JP4204295B2

JP4204295B2 - Manufacturing method of aluminum alloy hot-rolled sheet for automobile undercarriage parts

Info

Publication number: JP4204295B2
Application number: JP2002292495A
Authority: JP
Inventors: 尚幸佐久間; 俊樹村松; 宗太郎関田
Original assignee: Furukawa Sky Aluminum Corp
Current assignee: Furukawa Sky Aluminum Corp
Priority date: 2002-10-04
Filing date: 2002-10-04
Publication date: 2009-01-07
Anticipated expiration: 2022-10-04
Also published as: JP2004124211A

Description

【０００１】
【発明が属する技術分野】
この発明は自動車の足廻り部品、例えばアッパーアーム、ロアーアーム等のアーム類、リンク類、あるいはサスペンションメンバー（サブフレーム）等に使用されるアルミニウム合金熱延板の製造方法に関するものである。
【０００２】
【従来の技術】
自動車の足廻り部品、例えばアッパーアーム、ロアーアーム等としては、従来は鋼材を使用するのが一般的であったが、最近では自動車車体を軽量化し、これにより排ガス量を低減して地球温暖化防止に努めたり、低燃費化して省エネルギ化を図るべく、足廻り部品についても鋼材に代えてアルミニウム材料を用いることが押し進められている。
【０００３】
従来、自動車の足廻り部品にアルミニウム合金材料を使用した例としては、溶湯鍛造材を含む鍛造材を使用した場合がほとんどである。しかしながら溶湯鍛造材やその他の鍛造材は、一般に製造コストが高く、そのためこのような鍛造材を使用した場合、足廻り部品の高コスト化を招かざるを得なかったのが実情である。
【０００４】
【発明が解決しようとする課題】
アッパーアーム、ロアーアーム等の自動車の足廻り部品の軽量化を図るために鋼材に代えてアルミニウム材料を使用し、しかもコスト上昇を抑えるべく、本発明者等はアルミニウム合金の板材（圧延板）を自動車足廻り部品に使用することを検討している。このように板材をアッパーアーム、ロアーアーム等の自動車足廻り部品に使用する場合、板材としては通常は３〜６ｍｍ程度の厚板が必要となり、そこで熱間圧延板（熱延板）を用いることが考えられる。またこの場合、熱延板に対して曲げ加工で代表される成形加工を施すことが必須であり、したがってその熱延板には、成形性、特に曲げ加工性が優れていることが要求され、また強度も自動車足廻り部品として必要な程度に高いことが要求される。
【０００５】
ところでアルミニウム合金のうちでもＡｌ−Ｍｇ系合金は、強度と延性バランスに優れていて、良好な成形性を有するところから、従来から種々の用途に使用されており、自動車足廻り部品にも適用し得る可能性があると考えられる。またＡｌ−Ｍｇ系合金の熱延板については、本発明者等は熱延後の焼鈍を省略しながらも、強度と延性のバランスに優れたものを得るプロセスを既に開発しており、このように焼鈍を省略したプロセスによれば、省エネルギ化を図ることができるばかりでなく、地球環境保護にもつながると考えられる。
【０００６】
しかしながら従来のＡｌ−Ｍｇ系合金の熱延板では、成形性、特に曲げ加工性が必ずしも充分ではなかったのが実情である。そこで本発明者等は、Ａｌ−Ｍｇ系合金のうちでも特に熱延板として自動車の足廻り部品に適した材料を得る方法を見出すべく、種々実験・検討を重ねた結果、この発明をなすに至った。
【０００７】
したがってこの発明は、自動車足廻り部品用の材料として、軽量なアルミニウム合金を用い、かつコスト上昇を招かないように板材（熱延板）を用いることを前提とし、その場合に成形性、特に曲げ加工性および強度に優れていて自動車の足廻り部品に最適な性能を有するＡｌ−Ｍｇ系合金熱延板の製造方法を提供することを目的とするものである。
【０００８】
【課題を解決するための手段】
本発明者等が自動車足廻り部品に適したＡｌ−Ｍｇ系合金熱延板の成分組成、製造プロセス、金属組織について鋭意実験・検討を重ねた結果、合金成分としてＭｇ、Ｍｎ、Ｆｅ、Ｓｉ、Ｃｒの各量を適切に調整すると同時に、製造プロセスとして熱間圧延条件、特に熱間圧延の最終パスの圧延率および圧延速度、熱間圧延終了温度を適切に制御して、再結晶組織、特に板の表面近くの部分の再結晶率と再結晶粒のアスペクト比を適切に調整することにより、成形性特に曲げ加工性が優れかつ強度も自動車足廻り部品として充分なアルミニウム合金熱延板が得られること見出し、この発明をなすに至った。
【０００９】
具体的には、請求項１の発明の自動車足廻り部品向け成形加工用アルミニウム合金熱延板は、Ｍｇ２．２〜３．３％、Ｍｎ０．２〜１．０％、Ｆｅ０．０５〜０．５％、Ｓｉ０．０５〜０．４％、Ｃｒ０．０１〜０．２％を含有し、残部がＡｌおよび不可避的不純物によりなるアルミニウム合金の鋳塊に対し、均質化処理を兼ねた加熱処理を行なってから熱間圧延を行なうにあたり、最終パスの圧延率が３０〜５５％の範囲内、最終パスの圧延速度が２１５ｍ／分以上、熱間圧延終了温度が３００〜３６０℃の範囲内となるように圧延し、圧延方向断面から組織観察した場合に板表面から３００μｍの深さまでの領域における再結晶率が８５％以上でかつ再結晶粒の平均アスペクト比が１０以下であり、しかも耐力が８０〜１７５ＭＰａの範囲内でかつ引張強さＴＳと耐力ＹＳとの差（ＴＳ−ＹＳ）が７５ＭＰａ以上である熱延板を得ることを特徴とするものである。
【００１０】
