JP3785271B2

JP3785271B2 - High weldability and weatherproof steel

Info

Publication number: JP3785271B2
Application number: JP10254198A
Authority: JP
Inventors: 明宇佐見; 康児田辺; 岳史都築
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1997-10-01
Filing date: 1998-04-14
Publication date: 2006-06-14
Anticipated expiration: 2018-04-14
Also published as: JPH11172370A

Description

【０００１】
【発明の属する技術分野】
本発明は、海浜地区や融雪塩を散布する地区など、高飛来海塩粒子環境で塩害が懸念される大気環境における橋梁、鉄塔などの鋼構造物などに使用される、優れた溶接性と環境の飛来海塩粒子量に応じた耐候性を有する鋼材に関するものである。
【０００２】
【従来の技術】
従来、海岸地帯などの塩害が発生する場所で使用する鋼構造部材の防食としては、普通鋼材の塗装使用、めっき鋼板の使用、溶射やモルタルライニングなどの表面被覆の使用、ステンレスやチタンなどの高合金高耐食材料の使用が挙げられる。さらに、鋼構造物の維持管理費を低減する技術として、耐候性鋼材（ＪＩＳＧ３１４１溶接構造用耐候性鋼）の無塗装使用が挙げられる。
【０００３】
塗装の場合、塗り替えが必然的に必要なために維持管理費がかかるといった問題があった。また、めっきの場合、構造体の溶融めっきでは熱応力による変形やめっきの剥離などといった問題があった。溶射やモルタルライニングの場合も、防食皮膜の剥離や劣化などが問題であった。さらに高合金の耐食材料の場合、材料コストが高く主要構造部材として広く使えないといった問題があった。
【０００４】
耐候性鋼材は、無塗装使用の場合、使用後数年〜１０年で鋼材表面に防食性に優れた緻密な安定さびが形成し、この安定さびがその後の鋼材の腐食の進行を防ぐという鋼材である。鋼構造物には、溶接性を考慮した耐候性溶接構造用鋼が、橋梁や建築物を中心にこれまで多く使用されてきた。しかしながら、「無塗装耐候性橋梁の設計・施工要領（改訂案）：建設省土木研究所、鋼材倶楽部、日本橋梁建設協会、平成５年３月」に示されるように、海浜地区や融雪塩を散布する地区など飛来海塩粒子量が多い地域では、鋼材表面に付着した塩分によって保護性に優れた安定さびの形成が阻害されるため、無塗装使用に適さないといった問題点があった。
【０００５】
耐候性鋼の海浜地区での鋼材の耐候性向上については、例えば特公昭５６−９３５６号公報の発明では、含Ｐ（０．０３〜０．２０％）で溶接性に優れ、かつ海水が関与した腐食環境や一般大気環境で優れた耐候性を有する鋼材が開示されている。また、特開平２−１２５８３９号公報に記載の発明では、低Ｓｉ−Ｐ−Ｃｕ−Ｎｉの複合添加にＣａとＡｌの複合酸化物の添加が有効であるとしている。また、特開平５−５１６６８号公報の発明では、酸化物を鋼材中に微細分散させて鋼材表面のｐＨ低下を抑制することが有効であるとしている。また、特開平７−２４２９９３号公報に記載の発明では、Ｎｉ−Ｃｒ−Ａｌの複合添加が有効であるとしている。
【０００６】
このように、従来の耐候性鋼の欠点である海浜地区での耐候性が優れた鋼材は開発されているが、溶接構造用鋼材として必須である溶接性は、必ずしも十分ではなかった点が問題であった。また、特開平２−１２５８３９号公報に記載の発明および特開平５−５１６６８号公報に記載の発明は、いずれも数％のＮｉ添加鋼を基本成分とした発明であるが、これらの鋼材は海浜地区での耐候性は優れているものの、飛来海塩粒子量の適用限界が不明であったため、実構造物への適用可否を判断することが難しいといった課題があった。
また、発明者らによる研究調査の結果、例えば特公昭５６−９３５６号公報の発明では、含Ｐ（０．０３〜０．２０％）の鋼材でも、溶接継手部の機械的特性に一定以上の品質を求める場合、溶接時の入熱制限や予熱などが不可欠であるといった問題点があった。
【０００７】
一方、耐候性については言及していないが、溶接性を改善した鋼材としては、例えば特公昭６０−２４５７６号公報の発明では、Ｍｎ等を基本成分として含有する鋼に、粒子径を特定したＴｉ酸化物、Ｔｉ酸化物とＴｉ窒化物の複合体を含有させることにより、溶接熱影響部における切欠靭性を改善した溶接用高靭性鋼が得られるとしている。また、特公昭６１−１１７２４５号公報の発明では、特定量のＣ，Ｓｉ，Ｍｎ，Ｐ，Ｎ，Ａｌ，Ｓ，Ｂを含有させ、かつ特定粒子径、特定粒子数のＴｉ酸化物およびＴｉ窒化物＋ＭｎＳの複合体の両者を同時に含有させることにより、溶接熱影響部の低温切欠靭性の優れた鋼材を得られるとしている。
【０００８】
しかし、これらの鋼材は、Ｃｕ，ＮｉおよびＴｉを添加している場合でも、鋼材の海浜地区での耐候性能およびその性能に基づく用途については一切述べられていない。また、使用される環境の飛来海塩粒子量に応じて、好ましいＮｉ添加量の範囲が異なることについても述べられていない。それゆえ、これらの溶接性に優れた鋼板を海浜地区での無塗装使用に適用するためには、海浜地区での耐候性に及ぼす化学成分の影響を改めて別途に検討する必要があった。
