JP2004332078A - Free-cutting steel for machine structure use excellent in scrap disposal - Google Patents

Free-cutting steel for machine structure use excellent in scrap disposal Download PDF

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Publication number
JP2004332078A
JP2004332078A JP2003132094A JP2003132094A JP2004332078A JP 2004332078 A JP2004332078 A JP 2004332078A JP 2003132094 A JP2003132094 A JP 2003132094A JP 2003132094 A JP2003132094 A JP 2003132094A JP 2004332078 A JP2004332078 A JP 2004332078A
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Japan
Prior art keywords
free
mass
steel
cutting steel
cutting
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JP2003132094A
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Japanese (ja)
Inventor
Norimasa Tokokage
典正 常陰
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Sanyo Special Steel Co Ltd
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Sanyo Special Steel Co Ltd
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Priority to JP2003132094A priority Critical patent/JP2004332078A/en
Priority to US10/841,904 priority patent/US20040223867A1/en
Priority to CN200410034730.XA priority patent/CN1271232C/en
Publication of JP2004332078A publication Critical patent/JP2004332078A/en
Pending legal-status Critical Current

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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/60Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese

Abstract

<P>PROBLEM TO BE SOLVED: To provide a free-cutting steel for machine structure use which can attain very good scrap disposal without containing toxic substance such as Pb. <P>SOLUTION: The free-cutting steel comprises 0.01 to 0.70 mass% C, 0.05 to 2.00 mass% Si, 0.20 to 3.50 mass% Mn; 0.0003 to 0.01 mass% Ca, 0.020 to 0.300 mass% S, 0.002 to 0.300 mass% Al, 0.003 to 0.035 mass% N, and 0.0010 to 0.0080 mass% O, with the balance comprising Fe and unavoidable impurities. In a material prepared by subjecting the steel to hot rolling or hot swaging, MnS-based sulfide particles of below 20 μm account for at least 30% of the total sulfide particles, and at least 10 oxide contaminant particles being present by themselves or as a composite with a sulfide and having a major diameter of 0.5 to below 50 μm are present per mm<SP>2</SP>of a specimen. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【0001】
【発明の属する技術分野】
切削加工コスト削減を目的として種々の快削物質を含有させた自動車用部品をはじめとする部品用の機械構造用快削鋼に関する。
【0002】
【従来の技術】
