JP2014074209A - Duplex stainless steel material and duplex stainless steel pipe - Google Patents
Duplex stainless steel material and duplex stainless steel pipe Download PDFInfo
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本発明は、硫化水素などの腐食性物質を含有する環境において使用される二相系ステンレス鋼材および二相系ステンレス鋼管に関するものである。 The present invention relates to a duplex stainless steel material and a duplex stainless steel pipe used in an environment containing a corrosive substance such as hydrogen sulfide.
二相系ステンレス鋼材はフェライト相とオーステナイト相の二相からなるステンレス鋼材で、Ｃｒを多量に含有するため、耐食性および強度に優れた耐食材料である。そのため、二相系ステンレス鋼管は、油井管、化学プラント、アンビリカルチューブ等の腐食性の高い環境の構造材料として広く使用されている。しかし、使用環境がより苛酷なものになってくるにつれて、耐食材料全般に要求される耐食性が高度になってきている。 The duplex stainless steel material is a stainless steel material composed of two phases of a ferrite phase and an austenite phase, and contains a large amount of Cr, and is a corrosion resistant material having excellent corrosion resistance and strength. For this reason, duplex stainless steel pipes are widely used as structural materials for highly corrosive environments such as oil well pipes, chemical plants, and umbilical tubes. However, as the usage environment becomes more severe, the corrosion resistance required for the general corrosion resistant materials is becoming higher.
そのため、スーパー二相系ステンレス鋼材と呼ばれるものが種々提案されている。例えば、特許文献１には、所定量のＣ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ｃｒ、Ｍｏ、Ｎｉ、Ｗ、Ｎを含有し、残部がＦｅおよび不純物からなり、Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎ≧４０、Ｍｏ＋１．１Ｎｉ≦１２．５、Ｍｏ−０．８Ｎｉ≦−１．６なる関係を満たす化学組成を有し、粗大介在物の数密度が１０個／ｍｍ２以下である二相系ステンレス鋼材が提案されている。 For this reason, various so-called super duplex stainless steel materials have been proposed. For example, Patent Document 1 contains a predetermined amount of C, Si, Mn, P, S, Cr, Mo, Ni, W, and N, with the balance being Fe and impurities, Cr + 3.3 (Mo + 0.5W) + 16N ≧ 40, Mo + 1.1Ni ≦ 12.5, Mo—0.8Ni ≦ −1.6, a chemical composition satisfying the relationship, and the number density of coarse inclusions is 10 pieces / mm 2 or less. Has been proposed.
また、特許文献２には、所定量のＣ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ｎｉ、Ｃｒ、Ｍｏ、Ｎ、Ａｌ、Ｍｇ、Ｃａ、Ｏを含有し、残部がＦｅおよび不可避的不純物からなり、鋼中に含まれる全非金属介在物に対するＣａＯ−Ａｌ２Ｏ３系酸化物の個数比率が４０％以下、全非金属介在物におけるＣａＯ濃度が１０質量％以下であり、孔食電位が６００ｍＶ以上である二相系ステンレス鋼材が提案されている。 Patent Document 2 contains a predetermined amount of C, Si, Mn, P, S, Ni, Cr, Mo, N, Al, Mg, Ca, O, with the balance being Fe and inevitable impurities, The number ratio of CaO—Al 2 O 3 -based oxide to all nonmetallic inclusions contained in the steel is 40% or less, the CaO concentration in all nonmetallic inclusions is 10% by mass or less, and the pitting potential is 600 mV or more. A duplex stainless steel material has been proposed.
硫化水素環境のようなより厳しい腐食環境に二相系ステンレス鋼材を適用するには、熱間加工性と耐食性を同時に向上させる必要がある。しかしながら、特許文献１、２の二相系ステンレス鋼材においては、熱間加工性の改善を重視し、耐食性の改善はＣｒ、Ｍｏ、Ｗの増加に任せているため、耐食性において満足できるものではない。特に、特許文献１の二相系ステンレス鋼材においては、成分組成として、σ相（金属間化合物）の生成を促進するＷを含有しているため、σ相の生成によって耐食性および熱間加工性が低下する。 In order to apply a duplex stainless steel material to a more severe corrosive environment such as a hydrogen sulfide environment, it is necessary to improve hot workability and corrosion resistance at the same time. However, in the duplex stainless steel materials of Patent Documents 1 and 2, emphasis is placed on improving hot workability, and the improvement in corrosion resistance is left to increase in Cr, Mo, and W, so that the corrosion resistance is not satisfactory. . In particular, the duplex stainless steel material of Patent Document 1 contains W that promotes the generation of a σ phase (intermetallic compound) as a component composition, so that the corrosion resistance and hot workability are improved by the generation of the σ phase. descend.
本発明は、このような状況に鑑みてなされたものであり、その課題は熱間加工性を維持したまま、硫化水素環境等にも耐え得る優れた耐食性を有する二相系ステンレス鋼材および二相系ステンレス鋼管を提供することにある。 The present invention has been made in view of such a situation, and the problem thereof is a duplex stainless steel material having excellent corrosion resistance that can withstand a hydrogen sulfide environment or the like while maintaining hot workability, and a duplex phase. It is to provide a stainless steel pipe.
本発明者らは、二相系ステンレス鋼材の耐食性を向上させるために、組織中の介在物に着目した。二相系ステンレス鋼材で問題となる孔食、すきま腐食、ＳＣＣ（応力腐食割れ）は、粗大な酸化物系介在物（Ａｌ２Ｏ３等）および窒化物を起点として発生する。本発明者らは、介在物および窒化物起点の腐食を抑制することに着目し、耐食性および熱間加工性を向上させる技術検討を行った。 The present inventors paid attention to inclusions in the structure in order to improve the corrosion resistance of the duplex stainless steel material. Pitting corrosion, crevice corrosion, and SCC (stress corrosion cracking), which are problems in the duplex stainless steel material, are generated starting from coarse oxide inclusions (such as Al 2 O 3 ) and nitride. The present inventors paid attention to suppressing corrosion of inclusions and nitride starting points, and conducted a technical study to improve corrosion resistance and hot workability.
