JP4173611B2

JP4173611B2 - Austenitic stainless steel for inner pipe of double structure exhaust manifold

Info

Publication number: JP4173611B2
Application number: JP27675699A
Authority: JP
Inventors: 学奥; 佳幸藤村; 敏郎名越
Original assignee: Nippon Steel Nisshin Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1999-09-29
Filing date: 1999-09-29
Publication date: 2008-10-29
Anticipated expiration: 2019-09-29
Also published as: JP2001098344A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車の二重構造エキゾーストマニホールドを構成する内側の管（内管）に好適な高温強度，高温酸化特性，加工性および溶接性に優れたオーステナイト系ステンレス鋼に関するものである。
【０００２】
【従来の技術】
近年、地球環境問題に係る関心の高まりから、厳しい排ガス規制をクリアできる自動車エンジンが求められている。特に、エンジン始動時は、排ガス浄化装置の温度が低く通常運転時よりも有害ガスの浄化効率が低下するため、有害ガスの効率的な除去が重要となる。この問題を解消するために、何らかの手段により排ガス浄化装置に達する排ガス温度を高くし、触媒との反応を促進させる対策が種々検討されてきた。
【０００３】
その手段として、燃焼ガスそのものの温度を上昇させるという最も効果的な方法の他、例えば、エンジンから浄化装置までの排ガス経路（エキゾーストマニホールド，フロントパイプ等）の全体または一部を断熱材で覆う方法、浄化装置自体をエンジン始動前に加熱する方法、浄化装置をエキゾーストマニホールド直下に追加設置する方法等が検討されてきた。
【０００４】
ところが最近では、排ガスの温度低下抑制にかなり効果的な手段として、エキゾーストマニホールドやフロントパイプそのものを二重構造にする方法が浮上し、一部の車種で採用されている。この方法によると、従来の単構造パイプより部品単価は高くなるものの、断熱材，加熱装置，更なる浄化装置等を取り付ける必要がなく、組み立て時の部品点数が増加しないというメリットがある。
【０００５】
単構造のエキゾーストマニホールドでは、加熱および冷却の繰り返しによる熱疲労破壊を避けるために、オーステナイト系ステンレス鋼よりも熱膨張係数の小さいフェライト系ステンレス鋼が使用されてきた。一方、二重構造のエキゾーストマニホールドでは、外側の管（外管）は単構造の場合と同様に拘束された状態で加熱冷却の繰り返しを受けるため、やはり熱膨張の小さいフェライト系ステンレス鋼が使用される。しかし内管は、肉厚が1mm以下と薄いため外管より一層優れた加工性を有する素材が要求され、また材料が拘束されないように設計することが可能なことから、オーステナイト系ステンレス鋼を使用する方が有利な場合が多くなる。
【０００６】
エキゾーストマニホールドの内管部材は排ガスに直接曝されるため、材料温度は排ガスと同程度の800〜1000℃に達する。このため、この温度域で酸化増量の少ない、すなわち高温酸化特性に優れることが重要になる。例えば18Cr−8Niを基本組成とするSUS304では基本的にこの特性が不十分である。また、一般にオーステナイト系ステンレス鋼は、フェライト系ステンレス鋼よりも酸化スケールの密着性が劣るため、繰り返し加熱冷却における耐酸化性も重要となる。
【０００７】
さらに、エキゾーストマニホールドの内管用材料としては、高温強度，加工性，溶接性に優れることも要求される。すなわち、高温強度については、材料が拘束されないよう設計することで加熱冷却の繰り返しによる熱疲労破壊は回避し得るものの、エンジンの振動による疲労が問題となってくる。このため内管用材料には高温高サイクル疲労特性に優れることが望まれる。加工性については、プレス成形，バルジ成形，フランジ成形など種々の加工が想定され、それらの特性を最適に評価する指標は必ずしも明確でないが、少なくともオーステナイト系ステンレス鋼では全伸び，均一伸び，局所変形能に優れることが重要となる。特に穴拡げ加工の場合、加工部の加工硬化が極端に大きいと穴拡げ率が低下するので、耐熱性を向上させるための種々の合金元素の過剰添加は避けるべきである。溶接性については、個々の部品を溶接する際にTIG溶接，MIG溶接等を行うため、溶接割れ感受性の低い材料が好ましい。
【０００８】
【発明が解決しようとする課題】
JIS G 4305に規定されるオーステナイト系ステンレス鋼では、SUS302B，SUS310S，SUSXM15J1等が高温酸化特性に優れ、SUS316，SUS321，SUS347等が高温強度に比較的優れることが知られている。しかし、上述したような耐熱性，加工性，溶接性に優れ、かつ素材コストをも考慮したオーステナイト系ステンレス鋼の化学組成範囲を見出した例は見当たらない。本発明は、このような現状に対応すべく、二重構造エキゾーストマニホールドの内管用材料として要求される耐熱性を確保し、かつ厳しい加工にも十分耐え得る優れた加工性を有し、溶接性にも優れるオーステナイト系ステンレス鋼を提供することを目的とする。
