JP2004003000A

JP2004003000A - Austenitic stainless steel excellent in high-temperature strength and corrosion resistance, heat- and pressure-resistant member made of this and its manufacturing process

Info

Publication number: JP2004003000A
Application number: JP2003105933A
Authority: JP
Inventors: Atsuro Iseda; 伊勢田　敦朗; Mitsuyuki Senba; 仙波　潤之
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2002-04-17
Filing date: 2003-04-10
Publication date: 2004-01-08
Anticipated expiration: 2023-04-10
Also published as: JP4007241B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel which is suitable as a constituent material for ultrasupercritical pressure boilers, etc., with a steam temperature of ≥700°C, a heat- and pressure-resistant member having a coarse grain structure which is made of this steel and shows an excellent thermal fatigue resistance and structural stability at high-temperature range and its manufacturing process. <P>SOLUTION: The austenitic stainless steel comprises 0.03-0.12% C, 0.1-1% Si, 0.1-2% Mn, ≥20 and <28% Cr, >35% and ≤50% Ni, 4-10% W, 0.01-0.3% Ti, 0.01-1% Nb, 0.0005-0.04% sol. Al., 0.0005-0.01% B and the balance being Fe and impurities comprising ≤0.04% P, ≤0.010% S, <0.5% Mo, <0.02% N and ≤0.005% O. The heat- and pressure-resistant member made of this steel has a coarse grain structure with an austenite grain size number of ≤6 and a mixed grain size ratio of ≤10%. This member can be manufactured through steps wherein the steel is (1) heated at ≥1,100°C at least once before final processing, (2) subjected to plastic working with reduction of area of ≥10% and (3) subjected to final heat-treatment at ≥1,050°C. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、発電ボイラや化学工業用加熱炉等を構成する鋼管、鋼板、棒鋼および鍛鋼品等（以下、これらを総称して「耐熱耐圧部材」という。）の素材として好適なオーステナイト系ステンレス鋼、その鋼からなる高温強度と高温耐食性に優れた耐熱耐圧部材、およびその製造方法に関する。この耐熱耐圧部材は、高い高温強度と優れた高温耐食性を有するのに加えて、耐熱疲労特性と金属組織の安定性（以下、単に組織安定性という）にも優れている。
【０００２】
【従来の技術】
近年、高効率化のために蒸気の温度と圧力を高めた超々臨界圧ボイラの新設が世界中で進められている。蒸気の温度に関しては、これまでの６００℃前後から６５０℃以上、将来的には７００℃以上にまで高めることが計画されている。これは、省エネルギーと資源の有効活用、および環境保全のためのＣＯ_２ガス排出量削減が大きな課題となっており、この課題の解決には、化石燃料を燃焼させる高効率の超々臨界圧ボイラが有利なためである。
【０００３】
蒸気の高温高圧化、中でも高温化は、ボイラや化学工業用の加熱炉を構成する耐熱耐圧部材の温度を上昇させ、その温度は６５０℃以上に達する。このため、これらの耐熱耐圧部材には、高温強度と高温耐食性に加えて、耐熱疲労特性や長期にわたる組織安定性が要求される。
【０００４】
オーステナイト系ステンレス鋼は、フェライト系鋼に比べて高温強度と高温耐食性が優れる。このため、強度と耐食性の観点からフェライト系鋼が使えなくなる６５０℃以上の高温域では、オーステナイト系ステンレス鋼が使われる。
【０００５】
高温高圧用のオーステナイト系ステンレス鋼としては、ＳＵＳ３４７ＨやＳＵＳ３１６等の１８−８系のオーステナイト系ステンレス鋼が広く用いられているが、高温強度と耐食性において限界がある。また、耐食性を高めた２５Ｃｒ系のＳＵＳ３１０もあるが、６００℃以上の高温強度がＳＵＳ３１６よりも低い。
【０００６】
このため、１８−８系鋼以上の耐食性を有する２０Ｃｒ以上のオーステナイト系ステンレス鋼をベースにして高温強度と高温耐食性を高めた多くの鋼が提案されている。これらの鋼は次の３つに大別される。
【０００７】
（１）Ｃｒ量を２０％以上に高めるとともに、固溶強化元素のＷやＭｏ等を複合添加して粒内強化を図った鋼（例えば、特開昭６１−１７９８３３号公報および特開昭６１−１７９８３５号公報）。
【０００８】
（２）Ｗ、Ｍｏに加え、Ｎを積極的に添加して窒化物による析出強化を図った鋼（例えば、特開昭６３−１８３１５５号公報）。
【０００９】
（３）ＴｉやＡｌの金属間化合物による析出強化を図った鋼（例えば、特開平７−２１６５１１号公報）。
【００１０】
しかし、前記（１）の鋼は、高温域におけるクリープの主体が粒内の転位クリープから粒界すべりクリープとなるために、７００℃以上での高温クリープ強度が低い。（２）および（３）の鋼は、強度は十分なものの、延性が著しく低く、しかも高温域における耐熱疲労特性と組織安定性が劣り、７００℃以上でのクリープ強度とクリープ延性が低い。
【００１１】
また、（３）の鋼は、ＴｉやＡｌの金属間化合物が結晶粒の成長を抑制するために混粒組織となって粒界すべりクリープや不均一なクリープ変形が生じ、強度および靱性が大きく損なわれる。従って、これらの従来鋼は、７００℃以上の高温で用いる耐熱耐圧部材、中でも組織が著しい混粒になりやすい肉厚が２０ｍｍ以上の耐熱耐圧部材としては使用できない。
【００１２】
【発明が解決しようとする課題】
本発明の第１の課題は、７００℃以上の高温域において優れた耐熱疲労特性と組織安定性を示す耐熱耐圧部材の素材に適するオーステナイト系ステンレス鋼を提供することにある。
【００１３】
本発明の第２の課題は、高温強度と耐熱疲労性に優れた耐熱耐圧部材を提供することにある。とりわけ、７５０℃、１０，０００時間のクリープ破断強度と絞り率がそれぞれ　８０ＭＰａ以上、５５％以上という特性を持つ耐熱耐圧部材を提供することにある。
【００１４】
本発明の第３の課題は、上記の特性を持つ耐熱耐圧部材の製造方法を提供することにある。
【００１５】
【課題を解決するための手段】
本発明のオーステナイト系ステンレス鋼は、下記（１）と（２）の鋼である。また、本発明の耐熱耐圧部材は下記（３）の部材である。さらに、本発明の耐熱耐圧部材を得るのに好適なその製造方法は下記（４）の方法である。
【００１６】
（１）質量％で、Ｃ：０．０３〜０．１２％、Ｓｉ：０．１〜１％、Ｍｎ：０．１〜２％、Ｃｒ：２０％以上２８％未満、Ｎｉ：３５％を超え５０％以下、Ｗ：４〜１０％、Ｔｉ：０．０１〜０．３％、Ｎｂ：０．０１〜１％、ｓｏｌ．Ａｌ：０．０００５〜０．０４％、Ｂ：０．０００５〜０．０１％を含み、残部はＦｅおよび不純物からなり、不純物としてのＰが０．０４％以下、Ｓが０．０１０％以下、Ｍｏが０．５％未満、Ｎが０．０２％未満、Ｏ（酸素）が０．００５％以下であることを特徴とするオーステナイト系ステンレス鋼。
【００１７】
（２）上記（１）に記載の成分に加えて、さらに下記の第１群から第３群までの少なくとも１群の中から選んだ少なくとも１種の成分を含み、残部はＦｅおよび不純物で、不純物としてのＰ０．０４％以下、Ｓが０．０１０％以下、Ｍｏが０．５％未満、Ｎが０．０２％未満、Ｏ（酸素）が０．００５％以下であることを特徴とするオーステナイト系ステンレス鋼。
第１群…質量％で、０．０００５〜０．１％のＺｒ。
第２群…質量％で、０．０００５〜０．０５％のＣａおよび０．０００５〜０．０１％のＭｇ。
第３群…質量％で、それぞれ０．０００５〜０．２％の希土類元素、ＨｆおよびＰｄ。
【００１８】
ここで、希土類元素とは、原始番号５７のＬａから同７１のＬｕまでの１５元素と、ＹおよびＳｃを含めた１７元素のことである。
【００１９】
（３）上記（１）または（２）に記載のオーステナイト系ステンレス鋼からなり、オーステナイト平均粒度番号が６以下、混粒率が１０％以下であることを特徴とする高温域での耐熱疲労特性と組織安定性に優れた耐熱耐圧部材。特に、７５０℃、１０，０００時間のクリープ破断強度と絞り率がそれぞれ　８０ＭＰａ以上、５５％以上である耐熱耐圧部材。
【００２０】
ここで、上記のオーステナイト結晶粒度番号は、ＡＳＴＭ（Ａｍｅｒｉｃａｎ　Ｓｏｃｉｅｔｙ　ｆｏｒＴｅｓｔｉｎｇ　ａｎｄ　Ｍａｔｅｒｉａｌ：アメリカ材料試験協会）に規定される粒度番号を意味する。
【００２１】
