JP4265605B2 - Duplex stainless steel - Google Patents
Duplex stainless steel Download PDFInfo
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- JP4265605B2 JP4265605B2 JP2005511147A JP2005511147A JP4265605B2 JP 4265605 B2 JP4265605 B2 JP 4265605B2 JP 2005511147 A JP2005511147 A JP 2005511147A JP 2005511147 A JP2005511147 A JP 2005511147A JP 4265605 B2 JP4265605 B2 JP 4265605B2
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- 229910001039 duplex stainless steel Inorganic materials 0.000 title claims description 37
- 239000000203 mixture Substances 0.000 claims description 21
- 229910052750 molybdenum Inorganic materials 0.000 claims description 17
- 239000000126 substance Substances 0.000 claims description 13
- 229910052804 chromium Inorganic materials 0.000 claims description 11
- 239000012535 impurity Substances 0.000 claims description 10
- 229910052759 nickel Inorganic materials 0.000 claims description 10
- 229910052757 nitrogen Inorganic materials 0.000 claims description 7
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 5
- 229910052802 copper Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 229910052782 aluminium Inorganic materials 0.000 claims description 3
- 229910052742 iron Inorganic materials 0.000 claims description 3
- 229910052717 sulfur Inorganic materials 0.000 claims description 3
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 229910052698 phosphorus Inorganic materials 0.000 claims description 2
- 229910052721 tungsten Inorganic materials 0.000 claims description 2
- 229910052799 carbon Inorganic materials 0.000 claims 1
- 238000005260 corrosion Methods 0.000 description 63
- 230000007797 corrosion Effects 0.000 description 62
- 229910000831 Steel Inorganic materials 0.000 description 28
- 239000010959 steel Substances 0.000 description 28
- 238000001556 precipitation Methods 0.000 description 20
- 238000003466 welding Methods 0.000 description 19
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 13
- 230000006911 nucleation Effects 0.000 description 13
- 238000010899 nucleation Methods 0.000 description 13
- 229910000859 α-Fe Inorganic materials 0.000 description 13
- 229910001566 austenite Inorganic materials 0.000 description 12
- 230000000694 effects Effects 0.000 description 12
- 230000015572 biosynthetic process Effects 0.000 description 11
- 150000004767 nitrides Chemical class 0.000 description 11
- 229910000765 intermetallic Inorganic materials 0.000 description 7
- 238000002844 melting Methods 0.000 description 7
- 230000008018 melting Effects 0.000 description 7
- 229910001220 stainless steel Inorganic materials 0.000 description 7
- 229910052791 calcium Inorganic materials 0.000 description 6
- 229910052749 magnesium Inorganic materials 0.000 description 6
- 239000002893 slag Substances 0.000 description 6
- 239000002253 acid Substances 0.000 description 5
- 230000007423 decrease Effects 0.000 description 5
- 239000010935 stainless steel Substances 0.000 description 5
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 4
- 239000000956 alloy Substances 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 238000000034 method Methods 0.000 description 4
- 229910052760 oxygen Inorganic materials 0.000 description 4
- 239000002244 precipitate Substances 0.000 description 4
- 238000005096 rolling process Methods 0.000 description 4
- 239000013535 sea water Substances 0.000 description 4
- 238000005266 casting Methods 0.000 description 3
- 238000001816 cooling Methods 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 239000007787 solid Substances 0.000 description 3
- 230000001629 suppression Effects 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- 150000007513 acids Chemical class 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 239000010953 base metal Substances 0.000 description 2
- PMHQVHHXPFUNSP-UHFFFAOYSA-M copper(1+);methylsulfanylmethane;bromide Chemical compound Br[Cu].CSC PMHQVHHXPFUNSP-UHFFFAOYSA-M 0.000 description 2
- 238000006477 desulfuration reaction Methods 0.000 description 2
- 230000023556 desulfurization Effects 0.000 description 2
- 238000002149 energy-dispersive X-ray emission spectroscopy Methods 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 239000000047 product Substances 0.000 description 2
- 238000009991 scouring Methods 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- 229910052684 Cerium Inorganic materials 0.000 description 1
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 1
- 229910004298 SiO 2 Inorganic materials 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 230000004913 activation Effects 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 229910052784 alkaline earth metal Inorganic materials 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000004453 electron probe microanalysis Methods 0.000 description 1
- 238000010828 elution Methods 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 238000001125 extrusion Methods 0.000 description 1
- 238000005242 forging Methods 0.000 description 1
- 230000005484 gravity Effects 0.000 description 1
- 229910000037 hydrogen sulfide Inorganic materials 0.000 description 1
- 238000010191 image analysis Methods 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 238000003754 machining Methods 0.000 description 1
- 210000001161 mammalian embryo Anatomy 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 238000004663 powder metallurgy Methods 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 150000002910 rare earth metals Chemical class 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000002407 reforming Methods 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 238000005245 sintering Methods 0.000 description 1
- 238000003756 stirring Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 150000003568 thioethers Chemical class 0.000 description 1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Treatment Of Steel In Its Molten State (AREA)
- Heat Treatment Of Steel (AREA)
Description
この発明は、二相ステンレス鋼、特に優れた溶接性および耐孔食性を有する二相ステンレス鋼に関する。 The present invention relates to a duplex stainless steel, particularly a duplex stainless steel having excellent weldability and pitting corrosion resistance.
二相ステンレス鋼は、強度および耐食性、特に耐海水腐食性に優れているため熱交換器用鋼管等として古くから広範囲の技術分野で使用されている。従来にあっても、耐食性、強度、加工性等を改善した二相ステンレス鋼についてはすでに多くの組成例が提案されている。
例えば、特開平５−１３２７４１号公報には、Ｗを１．５質量％〜５質量％を含みＰＲＥＷ［ＰＲＥＷ＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎ］が４０以上である高強度二相ステンレス鋼が開示されており、これにはＷの多量添加により耐食性が飛躍的に向上し、しかも金属間化合物（シグマ相等）の析出による機械的性質、耐食性の劣化が小さいことが示されている。Duplex stainless steel has been used in a wide range of technical fields for a long time as a steel pipe for heat exchangers because it is excellent in strength and corrosion resistance, particularly seawater corrosion resistance. Even in the past, many composition examples have already been proposed for duplex stainless steels with improved corrosion resistance, strength, workability, and the like.
For example, Japanese Patent Laid-Open No. 5-137274 discloses a high-strength duplex stainless steel containing W in an amount of 1.5 mass% to 5 mass% and PREW [PREW = Cr + 3.3 (Mo + 0.5 W) + 16N] of 40 or more. It is shown that the corrosion resistance is drastically improved by adding a large amount of W, and that mechanical properties and corrosion resistance are not significantly deteriorated due to precipitation of an intermetallic compound (sigma phase or the like).