本発明に係る二相系ステンレス鋼材は、フェライト相とオーステナイト相とからなる二相系ステンレス鋼材であって、前記二相系ステンレス鋼材の成分組成は、Ｃ：０．１質量％以下、Ｓｉ：０．１〜２．０質量％、Ｍｎ：０．１〜２．０質量％、Ｐ：０．０４質量％以下、Ｓ：０．０１質量％以下、Ａｌ：０．００１〜０．０５質量％、Ｃａ：０．０００５〜０．０２０質量％、Ｎｉ：３〜１０質量％、Ｃｒ：２３〜２８質量％、Ｍｏ：２〜５質量％、Ｎ：０．２〜０．４質量％、Ｖ：０．１〜０．６質量％を含有し、残部がＦｅおよび不可避的不純物であって、Ｃｒ＋３．３Ｍｏ＋１６Ｎ≧４０、Ｖ−２．５Ｎ＜−０．２なる関係を満足することを特徴とする。 The duplex stainless steel material according to the present invention is a duplex stainless steel material composed of a ferrite phase and an austenite phase. The component composition of the duplex stainless steel material is C: 0.1% by mass or less, Si: 0.1 to 2.0 mass%, Mn: 0.1 to 2.0 mass%, P: 0.04 mass% or less, S: 0.01 mass% or less, Al: 0.001 to 0.05 mass% %, Ca: 0.0005 to 0.020 mass%, Ni: 3 to 10 mass%, Cr: 23 to 28 mass%, Mo: 2 to 5 mass%, N: 0.2 to 0.4 mass%, V: 0.1 to 0.6% by mass, the balance being Fe and inevitable impurities, satisfying the relationship of Cr + 3.3Mo + 16N ≧ 40 and V−2.5N <−0.2 And
前記のように、二相系ステンレス鋼材の成分組成が、所定量のＣ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ａｌ、Ｃａ、Ｎｉ、Ｃｒ、Ｍｏ、Ｎ、Ｖを含有し、Ｃｒ、Ｍｏ、ＮがＣｒ＋３．３Ｍｏ＋１６Ｎ≧４０なる関係を満足することによって、二相系ステンレス鋼材の耐食性および強度が向上した。また、所定量のＡｌ、Ｃａ、Ｖを含有することによって、酸化物系化合物の大きさ、数密度が制御され、二相系ステンレス鋼材における介在物起点の腐食が抑制された。またＷを添加せず、耐食性の向上にＶを使用することで、σ相の生成が抑制され、二相系ステンレス鋼材の熱間加工性が向上した。さらに、ＶとＮがＶ−２．５Ｎ＜−０．２なる関係を満足することによって、ＶとＮの成分バランスが適切となり、窒化物の生成を抑制しつつ、微細な酸化物系介在物を形成させることができた。その結果、二相系ステンレス鋼材の耐食性が向上した。 As described above, the component composition of the duplex stainless steel material contains a predetermined amount of C, Si, Mn, P, S, Al, Ca, Ni, Cr, Mo, N, V, Cr, Mo, N Satisfies the relationship Cr + 3.3Mo + 16N ≧ 40, the corrosion resistance and strength of the duplex stainless steel material are improved. Further, by containing a predetermined amount of Al, Ca, and V, the size and number density of the oxide-based compound were controlled, and the corrosion of the inclusion starting point in the duplex stainless steel material was suppressed. Further, by using V for improving corrosion resistance without adding W, the generation of the σ phase was suppressed, and the hot workability of the duplex stainless steel material was improved. Furthermore, when V and N satisfy the relationship of V−2.5N <−0.2, the component balance of V and N becomes appropriate, and the formation of fine oxide inclusions is suppressed while suppressing the formation of nitrides. Could be formed. As a result, the corrosion resistance of the duplex stainless steel material was improved.
また、本発明に係る二相系ステンレス鋼材は、前記成分組成が、さらにＣｏ：０．０１〜４．０質量％、Ｔｉ：０．０１〜０．５質量％のうちの一種または２種を含むことを特徴とする。また、前記成分組成が、さらにＢ：０．００１〜０．０１質量％を含むことを特徴とする。
前記のように、二相系ステンレス鋼材が、所定量のＣｏ、Ｔｉのうちの１種または２種、または、Ｂを含有することによって、耐食性、熱間加工性が一層向上した。
In the duplex stainless steel material according to the present invention, the component composition further includes one or two of Co: 0.01 to 4.0% by mass and Ti: 0.01 to 0.5% by mass. It is characterized by including. Moreover, the said component composition contains B: 0.001-0.01 mass% further, It is characterized by the above-mentioned.
As described above, when the duplex stainless steel material contains one or two of a predetermined amount of Co and Ti, or B, corrosion resistance and hot workability are further improved.
さらに、本発明に係る二相系ステンレス鋼管は、前記の二相系ステンレス鋼材からなることを特徴とする。
前記のように、二相系ステンレス鋼管が、前記の二相系ステンレス鋼材で構成されることによって、耐食性、熱間加工性が向上する。
Furthermore, the duplex stainless steel pipe according to the present invention is characterized by comprising the aforementioned duplex stainless steel material.
As described above, when the duplex stainless steel pipe is composed of the duplex stainless steel material, corrosion resistance and hot workability are improved.
本発明の二相系ステンレス鋼材および二相系ステンレス鋼管によれば、熱間加工性を維持したまま、硫化水素環境に耐え得る耐食性を有する。それにより、本発明の二相系ステンレス鋼材および二相系ステンレス鋼管は、油井管、化学プラント、アンビリカルチューブ等に使用できる。 The duplex stainless steel material and duplex stainless steel pipe of the present invention have corrosion resistance that can withstand a hydrogen sulfide environment while maintaining hot workability. Thereby, the duplex stainless steel material and duplex stainless steel pipe of the present invention can be used for oil well pipes, chemical plants, umbilical tubes, and the like.
