JP4712159B2

JP4712159B2 - Aluminum alloy plate excellent in strength and corrosion resistance and method for producing the same

Info

Publication number: JP4712159B2
Application number: JP2000150902A
Authority: JP
Inventors: 宏樹田中; 正箕田; 宏樹江崎
Original assignee: Kobe Steel Ltd; Nippon Light Metal Co Ltd; Furukawa Sky Aluminum Corp; Sumitomo Light Metal Industries Ltd
Current assignee: Kobe Steel Ltd; Nippon Light Metal Co Ltd; Furukawa Sky Aluminum Corp; Sumitomo Light Metal Industries Ltd
Priority date: 2000-05-23
Filing date: 2000-05-23
Publication date: 2011-06-29
Anticipated expiration: 2020-05-23
Also published as: JP2001335874A

Description

【０００１】
【発明の属する技術分野】
本発明は、強度と耐食性に優れたアルミニウム合金板、とくに航空機用、車両用として好適に使用される強度と耐食性に優れたアルミニウム合金板およびその製造方法に関する。
【０００２】
【従来の技術】
従来、構造用アルミニウム合金板、とくに航空機用アルミニウム合金板の一例として航空機用ストリンガー材の製造手法が提案されている（特許第１３３７６４６号〜１３３７６４９号公報、特許第１３３９９２７号公報、特許第１４０５１３６号公報など）。
【０００３】
具体的な製造手法としては、例えば、ＪＩＳＡ７０７５合金の鋳塊を４５０℃付近の温度で１０〜２０時間均質化処理したのち、４００〜４５０℃の温度で熱間圧延を開始して厚さ６ｍｍ程度の板材とし、ついで約４１０℃で１時間程度の中間熱処理を行ったのち、１００℃以下の温度域で冷間圧延を行って３〜４ｍｍ厚さの冷延板とし、この冷延板について、３２０〜５００℃の温度への急速加熱による溶体化処理を行い、１２０℃付近の温度で数時間〜２４時間程度の時効処理を施すことにより所定の強度を得るものである。
【０００４】
上記の工程において、時効処理工程では、結晶粒径の変化を生じることなしに析出硬化を図ることができ、得られた板材は２５μｍ以下の平均結晶粒径を有し、強度、成形性において実用上十分な特性をそなえたものとなる。しかしながら、耐食性、とくに耐応力腐食割れ性の面においては、実験室レベルの耐食性評価では十分と判断された場合でも、実使用環境下では耐応力腐食割れ性の点で必ずしも十分でない場合もあり、なお一層の耐食性の改善が求められている。
【０００５】
金属材料の機械的強度および成形性に関しては結晶粒径を微細にすることが好ましいことがよく知られているが、耐食性に関しては、結晶粒径を微細化することはむしろ耐食性を劣化させることも報告されており、発明者らは、ＺｎとＭｇを含有する７０００系のアルミニウム合金における結晶粒微細化と耐応力腐食割れ性との関連について種々の観点から実験、検討を行い、その過程において、隣り合う結晶粒の方位差（ミスオリエンテーション）が耐応力腐食割れ性に影響を与えることを知見した。
【０００６】
隣り合う結晶粒の方位差とは、図１に示すように、結晶粒１と結晶粒２に共通な回転軸に対してどの程度の角度差（方位差θ）があるかを示すものである。前記航空機用ストリンガー材の製造における溶体化処理後の結晶粒について調査した結果、方位差が２０°以上の大傾角粒界が形成されることが判明した。この場合には、それに続く時効処理において第２相化合物の粒界偏析が多くなり、粒内と粒界の電気化学的特性が異なることとなって耐食性が低下する。
【０００７】
【発明が解決しようとする課題】
本発明は、上記の知見に基づいてなされたものであり、その目的は、構造用アルミニウム合金板における従来の問題点を解消し、強度特性に優れ、耐食性、とくに耐応力腐食割れ性についてさらに改善を図った構造用アルミニウム合金板およびその製造方法を提供することにある。当該アルミニウム合金板を使用した構造物はコストダウンが可能となり、信頼性の向上を得ることができる。
【０００８】
