JP3778107B2 - High strength hot-rolled steel sheet and manufacturing method thereof - Google Patents
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Description
【０００１】
【発明の属する技術分野】
この発明は、張り出し加工や絞り加工等により様々な形状に成形されて自動車あるいは産業機器等の構造部材として用いられるところの、低降伏比であって加工性（伸び，伸びフランジ性，張り出し性，絞り性等），耐衝撃性，塗装性に優れる高強度複合組織熱延鋼板及びその製造方法に関するものである。
【０００２】
【従来の技術】
連続熱間圧延によって製造される所謂“熱延鋼板”は、比較的安価な構造材料として、前記の自動車を始めとする各種の産業機器に広く使用されている。
特に、近年になって、自動車のボディ部品など、張り出し加工や絞り加工等といった加工の厳しい部品であって、かつ軽量化のための高強度化，衝突安全性を確保するための耐衝撃性、更に長期防錆保証のための耐食性が要求される部品への熱延鋼板の適用が益々増加しつつある。更に、自動車組み立てに際しては寸法精度が重要であるが、その用途においては低降伏比の熱延鋼板が望まれることも多い。
【０００３】
従来、熱延鋼板の加工性や耐衝撃性を改善する方法として残留オ−ステナイトを活用する方法が知られている。
しかしながら、この方法の多くはSiを多量に添加するものであり、この方法で得られる熱延鋼板は塗装性に問題を有するものであった。また、この方法に係る熱延鋼板では寸法精度を得るのに必要な低降伏比を得ることができなかった。
【０００４】
そこで、残留オ−ステナイトを活用する熱延鋼板の塗装性を悪化させるSiの代わりにAlを添加する方法が、例えば特許第３０６６６８９号公報に開示されている。
しかし、この方法によって得られた熱延鋼板は降伏比が高く、厳しい寸法精度が要求される成形部材用としては満足できるものではなかった。
Al添加による残留オ−ステナイトの活用を図った熱延鋼板に関する技術は特開平１１−１０６８６２号公報にも開示されているが、この公報に開示された熱延鋼板はα＋γ２相域で仕上圧延を終了するという手法によって製造されるものであり、この手法で得られた材料はα＋γ２相域での圧延が施されるが故に耐衝撃性が不十分で降伏比も高いという問題があった。
【０００５】
他に、残留オ−ステナイトを活用するためにAlを添加した熱延鋼板としては、特開平１０−３１７０９６号公報，特開平１１−３５００６３号公報，特開平１１−６１３２７号公報あるいは特許第２８６４９６０号公報等に記載が見られるが、これらは要望が高まっている低降伏比で十分な加工性，耐衝撃性，塗装性を両立できる鋼性能を意図するものではない。
【０００６】
【発明が解決しようとする課題】
このようなことから、本発明が目的としたのは、前述した従来技術の問題点を解消し、低降伏比で加工性，耐衝撃性，塗装性に優れる高強度複合組織熱延鋼板を提供すると共に、その安定した製造方法を確立することであった。
【０００７】
【課題を解決するための手段】
本発明者らは、上記目的を達成すべく鋭意研究を行ったところ、熱延鋼板の構成成分としてのMnはフェライトの生成を抑える傾向があり、また耐衝撃性，塗装性の確保のためにSi量を規制した場合には延性が不十分となることを解明した。更に、これらの問題については適量のAlを添加することで解決できることも明らかとなった。
そこで更に研究を重ねた結果、低降伏比で十分な加工性，耐衝撃性，塗装性を兼備する熱延鋼板を実現するためには、Mn，Al，Siの量を適度にバランスさせ、かつ組織制御を行うことが重要であるとの知見を得ることができた。
【０００８】
本発明は、上記知見事項等を基に完成されたものであり、次の加工性，耐衝撃性，塗装性に優れる高強度熱延鋼板及びその製造方法を提供するものである。
1 ） Ｃ：0.001 〜0.15％（本発明において成分割合を表す％は重量％とする），Si： 1.4 ％未満，Mn：0.5 〜３％，Ｐ：0.1 ％以下，Ｓ：0.03％以下，Al：0.1 ％超〜 1.5 ％未満，Ｎ：0.0005〜0.02％を含むと共に残部が Fe及び不可避的不純物から成 り、かつ板厚の中央部から表層までの全ての金属組織が、フェライトの面積率：６ ０％以上、セメンタイト及びベイナイトの面積率：４０％以下、マルテンサイトの 面積率：２〜２０％、そして残留オ−ステナイトの面積率：５％以下であって、か つ結晶粒の長片／短片の比が５以下で、結晶粒径の平均粒径と最大粒径との差が10 μｍ以内であることを特徴とする高強度熱延鋼板。