次に、混粒率（％）の計算方法を説明する。光学顕微鏡による上記オーステナイト結晶粒度番号の判定に際して観察した視野数をＮとして、その１視野毎にその視野内に存在する結晶粒の数を数えることによって、オーステナイト結晶粒度番号を−３（粗粒）から＋１０（細粒）までのいずれかの粒度番号であると判定し、Ｎ個の判定結果を得て、粒度番号毎に頻度を計算する。そして、そのうち最大頻度を有する粒度番号Ｇを特定し、特定された粒度番号Ｇより３以上小さい粒度番号を有する視野数ｎ１と、特定された粒度番号Ｇより３以上大きい粒度番号を有する視野数ｎ２とを求める。この視野数ｎ１とｎ２の合計数を全視野数Ｎで除したものの百分率、すなわち、１００×（ｎ１＋ｎ２）／Ｎが、混粒率である。
【００２２】
（４）上記（１）または（２）に記載の化学組成を有する鋼を下記の工程▲１▼、▲２▼および▲３▼で順次処理することを特徴とする上記（３）に記載の高温域での耐熱疲労特性と組織安定性に優れた耐熱耐圧部材の製造方法。
工程▲１▼：熱間または冷間による最終加工前に、少なくとも１回、１１００℃以上に加熱する。
工程▲２▼：断面減少率１０％以上の塑性加工を行う。
工程▲３▼：１０５０℃以上で最終熱処理を行う。
【００２３】
【発明の実施の形態】
本発明者らは、高温域での耐食性を確保するためにＣｒ量を２０％以上に高めたオーステナイト系ステンレス鋼の７００℃以上におけるクリープや金属組織等に及ぼす合金元素の影響を詳細に調べた結果、以下の新たな知見を得た。
【００２４】
（ａ）Ｍｏは７００℃以上の高温域での高強度化にはほとんど効果がないだけでなく、かえって高温耐食性を低下させるので、不純物として含まれる場合でもその含有量は０．５％未満に制限する必要がある。
【００２５】
（ｂ）ＷはＭｏとは異なり７００℃以上の高温域での強度を向上させ、しかも高温耐食性を低下させることがないので、Ｍｏの積極的添加を行わないことによる強度不足はＷの多量添加により補える。
【００２６】
（ｃ）従来技術において高強度化のために利用されている多量のＴｉを含む炭窒化物や金属間化合物は、前述したように、粒界すべりクリープと不均一なクリープ変形を助長し、高温域での強度と延性を著しく低下させるので、できるだけ利用しない方がよい。
【００２７】
（ｄ）粒界すべりクリープと不均一なクリープ変形は、細粒組織よりも粗粒組織の方が生じにくく、特にオーステナイト結晶粒度番号が６以下で、しかも混粒率が１０％以下の粗粒組織、好ましくは混粒率が０（ゼロ）の粗粒組織の場合に生じにくくなる。
【００２８】
（ｅ）オーステナイト結晶粒度番号が６以下、混粒率が１０％以下の粗粒組織は、鋼中のＴｉ含有量を０．０１〜０．３％に制限するとともに、ＮとＯ（酸素）の含有量をそれぞれ０．０２％未満、０．００５％以下に制限し、かつ適量（０．０００５〜０．０１％）のＢを含有させた前記（１）または（２）の化学組成の鋼を素材とし、この鋼を例えば上記の工程▲１▼から▲３▼を経て処理すれば得られる。
【００２９】
即ち、Ｔｉ、Ｎ、ＯおよびＢの含有量を上記の範囲に制限した場合には、上記の工程▲１▼の後において鋼中に安定なＴｉやＢを含む未固溶炭窒化物や酸化物が存在せず、工程▲２▼において均一な歪みが蓄積され、工程▲３▼において再結晶が均一に進行して、オーステナイト結晶粒度番号６以下、混粒率１０％以下の粗粒組織を有する耐熱耐圧部材が得られる。
【００３０】
（ｆ）前記の量のＴｉおよびＮｂは、その組織がオーステナイト結晶粒度番号６以下、混粒率１０％以下とされた耐熱耐圧部材を実際に使用した場合におけるクリープ中に、微細な炭化物として粒内と粒界に均一に析出し、高温クリープ強度を向上させる。その結果、この部材の７５０℃、１０，０００時間のクリープ破断強度が　８０ＭＰａ以上、絞り率が５５％以上になる。このような特性を持つ部材は、耐熱疲労特性にも優れる。
【００３１】
以下、本発明のオーステナイト系ステンレス鋼の化学組成、この鋼からなる耐熱耐圧部材の結晶粒度および混粒率、ならびに好ましい製造方法の諸条件を上記のように定めた理由について詳細に説明する。なお、以下において「％」は特に断らない限り「質量％」を意味する。
【００３２】
１．オーステナイト系ステンレス鋼の化学組成
Ｃ：０．０３〜０．１２％
Ｃは炭化物を形成して高温用オーステナイト系ステンレス鋼として必要な高温引張強さ、高温クリープ強度を確保する上で必要な成分であり、０．０３％以上の含有量が必要である。しかし、その含有量が０．１２％を超えると、未固溶炭化物が生じたり、Ｃｒの炭化物が増えて溶接性が低下するので上限は０．１２％とした。望ましいＣ含有量は０．０５〜０．１０％である。
【００３３】
Ｓｉ：０．１〜１％
Ｓｉは、製鋼時に脱酸剤として添加されるが、鋼の耐水蒸気酸化性を高めるためにも必要な元素であり、最低でも０．１％の含有量が必要である。しかし、その含有量が過剰になると鋼の加工性が悪くなるので上限は１％とした。好ましい範囲は０．１〜０．５％である。
【００３４】
Ｍｎ：０．１〜２％
Ｍｎは、鋼中に含まれる不純物のＳと結合してＭｎＳを形成し、熱間加工性を向上させるが、その含有量が０．１％未満ではこの効果が得られない。一方、その含有量が過剰になると、鋼が硬くなって脆くなり、かえって加工性や溶接性を損なうので上限は２％とした。望ましいＭｎ含有量は０．５〜１．２％である。
【００３５】
Ｐ：０．０４％以下
Ｐは不純物として不可避的に混入するが、過剰なＰは溶接性および加工性を害するので、上限は０．０４％とする。好ましい上限は０．０３％である。なお、Ｐ含有量は少ないほどよい。
【００３６】
Ｓ：０．０１０％以下
Ｓも上記のＰと同様に不純物として不可避的に混入するが、過剰なＳは溶接性および加工性を害するため、上限は０．０１０％とする。好ましい上限は０．００８％である。なお、Ｓ含有量は加工性を向上させる上では少ないほどよいが、溶接時の湯流れ性を確保する上では０．００４〜０．００８％程度含有させるのがよい。
【００３７】
Ｃｒ：２０％以上、２８％未満
Ｃｒは、耐酸化性、耐水蒸気酸化性および耐食性を確保するための重要な元素である。７００℃以上の高温下での耐食性を１８−８系鋼以上にするためには最低限２０％の含有量が必要である。前記の耐食性はＣｒ含有量が多いほど向上するが、その含有量が２８％以上になると、組織安定性が低下してクリープ強度を損なう。また、オーステナイト組織を安定にするために高価なＮｉ含有量の増加を余儀なくされるだけでなく、溶接性も低下する。よって、Ｃｒ含有量は２０％以上で２８％未満とする。好ましい範囲は２２〜２６％である。
【００３８】
Ｎｉ：３５％を超えて５０％以下
Ｎｉは、オーステナイト組織を安定にする元素であり、耐食性の確保にも重要な合金元素である。上記のＣｒ量とのバランスからＮｉは３５％を超える量が必要である。一方、過剰なＮｉはコスト上昇を招くだけでなく、クリープ強度の低下を招くので、その上限は５０％とする。望ましいのは４０〜４８％である。
【００３９】
Ｍｏ：０．５％未満
Ｍｏは前述したように７００℃以上の使用環境下で脆化相を生じたり耐食性を劣化させることがあるだけでなく、後述するＷとの複合添加ではＷの単独添加に比べて強度向上効果がほとんどない。このため、本発明ではＭｏは積極的には添加しない。しかし、不純物量であっても、その含有量が０．５％以上になると、７００℃以上の高温域で使用した場合、脆化相の生成および耐食性の低下が著しくなる。従って、不純物としてのＭｏ含有量は０．５％未満とした。好ましいのは０．３％以下、より好ましいのは分析の検出限界値未満である。なお、Ｍｏの検出限界値は、通常、０．０１％である。
【００４０】
Ｗ：４〜１０％
Ｗも重要な元素の一つで、固溶強化作用によって７００℃以上の高温域において優先する粒界すべりクリープを抑制するが、そのためには最低でも４％の含有量が必要である。一方、過剰なＷはＭｏのように脆化相は生成させないものの、鋼を著しく硬化させ、加工性および溶接性を劣化させるので、上限は１０％とする。望ましいのは６〜８％である。
【００４１】
Ｔｉ：０．０１〜０．３％
Ｔｉは、未固溶炭窒化物や酸化物を形成してオーステナイト結晶粒の混粒化を助長したり、不均一なクリープ変形や延性低下の原因となるので、その含有量は０．３％以下とした。一方、その含有量が０．０１％未満では、高温域での使用中における炭化物の析出による高温強度の向上が望めない。このため、Ｔｉ含有量は０．０１〜０．３％とした。好ましいのは０．０３〜０．２％である。
【００４２】
Ｎｂ：０．０１〜１％
Ｎｂは、Ｔｉのように有害な酸化物にはならないが、炭化物によるクリープ強度の向上のためには最低限０．０１％の含有量が必要である。一方、過剰なＮｂは溶接性を害するので上限は１％とする。好ましいのは０．１〜０．５％である。
【００４３】
ｓｏｌ．Ａｌ：０．０００５〜０．０４％
Ａｌは、脱酸剤として添加させるが、多量に添加すると組織安定性が悪くなるので、その含有量はｓｏｌ．Ａｌ含有量で０．０４％以下とする。一方、十分な脱酸効果を得るには０．０００５％以上のｓｏｌ．Ａｌ含有量が必要である。好ましいのは０．００５〜０．０２％である。
【００４４】
Ｂ：０．０００５〜０．０１％
Ｂは、後述するＮおよびＯ（酸素）の含有量を低減して酸化物や窒化物を極力排除するようにした本発明の鋼においては極めて有効な粒界すべりクリープ抑制作用を有する元素であるが、その含有量が０．０００５％未満ではこの効果が得られない。一方、０．０１％を超えて含有させると溶接性を損なう。このため、Ｂ含有量は０．０００５〜０．０１％とした。好ましいのは０．００１〜０．００５％である。
【００４５】
Ｎ：０．０２％未満
Ｎおよび次に述べるＯの含有量の低減が本発明の重要な要件の一つである。Ｎは、従来、炭窒化物による析出強化と高価なＮｉの一部に代える元素として積極的に添加されている。しかし、多量のＮはＴｉやＢの未固溶炭窒化物を生成し、これが組織を混粒にし、７００℃以上の高温域での粒界すべりクリープおよび不均一なクリープ変形を助長して強度を損なう。従って、Ｎ含有量は極力低減する必要がある。ＮはＣｒとの親和力が強く、不純物として混入することが避けられない。しかし、その含有量が０．０２％未満であれば前記の未固溶炭窒化物が生成しなくなるので、Ｎ含有量は０．０２％未満とした。好ましいのは０．０１６％以下、より好ましいのは０．０１％以下である。なお、Ｎ含有量は低いほどよい。
