JP4197139B2

JP4197139B2 - Martensitic stainless steel sheet for welding, manufacturing method thereof and structural member

Info

Publication number: JP4197139B2
Application number: JP2003156108A
Authority: JP
Inventors: 宏紀冨村; 誠一磯崎
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2003-06-02
Filing date: 2003-06-02
Publication date: 2008-12-17
Anticipated expiration: 2023-06-02
Also published as: JP2004359971A

Description

【０００１】
【発明の属する技術分野】
本発明は、機械構造材，自転車・自動車その他の輸送機器，建材，屋内・屋外配管などのうち、特に溶接部の強度と耐食性が要求される用途、例えばスチールベルト，自転車リム，屋内・屋外配管用ステンレス鋼パイプなどの素材として好適な高強度ステンレス鋼板、およびその製造方法、並びに前記鋼板を溶接してなる構造部材に関する。
【０００２】
【従来の技術】
従来の高強度ステンレス鋼として以下のものが挙げられる。
（Ａ）SUS301やSUS304等のオーステナイト系ステンレス鋼を冷間圧延によって硬化させた加工硬化型ステンレス鋼。これは、冷間加工によって誘起されたマルテンサイト自体の硬さを利用するものである。
しかし、（Ａ）タイプの鋼種は溶接すると加工誘起マルテンサイトがオーステナイトに逆変態するため、軟化が避けられない。したがって、溶接用途には適していない。
【０００３】
（Ｂ）SUS630あるいはSUS631に代表される析出硬化型ステンレス鋼。これは、時効処理前には硬さが低く加工性に優れ、時効処理後は析出強化による高強度を発現する。また（Ａ）タイプに比べると一般的に溶接軟化抵抗も高い。これらの特性を活かし、各種ばね材やスチールベルト等に用いられている。本出願人は、この種のステンレス鋼において靱性やねじり特性を改善した鋼を提案し、下記特許文献１，２に開示した。
【０００４】
しかし、（Ｂ）タイプの鋼種は硬化に寄与する時効析出物が溶接により母相へ固溶したり過時効の状態になったりするため、用途によっては溶接部での軟化が問題になることがある。この場合、溶接部を再度時効処理すれば強度回復は可能であるが、コスト増等により現実的には困難な場合が多い。また、このタイプはＣu，Ａl，Ｔi，Ｍoといった時効硬化元素を含有させる必要があるため、原料コストも高くなる。
【０００５】
（Ｃ）焼鈍状態あるいは圧延率数％の調質圧延状態で高強度を有する焼入れ硬化型ステンレス鋼。これは、オーステナイト相あるいはオーステナイト相＋フェライト相の温度領域から室温へ冷却する際に生成するマルテンサイト相を利用して高強度を図るものである。SUS420J2のように更に焼戻し処理を行って高強度化する鋼種も含まれる。高価な析出硬化元素を要せず製造工程も比較的少ないことから、原料コスト・製造コスト共に比較的安価である。本出願人はこの種のステンレス鋼として、スチールベルト用低炭素マルテンサイト系ステンレス鋼を特許文献３に、また面内異方性の小さい高延性高強度の複相組織ステンレス鋼を特許文献４にそれぞれ紹介した。
【０００６】
しかし、（Ｃ）タイプの鋼種は一般的に（Ａ）や（Ｂ）のタイプに比べ強度が低い。強度向上のために調質圧延したりＣ，Ｎを多量に含有させたりする手法を採ると靱性が損なわれ易いため、この種のステンレス鋼で靱性を確保しながら強度向上を図ることは難しいとされていた。
【０００７】
本出願人は、Ｃ，Ｎ，Ｎi等の添加量を調整し、かつ各元素の含有量をバランスさせ組織制御することにより、この（Ｃ）タイプの鋼種において高強度・高靱性・高ばね特性を具備する安価な鋼を開発し、特許文献５，６に開示した。特許文献５，６に示した開発鋼は（Ａ）タイプよりも製造性に優れかつ製品特性のバラツキも小さく、（Ｂ）タイプよりも安価であり、性能，コストの両面において昨今のユーザーの厳しい要求に応えるものである。
【０００８】
【特許文献１】
特開平７−１５７８５０号公報
【特許文献２】
特開平８−７４００６号公報
【特許文献３】
特公昭５１−３１０８５号公報
【特許文献４】
特開昭６３−７３３８号公報
【特許文献５】
特開２０００−１２９４０１号公報
【特許文献６】
特開２００１−２７１１４０号公報
【０００９】
【発明が解決しようとする課題】
しかし、（Ｃ）タイプの鋼種には、溶接部（特に溶接熱影響部）において炭化物が析出し、Ｃr欠乏層生成により耐食性低下が生じるという問題が内在する。場合によっては強度低下が生じることもある。スチールベルトなどの機械構造部材や配管用ステンレス鋼パイプをはじめとする種々の用途において、溶接が施されても優れた強度特性を維持し、かつ厳しい環境下で優れた耐食性を維持し得る金属材料が求められている。特に昨今、このような厳しい要求は高まりつつある。特許文献５，６に開示される開発鋼においても、溶接部の耐食性維持という点では必ずしもユーザーの要求を満たしているとは言えない。
【００１０】
本発明はこのような現状に鑑み、本来安価な製造が可能な（Ｃ）タイプのマルテンサイト系鋼種において、特殊元素を添加することなく上記開発鋼に不足していた「溶接部で高耐食性を維持し得る性質」を付与し、各種溶接用途に高い信頼性をもって使用できる高強度ステンレス鋼板を提供することを目的とする。併せて、溶接部を有する構造部材であって、母材部・溶接部ともステンレス鋼本来の高耐食性を呈するものを提供することを目的とする。
【００１１】
【課題を解決するための手段】
一般的にステンレス鋼溶接部の耐食性を改善するには、Ｃrの増量やＭoの添加等、強固な不動態皮膜を形成する元素を添加する手法、あるいは、炭化物析出に伴うＣr欠乏層の生成を防止するために低Ｃ化する手法が考えられる。しかし、前者はコスト増を伴い、後者は強度低下を伴うため、いずれも高強度鋼を安価に提供する上で採用し難い。特にマルテンサイト系鋼種ではＣによる固溶強化を利用して強度向上を図っているため、溶接熱影響部において炭化物析出に伴う耐食性低下を防止することは本質的に難しい。
【００１２】
発明者らは、マルテンサイト相の強度向上を図る手法として、従来の常識である「Ｃの固溶強化」に代わる新たな手法の可能性を研究してきた。その結果、マルテンサイト相のマトリクス中に炭窒化物を均一に分散させたとき、固溶Ｃを大幅に減らした場合でも実用上十分な強度が確保できることを知見した。固溶Ｃと共に固溶Ｎも大幅に低減すれば、溶接熱影響部での新たな炭窒化物の析出を抑えることができ、耐食性劣化を防止できる。また、炭窒化物の均一分散を実現すれば母材部でのＣr欠乏層生成も解消される。詳細な実験の結果、そのような組織状態は、例えば30％以上の冷間圧延と600〜800℃という低温での最終焼鈍を組み合わせることにより実現できた。本発明はこれらの知見に基づいて完成したものである。
【００１３】
すなわち、本発明の目的を達成するために、請求項１の発明は、質量％で、Ｃ：0.05〜0.10％，Ｓi：0.2〜2.0％，Ｍn：1.0％以下，Ｐ：0.06％以下，Ｓ：0.006％以下，Ｎi：2.0〜5.0％，Ｃr：14.0〜17.0％，Ｎ：0.04〜0.10％，Ｂ：0（無添加）〜0.0070％を含み、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Ｌ値が320以上380未満である化学組成を有し、固溶(Ｃ＋Ｎ)量が0.04質量％以下であり、炭窒化物がマルテンサイト相のマトリクス中に分散して存在する溶接用マルテンサイト系ステンレス鋼板である。
Ｌ＝192Ｃ−11Ｓi＋12Ｍn＋17Ｎi−14Ｃr＋247Ｎ＋476 ……(1)
ここで、炭窒化物が分散して存在しているとは、粒界・粒内を問わず、局所的に分布量の多い箇所がないことを意味する。なお、(1)式右辺の元素記号の箇所には各元素の含有量を質量％で表した値が代入される。
【００１４】
請求項２の発明は、請求項１の発明において、さらに、「下記(a)に示す母材試験片について下記(b)に示す試験液を用いてJIS H 8502に準じたキャス試験を50±2℃で200時間行ったとき試験片表面に発銹が認められない耐食性を有する」点を規定したものである。
(a) 当該鋼板の表面を＃400研磨仕上げした試験片。
(b) 5％ＮaＣl水溶液１リットル当たりＣu₂Ｃlを0.268ｇ溶解させた液に酢酸を加えてpＨを3.0〜3.1に調整した試験液。
このような良好な耐食性を有することは、当該鋼板が事実上Ｃr欠乏層のない組織状態を有することを意味する。
【００１５】
請求項３の発明は、請求項１の発明において、さらに、「下記(a')の溶接試験片について前記(b)に示した試験液を用いてJIS H 8502に準じたキャス試験を50±2℃で200時間行ったとき、母材部表面に発銹が認められず、かつ溶接部表面に1.0mmを超える径の発銹が認められない耐食性を有する」点を規定したものである。
(a') 当該鋼板に、溶加材を用いないビードオンプレートのTIG溶接を溶融部が母材板厚を貫通する条件で行った後、溶接ビード部の凸部をグラインダーで平滑化し、母材部と共に＃400研磨仕上げした試験片。
ここで、ビードオンプレートのTIG溶接とは、板の表面上にTIG溶接のトーチを移動させながら溶接ビードを形成する溶接実験である。溶接部とは「溶融部」と「熱影響部」を合わせた部分をいう。母材部とは溶接部以外の部分をいう。
溶接部の耐食性が良好であることは、固溶(Ｃ＋Ｎ)量が新たな炭窒化物の析出を招かない程度に十分低減されていることを意味する。また、母材部の耐食性が良好であることは、当該鋼板が事実上Ｃr欠乏層のない組織状態を有することを意味する。
【００１６】
請求項４の発明は、請求項１に記載の化学組成を有する熱延焼鈍鋼板に圧延率30％以上の冷間圧延を施し、次いで最終焼鈍を600〜800℃で行う、溶接用マルテンサイト系ステンレス鋼板の製造方法である。
請求項５の発明は、上記製造方法において更に圧延率30％以下の調質圧延を施すものである。
このようにして得られる鋼板は、後述の実施例で示すように、母材部および溶接部において良好な耐食性を示すものである。
【００１７】
請求項６の発明は、請求項１〜３のいずれかに記載の鋼板において、特に硬さが320HV以上380HV未満のものである。
請求項７の発明は、請求項１、２、３、６のいずれかに記載の鋼板を溶接してなる溶接部の強度および耐食性に優れた構造部材である。構造部材には機械や建築物等を構成する「部品」が含まれる。
【００１８】
【発明の実施の形態】
炭化物，窒化物の構成には以下の態様が挙げられる。
▲１▼ ＣとＮが複合して金属元素と結合した析出物。例えばＭ₂₃(Ｃ,Ｎ)₆。
▲２▼ Ｃが単独で金属元素と結合した析出物。例えばＭ₂₃Ｃ₆。
▲３▼ Ｎが単独で金属元素と結合した析出物。例えばＭ₂₃Ｎ₆。
ここで、ＭはＦe，Ｃr等の金属元素である。本明細書で「炭窒化物」とは主として▲１▼を意味するが、▲２▼あるいは▲３▼が存在するときにはそれらも含む。また「炭化物」とは▲１▼と▲２▼を意味し、「窒化物」とは▲１▼と▲３▼を意味する。
【００１９】
以下、本発明を特定するための事項について説明する。
Ｃは、マルテンサイト相マトリクス中で炭化物を生成することにより強度に寄与する重要な元素である。また、δフェライトの生成を抑制する。これらの作用を十分に得るには0.05質量％以上のＣ含有が必要である。しかし、Ｃ含有量が多くなると炭化物を構成するＣr量も多くなるため、固溶Ｃr量が減少して耐食性レベルが低下する。また、炭化物が増加して延性や靱性も低下する。このような弊害はＣ含有量が0.10質量％を超えると顕著に現れるようになる。したがって、Ｃ含有量は0.05〜0.10質量％の狭い範囲に規定する。
【００２０】
Ｓiは、固溶強化能が大きく、マトリクスを強化する。この作用はＳi含有量が0.2質量％以上で顕著に現れる。しかし、2.0質量％を超えてＳiを含有させても、固溶強化作用は飽和すると共に、δフェライト相の生成が助長されることによる延性・靱性の劣化が目立つようになる。したがって、Ｓi含有量は0.2〜2.0質量％に規定する。
【００２１】
Ｍnは、高温域でのδフェライト相の生成を抑制する。しかし、多量のＭn含有は最終焼鈍後の残留オーステナイト量を多くし、強度低下の原因となる。このため、Ｍn含有量は1.0質量％以下に規定する。より好ましいＭn含有量は0.2〜0.6質量％である。
【００２２】
Ｐは、耐食性を悪化させる原因となるので、少ないほど望ましい。本発明ではＰ含有量は0.06質量％まで許容できる。
【００２３】