しかしながら、今日のように、各種溶接構造物が広く利用されるようになり、例えば、温度の高い海水環境で使用される熱交換器、ポンプ等へ溶接施行で二相ステンレス鋼が用いられる場合、耐食性、特に耐孔食性が問題となってきた。溶接熱影響部に生成する微細なシグマ相が孔食の起点や金属疲労の起点となることがわかってきたことから、二相ステンレス鋼においてそのようなシグマ相の生成防止の必要性が認識されてきた。
このような微細なシグマ相の生成抑制には、まず、溶接入熱量を低減するなど溶接施工法を改善することが考えられる。しかし、溶接入熱量の低減は確かに有効であるが、入熱量を低減すると、溶接施工能率を低下させるため、今日のようにコスト低減が強く求められる状況下からは、好ましい解決手段とは言えない。
したがって、二相ステンレス鋼それ自体を改善することが求められる。
ここに、本発明の課題は、優れた耐孔食性および溶接性を有する二相ステンレス鋼、特に溶接熱影響部においても微細なシグマ相などの金属間化合物が生成しない、優れた耐孔食性および溶接性を有する二相ステンレス鋼を提供することにある。
本発明者は、上述のような課題を達成すべく、種々検討を重ねた結果、次のような知見を得た。
すなわち、溶接熱影響部においても優れた耐食性、特に耐孔食性を得るポイントは、次の２点にある。
１）溶接熱影響部でのシグマ相と呼ばれる金属間化合物の生成抑制、および２）溶接熱影響部での粗大な析出物である窒化物の生成抑制である。
ここに、溶接のような短時間での急速加熱・急速冷却を行って得た組織（以下、単に「急熱急冷組織」という）では、シグマ相の生成は、シグマ相の核生成と核の成長とにより左右される。本発明者らはシグマ相の核生成はＷを２％程度添加することにより抑えられること、またその条件下ではＮｉ、Ｍｏ量にも依存することを知見した。一方では、Ｎｉ、Ｍｏは、耐隙間腐食性、耐孔食性等の一般的な耐食性を確保するのに必須の元素である。
さらに本発明者らは下記（１）式に示すように各元素の影響度を考慮したシグマ相の核生成抑制条件を定量的に明らかにした。
Ｍｏ＋１．１Ｎｉ≦１２．５ （１）
上記式（１）の冶金学的意味は以下の通りである。
すなわち、シグマ相はＣｒとＦｅがほぼ１：１の組成の金属間化合物であるため、溶接等の加熱でシグマ相の核が生じるには、Ｃｒの濃化が必要となる。Ｍｏは必ずしもシグマ相の主要構成元素ではない。しかし、Ｍｏが存在することで核生成のための活性化エネルギーが低くなり、より小さなエンブリオ（核の萌芽）であっても消滅することなく、安定な核となる。一方、Ｎｉはシグマ相析出温度では、フェライト相を不安定にするため、その結果として、フェライト相がシグマ相とオーステナイト相に分解する反応の駆動力を高める。
このように、Ｍｏ、Ｎｉはシグマ相の核生成ポテンシャルを高め、その寄与度は本発明者らの研究により、ＮｉはＭｏの１．１倍である。かかる知見により、（１）式の左辺を求めた。（１）式の左辺は、核生成頻度の相対的な大きさを記述するパラメータとなっている。
本発明によれば、このパラメータを１２．５以下となるようにＮｉ、Ｍｏ含有量を規定することで、シグマ相の生成を耐孔食性に影響しない程度にまで抑えることができる。
ところで、一方、シグマ相の核生成は、母材における酸化物系介在物の存在によっても影響を大きく受ける。シグマ相は、鋼の融点より４００℃以上低い温度範囲の７００℃〜１０００℃に加熱された低温ＨＡＺで析出しやすい。ここに、鋼の融点直下まで加熱される部分を高温ＨＡＺと呼ぶことに対して相対的に低温に加熱されたＨＡＺを低温ＨＡＺという。低温ＨＡＺの温度域では、オーステナイト相の形態そのものは変化しないため、シグマ相の核生成は母材における介在物の存在の影響を大きく受ける。すなわち、介在物と鋼マトリックスとの境界では自由エネルギーが高いため、析出によってエネルギーが下がる核生成は生じやすい。
これらを総合的に検討した結果、Ａｌ、Ｍｇ、Ｃａを含む酸化物系介在物、特にＡｌ含有介在物が特に界面エネルギーが高く、ある大きさ以上のそれらの粗大介在物がシグマ相析出を促す有害介在物であり、その密度を低減することがＨＡＺでのシグマ相の析出抑制に有効であることを知見した。
図１は、ＨＡＺにおけるＡｌを２０質量％以上含み長径が５μｍ以上の粗大介在物の密度と孔食発生温度との関係を示したものである。ここで、孔食発生温度が高いものほど、通常の使用環境下の温度（すなわち常温）との温度差があることを意味するため、高い孔食発生温度を有する鋼は耐孔食性が高いといえる。従来の二相ステンレス鋼では、このようなアルミナ系粗大介在物は１平方ｍｍ当たり２０個以上存在した。
図１に示す結果より、アルミナ系粗大介在物の密度が１０個／ｍｍ２以下の鋼は高い耐食性を示すが、１０個／ｍｍ２超えると孔食発生温度は急激に低下する。
一方、（１）式からはＭｏ、Ｎｉ量を低減すれば、ＨＡＺでのシグマ相の核生成が抑えられ、シグマ相の不存在による良好な耐孔食性が得られるはずであるが、過度のＮｉ量の低減は、融点直下まで加熱される高温ＨＡＺにおいて窒化物の生成を助長する。そのような窒化物の生成はシグマ相の生成と同様に孔食の発生をもたらす。
本発明によれば、これを抑える要件は（２）式に示す定量式で示される。
Ｍｏ−０．８Ｎｉ≦−１．６ （２）
窒化物の析出駆動力は、Ｎが短時間で拡散しうる５００℃以上の温度域での母材におけるＮの固溶度と拡散速度とに左右される。Ｎｉの添加は、フェライト相のみとなる融点直下に加熱された状態から冷却される過程で析出するオーステナイト相の析出開始温度を高める。高温でオーステナイト相が析出することは、過飽和に存在するフェライト相中のＮが、より短時間でＮの固溶度の高いオーステナイト相側に移動することを意味する。このことはさらに、オーステナイト相の成長を促し、冷却の進行とともに高まるフェライト相中のＮの過飽和度の緩和に、有効に寄与する。その結果として、窒化物の析出を抑制するのである。
ただし、Ｍｏが存在すると逆に、Ｍｏがオーステナイト相の析出開始温度を低下させる。本発明者らの研究の結果によれば、それに対するＭｏの寄与度は、Ｎｉの０．８倍であり、かかる知見により（２）式の左辺を求めた。（２）式の左辺は、オーステナイト相生成温度の変化によるフェライト相中のＮ過飽和度の相対的な大きさを記述するパラメータとなっている。
本発明によれば、このパラメータを−１．６以下とすることで、窒化物の生成を抑制すれば、それに起因する孔食の発生をほぼ完全に抑えることができる。
以上のような各知見に基づいて、上記（１）、（２）式を満たすように成分設計し、酸化物系介在物の制御をすることで、溶接能率を落とすことなくＨＡＺで微細シグマ相、窒化物が生じない、ＨＡＺでも耐食性、特に耐孔食性の優れた二相ステンレス鋼が得られることを見出した。
このような母材における酸化物系介在物の制御には、従来とは異なる新しい方法が必要で、溶製の際のスラグの塩基度および脱硫回数、取鍋でのキリング温度と時間、鋳造後のトータルの加工度を最適に組み合わせることによりその制御が可能となる。
なお、本発明においてもＰＲＥＷは４０以上とする。
ここに本発明は次の通りである。
（１）質量％で、
Ｃ：０．０３％以下、Ｓｉ：１．０％以下、Ｍｎ：１．５％以下、Ｐ：０．０４０％以下、
Ｓ：０．００８％以下、Ｃｒ：２３．０〜２７．０％、Ｍｏ：２．０〜４．０％、Ｎｉ：５．０〜９．０％、
Ｗ：１．５％超５．０％以下、Ｎ：０．２４〜０．３５％、Ｆｅおよび不純物：残部
かつ、ＰＲＥＷ＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎが４０以上で
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、Ａｌを２０％以上含み長径が５μｍ以上の介在物と定義される粗大介在物が断面観察で１平方ｍｍ当たり１０個以下であることを特徴とする二相ステンレス鋼。
（２）前記化学組成が、更に０．２〜２．０質量％のＣｕと０．０５〜１．５質量％のＶの一方または両方を含む、上記（１）に記載の二相ステンレス鋼。
（３）前記化学組成が、更に０．０００５〜０．００５質量％のＢおよび０．０００５〜０．２質量％の希土類元素の中の１種または２種以上を含む、上記（１）または（２）に記載の二相ステンレス鋼。
（４）前記化学組成が、さらにｓｏｌ．Ａｌ：０．０４０％以下を含む上記（１）〜（３）のいずれかに記載の二相ステンレス鋼。However, as today, various welded structures are widely used. For example, when duplex stainless steel is used for welding to heat exchangers and pumps used in high temperature seawater environments, Corrosion resistance, particularly pitting corrosion resistance, has become a problem. Since it has been found that the fine sigma phase generated in the heat affected zone is the starting point of pitting corrosion and the starting point of metal fatigue, the need to prevent such sigma phase generation in duplex stainless steel has been recognized. I came.
