JP6186289B2 - Case-hardened steel capable of suppressing the occurrence of abnormal grains during carburizing treatment and machine structural parts using the same - Google Patents

Case-hardened steel capable of suppressing the occurrence of abnormal grains during carburizing treatment and machine structural parts using the same Download PDF

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JP6186289B2
JP6186289B2 JP2014036210A JP2014036210A JP6186289B2 JP 6186289 B2 JP6186289 B2 JP 6186289B2 JP 2014036210 A JP2014036210 A JP 2014036210A JP 2014036210 A JP2014036210 A JP 2014036210A JP 6186289 B2 JP6186289 B2 JP 6186289B2
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成朗 岡本
成朗 岡本
新堂　陽介
陽介 新堂
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Description

本発明は、自動車等の輸送機器、建設機械、その他の産業機械等において、浸炭処理や浸炭窒化処理（以下、これらをまとめて「浸炭処理」と呼ぶ場合がある）等の表面硬化熱処理をして製造される機械構造部品、及びその素材となる肌焼鋼に関する。より詳細には、浸炭処理時の異常粒発生が抑制可能な肌焼鋼に関する。   In the present invention, surface hardening heat treatment such as carburizing treatment or carbonitriding treatment (hereinafter sometimes collectively referred to as “carburizing treatment”) is performed on transportation equipment such as automobiles, construction machinery, and other industrial machines. In particular, the present invention relates to a machine structural part manufactured by the method and a case-hardened steel as a material for the machine structural part. More specifically, the present invention relates to a case hardening steel that can suppress the occurrence of abnormal grains during carburizing.

輸送機器、建設機械、その他の産業機械等において、高強度が要求される機械構造部品の素材には、ＪＩＳ規格で定められたＳＣｒ、ＳＣＭ、ＳＮＣＭ等の機械構造用合金鋼鋼材（いわゆる、肌焼鋼）が使用されるのが一般的である。この肌焼鋼を、鍛造や切削等の機械加工により所望の部品形状に成形した後、浸炭処理や浸炭窒化処理等の表面硬化熱処理を施し、その後研磨等を行うことによって機械構造部品が製造される。   For structural materials that require high strength in transportation equipment, construction machinery, other industrial machines, etc., steel materials for machine structures such as SCr, SCM, SNCM, etc., defined by JIS standards (so-called skin) It is common to use baked steel). After this case-hardened steel is formed into a desired part shape by machining such as forging or cutting, surface-hardening heat treatment such as carburizing or carbonitriding is performed, followed by polishing and the like to produce mechanical structural parts. The

上記のような表面硬化熱処理においては、製造時のリードタイムを短縮するため、高温化を図ることによって、熱処理時間の短縮化等が行われている。しかしながら、表面硬化熱処理を高温化すると、機械構造部品の結晶粒が粗大化し、機械的特性が劣化するという問題が生じる。   In the surface hardening heat treatment as described above, in order to shorten the lead time at the time of manufacture, the heat treatment time is shortened by increasing the temperature. However, when the temperature of the surface hardening heat treatment is increased, there is a problem that the crystal grains of the mechanical structural component are coarsened and the mechanical characteristics are deteriorated.

このような結晶粒粗大化を防止する技術として、例えば特許文献１、２が提案されている。これらの技術では、ＮｂおよびＴｉを含む複合窒化物（特許文献１）や、Ｎｂ及び／又はＴｉ含有析出物（特許文献２）を鋼中に分散させることによってピンニング効果を発揮させ、結晶粒の粗大化を防止している。   For example, Patent Documents 1 and 2 have been proposed as techniques for preventing such coarsening of crystal grains. In these technologies, a composite nitride containing Nb and Ti (Patent Document 1) and a Nb and / or Ti-containing precipitate (Patent Document 2) are dispersed in steel to exert a pinning effect, Prevents coarsening.

特開２００７−１６２１２８号公報JP 2007-162128 A 特開２００７−３２１２１１号公報JP 2007-321211 A

これまで提案されている技術のように、析出物によるピンニング効果を利用した結晶粒粗大化防止技術では、１０ｎｍ以上の微細な析出物を利用していると考えられる。しかしながら、本発明者らが調査したところ、これまで提案されてきたピンニング効果を利用した技術では、近年の高温化する浸炭条件においては析出物密度が不足し、部分的に結晶粒が粗大化して異常粒が発生することが判明した。   Like the techniques proposed so far, it is considered that the crystal grain coarsening prevention technology using the pinning effect by the precipitates uses fine precipitates of 10 nm or more. However, as a result of investigations by the present inventors, in the technology using the pinning effect that has been proposed so far, the density of precipitates is insufficient under carburizing conditions in recent years, and crystal grains are partially coarsened. It was found that abnormal grains were generated.

本発明は上記のような事情に鑑みてなされたものであり、その目的は、浸炭処理や浸炭窒化処理等の表面硬化熱処理において、異常粒発生を防止できる肌焼鋼及びこれを用いた機械構造部品を提供することにある。   The present invention has been made in view of the circumstances as described above, and its purpose is a case-hardened steel capable of preventing the occurrence of abnormal grains in a surface hardening heat treatment such as a carburizing process or a carbonitriding process, and a mechanical structure using the same. To provide parts.

上記課題を達成した本発明の肌焼鋼は、
Ｃ ：０．１０〜０．３０％（質量％の意味。以下、同じ。）、
Ｓｉ：０．０１〜０．５０％、
Ｍｎ：０．８０〜２．００％、
Ｐ ：０．０３０％以下（０％を含まない）、
Ｓ ：０．０３０％以下（０％を含まない）、
Ｃｒ：０．５０〜１．５０％、
Ａｌ：０．０１〜０．１０％、
Ｎ ：０．００１０〜０．０１０％、
Ｎｂ：０．０４０〜０．１５０％、
Ｔｉ：０．０４０〜０．１５０％
を含有し、残部が鉄及び不可避不純物であって、
Ｔｉ及びＮｂを含有する円相当径１０ｎｍ以上２００ｎｍ未満の炭化物及び炭窒化物の密度が１０個／μｍ2以上であり、
Ｔｉ及びＮｂを含有する円相当径１０ｎｍ未満の炭化物及び炭窒化物の密度が５０個／μｍ2以上であり、更に
Ｔｉ及びＳを含有する円相当径２００ｎｍ以上の析出物の密度が０．２０個／μｍ2以下であることを特徴とし、浸炭処理時の異常粒発生が抑制可能である。 The case-hardened steel of the present invention that has achieved the above-mentioned problems is
C: 0.10 to 0.30% (meaning mass%, hereinafter the same),
Si: 0.01 to 0.50%,
Mn: 0.80 to 2.00%
P: 0.030% or less (excluding 0%),
S: 0.030% or less (excluding 0%),
Cr: 0.50 to 1.50%,
Al: 0.01 to 0.10%,
N: 0.0010 to 0.010%,
Nb: 0.040 to 0.150%,
Ti: 0.040 to 0.150%
The balance is iron and inevitable impurities,
The density of carbide and carbonitride having an equivalent circle diameter of 10 nm or more and less than 200 nm containing Ti and Nb is 10 pieces / μm 2 or more,
The density of carbide and carbonitride having an equivalent circle diameter of less than 10 nm containing Ti and Nb is 50 / μm 2 or more, and the density of precipitates having an equivalent circle diameter of 200 nm or more containing Ti and S is 0.20. The number of particles / μm 2 or less, and the occurrence of abnormal particles during carburizing treatment can be suppressed.

本発明の肌焼鋼は、必要に応じて更に（ａ）Ｍｏ：２．０％以下（０％を含まない）、（ｂ）Ｃｕ：０．１０％以下（０％を含まない）及びＮｉ：３．０％以下（０％を含まない）の少なくとも１種、を含有することが好ましい。   The case-hardened steel of the present invention may further include (a) Mo: 2.0% or less (not including 0%), (b) Cu: 0.10% or less (not including 0%), and Ni as necessary. : It is preferable to contain 3.0% or less (excluding 0%).