Ｓは、ＭnＳ等の非金属介在物として鋼中に存在し、耐食性に悪影響を及ぼす。また、熱間圧延時には粒界に偏析して熱間加工割れや肌荒れを引き起こすと共に、冷間圧延で耳切れを起こす要因となる。Ｓは少ないほど好ましく、0.006質量％以下に制限する。
【００２４】
Ｎiは、同じオーステナイト生成元素であるＣおよびＮの一部を置換して、多量のＣ，Ｎ添加による耐食性低下を防止する上で有効である。また、δフェライト相の生成を抑制し、最終焼鈍時にマルテンサイト組織を得て、母材部および溶接部で高強度を維持するのに重要である。本発明で対象とする合金系でこれらの効果を有効に得るには、少なくとも2.0質量％以上のＮi含有が必要である。しかし、5.0質量％を超えると残留オーステナイト量が多くなりすぎ、母材部および熱影響部で強度低下を招く。したがって、Ｎi含有量は2.0〜5.0質量％に規定する。
【００２５】
Ｃrは、優れた耐食性を得る上で、本発明では14.0質量％以上必要である。しかし、16.5質量％を超えると最終製品のδフェライト量が多くなる。若干のδフェライトは強度にそれほど悪影響を及ぼさないが、Ｃr含有量が17.0質量％を超えるとδフェライト相の増加に起因して十分な強度を得るのが困難になると共に、延性，靱性，冷間圧延性なども低下する。この場合、δフェライト相の生成を抑制するためにオーステナイト生成元素を多量添加する方法もあるが、最終焼鈍後に多量のオーステナイト相が残留して強度低下を招くことになり好ましくない。したがって、Ｃr含有量は14.0〜17.0質量％に規定する。特に望ましいＣr含有量の上限は16.5質量％である。
【００２６】
Ｎは、窒化物生成により強度向上に寄与すると共に、δフェライトの生成を抑制する。また、Ｃの一部をＮで置換してＣの多量添加を抑制することにより、延性および靱性の劣化を回避することができる。このようなＮの作用を有効に得るためには、少なくとも0.04質量％のＮ含有が必要である。しかし、0.10質量％を超えると、残留オーステナイト量が多くなりすぎるために、十分な強度が得られなくなる。したがって、Ｎ含有量は0.04〜0.10質量％に規定する。
【００２７】
Ｂは、冷間圧延時の耳切れ発生を抑制する効果がある。また、焼鈍後の冷却過程では場合によってＳが粒界に偏析し、室温での延性および靱性が低下することがあるが、Ｂはこの現象を改善する効果がある。このため、本発明では必要に応じてＢを添加する。ただし、0.0070質量％を超えて多量にＢを添加すると、Ｂ系析出物が粒界に生成して最終製品の靱性を劣化させる要因になる。したがって、Ｂを添加する場合は0.0070質量％以下の範囲で行う。
【００２８】
以上の元素に加え、ＭoやＣuを添加して更に耐食性を向上させることができる。ただし、Ｍo＋Ｃuの合計が2.0質量％を超えると、母材部または溶接部で残留オーステナイトあるいはδフェライトが生成して強度低下を招く場合がある。したがってこれらの元素を添加する場合には合計含有量が2.0質量％以下の範囲で行うことが望ましい。
【００２９】
以上の化学組成を有する本発明の対象鋼は、高温域から特別に急冷処理を施すことなくマルテンサイト組織が得られる。すなわち、熱延焼鈍鋼板，最終焼鈍後の鋼板，および更に調質圧延を施した鋼板は、いずれもマルテンサイト主体の組織を呈する。本発明の鋼板は多くの場合、マトリクスは実質的にマルテンサイト単相組織を呈する。析出物や介在物の他にごく少量のδフェライトや残留オーステナイトを含むことはあるが、少なくとも95体積％以上がマルテンサイト相である。
【００３０】
本発明の鋼板は、硬さが320〜380HV未満に調整されていることが望ましい。320HV未満では溶接後に母材部，溶接部とも十分な強度が確保されない。また、この成分系では固溶(Ｃ＋Ｎ)量が0.04質量％を超える組織状態にしないと最終焼鈍後の硬さを380HV以上に調整することは難しい。その場合は後述のように溶接後の耐食性低下を招く。したがって、最終焼鈍後の鋼板硬さは320〜380HV未満に調整することが好ましい。なお、最終焼鈍後に調質圧延を施す場合は、調質圧延後の硬さが320〜380HV未満になるように最終焼鈍後の硬さを調整することが望ましい。
【００３１】
前記(1)式で定義されるＬ値は、各元素の含有量が上記の範囲にある鋼板を700℃で最終焼鈍を施した場合に得られる硬さと良い対応関係を示す指標である。Ｌ値が320〜380未満となるように成分調整しておくと、最終焼鈍を後述のように600〜800℃で行ったときに鋼板硬さを320〜380HV未満の範囲にコントロールすることが可能となる。したがって、本発明ではＬ値が320〜380未満となる化学組成を採用する。
【００３２】
本発明対象鋼種における溶接部の耐食性低下は溶接熱影響部での炭窒化物析出に伴うＣr欠乏層の形成が主たる原因である。そこで本発明では、溶接時における新たな炭窒化物の生成を抑止するために、母材鋼板中の固溶Ｃ量および固溶Ｎ量を低減しておく。種々検討の結果、固溶Ｃ量と固溶Ｎ量の合計量、すなわち固溶(Ｃ＋Ｎ)量を0.04質量％以下に低減したとき、溶接熱影響部での耐食性低下は多くの用途でほとんど問題にならないことがわかった。
【００３３】
単に固溶(Ｃ＋Ｎ)量を低減するだけなら、低Ｃ化，低Ｎ化した鋼を使用し、必要に応じてＴi等のＣ，Ｎ固定元素を添加すれば間に合う。しかし、それでは高強度を維持することができない。本発明では、鋼中のＣ含有量は少なくとも0.05質量％以上、Ｎ含有量は0.04質量％以上を確保し、製造過程でこれらの大部分を炭窒化物の形にして存在させる。その炭窒化物は析出強化作用を発揮し、鋼板の強度は高レベルに維持される。そして、炭窒化物の析出に伴って固溶(Ｃ＋Ｎ)量を0.04質量％以下に低減させたとき、溶接部での耐食性低下が防止できるのである。
【００３４】
ただし、ここで解決しなければならない新たな問題が生じた。炭窒化物は熱延板焼鈍時にマルテンサイトの粒界に優先的に析出するので、その周囲にＣr欠乏層が生じてしまうのである。また、最終焼鈍時にも炭窒化物が元のマルテンサイト粒界に優先的に析出することがある。この不均一な炭窒化物の析出を克服し、最終的に粒界・粒内を問わず、局所的に分布量の多い箇所がないような組織状態にしなければ母材鋼板自体が耐食性に劣るものとなってしまう。
【００３５】
そこで、発明者らは種々実験を繰り返した結果、以下の(i)〜(iii)のプロセスにより、この問題を解消し得ることがわかった。
(i) 熱延板焼鈍を行う。
(ii) その後、圧延率30％以上の冷間圧延を行う。
(iii) 次いで、600〜800℃で最終焼鈍を行う。
これにより、炭窒化物をマルテンサイト相のマトリクス中に分散して存在させ、かつ固溶(Ｃ＋Ｎ)量を0.04質量％以下に低減させることができる。
【００３６】
(i)の熱延板焼鈍は、特に厳密な条件コントロールを要しないが、熱延板の歪みを除去するために600℃以上に加熱することが望ましい。ただし、加熱温度が高すぎると結晶粒径の粗大化を招き、靱性が低下しやすくなって製造しにくくなるので、1100℃以下で行うのがよい。加熱時間は均熱0〜120分程度が望ましい。材料加熱温度が600〜800℃の場合、この過程で炭窒化物はほぼ全量が析出し尽くす。800℃を超える高温加熱の場合は、加熱温度が高いほど析出量は少なくなる。熱延板焼鈍では炭窒化物はマルテンサイトの粒界に優先的に析出しやすいので、特に加熱温度が600〜800℃の場合はＣr欠乏層の形成が顕著になりやすい。このＣr欠乏層は(ii)(iii)の過程により消失させることができる。
【００３７】
なお、特許文献６の段落0043の記載によると、熱延板焼鈍（中間焼鈍）を600〜800℃の範囲で行うと炭窒化物の析出が抑えられると教示されるが、その後、本発明者らが詳細に調査した結果、上記の通り600〜800℃の温度範囲で炭窒化物の生成が認められることが判った。特許文献６では炭窒化物の析出が十分に抑えられているとき冷延耳切れが顕著に抑止できるとしているが、結果的に炭窒化物が生成してもＢ添加等により特許文献６で目的とする冷延耳切れの防止効果は十分得られるようである。
【００３８】
(ii)の冷間圧延は、所定の板厚を得ると共に、加工歪みの導入を目的とする。この加工歪みは、(iii)の最終焼鈍における組織均一化に必要となる。そのためには30％以上の圧延率を確保しなければならない。圧延率の上限は特に制限されないが、現実的には80％以下が望ましい。
【００３９】
(iii)の最終焼鈍は、再結晶化・軟質化の他、更に以下の２つの重要な目的を有する。１つは、熱延板焼鈍で生じたＣr欠乏層を消失させることである。30％以上の冷間圧延により導入された加工歪みを駆動力として熱延組織を崩壊することで、旧マルテンサイト粒界に局在化していた炭窒化物はマトリクス中に分散し、このとき同時にマトリクス中のＣrが析出物周辺に補給され、Ｃr欠乏層も消失する。もう１つは、特に熱延板焼鈍が800℃を超えて高かった場合、析出しきれずに固溶状態で残ったＣおよびＮを十分に析出させることである。その際、前記の加工歪みが駆動力となって炭窒化物の析出サイトが増加し、この段階で新たに生成する炭窒化物はマトリクス中の粒界・粒内を問わずほぼ均一に生成する。このようにして最終焼鈍後には、炭窒化物がマルテンサイト中に分散して存在し、固溶(Ｃ＋Ｎ)量が0.04％以下に低減したＣr欠乏層のない組織状態が得られるのである。熱延板焼鈍で粒界に析出した炭窒化物も、結果的に最終焼鈍で生じたものと区別できない程度に分散して存在する。
【００４０】
上記のような最終焼鈍の効果は600℃以上の加熱によって得られるが、800℃を超えるとすでに析出した炭窒化物が再固溶し、固溶(Ｃ＋Ｎ)量を安定して0.04質量％以下にすることが困難となる。したがって、最終焼鈍温度は600〜800℃に規定する。加熱時間は、材料温度が600〜800℃の範囲内に設定した所定温度に達したのちすぐに冷却するいわゆる均熱0秒の焼鈍も採用できる。鋼帯での連続処理を考慮すると均熱0〜180秒程度が好ましい。
なお、前記（ｃ）タイプの鋼種では、例えば950〜1050℃といった高温で最終焼鈍を行うのが一般的である。これに対し本発明では、600〜800℃という低温で最終焼鈍を行い、その後に鋼板素材としての熱処理は施さない。これは、本発明では固溶(Ｃ＋Ｎ)量を低減させるという、従来材とは異なる組織状態を得るためである。
【００４１】
炭窒化物がマルテンサイト相のマトリクス中に分散して存在する組織状態であるかどうかは、透過型電子顕微鏡等を用いた直接的観察により確認できる。しかし、そのような機器を用いなくても、間接的に組織状態の良否を知ることができる。例えば、当該鋼板から下記(a)の試験片を作製し、下記(b)に示す試験液を用いてJIS H 8502に準じたキャス試験を50±2℃で200時間行ってみればよい。
(a) 当該鋼板の表面を＃400研磨仕上げした試験片。
(b) 5％ＮaＣl水溶液１リットル当たりＣu₂Ｃlを0.268ｇ溶解させた液に酢酸を加えてpＨを3.0〜3.1に調整した試験液。
このとき、試験片表面に発銹が認められない耐食性を有していれば、炭窒化物は均一に分散し、Ｃr欠乏層は事実上ないとみてよい。
【００４２】
また、固溶(Ｃ＋Ｎ)量が0.04質量％以下であるかどうかは、後述の実施例に示すような抽出残渣の分析により確認することができる。しかし、そのような分析を行わなくても、間接的に固溶(Ｃ＋Ｎ)量が十分に低減されているかどうかを知ることができる。例えば、当該鋼板から下記(a')の溶接試験片を作製し、上記(b)に示す試験液を用いてJIS H 8502に準じたキャス試験を50±2℃で200時間行ってみればよい。
(a') 当該鋼板に、溶加材を用いないビードオンプレートのTIG溶接を溶融部が母材板厚を貫通する条件で行った後、溶接ビード部の凸部をグラインダーで平滑化し、母材部と共に＃400研磨仕上げした試験片。
このとき、溶接部表面に1.0mmを超える径の発銹が認められない耐食性を有していれば、固溶(Ｃ＋Ｎ)量は例えば0.04質量％以下に十分低減されているとみてよい。また、同時に母材部表面に発銹が認められない耐食性を有していれば、炭窒化物は均一に分散し、Ｃr欠乏層は事実上ないとみてよい。
【００４３】
最終焼鈍後にはさらに調質圧延を施すことができる。調質圧延は母材部の強度やばね性を高めるのに有効である。ただし、調質圧延率が30％を超えると、母材部の延性低下および靱性低下の問題が顕在化しやすくなる。したがって、調質圧延を行う場合は圧延率30％以下の範囲で行うことが望ましい。
【００４４】
【実施例】
〔実施例１〕
表１に示す化学組成の鋼を溶製し、各鋼とも約100kgの鋼塊から熱間圧延を経て板厚4.0mmの熱延板を製造した。表１中、A1〜A5が本発明で規定する化学組成を有する発明対象鋼、B1〜B5が比較鋼、C1が従来鋼のSUS301である。表１には前記(1)式で定義されるＬ値も示した。なお、B1はＣ含有量、B2はＮ含有量、B3およびB4はＬ値、B5はＣr含有量がそれぞれ本発明規定範囲を外れる。
【００４５】
【表１】

