JP4032915B2 - Wire for machine structure or steel bar for machine structure and manufacturing method thereof - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、自動車、電気機器等の機械部品用として好適な機械構造用鋼材およびその製造方法に関する。なお、本発明でいう鋼材は、 線材または棒鋼である。
【０００２】
【従来の技術】
自動車用部品や電気機器用部品などは、ほとんどが切削加工によって製造されている。そのため、材料となる鋼材については、機械的特性はもちろん被削性が優れていることが要求されている。被削性向上の最も一般的な方法としては、鋼中に被削性改善成分としてＳやPb等を含有させる方法が採用されており、特にPbは少量の添加で効果があることが知られている。例えば、特許文献１や、特許文献２には、被削性改善のために、Ｓ、Pbを添加した快削鋼が提案されている。
【０００３】
しかし、近年、環境問題が大きくクローズアップされ、Pbによる環境汚染が懸念されるようになった。そこで、鉄鋼等の材料分野においても、Pbを使用しない、いわゆるPbフリー化が推進されている。しかし、Pbフリーの鋼材で、Pb系快削鋼と同レベルまたはそれ以上の被削性を有する鋼材はまだ得られていないのが現状である。Pbと同様に被削性を向上する元素であるＳを添加したＳ系快削鋼でも、Pb系快削鋼と同レベルの被削性を有する材料はまだ実現していない。このため、Pbフリーで、Pb系快削鋼と同レベルまたはそれ以上の被削性を有する鋼材の開発が熱望されている。
【０００４】
一方、Ｓ系快削鋼では、鋼材を圧延や鍛造等で熱間加工する際に、母材の塑性変形とともにMnＳ等の硫化物が圧延方向に長く展伸し、このため、機械的特性に大きな異方性が生じる。具体的には、圧延方向のシャルピー衝撃値が大きく低下する。また、冷間鍛造に際し、展伸したMnＳの存在によりMnＳからミクロクラックが発生し、最終的に割れにつながるという問題も指摘されている。
【０００５】
このようなＳ系快削鋼における問題に対し、MnＳ等の硫化物系介在物の形態（大きさや縦横比（アスペクト比）等）を制御し、被削性を改善する技術が数多く提案されている。例えば、特許文献３には、MnＳの大きさおよびアスペクト比を適正範囲に制御した快削鋼が提案されている。特許文献３に記載された技術では、MnＳの形態制御により、切り屑分断性により評価した被削性が、また、靭性値により評価した機械的特性の向上に効果があるとしている。
【０００６】
また、特許文献４には、Ｓ：0.08〜0.4 重量％、Ca：0.01重量％以下含有する硫黄快削鋼を、1100〜1300℃に加熱し、平均圧延温度が1050℃以上となる圧延温度で行なう粗圧延を加工量全体の70％以上行い、800 〜1000℃の仕上圧延を加工量全体の５〜30％行なう硫黄快削鋼の圧延方法が記載されている。特許文献４に記載された技術では、MnS の伸長が抑えられ微視的組織は細粒で均一となり、安価で被削性に優れた硫黄快削鋼が得られるとしている。
【０００７】
また、特許文献５には、Mgを0.0005〜0.02質量％含有し、さらに硫化物系介在物粒子の分布状態を制御することにより、被削性、特に切り屑分断性と工具寿命、および機械的特性、とくに横方向衝撃値を向上させた機械構造用快削鋼が提案されている。特許文献５に記載された技術によれば、Pbフリーでも従来のPb含有鋼と匹敵する機械的特性や切り屑分断性を有する快削鋼が実現できるとしている。
【０００８】
【特許文献１】
特開昭59−205453号公報
【特許文献２】
特開昭62−23970 号公報
【特許文献３】
特開2001−152279号公報
【特許文献４】
特開平１−224103号公報
【特許文献５】
特開2002-69569号公報
【０００９】
【発明が解決しようとする課題】
しかしながら、本発明者らの検討によれば、特許文献３に記載された技術では、MnＳのサイズ効果にのみに注目しており、耐疲労特性や冷間鍛造性の顕著な向上までは得られないという問題がある。また、特許文献４に記載された技術では、粗圧延における加工量が多すぎて、MnS がある程度伸長してしまい、耐疲労特性、冷間鍛造性の顕著な向上が得られるまでには至っていないという問題がある。
【００１０】
また、特許文献５に記載された技術で製造された製品（鋼材）でも、必ずしも十分な耐疲労特性や冷間鍛造性を保持していない場合があり、 安定した特性を有する鋼材となっていないという問題を残していた。
本発明は、上記したような従来技術の状況に鑑みてなされたものであり、Pbフリーで従来のPb快削鋼の被削性に匹敵する被削性を有し、さらに耐疲労特性に優れ、被削性−耐疲労特性バランスに優れた機械構造用鋼材を提供することを目的とし、さらには、鍛造性とくに冷間鍛造性にも優れた機械構造用鋼材を提供することを目的とする。
【００１１】
【課題を解決するための手段】
本発明者らは、上記した課題を達成するために、鋼材の被削性、耐疲労特性および冷間鍛造性に及ぼす各種要因について、鋭意研究した。その結果、鋼材中の硫化物系介在物の形態と硬さとの関係が特定の範囲内となる場合にはじめて、鋼材の被削性、耐疲労特性、冷間鍛造性が同時に著しく向上することを見出した。
【００１２】
まず、本発明者らが行った基礎的な実験結果について、 説明する。
Ｃ：0.48質量％、Ｓ：0.02質量％、Te：0.0019質量％を含有するＳ４８Ｃ系鋼製ビレットを分塊圧延と、圧延条件を種々変化させた仕上圧延とにより、棒鋼（φ60mm）とした。得られた棒鋼について、組織試験、工具寿命試験 (被削性試験）、疲労試験、冷間鍛造試験を実施し、被削性、耐疲労特性、冷間鍛造性を評価した。
【００１３】
組織試験は、得られた棒鋼から試験片を採取し、試験片の圧延方向に平行な断面についてエッチングなしで、光学顕微鏡を用いて倍率400 倍で10視野観察し、視野中に観察される全ての硫化物系介在物について、画像解析装置を用いてアスペクト比を測定し、その平均値を求めた。また、圧延方向に平行な断面について、超マイクロヴィッカース硬度計（荷重：9.8 ×10-4Ｎ）用いて、硫化物１００個について硬さを測定し、その平均値を求めた。
【００１４】
工具寿命試験 (被削性試験）は、得られた棒鋼について、超硬工具P10 を使用し、
切り込み：２mm、
送り：0.25mm／rev 、
切削速度：200 ｍ／min 、
潤滑：なし、
の条件で切削を行い、工具の逃げ面摩耗幅ＶＢが0.2 mmとなるまでの時間（sec ）を工具寿命とし、被削性を評価した。
【００１５】
また、疲労試験は、得られた棒鋼（φ60mm）を熱間圧延により20mm厚の鋼板とし、これら鋼板の圧延方向から、回転曲げ疲労試験片（１号試験片）を採取し、JIS Z 2274の規定に準拠して回転曲げ疲労試験を実施した。試験回転数を3000rpm とし、繰り返し数107 回に達する繰り返し応力をもとめ疲れ限度とし、耐疲労特性を評価した。
【００１６】
また、冷間鍛造試験は、得られた棒鋼から試験片（タブレット：φ15mm、高さ22.5mm）を高さが圧延方向に一致するように切り出し、圧縮率を種々変化して圧縮鍛造を行った。各圧縮率で10個のタブレットを圧縮鍛造した。圧縮鍛造後、割れの有無を目視で測定し、各圧縮率での割れ発生率と圧縮率の関係をグラフにプロットし、試験片の50％（５個）が割れる圧縮率を求めた。試験片の50％（５個）が割れる圧縮率を冷間鍛造性の指数とし、鍛造性を評価した。この値が大きいほど鍛造性が良いことになる。
【００１７】
本発明者らは、被削性、耐疲労特性、冷間鍛造性は、硫化物系介在物のアスペクト比ａが小さいほうが有利であるが、これら特性はさらに硫化物系介在物の硬さＨにも影響されるであろうという考えのもとに、得られた被削性 (工具寿命）、耐疲労特性（疲れ限度）、冷間鍛造性（50％圧縮割れ発生率）を、硫化物系介在物の硬さＨとアスペクト比の逆数１／ａとの関係で図示し、図１に示す。図１から、硫化物系介在物の硬さＨとアスペクト比の逆数１／ａの関係が、次（１）式
