JP4299430B2 - High-strength thin steel sheet with excellent galvanizing adhesion and formability and method for producing the same - Google Patents
High-strength thin steel sheet with excellent galvanizing adhesion and formability and method for producing the same Download PDFInfo
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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亜鉛メッキ密着性および成形性の優れた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。
【０００３】
▲１▼については、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や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、特開昭５８−６９３７号公報や特開昭６０−１２１２２５号公報ではフェライト＋マルテンサイトを主相とすることで低降伏比かつ高延性を達成した高強度鋼板を、特開平１−２３０７１５号公報、特開平２−２１７４２５号公報や特開平１−７９３４５号公報では、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高延性を達成した高強度鋼板を得ることが開示されている。
ここで、マルテンサイトや残留オーステナイトを充分生成させるためにはパーライトや炭化物の生成を抑制するため焼鈍後の冷却速度を充分に確保することが必要である。さらに、残留オーステナイトの確保には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。
【０００４】
また、▲２▼については、溶融亜鉛メッキの活用が工業的には最も有用な手段の１つである。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、特にＳｉを多く（１〜２質量％）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。これに対し、特公昭６１−９３８６号公報では、溶融メッキ処理に先立ちＮｉを予め下地メッキする手法や特開平４−２５４５３１号公報では鋼板を予め酸化処理する手法などが開発されている。しかしながら、これらの手法は付加的な処理及びそれを行う設備追加が必要となり、コスト高は必携である。
【０００５】
【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溶融メッキ処理前にＮｉ下地メッキや酸化処理などなしに高強度・高延性の薄鋼板の製造は未だ困難であると言わざるを得ない。
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亜鉛メッキ密着性および成形性の優れた高強度薄鋼板およびその製造方法を提供することを目的とする。
【０００６】
【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、亜鉛メッキ密着性および成形性の優れた高強度薄鋼板成分およびその製造方法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：０．０１〜０．３％未満、Ｍｎ：０．５〜３．０％、Ｐ ：０．００５〜０．１％、Ａｌ：０．００１〜２．０％、Ｃｏ：０．００５〜２．０％、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、０．９％以上の炭素を含む残留オーステナイト量が体積％で３％以上であり、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％以上含むことを特徴とする亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
【０００７】
（２）質量％で、Ｍｏ：０．０１〜０．５０％を含むことを特徴とする前記（１）記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
（３）質量％で、Ｎｂ、ＴｉおよびＶの１種又は２種以上を合計で０．０１〜０．３％を含むことを特徴とする前記（１）または（２）記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
（４）質量％で、Ｂを０．０００１〜０．０１％を含むことを特徴とする前記（１）〜（３）のいずれか１項に記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
【０００８】
（５）質量％で、Ｃｒ，ＣｕおよびＮｉの１種又は２種以上を合計で０．０１〜１．５％を含むことを特徴とする前記（１）〜（４）のいずれか１項に記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
（６）質量％で、Ｃａおよび希土類元素の１種又は２種を合計で０．０００１〜０．５％含むことを特徴とする前記（１）〜（５）のいずれか１項に記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
【０００９】
（７）前記（１）〜（６）のいずれか１項に記載の薄鋼板に溶融亜鉛メッキを施した鋼板であって、該メッキ相中において質量％でＦｅを５〜２０％含有することを特徴とする亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。
【００１０】
（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、もしくは一旦冷却した後に１０００〜１３００℃の範囲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、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することを特徴とする亜鉛メッキ密着性および成形性の優れた高強度薄鋼板の製造方法。
【００１１】
（９）溶融亜鉛メッキを施した後３５０〜５５０℃で合金化処理することを特徴とする前記（８）に記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板の製造方法。
（１０）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（８）または（９）に記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板の製造方法。
（１１）熱延後巻取った熱延鋼板を巻き戻し、酸洗後冷延し、その後連続焼鈍亜鉛メッキを施すことを特徴とする前記（８）〜（１０）のいずれか１項に記載の亜鉛メッキ密着性および成形性の優れた高強度薄鋼板とその製造方法にある。
【００１２】
【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こと、およびマルテンサイト生成にも有効な元素で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やマルテンサイトの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質量％未満の場合には、最終的に得られるマルテンサイト量や炭素濃度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。
【００１３】
一方、鋼材の平均Ｃ量が増加するに従って確保可能なマルテンサイトまたは残留オーステナイト体積分率は増加し、特に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も可能となる。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。
【００１４】
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し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マルテンサイトまたは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、０．０１％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質量％で０．０１〜０．３％未満とした。
【００１５】
Ｍｎ：Ｍｎはオーステナイト安定化元素であり、焼入れ性を向上させてマルテンサイトを生成させることや、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、またはマルテンサイトを生成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。
【００１６】
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るが、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、質量％で０．００５〜０．１％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る。しかしながら、過剰添加はメッキ性を著しく損なうため質量％で０．００１〜２．０％とした。
【００１７】
