JP2004176176A - Steel superior in machinability - Google Patents
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- JP2004176176A JP2004176176A JP2003374511A JP2003374511A JP2004176176A JP 2004176176 A JP2004176176 A JP 2004176176A JP 2003374511 A JP2003374511 A JP 2003374511A JP 2003374511 A JP2003374511 A JP 2003374511A JP 2004176176 A JP2004176176 A JP 2004176176A
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 80
- 239000010959 steel Substances 0.000 title claims abstract description 80
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 21
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 8
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- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 229910052758 niobium Inorganic materials 0.000 claims description 3
- 229910052721 tungsten Inorganic materials 0.000 claims description 3
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- 229910017231 MnTe Inorganic materials 0.000 description 2
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
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Abstract
Description
本発明は、自動車や一般機械などに用いられる鋼に関するもので、特に切削時の工具寿命と切削表面粗さおよび切り屑処理性に優れた被削性に優れた鋼に関する。 The present invention relates to steel used for automobiles, general machines, and the like, and more particularly to steel excellent in machinability, which is excellent in tool life, cutting surface roughness, and chip disposal during cutting.
一般機械や自動車は多種の部品を組み合わせて製造されているが、その部品は要求精度と製造効率の観点から、多くの場合、切削工程を経て製造されている。その際、コスト低減と生産能率の向上が求められ、鋼にも被削性の向上が求められている。特に従来ＳＵＭ２３やＳＵＭ２４Ｌは被削性を重要視して開発されてきた。これまで被削性を向上させるためにＳ，Ｐｂなどの被削性向上元素を添加するのが有効であることが知られている。しかし、需要家によってはＰｂは環境負荷として使用を避ける場合も有り、その使用量を低減する方向にある。 General machines and automobiles are manufactured by combining various types of parts, and the parts are often manufactured through a cutting process from the viewpoint of required accuracy and manufacturing efficiency. At that time, cost reduction and improvement in production efficiency are required, and steel is also required to have improved machinability. In particular, conventionally, SUM23 and SUM24L have been developed with emphasis on machinability. It has been known that it is effective to add a machinability improving element such as S or Pb in order to improve machinability. However, some customers avoid using Pb as an environmental load, and the amount of Pb used is being reduced.
これまでもＰｂを添加しない場合にはＳのようにＭｎＳのような切削環境下で軟質となる介在物を形成して被削性を向上させる手法が使われている。しかし、いわゆる低炭鉛快削鋼ＳＵＭ２４Ｌにも低炭硫黄快削鋼ＳＵＭ２３と同量のＳが添加されており、被削性向上には従来以上のＳ量を添加する必要がある。しかし、多量のＳ添加ではＭｎＳを単に粗大にするだけで、被削性向上に有効なＭｎＳ分布にならないだけでなく、圧延、鍛造等において破壊起点になって圧延疵等の製造上の問題を多く引き起こす。さらに、ＳＵＭ２３をベースとする硫黄快削鋼では構成刃先が付着しやすく、構成刃先の脱落および切り屑分離現象に伴う、切削表面に凹凸が生じ、表面粗さが劣化する。切り屑処理性においても、切り屑が短く分断しやすい方が良好とされているが、単なるＳ増量添加だけではマトリックスの延性が大きいため、十分に分断されず、大きく改善できなかった。 Until now, when Pb is not added, a method of forming inclusions that are soft under a cutting environment such as MnS like S and improving machinability has been used. However, the same amount of S as that of the low-carbon sulfur free-cutting steel SUM23 is added to the so-called low-carbon lead free-cutting steel SUM24L, and it is necessary to add more S than before to improve machinability. However, when a large amount of S is added, merely making MnS coarse does not only result in MnS distribution effective for improving machinability, but also causes a problem in production such as rolling flaws as a starting point of fracture in rolling and forging. Cause a lot. Further, in the case of the sulfur free-cutting steel based on SUM23, the component cutting edge easily adheres, and the cutting surface becomes uneven due to the falling of the component cutting edge and the chip separation phenomenon, and the surface roughness deteriorates. In terms of the chip disposability, it is considered better that the chips are short and easy to be divided. However, the simple addition of an increased amount of S does not sufficiently disperse the matrix because the ductility of the matrix is large, and the chip cannot be significantly improved.
さらに、Ｓ以外の元素、Ｔｅ，Ｂｉ，Ｐ等も被削性向上元素として知られているが、ある程度被削性を向上させることができても、圧延や熱間鍛造時に割れを生じ易くなるため、極力少ない方が望ましいとされている。（例えば、特許文献１、特許文献２、特許文献３、特許文献４参照。）。
Further, elements other than S, such as Te, Bi, and P, are also known as machinability improving elements. However, even if machinability can be improved to some extent, cracks are likely to occur during rolling or hot forging. Therefore, it is said that it is desirable to have as little as possible. (For example, see
本発明は、圧延や熱間鍛造における不具合を避けつつ工具寿命と表面粗さの両者を改善し、従来の低炭鉛快削鋼と同等以上の被削性を有する鋼を提供する。 The present invention provides a steel which has both improved tool life and surface roughness while avoiding problems in rolling and hot forging, and has machinability equal to or higher than that of a conventional low-carbon lead free-cutting steel.
切削は切り屑を分離する破壊現象であり、それを促進させることが一つのポイントとなる。この効果はＳを単純に増量するだけでは限界がある本発明者らは、Ｓを増量するだけでなく、マトリックスを脆化させることで破壊を容易にして工具寿命を延長するとともに切削表面の凹凸を抑制することで被削性が向上することを知見した。 Cutting is a breaking phenomenon that separates chips, and promoting it is one point. This effect is limited by simply increasing the amount of S. The present inventors not only increase the amount of S, but also make the matrix brittle to facilitate fracture and prolong the tool life and to reduce the roughness of the cutting surface. It has been found that the machinability is improved by suppressing the amount.
本発明は上記知見に基づいてなされたもので、その要旨は次のとおりである。 The present invention has been made based on the above findings, and the gist is as follows.
（１）質量％で、Ｃ：０．００５〜０．２％，Ｓ：０．１〜０．５％，Ｂ：０．０００５〜０．０５％を含み、Ｍｎ／Ｓ：１．２〜２．８で、かつミクロ組織においてパーライト面積率が５％以下であることを特徴とする被削性に優れる鋼。 (1) In mass%, C: 0.005 to 0.2%, S: 0.1 to 0.5%, B: 0.0005 to 0.05%, Mn / S: 1.2 to A steel excellent in machinability characterized by having a pearlite area ratio of 5% or less in a microstructure.