本発明は、上記したいずれかの肌焼鋼を用いて得られた、表面が浸炭処理されている機械構造部品であって、浸炭層の旧オーステナイト粒度番号の最小値が６．０以上である機械構造部品も包含する。   The present invention is a machine structural part obtained by using any of the above-described case-hardened steels, the surface of which is carburized, and the minimum value of the prior austenite grain size number of the carburized layer is 6.0 or more. Also includes mechanical structural parts.

本発明によれば、化学組成を適切に調整し、Ｔｉ及びＮｂを含有する炭化物及び炭窒化物のうち、円相当径１０ｎｍ以上２００ｎｍ未満のもの、及び円相当径１０ｎｍ未満のものをそれぞれ所定量以上確保するとともに、Ｔｉ及びＳを含有する所定の大きさの析出物の密度を抑制しているため、浸炭処理時の異常粒発生を防止できる。   According to the present invention, the chemical composition is appropriately adjusted, and Ti and Nb-containing carbides and carbonitrides each having a predetermined amount of a circle equivalent diameter of 10 nm or more and less than 200 nm and a circle equivalent diameter of less than 10 nm. While ensuring the above, since the density of the precipitate of the predetermined magnitude | size containing Ti and S is suppressed, generation | occurrence | production of the abnormal grain at the time of a carburizing process can be prevented.

図１は、浸炭処理前後の析出物の挙動の概念を示した模式図である。FIG. 1 is a schematic diagram showing the concept of the behavior of precipitates before and after carburizing treatment. 図２は、浸炭処理時の熱処理パターンを示した模式図である。FIG. 2 is a schematic diagram showing a heat treatment pattern during carburizing treatment.

上述した特許文献１、２に開示されるように、Ｔｉ、Ｎｂを含有する微細析出物は結晶粒粗大化防止に有効であるが、その密度が不足すると、不足した部分で結晶粒の粗大化が生じ、異常粒が発生した状態となる。特に近年の浸炭温度の高温化により、従来までに提案されてきた微細析出物によるピンニング効果では、異常粒発生を十分に抑制することができなかった。   As disclosed in Patent Documents 1 and 2 described above, fine precipitates containing Ti and Nb are effective in preventing crystal grain coarsening. However, when the density is insufficient, the crystal grains are coarsened at the lacking portion. And abnormal particles are generated. In particular, due to the recent increase in carburizing temperature, the pinning effect by fine precipitates that has been proposed so far has not been able to sufficiently suppress the occurrence of abnormal grains.

そこで、本発明者らは、微細析出物が異常粒発生に与える影響を検討し、異常粒発生を抑制できる微細析出物の析出状態について鋭意研究を重ねた。その結果、（i）微細析出物のうち、結晶粒の粗大化を防止して異常粒発生を抑制するのに最も有効であるのは、Ｔｉ及びＮｂを含有する炭化物及び炭窒化物（以下、「（Ｔｉ、Ｎｂ）炭化物等」と呼ぶ場合がある。）のうち、サイズが１０ｎｍ以上２００ｎｍ未満のものであること、また前記した（i）の炭化物等を所定量確保するためには、(ii)Ｔｉ及びＳを含有する粗大な析出物（以下、「粗大な（Ｔｉ、Ｓ）析出物」と呼ぶ場合がある。）を抑制することが重要であることが判明した。更に、浸炭処理時に前記した(i)の炭化物等がマトリックス中に固溶することなく、有効にピンニング効果を発揮して異常粒発生を抑制するためには、（iii）（Ｔｉ、Ｎｂ）炭化物等のうち、サイズが１０ｎｍ未満であるものを所定量確保する必要があることも明らかになった。なお、本明細書において、炭化物等及び析出物のサイズは全て円相当径を意味する。   Therefore, the present inventors have studied the influence of fine precipitates on the generation of abnormal grains, and conducted extensive research on the precipitation state of fine precipitates that can suppress the generation of abnormal grains. As a result, (i) among the fine precipitates, the most effective for preventing the coarsening of crystal grains and suppressing the occurrence of abnormal grains are carbides and carbonitrides containing Ti and Nb (hereinafter, In order to secure a predetermined amount of the above-described (i) carbide, etc., the size may be 10 nm or more and less than 200 nm (sometimes referred to as “(Ti, Nb) carbide, etc.”). ii) It has been found that it is important to suppress coarse precipitates containing Ti and S (hereinafter sometimes referred to as “coarse (Ti, S) precipitates”). Furthermore, (iii) (Ti, Nb) carbide in order to effectively exhibit the pinning effect and suppress the occurrence of abnormal particles without causing the above-described carbide (i) or the like to dissolve in the matrix during the carburizing process. It has also become clear that it is necessary to secure a predetermined amount of those having a size of less than 10 nm. In the present specification, the size of the carbide and the like and the precipitate all mean the equivalent circle diameter.

本発明者らは、浸炭処理時における異常粒発生が起こる原因について、図１のように考えた。図１は、浸炭処理前後の析出物の挙動の概念を示した模式図である。   The present inventors considered the cause of occurrence of abnormal grains during carburization as shown in FIG. FIG. 1 is a schematic diagram showing the concept of the behavior of precipitates before and after carburizing treatment.

ＴｉとＳは結合して析出物を形成するが、粗大な（Ｔｉ、Ｓ）析出物の密度が高いと、異常粒発生抑制に有効な１０ｎｍ以上の（Ｔｉ、Ｎｂ）炭化物等に、Ｔｉが有効に利用されない。そこで、粗大な（Ｔｉ、Ｓ）析出物の密度は低減しなければならない。   Ti and S combine to form precipitates, but if the density of coarse (Ti, S) precipitates is high, Ti is effective in (Ti, Nb) carbides of 10 nm or more that are effective in suppressing abnormal grain generation. It is not used effectively. Therefore, the density of coarse (Ti, S) precipitates must be reduced.

また、浸炭処理時には常温に比べてＴｉ及びＮｂの固溶限が上昇するため、ナノオーダーのサイズの（Ｔｉ、Ｎｂ）炭化物等はマトリックス中に固溶しやすくなる。このとき、サイズの小さい（Ｔｉ、Ｎｂ）炭化物等の方がマトリックスに固溶しやすいため、１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等を十分に確保し、浸炭時にこれらを優先的に固溶させてマトリックスの固溶限までの固溶量を補う。これによって異常粒発生を抑制するのに有効な１０ｎｍ以上の（Ｔｉ、Ｎｂ）炭化物等が、浸炭処理時にマトリックス中へ固溶するのを防止できると考えられる。   In addition, since the solid solubility limit of Ti and Nb is increased at the time of carburizing treatment as compared to normal temperature, (Ti, Nb) carbide or the like having a nano-order size is easily dissolved in the matrix. At this time, (Ti, Nb) carbides and the like with smaller sizes are more easily dissolved in the matrix, so (Ti, Nb) carbides of less than 10 nm are sufficiently secured, and these are preferentially dissolved during carburization. To compensate for the solid solution amount up to the solid solution limit of the matrix. Thus, it is considered that 10 nm or more (Ti, Nb) carbides and the like effective for suppressing the occurrence of abnormal grains can be prevented from dissolving in the matrix during the carburizing process.

以上のような考えに基づき、本発明では、浸炭処理時の異常粒発生を抑制するために、具体的に、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を１０個／μｍ2以上、１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度が５０個／μｍ2以上とし、更に２００ｎｍ以上の（Ｔｉ、Ｓ）析出物の密度が０．２０個／μｍ2以下とする。 Based on the above idea, in the present invention, in order to suppress the occurrence of abnormal particles during the carburizing process, the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm is specifically 10 pieces / μm 2 or more. The density of (Ti, Nb) carbide or the like less than 10 nm is 50 pieces / μm 2 or more, and the density of (Ti, S) precipitates of 200 nm or more is 0.20 pieces / μm 2 or less.

