JPWO2016052271A1 - Steel and pipe for oil expansion - Google Patents

Steel and pipe for oil expansion Download PDF

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JPWO2016052271A1
JPWO2016052271A1 JP2016551945A JP2016551945A JPWO2016052271A1 JP WO2016052271 A1 JPWO2016052271 A1 JP WO2016052271A1 JP 2016551945 A JP2016551945 A JP 2016551945A JP 2016551945 A JP2016551945 A JP 2016551945A JP WO2016052271 A1 JPWO2016052271 A1 JP WO2016052271A1
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JP6213683B2 (en
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小林　憲司
憲司 小林
悠索 富尾
悠索 富尾
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Nippon Steel Corp
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Abstract

化学組成が、質量％で、Ｃ：0.6〜1.8％、Si：0.05〜1.00％、Mn：25.0%超45.0％以下、Al：0.003〜0.06％、Ｐ：0.03％以下、Ｓ：0.03％以下、Cu：0.5〜3.0％、Ｎ：0.10％以下、Ｖ：0〜2.0％、Cr：0〜3.0％、Mo：0〜3.0％、Ni：0〜1.5％、Nb：0〜0.5％、Ta：0〜0.5％、Ti：0〜0.5％、Zr：0〜0.5％、Ca：0〜0.005％、Mg：0〜0.005％、REM：0〜0.01％、Ｂ：0〜0.015％、残部：Feおよび不純物であり、[0.6＜Ｃ−0.18Ｖ＜1.44]を満足し、金属組織が、オーステナイト単相からなり、降伏強度が241MPa以上であり、均一伸びが40％以上である、鋼材。Chemical composition is mass%, C: 0.6-1.8%, Si: 0.05-1.00%, Mn: more than 25.0%, 45.0% or less, Al: 0.003-0.06%, P: 0.03% or less, S: 0.03% or less, Cu: 0.5 to 3.0%, N: 0.10% or less, V: 0 to 2.0%, Cr: 0 to 3.0%, Mo: 0 to 3.0%, Ni: 0 to 1.5%, Nb: 0 to 0.5%, Ta: 0 to 0.5%, Ti: 0 to 0.5%, Zr: 0 to 0.5%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, REM: 0 to 0.01%, B: 0 to 0.015%, balance: Fe And a steel material which is an impurity, satisfies [0.6 <C-0.18V <1.44], has a metal structure composed of austenite single phase, yield strength is 241 MPa or more, and uniform elongation is 40% or more.

Description

本発明は、鋼材および拡管用油井鋼管に係り、特に、高い拡管性を備えるとともに、硫化水素（Ｈ２Ｓ）を含む油井およびガス井環境等で使用される耐硫化物応力割れ性に優れる鋼材およびそれを用いた拡管用油井鋼管に関する。The present invention relates to steel materials and oil well steel pipes for pipe expansion, and in particular, steel materials having high pipe expandability and excellent resistance to sulfide stress cracking used in oil well and gas well environments containing hydrogen sulfide (H 2 S). Further, the present invention relates to an oil well steel pipe for expanding the pipe.

油井およびガス井（以下、油井およびガス井を総称して、単に「油井」という。）の掘削においては、掘削孔が所定の深度まで到達した後にケーシングを挿入して埋設することで、坑井壁の崩落を防止する方法が一般的に採用されている。そして、さらに掘削しながらより外径の小さいケーシングを順次挿入する作業を繰り返す。そのため、従来は、高深度まで掘削する必要がある場合、ケーシングを挿入する回数が増加するため、地層表層部における油井の外径方向の掘削面積が広くなり、掘削費用および工期が嵩み、経済的に不利であった。そこで近年、油井に挿入されたケーシングを油井内で拡管することによって、地層表層部における掘削面積を低減し、掘削工期を大幅に短縮することが可能な工法が提案されている（例えば、特許文献１を参照。）。   In the drilling of oil wells and gas wells (hereinafter, oil wells and gas wells are simply referred to as “oil wells”), after a drill hole reaches a predetermined depth, a casing is inserted and buried, A method for preventing the wall from collapsing is generally adopted. And the operation | work which inserts a casing with a smaller outer diameter sequentially is further repeated while excavating. Therefore, conventionally, when it is necessary to dig deeper, the number of times the casing is inserted increases, so that the drilling area in the outer diameter direction of the oil well in the surface layer of the formation becomes wider, the excavation cost and construction period increase, the economy It was disadvantageous. In recent years, therefore, there has been proposed a construction method capable of reducing the excavation area in the surface layer of the formation layer by expanding the casing inserted in the oil well in the oil well, and greatly shortening the excavation work period (for example, Patent Documents) 1).

Ｈ２Ｓを含有する原油、天然ガス等の油井では、湿潤硫化水素環境における鋼の硫化物応力割れ（以下、「ＳＳＣ」という。）が問題となることから、耐ＳＳＣ性に優れるケーシング用鋼管が必要となる。上記の工法では、ケーシングは、拡管のための加工を受けたまま、熱処理等が施されることなく、腐食環境に曝されることになる。そのため、ケーシングに用いられる材料は、拡管性に優れるとともに、冷間加工後の耐食性に優れている必要がある。例えば、特許文献１〜３には、拡管性能および耐食性に優れた材料が提案されている。In oil wells such as crude oil and natural gas containing H 2 S, steel sulfide stress cracking (hereinafter referred to as “SSC”) in a wet hydrogen sulfide environment becomes a problem. Is required. In the above construction method, the casing is exposed to a corrosive environment without being subjected to heat treatment or the like while being subjected to processing for expanding the pipe. Therefore, the material used for the casing needs to be excellent in pipe expandability and excellent in corrosion resistance after cold working. For example, Patent Documents 1 to 3 propose materials having excellent tube expansion performance and corrosion resistance.

特開２００８−２０２１２８号公報JP 2008-202128 A 特開２００２−２６６０５５号公報JP 2002-266055 A 特開２００６−９０７８号公報Japanese Patent Laid-Open No. 2006-9078

上記の工程に用いるのに不可欠な鋼管の拡管性を確保するためには、高い均一伸びが要求される。特許文献１および２には、耐ＳＳＣ性に優れる鋼管が開示されているものの、均一伸びについては検討されておらず、改善の余地が残されている。また、特許文献３では均一伸びの値が開示されているが、２１％以下という結果であり、加えて耐ＳＳＣ性に関しては検討されていない。油井内で拡管する鋼管の適用機会をさらに増やしていくためには、例えば４０％以上の均一伸びを持つとともに、拡管後の耐ＳＳＣ性が確保されている必要がある。   High uniform elongation is required to ensure the expandability of the steel pipe that is indispensable for use in the above process. Although Patent Documents 1 and 2 disclose steel pipes having excellent SSC resistance, uniform elongation has not been studied, and there remains room for improvement. Moreover, although the value of uniform elongation is disclosed by patent document 3, it is a result of 21% or less, and also about SSC resistance is not examined. In order to further increase the application opportunities of steel pipes expanded in the oil well, it is necessary to have a uniform elongation of, for example, 40% or more and to ensure SSC resistance after the expansion.

本発明は、高い拡管性を有するとともに、冷間加工後の耐ＳＳＣ性に優れ、しかも経済性が高い鋼材およびそれを用いた拡管用油井鋼管を提供することを目的とする。   It is an object of the present invention to provide a steel material having high pipe expandability, excellent SSC resistance after cold working, and high economic efficiency, and an oil well steel pipe for pipe expansion using the same.

発明者らは、上記の条件を満足する鋼材の化学組成について検討した結果、以下の知見を得るに至った。   As a result of examining the chemical composition of the steel material that satisfies the above conditions, the inventors have obtained the following knowledge.

（Ａ）高い耐ＳＳＣ性および均一伸びを確保するためには、オーステナイト安定化元素であるＭｎおよびＣを含有させることが有効であり、特に多量のＭｎを含有させることが有効である。オーステナイト組織はＳＳＣに対する高い抵抗性を有するが、ＣおよびＭｎの含有量を適切に選択すれば、オーステナイト組織は、冷間加工に対しても安定で、加工誘起マルテンサイト変態が生じにくくなる。そして、それゆえＢＣＣ（体心立方）組織が存在した場合に生じやすいＳＳＣの発生を抑制できる。   (A) In order to ensure high SSC resistance and uniform elongation, it is effective to contain Mn and C which are austenite stabilizing elements, and it is particularly effective to contain a large amount of Mn. Although the austenite structure has high resistance to SSC, if the contents of C and Mn are appropriately selected, the austenite structure is stable to cold working and hardly causes work-induced martensitic transformation. Therefore, it is possible to suppress the occurrence of SSC that tends to occur when a BCC (body-centered cubic) structure exists.

（Ｂ）Ｍｎは、湿潤硫化水素環境中では、耐全面腐食性の悪化を引き起こすという問題がある。しかしながら、耐全面腐食性の悪化は、鋼材中にＣｕを含有させることによって改善することが可能である。   (B) Mn has a problem that it causes deterioration of the general corrosion resistance in a wet hydrogen sulfide environment. However, the deterioration of the general corrosion resistance can be improved by including Cu in the steel material.

（Ｃ）Ｃ含有量を適切に管理するに際して、炭化物形成元素であるＶが含有される場合、Ｃは炭化物として消費される。そのため、炭化物として消費される量も考慮してＣ含有量を調整する必要がある。   (C) In appropriately managing the C content, when V, which is a carbide forming element, is contained, C is consumed as a carbide. Therefore, it is necessary to adjust the C content in consideration of the amount consumed as carbide.

本発明は、上記の知見に基づいて完成されたものであり、下記の鋼材および拡管用油井鋼管を要旨とする。   The present invention has been completed on the basis of the above findings, and the gist thereof is the following steel materials and oil well steel pipes for pipe expansion.