【００４６】
Ｏ（酸素）：０．００５％以下
Ｏは、上記のＮと同様に、ＴｉやＡｌの未固溶酸化物を生成し、これが組織を混粒にし、７００℃以上の高温域での粒界すべりクリープおよび不均一なクリープ変形を助長して強度を損なう。従って、Ｏ含有量も極力低減する必要がある。Ｏも不純物として混入することが避けられないが、その含有量が０．００５％以下であれば前記の未固溶酸化物が生成しなくなるので、Ｏ含有量は０．００５％以下とした。好ましいのは０．００３％以下である。なお、Ｏ含有量も低いほどよい。
【００４７】
本発明のオーステナイト系ステンレス鋼の残部は実質的にＦｅ、言いかえればＦｅと上記以外の不純物である。
【００４８】
本発明のオーステナイト系ステンレス鋼のもう一つは、前記の第１群から第３群までの少なくとも１群の中から選んだ少なくとも１種の成分を含む鋼である。以下、これらの成分について説明する。
【００４９】
第１群（Ｚｒ）
Ｚｒは、粒界を強化して高温強度を向上させる作用を有する。従って、その効果を得たい場合には積極的に添加含有させてもよい。その効果は、０．０００５％以上の含有量で顕著になる。しかし、その含有量が０．１％を超えると、前記のＴｉと同様に未固溶の酸化物や窒化物を生成し、粒界すべりクリープおよび不均一なクリープ変形を助長するだけでなく鋼質をも劣化させ、高温域でのクリープ強度および延性を損なう。このため、添加する場合のＺｒ含有量は０．０００５〜０．１％とするのがよい。さらに好ましいのは０．００１〜０．０６％である。
【００５０】
第２群（ＣａおよびＭｇ）
これらの元素は、いずれもＳと結合してＳを安定化し、加工性を向上させる作用を有する。従って、その効果を得たい場合には１種以上を積極的に添加含有させてもよく、その場合、それぞれ、０．０００５％以上の含有量で上記の効果が顕著になる。しかし、Ｃａについては０．０５％、Ｍｇについては０．０１％を超えると、靱性、延性及び鋼質を損なう。従って、添加する場合のＣａ含有量は０．０００５〜０．０５％、Ｍｇ含有量は０．０００５〜０．０１％とするのがよい。一層好ましいＣａ含有量は０．０００５〜０．０１％、Ｍｇ含有量は０．００１〜０．００５％である。
【００５１】
第３群（希土類元素、ＨｆおよびＰｄ）
これらの元素は、いずれも無害で安定な酸化物や硫化物を形成して、ＯおよびＳの好ましくない影響を小さくし、耐食性、加工性、クリープ強度およびクリープ延性を向上させる作用を有する。従って、その効果を得たい場合には１種以上を積極的に添加含有させてもよく、その場合、それぞれ０．０００５％以上の含有量で上記の効果が顕著になる。しかし、それぞれの含有量が０．２％を超えると、酸化物等の介在物が多くなり、加工性および溶接性を損なうだけでなく、コストの上昇を招く。従って、添加する場合のこれら元素の含有量は、それぞれ０．０００５〜０．２％とするのがよい。一層好ましい範囲はそれぞれ０．００１〜０．１％である。
【００５２】
なお、Ｐ、Ｓ、Ｍｏ、ＮおよびＯ以外の不純物としては、スクラップ等から混入することがあるＣｏおよびＣｕが挙げられる。しかし、Ｃｏは、本発明の鋼および耐熱耐圧部材の特性に特別な悪影響を及さない。従って、不純物として混入する場合のＣｏ含有量は特に制限しない。ただし、Ｃｏは放射化元素でもあるから、混入する場合のＣｏ含有量は０．８％以下、望ましくは０．５％以下にするのがよい。
【００５３】
Ｃｕは強度を向上させるものの、７００℃以上の高温域での粒界すべりクリープを著しく助長させる。従って、不純物として混入する場合のＣｕ含有量は０．５％以下、望ましくは０．２％以下にするのがよい。
【００５４】
２．耐熱耐圧部材
本発明の耐熱耐圧部材は、上記に説明したとおりの化学組成を有するオーステナイト系ステンレス鋼からなるが、その金属組織はオーステナイト結晶粒度番号で６以下、混粒率１０％以下の粗粒組織でなければならない。その理由は次のとおりである。
【００５５】
前述したように、７００℃以上の高温域でのクリープ強度は、オーステナイト結晶粒の大きさと整粒の程度に大きく依存し、粒度番号が６を超える細粒組織の場合には粒界すべりクリープが生じる。また、粒度番号が６以下の粗粒組織であっても混粒率が１０％を超える場合には不均一なクリープ変形が生じる。その結果、耐熱疲労特性と組織安定性が劣り、７５０℃、１０，０００時間のクリープ破断時間で８０ＭＰａ以上、絞り率で５５％以上が確保できなくなる。
【００５６】
このため、本発明では、オーステナイト結晶粒度番号６以下、混粒率１０％以下と定めた。好ましいオーステナイト結晶粒度番号は５．５〜３である。また、好ましい混粒率は０（ゼロ）％、即ち、粒度番号が６以下の粗粒でかつ整粒の組織である。なお、オーステナイト結晶粒度番号の下限は特に制限しないが、粒度番号が０未満の粗粒組織は、超音波探傷法による内部欠陥や表面疵の検査ができなくなるので、その下限は０番とするのがよい。
【００５７】
３．耐熱耐圧部材の製造方法
次に、上記のオーステナイト結晶粒度番号が６以下、混粒率が１０％以下の粗粒組織を有する本発明の耐熱耐圧部材を得るための好ましい製造方法について説明する。この製造方法は、先に述べた▲１▼から▲３▼までの工程を順次経ることを特徴とする。
【００５８】
工程▲１▼：
本発明の方法においては、熱間または冷間による最終加工の前に少なくとも１回の加熱を行って、加工中に析出した鋼中の析出物を十分に固溶させる必要がある。しかし、その加熱温度が１１００℃未満の場合には、加熱後の鋼中に安定なＴｉやＢを含む未固溶炭窒化物や酸化物が存在するようになる。その結果、これが次の工程▲２▼において不均一な歪みを蓄積させる原因となり、工程▲３▼の最終熱処理において再結晶を不均一にする。また、未固溶炭窒化物や酸化物それ自体が均一な再結晶を阻害し、上記所定の粗粒組織が確保できなくなる。このため、本発明の好ましい方法においては、熱間または冷間による最終加工前に少なくとも１回、１１００℃以上に加熱する。なお、加熱温度の上限は特に制限しないが、１３５０℃を超える温度に加熱すると、高温粒界割れや延性低下を引き起こすことがあるので、加熱温度の上限は１３５０℃とするのがよい。
【００５９】
加熱後は直ちに熱間または冷間による最終加工を行ってもよい。加熱後および最終加工が熱間加工の場合における加工後の冷却条件には特別な制約はない。しかし、８００℃から５００℃までの間を冷却速度０．２５℃／秒以上で冷却するのが望ましい。これは、冷却中に粗大な析出物をつくらせないためである。
【００６０】
工程▲２▼：
工程▲２▼の塑性加工は、工程▲１▼における最終加工が熱間加工の場合には熱間加工または温度５００℃以下の温間加工を含む冷間加工のいずれであってもよい。また、工程▲１▼における最終加工が温度５００℃以下の温間加工を含む冷間加工の場合には最終加工と同じ条件の冷間加工のことである。
【００６１】
この工程の塑性加工は、次の最終熱処理において再結晶を促進させるために歪みを付与する目的で行う。この加工の断面減少率が１０％未満の場合は、再結晶に必要な歪みを付与することができず、次の最終熱処理を行っても所望の粗粒組織は得られない。このため、塑性加工は断面減少率１０％以上で行う。望ましい断面減少率の下限は２０％である。なお、断面減少率は大きいほどよいので上限は規定しないが、通常の加工での最大値は９０％程度である。また、この加工工程は製品の寸法を決定する工程でもある。
【００６２】
工程▲３▼：
所望の粗粒組織を得るための熱処理である。この熱処理の加熱温度が１０５０℃よりも低いと、十分な再結晶が起こらず、所望の粗粒組織が得られない。また、結晶粒が扁平な加工組織となり、クリープ強度が低くなる。このため、最終熱処理は１０５０℃以上で行う。好ましい熱処理温度は、工程▲１▼における加熱温度よりも１０℃以上低い温度である。なお、最終熱処理温度の上限は特に制限しないが、工程▲１▼の場合と同じ理由からその上限は１３５０℃とするのがよい。また、最終熱処理後は、工程▲１▼の場合と同じ理由から８００℃から５００℃までの間を冷却速度０．２５℃／秒以上で冷却するのが望ましい。
【００６３】
【実施例】
表１に示す化学組成を有する２９種類の鋼を溶製した。なお、比較例中のＮｏ．２１は　ＳＵＳ３１０相当の鋼、Ｎｏ．２２は　ＳＵＳ３１６相当の鋼である。
【００６４】
Ｎｏ．１〜２０の鋼は、容量５０ｋｇの真空溶解炉を用いて溶製して鋼塊にした。そして、Ｎｏ．１〜４およびＮｏ．１１〜１４の鋼塊は、下記の製造法Ａにより板に仕上げ、Ｎｏ．５〜７およびＮｏ．１５〜１７の鋼塊は下記の製造法Ｂにより冷延板に仕上げた。また、Ｎｏ．８〜１０およびＮｏ．１８〜２０の鋼塊は下記の製造法Ｃにより鋼管に仕上げた。
【００６５】
Ｎｏ．２１〜２９の鋼は、容量１５０ｋｇの真空溶解炉を用いて溶製し、得られた鋼塊からそれぞれ表２に示すように下記の製造法Ａ、Ｂ、Ｃで処理した。なお、これらの製造法はいずれも本発明の製造方法に属する。
【００６６】
（１）製造法Ａ
工程１（工程▲１▼に相当）：１２２０℃に加熱、
工程２（工程▲２▼に相当）：断面減少率６７％の熱間鍛造にて厚さ２５ｍｍの板材
に成形、
工程３：８００℃から５００℃以下まで０．５５℃／秒で冷却、
工程４（工程▲３▼に相当）：１２１０℃に１５分間保持後水冷。
【００６７】
（２）製造法Ｂ
工程１（工程▲１▼に相当）：１２２０℃に加熱、
工程２：断面減少率６７％の熱間鍛造にて厚さ２５ｍｍの板材に成形、
工程３：８００℃から５００℃以下まで０．５５℃／秒で冷却
工程４：外面切削にて厚さ２０ｍｍの板材に成形、
工程５（工程▲２▼に相当）：室温下にて断面減少率３０％のロール圧延を行い厚さ
１４ｍｍの板材に成形、
工程６（工程▲３▼に相当）：１２００℃に１５分間保持後水冷。
【００６８】
（３）製造法Ｃ
工程１：熱間鍛造と外削にて外径１７５ｍｍの丸鋼に成形、
工程２（工程▲１▼に相当）：丸鋼を１２５０℃に加熱、
工程３：加熱丸鋼を熱間押出し、外径６４ｍｍ、肉厚１０ｍｍの鋼管に成形、
工程４（工程▲１▼に相当）：鋼管を１２２０℃に１０分間加熱後１℃／秒で冷却、
工程５（工程▲２▼に相当）：室温下にて断面減少率３３％の引抜き加工を行い外径
５０．８ｍｍ、肉厚８．５ｍｍの鋼管に成形、
工程６（工程▲３▼に相当）：１２１０℃に１０分間保持後水冷。
【００６９】
【表１】

