JP4264174B2

JP4264174B2 - Steel bar for machine structure with excellent chip separation and its manufacturing method

Info

Publication number: JP4264174B2
Application number: JP2000020651A
Authority: JP
Inventors: 高裕工藤; 守賀金丸; 浩家口; 武広土田; 勝彦尾崎; 雅実染川
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2000-01-28
Filing date: 2000-01-28
Publication date: 2009-05-13
Anticipated expiration: 2020-01-28
Also published as: JP2001214239A

Description

【０００１】
【発明の属する技術分野】
本発明は切屑分断性に優れた機械構造用鋼とその製法に関し、より詳細には、鋼材中に特定の金属酸化物を特定量含有させることによって、切屑分断性を高めた機械構造用鋼とその製法に関するものである。
【０００２】
【従来の技術】
鉄鋼材料は、様々の製品形状に加工するため切削加工されることが多いことから、これまでにも多くの快削鋼が開発されてきた。そして切削加工性の中でも、切削加工の自動化を支障なく円滑に進めるうえで、切削片が速やかに分断されて工具から離脱していく切屑分断性は重要な特性とされている。
【０００３】
ところで従来から最も多用されてきた鉛快削鋼は、鋼中に添加された低融点の鉛が切削加工時の被削材の昇温により溶融して切屑分断性を高める作用を発揮し、且つ機械的特性の劣化も比較的少ないことから、機械構造用鋼として広く使用されてきた。しかし近年、環境問題に対する認識が高まってくるにつれて、有害重金属である鉛を被削性成分として含む鉛快削鋼は忌避される傾向にある。
【０００４】
これに対し硫黄快削鋼は、鉛快削鋼に指摘される公害問題を起こすことがなく、しかも工具寿命や切屑分断性などにも優れていることから、鉛快削鋼に代わる快削鋼としての需要が高まってきている。ところが硫黄快削鋼は、圧延工程で鋼中の硫化物が圧延方向に伸張する傾向があり、圧延方向に対して垂直方向（以下、Ｃ方向と略記する）の機械的性質に悪影響を及ぼすばかりでなく、熱間鍛造時に硫化物が起点となって割れを起こし易くなるという問題があるため、Ｓ添加量には限界があり、その適用範囲も自ずと制限される。
【０００５】
更にＣａ快削鋼は、Ｓｉ，Ａ１，Ｃａの複合酸化物を制御することにより切削工具表面にベラーグと呼ばれる保護膜を形成させるものであるが、これらの酸化物は融点が１４００℃前後と非常に高く、超硬工具を用いた高切削速度領域での工具寿命の改善には有効であるが、低切削速度領域では満足のいく被削性改善効果を示さず、また切屑分断性も良好とはいえない。
【０００６】
【発明が解決しようとする課題】
上記の様に従来の快削鋼は、環境問題から使用が忌避されたり、あるいは切削条件等の制限を受けるなど、更なる改善が求められる。本発明はこの様な実状に鑑みてなされたもので、環境汚染の問題を起こす恐れがなく、しかも切削条件や用途制限などを受けることなく幅広く適用することができ、特に切削作業を自動化するうえで極めて重要な切屑分断性に優れた機械構造用鋼を提供することにある。
【０００７】
【課題を解決するための手段】
上記課題を解決することのできた本発明にかかる切屑分断性に優れた機械構造用鋼とは、質量％で
Ｃ：０．１〜０．６％
Ｓｉ：２．５％以下（０％を含まない）
Ｍｎ：０．２〜３．０％
Ｓ：０．１５０％未満（０％を含まない）
Ｏ：０．００１〜０．０１５％
を含む機械構造用鋼において、縦断面に現われる長径１μｍ以上の介在物が断面積１ｍｍ²当たり５０〜１５００個で、該介在物の全量中に占める酸化物の個数の割合が１０％以上で、且つ、該酸化物のうち、Ｎａ，ＬｉおよびＢよりなる群から選択される少なくとも１種の元素の酸化物の個数が５％以上であり、あるいは
上記酸化物のうち、Ｎａ，Ｌｉ，ＢおよびＳｉよりなる群から選択される少なくとも２種の元素の酸化物からなる複合酸化物の個数割合が５％であるところに要旨を有している。
【０００８】
また本発明に係る製法の構成は、上記切屑分断性に優れた機械構造用鋼を製造する方法であって、Ｎａ，ＬｉおよびＢよりなる群から選択される少なくとも１種の元素の酸化物からなる融点が１０００℃以下の酸化物を、溶鋼中に１００ｐｐｍ以上添加し、あるいはＮａ，Ｌｉ，ＢおよびＳｉよりなる群から選択される少なくとも２種の元素の酸化物からなる融点が１０００℃以下の複合酸化物を、溶鋼中に１００ｐｐｍ以上添加するところに特徴を有している。これら融点が１０００℃以下である上記酸化物は、レードル、タンディッシュおよび鋳型の少なくとも１個所で溶鋼に添加することにより、鋼中に均一に混入・分散させることができる。
【０００９】
【発明の実施の形態】
本発明者らは前述した様な課題の下で、従来の鉛や硫黄、Ｃａ酸化物などに代わる快削成分を模索し、特に切屑分断性の向上を期して鋭意研究を進めてきた。その結果、機械構造用鋼中にＮａ，ＬｉおよびＢよりなる群から選択される少なくとも１種の元素の酸化物、あるいは、Ｎａ，Ｌｉ，ＢおよびＳｉよりなる群から選択される少なくとも２種の元素の複合酸化物を適量含有させたものは、安定して優れた切屑分断性を示すことを確認し、上記本発明に想到したものである。
【００１０】
切屑分断性を高めるには、当然のことながら切屑を分断するための起点が必要となる。その起点は、鉛快削鋼においては鋼中に分散した鉛による金属溶融脆化であり、鉛は低融点（約３２７℃）であるため、切削加工中の被削材の温度上昇に伴う鉛の溶融によって切屑が脆化し、切屑分断性が向上すると考えられている。
【００１１】
一方硫黄快削鋼では、特に伸展したＭｎＳが切屑分断の起点となるが、一般に硫黄量の増加に伴ってＭｎＳ量は増大するので、切屑分断性も向上すると考えられている。
【００１２】
しかしながら、前述の如く鉛は環境汚染の観点から今後その使用は回避される傾向にあり、また硫黄は、Ｃ方向の機械的性質劣化の観点からその使用量が制限されるため、被削性の一層の改善が求められる。