また請求項２の発明は、Ｍｇ２．２〜３．３％、Ｍｎ０．２〜１．０％、Ｆｅ０．０５〜０．５％、Ｓｉ０．０５〜０．４％、Ｃｒ０．０１〜０．２％を含有し、さらにＴｉ０．００５〜０．２％、Ｃｕ０．０１〜０．２％、Ｚｎ０．０１〜１．０％のうちから選ばれた１種または２種以上を含有し、残部がＡｌおよび不可避的不純物によりなるアルミニウム合金の鋳塊に対し、均質化処理を兼ねた加熱処理を行なってから熱間圧延を行なうにあたり、最終パスの圧延率が３０〜５５％の範囲内、最終パスの圧延速度が２１５ｍ／分以上、熱間圧延終了温度が３００〜３６０℃の範囲内となるように圧延し、圧延方向断面から組織観察した場合に板表面から３００μｍの深さまでの領域における再結晶率が８５％以上でかつ再結晶粒の平均アスペクト比が１０以下であり、しかも耐力が８０〜１７５ＭＰａの範囲内でかつ引張強さＴＳと耐力ＹＳとの差（ＴＳ−ＹＳ）が７５ＭＰａ以上である熱延板を得ることを特徴とするものである。
【００１２】
【発明の実施の形態】
先ずこの発明におけるアルミニウム合金の成分組成の限定理由について説明する。
【００１３】
Ｍｇ：
Ｍｇの添加は、Ｍｇそれ自体の固溶による強度向上効果があり、またＭｇは転位との相互作用が大きいため、加工硬化による強度向上が期待でき、したがって自動車足廻り部品として必要とされる強度を得るために不可欠な元素である。但し、Ｍｇ量が２．２％未満では必要強度を得ることができないこともある。一方３．３％を越える多量のＭｇの添加により得られる高強度は、自動車足廻り部品では必要としないのが通常であり、また高Ｍｇ合金では耐ＳＣＣ性が低下してしまう。そこでＭｇ添加量は２．２〜３．３％の範囲内とした。
【００１４】
Ｍｎ：
Ｍｎの添加は熱延板の再結晶率の調整と強度調整に不可欠である。しかしながら、Ｍｎの添加量が０．２％未満では必要強度を得ることが困難となる。一方Ｍｎ添加量が１．０％を越えれば、Ａｌ−Ｍｎ−（Ｓｉ）−（Ｆｅ）系の微細金属間化合物が多くなり過ぎ、再結晶に対する抵抗が大きくなって再結晶しにくくなり、後述する再結晶率の条件を満たせなくなるおそれがあり、さらには再結晶した結晶粒でも平均アスペクト比が１０を越えてしまって、成形性を大きく低下させてしまう。またＭｎ量が多ければＡｌ−Ｍｎ−（Ｆｅ）−（Ｓｉ）系の粗大金属間化合物が多くなって、その点からも成形性を低下させてしまう。そこでＭｎ添加量は、０．２〜１．０％の範囲内とした。なおＭｎ添加量は、上述の範囲内でも、特に０．７１〜１．００％の範囲内とすることが好ましい。
【００１５】
Ｆｅ：
Ｆｅの添加も熱延板の再結晶率の調整と強度調整に不可欠である。しかしながらＦｅ添加量が０．０５％未満では必要強度を得ることが困難となる。また高純度の地金を使用しなければならなくなって、生産コストが高くなる。一方Ｆｅ量が０．５％を越えれば、熱延板の再結晶率の条件を満たすことが困難となり、さらにＡｌ−Ｆｅ−（Ｍｎ）−（Ｓｉ）系の粗大金属間化合物が多くなって成形性を低下させてしまう。そこでＦｅ添加量は、０．０５〜０．５％の範囲内とした。
【００１６】
Ｓｉ：
Ｓｉの添加も熱延板の再結晶率の調整と強度調整に不可欠である。しかしながら、Ｓｉ添加量が０．０５％未満では必要強度を得ることが困難となり、また高純度の地金を使用しなければならなくなって、生産コストが高くなる。一方Ｓｉ添加量が０．４％を越えれば、熱間圧延板の再結晶率の条件を満たすことが困難となる。またＡｌ−Ｆｅ−Ｓｉ−（Ｍｎ）系の粗大金属間化合物が多くなって成形性を低下させてしまう。そこでＳｉ添加量は０．０５〜０．４％の範囲内とした。
【００１７】
Ｃｒ：
Ｃｒの添加も熱延板の再結晶率の調整と強度調整に不可欠である。しかしながらＣｒ添加量が０．０１％未満では必要強度を得ることが困難となる。一方Ｃｒ添加量が０．２％を越えれば、熱間圧延板の再結晶率の条件を満たすことが困難となる。またＡｌ−Ｃｒ系の粗大金属間化合物が多くなって成形性を低下させてしまう。そこでＣｒ添加量は０．０１〜０．２％の範囲内とした。
【００１８】
以上の各元素のほかは、基本的にはＡｌおよび不可避的不純物とすれば良いが、請求項２の発明の熱延板の場合は、上述の各元素のほか、さらにＴｉ、Ｃｕ、Ｚｎのうちの１種または２種以上を添加する。これらの添加理由および添加量限定理由は次の通りである。
【００１９】
Ｔｉ：
Ｔｉは結晶粒微細化に有効である。Ｔｉの添加量が０．００５％未満では、その効果が現れにくく、一方０．２％を越えて添加すれば粗大金属間化合物が多くなって成形性を低下させてしまう。そこでＴｉを添加する場合には、Ｔｉ量は０．００５〜０．２％の範囲内とする。なおＴｉに併せてＢを加えて添加する場合もあるが、その場合のＢ量は３００ｐｐｍ以下にすることが望ましい。
【００２０】
Ｃｕ：
Ｃｕの添加は熱延板の再結晶率の調整と強度調整に有効である。しかしながらＣｕ添加量が０．０１％未満では強度に対してほとんど影響が現れず、添加する意味がない。一方Ｃｕ量が０．２％を越えれば、熱延板の再結晶率の条件を満たすことが困難となる。そこでＣｕ添加量は、０．０１〜０．２％の範囲内とした。
【００２１】
Ｚｎ：
Ｚｎの添加も熱延板の再結晶率の調整と強度調整に効果的である。しかしながら、Ｚｎ添加量が０．０１％未満では強度に対してほとんど影響が現れず、添加する意味がない。一方Ｚｎ添加量が１．０％を越えれば。熱延板の再結晶率の条件を満たすことが困難となり、また耐食性が低下する。そこでＺｎ添加量は０．０１〜１．０％の範囲内とした。
【００２２】
さらにこの発明の自動車足廻り部品用熱延板の製造方法では、合金の成分組成を前述のように調整するばかりでなく、後述するように製造プロセス条件、とりわけ熱間圧延条件を適切に制御することにより、熱延板の再結晶状態、特に再結晶率と再結晶粒のアスペクト比を適切に調整する必要がある。
【００２３】
すなわち、先ず再結晶率については、板表面から板厚方向へ３００μｍの深さの位置までの領域における再結晶率が８５％以上である必要がある。その理由は次の通りである。
【００２４】
熱延板を自動車足廻り部品に使用するにあたっては、種々の成形加工を施すのが通常であるが、代表的なものは曲げ加工である。曲げ加工においては、曲げＲ部の先端に張力が働いて、その部位に多量の転位が導入される。一方、熱延板の製造過程、特に熱間圧延の初期段階では材料に転位が導入され、その転位は熱間圧延後期や巻取り中に再結晶が生じれば消滅するが、再結晶率が８５％未満では、材料内の残存転位密度が高く、そのため曲げ加工で導入された転位密度と合わせると過多になり、曲げ加工時に直ちに破断応力に達してしまう。その結果、所望とする曲げＲまで成形できずに材料に割れが発生してしまう。