従来から自動車用部品をはじめとする機械構造用鋼は、切削加工コスト削減を目的として種々の快削物質を含有させる場合が多い。代表的な快削鋼としてPb快削鋼、S快削鋼、Ca脱酸快削鋼、および、これらの複合快削鋼がある。Pb快削鋼はその基本となる鋼と比較して機械的性質の劣化が小さく、被削性改善効果、特に低速切削時の工具寿命や切屑処理性が良好であることから、最も一般的に用いられている。しかし、Pbは人体に有害であるため、近年の環境問題への関心の高まりから、世界的に使用量削減の方向にあり、Pb快削鋼においてもそれに代わる快削鋼の要求が高まっている。その場合、S快削鋼への移行が考えられるが、Sは圧延方向に延伸するMnS介在物として鋼中に存在するため多量のSを添加させると機械的性質の異方性が増大するという欠点がある。また、Ca脱酸快削鋼は、鋼中に低融点のCaO−Al−SiO系酸化物を含有しており、この酸化物が工具刃先に保護膜を生成して切屑と工具の直接接触を妨げることにより被削性を改善するものである。しかし、Ca脱酸快削鋼は超硬工具旋削等の比較的高速切削時にしか効果が認められない。さらに、Pb、S、Caをすべて複合したPb三元快削鋼も多く使用されているが、被削性は非常に優れているものの上述のPbとSの欠点は改善されたものではなく、新たな快削鋼が要求されている。
【0003】
上記のS快削鋼の機械的性質を改善するために、Caを含有させている(例えば、特許文献1、特許文献2、特許文献3参照)。この場合、さらに硬質のAlをCaO−Alに変化させたり硫化物で覆うため無害化されることも報告されている。
【0004】
また、六方晶BN、CaO−Al、Ca−Mn−Sを含有させ、被削性改善を図っているものもある(例えば、特許文献4参照)。しかし、これらの場合、機械的性質の異方性は硫化物の形態制御により基本鋼からの劣化度合は改善されるが、被削性については種々の切削条件において必ずしも十分な結果が得られるものではなく、特にPb快削鋼の代替において最も重要な特性である切屑処理性については、必ずしもPb鋼レベルに到達していないのが現状である。
【0005】
さらに、硫化物の大きさを制御することにより、ドリル寿命を主とした被削性と強度異方性を改善した構造用鋼が発明されている(例えば、特許文献5参照)が、切屑処理性の点ではPb快削鋼代替には十分な特性が得られているとはいえない。
【0006】
【特許文献1】
特許第1981560号公報参照
【特許文献2】
特開平11−350065号公報参照
【特許文献3】
特開2000−34538号公報参照
【特許文献4】
特開平6−145889号公報参照
【特許文献5】
特開2002−180184号公報参照
【0007】
【発明が解決しようとする課題】
本発明が解決しようとする課題は、Pbのような有害物質を含有することなく、非常に良好な切屑処理性を得ることが可能な機械構造用快削鋼を提供することである。
【0008】
【課題を解決するための手段】
上記の課題を解決するための本発明の手段は、請求項1の発明では、質量%で、C:0.01〜0.70%、Si:0.05〜2.00%、Mn:0.20〜3.50%、Ca:0.0003〜0.01%、S:0.020〜0.300%、Al:0.002〜0.300%、N:0.003〜0.035%、O:0.0010〜0.0080%、残部がFeおよび不可避不純物からなり、熱間圧延あるいは熱間鍛伸を実施した材料において、長径が0.5μm以上、かつ、20μm未満のMnSを主体とする硫化物の全硫化物個数に占める割合が30%以上であり、さらに硫化物と複合あるいは単独で存在する長径が0.5μm以上、かつ、50μm未満の酸化物系介在物の個数が被検面積1mm中において10個以上含有することを特徴とする切屑処理性に優れる快削鋼である。
【0009】
請求項2の発明では、請求項1の手段の快削鋼において、該鋼成分に加えて、質量%で、Cr:0.20〜2.50%、Mo:0.05〜1.50%、Ni:0.05〜3.50%、V:0.01〜0.50%、Nb:0.01〜0.10%、Ti:0.01〜0.50%のうち1種または2種以上を含有することを特徴とする切屑処理性に優れる快削鋼である。
【0010】
請求項3の発明では、請求項1または2の手段の快削鋼において、該鋼成分加えて、質量%で、Mg:0.0003〜0.01%、Zr:0.0005〜0.30%、Bi:0.01〜0.30%、B:0.0003〜0.015%のうち1種または2種以上を含有することを特徴とする切屑処理性に優れる快削鋼である。
【0011】
上記の本発明の手段の切屑処理性に優れた機械構造用快削鋼の成分限定理由を次に述べる。なお、成分の%は質量%を示す。
【0012】
(必須元素)
C:0.01〜0.70%
Cは、鋼の強度を確保するために添加する元素であるが、0.01%未満では不十分であり、0.70%超えると靭性が低下する。そこでCは0.01〜0.70%とする。
【0013】
Si:0.05〜2.00%
Siは、製鋼での脱酸のためおよび強度確保のために添加するが、0.05%未満では脱酸効果が不十分であり、2.00%超えると熱間加工性が低下する。そこでSiは0.05〜2.00%とする。
【0014】
Mn:0.20〜3.50%
Mnは、焼入性を向上させるために添加し、また、Sと硫化物を生成して被削性を向上させるために不可欠な元素であり、また、MnSはオーステナイト粒成長を抑制し組織を微細化する効果もある。しかし、0.20%未満ではこの効果が小さく、3.50%を超えると加工性が低下する。そこでMnは0.20〜3.50%とする。
【0015】
Ca:0.0003〜0.01%
Caは、硫化物形態制御による異方性改善の効果、切屑処理性改善の効果および工具上に(Mn、Ca)SとAINの保護膜を付着させることによる工具寿命の改善効果を有する。さらに、Ca系酸化物を生成し切屑処理性を改善する効果を有する。この効果は0.0003%以上で得られ、望ましくは0.001%以上であり、0.01%を超えて含有させても効果は飽和し、むしろCa添加歩留が悪くなる。そこでCaは0.0003〜0.01%とする。
【0016】
S:0.020〜0.300%