１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等（すなわち、Ｔｉ及びＮｂを含有する炭化物及び炭窒化物）は、浸炭処理時の結晶粒粗大化防止に有効に働き、異常粒発生を抑制できる。このような効果を有効に発揮させるため、その密度は１０個／μｍ2以上必要であり、好ましくは１５個／μｍ2以上、より好ましくは２０個／μｍ2以上である。１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度の上限は特に限定されないが、通常１５０個／μｍ2程度であり、１２０個／μｍ2以下が好ましく、より好ましくは１００個／μｍ2以下である。 (Ti, Nb) carbides and the like (ie, carbides and carbonitrides containing Ti and Nb) of 10 nm or more and less than 200 nm effectively work to prevent crystal grain coarsening during carburizing treatment, and can suppress the occurrence of abnormal grains. In order to effectively exhibit such an effect, the density needs to be 10 / μm 2 or more, preferably 15 / μm 2 or more, and more preferably 20 / μm 2 or more. The upper limit of the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm is not particularly limited, but is usually about 150 pieces / μm 2 , preferably 120 pieces / μm 2 or less, more preferably 100 pieces / μm 2 or less. It is.

１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等（すなわち、Ｔｉ及びＮｂを含有する炭化物及び炭窒化物）は、浸炭処理時にマトリックス中に固溶するとともに、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等をオストワルド成長させ、マトリックス中への固溶を防止する。このような効果を有効に発揮させるため、１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度は、５０個／μｍ2以上必要であり、好ましくは１００個／μｍ2以上、より好ましくは１５０個／μｍ2以上である。１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度の上限は特に限定されないが、通常３００個／μｍ2程度である。なお、１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等として測定される（Ｔｉ、Ｎｂ）炭化物等のサイズの下限は特に限定されないが、電子顕微鏡等の測定装置の測定限界があるため、通常２ｎｍ程度である。 (Ti, Nb) carbides and the like less than 10 nm (ie, carbides and carbonitrides containing Ti and Nb) are dissolved in the matrix during the carburizing process, and (Ti, Nb) carbides and the like of 10 nm or more and less than 200 nm. To prevent solute growth in the matrix. In order to effectively exhibit such an effect, the density of (Ti, Nb) carbide or the like of less than 10 nm needs to be 50 / μm 2 or more, preferably 100 / μm 2 or more, more preferably 150 / μm 2 or more. The upper limit of the density of (Ti, Nb) carbide or the like less than 10 nm is not particularly limited, but is usually about 300 / μm 2 . In addition, the lower limit of the size of (Ti, Nb) carbide or the like measured as (Ti, Nb) carbide or the like of less than 10 nm is not particularly limited, but is usually about 2 nm because of the measurement limit of a measuring device such as an electron microscope. is there.

本発明における（Ｔｉ、Ｎｂ）炭化物等は、エネルギー分散型Ｘ線分光法（Ｅｎｅｒｇｙ Ｄｉｓｐｅｒｓｉｖｅ Ｘ−ｒａｙ Ｓｐｅｃｔｒｏｓｃｏｐｙ、ＥＤＸ）等を用いた元素分析により、Ｃ又はＮを示すピークが検出されるとともに、Ｔｉ及びＮｂのピークが検出される析出物を意味する。   In the present invention, (Ti, Nb) carbide and the like have a peak indicating C or N detected by elemental analysis using energy dispersive X-ray spectroscopy (EDX) or the like, and Ti And a precipitate in which a peak of Nb is detected.

２００ｎｍ以上の（Ｔｉ、Ｓ）析出物の密度が多くなりすぎると、異常粒発生の抑制に有効な１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の個数を確保することができない。そこで、２００ｎｍ以上の（Ｔｉ、Ｓ）析出物の密度は０．２０個／μｍ2以下とする必要があり、好ましくは０．１５個／μｍ2以下、より好ましくは０．１０個／μｍ2以下である。２００ｎｍ以上の（Ｔｉ、Ｓ）析出物は少なければ少ない程よいが、通常０個／μｍ2を超える値である。本発明における（Ｔｉ、Ｓ）析出物は、ＥＤＸ等を用いた元素分析により、Ｔｉ及びＳのピークが検出される析出物を意味する。 If the density of (Ti, S) precipitates of 200 nm or more is too large, it is not possible to ensure the number of (Ti, Nb) carbides, etc. of 10 nm or more and less than 200 nm that are effective in suppressing the generation of abnormal grains. Therefore, the density of (Ti, S) precipitates of 200 nm or more needs to be 0.20 pieces / μm 2 or less, preferably 0.15 pieces / μm 2 or less, more preferably 0.10 pieces / μm 2. It is as follows. The smaller the number of (Ti, S) precipitates of 200 nm or more, the better. However, it is usually a value exceeding 0 / μm 2 . The (Ti, S) precipitate in the present invention means a precipitate from which Ti and S peaks are detected by elemental analysis using EDX or the like.

本発明は、上述したような炭化物等及び析出物の制御に加えて、肌焼鋼としての基本的な特性を発揮させるため、その化学組成も適切に調整する必要がある。以下に説明する。   In addition to the above-described control of carbides and precipitates as described above, the present invention needs to appropriately adjust its chemical composition in order to exhibit basic characteristics as case hardening steel. This will be described below.

Ｃ：０．１０〜０．３０％
Ｃは、浸炭部品として必要な芯部硬さを確保するために必要な元素である。Ｃ含有量が０．１０％未満では、硬さ不足により浸炭部品としての静的強度が不足する。従ってＣ含有量は０．１０％以上とする必要があり、好ましくは０．１２％以上、より好ましくは０．１５％以上である。しかしながら、Ｃを過剰に含有させると、硬さが過度に高くなるため靭性が低下し、衝撃特性が劣化するとともに、冷間鍛造性も低下する。従って、Ｃ含有量は０．３０％以下とする必要があり、好ましくは０．２８％以下、より好ましくは０．２５％以下である。 C: 0.10 to 0.30%
C is an element necessary for ensuring the core hardness necessary for carburized parts. If the C content is less than 0.10%, the static strength as a carburized part is insufficient due to insufficient hardness. Therefore, the C content needs to be 0.10% or more, preferably 0.12% or more, more preferably 0.15% or more. However, when C is excessively contained, the hardness becomes excessively high, so that toughness is reduced, impact characteristics are deteriorated, and cold forgeability is also reduced. Therefore, the C content needs to be 0.30% or less, preferably 0.28% or less, more preferably 0.25% or less.

Ｓｉ：０．０１〜０．５０％
Ｓｉは、焼戻し硬さの低下を抑えて機械構造部品の面疲労特性を改善するのに有効な元素である。こうした効果を有効に発揮させるため、Ｓｉは０．０１％含有させる必要がある。Ｓｉ含有量は、好ましくは０．０３％以上、より好ましくは０．０５％以上である。しかしながら、Ｓｉを過剰に含有させると、被削性や鍛造性等の部品成形性に悪影響を及ぼす。こうした観点からＳｉ含有量は０．５０％以下とする必要があり、好ましくは０．４５％以下、より好ましくは０．４０％以下である。 Si: 0.01 to 0.50%
Si is an effective element for improving the surface fatigue characteristics of mechanical structural parts by suppressing the decrease in tempering hardness. In order to exhibit such an effect effectively, it is necessary to contain Si 0.01%. The Si content is preferably 0.03% or more, more preferably 0.05% or more. However, if Si is contained excessively, it adversely affects the part formability such as machinability and forgeability. From such a viewpoint, the Si content needs to be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