【００４６】
本発明対象鋼および比較鋼の熱延板（板厚4.0mm）については、700℃×均熱１時間の熱延板焼鈍を行い、スケールを除去した後、板厚1.0mmまで圧延率75％の冷間圧延を施した。次いで700℃×均熱60秒の最終焼鈍を施し、焼鈍鋼板を得た。従来鋼のC1は加工硬化型ステンレス鋼であるため、1070℃の焼鈍後に圧延率45％の冷間圧延を施し、板厚1.0mmの調質圧延板とした。これらについて、鋼板中の固溶(Ｃ＋Ｎ)量を調べた。また、溶接ビードを形成した試験片（以下「溶接試験片」という）について、母材部および溶接部の耐食性，硬さを調べた。
【００４７】
固溶(Ｃ＋Ｎ)量は抽出残渣の分析により求めた。すなわち、鋼板から切り出した分析試験片を10％アセチルアセトン＋１％テトラメチルアンモニウムクロライド＋メタノール溶液中で溶解電圧40〜70mVにて溶解し、採取した残渣について質量測定およびＥＰＭＡの定量分析を行って残渣中のＣ，Ｎ量（未固溶のＣ，Ｎ量に相当する）を求め、その値と予め判っている全(Ｃ＋Ｎ)量の値を用いて固溶(Ｃ＋Ｎ)量を算出した。
【００４８】
溶接試験片は厚さ1.0mmの鋼板に溶加材を用いないビードオンプレートのTIG溶接を施すことにより作製した。TIG溶接条件は、電極：1.2mm径のタングステン，溶接電流：約70Ａ，トーチ移動速度：300mm／min，シールガス：流量10Ｌ／minのアルゴンとした。この条件で母材板厚を貫通する溶融部が形成された。
【００４９】
耐食性はキャス試験で評価した。溶接ビード部を含む150×100mmの溶接試験片について、溶接ビード部の凸部をグラインダーで平滑化し、その部分と母材部を共に＃400研磨仕上げした後、キャス試験に供した。キャス試験はJIS H 8502のキャス試験方法に準じて、5％ＮaＣl水溶液１リットル当たりＣu₂Ｃlを0.268ｇ溶解させた液に酢酸を加えてpＨを3.0〜3.1に調整した試験液を用い、50±2℃で200時間行った。母材部，溶接部について、それぞれ以下の基準で評価した。
○：発銹が認められない。
△：点状の発銹が認められるが、発銹点の径は1.0mm以下である。
×：1.0mmを超える径の発銹が認められる。
この耐食性試験で母材部に発銹が見られず、かつ溶接部に1.0mmを超える径の発銹が生じなければ、スチールベルトや配管用ステンレス鋼パイプをはじめとする多くの構造部材において実用上十分な優れた耐食性を有するとみてよい。そこで、母材部が○評価、かつ溶接部が△以上の評価であるものを合格と判定した。なお、耐食性試験は各鋼種とも２枚の試験片について行った（ｎ＝２）。
【００５０】
母材部の硬さは、鋼板母材部表面のHV₂₀値にて表示した。溶接部の硬さは、溶接試験片の溶接ビードに垂直な断面について、HV_0.1値（荷重0.1kg）を母材部から溶接部を経て母材部に至る直線上を0.2mm間隔で測定し、溶接部における最も低いHV_0.1測定値を「溶接部の最低硬さ」として表示した。
これらの結果を表２に示す。耐食性についてはｎ＝２の結果を併記してある。
【００５１】
【表２】

