JP4494676B2

JP4494676B2 - Machine structural steel with excellent machinability

Info

Publication number: JP4494676B2
Application number: JP2001222249A
Authority: JP
Inventors: 雅之橋村; 真小此木; 秀雄蟹澤
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-07-23
Filing date: 2001-07-23
Publication date: 2010-06-30
Anticipated expiration: 2021-07-23
Also published as: JP2003034840A

Description

【０００１】
【発明の属する技術分野】
本発明は被削性および冷間鍛造性に優れた機械構造用鋼に関わる。ここでいう被削性とは切削工具寿命、切削表面粗さおよび切り屑処理性である。
【０００２】
【従来の技術】
通常の炭素鋼レベルの炭素量を有する鋼でも炭素を黒鉛化し、フェライト＋グラファイトの２相組織とすることで、冷間加工性と被削性が向上することが特開平３−１４０４１１号公報などに見られる。しかしそのような組織を実現するためには長時間の焼鈍が必要であり、生産能率とコストの点で問題があった。従って焼鈍時間の短縮が課題であった。
【０００３】
切削に関しても切削工具寿命は同一Ｃ量で黒鉛を有しない鋼や、軟質の快削鋼に比べて良好である。その理由は切削工具刃先に強固な構成刃先が生成し、それが切削工具の保護膜となるためと考えられる。黒鉛鋼は他の鋼に比べ構成刃先が生成しやすく、その大きさは大きい。しかしこのことは逆に安定した表面創成が困難であり、表面粗さが劣化するという問題を引き起こす。さらに黒鉛鋼は母材が軟質であるため、切り屑も延性に富むので分断されにくく、切り屑処理性に劣る。このようなことから黒鉛鋼はＳＡＥ１２Ｌ１４やＳＵＭ２３に代表される良好な切削工具寿命と表面品質を兼備した従来の低炭快削鋼の代替にはなっていなかった。つまり黒鉛鋼は特に表面品質の点で大きく従来の快削鋼に劣っていた。
【０００４】
この表面粗さの改善には鋼を硬質にすることが有効であるが、それは逆に切削工具寿命を低下させることになるので、切削工具寿命と表面品質、切り屑処理性のすべてを兼ね備えることはできなかった。
【０００５】
また切り屑処理性についてもＭｎＳの活用やＣａ等による軟質酸化物等の活用が考えられるが、Ｍｎは黒鉛化を阻害する代表的元素であり、黒鉛鋼のための焼鈍時間が長くなり、実用工業レベルでは実用化できなかった。また黒鉛化焼鈍の時間はＡｌの添加によって短縮できることが知られているが、Ａｌを添加するとＣａ等による軟質酸化物が生成しない。さらにはＡｌ添加によって生じるＡｌ₂Ｏ₃系酸化物は硬質のため、切削工具寿命を低下させる。
【０００６】
このように短時間の焼鈍時間で主にフェライト＋グラファイトの２相組織とすることが可能で、長い切削工具寿命と高品質の表面品質をした良好な被削性を有する鋼の開発が課題であった。
【０００７】
【発明が解決しようとする課題】
本発明は短時間の焼鈍で軟質化するとともに黒鉛を含む組織となって冷間鍛造性に優れ、さらに切削においては切削工具寿命、切削面の表面粗さおよび切り屑の処理性に優れた機械構造用鋼を提供しようとするものである。
【０００８】
【課題を解決するための手段】
本発明は上記の課題を解決するためになされ、その要旨は
（１）質量％で、
Ｃ：０．３〜２．０％、
Ｓｉ：０．５〜２．０％、
Ｍｎ：０．０５〜３．０％、
Ｐ：０．００１〜０．１％、
Ｓ：０．０１〜０．７％、
Ａｌ：０．００１〜０．０１％、
Ｎ：０．００１〜０．００６％
を含み、残部はＦｅおよび不可避的不純物からなり、ＭｎとＳの含有比率が質量％で２≦［Ｍｎ％］／［Ｓ％］≦６かつ鋼中Ｃが黒鉛として存在する比率（黒鉛率：黒鉛として析出した炭素量／鋼中炭素含有量）が８０％を超える組織を有することを特徴とする被削性に優れた機械構造用鋼。
【０００９】
（２）さらに質量％で、
Ｚｒ：０．０００５〜０．００４％、
Ｃａ：０．０００３〜０．００４％、
Ｌａ：０．０００５〜０．００２％、
Ｃｅ：０．０００５〜０．００２％、
Ｍｇ：０．０００５〜０．００４％
の１種または２種以上を含むことを特徴とする上記（１）記載の被削性に優れた機械構造用鋼。
【００１０】
（３）さらに質量％で、
Ｂ：０．０００１〜０．００６％
を含むことを特徴とする上記（１）または（２）に記載の被削性に優れた機械構造用鋼。
【００１１】
（４）さらに質量％で、
Ｃｒ：０．０５〜０．５％、
Ｔｉ：０．００５〜０．０１％、
Ｖ：０．０１〜０．１％、
Ｎｂ：０．００５〜０．０４％、
Ｍｏ：０．０５〜０．５％、
Ｗ：０．０５〜０．５％
の１種または２種以上を含むことを特徴とする上記（１）〜（３）の内のいずれかに記載の被削性に優れた機械構造用鋼。
【００１２】
（５）さらに質量％で、
Ｐｂ：０．０１〜０．０５％、
Ｂｉ：０．０１〜０．１％、
Ｔｅ：０．０００５〜０．０１％、
Ｓｅ：０．０００５〜０．０１％、
Ｓｎ：０．０１〜０．５％
の１種または２種以上を含むことを特徴とする上記（１）〜（４）のいずれかに記載の被削性に優れた機械構造用鋼。
【００１３】
（６）さらに質量％で、
Ｎｉ：０．０５〜３．０％、
Ｃｕ：０．１〜３．０％、
Ｃｏ：０．１〜３．０％
の１種または２種以上を含むことを特徴とする上記（１）〜（５）の内のいずれかに記載の被削性に優れた機械構造用鋼。
【００１４】
【発明の実施の形態】
以下に本発明を詳細に説明する。
【００１５】
まず、本発明で鋼成分を限定した理由を説明する。
【００１６】
Ｃ含有量は黒鉛量を確保して被削性を向上させるために０．３％以上必要であり、これ未満の場合には黒鉛量が少なく、被削性、特に切削工具寿命延長効果が少ない。また強度が要求される部材に使用する場合には、焼入れ焼き戻しを施して部品としての強度を確保するが、その際の強度はＣ量に依存するため、強度部材に使用する場合でも０．３％以上の含有量が必要である。また被削性の観点からは黒鉛は刃先近傍での鋼の変形を容易にするとともに、切り屑−母材分離時の破壊起点となる。従ってＣ量を多くして黒鉛量が多くすることが好ましいが、多すぎると熱間延性が低下し、鋳造や圧延等の製造工程にて割れを生じやすくなる。そのため製造工程における割れを防止するために２．０％を上限とした。
【００１７】
Ｓｉは鋼中の炭素活量を大きくすることにより、黒鉛化を促進する作用がある。０．５％未満ではその効果が小さいので、その下限値を０．５％とした。また２．０％を超えると、フェライト硬さが大きくなって硬化したり、硬質なＳｉ系酸化物を生成するので工具寿命を損なう。さらに鋼の靭性が損なわれるなどの弊害が顕著となる。そこで上限値を２．０％とした。またＳｉは黒鉛化率を調整する元素として使用でき、含有量が低いほど焼鈍後の黒鉛化率が小さくなる。
【００１８】