【０００９】
本発明は、かかる課題を解決するためになされたもので、塩害が懸念される環境で安定さびを形成して優れた耐候性を示し、かつ溶接構造用鋼として十分な母材強度、靭性および溶接性を有する鋼材を提供するものである。
【００１０】
【課題を解決するための手段】
本発明者らは、耐候性鋼が飛来海塩粒子の多い大気環境で、耐食性に優れた保護さび膜を形成しにくいことに着目し、従来の耐候性鋼の低合金鋼の成分系を基にして研究を重ねてきた。その結果、飛来海塩粒子の少ない内陸部においては鋼材の耐候性向上に有効であるＣｒは、海浜地区や融雪塩を散布する地区などの高飛来海塩粒子環境では、耐候性に対して顕著な悪影響があることが判明した。
【００１１】
また、種々の合金元素について耐候性向上の検討を実施した結果、Ｃｕ−１％以上のＮｉの複合添加が海浜地区での安定さび生成に顕著に作用することが明らかになった。さらに、Ｃｕ−Ｎｉ系の適用限界（安定さびが十分形成する上限の年平均飛来海塩粒子量）は、Ｎｉ添加量でほぼ整理できることが判明した。Ｎｉは高価な添加元素であることから、この知見によって腐食環境の厳しさに応じて最も経済性に優れた鋼材を提供することが可能となった。
【００１２】
次に、Ｃｕ−Ｎｉ系の母材特性について検討した。一般に、Ｃ，Ｐ，Ｓを低減し、Ｓｉ，ＭｎおよびＡｌで脱酸を行うことで、母材の強度、靭性に優れた鋼材が得られるという知見を利用した。さらに、Ｎｉを１％以上添加する本発明鋼の場合でも、耐候性向上を目的としてＴｉを添加することが好ましい。この場合、添加するＴｉに対してＮ含有量を制御することにより、フェライト相中にＴｉＮを微細分散析出することができ、入熱量の大きい溶接条件で生じる溶接熱影響部の靭性劣化を低減することができることが判明した。
【００１３】
さらに、本発明で完成した鋼材は、塗装や溶射などの防食被覆された状態でも、普通鋼や従来の耐候性鋼に比較して、遥かに優れた皮膜耐久性を有することが判明した。これは、皮膜の局所的な欠陥部から下地鋼板の腐食が進行しても、生成したさびが緻密で保護性に優れるため、防食皮膜の更なる剥離や皮膜下腐食の進展を抑制するものと推察される。
【００１４】
本発明は上記知見に基づくものであって、その要旨は下記の通りである。
（１）重量％で、
Ｃ：０．０３〜０．１５％、Ｓｉ：０．０５〜０．５５％、
Ｍｎ：０．３〜２．０％、Ｃｕ：０．３０〜１．００％、
Ｎｉ：１．０〜５．５％、Ａｌ：０．０１０〜０．０９０％、
Ｎ：０．００１０〜０．００７０％、Ｐ：０．０３０％以下、
Ｓ：０．０１０％以下、Ｃｒ：０．１％以下
を含有し、残部がＦｅおよび不可避的不純物からなることを特徴とする高溶接性高耐候性鋼。
（２）さらに重量％で、Ｔｉ：０．００５〜０．０２％を含み、Ｔｉ／Ｎ：２．０〜３．５であることを特徴とする前記（１）記載の高溶接性高耐候性鋼。
（３）さらに重量％で、Ｃａ：０．０００５〜０．０１００％、Ｍｇ：０．０００５〜０．０１００％、ＲＥＭ：０．０００５〜０．０１００％の１種または２種以上を含むことを特徴とする前記（１）または（２）記載の高溶接性高耐候性鋼。
（４）さらに重量％で、Ｍｏ：０．１〜１．０％、Ｗ：０．１〜１．０％の１種または２種を含むことを特徴とする前記（１）乃至（３）のいずれかに記載の高溶接性高耐候性鋼。
（５）さらに重量％で、Ｎｂ：０．００２〜０．０２０％、Ｖ：０．０１〜０．０５％、Ｂ：０．０００３〜０．００５０％のうち１種または２種以上を含むことを特徴とする前記（１）乃至（４）のいずれかに記載の高溶接性高耐候性鋼。
（６）表面が有機樹脂、金属または無機物で被覆されたことを特徴とする前記（１）乃至（５）のいずれかに記載の高溶接性高耐候性鋼。
【００１５】
【発明の実施の形態】
以下、本発明の実施する形態について説明する。まず、本発明の鋼における化学組成の限定理由とその作用について述べる。
Ｃ：Ｃは、構造材料としての強度を確保するために必要であり、０．０３％以上添加するが、０．１５％を超えて含まれると溶接継手部のマトリックスの靭性が低下し、溶接性が阻害されるため、その上限を０．１５％とした。
【００１６】
Ｓｉ：Ｓｉは脱酸のための必須元素で、０．０５％以上添加するが、０．５５％を超えて添加すると、溶接部に高炭素島状マルテンサイトが生成し、溶接性が阻害されるため、その上限を０．５５％とした。
【００１７】
Ｍｎ：Ｍｎは脱酸、強度調整および不純物であるＳをＭｎＳとして固定し、Ｓによる熱間脆性の防止、および後述するＴｉＮの微細分散析出サイトであるＭｎＳの生成のために０．３％以上添加するが、２．０％を超えて添加すると溶接性が阻害されるので、その範囲を０．３〜２．０％とした。
【００１８】
Ｃｕ：Ｃｕは鋼中Ｆｅと共に溶出し、さび層の形成時にさび粒子の結晶・粗大化を抑制し、さびの緻密さを保持するため、飛来海塩粒子の多い環境での耐候性を向上させるのに必須の元素であり、０．３０％以上の添加で有効である。その効果は多いほどよいが、１．０％を超えると溶接性低下や熱間加工における割れが問題となるので、その範囲を０．３〜１．０％とした。溶接性を優先的に考慮すれば、０．３０〜０．５０％が好ましい。
