JP4283408B2

JP4283408B2 - Hot-dip galvanized high-strength thin steel sheet with excellent formability and its manufacturing method

Info

Publication number: JP4283408B2
Application number: JP2000034777A
Authority: JP
Inventors: 展弘藤田; 学高橋; 康秀森本; 明博宮坂
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2000-02-14
Filing date: 2000-02-14
Publication date: 2009-06-24
Anticipated expiration: 2020-02-14
Also published as: JP2001226742A

Description

【０００１】
【発明の属する技術分野】
本発明は、高強度化かつ耐食性を必要とする部材、例えば自動車の足廻り、メンバーや内外板などに用いられる成形性の優れた溶融亜鉛メッキ高強度薄鋼板およびその製造方法に関するものである。
【０００２】
【従来の技術】
地球温暖化問題からＣＯ₂排出量の削減が強く求められている。これに伴い自動車の燃費規制が世界各国で強化される趨勢にある。燃費の向上には車体重量の減量化が必要となり、主要な構成材料である鋼板の薄手化が求められている。鋼板の薄手化を進める場合に重要なのは、▲１▼加工性を損なわずに高強度化すること▲２▼耐食性を向上させること等が挙げられる。
▲１▼については、残留オーステナイトの変態誘起塑性の活用により高強度・高延性化が実現可能であることが示されている。例えば、特開平１−２３０７１５号公報、特開平２−２１７４２５号公報や特開平１−７９３４５号公報に記載の発明がこれにあたる。これらは、Ｃ−Ｓｉ−Ｍｎ系成分を基本組成とした鋼で、二相域焼鈍後ベイナイト変態を活用した熱処理を施すことや熱延後の冷却と巻取りを制御することで残留オーステナイトを生成させることを特徴としている。残留オーステナイトを生成させる熱処理制御として重要なのがパーライトや炭化物の生成を抑制することである。具体的には、オーステナイトが分解しやすい温度域である６００〜４５０℃での滞留時間を短くすることやベイナイト変態温度である３５０〜４５０℃である程度保持することが必要となる。
【０００３】
▲２▼については、メッキが挙げられる。しかし、▲１▼にあるような比較的厳密な熱処理パターンでコストが低く目付量も厚くできる溶融亜鉛メッキを行うことは現状のメッキ設備では極めて困難である。また、Ｓｉを比較的多く（〜２ｗｔ％）含むことからもメッキの密着性向上は大きな課題である。例えば、マテリア（日本金属学会発行、第３８巻、第２号、１９９９年、１６６頁）などではフェライト＋マルテンサイト＋ベイナイトのいわゆる複相鋼板は現状の溶融亜鉛メッキ設備で製造可能であるがオーステナイト相を含む鋼板の製造が難しいことを述べている。成形性に寄与する残留オーステナイト量の確保とそのメッキ性の改善の両立については、例えば、特開平６−１４５８９２号公報に有るようにＡｌを適量添加する事で達成する発明がある。しかし、この中でもオーステナイトの分解を避けるべく焼鈍後の冷却時に６００〜４５０℃の温度域を５℃／ｓ以上の冷却速度とすることが望ましいと記され、依然現状の溶融亜鉛メッキ設備での製造は容易ではない。
【０００４】
【発明が解決しようとする課題】
以上のように、現状の溶融亜鉛メッキラインにて成形性向上に寄与する残留オーステナイト量を確保したメッキ鋼板およびその製造方法は未だ充分には見出されているとは言い難い。
本発明は、上記課題を解決し、現状の溶融亜鉛メッキラインの通常の処理条件にて成形性の優れた残留オーステナイトを含む高強度鋼板とその製造方法を提供する事を目的とする。
【０００５】
【課題を解決するための手段】
本発明者らは、現状の溶融亜鉛メッキラインの通常の処理条件にて処理しても、成形性に寄与する残留オーステナイト量及びその安定性を確保し得るための成分を見出したものである。この成分限定により、薄肉化に対応し得る成形性と耐食性の優れた薄鋼板を供給できる。
即ち、本発明の要旨とするところは、
（１）質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．３〜２．５％、Ｍｎ：０．５〜３．０％、Ｐ：０．１％以下、Ａｌ：０．０４０％以下、Ｍｏ：０．０１〜０．２０％、Ｎ：０．００１０〜０．０１００％を含有し、残部がＦｅおよび不可避的不純物からなり、Ｍｎ，ＭｏおよびＳｉが質量％で３．３−１．１Ｓｉ＞Ｍｎ＞２．３−１．１Ｓｉ、かつ８．３３×１０^-2−０．０１Ｍｎ−４．４４×１０^-2×Ｓｉ＜Ｍｏ＜０．３０３−０．０５Ｍｎ−４．４４×１０^-2×Ｓｉを満たし、炭素を平均濃度で０．９％以上含む残留オーステナイトを体積率で３％以上含有し、アスペクト比で０．５〜３．０の等軸フェライトを体積率で５０％以上含有することを特徴とする成形性の優れた溶融亜鉛メッキ高強度薄鋼板。
【０００６】
（２）Ｎｂ、Ｔi およびＶの１種又は２種以上を合計で０．０１〜０．３質量％以下含む事を特徴とした前記（１）記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板。
（３）Ｂを０．０００１〜０．０１質量％以下含むことを特徴とした前記（１）または（２）記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板。
（４）Ｃｒ，ＣｕおよびＮｉの１種又は２種以上を合計で０．０１〜１．５質量％以下含む事を特徴とした前記（１）〜（３）のいずれか１項に記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板。
【０００７】
（５）Ｃｏを合計で０．００５〜２．０質量％以下含む事を特徴とした前記（１）〜（４）のいずれか１項に記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板。
（６）Ｃａ及び希土類元素の１種又は２種を合計で０．０００１〜０．５質量％以下含む事を特徴とした前記（１）〜（５）のいずれか１項に記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板。
【０００８】