【００１３】
そこで本発明者らは、これら鉛や硫黄に代わる切屑分断性向上成分を見出すべく、特に、切削時の工具刃先の温度上昇による介在物の脆化作用により切屑分断性を高める成分について模索した。その結果、上記の様にＮａ，ＬｉおよびＢよりなる群から選択される少なくとも１種の元素の酸化物、あるいは、Ｎａ，Ｌｉ，ＢおよびＳｉよりなる群から選択される少なくとも２種の元素の複合酸化物は、機械構造用鋼の切屑分断性を著しく高める作用を有していることが確認された。
【００１４】
金属酸化物は一般に硬質であり、これまでは被削性を劣化させるものと考えられており、金属酸化物を被削性向上成分として積極的に利用するといったことはあまり考えられなかった。ところが、機械構造用鋼中にＮａ，ＬｉおよびＢよりなる群から選択される少なくとも１種の元素の酸化物、あるいは、Ｎａ，Ｌｉ，ＢおよびＳｉよりなる群から選択される少なくとも２種の元素の複合酸化物［以下、これらをまとめて（複合）酸化物ということがある］を適量含有させると、これらの（複合）酸化物が高速切削加工中の刃先温度域で溶融・軟化し、それに伴う溶融脆化によって切屑分断性が著しく改善されることを見出したのである。
【００１５】
本発明では、上記の如く機械構造用鋼中に、切屑分断性改善成分として前記（複合）酸化物を適量含有させるところに特徴を有しているが、その前提として、鋼材の縦断面に現われる長径１〜１５μｍの介在物が、断面積１ｍｍ²当たり５０〜１５００個、より好ましくは１５０〜１０００個分散していることが必要となる。ちなみに、鋼中に存在し得る介在物としては、酸化物、硫化物、窒化物などが挙げられるが、それら介在物の中でも長径が１μｍ未満の微細なものは、切屑分断性や機械的特性に及ぼす影響は少ない。一方、長径が１５μｍを超える介在物が鋼中に多数存在すると、機械構造用鋼としての機械的特性、特に靭性や延性等に顕著な悪影響を及ぼす可能性もあるが、現実的にはその様な大サイズの介在物は少ない。
【００１６】
そして本発明で意図する優れた切屑分断性を確保するには、上記介在物のサイズと個数を満たす要件の下で、長径が１μｍ以上である全介在物中に占める酸化物の個数の占める比率が１０％以上であり、且つ、該酸化物中の前記（複合）酸化物の占める個数の割合が５％以上であることが必須の要件となる。
【００１７】
即ち本発明では、基本的に鋼材中に分散している介在物のうち酸化物の個数を特定することに加えて、該酸化物中の前記（複合）酸化物の占める個数の割合を特定することによって、従来の快削鋼に優るとも劣ることのない優れた切屑分断性を確保することに成功したものである。
【００１８】
ちなみに、鋼断面に現われる全介在物の中には、酸化物、硫化物、窒化物、炭化物、およびそれらの複合物などが含まれ、また酸化物の中にはアルミナ、シリカ、酸化マンガン、酸化クロム、およびそれらの複合物、などが含まれるが、本発明で意図するレベルの優れた切屑分断性を確保するには、全介在物中に占める長径１μｍ以上の酸化物個数の比率が１０％以上で、且つ該酸化物のうち、前記（複合）酸化物の占める個数の割合が５％以上であることが必須となる。
【００１９】
そして、全介在物中に占める酸化物の個数割合が１０％未満で、しかも該酸化物中に占める前記（複合）酸化物の占める個数割合が５％未満では、これら特定の（複合）酸化物に期待される切屑分断性改善効果が有効に発揮されない。即ちこれら特定の（複合）酸化物は、融点が１０００℃以下であり、好ましくは、後述する方法によって溶鋼中に添加されるそれら（複合）酸化物の成分組成を融点が８００℃以下となる様に調整し、長径が１μｍ以上である該特定（複合）酸化物の個数が上記要件を満たす様にコントロールすれば、切削加工時の加工発熱による該特定（複合）酸化物の溶融脆化作用によって切屑分断性が大幅に高められる。
【００２０】
しかも、上記（複合）酸化物の多くは球状であるため、機械的特性を劣化させることはなく、酸化物の存在形態によってはむしろ機械的特性の向上に寄与する。
【００２１】
尚、上記酸化物による切屑分断性改善作用は、Ｎａ，ＬｉおよびＢの各酸化物については、それぞれ単独で有効に発揮される他、２種もしくは３種の複合酸化物としても有効に発揮されるが、Ｓｉの酸化物については、単独酸化物として所定量存在していても本発明の意図する様な切屑分断性を得ることはできず、前記Ｎａ，Ｌｉ，Ｂの少なくとも１種の酸化物との複合酸化物として存在させることが必須となる。これは、Ｓｉの単独酸化物はＮａ酸化物、Ｌｉ酸化物、Ｂ酸化物に比べて融点が高いため、高速切削時の昇温による溶融脆化作用が有効に発揮されないからである。
【００２２】
尚、切屑分断性の向上に寄与する前記Ｎａ，Ｌｉ，Ｂの酸化物源となるＮａ，Ｌｉ，Ｂは、通常の溶鋼中には殆ど含まれていない。従って本発明の上記目的を果たすには、溶鋼中にＮａ酸化物、Ｌｉ酸化物、Ｂ酸化物あるいはそれらの複合酸化物を添加することが必要であり、具体的には、レードル、タンディッシュおよび鋳型の少なくとも１個所で、溶鋼中にそれらの酸化物、もしくはＳｉとの複合酸化物を添加し、これらを溶鋼中に含有させる方法が採用される。
【００２３】
この時、添加される上記酸化物や複合酸化物は、添加前の状態で融点が１０００℃以下、より好ましくは８００℃以下となる様に成分調整しておくことが望ましい。溶鋼内に添加された上記酸化物や複合酸化物は高温の溶鋼中で形態変化を起こすが、前述した位置で添加する方法を採用すれば、最終的に得られる鋼材内においても、溶融温度１２００℃程度以下の（複合）酸化物として存在させることができる。
【００２４】
尚上記酸化物または複合酸化物の添加量は、溶鋼に対して１００ｐｐｍ以上、より好ましくは３００ｐｐｍ以上とすべきであり、１００ｐｐｍ未満では、最終的に得られる鋼中に、十分なサイズ（長径１μｍ以上）と量（個数）の（複合）酸化物を存在させることができず、本発明で意図する優れた切屑処理性が得られ難くなる。上記酸化物や複合酸化物は過剰量添加しても、その大部分は溶鋼中に歩留まることなくスラグとなって湯面上に浮上分離されるので実害は生じないが、その効果は２０００ｐｐｍ程度で飽和するので、それ以上の添加は経済的に無駄であり、好ましくは１０００ｐｐｍ程度以下で十分である。尚これらの酸化物は、単体酸化物として添加するよりも複合酸化物として溶鋼に添加した方が、鋼中に歩留まり易いので有利である。添加する酸化物の融点は、状態図の値を基準にして予め決めておけばよい。
