JP3790398B2

JP3790398B2 - Coated steel with excellent cross section corrosion resistance

Info

Publication number: JP3790398B2
Application number: JP35124999A
Authority: JP
Inventors: 謙治加藤; 一実西村; 亮介和気
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1999-12-10
Filing date: 1999-12-10
Publication date: 2006-06-28
Anticipated expiration: 2019-12-10
Also published as: JP2001164336A

Description

【０００１】
【発明の属する技術分野】
本発明は、加工部断面耐食性に優れた被覆鋼に係り、さらに詳しくは、製品の構造あるいは形状を得るに際して加えられる種々の加工（切断、穴あけ等）部について、例えば、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、橋梁、支柱、鉄塔、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、各種貯蔵タンク、支柱、杭、矢板等の土壌腐食環境、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）、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、缶容器、各種容器、食器、調理機器等の飲料水腐食環境、各種鉄筋構造物、支柱等のコンクリート腐食環境、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加工部断面耐食性に優れた被覆鋼に関する。
【０００２】
【従来の技術】
従来、様々な環境での耐食性の向上を目的に表面処理を施した鋼が使用されてきた。使用環境に応じた表面処理金属を使用することで、処理後表面の耐食性は著しく向上される。
一方で、目的とする製品や部材となすものに際しては様々な加工の行われることが一般的である。たとえば、切断、切削、溶接、穴あけ、プレス、曲げ等の加工が施される。このような加工によって、表面処理層の消失や表面処理層の損傷等が引き起され、加工部の耐食性は加工前の表面処理を施した部分と比較して著しく劣化することが一般的である。すなわち、加工部の耐食性が目的とする製品や部材の耐食寿命を著しく劣化させている。
【０００３】
このような問題に対して、様々な加工部表面処理部の補修技術や表面処理工程の大幅な見直し等の対策が行われている。すなわち、補修では加工部への塗装によるタッチアップ、めっき補修あるいは溶射補修の行われることが多い。しかしながらこれらの方法では加工部以外の本来の表面処理部分と同等の耐食性を得ることは困難な場合が多く、さらには、付加的な工程の増大によって経済性が大きく損われる。一方、表面処理工程の見直しでは、表面処理を施す以前に目的とする製品や部材への加工を行い、加工の完了後に表面処理を施すことが一般的である。この方法では加工部を含めて表面処理による比較的均一な耐食性の確保が可能であるが、加工後の形状によっては本対策が適用できない場合があり、さらには、前処理や表面処理工程が一品ごとの非連続工程となるために、生産性や経済性が阻害される。また、加工後の表面処理によって製品や部材の形状が損われる等の副次的問題を惹起することも少なくない。
【０００４】
このような情況にあって、近年高級ステンレス鋼等の基材に表面処理を施し加工部の耐食性を向上させた鋼が知られている。これらは加工部耐食性の改善には優れた方法と考えられるが、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素材コストも高くなるという難点がある。またこうした高級ステンレス鋼はＣｒやＭｏを大量に含有するために加工性に劣る。そのために、本技術はきわめて特殊な用途に限定して使用されている実態にある。
【０００５】
【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、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様々な腐食環境における加工部のうち、特に切断、穴あけ等の加工部断面（以下、切断、穴あけ等の加工部断面を、単に加工部ともいう。）の耐食性に優れた低コストの被覆鋼を提供することを目的としている。
【０００６】
【課題を解決するための手段】
発明者らは上記の目的を達成すべく、高温湿潤腐食環境、結露腐食環境、さらには大気腐食環境、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加工部断面耐食性を有する被覆鋼を開発するべく、各種金属を表面処理した被覆鋼の機械的切断端面について種々の観点から検討してきた。
【０００７】
まず、発明者らは、表面処理鋼の切断加工部の腐食にもっとも厳しい環境条件は、従来表面処理表面に対してもっとも厳しい腐食環境と考えられてきた塩害環境や大量の塩を含む海水飛沫などの環境ではなく、これらに比較してきわめて低濃度の塩を含む結露−乾燥が繰返される腐食環境であることを見いだした。これは塩害環境等の大量の塩を含む環境では、加工部と表面処理層の間で高い導電率を有する腐食環境が生成され、表面処理層の切断加工部への電気化学的防食作用が生じ、加工部が防食されるのに対して、きわめて低濃度の塩を含む結露−乾燥腐食環境では加工部と表面処理層の間の導電率がきわめて低いために、表面処理層の切断加工部への電気化学的防食作用が生じにくいことによることを明らかにした。
【０００８】
発明者らは、このようなきわめて低濃度の塩を含む結露−乾燥腐食環境で加工部の腐食抵抗を高めた表面処理鋼材を得る手段について検討した結果、Ｃｒを０．１〜９．９％含有し、Ａｌを０．１％以上、Ｍｇを０．０００３％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、優れた切断加工部耐食性が得られることを見出した。見出した著しい切断加工部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とＭｇを添加することで、きわめて低濃度の塩を含む結露−乾燥腐食環境であっても、電位が基材よりも卑なる金属の層の切断加工部に対する電気化学的なものと考えられる保護作用が発現しこれ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。
【０００９】
以上の基本的な新たな知見に基づいて、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Crを含有する基材金属中へのＡｌとＭｇの同時添加による加工部耐食性向上効果をさらに高め、さらに、加工性を向上させるための手段について種々検討を重ね、基材へのＳｉ，Ｍｎ，Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆ，Ｃｕ，Ｍｏ，Ｓｂ，Ｎｉ，Ｗの添加がより一層の加工部耐食性向上に有効であることを見いだした。
さらに、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、亜鉛にＡｌを０．１〜５５％含有する合金、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。
【００１０】
これらの被覆金属についても、基材との相互作用による加工部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、亜鉛にＡｌを０．１〜５５％含有する合金、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好ましくはＭｇ，Ｓｉ，Ｉｎの１種以上を、重量％でそれぞれ０．０５％以上、１０％以下含有せしめたものがより一層優れた加工部耐食性を実現することを見出した。
【００１１】
さらに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０．５〜９．９％、Ａｌを０．１〜１０％、Ｍｇを０．０００３〜０．１％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、加工部耐食性の改善を損うことなく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が適切であることを見出した。
【００１２】
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、その発明の要旨とするところは、以下のとおりである。
（１）質量％で、Ｃ：０．０２％以下、Ｓｉ：０．０１〜３％、Ｍｎ：０．１〜３％、Ｃｒ：０．１〜９．９％、Ａｌ：０．１〜１０％、Ｍｇ：０．０００３〜０．１％、Ｐ：０．０３％以下、Ｓ：０．０１％以下、Ｎ：０．０２％以下を含有し、残部Ｆｅおよび不可避的不純物からなる鋼を基材として、少なくとも使用環境に曝される面上に、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、アルミニウムもしくはアルミニウム基合金のいずれか、または亜鉛、亜鉛基合金もしくはＡｌを０．１〜５５％含有する亜鉛合金のいずれか、またはマンガンもしくはマンガン基合金のいずれかからなる厚さ０．５〜５００μｍの表面処理層を有することを特徴とする、表面処理層の消失、損傷が引き起こされた加工部断面耐食性に優れた被覆鋼。
【００１３】
（２）さらに基材に、質量％で、Ｃｕ：０．０５〜５％、Ｍｏ：０．０５〜１０％、Ｓｂ：０．０１〜０．５％、Ｎｉ：０．０１〜１０％、Ｗ：０．０５〜３％の１種または２種以上を含有することを特徴とする、前記（１）に記載の加工部断面耐食性に優れた被覆鋼。
（３）さらに基材に、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、前記（１）または（２）に記載の加工部断面耐食性に優れた被覆鋼。
【００１４】
（４）さらに、質量％で、Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆの１種または２種以上をそれぞれ０．０１〜１％含有し、かつ次式を満足することを特徴とする、前記（１）〜（３）に記載の加工部断面耐食性に優れた鋼。
Ｎｂ／９３＋Ｖ／５１＋Ｔｉ／４８＋Ｚｒ／９１＋Ｔａ／１８１＋Ｈｆ／１７９≧０．８×（Ｃ／１２＋Ｎ／１４）
【００１５】
（５）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、さらにＭｇ、Ｓｉ、Ｉｎの１種以上を、質量％でそれぞれ０．０５％以上、１５％以下含有することを特徴とする、前記（１）〜（４）のいずれかに記載の加工部断面耐食性に優れた被覆鋼。
【００１６】