（１）化学組成が、質量％で、
Ｃ：０．６〜１．８％、
Ｓｉ：０．０５〜１．００％、
Ｍｎ：２５．０％を超えて４５．０％以下、
Ａｌ：０．００３〜０．０６％、
Ｐ：０．０３％以下、
Ｓ：０．０３％以下、
Ｃｕ：０．５〜３．０％、
Ｎ：０．１０％以下、
Ｖ：０〜２．０％、
Ｃｒ：０〜３．０％、
Ｍｏ：０〜３．０％、
Ｎｉ：０〜１．５％、
Ｎｂ：０〜０．５％、
Ｔａ：０〜０．５％、
Ｔｉ：０〜０．５％、
Ｚｒ：０〜０．５％、
Ｃａ：０〜０．００５％、
Ｍｇ：０〜０．００５％、
ＲＥＭ：０〜０．０１％、
Ｂ：０〜０．０１５％、
残部：Ｆｅおよび不純物であり、
下記（ｉ）式を満足し、
金属組織が、オーステナイト単相からなり、
降伏強度が２４１ＭＰａ以上であり、均一伸びが４０％以上である、鋼材。
０．６＜Ｃ−０．１８Ｖ＜１．４４ ・・・（ｉ）
但し、式中の各元素記号は、鋼材中に含まれる各元素の含有量（質量％）を表し、含有されない場合はゼロとする。(1) The chemical composition is mass%,
C: 0.6-1.8%
Si: 0.05-1.00%,
Mn: more than 25.0% and 45.0% or less,
Al: 0.003-0.06%,
P: 0.03% or less,
S: 0.03% or less,
Cu: 0.5 to 3.0%,
N: 0.10% or less,
V: 0 to 2.0%,
Cr: 0 to 3.0%,
Mo: 0 to 3.0%,
Ni: 0 to 1.5%,
Nb: 0 to 0.5%,
Ta: 0 to 0.5%
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
REM: 0 to 0.01%
B: 0 to 0.015%
Balance: Fe and impurities,
Satisfying the following formula (i)
The metal structure consists of austenite single phase,
A steel material having a yield strength of 241 MPa or more and a uniform elongation of 40% or more.
0.6 <C-0.18V <1.44 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained.

（２）前記化学組成が、質量％で、
Ｖ：０．０３〜２．０％
を含有する、上記（１）に記載の鋼材。(2) The chemical composition is mass%,
V: 0.03-2.0%
The steel material as described in said (1) containing.

（３）前記化学組成が、質量％で、
Ｃｒ：０．１〜３．０％、
Ｍｏ：０．１〜３．０％および
Ｎｉ：０．１〜１．５％
から選択される１種以上を含有する、上記（１）または（２）に記載の鋼材。(3) The chemical composition is mass%,
Cr: 0.1 to 3.0%
Mo: 0.1-3.0% and Ni: 0.1-1.5%
The steel material according to (1) or (2) above, which contains one or more selected from the above.

（４）前記化学組成が、質量％で、
Ｎｂ：０．００５〜０．５％、
Ｔａ：０．００５〜０．５％、
Ｔｉ：０．００５〜０．５％、
Ｚｒ：０．００５〜０．５％、
Ｃａ：０．０００３〜０．００５％、
Ｍｇ：０．０００３〜０．００５％、
ＲＥＭ：０．００１〜０．０１％および
Ｂ：０．０００１〜０．０１５％
から選択される１種以上を含有する、上記（１）から（３）までのいずれかに記載の鋼材。(4) The chemical composition is mass%,
Nb: 0.005 to 0.5%,
Ta: 0.005 to 0.5%,
Ti: 0.005 to 0.5%,
Zr: 0.005 to 0.5%,
Ca: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%,
REM: 0.001 to 0.01% and B: 0.0001 to 0.015%
The steel material in any one of said (1) to (3) containing 1 or more types selected from.

（５）上記（１）から（４）までのいずれかに記載の鋼材からなる、拡管用油井鋼管。   (5) An oil well steel pipe for pipe expansion comprising the steel material according to any one of (1) to (4) above.

（６）前記鋼管が継目無鋼管である、上記（５）に記載の拡管用油井鋼管。   (6) The oil well steel pipe for pipe expansion according to the above (5), wherein the steel pipe is a seamless steel pipe.

本発明によれば、高い均一伸びを有するため拡管性に優れ、かつ冷間加工後であっても耐ＳＳＣ性に優れる鋼材を得ることが可能となる。したがって、本発明に係る鋼材は、湿潤硫化水素環境下における拡管用油井鋼管として好適に用いることができる。   According to the present invention, it is possible to obtain a steel material that has a high uniform elongation and is excellent in tube expandability and excellent in SSC resistance even after cold working. Therefore, the steel material according to the present invention can be suitably used as an oil well steel pipe for pipe expansion in a wet hydrogen sulfide environment.

Ｍｎ含有量と均一伸びとの関係を示した図である。It is the figure which showed the relationship between Mn content and uniform elongation. Ｃｕ含有量と腐食速度との関係を示した図である。It is the figure which showed the relationship between Cu content and a corrosion rate.

以下、本発明の各要件について詳しく説明する。   Hereinafter, each requirement of the present invention will be described in detail.

１．化学組成
各元素の限定理由は下記のとおりである。なお、以下の説明において含有量についての「％」は、「質量％」を意味する。1. Chemical composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

Ｃ：０．６〜１．８％
炭素（Ｃ）は、ＭｎまたはＮｉの含有量を低減しても、安価にオーステナイト相を安定化させる効果を有するとともに、双晶変形を促進し加工硬化特性と均一伸びとを向上させることができるため、本発明において極めて重要な元素である。そのため、Ｃを０．６％以上含有させる必要がある。一方、Ｃの含有量が多すぎると、セメンタイトが析出し粒界強度を低下させて応力腐食割れ感受性を増大させるだけでなく、材料の融点が顕著に低下し熱間加工性が悪化するため、Ｃ含有量は１．８％以下とする。Ｃ含有量は０．６５％以上であるのが好ましく、０．７％以上であるのがより好ましい。また、Ｃ含有量は１．６％以下であるのが好ましく、１．４％以下であるのがより好ましい。C: 0.6 to 1.8%
Carbon (C) has the effect of stabilizing the austenite phase at low cost even when the content of Mn or Ni is reduced, and can promote twin deformation and improve work hardening characteristics and uniform elongation. Therefore, it is an extremely important element in the present invention. Therefore, it is necessary to contain 0.6% or more of C. On the other hand, if the content of C is too large, not only cementite precipitates and lowers the grain boundary strength to increase the stress corrosion cracking susceptibility, but also the melting point of the material is significantly lowered and the hot workability deteriorates. The C content is 1.8% or less. The C content is preferably 0.65% or more, and more preferably 0.7% or more. Moreover, it is preferable that C content is 1.6% or less, and it is more preferable that it is 1.4% or less.

Ｓｉ：０．０５〜１．００％
シリコン（Ｓｉ）は、鋼の脱酸に必要な元素であり、その含有量が０．０５％未満であると、脱酸が不十分となって非金属介在物が多く残存し、所望の耐ＳＳＣ性が得られない。一方、その含有量が１．００％を超えると、粒界強度を弱め、耐ＳＳＣ性が低下する。したがって、Ｓｉ含有量は、０．０５〜１．００％とする。Ｓｉ含有量は０．１０％以上であるのが好ましく、０．２０％以上であるのがより好ましい。また、Ｓｉ含有量は０．８０％以下であるのが好ましく、０．６０％以下であるのがより好ましい。Si: 0.05-1.00%
Silicon (Si) is an element necessary for deoxidation of steel, and if its content is less than 0.05%, deoxidation is insufficient and a lot of non-metallic inclusions remain, and the desired resistance. SSC property cannot be obtained. On the other hand, when the content exceeds 1.00%, the grain boundary strength is weakened, and the SSC resistance is lowered. Therefore, the Si content is set to 0.05 to 1.00%. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. Moreover, it is preferable that Si content is 0.80% or less, and it is more preferable that it is 0.60% or less.

Ｍｎ：２５．０％を超えて４５．０％以下
マンガン（Ｍｎ）は、安価にオーステナイト相を安定化させることができる元素であるとともに、高い均一伸びを確保するために重要な元素である。それらの効果を十分に発揮させるためには、２５．０％を超える量のＭｎを含有させる必要がある。一方、湿潤硫化水素環境中ではＭｎは優先的に溶解し、材料表面に安定な腐食生成物は形成されない。その結果、Ｍｎ含有量が増加するのに伴い、耐全面腐食性が低下する。本発明においては、４５．０％を超える量のＭｎを含有させると、Ｃｕを一定量以上含有させたとしても低合金油井管の標準的な腐食速度を上回るため、Ｍｎ含有量は４５．０％以下とする必要がある。Ｍｎ含有量は４０．０％以下とするのが好ましい。Mn: more than 25.0% and not more than 45.0% Manganese (Mn) is an element that can stabilize the austenite phase at low cost and is an important element for ensuring high uniform elongation. In order to fully exhibit these effects, it is necessary to contain Mn in an amount exceeding 25.0%. On the other hand, Mn is preferentially dissolved in a wet hydrogen sulfide environment, and a stable corrosion product is not formed on the material surface. As a result, the overall corrosion resistance decreases as the Mn content increases. In the present invention, if Mn is contained in an amount exceeding 45.0%, even if Cu is contained in a certain amount or more, it exceeds the standard corrosion rate of the low alloy oil country tubular goods. Therefore, the Mn content is 45.0. % Or less is required. The Mn content is preferably 40.0% or less.

なお、本発明において、上記の「低合金油井管の標準的な腐食速度」とは、ＮＡＣＥ ＴＭ０１７７−２００５に規定される溶液Ａ（５％ＮａＣｌ＋０．５％ＣＨ３ＣＯＯＨ水溶液、１ｂａｒ Ｈ２Ｓ飽和）に３３６ｈ浸漬させた際の腐食量から換算される腐食速度で１．５ｇ/（ｍ２・ｈ）であることを意味する。In the present invention, the above-mentioned “standard corrosion rate of low alloy oil country tubular goods” refers to solution A (5% NaCl + 0.5% CH 3 COOH aqueous solution, 1 bar H 2 S saturation specified in NACE TM0177-2005). ) Is a corrosion rate converted from the amount of corrosion when immersed in 336h, and it is 1.5 g / (m 2 · h).