【００２５】
本発明は、上記の様に機械構造用鋼に含まれる（複合）酸化物のサイズと量を規定したところに特徴を有するもので、鋼材の種類には特に制限がなく、例えばＪＩＳＧ４０５１に規定される機械構造用炭素鋼、ＪＩＳＧ４１０２に規定されるニッケル・クロム鋼、ＪＩＳＧ４１０３に規定されるニッケル・クロム・モリブデン鋼、ＪＩＳＧ４１０６に適用されている機械構造用マンガン鋼、マンガン・クロム鋼などに幅広く適用することができ、その他の元素を適宜含有させた全ての機械構造用鋼に適用できるが、本発明の特徴が最も有効に発揮される機械構造用鋼の標準的な化学成分を例示すると下記の通りである。
【００２６】
Ｃ：０．１〜０．６％
Ｃは鋼の強度向上元素として重要な元素であるが、反面、延性を低下させる元素でもあり、その含有量が極めて低い低炭素鋼領域では、鋼の延性を適度に低下させて切屑分断性を高める作用を発揮する。そのため機械構造用鋼として用いるにはＣ量を０．１％以上、より好ましくは０．２％以上とすべきであるが、Ｃ量が多くなり過ぎると、鋼が高質化して工具寿命を低下させる原因になるので０．６％以下、より好ましくは０．５％以下に抑えるのがよい。
するので、０．１〜０．６％とした。
【００２７】
Ｓｉ：２．５％以下（０％を含まない）
Ｓｉは通常、溶製時に使用する脱酸剤として混入してくるが、Ｓｉは酸素との反応性が非常に速く、２．５％を超えると、鋼中の必要酸素量を確保することが困難になるばかりでなく、Ｎａ，Ｌｉ，Ｂ酸化物の酸素を奪って切屑分断性改質作用を発現し難くすることがあるので、２．５％以下、より好ましくは１．５％以下に抑えることが望ましい。
【００２８】
Ｍｎ：０．２〜３．０％
Ｍｎは、切屑分断性の向上に有効なＭｎＳを生成させる他、熱間加工性を高める上でも有効に作用する元素であり、これらの作用を有効に発揮させるには０．２％以上、より好ましくは０．５％以上含有させることが望ましい。しかし、Ｍｎ含有量が多過ぎると鋼材の加工硬化が顕著になり、工具寿命を短縮させる原因になるので３．０％以下、より好ましくは２．５％以下に抑えることが望ましい。
【００２９】
Ｓ：０．１５０％未満（０％を含まない）
Ｓは、切屑分断性を含めた被削性全般の向上に有効なＭｎＳを形成する元素であるが、０．１５０％を超えると熱間加工性や延性を著しく劣化させる。よって、Ｓ含有量は０．１５０％未満に抑えることが望ましく、特に機械的特性が重視される機械構造用部品として適用する際には、Ｓ量は０．１２％以下、より好ましくは０．０８％以下に抑えることが望まれる。
【００３０】
Ｏ：０．００１〜０．０１５％
Ｏは、前記（複合）酸化物の析出形態を左右する重要な元素であり、トータル酸素量が０．００１％未満では、本発明で定める前記サイズと量の（複合）酸化物を存在させることができず、本発明で意図するレベルの切屑分断性を確保するには０．００１％以上のＯを必須とする。しかしＯ量が多過ぎると、溶製時に酸化鉄の生成を促して溶製炉の内張り耐火物を損傷し炉寿命の短縮を招く。従って、Ｏの含有量は、炉寿命の観点から０．０１５％以下、より好ましくは０．０１０％以下に抑えることが望ましい。
【００３１】
本発明で使用される切屑分断性に優れた機械構造用鋼の好ましい含有元素は上記の通りであり、残部成分は実質的にＦｅであるが、該構造用鋼中には微量の不可避不純物の含有が許容されることは勿論のこと、前記本発明の作用に悪影響を与えない範囲で更に他の元素を積極的に含有させた鋼を使用することも可能である。積極添加が許容される他の元素の例としては、切屑分断性改善効果を有するＰｂ，Ｂｉ，Ｔｅなどが挙げられ、それらは１種又は２種以上を含有させることができるが、それらは通常合計で０．５％程度以下に抑えることが望ましい。
【００３２】
次に、前記（複合）酸化物の分析法について説明する。供試材は、１５５ｍｍ×１５５ｍｍの鋳片を１２００℃にて直径５０ｍｍの丸棒状に鍛造した後、８５０℃×１ｈｒ油冷→５００℃×２ｈｒ水冷にて焼入れ焼戻し処理を施し、ビッカース硬さで２７０±１５に揃えた。該鍛造材の鍛造方向に平行な断面のＤ／４位置を研磨した後、供試材の観察位置を特定するため荷重５ｋｇで圧痕を打った。
【００３３】
そして介在物の個数測定は、光学顕微鏡を用いて圧痕近傍を倍率１００倍で１視野当たり０．５ｍｍ×０．５ｍｍの面積を４視野づつ観察し、長径が１μｍ以上の介在物について画像解析した。次いで、日本電子製「ＪＸＡ−８８００ＲＬ」のＥＰＭＡにより加速電圧１５ｋＶ、倍率５００倍で４視野を観察して介在物の組成を特定した。この観察では、Ｎａ，Ｌｉ，Ｂ，Ｓｉ，ＯおよびＭｎ，Ｓ，Ｃｒのマッピングを行なって各元素の存在を確認した。ただしＥＰＭＡでは、ＬｉやＢなどの軽元素は検出し難いので、ＥＰＭＡ観察と同じ領域をＣＡＭＥＣＡ−ｉｍｓｆ５ｆのＳＩＭＳにより一次イオン条件Ｏ₂ ⁺−８ｋｅＶ−１ｎＡで倍率５００倍にて４視野を観察した。ただし、ＳＩＭＳでは一次イオンとしてＯ₂ ⁺を用いており、Ｏの存在がはっきりしないが、ＥＰＭＡでＯが観察されたものは酸化物と判断した。ＥＰＭＡおよびＳＩＭＳ観察で、Ｎａ，Ｌｉ，Ｂ，Ｓｉから選択される１種以上および酸素Ｏの存在が確認できたものを（複合）酸化物とした。この観察で、長径１μｍ以上の全介在物のうち観察された酸化物および（複合）酸化物の数からそれらの割合を求めた。酸化物とＭｎＳとからなる介在物は酸化物としてカウントした。
【００３４】
次に評価法について説明すると、評価項目は切屑分断性と横目靱性値の２つとし、切屑分断性は超硬旋削試験により評価した。試験条件は、表１に示す如く、切削速度１５０（ｍ／ｍｉｎ）で、送りを０．０５，０．５，０．２，０．３（ｍｍ／ｒｅｖ）の４水準、切込みを０．５，１．０，２．０（ｍｍ）の３水準に変化させ、各鋼種につき１２条件で行なった。そして、図１に示す評価点を基準に各切削条件における切屑を配分し、その基準の評価点と切屑の配分割合を掛け合わせ、１２条件の合計を切屑処理性指数とした。仮に全条件で表２右端の最も細かい切屑状態であったとすると、切屑処理性指数は１００（正確には９９．９６）となる。
【００３５】
【表１】