＜二相系ステンレス鋼材＞
本発明に係る二相系ステンレス鋼材の実施形態について詳細に説明する。
本発明の二相系ステンレス鋼材は、フェライト相とオーステナイト相とからなる二相系ステンレス鋼材であって、前記二相系ステンレス鋼材の成分組成は、Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ａｌ、Ｃａ、Ｎｉ、Ｃｒ、Ｍｏ、Ｎ、Ｖを所定量含有し、Ｃｒ＋３．３Ｍｏ＋１６Ｎ≧４０、Ｖ−２．５Ｎ＜−０．２なる関係を満足し、残部がＦｅおよび不可避的不純物からなる。以下、各構成について説明する。
<Duplex stainless steel>
An embodiment of a duplex stainless steel material according to the present invention will be described in detail.
The duplex stainless steel material of the present invention is a duplex stainless steel material composed of a ferrite phase and an austenite phase, and the component composition of the duplex stainless steel material is C, Si, Mn, P, S, Al, A predetermined amount of Ca, Ni, Cr, Mo, N, and V is contained, the relationship of Cr + 3.3Mo + 16N ≧ 40, V−2.5N <−0.2 is satisfied, and the balance is made of Fe and inevitable impurities. Each configuration will be described below.
（鋼材組織）
本発明の二相系ステンレス鋼材は、フェライト相とオーステナイト相の二相からなるものである。フェライト相とオーステナイト相からなる二相系ステンレス鋼材においては、ＣｒやＭｏなどのフェライト相安定化元素はフェライト相に濃縮し、ＮｉやＮなどのオーステナイト相安定化元素はオーステナイト相に濃縮する傾向にある。このとき、フェライト相のオーステナイト相に対する面積率が３０％未満または７０％を超える場合には、Ｃｒ、Ｍｏ、Ｎｉ、Ｎなどの耐食性に寄与する元素のフェライト相とオーステナイト相における濃度差異が大きくなりすぎて、フェライト相とオーステナイト相のいずれか耐食性に劣る側が選択腐食されて耐食性が劣化する傾向が大きくなる。したがって、フェライト相とオーステナイト相との比率も最適化することが推奨され、フェライト相の面積率は、耐食性の観点から３０〜７０％が好ましく、４０〜６０％がさらに好ましい。このようなフェライト相とオーステナイト相の面積率は、フェライト相安定化元素とオーステナイト相安定化元素の含有量を調整することによって適正化することが可能である。
(Steel structure)
The duplex stainless steel material of the present invention is composed of two phases of a ferrite phase and an austenite phase. In a duplex stainless steel material composed of a ferrite phase and an austenite phase, ferrite phase stabilizing elements such as Cr and Mo concentrate in the ferrite phase, and austenite phase stabilizing elements such as Ni and N tend to concentrate in the austenite phase. is there. At this time, when the area ratio of the ferrite phase to the austenite phase is less than 30% or more than 70%, the concentration difference between the ferrite phase and the austenite phase of elements contributing to the corrosion resistance such as Cr, Mo, Ni, and N becomes large. Too much, either the ferrite phase or the austenite phase, which is inferior in corrosion resistance, is selectively corroded, and the tendency of the corrosion resistance to deteriorate increases. Therefore, it is recommended that the ratio between the ferrite phase and the austenite phase is also optimized, and the area ratio of the ferrite phase is preferably 30 to 70%, more preferably 40 to 60% from the viewpoint of corrosion resistance. Such an area ratio of the ferrite phase and the austenite phase can be optimized by adjusting the contents of the ferrite phase stabilizing element and the austenite phase stabilizing element.
また、本発明の二相系ステンレス鋼材は、フェライト相とオーステナイト相以外にσ相やＣｒの炭窒化物などの異相も耐食性や機械特性などの諸特性を害さない程度に許容できる。フェライト相とオーステナイト相との面積率の合計は、９５％以上とすることが好ましく、９７％以上とすることがさらに好ましい。 Further, in the duplex stainless steel material of the present invention, other phases such as σ phase and Cr carbonitride as well as ferrite phase and austenite phase can be tolerated to such an extent that various properties such as corrosion resistance and mechanical properties are not harmed. The total area ratio of the ferrite phase and the austenite phase is preferably 95% or more, and more preferably 97% or more.
二相系ステンレス鋼材の成分組成の数値限定理由について説明する。
（Ｃ：０．１質量％以下）
Ｃは、オーステナイト相を安定化させ、二相系ステンレス鋼材の強度を向上させる。しかし、Ｃ量が０．１質量％を超えると、炭化物が析出し耐孔食性が劣化するので、Ｃ量は０．１質量％以下（ただし、０質量％は含まれないことが好ましい）とした。
The reason for limiting the numerical values of the component composition of the duplex stainless steel material will be described.
(C: 0.1% by mass or less)
C stabilizes the austenite phase and improves the strength of the duplex stainless steel material. However, if the amount of C exceeds 0.1% by mass, carbide precipitates and the pitting corrosion resistance deteriorates, so the amount of C is 0.1% by mass or less (however, 0% by mass is preferably not included) and did.
（Ｓｉ：０．１〜２．０質量％）
Ｓｉは、溶製時に脱酸剤として使用される。このような効果を得るためには、Ｓｉ量が０．１質量％以上である必要がある。しかし、Ｓｉは金属間化合物の生成を促進する効果があるため、Ｓｉ量が２．０質量％を超えると耐食性および熱間加工性が低下する。したがって、Ｓｉ量は０．１〜２．０質量％とした。好ましくは、０．１〜０．４質量％である。
(Si: 0.1 to 2.0% by mass)
Si is used as a deoxidizer during melting. In order to obtain such an effect, the Si amount needs to be 0.1% by mass or more. However, since Si has an effect of accelerating the formation of intermetallic compounds, the corrosion resistance and hot workability deteriorate when the Si content exceeds 2.0 mass%. Therefore, the Si amount is set to 0.1 to 2.0% by mass. Preferably, it is 0.1-0.4 mass%.