Ａｌ：０．００３〜０．０６％
アルミニウム（Ａｌ）は、鋼の脱酸に必要な元素であるため、０．００３％以上含有させる必要がある。しかしながら、Ａｌの含有量が０．０６％を超えると、酸化物が介在物として混入しやすくなり、靭性および耐食性に悪影響を与えるおそれがある。したがって、Ａｌ含有量は０．００３〜０．０６％とする。Ａｌ含有量は０．００８％以上であるのが好ましく、０．０１２％以上であるのがより好ましい。また、Ａｌ含有量は０．０５％以下であるのが好ましく、０．０４％以下であるのがより好ましい。本発明では、Ａｌは酸可溶Ａｌ（ｓｏｌ．Ａｌ）を意味する。Al: 0.003 to 0.06%
Since aluminum (Al) is an element necessary for deoxidation of steel, it is necessary to contain 0.003% or more. However, if the Al content exceeds 0.06%, the oxide tends to be mixed as inclusions, which may adversely affect toughness and corrosion resistance. Therefore, the Al content is set to 0.003 to 0.06%. The Al content is preferably 0.008% or more, and more preferably 0.012% or more. Further, the Al content is preferably 0.05% or less, and more preferably 0.04% or less. In the present invention, Al means acid-soluble Al (sol. Al).

Ｐ：０．０３％以下
リン（Ｐ）は、不純物として鋼中に不可避的に存在する元素である。しかし、その含有量が０．０３％を超えると、粒界に偏析して耐ＳＳＣ性を劣化させる。したがって、Ｐ含有量は、０．０３％以下とする必要がある。なお、Ｐの含有量は、低ければ低いほど望ましく、０．０２％以下とするのが好ましく、０．０１２％以下とするのがより好ましい。しかし、過度の低下は、鋼材の製造コスト上昇を招くため、その下限は、０．００１％とするのが好ましく、０．００５％とするのがより好ましい。P: 0.03% or less Phosphorus (P) is an element unavoidably present in steel as an impurity. However, if its content exceeds 0.03%, it segregates at the grain boundaries and degrades the SSC resistance. Therefore, the P content needs to be 0.03% or less. The P content is preferably as low as possible, preferably 0.02% or less, and more preferably 0.012% or less. However, excessive reduction causes an increase in the manufacturing cost of the steel material, so the lower limit is preferably 0.001%, and more preferably 0.005%.

Ｓ：０．０３％以下
硫黄（Ｓ）は、Ｐと同様に不純物として鋼中に不可避的に存在するが、０．０３％を超えると粒界に偏析するとともに、硫化物系の介在物を生成して耐ＳＳＣ性を低下させる。したがって、Ｓ含有量は、０．０３％以下とする必要がある。なお、Ｓの含有量は、低ければ低いほど望ましく、０．０１５％以下とするのが好ましく、０．０１％以下とするのがより好ましい。しかし、過度の低下は、鋼材の製造コスト上昇を招くため、その下限は、０．００１％とするのが好ましく、０．００２％とするのがより好ましい。S: 0.03% or less Sulfur (S) is unavoidably present in the steel as an impurity in the same manner as P. However, if it exceeds 0.03%, it segregates at the grain boundaries and contains sulfide inclusions. To reduce SSC resistance. Therefore, the S content needs to be 0.03% or less. The S content is preferably as low as possible, preferably 0.015% or less, and more preferably 0.01% or less. However, excessive reduction leads to an increase in the manufacturing cost of the steel material. Therefore, the lower limit is preferably 0.001%, and more preferably 0.002%.

Ｃｕ：０．５〜３．０％
銅（Ｃｕ）は、Ｍｎ含有量が低い鋼材の場合には局部腐食を促進し、鋼材表面に応力集中部を形成しやすい元素である。しかし、Ｃｕは鋼材中の母相の腐食速度が大きい場合には、湿潤硫化水素環境中で材料表面に硫化物を形成し、その後の腐食を抑制する効果を有する。本発明においては、Ｍｎ含有量が高く母相の腐食速度の増加を招きやすいため、Ｃｕを０．５％以上含有させる必要がある。一方、Ｃｕ含有量が過剰であると上記の効果が飽和するだけでなく、局部腐食を促進し鋼材表面に応力集中部を形成するおそれがある。そのため、Ｃｕ含有量は３．０％以下とする。Ｃｕ含有量は０．６％以上であるのが好ましく、０．７％以上であるのがより好ましい。また、Ｃｕ含有量は２．５％以下であるのが好ましく、２．０％以下であるのがより好ましく、１．５％以下であるのがさらに好ましい。Cu: 0.5 to 3.0%
Copper (Cu) is an element that promotes local corrosion in the case of a steel material having a low Mn content and easily forms a stress concentration portion on the surface of the steel material. However, Cu has the effect of suppressing the subsequent corrosion by forming sulfides on the material surface in a wet hydrogen sulfide environment when the corrosion rate of the parent phase in the steel material is high. In the present invention, since the Mn content is high and the corrosion rate of the matrix phase is likely to increase, it is necessary to contain Cu by 0.5% or more. On the other hand, if the Cu content is excessive, not only the above effects are saturated, but also local corrosion is promoted and a stress concentration portion may be formed on the steel material surface. Therefore, the Cu content is 3.0% or less. The Cu content is preferably 0.6% or more, and more preferably 0.7% or more. Further, the Cu content is preferably 2.5% or less, more preferably 2.0% or less, and further preferably 1.5% or less.

Ｖ：０〜２．０％
バナジウム（Ｖ）は、適切な温度および時間で熱処理を行うことにより、鋼中に微細な炭化物（Ｖ４Ｃ３）を析出させ、鋼材を高強度化させることのできる元素であるため、必要に応じて含有させても良い。しかしながら、Ｖ含有量が過剰であると上記の効果が飽和するだけでなく、オーステナイト相を安定化させるＣを多量に消費してしまう。そのため、Ｖ含有量は２．０％以下とする。Ｖ含有量は１．８％以下であるのが好ましく、１．６％以下であるのがより好ましい。なお、本発明においては、高い均一伸びを確保するために顕著な強度上昇は控えるべきであり、また、Ｖ含有量の増加に伴い製造性の低下が懸念されるため、Ｖ含有量は０．５％未満であるのがさらに好ましい。上記の効果を得るためには、Ｖ含有量は０．０３％以上であるのが好ましい。V: 0 to 2.0%
Vanadium (V) is an element capable of precipitating fine carbides (V 4 C 3 ) in steel by heat treatment at an appropriate temperature and time, and increasing the strength of the steel material. You may make it contain according to it. However, if the V content is excessive, not only the above effect is saturated, but also a large amount of C that stabilizes the austenite phase is consumed. Therefore, the V content is 2.0% or less. The V content is preferably 1.8% or less, and more preferably 1.6% or less. In the present invention, a significant increase in strength should be refrained in order to ensure a high uniform elongation, and a decrease in manufacturability with increasing V content is a concern. More preferably, it is less than 5%. In order to obtain the above effect, the V content is preferably 0.03% or more.

Ｎ：０．１０％以下
窒素（Ｎ）は、鉄鋼材料においては、通常は不純物元素として扱われ、脱窒により低減させる。しかし、Ｎはオーステナイト相を安定化させる元素であるため、オーステナイト安定化のためにＮが多く含有されていても良い。しかし、本発明ではＣおよびＭｎによりオーステナイトの安定化を意図しているため、積極的にＮを含有させる必要はない。また、Ｎを過剰に含有させると、高温強度を上昇させて高温での加工応力を増大させ、熱間加工性の低下を招く。したがって、Ｎ含有量は０．１０％以下とする必要がある。なお、精錬コストの観点から不必要に脱窒する必要はなく、Ｎ含有量の下限は０．００１５％とするのが好ましい。N: 0.10% or less Nitrogen (N) is usually treated as an impurity element in steel materials and is reduced by denitrification. However, since N is an element that stabilizes the austenite phase, a large amount of N may be contained for stabilizing austenite. However, since the present invention intends to stabilize austenite with C and Mn, it is not necessary to positively contain N. Further, if N is contained excessively, the high-temperature strength is increased, the processing stress at high temperature is increased, and the hot workability is lowered. Therefore, the N content needs to be 0.10% or less. In addition, it is not necessary to denitrify unnecessarily from the viewpoint of refining costs, and the lower limit of the N content is preferably 0.0015%.

Ｃｒ：０〜３．０％
クロム（Ｃｒ）は、耐全面腐食性を向上させる元素であるので、必要に応じて含有させても良い。しかしながら、その含有量が３．０％を超えると、粒界に偏析し耐ＳＳＣ性の低下を招くおそれもあるので、含有させる場合のＣｒ含有量は３．０％以下とする。また、上述のように、本発明においては、Ｍｎ含有量を高めることにより腐食を促進させるとともに、Ｃｕ硫化物を生成させることによって、その後の腐食を抑制する。そのため、Ｃｒ含有量は積極的に含有させる必要はなく、１．０％未満であるのが好ましい。なお、上記の効果を得たい場合は、Ｃｒ含有量を０．１％以上とするのが好ましく、０．２％以上とするのがより好ましく、０．５％以上とするのがさらに好ましい。Cr: 0 to 3.0%
Chromium (Cr) is an element that improves the overall corrosion resistance, and may be contained as necessary. However, if its content exceeds 3.0%, it may segregate at grain boundaries and cause a decrease in SSC resistance, so the Cr content in the case of inclusion is 3.0% or less. Further, as described above, in the present invention, the corrosion is accelerated by increasing the Mn content, and the subsequent corrosion is suppressed by generating Cu sulfide. Therefore, the Cr content does not need to be positively contained, and is preferably less than 1.0%. In addition, when obtaining said effect, it is preferable that Cr content shall be 0.1% or more, it is more preferable to set it as 0.2% or more, and it is further more preferable to set it as 0.5% or more.