１／ａ≧5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ………（１）
（ここで、ａ：硫化物系介在物の平均アスペクト比、Ｈ：硫化物系介在物の平均ヴィッカース硬さ）
を満足する範囲となる場合にはじめて、被削性、耐疲労特性、冷間鍛造性が同時に著しく向上することがわかる。ヴィッカース硬さＨとアスペクト比の逆数１／ａとの関係が（１）式を満足しない場合には、被削性、耐疲労特性、冷間鍛造性ともに劣化している。
【００１８】
本発明は、上記した知見に基づき、さらに検討を加え完成されたものである。すなわち、本発明は、硫化物系介在物を含む鋼材であって、質量％で、Ｃ：0.01〜0.8 ％、Si：2.0 ％以下、Mn：0.1 〜2.0 ％、Ｐ：0.1 ％以下、Ｓ：0.004 〜0.1 ％（但し、0.010 ％以下は除く）、Al：0.1 ％以下を含み、さらにTe：0.0005〜0.2 ％、Se：0.0005〜0.2 ％、Zr：0.0005〜0.02％、Mg：0.0005〜0.02％、Ca：0.0005〜0.02％、REM ：0.0005〜0.02％のうちから選ばれた１種または２種以上を含有し、残部Feおよび不可避的不純物からなる組成を有し、前記硫化物系介在物の平均ヴィッカース硬さＨが210 以下であり、かつ、前記硫化物系介在物の、加工方向（圧延方向または鍛造方向）に平行な断面における平均アスペクト比ａおよび平均ヴィッカース硬さＨが、次（１）式
１／ａ≧5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ………（１）
（ここで、ａ：硫化物系介在物の平均アスペクト比、Ｈ：硫化物系介在物の平均ヴィッカース硬さ）
を満足することを特徴とする機械構造用線または機械構造用棒鋼であり、また、本発明では、前記組成に加えてさらに、質量％で、Cu：2.0 ％以下、Ni：2.0 ％以下、Cr：3.0 ％以下、Mo：2.0 ％以下、Nb：0.10％以下、Ｂ：0.004 ％以下のうちから選ばれた１種または２種以上を含有する組成とすることが好ましく、また、前記各組成に加えてさらに、質量％で、Ｗ：0.1 ％以下、Ｖ：0.5 ％以下のうちから選ばれた１種または２種を含有する組成とすることが好ましい。
【００１９】
また、本発明で言う硫化物系介在物は、MnS や、MnS とTe、Se、Zr、Mg、Ca、REM 、Cr等との複合硫化物を指すものとする。
また、本発明は、質量％で、Ｃ：0.01〜0.8 ％、Si：2.0 ％以下、Mn：0.1 〜2.0 ％、Ｐ：0.1 ％以下、Ｓ：0.004 〜0.1 ％（但し、0.010 ％以下を除く）、Al：0.1 ％以下を含み、かつTe：0.0005〜0.2 ％、Se：0.0005〜0.2 ％、Zr：0.0005〜0.02％、Mg：0.0005〜0.02％、Ca：0.0005〜0.02％、REM ：0.0005〜0.02％のうちから選ばれた１種または２種以上を含有し、あるいはさらに、Cu：2.0 ％以下、Ni：2.0 ％以下、Cr：3.0 ％以下、Mo：2.0 ％以下、Nb：0.10％以下、Ｂ：0.004 ％以下のうちから選ばれた１種または２種以上、および／または、Ｗ：0.1 ％以下、Ｖ：0.5 ％以下のうちから選ばれた１種または２種を含有し、残部Feおよび不可避的不純物からなる組成の鋼素材を、1200℃以上に加熱した後、850 ℃以上の温度域で、1 パス当たりの圧下率が20％以下の熱間加工を所定の寸法形状となるまで施し、該熱間加工終了後、直ちに冷却することを特徴とする機械構造用線または機械構造用棒鋼の製造方法である。
【００２０】
【発明の実施の形態】
本発明鋼材では、含まれる硫化物系介在物は、加工方向（圧延方向または鍛造方向）に平行な断面における平均アスペクト比ａおよび平均ヴィッカース硬さＨが、次（１）式
１／ａ≧5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ………（１）
（ここで、ａ：硫化物系介在物の平均アスペクト比、Ｈ：硫化物系介在物の平均ヴィッカース硬さ）
を満足し、かつ硫化物系介在物の平均ヴィッカース硬さＨが 210 以下である。含まれる硫化物系介在物が（１）式を満足しない場合には、展伸したMnＳ等の硫化物系介在物が多いことになり、被削性、耐疲労特性、冷間鍛造性が劣ることになる。なお、本発明でいう「硫化物系介在物の平均アスペクト比」とは、鋼材の加工方向（圧延方向あるいは鍛造方向）に平行な断面についてエッチングなしで、倍率400 倍の光学顕微鏡を用いて10視野観察し、視野中に観察される全ての硫化物系介在物について、画像解析装置を用いてアスペクト比（加工方向とそれに直交する方向の長さ比）を測定し求めた平均値をいうものとする。また、本発明でいう「硫化物系介在物の平均ヴィッカース硬さ」とは、鋼材の加工方向（圧延方向あるいは鍛造方向）に平行な断面について超マイクロヴィッカース硬度計（荷重：9.8 ×10-4Ｎ）用いて、硫化物（MnＳ）１００個についてヴィッカース硬さを測定し求めた平均値をいうものとする。
【００２１】
つぎに、本発明鋼材の組成範囲について説明する。なお、以下、組成についての質量％は単に％と記する。
Ｃ：0.01〜0.8 ％
Ｃは、強度を確保するために必要な元素であり、所定の強度を確保するためには0.01％以上含有する。一方、0.8 ％を超えて含有すると、被削性、耐疲労特性、冷間鍛造性が劣化する。このため、Ｃは0.01〜0.8 ％の範囲に限定する。なお、好ましくは0.3 〜0.8 ％である。
【００２２】
Si：2.0 ％以下
Siは、脱酸剤として作用するとともに強度を増加させる有効な元素であり、0.04％以上含有することが好ましいが、2.0 ％を超えて含有すると、被削性の低下および冷間鍛造性の低下が著しくなる。このため、Siは2.0 ％以下に限定する。なお、好ましくは、0.1 〜1.0 ％である。
【００２３】
Mn：0.1 〜2.0 ％
Mnは、硫化物形成元素であり、Ｓによる延性低下を防止する作用を有する元素であり、このような効果は0.1 ％以上の含有で認められる。 一方、2.0 ％を超える含有は、冷間鍛造性を著しく低下させる。このため、Mnは0.1 〜2.0 ％の範囲に限定する。なお、好ましくは、0.3 〜1.5 ％である。
【００２４】
Ｐ：0.1 ％以下
Ｐは、強度を増加させる元素であるが、0.1 ％を超えて含有すると、熱間加工性が劣化する。このため、Ｐは0.1 ％以下に限定する。なお、好ましくは、0.05％以下である。
Ｓ：0.004 〜0.1 ％（但し、 0.010 ％以下は除く）
Ｓは、被削性の向上に有効な元素であり、本発明では0.004 ％以上の含有を必要とする。一方、0.1 ％を超えて含有すると、疲労強度や冷間鍛造性の低下が顕著となる。このため、Ｓは0.004 ％〜0.1 ％の範囲（但し、 0.010 ％以下は除く）に限定する。なお、好ましくは、0.01〜0.06％（但し、 0.010 ％以下は除く）である。
【００２５】
Al：0.1 ％以下
Alは、脱酸剤として有効に作用する元素であり、また、アルミナ系酸化物は硫化物生成核として、硫化物の均一分散にも有効に作用する。このような効果は0.01％以上の含有で顕著となるが、0.1 ％を超えて過剰に含有すると、硬質のアルミナクラスターが多量に生成され被削性がかえって低下する。このため、Alは0.1 ％以下に限定する。なお、好ましくは、0.08％以下、より好ましくは0.01〜0.06％である。
【００２６】
本発明鋼材は、上記した成分に加えてさらに、Te：0.0005〜0.2 ％、Se：0.0005〜0.2 ％、Zr：0.0005〜0.02％、Mg：0.0005〜0.02％、Ca：0.0005〜0.02％、REM ：0.0005〜0.02％のうちから選ばれた１種または２種以上を含有することが好ましい。