Ｍｎは鋼中硫黄をＭｎＳとして固定・分散させるために必要な量およびマトリックスに固溶させて焼入れ後の強度を確保するために必要な量を加算した量が必要であり、その下限値は０．０５％である。しかしＭｎ量が大きくなると素地の硬さが大きくなり冷間加工性が低下する上、Ｍｎは黒鉛化阻害元素であり、後述するようにＳとの関係を考慮する必要がある。Ｓ添加量との関係を考慮する必要があるが、Ｓ添加量の上限を考慮するとＭｎ添加量の上限値は３．０％である。これ以上のＭｎ添加は黒鉛化に要する焼鈍時間が長くなり、工業的に成り立たなくなる。また素地の硬さが大きくなり冷間鍛造性を低下させる。
【００１９】
Ｐは鋼中において素地の硬さが大きくなり、冷間加工性が低下するので、その上限を０．１％にしなければならない。一方、表面粗さを必要とする鋼の場合には０．００１％未満ではその効果が認められないので０．００１％を下限とした。
【００２０】
Ｓは一般に被削性を向上させる元素として知られている。しかし被削性を向上させるにはＭｎＳ介在物として存在することが重要で、Ｓと結合して硫化物を生成するに十分なＭｎが添加されていることが必要である。Ｓ添加量は０．０１％未満では被削性向上効果が認められず、０．７％を超えると鋳造や圧延等の製造工程で割れを生じやすくなったり、冷間加工性を低下させたりするので、Ｓ添加量を０．０１〜０．７％とした。
【００２１】
さらにＭｎおよびＳと被削性および黒鉛化挙動との関係を述べる。またその関係をもとに２≦Ｍｎ／Ｓ≦６と規定した理由を詳しく述べる。
【００２２】
鋼にＭｎおよびＳを添加すると、その一部がＭｎＳを生成する。鋼中ＭｎＳは切削工具刃先近傍で切り屑生成のための破壊起点となったり、工具すくい面上での潤滑効果により鋼の切削工具寿命を向上させる。一方、黒鉛鋼は黒鉛が変形と破壊の点から被削性を向上させることは前述したとおりである。その黒鉛の介在した切り屑分離では切削工具上に構成刃先を生成しやすくなり、さらに構成刃先が強固に工具と凝着することで工具保護膜となり、構成刃先が実質の刃先となるために切削工具寿命を延長すると考えられる。
【００２３】
しかし構成刃先による切削では安定した表面創成が困難であり、表面粗さが劣化し、構成刃先が大きい方が表面粗さが粗い。従って表面粗さの小さな良好な表面を得るためには構成刃先の成長を抑制することが重要で、さらに構成刃先の成長を抑制するためには工具すくい面上の潤滑を大きくすることが重要との結論に至った。そこで研究の結果、構成刃先を制御する手法として黒鉛鋼においても黒鉛とともにＭｎＳを十分に存在させることで切削表面粗さが改善されることを見いだした。
【００２４】
ＭｎＳを十分に存在させるには、ＭｎとＳを多量に添加することが重要であるが、Ｍｎ、Ｓの両者とも黒鉛化阻害元素であることが知られ、ＭｎまたはＳを単独で多く添加すると黒鉛化できないことが知られている。
【００２５】
しかし研究の結果、ＭｎおよびＳを多量に添加しても、そのほとんどをＭｎＳとして鋼中に存在させ、マトリックス中に残存するＭｎおよびＳ量を抑制することで黒鉛化を阻害しないことを見いだした。黒鉛鋼の製造工程を考えた場合、ＭｎＳは鋳造末期に生成する。そのため添加量を適正に調整することで黒鉛化焼鈍時には添加したＭｎとＳのほとんどをＭｎＳとしておくことが可能であり、黒鉛化を短時間で終了させることが可能である。すなわちＭｎとＳの添加量比率を２≦Ｍｎ／Ｓ≦６とすることで添加したＭｎとＳのほとんどをＭｎＳとして鋼中に分布させるとともに、黒鉛化を短時間で終了させ、黒鉛とＭｎＳを多量に鋼中に含む鋼を作れることを見いだした。
【００２６】
このように鋼中に黒鉛とＭｎＳを分布させることによって、良好な切削工具寿命、切削表面粗さおよび切り屑処理性を達成できることを見いだした。ここで重要なことは通常ＭｎＳが存在すると切削工具寿命が向上すると考えられているが、本発明の黒鉛鋼の場合、ＭｎＳは工具保護膜である構成刃先の成長を抑制するため、切削工具寿命はむしろ低下する傾向にある。
【００２７】
Ａｌは鋼の脱酸元素であり脱酸して圧延時の表面キズを防止するために必要であるが、その添加量が多くなると鋼中で多量のＡｌ₂Ｏ₃系の酸化物を生成する。しかしＡｌ₂Ｏ₃は硬質であり、切削工具寿命を低下させる。従ってその上限を０．０１％とした。一方Ａｌは黒鉛化促進元素であり、黒鉛化阻害元素であるＮをＡｌＮとして固定することで黒鉛化を促進できる。また添加量が微量の場合にはＳｉ、Ｍｎ等と複合酸化物を生成して切削工具寿命に対して無害であるため、下限を０．００１％とした。
【００２８】
Ｎに関しては窒化物として存在しない固溶窒素はセメンタイト中に溶けこみ、セメンタイトの分解を阻害することから、黒鉛化阻害元素となる。また被削性の観点からも固溶窒素はマトリックスを硬化させるため、切削工具寿命を低下させる。一方、ＡｌＮ等の窒化物は黒鉛析出核となるため、黒鉛化を促進する。０．００６％を超えると固溶窒素が多くなり黒鉛化を阻害し、切削工具寿命を低下させるためこれを上限とした。窒化物を生成する０．００１％を下限とした。
【００２９】
Ｚｒは酸化物、窒化物、炭化物、硫化物を形成する。それらは析出核として黒鉛化焼鈍時間を短縮する。また窒化物生成時には固溶Ｎを低減させる。またＭｎＳなどの硫化物の形状を球状化させ、機械的性質の圧延異方性を緩和することができる。さらに焼入れ性も向上させることができる。Ｚｒが０．０００５％未満ではその効果が小さく、０．００４％を超えるとその効果が飽和するだけでなく、Ｚｒ系硫化物、窒化物等も生成し、それらがクラスターを造ることによって機械的性質を損なったり切削工具寿命を低下させたりする。
【００３０】
ＣａはＭｎＳの球状化による圧延異方性の緩和と被削性向上を必要とする場合に有効である。また析出したＣａ系介在物は黒鉛の析出核として作用する。しかしＣａは多量に添加しすぎると硬質な酸化物、硫化物を生じ、被削性や機械的性質を低下させる。従って適正量を添加することが重要である。被削性向上等の効果は０．０００３％未満では効果が小さく、０．００４％を超えると析出物によって被削性や機械的性質を損なうおそれがあるため、これを上限とした。
【００３１】
ＬａおよびＣｅはいわゆるＲＥＭともよばれ、脱酸効果がある。黒鉛化には酸化物として黒鉛の析出核になるため適度の添加は好ましい。しかし多量の添加では酸化物が硬質化し、切削工具寿命を低下させたり、クラスターを生じて延性を損なうなどの弊害を多く生じる。そこでＬａは析出核としての効果が期待できる０．０００５％を下限とし、硬質な酸化物によって弊害を生じない０．００２％を上限とした。またＣｅに関しても析出核としての効果が期待できる０．０００５％を下限とし、硬質な酸化物によって弊害を生じない０．００２％を上限とした。
【００３２】
ＭｇはＭｇＯなどの酸化物生成元素であるとともに、硫化物を生成する。ＭｇＳはＭｎＳなどと共存することも多い。このような酸化物、硫化物は黒鉛析出核になり、黒鉛の微細分散と焼鈍時間の短縮に有用である。その効果はＭｇ０．０００５％未満では認められず、０．００４％を超えると酸化物、硫化物を多く生成し、鋼の被削性や機械的性質に悪影響を及ぼす。従ってＭｇ０．０００５〜０．００４％とした。