（２）質量％で、Ｃ：０．００５〜０．２％，Ｓｉ：０．００１〜０．５％，Ｍｎ：０．５〜３．０％，Ｐ：０．００１〜０．２％，Ｓ：０．１〜０．５％，ｔｏｔａｌ−Ｎ：０．００２〜０．０２％，ｔｏｔａｌ−Ｏ：０．０００５〜０．０３５％を含有し、残部がＦｅおよび不可避的不純物よりなり、ミクロ組織においてパーライト面積率が５％以下であることを特徴とする被削性に優れる鋼。 (2) In mass%, C: 0.005 to 0.2%, Si: 0.001 to 0.5%, Mn: 0.5 to 3.0%, P: 0.001 to 0.2% , S: 0.1-0.5%, total-N: 0.002-0.02%, total-O: 0.0005-0.035%, with the balance being Fe and unavoidable impurities A steel excellent in machinability, characterized in that the pearlite area ratio in the microstructure is 5% or less.
（３）前記鋼において、質量％で、さらに、Ｂ：０．０００５〜０．０５％を含有することを特徴とする（２）記載の被削性に優れる鋼。 (3) The steel excellent in machinability according to (2), wherein the steel further contains B: 0.0005 to 0.05% by mass%.
（４）前記鋼において、Ｍｎ／Ｓ：１．２〜２．８であることを特徴とする（２）または（３）記載の被削性に優れる鋼。 (4) The steel excellent in machinability according to (2) or (3), wherein Mn / S is 1.2 to 2.8.
（５）（１）〜（４）のいずれかの項に記載の鋼で、抽出レプリカ法にて採取して透過型電子顕微鏡で観察するＭｎＳに関し、鋼材の圧延方向と平行な断面において円相当径にて０．１〜０．５μｍのものの存在密度が１０，０００個／mm2 以上であることを特徴とする被削性に優れる鋼。 (5) MnS sampled by the extraction replica method and observed with a transmission electron microscope in the steel according to any one of (1) to (4), which is equivalent to a circle in a cross section parallel to the rolling direction of the steel material. A steel excellent in machinability, characterized in that the density of particles having a diameter of 0.1 to 0.5 μm is not less than 10,000 pieces / mm 2 .
（６）前記鋼において、Ａｌ：０．０１％以下に制限することを特徴とする（１）〜（５）のいずれかの項に記載の被削性に優れる鋼。 (6) The steel according to any one of (1) to (5), wherein the steel is limited to Al: 0.01% or less.
（７）前記鋼が、質量％で、さらに、Ｖ：０．０５〜１．０％，Ｎｂ：０．００５〜０．２％，Ｃｒ：０．０１〜２．０％，Ｍｏ：０．０５〜１．０％，Ｗ：０．０５〜１．０％の１種または２種以上を含有することを特徴とする（１）〜（６）のいずれかの項に記載の被削性に優れる鋼。 (7) The steel further contains, by mass%, V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, Cr: 0.01 to 2.0%, Mo: 0. Machinability according to any one of (1) to (6), wherein one or more of 0.05 to 1.0% and W: 0.05 to 1.0% are contained. Excellent steel.
（８）前記鋼が、質量％で、さらに、Ｎｉ：０．０５〜２．０％，Ｃｕ：０．０１〜２．０％の１種または２種を含有することを特徴とする（１）〜（７）のいずれかの項に記載の被削性に優れる鋼。 (8) The steel is characterized in that the steel further contains one or two of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% by mass% (1). The steel excellent in machinability according to any one of the items (1) to (7).
（９）前記鋼が、質量％で、さらに、Ｓｎ：０．００５〜２．０％，Ｚｎ：０．０００５〜０．５％の１種または２種を含有することを特徴とする（１）〜（８）のいずれかの項に記載の被削性に優れる鋼。 (9) The steel is characterized in that the steel further contains one or two of Sn: 0.005 to 2.0% and Zn: 0.0005 to 0.5% by mass. ) The steel excellent in machinability according to any one of the above items (8) to (8).
（１０）前記鋼が、質量％で、さらに、Ｔｉ：０．０００５〜０．１％，Ｃａ：０．０００２〜０．００５％，Ｚｒ：０．０００５〜０．１％，Ｍｇ：０．０００３〜０．００５％の１種または２種以上を含有することを特徴とする（１）〜（９）のいずれかの項に記載の被削性に優れる鋼。 (10) The steel further contains, by mass%, Ti: 0.0005 to 0.1%, Ca: 0.0002 to 0.005%, Zr: 0.0005 to 0.1%, and Mg: 0. The steel excellent in machinability according to any one of (1) to (9), which contains one or more of 0003 to 0.005%.
（１１）前記鋼が、質量％で、さらに、Ｔｅ：０．０００３〜０．０５％，Ｂｉ：０．００５〜０．５％，Ｐｂ：０．０１〜０．５％の１種または２種以上を含有することを特徴とする（１）〜（１０）のいずれかの項に記載の被削性に優れる鋼。 (11) The steel is, in mass%, one or more of Te: 0.0003 to 0.05%, Bi: 0.005 to 0.5%, Pb: 0.01 to 0.5%. A steel excellent in machinability according to any one of (1) to (10), which contains at least one kind.
以上説明したように、本発明は切削時の工具寿命と切削表面粗さおよび切り屑処理性に優れた特性を有するため自動車用部材、一般機械用部材に用いることが可能となる。 As described above, the present invention has excellent properties such as tool life during cutting, cutting surface roughness, and chip disposability, so that it can be used for members for automobiles and members for general machinery.
本発明は、鉛を添加することなく、十分な被削性、特に良好な表面粗さを有する鋼を得るものである。そのため、鋼のミクロ組織は極力均一な方がよく、鋼中のパーライトの分布ですら、不均一の原因となり、表面粗さを低下させる原因であることを見出した。そのため、均質化を徹底するためにパーライトの面積率を制限することで良好な表面粗さと工具寿命特性を得るものである。次に、本発明で規定する鋼成分の限定理由を説明する。 The present invention is to obtain a steel having sufficient machinability, particularly good surface roughness, without adding lead. Therefore, it has been found that the microstructure of the steel is preferably as uniform as possible, and even the distribution of pearlite in the steel is a cause of non-uniformity and a cause of a reduction in surface roughness. Therefore, good surface roughness and tool life characteristics are obtained by limiting the area ratio of pearlite in order to thoroughly homogenize. Next, the reasons for limiting the steel components specified in the present invention will be described.
