JP2004315857A

JP2004315857A - High-strength hot-rolled steel sheet superior in stampability, and manufacturing method therefor

Info

Publication number: JP2004315857A
Application number: JP2003108650A
Authority: JP
Inventors: Satoshi Akamatsu; 聡赤松; Hiroyuki Tanahashi; 浩之棚橋; Manabu Takahashi; 学高橋
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2003-04-14
Filing date: 2003-04-14
Publication date: 2004-11-11

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel sheet with a strength of 690 MPa or higher superior in hole-expandability and ductility, which prevents defects of a stamped end face from occurring in stamping, and to provide a manufacturing method therefor. <P>SOLUTION: The high-strength hot-rolled steel sheet superior in stampability comprises 0.01-0.07% C, 0.005% or less N, 0.005% or less S, 0.03-0.2% Ti, 0.0002-0.002% B, further one ore more elements among 0.01-0.2% V, 0.01-0.2% Nb and 0.01-0.2% Mo; has a ferritic or bainitic ferrite structure for a phase occupying a maximum area rate, and a hard secondary phase of 3% or less by an area rate; and has a tensile strength of 690 MPa or higher. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、穴拡げ性に代表されるバーリング加工性や伸びフランジ性と延性に優れた引張強さ６９０ＭＰａ以上の高強度熱延鋼板に関し、その優れた加工性を活かして自動車部品、例えばメンバー類やアーム類などの足周り部品やシャーシなどの材料として有効に活用できる。
【０００２】
【従来の技術】
近年、自動車の燃費向上などのために軽量化を目的として、Ａｌ合金等の軽金属や高強度鋼板の自動車部材への適用が進められている。ただ、Ａｌ合金等の軽金属は比強度が高いという利点があるものの、鋼に比較して著しく高価であるため、その適用は特殊な用途に限られてきた。より広い範囲で自動車の軽量化を推進するためには、安価な高強度鋼板の適用が強く求められている。
【０００３】
一般に材料は高強度になるほど延性が低下して加工性（成形性）が悪くなる。
鉄鋼材においても例外ではなく、これまでに高強度と高延性の両立の試みがなされてきた。一方、自動車のサスペンションアーム等の足廻り部品に使用される材料には、これらの特性に加えて高いバーリング加工性（穴拡げ性）が求められている。しかし、高強度化に伴って穴拡げ性は延性と同様に低下する傾向を示し、複雑な形状をしている自動車の足廻り部品等への高強度鋼板の適用にあたっては、その穴拡げ性が重要な検討課題となる。このような背景からフェライトやベイニティックフェライトなどの組織を主体とし、パーライトやマルテンサイト組織又はセメンタイト相などのいわゆる硬質第２相を低減した延性及び穴拡げ性に優れた熱延鋼板が開発されている。
【０００４】
ところで最近これらの高強度熱延鋼板の適用拡大に伴って、素材からプレス加工の際に打ち抜かれた板端面にハガレやメクレ状の欠陥が発生することが問題となっている。これらの欠陥は製品端面の意匠性を著しく損なうばかりか、高強度鋼板ゆえに切り欠きなどの応力集中部となって疲労強度などに影響を及ぼす危険性があるものである。普通鋼の打ち抜き性に関してはそれほど多くの検討はなされていないが、例えば特許文献１（特開平９−４９０５３号公報）には鋼中析出物の制御によって、また、特許文献２（特開２００３−４１３４２号公報）には鋼板表層部の組織を微細化することで改善する発明が開示されている。
【０００５】
【引用文献】
（１）特許文献１（特開平９−４９０５３号公報）
（２）特許文献２（特開２００３−４１３４２号公報）
【０００６】
【発明が解決しようとする課題】
しかしながら、上記従来技術は基本的に低強度の極低炭素鋼板の打ち抜き性に関する技術であり、本発明が対象とする引張強さ６９０ＭＰａ以上の高強度熱延鋼板に適用できるとは言い難い。すなわち、特許文献１に記載の発明は、軟質鋼板が故に発生し易い打ち抜き時のバリ発生に関するものであり、具体的には鋼中にＴｉ_４Ｃ_２Ｓ_２を析出せしめてこれにより打ち抜き端面内の剪断面部分にボイドを発生させてクラックを進展させることでバリを低減させる技術である。すなわち、特許文献１に記載された発明ではバリは低減できてもかえって端面の欠陥は助長させている可能性がある。
