態で７２０時間（約１ヶ月）試験を行った後の試験片の破断またはクラックの発生有無で試験片のＳＳＣを評価する方法である。この試験方法では、試験片に定荷重を負荷することのできる試験機が必要であり、また、比較的時間を要するため、より簡易に、短時間でＳＳＣを評価できる新たな指標があれば開発スピードの大幅な加速が見込まれる。ＳＳＣ起点となる孔食の発生過程は、金属表面の不動態皮膜の劣化・破壊や再不動態化の中で、孔食萌芽（孔食の初期過程）の発生と成長が過渡的かつ複雑に起こっていると考えられる。孔食萌芽に相当する腐食の前駆体（以下、「腐食前駆体」ともいう）が判別できれば、不動態皮膜の保護性が局所的に失われることを発端とする孔食の発生メカニズムが解明できる上、ＳＳＣ発生の判定に活用できるものと考えられる。 As a test method for SSC generated due to such pitting corrosion, a constant load test defined in NACE ™ 0177 Method A is known. This test was conducted after 720 hours (about 1 month) of the test piece with load stress applied under the conditions of pH and H 2 S concentration of the test solution simulating formation water or condensed water. This is a method for evaluating the SSC of a test piece based on whether or not the test piece is broken or cracked. This test method requires a testing machine that can apply a constant load to the test piece, and it takes a relatively long time. Therefore, if there is a new index that can evaluate SSC more easily and in a short time, it will be developed. Significant speed increase is expected. The generation process of pitting corrosion starting from SSC is a transitional and complicated occurrence and growth of pitting corrosion (initial process of pitting corrosion) in the deterioration / destruction and repassivation of the passive film on the metal surface. It is thought that. If the precursor of corrosion corresponding to pitting corrosion sprouting (hereinafter also referred to as “corrosion precursor”) can be identified, the mechanism of pitting corrosion starting from the local loss of protection of the passive film can be elucidated. In addition, it can be used to determine the occurrence of SSC.
応力腐食割れの試験方法に関して、例えば、特許文献１には、応力腐食割れの加速試験方法が開示されている。
また、孔食萌芽の判別方法に関して、例えば、特許文献２には、金属材料の主要成分の陽イオンと発色反応する金属陽イオン反応性発色剤を用いて、皮膜破壊箇所から溶出した金属イオンによる発色反応をもとに金属材料の応力腐食割れ及び／又は孔食の発生過程を視覚化・検知する方法が開示されている。
一方、非特許文献１には、M. T. Ghasrらにより、ミリ波プローブを用いて塗膜下のアルミニウム表面に形成される孔食の前駆体を検出する方法が開示されている。
Regarding a stress corrosion cracking test method, for example, Patent Document 1 discloses an accelerated test method for stress corrosion cracking.
Regarding the method for discriminating pitting sprouting, for example, Patent Document 2 discloses a method using a metal cation reactive color former that develops a color reaction with a cation as a main component of a metal material, and a metal ion eluted from a film breaking point. A method for visualizing and detecting the stress corrosion cracking and / or pitting corrosion process of a metal material based on a coloring reaction is disclosed.
On the other hand, Non-Patent Document 1 discloses a method for detecting a pitting corrosion precursor formed on an aluminum surface under a coating film using a millimeter wave probe by MT Ghasr et al.
上記特許文献１に記載の応力腐食割れの加速試験方法は、早期診断に活用する粒界を生じさせるため孔食を抑制する作用を持つ薬剤を用い、前記薬剤と塩化物イオンとを含む水溶液中に、応力を負荷した試験片を設置し、当該試験片への通電による電解作用を利用して応力腐食割れ試験を加速して行うものであるが、試験片の種類に対して薬剤の効能が必ずしも一律でないという課題があった。 The stress corrosion cracking acceleration test method described in Patent Document 1 uses a drug having an action of suppressing pitting corrosion in order to generate grain boundaries used for early diagnosis, and in an aqueous solution containing the drug and chloride ions. In addition, a test piece loaded with stress is installed, and the stress corrosion cracking test is accelerated by utilizing the electrolytic action caused by energizing the test piece. There was a problem that it was not necessarily uniform.
上記特許文献２に記載の応力腐食割れ及び孔食の発生過程を検知する方法では、リアルタイムで応力腐食割れ及び孔食の発生過程を観察できることが期待できるが、金属陽イオン反応性発色剤を試験環境の水溶液に添加する必要があるため、実腐食環境を模擬し難い問題があった。また、特許文献２の方法では、増粘剤の使用により改善されるものの発色箇所が水溶液中で拡散する点や、アノード溶解反応を利用しているため、検出にはある程度の溶解（腐食）が進行しなければならない点等から、孔食の初期過程のような微小な発生点を検出し難いという課題があった。 In the method for detecting the occurrence of stress corrosion cracking and pitting corrosion described in Patent Document 2, it can be expected that the process of stress corrosion cracking and pitting corrosion can be observed in real time. Since it is necessary to add to the aqueous solution of the environment, there is a problem that it is difficult to simulate the actual corrosive environment. In addition, although the method of Patent Document 2 is improved by the use of a thickener, the development of the colored portion diffuses in an aqueous solution and the use of an anodic dissolution reaction. There is a problem that it is difficult to detect a minute generation point such as an initial process of pitting corrosion because it has to proceed.
上記非特許文献１に記載のミリ波プローブを用いた孔食の前駆体の検出方法では、塗膜下の表面状態を可視化できるメリットがあるが、測定対象の孔食サイズが０．５ｍｍかつ深さが１００〜５００μｍで、光学顕微鏡の低倍（または肉眼）で認識できるくらいの比較的大きな孔食であり、数μｍオーダーのミクロな変化点を捉えるには空間分解能が十分でないという課題があった。 The method for detecting a pitting corrosion precursor using the millimeter-wave probe described in Non-Patent Document 1 has an advantage that the surface state under the coating film can be visualized. This is a relatively large pitting corrosion that can be recognized with a low magnification (or the naked eye) of an optical microscope at 100 to 500 μm, and there is a problem that the spatial resolution is not sufficient to capture micro change points on the order of several μm. It was.
本発明は、より簡易に鋼材の硫化物応力腐食割れ性を評価できる鋼材の硫化物応力腐食割れ性の評価方法および評価装置を提供することを目的とする。 An object of this invention is to provide the evaluation method and evaluation apparatus of the sulfide stress corrosion cracking property of steel materials which can evaluate the sulfide stress corrosion cracking property of steel materials more simply.
本発明者らは、上記課題を解決するために、ＳＳＣ判定の指標に成り得る孔食発生のごく初期段階を捉えるべく、その判別方法を鋭意研究した。その結果、高い空間分解能を保ちつつ不動態皮膜の僅かな表面性状の変化を捉え、孔食萌芽発生を可視化するためには、走査電子顕微鏡による低加速電圧観察技術が有効であるとの考えに至った。 In order to solve the above-mentioned problems, the present inventors diligently studied a method for discriminating them in order to grasp the very initial stage of occurrence of pitting corrosion that can serve as an index for SSC determination. As a result, low acceleration voltage observation technology using a scanning electron microscope is effective for capturing slight changes in the surface properties of the passive film while maintaining high spatial resolution and visualizing the occurrence of pitting sprouting. It came.
殆どの試料の表面には、試料準備・研磨・作製過程等において、不可避的に混入・付着するコンタミネーション（カーボン付着汚れ）が少なからず存在する。その大きさは数μｍの場合があり、初期のミクロ腐食を探索する上で支障に成り得る。さらに、汎用的に使用されるＳＥＭ像（ＥＴ検出器による二次電子像、例：加速電圧１５ｋＶ）では、これらコンタミネーションと、試料の表面に発生した孔食萌芽（腐食前駆体）の見え方が同じであり画像では区別できない。 On the surface of most samples, there are not a few contaminations (carbon adhering stains) that are inevitably mixed in and adhering during sample preparation, polishing, and production processes. The size may be several μm, which can be a hindrance in searching for initial micro-corrosion. In addition, in general-purpose SEM images (secondary electron images from ET detectors, eg acceleration voltage 15 kV), these contaminations and how pitting corrosion germs (corrosion precursors) appear on the surface of the sample are visible. Are the same and cannot be distinguished by images.