【０００９】
【課題を解決するための手段】
発明者らは、オーステナイト系ステンレス鋼の耐熱性，加工性および溶接性に及ぼす合金元素の影響について詳細な検討を行い、次のような知見を得た。i)Cr含有量，Si含有量の適正化による高温酸化特性の改善とNbの微量添加による高温強度の改善により、エキゾーストマニホールド内管に必要な耐熱性が確保されること、ii)オーステナイト相の相バランスの適正化により、加工性と溶接性の双方に優れた化学組成範囲を明確に特定することが可能であること。本発明はこのような知見に基づき案出されたものである。
【００１０】
上記目的を達成するために、請求項１の発明は、質量％で、C：0.08％以下，Si：2.0超え〜4.0％，Mn：3.0％以下，Ni：6〜12％，Cr：15〜20％，Nb：0.05〜0.30％，Cu：3％以下，Mo：0〜3％（無添加を含む），N：0.08％以下を含有し、残部がFeおよび不可避的不純物からなり、下記(1)式で定義されるＭ値が−40〜0となり、下記(2)式で定義されるＤ値が7〜10となる化学組成を有する、高温強度，高温酸化特性，加工性および溶接性に優れた二重構造エキゾーストマニホールドの内管用オーステナイト系ステンレス鋼である。
Ｍ＝551−462(C＋N)−9.2Si−8.1Mn−29(Ni＋Cu)−13.7Cr−18.5Mo ・・(1)
Ｄ＝(Cr＋1.5Si＋0.5Nb＋Mo)−(Ni＋0.5Mn＋0.3Cu＋30C＋30N) ・・(2)
ここで、Mo含有量の0％とはMoを添加しない場合を意味する。
【００１１】
また、上記目的は、質量％で、C：0.08％以下，Si：2.0超え〜4.0％，Mn：3.0％以下，Ni：6〜12％，Cr：15〜20％，Nb：0.05〜0.30％，Cu：3％以下，Mo：0〜3％（無添加を含む），N：0.08％以下を含有し、さらに、
▲１▼Ti，V，W，Zrの１種または２種以上：合計0.1〜3％
▲２▼REM，Ca，Yの１種または２種以上：合計0.005〜0.1％，
▲３▼Al：0.1〜3.0％，
の▲１▼〜▲３▼の構成のうち必要に応じて１種以上の構成を有し、残部がFeおよび不可避的不純物からなり、前記(1)式で定義されるＭ値が−40〜0となり、前記(2)式で定義されるＤ値が7〜10となる化学組成を有する、高温強度，高温酸化特性，加工性および溶接性に優れた二重構造エキゾーストマニホールドの内管用オーステナイト系ステンレス鋼によって達成される。
【００１２】
請求項２の発明は▲１▼の構成を有するもの、請求項３の発明は▲２▼の構成を有するもの、請求項４は▲１▼＋▲２▼の構成を有するもの、請求項５は▲３▼の構成を有するもの、請求項６は▲１▼＋▲３▼の構成を有するもの、請求項７は▲２▼＋▲３▼の構成を有するもの、請求項８は▲１▼＋▲２▼＋▲３▼の構成を有するものである。
【００１３】
【発明の実施の形態】
以下に、本発明を特定するための事項について説明する。
CとNは、一般にオーステナイト系ステンレス鋼の高温強度を向上させるのに有効であるとされる。また、オーステナイト生成元素として、オーステナイトの相バランスを調整するために添加されることがある。しかし、C，Nを過剰に添加すると、使用中にCr炭化物を生成し、鋼の靱性を低下させるとともに、高温酸化特性に有効な固溶Cr量を減少させる。したがって、必要な高温強度を得るためにC，Nの含有量上限は最小限に規定すべく、C：0.08質量％以下，N：0.08質量％以下とした。好ましいC，Nの含有量範囲は、それぞれC：0.02〜0.08質量％，N：0.01〜0.05質量％である。
【００１４】
Siは、高温酸化特性の改善に非常に有効な元素である。Si含有量が1質量％以下であるSUS304では、800℃という比較的低温での繰り返し酸化においてもスケール剥離が起こる。本発明では前述のように900℃以上の耐酸化性を付与しなくてはならず、そのためには2.0質量％を超えるSi含有量が必要となる。一方、オーステナイト系ステンレス鋼にSiを多量に添加すると硬質になり、また、使用中の加熱によってσ脆化を起こし易くなる。したがって、Si含有量は2.0超え〜4.0質量％とした。好ましいSi含有量は2.0超え〜3.0質量％である。
【００１５】
Mnは、オーステナイト相安定化元素であり、本発明では主として相バランス調整のために添加される。また、高価なNiの代替として使用できる。しかし、過剰なMn添加は高温酸化特性の低下を招く。そこでMn含有量は3.0質量％以下とした。好ましいMn含有量の下限は1.5質量％、上限は2.5質量％である。
【００１６】
Niは、オーステナイト相安定化元素であり、オーステナイト系ステンレス鋼において必要不可欠な元素である。Niは高価だから含有量はできるだけ少量に抑えたいが、本発明においては後述するオーステナイトバランスの調整に必要な量を確保しなければならない。この点を考慮して、Ni含有量は6〜12質量％とする。好ましいNi含有量は6〜9質量％である。
【００１７】
Crは、耐高温酸化性を付与するのに必要不可欠な元素である。特にSiを約2％以上含有するオーステナイト系ステンレス鋼において良好な高温酸化特性を付与するには、15質量％以上のCr含有が必要となる。一方、過剰なCr添加は加熱によるσ脆化を招き、また原料コストも高くなる。したがって、Cr含有量は15〜20質量％とした。好ましいCr含有量は15〜18質量％である。
【００１８】
