JP2004285390A

JP2004285390A - Al-Mg ALUMINUM ALLOY SHEET FOR HIGH STRAIN-RATE SUPERPLASTIC FORMING

Info

Publication number: JP2004285390A
Application number: JP2003077885A
Authority: JP
Inventors: Kazunori Kobayashi; 一徳小林; Tetsuya Masuda; 哲也増田
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2003-03-20
Filing date: 2003-03-20
Publication date: 2004-10-14
Anticipated expiration: 2023-03-20
Also published as: JP4035465B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Al-Mg aluminum alloy sheet having high strength after being superplastically formed with high strain rate, and superior high strain rate superplastic formability, particularly for a use in a vehicle body panel. <P>SOLUTION: The Al-Mg aluminum alloy sheet for high strain rate superplastic-forming comprises 3.5-7.0% Mg, more than 0.5% but 1.0% or less Cu, 0.001-0.1% Ti, Si and Fe each limited to 0.2% or less, 0.1% or less Mn and the balance Al with unavoidable impurities; and has a 0.2% yield stress of 150 MPa or higher, after having been subjected to heat treatment of holding the sheet at 500°C for three minutes and radiationally cooling it to room temperature and then further to an artificial aging. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、高速超塑性成形後の強度が高いＡｌ−Ｍｇ系アルミニウム合金板（以下、アルミニウムをＡｌとも言う）に関するものである。
【０００２】
【従来の技術】
Ａｌ−Ｍｇ系アルミニウム合金において、例えば、４６０〜５５０ ℃の高温領域で高い伸びの特性を生じ、この高温領域での成形性に優れるような、高速超塑性成形用Ａｌ−Ｍｇ系（ＡＡ乃至ＪＩＳ規格でいう５０００系）アルミニウム合金が従来から開発されている。
【０００３】
これまで提案された高速超塑性成形用Ａｌ−Ｍｇ系アルミニウム合金としては、Ｍｇを３．０〜８．０％程度必須に含むとともに、Ｂｅ：０．０００１〜０．０１％程度と、Ｍｎ（０．３〜２．５％程度）、Ｃｒ、Ｖ、Ｚｒの一種以上、Ｔｉ：０．００１〜０．１程度、などを必須に含み、Ｓｉ、Ｆｅを各々０．２％以下程度に規制した、Ａｌ−Ｍｇ系アルミニウム合金が提案されている（例えば、特許文献１〜５参照）。
【０００４】
【特許文献１】
特許第２６４０９９３号公報
【特許文献２】
特開平６−２４０３９５号公報
【特許文献３】
特許第２８４４４１１号公報
【特許文献４】
特許第２９２１８２０号公報
【特許文献５】
特開平７−１９７１７７号公報
【０００５】
これらの特許文献１〜５において開示されているように、Ｍｇは高速超塑性成形性を向上させる。Ｂｅは高速超塑性成形時のアルミニウム合金板のキャビテーション（空孔）発生を防止する。Ｍｎ、Ｃｒ、Ｖ、Ｚｒは、高速超塑性成形のための昇温過程で生じる再結晶粒を微細化し、高速超塑性成形時の結晶粒の粗大化を防止する。Ｓｉ、Ｆｅは高速超塑性成形時のアルミニウム合金板のキャビテーションの原因となり、高速超塑性成形時の伸びを低下させるために規制する。
【０００６】
また、同じくＭｇを３．０〜８．０％程度必須に含むとともに、Ｔｉ：０．００１〜０．１％程度、Ｃｕ：０．０５〜０．５％程度を必須に含み、Ｓｉ、Ｆｅを同じく各々０．２％以下程度に規制した、Ａｌ−Ｍｇ系アルミニウム合金が提案されている（例えば、特許文献６〜８参照）。
【０００７】
【特許文献６】
特公平４−６３１４０号公報
【特許文献７】
特許第３１４５９０４号公報
【特許文献８】
特開平１０−２５９４４１号公報
【０００８】
これらの特許文献６〜８において開示されているように、Ｔｉは結晶粒を微細化し、高速超塑性成形能を向上させる。Ｃｕは高速超塑性成形時のアルミニウム合金板のキャビテーション（空孔）発生を防止する。
【０００９】
したがって、従来から、この種高速超塑性成形用Ａｌ−Ｍｇ系アルミニウム合金において、高速超塑性成形時のアルミニウム合金板のキャビテーション（空孔）発生を防止するために、ＢｅかＣｕを用い、高速超塑性成形時のアルミニウム合金板のキャビテーションの原因となるＳｉ、Ｆｅを規制することが公知である。
【００１０】
【発明が解決しようとする課題】
これら従来のＡｌ−Ｍｇ系アルミニウム合金は、４６０〜５５０ ℃の高温領域で、歪み速度が１０^−２〜１０^０／ｓに達するような高速超塑性成形性は確かに優れる。しかし、この高速超塑性成形後の強度、高速超塑性成形後に塗装焼き付け硬化処理される板であれば、塗装焼き付け硬化処理した後の強度、がいずれも低い。