【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請求項１による強度と耐食性に優れたアルミニウム合金板は、Ｚｎ：４．８〜７％、Ｍｇ：１〜３％、Ｃｕ：１〜２．５％、Ｚｒ：０．０５〜０．２５％を含有し、残部Ａｌおよび不純物からなる組成を有するアルミニウム合金板であって、該アルミニウム合金板の板面からみた平均結晶粒径が１０μｍ以下であり、該アルミニウム合金板の板面において結晶方位差が３〜１０°の結晶粒界を２５％以上含む組織を有することを特徴とする。
【００１０】
請求項２による強度と耐食性に優れたアルミニウム合金板の製造方法は、請求項１記載のアルミニウム合金板を製造する方法であって、請求項１に記載の組成を有するアルミニウム合金の鋳塊を均質化処理後熱間鍛造し、その後、４００〜１５０℃の温度域において、加工度が７０％以上になるよう繰り返し圧延して所定の板厚としたのち、４５０〜４９０℃の温度で５分以上の溶体化処理を行い、１０℃／秒以上の冷却速度で冷却後、時効処理することを特徴とする。
【００１１】
【発明の実施の形態】
本発明は、７０００系アルミニウム合金の合金組成と結晶方位差の最適の組合わせによって高強度と高耐食性を得ることを特徴とするものであるが、まず、本発明における含有成分の意義および限定理由について説明すると、Ｚｎは、時効処理時にＺｎ−Ｍｇ系の微細析出を生じ、析出硬化によって材料強度を向上させるよう機能する。Ｚｎの好ましい含有量は４．８〜７％の範囲であり、４．８％未満では従来合金のＪＩＳＡ７０７５合金やＡ７４７５合金並の強度が得られない。７％を越えて含有すると、熱間加工性が劣化して割れなどの問題が生じる。Ｚｎはまた、溶体化処理時に結晶粒成長を抑制する効果をも有する。Ｚｎのさらに好ましい含有範囲は５．０〜６．５％である。
【００１２】
Ｍｇは、Ｚｎと同様、強度の向上に寄与する元素であり、Ｍｇの好ましい含有量は１〜３％の範囲である。１％未満では従来合金並の強度を得ることが難しく、３％を越えて含有すると熱間加工性が低下して割れなどの問題が生じる。
【００１３】
Ｃｕは、時効処理時にＡｌ−Ｃｕ−Ｍｇ系化合物の微細析出が生じ、析出硬化によって材料強度を向上させるよう機能する。Ｃｕの好ましい含有量は１〜２．５％の範囲であり、１％未満では従来合金並の強度が得難く、２．５％を越えると、熱間加工性が低下して割れなどの問題が生じる。
【００１４】
Ｚｒは、溶体化処理時に結晶粒の成長を抑制する元素であり、結果的に小傾角粒界を多く留める効果を有する。Ｚｒの好ましい含有量は０．０５〜０．２５％の範囲であり、０．０５％未満ではその効果が小さく、０．２５％を越えると、鋳造時に粗大なＡｌ−Ｚｒ系化合物を形成して最終板の成形性が低下する。０．２５％を越えて含有しても、溶体化処理時の結晶粒成長を抑制する効果が飽和し、それ以上の抑制効果が得られない。Ｚｒのさらに好ましい含有範囲は０．０８〜０．２０％である。
【００１５】
本発明においては、通常、７０００系アルミニウム合金に含有される程度の量のＭｎ、Ｃｒ、Ｔｉ、Ｂ、Ｆｅ、Ｓｉを含有することができるが、Ｆｅ、Ｓｉは成形性の観点から各々０．５％以下に制限するのが好ましく、また、Ｃｒも０．０５％以下に制限するのが好ましい。
【００１６】
上記の組成との組み合わせにおいては、方位差が１０°以下で時効処理後の粒界偏析が少なくなることが見出され、また、このような小傾角粒界をもつ組織は微細粒化するため、粒界面積が大きくなって、さらに粒界偏析の程度が少なくなり、耐食性が向上することがわかった。さらに検討を行い、板面において０．０２ｍｍ²以上の範囲の方位差の分布を調査した結果、３〜１０°の小傾角粒界が全粒界の２５％以上あれば、耐応力腐食割れ性が著しく向上することが認められた。
【００１７】
方位差が３〜１０°の小傾角粒界が全粒界の２５％以上となる組織は、平均結晶粒径も微細になる。但し、平均結晶粒径が１０μｍを越えると、耐応力腐食割れ性がわるくなり、材料強度も低下するから、平均結晶粒径は１０μｍ以下とするのが好ましい。
【００１８】
方位差（ミスオリエンテーション）は、走査型電子顕微鏡（ＳＥＭ）とＣＣＤカメラの組み合わせからなる自動測定装置を使用する。この装置は、試料表面に現れた結晶面に電子線を入射させて菊地パターンをＣＣＤカメラに取り込み、コンピュータで結晶面を特定するもので、隣り合う結晶粒の方位差は、各々の結晶面がわかれば共通の回転軸が特定でき、その回転軸に対する角度差（方位差＝ミスオリエンテーション）が判明するというものである。なお、本発明においては、測定装置の分解能、誤差などを考慮して方位差の下限値を３°とした。
【００１９】
以下、本発明による強度と耐食性に優れたアルミニウム合金板の製造方法について説明する。上記の組成を有するアルミニウム合金を、例えば、通常のＤＣ鋳造によって造塊し、得られた鋳塊を常法に従って均質化処理後熱間鍛造する。熱間鍛造後の中間熱処理は常法に従って行ってもよいが、省略することもできる。