Sは、MnSや(Mn、Ca)Sなどの硫化物を形成して被削性を改善させる効果を有する。また、熱間加工のために1000℃以上に加熱した場合、オーステナイト粒成長を抑制するため非調質鋼では靭性を高める効果もある。これらの効果を得るには最低0.020%以上必要であり、望ましくは0.050%以上必要である。しかし、0.300%を超えると硫化物の応力集中効果により靭性を劣化させる。そこでSは0.020〜0.300%とする。
【0017】
Al:0.002〜0.300%
Alは、Siと同様に製鋼での脱酸のために添加し、さらに生成した複合酸化物は切屑処理性に有効に作用する。また、AlNを形成しオーステナイト粒微細化に寄与する。それらの効果を得るには0.002%以上必要であり、0.300%を超えて添加するとAl酸化物により靭性や被削性が劣化する。そこでAlは0.002〜0.300%とする。望ましくは0.003〜0.015%とする。
【0018】
N:0.003〜0.035%
Nは、強靭化のために添加する。さらにAlNなどの窒化物を生成してオーステナイト粒微細化の効果がある。その効果を得るには0.003%以上必要であり、0.035%を超えて添加してもその効果は飽和する。そこでNは0.003〜0.035%とする。望ましくは0.005〜0.020%とする。
【0019】
O:0.0010〜0.0080%
Oは、被削性に有効な酸化物を生成し、さらに硫化物の核として微細分散化させるためにも添加する。この効果を得るには0.0010%以上必要であり、0.0080%を超えて添加すると機械的性質が低下する。そこでOは0.0010〜0.0080%とする。望ましくは0.0015〜0.0050%とする。
【0020】
(選択元素)
Cr:0.20〜2.50%
Crは、Mnと同様の働きをし、焼入性を高め強度を向上させる。0.20%未満ではその効果が小さく、2.50%を超えるとコスト高となる。そこでCrは0.20〜2.50%とする。
【0021】
Mo:0.05〜1.50%
Moは、Crと同様の働きをし、焼入性を高め強度を向上させる。0.05%未満ではその効果が小さく、1.50%を超えるとコスト高となる。そこでMoは0.05〜1.50%とする。
【0022】
Ni:0.05〜3.50%
Niは、Moと同様の働きをし、焼入性を高め強度を向上させる。0.05%未満ではその効果が小さく、3.50%を超えるとコスト高となる。そこでNiは0.05〜3.50%とする。
【0023】
V:0.01〜0.50%
Nb:0.01〜0.10%
Ti:0.01〜0.50%
V、Nb、Tiは、鋼中に微細な炭窒化物を生成し、これらの析出物により熱間加工時のオーステナイト粒径を微細化し靭性を向上させる。さらにこれらの析出物の分散強化あるいは析出強化による強度向上効果もある。この効果はV、Nb、Tiともに0.01%以下では効果がない。ただし、多量に添加すると靭性が劣化するため、Vは0.50%、Nbは0.10%、Tiは0.50%を上限とする。そこでVは0.01〜0.50%、Nbは0.01〜0.10%、Tiは0.01〜0.50%とする。
【0024】
Mg:0.0003〜0.01%
Zr:0.0005〜0.30%
Bi:0.01〜0.30%
B:0.0003〜0.015%
Mg、Zrは、Caと同様に硫化物あるいは酸化物として、Biは単独または他の介在物と共存で、Bは窒化物として存在し、本発明鋼の切屑処理性をさらに改善する。さらに、Mg、Zrは硫化物形態制御元素であり、機械的異方性を改善する効果もある。これらの効果は、それぞれMg0.0003%、Zr0.0005%、Bi0.01%、B0.0003%未満では効果が小さく、Mg0.01%、Zr0.30%、Bi0.30%、B0.015%を超えて含有させても効果は飽和し、コスト高となる。そこでMg:0.0003〜0.01%、Zr:0.0005〜0.30%、Bi:0.01〜0.30%、B:0.0003〜0.015%とする。
【0025】
本発明は、Pb快削鋼と比較した場合に機械的性質はほぼ同等で、良好な被削性が得られる、特に切屑処理性がPb鋼と同等である機械構造用快削鋼を得るものであり、産業上非常に有益である。
【0026】
本発明の被削性改善効果について説明する。本発明では、製鋼工程において成分調整の他にAlやSi脱酸で溶存酸素量を15〜90ppmの間に調整し、Caを添加することで、その後の熱間圧延あるいは熱間鍛伸を施した材料中において長径が0.5μm以上、かつ、20μm未満の硫化物が全硫化物個数に占める割合が30%以上となり、さらに、硫化物と複合あるいは単独で存在する長径が0.5μm以上、かつ、50μm未満の酸化物系介在物の個数が被検面積1mm中において10個以上含有させることが可能となる。この材料を切削加工する際、長径が20μm以上の比較的大型のMnSを主体とする硫化物で亀裂を発生させて、その周辺に存在する長径が0.5μm以上、かつ、20μm未満の比較的微細で数の多い硫化物(全硫化物中で30%以上を占める)で亀裂を伝播させる。この際、硫化物のみでは効果は小さいが、長径が0.5μm以上、かつ、50μm未満の酸化物系介在物が被検面積1mm中において10個以上存在する場合に、飛躍的に切屑中の亀裂伝播が促進され、切屑処理性が上昇する。
【0027】
なお、本発明鋼中の酸化物は、CaOやAlを含有しており、さらには、酸化物形成能の高い不純物元素あるいは合金元素の存在によって生成するMgO、MnO、SiO、TiO、ZrO、REM酸化物等を複合で含有する場合もある。
【0028】
【発明の実施の形態】
本発明の発明の実施の形態を以下に示す実施例を通じて説明する。
【0029】
【実施例】
100kg真空溶解炉で、表1に示す成分からなる、No.1〜20の鋼種を溶製した。常法により得られたこれらの鋼塊は、1200℃で直径45mmの棒鋼に鍛伸した。
【0030】
【表1】
【0031】
表1の比較鋼における網掛け部は、本発明の各請求項の範囲からから外れている成分を示す。
【0032】