Ｃｏ：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％を下限とする。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。
Ｎ：Ｎは、Ｃ同様、オーステナイトを室温で安定化させて残留させることやマルテンサイト生成のために有効であることから、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原因となるため、上限を０．０１００％とした。
【００１８】
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。また、焼入れ性を向上させマルテンサイト生成を促す。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、必要なマルテンサイトまたは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、０．５０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。
【００１９】
Ｎｂ、Ｔｉ、Ｖ：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、合計の含有量として０．０１質量％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延性を劣化させる場合がある。また、このような、炭化物の生成は、本発明にとって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：必要に応じて添加するＢは、粒界の強化や鋼材の高強度化に有効ではあるので０．０００１質量％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。
【００２０】
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、およびマルテンサイト生成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や焼入れ性を向上させるマルテンサイトの生成を促すことが可能となる。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質量％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ためこれを上限とした。
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、適量添加は熱間加工性を向上させるので合計で０．０００１質量％以上添加するが、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。
【００２１】
残留オーステナイト中の平均炭素濃度およ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％以上含有することが必要である。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質量％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。
【００２２】
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［１×１０-10 ｍ］で得ることが出来る。
【００２３】
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。メッキ相中のＦｅ濃度：メッキ相と地鉄の密着性を確保する上でメッキ相／母相界面の組成やメッキ相自体の組成は重要な因子である。実際、種々のＦｅ−Ｚｎの化合物が形成され、メッキ相中のＦｅ濃度を規定することで密着性を規定できる。質量％で５％以上２０％以下であると良好な密着性が得られる。
【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。
【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。さらに、合金化処理として３５０℃以上とした。３５０℃未満では十分な合金化がなされずに、合金化の目的が達せられない。また、炭化物析出抑制の観点からは５５０℃以下が望ましい。
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で行うことが望ましい。
【００２８】
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は以下のように記述される。（「鉄と鋼」１９９１ ｖｏｌ．７７Ｎｏ．９ ｐ１４５０参照）
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
【００２９】
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。
【００３０】
【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高圧水を鋼板表面に噴射することで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。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４５０℃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、熱延板および冷延板について、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。
【００３１】
【表１】
表２に各鋼の機械的性質を示す。発明鋼であるＡ〜Ｊ鋼の熱延板および冷延板共にＴＳ×Ｅｌ．＞２１０００ ＭＰａ・％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Ｋ，Ｍ〜Ｒは、ＴＳ×Ｅｌ．＜２００００ ＭＰａ・％にとどまることが判る。一方で、Ａｌを多量添加したＬ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。
【００３３】
【表２】
【表３】
【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亜鉛メッキ密着性および成形性の優れた高強度薄鋼板を得ることができる極めて優れた効果を奏するものである。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a high-strength thin steel sheet having excellent galvanizing adhesion and formability used for members that require high strength and corrosion resistance, such as automobile suspensions, members and inner and outer plates, and a method for producing the same. is there.
[0002]
[Prior art]
There is a strong demand for reducing CO 2 emissions due to global warming issues. 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.
[0003]
Regarding (1), it is shown that high strength and high ductility can be realized by utilizing the so-called double phase steel plate of ferrite + martensite + bainite and transformation induced plasticity of retained austenite. For example, in Japanese Patent Application Laid-Open Nos. 58-6937 and 60-121225, a high strength steel sheet that achieves a low yield ratio and high ductility by using ferrite + martensite as a main phase is disclosed in Japanese Patent Application Laid-Open No. 1-230715. In Japanese Patent Application Laid-Open No. 2-217425 and Japanese Patent Application Laid-Open No. 1-279345, a steel having a C—Si—Mn-based component as a basic composition is subjected to heat treatment utilizing bainite transformation after two-phase annealing. It is disclosed that a high-strength steel sheet that achieves high ductility by generating retained austenite by controlling cooling and winding after hot rolling is disclosed.