そこで発明者らは、低加速電圧観察技術で捉えられる構造体とそれらの組成を相互解析した結果、試料の表面に発生した特定の構造体が、ＳＳＣ試験の試験結果と関連しており、前記特定の構造体を指標とすることでＳＳＣ試験の試験結果を短時間で予測できることを突き止めた。 Therefore, as a result of mutual analysis of the structures captured by the low acceleration voltage observation technique and their compositions, the specific structures generated on the surface of the sample are related to the test results of the SSC test. It was found that the test result of the SSC test can be predicted in a short time by using a specific structure as an index.
本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨は次の通りである。
［１］塩化物イオンを含み硫化水素ガスを通気した水溶液中に鋼材の試験片を応力を負荷せずに浸漬し、
走査型電子顕微鏡により、０．１ｋＶ以上５ｋＶ以下の加速電圧で、前記浸漬後の試験片表面の二次電子像を取得し、当該二次電子像から、中心部と、中心部を囲繞する輪郭部から構成され、最大長さが１μｍ以上１０μｍ以下であり、前記試験片表面のバルク部の輝度値Ａｂと前記輪郭部の輝度値Ａｓとの比（Ａｂ／Ａｓ）が１．００未満である構造体を抽出するとともに、
前記バルク部の輝度値Ａｂと前記中心部の輝度値Ａｃとの比（Ａｂ／Ａｃ）を用いて、前記構造体のなかから腐食前駆体を判別し、
前記判別した腐食前駆体の個数密度で硫化物応力腐食割れ性を評価することを特徴とする、鋼材の硫化物応力腐食割れ性の評価方法。
［２］前記鋼材が、質量％で、Ｃ：０．００５〜０．５％、Ｓｉ：０．０５〜１．０％、Ｍｎ：０．１〜２．０％、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．００１〜０．１％、Ｃｒ：１０〜２５％、Ｎｉ：１．５〜１０％、Ｎ：０．１５％以下を含有し、残部はＦｅおよび不可避的不純物からなる組成を有するステンレス鋼であることを特徴とする、［１］に記載の鋼材の硫化物応力腐食割れ性の評価方法。
［３］前記鋼材が、さらに、質量％で、Ｍｏ：１〜５％、Ｃｕ：０．０３〜４％、Ｖ：０．０１〜０．５％のうちから選んだ１種以上を含有する組成を有するステンレス鋼であることを特徴とする、［２］に記載の鋼材の硫化物応力腐食割れ性の評価方法。
［４］前記鋼材が、フェライト相とマルテンサイト相を主相とする組織を有するステンレス鋼であることを特徴とする、［１］〜［３］のいずれかに記載の鋼材の硫化物応力腐食割れ性の評価方法。
［５］前記構造体のなかからＡｂ／Ａｃが１．３以上のものを腐食前駆体として判別することを特徴とする、［１］〜［４］のいずれかに記載の鋼材の硫化物応力腐食割れ性の評価方法。
［６］前記腐食前駆体の個数密度が４５０（個／ｍｍ２）以下の場合に、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに規定されるＳＳＣ試験で硫化物応力腐食割れが発生しないと評価することを特徴とする、［１］〜［５］のいずれかに記載の鋼材の硫化物応力腐食割れ性の評価方法。
［７］上記［１］〜［６］のいずれかに記載の鋼材の硫化物応力腐食割れ性の評価方法に用いる鋼材の硫化物応力腐食割れ性の評価装置であって、
応力を負荷せずに鋼材の試験片を浸漬する、塩化物イオンを含み硫化水素ガスを通気した水溶液を収容する容器と、
０．１ｋＶ以上５ｋＶ以下の加速電圧で、前記浸漬後の試験片表面の二次電子像を取得する走査型電子顕微鏡と、
当該二次電子像から、中心部と、中心部を囲繞する輪郭部から構成され、最大長さが１μｍ以上１０μｍ以下であり、前記試験片表面のバルク部の輝度値Ａｂと前記輪郭部の輝度値Ａｓとの比（Ａｂ／Ａｓ）が１．００未満である構造体を抽出するとともに、
前記バルク部の輝度値Ａｂと前記中心部の輝度値Ａｃとの比（Ａｂ／Ａｃ）を求める画像処理手段と、を備えることを特徴とする、鋼材の硫化物応力腐食割れ性の評価装置。
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
[1] A steel specimen is immersed in an aqueous solution containing chloride ions and aerated with hydrogen sulfide gas without applying stress.
Using a scanning electron microscope, a secondary electron image of the surface of the test specimen after immersion is acquired at an acceleration voltage of 0.1 kV or more and 5 kV or less, and a center portion and a contour surrounding the center portion are obtained from the secondary electron image. And the maximum length is 1 μm or more and 10 μm or less, and the ratio (Ab / As) of the luminance value Ab of the bulk portion on the surface of the test piece to the luminance value As of the contour portion is less than 1.00. While extracting the structure,
Using the ratio (Ab / Ac) between the luminance value Ab of the bulk part and the luminance value Ac of the central part, a corrosion precursor is determined from the structure,
A method for evaluating sulfide stress corrosion cracking property of a steel material, comprising evaluating sulfide stress corrosion cracking property based on the number density of the identified corrosion precursor.
[2] The steel material is mass%, C: 0.005 to 0.5%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.05% Hereinafter, S: 0.01% or less, Al: 0.001-0.1%, Cr: 10-25%, Ni: 1.5-10%, N: 0.15% or less, the balance is The method for evaluating sulfide stress corrosion cracking property of a steel material according to [1], wherein the steel material is a stainless steel having a composition comprising Fe and inevitable impurities.
[3] The steel material further contains, in mass%, one or more selected from Mo: 1 to 5%, Cu: 0.03 to 4%, and V: 0.01 to 0.5%. The method for evaluating sulfide stress corrosion cracking property of a steel material according to [2], characterized by being stainless steel having a composition.
[4] The sulfide stress corrosion of a steel material according to any one of [1] to [3], wherein the steel material is a stainless steel having a structure whose main phase is a ferrite phase and a martensite phase. Evaluation method of crackability.
[5] The sulfide stress of the steel material according to any one of [1] to [4], wherein Ab / Ac of 1.3 or more among the structures is identified as a corrosion precursor. Corrosion cracking evaluation method.
[6] When the number density of the corrosion precursor is 450 (pieces / mm 2 ) or less, it is evaluated that sulfide stress corrosion cracking does not occur in the SSC test specified in NACE ™ 0177 Method A. The method for evaluating sulfide stress corrosion cracking of a steel material according to any one of [1] to [5].
[7] An evaluation apparatus for sulfide stress corrosion cracking of steel used in the method for evaluating sulfide stress corrosion cracking of steel according to any one of [1] to [6],
A container containing an aqueous solution containing chloride ions and aerated with hydrogen sulfide gas, in which a steel specimen is immersed without applying stress,
A scanning electron microscope that acquires a secondary electron image of the surface of the test specimen after immersion at an acceleration voltage of 0.1 kV to 5 kV;
The secondary electron image is composed of a central portion and a contour portion surrounding the central portion, the maximum length is 1 μm or more and 10 μm or less, and the luminance value Ab of the bulk portion on the surface of the test piece and the luminance of the contour portion While extracting the structure whose ratio (Ab / As) to the value As is less than 1.00,
An image processing means for obtaining a ratio (Ab / Ac) between the luminance value Ab of the bulk part and the luminance value Ac of the central part, and an evaluation apparatus for sulfide stress corrosion cracking of steel.
本発明によれば、より簡易に鋼材の硫化物応力腐食割れ性を評価できる鋼材の硫化物応力腐食割れ性の評価方法および評価装置を提供することができる。
本発明によれば、所定の水溶液に浸漬した後の試験片表面を観察するだけでＳＳＣ試験の試験結果を予測でき、試験片に定荷重を負荷しながら試験を行う必要がない。また、より短時間でＳＳＣ試験の試験結果を予測することができる。さらに、本発明によれば、ＳＳＣの評価に留まらず、孔食が懸念される環境における材料の早期診断への適用、耐孔食性に優れた高性能材料の開発の迅速化が可能となる。
ADVANTAGE OF THE INVENTION According to this invention, the evaluation method and evaluation apparatus of the sulfide stress corrosion cracking property of steel materials which can evaluate the sulfide stress corrosion cracking property of steel materials more simply can be provided.