その後、上記の鍛伸した棒鋼を焼入焼戻し処理を行い、全鋼種共に硬さを25±2HRCに調整し、下記の切屑処理性試験に供した。その結果を硫化物および酸化物の大きさと個数、切屑処理性試験結果として以下の表2に示す。
【0033】
なお、上記の切屑処理性試験は、旋削超硬工具のP20工具により、切削速度:150m/min、送り:0.1mm/rev、切込み:0.5mmで、行った。評価方法は切屑1g当たりの個数で切屑処理性指数として示す。
【0034】
【表2】
【0035】
上記の表2において、小型硫化物とは長径が0.5μm以上、かつ、5μm未満のMnSを主体とする硫化物で、酸化物とは長径が0.5μm以上、かつ、50μm未満の酸化物系介在物をいう。
【0036】
発明鋼1〜13は本発明の請求項の範囲にあるものである。本発明鋼中に存在する小型硫化物の割合は30%以上であり、酸化物個数は10個/mm以上である。
【0037】
本発明鋼の切屑処理性指数は少なくとも10以上であるが、比較材のNo.14、15、17、18、20はこれに達しない。No.16、19の切屑処理性指数は10以上であるが、これは有害物質であるPbを含有しているためである。
以上に示したように、本発明の機械構造用快削鋼は、Pbのような有害物質を含有すること無く非常に良好な切屑処理性を得ることが可能である。
【0038】
【発明の効果】
以上に説明したように、本発明の機械構造用快削鋼は、Pbのような有害物質を含有すること無く非常に良好な切屑処理性を得ることができる機械構造用快削鋼である。
[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a free-cutting steel for a machine structure, such as a part for an automobile, containing various free-cutting substances for the purpose of cutting cost reduction.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, steel for machine structural use such as automobile parts often contains various free-cutting substances for the purpose of cutting cost reduction. Typical free-cutting steels include Pb free-cutting steel, S free-cutting steel, Ca deoxidized free-cutting steel, and composite free-cutting steels of these. Pb free-cutting steel is most commonly used because its mechanical properties are less deteriorated and its machinability improving effect, especially tool life and chip disposability at low speed cutting, is better than the base steel. Used. However, since Pb is harmful to the human body, there has been an increasing interest in environmental issues in recent years, and there is a worldwide trend toward reducing the amount of used Pb. . In that case, the transition to S free-cutting steel is conceivable, but since S exists in the steel as MnS inclusions extending in the rolling direction, the addition of a large amount of S increases the anisotropy of the mechanical properties. There are drawbacks. Further, Ca deoxidized free-cutting steel contains a low-melting CaO—Al 2 O 3 —SiO 2 -based oxide in the steel, and this oxide forms a protective film on the cutting edge of the tool to generate chips and tool. To improve the machinability by preventing direct contact of the steel. However, Ca deoxidized free-cutting steel has an effect only at relatively high speed cutting such as turning of a carbide tool. Furthermore, Pb ternary free-cutting steels, in which Pb, S, and Ca are all combined, are also often used. However, although the machinability is very excellent, the above-mentioned disadvantages of Pb and S are not improved. New free-cutting steel is required.
