JP2004068076A

JP2004068076A - Aluminum alloy forging material for structure having excellent corrosion resistance and method for producing the same

Info

Publication number: JP2004068076A
Application number: JP2002228279A
Authority: JP
Inventors: Tomoyuki Kanda; 神田　知之; Shinichi Matsuda; 松田　眞一; Hideo Yoshida; 吉田　英雄; Koichi Fujita; 藤田　浩一; Takeshi Seguchi; 瀬口　剛; Motohide Mori; 森　元秀; Hitoshi Sakuma; 佐久間　均
Original assignee: Sumitomo Light Metal Industries Ltd; Toyota Motor Corp; Somic Ishikawa KK
Current assignee: Sumitomo Light Metal Industries Ltd; Toyota Motor Corp; Somic Ishikawa KK
Priority date: 2002-08-06
Filing date: 2002-08-06
Publication date: 2004-03-04
Anticipated expiration: 2022-08-06
Also published as: JP3929850B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy forging material for structure which is excellent in a balance of strength, toughness and corrosion resistance, and to provide a method for producing the same. <P>SOLUTION: The aluminum alloy forging material comprises 0.40 to 0.8% Si, 0.8 to 1.2% Mg, ≤0.40% Cu, 0.08 to 0.15% Mn and 0.10 to 0.35% Cr, and the balance Al with impurities. In the vertical cross-section of the forging material, the surface layer part has a recrystallized structure, and in the part other than the surface layer part, a Hypo-crystal structure with a mean crystal grain size of ≤10 μm occupying the region of 50 to 95% in the vertical cross-section is present. The aluminum alloy has the composition within the componential standards of a JIS 6061 alloy, thus is also excellent in productivity and recycling properties, and can expand the applying range to automobile structural members. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、耐食性に優れた構造用アルミニウム合金鍛造材、とくに車両用の構造部材として好適に使用される耐食性に優れた構造用アルミニウム合金鍛造材およびその製造方法に関する。
【０００２】
【従来の技術】
近年、自動車など車両分野においては、地球環境保全の観点から排ガス規制が厳しくなりつつあり、燃費向上が重要な課題となっている。この課題を解決する手段として車両重量の軽量化が急速に進行しており、車両用構造部材についても、従来の鉄系材料に替えてアルミニウム系材料が実用化されている。
【０００３】
車両用構造部材のうち足回り部品や複雑形状の部品にはアルミニウム合金の鍛造部材が採用され、材質としてはＡｌ−Ｍｇ−Ｓｉ系（６０００系）アルミニウム合金が一般的に使用されている。とくに、足回り部品は、使用環境が過酷であるため、強度、靱性とともに耐食性が要求されているため、これらの特性に優れ生産性も良いＪＩＳ６０６１合金が適用されている。
【０００４】
ＪＩＳ６０６１合金の構造部材、鋳塊を熱間鍛造し、あるいは鋳塊を押出加工したのち熱間鍛造し、その後Ｔ６調質を行うことにより製造されるが、その強度特性は引張強さで２７０〜３２０ＭＰａ程度であるため、車両構造の十分な軽量化が得られず、さらに改善された強度をそなえたアルミニウム合金材が要請されている。
【０００５】
強度を向上させた６０００系のアルミニウム合金として、Ｍｎ、Ｃｒ、Ｚｒを添加し、主要成分のＭｇ、Ｓｉ量を調整して結晶粒の粗大化を抑制し、焼入れ感受性に優れた押出鍛造用Ａｌ−Ｍｇ−Ｓｉ系合金（特公平５−４７６１３号公報）、Ｍｇ、Ｓｉ、Ｃｕを増量し、Ｍｎ、Ｃｒを併用添加した鍛造用アルミニウム合金（特開平５−５９４７７号公報）が提案されている。
【０００６】
しかしながら、これらのアルミニウム合金においては、強度は優れているが、靱性、耐食性に対する配慮がなされておらず、添加元素の増加は熱間変形抵抗を増大させる傾向があるため、押出性や鍛造性を低下させ、押出速度の低下や鍛造回数増により製造コストを増大させるという問題があり、とくにＣｕ量の増加は耐食性を低下させるという難点がある。
【０００７】