【０００７】
また、特許文献２に記載の発明も、軟質な冷延鋼板と対象としたバリ発生防止に関するものであり、打ち抜き端面そのものの性状については何ら配慮されていない。そこで本発明は、上記従来技術では検討されているとは言い難い、高強度熱延鋼、とりわけ自動車足回り部品へ適用が期待される延性と穴拡げ性に優れた引張強さが６９０ＭＰａ以上の高強度熱延鋼板を前提とした、打ち抜き端面の欠陥を防止する技術を提供することを目的とするものである。
【０００８】
【課題を解決するための手段】
本発明者らは、延性と穴拡げ性に優れた引張強さが６９０ＭＰａ以上の高強度熱延鋼板を用いてまず種々のクリアランスにて打ち抜き加工を行い、その端面性状について目視で調査した。その結果、通常穴拡げ試験で推奨されている１２．５％前後のクリアランスで打ち抜いた場合には確認できないハガレやメクレ状の欠陥が、クリアランスを増加させることで明瞭に発生することを確認した。そこで以下では１７〜２３％のクリアランスを用いて調査を進めた。
【０００９】
次に、熱延鋼板の化学組成や現在通常に採用されている連続熱間圧延設備により工業的規模で生産されている熱延鋼板の製造プロセスを念頭において、この打ち抜き端面の欠陥に及ぼす製造条件の影響を鋭意研究した結果、鋼中に微量なＢを添加することで基本的な鋼組成や製造プロセスは従来の高強度熱延鋼板のそれに何ら変更を加えることなく欠陥の発生を防止できることを見出し、本発明をなしたものである。
【００１０】
即ち、本発明の要旨は以下の通りである。
（１）質量％にて（以下同じ）、Ｃ：０．０１〜０．０７％、Ｎ：≦０．００５％、Ｓ：≦０．００５％、Ｔｉ：０．０３〜０．２％、Ｂ：０．０００２〜０．００２％を含み、残部がＦｅ及び不可避的不純物からなる鋼であって、フェライト又はベイニティックフェライト組織を面積率最大の相とし、硬質第２相及びセメンタイトが面積率で３％以下であり、引張強さが６９０ＭＰａ以上であることを特徴とする打ち抜き加工性に優れた高強度熱延鋼板。
【００１１】
（２）さらに、質量％にて、Ｖ：０．０１〜０．２％、Ｎｂ：０．０１〜０．２％、Ｍｏ：０．０１〜０．２％の一種または二種以上を含有し、かつ、Ｔｉ／４８＋Ｖ／５１＋Ｎｂ／９３＋Ｍｏ／９６−Ｃ／１２−Ｎ／１４−Ｓ／３２≧０％を満足することを特徴とする前記（１）に記載の打ち抜き加工性に優れた高強度熱延鋼板。
（３）さらに、質量％にて、Ｓｉ：０．０１〜２％、Ｍｎ：０．０５〜２％、Ｐ：≦０．１％、Ａｌ：０．００５〜０．５％を含有することを特徴とする前記（１）又は（２）記載の打ち抜き加工性に優れた高強度熱延鋼板。
【００１２】
（４）さらに、質量％にて、Ｃａ：０．０００５〜０．０２％、ＲＥＭ：０．０００５〜０．２％の一種または二種を含有することを特徴とする前記（１）〜（３）のいずれか１項に記載の打ち抜き加工性に優れた高強度熱延鋼板。
（５）前記（１）〜（４）のいずれか１項に記載の鋼板を製造する方法であって、請求項１〜４のいずれか１項に記載の成分を有する鋼片の熱間圧延に際し、熱延加熱温度を１２００℃以上とし、Ａｒ_３変態点以上で熱間仕上圧延を終了した後、４５０℃から６５０℃で巻き取ることを特徴とする打ち抜き加工性に優れた高強度熱延鋼板の製造方法である。
【００１３】
【発明の実施の形態】
以下に本発明を更に詳細に説明する。まず、本発明における鋼のミクロ組織の限定理由を記述する。
本発明のミクロ組織はフェライト又はベイニティックフェライト組織を面積率最大の相とするものであり、穴拡げ性を大きく低下させる硬質第２相は面積率で３％以下（０％を含む）とする。ここで硬質第２相とはマルテンサイトやパーライトが代表的なものであるが、穴拡げ加工の際これに先行して行われる打ち抜き加工においてマルテンサイトに変態する可能性のある残留オーステナイトなども含む。更にセメンタイトもやはり硬質であり穴拡げ性を劣化させる。そこで本発明ではフェライト又はベイニティックフェライト組織を面積率最大の相とし、硬質第２相及びセメンタイトが面積率で３％以下と規定した。なお好ましくはセメンタイトの面積率は１％以下である。ここで言うセメンタイトの存在状態はフェライト粒界などに塊状に存在するものの他、ベイナイト組織内のラス状の境界に生成するものも含まれる。
【００１４】
次に、本発明の化学成分の限定理由について説明する。化学成分の量は質量％である。
Ｃは、０．０７％超含有していると加工性が劣化するので、０．０７％以下とする。また、０．０１％未満では強度が低下するので０．０１％以上とする。
Ｎは、鋼中にて粗大なＴｉＮを形成する元素であり、基本的には極力低減させるべきであるが、いたずらな低減は製鋼コストの上昇を招くこと、また製鋼能力も勘案して０．００５％以下とする。
【００１５】
Ｓは、多すぎるとＭｎＳなどの介在物として穴拡げ性を劣化させ、更に熱間圧延時の割れを引き起こすので極力低減させるべきであるが、０．００５％以下ならば許容できる範囲である。
Ｔｉは、析出強化により鋼板の強度上昇に寄与する。ただし、０．０３％未満ではこの効果が不十分であり、一方、０．２％超含有すると粗大なＴｉＮの生成が避けられない。従ってＴｉの含有量は０．０３％以上、０．２％以下とする。