Ｍｏ：０〜３．０％
モリブデン（Ｍｏ）は、Ｃｕと同様に、鋼材中の母相の腐食速度が大きい場合には、湿潤硫化水素環境中で材料表面に硫化物を形成し、その後の腐食を抑制する効果を有する元素であるため、必要に応じて含有させても良い。ただし、その効果はＣｕと比較して小さく、また極めて高価な元素であるため過剰に含有させることは好ましくない。Ｍｏ含有量が３．０％を超えると上記の効果が飽和するだけでなく、経済性が悪化するため、含有させる場合のＭｏ含有量は３．０％以下とする。なお、上記の効果を得たい場合は、Ｍｏ含有量を０．１％以上とするのが好ましく、０．２％以上とするのがより好ましく、０．５％以上とするのがさらに好ましい。Mo: 0 to 3.0%
Molybdenum (Mo) is an element that has the effect of inhibiting the subsequent corrosion by forming sulfides on the surface of the material in a wet hydrogen sulfide environment when the corrosion rate of the parent phase in the steel material is high, like Cu. Therefore, you may make it contain as needed. However, the effect is small compared to Cu, and since it is an extremely expensive element, it is not preferable to contain it excessively. When the Mo content exceeds 3.0%, not only the above effects are saturated but also the economic efficiency is deteriorated. Therefore, the Mo content in the case of inclusion is 3.0% or less. In addition, when obtaining said effect, it is preferable to make Mo content into 0.1% or more, it is more preferable to set it as 0.2% or more, and it is further more preferable to set it as 0.5% or more.

Ｎｉ：０〜１．５％
ニッケル（Ｎｉ）もＣｕと同様に、オーステナイト相を安定化させることのできる元素であり、さらに、Ｃｕ含有鋼に生じることのある熱間圧延時のひび割れを抑制する効果があるので必要に応じて含有させることができる。しかしながら、Ｎｉは局部腐食を促進し、鋼材表面に応力集中部を形成しやすい元素であるため、過剰に含有させると耐ＳＳＣ性を低下させるおそれがある。したがって、含有させる場合のＮｉ含有量は１．５％以下とする。上記のひび割れを抑制する効果は少量の含有量でも認められるが、０．１％以上含有させるのが好ましく、０．２％以上含有させるのがより好ましい。Ni: 0 to 1.5%
Nickel (Ni) is an element that can stabilize the austenite phase as well as Cu, and further has the effect of suppressing cracking during hot rolling that may occur in Cu-containing steel. It can be included. However, since Ni is an element that promotes local corrosion and easily forms a stress concentration portion on the surface of the steel material, if it is excessively contained, the SSC resistance may be lowered. Therefore, the Ni content when contained is 1.5% or less. The effect of suppressing the above cracks is recognized even in a small amount, but it is preferably contained at 0.1% or more, more preferably 0.2% or more.

Ｎｂ：０〜０．５％
Ｔａ：０〜０．５％
Ｔｉ：０〜０．５％
Ｚｒ：０〜０．５％
ニオブ（Ｎｂ）、タンタル（Ｔａ）、チタン（Ｔｉ）およびジルコニウム（Ｚｒ）は、ＣまたはＮと結びつき微小な炭化物または炭窒化物を形成することで、鋼の強化に寄与する元素であり、必要に応じて含有させても良い。加えて、これら炭化物、炭窒化物形成能を有する元素を含有させておけば、時効熱処理を行うことにより、時効熱処理による析出強化を図ることができる。しかしながら、これらの元素を多量に含有させても効果が飽和する上、靭性の低下およびオーステナイト相の不安定化を引き起こすことがあるため、各元素ともその含有量を０．５％以下とする必要があり、０．３５％以下とするのが好ましい。上記の効果を得るためには、これらの元素から選択される１種以上を０．００５％以上含有させることが好ましく、０．１％以上含有させることがより好ましい。Nb: 0 to 0.5%
Ta: 0 to 0.5%
Ti: 0 to 0.5%
Zr: 0 to 0.5%
Niobium (Nb), Tantalum (Ta), Titanium (Ti), and Zirconium (Zr) are elements that contribute to strengthening steel by forming fine carbides or carbonitrides by combining with C or N. You may make it contain according to. In addition, if these elements having the ability to form carbides and carbonitrides are contained, precipitation strengthening by aging heat treatment can be achieved by performing aging heat treatment. However, even if these elements are contained in a large amount, the effect is saturated, and the toughness is lowered and the austenite phase is destabilized. Therefore, the content of each element needs to be 0.5% or less. And is preferably 0.35% or less. In order to acquire said effect, it is preferable to contain 0.005% or more of 1 or more types selected from these elements, and it is more preferable to contain 0.1% or more.

Ｃａ：０〜０．００５％
Ｍｇ：０〜０．００５％
カルシウム（Ｃａ）およびマグネシウム（Ｍｇ）は、介在物の形態を制御することで靭性および耐食性を改善する効果があり、さらに、鋳込み時のノズル詰まりを抑制して鋳込み特性を改善する効果もあるため、必要に応じて含有させても良い。しかしながら、これらの元素を多量に含有させても効果が飽和するだけでなく、介在物がクラスター化し易くなり、かえって靱性および耐食性が低下する。したがって、各元素ともその含有量を０．００５％以下とする。各元素の含有量は０．００３％以下であるのが好ましい。上記の効果を得るためには、ＣａおよびＭｇの１種または２種を０．０００３％以上含有させることが好ましく、０．０００５％以上含有させることがより好ましい。Ca: 0 to 0.005%
Mg: 0 to 0.005%
Calcium (Ca) and magnesium (Mg) have the effect of improving toughness and corrosion resistance by controlling the form of inclusions, and also have the effect of suppressing nozzle clogging during casting and improving casting characteristics. Further, it may be contained as necessary. However, even if these elements are contained in a large amount, not only the effect is saturated, but also inclusions are easily clustered, and the toughness and corrosion resistance are reduced. Therefore, the content of each element is set to 0.005% or less. The content of each element is preferably 0.003% or less. In order to acquire said effect, it is preferable to contain 1 type or 2 types of Ca and Mg 0.0003% or more, and it is more preferable to contain 0.0005% or more.

ＲＥＭ：０〜０．０１％
希土類元素（ＲＥＭ）も、ＣａおよびＭｇと同様に、介在物の形態を制御することで靭性および耐食性を改善する効果があるため、必要に応じて含有させても良い。しかしながら、ＲＥＭを多量に含有させても効果が飽和するだけでなく、介在物がクラスター化し易くなり、かえって靱性および耐食性が低下する。したがって、ＲＥＭの含有量は、０．０１％以下とする。ＲＥＭ含有量は０．００５％以下であるのが好ましい。上記の効果を得るためには、ＲＥＭは０．００１％以上含有させることが好ましく、０．００２％以上含有させることがより好ましいREM: 0 to 0.01%
Similarly to Ca and Mg, rare earth elements (REM) may be contained as needed because they have the effect of improving toughness and corrosion resistance by controlling the form of inclusions. However, even if a large amount of REM is contained, not only the effect is saturated, but also inclusions are easily clustered, and the toughness and corrosion resistance are reduced. Therefore, the content of REM is set to 0.01% or less. The REM content is preferably 0.005% or less. In order to obtain the above effect, REM is preferably contained in an amount of 0.001% or more, and more preferably 0.002% or more.

なお、「ＲＥＭ」とは、Ｓｃ、Ｙおよびランタノイドの合計１７元素の総称であり、ＲＥＭの含有量はＲＥＭのうちの１種または２種以上の元素の合計含有量を指す。   “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM.

Ｃａ、ＭｇおよびＲＥＭの２種以上を複合的に含有させる場合、その含有量の合計を０．００８％以下とすることが好ましい。   When two or more of Ca, Mg, and REM are contained in a composite manner, the total content is preferably 0.008% or less.

Ｂ：０〜０．０１５％
ホウ素（Ｂ）は、析出物を微細化する作用とオーステナイト結晶粒径を微細化する作用とを有するので必要に応じて含有させても良い。しかしながら、Ｂを多量に含有させると低融点の化合物を形成して熱間加工性が低下することがあり、特にＢの含有量が０．０１５％を超えると熱間加工性の低下が著しくなる場合がある。したがって、Ｂの含有量は、０．０１５％以下とする。上記の効果を得るためには、Ｂは０．０００１％以上含有させることが好ましい。B: 0 to 0.015%
Since boron (B) has an effect of refining the precipitate and an effect of refining the austenite crystal grain size, it may be contained as necessary. However, when B is contained in a large amount, a low melting point compound may be formed and the hot workability may be deteriorated. In particular, when the B content exceeds 0.015%, the hot workability is significantly deteriorated. There is a case. Therefore, the B content is 0.015% or less. In order to acquire said effect, it is preferable to contain B 0.0001% or more.

本発明の鋼材は、上記のＣからＢまでの元素と、残部Ｆｅおよび不純物とからなる化学組成を有する。   The steel material of the present invention has a chemical composition comprising the above-described elements C to B, the remaining Fe and impurities.

ここで「不純物」とは、鋼を工業的に製造する際に、鉱石、スクラップ等の原料、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。   Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something.