2 ） Ｃ：0.001 〜0.15％，Si：1.4 ％未満，Mn：0.5 〜３％，Ｐ：0.1 ％以下，Ｓ： 0.03％以下，Al： 0.1％超〜 1.5％未満，Ｎ：0.0005〜0.02％を含み、更にCr：0. 001 〜１％，Mo：0.001 〜１％，Ti：0.001 〜0.1 ％，Nb：0.001 〜0.1 ％，Ｖ： 0.001 〜0.1 ％，Cu：0.002 〜１％，Ni：0.002 〜１％，Ca：0.0001〜0.01％及び Ｂ：0.0001〜0.01％なる群から選ばれた１種以上をも含むと共に残部が Fe及び不可 避的不純物から成り、かつ板厚の中央部から表層までの全ての金属組織が、フェラ イトの面積率：６０％以上、セメンタイト及びベイナイトの面積率：４０％以下、 マルテンサイトの面積率：２〜２０％、そして残留オ−ステナイトの面積率：５％ 以下であって、かつ結晶粒の長片／短片の比が５以下で、結晶粒径の平均粒径と最 大粒径との差が10μｍ以内であることを特徴とする高強度熱延鋼板。
3 ） 前記 1) 項又は 2) 項に記載の成分組成を有する鋼片を、鋳造後、１０８０〜１ ４２０℃の区間を平均冷却速度３〜７０℃／min で冷却し、その後に１０５０℃以 上に加熱してから粗圧延を開始し、粗圧延終了後、粗バ−の加熱あるいは温度保定 を実施して仕上入側温度９００〜１２００℃にて仕上圧延を開始し、仕上出側温度 ７８０〜１０３０℃で圧延を終了した後、巻取までを平均冷却速度５℃／s 以上で 冷却し、６００℃以下で巻き取ることを特徴とする高強度熱延鋼板の製造方法。
【０００９】
【発明の実施の形態】
次に、本発明において鋼片乃至熱延鋼板の成分組成，熱延鋼板の金属組織並びに製造条件を前記の如くに限定した理由を説明する。
(A) 鋼片乃至熱延鋼板の成分組成
Ｃ： Ｃ含有量が0.15％を超えた場合には伸びフランジ性及び溶接性が低下する。一方、Ｃ含有量が 0.001％未満であると低降伏比の特性を得るのに必要なマルテンサイトが十分に得られなくなる。従って、Ｃ含有量は 0.001〜0.15％と定めた。
【００１０】
Si： Siは鋼板の強化に有効でしかも延性に悪影響を及ぼすことが少ないので機械的性質の面からは多量に添加したい成分であるが、過剰に添加すると赤スケ−ルの生成による外観悪化のほか、本発明が意図する塗装性，耐衝撃性を著しく阻害する。従って、Si含有量は 1.4％未満と定めたが、好ましくは 1.0％以下に調整するのが良い。
【００１１】
Mn： Mnは、ＣやSi等には及ばないものの有効な強化成分である。しかも、Mnにはパ−ライトの過剰な生成を抑え、結晶粒を微細にする作用がある。そして、これらの作用を発揮させるためにはMnを 0.5％以上含有させる必要があるが、３％を超えてMnを含有させても強化能の向上効果が飽和する。
Ｐ： Ｐは鋼板の強化に有効であるので多量に添加したい成分であるが、Ｐ含有量が 0.1％を超えると脆化しやすくなり、本発明が意図する耐衝撃性が悪化する。従って、Ｐ含有量は 0.1％以下と定めた。
【００１２】
Ｓ： Ｓは、0.03％を超えて含有されるとMnとＡ系介在物を作って加工性（伸びフランジ性）を低下させる不純物元素であるので、極力低減することが望ましい。そこで、その許容範囲を検討した結果、Ｓ含有量は0.03％以下と定めた。
【００１３】
Al： Alは本発明に係る熱延鋼板において重要な成分であり、Siと同様にフェライト生成を促進することにより熱延鋼板の延性向上に寄与する。しかも、Siのように塗装性や耐衝撃性の低下を生じない有益な成分である。そして、その効果を発揮させるためには 0.1％超のAl含有量が必要であるが、1.5 ％以上含有させてもその効果は飽和するので、Al含有量は 0.1％超〜 1.5％未満と定めた。より好ましいAl含有量の範囲は 0.2％超〜１％である。
【００１４】
Ｎ： Ｎは各種の窒化物となって熱延に際しての加熱時の粗粒化を防止する働きがある。しかしながら、窒化物が多すぎると割れの起点となってしまう。従って、Ｎ含有量は0.0005〜0.02％と定めた。
【００１５】
Cr，Mo，Ti，Nb，Ｖ，Cu，Ni及びＢ： これらは何れも熱延鋼板の高張力化に有効な成分であるほか、所要量を添加することにより実製造上において冷却速度の制御が困難な比較的低温域での巻き取り等の複雑な冷却パタ−ンを採らなくても硬質相が生じやすくなる成分でもあるので、必要に応じて１種以上が添加される。しかしながら、その効果はそれぞれの含有量が下限未満では不十分であり、またそれぞれの上限を超えて含有させると塗装性が低下し、降伏比も高くなってしまう。
【００１６】
Ca： Caは、Ｓと結合して硫化物を球状化させ、熱延鋼板の伸びフランジ性を改善させるのに有効な成分であるので、必要に応じて含有せしめられる。しかしながら、その含有量が0.0001％未満では前記効果は不十分であり、また0.01％を超えて含有させてもその効果は飽和してコスト的に不利となることから、Ca含有量は0.0001〜0.01％と定めた。
