JP4016770B2 - Seawater resistant steel and manufacturing method thereof - Google Patents
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Description
【０００１】
【産業上の利用分野】
本発明は、海上橋梁の橋脚、その他の港湾構造物、または船舶の内外構造部材や海水タンクなど、海水に直接さらされる部分に使用するのに好適な耐海水性鋼とその製造方法に関する。
【０００２】
【従来の技術】
鋼構造物の使用環境は幅広いが、腐食が特に問題となるのは、海水環境をはじめとする塩化物を含む環境である。
【０００３】
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)のP.123に記載されているように、マリーナースチールをはじめ、Cu、Ni、Cr、Ｐなどの合金成分を添加、増量して耐海水性を高めた鋼がこれまで開発されてきた。また、従来、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ほとんどないとされていた。
【０００４】
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、増量が必要なため、コストが上昇するほか、溶接性や加工性が低下するという問題があった。
【０００５】
耐海水性に優れた鋼については、例えば特開平7-3388号公報や同11-1745 号公報に示される鋼がある。即ち、前者に示される鋼は、CrとAlを必須成分として含有させることにより耐海水性を向上させたものであるが、Al含有量が多いために熱間加工性や溶接性が著しく劣る。また、後者に示される鋼は、MoとNiを必須成分として含有させることにより耐海水性を向上させたものであるが、高価なNiとMoの両方を含有しているためにコスト増が避けられない。
【０００６】
【発明が解決しようとする課題】
本発明は、上記のような実状に鑑みてなされたもので、第１の目的は、合金元素の種類と含有量が少ないにもかかわらず良好な耐食性を示す安価な耐海水性鋼を提供することにある。また、第２の目的は、塩化物を含む海水環境での耐食性が一段と良好な耐海水性鋼とその製造方法を提供することにある。
【０００７】
【課題を解決するための手段】
本発明者らは、上記の課題を達成するために、海水環境での耐食性について祥細に検討した結果、以下のことを知得した。
【０００８】
(a) 耐海水性は、鋼中のＣ、Ｓ、Cr、ＮおよびＯ（酸素）の５元素を適切な範囲、特にCrを1.0 ％以上含有させる一方、不純物としてのＳを0.003 ％以下、Ｎを0.008 ％以下、Ｏを0.005 ％以下に制限すると、大幅に向上する。
【０００９】
(b) 上記の耐海水性は、鋼の表層、特に少なくとも表面下0.5 mmまでの表層部分の金属組織を、ベイナイト相の面積割合が50％以上である平均粒径８μｍ以下の細粒組織にすると、一段と向上する。
【００１０】
(c) 上記の金属組織は、熱間圧延後、鋼の表面温度が600 ℃以下になるまで強制冷却し、内部の顕熱によって鋼の表面温度が750 ℃以上にまで復熱した後、終了温度700 ℃以上の仕上圧延を行い、その後所定の冷却処理を施すことにより得られる。
【００１１】
上記の知見に基づく本発明の要旨は、下記（１）〜（５）の耐海水性鋼および下記（６）の耐海水性鋼の製造方法にある。
【００１２】
（１）質量％で、Ｃ：0.04％以下、Si：1.5％以下、Mn：5.0％以下、Cr：1.0〜5.0％およびsol.Al：0.003〜0.05％を含有し、残部はFeおよび不純物からなり、不純物としてのＳが0.003％以下、Ｎが0.008％以下、Ｏ（酸素）が0.005％以下であり、少なくとも表面下 0.5mm までの表層部分の金属組織が、ベイナイト相の面積割合が 50 ％以上である平均粒径８μｍ以下の細粒組織であることを特徴とする耐海水性鋼。
【００１３】
（２）さらに、質量％で、Ｐ：0.01 〜0.3 ％を含有する上記（１）に記載の耐海水性鋼。
【００１４】
（３）さらに、質量％で、Cu：0.05〜1.0 ％、Ni：0.05〜3.0 ％、Mo：0.05〜1.0 ％およびＷ：0.05〜1.0 ％のうちの１種以上を含有する上記（１）または（２）に記載の耐海水性鋼。
【００１５】
（４）さらに、質量％で、Nb：0.002 〜0.10％、Ｖ：0.005 〜0.10％およびＢ：0.0003〜0.0050％の１種以上を含有する上記（１）から（３）までのいずれかに記載の耐海水性鋼。
【００１６】
（５）さらに、質量％で、Ti：0.005 〜0.10％、Ca：0.0001〜0.02％、Mg：0.0001〜0.02％およびREM ：0.0001〜0.02％のうちの１種以上を含有する上記（１）から（４）までのいずれかに記載の耐海水性鋼。
