JP4161090B2

JP4161090B2 - High carbon steel plate with excellent punchability

Info

Publication number: JP4161090B2
Application number: JP06949899A
Authority: JP
Inventors: 雅人鈴木; 直人大久保; 昭史平松
Original assignee: Nippon Steel Nisshin Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1999-03-16
Filing date: 1999-03-16
Publication date: 2008-10-08
Anticipated expiration: 2019-03-16
Also published as: JP2000265239A

Description

【０００１】
【産業上の利用分野】
本発明は、打抜き加工性に優れ、各種機械部品等に使用される高炭素鋼板に関する。
【０００２】
【従来の技術】
複雑形状をもち高い寸法精度，耐摩耗性が要求されるギア等の機械部品は、高炭素鋼を素材とし、切削加工等によって成形仕上げされた後、焼入れ焼戻し等の必要な熱処理を施すことにより製造されてきた。切削加工では製造コストが高くつくため、切削加工を打抜き加工に置き換えることが検討されている。打抜き加工による場合、特にギア歯等の部品と接触する部分では、打抜き加工に起因する割れが抑制又は解消され、剪断面が多いことが要求される。また、複雑形状で高寸法精度の製品を得るためには、打抜き加工技術の向上に加え、優れた打抜き性を示す高炭素鋼板が素材として要求される。
【０００３】
鋼帯の熱延条件を制御してベイナイトの生成を抑制するとき、熱延材のままであっても打抜きによる割れが発生しない鋼板が得られることが特開昭５６−９３２９号公報で紹介されている。しかし、対象鋼種はＣ含有量が０．６０重量％以下の鋼板であり、Ｃ含有量が０．７０重量％以上の高炭素鋼板には適用できない。
高炭素鋼の打抜き性に関しては、炭化物粒径の微細化及びフェライトの粒径制御によりバリ高さの低減や金型の長寿命化が図られること（特開平９−３１６５９５号公報）, 特殊元素の添加によりバリ高さの低減や金型の長寿命化が図られること（特開昭５７−１１０６２２号公報，特開平３−４４４４７号公報，特開平４−２３５２５２号公報），打抜き時の騒音低減や金型の長寿命化に炭化物の黒鉛化が有効であること（特開昭５６−１１９７５８号公報）等が知られている。
【０００４】
【発明が解決しようとする課題】
従来の打抜き性改善策は、何れもバリ高さの低減や金型の長寿命化を狙ったものに過ぎない。また、打抜き性の改善に使用される特殊元素は、一般的な高炭素鋼に適用できず、製造コストを上昇させる原因にもなる。
このように、打抜き性の中でも特に打抜き面性状に優れた高炭素鋼板のニーズが高まっているにも拘わらず、一般的な高炭素鋼において割れが小さいこと，剪断面が多いこと等、打抜き面性状を改善する有効な方法が確立されていない。
本発明は、このような問題を解消すべく案出されたものであり、０．７０〜１．２０重量％のＣを含む高炭素鋼板において炭化物を粒径制御することにより、打抜き性に密接な関係をもつ切欠き引張伸びを改善し、打抜き性に優れた高炭素鋼板を提供することを目的とする。
【０００５】
【課題を解決するための手段】
本発明の高炭素鋼板は、その目的を達成するため、Ｃ：０．７０〜１．２０重量％，Ｓｉ：０．４０重量％以下，Ｍｎ：１．０重量％以下，Ｐ：０．０３重量％以下，全Ａｌ：０．１０重量％以下，Ｃｒ：１．６重量％以下を含み、残部がＦｅ及び不可避的不純物の組成をもち、平均粒径０．３〜１．２μｍで炭化物球状化率８０％以上の炭化物がフェライト粒径５μｍ以上のフェライトマトリックスに分散した組織をもち、ＪＩＳ５号引張試験片の平行部長手方向中央位置における幅方向両サイドに開き角４５度，深さ２ｍｍのＶノッチを入れた試験片を用いて引張試験し、平行部長手方向中央部の標点間距離１０ｍｍに対する破断後の伸び率として表わされる切欠き引張伸びが１０％以上であることを特徴とする。
【０００６】
高炭素鋼板は、更にＣｒ：１．６重量％以下，Ｍｏ：０．３重量％以下，Ｃｕ：０．３重量％以下，Ｎｉ：２．０重量％以下，Ｃａ：０．０１重量％以下の１種又は２種以上を含むことができる。不純物として含まれるＳは、０．０１重量％以下に規制することが好ましい。
炭化物球状化率は、鋼板断面の金属組織を観察するとき、炭化物総数が３００個以上の領域を観察視野にとり、最大長さｐを定め、最大長さｐと直角方向の最大長さｑとの比ｐ／ｑが３未満の炭化物（以下、球状化炭化物という）の個数が観察視野内の炭化物総数に占める割合（％）で表わされる。また、炭化物平均粒径は、同じ炭化物総数３００個以上の観察視野において個々の炭化物について測定した円相当径を全測定炭化物で平均した値で表わされる。