【００３６】
横目靱性値はＪＩＳ３号の衝撃試験により行った。衝撃試験片は鍛伸方向と直角の方向から切り出して試験に供した。
【００３７】
【実施例】
以下、実施例を挙げて本発明の構成と作用効果をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に包含される。
【００３８】
実施例
本発明鋼は、５００ｋｇ高周波溶解炉で溶解した低炭素鋼に、組成調整した（複合）酸化物を添加し、１５５ｍｍに鋳造した。表２に本発明鋼（Ｎｏ．１〜１１）および比較鋼（Ｎｏ．１２〜２０）の主成分、添加酸化物の組成、添加量、融点を示す。なお、Ｎｏ．１２，１３は現用の鉛快削鋼、Ｎｏ．１４はべース鋼、Ｎｏ．１５，１６はＳ添加量を変えたものである。
【００３９】
表３および図２〜４に供試材の評価結果を示す。表４にベース鋼（Ｎｏ．１４）の切屑分断性判定の結果を示す。図２に見られる様に、（複合）酸化物の割合が５％を超えると切屑分断性が大幅に向上し、鉛快削鋼や硫黄快削鋼以上となっている。図３に示す通り、切屑分断性を向上させるには、（複合）酸化物を１００ｐｐｍ以上添加しなければならないことが分かる。図４に示す通り本発明鋼は、ベース鋼やＳ減量もしくはＳ増量鋼などに比べれば、切屑分断性と横目衝撃値が大きく改善され、鉛快削鋼並の特性が得られている。
【００４０】
【表２】