（Ｍｎ：０．１〜２．０質量％）
Ｍｎは、脱酸元素であると同時に、Ｎの固溶量を増加させる効果がある。このような効果を得るためには、Ｍｎ量が０．１質量％以上である必要がある。しかし、Ｍｎ量が２．０質量％を超えると、粗大な硫化物系介在物が析出し、耐孔食性、割れ性が劣化する。そのため、Ｍｎ量は０．１〜２．０質量％とした。好ましくは０．１〜１．０質量％である。
(Mn: 0.1 to 2.0% by mass)
Mn is a deoxidizing element and at the same time has an effect of increasing the solid solution amount of N. In order to obtain such an effect, the amount of Mn needs to be 0.1% by mass or more. However, if the amount of Mn exceeds 2.0% by mass, coarse sulfide inclusions are precipitated, and the pitting corrosion resistance and cracking properties are deteriorated. Therefore, the amount of Mn is set to 0.1 to 2.0% by mass. Preferably it is 0.1-1.0 mass%.
（Ｐ：０．０４質量％以下）
Ｐは、溶製時に混入する不純物であり、耐食性や熱間加工性を劣化させるため、Ｐ量は０．０４質量％以下、好ましくは０．０３質量％以下とした。しかし、Ｐ量の過度の低減は、製造コストの上昇をもたらすので、Ｐ量は０．０１５質量％以上が好ましい。
(P: 0.04 mass% or less)
P is an impurity mixed during melting and deteriorates corrosion resistance and hot workability. Therefore, the amount of P is set to 0.04% by mass or less, preferably 0.03% by mass or less. However, excessive reduction of the amount of P brings about an increase in manufacturing cost, so the amount of P is preferably 0.015% by mass or more.
（Ｓ：０．０１質量％以下）
Ｓは、Ｐと同様に溶製時に混入する不純物であり、Ｍｎ等と結合して硫化物系介在物を形成させると共に、耐食性や熱間加工性を劣化させる。したがって、Ｓ量は０．０１質量％以下、好ましくは０．００３質量％以下である。
(S: 0.01% by mass or less)
S, like P, is an impurity mixed during melting, and combines with Mn to form sulfide inclusions and degrades corrosion resistance and hot workability. Therefore, the amount of S is 0.01% by mass or less, preferably 0.003% by mass or less.
（Ａｌ：０．００１〜０．０５質量％）
Ａｌは、脱酸元素であり、溶製時の酸素量低減に必要な元素である。このような効果を得るためには、Ａｌ量が０．００１質量％以上である必要がある。しかし、Ａｌ量が０．０５質量％を超えると、酸化物系介在物を生成し耐孔食性に悪影響を及ぼす。そのため、Ａｌ量は０．００１〜０．０５質量％とした。好ましくは０．００１〜０．０２質量％である。
(Al: 0.001 to 0.05 mass%)
Al is a deoxidizing element and is an element necessary for reducing the amount of oxygen during melting. In order to obtain such an effect, the amount of Al needs to be 0.001% by mass or more. However, if the amount of Al exceeds 0.05% by mass, oxide inclusions are generated and the pitting corrosion resistance is adversely affected. Therefore, the Al amount is set to 0.001 to 0.05% by mass. Preferably it is 0.001-0.02 mass%.
（Ｃａ：０．０００５〜０．０２０質量％）
Ｃａは、鋼中に不純物として含まれるＳやＯと結合して粒界に偏析するのを抑制して熱間加工性を向上させる効果がある。このような効果を得るためには、Ｃａ量が０．０００５質量％以上である必要がある。しかし、Ｃａ量が０．０２０質量％を超えると、酸化物系介在物量が増加し耐食性が劣化する。そのため、Ｃａ量は０．０００５〜０．０２０質量％とした。好ましくは０．００２〜０．０２０質量である。
(Ca: 0.0005 to 0.020 mass%)
Ca is effective in improving hot workability by suppressing the segregation at grain boundaries by combining with S and O contained as impurities in the steel. In order to obtain such an effect, the Ca amount needs to be 0.0005% by mass or more. However, if the Ca content exceeds 0.020 mass%, the amount of oxide inclusions increases and the corrosion resistance deteriorates. Therefore, the Ca content is set to 0.0005 to 0.020 mass%. Preferably it is 0.002-0.020 mass.
（Ｎｉ：３〜１０質量％）
Ｎｉは、オーステナイト生成元素であり、二相系構造を維持して鋼材に耐食性を発現させる効果がある。このような効果を得るためには、Ｎｉ量が３質量％以上である必要がある。しかし、Ｎｉ量が１０質量％を超えると、オーステナイト相が多くなりすぎて、強度が低下する。そのため、Ｎｉ量は３〜１０質量％とした。好ましくは５〜８質量％である。
(Ni: 3 to 10% by mass)
Ni is an austenite-forming element and has an effect of maintaining the two-phase structure and causing the steel material to exhibit corrosion resistance. In order to obtain such an effect, the amount of Ni needs to be 3% by mass or more. However, when the amount of Ni exceeds 10% by mass, the austenite phase becomes excessive and the strength is lowered. Therefore, the amount of Ni is 3 to 10% by mass. Preferably it is 5-8 mass%.
（Ｃｒ：２３〜２８質量％）
Ｃｒは、フェライト生成元素であり、二相系構造を維持して鋼材に耐食性を発現させる効果がある。このような効果を得るためには、Ｃｒ量が２３質量％以上である必要がある。しかし、Ｃｒ量が２８質量％を超えると、σ相等の金属間化合物の生成を助長し、耐食性および熱間加工性が低下する。そのため、Ｃｒ量は２３〜２８質量％とした。好ましくは２４〜２６質量％である。
(Cr: 23 to 28% by mass)
Cr is a ferrite-forming element, and has the effect of maintaining the two-phase structure and exhibiting corrosion resistance in the steel material. In order to obtain such an effect, the Cr amount needs to be 23% by mass or more. However, if the Cr content exceeds 28% by mass, the formation of intermetallic compounds such as the σ phase is promoted, and the corrosion resistance and hot workability are reduced. Therefore, the Cr amount is set to 23 to 28% by mass. Preferably it is 24-26 mass%.