Ｃは、鋼材の基本強度と鋼中の酸素量に関係するので被削性に大きな影響を及ぼす。Ｃを多量に添加して強度を高めると被削性を低下させるのでその上限を０．２％とした。一方、被削性を低下させる硬質酸化物生成を防止しつつ、凝固過程でのピンホール等の高温での固溶酸素の弊害を抑制するため、酸素量を適量に制御する必要がある。単純に吹錬によってＣ量を低減させすぎるとコストが嵩むだけでなく、鋼中酸素量が多量に残留してピンホール等の不具合の原因となる。従って、ピンホール等の不具合を容易に防止できるＣ量０．００５％を下限とした。Ｃ量の好ましい下値は０．０５％である。 C has a significant effect on machinability because it relates to the basic strength of the steel material and the amount of oxygen in the steel. If the strength is increased by adding a large amount of C, the machinability decreases, so the upper limit was made 0.2%. On the other hand, it is necessary to control the amount of oxygen to an appropriate amount in order to prevent the formation of hard oxides that reduce machinability and to suppress the adverse effects of dissolved oxygen at high temperatures such as pinholes during the solidification process. If the amount of C is simply reduced too much by blowing, not only the cost increases, but also a large amount of oxygen in the steel remains and causes problems such as pinholes. Therefore, the lower limit of the C content of 0.005% at which inconveniences such as pinholes can be easily prevented is set. A preferred lower value of the C content is 0.05%.
Ｓｉの過度な添加は硬質酸化物を生じて被削性を低下させるが、適量の添加は酸化物を軟質化させ、被削性を低下させることが多いのでその上限は０．５％であり、それ以上では硬質酸化物を生じる。０．００１％以下では酸化物の軟質化が困難になるとともに工業的にはコストがかかる。 Excessive addition of Si produces hard oxides and reduces machinability, but the addition of an appropriate amount softens oxides and reduces machinability, so the upper limit is 0.5%. Above which hard oxides are formed. If the content is less than 0.001%, it becomes difficult to soften the oxide, and the cost is industrially high.
Ｍｎは、鋼中硫黄をＭｎＳとして固定・分散させるために必要である。また鋼中酸化物を軟質化させ、酸化物を無害化させるために必要である。その効果は添加するＳ量にも依存するが、０．２％以下では添加ＳをＭｎＳとして十分に固定できず、ＳがＦｅＳとなり脆くなる。Ｍｎ量が大きくなると素地の硬さが大きくなり被削性や冷間加工性が低下するので、３．０％を上限とした。 Mn is necessary for fixing and dispersing sulfur in steel as MnS. In addition, it is necessary to soften oxides in steel and make the oxides harmless. Although the effect also depends on the amount of S added, if it is 0.2% or less, the added S cannot be sufficiently fixed as MnS, and S becomes FeS and becomes brittle. When the amount of Mn increases, the hardness of the substrate increases, and machinability and cold workability decrease. Therefore, the upper limit is set to 3.0%.
Ｐは、鋼中において素地の硬さが大きくなり、冷間加工性だけでなく、熱間加工性や鋳造特性が低下するので、その上限を０．２％にしなければならない。一方、被削性向上に効果がある元素で下限値を０．００１％とした。 P increases the hardness of the base material in steel, and deteriorates not only cold workability but also hot workability and casting properties. Therefore, the upper limit of P must be set to 0.2%. On the other hand, the lower limit of elements that are effective in improving machinability is set to 0.001%.
Ｓは、Ｍｎと結合してＭｎＳ介在物として存在する。ＭｎＳは被削性を向上させるが、伸延したＭｎＳは鍛造時の異方性を生じる原因の一つである。大きなＭｎＳは避けるべきであるが、被削性向上の観点からは多量の添加が好ましい。従ってＭｎＳを微細分散させることが好ましい。Ｐｂを添加しない場合の被削性の向上には０．１％以上の添加が必要である。一方、０．５％を超えると粗大ＭｎＳ生成の確率が高くなり、更に熱間変形特性の低下から製造中の疵の発生が増加する恐れがあるので、これを上限とした。 S bonds with Mn and exists as MnS inclusions. MnS improves machinability, but elongated MnS is one of the causes of anisotropy during forging. Large MnS should be avoided, but a large amount is preferable from the viewpoint of improving machinability. Therefore, it is preferable to finely disperse MnS. To improve machinability when Pb is not added, it is necessary to add 0.1% or more. On the other hand, if it exceeds 0.5%, the probability of generation of coarse MnS becomes high, and furthermore, there is a possibility that the generation of flaws during the production may increase due to the deterioration of the hot deformation characteristics.
Ｎ（ｔｏｔａｌ−Ｎ）は、固溶Ｎの場合、鋼を硬化させる。特に切削においては動的ひずみ時効によって刃先近傍で硬化し、工具の寿命を低下させるが、切削表面粗さを改善する効果もある。またＢと結びついてＢＮを生成して被削性を向上させる。０．００２％以下では固溶窒素による表面粗さ向上効果やＢＮによる被削性改善効果が認められないので、これを下限とした。また０．０２％を越えると固溶窒素が多量に存在するためかえって工具寿命を低下させる。また鋳造途中に気泡を生成し、疵などの原因となる。従って本発明ではそれらの弊害が顕著になる０．０２％を上限とした。 N (total-N) hardens steel in the case of solid solution N. In particular, in cutting, it hardens near the cutting edge due to dynamic strain aging and shortens the life of the tool, but also has the effect of improving the cutting surface roughness. In addition, BN is generated in combination with B to improve machinability. If the content is 0.002% or less, the effect of improving the surface roughness due to solid solution nitrogen and the effect of improving the machinability due to BN are not recognized. On the other hand, if the content exceeds 0.02%, a large amount of solute nitrogen is present, so that the tool life is rather shortened. In addition, bubbles are generated during casting, which causes flaws and the like. Therefore, in the present invention, the upper limit is set to 0.02% at which these adverse effects become remarkable.