（７）前記（１）〜（６）のいずれか１項に記載の成分を有する鋳造スラブを鋳造まま、もしくは鋳造後一旦冷却した後に１０００〜１３００℃に再度加熱したのち、Ａｒ₃変態温度−１０℃以上、Ａｒ₃変態温度＋１２０℃未満で熱延を完了し、その後２℃／秒以上１００℃／秒以下で鋼板を冷却し、２５０℃以上４２０℃未満で巻き取り、前記鋼板を巻き戻した後、酸化スケールを除去し、Ｎｉ、ＦｅおよびＣｕの何れか一種以上を０．０２〜１０ｇ／ｍ² をあらかじめメッキしたのちに連続焼鈍亜鉛メッキ工程で、０．１×（Ａｃ₃変態温度−Ａｃ₁変態温度＋Ａｃ₁変態温度［℃］以上、Ａc₃変態温度＋５０［℃］以下で１０秒〜３分間焼鈍した後平均冷却速度１〜１００℃／ｓでメッキ浴温度以上５００℃以下に冷却し、引き続き溶融亜鉛メッキを施して、炭素を平均質量濃度で０．９％以上含む残留オーステナイトを体積率で３％以上含有し、アスペクト比で０．５〜３．０の等軸フェライトを体積率で５０％以上含有する鋼板を得ることを特徴とする成形性の優れた溶融亜鉛メッキ高強度薄鋼板の製造方法。
【０００９】
（８）前記（１）〜（６）のいずれか１項に記載の成分を有する鋳造スラブを鋳造まま、もしくは鋳造後一旦冷却した後に１０００〜１３００℃に再度加熱したのち、Ａｒ₃変態温度−１０℃以上、Ａｒ₃変態温度＋１２０℃未満で熱延を完了し、その後２℃／秒以上１００℃／秒以下で鋼板を冷却し、２５０℃以上４２０℃未満で巻き取り、前記鋼板を巻き戻した後、酸化スケールを除去し、還元焼鈍前に、燃焼空気比０．９〜１．２にて酸化し、鋼板表面に１００〜１０００ｎｍのＦｅ酸化物を付与させたのちに連続焼鈍亜鉛メッキ工程で、０．１×（Ａc₃変態温度−Ａｃ₁変態温度）＋Ａｃ₁変態温度［℃］以上、Ａｃ₃変態温度＋５０［℃］以下で１０秒〜３分間焼鈍した後平均冷却速度１〜１００℃／ｓ以上でメッキ浴温度以上５００℃以下に冷却し、引き続き溶融亜鉛メッキを施し、炭素を平均質量濃度で０．９％以上含む残留オーステナイトを体積率で３％以上含有し、アスペクト比で０．５〜３．０の等軸フェライトを体積率で５０％以上含有する鋼板を得ることを特徴とする成形性の優れた溶融亜鉛メッキ高強度薄鋼板の製造方法。
【００１０】
（９）熱延後巻き取った鋼板を巻き戻し、酸洗冷延してその後Ｎｉ、ＦｅおよびＣｕの１種以上の予メッキまたは還元焼鈍を行い、連続焼鈍亜鉛メッキを施すことを特徴とする前記（７）又は（８）記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板の製造方法。
（１０）溶融亜鉛メッキを施した後３５０〜５５０℃で合金化処理する事を特徴とする前記（７）〜（９）のいずれか１項に記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板の製造方法。
（１１）熱間圧延時の仕上げ圧延前に、高圧水を鋼板表面に吹き付けて酸化スケールを除去することを特徴とする前記（７）〜（１０）の何れか１項に記載の成形性の優れた溶融亜鉛メッキ高強度薄鋼板の製造方法にある。
【００１１】
【発明の実施の形態】
以下に、本発明を詳細に説明する。
Ｃ：Ｃは室温で残留するオーステナイトの安定化に貢献する最も安価な元素であるために、本発明において最も重要な元素といえる。鋼材の平均Ｃ量は、室温で確保できる残留オーステナイト体積分率に影響を及ぼすのみならず、製造の加工熱処理中に未変態オーステナイト中に濃化する事で、残留オーステナイトの加工に対する安定性を向上させることが出来る。しかしながら、この添加量が０．０５重量％未満の場合には、最終的に得られる炭素濃度０．９％以上の残留オーステナイト体積分率が３％以上を確保することが出来ないので０．０５％を下限とした。
【００１２】
一方、鋼材の平均Ｃ量が増加するに従って確保可能な残留オーステナイト体積分率は増加し、残留オーステナイト体積率を確保しつつ残留オーステナイトの安定性を確保することが可能となる。しかしながら、鋼材のＣ添加量が過大になると、必要以上に鋼材の強度を上昇させ、プレス加工等の成形性を阻害するのみならず、静的な強度上昇に比して動的な応力上昇が阻害されると共に、溶接性を低下させることによって部品としての鋼材の利用が制限されるようになる。従って鋼材のＣ量の上限を０．２％とした。
【００１３】
Ｓｉ：Ｓｉはフェライトの安定化元素であり、フェライト体積率を増加させることによって鋼材の加工性を向上させる動きがある。また、セメンタイトの生成を抑制することから、効果的にオーステナイト中へのＣを濃化させることを可能とすることから、室温で適当な体積分率のオーステナイトを残留させるためには不可避的な元素であり、０．３％以上添加することが必要である。この様な機能を持つ添加元素としては、Ｓｉ以外に、Ａｌ、ＰやＣｕ、Ｃｒ、Ｍｏ等があげられ、この様な元素を適当に添加することも同様な効果が期待される。しかしながら、Ｓｉの過剰添加はメッキ性を損なうため上限を２．５％とした。
【００１４】
Ｍｎ：Ｍｎはオーステナイト安定化元素であり、室温でオーステナイトを安定化させるためには有効な元素である。特に、溶接性の観点からＣの添加量が制限される場合には、この様なオーステナイト安定化元素を適量添加することによって効果的にオーステナイトを残留させることが可能となるため、０．５％を下限とした。また、ＭｎはＡｌやＳｉ程ではないがセメンタイトの生成を抑制する効果があり、オーステナイトへのＣの濃化を助ける働きもする。しかしながら、３．０質量％を越える場合には、母相であるフェライトの硬質化を招くためこれを上限とした。
【００１５】
Ｐ：Ｐは、鋼材の高強度化や前述のように残留オーステナイトの確保に有効であり、０．００５％以上含有しても良いが、０．１質量％を越えて添加された場合には鋼材のコストの上昇を招くばかりでなく、耐置き割れ性の劣化や疲労特性、靱性の劣化を招くことから、０．１質量％をその上限とした。
Ａｌ：Ａｌは、Ｓｉ同様、フェライト体積率を増加させることによって鋼材の加工性を向上させる働きとセメンタイトの生成を抑制する効果があり０．００５％含有しても良いが、過剰添加はメッキ性を著しく損なうため上限を０．０４０％とした。
【００１６】
Ｍｏ：ＭｏはＡｌやＳｉ程ではないがセメンタイトの生成を抑制する効果があり、オーステナイトへのＣの濃化を助ける働きもする。更に、マトリックスであるフェライトやベイナイトを固溶強化させる。しかしながら、添加が０．０１質量％未満の場合には、必要な残留オーステナイトの確保が出来なくなるとともに、鋼材の強度が低くなり、炭化物抑制効果も十分でない。一方、０．２０質量％を越える場合には、母相であるフェライトの硬質化を招くだけでなく、焼き入れ性が極めて良好になり冷却中にオーステナイトがマルテンサイトに変態してしまうため、これを上限とした。