【００１１】
上記高速超塑性成形用Ａｌ−Ｍｇ系アルミニウム合金に関する各特許文献は、高速超塑性成形での成形性は主要な課題にしているものの、この高速超塑性成形後の低強度については明確な課題としていない。
【００１２】
この種高速超塑性成形は自動車の車体パネルの成形に用いられることが多い。自動車の車体パネルでは、これらＡｌ−Ｍｇ系アルミニウム合金板を、上記高温、高歪み速度条件で高速超塑性成形し、フード、フェンダー、ドア、ルーフ、トランクリッドなどの、アウタパネル（外板）やインナパネル（内板）等のパネルとして製作して用いる。
【００１３】
周知の通り、自動車の車体は、組み立て後、塗装され、塗布された塗料の焼き付け硬化処理が行なわれる。この塗装焼き付け硬化処理は、例えば、１６０ ℃×２０分などの低温短時間の条件から、１８０ ℃×６０分の高温長時間の条件までなど、種々の条件がある。これら塗装焼き付け硬化処理は、アルミニウム合金にとっては人工時効硬化処理となり、時効硬化型合金のＡｌ−Ｍｇ−Ｓｉ系の６０００系アルミニウム合金などでは、塗装焼き付け硬化処理後の強度が著しく向上する。
【００１４】
しかし、高速超塑性成形用のＡｌ−Ｍｇ系アルミニウム合金は、上記６０００系のような時効硬化型合金ではなく、その時効硬化能は小さい。このため、Ａｌ−Ｍｇ系アルミニウム合金の高速超塑性成形後の強度が元々低ければ、その後の上記種々の条件の塗装焼き付け硬化処理（人工時効硬化処理）によっても、強度の実質的な上昇は見込めない。
【００１５】
このため、必然的に、高速超塑性成形後のＡｌ−Ｍｇ系アルミニウム合金車体パネルでは必要な強度が不足し、特に、アウタパネルではデント性が低下したり、インナパネルでもアウタパネルの補強効果が低下する事態が起こりうる。したがって、この強度不足を補うためには、どうしても、パネルに用いるＡｌ−Ｍｇ系アルミニウム合金板の板厚を厚くする必要が生じる。この結果、鋼板に代わる材料として、車体パネル用アルミニウム合金板に求められる、薄肉、軽量でかつ高強度である利点が大きく損なわれ、Ａｌ−Ｍｇ系アルミニウム合金板採用の意義が失われる可能性も生じる。
【００１６】
本発明はこの様な事情に着目してなされたものであって、その目的は、特に車体パネル用途として、高速超塑性成形後の強度が高く、しかも高速超塑性成形性にも優れた、Ａｌ−Ｍｇ系アルミニウム合金板を提供しようとするものである。
【００１７】
【課題を解決するための手段】
この目的を達成するために、本発明高速超塑性成形用Ａｌ−Ｍｇ系アルミニウム合金板の要旨は、Ｍｇ：３．５〜７．０％、Ｃｕ：０．５％を越え１．０％以下、Ｔｉ：０．００１〜０．１％を含み、かつＳｉ、Ｆｅを各々０．２％以下に規制するとともに、Ｍｎ：０．１％以下であり、残部Ａｌおよび不可避的不純物からなり、５００ ℃で３分間保持して室温まで放冷する熱処理後、更に人工時効硬化処理を施した後の０．２％耐力が１５０ＭＰａ以上である特性を有することである。なお、前記熱処理後の人工時効硬化処理の条件は、好ましくは１８０ ℃で６０分間保持するものとする。
【００１８】
前記した通り、高速超塑性成形用のＡｌ−Ｍｇ系アルミニウム合金は時効硬化能が元々は小さい。しかし、本発明Ａｌ−Ｍｇ系アルミニウム合金板のように、特にＣｕを比較的多く含有させる化学成分組成とすることで、高速超塑性成形用のＡｌ−Ｍｇ系アルミニウム合金の時効硬化能を著しく高めることができる。このため、高速超塑性成形後のパネルとして、塗装焼き付け硬化処理（人工時効硬化処理）を施された場合の強度を高めることが可能となる。この点、４６０〜５５０ ℃の高温領域での高速超塑性成形であっても、この成形後（人工時効硬化処理後）の強度が、車体アウタパネルなどとして最低限必要な、０．２％耐力で１５０ＭＰａ以上を確保することができる。
【００１９】
このような高速超塑性成形およびその後の塗装焼き付け硬化処理後の強度は、実際に高速超塑性成形およびその後の塗装焼き付け硬化処理などを行なわずともこれを模擬した試験によって一定程度評価できる。即ち、上記組成を有するアルミニウム合金板に対し、実際の自動車などのパネルを想定し、高速超塑性成形を模擬した熱処理と、成形後の塗装焼き付け硬化処理を模擬した人工時効硬化処理を施した後の０．２％耐力によって評価できる。より具体的には、先ず、高速超塑性成形を模擬した熱処理条件として、５００ ℃で３分間保持して室温まで放冷する条件を選択する。更にその後の塗装焼き付け硬化処理を模擬した熱処理として、好ましくは１８０ ℃で６０分間保持する人工時効硬化処理条件を選択する。そして、これら熱処理を施された板を常温で引張試験した際の０．２％耐力によって、上記塗装焼き付け硬化処理後の強度が評価できる。そして、後述する通り、耐デント性など、高速超塑性成形された車体アウタパネルなどとして必要な強度を得るためには、上記条件での熱処理後の板の特性として、上記０．２％耐力が１５０ＭＰａ以上であることが必要である。
【００２０】
【発明の実施の形態】
次に、本発明Ａｌ−Ｍｇ系アルミニウム合金板の化学成分組成の実施形態につき、以下に説明する。本発明Ａｌ−Ｍｇ系アルミニウム合金板の基本組成は、Ｍｇ：３．５〜７．０％、Ｃｕ：０．５％を越え１．０％以下、Ｔｉ：０．００１〜０．１％を含み、かつＳｉ、Ｆｅを各々０．２％以下に規制するとともに、Ｍｎ：０．１％以下であり、残部Ａｌおよび不可避的不純物からなるものとする。なお、本発明での化学成分組成の％表示は、前記請求項の％表示も含めて、全て質量％の意味である。
【００２１】
上記合金元素以外の、Ｚｒ、Ｂ、Ｚｎ、Ｎｉ、Ｖなど、その他の元素は、基本的には不純物元素であり、含有量をできるだけ少なくすることが好ましい。しかし、リサイクルや経済性の観点から、溶解材として、高純度Ａｌ地金だけではなく、５０００系合金やその他のＡｌ合金スクラップ材、低純度Ａｌ地金などを溶解原料として使用して、本発明Ａｌ合金組成を溶製する場合には、これら他の合金元素は必然的に含まれる可能性が高い。したがって、本発明では、目的とする本発明アルミニウム合金板の特性乃至効果を阻害しない範囲で、これら他の元素が含有されることを許容する。