Ｍｎ：０．８０〜２．００％
Ｍｎは、浸炭処理時の焼入れ性を高めるのに有効な元素である。また、Ｍｎは脱酸剤としても作用し、鋼中の酸化物系介在物量を低減して内部品質を高める作用を有する元素である。更に、Ｍｎは赤熱脆性を防止する作用も有する。こうした作用を有効に発揮させるため、Ｍｎは０．８０％以上含有させる必要がある。Ｍｎ含有量は、好ましくは０．８５％以上であり、より好ましくは０．９０％以上である。しかしながら、Ｍｎを過剰に含有させると、鍛造性が悪化しやすくなると共に、材質のばらつきが大きくなる。従って、Ｍｎ含有量は２．００％以下とする必要があり、好ましくは１．８％以下であり、より好ましくは１．７％以下である。 Mn: 0.80 to 2.00%
Mn is an element effective for improving the hardenability during the carburizing process. Mn also acts as a deoxidizer, and is an element that has the effect of increasing the internal quality by reducing the amount of oxide inclusions in the steel. Furthermore, Mn also has an effect of preventing red heat embrittlement. In order to exhibit such an action effectively, it is necessary to contain 0.80% or more of Mn. The Mn content is preferably 0.85% or more, more preferably 0.90% or more. However, when Mn is contained excessively, the forgeability is likely to be deteriorated and the variation of the material is increased. Accordingly, the Mn content needs to be 2.00% or less, preferably 1.8% or less, and more preferably 1.7% or less.

Ｐ：０．０３０％以下（０％を含まない）
Ｐは、鋼中に不可避不純物として含まれる元素であり、結晶粒界に偏析して機械構造部品の衝撃疲労特性を劣化させる。従って、Ｐ含有量は０．０３０％以下とする必要があり、好ましくは０．０２５％以下、より好ましくは０．０２０％以下である。Ｐ含有量は少なければ少ない程好ましいが、製造工程の制約上０％とすることは難しく、通常０．００１％程度は含まれる。 P: 0.030% or less (excluding 0%)
P is an element contained in the steel as an inevitable impurity, and segregates at the grain boundary to deteriorate the impact fatigue characteristics of the mechanical structural component. Therefore, the P content needs to be 0.030% or less, preferably 0.025% or less, more preferably 0.020% or less. The smaller the P content, the better. However, it is difficult to make it 0% due to restrictions on the manufacturing process, and usually about 0.001% is included.

Ｓ：０．０３０％以下（０％を含まない）
Ｓは、Ｍｎと結合してＭｎＳを形成し、切削加工するときの被削性を改善する元素である。こうした作用を有効に発揮させるため、Ｓは０．００１％以上含有させることが好ましく、より好ましくは０．００２％以上である。しかしながら、Ｓ含有量が過剰になってＭｎＳの生成量が多くなると、Ｔｉ及びＳを含有する析出物密度の増大や、機械構造部品としての強度劣化を引き起こす。こうした観点から、Ｓ含有量は０．０３０％以下とする必要があり、好ましくは０．０２５％以下、より好ましくは０．０２０％以下である。 S: 0.030% or less (excluding 0%)
S is an element that combines with Mn to form MnS and improves machinability when cutting. In order to effectively exhibit such an action, S is preferably contained in an amount of 0.001% or more, more preferably 0.002% or more. However, when the S content becomes excessive and the amount of MnS produced increases, the density of precipitates containing Ti and S increases and the strength deteriorates as a mechanical structural component. From such a viewpoint, the S content needs to be 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less.

Ｃｒ：０．５０〜１．５０％
Ｃｒは、浸炭を促進し、鋼の表面に硬化層を形成するために必要な元素である。こうした作用を有効に発揮させるため、Ｃｒは０．５０％以上含有させる必要があり、好ましくは０．６０％以上、より好ましくは０．７０％以上である。しかしながら、Ｃｒを過剰に含有させると、過剰浸炭を引き起こし、機械構造部品の強度を低下させる。こうした観点から、Ｃｒ含有量は１．５０％以下とする必要があり、好ましくは１．４５％以下、より好ましくは１．４０％以下である。 Cr: 0.50 to 1.50%
Cr is an element necessary for promoting carburization and forming a hardened layer on the steel surface. In order to exhibit such an action effectively, Cr needs to be contained in an amount of 0.50% or more, preferably 0.60% or more, and more preferably 0.70% or more. However, when Cr is excessively contained, excessive carburization is caused and the strength of the mechanical structural component is lowered. From such a viewpoint, the Cr content needs to be 1.50% or less, preferably 1.45% or less, more preferably 1.40% or less.

Ａｌ：０．０１〜０．１０％
Ａｌは、脱酸剤として作用する元素であり、こうした作用を有効に発揮させるため、Ａｌは０．０１％以上含有させる必要がある。Ａｌ含有量は、好ましくは０．０１５％以上であり、より好ましくは０．０２０％以上である。しかしながら、Ａｌを過剰に含有させると、鋼の変形抵抗が増大し、冷間鍛造性が劣化する。従って、Ａｌ含有量は０．１０％以下とする必要があり、好ましくは０．０８０％以下であり、より好ましくは０．０６０％以下である。 Al: 0.01-0.10%
Al is an element that acts as a deoxidizing agent. In order to effectively exhibit such action, Al needs to be contained in an amount of 0.01% or more. The Al content is preferably 0.015% or more, and more preferably 0.020% or more. However, when Al is contained excessively, the deformation resistance of steel increases and the cold forgeability deteriorates. Therefore, the Al content needs to be 0.10% or less, preferably 0.080% or less, and more preferably 0.060% or less.

Ｎ：０．００１０〜０．０１０％
Ｎは、機械構造部品の結晶粒度を適切に調整するために作用するＴｉＮｂ炭窒化物を形成するために必要な元素である。こうした効果を発揮させるためには、Ｎは０．００１０％以上含有させる必要があり、好ましくは０．００２０％以上、より好ましくは０．００３０％以上である。しかしながら、Ｎを過剰に含有させると、鋼中にＡｌＮ、ＴｉＮなどの窒化物が多量に形成され、切削性や鍛造性を劣化させる。こうした観点から、Ｎ含有量は０．０１０％以下とする必要があり、好ましくは０．００９％以下、より好ましくは０．００８％以下である。 N: 0.0010 to 0.010%
N is an element necessary for forming TiNb carbonitride that acts to appropriately adjust the crystal grain size of the mechanical structural component. In order to exert such an effect, N needs to be contained in an amount of 0.0010% or more, preferably 0.0020% or more, and more preferably 0.0030% or more. However, when N is excessively contained, a large amount of nitrides such as AlN and TiN are formed in the steel, and the machinability and forgeability deteriorate. From such a viewpoint, the N content needs to be 0.010% or less, preferably 0.009% or less, more preferably 0.008% or less.

Ｎｂ：０．０４０〜０．１５０％
Ｎｂは、機械構造部品の結晶粒度を適切に調整するために作用するＴｉＮｂ炭化物及び炭窒化物を形成するために必要な元素である。こうした効果を発揮させるためには、Ｎｂは０．０４０％以上含有させる必要があり、好ましくは０．０４５％以上、より好ましくは０．０５０％以上である。しかしながら、Ｎｂを過剰に含有させると、鋼中に過剰にＴｉＮｂ炭化物及び炭窒化物を形成し、切削性や鍛造性を劣化させる。こうした観点からＮｂ含有量は０．１５０％以下とする必要があり、好ましくは０．１２０％以下、より好ましくは０．１００％以下である。 Nb: 0.040 to 0.150%
Nb is an element necessary for forming TiNb carbides and carbonitrides that function to appropriately adjust the crystal grain size of mechanical structural components. In order to exert such an effect, Nb needs to be contained in an amount of 0.040% or more, preferably 0.045% or more, more preferably 0.050% or more. However, when Nb is contained excessively, TiNb carbide and carbonitride are excessively formed in the steel, and the machinability and forgeability are deteriorated. From such a viewpoint, the Nb content needs to be 0.150% or less, preferably 0.120% or less, and more preferably 0.100% or less.