【００２０】
本発明の特徴は、その後、４００〜１５０℃の温度域、さらに好ましくは３５０〜１８０℃の温度域において、加工度が７０％以上になるよう繰り返し圧延を行うことにある。特定温度域での繰り返し圧延により、その後の溶体化処理時に結晶粒成長を抑制し得る下部組織を形成することができる。加工度が７０％未満では、Ｚｒの微細析出が不十分となり、溶体化処理時の結晶粒成長を抑制することが困難となる。４００℃を越える温度で繰り返し圧延を開始すると、Ｚｒの析出が阻害されて溶体化処理時の結晶粒成長抑制効果が薄れ、１５０℃未満では、Ｚｒの析出が遅れ、溶体化処理時の結晶粒成長抑制効果が薄れる。
【００２１】
溶体化繰り返し圧延により所定の板厚としたのち、４５０〜４９０℃の温度で５分以上の溶体化処理を行い、１０℃／秒以上の冷却速度で冷却する。溶体化処理温度が４５０℃未満では合金元素の固溶が不十分となり、時効処理後に所定の強度が得られない。４９０℃を越えると、結晶粒成長が抑制できず１０°以下の小傾角粒界の割合が少なくなる。
【００２２】
溶体化処理後の冷却速度が１０℃／秒未満では、冷却途中において第２相の析出が生じ、溶体化の効果が薄れて時効処理後の所定の強度が得られなくなる。溶体化処理、冷却後は、常法に従って時効処理を行う。
【００２３】
本発明においては、合金元素として、遷移元素のＺｒを添加することにより、４００〜１５０℃の温度域での圧延時にＺｒを微細に析出させ、溶体化処理での結晶粒成長（大傾角化）を防ぐことが重要である。アルミニウム合金における結晶粒微細化のために同じ遷移元素のＣｒの添加が行われることがあるが、本発明では有効ではなく、ＣｒとＺｒの併用も溶体化処理での結晶粒成長を抑制することができなかった。
【００２４】
また、これまでの研究により、強加工されたアルミニウム合金に１００〜３００℃の中温度域で熱処理を行うことにより小傾角粒界をもつ組織（サブグレイン組織）が得られることは知られているが、本発明の７０００系アルミニウム合金においては、４５０℃以上の温度での溶体化処理が必須であり、溶体化処理後も小傾角粒界を多く含む組織を維持なければならない。そのための製造方法について多くの実験、検討を行った結果、上記４００〜１５０℃の温度域で加工度が７０％以上になるよう繰り返し圧延を行う手法が有効であることを知見し、本発明に至ったものである。
【００２５】
【実施例】
以下、本発明の実施例を比較例と対比して説明するとともに、それに基づいてその効果を実証する。なお、これらの実施例は、本発明の好ましい一実施態様を説明するためのものであって、これにより本発明が制限されるものではない。
【００２６】
実施例１
ＤＣ鋳造法により表１に示す組成を有するアルミニウム合金を造塊し、得られたビレット（直径９０ｍｍ）を１００ｍｍ長さに切断し、これについて、４７０℃で１０時間の均質化処理を行ったのち、４００℃の温度で鍛造を行い、３０ｍｍ厚さの試料を作製した。
【００２７】
上記の試料を面削して２０ｍｍ厚さとし、３５０〜２００℃の温度域で繰り返し圧延を行い、厚さ１．５ｍｍの板材とした。圧延繰り返し数は１２回である。ついで、板材にソルトバス中において４８０℃で５分の溶体化処理を施し、水冷したのち、１２０℃で２４時間の時効処理を行い、試験材を得た。
【００２８】
得られた試験材について、以下の方法に従って結晶粒組織の調査、引張試験、耐応力腐食割れ試験を行った。
結晶粒組織の調査：板面の結晶粒組織を（株）日立製作所製ＳＥＭ、Oxford社製ＥＢＳＤ（Electron backscatter diffraction) 装置を用いて調査し、結晶方位差（ミスオリエンテーション）分布を示すヒストグラムから傾角３〜１０°を示す結晶粒界の比率を求めた
【００２９】
引張試験：試験材の圧延方向に対して９０°方向に試験片を採取し、試験片の標点間距離を１０ｍｍとして、インストロン型引張試験機を用いて引張試験を行い、引張強さ（σ_B) 、０．２％耐力（σ_0.2)、伸び（δ）を測定した。
【００３０】
耐応力腐食割れ試験：試験材の圧延方向に対して９０°方向に試験片を採取し、試験片に０．２％耐力の８２％の負荷を与え、温度３０℃の３．５％ＮａＣｌ溶液に１０分間浸漬後、２５℃で５０分乾燥させるサイクルを繰り返す乾湿交互試験を行い、試験時間２００時間内の破断数を調査した。なお、試験片として、各合金について５本づつの試験片を準備して耐応力腐食割れ試験を行った。
【００３１】
これらの調査、試験結果を表２に示す。表２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、５００ＭＰａを越える優れた耐力をそなえているとともに、耐応力腐食割れ試験において破断が生じることなく、優れた耐応力腐食割れ性を示した。
【００３２】
【表１】