【００１６】
Ｂは、本発明における最も重要な元素であり、この元素を添加することで打ち抜き端面の欠陥が防止される。この理由は明らかではないが、上述したように本発明が対象とする延性と穴拡げ性に優れた高強度熱延鋼板においてはＴｉを始めとするＶ，Ｎｂ，Ｍｏなどの析出物構成元素が鋼中のＣ，Ｎ，Ｓを十分固定しているため粒界への元素の偏析が少なく、Ｂはこれを補完する役割として機能するものと思われる。ところでこのような機能はＢが固溶状態で存在していなければ発現せず、炭化物や窒化物としての析出は避けなければならない。しかし、本発明ではＴｉを始めとするＶ，Ｎｂ，Ｍｏが、Ｂと結合する恐れのあるＣ，Ｎを固定しているため、微量添加であっても固溶状態が維持される特徴を有している。
ただし、０．０００２％未満ではこの効果が不十分であり、一方、０．００２％超含有すると延性などの加工性の低下を招く。従って、Ｂの含有量は０．０００２％以上、０．００２％以下とする。
【００１７】
Ｖ，Ｎｂ，Ｍｏは、共にＴｉを補完する析出強化元素として、またこの析出によりセメンタイト生成に寄与するＣを固着する目的として添加される。ただし、それぞれの含有量が０．０１％未満ではこの効果が不十分であり、一方、０．２％超含有するとＴｉＮを形成する際にもＴｉを補完してしまい粗大析出物の生成に寄与してしまう。従って、Ｖ，Ｎｂ，Ｍｏの含有量は共に０．０１％以上、０．２％以下とするが、これらは一種のみならず二種以上を上記範囲において含むことを許容する。
【００１８】
更に、これらの元素は下記式を満足することを要件とする（元素名は各元素の化学成分の質量％を意味する）。
Ｔｉ／４８＋Ｖ／５１＋Ｎｂ／９３＋Ｍｏ／９６−Ｃ／１２−Ｎ／１４−Ｓ／３２≧０
本式は、Ｔｉを始めとするＶ，Ｎｂ，Ｍｏなどの析出物構成元素が鋼中のＣ，Ｎ，Ｓを十分固定するに必要なだけ添加されていることを規定するものとして公知な式であり、これによりセメンタイト量を低減することが可能となる。
【００１９】
Ｓｉは、固溶強化元素として強度上昇に有効であるので、必要に応じて添加する。所望の強度を得るためには０．０１％以上含有する必要がある。しかし、２％超含有すると加工性が劣化する。そこで、Ｓｉの含有量は、０．０１％以上、２％以下とする。
Ｍｎは、固溶強化元素として強度上昇に有効であるので、必要に応じて添加する。所望の強度を得るためには０．０５％以上必要である。また、２％超添加するとスラブ割れを生ずるため、２％以下とする。
Ｐは、０．１％超含有すると加工性や溶接性に悪影響を及ぼすので、０．１％以下とする。
【００２０】
Ａｌは、溶鋼脱酸のために必要に応じて添加する。０．００５％以上添加する必要があるが、コストの上昇を招くため、その上限を０．５％とする。また、あまり多量に添加すると非金属介在物を増大させ伸びを劣化させるので、好ましくは０．３％以下とする。
ＣａおよびＲＥＭは、破壊の起点となったり、加工性を劣化させる非金属介在物の形態変化させて無害化する元素である。ただし、それぞれ０．０００５％未満では添加してもその効果がなく、Ｃａならば０．０２％超、ＲＥＭならば０．２％超添加してもその効果が飽和するので、Ｃａ：０．０００５〜０．０２％、ＲＥＭ：０．０００５〜０．２％の添加とする。
【００２１】
次に、本発明の製造方法の限定理由について以下に述べる。
本発明では、目的の成分含有量になるように成分調整した溶鋼を鋳込むことによって得たスラブを熱間圧延するに際し、その加熱温度を１２００℃以上とする。これは１２００℃未満であると鋼中のＴｉを始めとするＶ，Ｎｂ，Ｍｏなどの析出物構成元素が十分に再固溶されないために、析出強化が不十分となるためである。加熱温度の上限は特に設けないが１４００℃以上であるとスケールオフ量が多量になり歩留まりが低下するので、再加熱温度は１４００℃未満が望ましい。なお、本発明は上記の鋳片を冷却後に加熱炉にて再加熱する場合のほか、高温鋳片のまま熱間圧延機に直送してもかまわない。
【００２２】
熱間圧延工程は、粗圧延を終了後、仕上圧延を行うが、仕上温度がＡｒ_３変態点以上の温度域で終了する必要がある。これは、熱間圧延中に圧延温度がＡｒ_３変態点を切るとひずみが残留して延性が低下するためである。仕上温度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、操業上スケール疵が発生する可能性があるのため、１０００℃以下とすることが望ましい。
【００２３】
仕上圧延を終了した後は、指定の巻取温度まで冷却するが、その冷却速度は本発明の効果を得るためには特に定める必要はない。ただし冷却速度があまりに遅いと、熱間圧延後の冷却中に析出する析出物のサイズが粗大化して析出強化による強度上昇に寄与しなくなるばかりか、穴拡げ性に有害なセメンタイトのような硬質相やパーライトなどの有害組織が発生する可能性があることから、冷却速度の下限は２０℃／ｓ以上が望ましい。一方、冷却の途中のフェライト生成域にて短時間の空冷のような低冷速領域を冷却帯内に設けることで、フェライト変態を促進させるなどの冷却条件の調整を施しても良い。
【００２４】
巻取温度は４５０℃未満では巻取後の析出が不十分となり、析出強化が得られず、更に鋼中に固溶Ｃが残留して加工性を低下させる恐れがあるばかりか、穴拡げ性に有害な硬質のマルテンサイトが発生する可能性がある。逆に、６５０℃超では前述の冷却速度があまりに遅い場合に懸念されるものと同様の現象、即ち、析出する析出物のサイズが粗大化して析出強化による強度上昇に寄与しなくなるばかりか、穴拡げ性に有害なセメンタイトのような硬質相やパーライトなどの有害組織が発生する可能性がある。従って、巻取温度は４５０〜６５０℃とする。