【００１８】
（６）上記（１）から（５）までのいずれかに記載の化学組成を有する鋼を、1000〜1200℃に加熱した後、Ａｒ3点以上の温度域で累積圧下率10％以上の熱間圧延を行い、次いで、15℃／秒以上の冷却速度で鋼の表面温度が600℃以下になるまで一旦冷却し、内部の顕熱によって鋼の表面温度が750℃以上になるまで復熱させた後、終了温度700℃以下の仕上圧延を行い、その後、冷却開始温度650℃以上、冷却速度５℃／秒以上、冷却終了温度550℃以下の冷却処理を行うことを特徴とする上記（１）から（５）までのいずれかに記載の耐海水性鋼の製造方法。
【００１９】
【発明の実施の形態】
以下、本発明を上記のように定めた理由について説明する。
【００２０】
１．化学組成について
Ｃ：0.04％以下
Ｃは海水中でカソードサイトとなりやすいクロム炭化物の生成を低減させるためにはなるべく低い方がよく、0.04％以下とする。好ましいのは0.03％以下、より好ましいのは0.02％以下である。なお、下限は特に規定しないが過度な低減はコスト上昇を招くので、その下限は0.001 ％程度とするのがよい。
【００２１】
Si：1.5 ％以下
Siは脱酸剤として添加される。その場合の望ましい下限含有量は0.03％である。しかし、その含有量が1.5 ％を超えると、靱性が劣化するので、Si含有量は1.5 ％以下とする。好ましい上限は1.0 ％、より好ましい上限は0.8 ％である。
【００２２】
Mn：5.0 ％以下
Mnは脱酸剤として、また耐海水性および強度を向上させるのに有効な元素であり、その効果は0.2 ％以上の含有量で顕著になるが、5.0 ％を超えると靱性の劣化を招く。このため、Mn含有量は5.0 ％以下とした。好ましいのは4.0 ％以下、より好ましいのは3.0 ％以下である。
【００２３】
Cr：1.0 〜5.0 ％
Crは本発明では必要不可欠な元素であり、その含有量を1.0 〜5.0 ％としたのは以下の理由による。即ち、1.0 ％以上で耐海水性が顕著に向上し、5.0 ％を超えると孔食感受性が著しく高くなる。なお、Crはさび層中に濃化し、さびの保護性を高める作用も有する。好ましいのは2.0 〜4.0 ％、より好ましいのは2.0 〜3.0 ％である。
【００２４】
sol.Al：0.003 〜0.05％
Alは脱酸剤として添加される元素であり、その効果を確実に得るためには最低でもsol.Al含有量で0.003 ％以上が必要である。一方、0.05％を超えると溶接性が低下する。このため、Alの含有量はsol.Alで0.003 〜0.05％とした。好ましいのは0.003〜0.045％、より好ましいのは0.003〜0.040％である。
【００２５】
Ｓ：0.003 ％以下
Ｓは耐食性の観点から0.003 ％以下とする。即ち、海水環境下では非金属介在物のMnＳが溶解し、選択的な腐食、特にCr含有鋼では孔食の起点となる。このため、Ｓ含有量は低ければ低いほどよい。
【００２６】
Ｎ：0.008 ％以下
Ｎは本発明では重要な元素であり、Ｃと同様に、Crと結合してCrＮなどの窒化物として析出し、耐食性に有効な固溶Cr濃度を低下させて耐海水性を劣化させるだけでなく、靱性をも劣化させる。このため、Ｎ含有量は少なければ少ないほどよいが、0.008 ％以下であればその有害性が小さく、許容できる。このため、Ｎ含有量は、0.008 ％以下に制限することとした。好ましい上限は0.006 ％、より好ましい上限は0.004 ％である。
【００２７】
Ｏ（酸素）：0.005 ％以下
Ｏは、上記のＮと同様に、本発明では重要な元素であり、Cr含有鋼においては孔食の起点となりやすいCaＯ、MgＯ、Al２Ｏ３などの酸化物系非金属介在物を形成して耐食性を劣化させる。また、過剰なＯは靱性をも劣化させる。しかし、その含有量が0.005 ％までであれば影響が小さいので、0.005 ％以下とした。好ましいのは0.004 ％以下、より好ましいのは0.003 ％以下である。
【００２８】
Ｐ：
Ｐは添加しなくてもよい。添加すれば、耐海水性を向上させる作用がある。このため、この効果を得たい場合には積極的に添加してもよく、その効果は0.01％以上の含有量で得られる。しかし、含有量が0.3 ％を超えると、靱性および溶接性が著しく低下する。このため、添加する場合のＰ含有量は0.01〜0.3 ％とするのがよい。
【００２９】
Cu、Ni、Mo、Ｗ：
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も耐海水性を向上させる作用がある。このため、この効果を得たい場合には、いずれか１種または２種以上を積極的に添加してもよい。しかし、いずれの元素も0.05％以上の含有量でないとその効果が得られない。一方、Niは3.0 ％、その他の元素は1.0 ％でその効果は飽和し、それ以上の添加はコスト上昇を招くだけである。従って、添加する場合のこれら元素の含有量は、Cu、MoおよびＷは0.05〜1.0 ％、Niは0.05〜3.0 ％とするのがよい。