【００１７】
なお、上記以外の成分はFe及び不可避的不純物である。不可避的不純物としては例えばＯ，ＲＥＭ，Sn，Sb，Ｗが挙げられるが、それぞれ0.02％以下の含有量であれば許容できる。
また、上記組成の鋼は、例えば転炉，電気炉又は平炉等により溶製される。そして、鋼種もリムド鋼，キャップド鋼，セミキルド鋼又はキルド鋼の何れでも良い。更に、鋼片の鋳造は“造塊−分塊圧延”あるいは“連続鋳造”の何れの手段によっても良い。
【００１８】
(B) 金属組織
本発明に係る熱延鋼板は、板厚の中央部から表層までの全ての金属組織が、フェライトの面積率：６０％以上、セメンタイト及びベイナイトの面積率：４０％以下、マルテンサイトの面積率：２〜２０％、そして残留オ−ステナイトの面積率：５％以下であって、かつ結晶粒の長片／短片の比が５以下で、結晶粒径の平均粒径と最大粒径との差が１０μｍ以内であることを特徴とする。
フェライトは延性に寄与し、マルテンサイトは強度に寄与するが、フェライトとマルテンサイトの強度差により低降伏比が達成される。
【００１９】
ここで、熱延鋼板に所望の延性を得るためにはフェライトは面積率で６０％以上、好ましくは８５〜９５％であることが必要である。また、フェライトはポリゴナルフェライトであるのが望ましい。
マルテンサイトは、面積率で２０％を超えると熱延鋼板が硬質化して加工性が低下するので、面積率にて２〜２０％とする。
残部組織は、不可避的に生じるセメンタイト，パ−ライト，残留オ−ステナイト，ベイナイトの何れでも構わないが、降伏比の低下を妨げるのでベイナイトとセメンタイトは面積率で４０％以下、好ましくは５〜１５％とすべきである。ここで、ベイナイトには、上部ベイナイト，下部ベイナイト以外にアシキュラフェライトを含む。
残留オ−ステナイトも同様の理由で面積率にて５％以下とすべきであり、望ましくは３％未満とするのが良い。
【００２０】
なお、熱延鋼板において、その板厚方向部分で部分的に結晶粒が展伸していたり混粒であったりすると加工性，耐衝撃性が低下するので、結晶粒の長片／短片の比が５以下、結晶粒の平均粒径に対する最大粒径の差が１０μｍ以内の組織とする。
そして、板厚の中央部及び表層の金属組織は、共に前記の組織にする必要がある。
【００２１】
一般に、本発明熱延鋼板のように、加工性を求める鋼板では加工性の良好なフェライト組織を積極的に生成させるため、オ−ステナイト温度域の下限温度であるＡr3変態点直上での熱延仕上圧延が行われる。しかし、Ａr3変態点直上圧延を追求すると、圧延中の温度変動によって部分的にあるいは冷えやすい板厚表層で変態点を下回り、混粒組織や展伸粒組織が生じ、加工性や耐衝撃性の低下を招くため、本発明の狙い性能を得るためには、板厚全域にわたって前述の組織にする必要がある。
なお、板厚の中央部とは、板厚表層から 1/4厚〜 3/4厚の範囲とする。また、表層とは、板厚表層０〜３０μｍの範囲とする。
【００２２】
(C) 製造条件
本発明に係る熱延鋼板は、鋼片の鋳造後、１０８０〜１４２０℃の区間を平均冷却速度３〜７０℃/minで冷却し、その後に１０５０℃以上に加熱してから粗圧延を開始し、粗圧延終了後、粗バ−の加熱あるいは温度保定を実施して仕上入側温度９００〜１２００℃にて仕上圧延を開始し、仕上出側温度７８０〜１０３０℃で圧延を終了した後、巻取までを平均冷却速度５℃/s以上で冷却し、６００℃以下で巻き取ることによって安定に製造することができる。
【００２３】
ここで、鋳造後、１０８０〜１４２０℃の区間を３〜７０℃/minの平均冷却速度で冷却し、１０５０℃以上に加熱あるいは保定するのは、SiやTi，Nb等の添加元素を十分に固溶させ、細粒化と高強度化のために無駄なく活用するのに必要だからである。
そして、粗圧延終了後、仕上入側温度及び仕上出側温度を高精度に制御して板厚全域にわたって前述の組織にすることを目的とし、粗バ−を加熱あるいは保定する。加熱あるいは保定の方法としては、粗バ−をコイル状に巻き取る方法、あるいは粗バ−を誘導加熱方法で加熱する粗バ−ヒ−タ−，ガスバ−ナ−加熱、直接粗バ−に電流を流す通電加熱方法などがある。特に、粗バ−ヒ−タ−が好適である。
また、仕上圧延前に粗バ−を接合して連続的に圧延する方法も、速度を余り落とさずに高速で短時間に仕上圧延を完了できるので有効である。
【００２４】
仕上入側温度を９００〜１２００℃とするのは、当該温度が９００℃未満では仕上圧延中にフェライト変態を生じ、バンド状組織や混粒組織が生じて延性と耐衝撃性が低下し、一方、１２００℃を超える温度ではスケ−ルの生成が活発になるため表面疵不良が生じるからである。好ましくは、仕上入側温度は９５０〜１１００℃とするのが良い。
【００２５】