【００２５】
またここで、板の表層領域の方が板の中心部（板厚方向内部）よりも再結晶が起こりやすいから、表面から３００μｍの深さの領域での再結晶率が８５％以上となっていなければ、板厚内部では再結晶率がより低くなってしまって、板の耐力が１７５ＭＰａを越えてしまい、また引張強さ−耐力の差の値が７５ＭＰａ未満になることがあり、そのため良好な成形性を得ることができなくなる。また曲げ加工では板の曲げＲ部の表面付近から割れが発生するの通常である。そこで熱延板の再結晶率については、圧延方向断面から組織観察した場合における、板の表面から３００μｍ板厚方向に入った位置までの領域での再結晶率を８５％以上と限定した。
【００２６】
次に再結晶粒のアスペクト比（長径と短径との比）については、前記と同じ表層領域（板表面から３００μｍの深さの位置までの領域）の再結晶粒について、平均アスペクト比が１０以下である必要がある。このように平均アスペクト比を定めた理由は次の通りである。
【００２７】
成形性に優れる条件の一つとして、結晶粒には等方的な応力が負荷されることが望まれる。再結晶粒のアスペクト比が大きいことは、再結晶粒の異方性が強いことを意味するが、平均アスペクト比が１０を越えるような異方性の大きい再結晶粒からなる組織では、等方的に応力が負荷されないため、成形時に材料が割れやすくなってしまう。そこで再結晶粒の平均アスペクト比を１０以下と規定した。
【００２８】
さらにこの発明の方法により得られる熱延板では、機械的性能として、耐力が８０〜１７５ＭＰａの範囲内、引張強さＴＳと耐力ＹＳとの差（ＴＳ−ＹＳ）の値が７５ＭＰａ以上である必要がある。その理由は次の通りである。
【００２９】
耐力が８０ＭＰａ未満では、自動車足廻り材として強度が不足する。一方１７５ＭＰａを越えるような高強度では、熱延板に残留している転位密度が高いため、成形時、例えば曲げ加工時において直ちに破断応力に到達して、材料が割れてしまう。また、引張強さと耐力との差の値が７５ＭＰａ未満でも、成形時に直ちに破断応力に到達してしまい、良好な成形性を得ることができない。
【００３０】
以上のように、熱延板の成分組成を適切に調整するのみならず、後述するように製造プロセス条件、とりわけ熱間圧延条件を適切に制御して、再結晶状態、特に板表層領域における再結晶率と再結晶粒の平均アスペクト比を適切に規制し、併せて機械的特性を適切に調整することによって、自動車足廻り部品として望まれる成形性、特に曲げ加工性が、３〜６ｍｍ程度の厚板でも優れていて、しかも自動車足廻り部品として必要な強度を有することとなるのである。
【００３１】
次にこの発明の熱延板の製造方法について説明する。
【００３２】
先ず前述のような成分組成の合金を常法に従って鋳造し、得られた鋳塊に対し均質化処理を兼ねた加熱処理を行なって熱間圧延に供する。
【００３３】
ここで、熱間圧延前に行なう均質化処理を兼ねた加熱処理としては、次の▲１▼〜▲３▼の手法があり、いずれを適用しても良いが、再結晶粒サイズを小さくして、より成形性を高めるには、▲１▼、▲２▼のいずれかの手法を適用することが好ましい。
▲１▼ ２ステージ法：均質化処理を施したスラブを冷却して、面削等を行ない再度熱処理（加熱処理）する。
▲２▼ インライン２ステージ法：均質化処理を施したスラブを、同じ炉内で炉温を変えて熱処理（加熱処理）する。
▲３▼ １ステージ法：均質化処理と加熱処理を同時に行なう。なおこの場合は炉温は同じである。
【００３４】
なお均質化処理を兼ねた加熱処理の温度、時間は特に限定されるものではないが、前記▲１▼、▲２▼の手法の場合は、第１ステージ（均質化処理）を５００〜５８０℃程度で３〜１５間程度行ない、第２ステージ（熱間圧延前予備加熱）を４６０〜５３０℃程度で０．５〜１３時間程度行なうことが好ましく、また▲３▼の手法の場合、４６０〜５８０℃程度で３〜１５時間加熱することが好ましい。
【００３５】
次いで熱間圧延を行なうが、この発明で規定している成分組成付近の合金では、従来の一般的な条件で熱間圧延を行なっても熱間圧延上りの状態で再結晶組織を得ることは困難である。しかしながら、この発明の場合、熱間圧延条件、特に最終パスの圧延率および圧延速度、熱間圧延上り温度を適切に制御することにより、厚板化した場合でも熱間圧延末期から熱延コイル巻取中およびその後のコイル保持中に再結晶が生じて、熱間圧延上りで再結晶組織を得ることが可能となった。またこのように熱間圧延上りで再結晶組織を得ることができるため、熱間圧延後に改めて再結晶のための焼鈍を行なう必要がなくなったのである。次に熱間圧延の条件について説明する。
【００３６】
先ず熱間圧延の最終パスの圧延率は、３０〜５５％の範囲内とする必要がある。最終パスの圧延率が３０％未満では、熱間圧延時の加工発熱量が小さいため、熱間圧延終了温度が３００℃を下回ってしまう場合があり、熱延板再結晶率の条件を満たすことが困難となる。また最終パスの圧延率が３０％未満の場合、再結晶への駆動力が小さいため、再結晶した結晶粒でも、結晶粒形状がパンケーキ状になって、アスペクト比が１０を越えてしまうことが多い。一方最終パスの圧延率が５５％を越える場合、再結晶への駆動力が大きいため、再結晶率条件を容易に満たすことは可能であり、また再結晶粒形状も等軸状になってアスペクト比条件を満たすことが容易となる。しかしながらこの場合、材料の加工発熱量が過大となるため、熱延ロール温度が高くなってロール焼けが生じ、熱延板表面にコーティングが発生してしまうおそれがある。このようにコーティングが発生すれば、製品としての価値はなくなってしまう。そこで熱間圧延における最終パスの圧延率は３０〜５５％の範囲内に限定した。
【００３７】
次に熱間圧延における最終パスの圧延速度は、２１５ｍ／分以上とする必要がある。最終パスの圧延速度が２１５ｍ／分未満であれば、熱間圧延時の加工発熱量が小さいため、熱間圧延の終了温度が３００℃を下回ってしまう場合があり、熱延板再結晶率の条件を満たすことが困難となる。またこの場合、再結晶への駆動力が小さいため、再結晶した結晶粒でも、結晶粒形状がパンケーキ状になって、アスペクト比が１０を越えてしまうことが多い。そこで最終パスの圧延速度は２１５ｍ／分以上とした。
【００３８】