【００３３】
ＢはＮと反応してオーステナイト結晶粒界にＢＮとして析出するので黒鉛化を阻害する固溶Ｎ低減に役立つ。またＢＮの結晶構造は黒鉛と同じく六方晶系であり、黒鉛の析出核となる。また固溶Ｂは焼入れ性を向上させる元素であり、焼入れ性を必要とする場合に添加することが望ましい。その効果は０．０００１％未満では認められず、０．００６％を超えるとＢＮを析出させる効果や焼入れ性向上効果が飽和するので上限を０．００６％以下とした。
【００３４】
Ｃｒは焼入れ性向上元素であるが、同時に黒鉛化阻害元素である。そのため焼入れ性向上が必要な場合には０．０５％以上の添加を必要とする。しかし多量に添加すると黒鉛化を阻害するので焼鈍時間が長くなるため、０．５％を上限とした。
【００３５】
Ｔｉは鋼中でＴｉＮを形成し、オーステナイト粒径を小さくする。黒鉛は旧オーステナイト粒界や析出物という、いわば格子の不均一部に析出する傾向にあり、Ｔｉの炭窒化物は黒鉛の析出核としての役割と、オーステナイト粒径微細化による黒鉛析出核の創出という役割を担う。さらにＮを窒化物として固定するために、固溶Ｎを低減させる。Ｔｉが０．００５％未満ではその効果が小さく、０．０１％を超えるとその効果が飽和するとともに、多くのＴｉＮが析出して機械的性質を損なう。またＴｉは炭化物を形成し、セメンタイトを安定化するため、過剰なＴｉ添加は焼鈍による黒鉛化および軟質化を阻害する。そのため上限を０．０１％とした。
【００３６】
Ｖは炭窒化物を形成し、オーステナイト粒径微細化と析出核の両面で黒鉛化焼鈍時間を短縮する。また窒化物生成時に黒鉛化を阻害する固溶Ｎを低減させる。Ｖが０．０１％未満ではその効果が小さく、０．１％を超えるとＴｉと同様、炭化物を形成し、セメンタイトを安定化するため、過剰なＶ添加は焼鈍による黒鉛化および軟質化を阻害する。そのため黒鉛化を阻害しない０．１％を上限とした。
【００３７】
Ｎｂは炭窒化物を形成し、オーステナイト粒径微細化と析出核の両面で黒鉛化焼鈍時間を短縮する。また窒化物生成時に固溶Ｎを低減させる。Ｎｂが０．００５％未満ではその効果が小さく、０．０４％を超えるとその効果が飽和するとともに、多くの未溶解炭化物が残留するために機械的性質を損なう。またＴｉおよびＶと同様、炭化物を形成し、セメンタイトを安定化するため、過剰なＮｂ添加は焼鈍による黒鉛化および軟質化を阻害する。そのため黒鉛化を阻害しない０．０４％を上限とした。
【００３８】
Ｍｏは焼入れ後の強度を増加させるが、０．０５％未満ではその効果が小さくいので、０．０５％を下限とした。Ｍｏは炭化物を生じやすく炭素の活量を低下させる元素で黒鉛化を阻害する元素である。ただし黒鉛化阻害効果はＴｉ、Ｖ等よりも小さい。そこで黒鉛化阻害効果が顕著となる０．５％を上限とし、黒鉛の核生成を大きく阻害しない添加量にとどめた。ただし他の焼入れ性向上元素に比べ、黒鉛化阻害の程度が小さいので、焼入れ性を向上させるために指定した範囲内でＭｏ添加量を多くすればよい。
【００３９】
Ｗは焼入れ後の強度を増加させる。炭化物を生成しやすく、炭素の活量を低下させるため、黒鉛化を阻害する。しかし黒鉛化阻害の程度が小さいので焼入れ性を向上させる場合には指定した範囲で添加すればよい。０．０５％未満では焼入れ性および焼戻し軟化抵抗の増加に寄与しない。また０．５％を超えて添加すると黒鉛化が阻害させるので０．５％を上限とした。
【００４０】
Ｐｂは被削性向上元素である。被削性を必要とする場合には０．０１％以上必要であり、０．０５％を超えると黒鉛化を阻害するとともに圧延きずなどの製造上の問題を生じるため、これを上限とした。
【００４１】
Ｂｉは被削性向上に有効で、０．０１％未満ではその効果が小さく、０．１％以上ではその効果が飽和するのでこれを上限とした。
【００４２】
Ｔｅは被削性向上元素であるとともいに、ＭｎＳの球状化による圧延異方性の緩和に役立つ。０．０００５％未満では効果が小さく、０．０１％を超えると黒鉛化阻害や圧延きずなどの問題を引き起こすので、これを上限とした。
【００４３】
Ｓｅは被削性向上に有効で、０．０００５％未満ではその効果が小さく、０．０１％以上ではその効果が飽和するのでこれを上限とした。
【００４４】
Ｓｎは鋼を脆化させるために切り屑処理性向上に効果がある。しかし黒鉛化を阻害する元素であり、０．０１％未満では効果が無く、０．５％を超えると黒鉛化阻害効果が顕著になる。そこでＳｎ添加量を０．０１〜０．５％と規定した。
【００４５】
Ｎｉは黒鉛化挙動にほとんど影響せず、黒鉛化を阻害させずに焼入れ性や耐食性を向上させることができる。またＣｕを添加する場合には熱間延性を確保するために添加しても製造工程における割れの発生等を抑制できる。０．０５％未満ではその効果が小さいので下限を０．０５％とした。しかしマトリックスを硬化させることにくわえて延性が増すために、切削においては切削工具寿命と切り屑処理性を劣化させる。３．０％以上では特にその効果が顕著なため、上限を３．０％とした。
【００４６】
Ｃｕは黒鉛化挙動にほとんど影響せず耐食性の向上に効果がある。０．１％未満ではその効果が認められず、３．０％以上では鋳造、圧延等の製造工程で粒界を劣化させ、割れを生じ強卯になるので、これを上限とした。
【００４７】
Ｃｏは高温強度を向上させるとともに黒鉛化を促進させる効果がある。０．１％未満ではその効果は認められない。添加量が３．０％を超えると高温強度や延性の向上のための工具寿命低下が顕著になるのでこれを上限とした。
【００４８】
鋼中Ｃは大部分がセメンタイトまたは黒鉛として存在するが、黒鉛は劈開性を有するので容易に変形できる。マトリックスが軟質であれば冷間鍛造性に優れ、切削時には内部潤滑剤と破壊起点の両方の機能から被削性を向上させる。黒鉛化率に関しては焼鈍後に次式で示される黒鉛化率を求める。
黒鉛化率（％）＝（鋼中黒鉛含有量／鋼の炭素含有量）×１００
ここで、炭素含有量および黒鉛含有量は化学分析により定量分析結果である。この黒鉛化率が８０％以下では黒鉛化率が不十分であり、軟質化していないだけでなく、黒鉛の有する切削切り屑分離特性等の被削性向上メカニズムが機能しなくなる。そこで黒鉛化率の下限を８０％を超えるものとした。この結果、従来鋼ではセメンタイトとして強度に寄与していたCの大半が黒鉛として鋼中に存在するため、軟質化し、ビッカース硬さではＨＶ１４０程度かそれ以下となる。
【００４９】
【実施例】
表１、表２に示す化学成分を有する鋼を溶製し、７５０〜８５０℃でφ５０ｍｍに圧延した。比較例を含む一部の試験片については１２００℃以上で鍛造した。圧延材はＣ量が０．５％以下のものに関しては圧延直後に８００〜９００℃からオンライン水冷装置によって水冷した。またその他の実施例に関しては空冷した。このように冷却した熱処理材を再度６９０℃に加熱し、２４時間焼鈍した。
【００５０】
黒鉛化が進行するに従って硬度が低下する。表には６９０℃によって２４時間焼鈍後のビッカース硬度を表記した。
【００５１】
【表１】