Nbは、微量添加でCr炭窒化物を微細に析出させ、オーステナイト系ステンレス鋼の高温強度を改善する効果を有する。この効果を十分に発揮させるためには0.05質量％以上のNb含有が必要となる。一方、Nbを過剰に添加すると、加熱によってNb炭窒化物を生成してしまうため微細なCr炭窒化物による高温強度の改善が見込めなくなる他、鋼の靱性も劣化する。十分な高温強度を維持し、かつ靱性を阻害しないよう、Nb含有量は0.05〜0.30質量％とした。好ましいNb含有量は0.05〜0.15質量％である。
【００１９】
Moは、フェライト生成元素であり、高温強度の改善に有効であるが、本発明では必ずしも添加する必要はない。Moを過剰に含有させると加熱中のσ相の生成を促進させ、鋼の靱性を劣化させる。このためMoを添加する場合は3質量％以下の含有量とする。Moを添加する場合の好ましいMo含有量の上限は2.5質量％である。
【００２０】
Cuは、オーステナイト生成元素であり、高温強度の改善に有効である。このため、後述するオーステナイトバランスの調整元素として積極的に添加することが望ましい。一方、過剰の添加は高温酸化特性の劣化を招く。そこでCu含有量は3質量％以下とした。好ましいCu含有量の範囲は1〜3質量％である。
【００２１】
W，TiおよびVは、高温強度の改善に有効であり、その効果を十分に発揮させるにはこれらの元素の合計量が0.1質量％以上となるように添加することが望ましい。これらの元素は単独で用いてもよく、また２種以上を複合で添加してもよい。一方、多量に添加すると鋼が硬質になり、また原料コストも高くなる。このため、W，Ti，Vを添加する場合は、これらの合計量が0.1〜3質量％となるようにする。より好ましいW，Ti，Vの合計含有量範囲は、下限が合計0.5質量％、上限が合計2.5質量％である。
【００２２】
REM，YおよびCaは、高温酸化特性の改善に有効であり、その効果を十分に発揮させるためにはこれらの元素の合計量が0.005質量％以上となるように添加することが望ましい。これらの元素は単独で用いてもよく、また２種以上を複合で添加してもよい。一方、多量に添加すると鋼が硬質になり、また原料コストも高くなる。このため、REM，Y，Caを添加する場合は、これらの合計量が0.005〜0.1質量％となるようにする。より好ましいREM，Y，Caの合計含有量範囲は0.01〜0.1質量％である。
【００２３】
Alも、高温酸化特性の改善に有効な元素であり、その効果を十分に発揮させるには0.1質量％以上含有させることが望ましい。一方、多量に添加すると鋼が硬質になり、また原料コストも高くなる。このため、Alを添加する場合は、0.1〜3.0質量％となるようにする。より好ましいAl含有量の下限は0.5質量％、上限は2.5質量％である。なお、Alは、REM，Y，Caの１種以上と複合で含有させてもよい。
【００２４】
本発明では、以上の合金元素を含有して耐熱性を高めたオーステナイト系ステンレス鋼に、二重構造のエキゾーストマニホールド内管用材料として要求される優れた「加工性」および「溶接性」をも付与することを重要な課題としている。発明者らの研究によれば、加工性と溶接性は、それぞれ特有のオーステナイト安定度の指標に大きく依存することがわかった。そして、加工性と溶接性の両者を高レベルで具備するオーステナイト系ステンレス鋼の化学組成範囲を見出すに至ったのである。前記(1)式で定義されるＭ値は、「加工性」の化学組成依存性を顕著に表すオーステナイト安定度の指標である。前記(2)式で定義されるＤ値は、「溶接性」の化学組成依存性を表すとともに、Ｍ値が同程度の材料間では「加工性」の化学組成依存性をも顕著に表すオーステナイト安定度の指標である。以下にＭ値，Ｄ値の限定理由を説明する。
【００２５】
〔Ｍ値〕
図１は、SUS304より高温酸化特性に優れる16Cr−8Ni−2Si−2Mn鋼をベースとして、Crは14〜20質量％，Niは6〜10質量％，Siは1〜3質量％，Mnは1〜4質量％の範囲でそれぞれ変化させ、C，N，Cu，Moの含有量も適宜変化させた種々のＭ値を有するオーステナイト系ステンレス鋼について、板の「穴拡げ比」に及ぼすＭ値の影響を示したものである。ただし、いずれもδフェライト相の存在しない試料を用いている。ここで、穴拡げ比とは、板厚1.2mm、サイズ90mm角の冷延焼鈍板の中央に直径10mmの打ち抜き穴（d₀＝10mm）をあけ、直径40mmのダイスにて穴を拡げていき、穴縁に亀裂が生じたときの穴直径d₁(mm)を測定して求まるd₁/d₀値を意味している。加工性を穴拡げ比で評価したのは、エキゾーストマニホールドの製造時にはバーリング加工等の穴拡げ加工が多用されるからである。
【００２６】
図１から、穴拡げ比はＭ値に大きく依存することがわかる。しかも、Ｍ値が−40〜0の付近に穴拡げ比のピークが存在するのである。この挙動については以下のように推察される。すなわち、Ｍ値が小さい場合には、オーステナイト相が安定なため加工誘起マルテンサイト変態が生じにくいうえ、本成分系ではオーステナイト相の加工硬化量が大きいことから、良好な穴拡げ比は得られないものと考えられる。Ｍ値が大きくなるに従ってこの加工性阻害要因は解消され、加工性は改善される。しかし、Ｍ値がある程度以上大きくなるとオーステナイト相が不安定になり、穴あけせん断加工時やダイスでの穴拡げ初期にマルテンサイト相が容易に生成してしまう。このため十分な延性が得られず、穴拡げ比は逆に低下するものと考えられる。
【００２７】