Ｎ：Ｎは、Ｃ同様、室温で残留するオーステナイトの安定化に貢献する安価な元素で、０．００１０％以上とした。一方で、過剰添加は溶接時のブローホール発生の直接の原因となるため、上限を０．０１００％とした。
【００１７】
さらに、溶融亜鉛メッキラインでの成形性の向上に寄与する残留オーステナイト量を確保するための範囲として、Ｍｎ、ＭｏおよびＳｉが質量％で３．３−１．１Ｓｉ＞Ｍｎ＞２．３−１．１Ｓｉ、かつ８．３３×１０^-2−０．０１Ｍｎ−４．４４×１０^-2×Ｓｉ＜Ｍｏ＜０．３０３−０．０５Ｍｎ−４．４４×１０^-2×Ｓｉを満たすことを必要とする。ＭｎまたはＭｏ量が上記関係式の下限よりも低いと、炭化物生成が促進されて残留オーステナイトが生成困難になり、上限よりも多いと焼き入れ性が上がり、マルテンサイトが生成するため残留オーステナイトが生成困難になるため、上記の範囲に限定した。
【００１８】
残留オーステナイト中の平均炭素濃度及び残留オーステナイト体積率：残留オーステナイト中の平均炭素量はその安定性を高めて成形加工時に残留オーステナイトの変態誘起塑性を十分に活用するために重要であり、０．９質量％以上含む残留オーステナイトを体積率で３％以上含有する事が必要である。残留オーステナイト中の平均炭素濃度が０．９質量％より小さいと残留オーステナイトが極めて不安定で延性向上には寄与しないため、下限を０．９重量％とした。残留オーステナイト中の平均炭素濃度の上限についても特に限定することなく本発明の効果が得られるが、Ｃのオーステナイトの固溶限は概ね２質量％でありこれ以上の濃化は不可能で炭化物析出を伴うので好ましくない。また、オーステナイトの体積率の上限は特に限定することなく本発明の効果を得ることが出来るが体積率増加には合金添加量を増加させることが必要となり経済的に不利となるため５０％未満が望ましい。
【００１９】
なお、残留オーステナイトの体積率およびその炭素濃度は特開平１１−１９３４３９号公報にあるようにＸ線解析により実験的に求められるもので、Ｍｏ−Ｋα線およびＣｕ−Ｋα線を用いて得たデータから次式によりそれぞれ算出できる。残留オーステナイトの体積率＝（２／３）［１００／｛０．７×（フェライトの２１１面のＸ線強度）／（オーステナイトの２２０面のＸ線強度＋１）｝＋１］＋（１／３）［１００／｛０．７８×（フェライトの２１１面のＸ線強度）／（オーステナイトの３１１面のＸ線強度）｝＋１］
また、オーステナイトの（２００）、（２２０）および（３１１）の各面の反射角から格子定数を求め、炭素濃度＝（格子定数−３．５７２）／０．０３３［１×１０^-10ｍ］で得ることが出来る。
【００２０】
フェライトのアスペクト比と体積率：残留オーステナイトばかりでなく主相であるフェライトも充分な変形能を持たなければ、素材全体の延性は確保されない。延性確保には粒の等軸化が有効で、Ｌ断面でのフェライト主相の平均のアスペクト比（Ｌ断面の２００〜１０００倍の１０〜２０視野の光顕観察により、圧延方向と厚さ方向の粒の長さの比を取った値の平均値）を０．５〜３．０とし、これらフェライトが体積率で５０％以上含む事が必要である。アスペクト比が０．５未満であったり３．０以上であると延性が低下し強度が増加し、結果強度−延性バランスが劣化するため、０．５〜３．０に限定した。また、軟質のフェライト相は延性向上に効果的であるため体積率で５０％以上とした。上限は特に定めないが、残留オーステナイトの体積率を確保する点から９７％未満が望ましい。
【００２１】
Ｎｂ、Ｔｉ、Ｖ：また、必要に応じて添加するＮｂ、Ｔｉ、Ｖは、炭化物、窒化物もしくは炭窒化物を形成することによって鋼材を高強度化する事が出来るので、合計０．０１％以上添加する。一方、その合計が０．３％を越えた場合には母相であるフェライト粒内もしくは粒界に多量の炭化物、窒化物もしくは炭窒化物として析出してしまう。このような、炭化物の生成は、本発明にとって最も重要な残留オーステナイト中へのＣの濃化を阻害し、Ｃを浪費することから上限を０．３重量％とした。
Ｂ：また、必要に応じて添加するＢは、粒界の強度や鋼材の高強度化に有効ではあるので０．０００１％以上添加する。一方、その添加量が０．０１質量％を越えるとその効果が飽和するばかりでなく、必要以上に鋼板強度を上昇させ、加工性も低下させることから、上限を０．０１質量％とした。
【００２２】
Ｎｉ、Ｃｒ、Ｃｕ：必要に応じて添加するＮｉ、Ｃｒ、Ｃｕは全てオーステナイト安定化元素であり、室温でオーステナイトを安定化させるためには有効な元素である。特に、溶接性の観点からＣの添加量が制限される場合には、この様なオーステナイト安定化元素を適量添加することによって効果的にオーステナイトを残留させることが可能となる。また、これらの元素はＡｌやＳｉ程ではないがセメンタイトの生成を抑制する効果があり、オーステナイトへのＣの濃化を助ける働きもするので合計で０．０１％以上添加する。一方、これらの合計が１．５質量％を越える場合には、母相であるフェライトの硬質化を招くと共に、焼き入れ性の向上から冷却時のオーステナイトからマルテンサイトへの変態を促進して、残留オーステナイト量を確保できなくなる。したがって、これを条件とした。
【００２３】
Ｃｏ：必要に応じて添加するＣｏはオーステナイト中のＣ濃度を高めるのに有効な元素であり、安定なオーステナイト形成のためには特に有効であるので０．００５％以上添加する。一方で、高価であるため、実用上十分な炭素濃化が図れる添加量として２．０％を上限とした。
Ｃａ，Ｒｅｍ：
必要に応じて添加するＣａ，ＲＥＭは介在物制御に有効な元素で、合計で０．０００１％以上添加することにより熱間加工性を向上させるが、過剰添加は逆に熱間脆化を助長させるため上限を０．５％とした。
【００２４】
熱延条件：熱延ままで本発明の鋼板を製造する場合には、所定の成分に調整されたスラブを鋳造まま、もしくは一旦冷却した後に１０００〜１３００℃の範囲に再度加熱し、熱間圧延を行う。再加熱温度を１０００℃未満とすると、スラブの均一加熱が困難となり、表面キズ発生等の問題を生じるので、再加熱温度の下限を１０００℃とした。また、再加熱温度が１３００℃超では、スラブの変形が激しくなると同時にコスト高となることから、これを上限とした。また、熱延完了温度ＦＴが鋼材の化学成分で決まるＡｒ₃変態温度−１０℃未満である場合には時に鋼板の表層部及びその近傍に加工フェライト層が生成し、加工性を著しく劣化させると同時に、動的な変形抵抗を下げる。従ってこれを熱延完了温度の下限値とする。また熱延完了温度がＡｒ₃変態温度＋１２０℃以上の場合には必要以上に鋼板の強度が上昇するのみならず、組織の粗大化が起こり、鋼板動的変形抵抗の上昇を阻害する。またこの様な高温で熱延が完了された場合には鋼板の表面粗度が大きくなり、表面品位を落とす。従って、これを熱延完了温度の上限値とする。尚、Ａｒ₃変態温度はＡｒ₃＝９０１−３２５×％Ｃ＋３３×％Ｓｉ−９２×（％Ｍｎ＋％Ｎｉ／２＋％Ｃｒ／２＋％Ｃｕ／２＋％Ｍｏ／２）で計算される。
