JP4264175B2

JP4264175B2 - Free-cutting steel bar with excellent machinability and its manufacturing method

Info

Publication number: JP4264175B2
Application number: JP2000020652A
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: JP2001214240A

Description

【０００１】
【発明の属する技術分野】
本発明は、被削性に優れた快削鋼とその製法に関し、より詳細には、鋼中に特定の酸化物を積極的に特定量含有させることにより、安定して優れた被削性を示す様に改善された快削鋼とその製法に関するものである。
【０００２】
【従来の技術】
近年、工業製品の大量生産に伴い鋼材の被削性が大きな問題となっており、切削加工の低コスト化および部品仕上げ面精度の向上などから被削性改善が強く求められている。
【０００３】
こうした状況の下で、被削性改善手段についても多くの研究が進められており、例えば鉛快削鋼、硫黄快削鋼、Ｃａ快削鋼、更にはこれらを組合わせた複合快削鋼等が提案され、実用化されている。
【０００４】
このうち鉛快削鋼は、工具寿命および仕上げ面精度等において優れたものであるが、近年環境問題に関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含む鉛快削鋼は忌避される傾向が強い。
【０００５】
一方硫黄快削鋼は、鉛快削鋼にみられる環境汚染の問題は生じないが、鋼中に存在する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の機械的特性に悪影響を及ぼしたり、鍛造時に硫化物を起点とする割れを起こすといった問題を起こすため硫黄添加量には自ずと限度があり、必ずしも満足のいく被削性を得ることができない。
【０００６】
また、硫黄快削鋼の被削性改善効果は鋼中に含まれる硫化物の形態によって左右され、該硫化物の形態は鋼中の酸素によって影響を受けるとされている。即ち、鋼中の酸素量が多いと硫化物が球状化し、工具寿命が向上すると考えられている。これは、鋼の凝固時に鋼中で硫化物が析出する際に、鋼中の酸化物がその核となって硫化物の析出に関与するためと考えられており、酸素量が多い程、被削性に有効な大径の硫化物が析出し易くなるとされている。つまり、酸素の存在によって大きな硫化物が生成し、これにより工具の摩耗が抑えられるのである。こうした意味において、鋼中の酸素は鋼材の被削性に対し好影響を与えていると考えられる。
【０００７】
ところが反面で、鋼中の酸素量が多いと鋼中に多くの酸化物が生成するが、該酸化物は硬質でそれ自身が工具摩耗を促進する方向に作用する。即ち全体としてみれば、鋼中の酸素は鋼材の被削性に対し好影響と悪影響を及ぼしていると考えられる。
【０００８】
またＣａ快削鋼は、酸化物の形態（組成）制御により切削工具表面にベラーグと呼ばれる保護層を形成し、工具の拡散摩耗を抑制するタイプの快削鋼である。この保護層の形成は、工具表面近傍における温度と酸化物の融点に影響を受ける。現在一般に製造されているＣａ酸化物系快削鋼は、Ｓｉ，Ａ１，Ｃａの複合酸化物において最も低融点となるアノルサイトやゲーレナイトと呼ばれる酸化物を生成させるものであり、超硬工具による高速切削においては工具寿命を延長する効果が認められている。しかし、低切削速度領域での工具寿命改善効果は乏しい。また、結晶粒度を調整するためＡ１を添加したＣａ快削鋼では、酸化物としてＡ１₂Ｏ₃を多く含む高融点のものが生成し易いため、切削条件によっては工具摩耗を促進する恐れもある。
【０００９】
【発明が解決しようとする課題】
前述の如く従来の快削鋼は、切削条件の制限や環境汚染の問題を生じるなど、必ずしも満足し得るものとは言い難い。すなわち鉛快削鋼は、重金属である鉛を使用しているため環境保全の観点から充分留意しなければなず、硫黄快削鋼は、鋼中の酸素が鋼材の被削性に対して好影響と悪影響とを与える。Ｃａ快削鋼は、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、低切削速度領域での被削性改善効果は乏しい。
【００１０】
本発明はかかる実状を鑑み、環境にも配慮し、低切削速度領域から高切削速度領域の広い加工領域において安定して優れた被削性を示し、工具寿命延長作用や面粗度改善効果に優れた快削鋼とその製法を提供することにある。
【００１１】
【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係る快削鋼とは、質量％で
Ｃ：０．０２〜０．１５％、
Ｓｉ：０．５％以下、
Ｍｎ：０．５〜１．７５％、
Ｐ：０．００４〜０．２０％、
Ｓ：０．１５〜０．５０％、
Ｏ：０．００５〜０．０２８％を含む鋼からなり、該鋼の縦断面に現われる長径１μｍ以上の介在物が断面１ｍｍ²当たり１００〜２０００個であり、更に
該介在物の全量中に占める酸化物の個数割合が４％以上で、且つ、該酸化物のうち、Ｎａ，Ｌｉ，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割合が５％以上であるか、あるいは
該介在物の全量中に占める酸化物の個数割合が４％以上であり、且つ、該酸化物のうちＮａ，Ｌｉ，Ｂ，Ｓｉよりなる群から選択される少なくとも２種の酸化物の個数割合が５％以上である
被削性に優れた快削鋼である。
【００１２】
また本発明に係る製法は、上記被削性に優れた快削鋼を製造する方法を提供するもので、該快削鋼を製造する際に、
Ｎａ，Ｌｉ，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，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
ところに要旨を有している。尚、融点が１０００℃以下である前記酸化物は、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。
【００１３】
【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、切削性や仕上げ面精度の一層の改善を期して鋭意研究を進めてきた。その結果、Ｎａ，Ｌｉ，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、もしくは、Ｎａ，Ｌｉ，Ｂ，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、溶鋼中に適量含有させたものは、安定して優れた被削性を示すと共に、良好な仕上げ面精度を与えることを知り、上記本発明に想到したものである。
【００１４】
鋼材の被削性を低下させているのは硬質な酸化系介在物であり、鋼中に介在するＡ１系やＳｉ系の硬質な酸化物は工具摩耗を促進して被削性を阻害し、工具寿命を短縮すると共に仕上げ面粗度を悪化させている。
【００１５】
そこで、本発明者らは低融点酸化物に着目して鋭意検討の結果、１０００℃以下の融点を持った低融点酸化物を含有させれば、切削時における鋼中の該低融点酸化物が溶融・軟化することによって、工具寿命および仕上げ面精度が高められることを知った。ちなみに、融点が１０００℃以下の酸化物を鋼中に分散させれば、切削中の刃先温度上昇により鋼中の該酸化物が溶融もしくは軟化し、工具の摩耗が抑制されると共に、該酸化物が工具表面を保護して工具寿命の向上にも有効に作用することが判明した。
【００１６】
前述した通り鋼中に含まれる金属酸化物は一般的に硬質であり、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、鋼中にＮａ，Ｌｉ，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低融点の酸化物、もしくは、Ｎａ，Ｌｉ，Ｂ，Ｓｉよりなる群から選択される少なくとも２種の元素の低融点の複合酸化物［以下、これらをまとめて（複合）酸化物ということがある］を溶鋼中に適量含有させたものは、安定して優れた被削性を示すと共に、良好な仕上げ面精度を与えることを見出したのである。
【００１７】
従って本発明では、上記の如く鋼中に被削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１００〜２０００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、被削性や仕上げ面精度に殆ど影響を及ぼすことがない。一方、長径が１５μｍを超える介在物が鋼中に多数存在すると、鋼材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を及ぼす可能性があるが、現実にはその様な大きなサイズの介在物は殆ど見られない。