In order to suppress the formation of such a fine sigma phase, it is conceivable to first improve the welding method such as reducing the amount of heat input to welding. However, reduction of the welding heat input is certainly effective, but if the heat input is reduced, the welding work efficiency is lowered, so it can be said that it is a preferable solution from the situation where cost reduction is strongly demanded like today. Absent.
Therefore, it is required to improve the duplex stainless steel itself.
Here, the problem of the present invention is that the duplex stainless steel having excellent pitting corrosion resistance and weldability, particularly excellent pitting corrosion resistance in which an intermetallic compound such as a fine sigma phase is not generated even in the heat affected zone of welding, and The object is to provide a duplex stainless steel having weldability.
As a result of various studies to achieve the above-described problems, the present inventor has obtained the following knowledge.
That is, there are the following two points for obtaining excellent corrosion resistance, particularly pitting corrosion resistance even in the weld heat affected zone.
1) Suppression of the formation of intermetallic compounds called sigma phases in the weld heat affected zone, and 2) Suppression of the formation of nitrides as coarse precipitates in the weld heat affected zone.
Here, in the structure obtained by performing rapid heating / cooling in a short time such as welding (hereinafter simply referred to as “rapidly quenched structure”), the sigma phase is generated by the nucleation of the sigma phase and the nucleation of the nuclei. It depends on growth. The present inventors have found that the nucleation of the sigma phase can be suppressed by adding about 2% of W, and that it also depends on the amounts of Ni and Mo under the conditions. On the other hand, Ni and Mo are elements essential for ensuring general corrosion resistance such as crevice corrosion resistance and pitting corrosion resistance.
Furthermore, the present inventors quantitatively clarified the sigma phase nucleation suppression conditions considering the influence of each element as shown in the following formula (1).
Mo + 1.1Ni ≦ 12.5 (1)
The metallurgical meaning of the above formula (1) is as follows.
That is, since the sigma phase is an intermetallic compound having a composition of Cr and Fe of approximately 1: 1, Cr concentration is necessary to produce sigma phase nuclei by heating such as welding. Mo is not necessarily the main constituent element of the sigma phase. However, the presence of Mo lowers the activation energy for nucleation, and even a smaller Embryo (nuclear sprouting) does not disappear and becomes a stable nucleus. On the other hand, Ni makes the ferrite phase unstable at the sigma phase precipitation temperature. As a result, the driving force of the reaction in which the ferrite phase decomposes into the sigma phase and the austenite phase is increased.
As described above, Mo and Ni increase the nucleation potential of the sigma phase, and the contribution thereof is 1.1 times that of Mo according to the study by the present inventors. Based on this finding, the left side of equation (1) was determined. The left side of equation (1) is a parameter that describes the relative magnitude of the nucleation frequency.
According to the present invention, by defining the contents of Ni and Mo so that this parameter is 12.5 or less, generation of sigma phase can be suppressed to a level that does not affect pitting corrosion resistance.
On the other hand, the nucleation of the sigma phase is greatly affected by the presence of oxide inclusions in the base material. The sigma phase is likely to precipitate at a low temperature HAZ heated to 700 ° C. to 1000 ° C. in a temperature range 400 ° C. or more lower than the melting point of steel. Here, the part heated to just below the melting point of steel is called high-temperature HAZ, whereas HAZ heated to a relatively low temperature is called low-temperature HAZ. Since the austenite phase itself does not change in the temperature range of the low temperature HAZ, the nucleation of the sigma phase is greatly influenced by the presence of inclusions in the base material. That is, since the free energy is high at the boundary between the inclusion and the steel matrix, nucleation in which the energy decreases due to precipitation is likely to occur.
As a result of comprehensively examining these, oxide-based inclusions containing Al, Mg, and Ca, particularly Al-containing inclusions have particularly high interfacial energy, and those coarse inclusions of a certain size or more promote sigma phase precipitation. It was found that it is a harmful inclusion and reducing its density is effective in suppressing precipitation of sigma phase in HAZ.
FIG. 1 shows the relationship between the density of coarse inclusions containing 20 mass% or more of Al in HAZ and having a major axis of 5 μm or more and the pitting corrosion occurrence temperature. Here, the higher the pitting corrosion occurrence temperature, the higher the temperature of the normal use environment (that is, normal temperature) means that there is a temperature difference. I can say that. In the conventional duplex stainless steel, there are 20 or more such alumina-based coarse inclusions per square mm.
From the results shown in FIG. 1, the steel having a density of coarse alumina inclusions of 10 pieces / mm 2 or less shows high corrosion resistance, but if it exceeds 10 pieces / mm 2 , the pitting corrosion temperature rapidly decreases.
On the other hand, if the amount of Mo and Ni is reduced from the formula (1), nucleation of the sigma phase in the HAZ should be suppressed, and good pitting corrosion resistance due to the absence of the sigma phase should be obtained. The reduction of the Ni content promotes the formation of nitrides in the high-temperature HAZ heated to just below the melting point. Such nitride formation leads to the occurrence of pitting corrosion as well as sigma phase formation.
According to the present invention, the requirement to suppress this is shown by the quantitative formula shown in formula (2).
Mo-0.8Ni ≦ -1.6 (2)
The driving force for precipitation of nitride depends on the solid solubility and diffusion rate of N in the base material in a temperature range of 500 ° C. or higher where N can diffuse in a short time. The addition of Ni increases the precipitation start temperature of the austenite phase that precipitates in the process of cooling from the state of being heated just below the melting point, which is only the ferrite phase. The precipitation of the austenite phase at a high temperature means that N in the supersaturated ferrite phase moves to the austenite phase side where the solid solubility of N is higher in a shorter time. This further promotes the growth of the austenite phase and effectively contributes to the relaxation of the degree of supersaturation of N in the ferrite phase, which increases with the progress of cooling. As a result, the precipitation of nitride is suppressed.
However, when Mo is present, Mo lowers the precipitation start temperature of the austenite phase. According to the results of the study by the present inventors, the contribution of Mo to that is 0.8 times that of Ni, and the left side of the equation (2) was obtained based on this finding. The left side of equation (2) is a parameter that describes the relative magnitude of N supersaturation in the ferrite phase due to changes in the austenite phase formation temperature.
According to the present invention, by setting this parameter to −1.6 or less, if the formation of nitrides is suppressed, the occurrence of pitting corrosion resulting therefrom can be suppressed almost completely.
Based on the above findings, the components are designed so as to satisfy the above formulas (1) and (2), and the oxide inclusions are controlled, so that the fine sigma phase can be obtained in HAZ without reducing the welding efficiency. It has been found that a duplex stainless steel excellent in corrosion resistance, particularly pitting corrosion resistance can be obtained even in HAZ, in which no nitride is formed.
