JP4291480B2 - Structural steel with excellent corrosion resistance and corrosion fatigue resistance - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は高張力棒鋼・線材・機械構造用鋼、又は造船、建築、橋梁・橋脚、タンク、圧力容器、海洋・港湾構造物、及び化学プラント等の大型鋼構造物向け溶接構造用鋼等に適用される耐食性と耐腐食疲労特性に優れた構造用鋼に関する。
【０００２】
【従来の技術】
腐食は単独で、あるいは、疲労破壊、不安定破壊、脆性破壊の起点となって、鋼構造物の重大損傷を引き起こす。腐食及び腐食を起点とする損傷事例は鋼構造物全体の損傷事例の大きな割合を占めるため、その改善は極めて重要である。
【０００３】
鋼構造物の使用環境は幅広いが、特に腐食、腐食疲労が問題となるのは、海水環境はじめとする塩素あるいは塩化物を含む水環境である。これに対して、例えば日本鉄鋼協会第１５９回西山記念講座（１９９６）ｐ．１２３にまとめられているように、従来、マリーナースチールはじめ、Ｃｕ、Ｎｉ、Ｃｒ、Ｐなどの合金成分を添加・増量し耐海水性を高めた鋼材がこれまで開発されてきた。更に、鋼の耐食性は、鋼中の合金成分によって決まり、鋼の組織への依存性はないというのが、これまでの知見であった。従って鋼に耐食性を付与するためには前述のような合金元素の添加が必要となるが、それによって、溶接用構造用鋼（以下単に構造用鋼と称す）としてコスト上昇すると共に、多量の合金元素の含有により、構造用鋼として必要な溶接性や加工性が低下する問題があった。
【０００４】
【発明が解決しようとする課題】
このような背景から、本発明の課題は、鋼材組織を制御することによって、構造用鋼の耐食性、特に塩素あるいは塩化物を含む水環境での耐食性を向上させることにある。即ち、従来の様に耐食性に有効な合金元素に頼ることなく、溶接性を確保しながら、耐食性を向上し、また、前記合金元素を加えた場合には、その耐食性を大幅に向上させることを課題とする。
【０００５】
【課題を解決するための手段】
本発明は前記課題を解決するためになされたものであり、その要旨は以下に示す通りである。
【０００６】
（１）重量％で、
Ｃ：０．０４〜０．２５％、
Ｓｉ：０．０１〜１．０％、
Ｍｎ：０．３〜２．０％、
Ａｌ：０．００５〜０．６％、
Ｐ：０．１５％以下、
Ｓ：０．０１％以下
の成分を有し、残部鉄及び不可避的不純物から成る鋼又は鋼板で、該鋼の表層部又は鋼板の表・裏層部からそれぞれで鋼の径又は鋼板の厚さの５％以上の表層領域において、結晶粒界及び／又は結晶亜粒界に０．５μｍ以下のセメンタイト相を有すると共にフェライトが９５％以上を占め、且つ、そのフェライト組織の硬さＨが、下記（１）式を満足し、更に、圧延面に平行な集合組織の（１００）面強度比が１．５以上３以下を有することを特徴とする耐食性と耐腐食疲労特性に優れた構造用鋼。
Ｈ≦２００［Ｃｅｑ％］＋２０＋（９［Ｃｅｑ％］＋３．７）／√（ｄ）
・ ・ ・ （１）
但し、［Ｃｅｑ％］＝Ｃ％＋Ｓｉ％／２４＋Ｍｎ％／６であり、このＣ％、Ｓｉ％、Ｍｎ％はそれぞれＣ、Ｓｉ、Ｍｎの重量％であり、更に、ｄはフェライトの平均円相当粒径である。
【０００７】
（２） 重量％で、更に、
Ｎｂ：０．００５〜０．１％、
Ｔｉ：０．００５〜０．０５％、
Ｔａ：０．００５〜０．０５％
の１種又は２種以上を含有することを特徴とする上記（１）項記載の耐食性と耐腐食疲労特性に優れた構造用鋼。
【０００８】
（３） 重量％で、更に、
Ｃｕ：０．０５〜１．０％、
Ｎｉ：０．１〜２．０％、
Ｃｒ：０．０３〜３．０％、
Ｍｏ：０．０５〜１．０％、
Ｖ：０．０１〜０．４％、
Ｂ：０．０００２〜０．００２％、
Ｃａ：０．０００１〜０．０２％、
Ｍｇ：０．０００１〜０．０２％、
ＲＥＭ：０．００１％〜０．２％
の１種又は２種以上を含有し、且つ、フェライト組織の硬さＨが、下記（２）式を満足することを特徴とする上記（１）項又は（２）項記載の耐食性と耐腐食疲労特性に優れた構造用鋼。
Ｈ≦２００［Ｃｅｑ％］＋２０＋（９［Ｃｅｑ％］＋３．７）／√（ｄ）
・ ・ ・ （２）
但し、［Ｃｅｑ％］＝Ｃ％＋Ｓｉ％／２４＋Ｍｎ％／６＋（Ｃｕ％＋Ｎｉ％）／１５であり、このＣ％、Ｓｉ％、Ｍｎ％、Ｃｕ％、Ｎｉ％はそれぞれＣ、Ｓｉ、Ｍｎ、Ｃｕ、Ｎｉの重量％で、更に、ｄはフェライトの平均円相当粒径である。
【００１４】
【発明の実施の形態】
本発明者は種々の鋼の塩素を含む水環境、湿潤環境、乾湿繰り返し環境での耐食性を詳細に検討した結果、鋼組織において、転位密度の低いフェライトを主体とし、集合組織の（１００）面強度比を１．５以上で、且つ、フェライト結晶粒界及び／又は結晶亜粒界に０．５μｍ以下のセメンタイト相又はセメンタイト相とＮｂ、Ｔｉ、Ｔａの１種又は２種以上の炭窒化物相を析出させることで鋼の耐食性が大きく向上することを見出した。
【００１５】
このフェライトの転位密度が低いと耐食性が向上する機構は明確ではないが、転位密度の高い組織の方が、転位による歪みエネルギーが大きいため、腐食反応の際にも活性化するのではないかと推察される。
【００１６】
フェライトは比較的転位密度の低い組織ではあるが、加工フェライト等、転位密度の高い組織も存在するため、フェライト組織で、且つ転位密度の低いことを必要条件としなければならない。
【００１７】
このため、先ず、フェライト内部の転位密度を評価する必要がある。この転位密度を定量的に測定すことは極めて難しい。
【００１８】
そこで、粒界の特性により、粒径と降伏応力の関係が変化することを利用して、フェライト内部の転位密度の大小関係を評価する方法がある。これは、オーステナイト／フェライト変態により生成した通常のフェライト組織では、フェライト粒界によって定義される粒径と硬さの間には一定の関係があり、ホールペッチの関係とも関連づけることが出来る。しかしながら、フェライト内部の転位密度が高くなると、粒径と硬さの関係が変化してくる。この実験事実に着目し、フェライト内部の転位密度を評価した。これには、先ず、結晶粒径を測定する必要がある。本発明では、通常のオーステナイト／フェライト変態による粒界だけでなく、加工再結晶による粒界も対象とするので、通常のナイタール腐食による粒界現出では不十分である。そこで、加工組織でも明瞭な粒界を現出させるためには、蓚酸水溶液、過酸化水素水、硫酸水溶液を主体とする腐食液であるマーシャル試薬が適していることを知見し、本試薬により腐食させて現出させた結晶粒径を測定した。
【００１９】
このような評価方法を用いることにより、転位密度の低い組織におけるフェライトの硬さＨと各成分、結晶粒径ｄの間で、下記式の関係が成立することの知見を得た。
【００２０】
Ｈ≦２００［Ｃｅｑ％］＋２０＋（９［Ｃｅｑ％］＋３．７）／√(ｄ)
但し、［Ｃｅｑ％］＝Ｃ％＋Ｓｉ％／２４＋Ｍｎ％／６＋（Ｃu％＋Ｎｉ％）／１５であり、このＣ％、Ｓｉ％、Ｍｎ％、Ｃ％、Ｎｉ％はそれぞれＣ、Ｓｉ、Ｍｎ、Ｃｕ、Ｎｉの重量％である。
【００２１】
この式は、転位密度の状態を示す指標であり、化学成分（Ｃｅｑ）と結晶粒径ｄにより決定される指標である。この粒径ｄの測定は通常のナイタール腐食による組織現出でよいが、転位密度が高い組織では組織現出が不鮮明であるので、マーシャル試薬による粒界現出の方が適切である。その現出方法を以下に示す。