[0003]
In order to improve the mechanical properties of the above S free-cutting steel, Ca is contained (for example, see Patent Document 1, Patent Document 2, and Patent Document 3). In this case, it is also reported that the hardened Al 2 O 3 is converted into CaO—Al 2 O 3 or is made harmless by covering it with a sulfide.
[0004]
Further, there is hexagonal BN, is contained CaO-Al 2 O 3, Ca -Mn-S, some are working to improve machinability (for example, see Patent Document 4). However, in these cases, the anisotropy of the mechanical properties can be improved from the basic steel by controlling the sulfide morphology, but the machinability is not necessarily sufficient under various cutting conditions. However, in particular, the chip handling property, which is the most important property in the replacement of Pb free-cutting steel, has not always reached the Pb steel level.
[0005]
Further, a structural steel having improved machinability and strength anisotropy mainly by drill life by controlling the size of sulfide has been invented (for example, see Patent Document 5). In terms of properties, it cannot be said that sufficient characteristics have been obtained as a substitute for Pb free-cutting steel.
[0006]
[Patent Document 1]
See Japanese Patent No. 1981560 [Patent Document 2]
See JP-A-11-350065 [Patent Document 3]
See JP-A-2000-34538 [Patent Document 4]
See JP-A-6-145889 [Patent Document 5]
See JP-A-2002-180184
[Problems to be solved by the invention]
The problem to be solved by the present invention is to provide a free-cutting steel for machine structures capable of obtaining very good chip controllability without containing a harmful substance such as Pb.
[0008]
[Means for Solving the Problems]
Means of the present invention for solving the above-mentioned problems are, in the invention of claim 1, C: 0.01 to 0.70%, Si: 0.05 to 2.00%, and Mn: 0% by mass. .20 to 3.50%, Ca: 0.0003 to 0.01%, S: 0.020 to 0.300%, Al: 0.002 to 0.300%, N: 0.003 to 0.035 %, O: 0.0010 to 0.0080%, the balance being Fe and unavoidable impurities, and in a material subjected to hot rolling or hot forging, MnS having a major axis of 0.5 μm or more and less than 20 μm is used. The ratio of the main sulfide to the total number of sulfides is 30% or more, and the number of oxide-based inclusions having a major axis of 0.5 μm or more and less than 50 μm which is combined with or independently of the sulfide is 50% or more. characterized in that it contains 10 or more in a subject area 1mm 2 Is a free-cutting steel having excellent chip disposability to.
[0009]
According to the second aspect of the present invention, in the free-cutting steel according to the first aspect, in addition to the steel components, Cr: 0.20 to 2.50%, Mo: 0.05 to 1.50% by mass%. , Ni: 0.05 to 3.50%, V: 0.01 to 0.50%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.50% It is a free-cutting steel with excellent chip controllability characterized by containing more than one kind.
[0010]
According to a third aspect of the present invention, in the free-cutting steel according to the first or second aspect, in addition to the steel component, Mg: 0.0003 to 0.01%, Zr: 0.0005 to 0.30 by mass%. %, Bi: 0.01 to 0.30%, and B: 0.0003 to 0.015%.