（Ｍｏ：２〜５質量％）
Ｍｏは、フェライト生成元素であり、鋼材の耐孔食性・耐割れ性を改善させる効果がある。このような効果を得るためには、Ｍｏ量が２質量％以上である必要がある。しかしＭｏ量が５質量％を超えると、σ相等の金属間化合物の生成を助長し、耐食性および熱間加工性が低下する。そのため、Ｍｏ量は２〜５質量％とした。 好ましくは２〜４質量％である。
(Mo: 2 to 5% by mass)
Mo is a ferrite-forming element and has an effect of improving the pitting corrosion resistance and crack resistance of the steel material. In order to obtain such an effect, the Mo amount needs to be 2% by mass or more. However, when the amount of Mo exceeds 5% by mass, the formation of intermetallic compounds such as the σ phase is promoted, and the corrosion resistance and hot workability are reduced. Therefore, the Mo amount is set to 2 to 5% by mass. Preferably it is 2-4 mass%.
（Ｎ：０．２〜０．４質量％）
Ｎは、オーステナイト生成元素であり、σ相の生成感受性を増加させずに耐食性を向上させる効果がある。このような効果を得るためには、Ｎ量が０．２質量％以上である必要がある。しかし、Ｎ量が０．４質量％を超えると、窒化物が形成され靭性や耐食性が低下する。また、熱間加工性を劣化させ、鍛造・圧延時に耳割れや表面欠陥を生じさせる。そのため、Ｎ量は０．２〜０．４質量％とした。
(N: 0.2-0.4 mass%)
N is an austenite-forming element and has an effect of improving the corrosion resistance without increasing the susceptibility of the σ phase. In order to obtain such an effect, the N amount needs to be 0.2% by mass or more. However, if the amount of N exceeds 0.4 mass%, nitrides are formed and toughness and corrosion resistance are reduced. In addition, hot workability is deteriorated, and ear cracks and surface defects are generated during forging and rolling. Therefore, the N amount is set to 0.2 to 0.4% by mass.
（Ｖ：０．１〜０．６質量％）
Ｖは、粗大な酸化物系介在物の生成を抑制し、耐食性を向上させる効果がある。このような効果を得るためには、Ｖ量は０．１質量％以上である必要がある。しかし、Ｖ量が０．６質量％を超えると、窒化物が形成され、組織中のＮ濃度が減少し耐食性が低下する。そのため、Ｖ量は０．１〜０．６質量％とした。
(V: 0.1-0.6% by mass)
V has the effect of suppressing the formation of coarse oxide inclusions and improving the corrosion resistance. In order to obtain such an effect, the V amount needs to be 0.1% by mass or more. However, if the amount of V exceeds 0.6% by mass, nitrides are formed, the N concentration in the structure decreases, and the corrosion resistance decreases. Therefore, the V amount is set to 0.1 to 0.6% by mass.
（Ｃｒ＋３．３Ｍｏ＋１６Ｎ≧４０）
（Ｃｒ＋３．３Ｍｏ＋１６Ｎ）は、鋼材の耐食性を表す指標として従来知られている耐孔食性指数（ＰＲＥ：Ｐｉｔｔｉｎｇ Ｒｅｓｉｓｔａｎｃｅ Ｅｑｕｉｖａｌｅｎｔ）である。本発明では、ＰＲＥ≧４０とすることによって、組織中のＣｒ量、Ｍｏ量、Ｎ量のバランスが適切なものとなり、鋼材の耐食性および強度が向上する。
(Cr + 3.3Mo + 16N ≧ 40)
(Cr + 3.3Mo + 16N) is a pitting corrosion resistance (PRE) conventionally known as an index representing the corrosion resistance of steel materials. In the present invention, by setting PRE ≧ 40, the balance of Cr content, Mo content, and N content in the structure becomes appropriate, and the corrosion resistance and strength of the steel material are improved.
（Ｖ−２．５Ｎ＜−０．２）
二相系ステンレス鋼材では、前記したように、Ｖは酸化物系介在物を微細化するが、Ｖ量が多いと窒化物の生成を促進すると共に、組織中のＮ濃度も低下させる。そして、粗大な窒化物の生成とＮ濃度の低下によって、耐食性が低下する。したがって、Ｖ量とＮ量のバランスを適切に制御することが重要である。そこで、Ｖ量とＮ量とのバランスを鋭意検討した結果、Ｖ−２．５Ｎ＜−０．２となるように制御することで、窒化物の生成を抑制しつつ、微細な酸化物系介在物を形成させることができることを見出した。具体的には、Ｖ量とＮ量とがＶ−２．５Ｎ＜−０．２を満足することによって、Ａｌ、Ｃａ、Ｖのいずれかを含む酸化物系介在物、および、窒化物を平均長径で１０μｍ以下、平均アスペクト比で５０以下に制御できると共に、長径が１μｍ以上の酸化物系介在物および窒化物の数密度を１０個／ｍｍ２以下に制御できる。それにより、二相系ステンレス鋼材の耐食性および熱間加工性が向上する。
(V-2.5N <-0.2)
In the duplex stainless steel material, as described above, V refines oxide inclusions. However, if the amount of V is large, the formation of nitride is promoted and the N concentration in the structure is also lowered. And corrosion resistance falls by the production | generation of a coarse nitride and the fall of N density | concentration. Therefore, it is important to appropriately control the balance between the V amount and the N amount. Therefore, as a result of diligent examination of the balance between the V amount and the N amount, by controlling so that V−2.5N <−0.2, it is possible to suppress the formation of nitrides and fine oxide-based intervening. It was found that an object can be formed. Specifically, when the amount of V and the amount of N satisfy V−2.5N <−0.2, the oxide inclusions containing any of Al, Ca, and V, and the nitride are averaged. The major axis can be controlled to 10 μm or less and the average aspect ratio to 50 or less, and the number density of oxide inclusions and nitrides having a major axis of 1 μm or more can be controlled to 10 pieces / mm 2 or less. Thereby, the corrosion resistance and hot workability of the duplex stainless steel material are improved.