このように、Ｍ値によって、エキゾーストマニホールド内管に適した加工性を有するオーステナイト系ステンレス鋼の化学組成範囲を規定することができ、そのＭ値が−40〜0の範囲でほぼピークに近い優れた加工性を呈することがわかった。したがって本発明ではＭ値を−40〜0の範囲に規定した。より好ましいＭ値の下限は−40、上限は−20である。
【００２８】
〔Ｄ値〕
図２は、図１と同様の成分系のうちＭ値が−40〜−10の範囲の鋼について、溶接高温割れ感受性を表す「臨界ひずみ(％)」と、「穴拡げ比」に及ぼすＤ値の影響を示したものである。ここで、臨界ひずみは次のような溶接高温割れ試験によって求めた。すなわち、板厚1.2mm，幅40mm，長さ200mmの冷延焼鈍板を試験片として用い、この試験片の長手方向に最大300N/mm²までの範囲で一定の引張荷重をを加え、この状態で試験片長手方向にTIGのなめづけ溶接をビード長さが100mmとなるように施したのち、予め設定した標点間50mm内に生じた割れ個数を測定する。また、試験後の標点間距離を測定し、試験前からの増分より「溶接時に付与されるひずみ」を求める。このような試験を引張荷重を段階的に変えて行い、割れが発生し始めるときのひずみ値をその材料の臨界ひずみとする。
【００２９】
図２から、臨界ひずみはＤ値＝約7を境に急変し、Ｄ値が約7以上の領域で高い臨界ひずみが得られることがわかる。すなわちＤ値が約7以上の領域で材料の溶接高温割れ感受性は小さくなり、溶接性の顕著な改善が達成される。これは、Ｄ値が大きくなると溶接時にδフェライト相が生成するようになり、Ｄ値が約7で溶接高温割れ感受性を抑止するに足るδフェライト生成量に達するためだと考えられる。
一方、穴拡げ比はＤ値が約10を超えると急激に低下することがわかる。これは、Ｄ値が約10を超えると冷延焼鈍板においてもδフェライト相が残存するするようになることが原因だと推察される。
【００３０】
このように、Ｄ値によって、オーステナイト系ステンレス鋼の溶接性と加工性をともに高レベルに改善する化学組成範囲を規定することができ、その範囲はＤ値が7〜10の範囲であることがわかった。したがって本発明ではＤ値を7〜10の範囲に規定した。より好ましいＤ値の範囲は9〜10である。
【００３１】
【実施例】
表１に供試材の化学組成を示す。表１中のNo.1〜13は発明鋼、No.14〜20は比較鋼である。このうちNo.１〜18の鋼は、真空溶解炉で溶製し、30kgのインゴットに鋳造した。その後、鍛造→熱間圧延→焼鈍→冷間圧延→仕上焼鈍の工程で、板厚1.2mmの冷延焼鈍板を製造した。また、比較鋼のうち、No.19はSUS302B相当鋼、No.20はSUSXM15J1相当鋼であり、これらは板厚1.2mmの市販品（冷延焼鈍板）を試験に用いている。
【００３２】
【表１】

【００３３】
各冷延焼鈍板から試験片を切り出し、穴拡げ試験，溶接高温割れ試験，高温引張試験，高温酸化試験に供した。
穴拡げ試験および溶接高温割れ試験は前述した方法で実施した。高温引張試験はJIS G 0567に準拠して900℃で行い、0.2％耐力を求めて高温強度の指標とした。高温酸化試験はJIS Z 2282に準拠して「大気中1000℃×25分間の加熱→５分間の空冷」を１サイクルとし、これを100サイクル繰り返し、試験前後の重量変化で評価した。これらの結果を表２に示す。
【００３４】
【表２】

【００３５】
本発明に従ったNo.1〜13の鋼は、いずれも穴拡げ性および耐溶接高温割れ性に優れており、しかも高温強度，高温酸化特性といった耐熱性も良好である。このことから、オーステナイト安定度（Ｍ値，Ｄ値）およびCr，Si，Nbの含有量を厳密に制御した効果が確認される。
【００３６】
これに対し、No.14，15，19，20の鋼はＭ値が本発明規定範囲を外れるため、耐熱性は優れるものの、穴拡げ性が発明鋼より劣る。No.16鋼はＤ値が本発明規定範囲より大きいため、臨界ひずみ値は高い（溶接高温割れ感受性は良好である）が、穴拡げ性に劣る。No.17鋼はＤ値が本発明規定範囲より小さいため、臨界ひずみ値が非常に低く溶接高温割れ感受性が高い。さらにNo.17鋼ではNb含有量が本発明規定範囲を外れて低いため、高温強度が低い。No.18鋼はSi含有量が本発明規定範囲を外れて低いため、穴拡げ性，耐溶接高温割れ性，高温強度は良好であっても耐高温酸化性に劣る。
【００３７】
【発明の効果】
本発明では、オーステナイト系ステンレス鋼において、二重構造のエキゾーストマニホールド内管用材料に適した耐熱性，加工性および溶接性を高レベルで呈する、従来知られていなかった組成範囲の存在を明らかにした。したがって、本発明は、当該用途に対して、品質の安定した高性能材料の提供を可能にするものである。
【図面の簡単な説明】
【図１】オーステナイト系ステンレス鋼のＭ値と穴拡げ比の関係を示すグラフである。
【図２】オーステナイト系ステンレス鋼のＤ値と臨界ひずみおよび穴拡げ比の関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an austenitic stainless steel excellent in high-temperature strength, high-temperature oxidation characteristics, workability, and weldability suitable for an inner pipe (inner pipe) constituting a dual structure exhaust manifold of an automobile.