【００１８】
そして本発明で意図する優れた被削性と仕上げ面精度を確保するには、上記介在物のサイズと個数を満たす条件の下で、長径が１μｍ以上である全介在物中に占める酸化物の個数比率が４％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。
【００１９】
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割合を特定することによって、卓越した被削性と仕上げ面精度を与える快削鋼を得ることに成功したものである。
【００２０】
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被削性と仕上げ面精度を確保するには、全介在物中に占める長径１μｍ以上の酸化物の個数割合が４％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割合が５％以上であることが必須となる。
【００２１】
そして、全介在物中に占める酸化物の個数割合が４％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被削性および仕上げ面精度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、切削加工時の摩擦熱による該特定（複合）酸化物の軟化・溶融作用によって被削性が大幅に高められると共に、仕上げ面精度を著しく改善することができるのである。
【００２２】
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。
【００２３】
尚、こうした酸化物による改善作用は、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被削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、切削加工時の昇温による軟化・溶融作用が有効に発揮されないからと思われる。
【００２４】
尚、被削性や仕上げ面精度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。
【００２５】
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。
【００２６】
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被削性と仕上げ面精度が得られ難くなる。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。
【００２７】
本発明は、上記のように快削鋼中に含まれる（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、鋼材の種類は特に制限されないが、本発明の前記特徴が最も有効に発揮される快削鋼の標準的な化学成分を例示すると下記の通りである。
【００２８】
Ｃ：０．０２〜０．１５％
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被削性を高める作用を発揮する。そのためにはＣ量を０．０２％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０．１５％以下に抑えるのがよい。
【００２９】
Ｓｉ：０．５％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、０．５％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被削性改質作用を発現し難くすることがあるので、０．５％以下、より好ましくは０．１５％以下に抑えることが望ましい。
【００３０】
Ｍｎ：０．５〜１．７５％
Ｍｎは、被削性の向上に有効な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０．５％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１．７５％以下、より好ましくは１．５％以下に抑えることが望ましい。
【００３１】
Ｐ：０．００４〜０．２％
Ｐは鋼の延性を低下させ、切削加工時の切屑処理性を向上すると共に仕上げ面粗さを低減するのに有効な元素であり、それらの作用を有効に発揮させるには０．００４％以上のＰを含有させることが望ましい。しかし、０．２％を超えて過度に含有させると、熱間加工時に表面欠陥を起こし易くなる。よって、Ｐの含有量は０．００４％以上、より好ましくは０．０１％以上で、０．２％以下、より好ましくは０．１５％以下に抑えることが望ましい。
【００３２】
Ｓ：０．１５〜０．５％
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、０．１５％未満ではその効果が十分に発揮されず、一方０．５％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０．１５％以上、より好ましくは０．２％以上で、０．５％以下、より好ましくは０．４％以下に抑えることが望ましい。
【００３３】
Ｏ：０．００５〜０．０２８％
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０．００５％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被削性と仕上げ面精度を確保するには０．００５％以上、より好ましくは０．０１％以上のＯを含むものが望ましい。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ばかりでなく、溶損した耐火物が鋼中に混入すると切削工具寿命を低下させる大きな原因になる。従って、Ｏの含有量は０．０２８％以下、より好ましくは０．０２５％以下に抑えることが望ましい。
【００３４】
本発明で使用される快削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、該快削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快削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、工具寿命や面粗度改善効果を有するＰｂ，Ｂｉ，Ｔｅ等が挙げられ、それらは単独で或いは２種以上を複合添加できるが、それらは合計量で０．５％程度以下に抑えることが望ましい。
【００３５】
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℃で直径８０ｍｍの丸棒状に鍛造した後、８５０℃×１ｈｒ空冷で焼ならし処理し、該鍛造材の鍛造方向に平行な断面のＤ／４位置を研磨する。次いで、供試材の観察位置を特定するため荷重５ｋｇで圧痕を打つ。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する。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、倍率５００倍で４視野を観察する。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の観察されているものは酸化物と判断する。ＥＰＭＡおよびＳＩＭＳ観察でＮａ，Ｌｉ，Ｂ，Ｓｉから選択される１種以上と酸素（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、観察された全酸化物および（複合）酸化物の数を求めて夫々の個数割合を算出した。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。
【００３６】
次に評価法について説明すると、評価項目は工具寿命と仕上げ面粗度の２つとし、工具寿命は、１５５ｍｍ×１５５ｍｍ鋳片を直径２２ｍｍ×３ｍの棒鋼に熱間圧延し、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、Ｐ１０チップを用い、切削速度：２００（ｍ／ｍｉｎ）、送り：０．２５（ｍｍ／ｒｅｖ）、切込み：１．５（ｍｍ）とした。工具寿命の判断基準は、ＶＢ摩耗０．２ｍｍとした。
【００３７】
【表１】

