JP3930643B2

JP3930643B2 - Corrosion resistant steel

Info

Publication number: JP3930643B2
Application number: JP16347698A
Authority: JP
Inventors: 謙治加藤; 英幸中村; 英俊新頭
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1998-06-11
Filing date: 1998-06-11
Publication date: 2007-06-13
Anticipated expiration: 2018-06-11
Also published as: JPH11350084A

Description

【０００１】
【発明の属する技術分野】
本発明は耐食鋼に係り、更に詳しくは、例えば、（１）自動車や船舶等の内燃機関排気系統、ボイラ排気系統、低温熱交換機、焼却炉床等の高温湿潤腐食環境、（２）橋梁、支柱、建築内外装材、屋根材、建具、厨房部材、各種手すり、ガードレール、各種フック、ルーフドレイン、鉄道車両等の大気腐食環境、（３）各種貯蔵タンク、支柱、杭、矢板等の土壌腐食環境、（４）缶容器、各種容器、低温熱交換機、浴室部材、自動車構造部材等の結露腐食環境（冷凍、湿潤、乾燥が複合する腐食環境を含む）、（５）貯水槽、給水管、給湯管、缶容器、各種容器、食器、調理機器、浴槽、プール、洗面化粧台等の水道水腐食環境、（６）缶容器、各種容器、食器、調理機器等の飲料水腐食環境、（７）各種鉄筋構造物、支柱等のコンクリート腐食環境、（８）船舶、橋梁、杭、矢板、海洋構造物等の海水腐食環境等の、種々の腐食環境において優れた耐食を有する鋼に関する。
【０００２】
【従来の技術】
従来、自動車を中心とする内燃機関の排気系統には、内面あるいは外面からの腐食を抑制するために普通鋼にアルミニウムメッキや亜鉛メッキを施した鋼が使用されてきた。環境汚染を抑制するために排気ガス浄化の目的で触媒等が排気系統に具備されたためにこうしたメッキ鋼材では耐食性が充分ではなくなり、鋼素地の耐食性向上を目的として５〜１０％のＣｒを含有させた鋼が、特開昭６３−１４３２４０号公報や特開昭６３−１４３２４１号公報で開示されている。しかし、近年の車両の使用期間および保証期間の延長に伴って、更にＣｒを１８％程度まで含有させ、あるいは更にＭｏを添加した高級ステンレス鋼が排気系統に多く使用されている。
【０００３】
しかし、このような高級ステンレス鋼であっても孔食状の局部腐食が発生する場合があるなど、耐食性は必ずしも充分ではない。また、こうした高級ステンレス鋼はＣｒやＭｏを多量に含有するために加工性が悪く、排気系部材のような複雑な形状を形成するためには、製造に非常な困難を伴い、製造工程が著しく複雑になるために加工コストも高くなるという難点がある。かつ、素材コストも高い。
【０００４】
上記の排気系統を代表として、一般にＣｒをある程度含有する鋼では使用腐食環境が厳しくなると局部腐食が発生し易く、これに対する手段として腐食に対する抵抗を向上させるためには、更にＣｒあるいはＭｏの含有量を増加させるのが極めて一般的な技術的手段であった。また、ＣｒおよびＭｏを用いて耐食性を保有させる場合、排気ガス環境に対しては充分な耐食性を有する場合でも、米国やカナダの寒冷地のように、冬季に道路路面の凍結を防止する目的で多量の塩を散布する場合には、かかる塩分によって外面から排気系部材が侵食されることも問題となっている。
【０００５】
近年、特開平５−２７９７９１号公報、特開平６−１７９９４９号公報、特開平６−１７９９５０号公報、特開平６−１７９９５１号公報、特開平６−２１２２５６号公報、特開平６−２１２２５７号公報、特開平７−３３８８号公報において、耐食性の向上あるいは耐食性と加工性の向上を目的としたＣｒにＡｌを添加した鋼が開示されている。これらの鋼は、排気系内面耐食性あるいは排気系内面耐食性と加工性の向上にはある程度有効と認められるが、塩害耐食性を中心とする湿潤耐食性に関しては改善の余地を残しているのが現状である。
【０００６】
【発明が解決しようとする課題】
本発明は、こうした現状に鑑みて、内燃機関の排気系統をはじめとする高温湿潤腐食環境、結露腐食環境、更には大気腐食環境、水道水腐食環境、土壌腐食環境、コンクリート腐食環境、海水腐食環境、飲料水腐食環境等の様々な腐食環境における耐食性の優れた低コストの耐食鋼を提供することを目的としている。
【０００７】
【課題を解決するための手段】
本発明者らは上記の目的を達成すべく、内燃機関の排気系統をはじめとする高温湿潤腐食環境、結露腐食環境、更には大気腐食環境、水道水腐食環境、土壌腐食環境、コンクリート腐食環境、海水腐食環境、飲料水腐食環境等の様々な腐食環境において優れた耐食性を有する鋼を開発するべく、種々の観点から検討してきた。
【０００８】
まず、本発明者らは最も腐食に対して厳しい排気系統の内面腐食環境について検討し、内燃機関排気系統の腐食は排気ガス中に含まれる塩化物、硫酸イオン等が８０〜１５０℃に加熱された環境において起こることを見出した。
更に、該腐食環境において耐食性を向上させる手段を種々検討した結果、Ａｌを０．１％〜１０％以下添加したＣｒを１０〜３０％含む鋼が排気系統をはじめとする腐食環境で非常に優れた耐食性を示すことを見出した。
【０００９】
更に本発明者らはより優れた鋼にせんとして検討を続けた結果、Ｃｒを１０〜３０％、Ａｌを０．１％〜１０％以下含有する鋼のＣおよびＮを低減した上でＮｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆを特定の条件を満足するように添加すると、耐食性の改善と加工性の向上に効果があること、脱酸および強化元素としてはＳｉおよびＭｎが適切であること、上記の鋼にＣｕ，Ｍｏ，Ｓｂ，Ｎｉ，Ｗを単独あるいは組み合わせて添加するとより優れた耐食性が得られることを見出した。
【００１０】
一方で本発明者らは、排気系統の内面腐食環境に次ぐ厳しい腐食環境である塩害腐食や塩水等の乾湿繰り返しに対する腐食抵抗を高めた鋼材を得る手段についても別途並行して検討した結果、Ｃｒを１０〜３０％含有し、Ａｌを０．１％以上含有する鋼を基材として、その表面に、水溶液環境における電位が基材よりも卑なる金属の層を形成すると、優れた耐食性、特に優れた耐塩害腐食性が得られることを見い出した。特に基材中へのＡｌの添加は、基材表面に被覆した電位が基材よりも卑なる金属の層が部分的に消失し、ごくわずかに基材表面が腐食環境に露出した後の耐食性向上に効果が顕著であることを見出した。
【００１１】
このＡｌの基材中への添加による耐食性向上挙動は、上述した基材の耐食性とは全く異なる現象であり、基材表面に被覆した電位が基材よりも卑なる金属の層が存在し、かつ、この金属層が部分的に消失したときに初めて認められる現象であることを確認している。
見出した著しい耐食性向上の理由には現状では不明点が多いが、基材にＡｌを添加することで、基材表面が腐食環境に露出した後の残存する基材表面に被覆した、電位が基材よりも卑なる金属の層の消失速度が著しく低下し、従って基材表面の腐食環境に露出される面積の拡大速度が著しく低下し、同時に、露出した基材部分に対する基材表面に被覆した電位が基材よりも卑なる金属の層の基材露出部分に対する保護作用が長期にわたって継続することによって、耐食性が向上していることを確認している。
【００１２】
このような効果が認められる基材表面の腐食環境への露出部分は顕微鏡観察で初めて確認できるもので、絶対値では約０．０５mm²程度以下の微小面積で、全腐食面積に対する比率では０．２％以下のわずかなものであり、実使用を想定した巨視的な肉眼外観上は全く赤錆の発生がないことはもとより、一般的には基材の腐食が認められないと判断される外観を呈している。しかもこのような状態が長期間維持されることが特徴である。
【００１３】
従来の知見では、基材表面に被覆した電位が基材よりも卑なる金属の層が存在し、かつ、この金属層が部分的に消失したときには、表面に被覆した金属層の腐食速度は基材金属の腐食を抑制するために増大し、従って基材金属の腐食環境への露出面積は急速に拡大し、速やかに基材金属の腐食に移行すると考えるのが一般的であることを考えると、基材金属中へのＡｌの添加による上述した耐食性向上は、従来全く知られていなかった本願発明の根本となる耐食性向上手段であり、本願発明はこのような新たな発見に基づいてなされたものである。
【００１４】
更に耐食性の向上を目的に引き続き検討を行い、このような基材中へのＡｌの添加に伴い、鋼中に従来安定的に添加することが困難とされてきた各種元素を添加することが可能となることを新たに見出した。特に、Ａｌを０．１〜１０％含有する鋼に、Ｍｇを添加することでより一層、上述した基材と被覆金属の相互作用による耐食性向上が著しいことを新たに見出した。
【００１５】
本発明者らは、基材表面に被覆した電位が基材よりも卑なる金属の層が存在する基材金属中へのＡｌとＭｇの同時添加による耐食性向上効果を更に高めるための手段について種々検討を重ね、基材へのＳｉ，Ｍｎ，Ｎｂ，ＶＴｉ，Ｚｒ，Ｔａ，Ｈｆ，Ｃｕ，Ｍｏ，Ｓｂ，Ｎｉ，Ｗの添加が有効であることを明らかにした。
【００１６】
更に、本発明者らは検討を続け、水溶液環境における電位が基材よりも卑なる金属として、アルミニウム、アルミニウムを主体とする合金、亜鉛、亜鉛を主体とする合金、クロム、クロムを主体とする合金、マンガン、マンガンを主体とする合金、が本発明の目的に適する金属であることをも見出した。
これらの被覆金属についても、基材との相互作用による耐食性向上の観点から種々の検討を重ねた結果、アルミニウム、アルミニウムを主体とする合金、亜鉛、亜鉛を主体とする合金、クロム、クロムを主体とする合金、マンガン、マンガンを主体とする合金、といった金属のいずれかにＭｇ，Ｉｎのうちいずれか一種以上を、質量％で０．０５％以上、１０％以下含有せしめたものがより一層優れた耐食性を実現することを見出した。
【００１７】
本発明は主に上記の知見に基づいてなされたものであり、本願第１発明の要旨は、質量％で、
Ｓｉ：０．０１〜３．０％、
Ｍｎ：０．０１〜３．０％、
Ｃｒ：１０〜３０％、
Ａｌ：０．１〜１０％、
Ｍｇ：０．０００３〜０．０５％、
残部Ｆｅおよび不可避的不純物からなり、該不可避的不純物のうちＣを０．０２％以下、Ｐを０．０３％以下、Ｓを０．０１％以下、Ｎを０．０２％以下、に制限した鋼を基材とし、該基材の表面に、水溶液環境における電位が該基材よりも卑なる金属の被覆層を０．０５〜５００μｍの厚さに形成したことを特徴とする耐食鋼にある。
【００１８】
第２発明の要旨は、第１発明の鋼において、基材が付加成分として更に、質量％で、
Ｃｕ：０．０１〜５．０％、
Ｍｏ：０．０５〜１０％、
Ｓｂ：０．０１〜０．５％、
Ｎｉ：０．０１〜１０％、
Ｗ：０．０５〜３．０％、
の１種または２種以上を含有することを特徴とする記載の耐食鋼にある。
【００１９】
第３発明の要旨は、第１発明、第２発明の鋼において、基材が付加成分として更に、質量％で、
希土類元素：０．００１〜０．１％、Ｃａ：０．０００１〜０．０５％の１種または２種以上を含有することを特徴とする耐食鋼にある。
第４発明の要旨は、第１発明、第２発明、第３発明の鋼において、基材が付加成分として更に、質量％で、Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆの中から選ばれる１種あるいは２種以上の元素を単独含有量で０．０１〜１％含有し、かつ次式を満足することを特徴とする耐食性および加工性の優れた鋼にある。
【００２０】
Ｎｂ／９３＋Ｖ／５１＋Ｔｉ／４８＋Ｚｒ／９１＋Ｔａ／１８１＋Ｈｆ／１７９≧０．８×（Ｃ／１２＋Ｎ／１４）
第５発明の要旨は、第１発明、第２発明、第３発明、第４発明において、被覆層の金属が、アルミニウムあるいはアルミニウムを主体とする合金のいずれかである耐食性並びに耐食鋼にある。
【００２１】
第６の発明の要旨は、第１発明、第２発明、第３発明、第４発明において、被覆層の金属が、亜鉛あるいは亜鉛を主体とする合金のいずれかである耐食鋼にある。