【００１９】
Ｎｉ：Ｎｉは、さび層中に０．５％以上含まれると鋼中Ｆｅと共に溶出し、さび層中にほぼ均一に含まれることにより、さび層表面に飛来海塩粒子として付着したＣｌイオンのさび層／地鉄界面への浸透を抑制し、さび層内部を低Ｃｌ環境としてさび粒子の結晶化・粗大成長を抑制することにより、さび層の緻密さを保持する作用がある。また、鋼中Ｎｉ添加量の増加に従って、Ｃｌイオンを含む水溶液中での乾湿繰り返し腐食環境で鋼材の耐食性を向上する。
本発明者らの研究によれば、さび層中に０．５％以上Ｎｉが含まれるためには、１．０％以上のＮｉ添加が有効であることが明らかとなった。また、５．５％を超えるとコスト高となるので、１．０％〜５．５％とした。好ましくは、使用環境の飛来海塩粒子量に応じてＮｉ添加量をさらに限定すれば、経済性、溶接性共に好ましいことが明らかとなった。使用環境の年平均飛来海塩粒子量が、０．０５〜０．２mg／１００cm²/dayの場合、１．０〜２．５％のＮｉ添加が好ましい。０．２〜０．５mg／１００cm²/dayの場合、２．５〜３．５％のＮｉ添加が好ましい。０．５〜０．８mg／１００cm²/dayの場合、２．５〜５．５％のＮｉ添加が好ましく、０．８mg／１００cm²/dayを超える場合、１．０％〜３．５％のＮｉ添加鋼への塗装などの防食被覆使用が好ましい。
【００２０】
Ａｌ：Ａｌは脱酸元素として０．０１０％以上必要であるが、添加量が多いと介在物が増加するため、上限を０．０９０％とする。
【００２１】
Ｔｉ，Ｎ：Ｔｉは必要に応じて、鋼中に一定量以上かつＮと一定の割合の範囲でＴｉとＮが同時に含まれる場合、ＴｉＮとしてフェライト相中に微細分散析出し、鋼中のＣｕおよびＮｉがＦｅと共に均一に溶出する反応を促進し、Ｃｌイオンの浸透を抑制する緻密なさび層の形成を助長する。この効果を得るには０．００１０％以上のＮと０．００５０％以上のＴｉ添加が必要である。また、０．０２％超のＴｉ、または０．００７０％超のＮを添加すると、粗大な析出物の析出が起こり、靭性を劣化させるのでＴｉ，Ｎの範囲をそれぞれ０．００５〜０．０２％、０．００１０〜０．００７０％とした。
【００２２】
さらにＴｉ／Ｎが２．０未満の場合、ＴｉＮ粒子が十分フェライト相中に析出せず、３．５を超える場合、ＴｉＮ粒子の粗大凝集が生じて微細、分散状態が得られず、耐候性および溶接性の向上が得られないため、Ｔｉ／Ｎの範囲を２．０〜３．５とした。
【００２３】
Ｐ：Ｐは耐候性を向上するのに有効な元素であるが，０．０３０％を超えて含まれると溶接性が劣化するので、その範囲を０．０３０％以下とした。特に大入熱溶接特性を十分確保する場合、０．００５％以下が好ましい。
【００２４】
Ｓ：Ｓは鋼材の靭性や耐候性を劣化させる不可避的不純物であり、少ないほど好ましい。特に０．０１％を超えて含まれると介在物が増加すると共に、継手部フェライト相の靭性を著しく劣化させるので、その範囲を０．０１％以下とした。十分な耐候性を確保するためには、０．００５％以下が好ましい。
【００２５】
Ｃｒ：ＣｒはＦｅよりも卑な金属のため、数％の添加では海塩粒子の多い環境中での耐候性を阻害するほか、溶接性を阻害するため、少なければ少ないほどよい。０．１％以下であれば、耐候性や溶接性への阻害作用はほぼ無視できるので、その範囲を０．１％以下とした。
【００２６】
Ｃａ，Ｍｇ：ＣａおよびＭｇは必要に応じて添加するものであり、これらを添加すると、鋼中に酸化物または硫化物として存在し、地鉄から溶出することにより、Ｃａ（ＯＨ）₂やＭｇ（ＯＨ）₂などを形成して、さびコロイド粒子生成初期の成長を抑制するため、さび粒子の微細析出、凝集を促進する。その効果は０．０００５％以上の添加で有効であり、０．０１００％で飽和するので、それらの元素の範囲を０．０００５％〜０．０１００％とした。
【００２７】
ＲＥＭ：ＲＥＭも必要に応じて添加するものであり、介在物の形態を制御して板厚方向の引張特性を改善し、ラメラーティアの軽減や低温靭性の向上に有効である。このために０．０００５％以上含有するが、添加量が多すぎると介在物が増加するため、上限を０．０１００％とした。
【００２８】
Ｍｏ，Ｗ：ＭｏおよびＷは、必要に応じて０．１％以上添加すると地鉄から溶出したのち、モリブデン酸およびタングステン酸を生成し、さび粒子表面に吸着して凝集したさび粒子間で生じた空隙を負電荷過剰として、Ｃｌイオンや硫酸イオンなどの陰イオンの地鉄界面への浸透を抑制し、耐候性または安定さびを形成する限界飛来海塩粒子量をさらに向上させる作用があるが、その効果は１．０％で飽和する。それゆえ、耐候性の向上を優先する場合には、０．１〜１．０％が好ましい。
【００２９】
また、Ｍｏは母材強度を上昇させる効果があり、厚手材、高強度材などに対して０．０２％以上添加するが、０．２０％を超えて添加すると溶接継手部に上部マルテンサイトや島状マルテンサイトなどを生成して、継手靭性を著しく低下する。それゆえ、溶接性を十分に確保する場合は、０．０２０〜０．２０％が好ましい。
【００３０】