【００３８】
部品の仕上げ面粗度は、上記焼きならし処理材を用いて超硬旋削試験により評価した。試験条件は、表２に示す如くＰ１０チップを用い、切削速度：１５０（ｍ／ｍｉｎ）、送り：０．１０（ｍｍ／ｒｅｖ）、切込み：１．０（ｍｍ）、切削時間：１（ｍｉｎ）とした。仕上げ面粗度は切削終了間際の領域を評価した。
【００３９】
【表２】

【００４０】
【実施例】
次に実施例を挙げて本発明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。
【００４１】
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角に鋳造した。表３に本発明鋼（Ｎｏ．１〜１２）および比較鋼（Ｎｏ．１３〜１８）の主成分、添加酸化物の組成、添加量、融点を示す。なおＮｏ．１３は現用の鉛快削鋼、Ｎｏ．１４はべース鋼、Ｎｏ．１５〜１８は本発明で定めるいずれかの規定要件を欠く比較例である。
【００４２】
表４および図１〜４に供試材の評価結果を示す。図１に見られる様に、（複合）酸化物の個数割合が５％以上になると、工具寿命が大幅に向上し、Ｎｏ．１３の鉛快削鋼と同レベルの被削性が得られている。また図２に見られる様に、（複合）酸化物の個数割合が５％が以上になると、仕上げ面粗度も鉛快削鋼レベル以上に向上している。
【００４３】
更に、図３，４からも明らかな様に、工具寿命の向上効果を有効に発揮させるには、成分調整された（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。
【００４４】
これらの実験結果からも明らかな様に、本発明によれば、従来の鉛快削鋼に較べて全く遜色のない工具寿命と仕上げ面精度を示す快削鋼を得ることができることが分かる。
【００４５】
【表３】