【００２２】
前記マーシャル試薬は、蓚酸水溶液、過酸化水素水、硫酸液７ｍｌを主体とする腐食液であり、通常、８％蓚酸水溶液５０ｍｌ、過酸化水素水５０ｍｌ、５０％硫酸液７ｍｌから成るが、粒界をより現出しやすくするため、８％蓚酸水溶液５０ｍｌ、過酸化水素水５０ｍｌ、５０％硫酸液１４ｍｌ、エチルアルコール１ｍｌから成る改良型マーシャル試薬を用いた。試料を先ず５％塩酸液に３〜４秒浸漬の後、水洗、乾燥させ、改良マーシャル試薬を用いて、室温にて３〜５秒腐食させ、水洗、乾燥させることにより、粒界を現出させるものである。この腐食方法は、代表例であり、腐食液の成分を多少変化させても粒界の観察は行い難くなるものの、観察しようとする粒界は腐食現出するので、本発明の範囲にはいるものである。
【００２３】
更に、フェライト結晶粒界及び／又は結晶亜粒界に０．５μｍ以下のセメンタイト又は、セメンタイトとＮｂ、Ｔｉ、Ｔａの１種又は２種以上の炭窒化物相を析出させるためには、Ｃ又はＣ及びＮｂ、Ｔｉ、Ｔａの１種又は２種以上を含有する鋼又は鋼板の素材（例えばビレット、スラブ）をＡｃ3点以上に加熱して、Ｃ又はＣとＮｂ、Ｔｉ、Ｔａの１種又は２種以上を固溶させた状態で、制御圧延等の熱間加工の前又は途中でフェライト分率が５０％以上となる温度まで急冷して、Ｃ又はＣとＮｂ、Ｔｉ、Ｔａの１種又は２種以上を過飽和に固溶させ、そして、その後の復熱過程において熱間加工を開始又は再開してＡｃ3点以下で熱間加工を終了し、引き続いてＡｃ3点以上に復熱させないで冷却することがェライトを主体とし、且つ、圧延面に平行な集合組織の（１００）面強度比が１．５以上を有する組織を効果的に確保する上で不可欠であることが判明した。
【００２４】
以下に本発明で成分を規定した理由を詳細に説明する。
【００２５】
Ｃは本発明では過飽和固溶状態から０．５μｍ以下にフェライト結晶粒界又は結晶亜粒界に析出させたセメンタイトによって超微細粒フェライトをピンニングする必須元素であり安価に強度を向上するのに最も有効な元素であるが、０．２５％を超えると低温靱性を阻害する。更に、鋼又は鋼板の表層領域においてもパーライト分率が１０％を超え、０．０４％未満ではピンニングに必要なセメンタイト量が不足するために、０．０４〜０．２５％に限定する。尚、溶接用の構造用鋼の場合には０．２％を超えると溶接性（溶接部靱性）が劣化するために０．０４〜０．２％にするのが好ましい。
【００２６】
Ｓｉは強度向上元素として有効であり安価な溶鋼の脱酸元素としても有用であるが、１．０％を超えると溶接性が劣化し、０．０１％未満では脱酸効果が不十分でＴｉやＡｌ等の高価な脱酸元素を多用する必要があるために、０．０１〜１．０％に限定する。
【００２７】
Ｍｎは強度を向上するに必要な元素であり、その必要性から０．３％以上として、２．０％超の添加は母材靱性・溶接性を阻害すると共にＡｒ3変態点を低下させる結果、二相域圧延等の熱間圧延をを困難にするために０．３〜２．０％に限定した。
【００２８】
Ｓは耐食性、靭性の観点から０．０１％以下に限定した。ＭｎＳが塩素あるいは塩化物を含む水環境で溶解し、選択的な腐食起点となることはよく知られており、その観点から、Ｓは出来るだけ低いほど好ましい。
【００２９】
Ｎｂは加工熱処理（ＴＭＣＰ）鋼においてＴｉと共に最も有用な元素であり、ＮｂＣ又はＮｂ（Ｃ，Ｎ）（Ｃａｒｂｏ−ｎｉｔｒｉｄｅ）として鋼材の再加熱時のγ粒成長の抑制・制御圧延時の未再結晶域温度域の拡大・圧延時の変形帯における析出強化・大入熱溶接時の溶接熱影響部（ＨＡＺ）におけるＨＡＺ軟化の防止の効果が一般的に知られている。更に、本発明者の仔細な検討から超微細析出させたセメンタイトの熱的な安定性及びフェライト粒の成長抑制効果が著しく増加することを知見した。従って、０．００５％未満では過飽和固溶状態から０．５μｍ以下にフェライト結晶粒界又は結晶亜粒界に析出させるＮｂＣ又はＮｂ（Ｃ，Ｎ）量が不足すると共に０．５μｍ以下に析出させたセメンタイトの熱的な安定性も不足して、０．１％以上では溶接性を損なうために０．００５〜０．１％に限定する。
【００３０】
ＴｉもまたＴＭＣＰ鋼においてＮｂと共に最も有用な元素であり、ＴｉＣ又はＴｉ（Ｃ，Ｎ）として鋼材の再加熱時のγ粒成長の抑制・制御圧延時の未再結晶域温度域の拡大・圧延時の析出強化・大入熱溶接時のＨＡＺ靭性の向上を図れることが一般的に知られている。更に、本発明者の仔細な検討からＮｂと同様に超微細析出させたセメンタイトの熱的な安定性及びフェライト粒の成長抑制効果が改善することを見出した。従って、０．００５％未満では過飽和固溶状態から０．５μｍ以下にフェライト結晶粒界又は結晶亜粒界に析出させるＴｉＣ又はＴｉＣＮ量が不足すると共に０．５μｍ以下に析出させたセメンタイトの熱的な安定性も不足して、０．０５％以上では溶接性を損なうために、０．００５〜０．０５％に限定する。
【００３１】
ＴａはＴａＣ又はＴａ（Ｃ，Ｎ）として鋼材の再加熱時のγ粒成長の抑制・大入熱時のＨＡＺ靱性向上の効果が知られているが、高価なためにそれ程一般的に使われてはいない。然し、本発明者の仔細な検討からＮｂ・Ｔｉと同様に超微細析出させたセメンタイトの熱的な安定性及びフェライト粒の成長抑制効果が改善することを見出した。従って、０．００５％未満では過飽和固溶状態から０．５μｍ以下にフェライト結晶粒界又は結晶亜粒界に析出させるＴａＣ又はＴａＣＮ量が不足すると共に０．５μｍ以下に析出させたセメンタイトの熱的な安定性も不足して、０．０５％以上では溶接性を損なうために、０．００５〜０．０５％に限定する。
【００３２】
ＡｌはＳｉ同様に脱酸上必要な元素であり、本発明の技術思想からＴｉ・Ｔａ又はＮｂを微量添加する時にはその酸化を防止するのにＳｉ単独の脱酸では不十分なために０．００５％以上添加が必要である。更に本発明者はＡｌの添加が本発明鋼の耐食性に対しても有効であることを知見した。但し０．６％以上の過度の添加はＨＡＺ靭性を損なうために、０．００５〜０．６％に限定した。
【００３３】
以上が本発明が対象とする鋼の基本成分であるが、更に、母材強度の向上や低温靱性・溶接性の改善を目的とした低炭素等量化のために、要求される品質特性又は鋼材の大きさ・鋼板厚に応じてＣｕを０．０５〜１．０％、Ｎｉを０．１〜２．０％、Ｃｒを０．０３〜３．０％、Ｍｏを０．０５〜１．０％、Ｖを０．０１〜０．４％、Ｂを０．０００２〜０．００２％の範囲で添加することが強度・低温靱性・溶接性を向上する観点から好ましく、しかも、この添加により本発明の効果は何ら損なわれることはない。また、Ｃｕ、Ｎｉ、Ｃｒは従来から、海水など塩素あるいは塩化物を含む水環境で鋼の耐食性を向上させる元素として知られているが、これら元素を含有する鋼に本発明を適用することにより、さらなる耐食性の向上が図られる。更に、これら元素と併せてP添加も耐食性に有効であり、本発明においても添加することが好ましい。但し、０．１５％を超える添加は、靭性、溶接性を著しく低下させることから、０．１５％以下とする。
【００３４】
更に、前述のように塩素あるいは塩化物を含む水環境ではＭｎＳは腐食の起点として有害であり、これを低減するために、鋼中硫化物の形態・分散制御の観点からＣａを０．０００１〜０．０２％、Ｍｇを０．０００１〜０．０２％、ＲＥＭを０．００１％〜０．２％の範囲で添加することが本発明効果と重畳して有効である。