０．６＜Ｃ−０．１８Ｖ＜１．４４ ・・・（ｉ）
但し、式中の各元素記号は、鋼材中に含まれる各元素の含有量（質量％）を表し、含有されない場合はゼロとする。
本発明においては、オーステナイト相を安定化させるため、Ｃ含有量を上記の範囲に規定しているが、Ｖ炭化物を析出させることによって鋼材を強化した場合には、Ｃの一部が消費され、オーステナイト安定性が低下し、均一伸びが低下するおそれがある。Ｖ炭化物は全てＶ４Ｃ３であるとすると、オーステナイトの安定化に寄与する有効Ｃ量は上記（ｉ）式に示すようにＣ−０．１８Ｖで表され、該有効Ｃ量が０．６を超えるようにＣおよびＶの含有量を調整する必要がある。一方、有効Ｃ量が１．４４以上となるとセメンタイトの生成に伴う組織の不均一化および熱間加工性の低下の問題が生じるため、有効Ｃ量が１．４４未満となるようにＣおよびＶの含有量を調整する必要がある。有効Ｃ量は０．６５以上であるのが好ましく、０．７以上であるのがより望ましい。また、有効Ｃ量は１．４以下であるのが好ましく、１．３以下であるのがより好ましい。0.6 <C-0.18V <1.44 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained.
In the present invention, in order to stabilize the austenite phase, the C content is defined in the above range, but when the steel material is strengthened by precipitating V carbide, a part of C is consumed, Austenite stability may be reduced, and uniform elongation may be reduced. Assuming that all V carbides are V 4 C 3 , the effective C amount contributing to the stabilization of austenite is represented by C−0.18 V as shown in the above formula (i), and the effective C amount is 0.6. It is necessary to adjust the contents of C and V so as to exceed. On the other hand, when the effective C amount is 1.44 or more, there arises a problem of non-uniform structure due to the formation of cementite and a decrease in hot workability. Therefore, the effective C amount is less than 1.44 so that the effective C amount is less than 1.44. It is necessary to adjust the content of. The effective C amount is preferably 0.65 or more, and more preferably 0.7 or more. Further, the effective C amount is preferably 1.4 or less, and more preferably 1.3 or less.

２．金属組織
上述のように、金属組織中にＢＣＣ構造であるα’マルテンサイトおよびフェライトが混在すると、均一伸びが低下するだけでなく、耐ＳＳＣ性の低下を招くおそれがある。そのため、本発明では、金属組織をＦＣＣ（面心立方）構造であるオーステナイト単相とする。2. Metal Structure As described above, when α ′ martensite and ferrite having a BCC structure are mixed in the metal structure, not only the uniform elongation is lowered, but also the SSC resistance may be lowered. Therefore, in the present invention, the metal structure is an austenite single phase having an FCC (face centered cubic) structure.

なお、ＢＣＣ構造であるα’マルテンサイトおよびフェライトの混入量が、Ｘ線回折（ＸＲＤ）では検出できないほどに微量であっても、均一伸びおよび耐ＳＳＣ性が劣化するおそれがある。そこで、本発明においては、Ｈｅｌｍｕｔ Ｆｉｓｃｈｅｒ製のフェライトメーター（型番：ＦＥ８ｅ３）を用いてＢＣＣ構造を有するフェライトおよびα’マルテンサイトの体積量を測定し、評価するものとする。   Even if the amount of α 'martensite and ferrite having a BCC structure is so small that it cannot be detected by X-ray diffraction (XRD), the uniform elongation and SSC resistance may be deteriorated. Therefore, in the present invention, the volume of ferrite and α ′ martensite having a BCC structure is measured and evaluated using a ferrite meter (model number: FE8e3) manufactured by Helmut Fischer.

３．機械的性質
本発明に係る鋼材は、２４１ＭＰａ以上の降伏強度を有する。一方、拡管性を確保するためには、鋼材の降伏強度は８６２ＭＰａ未満であることが望ましい。特に、本発明に係る鋼材を拡管用油井鋼管として用いる場合には、鋼材の降伏強度は７５８ＭＰａ未満であることが望ましく、６５４ＭＰａ未満であることがより望ましい。3. Mechanical properties The steel material according to the present invention has a yield strength of 241 MPa or more. On the other hand, in order to ensure pipe expandability, it is desirable that the yield strength of the steel material is less than 862 MPa. In particular, when the steel material according to the present invention is used as an oil well steel pipe for expansion, the yield strength of the steel material is desirably less than 758 MPa, and more desirably less than 654 MPa.

また、本発明に係る鋼材は、良好な拡管性を確保するために、高い均一伸びが必要である。通常の油井における拡管法では、拡管率は２５％程度であるが、２５％の冷間加工が加えられた後にも十分な伸びを示す材料が実用上は望ましい。そのため、本発明における鋼材は４０％以上の均一伸びを有するものとする。   In addition, the steel material according to the present invention requires high uniform elongation in order to ensure good tube expansion. In a normal oil well expansion method, the expansion rate is about 25%, but a material that exhibits sufficient elongation even after 25% cold work is applied is desirable in practice. Therefore, the steel material in the present invention has a uniform elongation of 40% or more.

鋼材の均一伸びは、概して降伏強度と反比例する傾向にある。そのため、降伏強度の低い鋼材においては、その降伏強度に応じてより高い均一伸びを有することが望ましい。したがって、本発明に係る鋼材は下記（ii）式を満たすことが望ましい。
ｕＥｌ（％）＞７０−０．０６×ＹＳ（ＭＰａ） ・・・（ii）
但し、式中のｕＥｌは鋼材の均一伸び（％）、ＹＳは降伏強度（ＭＰａ）を意味する。The uniform elongation of the steel material generally tends to be inversely proportional to the yield strength. Therefore, it is desirable that a steel material having a low yield strength has a higher uniform elongation depending on the yield strength. Therefore, it is desirable that the steel material according to the present invention satisfies the following formula (ii).
uEl (%)> 70-0.06 × YS (MPa) (ii)
However, uEl in the formula means uniform elongation (%) of the steel material, and YS means yield strength (MPa).

特に、降伏強度が５００ＭＰａ未満の場合、固溶化熱処理された鋼管を予め冷間加工により強化して出荷することも想定されるので、上記式（ii）を満たすことが望ましいといえる。   In particular, when the yield strength is less than 500 MPa, it is assumed that the solution-treated heat-treated steel pipe is strengthened by cold working in advance, so that it is desirable to satisfy the above formula (ii).

４．用途
上述のように本発明に係る鋼材は、拡管性に優れるだけでなく、拡管後に熱処理を施さなくても耐食性が悪化しないという特長を有する。そのため、本発明の鋼材は拡管用油井鋼管として用いるのに好適である。鋼管の種類については特に限定されず、継目無鋼管、電縫鋼管またはアーク溶接鋼管等を用いることができる。4). Applications As described above, the steel material according to the present invention has not only excellent tube expandability, but also has a feature that corrosion resistance does not deteriorate even if heat treatment is not performed after tube expansion. Therefore, the steel material of the present invention is suitable for use as an oil well steel pipe for pipe expansion. The type of the steel pipe is not particularly limited, and a seamless steel pipe, an ERW steel pipe, an arc welded steel pipe, or the like can be used.

通常、拡管する上では、厚さに多少のばらつきがある継目無鋼管を用いるより、厚さが均一である鋼帯または鋼板を管状に加工した後に接合させた鋼管を用いる方が望ましい。しかしながら、本発明に係る鋼材は、加工を施すことによって著しく硬化される特徴を有する。そのため、厚さのばらつきがある鋼管を拡管した場合、薄い部分がまず拡張されることにより硬化され、それ以上の伸びが制限される。そして、次いで厚い部分が拡張されることになるため、結果的には均一に拡張されることとなる。したがって、本発明に係る鋼材は、継目無鋼管として好適に用いることができる。加えて、良好な耐ＳＳＣ性を安定的に発揮するためには溶接部を含まない継目無鋼管がより望ましい。   In general, when expanding the pipe, it is preferable to use a steel pipe joined after processing a steel strip or steel plate having a uniform thickness into a tubular shape, rather than using a seamless steel pipe with some variation in thickness. However, the steel material according to the present invention has a characteristic that it is remarkably hardened by processing. Therefore, when a steel pipe having a variation in thickness is expanded, the thin portion is first expanded to be hardened, and further elongation is limited. And since a thick part will be expanded next, as a result, it will be expanded uniformly. Therefore, the steel material according to the present invention can be suitably used as a seamless steel pipe. In addition, in order to stably exhibit good SSC resistance, a seamless steel pipe that does not include a weld is more desirable.

５．製造方法
本発明に係る鋼材は、例えば、以下の方法により製造することができるが、この方法には限定されない。5. Manufacturing Method The steel material according to the present invention can be manufactured, for example, by the following method, but is not limited to this method.

＜溶解および鋳造＞
溶解および鋳造については一般的なオーステナイト系鋼材の製造方法で行われる方法を用いることができ、鋳造はインゴット鋳造でも連続鋳造でも良い。継目無鋼管を製造する場合には、ラウンドＣＣにより、製管用ラウンドビレットの形状に鋳造しても良い。<Melting and casting>
For melting and casting, a method performed by a general method for producing austenitic steel materials can be used, and the casting may be ingot casting or continuous casting. When producing a seamless steel pipe, it may be cast into the shape of a round billet for pipe making by round CC.

＜熱間加工（鍛造、穿孔、圧延）＞
鋳造後は、鍛造、穿孔、圧延等の熱間加工が施される。なお、継目無鋼管の製造では、上述のラウンドＣＣによって円形ビレットを鋳造した場合、円形ビレットに成形するための鍛造、分塊圧延等の工程は必要ない。鋼材が継目無鋼管の場合は、上記の穿孔工程の後、マンドレルミルまたはプラグミルを使用して圧延が行われる。また、鋼材が板材の場合は、スラブを粗圧延した後、仕上げ圧延するという工程になる。穿孔、圧延等の熱間加工の望ましい条件は、以下の通りである。<Hot processing (forging, drilling, rolling)>
After casting, hot working such as forging, drilling and rolling is performed. In the manufacture of seamless steel pipes, when a round billet is cast by the above-described round CC, processes such as forging and split rolling for forming the round billet are not necessary. When the steel material is a seamless steel pipe, rolling is performed using a mandrel mill or a plug mill after the drilling step. Further, when the steel material is a plate material, the slab is roughly rolled and then finish-rolled. Desirable conditions for hot working such as piercing and rolling are as follows.