仕上出側温度を７８０〜１０３０℃とするのは、当該温度が７８０℃未満では熱延終了前に変態したフェライトに加わる圧延で加工量が多くなってバンド組織や混粒組織が生じ、延性と耐衝撃性が低下し、一方、１０３０℃を超える温度ではスケ−ルの生成が活発になるために表面疵不良が生じるからである。仕上出側温度は、好ましくは８２０〜９００℃とするのが良い。
【００２６】
熱延後における冷却速度５℃/s以上の冷却はマルテンサイトを生成させるのに必要である。ここで、冷却停止温度が高いとパ−ライトが生成して低降伏比が得られず、耐衝撃性も低下する。
巻取温度は、セメンタイトの析出を抑制するため６００℃以下とする。
【００２７】
製造された熱延鋼板については、通常、スキンパス圧延による形状矯正や酸洗によるスケ−ルの除去が行われ、表面には防錆油が塗布される。
【００２８】
以下、実施例によって本発明を更に具体的に説明する。
【実施例】
表１に示す成分組成の鋼を鋳造した後、１０８０〜１４２０℃の区間を平均冷却速度３〜７０℃/minで冷却し、次いで１０５０℃以上に加熱してから粗圧延を開始し、粗圧延終了後、粗バ−を加熱するかあるいは温度保定を施して仕上圧延を開始し、仕上温度７８０〜１０３０℃で圧延終了後、平均冷却速度５℃/s以上で冷却し、６００℃以下で巻き取り、板厚 2.6mmの熱延鋼板を製造した。
この時の熱延条件を表２に示す。
【００２９】
【表１】
【表２】
次に、得られた熱延鋼板につき、スキンパスでの平坦矯正及び酸洗でのスケ−ル除去を行った後、鋼の圧延方向断面の板厚中央部及び表層の金属組織を、それぞれナイタル腐食し光学顕微鏡（×５００倍）及びＳＥＭ（×１０００倍）にて観察を行い、フェライト及びマルテンサイト，セメンタイト，ベイナイトの面積を面積百分率（ＪＩＳ Ｇ ０５５２）で算出した。
結晶粒の長片／短片の比は、上記の組織観察から求めた。
結晶粒径の平均粒径に対する最大粒径の差は、切断法を用いて求めた。
残留オ−ステナイト量はＸ線にて測定し、一部の組織についてはＳＥＭ，透過電子顕微鏡で種類を確定した。
【００３２】
また、圧延直角方向に採取したＪＩＳ５号試験片での引張試験（ＪＩＳ Ｚ ２２４１）、６０°円錐ポンチでの穴拡げ試験（ＪＩＳ Ｚ ２２４２，Ｕノッチ，ｔ2.5mm ）、並びに塗装性試験も行った。
なお、塗装性は化成処理試験で評価した。この時の化成処理試験条件は下記の通りであった。
化成処理剤…日本ペイント製のＳＤ２５００，
処理時間… １２０秒，
評価法… 結晶サイズをＳＥＭ×１００倍で確認し、付着量はクロム酸で皮膜を溶解し重量差から算出。
これらの測定結果を表３及び表４に示す。
【００３３】
【表３】
【表４】
表３及び表４に示される結果からも、本発明で規定する製造条件に従うと本発明の規定条件を満足する熱延鋼板が得られ、高強度であってかつ低降伏比の加工性，耐衝撃性，塗装性に優れるという特性を満足することが分かる。
【００３６】
これに対して、試験番号７〜11は製造条件の何れかが本発明の規定条件から外れているため、得られた熱延鋼板は性能が劣ったものとなっている。
また、試験番号23〜26，28，29で得られた熱延鋼板は、鋼板組成が本発明の規定条件を満たしていないために性能が劣ったものとなっている。
【００３７】
【発明の効果】
以上に説明した如く、この発明によれば、低降伏比で加工性，耐衝撃性，塗装性に優れる高張力熱延鋼板（特に圧延直角方向の引張強度で４９０〜７８０ＭPa級の高強度鋼板）を提供することができ、張り出し加工や絞り加工等により様々な形状に成形されてロアア−ムやホイ−ル等といった自動車の足廻り部品、あるいは産業機器の構造部材等といった“加工性，耐衝撃性，塗装性を求められる用途”における近年の要求特性を満たすことが可能となるなど、産業上有用な効果がもたらされる。[0001]
BACKGROUND OF THE INVENTION
This invention has a low yield ratio and processability (elongation, stretch flangeability, stretchability, etc.) that is formed into various shapes by overhanging or drawing, etc. and used as a structural member for automobiles or industrial equipment. The present invention relates to a hot rolled steel sheet having a high strength composite structure excellent in drawability, impact resistance, and paintability, and a method for producing the same.