さらに熱間圧延の終了温度は３００〜３６０℃の範囲内とする必要がある。すなわち熱間圧延の終了温度が３００℃未満では、熱延上りで再結晶率の条件を満たすことが困難となり、また再結晶への駆動力が小さいため、再結晶した結晶粒でも結晶粒形状がパンケーキ状になって、アスペクト比が１０を越えてしまうことがある。一方熱間圧延の終了温度が３６０℃を越えれば、再結晶には効果的であるが、熱延ロール温度が高くなってロール焼けが生じ、熱延板表面にコーティングが発生してしまうおそれがある。このようにコーティングが発生すれば、製品としての価値がなくなってしまう。そこで熱間圧延の終了温度は３００〜３６０℃の範囲内に規制することとした。
【００３９】
なお上記以外の熱間圧延条件、例えば熱間圧延開始温度等は特に限定されるものではなく、要は最終パスの圧延率および圧延速度、熱間圧延終了温度が上記の条件を満たすようにその他の条件も定めれば良いが、熱間圧延開始温度は通常は４５０〜５３０℃程度とすることが好ましい。また熱間圧延上り板厚（製品板厚）は適用される自動車足廻り部品によっても異なるが、通常は３〜６ｍｍ程度とすることが多い。
【００４０】
前述のように熱間圧延の条件を厳しく制御することによって、熱間圧延末期から熱延板コイル巻取の段階において材料に再結晶を充分に生起させて、前述の再結晶率条件を満たしかつ平均アスペクト比も前述の条件を満たす熱延板を得ることができる。
【００４１】
なお上述のようにして得られた熱延板は、熱間圧延上りで前述の条件を満たすように再結晶しているため、改めて再結晶のための焼鈍を行なう必要はない。
【００４２】
【実施例】
表１の合金Ｎｏ．１〜Ｎｏ．５に示す種々の化学成分のＡｌ合金鋳塊を、表２の製造番号１〜７に示す種々のプロセスで熱間圧延し、熱延板とした。以上のようにして得られた各熱延板から、圧延方向に対して平行な方向に試験片を切り出して、機械的性質（引張強さＴＳおよび耐力ＹＳ）と曲げ加工性を調べたので、その結果を表３に示す。なお熱間圧延後には板を平坦にするためにレベリングを行なうのが通常であり、ここで調べた熱延板の機械的性質も、レベリング後の特性である。
【００４３】
また曲げ加工性については、圧延方向に対して平行な方向および直角な方向からそれぞれ試験片を切り出し、Ｒ／ｔ（Ｒ：曲げ治具先端の曲率、ｔ：試験材の板厚）が２．０の条件で１８０°曲げを行なって、目視で割れの有無を調べた。割れが全く生じていない場合を合格として○印を付し、１ケ所でも割れが発生した場合を不合格として×印を付した。
【００４４】
さらに、各熱延板からＯＭ観察用試料を切り出してバーカーエッチングを行ない、圧延方向でかつ板厚方向に沿った平行でかつ板厚方向に沿った断面で組織観察を行なって、板表面から３００μｍの深さの領域における再結晶率と平均アスペクト比を調べたので、その結果を表３中に併せて示す。
【００４５】
ここで再結晶率は、画像解析処理装置を用いて５０視野の平均値を求めた。また再結晶粒の平均アスペクト比は、圧延方向に対して平行な方向の結晶粒長さＸＬと、圧延方向に対し直角な方向の結晶粒の長さ（板厚方向の結晶粒の長さ）ＹＬとを測定し、ＸＬ／ＹＬを１００個の再結晶粒に対してランダムに測定した。そしてＸＬ／ＹＬが１０以下の場合を合格として○印を付し、ＸＬ／ＹＬが１０を越える場合を不合格として×印を付した。なお等軸再結晶粒の場合にはＸＬ／ＹＬは、ほぼ１となる。
【００４６】
【表１】

【００４７】
【表２】

【００４８】
【表３】

【００４９】
表３に示すように、この発明で規定する成分組成範囲内の合金を用いかつ熱間圧延条件もこの発明で規定する条件を満たした製造番号１，３の場合は、再結晶率、平均アスペクト比がこの発明で規定する条件を満たし、そのため曲げ加工性が良好でかつ機械的性質も適切な範囲内であることが確認できた。
【００５０】
一方製造番号２は、合金成分組成はこの発明で規定する範囲内であるが、熱間圧延最終パスの圧下率が小さくかつ熱間圧延終了温度も低かった例であり、この場合は再結晶が生起されず、強度が高過ぎて曲げ加工性が著しく悪くなってしまった。
【００５１】
また製造番号４は、合金成分組成はこの発明で規定する範囲内であるが、熱間圧延の最終パスの圧延速度が低くかつ熱間圧延終了温度も低かった例であり、この場合は再結晶はある程度生じたものの、再結晶率が低く、かつ再結晶粒の平均アスペクト比が高過ぎ、その結果、熱延板の強度が高過ぎ、曲げ加工性が劣っていた。
【００５２】
さらに製造番号５，６，７は、成分組成がこの発明で規定する範囲を外れた比較合金を用いた例であり、熱間圧延条件はこの発明で規定する範囲内としたが、強度が高過ぎて曲げ加工性に劣るか、または逆に強度が低過ぎ、いずれにしても自動車足廻り部品用材料として不適当となってしまった。
【００５３】
【発明の効果】
この発明の製造方法により得られた自動車足廻り部品用アルミニウム合金熱延板は、３〜６ｍｍ程度の厚板でも、成形性、特に曲げ加工性に優れており、また強度面も自動車足廻り部品に必要な強度を備えていて、アッパーアーム、ロアーアーム等の自動車足廻り部品向けに成形加工、特に曲げ加工を施して使用される用途に最適である。またこの発明の製造方法により得られた自動車足廻り用アルミニウム合金熱延板は、アルミニウム材料であるため軽量であって、自動車の車体軽量化を通じて省エネルギ、地球環境保護等にも貢献することができ、しかも鍛造等の高コストの製造プロセスを必要とせず、また熱間圧延後の焼鈍も省略しているため、自動車足廻り部品の低コスト化を図れるのみならず、製造プロセス面からも省エネルギ化を図って地球環境保護に貢献することができる。[0001]
[Technical field to which the invention belongs]
The present invention relates to a method for manufacturing an aluminum alloy hot-rolled sheet used for undercar parts of an automobile, for example, arms such as an upper arm and a lower arm, links, or suspension members (subframes).