鋳塊を直接熱間鍛造するＡｌ−Ｍｇ−Ｓｉ系合金で、晶析出物の粒径、間隔を規定し、靱性、耐食性の向上を図ったアルミニウム合金鍛造材も提案されているが（特開２００１−１０７１６８号公報）、このものにおいてもシャルピー衝撃値は１１〜１３Ｊ／ｃｍ^２で、自動車の足回り部品として使用するためにはなお十分なものではない。また、押出工程を経ることなく鋳造材を直接熱間鍛造した場合、鍛造比が小さい部位には鋳塊組織が残存して靱性の低下が生じ易く、鍛造品内部の機械的特性のばらつきも大きくなる可能性が高い。
【０００８】
Ｍｇ０．８〜１．２％、Ｓｉ０．４〜１．０％、Ｃｕ０．２〜０．４％、Ｍｎ０．１５〜０．４０％、Ｃｒ０．１〜０．２０％、Ｚｒ０．１〜０．２％を含有し、残部Ａｌおよび不純物からなり、粗大結晶粒の発生を抑え強度を高めた押出鍛造用Ａｌ−Ｍｇ−Ｓｉ系合金も提案されているが（特開平１−２８３３３７号公報）、この合金を当該公報に開示されている通常の４５０℃の鍛造温度で鍛造加工したものは自動車用構造部材として十分な強度が得られていない。
【０００９】
Ｍｇ０．４〜０．８％、Ｓｉ０．４〜１．０％、Ｃｕ０．１５〜０．５％、Ｔｉ０．００５〜０．２％を含有し、さらにＭｎ０．０５〜０．６％、Ｃｒ０．０５〜０．３％、Ｚｒ０．０５〜０．３％のうちの１種以上を合計で０．９％以下の範囲で含み、残部Ａｌおよび不純物からなる熱間鍛造用アルミニウム合金も提案されているが（特開平１１−１２６７５号公報）、この合金においても、溶体化処理、時効処理後の引張強さは３３５ＭＰａ以下で、自動車部品として強度的に必ずしも十分なものではなく、また、押出工程を経ることなく鋳造材を直接熱間鍛造するものであるため、前記のように、鋳塊組織の残存による靱性の低下、機械的特性のばらつきなどの問題もある。
【００１０】
さらに、車両用構造部材にアルミニウム合金材料を適用する場合、コスト低減の観点からリサイクル性が重要な課題であり、既存の規格合金成分範囲を大きく外れた材料を使用した場合、使用成分範囲によっては他の規格合金と識別する必要性が生じ、一般的に、添加元素の種類や量を多くすることはリサイクルの点で好ましくない。
【００１１】
【発明が解決しようとする課題】
本発明は、車両構造用Ａｌ−Ｍｇ−Ｓｉ系合金における上記従来の問題点を解消するとともに、リサイクル性も考慮した６０００系の押出鍛造アルミニウム合金材を得るために、合金材の諸特性と合金成分の組合わせ、組織の関係についてさらに試験、検討を重ねた結果としてなされたものであり、その目的は、強度、靱性および耐食性のバランスに優れ、生産性、リサイクル性にも優れた構造用アルミニウム合金鍛造材およびその製造方法を提供することにある。
【００１２】
【課題を解決するための手段】
上記の目的を達成するための本発明による耐食性に優れた構造用アルミニウム合金鍛造材は、Ｓｉ：０．４０〜０．８％、Ｍｇ：０．８〜１．２％、Ｃｕ：０．４０％以下、Ｍｎ：０．０８〜０．１５％、Ｃｒ：０．１０〜０．３５％を含有し、残部Ａｌおよび不可避不純物からなる組成を有するアルミニウム合金の鍛造材であって、該鍛造材の直角断面において表層部は再結晶組織で、表層部以外の部分に前記直角断面の５０〜９５％の領域を占める平均結晶粒径１０μｍ以下の亜結晶粒組織が存在することを特徴とする。
【００１３】
また、本発明による耐食性に優れた構造用アルミニウム合金鍛造材の製造方法は、上記請求項１記載のアルミニウム合金の鋳塊を、４００〜４９０℃で均質化処理した後、４８０〜５４０℃で熱間押出および４８０〜５４０℃で熱間鍛造を行い、５００〜５４０℃で溶体化処理し、ついで１５０〜２００℃で時効処理することを特徴とする。
【００１４】
【発明の実施の形態】
本発明のアルミニウム合金鍛造材における合金成分の意義および限定理由につて説明すると、Ｓｉは、Ｍｇと共存して合金マトリックス中にＭｇ_２Ｓｉ粒子を析出させ、合金の強度を向上するよう機能する。好ましい含有量は０．４０〜０．８％の範囲であり、０．４０％未満では十分な強度が得難く、０．８％を越えて含有されると加工性が低下するとともに、伸びの低下も生じる。とくに過剰Ｓｉが多くなるため耐食性も低下する。
【００１５】
Ｍｇは、Ｓｉと共存して合金マトリックス中にＭｇ_２Ｓｉ粒子を析出させ、合金の強度を高めるよう機能する。Ｍｇの好ましい含有量は０．８〜１．２％の範囲であり、０．８％未満ではその効果が小さく、１．２％を越えると、加工性、焼入れ性が低下する。
【００１６】
Ｃｕは、合金マトリックス中に固溶して強度を向上させるよう機能する。好ましい含有量は０．４０％以下範囲であり、０．４０％を越えて含有すると、耐食性が顕著に低下し、加工性の低下も生じる。Ｃｕのさらに好ましい含有範囲は０．２０〜０．４０％である。
【００１７】
ＭｎおよびＣｒは、合金マトリックス中に、それぞれＡｌ−Ｍｎ（−Ｓｉ）系およびＡｌ−Ｃｒ系の化合物を析出させ、亜結晶粒の形成と形成された亜結晶粒組織を維持する役割を果たし、さらに両者を複合して添加することによりその効果がさらに向上する。好ましい含有量は、Ｍｎ：０．０８〜０．１５％、Ｃｒ：０．１０〜０．３５％の範囲であり、それぞれ下限未満では、亜結晶粒の形成、維持効果が十分でなく、それぞれ上限を越えて含有すると、巨大な金属間化合物が生成して靱性、延性が低下する。ＭｎおよびＣｒのさらに好ましい含有範囲は、Ｍｎ：０．１０〜０．１５％、Ｃｒ：０．２０〜０．３０％である。
【００１８】
なお、本発明のアルミニウム合金鍛造材には、１％以下のＦｅ、０．３％以下のＺｎ、０．２％以下のＴｉ、０．１％以下のＢが含有されていても本発明の効果が害されることはない。
【００１９】
本発明においては、溶体化処理および時効処理（Ｔ６処理）後のアルミニウム合金鍛造材の直角断面において表層部は再結晶組織で、表層部以外の部分に該直角断面の５０〜９５％の領域を占める平均結晶粒径１０μｍ以下の亜結晶粒組織が存在することが重要である。この組織とすることにより高強度、高靱性を達成することができる。
【００２０】