【０００７】
【作用】
本発明者等は、一般的な高炭素鋼板の打抜き性を向上させる方法を種々検討したところ、局部延性の指標の一つである切欠き引張伸び及び打抜き時の剪断面率に打抜き性が良好な相関関係をもつことを見出した。また、鋼板に分散している炭化物の形態が切欠き引張伸びに大きな影響を及ぼし、炭化物の球状化及び平均粒径の増大によって切欠き引張伸びが改善されることが判った。
割れや亀裂は、打抜き加工時に鋼板が加工変形する際に発生した非常に局所的な欠陥を起点とし、加工変形の進行に伴って素材内部を伝播した結果であると考えられる。高炭素鋼板においては、欠陥生成原因として炭化物（セメンタイト），ＭｎＳ系介在物等を起点とするミクロボイドの発生・成長が挙げられる。
【０００８】
このような前提に立つとき、加工変形時にミクロボイドの発生・成長を可能な限り抑制できる金属組織の調整及び介在物の低減が打抜き性の改善に有効であるといえる。ミクロボイドの発生・成長を抑制することは、局部延性を向上させることにもなる。実際に切欠き引張試験に供した試験片のミクロボイドを観察すると、ミクロボイドの発生・成長が金属組織の形態に大きく影響され、打抜き加工時のミクロボイドの発生・成長に酷似していた。このことからしても、打抜き性と切欠き引張伸びとの間に密接な関係があることが窺がわれる。
【０００９】
［成分・組成］
本発明では、Ｃ：０．７０〜１．２０重量％を含む高炭素鋼を対象としている。Ｃは、高炭素鋼において最も基本となる合金成分であり、含有量の如何に応じて加工性，焼入れ硬さ，炭化物量等が大きく変動する。Ｃ含有量が０．７０重量％未満では、焼入れ後に十分な量の未溶解炭化物が残存せず、所望の耐摩耗性が得られない。逆に１．２０重量％を超えるＣ含有量では、熱延後の靭性低下により鋼帯の製造性・取扱い性が悪化すると共に、焼鈍後においても十分な延性が得られないため、本発明が狙っている優れた打抜き性が期待できない。
Ｓｉは、局部延性に対し大きな影響を及ぼす合金成分である。過剰量のＳｉを添加すると、固溶強化作用によってフェライトが硬化し、成形加工時に割れを発生させる原因になる。過剰なＳｉ添加は、製造過程で鋼板表面におけるスケール疵の発生を助長し、表面品質を低下させる原因にもなる。そこで、Ｓｉ含有量の上限を０．４０重量％に規定し、特に加工性が要求される用途では０．２０重量％以下に規制することが好ましい。
【００１０】
Ｍｎは、耐摩耗性改善に有効な合金成分であるが、１．０重量％を超える多量のＭｎが含まれるとフェライトが硬化し、加工性が劣化する。
Ｐは、延性及び靭性に悪影響を及ぼす成分であることから、上限を０．０３重量％に規定する。
Ａｌは、溶鋼の脱酸剤として添加される成分であるが、鋼中の全Ａｌ量が０．１重量％を超えると鋼材の清浄度が損われ、鋼板表面に疵が発生し易くなる。
【００１１】
熱処理性を更に改善するため、Ｃｒ，Ｍｏ，Ｃｕ，Ｎｉ，Ｃａの１種又は２種以上が必要に応じて添加される。
Ｃｒは、焼入れ性の改善に有効な合金成分であり、焼戻し軟化抵抗を大きくする作用を呈する。しかし、１．６重量％を超える多量のＣｒが含まれると、Ａ₁ 変態点以下での長時間焼鈍やＡ₁ 変態点以上の加熱を伴った焼鈍を施しても軟質化し難く、却って打抜き性が低下する。したがって、Ｃｒを添加する場合には、Ｃｒ含有量の上限を１．６重量％に設定する。
Ｍｏは、少量の添加でＣｒと同様に焼入れ性及び焼戻し軟化抵抗を改善する作用を呈する。しかし、０．３重量％を超える多量のＭｏが含まれると、Ａ₁ 変態点以下での長時間焼鈍やＡ₁ 変態点以上の加熱を伴った焼鈍を施しても軟質化し難く、却って打抜き性が低下する。したがって、Ｍｏを添加する場合には、Ｍｏ含有量の上限を０．３重量％に設定する。
Ｃｕは、熱延中に生成される酸化スケールの剥離性を向上させ、鋼板の表面品質を改善する作用を呈する。しかし、０．３重量％を超える多量のＣｕが含まれると、溶融金属脆化に起因して鋼板表面に微細なクラックが発生し易くなる。Ｃｕを添加する場合、０．１０〜０．１５重量％の範囲が好ましい。
【００１２】
Ｎｉは、焼入れ性を改善すると共に、低温靭性の向上に有効な合金成分である。また、Ｃｕ添加に起因する溶融金属脆化の悪影響を打ち消す作用も呈する。溶融金属脆化の防止には、０．２重量％以上のＣｕを添加する場合、Ｃｕ添加量と当量程度のＮｉを添加することが有効である。しかし、２．０重量％を超える多量のＮｉを添加すると、Ａ₁ 変態点以下での長時間焼鈍やＡ₁ 変態点以上の加熱を伴った焼鈍を施しても軟質化し難く、却って打抜き性が低下する。
【００１３】
打抜き性は、Ｓ含有量を規制し、Ｃａを添加することによっても改善される。
Ｓは、ＭｎＳ系介在物を生成する成分である。ＭｎＳ系介在物の量が多くなると局部延性が劣化するので、鋼中のＳ量は可能な限り低減することが好ましいが、本発明で規定する炭化物形態が得られる限り、極低Ｓ化を要することなく一般的な市販鋼に対しても打抜き性改善の効果は得られる。しかし、Ｃ含有量が１．２０重量％近くまで高くなった場合でも高い局部延性を安定して確保するためには、Ｓ含有量を０．０１重量％以下に低減した鋼を使用することが好ましい。