Ｏ（ｔｏｔａｌ−Ｏ）は、フリーで存在する場合には冷却時に気泡となり、ピンホールの原因となる。また酸化物を軟質化し、被削性に有害な硬質酸化物を抑制するためにも制御が必要である。さらにＭｎＳの微細分散させる際にも析出核として酸化物を利用する。０．０００５％未満では十分にＭｎＳを微細分散させることができず、粗大なＭｎＳを生じ、機械的性質にも悪影響を及ぼす。従って０．０００５％を下限とした。さらに酸素量０．０３５％を越えると鋳造中に気泡となりピンホールとなるため、０．０３５％以下とした。 When O (total-O) is present in a free state, it becomes bubbles at the time of cooling and causes pinholes. Control is also required to soften oxides and suppress hard oxides harmful to machinability. Further, when finely dispersing MnS, an oxide is used as a precipitation nucleus. If the content is less than 0.0005%, MnS cannot be sufficiently finely dispersed, coarse MnS is generated, and the mechanical properties are adversely affected. Therefore, 0.0005% was made the lower limit. Further, if the oxygen content exceeds 0.035%, bubbles are formed during casting and pinholes are formed.
パーライト面積率を５％以下とする理由について説明する。一般に炭素を含む鋼を変態点以上の温度から冷却すると、フェライト−パーライト組織となる。本発明の対処となるＣ量の比較的少ない鋼の場合、変態点（Ａ３点）以上の温度から空冷後、切り出してその内部を鏡面研磨してナイタールでエッチングすると、図１のようなミクロ組織を観察することができる。黒い粒がパーライトと呼ばれるフェライトとセメンタイトの複合組織であるが、通常、このようにナイタールによって黒く見える粒は白くみえるフェライト粒よりも硬質であり、鋼の変形／破断挙動において局部的にフェライト粒とは異なる挙動を示す。このことは切削において切りくずの破断挙動において、均一変形／破断を阻害するため、構成刃先の生成に大きく関与し、さらには切削面の表面粗さを劣化させる。従って、Ｃに起因する組織的不均一を極力排除することが重要である。そこでナイタールでエッチングされる黒い粒をパーライト粒とみなし、このパーライト粒が多すぎると組織不均一を引き起こし、表面粗さ劣化の原因になるのでその面積率を５％以下に制限した。図４にパーライト面積率と表面粗さの関係を示した。 The reason for setting the pearlite area ratio to 5% or less will be described. Generally, when a steel containing carbon is cooled from a temperature equal to or higher than the transformation point, a ferrite-pearlite structure is formed. In the case of steel having a relatively small amount of C, which is to be dealt with by the present invention, after cooling from a temperature above the transformation point (point A3), cutting out, mirror-polishing the inside and etching with nital, the microstructure as shown in FIG. Can be observed. The black grains are a composite structure of ferrite and cementite called pearlite. Generally, grains that appear black due to nital are harder than ferrite grains that appear white, and the ferrite grains appear locally in the deformation / fracture behavior of steel. Shows different behavior. This hinders uniform deformation / rupture in the chip breaking behavior during cutting, and thus greatly contributes to the formation of a cutting edge, and further deteriorates the surface roughness of the cut surface. Therefore, it is important to eliminate systematic nonuniformity caused by C as much as possible. Therefore, black particles etched with nital were regarded as pearlite particles, and if the number of pearlite particles was too large, the structure became nonuniform and the surface roughness deteriorated. Therefore, the area ratio was limited to 5% or less. FIG. 4 shows the relationship between the pearlite area ratio and the surface roughness.
ここで測定方法の詳細に関して述べる。圧延または鍛造後の鋼の長手方向断面（Ｌ断面）に切断、樹脂埋め込みサンプルを鏡面研磨し、ナイタールエッチングした。ナイタールにて黒色にエッチングされたものの内、灰色のＭｎＳを除いた粒径（円相当径）１μｍ以上の粒を画像処理装置で解析し、その面積率を求めた。面積率測定の画像処理時に、黒色に見えるパーライトに合わせた“しきい値”設定で画像濃淡を合わせ、グレーに見える介在物（ＭｎＳ等）を画面上から消すことで、パーライトのみを測定対象とした。この時の認識最小パーライトは約１μｍであるが、１μｍ未満のパーライトは被削性に影響を及ぼさないので、認識されなくても影響はない。 Here, the details of the measurement method will be described. The rolled or forged steel was cut into a longitudinal section (L section), and the resin-embedded sample was mirror-polished and nital etched. Of those etched black with Nital, particles having a particle size (circle equivalent diameter) of 1 μm or more, excluding gray MnS, were analyzed with an image processing apparatus, and the area ratio was determined. At the time of image processing of area ratio measurement, adjust the image density by setting the "threshold" according to the pearlite that looks black, and eliminate grayish inclusions (MnS, etc.) from the screen, so that only pearlite can be measured. did. The minimum perlite recognized at this time is about 1 μm, but perlite smaller than 1 μm does not affect the machinability, so there is no effect even if it is not recognized.
本発明での測定視野は、１視野０．２mm2 （０．４mm×０．５mm）を４００倍以上の倍率で２０視野測定し、計４mm2 の面積について、パーライト面積率を算出した。 In the present invention, 20 visual fields were measured at a magnification of 400 times or more in one visual field of 0.2 mm 2 (0.4 mm × 0.5 mm), and the pearlite area ratio was calculated for a total area of 4 mm 2 .
ＢはＢＮとして析出すると被削性向上に効果がある。これらの効果は０．０００５％以下では顕著でなく、０．０５％を超えて添加してもその効果が飽和し、ＢＮが多く析出しすぎるとかえって鋳造特性、熱間変形特性の劣化から製造中に割れを生じる。そこで０．０００５〜０．０５％を範囲とした。 When B is precipitated as BN, B is effective in improving machinability. These effects are not remarkable at 0.0005% or less, and even if added over 0.05%, the effects are saturated, and if too much BN is precipitated, the casting properties and hot deformation properties are rather deteriorated. Cracks occur inside. Therefore, the range is 0.0005 to 0.05%.