[0011]
The reasons for limiting the components of the free-cutting steel for machine structures excellent in the chip controllability of the above-mentioned means of the present invention are described below. In addition,% of a component shows mass%.
[0012]
(Essential element)
C: 0.01-0.70%
C is an element added to secure the strength of steel, but if it is less than 0.01%, it is insufficient, and if it exceeds 0.70%, toughness is reduced. Therefore, C is set to 0.01 to 0.70%.
[0013]
Si: 0.05-2.00%
Si is added for deoxidation in steel making and for securing strength. However, if it is less than 0.05%, the deoxidizing effect is insufficient, and if it exceeds 2.00%, hot workability decreases. Therefore, Si is set to 0.05 to 2.00%.
[0014]
Mn: 0.20 to 3.50%
Mn is added to improve hardenability, and is an element indispensable for generating S and sulfide to improve machinability, and MnS suppresses austenite grain growth and changes the structure. There is also an effect of miniaturization. However, if it is less than 0.20%, this effect is small, and if it exceeds 3.50%, the workability is reduced. Therefore, Mn is set to 0.20 to 3.50%.
[0015]
Ca: 0.0003-0.01%
Ca has an effect of improving anisotropy by controlling the sulfide form, an effect of improving chip controllability, and an effect of improving tool life by attaching a protective film of (Mn, Ca) S and AIN on the tool. Further, it has the effect of generating Ca-based oxides and improving the chip disposability. This effect is obtained at 0.0003% or more, desirably 0.001% or more. Even if the content exceeds 0.01%, the effect is saturated, and the Ca addition yield is rather deteriorated. Therefore, Ca is set to 0.0003 to 0.01%.
[0016]
S: 0.020 to 0.300%
S has an effect of forming a sulfide such as MnS or (Mn, Ca) S to improve machinability. Further, when heated to 1000 ° C. or higher for hot working, non-heat treated steel has an effect of increasing toughness in order to suppress austenite grain growth. To obtain these effects, at least 0.020% or more is required, and preferably 0.050% or more. However, if it exceeds 0.300%, the toughness is deteriorated due to the stress concentration effect of sulfide. Therefore, S is set to 0.020 to 0.300%.
[0017]
Al: 0.002 to 0.300%
Al is added for deoxidation in steelmaking as in the case of Si, and the resulting composite oxide effectively affects the chip disposability. In addition, it forms AlN and contributes to refinement of austenite grains. To obtain these effects, 0.002% or more is necessary. If it exceeds 0.300%, toughness and machinability deteriorate due to Al oxide. Therefore, the content of Al is set to 0.002 to 0.300%. Desirably, the content is 0.003 to 0.015%.
[0018]
N: 0.003 to 0.035%
N is added for toughening. Further, a nitride such as AlN is generated to provide an effect of miniaturizing austenite grains. To obtain the effect, 0.003% or more is necessary, and even if it exceeds 0.035%, the effect is saturated. Therefore, N is set to 0.003 to 0.035%. Desirably, it is 0.005 to 0.020%.
[0019]
O: 0.0010 to 0.0080%
O is added for generating an oxide effective for machinability and for finely dispersing it as a sulfide nucleus. To obtain this effect, 0.0010% or more is necessary, and if added in excess of 0.0080%, the mechanical properties deteriorate. Therefore, O is set to 0.0010 to 0.0080%. Desirably, it is 0.0015 to 0.0050%.
[0020]
(Selected element)
Cr: 0.20 to 2.50%
Cr functions similarly to Mn and enhances hardenability and improves strength. If it is less than 0.20%, the effect is small, and if it exceeds 2.50%, the cost is high. Therefore, Cr is set to 0.20 to 2.50%.
[0021]
Mo: 0.05 to 1.50%
Mo has the same function as Cr, and enhances hardenability and improves strength. If it is less than 0.05%, the effect is small, and if it exceeds 1.50%, the cost increases. Therefore, Mo is set to 0.05 to 1.50%.