【００３５】
次に、本発明の技術思想である結晶組織を規定する理由について述べる。
【００３６】
ベイナイトを含むフェライト・パーライト鋼ではフェライト組織が主体となっても耐食性は必ずしも改善しない。本発明者の仔細な調査により、フェライト粒が加工を受けて転位密度が高い状態にある場合には、腐食環境下で腐食反応が活発化し、塩素あるいは塩化物を含む水環境での腐食孔が発生する頻度が高く、且つ腐食深さが大きいことが判明した。更に、微細なセメンタイト相を有するフェライト組織の分率を９５％以上にすると、耐食性は特段に向上することも知見した。
【００３７】
一方、単に微細なセメンタイト又は炭窒化物相から構成される組織だけでは、平均粒径３μｍ以下のフェライト若しくはベイナイトを主体とする組織を安定して得ることが出来ず、フェライト結晶粒の成長抑制が必要不可欠であることも見い出した。即ち、フェライト結晶粒界又は結晶亜粒界に０．５μｍ以下のセメンタイトを析出させることによって初めてフェライト若しくはベイナイトをピーニングして、その成長を効果的に抑制出来る。
【００３８】
また、０．５μｍ以下のＮｂ、Ｔｉ、Ｔａの炭窒化物をフェライト結晶粒界又は結晶亜粒界に析出させると、セメンタイトと同様のピーニング効果が認められると共に、更にフェライト結晶粒界又は結晶亜粒界に超微細に析出させたセメンタイト自体の熱的な安定性が増すことも分かった。
【００３９】
次に、耐腐食疲労は発生した腐食ピットから疲労亀裂が進展するものであり、この耐腐食疲労特性を良好にするには、（１）発生する腐食ピットの大きさを小さく抑制することが必要である。これには、腐食ピットの原因の一つであるフェライト相以外のセメンタイト相又は炭化物相である第二相組織を０．５μｍ以下に微細化することがポイントである。更に、（２）前記腐食ピットから疲労亀裂として成長することを抑制することが重要であり、この疲労亀裂の板厚方向への進展を阻止するには、圧延面に平行な面での（１００）面集合組織を強化することが効果的であり、顕著な効果を得るためには（１００）面強度比を１．５以上３以下にすることが必要である。
【００４０】
他方、超細粒組織の割合が鋼又は鋼板の表層領域、即ち、鋼の径又は鋼板の厚さの５％未満では、長時間側の耐食性にばらつきがみられ顕著に改善しないために５％以上に限定した。超細粒組織の占める割合が大きいほど耐食性が向上して好ましく、その上限は特に規定しないが、過度の増加は製造コストの上昇につながるため３０％迄が好ましい。
【００４１】
次に、本発明で鋼又は鋼板の裏層領域における前記組織を実現する製造方法を規定する理由について述べる。
【００４２】
鋼の素材又は鋼を再加熱時においてＣ及びＣとＮｂ、Ｔｉ、Ｔａの１種又は２種以上を固溶させる加熱温度はＡｃ3点以上に限定される。Ｎｂ、Ｔｉ、Ｔａの１種又は２種以上を充分に固溶させる加熱温度としては１０００℃以上が好ましく、加熱時におけるγ粒の粗大化を防止するためには加熱温度を１２００℃以下とすることが好ましい。
鋼又は鋼板の表層領域において、フェライト結晶粒界及び／又は結晶亜粒界に０．５μｍ以下のセメンタイト及びセメンタイトとはＮｂ、Ｔｉ、Ｔａの１種又は２種以上の炭窒化物を析出させるには、Ｃ又はＣとＮｂ、Ｔｉ、Ｔａの１種又は２種以上を鋼中に固溶させた状態で、表層領域を３℃／秒以上の冷却速度で冷却することによってＣ、Ｎｂ、Ｔｉ、Ｔａを鋼中に過飽和に固溶せしめたる後に、冷却によっても温度低下の少ない鋼中の中心部の顕熱を利用して復熱させる過程で超微細に析出させるものである。
【００４３】
鋼又は鋼板の裏層領域において、転位密度の低いフェライトを主体とする組織（平均粒径３μｍ以下のフェライト又はベイナイトを主体とする組織）となすには、鋼又は鋼の素材をＡｃ3点以上に加熱してから熱間加工の前又は途中で表層領域を３℃／秒以上の冷却速度でフェライト分率が５０％以上となる温度まで急冷した後に、この冷却によっても温度低下の少ない鋼の中心部の顕熱を利用して復熱させる過程で、（Ａｃ1点−１５０℃）以上の温度から熱間加工を開始又は再開して、（Ａｃ1点＋５０℃）〜Ａｃ3点の範囲で熱間加工を終了することによってフェライトの回復・再結晶を惹起せしめて超微細粒化し、且つ、Ａｃ3点以上に復熱することなく冷却すると共にフェライト結晶粒界及び／又は結晶亜粒界に析出させた０．５μｍ以下のセメンタイト及び／又はＮｂ、Ｔｉ、Ｔａの１種又は２種以上の炭窒化物によるピンニング効果を効果的に活用して超微細粒組織の成長を防止するものである。
【００４４】
更に、一次加工後に前記表層領域をＡｒ3点以下に冷却した後に、鋼内部に顕熱による復熱時に二次加工を実施すると、鋼の中心部までは未再結晶温度域での熱間加工となって、鋼の低温靱性は著しく向上する。
【００４５】
この熱間加工としては圧延・押し出し・引き抜き等の一般的なを対象とする。また、鋼の素材寸法が大きくて加熱温度が１１７０℃以上の場合や、低温靱性の要求が厳しい場合には鋼又は鋼板の表層領域を冷却する前の初期γ粒を細かくしておくために、Ｎｂ、Ｔｉ、Ｔａの添加及び制御圧延等の熱間加工を行うことが好ましい。更に、鋼の加熱に引き続く冷却前に熱間加工を行わない場合には鋼の初期γ粒を細かくしておくために低温加熱及びＮｂ、Ｔｉ、Ｔａの添加又は初期γ粒の細かな熱間加工半製品の使用が好ましい。
【００４６】
鋼又は鋼板の表層領域を超微細化した後に、鋼又は鋼板中心部の顕熱によってＡｃ3点以上に復熱すると該表層領域を超微細化した効果が損なわれるばかりでなく、フェライト結晶粒界又は結晶亜粒界に微細析出させたセメンタイトがγに再固溶してピンニング効果が失われてしまう。従って、表層領域がＡｃ3点以上に復熱することのないようにすることが必要であり、圧延中の温度を監視しつつ、必要に応じてＡｃ3点以上に復熱しないように、圧延後、冷却しても差し支えない。
【００４７】
また、鋼又は鋼板を高強度化するためには、要求強度レベルに応じて添加成分を調整すると共に、熱間加工の終了後に引き続いてＡｃ3点以上に復熱させることなく５℃／秒以上の冷却速度でＴＭＣＰ設備による加速冷却又はＤＱ設備による直接焼き入れを実施すればよい。
【００４８】
加速冷却又は直接焼き入れに引き続いて、鋼又は鋼板を焼戻しするには通常の熱処理設備による焼戻しを行う。尚、ＴＭＣＰ設備による加速冷却やＤＱ設備による直接焼き入れの場合には加速冷却又は直接焼き入れ時の水冷を途中停止するオートテンパーで代替しても構わない。
【００４９】
【実施例】
本発明の実施例を表１〜表３を参照しつつ説明する。
【００５０】
先ず、表１は鋼の成分を示すものであり、鋼Ａ〜鋼Ｅが本発明例であり、鋼ＦはＣ、Ｓが本発明の範囲外となる比較例である。
【００５１】
【表１】
次に、表２は鋼板の製造条件を示すものであり、表３に製造した鋼板表層領域の組織及び特性を示す。表３中の鋼Ａ−１、鋼Ｂ−１、鋼Ｃ−１、鋼Ｄ−１、鋼Ｅ−１が本発明例であり、鋼Ａ−２、鋼Ａ−３、鋼Ｂ−２、鋼Ｃ−２、鋼Ｄ−２、鋼Ｅ−２、鋼Ｆ−１が比較例である。
【００５３】
【表２】