【００４１】
【表３】

【００４２】
【表４】

【００４３】
【発明の効果】
本発明は以上の様に構成されており、縦断面の長径１μｍ以上の全介在物に占める酸化物の個数の割合が１０％以上であり、かつ、その酸化物のうちＮａ，ＬｉおよびＢよりなる群から選択される少なくとも１種の元素の酸化物、もしくはＮａ，Ｌｉ，ＢおよびＳｉよりなる群から選択される少なくとも２種の元素の複合酸化物の個数の割合が５％以上である本発明鋼によれば、Ｃ方向の機械的性質も低下せずに切屑分断性に優れた機械構造用を提供することができる。
【図面の簡単な説明】
【図１】実験で採用した切屑処理性の評価基準を示す図である。
【図２】供試鋼中の（複合）酸化物の割合と切屑分断性指数の関係を示すグラフである。
【図３】鋼への酸化物の添加量と切屑分断性指数の関係を示すグラフである。
【図４】供試鋼のうち、本発明の規定要件を満たす鋼と比較鋼の横目靭性値と切屑分断性指数を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a machine structural steel excellent in chip breaking property and a manufacturing method thereof, and more specifically, to a machine structural steel having improved chip breaking property by containing a specific amount of a specific metal oxide in a steel material. It relates to the manufacturing method.
[0002]
[Prior art]
Many free-cutting steels have been developed so far because steel materials are often machined to be processed into various product shapes. Among the machinability, the chip breaking property, in which the cutting piece is quickly divided and separated from the tool, is an important characteristic for smoothly proceeding with automation of the cutting operation without any trouble.
[0003]
By the way, the lead free-cutting steel that has been used most frequently in the past, the low melting point lead added to the steel melts due to the temperature rise of the work material at the time of cutting, and exhibits the effect of increasing the chip breaking property, and Since deterioration of mechanical properties is relatively small, it has been widely used as mechanical structural steel. However, in recent years, as the awareness of environmental problems increases, lead free cutting steel containing lead, which is a harmful heavy metal, as a machinable component tends to be avoided.
[0004]
In contrast, sulfur free-cutting steel does not cause the pollution problems pointed out by lead free-cutting steel, and also has excellent tool life and chip breaking properties. As demand grows. However, in the free-cutting steel, the sulfide in the steel tends to extend in the rolling direction in the rolling process, and the mechanical properties in the direction perpendicular to the rolling direction (hereinafter abbreviated as C direction) are adversely affected. In addition, there is a problem that sulfide is likely to cause cracking during hot forging, so there is a limit to the amount of S added, and the application range is naturally limited.
[0005]
Furthermore, the Ca free-cutting steel forms a protective film called belag on the cutting tool surface by controlling the complex oxides of Si, A1, and Ca. These oxides have a melting point of around 1400 ° C. It is effective in improving the tool life in the high cutting speed region using carbide tools, but does not show a satisfactory machinability improvement effect in the low cutting speed region, and also has good chip separation. I can't say that.
[0006]
[Problems to be solved by the invention]
As described above, the conventional free-cutting steel is required to be further improved, such as the use of which is avoided due to environmental problems or the cutting conditions are restricted. The present invention has been made in view of such a situation, and can be widely applied without causing a problem of environmental pollution and without being subjected to cutting conditions or application restrictions. It is an object of the present invention to provide a steel for machine structural use that is extremely important for chip breaking.
[0007]
[Means for Solving the Problems]
The steel for machine structural use having excellent chip breaking properties according to the present invention that has solved the above-mentioned problems is C: 0.1 to 0.6% in mass%.
Si: 2.5% or less (excluding 0%)
Mn: 0.2 to 3.0%
S: Less than 0.150% (excluding 0%)
O: 0.001 to 0.015%
In the steel for machine structural use, inclusions with a major axis of 1 μm or more appearing in the longitudinal section are 50-1500 per 1 mm ^{2 in} cross-sectional area, and the ratio of the number of oxides in the total amount of the inclusions is 10% or more, In addition, the number of oxides of at least one element selected from the group consisting of Na, Li and B among the oxides is 5% or more, or among the above oxides, Na, Li, B and The main point is that the number ratio of the composite oxide composed of oxides of at least two elements selected from the group consisting of Si is 5%.
[0008]
Further, the constitution of the production method according to the present invention is a method for producing a steel for machine structural use having excellent chip breaking properties, and is made of an oxide of at least one element selected from the group consisting of Na, Li and B. An oxide having a melting point of 1000 ° C. or less is added to the molten steel at 100 ppm or more, or a melting point of an oxide of at least two elements selected from the group consisting of Na, Li, B and Si is 1000 ° C. or less. The composite oxide is characterized in that 100 ppm or more is added to the molten steel. These oxides having a melting point of 1000 ° C. or lower can be uniformly mixed and dispersed in the steel by adding them to the molten steel at at least one of a ladle, tundish and mold.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Under the problems as described above, the present inventors have sought a free-cutting component in place of conventional lead, sulfur, Ca oxide, and the like, and have been carrying out intensive studies particularly for improving chip breaking. As a result, an oxide of at least one element selected from the group consisting of Na, Li and B in the steel for mechanical structure, or at least two types selected from the group consisting of Na, Li, B and Si It has been conceived of the present invention that it has been confirmed that a compound oxide containing an appropriate amount of elemental oxide exhibits stable and excellent chip breaking properties.
[0010]
In order to improve the chip dividing property, it is a matter of course that a starting point for dividing the chip is required. The starting point is the metal melting embrittlement caused by lead dispersed in steel in lead free-cutting steel, and lead has a low melting point (about 327 ° C). Therefore, lead accompanying the temperature rise of the work material during cutting It is considered that chip melting becomes brittle and the chip breaking property is improved.
[0011]
On the other hand, in sulfur free-cutting steel, the extended MnS is the starting point for chip breaking, but generally, the amount of MnS increases with an increase in the amount of sulfur, so it is considered that chip breaking is also improved.
[0012]
However, as described above, the use of lead tends to be avoided in the future from the viewpoint of environmental pollution, and the use of sulfur is limited from the viewpoint of deterioration of mechanical properties in the C direction. Further improvement is required.
[0013]
Therefore, the present inventors have sought for a component that improves chip fragmentation by embrittlement of inclusions due to an increase in the temperature of the cutting edge of the tool at the time of cutting in order to find a component for improving chip fragmentation in place of lead and sulfur. As a result, as described above, an oxide of at least one element selected from the group consisting of Na, Li and B , or at least two elements selected from the group consisting of Na, Li, B and Si. It was confirmed that the composite oxide has an effect of significantly improving the chip breaking property of the steel for machine structural use.
[0014]
Metal oxides are generally hard and have been considered to deteriorate machinability so far, and it has not been considered that metal oxides are actively used as machinability improving components. However, at least one element selected from the group consisting of Na, Li, B, and Si, or an oxide of at least one element selected from the group consisting of Na, Li, and B in the steel for machine structural use When an appropriate amount of these composite oxides (hereinafter sometimes referred to collectively as (composite) oxides) is contained, these (composite) oxides melt and soften in the temperature range of the cutting edge during high-speed cutting, It has been found that the chip breaking property is remarkably improved by the accompanying melt embrittlement.
[0015]
The present invention is characterized in that an appropriate amount of the (composite) oxide is contained in the steel for machine structural use as a chip breaking property improving component as described above, but as a premise, it appears in the longitudinal section of the steel material. It is necessary that 50 to 1500, more preferably 150 to 1000, inclusions having a major axis of 1 to 15 μm are dispersed per 1 mm ^{2 of} the cross-sectional area. By the way, inclusions that can exist in steel include oxides, sulfides, nitrides, etc. Among these inclusions, fine inclusions having a major axis of less than 1 μm have good chip separation and mechanical properties. The effect is small. On the other hand, if there are many inclusions in the steel whose major axis exceeds 15 μm, the mechanical properties as mechanical structural steel, particularly toughness and ductility, etc. may be adversely affected. There are few large inclusions.
[0016]
And in order to ensure the excellent chip | tip cutting | disconnection property which is intended by this invention, the ratio which the number of oxides occupies in all the inclusions whose major axis is 1 micrometer or more under the requirements which satisfy | fill the size and number of the said inclusions Is 10% or more, and the ratio of the number of the (composite) oxides in the oxide is 5% or more.
[0017]
That is, in the present invention, in addition to specifying the number of oxides among the inclusions dispersed in the steel material, the ratio of the number of the (composite) oxides in the oxide is specified. By this, it succeeded in ensuring the outstanding chip | tip division | segmentation property which is not inferior to the conventional free-cutting steel.
[0018]
Incidentally, all the inclusions that appear in the steel cross section include oxides, sulfides, nitrides, carbides, and composites thereof, and among oxides are alumina, silica, manganese oxide, oxidation In order to ensure excellent chip fragmentation at the level intended in the present invention, the ratio of the number of oxides having a major axis of 1 μm or more in all inclusions is 10%. As described above, it is essential that the ratio of the number of the (composite) oxide in the oxide is 5% or more.
[0019]