[0022]
Ni: 0.05 to 3.50%
Ni has the same function as Mo, and enhances hardenability and improves strength. If it is less than 0.05%, the effect is small, and if it exceeds 3.50%, the cost increases. Therefore, Ni is set to 0.05 to 3.50%.
[0023]
V: 0.01 to 0.50%
Nb: 0.01 to 0.10%
Ti: 0.01 to 0.50%
V, Nb, and Ti generate fine carbonitrides in the steel, and these precipitates reduce the austenite grain size during hot working and improve toughness. Further, there is an effect of enhancing the strength by dispersion strengthening or precipitation strengthening of these precipitates. This effect is ineffective when V, Nb and Ti are all 0.01% or less. However, if added in a large amount, the toughness is degraded. Therefore, the upper limit is 0.50% for V, 0.10% for Nb, and 0.50% for Ti. Therefore, V is 0.01 to 0.50%, Nb is 0.01 to 0.10%, and Ti is 0.01 to 0.50%.
[0024]
Mg: 0.0003-0.01%
Zr: 0.0005 to 0.30%
Bi: 0.01 to 0.30%
B: 0.0003 to 0.015%
Mg and Zr are present as sulfides or oxides like Ca, Bi is present alone or in combination with other inclusions, and B is present as a nitride, which further improves the chip controllability of the steel of the present invention. Further, Mg and Zr are sulfide form controlling elements and have an effect of improving mechanical anisotropy. These effects are small when Mg is less than 0.0003%, Zr 0.0005%, Bi 0.01%, and B 0.0003%, respectively, and are less than Mg 0.01%, Zr 0.30%, Bi 0.30%, and B 0.015%. The effect is saturated even if it is contained in excess of, and the cost increases. Therefore, Mg: 0.0003 to 0.01%, Zr: 0.0005 to 0.30%, Bi: 0.01 to 0.30%, and B: 0.0003 to 0.015%.
[0025]
The present invention is intended to provide a free-cutting steel for machine structures which has substantially the same mechanical properties as Pb free-cutting steel and good machinability, and in particular, has the same chip controllability as Pb steel. And is very useful in industry.
[0026]
The machinability improving effect of the present invention will be described. In the present invention, in addition to the component adjustment in the steelmaking process, the amount of dissolved oxygen is adjusted to 15 to 90 ppm by deoxidation of Al and Si, and Ca is added to perform subsequent hot rolling or hot forging. In the material obtained, the ratio of sulfides having a major axis of 0.5 μm or more and less than 20 μm to the total number of sulfides is 30% or more, and further, the major axis which is a complex or alone with a sulfide is 0.5 μm or more, In addition, it becomes possible to include 10 or more oxide-based inclusions having a size of less than 50 μm in a test area of 1 mm 2 . When this material is cut, a crack is generated by a relatively large sulfide mainly composed of MnS having a major axis of 20 μm or more, and a major axis present in the periphery of the sulfide is 0.5 μm or more and less than 20 μm. Fine and abundant sulfides (occupying more than 30% of total sulfides) propagate cracks. At this time, the effect is small by sulfide alone, but when there are 10 or more oxide-based inclusions having a major axis of 0.5 μm or more and less than 50 μm in the test area 1 mm 2 , the sulphide is drastically reduced. Crack propagation is promoted, and chip disposability is increased.
[0027]
The oxide in the steel of the present invention contains CaO and Al 2 O 3 , and further contains MgO, MnO, SiO, TiO, which is generated by the presence of an impurity element or alloy element having high oxide forming ability. ZrO, REM oxide, etc. may be contained in a composite.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will be described with reference to the following examples.
[0029]
【Example】
In a 100 kg vacuum melting furnace, No. 1 to 20 steel types were melted. These ingots obtained by a conventional method were forged at 1200 ° C. into steel bars having a diameter of 45 mm.
[0030]
[Table 1]
[0031]
The shaded portions in the comparative steels in Table 1 indicate components outside the scope of each claim of the present invention.
[0032]
Thereafter, the above-forged and drawn steel bars were subjected to quenching and tempering treatment, and the hardness of all steel types was adjusted to 25 ± 2 HRC, and subjected to the following chip disposal test. The results are shown in Table 2 below as the size and number of sulfides and oxides and the results of chip disposal test.