【００４６】
【表４】

【００４７】
【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割合が４％以上であり、かつ、その酸化物のうちＮａ、Ｌｉ，Ｂから選ばれる１種以上の酸化物、もしくは、Ｌｉ，Ｎａ，Ｂ，Ｓｉから選ばれる２種以上の複合酸化物の個数割合が５％以上である本発明鋼の快削鋼は、公害原因を生じることなく優れた工具寿命と仕上げ面粗度を得ることができる。
【図面の簡単な説明】
【図１】実験で用いた供試快削鋼中の（複合）酸化物の個数割合と工具寿命の関係を示すグラフである。
【図２】実験で用いた供試快削鋼中の（複合）酸化物の割合と仕上げ面粗度の関係を示すグラフである。
【図３】鋼への（複合）酸化物の添加量と工具寿命の関係を示すグラフである。
【図４】鋼への（複合）酸化物の添加量と仕上げ面粗度の関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a free-cutting steel excellent in machinability and a manufacturing method thereof, and more specifically, by positively containing a specific amount of a specific oxide in the steel, stable and excellent machinability is achieved. As shown, it relates to an improved free-cutting steel and its manufacturing method.
[0002]
[Prior art]
In recent years, machinability of steel materials has become a major problem with mass production of industrial products, and there is a strong demand for machinability improvement due to cost reduction of cutting work and improvement of part finish surface accuracy.
[0003]
Under these circumstances, much research has been conducted on machinability improvement means, such as lead free-cutting steel, sulfur free-cutting steel, Ca free-cutting steel, and composite free-cutting steel combining these. Has been proposed and put into practical use.
[0004]
Among these, lead free-cutting steel is excellent in tool life and finished surface accuracy, etc. However, as the recognition of environmental problems has increased in recent years, lead free-cutting steel containing lead, which is a harmful heavy metal, tends to be avoided. Is strong.
[0005]
Sulfur free-cutting steel, on the other hand, does not cause the environmental pollution problem seen in lead free-cutting steel, but the sulfides present in the steel tend to stretch in the rolling direction, and the mechanical direction is perpendicular to the rolling direction. There is a limit to the amount of sulfur added because it adversely affects the properties and causes cracking starting from sulfide during forging, and satisfactory machinability cannot always be obtained.
[0006]
Further, the machinability improving effect of sulfur free-cutting steel depends on the form of sulfide contained in the steel, and the form of the sulfide is considered to be affected by oxygen in the steel. That is, it is considered that when the amount of oxygen in the steel is large, the sulfide is spheroidized and the tool life is improved. This is thought to be due to oxides in the steel becoming the core of sulfide precipitation when sulfides precipitate in the steel during solidification of the steel. It is said that large-diameter sulfide effective for machinability is likely to precipitate. In other words, large sulfides are generated due to the presence of oxygen, which suppresses tool wear. In this sense, oxygen in steel is considered to have a positive effect on the machinability of steel.
[0007]
However, when the amount of oxygen in the steel is large, many oxides are produced in the steel, but the oxides are hard and act in a direction that promotes tool wear. In other words, as a whole, oxygen in steel is considered to have a positive and negative effect on the machinability of steel.
[0008]
Ca free-cutting steel is a type of free-cutting steel that forms a protective layer called belag on the cutting tool surface by controlling the form (composition) of oxides and suppresses diffusion wear of the tool. The formation of this protective layer is affected by the temperature near the tool surface and the melting point of the oxide. The Ca oxide-based free-cutting steel that is generally manufactured at present produces an oxide called anorsite or gehlenite that has the lowest melting point in a complex oxide of Si, A1, and Ca. Is effective in extending tool life. However, the tool life improvement effect in the low cutting speed region is poor. In addition, in Ca free cutting steel to which A1 is added to adjust the crystal grain size, a high melting point material containing a large amount of A1 ₂ O ₃ as an oxide is likely to be formed. Therefore, tool wear may be accelerated depending on cutting conditions. .
[0009]
[Problems to be solved by the invention]
As described above, the conventional free-cutting steel is not necessarily satisfactory because it causes cutting conditions and environmental pollution problems. In other words, lead free-cutting steel uses lead, which is a heavy metal, so care must be taken from the viewpoint of environmental protection. In sulfur free-cutting steel, oxygen in the steel is favorable for the machinability of the steel. Influences and adverse effects. Ca free-cutting steel is effective in improving the tool life in a high cutting speed region using a carbide tool, but the machinability improving effect in a low cutting speed region is poor.
[0010]
In view of the actual situation, the present invention considers the environment, shows stable and excellent machinability in a wide machining range from a low cutting speed region to a high cutting speed region, and is effective in extending the tool life and improving the surface roughness. It is to provide excellent free-cutting steel and its manufacturing method.
[0011]
[Means for Solving the Problems]
The free-cutting steel according to the present invention capable of solving the above-mentioned problems is C 2: 0.02 to 0.15% in mass%.
Si: 0.5% or less,
Mn: 0.5 to 1.75%,
P: 0.004 to 0.20%
S: 0.15-0.50%,
O: It is made of steel containing 0.005 to 0.028%, and inclusions with a major axis of 1 μm or more appearing in the longitudinal section of the steel are 100 to 2000 per 1 mm ^{2 of the} cross section, and further occupy in the total amount of the inclusions. The number ratio of the oxide is 4% or more, and the number ratio of the oxide of at least one element selected from the group consisting of Na, Li, and B among the oxides is 5% or more, Alternatively, the number of oxides in the total amount of inclusions is 4% or more, and the number of at least two oxides selected from the group consisting of Na, Li, B, and Si among the oxides It is a free-cutting steel excellent in machinability with a ratio of 5% or more.
[0012]
Further, the production method according to the present invention provides a method of producing a free-cutting steel excellent in the machinability, and when producing the free-cutting steel,