[0002]
[Prior art]
A so-called “hot rolled steel sheet” produced by continuous hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment including the automobile.
In particular, in recent years, parts that are severely processed such as overhanging and drawing, such as body parts of automobiles, and have high strength for weight reduction, impact resistance to ensure collision safety, Furthermore, the application of hot-rolled steel sheets to parts that require corrosion resistance for long-term corrosion prevention is increasing. Furthermore, dimensional accuracy is important when assembling automobiles, but a hot-rolled steel sheet having a low yield ratio is often desired for its use.
[0003]
Conventionally, a method of utilizing retained austenite is known as a method for improving the workability and impact resistance of a hot-rolled steel sheet.
However, many of these methods add a large amount of Si, and the hot-rolled steel sheet obtained by this method has a problem in paintability. Further, the hot-rolled steel sheet according to this method cannot obtain a low yield ratio necessary for obtaining dimensional accuracy.
[0004]
Therefore, for example, Japanese Patent No. 3066689 discloses a method of adding Al instead of Si, which deteriorates the paintability of a hot rolled steel sheet utilizing residual austenite.
However, the hot-rolled steel sheet obtained by this method has a high yield ratio, and is not satisfactory for a molded member requiring strict dimensional accuracy.
The technology related to hot rolled steel sheet utilizing residual austenite by addition of Al is also disclosed in Japanese Patent Application Laid-Open No. 11-106862, but the hot rolled steel sheet disclosed in this publication is subjected to finish rolling in the α + γ2 phase region. The material obtained by this method is rolled in the α + γ2 phase region, so that there is a problem that the impact resistance is insufficient and the yield ratio is high.
[0005]
In addition, as a hot-rolled steel sheet to which Al is added in order to utilize retained austenite, JP-A-10-317096, JP-A-11-350063, JP-A-11-61327, or Japanese Patent No. 2864960 Although described in the publications, these are not intended to achieve steel performance that can achieve sufficient workability, impact resistance, and paintability at a low yield ratio that is increasing in demand.
[0006]
[Problems to be solved by the invention]
For these reasons, the present invention aims to solve the above-mentioned problems of the prior art and provide a high-strength hot rolled steel sheet having a low yield ratio and excellent workability, impact resistance, and paintability. And to establish a stable manufacturing method.
[0007]
[Means for Solving the Problems]
As a result of intensive research aimed at achieving the above object, the present inventors have found that Mn as a constituent component of a hot-rolled steel sheet tends to suppress the formation of ferrite, and in order to ensure impact resistance and paintability. It was clarified that the ductility becomes insufficient when the amount of Si is regulated. Furthermore, it became clear that these problems can be solved by adding an appropriate amount of Al.
As a result of further research, in order to realize a hot-rolled steel sheet that has sufficient workability, impact resistance, and paintability at a low yield ratio, the amount of Mn, Al, and Si must be balanced appropriately, and We were able to obtain knowledge that it is important to control the tissue.
[0008]
The present invention has been completed based on the above knowledge and the like, and provides the following high-strength hot-rolled steel sheet excellent in workability, impact resistance, and paintability, and a method for producing the same.
1 ) C: 0.001 to 0.15% (in the present invention,% representing the component ratio is weight%), Si: less than 1.4%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.03% or less, Al : More than 0.1% to less than 1.5%, N: 0.0005 to 0.02%, the balance is composed of Fe and inevitable impurities, and the entire metal structure from the center to the surface of the plate thickness is the area ratio of ferrite: 60% or more, cementite and bainite area ratio: 40% or less, martensite area ratio: 2 to 20%, and residual austenite area ratio: 5% or less, and long pieces of crystal grains / High-strength hot-rolled steel sheet characterized in that the ratio of short pieces is 5 or less and the difference between the average grain size and the maximum grain size is within 10 μm.