（不可避的不純物）
不可避的不純物は、溶製時に不可避的に混入するＯ等の不純物であり、二相系ステンレス鋼材の諸特性を害さないものである。また、不可避的不純物は、その含有量を合計で０．１質量％以下、好ましくは０．０９質量％以下に抑えることが好ましく、それによって、本発明の耐食性発現効果を極大化することができる。
(Inevitable impurities)
Inevitable impurities are impurities such as O which are inevitably mixed during melting, and do not impair various properties of the duplex stainless steel material. Moreover, it is preferable to suppress the content of inevitable impurities to 0.1% by mass or less, preferably 0.09% by mass or less in total, thereby maximizing the corrosion resistance expression effect of the present invention. .
また、本発明の二相系ステンレス鋼材は、前記成分組成が、さらに、所定量のＣｏ、Ｔｉの１種または２種を含有することが好ましい。 In the duplex stainless steel material of the present invention, the component composition preferably further contains one or two kinds of Co and Ti in predetermined amounts.
（Ｃｏ：０．０１〜４．０質量％、Ｔｉ：０．０１〜０．５質量％）
Ｃｏ， Ｔｉは、いずれも耐食性を向上させる効果がある。このような効果を得るためには、Ｃｏ量は０．０１質量％以上、Ｔｉ量も０．０１質量％である必要がある。しかし、Ｃｏ量が４．０質量％を超えると、または、Ｔｉ量が０．５質量％を超えると、金属間化合物の析出を助長して、耐食性および熱間加工性が低下する。そのため、Ｃｏ：０．０１〜４．０質量％、Ｔｉ：０．０１〜０．５質量％とした。なお、Ｃｏ、Ｔｉのうちの１種または２種を含有できる。
(Co: 0.01-4.0 mass%, Ti: 0.01-0.5 mass%)
Co and Ti have the effect of improving the corrosion resistance. In order to obtain such an effect, the Co amount needs to be 0.01% by mass or more, and the Ti amount needs to be 0.01% by mass. However, when the Co content exceeds 4.0 mass% or the Ti content exceeds 0.5 mass%, precipitation of intermetallic compounds is promoted, and corrosion resistance and hot workability are deteriorated. Therefore, it was set as Co: 0.01-4.0 mass%, Ti: 0.01-0.5 mass%. One or two of Co and Ti can be contained.
また、本発明の二相系ステンレス鋼材は、前記成分組成が、さらに、所定量のＢを含有することが好ましい。
（Ｂ：０．００１〜０．０１質量％）
Ｂは、熱間加工性を向上させる効果がある。このような効果を得るためには、Ｂ量は０．００１質量％以上である必要がある。しかし、Ｂ量が０．０１質量％を超えると、熱間加工性を低下させる。そのため、Ｂ量は０．００１〜０．０１質量％とした。
In the duplex stainless steel material of the present invention, the component composition preferably further contains a predetermined amount of B.
(B: 0.001 to 0.01% by mass)
B has an effect of improving hot workability. In order to obtain such an effect, the amount of B needs to be 0.001% by mass or more. However, when the amount of B exceeds 0.01 mass%, hot workability will be reduced. Therefore, the B amount is set to 0.001 to 0.01% by mass.
（二相系ステンレス鋼材の製造方法）
本発明の二相系ステンレス鋼材は、通常のステンレス鋼の量産に用いられている製造設備および製造方法によって製造することができる。例えば、転炉あるいは電気炉にて溶解した溶鋼に対して、ＡＯＤ法やＶＯＤ法などによる精錬を行って成分調整した後、連続鋳造法や造塊法などの鋳造方法で鋼塊とする。得られた鋼塊を１０００℃〜１２００℃程度の温度域にて熱間加工を行い、次いで冷間加工を行って所望の寸法形状にすることができる。
(Method for producing duplex stainless steel)
The duplex stainless steel material of the present invention can be manufactured by a manufacturing facility and a manufacturing method used for mass production of ordinary stainless steel. For example, a molten steel melted in a converter or an electric furnace is refined by an AOD method, a VOD method, or the like to adjust the components, and then formed into a steel ingot by a casting method such as a continuous casting method or an ingot-making method. The obtained steel ingot can be hot-worked in a temperature range of about 1000 ° C. to 1200 ° C., and then cold-worked to obtain a desired dimensional shape.
本発明においては、機械特性に有害な析出物をなくすため、必要に応じて固溶化熱処理を施して急冷することが好ましい。固溶化熱処理の温度は、１０００℃〜１１００℃が好ましく、保持時間は１０分から３０分が好ましく、急冷は１０℃／秒以上の冷却速度で冷却することが好ましい。また、必要に応じてスケール除去などの表面調整のための酸洗を行うことができる。 In the present invention, in order to eliminate precipitates detrimental to mechanical properties, it is preferable to quench by applying a solution heat treatment as necessary. The temperature of the solution heat treatment is preferably 1000 ° C. to 1100 ° C., the holding time is preferably 10 minutes to 30 minutes, and the rapid cooling is preferably performed at a cooling rate of 10 ° C./second or more. Moreover, the pickling for surface adjustments, such as scale removal, can be performed as needed.
＜二相系ステンレス鋼管＞
本発明に係る二相系ステンレス鋼管の実施形態について説明する。
本発明の二相系ステンレス鋼管は、前記二相系ステンレス鋼材からなるもので、通常のステンレス鋼管の量産に用いられる製造設備および製造方法によって製造することができる。例えば、丸棒を素材とした押出製管やマンネスマン製管、板材を素材として成形後に継ぎ目を溶接する溶接製管などによって、所望の寸法にすることができる。また、二相系ステンレス鋼管の寸法は、鋼管が使用される油井管、化学プラント、アンビリカルチューブ等に応じて適宜設定することができる。なお、二相系ステンレス鋼管は、海水淡水化プラント、ＬＮＧ気化器等にも使用できる。
<Duplex stainless steel pipe>
An embodiment of a duplex stainless steel pipe according to the present invention will be described.