When the number ratio of oxides in all the inclusions is less than 10% and the number ratio of the (composite) oxides in the oxide is less than 5%, these specific (composite) oxides Therefore, the expected chip breaking improvement effect is not exhibited effectively. That is, these specific (composite) oxides have a melting point of 1000 ° C. or less, and preferably the component composition of those (composite) oxides added to molten steel by the method described later is such that the melting point is 800 ° C. or less. If the number of the specific (composite) oxide having a major axis of 1 μm or more is controlled so as to satisfy the above requirements, the specific (composite) oxide is melted and embrittled by processing heat generated during cutting. Chip separation is greatly improved.
[0020]
Moreover, since many of the above (composite) oxides are spherical, the mechanical properties are not deteriorated, and rather contribute to the improvement of the mechanical properties depending on the form of the oxide.
[0021]
In addition, the chip breaking property improving effect by the oxide is effectively exhibited independently for each of the oxides of Na, Li and B, as well as 2 or 3 complex oxides. However, with respect to the oxide of Si, even if a predetermined amount exists as a single oxide, the chip fragmentation as intended by the present invention cannot be obtained, and at least one kind of oxidation of Na, Li and B is not possible. It is essential to exist as a complex oxide with a product. This is because the single oxide of Si has a higher melting point than Na oxide, Li oxide, and B oxide, so that the melt embrittlement effect due to the temperature rise during high-speed cutting cannot be exhibited effectively.
[0022]
In addition, Na, Li, and B, which are oxide sources of Na, Li, and B that contribute to the improvement of chip breaking property, are hardly contained in ordinary molten steel. Therefore, in order to achieve the above object of the present invention, it is necessary to add Na oxide, Li oxide, B oxide or a composite oxide thereof into the molten steel, specifically, ladle, tundish and A method of adding these oxides or complex oxides with Si into the molten steel at least at one place of the mold and incorporating these into the molten steel is adopted.
[0023]
At this time, it is desirable to adjust the components of the oxide or composite oxide to be added so that the melting point is 1000 ° C. or less, more preferably 800 ° C. or less before the addition. The oxides and composite oxides added to the molten steel cause a change in shape in the high-temperature molten steel, but if the method of adding at the position described above is adopted, the melting temperature 1200 is also obtained in the steel material finally obtained. It can be present as a (composite) oxide having a temperature of about 0 ° C. or less.
[0024]
The addition amount of the oxide or composite oxide should be 100 ppm or more, more preferably 300 ppm or more with respect to the molten steel. If it is less than 100 ppm, the steel obtained finally has a sufficient size (major axis 1 μm). The above (comparative) and amount (number) of (composite) oxides cannot be present, and it is difficult to obtain the excellent chip treatability intended in the present invention. Even if an excessive amount of the above oxides or composite oxides are added, most of the oxides and slags are floated and separated on the surface of the molten metal as slag without yielding in the molten steel. Therefore, addition beyond that is economically wasteful, and preferably about 1000 ppm or less is sufficient. In addition, it is more advantageous to add these oxides to the molten steel as a composite oxide than to add them as a single oxide because the yield is easily increased in the steel. The melting point of the oxide to be added may be determined in advance based on the values in the phase diagram.
[0025]
The present invention is characterized in that the size and amount of the (composite) oxide contained in the machine structural steel is defined as described above, and the type of steel material is not particularly limited. For example, it is defined in JIS G4051. Carbon steel for machine structure, nickel / chromium steel specified by JIS G4102, nickel / chromium / molybdenum steel specified by JIS G4103, manganese steel for machine structure applied to JIS G4106, manganese / chromium steel, etc. It is applicable to all mechanical structural steels that contain other elements as appropriate, but the typical chemical components of mechanical structural steels that exhibit the features of the present invention most effectively are illustrated. Then it is as follows.
[0026]
C: 0.1 to 0.6%
C is an important element for improving the strength of steel. On the other hand, it is also an element that lowers the ductility. In the low-carbon steel region, where the content is extremely low, the ductility of the steel is moderately lowered to reduce chip breaking. Demonstrate the effect of increasing For this reason, the C content should be 0.1% or more, more preferably 0.2% or more for use as steel for machine structural use. However, if the C content is excessive, the quality of the steel will be improved and the tool life will be shortened. Since it causes a decrease, it is preferable to keep it at 0.6% or less, more preferably 0.5% or less.
Therefore, the content is set to 0.1 to 0.6%.
[0027]
Si: 2.5% or less (excluding 0%)
Si is usually mixed as a deoxidizer used during melting, but Si has a very fast reactivity with oxygen, and if it exceeds 2.5%, the necessary amount of oxygen in the steel can be secured. Not only is it difficult, but it may make it difficult to develop the chip breaking property modification by depriving the oxygen of Na, Li, B oxide, so 2.5% or less, more preferably 1.5% or less. It is desirable to suppress.
[0028]
Mn: 0.2 to 3.0%
Mn is an element that works effectively in improving hot workability in addition to generating MnS effective in improving chip breaking, and 0.2% or more in order to exert these actions effectively. Preferably it is 0.5% or more. However, if the Mn content is too large, work hardening of the steel material becomes prominent and causes the tool life to be shortened, so it is desirable to keep it to 3.0% or less, more preferably 2.5% or less.
[0029]
S: Less than 0.150% (excluding 0%)
S is an element that forms MnS that is effective in improving the overall machinability including chip breaking properties, but when it exceeds 0.150%, hot workability and ductility are significantly deteriorated. Therefore, the S content is desirably suppressed to less than 0.150%, and particularly when applied as a machine structural component in which mechanical characteristics are emphasized, the S content is 0.12% or less, more preferably 0.8. It is desired to keep it below 08%.
[0030]
O: 0.001 to 0.015%
O is an important element that influences the precipitation form of the (composite) oxide. When the total oxygen content is less than 0.001%, the (composite) oxide of the size and amount defined in the present invention must be present. Therefore, 0.001% or more of O is essential in order to ensure the chip breaking property at the level intended by the present invention. However, if the amount of O is too large, the production of iron oxide is promoted at the time of melting, and the refractory material of the melting furnace is damaged and the life of the furnace is shortened. Therefore, the O content is desirably 0.015% or less, more preferably 0.010% or less from the viewpoint of the life of the furnace.
[0031]
The preferred contained elements of the machine structural steel excellent in chip breaking properties used in the present invention are as described above, and the remaining component is substantially Fe, but a small amount of inevitable impurities are contained in the structural steel. Needless to say, it is also possible to use steel in which other elements are actively contained within a range that does not adversely affect the action of the present invention. Examples of other elements that are allowed to be positively added include Pb, Bi, Te, etc., which have an effect of improving chip breaking property, and they can contain one or more kinds, but they are usually It is desirable to keep the total to about 0.5% or less.
[0032]
Next, a method for analyzing the (composite) oxide will be described. The specimen was forged into a round bar with a diameter of 50 mm at 1200 ° C. and then subjected to quenching and tempering at 850 ° C. × 1 hr oil cooling → 500 ° C. × 2 hr water cooling, and with Vickers hardness 270 ± 15. After polishing the D / 4 position of the cross-section parallel to the forging direction of the forged material, an indentation was made with a load of 5 kg in order to specify the observation position of the test material.
[0033]
Then, the number of inclusions was measured by observing the area of 0.5 mm × 0.5 mm per field of view at four magnifications in the vicinity of the indentation at a magnification of 100 using an optical microscope, and analyzing the image of the inclusion having a major axis of 1 μm or more. . Next, the composition of inclusions was identified by observing 4 fields of view with an acceleration voltage of 15 kV and a magnification of 500 times with EPMA of “JXA-8800RL” manufactured by JEOL. In this observation, the presence of each element was confirmed by mapping Na, Li, B, Si, O and Mn, S, Cr. However, in EPMA, since light elements hardly detected such as Li or B, was observed 4 field at 500X magnification in the primary ionic conditions O ₂ ⁺ -8keV-1nA by SIMS of CAMECA-imsf5f the same area as EPMA observation . However, SIMS uses O ₂ ⁺ as the primary ion, and the presence of O is not clear, but those in which O was observed by EPMA were judged to be oxides. One or more selected from Na, Li, B, and Si and the presence of oxygen O were confirmed by EPMA and SIMS observations as (composite) oxides. In this observation, the ratio was determined from the number of oxides and (composite) oxides observed among all the inclusions having a major axis of 1 μm or more. Inclusions composed of oxides and MnS were counted as oxides.
[0034]
Next, the evaluation method will be described. The evaluation items are two values, chip breaking property and transverse toughness value, and the chip breaking property was evaluated by a carbide turning test. As shown in Table 1, the test conditions were a cutting speed of 150 (m / min), a feed of 4 levels of 0.05, 0.5, 0.2, and 0.3 (mm / rev), and a cut of 0. It was changed to 3 levels of 5, 1.0, and 2.0 (mm), and each steel type was performed under 12 conditions. And the chip | tip in each cutting condition was allocated on the basis of the evaluation point shown in FIG. 1, the evaluation point of the reference | standard and the distribution ratio of the chip | tip were multiplied, and the total of 12 conditions was made into the chip processing index. If it is the finest chip state at the right end of Table 2 under all conditions, the chip disposability index is 100 (99.96 to be exact).
[0035]
[Table 1]