2 ) C: 0.001 to 0.15%, Si: less than 1.4%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.03% or less, Al: more than 0.1% to less than 1.5%, N: 0.0005 to 0.02% Further, Cr: 0.001 to 1%, Mo: 0.001 to 1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, Cu: 0.002 to 1%, Ni: One or more selected from the group consisting of 0.002 to 1%, Ca: 0.0001 to 0.01%, and B: 0.0001 to 0.01%, the balance being Fe and inevitable impurities, and the surface layer from the center of the plate thickness All the metal structures up to and including ferrite area ratio: 60% or more, cementite and bainite area ratio: 40% or less, martensite area ratio: 2 to 20%, and residual austenite area ratio: 5 %, The ratio of the long piece / short piece of the crystal grains is 5 or less, and the difference between the average grain size and the maximum grain size is within 10 μm. High strength hot rolled steel sheet.
3 ) After casting the steel slab having the component composition described in the above 1) or 2) , after cooling, the zone of 1080 to 1420 ° C. is cooled at an average cooling rate of 3 to 70 ° C./min. After starting the rough rolling, after the rough rolling, the rough bar is heated or the temperature is maintained, and the finish rolling is started at a finishing input temperature of 900 to 1200 ° C., and the finishing temperature is 780. A method for producing a high-strength hot-rolled steel sheet, characterized in that after rolling is finished at -1030 ° C., the winding is cooled at an average cooling rate of 5 ° C./s or more and taken up at 600 ° C. or less.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, the reason why the composition of the steel slab or hot-rolled steel sheet, the metal structure of the hot-rolled steel sheet, and the production conditions are limited as described above in the present invention will be described.
(A) Component composition C of billet or hot-rolled steel sheet: When the C content exceeds 0.15%, stretch flangeability and weldability deteriorate. On the other hand, if the C content is less than 0.001%, sufficient martensite for obtaining the characteristics of a low yield ratio cannot be obtained. Therefore, the C content is determined to be 0.001 to 0.15%.
[0010]
Si: Si is effective for strengthening steel sheets and has little adverse effect on ductility. Therefore, it is a component that should be added in a large amount from the viewpoint of mechanical properties. However, if added excessively, the appearance deteriorates due to the formation of red scale. In addition, the paintability and impact resistance intended by the present invention are significantly impaired. Therefore, the Si content is determined to be less than 1.4%, but is preferably adjusted to 1.0% or less.
[0011]
Mn: Mn is an effective reinforcing component although it does not reach C and Si. In addition, Mn has the effect of suppressing excessive formation of pearlite and making the crystal grains fine. And in order to exhibit these actions, it is necessary to contain 0.5% or more of Mn. However, even if Mn is contained in excess of 3%, the enhancement effect of the strengthening ability is saturated.
P: P is a component that is desired to be added in a large amount because it is effective for strengthening the steel sheet. However, when the P content exceeds 0.1%, embrittlement tends to occur and the impact resistance intended by the present invention deteriorates. Therefore, the P content is set to 0.1% or less.
[0012]
S: If S is contained in an amount exceeding 0.03%, it is an impurity element that forms an M-based inclusion with Mn and lowers workability (stretch flangeability), so it is desirable to reduce it as much as possible. Therefore, as a result of examining the allowable range, the S content was determined to be 0.03% or less.
[0013]
Al: Al is an important component in the hot-rolled steel sheet according to the present invention, and contributes to improving the ductility of the hot-rolled steel sheet by promoting the formation of ferrite in the same manner as Si. Moreover, it is a useful component that does not cause deterioration in paintability and impact resistance like Si. And in order to bring out the effect, an Al content of more than 0.1% is necessary, but the effect is saturated even if it is contained more than 1.5%, so the Al content is determined to be more than 0.1% to less than 1.5%. It was. A more preferable range of Al content is more than 0.2% to 1%.
[0014]
N: N serves as various nitrides and serves to prevent coarsening during heating during hot rolling. However, if there is too much nitride, it becomes the starting point of cracking. Therefore, the N content is determined to be 0.0005 to 0.02%.
[0015]
Cr, Mo, Ti, Nb, V, Cu, Ni and B: All of these are effective components for increasing the tensile strength of hot-rolled steel sheets, and the cooling rate can be controlled in actual production by adding the required amount. Since it is also a component in which a hard phase is likely to occur without taking a complicated cooling pattern such as winding in a relatively low temperature range, which is difficult to achieve, one or more types are added as necessary. However, the effect is insufficient if the respective contents are less than the lower limit, and if the content exceeds each upper limit, the paintability is lowered and the yield ratio is also increased.
[0016]
Ca: Ca is an effective component to combine with S to spheroidize the sulfide and improve the stretch flangeability of the hot-rolled steel sheet. Therefore, Ca is contained as necessary. However, if the content is less than 0.0001%, the effect is insufficient, and even if contained over 0.01%, the effect is saturated and disadvantageous in terms of cost, so the Ca content is 0.0001 to 0.01%. %.
[0017]
Components other than the above are Fe and inevitable impurities. Inevitable impurities include, for example, O, REM, Sn, Sb, and W. Each content is acceptable if the content is 0.02% or less.