本発明のアルミニウム合金鍛造材において、強度および靱性の向上は、前記Ｍｇ_２Ｓｉなどの析出硬化、Ｃｕなどの固溶強化による以外に、微細な亜結晶粒組織を形成することが重要な要素となる。好ましい組織性状は、亜結晶粒組織が鍛造材の直角断面において、該断面の５０〜９５％を占めるようにし、亜結晶粒の平均粒径を１０μｍ以下とする。
【００２１】
亜結晶粒組織が鍛造材の直角断面において、該断面の５０％未満では鍛造材全体の強度が不十分となり、９５％を越えると鍛造材全体の靱性が不足する。亜結晶粒の平均粒径が１０μｍを越えると、強度向上の効果が十分でなくなる。
【００２２】
亜結晶粒組織は繊維状に発達するため、材料中に異方性が生じ、繊維状長手方向と比較して垂直方向の靱性が低くなる傾向がある。従って、鍛造材全体の靱性を向上させるために、鍛造材の表層部は再結晶組織として、表層部を延性および耐衝撃性に優れたものとするのが好ましく、内部の亜結晶粒組織との組合わせによって改善された強度、靱性が達成される。
【００２３】
亜結晶粒の粒径の測定は、鍛造材の直角断面の組織を、透過型電子顕微鏡で倍率５０００倍で写真撮影し、切断法（既知の長さの線分によって完全に切断される結晶粒の数を数え、その切断長さの平均値をもって表示するもので、ＪＩＳ　Ｇ０５２２に規定される鋼のフェライト結晶粒度試験方法に準拠する）により測定する。また、亜結晶粒組織の直角断面の面積率は、鍛造材サンプルの直角断面を１０％ＮａＯＨ水溶液でエッチング処理（液温：２０℃、浸漬時間：４５分）し、サンプルの大きさに応じて１〜１０倍で写真撮影し、画像解析装置を使用して測定する。
【００２４】
つぎに、本発明のアルミニウム合金鍛造材の製造について説明すると、前記のように、本発明においては、微細な亜結晶粒組織の形成を特徴とするものであるが、鍛造材の組織が粗大な再結晶組織となるのは、熱間押出または熱間鍛造時に蓄積された加工歪みが最終溶体化処理時に回復、再結晶することに起因することが多く、溶体化処理温度が高いほど再結晶粒が粗大化する傾向がある。所定の強度が得られる溶体化処理温度は合金組成で決まっており、本発明による合金組成の場合、十分な固溶状態を得るためには５００℃以上の溶体化処理を行う必要があるため、粗大再結晶粒の形成を抑制するために、溶体化処理温度を下げることはできない。これを解消する手段として、本発明においては、熱間加工温度を溶体化処理温度域あるいはその近傍として、溶体化処理時に再結晶しない熱的に安定した亜結晶粒組織を形成するとともに、Ｍｎ系化合物、Ｃｒ系化合物を微細、均一に析出させて亜結晶粒の形成を促進し、形成された亜結晶粒組織の維持を図る。
【００２５】
Ｍｎ系化合物、Ｃｒ系化合物を微細、均一に析出させるために、前記の組成を有するアルミニウム合金の鋳塊について、４００〜４９０℃の温度で均質化処理を行う。通常、６０００系アルミニウム合金の均質化処理は４９０℃を越える温度（且つ固相線未満の温度）で行われることが多いが、４９０℃を越える温度で均質化処理を行った場合、鋳造時に過飽和に固溶していたＭｎ、Ｃｒ原子が均質化処理中に析出、粗大化してピンニング効果が小さくなり、熱間加工時に安定した亜結晶粒組織が形成し難くなる。４００℃未満ではＭｎ、Ｃｒがほとんど析出しないため、熱間加工時に導入される歪みの回復サイトが不足し、安定した亜結晶粒組織の形成が困難となる。さらに好ましい均質化処理温度は４５０〜４９０℃である。
【００２６】
ついで、４８０〜５４０℃で熱間押出および４８０〜５４０℃で熱間鍛造を行う。熱間押出および熱間鍛造を４８０℃未満の温度で行った場合には、Ｍｇ_２Ｓｉの固溶が不十分となり、熱間加工中にＭｇ_２Ｓｉが凝集化し易くなり、その結果、最終溶体化処理時に凝集化したＭｇ_２Ｓｉが再固溶し難くなって十分な強度が得られなくなる。また、熱間加工時に蓄積される歪み量が多くなり、溶体化処理時に粗大な再結晶粒が発生し易くなる。５４０℃を越える温度で熱間押出、熱間鍛造を行うと、亜結晶粒組織の形成、維持が困難となり、熱間加工時に粗大な再結晶組織が形成し易くなる。さらに好ましい熱間押出および熱間鍛造の温度は５００〜５４０℃である。
【００２７】
上記の熱間加工後、溶体化処理を行い。好ましい溶体化処理温度は５００〜５４０℃であり、５００℃未満ではＭｇ_２Ｓｉの固溶が不十分となり、所望の強度が得難い。５４０℃を越えると、粗大再結晶が生じ易くなり強度低下を招く。さらに好ましい溶体化処理温度は５１０〜５３０℃である。
【００２８】
溶体化処理に続いて時効処理（焼戻し処理）を行う。好ましい焼戻し処理温度は１５０〜２００℃の範囲であり、１５０℃未満ではＭｇ_２Ｓｉによる析出硬化が十分でなく、２００℃を越えると、Ｍｇ_２Ｓｉが粗大析出して十分な析出硬化が得難くなる。さらに好ましい焼戻し処理温度は１７０〜１９０℃である。
【００２９】
【実施例】
以下、本発明の実施例を比較例と対比して説明し、本発明の効果を実証する。これらに実施例は、本発明の一実施態様であり、本発明はこれに限定されるものではない。
【００３０】
実施例１
表１に示す組成のアルミニウム合金を溶解し、半連続鋳造により直径９０ｍｍの押出用ビレットに造塊した。このビレットを表２に示す条件で、均質化処理、熱間押出（押出寸法：直径２０ｍｍ、丸棒材）、熱間鍛造（鍛造材：図１に示すようにノッチ部Ｎを有する形状、寸法）し、その後、表２に示す温度で溶体化処理を行い、１７０℃で１０ｈの焼戻し処理を施した。
【００３１】
得られた鍛造品（Ｔ６調質材）を試験材として、図１のＡ−Ａ断面中心部の平均結晶粒径、断面に占める亜結晶粒組織の面積率を前記の方法で調査し、機械的性質を評価し、また、鍛造品全体の靱性を評価するために、ノッチ部Ｎを含むＪＩＳ３号衝撃試験片を採取し、室温でシャルピー衝撃試験を実施した。なお、試験片において、ノッチ部Ｎは鍛造バリのみを機械加工で除去した状態とした。さらに、耐食性の評価として、ＪＩＳ　Ｚ２３７１に準拠する塩水噴霧試験を１０００時間行い、最大孔食深さを測定した。結果を表３に示す。
【００３２】
表３にみられるように、本発明に従う試験材Ｎｏ．１〜４はいずれも、耐力が３４０ＭＰａ以上の優れた強度、３０Ｊ／ｃｍ^２を越える衝撃特性をそなえ、耐食性も最大孔食深さが２００μｍ以下であった。
【００３３】
【表１】