ＭｎＳ系介在物は、Ｃａ添加により効果的に形態制御される。通常のＭｎＳ系介在物は、細長い形状を呈し、打抜き時にミクロボイド生成の起点になり易い。これに対し、Ｃａ添加した鋼材ではＭｎ，Ｓ，Ｃａの複合介在物となり、介在物が球状化するためミクロボイドの発生が抑えられる。しかし、０．０１重量％を超える過剰量のＣａを添加すると、介在物の粗大化に起因する弊害が現れるようになる。したがって、Ｃａを添加する場合、Ｃａ含有量の上限を０．０１重量％に設定する。
【００１４】
［炭化物の球状化率］
炭化物球状化率は、「球状化した炭化物」が全炭化物に占める割合を示す。本件明細書では、鋼板断面の金属組織観察視野で最大長さｐとそれに直交する方向の最大長さｑの比ｐ／ｑが３未満の炭化物を「球状化した炭化物」として扱った。たとえば、再生パーライトにおける炭化物では、ほとんどｐ／ｑ≧３の炭化物である。他方，Ａｃ₁ 変態点以上の加熱で残留した未溶解炭化物を起点として成長した炭化物では、比ｐ／ｑが３未満になる。
炭化物の形状を立体的に正確に捉えて規定することは難しく、製品鋼板の適否を判定する上でも煩雑である。これに対し、鋼板断面の平面的な金属組織を観察することは容易である。本発明者等は、鋼板断面の金属組織の中で観察される炭化物形状について比ｐ／ｑを用いて球状化の程度を捉えたとき、鋼板の局部延性に対する炭化物形状の影響を適切に評価できることを確認した。そして、種々の実験結果から、比ｐ／ｑが３未満の「球状化した炭化物」の数が全炭化物数の８０％以上を占め、更には平均炭化物粒径を特定範囲に調整するとき、鋼板が高い打抜き性を示すことを見出した。
【００１５】
炭化物球状化率を高めると打抜き性が向上することは、球状化率の高い炭化物は加工時にミクロボイドの生成起点になりにくいことが原因であると推察される。炭化物球状化率の低い鋼板では、分散している炭化物のうち、たとえば再生パーライトの炭化物のように球状化が不充分な炭化物は、周囲のフェライト粒との変形能が異なる。そのため、球状化不充分な炭化物がミクロボイドの生成起点となり、ミクロボイドの生成・連結を助長させて割れ発生に至るものと考えられる。したがって、打抜き性の改善には、平均炭化物粒径の調整と相俟って鋼板の炭化物球状化率を８０％以上にすることが有効である。
【００１６】
［炭化物の平均粒径］
打抜き性や局部延性は、炭化物の平均粒径を大きくすることによっても顕著に改善される。平均粒径の増大は、鋼中の炭素量は一定であることから炭化物総数の減少を意味する。炭化物総数の減少は、個々の炭化物を起点として生成したミクロボイドの連結を抑制し、結果として打抜き性及び局部延性の顕著な向上に寄与するものと推察される。
平均炭化物粒径は、鋼板断面の金属組織を観察するとき、観察視野にある個々の炭化物について測定した円相当径を全測定炭化物で平均した値で示される。具体的には、個々の炭化物について面積を測定し、得られた面積から円相当径を算出する。炭化物の面積は、画像処理装置を用いて容易に測定できる。測定した全ての炭化物の円相当径の総和を求め、総和を測定炭化物の総数で除した値を平均炭化物粒径とする。数値の信頼性を高めるためには、測定炭化物総数が３００個以上となる観察視野を選定することが好ましい。
本発明者等による詳細な打抜き実験の結果、炭化物球状化率を８０％以上，平均炭化物粒径を０．３μｍ以上とするとき、優れた打抜き性及び局部延性を示す鋼板が得られることが判った。しかし、１．２μｍ以上に平均炭化物粒径を粗大化させても、炭化物粒径の増大に見合った局部延性向上の効果が小さく、長時間の焼鈍を施す必要があるため経済的なデメリットが大きくなる。したがって、本発明では、鋼板中の平均炭化物粒径を０．３〜１．２μｍの範囲に規定した。
【００１７】
［フェライト粒径］
焼鈍後のフェライト粒径も、打抜き性や局部延性の改善に影響を及ぼす因子である。フェライト粒径が５μｍ未満になると、材料の局部延性が低下する傾向がみられる。この点、炭化物分散形態適正化の効果を最大限発揮させるためには、フェライトの結晶粒径（平均粒径）を５μｍ以上にすることが好ましい。更には、フェライト結晶粒径が不揃いの混粒組織は加工性に悪影響を及ぼすので、可能な限りフェライト結晶粒径が揃った整粒組織にすることが好ましい。整粒組織を得るためにはフェライト結晶粒径（平均粒径）を５〜３５μｍの範囲に調整することが好ましく、平均粒径が３５μｍを超えるフェライト結晶粒では混粒組織になり易い。
以上のような特性をもつ鋼板は、Ａｃ₁ 変態点以下の長時間焼鈍によっても得られるが、焼鈍方法の工夫により比較的短時間の焼鈍で得ることができる。たとえば、Ａｃ₁ 変態点直下及び直上の特定温度範囲における加熱を適切に組み合わせた焼鈍等が採用される。具体的には、（Ａ_C1−５０℃）〜（Ａ_C1未満の温度）の温度域に熱延鋼板又は冷延鋼板を０．５時間以上保持する１段目の加熱、Ａ_C1〜（Ａ_C1＋１００℃）の温度域に０．５〜２０時間保持する２段目の加熱、次いで（Ａ_r1−８０℃）〜Ａ_r1の温度域に２〜６０時間保持する３段目の加熱を連続させ、２段目の保持温度から３段目の保持温度への冷却速度を５〜３０℃／時間とする３段階焼鈍によって、炭化物分散形態が適正に制御された鋼板が製造される。