[0036]
The transverse toughness value was determined by an impact test of JIS No. 3. The impact test piece was cut out from the direction perpendicular to the forging direction and used for the test.
[0037]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out by modifying the above, and they are all included in the technical scope of the present invention.
[0038]
Example Steel of the present invention was cast to 155 mm by adding (composite) oxide whose composition was adjusted to low carbon steel melted in a 500 kg high frequency melting furnace. Table 2 shows the main components of the present invention steel (No. 1 to 11) and the comparative steel (No. 12 to 20), the composition of the added oxide, the added amount, and the melting point. In addition, No. Nos. 12 and 13 are lead free-cutting steels currently in use. 14 is base steel, No. 14; 15 and 16 are obtained by changing the addition amount of S.
[0039]
The evaluation results of the test materials are shown in Table 3 and FIGS. Table 4 shows the result of the chip breaking property judgment of the base steel (No. 14). As seen in FIG. 2, when the ratio of the (composite) oxide exceeds 5%, the chip breaking property is greatly improved, which is higher than that of lead free cutting steel or sulfur free cutting steel. As shown in FIG. 3, it can be seen that (composite) oxide must be added in an amount of 100 ppm or more in order to improve chip separability. As shown in FIG. 4, the steel according to the present invention is greatly improved in chip breaking property and transverse impact value as compared with base steel, S weight-reduced or S-weighted steel, and has characteristics similar to lead free-cutting steel.
[0040]
[Table 2]