【００３４】
【表２】

【００３５】
【表３】

【００３６】
比較例１
表１に示す組成のアルミニウム合金を溶解し、半連続鋳造により直径９０ｍｍの押出用ビレットに造塊した。このビレットを表４に示す条件で、均質化処理、熱間押出（押出寸法：直径２０ｍｍ、丸棒材）、熱間鍛造（鍛造材：図１に示すようにノッチ部Ｎを有する形状、寸法）し、その後、表４に示す温度で溶体化処理を行い、１７０℃で１０ｈの焼戻し処理を施した。
【００３７】
得られた鍛造品（Ｔ６調質材）を試験材として、実施例１と同様、図１のＡ−Ａ断面中心部の平均結晶粒径、断面に占める亜結晶粒組織の面積率を前記の方法で調査し、機械的性質を評価し、また、実施例１と同様にして、衝撃特性、耐食性を評価した。結果を表５に示す。なお、表４〜５において、本発明の条件を外れたものには下線を付した。
【００３８】
表５に示すように、試験材Ｎｏ．５は均質化処理温度が高く熱間押出温度が低く、試験材Ｎｏ．６は熱間鍛造温度が低く、試験材Ｎｏ．７は溶体化処理温度が高く、また試験材Ｎｏ．８は熱間鍛造温度が低いため、いずれも粗大な再結晶組織となり十分な強度が得られなかった。
【００３９】
【表４】

【００４０】
【表５】

【００４１】
比較例２
表６に示す組成のアルミニウム合金を溶解し、半連続鋳造により直径９０ｍｍの押出用ビレットに造塊した。このビレットを表７に示す条件で、均質化処理、熱間押出（押出寸法：直径２０ｍｍ、丸棒材）、熱間鍛造（鍛造材：図１に示すようにノッチ部Ｎを有する形状、寸法）し、その後、表７に示す温度で溶体化処理を行い、１７０℃で１０ｈの焼戻し処理を施した。
【００４２】
得られた鍛造品（Ｔ６調質材）を試験材として、実施例１と同様、図１のＡ−Ａ断面中心部の平均結晶粒径、断面に占める亜結晶粒組織の面積率を前記の方法で調査し、機械的性質を評価し、また、実施例１と同様にして、衝撃特性、耐食性を評価した。結果を表８に示す。なお、表６〜８において、本発明の条件を外れたものには下線を付した。
【００４３】
表８に示すように、試験材Ｎｏ．９はＭｎ量、Ｃｒ量が多いため、亜結晶粒組織の面積率が大きく、衝撃特性が劣る。試験材Ｎｏ．１０はＳｉ量、Ｍｇ量が多いため、衝撃特性が劣るとともに耐食性も低下している。試験材Ｎｏ．１１〜１３はＭｎ量、Ｃｒ量が少ないため、鍛造材の組織が粗大化した再結晶組織となり強度が低い。試験材Ｎｏ．１４はＣｕ量が多いため、耐食性が顕著に低下した。
【００４４】
【表６】

