JP4964063B2 - Case-hardened steel with excellent cold forgeability and grain coarsening prevention properties and machine parts obtained therefrom - Google Patents
Case-hardened steel with excellent cold forgeability and grain coarsening prevention properties and machine parts obtained therefrom Download PDFInfo
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本発明は、自動車などの輸送機器や、建設機械、その他の産業機械などにおいて、浸炭処理して使用される機械部品の素材となる肌焼鋼、およびそれから得られる機械部品に関するものである。殊に本発明の肌焼鋼は、歯車などの素材として用いた場合に、冷間鍛造の際の変形抵抗が小さく、且つ冷間鍛造後に浸炭処理を行なった場合でも結晶粒が粗大化しないという特性（以下、「結晶粒粗大化防止特性」と省略することがある。）に優れている。 The present invention relates to case-hardened steel as a material for machine parts used by carburizing in transport equipment such as automobiles, construction machines, and other industrial machines, and machine parts obtained therefrom. In particular, when the case-hardened steel of the present invention is used as a material such as a gear, the deformation resistance during cold forging is small, and even when carburizing is performed after cold forging, the crystal grains are not coarsened. It has excellent characteristics (hereinafter, may be abbreviated as “crystal grain coarsening prevention characteristic”).
自動車、建設機械、その他の各種産業機械用として用いられる機械部品において、特に耐摩耗性、高疲労強度が要求される部品には、従来から浸炭、窒化および浸炭窒化などの表面硬化熱処理（肌焼き処理）が行なわれている。これらの用途には、通常、ＪＩＳ規格で定められた肌焼鋼、例えばＳＣＭ４２０Ｈが使用され、鍛造・切削などの機械加工により所望の部品形状に成形した後、浸炭、浸炭窒化などの表面硬化熱処理を施し、その後、研磨などの仕上工程を経て製造される。 In machine parts used for automobiles, construction machinery, and other various industrial machines, surface hardening heat treatments such as carburizing, nitriding and carbonitriding (case hardening) have been conventionally applied to parts that require particularly high wear resistance and high fatigue strength. Processing). For these applications, case-hardened steel defined by JIS standards, such as SCM420H, is usually used. After forming into a desired part shape by machining such as forging and cutting, surface hardening heat treatment such as carburizing and carbonitriding After that, it is manufactured through a finishing process such as polishing.
自動車、建設機械、産業機械等に使用される部品、殊に歯車の製造では、切削コストが、製造コストの大部分を占めている。そこで製造コストを削減するために、切削から鍛造への変更が進められている。しかし熱間鍛造だけでは、歯車などの精密部品を精度良く製造することが難しい。そのため切削工程を省略して冷間鍛造のみで、歯車などの精密部品を製造できるようにすることが望まれている。 In the production of parts used in automobiles, construction machines, industrial machines, etc., especially gears, the cutting costs account for the majority of the production costs. Therefore, in order to reduce the manufacturing cost, a change from cutting to forging is in progress. However, it is difficult to manufacture precision parts such as gears with high precision only by hot forging. Therefore, it is desired to be able to manufacture precision parts such as gears by omitting the cutting process and performing only cold forging.
しかし熱間鍛造に比べて冷間鍛造では、変形抵抗が高いという問題がある。そこで近年、冷間鍛造性に優れた肌焼鋼、殊に金型寿命を改善するために冷間鍛造時の変形抵抗が低い肌焼鋼が求められている。この点、Ｂ（ボロン）を添加して、他の合金元素を低減した低合金ボロン鋼が、冷間鍛造性に優れていることは知られている。しかし低合金ボロン鋼は、浸炭処理時にオーステナイト結晶粒が粗大化しやすいという問題があった。そこで冷間鍛造性だけでなく、結晶粒粗大化防止特性にも優れた肌焼鋼が求められている。 However, cold forging has a problem of higher deformation resistance than hot forging. Therefore, in recent years, there has been a demand for case-hardened steel having excellent cold forgeability, in particular, case-hardened steel having low deformation resistance during cold forging in order to improve the die life. In this respect, it is known that the low alloy boron steel added with B (boron) to reduce other alloy elements is excellent in cold forgeability. However, the low alloy boron steel has a problem that austenite crystal grains are likely to be coarsened during the carburizing process. Therefore, there is a need for a case-hardened steel that is excellent not only in cold forgeability but also in preventing grain coarsening.