【００５２】
表２に示されるように、本発明対象鋼は母材部，溶接部ともに耐食性は良好である。また、溶接部での著しい硬度低下はみられず、母材部，溶接部ともに320HV以上の硬さを維持した。
これに対し、比較鋼B1およびB4はそれぞれＣ含有量およびＬ値が本発明規定範囲より高いため、溶接部の耐食性が良くない。B2およびB3はそれぞれＮ含有量およびＬ値が本発明規定範囲より低いため、母材部で320HV以上の硬さが得られず、溶接部ではさらに硬度低下が生じた。B5はＣr含有量が高いため母材鋼板にδフェライトが残留し、母材部，溶接部とも320HV以上の硬さは得られていない。従来鋼C1はSUS301Hに相当し、溶接により溶融部がオーステナイト単相となったため、硬さが200HVと低い。
【００５３】
〔実施例２〕
表１のA1鋼を用いて、熱延板焼鈍後の冷間圧延率および最終焼鈍温度を種々変化させ、最終焼鈍後の固溶(Ｃ＋Ｎ)量，並びに母材部および溶接部の耐食性，硬さを調べた。各試験方法は実施例１と同様である。なお、熱延板焼鈍は700℃×均熱１時間で行い、熱延板焼鈍前の板厚を調整して冷間圧延後の板厚が1mmになるようにし、最終焼鈍の時間は均熱60秒とした。
製造条件および結果を表３に示す。このうちR8は5％の調質圧延を施したものである。また、図１には冷間圧延率を60％と一定にしたR3〜R7について、母材部の硬さと溶接部の耐食性に及ぼす最終焼鈍温度の影響を示す。
【００５４】
【表３】