第７発明の要旨は、第１発明、第２発明、第３発明、第４発明において、被覆層の金属が、クロムあるいはクロムを主体とする合金のいずれかである耐食鋼にある。
【００２２】
第８発明の要旨は、第１発明、第２発明、第３発明、第４発明において、被覆層の金属が、マンガンあるいはマンガンを主体とする合金のいずれかである耐食鋼にある。
第９発明の要旨は、第５発明、第６発明、第７発明、第８発明の鋼において、被覆層の金属が更に、質量％で、ＭｇおよびＩｎのうちの少なくとも一種を単独含有量で０．０５〜１０％含有することを特徴とする耐食鋼にある。
【００２３】
【発明の実施の形態】
以下に本発明において、基材の各成分の範囲を限定した理由を述べる。
Ｓｉ：Ｓｉは、基材表面に、水溶液環境における電位が基材よりも卑なる金属の層を０．０５〜５００μｍ厚さに形成せしめた場合の耐食性を向上する効果をもたらすが、０．０１％未満では効果が認められず、３％を超えて添加してもその効果が飽和する。従って、含有量範囲を０．０１％以上３％以下に限定する。更にＣｒを１０％以上含有する鋼にＳｉを添加することで脱酸剤および強化元素としての添加が有効であるが、含有量が０．０１５％未満ではその脱酸効果が充分ではなく、１．５％以上を含有するともはやその効果は飽和している上に加工性をやや低下させる。従って、０．０１５％以上１．５％以下の範囲で添加することがより望ましい。
【００２４】
Ｍｎ：Ｍｎは、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、０．０１％未満では効果が認められず、３％を超えて添加してもその効果が飽和する。従って、含有量範囲を０．０１％以上３％未満に限定する。更にＭｎは鋼の脱酸剤として有効で、０．０５％以上を含有させる必要があるが、１．２％を超えて含有させてもその効果はもはや飽和しているばかりか、過剰にＭｎを含有させると加工性が低下する。従って、０．０５％以上１．２％以下の範囲で添加することがより望ましい。
【００２５】
Ｃｒ：Ｃｒは、Ａｌを０．１％以上含有する鋼にＣｒを添加することで、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、１０％未満では効果が十分ではなく、一方３０％以上添加してもその効果が飽和する。従ってＣｒの含有量は１０％以上３０％以下に限定する。更に排気ガス環境等に対する基材単体での耐食性を確保するためにＡｌを０．１％以上含有する鋼に１０％以上を含有させることが必要であるが、２５％を超えて含有させても加工性が低下する。従って、１０％以上２５％以下の範囲で添加することがより望ましい。
【００２６】
Ａｌ：Ａｌは、本発明において耐食性を確保するために最も重要な元素であって、Ｃｒを１０％以上３０％以下含有する鋼にＡｌを添加することで、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、０．１％未満では効果が十分ではなく、１０％を超えて添加してもその効果が飽和するものであるから、Ａｌの含有量は０．１％以上１０％以下に限定する。より安定な効果を得るためにはＡｌ含有量を０．３％以上とすることが望ましいが、加工性を低下させないために４％以下とすることが望ましい。したがって、Ａｌ含有量は０．３％以上４％以下とすることが望ましい。
【００２７】
Ｍｇ：Ｍｇは、本発明において耐食性を確保するためにＡｌにつぐ重要な元素であって、Ａｌを０．１％〜１０％、Ｃｒを１０％以上３０％以下含有する鋼に、Ｍｇを添加することで、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、０．０００３％未満では効果が十分ではなく、０．０５％を超えて添加してもその効果が飽和するものであるから、Ｍｇの含有量は０．０００３％以上０．０５％以下に限定する。
【００２８】
Ｃ，Ｎ：ＣおよびＮは、鋼板の加工性を低下させる上に、ＣはＣｒと炭化物を生成して耐食性を低下させるので、またＮは靱性を低下させるので、ＣおよびＮ量は少ない方が望ましく、上限含有量はいずれも０．０２％とし、いずれも少ないほど好ましい。更に、優れた加工性を確保するためには、Ｃ＋Ｎの合計量を低減する必要があり、本発明の望ましい態様による鋼としては、Ｃ＋Ｎを０．０３％以下とする。
【００２９】
Ｐ：Ｐは、多量に存在すると靱性を低下させるので少ない方が望ましく、上限含有量は０．０３％とする。
Ｓ：Ｓも、多量に存在すると耐孔食性を低下させるので少ない方が望ましく、上限含有量は０．０１％とする。
以上が本発明が対象とする耐食性に優れた鋼の基材の基本的成分であるが、本発明においては、必要に応じて更に以下の元素を添加して耐食性を一段と向上させた鋼材も対象としている。
【００３０】
Ｃｕ：Ｃｕは、Ａｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、０．０１％未満では効果が認められず、一方５％を超えて添加してもその効果が飽和する。従って、その範囲を０．０１％以上５％以下の範囲に限定する。更に０．０５％以上添加すると、基材単体での全面腐食に対する抵抗を向上させる効果があり、２．５％を超えて添加するとその効果は飽和する。従って、０．０５％以上２．５％以下の範囲で添加することがより望ましい。
【００３１】
Ｍｏ：ＭｏはＡｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼に添加することで、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、０．０５％未満では効果が認められず、一方１０％を超えて添加してもその効果が飽和する。従って、その範囲を０．０５％以上１０％以下に限定する。更にＭｏは０．１％以上添加すると、基材単体での孔食の発生と成長を抑制する効果があるが、３．０％を超えて添加してもその効果は飽和するばかりか加工性を低下させる。従って、０．１％以上３％以下の範囲で添加することがより望ましい。
【００３２】
Ｓｂ：Ｓｂは、Ａｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼に０．０１％以上添加すると、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、０．０１％未満では効果が認められず、一方０．５％を超えて添加してもその効果が飽和する。従って、その範囲を０．０１％以上０．５％以下に限定する。更にＳｂを０．０１５％以上添加することで、基材単体での孔食および全面腐食に対する抵抗を向上させる効果があるが、０．３％を超えて添加すると熱間加工性をやや低下させる。従って、０．０１５％以上０．３％以下の範囲で添加することがより望ましい。
【００３３】
Ｎｉ：Ｎｉは、Ａｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼に０．０１％以上添加すると、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、一方１０％を超えて添加してもその効果が飽和する。従って、その範囲を０．０１％以上１０％以下に限定する。更にＮｉを０．１％以上添加することで、基材単体での孔食を抑制する効果があるが、６％を超えて添加しても効果が飽和する。従って、０．１％以上６％以下の範囲で添加することがより望ましい。
【００３４】
Ｗ：Ｗは、Ａｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼に０．０５％以上添加すると、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、一方３％を超えて添加してもその効果が飽和する。従って、その範囲を０．０５％以上３％以下に限定する。更にＷを添加することで、基材単体での孔食の発生と成長を抑制する効果があるが、０．１％未満では効果は十分ではなく、一方２．０％を超えて添加しても効果が飽和するばかりか加工性を低下させる。従って、０．１％以上２％以下の範囲で添加することがより望ましい。
【００３５】
希土類元素（ＲＥＭ）、Ｃａ：希土類金属（ＲＥＭ）やＣａはＡｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼に添加することで、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、ＲＥＭでは０．００１％未満では効果が認められず、Ｃａでは０．０００１％未満では効果が認められず、一方ＲＥＭでは０．１％を超えて、Ｃａでは０．０５％を超えて添加してもその効果が飽和する。従って、ＲＥＭの範囲を０．００１％以上０．１％以下、Ｃａの範囲を０．０００１％以上０．０５％以下にそれぞれ限定する。更にＲＥＭおよびＣａは熱間加工性の向上と基材単体での耐孔食性の改善に効果のある元素であるが、添加量がＲＥＭでは０．０１％未満、Ｃａでは０．００５％未満ではその効果が充分ではなく、Ｃａでは０．０１％を、ＲＥＭでは０．０５％を超えて添加すると、それぞれ粗大な非金属介在物を生成して逆に熱間加工性や耐孔食性を劣化させるので、上限含有量はＲＥＭでは０．１％、Ｃａでは０．０３％とした。従って、Ｃａは０．００５％以上０．０１％以下の範囲で、ＲＥＭは０．０１％以上０．０５％以下の範囲でそれぞれ添加することがより望ましい。なお、本発明において希土類元素とは原子番号が５７〜７１番および８９〜１０３番の元素およびＹを指す。
【００３６】
Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆ：Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，ＨｆはＡｌを０．１％以上含有しＣｒを１０％以上３０％以下含有する鋼に添加することで、基材表面に水溶液環境における電位が基材よりも卑なる金属の層を０．５〜５００μｍ厚さに形成せしめた場合に耐食性を向上する効果をもたらすが、各元素共に０．０１％未満では効果が認められず、一方約１．０％を超えて添加してもその効果が飽和する。従ってＮｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆの含有量は０．０１％以上１．０％以下に限定する。更に、Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆは含Ｃｒ鋼中のＣおよびＮを炭化物として固定することによって基材単体での耐食性の向上や加工性の改善に顕著な効果があり、各元素単独の添加あるいは２種以上の元素を複合して添加することができるが、単独での添加量が０．０５％未満では効果がなく、０．８％を超えて添加するといたずらにコストを上昇させるとともに圧延疵等の原因となる。従って、Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆは０．０５％以上０．８％以下の範囲で添加することがより望ましい。かつ、加工性を有効に改善するためには、Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆの添加量の合計が次式を満足することが必要である。
【００３７】
Ｎｂ／９３＋Ｖ／５１＋Ｔｉ／４８＋Ｚｒ／９１＋Ｔａ／１８１＋Ｈｆ／１７９≧０．８×（Ｃ／１２＋Ｎ／１４）
本発明においては、鋼基材表面、特に、少なくとも腐食環境に曝される面を、基材よりも電位が卑なる金属で被覆する。基材よりも電位が卑なる金属で被覆する厚さが０．５μｍ以下では、基材にＡｌ添加することによる、基材表面が腐食環境に露出した後の残存する基材表面に被覆した、電位が基材よりも卑なる金属の層の消失速度の低下と、基材表面の腐食環境に露出される面積の拡大速度の著しい低下、同時に、露出した基材部分に対する基材表面に被覆した電位が基材よりも卑なる金属の層の基材露出部分に対する保護作用が長期にわたって継続するという効果の発現が充分ではなく、５００μｍを超える厚さまで被覆しても、もはやその効果は飽和しているのに対して、生産性を低下させて徒にコストを上昇させるだけであるから、被覆層の厚さは０．５〜５００μｍとする。
【００３８】