なお、本発明鋼は表面に表面処理（例えば亜鉛メッキ等）が施されていても同様の効果を有し、本発明を逸脱するものではない。
【００２５】
【実施例】
以下に、実施例により本発明をさらに説明する。表１に示す化学成分を有するＡ〜Ｕの鋼を、転炉にて溶製して連続鋳造し熱間圧延用のスラブを得た。ただし、表中の化学組成についての表示は質量％である。また、表１中には本発明の要件である下記式の左辺の値を併記した。
Ｔｉ／４８＋Ｖ／５１＋Ｎｂ／９３＋Ｍｏ／９６−Ｃ／１２−Ｎ／１４−Ｓ／３２≧０
これらを表２に示す加熱温度（ＳＲＴ）で再加熱し、粗圧延後に仕上温度（ＦＴ）で１．２〜６．０ｍｍの板厚に圧延した後、巻取温度（ＣＴ）でそれぞれ巻き取った。
【００２６】
【表１】

【００２７】
このようにして得られた熱延板の鋼板板幅の１／４Ｗ位置から切出した試料を圧延方向断面に研磨し、ナイタール試薬にてエッチングし、光学顕微鏡を用い２００〜５００倍の倍率で観察されたミクロ組織から、面積率最大の相の組織名と硬質第２相が存在する場合はその組織名と各面積率を、また同試料をピクラール試薬にてエッチングし観察された組織から画像解析によって算出したセメンタイトの面積率を表２に併記する。引張特性は供試材をＪＩＳＺ２２０１記載の５号試験片に加工してＪＩＳＺ２２４１記載の試験方法に従って評価した。
穴拡げ試験は日本鉄鋼連盟規格ＪＦＳＴ１００１−１９９６記載の試験方法に従って評価した。また、打ち抜き端面の欠陥の発生有無は、前述したように打ち抜き時のクリアランスを１７〜２３％として穴拡げ試験と同様に１０ｍｍ径の穴を打ち抜き、その端面状態を目視で観察した。表２にその試験結果を示す。
【００２８】
【表２】

【００２９】
本発明に沿うＮｏ．１〜１０は、引張強さＴＳが６９０ＭＰａ以上で、伸びＥｌが２０％以上また穴拡げ値λも９０％以上と、いわゆる延性と穴拡げ性に優れた高強度熱延鋼板であり、打ち抜き端面の欠陥も生じていない。
上記以外は以下の理由によって本発明の範囲外である。すなわち、比較例Ｎｏ．１１〜１６は各々の鋼Ｋ〜ＯでＢ無添加である以外は、その他の成分や製造条件が本発明の要件を満たしているため、引張特性や穴拡げ性は本発明と遜色ない特性を有するにもかかわらず、打ち抜き端面欠陥は全て発生した。
【００３０】
一方、比較例Ｎｏ．１６はその鋼ＰにおいてＢ添加量が３５ｐｐｍと本発明の上限を超えており、これによってフェライト変態が遅延された結果、セメンタイトが析出しており穴拡げ性が劣位である。比較例Ｎｏ．１７はその鋼ＱのＣ量が少なく本発明外のため、引張強さが６９０ＭＰａ以下と劣位である。逆に、比較例Ｎｏ．１８はその鋼ＲのＣ量が多く本発明外のため、硬質第２相であるパーライトが生成しており、延性及び穴拡げ性が劣位となっている。比較例Ｎｏ．１９〜２１は各々の鋼Ｓ〜Ｕにおいて本発明で規定している成分式の条件を満足しないものであり、特にＳはＴｉ量そのものも少なく本発明の範囲外であるが、これらは全てセメンタイトを３％超含む組織となっており、特に穴拡げ性が劣化している。なおＵはＢ無添加のためやはり打ち抜き端面に欠陥が見られる。
【００３１】
また、鋼成分としては本発明範囲であるにも関わらず製造条件が外れているため特性が不十分となったものが比較例Ｎｏ．２２〜２６である。即ち、比較例Ｎｏ．２２及び比較例Ｎｏ．２３では加熱温度ＳＲＴが低く本発明外のため十分にＴｉなどの析出物構成元素を固溶できておらず、析出強化が不十分で本発明条件のＮｏ．１や比較例Ｎｏ．２３と比較して引張強さＴＳが低下しているにもかかわらずセメンタイトが多く、穴拡げ性が低下している。比較例Ｎｏ．２４は仕上温度ＦＴがＡｒ_３温度以下のためひずみが残留した結果、やはりセメンタイトが多く、延性が低下している。比較例Ｎｏ．２５は巻取温度ＣＴが高く本発明外のため、パーライトが生成しており、この結果穴拡げ性に劣るものとなっている。これに対し比較例Ｎｏ．２６は逆に巻取温度ＣＴが低く硬質なマルテンサイトが生成しており、延性は優れているもののやはり穴拡げ性が劣位である。
【００３２】
【発明の効果】
以上詳述したように、本発明は大きなクリアランスで打ち抜かれた場合でも打ち抜き端面に欠陥が発生しない、引張強さが６９０ＭＰａ以上の穴拡げ性と延性に優れた高強度熱延鋼板及びその製造方法を提供するものであり、その優れた加工性を活かして自動車部品、例えばメンバー類やアーム類などの足周り部品やシャーシなどの材料として有効な、工業的価値が高い発明である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more, which is excellent in burring workability represented by hole expandability, stretch flangeability and ductility, and makes use of its excellent workability in automobile parts such as members. It can be effectively used as a material for chassis parts, such as foot parts and arms.