【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、Ａｌを０．１％以上，Ｍｇを０．０００３％以上、Ｃｒを０．１％以上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の加工部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。さらにＣｒを０．１％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０．０１％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。
【００１７】
Ｍｎ：Ｍｎは、Ａｌを０．１％以上，Ｍｇを０．０００３％以上、Ｃｒを０．１％以上含有する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加工部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。さらに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。
【００１８】
Ｃｒ：Ｃｒは、Ａｌを０．１％以上、Ｍｇを０．０００３％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加工部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、一方１０％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０．１％以上９．９％以下に限定する。
【００１９】
Ａｌ：Ａｌは、本発明において加工部耐食性を確保するためにＭｇとともに最も重要な元素であって、Ｍｇを０．０００３％以上、Ｃｒを０．１％以上９．９％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加工部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。
【００２０】
Ｍｇ：Ｍｇは、本発明において加工部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０．１％以上９．９％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、０．１％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０．１％以下に限定する。
【００２１】
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は靭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。
Ｐ：Ｐは、多量に存在すると靭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。
【００２２】
Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，Ｈｆ：Ｎｂ，Ｖ，Ｔｉ，Ｚｒ，Ｔａ，ＨｆはＡｌを０．１％以上、Ｍｇを０．０００３％以上、Ｃｒを０．１％以上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加工部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、一方それぞれ１％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の含有量はそれぞれ０．０１％以上１．０％以下に限定する。
【００２３】
さらに、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鋼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はそれぞれ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。
Ｎｂ／９３＋Ｖ／５１＋Ｔｉ／４８＋Ｚｒ／９１＋Ｔａ／１８１＋Ｈｆ／１７９≧０．８×（Ｃ／１２＋Ｎ／１４）
以上が本発明が対象とする加工部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、必要に応じてさらに以下の元素を添加して加工部耐食性を一段と向上させた鋼材も対象としている。
【００２４】
Ｃｕ：Ｃｕは、Ａｌを０．１％以上、Ｍｇを０．０００３％以上含有しＣｒを０．１％以上９．９％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加工部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。さらに０．１％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、０．１％以上２．５％以下の範囲で添加することがより望ましい。
【００２５】
Ｍｏ：ＭｏはＡｌを０．１％以上、Ｍｇを０．０００３％以上含有しＣｒを０．１％以上９．９％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加工部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。さらに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％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。
【００２６】
Ｗ：ＷはＡｌを０．１％以上、Ｍｇを０．０００３％以上含有しＣｒを０．１％以上９．９％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加工部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。さらに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％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。
【００２７】
Ｓｂ：Ｓｂは、Ａｌを０．１％以上、Ｍｇを０．０００３％以上含有しＣｒを０．１％以上９．９％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加工部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。さらにＳｂを添加することで、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、０．０１％以上０．３％以下の範囲で添加することがより望ましい。
【００２８】
Ｎｉ：Ｎｉは、Ａｌを０．１％以上、Ｍｇを０．０００３％以上含有しＣｒを０．１％以上９．９％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加工部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。さらにＮｉを添加することで、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。
【００２９】
希土類元素（ＲＥＭ），Ｃａ：ＲＥＭやＣａはＡｌを０．１％以上、Ｍｇを０．０００３％以上含有しＣｒを０．１％以上９．９％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加工部耐食性を向上する効果をもたらすが、Ｃａでは０．０００５％未満では効果が認められず、ＲＥＭでは０．００１％未満では効果が認められず、一方希土類元素では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０．０００５％以上０．０５％以下にそれぞれ限定する。
【００３０】
さらに希土類元素および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、添加量が希土類元素では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０．０３％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、上限含有量は希土類元素では０．０５％、Ｃａでは０．０３％とした。従って、Ｃａは０．００５％以上０．０３％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。
【００３１】
本発明においては、本発明に係る鋼が使用される場合において、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ものである。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とＭｇを添加することによる、電位が基材よりも卑なる金属の層の基材加工部に対する保護作用が長期にわたって継続するといった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、被覆の厚さは０．５〜５００μｍとする。
【００３２】
被覆に供される、水溶液環境における電位が基材よりも卑なる金属の実施態様としては、アルミニウム、亜鉛、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。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、塗装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、本発明の被覆および処理の前後にいかなる処理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。
【００３３】
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０．０１〜０．３％程度の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。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。
【００３４】
さらに、これらの被覆金属中にＭｇ，Ｓｉ，Ｉｎの１種以上を含有させることで、加工部耐食性が、さらに向上することを見いだしている。これらの元素の加工部耐食性向上の理由については不明点が多いが、それぞれ質量％で０．０５％未満では効果が顕著ではなく、一方１５％を越えて添加しても効果が飽和するばかりか経済性、製造性を損なうことから、これらのそれぞれの元素の添加量は０．０５〜１５％とする。
【００３５】