Here, in order to sufficiently generate martensite and retained austenite, it is necessary to sufficiently secure the cooling rate after annealing in order to suppress the formation of pearlite and carbide. Furthermore, in order to secure retained austenite, it is necessary to maintain the bainite transformation temperature at 350 to 450 ° C. to some extent.
[0004]
Regarding (2), the utilization of hot dip galvanizing is one of the most useful means industrially. 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 a big subject also from containing especially much Si (1-2 mass%). On the other hand, Japanese Patent Publication No. 61-9386 has developed a technique in which Ni is pre-plated prior to the hot dipping process, and Japanese Patent Laid-Open No. 4-254531 has been developed in which a steel sheet is previously oxidized. However, these methods require additional processing and additional equipment for performing such processing, and are expensive.
[0005]
[Problems to be solved by the invention]
As described above, I have to say the production of thin steel sheet having high strength and high ductility Shi Donna N i underlayer plating or oxidation treatment in the state of hot-dip galvanizing line before molten plating is still difficult Absent.
The present invention is to solve the above problems, and an object thereof is to provide a high strength thin steel sheet and a manufacturing method thereof excellent zinc plating adhesion and moldability at normal processing conditions of the state of molten zinc plating line.
[0006]
[Means for Solving the Problems]
The present inventors have found a high-strength thin steel plate component excellent in galvanizing adhesion and formability and a method for producing the same, even when processed under 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: 0.01 to less than 0.3%, Mn: 0.5 to 3.0%, P: 0.005 to 0.1 %, Al: 0.001~2.0%, Co : 0.005~2.0%, N: contains 0.0010 to 0.0100 percent, Ri Do from the balance Fe and unavoidable impurities, 0 Zinc plating characterized in that the amount of retained austenite containing 9% or more of carbon is 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. High-strength thin steel sheet with excellent adhesion and formability.
[0007]
(2) in mass%, Mo: 0.01~0.50% above, wherein the free Mukoto a (1) of zinc plating adhesion and formability according excellent high strength thin steel sheet.
(3)% by weight, Nb, wherein (1) or (2) zinc plating adhesion according to characterized in that it comprises from 0.01 to 0.3% one or more in total of Ti and V High-strength thin steel sheet with excellent properties and formability.
(4) in mass%, excellent zinc plating adhesion and formability according to any one of the characterized in that it comprises a 0.0001 to 0.01 percent of B (1) ~ (3) High strength thin steel sheet.
[0008]
(5) in mass%, Cr, said characterized in that it comprises a 0.01 to 1.5% one or more in total of Cu and Ni (1) any one of - (4) A high-strength thin steel sheet having excellent galvanizing adhesion and formability as described in 1.
(6) mass%, the characterized in that it comprises from 0.0001 to 0.5% one or two of Ca and rare earth elements in a total (1) to according to any one of (5) High strength thin steel sheet with excellent galvanized adhesion and formability.
[0009]
(7) the (1) to a steel sheet galvanized thin steel sheet according to any one of (6), which contains 5-20% of Fe in terms of mass% in the plating phase A high-strength thin steel sheet with excellent galvanized adhesion and formability.
[0010]
(8) the (1) to (6) while casting a cast slab having a component according to any one of, or after heated again in the range of 1000 to 1300 ° C. After once cooled, Ar 3 transformation temperature - Hot rolling is completed at 10 ° C. or more and Ar 3 transformation temperature + 120 ° C., and then the steel plate is cooled at 2 ° C./second or more and 100 ° C./second or less, wound up at 250 ° C. or more and less than 420 ° C., and the steel sheet is rewound. Thereafter, the oxide scale is removed, and in the continuous annealing galvanizing step, 0.1 × (Ac 3 transformation temperature−Ac 1 transformation temperature) + Ac 1 transformation temperature [° C.] or more and Ac 3 transformation temperature + 50 [° C.] or less is 10 Highly excellent galvanizing adhesion and formability characterized by annealing at a cooling rate of 1-100 ° C./s at an average cooling rate of 1-100 ° C./s and then hot-dip galvanizing at an average cooling rate of 1-100 ° C./s. Production of high strength thin steel sheet Law.