【００５５】
表３，図１に示されるように、熱延板焼鈍後に30％以上の冷間圧延を施し、かつ最終焼鈍を600〜800℃の範囲で行った本発明例のものは、鋼板中の固溶(Ｃ＋Ｎ)量が0.04質量％以下となり、その鋼板は溶接後に母材部，溶接部とも優れた耐食性を示し、かつ溶接部においても320HV以上の硬さを維持することが確認された。透過型電子顕微鏡観察の結果、これらの鋼板には炭窒化物がマルテンサイト相のマトリクス中に粒内・粒界を問わずほぼ均一に分散していた。したがって、単に固溶(Ｃ＋Ｎ)量が低減しているだけでなく、Ｃr欠乏層も消失していると考えられる。この点は母材部の耐食性が非常に良好であることによって肯定されるところである。
【００５６】
これに対し、比較例のR1は冷間圧延率が30％未満で冷延歪みが小さいため、最終焼鈍においては、熱延板焼鈍時の局所的な炭窒化物析出に伴うＣr欠乏層を消失させることができなかったこと、および、新たな炭窒化物が冷間圧延前にマルテンサイト粒界であったサイトに局所的に析出したことが考えられ、母材部，溶接部とも耐食性が悪かった。R4は最終焼鈍温度が600℃未満のため、熱延板焼鈍時に生じたＣr欠乏層を消失させることができなかったと考えられ、母材部，溶接部の耐食性に劣った。R7は最終焼鈍温度が高すぎたため、鋼板中の固溶(Ｃ＋Ｎ)量が0.04質量％を超え、溶接時に熱影響部で新たな炭窒化物が生成して鋭敏化したと考えられ、溶接部の耐食性が劣化した。
【００５７】
【発明の効果】
本発明によれば、本来的に安価なマルテンサイト系ステンレス鋼において、特殊な元素を添加することなく、溶接に供したときに耐食性および強度が高く維持できる鋼板が提供可能になった。
【図面の簡単な説明】
【図１】母材部の硬さと溶接部の耐食性に及ぼす最終焼鈍温度の影響を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention is a machine structure material, bicycle / automobile or other transport equipment, building material, indoor / outdoor piping, etc., particularly where the strength and corrosion resistance of the welded portion are required, such as steel belt, bicycle rim, indoor / outdoor piping. High-strength stainless steel plate suitable as a material for stainless steel pipes, and Manufacturing method thereof and the steel sheet It is related with the structural member formed by welding.
[0002]
[Prior art]
Examples of conventional high-strength stainless steel include the following.
(A) Work hardening type stainless steel obtained by hardening austenitic stainless steel such as SUS301 or SUS304 by cold rolling. This utilizes the hardness of martensite itself induced by cold working.
However, when the steel type (A) is welded, the work-induced martensite reversely transforms to austenite, so softening is inevitable. Therefore, it is not suitable for welding applications.
[0003]
(B) Precipitation hardening type stainless steel represented by SUS630 or SUS631. This is low in hardness before aging treatment and excellent in workability, and exhibits high strength by precipitation strengthening after aging treatment. Moreover, compared with the (A) type, generally the welding softening resistance is also high. Taking advantage of these characteristics, it is used in various spring materials and steel belts. The present applicant has proposed a steel having improved toughness and torsional characteristics in this type of stainless steel, and disclosed it in the following Patent Documents 1 and 2.
[0004]
However, (B) type steel grades may cause aging precipitates that contribute to hardening to form a solid solution in the parent phase or become over-aged by welding. is there. In this case, the strength can be recovered by re-aging the welded portion, but it is often difficult in practice due to an increase in cost. Further, since this type needs to contain an age hardening element such as Cu, Al, Ti, and Mo, the raw material cost is also increased.
[0005]
(C) A quench-hardening stainless steel having high strength in an annealed state or a temper rolled state with a rolling rate of several percent. This is intended to increase the strength by utilizing the martensite phase generated when cooling from the temperature range of the austenite phase or austenite phase + ferrite phase to room temperature. Also included is a steel grade that is further strengthened by tempering, such as SUS420J2. Since expensive precipitation hardening elements are not required and the number of manufacturing processes is relatively small, both raw material costs and manufacturing costs are relatively low. The present applicant, as this kind of stainless steel, in Patent Document 3 is a low carbon martensitic stainless steel for steel belts, and in Patent Document 4 is a high ductility high strength multiphase stainless steel with small in-plane anisotropy. Each was introduced.
[0006]
However, the steel type (C) generally has lower strength than the types (A) and (B). It is difficult to improve strength while securing toughness with this kind of stainless steel because toughness is likely to be lost if a method of temper rolling to improve strength or incorporating a large amount of C and N is used. It had been.
[0007]
The present applicant adjusts the addition amount of C, N, Ni, etc., and balances the content of each element to control the structure, thereby providing high strength, high toughness, and high spring characteristics in this (C) type steel type. Inexpensive steel having the above has been developed and disclosed in Patent Documents 5 and 6. The developed steels shown in Patent Documents 5 and 6 are more manufacturable than the (A) type and have less variation in product characteristics, are less expensive than the (B) type, and are strict by recent users in terms of both performance and cost. It meets the demand.
[0008]
[Patent Document 1]
JP-A-7-157850
[Patent Document 2]
JP-A-8-740006
[Patent Document 3]
Japanese Patent Publication No. 51-31085
[Patent Document 4]
JP 63-7338 A
[Patent Document 5]
JP 2000-129401 A
[Patent Document 6]
JP 2001-271140 A
[0009]
[Problems to be solved by the invention]
However, the (C) type steel type has a problem that carbide is precipitated in the welded portion (particularly the weld heat affected zone), and the corrosion resistance is reduced due to the Cr-deficient layer formation. In some cases, the strength may decrease. Metal materials that can maintain excellent strength characteristics even under welding in various applications including mechanical structural members such as steel belts and stainless steel pipes for piping, and can maintain excellent corrosion resistance in harsh environments Is required. Especially in recent years, such strict requirements are increasing. Even in the developed steels disclosed in Patent Documents 5 and 6, it cannot always be said that the user's request is satisfied in terms of maintaining the corrosion resistance of the welded portion.
[0010]
In view of the current situation, the present invention is a (C) type martensitic steel type that can be manufactured at low cost, and lacks the above-mentioned developed steel without adding special elements. The object is to provide a high-strength stainless steel sheet that can be used with high reliability in various welding applications. In addition, it is an object of the present invention to provide a structural member having a welded portion, in which both the base material portion and the welded portion exhibit high corrosion resistance inherent to stainless steel.
[0011]
[Means for Solving the Problems]
In general, in order to improve the corrosion resistance of stainless steel welds, a method of adding an element that forms a strong passive film, such as increasing the amount of Cr or adding Mo, or generation of a Cr-depleted layer accompanying carbide precipitation In order to prevent this, a method of reducing the C can be considered. However, since the former is accompanied by an increase in cost and the latter is accompanied by a decrease in strength, both of them are difficult to adopt for providing high-strength steel at low cost. In particular, martensitic steel types use solid solution strengthening due to C to improve strength, and therefore it is essentially difficult to prevent a decrease in corrosion resistance associated with carbide precipitation in the weld heat affected zone.
[0012]
The inventors have studied the possibility of a new technique as a technique for improving the strength of the martensite phase, replacing the conventional common sense “solid solution strengthening of C”. As a result, it was found that when carbonitride was uniformly dispersed in the matrix of the martensite phase, a practically sufficient strength could be secured even when the solid solution C was greatly reduced. If the solid solution N is significantly reduced together with the solid solution C, precipitation of new carbonitrides at the weld heat affected zone can be suppressed, and deterioration of corrosion resistance can be prevented. In addition, if a uniform dispersion of carbonitride is realized, the generation of a Cr-deficient layer in the base material portion is also eliminated. As a result of detailed experiments, such a structural state can be realized by combining, for example, cold rolling of 30% or more and final annealing at a low temperature of 600 to 800 ° C. The present invention has been completed based on these findings.
[0013]
That is, in order to achieve the object of the present invention, the invention of claim 1 is mass%, C: 0.05 to 0.10%, Si: 0.2 to 2.0%, Mn: 1.0% or less, P: 0.06% or less, S : 0.006% or less, Ni: 2.0 to 5.0%, Cr: 14.0 to 17.0%, N: 0.04 to 0.10%, B: 0 (no addition) to 0.0070%, the balance consisting of Fe and inevitable impurities, (1) It has a chemical composition whose L value defined by the formula is 320 or more and less than 380, the solid solution (C + N) amount is 0.04% by mass or less, and carbonitride is dispersed in the matrix of martensite phase. It is a martensitic stainless steel sheet for welding.
L = 192C-11Si + 12Mn + 17Ni-14Cr + 247N + 476 (1)
Here, the fact that carbonitrides are dispersed means that there is no portion where the distribution amount is locally large regardless of the grain boundary or inside the grain. Note that a value representing the content of each element in mass% is substituted for the element symbol on the right side of the equation (1).
[0014]
The invention of claim 2 is the invention of claim 1, ,further `` When the cast test according to JIS H 8502 was performed at 50 ± 2 ° C. for 200 hours using the test solution shown in (b) below for the base material test piece shown in (a) below, the surface of the test piece was rusted. "It has an unacceptable corrosion resistance."
(a) A test piece obtained by polishing the surface of the steel plate with # 400.
(b) Cu per liter of 5% NaCl aqueous solution ₂ A test solution in which pH was adjusted to 3.0 to 3.1 by adding acetic acid to a solution in which 0.268 g of Cl was dissolved.
Having such good corrosion resistance means that the steel sheet has a structure state that is virtually free of a Cr-deficient layer.
[0015]
The invention of claim 3 is the invention of claim 1. ,further "When a casting test according to JIS H 8502 was conducted for 200 hours at 50 ± 2 ° C using the test solution shown in (b) above for the weld test piece of (a ') below, It has a corrosion resistance in which no flaws are observed and no flaws having a diameter exceeding 1.0 mm are observed on the surface of the weld zone.
(a ') After performing TIG welding of a bead-on-plate without using a filler metal on the steel sheet under the condition that the molten part penetrates the base metal plate thickness, the convex part of the weld bead part is smoothed with a grinder, A specimen finished with # 400 polished together with the material.
Here, bead-on-plate TIG welding is a welding experiment in which a weld bead is formed while moving the TIG welding torch on the surface of the plate. The welded portion refers to the portion where the “melted portion” and the “heat affected zone” are combined. A base material part means parts other than a welding part.
The good corrosion resistance of the weld zone means that the amount of solid solution (C + N) is sufficiently reduced to the extent that no new carbonitride precipitates. Moreover, that the corrosion resistance of the base metal part is good means that the steel sheet has a structure state that is virtually free of a Cr-deficient layer.
[0016]
In the invention of claim 4, the hot-rolled annealed steel sheet having the chemical composition according to claim 1 is subjected to cold rolling with a rolling rate of 30% or more, and then the final annealing is performed at 600 to 800 ° C. , Martensitic stainless steel sheet for welding Manufacturing method It is.
The invention of claim 5 In the above manufacturing method Further Pressure Apply temper rolling with a rolling ratio of 30% or less Is a thing .
like this Can be obtained The steel sheet exhibits good corrosion resistance in the base metal part and the welded part, as shown in Examples described later.
[0017]
The invention of claim 6 Claim 1 to 3 In particular, the steel plate has a hardness of 320HV or more and less than 380HV.
The invention of claim 7 Claim 1, 2, 3, or 6 It is the structural member excellent in the intensity | strength and corrosion resistance of the welding part formed by welding this steel plate. The structural member includes “parts” that constitute a machine, a building, or the like.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The configuration of carbide and nitride includes the following modes.
(1) A precipitate in which C and N are combined and combined with a metal element. For example, M _{twenty three} (C, N) ₆ .
(2) A precipitate in which C is bonded to a metal element alone. For example, M _{twenty three} C ₆ .
(3) A precipitate in which N is bonded to a metal element alone. For example, M _{twenty three} N ₆ .
Here, M is a metal element such as Fe or Cr. In this specification, “carbonitride” mainly means (1), but includes (2) or (3) when there is (2) or (3). “Carbide” means (1) and (2), and “nitride” means (1) and (3).
[0019]
Hereinafter, matters for specifying the present invention will be described.
C is an important element that contributes to strength by forming carbides in the martensite phase matrix. In addition, the formation of δ ferrite is suppressed. In order to obtain these effects sufficiently, it is necessary to contain 0.05% by mass or more of C. However, if the C content increases, the amount of Cr constituting the carbide also increases, so the amount of solid solution Cr decreases and the corrosion resistance level decreases. Further, the carbides increase and the ductility and toughness also decrease. Such an adverse effect appears remarkably when the C content exceeds 0.10% by mass. Therefore, the C content is specified in a narrow range of 0.05 to 0.10% by mass.
[0020]
Si has a large solid solution strengthening ability and reinforces the matrix. This effect is prominent when the Si content is 0.2% by mass or more. However, even if Si is contained in an amount exceeding 2.0% by mass, the solid solution strengthening action is saturated and the ductility and toughness are deteriorated due to the promotion of the formation of the δ ferrite phase. Therefore, the Si content is specified to be 0.2 to 2.0 mass%.
[0021]
Mn suppresses the formation of the δ ferrite phase in the high temperature range. However, when a large amount of Mn is contained, the amount of retained austenite after the final annealing is increased, which causes a decrease in strength. For this reason, Mn content is prescribed | regulated to 1.0 mass% or less. A more preferable Mn content is 0.2 to 0.6% by mass.
[0022]
Since P causes deterioration of corrosion resistance, the smaller the amount, the better. In the present invention, the P content is acceptable up to 0.06% by mass.
[0023]
S exists in steel as non-metallic inclusions such as MnS, and adversely affects corrosion resistance. Further, during hot rolling, it segregates at the grain boundaries to cause hot work cracking and rough skin, and also causes cutting off at cold rolling. S is preferably as small as possible, and is limited to 0.006% by mass or less.
[0024]
Ni is effective in substituting a part of C and N, which are the same austenite-forming elements, to prevent a decrease in corrosion resistance due to the addition of a large amount of C and N. Further, it is important to suppress the formation of δ ferrite phase, obtain a martensite structure at the time of final annealing, and maintain high strength in the base metal part and the welded part. In order to effectively obtain these effects in the alloy system targeted by the present invention, Ni content of at least 2.0% by mass or more is necessary. However, if it exceeds 5.0% by mass, the amount of retained austenite is excessively increased, leading to a decrease in strength at the base material portion and the heat affected zone. Therefore, the Ni content is specified to be 2.0 to 5.0 mass%.
[0025]
In order to obtain excellent corrosion resistance, Cr is required to be 14.0% by mass or more in the present invention. However, if it exceeds 16.5% by mass, the amount of δ ferrite in the final product increases. Some δ-ferrites do not adversely affect the strength, but if the Cr content exceeds 17.0% by mass, it becomes difficult to obtain sufficient strength due to the increase in δ-ferrite phase, and ductility, toughness, cold The rolling property is also lowered. In this case, there is a method of adding a large amount of austenite generating elements in order to suppress the formation of the δ ferrite phase, but a large amount of austenite phase remains after the final annealing, which is not preferable. Therefore, the Cr content is specified to be 14.0 to 17.0 mass%. A particularly desirable upper limit of the Cr content is 16.5% by mass.
[0026]
N contributes to strength improvement by forming nitrides and suppresses the formation of δ ferrite. Moreover, deterioration of ductility and toughness can be avoided by substituting part of C with N to suppress the addition of a large amount of C. In order to effectively obtain such an action of N, it is necessary to contain at least 0.04% by mass of N. However, if it exceeds 0.10% by mass, the amount of retained austenite becomes excessive, and sufficient strength cannot be obtained. Therefore, the N content is specified to be 0.04 to 0.10% by mass.
[0027]
B has the effect of suppressing the occurrence of edge breaks during cold rolling. Further, in the cooling process after annealing, S may segregate at the grain boundary in some cases, and the ductility and toughness at room temperature may be lowered, but B has an effect of improving this phenomenon. For this reason, B is added as needed in the present invention. However, when B is added in a large amount exceeding 0.0070% by mass, a B-based precipitate is generated at the grain boundary, which causes deterioration of the toughness of the final product. Therefore, when adding B, it is performed in the range of 0.0070 mass% or less.
[0028]
In addition to the above elements, Mo and Cu can be added to further improve the corrosion resistance. However, if the sum of Mo + Cu exceeds 2.0% by mass, residual austenite or δ ferrite may be generated in the base metal part or welded part, leading to a decrease in strength. Therefore, when these elements are added, it is desirable that the total content is within a range of 2.0% by mass or less.
[0029]
The target steel of the present invention having the above chemical composition has a martensitic structure without being subjected to a special quenching treatment from a high temperature range. That is, the hot-rolled annealed steel sheet, the steel sheet after final annealing, and the steel sheet further subjected to temper rolling all exhibit a martensite-based structure. In many cases, the steel sheet of the present invention has a substantially martensitic single-phase structure. In addition to precipitates and inclusions, a very small amount of δ ferrite and retained austenite may be contained, but at least 95% by volume or more is the martensite phase.
[0030]
The steel sheet of the present invention desirably has a hardness adjusted to 320 to less than 380 HV. If it is less than 320 HV, sufficient strength is not secured in both the base metal part and the welded part after welding. Moreover, in this component system, it is difficult to adjust the hardness after the final annealing to 380 HV or more unless the solid solution (C + N) amount exceeds 0.04 mass%. In that case, the corrosion resistance after welding is reduced as described later. Therefore, the steel sheet hardness after the final annealing is preferably adjusted to less than 320 to 380 HV. In addition, when performing temper rolling after final annealing, it is desirable to adjust the hardness after final annealing so that the hardness after temper rolling may be less than 320-380HV.
[0031]
The L value defined by the formula (1) is an index indicating a good correspondence with the hardness obtained when the steel sheet having the content of each element in the above range is subjected to final annealing at 700 ° C. If the components are adjusted so that the L value is less than 320 to 380, the steel sheet hardness can be controlled in the range of less than 320 to 380 HV when the final annealing is performed at 600 to 800 ° C. as described later. It becomes. Therefore, in the present invention, a chemical composition having an L value of 320 to less than 380 is employed.
[0032]
The decrease in the corrosion resistance of the welded portion in the steel grade subject to the present invention is mainly caused by the formation of a Cr-deficient layer accompanying carbonitride precipitation in the weld heat affected zone. Therefore, in the present invention, in order to suppress the formation of new carbonitride during welding, the amount of solute C and the amount of solute N in the base steel plate are reduced. As a result of various studies, when the total amount of solute C and solute N, that is, the amount of solute (C + N) is reduced to 0.04% by mass or less, the corrosion resistance degradation at the weld heat affected zone is almost a problem in many applications. I found out that
[0033]
If the amount of solute (C + N) is simply reduced, steel with low C and low N is used, and if necessary, C and N fixed elements such as Ti are added in time. However, it cannot maintain high strength. In the present invention, the C content in the steel is at least 0.05% by mass and the N content is at least 0.04% by mass, and most of these are present in the form of carbonitrides during the production process. The carbonitride exhibits a precipitation strengthening action, and the strength of the steel sheet is maintained at a high level. And when the amount of solid solution (C + N) is reduced to 0.04 mass% or less with the precipitation of carbonitride, the corrosion-resistance fall in a welded part can be prevented.
[0034]
However, there was a new problem that had to be solved here. Since carbonitride precipitates preferentially at the grain boundaries of martensite during the hot-rolled sheet annealing, a Cr-deficient layer is generated around the carbon nitride. In addition, carbonitrides may be preferentially precipitated at the original martensite grain boundaries during the final annealing. Overcoming this non-uniform carbonitride precipitation, and ultimately not having a microstructure where there are no locally distributed parts, regardless of grain boundaries or within grains, the base steel sheet itself is inferior in corrosion resistance. It becomes a thing.
[0035]
As a result of repeating various experiments, the inventors have found that this problem can be solved by the following processes (i) to (iii).
(i) Hot-rolled sheet annealing is performed.
(ii) Thereafter, cold rolling with a rolling rate of 30% or more is performed.
(iii) Next, final annealing is performed at 600 to 800 ° C.
Thereby, carbonitride can be dispersed and present in the matrix of the martensite phase, and the amount of solid solution (C + N) can be reduced to 0.04% by mass or less.
[0036]
The hot-rolled sheet annealing (i) does not require strict condition control, but it is desirable to heat to 600 ° C. or higher in order to remove the distortion of the hot-rolled sheet. However, if the heating temperature is too high, the crystal grain size becomes coarse, and the toughness tends to be reduced, making it difficult to produce. The heating time is desirably about 0 to 120 minutes. When the material heating temperature is 600 to 800 ° C., almost the entire amount of carbonitride is precipitated in this process. In the case of high temperature heating exceeding 800 ° C., the amount of precipitation decreases as the heating temperature increases. In hot-rolled sheet annealing, carbonitride tends to precipitate preferentially at the grain boundaries of martensite, and particularly when the heating temperature is 600 to 800 ° C., formation of a Cr-deficient layer tends to be remarkable. This Cr-deficient layer can be eliminated by the processes (ii) and (iii).
[0037]
According to the description in paragraph 0043 of Patent Document 6, it is taught that precipitation of carbonitride is suppressed when hot-rolled sheet annealing (intermediate annealing) is performed in the range of 600 to 800 ° C. As a result of their detailed investigation, it was found that carbonitride formation was observed in the temperature range of 600 to 800 ° C. as described above. Patent Document 6 states that when the precipitation of carbonitride is sufficiently suppressed, it is possible to remarkably suppress cold-rolled edge cutting. However, even if carbonitride is formed as a result, the purpose of Patent Document 6 is to add B or the like. It seems that the effect of preventing cold-rolled ear cutting is sufficiently obtained.
[0038]
The cold rolling of (ii) aims at obtaining a predetermined plate thickness and introducing processing strain. This processing strain is necessary for homogenizing the structure in the final annealing of (iii). For that purpose, a rolling rate of 30% or more must be secured. The upper limit of the rolling rate is not particularly limited, but practically 80% or less is desirable.
[0039]
The final annealing of (iii) has the following two important purposes in addition to recrystallization and softening. One is to eliminate the Cr-deficient layer produced by hot-rolled sheet annealing. By breaking the hot-rolled structure using the processing strain introduced by cold rolling of 30% or more as a driving force, the carbonitrides localized at the former martensite grain boundaries are dispersed in the matrix. Cr in the matrix is replenished around the precipitate, and the Cr-deficient layer also disappears. The other is to sufficiently precipitate C and N remaining in a solid solution state without being completely precipitated, particularly when the hot-rolled sheet annealing is higher than 800 ° C. At that time, the above-mentioned processing strain becomes a driving force and the number of carbonitride precipitation sites increases, and carbonitrides newly generated at this stage are generated almost uniformly regardless of grain boundaries and grains in the matrix. . In this way, after the final annealing, carbonitride is dispersed in the martensite, and a structure state without a Cr-deficient layer in which the amount of solid solution (C + N) is reduced to 0.04% or less is obtained. Carbonitrides precipitated at the grain boundaries by hot-rolled sheet annealing are also dispersed and present so as to be indistinguishable from those produced by final annealing.
[0040]
The effect of the final annealing as described above can be obtained by heating at 600 ° C or higher. However, when it exceeds 800 ° C, the carbonitride that has already precipitated re-dissolves, and the amount of solid solution (C + N) is stabilized to 0.04 mass% or less. It becomes difficult to make. Therefore, the final annealing temperature is specified at 600 to 800 ° C. As the heating time, so-called annealing of 0 seconds soaking after the material temperature reaches a predetermined temperature set in the range of 600 to 800 ° C. can be employed. Considering continuous treatment with a steel strip, soaking is preferably about 0 to 180 seconds.
In the steel type (c), the final annealing is generally performed at a high temperature of 950 to 1050 ° C., for example. In contrast, in the present invention, the final annealing is performed at a low temperature of 600 to 800 ° C., and the heat treatment as a steel plate material is not performed thereafter. This is to obtain a structure state different from that of the conventional material in which the solid solution (C + N) amount is reduced in the present invention.
[0041]
It can be confirmed by direct observation using a transmission electron microscope or the like whether the carbonitride is in a textured state in which it is dispersed in the matrix of martensite phase. However, it is possible to know the quality of the tissue state indirectly without using such a device. For example, a test piece of the following (a) may be prepared from the steel plate, and a cast test according to JIS H 8502 may be performed at 50 ± 2 ° C. for 200 hours using the test solution shown in (b) below.
(a) A test piece obtained by polishing the surface of the steel plate with # 400.
(b) Cu per liter of 5% NaCl aqueous solution ₂ A test solution in which pH was adjusted to 3.0 to 3.1 by adding acetic acid to a solution in which 0.268 g of Cl was dissolved.
At this time, if the surface of the test piece has corrosion resistance that does not cause rusting, the carbonitride may be uniformly dispersed and the Cr-deficient layer may be virtually absent.
[0042]
Whether or not the solid solution (C + N) amount is 0.04% by mass or less can be confirmed by analysis of the extraction residue as shown in Examples described later. However, it is possible to know whether or not the amount of solid solution (C + N) is indirectly reduced without performing such analysis. For example, a welding test piece of the following (a ′) is produced from the steel plate, and a casting test according to JIS H 8502 is performed for 200 hours at 50 ± 2 ° C. using the test solution shown in (b) above. .
(a ') After performing TIG welding of a bead-on-plate without using a filler metal on the steel sheet under the condition that the molten part penetrates the base metal plate thickness, the convex part of the weld bead part is smoothed with a grinder, A specimen finished with # 400 polished together with the material.
At this time, if the surface of the weld has corrosion resistance that does not show a crack of a diameter exceeding 1.0 mm, the amount of solid solution (C + N) may be considered to be sufficiently reduced to, for example, 0.04% by mass or less. At the same time, if the surface of the base material portion has corrosion resistance that does not cause rusting, the carbonitride may be uniformly dispersed and the Cr-deficient layer may be virtually absent.
[0043]
After the final annealing, temper rolling can be further performed. Temper rolling is effective in increasing the strength and springiness of the base material. However, if the temper rolling ratio exceeds 30%, the problems of reduced ductility and toughness of the base metal portion are likely to become obvious. Therefore, when temper rolling is performed, it is desirable to perform the rolling rate within a range of 30% or less.
[0044]
【Example】
[Example 1]
Steels having the chemical composition shown in Table 1 were melted and hot rolled sheets having a thickness of 4.0 mm were manufactured from each ingot by hot rolling from an ingot of about 100 kg. In Table 1, A1 to A5 are subject steels having the chemical composition defined in the present invention, B1 to B5 are comparative steels, and C1 is SUS301, which is a conventional steel. Table 1 also shows the L value defined by the equation (1). In addition, B1 is C content, B2 is N content, B3 and B4 are L value, and B5 is Cr content outside the range specified in the present invention.
[0045]
[Table 1]