[0002]
[Prior art]
Conventionally, steel parts are generally used as the undercarriage parts of automobiles, such as the upper arm and lower arm, but recently, the weight of the automobile body has been reduced, thereby reducing the amount of exhaust gas and preventing global warming. In order to reduce energy consumption by reducing fuel consumption, the use of aluminum materials for undercarriage parts instead of steel is being promoted.
[0003]
Conventionally, as an example of using an aluminum alloy material for an automobile undercarriage part, a case where a forging material including a molten forging material is used is almost all. However, molten forgings and other forgings generally have high production costs, and when such forgings are used, the cost of undercarriage parts must be increased.
[0004]
[Problems to be solved by the invention]
In order to reduce the weight of undercarriage parts of automobiles such as upper arms and lower arms, the present inventors use aluminum materials instead of steel, and in order to suppress the cost increase, the inventors have used aluminum alloy sheets (rolled sheets) for automobiles. We are considering using it for suspension parts. Thus, when using plate materials for automobile undercarriage parts such as upper arms and lower arms, it is usually necessary to use thick plates of about 3 to 6 mm as plate materials, and hot rolled plates (hot rolled plates) should be used there. Conceivable. In this case, it is essential that the hot-rolled sheet is subjected to a forming process represented by bending. Therefore, the hot-rolled sheet is required to have excellent formability, particularly bending workability, Also, the strength is required to be as high as necessary for automobile undercarriage parts.
[0005]
By the way, among aluminum alloys, Al-Mg alloy is excellent in strength and ductility balance and has good formability, so it has been used for various purposes from the past, and it can also be applied to automobile undercarriage parts. There is a possibility to get. In addition, for hot-rolled sheets of Al-Mg alloy, the present inventors have already developed a process for obtaining a material having an excellent balance between strength and ductility while omitting annealing after hot rolling. According to the process in which annealing is omitted, it is considered that not only energy saving can be achieved, but also global environmental protection is achieved.
[0006]
However, the conventional Al-Mg alloy hot-rolled sheet is not always satisfactory in formability, particularly bending workability. Accordingly, the present inventors made various experiments and examinations in order to find out a method for obtaining a material suitable for an automobile undercarriage part as an especially hot-rolled sheet among Al-Mg based alloys. It came.
[0007]
Therefore, the present invention is based on the premise that a lightweight aluminum alloy is used as a material for automobile undercarriage parts and that a plate material (hot rolled sheet) is used so as not to increase the cost. It is an object of the present invention to provide a method for producing an Al-Mg alloy hot-rolled sheet having excellent workability and strength and having optimum performance for automobile undercarriage parts.
[0008]
[Means for Solving the Problems]
The present inventors have a component composition of Al-Mg-based alloy hot-rolled sheet suitable for automotive underbody parts, the manufacturing process, the results of extensive experiments and studied metallic tissue, Mg as an alloying element, Mn, Fe, Si In addition, the amount of Cr is adjusted appropriately, and at the same time, the hot rolling conditions as a manufacturing process, particularly the rolling rate and rolling speed of the final pass of hot rolling, the hot rolling finish temperature is appropriately controlled, and the recrystallized structure, In particular, by appropriately adjusting the recrystallization rate and the aspect ratio of the recrystallized grains in the vicinity of the surface of the plate, an aluminum alloy hot-rolled plate that has excellent formability, particularly bending workability, and sufficient strength as an automobile undercarriage component can be obtained. It was found that it was obtained and the present invention was made.
[0009]
Specifically, the aluminum alloy hot-rolled sheet for forming for automobile undercarriage parts of the invention of claim 1 is Mg 2.2 to 3.3%, Mn 0.2 to 1.0%, Fe 0.05 to 0.00. 5%, Si 0.05-0.4%, Cr 0.01-0.2%, the heat treatment that also serves as a homogenization treatment for the ingot of aluminum alloy that the balance is made of Al and inevitable impurities When performing the hot rolling after performing, the rolling rate of the final pass is in the range of 30 to 55%, the rolling speed of the final pass is in the range of 215 m / min or more, and the hot rolling finish temperature is in the range of 300 to 360 ° C. When the structure is observed from the cross section in the rolling direction, the recrystallization rate in the region from the plate surface to a depth of 300 μm is 85% or more, the average aspect ratio of the recrystallized grains is 10 or less, and the yield strength is 80 ~ 175 MPa range The difference between the inner and and tensile strength TS and yield strength YS (TS-YS) is characterized in that to obtain a hot rolled sheet not less than 75 MPa.
[0010]
In the invention of claim 2, Mg 2.2-3.3%, Mn 0.2-1.0%, Fe 0.05-0.5%, Si 0.05-0.4%, Cr 0.01-0. Containing 2%, further containing one or more selected from Ti 0.005-0.2%, Cu 0.01-0.2%, Zn 0.01-1.0% , the balance In the case of hot rolling after performing a heat treatment also serving as a homogenization treatment for an ingot of aluminum alloy consisting of Al and inevitable impurities, the rolling rate of the final pass is in the range of 30 to 55%. When rolling is performed so that the rolling speed of the pass is 215 m / min or more and the end temperature of hot rolling is within the range of 300 to 360 ° C., and the structure is observed from the cross section in the rolling direction, The crystallinity is 85% or more and the average grain size of recrystallized grains And aspect ratio is 10 or less, yet which is characterized in that strength is the difference between the strength TS and yield strength YS tensile and within the scope of 80~175MPa (TS-YS) to obtain a hot-rolled sheet is more than 75MPa It is.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the component composition of the aluminum alloy in the present invention will be described.
[0013]
Mg:
The addition of Mg has the effect of improving strength due to solid solution of Mg itself, and since Mg has a large interaction with dislocations, it can be expected to improve the strength by work hardening, and therefore the strength required as an automobile undercarriage part. It is an indispensable element for obtaining. However, if the Mg content is less than 2.2%, the required strength may not be obtained. On the other hand, the high strength obtained by adding a large amount of Mg exceeding 3.3% is usually not required for automobile undercarriage parts, and the SCC resistance is lowered with a high Mg alloy. Therefore, the amount of Mg added is set in the range of 2.2 to 3.3%.