【００２２】
各合金元素の含有範囲と意義、あるいは許容量について以下に説明する。
【００２３】
Ｍｇ：３．５〜７．０％
Ｍｇは、高速超塑性成形時に動的再結晶を促進し、超塑性成形性を向上させる。また、高速超塑性成形成形後の強度を向上させ、塗装焼き付け硬化処理などの人工時効処理後の、車体アウタパネルとして必要な１５０ＭＰａ以上の０．２％耐力などの、強度を保証するためにも、必須の元素である。Ｍｇの３．５％未満の含有では、Ｍｇの絶対量が不足するため、超塑性成形性が低下する。また、高速超塑性成形後の強度を、車体アウタパネルなどとして必要な、０．２％耐力で１５０ＭＰａ以上に確保させることができない。一方、Ｍｇが７．０％を越えて含有されると、熱間圧延や冷間圧延などの圧延性が低下し、板の製造が困難となる。したがって、Ｍｇの含有量は３．５〜７．０％の範囲とする。
【００２４】
Ｍｎ：０．１％以下
Ｍｎは、本来であれば、超塑性特性を向上させ、高速超塑性成形時の結晶粒の粗大化を防止する。また、母相に固溶することにより、高速超塑性成形後の強度を高める。しかし、本発明では、Ｃｕの含有量を比較的多くしているので、これらのＭｎ含有の効果は不要である。このため、Ｍｎ含有量は０．１％以下とする。
【００２５】
Ｃｕ：０．５％を越え、１．０％以下
Ｃｕは、高速超塑性成形時のＡｌ−Ｍｇ系アルミニウム合金板のキャビテーション（空孔）発生を防止し、超塑性伸びを向上させる。また、重要には、人工時効硬化能を増して、高速超塑性成形および人工時効硬化処理後の強度を、車体アウタパネルなどとして必要な０．２％耐力で１５０ＭＰａ以上に確保させる。この点、Ｃｕ含有量が０．５％以下では、これらの効果、特に、人工時効硬化能向上効果が無い。一方、Ｃｕ含有量が１．０％を越えて多くなった場合、車体パネルなどの用途での塗装後の耐蝕性、溶接性を著しく劣化させる。また、熱間圧延や冷間圧延などの圧延性が著しく低下する。このため、Ｃｕは０．５％を越え、１．０％以下の範囲とする。
【００２６】
Ｔｉ：０．００１〜０．１％
Ｔｉは、鋳塊の結晶粒を微細化し、高速超塑性成形性を向上させる効果がある。Ｔｉ：０．００１％未満ではこの効果が無く、０．１％を越えて含有すると、粗大な晶出物を形成し、却って、高速超塑性成形性を低下させる。したがって、Ｔｉは０．００１〜０．１％の範囲とする。また、ＢにもＴｉと同様の効果があるが、３００ｐｐｍを越えて含有すると、粗大な晶出物を形成し、却って、高速超塑性成形性を低下させる。したがって、Ｂ：３００ｐｐｍ以下までの含有は許容する。
【００２７】
Ｃｒ：０．０５〜０．５％
Ｃｒは、Ｍｎと同様、超塑性特性を向上させる効果があり、高速超塑性成形のための昇温過程で生じる再結晶粒を微細化し、高速超塑性成形時の結晶粒の粗大化を防止する。Ｃｒ含有量が０．０５％未満では、これらの効果が無い。一方、Ｃｒ含有量が０．５％を越えて多くなった場合、溶解、鋳造時に粗大な金属間化合物や晶析出物を生成しやすく、破壊の起点となり易いため、高速超塑性成形を却って低下させる原因となる。したがって、Ｃｒは選択的に含有させ、含有させる場合には、０．０５〜０．５％の範囲とする。、
【００２８】
Ｓｉ、Ｆｅ：各々０．２％以下
Ｓｉ、Ｆｅは高速超塑性成形時のアルミニウム合金板のキャビテーションの原因となり、高速超塑性成形時の伸びを著しく低下させる。また、Ｓｉ、Ｆｅは溶解の際にＡｌ−Ｍｇ系アルミニウム合金スクラップを使用した場合、必然的に含有される。したがって、Ｓｉ、Ｆｅは各々０．２％以下のできるだけ少ない含有量に規制する。
【００２９】
以下に、本発明におけるＡｌ−Ｍｇ系アルミニウム合金板の製造方法につき説明する。本発明における製造方法は基本的に常法により製造可能である。先ず、溶解、鋳造工程は、本発明成分規格範囲内に溶解調整されたアルミニウム合金溶湯を、連続鋳造圧延法、半連続鋳造法（ＤＣ鋳造法）等の通常の溶解鋳造法を適宜選択して鋳造する。次いで、このアルミニウム合金鋳塊に均質化熱処理を施す。
【００３０】
均質化熱処理後の前記した熱間圧延を経て、必要により中間焼鈍が施された後に、Ａｌ合金板は冷間圧延され、所望の板厚とされる。パネルによって、３．０ｍｍ以上の厚板が必要な場合には、冷間圧延を省略して、アルミニウム合金板を熱間圧延上がりとしても良い。
【００３１】
これら熱間圧延板、冷間圧延板は、最終焼鈍などの調質処理を必要により施されるか、または調質処理無しで、高速超塑性成形され、車体パネルとされる。
【００３２】
本発明で言うアルミニウム合金板とは、熱延（熱間圧延上がり）板、冷延（冷間圧延上がり）板などの未調質処理（熱処理）の板や、あるいは、これらの板を焼鈍などの調質処理を施した後の板のことを言う。そして、本発明で言う板とは高速超塑性成形前の、板、コイル、切り板、などの適宜の形状状態を含む。
【００３３】
また、本発明で言う高速超塑性成形とは、４６０〜５５０ ℃の高温領域で、歪み速度が１０^−２〜１０^０／ｓに達する条件での、板をパネルに成形するための任意のプレス成形方法、金型成形方法を含みうる。更に、車体パネルの用途や形状に応じて、高速超塑性成形前に、あるいは高速超塑性成形後に、冷間でプレス成形されたり、アウタパネルとしてのフラットヘムなどのヘミング加工や、曲げ加工、トリミング等の加工を適宜付加される場合を含む。
【００３４】
【実施例】
次に、本発明の実施例を説明する。表１に示すＡ〜Ｆの本発明範囲内のＡｌ−Ｍｇ系アルミニウム合金と、Ｇ〜Ｊの本発明範囲外のＡｌ−Ｍｇ系アルミニウム合金とのアルミニウム合金鋳塊をＤＣ鋳造法にて溶製し、以下の同じ条件で板を製造した。即ち、面削後に４７０ｍｍｔの厚みとして、昇温速度４０°／ｈにて加熱して４８０ ℃×４時間の均質化熱処理後、熱間圧延し、４．５ｍｍｔの熱延板とした。この熱延板を４００ ℃×３時間中間焼鈍した後、冷間圧延して、１．０ｍｍｔの冷延板とした。そして、この冷延板を５２０ ℃×２０秒最終焼鈍し、供試板とした。