Ｍｎ／Ｓに関してはすでに熱間延性に大きく影響し、通常、Ｍｎ／Ｓ＞３でなければ製造性を大きく低下させることが知られている。その原因はＦｅＳの生成であるが、本発明においては、低Ｃかつ高Ｓの領域ではその比率をさらに低下させることができることを見出しＭｎ／Ｓ：１．２〜２．８に規制した。Ｍｎ／Ｓ：１．２以下ではＦｅＳが多く生成し、熱間延性を極端に低下させ、製造性を大きく低下させる。Ｍｎ／Ｓ：２．８以上では硬さが上昇し、更に微細ＭｎＳが生成しずらくなるので被削性が低下する。従って、Ｍｎ／Ｓの値は１．２〜２．８とすることが好ましい。 It is known that Mn / S has a great influence on hot ductility, and usually, unless Mn / S> 3, the productivity is greatly reduced. The cause is the formation of FeS, but in the present invention, it has been found that the ratio can be further reduced in the region of low C and high S, and Mn / S is regulated to 1.2 to 2.8. If Mn / S is 1.2 or less, a large amount of FeS is generated, and the hot ductility is extremely reduced, and the productivity is greatly reduced. When Mn / S is 2.8 or more, the hardness increases, and fine MnS is hardly generated, so that the machinability decreases. Therefore, the value of Mn / S is preferably set to 1.2 to 2.8.
図２に発明鋼のＭｎＳをレプリカ法を用い、透過型電子顕微鏡にて観察した例を示す。従来鋼の成分範囲で従来どおりの熱履歴であれば図２（ｂ）に示すような、粗大なＭｎＳのみとなり、表面粗さを小さくすることができない。本発明では、Ｍｎ／Ｓ：１．２〜２．８と規定することで図２（ａ）に示すような微細なＭｎＳを生成させることができる。この微細なＭｎＳは連続鋳造やインゴットによる鋳造後、９００℃以上の加熱を繰り返すことにより、個数を増加させることができる。 FIG. 2 shows an example of MnS of the invention steel observed by a transmission electron microscope using a replica method. If the heat history is the same as in the conventional steel within the component range of the conventional steel, only coarse MnS as shown in FIG. 2B is obtained, and the surface roughness cannot be reduced. In the present invention, by defining Mn / S as 1.2 to 2.8, fine MnS as shown in FIG. 2A can be generated. The number of the fine MnS can be increased by repeating heating at 900 ° C. or more after continuous casting or casting by ingot.
次に、本発明では、抽出レプリカ法にて採取して透過型電子顕微鏡で観察するＭｎＳに関し、鋼材の圧延方向と平行な断面において、円相当径で、０．１〜０．５μｍ以下のものの存在密度が１０，０００個／mm2 以上であることが好ましい。 Next, in the present invention, regarding the MnS sampled by the extraction replica method and observed with a transmission electron microscope, the cross-section parallel to the rolling direction of the steel material has an equivalent circle diameter of 0.1 to 0.5 μm or less. It is preferable that the existence density is 10,000 pieces / mm 2 or more.
なお、ＭｎＳとは、純粋なＭｎＳのみならず、ＭｎＳを主体に含み、Ｆｅ，Ｃａ，Ｔｉ，Ｚｒ，Ｍｇ，ＲＥＭ等の硫化物がＭｎＳと固溶したり結合して共存している介在物や、ＭｎＴｅのようにＳ以外の元素がＭｎと化合物を形成してＭｎＳと固溶・結合して共存している介在物や、酸化物を核として析出した上記介在物が含まれるものであり、化学式では、（Ｍｎ，Ｘ）（Ｓ，Ｙ）（ここで、Ｘ：Ｍｎ以外の硫化物形成元素、Ｙ：Ｓ以外でＭｎと結合する元素）として表記できるＭｎ硫化物系介在物を総称して言うものである。 In addition, MnS means not only pure MnS but also an inclusion mainly containing MnS, and sulfides such as Fe, Ca, Ti, Zr, Mg and REM coexist with MnS by solid solution or bonding. And inclusions such as MnTe in which an element other than S forms a compound with Mn to form a compound with MnS to form a solid solution / bond and coexist with the MnS, or includes the above-mentioned inclusions precipitated with an oxide as a nucleus. In the chemical formula, Mn sulfide-based inclusions that can be expressed as (Mn, X) (S, Y) (here, X: an element other than Mn and a sulfide-forming element other than Y: S that binds to Mn) are collectively referred to. That's what they say.
Ａｌは脱酸元素で鋼中ではＡｌ2 Ｏ3 やＡＩＮを形成する。しかし、Ａｌ2 Ｏ3 は硬質なので切削時に工具損傷の原因となり、摩耗を促進させる。そこでＡｌ2 Ｏ3を多量に生成しない０．０１％以下に制限した。特に工具寿命を優先させる場合には０．００５％以下が好ましい。 Al is a deoxidizing element and forms Al 2 O 3 and AIN in steel. However, since Al 2 O 3 is hard, it causes tool damage during cutting and promotes wear. Therefore, the content is limited to 0.01% or less, which does not produce a large amount of Al 2 O 3 . In particular, when giving priority to the tool life, 0.005% or less is preferable.
次に、本発明においては、上述した成分に加え、Ｖ，Ｎｂ，Ｃｒ，Ｍｏ，Ｗ，Ｎｉ，Ｃｕ，Ｓｕ，Ｚｎ，Ｔｉ，Ｃａ，Ｚｒ，Ｍｇ，Ｔｅ，Ｂｉ，Ｐｂの１種または２種以上を必要に応じて添加することができる。 Next, in the present invention, in addition to the above-mentioned components, one or two of V, Nb, Cr, Mo, W, Ni, Cu, Su, Zn, Ti, Ca, Zr, Mg, Te, Bi, and Pb. More than one species can be added as needed.
Ｖは、炭窒化物を形成し、二次析出硬化により鋼を強化することができる。０．０５％以下では高強度化に効果はなく、１．０％を超えて添加すると多くの炭窒化物を析出し、かえって機械的性質を損なうので、これを上限とした。 V forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on increasing the strength, and if it exceeds 1.0%, a large amount of carbonitride precipitates and mechanical properties are rather impaired.
Ｎｂも炭窒化物を形成し、二次析出硬化により鋼を強化することができる。０．００５％以下では高強度化に効果はなく、０．２％を超えて添加すると多くの炭窒化物を析出し、かえって機械的性質を損なうので、これを上限とした。 Nb also forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.005%, there is no effect on increasing the strength, and if it exceeds 0.2%, a large amount of carbonitride precipitates and mechanical properties are rather impaired.