According to the present invention, the test result of the SSC test can be predicted only by observing the surface of the test piece after being immersed in a predetermined aqueous solution, and it is not necessary to perform the test while applying a constant load to the test piece. Moreover, the test result of the SSC test can be predicted in a shorter time. Furthermore, according to the present invention, it is possible not only to evaluate SSC but also to speed up the development of a high-performance material excellent in pitting corrosion resistance and application to early diagnosis of materials in an environment where pitting corrosion is a concern.
本発明では評価したい試験溶液に試験片を浸漬し、その後以下に記述するＳＥＭによりＳＳＣの指標となる腐食前駆体を判別し、その個数密度で硫化物応力腐食割れ性を評価し、ＳＳＣ試験結果を予測する。以下、本発明の一実施形態について説明する。ただし、本発明は、以下に示す実施形態に限定されない。 In the present invention, a test piece is immersed in a test solution to be evaluated, and thereafter a corrosion precursor serving as an index of SSC is discriminated by SEM described below, and the sulfide stress corrosion cracking property is evaluated based on the number density. Predict. Hereinafter, an embodiment of the present invention will be described. However, the present invention is not limited to the embodiments shown below.
本実施形態では、ＳＳＣ試験として、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに規定される定荷重試験を想定しており、本実施形態の鋼材の硫化物応力腐食割れ性の評価方法は、試験時間、応力負荷の有無、試験片の形状以外は、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａの規定に準拠する。
以下、本実施形態で用いる試験片、試験溶液、走査電子顕微鏡による二次電子像の取得方法、構造体、腐食前駆体、ＳＳＣ試験について順次詳細に記述する。
In the present embodiment, a constant load test defined in NACE TM 0177 Method A is assumed as the SSC test. The method for evaluating sulfide stress corrosion cracking of the steel material of the present embodiment is based on the test time and stress load. Except for the presence / absence and the shape of the test piece, it conforms to the rules of NACE ™ 0177 Method A.
Hereinafter, a test piece, a test solution, a method for acquiring a secondary electron image by a scanning electron microscope, a structure, a corrosion precursor, and an SSC test used in the present embodiment will be described in detail.
（１）試験片
試験片は、硫化物応力腐食割れを評価する材料である。前記材料としては、特に限定されないが、本実施形態では、鋼材、特に鋼管を想定している。前記試験片としては、鋼材そのものを用いてもよいし、例えば、鋼管から所定形状に切り出した鋼片を用いてもよい。本実施形態で想定する鋼材（以下、特に断らない限り％は質量％を意味する）としては、Ｃ：０．００５〜０．５％、Ｓｉ：０．０５〜１．０％、Ｍｎ：０．１〜２．０％、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．００１〜０．１％、Ｃｒ：１０〜２５％、Ｎｉ：１．５〜１０％、Ｎ：０．１５％以下を含有し、残部はＦｅおよび不可避的不純物からなる組成を有するステンレス鋼が挙げられる。前記ステンレス鋼は、さらに、Ｍｏ：１〜５％、Ｃｕ：０．０３〜４％、Ｖ：０．０１〜０．５％のうちから選んだ１種以上を含有する組成を有するものであってもよい。また、鋼材は、フェライト相とマルテンサイト相を主相とする組織を有するステンレス鋼が好ましい。なお、ここでフェライト相とマルテンサイト相を主相とする組織とは、フェライト相とマルテンサイト相の合計が、体積率で、７０％以上である組織をいい、９０％以上が好ましく、１００％であってもよい。また、フェライト相とマルテンサイト相の合計が体積率で１００％未満である場合の残部としては、残留オーステナイト相が挙げられる。
(1) Test piece The test piece is a material for evaluating sulfide stress corrosion cracking. Although it does not specifically limit as said material, In this embodiment, steel materials, especially a steel pipe are assumed. As the test piece, a steel material itself may be used, for example, a steel piece cut out from a steel pipe into a predetermined shape may be used. As steel materials assumed in this embodiment (hereinafter, unless otherwise specified,% means mass%) C: 0.005 to 0.5%, Si: 0.05 to 1.0%, Mn: 0 0.1-2.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.001-0.1%, Cr: 10-25%, Ni: 1.5-10% , N: 0.15% or less, with the balance being stainless steel having a composition consisting of Fe and inevitable impurities. The stainless steel further has a composition containing one or more selected from Mo: 1 to 5%, Cu: 0.03 to 4%, and V: 0.01 to 0.5%. May be. Further, the steel material is preferably stainless steel having a structure whose main phase is a ferrite phase and a martensite phase. In addition, the structure | tissue which has a ferrite phase and a martensite phase as a main phase here means the structure | tissue in which the sum total of a ferrite phase and a martensite phase is 70% or more by volume ratio, 90% or more is preferable and 100% It may be. Moreover, a residual austenite phase is mentioned as a remainder in case the sum total of a ferrite phase and a martensite phase is less than 100% by a volume ratio.
鋼材の製造方法は、特に限定されず、通常公知の鋼材の製造方法が適用できる。例えば、鋼管であれば、通常公知の継目無鋼管や電縫鋼管の製造方法が適用でき、上記組成を有する溶鋼を、転炉、電気炉、真空溶解炉等の通常公知の溶製方法で溶製し、連続鋳造法、造塊−分塊圧延法等、通常の方法でビレット等の鋼管素材とすることができる。ついで、これら鋼管素材を所望に応じて加熱し、通常公知の造管方法であるマンネスマン−プラグミル方式、あるいはマンネスマン−マンドレルミル方式の造管工程を用いて熱間で造管し、所定寸法の継目無鋼管とする。また、電縫鋼管の場合は、通常公知の方法で製造した鋼管素材を通常公知の方法で造管し、電縫鋼管としてもよい。また、所望の強度、靭性等を達成するための組織（マルテンサイト、フェライト、オーステナイト等）を得るため焼入処理、焼戻処理等を適宜実施してよい。 The manufacturing method of steel materials is not particularly limited, and generally known methods for manufacturing steel materials can be applied. For example, if it is a steel pipe, the manufacturing method of a well-known seamless steel pipe or a ERW steel pipe can be applied, and the molten steel which has the above-mentioned composition is melted by the usual publicly known melting methods, such as a converter, an electric furnace, and a vacuum melting furnace. It can be made into a steel pipe material such as a billet by a usual method such as continuous casting method, ingot-making and ingot rolling method. Next, these steel pipe materials are heated as desired, and are piped hot using a Mannesmann-plug mill method, or Mannesmann-Mandrel mill method, which is a generally known pipe making method, and joints of predetermined dimensions are produced. Steel-free pipe. In the case of an electric resistance steel pipe, a steel pipe material manufactured by a generally known method may be formed by a generally known method to form an electric resistance steel pipe. Further, in order to obtain a structure (martensite, ferrite, austenite, etc.) for achieving desired strength, toughness, etc., quenching treatment, tempering treatment, etc. may be appropriately performed.
本実施形態の鋼材の硫化物応力腐食割れ性の評価方法に用いる試験片は、必要に応じて上記試験片を数ｃｍ程度に切り出した後、エポキシ樹脂に包埋し、ＳｉＣ研磨紙＃４００〜＃４０００を用いて湿式研磨を行う。微小な構造体を捉えるため表面粗さは小さい＃４０００研磨が好ましく、より好ましくはダイヤモンド、コロイダルシリカ等の研磨剤を用いた鏡面研磨である。 The test piece used for the method for evaluating the sulfide stress corrosion cracking property of the steel material of the present embodiment is cut out to about several centimeters, if necessary, and then embedded in an epoxy resin, and the SiC abrasive paper # 400- Wet polishing is performed using # 4000. In order to capture a minute structure, # 4000 polishing having a small surface roughness is preferable, and mirror polishing using an abrasive such as diamond or colloidal silica is more preferable.