【００３０】
Nb、Ｖ、Ｂ：
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も強度および靱性を向上させる作用がある。このため、この効果を得たい場合には、いずれか１種または２種以上を積極的に添加してもよい。しかし、Nbは0.002 ％以上、Ｖは0.005 ％、Ｂは0.0003％以上の含有量でないとその効果が得られない。一方、NbおよびＶは0.10％、Ｂは0.0050％でその効果は飽和し、それ以上の添加はかえって靱性低下を招く。従って、添加する場合のこれら元素の含有量は、Nbは0.002 〜0.10％、Ｖは0.005 〜0.10％、Ｂは0.0003〜0.0050％とするのがよい。
【００３１】
Ti、Ca、Mg、REM：
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も鋼中の硫化物の形態を制御し、孔食の発生起点となるMnＳを低減させ、耐食性を向上させる作用がある。このため、この効果を得たい場合には、いずれか１種または２種以上を積極的に添加してもよい。しかし、Tiは0.005 ％以上、Ca、MgおよびREM はいずれも0.0001％以上の含有量でないとその効果が得られない。一方、Tiは0.10％、Ca、MgおよびREM はいずれも0.02％でその効果は飽和し、それ以上の添加はかえって靱性低下を招く。従って、添加する場合のこれら元素の含有量は、Tiは0.005 〜0.10％、Ca、MgおよびREM はいずれも0.0001〜0.02％とするのがよい。
【００３２】
上記の成分以外は実質的にFe、言い換えれば残部はFeおよび上記以外の不純物である。
【００３３】
２．金属組織について
上記の化学組成を有する本発明の耐海水性鋼は、この種の鋼の一般的な金属組織であるフェライト組織やフェライトとパーライトの混合組織およびマルテンサイト組織であっても十分な耐海水性を備える。
【００３４】
しかし、その耐海水性は、ベイナイト相の面積割合が50％以上で、しかも結晶粒の平均粒径が８μｍ以下の金属組織の場合、一段と良好になることは前述したとおりである。
【００３５】
なお、フェライト組織やフェライトとパーライトの混合組織およびマルテンサイト組織よりもベイナイト組織の方が耐海水性が良好になる理由の詳細は不明であるが、結晶が細粒であるほど耐海水性がより良好となるのは、細粒化によりアノード溶解反応の交換電流密度が大きくなってFeの溶解が抑制されるためである。
【００３６】
鋼の中心部は、必ずしも上記したベイナイトを主体とする細粒組織にする必要はなく、少なくとも表面下0.5 mmまでの表層部分のみをベイナイト主体の細粒組織にすれば十分であり、この場合には構造用材料として必要な靱性の確保が容易になる。
【００３７】
ここで、表層部分のみをベイナイト主体の細粒組織にする場合の最低深さを0.5 mmと定めたのは次の理由による。即ち、耐海水性鋼には、通常、20年間での腐食量が0.5 mm以下であることが望まれるが、本発明の鋼の腐食速度は、後述する実施例からもわかるように、およそ25μｍ／年であり、表面下0.5 mmまでが25μｍ／年の腐食速度であれば20年間の使用に確実に耐え得るからである。
【００３８】
なお、表層部分の組織は扁平粒組織であっても問題ない。従って、扁平粒組織の場合の粒径は、板厚方向の粒切片（長径）と圧延方向の粒切片（短径）の平均値で求められ、この平均値の総和を結晶粒の数で除して求められる値が平均粒径である。
【００３９】
３．表層部分が上記の状態の耐海水性鋼を得る製造条件について
３-1．素材鋼の加熱温度
加熱時にはＣを固溶させる必要があるが1000℃未満ではＣを確実に固溶させることができない。一方、1200℃を超えるとγ粒の粗大化を招く。このため、加熱温度は1000〜1200℃とするのが望ましい。好ましいのは1050〜1150℃である。
【００４０】
３-2．途中冷却とその前の熱間圧延
加熱後の熱間圧延前または圧延途中において鋼の表層部をＡr３点以下に冷却し、その後に、鋼内部の顕熱により表層部をＡc１点以上に復熱させると、鋼の表層部に逆変態現象が生じて結晶粒が細粒となる。また、その後、さらに圧延を行うと、表層部のγ粒、ベイナイトパケットおよびその内部の組織も微細化し、これによってγ粒界、パケットおよびべイナイトラス界面での炭化物の析出密度が減少する。
【００４１】
しかし、途中冷却前のＡr３点以上の温度域における累積圧下率が10％未満、途中冷却時の冷却速度が15℃／秒未満、鋼の表面温度が600 ℃超、復熱時の鋼の表面温度が750 ℃未満では、表面下0.5 mmの表層部分の金属組織を安定してベイナイト相の面積割合が50％以上、平均粒径８μｍ以下の細粒組織にすることができない。このため、途中冷却前のＡr３点以上の温度域における累積圧下率は10％以上、途中冷却時の冷却速度は15℃／秒以上、鋼の表面温度は600 ℃以下、復熱後の鋼の表面温度は750 ℃以上とするのが望ましい。