[0002]
[Prior art]
BACKGROUND ART In recent years, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of weight reduction in order to improve fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength, their application has been limited to special applications because they are significantly more expensive than steel. In order to promote the reduction of the weight of automobiles in a wider range, the use of inexpensive high-strength steel sheets is strongly required.
[0003]
In general, the higher the strength of a material, the lower the ductility and the worse the workability (moldability).
Steel materials are no exception, and attempts have been made to achieve both high strength and high ductility. On the other hand, materials used for suspension parts such as suspension arms of automobiles are required to have high burring workability (hole expanding property) in addition to these characteristics. However, as the strength increases, the hole expandability tends to decrease as well as the ductility, and when applying a high-strength steel sheet to undercarriage parts of automobiles with complicated shapes, the hole expandability becomes poor. This is an important consideration. From such a background, a hot rolled steel sheet mainly having a structure such as ferrite or bainitic ferrite, and having excellent ductility and hole expanding property in which a so-called hard second phase such as a pearlite or martensite structure or a cementite phase is reduced has been developed. ing.
[0004]
By the way, recently, with the expansion of application of these high-strength hot-rolled steel sheets, there has been a problem that peeling or square-shaped defects are generated on a plate end face punched from a material at the time of press working. These defects not only significantly impair the design of the end face of the product, but also have a risk of becoming a stress concentration portion such as a notch due to the high strength steel plate and affecting the fatigue strength. Although not much has been studied regarding the punching properties of ordinary steel, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 9-49053) discloses a method of controlling precipitates in steel and Patent Document 2 (Japanese Patent Application Laid-Open No. 2003-49053). No. 41342) discloses an invention which is improved by making the structure of the surface layer portion of a steel sheet finer.
[0005]
[References]
(1) Patent Document 1 (JP-A-9-49053)
(2) Patent Document 2 (JP-A-2003-41342)
[0006]
[Problems to be solved by the invention]
However, the above-mentioned prior art is basically a technique relating to the punching property of a low-strength ultra-low carbon steel sheet, and it cannot be said that the present invention can be applied to a high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more, which is the target of the present invention. That is, the invention described in Patent Document 1 relates to the generation of burrs at the time of punching, which is likely to occur due to the use of a soft steel plate. Specifically, Ti ₄ C ₂ S ₂ is precipitated in the steel, whereby the punched end face is formed. This is a technique for reducing burrs by generating voids in the sheared surface portion and causing cracks to propagate. That is, in the invention described in Patent Literature 1, burrs can be reduced, but defects on the end face may be promoted.
[0007]
The invention described in Patent Document 2 also relates to the prevention of burr generation for a soft cold-rolled steel sheet and does not consider the properties of the punched end face itself at all. Therefore, the present invention is hard to say that it has been studied in the above prior art, high tensile strength of hot rolled steel, particularly excellent ductility and hole expandability expected to be applied to automotive undercarriage parts of 690 MPa or more. It is an object of the present invention to provide a technology for preventing a defect on a punched end face on the assumption that a high-strength hot-rolled steel sheet is used.
[0008]
[Means for Solving the Problems]
The present inventors first performed punching with various clearances using a high-strength hot-rolled steel sheet excellent in ductility and hole expandability and having a tensile strength of 690 MPa or more, and visually inspected the end surface properties. As a result, it was confirmed that peeling or a chip-like defect, which cannot be confirmed when punching with a clearance of about 12.5%, which is normally recommended in a hole expanding test, clearly occurs by increasing the clearance. In the following, the investigation was conducted using a clearance of 17 to 23%.
[0009]
Next, considering the chemical composition of the hot-rolled steel sheet and the manufacturing process of the hot-rolled steel sheet that is currently manufactured on an industrial scale by the continuous hot rolling equipment currently used, the manufacturing conditions that affect the defects at the punched end face are considered. As a result of diligent research into the effects of steel, it has been found that the addition of a small amount of B to steel can prevent the basic steel composition and manufacturing process from producing defects without making any changes to conventional high-strength hot-rolled steel sheets. Heading, the present invention.