Te、Se、Zr、Mg、Ca、REM はいずれも、MnＳ等の硫化物系介在物を球状化させ、被削性向上、耐疲労性向上に大きく寄与する元素であり、選択して１種以上含有することが好ましい。このような効果を得るためには、Te、Se、Zr、Mg、Ca、REM はいずれも0.0005％以上の含有を必要とする。一方、Te、Seでは0.2 ％を超えて含有すると熱間加工性を劣化させる。また、Zr、Mg、Ca、REM では0.02％を超えて含有しても効果が飽和し、含有量に見合う効果が期待できず経済的に不利となる。このため、Te：0.0005〜0.2 ％、Se：0.0005〜0.2 ％、Zr：0.0005〜0.02％、Mg：0.0005〜0.02％、Ca：0.0005〜0.02％、REM ：0.0005〜0.02％に限定することが好ましい。
【００２７】
なお、Mg、Caは、MnＳ等の硫化物系介在物を球状化させ、被削性向上および耐疲労性向上に大きく寄与するとともに、さらにMnＳ等の硫化物系介在物を均一に分散させる作用が強く、とくにMgとCaを複合含有した場合にその作用がより大きくなる。
また、上記した成分に加えてさらに、Cu：2.0 ％以下、Ni：2.0 ％以下、Cr：3.0 ％以下、Mo：2.0 ％以下、Nb：0.10％以下、Ｂ：0.004 ％以下のうちから選ばれた１種または２種以上を含有することができる。
【００２８】
Cu、Ni、Cr、Mo、Nb、Ｂはいずれも、焼入れ性の向上を介して、さらに強度を上昇させる元素であり、必要に応じ選択して含有できる。
Cu、Ni、Moは、焼入れ性を向上させ、ベイナイト組織形成により強度増加に寄与するうえ、さらに被削性を向上させる。しかし、2.0 ％を超える含有は強度が増加しすぎて、被削性や鍛造性が低下する。このため、Cu、Ni、Moはそれぞれ2.0 ％以下に限定することが好ましい。
【００２９】
Crは、焼入れ性を向上させ、ベイナイト組織形成により強度増加に寄与するうえ、さらに被削性を向上させる。しかし、3.0 ％を超える含有は強度が増加しすぎて、被削性や鍛造性が低下する。このため、Crは3.0 ％以下に限定することが好ましい。
Nbは、焼入れ性を向上させ、ベイナイト組織形成により強度増加に寄与するうえ、さらに被削性を向上させる。しかし、0.10％を超える含有は強度が増加しすぎて、被削性や鍛造性が低下する。このため、Nbは0.10％以下に限定することが好ましい。
【００３０】
Ｂは、焼入れ性を向上させ、ベイナイト組織形成により強度増加に寄与するうえ、さらに被削性を向上させる。しかし、0.004 ％を超える含有は強度が増加しすぎて、被削性や鍛造性が低下する。このため、Ｂは0.004 ％以下に限定することが好ましい。
また、本発明では、上記した成分に加えてさらに、強度上昇を図るために、Ｗ：0.1 ％以下およびＶ：0.5 ％以下のうちから選ばれた１種または２種を含有できる。Ｗ、Ｖはいずれも強度を増加させる作用を有する元素であり、必要に応じ選択して含有できる。
【００３１】
Ｗは、固溶強化により、強度を増加させる作用を有するが、0.1 ％を超えて含有すると被削性や鍛造性が劣化する。このため、Ｗは0.1 ％以下に限定することが好ましい。
Ｖは、Ｖ炭窒化物として析出し、析出強化により、強度を増加させる作用を有するが、0.5 ％を超えて含有する被削性や鍛造性が低下する。このため、Ｖは、0.5 ％以下に限定することが好ましい。
【００３２】
上記した成分以外の残部はFeおよび不可避的不純物である。不可避的不純物としては、Ｏ：0.0100％以下、Ｎ：0.0100％以下が許容できる。
ついで、本発明鋼材の製造方法について説明する。
上記した組成の溶鋼を、 転炉、電気炉等の通常公知の溶製方法を用いて溶製し、連続鋳造法、あるいは造塊−分塊法等の通常公知の鋳造方法で、ビレット、スラブ等の鋼素材とすることが好ましい。
【００３３】
鋼素材は、ついで、1200℃以上に加熱されたのち、熱間加工を施されて所定寸法の鋼材とされる。熱間加工としては、圧延、鍛造とすることが好ましい。
熱間加工の加工温度が低いほど、硫化物系介在物のアスペクト比ａが大きくなり、また熱間加工の１パスあたりの圧下率が大きいほど、硫化物系介在物の硬さＨが高くなる。本発明では、硫化物系介在物が、前記（１）式を満足するように、熱間圧延温度、圧下率を調整することが好ましい。
【００３４】
本発明では、熱間加工は、850 ℃以上の温度域で、1 パス当たりの圧下率が20％以下の熱間加工を所定の寸法形状となるまで施すことが好ましい。熱間加工は、850 ℃以上のできるだけ高温、好ましくは900 〜1100℃で行なうことが好ましい。熱間加工の温度域が850 ℃未満では、MnＳ等の硫化物系介在物の形態が望ましい形態とならない。また、熱間加工は、1 パス当たりの圧下率が20％以下とすることが好ましい。1 パス当たりの圧下率が20％を超えて大きくなると、MnＳ等の硫化物系介在物が展伸しやすくなるとともに、硫化物系介在物の硬さが高くなり、 被削性、耐疲労特性、冷間鍛造性をともに向上させることができなくなる。なお、好ましくは、1 パス当たりの圧下率は10〜20％である。
【００３５】
上記したような製造方法で得られた鋼材では、硫化物系介在物の加工方向（圧延方向または鍛造方向）に平行な断面における平均アスペクト比ａおよび平均ヴィッカース硬さＨが、前記（１）式を満足し、被削性、耐疲労特性および冷間鍛造性がともに優れ、かつ被削性−耐疲労特性バランスに優れた機械構造用鋼材となる。
【００３６】
【実施例】
以下、本発明をさらに実施例に基づいて詳細に説明する。
(実施例１）
表１に示す組成を有するＳ４８Ｃベース鋼を、２ｔ真空溶解炉にて溶製し、造塊ー分塊法でビレット（断面：80mm×600 mm）とした。これらビレットを、表２に示す、加熱温度、熱間加工終了温度、１パス当たりの圧下率、全圧下率の条件で熱間加工（圧延加工）し、φ60mmの棒鋼とし、圧延後直ちに２℃／ｓの冷却速度で500 ℃まで冷却した。得られた棒鋼について、組織試験、工具寿命（被削性）試験、疲労試験、冷間鍛造試験を実施した。
【００３７】
試験方法はつぎのとおりである。
（１）組織試験
得られた棒鋼から試験片を採取し、試験片の圧延方向に平行な断面についてエッチングなしで、光学顕微鏡を用いて倍率400 倍で10視野観察し、視野中に観察される全ての硫化物について、画像解析装置を用いてアスペクト比（圧延方向の介在物長さと圧延方向に垂直方向の介在物長さの比）ａを測定し、その平均値を求めた。また、圧延方向に平行な断面について、超マイクロヴィッカース硬度計（荷重：9.8 ×10-4Ｎ）を用いて、硫化物１００個についてヴィッカース硬さＨを測定し、その平均値を求めた。
（２）工具寿命試験 (被削性試験）
得られた棒鋼について、超硬工具P10 を使用し、
切り込み：２mm、
送り：0.25mm／rev 、
切削速度：200 ｍ／min 、
潤滑：なし、
の条件で切削を行い、工具の逃げ面摩耗幅ＶＢが0.2 mmとなるまでの時間（sec ）を求め、工具寿命とし、被削性を評価した。
（３）疲労試験
得られた棒鋼（φ60mm）を熱間圧延により20mm厚の鋼板とし、これら鋼板の圧延方向（Ｌ）と圧延方向に直角方向（Ｃ）とから、回転曲げ疲労試験片（１号試験片）を採取し、JIS Z 2274の規定に準拠して回転曲げ疲労試験を実施した。試験回転数を3000rpm とし、繰り返し数107 回に達する繰り返し応力をもとめ疲れ限度とし、耐疲労特性を評価した。
（４）冷間鍛造試験
得られた棒鋼から，図２に示すように、試験片（タブレット：φ15mm、高さ22.5mm）を高さが圧延方向に一致するように切り出し、圧縮率を種々変化して圧縮鍛造を行った。圧縮鍛造は、各圧縮率で10個のタブレットを圧縮した。圧縮鍛造後、割れの有無を目視で測定し、各圧縮率での割れ発生率と圧縮率の関係をグラフにプロットし、試験片の50％（５個）が割れる圧縮率を求めた。試験片の50％（５個）が割れる圧縮率を冷間鍛造性の指数とし、鍛造性を評価した。