Ｃｒは、焼入れ性向上、焼戻し軟化抵抗付与元素である。そのため高強度化が必要な鋼には添加される。その場合、０．０１％以上の添加を必要とする。しかし多量に添加するとＣｒ炭化物を生成し脆化させるため、２．０％を上限とした。 Cr is an element that imparts hardenability and temper softening resistance. Therefore, it is added to steels requiring high strength. In that case, 0.01% or more must be added. However, if added in a large amount, Cr carbides are formed and embrittled, so the upper limit is 2.0%.
Ｍｏは、焼戻し軟化抵抗を付与するとともに、焼入れ性を向上させる元素である。０．０５％未満ではその効果が認められず、１．０％を超えて添加してもその効果が飽和しているので、０．０５％〜１．０％を添加範囲とした。 Mo is an element that imparts temper softening resistance and improves hardenability. If the content is less than 0.05%, the effect is not recognized, and even if added over 1.0%, the effect is saturated. Therefore, the addition range is set to 0.05% to 1.0%.
Ｗは、炭化物を形成し、二次析出硬化により鋼を強化することができる。０．０５％以下では高強度化に効果はなく、１．０％を超えて添加すると多くの炭化物が析出し、かえって機械的性質を損うので、これを上限とした。 W forms carbides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on increasing the strength, and if it exceeds 1.0%, a large amount of carbides precipitates out, which impairs the mechanical properties.
Ｎｉは、フェライトを強化し、延性を延性向上させるとともに焼入れ性向上、耐食性向上にも有効である。０．０５％未満ではその効果は認められず、２．０％を超えて添加しても、機械的性質の点では効果が飽和するので、これを上限とした。 Ni is effective in strengthening ferrite, improving ductility, improving hardenability, and improving corrosion resistance. If the content is less than 0.05%, the effect is not recognized. If the content exceeds 2.0%, the effect is saturated in terms of mechanical properties. Therefore, the upper limit is set.
Ｃｕは、フェライトを強化し、焼入れ性向上、耐食性向上にも有効である。０．０１％未満ではその効果は認められず、２．０％を超えて添加しても、機械的性質の点では効果が飽和するので、これを上限とした。特に熱間延性を低下させ、圧延時の疵の原因となりやすいので、Ｎｉと同時に添加することが好ましい。 Cu strengthens ferrite and is also effective for improving hardenability and corrosion resistance. If the content is less than 0.01%, the effect is not recognized. If the content exceeds 2.0%, the effect is saturated in terms of mechanical properties. In particular, it is preferable to add it at the same time as Ni because it reduces the hot ductility and easily causes flaws during rolling.
Ｓｎは、フェライトを脆化させ、工具寿命を延ばすとともに、表面粗さ向上に効果がある。０．００５％未満ではその効果は認められず、２．０％を超えて添加しても、機械的性質の点では効果が飽和するので、これを上限とした。 Sn has the effect of making the ferrite brittle, extending the tool life, and improving the surface roughness. If the content is less than 0.005%, the effect is not recognized. If the content exceeds 2.0%, the effect is saturated in terms of mechanical properties, so the upper limit is set.
Ｚｎは、フェライトを脆化させ、工具寿命を延ばすとともに、表面粗さ向上に効果がある。０．０００５％未満ではその効果は認められず、０．５％を超えて添加しても、機械的性質の点では効果が飽和するので、これを上限とした。 Zn has the effect of making the ferrite brittle, extending the tool life, and improving the surface roughness. If the content is less than 0.0005%, the effect is not recognized. Even if the content exceeds 0.5%, the effect is saturated in terms of mechanical properties. Therefore, the upper limit is set.
Ｔｉも炭窒化物を形成し、鋼を強化する。また脱酸元素でもあり、軟質酸化物を形成させることで被削性を向上させることが可能である。０．０００５％以下ではその効果が認められず、０．１％を超えて添加してもその効果が飽和する。また、Ｔｉは高温でも窒化物となりオーステナイト粒の成長を抑制するので上限を０．１％とした。尚、ＴｉはＮと化合してＴｉＮを形成するが、ＴｉＮは硬質物質で被削性を低下させる。また被削性向上に有効なＢＮを造るのに必要なＮ量を低減させる。そのためＴｉ添加量は０．０１０％以下が好ましい。 Ti also forms carbonitrides and strengthens the steel. It is also a deoxidizing element, and can improve machinability by forming a soft oxide. If the content is less than 0.0005%, the effect is not recognized, and even if added over 0.1%, the effect is saturated. The upper limit is set to 0.1%, since Ti becomes a nitride even at a high temperature and suppresses the growth of austenite grains. Note that Ti combines with N to form TiN, but TiN is a hard substance and reduces machinability. Further, the amount of N required to produce BN effective for improving machinability is reduced. Therefore, the addition amount of Ti is preferably 0.010% or less.
Ｃａは、脱酸元素であり、軟質酸化物を生成し、被削性を向上させるだけでなく、ＭｎＳに固溶してその変形能を低下させ、圧延や熱間鍛造してもＭｎＳ形状の伸延を抑制する働きがある。したがって異方性の低減に有効な元素である。０．０００２％未満ではその効果は顕著ではなく、０．００５％以上添加しても歩留まりが極端に悪くなるばかりでなく、硬質のＣａＯを大量に生成し、かえって被削性を低下させる。したがって成分範囲を０．０００２〜０．００５％と規定した。 Ca is a deoxidizing element, generates a soft oxide and not only improves the machinability, but also dissolves in MnS to reduce its deformability, and the MnS shape is formed even by rolling or hot forging. It works to control distraction. Therefore, it is an element effective for reducing anisotropy. If the content is less than 0.0002%, the effect is not remarkable. Even if 0.005% or more is added, not only the yield is extremely deteriorated, but also a large amount of hard CaO is generated, and the machinability is rather reduced. Therefore, the component range was specified as 0.0002 to 0.005%.