Ｎｂ，Ｖ，Ｂ：Ｎｂ，Ｖ，Ｂは、母材強度を上昇させる効果があり、厚手材、高強度材に対して、Ｎｂは０．００２％以上、Ｖは０．０１％以上、Ｂは０．０００３％以上添加するが、いずれの元素も次の範囲を超えて添加すると靭性が劣化するため、Ｎｂ：０．０２％以下、Ｖ：０．０５％以下およびＢ：０．００５０％以下と限定した。
【００３１】
防食被覆：本発明鋼に、有機樹脂による塗装、金属溶射、またはめっきを施して、塩害が懸念される大気環境で使用した場合、普通鋼や従来の耐候性鋼に同様の防食被覆を施した場合に比べて遥かに優れた耐候性、耐久性を示す。有機樹脂としては、エポキシ樹脂系、フタル酸系、ウレタン樹脂系、ビニルブチラール樹脂系およびその他の樹脂系でいずれも塗装耐久性が向上する。
【００３２】
また、金属被覆では、Ｚｎ，Ｚｎ−Ａｌ，Ａｌめっきおよび溶射などで優れた耐候性を示す。いずれの場合も、被覆層の微視的あるいは巨視的な欠陥から地鉄の腐食が進行した際、Ｎｉ，Ｃｕなどを含有した緻密なさび層が形成され、それ以降の腐食の進展を抑制する。また、特開昭５３−６５２３２号公報、特開昭５５−９７４７７号公報および特開昭５５−９７４７８号公報などに開示されている耐候性鋼の初期さび汁流出防止技術としてのさび安定化処理皮膜を本発明鋼に塗布することにより、高海塩粒子環境でも初期さび汁を防止しながら、安定さびが形成される。
【００３３】
【実施例】
表１（表１−１）及び表２（表１−２）に示す化学組成の鋼を溶製し、熱間圧延および必要に応じて熱処理を施して厚さ２５mmの厚鋼板を試作した。中入熱溶接（５０kJ／cm）および大入熱溶接（１２０kJ／cm）で実継手を製作し、溶接継手部の靭性を−４０℃でのシャルピー衝撃試験の吸収エネルギーで評価した。
【００３４】
また、試作鋼の高海塩粒子環境での耐候性を評価するために、千葉県富津市臨海部の４カ所で暴露試験を１，３，７年実施した。なお、その暴露地点は、離岸距離（平均飛来海塩粒子量）が各々地点Ｖ：５ｍ（１．３ｍｄｄ）、地点Ｗ：５０ｍ（０．８ｍｄｄ）、地点Ｘ：２００ｍ（０．５ｍｄｄ）、地点Ｙ：８００ｍ（０．２ｍｄｄ）とした。
【００３５】
試作鋼の耐候性および適用限界飛来海塩粒子量を次の評価で求めた。すなわち、さびの外観評点評価、さびのイオン透過抵抗測定、腐食量から求めた平均板厚減少量の３項目である。
【００３６】
さびの安定化の状況を、さび層の外観評点１〜４で評価し、４が最も良く、安定さび形成を示し、１が層状の剥離錆が認められ、さびの安定化および腐食進展の防止が期待できない状態を示すという指標により評価を行った。
【００３７】
さびのイオン透過抵抗測定では、交流インピーダンス法によるさびのイオン透過抵抗値を測定し、３ｋΩ以上で緻密な安定さび形成と判断した。平均板厚減少量は、４カ所での腐食量−時間曲線から５０年後の推定板厚減少量を、平均板厚減少量と時間の両対数プロット上で外挿して求め、無塗装橋梁使用の基準である腐食量０．４mm／５０年を無塗装使用可否の目安として、４カ所の暴露試験結果から適用限界飛来海塩粒子量を求めた。
【００３８】
表３（表２−１）及び表４（表２−２）に、試作鋼の特性値、すなわち溶接性および海浜地区での耐候性を示す。表中、さび評点、イオン透過抵抗値は、４カ所のうち飛来海塩粒子量が最も少ない地点Ｙにおける評価結果である。
【００３９】
比較例Ａ１〜Ａ１１は、それぞれＣ，Ｓｉ，Ｍｎ，Ｐ，Ｃｕ，Ｎｉ，Ａｌ，Ｔｉ，Ｎが過剰のため継手靭性値が低く、大入熱溶接性が悪い。また、比較例Ｃ１〜Ｃ７は、Ｃ，Ｓ，Ｃｕ，Ｎｉ，Ｃｒが本発明の範囲外のため、本発明例と比較すると耐候性が不十分である。比較例Ｃ８は、従来の耐候性鋼（ＪＩＳＧ３１４１溶接構造用耐候性鋼板ＳＭＡ４９０）であるが、地点Ｖ〜Ｙの４カ所の中で最も飛来海塩粒子量が少ない地点Ｙでも安定さびが形成されず、無塗装使用に適さないことがわかる。
【００４０】
Ｄ１〜Ｄ４０は本発明鋼の結果であり、いずれの試作鋼も地点Ｙの環境では、さび評点、イオン透過抵抗および５０年後の推定腐食量は、いずれの評価においても優れた耐候性を有することがわかる。また、４カ所の暴露試験結果から推定した限界海塩粒子量は、Ｎｉ添加量でほぼ分類できることがわかる。すなわち、環境の年平均飛来海塩粒子量が
（１）０．０５〜０．２mg／１００cm²/dayの場合、１．０〜２．５％Ｎｉ添加が好ましい。
（２）０．２〜０．５mg／１００cm²/dayの場合、２．５〜３．５％Ｎｉ添加が好ましい。
（３）０．５〜０．８mg／１００cm²/dayの場合、２．５〜５．５％Ｎｉ添加が好ましい。
【００４１】
表５（表３）に、試作鋼に有機樹脂塗装を施し、表面にカッタナイフで１片７０mmのクロスカット傷をつけて地点Ｖ（年平均飛来海塩粒子量１．３ｍｄｄ）に５年間暴露し、クロスカットからの最大塗膜膨れ幅（塗膜下腐食の最大進展幅）を評価して、塗装した試作鋼の当該環境における耐候性の評価を示した。
【００４２】
耐候性向上に必要な成分が本発明範囲外である比較例Ｃ１〜Ｃ８に塗装した供試材は、いずれも最大膨れ幅が最小のものでも３２mmであり、本発明例Ｄ１〜Ｄ４０に塗装した供試材は、最大のものでも１４mmであることから、本発明鋼が塗装を施して使用しても、優れた耐候性を有することがわかる。
【００４３】
【表１】