[0046]
Hot rolled sheets (thickness 4.0 mm) of the subject steel and comparative steel of the present invention were subjected to hot rolled sheet annealing at 700 ° C. × soaking for 1 hour, the scale was removed, and the rolling rate was 75% to a sheet thickness of 1.0 mm. Was subjected to cold rolling. Next, final annealing was performed at 700 ° C. × soaking for 60 seconds to obtain an annealed steel plate. Since conventional steel C1 is a work hardening type stainless steel, it was annealed at 1070 ° C. and then cold-rolled at a rolling rate of 45% to obtain a temper rolled sheet having a thickness of 1.0 mm. About these, the amount of solid solution (C + N) in a steel plate was investigated. Further, the corrosion resistance and hardness of the base metal part and the welded part were examined for the test piece on which the weld bead was formed (hereinafter referred to as “welded test piece”).
[0047]
The amount of solid solution (C + N) was determined by analyzing the extraction residue. That is, an analytical test piece cut out from a steel plate was dissolved in a 10% acetylacetone + 1% tetramethylammonium chloride + methanol solution at a dissolution voltage of 40 to 70 mV, and the collected residue was subjected to mass measurement and EPMA quantitative analysis. The amount of C and N (corresponding to the amount of undissolved C and N) was determined, and the amount of solid solution (C + N) was calculated using this value and the previously known value of the total (C + N) amount.
[0048]
A weld specimen was prepared by performing TIG welding of a bead-on-plate without using a filler metal on a steel plate having a thickness of 1.0 mm. TIG welding conditions were as follows: electrode: 1.2 mm diameter tungsten, welding current: about 70 A, torch moving speed: 300 mm / min, sealing gas: argon at a flow rate of 10 L / min. Under these conditions, a melted portion penetrating the base metal plate thickness was formed.
[0049]
Corrosion resistance was evaluated by a cast test. The 150 × 100 mm weld test piece including the weld bead portion was smoothed with a grinder on the convex portion of the weld bead portion, and both the portion and the base material portion were polished by # 400, and then subjected to a cast test. The cast test was conducted according to the cast test method of JIS H 8502 per liter of 5% NaCl aqueous solution. ₂ Using a test solution in which acetic acid was added to a solution in which 0.268 g of Cl was dissolved to adjust the pH to 3.0 to 3.1, the test was performed at 50 ± 2 ° C. for 200 hours. The base metal part and the welded part were evaluated according to the following criteria.
○: No sprout is observed.
Δ: Point-like wrinkles are observed, but the diameter of the wrinkles is 1.0 mm or less.
X: A crack with a diameter exceeding 1.0 mm is observed.
If no corrosion is observed in the base metal part in this corrosion resistance test and if the weld part does not have a diameter exceeding 1.0 mm, it is practical for many structural members including steel belts and stainless steel pipes for piping. It may be considered that it has a sufficiently excellent corrosion resistance. Therefore, it was determined that the base material part was evaluated as “good” and the welded part was evaluated as “good” or better. The corrosion resistance test was performed on two test pieces for each steel type (n = 2).
[0050]
The hardness of the base metal part is the HV on the surface of the steel base metal part. ₂₀ Displayed by value. The hardness of the weld is HV for the cross section perpendicular to the weld bead of the weld specimen. _0.1 The value (load 0.1 kg) is measured at 0.2 mm intervals on the straight line from the base metal part to the base metal part through the welded part, and the lowest HV at the welded part _0.1 The measured value was displayed as “the minimum hardness of the weld”.
These results are shown in Table 2. As for corrosion resistance, the result of n = 2 is also shown.
[0051]
[Table 2]