【００３８】
また、（工具寿命（sec ））×（平均疲れ限度（Ｎ／mm2 ））で定義される値を、被削性−耐疲労特性バランスとして、被削性と耐疲労特性とのバランスを評価した。なお、平均疲れ限度は、｛（圧延方向（Ｌ方向）の平均疲れ限度）＋（圧延と直角方向（Ｃ方向）の平均疲れ限度）｝／２で定義される値とする。
得られた結果を表３に示す。
【００３９】
【表１】
【表２】
【表３】
本発明例は、いずれも被削性−耐疲労特性バランスが高く、被削性、耐疲労特性、冷間鍛造性が同時に顕著に向上している。また耐疲労特性の異方性も低減している。一方、本発明の範囲を外れる比較例は、被削性、耐疲労特性、冷間鍛造性がいずれも劣化している。
(実施例２）
表４に示す組成の鋼塊を100 kg真空溶解炉で製造した。得られた鋼塊を1200℃に加熱したのち、表５に示す条件で熱間圧延を行い、φ60mmの棒鋼とし、圧延後直ちに２℃／ｓの冷却速度で500 ℃まで冷却した。得られた棒鋼について、組織試験、工具寿命（被削性）試験、疲労試験、冷間鍛造試験を実施例１と同様に行なった。
【００４３】
得られた結果を表６に示す。
【００４４】
【表４】
【表５】
【表６】
本発明例は、いずれも被削性−耐疲労特性バランスが高く、被削性、耐疲労特性が同時に顕著に向上している。なお、Ｃ含有量が低い発明例８では被削性が非常に良好であり、冷間鍛造性に優れるものの疲れ限界が低く、逆にＣ含有量が高い発明例12では被削性と疲れ限界には優れるものの冷間鍛造性が低い。Mn含有量が低い発明例13では有効なMnS の形成が抑制されるため被削性が若干悪い。一方、Mn含有量が高い発明例16では冷間鍛造性が低下している。Ｓ含有量が低い参考例17では有効なMnS の形成が抑制されるため、被削性が若干悪い。一方、Ｓ含有量が高い発明例20では、耐疲労特性が若干悪く、異方性も認められる。さらに冷間鍛造性が低下している。
【００４８】
(実施例３）
表７に示す組成の鋼塊を100 kg真空溶解炉で製造した。得られた鋼塊を1200℃に加熱したのち、表８に示す条件で熱間圧延を行い、φ60mmの棒鋼とし、圧延後直ちに２℃／ｓの冷却速度で500 ℃まで冷却した。得られた棒鋼について、組織試験、工具寿命（被削性）試験、疲労試験、冷間鍛造試験を実施例１と同様に行なった。
【００４９】
得られた結果を表９に示す。
【００５０】
【表７】
【表８】
【表９】
本発明例は、いずれも被削性−耐疲労特性バランスが高く、被削性、耐疲労特性、冷間鍛造性が同時に顕著に向上している。一方、本発明範囲を外れる比較例は、被削性、耐疲労特性、冷間鍛造性のうちの少なくとも一つが低下している。なお、Teが高い発明例23では熱間圧延中に割れが発生した。
(実施例４）
表10に示す組成の鋼塊を100 kg真空溶解炉で製造した。得られた鋼塊を1200℃に加熱したのち、表11に示す条件で熱間圧延を行い、φ60mmの棒鋼とし、圧延後直ちに２℃／ｓの冷却速度で500 ℃まで冷却した。得られた棒鋼について、組織試験、工具寿命（被削性）試験、疲労試験、冷間鍛造試験を実施例１と同様に行なった。
【００５４】
得られた結果を表12に示す。
【００５５】
【表１０】
【表１１】
【表１２】
本発明例は、いずれも被削性−耐疲労特性バランスが高く、被削性、耐疲労特性、冷間鍛造性が同時に顕著に向上している。
(実施例５）
表13に示す組成を有するＳ４８Ｃベース鋼を、２ｔ真空溶解炉にて溶製し、造塊−分塊法でビレット（断面：80mm×600 mm）とした。これらビレットを、表14に示す、加熱温度、熱間加工終了温度、１パス当たりの圧下率、全圧下率の条件で熱間加工（圧延加工）し、φ60mmの棒鋼とし、圧延後直ちに２℃／ｓの冷却速度で500 ℃まで冷却した。得られた棒鋼について、実施例１と同様に組織試験、工具寿命（被削性）試験、疲労試験、冷間鍛造試験を実施した。
【００５９】
得られた結果を表15に示す。
【００６０】
【表１３】
【表１４】
【表１５】
本発明例は、いずれも被削性、耐疲労特性が同時に顕著に向上し、高い被削性−耐疲労特性バランスを有している。なお、本発明の好適範囲に比べ、発明例44はＣ含有量が高く、発明例45はSi含有量が高く、発明例46はMn含有量が高く、発明例47はＰ含有量が高く、発明例50はＳ含有量が高く、発明例51はAl含有量が低く、発明例55はCr含有量が高く、それぞれ冷間鍛造性が低下している。
【００６４】
(実施例６）
表16に示す組成の鋼塊を100 kg真空溶解炉で製造した。得られた鋼塊を1200℃に加熱したのち、表17に示す条件で熱間圧延を行い、φ60mmの棒鋼とし、圧延後直ちに２℃／ｓの冷却速度で500 ℃まで冷却した。得られた棒鋼について、組織試験、工具寿命（被削性）試験、疲労試験、冷間鍛造試験を実施例１と同様に行なった。
【００６５】
得られた結果を表18に示す。
【００６６】
【表１６】
【表１７】
【表１８】
本発明例は、いずれも被削性−耐疲労特性バランスが高く、被削性、耐疲労特性、冷間鍛造性が同時に顕著に向上している。
表３、表６、 表９、 表12、表15に記載されたデータ（Ｈ、ａ、工具寿命、平均疲れ限界）を用いて、被削性−耐疲労特性バランスと｛（5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ）×ａ｝との関係をプロットし図３に示す。被削性−耐疲労特性バランスは、（工具寿命）×（平均疲れ限界）であり、平均疲れ限界は｛（Ｌ方向疲れ限界）＋（Ｃ方向疲れ限界）｝／２から計算した。なお、（5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ）×ａは、前記（１）式
１／ａ≧5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ………（１）
を変形して得られた、次式
１≧（5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ）×ａ
の右辺である。
【００７０】
図３から、｛（5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ）×ａ｝が、１以下となる場合には、被削性−耐疲労特性バランス、（工具寿命）×（平均疲れ限界）が顕著に増大している。すなわち、（１）式を満足する鋼材であれば、被削性−耐疲労特性バランスが高い、被削性と耐疲労特性がバランスよく向上した鋼材となる。
【００７１】
【発明の効果】
以上のように、本発明によれば、被削性、疲労強度、さらには冷間鍛造性に優れた機械構造用鋼材を安定して安価に製造でき、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】被削性、耐疲労特性、冷間鍛造性に及ぼす硫化物系介在物のアスペクト比、硬さの関係を示すグラフである。
【図２】冷間鍛造試験用試験片の採取方法および圧縮方法を示す説明図である。
【図３】被削性−耐疲労特性バランスと（5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ）×ａの関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention is a steel material for machine structure suitable for machine parts such as automobiles and electrical equipment.And manufacturing method thereofAbout. The steel material referred to in the present invention is a wire material.OrSteel barIs.