また、表３に示した鋼板の耐食性評価結果の評価法は、塩水散布暴露試験及び海水浸漬試験を行ったものである。この塩水散布暴露試験は鋼板表層から採取した１５０ｍｍ長×５０ｍｍ幅×５ｍｍ厚さの試験片を屋外暴露し、５％ＮａＣｌ水溶液を一日一回噴霧器にて試験面に散布して、試験面の腐食の発生に伴う板厚減、重量減を測定するものである。暴露期間は３ヶ月と６ヶ月、それぞれの期間で各鋼種別に３試験片ずつ供試した。また、海水浸漬試験は海水相当の３．５％ＮａＣｌの５０℃の水溶液に１５０ｍｍ長×５０ｍｍ幅×５ｍｍ厚さの試験片を浸漬し、腐食の発生に伴う板厚減、重量減を測定するものである。浸漬期間は１ヶ月と３ヶ月、それぞれの期間で各鋼種別に３試験片ずつ供試した。そして、表３の結果はいずれの試験も３試験片の平均値である。
【００５５】
更に、表３に示す耐腐食疲労性評価結果の評価法は、全厚平板の引張試験片（平滑、応力集中係数Ｋｔ＝１．１、板厚部分はポリマーでシールして鋼板表面からの疲労亀裂発生を評価）を用いて、２５℃のＡＳＴＭの規定の人工海水中で片振り引張りで０．１Ｈｚで繰返応力を付加したものである。そして、種々の応力範囲で試験を行い、応力破断線図（Ｓ−Ｎｆ曲線）を測定した。そして、腐食疲労強度の指標として、Ｎｆ＝５×１０5での疲労強度を引張強度で除した。
【００５６】
また、表３のフェライト硬さＨｖは１０ｋｇのビッカース硬度計を用いて測定し、結晶粒径は前述のマーシャル試薬により組織を現出させ、顕微鏡、画像解析装置を利用して測定した。
【００５７】
【表３】
前記比較例の鋼Ａ−２、鋼Ａ−３は鋼板表層領域の途中冷却で、その冷却速度が本発明の範囲より遅かったため、鋼板表層領域がＡｃ3点以上に復熱しなかったにもかかわらず析出セメンタイト寸法が粗大化してしまった例である。鋼Ｂ−２、Ｅ−２は途中冷却で本発明の範囲内での冷却速度はあったが、途中冷却時間が短くフェライト分率が５０％以上となる表層領域の厚さが鋼板の５％未満と小さかった例である。鋼Ｃ−２及び鋼Ｄ−２は、それぞれ途中冷却を実施しなかったため、表層領域にフェライト層の形成がなかった例であり、鋼Ｆ−１は本発明例の鋼Ｃ−１と概ね同じ製造条件であるが、その主要な成分であるＣ、Ｓが本発明の範囲から外れた例である。
【００５９】
この結果から、Ａ、Ｂ、Ｃ、Ｄ、Ｅいずれの鋼板においても、本発明例の方が、表層領域の組織の状態が本発明条件を満足する結果、同一組成の比較例と比べて暴露試験、浸漬試験とも明らかに耐食性に優れ、更に、耐腐食疲労性も優れている。
【００６０】
例えば、本発明例のＡ−１鋼においては、比較例のＡ−２鋼と比べて表層領域のフェライト分率、析出物とも本発明の条件を満足しており、それに伴い腐食減量とも半分以下に改善され、腐食疲労強度も絶対値で約１．５倍であり、引張強度で除しても、約１．６８倍と大幅に改善されている。更に、比較例の鋼Ａ−２はＡｃ3点以上に復熱したことによって微細化したα粒がγに逆変態すると共に超微細析出したセメンタイトもγに再固溶する結果、表層領域のα粒・セメンタイトも粗大化すると共にフェライト分率が９０％以下になったものである。それに対応して、鋼板表面に発生した腐食ピットは鋼Ａ−１より鋼Ａ−２の方が大きく、且つ深く、腐食疲労性も劣ていることが判る。
【００６１】
また、Ｎｂ、Ｔｉ、Ｔａを添加した鋼Ｂ−１、Ｃ−１ではフェライト結晶粒界及び結晶亜粒界にセメンタイト又は炭窒化物が極めて微細に析出してフェライト若しくは一部ベイナイトの成長を効果的に抑制する結果、本発明例である鋼Ａ−１に比較して、炭素等量が大きくなっているにもかかわらずフェライト分率の確保が安定し、腐食ピットも更に微細化し、腐食減量の点、腐食疲労強度の点でも一段と優れる。一方、比較例の鋼Ｂ−２は仕上圧延前の途中冷却条件が不十分で細粒層の厚さが５％未満と本発明の条件を満足しないために、表層領域でのフェライト分率は充分に確保出来ているもののフェライト硬さ、析出物寸法、（１００）面強度が本発明条件を満足出来ず耐食性、腐食疲労強度は本発明例よりも大きく劣っている。鋼Ｃ−２は途中冷却を実施しなかったため、本発明例よりも耐食性、腐食疲労強度が劣っている。同様の傾向は、鋼Ｄ−１とＤ−２、鋼Ｅ−１とＥ−２の間にも認められた。
【００６２】
また、本発明例同志、例えば鋼Ａ−１と鋼Ｂ−１〜鋼Ｅ−１を比較すると、鋼材成分にＣｕ、Ｎｉ、Ｃｒ及びＣａ、ＲＥＭ、Ｍｇを添加した方が絶対的なレベル比較では耐食性に優れている。このことは、これら添加元素の耐食性への効果（従来知見）と本発明が重畳出来ることを示している。本発明により、通常の構造用鋼の耐食性を向上出来るばかりでなく、更に従来の耐食構造用鋼の耐食性も大幅に向上出来る。
【００６３】
最後に、本発明例の鋼ＥＡ−２と概ね製造条件が同じでありながら、Ｃ、Ｓが本発明例の高め側に外れている比較例の鋼ＥＦ−１はフェライト分率、フェライト硬さは本発明の条件を満足しているが、パーラート分率が高く、且つ高Ｓの結果、耐食性が本発明例よりも劣っている。
【００６４】
【発明の効果】
本発明は鋼又は鋼板の表層領域を転位密度の低いフェライト組織を主体として構成させ、しかも、（１００）面強度比を１．５以上３以下とすることによって、化学成分面だけでなく、鋼材組織の点からも、海水等塩化物を含む水環境での構造用鋼又は溶接用構造用鋼の耐食性と耐腐食疲労強度特性を向上することが可能となり、機械部品又は鋼構造物の寿命を延長することが出来るものである。更に、Ｃｕ、Ｎｉ等の高価な元素の多量の添加をしなくても本発明により耐食性の向上が可能となり、産業界が享受可能な経済的利益は多大なものがあると思料される。更に、本発明鋼の優れた機械的性質と相まって、本発明は、腐食を起点とする腐食疲労、ＳＣＣに対しても抵抗力の高い鋼材のベースとなるものである。
【図面の簡単な説明】
【図１】図１は（フェライト硬さ／（１）式の値）と腐食量の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is applied to high-strength steel bars, wire rods, mechanical structural steels, or welded structural steels for large steel structures such as shipbuilding, architecture, bridges / piers, tanks, pressure vessels, marine / port structures, and chemical plants. Applied corrosion resistance and corrosion fatigue resistanceExcellentFor structuredOn steelRelated.
[0002]
[Prior art]
Corrosion alone or as a starting point for fatigue, unstable and brittle fractures can cause serious damage to steel structures. Since corrosion and damage cases originating from corrosion account for a large percentage of damage cases in the entire steel structure, improvement is extremely important.