Ｔｉ：０．０４０〜０．１５０％
Ｔｉは、機械構造部品の結晶粒度を適切に調整するために作用するＴｉＮｂ炭化物及び炭窒化物を形成するために必要な元素である。こうした効果を発揮させるためには、Ｔｉは０．０４０％以上含有させる必要があり、好ましくは０．０４５％以上、より好ましくは０．０５０％以上である。しかしながら、Ｔｉを過剰に含有させると、鋼中に過剰にＴｉＮやＴｉＮｂ炭化物及び炭窒化物を形成し、切削性や鍛造性を劣化させる。こうした観点からＴｉ含有量は０．１５０％以下とする必要があり、好ましくは０．１２０％以下、より好ましくは０．１００％以下である。 Ti: 0.040 to 0.150%
Ti is an element necessary for forming TiNb carbides and carbonitrides that function to appropriately adjust the crystal grain size of mechanical structural components. In order to exhibit such an effect, Ti needs to be contained by 0.040% or more, preferably 0.045% or more, and more preferably 0.050% or more. However, when Ti is excessively contained, TiN, TiNb carbides and carbonitrides are excessively formed in the steel, and the machinability and forgeability are deteriorated. From such a viewpoint, the Ti content needs to be 0.150% or less, preferably 0.120% or less, more preferably 0.100% or less.

本発明に係る肌焼鋼の基本成分は上記の通りであり、残部は実質的に鉄である。但し、原材料、資材、製造設備等の状況によって持ち込まれる不可避不純物が鋼中に含まれることは当然に許容される。   The basic components of the case-hardened steel according to the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel.

さらに本発明では、必要に応じて更に（ａ）Ｍｏ：２．０％以下（０％を含まない）、（ｂ）Ｃｕ：０．１０％以下（０％を含まない）及びＮｉ：３．０％以下（０％を含まない）の少なくとも１種を含有することも有用であり、含有される元素の種類に応じて肌焼鋼の特性が更に改善される。   Furthermore, in the present invention, if necessary, (a) Mo: 2.0% or less (not including 0%), (b) Cu: 0.10% or less (not including 0%), and Ni: 3. It is also useful to contain at least one of 0% or less (excluding 0%), and the characteristics of the case-hardened steel are further improved according to the type of element contained.

Ｍｏ：２．０％以下（０％を含まない）
Ｍｏは、浸炭処理における焼入性を向上する元素である。こうした作用を有効に発揮させるため、Ｍｏを０．０５％以上含有させることが好ましく、より好ましくは０．０８％以上、更に好ましくは０．１０％以上である。しかしながら、Ｍｏを過剰に含有させると、切削性や鍛造性を劣化させる。従って、Ｍｏ含有量は２．０％以下であることが好ましく、より好ましくは１．５％以下、更に好ましくは１．２％以下である。 Mo: 2.0% or less (excluding 0%)
Mo is an element that improves the hardenability in the carburizing process. In order to effectively exhibit such an action, it is preferable to contain 0.05% or more of Mo, more preferably 0.08% or more, and still more preferably 0.10% or more. However, when Mo is contained excessively, machinability and forgeability are deteriorated. Therefore, the Mo content is preferably 2.0% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

Ｃｕ：０．１０％以下（０％を含まない）及びＮｉ：３．０％以下（０％を含まない）の少なくとも１種
Ｃｕ及びＮｉは、上記Ｍｏと同様に、浸炭処理における焼入性を高めるのに有効な元素である。また、ＣｕとＮｉは、Ｆｅよりも酸化され難い元素であるため、機械構造部品の耐食性を改善するのにも作用する。これらの作用を有効に発揮させるには、Ｃｕは０．０３％以上含有することが好ましく、より好ましくは０．０４％以上である。Ｎｉは０．０３％以上含有することが好ましく、より好ましくは０．０５％以上、更に好ましくは０．０８％以上である。しかしながら、Ｃｕを過剰に含有させると、熱間鍛造性が低下し、割れなどの問題が発生しやすくなる。従って、Ｃｕ含有量は０．１０％以下とすることが好ましく、より好ましくは０．０８％以下である。またＮｉを過剰に含有させるとコスト高となるため、Ｎｉ含有量は３．０％以下とすることが好ましく、より好ましくは２．５％以下、更に好ましくは２．０％以下である。ＣｕとＮｉは、何れか一方を含有しても良いし、両方を含有しても良い。 Cu: 0.10% or less (excluding 0%) and Ni: 3.0% or less (not including 0%) Cu and Ni are hardenability in carburizing treatment as in the case of Mo. It is an effective element for enhancing Further, since Cu and Ni are elements that are less likely to be oxidized than Fe, they also act to improve the corrosion resistance of mechanical structural parts. In order to effectively exhibit these actions, Cu is preferably contained in an amount of 0.03% or more, and more preferably 0.04% or more. Ni is preferably contained in an amount of 0.03% or more, more preferably 0.05% or more, and further preferably 0.08% or more. However, when Cu is excessively contained, hot forgeability is lowered, and problems such as cracking are likely to occur. Therefore, the Cu content is preferably 0.10% or less, more preferably 0.08% or less. Moreover, since it will become expensive if Ni is contained excessively, it is preferable to make Ni content into 3.0% or less, More preferably, it is 2.5% or less, More preferably, it is 2.0% or less. Cu and Ni may contain either one or both.

本発明の肌焼鋼を製造するためには、所定の化学組成の鋼を通常の溶製法に従って溶製し、鋳造、分塊圧延した後、棒鋼圧延するという一連の工程において、特に分塊圧延時及び棒鋼圧延時の加熱温度と加熱保持時間を適切に調整することが好ましい。具体的には、分塊圧延時の加熱を１０００〜１２５０℃で３０分未満とし、棒鋼圧延時の加熱を８００〜１０００℃で６０分以下とする。   In order to produce the case-hardened steel of the present invention, a steel having a predetermined chemical composition is melted in accordance with a normal melting method, cast, split-rolled, and then subjected to bar rolling, in particular, in the series of rolling. It is preferable to appropriately adjust the heating temperature and the heating holding time during rolling and bar rolling. Specifically, heating at the time of the ingot rolling is set to 1000 to 1250 ° C. for less than 30 minutes, and heating at the time of rolling the steel bar is set to 800 to 1000 ° C. for 60 minutes or less.

本発明において、分塊圧延では、（Ｔｉ、Ｓ）析出物の生成を抑制しすると共に、鋳造段階で生成した（Ｔｉ、Ｎｂ）炭化物等をできるだけマトリックス中に固溶させず、異常粒発生の抑制に有効な１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の核となる析出物を確保する。また、棒鋼圧延では、分塊圧延にて残存させた（Ｔｉ、Ｎｂ）炭化物等をオストワルド成長させ、異常粒発生の抑制に有効な１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等を上述した密度に到達させる。   In the present invention, in the bulk rolling, the generation of (Ti, S) precipitates is suppressed, and (Ti, Nb) carbides and the like generated in the casting stage are not dissolved in the matrix as much as possible, and abnormal grains are generated. Precipitates that become nuclei such as (Ti, Nb) carbides of 10 nm or more and less than 200 nm that are effective for suppression are secured. Further, in the steel bar rolling, the density of (Ti, Nb) carbide, etc., which is effective for suppressing the occurrence of abnormal grains by causing Ostwald growth of (Ti, Nb) carbide, etc. remaining in the partial rolling, is described above. To reach.