被覆層を形成する、水溶液環境における電位が基材よりも卑なる金属としては、アルミニウム、亜鉛、クロム、マンガン、およびこれらを主体とする合金を使用することができる。
上記被覆層の金属が更に、ＭｇおよびＩｎのうちの少なくとも一種を単独含有量で０．０５〜１０％含有することにより、より一層優れた耐食性が得られる。
【００３９】
Ｍｇ，Ｉｎの含有量を上記範囲に限定した理由は下記のとおりである。すなわち、それぞれの添加量が、０．０５％未満では耐食性を向上させる効果が見られず、逆に１０％を超えて添加しても効果が飽和するばかりで、本発明鋼の特徴である低コストを損なう。従って、Ｍｇ，Ｉｎはそれぞれ、０．０５〜１０％の範囲で添加する。
【００４０】
また、被覆のプロセスは該金属が基材に充分に固着されていればそのプロセスを限定するものではない。用途やコスト等を考慮した上で選択すれば良く、溶融メッキ、電着メッキ、溶融塩電解メッキ、真空蒸着、スパッタリング、イオンプレーティング、溶射等を使用することができ、それらを併用することも可能である。また、被覆およびそのための処理の前後にいかなる処理を行なってもよい。
【００４１】
亜鉛を主体とする合金とは、合金成分のうち最大量を占める成分が亜鉛である合金すなわち亜鉛基合金であり、一般に亜鉛基合金に含有されるアルミニウム等の合金成分および不純物成分を含んでよい。
アルミニウムを主体とする合金とは、合金成分のうち最大量を占める成分がアルミニウムである合金すなわちアルミニウム基合金であり、一般にアルミニウム基合金に含有されるシリコン、亜鉛等の合金成分および不純物成分を含んでよい。
【００４２】
クロムを主体とする合金とは、合金成分のうち最大量を占める成分がクロムである合金すなわちクロム基合金であり、一般にクロム基合金に含有されるシリコン等の合金成分および不純物成分を含んでよい。
マンガンを主体とする合金とは、合金成分のうち最大量を占める成分がマンガンである合金すなわちマンガン基合金であり、一般にマンガン基合金に含有されるアルミニウム等の合金成分および不純物成分を含んでよい。
【００４３】
また、使用上の目的から鋼管や板材等のように表裏面を有する材料の一方の面だけに被覆されていれば良い場合には、卑なる金属を被覆するプロセスから片面のみが被覆される鋼を使用してもよい。このような場合において片面だけの被覆を使用するか、あるいは両面に被覆された鋼を使用するかは、コストや溶接性等の他の要因を考慮して選択すれば良い。
【００４４】
上記被覆を施す実期については、コイル、板、棒、ケーブル、穿孔鋼管等の鋼材の一般的な形状とした後に、本発明の被覆やそのための処理を行ってもよいし、被覆・処理後の本発明鋼をプレスやロール成形等で所定の形状に成形し、更に加工・溶接して製品として製造しても良いし、本発明の鋼を例えば電縫鋼管等としてまず鋼管の形状にした後に、２次加工および溶接等によって製品としても良く、更に、本発明の被覆・処理を施す前に鋼材を上述したようなプロセスによって目的の形状とした後に本発明の表面被覆処理を施すことも可能であり、その他のプロセスも含めて本発明で限定する組成および処理条件の組み合わせを有する鋼は、いずれも本発明の対象とするところであって、コストや既存製品設備の制約等によって最適な製品製造工程を選択することができ、どの製造工程を選択したとしてもそれをもって本発明の範囲を逸脱するものではない。
【００４５】
以上の本発明において提案する鋼は、内燃機関の排気系統をはじめとする高温湿潤腐食環境、結露腐食環境はもとより、大気腐食環境、水道水腐食環境、土壌腐食環境、コンクリート腐食環境、海水腐食環境、飲料水腐食環境等の種々の腐食環境に適用することができる。
【００４６】
【実施例】
以下に本発明の実施例について説明する。
〔耐食性の評価〕
表１，２に成分を示す鋼を溶製し、熱延、冷延等の通常の鋼板製造工程によって肉厚１mmの鋼板とし、９００℃にて焼鈍を施した後、両面それぞれに、片面あたり１５±２μｍの条件で被覆を施した。表１，２に示した被覆１はアルミニウム被覆、被覆２は亜鉛被覆、被覆３はマンガン被覆、被覆４はクロム被覆をそれぞれ示す。
【００４７】
次に、これらの鋼板から幅５０mm、長さ７０mmの試験片を採取して、以下に述べる各種の腐食試験に供した。
高温湿潤腐食試験は、硫酸イオン２０００ppm 、塩化物イオン２０００ppm 、重炭酸イオン１００００ppm をアンモニウム塩の形で添加した水溶液５０cc中に試験片を半分まで浸漬し、試験容器ごと１３０℃の雰囲気に保持して試験溶液が完全に蒸発・揮散することを１００回繰り返す試験とした。本試験は自動車排気系の内面環境に相当する腐食試験であり、実車の約４年以上の走行に対応する厳しい試験方法である。試験結果を表１，２に併せて示した。腐食試験結果の◎は最大腐食深さが０．１０mm未満、○は０．２mm未満、△は０．３mm未満、×は０．３mm以上であったことをそれぞれ示す。
【００４８】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は塩化物を含む高温湿潤という非常に厳しい腐食環境であっても良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
また、大気環境や自動車排気系外面の塩害腐食を想定した試験としては、５０℃−１時間の塩水噴霧後、６０℃で湿度９６％の環境に５時間保持した後、更に１時間の冷凍保持を行うことを１５００回繰り返す塩害腐食試験とした。試験後の試験片について最大孔食深さを測定し、試験結果とした。得られた結果を表１，２に併せて示した。最大孔食深さが０．２mm以下のものは◎、最大孔食深さが０．２mmを超え０．４mm以下のものは○、最大孔食深さが０．４mmを超え０．８mm以下のものは×、最大孔食深さが０．８mmを超えるものは××で表示することとした。
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は塩害腐食という非常に厳しい腐食環境であっても良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
【００４９】
土壌腐食試験は、含水率１５％、比抵抗２００Ω・cmに塩化ナトリウム含有量で調整した砂中に試験片を埋め込み、６０℃に保持して約９００日放置する試験とした。試験結果を表１，２に併せて示した。腐食試験結果の◎は最大腐食深さが０．０５mm未満、○は０．１mm未満、△は０．５mm未満、×は０．５mm以上であったことをそれぞれ示す。
【００５０】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は土壌腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
コンクリート中腐食試験は、塩化物を含む海砂を用いて混練したポルトランドセメント中に試験片を埋め込みサンプルとなし、凝固させた後、人工海水中にサンプルを半分まで浸漬し、６０℃の環境に約１５００日放置する試験とした。試験結果を表１，２に併せて示した。腐食試験結果の◎は腐食の発生が認められなかったもの、○は発錆面積率が５％未満、△は発錆面積率が１０％未満、×は１０％以上であったことをそれぞれ示す。
【００５１】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）はコンクリート中腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
水道水環境腐食試験は、水道水中に試験片を浸漬し、６０℃の雰囲気に３０ヶ月間保持する試験とした。試験結果を表１，２に併せて示した。腐食試験結果の◎は腐食の発生が認められなかったもの、○は発錆面積率が５％未満、△は発錆面積率が１０％未満、×は１０％以上であったことをそれぞれ示す。
【００５２】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は水道水腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
海水環境腐食試験は、海岸飛沫帯に試験片を３６ヶ月間暴露する試験とした。試験結果を表１，２に併せて示した。腐食試験結果の◎は腐食深さ０．０５mm未満だったもの、○は０．１mm未満、△は０．３mm未満、×は０．３mm以上であったことをそれぞれ示す。
【００５３】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は海水腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
結露腐食試験は、−２０℃の環境に２時間保持後湿度９８％、５０℃の環境に４時間保持することを３０００回繰り返す試験とした。試験結果を表１，２に併せて示した。腐食試験結果の◎は腐食の発生が認められなかったもの、○は発錆面積率が５％未満、△は発錆面積率が１０％未満、×は１０％以上であったことをそれぞれ示す。
【００５４】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は結露腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
大気腐食試験は、海岸から約１０ｍの位置に試験片を約９００日暴露する試験とした。試験結果を表１，２に併せて示した。腐食試験結果の◎は腐食が認められなかったもの、○は発錆面積率が３％未満、△は発錆面積率が１０％未満、×は１０％以上であったことをそれぞれ示す。
【００５５】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は大気腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
飲料水環境腐食試験は、水酸化ナトリウムを用いてpHを２．３に調整し、高純度窒素ガスを通気して脱気し、２７℃に保持した、（ａ）０．５％リン酸溶液、（ｂ）０．５％クエン酸溶液（Ｃ）０．５％クエン酸−０．５％塩化ナトリウム溶液等の溶液８５０cc中に試験片を６０日間浸漬し、溶液中に溶出した鉄イオン量を分析する試験とした。なお本試験のみ、被覆１のアルミニウム被覆、被覆４のクロム被覆について試験を実施した。試験結果を表１，２に併せて示した。腐食試験結果の◎は溶液中への鉄イオンの溶出量が１ppm 以下、○は３ppm 未満、△は５ppm 未満、×は５ppm 以上であったことをそれぞれ示す。
【００５６】
表１，２から明らかなように、本発明鋼（番号１〜４０，５１〜９０）は飲料水腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
すなわち本発明鋼（番号１〜４０，５１〜９０）は高温湿潤腐食環境、結露腐食環境、大気腐食環境、水道水腐食環境、土壌腐食環境、コンクリート腐食環境、海水腐食環境、飲料水腐食環境等の種々の腐食環境で良好な耐食性を示しているのに対して、比較鋼（番号４１〜５０，１００〜１０９）は耐食性に劣ることがわかる。
〔加工性の評価〕
表２に成分を示す鋼を溶製し、熱延、冷延など通常の鋼板製造工程によって、厚さ１．０mmの鋼板とし、８５０℃にて焼鈍を施した。これらの鋼板から幅１００mm長さ１００mmの試験片を採取し、絞り比１．８の円筒絞り試験を行なって割れの有無で判定した。試験結果を表２に併せて示した。表２の加工性において○は円筒絞り試験結果が良好であったことを示し、×は円筒絞り試験で割れを生じたことを示している。尚、表２中のＸ値は、次式によって算出したものを記載した。
【００５７】
Ｘ＝Ｎｂ／９３＋Ｖ／５１＋Ｔｉ／４８＋Ｚｒ／９１＋Ｔａ／１８１＋Ｈｆ／１７９−０．８×（Ｃ／１２＋Ｎ／１４）
表２から明らかなように、本発明鋼において特に加工性を向上させたもの（番号５１〜９０）は良好な耐食性を示し、かつ加工性も良好である。すなわち、高温湿潤腐食環境、結露腐食環境、大気腐食環境、水道水腐食環境、土壌腐食環境、コンクリート腐食環境、海水腐食環境、飲料水腐食環境等の種々の腐食環境で良好な耐食性を示し、かつ加工性も優れている。これに対して、比較鋼（番号１００〜１０９）耐食性と加工性が同時に達成できないことがわかる。
【００５８】
【表１】