An oxide having a melting point of 1000 ° C. or less made of an oxide of at least one element selected from the group consisting of Na, Li, and B is added to the molten steel at 100 ppm or more, or made of Na, Li, B, and Si. The gist is that a composite oxide having an melting point of 1000 ° C. or less composed of oxides of at least two elements selected from the group is added to molten steel at 100 ppm or more. In addition, the said oxide whose melting | fusing point is 1000 degrees C or less can be uniformly mixed and disperse | distributed in steel by adding to molten steel in at least one place of a ladle, a tundish, and a casting_mold | template.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Under the above-mentioned problems, the present inventors searched for a free-cutting component to replace conventional lead, sulfur, Ca oxide, etc., and proceeded with earnest research for further improvement of machinability and finished surface accuracy. I came. As a result, the melting point made of an oxide of at least one element selected from the group consisting of Na, Li and B is selected from the group consisting of an oxide having a melting point of 1000 ° C. or lower, or the group consisting of Na, Li, B and Si. Knowing that a compound oxide containing at least two kinds of complex oxides contained in a suitable amount in molten steel shows stable and excellent machinability and gives good finished surface accuracy, the present invention has been conceived. Is.
[0014]
It is hard oxide inclusions that reduce the machinability of the steel material, and A1 and Si hard oxides intervening in the steel promote tool wear and inhibit machinability. The tool life is shortened and the finished surface roughness is deteriorated.
[0015]
Accordingly, as a result of intensive studies focusing on the low melting point oxide, the present inventors have included a low melting point oxide having a melting point of 1000 ° C. or less, and the low melting point oxide in the steel at the time of cutting is reduced. It has been found that tool life and finished surface accuracy can be improved by melting and softening. Incidentally, if an oxide having a melting point of 1000 ° C. or less is dispersed in the steel, the oxide in the steel is melted or softened due to an increase in the temperature of the cutting edge during cutting, and wear of the tool is suppressed, and the oxide Has been found to effectively protect the tool surface and improve tool life.
[0016]
As described above, metal oxides contained in steel are generally hard and are considered to deteriorate machinability so far, and metal oxides are actively used as machinability improving components. I could n’t think of that. However, at least one selected from the group consisting of oxides of at least one element selected from the group consisting of Na, Li, and B in the steel, or from the group consisting of Na, Li, B, and Si. A composite oxide having a low melting point of two kinds of elements [hereinafter, these may be collectively referred to as (composite) oxides] contained in an appropriate amount in molten steel exhibits stable and excellent machinability. It was found that it gives good finished surface accuracy.
[0017]
Therefore, in the present invention, as described above, the steel is characterized in that an appropriate amount of the (composite) oxide is contained as a machinability improving component in the steel. It is necessary that 15-μm inclusions are dispersed in an amount of 100 to 2000, more preferably 150 to 1000, per 1 mm ² in cross-sectional area. Incidentally, inclusions that can be present in steel include oxides, sulfides, nitrides, etc. Among these inclusions, fine inclusions with a major axis of less than 1 μm are excellent in machinability and finished surface accuracy. Almost no effect. On the other hand, if there are many inclusions in the steel whose major axis exceeds 15 μm, the mechanical properties of the steel material, particularly toughness and ductility, etc., may be significantly adversely affected. There are almost no inclusions.
[0018]
In order to secure the excellent machinability and finished surface accuracy intended in the present invention, the oxides occupying in all inclusions having a major axis of 1 μm or more under the conditions satisfying the size and number of the inclusions. It is essential that the number ratio is 4% or more and the ratio of the number of the (composite) oxide in the oxide is 5% or more.
[0019]
That is, in the present invention, in addition to specifying the number of oxides among inclusions dispersed in the steel material, the number ratio of the (composite) oxide in the oxide is specified. Has succeeded in obtaining free-cutting steel that gives outstanding machinability and finished surface accuracy.
[0020]
Incidentally, all the inclusions that appear in the steel cross section include oxides, sulfides, nitrides, carbides and their composites, and among oxides are alumina, silica, manganese oxide, chromium oxide. In order to ensure the level of machinability and finished surface accuracy intended by the present invention, the number ratio of oxides having a major axis of 1 μm or more in all inclusions is 4% or more. In addition, it is essential that the number ratio of the (composite) oxide in the oxide is 5% or more.
[0021]
When the number ratio of oxides in all the inclusions is less than 4% and the number ratio of the (composite) oxides in the oxide is less than 5%, these specific (composite) oxides The effect of improving the machinability and the finished surface accuracy expected for is not effectively exhibited. 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 softening / melting action of the specific (composite) oxide due to frictional heat during cutting The machinability is greatly improved and the finished surface accuracy can be remarkably improved.
[0022]
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.
[0023]
In addition, although the improvement effect by such an oxide is effectively exhibited independently for each of the oxides of Na, Li and B, it is also effectively exhibited as two or three complex oxides. With regard to the oxide of Si, even if a predetermined amount exists as a single oxide, the machinability as intended by the present invention cannot be obtained, and it is difficult to obtain at least one kind of oxide of Na, Li, and B. It is essential to exist as a complex oxide. This is presumably because the single oxide of Si has a higher melting point than Na oxide, Li oxide, and B oxide, so that the softening / melting action due to the temperature rise during the cutting process is not exhibited effectively.
[0024]
Note that Na, Li, and B, which are oxide sources of Na, Li, and B that contribute to improving machinability and finished surface accuracy, 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.
[0025]
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 shape change in the high-temperature molten steel, but if the method of adding at the above-mentioned position is adopted, the melting temperature can be obtained even in the steel material finally obtained. It can be present as a (composite) oxide of about 1200 ° C. or lower.