Moreover, the steel of the said composition is smelted by a converter, an electric furnace, a flat furnace, etc., for example. The steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the casting of the steel slab may be performed by any means of “ingot-bundling rolling” or “continuous casting”.
[0018]
(B) Metal structure In the hot-rolled steel sheet according to the present invention, all metal structures from the central part of the plate thickness to the surface layer have a ferrite area ratio of 60% or more, a cementite and bainite area ratio of 40% or less, martensite. The area ratio of the site: 2 to 20%, and the area ratio of residual austenite: 5% or less, and the ratio of long / short pieces of crystal grains is 5 or less. The difference from the particle size is within 10 μm.
Ferrite contributes to ductility and martensite contributes to strength, but a low yield ratio is achieved due to the strength difference between ferrite and martensite.
[0019]
Here, in order to obtain desired ductility in the hot-rolled steel sheet, the ferrite needs to be 60% or more, preferably 85 to 95% in terms of area ratio. The ferrite is preferably polygonal ferrite.
When martensite exceeds 20% in area ratio, the hot-rolled steel sheet becomes hard and the workability decreases, so the area ratio is set to 2 to 20 %.
The remaining structure may be any of cementite, pearlite, retained austenite, and bainite that are inevitably generated. However, since the reduction of the yield ratio is prevented, bainite and cementite are 40% or less in area ratio, preferably 5 to 15 Should be%. Here, the bainite contains acicular ferrite in addition to the upper bainite and the lower bainite.
Residual austenite should be 5% or less in area ratio for the same reason, and preferably less than 3%.
[0020]
In the case of hot-rolled steel sheets, if the crystal grains are partially expanded or mixed in the thickness direction, the workability and impact resistance deteriorate, so the ratio of long / short pieces of crystal grains. Is a structure in which the difference between the maximum grain size and the average grain size of the crystal grains is within 10 μm.
Further, both the central portion of the plate thickness and the metal structure of the surface layer need to have the above-described structure.
[0021]
In general, in a steel sheet that requires workability, such as the hot-rolled steel sheet of the present invention, a ferrite structure having good workability is positively generated. Therefore, heat immediately above the Ar 3 transformation point, which is the lower limit temperature of the austenite temperature range. Rolling finish is performed. However, when the rolling immediately above the Ar 3 transformation point is pursued, it is partly or easily below the transformation point due to temperature fluctuations during rolling, resulting in a mixed grain structure and a stretched grain structure, resulting in workability and impact resistance. Therefore, in order to obtain the targeted performance of the present invention, it is necessary to have the above-described structure over the entire plate thickness.
Note that the thickness of the central portion, and 1/4 thickness ~ 3/4 of the thickness range from a thickness surface layer. Further, the surface layer is in the range of the plate thickness surface layer of 0 to 30 μm.
[0022]
(C) Manufacturing conditions The hot-rolled steel sheet according to the present invention is cooled at an average cooling rate of 3 to 70 ° C / min after the casting of the steel slab at an average cooling rate of 3 to 70 ° C / min, and then heated to 1050 ° C or higher. Rough rolling is started, and after the rough rolling is finished, heating of the rough bar or temperature holding is performed and finishing rolling is started at a finishing input temperature of 900 to 1200 ° C., and rolling is performed at a finishing outlet temperature of 780 to 1030 ° C. After finishing, it can be stably manufactured by cooling up to winding up at an average cooling rate of 5 ° C./s or more and winding up at 600 ° C. or less.
[0023]
Here, after casting, the section of 1080 to 1420 ° C. is cooled at an average cooling rate of 3 to 70 ° C./min, and heated or held at 1050 ° C. or higher is sufficient for adding elements such as Si, Ti, and Nb. This is because it is necessary for solid solution and use without waste for finer grain and higher strength.
Then, after finishing the rough rolling, the rough bar is heated or held for the purpose of controlling the finishing side temperature and the finishing side temperature with high accuracy so as to obtain the above-described structure over the entire plate thickness. As a heating or holding method, a method of winding a coarse bar in a coil shape, a coarse bar heater for heating the coarse bar by an induction heating method, a gas burner heating, or a direct current to the coarse bar is used. There is an electric heating method etc. In particular, a coarse bar heater is preferred.
In addition, a method of continuously rolling by joining a rough bar before finish rolling is effective because finish rolling can be completed in a short time at a high speed without reducing the speed.
[0024]
The finishing side temperature is set to 900 to 1200 ° C. If the temperature is less than 900 ° C., ferrite transformation occurs during finish rolling, and a band-like structure and mixed grain structure are formed, resulting in reduced ductility and impact resistance. This is because, when the temperature is higher than 1200 ° C., the generation of scale becomes active, resulting in surface defects. Preferably, the finish entry temperature is 950 to 1100 ° C.