【００２５】
鋼板は熱延完了後に冷却されるが、このときの冷却速度を２℃／秒未満もしくは１００℃／秒超とすることは、大量生産の工程条件上困難であることから、これを下限、上限とした。また冷却の方法は一定の冷却速度で行っても、途中で低冷却速度の領域を含むような複数種類の冷却速度の組み合わせであってもよい。冷却後鋼板は巻き取り処理が行われるが、巻き取り温度が２５０℃未満ではマルテンサイトの生成が過多となって加工性を損なうので下限を２５０℃とした。また、炭化物析出を抑制する目的で低温巻き取りとして巻取温度を４２０℃未満とした。巻き戻し後、メッキぬれ性を十分確保するため酸化スケールを除去する。酸化スケールは酸洗や、メカデスケ等により除去できる。
Ｎｉ，Ｃｕ，Ｆｅの下地メッキ：下地メッキは鋼板表面とメッキ相のぬれ性を確保する目的で行う。したがって、ぬれ性確保のため下限を０．０２ｇ／ｍ² とした。また、それぞれ１０ｇ／ｍ² 以上のメッキはそのぬれ性向上効果が飽和するうえ、経済的に不利になることからこれを上限とした。メッキは電機メッキ装置などにより実施できる。
【００２６】
連続焼鈍亜鉛メッキ条件：熱延又は冷延後の溶融亜鉛メッキに於いては、充分な２相域での焼鈍、すなわち、焼鈍温度が鋼の化学成分によって決まるＡｃ₁変態温度及びＡｃ₃変態温度（例えば「鉄鋼材料学」：Ｗ．Ｃ．Ｌｅｓｌｉｅ著、幸田成康監訳、丸善Ｐ２７３）で表現される０．１×（Ａｒ₃−Ａｃ₁）＋Ａｃ₁［℃］未満の場合には、焼鈍温度で得られるオーステナイト量が少ないので、最終的な鋼板中に安定して残留オーステナイトを残すことができないためにこれを焼鈍温度の下限とした。また焼鈍温度がＡｃ₃変態温度＋５０［℃］を越えても何ら鋼板の特性を改善することができない一方で製造コストの上昇をまねくために、焼鈍温度の上限をＡc₃変態温度＋５０［℃］とした。この温度での焼鈍時間は鋼板の温度均一化とオーステナイト量の確保のために最低１０秒以上必要である。しかし、３分超では効果が飽和するのみならずコストアップにつながることから、これを上限とした。
【００２７】
その後の一次冷却はオーステナイトからフェライトへの変態を促して、未変態のオーステナイト中にＣを濃化させて最終的に残留するオーステナイトの安定化をはかるのに重要である。この冷却速度を１℃／秒未満にすることは、必要な生産ライン長を長くしたり、生産速度を極めて遅くするといった製造上のデメリットを生じるために、この冷却速度の下限を１℃／秒とした。また、設備能力上の冷速の上限として１００℃／ｓとした。また、冷却停止温度は炭化物析出を抑制するため５００℃以下とし、メッキ浴温度まで冷却しても本発明の効果を得ることが出来る。
【００２８】
Ｆｅ酸化物の事前付与：Ｆｅ酸化物形成の燃焼雰囲気として燃焼空気比が０．９に満たない場合、Ｆｅ酸化物が完全に鋼板表面を被覆しないため、焼鈍中にＦｅ酸化物未生成部において、Ｓｉ、Ｍｎ等の酸化物が生成し、メッキ濡れ性を低下させる。一方、１．２を越える場合、生成したＦｅ酸化物の密着性が確保できず、焼鈍炉のハースロールにＦｅ酸化物がビルドアップし、操業性に悪影響を及ぼす。以上の条件にてＦｅ酸化物の厚みを１０００ｎｍ以下とすることでその密着性を確保することとした。一方、酸化物の厚みが２０ｎｍ未満ではメッキぬれ性が確保できずこれを下限とした。なお、酸化物の厚みは走査型電顕を用いて測定できる。
【００２９】
また、冷延は加工性確保のため圧下率５０％以上の冷延が望ましい。合金化処理は、３５０℃以下では反応が進まずこれを下限とした。また、５５０℃を越えると炭化物が析出して充分な残留オーステナイト量を確保できなくなることからこれを上限とした。
再加熱後、酸化スケール除去のため、高圧水によるデスケーリングを以下の条件て行うことが望ましい。
【００３０】
仕上げ熱延前のデスケーリング：仕上げ圧延前の鋼板表面に、衝突圧Ｐ（ＭＰａ）×流量Ｌ（リットル／ｃｍ² ）≧０．００２５の条件を満たす高圧水で、酸化スケール除去を行うこととする。ここで酸化スケールを十分除去することは、メッキの密着性を十分確保する上で重要となる。ここで、鋼板表面での高圧水の衝突圧Ｐは以下のように極めて記述される。（「鉄と鋼」１９９１ｖｏｌ．７７Ｎｏ．９ｐ１４５０参照）
Ｐ（ＭＰａ）＝５．６４×Ｐ₀×Ｖ／Ｈ²
ただし、
Ｐ₀（ＭＰａ）：液圧力
Ｖ（リットル／ｍｉｎ）：ノズル流液量
Ｈ（ｃｍ）：鋼板表面とノズル間の距離
【００３１】
流量Ｌは以下のように記述される。
Ｌ（リットル／ｃｍ²）＝Ｖ／（Ｗ×ｖ）
ただし、
Ｖ（リットル／ｍｉｎ）：ノズル流液量
Ｗ（ｃｍ）：ノズル当たり噴射液が鋼板表面に当たっている幅
ｖ（ｃｍ／ｍｉｎ）：通板速度
衝突圧Ｐ×流量Ｌの上限は本発明の効果を得るためには特に定める必要はないが、ノズル流液量を増加させるとノズルの摩耗が激しくなる等の不都合が生じるため、０．０２以下とすることが好ましい。
【００３２】
【実施例】
表１に示す成分の各鋼を、２５ｋｇインゴットに鋳造して１２００℃に加熱後、熱間圧延して各鋼の成分で決まるＡｒ₃変態点−１０［℃］以上、Ａｒ₃変態点＋１２０［℃］未満（概ね９００℃）で熱間圧延を終了して、５０℃／ｓで３７０℃まで冷却した後３７０℃×１ｈ保定後炉冷の巻き取り処理を行った。また、一部鋼種については、仕上げ熱延前に、衝撃圧２．７ＭＰａ、流量０．００１リットル／ｃｍ²の条件で高圧デスケーリングを行った。得られた熱延板はメッキ試験および冷間圧延に使用した。圧下率６０％の冷延後、０．５×（Ａｃ₃−Ａc₁）＋Ａc₁［℃］の温度で１分焼鈍の２相域加熱した後平均冷却速度２．５℃／ｓで４６０℃まで冷却してその後５３０℃で１０秒保定し空冷したのち機械的性質を調査した。亜鉛メッキ試験は、熱延板、冷延板、あらかじめＮｉを３ｇ／ｍ² 、Ｃｕを３ｇ／ｍ² またはＦｅを８ｇ／ｍ² をメッキした各鋼板および燃焼雰囲気でＦｅ酸化物を１００〜２００ｎｍ形成させた各鋼板について、上記と同じ熱処理に加えて焼鈍・冷却後亜鉛メッキ浴に浸漬したのち一部は５３０℃で１０秒の合金化処理を行い空冷後、６０度曲げ・戻し試験を先端曲率半径３ｍｍで行い、その内側にテープ着脱を行い外観検査にて評価した。
【００３３】
【表１】