[0033]
In addition, the said chip processing test was performed by the turning speed: 150 m / min, feed: 0.1 mm / rev, and cutting: 0.5 mm using the turning super hard tool P20 tool. The evaluation method is shown as a chip disposal index in terms of the number per 1 g of chips.
[0034]
[Table 2]
[0035]
In Table 2 above, small sulfide is a sulfide mainly composed of MnS having a major axis of 0.5 μm or more and less than 5 μm, and oxide is an oxide having a major axis of 0.5 μm or more and less than 50 μm. Refers to system inclusions.
[0036]
Invention steels 1 to 13 fall within the scope of the claims of the present invention. The proportion of small sulfides present in the steel of the present invention is 30% or more, and the number of oxides is 10 / mm 2 or more.
[0037]
Although the chip treatability index of the steel of the present invention is at least 10 or more, the comparative material No. 14, 15, 17, 18, 20 do not reach this. No. The chip disposability index of 16 and 19 is 10 or more because it contains harmful substance Pb.
As described above, the free-cutting steel for machine structures of the present invention can obtain very good chip controllability without containing harmful substances such as Pb.
[0038]
【The invention's effect】
As described above, the free-cutting steel for machine structures of the present invention is a free-cutting steel for machine structures capable of obtaining extremely good chip disposability without containing harmful substances such as Pb.

Claims (3)

質量%で、C:0.01〜0.70%、Si:0.05〜2.00%、Mn:0.20〜3.50%、Ca:0.0003〜0.01%、S:0.020〜0.300%、Al:0.002〜0.300%、N:0.003〜0.035%、O:0.0010〜0.0080%、残部がFeおよび不可避不純物からなり、熱間圧延あるいは熱間鍛伸を実施した材料において、長径が0.5μm以上、かつ、20μm未満のMnSを主体とする硫化物の全硫化物個数に占める割合が30%以上であり、さらに、硫化物と複合あるいは単独で存在する長径が0.5μm以上、かつ、50μm未満の酸化物系介在物の個数が被検面積1mm中において10個以上含有することを特徴とする切屑処理性に優れる快削鋼。In mass%, C: 0.01 to 0.70%, Si: 0.05 to 2.00%, Mn: 0.20 to 3.50%, Ca: 0.0003 to 0.01%, S: 0.020 to 0.300%, Al: 0.002 to 0.300%, N: 0.003 to 0.035%, O: 0.0010 to 0.0080%, with the balance being Fe and unavoidable impurities In a material that has been subjected to hot rolling or hot forging, the ratio of the sulfide mainly composed of MnS having a major axis of 0.5 μm or more and less than 20 μm to the total number of sulfides is 30% or more, and A chip treating property, characterized in that the number of oxide-based inclusions having a major axis of 0.5 μm or more and less than 50 μm, which are present alone or in combination with sulfides, is contained at least 10 per 1 mm 2 of the test area. Excellent free-cutting steel. 請求項1に記載の快削鋼において、該鋼成分に加えて、質量%で、Cr:0.20〜2.50%、Mo:0.05〜1.50%、Ni:0.05〜3.50%、V:0.01〜0.50%、Nb:0.01〜0.10%、Ti:0.01〜0.50%のうち1種または2種以上を含有することを特徴とする切屑処理性に優れる快削鋼。The free-cutting steel according to claim 1, wherein in addition to the steel components, in mass%, Cr: 0.20 to 2.50%, Mo: 0.05 to 1.50%, Ni: 0.05 to 0.05%. 3.50%, V: 0.01 to 0.50%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.50% Free-cutting steel with excellent chip controllability. 請求項1または2に記載の快削鋼において、該鋼成分加えて、質量%で、Mg:0.0003〜0.01%、Zr:0.0005〜0.30%、Bi:0.01〜0.30%、B:0.0003〜0.015%のうち1種または2種以上を含有することを特徴とする切屑処理性に優れる快削鋼。3. The free-cutting steel according to claim 1, wherein, in addition to the steel components, Mg: 0.0003 to 0.01%, Zr: 0.0005 to 0.30%, and Bi: 0.01 by mass%. A free-cutting steel having excellent chip controllability, characterized in that it contains one or more of B-0.000 to 0.015%.
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