[0025]
The finishing side temperature is set to 780 to 1030 ° C. If the temperature is less than 780 ° C., the amount of work increases due to rolling applied to the ferrite transformed before the end of hot rolling, resulting in a band structure or mixed grain structure. This is because the impact resistance is lowered, and on the other hand, at a temperature exceeding 1030 ° C., the generation of scale becomes active, resulting in surface defects. The finish delivery temperature is preferably 820 to 900 ° C.
[0026]
Cooling at a cooling rate of 5 ° C./s or higher after hot rolling is necessary to produce martensite. Here, when the cooling stop temperature is high, pearlite is generated, a low yield ratio cannot be obtained, and impact resistance is also lowered.
The coiling temperature is set to 600 ° C. or lower in order to suppress cementite precipitation.
[0027]
The manufactured hot-rolled steel sheet is usually subjected to shape correction by skin pass rolling and scale removal by pickling, and rust preventive oil is applied to the surface.
[0028]
Hereinafter, the present invention will be described more specifically with reference to examples.
【Example】
After casting the steel having the component composition shown in Table 1, the section of 1080 to 1420 ° C. is cooled at an average cooling rate of 3 to 70 ° C./min, then heated to 1050 ° C. or higher, and then rough rolling is started. After finishing, the coarse bar is heated or the temperature is kept and finish rolling is started. After rolling at a finish temperature of 780 to 1030 ° C., the steel is cooled at an average cooling rate of 5 ° C./s or more and wound at 600 ° C. or less. A hot rolled steel sheet with a thickness of 2.6 mm was manufactured.
Table 2 shows the hot rolling conditions at this time.
[0029]
[Table 1]
[Table 2]
Next, the obtained hot-rolled steel sheet was subjected to flattening with a skin pass and scale removal by pickling, and then the central thickness of the cross section in the rolling direction of the steel and the metal structure of the surface layer were each subjected to nital corrosion. Observation was performed with an optical microscope (× 500 times) and SEM (× 1000 times), and the areas of ferrite, martensite, cementite, and bainite were calculated as area percentages (JIS G 0552).
The ratio of the long piece / short piece of crystal grains was obtained from the above-mentioned structure observation.
The difference of the maximum grain size with respect to the average grain size of the crystal grain size was determined using a cutting method.
The amount of retained austenite was measured by X-ray, and the type of some structures was determined by SEM and transmission electron microscope.
[0032]
In addition, a tensile test (JIS Z 2241) with a JIS No. 5 specimen taken in the direction perpendicular to the rolling, a hole expansion test with a 60 ° conical punch (JIS Z 2242, U notch, t2.5 mm), and a paintability test are also performed. It was.
The paintability was evaluated by a chemical conversion treatment test. The chemical conversion test conditions at this time were as follows.
Chemical conversion agent ... Nippon Paint SD2500,
Processing time ... 120 seconds,
Evaluation method: The crystal size is confirmed by SEM × 100 times, and the adhesion amount is calculated from the difference in weight by dissolving the film with chromic acid.
These measurement results are shown in Tables 3 and 4.
[0033]
[Table 3]
[Table 4]
From the results shown in Tables 3 and 4, a hot-rolled steel sheet that satisfies the specified conditions of the present invention is obtained according to the manufacturing conditions specified in the present invention, and has high strength and low yield ratio. It can be seen that it satisfies the characteristics of excellent impact and paintability.
[0036]
On the other hand, test numbers 7 to 11 are inferior in performance of the obtained hot-rolled steel sheet because any of the manufacturing conditions is out of the specified conditions of the present invention.
In addition, the hot-rolled steel sheets obtained in test numbers 23 to 26, 28, and 29 have inferior performance because the steel sheet composition does not satisfy the prescribed conditions of the present invention.
[0037]
【The invention's effect】
As described above, according to the present invention, a high-tensile hot-rolled steel sheet having a low yield ratio and excellent workability, impact resistance, and paintability (particularly a high-strength steel sheet having a tensile strength in the direction perpendicular to the rolling of 490 to 780 MPa). It is formed into various shapes by overhanging or drawing, etc., such as automobile undercarriage parts such as lower arms and wheels, or structural members of industrial equipment, etc. Industrially useful effects, such as being able to satisfy the recently required characteristics in “uses that require high performance and paintability”.
Claims (3)
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002056430A JP3778107B2 (en) | 2002-03-01 | 2002-03-01 | High strength hot-rolled steel sheet and manufacturing method thereof |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002056430A JP3778107B2 (en) | 2002-03-01 | 2002-03-01 | High strength hot-rolled steel sheet and manufacturing method thereof |
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| JP3778107B2 true JP3778107B2 (en) | 2006-05-24 |
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| JP5390922B2 (en) * | 2009-04-20 | 2014-01-15 | 株式会社神戸製鋼所 | Low yield ratio high toughness steel plate |
| KR101449126B1 (en) | 2012-09-27 | 2014-10-08 | 주식회사 포스코 | Formable hot-rolled steel sheet having excellent formability and method for manufacturing the same |
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