【００３４】
表２に各鋼の機械的性質を示す。発明鋼であるＡ，Ｂ，Ｃ，Ｇ，Ｈ，Ｉ，Ｊ，Ｋ，Ｌ，Ｍ，Ｎは熱延板および冷延板共にＴＳ×Ｅｌ＞２１０００ＭＰａ・％と良好な強度延性バランスを示すことが判る。一方、比較鋼のＤ，Ｅ，Ｆ，Ｏ，Ｐ，Ｑ，Ｒ，Ｓ，Ｔは残留オーステナイト量及びその炭素量が低くＴＳ×Ｅｌ．＜２００００ＭＰａ・％にとどまることが判る。これは、焼鈍後の冷却中にオーステナイトが分解してパーライトもしくはマルテンサイトが生成したことに起因すると考えられる。一方で、Ａｌを多量添加したＶ鋼は良好な機械的性質を示すが、メッキ性が良好でないことが表３から判る。
【００３５】
【表２】

【００３６】
【表３】

【００３７】
【発明の効果】
本発明により、現状の溶融亜鉛メッキラインの通常の処理条件にて成形性の優れた残留オーステナイトを含む高強度鋼板を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-dip galvanized high-strength thin steel sheet having excellent formability and used for a member requiring high strength and corrosion resistance, such as an automobile suspension, a member, and an inner and outer plate.
[0002]
[Prior art]
CO from global warming₂There is a strong need to reduce emissions. As a result, automobile fuel efficiency regulations are on the rise in countries around the world. In order to improve fuel consumption, it is necessary to reduce the weight of the vehicle body, and it is required to reduce the thickness of the steel sheet, which is the main constituent material. When proceeding to make the steel sheet thinner, it is important to (1) increase the strength without impairing the workability (2) improve the corrosion resistance.
Regarding (1), it is shown that high strength and high ductility can be realized by utilizing transformation-induced plasticity of retained austenite. For example, the inventions described in JP-A-1-230715, JP-A-2-217425, and JP-A-1-79345 correspond to this. These are steels with a C-Si-Mn-based component as the basic composition. Residual austenite is produced by performing heat treatment utilizing bainite transformation after two-phase annealing and by controlling cooling and winding after hot rolling. It is characterized by letting. What is important as heat treatment control for generating retained austenite is to suppress the formation of pearlite and carbides. Specifically, it is necessary to shorten the residence time at 600 to 450 ° C., which is a temperature range in which austenite is easily decomposed, and to maintain it at 350 to 450 ° C., which is a bainite transformation temperature.
[0003]
As for (2), plating is mentioned. However, it is extremely difficult to perform hot dip galvanizing with a relatively strict heat treatment pattern as described in (1), which can reduce the cost and increase the basis weight, with the current plating equipment. Moreover, the improvement of the adhesiveness of plating is also a big subject from containing comparatively much Si (~ 2 wt%). For example, in Materia (published by the Japan Institute of Metals, Vol. 38, No. 2, 1999, page 166), so-called dual-phase steel sheets of ferrite + martensite + bainite can be manufactured with current hot dip galvanizing equipment, but austenite It states that it is difficult to manufacture steel sheets containing phases. For example, Japanese Patent Application Laid-Open No. 6-145892 discloses an invention that achieves both the securing of the retained austenite amount contributing to the formability and the improvement of the plating property by adding an appropriate amount of Al. However, among these, in order to avoid decomposition of austenite, it is described that it is desirable to set the temperature range of 600 to 450 ° C. to a cooling rate of 5 ° C./s or more during cooling after annealing, and it is still manufactured with the current hot dip galvanizing equipment. Is not easy.
[0004]
[Problems to be solved by the invention]
As described above, it is difficult to say that a plated steel sheet having a retained austenite amount that contributes to improvement of formability in the current hot dip galvanizing line and a method for producing the same are still not sufficiently found.
An object of the present invention is to solve the above-mentioned problems and to provide a high-strength steel sheet containing retained austenite having excellent formability under normal processing conditions of a current hot-dip galvanizing line and a method for producing the same.
[0005]
[Means for Solving the Problems]
The present inventors have found a component for ensuring the amount of retained austenite that contributes to formability and its stability even if the processing is performed under the normal processing conditions of the current hot dip galvanizing line. By this component limitation, it is possible to supply a thin steel sheet having excellent formability and corrosion resistance that can cope with thinning.
That is, the gist of the present invention is that
(1) By mass%, C: 0.05 to 0.2%, Si: 0.3 to 2.5%, Mn: 0.5 to 3.0%, P: 0.1% or less, Al: 0.040% or less, Mo: 0.01 to 0.20%, N: 0.0010 to 0.0100%, the balance is made of Fe and inevitable impurities, and Mn, Mo and Si are in mass%. 3.3-1.1Si> Mn> 2.3-1.1Si and 8.33 × 10^-2-0.01Mn-4.44 × 10^-2× Si <Mo <0.303-0.05Mn-4.44 × 10^-2X Residual austenite containing Si at an average concentration of 0.9% or more is contained at a volume ratio of 3% or more, and equiaxed ferrite having an aspect ratio of 0.5 to 3.0 is contained at a volume ratio of 50% or more. A hot-dip galvanized high-strength thin steel sheet with excellent formability.
[0006]
(2) Hot-dip galvanized high strength excellent in formability as described in (1) above, wherein one or more of Nb, Ti and V are contained in a total amount of 0.01 to 0.3% by mass or less. Thin steel plate.
(3) The hot-dip galvanized high-strength thin steel sheet having excellent formability as described in (1) or (2) above, containing B in an amount of 0.0001 to 0.01% by mass or less.
(4) The composition according to any one of (1) to (3), wherein one or more of Cr, Cu, and Ni are included in a total amount of 0.01 to 1.5% by mass or less. Hot-dip galvanized high-strength thin steel sheet with excellent formability.
[0007]
(5) Hot-dip galvanized high-strength thin film excellent in formability according to any one of (1) to (4), characterized in that it contains 0.005 to 2.0% by mass or less of Co in total steel sheet.
(6) Formability as described in any one of (1) to (5) above, wherein one or two of Ca and rare earth elements are contained in total of 0.0001 to 0.5 mass% or less. Excellent hot dip galvanized high strength steel sheet.
[0008]
(7) The cast slab having the component described in any one of the above (1) to (6) is cast as it is or after cooling once after casting, and then heated again to 1000 to 1300 ° C., and then Ar_ThreeTransformation temperature -10 ° C or higher, Ar_ThreeAfter completion of hot rolling at the transformation temperature + 120 ° C., the steel plate is cooled at 2 ° C./second or more and 100 ° C./second or less, wound at 250 ° C. or more and less than 420 ° C. Remove one or more of Ni, Fe and Cu from 0.02 to 10 g / m² In the continuous annealing galvanizing process after pre-plating, 0.1 × (Ac_ThreeTransformation temperature-Ac₁Transformation temperature + Ac₁Transformation temperature [° C] or higher, Ac_ThreeAfter annealing for 10 seconds to 3 minutes at a transformation temperature of +50 [° C.] or less, the steel is cooled to a plating bath temperature of 500 ° C. or more at an average cooling rate of 1 to 100 ° C./s, followed by hot dip galvanization to obtain an average mass concentration of carbon. A steel sheet containing 3% or more of retained austenite containing 0.9% or more by volume and containing equiaxed ferrite having an aspect ratio of 0.5 to 3.0 or more by volume ratio of 50% or more is obtained. A method for producing hot-dip galvanized high-strength thin steel sheets with excellent formability.
[0009]
(8) The cast slab having the component described in any one of the above (1) to (6) is cast as it is or after cooling once after casting, and then heated again to 1000 to 1300 ° C., and then Ar_ThreeTransformation temperature -10 ° C or higher, Ar_ThreeAfter completion of hot rolling at the transformation temperature + 120 ° C., the steel plate is cooled at 2 ° C./second or more and 100 ° C./second or less, wound at 250 ° C. or more and less than 420 ° C. Remove and oxidize at a combustion air ratio of 0.9 to 1.2 before reduction annealing, and give 100 to 1000 nm Fe oxide on the steel sheet surface, then in the continuous annealing galvanizing step, 0.1 × (Ac_ThreeTransformation temperature-Ac₁Transformation temperature) + Ac₁Transformation temperature [° C] or higher, Ac_ThreeAfter annealing for 10 seconds to 3 minutes at a transformation temperature of +50 [° C.] or less, cooling to a plating bath temperature of 500 ° C. or less at an average cooling rate of 1 to 100 ° C./s or higher, followed by hot dip galvanization, and the average mass concentration of carbon A steel sheet containing 3% or more of retained austenite containing 0.9% or more by volume and containing equiaxed ferrite having an aspect ratio of 0.5 to 3.0 or more by volume ratio of 50% or more is obtained. A method for producing hot-dip galvanized high-strength thin steel sheets with excellent formability.
[0010]
(9) The steel sheet taken up after hot rolling is rewound, pickled and cold rolled, and then subjected to one or more types of pre-plating or reduction annealing of Ni, Fe and Cu, followed by continuous annealing galvanization. The method for producing a hot-dip galvanized high-strength thin steel sheet having excellent formability according to (7) or (8).
(10) Hot-dip galvanized high strength excellent in formability as set forth in any one of (7) to (9), characterized by being alloyed at 350 to 550 ° C. after hot dip galvanizing Manufacturing method of thin steel sheet.
(11) Before the finish rolling at the time of hot rolling, high-pressure water is sprayed on the surface of the steel plate to remove the oxide scale, and the formability according to any one of (7) to (10) above It is in the manufacturing method of the excellent hot-dip galvanized high strength thin steel sheet.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