また、使用上の目的からは、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おいて、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、本発明の目的と効果を何等逸脱するものではなく、かかる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特性を考慮して選択すれば良い。
【００３６】
上記被覆の実施様態としては、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、本発明の被覆や処理を行うことはもちろんのこと、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、さらに加工・溶接して製品として製造しても良い。また、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に使用しても良い。さらに、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る。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、コストや既存製造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。
以上の本発明において提案する鋼は、結露腐食環境、高温湿潤腐食環境、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に適用することができる。
【００３７】
【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、表５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厚さ５ｍｍ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２〜４、表６〜８に示した被覆１はアルミニウム被覆、被覆２は亜鉛被覆（０．２％Ａｌ）、被覆３はＺｎ−Ａｌ被覆（５５％Ａｌ）、被覆４はマンガン被覆をそれぞれ示す。
次に、これらの鋼板から幅５０ｍｍ、長さ７０ｍｍのシャー切断試験片を採取して、シャー切断ままの加工部を露出したままの状態で、以下に述べる各種の腐食試験に供し、厚さ５ｍｍの切断加工部の耐食性を評価した。
【００３８】
高温湿潤腐食試験は、硫酸イオン１０００ｐｐｍ、塩化物イオン１０００ｐｐｍ、重炭酸イオン５０００ｐｐｍ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２、表６に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○は発錆面積率が１０％未満、△は発錆面積率が２０％未満、×は２０％以上であったことをそれぞれ示す。
表２、表６から明らかなように、本発明鋼である表２、表６中のＮｏ１〜４０、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、比較鋼である表２、表６中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００３９】
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０．５％ＮａＣｌを含む塩水噴霧後、６０℃で湿度９６％の環境に５時間保持した後、さらに１時間の冷凍保持を行うことを３００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。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○は発錆面積率が５％未満、△は発錆面積率が１５％未満、×は１５％以上であったことをそれぞれ示す。
表２、表６から明らかなように、本発明鋼である表２、表６中のＮｏ１〜４０、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、比較鋼である表２、表６中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４０】
土壌腐食試験は、含水率１５％、比抵抗２８０Ω・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、５５℃に保持して約１２００日放置する試験とした。試験結果を表２、表６に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○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。
表２、表６から明らかなように、本発明鋼である表２、表６中のＮｏ１〜４０、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、比較鋼である表２、表６中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４１】
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コンクリート中に試験片を埋め込みサンプルとなし、凝固させた後、純水中にサンプルを半分まで浸漬し、６０℃の環境に約１０００日放置し、試験期間の終了後コンクリートを破壊してサンプルを観察する試験とした。試験結果を表２、表６に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○は発錆面積率が５％未満、△は発錆面積率が２０％未満、×は２０％以上であったことをそれぞれ示す。
表２、表６から明らかなように、本発明鋼である表２、表６中のＮｏ１〜４０、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、比較鋼である表２、表６中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４２】
水道水環境腐食試験は、流動加温水道水中に試験片を浸漬し、５５℃の雰囲気に２４ケ月間保持する試験とした。試験結果を表３、表７に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○は発錆面積率が１０％未満、△は発錆面積率が２０％未満、×は２０％以上であったことをそれぞれ示す。
表３、表７から明らかなように、本発明鋼である表３、表７中のＮｏ１〜４０、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、比較鋼である表３、表７中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４３】
海水環境腐食試験は、海岸の海水飛沫帯に試験片を３０ケ月間暴露する試験とした。試験結果を表３、表７に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○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。
表３、表７から明らかなように、本発明鋼である表３、表７中のＮｏ１〜４０、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、比較鋼である表３、表７中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４４】
結露腐食試験は、−２０℃の環境に２時間保持後湿度９８％、４０℃の環境に４時間保持することを１８００回繰り返す試験とした。試験結果を表３、表７に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○は発錆面積率が５％未満、△は発錆面積率が２０％未満、×は２０％以上であったことをそれぞれ示す。
表３、表７から明らかなように、本発明鋼である表３、表７中のＮｏ１〜４０、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、比較鋼である表３、表７中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４５】
大気腐食試験は、海岸から約１．５ｋｍ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３、表７に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○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。
表３、表７から明らかなように、本発明鋼である表３、表７中のＮｏ１〜４０、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、比較鋼である表３、表７中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４６】
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５０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のマンガン被覆について試験を実施した。試験結果を表４、表８に併せて示した。腐食試験結果の◎は切断加工部に腐食の発生が認められなかったもの、さらに切断加工部の全面積に対して○は発錆面積率が１％未満、△は発錆面積率が５％未満、×は５％以上であったことをそれぞれ示す。
表４、表８から明らかなように、本発明鋼である表４、表８中のＮｏ１〜４０、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、比較鋼である表４、表８中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４７】
すなわち本発明鋼である表１〜表８中のＮｏ１〜４０、５１〜９０および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、比較鋼である表１〜表８中のＮｏ４１〜５０、９１〜１００は耐食性に劣ることがわかる。
【００４８】
［加工性の評価］
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５に併せて示した。表５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５中のＸ値は、次式によって算出したものを記載した。
Ｘ＝Ｎｂ／９３＋Ｖ／５１＋Ｔｉ／４８＋Ｚｒ／９１＋Ｔａ／１８１＋Ｈｆ／１７９−０．８×（Ｃ／１２＋Ｎ／１４）
【００４９】
表５から明らかなように、本発明鋼である表５中のＮｏ５１〜９０は良好な酸洗性を示し、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のに対して、比較鋼である表５中のＮｏ９１〜１００は耐食性と加工性が同時に達成できないことがわかる。
【００５０】
【表１】