【００４５】
【表７】

【００４６】
【表８】

【００４７】
【発明の効果】
本発明によれば、強度、靱性および耐食性のバランスに優れた構造用アルミニウム合金鍛造材およびその製造方法が提供される。本発明のアルミニウム合金はＪＩＳ６０６１合金の成分規格内の組成を有することから、生産性、リサイクル性に優れ、自動車用構造部材への適用範囲を拡大することが可能であり、工業上の価値が大きい。
【図面の簡単な説明】
【図１】本発明の実施例、比較例の試験材として使用する鍛造材の形状、寸法を示す正面、Ａ−Ａ断面図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a structural aluminum alloy forging having excellent corrosion resistance, and more particularly to a structural aluminum alloy forging having excellent corrosion resistance suitably used as a structural member for a vehicle and a method for producing the same.
[0002]
[Prior art]
2. Description of the Related Art In recent years, in the field of vehicles such as automobiles, exhaust gas regulations have been becoming stricter from the viewpoint of global environmental protection, and improving fuel efficiency has become an important issue. As means for solving this problem, the weight of vehicles has been rapidly reduced, and aluminum-based materials have been put into practical use also for structural members for vehicles in place of conventional iron-based materials.
[0003]
Among the structural members for vehicles, forged members of aluminum alloy are used for underbody parts and parts of complicated shapes, and Al-Mg-Si (6000-based) aluminum alloy is generally used as the material. In particular, since the undercarriage parts are required to have corrosion resistance as well as strength and toughness due to severe use environment, JIS6061 alloy which is excellent in these characteristics and has good productivity is applied.
[0004]
It is manufactured by hot forging a JIS6061 alloy structural member or ingot or extruding an ingot and then hot forging, and then performing T6 tempering. Its strength characteristics are 270 to 270 in tensile strength. Since the pressure is about 320 MPa, sufficient weight reduction of the vehicle structure cannot be obtained, and an aluminum alloy material having further improved strength has been demanded.
[0005]
As a 6000 series aluminum alloy with improved strength, Mn, Cr, and Zr are added, and the amounts of main components Mg and Si are adjusted to suppress coarsening of crystal grains, and Al for extrusion forging is excellent in quenching sensitivity. -Mg-Si based alloy (Japanese Patent Publication No. 5-47613), and an aluminum alloy for forging in which Mg, Si, and Cu are increased and Mn and Cr are added in combination (Japanese Patent Application Laid-Open No. 5-59477) have been proposed. .
[0006]
However, these aluminum alloys have excellent strength, but no consideration is given to toughness and corrosion resistance, and an increase in added elements tends to increase hot deformation resistance. However, there is a problem that the production cost is increased due to a decrease in the extrusion rate or an increase in the number of times of forging. Particularly, an increase in the amount of Cu has a problem in that the corrosion resistance is reduced.
[0007]
There has been proposed an aluminum-alloy forging material which is an Al-Mg-Si-based alloy for directly hot forging an ingot, which regulates the grain size and interval of crystal precipitates and improves toughness and corrosion resistance (Japanese Patent Application Laid-Open (JP-A) No. 2002-287, 1988). 2001-107168), which has a Charpy impact value of 11 to 13 J / cm ² , which is still insufficient for use as a vehicle underbody part. In addition, when the cast material is directly hot forged without going through the extrusion process, the ingot structure remains in a portion where the forging ratio is small, and the toughness is likely to decrease, and the variation in the mechanical properties inside the forged product is large. Likely to be.
[0008]
Mg 0.8-1.2%, Si 0.4-1.0%, Cu 0.2-0.4%, Mn 0.15-0.40%, Cr 0.1-0.20%, Zr 0.1-0 An Al-Mg-Si alloy for extrusion forging has been proposed which contains 0.2%, the balance being Al and impurities, suppressing the generation of coarse crystal grains and increasing the strength (Japanese Patent Application Laid-Open No. 1-283337). When this alloy is forged at a normal forging temperature of 450 ° C. disclosed in this publication, sufficient strength as a structural member for automobiles is not obtained.
[0009]
Mg 0.4-0.8%, Si 0.4-1.0%, Cu 0.15-0.5%, Ti 0.005-0.2%, Mn 0.05-0.6%, Cr0 Also, an aluminum alloy for hot forging containing at least one of 0.05 to 0.3% and Zr of 0.05 to 0.3% in a range of 0.9% or less in total and the balance of Al and impurities has been proposed. However, even in this alloy, the tensile strength after solution treatment and aging treatment is 335 MPa or less, which is not always sufficient in terms of strength as an automobile part. Since the cast material is directly hot-forged without going through a process, there are problems such as a decrease in toughness due to the residual ingot structure and a variation in mechanical properties as described above.
[0010]
Furthermore, when aluminum alloy material is applied to structural members for vehicles, recyclability is an important issue from the viewpoint of cost reduction.If a material that greatly deviates from the existing standard alloy component range is used, depending on the used component range, There is a need to distinguish the alloy from other standard alloys, and it is generally not preferable to increase the types and amounts of the added elements from the viewpoint of recycling.
[0011]
[Problems to be solved by the invention]
The present invention solves the above-mentioned conventional problems in Al-Mg-Si alloys for vehicle structures and obtains 6000 series extruded and forged aluminum alloy materials in consideration of recyclability, by using various properties of alloy materials and alloys. This was the result of further testing and examination of the combination of components and the relationship between the structures, and the purpose was to achieve a structural aluminum with an excellent balance of strength, toughness and corrosion resistance, as well as excellent productivity and recyclability. An object of the present invention is to provide an alloy forging and a method for manufacturing the same.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, a structural aluminum alloy forging having excellent corrosion resistance according to the present invention comprises Si: 0.40 to 0.8%, Mg: 0.8 to 1.2%, and Cu: 0.40. % Or less, Mn: 0.08 to 0.15%, Cr: 0.10 to 0.35%, forged aluminum alloy having a composition consisting of the balance of Al and inevitable impurities, Is characterized in that the surface layer portion has a recrystallized structure, and a sub-crystal structure having an average crystal grain size of 10 μm or less occupying a region of 50 to 95% of the rectangular cross section exists in a portion other than the surface layer portion.