C: Since C is the cheapest element that contributes to the stabilization of austenite remaining at room temperature, it can be said to be the most important element in the present invention. The average C content of the steel material not only affects the retained austenite volume fraction that can be secured at room temperature, but also improves the stability of residual austenite to processing by concentrating in the untransformed austenite during the manufacturing heat treatment. It can be made. However, if this addition amount is less than 0.05% by weight, the final austenite volume fraction with a carbon concentration of 0.9% or more that is finally obtained cannot be ensured to be 3% or more. % Was the lower limit.
[0012]
On the other hand, the retained austenite volume fraction that can be secured increases as the average C content of the steel material increases, and the stability of retained austenite can be secured while securing the retained austenite volume fraction. However, when the amount of C added to the steel material is excessive, the strength of the steel material is increased more than necessary, and not only the formability such as press working is hindered, but also the dynamic stress increase compared to the static strength increase. In addition to being hindered, the use of steel as a part is limited by reducing weldability. Therefore, the upper limit of the C content of the steel material is set to 0.2%.
[0013]
Si: Si is a stabilizing element of ferrite, and there is a movement to improve the workability of the steel material by increasing the ferrite volume fraction. In addition, since the formation of cementite is suppressed, it is possible to effectively concentrate C in austenite. Therefore, it is an inevitable element to leave austenite having an appropriate volume fraction at room temperature. It is necessary to add 0.3% or more. In addition to Si, additive elements having such a function include Al, P, Cu, Cr, Mo, and the like, and a similar effect can be expected by appropriately adding such elements. However, excessive addition of Si impairs plating properties, so the upper limit was made 2.5%.
[0014]
Mn: Mn is an austenite stabilizing element, and is an effective element for stabilizing austenite at room temperature. In particular, when the addition amount of C is limited from the viewpoint of weldability, it is possible to effectively retain austenite by adding an appropriate amount of such an austenite stabilizing element. Was the lower limit. In addition, Mn has an effect of suppressing the formation of cementite, although not as much as Al and Si, and also functions to help enrich C in austenite. However, when it exceeds 3.0% by mass, the upper limit is set because it leads to hardening of the ferrite as the parent phase.
[0015]
P: P is effective in increasing the strength of steel materials and securing retained austenite as described above, and may be contained in an amount of 0.005% or more, but when added in excess of 0.1% by mass, Not only the cost of the steel material is increased, but also the crack resistance, fatigue characteristics, and toughness are deteriorated. Therefore, the upper limit is set to 0.1% by mass.
Al: Al, like Si, has the effect of improving the workability of steel by increasing the ferrite volume fraction and the effect of suppressing the formation of cementite, and may be contained in an amount of 0.005%. The upper limit was made 0.040%.
[0016]
Mo: Although Mo is not as much as Al or Si, it has an effect of suppressing the formation of cementite, and also serves to assist the concentration of C into austenite. Further, the matrix ferrite and bainite are strengthened by solid solution. However, if the addition is less than 0.01% by mass, the necessary retained austenite cannot be secured, the strength of the steel material is lowered, and the carbide suppressing effect is not sufficient. On the other hand, if it exceeds 0.20% by mass, not only does the ferrite as the matrix phase harden, but also the hardenability becomes extremely good and austenite transforms into martensite during cooling. Was the upper limit.
N: N, like C, is an inexpensive element that contributes to the stabilization of austenite remaining at room temperature, and is made 0.0010% or more. On the other hand, excessive addition directly causes blowholes during welding, so the upper limit was made 0.0100%.
[0017]
Furthermore, as a range for securing the amount of retained austenite that contributes to improvement of formability in the hot dip galvanizing line, 3.3 to 1.1 Si> Mn> 2.3-1 in terms of mass% of Mn, Mo, and Si. .1Si and 8.33 × 10^-2-0.01Mn-4.44 × 10^-2× Si <Mo <0.303-0.05Mn-4.44 × 10^-2It is necessary to satisfy xSi. If the amount of Mn or Mo is lower than the lower limit of the above relational expression, carbide formation is promoted and it becomes difficult to generate retained austenite. If it exceeds the upper limit, hardenability is improved and martensite is generated, so that residual austenite is generated. Since it becomes difficult, it was limited to the above range.
[0018]
Average carbon concentration and residual austenite volume fraction in retained austenite: The average carbon content in retained austenite is important for enhancing its stability and fully utilizing the transformation-induced plasticity of retained austenite during molding. It is necessary to contain 3% or more of retained austenite containing at least mass% by volume. If the average carbon concentration in the retained austenite is less than 0.9% by mass, the retained austenite is extremely unstable and does not contribute to the improvement of ductility, so the lower limit was made 0.9% by weight. The upper limit of the average carbon concentration in the retained austenite is not particularly limited, but the effect of the present invention can be obtained. However, the solid solubility limit of C austenite is approximately 2% by mass, and no further enrichment is possible. Is not preferable. Further, the upper limit of the volume ratio of austenite is not particularly limited, but the effect of the present invention can be obtained. However, an increase in the volume ratio requires an increase in the amount of alloy addition, which is economically disadvantageous, so less than 50%. desirable.
[0019]
The volume fraction of retained austenite and its carbon concentration are obtained experimentally by X-ray analysis as described in JP-A-11-193439, and data obtained using Mo-Kα rays and Cu-Kα rays. From the following equations. Volume ratio of retained austenite = (2/3) [100 / {0.7 × (X-ray intensity of 211 face of ferrite) / (X-ray intensity of 220 face of austenite + 1)} + 1] + (1/3) [100 / {0.78 × (X-ray intensity of 211 face of ferrite) / (X-ray intensity of 311 face of austenite)} + 1]
Also, the lattice constant was determined from the reflection angles of the respective surfaces of (200), (220) and (311) of austenite, and the carbon concentration = (lattice constant−3.572) /0.033 [1 × 10^-Tenm].
[0020]
Aspect ratio and volume ratio of ferrite: If not only retained austenite but also ferrite as a main phase does not have sufficient deformability, ductility of the entire material cannot be secured. Grain equiaxing is effective for ensuring ductility, and the average aspect ratio of the ferrite main phase in the L cross section (by light microscopy of 10 to 20 fields of view of 200 to 1000 times the L cross section, in the rolling direction and the thickness direction) The average value of the ratio of the grain lengths) is 0.5 to 3.0, and it is necessary that these ferrites are contained in a volume ratio of 50% or more. When the aspect ratio is less than 0.5 or 3.0 or more, the ductility is lowered and the strength is increased. As a result, the strength-ductility balance is deteriorated, so the content is limited to 0.5 to 3.0. Further, since the soft ferrite phase is effective in improving ductility, the volume ratio is set to 50% or more. Although the upper limit is not particularly defined, it is preferably less than 97% from the viewpoint of securing the volume ratio of retained austenite.
[0021]
Nb, Ti, V: Nb, Ti, V added as necessary can increase the strength of the steel material by forming carbide, nitride, or carbonitride, so a total of 0.01% Add more. On the other hand, when the total exceeds 0.3%, a large amount of carbide, nitride, or carbonitride precipitates in the ferrite grains or grain boundaries as the parent phase. Such carbide formation inhibits the concentration of C in the retained austenite, which is most important for the present invention, and wastes C, so the upper limit was made 0.3 wt%.
B: Further, B added if necessary is effective for increasing the grain boundary strength and the strength of the steel material, so 0.0001% or more is added. On the other hand, when the addition amount exceeds 0.01% by mass, not only the effect is saturated but also the steel sheet strength is increased more than necessary, and the workability is also decreased, so the upper limit was made 0.01% by mass.
[0022]
Ni, Cr, Cu: Ni, Cr, and Cu, which are added as necessary, are all austenite stabilizing elements and are effective elements for stabilizing austenite at room temperature. In particular, when the amount of addition of C is limited from the viewpoint of weldability, austenite can effectively remain by adding an appropriate amount of such an austenite stabilizing element. Further, although these elements are not as much as Al and Si, they have an effect of suppressing the formation of cementite, and also serve to assist the concentration of C into austenite, so a total of 0.01% or more is added. On the other hand, when the total of these exceeds 1.5% by mass, it leads to hardening of the ferrite that is the parent phase, and promotes transformation from austenite to martensite during cooling from improving hardenability, The amount of retained austenite cannot be secured. Therefore, this was a condition.
[0023]