Control of such oxide inclusions in the base metal requires a new method different from the conventional ones. Basicity and desulfurization frequency of slag during melting, killing temperature and time in ladle, after casting The total processing degree can be controlled by optimally combining them.
In the present invention, PREW is 40 or more.
Here, the present invention is as follows.
(1) In mass%,
C: 0.03% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.040% or less,
S: 0.008% or less, Cr: 23.0 to 27.0%, Mo: 2.0 to 4.0%, Ni: 5.0 to 9.0%,
W: more than 1.5% and 5.0% or less, N: 0.24 to 0.35%, Fe and impurities: remainder, and PREW = Cr + 3.3 (Mo + 0.5W) + 16N is 40 or more and Mo + 1.1Ni ≦ 12.5
Mo-0.8Ni ≦ −1.6
A two-phase characterized in that the number of coarse inclusions defined as inclusions having a chemical composition satisfying the following relationship and containing 20% or more of Al and having a major axis of 5 μm or more is 10 or less per square mm in cross-sectional observation Stainless steel.
(2) The duplex stainless steel according to (1), wherein the chemical composition further includes one or both of 0.2 to 2.0 mass% Cu and 0.05 to 1.5 mass% V. .
(3) The above (1) or wherein the chemical composition further comprises one or more of 0.0005 to 0.005 mass% B and 0.0005 to 0.2 mass% rare earth element The duplex stainless steel according to (2).
(4) The chemical composition is further sol. Al: The duplex stainless steel according to any one of (1) to (3), including 0.040% or less.
図１は、Ａｌ２０％以上含む長径５μｍ以上の酸化物系介在物の密度と孔食発生温度との関係を示すグラフである。
図２は、酸化物系介在物の長径および組成の測定箇所を定義する酸化物系介在物の模式図である。FIG. 1 is a graph showing the relationship between the density of oxide inclusions having a major axis of 5 μm or more containing 20% or more of Al and the pitting corrosion occurrence temperature.
FIG. 2 is a schematic diagram of an oxide inclusion that defines the measurement points of the major axis and composition of the oxide inclusion.
次に、本発明において二相ステンレス鋼の化学組成を上述のように限定した理由について説明するが、本明細書において、鋼および介在物の化学組成を示す「％」は、とくにことわりがない限り、「質量％」を意味する。
本発明にかかる二相ステンレス鋼は、上記の多種類の合金成分の総合的な効果と組織形態の制御によって優れた溶接性（溶接能率を低下させずに耐孔食性確保）、その他の特性を発揮するが、最も大きな特徴は、Ｎｉ、Ｍｏ量の組み合わせの適正化と、アルミナ系粗大介在物の制御にある。
Ｃ：Ｃは、後述するＮと同様にオーステナイト相を安定化するのに有効であるが、その含有量が０．０３％を超えると炭化物が析出しやすくなり、耐食性が劣化するため０．０３％以下とする。好ましくは０．０２％以下である。本発明においてＣは不純物として含有する場合も包含する。
Ｓｉ：Ｓｉは鋼の脱酸成分として有効であるが、金属間化合物（シグマ相等）の生成を促進する元素であるから本発明では１．０％以下に限定する。好ましくは０．５％以下である。本発明においてＳｉは不純物として含有する場合も包含する。
Ｍｎ：Ｍｎは二相ステンレス鋼の溶製時の脱硫および脱酸効果によって熱間加工性を向上させる。また、Ｎの溶解度を大きくする作用もある。これらの効果を狙って通常はその含有量を２．０％までとすることが多い。しかし、Ｍｎは耐食性を劣化させる元素でもあるため、本発明では１．５％以下と定めた。好ましくは１．０％以下である。本発明においてＭｎは不純物として含有する場合も包含する。
Ｐ：Ｐは鋼中に不可避的に混入する不純物元素であるが、その含有量が０．０４０％を超えると耐食性、靱性の劣化が著しくなるから０．０４０％を上限とする。
Ｓ：Ｓも鋼中に不可避的に混入する不純物元素で、鋼の熱間加工性を劣化させる。また、硫化物は孔食の発生起点となり耐孔食性を損なう。これらの悪影響を避けるため、その含有量を０．００８％以下に抑える。これ以下でできるだけ少ない方がよく、特に０．００５％以下が望ましい。
Ｃｒ：Ｃｒは耐食性を維持するために有効な基本成分である。その含有量が２３．０％未満では、いわゆるスーパ二相ステンレス鋼と言えるだけの耐食性が母材で得られない。一方、Ｃｒの含有量が２７．０％を超えると金属間化合物（シグマ相等）の析出が顕著になり、熱間加工性の低下および溶接性の劣化を招く。
Ｍｏ：ＭｏはＣｒと同様にＰＲＥＷの向上に寄与し、耐食性を向上させるのに非常に有効な成分である。特に耐孔食性および耐隙間腐食性を高めるため、本発明ではその含有量を２．０％以上とする。一方、Ｍｏの過剰添加は製造中の素材の脆化の原因になり、Ｃｒと同様に金属間化合物の析出を容易にする作用が強い。従って、Ｍｏの含有量は４．０％までにとどめる。
Ｎｉ：Ｎｉはオーステナイトを安定化するために必須の成分であるが、その含有量が９．０％を超えるとフェライト量の減少により二相ステンレス鋼の基本的な性質が確保しにくくなり、またシグマ相等の析出が容易になる。一方、Ｎｉの含有量が５．０％より少ないとフェライト量が多くなり過ぎて同じく二相ステンレス鋼の特徴が失われる。また、フェライト中へのＮの固溶度が小さいため窒化物が析出して耐食性が劣化する。
ただし、Ｎｉ、Ｍｏの範囲についてはこれらのみの規定では不十分で、前述の通り本発明の特徴である下記式（１）、（２）を満足するように制限される。
Ｍｏ＋１．１Ｎｉ≦１２．５ （１）
Ｍｏ−０．８Ｎｉ≦−１．６ （２）
ここに上記式における「Ｍｏ」および「Ｎｉ」はそれぞれの含有量（質量％）を表わす。
（Ｍｏ＋１．１Ｎｉ）の値が１２．５を超えると、低温ＨＡＺでの微量シグマ相析出が、そして（Ｍｏ−０．８Ｎｉ）の値が−１．６を超えると高温ＨＡＺでの窒化物析出が、それぞれ生じるため、上記範囲内に抑えるのである。
二相ステンレス鋼の耐食性、特に耐海水腐食性を表すパラメーターとして下記の耐孔食性指数（ＰＲＥＷ）を４０以上とする。
ＰＲＥＷ（Ｐｉｔｔｉｎｇ Ｒｅｓｉｓｔａｎｃｅ Ｅｑｕｉｖａｌｅｎｔ Ａｆｔｅｒ Ｗｅｌｄｉｎｇ）＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎ
一般には、このＰＲＥＷが３５以上となるようにＣｒ、Ｍｏ、Ｎの含有量を調整するのであるが、本発明にかかるスーパ二相ステンレス鋼では、Ｃｒ、Ｍｏ、Ｎをさらに高めてＰＲＥＷを４０以上としたもので、著しく優れた耐海水腐食性を示す。Ｃｒ、Ｍｏ、Ｎの増加は鋼の高強度化にも寄与するから、元来、フェライトあるいはオーステナイト単相の鋼に比較して高強度である二相ステンレス鋼が、さらに高強度化されたスーパ二相ステンレス鋼が得られる。
ＷはＭｏと同様に耐食性、特に孔食および隙間腐食への抵抗性を向上させる元素であり、就中、ｐＨの低い環境で耐食性を向上させる安定な酸化物を形成する元素である。したがって、１．５％を超えるＷを含有させる。１．５％以下では、ＰＲＥＷを４０以上とするのに、Ｃｒ、Ｍｏ、Ｎ等の添加を増さなければならず、Ｗを利用する効果が小さくなる。Ｗ含有量を増すほどＰＲＥＷを４０以上とするためのＣｒ、Ｍｏの含有量を少なくすることができ、これらの元素のシグマ相等の生成促進の害を小さくできる。望ましいＷの含有量は、２．０％を超える量である。しかし、５．０％を超える量のＷを添加してもそれに見合うだけの効果の増大はなく、徒にコストが嵩むだけであるから上限は５．０％とする。
Ｎ（窒素）：Ｎは強力なオーステナイト生成元素で、二相ステンレス鋼の熱的安定性と耐食性の向上に有効である。本発明鋼のようにフェライト生成元素であるＣｒ、Ｍｏが多量に添加される場合には、フェライトとオーステナイトの二相のバランスを適正なものにするためにも０．２４％以上のＮを含有させる。
さらにＮは、ＰＲＥＷの向上に寄与してＣｒ、ＭｏおよびＷと同様に合金の耐食性を向上させる。しかし、本発明鋼のような２５％Ｃｒ系の二相ステンレス鋼では、Ｎを０．３５％を超えて含有させようとするとブローホールの発生による欠陥、あるいは溶接の際の熱影響による窒化物生成等により鋼の靱性、耐食性を劣化させる。そのためＮの上限は０．３５％とする。
ｓｏｌ．Ａｌ：Ａｌは鋼の脱酸剤として有効であるが、鋼中のＮ量が高い場合にはＡｌＮ（窒化アルミニウム）として析出し、靱性および耐食性を劣化させる。さらには、酸化物を形成し、シグマ相の核生成サイトとなる。従って、本発明ではＡｌ含有量をｓｏｌ．Ａｌとして０．０４０％以下に抑える。本発明鋼ではＳｉの多量添加は避けているので、脱酸剤としてＡｌを用いることが多いが、真空溶解を行う場合には必ずしもＡｌの添加を要しない。
本発明の二相ステンレス鋼は、以上のような成分に加えて、さらに下記の第１群および第２群の元素のうちの１種以上を必要に応じて含むことができる。
第１群元素（Ｃｕ、Ｖ）：ＣｕとＶは、本発明の二相ステンレス鋼においては少なくとも１種含有され、耐食性、特に硫酸等の酸に対する耐酸性を向上させるという点で均等な作用効果をもつ。
Ｃｕは、還元性の低ｐＨ環境、例えばＨ２ＳＯ４あるいは硫化水素環境での耐酸性向上に特に有効で、その効果を得るためには０．２％以上の含有量とする。しかし、Ｃｕの多量添加は鋼の熱間加工性を劣化させるから上限を２．０％とする。
Ｖは、０．０５％以上添加することで硫酸等の酸に対する耐酸性を向上させ、特にＷと複合添加した場合、耐隙間腐食性をも向上させる。しかし、Ｖの添加が過多になるとフェライト量が過度に増加し、靱性および耐食性の低下が生じるからその上限を１．５％とする。
第２群元素（Ｂおよび希土類元素）：いずれもＳあるいはＯ（酸素）を固定し熱間加工性を向上させる元素である。
本発明鋼ではＳを低く抑えており、Ｗを多量添加しているとはいえ、これはシグマ相等の生成を促進しないから、元来熱間加工性は良好である。
また、本発明の二相ステンレス鋼は、鋳物として使用することが可能であり、更に、粉末にしてプレス、焼結等の粉末冶金法で管等の製品にすることも可能である。
このような製造方法をとる場合には、熱間加工性はさして問題にならない。従って、第２群元素の添加は必ずしも必要でない。しかし、鍛造、圧延、押出し等の工程を経て製品にする場合に熱間加工性が優れていることは望ましいので、このような場合、必要に応じて、Ｂ：０．０００５％以上、Ｌａ、Ｃｅ等の希土類：それぞれ０．０００５％以上１種または２種以上の添加を行えばよい。ただし、これらの元素も多量に添加されるとそれらの酸化物、硫化物の非金属介在物が増加し、シグマ相の析出核生成サイトとなったり、孔食の起点となり耐食性の劣化を招く。従って、含有量としてＢは０．００５％以下、希土類（主に、Ｌａ、Ｃｅ）はそれぞれ０．２％以下とするのがよい。
なお、これらＢおよび希土類元素の下限値の合計量はいずれも不純物元素であるＳとＯの算術和（Ｓ＋１／２・Ｏ）の値以上とすることが推奨される。
次に、本発明にあっては、下記に定義される粗大介在物、特にアルミナ系粗大介在物が断面観察で１平方ｍｍ当たり１０個以下に制限される。
ここで粗大介在物は、「当該介在物にもしＡｌとともにＣａおよび／またはＭｇが不純物として含まれるときは、ＣａとＭｇを含めて質量％の和で２０％以上含み長径が５μｍ以上の介在物」と定義とする。その理由は、ＡｌとＣａとＭｇを質量％の和で２０％以上含む介在物は、結晶格子の母相（フェライト相）とのずれが大きくなり界面エネルギーを高めるためである。なお、本明細書ではこのような粗大介在物を便宜上「Ａｌを２０質量％以上含み長径が５μｍ以上の介在物」と記述する。
本発明にかかる二相ステンレス鋼における粗大介在物は酸化物系介在物、特にアルミナ系介在物が主要なもので、本明細書においては粗大介在物を便宜上アルミナ系粗大介在物とも云う。
長径が５μｍ未満では、母相と介在物との界面の面積そのものが十分大きいため、界面がシグマ相の析出サイトとなる確率が小さくなる。
介在物の長径とは、図２（ａ）、（ｂ）に示すように、母材と介在物１との界面上の異なる二点を結んだ直線のうち、最も長くなる直線の長さを意味する。図２（ａ）、（ｂ）ではそれぞれａ１またはａ２となる。また、酸化物系介在物の組成は、介在物１の中心部近傍（図２（ａ）、（ｂ）に示す例ではそれぞれｂ１およびｂ２）、即ち、介在物１の断面形状の重心部近傍をＥＤＸ（エネルギー分散型Ｘ線分析）を用いて、Ｏ（酸素）以外の合金元素の含有量を求め、これにより決定する。よって、本明細書において、「Ａｌを２０質量％以上含み」とは、Ｏ以外の合金元素に占めるＡｌ（＋Ｃａ＋Ｍｇ）含有率を意味する。
実用上はこれらのアルミナ系粗大介在物の密度の影響が大きく、断面観察で１平方ｍｍ当たり１０個超あると粗大介在物と母相との界面においてばかりでなく、自由エネルギーの高いフェライト／オーステナイト界面上にも粗大介在物が存在してシグマ相の析出を助長する確率が高くなる。そのためそのような粗大介在物の存在はＨＡＺ部でのシグマ相析出に有害であり、密度をこれ以下にすることがＨＡＺ部でのシグマ相析出抑制に有効となる。
本発明においてアルミナ系粗大介在物の密度を上述のように１平方ｍｍ当たり１０個以下に限定する。
本発明にしたがってそのような二相ステンレス鋼を製造するには、例えば真空精錬による二次精錬を行い、その際のスラグ塩基度を例えば０．３〜３．０に調整し、十分な溶鋼撹拌およびスラグ改質を行えばよい。
本発明にかかる鋼組成においては介在物としては主としてアルミナ系介在物が生成し、一部Ｃａ、Ｍｇ等が不純物として混合しているときにはＣａおよびＭｇを含む介在物が存在する可能性がある。
ここに、本発明においてアルミナ系粗大介在物のＡｌ含有量を２０％以上、Ｃａ、Ｍｇ系介在物が混入しているときには、アルミナ系粗大介在物に加え、Ｃａ系粗大介在物およびＭｇ系粗大介在物の（Ａｌ＋Ｃａ＋Ｍｇ）の合計量を２０％以上に限定した理由は腐食環境での溶出を生じにくくすることで耐孔食性を確保するためである。なお、そのようなＭｇ系、Ｃａ系介在物も形態的には酸化物であって、アルミナ系介在物と複合化している。
次に実施例によって本発明の作用効果についてさらに具体的に説明する。Next, the reason why the chemical composition of the duplex stainless steel is limited as described above in the present invention will be described. In the present specification, “%” indicating the chemical composition of the steel and inclusions, unless otherwise specified. , “Mass%”.