【００４２】
ここで、途中冷却前の熱間圧延における加工温度と累積圧下率は、いずれも高いほどよいのでその上限は特に限定しないが、加工温度があまり高すぎると靱性が劣化しやすく、累積圧下率が大きすぎると異方性がでやすいため、加工温度の上限は1000℃程度、累積圧下率の上限は80％程度とするのがよい。
【００４３】
また、途中冷却後における復熱後の鋼の表面温度は高いほどよく、逆に途中冷却後の鋼の表面温度は低いほどよいので、いずれも、その上限温度および下限温度は特に限定しない。しかし、復熱後の鋼の表面温度があまり高すぎると組織が粗大化しやすいので、復熱後の鋼の表面温度の上限は950 ℃程度とするのがよく、途中冷却後の鋼の表面温度をあまり低くしすぎると復熱後の表面温度750 ℃以上の確保ができなくなるため、途中冷却後の鋼の表面温度の下限温度は400 ℃程度とするのがよい。
【００４４】
さらに、途中冷却時の冷却速度は、速ければ速いほど鋼の表層部と内部の温度差が大きくなり、復熱が十分にできるため、その上限は特に限定しないが、あまり速すぎると冷却が不均一となりやすので、その上限は60℃／秒程度とするのがよい。
【００４５】
３-3．復熱後の仕上熱間圧延と圧延後の冷却
復熱後の仕上熱間圧延には特別な制約はない。しかし、仕上熱間圧延の終了温度が700 ℃を上回ったり、圧延後の冷却開始温度が650 ℃未満、冷却速度が５℃／秒未満および冷却終了温度が550 ℃を上回ると、鋼内部の靱性が劣化することがある。このため、仕上熱間圧延の終了温度は700 ℃以下、圧延後の冷却開始温度は650 ℃以上、冷却速度は５℃／秒以上、冷却終了温度は550 ℃以下とするのが望ましい。
【００４６】
ここで、仕上熱間圧延の冷却における冷却速度は、速いほどよいのでその上限は特に限定しないが、あまり速すぎると、強度が高くなりすぎて靱性が劣化するので、その上限は20℃／秒程度とするのがよい。
【００４７】
【実施例】
表１および表２に示す化学組成を有する１３種類の鋼を準備し、表３に示す製造条件のうちの１または２以上の方法で厚さ12mmの鋼板を製造した。
【００４８】
得られた鋼板から、幅100 mm、長さ100 mmの試験片を各３枚採取し、周囲の端面をタールエポキシ樹脂によりシールして腐食試験片とした。
【００４９】
腐食試験としては、自然海水中に腐食試験片を完全に浸漬する試験を行った。試験期間は0.5 年間とした。
【００５０】
腐食速度は、試験後の腐食生成物を除去し、重量減少から板厚減少量を計算することにより求めた。なお、各鋼板の表層部と中央部の金属組織とその面積割合は光学顕微鏡によって求め、平均粒径はSEM 観察によって個々のラス状組織の大きさを測定することにより求めた（３枚の試験片の平均値）。
【００５１】
試験の結果は、表４にまとめて示した。なお、表４に示した結果の値はいずれも試験片３枚の平均値である。
【００５２】
表４からわかるように、本発明例の鋼板（試験番号１〜24）は腐食速度が37μｍ／年以下と優れた耐食性を示す。特に、表面下0.5 mmまでの表層部分が平均粒径８μｍ以下のベイナイト主体組織である試験番号１、２、13〜15、17〜20、22および24の鋼板の腐食速度は21μｍ／年以下と遅い。
【００５３】
これに対し、比較例の鋼板（試験番号25〜30）は腐食速度が107μm／年以上と本発明の鋼板の約３倍強と早く、耐食性が劣る。即ち、試験番号25の鋼板はＣ量が高く、試験番号26の鋼板はCr量が不足し、試験番号27の鋼板はＳ量が高いためにいずれも耐食性が不十分である。また、試験番号28〜30の鋼板は、本発明の望ましい製造方法で製造されていて結晶粒は微細であるが、上記のように化学組成が本発明で規定する範囲を外れているために微細化の効果がなく、耐食性に劣る。
【００５４】
【表１】
【表２】
【表３】
【表４】
【発明の効果】
本発明の耐海水性鋼は腐食速度が遅い。このため、本発明の耐海水性鋼で構成された海洋構造物は長期にわたり使用可能である。また、本発明の望ましい製造方法によれば、腐食速度が一段と遅い耐海水性鋼を安価かつ高能率に安定して製造可能である。[0001]
[Industrial application fields]
The present invention relates to a seawater resistant steel suitable for use in a portion directly exposed to seawater, such as a bridge pier of a marine bridge, other port structures, a ship's internal and external structural members, a seawater tank, etc.