そのため肌焼鋼の技術分野では、冷間鍛造性と結晶粒粗大化防止特性との両立を図るため、様々な研究がなされている。例えば特許文献１は、（１）合金元素量を適正範囲に調整すること、（２）Ｔｉの微細析出物を一定量以上で確保すること、（３）硬さを制限すること、（４）脱炭深さを一定以下に抑えることにより、肌焼鋼の冷間加工性と結晶粒粗大化防止特性とを向上させることを開示している（特許請求の範囲参照）。特許文献１は、冷間加工性を向上させるために、Ｃｒ量を１．２５％以下、好適には１．０％以下に抑えることを提案している（段落［００２５］参照）。
本発明の目的は、冷間鍛造性および結晶粒粗大化防止特性の両方に優れた肌焼鋼を提供することにあり、詳しくは、従来鋼であるＪＩＳ規格のＳＣＭ４２０Ｈに比べて、冷間鍛造時の変形抵抗が低減されているが、結晶粒粗大化防止特性は良好に維持されている肌焼鋼を提供することにある。 An object of the present invention is to provide a case-hardened steel that is excellent in both cold forgeability and grain coarsening prevention characteristics. Specifically, it is cold forged as compared to JIS standard SCM420H, which is a conventional steel. An object of the present invention is to provide a case-hardened steel in which the deformation resistance at the time is reduced, but the grain coarsening prevention property is maintained well.
上記目的を達成することができた本発明の肌焼鋼とは、Ｃ：０．１〜０．３％（質量％の意味、以下同じ）、Ｓｉ：０．１％以下（０％を含まない）、Ｍｎ：０．６％以下（０％を含まない）、Ｐ：０．０３％以下（０％を含まない）、Ｓ：０．０２％以下（０％を含まない）、Ｃｒ：１．２５〜２％、Ａｌ：０．１％以下（０％を含まない）、Ｔｉ：０．０７％以下（０％を含まない）、Ｂ：０．０００５〜０．００５％、およびＮ：０．００８％以下（０％を含まない）を含有し、且つ下記式（１）：
０．０１≦［Ｔｉ］−３．４２［Ｎ］≦０．０５ ・・・ （１）
〔式中、［Ｔｉ］および［Ｎ］は、それぞれ鋼中のＴｉおよびＮ含有量（質量％）を表す。〕
を満たし、残部がＦｅおよび不可避不純物からなり、直径が０．０１〜０．２μｍであるＴｉＣ析出物の個数が５〜３０個／μｍ2であることを特徴とするものである。
The case-hardened steel of the present invention capable of achieving the above object is C: 0.1 to 0.3% (meaning mass%, the same shall apply hereinafter), Si: 0.1% or less (including 0%) Mn: 0.6% or less (not including 0%), P: 0.03% or less (not including 0%), S: 0.02% or less (not including 0%), Cr: 1.25 to 2%, Al: 0.1% or less (not including 0%), Ti: 0.07% or less (not including 0%), B: 0.0005 to 0.005%, and N : 0.008% or less (not including 0%), and the following formula (1):
0.01 ≦ [Ti] −3.42 [N] ≦ 0.05 (1)
[In formula, [Ti] and [N] represent Ti and N content (mass%) in steel, respectively. ]
And the balance is Fe and inevitable impurities, and the number of TiC precipitates having a diameter of 0.01 to 0.2 μm is 5 to 30 / μm 2 .
本発明の肌焼鋼は、さらに（１）Ｃａ：０．００５％以下（０％を含まない）、および／または（２）Ｎｂ：０．０１５％以下（０％を含まない）を含有していても良い。また冷間鍛造時の割れを防止するために、鋼中に存在するＴｉＮ析出物の最大直径が３０μｍ以下であることが好ましい。 The case-hardened steel of the present invention further contains (1) Ca: 0.005% or less (excluding 0%) and / or (2) Nb: 0.015% or less (excluding 0%). May be. In order to prevent cracking during cold forging, the maximum diameter of TiN precipitates present in the steel is preferably 30 μm or less.
本発明は、さらに上記肌焼鋼から得られた機械部品、例えば歯車、無段変速機（ＣＶＴ）用プーリー、シャフトも提供する。 The present invention further provides mechanical parts obtained from the case-hardened steel, such as gears, continuously variable transmission (CVT) pulleys, and shafts.