（２）試験溶液
本実施形態の鋼材の硫化物応力腐食割れ性の評価方法に用いる試験溶液（水溶液）は、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに準拠する。具体的には、２０質量％ＮａＣｌ水溶液に、０．５質量％ＣＨ３ＣＯＯＨと、ＣＨ３ＣＯＯＮａを添加し、所定圧でＨ２Ｓを飽和させた水溶液、または０．１６５質量％ＮａＣｌ水溶液に、０．０４質量％ＣＨ３ＣＯＯＮａと、ＨＣｌを添加し、所定圧でＨ２Ｓを飽和させた水溶液が挙げられる。前記水溶液のｐＨおよびＨ２Ｓ濃度は所望の条件に応じて適宜変えて良い。
(2) Test Solution The test solution (aqueous solution) used in the method for evaluating the sulfide stress corrosion cracking property of the steel material of the present embodiment conforms to NACE ™ 0177 Method A. Specifically, an aqueous solution obtained by adding 0.5% by mass CH 3 COOH and CH 3 COONa to a 20% by mass NaCl aqueous solution and saturating H 2 S at a predetermined pressure, or 0.165% by mass NaCl aqueous solution. , 0.04 mass% CH 3 COONa, and an aqueous solution in which HCl is added and H 2 S is saturated at a predetermined pressure. The pH and H 2 S concentration of the aqueous solution may be appropriately changed according to desired conditions.
試験片を水溶液に浸漬させる時間は、構造体を捉える上で２４時間以上１２０時間以内の任意の時間であることが好ましく、４８時間以上１００時間以内の任意の時間であることがより好ましい。ただし浸漬時間は試験片および腐食環境（溶液ｐＨ、Ｈ２Ｓ濃度）に依るため、この時間のみに限定されるものではない。また、構造体の抽出や腐食前駆体の判別に支障がでないよう、上記浸漬時間内で、外観上、全面腐食または０．１ｍｍ程度の大きな孔食が多数発生しない溶液ｐＨを選定することがより好ましい。試験溶液の温度は、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに準拠した２５℃が好ましいが、構造体の抽出や腐食前駆体の判別に支障がでない範囲で適宜変えて良い。 The time for immersing the test piece in the aqueous solution is preferably an arbitrary time of 24 hours or more and 120 hours or less, and more preferably an arbitrary time of 48 hours or more and 100 hours or less in capturing the structure. However, since the immersion time depends on the test piece and the corrosive environment (solution pH, H 2 S concentration), it is not limited to this time alone. In addition, it is more preferable to select a solution pH that does not cause a general corrosion or a large number of large pitting corrosion of about 0.1 mm within the above immersion time so as not to hinder the extraction of the structure and the determination of the corrosion precursor. preferable. The temperature of the test solution is preferably 25 ° C. in accordance with NACE ™ 0177 Method A, but may be appropriately changed within a range that does not hinder the extraction of the structure and the identification of the corrosion precursor.
（３）走査電子顕微鏡（Ｓｃａｎｎｉｎｇ Ｅｌｅｃｔｒｏｎ Ｍｉｃｒｏｓｃｏｐｙ：ＳＥＭ）による二次電子像の取得方法
本実施形態の鋼材の硫化物応力腐食割れ性の評価方法では、上記試験片を上記水溶液に浸漬した後の試験片表面を観察面として、ＳＥＭによる二次電子像を取得する。
ＳＥＭで使用する電子銃は、空間分解能の観点から電界放出型（フィールドエミッション）電子銃またはショットキー型電子銃が望ましい。また、対物レンズのレンズ方式は、インレンズ方式、アウトレンズ方式、その間のセミインレンズ方式のいずれの方式でもよい。対物レンズとしては、例えば、Ｃａｒｌ ＺＥＩＳＳ製ＵＬＴＲＡ５５が挙げられる。
(3) Method for obtaining secondary electron image by scanning electron microscope (SEM) In the method for evaluating the sulfide stress corrosion cracking property of the steel material of the present embodiment, a test after immersing the test piece in the aqueous solution. A secondary electron image by SEM is acquired using one surface as an observation surface.
The electron gun used in the SEM is preferably a field emission electron gun or a Schottky electron gun from the viewpoint of spatial resolution. Further, the lens system of the objective lens may be any of an in-lens system, an out-lens system, and a semi-in-lens system therebetween. Examples of the objective lens include ULTRA55 manufactured by Carl ZEISS.
本実施形態では、試験片表面の不動態皮膜に関する情報を得るため、ＳＥＭの加速電圧を低加速条件とし、侵入深さを抑える必要がある。加速電圧は、０．１ｋＶ〜５ｋＶとし、０．１ｋＶ〜１ｋＶがより好ましい形態である。なお、前述したように、汎用的に使用されるＳＥＭ像（ＥＴ検出器による二次電子像、例：加速電圧１５ｋＶ）では、コンタミネーションと腐食前駆体の見え方が同じであり判別できない。前記の低加速ＳＥＭ条件を適用すれば、二次電子像上で、中心部と輪郭部から構成される腐食前駆体を、コンタミネーション等と区別して判別できる。 In this embodiment, in order to obtain information on the passive film on the surface of the test piece, it is necessary to set the acceleration voltage of the SEM to a low acceleration condition and to suppress the penetration depth. The acceleration voltage is 0.1 kV to 5 kV, more preferably 0.1 kV to 1 kV. As described above, in the SEM image (secondary electron image obtained by the ET detector, for example, acceleration voltage 15 kV) used for general purposes, the appearance of the contamination and the corrosion precursor are the same and cannot be discriminated. When the low acceleration SEM condition is applied, the corrosion precursor composed of the center portion and the contour portion can be distinguished from the contamination and the like on the secondary electron image.
また、ＳＥＭの加速電圧を小さくする機能として、試料ステージに負バイアスを印加し、電子ビームを試料直前で減速させ、実効的な電子ビームの照射エネルギーを低下させる方法（リターディング機能）を用いても良い。 In addition, as a function of reducing the acceleration voltage of the SEM, a method (retarding function) is used that applies a negative bias to the sample stage, decelerates the electron beam immediately before the sample, and reduces the effective electron beam irradiation energy. Also good.
検出器は、鏡筒内の二次電子検出器（インレンズ検出器）または、鏡筒外に設置された二次電子検出器（Ｅｖｅｒｈａｒｔ−Ｔｈｏｒｎｌｅｙ：ＥＴ検出器）を用いることが望ましい。前者のインレンズ検出器は、試料表面より発生した二次電子のうち、低エネルギー側の二次電子を検出するため、物質の表面電位や化学状態の違いに対してコントラスト差を生じる。このため、より幅広い加速電圧条件で観察され易いという観点からインレンズ検出器を用いることがより好ましい形態である。
対物レンズと試料ステージの距離（Ｗｏｒｋｉｎｇ Ｄｉｓｔａｎｃｅ：ＷＤ）は、メーカー毎の装置や試料状態に応じて適宜調整して良い。
倍率は測定対象の腐食部の大きさに応じて調節してよく、例えば、１０００倍〜５万倍である。
The detector is preferably a secondary electron detector (in-lens detector) in the lens barrel or a secondary electron detector (Everhart-Thornley: ET detector) installed outside the lens barrel. Since the former in-lens detector detects secondary electrons on the low energy side among the secondary electrons generated from the sample surface, a contrast difference is generated with respect to a difference in surface potential or chemical state of the substance. For this reason, it is a more preferable form to use an in-lens detector from the viewpoint of being easily observed under a wider range of acceleration voltage conditions.
The distance between the objective lens and the sample stage (working distance: WD) may be appropriately adjusted according to the apparatus and sample state of each manufacturer.
The magnification may be adjusted according to the size of the corroded portion to be measured, and is, for example, 1000 to 50,000 times.
以上が、本発明の構造体を抽出し、腐食前駆体を判別するために必要なＳＥＭの要件である。だだし、二次電子像取得の際、低加速ＳＥＭ観察技術の性質上、極表層に敏感であることから、取り込みスキャンを同一箇所で繰り返すと試料表面へのコンタミネーション、電子ビームによる試料ダメージの影響で、後述する構造体の輪郭部が捉え難くなり構造体の抽出が困難になる場合があるので注意が必要である。 The above is the SEM requirement necessary for extracting the structure of the present invention and discriminating the corrosion precursor. However, due to the nature of the low-acceleration SEM observation technique when acquiring a secondary electron image, it is sensitive to the extreme surface layer. Therefore, if the capture scan is repeated at the same location, contamination of the sample surface and sample damage caused by the electron beam Note that it is difficult to detect the outline of the structure, which will be described later, and extraction of the structure may be difficult due to the influence.