Ｚｒは、脱酸元素であり、酸化物を生成する。酸化物はＭｎＳの析出核になりＭｎＳの微細均一分散に効果がある。またＭｎＳに固溶してその変形能を低下させ、圧延や熱間鍛造してもＭｎＳ形状の伸延を抑制する働きがある。したがって異方性の低減に有効な元素である。０．０００５％未満ではその効果は顕著ではなく、０．１％以上添加しても歩留まりが極端に悪くなるばかりでなく、硬質のＺｒＯ2 やＺｒＳなどを大量に生成し、かえって被削性を低下させる。従ってＺｒの添加の範囲を０．０００５〜０．１％と規定した。なお、ＭｎＳの微細分散を図る場合には、ＺｒとＣａとの複合添加が好ましい。 Zr is a deoxidizing element and generates an oxide. The oxide serves as a precipitation nucleus of MnS, and is effective in fine and uniform dispersion of MnS. Further, it has the function of dissolving in MnS to reduce its deformability and suppressing the elongation of the MnS shape even in rolling or hot forging. Therefore, it is an element effective for reducing anisotropy. If it is less than 0.0005%, the effect is not remarkable. Even if it is added at 0.1% or more, not only the yield is extremely deteriorated, but also hard ZrO 2 , ZrS and the like are generated in large amounts, and the machinability is rather reduced. Lower. Therefore, the range of Zr addition is defined as 0.0005 to 0.1%. In order to achieve fine dispersion of MnS, it is preferable to add Zr and Ca in combination.
Ｍｇは、脱酸元素であり、酸化物を生成する。酸化物はＭｎＳの析出核になりＭｎＳの微細均一分散に効果があり異方性の低減に有効な元素である。０．０００３％未満ではその効果は顕著ではなく、０．００５％以上添加しても歩留まりが極端に悪くなるばかりで効果は飽和する。従ってＭｇの添加範囲を０．０００３〜０．００５％と規定した。 Mg is a deoxidizing element and generates an oxide. The oxide is an element that becomes a precipitation nucleus of MnS, has an effect on fine and uniform dispersion of MnS, and is effective in reducing anisotropy. If it is less than 0.0003%, the effect is not remarkable. Even if 0.005% or more is added, the yield is extremely deteriorated and the effect is saturated. Therefore, the range of addition of Mg is specified as 0.0003 to 0.005%.
Ｔｅは、被削性向上元素である。またＭｎＴｅを生成したり、ＭｎＳと共存することでＭｎＳの変形能を低下させてＭｎＳ形状の伸延を抑制する働きがある。したがって異方性の低減に有効な元素である。この効果は０．０００３％未満では認められず、０．０５％を超えると効果が飽和する。 Te is a machinability improving element. In addition, by producing MnTe or coexisting with MnS, it has a function of reducing the deformability of MnS and suppressing the elongation of the MnS shape. Therefore, it is an element effective for reducing anisotropy. This effect is not recognized at less than 0.0003%, and the effect is saturated at more than 0.05%.
ＢｉおよびＰｂは、被削性向上に効果のある元素である。その効果は０．００５％以下では認められず、０．５％を超えて添加しても被削性向上効果が飽和するだけでなく、熱間鍛造特性が低下して疵の原因となりやすい。 Bi and Pb are elements that are effective in improving machinability. The effect is not recognized at 0.005% or less, and even if added over 0.5%, not only the machinability improving effect is saturated, but also the hot forging property is reduced, which is likely to cause flaws.
Ｐｂは、被削性向上に効果のある元素である。その効果は０．０１％以下では認められず、０．５％を超えて添加しても被削性向上効果が飽和するだけでなく、熱間鍛造特性が低下して疵の原因となりやすい。 Pb is an element effective in improving machinability. The effect is not recognized at 0.01% or less, and even if added over 0.5%, not only the machinability improving effect is saturated, but also the hot forging property is reduced, which is likely to cause a flaw.
本発明の効果を実施例によって説明する。表１、表２（表１のつづき１）、表３（表１のつづき２）、表４（表１のつづき３）、表５、表６（表５のつづき）に示す供試材のうち、実施例Ｎｏ．６は２７０ｔ転炉で、その他は２ｔ真空溶解炉で溶製後、ビレットに分解圧延、さらにφ６０mmに圧延した。
The effects of the present invention will be described with reference to examples. Table 1 and Table 2 (
材料は熱処理され、発明例に関してはその成分によって熱処理条件を変更した。 The material was heat-treated, and the heat-treatment conditions were changed depending on the components for the invention examples.
表１〜表６においての熱処理の項で、焼準と記された実施例は９２０℃で１０min 以上保持し、空冷したものである。ＱＴと記された発明例は９２０℃から圧延ライン後端の水槽に投入して急冷後、焼鈍にて７００℃で１時間以上保持した。これによりパーライト面積率を調整した。発明例でもＣ量が低いものは焼準でもパーライト面積率を低減することができる。 In the examples of heat treatment in Tables 1 to 6, the examples described as normalization were held at 920 ° C. for 10 minutes or more and air-cooled. The invention example described as QT was put into a water tank at the rear end of the rolling line from 920 ° C., rapidly cooled, and then kept at 700 ° C. for 1 hour or more by annealing. Thereby, the pearlite area ratio was adjusted. Even in the invention examples, those having a low C content can reduce the pearlite area ratio even in normalizing.
ＭｎＳ密度は凝固時の冷却速度を制御することにより調整した。冷却速度を増大させると微細ＭｎＳが生成しやすくなる。 The MnS density was adjusted by controlling the cooling rate during solidification. When the cooling rate is increased, fine MnS is easily generated.
表１〜表６の実施例１〜４７に示す材料の被削性評価はドリル穿孔試験で表７に切削条件を示す。累積穴深さ１０００mmまで切削可能な最高の切削速度（いわゆるＶＬ１０００、単位はｍ／min ）で被削性を評価した。 For the evaluation of the machinability of the materials shown in Examples 1 to 47 in Tables 1 to 6, cutting conditions are shown in Table 7 in a drilling test. The machinability was evaluated at the highest cutting speed (so-called VL1000, unit: m / min) capable of cutting to a cumulative hole depth of 1000 mm.