[0011]
(9) A method of manufacturing a high-strength thin steel sheet excellent zinc plating adhesion and formability according to the above (8), characterized in that the alloying at 350 to 550 ° C. After galvanized.
( 10 ) The galvanized adhesiveness and formability described in ( 8 ) or ( 9 ) above, wherein the oxide scale is removed by spraying high-pressure water on the surface of the steel sheet before the finish rolling at the time of hot rolling. An excellent method for producing high strength thin steel sheets.
(11) to rewind the wound hot rolled steel sheet after hot rolling, cold rolled after pickling, the subsequently described any one of the, characterized in that the continuous annealing galvanizing subjecting (8) - (10) Are high-strength thin steel sheets having excellent galvanizing adhesion and formability, and a method for producing the same.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
C: C contributes to the stabilization of austenite remaining at room temperature, and is an element effective for martensite formation, and 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 volume fraction of retained austenite and martensite that can be secured at room temperature, but also concentrates in the untransformed austenite during the processing heat treatment of the manufacturing process. Stability can be improved. However, when the added amount is less than 0.05% by mass, the finally obtained martensite amount or the retained austenite volume fraction with a carbon concentration of 0.9% or more cannot be ensured to be 3% or more. Therefore, 0.05% was made the lower limit.
[0013]
On the other hand, the martensite or retained austenite volume fraction that can be secured increases as the average C content of the steel material increases, and in particular, 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%.
[0014]
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 it is possible to effectively concentrate C in austenite by suppressing the formation of cementite, it is an unavoidable element to leave martensite or austenite having an appropriate volume fraction at room temperature. It is necessary to add 0.01% 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 the plateability, so the content is set to 0.01 to less than 0.3% by mass.
[0015]
Mn: Mn is an austenite stabilizing element, and is an effective element for improving the hardenability to generate martensite and 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 or to generate martensite by adding an appropriate amount of such an austenite stabilizing element. Therefore, 0.5% was made the lower limit. Mn is less effective than Al and Si, but has an effect of suppressing the formation of cementite, and also serves 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.
[0016]
P: P is effective in increasing the strength of the steel material and securing retained austenite as described above, but not only incurs an increase in the cost of the steel material when added over 0.1 mass%, From the standpoint of causing cracking resistance deterioration, fatigue characteristics, and toughness deterioration, the content is set to 0.005 to 0.1% by mass.
Al: Like Si, Al has the effect of improving the workability of steel by increasing the ferrite volume fraction and the effect of suppressing the formation of cementite. However, excessive addition significantly impairs the plateability, so 0.001 to 2.0% by mass was set.
[0017]
Co: Co is an element effective for increasing the C concentration in austenite, and is particularly effective for stable austenite formation, so 0.005% is made the lower limit. 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.
N: Like C, N: N is set to 0.0010% or more because it is effective for stabilizing and retaining austenite at room temperature and for generating martensite. On the other hand, excessive addition causes blowholes during welding, so the upper limit was made 0.0100%.
[0018]
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. It also improves hardenability and promotes martensite formation. Further, the matrix ferrite and bainite are strengthened by solid solution. However, if the addition is less than 0.01% by mass, the required martensite or retained austenite cannot be secured, the strength of the steel material is lowered, and the carbide suppression effect is not sufficient. On the other hand, if it exceeds 0.50% by mass, the upper limit is set because it leads to hardening of the ferrite as the parent phase.
[0019]
Nb, Ti, V: Nb, Ti, V added as necessary can increase the strength of the steel material by forming carbides, nitrides, or carbonitrides. Add 01% by mass or more. On the other hand, when the total exceeds 0.3%, it may precipitate as a large amount of carbide, nitride, or carbonitride in the ferrite grain or grain boundary which is the parent phase and deteriorate ductility. Further, the formation of such carbides inhibits the concentration of C in retained austenite, which is important for the present invention, and wastes C, so the upper limit was made 0.3% by weight.
B: B added as needed is effective for strengthening grain boundaries and increasing the strength of steel, so 0.0001% by mass 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.