【００３３】
【表２】

【００３４】
比較例１
ＤＣ鋳造法により表３に示す組成を有するアルミニウム合金を造塊し、得られたビレット（直径９０ｍｍ）を１００ｍｍ長さに切断し、これについて、４７０℃で１０時間の均質化処理を行ったのち、４００℃の温度で鍛造を行い、３０ｍｍ厚さの試料を作製した。この試料を実施例１と同一の工程で処理して試験材を作製し、得られた試験材について、実施例１と同じ方法に従って結晶粒組織の調査、引張試験、耐応力腐食割れ試験を行った。結果を表４に示す。
【００３５】
【表３】

《表注》合金I:JIS A7475
【００３６】
【表４】

【００３７】
表４に示すように、試験材Ｎｏ．５は、Ｚｎの含有量が少ないため強度が低く、小傾角結晶粒界の比率が低いため耐応力腐食割れ性が劣っている。試験材Ｎｏ．６はＭｇ量、Ｃｕ量が少ないため、強度が劣っている。試験材Ｎｏ．７はＺｒの含有量が少ないため、溶体化処理時に結晶粒成長の抑制効果が小さく、小傾角結晶粒界の比率が低くなって耐応力腐食割れ性が劣る。試験材Ｎｏ．８は、Ｚｎ含有量が上限を越えているため、熱間圧延時に割れが生じ最終板の製造ができなかった。試験材Ｎｏ．９は従来のＪＩＳＡ７４７５合金であり、小傾角結晶粒界の比率が低くなって耐応力腐食割れ性が劣る。
【００３８】
実施例２
実施例１のＡ合金を使用し、製造条件を変えて特性を評価した。造塊、均質化処理、熱間鍛造および面削条件は実施例１と同一とし、繰り返し圧延以降の工程を表５に示す条件で行い、試験材を作製した。圧延繰り返し数は８〜１２回とし、時効処理は全て１２０℃で２４時間とした。
【００３９】
得られた試験材について、実施例１と同じ方法に従って結晶粒組織の調査、引張試験、耐応力腐食割れ試験を行った。結果を表６に示す。表６にみられるように、本発明に従う試験材Ｎｏ．１０〜１４はいずれも、５００ＭＰａを越える優れた耐力をそなえているとともに、耐応力腐食割れ試験において破断が生じることなく、優れた耐応力腐食割れ性を示した。
【００４０】
【表５】

【００４１】
【表６】

【００４２】
比較例２
実施例１のＡ合金を使用し、製造条件を変えて特性を評価した。造塊、均質化処理、熱間鍛造および面削条件は実施例１と同一とし、繰り返し圧延以降の工程を表７に示す条件で行い、試験材を作製した。圧延繰り返し数は８〜１２回とし、時効処理は全て１２０℃で２４時間とした。得られた試験材について、実施例１と同じ方法に従って結晶粒組織の調査、引張試験、耐応力腐食割れ試験を行った。結果を表８に示す。
【００４３】
【表７】