[0026]
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). Thus, the (composite) oxide in the amount (number) cannot be present, and it becomes difficult to obtain the excellent machinability and finished surface accuracy 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.
[0027]
The present invention is characterized in that the size and amount of the (composite) oxide contained in the free-cutting steel is defined as described above, and the type of the steel material is not particularly limited. Examples of the standard chemical components of free-cutting steel that are most effective are as follows.
[0028]
C: 0.02-0.15%
C is an important element for improving the strength of steel, but it is also an element that lowers ductility. In low-carbon steel regions where its content is extremely low, the ductility of steel is moderately lowered to improve machinability. Demonstrate the effect of increasing. For that purpose, the C content should be 0.02% or more. However, if the C content becomes too large, the steel becomes of high quality and causes a reduction in tool life. Good.
[0029]
Si: 0.5% 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 0.5%, the necessary amount of oxygen in the steel can be secured. Not only is this difficult, but oxygen in the Na, Li and B oxides may be taken away to make it difficult to develop the machinability modifying action, so it is 0.5% or less, more preferably 0.15% or less. It is desirable to suppress.
[0030]
Mn: 0.5 to 1.75%
Mn generates MnS effective for improving machinability, and is also an element that works effectively to improve hot workability. In order to effectively exhibit these functions, 0.5% or more is contained. It is desirable. However, if the Mn content is too large, the work hardening of the steel material becomes prominent and causes the tool life to be shortened, so it is desirable to keep it to 1.75% or less, more preferably 1.5% or less.
[0031]
P: 0.004 to 0.2%
P is an element effective in reducing the ductility of steel, improving chip disposal during cutting, and reducing the roughness of the finished surface, and 0.004% or more for effectively exerting these effects. It is desirable to contain P. However, if it is excessively contained exceeding 0.2%, surface defects are likely to occur during hot working. Therefore, the P content is preferably 0.004% or more, more preferably 0.01% or more, and is preferably 0.2% or less, more preferably 0.15% or less.
[0032]
S: 0.15-0.5%
S is an element that forms MnS that is effective for improving the overall machinability including chip breaking properties, but if it is less than 0.15%, the effect is not sufficiently exhibited, while if it exceeds 0.5% Deteriorates hot workability and ductility significantly. Therefore, the S content is desirably 0.15% or more, more preferably 0.2% or more, and 0.5% or less, and more preferably 0.4% or less.
[0033]
O: 0.005 to 0.028%
O is an important element that influences the form of precipitation of the (composite) oxide. When the total oxygen content is less than 0.005%, the (composite) oxide having the size and amount defined in the present invention must be present. In order to ensure the machinability and finished surface accuracy as intended in the present invention, it is desirable to contain 0.005% or more, more preferably 0.01% or more. However, if the amount of O is too large, not only will the production of iron oxide be promoted during melting to damage the inner refractory of the melting furnace and shorten the furnace life, but cutting will occur if the molten refractory enters the steel. This is a major cause of reduced tool life. Therefore, it is desirable that the O content be 0.028% or less, more preferably 0.025% or less.
[0034]
The preferred contained elements of the free-cutting steel used in the present invention are as described above, and the remaining component is substantially Fe, but the free-cutting steel is allowed to contain a trace amount of inevitable impurities. Of course, it is also possible to use free-cutting steel in which other elements are positively contained within a range that does not adversely affect the operation 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 tool life and surface roughness, and these can be added alone or in combination of two or more. It is desirable to suppress the total amount to about 0.5% or less.
[0035]
Next, a method for analyzing the (composite) oxide will be described. The specimen was forged into a round bar with a diameter of 155 mm × 155 mm at 1200 ° C. and a diameter of 80 mm, and then normalized by air cooling at 850 ° C. × 1 hr. Polish 4 positions. Next, an indentation is made with a load of 5 kg in order to specify the observation position of the specimen. In the measurement of the number of inclusions, an optical microscope was used to observe the vicinity of the indentation at a magnification of 100 at an area of 0.5 mm × 0.5 mm per visual field in four visual fields, and image analysis was performed on inclusions having a major axis of 1 μm or more. Next, the composition of inclusions is specified by observing four visual fields with an EPMA of “JXA-8800RL” manufactured by JEOL at an acceleration voltage of 15 kV and a magnification of 500 times. In this observation, the presence of each element was confirmed by mapping Na, Li, B, Si, O and Mn, S, Cr. However, since it is difficult to detect light elements such as Li and B in EPMA, four fields of view are observed in the same region as in EPMA observation under SIMA of CAMECA-imsf5f under the primary ion condition O ₂ ⁺ -8 keV-1nA and 500 times magnification. In SIMS, O ₂ ⁺ is used as a primary ion, and the presence of O is not clear, but what is observed in O in EPMA is determined to be an oxide. One (one or more) selected from Na, Li, B, and Si and the presence of oxygen (O) were confirmed by EPMA and SIMS observations as a (composite) oxide. In this observation, among all the inclusions having a major axis of 1 μm or more, the number of all oxides and (composite) oxides observed were calculated and the respective number ratios were calculated. Inclusions composed of oxides and MnS were counted as oxides.
[0036]
Next, the evaluation method will be described. The evaluation items are tool life and finished surface roughness, and the tool life is hot rolled from a 155 mm × 155 mm slab to a steel bar having a diameter of 22 mm × 3 m, and a carbide turning test is performed. evaluated. As shown in Table 1, P10 tips were used as shown in Table 1, cutting speed: 200 (m / min), feed: 0.25 (mm / rev), and cutting: 1.5 (mm). The criterion for determining the tool life was VB wear of 0.2 mm.
[0037]
[Table 1]