[0013]
The method for producing a structural aluminum alloy forged material having excellent corrosion resistance according to the present invention is characterized in that the ingot of the aluminum alloy according to claim 1 is homogenized at 400 to 490 ° C and then heated at 480 to 540 ° C. Hot extrusion at 480-540 ° C, solution treatment at 500-540 ° C, and aging at 150-200 ° C.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Explaining the significance of alloy components in the aluminum alloy forged material of the present invention and the reasons for limitation, Si functions to coexist with Mg to precipitate Mg ₂ Si particles in an alloy matrix, thereby improving the strength of the alloy. The preferred content is in the range of 0.40 to 0.8%. If the content is less than 0.40%, it is difficult to obtain sufficient strength. If the content is more than 0.8%, the processability is reduced and the elongation is low. Degradation also occurs. In particular, the excess Si increases, so that the corrosion resistance also decreases.
[0015]
Mg coexists with Si to precipitate Mg ₂ Si particles in the alloy matrix and functions to increase the strength of the alloy. The preferred content of Mg is in the range of 0.8 to 1.2%. If the content is less than 0.8%, the effect is small, and if it exceeds 1.2%, workability and hardenability are reduced.
[0016]
Cu functions as a solid solution in the alloy matrix to improve the strength. The preferred content is in the range of 0.40% or less. When the content exceeds 0.40%, the corrosion resistance is significantly reduced and the workability is also reduced. A more preferable content range of Cu is 0.20 to 0.40%.
[0017]
Mn and Cr precipitate Al-Mn (-Si) -based and Al-Cr-based compounds, respectively, in the alloy matrix, and play a role of forming sub-crystal grains and maintaining the formed sub-crystal grain structure, The effect is further improved by adding both in combination. The preferred content is in the range of Mn: 0.08 to 0.15% and Cr: 0.10 to 0.35%. If the content is less than the lower limit, the effect of forming and maintaining subcrystal grains is not sufficient. If the content exceeds the upper limit, a huge intermetallic compound is formed, and the toughness and ductility decrease. More preferred ranges of Mn and Cr are Mn: 0.10 to 0.15% and Cr: 0.20 to 0.30%.
[0018]
In addition, even if the forged aluminum alloy of the present invention contains 1% or less of Fe, 0.3% or less of Zn, 0.2% or less of Ti, and 0.1% or less of B, The effect is not impaired.
[0019]
In the present invention, in the right-angled cross section of the aluminum alloy forged material after the solution treatment and the aging treatment (T6 treatment), the surface layer portion has a recrystallized structure, and a region other than the surface layer portion has a region of 50 to 95% of the right-angled cross section. It is important that a sub-crystalline structure having an average crystal grain size of 10 μm or less is present. With this structure, high strength and high toughness can be achieved.
[0020]
In the aluminum alloy forged material of the present invention, the improvement in strength and toughness is not only due to the precipitation hardening of Mg ₂ Si or the like, but also the solid solution strengthening of Cu or the like, and it is important to form a fine sub-crystal grain structure. Become. The preferred texture is such that the sub-crystal grain structure occupies 50 to 95% of the cross section of the forged material in a right-angled cross section, and the average grain size of the sub-crystal grains is 10 μm or less.
[0021]
If the subgrain structure is a perpendicular cross section of the forged material, if the cross section is less than 50%, the strength of the whole forged material is insufficient, and if it exceeds 95%, the toughness of the whole forged material is insufficient. If the average grain size of the sub-crystal grains exceeds 10 μm, the effect of improving the strength becomes insufficient.
[0022]
Since the subcrystalline structure develops in a fibrous form, anisotropy occurs in the material, and the toughness in the vertical direction tends to be lower than that in the longitudinal direction of the fibrous form. Therefore, in order to improve the toughness of the forged material as a whole, it is preferable that the surface layer portion of the forged material has a recrystallized structure, and that the surface layer portion has excellent ductility and impact resistance. Improved strength and toughness are achieved by the combination.
[0023]
The grain size of the sub-crystal grains is measured by taking a photograph of the structure of a right-angled cross section of the forged material with a transmission electron microscope at a magnification of 5000 times, and cutting it (a crystal grain completely cut by a line segment of a known length). And the average value of the cut lengths is used, and it is measured according to the ferrite grain size test method for steel specified in JIS G0522). The area ratio of the right-angle cross section of the sub-crystal grain structure is determined by etching the right-angle cross section of the forged material sample with a 10% NaOH aqueous solution (liquid temperature: 20 ° C., immersion time: 45 minutes), and according to the size of the sample. Photograph at 1-10x and measure using image analyzer.
[0024]