【００１８】
【実施例】
表１の組成をもつ各種鋼を溶製し、連鋳で得られたスラブを熱間圧延した。このとき、巻取り温度を調整して、熱延鋼帯の組織を変化させた。熱延鋼帯を酸洗した後、鋼板の炭化物球状化率，平均炭化物粒径が異なるように種々の条件で焼鈍した。表２の試験番号１，３〜９，１１〜１４では、巻取り温度４５０〜６００℃で熱延した後、Ａｃ₁ 変態点より低い温度に４時間保持し、Ａｃ₁ 変態点以上の７３０〜７７０℃の一定温度に４時間保持し、冷却速度１０℃／時で冷却し、Ａｃ₁ 変態点以下の６９０℃に４〜４０時間保持し、次いで冷却速度１０℃／時で７６０℃まで冷却した後、空冷する熱処理を施した。また、試験番号２，１０では、巻取り温度５５０℃で得られた熱延鋼帯を酸洗した後、３０〜６０％で冷延し、Ａｃ₁ 変態点より低い７００℃に１０〜３０時間保持する熱処理を施した。
【００１９】

【００２０】
板厚２．０ｍｍの鋼帯から引張試験，切欠き引張試験及び打抜き性評価試験用の試験片を切り出した。
炭化物球状化率は、走査型電子顕微鏡を用いて鋼板断面の一定領域を観察し、総数３００〜１０００個の炭化物が析出している部分を観察領域として選定した。炭化物の最大長さｐとその直角方向の最大長さｑとの比ｐ／ｑが３未満となるものを「球状化した炭化物」としてカウントし、測定炭化物総数に占める「球状化した炭化物」の数の割合を炭化物球状化率として算出した。
平均炭化物粒径は、炭化物球状化率を測定した観察視野を画像処理して個々の炭化物の円相当径を算出し、算出値を全測定炭化物で平均化することにより求めた。
フェライト粒径は、ＪＩＳＧ０５２２に規定されている切断法に従って、直交する二つの線分で切断されるフェライト結晶粒の数を測定し、１０視野測定の結果を平均化して求めた。
【００２１】
引張試験にはＪＩＳ５号試験片を用い、平行部の標点間距離を５０ｍｍに設定した。切欠き引張試験では、ＪＩＳ５号引張り試験片の平行部長手方向中央位置における幅方向両側に開き角４５度，深さ２ｍｍのＶノッチを入れた試験片を使用した。そして、平行部長手方向中央部の標点間距離１０ｍｍに対する伸び率を破断後に測定し、得られた伸び率を切欠き引張伸びＥｌ_V とした。Ｅｌ_V 値は、局部延性を示す指標であり、通常の引張試験で（全伸び）−（均一伸び）として求められる局部伸びに比較して、より適切に局部延性を定量的に評価できる。
打抜き性評価試験では、径１０ｍｍのポンチを用い、クリアランスを板厚の５％に設定して鋼板を打ち抜き、試験片１００個について剪断面率及び割れ深さの平均を求めた。剪断面率は、打ち抜かれた試験片の加工面において［板厚方向の剪断面長さ］／［板厚］×１００（％）として算出した。割れ深さは、各試験片について打抜き面近傍の圧延方向断面を顕微鏡観察し、破断部の最も深い割れの深さを剪断面からの長さで測定した。
【００２２】
表２の調査結果にみられるように、試験番号１４では、Ｃ含有量が１．２０重量％を超えるＦ鋼を素材としているため、炭化物粒径，炭化物球状化率及びフェライト粒径が本発明で規定した範囲にあるものの、切欠き引張伸びＥｌ_V が低く、打抜き性も剪断面率８．８％，割れ深さ０．２６ｍｍと劣っていた。試験番号２は炭化物粒径が０．３μｍに達しておらず、試験番号８は炭化物の球状化率が８０％未満，試験番号１０はフェライト粒径が５μｍ未満であり、何れも切欠き引張伸びＥｌ_V が１０％に達しておらず、打抜き性も剪断面率１０％未満，割れ深さ０．２５ｍｍ以上と劣っていた。
これに対し、炭化物平均粒径，炭化物球状化率及びフェライト粒径を本発明で規定した範囲に調整した試験番号１，３〜７，９，１１〜１３では、１５％以上の高い剪断面率で割れ深さも０．１５ｍｍ以下と小さくなっており、良好な打抜き性が示された。
【００２３】

【００２４】
【発明の効果】
以上に説明したように、本発明の高炭素鋼板は、炭化物球状化率，平均炭化物粒径及びフェライト粒径を適正範囲に設定することにより、打抜き性及び局部延性が改善されている。この高炭素鋼板は、従来の高炭素鋼板に比較して打抜き性が格段に改善されているため、複雑な形状をもつ自動車部品，各種機械部品等の素材として広範な分野で使用される。しかも、軟質化されているので、プレス金型の長寿命化にも有効である。[0001]
[Industrial application fields]
The present invention relates to a high carbon steel sheet that is excellent in punching workability and used for various machine parts.