[0002]
[Prior art]
In recent years, an automobile engine that can meet strict exhaust gas regulations has been demanded due to the growing interest in global environmental problems. In particular, when the engine is started, the temperature of the exhaust gas purification device is low and the purification efficiency of harmful gas is lower than that during normal operation. Therefore, efficient removal of harmful gas is important. In order to solve this problem, various measures for increasing the exhaust gas temperature reaching the exhaust gas purification device by some means and promoting the reaction with the catalyst have been studied.
[0003]
As its means, in addition to the most effective method of raising the temperature of the combustion gas itself, for example, a method of covering the whole or part of the exhaust gas path (exhaust manifold, front pipe, etc.) from the engine to the purification device with a heat insulating material, A method of heating the purification device itself before starting the engine and a method of additionally installing the purification device directly under the exhaust manifold have been studied.
[0004]
However, recently, a method of making the exhaust manifold and the front pipe itself a double structure has emerged as a very effective means for suppressing the temperature drop of the exhaust gas, and has been adopted in some vehicle models. According to this method, although the unit cost is higher than that of the conventional single structure pipe, there is no need to install a heat insulating material, a heating device, a further purification device, etc., and there is an advantage that the number of components at the time of assembly does not increase.
[0005]
In a single structure exhaust manifold, ferritic stainless steel having a smaller thermal expansion coefficient than austenitic stainless steel has been used to avoid thermal fatigue failure due to repeated heating and cooling. On the other hand, in a double structure exhaust manifold, the outer pipe (outer pipe) is subjected to repeated heating and cooling in the same manner as in the case of a single structure, so ferritic stainless steel with low thermal expansion is also used. The However, since the inner pipe is thinner than 1 mm, a material with better workability than the outer pipe is required, and it can be designed so that the material is not constrained, so austenitic stainless steel is used. In many cases, it is more advantageous to do this.
[0006]
Since the inner pipe member of the exhaust manifold is directly exposed to the exhaust gas, the material temperature reaches 800 to 1000 ° C., which is the same as that of the exhaust gas. For this reason, it is important that there is little increase in oxidation in this temperature range, that is, excellent high-temperature oxidation characteristics. For example, SUS304 having a basic composition of 18Cr-8Ni basically has insufficient properties. In general, austenitic stainless steel is inferior in the adhesion of oxide scale to ferritic stainless steel, so oxidation resistance in repeated heating and cooling is also important.
[0007]
Further, the material for the inner pipe of the exhaust manifold is required to be excellent in high temperature strength, workability and weldability. That is, with respect to the high temperature strength, thermal fatigue failure due to repeated heating and cooling can be avoided by designing the material not to be constrained, but fatigue due to engine vibration becomes a problem. For this reason, it is desired that the inner pipe material is excellent in high temperature and high cycle fatigue characteristics. As for workability, various processes such as press forming, bulge forming, and flange forming are assumed, and the index for optimal evaluation of these characteristics is not always clear, but at least for austenitic stainless steel, total elongation, uniform elongation, local deformation It is important to have excellent performance. In particular, in the case of hole expansion processing, if the work hardening of the processed portion is extremely large, the hole expansion ratio is lowered. Therefore, excessive addition of various alloy elements for improving heat resistance should be avoided. With respect to weldability, TIG welding, MIG welding, etc. are performed when individual parts are welded, and therefore materials with low weld crack sensitivity are preferred.
[0008]
[Problems to be solved by the invention]
Among austenitic stainless steels defined in JIS G 4305, it is known that SUS302B, SUS310S, SUSXM15J1, etc. are excellent in high temperature oxidation characteristics, and SUS316, SUS321, SUS347, etc. are relatively excellent in high temperature strength. However, no examples have been found that have found the chemical composition range of austenitic stainless steels that are excellent in heat resistance, workability, and weldability as described above, and that take material costs into consideration. In order to respond to such a current situation, the present invention ensures the heat resistance required as a material for the inner pipe of the double structure exhaust manifold, has excellent workability sufficient to withstand severe processing, and has weldability. Another object of the present invention is to provide an austenitic stainless steel that is also excellent.
[0009]
[Means for Solving the Problems]
The inventors conducted a detailed study on the influence of alloying elements on the heat resistance, workability, and weldability of austenitic stainless steel, and obtained the following knowledge. i) The heat resistance required for the exhaust manifold inner pipe is secured by improving the high-temperature oxidation characteristics by optimizing the Cr and Si contents and improving the high-temperature strength by adding a small amount of Nb. ii) The austenite phase By optimizing the phase balance, it is possible to clearly specify a chemical composition range excellent in both workability and weldability. The present invention has been devised based on such knowledge.
[0010]
In order to achieve the above object, the invention of claim 1 is, in mass%, C: 0.08% or less, Si: 2.0 to 4.0%, Mn: 3.0% or less, Ni: 6 to 12%, Cr: 15 to Containing 20%, Nb: 0.05 to 0.30%, Cu: 3% or less, Mo: 0 to 3% (including no additive), N: 0.08% or less, the balance consisting of Fe and inevitable impurities, High-temperature strength, high-temperature oxidation characteristics, workability and weldability having a chemical composition in which the M value defined by the formula (1) is -40 to 0 and the D value defined by the formula (2) is 7 to 10. This is an austenitic stainless steel for the inner pipe of the dual structure exhaust manifold.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)
Here, 0% of the Mo content means a case where Mo is not added.