[0003]
Although steel structures are used in a wide range of environments, corrosion and corrosion fatigue are particularly problematic in water environments that contain chlorine or chloride, including seawater environments. On the other hand, for example, the Japan Steel Association 159th Nishiyama Memorial Lecture (1996) p. As summarized in No. 123, steel materials have been developed so far in which marine steel and other alloy components such as Cu, Ni, Cr, and P are added and increased to increase seawater resistance. Furthermore, it has been known so far that the corrosion resistance of steel is determined by the alloy components in the steel and does not depend on the structure of the steel. Therefore, in order to impart corrosion resistance to the steel, it is necessary to add the alloy elements as described above, which increases the cost as a structural steel for welding (hereinafter simply referred to as structural steel) and a large amount of alloys. There was a problem that the weldability and workability required for structural steels deteriorated due to the inclusion of elements.
[0004]
[Problems to be solved by the invention]
From such a background, an object of the present invention is to improve the corrosion resistance of structural steel, particularly in a water environment containing chlorine or chloride, by controlling the steel structure. That is, without relying on an alloy element effective for corrosion resistance as in the prior art, the corrosion resistance is improved while ensuring weldability, and when the alloy element is added, the corrosion resistance is greatly improved. Let it be an issue.
[0005]
[Means for Solving the Problems]
The present invention has been made to solve the above-mentioned problems, and the gist thereof is as follows.
[0006]
(1) By weight%
C: 0.04 to 0.25%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.0%,
Al: 0.005 to 0.6%,
P: 0.15% or less,
S: 0.01% or less
A steel or steel plate comprising the remaining iron and unavoidable impurities, and a surface layer region of 5% or more of the steel diameter or steel plate thickness from the surface layer portion of the steel or the front and back layer portions of the steel plate, respectively. In which the ferrite has a cementite phase of 0.5 μm or less at the crystal grain boundary and / or the crystal sub-grain boundary, and the ferrite accounts for 95% or more, and the hardness H of the ferrite structure satisfies the following formula (1): Furthermore, the (100) plane strength ratio of the texture parallel to the rolling surface is 1.5 or more.3 or lessStructural steel excellent in corrosion resistance and corrosion fatigue resistance, characterized by having
H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d)
(1)
However, [Ceq%] = C% + Si% / 24 + Mn% / 6, where C%, Si%, and Mn% are weight percentages of C, Si, and Mn, respectively, and d is equivalent to the average circle of ferrite. The particle size.
[0007]
(2) By weight%,In addition,
Nb: 0.005 to 0.1%,
Ti: 0.005 to 0.05%,
Ta: 0.005 to 0.05%
The structural steel having excellent corrosion resistance and corrosion fatigue resistance as described in the above item (1), comprising one or more of the above.
[0008]
(3) By weight%,
Cu: 0.05 to 1.0%,
Ni: 0.1 to 2.0%,
Cr: 0.03-3.0%,
Mo: 0.05-1.0%,
V: 0.01 to 0.4%
B: 0.0002 to 0.002%,
Ca: 0.0001 to 0.02%,
Mg: 0.0001 to 0.02%,
REM: 0.001% to 0.2%
The corrosion resistance and corrosion resistance according to (1) or (2) above, wherein the hardness H of the ferrite structure satisfies the following formula (2): Structural steel with excellent fatigue properties.
H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d)
(2)
However, [Ceq%] = C% + Si% / 24 + Mn% / 6 + (Cu% + Ni%) / 15, where C%, Si%, Mn%, Cu%, and Ni% are C, Si, Mn, Cu is the weight percent of Ni, and d is the average equivalent-circle grain size of ferrite.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
As a result of detailed examination of the corrosion resistance of various steels in a water environment containing chlorine, in a wet environment, and in a dry and wet repeated environment, the present inventors have found that the steel structure is mainly composed of ferrite with a low dislocation density, and the (100) plane of the texture. A strength ratio of 1.5 or more, and a ferrite grain boundary and / or a sub-grain boundary of 0.5 μm or less cementite phase or cementite phase and one or more carbon nitrides of Nb, Ti, Ta It has been found that the corrosion resistance of steel is greatly improved by precipitating phases.
[0015]
The mechanism by which the corrosion resistance is improved when the ferrite dislocation density is low is not clear, but the structure with a high dislocation density is presumed to be activated during the corrosion reaction because the strain energy due to the dislocation is large. Is done.
[0016]
Ferrite is a structure having a relatively low dislocation density, but there are structures having a high dislocation density such as processed ferrite. Therefore, a ferrite structure and a low dislocation density must be required.
[0017]
For this reason, first, it is necessary to evaluate the dislocation density inside the ferrite. It is extremely difficult to quantitatively measure the dislocation density.
[0018]
Therefore, there is a method for evaluating the magnitude relationship of the dislocation density inside the ferrite by utilizing the fact that the relationship between the grain size and the yield stress changes depending on the grain boundary characteristics. This is because a normal ferrite structure formed by the austenite / ferrite transformation has a certain relationship between the grain size and the hardness defined by the ferrite grain boundary, and can also be related to the Hall Petch relationship. However, as the dislocation density inside the ferrite increases, the relationship between the grain size and the hardness changes. Focusing on this experimental fact, the dislocation density inside the ferrite was evaluated. For this purpose, it is first necessary to measure the crystal grain size. In the present invention, not only the grain boundary caused by normal austenite / ferrite transformation but also the grain boundary caused by work recrystallization, the grain boundary appearance caused by normal nital corrosion is insufficient. Therefore, in order to reveal a clear grain boundary even in the processed structure, it was discovered that a marshall reagent, which is a corrosive solution mainly composed of an aqueous oxalic acid solution, a hydrogen peroxide solution, and an aqueous sulfuric acid solution, was suitable. The crystal grain size revealed was measured.
[0019]
By using such an evaluation method, it was found that the relationship of the following formula is established between the hardness H of the ferrite in the structure having a low dislocation density, each component, and the crystal grain size d.
[0020]
H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d)
However, [Ceq%] = C% + Si% / 24 + Mn% / 6 + (Cu% + Ni%) / 15, and this C%, Si%, Mn%, C%, and Ni% are C, Si, Mn, It is the weight% of Cu and Ni.
[0021]
This equation is an index indicating the state of dislocation density, and is an index determined by the chemical component (Ceq) and the crystal grain size d. The measurement of the particle size d may be the appearance of the structure by normal nital corrosion, but the appearance of the grain boundary by the Marshall reagent is more appropriate because the appearance of the structure is unclear in the structure having a high dislocation density. The appearance method is shown below.
[0022]
The Marshall reagent is a corrosive solution mainly composed of an aqueous oxalic acid solution, a hydrogen peroxide solution and 7 ml of a sulfuric acid solution, and usually comprises 50 ml of an 8% aqueous oxalic acid solution, 50 ml of hydrogen peroxide solution and 7 ml of 50% sulfuric acid solution. In order to make it easier to appear, an improved Marshall reagent comprising 50 ml of 8% oxalic acid aqueous solution, 50 ml of hydrogen peroxide solution, 14 ml of 50% sulfuric acid solution, and 1 ml of ethyl alcohol was used. First, the sample was immersed in 5% hydrochloric acid for 3-4 seconds, then washed with water, dried, then corroded for 3-5 seconds at room temperature using an improved Marshall reagent, washed with water, and dried to reveal grain boundaries. It is something to be made. This corrosion method is a representative example, and although it becomes difficult to observe the grain boundary even if the composition of the corrosion solution is changed to some extent, the grain boundary to be observed appears to corrode, and thus falls within the scope of the present invention. Is.
[0023]
Further, in order to precipitate cementite of 0.5 μm or less or one or more carbonitride phases of cementite and Nb, Ti, Ta at ferrite grain boundaries and / or crystal sub-boundaries, C or Ac or material of steel or steel plate (eg billet, slab) containing one or more of C and Nb, Ti, TaThreeIn a state in which one or two or more of C or C and Nb, Ti, and Ta are heated in a solid solution, the ferrite fraction is 50% or more before or during hot working such as controlled rolling. Is rapidly cooled to a temperature at which C or C and one or more of Nb, Ti, and Ta are dissolved in a supersaturated state.ThreeThe hot working is finished below the point, and then AcThreeCooling without reheating beyond the point is indispensable for effectively ensuring a structure having a (100) plane strength ratio of 1.5 or more in the texture parallel to the rolling surface and mainly composed of cerite. It turned out to be.
[0024]
The reason why the components are defined in the present invention will be described in detail below.