【００３５】
これら供試板から各例とも試験片を採取し、板の圧延（Ｌ）方向の平均結晶粒径を測定した。結果は、各発明例、比較例とも平均結晶粒径は５０μｍ以下であった。この平均結晶粒径の測定は、アルミニウム合金板を０．０５〜０．１ｍｍ機械研磨した後電解エッチングした表面を、２００倍の光学顕微鏡を用いて観察し、前記Ｌ方向に、ラインインターセプト法で測定する。１測定ライン長さは０．９５ｍｍとし、１視野当たり各３本で合計５視野を観察することにより、全測定ライン長さを０．９５×１５ｍｍとした。
【００３６】
各例とも上記供試板から試験片を採取し、高速超塑性成形を評価するために、高温特性を供試板の高温引張試験により評価した。高温特性の内の高温伸びの測定のための高温引張試験は温度５００ ℃で行なった。そして、この温度で、昇温速度：１００℃／分、歪み速度：１０ ^−１／ｓ、評点間距離：１５ｍｍ、試験片形状はＪＩＳ５号試験片、の条件で行った。また、試験片が破断するまで一定の上記歪み速度で行った。そして、各供試板の圧延方向に対し平行方向の伸び（δ、％）を測定した。
【００３７】
また、高温特性の内のキャビティ面積率の測定は、上記温度５００ ℃で高温引張試験を行なった後の試験片について行い、高温引張試験後の板厚が１／２に減じた箇所の試験片組織を、２００倍の光学顕微鏡を用いて観察し、圧延方向に対し平行方向のキャビティ面積率を、４視野の平均値で測定した。これらの結果も表２に示す。
【００３８】
更に、上記各試験片の高速超塑性成形後の強度を評価した。このため、各例とも高速超塑性成形を模擬して上記供試板から試験片を採取し、この試験片を５００ ℃で３分間保持して室温まで放冷する熱処理の後の試験片の０．２％耐力（σ_０．２、ＭＰａ、表２のＡ）を測定した。更に、実際の自動車パネルを想定し、高速超塑性成形後に塗装焼き付け硬化処理されることを模擬して、この熱処理後の試験片を更に１８０ ℃×６０分の人工時効硬化処理した後（ベークハード後）の試験片の０．２％耐力（σ_０．２、ＭＰａ、表２のＢ）を測定した。
【００３９】
そして、上記１８０ ℃×６０分の人工時効硬化処理した後の試験片の耐デント性も更に評価した。耐デント性試験は、この試験片の中央部に対し、先端のＲが５０ｍｍΦの球頭ポンチにて、２４５ＭＰａの荷重を加えた際の、荷重点の凹み量を測定することにより行なった。そして、凹み量が０．３ｍｍ未満のものを〇、凹み量が０．３ｍｍ以上のものを×として評価した。これらの結果も表２に示す。
【００４０】
表１、２から明らかな通り、本発明組成範囲内である表１のＡ〜Ｆの本発明範囲内のＡｌ−Ｍｇ系アルミニウム合金を用いた発明例１〜６は、高温伸び（δ、％）が比較例７〜９に比して、同等か高く、また、キャビティ面積率も小さい。したがって、比較例７〜９に比して、高速超塑性成形性が同等か優れている。
【００４１】
そして、発明例１〜６は、高速超塑性成形を模擬した高温熱処理後に人工時効硬化処理した後の０．２％耐力Ｂが１５０ＭＰａ以上であり、高温熱処理および人工時効硬化処理後の強度が高い。また、比較例７、８に比して、耐デント性にも著しく優れている。
【００４２】
これら、発明例１〜６の結果は、比較例７〜９の結果とも合わせて、高速超塑性成形およびその後の塗装焼き付け硬化処理される自動車車体アウタパネルなどに好適であることを示している。
【００４３】
なお、発明例１〜６の内でも、Ｓｉ、Ｆｅの含有量が比較的多い合金Ｃを用いた発明例３は、Ｓｉ、Ｆｅの含有量が比較的低い他の発明例に比して、高温伸び（δ％）が低く、またキャビティ面積率も大きい。したがって、他の発明例に比して高速超塑性成形性が比較的劣る。また、Ｓｉ、Ｆｅが多めに外れた合金Ｉを用いた比較例９は、高温熱処理および人工時効硬化処理した後の０．２％耐力Ｂは１５０ＭＰａ以上であるものの、高温伸び（δ％）が著しく低くく、キャビティ面積率も著しく大きい。これらの結果から、Ｓｉ、Ｆｅを０．２％以下に規制する意義が裏付けられる。
【００４４】
発明例１〜６の内でも、Ｍｇの含有量が比較的低い合金Ｆを用いた発明例６は、Ｍｇの含有量が比較的高い他の発明例１、２に比して、高温熱処理および人工時効硬化処理した後の０．２％耐力Ｂが比較的低い。この結果や、Ｍｇの含有量が低めに外れた合金Ｈを用いた比較例８の高温熱処理および人工時効硬化処理した後の０．２％耐力Ｂが共に１５０ＭＰａ未満と低くい結果などと合わせて、強度に対するＭｇの寄与と数値範囲の意義を裏付けられる。
【００４５】
発明例１〜６の内でも、Ｃｕの含有量が比較的低い合金Ｅを用いた発明例５は、Ｃｕの含有量が比較的高い他の発明例１、２に比して、高温熱処理および人工時効硬化処理した後の０．２％耐力Ｂが比較的低い。この結果や、Ｃｕの含有量が０．５％と比較的低い低めに外れた合金Ｇを用いた比較例７の高温熱処理および人工時効硬化処理した後の０．２％耐力Ｂが共に１５０ＭＰａ未満と著しく低い結果などと合わせて、Ｃｕを０．５％を越えて多く含有させることの意義が裏付けられる。
【００４６】
【表１】

【００４７】
【表２】

【００４８】
【発明の効果】
本発明によれば、車体パネル用途として、高速超塑性成形後の強度が高く、しかも高速超塑性成形性にも優れた、Ａｌ−Ｍｇ系アルミニウム合金板を提供することができる。しかも、このＡｌ−Ｍｇ系アルミニウム合金板を従来の板製造工程を変更せずに製造することができる。したがって、５０００系アルミニウム合金板の高速超塑性成形および車体パネル用途への拡大を図ることができる点で、多大な工業的な価値を有するものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an Al-Mg based aluminum alloy plate (hereinafter, aluminum is also referred to as Al) having high strength after high-speed superplastic forming.