【００５９】
【表２】

【００６０】
【表３】

【００６１】
【表４】

【００６２】
【表５】

【００６３】
【表６】

【００６４】
【表７】

【００６５】
【表８】

【００６６】
【発明の効果】
以上述べたように、本発明によれば、例えば自動車等の内燃機関の排気系統といった高温湿潤腐食環境、結露腐食環境をはじめとして、大気腐食環境、水道水腐食環境、土壌腐食環境、コンクリート腐食環境、海水腐食環境、飲料水腐食環境等の種々の腐食環境において耐食性に優れる耐食鋼が低コストで提供される。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to corrosion-resistant steel, and more specifically, for example, (1) High-temperature and wet-corrosion environments such as internal combustion engine exhaust systems such as automobiles and ships, boiler exhaust systems, low-temperature heat exchangers, incinerator floors, (2) bridges, Prop, exterior / interior building materials, roofing materials, joinery, kitchen components, various handrails, guardrails, various hooks, roof drains, railcars, etc., (3) soil corrosion of various storage tanks, columns, piles, sheet piles, etc. Environment, (4) Canned containers, various containers, low-temperature heat exchangers, bathroom members, automobile structural members, and other condensed and corrosive environments (including corrosive environments where freezing, wetting and drying are combined), (5) water storage tanks, water supply pipes, Hot water pipes, can containers, various containers, tableware, cooking equipment, bathtubs, pools, bathroom vanities, and other tap water corrosive environments, (6) can containers, various containers, tableware, cooking equipment corrosive drinking water environments, (7 ) Concrete such as various reinforcing steel structures and supports Food environment, 8 ship, bridges, piles, sheet piles, seawater corrosive environments such as marine structures, to a steel having excellent corrosion in various corrosive environments.
[0002]
[Prior art]
Conventionally, in an exhaust system of an internal combustion engine centering on an automobile, steel obtained by subjecting ordinary steel to aluminum plating or zinc plating to suppress corrosion from the inner surface or the outer surface has been used. Since the exhaust system is equipped with a catalyst for the purpose of purifying exhaust gas in order to suppress environmental pollution, the corrosion resistance of such plated steel is not sufficient, and 5-10% Cr is included for the purpose of improving the corrosion resistance of the steel substrate. Steels disclosed in Japanese Patent Laid-Open Nos. 63-143240 and 63-143241 are disclosed. However, with the extension of the vehicle use period and warranty period in recent years, high-grade stainless steel further containing Cr up to about 18% or further containing Mo is used in the exhaust system.
[0003]
However, even such high-grade stainless steel does not always have sufficient corrosion resistance, such as pitting corrosion-like local corrosion. In addition, these high-grade stainless steels contain a large amount of Cr and Mo, so that the processability is poor, and in order to form a complicated shape such as an exhaust system member, it is very difficult to manufacture and the manufacturing process is remarkably high. Due to the complexity, the processing cost is also high. In addition, the material cost is high.
[0004]
As a representative of the above-mentioned exhaust system, in general, steel containing a certain amount of Cr tends to generate local corrosion when the corrosive environment becomes severe. In order to improve the resistance against corrosion as a means against this, the content of Cr or Mo is further increased. It was a very common technical measure to increase. In addition, when Cr and Mo are used to maintain corrosion resistance, even if they have sufficient corrosion resistance to the exhaust gas environment, the purpose is to prevent freezing of road surfaces in the winter, as in the cold regions of the United States and Canada. When spraying a large amount of salt, the exhaust system member is eroded from the outer surface by such salt.
[0005]
In recent years, Japanese Patent Application Laid-Open Nos. Hei 5-279791, Hei 6-179949, Hei 6-179950, Hei 6-179951, Hei 6-212256, Hei 6-212257, Japanese Patent Application Laid-Open No. 7-3388 discloses steel in which Al is added to Cr for the purpose of improving corrosion resistance or improving corrosion resistance and workability. These steels are recognized to be effective to some extent in improving exhaust system inner surface corrosion resistance or exhaust system inner surface corrosion resistance and workability, but there is still room for improvement in terms of wet corrosion resistance centering on salt corrosion resistance. .
[0006]
[Problems to be solved by the invention]
In view of such a current situation, the present invention is a high temperature wet corrosion environment including an exhaust system of an internal combustion engine, a condensation corrosion environment, an air corrosion environment, a tap water corrosion environment, a soil corrosion environment, a concrete corrosion environment, a seawater corrosion environment. Another object of the present invention is to provide a low-cost corrosion-resistant steel having excellent corrosion resistance in various corrosive environments such as drinking water corrosive environments.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have achieved a high temperature and wet corrosion environment including an exhaust system of an internal combustion engine, a condensation corrosion environment, an air corrosion environment, a tap water corrosion environment, a soil corrosion environment, a concrete corrosion environment, In order to develop steel having excellent corrosion resistance in various corrosive environments such as seawater corrosive environment and drinking water corrosive environment, investigation has been made from various viewpoints.
[0008]
First, the present inventors examined the internal corrosion environment of the exhaust system that is the most severe against corrosion. Corrosion of the internal combustion engine exhaust system involves heating chloride, sulfate ions, etc. contained in the exhaust gas to 80 to 150 ° C. Found what happens in a different environment.
Furthermore, as a result of various studies on means for improving the corrosion resistance in the corrosive environment, steel containing 10 to 30% of Cr with 0.1% to 10% or less of Al added is excellent in corrosive environments including exhaust systems. It was found that it exhibits high corrosion resistance.
[0009]
Furthermore, as a result of continuing the study as a superior steel, the present inventors have reduced C and N of steel containing 10 to 30% Cr and 0.1% to 10% Al, and Nb, Addition of V, Ti, Zr, Ta, Hf to satisfy specific conditions is effective in improving corrosion resistance and workability, and Si and Mn are suitable as deoxidation and strengthening elements It has been found that when Cu, Mo, Sb, Ni, and W are added to the above steel alone or in combination, more excellent corrosion resistance can be obtained.
[0010]
On the other hand, as a result of separately examining in parallel the means for obtaining a steel material having enhanced corrosion resistance against repeated dry and wet conditions such as salt damage corrosion and salt water, which is a severe corrosion environment next to the internal corrosion environment of the exhaust system, When a steel layer containing 10 to 30% of Al and 0.1% or more of Al is used as a base material and a metal layer whose potential in an aqueous solution environment is lower than that of the base material is formed on the surface, excellent corrosion resistance, particularly It has been found that excellent salt corrosion resistance can be obtained. In particular, the addition of Al to the base material has a corrosion resistance after the surface of the base metal exposed to the corrosive environment is slightly lost due to the partial disappearance of the metal layer whose potential is lower than that of the base material. It has been found that the effect is significant for improvement.
[0011]
The corrosion resistance improvement behavior due to the addition of Al to the base material is a phenomenon that is completely different from the above-described base material corrosion resistance, and there is a metal layer in which the potential coated on the base material surface is lower than the base material, In addition, it has been confirmed that this is the first phenomenon observed when this metal layer partially disappears.
The reasons for the significant improvement in corrosion resistance that we have found are currently unknown, but by adding Al to the base material, the potential of the base material surface covered after the base material surface is exposed to the corrosive environment is covered. The disappearance rate of the metal layer that is lower than the material is significantly reduced, and therefore the expansion rate of the area exposed to the corrosive environment of the substrate surface is significantly reduced, and at the same time, the substrate surface is coated on the exposed substrate part. It has been confirmed that the corrosion resistance has been improved by continuing the protective action for the exposed portion of the base material of the metal layer whose potential is lower than that of the base material.
[0012]
The part exposed to the corrosive environment on the substrate surface where such an effect is recognized can be confirmed for the first time by microscopic observation, and the absolute value is about 0.05 mm. ² It is a small area less than or equal to about 0.2% of the total corrosion area, and in general there is no red rust on the macroscopic macroscopic appearance assuming actual use. Has the appearance that no corrosion of the substrate is observed. Moreover, such a state is characterized by being maintained for a long time.
[0013]
According to the conventional knowledge, when there is a metal layer with a lower potential applied to the substrate surface than the substrate, and the metal layer partially disappears, the corrosion rate of the metal layer coated on the surface is Considering that it is common to think that the area exposed to the corrosive environment of the base metal will rapidly expand and quickly shift to the corrosion of the base metal. The above-described improvement in corrosion resistance due to the addition of Al to the base metal is a means for improving corrosion resistance, which has never been known so far, and is based on such a new discovery. Is.
[0014]
Furthermore, we will continue to study for the purpose of improving corrosion resistance, and with the addition of Al to such a base material, it is possible to add various elements that have conventionally been difficult to add stably to steel Newly found to be. In particular, it has been newly found that the corrosion resistance is significantly improved by adding Mg to steel containing 0.1 to 10% of Al by the interaction between the base material and the coated metal described above.