分塊圧延時の加熱温度が１０００℃を下回ると、分塊圧延時の圧延機への負荷が大きくなり、所望形状への圧延が困難になる。このため、加熱温度は１０００℃以上が好ましい。加熱温度は１１２０℃以上がより好ましく、更に好ましくは１１５０℃以上である。しかしながら、加熱温度が高くなりすぎると、鋳造段階で生成した（Ｔｉ、Ｎｂ）炭化物等がマトリックス中に固溶するとともに、２００ｎｍ以上の（Ｔｉ、Ｓ）析出物の密度が増大する。従って、分塊圧延時の加熱温度は１２５０℃以下が好ましく、より好ましくは１２３０℃以下であり、更に好ましくは１２００℃以下である。また前記温度範囲での加熱保持時間が長すぎると、鋳造段階で生成した（Ｔｉ、Ｎｂ）炭化物等がマトリックス中へ固溶する。従って、加熱保持時間は３０分未満が好ましく、より好ましくは２５分未満である。一方、加熱保持時間が短すぎると鋼材の温度ムラができ、材質のばらつきにつながるため、加熱保持時間は５分以上が好ましく、より好ましくは１０分以上である。   When the heating temperature at the time of the block rolling is below 1000 ° C., the load on the rolling mill at the time of the block rolling becomes large and it becomes difficult to perform the rolling to a desired shape. For this reason, the heating temperature is preferably 1000 ° C. or higher. The heating temperature is more preferably 1120 ° C or higher, and still more preferably 1150 ° C or higher. However, when the heating temperature becomes too high, (Ti, Nb) carbides and the like generated in the casting stage are dissolved in the matrix, and the density of (Ti, S) precipitates of 200 nm or more increases. Therefore, the heating temperature at the time of the block rolling is preferably 1250 ° C. or less, more preferably 1230 ° C. or less, and further preferably 1200 ° C. or less. On the other hand, if the heating and holding time in the temperature range is too long, (Ti, Nb) carbides and the like generated in the casting stage are dissolved in the matrix. Therefore, the heating and holding time is preferably less than 30 minutes, more preferably less than 25 minutes. On the other hand, if the heating and holding time is too short, temperature unevenness of the steel material is generated, leading to variations in the material. Therefore, the heating and holding time is preferably 5 minutes or more, and more preferably 10 minutes or more.

棒鋼圧延時の加熱温度が８００℃を下回ると、棒鋼圧延機への負荷が大きくなり、所望形状への圧延が困難となる。このため、棒鋼圧延時の加熱温度は８００℃以上が好ましく、より好ましくは８２０℃以上であり、更に好ましくは８５０℃以上である。しかしながら、棒鋼圧延時の加熱温度が１０００℃を超えると１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度が低下する。このため、加熱温度は１０００℃以下が好ましく、より好ましくは９８０℃以下であり、更に好ましくは９５０℃以下である。また、前記温度範囲での加熱保持時間が長すぎると１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度が低下する。このため、加熱保持時間は６０分以下が好ましく、より好ましくは４５分以下である。加熱保持時間が短すぎると鋼材の温度ムラができ、材質のばらつきにつながるため、加熱保持時間は５分以上が好ましく、より好ましくは１０分以上である。   When the heating temperature at the time of steel bar rolling is below 800 ° C., the load on the steel bar rolling machine becomes large, and rolling to a desired shape becomes difficult. For this reason, the heating temperature at the time of steel bar rolling is preferably 800 ° C. or higher, more preferably 820 ° C. or higher, and further preferably 850 ° C. or higher. However, when the heating temperature at the time of steel bar rolling exceeds 1000 ° C., the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm decreases. For this reason, the heating temperature is preferably 1000 ° C. or lower, more preferably 980 ° C. or lower, and further preferably 950 ° C. or lower. On the other hand, if the heating and holding time in the above temperature range is too long, the density of (Ti, Nb) carbide or the like less than 10 nm decreases. For this reason, the heat holding time is preferably 60 minutes or less, more preferably 45 minutes or less. If the heating and holding time is too short, the temperature of the steel material will be uneven, leading to variations in material quality. Therefore, the heating and holding time is preferably 5 minutes or more, more preferably 10 minutes or more.

上記のように分塊圧延時及び棒鋼圧延時の加熱温度及び加熱保持時間の条件を満足させることによって、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を１０個／μｍ2以上、１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を５０個／μｍ2以上とし、更に２００ｎｍ以上の（Ｔｉ、Ｓ）析出物の密度を０．２０個／μｍ2以下とできる。本発明の肌焼鋼の形状は特に限定されないが、例えばφ１０〜１５０ｍｍの棒鋼である。このような要件を満足する本発明の肌焼鋼に、浸炭処理を施して得られる、すなわち表面が浸炭処理されている機械構造部品は、異常粒発生を抑制でき、このような機械構造部品も本発明に含まれる。特に、本発明の機械構造部品は、ＪＩＳ Ｇ０５５１（２００５）に基づいて測定される旧オーステナイト結晶粒度番号の最小値を６番以上とでき、すなわち最も大きな旧オーステナイト結晶粒径を粒度番号で６番以上とでき、異常粒の発生を抑制できる。前記結晶粒度番号の最小値の上限は限定されないが、通常９番程度である。前記浸炭処理条件は特に限定されず、例えば９００〜１０００℃の温度で行えば良く、特に９３０℃以上（更に好ましくは９５０℃以上）のような高温で浸炭を行った場合でも、本発明の肌焼鋼は異常粒発生を抑制でき、異常粒発生の抑制された機械構造部品を得ることができる。 By satisfying the conditions of the heating temperature and heating holding time at the time of the ingot rolling and the bar rolling as described above, the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm is 10 / μm 2 or more, 10 nm. The density of less than (Ti, Nb) carbide or the like can be 50 pieces / μm 2 or more, and the density of (Ti, S) precipitates of 200 nm or more can be 0.20 pieces / μm 2 or less. Although the shape of the case hardening steel of this invention is not specifically limited, For example, it is a bar steel of (phi) 10-150mm. Machine structural parts obtained by carburizing the case-hardened steel of the present invention that satisfies such requirements, that is, the surface of which is carburized, can suppress the occurrence of abnormal grains. It is included in the present invention. In particular, the mechanical structural component of the present invention can have a minimum prior austenite grain size number measured in accordance with JIS G0551 (2005) of 6 or more, that is, the largest prior austenite grain size is 6 in grain size number. Thus, the occurrence of abnormal particles can be suppressed. The upper limit of the minimum value of the crystal grain size number is not limited, but is usually about 9th. The carburizing treatment conditions are not particularly limited, and may be performed at a temperature of 900 to 1000 ° C., for example, even when carburizing is performed at a high temperature such as 930 ° C. or higher (more preferably 950 ° C. or higher). Hardened steel can suppress the generation of abnormal grains, and can obtain a machine structural component in which the generation of abnormal grains is suppressed.

本発明の肌焼鋼を用いた機械構造部品としては、具体的に、歯車、シャフト類、無段変速機（Ｃｏｎｔｉｎｕｏｕｓｌｙ Ｖａｒｉａｂｌｅ Ｔｒａｎｓｍｉｓｓｉｏｎ、ＣＶＴ）プーリ、等速ジョイント（Ｃｏｎｓｔａｎｔ Ｖｅｌｏｃｉｔｙ Ｊｏｉｎｔ、ＣＶＪ）、軸受等が挙げられる。   Specific examples of mechanical structural parts using the case-hardened steel of the present invention include gears, shafts, continuously variable transmission (CVT) pulleys, constant velocity joints (CVJ), bearings, and the like. Is mentioned.

以下、実施例を挙げて本発明をより具体的に説明する。本発明は以下の実施例によって制限を受けるものではなく、前記、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勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。   Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

下記表１に示す化学組成を満たす鋼を、通常の溶製法に従って溶製炉で溶製し、鋼片を製造した。   Steel satisfying the chemical composition shown in Table 1 below was melted in a melting furnace in accordance with a normal melting method to produce a steel piece.