[0002]
[Prior art]
In an Al—Mg series aluminum alloy, for example, an Al—Mg series (AA to JIS) for high-speed superplastic forming that produces high elongation characteristics at a high temperature range of 460 to 550 ° C. and is excellent in formability at this high temperature range. 5000 series) Aluminum alloys have been developed in the past.
[0003]
Al-Mg based aluminum alloys for high-speed superplastic forming that have been proposed so far include Mg in an essential amount of about 3.0 to 8.0%, and Be: about 0.0001 to 0.01%, and Mn ( About 0.3 to 2.5%), one or more of Cr, V, and Zr, Ti: about 0.001 to 0.1, etc. are essential, and Si and Fe are restricted to about 0.2% or less respectively. Al-Mg based aluminum alloys have been proposed (see, for example, Patent Documents 1 to 5).
[0004]
[Patent Document 1]
Japanese Patent No. 2640993 [Patent Document 2]
JP-A-6-240395 [Patent Document 3]
Japanese Patent No. 2844411 [Patent Document 4]
Japanese Patent No. 2921820 [Patent Document 5]
JP-A-7-197177 [0005]
As disclosed in these patent documents 1 to 5, Mg improves high-speed superplastic formability. Be prevents cavitation (holes) in the aluminum alloy sheet during high-speed superplastic forming. Mn, Cr, V 2 and Zr refine the recrystallized grains generated in the temperature raising process for high-speed superplastic forming, and prevent the crystal grains from becoming coarse during the high-speed superplastic forming. Si and Fe cause cavitation of the aluminum alloy plate during high-speed superplastic forming, and are regulated to reduce the elongation during high-speed superplastic forming.
[0006]
Similarly, Mg is essentially contained in an amount of about 3.0 to 8.0%, Ti is about 0.001 to about 0.1%, Cu is essentially about 0.05 to 0.5%, Si, Fe Al-Mg-based aluminum alloys are also proposed, each of which is regulated to about 0.2% or less (see, for example, Patent Documents 6 to 8).
[0007]
[Patent Document 6]
Japanese Patent Publication No. 4-63140 [Patent Document 7]
Japanese Patent No. 3145904 [Patent Document 8]
Japanese Patent Laid-Open No. 10-259441
As disclosed in these Patent Documents 6 to 8, Ti refines crystal grains and improves high-speed superplastic forming ability. Cu prevents cavitation (voids) in the aluminum alloy sheet during high-speed superplastic forming.
[0009]
Therefore, conventionally, in this type of Al-Mg based aluminum alloy for high-speed superplastic forming, Be or Cu is used to prevent cavitation (vacancy) of the aluminum alloy plate during high-speed superplastic forming. It is known to regulate Si and Fe that cause cavitation of an aluminum alloy plate during plastic forming.
[0010]
[Problems to be solved by the invention]
These conventional Al—Mg-based aluminum alloys have excellent high-speed superplastic formability such that the strain rate reaches 10 ⁻² to 10 ⁰ / s in a high temperature region of 460 to 550 ° C. However, the strength after the high-speed superplastic forming and the strength after the paint bake-hardening treatment are low if the plate is subjected to the paint bake-hardening treatment after the high-speed superplastic forming.
[0011]
Although each patent document relating to the Al-Mg based aluminum alloy for high-speed superplastic forming has a main issue of formability in high-speed superplastic forming, the low strength after high-speed superplastic forming is a clear issue. Not in.
[0012]
This type of high-speed superplastic forming is often used for forming automobile body panels. In automobile body panels, these Al-Mg aluminum alloy plates are formed by high-speed superplastic forming under the above-mentioned high temperature and high strain rate conditions, and outer panels (outer plates) and inner parts such as hoods, fenders, doors, roofs, and trunk lids. It is manufactured and used as a panel (inner plate).
[0013]
As is well known, the body of an automobile is painted after being assembled, and the applied paint is baked and cured. This paint baking and curing treatment has various conditions, for example, from a low temperature short time condition such as 160 ° C. × 20 minutes to a high temperature long time condition of 180 ° C. × 60 minutes. These coating bake hardening treatments are artificial age hardening treatments for aluminum alloys, and in the case of Al-Mg-Si based 6000 series aluminum alloys such as age hardening type alloys, the strength after the paint bake hardening treatment is remarkably improved.
[0014]
However, the Al—Mg 2 type aluminum alloy for high-speed superplastic forming is not an age hardening type alloy such as the above-mentioned 6000 type, and its age hardening ability is small. For this reason, if the strength after high-speed superplastic forming of an Al-Mg-based aluminum alloy is originally low, a substantial increase in strength can be expected even by the subsequent paint bake hardening treatment (artificial age hardening treatment) under the various conditions described above. Absent.