[0010]
That is, the gist of the present invention is as follows.
(1) In mass% (the same applies hereinafter), C: 0.01 to 0.07%, N: ≤ 0.005%, S: ≤ 0.005%, Ti: 0.03 to 0.2%, B: A steel containing 0.0002 to 0.002%, with the balance being Fe and unavoidable impurities, with a ferrite or bainitic ferrite structure having the largest area ratio phase, and a hard second phase and cementite having an area of A high-strength hot-rolled steel sheet excellent in punching workability, wherein the tensile strength is 3% or less and the tensile strength is 690 MPa or more.
[0011]
(2) Further, one or more of V: 0.01 to 0.2%, Nb: 0.01 to 0.2%, and Mo: 0.01 to 0.2% are contained in mass%. And satisfying Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0%, and having excellent punching workability according to the above (1), High strength hot rolled steel sheet.
(3) Further, in terms of mass%, Si: 0.01 to 2%, Mn: 0.05 to 2%, P: ≦ 0.1%, Al: 0.005 to 0.5%. A high-strength hot-rolled steel sheet excellent in punching workability according to the above (1) or (2), characterized in that:
[0012]
(4) Further, the above-mentioned (1) to (1) to (1) to (2) further containing one or two kinds of Ca: 0.0005 to 0.02% and REM: 0.0005 to 0.2% by mass%. The high-strength hot-rolled steel sheet excellent in punching workability according to any one of 3).
(5) A method for producing the steel sheet according to any one of (1) to (4), wherein hot rolling of a steel slab having the component according to any one of claims 1 to 4. In this case, the hot-rolling heating temperature is set to 1200 ° C. or higher, and after hot finish rolling at an Ar ₃ transformation point or higher, winding is performed at 450 ° C. to 650 ° C. High-strength hot rolling excellent in punching workability This is a method for manufacturing a steel sheet.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail. First, the reasons for limiting the microstructure of steel in the present invention will be described.
The microstructure of the present invention uses a ferrite or bainitic ferrite structure as a phase having the largest area ratio. The hard second phase, which greatly reduces hole expandability, has an area ratio of 3% or less (including 0%). I do. Here, the hard second phase is typically martensite or pearlite, but also includes retained austenite which may be transformed into martensite in a punching process performed prior to the hole expanding process. . Furthermore, cementite is also hard and deteriorates the hole expandability. Therefore, in the present invention, the ferrite or bainitic ferrite structure is defined as the phase having the maximum area ratio, and the hard second phase and the cementite are specified to have an area ratio of 3% or less. Preferably, the area ratio of cementite is 1% or less. The presence state of the cementite mentioned here includes not only a state existing in a lump at a ferrite grain boundary or the like, but also a state generated at a lath-like boundary in a bainite structure.
[0014]
Next, the reasons for limiting the chemical components of the present invention will be described. The amounts of the chemical components are% by weight.
If the content of C exceeds 0.07%, the workability is deteriorated, so the content of C is set to 0.07% or less. If it is less than 0.01%, the strength is reduced.
N is an element that forms coarse TiN in steel and should be basically reduced as much as possible. However, an unreasonable reduction leads to an increase in steelmaking cost and is considered in consideration of steelmaking capacity. 005% or less.
[0015]
If S is too large, it deteriorates hole expandability as inclusions such as MnS and further causes cracking during hot rolling. Therefore, S should be reduced as much as possible. However, if it is 0.005% or less, it is in an allowable range.
Ti contributes to an increase in the strength of the steel sheet by precipitation strengthening. However, if the content is less than 0.03%, this effect is insufficient. On the other hand, if the content exceeds 0.2%, formation of coarse TiN is inevitable. Therefore, the content of Ti is set to 0.03% or more and 0.2% or less.
[0016]
B is the most important element in the present invention, and the addition of this element prevents defects at the punched end face. Although the reason for this is not clear, as described above, in a high-strength hot-rolled steel sheet excellent in ductility and hole expandability targeted by the present invention, precipitate constituent elements such as V, Nb, and Mo, including Ti, are contained. Since C, N, and S in the steel are sufficiently fixed, segregation of elements at grain boundaries is small, and B seems to function as a complement to this. Incidentally, such a function is not exhibited unless B is present in a solid solution state, and precipitation as carbide or nitride must be avoided. However, in the present invention, since V, Nb, and Mo, including Ti, fix C and N, which may combine with B, a solid solution state is maintained even when a small amount is added. are doing.
However, if the content is less than 0.0002%, this effect is insufficient, while if it exceeds 0.002%, the workability such as ductility is reduced. Therefore, the content of B is set to 0.0002% or more and 0.002% or less.
[0017]
V, Nb, and Mo are all added as precipitation strengthening elements that complement Ti, and for the purpose of fixing C that contributes to the formation of cementite by this precipitation. However, if the content of each is less than 0.01%, this effect is insufficient. On the other hand, if the content is more than 0.2%, Ti is complemented when forming TiN and contributes to the formation of coarse precipitates. Resulting in. Therefore, the contents of V, Nb, and Mo are set to 0.01% or more and 0.2% or less, respectively. However, not only one kind but also two or more kinds in the above range are allowed.