得られた各種鋼片を、下記表２に示す加熱温度及び加熱保持時間で加熱した後、分塊圧延を行い、室温まで冷却した。次いで、下記表２に示す加熱温度及び加熱保持時間に加熱して棒鋼圧延を行い、直径２３ｍｍの棒鋼を製造した。   The various steel pieces obtained were heated at the heating temperature and the heating holding time shown in Table 2 below, then subjected to block rolling and cooled to room temperature. Next, the steel bar was rolled by heating at the heating temperature and the heating and holding time shown in Table 2 below to produce a steel bar having a diameter of 23 mm.

Ｔｉ及びＮｂを含有する炭化物及び炭窒化物、ならびにＴｉ及びＳを含有する析出物の観察を次の手順で行った。   Observation of carbides and carbonitrides containing Ti and Nb, and precipitates containing Ti and S was performed in the following procedure.

（１）各析出物の密度の測定
得られた棒鋼の横断面（すなわち、棒鋼の軸心と垂直な断面）を機械研磨した後、電解研磨を行い、ナイタール液（エタノールと３％硝酸との混合液）によるエッチングの後、カーボン蒸着を行う抽出レプリカ法によりレプリカ膜を作製した。析出物の観察は、前記横断面のＤ／４（Ｄは棒鋼の直径を表す）位置を、日立製作所製の透過電子顕微鏡Ｈ−８００を用いて７５０００倍で観察することにより行った。観察された析出物の構成元素は、堀場製作所製ＥＤＸ分析装置ＥＭＡＸ−７０００による点分析により測定した。Ｃ又はＮを示すピークが検出されるとともに、Ｔｉ及びＮｂのピークが検出される析出物を「Ｔｉ及びＮｂを含有する炭化物及び炭窒化物」と判断し、Ｔｉ及びＳのピークが検出される析出物を「Ｔｉ及びＳを含有する析出物」と判断した。なお、透過画像にて同様の様相を呈する析出物は同一の構成元素と判定することにより析出物の判定を行った。また、各析出物の密度は、住友金属テクノロジー社製粒子解析Ｖｅｒ．３．０により測定した。測定視野は１．３５μｍ×１．６０μｍとして、５視野観察し、その算術平均値を各析出物の密度とした。結果を下記表３に示す。 (1) Measurement of the density of each precipitate The cross section of the obtained steel bar (that is, the cross section perpendicular to the axis of the steel bar) was mechanically polished and then electropolished to obtain a nital solution (ethanol and 3% nitric acid). After etching with the mixed solution, a replica film was prepared by an extraction replica method in which carbon deposition was performed. The precipitate was observed by observing the position of D / 4 (D represents the diameter of the steel bar) at the cross section at 75000 times using a transmission electron microscope H-800 manufactured by Hitachi, Ltd. The constituent elements of the observed deposits were measured by point analysis using an EDX analyzer EMAX-7000 manufactured by Horiba. A peak indicating C or N is detected, and a precipitate in which a peak of Ti and Nb is detected is determined as “a carbide and carbonitride containing Ti and Nb”, and a peak of Ti and S is detected. The precipitate was judged as “precipitate containing Ti and S”. In addition, the precipitate which showed the same aspect in a transmission image determined the precipitate by determining with the same structural element. In addition, the density of each precipitate was measured by Sumitomo Metal Technology's Particle Analysis Ver. Measured by 3.0. The measurement visual field was 1.35 μm × 1.60 μm, and five visual fields were observed. The arithmetic average value was defined as the density of each precipitate. The results are shown in Table 3 below.

次に、得られた棒鋼より、長手方向が圧延方向と平衡になるようにφ２０ｍｍ×Ｌ３０ｍｍの円柱試験片を作製し、該円柱試験片の長手方向に５０％の冷間圧縮（冷間鍛造）を行った。この冷間圧縮を行った試験片を、図２で示す通り、ＣＰ（カーボンポテンシャル）：０．８％、温度：９３０、９５０、９８０℃の各温度で６０分間浸炭して１００℃の油浴に浸漬した後、１７０℃で１２０分間の焼戻し処理を行って、結晶粒度測定用の試験片とした。結晶粒度測定の手順は以下の通りである。   Next, a cylindrical test piece of φ20 mm × L30 mm is prepared from the obtained steel bar so that the longitudinal direction is in equilibrium with the rolling direction, and 50% cold compression (cold forging) is performed in the longitudinal direction of the cylindrical test piece. Went. As shown in FIG. 2, this cold-compressed test piece was carburized at CP (carbon potential): 0.8%, temperature: 930, 950, and 980 ° C. for 60 minutes to obtain an oil bath at 100 ° C. After soaking, the specimen was tempered at 170 ° C. for 120 minutes to obtain a test piece for measuring crystal grain size. The procedure for measuring the grain size is as follows.

（２）旧オーステナイト粒の結晶粒度番号の判定
前記した結晶粒度測定用の試験片の、圧縮方向に平行な面を切出し、ナイタール液でエッチングした後、光学顕微鏡を用いて倍率１００倍で観察し、ＪＩＳ Ｇ０５５１（２００５）に従って旧オーステナイト粒の粒度番号を測定した。粒度番号の測定は、圧縮端部の表層部で行い、結晶粒が最も大きくなった部分の粒度番号（最大γ粒度）を測定した。そして最大γ粒度が６．０以上であるものを、異常粒の「発生なし」と評価した。結果を表３に示す。 (2) Judgment of grain size number of prior austenite grains After cutting out the plane parallel to the compression direction of the above test piece for grain size measurement and etching it with a nital solution, it was observed at a magnification of 100 using an optical microscope. The particle size number of the prior austenite grains was measured according to JIS G0551 (2005). The particle size number was measured at the surface layer portion at the compression end, and the particle size number (maximum γ particle size) of the portion where the crystal grains became the largest was measured. Those having a maximum γ particle size of 6.0 or more were evaluated as “no occurrence” of abnormal particles. The results are shown in Table 3.

表３の試験Ｎｏ．１〜３０は、本発明で規定する化学組成を満足する鋼を、適切な製造条件（分塊圧延時及び棒鋼圧延時の加熱温度及び加熱保持時間）で製造したため、（Ｔｉ、Ｎｂ）炭化物等や（Ｔｉ、Ｓ）析出物の密度を本発明の要件を満足するように調整でき、浸炭処理時の異常粒発生を抑制できた。   Test No. in Table 3 1 to 30 are steels that satisfy the chemical composition defined in the present invention, manufactured under appropriate manufacturing conditions (heating temperature and heating holding time during mass rolling and bar rolling), (Ti, Nb) carbide, etc. Moreover, the density of the (Ti, S) precipitates could be adjusted so as to satisfy the requirements of the present invention, and abnormal grain generation during carburizing treatment could be suppressed.

一方、表３の試験Ｎｏ．３１〜４４は、鋼の化学組成又は製造条件が不適切だったため、（Ｔｉ、Ｎｂ）炭化物等や（Ｔｉ、Ｓ）析出物の密度を、本発明で規定する範囲に調整することができず、浸炭処理時の異常粒が発生した。   On the other hand, test No. Nos. 31 to 44 were unable to adjust the density of (Ti, Nb) carbide or the like or (Ti, S) precipitates to the range specified in the present invention because the chemical composition or manufacturing conditions of the steel was inappropriate. Abnormal grains were generated during carburization.