[0011]
In addition, the above purpose is mass%, C: 0.08% or less, Si: more than 2.0 to 4.0%, Mn: 3.0% or less, Ni: 6-12%, Cr: 15-20%, Nb: 0.05-0.30% , Cu: 3% or less, Mo: 0 to 3% (including no addition), N: 0.08% or less,
(1) One or more of Ti, V, W and Zr: 0.1 to 3% in total
(2) One or more types of REM, Ca, Y: Total 0.005-0.1%,
(3) Al: 0.1-3.0%
(1) to (3) having at least one type of configuration as required, the balance being made of Fe and inevitable impurities, and the M value defined by the formula (1) being −40 to Austenite for inner pipe of dual structure exhaust manifold having a chemical composition in which the D value defined by the formula (2) is 7 to 10 and excellent in high temperature strength, high temperature oxidation characteristics, workability and weldability Achieved with stainless steel.
[0012]
The invention of claim 2 has a configuration of (1), the invention of claim 3 has a configuration of (2), claim 4 has a configuration of (1) + (2), and claim 5 Has a configuration of (3), Claim 6 has a configuration of (1) + (3), Claim 7 has a configuration of (2) + (3), and Claim 8 has a configuration of (1) It has the structure of ▼ + ▲ 2 ▼ + ▲ 3 ▼.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Below, the matter for specifying this invention is demonstrated.
C and N are generally considered effective for improving the high temperature strength of austenitic stainless steel. Moreover, it may be added as an austenite generating element in order to adjust the phase balance of austenite. However, when C and N are added excessively, Cr carbide is generated during use, and the toughness of the steel is lowered, and the amount of solute Cr effective for high-temperature oxidation characteristics is reduced. Therefore, in order to obtain the necessary high temperature strength, the upper limit of the C and N content is set to C: 0.08% by mass or less and N: 0.08% by mass or less in order to define the minimum. Preferable content ranges of C and N are C: 0.02 to 0.08 mass% and N: 0.01 to 0.05 mass%, respectively.
[0014]
Si is an extremely effective element for improving high-temperature oxidation characteristics. In SUS304 having a Si content of 1% by mass or less, scale peeling occurs even in repeated oxidation at a relatively low temperature of 800 ° C. In the present invention, as described above, oxidation resistance of 900 ° C. or higher must be imparted, and for that purpose, a Si content exceeding 2.0 mass% is required. On the other hand, when a large amount of Si is added to austenitic stainless steel, it becomes hard, and σ embrittlement is likely to occur due to heating during use. Therefore, the Si content is more than 2.0 to 4.0% by mass. A preferable Si content is more than 2.0 to 3.0% by mass.
[0015]
Mn is an austenite phase stabilizing element and is added mainly for adjusting the phase balance in the present invention. It can also be used as an alternative to expensive Ni. However, excessive addition of Mn causes deterioration of high temperature oxidation characteristics. Therefore, the Mn content is set to 3.0% by mass or less. The lower limit of the preferable Mn content is 1.5% by mass, and the upper limit is 2.5% by mass.
[0016]
Ni is an austenitic phase stabilizing element and is an indispensable element in austenitic stainless steel. Since Ni is expensive, it is desired to keep the content as small as possible. However, in the present invention, the amount necessary for adjusting the austenite balance described later must be ensured. Considering this point, the Ni content is 6-12% by mass. A preferable Ni content is 6 to 9% by mass.
[0017]
Cr is an indispensable element for imparting high-temperature oxidation resistance. In particular, an austenitic stainless steel containing about 2% or more of Si needs to contain 15% by mass or more of Cr in order to provide good high-temperature oxidation characteristics. On the other hand, excessive addition of Cr causes σ embrittlement due to heating, and increases the raw material cost. Therefore, the Cr content is 15 to 20% by mass. A preferable Cr content is 15 to 18% by mass.
[0018]
Nb has an effect of improving the high temperature strength of austenitic stainless steel by precipitating Cr carbonitride finely by adding a small amount. In order to fully exhibit this effect, 0.05% by mass or more of Nb is required. On the other hand, if Nb is added excessively, Nb carbonitride is produced by heating, so that improvement in high-temperature strength due to fine Cr carbonitride cannot be expected, and the toughness of the steel also deteriorates. The Nb content was set to 0.05 to 0.30% by mass so that sufficient high-temperature strength was maintained and toughness was not hindered. A preferable Nb content is 0.05 to 0.15% by mass.
[0019]
Mo is a ferrite-forming element and is effective in improving the high-temperature strength, but it is not necessarily added in the present invention. If Mo is excessively contained, the formation of σ phase during heating is promoted, and the toughness of the steel is deteriorated. Therefore, when adding Mo, the content is 3% by mass or less. A preferable upper limit of the Mo content when Mo is added is 2.5 mass%.
[0020]
Cu is an austenite-forming element and is effective in improving high temperature strength. For this reason, it is desirable to add positively as an austenite balance adjusting element described later. On the other hand, excessive addition leads to deterioration of high temperature oxidation characteristics. Therefore, the Cu content is set to 3% by mass or less. The range of preferable Cu content is 1-3 mass%.