[0018]
Further, these elements are required to satisfy the following formula (element names mean mass% of chemical components of each element).
Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0
This formula is a well-known formula that defines that precipitate constituent elements such as Ti, V, Nb, and Mo are added as necessary to sufficiently fix C, N, and S in steel. Thus, the amount of cementite can be reduced.
[0019]
Since Si is effective as a solid solution strengthening element for increasing the strength, Si is added as necessary. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if the content exceeds 2%, the workability deteriorates. Therefore, the content of Si is set to 0.01% or more and 2% or less.
Since Mn is effective as a solid solution strengthening element for increasing the strength, Mn is added as necessary. To obtain the desired strength, 0.05% or more is required. Further, if added over 2%, slab cracks occur, so the content is set to 2% or less.
If P exceeds 0.1%, workability and weldability are adversely affected. Therefore, P is set to 0.1% or less.
[0020]
Al is added as needed for deoxidation of molten steel. Although it is necessary to add 0.005% or more, the upper limit is set to 0.5% to increase the cost. Further, if added in an excessively large amount, nonmetallic inclusions increase and elongation deteriorates. Therefore, the content is preferably set to 0.3% or less.
Ca and REM are elements that become the starting point of destruction or change the form of nonmetallic inclusions that degrade workability and render them harmless. However, if the addition is less than 0.0005%, there is no effect even if added, and if Ca is added more than 0.02%, and if REM is added more than 0.2%, the effect is saturated. 0005 to 0.02%, REM: 0.0005 to 0.2%.
[0021]
Next, the reasons for limiting the production method of the present invention will be described below.
In the present invention, when hot rolling a slab obtained by casting molten steel whose components have been adjusted to the target component content, the heating temperature is set to 1200 ° C. or higher. This is because if the temperature is lower than 1200 ° C., precipitation constituent elements such as V, Nb, and Mo in the steel, such as Ti, are not sufficiently re-dissolved, so that precipitation strengthening becomes insufficient. Although the upper limit of the heating temperature is not particularly set, if it is 1400 ° C. or more, the scale-off amount becomes large and the yield decreases, so the reheating temperature is desirably less than 1400 ° C. In the present invention, in addition to the case where the above slab is cooled and then reheated in a heating furnace, the slab may be directly sent to a hot rolling mill as it is as a high temperature slab.
[0022]
In the hot rolling step, finish rolling is performed after rough rolling is completed, but it is necessary to finish in a temperature range where the finishing temperature is equal to or higher than the Ar ₃ transformation point. This is because if the rolling temperature falls below the Ar ₃ transformation point during hot rolling, strain remains and ductility decreases. The upper limit of the finishing temperature is not particularly required to obtain the effects of the present invention, but is desirably set to 1000 ° C. or less because scale flaws may occur during operation.
[0023]
After the finish rolling, it is cooled to a specified winding temperature, but the cooling rate does not need to be particularly determined to obtain the effect of the present invention. However, if the cooling rate is too slow, the size of precipitates precipitated during cooling after hot rolling becomes coarse, not only not contributing to the increase in strength due to precipitation strengthening, but also hard phase such as cementite, which is harmful to hole expandability. The lower limit of the cooling rate is desirably 20 ° C./s or more, since harmful tissues such as pearl and pearlite may be generated. On the other hand, by providing a low cooling speed region such as air cooling for a short time in the cooling zone in the ferrite generation region during cooling, cooling conditions may be adjusted such as to promote ferrite transformation.
[0024]
If the winding temperature is less than 450 ° C., precipitation after winding becomes insufficient, so that precipitation strengthening cannot be obtained. In addition, solid solution C may remain in the steel to lower workability, and hole expandability may be reduced. Harmful hard martensite may be generated. Conversely, if the cooling rate is too high at 650 ° C., a phenomenon similar to that which is a concern when the cooling rate is too slow, that is, not only does the size of the deposited precipitate become coarse and does not contribute to the increase in strength due to precipitation strengthening, There is a possibility that a harmful tissue such as a hard phase such as cementite or pearlite which is harmful to the spreading property is generated. Therefore, the winding temperature is set to 450 to 650 ° C.
The steel of the present invention has the same effect even if the surface is subjected to a surface treatment (for example, galvanizing) and does not deviate from the present invention.
[0025]
【Example】
Hereinafter, the present invention will be further described with reference to examples. Steels A to U having the chemical components shown in Table 1 were melted in a converter and continuously cast to obtain slabs for hot rolling. However, the indication of the chemical composition in the table is% by mass. In Table 1, the values on the left side of the following equation, which are requirements of the present invention, are also shown.
Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0
These were reheated at the heating temperature (SRT) shown in Table 2, rolled to a sheet thickness of 1.2 to 6.0 mm at the finishing temperature (FT) after rough rolling, and then wound at the winding temperature (CT). Was.
[0026]
[Table 1]

[0027]
A sample cut out from the 1 / 4W position of the steel sheet width of the hot-rolled sheet thus obtained was polished to a cross section in the rolling direction, etched with a nital reagent, and observed at a magnification of 200 to 500 times using an optical microscope. From the obtained microstructure, the structure name of the phase with the largest area ratio and the hard second phase, if any, are analyzed with the name of each structure and each area ratio. Table 2 also shows the area ratio of cementite calculated by the above. The tensile properties were determined by processing the test material into a No. 5 test piece described in JIS Z 2201 and evaluating it according to the test method described in JIS Z 2241.