【００７０】
前記の工程Ａ、ＢまたはＣによって得られた熱間加工鋼板、冷間圧延鋼板および冷間加工鋼管について、オーステナイト結晶粒度番号と混粒率を調べた。オーステナイト結晶粒度番号は、ＡＳＴＭに規定される方法に従って測定し、混粒率は前述した方法により求めた。その際、いずれの場合も２０視野を観察した。
【００７１】
同じく、工程Ａ、ＢまたはＣによって得られた熱間加工鋼板、冷間圧延鋼板および冷間加工鋼管から、外径６ｍｍ、標点距離３０ｍｍのクリープ試験片を採取してクリープ試験に供し、７５０℃、１０，０００時間のクリープ破断強度（ＭＰａ）と絞り率（内挿値：％）を調べた。以上の結果を、表２にまとめて示す。
【００７２】
【表２】

【００７３】
表２からわかるように、化学組成が本発明で規定する範囲内の鋼（Ｎｏ．１〜２０）では、Ａ、Ｂ、Ｃのどの方法で加工しても、オーステナイト結晶粒度番号と混粒率が本発明で規定する範囲になっている。その結果、７５０℃、１０，０００時間のクリープ破断強度が　８７ＭＰａ以上、絞り率が　５７％以上と高く、耐熱疲労特性と組織安定性に優れた耐熱耐圧部材が得られることが明らかである。
【００７４】
Ｎｏ．２１（ＳＵＳ３１０）およびＮｏ．２２（ＳＵＳ３１６）は、組織は本発明で規定する条件を満たす粗粒組織になっているが、化学組成が本発明で規定する範囲外であるため、クリープ破断強度がそれぞれ４１ＭＰａおよび５５ＭＰａと著しく低い。
【００７５】
化学組成が本発明で規定する範囲外の鋼（Ｎｏ．２３〜２９）では、本発明の製造方法により加工熱処理しても、オーステナイト結晶粒度番号と混粒率の両方が本発明で規定する範囲内にある粗粒組織は得られていない。その結果、クリープ破断強度が６８〜７８ＭＰａ、絞り率が４〜２３％と低い。Ｎｏ．２５はＯ（酸素）含有量が高すぎ、また、Ｎｏ．２６はＮの含有量が高すぎるものである。Ｎｏ．２９は、Ｏ含有量およびＮ含有量が両方とも高すぎる。これらのクリープ破断強度および絞り率が目標値をはるかに下回ることから、ＯとＮの含有量を低く抑えることの重要性がわかる。即ち、これらの比較鋼では、７００℃以上の高温域において優れた耐熱疲労特性と組織安定性を発揮する耐熱耐圧部材は得られない。
【００７６】
【発明の効果】
本発明のオーステナイト系ステンレス鋼は、高温強度と高温耐食性が良好なだけでなく、オーステナイト結晶粒度番号と混粒率がそれぞれ６以下、１０％以下の粗粒組織で、７００℃以上の高温域での耐熱疲労特性と組織安定性に優れた耐熱耐圧部材の素材として好適である。また、本発明の耐熱耐圧部材は、７５０℃、１０，０００時間のクリープ破断強度が８７ＭＰａ以上、絞り率が５７％以上と高いので、蒸気温度が７００℃以上というような超々臨界圧ボイラ等の構成部材として使用可能である。さらに、本発明の方法によれば、本発明の耐熱耐圧部材を低コストで製造することが可能である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an austenitic stainless steel suitable as a material for steel pipes, steel plates, steel bars, forged products, etc. (hereinafter collectively referred to as “heat-resistant and pressure-resistant members”) constituting a power generation boiler, a heating furnace for the chemical industry, and the like. The present invention also relates to a heat-resistant and pressure-resistant member made of the steel and having excellent high-temperature strength and high-temperature corrosion resistance, and a method for producing the same. This heat-resistant and pressure-resistant member has not only high temperature strength and excellent high-temperature corrosion resistance, but also excellent heat-resistant fatigue characteristics and metal structure stability (hereinafter simply referred to as structure stability).
[0002]
[Prior art]
In recent years, ultra-supercritical boilers in which the temperature and pressure of steam are increased for higher efficiency are being promoted worldwide. It is planned to increase the temperature of the steam from about 600 ° C. to 650 ° C. or more, and to 700 ° C. or more in the future. This is because CO for energy conservation, effective use of resources, and environmental conservation ₂ Gas emission reduction has become a major issue, and a high-efficiency ultra-supercritical boiler that burns fossil fuels is advantageous for solving this issue.
[0003]
High temperature and high pressure of steam, especially high temperature, raises the temperature of heat-resistant and pressure-resistant members constituting a boiler and a heating furnace for the chemical industry, and the temperature reaches 650 ° C. or more. Therefore, these heat-resistant and pressure-resistant members are required to have high-temperature strength and high-temperature corrosion resistance, as well as heat-resistant fatigue characteristics and long-term structural stability.
[0004]
Austenitic stainless steel has higher high-temperature strength and high-temperature corrosion resistance than ferritic steel. For this reason, austenitic stainless steel is used in a high temperature range of 650 ° C. or higher where ferritic steel cannot be used from the viewpoint of strength and corrosion resistance.
[0005]
As austenitic stainless steel for high temperature and high pressure, 18-8 austenitic stainless steel such as SUS347H and SUS316 is widely used, but there is a limit in high temperature strength and corrosion resistance. There is also a 25Cr-based SUS310 with improved corrosion resistance, but its high-temperature strength at 600 ° C. or higher is lower than that of SUS316.
[0006]
For this reason, many steels have been proposed in which high-temperature strength and high-temperature corrosion resistance are enhanced based on austenitic stainless steel of 20Cr or more having corrosion resistance of 18-8 or more. These steels are roughly classified into the following three.
[0007]
(1) A steel in which the amount of Cr is increased to 20% or more, and a solid solution strengthening element such as W or Mo is added in a complex manner to achieve intragranular strengthening (for example, Japanese Patent Application Laid-Open Nos. -179835).
[0008]
(2) Steel in which N is positively added in addition to W and Mo to strengthen precipitation by nitride (for example, JP-A-63-183155).
[0009]
(3) Steel with precipitation strengthening by an intermetallic compound of Ti or Al (for example, JP-A-7-216511).
[0010]
However, the steel of the above (1) has a low high-temperature creep strength at 700 ° C. or higher because the main component of creep in the high-temperature region is from dislocation creep in grains to intergranular slip creep. Although the steels of (2) and (3) have sufficient strength, they have extremely low ductility, are inferior in heat-resistant fatigue properties and microstructure stability in a high temperature range, and have low creep strength and creep ductility at 700 ° C. or higher.
[0011]
Further, in the steel of (3), the intermetallic compound of Ti or Al suppresses the growth of crystal grains to form a mixed grain structure, causing grain boundary slip creep and non-uniform creep deformation, and large strength and toughness. Be impaired. Therefore, these conventional steels cannot be used as a heat-resistant and pressure-resistant member used at a high temperature of 700 ° C. or higher, and particularly a heat-resistant and pressure-resistant member having a thickness of 20 mm or more, in which the structure is apt to be mixed.
[0012]
[Problems to be solved by the invention]
A first object of the present invention is to provide an austenitic stainless steel suitable for a material of a heat-resistant and pressure-resistant member exhibiting excellent heat-resistant fatigue characteristics and structural stability in a high temperature range of 700 ° C. or higher.
[0013]
A second object of the present invention is to provide a heat and pressure resistant member excellent in high temperature strength and heat fatigue resistance. In particular, it is an object of the present invention to provide a heat and pressure resistant member having characteristics of a creep rupture strength at 750 ° C. for 10,000 hours and a draw ratio of 80 MPa or more and 55% or more, respectively.
[0014]
A third object of the present invention is to provide a method for manufacturing a heat-resistant and pressure-resistant member having the above characteristics.
[0015]
[Means for Solving the Problems]
The austenitic stainless steel of the present invention is the following steels (1) and (2). Further, the heat and pressure resistant member of the present invention is a member of the following (3). Further, a manufacturing method suitable for obtaining the heat and pressure resistant member of the present invention is the following method (4).
[0016]
(1) In mass%, C: 0.03 to 0.12%, Si: 0.1 to 1%, Mn: 0.1 to 2%, Cr: 20% or more and less than 28%, Ni: 35% Over 50%, W: 4 to 10%, Ti: 0.01 to 0.3%, Nb: 0.01 to 1%, sol. Al: 0.0005-0.04%, B: 0.0005-0.01%, the balance being Fe and impurities, P as impurities is 0.04% or less, and S is 0.010% or less. , Mo is less than 0.5%, N is less than 0.02%, and O (oxygen) is 0.005% or less.
[0017]
(2) In addition to the components described in the above (1), the composition further contains at least one component selected from at least one of the following first to third groups, with the balance being Fe and impurities, P as an impurity is 0.04% or less, S is 0.010% or less, Mo is less than 0.5%, N is less than 0.02%, and O (oxygen) is 0.005% or less. Austenitic stainless steel.
First group: 0.0005 to 0.1% Zr in mass%.
Second group: 0.0005 to 0.05% Ca and 0.0005 to 0.01% Mg by mass%.
Third group: Rare earth elements, Hf and Pd of 0.0005 to 0.2% by mass, respectively.
[0018]
Here, the rare earth elements are 15 elements from La of primitive number 57 to Lu of 71 and 17 elements including Y and Sc.
[0019]
(3) A heat-resistant fatigue property in a high-temperature region, comprising the austenitic stainless steel according to (1) or (2) above, having an austenite average grain size number of 6 or less and a mixed grain ratio of 10% or less. Heat and pressure resistant member with excellent tissue stability. Particularly, a heat-resistant and pressure-resistant member having a creep rupture strength at 750 ° C. for 10,000 hours and a draw ratio of 80 MPa or more and 55% or more, respectively.
[0020]
Here, the austenitic crystal grain size number means a grain size number defined by ASTM (American Society for Testing and Material: American Society for Testing Materials).
[0021]
Next, a method of calculating the particle mixture ratio (%) will be described. The number of visual fields observed in the determination of the austenite crystal grain size number by an optical microscope is set to N, and the number of crystal grains present in the visual field is counted for each visual field, whereby the austenite crystal grain size number is -3 (coarse grain). Is determined to be any one of the particle size numbers from to +10 (fine grain), N determination results are obtained, and the frequency is calculated for each particle number. Then, the particle size number G having the maximum frequency is specified, and the field number n1 having a particle size number smaller than the specified particle number G by 3 or more and the field number n2 having a particle size number larger than the specified particle number G by 3 or more. And ask. The percentage of the total number of the visual field numbers n1 and n2 divided by the total visual field number N, that is, 100 × (n1 + n2) / N is the mixed particle rate.
[0022]
(4) The steel according to the above (3), wherein the steel having the chemical composition described in the above (1) or (2) is sequentially treated in the following steps (1), (2) and (3). A method for producing a heat-resistant and pressure-resistant member having excellent heat-resistant fatigue characteristics and structural stability at high temperatures.
Step (1): Before final processing by hot or cold, heat at least once to 1100 ° C. or more.
Step (2): Plastic working with a cross-sectional reduction rate of 10% or more is performed.
Step (3): Final heat treatment is performed at 1050 ° C. or higher.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have investigated in detail the effects of alloying elements on creep, metallographic structure, and the like of austenitic stainless steel at 700 ° C. or higher in which the amount of Cr is increased to 20% or more in order to secure corrosion resistance in a high temperature range. As a result, the following new knowledge was obtained.
[0024]
(A) Mo not only has little effect on increasing strength in a high temperature range of 700 ° C. or more, but also lowers high-temperature corrosion resistance. Therefore, even if it is contained as an impurity, its content is less than 0.5%. It needs to be restricted.