[0020]
Ni, Cr, Cu: Ni, Cr, and Cu added as necessary are all austenite stabilizing elements, and are elements effective for stabilizing austenite at room temperature and for martensite formation. In particular, when the amount of addition of C is limited from the viewpoint of weldability, the addition of an appropriate amount of such an austenite stabilizing element can effectively retain austenite and generate martensite that improves hardenability. Can be encouraged. Moreover, although these elements are not as much as Al and Si, they have the effect of suppressing the formation of cementite, and also serve to assist the concentration of C in austenite, so a total of 0.01% by mass or more is added. On the other hand, when the total of these exceeds 1.5% by mass, the upper limit is set because it leads to hardening of the ferrite as the parent phase.
Ca, Rem: Ca and REM added as necessary are elements effective for inclusion control. Addition of a suitable amount improves hot workability, so a total of 0.0001% by mass or more is added. In order to promote hot embrittlement, the upper limit was made 0.5%.
[0021]
Average carbon concentration in retained austenite and volume fraction of 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%. 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 set to 0.9% by mass. 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. This is not preferable because it involves carbide precipitation. 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.
[0022]
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 (200), (220) and (311) surfaces of austenite, and the carbon concentration = (lattice constant−3.572) /0.033 [1 × 10 −10 m]. Can be obtained.
[0023]
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. Fe concentration in the plating phase: The composition of the plating phase / matrix phase interface and the composition of the plating phase itself are important factors in ensuring the adhesion between the plating phase and the ground iron. Actually, various Fe—Zn compounds are formed, and the adhesion can be regulated by regulating the Fe concentration in the plating phase. Good adhesion can be obtained when the mass% is 5% or more and 20% or less.
[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. Further, when the reheating temperature exceeds 1300 ° C., the deformation of the slab becomes severe and the cost increases at the same time. Further, when the hot rolling completion temperature FT is less than Ar 3 transformation temperature −10 ° C. determined by the chemical composition of the steel material, a work ferrite layer is sometimes formed in the surface layer portion of the steel plate and its vicinity, and the workability is remarkably deteriorated. At the same time, the dynamic deformation resistance is lowered. Therefore, this is the lower limit of the hot rolling completion temperature. Further, when the hot rolling completion temperature is Ar 3 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 to hinder the increase in the dynamic deformation resistance of the steel sheet. 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. The Ar 3 transformation temperature is calculated as Ar 3 = 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.
[0026]
Continuous annealing galvanizing conditions: In hot dip galvanizing after hot rolling or cold rolling, annealing in a sufficient two-phase region, that is, the Ac 1 transformation temperature and Ac 3 transformation temperature where the annealing temperature is determined by the chemical composition of the steel (For example, “Steel Material Science” written by W. C. Leslie, translated by Koyasu Naruse, Maruzen P273) When the temperature is less than 0.1 × (Ar 3 −Ac 1 ) + Ac 1 [° C.], the annealing temperature Since the amount of austenite obtained by the above method is small, residual austenite cannot be stably left in the final steel sheet, and this is set as the lower limit of the annealing temperature. Further, even if the annealing temperature exceeds the Ac 3 transformation temperature +50 [° C.], the properties of the steel sheet cannot be improved, but the upper limit of the annealing temperature is set to Ac 3 transformation temperature +50 [° C.] in order to increase the manufacturing cost. It was. 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 and concentrating C in the untransformed austenite to finally stabilize 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. Further, the alloying treatment was performed at 350 ° C. or higher. If it is less than 350 ° C., sufficient alloying is not performed, and the purpose of alloying cannot be achieved. Moreover, from a viewpoint of carbide precipitation suppression, 550 degrees C or less is desirable.
After reheating, for oxidation scale removal, it is preferable to perform descaling by high pressure water under the following conditions.
[0028]
Descaling before finish hot rolling: Oxidation scale removal is performed on the steel plate surface before finish rolling with high-pressure water that satisfies the condition of collision pressure P (MPa) × flow rate L (liter / cm 2 ) ≧ 0.0025. To do. 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 the high-pressure water on the steel plate surface is described as follows. (Refer to "Iron and Steel" 1991 vol. 77 No. 9 p1450)
P (MPa) = 5.64 × P 0 × V / H 2
However,
P 0 (MPa): Fluid pressure V (L / min): Nozzle flow rate H (cm): Distance between steel plate surface and nozzle
The flow rate L is described as follows.