[0002]
[Prior art]
Most automotive parts and electrical equipment parts are manufactured by cutting. For this reason, the steel material used as a material is required to have excellent machinability as well as mechanical properties. As the most general method for improving machinability, a method of incorporating S, Pb or the like as a machinability improving component in steel is adopted, and it is known that Pb is particularly effective when added in a small amount. ing. For example,
[0003]
However, in recent years, environmental issues have been greatly highlighted, and environmental pollution due to Pb has become a concern. Therefore, in the field of materials such as steel, so-called Pb-free, which does not use Pb, is being promoted. However, the present situation is that a Pb-free steel material having a machinability equivalent to or higher than that of Pb-based free-cutting steel has not yet been obtained. Even with S-based free-cutting steel to which S, which is an element that improves machinability, is added as in Pb, a material having the same level of machinability as Pb-based free-cutting steel has not yet been realized. For this reason, development of a steel material that is Pb-free and has machinability at the same level as or higher than that of Pb-based free-cutting steel is eagerly desired.
[0004]
On the other hand, in S-based free-cutting steel, when hot working a steel material by rolling or forging, sulfides such as MnS are elongated in the rolling direction along with the plastic deformation of the base material. Large anisotropy occurs. Specifically, the Charpy impact value in the rolling direction is greatly reduced. In cold forging, there is a problem that microcracks are generated from MnS due to the presence of expanded MnS, which eventually leads to cracking.
[0005]
Many technologies have been proposed to improve the machinability by controlling the morphology (size, aspect ratio, etc.) of sulfide inclusions such as MnS in order to solve such problems in S-based free-cutting steels. Yes. For example, Patent Document 3 proposes a free-cutting steel in which the size and aspect ratio of MnS are controlled within an appropriate range. In the technique described in Patent Document 3, the machinability evaluated by the chip dividing property by the form control of MnS is also effective in improving the mechanical properties evaluated by the toughness value.
[0006]
In Patent Document 4, sulfur free-cutting steel containing S: 0.08 to 0.4% by weight and Ca: 0.01% by weight or less is heated to 1100 to 1300 ° C, and the average rolling temperature is 1050 ° C or higher. A rolling method of sulfur free-cutting steel is described in which rough rolling is performed for 70% or more of the entire processing amount, and finish rolling at 800 to 1000 ° C. is performed for 5 to 30% of the entire processing amount. According to the technique described in Patent Document 4, it is said that sulfur free cutting steel excellent in machinability can be obtained at a low cost by suppressing the elongation of MnS and making the microscopic structure uniform with fine grains.
[0007]
Patent Document 5 contains 0.0005 to 0.02% by mass of Mg, and further controls the distribution state of sulfide inclusion particles, so that machinability, in particular, chip breaking property and tool life, and mechanical properties are improved. Free-cutting steels for machine structures with improved properties, particularly lateral impact values, have been proposed. According to the technique described in Patent Document 5, it is said that a free-cutting steel having mechanical properties and chip breaking properties comparable to conventional Pb-containing steel can be realized even without Pb.
[0008]
[Patent Document 1]
JP 59-205453 A
[Patent Document 2]
JP 62-23970
[Patent Document 3]
JP 2001-152279 A
[Patent Document 4]
JP-A-1-224103
[Patent Document 5]
JP 2002-69569 A
[0009]
[Problems to be solved by the invention]
However, according to the study by the present inventors, the technique described in Patent Document 3 focuses only on the size effect of MnS, and it is possible to obtain a remarkable improvement in fatigue resistance and cold forgeability. There is no problem. Moreover, in the technique described in Patent Document 4, the amount of processing in rough rolling is too large, and MnS is stretched to some extent, so that the fatigue resistance and cold forgeability are not significantly improved. There is a problem.
[0010]
In addition, even products (steel materials) manufactured with the technology described in Patent Document 5 may not always have sufficient fatigue resistance and cold forgeability, and are not steel materials with stable characteristics. The problem was left.
The present invention has been made in view of the state of the prior art as described above, has a machinability comparable to the machinability of Pb-free and conventional Pb free-cutting steel, and has excellent fatigue resistance. The purpose of the present invention is to provide a machine structural steel material excellent in the machinability-fatigue resistance balance, and further to provide a steel for machine structure excellent in forgeability, particularly cold forgeability. .
[0011]
[Means for Solving the Problems]
In order to achieve the above-described problems, the present inventors diligently studied various factors affecting the machinability, fatigue resistance, and cold forgeability of steel materials. As a result, the machinability, fatigue resistance, and cold forgeability of steel materials will be significantly improved only when the relationship between the form and hardness of sulfide inclusions in steel materials is within a specific range. I found it.
[0012]
First, basic experimental results conducted by the present inventors will be described.
A billet made of S48C steel containing C: 0.48% by mass, S: 0.02% by mass, and Te: 0.0019% by mass was made into bar steel (φ60 mm) by split rolling and finish rolling with various rolling conditions changed. The obtained steel bar was subjected to a structure test, a tool life test (machinability test), a fatigue test, and a cold forging test to evaluate machinability, fatigue resistance, and cold forgeability.
[0013]
In the structure test, specimens were taken from the obtained steel bars, and the cross section parallel to the rolling direction of the specimens was observed without etching, using a light microscope to observe 10 fields at 400x magnification. For the sulfide inclusions, the aspect ratio was measured using an image analyzer, and the average value was obtained. Also, for the cross section parallel to the rolling direction, ultra micro Vickers hardness tester (load: 9.8 × 10-FourN) Using 100 sulfides, the hardness was measured and the average value was obtained.
[0014]
The tool life test (machinability test) uses the carbide tool P10 for the obtained steel bar.
Cutting depth: 2mm
Feed: 0.25mm / rev,
Cutting speed: 200 m / min
Lubrication: None,
Cutting was performed under the conditions described above, and the time (sec) until the flank wear width VB of the tool reached 0.2 mm was defined as the tool life, and machinability was evaluated.
[0015]
In the fatigue test, the obtained bar steel (φ60mm) was hot rolled into 20mm-thick steel plates, and a rotating bending fatigue test piece (No. 1 test piece) was taken from the rolling direction of these steel plates, and JIS Z 2274 A rotating bending fatigue test was conducted in accordance with the regulations. The test speed is 3000 rpm and the number of repetitions is 10.7Fatigue resistance was evaluated by determining the repeated stress reaching the rotation and setting it as the fatigue limit.
[0016]
In the cold forging test, a test piece (tablet: φ15mm, height 22.5mm) was cut from the obtained steel bar so that the height coincided with the rolling direction, and compression forging was performed with various changes in the compression ratio. . Ten tablets were compression-forged at each compression ratio. After compression forging, the presence or absence of cracks was visually measured, the relationship between the crack generation rate and the compression rate at each compression rate was plotted on a graph, and the compression rate at which 50% (5 pieces) of the test piece was broken was determined. The compressibility at which 50% (5 pieces) of the test piece breaks was used as an index of cold forgeability, and forgeability was evaluated. The larger this value, the better the forgeability.
[0017]
The inventors of the present invention are advantageous in terms of machinability, fatigue resistance, and cold forgeability when the aspect ratio a of the sulfide inclusions is small. These characteristics are further improved by the hardness H of the sulfide inclusions. The resulting machinability (tool life), fatigue resistance (fatigue limit), cold forgeability (50% compression cracking rate), sulfide FIG. 1 shows the relationship between the hardness H of the system inclusions and the reciprocal 1 / a of the aspect ratio. From FIG. 1, the relationship between the hardness H of sulfide inclusions and the reciprocal 1 / a of the aspect ratio is expressed by the following equation (1).
1 / a ≧ 5.3 × 10-Five× H2-0.0187 × H + 1.849 (1)
(Where, a: average aspect ratio of sulfide inclusions, H: average Vickers hardness of sulfide inclusions)
It is understood that the machinability, fatigue resistance, and cold forgeability are remarkably improved only when the range satisfies the above. When the relationship between the Vickers hardness H and the reciprocal 1 / a of the aspect ratio does not satisfy the formula (1), machinability, fatigue resistance, and cold forgeability are all deteriorated.