【００５１】
【表２】

【００５２】
【表３】

【００５３】
【表４】

【００５４】
【表５】

【００５５】
【表６】

【００５６】
【表７】

【００５７】
【表８】

【００５８】
【発明の効果】
以上述べたように、本発明は、結露腐食環境をはじめとして、高温湿潤腐食環境、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において、加工部断面耐食性および加工性に優れる被覆鋼を低コストで提供することを可能としたものであり、産業の発展に貢献するところ極めて大である。[0001]
BACKGROUND OF THE INVENTION
The present invention provides a machining section cross section Excellent corrosion resistance Coating It relates to steel, and more specifically, various processes (cutting, drilling, etc.) that are added when obtaining the structure or shape of the product Such as ), For example, internal combustion engine exhaust systems such as automobiles and ships, boiler exhaust systems, low-temperature heat exchangers, incinerator floors and other hot and humid corrosive environments, bridges, struts, steel towers, building interior and exterior materials, roofing materials, fittings, Kitchen parts, various handrails, guardrails, various hooks, roof drains, atmospheric corrosion environments such as railway vehicles, various storage tanks, struts, piles, sheet piles and other soil corrosion environments, can containers, various containers, low-temperature heat exchangers, bathroom members, Condensation corrosive environment such as automobile structural members (including corrosive environment where freezing, wetting and drying are combined) , Corrosion environment for tap water such as water storage tanks, water supply pipes, hot water supply pipes, can containers, various containers, tableware, cooking equipment, bathtubs, pools, vanities, etc., drinking water corrosive environments such as can containers, various containers, tableware, cooking equipment Excellent processed parts in various corrosive environments such as various corrosive environments such as various reinforcing steel structures, concrete corrosive environments such as props, seawater corrosive environments such as ships, bridges, piles, sheet piles, marine structures cross section Excellent corrosion resistance Coating Related to steel.
[0002]
[Prior art]
Conventionally, steel that has been surface-treated for the purpose of improving corrosion resistance in various environments has been used. By using a surface-treated metal according to the usage environment, the corrosion resistance of the treated surface is significantly improved.
On the other hand, it is common that various processing is performed for a product or member to be obtained. For example, processes such as cutting, cutting, welding, drilling, pressing, and bending are performed. Such processing causes disappearance of the surface treatment layer, damage to the surface treatment layer, and the like, and the corrosion resistance of the processed portion is generally significantly deteriorated compared to the portion subjected to the surface treatment before processing. . That is, the corrosion resistance life of the product or member intended for the corrosion resistance of the processed portion is significantly deteriorated.
[0003]
In order to deal with such problems, various countermeasures such as a repair technique for various surface treatment parts and a substantial review of the surface treatment process have been taken. That is, in the repair, touch-up by painting on the processed part, plating repair or thermal spray repair is often performed. However, in these methods, it is often difficult to obtain the same corrosion resistance as that of the original surface treatment portion other than the processed portion, and furthermore, the economical efficiency is greatly impaired due to an increase in additional steps. On the other hand, in reviewing the surface treatment process, it is common to perform processing on a target product or member before performing the surface treatment and to perform the surface treatment after the processing is completed. In this method, it is possible to ensure relatively uniform corrosion resistance by surface treatment including the processed part, but this measure may not be applied depending on the shape after processing, and furthermore, pretreatment and surface treatment processes are one item. Therefore, productivity and economy are hindered. Moreover, secondary problems such as the shape of products and members being damaged by surface treatment after processing are often caused.
[0004]
Under such circumstances, a steel in which a surface treatment is performed on a base material such as high-grade stainless steel to improve the corrosion resistance of a processed part is known in recent years. Although these are considered to be excellent methods for improving the corrosion resistance of the processed part, they are accompanied by a very difficult manufacturing process, and the manufacturing process is significantly complicated, resulting in a problem that the material cost increases. Further, such high-grade stainless steel contains a large amount of Cr and Mo, so that it is inferior in workability. For this reason, the present technology is actually used only for a very special purpose.
[0005]
[Problems to be solved by the invention]
In view of the present situation, the present invention provides various corrosive environments such as a high temperature and wet corrosive environment, a dew corrosive environment, an atmospheric corrosive environment, a tap water corrosive environment, a soil corrosive environment, a concrete corrosive environment, a seawater corrosive environment, and a drinking water corrosive environment. Machining part Among them, in particular, a cross section of a processed part such as cutting and drilling (hereinafter, a cross section of a processed part such as cutting and drilling is also simply referred to as a processed part). Low cost with excellent corrosion resistance Coating The aim is to provide steel.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the inventors have developed a high temperature and wet corrosion environment, 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, a drinking water corrosion environment, and the like. Excellent machined parts in various corrosive environments cross section Has corrosion resistance Coating Surface treatment of various metals to develop steel Coating We examined the mechanically cut end surface of steel from various viewpoints. The Came.
[0007]
First, the inventors have found that the most severe environmental conditions for the corrosion of the cut portion of the surface-treated steel are the salt damage environment that has been considered to be the most severe corrosive environment for the conventional surface-treated surface, and seawater droplets containing a large amount of salt. It was found that this is a corrosive environment in which condensation-drying containing extremely low concentration of salt is repeated. This is because, in an environment containing a large amount of salt, such as a salt damage environment, a corrosive environment having high conductivity is generated between the processed part and the surface treatment layer, and an electrochemical anticorrosive action occurs on the cut part of the surface treatment layer. In contrast, the processed part is protected against corrosion, but in a condensation-dry corrosion environment containing a very low concentration of salt, the conductivity between the processed part and the surface treatment layer is extremely low. It was clarified that the electrochemical anticorrosive action of was difficult to occur.
[0008]
As a result of studying means for obtaining a surface-treated steel material having an increased corrosion resistance of a processed part in a condensation-dry corrosion environment containing such a very low salt concentration, the inventors have found Cr to be 0.1 to 9.9%. When a steel layer containing 0.1% or more of Al and 0.0003% or more of Mg is used as a base material, a metal layer whose potential in an aqueous solution environment is lower than that of the base material is formed on the surface. The corrosion resistance of the cut part can be obtained. Find did. There are many unclear points at present regarding the reason for the significant improvement in corrosion resistance of the cut parts found, but by adding Al and Mg to the base material, even in a condensation-dry corrosion environment containing extremely low concentrations of salt, the potential However, the protective action considered to be electrochemical with respect to the cut processed portion of the metal layer which is lower than the base material is developed, and it is confirmed that the corrosion resistance is improved by continuing this over a long period of time.
[0009]
Based on the above basic new findings, the inventors have found that Al and Mg in a base metal containing Cr in which a metal layer having a potential lower than that of the base exists on the surface of the base. Further improving the effect of improving the corrosion resistance of the processed part by the simultaneous addition of N and various studies on means for improving the workability, Si, Mn, Nb, V, Ti, Zr, Ta, Hf, Cu to the substrate , Mo, Sb, Ni and W were found to be effective for further improving the corrosion resistance of the processed part.
Furthermore, the inventors continue to study, 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, 0.1 to 0.1% in Al. It has also been found that an alloy containing 55%, manganese and an alloy mainly composed of manganese are suitable metals for the purpose of the present invention.