[0025]
(B) Unlike Mo, W improves strength in a high-temperature region of 700 ° C. or higher and does not lower high-temperature corrosion resistance. Can be supplemented by
[0026]
(C) Carbonitrides and intermetallic compounds containing a large amount of Ti used for increasing strength in the prior art promote grain boundary slip creep and non-uniform creep deformation as described above, It is better not to use as much as possible, as it significantly reduces the strength and ductility in the region.
[0027]
(D) Grain boundary slip creep and non-uniform creep deformation are less likely to occur in a coarse-grained structure than in a fine-grained structure. In particular, coarse particles having an austenite crystal grain size number of 6 or less and a mixture ratio of 10% or less. It is less likely to occur in the case of a structure, preferably a coarse-grained structure having a mixing ratio of 0 (zero).
[0028]
(E) The coarse-grained structure having an austenite grain size number of 6 or less and a mixed grain rate of 10% or less restricts the Ti content in steel to 0.01 to 0.3%, and also contains N and O (oxygen). Of the chemical composition according to (1) or (2), wherein the content of B is limited to less than 0.02% and 0.005% or less, respectively, and an appropriate amount (0.0005 to 0.01%) of B is contained. It can be obtained by using steel as a material and treating the steel through, for example, the above steps (1) to (3).
[0029]
That is, when the contents of Ti, N, O, and B are limited to the above ranges, after the above-mentioned step (1), undissolved carbonitride or oxidized carbon containing stable Ti or B in steel is obtained. In the step (2), uniform strain is accumulated, and in the step (3), recrystallization proceeds uniformly, and a coarse-grained structure having an austenite crystal grain size number of 6 or less and a grain size of 10% or less is formed. A heat-resistant and pressure-resistant member having the same is obtained.
[0030]
(F) The amounts of Ti and Nb are reduced to fine carbides during creep when a heat-resistant and pressure-resistant member having an austenite crystal grain size number of 6 or less and a grain size ratio of 10% or less is actually used. Precipitates uniformly inside and at grain boundaries, improving high-temperature creep strength. As a result, the creep rupture strength of this member at 750 ° C. for 10,000 hours becomes 80 MPa or more, and the drawing ratio becomes 55% or more. A member having such characteristics is also excellent in thermal fatigue characteristics.
[0031]
Hereinafter, the chemical composition of the austenitic stainless steel of the present invention, the crystal grain size and the mixing ratio of the heat-resistant and pressure-resistant member made of this steel, and the reasons for determining the various conditions of the preferred production method as described above will be described in detail. In the following, “%” means “% by mass” unless otherwise specified.
[0032]
1. Chemical composition of austenitic stainless steel
C: 0.03 to 0.12%
C is a component necessary for forming carbides and ensuring high-temperature tensile strength and high-temperature creep strength required for high-temperature austenitic stainless steel, and requires a content of 0.03% or more. However, if the content exceeds 0.12%, undissolved carbides are generated, or the carbides of Cr increase to deteriorate weldability. Therefore, the upper limit is set to 0.12%. Desirable C content is 0.05 to 0.10%.
[0033]
Si: 0.1-1%
Si is added as a deoxidizing agent at the time of steel making, but is also an element necessary for enhancing the steam oxidation resistance of steel, and requires a content of at least 0.1%. However, if the content is excessive, the workability of steel deteriorates, so the upper limit was set to 1%. The preferred range is 0.1-0.5%.
[0034]
Mn: 0.1 to 2%
Mn combines with impurity S contained in steel to form MnS and improves hot workability. However, if the content is less than 0.1%, this effect cannot be obtained. On the other hand, if the content is excessive, the steel becomes hard and brittle, impairing the workability and weldability, so the upper limit was made 2%. Desirable Mn content is 0.5 to 1.2%.
[0035]
P: 0.04% or less
P is inevitably mixed as an impurity, but an excessive P impairs weldability and workability, so the upper limit is made 0.04%. A preferred upper limit is 0.03%. The smaller the P content, the better.
[0036]
S: 0.010% or less
S is inevitably mixed as an impurity similarly to the above-described P, but the upper limit is set to 0.010% because excessive S impairs weldability and workability. A preferred upper limit is 0.008%. The S content is preferably as small as possible in order to improve workability, but is preferably about 0.004 to 0.008% in order to secure the flowability of molten metal during welding.
[0037]
Cr: 20% or more and less than 28%
Cr is an important element for ensuring oxidation resistance, steam oxidation resistance, and corrosion resistance. In order to make the corrosion resistance at a high temperature of 700 ° C. or higher at least 18-8 series steel, a content of at least 20% is required. The above-mentioned corrosion resistance improves as the Cr content increases, but when the Cr content is 28% or more, the structural stability decreases and the creep strength is impaired. Further, in order to stabilize the austenite structure, not only the expensive Ni content must be increased, but also the weldability decreases. Therefore, the Cr content is set to 20% or more and less than 28%. The preferred range is 22-26%.
[0038]
Ni: more than 35% and 50% or less
Ni is an element that stabilizes the austenite structure, and is also an important alloy element for ensuring corrosion resistance. From the balance with the above-mentioned Cr amount, Ni needs to exceed 35%. On the other hand, excessive Ni not only causes an increase in cost but also a decrease in creep strength, so the upper limit is set to 50%. Desirable is 40-48%.
[0039]
Mo: less than 0.5%
As described above, Mo not only causes an embrittlement phase or deteriorates the corrosion resistance under a use environment of 700 ° C. or more, but also has an effect of improving the strength in the composite addition with W described later as compared with the single addition of W. rare. Therefore, in the present invention, Mo is not positively added. However, even if the content of impurities is 0.5% or more, when used in a high temperature range of 700 ° C. or more, generation of an embrittlement phase and reduction in corrosion resistance become remarkable. Therefore, the content of Mo as an impurity is set to less than 0.5%. It is preferably at most 0.3%, more preferably below the detection limit of the assay. The detection limit value of Mo is usually 0.01%.
[0040]
W: 4 to 10%
W is also an important element and suppresses grain boundary slip creep, which is preferential in a high temperature range of 700 ° C. or higher, by a solid solution strengthening action, and for this purpose, a content of at least 4% is required. On the other hand, excessive W does not generate an embrittlement phase unlike Mo, but remarkably hardens steel and deteriorates workability and weldability. Therefore, the upper limit is set to 10%. Desirable is 6 to 8%.
[0041]
Ti: 0.01-0.3%
Ti forms undissolved carbonitrides and oxides to promote austenite crystal grain mixing, and causes non-uniform creep deformation and reduced ductility. Therefore, the content of Ti is 0.3%. The following was set. On the other hand, if the content is less than 0.01%, improvement in high-temperature strength due to precipitation of carbide during use in a high-temperature region cannot be expected. For this reason, the Ti content is set to 0.01 to 0.3%. Preferred is 0.03-0.2%.
[0042]
Nb: 0.01 to 1%
Nb does not become a harmful oxide like Ti, but a minimum content of 0.01% is necessary for improving the creep strength of carbides. On the other hand, excessive Nb impairs weldability, so the upper limit is 1%. Preferred is 0.1-0.5%.
[0043]
sol. Al: 0.0005 to 0.04%
Al is added as a deoxidizing agent, but if added in a large amount, the stability of the tissue deteriorates. The Al content is set to 0.04% or less. On the other hand, in order to obtain a sufficient deoxidizing effect, sol. Al content is required. Preferred is 0.005 to 0.02%.
[0044]
B: 0.0005 to 0.01%
B is an element having an extremely effective grain boundary slip creep suppressing action in the steel of the present invention in which oxides and nitrides are eliminated as much as possible by reducing the contents of N and O (oxygen) described below. However, if the content is less than 0.0005%, this effect cannot be obtained. On the other hand, when the content exceeds 0.01%, the weldability is impaired. Therefore, the B content is set to 0.0005 to 0.01%. Preferred is 0.001 to 0.005%.
[0045]
N: less than 0.02%
Reduction of the content of N and the following O is one of the important requirements of the present invention. Conventionally, N has been actively added as an element to replace precipitation strengthening by carbonitride and a part of expensive Ni. However, a large amount of N forms undissolved carbonitrides of Ti and B, which mix the structure and promote grain boundary slip creep and non-uniform creep deformation in a high temperature region of 700 ° C. or higher. Impair. Therefore, it is necessary to reduce the N content as much as possible. N has a strong affinity with Cr and cannot be avoided as an impurity. However, if the content is less than 0.02%, the undissolved carbonitride is not generated, so the N content is set to less than 0.02%. It is preferably at most 0.016%, more preferably at most 0.01%. The lower the N content, the better.
[0046]
O (oxygen): 0.005% or less
O forms an undissolved oxide of Ti or Al similarly to the above-mentioned N, and this forms a mixed grain, which promotes grain boundary sliding creep and non-uniform creep deformation in a high temperature region of 700 ° C. or higher. And loses strength. Therefore, it is necessary to reduce the O content as much as possible. O is inevitably mixed as an impurity, but if the content is 0.005% or less, the above-mentioned undissolved oxide is not generated, so the O content is 0.005% or less. It is preferably at most 0.003%. The lower the O content, the better.
[0047]
The balance of the austenitic stainless steel of the present invention is substantially Fe, in other words, Fe and impurities other than the above.
[0048]
Another of the austenitic stainless steels of the present invention is a steel containing at least one component selected from at least one of the first to third groups. Hereinafter, these components will be described.
[0049]
First group (Zr)