[0015]
Therefore, inevitably, the Al-Mg-based aluminum alloy body panel after high-speed superplastic forming lacks the required strength. In particular, the dent property is lowered in the outer panel, and the reinforcing effect of the outer panel is also lowered in the inner panel. Things can happen. Therefore, in order to make up for this lack of strength, it is necessary to increase the thickness of the Al—Mg based aluminum alloy plate used for the panel. As a result, the advantage of being thin, lightweight and high strength, which is required for aluminum alloy plates for car body panels as a material to replace steel plates, is greatly impaired, and the significance of adopting Al-Mg based aluminum alloy plates may be lost. Arise.
[0016]
The present invention has been made by paying attention to such circumstances, and the purpose thereof is Al, which is high in strength after high-speed superplastic forming, and also excellent in high-speed superplastic formability, particularly for vehicle body panel applications. -It is going to provide a Mg type | system | group aluminum alloy plate.
[0017]
[Means for Solving the Problems]
In order to achieve this object, the gist of the Al-Mg-based aluminum alloy sheet for high-speed superplastic forming of the present invention is Mg: 3.5 to 7.0%, Cu: more than 0.5% and 1.0% or less. , Ti: 0.001 to 0.1%, and Si and Fe are controlled to 0.2% or less, Mn: 0.1% or less, the balance being Al and inevitable impurities, 500 It has a property that the 0.2% proof stress is 150 MPa or more after the heat treatment of holding at 3 ° C. for 3 minutes and allowing to cool to room temperature, and further after the artificial age hardening treatment. The condition for the artificial age hardening after the heat treatment is preferably maintained at 180 ° C. for 60 minutes.
[0018]
As described above, the Al—Mg-based aluminum alloy for high-speed superplastic forming originally has a low age hardening ability. However, like the Al-Mg-based aluminum alloy plate of the present invention, the age-hardening ability of the Al-Mg-based aluminum alloy for high-speed superplastic forming is remarkably increased by using a chemical component composition that contains a relatively large amount of Cu. be able to. For this reason, it is possible to increase the strength of the panel after high-speed superplastic forming when subjected to a paint bake hardening process (artificial age hardening process). In this regard, even in high-speed superplastic forming at a high temperature range of 460 to 550 ° C., the strength after this forming (after artificial age hardening) is 0.2% proof stress, which is the minimum required for an outer panel of a vehicle body. 150 MPa or more can be secured.
[0019]
The strength after such high-speed superplastic forming and subsequent paint bake-hardening treatment can be evaluated to a certain degree by a test simulating it without actually performing high-speed superplastic forming and subsequent paint bake-hardening treatment. That is, after an aluminum alloy plate having the above composition is subjected to heat treatment simulating high-speed superplastic forming and artificial age hardening processing simulating paint baking hardening after forming, assuming an actual panel such as an automobile. The 0.2% proof stress can be evaluated. More specifically, first, as a heat treatment condition simulating high-speed superplastic forming, a condition of holding at 500 ° C. for 3 minutes and allowing to cool to room temperature is selected. Further, as the heat treatment simulating the subsequent paint baking and hardening treatment, an artificial age hardening treatment condition that is preferably maintained at 180 ° C. for 60 minutes is selected. And the intensity | strength after the said paint bake hardening process can be evaluated by the 0.2% yield strength at the time of carrying out the tension test at normal temperature of the board which gave these heat processing. And, as will be described later, in order to obtain the strength required for a car body outer panel formed by high-speed superplastic molding, such as dent resistance, the 0.2% proof stress is 150 MPa as a characteristic of the plate after the heat treatment under the above conditions. That is necessary.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the chemical component composition of the Al—Mg 2 aluminum alloy sheet of the present invention will be described below. The basic composition of the Al-Mg-based aluminum alloy plate of the present invention is Mg: 3.5 to 7.0%, Cu: more than 0.5% and 1.0% or less, Ti: 0.001 to 0.1%. In addition, each of Si and Fe is regulated to 0.2% or less, and Mn: 0.1% or less, and the balance is Al and inevitable impurities. In the present invention, “%” in the chemical component composition means “% by mass” including “%” in the above claims.
[0021]
Other elements such as Zr, B 2, Zn, Ni, and V other than the above alloy elements are basically impurity elements, and it is preferable to reduce the content as much as possible. However, from the viewpoint of recycling and economy, the present invention uses not only high-purity Al bullion but also 5000 series alloys, other Al alloy scrap materials, low-purity Al bullion, and the like as melting materials. In the case of melting the Al alloy composition, these other alloy elements are inevitably contained. Therefore, in the present invention, it is allowed that these other elements are contained within a range not impairing the characteristics or effects of the intended aluminum alloy sheet of the present invention.
[0022]
The content range and significance of each alloy element, or the allowable amount will be described below.
[0023]
Mg: 3.5-7.0%
Mg promotes dynamic recrystallization during high-speed superplastic forming and improves superplastic formability. Also, in order to improve the strength after high-speed superplastic molding, and to guarantee the strength, such as 0.2% proof stress of 150 MPa or more required as a vehicle body outer panel after artificial aging treatment such as paint baking hardening treatment, It is an essential element. If the Mg content is less than 3.5%, the absolute amount of Mg is insufficient, so the superplastic formability is reduced. Further, the strength after high-speed superplastic forming cannot be ensured to 150 MPa or more with a 0.2% proof stress necessary for a vehicle body outer panel or the like. On the other hand, if Mg is contained in excess of 7.0%, rollability such as hot rolling or cold rolling deteriorates, and it becomes difficult to produce a plate. Therefore, the Mg content is in the range of 3.5 to 7.0%.
[0024]
Mn: 0.1% or less Mn originally improves superplastic characteristics and prevents coarsening of crystal grains during high-speed superplastic forming. Moreover, the strength after high-speed superplastic forming is increased by dissolving in the matrix. However, in the present invention, since the Cu content is relatively large, these effects of containing Mn are unnecessary. For this reason, Mn content shall be 0.1% or less.