[0015]
The inventors of the present invention have various means for further enhancing the corrosion resistance improvement effect by the simultaneous addition of Al and Mg into a base metal in which a metal layer having a lower potential than that of the base is present. After repeated studies, it was clarified that the addition of Si, Mn, Nb, VTi, Zr, Ta, Hf, Cu, Mo, Sb, Ni, and W to the base material is effective.
[0016]
Further, the present inventors continue to study, and as a metal whose potential in an aqueous solution environment is lower than that of the base material, aluminum, an alloy mainly composed of aluminum, zinc, an alloy mainly composed of zinc, chromium, and chromium are mainly used. It has also been found that alloys, manganese, and manganese-based alloys are suitable metals for the purposes of the present invention.
As for these coated metals, as a result of repeated studies from the viewpoint of improving corrosion resistance by interaction with the base material, aluminum, alloys mainly composed of aluminum, zinc, alloys mainly composed of zinc, chromium, and chromium are mainly used. Any one or more of Mg and In to any of the metals such as alloys, manganese, alloys mainly composed of manganese, quality It has been found that a material containing 0.05% or more and 10% or less in an amount of% realizes even better corrosion resistance.
[0017]
The present invention was made mainly based on the above findings, and the gist of the first invention of the present application is as follows. quality In%
Si: 0.01-3.0%,
Mn: 0.01 to 3.0%,
Cr: 10-30%,
Al: 0.1 to 10%,
Mg: 0.0003 to 0.05%,
It consists of the balance Fe and unavoidable impurities, of which C is 0.02% or less, P is 0.03% or less, S is 0.01% or less, and N is 0.02% or less. A corrosion-resistant steel comprising a base material of steel, and a metal coating layer having a lower potential in an aqueous solution environment than the base material is formed on the surface of the base material in a thickness of 0.05 to 500 μm. .
[0018]
The gist of the second invention is that in the steel of the first invention, the base material is further added as an additional component. quality In%
Cu: 0.01 to 5.0%,
Mo: 0.05 to 10%,
Sb: 0.01 to 0.5%,
Ni: 0.01 to 10%,
W: 0.05-3.0%
The corrosion-resistant steel according to the invention is characterized by containing one or more of the above.
[0019]
The gist of the third invention is that in the steel of the first invention and the second invention, the base material is further added as an additional component. quality In%
It exists in the corrosion-resistant steel characterized by containing 1 type, or 2 or more types of rare earth elements: 0.001-0.1%, Ca: 0.0001-0.05%.
The gist of the fourth invention is the steel of the first invention, the second invention, and the third invention, wherein the base material is further added as an additional component, quality 1% or more of elements selected from Nb, V, Ti, Zr, Ta, and Hf are contained in an amount of 0.01 to 1% in a single content, and the following formula is satisfied. It is a steel with excellent corrosion resistance and workability.
[0020]
Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179 ≧ 0.8 × (C / 12 + N / 14)
The gist of the fifth invention resides in the corrosion resistance and corrosion resistant steel according to the first invention, the second invention, the third invention and the fourth invention, wherein the metal of the coating layer is either aluminum or an alloy mainly composed of aluminum.
[0021]
The gist of the sixth invention resides in the corrosion resistant steel according to the first invention, the second invention, the third invention, and the fourth invention, wherein the metal of the coating layer is either zinc or an alloy mainly composed of zinc.
The gist of the seventh invention resides in the corrosion-resistant steel according to the first invention, the second invention, the third invention, and the fourth invention, wherein the metal of the coating layer is either chromium or a chromium-based alloy.
[0022]
The gist of the eighth invention resides in the corrosion resistant steel according to the first invention, the second invention, the third invention, and the fourth invention, wherein the metal of the coating layer is either manganese or an alloy mainly composed of manganese.
The gist of the ninth invention is the steel of the fifth invention, sixth invention, seventh invention, and eighth invention, wherein the metal of the coating layer is further quality The corrosion-resistant steel is characterized by containing at least one of Mg and In in an amount of 0.05 to 10% as a single content.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
The reason why the range of each component of the substrate is limited in the present invention will be described below.
Si: Si has an effect of improving the corrosion resistance when a metal layer having a lower potential in an aqueous solution environment than the base material is formed on the surface of the base material to a thickness of 0.05 to 500 μm. If it is less than%, no effect is observed, and even if added over 3%, the effect is saturated. Therefore, the content range is limited to 0.01% or more and 3% or less. Furthermore, addition of Si as a steel to steel containing 10% or more of Cr is effective as a deoxidizer and strengthening element. However, if the content is less than 0.015%, the deoxidation effect is not sufficient. When the content is 5% or more, the effect is no longer saturated and the workability is slightly reduced. Therefore, it is more desirable to add in the range of 0.015% to 1.5%.
[0024]
Mn: Mn brings about the effect of improving the corrosion resistance when a metal layer whose potential in an aqueous solution environment is lower than that of the base material is formed to a thickness of 0.5 to 500 μm on the surface of the base material. If it is less than 3%, the effect is not recognized, and even if added in excess of 3%, the effect is saturated. Therefore, the content range is limited to 0.01% or more and less than 3%. Further, Mn is effective as a deoxidizer for steel, and it is necessary to contain 0.05% or more, but even if it exceeds 1.2%, the effect is no longer saturated, but excessively Mn If it contains, workability will fall. Therefore, it is more desirable to add in the range of 0.05% or more and 1.2% or less.
[0025]
Cr: By adding Cr to steel containing 0.1% or more of Al, Cr has a thickness of 0.5 to 500 μm on the surface of the base material with a metal layer whose potential in an aqueous solution environment is lower than that of the base material. When it is formed, the effect of improving the corrosion resistance is brought about. However, if it is less than 10%, the effect is not sufficient. Therefore, the Cr content is limited to 10% to 30%. Furthermore, in order to ensure the corrosion resistance of the base material alone against the exhaust gas environment or the like, it is necessary to contain 10% or more in steel containing 0.1% or more of Al, but even if it contains more than 25% Workability is reduced. Therefore, it is more desirable to add in the range of 10% to 25%.
[0026]
Al: Al is the most important element for securing corrosion resistance in the present invention, and by adding Al to steel containing Cr of 10% or more and 30% or less, the potential in the aqueous solution environment is increased on the substrate surface. When a metal layer that is lower than the base material is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance is brought about. However, if it is less than 0.1%, the effect is not sufficient, and the addition is more than 10%. Even if the effect is saturated, the Al content is limited to not less than 0.1% and not more than 10%. In order to obtain a more stable effect, the Al content is preferably set to 0.3% or more, but is preferably set to 4% or less so as not to deteriorate the workability. Therefore, the Al content is desirably 0.3% or more and 4% or less.
[0027]
Mg: Mg is an important element next to Al in order to ensure corrosion resistance in the present invention. Mg is added to steel containing 0.1% to 10% Al and 10% to 30% Cr. Thus, when a metal layer whose potential in an aqueous solution environment is lower than that of the base material is formed on the surface of the base material to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance is brought about, but less than 0.0003% Then, the effect is not sufficient, and even if added over 0.05%, the effect is saturated, so the Mg content is limited to 0.0003% or more and 0.05% or less.
[0028]
C, N: C and N lower the workability of the steel sheet, and C generates Cr and carbides to lower the corrosion resistance. Also, N lowers the toughness, so the amount of C and N is smaller. Desirably, the upper limit content is 0.02% for all, and the lower the content, the better. Furthermore, in order to ensure excellent workability, it is necessary to reduce the total amount of C + N. In the steel according to a desirable aspect of the present invention, C + N is set to 0.03% or less.
[0029]
P: When P is present in a large amount, the toughness is lowered, so the smaller one is desirable, and the upper limit content is 0.03%.
S: If S is present in a large amount, the pitting corrosion resistance is deteriorated, so that it is desirable that the content is small. The upper limit is 0.01%.
The above is the basic component of the steel base material excellent in corrosion resistance targeted by the present invention, but in the present invention, steel materials whose corrosion resistance is further improved by adding the following elements as necessary are also targeted. It is said.
[0030]
Cu: Cu has a thickness of 0.5 to 500 μm on the surface of a steel base containing 0.1% or more of Al and containing 10% or more and 30% or less of Cr. When it is formed, the effect of improving the corrosion resistance is brought about. However, when the content is less than 0.01%, the effect is not recognized, and when the content exceeds 5%, the effect is saturated. Therefore, the range is limited to a range of 0.01% to 5%. Further, when added in an amount of 0.05% or more, there is an effect of improving the resistance against the general corrosion of the base material alone, and when added over 2.5%, the effect is saturated. Therefore, it is more desirable to add in the range of 0.05% to 2.5%.
[0031]