本発明によれば、鋼の化学成分量（合金元素量）およびＴｉＣ析出物の個数を適正範囲に調整することにより、優れた冷間鍛造性を有しながら、結晶粒粗大化防止特性が良好に維持された肌焼鋼を得ることができた。本発明の肌焼鋼は、各種機械部品、殊に歯車の素材として有用である。 According to the present invention, by adjusting the chemical component amount (alloy element amount) of steel and the number of TiC precipitates to an appropriate range, the crystal grain coarsening prevention property is excellent while having excellent cold forgeability. The case-hardened steel was maintained. The case-hardened steel of the present invention is useful as a material for various machine parts, particularly gears.
本発明者らは、従来鋼であるＪＩＳ規格のＳＣＭ４２０Ｈよりも冷間鍛造時の変形抵抗が低い肌焼鋼を製造するため、ＳＣＭ４２０Ｈ中に含まれる合金元素（Ｃ、Ｓｉ、Ｍｎ、ＣｒおよびＭｏ）、または含まれ得る合金元素（Ｎｉ）が変形抵抗に及ぼす影響に着目した。これらの元素の含有量、特にＭｎ、Ｎｉ、ＣｒおよびＭｏ量を低減させれば、肌焼鋼の変形抵抗を低減させることができる。しかしこれらの合金元素量を単に低減させるだけでは焼入性が不充分になり、浸炭焼入れを行っても、部品として求められる強度を得ることができない。そこで本発明者らは、これらの合金元素の中でもＣｒに着目した。なぜなら以下に示すように、Ｃｒは変形抵抗の増加に寄与する割合が低いからである。 In order to produce a case-hardened steel having a lower deformation resistance during cold forging than the conventional JIS standard SCM420H, which is a conventional steel, the present inventors have prepared alloy elements (C, Si, Mn, Cr and Mo contained in SCM420H). ), Or the influence of alloy elements (Ni) that may be included on deformation resistance. If the content of these elements, particularly the amount of Mn, Ni, Cr and Mo, is reduced, the deformation resistance of the case-hardened steel can be reduced. However, simply reducing the amount of these alloy elements results in insufficient hardenability, and even when carburizing and quenching is performed, the strength required as a part cannot be obtained. Therefore, the present inventors paid attention to Cr among these alloy elements. This is because, as will be described below, Cr has a low rate of contribution to an increase in deformation resistance.
具体的には、以下の表１に示す化学成分組成の鋼材試料Ｎｏ．１〜１０の変形抵抗を測定し（表１参照）、該変形抵抗（ｋｇｆ／ｍｍ2）を従属変数とし、Ｃ、Ｓｉ、ＭｎおよびＣｒ含有量（質量％）を独立変数として、重回帰分析することにより、下記式（２）で示す重回帰式を求めた。なお各試料の変形抵抗は、下記表１の化学成分組成の棒鋼圧延材（直径２５ｍｍ）を球状化焼鈍した後、旋盤切削して、直径１５ｍｍ×高さ２２．５ｍｍである円柱状の試験片を作製し、３００トンのプレス機を用いて、端面拘束圧縮で室温下、７０％の加工率（＝（ｈ1−ｈ2）／ｈ1×１００、ｈ1：圧縮前の試験片高さ、ｈ2：圧縮後の試験片高さ）での変形抵抗を測定した。 Specifically, steel sample Nos. Having chemical composition shown in Table 1 below. Measurement of deformation resistance of 1 to 10 (see Table 1), multiple regression analysis using the deformation resistance (kgf / mm 2 ) as a dependent variable and the contents (mass%) of C, Si, Mn and Cr as independent variables Thus, a multiple regression equation represented by the following equation (2) was obtained. The deformation resistance of each sample is a cylindrical test piece having a diameter of 15 mm and a height of 22.5 mm after spheroidizing and annealing a rolled steel bar (diameter 25 mm) having the chemical composition shown in Table 1 below. , And using a 300-ton press, 70% processing rate (= (h 1 −h 2 ) / h 1 × 100, h 1 : test piece height before compression at room temperature with end face constrained compression , H 2 : test piece height after compression).