[0025]
In the present invention, C is an essential element for pinning ultrafine-grained ferrite with cementite precipitated at a ferrite grain boundary or a sub-grain boundary from a supersaturated solid solution state to 0.5 μm or less. Although it is an effective element, when it exceeds 0.25%, low temperature toughness is inhibited. Further, even in the surface layer region of steel or steel plate, the pearlite fraction exceeds 10%, and if it is less than 0.04%, the amount of cementite necessary for pinning is insufficient, so it is limited to 0.04 to 0.25%. In the case of structural steel for welding, if over 0.2%, weldability (weld portion toughness) deteriorates, so 0.04 to 0.2% is preferable.
[0026]
Si is effective as an element for improving strength and is also useful as a deoxidizing element for inexpensive molten steel. However, if it exceeds 1.0%, the weldability deteriorates, and if it is less than 0.01%, the deoxidizing effect is insufficient. Since expensive deoxidizing elements such as Al and Al need to be used frequently, the content is limited to 0.01 to 1.0%.
[0027]
Mn is an element necessary for improving the strength. From the necessity, Mn is 0.3% or more, and if it exceeds 2.0%, the base material toughness and weldability are inhibited and ArThreeAs a result of lowering the transformation point, the content is limited to 0.3 to 2.0% in order to make hot rolling such as two-phase rolling difficult.
[0028]
S was limited to 0.01% or less from the viewpoint of corrosion resistance and toughness. It is well known that MnS dissolves in an aqueous environment containing chlorine or chloride and becomes a selective corrosion starting point. From that viewpoint, S is preferably as low as possible.
[0029]
Nb is the most useful element together with Ti in thermomechanical processing (TMCP) steel, and as NbC or Nb (C, N) (Carbo-nitride), suppression of γ grain growth during reheating of steel and control of unrecycled during rolling The effect of preventing HAZ softening in the weld heat affected zone (HAZ) at the time of expansion of the crystal zone temperature, precipitation strengthening in the deformation zone during rolling, and high heat input welding is known. Furthermore, it has been found from the detailed study of the present inventor that the thermal stability of cementite ultra-precipitated and the effect of suppressing the growth of ferrite grains are remarkably increased. Therefore, if it is less than 0.005%, the amount of NbC or Nb (C, N) to be precipitated at the ferrite grain boundary or the sub-grain boundary from the supersaturated solid solution state to 0.5 μm or less is insufficient, and at 0.5 μm or less. Further, the thermal stability of cementite is also insufficient, and if it is 0.1% or more, the weldability is impaired, so the content is limited to 0.005 to 0.1%.
[0030]
Ti is also the most useful element together with Nb in TMCP steel. TiC or Ti (C, N) suppresses γ grain growth during reheating of steel, and controls and expands non-recrystallized temperature range during rolling. It is generally known that the HAZ toughness can be improved during precipitation strengthening and high heat input welding. Further, from the detailed study of the present inventor, it was found that the thermal stability of cementite ultrafinely precipitated and the effect of suppressing the growth of ferrite grains are improved in the same manner as Nb. Therefore, if it is less than 0.005%, the amount of TiC or TiCN precipitated at the ferrite crystal grain boundary or the sub-grain boundary from the supersaturated solid solution state to 0.5 μm or less is insufficient, and the cementite deposited to 0.5 μm or less The stability is also insufficient, and if it is 0.05% or more, the weldability is impaired, so the content is limited to 0.005 to 0.05%.
[0031]
Ta is known as TaC or Ta (C, N), which has the effect of suppressing γ grain growth during reheating of steel materials and improving HAZ toughness during large heat input, but is generally used because it is expensive. Not. However, from the detailed study of the present inventor, it has been found that the thermal stability of cementite ultrafinely precipitated and the effect of suppressing the growth of ferrite grains are improved in the same manner as Nb · Ti. Therefore, if it is less than 0.005%, the amount of TaC or TaCN precipitated at a ferrite grain boundary or a crystal sub-grain boundary from the supersaturated solid solution state to 0.5 μm or less is insufficient, and the cementite deposited to 0.5 μm or less The stability is also insufficient, and if it is 0.05% or more, the weldability is impaired, so the content is limited to 0.005 to 0.05%.
[0032]
Al is an element necessary for deoxidation like Si. From the technical idea of the present invention, when a small amount of Ti.Ta or Nb is added, deoxidation of Si alone is insufficient to prevent its oxidation. Addition of 005% or more is necessary. Furthermore, the present inventor has found that the addition of Al is also effective for the corrosion resistance of the steel of the present invention. However, excessive addition of 0.6% or more is limited to 0.005 to 0.6% in order to impair the HAZ toughness.
[0033]
The above are the basic components of steel targeted by the present invention, but the quality characteristics or steel materials required for lowering carbon equivalents for the purpose of improving the strength of the base metal and improving low temperature toughness and weldability. Depending on the size and thickness of the steel sheet, Cu is 0.05 to 1.0%, Ni is 0.1 to 2.0%, Cr is 0.03 to 3.0%, and Mo is 0.05 to 1.%. It is preferable to add 0%, V in the range of 0.01 to 0.4%, and B in the range of 0.0002 to 0.002% from the viewpoint of improving strength, low temperature toughness, and weldability. The effect of the present invention is not impaired at all. Further, Cu, Ni, and Cr are conventionally known as elements that improve the corrosion resistance of steel in an aqueous environment containing chlorine or chloride such as seawater. By applying the present invention to steel containing these elements, Further improvement of corrosion resistance is achieved. Furthermore, addition of P in combination with these elements is also effective for corrosion resistance, and is preferably added also in the present invention. However, addition over 0.15% significantly reduces toughness and weldability, so it is made 0.15% or less.
[0034]
Furthermore, as described above, MnS is harmful as a starting point of corrosion in an aqueous environment containing chlorine or chloride, and in order to reduce this, in order to control the form and dispersion of sulfides in steel, Ca is added in an amount of 0.0001 to It is effective to add 0.02%, Mg in the range of 0.0001 to 0.02%, and REM in the range of 0.001% to 0.2% in combination with the effects of the present invention.
[0035]
Next, the reason for defining the crystal structure which is the technical idea of the present invention will be described.
[0036]
Ferrite and pearlite steel containing bainite does not necessarily improve the corrosion resistance even if the ferrite structure is the main component. According to the inventor's detailed investigation, when ferrite grains are processed and have a high dislocation density, the corrosion reaction is activated in a corrosive environment, and corrosive pores in an aqueous environment containing chlorine or chloride are formed. It was found that the frequency of occurrence was high and the corrosion depth was large. Furthermore, it has also been found that when the fraction of the ferrite structure having a fine cementite phase is 95% or more, the corrosion resistance is particularly improved.
[0037]
On the other hand, a structure composed mainly of fine cementite or carbonitride phase cannot stably obtain a structure mainly composed of ferrite or bainite having an average particle diameter of 3 μm or less, and suppresses the growth of ferrite crystal grains. I also found it essential. That is, it is possible to effectively suppress the growth of peening ferrite or bainite for the first time by precipitating cementite of 0.5 μm or less at the ferrite crystal grain boundary or crystal sub-grain boundary.
[0038]
Moreover, when Nb, Ti, and Ta carbonitrides of 0.5 μm or less are precipitated at ferrite grain boundaries or crystal subgrain boundaries, a peening effect similar to that of cementite is observed, and ferrite grain boundaries or crystal sub- It was also found that the cementite itself deposited ultrafinely at the grain boundaries increases the thermal stability.
[0039]
Next, anti-corrosion fatigue is the one in which fatigue cracks develop from the generated corrosion pits.(1)It is necessary to reduce the size of the generated corrosion pits. The point is to refine the second phase structure, which is a cementite phase or carbide phase other than the ferrite phase, which is one of the causes of corrosion pits, to 0.5 μm or less. Furthermore,(2)It is important to suppress the growth from the corrosion pits as fatigue cracks, and in order to prevent the progress of the fatigue cracks in the thickness direction, the (100) plane texture in a plane parallel to the rolling surface is required. It is effective to strengthen, and in order to obtain a remarkable effect, the (100) plane strength ratio is 1.5 or more.3 or lessIt is necessary to make it.
[0040]
On the other hand, if the ratio of the ultrafine grain structure is less than 5% of the surface layer region of the steel or steel plate, that is, the diameter of the steel or the thickness of the steel plate, the corrosion resistance on the long time side varies and does not significantly improve. Limited to the above. The larger the proportion of the ultrafine grain structure, the better the corrosion resistance, and the upper limit is not particularly specified, but an excessive increase leads to an increase in production cost, so 30% is preferable.