【００５２】
【表２】

【００５３】
切削試験はφ５ｍｍの高速度鋼ドリルによる孔あけ加工で、切削条件は切削速度を変化させ、工具寿命１０００ｍｍ以上となるドリル周速度いわゆるＶＬ１０００（ｍ／ｍｉｎ）を被削性の指標として用いた。なお送り量は０．３３ｍｍ／ｒｅｖで不水溶性油を用いた湿式切削である。
【００５４】
さらに切削表面粗さを評価するため、被削材を回転させ、工具を半径方向にのみ送ることで丸棒に溝加工を施す切削、いわゆるプランジ切削加工を行った。その概要は図１に示す。切削条件は切削速度８０ｍ／ｍｉｎ、工具送り０．０５ｍｍ／ｒｅｖで、２．５ｓ切削後、工具を引き抜き６ｓ間空転させる操作を１サイクルとし、切削により次々と溝が丸棒表面に創成されるので、その１００サイクル目の溝底の切削表面粗さを測定した。表面粗さはＪＩＳＢ０６０１に準拠した十点平均粗さＲｚを用いた。
【００５５】
また１２００℃まで加熱して９００℃まで放冷して据え込み鍛造したときの割れの有無を目視によって判定した。熱間における据え込み試験片はφ２０ｍｍ×３０ｍｍで熱電対を取り付けてあり、高周波により１２００℃まで加熱し、加熱終了後鍛造用平面ダイス上で９００℃まで温度が下がるのを待ってひずみ８０％で据え込み鍛造を行った。ここでひずみとは下記式（１）で定義される、いわゆる公称ひずみである。
【００５６】
ε＝（Ｈ₀−Ｈ）／Ｈ₀ ・・・（１）
ここでε：ひずみ、Ｈ₀：変形前の試験片高さ、Ｈ：変形後の試験片高さである。
【００５７】
表中にはその判定結果を○×によって示し、×は外周部に大きな割れが生じ不適と判定された例である。
【００５８】
表１および２中の化学成分に関する網がけは本発明の規定外の部分であり、評価結果（硬さ、ドリル被削性、切削表面粗さ、熱間延性割れ有無）の網がけはその結果生じた不適である。実施例１０８には従来の硫黄快削鋼ＳＵＭ２３を比較のために示した。
【００５９】
発明例と比較例の比較が示すように、Ｓｉ、Ｎ、Ｍｎ／Ｓ、Ｃｒ、Ｔｉ、Ｖ、Ｐｂ等の規定が本発明の規定から外れると黒鉛化が遅れ、２４時間の焼鈍では未だ硬質のため、工具寿命の点で大きく劣る。これらを発明例と同様の硬度にするにはさらに長時間の焼鈍が必要となる。このように所定時間で軟質化できなかった比較例に関しては切削表面粗さや熱間延性に関して評価しなかった。
【００６０】
さらにＡｌ、Ｚｒ、Ｌａ、Ｃｅ等の規定が本発明と異なると、鋼中に生成される酸化物が硬質であったり、窒化物、硫化物等のクラスターを生じることから、２４時間程度の焼鈍で軟質化は可能であるものの、ドリル被削性のような工具寿命はやはり発明例に大きく劣る。
【００６１】
切削表面粗さの点では適度のＳ添加が重要である。実施例３７および８９に見られるようなＳ添加量を抑制して軟質化の焼鈍温度短縮を図ると、表面粗さの点で劣る。
【００６２】
しかし表面粗さの改善を目的としてＳを添加した場合でもＭｎ／Ｓが規定外の場合には黒鉛化が遅れ、２４時間程度の焼鈍では軟質化できない。このことは切削性能だけでなく冷間鍛造性に劣ることを意味する。図２にＭｎ／Ｓと２４時間焼鈍後の硬さの関係を示す。Ｍｎ／Ｓ＞６の場合、すなわちＭｎが過剰な場合、焼鈍時間２４時間ではＨＶ１４０以下への軟質化は達成できず、軟質化するにはかなりの焼鈍時間延長が必要になる。またＭｎ／Ｓ＜２の場合、すなわちＳが過剰な場合も同様に焼鈍時間２４時間ではＨＶ１４０以下への軟質化は達成できず、軟質化するにはかなりの焼鈍時間延長が必要になる。さらにＳが過剰な場合には熱間延性も極端に低下させ、割れが発生し、鋳造、圧延時の割れ発生の原因となる。このようにＳ添加によって高性能化を図るにはＭｎ／Ｓが非常に重要であることがわかる。
【００６３】
【発明の効果】
本発明の鋼はＭｎＳと黒鉛の効果により優れた切削工具寿命と高品質の切削表面粗さの兼備を可能にした。さらにＭｎ／Ｓを適正にすることで軟質化の焼鈍時間を短縮でき、容易に良好な被削性、冷間鍛造性を得ることができる。さらに熱間延性にも優れるため、実工程での製造が容易になり、従来より高性能な快削鋼を供することができる。
【図面の簡単な説明】
【図１】プランジ切削方法を示す図である。
【図２】Ｍｎ／Ｓと２４時間焼鈍後の硬さの関係を示す図である。
【符号の説明】
１切削工具
２切削面[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a machine structural steel excellent in machinability and cold forgeability. The machinability here refers to cutting tool life, cutting surface roughness, and chip disposal.
[0002]
[Prior art]
Japanese Patent Application Laid-Open No. 3-140411 discloses that even a steel having a carbon amount at a normal carbon steel level graphitizes carbon to form a ferrite + graphite two-phase structure, thereby improving cold workability and machinability. Seen in. However, in order to realize such a structure, annealing for a long time is necessary, and there are problems in terms of production efficiency and cost. Therefore, shortening the annealing time has been a problem.
[0003]
Regarding cutting, the cutting tool life is better than steel with the same C content and no graphite or soft free-cutting steel. The reason is considered to be that a strong component cutting edge is generated at the cutting tool cutting edge, which becomes a protective film of the cutting tool. Compared to other steels, graphite steel is easier to produce a cutting edge and its size is large. However, this causes a problem that it is difficult to create a stable surface and the surface roughness is deteriorated. Further, since graphite steel has a soft base material, chips are also highly ductile and therefore difficult to be cut, resulting in poor chip disposal. For this reason, graphite steel has not been a substitute for conventional low-carbon free-cutting steel that combines good cutting tool life and surface quality, such as SAE12L14 and SUM23. In other words, graphite steel is greatly inferior to conventional free-cutting steel, particularly in terms of surface quality.
[0004]
To improve the surface roughness, it is effective to harden the steel, but it will reduce the cutting tool life, so it has both cutting tool life, surface quality and chip disposal. I couldn't.
[0005]
As for chip disposal, it is conceivable to use MnS or soft oxides such as Ca, but Mn is a typical element that inhibits graphitization, and the annealing time for graphite steel becomes longer, making it practical. It could not be put into practical use at the industrial level. Further, it is known that the graphitization annealing time can be shortened by adding Al, but when Al is added, a soft oxide such as Ca is not generated. Furthermore, since the Al ₂ O ₃ oxide generated by the addition of Al is hard, it reduces the cutting tool life.
[0006]
Thus, the development of steel with good machinability with a long cutting tool life and high surface quality that can be mainly made of ferrite + graphite two-phase structure in a short annealing time. there were.
[0007]
[Problems to be solved by the invention]
The present invention is a machine that is softened by annealing in a short time and has a structure containing graphite and is excellent in cold forgeability. Further, in cutting, it is a machine with excellent cutting tool life, surface roughness of the cutting surface and chip disposability. It is intended to provide structural steel.
[0008]
[Means for Solving the Problems]
The present invention has been made to solve the above problems, and the gist thereof is (1) mass%,
C: 0.3 to 2.0%,
Si: 0.5 to 2.0%,
Mn: 0.05 to 3.0%
P: 0.001 to 0.1%,
S: 0.01-0.7%,
Al: 0.001 to 0.01%
N: 0.001 to 0.006%
The balance consists of Fe and inevitable impurities, and the content ratio of Mn and S is 2 ≦ [Mn%] / [S%] ≦ 6 and the ratio of C in the steel as graphite (graphite ratio: A machine structural steel excellent in machinability, characterized by having a structure in which the amount of carbon precipitated as graphite / the carbon content in the steel exceeds 80%.
[0009]
(2) Furthermore, in mass%,
Zr: 0.0005 to 0.004%,
Ca: 0.0003 to 0.004%,
La: 0.0005 to 0.002%,
Ce: 0.0005 to 0.002%,
Mg: 0.0005 to 0.004%
The machine structural steel having excellent machinability according to the above (1), comprising one or more of the above.
[0010]
(3) Furthermore, in mass%,
B: 0.0001 to 0.006%
The machine structural steel excellent in machinability according to the above (1) or (2), characterized by comprising
[0011]
(4) Furthermore, in mass%,
Cr: 0.05 to 0.5%,
Ti: 0.005-0.01%,
V: 0.01 to 0.1%
Nb: 0.005 to 0.04%,
Mo: 0.05-0.5%
W: 0.05-0.5%
1 or 2 types or more of said (1)-(3) characterized by the above-mentioned in machinability steel excellent in machinability characterized by the above-mentioned.
[0012]
(5) Furthermore, in mass%,
Pb: 0.01 to 0.05%,
Bi: 0.01 to 0.1%
Te: 0.0005 to 0.01%,
Se: 0.0005 to 0.01%,
Sn: 0.01-0.5%