[0041]
[Table 3]

[0042]
[Table 4]

[0043]
【The invention's effect】
The present invention is configured as described above, and the ratio of the number of oxides in all inclusions having a major axis of 1 μm or more in the longitudinal section is 10% or more, and among the oxides, Na, Li and B A book in which the ratio of the number of oxides of at least one element selected from the group consisting of, or the composite oxide of at least two elements selected from the group consisting of Na, Li, B and Si is 5% or more According to the invention steel, it is possible to provide a machine structure excellent in chip breaking property without deteriorating the mechanical properties in the C direction.
[Brief description of the drawings]
FIG. 1 is a diagram showing evaluation criteria for chip disposal adopted in an experiment.
FIG. 2 is a graph showing the relationship between the (composite) oxide ratio in the test steel and the chip breaking index.
FIG. 3 is a graph showing the relationship between the amount of oxide added to steel and the chip breaking index.
FIG. 4 is a graph showing the transverse toughness value and chip fraction index of steels that satisfy the specified requirements of the present invention and the comparative steel among the test steels.

Claims

In mass% C: 0.1-0.6%
Si: 2.5% or less (excluding 0%)
Mn: 0.2 to 3.0%
S: Less than 0.150% (excluding 0%)
O: 0.001 to 0.015% is contained,
In the balance being Fe and inevitable impurities for machine structural use bar steel,
The steel bar further contains at least one element selected from the group consisting of Na, Li and B, and
In inclusions major axis 1 15 m appearing in cross section parallel to the forging direction of the bar steel with 50 to 1,500 per cross-sectional area 1 mm ^2, the ratio of the number of oxide occupied in the total amount of the inclusion of 10% or more And the mechanical structure excellent in chip breaking property, wherein the number of oxides of at least one element selected from the group consisting of Na, Li and B among the oxides is 5% or more use bar steel.

The Na, Li, and B are oxides of at least one element selected from the group consisting of Na, Li, and B in molten steel in which C, Si, Mn, and S satisfy the range of claim 1. The steel bar for machine structure according to claim 1, which is supplied by adding 100 ppm or more of an oxide having a melting point of 1000 ° C. or less.

In mass% C: 0.1-0.6%
Si: 2.5% or less (excluding 0%)
Mn: 0.2 to 3.0%
S: Less than 0.150% (excluding 0%)
O: 0.001 to 0.015% is contained,
In the balance being Fe and inevitable impurities for machine structural use bar steel,
The steel bar further contains at least one element selected from the group consisting of Na, Li and B, and
In inclusions major axis 1 15 m appearing in cross section parallel to the forging direction of the bar steel with 50 to 1,500 per cross-sectional area 1 mm ^2, the ratio of the number of oxide occupied in the total amount of the inclusion of 10% or more And the ratio of the number of complex oxides composed of oxides of at least two elements selected from the group consisting of Na, Li, B and Si among the oxides is 5% or more. chip divided excellent in mechanical structure for bar steel.

The Na, Li, and B are oxides of at least one element selected from the group consisting of Na, Li, B, and Si in molten steel in which C, Mn, and S satisfy the range of claim 3. The steel bar for machine structure according to claim 3, which is supplied by adding 100 ppm or more of an oxide having a melting point of 1000 ° C or less.

A method of manufacturing a machine structural bar steel according to claim 1 or 2,
C, Si, Mn and S prepare molten steel satisfying the range of claim 1,
During solution steel, Na, the chip cutting property, wherein a melting point of an oxide of at least one element selected from the group consisting of Li and B is added an oxide of 1000 ° C. or less 1 00Ppm more preparation of excellent mechanical structure for the bar steel.

A method of manufacturing a machine structural bar steel according to claim 3 or 4,
C, Mn and S prepare a molten steel satisfying the range of claim 3,
During solution steel, chips, wherein the Na, Li, that the melting point of an oxide of at least two elements selected from the group consisting of B and Si is added a composite oxide of 1000 ° C. or less 1 00Ppm more preparation of excellent mechanical structure for bar steel to divide property.

The manufacturing method of Claim 5 or 6 which adds the said oxide whose melting | fusing point is 1000 degrees C or less to molten steel in at least 1 place of a ladle, a tundish, and a casting_mold | template.