[0018]
The present invention has been completed with further studies based on the above findings. That is, the present invention is a steel material containing sulfide inclusions, in mass%, C: 0.01 to 0.8%, Si: 2.0% or less, Mn: 0.1 to 2.0%, P: 0.1% or less, S: 0.004 to 0.1% (excluding 0.010% or less), Al: 0.1% or less, Te: 0.0005-0.2%, Se: 0.0005-0.2%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02% , Ca: 0.0005 to 0.02%, REM: One or more selected from 0.0005 to 0.02%, having a composition consisting of the balance Fe and inevitable impurities, and containing the sulfide inclusions The average aspect ratio a and the average Vickers hardness H in the cross section parallel to the processing direction (rolling direction or forging direction) of the sulfide inclusions are 210 or less and the average Vickers hardness H is (1 )formula
1 / a ≧ 5.3 × 10-Five× H2-0.0187 × H + 1.849 (1)
(Where, a: average aspect ratio of sulfide inclusions, H: average Vickers hardness of sulfide inclusions)
A mechanical structural wire characterized by satisfyingFor machine structureFurther, in the present invention, in addition to the above composition, in the present invention, by mass, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.10% or less , B: It is preferable to use a composition containing one or more selected from 0.004% or less, and in addition to the above-mentioned compositions, W: 0.1% or less, and V: A composition containing one or two selected from 0.5% or less is preferable.
[0019]
In addition, the sulfide inclusions referred to in the present invention refer to complex sulfides of MnS or MnS and Te, Se, Zr, Mg, Ca, REM, Cr, or the like.
In the present invention, C: 0.01 to 0.8%, Si: 2.0% or less, Mn: 0.1 to 2.0%, P: 0.1% or less, S: 0.004 to 0.1% (however, excluding 0.010% or less) ), Al: 0.1% or less, and Te: 0.0005-0.2%, Se: 0.0005-0.2%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02%, Ca: 0.0005-0.02%, REM: 0.0005- Contains one or more selected from 0.02%, or Cu: 2.0% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.10% or less B: One or two or more selected from 0.004% or less and / or W: 0.1% or less, V: One or two selected from 0.5% or less, the balance After heating a steel material composed of Fe and inevitable impurities to 1200 ° C or higher, hot working with a reduction rate of 20% or less per pass is performed at a temperature range of 850 ° C or higher to a specified size and shape. Subjected to that, after the heat-working ends immediately line for machine structural, characterized in that cooling orFor machine structureIt is a manufacturing method of a steel bar.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
In the steel material of the present invention, the sulfide inclusions contained therein have an average aspect ratio a and an average Vickers hardness H in a cross section parallel to the processing direction (rolling direction or forging direction), expressed by the following formula (1):
1 / a ≧ 5.3 × 10-Five× H2-0.0187 × H + 1.849 (1)
(Where, a: average aspect ratio of sulfide inclusions, H: average Vickers hardness of sulfide inclusions)
SatisfiedAnd the average Vickers hardness H of sulfide inclusions is 210 IsThe When the sulfide inclusions contained do not satisfy the formula (1), there are many sulfide inclusions such as expanded MnS, and the machinability, fatigue resistance, and cold forgeability are inferior. It will be. The “average aspect ratio of sulfide inclusions” as used in the present invention means that a cross section parallel to the processing direction (rolling direction or forging direction) of a steel material is not etched, and is measured using an optical microscope with a magnification of 400 times. An average value obtained by observing the field of view and measuring the aspect ratio (length ratio of the machining direction and the direction perpendicular to it) using an image analyzer for all sulfide inclusions observed in the field of view. And The “average Vickers hardness of sulfide inclusions” as used in the present invention is an ultra micro Vickers hardness meter (load: 9.8 × 10 6) for a cross section parallel to the processing direction (rolling direction or forging direction) of the steel material.-FourN) Used to mean the average value obtained by measuring the Vickers hardness of 100 sulfides (MnS).
[0021]
Next, the steel material of the present inventionSet ofThe formation range will be described. Hereinafter, the mass% of the composition is simply referred to as%.
C: 0.01 to 0.8%
C is an element necessary for securing strength, and is contained in an amount of 0.01% or more for securing a predetermined strength.TheOn the other hand, if the content exceeds 0.8%, the machinability, fatigue resistance, and cold forgeability deteriorate. For this reason, C is limited to the range of 0.01 to 0.8%.TheIn additionGoodIt is preferably 0.3 to 0.8%.
[0022]
Si: 2.0% or less
Si is an effective element that acts as a deoxidizer and increases strength, and is preferably contained in an amount of 0.04% or more, but if contained over 2.0%, machinability and cold forgeability are reduced. Becomes remarkable. For this reason, Si is limited to 2.0% or less.TheIn additionGoodPreferably, it is 0.1 to 1.0%.
[0023]
Mn: 0.1 to 2.0%
Mn is a sulfide-forming element and is an element having a function of preventing ductility deterioration due to S, and such an effect is recognized with a content of 0.1% or more. On the other hand, if the content exceeds 2.0%, the cold forgeability is significantly reduced. For this reason, Mn is limited to the range of 0.1 to 2.0%.TheIn additionGoodPreferably, it is 0.3 to 1.5%.
[0024]
P: 0.1% or less
P is an element that increases the strength, but if it exceeds 0.1%, hot workability deteriorates. For this reason, P is limited to 0.1% or less.TheIn additionGoodPreferably, it is 0.05% or less.
S: 0.004 to 0.1%(However, 0.010 % Or less are excluded)
S is an element effective for improving machinability, and in the present invention, S is required to be contained in an amount of 0.004% or more. On the other hand, if the content exceeds 0.1%, the fatigue strength and cold forgeability are significantly reduced. For this reason, S ranges from 0.004% to 0.1%.(However, 0.010 % Or less are excluded)Limited toTheIn additionGoodPreferably, 0.01-0.06%(However, 0.010 % Or less are excluded)It is.
[0025]
Al: 0.1% or less
Al is an element that effectively acts as a deoxidizer, and alumina-based oxides also act effectively on uniform dispersion of sulfides as sulfide nuclei. Such an effect becomes remarkable when the content is 0.01% or more. However, if the content exceeds 0.1%, a large amount of hard alumina clusters are formed, and the machinability is lowered. For this reason, Al is limited to 0.1% or less.TheIn additionGoodPreferably, 0.08% or less,ThanPreferably it is 0.01 to 0.06%.
[0026]
In addition to the above-described components, the steel of the present invention further includes Te: 0.0005 to 0.2%, Se: 0.0005 to 0.2%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, Ca: 0.0005 to 0.02%, REM: It is preferable to contain one or more selected from 0.0005 to 0.02%.
Te, Se, Zr, Mg, Ca, and REM are elements that greatly contribute to improving machinability and fatigue resistance by spheroidizing sulfide inclusions such as MnS. It is preferable to contain above. In order to obtain such an effect, Te, Se, Zr, Mg, Ca, and REM must each contain 0.0005% or more. On the other hand, if Te and Se are contained in excess of 0.2%, hot workability is deteriorated. Further, if Zr, Mg, Ca, and REM are contained over 0.02%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit to Te: 0.0005-0.2%, Se: 0.0005-0.2%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02%, Ca: 0.0005-0.02%, REM: 0.0005-0.02% .
[0027]
Mg and Ca spheroidize sulfide inclusions such as MnS, greatly contribute to improving machinability and fatigue resistance, and further uniformly disperse sulfide inclusions such as MnS. In particular, when Mg and Ca are combined, the effect is greater.
In addition to the above components, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.10% or less, B: 0.004% or less 1 type or 2 types or more can be contained.
[0028]
Cu, Ni, Cr, Mo, Nb, and B are all elements that further increase the strength through improvement of hardenability, and can be selected and contained as necessary.
Cu, Ni, and Mo improve hardenability, contribute to increasing strength by forming a bainite structure, and further improve machinability. However, if the content exceeds 2.0%, the strength increases excessively and the machinability and forgeability deteriorate. For this reason, Cu, Ni, and Mo are each preferably limited to 2.0% or less.