（４）構造体
次に、上記条件で取得した二次電子像から抽出する構造体について、図面を参照しながら説明する。
図１は、本発明で抽出する構造体１の構造を示す概略図である。この構造体１は、上記試験片を上記水溶液に浸漬した際に、試験片表面に発生するものである。本発明の鋼材の硫化物応力腐食割れ性の評価方法では、この構造体１を抽出し、抽出した構造体１から腐食前駆体を判別し、判別した腐食前駆体の個数密度で硫化物応力腐食割れ性を評価しＳＳＣ試験の試験結果を予測する。構造体１は、前記浸漬後の試験片表面を観察面として、上記ＳＥＭの要件で二次電子像を取得することで捉えることができる。
(4) Structure Next, a structure extracted from the secondary electron image acquired under the above conditions will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing the structure of a structure 1 extracted in the present invention. The structure 1 is generated on the surface of the test piece when the test piece is immersed in the aqueous solution. In the method for evaluating sulfide stress corrosion cracking of steel according to the present invention, the structure 1 is extracted, a corrosion precursor is identified from the extracted structure 1, and sulfide stress corrosion is performed at the number density of the identified corrosion precursor. Evaluate the cracking property and predict the test result of the SSC test. The structure 1 can be captured by acquiring a secondary electron image with the SEM requirement using the surface of the test piece after immersion as an observation surface.
図１に示すように、構造体１は、二次電子像において、中心部１０と、前記中心部１０を囲繞する輪郭部２０から構成される。構造体１の中心部１０は、試験片のバルク部Ｂ、すなわち、試験片表面において構造体１やコンタミネーション等が存在しない正常部に対して、低い輝度値を有する。一方、構造体１の輪郭部２０は、前記バルク部Ｂに対して、高い輝度値を有する。構造体１は、このような特徴的な明暗のコントラストを有するため、二次電子像において、バルク部Ｂやコンタミネーション等と区別して抽出することができる。 As shown in FIG. 1, the structure 1 includes a central portion 10 and a contour portion 20 surrounding the central portion 10 in a secondary electron image. The central portion 10 of the structure 1 has a low luminance value with respect to the bulk portion B of the test piece, that is, the normal portion where the structure 1 or contamination or the like does not exist on the surface of the test piece. On the other hand, the outline portion 20 of the structure 1 has a high luminance value with respect to the bulk portion B. Since the structure 1 has such characteristic contrast of light and dark, the secondary electron image can be extracted separately from the bulk portion B, contamination, and the like.
二次電子像上で構造体１の大きさ（最大長さＬ）は１０μｍ以下である。本実施形態において、構造体１の大きさは、１μｍ〜１０μｍの範囲である。ただし、腐食の進行度の違いなどによって、最大長さＬが１μｍ未満または１０μｍ超となることは十分予想される。この場合、最大長さＬが１μｍ〜１０μｍの範囲のものを構造体１として抽出する。構造体１の大きさは、取得した二次電子像から容易に測られる。 The size (maximum length L) of the structure 1 on the secondary electron image is 10 μm or less. In the present embodiment, the size of the structure 1 is in the range of 1 μm to 10 μm. However, it is sufficiently expected that the maximum length L will be less than 1 μm or more than 10 μm due to the difference in the degree of progress of corrosion. In this case, the structure 1 having a maximum length L in the range of 1 μm to 10 μm is extracted. The size of the structure 1 can be easily measured from the acquired secondary electron image.
本発明で述べる輝度値は、画像処理手段、具体的には、Ｐｈｏｔｏｓｈｏｐ（登録商標）またはＰｈｏｔｏｓｈｏｐ ｅｌｅｍｅｎｔｓ（登録商標）等のソフトウェアを使用し、各領域を指定した際の輝度の平均値として求めることができる。 The luminance value described in the present invention is obtained as an average value of luminance when each region is specified by using image processing means, specifically, software such as Photoshop (registered trademark) or Photoshop elements (registered trademark). Can do.
構造体１は輪郭部２０を呈しており、輪郭部２０の輝度値Ａｓと正常部のバルク部Ｂの輝度値Ａｂの比Ａｓ／Ａｂが１．００未満である。スキャンを繰り返した後で取り込んだ二次電子像の場合、前述の通りコンタミネーション・ダメージの影響により輪郭部２０の輝度値がバルク部Ｂの輝度値と差異が小さく、すなわちＡｓ／Ａｂ比が１．００に近づくため、最低限のスキャンにて取得した二次電子像を用いることが望ましい。 The structure 1 has a contour portion 20, and the ratio As / Ab between the luminance value As of the contour portion 20 and the luminance value Ab of the bulk portion B of the normal portion is less than 1.00. In the case of a secondary electron image captured after repeated scanning, as described above, the brightness value of the contour portion 20 is less different from the brightness value of the bulk portion B due to the influence of contamination damage, that is, the As / Ab ratio is 1. Since it approaches 0.000, it is desirable to use a secondary electron image acquired by a minimum scan.
（５）腐食前駆体
腐食前駆体は、上述の構造体１のなかから判別される。この判別には、上記バルク部Ｂの輝度値Ａｂと中心部１０の輝度値Ａｃの比（Ａｂ／Ａｃ）を用いる。具体的には、構造体のなかからＡｂ／Ａｃが所定の値以上のものを腐食前駆体として判別する。前記所定の値は、試験に供する鋼材の鋼種等によって変わり得るため、鋼材の鋼種等に応じて決定される。後述するように、前記所定の値は、予め、試験に供する鋼材と同種の鋼材を試験片とした試験を行い、この試験片の二次電子像の、構造体中心部の元素分析結果と、試験片バルク部の輝度値Ａｂと構造体中心部の輝度値Ａｃの比との関係から決定することができる。後述するように、本実施形態の鋼種の鋼材では、前記所定の値は１．３である。すなわち、本実施形態では、抽出した構造体のなかからＡｂ／Ａｃが１．３以上のものを腐食前駆体として判別する。なお、上述したように、この腐食前駆体は孔食萌芽に相当するものである。腐食前駆体の中心部の輝度値Ａｃは０以上１００．０未満となり、輝度値が小さい方がより腐食が進行していると考えられる。
(5) Corrosion precursor The corrosion precursor is discriminated from the structure 1 described above. For this determination, the ratio (Ab / Ac) of the luminance value Ab of the bulk portion B and the luminance value Ac of the central portion 10 is used. Specifically, a structure in which Ab / Ac is equal to or greater than a predetermined value is determined as a corrosion precursor. Since the predetermined value may vary depending on the steel type of the steel material used for the test, it is determined according to the steel type of the steel material. As will be described later, the predetermined value is a test performed in advance using a steel material of the same type as the steel material to be subjected to the test, and a secondary electron image of the test piece, the result of elemental analysis in the center of the structure, It can be determined from the relationship between the luminance value Ab of the test piece bulk part and the ratio of the luminance value Ac of the central part of the structure. As will be described later, in the steel material of this embodiment, the predetermined value is 1.3. That is, in this embodiment, Ab / Ac of 1.3 or more is determined as a corrosion precursor from the extracted structures. As described above, this corrosion precursor corresponds to pitting corrosion. The luminance value Ac of the central portion of the corrosion precursor is 0 or more and less than 100.0, and it is considered that the smaller the luminance value, the more the corrosion proceeds.
（６）構造体の組成分析
構造体１の中心部１０または輪郭部２０の組成を分析する方法として、ＳＥＭに搭載されるエネルギー分散型Ｘ線分析（Ｅｎｅｒｇｙ Ｄｉｓｐｅｒｓｉｖｅ Ｘ−ｒａｙ Ｓｐｅｃｔｒｏｓｃｏｐｙ：ＥＤＳ）が挙げられる。検出対象となる元素に依るが、測定時の加速電圧は、低い方が侵入深さは浅くなり、表面近傍の信号量が相対的に増加するため有効である。例えば、好ましい加速電圧として５ｋＶが挙げられる。
(6) Composition analysis of structure As a method of analyzing the composition of the central portion 10 or the contour portion 20 of the structure 1, energy dispersive X-ray spectroscopy (EDS) mounted on the SEM can be cited. It is done. Although it depends on the element to be detected, the lower the acceleration voltage during measurement, the lower the penetration depth and the more effective the signal amount in the vicinity of the surface. For example, a preferable acceleration voltage is 5 kV.