[0021]
W, Ti, and V are effective in improving the high-temperature strength, and it is desirable to add them so that the total amount of these elements is 0.1% by mass or more in order to fully exhibit the effect. These elements may be used alone or in combination of two or more. On the other hand, when added in a large amount, the steel becomes hard and the raw material cost increases. For this reason, when adding W, Ti, and V, it is made for these total amount to be 0.1-3 mass%. A more preferable total content range of W, Ti, and V has a lower limit of 0.5% by mass and an upper limit of 2.5% by mass.
[0022]
REM, Y, and Ca are effective in improving the high-temperature oxidation characteristics, and it is desirable to add them so that the total amount of these elements is 0.005% by mass or more in order to fully exhibit the effect. These elements may be used alone or in combination of two or more. On the other hand, when added in a large amount, the steel becomes hard and the raw material cost increases. For this reason, when adding REM, Y, and Ca, the total amount of these is 0.005 to 0.1 mass%. A more preferable total content range of REM, Y, and Ca is 0.01 to 0.1% by mass.
[0023]
Al is also an element effective for improving the high-temperature oxidation characteristics, and is desirably contained in an amount of 0.1% by mass or more in order to fully exhibit the effect. On the other hand, when added in a large amount, the steel becomes hard and the raw material cost increases. For this reason, when adding Al, it will be 0.1-3.0 mass%. The lower limit of the Al content is more preferably 0.5% by mass, and the upper limit is 2.5% by mass. Al may be contained in combination with one or more of REM, Y, and Ca.
[0024]
In the present invention, the austenitic stainless steel containing the above alloy elements and having improved heat resistance is also provided with excellent “workability” and “weldability” required as a material for a double-structure exhaust manifold inner pipe. Doing so is an important issue. According to the studies by the inventors, it has been found that workability and weldability are largely dependent on specific austenite stability indicators. And it came to find the chemical composition range of the austenitic stainless steel which has both workability and weldability at a high level. The M value defined by the formula (1) is an austenite stability index that significantly represents the chemical composition dependence of “workability”. The D value defined by the formula (2) represents the chemical composition dependence of “weldability”, and austenite that also significantly represents the chemical composition dependence of “workability” between materials having the same M value. It is an indicator of stability. The reason for limiting the M and D values will be described below.
[0025]
[M value]
Figure 1 is based on 16Cr-8Ni-2Si-2Mn steel, which has better high-temperature oxidation characteristics than SUS304, Cr is 14-20% by mass, Ni is 6-10% by mass, Si is 1-3% by mass, and Mn is 1 For austenitic stainless steels with various M values that were varied in the range of ~ 4% by mass, and the contents of C, N, Cu, and Mo were appropriately changed, the M value that affects the “hole expansion ratio” of the plate It shows the impact. However, in each case, a sample having no δ ferrite phase is used. Here, the hole expansion ratio refers to a punched hole (d ₀ = 10 mm) with a diameter of 10 mm in the center of a cold rolled annealed sheet with a thickness of 1.2 mm and a size of 90 mm square, and the hole is expanded with a 40 mm diameter die. It means the d ₁ / d ₀ value obtained by measuring the hole diameter d ₁ (mm) when a crack occurs in the hole edge. The reason why the workability was evaluated by the hole expansion ratio is that hole expansion processing such as burring processing is frequently used when manufacturing the exhaust manifold.
[0026]
FIG. 1 shows that the hole expansion ratio greatly depends on the M value. In addition, there is a hole expansion ratio peak in the vicinity of the M value of −40 to 0. This behavior is inferred as follows. That is, when the M value is small, since the austenite phase is stable, work-induced martensite transformation is unlikely to occur, and since the work hardening amount of the austenite phase is large in this component system, a good hole expansion ratio cannot be obtained. It is considered a thing. As the M value increases, the workability hindering factor is eliminated and the workability is improved. However, when the M value becomes larger than a certain level, the austenite phase becomes unstable, and a martensite phase is easily generated at the time of drilling shearing or at the initial stage of hole expansion with a die. For this reason, it is considered that sufficient ductility cannot be obtained and the hole expansion ratio is decreased.
[0027]
In this way, the chemical composition range of austenitic stainless steel having workability suitable for the exhaust manifold inner pipe can be defined by the M value, and the M value is in the range of −40 to 0 and is almost close to the peak. It was found to exhibit excellent workability. Therefore, in this invention, M value was prescribed | regulated in the range of -40-0. The lower limit of the M value is more preferably −40, and the upper limit is −20.
[0028]
[D value]
FIG. 2 is a graph showing the effect of D on the “critical strain (%)” representing the weld hot cracking susceptibility and the “hole expansion ratio” for steels having an M value in the range of −40 to −10 in the same component system as FIG. It shows the effect of the value. Here, the critical strain was determined by the following welding hot cracking test. In other words, a cold-rolled annealed plate with a thickness of 1.2 mm, width 40 mm, and length 200 mm was used as a test piece, and a constant tensile load was applied in the longitudinal direction of this test piece up to 300 N / mm ² in this state. Then, TIG tanning welding is performed in the longitudinal direction of the test piece so that the bead length becomes 100 mm, and then the number of cracks generated within a preset gauge point of 50 mm is measured. Further, the distance between the test marks after the test is measured, and the “strain applied during welding” is obtained from the increment from before the test. Such a test is performed by changing the tensile load step by step, and the strain value when cracks start to occur is defined as the critical strain of the material.
[0029]
From FIG. 2, it can be seen that the critical strain suddenly changes at a D value = about 7, and a high critical strain can be obtained in a region where the D value is about 7 or more. That is, in the region where the D value is about 7 or more, the weld hot cracking susceptibility of the material is reduced, and a remarkable improvement in weldability is achieved. This is presumably because when the D value is increased, a δ ferrite phase is generated during welding, and a D value of about 7 reaches a δ ferrite generation amount sufficient to suppress weld hot cracking susceptibility.