[0052]
As shown in Table 2, the steel according to the present invention has good corrosion resistance in both the base metal part and the welded part. In addition, no significant decrease in hardness was observed at the welds, and both the base metal and welds maintained a hardness of 320HV or higher.
On the other hand, since the comparative steels B1 and B4 have higher C content and L value than the specified range of the present invention, the corrosion resistance of the welded portion is not good. Since B2 and B3 each have an N content and an L value lower than the specified range of the present invention, a hardness of 320 HV or more cannot be obtained in the base material portion, and the hardness is further reduced in the welded portion. Since B5 has a high Cr content, δ ferrite remains on the base steel sheet, and the hardness of the base metal part and the welded part is not higher than 320HV. Conventional steel C1 is equivalent to SUS301H, and the weld became austenite single phase by welding, so the hardness is as low as 200HV.
[0053]
[Example 2]
Using the A1 steel of Table 1, the cold rolling rate and the final annealing temperature after hot-rolled sheet annealing were changed in various ways, the amount of solid solution (C + N) after the final annealing, and the corrosion resistance and hardness of the base metal part and the welded part. I investigated. Each test method is the same as in Example 1. In addition, hot-rolled sheet annealing is performed at 700 ° C x 1-hour soaking, adjusting the thickness before hot-rolling sheet annealing so that the thickness after cold rolling becomes 1 mm, and the final annealing time is soaking. 60 seconds.
Production conditions and results are shown in Table 3. R8 is temper rolled at 5%. FIG. 1 shows the influence of the final annealing temperature on the hardness of the base metal part and the corrosion resistance of the welded part for R3 to R7 with a constant cold rolling rate of 60%.
[0054]
[Table 3]