[0010]
As for these coated metals, as a result of various investigations from the viewpoint of improving the corrosion resistance of the processed part due to the interaction with the base material, aluminum, an alloy mainly composed of aluminum, zinc, an alloy mainly composed of zinc, and Al in zinc. Any metal such as an alloy containing 0.1 to 55% of manganese, an alloy mainly composed of manganese, or manganese, preferably one or more of Mg, Si, and In, 0.05% or more and 10% by weight, respectively. It has been found that what is contained below realizes a further excellent processed portion corrosion resistance.
[0011]
Furthermore, as a result of continuous investigations by the inventors as a superior steel, Cr contains 0.5 to 9.9%, Al contains 0.1 to 10%, and Mg contains 0.0003 to 0.1%. Addition of Nb, V, Ti, Zr, Ta, and Hf to satisfy specific conditions while reducing C and N in steel is effective in improving workability without impairing improvement in corrosion resistance of the processed part. It has been found that Si and Mn are suitable as deoxidation and strengthening elements.
[0012]
The present invention has been made mainly based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.02% or less, Si: 0.01 to 3%, Mn: 0.1 to 3%, Cr: 0.1 to 9.9%, Al: 0.1 Steel containing 10%, Mg: 0.0003 to 0.1%, P: 0.03% or less, S: 0.01% or less, N: 0.02% or less, the balance being Fe and inevitable impurities As a base material, at least on the surface exposed to the use environment, as a metal whose electric potential in an aqueous solution environment is lower than that of the base material, either aluminum or an aluminum base alloy, or zinc, a zinc base alloy or Al is added to the base. It has a surface treatment layer having a thickness of 0.5 to 500 μm made of either zinc alloy containing 1 to 55% or manganese or a manganese-based alloy. The Coated steel with excellent cross-section corrosion resistance due to disappearance and damage of the surface treatment layer.
[0013]
(2) Further, the base material is, in mass%, Cu: 0.05 to 5%, Mo: 0.05 to 10%, Sb: 0.01 to 0.5%, Ni: 0.01 to 10%, W: 0.05 to 3% of one or more kinds are contained , Processed part according to (1) cross section Coated steel with excellent corrosion resistance.
(3) The base material further contains one or more rare earth elements: 0.001 to 0.1% and Ca: 0.0001 to 0.05% by mass%. , Processed portion according to (1) or (2) cross section Coated steel with excellent corrosion resistance.
[0014]
(4) Further, it is characterized by containing 0.01 to 1% of one or more of Nb, V, Ti, Zr, Ta, and Hf by mass% and satisfying the following formula: , Processed part as described in said (1)-(3) cross section Steel with excellent corrosion resistance.
Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179 ≧ 0.8 × (C / 12 + N / 14)
[0015]
(5) As a metal whose electric potential in an aqueous solution environment is lower than that of a base material, it further contains one or more kinds of Mg, Si, and In by 0.05% by mass and 15% or less, respectively. , Processed part in any one of said (1)-(4) cross section Coated steel with excellent corrosion resistance.
[0016]
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 is a layer of metal whose electric potential in an aqueous solution environment is lower than that of a base material on the surface of a steel base material containing 0.1% or more of Al, 0.0003% or more of Mg, and 0.1% or more of Cr Brings about the effect of improving the corrosion resistance of the processed part when formed to a thickness of 0.5 to 500 μm, but the effect is not observed if it is less than 0.01%, and the effect is saturated even if added over 3% To do. Therefore, the content range is limited to 0.01% or more and 3% or less. Furthermore, it is effective to add Si as a deoxidizer and strengthening element by adding Si to steel containing 0.1% or more of Cr. However, if the content is less than 0.01%, the deoxidation effect is not sufficient. If the content is 1.5% 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%.
[0017]
Mn: Mn is a metal layer whose potential in an aqueous solution environment is lower than that of the substrate on the surface of the substrate containing 0.1% or more of Al, 0.0003% or more of Mg and 0.1% or more of Cr. When it is formed to have a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance of the processed part is brought about. However, the effect is not observed when it is less than 0.01%, and the effect is saturated even if it is added in excess of 3%. 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.
[0018]
Cr: Cr is a steel layer containing 0.1% or more of Al and 0.0003% or more of Mg. By adding Cr to the surface of the base material, a metal layer whose potential in an aqueous solution environment is lower than that of the base material. When it is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance of the processed part is brought about. However, if it is less than 0.1%, the effect is not sufficient. Therefore, the Cr content is limited to 0.1% or more and 9.9% or less.
[0019]
Al: Al is the most important element together with Mg in order to ensure the corrosion resistance of the processed part in the present invention, and is a steel containing Mg of 0.0003% or more and Cr of 0.1% or more and 9.9% or less. By adding Al, when the metal layer whose potential in the 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 of the processed part is brought about. If it is less than 0.1%, the effect is not sufficient, and even if added over 10%, the effect is saturated, so the Al content is limited to 0.1% or more and 10% or less.
[0020]
Mg: Mg is an important element next to Al in order to ensure the corrosion resistance of the processed part in the present invention, and contains 0.1 to 10% Al and 0.1 to 9.9% Cr. In addition, the addition of Mg brings about the effect of improving the corrosion resistance when a metal layer having a lower potential in the aqueous solution environment on the surface of the substrate is formed to a thickness of 0.5 to 500 μm, If less than 0.0003%, the effect is not sufficient, and even if added over 0.1%, the effect is saturated, so the Mg content is 0.000. 3 % To 0.1% or less.
[0021]
C, N: C and N lower the workability of the steel sheet, and C generates Cr and carbides to lower the corrosion resistance. N also lowers the toughness. Desirably, the upper limit content is 0.02% for all, and the lower the content, the better.
P: If P is present in a large amount, the toughness is lowered, so a smaller amount 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, and the upper limit content is 0.01%.
[0022]
Nb, V, Ti, Zr, Ta, Hf: Nb, V, Ti, Zr, Ta, and Hf are steels containing Al of 0.1% or more, Mg of 0.0003% or more, and Cr of 0.1% or more. Is added to the surface of the base material, the effect of improving the corrosion resistance of the processed part when a metal layer having a lower potential in the aqueous solution environment than the base material is formed to a thickness of 0.5 to 500 μm. If both elements are less than 0.01%, the effect is not recognized. On the other hand, even if each element is added in excess of 1%, the effect is saturated. Therefore, the contents of Nb, V, Ti, Zr, Ta, and Hf are limited to 0.01% or more and 1.0% or less, respectively.
[0023]
Further, Nb, V, Ti, Zr, Ta, and Hf have a remarkable effect in improving the corrosion resistance and workability of the base steel by fixing C and N in the Cr-containing steel as carbides. Single addition or a combination of two or more elements can be added, but if the addition amount alone is less than 0.05%, there is no effect, and adding more than 0.8% unnecessarily increases the cost. And cause rolling wrinkles. Therefore, it is more desirable to add Nb, V, Ti, Zr, Ta, and Hf 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.
Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179 ≧ 0.8 × (C / 12 + N / 14)
The above is the basic component of the steel base material excellent in processed part corrosion resistance targeted by the present invention.In the present invention, the following elements are further added as necessary to further improve the processed part corrosion resistance. Steel materials are also targeted.
[0024]
Cu: Cu contains 0.1% or more of Al, 0.0003% or more of Mg, and 0.1% or more and 9.9% or less of Cr. When a base metal layer is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance of the processed part is brought about. However, when the content is less than 0.01%, the effect is not recognized, while the content exceeding 5% is added. But the effect is saturated. Therefore, the range is limited to a range of 0.01% to 5%. Further, when 0.1% or more is added, there is an effect of improving the resistance against the general corrosion of the base material alone, and when the content exceeds 2.5%, the effect is saturated. Therefore, it is more desirable to add in the range of 0.1% to 2.5%.
[0025]