The duplex stainless steel according to the present invention has excellent weldability (ensuring pitting corrosion resistance without reducing the welding efficiency) and other characteristics by controlling the overall effect and structure of the above-mentioned various types of alloy components. The most significant feature is in optimizing the combination of the amounts of Ni and Mo and controlling the alumina-based coarse inclusions.
C: C is effective for stabilizing the austenite phase, as is the case with N described later. However, if its content exceeds 0.03%, carbide is likely to precipitate, and the corrosion resistance deteriorates. % Or less. Preferably it is 0.02% or less. In the present invention, the case where C is contained as an impurity is also included.
Si: Si is effective as a deoxidizing component of steel, but is an element that promotes the formation of intermetallic compounds (such as sigma phase), so it is limited to 1.0% or less in the present invention. Preferably it is 0.5% or less. In the present invention, Si is included as an impurity.
Mn: Mn improves hot workability by desulfurization and deoxidation effects during the melting of duplex stainless steel. It also has the effect of increasing the solubility of N. Aiming at these effects, the content is usually up to 2.0% in many cases. However, since Mn is an element that deteriorates the corrosion resistance, it is set to 1.5% or less in the present invention. Preferably it is 1.0% or less. In the present invention, Mn is included as an impurity.
P: P is an impurity element inevitably mixed in the steel, but if its content exceeds 0.040%, the corrosion resistance and toughness deteriorate significantly, so 0.040% is made the upper limit.
S: S is an impurity element inevitably mixed in steel, and deteriorates the hot workability of steel. In addition, the sulfide becomes a starting point of pitting corrosion and impairs pitting corrosion resistance. In order to avoid these adverse effects, the content is limited to 0.008% or less. Less than this is preferable, and 0.005% or less is particularly desirable.
Cr: Cr is a basic component effective for maintaining corrosion resistance. When the content is less than 23.0%, the base metal cannot provide corrosion resistance enough to be called a so-called super duplex stainless steel. On the other hand, if the Cr content exceeds 27.0%, precipitation of intermetallic compounds (such as sigma phase) becomes prominent, resulting in a decrease in hot workability and weldability.
Mo: Mo, like Cr, contributes to the improvement of PREW and is a very effective component for improving the corrosion resistance. In particular, in order to improve pitting corrosion resistance and crevice corrosion resistance, the content is set to 2.0% or more in the present invention. On the other hand, excessive addition of Mo causes embrittlement of the raw material during manufacture, and has a strong effect of facilitating the precipitation of intermetallic compounds like Cr. Therefore, the Mo content is limited to 4.0%.
Ni: Ni is an essential component for stabilizing austenite, but if its content exceeds 9.0%, it becomes difficult to secure the basic properties of duplex stainless steel due to the decrease in ferrite content, Precipitation of sigma phase and the like is facilitated. On the other hand, if the Ni content is less than 5.0%, the amount of ferrite becomes too large and the characteristics of the duplex stainless steel are lost. In addition, since the solid solubility of N in ferrite is small, nitride precipitates and the corrosion resistance deteriorates.
However, with respect to the ranges of Ni and Mo, it is not sufficient to define only these, and as described above, they are limited so as to satisfy the following formulas (1) and (2) that are the features of the present invention.
Mo + 1.1Ni ≦ 12.5 (1)
Mo-0.8Ni ≦ -1.6 (2)
Here, “Mo” and “Ni” in the above formula represent respective contents (mass%).
When the value of (Mo + 1.1Ni) exceeds 12.5, a small amount of sigma phase precipitation at low temperature HAZ, and when the value of (Mo-0.8Ni) exceeds -1.6, nitride precipitation at high temperature HAZ Since each occurs, it is suppressed within the above range.
The following pitting corrosion resistance index (PREW) is set to 40 or more as a parameter representing the corrosion resistance of the duplex stainless steel, particularly the seawater corrosion resistance.
PREW (Pitting Resistance Equivalent After Welding) = Cr + 3.3 (Mo + 0.5W) + 16N
Generally, the content of Cr, Mo, and N is adjusted so that the PREW is 35 or more. However, in the super duplex stainless steel according to the present invention, the PRE, 40, and 40 are further increased by increasing Cr, Mo, and N. As described above, it exhibits extremely excellent seawater corrosion resistance. Since the increase in Cr, Mo, and N contributes to the strengthening of steel, duplex stainless steel, which is originally stronger than ferritic or austenitic single-phase steel, is a super-strength that is further strengthened. Duplex stainless steel is obtained.
W, like Mo, is an element that improves corrosion resistance, particularly resistance to pitting corrosion and crevice corrosion. In particular, W is an element that forms a stable oxide that improves corrosion resistance in a low pH environment. Therefore, W exceeding 1.5% is contained. If it is 1.5% or less, the addition of Cr, Mo, N, etc. must be increased in order to make PREW 40 or more, and the effect of using W becomes small. As the W content is increased, the content of Cr and Mo for increasing PREW to 40 or more can be reduced, and the harm of promoting the generation of sigma phase or the like of these elements can be reduced. A desirable W content is an amount exceeding 2.0%. However, even if an amount of W exceeding 5.0% is added, there is no increase in the effect commensurate with it, and the cost is simply increased, so the upper limit is made 5.0%.
N (nitrogen): N is a strong austenite-forming element and is effective in improving the thermal stability and corrosion resistance of the duplex stainless steel. When a large amount of Cr or Mo, which is a ferrite-forming element, is added as in the steel of the present invention, it contains 0.24% or more of N in order to properly balance the two phases of ferrite and austenite. Let
Further, N contributes to the improvement of PREW and improves the corrosion resistance of the alloy in the same manner as Cr, Mo and W. However, in a 25% Cr duplex stainless steel such as the steel of the present invention, if N exceeds 0.35%, a nitride due to defects due to blowholes or thermal effects during welding It deteriorates the toughness and corrosion resistance of steel due to formation. Therefore, the upper limit of N is set to 0.35%.
sol. Al: Al is effective as a deoxidizer for steel, but when the amount of N in the steel is high, it precipitates as AlN (aluminum nitride) and deteriorates toughness and corrosion resistance. Furthermore, an oxide is formed and becomes a nucleation site of a sigma phase. Therefore, in the present invention, the Al content is set to sol. Al is suppressed to 0.040% or less. Since a large amount of Si is avoided in the steel of the present invention, Al is often used as a deoxidizing agent. However, when vacuum melting is performed, it is not always necessary to add Al.
In addition to the above components, the duplex stainless steel of the present invention may further contain one or more of the following elements of the first group and the second group as necessary.
Group 1 elements (Cu, V): Cu and V are contained in the duplex stainless steel of the present invention in at least one kind, and are equally effective in improving corrosion resistance, particularly acid resistance against acids such as sulfuric acid. It has.