Mo: Mo is added to steel containing 0.1% or more of Al and containing 10% or more and 30% or less of Cr, so that the surface of the substrate has a metal layer whose potential in aqueous solution environment is lower than that of the substrate. When it is formed to have a thickness of 5 to 500 μm, the effect of improving the corrosion resistance is brought about. However, when the content is less than 0.05%, the effect is not recognized, and when the content exceeds 10%, the effect is saturated. Therefore, the range is limited to 0.05% or more and 10% or less. Furthermore, when Mo is added in an amount of 0.1% or more, it has the effect of suppressing the occurrence and growth of pitting corrosion on the base material alone, but adding over 3.0% not only saturates the effect but also the workability. Reduce. Therefore, it is more desirable to add in the range of 0.1% to 3%.
[0032]
Sb: Sb is a metal whose potential in the aqueous solution environment is lower than that of the base material when 0.01% or more is added to steel containing 0.1% or more of Al and 10% or more and 30% or less of Cr. When the layer is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance is brought about. However, if the layer is less than 0.01%, the effect is not recognized. The effect is saturated. Therefore, the range is limited to 0.01% or more and 0.5% or less. Furthermore, by adding Sb in an amount of 0.015% or more, there is an effect of improving resistance to pitting corrosion and overall corrosion of the base material alone, but when it exceeds 0.3%, hot workability is slightly reduced. . Therefore, it is more desirable to add in the range of 0.015% to 0.3%.
[0033]
Ni: Ni is a metal whose electric potential in an aqueous solution environment is lower than that of a base material when added to a steel containing 0.1% or more of Al and containing 10% or more and 30% or less of Cr. When the layer is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance is brought about. On the other hand, the addition of more than 10% saturates the effect. Therefore, the range is limited to 0.01% or more and 10% or less. Further, by adding 0.1% or more of Ni, there is an effect of suppressing pitting corrosion in the base material alone, but even if added over 6%, the effect is saturated. Therefore, it is more desirable to add in the range of 0.1% to 6%.
[0034]
W: W is a metal whose electric potential in an aqueous solution environment is lower than that of a base material when 0.05% or more is added to steel containing 0.1% or more of Al and 10% or more and 30% or less of Cr. When the layer is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance is brought about. On the other hand, the addition of more than 3% saturates the effect. Therefore, the range is limited to 0.05% or more and 3% or less. Further, by adding W, there is an effect of suppressing the occurrence and growth of pitting corrosion in the base material alone, but if it is less than 0.1%, the effect is not sufficient, while adding over 2.0% Not only saturates the effect, but also reduces workability. Therefore, it is more desirable to add in the range of 0.1% to 2%.
[0035]
Rare earth elements (REM), Ca: Rare earth metals (REM) and Ca are added to steel containing 0.1% or more of Al and 10% or more and 30% or less of Cr, so that the potential in the aqueous solution environment is added to the substrate surface. Has an effect of improving the corrosion resistance when a metal layer that is lower than the base material is formed to a thickness of 0.5 to 500 μm, but in REM less than 0.001%, no effect is observed in Ca, and 0 in Ca If it is less than 0.0001%, no effect is observed. On the other hand, even if it exceeds 0.1% in REM and 0.05% in Ca, the effect is saturated. Therefore, the range of REM is limited to 0.001% or more and 0.1% or less, and the range of Ca is limited to 0.0001% or more and 0.05% or less. Furthermore, REM and Ca are elements that are effective in improving hot workability and improving the pitting corrosion resistance of the base material alone, but if the addition amount is less than 0.01% for REM and less than 0.005% for Ca. The effect is not sufficient. When Ca is added in an amount of 0.01% or more than 0.05% in REM, coarse non-metallic inclusions are formed, and hot workability and pitting corrosion resistance are deteriorated. Therefore, the upper limit content is 0.1% for REM and 0.03% for Ca. Therefore, it is more desirable to add Ca in the range of 0.005% to 0.01% and REM in the range of 0.01% to 0.05%. In the present invention, the rare earth element refers to an element having atomic numbers 57 to 71 and 89 to 103 and Y.
[0036]
Nb, V, Ti, Zr, Ta, Hf: Nb, V, Ti, Zr, Ta, Hf are added to steel containing 0.1% or more of Al and 10% or more and 30% or less of Cr. When a metal layer whose potential in an aqueous solution environment is lower than that of the base material is formed on the surface of the base material to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance is brought about. The effect is not recognized, but the effect is saturated even if added over about 1.0%. Therefore, the contents of Nb, V, Ti, Zr, Ta, and Hf are limited to 0.01% or more and 1.0% or less. Furthermore, Nb, V, Ti, Zr, Ta, and Hf have a remarkable effect in improving the corrosion resistance and workability of the base material alone by fixing C and N in the Cr-containing steel as carbides. Although the addition of an element alone or a combination of two or more elements can be added, there is no effect if the addition amount alone is less than 0.05%, and if it exceeds 0.8%, the cost is unnecessarily high. It raises and causes rolling wrinkles. Therefore, Nb, V, Ti, Zr, Ta, and Hf are more preferably added in the range of 0.05% to 0.8%. And in order to improve workability effectively, it is necessary for the sum total of the addition amount of Nb, V, Ti, Zr, Ta, and Hf to satisfy the following formula.
[0037]
Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179 ≧ 0.8 × (C / 12 + N / 14)
In the present invention, the steel substrate surface, particularly at least the surface exposed to the corrosive environment, is coated with a metal having a lower potential than the substrate. With a thickness of 0.5 μm or less covering with a metal whose potential is lower than that of the substrate, the remaining substrate surface was coated after the substrate surface was exposed to a corrosive environment by adding Al to the substrate. Decrease in the disappearance rate of the metal layer whose potential is lower than that of the substrate, and a significant decrease in the expansion rate of the area exposed to the corrosive environment on the substrate surface, at the same time, coating the substrate surface with respect to the exposed substrate part The effect that the protective action against the exposed portion of the base layer of the metal layer whose potential is lower than that of the base material continues for a long period of time is not sufficient, and even if it is coated to a thickness exceeding 500 μm, the effect is no longer saturated. On the other hand, the thickness of the coating layer is set to 0.5 to 500 μm because productivity is lowered and the cost is increased.
[0038]
Aluminum, zinc, chromium, manganese, and alloys based on these can be used as the metal that forms the coating layer and has a lower potential in the aqueous solution environment than the base material.
Further excellent corrosion resistance can be obtained when the metal of the coating layer further contains at least one of Mg and In in an independent content of 0.05 to 10%.
[0039]
The reason why the contents of Mg and In are limited to the above range is as follows. That is, when the addition amount is less than 0.05%, the effect of improving the corrosion resistance is not seen, and conversely, the addition of more than 10% only saturates the effect, which is a feature of the steel of the present invention. Reduce costs. Therefore, Mg and In are added in a range of 0.05 to 10%, respectively.
[0040]
The coating process is not limited as long as the metal is sufficiently fixed to the substrate. Selection may be made in consideration of the use and cost, etc., and hot-dip plating, electrodeposition plating, hot-salt electrolytic plating, vacuum deposition, sputtering, ion plating, thermal spraying, etc. can be used. Is possible. Further, any treatment may be performed before and after the coating and the treatment therefor.
[0041]
The alloy mainly composed of zinc is an alloy in which the component occupying the maximum amount among the alloy components is zinc, that is, a zinc-based alloy, and may generally include an alloy component such as aluminum and an impurity component contained in the zinc-based alloy. .
An alloy mainly composed of aluminum is an alloy in which the largest component of the alloy components is aluminum, that is, an aluminum-based alloy, and generally includes alloy components such as silicon and zinc and impurity components contained in the aluminum-based alloy. It's okay.
[0042]
The alloy mainly composed of chromium is an alloy in which the component occupying the largest amount among the alloy components is chromium-based alloy, that is, a chromium-based alloy, and may generally include an alloy component such as silicon and an impurity component contained in the chromium-based alloy. .
The manganese-based alloy is an alloy in which the component occupying the maximum amount among the alloy components is manganese, that is, a manganese-based alloy, and may generally include an alloy component such as aluminum and an impurity component contained in the manganese-based alloy. .
[0043]
In addition, if it is sufficient to cover only one surface of a material having front and back surfaces, such as steel pipes and plate materials, for the purpose of use, steel that is coated on only one surface from the process of covering a base metal May be used. In such a case, whether to use the coating on only one side or the steel coated on both sides may be selected in consideration of other factors such as cost and weldability.
[0044]
For the actual period of applying the coating, after the general shape of the steel material such as a coil, a plate, a rod, a cable, a perforated steel pipe, etc., the coating of the present invention and the processing for the same may be performed. The steel of the present invention may be formed into a predetermined shape by pressing or roll forming, and further processed and welded to produce a product. The steel of the present invention is first made into a shape of a steel pipe, for example, as an electric resistance steel pipe Later, it may be made into a product by secondary processing, welding, or the like. Further, before the coating / treatment of the present invention is performed, the steel material may be subjected to the surface coating treatment of the present invention after having been formed into the target shape by the above-described process. Steels having a combination of composition and processing conditions limited by the present invention, including other processes, are all objects of the present invention, and are optimally manufactured depending on costs, constraints on existing product facilities, etc. Can select the manufacturing process, it with a do not depart from the scope of the present invention even on the choice of manufacturing process.