The machine structural steel having excellent machinability according to any one of the above (1) to (4), comprising one or more of the above.
[0013]
(6) Furthermore, in mass%,
Ni: 0.05-3.0%,
Cu: 0.1 to 3.0%,
Co: 0.1-3.0%
The machine structural steel having excellent machinability according to any one of the above (1) to (5), comprising one or more of the above.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
[0015]
First, the reason why the steel components are limited in the present invention will be described.
[0016]
The C content is required to be 0.3% or more in order to secure the graphite amount and improve the machinability. When the C content is less than this, the graphite amount is small, and the machinability, particularly the cutting tool life extension effect is small. . In addition, when used for a member that requires strength, quenching and tempering are performed to ensure the strength as a part. However, since the strength at that time depends on the amount of C, even when used for a strength member, the strength is 0. A content of 3% or more is necessary. From the viewpoint of machinability, graphite facilitates deformation of the steel in the vicinity of the cutting edge, and also serves as a starting point for fracture during chip-base material separation. Therefore, it is preferable to increase the amount of graphite by increasing the amount of C. However, if the amount is too large, the hot ductility is lowered, and cracking is likely to occur in a manufacturing process such as casting or rolling. Therefore, 2.0% was made the upper limit to prevent cracks in the manufacturing process.
[0017]
Si has the effect of promoting graphitization by increasing the carbon activity in the steel. If less than 0.5%, the effect is small, so the lower limit was set to 0.5%. On the other hand, if it exceeds 2.0%, the hardness of the ferrite is increased and hardened, or a hard Si-based oxide is produced, so that the tool life is impaired. Furthermore, harmful effects such as loss of the toughness of steel become remarkable. Therefore, the upper limit is set to 2.0%. Si can be used as an element for adjusting the graphitization rate, and the lower the content, the smaller the graphitization rate after annealing.
[0018]
Mn requires an amount required to fix and disperse sulfur in steel as MnS, and an amount obtained by adding a necessary amount to ensure strength after quenching by solid solution in the matrix, and its lower limit is 0. .05%. However, as the amount of Mn increases, the hardness of the substrate increases and cold workability decreases, and Mn is a graphitization-inhibiting element, and it is necessary to consider the relationship with S as described later. Although it is necessary to consider the relationship with the S addition amount, considering the upper limit of the S addition amount, the upper limit value of the Mn addition amount is 3.0%. When Mn is added more than this, the annealing time required for graphitization becomes longer, and it is not industrially feasible. Moreover, the hardness of the substrate is increased and the cold forgeability is lowered.
[0019]
P increases the hardness of the substrate in the steel and decreases the cold workability, so the upper limit must be 0.1%. On the other hand, in the case of steel requiring surface roughness, if less than 0.001%, the effect is not recognized, so 0.001% was made the lower limit.
[0020]
S is generally known as an element that improves machinability. However, in order to improve machinability, it is important to exist as MnS inclusions, and it is necessary to add Mn enough to combine with S to produce sulfide. If the amount of S added is less than 0.01%, the machinability improving effect is not recognized. If it exceeds 0.7%, cracks are likely to occur in the manufacturing process such as casting and rolling, and cold workability is reduced. Therefore, the amount of S added is set to 0.01 to 0.7%.
[0021]
Further, the relationship between Mn and S, machinability and graphitization behavior will be described. The reason why it is defined as 2 ≦ Mn / S ≦ 6 based on the relationship will be described in detail.
[0022]
When Mn and S are added to the steel, some of them produce MnS. MnS in steel serves as a fracture starting point for chip generation in the vicinity of the cutting tool cutting edge, and improves the cutting tool life of steel by a lubricating effect on the tool rake face. On the other hand, as described above, graphite steel improves machinability in terms of deformation and fracture. The graphite-containing chip separation makes it easy to generate a component edge on the cutting tool, and the component edge firmly adheres to the tool to form a tool protection film, and the component edge becomes a substantial edge. It is thought to extend the tool life.
[0023]
However, it is difficult to create a stable surface by cutting with the constituent cutting edge, the surface roughness is deteriorated, and the larger the constituent cutting edge is, the rougher the surface roughness is. Therefore, in order to obtain a good surface with a small surface roughness, it is important to suppress the growth of the constituent cutting edges, and in order to further suppress the growth of the constituent cutting edges, it is important to increase the lubrication on the tool rake face. The conclusion was reached. As a result of the research, it was found that the cutting surface roughness was improved by making MnS sufficiently present together with graphite in the graphite steel as a method for controlling the constituent cutting edges.
[0024]
In order to make MnS sufficiently present, it is important to add a large amount of Mn and S. However, it is known that both Mn and S are graphitization inhibiting elements. It is known that it cannot be graphitized.
[0025]
However, as a result of research, even when a large amount of Mn and S was added, it was found that most of them existed in the steel as MnS, and the graphitization was not inhibited by suppressing the amount of Mn and S remaining in the matrix. . When considering the manufacturing process of graphite steel, MnS is produced at the end of casting. Therefore, by adjusting the addition amount appropriately, most of Mn and S added during graphitization annealing can be made MnS, and graphitization can be completed in a short time. That is, most of the added Mn and S are distributed in the steel as MnS by setting the ratio of addition amount of Mn and S to 2 ≦ Mn / S ≦ 6, and the graphitization is completed in a short time. I found that I can make a large amount of steel.