On the other hand, it can be seen that the hole expansion ratio rapidly decreases when the D value exceeds about 10. It is presumed that this is because when the D value exceeds about 10, the δ ferrite phase remains even in the cold-rolled annealed sheet.
[0030]
Thus, the D value can define a chemical composition range that improves both the weldability and workability of the austenitic stainless steel to a high level, and the range is that the D value is in the range of 7 to 10. all right. Therefore, in this invention, D value was prescribed | regulated in the range of 7-10. A more preferable range of the D value is 9-10.
[0031]
【Example】
Table 1 shows the chemical composition of the test materials. In Table 1, Nos. 1 to 13 are invention steels and Nos. 14 to 20 are comparative steels. Among these, No. 1-18 steel was melted in a vacuum melting furnace and cast into a 30 kg ingot. Thereafter, a cold-rolled annealed sheet having a thickness of 1.2 mm was manufactured in a process of forging → hot rolling → annealing → cold rolling → finish annealing. Of the comparative steels, No. 19 is SUS302B equivalent steel, and No. 20 is SUSXM15J1 equivalent steel. These products use a commercial product (cold-rolled annealed plate) with a thickness of 1.2 mm for the test.
[0032]
[Table 1]

[0033]
A test piece was cut out from each cold-rolled annealed plate and subjected to a hole expansion test, a weld hot cracking test, a high temperature tensile test, and a high temperature oxidation test.
The hole expansion test and the weld hot cracking test were performed by the methods described above. The high temperature tensile test was conducted at 900 ° C. in accordance with JIS G 0567, and 0.2% proof stress was obtained as an index of high temperature strength. The high-temperature oxidation test was performed in accordance with JIS Z 2282 with “heating in the atmosphere at 1000 ° C. × 25 minutes → air cooling for 5 minutes” as one cycle, and this was repeated 100 cycles, and the change in weight before and after the test was evaluated. These results are shown in Table 2.
[0034]
[Table 2]

[0035]
The steels Nos. 1 to 13 according to the present invention are all excellent in hole expansibility and weld hot cracking resistance, and also have good heat resistance such as high temperature strength and high temperature oxidation characteristics. This confirms the effect of strictly controlling the austenite stability (M value, D value) and the contents of Cr, Si, and Nb.
[0036]
On the other hand, steels No. 14, 15, 19, and 20 have M values that are out of the range specified in the present invention, so that although the heat resistance is excellent, the hole expandability is inferior to that of the inventive steel. Since No. 16 steel has a D value larger than the specified range of the present invention, the critical strain value is high (the weld hot cracking sensitivity is good), but the hole expandability is inferior. Since No. 17 steel has a D value smaller than the specified range of the present invention, the critical strain value is very low and the weld hot cracking sensitivity is high. Further, in No. 17 steel, the Nb content is low outside the scope of the present invention, so the high temperature strength is low. Since No. 18 steel has a low Si content outside the scope of the present invention, hole expandability, weld hot crack resistance and high temperature strength are good, but it is inferior in high temperature oxidation resistance.
[0037]
【The invention's effect】
In the present invention, in the austenitic stainless steel, the existence of a composition range that has not been known so far, which exhibits a high level of heat resistance, workability and weldability suitable for a dual-structure exhaust manifold inner pipe material, was clarified. . Therefore, the present invention makes it possible to provide a high-performance material with stable quality for the application.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between M value and hole expansion ratio of austenitic stainless steel.
FIG. 2 is a graph showing the relationship between D value, critical strain and hole expansion ratio of austenitic stainless steel.

Claims

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less, with the balance being Fe and inevitable impurities, M value defined by the following formula (1) is -40 to 0, and D value defined by the following formula (2) is 7 Austenitic stainless steel for inner pipe of dual structure exhaust manifold with chemical composition of ~ 10 and excellent in high temperature strength, high temperature oxidation characteristics, workability and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less,
One or more of Ti, V, W, Zr: 0.1 to 3% in total
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less
One or more of REM, Ca, Y: Total 0.005-0.1%
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less
One or more of REM, Ca, Y: Total 0.005-0.1%,
One or more of Ti, V, W, Zr: 0.1 to 3% in total
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less,
Al: 0.1-3.0%
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less,
Al: 0.1-3.0%,
One or more of Ti, V, W, Zr: 0.1 to 3% in total
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less,
One or more of REM, Ca, Y: Total 0.005-0.1%,
Al: 0.1-3.0%
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)

% By mass
C: 0.08% or less,
Si: more than 2.0 to 4.0%,
Mn: 3.0% or less,
Ni: 6-12%,
Cr: 15-20%,
Nb: 0.05-0.30%,
Cu: 3% or less,
Mo: 0 to 3% (including no additive),
N: 0.08% or less,
One or more of REM, Ca, Y: Total 0.005-0.1%,
Al: 0.1-3.0%,
One or more of Ti, V, W, Zr: 0.1 to 3% in total
In which the balance is Fe and inevitable impurities, the M value defined by the following formula (1) is −40 to 0, and the D value defined by the following formula (2) is 7 to 10. Austenitic stainless steel for inner pipes of dual-structure exhaust manifolds with a composition that has excellent high-temperature strength, high-temperature oxidation characteristics, workability, and weldability.
M = 551−462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo ・・ (1)
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N) (2)