Cu is particularly effective for improving acid resistance in a reducing low pH environment such as H 2 SO 4 or a hydrogen sulfide environment, and in order to obtain the effect, the content is made 0.2% or more. However, addition of a large amount of Cu deteriorates the hot workability of steel, so the upper limit is made 2.0%.
V improves the acid resistance against acids such as sulfuric acid by adding 0.05% or more, and also improves the crevice corrosion resistance, especially when added in combination with W. However, when V is excessively added, the amount of ferrite is excessively increased, and the toughness and corrosion resistance are lowered. Therefore, the upper limit is made 1.5%.
Group 2 elements (B and rare earth elements): All are elements that fix S or O (oxygen) and improve hot workability.
In the steel of the present invention, S is kept low and a large amount of W is added, but this does not promote the formation of sigma phase or the like, so that the hot workability is originally good.
Further, the duplex stainless steel of the present invention can be used as a casting, and can be made into a product such as a tube by powder metallurgy such as pressing and sintering.
When such a manufacturing method is adopted, hot workability is not a problem. Therefore, the addition of the second group element is not always necessary. However, since it is desirable that the hot workability is excellent when making the product through processes such as forging, rolling, and extrusion, in such a case, if necessary, B: 0.0005% or more, La, Rare earth such as Ce: Addition of 0.0005% or more, one kind or two kinds or more, respectively. However, when these elements are also added in a large amount, their non-metallic inclusions of oxides and sulfides increase, resulting in sigma phase precipitation nucleation sites and pitting corrosion, leading to deterioration of corrosion resistance. Accordingly, the B content is preferably 0.005% or less, and the rare earths (mainly La and Ce) are each preferably 0.2% or less.
It is recommended that the total amount of the lower limit values of these B and rare earth elements is not less than the value of the arithmetic sum (S + 1/2 · O) of S and O, which are impurity elements.
Next, in the present invention, the coarse inclusions defined below, particularly alumina-based coarse inclusions, are limited to 10 or less per square mm by cross-sectional observation.
Here, coarse inclusions are “inclusions that contain Ca and / or Mg as well as Al as impurities when the inclusions contain 20% or more in terms of the sum of mass% including Ca and Mg, and the major axis is 5 μm or more. As a definition. The reason for this is that inclusions containing 20% or more of Al, Ca, and Mg in a sum of mass% increase the deviation of the crystal lattice from the parent phase (ferrite phase) and increase the interfacial energy. In this specification, such coarse inclusions are described as “inclusions containing 20% by mass or more of Al and having a major axis of 5 μm or more” for convenience.
The coarse inclusions in the duplex stainless steel according to the present invention are mainly oxide inclusions, particularly alumina inclusions. In the present specification, the coarse inclusions are also referred to as alumina coarse inclusions for convenience.
If the major axis is less than 5 μm, the area itself of the interface between the parent phase and the inclusion is sufficiently large, so the probability that the interface becomes a precipitation site for the sigma phase decreases.
As shown in FIGS. 2 (a) and 2 (b), the major axis of the inclusion means the length of the longest straight line among two straight lines connecting two different points on the interface between the base material and the inclusion 1. means. In FIG. 2 (a) and (b), it becomes a1 or a2, respectively. Further, the composition of the oxide inclusions is near the center of the inclusion 1 (b1 and b2 in the examples shown in FIGS. 2A and 2B, respectively), that is, near the center of gravity of the cross-sectional shape of the inclusion 1 Using EDX (energy dispersive X-ray analysis), the content of alloy elements other than O (oxygen) is obtained and determined. Therefore, in this specification, “containing 20 mass% or more of Al” means the Al (+ Ca + Mg) content in the alloy elements other than O.
In practice, the density of these alumina-based coarse inclusions is greatly affected. If the number of cross-sectional observations exceeds 10 per square mm, not only at the interface between the coarse inclusions and the parent phase, but also ferrite / austenite having a high free energy. There is a high probability that coarse inclusions exist on the interface and promote the precipitation of the sigma phase. Therefore, the presence of such coarse inclusions is detrimental to sigma phase precipitation in the HAZ part, and it is effective to suppress the sigma phase precipitation in the HAZ part if the density is lower than this.
In the present invention, the density of coarse alumina inclusions is limited to 10 or less per square mm as described above.
In order to produce such a duplex stainless steel according to the present invention, for example, secondary refining by vacuum refining is performed, the slag basicity at that time is adjusted to, for example, 0.3 to 3.0, and sufficient molten steel stirring is performed. And slag reforming.
In the steel composition according to the present invention, mainly alumina-based inclusions are generated as inclusions. When some Ca, Mg, etc. are mixed as impurities, inclusions containing Ca and Mg may exist.
Here, in the present invention, when the Al content of the alumina-based coarse inclusion is 20% or more and Ca and Mg-based inclusions are mixed, in addition to the alumina-based coarse inclusion, the Ca-based coarse inclusion and the Mg-based coarse inclusion The reason for limiting the total amount of inclusions (Al + Ca + Mg) to 20% or more is to ensure pitting corrosion resistance by making it difficult for elution in a corrosive environment. Note that such Mg-based and Ca-based inclusions are also oxides in form and are combined with alumina-based inclusions.
Next, the effects of the present invention will be described more specifically with reference to examples.
表１に示す化学組成の鋼を電気炉にて溶解し、ＡＯＤ炉に移して二次精錬を実施した。ただし、代符Ｂ６の場合は、二次精練を行わなかった。二次精練においては、スラグ中の（ＣａＯ＋ＭｇＯ）重量／スラグ中の（Ａｌ２Ｏ３＋ＳｉＯ２）重量で定義されるスラグ塩基度を−１から３の範囲の異なる値とすることで、介在物の組成、形態、密度の異なる溶鋼を作製した。鋳造後、１２００℃に加熱して鍛造により厚さ４０ｍｍの板材とした。
得られた板材を、１２５０℃に加熱し、圧延により、厚さ１０ｍｍとした。得られた鋼板の一部を切り出し、圧延面と直交する断面を上にして樹脂中に埋め込んだ後、この断面を鏡面研磨した。その後、粗大介在物を２００倍の倍率で５視野、ＳＥＭ観察を行って、そのサイズを評価した。
アルミナ系粗大介在物の長径は、図２の定義に従って測定し、粗大介在物の中心部近傍（図２のｂ１およびｂ２）をＥＰＭＡにより組成分析して、前述の粗大介在物を同定し、その密度を測定した。密度は１ｍｍ２当たりの粗大介在物の個数の５視野の平均値をもって評価した。
供試鋼板を機械加工により、厚さ８ｍｍ×幅１００ｍｍ×長さ２００ｍｍ、長辺の端部に開先角度３０度のＶ開先を設け試験材とした。代符Ａｌの鋼から作成した外径２ｍｍの溶接棒材を共通に用いて、供試材同志を突き合わせて、一般のステンレス鋼よりも高グレードの高耐食ステンレス鋼で用いられる入熱量１０ｋＪ／ｃｍ（溶接条件１）および一般的なステンレス鋼の溶接施工としては特に能率に問題が生じない入熱量である２０ｋＪ／ｃｍ（溶接条件２）の二つの条件にて片側からＴＩＧ溶接にて多層溶接して二種類の溶接継手を作製した。
得られた溶接継手から、溶接線に直交方向が４０ｍｍの辺で３×１０ｍｍの面が圧延面と平行となるように厚さ３ｍｍ、幅１０ｍｍ、長さ４０ｍｍの腐食試験片を採取し、１０％ＦｅＣｌ３・６Ｈ２Ｏ（６５℃）の溶液に２４時間浸漬し、５００倍の視野にてＨＡＺ部での孔食発生の有無を評価した。
また、溶接線と圧延面に直交する断面を検鏡エッチングして、５００倍の視野で、画像解析を行いＨＡＺ部での微細シグマ相の面積率を測定した。シグマ相の面積率が１％あれば微量のシグマ相あり、と判定した。
これらの結果を表２にまとめて示す。表２に示す結果から明らかなように、化学組成と粗大介在物の密度が本発明の範囲を満足している試験体では、一般的なステンレス鋼の溶接施工としては特に能率に問題が生じない高入熱量での評価にもかかわらず、微量のシグマ相の析出も認められず、優れた耐孔食性を示している。一方、代符Ｂ１、Ｂ２のように、ここの元素が化学組成範囲を満たしてもＮｉ、Ｍｏの組み合わせ範囲が本発明要件を満たさなければ、代符Ｂ１のように微量のシグマ相が生じたり、代符Ｂ２のようにシグマ相が生じなくて窒化物が生じ、耐孔食性が劣化していた。また、代符Ｂ３〜Ｂ５のように鋼組成そのものはそれぞれ代符Ａ１、Ａ３と同一であっても、粗大介在物の密度が本発明の範囲内のものでないものでは、微量のシグマ相が生じたり、耐孔食性が劣化していた。
The obtained plate was heated to 1250 ° C. and rolled to a thickness of 10 mm. A part of the obtained steel plate was cut out and embedded in the resin with the cross section orthogonal to the rolling surface facing up, and then this cross section was mirror-polished. Thereafter, the coarse inclusions were observed with five fields of view and SEM at a magnification of 200 times to evaluate the size.