[0014]
Mn:
The addition of Mn is indispensable for adjusting the recrystallization rate and strength of the hot rolled sheet. However, if the amount of Mn added is less than 0.2%, it will be difficult to obtain the required strength. On the other hand, if the amount of Mn added exceeds 1.0%, the amount of Al-Mn- (Si)-(Fe) -based fine intermetallic compound increases, resistance to recrystallization increases, and recrystallization hardly occurs. The recrystallization rate condition may not be satisfied, and even the recrystallized crystal grains have an average aspect ratio exceeding 10, and the formability is greatly reduced. Moreover, if there is much Mn amount, the Al-Mn- (Fe)-(Si) type coarse intermetallic compound will increase, and the moldability will also fall from that point. Therefore, the amount of Mn added is set in the range of 0.2 to 1.0%. In addition, it is preferable to make Mn addition amount into the range of 0.71-1.00% especially also in the above-mentioned range.
[0015]
Fe:
The addition of Fe is also indispensable for adjusting the recrystallization rate and strength of the hot rolled sheet. However, if the amount of Fe added is less than 0.05%, it will be difficult to obtain the required strength. In addition, high-purity bullion must be used, resulting in high production costs. On the other hand, if the amount of Fe exceeds 0.5%, it becomes difficult to satisfy the condition of the recrystallization rate of the hot-rolled sheet, and more Al-Fe- (Mn)-(Si) -based coarse intermetallic compounds increase. Formability will be reduced. Therefore, the amount of Fe added is set in the range of 0.05 to 0.5%.
[0016]
Si:
The addition of Si is also essential for adjusting the recrystallization rate and strength of the hot rolled sheet. However, if the amount of Si added is less than 0.05%, it is difficult to obtain the required strength, and a high-purity metal must be used, resulting in an increase in production cost. On the other hand, if the Si addition amount exceeds 0.4%, it becomes difficult to satisfy the condition of the recrystallization rate of the hot rolled sheet. In addition, the Al—Fe—Si— (Mn) -based coarse intermetallic compound increases and the formability is lowered. Therefore, the amount of Si added is in the range of 0.05 to 0.4%.
[0017]
Cr:
Addition of Cr is also indispensable for adjusting the recrystallization rate and strength of the hot-rolled sheet. However, if the Cr addition amount is less than 0.01%, it is difficult to obtain the required strength. On the other hand, if the Cr addition amount exceeds 0.2%, it becomes difficult to satisfy the condition of the recrystallization rate of the hot rolled sheet. In addition, the Al—Cr-based coarse intermetallic compound increases and the formability is lowered. Therefore, the amount of Cr added is set in the range of 0.01 to 0.2%.
[0018]
In addition to the above elements, Al and unavoidable impurities may be basically used. However, in the case of the hot-rolled sheet of the invention of claim 2, in addition to the above elements, Ti, Cu, Zn Add one or more of them. The reason for these additions and the reason for limiting the addition amount are as follows.
[0019]
Ti:
Ti is effective for crystal grain refinement. If the addition amount of Ti is less than 0.005%, the effect is difficult to appear. On the other hand, if the addition amount exceeds 0.2%, coarse intermetallic compounds increase and formability deteriorates. Therefore, when adding Ti, the amount of Ti is within the range of 0.005 to 0.2%. In some cases, B is added in addition to Ti, and the amount of B in that case is preferably 300 ppm or less.
[0020]
Cu:
The addition of Cu is effective for adjusting the recrystallization rate and strength of the hot-rolled sheet. However, if the amount of Cu added is less than 0.01%, there is almost no effect on strength, and there is no point in adding. On the other hand, if the amount of Cu exceeds 0.2%, it becomes difficult to satisfy the condition of the recrystallization rate of the hot-rolled sheet. Therefore, the amount of Cu added is set in the range of 0.01 to 0.2%.
[0021]
Zn:
Addition of Zn is also effective in adjusting the recrystallization rate and strength of the hot-rolled sheet. However, if the amount of Zn added is less than 0.01%, there is almost no effect on the strength, and there is no point in adding it. On the other hand, if the Zn content exceeds 1.0%. It becomes difficult to satisfy the condition of the recrystallization rate of the hot-rolled sheet, and the corrosion resistance is lowered. Therefore, the amount of Zn added is set in the range of 0.01 to 1.0%.
[0022]
Furthermore, in the method of manufacturing a hot rolled sheet for automobile undercarriage parts according to the present invention, not only the composition of the alloy is adjusted as described above, but also the manufacturing process conditions, particularly hot rolling conditions, are appropriately controlled as described later. Thus, it is necessary to appropriately adjust the recrystallization state of the hot- rolled sheet, particularly the recrystallization rate and the aspect ratio of the recrystallized grains.
[0023]
That is, first, regarding the recrystallization rate, the recrystallization rate in the region from the plate surface to the depth of 300 μm in the plate thickness direction needs to be 85% or more. The reason is as follows.
[0024]
When hot-rolled sheets are used for automobile undercarriage parts, various forming processes are usually performed, but a typical one is bending. In the bending process, a tension acts on the tip of the bending R portion, and a large amount of dislocations are introduced into that portion. On the other hand, dislocations are introduced into the material during the hot-rolled sheet manufacturing process, particularly in the initial stage of hot rolling, and the dislocations disappear when the recrystallization occurs during the later stage of hot rolling or during winding. If it is less than 85%, the residual dislocation density in the material is high, so when combined with the dislocation density introduced by bending, it becomes excessive, and the breaking stress is reached immediately during bending. As a result, it is impossible to form the desired bending R, and the material is cracked.
[0025]
Here, since the surface layer region of the plate is more likely to be recrystallized than the central portion of the plate (inside the plate thickness direction), the recrystallization rate in the region 300 μm deep from the surface is 85% or more. Otherwise, the recrystallization rate is lower inside the plate thickness, the proof stress of the plate exceeds 175 MPa, and the value of the difference between tensile strength and proof strength may be less than 75 MPa. Formability cannot be obtained. Further, in bending, cracks usually occur from the vicinity of the surface of the bent R portion of the plate. Therefore, with respect to the recrystallization rate of the hot-rolled sheet, the recrystallization rate in the region from the surface of the plate to the position entering the 300 μm thickness direction when the structure is observed from the cross section in the rolling direction is limited to 85% or more.
[0026]
Next, regarding the aspect ratio of the recrystallized grains (ratio of major axis to minor axis), the average aspect ratio of the recrystallized grains in the same surface layer region (region from the plate surface to a depth of 300 μm) is 10 Must be: The reason for determining the average aspect ratio in this way is as follows.