[0002]
[Prior art]
Machine parts such as gears, which have complex shapes and require high dimensional accuracy and wear resistance, are made of high carbon steel as a raw material, and are subjected to necessary heat treatment such as quenching and tempering after being finished by cutting. Has been manufactured. Since cutting is expensive to manufacture, replacement of cutting with punching has been considered. In the case of the punching process, it is required that cracks due to the punching process are suppressed or eliminated and that there are many shearing surfaces, particularly in a portion that comes into contact with a part such as a gear tooth. In addition, in order to obtain a product having a complicated shape and high dimensional accuracy, a high carbon steel sheet exhibiting excellent punchability is required as a material in addition to improvement of punching technology.
[0003]
Japanese Patent Application Laid-Open No. 56-9329 discloses that when the hot rolling conditions of a steel strip are controlled to suppress the formation of bainite, a steel plate that does not cause cracking due to punching even when it is a hot rolled material is obtained. ing. However, the target steel type is a steel plate having a C content of 0.60% by weight or less and cannot be applied to a high carbon steel plate having a C content of 0.70% by weight or more.
With regard to the punchability of high carbon steel, the burr height can be reduced and the life of the mold can be extended by refining the grain size of carbide and controlling the grain size of ferrite (JP 9-316595 A), special elements Can reduce the burr height and extend the life of the mold (Japanese Patent Laid-Open Nos. 57-110622, 3-44447, 4-235252), and noise during punching It is known that graphitization of carbides is effective for reduction and extending the life of molds (Japanese Patent Laid-Open No. 56-119758).
[0004]
[Problems to be solved by the invention]
All of the conventional punchability improvement measures are aimed at reducing the burr height and extending the life of the mold. In addition, special elements used for improving punchability cannot be applied to general high-carbon steel, which also increases production costs.
Thus, despite the growing need for high carbon steel sheets with particularly excellent punching surface properties among punching properties, punching surfaces such as small cracks and large shear surfaces in general high carbon steels. An effective method for improving the properties has not been established.
The present invention has been devised to solve such a problem. By controlling the grain size of carbides in a high carbon steel sheet containing 0.70 to 1.20% by weight of C, it is closely related to punchability. It is an object of the present invention to provide a high-carbon steel sheet that improves notch tensile elongation and has excellent punchability.
[0005]
[Means for Solving the Problems]
In order to achieve the object, the high carbon steel sheet of the present invention has C: 0.70 to 1.20% by weight, Si: 0.40% by weight or less, Mn: 1.0% by weight or less, P: 0.03 Weight% or less, Total Al: 0.10% by weight or less , Cr: 1.6% by weight or less , the balance having the composition of Fe and inevitable impurities , with an average particle size of 0.3 to 1.2 μm and carbide spheres It has a structure in which carbides with a conversion rate of 80% or more are dispersed in a ferrite matrix with a ferrite grain size of 5 μm or more. A tensile test is performed using a test piece having a V-notch, and a notch tensile elongation expressed as an elongation percentage after breaking with respect to a distance between gauge points of 10 mm in the central portion in the longitudinal direction of the parallel portion is 10% or more. .
[0006]
High carbon steel sheet is further Cr: 1.6 wt% or less, Mo: 0.3 wt% or less, Cu: 0.3 wt% or less, Ni: 2.0 wt% or less, Ca: 0.01 wt% or less 1 type (s) or 2 or more types can be included. S contained as an impurity is preferably regulated to 0.01% by weight or less.
The carbide spheroidization rate is determined by observing a region where the total number of carbides is 300 or more when observing the metal structure of the cross section of the steel sheet, defining the maximum length p, and the maximum length p and the maximum length q in the perpendicular direction. The number of carbides having a ratio p / q of less than 3 (hereinafter referred to as spheroidized carbides) is represented by a ratio (%) to the total number of carbides in the observation field. Further, the carbide average particle diameter is represented by a value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field of 300 or more of the same total number of carbides with all the measured carbides.
[0007]
[Action]
The inventors of the present invention have studied various methods for improving the punchability of general high carbon steel sheets, and have good punchability in notch tensile elongation and shear surface ratio at the time of punching, which are one of local ductility indexes. We found that there is a good correlation. Further, it has been found that the form of carbide dispersed in the steel plate has a great influence on the notch tensile elongation, and the notch tensile elongation is improved by increasing the spheroidization of carbide and increasing the average particle diameter.
Cracks and cracks are thought to be the result of propagation of the inside of the material as the work deformation progresses, starting from a very local defect that occurs when the steel sheet is deformed during punching. In high carbon steel sheets, the generation and growth of microvoids starting from carbides (cementite), MnS inclusions, etc. can be cited as the cause of defect generation.
[0008]
Based on this premise, it can be said that adjustment of the metal structure and reduction of inclusions that can suppress the generation and growth of microvoids as much as possible during processing deformation are effective in improving punchability. Suppressing the generation and growth of microvoids also improves local ductility. When the microvoids of the specimens actually subjected to the notch tensile test were observed, the generation / growth of microvoids was greatly influenced by the form of the metal structure, which was very similar to the generation / growth of microvoids during punching. This suggests that there is a close relationship between punchability and notch tensile elongation.