さらに、切削における表面品質を示す切削表面粗さを評価した。その切削条件を表８に、その評価方法（以後、プランジ切削試験と記す）の概要を図３（ａ），（ｂ）に示す。プランジ切削試験では工具は短時間切削を繰り返す。一回の切削で工具は被削材長手方向に動かず、回転している被削材中心に向かって動くため、短時間の切削後、工具は引き抜かれるが、その形状は基本的には工具は刃先形状が被削材表面に転写される。構成刃先の付着や工具の磨耗損傷によりこの転写された切削面の表面粗さは影響を受ける。この表面粗さを表面粗さ計で測定した。その結果を図４に示す。１０点表面粗さＲｚ（μｍ）を表面粗さを示す指標とした。 Furthermore, the cutting surface roughness indicating the surface quality in cutting was evaluated. The cutting conditions are shown in Table 8, and the outline of the evaluation method (hereinafter referred to as plunge cutting test) is shown in FIGS. 3 (a) and 3 (b). In the plunge cutting test, the tool repeats cutting for a short time. In a single cut, the tool does not move in the longitudinal direction of the work material but moves toward the center of the rotating work material, so the tool is pulled out after a short cut, but the shape is basically the tool Is transferred to the surface of the work material. The surface roughness of the transferred cutting surface is affected by the adhesion of the constituent cutting edges and wear damage of the tool. The surface roughness was measured with a surface roughness meter. The result is shown in FIG. Ten-point surface roughness Rz (μm) was used as an index indicating the surface roughness.
円相当径にて０．１〜０．５μｍの寸法のＭｎＳの測定は、φ５０mm圧延後の圧延方向と平行な断面のＱ部より抽出レプリカ法にて採取して過型電子顕微鏡にて行った。測定は１００００倍で１視野８０μｍ2 を４０視野以上行い、それを１平方ミリメートル当たりのＭｎＳ数に換算して算出した。 The measurement of MnS having a circle equivalent diameter of 0.1 to 0.5 μm was carried out by an extraction replica method by sampling from the Q portion of a cross section parallel to the rolling direction after φ50 mm rolling by an extraction replica method. . The measurement was carried out at a magnification of 10,000 times and a visual field of 80 μm 2 was measured in 40 visual fields or more, and the calculated value was converted into the number of MnS per square millimeter.
切り屑処理性に関しては切り屑のカール時の曲率が小さいもの、あるいは分断されているものが好ましい。そこで切り屑が２０mmを超えた曲率半径で３巻き以上連続してカールして長く延びた切り屑を不良とした。巻数が多くとも曲率半径が小さいもの、あるいは曲率半径が大きくとも切り屑長さが１００mmに達しなかったものは良好とした。 With respect to the chip handling property, it is preferable that the chip has a small curvature at the time of curling or that the chip is divided. Accordingly, chips that were continuously curled and extended longer than three turns with a radius of curvature exceeding 20 mm were regarded as defective. If the number of windings was large and the radius of curvature was small, or if the chip length did not reach 100 mm even if the radius of curvature was large, it was regarded as good.
熱間延性の評価は高温引張試験により行った。６００℃〜１２００℃の各温度で引張試験を行い、８００〜１０００℃付近での絞り値が５０％未満のものを×、５０％以上のものを○とした。 The evaluation of hot ductility was performed by a high temperature tensile test. Tensile tests were performed at each temperature of 600 ° C to 1200 ° C, and those with an aperture value of less than 50% near 800 to 1000 ° C were rated as x, and those with 50% or more were rated as good.
発明例１〜３８および４５〜４７はいずれも比較例３９〜４４に対してドリル工具寿命に優れるとともに、プランジ切削における表面粗さが良好であった。これはＢによってフェライトが局部的に脆化され、表面創成がスムーズに行われたために良好な表面粗さを得られたと考えられる。 Inventive Examples 1 to 38 and 45 to 47 all had better drill tool life than Comparative Examples 39 to 44, and had good surface roughness in plunge cutting. This is considered to be because the ferrite was locally embrittled by B and the surface was created smoothly, so that good surface roughness was obtained.
これらの表面粗さの改善効果はＳが０．５％超の場合に顕著であるが、Ｓ量がそれより少ない場合でも切り屑処理性に効果が見られた。 The effect of improving the surface roughness is remarkable when S is more than 0.5%. However, even when the amount of S is smaller than that, the effect on the chip controllability was observed.
さらにＭｎとＳの比率が従来鋼によく見られる３程度でも効果が認められるが、Ｍｎ／Ｓを小さくすると、より工具寿命が向上するとともに、表面粗さも向上する。ただし比較例３６のようにＭｎ／Ｓが小さすぎるとＦｅＳが生成するため、圧延割れを生じる。Ｃ量を若干変更した場合（表１、表２および表５、表６の実施例３５〜３８および４５〜４７）でもＢを大量に添加すること、さらにパーライト面積率を制御することで良好な工具寿命と切削表面粗さを得ることができた。 Further, even when the ratio of Mn to S is about 3, which is often found in conventional steels, the effect is recognized. However, when Mn / S is reduced, the tool life is further improved and the surface roughness is also improved. However, when Mn / S is too small as in Comparative Example 36, FeS is generated, so that rolling cracks occur. Even when the amount of C is slightly changed (Examples 35 to 38 and 45 to 47 in Tables 1, 2, and 5, and 6), it is preferable to add a large amount of B and further control the pearlite area ratio. Tool life and cutting surface roughness could be obtained.
Claims (11)
Ｃ：０．００５〜０．２％，
Ｓｉ：０．００１〜０．５％，
Ｍｎ：０．２〜３．０％，
Ｐ：０．００１〜０．２％，
Ｓ：０．１〜０．５％，
ｔｏｔａｌ−Ｎ：０．００２〜０．０２％，
ｔｏｔａｌ−Ｏ：０．０００５〜０．０３５％を含有し、残部がＦｅおよび不可避的不純物よりなり、ミクロ組織においてパーライト面積率が５％以下であることを特徴とする被削性に優れる鋼。 In mass%,
C: 0.005 to 0.2%,
Si: 0.001-0.5%,
Mn: 0.2-3.0%,
P: 0.001-0.2%,
S: 0.1-0.5%,
total-N: 0.002 to 0.02%,
Total-O: steel excellent in machinability, containing 0.0005 to 0.035%, the balance being Fe and unavoidable impurities, and having a pearlite area ratio of 5% or less in a microstructure.
The steel further contains, by mass%, one or more of Te: 0.0003 to 0.05%, Bi: 0.005 to 0.5%, and Pb: 0.01 to 0.5%. The steel excellent in machinability according to any one of claims 1 to 10, characterized by containing.
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