【００４４】
【表２】

【００４５】
【表３】

【００４６】
【表４】

【００４７】
【表５】

【００４８】
【発明の効果】
上記実施例からも明らかなように、本発明は海浜地区や融雪塩の散布などにより塩害が懸念される地区における橋梁、鉄塔をはじめとする鋼構造物に対して、構造用鋼材として必須特性である溶接性と高飛来海塩粒子でも安定したさびを形成して優れた耐候性を共に有する鋼材を提供するものであり、また、無塗装使用および塗装使用においても優れた耐候性を有することから、いずれの使用方法でも鋼構造物の維持管理費の低減を可能とする。産業上その効果は極めて顕著である。[0001]
BACKGROUND OF THE INVENTION
The present invention has excellent weldability and environment used for steel structures such as bridges and steel towers in an atmospheric environment where salt damage is a concern in high-flying sea salt particle environments, such as beach areas and areas where snow melting salt is sprayed. It relates to a steel material having weather resistance according to the amount of flying sea salt particles.
[0002]
[Prior art]
Conventionally, corrosion protection of steel structural members used in locations where salt damage occurs, such as coastal areas, includes the use of ordinary steel materials, the use of plated steel sheets, the use of surface coatings such as thermal spraying and mortar lining, and the high use of stainless steel and titanium. Use of alloy high corrosion resistance material is mentioned. Furthermore, as a technique for reducing the maintenance cost of the steel structure, there is a non-painting use of weathering steel (JISG 3141 weathering steel for welded structure).
[0003]
In the case of painting, there is a problem that maintenance and management costs are required because repainting is inevitably necessary. In the case of plating, there are problems such as deformation due to thermal stress and peeling of the plating in the hot-dip plating of the structure. In the case of thermal spraying or mortar lining, peeling or deterioration of the anticorrosive film was a problem. Further, in the case of a high alloy corrosion resistant material, there is a problem that the material cost is high and it cannot be widely used as a main structural member.
[0004]
Weather-resistant steel is a steel material that, when used unpainted, forms a precise stable rust with excellent corrosion resistance on the surface of the steel within a few years to 10 years after use, and this stable rust prevents the subsequent corrosion of the steel. It is. For steel structures, weather-resistant welded structural steels that take weldability into account have been used so far, mainly in bridges and buildings. However, as shown in “Design and Construction Guidelines for Unpainted Weatherproof Bridges (Revised Draft): Ministry of Construction, Public Works Research Institute, Steel Club, Japan Bridge Construction Association, March 1993” In areas where the amount of flying sea salt particles is large, such as the area where it is sprayed, the formation of stable rust with excellent protective properties is hindered by the salt attached to the surface of the steel material, which makes it unsuitable for unpainted use.
[0005]
For improving the weather resistance of weathering steel in the beach area, for example, in the invention of Japanese Examined Patent Publication No. 56-9356, it contains P (0.03 to 0.20%) and has excellent weldability, and sea water is involved. Steel materials having excellent weather resistance in a corrosive environment and a general atmospheric environment are disclosed. In addition, in the invention described in Japanese Patent Laid-Open No. 2-125839, the addition of a complex oxide of Ca and Al is effective for the complex addition of low Si—P—Cu—Ni. Further, in the invention of Japanese Patent Laid-Open No. 5-51668, it is said that it is effective to finely disperse the oxide in the steel material to suppress the pH drop on the steel material surface. In addition, in the invention described in Japanese Patent Laid-Open No. 7-242993, the combined addition of Ni—Cr—Al is effective.
[0006]
In this way, steel materials with excellent weather resistance in the beach area, which is a drawback of conventional weather resistant steels, have been developed, but the weldability that is essential as a steel material for welded structures has not always been sufficient. Met. The invention described in Japanese Patent Laid-Open No. 2-12539 and the invention described in Japanese Patent Laid-Open No. 5-51668 are both inventions based on Ni-added steel of several%, but these steel materials are Although the weather resistance in the area is excellent, there was a problem that it was difficult to determine whether to apply to actual structures because the application limit of the amount of flying sea salt particles was unknown.
In addition, as a result of research and investigation by the inventors, for example, in the invention of Japanese Patent Publication No. 56-9356, even with steel containing P (0.03 to 0.20%), the mechanical characteristics of the welded joint portion are more than a certain level. When quality is required, there is a problem that it is indispensable to limit heat input during welding or to preheat.
[0007]
On the other hand, although the weather resistance is not mentioned, as a steel material with improved weldability, for example, in the invention of Japanese Examined Patent Publication No. 60-24576, a steel containing Mn or the like as a basic component, Ti having a specified particle diameter is used. It is said that high toughness steel for welding with improved notch toughness in the weld heat affected zone can be obtained by including a composite of oxide, Ti oxide and Ti nitride. Further, in the invention of Japanese Patent Publication No. 61-117245, a specific amount of C, Si, Mn, P, N, Al, S, B is contained, and a specific particle diameter, a specific number of particles of Ti oxide and Ti nitriding are included. It is said that the steel material excellent in the low temperature notch toughness of the weld heat affected zone can be obtained by simultaneously containing both the composite of the product + MnS.
[0008]
However, even when Cu, Ni, and Ti are added to these steel materials, the weather resistance performance of the steel materials in the beach area and uses based on the performance are not described at all. Moreover, it is not described that the range of the preferable Ni addition amount varies depending on the amount of flying sea salt particles in the environment used. Therefore, in order to apply these steel sheets having excellent weldability to uncoated use in the beach area, it was necessary to separately examine the influence of chemical components on the weather resistance in the beach area.
[0009]
The present invention has been made in order to solve such problems, and exhibits excellent weather resistance by forming stable rust in an environment in which salt damage is a concern, and sufficient base material strength, toughness, and A steel material having weldability is provided.
[0010]
[Means for Solving the Problems]
The present inventors paid attention to the fact that weathering steel is difficult to form a protective rust film with excellent corrosion resistance in an atmospheric environment with a lot of incoming sea salt particles, and based on the low-alloy steel component system of conventional weathering steel. I have been doing research. As a result, Cr, which is effective in improving the weather resistance of steel in inland areas where there are few flying sea salt particles, is prominent in terms of weather resistance in high flying sea salt particle environments such as beach areas and areas where snow melting salt is sprayed. It turns out that there is a bad influence.
[0011]
Moreover, as a result of investigating the improvement in weather resistance of various alloy elements, it has been clarified that the combined addition of Ni of Cu-1% or more significantly affects the formation of stable rust in the beach area. Furthermore, it has been found that the application limit of Cu-Ni system (the amount of annual average sea salt particles that can be sufficiently formed by stable rust) can be roughly arranged by adding Ni. Since Ni is an expensive additive element, this knowledge makes it possible to provide the most economical steel material according to the severity of the corrosive environment.
[0012]