[0045]
The steel proposed in the present invention described above is not only high-temperature humid corrosion environment, condensation corrosion environment including exhaust system of internal combustion engine, but also atmospheric corrosion environment, tap water corrosion environment, soil corrosion environment, concrete corrosion environment, seawater corrosion environment It can be applied to various corrosive environments such as drinking water corrosive environments.
[0046]
【Example】
Examples of the present invention will be described below.
[Evaluation of corrosion resistance]
Steels with the components shown in Tables 1 and 2 are melted and made into a steel plate with a thickness of 1 mm by a normal steel plate manufacturing process such as hot rolling and cold rolling. After annealing at 900 ° C., each side of each side The coating was applied under the condition of 15 ± 2 μm. The coating 1 shown in Tables 1 and 2 is an aluminum coating, the coating 2 is a zinc coating, the coating 3 is a manganese coating, and the coating 4 is a chromium coating.
[0047]
Next, test pieces having a width of 50 mm and a length of 70 mm were collected from these steel plates and subjected to various corrosion tests described below.
In the high temperature wet corrosion test, the test piece was immersed in 50 cc of an aqueous solution to which 2000 ppm of sulfate ion, 2000 ppm of chloride ion and 10000 ppm of bicarbonate ion were added in the form of ammonium salt, and the test vessel was kept in an atmosphere of 130 ° C. It was set as the test which repeats 100 times that test solution evaporates and volatilizes completely. This test is a corrosion test that corresponds to the internal environment of the automobile exhaust system, and is a rigorous test method that can be used for running a real vehicle for about four years or more. The test results are also shown in Tables 1 and 2. In the corrosion test results, ◎ indicates that the maximum corrosion depth was less than 0.10 mm, ○ was less than 0.2 mm, Δ was less than 0.3 mm, and x was 0.3 mm or more.
[0048]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) show good corrosion resistance even in a very severe corrosive environment of high temperature humidity containing chloride. Thus, it can be seen that the comparative steels (numbers 41 to 50, 100 to 109) are inferior in corrosion resistance.
In addition, as a test assuming salt damage corrosion of the air environment and the outer surface of an automobile exhaust system, after spraying salt water at 50 ° C. for 1 hour, holding it in an environment of 96% humidity at 60 ° C. for 5 hours, and then holding it frozen for another hour The salt damage corrosion test was repeated 1500 times. The maximum pitting corrosion depth of the test piece after the test was measured and used as the test result. The obtained results are also shown in Tables 1 and 2. The maximum pitting depth is 0.2 mm or less, ◎, the maximum pitting depth is more than 0.2 mm and less than 0.4 mm, and the maximum pitting depth is more than 0.4 mm and less than 0.8 mm. Those with a maximum pitting depth exceeding 0.8 mm are indicated with xx.
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) show good corrosion resistance even in a very severe corrosive environment called salt corrosion, whereas the comparative steels (Nos. 41 to 50, 100 to 109) are inferior in corrosion resistance.
[0049]
The soil corrosion test was a test in which a test piece was embedded in sand adjusted to have a water content of 15%, a specific resistance of 200 Ω · cm and a sodium chloride content, and kept at 60 ° C. for about 900 days. The test results are also shown in Tables 1 and 2. In the corrosion test results, ◎ indicates that the maximum corrosion depth was less than 0.05 mm, ◯ was less than 0.1 mm, Δ was less than 0.5 mm, and x was 0.5 mm or more.
[0050]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) show good corrosion resistance in a soil corrosive environment, whereas the comparative steels (Nos. 41 to 50, 100 to 100) are shown. 109) is inferior in corrosion resistance.
In the concrete corrosion test, test specimens were embedded in Portland cement kneaded with sea sand containing chloride, solidified, and then half-immersed in artificial seawater in an environment of 60 ° C. The test was allowed to stand for about 1500 days. The test results are also shown in Tables 1 and 2. Corrosion test results ◎ indicate that no corrosion was observed, ○ indicates that the rusting area ratio is less than 5%, △ indicates that the rusting area ratio is less than 10%, and X indicates that it is 10% or more. .
[0051]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) show good corrosion resistance in a corrosive environment in concrete, whereas the comparative steels (Nos. 41 to 50, 100). ~ 109) are found to be inferior in corrosion resistance.
The tap water environmental corrosion test was a test in which a test piece was immersed in tap water and kept in an atmosphere of 60 ° C. for 30 months. The test results are also shown in Tables 1 and 2. Corrosion test results ◎ indicate that no corrosion was observed, ○ indicates that the rusting area ratio is less than 5%, △ indicates that the rusting area ratio is less than 10%, and X indicates that it is 10% or more. .
[0052]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) show good corrosion resistance in a tap water corrosive environment, whereas the comparative steels (Nos. 41 to 50, 100). ~ 109) are found to be inferior in corrosion resistance.
The seawater environment corrosion test was a test in which the test piece was exposed to the coastal splash zone for 36 months. The test results are also shown in Tables 1 and 2. In the corrosion test results, ◎ indicates that the corrosion depth was less than 0.05 mm, ○ indicates less than 0.1 mm, Δ indicates less than 0.3 mm, and x indicates 0.3 mm or more.
[0053]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) exhibit good corrosion resistance in a seawater corrosive environment, whereas the comparative steels (Nos. 41 to 50 and 100 to 100) are shown. 109) is inferior in corrosion resistance.
The dew condensation corrosion test was a test that was repeated 3000 times for 2 hours in an environment of −20 ° C. and then in an environment of 98% humidity and 50 ° C. for 4 hours. The test results are also shown in Tables 1 and 2. Corrosion test results ◎ indicate that no corrosion was observed, ○ indicates that the rusting area ratio is less than 5%, △ indicates that the rusting area ratio is less than 10%, and X indicates that it is 10% or more. .
[0054]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) exhibit good corrosion resistance in a condensation corrosion environment, whereas the comparative steels (Nos. 1 to 50 and 100 to 100). 109) is inferior in corrosion resistance.
The atmospheric corrosion test was a test in which the test piece was exposed for about 900 days at a position of about 10 m from the coast. The test results are also shown in Tables 1 and 2. In the corrosion test results, “◎” indicates that no corrosion was observed, “◯” indicates that the rusting area ratio is less than 3%, “Δ” indicates that the rusting area ratio is less than 10%, and “x” indicates 10% or more.
[0055]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) exhibit good corrosion resistance in an atmospheric corrosion environment, whereas the comparative steels (Nos. 41 to 50, 100 to 100) are shown. 109) is inferior in corrosion resistance.
In the drinking water environmental corrosion test, pH was adjusted to 2.3 using sodium hydroxide, high-purity nitrogen gas was vented and degassed, and maintained at 27 ° C. (a) 0.5% phosphoric acid solution (B) A 0.5% citric acid solution (C) A test piece was immersed in 850 cc of a 0.5% citric acid-0.5% sodium chloride solution for 60 days, and the amount of iron ions eluted in the solution The test was analyzed. Only in this test, the test was conducted on the aluminum coating of the coating 1 and the chromium coating of the coating 4. The test results are also shown in Tables 1 and 2. In the corrosion test results, “◎” indicates that the elution amount of iron ions in the solution was 1 ppm or less, “◯” was less than 3 ppm, “Δ” was less than 5 ppm, and “x” was 5 ppm or more.
[0056]
As is clear from Tables 1 and 2, the steels of the present invention (Nos. 1 to 40 and 51 to 90) exhibit good corrosion resistance in drinking water corrosive environments, whereas the comparative steels (Nos. 41 to 50, 100). ~ 109) are found to be inferior in corrosion resistance.
That is, the steel of the present invention (Nos. 1 to 40, 51 to 90) is a hot and humid corrosive environment, dew corrosive environment, atmospheric corrosive environment, tap water corrosive environment, soil corrosive environment, concrete corrosive environment, seawater corrosive environment, drinking water corrosive environment, etc. It can be seen that the comparative steels (Nos. 41 to 50, 100 to 109) are inferior in corrosion resistance, while exhibiting good corrosion resistance in various corrosive environments.
[Evaluation of workability]
Steels having the components shown in Table 2 were melted and made into steel plates having a thickness of 1.0 mm by normal steel plate manufacturing processes such as hot rolling and cold rolling, and were annealed at 850 ° C. Test pieces having a width of 100 mm and a length of 100 mm were collected from these steel plates, and subjected to a cylindrical drawing test with a drawing ratio of 1.8 to determine whether or not there were cracks. The test results are also shown in Table 2. In the workability in Table 2, ◯ indicates that the result of the cylindrical drawing test was good, and × indicates that cracking occurred in the cylindrical drawing test. The X values in Table 2 are those calculated by the following formula.
[0057]
X = Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179−0.8 × (C / 12 + N / 14)
As is apparent from Table 2, the steels of the present invention with particularly improved workability (Nos. 51 to 90) exhibit good corrosion resistance and good workability. That is, it shows good corrosion resistance in various corrosive environments such as high temperature and wet corrosive environment, condensation corrosive environment, air corrosive environment, tap water corrosive environment, soil corrosive environment, concrete corrosive environment, seawater corrosive environment, drinking water corrosive environment, and Excellent workability. In contrast, it can be seen that the corrosion resistance and workability of the comparative steel (numbers 100 to 109) cannot be achieved at the same time.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
[Table 3]