【００４４】
【表８】

【００４５】
表８に示すように、試験材Ｎｏ．１５は、圧延開始温度が高くＺｒの効果が十分でないため、溶体化処理時の結晶粒成長を抑制できず耐応力腐食割れ性が劣っている。試験材Ｎｏ．１６は繰り返し圧延の温度の下限が低くＺｒの効果が十分でないため、溶体化処理時の結晶粒成長を抑制できず耐応力腐食割れ性が劣る。試験材Ｎｏ．１７は圧延加工度が低くＺｒの析出が十分でないため、溶体化処理時の結晶粒成長を抑制できず耐応力腐食割れ性が劣る。試験材Ｎｏ．１８は、溶体化処理温度が高いため結晶粒成長が生じ、耐応力腐食割れ性が劣るものとなった。試験材Ｎｏ．１９は、溶体化処理後の冷却速度が低いため冷却途中で第２相の析出が生じ、時効処理において十分な析出硬化が得られなかった。また耐応力腐食割れ試験においても破断が生じた。
【００４６】
【発明の効果】
本発明によれば、強度と耐食性とくに耐応力腐食割れ性に優れた構造用アルミニウム合金板が提供される。当該アルミニウム合金板を使用することにより、材料の薄肉化が可能となり、構造物の軽量化、コストダウンを達成することができる。また、優れた耐応力腐食割れ性により構造物に対する信頼性向上の効果も達成できる。
【図面の簡単な説明】
【図１】結晶粒の方位差を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an aluminum alloy plate excellent in strength and corrosion resistance, in particular, an aluminum alloy plate excellent in strength and corrosion resistance that is suitably used for aircraft and vehicles, and a method for producing the same.
[0002]
[Prior art]
Conventionally, a manufacturing method of a stringer material for an aircraft has been proposed as an example of a structural aluminum alloy plate, particularly an aircraft aluminum alloy plate (Patent Nos. 1337646 to 1337649, Nos. 1339927, 1405136). Such).
[0003]
As a specific manufacturing method, for example, a JIS A7075 ingot is homogenized at a temperature of about 450 ° C. for 10 to 20 hours, then hot rolling is started at a temperature of 400 to 450 ° C., and the thickness is about 6 mm. Next, after performing an intermediate heat treatment at about 410 ° C. for about 1 hour, cold rolling in a temperature range of 100 ° C. or less to obtain a cold rolled plate having a thickness of 3 to 4 mm, A solution treatment is performed by rapid heating to a temperature of 320 to 500 ° C., and an aging treatment is performed at a temperature around 120 ° C. for several hours to 24 hours to obtain a predetermined strength.
[0004]
In the above aging treatment step, precipitation hardening can be achieved without causing a change in crystal grain size, and the obtained plate material has an average crystal grain size of 25 μm or less and is practical in strength and formability. In addition, it has sufficient characteristics. However, in terms of corrosion resistance, especially stress corrosion cracking resistance, even if it is judged sufficient in the laboratory level corrosion resistance evaluation, it may not always be sufficient in terms of stress corrosion cracking resistance under the actual use environment. Further improvement in corrosion resistance is required.
[0005]
It is well known that it is preferable to make the crystal grain size fine with respect to the mechanical strength and formability of the metal material. However, with respect to the corrosion resistance, making the crystal grain size finer may rather deteriorate the corrosion resistance. The inventors have conducted experiments and examinations from various viewpoints on the relationship between grain refinement and stress corrosion cracking resistance in a 7000 series aluminum alloy containing Zn and Mg. It was found that the misorientation of adjacent grains affects the stress corrosion cracking resistance.
[0006]
As shown in FIG. 1, the orientation difference between adjacent crystal grains indicates how much angle difference (orientation difference θ) is present with respect to the rotation axis common to the crystal grains 1 and 2. . As a result of investigating the crystal grains after the solution treatment in the production of the aircraft stringer material, it was found that a large-angle grain boundary having an orientation difference of 20 ° or more was formed. In this case, the grain boundary segregation of the second phase compound increases in the subsequent aging treatment, and the electrochemical characteristics of the grain interior and grain boundary differ, resulting in a decrease in corrosion resistance.
[0007]
[Problems to be solved by the invention]
The present invention has been made on the basis of the above knowledge, and its purpose is to eliminate the conventional problems in structural aluminum alloy sheets, to improve strength characteristics, and to further improve corrosion resistance, particularly stress corrosion cracking resistance. An object of the present invention is to provide a structural aluminum alloy plate and a method for producing the same. A structure using the aluminum alloy plate can be reduced in cost and can be improved in reliability.
[0008]
[Means for Solving the Problems]
The aluminum alloy plate excellent in strength and corrosion resistance according to claim 1 for achieving the above object is Zn: 4.8-7%, Mg: 1-3%, Cu: 1-2.5%, Zr: An aluminum alloy plate containing 0.05 to 0.25% and having a composition comprising the balance Al and impurities, the average crystal grain size as viewed from the plate surface of the aluminum alloy plate being 10 μm or less, and the aluminum alloy It has a structure including 25% or more of crystal grain boundaries having a crystal orientation difference of 3 to 10 ° on the plate surface of the plate.
[0010]