[0029]
Cr improves hardenability, contributes to increasing strength by forming a bainite structure, and further improves machinability. However, if the content exceeds 3.0%, the strength increases too much and the machinability and forgeability deteriorate. For this reason, Cr is preferably limited to 3.0% or less.
Nb improves hardenability, contributes to an increase in strength by forming a bainite structure, and further improves machinability. However, if the content exceeds 0.10%, the strength increases too much, and the machinability and forgeability deteriorate. For this reason, it is preferable to limit Nb to 0.10% or less.
[0030]
B improves hardenability, contributes to an increase in strength by forming a bainite structure, and further improves machinability. However, if the content exceeds 0.004%, the strength increases excessively and the machinability and forgeability deteriorate. For this reason, it is preferable to limit B to 0.004% or less.
Further, in the present invention, in addition to the above-described components, one or two selected from W: 0.1% or less and V: 0.5% or less can be further contained in order to increase the strength. W and V are both elements that have the effect of increasing the strength, and can be selected and contained as necessary.
[0031]
W has the effect of increasing the strength by solid solution strengthening, but if it exceeds 0.1%, the machinability and forgeability deteriorate. For this reason, it is preferable to limit W to 0.1% or less.
V precipitates as V carbonitride and has the effect of increasing the strength by precipitation strengthening, but the machinability and forgeability contained in excess of 0.5% are reduced. For this reason, it is preferable to limit V to 0.5% or less.
[0032]
The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, O: 0.0100% or less and N: 0.0100% or less are acceptable.
Subsequently, the manufacturing method of this invention steel material is demonstrated.
The molten steel having the above composition is melted by using a generally known melting method such as a converter or an electric furnace, and the billet or slab is obtained by a generally known casting method such as a continuous casting method or an ingot-bundling method. It is preferable to use a steel material such as
[0033]
The steel material is then heated to 1200 ° C. or higher and then hot-worked to obtain a steel material having a predetermined size. As hot working, rolling and forging are preferable.
The lower the processing temperature of hot working, the larger the aspect ratio a of sulfide inclusions, and the higher the rolling reduction per pass of hot working, the higher the hardness H of sulfide inclusions. . In the present invention, it is preferable to adjust the hot rolling temperature and the rolling reduction so that the sulfide inclusions satisfy the above formula (1).
[0034]
In the present invention, the hot working is preferably performed in a temperature range of 850 ° C. or higher until the rolling reduction per pass is 20% or less until a predetermined dimensional shape is obtained. The hot working is preferably performed at a temperature as high as possible of 850 ° C. or higher, preferably 900 to 1100 ° C. When the temperature range of hot working is less than 850 ° C., the form of sulfide inclusions such as MnS is not a desirable form. In the hot working, it is preferable that the rolling reduction per pass is 20% or less. If the rolling reduction per pass exceeds 20%, sulfide inclusions such as MnS will easily expand, and the hardness of the sulfide inclusions will increase, resulting in machinability and fatigue resistance. Both cold forgeability cannot be improved. In addition, Preferably, the rolling reduction per pass is 10 to 20%.
[0035]
In the steel material obtained by the manufacturing method as described above, the average aspect ratio a and the average Vickers hardness H in the cross section parallel to the processing direction (rolling direction or forging direction) of the sulfide inclusions are expressed by the above formula (1). Is satisfied, and the steel for machine structural use is excellent in machinability, fatigue resistance and cold forgeability and excellent in machinability-fatigue resistance balance.
[0036]
【Example】
Hereinafter, the present invention will be further described in detail based on examples.
Example 1
S48C base steel having the composition shown in Table 1 was melted in a 2t vacuum melting furnace, and billet (cross section: 80 mm × 600 mm) was formed by the ingot-making and lump method. These billets are hot-worked (rolled) under the conditions of heating temperature, hot-working end temperature, reduction rate per pass, and total reduction rate shown in Table 2 to obtain a φ60 mm steel bar, which is 2 ° C immediately after rolling. It cooled to 500 degreeC with the cooling rate of / s. The obtained steel bar was subjected to a structure test, a tool life (machinability) test, a fatigue test, and a cold forging test.
[0037]
The test method is as follows.
(1) Tissue test
Specimens were collected from the obtained steel bars, and the cross-section parallel to the rolling direction of the specimens was not etched, and was observed in 10 visual fields using an optical microscope at a magnification of 400x, and all sulfides observed in the visual field were observed. The aspect ratio (ratio of inclusion length in the rolling direction to inclusion length in the direction perpendicular to the rolling direction) a was measured using an image analyzer, and the average value was obtained. Also, for the cross section parallel to the rolling direction, ultra micro Vickers hardness tester (load: 9.8 × 10-FourN), Vickers hardness H was measured for 100 sulfides, and the average value was obtained.
(2) Tool life test (Machinability test)
For the obtained steel bar, use carbide tool P10,
Cutting depth: 2mm
Feed: 0.25mm / rev,
Cutting speed: 200 m / min
Lubrication: None,
Cutting was performed under the conditions described above, and the time (sec) until the flank wear width VB of the tool reached 0.2 mm was determined to determine the tool life and to evaluate the machinability.
(3) Fatigue test
The obtained steel bars (φ60mm) were hot rolled into 20mm thick steel plates, and the rotating bending fatigue test piece (No. 1 test piece) was formed from the rolling direction (L) of these steel plates and the direction perpendicular to the rolling direction (C). The samples were collected and subjected to a rotating bending fatigue test in accordance with the provisions of JIS Z 2274. The test speed is 3000 rpm and the number of repetitions is 10.7Fatigue resistance was evaluated by determining the repeated stress reaching the rotation and setting it as the fatigue limit.
(4) Cold forging test
As shown in FIG. 2, a test piece (tablet: φ15 mm, height 22.5 mm) was cut out from the obtained steel bar so that the height coincided with the rolling direction, and compression forging was performed with various compression ratios. . In compression forging, 10 tablets were compressed at each compression rate. After compression forging, the presence or absence of cracks was visually measured, the relationship between the crack generation rate and the compression rate at each compression rate was plotted on a graph, and the compression rate at which 50% (5 pieces) of the test piece was broken was determined. The compressibility at which 50% (5 pieces) of the test piece breaks was used as an index of cold forgeability, and forgeability was evaluated.
[0038]
Also, (tool life (sec)) x (average fatigue limit (N / mm)2)) Was used as the balance between machinability and fatigue resistance, and the balance between machinability and fatigue resistance was evaluated. The average fatigue limit is a value defined by {(average fatigue limit in the rolling direction (L direction)) + (average fatigue limit in the direction perpendicular to the rolling (C direction))} / 2.
The obtained results are shown in Table 3.
[0039]
[Table 1]
[Table 2]
[Table 3]
Each of the inventive examples has a high machinability-fatigue resistance balance, and the machinability, fatigue resistance, and cold forgeability are significantly improved at the same time. Also, the anisotropy of fatigue resistance is reduced. On the other hand, in the comparative examples that are out of the scope of the present invention, the machinability, fatigue resistance, and cold forgeability are all deteriorated.
(Example 2)
Steel ingots having the compositions shown in Table 4 were produced in a 100 kg vacuum melting furnace. The obtained steel ingot was heated to 1200 ° C. and then hot-rolled under the conditions shown in Table 5 to obtain a φ60 mm bar steel, which was immediately cooled to 500 ° C. at a cooling rate of 2 ° C./s. The obtained steel bar was subjected to a structure test, a tool life (machinability) test, a fatigue test, and a cold forging test in the same manner as in Example 1.
[0043]
The results obtained are shown in Table 6.
[0044]
[Table 4]
[Table 5]
[Table 6]
In all of the examples of the present invention, the machinability-fatigue resistance balance is high, and the machinability and fatigue resistance characteristics are significantly improved at the same time. Inventive Example 8 with a low C content has very good machinability and excellent cold forgeability, but the fatigue limit is low. Conversely, in Inventive Example 12 with a high C content, machinability and fatigue limit are low. However, the cold forgeability is low. In Invention Example 13 having a low Mn content, the formation of effective MnS is suppressed, so that the machinability is slightly poor. On the other hand, in the invention example 16 having a high Mn content, the cold forgeability is lowered. Low S contentreferenceIn Example 17, since the formation of effective MnS is suppressed, the machinability is slightly poor. On the other hand, in Invention Example 20 with a high S content, the fatigue resistance is slightly poor and anisotropy is also observed. Furthermore, the cold forgeability is reduced.