[0041]
Next, the reason for prescribing the manufacturing method for realizing the structure in the back layer region of steel or steel plate in the present invention will be described.
[0042]
The heating temperature at which one or more of C and C and Nb, Ti, and Ta are dissolved at the time of reheating the steel material or steel is Ac.ThreeLimited to points or more. The heating temperature at which one or more of Nb, Ti and Ta are sufficiently dissolved is preferably 1000 ° C. or higher, and the heating temperature is 1200 ° C. or lower in order to prevent coarsening of γ grains during heating. It is preferable.
In the surface layer region of steel or steel plate, cementite and cementite of 0.5 μm or less are precipitated at one or two or more carbonitrides of Nb, Ti and Ta at the ferrite grain boundaries and / or crystal subgrain boundaries. Is a state in which one or more of C or C and Nb, Ti, Ta are dissolved in steel, and the surface layer region is cooled at a cooling rate of 3 ° C./second or more to cool C, Nb, Ti. After Ta is dissolved in supersaturated form in the steel, it is precipitated in a very fine manner in the process of reheating using sensible heat in the center of the steel in which the temperature does not decrease much even by cooling.
[0043]
In order to form a structure mainly composed of ferrite having a low dislocation density (structure mainly composed of ferrite or bainite having an average particle diameter of 3 μm or less) in the back layer region of steel or steel sheet, the material of steel or steel is Ac.ThreeThe surface area is rapidly cooled to a temperature at which the ferrite fraction becomes 50% or more at a cooling rate of 3 ° C./second or more after heating to the point or before or during hot working, and this cooling causes little temperature decrease. In the process of reheating using sensible heat in the center of the steel, (Ac1Start or resume hot working from a temperature higher than (point -150 ° C), and (Ac1Point + 50 ° C) to AcThreeBy finishing the hot working within the range of points, ferrite recovery and recrystallization are induced to make ultrafine grains, and
[0044]
Furthermore, after the primary processing, the surface layer region is changed to Ar.ThreeAfter cooling below the point, if secondary processing is performed during sensible heat recovery inside the steel, hot working in the non-recrystallization temperature range is performed up to the center of the steel, and the low temperature toughness of the steel is significantly improved To do.
[0045]
This hot working is intended for general processes such as rolling, extrusion, and drawing. In addition, when the material size of the steel is large and the heating temperature is 1170 ° C. or higher, or when the requirement for low temperature toughness is severe, in order to keep the initial γ grains before cooling the surface layer region of the steel or steel plate, It is preferable to perform hot working such as addition of Nb, Ti, Ta and controlled rolling. Furthermore, in the case where hot working is not performed prior to cooling following the heating of the steel, low temperature heating and addition of Nb, Ti, Ta or fine hot of the initial γ grains are performed in order to make the initial γ grains of the steel finer. The use of processed semi-finished products is preferred.
[0046]
After the surface layer region of steel or steel plate is made ultrafine, Ac is generated by sensible heat at the center of steel or steel plate.ThreeReheating beyond the point not only impairs the effect of making the surface layer ultrafine, but cementite finely precipitated at the ferrite grain boundaries or crystal subgrain boundaries re-dissolves in γ and loses the pinning effect. End up. Therefore, the surface layer region is AcThreeIt is necessary not to reheat more than the point, and while monitoring the temperature during rolling, if necessary AcThreeIn order not to reheat beyond the point, it may be cooled after rolling.
[0047]
In order to increase the strength of steel or steel plate, the additive component is adjusted according to the required strength level, and subsequently, after the hot working is completed, AcThreeAccelerated cooling by the TMCP facility or direct quenching by the DQ facility may be performed at a cooling rate of 5 ° C./second or more without reheating beyond the point.
[0048]
Subsequent to accelerated cooling or direct quenching, the steel or steel plate is tempered by normal heat treatment equipment. In the case of accelerated cooling by the TMCP facility or direct quenching by the DQ facility, it may be replaced by an auto temper that stops the water cooling at the time of accelerated cooling or direct quenching.
[0049]
【Example】
Examples of the present invention will be described with reference to Tables 1 to 3.
[0050]
First, Table 1 shows the components of steel, Steel A to Steel E are examples of the present invention, and Steel F is a comparative example in which C and S are outside the scope of the present invention.
[0051]
[Table 1]
Next, Table 2 shows the manufacturing conditions of the steel sheet, and Table 3 shows the structure and characteristics of the manufactured steel sheet surface layer region. Steel A-1, Steel B-1, Steel C-1, Steel D-1 and Steel E-1 in Table 3 are examples of the present invention, Steel A-2, Steel A-3, Steel B-2, Steel C-2, steel D-2, steel E-2, and steel F-1 are comparative examples.
[0053]
[Table 2]
Moreover, the evaluation method of the corrosion resistance evaluation result of the steel plate shown in Table 3 is a salt spray exposure test and a seawater immersion test. In this salt water spray exposure test, a test piece of 150 mm length × 50 mm width × 5 mm thickness collected from the surface layer of the steel sheet was exposed outdoors, and a 5% NaCl aqueous solution was sprayed on the test surface once a day with a sprayer. It measures the thickness reduction and weight loss due to the occurrence of corrosion. The exposure period was 3 months and 6 months, and 3 specimens were used for each steel type in each period. In the seawater immersion test, a test piece of 150 mm length x 50 mm width x 5 mm thickness is immersed in an aqueous solution of 3.5% NaCl equivalent to seawater at 50 ° C, and the thickness reduction and weight loss associated with the occurrence of corrosion are measured. Is. The immersion period was 1 month and 3 months, and three test pieces were used for each steel type in each period. And the result of Table 3 is an average value of 3 test pieces in any test.
[0055]
Furthermore, the evaluation method of the corrosion fatigue resistance evaluation results shown in Table 3 is that a tensile test piece of a full thickness flat plate (smooth, stress concentration factor Kt = 1.1, the plate thickness portion is sealed with a polymer, and fatigue from the steel plate surface is observed. (Evaluation of generation of cracks) is used, and repeated stress is applied at 0.1 Hz by swinging and pulling in artificial seawater specified by ASTM at 25 ° C. Then, tests were performed in various stress ranges, and a stress fracture diagram (S-Nf curve) was measured. As a measure of corrosion fatigue strength, Nf = 5 × 10FiveThe fatigue strength at was divided by the tensile strength.
[0056]
Further, the ferrite hardness Hv in Table 3 was measured using a 10 kg Vickers hardness meter, and the crystal grain size was measured using a microscope and an image analysis device by revealing the structure with the aforementioned Marshall reagent.
[0057]
[Table 3]
Steel A-2 and Steel A-3 of the comparative example were cooled in the middle of the steel plate surface region, and the cooling rate was slower than the range of the present invention.ThreeThis is an example in which the cementite size was coarsened although the recuperation was not more than the point. Steels B-2 and E-2 were cooled in the middle and had a cooling rate within the range of the present invention, but the thickness of the surface layer region where the cooling time was short and the ferrite fraction was 50% or more was 5% of the steel plate. It was an example that was less than less. Steel C-2 and Steel D-2 were examples in which no ferrite layer was formed in the surface layer region because no cooling was performed on the way, and Steel F-1 was almost the same as Steel C-1 of the present invention example. Although it is a manufacturing condition, C and S which are the main components are examples which are out of the scope of the present invention.
[0059]
From this result, in any steel sheet of A, B, C, D, and E, the inventive example was exposed as compared with the comparative example having the same composition as a result of the structure state of the surface layer region satisfying the inventive condition. Both the test and the immersion test are clearly excellent in corrosion resistance, and are also excellent in corrosion fatigue resistance.
[0060]
For example, in the A-1 steel of the present invention example, the ferrite fraction and precipitates in the surface layer region satisfy the conditions of the present invention as compared with the A-2 steel of the comparative example, and accordingly the corrosion weight loss is less than half. Corrosion fatigue strength is about 1.5 times in absolute value, and even if divided by tensile strength, it is greatly improved to about 1.68 times. Furthermore, steel A-2 of the comparative example is Ac.ThreeThe α grains refined by reheating beyond the point reversely transform into γ, and the cementite that has been ultrafinely precipitated re-dissolves in γ. As a result, the α grains and cementite in the surface region also coarsen and the ferrite fraction increases. 90% or less. Correspondingly, it can be seen that the corrosion pits generated on the surface of the steel sheet are larger and deeper in the steel A-2 than the steel A-1 and inferior in corrosion fatigue.