[0002]
[Prior art]
Although steel structures are used in a wide range of environments, corrosion is particularly problematic in environments containing chlorides, including the seawater environment.
[0003]
On the other hand, as described in P.123 of the 159th Nishiyama Memorial Course (1996) of the Japan Iron and Steel Institute, for example, Marina Steel and other alloy components such as Cu, Ni, Cr, and P are added. Steels with increased seawater resistance have been developed so far. Conventionally, the corrosion resistance of steel is determined by the alloy components in the steel, and it is said that there is almost no dependence on the structure of the steel.
[0004]
Therefore, in order to impart corrosion resistance to the steel, it is necessary to add and increase the amount of alloy elements as described above, so that there are problems that costs are increased and weldability and workability are reduced.
[0005]
Examples of steel having excellent seawater resistance include steels disclosed in JP-A-7-3388 and 11-1745. That is, the steel shown in the former has improved seawater resistance by containing Cr and Al as essential components, but the hot workability and weldability are remarkably inferior because of the high Al content. In addition, the steel shown in the latter has improved seawater resistance by containing Mo and Ni as essential components, but avoids an increase in cost because it contains both expensive Ni and Mo. I can't.
[0006]
[Challenges that the Invention is to Solve]
The present invention has been made in view of the actual situation as described above, and a first object is to provide an inexpensive seawater-resistant steel that exhibits good corrosion resistance despite a small type and content of alloy elements. There is. A second object is to provide a seawater-resistant steel having a much better corrosion resistance in a seawater environment containing chloride and a method for producing the same.
[0007]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have studied carefully about the corrosion resistance in the seawater environment, and as a result, have learned the following.
[0008]
(a) As for seawater resistance, 5 elements of C, S, Cr, N and O (oxygen) in steel are included in an appropriate range, particularly 1.0% or more of Cr, while S as an impurity is 0.003% or less. When N is limited to 0.008% or less and O is limited to 0.005% or less, the efficiency is greatly improved.
[0009]
(b) The seawater resistance described above is that the steel surface, particularly the metal structure of the surface layer part of at least 0.5 mm below the surface is turned into a fine-grained structure with an average particle size of 8 μm or less with an area ratio of bainite phase of 50% or more. Then, it improves further.
[0010]
(c) After the hot rolling, the above metal structure is forcibly cooled until the steel surface temperature becomes 600 ° C or lower, and the steel surface temperature is reheated to 750 ° C or higher by internal sensible heat, and then finished. It is obtained by performing finish rolling at a temperature of 700 ° C. or higher and then subjecting it to a predetermined cooling treatment.
[0011]
The gist of the present invention based on the above findings resides in the following (1) to (5) seawater-resistant steel and the following (6) seawater-resistant steel manufacturing method.
[0012]
(1) By mass%, C: 0.04% or less, Si: 1.5% or less, Mn: 5.0% or less, Cr: 1.0-5.0% and sol.Al: 0.003-0.05%, the balance from Fe and impurities becomes, S is 0.003% as impurities less, N is 0.008% or less, O (oxygen) Ri der 0.005% or less, the metal structure of the surface layer portion up to at least subsurface 0.5mm is, the area ratio of the bainite phase 50 % Seawater-resistant steel, characterized by having a fine-grained structure with an average particle size of 8 μm or less.
[0013]
(2) The seawater resistant steel according to (1), further containing P: 0.01 to 0.3% by mass.
[0014]
(3) The above (1) or further containing at least one of Cu: 0.05-1.0%, Ni: 0.05-3.0%, Mo: 0.05-1.0% and W: 0.05-1.0% by mass% The seawater resistant steel according to (2).
[0015]
(4) Further described in any one of (1) to (3) above, further containing at least one of Nb: 0.002 to 0.10%, V: 0.005 to 0.10% and B: 0.0003 to 0.0050% by mass% Seawater resistant steel.
[0016]
(5) From the above (1), which further contains at least one of Ti: 0.005 to 0.10%, Ca: 0.0001 to 0.02%, Mg: 0.0001 to 0.02% and REM: 0.0001 to 0.02% by mass% (4) The seawater resistant steel according to any one of the items up to.
[0018]
(6) After heating the steel having the chemical composition according to any one of (1) to (5 ) above to 1000 to 1200 ° C., heat having a cumulative reduction ratio of 10% or more in a temperature range of Ar 3 points or more. Then, the steel is cooled once at a cooling rate of 15 ° C / second or more until the surface temperature of the steel becomes 600 ° C or less, and reheated until the surface temperature of the steel becomes 750 ° C or more by internal sensible heat. Then, finish rolling at an end temperature of 700 ° C. or lower is performed, and then a cooling process is performed at a cooling start temperature of 650 ° C. or higher, a cooling rate of 5 ° C./second or higher, and a cooling end temperature of 550 ° C. or lower (1 ) To (5) .