Mo: Mo is added to steel containing 0.1% or more of Al, 0.0003% or more of Mg and 0.1% or more and 9.9% or less of Cr. Has an effect of improving the corrosion resistance of the processed part when a metal layer that is lower than the base material is formed to a thickness of 0.5 to 500 μm. However, if it is less than 0.05%, the effect is not recognized, whereas 10% The effect is saturated even if added in excess of. Therefore, the range is limited to 0.05% or more and 10% or less. Furthermore, if 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 more than 3% not only saturates the effect but also reduces the workability. Let Therefore, it is more desirable to add in the range of 0.1% to 3%.
[0026]
W: When 0.05% or more is added to steel containing 0.1% or more of Al, 0.0003% or more of Mg and 0.1% or more and 9.9% or less of Cr, W is an aqueous solution on the substrate surface. When the metal layer whose potential in the environment is lower than that of the base material is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance of the processed part is brought about. Is saturated. 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 if it exceeds 2%, it is also effective Not only saturates, but also reduces workability. Therefore, it is more desirable to add in the range of 0.1% to 2%.
[0027]
Sb: When Sb is added to a steel containing 0.1% or more of Al, 0.0003% or more of Mg and 0.1% or more and 9.9% or less of Cr, When the metal layer whose potential in the aqueous solution environment is lower than that of the base material is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance of the processed portion is brought about. However, the effect is not recognized at less than 0.01%. On the other hand, even if added over 0.5%, the effect is saturated. Therefore, the range is limited to 0.01% or more and 0.5% or less. Further, by adding Sb, there is an effect of improving the resistance against pitting corrosion and overall corrosion of the base material alone, but when it exceeds 0.3%, the hot workability is slightly lowered. Therefore, 0.0 1% It is more desirable to add in the range of 0.3% or less.
[0028]
Ni: When Ni is added to steel containing 0.1% or more Al, 0.0003% or more Mg and 0.1% or more and 9.9% or less Cr, When the metal layer whose potential in the aqueous solution environment is lower than that of the base material is formed to a thickness of 0.5 to 500 μm, the effect of improving the corrosion resistance of the processed part is brought about. The effect is saturated. Therefore, the range is limited to 0.01% or more and 10% or less. Further, by adding Ni, there is an effect of suppressing pitting corrosion of 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%.
[0029]
Rare earth elements (REM), Ca: REM and Ca are added to steel containing 0.1% or more of Al, 0.0003% or more of Mg and 0.1% or more and 9.9% 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 of the processed part is brought about. However, in Ca, less than 0.0005% In REM, the effect is not recognized if it is less than 0.001% in REM, while the effect is saturated even if it is added in excess of 0.1% for rare earth elements and 0.05% for Ca. To do. 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 0.0005% or more and 0.05% or less.
[0030]
Furthermore, rare earth elements and Ca are elements that are effective in improving hot workability and improving the pitting corrosion resistance of the base material alone, but the addition amount is less than 0.01% for rare earth elements and 0.005% for Ca. If the ratio is less than 0.0, the effect is not sufficient. 3 %, In the case of adding more than 0.05% in REM, each produces coarse non-metallic inclusions and conversely degrades hot workability and pitting corrosion resistance. 0.05 %, Ca was 0.03%. Therefore, it is more preferable to add Ca in the range of 0.005% to 0.03% 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.
[0031]
In the present invention, when the steel according to the present invention is used, at least the surface exposed to the corrosive environment is covered with a metal having a lower potential than the base material. With a thickness of 0.5 μm or less covering with a metal whose potential is lower than that of the base material, by adding Al and Mg to the base material, the base material processed portion of the metal layer whose potential is lower than that of the base material The effect of the protective action lasting for a long time is not enough, and even if it is coated to a thickness exceeding 500 μm, the effect is no longer saturated, but the productivity is lowered and the cost is increased. Therefore, the thickness of the coating is 0.5 to 500 μm.
[0032]
As an embodiment of the metal that is used for coating and has a lower potential in the aqueous solution environment than the base material, aluminum, zinc, manganese, and alloys based on these can be used. 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 application and cost, and hot-dip plating, electrodeposition plating, hot-salt electroplating, vacuum deposition, sputtering, ion plating, thermal spraying, coating, etc. can be used, and they are used in combination. It is also possible. Also, no matter what treatment is selected before or after the coating and treatment of the present invention, it does not depart from the scope of the present invention.
[0033]
An 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 generally 0.01 to 0.3% aluminum or the like contained in the zinc-based alloy. These alloy components and impurity components may be included.
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. 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. .
[0034]
Furthermore, it has been found that the corrosion resistance of the processed part is further improved by including one or more of Mg, Si, and In in these coated metals. Corrosion resistance of processed parts of these elements Improvement There are many unclear points about the reasons, quality If the amount is less than 0.05%, the effect is not remarkable. On the other hand, the addition of more than 15% not only saturates the effect but also impairs the economy and manufacturability. 0.05 to 15%.
[0035]
In addition, for the purpose of use, only one side is covered from the process of covering the base metal when it is sufficient to cover only one side of the material having front and back surfaces such as steel pipes and plate materials. The use of steel does not depart from the purpose and effect of the present invention. In such a case, whether to use a coating on only one side or to use a steel coated on both sides is cost or weldability. It may be selected in consideration of other characteristics.
[0036]
The embodiment of the coating is not limited to the general shape of steel materials such as coils, plates, rods, cables, perforated steel pipes, and the coating and processing of the present invention, as well as the coated and processed book. Invention steel may be formed into a predetermined shape by pressing or roll forming, and then processed and welded to produce a product. . Also The steel of the present invention may be used as a product by secondary processing, welding, etc., after first forming the shape of a steel pipe, for example, as an electric resistance steel pipe . Furthermore, it is also possible to apply the surface coating treatment of the present invention after making the steel material into the target shape by the above-described process before performing the coating and processing of the present invention. The Steels having a combination of composition and processing conditions limited in the present invention, including other processes, are all subject to the present invention, and an optimal product manufacturing process is selected depending on costs, constraints on existing manufacturing facilities, etc. Whichever manufacturing process is selected does not depart from the scope of the present invention.
The steel proposed in the present invention has various corrosive environments such as a dew corrosive environment, a hot and humid corrosive environment, an atmospheric corrosive environment, a tap water corrosive environment, a soil corrosive environment, a concrete corrosive environment, a seawater corrosive environment, and a drinking water corrosive environment. Can be applied to.
[0037]
【Example】
Examples of the present invention will be described below.
[Evaluation of corrosion resistance]
After melting steels having the components shown in Tables 1 and 5 and performing annealing at 900 ° C. with a steel sheet having a thickness of 5 mm by a normal steel sheet manufacturing process such as hot rolling and cold rolling, The coating was applied under the condition of 15 ± 2 μm. The coating 1 shown in Tables 2 to 4 and Tables 6 to 8 is an aluminum coating, the coating 2 is a zinc coating (0.2% Al), the coating 3 is a Zn-Al coating (55% Al), and the coating 4 is a manganese coating. Each is shown.
Next, a shear cutting test piece having a width of 50 mm and a length of 70 mm was collected from these steel plates, and subjected to various corrosion tests described below in a state in which the processed part as it was cut from the shear was exposed, and the thickness was 5 mm. The corrosion resistance of the cut processed part was evaluated.
[0038]
In the high temperature wet corrosion test, the test piece was immersed in half in 50 cc of an aqueous solution in which 1000 ppm of sulfate ions, 1000 ppm of chloride ions and 5000 ppm of bicarbonate ions 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 shown in Tables 2 and 6. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ◯ indicates a rust area ratio of less than 10% and △ indicates a rust area ratio of 20% with respect to the entire area of the cut part. Less than, x indicates 20% or more, respectively.