A method for producing an aluminum alloy plate excellent in strength and corrosion resistance according to claim 2 is a method for producing an aluminum alloy plate according to claim 1, wherein an ingot of the aluminum alloy having the composition according to claim 1 is homogenized. After forging , hot forging , and then repeatedly rolling to a predetermined sheet thickness in a temperature range of 400 to 150 ° C. so that the degree of processing becomes 70% or more, then at a temperature of 450 to 490 ° C. for 5 minutes or more A solution treatment is performed, and an aging treatment is performed after cooling at a cooling rate of 10 ° C./second or more.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is characterized in that high strength and high corrosion resistance are obtained by an optimal combination of alloy composition and crystal orientation difference of 7000 series aluminum alloy. As for Zn, Zn-Mg based fine precipitation occurs during the aging treatment, and functions to improve the material strength by precipitation hardening. The preferable content of Zn is in the range of 4.8 to 7%, and if it is less than 4.8%, the strength comparable to that of the conventional alloys JIS A7075 alloy and A7475 alloy cannot be obtained. If the content exceeds 7%, hot workability deteriorates and problems such as cracking occur. Zn also has the effect of suppressing crystal grain growth during the solution treatment. The more preferable content range of Zn is 5.0 to 6.5%.
[0012]
Mg, like Zn, is an element that contributes to improving the strength, and the preferred Mg content is in the range of 1 to 3%. If it is less than 1%, it is difficult to obtain the same strength as that of conventional alloys, and if it exceeds 3%, hot workability deteriorates and problems such as cracking occur.
[0013]
Cu functions as a fine precipitation of Al—Cu—Mg compound during aging treatment, and improves the material strength by precipitation hardening. The preferable content of Cu is in the range of 1 to 2.5%. If it is less than 1%, it is difficult to obtain the same strength as conventional alloys, and if it exceeds 2.5%, hot workability deteriorates and cracks and other problems occur. Occurs.
[0014]
Zr is an element that suppresses the growth of crystal grains during the solution treatment, and as a result, has an effect of retaining a large number of low-angle grain boundaries. The preferable content of Zr is in the range of 0.05 to 0.25%. If the content is less than 0.05%, the effect is small, and if it exceeds 0.25%, a coarse Al—Zr compound is formed during casting. As a result, the formability of the final plate decreases. Even if it contains exceeding 0.25%, the effect which suppresses the crystal grain growth at the time of solution treatment is saturated, and the inhibitory effect beyond it is not acquired. A more preferable content range of Zr is 0.08 to 0.20%.
[0015]
In the present invention, Mn, Cr, Ti, B, Fe, and Si can be contained in an amount that is usually contained in a 7000 series aluminum alloy. It is preferable to limit it to 5% or less, and Cr is also preferably limited to 0.05% or less.
[0016]
In combination with the above composition, it is found that the grain boundary segregation after the aging treatment is reduced when the orientation difference is 10 ° or less, and a structure having such a low-angle grain boundary is finely grained. It has been found that the grain boundary area is increased, the degree of grain boundary segregation is further reduced, and the corrosion resistance is improved. As a result of further investigation and investigation of the orientation difference distribution in the range of 0.02 mm ² or more on the plate surface, if the 3 ° to 10 ° low-angle grain boundary is 25% or more of the total grain boundary, the stress corrosion cracking resistance Was found to be significantly improved.
[0017]
In a structure in which a low-angle grain boundary with an orientation difference of 3 to 10 ° is 25% or more of all grain boundaries, the average crystal grain size is also fine. However, if the average crystal grain size exceeds 10 μm, the stress corrosion cracking resistance is deteriorated and the material strength is also lowered. Therefore, the average crystal grain size is preferably 10 μm or less.
[0018]
For the misorientation, an automatic measuring device comprising a combination of a scanning electron microscope (SEM) and a CCD camera is used. In this device, an electron beam is incident on the crystal plane appearing on the surface of the sample, the Kikuchi pattern is taken into a CCD camera, and the crystal plane is specified by a computer. If it is known, a common rotation axis can be specified, and an angle difference (azimuth difference = misorientation) with respect to the rotation axis can be determined. In the present invention, the lower limit value of the azimuth difference is set to 3 ° in consideration of the resolution and error of the measuring apparatus.
[0019]
Hereinafter, the manufacturing method of the aluminum alloy plate excellent in strength and corrosion resistance according to the present invention will be described. The aluminum alloy having the above composition is ingoted by, for example, ordinary DC casting, and the obtained ingot is homogenized and hot forged according to a conventional method. The intermediate heat treatment after hot forging may be performed according to a conventional method, but may be omitted.
[0020]
The feature of the present invention is to perform repeated rolling so that the degree of processing becomes 70% or more in a temperature range of 400 to 150 ° C, more preferably in a temperature range of 350 to 180 ° C. By repeating rolling in a specific temperature range, a substructure capable of suppressing crystal grain growth during the subsequent solution treatment can be formed. If the degree of work is less than 70%, fine precipitation of Zr becomes insufficient, and it becomes difficult to suppress crystal grain growth during the solution treatment. When rolling is repeatedly started at a temperature exceeding 400 ° C., the precipitation of Zr is inhibited and the effect of suppressing the growth of crystal grains at the time of solution treatment is diminished. Below 150 ° C., the precipitation of Zr is delayed and the crystal grains at the time of solution treatment are delayed. Growth suppression effect fades.