Next, the characteristics of the Cu-Ni base material were examined. In general, the knowledge that a steel material excellent in strength and toughness of a base material can be obtained by reducing C, P and S and deoxidizing with Si, Mn and Al was utilized. Furthermore, even in the case of the steel of the present invention to which Ni is added at 1% or more, it is preferable to add Ti for the purpose of improving the weather resistance. In this case, by controlling the N content with respect to Ti to be added, TiN can be finely dispersed and precipitated in the ferrite phase, thereby reducing the toughness deterioration of the weld heat affected zone occurring under welding conditions with a large heat input. It turns out that you can.
[0013]
Furthermore, it has been found that the steel material completed in the present invention has a much superior film durability compared to ordinary steel and conventional weathering steel even in a state where the steel material is coated with anticorrosion such as painting or spraying. This is because even if corrosion of the underlying steel plate proceeds from local defects in the film, the generated rust is dense and excellent in protection, so that further peeling of the anticorrosion film and progress of subfilm corrosion are suppressed. Inferred.
[0014]
The present invention is based on the above findings, and the gist thereof is as follows.
(1) By weight%
C: 0.03-0.15%, Si: 0.05-0.55%,
Mn: 0.3 to 2.0%, Cu: 0.30 to 1.00%,
Ni: 1.0 to 5.5%, Al: 0.010 to 0.090%,
N: 0.0010 to 0.0070%, P: 0.030% or less,
S: 0.010% or less, Cr: 0.1% or less, with the balance being Fe and inevitable impurities, high weldability and weatherability steel.
(2) Further, by weight%, Ti: 0.005 to 0.02%, Ti / N: 2.0 to 3.5, high weldability and high weather resistance according to (1) above Steel.
(3) Further, by weight%, one or more of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100% are included. The high weldability and weatherproof steel according to (1) or (2) above.
(4) The above (1) to (3), further comprising one or two of Mo: 0.1 to 1.0% and W: 0.1 to 1.0% by weight% The high weldability and weatherproof steel according to any one of the above.
(5) Further, by weight%, Nb: 0.002 to 0.020%, V: 0.01 to 0.05%, B: 0.0003 to 0.0050%, including one or more The high weldability and weatherproof steel according to any one of (1) to (4), wherein:
(6) The high weldability and weatherable steel according to any one of (1) to (5), wherein the surface is coated with an organic resin, a metal, or an inorganic substance.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described. First, the reason for limiting the chemical composition of the steel of the present invention and the action thereof will be described.
C: C is necessary to ensure the strength as a structural material, and is added in an amount of 0.03% or more, but if it exceeds 0.15%, the toughness of the matrix of the welded joint portion decreases, and welding The upper limit was made 0.15% because the property is inhibited.
[0016]
Si: Si is an essential element for deoxidation, and is added at 0.05% or more, but if added over 0.55%, high-carbon island martensite is generated in the weld zone and weldability is hindered. Therefore, the upper limit was made 0.55%.
[0017]
Mn: Mn is 0.3% or more for deoxidation, strength adjustment, fixing S as an impurity as MnS, prevention of hot brittleness due to S, and generation of MnS which is a finely dispersed precipitation site of TiN described later. However, if it exceeds 2.0%, weldability is impaired, so the range was made 0.3 to 2.0%.
[0018]
Cu: Cu elutes together with Fe in steel, suppresses rust particle crystallization and coarsening when forming a rust layer, and maintains the fineness of rust, thus improving the weather resistance in an environment with a lot of incoming sea salt particles It is an essential element, and is effective when added in an amount of 0.30% or more. The greater the effect, the better. However, if it exceeds 1.0%, weldability deterioration and cracks in hot working become problems, so the range was made 0.3 to 1.0%. Considering weldability preferentially, 0.30 to 0.50% is preferable.
[0019]
Ni: When Ni is contained in the rust layer in an amount of 0.5% or more, it elutes together with Fe in the steel, and is almost uniformly contained in the rust layer, so that Cl ions adhered to the surface of the rust layer as flying sea salt particles. It suppresses the penetration of the rust layer / base metal interface, and suppresses crystallization and coarse growth of rust particles by setting the inside of the rust layer as a low Cl environment, thereby maintaining the denseness of the rust layer. Further, the corrosion resistance of the steel material is improved in a dry and wet repeated corrosion environment in an aqueous solution containing Cl ions as the amount of Ni added in the steel increases.
According to the study by the present inventors, it has been clarified that addition of 1.0% or more of Ni is effective for containing 0.5% or more of Ni in the rust layer. Moreover, since it will become expensive if it exceeds 5.5%, it was set as 1.0%-5.5%. Preferably, it is clear that both the economical efficiency and the weldability are preferable if the Ni addition amount is further limited according to the amount of flying sea salt particles in the use environment. When the annual average amount of sea salt particles used is 0.05 to 0.2 mg / 100 cm ² / day, 1.0 to 2.5% Ni addition is preferable. In the case of 0.2 to 0.5 mg / 100 cm ² / day, Ni addition of 2.5 to 3.5% is preferable. For 0.5~0.8mg / 100cm ² / day, preferably Ni addition of 2.5 to 5.5%, if it exceeds ^{0.8mg / 100cm 2 / day, 1.0} % ~3.5% It is preferable to use an anticorrosion coating such as painting on Ni-added steel.
[0020]
Al: Al needs to be 0.010% or more as a deoxidizing element. However, since the inclusion increases when the addition amount is large, the upper limit is made 0.090%.
[0021]
Ti, N: Ti is optionally dispersed in the ferrite phase as TiN when Ti and N are simultaneously contained in the steel in a certain amount or more and in a certain ratio range with N, if necessary. And promotes the reaction of Ni eluting uniformly with Fe and promotes the formation of a dense rust layer that suppresses the penetration of Cl ions. To obtain this effect, 0.0010% or more of N and 0.0050% or more of Ti need to be added. Further, if adding more than 0.02% Ti or more than 0.0070% N, coarse precipitates are precipitated and the toughness is deteriorated, so the range of Ti and N is 0.005 to 0.02 respectively. %, 0.0010 to 0.0070%.
[0022]
Further, when Ti / N is less than 2.0, TiN particles do not sufficiently precipitate in the ferrite phase, and when it exceeds 3.5, coarse aggregation of TiN particles occurs and a fine and dispersed state cannot be obtained, and weather resistance And since the improvement of weldability is not obtained, the range of Ti / N was set to 2.0 to 3.5.
[0023]
P: P is an element effective for improving the weather resistance, but if included over 0.030%, the weldability deteriorates, so the range was made 0.030% or less. In particular, when sufficiently ensuring the high heat input welding characteristics, 0.005% or less is preferable.
[0024]
S: S is an unavoidable impurity that deteriorates the toughness and weather resistance of the steel material, and the smaller the better. In particular, inclusions exceeding 0.01% increase inclusions and significantly deteriorate the toughness of the joint ferrite phase, so the range was made 0.01% or less. In order to ensure sufficient weather resistance, 0.005% or less is preferable.
[0025]
Cr: Since Cr is a base metal than Fe, addition of a few percent inhibits the weather resistance in an environment where there are many sea salt particles, and also inhibits weldability. If it is 0.1% or less, the inhibitory effect on weather resistance and weldability can be almost ignored, so the range was made 0.1% or less.
[0026]