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason for defining the present invention as described above will be described.
[0020]
1. Regarding chemical composition C: 0.04% or less C is preferably as low as possible in order to reduce the formation of chromium carbide that tends to be a cathode site in seawater, and is 0.04% or less. Preferred is 0.03% or less, and more preferred is 0.02% or less. Although the lower limit is not specified, excessive reduction leads to an increase in cost, so the lower limit is preferably about 0.001%.
[0021]
Si: 1.5% or less
Si is added as a deoxidizer. The desirable lower limit content in that case is 0.03%. However, if the content exceeds 1.5%, the toughness deteriorates, so the Si content should be 1.5% or less. A preferable upper limit is 1.0%, and a more preferable upper limit is 0.8%.
[0022]
Mn: 5.0% or less
Mn is an element effective as a deoxidizer and for improving seawater resistance and strength. The effect becomes significant when the content is 0.2% or more, but if it exceeds 5.0%, the toughness is deteriorated. For this reason, Mn content was made into 5.0% or less. It is preferably 4.0% or less, more preferably 3.0% or less.
[0023]
Cr: 1.0-5.0%
Cr is an indispensable element in the present invention, and the content thereof is set to 1.0 to 5.0% for the following reason. That is, the seawater resistance is remarkably improved at 1.0% or more, and the pitting corrosion sensitivity is remarkably increased when it exceeds 5.0%. Note that Cr has a function of concentrating in the rust layer and enhancing the protection of rust. Preferable is 2.0 to 4.0%, and more preferable is 2.0 to 3.0%.
[0024]
sol.Al: 0.003 to 0.05%
Al is an element added as a deoxidizer, and at least 0.003% or more of the sol.Al content is necessary to obtain the effect reliably. On the other hand, if it exceeds 0.05%, the weldability decreases. For this reason, the content of Al is set to 0.003 to 0.05% in sol.Al. Preferable is 0.003 to 0.045%, and more preferable is 0.003 to 0.040%.
[0025]
S: 0.003% or less S is made 0.003% or less from the viewpoint of corrosion resistance. That is, non-metallic inclusion MnS dissolves in a seawater environment, and becomes a starting point of selective corrosion, particularly pitting corrosion in Cr-containing steel. For this reason, the lower the S content, the better.
[0026]
N: 0.008% or less N is an important element in the present invention, and like C, it is combined with Cr and precipitated as nitrides such as CrN, reducing the concentration of solid solution Cr effective in corrosion resistance and seawater resistance. Not only deteriorates, but also toughness. For this reason, the smaller the N content, the better. However, if it is 0.008% or less, its harmfulness is small and acceptable. Therefore, the N content is limited to 0.008% or less. A preferable upper limit is 0.006%, and a more preferable upper limit is 0.004%.
[0027]
O (oxygen): 0.005% or less O is an important element in the present invention, as in the case of N described above, and is an oxide such as CaO, MgO, Al 2 O 3 and the like, which is a starting point of pitting corrosion in Cr-containing steel. Corrosion resistance is deteriorated by forming non-metallic inclusions. Excessive O also deteriorates toughness. However, since the effect is small if the content is up to 0.005%, it was set to 0.005% or less. It is preferably 0.004% or less, and more preferably 0.003% or less.
[0028]
P:
P may not be added. If added, it has the effect of improving seawater resistance. For this reason, when it is desired to obtain this effect, it may be added positively, and the effect is obtained with a content of 0.01% or more. However, if the content exceeds 0.3%, the toughness and weldability are significantly reduced. For this reason, the P content when added is preferably 0.01 to 0.3%.
[0029]
Cu, Ni, Mo, W:
These elements need not be added. If added, any element has the effect of improving seawater resistance. For this reason, when it is desired to obtain this effect, one or more of them may be positively added. However, the effect cannot be obtained unless the content of each element is 0.05% or more. On the other hand, the effect of Ni is 3.0% and the other elements are 1.0%, and the effect is saturated. Therefore, the content of these elements when added is preferably 0.05 to 1.0% for Cu, Mo and W, and 0.05 to 3.0% for Ni.
[0030]
Nb, V, B:
These elements need not be added. If added, any element has the effect of improving strength and toughness. For this reason, when it is desired to obtain this effect, one or more of them may be positively added. However, the effect cannot be obtained unless the content of Nb is 0.002% or more, V is 0.005%, and B is 0.0003% or more. On the other hand, when Nb and V are 0.10% and B is 0.0050%, the effect is saturated, and addition beyond that causes a decrease in toughness. Therefore, the content of these elements when added is preferably 0.002 to 0.10% for Nb, 0.005 to 0.10% for V, and 0.0003 to 0.0050% for B.