[0021]
After a predetermined plate thickness is obtained by repeated solution rolling, solution treatment is performed at a temperature of 450 to 490 ° C. for 5 minutes or more, and cooling is performed at a cooling rate of 10 ° C./second or more. When the solution treatment temperature is less than 450 ° C., the alloy elements are not sufficiently dissolved, and a predetermined strength cannot be obtained after the aging treatment. If the temperature exceeds 490 ° C., the growth of crystal grains cannot be suppressed, and the proportion of low-angle grain boundaries of 10 ° or less decreases.
[0022]
When the cooling rate after the solution treatment is less than 10 ° C./second, precipitation of the second phase occurs during the cooling, so that the effect of the solution treatment is diminished and the predetermined strength after the aging treatment cannot be obtained. After solution treatment and cooling, an aging treatment is performed according to a conventional method.
[0023]
In the present invention, by adding Zr as a transition element as an alloy element, Zr is finely precipitated at the time of rolling in a temperature range of 400 to 150 ° C., and crystal grain growth (large tilting) in solution treatment is performed. It is important to prevent. The same transition element, Cr, may be added for grain refinement in an aluminum alloy, but it is not effective in the present invention, and the combined use of Cr and Zr also suppresses grain growth during solution treatment. I could not.
[0024]
In addition, it has been known from previous studies that a microstructure (subgrain structure) having a low-angle grain boundary can be obtained by heat-treating a hard-worked aluminum alloy at an intermediate temperature range of 100 to 300 ° C. However, in the 7000 series aluminum alloy of the present invention, a solution treatment at a temperature of 450 ° C. or higher is essential, and a structure containing a large number of low-angle grain boundaries must be maintained after the solution treatment. As a result of many experiments and examinations on the production method therefor, it has been found that a technique of repeatedly rolling so that the degree of work is 70% or more in the temperature range of 400 to 150 ° C. is effective, and the present invention is effective. It has come.
[0025]
【Example】
Examples of the present invention will be described below in comparison with comparative examples, and the effects will be demonstrated based on the examples. These examples are for explaining a preferred embodiment of the present invention, and the present invention is not limited thereby.
[0026]
Example 1
An aluminum alloy having the composition shown in Table 1 was ingoted by a DC casting method, and the obtained billet (diameter 90 mm) was cut into a length of 100 mm, and this was subjected to a homogenization treatment at 470 ° C. for 10 hours. Forging was performed at a temperature of 400 ° C. to prepare a sample having a thickness of 30 mm.
[0027]
The sample was chamfered to a thickness of 20 mm and repeatedly rolled in a temperature range of 350 to 200 ° C. to obtain a plate material having a thickness of 1.5 mm. The number of rolling repetitions is 12 times. Next, the plate material was subjected to a solution treatment at 480 ° C. for 5 minutes in a salt bath, water-cooled, and then subjected to an aging treatment at 120 ° C. for 24 hours to obtain a test material.
[0028]
The obtained test material was subjected to a crystal grain structure investigation, a tensile test, and a stress corrosion cracking test in accordance with the following methods.
Investigation of crystal grain structure: The grain structure of the plate surface was investigated using SEM manufactured by Hitachi, Ltd. and EBSD (Electron backscatter diffraction) device manufactured by Oxford, and the inclination was determined from a histogram showing the crystal orientation difference (misorientation) distribution. The ratio of grain boundaries showing 3 to 10 ° was determined.
Tensile test: A specimen is taken in the direction of 90 ° with respect to the rolling direction of the test material, the distance between the test specimens is set to 10 mm, and a tensile test is performed using an Instron type tensile tester. σ _B ), 0.2% yield strength (σ _0.2 ), and elongation (δ) were measured.
[0030]
Stress corrosion cracking resistance test: Specimens were taken in the direction of 90 ° with respect to the rolling direction of the test material. The test was repeated for 10 minutes, followed by a cycle of drying at 25 ° C. for 50 minutes, and an alternating dry / wet test was conducted to investigate the number of breaks within the test time of 200 hours. As test pieces, five test pieces were prepared for each alloy, and a stress corrosion cracking test was performed.
[0031]
These investigations and test results are shown in Table 2. As can be seen in Table 2, the test material No. Each of Nos. 1 to 4 had excellent proof stress exceeding 500 MPa, and also exhibited excellent stress corrosion cracking resistance without breaking in the stress corrosion cracking test.
[0032]
[Table 1]