As is apparent from Tables 2 and 6, Nos. 1 to 40 and 51 to 90 in Tables 2 and 6 which are the steels of the present invention have good corrosion resistance even in a very severe corrosive environment of high temperature humidity containing chloride. It can be seen that No. 41 to 50 and 91 to 100 in Tables 2 and 6 which are comparative steels are inferior in corrosion resistance.
[0039]
In addition, as a test assuming salt damage corrosion of the air environment or the exterior surface of an automobile exhaust system, after spraying with salt water containing 0.5% NaCl at 50 ° C. for 1 hour, it is kept at 60 ° C. in a 96% humidity environment for 5 hours. In addition, a salt damage corrosion test was repeated 300 times to keep frozen for 1 hour. The maximum pitting corrosion depth of the test piece after the test was measured and used as the test result. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ○ indicates a rust area ratio of less than 5% and △ indicates a rust area ratio of 15% with respect to the entire area of the cut part. Less than, x indicates 15% or more.
As is apparent from Tables 2 and 6, Nos. 1 to 40 and 51 to 90 in Tables 2 and 6 of the present invention steel show good corrosion resistance even in a very severe corrosive environment called salt damage corrosion. On the other hand, it is understood that No. 41 to 50 and 91 to 100 in Tables 2 and 6 which are comparative steels are inferior in corrosion resistance.
[0040]
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 280 Ω · cm and a sodium chloride content, and kept at 55 ° C. for about 1200 days. The test results are shown in Tables 2 and 6. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ○ indicates a rust area ratio of less than 5% and △ indicates a rust area ratio of 10% with respect to the entire area of the cut part. Less than, x indicates 10% or more, respectively.
As is clear from Tables 2 and 6, Nos. 1 to 40 and 51 to 90 in Tables 2 and 6 which are steels of the present invention show good corrosion resistance in a soil corrosive environment, while they are comparative steels. It can be seen that Nos. 41 to 50 and 91 to 100 in Tables 2 and 6 are inferior in corrosion resistance.
[0041]
In the concrete corrosion test, the test piece was embedded in Portland concrete kneaded with sea sand containing chloride, solidified, and then solidified, and then immersed in pure water by half, and the environment at 60 ° C. For about 1000 days, and after the test period, the concrete was destroyed and the sample was observed. The test results are shown in Tables 2 and 6. Corrosion test result ◎ indicates that no corrosion occurred in the cut part, and ○ indicates a rust area ratio of less than 5% and △ indicates a rust area ratio of 20% with respect to the entire area of the cut part. Less than, x indicates 20% or more, respectively.
As is clear from Tables 2 and 6, Nos. 1 to 40 and 51 to 90 in Tables 2 and 6 which are steels of the present invention show good corrosion resistance in a corrosive environment in concrete, whereas comparative steels. It can be seen that Nos. 41 to 50 and 91 to 100 in Tables 2 and 6 are inferior in corrosion resistance.
[0042]
The tap water environmental corrosion test was a test in which a test piece was immersed in flowing warm tap water and kept in an atmosphere at 55 ° C. for 24 months. The test results are shown in Tables 3 and 7. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ◯ indicates a rust area ratio of less than 10% and △ indicates a rust area ratio of 20% with respect to the entire area of the cut part. Less than, x indicates 20% or more, respectively.
As is clear from Tables 3 and 7, Nos. 1 to 40 and 51 to 90 in Tables 3 and 7 which are steels of the present invention show good corrosion resistance in a tap water corrosive environment, while comparative steels. It can be seen that Nos. 41 to 50 and 91 to 100 in Tables 3 and 7 are inferior in corrosion resistance.
[0043]
The seawater environment corrosion test was a test in which a test piece was exposed to a seawater splash zone on the coast for 30 months. The test results are shown in Tables 3 and 7. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ○ indicates a rust area ratio of less than 5% and △ indicates a rust area ratio of 10% with respect to the entire area of the cut part. Less than, x indicates 10% or more, respectively.
As is apparent from Tables 3 and 7, Nos. 1 to 40 and 51 to 90 in Tables 3 and 7 which are the steels of the present invention show good corrosion resistance in a seawater corrosive environment, but are comparative steels. It can be seen that No. 41 to 50 and 91 to 100 in Tables 3 and 7 are inferior in corrosion resistance.
[0044]
The dew condensation corrosion test was a test that was repeated 1800 times that it was kept in an environment of −20 ° C. for 2 hours and then kept in a humidity of 98% and 40 ° C. for 4 hours. The test results are shown in Tables 3 and 7. Corrosion test result ◎ indicates that no corrosion occurred in the cut part, and ○ indicates a rust area ratio of less than 5% and △ indicates a rust area ratio of 20% with respect to the entire area of the cut part. Less than, x indicates 20% or more, respectively.
As is clear from Tables 3 and 7, Nos. 1 to 40 and 51 to 90 in Tables 3 and 7 which are steels of the present invention show good corrosion resistance in a condensation corrosion environment, whereas they are comparative steels. It can be seen that No. 41 to 50 and 91 to 100 in Tables 3 and 7 are inferior in corrosion resistance.
[0045]
The atmospheric corrosion test was a test in which the test piece was exposed to a position about 1.5 km from the coast for about 900 days. The test results are shown in Tables 3 and 7. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ○ indicates a rust area ratio of less than 5% and △ indicates a rust area ratio of 10% with respect to the entire area of the cut part. Less than, x indicates 10% or more, respectively.
As is apparent from Tables 3 and 7, Nos. 1 to 40 and 51 to 90 in Tables 3 and 7 which are steels of the present invention show good corrosion resistance in an atmospheric corrosive environment, but are comparative steels. It can be seen that No. 41 to 50 and 91 to 100 in Tables 3 and 7 are inferior in corrosion resistance.
[0046]
In the drinking water environmental corrosion test, the pH was adjusted to 2.3 using sodium hydroxide, degassed by aeration with high-purity nitrogen gas, and maintained at 27 ° C. (a) 0.5% phosphoric acid solution , (B) 0.5% citric acid solution, (c) 0.5% citric acid-0.5% sodium chloride solution, etc. It was a test to analyze the amount. Only in this test, the test was conducted on the aluminum coating of the coating 1 and the manganese coating of the coating 4. The test results are shown in Tables 4 and 8. Corrosion test result ◎ indicates that no corrosion was observed in the cut part, and ○ indicates a rust area ratio of less than 1% and △ indicates a rust area ratio of 5% with respect to the entire area of the cut part. Less than, x indicates 5% or more, respectively.
As apparent from Tables 4 and 8, Nos. 1 to 40 and 51 to 90 in Tables 4 and 8 which are steels of the present invention show good corrosion resistance in a drinking water corrosive environment, whereas comparative steels. It can be seen that Nos. 41 to 50 and 91 to 100 in Tables 4 and 8 are inferior in corrosion resistance.
[0047]
That is, Nos. 1 to 40, 51 to 90 in Tables 1 to 8 which are steels of the present invention are high temperature wet corrosion environment, condensation corrosion environment, atmospheric corrosion environment, tap water corrosion environment, soil corrosion environment, concrete corrosion environment, seawater corrosion. While it shows good corrosion resistance in various corrosive environments such as environment and drinking water corrosive environment, it is understood that No. 41 to 50 and 91 to 100 in Tables 1 to 8 which are comparative steels are inferior in corrosion resistance. .
[0048]
[Evaluation of workability]
A cylindrical drawing test with a drawing ratio of 1.8 was performed and judged by the presence or absence of cracks. The test results are also shown in Table 5. In the workability of Table 5, ◯ indicates that the result of the cylindrical drawing test was good, and × indicates that a crack was generated in the cylindrical drawing test. The X values in Table 5 are those calculated by the following formula.
X = Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179−0.8 × (C / 12 + N / 14)
[0049]
As apparent from Table 5, Nos. 51 to 90 in Table 5 which are steels of the present invention show good pickling properties, high temperature wet corrosion environment, condensation corrosion environment, air corrosion environment, tap water corrosion environment, soil corrosion environment No. 91 to 100 in Table 5, which are comparative steels, show good corrosion resistance in various corrosive environments such as concrete corrosive environment, seawater corrosive environment, drinking water corrosive environment, and excellent workability. It can be seen that corrosion resistance and workability cannot be achieved simultaneously.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
[Table 3]