[0061]
[Table 4]

[0062]
[Table 5]

[0063]
[Table 6]

[0064]
[Table 7]

[0065]
[Table 8]

[0066]
【The invention's effect】
As described above, according to the present invention, for example, an exhaust system of an internal combustion engine such as an automobile, a high temperature wet corrosion environment, a condensation corrosion environment, an air corrosion environment, a tap water corrosion environment, a soil corrosion environment, a concrete corrosion environment, etc. Corrosion-resistant steel having excellent corrosion resistance in various corrosive environments such as seawater corrosive environment and drinking water corrosive environment is provided at low cost.

Claims

In mass%,
Si: 0.01-3.0%,
Mn: 0.01 to 3.0%,
Cr: 10-30%,
Al: 0.1 to 10%,
Mg: 0.0003 to 0.05%,
It consists of the balance Fe and unavoidable impurities, of which C is 0.02% or less, P is 0.03% or less, S is 0.01% or less, and N is 0.02% or less. Excellent corrosion resistance, characterized by forming a metal coating layer having a thickness of 0.5 to 500 μm on the surface of the base material and having a lower potential in the aqueous solution environment than the base material on the surface of the base material. steel.

Steel of the substrate further, in mass%,
Cu: 0.01 to 5.0%,
Mo: 0.05 to 10%,
Sb: 0.01 to 0.5%,
Ni: 0.01 to 10%,
W: 0.05-3.0%
The corrosion-resistant steel according to claim 1, comprising one or more of the following.

Steel of the substrate further, in mass%,
Rare earth elements: 0.001 to 0.1%, Ca: 0.0001 to 0.05%
The corrosion-resistant steel according to claim 1 or 2, comprising one or more of the following.

The steel substrate is further in mass%, Nb, V, Ti, Zr, Ta, containing 0.01% to 1% alone content one or any two or more selected from among Hf The corrosion resistance and corrosion resistant steel according to claim 1, 2 or 3, wherein the following formula is satisfied.
Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179 ≧ 0.8 × (C / 12 + N / 14)

The corrosion-resistant steel according to claim 1, 2, 3, or 4, wherein the metal of the coating layer is aluminum or an alloy mainly composed of aluminum.

The corrosion-resistant steel according to claim 1, 2, 3 or 4, wherein the metal of the coating layer is zinc or an alloy mainly composed of zinc.

The corrosion-resistant steel according to claim 1, 2, 3, or 4, wherein the metal of the coating layer is chromium or an alloy mainly composed of chromium.

The corrosion-resistant steel according to claim 1, 2, 3, or 4, wherein the metal of the coating layer is manganese or an alloy mainly composed of manganese.

Wherein the metal coating layer is further in mass%, according to claim 5, 6, 7 or 8, characterized in that it contains 0.05 to 10% alone content to at least one of Mg and In Corrosion resistant steel.