[0038]
The finished surface roughness of the parts was evaluated by a carbide turning test using the above-mentioned normalizing material. As shown in Table 2, P10 inserts were used as the test conditions, cutting speed: 150 (m / min), feed: 0.10 (mm / rev), depth of cut: 1.0 (mm), cutting time: 1 (min ). As for the finished surface roughness, an area just before the end of cutting was evaluated.
[0039]
[Table 2]

[0040]
【Example】
EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples as a matter of course, and appropriate modifications are made within a range that can meet the purpose described above and below. It is also possible to carry out and they are all included in the technical scope of the present invention.
[0041]
EXAMPLE Steel of the present invention was cast into a 155 mm square by adding a (composite) oxide whose composition was adjusted to low carbon steel melted in a 500 kg high frequency melting furnace. Table 3 shows the main components of the present invention steel (No. 1 to 12) and the comparative steel (No. 13 to 18), the composition of the added oxide, the added amount, and the melting point. No. No. 13 is the current lead free-cutting steel. 14 is base steel, No. 14; 15 to 18 are comparative examples lacking any of the defining requirements defined in the present invention.
[0042]
The evaluation results of the test materials are shown in Table 4 and FIGS. As shown in FIG. 1, when the number ratio of (composite) oxide is 5% or more, the tool life is significantly improved. The same level of machinability as 13 lead free-cutting steels is obtained. Further, as seen in FIG. 2, when the number ratio of (composite) oxide is 5% or more, the finished surface roughness is also improved to the level of lead free-cutting steel.
[0043]
Further, as is apparent from FIGS. 3 and 4, it is understood that the component-adjusted (composite) oxide must be added in an amount of 100 ppm or more in order to effectively exhibit the effect of improving the tool life.
[0044]
As is clear from these experimental results, according to the present invention, it can be seen that a free-cutting steel having tool life and finished surface accuracy that is completely inferior to conventional lead free-cutting steel can be obtained.
[0045]
[Table 3]