[0009]
[Ingredients / Composition]
In the present invention, high carbon steel containing C: 0.70 to 1.20% by weight is targeted. C is the most basic alloy component in high-carbon steel, and the workability, quenching hardness, carbide content, etc. vary greatly depending on the content. If the C content is less than 0.70% by weight, a sufficient amount of undissolved carbide does not remain after quenching, and the desired wear resistance cannot be obtained. On the contrary, if the C content exceeds 1.20% by weight, the steel strip manufacturability and handleability deteriorate due to toughness reduction after hot rolling, and sufficient ductility cannot be obtained even after annealing. We cannot expect the excellent punching performance we are aiming for.
Si is an alloy component that greatly affects local ductility. If an excessive amount of Si is added, the ferrite is hardened by the solid solution strengthening action, which causes cracks during molding. Excessive Si addition promotes the generation of scale flaws on the steel sheet surface during the manufacturing process, and also causes the surface quality to deteriorate. Therefore, it is preferable that the upper limit of the Si content is regulated to 0.40% by weight and restricted to 0.20% by weight or less particularly in applications where workability is required.
[0010]
Mn is an alloy component effective for improving the wear resistance. However, if a large amount of Mn exceeding 1.0% by weight is contained, the ferrite is cured and the workability is deteriorated.
Since P is a component that adversely affects ductility and toughness, the upper limit is specified to be 0.03% by weight.
Al is a component added as a deoxidizer for molten steel, but if the total amount of Al in the steel exceeds 0.1% by weight, the cleanliness of the steel material is impaired and wrinkles are likely to occur on the steel sheet surface.
[0011]
In order to further improve the heat treatment properties, one or more of Cr, Mo, Cu, Ni, and Ca are added as necessary.
Cr is an alloy component effective for improving hardenability, and exhibits an effect of increasing temper softening resistance. However, when contains a large amount of Cr exceeding 1.6% by weight, hardly softened even annealed accompanied by prolonged annealing and A ₁ transformation point or more heating below the A ₁ transformation point, rather punching properties Decreases. Therefore, when adding Cr, the upper limit of Cr content is set to 1.6% by weight.
Mo exhibits the effect of improving hardenability and temper softening resistance in the same manner as Cr when added in a small amount. However, when contains a large amount of Mo exceeding 0.3% by weight, hardly softened even annealed accompanied by prolonged annealing and A ₁ transformation point or more heating below the A ₁ transformation point, rather punching properties Decreases. Therefore, when adding Mo, the upper limit of Mo content is set to 0.3 weight%.
Cu exhibits the effect of improving the peelability of the oxide scale produced during hot rolling and improving the surface quality of the steel sheet. However, when a large amount of Cu exceeding 0.3% by weight is contained, fine cracks are likely to be generated on the surface of the steel sheet due to molten metal embrittlement. When adding Cu, the range of 0.10 to 0.15 weight% is preferable.
[0012]
Ni is an alloy component effective for improving hardenability and improving low temperature toughness. Moreover, the effect which counteracts the bad influence of the molten metal embrittlement resulting from Cu addition is also exhibited. In order to prevent molten metal embrittlement, when 0.2 wt% or more of Cu is added, it is effective to add Ni equivalent to the Cu addition amount. However, if adding a large amount of Ni in excess of 2.0 wt%, it is difficult to softening even annealed accompanied by prolonged annealing and A ₁ transformation point or more heating below the A ₁ transformation point, rather punching property descend.
[0013]
The punchability is improved by regulating the S content and adding Ca.
S is a component that generates MnS inclusions. Since the local ductility deteriorates when the amount of MnS inclusions increases, the amount of S in the steel is preferably reduced as much as possible. However, as long as the carbide form defined in the present invention is obtained, extremely low S is required. The effect of improving punchability can be obtained even for general commercial steels. However, in order to stably ensure high local ductility even when the C content increases to near 1.20% by weight, it is necessary to use steel with the S content reduced to 0.01% by weight or less. preferable.
MnS inclusions are effectively controlled in form by the addition of Ca. Ordinary MnS-based inclusions have an elongated shape and are likely to be the starting point for microvoid formation during punching. On the other hand, in the steel material added with Ca, a composite inclusion of Mn, S, and Ca is formed, and the inclusion is spheroidized, so that generation of microvoids is suppressed. However, when an excessive amount of Ca exceeding 0.01% by weight is added, adverse effects due to the coarsening of inclusions appear. Therefore, when adding Ca, the upper limit of Ca content is set to 0.01 wt%.
[0014]
[Carbide spheroidization rate]
The carbide spheroidization rate indicates the ratio of “spheroidized carbide” to the total carbides. In this specification, a carbide having a ratio p / q of less than 3 between the maximum length p and the maximum length q in the direction orthogonal to the metal structure observation field of the cross section of the steel sheet is treated as “spheroidized carbide”. For example, carbides in recycled perlite are mostly carbides with p / q ≧ 3. On the other hand, the ratio p / q is less than 3 for carbides grown starting from undissolved carbides remaining after heating above the Ac ₁ transformation point.
It is difficult to accurately define and define the shape of the carbide three-dimensionally, and it is complicated to determine the suitability of the product steel plate. On the other hand, it is easy to observe the planar metal structure of the cross section of the steel plate. The present inventors can appropriately evaluate the influence of the carbide shape on the local ductility of the steel sheet when capturing the degree of spheroidization using the ratio p / q for the carbide shape observed in the metal structure of the steel sheet cross section. It was confirmed. From various experimental results, the number of “spheroidized carbides” having a ratio p / q of less than 3 occupies 80% or more of the total number of carbides, and when the average carbide particle size is adjusted to a specific range, Was found to show high punchability.