Next, the production of the aluminum alloy forged material of the present invention will be described. As described above, the present invention is characterized by the formation of a fine subcrystalline structure, but the structure of the forged material is coarse. The recrystallized structure often results from the processing strain accumulated during hot extrusion or hot forging being recovered and recrystallized during the final solution treatment, and the higher the solution treatment temperature, the higher the recrystallized grains. Tends to be coarse. The solution treatment temperature at which a predetermined strength is obtained is determined by the alloy composition, and in the case of the alloy composition according to the present invention, it is necessary to perform a solution treatment at 500 ° C. or more to obtain a sufficient solid solution state. In order to suppress the formation of coarse recrystallized grains, the solution treatment temperature cannot be lowered. As means for solving this, in the present invention, the hot working temperature is set at or near the solution treatment temperature range to form a thermally stable subcrystal grain structure that does not recrystallize during the solution treatment, A compound and a Cr-based compound are finely and uniformly precipitated to promote the formation of sub-crystal grains and to maintain the formed sub-crystal grain structure.
[0025]
In order to precipitate the Mn-based compound and the Cr-based compound finely and uniformly, the ingot of the aluminum alloy having the above composition is subjected to a homogenization treatment at a temperature of 400 to 490 ° C. Usually, the homogenization treatment of a 6000 series aluminum alloy is often performed at a temperature exceeding 490 ° C. (and below the solidus), but when the homogenization treatment is performed at a temperature exceeding 490 ° C., supersaturation occurs during casting. The Mn and Cr atoms that were dissolved as a solid solution precipitate and coarsen during the homogenization treatment, so that the pinning effect is reduced, and it is difficult to form a stable sub-crystal grain structure during hot working. If the temperature is lower than 400 ° C., since Mn and Cr hardly precipitate, the recovery site of the strain introduced during hot working is insufficient, and it is difficult to form a stable subcrystalline structure. A more preferred homogenization treatment temperature is 450 to 490 ° C.
[0026]
Next, hot extrusion is performed at 480 to 540 ° C and hot forging is performed at 480 to 540 ° C. When hot extrusion and hot forging are performed at a temperature lower than 480 ° C., the solid solution of Mg ₂ Si becomes insufficient, and Mg ₂ Si is easily aggregated during hot working, and as a result, the final solution Mg ₂ Si that is agglomerated during the crystallization treatment is difficult to solid-dissolve again, and sufficient strength cannot be obtained. Further, the amount of strain accumulated during hot working increases, and coarse recrystallized grains are likely to be generated during solution treatment. When hot extrusion or hot forging is performed at a temperature exceeding 540 ° C., formation and maintenance of a sub-crystalline structure becomes difficult, and a coarse recrystallized structure is easily formed during hot working. More preferred hot extrusion and hot forging temperatures are 500-540 ° C.
[0027]
After the above hot working, a solution treatment is performed. A preferable solution treatment temperature is 500 to 540 ° C. If the temperature is less than 500 ° C, the solid solution of Mg ₂ Si becomes insufficient, and it is difficult to obtain a desired strength. If it exceeds 540 ° C., coarse recrystallization is likely to occur, resulting in a decrease in strength. A more preferred solution treatment temperature is 510-530 ° C.
[0028]
After the solution treatment, an aging treatment (tempering treatment) is performed. The preferred tempering temperature is in the range of 150 to 200 ° C. If it is lower than 150 ° C, precipitation hardening by Mg ₂ Si is not sufficient, and if it exceeds 200 ° C, Mg ₂ Si coarsely precipitates and it is difficult to obtain sufficient precipitation hardening. Become. A more preferred tempering temperature is 170 to 190 ° C.
[0029]
【Example】
Hereinafter, examples of the present invention will be described in comparison with comparative examples to demonstrate the effects of the present invention. These examples are one embodiment of the present invention, and the present invention is not limited thereto.
[0030]
Example 1
An aluminum alloy having the composition shown in Table 1 was melted and formed into a 90 mm diameter extrusion billet by semi-continuous casting. Under the conditions shown in Table 2, the billet was homogenized, hot extruded (extrusion size: diameter 20 mm, round bar), and hot forged (forged material: shape and size having a notch N as shown in FIG. 1). Then, a solution treatment was performed at the temperature shown in Table 2, and a tempering treatment was performed at 170 ° C. for 10 hours.
[0031]
Using the obtained forged product (T6 tempered material) as a test material, the average crystal grain size at the center of the AA cross section in FIG. 1 and the area ratio of the sub-crystal grain structure occupying the cross section were investigated by the method described above. In order to evaluate the mechanical properties and to evaluate the toughness of the whole forged product, a JIS No. 3 impact test piece including the notch portion N was collected and subjected to a Charpy impact test at room temperature. In the test piece, the notch portion N was in a state where only forged burrs were removed by machining. Furthermore, as an evaluation of corrosion resistance, a salt spray test based on JIS Z2371 was performed for 1000 hours to measure the maximum pit depth. Table 3 shows the results.
[0032]
As can be seen in Table 3, the test material No. All of Nos. 1 to 4 had excellent strength with a proof stress of 340 MPa or more, impact characteristics exceeding 30 J / cm ^2, and had a maximum pit depth of 200 μm or less in corrosion resistance.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
[Table 3]