ＥＤＳにより構造体１を解析すると、構造体１のうち、腐食前駆体の中心部は、ＦｅおよびＳを含む組成を有する。このことから、腐食前駆体の中心部は、主に母材（鉄）の腐食生成物から構成されると考えられる。ただし、腐食生成物は腐食環境によって構成が変わることが十分考えられる。例えば、腐食環境が大気環境下等であれば腐食前駆体の中心部は、ＦｅおよびＯを含む組成となることが考えられる。一方、構造体１のうち、腐食前駆体に相当しないコンタミネーション等の中心部は、Ｃ主体の組成を有する。このことから、構造体１中心部の組成を分析することによって、構造体１が腐食前駆体に相当するか否かを知ることができる。 When the structure 1 is analyzed by EDS, the central part of the corrosion precursor in the structure 1 has a composition containing Fe and S. From this, it is considered that the central portion of the corrosion precursor is mainly composed of a corrosion product of the base material (iron). However, it is conceivable that the structure of the corrosion product varies depending on the corrosive environment. For example, if the corrosive environment is an atmospheric environment or the like, the central portion of the corrosion precursor may have a composition containing Fe and O. On the other hand, the central part of the structure 1 such as contamination that does not correspond to the corrosion precursor has a C-based composition. From this, it is possible to know whether or not the structure 1 corresponds to a corrosion precursor by analyzing the composition at the center of the structure 1.
（７）腐食前駆体の個数密度について
前記ＳＥＭ条件による二次電子像を倍率１０００倍で任意点を１０枚取得し、腐食前駆体の個数密度をそれぞれ求め、平均した値を腐食前駆体の個数密度とする。
(7) Number Density of Corrosion Precursors Ten arbitrary points were acquired at a magnification of 1000 times as secondary electron images under the SEM conditions, the number densities of the corrosion precursors were determined, and the average value was obtained as the number of corrosion precursors. Density.
（８）ＳＳＣ試験
比較のため、ＳＳＣ試験を実施する。本実施形態では、上記試験片を所定形状に加工し、規格化されているＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに準拠して定荷重試験を実施する。この定荷重試験では、同規格による試験溶液にＨ２Ｓ＋ＣＯ２バランスの混合ガスを通気しながら、応力負荷した試験片を２５℃で３０日間浸漬し、破断の有無を評価する。
(8) SSC test An SSC test is performed for comparison. In the present embodiment, the test piece is processed into a predetermined shape, and a constant load test is performed in accordance with the standardized NACE TM 0177 Method A. In this constant load test, a stress-loaded test piece is immersed at 25 ° C. for 30 days while a mixed gas of H 2 S + CO 2 balance is passed through a test solution according to the same standard, and the presence or absence of breakage is evaluated.
なお、本発明の鋼材の硫化物応力腐食割れ性の評価方法は、上述の実施形態に限定されない。例えば、上記実施形態においては、ＳＳＣ試験としてＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに規定される定荷重試験を想定しているが、ＳＳＣ試験は、これ以外の規格によるものであってもよい。この場合、試験片を浸漬する水溶液等の条件を、予測するＳＳＣ試験の規格に準拠したものとすればよい。 In addition, the evaluation method of the sulfide stress corrosion cracking property of the steel material of this invention is not limited to the above-mentioned embodiment. For example, in the above embodiment, a constant load test defined in NACE ™ 0177 Method A is assumed as the SSC test, but the SSC test may be based on other standards. In this case, the conditions such as the aqueous solution in which the test piece is immersed may be compliant with the predicted SSC test standard.
また、本発明の鋼材の硫化物応力腐食割れ性の評価方法に用いる鋼材の硫化物応力腐食割れ性の評価装置は、応力を負荷せずに鋼材の試験片を浸漬する、塩化物イオンを含み硫化水素ガスを通気した水溶液を収容する容器と、０．１ｋＶ以上５ｋＶ以下の加速電圧で、前記浸漬後の試験片表面の二次電子像を取得する走査型電子顕微鏡と、当該二次電子像から、中心部と、中心部を囲繞する輪郭部から構成され、最大長さが１μｍ以上１０μｍ以下であり、前記試験片表面のバルク部の輝度値Ａｂと前記輪郭部の輝度値Ａｓとの比（Ａｂ／Ａｓ）が１．００未満である構造体を抽出するとともに、前記バルク部の輝度値Ａｂと前記中心部の輝度値Ａｃとの比（Ａｂ／Ａｃ）を求める画像処理手段と、を備える。画像処理手段は、その一部または全部が、走査型電子顕微鏡に組み込まれていてもよいし、走査型電子顕微鏡とは別に、例えばパーソナルコンピュータやマイクロコンピュータ等の情報処理装置に組み込まれていてもよい。 Moreover, the sulfide stress corrosion cracking evaluation apparatus for steel used in the method for evaluating sulfide stress corrosion cracking of steel of the present invention includes chloride ions that immerse a test piece of steel without applying stress. A container containing an aqueous solution through which hydrogen sulfide gas has been passed; a scanning electron microscope that acquires a secondary electron image of the surface of the test specimen after immersion at an acceleration voltage of 0.1 kV to 5 kV; and the secondary electron image From the central portion and the contour portion surrounding the central portion, the maximum length is 1 μm or more and 10 μm or less, and the ratio between the luminance value Ab of the bulk portion on the surface of the test piece and the luminance value As of the contour portion Image processing means for extracting a structure having (Ab / As) less than 1.00 and obtaining a ratio (Ab / Ac) between the luminance value Ab of the bulk portion and the luminance value Ac of the central portion; Prepare. Part or all of the image processing means may be incorporated in a scanning electron microscope, or may be incorporated in an information processing apparatus such as a personal computer or a microcomputer separately from the scanning electron microscope. Good.
以下に、実施例を挙げて本発明を具体的に説明する。なお、本発明は以下の実施例に限定されるものではない。 Hereinafter, the present invention will be specifically described with reference to examples. In addition, this invention is not limited to a following example.
本実施例は、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに規定されるＳＳＣ試験の試験結果を予測したものである。 In this example, the test result of the SSC test defined in NACE ™ 0177 Method A is predicted.
鋼材の試験片として、フェライト相およびマルテンサイト相を主相とする組織（一部残留オーステナイト相含む）を有するステンレス鋼を用いた。この試験片を以下のように調製した。 As a steel specimen, stainless steel having a structure (including partially retained austenite phase) having a ferrite phase and a martensite phase as main phases was used. This test piece was prepared as follows.
質量％で、Ｃ：０．０２７％、Ｓｉ：０．２４％、Ｍｎ：０．３４％、Ｐ：０．０２１％、Ｓ：０．００３％、Ａｌ：０．０２４％、Ｃｒ：１６．６２％、Ｎｉ：３．８％、Ｍｏ：２．４６％、Ｖ：０．０６４％、Ｎ：０．０４６％を含有し、残部がＦｅおよび不可避的不純物からなる鋼素材を所定寸法の鋼管に造管し、９６０℃の温度に加熱し水冷する焼入処理と、６００℃の温度から空冷する焼戻処理とを施した。この鋼管から切り出し、バフ（鏡面）研磨を実施後、数日間、大気中に放置したものを試験片として用いた。 In mass%, C: 0.027%, Si: 0.24%, Mn: 0.34%, P: 0.021%, S: 0.003%, Al: 0.024%, Cr: 16. A steel pipe containing 62%, Ni: 3.8%, Mo: 2.46%, V: 0.064%, N: 0.046%, the balance being Fe and inevitable impurities, with a predetermined dimension Were subjected to a quenching process of heating to a temperature of 960 ° C. and water cooling, and a tempering process of cooling from a temperature of 600 ° C. to air. After cutting out from this steel pipe and carrying out buffing (mirror surface) polishing, it was allowed to stand in the atmosphere for several days and used as a test piece.