Ca, Mg: Ca and Mg are added as necessary. When these are added, they are present in the steel as oxides or sulfides, and are eluted from the ground iron to cause Ca (OH) ₂ and Mg. (OH) _{2 and the} like are formed to suppress the initial growth of rust colloidal particles, thereby promoting fine precipitation and aggregation of rust particles. The effect is effective when 0.0005% or more is added, and is saturated at 0.0100%. Therefore, the range of these elements is set to 0.0005% to 0.0100%.
[0027]
REM: REM is also added as necessary, and is effective in reducing lamellar tear and improving low-temperature toughness by controlling the form of inclusions to improve the tensile properties in the thickness direction. For this reason, it contains 0.0005% or more, but if the addition amount is too large, inclusions increase, so the upper limit was made 0.0100%.
[0028]
Mo, W: Mo and W are dissolved between the rust particles, which are eluted from the ground iron and added to molybdic acid and tungstic acid when adsorbed on the surface of the rust particles. However, it has the effect of further increasing the amount of limit sea salt particles that form weather resistance or stable rust by suppressing the penetration of negative ions such as Cl ions and sulfate ions into the iron-iron interface by making the voids excessively negatively charged. The effect is saturated at 1.0%. Therefore, when priority is given to improving the weather resistance, 0.1 to 1.0% is preferable.
[0029]
Mo has the effect of increasing the strength of the base metal and is added to 0.02% or more of thick materials, high strength materials, etc., but if added over 0.20%, upper martensite and Island martensite and the like are generated, and the joint toughness is significantly reduced. Therefore, when sufficient weldability is ensured, 0.020 to 0.20% is preferable.
[0030]
Nb, V, B: Nb, V, B has an effect of increasing the strength of the base material. Nb is 0.002% or more, V is 0.01% or more, and B is thick or high-strength material. 0.0003% or more is added, but if any element is added beyond the following range, toughness deteriorates, so Nb: 0.02% or less, V: 0.05% or less, and B: 0.0050% Limited to:
[0031]
Anti-corrosion coating: The steel of the present invention is coated with organic resin, metal sprayed or plated, and when used in an air environment where salt damage is a concern, the same anti-corrosion coating is applied to ordinary steel and conventional weather-resistant steel. It shows far better weather resistance and durability than the case. As the organic resin, an epoxy resin type, a phthalic acid type, a urethane resin type, a vinyl butyral resin type and other resin types all improve the coating durability.
[0032]
Further, the metal coating exhibits excellent weather resistance due to Zn, Zn-Al, Al plating, thermal spraying, and the like. In either case, a dense rust layer containing Ni, Cu or the like is formed when the corrosion of the iron core proceeds from microscopic or macroscopic defects in the coating layer, and the subsequent progress of corrosion is suppressed. . Further, rust stabilization treatment as a technique for preventing initial rust juice outflow of weathering steel disclosed in JP-A-53-65232, JP-A-55-97477, JP-A-55-97478, etc. By applying the film to the steel of the present invention, stable rust is formed while preventing initial rust juice even in a high sea salt particle environment.
[0033]
【Example】
Steels having the chemical compositions shown in Table 1 (Table 1-1) and Table 2 (Table 1-2) were melted and subjected to hot rolling and heat treatment as necessary to produce a 25 mm thick steel plate. Real joints were manufactured by medium heat input welding (50 kJ / cm) and large heat input welding (120 kJ / cm), and the toughness of the welded joints was evaluated by the absorbed energy of the Charpy impact test at -40 ° C.
[0034]
In addition, in order to evaluate the weather resistance of the prototype steel in a high sea salt particle environment, exposure tests were conducted for 1, 3 and 7 years at four locations in the coastal area of Futtsu City, Chiba Prefecture. In addition, the shoreline distance (average amount of incoming sea salt particles) is point V: 5 m (1.3 mdd), point W: 50 m (0.8 mdd), point X: 200 m (0.5 mdd), Point Y: 800 m (0.2 mdd).
[0035]
The weather resistance and the application limit flying sea salt particle amount of the prototype steel were obtained by the following evaluation. That is, there are three items: rust appearance rating evaluation, rust ion permeation resistance measurement, and average plate thickness reduction obtained from the amount of corrosion.
[0036]
The rust stabilization status was evaluated with the appearance score 1 to 4 of the rust layer, 4 being the best, indicating stable rust formation, 1 showing layered peeling rust, rust stabilization and prevention of corrosion progression The evaluation was based on an indicator that indicates a state that cannot be expected.
[0037]
In the rust ion permeation resistance measurement, the rust ion permeation resistance value was measured by the AC impedance method, and it was judged that a dense stable rust was formed at 3 kΩ or more. Average thickness reduction amount is obtained by extrapolating the estimated thickness reduction amount after 50 years from the corrosion amount-time curve at four locations on the logarithmic plot of average thickness reduction amount and time, and using unpainted bridges. Corrosion amount of 0.4 mm / 50 years, which is the standard for the above, was used as an indication of whether or not unpainted can be used.
[0038]
Table 3 (Table 2-1) and Table 4 (Table 2-2) show the characteristic values of the prototype steel, that is, the weldability and the weather resistance in the beach area. In the table, the rust score and the ion permeation resistance value are the evaluation results at the point Y where the amount of flying sea salt particles is the smallest among the four locations.
[0039]
In Comparative Examples A1 to A11, since C, Si, Mn, P, Cu, Ni, Al, Ti, and N are excessive, the joint toughness value is low and the high heat input weldability is poor. Moreover, since C, S, Cu, Ni, and Cr are outside the scope of the present invention in Comparative Examples C1 to C7, the weather resistance is insufficient when compared with the present invention. Comparative Example C8 is a conventional weathering steel (JIS G3141 weathering steel plate for welded structure SMA490), but stable rust is formed even at point Y where the amount of sea salt particles is the smallest among four points V to Y. It turns out that it is not suitable for unpainted use.
[0040]
D1 to D40 are the results of the steels of the present invention. In any of the prototype steels, the rust rating, ion permeation resistance, and estimated corrosion amount after 50 years have excellent weather resistance in any evaluation in the environment of point Y. I understand that. It can also be seen that the limit sea salt particle amount estimated from the exposure test results at four locations can be roughly classified by the amount of Ni added. That is, when the average amount of sea salt particles in the environment is (1) 0.05 to 0.2 mg / 100 cm ² / day, addition of 1.0 to 2.5% Ni is preferable.
(2) In the case of 0.2 to 0.5 mg / 100 cm ² / day, addition of 2.5 to 3.5% Ni is preferable.
(3) In the case of 0.5 to 0.8 mg / 100 cm ² / day, 2.5 to 5.5% Ni addition is preferable.
[0041]
In Table 5 (Table 3), the prototype steel is coated with an organic resin, the surface is cut with a cutter knife with a 70 mm crosscut, and exposed to point V (average amount of sea salt particles 1.3 mdd per year) for 5 years. Then, the maximum coating blister width from the crosscut (maximum progress width of corrosion under the coating) was evaluated, and an evaluation of the weather resistance of the coated prototype steel in the environment was shown.
[0042]
The test materials coated on Comparative Examples C1 to C8 whose components necessary for improving the weather resistance were outside the scope of the present invention were all 32 mm even when the maximum swollen width was the smallest, and were coated on Invention Examples D1 to D40. Since the maximum specimen is 14 mm, it can be seen that the steel of the present invention has excellent weather resistance even if it is used after being coated.
[0043]
[Table 1]