[0031]
Ti, Ca, Mg, REM:
These elements need not be added. If added, any element has the effect of controlling the form of sulfide in the steel, reducing MnS, which is the starting point of pitting corrosion, and improving the corrosion resistance. For this reason, when it is desired to obtain this effect, one or more of them may be positively added. However, the effect cannot be obtained unless the content of Ti is 0.005% or more, and Ca, Mg, and REM are all 0.0001% or more. On the other hand, Ti is 0.10%, Ca, Mg, and REM are all 0.02%, and the effect is saturated, and addition beyond this leads to a decrease in toughness. Therefore, the content of these elements when added is preferably 0.005 to 0.10% for Ti and 0.0001 to 0.02% for Ca, Mg and REM.
[0032]
The components other than the above components are substantially Fe, in other words, the balance is Fe and impurities other than those described above.
[0033]
2. The seawater-resistant steel of the present invention having the above-mentioned chemical composition with respect to the metal structure is sufficiently resistant to a ferrite structure, a mixed structure of ferrite and pearlite, and a martensite structure, which are general metal structures of this type of steel. Provide seawater.
[0034]
However, as described above, the seawater resistance is further improved in the case of a metal structure in which the area ratio of the bainite phase is 50% or more and the average grain size of the crystal grains is 8 μm or less.
[0035]
The details of why the bainite structure has better seawater resistance than the ferrite structure and the mixed structure of ferrite and pearlite and the martensite structure are unknown, but the finer the crystals, the more the seawater resistance. The reason for this is that the exchange current density of the anodic dissolution reaction is increased by the finer particles and the dissolution of Fe is suppressed.
[0036]
The central part of the steel does not necessarily have to have the fine grain structure mainly composed of bainite, and it is sufficient if only the surface layer portion at least up to 0.5 mm below the surface has a fine grain structure mainly composed of bainite. Ensures the toughness required as a structural material.
[0037]
Here, the reason why the minimum depth when only the surface layer portion is a fine grain structure mainly composed of bainite is set to 0.5 mm is as follows. That is, for seawater-resistant steel, it is usually desired that the corrosion amount in 20 years be 0.5 mm or less, but the corrosion rate of the steel of the present invention is about 25 μm as can be seen from the examples described later. This is because if the corrosion rate is 25 μm / year up to 0.5 mm below the surface, it can be reliably used for 20 years.
[0038]
In addition, there is no problem even if the structure of the surface layer portion is a flat grain structure. Therefore, the particle size in the case of a flat grain structure is obtained by the average value of the grain slices (major axis) in the plate thickness direction and the grain slices (minor axis) in the rolling direction, and the sum of these average values is divided by the number of crystal grains. The value obtained in this way is the average particle size.
[0039]
3. 3. Manufacturing conditions for obtaining seawater-resistant steel whose surface layer is in the above state 3-1. Although it is necessary to make C dissolve at the heating temperature of the material steel, if it is less than 1000 ° C., C cannot be reliably dissolved. On the other hand, when it exceeds 1200 ° C., the γ grains become coarse. For this reason, the heating temperature is desirably 1000 to 1200 ° C. Preferred is 1050 to 1150 ° C.
[0040]
3-2. Cooling on the way and before hot rolling after hot rolling heating before or during rolling, the steel surface layer part is cooled to Ar 3 points or less, and then the surface layer part is made Ac 1 point or more by sensible heat inside the steel. When recuperating, a reverse transformation phenomenon occurs in the surface layer of the steel and the crystal grains become fine. If the rolling is further performed thereafter, γ grains and bainite packets in the surface layer portion and the internal structure thereof are also refined, thereby reducing the precipitation density of carbides at the γ grain boundaries, packets, and bainitic lath interfaces.
[0041]
However, the cumulative rolling reduction in the temperature range of 3 points or more before Ar cooling is less than 10%, the cooling rate during cooling is less than 15 ° C / sec, the surface temperature of the steel exceeds 600 ° C, When the surface temperature is less than 750 ° C., the metal structure of the surface layer portion 0.5 mm below the surface cannot be stably formed into a fine-grained structure having an area ratio of bainite phase of 50% or more and an average particle diameter of 8 μm or less. For this reason, the cumulative reduction rate in the temperature range of 3 points or more before Ar cooling is 10% or more, the cooling rate during cooling is 15 ° C / second or more, the steel surface temperature is 600 ° C or less, and the steel after recuperation The surface temperature is preferably 750 ° C. or higher.
[0042]
Here, the processing temperature and cumulative rolling reduction in the hot rolling before cooling during the course are better as both are higher, so the upper limit is not particularly limited. If it is too large, anisotropy tends to occur, so the upper limit of the processing temperature is preferably about 1000 ° C., and the upper limit of the cumulative rolling reduction is preferably about 80%.