Ｎｏ．３１はＳ量が多い鋼Ｚ１を用いた例であり、２００ｎｍ以上の（Ｔｉ、Ｓ）析出物が増加し、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。Ｎｏ．３２はＮｂ量が少ない鋼Ｚ２を用いた例であり、１０ｎｍ未満、及び１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。Ｎｏ．３３はＴｉ量が少ない鋼Ｚ３を用いた例であり、２００ｎｍ以上の（Ｔｉ、Ｓ）析出物が増加し、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。Ｎｏ．３４はＮｂ及びＴｉ量が少ない鋼Ｚ４を用いた例であり、１０ｎｍ未満、及び１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。Ｎｏ．３５はＮ及びＮｂ量が多い鋼Ｚ５を用いた例であり、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。これは、過剰なＮ量の存在により粗大なＴｉＮが析出し、（Ｔｉ、Ｎｂ）炭化物等を形成するためのＴｉ量が不足したため、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度が不足したと考えられる。   No. 31 is an example using steel Z1 with a large amount of S, and (Ti, S) precipitates of 200 nm or more increase and density of (Ti, Nb) carbides of 10 nm or more and less than 200 nm cannot be secured. , Abnormal particles occurred. No. No. 32 is an example using steel Z2 with a small amount of Nb, and the density of (Ti, Nb) carbide or the like of less than 10 nm and not less than 10 nm and less than 200 nm could not be secured, and abnormal grains were generated. No. No. 33 is an example using steel Z3 with a small amount of Ti. (Ti, S) precipitates of 200 nm or more increase and density of (Ti, Nb) carbides of 10 nm or more and less than 200 nm cannot be secured. , Abnormal particles occurred. No. No. 34 is an example using steel Z4 with a small amount of Nb and Ti, and the density of (Ti, Nb) carbide or the like of less than 10 nm and not less than 10 nm and less than 200 nm could not be secured, and abnormal particles were generated. No. No. 35 is an example using steel Z5 with a large amount of N and Nb, and the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm could not be ensured, and abnormal grains were generated. This is because coarse TiN precipitates due to the presence of an excessive amount of N, and the amount of Ti for forming (Ti, Nb) carbide or the like is insufficient, so the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm is low. It is thought that it was insufficient.

Ｎｏ．３６〜３８は、分塊圧延時の加熱保持時間が長い製造条件Ｂを採用した例であり、２００ｎｍ以上の（Ｔｉ、Ｓ）析出物が増加し、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。Ｎｏ．３９〜４１は分塊圧延時の加熱保持時間が長い製造条件Ｃを採用した例であり、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。Ｎｏ．４２〜４４は、分塊圧延時の加熱温度が高いとともに加熱保持時間が長く、さらに棒鋼圧延時の加熱保持時間が長い製造条件Ｄを採用した例であり、２００ｎｍ以上の（Ｔｉ、Ｓ）析出物が増加し、１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を確保することができず、異常粒が発生した。なお、Ｎｏ．４２〜４４では棒鋼圧延時の加熱保持時間が長いものの、１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を十分に確保できている。これは、Ｎｏ．４２〜４４では、分塊圧延時の加熱温度が高いとともに加熱保持時間も長いため、鋳造時に生成した（Ｔｉ、Ｎｂ）炭化物等が十分に固溶し、その結果、分塊圧延時に微細な１０ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等が多く析出したためと考えられる。また、製造条件Ｄのように、鋳造時に生成した（Ｔｉ、Ｎｂ）炭化物等を分塊圧延時に一旦固溶させてしまうと、後の棒鋼圧延時に長時間加熱しても１０ｎｍ以上２００ｎｍ未満の（Ｔｉ、Ｎｂ）炭化物等の密度を十分に確保することができない。   No. 36 to 38 are examples in which the production condition B in which the heating and holding time at the time of the block rolling is long is adopted, (Ti, S) precipitates of 200 nm or more increase, and (Ti, Nb) carbides of 10 nm or more and less than 200 nm. Thus, abnormal density was generated. No. Nos. 39 to 41 are examples in which the production condition C is used, which has a long heating and holding time during the block rolling, and the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm cannot be secured, and abnormal grains are generated. did. No. Nos. 42 to 44 are examples in which production conditions D were employed, in which the heating temperature at the time of the block rolling was high and the heating and holding time was long, and the heating and holding time at the time of rolling the steel bar was long, and precipitation of (Ti, S) of 200 nm or more As a result, the density of (Ti, Nb) carbide or the like of 10 nm or more and less than 200 nm could not be secured, and abnormal particles were generated. In addition, No. In 42-44, although the heat holding time at the time of steel bar rolling is long, the density of (Ti, Nb) carbide or the like of less than 10 nm can be sufficiently secured. This is no. In Nos. 42 to 44, since the heating temperature at the time of the block rolling is high and the heating and holding time is also long, (Ti, Nb) carbide and the like generated at the time of casting are sufficiently dissolved, and as a result, a fine 10 nm at the time of the block rolling. This is probably because a large amount of (Ti, Nb) carbide or the like was precipitated. Moreover, if (Ti, Nb) carbide | carbonized_material etc. which were produced | generated at the time of casting were once made into solid solution at the time of block rolling like manufacturing condition D, even if it heats for a long time at the time of subsequent steel bar rolling, it is 10 nm or more and less than 200 nm ( The density of Ti, Nb) carbides, etc. cannot be secured sufficiently.

Claims (4)

Ｃ ：０．１０〜０．３０％（質量％の意味。以下、同じ。）、
Ｓｉ：０．０１〜０．５０％、
Ｍｎ：０．８０〜２．００％、
Ｐ ：０．０３０％以下（０％を含まない）、
Ｓ ：０．０３０％以下（０％を含まない）、
Ｃｒ：０．５０〜１．５０％、
Ａｌ：０．０１〜０．１０％、
Ｎ ：０．００１０〜０．０１０％、
Ｎｂ：０．０４０〜０．１５０％、
Ｔｉ：０．０４０〜０．１５０％
を含有し、残部が鉄及び不可避不純物であって、
Ｔｉ及びＮｂを含有する円相当径１０ｎｍ以上２００ｎｍ未満の炭化物及び炭窒化物の密度が１０個／μｍ2以上であり、
Ｔｉ及びＮｂを含有する円相当径１０ｎｍ未満の炭化物及び炭窒化物の密度が５０個／μｍ2以上であり、更に
Ｔｉ及びＳを含有する円相当径２００ｎｍ以上の析出物の密度が０．２０個／μｍ2以下であることを特徴とする浸炭処理時の異常粒発生が抑制可能な肌焼鋼。 C: 0.10 to 0.30% (meaning mass%, hereinafter the same),
Si: 0.01 to 0.50%,
Mn: 0.80 to 2.00%
P: 0.030% or less (excluding 0%),
S: 0.030% or less (excluding 0%),
Cr: 0.50 to 1.50%,
Al: 0.01 to 0.10%,
N: 0.0010 to 0.010%,
Nb: 0.040 to 0.150%,
Ti: 0.040 to 0.150%
The balance is iron and inevitable impurities,
The density of carbide and carbonitride having an equivalent circle diameter of 10 nm or more and less than 200 nm containing Ti and Nb is 10 pieces / μm 2 or more,
The density of carbide and carbonitride having an equivalent circle diameter of less than 10 nm containing Ti and Nb is 50 / μm 2 or more, and the density of precipitates having an equivalent circle diameter of 200 nm or more containing Ti and S is 0.20. A case-hardened steel capable of suppressing the generation of abnormal grains during carburizing treatment, characterized in that the number of particles / μm 2 or less. 更に、Ｍｏ：２．０％以下（０％を含まない）を含有する請求項１に記載の肌焼鋼。   Furthermore, the case hardening steel of Claim 1 containing Mo: 2.0% or less (excluding 0%). 更に、Ｃｕ：０．１０％以下（０％を含まない）及びＮｉ：３．０％以下（０％を含まない）の少なくとも１種を含有する請求項１または２に記載の肌焼鋼。   Furthermore, the case hardening steel of Claim 1 or 2 containing at least 1 sort (s) of Cu: 0.10% or less (0% is not included) and Ni: 3.0% or less (not including 0%). 請求項１〜３のいずれかに記載の肌焼鋼を用いて得られた、表面が浸炭処理されている機械構造部品であって、浸炭層の旧オーステナイト粒度番号の最小値が６．０以上であることを特徴とする機械構造部品。   A machine structural part obtained by using the case-hardened steel according to any one of claims 1 to 3 and having a carburized surface, wherein the minimum value of the prior austenite grain size number of the carburized layer is 6.0 or more. Mechanical structural parts characterized by being.
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