L (liter / cm 2 ) = V / (W × v)
However,
V (liter / min): Nozzle flow rate W (cm): Width of spray liquid per nozzle hitting the steel plate surface v (cm / min): Upper limit of plate speed collision pressure P × flow rate L is the effect of the present invention. Although it is not necessary to determine in particular in order to obtain it, it is preferable to make it 0.02 or less because an increase in the amount of nozzle flow causes problems such as increased wear on the nozzle.
[0030]
【Example】
Each steel having the components shown in Table 1 is cast into a 25 kg ingot, heated to 1200 ° C., hot-rolled, and Ar 3 transformation point determined by each steel component is −10 ° C. or higher, and Ar 3 transformation point is less than 120 ° C. ( The hot rolling was finished at about 900 ° C., and high pressure water was sprayed onto the steel sheet surface to cool it to 370 ° C. at 50 ° C./s, followed by holding at 370 ° C. × 1 h and furnace cooling winding. For some steel types, high-pressure descaling was performed under conditions of an impact pressure of 2.7 MPa and a flow rate of 0.001 liter / cm 2 before hot rolling. The obtained hot rolled sheet was used for plating test and cold rolling. After cold rolling, after heating in a two-phase region of 1 minute annealing at a temperature of 0.5 × (Ac 3 −Ac 1 ) + Ac 1 ° C., cooling to 450 ° C. at an average cooling rate of 2.5 ° C./s and then 530 ° C. Then, after holding for 10 seconds and air cooling, the mechanical properties were investigated. In the galvanizing test, a hot-rolled plate and a cold-rolled plate were subjected to a 60-degree bending / returning test with a radius of curvature of the tip of 3 mm, tape was attached to the inside, and evaluation was performed by appearance inspection.
[0031]
[Table 1]
Table 2 shows the mechanical properties of each steel. Both hot rolled and cold rolled sheets of steels A to J, which are invention steels, are TS × El. It can be seen that it shows a good strength ductility balance of> 21000 MPa ·%. On the other hand, the comparative steels K, M to R are TS × El. It can be seen that it remains at <20000 MPa ·%. On the other hand, L 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.
[0033]
[Table 2]
[Table 3]
【The invention's effect】
According to the present invention, it is possible to obtain a high strength thin steel sheet having excellent galvanizing adhesion and formability under normal processing conditions of the current hot dip galvanizing line.
Claims (11)
Ｃ ：０．０５〜０．２％、
Ｓｉ：０．０１〜０．３％未満、
Ｍｎ：０．５〜３．０％、
Ｐ ：０．００５〜０．１％、
Ａｌ：０．００１〜２．０％、
Ｃｏ：０．００５〜２．０％、
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、０．９％以上の炭素を含む残留オーステナイト量が体積％で３％以上であり、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％以上含むことを特徴とする亜鉛メッキ密着性および成形性の優れた高強度薄鋼板。% By mass
C: 0.05 to 0.2%
Si: 0.01 to less than 0.3%,
Mn: 0.5 to 3.0%
P: 0.005-0.1%,
Al: 0.001 to 2.0%,
Co: 0.005 to 2.0%,
N: 0.0010 to 0.0100%
Containing the balance Ri Do of Fe and unavoidable impurities, it is 3% or more in an amount of retained austenite volume percent containing more than 0.9% carbon, equiaxed 0.5-3.0 aspect ratio excellent high strength thin steel sheet galvanized adhesion and moldability you comprising the ferrite volume ratio of 50% or more.
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| JP2010043360A (en) * | 2004-03-01 | 2010-02-25 | Nippon Steel Corp | High-strength and high-ductility hot-dip galvanized steel sheet superior in hole expandability, and manufacturing method therefor |
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| KR100978734B1 (en) * | 2008-03-07 | 2010-08-30 | 현대하이스코 주식회사 | Dual Phase Steel Sheet And Method For Manufacturing The Same |
| KR101253869B1 (en) | 2010-12-15 | 2013-04-12 | 주식회사 포스코 | Method for manufacturing high strength hot dip galvanized steel sheet having excellent coating adhesion |
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