[0027]
As one of the conditions excellent in formability, it is desirable that an isotropic stress be applied to the crystal grains. A large aspect ratio of the recrystallized grains means that the anisotropy of the recrystallized grains is strong, but isotropic in a structure composed of recrystallized grains having a large anisotropy whose average aspect ratio exceeds 10. Since stress is not applied to the material, the material is easily cracked during molding. Therefore, the average aspect ratio of the recrystallized grains is defined as 10 or less.
[0028]
Furthermore, in the hot- rolled sheet obtained by the method of the present invention, as mechanical performance, the proof stress is in the range of 80 to 175 MPa, and the value of the difference between the tensile strength TS and the proof strength YS (TS−YS) needs to be 75 MPa or more. There is. The reason is as follows.
[0029]
If the proof stress is less than 80 MPa, the strength is insufficient as an automobile suspension material. On the other hand, at a high strength exceeding 175 MPa, the dislocation density remaining on the hot-rolled sheet is high, so that the material immediately breaks at the time of molding, for example, during bending, and cracks the material. Even if the value of the difference between the tensile strength and the proof stress is less than 75 MPa, the breaking stress is reached immediately at the time of molding, and good moldability cannot be obtained.
[0030]
As described above, not only the component composition of the hot-rolled sheet is appropriately adjusted, but also the production process conditions, particularly hot rolling conditions, are appropriately controlled as will be described later, so that By appropriately regulating the crystal ratio and the average aspect ratio of the recrystallized grains and adjusting the mechanical properties appropriately, the formability desired as an automobile undercarriage part, particularly bending workability, is about 3 to 6 mm. Even thick plates are excellent, and they have the strength required for automobile undercarriage parts.
[0031]
Next, the manufacturing method of the hot rolled sheet of this invention is demonstrated.
[0032]
First, an alloy having a component composition as described above is cast according to a conventional method, and the resulting ingot is subjected to a heat treatment that also serves as a homogenization treatment and subjected to hot rolling.
[0033]
Here, as the heat treatment that also serves as the homogenization treatment performed before hot rolling, the following methods (1) to (3) may be applied, and any of them may be applied, but the recrystallized grain size is reduced. In order to further improve the moldability, it is preferable to apply any one of the methods (1) and (2).
{Circle around (1)} Two-stage method: The slab subjected to the homogenization treatment is cooled, subjected to chamfering and the like, and heat treated (heat treatment) again.
(2) In-line two-stage method: A slab subjected to homogenization treatment is heat-treated (heat treatment) in the same furnace while changing the furnace temperature.
(3) One-stage method: A homogenization process and a heating process are performed simultaneously. In this case, the furnace temperature is the same.
[0034]
The temperature and time of the heat treatment also serving as a homogenization treatment are not particularly limited, but in the case of the above methods (1) and (2), the first stage (homogenization treatment) is performed at 500 to 580 ° C. About 3 to 15 hours are preferably performed, and the second stage (preheating before hot rolling) is preferably performed at about 460 to 530 ° C. for about 0.5 to 13 hours. It is preferable to heat at about 580 ° C. for 3 to 15 hours.
[0035]
Next, hot rolling is performed, but in the case of an alloy near the component composition specified in the present invention, it is possible to obtain a recrystallized structure in the state after hot rolling even if hot rolling is performed under conventional general conditions. Have difficulty. However, in the case of the present invention, hot rolling coil winding is performed from the end of hot rolling even when it is thickened by appropriately controlling the hot rolling conditions, particularly the rolling rate and rolling speed of the final pass, and the hot rolling up temperature. Recrystallization occurred during taking and subsequent coil holding, and it became possible to obtain a recrystallized structure after hot rolling. In addition, since a recrystallized structure can be obtained after hot rolling in this way, it is no longer necessary to perform recrystallization annealing after hot rolling. Next, conditions for hot rolling will be described.
[0036]
First, the rolling rate of the final pass of hot rolling needs to be in the range of 30 to 55%. If the rolling rate of the final pass is less than 30%, the processing heat generation amount during hot rolling is small, so the hot rolling end temperature may be lower than 300 ° C., and the condition of the hot rolled sheet recrystallization rate is satisfied. It becomes difficult. Also, when the rolling rate of the final pass is less than 30%, the driving force for recrystallization is small, so even with the recrystallized crystal grains, the crystal grain shape becomes pancake and the aspect ratio exceeds 10. There are many. On the other hand, when the rolling rate of the final pass exceeds 55%, the driving force for recrystallization is large, so it is possible to easily satisfy the recrystallization rate condition, and the recrystallized grain shape is also equiaxed and has an aspect ratio. It becomes easy to satisfy the ratio condition. However, in this case, since the processing calorific value of the material becomes excessive, the hot-rolling roll temperature becomes high and roll burning occurs, and there is a possibility that coating is generated on the surface of the hot-rolled sheet. If such a coating occurs, the value as a product is lost. Therefore, the rolling rate of the final pass in hot rolling is limited to a range of 30 to 55%.
[0037]
Next, the rolling speed of the final pass in hot rolling needs to be 215 m / min or more. If the rolling speed of the final pass is less than 215 m / min, the processing calorific value during hot rolling is small, so the end temperature of hot rolling may be lower than 300 ° C. It becomes difficult to satisfy the conditions. Further, in this case, since the driving force for recrystallization is small, even in the recrystallized crystal grain, the crystal grain shape becomes pancake and the aspect ratio often exceeds 10. Therefore, the rolling speed of the final pass was set to 215 m / min or more.
[0038]
Furthermore, the end temperature of hot rolling needs to be in the range of 300 to 360 ° C. That is, when the end temperature of hot rolling is less than 300 ° C., it becomes difficult to satisfy the condition of the recrystallization rate due to hot rolling, and since the driving force for recrystallization is small, the crystal grain shape of the recrystallized crystal grains is also small. It may become a pancake and the aspect ratio may exceed 10. On the other hand, if the end temperature of hot rolling exceeds 360 ° C., it is effective for recrystallization. However, the hot rolling roll temperature becomes high and roll burning may occur, which may cause coating on the surface of the hot rolled sheet. is there. If such a coating occurs, the value as a product is lost. Therefore, the end temperature of hot rolling is regulated within the range of 300 to 360 ° C.
[0039]
The hot rolling conditions other than the above, for example, the hot rolling start temperature, etc. are not particularly limited, and the other points are that the rolling rate and rolling speed of the final pass, and the hot rolling end temperature satisfy the above conditions. However, the hot rolling start temperature is usually preferably about 450 to 530 ° C. In addition, the hot rolled up board thickness (product board thickness) is usually about 3 to 6 mm, although it varies depending on the automobile undercarriage parts to be applied.