[0036]
Comparative Example 1
An aluminum alloy having the composition shown in Table 1 was melted and formed into a 90 mm diameter extrusion billet by semi-continuous casting. Under the conditions shown in Table 4, the billet was homogenized, hot extruded (extrusion size: diameter 20 mm, round bar), and hot forged (forged material: shape and size having a notch N as shown in FIG. 1). Then, a solution treatment was performed at the temperature shown in Table 4 and a tempering treatment was performed at 170 ° C. for 10 hours.
[0037]
Using the obtained forged product (T6 tempered material) as a test material, the average crystal grain size at the center of the AA cross section in FIG. Investigation was carried out by a method, mechanical properties were evaluated, and impact characteristics and corrosion resistance were evaluated in the same manner as in Example 1. Table 5 shows the results. In Tables 4 and 5, those outside the conditions of the present invention are underlined.
[0038]
As shown in Table 5, the test material No. Test material No. 5 has a high homogenization treatment temperature and a low hot extrusion temperature. No. 6 has a low hot forging temperature, and No. 7 has a high solution treatment temperature, and test material No. Sample No. 8 had a low hot forging temperature, and each had a coarse recrystallized structure and could not obtain sufficient strength.
[0039]
[Table 4]

[0040]
[Table 5]

[0041]
Comparative Example 2
An aluminum alloy having the composition shown in Table 6 was melted and ingot into a billet for extrusion having a diameter of 90 mm by semi-continuous casting. Under the conditions shown in Table 7, the billet was homogenized, hot extruded (extrusion size: diameter 20 mm, round bar material), and hot forged (forged material: shape and size having a notch N as shown in FIG. 1). Then, a solution treatment was performed at the temperature shown in Table 7 and a tempering treatment was performed at 170 ° C. for 10 hours.
[0042]
Using the obtained forged product (T6 tempered material) as a test material, the average crystal grain size at the center of the AA cross section in FIG. Investigation was carried out by a method, mechanical properties were evaluated, and impact characteristics and corrosion resistance were evaluated in the same manner as in Example 1. Table 8 shows the results. In Tables 6 to 8, the values out of the conditions of the present invention are underlined.
[0043]
As shown in Table 8, the test material No. 9 has a large Mn content and a large Cr content, so that the area ratio of the sub-crystal grain structure is large and the impact characteristics are poor. Test material No. Sample No. 10 has a large amount of Si and a large amount of Mg, so that the impact characteristics are poor and the corrosion resistance is low. Test material No. Since Nos. 11 to 13 have small amounts of Mn and Cr, the forged material has a coarse recrystallized structure and low strength. Test material No. 14 had a large amount of Cu, so that the corrosion resistance was significantly reduced.
[0044]
[Table 6]

[0045]
[Table 7]

[0046]
[Table 8]

[0047]
【The invention's effect】
According to the present invention, there is provided a structural aluminum alloy forging having an excellent balance of strength, toughness and corrosion resistance, and a method for producing the same. Since the aluminum alloy of the present invention has a composition within the component standard of JIS6061 alloy, it has excellent productivity and recyclability, can be applied to structural members for automobiles, and has great industrial value. .
[Brief description of the drawings]
FIG. 1 is a front view and a cross-sectional view taken along the line AA showing the shape and dimensions of a forged material used as a test material in Examples and Comparative Examples of the present invention.

Claims

Si: 0.40 to 0.8% (mass%, the same applies hereinafter), Mg: 0.8 to 1.2%, Cu: 0.40% or less (excluding 0%, the same applies hereinafter), Mn: 0 An aluminum alloy forging material containing 0.08 to 0.15% and Cr: 0.10 to 0.35% and having a balance of Al and unavoidable impurities, wherein a surface layer portion is formed at a right angle cross section of the forging material. Is a structural aluminum having excellent corrosion resistance, characterized by having a subcrystal structure having an average crystal grain size of 10 μm or less occupying a region of 50 to 95% of the right-angled cross section in a portion other than the surface layer portion. Alloy forgings.

After the aluminum alloy ingot according to claim 1 is homogenized at 400 to 490 ° C, hot extrusion is performed at 480 to 540 ° C and hot forging is performed at 480 to 540 ° C, and solution is performed at 500 to 540 ° C. A method for producing a structural aluminum alloy forged material having excellent corrosion resistance, which is followed by aging at 150 to 200 ° C.