変形抵抗＝４５．９［Ｃ］＋２１．５［Ｓｉ］＋１０．５［Ｍｎ］＋８．６［Ｃｒ］＋３９．１ ・・・ （２）
〔式中、［ ］は、鋼材試料中の各元素の含有量（質量％）を表す。自由度調製済み寄与率Ｒ*2は０．９９４である。〕
Deformation resistance = 45.9 [C] +21.5 [Si] +10.5 [Mn] +8.6 [Cr] +39.1 (2)
[In formula, [] represents content (mass%) of each element in a steel material sample. The degree of freedom adjusted contribution ratio R * 2 is 0.994. ]
上記式（２）、殊に回帰係数から示されるように、Ｃ（回帰係数：４５．９）、Ｓｉ（回帰係数：２１．５）およびＭｎ（回帰係数：１０．５）に比べて、Ｃｒ（回帰係数：８．６）は、変形抵抗の増加に寄与する割合が低い。このような知見に基づき本発明は、従来、例えば前述した特許文献１に記載されているように冷間鍛造性に悪影響を及ぼすと考えられていたＣｒ量を、あえて増やしたことを主要な特徴の１つとする。 As shown from the above formula (2), especially the regression coefficient, compared with C (regression coefficient: 45.9), Si (regression coefficient: 21.5) and Mn (regression coefficient: 10.5), Cr (Regression coefficient: 8.6) has a low rate of contribution to an increase in deformation resistance. Based on such knowledge, the present invention is characterized in that the amount of Cr, which has been conventionally considered to adversely affect cold forgeability as described in, for example, Patent Document 1 described above, has been intentionally increased. One of them.
さらに本発明は、Ｃｒ量を増加させたことに伴い、（Ｉ）従来鋼のＳＣＭ４２０Ｈに比べてＳｉおよびＭｎ量を低減させたこと、および（ＩＩ）ＳＣＭ４２０Ｈに含まれるＭｏおよび含まれ得るＮｉを、肌焼鋼中に添加しないことを特徴とする。そしてＭｎ、ＮｉおよびＭｏを低減ないし省略したために焼入性は低下するが、その低下を、鋼中にＢを含有させることで補っていることも本発明の特徴の１つである。 Furthermore, the present invention has the following advantages: (I) the amount of Si and Mn is reduced as compared with SCM420H of conventional steel, and (II) Mo contained in SCM420H and Ni that can be contained. It is characterized by not being added to case-hardened steel. And since Mn, Ni, and Mo are reduced or omitted, the hardenability is lowered, but it is also one of the features of the present invention that the decrease is compensated by containing B in the steel.
しかし合金元素を低減させたボロン鋼は、結晶粒粗大化防止特性が不充分であるという欠点を有する。そのため本発明者らは、微細なＴｉＣ析出物によるピンニング効果を利用することで、この欠点を補うことを試みた。しかし鋼中に微細なＴｉＣを充分に生成させるために、Ｔｉを過剰に添加すると、変形抵抗が大幅に向上し、本発明の目的である冷間鍛造性に優れた肌焼鋼を達成することができない。そこで本発明は、肌焼鋼中にＴｉを含有させるが、その含有量もある程度抑えていることを主要な特徴の１つとする。 However, boron steel with reduced alloying elements has the disadvantage of insufficient crystal grain coarsening prevention properties. Therefore, the present inventors tried to make up for this defect by utilizing the pinning effect by the fine TiC precipitates. However, in order to sufficiently produce fine TiC in the steel, when excessively adding Ti, the deformation resistance is greatly improved, and the purpose of the present invention is to achieve a case-hardened steel excellent in cold forgeability. I can't. Therefore, the present invention has one of the main features that Ti is contained in the case-hardened steel, but its content is also suppressed to some extent.
上記のように本発明では肌焼鋼中のＭｎ量を低減しているため、Ｍｎで充分にトラップされないＳがＴｉと結びつき、ＴｉＳまたはＴｉ4Ｃ2Ｓ2（以下、「Ｔｉ系炭硫化物」と省略することがある。）が生成されやすくなっている。これらのＴｉ系炭硫化物は、充分なピンニング効果を発揮しない。さらに本発明では、冷間鍛造性を向上させるために、Ｔｉ量をある程度制限しているため、ＴｉがＳにより消費されると、ピンニング効果を発揮する微細なＴｉＣが充分に生成せず、良好な結晶粒粗大化防止特性を達成することができなくなる。そこで本発明は、製造条件、殊に分塊圧延条件およびその後の圧延条件（例えば棒鋼を製造する場合は、分塊圧延後の棒鋼圧延条件）を調整することにより、Ｔｉ系炭硫化物の生成を抑制して、適正量の微細ＴｉＣを確保していることを主要な特徴の１つとする。 As described above, in the present invention, since the amount of Mn in the case-hardened steel is reduced, S that is not sufficiently trapped by Mn is combined with Ti, and TiS or Ti 4 C 2 S 2 (hereinafter referred to as “Ti-based carbon sulfide”). Is sometimes abbreviated as ")". These Ti-based carbon sulfides do not exhibit a sufficient pinning effect. Furthermore, in the present invention, in order to improve the cold forgeability, the amount of Ti is limited to some extent. Therefore, when Ti is consumed by S, fine TiC that exhibits a pinning effect is not sufficiently generated and is good. Therefore, it becomes impossible to achieve a good crystal grain coarsening preventing property. Therefore, the present invention produces Ti-based carbon sulfide by adjusting the production conditions, in particular, the partial rolling conditions and the subsequent rolling conditions (for example, when producing steel bars, the steel bar rolling conditions after the partial rolling). It is one of the main features that an appropriate amount of fine TiC is secured by suppressing the above.