[0043]
Moreover, since the surface temperature of the steel after recuperation after intermediate cooling is better as the surface temperature of the steel after intermediate cooling is lower, the upper limit temperature and the lower limit temperature are not particularly limited. However, if the surface temperature of the steel after reheating is too high, the structure tends to become coarser. Therefore, the upper limit of the surface temperature of the steel after reheating should be about 950 ° C. If the temperature is too low, it becomes impossible to secure a surface temperature of 750 ° C. or higher after recuperation. Therefore, the lower limit temperature of the surface temperature of steel after intermediate cooling is preferably about 400 ° C.
[0044]
Furthermore, the faster the cooling rate during the intermediate cooling, the greater the temperature difference between the steel surface layer and the interior, and sufficient recuperation is possible, so the upper limit is not particularly limited. Since it tends to be uniform, the upper limit is preferably about 60 ° C./second.
[0045]
3-3. There are no special restrictions on finish hot rolling after recuperation and finish hot rolling after cooling reheat after rolling. However, if the finish hot rolling finish temperature exceeds 700 ° C, the cooling start temperature after rolling is less than 650 ° C, the cooling rate is less than 5 ° C / second and the cooling finish temperature exceeds 550 ° C, the toughness inside the steel May deteriorate. Therefore, it is desirable that the finish hot rolling end temperature is 700 ° C. or lower, the cooling start temperature after rolling is 650 ° C. or higher, the cooling rate is 5 ° C./second or higher, and the cooling end temperature is 550 ° C. or lower.
[0046]
Here, the cooling rate in finishing hot rolling cooling is better as it is faster, so the upper limit is not particularly limited, but if it is too fast, the strength becomes too high and the toughness deteriorates, so the upper limit is 20 ° C./second. It is good to be about.
[0047]
【Example】
Thirteen kinds of steels having the chemical compositions shown in Table 1 and Table 2 were prepared, and a steel sheet having a thickness of 12 mm was produced by one or more of the production conditions shown in Table 3.
[0048]
Three test pieces each having a width of 100 mm and a length of 100 mm were sampled from the obtained steel sheet, and the peripheral end face was sealed with a tar epoxy resin to obtain a corrosion test piece.
[0049]
As the corrosion test, a test was conducted in which the corrosion test piece was completely immersed in natural seawater. The test period was 0.5 years.
[0050]
The corrosion rate was determined by removing the corrosion products after the test and calculating the thickness reduction from the weight reduction. In addition, the surface microstructure and the central portion of each steel plate were obtained by optical microscope, and the average grain size was obtained by measuring the size of each lath structure by SEM observation (three tests). Average value of the piece).
[0051]
The test results are summarized in Table 4. In addition, all the values of the results shown in Table 4 are average values of three test pieces.
[0052]
As can be seen from Table 4, the steel sheets of the present invention examples (test numbers 1 to 24) exhibit excellent corrosion resistance with a corrosion rate of 37 μm / year or less. In particular, the corrosion rate of the steel sheets of test numbers 1, 2, 13-15, 17-20, 22 and 24, whose surface layer portion up to 0.5 mm below the surface is a bainite-based structure with an average particle size of 8 μm or less is 21 μm / year or less. slow.
[0053]
On the other hand, the steel plate of the comparative example (test numbers 25 to 30) has a corrosion rate of 107 μm / year or more, which is about three times faster than the steel plate of the present invention, and is inferior in corrosion resistance. That is, the steel plate of test number 25 has a high C content, the steel plate of test number 26 has a short Cr content, and the steel plate of test number 27 has a high S content, so that both have insufficient corrosion resistance. In addition, the steel plates with test numbers 28 to 30 are manufactured by the preferable manufacturing method of the present invention, and the crystal grains are fine, but the fine chemical composition is out of the range defined by the present invention as described above. There is no effect of corrosion, and the corrosion resistance is inferior.
[0054]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
【The invention's effect】
The seawater resistant steel of the present invention has a slow corrosion rate. For this reason, the offshore structure comprised with the seawater-resistant steel of this invention can be used over a long period of time. Moreover, according to the desirable manufacturing method of the present invention, it is possible to stably manufacture a seawater-resistant steel having a slower corrosion rate at a low cost and with high efficiency.
Claims (6)
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| KR100723171B1 (en) | 2005-12-26 | 2007-05-30 | 주식회사 포스코 | Producing method of weather resistable steel having excellent toughness, high strength and low yield ratio for using at the seaside atmosphere |
| KR100723170B1 (en) | 2005-12-26 | 2007-05-30 | 주식회사 포스코 | Producing method of weather resistable steel having high strength for using at the seaside atmosphere |
| KR101129979B1 (en) * | 2009-10-29 | 2012-03-26 | 현대제철 주식회사 | steel having high strength, and method for producing the same |
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