[0026]
Thus, it has been found that by distributing graphite and MnS in steel, it is possible to achieve good cutting tool life, cutting surface roughness and chip disposal. What is important here is that it is considered that the life of the cutting tool is usually improved when MnS is present. However, in the case of the graphite steel of the present invention, MnS suppresses the growth of the component cutting edge which is a tool protective film. Rather tend to decline.
[0027]
Al is a deoxidizing element for steel and is necessary to deoxidize and prevent surface scratches during rolling. However, when the amount added is increased, a large amount of Al ₂ O ₃ oxide is produced in the steel. . However, Al ₂ O ₃ is hard and reduces the cutting tool life. Therefore, the upper limit was made 0.01%. On the other hand, Al is a graphitization promoting element, and graphitization can be promoted by fixing N, which is a graphitization inhibiting element, as AlN. In addition, when the addition amount is very small, a composite oxide is formed with Si, Mn, etc. and is harmless to the cutting tool life, so the lower limit was made 0.001%.
[0028]
Regarding N, solute nitrogen that does not exist as a nitride dissolves in cementite and inhibits the decomposition of cementite, so that it becomes a graphitization inhibiting element. In addition, from the viewpoint of machinability, solute nitrogen hardens the matrix, thereby reducing the cutting tool life. On the other hand, nitrides such as AlN serve as graphite precipitation nuclei and promote graphitization. If it exceeds 0.006%, the amount of dissolved nitrogen increases to inhibit graphitization, and the cutting tool life is shortened. The lower limit was 0.001% for forming nitride.
[0029]
Zr forms oxides, nitrides, carbides and sulfides. They reduce the graphitization annealing time as precipitation nuclei. Further, the solute N is reduced during the formation of nitride. Moreover, the shape of sulfides such as MnS can be spheroidized and the rolling anisotropy of mechanical properties can be relaxed. Furthermore, hardenability can also be improved. If Zr is less than 0.0005%, the effect is small, and if it exceeds 0.004%, the effect is not only saturated, but also Zr-based sulfides, nitrides, etc. are generated and mechanically formed by forming clusters. The properties are impaired and the cutting tool life is shortened.
[0030]
Ca is effective when it is necessary to reduce rolling anisotropy and improve machinability by spheroidizing MnS. The precipitated Ca-based inclusions act as graphite precipitation nuclei. However, if Ca is added in a large amount, hard oxides and sulfides are formed, and machinability and mechanical properties are deteriorated. Therefore, it is important to add an appropriate amount. The effect of improving the machinability is small if it is less than 0.0003%, and if it exceeds 0.004%, there is a possibility that the machinability and mechanical properties may be impaired by precipitates, so this was made the upper limit.
[0031]
La and Ce are also called so-called REM and have a deoxidizing effect. Appropriate addition is preferable for graphitization because it becomes a precipitation nucleus of graphite as an oxide. However, if added in a large amount, the oxide becomes hard, which causes many adverse effects such as shortening of the cutting tool life and loss of ductility by forming clusters. Therefore, La has a lower limit of 0.0005% at which the effect as precipitation nuclei can be expected, and an upper limit of 0.002% at which no harmful effects are caused by hard oxides. Regarding Ce, the lower limit was set to 0.0005% at which an effect as a precipitation nucleus can be expected, and the upper limit was set to 0.002% at which no harmful effect is caused by a hard oxide.
[0032]
Mg is an oxide-generating element such as MgO and generates sulfide. MgS often coexists with MnS or the like. Such oxides and sulfides become graphite precipitation nuclei and are useful for finely dispersing graphite and shortening the annealing time. The effect is not recognized when Mg is less than 0.0005%, and when it exceeds 0.004%, a large amount of oxides and sulfides are generated, which adversely affects the machinability and mechanical properties of steel. Therefore, Mg is set to 0.0005 to 0.004%.
[0033]
Since B reacts with N and precipitates as BN at the austenite grain boundary, it helps to reduce solute N which inhibits graphitization. The crystal structure of BN is a hexagonal system similar to that of graphite, and becomes a precipitation nucleus of graphite. Solid solution B is an element that improves hardenability, and it is desirable to add it when hardenability is required. The effect is not recognized if it is less than 0.0001%, and if it exceeds 0.006%, the effect of precipitating BN and the effect of improving hardenability are saturated, so the upper limit was made 0.006% or less.
[0034]
Cr is an element that improves hardenability, but at the same time is an element that inhibits graphitization. Therefore, when improvement of hardenability is required, addition of 0.05% or more is required. However, if added in a large amount, the graphitization is inhibited, so the annealing time becomes long, so 0.5% was made the upper limit.
[0035]
Ti forms TiN in the steel and reduces the austenite grain size. Graphite tends to precipitate in the former austenite grain boundaries and precipitates, in other words, in the heterogeneous part of the lattice, and Ti carbonitride plays a role as graphite precipitation nuclei and creates graphite precipitation nuclei by refining the austenite grain size. To play a role. Further, in order to fix N as a nitride, solid solution N is reduced. If Ti is less than 0.005%, the effect is small, and if it exceeds 0.01%, the effect is saturated and a large amount of TiN precipitates to impair mechanical properties. Further, Ti forms carbides and stabilizes cementite, so excessive addition of Ti inhibits graphitization and softening due to annealing. Therefore, the upper limit was made 0.01%.
[0036]