前記試験片を２０質量％ＮａＣｌ水溶液（液温：２５℃、Ｈ２Ｓ：０．１気圧、ＣＯ２：０．９気圧雰囲気）に酢酸と酢酸Ｎａを加えて、ｐＨ４．５に調整した水溶液（試験溶液）に７０時間浸漬した。 An aqueous solution prepared by adding acetic acid and Na acetate to a 20 mass% NaCl aqueous solution (liquid temperature: 25 ° C., H 2 S: 0.1 atm, CO 2 : 0.9 atm atmosphere) to adjust the pH to 4.5. It was immersed in (test solution) for 70 hours.
上記浸漬後の試験片において任意点の皮膜表面を走査型電子顕微鏡で観察した。測定条件として、電子ビームの加速電圧を０．１〜１５．０ｋＶとし、鏡筒内の二次電子検出器（インレンズ検出器）を用いて二次電子像を取得した。撮影倍率は１０００倍である。組成分析には、エネルギー分散型Ｘ線分析を用い、測定時の加速電圧は５ｋＶとした。
表１に、取得した二次電子像における腐食前駆体の判別結果を示す。
In the test piece after the immersion, the surface of the film at an arbitrary point was observed with a scanning electron microscope. As measurement conditions, the acceleration voltage of the electron beam was set to 0.1 to 15.0 kV, and a secondary electron image was obtained using a secondary electron detector (in-lens detector) in the lens barrel. The photographing magnification is 1000 times. For composition analysis, energy dispersive X-ray analysis was used, and the acceleration voltage during measurement was 5 kV.
Table 1 shows the discrimination results of the corrosion precursor in the acquired secondary electron image.
表１に示されるように、Ｎｏ．１〜６は、取得した二次電子像において、バルク部の輝度値Ａｂと輪郭部の輝度値Ａｓとの比（Ａｂ／Ａｓ）が１．００未満である。Ｎｏ．１〜６は、ＥＤＳ組成分析の結果、中心部がＦｅ、Ｓを含む組成を有しており、このことから、本発明の腐食前駆体に相当すると判定できる。このＮｏ．１〜６は、バルク部の輝度値Ａｂと中心部の輝度値Ａｃとの比（Ａｂ／Ａｃ）が１．３以上となっている。
これに対し、Ｎｏ．７〜１０は、ＥＤＳ組成分析の結果、中心部がＣ主体の組成を有しており、本発明の腐食前駆体に相当しないもの（コンタミネーション等）である。Ｎｏ．７〜１０は、Ａｂ／Ａｓが１．００以上であるか、Ａｂ／Ａｃが１．３未満となっている。
すなわち、本実施例の場合では、抽出した構造体（Ａｂ／Ａｓが１．００未満）のうちＡｂ／Ａｃが１．３以上のものを腐食前駆体として判別できる。また、予めこのような試験を行っておき、構造体中心部の元素分析結果と、Ａｂ／Ａｃの関係を把握し、腐食前駆体として抽出するＡｂ／Ａｃの下限を決めておけば、二次電子像から抽出した構造体のなかからＡｂ／Ａｃの値を用いて腐食前駆体を判別することができる。例えば、本実施例と同様の鋼種の鋼材を試験に供する場合では、抽出した構造体のなかからＡｂ／Ａｃが１．３以上のものを腐食前駆体として判別することができる。
As shown in Table 1, no. Nos. 1 to 6 have a ratio (Ab / As) between the luminance value Ab of the bulk portion and the luminance value As of the contour portion (Ab / As) of less than 1.00 in the acquired secondary electron image. No. As a result of EDS composition analysis, Nos. 1 to 6 have compositions containing Fe and S at the center, and from this, it can be determined that they correspond to the corrosion precursor of the present invention. This No. 1 to 6, the ratio (Ab / Ac) between the luminance value Ab of the bulk part and the luminance value Ac of the central part is 1.3 or more.
In contrast, no. As a result of EDS composition analysis, Nos. 7 to 10 have a composition mainly composed of C at the center and do not correspond to the corrosion precursor of the present invention (contamination etc.). No. As for 7-10, Ab / As is 1.00 or more, or Ab / Ac is less than 1.3.
That is, in the case of the present example, among the extracted structures (Ab / As is less than 1.00), those with Ab / Ac of 1.3 or more can be identified as corrosion precursors. Further, if such a test is performed in advance, the elemental analysis result in the center of the structure and the relationship between Ab / Ac are grasped, and the lower limit of Ab / Ac extracted as a corrosion precursor is determined, the secondary Corrosion precursors can be discriminated from the structures extracted from the electronic image using the value of Ab / Ac. For example, in the case where a steel material of the same steel type as in this example is used for the test, it is possible to discriminate an Ab / Ac of 1.3 or more from the extracted structures as a corrosion precursor.
図２に、Ｎｏ．４の腐食前駆体のインレンズ像を示す。図２に示されるように、本実施例の腐食前駆体は、Ａｂ／Ａｓ比が１．００未満、Ａｂ／Ａｃ比が１．３以上であり、中心部と輪郭部、さらに輪郭部とバルク部の間で明暗のコントラスト差を有しており、バルク部やコンタミネーションと区別して判別することができる。 In FIG. 4 shows an in-lens image of No. 4 corrosion precursor. As shown in FIG. 2, the corrosion precursor of this example has an Ab / As ratio of less than 1.00 and an Ab / Ac ratio of 1.3 or more. It has a contrast difference between light and dark between the parts, and can be distinguished from the bulk part and contamination.
次に、判別した腐食前駆体の個数密度と、ＳＳＣ試験の試験結果との関係について検討した。
試験溶液のｐＨを１．５〜４．５、Ｈ２Ｓを０．０１気圧、０．１気圧、０．２気圧にそれぞれ変えたこと以外は、上記と同様にして、上記試験片を試験溶液に浸漬し、浸漬後の試験片表面を観察面として、上記Ｎｏ．４の条件（加速電圧０．５ｋＶ、インレンズ像）と同様の条件で二次電子像を取得した。そして、任意点１０箇所で倍率１０００倍の二次電子像を取得し、腐食前駆体の個数密度を算出した。また、これと並行してＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに準拠したＳＳＣ試験を、試験溶液のｐＨ、Ｈ２Ｓ気圧をそれぞれ上記のように変えて実施した。
Next, the relationship between the number density of the identified corrosion precursor and the test result of the SSC test was examined.
The test piece was tested in the same manner as above except that the pH of the test solution was changed to 1.5 to 4.5, and H 2 S was changed to 0.01 atmosphere, 0.1 atmosphere, and 0.2 atmosphere, respectively. Immerse in the solution and use the surface of the test piece after immersion as the observation surface. A secondary electron image was obtained under the same conditions as in condition 4 (acceleration voltage 0.5 kV, in-lens image). And the secondary electron image of 1000-times multiplication factor was acquired in ten arbitrary points, and the number density of the corrosion precursor was computed. In parallel with this, an SSC test based on NACE ™ 0177 Method A was carried out by changing the pH and H 2 S atmospheric pressure of the test solution as described above.
図３に、腐食前駆体の個数密度と試験溶液ｐＨの関係を、Ｈ２Ｓ濃度毎でそれぞれ示す。また、並行して行った各条件におけるＳＳＣ試験の結果も合わせて示す。 FIG. 3 shows the relationship between the number density of the corrosion precursor and the test solution pH for each H 2 S concentration. Moreover, the result of the SSC test in each condition performed in parallel is also shown.
図３において、Ｈ２Ｓ濃度０．０１気圧の条件を三角（△、▲）、０．１気圧の条件を丸（○、●）、０．２気圧の条件を四角（□、■）のプロットで示す。また、黒塗りのプロット（▲、●、■）は、並行して行った各条件でのＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに準拠したＳＳＣ試験において硫化物応力腐食割れ（ＳＳＣ）が発生したこと（ＳＳＣ試験不合格、破断有）を示し、黒塗りしていないプロット（△、○、□）は、同ＳＳＣ試験においてＳＳＣが発生しなかったこと（ＳＳＣ試験合格、破断なし）を示す。 In FIG. 3, the conditions for the H 2 S concentration of 0.01 atm are triangular (Δ, ▲), the conditions for 0.1 atm are circles (◯, ●), and the conditions for 0.2 atm are squares (□, ■). Shown in the plot. In addition, black plots (▲, ●, ■) indicate that sulfide stress corrosion cracking (SSC) occurred in the SSC test in accordance with NACE TM 0177 Method A under each condition performed in parallel (SSC test). The plots (Δ, ○, □) that are not black and indicate no failure (with failure, with fracture) indicate that no SSC occurred in the same SSC test (SSC test passed, without fracture).