[0046]
[Table 4]

[0047]
【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 4% or more, and the oxide is selected from Na, Li, and B. The free-cutting steel of the present invention in which the number ratio of one or more oxides or two or more complex oxides selected from Li, Na, B, and Si is 5% or more causes pollution. Excellent tool life and finished surface roughness can be obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the number ratio of (composite) oxides in a test free-cutting steel used in an experiment and the tool life.
FIG. 2 is a graph showing the relationship between the ratio of (composite) oxide in the free cutting steel used in the experiment and the finished surface roughness.
FIG. 3 is a graph showing the relationship between the amount of (composite) oxide added to steel and the tool life.
FIG. 4 is a graph showing the relationship between the amount of (composite) oxide added to steel and the finished surface roughness.

Claims

C: 0.02 to 0.15% by mass%,
Si: 0.5% or less,
Mn: 0.5 to 1.75%,
P: 0.004 to 0.20%
S: 0.15-0.50%,
O: 0.005~0.028% to have free and
The balance being a bar steel for free-cutting which is Fe and unavoidable impurities,
The steel bar further contains at least one element selected from the group consisting of Na, Li and B, and
Inclusions major axis 1 15 m appearing in a cross section parallel to the forging direction of the bar steel with 100 to 2000 per cross-sectional area 1 mm ^2, the number of oxides occupied in the total amount of the inclusions is 4% or more And the number ratio of oxides of at least one element selected from the group consisting of Na, Li, and B among the oxides is 5% or more. cutting for bar steel.

The Na, Li, and B include at least one element selected from the group consisting of Na, Li, and B in molten steel in which C, Si, Mn, P, and S satisfy the scope of claim 1. The steel bar for free cutting according to claim 1, wherein the steel bar is supplied by adding 100 ppm or more of an oxide composed of an oxide and having a melting point of 1000 ° C or lower.

C: 0.02 to 0.15% by mass%,
Si: 0.5% or less,
Mn: 0.5 to 1.75%,
P: 0.004 to 0.20%
S: 0.15-0.50%,
O: 0.005~0.028% to have free and
The balance being a bar steel for free-cutting which is Fe and unavoidable impurities,
The steel bar further contains at least one element selected from the group consisting of Na, Li and B, and
Inclusions having a major axis of 1 to 15 μm appearing in a cross section parallel to the forging direction of the steel are 100 to 2000 per 1 mm ^{2 in} cross-sectional area, and the number ratio of oxides in the total amount of the inclusions is 4% or more,
In addition, the number ratio of at least two kinds of oxides selected from the group consisting of Na, Li, B, and Si among the oxides is 5% or more . bar steel.

The Na, Li, and B include at least one element selected from the group consisting of Na, Li, B, and Si in a molten steel in which C, Mn, P, and S satisfy the range of claim 3. The steel bar for free cutting 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 made of an oxide.

A method of manufacturing a free-cutting for bar steel according to claim 1 or 2,
C, Si, Mn, P and S prepare molten steel satisfying the range of claim 1;
To the machinability characterized by adding 100 ppm or more of an oxide having a melting point of 1000 ° C. or less composed of an oxide of at least one element selected from the group consisting of Na, Li and B to the molten steel. preparation of excellent free-cutting for the bar steel.

A method of manufacturing a free-cutting for bar steel according to claim 3 or 4,
C, Mn, P and S prepare molten steel satisfying the range of claim 3,
During solution steel, the features Na, Li, B, that the melting point of an oxide of at least two elements selected from the group consisting of Si is added a composite oxide of 1000 ° C. or less 1 00Ppm more preparation of free-cutting for the bar steel with excellent machinability.

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.