Zr has the effect of strengthening the grain boundaries and improving the high-temperature strength. Therefore, if it is desired to obtain the effect, it may be positively added and contained. The effect becomes remarkable at a content of 0.0005% or more. However, when the content exceeds 0.1%, undissolved oxides and nitrides are formed similarly to the above-mentioned Ti, which not only promotes grain boundary sliding creep and non-uniform creep deformation, but also increases the steel content. It also degrades quality and impairs creep strength and ductility at high temperatures. Therefore, when added, the Zr content is preferably set to 0.0005 to 0.1%. More preferably, it is 0.001 to 0.06%.
[0050]
Second group (Ca and Mg)
All of these elements have the effect of bonding with S to stabilize S and improve workability. Therefore, when it is desired to obtain the effect, one or more of them may be positively added and contained. In such a case, the above-mentioned effects become remarkable at a content of 0.0005% or more, respectively. However, if Ca exceeds 0.05% and Mg exceeds 0.01%, toughness, ductility and steel quality are impaired. Therefore, when added, the Ca content is preferably 0.0005 to 0.05%, and the Mg content is preferably 0.0005 to 0.01%. More preferably, the Ca content is 0.0005 to 0.01%, and the Mg content is 0.001 to 0.005%.
[0051]
Third group (rare earth elements, Hf and Pd)
All of these elements form harmless and stable oxides and sulfides, reduce the undesired effects of O and S, and have the effect of improving corrosion resistance, workability, creep strength and creep ductility. Therefore, when it is desired to obtain the effect, one or more kinds may be positively added and contained, and in such a case, the above-mentioned effects become remarkable at a content of 0.0005% or more. However, when the content of each exceeds 0.2%, inclusions such as oxides increase and not only impair workability and weldability but also increase costs. Therefore, the content of these elements when added is preferably 0.0005 to 0.2%. A more preferred range is each 0.001 to 0.1%.
[0052]
In addition, as impurities other than P, S, Mo, N, and O, Co and Cu which may be mixed in from scrap or the like can be mentioned. However, Co has no particular adverse effect on the properties of the steel and the heat-resistant and pressure-resistant member of the present invention. Therefore, the content of Co when mixed as an impurity is not particularly limited. However, since Co is also an activation element, the content of Co when mixed is preferably 0.8% or less, more preferably 0.5% or less.
[0053]
Although Cu improves the strength, it significantly promotes grain boundary sliding creep in a high temperature range of 700 ° C. or higher. Therefore, the Cu content when mixed as an impurity is preferably 0.5% or less, more preferably 0.2% or less.
[0054]
2. Heat and pressure resistant material
The heat and pressure resistant member of the present invention is made of austenitic stainless steel having the chemical composition as described above, and its metal structure must be a coarse structure having an austenite crystal grain size number of 6 or less and a mixed particle ratio of 10% or less. Must. The reason is as follows.
[0055]
As described above, the creep strength in a high temperature range of 700 ° C. or more greatly depends on the size of austenite crystal grains and the degree of sizing. In the case of a fine grain structure having a grain size number exceeding 6, grain boundary slip creep is low. Occurs. Further, even in the case of a coarse-grained structure having a particle size number of 6 or less, when the mixed particle ratio exceeds 10%, uneven creep deformation occurs. As a result, the thermal fatigue resistance and the structural stability are inferior, and the creep rupture time of 750 ° C. and 10,000 hours cannot be 80 MPa or more, and the draw ratio cannot be 55% or more.
[0056]
For this reason, in the present invention, the austenite grain size number is set to 6 or less, and the mixing ratio is set to 10% or less. Preferred austenite grain size numbers are 5.5-3. The preferred mixing ratio is 0 (zero)%, that is, a coarse and grain-size structure having a particle size number of 6 or less. The lower limit of the austenite grain size number is not particularly limited, but the coarse grained structure having a grain size number less than 0 cannot be inspected for internal defects or surface flaws by the ultrasonic flaw detection method. Is good.
[0057]
3. Manufacturing method of heat-resistant and pressure-resistant member
Next, a preferred manufacturing method for obtaining the heat-resistant and pressure-resistant member of the present invention having a coarse-grained structure having an austenite crystal grain size number of 6 or less and a mixed grain ratio of 10% or less will be described. This manufacturing method is characterized in that the above-mentioned steps (1) to (3) are sequentially performed.
[0058]
Step ▲ 1 ：
In the method of the present invention, it is necessary to perform heating at least once before final working by hot or cold so that precipitates in the steel precipitated during working are sufficiently dissolved. However, if the heating temperature is lower than 1100 ° C., undissolved carbonitrides and oxides containing stable Ti and B will be present in the heated steel. As a result, this causes non-uniform strain to accumulate in the next step (2), and makes recrystallization uneven in the final heat treatment in the step (3). Further, the undissolved carbonitride or oxide itself inhibits uniform recrystallization, and the above-mentioned predetermined coarse-grained structure cannot be secured. For this reason, in a preferred method of the present invention, heating to 1100 ° C. or more is performed at least once before final working by hot or cold. The upper limit of the heating temperature is not particularly limited, but heating to a temperature exceeding 1350 ° C. may cause high-temperature grain boundary cracking and a decrease in ductility. Therefore, the upper limit of the heating temperature is preferably 1350 ° C.
[0059]
Immediately after heating, hot or cold final processing may be performed. There are no particular restrictions on the cooling conditions after heating and after working when the final working is hot working. However, it is desirable to cool at a cooling rate of 0.25 ° C./sec or more between 800 ° C. and 500 ° C. This is because a coarse precipitate is not formed during cooling.
[0060]
Step (2):
When the final working in the step (1) is hot working, the plastic working in the step (2) may be either hot working or cold working including warm working at a temperature of 500 ° C. or lower. In the case where the final working in the step (1) is cold working including warm working at a temperature of 500 ° C. or lower, it means cold working under the same conditions as the final working.
[0061]
The plastic working in this step is performed for the purpose of imparting distortion in order to promote recrystallization in the next final heat treatment. When the cross-sectional reduction rate of this processing is less than 10%, the strain required for recrystallization cannot be imparted, and a desired coarse-grained structure cannot be obtained even if the next final heat treatment is performed. For this reason, plastic working is performed at a cross-sectional reduction rate of 10% or more. A desirable lower limit of the cross-sectional reduction rate is 20%. The upper limit is not specified because the larger the area reduction rate is, the better, but the maximum value in normal processing is about 90%. This processing step is also a step for determining the dimensions of the product.
[0062]
Process ③:
This is a heat treatment for obtaining a desired coarse-grained structure. If the heating temperature of this heat treatment is lower than 1050 ° C., sufficient recrystallization does not occur, and a desired coarse grain structure cannot be obtained. Further, the crystal grains have a flat work structure, and the creep strength is low. Therefore, the final heat treatment is performed at 1050 ° C. or higher. A preferable heat treatment temperature is a temperature lower by 10 ° C. or more than the heating temperature in the step (1). The upper limit of the final heat treatment temperature is not particularly limited, but is preferably 1350 ° C. for the same reason as in the step (1). Further, after the final heat treatment, it is preferable to cool at a cooling rate of 0.25 ° C./sec or more between 800 ° C. and 500 ° C. for the same reason as in the step (1).
[0063]
【Example】
29 types of steels having the chemical compositions shown in Table 1 were melted. In addition, No. in a comparative example. No. 21 is steel equivalent to SUS310. 22 is SUS316 equivalent steel.
[0064]
No. Steels 1 to 20 were melted into steel ingots using a vacuum melting furnace having a capacity of 50 kg. And, No. Nos. 1 to 4 and Nos. The steel ingots Nos. 11 to 14 were finished into plates by the following production method A. Nos. 5 to 7 and Nos. Steel ingots 15 to 17 were finished into cold-rolled sheets by the following production method B. No. Nos. 8 to 10 and Nos. Steel ingots 18 to 20 were finished into steel pipes by the following production method C.
[0065]
No. Steels Nos. 21 to 29 were melted using a vacuum melting furnace having a capacity of 150 kg, and the obtained steel ingots were subjected to the following manufacturing methods A, B, and C as shown in Table 2, respectively. All of these production methods belong to the production method of the present invention.
[0066]
(1) Manufacturing method A
Step 1 (corresponding to step (1)): heating to 1220 ° C.
Step 2 (corresponding to step (2)): Hot forging with a cross-sectional reduction rate of 67% and a plate material with a thickness of 25 mm
Molded into
Step 3: cooling from 800 ° C. to 500 ° C. or less at 0.55 ° C./sec.
Step 4 (corresponding to step {circle around (3)}): water was cooled after holding at 1210 ° C. for 15 minutes.
[0067]
(2) Manufacturing method B
Step 1 (corresponding to step (1)): heating to 1220 ° C.
Step 2: Forming into a 25 mm-thick plate by hot forging with a cross-sectional reduction rate of 67%
Step 3: cooling from 800 ° C. to 500 ° C. or less at 0.55 ° C./sec
Step 4: Forming into a 20 mm thick plate by external cutting
Step 5 (corresponding to step (2)): Roll rolling at room temperature at a reduction rate of 30% in thickness
Formed into a 14 mm plate,
Step 6 (corresponding to step {circle around (3)}): After cooling at 1200 ° C. for 15 minutes, water cooling.
[0068]
(3) Manufacturing method C
Step 1: Hot forging and external cutting to form round steel with an outer diameter of 175 mm
Step 2 (corresponding to step (1)): heating the round steel to 1250 ° C.
Step 3: The heated round steel is hot extruded and formed into a steel tube having an outer diameter of 64 mm and a wall thickness of 10 mm.
Step 4 (corresponding to step (1)): a steel pipe is heated to 1220 ° C. for 10 minutes and then cooled at 1 ° C./sec.
Step 5 (corresponding to step (2)): drawing at 33% cross-section reduction at room temperature, outer diameter
Formed into 50.8mm, 8.5mm thick steel pipe,
Step 6 (corresponding to step {circle around (3)}): water was cooled after holding at 1210 ° C. for 10 minutes.
[0069]
[Table 1]