ビレットの加熱は、穿孔圧延機での熱間穿孔が可能な程度に行えば良いが、望ましい温度範囲は１０００〜１２５０℃である。穿孔圧延およびマンドレルミル、プラグミル等のその他の圧延機による圧延に関しても特別の制約はないが、熱間加工性の上から、具体的には表面疵の防止のために、仕上げ温度を９００℃以上とするのが望ましい。仕上げ温度の上限にも特に制約はないが、１１００℃までに留めるのが良い。   The billet may be heated to such an extent that hot piercing with a piercing and rolling mill is possible, but a desirable temperature range is 1000 to 1250 ° C. There are no particular restrictions on piercing rolling and rolling by other rolling mills such as mandrel mills, plug mills, etc. However, in terms of hot workability, in order to prevent surface flaws, the finishing temperature should be 900 ° C or higher. Is desirable. Although there is no restriction | limiting in particular in the upper limit of finishing temperature, It is good to keep it to 1100 degreeC.

鋼板を製造する場合は、スラブ等の加熱温度は、熱間圧延が可能な温度範囲、例えば、１０００〜１２５０℃とすれば十分である。熱間圧延のパススケジュールは任意であるが、製品の表面疵、耳割れなどの発生を少なくするための熱間加工性を考慮して、仕上げ温度を９００℃以上とするのが望ましい。仕上げ温度は、上記継目無鋼管と同様に１１００℃までとするのが良い。   When manufacturing a steel plate, it is sufficient that the heating temperature of the slab or the like is in a temperature range in which hot rolling is possible, for example, 1000 to 1250 ° C. The hot rolling pass schedule is arbitrary, but it is desirable to set the finishing temperature to 900 ° C. or higher in consideration of hot workability for reducing the occurrence of surface flaws, ear cracks and the like of the product. The finishing temperature is preferably up to 1100 ° C. like the seamless steel pipe.

＜固溶化熱処理＞
熱間加工後の鋼材は、炭化物等を完全に固溶させるのに十分な温度に加熱してから急冷する。この場合、１０００〜１２００℃の温度範囲に１０ｍｉｎ以上保持した後、急冷するのが望ましい。すなわち、加熱温度が１０００℃未満であると、炭化物、特にＣｒおよびＭｏを含有させた場合にＣｒ−Ｍｏ系の炭化物を完全固溶させることができず、このＣｒ−Ｍｏ系炭化物周辺にＣｒおよびＭｏの欠乏層が形成され、孔食発生に伴う応力腐食割れを起こし、所望の耐ＳＳＣ性が得られなくなる場合がある。一方、加熱温度が１２００℃を超えると、フェライト等の異相が析出し、所望の耐ＳＳＣ性が得られなくなる場合がある。また、保持時間が１０ｍｉｎ未満であると、固溶化の効果が不十分となって炭化物を完全に固溶させられないために、加熱温度が１０００℃未満である場合と同様の理由により、所望の耐ＳＳＣ性が得られなくなる場合がある。<Solution heat treatment>
The steel material after hot working is rapidly cooled after being heated to a temperature sufficient to completely dissolve carbides and the like. In this case, it is desirable to rapidly cool after holding in the temperature range of 1000 to 1200 ° C. for 10 minutes or more. That is, when the heating temperature is less than 1000 ° C., when Cr, especially Cr and Mo are contained, Cr—Mo based carbide cannot be completely dissolved, and Cr and Mo around the Cr—Mo based carbide are not dissolved. A Mo deficient layer is formed, causing stress corrosion cracking accompanying the occurrence of pitting corrosion, and the desired SSC resistance may not be obtained. On the other hand, when the heating temperature exceeds 1200 ° C., a heterogeneous phase such as ferrite precipitates and the desired SSC resistance may not be obtained. Further, if the holding time is less than 10 min, the effect of solid solution is insufficient and the carbide cannot be completely dissolved, and therefore, for the same reason as when the heating temperature is less than 1000 ° C., the desired SSC resistance may not be obtained.

保持時間の上限は、鋼材のサイズ、形状にも依存し、一概には決められない。いずれにしても、鋼材全体が均熱される時間が必要であるが、製造コストを抑えるという観点からは長すぎる時間は望ましくなく、通常１ｈ以内とするのが適当である。また、冷却は、冷却中の炭化物（セメンタイトまたはＣｒ−Ｍｏ系炭化物）、その他の金属間化合物等の析出を防ぐために、油冷以上の冷却速度で冷却するのが望ましい。   The upper limit of the holding time depends on the size and shape of the steel material and cannot be determined unconditionally. In any case, a time for soaking the entire steel material is required, but from the viewpoint of suppressing the manufacturing cost, an excessively long time is not desirable, and it is appropriate that the time is usually within 1 h. The cooling is preferably performed at a cooling rate equal to or higher than that of oil cooling in order to prevent precipitation of carbide (cementite or Cr—Mo based carbide) during cooling and other intermetallic compounds.

なお、上記保持時間の下限値は、熱間加工後の鋼材を１０００℃未満の温度に一旦冷却した後、上記１０００〜１２００℃の温度範囲に再加熱する場合の保持時間である。しかし、熱間加工の終了温度（仕上がり温度）を１０００〜１２００℃の範囲にした場合、その温度でおよそ５ｍｉｎ以上の補熱を行えば上記の条件によった場合の固溶化熱処理と同じ効果が得られ、再加熱することなく、そのまま急冷することができる。したがって、本発明における上記保持時間の下限値は、熱間加工の終了温度（仕上がり温度）を１０００〜１２００℃の範囲とし、その温度でおよそ５ｍｉｎ以上の補熱を行う場合を含むものとする。   The lower limit of the holding time is a holding time when the steel material after hot working is once cooled to a temperature of less than 1000 ° C. and then reheated to the temperature range of 1000 to 1200 ° C. However, when the end temperature of hot working (finishing temperature) is in the range of 1000 to 1200 ° C., if the heat is supplemented for about 5 minutes or more at that temperature, the same effect as the solution heat treatment under the above conditions is obtained. It can be obtained and rapidly cooled without reheating. Therefore, the lower limit value of the holding time in the present invention includes a case where the end temperature (finished temperature) of hot working is in the range of 1000 to 1200 ° C., and the temperature is supplemented for about 5 minutes or more.

＜時効熱処理＞
本発明鋼材は、主に炭化物、炭窒化物の析出による析出強化を目的とした時効熱処理を行っても良い。特に、Ｖ、Ｎｂ、Ｔａ、ＴｉおよびＺｒの１種または２種以上を含有する場合に有効である。しかしながら、過度の時効熱処理は、過剰な炭化物の生成を招き、母相中のＣ濃度を低減させオーステナイトの不安定化を引き起こす。熱処理条件としては、６００〜８００℃の温度範囲で数１０ｍｉｎ〜数ｈ程度の時間加熱するのが好ましい。<Aging heat treatment>
The steel of the present invention may be subjected to aging heat treatment for the purpose of precipitation strengthening mainly by precipitation of carbides and carbonitrides. In particular, it is effective when one or more of V, Nb, Ta, Ti and Zr are contained. However, excessive aging heat treatment leads to the formation of excessive carbides, reducing the C concentration in the parent phase and causing destabilization of austenite. As heat treatment conditions, it is preferable to heat in the temperature range of 600 to 800 ° C. for several tens of minutes to several hours.

＜冷間加工＞
固溶化熱処理またはさらに時効熱処理を施した後の鋼材には、必要に応じて冷間加工を施しても良い。加工度（断面減少率）については特に制限は設けないが、特に４００ＭＰａ以上８６２ＭＰａ未満の降伏強度を得たい場合は、１０％程度の冷間加工を施すことが好ましい。一方、本発明に係る鋼材を拡管用油井鋼管として用いる場合は、高い拡管性を確保するため、過度に冷間加工を行うのは好ましくなく、加工度は２５％以下とするのが望ましい。加工度が高すぎれば、その分均一伸びは低下するとともに、強度が上昇するため現場での均一な拡管が困難となる。<Cold processing>
The steel material after being subjected to solution heat treatment or further aging heat treatment may be subjected to cold working as necessary. Although there is no particular limitation on the degree of work (cross-sectional reduction rate), it is preferable to perform cold work of about 10% particularly when it is desired to obtain a yield strength of 400 MPa or more and less than 862 MPa. On the other hand, when the steel material according to the present invention is used as an oil well steel pipe for pipe expansion, in order to ensure high pipe expandability, it is not preferable to perform cold working excessively, and the degree of work is desirably 25% or less. If the degree of processing is too high, the uniform elongation is reduced correspondingly, and the strength is increased, so that uniform tube expansion on the site becomes difficult.

冷間加工方法としては、鋼材を均一に加工できる方法であれば、特に制限されない。しかし、鋼材が鋼管の場合は、孔明きダイスとプラグを用いるいわゆる冷間抽伸機またはコールドピルガーミルと称される冷間圧延機等を用いるのが工業的に有利である。また、鋼材が板材の場合は、通常の冷延鋼板の製造に用いられる圧延機を用いるのが工業的に有利である。   The cold working method is not particularly limited as long as it is a method capable of uniformly processing a steel material. However, when the steel material is a steel pipe, it is industrially advantageous to use a so-called cold drawing machine using a perforated die and a plug or a cold rolling machine called a cold pilger mill. Further, when the steel material is a plate material, it is industrially advantageous to use a rolling mill that is used for manufacturing a normal cold-rolled steel sheet.

＜焼鈍＞
上記の冷間加工後、焼鈍を行っても良い。特に、冷間加工により狙い以上の強度が出た際に強度を低減させ、伸びを回復させる目的で適用することが可能である。焼鈍条件としては、３００〜５００℃の温度範囲で数ｍｉｎ〜１ｈ程度の時間加熱するのが好ましい。<Annealing>
Annealing may be performed after the cold working. In particular, it can be applied for the purpose of reducing the strength and recovering the elongation when the strength higher than the target is obtained by cold working. As annealing conditions, it is preferable to heat in the temperature range of 300 to 500 ° C. for several minutes to 1 hour.