The duplex stainless steel pipe of the present invention is made of the duplex stainless steel material and can be produced by a production facility and a production method used for mass production of ordinary stainless steel pipes. For example, the desired dimensions can be obtained by an extruded pipe or Mannesmann pipe made of a round bar, or a weld pipe made by welding a seam after forming a plate material. The dimensions of the duplex stainless steel pipe can be appropriately set according to the oil well pipe, the chemical plant, the umbilical tube, etc. in which the steel pipe is used. The duplex stainless steel pipe can also be used for a seawater desalination plant, an LNG vaporizer, and the like.
以下実施例を挙げて本発明を具体的に説明する。
電極アーク加熱機能を備える溶鋼処理設備によって、表１に示す成分組成の鋼（残部はＦｅおよび不可避的不純物）をそれぞれ溶製し、５０ｋｇの丸鋳型（本体：約φ１４０×３２０ｍｍ）を用いて鋳造した。また、各鋼の組織についてＰＲＥ＝Ｃｒ＋３．３Ｍｏ＋１６Ｎ、Ｖ−２．５Ｎを算出した結果も表１に示す。なお、表１において、空欄は該当成分が含有されていないことを示す。
凝固した鋼塊を１２００℃まで加熱し同温度で熱間鍛造を施し、その後切断し、１１００℃で３０分保持の固溶化熱処理を施し、水冷して６００×１２０×６０ｍｍの鍛鋼品（Ｎｏ．Ａ１〜Ａ５、Ｂ１〜Ｂ５）に仕上げた。
Hereinafter, the present invention will be specifically described with reference to examples.
Steels with the composition shown in Table 1 (the balance is Fe and inevitable impurities) are melted by molten steel processing equipment equipped with an electrode arc heating function, and cast using a 50 kg round mold (main body: about φ140 x 320 mm). did. Table 1 also shows the results of calculating PRE = Cr + 3.3Mo + 16N and V−2.5N for the structure of each steel. In Table 1, a blank indicates that the corresponding component is not contained.
The solidified steel ingot is heated to 1200 ° C., hot forged at the same temperature, then cut, subjected to a solution heat treatment held at 1100 ° C. for 30 minutes, water cooled, and a forged steel product of 600 × 120 × 60 mm (No. A1 to A5 and B1 to B5) were finished.
室温まで冷却して得られた鍛鋼品（Ｎｏ．Ａ１〜Ａ５、Ｂ１〜Ｂ５）について、加工方向に垂直な断面の金属組織をＥＰＭＡで観察し、Ａｌ、Ｃａ、Ｖのいずれかを含む酸化物系介在物と窒化物の長径、短径を測定し、アスペクト比（長径／短径）を算出した。また、Ａｌ、Ｃａ、Ｖのいずれかを含む酸化物系介在物と窒化物のうち長径が１μｍ以上のものについて数密度を測定した。１０視野の平均値を表２に示す。 For forged steel products (No. A1 to A5, B1 to B5) obtained by cooling to room temperature, the metal structure of the cross section perpendicular to the processing direction is observed with EPMA, and an oxide containing any one of Al, Ca, and V The major axis and minor axis of system inclusions and nitrides were measured, and the aspect ratio (major axis / minor axis) was calculated. Further, the number density was measured for oxide inclusions and nitrides containing any of Al, Ca, and V having a major axis of 1 μm or more. Table 2 shows the average of 10 fields of view.
次に、前記鍛鋼品から加工方向に平行に採取した試料（２０ｍｍ×３０ｍｍ×２ｍｍｔ）を用いて、以下に示す手順で耐孔食性および熱間加工性を評価した。
（耐孔食性の評価）
ＪＩＳ Ｇ ０５７７に基づき孔食電位測定で行った。まず、試料表面をＳｉＣ＃６００研磨紙で湿式研磨し、超音波洗浄した後５０℃の３０％硝酸に１時間浸漬し不働態化処理をした。次に、試料にスポット溶接で導線を取り付け試験面積１０ｍｍ×１０ｍｍを残してエポキシ樹脂で被覆した。その試料を８０℃に保持した２０％ＮａＣｌ水溶液中に１０分間浸漬した後、自然電位からアノード方向に２０ｍＶ／ｍｉｎの掃引速度で分極し、電流密度が１００μＡ／ｃｍ２を超えた最も貴な電位を孔食電位Ｖｃ’１００とした。また、参照電極には飽和カロメル電極（ＳＣＥ）を用い、Ａｒ脱気しながら実施した。その結果を表３に示す。なお、孔食電位ＶC’１００が６００ｍＶ以上のものを耐孔食性が良好と評価した。
（熱間加工性の評価）
前記鍛鋼品の表面を目視にて観察し、表面欠陥の有無を観察した。その結果を表３に示す。
Next, pitting corrosion resistance and hot workability were evaluated by the following procedure using a sample (20 mm × 30 mm × 2 mm t ) collected from the forged steel product in parallel to the processing direction.
(Evaluation of pitting corrosion resistance)
The pitting potential was measured based on JIS G 0577. First, the sample surface was wet-polished with SiC # 600 abrasive paper, subjected to ultrasonic cleaning, and then immersed in 30% nitric acid at 50 ° C. for 1 hour for passivation treatment. Next, a conductive wire was attached to the sample by spot welding, and the sample was covered with an epoxy resin leaving a test area of 10 mm × 10 mm. The sample was immersed in a 20% NaCl aqueous solution maintained at 80 ° C. for 10 minutes, and then polarized at a sweep rate of 20 mV / min from the natural potential toward the anode, and the most noble potential with a current density exceeding 100 μA / cm 2. Was pitting corrosion potential V c '100. In addition, a saturated calomel electrode (SCE) was used as a reference electrode, and Ar was deaerated. The results are shown in Table 3. A pitting corrosion resistance V C '100 of 600 mV or more was evaluated as good pitting corrosion resistance.
(Evaluation of hot workability)
The surface of the forged steel product was visually observed, and the presence or absence of surface defects was observed. The results are shown in Table 3.