The major axis of the alumina-based coarse inclusion is measured according to the definition of FIG. 2, and the composition of the vicinity of the center of the coarse inclusion (b1 and b2 in FIG. 2) is analyzed by EPMA to identify the coarse inclusion, Density was measured. The density was evaluated based on the average value of 5 fields of the number of coarse inclusions per 1 mm 2 .
The test steel sheet was subjected to machining to provide a V groove having a thickness of 8 mm, a width of 100 mm, a length of 200 mm, and a groove having a groove angle of 30 degrees at the end of the long side. A heat input of 10 kJ / cm used in high-corrosion-resistant stainless steel of a higher grade than general stainless steel, using a welding rod with an outer diameter of 2 mm made from steel of Al. (Welding condition 1) and general stainless steel welding work, multi-layer welding by TIG welding from one side under two conditions of 20 kJ / cm (welding condition 2), which is a heat input that does not cause any problem in efficiency. Two types of welded joints were prepared.
From the obtained welded joint, a corrosion test piece having a thickness of 3 mm, a width of 10 mm, and a length of 40 mm was sampled so that the side of the weld line was 40 mm in the direction perpendicular to the weld line and the surface of 3 × 10 mm was parallel to the rolling surface. % FeCl 3 · 6H 2 O ( 65 ℃) solution was immersed 24 hours, to evaluate the presence or absence of pitting in the HAZ at 500 times the field of view.
Further, the cross section orthogonal to the weld line and the rolling surface was spectroscopically etched, and image analysis was performed with a field of view of 500 times to measure the area ratio of the fine sigma phase in the HAZ part. If the area ratio of the sigma phase was 1%, it was determined that there was a trace amount of sigma phase.
These results are summarized in Table 2. As is clear from the results shown in Table 2, in the test specimens in which the chemical composition and the density of coarse inclusions satisfy the scope of the present invention, there is no problem in efficiency particularly for general stainless steel welding. Despite the evaluation with a high heat input, no trace of sigma phase was observed, indicating excellent pitting corrosion resistance. On the other hand, even if these elements satisfy the chemical composition range as in the case of B1 and B2, if the combination range of Ni and Mo does not satisfy the requirements of the present invention, a trace amount of sigma phase may be generated as in B1. As in the case of the symbol B2, no sigma phase was generated, nitride was formed, and the pitting corrosion resistance was deteriorated. Further, even if the steel composition itself is the same as that of the symbols A1 and A3, as in the symbols B3 to B5, a trace amount of sigma phase occurs when the density of coarse inclusions is not within the scope of the present invention. Or pitting corrosion resistance was deteriorated.
本発明によれば、溶接熱影響部におけるシグマ相の生成が防止できることから、また粗大介在物が生成量を大幅に低減できることから、得られる二相ステンレス鋼は、優れた耐孔食性を示すことになり、例えば今日その用途への適用が求められている優れた二相ステンレス鋼が提供される。 According to the present invention, since the generation of the sigma phase in the weld heat affected zone can be prevented and the amount of coarse inclusions can be greatly reduced, the resulting duplex stainless steel exhibits excellent pitting corrosion resistance. Thus, for example, excellent duplex stainless steels that are required to be applied to the application today are provided.
Claims (4)
かつ、ＰＲＥＷ＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎが４０以上で
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、Ａｌを２０質量％以上含み長い側の径が５μｍ以上の介在物と定義される粗大介在物が断面観察で１平方ｍｍ当たり１０個以下であることを特徴とする二相ステンレス鋼。In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.008% or less, Cr: 23.0 to 27.0%, Mo: 2.0-4.0%, Ni: 5.0-9.0%, W: more than 1.5% and 5.0% or less, N: 0.24-0.35% Fe and impurities: balance and PREW = Cr + 3.3 (Mo + 0.5W) + 16N is 40 or more and Mo + 1.1Ni ≦ 12.5
Mo-0.8Ni ≦ −1.6
It has a chemical composition satisfying the following relationship, and is characterized by having no more than 10 coarse inclusions per square mm in cross-sectional observation, defined as inclusions containing 20 mass% or more of Al and having a long side diameter of 5 μm or more. And duplex stainless steel.
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| SE530847C2 (en) * | 2006-12-14 | 2008-09-30 | Sandvik Intellectual Property | Plate for plate heat exchangers, plate heat exchangers made up of such plates and use of this plate heat exchanger |
| SE531593C2 (en) * | 2007-10-26 | 2009-06-02 | Sandvik Intellectual Property | Heat exchanger for phosphoric acid environment |
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| JP3758508B2 (en) * | 2001-02-13 | 2006-03-22 | 住友金属工業株式会社 | Manufacturing method of duplex stainless steel pipe |
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2004
- 2004-06-29 BR BRPI0412092-2A patent/BRPI0412092A/en not_active Application Discontinuation
- 2004-06-29 AU AU2004252373A patent/AU2004252373B2/en active Active
- 2004-06-29 CN CNB2004800188034A patent/CN100497704C/en active Active
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- 2004-06-29 KR KR1020057025084A patent/KR100704201B1/en active IP Right Grant
- 2004-06-29 EP EP04746980A patent/EP1645650A4/en not_active Withdrawn
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103602915A (en) * | 2013-11-22 | 2014-02-26 | 山东建筑大学 | High-carbon high-chromium duplex stainless steel |
| CN103602915B (en) * | 2013-11-22 | 2015-11-18 | 山东建筑大学 | High carbon and chromium duplex stainless steel |
Also Published As
| Publication number | Publication date |
|---|---|
| EP1645650A4 (en) | 2007-07-25 |
| CN1816640A (en) | 2006-08-09 |
| BRPI0412092A (en) | 2006-09-05 |
| CN100497704C (en) | 2009-06-10 |
| WO2005001151A1 (en) | 2005-01-06 |
| AU2004252373A1 (en) | 2005-01-06 |
| AU2004252373B2 (en) | 2007-02-22 |
| US20060191605A1 (en) | 2006-08-31 |
| EP1645650A1 (en) | 2006-04-12 |
| JPWO2005001151A1 (en) | 2006-08-10 |
| KR100704201B1 (en) | 2007-04-09 |
| NO20056009L (en) | 2006-01-11 |
| KR20060026898A (en) | 2006-03-24 |
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