[0053]
[Table 4]

[0054]
[Table 5]

[0055]
[Table 6]

[0056]
[Table 7]

[0057]
[Table 8]

[0058]
【The invention's effect】
As described above, the present invention , In various corrosive environments such as dew corrosive environments, high temperature wet corrosive environments, atmospheric corrosive environments, tap water corrosive environments, soil corrosive environments, concrete corrosive environments, seawater corrosive environments, drinking water corrosive environments, etc. , Machining section Excellent corrosion resistance and workability Coating Providing steel at low cost The It is possible, and it is extremely important to contribute to industrial development.

Claims

% By mass
C: 0.02% or less,
Si: 0.01 to 3%,
Mn: 0.1 to 3%,
Cr: 0.1 to 9.9%,
Al: 0.1 to 10%,
Mg: 0.0003 to 0.1%
P: 0.03% or less,
S: 0.01% or less,
N: 0.02% or less,
The steel consisting of the balance Fe and unavoidable impurities, at least on the surface exposed to the use environment, as the metal whose potential in the aqueous solution environment is lower than the base material, either aluminum or an aluminum-based alloy, or zinc And a surface treatment layer having a thickness of 0.5 to 500 μm made of either zinc-based alloy or zinc alloy containing 0.1 to 55% Al, or manganese or manganese-based alloy. Coated steel with excellent cross-section corrosion resistance due to disappearance and damage of the surface treatment layer.

Furthermore, in the base material,
Cu: 0.05 to 5%,
Mo: 0.05 to 10%,
Sb: 0.01 to 0.5%,
Ni: 0.01 to 10%,
W: 0.05-3%
The coated steel excellent in processed section cross-section corrosion resistance according to claim 1, characterized by containing one or more of the following.

Furthermore, in the base material,
Rare earth elements: 0.001 to 0.1%,
Ca: 0.0001 to 0.05%
The coated steel excellent in processed section cross-sectional corrosion resistance according to claim 1 or 2, characterized by containing one or more of the following.

Furthermore, the base material contains 0.01 to 1% of one or more of Nb, V, Ti, Zr, Ta, and Hf by mass%, respectively, and satisfies the following formula: Coated steel excellent in processed section cross-sectional corrosion resistance according to any one of claims 1 to 3.
Nb / 93 + V / 51 + Ti / 48 + Zr / 91 + Ta / 181 + Hf / 179 ≧ 0.8 × (C / 12 + N / 14)

The metal in which the electric potential in an aqueous solution environment is lower than that of the base material, and further contains one or more of Mg, Si, and In by 0.05% or more and 15% or less by mass%, respectively. The coated steel excellent in the cross-section corrosion resistance of the processed part according to any one of -4.