[0044]
[Table 2]

[0045]
[Table 3]

[0046]
[Table 4]

[0047]
[Table 5]

[0048]
【The invention's effect】
As is apparent from the above examples, the present invention has essential characteristics as a structural steel material for steel structures including bridges and steel towers in the coastal area and areas where salt damage is a concern due to the application of snowmelt salt. It provides a steel material that has both good weldability and stable rust even with high flying sea salt particles and has excellent weather resistance, and also has excellent weather resistance in both unpainted and painted applications Any method of use makes it possible to reduce the maintenance cost of steel structures. The effect is very remarkable in industry.

Claims

% By weight
C: 0.03-0.15%,
Si: 0.05 to 0.55%,
Mn: 0.3 to 2.0%,
Cu: 0.30 to 1.00%,
Ni: 1.0 to 5.5%,
Al: 0.010 to 0.090%,
N: 0.0010 to 0.0070%
Inevitably further,
P: 0.030% or less,
S: 0.010% or less,
Cr: Highly weldable and weatherable steel containing 0.1% or less, with the balance being Fe and inevitable impurities.

The steel of claim 1 further in weight percent,
Ti: 0.005-0.02%
And the ratio of Ti and N is 2.0 ≦ Ti / N ≦ 3.5
Highly weldable and weatherproof steel characterized by

The steel according to claim 1 or 2, further in wt%,
Ca: 0.0005 to 0.0100%,
Mg: 0.0005 to 0.0100%,
REM: 0.0005 to 0.0100%
High weldability high weatherability steel characterized by including 1 type or 2 types or more.

The steel according to any one of claims 1 to 3, further in wt%,
Mo: 0.1 to 1.0%,
W: 0.1 to 1.0%
A high-weldability and weather-resistant steel characterized by containing one or two of the following.

The steel according to any one of claims 1 to 4, further in wt%,
Nb: 0.002 to 0.020%,
V: 0.01 to 0.05%,
B: 0.0003 to 0.0050%
High weldability high weatherability steel characterized by including 1 type or 2 types or more.

The high weldability and weatherproof steel according to any one of claims 1 to 5, wherein the surface has an anticorrosion coating of an organic resin, a metal, or an inorganic substance.