以下、実施例によって本発明をより具体的に説明するが、本発明はこれらの実施例に限定されるものではない。   EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

表１に示す化学成分を有するＡ〜ＰおよびＡＡ〜ＡＧの２３種類の鋼を、５０ｋｇ真空炉で溶製し、インゴットに鋳造した。各インゴットを１１８０℃で３ｈ加熱した後、鍛造し、放電切断により分断した。その後さらに１１５０℃で１ｈ均熱し、熱間圧延して厚さ２０ｍｍの板材とした後、１１００℃、１ｈの固溶化熱処理を行い、試験材（試験番号１〜２３）を得た。加えて、試験番号１〜２３と同様にして作製した試験材に対して、固溶化熱処理後にさらに加工度１０％の冷間加工を施し、強度を高めた試験材（試験番号２４〜４６）を得た。   23 types of steels A to P and AA to AG having chemical components shown in Table 1 were melted in a 50 kg vacuum furnace and cast into ingots. Each ingot was heated at 1180 ° C. for 3 hours, forged, and divided by discharge cutting. Then, it was further soaked at 1150 ° C. for 1 h, hot rolled to obtain a plate material having a thickness of 20 mm, and then subjected to a solution heat treatment at 1100 ° C. for 1 h to obtain test materials (test numbers 1 to 23). In addition, the test materials manufactured in the same manner as in test numbers 1 to 23 were subjected to cold working with a workability of 10% after the solution heat treatment, and the test materials (test numbers 24 to 46) with increased strength were obtained. Obtained.

Figure 2016052271
Figure 2016052271

上記の試験材（試験番号１〜４６）を用いて機械的性質および金属組織を調査した後、それぞれの試験材に対して拡管を模擬した加工度２５％の冷間加工を施し、その後、機械的性質、金属組織、耐ＳＳＣ性および腐食速度を調査した。機械的性質については、降伏強度および均一伸びの測定を行った。各鋼から、外径６ｍｍ、長さ４０ｍｍの平行部を有する丸棒引張試験片を採取し、常温（２５℃）で引張試験を行い、降伏強度ＹＳ（０．２％耐力）（ＭＰａ）および均一伸び（％）を求めた。   After investigating the mechanical properties and metal structure using the above test materials (test numbers 1 to 46), each test material was subjected to cold working with a working degree of 25% simulating tube expansion. Physical properties, metallographic structure, SSC resistance and corrosion rate were investigated. For mechanical properties, yield strength and uniform elongation were measured. A round bar tensile test piece having a parallel part with an outer diameter of 6 mm and a length of 40 mm was taken from each steel, and subjected to a tensile test at room temperature (25 ° C.), yield strength YS (0.2% proof stress) (MPa) and Uniform elongation (%) was determined.

本実施例においては、均一伸びが４０％以上であり、かつ、降伏応力との関係で下記（ii）式を満たす場合を、均一伸びに優れるとした。なお、下記の表２には、４０％または７０−０．０６×ＹＳのいずれか大きい方の値を、要求伸び（％）として示した。
ｕＥｌ（％）＞７０−０．０６×ＹＳ（ＭＰａ） ・・・（ii）
但し、式中のｕＥｌは鋼材の均一伸び（％）、ＹＳは降伏強度（ＭＰａ）を意味する。In this example, the uniform elongation was excellent when the uniform elongation was 40% or more and the following formula (ii) was satisfied in relation to the yield stress. In Table 2 below, the larger value of 40% or 70-0.06 × YS is shown as the required elongation (%).
uEl (%)> 70-0.06 × YS (MPa) (ii)
However, uEl in the formula means uniform elongation (%) of the steel material, and YS means yield strength (MPa).

耐ＳＳＣ性は、板状の平滑試験片を採取し、４点曲げ法によって一方の面に降伏強度の９０％に相当する応力を付加した後、試験溶液として、ＮＡＣＥ ＴＭ０１７７−２００５に規定される溶液Ａ（５％ＮａＣｌ＋０．５％ＣＨ３ＣＯＯＨ水溶液、１ｂａｒ Ｈ２Ｓ飽和）に浸漬させ、２４℃で３３６ｈ保持して破断するか否かを判定し、破断しないものを耐ＳＳＣ性が良好（表２において「○」と表記する。）、破断したものを耐ＳＳＣ性が不良（表２において「×」と表記する。）として評価した。SSC resistance is specified in NACE TM0177-2005 as a test solution after taking a plate-like smooth test piece, applying a stress corresponding to 90% of the yield strength to one surface by a four-point bending method. Immerse in solution A (5% NaCl + 0.5% CH 3 COOH aqueous solution, 1 bar H 2 S saturated), hold at 336 h at 24 ° C. to determine whether or not to break, and those that do not break have good SSC resistance ( In Table 2, “◯” is indicated), and the fractured piece was evaluated as having poor SSC resistance (indicated as “x” in Table 2).

また、耐全面腐食性を評価するために腐食速度を以下の方法により求めた。上記の試験材を常温において上記の溶液Ａに３３６ｈ浸漬させ、腐食減量を求め、平均腐食速度に換算した。本発明においては、腐食速度が１．５ｇ／（ｍ２・ｈ）未満である場合を、耐全面腐食性に優れるとした。Further, in order to evaluate the overall corrosion resistance, the corrosion rate was determined by the following method. The above test material was immersed in the above solution A for 336 h at room temperature, the corrosion weight loss was determined, and converted to an average corrosion rate. In the present invention, when the corrosion rate is less than 1.5 g / (m 2 · h), the overall corrosion resistance is considered excellent.

試験番号１〜４６の、加工度２５％の冷間加工の前後の試験材それぞれに対して、フェライトメーターを用いてＢＣＣ構造を有するフェライトおよびα’マルテンサイトの体積量を測定した。なお、冷間加工前の試験材については、いずれの試料においてもＢＣＣ構造を有する組織は検出されず、オーステナイト単相であった。そのため、表には冷間加工後の試験材についてのＢＣＣ構造を有する組織の体積量のみを、ＢＣＣ率として体積％で示している。これらの結果を表２および３に示す。   With respect to each of the test materials before and after the cold working with a working degree of 25% for test numbers 1 to 46, the volume amounts of ferrite having a BCC structure and α ′ martensite were measured using a ferrite meter. In addition, about the test material before cold working, the structure | tissue which has a BCC structure was not detected in any sample, but was an austenite single phase. Therefore, only the volume amount of the structure | tissue which has a BCC structure about the test material after cold working is shown in the table | surface by volume% as a BCC rate. These results are shown in Tables 2 and 3.

Figure 2016052271
Figure 2016052271

Figure 2016052271
Figure 2016052271

表２に示すように、本発明例である試験番号１〜１６は、６０％以上という極めて高い均一伸びを有しており、その後、拡管を模擬した加工度２５％の冷間加工を施しても、耐ＳＳＣ性に優れ、また腐食速度も１．５ｇ／（ｍ２・ｈ）未満に抑制可能である。また、表３に示すように、本発明例である試験番号２４〜３９は加工度１０％の冷間加工を施すことによって、５１９ＭＰａ以上の降伏強度が得られているにもかかわらず４７％以上の均一伸びを有しており、強度と拡管性のバランスに優れることが分かる。また、その後に拡管を模擬した加工度２５％の冷間加工を施しても、耐ＳＳＣ性に優れ、また腐食速度も１．５ｇ／（ｍ２・ｈ）未満に抑制可能である。As shown in Table 2, Test Nos. 1 to 16, which are examples of the present invention, have an extremely high uniform elongation of 60% or more, and then subjected to cold working with a working degree of 25% simulating tube expansion. However, the SSC resistance is excellent, and the corrosion rate can be suppressed to less than 1.5 g / (m 2 · h). Moreover, as shown in Table 3, test numbers 24 to 39, which are examples of the present invention, are 47% or more even though yield strength of 519 MPa or more is obtained by performing cold working with a workability of 10%. It can be seen that it has a uniform elongation, and is excellent in the balance between strength and tube expandability. Further, even if cold working with a workability of 25% simulating pipe expansion is performed thereafter, the SSC resistance is excellent, and the corrosion rate can be suppressed to less than 1.5 g / (m 2 · h).

一方、Ｃ含有量、Ｍｎ含有量または有効Ｃ量が本発明で規定する下限に満たない試験番号１７、１８、２２、２３、４０、４１、４５および４６は、均一伸びが低く、拡管性が劣る結果となった。なお、試験番号２２および２３は、均一伸びはそれぞれ４２％および４９％であり、本発明の規定を一応満足するが、降伏強度が３０８ＭＰａおよび３１３ＭＰａと低いことを考慮すると、下記（ii）式を満足せず、拡管性が十分とはいえない。   On the other hand, the test numbers 17, 18, 22, 23, 40, 41, 45 and 46 in which the C content, the Mn content or the effective C amount is less than the lower limit specified in the present invention are low in uniform elongation and tube expandability. The result was inferior. In Test Nos. 22 and 23, the uniform elongations are 42% and 49%, respectively, which satisfy the provisions of the present invention. However, considering that the yield strength is as low as 308 MPa and 313 MPa, We are not satisfied and cannot say that pipe expandability is enough.

試験番号１７、２２、４０および４５については、有効Ｃ量が本発明の規定を満足しなかったため、オーステナイト安定性が低下し、わずかにＢＣＣ構造を有する組織が検出され、そのことに起因して均一伸びが低下したものと考えられる。一方、ＢＣＣ構造を有する組織の混入量がわずかであり、また強度もそれほど高くないため、本実施例においては、耐ＳＳＣ性に与える影響は認められなかった。   For Test Nos. 17, 22, 40 and 45, since the effective C amount did not satisfy the provisions of the present invention, the austenite stability was lowered, and a structure having a slight BCC structure was detected. It is considered that the uniform elongation was lowered. On the other hand, since the mixed amount of the tissue having the BCC structure is small and the strength is not so high, no influence on the SSC resistance was observed in this example.