[0070]
The austenitic grain size number and the grain size ratio of the hot-worked steel sheet, the cold-rolled steel sheet, and the cold-worked steel pipe obtained in the steps A, B, or C were examined. The austenite grain size number was measured according to the method specified in ASTM, and the grain size was determined by the method described above. At that time, 20 visual fields were observed in each case.
[0071]
Similarly, a creep test piece having an outer diameter of 6 mm and a gauge length of 30 mm was sampled from the hot-worked steel sheet, cold-rolled steel sheet, and cold-worked steel pipe obtained in step A, B or C, and subjected to a creep test. The creep rupture strength (MPa) at 10,000 ° C. for 10,000 hours and the draw ratio (interpolated value:%) were examined. Table 2 summarizes the above results.
[0072]
[Table 2]

[0073]
As can be seen from Table 2, in steels (No. 1 to 20) whose chemical composition is within the range specified in the present invention, the austenite grain size number and the grain size ratio are obtained regardless of the method of A, B or C processed. Is within the range specified in the present invention. As a result, it is clear that a creep rupture strength at 10,000 hours at 750 ° C. of 87 MPa or more and a draw ratio of 57% or more are high, and a heat-resistant pressure-resistant member excellent in heat-resistant fatigue characteristics and structural stability can be obtained.
[0074]
No. 21 (SUS310) and No. 21. 22 (SUS316) has a coarse-grained structure that satisfies the conditions specified in the present invention, but has a very low creep rupture strength of 41 MPa and 55 MPa, respectively, because the chemical composition is out of the range specified in the present invention. .
[0075]
In steels (Nos. 23 to 29) whose chemical composition is out of the range specified in the present invention, both the austenite grain size number and the mixed particle ratio are in the range specified in the present invention, even if the steel is processed and heat-treated by the manufacturing method of the present invention. No coarse-grained structure inside was obtained. As a result, the creep rupture strength is as low as 68 to 78 MPa and the draw ratio is as low as 4 to 23%. No. No. 25 has too high O (oxygen) content. 26 has too high N content. No. 29 has both O and N contents too high. Since the creep rupture strength and the draw ratio are far below the target values, it is understood that the importance of keeping the contents of O and N low is important. That is, these comparative steels cannot provide a heat-resistant and pressure-resistant member exhibiting excellent heat-resistant fatigue characteristics and structural stability in a high temperature range of 700 ° C. or higher.
[0076]
【The invention's effect】
The austenitic stainless steel of the present invention not only has good high-temperature strength and high-temperature corrosion resistance, but also has a coarse-grained structure having an austenite grain size number and a grain size ratio of 6% or less and 10% or less, respectively, in a high-temperature region of 700 ° C or more. It is suitable as a material for heat-resistant and pressure-resistant members having excellent heat-resistant fatigue characteristics and structural stability. Further, the heat and pressure resistant member of the present invention has a creep rupture strength at 750 ° C. and 10,000 hours of 87 MPa or more and a draw ratio of 57% or more, so that the steam temperature is 700 ° C. or more. It can be used as a component. Further, according to the method of the present invention, the heat-resistant and pressure-resistant member of the present invention can be manufactured at low cost.