[0040]
By strictly controlling the hot rolling conditions as described above, sufficient recrystallization occurs in the material from the end of hot rolling to the stage of coiling of the hot-rolled sheet to satisfy the above-mentioned recrystallization rate conditions and A hot-rolled sheet that satisfies the above-described conditions for the average aspect ratio can also be obtained.
[0041]
In addition, since the hot-rolled sheet obtained as described above is recrystallized so as to satisfy the above-described conditions after hot rolling, it is not necessary to perform annealing for recrystallization again.
[0042]
【Example】
Alloy No. 1 in Table 1 1-No. 5 were hot-rolled by various processes shown in production numbers 1 to 7 in Table 2 to obtain hot-rolled sheets. From each hot-rolled sheet obtained as described above, a test piece was cut out in a direction parallel to the rolling direction, and mechanical properties (tensile strength TS and yield strength YS) and bending workability were examined. The results are shown in Table 3. In addition, after hot rolling, leveling is usually performed to flatten the plate, and the mechanical properties of the hot-rolled plate examined here are also properties after leveling.
[0043]
Regarding bending workability, test pieces were cut out from a direction parallel to the rolling direction and a direction perpendicular to the rolling direction, and R / t (R: curvature of the bending jig tip, t: thickness of the test material) was 2. The sample was bent 180 ° under the condition of 0 and visually checked for cracks. A case where no cracks occurred was marked as acceptable, and a case where cracks occurred even at one location was marked as unacceptable.
[0044]
Further, a sample for OM observation was cut out from each hot-rolled sheet and subjected to Barker etching, and the structure was observed in a cross-section along the plate thickness direction in the rolling direction, parallel to the plate thickness direction, and 300 μm from the plate surface. The recrystallization rate and the average aspect ratio in the region of the depth of 5 were investigated, and the results are also shown in Table 3.
[0045]
Here, the recrystallization rate was determined as an average value of 50 visual fields using an image analysis processing apparatus. The average aspect ratio of the recrystallized grains is the crystal grain length XL in the direction parallel to the rolling direction and the crystal grain length in the direction perpendicular to the rolling direction (the length of the crystal grains in the plate thickness direction). YL was measured, and XL / YL was randomly measured for 100 recrystallized grains. A case where XL / YL was 10 or less was marked as acceptable, and a case where XL / YL exceeded 10 was marked as unacceptable. In the case of equiaxed recrystallized grains, XL / YL is approximately 1.
[0046]
[Table 1]

[0047]
[Table 2]

[0048]
[Table 3]

[0049]
As shown in Table 3, in the case of production numbers 1 and 3 that use an alloy within the component composition range specified in the present invention and the hot rolling conditions satisfy the conditions specified in the present invention, the recrystallization rate, the average aspect ratio It was confirmed that the ratio satisfied the conditions specified in the present invention, so that the bending workability was good and the mechanical properties were within an appropriate range.
[0050]
On the other hand, production number 2 is an example in which the alloy component composition is within the range specified in the present invention, but the rolling reduction in the final hot rolling pass is small and the hot rolling finish temperature is low. It did not occur, the strength was too high, and the bending workability was remarkably deteriorated.
[0051]
Production No. 4 is an example in which the alloy component composition is within the range specified in the present invention, but the rolling speed in the final pass of the hot rolling is low and the hot rolling finish temperature is low. However, the recrystallization rate was low and the average aspect ratio of the recrystallized grains was too high. As a result, the strength of the hot rolled sheet was too high and the bending workability was poor.
[0052]
Further, production numbers 5, 6 and 7 are examples using comparative alloys whose component compositions were outside the range specified in the present invention, and the hot rolling conditions were set in the range specified in the present invention, but the strength was high. It is too inferior in bending workability, or on the contrary, the strength is too low, and in any case, it becomes unsuitable as a material for automobile undercarriage parts.
[0053]
【The invention's effect】
Automotive underbody parts for aluminum alloy hot-rolled sheet obtained by the process of this invention, even in thick plate of about 3 to 6 mm, the moldability, in particular bending and excellent in workability, the strength plane also automobile underbody It has the necessary strength for the parts, and is ideal for applications that are used by molding, especially bending, for automobile suspension parts such as upper arms and lower arms. The automobile underbody aluminum alloy hot-rolled sheet obtained by the process of this invention is a light-weight because an aluminum material, energy saving through the body weight of automobiles, to contribute to global environmental protection, etc. It can be, moreover without requiring costly manufacturing process of forging, also because not also omitted annealing after hot rolling, not only attained the cost of the automotive underbody part, from the manufacturing process surface It can contribute to global environment protection by saving energy.

Claims

Mg 2.2 to 3.3% (mass%, the same applies hereinafter), Mn 0.2 to 1.0%, Fe 0.05 to 0.5%, Si 0.05 to 0.4%, Cr 0.01 to 0.2 %, And the remainder of the aluminum alloy ingot made of Al and inevitable impurities is subjected to a heat treatment that also serves as a homogenization treatment and then hot rolling, and the rolling rate of the final pass is 30 to 55 %, The rolling speed of the final pass is 215 m / min or more, the hot rolling finish temperature is 300 to 360 ° C., and the structure is observed from the cross section in the rolling direction, 300 μm from the plate surface. The recrystallization rate in the region up to the depth is 85% or more, the average aspect ratio of the recrystallized grains is 10 or less, and the yield strength is in the range of 80 to 175 MPa, and the difference between the tensile strength TS and the yield strength YS (TS -YS) is 75M characterized in that to obtain a hot rolled sheet is more than a, method for producing automotive underbody parts aluminum alloy hot-rolled plate.

Containing Mg 2.2-3.3%, Mn 0.2-1.0%, Fe 0.05-0.5%, Si 0.05-0.4%, Cr 0.01-0.2%, and Ti0 Contains one or more selected from 0.005 to 0.2%, Cu 0.01 to 0.2%, Zn 0.01 to 1.0%, with the balance being made of Al and inevitable impurities When hot rolling is performed on an aluminum alloy ingot after performing a heat treatment that also serves as a homogenization treatment, the rolling rate of the final pass is within a range of 30 to 55%, and the rolling speed of the final pass is 215 m / min. As described above, when the hot rolling finish temperature is rolled within the range of 300 to 360 ° C., and the structure is observed from the cross section in the rolling direction, the recrystallization rate in the region from the plate surface to the depth of 300 μm is 85% or more and The average aspect ratio of recrystallized grains is 10 or less And a hot rolled sheet having a yield strength in the range of 80 to 175 MPa and a difference between the tensile strength TS and the yield strength YS (TS-YS) of 75 MPa or more. Manufacturing method of alloy hot-rolled sheet.