Co: Co added if necessary is an element effective for increasing the C concentration in austenite, and is particularly effective for forming stable austenite, so 0.005% or more is added. On the other hand, since it is expensive, 2.0% was made the upper limit as the amount of addition that can achieve practically sufficient carbon concentration.
Ca, Rem:
Ca and REM, which are added as necessary, are effective elements for controlling inclusions. Adding 0.0001% or more in total improves hot workability, but excessive addition promotes hot embrittlement. Therefore, the upper limit is set to 0.5%.
[0024]
Hot rolling conditions: When producing the steel sheet of the present invention while hot rolling, the slab adjusted to a predetermined component is cast as it is or once cooled, it is heated again in the range of 1000 to 1300 ° C and hot rolled. I do. If the reheating temperature is less than 1000 ° C., uniform heating of the slab becomes difficult and problems such as surface scratches occur, so the lower limit of the reheating temperature is set to 1000 ° C. In addition, when the reheating temperature is higher than 1300 ° C., the deformation of the slab becomes severe and at the same time the cost is increased. In addition, the hot rolling completion temperature FT is determined by the chemical composition of the steel material._ThreeWhen the transformation temperature is lower than −10 ° C., a processed ferrite layer is sometimes formed in the surface layer portion of the steel plate and the vicinity thereof, and the workability is remarkably deteriorated, and at the same time, the dynamic deformation resistance is lowered. Therefore, this is the lower limit of the hot rolling completion temperature. The hot rolling completion temperature is Ar_ThreeIn the case of the transformation temperature + 120 ° C. or higher, not only the strength of the steel sheet is increased more than necessary, but also the structure is coarsened, and the increase in the steel sheet dynamic deformation resistance is inhibited. In addition, when hot rolling is completed at such a high temperature, the surface roughness of the steel sheet increases and the surface quality is degraded. Therefore, this is the upper limit value of the hot rolling completion temperature. Ar_ThreeTransformation temperature is Ar_Three= 901-325 *% C + 33 *% Si-92 * (% Mn +% Ni / 2 +% Cr / 2 +% Cu / 2 +% Mo / 2).
[0025]
The steel sheet is cooled after completion of hot rolling, but it is difficult to set the cooling rate at this time to less than 2 ° C./second or more than 100 ° C./second because of the process conditions for mass production. It was. The cooling method may be performed at a constant cooling rate, or may be a combination of a plurality of types of cooling rates including a low cooling rate region on the way. After cooling, the steel sheet is subjected to a winding process, but if the winding temperature is less than 250 ° C, the martensite is excessively generated and the workability is impaired, so the lower limit is set to 250 ° C. Further, the coiling temperature was set to less than 420 ° C. for low temperature coiling for the purpose of suppressing carbide precipitation. After rewinding, the oxide scale is removed to ensure sufficient plating wettability. The oxidized scale can be removed by pickling or mechanical deske.
Ni, Cu, Fe base plating: The base plating is performed for the purpose of ensuring the wettability of the steel plate surface and the plating phase. Therefore, to ensure wettability, the lower limit is 0.02 g / m.² It was. Also, 10g / m each² The above plating is the upper limit because the wettability improving effect is saturated and disadvantageous economically. Plating can be performed by an electroplating apparatus or the like.
[0026]
Continuous annealing galvanizing conditions: In hot dip galvanizing after cold rolling, annealing in a sufficient two-phase region, that is, the annealing temperature is determined by the chemical composition of the steel.₁Transformation temperature and Ac_Three0.1 × (Ar) expressed by transformation temperature (for example, “steel material science”: W. C. Leslie, translated by Koyasu Naruyasu, Maruzen P273)_Three-Ac₁) + Ac₁When the temperature is lower than [° C.], since the amount of austenite obtained at the annealing temperature is small, it is not possible to leave residual austenite stably in the final steel sheet, so this was made the lower limit of the annealing temperature. Also, the annealing temperature is Ac_ThreeEven if the transformation temperature exceeds +50 [° C.], the steel sheet characteristics cannot be improved at all, but the upper limit of the annealing temperature is set to Ac to increase the manufacturing cost._ThreeThe transformation temperature was +50 [° C.]. The annealing time at this temperature is required to be at least 10 seconds in order to make the temperature of the steel plate uniform and to secure the amount of austenite. However, over 3 minutes not only saturates the effect but also increases costs, so this was made the upper limit.
[0027]
Subsequent primary cooling is important for promoting the transformation from austenite to ferrite, concentrating C in untransformed austenite, and finally stabilizing the remaining austenite. If this cooling rate is less than 1 ° C./second, the production line length required or the production rate extremely slows down, so the lower limit of this cooling rate is 1 ° C./second. It was. The upper limit of the cooling speed on the equipment capacity was set to 100 ° C./s. In addition, the cooling stop temperature is set to 500 ° C. or lower in order to suppress carbide precipitation, and the effect of the present invention can be obtained even when cooling to the plating bath temperature.
[0028]
Prior application of Fe oxide: When the combustion air ratio is less than 0.9 as the combustion atmosphere for Fe oxide formation, since the Fe oxide does not completely cover the steel sheet surface, during the annealing in the Fe oxide non-production part , Si, Mn, and other oxides are generated and the plating wettability is lowered. On the other hand, when it exceeds 1.2, the adhesion of the produced Fe oxide cannot be secured, and the Fe oxide builds up on the hearth roll of the annealing furnace, which adversely affects operability. Under the above conditions, the adhesion was ensured by setting the thickness of the Fe oxide to 1000 nm or less. On the other hand, if the thickness of the oxide is less than 20 nm, the plating wettability cannot be secured, and this is set as the lower limit. The oxide thickness can be measured using a scanning electron microscope.
[0029]
Further, the cold rolling is preferably performed at a rolling reduction of 50% or more in order to ensure workability. In the alloying treatment, the reaction did not proceed at 350 ° C. or lower, and this was set as the lower limit. Further, if the temperature exceeds 550 ° C., carbides are precipitated, and a sufficient amount of retained austenite cannot be secured, so this was made the upper limit.
After reheating, descaling with high-pressure water is desirably performed under the following conditions in order to remove oxide scale.
[0030]
Descaling before finish hot rolling: Collision pressure P (MPa) x flow rate L (liters / cm) on the steel plate surface before finish rolling² ) Oxidation scale removal is performed with high-pressure water that satisfies the condition of ≧ 0.0025. Here, it is important to sufficiently remove the oxide scale in order to ensure sufficient adhesion of the plating. Here, the collision pressure P of high-pressure water on the surface of the steel sheet is extremely described as follows. (Refer to "Iron and Steel" 1991 vol. 77 No. 9 p1450)
P (MPa) = 5.64 × P₀× V / H²
However,
P₀(MPa): Fluid pressure
V (liter / min): Nozzle flow rate
H (cm): distance between the steel plate surface and the nozzle
[0031]
The flow rate L is described as follows.
L (liters / cm²) = V / (W × v)
However,
V (liter / min): Nozzle flow rate
W (cm): Width of spray liquid per nozzle hitting steel plate surface
v (cm / min): Feeding speed
The upper limit of the collision pressure P × the flow rate L is not particularly required to obtain the effects of the present invention, but increasing the nozzle flow rate causes problems such as severe wear of the nozzle, and therefore is 0.02 or less. It is preferable that
[0032]
【Example】
Each steel of the components shown in Table 1 is cast into a 25 kg ingot, heated to 1200 ° C., hot-rolled, and Ar determined by the components of each steel_ThreeTransformation point −10 [° C.] or higher, Ar_ThreeHot rolling was terminated at a transformation point of less than +120 [° C.] (generally 900 ° C.), cooled to 370 ° C. at 50 ° C./s, and then held at 370 ° C. × 1 h, followed by a furnace cooling winding process. For some steel types, the impact pressure is 2.7 MPa and the flow rate is 0.001 liter / cm before finishing hot rolling.²High pressure descaling was performed under the following conditions. The obtained hot rolled sheet was used for plating test and cold rolling. After cold rolling with a rolling reduction of 60%, 0.5 × (Ac_Three-Ac₁) + Ac₁After heating in a two-phase region of 1 minute annealing at a temperature of [° C.], it was cooled to 460 ° C. at an average cooling rate of 2.5 ° C./s, then held at 530 ° C. for 10 seconds and air-cooled, and then the mechanical properties were investigated. The galvanization test is performed by hot-rolled sheets, cold-rolled sheets, and Ni in advance 3 g / m² Cu 3g / m² Or Fe 8g / m² In addition to the same heat treatment as above, each steel plate plated with galvanized steel and each steel plate formed with an oxide of 100 to 200 nm was immersed in a galvanizing bath after annealing and cooling, and a part of the steel plate was 530 ° C. for 10 seconds. After alloying treatment and air cooling, a 60-degree bending / returning test was conducted with a radius of curvature of the tip of 3 mm, tape was attached to the inside thereof, and evaluation was performed by appearance inspection.
[0033]
[Table 1]