図３に示すように、Ｈ２Ｓ濃度０．０１気圧の条件では、腐食前駆体の個数密度が４５０（個／ｍｍ２）を閾値として超えた場合、並行して行った同条件でのＳＳＣ試験においてＳＳＣが発生した（ＳＳＣ試験不合格、破断有）。さらに腐食条件が異なるＨ２Ｓ濃度０．１気圧、０．２気圧においても腐食前駆体の個数密度が４５０（個／ｍｍ２）超で、共通してＳＳＣ試験において割れが認められた。 As shown in FIG. 3, when the number density of the corrosion precursor exceeds 450 (pieces / mm 2 ) as a threshold value under the condition of H 2 S concentration of 0.01 atm, SSC under the same condition performed in parallel is performed. SSC occurred in the test (failed SSC test, with fracture). Furthermore, the number density of the corrosion precursor was over 450 (pieces / mm 2 ) even at H 2 S concentrations of 0.1 atm and 0.2 atm with different corrosion conditions, and cracks were commonly observed in the SSC test.
上記実施例より、腐食前駆体の個数密度と、ＮＡＣＥ ＴＭ ０１７７ Ｍｅｔｈｏｄ Ａに準拠して行った定荷重試験の試験結果とが対応していることが分かる。すなわち、本実施例では、腐食前駆体の個数密度が４５０（個／ｍｍ２）以下であれば、ＳＳＣ試験において割れが発生しないと予測できる。このように、本発明によれば、試験片表面に発生した腐食前駆体を指標として用いることで、ＳＳＣ試験の試験結果を予測することができる。本発明によれば、所定の試験溶液に浸漬した後の試験片表面を観察するだけでＳＳＣ試験の試験結果を予測でき、より簡易に、かつ、短時間でＳＳＣ試験の試験結果を予測できる。 From the above examples, it can be seen that the number density of the corrosion precursor corresponds to the test result of the constant load test performed in accordance with NACE ™ 0177 Method A. That is, in this example, if the number density of the corrosion precursor is 450 (pieces / mm 2 ) or less, it can be predicted that no cracks will occur in the SSC test. Thus, according to the present invention, the test result of the SSC test can be predicted by using the corrosion precursor generated on the surface of the test piece as an index. According to the present invention, the test result of the SSC test can be predicted simply by observing the surface of the test piece after being immersed in the predetermined test solution, and the test result of the SSC test can be predicted more easily and in a short time.
さらに、本発明の予測方法を適用した場合に、腐食前駆体の個数密度が設定された上限値以下（例えば本実施例であれば４５０（個／ｍｍ２）以下）となる鋼材は、ＳＳＣ試験を行った場合にＳＳＣが発生しない、すなわち、耐ＳＳＣ性に優れるものと判定できる。そのため、孔食が懸念される環境における材料の早期診断への適用、耐孔食性に優れた高性能材料の開発の迅速化等が可能となる。 Furthermore, when the prediction method of the present invention is applied, a steel material in which the number density of corrosion precursors is equal to or lower than a set upper limit value (for example, 450 (pieces / mm 2 ) or less in this example) is an SSC test. It can be determined that SSC does not occur when the process is performed, that is, the SSC resistance is excellent. Therefore, it is possible to apply to early diagnosis of materials in an environment where pitting corrosion is a concern and to speed up the development of high performance materials with excellent pitting corrosion resistance.
なお、本実施例ではフェライト相とマルテンサイト相を主相とするステンレス鋼を用いた場合について説明したが、本発明はこれに限るものでなく、マルテンサイト単相のスレンレス鋼をはじめとして多種多様の鋼に適用が可能である。 In this example, the case of using a stainless steel having a ferrite phase and a martensite phase as a main phase has been described. However, the present invention is not limited to this, and various types including a martensite single phase stainless steel. It can be applied to other steels.
１ 構造体
１０ 中心部
２０ 輪郭部
Ｂ バルク部
Ｌ 構造体１の大きさ（最大長さ）
DESCRIPTION OF SYMBOLS 1 Structure 10 Center part 20 Outline part B Bulk part L Size (maximum length) of structure 1
Claims (7)
走査型電子顕微鏡により、０．１ｋＶ以上５ｋＶ以下の加速電圧で、前記浸漬後の試験片表面の二次電子像を取得し、当該二次電子像から、中心部と、中心部を囲繞する輪郭部から構成され、最大長さが１μｍ以上１０μｍ以下であり、前記試験片表面のバルク部の輝度値Ａｂと前記輪郭部の輝度値Ａｓとの比（Ａｂ／Ａｓ）が１．００未満である構造体を抽出するとともに、
前記バルク部の輝度値Ａｂと前記中心部の輝度値Ａｃとの比（Ａｂ／Ａｃ）を用いて、前記構造体のなかから腐食前駆体を判別し、
前記判別した腐食前駆体の個数密度で硫化物応力腐食割れ性を評価することを特徴とする、鋼材の硫化物応力腐食割れ性の評価方法。 Immerse the steel specimen in an aqueous solution containing chloride ions and aerated with hydrogen sulfide gas without applying stress.
Using a scanning electron microscope, a secondary electron image of the surface of the test specimen after immersion is acquired at an acceleration voltage of 0.1 kV or more and 5 kV or less, and a center portion and a contour surrounding the center portion are obtained from the secondary electron image. And the maximum length is 1 μm or more and 10 μm or less, and the ratio (Ab / As) of the luminance value Ab of the bulk portion on the surface of the test piece to the luminance value As of the contour portion is less than 1.00. While extracting the structure,
Using the ratio (Ab / Ac) between the luminance value Ab of the bulk part and the luminance value Ac of the central part, a corrosion precursor is determined from the structure,
A method for evaluating sulfide stress corrosion cracking property of a steel material, comprising evaluating sulfide stress corrosion cracking property based on the number density of the identified corrosion precursor.
応力を負荷せずに鋼材の試験片を浸漬する、塩化物イオンを含み硫化水素ガスを通気した水溶液を収容する容器と、
０．１ｋＶ以上５ｋＶ以下の加速電圧で、前記浸漬後の試験片表面の二次電子像を取得する走査型電子顕微鏡と、
当該二次電子像から、中心部と、中心部を囲繞する輪郭部から構成され、最大長さが１μｍ以上１０μｍ以下であり、前記試験片表面のバルク部の輝度値Ａｂと前記輪郭部の輝度値Ａｓとの比（Ａｂ／Ａｓ）が１．００未満である構造体を抽出するとともに、
前記バルク部の輝度値Ａｂと前記中心部の輝度値Ａｃとの比（Ａｂ／Ａｃ）を求める画像処理手段と、を備えることを特徴とする、鋼材の硫化物応力腐食割れ性の評価装置。 An evaluation apparatus for sulfide stress corrosion cracking of steel used in the method for evaluating sulfide stress corrosion cracking of steel according to any one of claims 1 to 6,
A container containing an aqueous solution containing chloride ions and aerated with hydrogen sulfide gas, in which a steel specimen is immersed without applying stress,
A scanning electron microscope that acquires a secondary electron image of the surface of the test specimen after immersion at an acceleration voltage of 0.1 kV to 5 kV;
The secondary electron image is composed of a central portion and a contour portion surrounding the central portion, the maximum length is 1 μm or more and 10 μm or less, and the luminance value Ab of the bulk portion on the surface of the test piece and the luminance of the contour portion While extracting the structure whose ratio (Ab / As) to the value As is less than 1.00,
An image processing means for obtaining a ratio (Ab / Ac) between the luminance value Ab of the bulk part and the luminance value Ac of the central part, and an evaluation apparatus for sulfide stress corrosion cracking of steel.
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