[0025]
Cu: More than 0.5% and not more than 1.0% Cu prevents the occurrence of cavitation (vacancy) in the Al—Mg-based aluminum alloy plate during high-speed superplastic forming and improves superplastic elongation. Also, importantly, the artificial age-hardening ability is increased, and the strength after high-speed superplastic forming and artificial age-hardening treatment is ensured to be 150 MPa or more with a 0.2% proof stress necessary for a vehicle body outer panel or the like. In this respect, when the Cu content is 0.5% or less, these effects, particularly, the effect of improving the artificial age hardening ability is not obtained. On the other hand, when the Cu content exceeds 1.0%, the corrosion resistance and the weldability after painting in applications such as body panels are remarkably deteriorated. In addition, rollability such as hot rolling and cold rolling is remarkably lowered. For this reason, Cu is made to exceed 0.5% and 1.0% or less.
[0026]
Ti: 0.001 to 0.1%
Ti has the effect of refining the crystal grains of the ingot and improving the high-speed superplastic formability. If Ti is less than 0.001%, this effect is not obtained. If it exceeds 0.1%, a coarse crystallized product is formed, and on the contrary, high-speed superplastic formability is lowered. Therefore, Ti is in the range of 0.001 to 0.1%. B 2 also has the same effect as Ti, but if it exceeds 300 ppm, a coarse crystallized product is formed, and on the contrary, high-speed superplastic formability is lowered. Therefore, the content of B: up to 300 ppm is allowed.
[0027]
Cr: 0.05-0.5%
Cr, like Mn, has the effect of improving the superplastic properties, refines the recrystallized grains that occur during the heating process for high-speed superplastic forming, and prevents the coarsening of the grains during high-speed superplastic forming. . If the Cr content is less than 0.05%, these effects are not obtained. On the other hand, when the Cr content exceeds 0.5%, coarse intermetallic compounds and crystal precipitates are likely to be generated during melting and casting, which tends to be the starting point of fracture. Cause it. Therefore, Cr is selectively contained, and when it is contained, the content is made 0.05 to 0.5%. ,
[0028]
Si and Fe: each 0.2% or less Si and Fe cause cavitation of the aluminum alloy plate during high-speed superplastic forming, and significantly reduce the elongation during high-speed superplastic forming. Si and Fe are inevitably contained when an Al-Mg based aluminum alloy scrap is used during melting. Therefore, Si and Fe are regulated to the smallest possible content of 0.2% or less.
[0029]
Below, the manufacturing method of the Al-Mg type aluminum alloy plate in this invention is demonstrated. The production method in the present invention can basically be produced by a conventional method. First, the melting and casting processes are performed by appropriately selecting a normal melting and casting method such as a continuous casting and rolling method or a semi-continuous casting method (DC casting method) for a molten aluminum alloy adjusted to be within the component specification range of the present invention. Cast. Next, the aluminum alloy ingot is subjected to a homogenization heat treatment.
[0030]
After the above-described hot rolling after the homogenization heat treatment and intermediate annealing as necessary, the Al alloy plate is cold-rolled to a desired thickness. When a thick plate of 3.0 mm or more is required depending on the panel, cold rolling may be omitted and the aluminum alloy plate may be hot rolled.
[0031]
These hot-rolled sheets and cold-rolled sheets are subjected to a tempering process such as final annealing as necessary, or are subjected to high-speed superplastic forming without a tempering process to form a body panel.
[0032]
The aluminum alloy sheet referred to in the present invention is an unheated (heat treated) sheet such as a hot-rolled (hot-rolled) sheet, a cold-rolled (cold-rolled) sheet, or annealing these sheets. This refers to the board after the tempering treatment. And the board said by this invention includes appropriate shape states, such as a board, a coil, and a cut board, before high-speed superplastic forming.
[0033]
The high-speed superplastic forming referred to in the present invention is an arbitrary press for forming a plate into a panel in a high temperature region of 460 to 550 ° C. under a condition that the strain rate reaches 10 ⁻² to 10 ⁰ / s. A molding method and a mold molding method can be included. Furthermore, depending on the use and shape of the body panel, it is cold-pressed before high-speed superplastic forming or after high-speed superplastic forming, hemming such as flat hem as outer panel, bending, trimming, etc. This includes the case where the above processing is added as appropriate.
[0034]
【Example】
Next, examples of the present invention will be described. Aluminum alloy ingots of Al-Mg-based aluminum alloys within the scope of the present invention of A to F shown in Table 1 and Al-Mg-based aluminum alloys outside the scope of the present invention of G to J are melted by DC casting. And the board was manufactured on the following same conditions. That is, after chamfering, the thickness was set to 470 mmt, heated at a heating rate of 40 ° / h, homogenized heat treatment at 480 ° C. × 4 hours, and hot-rolled to obtain a 4.5 mmt hot-rolled sheet. This hot-rolled sheet was subjected to intermediate annealing at 400 ° C. for 3 hours and then cold-rolled to obtain a 1.0 mmt cold-rolled sheet. And this cold-rolled sheet was finally annealed at 520 ° C. for 20 seconds to obtain a test plate.
[0035]
In each case, test pieces were collected from these test plates, and the average crystal grain size in the rolling (L) direction of the plates was measured. As a result, the average crystal grain size of each invention example and comparative example was 50 μm or less. The average crystal grain size is measured by observing the surface of the aluminum alloy plate that has been mechanically polished by 0.05 to 0.1 mm and then electrolytically etched using a 200 × optical microscope, and using the line intercept method in the L direction. taking measurement. The length of 1 measurement line was 0.95 mm, and the total measurement line length was 0.95 × 15 mm by observing a total of 5 fields with 3 lines per field.
[0036]
In each example, a test piece was taken from the test plate, and the high temperature characteristics were evaluated by a high temperature tensile test of the test plate in order to evaluate high-speed superplastic forming. A high temperature tensile test for measuring high temperature elongation among the high temperature properties was conducted at a temperature of 500 ° C. And at this temperature, it carried out on the conditions of temperature rising rate: 100 degree-C / min, strain rate: 10 ^<-1 > / s, distance between grades: 15 mm, and test piece shape JIS No. 5 test piece. Further, the test was performed at a constant strain rate until the test piece broke. Then, the elongation (δ,%) in the direction parallel to the rolling direction of each test plate was measured.