[0034]
Table 2 shows the mechanical properties of each steel. Invented steels A, B, C, G, H,I, J, K, L, M, NIt can be seen that both hot-rolled and cold-rolled plates show a good balance of strength and ductility as TS × El> 21000 MPa ·%. On the other hand, comparative steels D, E, F,O, P, Q, R, S, TIs low in retained austenite and carbon content, and TS × El. It can be seen that it remains at <20000 MPa ·%. This is considered to be due to austenite being decomposed and pearlite or martensite formed during cooling after annealing. On the other hand, V steel to which a large amount of Al is added shows good mechanical properties, but it can be seen from Table 3 that the plating property is not good.
[0035]
[Table 2]

[0036]
[Table 3]

[0037]
【The invention's effect】
According to the present invention, a high-strength steel sheet containing retained austenite having excellent formability can be obtained under normal processing conditions of the current hot dip galvanizing line.

Claims

% By mass
C: 0.05 to 0.2%
Si: 0.3-2.5%
Mn: 0.5 to 3.0%
P: 0.1% or less Al: 0.040% or less Mo: 0.01-0.20%
N: 0.0010 to 0.0100%
The balance is Fe and inevitable impurities, and Mn, Mo and Si are 3.3% by mass in terms of 3.3-1.1Si>Mn> 2.3-1.1Si, and 8.33 × 10 ⁻² −. 0.01Mn-4.44 × 10 ⁻² × Si <Mo <0.303-0.05Mn−4.44 × 10 ⁻² × Si
And containing a residual austenite containing 0.9% or more of carbon in an average concentration of 3% or more by volume and containing equiaxed ferrite having an aspect ratio of 0.5 to 3.0 by 50% or more by volume. Hot-dip galvanized high-strength thin steel sheet with excellent formability.

The hot-dip galvanized high-strength thin steel sheet with excellent formability according to claim 1, wherein one or more of Nb, Ti and V are contained in a total amount of 0.01 to 0.3% by mass or less.

The hot-dip galvanized high-strength steel sheet having excellent formability according to claim 1 or 2, wherein B is contained in an amount of 0.0001 to 0.01 mass% or less.

The melt having excellent formability according to any one of claims 1 to 3, characterized by containing one or more of Cr, Cu and Ni in a total amount of 0.01 to 1.5 mass% or less. Galvanized high strength thin steel sheet.

The hot-dip galvanized high-strength thin steel sheet with excellent formability according to any one of claims 1 to 4, wherein Co is contained in a total amount of 0.005 to 2.0 mass% or less.

The hot dip galvanizing with excellent formability according to any one of claims 1 to 5, characterized by containing one or two of Ca and rare earth elements in total of 0.0001 to 0.5 mass% or less. High strength thin steel sheet.

The cast slab having the component according to any one of claims 1 to 6 is cast as it is or after it is cooled once after casting and then heated again to 1000 to 1300 ° C, and then Ar ₃ transformation temperature is -10 ° C or higher, Ar ₃ The hot rolling is completed at a transformation temperature of less than + 120 ° C, and then the steel plate is cooled at 2 ° C / second to 100 ° C / second and wound up at 250 ° C to less than 420 ° C. Then, after plating at least one of Ni, Fe and Cu in the range of 0.02 to 10 g / m ² in advance, in a continuous annealing galvanizing step, 0.1 × (Ac ₃ transformation temperature−Ac ₁ transformation temperature) ) After annealing for 10 seconds to 3 minutes at + Ac ₁ transformation temperature [° C.] or higher and Ac ₃ transformation temperature +50 [° C.] or lower, cooling to plating bath temperature or higher and 500 ° C. or lower at an average cooling rate of 1 to 100 ° C./s. Molten zinc The residual austenite containing 0.9% or more of carbon in an average mass concentration is contained by 3% or more by volume, and equiaxed ferrite having an aspect ratio of 0.5 to 3.0 is contained by 50% or more by volume. A method for producing a hot-dip galvanized high-strength thin steel sheet having excellent formability, comprising obtaining a steel sheet to be contained.

The cast slab having the component according to any one of claims 1 to 6 is cast as it is or after it is cooled once after casting and then heated again to 1000 to 1300 ° C, and then Ar ₃ transformation temperature is -10 ° C or higher, Ar ₃ After completion of hot rolling at the transformation temperature + 120 ° C., the steel plate is cooled at 2 ° C./second or more and 100 ° C./second or less, wound at 250 ° C. or more and less than 420 ° C. Remove and oxidize at a combustion air ratio of 0.9 to 1.2 before reduction annealing, and give 20 to 1000 nm Fe oxide on the steel sheet surface, then in the continuous annealing galvanizing step, 0.1 × (Ac ₃ transformation temperature−Ac ₁ transformation temperature) + Ac ₁ transformation temperature [° C.] or more, Ac ₃ transformation temperature + 50 [° C.] or less, annealing for 10 seconds to 3 minutes, and then plating at an average cooling rate of 1 to 100 ° C./s or more Cool to bath temperature or higher and 500 ° C or lower Subsequently, hot dip galvanization was performed, and retained austenite containing 0.9% or more of carbon by average mass concentration was contained by 3% or more by volume, and equiaxed ferrite having an aspect ratio of 0.5 to 3.0 by volume. A method for producing a hot-dip galvanized high-strength thin steel sheet having excellent formability, characterized by obtaining a steel sheet containing at least%.

The steel sheet taken up after hot rolling is rewound, pickled and cold rolled, and then one or more kinds of pre-plating or reduction annealing of Ni, Fe and Cu are performed, and continuous annealing galvanization is performed. Or the manufacturing method of the hot dip galvanized high-strength thin steel plate excellent in the formability of 8.

The method for producing a hot-dip galvanized high-strength thin steel sheet with excellent formability according to any one of claims 7 to 9, wherein the alloying treatment is performed at 350 to 550 ° C after the hot dip galvanizing.

The hot-dip galvanized sheet having excellent formability according to any one of claims 7 to 10, wherein the oxide scale is removed by spraying high-pressure water onto the surface of the steel sheet before finish rolling during hot rolling. A manufacturing method for high strength steel sheet.