[0055]
As shown in Table 3 and FIG. 1, the examples of the present invention in which cold rolling of 30% or more was performed after hot-rolled sheet annealing and final annealing was performed in the range of 600 to 800 ° C. The amount of molten (C + N) was 0.04% by mass or less, and it was confirmed that the steel sheet exhibited excellent corrosion resistance after welding and maintained a hardness of 320 HV or higher in the welded part. As a result of observation with a transmission electron microscope, carbonitrides were dispersed almost uniformly in the martensite matrix regardless of the intragranular boundaries and grain boundaries. Therefore, it is considered that not only the amount of solid solution (C + N) is reduced, but also the Cr-deficient layer has disappeared. This point is affirmed by the very good corrosion resistance of the base metal part.
[0056]
On the other hand, since R1 of the comparative example has a cold rolling reduction of less than 30% and a small cold rolling strain, in the final annealing, the Cr-depleted layer disappears due to local carbonitride precipitation during hot-rolled sheet annealing. It was conceivable that the new carbonitride was locally deposited on the martensite grain boundaries before cold rolling, and both the base metal part and the welded part had poor corrosion resistance. It was. Since the final annealing temperature of R4 was less than 600 ° C., it was considered that the Cr-deficient layer generated during the hot-rolled sheet annealing could not be eliminated, and the corrosion resistance of the base metal part and the welded part was inferior. Since the final annealing temperature of R7 was too high, the amount of solid solution (C + N) in the steel sheet exceeded 0.04% by mass, and it was thought that new carbonitrides were generated in the heat-affected zone during welding and sensitized. The corrosion resistance of was deteriorated.
[0057]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it became possible to provide the steel plate which can maintain high corrosion resistance and intensity | strength when it uses for welding in the inherently cheap martensitic stainless steel, without adding a special element.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of the final annealing temperature on the hardness of a base metal part and the corrosion resistance of a welded part.

Claims

In mass%, C: 0.05 to 0.10%, Si: 0.2 to 2.0%, Mn: 1.0% or less, P: 0.06% or less, S: 0.006% or less, Ni: 2.0 to 5.0%, Cr: 14.0 to 17.0%, Chemical composition containing N: 0.04 to 0.10%, B: 0 (no addition) to 0.0070%, the balance being Fe and inevitable impurities, and L value defined by the following formula (1) being less than 320 to 380 A martensitic stainless steel plate for welding in which a solid solution (C + N) content is 0.04% by mass or less and carbonitrides are dispersed in a matrix of martensite phase.
L = 192C-11Si + 12Mn + 17Ni-14Cr + 247N + 476 (1)

In mass%, C: 0.05 to 0.10%, Si: 0.2 to 2.0%, Mn: 1.0% or less, P: 0.06% or less, S: 0.006% or less, Ni: 2.0 to 5.0%, Cr: 14.0 to 17.0%, Chemical composition containing N: 0.04 to 0.10%, B: 0 (no addition) to 0.0070%, the balance being Fe and inevitable impurities, and L value defined by the following formula (1) being less than 320 to 380 The solid solution (C + N) content is 0.04% by mass or less, and the carbonitride is dispersed in the matrix of the martensite phase. A martensitic stainless steel plate for welding having corrosion resistance with no cracking observed on the surface of the test piece when a cast test according to JIS H 8502 is conducted at 50 ± 2 ° C. for 200 hours using the test solution shown in FIG.
L = 192C-11Si + 12Mn + 17Ni-14Cr + 247N + 476 (1)
(a) A test piece obtained by polishing the surface of the steel plate with # 400.
(b) A test solution prepared by adding acetic acid to a solution prepared by dissolving 0.268 g of Cu ₂ Cl per liter of 5% NaCl aqueous solution to adjust the pH to 3.0 to 3.1.

In mass%, C: 0.05 to 0.10%, Si: 0.2 to 2.0%, Mn: 1.0% or less, P: 0.06% or less, S: 0.006% or less, Ni: 2.0 to 5.0%, Cr: 14.0 to 17.0%, Chemical composition containing N: 0.04 to 0.10%, B: 0 (no addition) to 0.0070%, the balance being Fe and inevitable impurities, and L value defined by the following formula (1) being less than 320 to 380 The solid solution ( C + N ) content is 0.04 % by mass or less, and the carbonitride is dispersed in the matrix of the martensite phase. When a casting test according to JIS H 8502 was performed at 50 ± 2 ° C for 200 hours using the test solution shown, no cracks were observed on the surface of the base metal and the surface of the welded part had a diameter exceeding 1.0 mm. A martensitic stainless steel sheet for welding that has corrosion resistance without flaws.
L = 192C-11Si + 12Mn + 17Ni-14Cr + 247N + 476 (1)
(a ') After performing TIG welding of a bead-on-plate without using a filler metal on the steel sheet under the condition that the molten part penetrates the base metal plate thickness, the convex part of the weld bead part is smoothed with a grinder, A specimen finished with # 400 polished together with the material.
(b) A test solution prepared by adding acetic acid to a solution prepared by dissolving 0.268 g of Cu ₂ Cl per liter of 5% NaCl aqueous solution to adjust the pH to 3.0 to 3.1.

In mass%, C: 0.05 to 0.10%, Si: 0.2 to 2.0%, Mn: 1.0% or less, P: 0.06% or less, S: 0.006% or less, Ni: 2.0 to 5.0%, Cr: 14.0 to 17.0%, N: 0.04 to 0.10%, B: 0 (no additive) to 0.0070%, the balance being Fe and inevitable impurities, L value defined by the following formula (1) is less than 320 to 380 hot-rolled annealed steel sheet subjected to rolling reduction ratio of 30% or more of cold, followed by a final annealing at 600 to 800 ° C., a manufacturing method of welding martensitic stainless steel.
L = 192C-11Si + 12Mn + 17Ni-14Cr + 247N + 476 (1)

In mass%, C: 0.05 to 0.10%, Si: 0.2 to 2.0%, Mn: 1.0% or less, P: 0.06% or less, S: 0.006% or less, Ni: 2.0 to 5.0%, Cr: 14.0 to 17.0%, N: 0.04 to 0.10%, B: 0 (no additive) to 0.0070%, the balance being Fe and inevitable impurities, L value defined by the following formula (1) is less than 320 to 380 hot-rolled annealed steel sheet subjected to rolling reduction ratio of 30% or more of cold, then perform a final annealing at 600 to 800 ° C., then subjected to rolling of 30% or less temper rolling, the manufacture of welding martensitic stainless steel sheet Way .

The martensitic stainless steel plate for welding according to any one of claims 1 to 3 , having a hardness of 320 to 380 HV.

The structural member excellent in the intensity | strength and corrosion resistance of the welding part formed by welding the steel plate in any one of Claim 1 , 2, 3, 6 .