[0061]
In addition, in steels B-1 and C-1 to which Nb, Ti, and Ta are added, cementite or carbonitride precipitates very finely at the ferrite grain boundaries and crystal sub-grain boundaries, thereby effecting the growth of ferrite or some bainite. As a result, the ferrite fraction can be secured stably even though the carbon equivalent is larger than that of the steel A-1 as an example of the present invention, and the corrosion pits are further refined, thereby reducing the corrosion weight. In this respect, the corrosion fatigue strength is further improved. On the other hand, the steel B-2 in the comparative example has insufficient cooling conditions before the finish rolling and the fine layer thickness is less than 5% and does not satisfy the conditions of the present invention. Although sufficiently secured, ferrite hardness, precipitate size, and (100) plane strength cannot satisfy the conditions of the present invention, and corrosion resistance and corrosion fatigue strength are greatly inferior to those of the examples of the present invention. Since steel C-2 was not cooled in the middle, the corrosion resistance and corrosion fatigue strength were inferior to those of the examples of the present invention. The same tendency was observed between steels D-1 and D-2 and steels E-1 and E-2.
[0062]
In addition, when the present invention examples are compared, for example, Steel A-1 and Steel B-1 to Steel E-1, it is absolute level comparison when Cu, Ni, Cr and Ca, REM, Mg are added to the steel material components. In, it is excellent in corrosion resistance. This indicates that the effect of these additive elements on the corrosion resistance (conventional knowledge) and the present invention can be superimposed. According to the present invention, not only the corrosion resistance of ordinary structural steel can be improved, but also the corrosion resistance of conventional corrosion resistant structural steel can be greatly improved.
[0063]
Finally, steel EF-1 of the comparative example in which the production conditions are substantially the same as the steel EA-2 of the present invention example, but C and S are out of the higher side of the present invention example, the ferrite fraction, the ferrite hardness Satisfies the conditions of the present invention, but as a result of the high perlate fraction and high S, the corrosion resistance is inferior to the examples of the present invention.
[0064]
【The invention's effect】
In the present invention, the surface layer region of steel or steel plate is mainly composed of a ferrite structure having a low dislocation density, and the (100) plane strength ratio is 1.5 or more.3 or lessBy improving the corrosion resistance and corrosion fatigue strength characteristics of structural steel or structural steel for welding in an aqueous environment containing chlorides such as seawater, not only in terms of chemical composition but also in terms of steel structure It is possible to extend the life of machine parts or steel structures. Furthermore, the corrosion resistance can be improved by the present invention without adding a large amount of expensive elements such as Cu and Ni, and it is considered that there are many economic benefits that the industry can enjoy. Furthermore, coupled with the excellent mechanical properties of the steel of the present invention, the present invention is a base of steel material having high resistance against corrosion fatigue and SCC starting from corrosion.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between (ferrite hardness / value of equation (1)) and the amount of corrosion.
Claims (3)
Ｃ：０．０４〜０．２５％、
Ｓｉ：０．０１〜１．０％、
Ｍｎ：０．３〜２．０％、
Ａｌ：０．００５〜０．６％、
Ｐ：０．１５％以下、
Ｓ：０．０１％以下
の成分を有し、残部鉄及び不可避的不純物から成る鋼又は鋼板で、該鋼の表層部又は鋼板の表・裏層部からそれぞれで鋼の径又は鋼板の厚さの５％以上の表層領域において、結晶粒界及び／又は結晶亜粒界に０．５μｍ以下のセメンタイト相を有すると共にフェライトが９５％以上を占め、且つ、そのフェライト組織の硬さＨが、下記（１）式を満足し、更に、圧延面に平行な集合組織の（１００）面強度比が１．５以上３以下を有することを特徴とする耐食性と耐腐食疲労特性に優れた構造用鋼。
Ｈ≦２００［Ｃｅｑ％］＋２０＋（９［Ｃｅｑ％］＋３．７）／√（ｄ）
・ ・ ・ （１）
但し、［Ｃｅｑ％］＝Ｃ％＋Ｓｉ％／２４＋Ｍｎ％／６であり、このＣ％、Ｓｉ％、Ｍｎ％はそれぞれＣ、Ｓｉ、Ｍｎの重量％であり、更に、ｄはフェライトの平均円相当粒径である。% By weight
C: 0.04 to 0.25%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.0%,
Al: 0.005 to 0.6%,
P: 0.15% or less,
S: A steel or steel plate having a component of 0.01% or less, the balance being iron and unavoidable impurities, the steel diameter or the steel plate thickness from the surface layer portion of the steel or the front and back layer portions of the steel plate, respectively. In the surface layer region of 5% or more, the ferrite has a cementite phase of 0.5 μm or less at the crystal grain boundaries and / or crystal sub-grain boundaries, and ferrite accounts for 95% or more, and the hardness H of the ferrite structure is A structural steel excellent in corrosion resistance and corrosion fatigue resistance characterized by satisfying the formula (1) and having a (100) plane strength ratio of the texture parallel to the rolling surface of 1.5 to 3 .
H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d)
(1)
However, [Ceq%] = C% + Si% / 24 + Mn% / 6, where C%, Si%, and Mn% are weight percentages of C, Si, and Mn, respectively, and d is equivalent to the average circle of ferrite. The particle size.
Ｎｂ：０．００５〜０．１％、
Ｔｉ：０．００５〜０．０５％、
Ｔａ：０．００５〜０．０５％
の１種又は２種以上を含有することを特徴とする請求項１記載の耐食性と耐腐食疲労特性に優れた構造用鋼。In weight percent, in further,
Nb : 0.005 to 0.1%,
Ti: 0.005 to 0.05%,
Ta: 0.005 to 0.05%
The structural steel having excellent corrosion resistance and corrosion fatigue resistance according to claim 1, comprising one or more of the following.
Ｃｕ：０．０５〜１．０％、
Ｎｉ：０．１〜２．０％、
Ｃｒ：０．０３〜３．０％、
Ｍｏ：０．０５〜１．０％、
Ｖ：０．０１〜０．４％、
Ｂ：０．０００２〜０．００２％、
Ｃａ：０．０００１〜０．０２％、
Ｍｇ：０．０００１〜０．０２％、
ＲＥＭ：０．００１％〜０．２％
の１種又は２種以上を含有し、且つ、フェライト組織の硬さＨが、下記（２）式を満足することを特徴とする請求項１又は２記載の耐食性と耐腐食疲労特性に優れた構造用鋼。
Ｈ≦２００［Ｃｅｑ％］＋２０＋（９［Ｃｅｑ％］＋３．７）／√（ｄ）
・ ・ ・ （２）
但し、［Ｃｅｑ％］＝Ｃ％＋Ｓｉ％／２４＋Ｍｎ％／６＋（Ｃｕ％＋Ｎｉ％）／１５であり、このＣ％、Ｓｉ％、Ｍｎ％、Ｃｕ％、Ｎｉ％はそれぞれＣ、Ｓｉ、Ｍｎ、Ｃｕ、Ｎｉの重量％で、更に、ｄはフェライトの平均円相当粒径である。% By weight,
Cu: 0.05 to 1.0%,
Ni: 0.1 to 2.0%,
Cr: 0.03-3.0%,
Mo: 0.05-1.0%,
V: 0.01 to 0.4%
B: 0.0002 to 0.002 %,
Ca : 0.0001 to 0.02%,
Mg: 0.0001 to 0.02%,
REM: 0.001% to 0.2%
1 or 2 or more, and the hardness H of the ferrite structure satisfies the following formula (2): Excellent corrosion resistance and corrosion fatigue resistance according to claim 1 or 2 Structural steel.
H ≦ 200 [Ceq%] + 20+ (9 [Ceq%] + 3.7) / √ (d)
(2)
However, [Ceq%] = C% + Si% / 24 + Mn% / 6 + (Cu% + Ni%) / 15, where C%, Si%, Mn%, Cu%, and Ni% are C, Si, Mn, Cu is the weight percent of Ni, and d is the average equivalent-circle grain size of ferrite.
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| JP34508799A JP4291480B2 (en) | 1999-12-03 | 1999-12-03 | Structural steel with excellent corrosion resistance and corrosion fatigue resistance |
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