Claims

In mass%, C: 0.03 to 0.12%, Si: 0.1 to 1%, Mn: 0.1 to 2%, Cr: 20% or more and less than 28%, Ni: more than 35% and 50% Hereinafter, W: 4 to 10%, Ti: 0.01 to 0.3%, Nb: 0.01 to 1%, sol. Al: 0.0005 to 0.04%, and B: 0.0005 to 0.01%, with the balance being Fe and impurities, P as an impurity is 0.04% or less, and S is 0.010%. Hereinafter, an austenitic stainless steel, wherein Mo is less than 0.5%, N is less than 0.02%, and O (oxygen) is 0.005% or less.

The steel according to claim 1, which has a coarse grain structure having an austenite grain size number of 6 or less and a mixed grain ratio of 10% or less, and is excellent in heat-resistant fatigue characteristics and structure stability in a high temperature range. Heat resistant pressure resistant member.

The heat-resistant and pressure-resistant member according to claim 2, wherein the creep rupture strength at 750 ° C for 10,000 hours is 80 MPa or more and the drawing ratio is 55% or more.

In mass%, C: 0.03 to 0.12%, Si: 0.1 to 1%, Mn: 0.1 to 2%, Cr: 20% or more and less than 28%, Ni: more than 35% and 50% Hereinafter, W: 4 to 10%, Ti: 0.01 to 0.3%, Nb: 0.01 to 1%, sol. Al: 0.0005 to 0.04%, B: 0.0005 to 0.01%, and at least one component selected from at least one of the following first to third groups, The balance consists of Fe and impurities, where P as impurities is 0.04% or less, S is 0.010% or less, Mo is less than 0.5%, N is less than 0.02%, and O (oxygen) is 0.1% or less. Austenitic stainless steel characterized by being not more than 005%.
Group 1: 0.0005 to 0.1% Zr by mass.
Group 2: 0.0005-0.05% Ca and 0.0005-0.01% Mg, by weight.
Third group: 0.0005 to 0.2% each of rare earth elements, Hf and Pd by mass%.

The steel according to claim 4, which has an austenitic crystal grain size number of 6 or less and a coarse grain structure with a mixed grain ratio of 10% or less, and is excellent in heat resistance fatigue characteristics and structure stability in a high temperature range. Heat resistant pressure resistant member.

The heat-resistant and pressure-resistant member according to claim 5, wherein the creep rupture strength at 750 ° C for 10,000 hours is 80 MPa or more, and the drawing ratio is 55% or more.

The steel having the chemical composition according to claim 1 is sequentially treated in the following steps (1), (2), and (3), wherein the steel is heat-resistant in a high temperature range according to claim 2 or 3. A method for manufacturing a heat-resistant and pressure-resistant member having excellent properties and structural stability.
Step (1): Before final processing by hot or cold, heating is performed at least once at 1100 ° C. or higher.
Step (2): Plastic working with a cross-sectional reduction rate of 10% or more is performed.
Step (3): Final heat treatment is performed at 1050 ° C. or higher.

The steel having the chemical composition according to claim 4 is sequentially treated in the following steps (1), (2), and (3), and the heat-resistant fatigue in a high temperature region according to claim 5 or 6, A method for manufacturing a heat-resistant and pressure-resistant member having excellent properties and structural stability.
Step (1): Before final processing by hot or cold, heat at least once to 1100 ° C. or more.
Step (2): Plastic working with a cross-sectional reduction rate of 10% or more is performed.
Step (3): Final heat treatment is performed at 1050 ° C. or higher.