[0015]
It is inferred that the improvement in punchability when the carbide spheroidization rate is increased is due to the fact that carbides with a high spheroidization rate are less likely to be the starting point of microvoid formation during processing. In a steel plate having a low carbide spheroidization rate, among the dispersed carbides, carbides that are insufficiently spheroidized, such as carbides of regenerated pearlite, have different deformability from surrounding ferrite grains. For this reason, it is considered that carbides that are insufficiently spheroidized serve as starting points for the formation of microvoids, which promotes the formation and connection of microvoids, leading to cracking. Therefore, in order to improve punchability, it is effective to set the carbide spheroidization rate of the steel sheet to 80% or more in combination with the adjustment of the average carbide particle size.
[0016]
[Average particle size of carbide]
The punchability and local ductility can be remarkably improved by increasing the average particle size of the carbide. An increase in average particle size means a decrease in the total number of carbides because the amount of carbon in the steel is constant. It is surmised that the decrease in the total number of carbides suppresses the connection of microvoids generated from individual carbides, and as a result, contributes to a marked improvement in punchability and local ductility.
When observing the metal structure of the cross section of the steel sheet, the average carbide particle diameter is indicated by a value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field with all the measured carbides. Specifically, the area of each carbide is measured, and the equivalent circle diameter is calculated from the obtained area. The area of the carbide can be easily measured using an image processing apparatus. The total sum of the equivalent circle diameters of all the measured carbides is obtained, and the value obtained by dividing the total by the total number of measured carbides is defined as the average carbide particle size. In order to increase the reliability of the numerical value, it is preferable to select an observation field in which the total number of measured carbides is 300 or more.
As a result of detailed punching experiments by the present inventors, it has been found that when the carbide spheroidization rate is 80% or more and the average carbide particle size is 0.3 μm or more, a steel sheet having excellent punchability and local ductility can be obtained. It was. However, even if the average carbide particle size is increased to 1.2 μm or more, the effect of improving the local ductility corresponding to the increase in the carbide particle size is small, and it is necessary to perform annealing for a long time, so the economic disadvantage is large. Become. Therefore, in the present invention, the average carbide particle size in the steel sheet is specified in the range of 0.3 to 1.2 μm.
[0017]
[Ferrite particle size]
The ferrite grain size after annealing is also a factor that affects the improvement of punchability and local ductility. When the ferrite particle size is less than 5 μm, the local ductility of the material tends to decrease. In this respect, in order to maximize the effect of optimization of the carbide dispersion form, it is preferable to set the crystal grain size (average grain size) of ferrite to 5 μm or more. Furthermore, since a mixed grain structure with irregular ferrite crystal grain sizes adversely affects workability, it is preferable to use a sized grain structure with ferrite crystal grain diameters as uniform as possible. In order to obtain a sized structure, it is preferable to adjust the ferrite crystal grain size (average grain size) to a range of 5 to 35 μm. A ferrite crystal grain having an average grain size exceeding 35 μm tends to have a mixed grain structure.
Although the steel plate having the above characteristics can be obtained by long-term annealing below the Ac ₁ transformation point, it can be obtained by annealing in a relatively short time by devising the annealing method. For example, annealing that appropriately combines heating in a specific temperature range immediately below and immediately above the Ac ₁ transformation point is employed. Specifically, the first stage of heating in which the hot-rolled steel sheet or the cold-rolled steel sheet is held in the temperature range of (A _C1 -50 ° C.) to (temperature less than A _C1 ) for 0.5 hour or longer, A _C1 to (A temperature range for heating the second stage to hold 0.5 to 20 hours _C1 + 100 ℃), then (a _r1 -80 ℃) continuous heating of the third stage to hold 2 to 60 hours at a temperature range of to a _r1 In addition, a steel sheet in which the carbide dispersion form is appropriately controlled is manufactured by three-stage annealing in which the cooling rate from the second-stage holding temperature to the third-stage holding temperature is 5 to 30 ° C./hour.
[0018]
【Example】
Various steels having the compositions shown in Table 1 were melted and slabs obtained by continuous casting were hot-rolled. At this time, the coiling temperature was adjusted to change the structure of the hot-rolled steel strip. After pickling the hot-rolled steel strip, it was annealed under various conditions so that the carbide spheroidization rate and the average carbide particle size of the steel plate were different. In Test Nos. 1, 3 to 9, and 11 to 14 in Table 2, after hot rolling at a coiling temperature of 450 to 600 ° C., the temperature is kept at a temperature lower than the Ac ₁ transformation point for 4 hours, and 730 to a temperature equal to or higher than the Ac ₁ transformation point. It was kept at a constant temperature of 770 ° C. for 4 hours, cooled at a cooling rate of 10 ° C./hour, held at 690 ° C. below the Ac ₁ transformation point for 4 to 40 hours, and then cooled to 760 ° C. at a cooling rate of 10 ° C./hour. Thereafter, a heat treatment for air cooling was performed. In Test Nos. 2 and 10, the hot-rolled steel strip obtained at a coiling temperature of 550 ° C. was pickled, then cold-rolled at 30 to 60%, and then at 700 ° C. lower than the Ac ₁ transformation point for 10 to 30 hours. A holding heat treatment was applied.