Ｎｉ含有量が本発明で規定する上限を超える試験番号２１および４４は、耐ＳＳＣ性が劣る結果となった。また、Ｃｕ含有量が本発明で規定する下限に満たない試験番号１９および４２、ならびにＣｒ含有量が本発明で規定する上限を超える試験番号２０および４３は、耐ＳＳＣ性は良好であるものの、腐食速度が大きく、耐全面腐食性が劣る結果となった。   Test numbers 21 and 44 in which the Ni content exceeds the upper limit defined in the present invention resulted in poor SSC resistance. Moreover, although the test numbers 19 and 42 in which the Cu content is less than the lower limit specified in the present invention and the test numbers 20 and 43 in which the Cr content exceeds the upper limit specified in the present invention, the SSC resistance is good, The corrosion rate was large and the overall corrosion resistance was poor.

図１は、本発明の規定を満足する鋼ＡおよびＢならびに規定範囲外の鋼ＡＢおよびＡＧについて、Ｍｎ含有量と、固溶化熱処理後および加工度１０％の冷間加工後の均一伸びとの関係を示した図である。なお、これらの鋼はＭｎ含有量以外の組成が類似している。図１から分かるように、Ｍｎ含有量が２５％を超える本発明に係る鋼材は、均一伸びが極めて高く良好な拡管性を有する。   FIG. 1 shows the Mn content and the uniform elongation after solution heat treatment and after cold working with a working degree of 10% for steels A and B satisfying the provisions of the present invention and steels AB and AG outside the prescribed range. It is the figure which showed the relationship. These steels have similar compositions except for the Mn content. As can be seen from FIG. 1, the steel material according to the present invention having a Mn content exceeding 25% has a very high uniform elongation and a good tube expandability.

また、図２は、本発明の規定を満足する鋼Ａ、ＣおよびＤならびに規定範囲外の鋼ＡＣについて、Ｃｕ含有量と、固溶化熱処理後および加工度１０％の冷間加工後の腐食速度との関係を示した図である。なお、これらの鋼はＣｕ含有量以外の組成が類似している。図２から明らかなように、Ｃｕ含有量を本発明で規定するように０．５％以上とすることで、顕著に腐食速度が低下し、耐全面腐食性に優れるようになることが分かる。   FIG. 2 shows the Cu content, the corrosion rate after solution heat treatment and after cold working at a working degree of 10% for steels A, C and D satisfying the provisions of the present invention and steel AC outside the prescribed range. It is the figure which showed the relationship. These steels have similar compositions except for the Cu content. As is apparent from FIG. 2, it can be seen that when the Cu content is 0.5% or more as specified in the present invention, the corrosion rate is remarkably lowered and the overall corrosion resistance is improved.

実施例１で準備した鋼Ｋ、Ｌ、ＯおよびＡＦを用いて、固溶化処理後の時効熱処理の影響を調査した。固溶化熱処理の条件は、実施例１と同様であり、その後の時効熱処理の条件は８００℃、１ｈである。なお、評価試験の方法は実施例１の場合と同様である。   Using the steels K, L, O and AF prepared in Example 1, the influence of aging heat treatment after solution treatment was investigated. The conditions for the solution heat treatment are the same as in Example 1, and the conditions for the subsequent aging heat treatment are 800 ° C. and 1 h. The evaluation test method is the same as in Example 1.

上記の時効熱処理後の金属組織についても、実施例１と同様に加工度２５％の冷間加工の前後の試験材それぞれに対して、フェライトメーターを用いて調査した。その結果、冷間加工前の試験材については、いずれの試料においてもＢＣＣ構造の組織が検出されず、オーステナイト単相であった。そのため、表には冷間加工後の試験材についてのＢＣＣ構造を有する組織の体積量のみを、ＢＣＣ率として体積％で示している。それらの結果を表４に示す。   The metal structure after the aging heat treatment was also investigated using a ferrite meter for each of the test materials before and after cold working with a workability of 25%, as in Example 1. As a result, regarding the test material before cold working, the structure of the BCC structure was not detected in any sample, and it was an austenite single phase. Therefore, only the volume amount of the structure | tissue which has a BCC structure about the test material after cold working is shown in the table | surface by volume% as a BCC rate. The results are shown in Table 4.

Figure 2016052271
Figure 2016052271

表４から分かるように、本発明例である試験番号４７〜４９では、Ｖを含有させた鋼に時効熱処理を施すことによって、降伏強度を５００ＭＰａ以上に強化させつつ、４０％以上の均一伸びを確保することができる。一方、比較例である試験番号５０では、時効熱処理によって、降伏強度を５００ＭＰａ以上にはできるものの、有効Ｃ量が本発明の規定を満足しなかったため、わずかにＢＣＣ構造を有する組織が検出された。そして、そのことに起因して均一伸びが３４％と低くなり、拡管性が劣る結果となった。   As can be seen from Table 4, in Test Nos. 47 to 49, which are examples of the present invention, by applying an aging heat treatment to the steel containing V, the yield strength is strengthened to 500 MPa or more, and a uniform elongation of 40% or more is achieved. Can be secured. On the other hand, in the test number 50 which is a comparative example, although the yield strength can be increased to 500 MPa or more by aging heat treatment, the effective C amount did not satisfy the provisions of the present invention, and thus a structure having a slight BCC structure was detected. . As a result, the uniform elongation was as low as 34%, resulting in poor tube expandability.

本発明によれば、高い均一伸びを有するため拡管性に優れ、かつ冷間加工後であっても耐ＳＳＣ性に優れる鋼材を得ることが可能となる。したがって、本発明に係る鋼材は、湿潤硫化水素環境下における拡管用油井鋼管として好適に用いることができる。   According to the present invention, it is possible to obtain a steel material that has a high uniform elongation and is excellent in tube expandability and excellent in SSC resistance even after cold working. Therefore, the steel material according to the present invention can be suitably used as an oil well steel pipe for pipe expansion in a wet hydrogen sulfide environment.

Claims (6)

化学組成が、質量％で、
Ｃ：０．６〜１．８％、
Ｓｉ：０．０５〜１．００％、
Ｍｎ：２５．０％を超えて４５．０％以下、
Ａｌ：０．００３〜０．０６％、
Ｐ：０．０３％以下、
Ｓ：０．０３％以下、
Ｃｕ：０．５〜３．０％、
Ｎ：０．１０％以下、
Ｖ：０〜２．０％、
Ｃｒ：０〜３．０％、
Ｍｏ：０〜３．０％、
Ｎｉ：０〜１．５％、
Ｎｂ：０〜０．５％、
Ｔａ：０〜０．５％、
Ｔｉ：０〜０．５％、
Ｚｒ：０〜０．５％、
Ｃａ：０〜０．００５％、
Ｍｇ：０〜０．００５％、
ＲＥＭ：０〜０．０１％、
Ｂ：０〜０．０１５％、
残部：Ｆｅおよび不純物であり、
下記（ｉ）式を満足し、
金属組織が、オーステナイト単相からなり、
降伏強度が２４１ＭＰａ以上であり、均一伸びが４０％以上である、鋼材。
０．６＜Ｃ−０．１８Ｖ＜１．４４ ・・・（ｉ）
但し、式中の各元素記号は、鋼材中に含まれる各元素の含有量（質量％）を表し、含有されない場合はゼロとする。Chemical composition is mass%,
C: 0.6-1.8%
Si: 0.05-1.00%,
Mn: more than 25.0% and 45.0% or less,
Al: 0.003-0.06%,
P: 0.03% or less,
S: 0.03% or less,
Cu: 0.5 to 3.0%,
N: 0.10% or less,
V: 0 to 2.0%,
Cr: 0 to 3.0%,
Mo: 0 to 3.0%,
Ni: 0 to 1.5%,
Nb: 0 to 0.5%,
Ta: 0 to 0.5%
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
REM: 0 to 0.01%
B: 0 to 0.015%
Balance: Fe and impurities,
Satisfying the following formula (i)
The metal structure consists of austenite single phase,
A steel material having a yield strength of 241 MPa or more and a uniform elongation of 40% or more.
0.6 <C-0.18V <1.44 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in steel materials, and is set to zero when not contained. 前記化学組成が、質量％で、
Ｖ：０．０３〜２．０％
を含有する、請求項１に記載の鋼材。The chemical composition is mass%,
V: 0.03-2.0%
The steel material according to claim 1, comprising: 前記化学組成が、質量％で、
Ｃｒ：０．１〜３．０％
Ｍｏ：０．１〜３．０％および
Ｎｉ：０．１〜１．５％
から選択される１種以上を含有する、請求項１または請求項２に記載の鋼材。The chemical composition is mass%,
Cr: 0.1-3.0%
Mo: 0.1-3.0% and Ni: 0.1-1.5%
The steel material of Claim 1 or Claim 2 containing 1 or more types selected from these. 前記化学組成が、質量％で、
Ｎｂ：０．００５〜０．５％、
Ｔａ：０．００５〜０．５％、
Ｔｉ：０．００５〜０．５％、
Ｚｒ：０．００５〜０．５％、
Ｃａ：０．０００３〜０．００５％、
Ｍｇ：０．０００３〜０．００５％、
ＲＥＭ：０．００１〜０．０１％および
Ｂ：０．０００１〜０．０１５％
から選択される１種以上を含有する、上記（１）から（３）までのいずれかに記載の鋼材。The chemical composition is mass%,
Nb: 0.005 to 0.5%,
Ta: 0.005 to 0.5%,
Ti: 0.005 to 0.5%,
Zr: 0.005 to 0.5%,
Ca: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%,
REM: 0.001 to 0.01% and B: 0.0001 to 0.015%
The steel material in any one of said (1) to (3) containing 1 or more types selected from. 請求項１から請求項４までのいずれかに記載の鋼材からなる、拡管用油井鋼管。   An oil well steel pipe for pipe expansion comprising the steel material according to any one of claims 1 to 4. 前記鋼管が継目無鋼管である、請求項５に記載の拡管用油井鋼管。   The oil well steel pipe for pipe expansion according to claim 5, wherein the steel pipe is a seamless steel pipe.
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