V forms carbonitride and shortens the graphitization annealing time on both sides of the austenite grain size refinement and precipitation nuclei. Moreover, the solid solution N which inhibits graphitization at the time of nitride production is reduced. When V is less than 0.01%, the effect is small, and when it exceeds 0.1%, carbide is formed and cementite is stabilized like Ti, so excessive addition of V inhibits graphitization and softening due to annealing. To do. Therefore, the upper limit is set to 0.1% which does not inhibit graphitization.
[0037]
Nb forms carbonitrides and shortens the graphitization annealing time on both the austenite grain size refinement and precipitation nuclei. Further, the solid solution N is reduced when the nitride is formed. If Nb is less than 0.005%, the effect is small, and if it exceeds 0.04%, the effect is saturated, and many undissolved carbides remain, and mechanical properties are impaired. Further, like Ti and V, carbides are formed and cementite is stabilized, so excessive Nb addition inhibits graphitization and softening by annealing. Therefore, the upper limit is set to 0.04% which does not inhibit graphitization.
[0038]
Mo increases the strength after quenching, but if less than 0.05%, the effect is small, so 0.05% was made the lower limit. Mo is an element that easily forms carbides and decreases the activity of carbon, and is an element that inhibits graphitization. However, the graphitization inhibitory effect is smaller than Ti, V and the like. Therefore, the upper limit is set to 0.5% at which the graphitization inhibitory effect becomes significant, and the addition amount is not limited so as not to significantly inhibit the nucleation of graphite. However, since the degree of inhibition of graphitization is small compared to other hardenability improving elements, the Mo addition amount may be increased within a specified range in order to improve hardenability.
[0039]
W increases the strength after quenching. It is easy to produce carbides and lowers the carbon activity, thus inhibiting graphitization. However, since the degree of graphitization inhibition is small, when the hardenability is improved, it may be added within the specified range. If it is less than 0.05%, it does not contribute to an increase in hardenability and temper softening resistance. Further, if added over 0.5%, graphitization is inhibited, so 0.5% was made the upper limit.
[0040]
Pb is a machinability improving element. When machinability is required, it is necessary to be 0.01% or more. If it exceeds 0.05%, graphitization is inhibited and production problems such as rolling flaws occur, so this was made the upper limit.
[0041]
Bi is effective for improving the machinability, and if it is less than 0.01%, the effect is small, and if it is 0.1% or more, the effect is saturated.
[0042]
Te is an element that improves machinability and helps to mitigate rolling anisotropy by spheroidizing MnS. If it is less than 0.0005%, the effect is small, and if it exceeds 0.01%, problems such as graphitization inhibition and rolling flaws are caused, so this was made the upper limit.
[0043]
Se is effective in improving the machinability, and its effect is small when it is less than 0.0005%, and the effect is saturated when it is 0.01% or more.
[0044]
Sn is effective in improving chip disposal because it embrittles steel. However, it is an element that inhibits graphitization, and if it is less than 0.01%, there is no effect, and if it exceeds 0.5%, the graphitization inhibition effect becomes significant. Therefore, the Sn addition amount is defined as 0.01 to 0.5%.
[0045]
Ni hardly affects graphitization behavior, and can improve hardenability and corrosion resistance without inhibiting graphitization. Further, when Cu is added, cracks and the like in the manufacturing process can be suppressed even if Cu is added to ensure hot ductility. If less than 0.05%, the effect is small, so the lower limit was made 0.05%. However, since the ductility increases in addition to hardening the matrix, cutting tool life and chip disposal are deteriorated in cutting. The effect is particularly remarkable at 3.0% or more, so the upper limit was made 3.0%.
[0046]
Cu has little effect on graphitization behavior and is effective in improving corrosion resistance. If it is less than 0.1%, the effect is not recognized, and if it is 3.0% or more, the grain boundary is deteriorated in the manufacturing process such as casting and rolling, and cracks are formed and become strong.
[0047]
Co has the effect of improving high temperature strength and promoting graphitization. If it is less than 0.1%, the effect is not recognized. When the addition amount exceeds 3.0%, the tool life for improving the high-temperature strength and ductility becomes remarkable, so this was made the upper limit.
[0048]
Most of C in steel exists as cementite or graphite, but graphite has a cleavage property and can be easily deformed. If the matrix is soft, it is excellent in cold forgeability and improves machinability from the functions of both the internal lubricant and the fracture starting point during cutting. Regarding the graphitization rate, the graphitization rate represented by the following formula is obtained after annealing.
Graphitization rate (%) = (graphite content in steel / carbon content of steel) × 100
Here, the carbon content and the graphite content are quantitative analysis results by chemical analysis. If the graphitization rate is 80% or less, the graphitization rate is not sufficient and not softened, and the machinability improving mechanism such as cutting chip separation characteristics of graphite does not function. Therefore, the lower limit of the graphitization rate is set to exceed 80%. As a result, most of the C that has contributed to the strength as cementite in the conventional steel is present in the steel as graphite, so it softens, and the Vickers hardness is about HV140 or less.
[0049]
【Example】
Steels having chemical components shown in Tables 1 and 2 were melted and rolled to Φ50 mm at 750 to 850 ° C. Some test pieces including comparative examples were forged at 1200 ° C. or higher. As for the rolled material, the C content of 0.5% or less was water-cooled from 800 to 900 ° C. by an on-line water cooling device immediately after rolling. Other examples were air-cooled. The heat-treated material thus cooled was again heated to 690 ° C. and annealed for 24 hours.
[0050]
Hardness decreases as graphitization proceeds. The table shows the Vickers hardness after annealing for 24 hours at 690 ° C.
[0051]
[Table 1]