[0019]

[0020]
Test pieces for a tensile test, a notch tensile test, and a punchability evaluation test were cut out from a steel strip having a thickness of 2.0 mm.
The carbide spheroidization ratio was determined by observing a certain region of the cross section of the steel sheet using a scanning electron microscope, and selecting a portion where 300 to 1000 carbides were deposited as the observation region. When the ratio p / q between the maximum length p of carbides and the maximum length q in the direction perpendicular thereto is less than 3, “spheroidized carbides” in the total number of measured carbides are counted as “spheroidized carbides”. The ratio of the numbers was calculated as the carbide spheroidization rate.
The average carbide particle size was determined by calculating the equivalent circle diameter of each carbide by performing image processing on the observation visual field in which the carbide spheroidization ratio was measured, and averaging the calculated values with all the measured carbides.
The ferrite grain size was determined by measuring the number of ferrite crystal grains cut at two orthogonal line segments according to the cutting method defined in JIS G0522 and averaging the results of 10 visual field measurements.
[0021]
In the tensile test, a JIS No. 5 test piece was used, and the distance between the gauge points of the parallel part was set to 50 mm. In the notch tensile test, a test piece having a V notch with an opening angle of 45 degrees and a depth of 2 mm on both sides in the width direction at the center position in the longitudinal direction of the parallel part of a JIS No. 5 tensile test piece was used. Then, the elongation measured after fracture for gauge distance 10mm parallel the longitudinal direction of the central portion, the resulting elongation was notched tensile elongation El _V. The El _V value is an index indicating local ductility, and the local ductility can be more appropriately quantitatively evaluated as compared with the local elongation obtained as (total elongation) − (uniform elongation) in a normal tensile test.
In the punchability evaluation test, a punch having a diameter of 10 mm was used, the steel sheet was punched with the clearance set to 5% of the plate thickness, and the average of the shear surface ratio and crack depth was obtained for 100 test pieces. The shear surface ratio was calculated as [shear surface length in the plate thickness direction] / [plate thickness] × 100 (%) on the processed surface of the punched specimen. The crack depth of each test piece was observed with a microscope in the rolling direction in the vicinity of the punched surface, and the depth of the deepest crack at the fracture portion was measured by the length from the shear plane.
[0022]
As can be seen from the results of the investigation in Table 2, in test No. 14, since the C content exceeds 1.20% by weight, the steel is made of F steel, the carbide particle size, carbide spheroidization rate and ferrite particle size are the present invention. However, the notch tensile elongation El _V was low, the punchability was inferior with a shear surface ratio of 8.8% and a crack depth of 0.26 mm. Test No. 2 has a carbide particle size of less than 0.3 μm, Test No. 8 has a carbide spheroidization rate of less than 80%, Test No. 10 has a ferrite particle size of less than 5 μm, and both are notched tensile elongations. The El _V did not reach 10%, and the punchability was inferior with a shear surface ratio of less than 10% and a crack depth of 0.25 mm or more.
On the other hand, in the test numbers 1, 3 to 7, 9, and 11 to 13 in which the carbide average particle diameter, the carbide spheroidization ratio, and the ferrite particle diameter are adjusted to the ranges specified in the present invention, a high shear surface ratio of 15% or more is obtained. The crack depth was also as small as 0.15 mm or less, indicating good punchability.
[0023]

[0024]
【The invention's effect】
As explained above, the punchability and local ductility of the high carbon steel sheet of the present invention are improved by setting the carbide spheroidization ratio, the average carbide particle size, and the ferrite particle size within appropriate ranges. This high-carbon steel sheet is used in a wide range of fields as materials for automobile parts, various machine parts and the like having complicated shapes because punchability is remarkably improved as compared with conventional high-carbon steel sheets. Moreover, since it is softened, it is effective for extending the life of the press die.

Claims

C: 0.70 to 1.20% by weight, Si: 0.40% by weight or less, Mn: 1.0% by weight or less, P: 0.03% by weight or less, Total Al: 0.10% by weight or less , Cr : A ferrite matrix containing 1.6% by weight or less , the balance being Fe and inevitable impurities , an average particle size of 0.3 to 1.2 μm, and a carbide having a spheroidization rate of 80% or more being a ferrite particle size of 5 μm or more Tensile test was conducted using a specimen having a V-notch with an opening angle of 45 degrees and a depth of 2 mm on both sides in the width direction at the center position in the longitudinal direction of the parallel part of the JIS No. 5 tensile test piece. A high-carbon steel sheet excellent in punchability having a notch tensile elongation of 10% or more expressed as an elongation percentage after breaking with respect to a distance of 10 mm between center marks in the center in the hand direction.

C: 0.70 to 1.20% by weight, Si: 0.40% by weight or less, Mn: 1.0% by weight or less, P: 0.03% by weight or less, Total Al: 0.10% by weight or less, Cr : 1.6% by weight or less, Mo: 0.3% by weight or less, Cu: 0.3% by weight or less, Ni: 2.0% by weight or less, the balance being Fe and inevitable It has a structure in which carbide having an average particle size of 0.3 to 1.2 μm and a carbide spheroidization rate of 80% or more is dispersed in a ferrite matrix having a ferrite particle size of 5 μm or more. Tensile test using a test piece with a V notch with an opening angle of 45 degrees and a depth of 2 mm on both sides in the width direction at the center in the hand direction. Notch tensile elongation expressed as a percentage is 10% or more A high carbon steel sheet with excellent punchability.