表２、表３の結果から、本発明の要件を満たす実施例（Ｎｏ．Ａ１〜Ａ５）は、酸化物系介在物および窒化物の平均長径が１０μｍ以下、平均アスペクト比が５０以下、数密度が１０個／ｍｍ２以下となった。また、実施例（Ｎｏ．Ａ１〜Ａ５）の金属組織は、フェライト相とオーステナイト相とからなる二相系構造であった。その結果、実施例（Ｎｏ．Ａ１〜Ａ５）の孔食電位（ｖｓ．ＳＣＥ：飽和カロメル電極基準電位）は、９００ｍＶを超える貴なものとなり、硫化水素環境等にも耐え得る優れた耐孔食性を有していることが分かる。また、実施例（Ｎｏ．Ａ１〜Ａ５）は、表面欠陥も観察されず、優れた熱間加工性を有していることが分かる。 From the results of Tables 2 and 3, Examples (No. A1 to A5) satisfying the requirements of the present invention have an average major axis of oxide inclusions and nitrides of 10 μm or less, an average aspect ratio of 50 or less, and a number density. Of 10 pieces / mm 2 or less. Moreover, the metal structure of the examples (No. A1 to A5) was a two-phase structure composed of a ferrite phase and an austenite phase. As a result, the pitting corrosion potential (vs. SCE: saturated calomel electrode reference potential) of Examples (No. A1 to A5) is noble exceeding 900 mV, and excellent pitting corrosion resistance that can withstand a hydrogen sulfide environment or the like. It can be seen that Moreover, it turns out that a surface defect is not observed but an Example (No. A1-A5) has the outstanding hot workability.
一方、Ｖ量が下限値未満の比較例（Ｎｏ．Ｂ１）、ＰＲＥ＝Ｃｒ＋３．３Ｍｏ＋１６Ｎが４０未満の比較例（Ｎｏ．Ｂ２）、Ｖ−２．５Ｎが−０．２より大きい比較例（Ｎｏ．Ｂ３）、Ｓ量が上限値超えの比較例（Ｎｏ．Ｂ４）は、酸化物系介在物および窒化物の平均長径が１０μｍを超え、平均アスペクト比が５０を超え、数密度が１０個／ｍｍ２を超えた。その結果、比較例（Ｎｏ．Ｂ１〜Ｂ４）は、孔食電位（ｖｓ．ＳＣＥ）が６００ｍＶ未満の卑なものとなり、硫化水素環境等にも耐え得る優れた耐孔食性を有していないことが分かる。また、比較例（Ｎｏ．Ｂ１〜Ｂ４）は、表面欠陥が観察され、熱間加工性が不足していることが分かる。なお、比較例（Ｎｏ．Ｂ１〜Ｂ４）の金属組織は、フェライト相とオーステナイト相とからなる二相系構造であった。Ｎｉ量が下限値未満の比較例（Ｎｏ．Ｂ５）は、金属組織が二相系構造ではなく、フェライト相のみの単相構造であった。その結果、比較例（Ｎｏ．Ｂ５）は、孔食電位（ｖｓ．ＳＣＥ）が−５０ｍＶと卑なものとなり、硫化水素等にも耐え得る優れた耐孔食性を有していないことが分かる。 On the other hand, the comparative example (No. B1) in which the V amount is less than the lower limit, the comparative example in which PRE = Cr + 3.3Mo + 16N is less than 40 (No. B2), and the comparative example in which V-2.5N is larger than −0.2 (No B3), the comparative example (No. B4) in which the S amount exceeds the upper limit value, the average major axis of oxide inclusions and nitrides exceeds 10 μm, the average aspect ratio exceeds 50, and the number density is 10 / mm 2 was exceeded. As a result, the comparative examples (No. B1 to B4) have a pitting corrosion potential (vs. SCE) of less than 600 mV and do not have excellent pitting corrosion resistance that can withstand a hydrogen sulfide environment or the like. I understand. Moreover, as for a comparative example (No. B1-B4), a surface defect is observed and it turns out that hot workability is insufficient. In addition, the metal structure of the comparative examples (No. B1 to B4) was a two-phase structure composed of a ferrite phase and an austenite phase. In the comparative example (No. B5) in which the Ni content was less than the lower limit, the metal structure was not a two-phase structure but a single-phase structure including only a ferrite phase. As a result, it can be seen that the comparative example (No. B5) has a pitting corrosion potential (vs. SCE) as low as −50 mV, and does not have excellent pitting corrosion resistance that can withstand hydrogen sulfide and the like.
Claims (4)
Ｃ：０．１質量％以下、
Ｓｉ：０．１〜２．０質量％、
Ｍｎ：０．１〜２．０質量％、
Ｐ：０．０４質量％以下、
Ｓ：０．０１質量％以下、
Ａｌ：０．００１〜０．０５質量％、
Ｃａ：０．０００５〜０．０２０質量％、
Ｎｉ：３〜１０質量％、
Ｃｒ：２３〜２８質量％、
Ｍｏ：２〜５質量％、
Ｎ：０．２〜０．４質量％、
Ｖ：０．１〜０．６質量％を含有し、残部がＦｅおよび不可避的不純物であって、
Ｃｒ＋３．３Ｍｏ＋１６Ｎ≧４０、Ｖ−２．５Ｎ＜−０．２なる関係を満足することを特徴とする二相系ステンレス鋼材。 It is a duplex stainless steel material composed of a ferrite phase and an austenite phase, and the component composition of the duplex stainless steel material is:
C: 0.1% by mass or less,
Si: 0.1 to 2.0% by mass,
Mn: 0.1 to 2.0% by mass,
P: 0.04 mass% or less,
S: 0.01% by mass or less,
Al: 0.001 to 0.05 mass%,
Ca: 0.0005 to 0.020 mass%,
Ni: 3 to 10% by mass,
Cr: 23 to 28% by mass,
Mo: 2 to 5% by mass,
N: 0.2-0.4 mass%,
V: 0.1 to 0.6% by mass, with the balance being Fe and inevitable impurities,
A duplex stainless steel material satisfying the relationship of Cr + 3.3Mo + 16N ≧ 40 and V−2.5N <−0.2.
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