[0052]
[Table 2]

[0053]
The cutting test was drilling with a high-speed steel drill of φ5 mm, and the cutting conditions were such that the cutting speed was changed, and the drill peripheral speed at which the tool life was 1000 mm or more, so-called VL1000 (m / min) was used as an index of machinability. The feed amount is 0.33 mm / rev and wet cutting using water-insoluble oil.
[0054]
Further, in order to evaluate the cutting surface roughness, the work material was rotated, and cutting in which a round bar was grooved by sending the tool only in the radial direction, so-called plunge cutting, was performed. The outline is shown in FIG. Cutting conditions are a cutting speed of 80 m / min, a tool feed of 0.05 mm / rev, and after 2.5 s cutting, the operation of drawing out the tool and idling for 6 s is one cycle, and grooves are successively created on the surface of the round bar by cutting. Therefore, the cutting surface roughness of the groove bottom at the 100th cycle was measured. As the surface roughness, ten-point average roughness Rz based on JISB0601 was used.
[0055]
Moreover, the presence or absence of the crack when heating to 1200 degreeC and standing to cool to 900 degreeC and carrying out upset forging was judged by visual observation. The hot upset test piece is φ20mm × 30mm and has a thermocouple attached, heated to 1200 ° C by high frequency, waited for the temperature to drop to 900 ° C on the flat die for forging, and strain was 80%. Upset forging was performed. Here, the strain is a so-called nominal strain defined by the following formula (1).
[0056]
ε = (H ₀ −H) / H ₀ (1)
Here, ε: strain, H ₀ : test piece height before deformation, H: test piece height after deformation.
[0057]
In the table, the determination result is indicated by ○ ×, where × is an example in which a large crack occurs in the outer peripheral portion and is determined to be inappropriate.
[0058]
The netting relating to the chemical components in Tables 1 and 2 is a part not specified in the present invention, and the netting of the evaluation results (hardness, drill machinability, cutting surface roughness, hot ductility crack presence / absence) is the result. Inadequate resulting. Example 108 shows a conventional sulfur free-cutting steel SUM23 for comparison.
[0059]
As the comparison between the inventive example and the comparative example shows, graphitization is delayed when the specifications of Si, N, Mn / S, Cr, Ti, V, Pb, etc. deviate from the specifications of the present invention, and still hard after 24 hours of annealing. Therefore, it is greatly inferior in terms of tool life. In order to obtain the same hardness as in the invention examples, it is necessary to perform annealing for a longer time. Thus, the comparative example which could not be softened in a predetermined time was not evaluated with respect to cutting surface roughness and hot ductility.
[0060]
Furthermore, if the specifications of Al, Zr, La, Ce, etc. are different from those of the present invention, the oxide produced in the steel is hard, or a cluster of nitride, sulfide, etc. is produced, so annealing for about 24 hours. However, the tool life such as drill machinability is still inferior to that of the invention example.
[0061]
Appropriate addition of S is important in terms of cutting surface roughness. When the amount of S added as in Examples 37 and 89 is suppressed to reduce the annealing temperature for softening, the surface roughness is inferior.
[0062]
However, even when S is added for the purpose of improving the surface roughness, if Mn / S is not specified, graphitization is delayed, and it cannot be softened by annealing for about 24 hours. This means that not only cutting performance but also cold forgeability is inferior. FIG. 2 shows the relationship between Mn / S and the hardness after 24-hour annealing. In the case of Mn / S> 6, that is, when Mn is excessive, softening to HV140 or less cannot be achieved with an annealing time of 24 hours, and considerable annealing time extension is required for softening. Similarly, when Mn / S <2, that is, when S is excessive, softening to HV140 or less cannot be achieved with an annealing time of 24 hours, and considerable annealing time extension is required for softening. Further, when S is excessive, the hot ductility is also extremely reduced, cracks are generated, which causes cracks during casting and rolling. Thus, it can be seen that Mn / S is very important for improving the performance by adding S.
[0063]
【The invention's effect】
The steel of the present invention can combine excellent cutting tool life and high quality cutting surface roughness due to the effects of MnS and graphite. Furthermore, by making Mn / S appropriate, the annealing time for softening can be shortened, and good machinability and cold forgeability can be easily obtained. Furthermore, since it is excellent also in hot ductility, manufacture in an actual process becomes easy and can provide free-cutting steel with higher performance than before.
[Brief description of the drawings]
FIG. 1 is a diagram showing a plunge cutting method.
FIG. 2 is a diagram showing a relationship between Mn / S and hardness after 24 hours annealing.
[Explanation of symbols]
1 Cutting tool 2 Cutting surface

Claims

% By mass
C: 0.3 to 2.0%,
Si: 0.5 to 2.0%,
Mn: 0.05 to 3.0%
P: 0.001 to 0.1%,
S: 0.01-0.7%,
Al: 0.001 to 0.01%
N: 0.001 to 0.006%
The balance consists of Fe and inevitable impurities, and the content ratio of Mn and S is 2 ≦ [Mn%] / [S%] ≦ 6 and the ratio of C in the steel as graphite (graphite ratio: A machine structural steel excellent in machinability, characterized by having a structure in which the amount of carbon precipitated as graphite / the carbon content in the steel exceeds 80%.

In addition,
Zr: 0.0005 to 0.004%,
Ca: 0.0003 to 0.004%,
La: 0.0005 to 0.002%,
Ce: 0.0005 to 0.002%,
Mg: 0.0005 to 0.004%
The steel for machine structure excellent in machinability according to claim 1, comprising one or more of the following.

In addition,
B: 0.0001 to 0.006%
The steel for machine structure excellent in machinability of Claim 1 or 2 characterized by the above-mentioned.

In addition,
Cr: 0.05 to 0.5%,
Ti: 0.005-0.01%,
V: 0.01 to 0.1%
Nb: 0.005 to 0.04%,
Mo: 0.05-0.5%
W: 0.05-0.5%
The machine structural steel excellent in machinability according to any one of claims 1 to 3, wherein one or more of the above are included.

In addition,
Pb: 0.01 to 0.05%,
Bi: 0.01 to 0.1%
Te: 0.0005 to 0.01%,
Se: 0.0005 to 0.01%,
Sn: 0.01-0.5%
The machine structural steel excellent in machinability according to any one of claims 1 to 4, comprising one or more of the following.

In addition,
Ni: 0.05-3.0%,
Cu: 0.1 to 3.0%,
Co: 0.1-3.0%
The steel for machine structure excellent in machinability in any one of Claims 1-5 characterized by including 1 type, or 2 or more types of these.