[0037]
The cavity area ratio in the high temperature characteristics is measured for the test piece after the high temperature tensile test is performed at the above temperature of 500 ° C., and the test piece at the position where the plate thickness after the high temperature tensile test is reduced to ½. The structure was observed using a 200-fold optical microscope, and the cavity area ratio in the direction parallel to the rolling direction was measured with an average value of four fields of view. These results are also shown in Table 2.
[0038]
Furthermore, the strength of each test piece after high-speed superplastic forming was evaluated. For this reason, in each example, a test piece was sampled from the above-mentioned test plate by simulating high-speed superplastic forming, and this test piece was held at 500 ° C. for 3 minutes and allowed to cool to room temperature. .2% yield strength (σ _0.2 , MPa, A in Table 2) was measured. Furthermore, assuming an actual automobile panel, the test piece after the heat treatment was further subjected to an artificial age hardening treatment at 180 ° C. for 60 minutes after being subjected to a paint bake hardening treatment after high-speed superplastic forming (Bake Hard The 0.2% proof stress (σ _0.2 , MPa, B in Table 2) of the test piece was measured.
[0039]
And the dent resistance of the test piece after the above-mentioned artificial age hardening treatment at 180 ° C. × 60 minutes was further evaluated. The dent resistance test was performed by measuring the amount of dent at the load point when a load of 245 MPa was applied to the central portion of the test piece with a ball head punch having a tip R 2 of 50 mmΦ. Then, evaluation was made with a dent amount of less than 0.3 mm 3 as ◯ and a dent amount of 0.3 mm or more as x. These results are also shown in Table 2.
[0040]
As apparent from Tables 1 and 2, Invention Examples 1 to 6 using Al—Mg based aluminum alloys within the scope of the present invention of A to F in Table 1 within the composition range of the present invention are high temperature elongation (δ,% ) Is equal to or higher than those of Comparative Examples 7 to 9, and the cavity area ratio is also small. Therefore, compared with Comparative Examples 7 to 9, the high-speed superplastic formability is equal or superior.
[0041]
In Invention Examples 1 to 6, the 0.2% proof stress B 1 after the artificial age hardening treatment after the high temperature heat treatment simulating high-speed superplastic forming is 150 MPa or more, and the strength after the high temperature heat treatment and the artificial age hardening treatment is high. . Further, compared to Comparative Examples 7 and 8, the dent resistance is remarkably excellent.
[0042]
These results of Invention Examples 1 to 6 together with the results of Comparative Examples 7 to 9 indicate that it is suitable for high-speed superplastic forming and subsequent automobile body outer panel subjected to paint baking and curing.
[0043]
Among Invention Examples 1 to 6, Invention Example 3 using an alloy C 2 having a relatively high content of Si and Fe is different from other Invention Examples having a relatively low content of Si and Fe. High temperature elongation (δ%) is low, and cavity area ratio is large. Therefore, the high-speed superplastic formability is relatively inferior compared to other invention examples. Further, in Comparative Example 9 using the alloy I with a large amount of Si and Fe removed, the 0.2% proof stress B after high temperature heat treatment and artificial age hardening treatment is 150 MPa or more, but the high temperature elongation (δ%) is high. The cavity area ratio is extremely large. These results support the significance of restricting Si and Fe to 0.2% or less.
[0044]
Among Invention Examples 1 to 6, Invention Example 6 using an alloy F 2 having a relatively low Mg content is higher in heat treatment and heat treatment than other Invention Examples 1 and 2 having a relatively high Mg content. The 0.2% yield strength B 2 after the artificial age hardening treatment is relatively low. Together with this result and the result that the 0.2% proof stress B 2 after the high-temperature heat treatment and the artificial age hardening treatment of Comparative Example 8 using the alloy H 2 with a slightly lower Mg content is less than 150 MPa, etc. This confirms the contribution of Mg to strength and the significance of the numerical range.
[0045]
Among Invention Examples 1 to 6, Invention Example 5 using the alloy E 2 having a relatively low Cu content is higher in heat treatment and heat treatment than other Invention Examples 1 and 2 having a relatively high Cu content. The 0.2% yield strength B 2 after the artificial age hardening treatment is relatively low. As a result, the 0.2% proof stress B 2 after the high-temperature heat treatment and artificial age hardening treatment of Comparative Example 7 using the alloy G 2 with a relatively low Cu content of 0.5% is less than 150 MPa. Together with the remarkably low results, the significance of containing a large amount of Cu exceeding 0.5% is supported.
[0046]
[Table 1]

[0047]
[Table 2]

[0048]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the Al-Mg type aluminum alloy plate which is high in strength after high-speed superplastic forming and excellent in high-speed superplastic formability can be provided as a body panel application. Moreover, this Al—Mg 2 aluminum alloy plate can be manufactured without changing the conventional plate manufacturing process. Therefore, it has a great industrial value in that it can be applied to high-speed superplastic forming of a 5000 series aluminum alloy plate and use for a body panel.

Claims

Mg: 3.5 to 7.0%, Cu: more than 0.5% and 1.0% or less, Ti: 0.001 to 0.1%, and Si and Fe are each 0.2% or less In addition to the regulation, Mn: 0.1% or less, consisting of the balance Al and inevitable impurities, maintained at 500 ° C. for 3 minutes and allowed to cool to room temperature, and further subjected to artificial age hardening treatment An Al—Mg-based aluminum alloy plate for high-speed superplastic forming characterized by having a characteristic that the 2% proof stress is 150 MPa or more.

Furthermore, the Al-Mg type aluminum alloy plate for high-speed superplastic forming of Claim 1 containing 0.05 to 0.5% of Cr.

The Al-Mg based aluminum alloy plate for high-speed superplastic forming according to claim 1 or 2, wherein the aluminum alloy plate is subjected to paint baking hardening after high-speed superplastic forming.

The Al-Mg based aluminum alloy plate for high-speed superplastic forming according to any one of claims 1 to 3, wherein the aluminum alloy plate is used for an automobile panel.

The Al-Mg based aluminum alloy plate for high-speed superplastic forming according to any one of claims 1 to 4, wherein the artificial age hardening treatment is performed at 180 ° C for 60 minutes.