The hole expansion test was evaluated according to the test method described in the Japan Iron and Steel Federation Standard JFS T1001-1996. As described above, the presence or absence of defects on the punched end face was determined by punching out a hole having a diameter of 10 mm in the same manner as in the hole expanding test, with the clearance at the time of punching being 17 to 23%, and visually observing the state of the end face. Table 2 shows the test results.
[0028]
[Table 2]

[0029]
No. 1 according to the present invention. Nos. 1 to 10 are high-strength hot-rolled steel sheets having tensile strength TS of 690 MPa or more, elongation El of 20% or more, and hole expansion value λ of 90% or more, so-called excellent ductility and hole expansion properties. No defects have occurred.
Other than the above, it is out of the scope of the present invention for the following reasons. That is, in Comparative Example No. 11 to 16 are steels K to O except that B is not added, and other components and production conditions satisfy the requirements of the present invention, so that tensile properties and hole expandability are properties comparable to those of the present invention. Despite having, all punched end face defects occurred.
[0030]
On the other hand, in Comparative Example No. In No. 16, the addition amount of B in the steel P is 35 ppm, which exceeds the upper limit of the present invention. As a result, the ferrite transformation is delayed, so that cementite is precipitated and the hole expandability is inferior. Comparative Example No. No. 17 is inferior in tensile strength to 690 MPa or less because the steel Q has a small C content and is out of the present invention. Conversely, in Comparative Example No. Steel No. 18 has a large amount of C in the steel R and is outside the scope of the present invention, so that pearlite, which is a hard second phase, is formed, and the ductility and hole expanding properties are inferior. Comparative Example No. Nos. 19 to 21 do not satisfy the conditions of the composition formula defined in the present invention in each of the steels S to U. In particular, S has a small Ti content itself and is out of the range of the present invention. Is contained in more than 3%, and particularly the hole expandability is deteriorated. Since U does not contain B, defects are found on the punched end face.
[0031]
In addition, although the steel composition was out of the production conditions despite being within the scope of the present invention, the properties of the steel composition were insufficient. 22 to 26. That is, in Comparative Example No. 22 and Comparative Example No. 22. In No. 23, since the heating temperature SRT was low and outside the present invention, the constituent elements of the precipitates such as Ti could not be sufficiently dissolved, and the precipitation strengthening was insufficient and No. 23 of the conditions of the present invention. 1 and Comparative Example No. Although the tensile strength TS is lower than that of No. 23, there is more cementite and the hole expandability is lower. Comparative Example No. In No. 24, since the finishing temperature FT was lower than the Ar ₃ temperature, the strain remained, and as a result, the cementite was also large and the ductility was lowered. Comparative Example No. Since No. 25 has a high winding temperature CT and is outside the scope of the present invention, pearlite is generated, and as a result, hole expandability is poor. On the other hand, in Comparative Example No. On the contrary, hard martensite 26 has a low winding temperature CT and hard martensite is formed. Although the ductility is excellent, the hole expandability is still inferior.
[0032]
【The invention's effect】
INDUSTRIAL APPLICABILITY As described above in detail, the present invention provides a high-strength hot-rolled steel sheet excellent in hole expandability and ductility having a tensile strength of 690 MPa or more, in which no defect is generated on a punched end face even when punched with a large clearance, and a method for producing the same. The present invention is an invention having high industrial value, which is effective as a material for automobile parts, for example, foot parts such as members and arms and a chassis by utilizing its excellent workability.

Claims

In mass%,
C: 0.01-0.07%,
N: ≦ 0.005%,
S: ≦ 0.005%,
Ti: 0.03 to 0.2%,
B: 0.0002-0.002%
, The balance being Fe and unavoidable impurities, having a ferrite or bainitic ferrite structure as a phase having a maximum area ratio, a hard second phase and cementite having an area ratio of 3% or less, and a tensile strength of A high-strength hot-rolled steel sheet excellent in punching workability, having a hardness of 690 MPa or more.

Further, in mass%,
V: 0.01-0.2%,
Nb: 0.01 to 0.2%,
Mo: 0.01 to 0.2%
And at least one of Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0%
2. The high-strength hot-rolled steel sheet according to claim 1, which satisfies the following.

Further, in mass%,
Si: 0.01 to 2%,
Mn: 0.05-2%,
P: ≦ 0.1%,
The high-strength hot-rolled steel sheet having excellent punching workability according to claim 1 or 2, wherein the steel sheet contains 0.005 to 0.5% of Al.

Further, in mass%,
Ca: 0.0005-0.02%,
REM: 0.0005-0.2%
The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the high-strength hot-rolled steel sheet has excellent punching workability.

A method for producing a steel sheet according to any one of claims 1 to 4, wherein hot rolling of a steel slab having the component according to any one of claims 1 to 4 is performed by hot rolling. A high-strength hot-rolled steel sheet excellent in punching workability, wherein after finishing hot finish rolling at an Ar ₃ transformation point or more, winding is performed at 450 ° C. to 650 ° C.