[0048]
Example 3
Steel ingots having the compositions shown in Table 7 were produced in a 100 kg vacuum melting furnace. The obtained steel ingot was heated to 1200 ° C., and then hot-rolled under the conditions shown in Table 8 to obtain a φ60 mm steel bar, which was immediately cooled to 500 ° C. at a cooling rate of 2 ° C./s. The obtained steel bar was subjected to a structure test, a tool life (machinability) test, a fatigue test, and a cold forging test in the same manner as in Example 1.
[0049]
Table 9 shows the obtained results.
[0050]
[Table 7]
[Table 8]
[Table 9]
Each of the inventive examples has a high machinability-fatigue resistance balance, and the machinability, fatigue resistance, and cold forgeability are significantly improved at the same time. On the other hand, in a comparative example outside the scope of the present invention, at least one of machinability, fatigue resistance, and cold forgeability is degraded. In Invention Example 23 having a high Te, cracking occurred during hot rolling.
Example 4
Steel ingots having the compositions shown in Table 10 were produced in a 100 kg vacuum melting furnace. The obtained steel ingot was heated to 1200 ° C. and then hot-rolled under the conditions shown in Table 11 to obtain a φ60 mm bar steel, and immediately after rolling, cooled to 500 ° C. at a cooling rate of 2 ° C./s. The obtained steel bar was subjected to a structure test, a tool life (machinability) test, a fatigue test, and a cold forging test in the same manner as in Example 1.
[0054]
The results obtained are shown in Table 12.
[0055]
[Table 10]
[Table 11]
[Table 12]
Each of the inventive examples has a high machinability-fatigue resistance balance, and the machinability, fatigue resistance, and cold forgeability are significantly improved at the same time.
(Example 5)
S48C base steel having the composition shown in Table 13 was melted in a 2t vacuum melting furnace, and billet (cross section: 80 mm × 600 mm) was formed by the ingot-bundling method. These billets are hot-worked (rolled) under the conditions of heating temperature, hot working finish temperature, rolling reduction per pass, and total rolling reduction shown in Table 14 to obtain a φ60mm steel bar, which is 2 ° C immediately after rolling. It cooled to 500 degreeC with the cooling rate of / s. The obtained steel bar was subjected to a structure test, a tool life (machinability) test, a fatigue test, and a cold forging test in the same manner as in Example 1.
[0059]
The results obtained are shown in Table 15.
[0060]
[Table 13]
[Table 14]
[Table 15]
In all of the examples of the present invention, the machinability and fatigue resistance are remarkably improved at the same time, and the machinability-fatigue resistance balance is high. Compared to the preferred range of the present invention, Invention Example 44 has a high C content, Invention Example 45 has a high Si content, Invention Example 46 has a high Mn content, Invention Example 47 has a high P content, Inventive Example 50 has a high S content, Inventive Example 51 has a low Al content, and Inventive Example 55 has a high Cr content, each having a low cold forgeability.
[0064]
Example 6
Steel ingots having the compositions shown in Table 16 were produced in a 100 kg vacuum melting furnace. The obtained steel ingot was heated to 1200 ° C. and then hot-rolled under the conditions shown in Table 17 to obtain a φ60 mm bar steel, which was immediately cooled to 500 ° C. at a cooling rate of 2 ° C./s. The obtained steel bar was subjected to a structure test, a tool life (machinability) test, a fatigue test, and a cold forging test in the same manner as in Example 1.
[0065]
The obtained results are shown in Table 18.
[0066]
[Table 16]
[Table 17]
[Table 18]
Each of the inventive examples has a high machinability-fatigue resistance balance, and the machinability, fatigue resistance, and cold forgeability are significantly improved at the same time.
Using the data (H, a, tool life, average fatigue limit) described in Table 3, Table 6, Table 9, Table 12, Table 15, the machinability-fatigue resistance balance and {(5.3 × 10-Five× H2-0.0187 * H + 1.849) * a} is plotted and shown in FIG. The machinability-fatigue resistance balance was (tool life) × (average fatigue limit), and the average fatigue limit was calculated from {(L direction fatigue limit) + (C direction fatigue limit)} / 2. (5.3 × 10-Five× H2−0.0187 × H + 1.849) × a is the formula (1)
1 / a ≧ 5.3 × 10-Five× H2-0.0187 × H + 1.849 (1)
Obtained by transforming
1 ≧ (5.3 × 10-Five× H2-0.0187 × H + 1.849) × a
The right side of
[0070]
From Fig. 3, {(5.3 × 10-Five× H2When −0.0187 × H + 1.849) × a} is 1 or less, the machinability-fatigue resistance balance, (tool life) × (average fatigue limit), is remarkably increased. That is, if it is steel materials which satisfy | fill (1) Formula, it will become a steel material with high machinability and fatigue resistance characteristics with a good balance between machinability and fatigue resistance characteristics.
[0071]
【The invention's effect】
As described above, according to the present invention, it is possible to stably and inexpensively manufacture a machine structural steel material excellent in machinability, fatigue strength, and cold forgeability, and achieve a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the aspect ratio and hardness of sulfide inclusions on machinability, fatigue resistance, and cold forgeability.
FIG. 2 is an explanatory diagram showing a sampling method and a compression method of a cold forging test specimen.
[Fig.3] Balance between machinability and fatigue resistance and (5.3 × 10-Five× H2-0.0187 × H + 1.849) × a is a graph showing the relationship.
Claims (6)
記
１／ａ≧5.3 ×10-5×Ｈ2 −0.0187×Ｈ＋1.849 ………（１）
ここで、ａ：硫化物系介在物の平均アスペクト比
Ｈ：硫化物系介在物の平均ヴィッカース硬さWire or steel bar containing sulfide inclusions in mass%, C: 0.01 to 0.8%, Si: 2.0% or less, Mn: 0.1 to 2.0%, P: 0.1% or less, S: 0.004 to 0.1% (Except 0.010% or less), Al: 0.1% or less, Te: 0.0005-0.2%, Se: 0.0005-0.2%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02%, Ca: 0.0005 -0.02%, REM: containing one or more selected from 0.0005-0.02%, having a composition consisting of the balance Fe and inevitable impurities, the average Vickers hardness of the sulfide inclusions A machine in which H is 210 or less, and an average aspect ratio a and an average Vickers hardness H of the sulfide inclusions in a cross section parallel to the processing direction satisfy the following expression (1): Structural wire or steel bar for machine structure .
Record
1 / a ≧ 5.3 × 10 −5 × H 2 −0.0187 × H + 1.849 (1)
Where a: average aspect ratio of sulfide inclusions
H: Average Vickers hardness of sulfide inclusions
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| JP4502126B2 (en) * | 2005-03-07 | 2010-07-14 | 住友金属工業株式会社 | Steel for machine structure |
| JP5200634B2 (en) * | 2007-04-11 | 2013-06-05 | 新日鐵住金株式会社 | Hot rolled steel bar for forging and carburizing |
| GB2452092B (en) | 2007-08-24 | 2010-06-23 | Gibbs Tech Ltd | Improvements in or relating to amphibians |
| CN113913704B (en) * | 2021-12-13 | 2022-03-11 | 北京科技大学 | Tellurium-sulfur co-processed aluminum deoxidized steel and preparation method and application thereof |
| CN114959501B (en) * | 2022-06-29 | 2023-04-07 | 马鞍山钢铁股份有限公司 | Steel for Te microalloying high-carbon expansion fracture connecting rod and manufacturing method and application thereof |
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| JP2550640B2 (en) * | 1988-03-01 | 1996-11-06 | 日本鋼管株式会社 | Rolling method for sulfur free-cutting steel |
| JPH06145783A (en) * | 1992-11-05 | 1994-05-27 | Sumitomo Metal Ind Ltd | Treatment for steel for pressure vessel |
| JPH06299235A (en) * | 1993-02-18 | 1994-10-25 | Kobe Steel Ltd | Manufacture of steel for die having hrc 27 or higher hardness and excellent machinability |
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