[0033]
[Table 2]

[0034]
Comparative Example 1
An aluminum alloy having the composition shown in Table 3 was ingoted by DC casting, and the obtained billet (diameter 90 mm) was cut into a length of 100 mm, and this was homogenized at 470 ° C. for 10 hours. Forging was performed at a temperature of 400 ° C. to prepare a sample having a thickness of 30 mm. This sample is processed in the same process as in Example 1 to prepare a test material, and the obtained test material is subjected to a crystal grain structure investigation, a tensile test, and a stress corrosion cracking test according to the same method as in Example 1. It was. The results are shown in Table 4.
[0035]
[Table 3]

<Table Note> Alloy I: JIS A7475
[0036]
[Table 4]

[0037]
As shown in Table 4, the test material No. No. 5 has a low strength because the Zn content is small, and the stress corrosion cracking resistance is inferior because the ratio of low-angle crystal grain boundaries is low. Test material No. 6 is inferior in strength because the amount of Mg and the amount of Cu are small. Test material No. No. 7 has a low Zr content, so the effect of suppressing crystal grain growth during solution treatment is small, and the ratio of small-angle crystal grain boundaries is low, resulting in poor stress corrosion cracking resistance. Test material No. In No. 8, since the Zn content exceeded the upper limit, cracking occurred during hot rolling, and the final plate could not be produced. Test material No. 9 is a conventional JIS A7475 alloy, the ratio of a low-angle crystal grain boundary becomes low, and the stress corrosion cracking resistance is inferior.
[0038]
Example 2
Using the alloy A of Example 1, the characteristics were evaluated by changing the production conditions. The ingot forming, homogenizing treatment, hot forging and chamfering conditions were the same as in Example 1, and the steps after repeated rolling were performed under the conditions shown in Table 5 to prepare test materials. The number of rolling repetitions was 8 to 12 times, and all aging treatments were performed at 120 ° C. for 24 hours.
[0039]
The obtained test material was subjected to a crystal grain structure investigation, a tensile test, and a stress corrosion cracking resistance test in accordance with the same method as in Example 1. The results are shown in Table 6. As seen in Table 6, the test material No. Each of Nos. 10 to 14 had excellent proof stress exceeding 500 MPa, and showed excellent stress corrosion cracking resistance without causing breakage in the stress corrosion cracking test.
[0040]
[Table 5]

[0041]
[Table 6]

[0042]
Comparative Example 2
Using the alloy A of Example 1, the characteristics were evaluated by changing the production conditions. The ingot forming, homogenizing treatment, hot forging, and chamfering conditions were the same as in Example 1, and the steps after repeated rolling were performed under the conditions shown in Table 7 to prepare test materials. The number of rolling repetitions was 8 to 12 times, and all aging treatments were performed at 120 ° C. for 24 hours. The obtained test material was subjected to a crystal grain structure investigation, a tensile test, and a stress corrosion cracking resistance test in accordance with the same method as in Example 1. The results are shown in Table 8.
[0043]
[Table 7]

[0044]
[Table 8]

[0045]
As shown in Table 8, the test material No. No. 15, since the rolling start temperature is high and the effect of Zr is not sufficient, the crystal grain growth during the solution treatment cannot be suppressed, and the stress corrosion cracking resistance is inferior. Test material No. No. 16 has a low lower limit of the temperature of repeated rolling, and the effect of Zr is not sufficient, so that the crystal grain growth during the solution treatment cannot be suppressed and the stress corrosion cracking resistance is inferior. Test material No. No. 17 has a low degree of rolling work and does not sufficiently precipitate Zr. Therefore, the crystal grain growth during the solution treatment cannot be suppressed, and the stress corrosion cracking resistance is inferior. Test material No. In No. 18, since the solution treatment temperature was high, crystal grain growth occurred, and the resistance to stress corrosion cracking was inferior. Test material No. In No. 19, since the cooling rate after the solution treatment was low, precipitation of the second phase occurred during cooling, and sufficient precipitation hardening was not obtained in the aging treatment. In the stress corrosion cracking test, fracture occurred.
[0046]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the structural aluminum alloy plate excellent in intensity | strength and corrosion resistance, especially stress corrosion cracking resistance is provided. By using the aluminum alloy plate, the thickness of the material can be reduced, and the weight of the structure can be reduced and the cost can be reduced. Moreover, the effect of the reliability improvement with respect to a structure can also be achieved by the outstanding stress corrosion cracking resistance.
[Brief description of the drawings]
FIG. 1 is a diagram showing the orientation difference of crystal grains.

Claims

Zn: 4.8-7% (mass%, hereinafter the same), Mg: 1-3%, Cu: 1-2.5%, Zr: 0.05-0.25%, the balance Al and impurities A crystal grain having an average crystal grain size as viewed from the plate surface of the aluminum alloy plate of 10 μm or less and having a crystal orientation difference of 3 to 10 ° on the plate surface of the aluminum alloy plate An aluminum alloy plate excellent in strength and corrosion resistance, characterized by having a structure containing 25% or more of the boundary.

The ingot of the aluminum alloy having the composition according to claim 1 is subjected to hot forging after homogenization, and then repeatedly rolled so that the degree of processing becomes 70% or more in a temperature range of 400 to 150 ° C. The strength according to claim 1 , wherein after the plate thickness is obtained, a solution treatment is performed at a temperature of 450 to 490 ° C. for 5 minutes or more, and after cooling at a cooling rate of 10 ° C./second or more , an aging treatment is performed. A method for producing an aluminum alloy plate having excellent corrosion resistance.