以上をまとめると、本発明の肌焼鋼は、（ｉ）焼入性をある程度確保しながら冷間鍛造性を向上させるために、Ｓｉ、Ｍｎ、ＮｉおよびＭｏを低減ないし省略させ、且つＣｒを増量し、Ｂを含有させたこと、（ｉｉ）冷間鍛造性および結晶粒粗大化防止特性を両立するために、Ｔｉをある程度抑えた量で含有させたこと、および（ｉｉｉ）分塊圧延条件およびその後の圧延条件を調整して、鋼中に適正量の微細ＴｉＣを形成させ、ピンニング効果により、良好な結晶粒粗大化防止特性を実現したことを特徴とする。以下、（ア）Ｃｒなどの鋼の化学成分（合金元素）の含有量、（イ）ＴｉＣなどのＴｉ系析出物、並びに（ウ）製造条件を、順に説明する。 In summary, the case-hardened steel of the present invention (i) reduces or omits Si, Mn, Ni and Mo, and improves Cr in order to improve cold forgeability while ensuring hardenability to some extent. Increased in amount and contained B, (ii) in order to achieve both cold forgeability and grain coarsening prevention characteristics, Ti was included in a certain amount, and (iii) ingot rolling conditions And the subsequent rolling conditions were adjusted, the appropriate amount of fine TiC was formed in the steel, and good grain coarsening prevention characteristics were realized by the pinning effect. Hereinafter, (a) the content of chemical components (alloy elements) of steel such as Cr, (b) Ti-based precipitates such as TiC, and (c) production conditions will be described in this order.
（ア）化学成分（合金元素）の含有量について
〈Ｃ：０．１〜０．３％〉
Ｃは、鋼の変形抵抗を低下させるために、低減することが好ましい。そこで本発明において、Ｃ量の上限を０．３％と定めた。Ｃ量は、好ましくは０．２８％以下、より好ましくは０．２３％以下である。しかしＣ量を、あまりに低減しすぎると、浸炭部品に要求される強度を確保することができなくなる。そこでＣ量の下限を０．１％と定めた。Ｃ量は、好ましくは０．１２％以上、より好ましくは０．１５％以上である。
(A) Content of chemical component (alloy element) <C: 0.1 to 0.3%>
C is preferably reduced in order to reduce the deformation resistance of the steel. Therefore, in the present invention, the upper limit of the C amount is set to 0.3%. The amount of C is preferably 0.28% or less, more preferably 0.23% or less. However, if the amount of C is reduced too much, the strength required for carburized parts cannot be secured. Therefore, the lower limit of the C amount is set to 0.1%. The amount of C is preferably 0.12% or more, more preferably 0.15% or more.
〈Ｓｉ：０．１％以下（０％を含まない）〉
Ｓｉは、鉄中に固溶し、材料の変形抵抗を増大させるために、低減することが好ましい。そこでＳｉ量の上限を０．１％と定めた。Ｓｉ量は、好ましくは０．０８％以下、さらに好ましくは０．０７％以下である。しかしＳｉは、脱酸剤としての作用を有し、また焼戻し処理時の硬さ低下を抑えて浸炭部品の表層硬さを確保する作用も持つ。このような作用を充分に発揮させるためにＳｉは、好ましくは０．０１％以上、より好ましくは０．０２％以上、さらに好ましくは０．０３％以上の量で鋼中に含まれていることが推奨される。
<Si: 0.1% or less (excluding 0%)>
Si is preferably reduced in order to dissolve in iron and increase the deformation resistance of the material. Therefore, the upper limit of Si content was set to 0.1%. The amount of Si is preferably 0.08% or less, more preferably 0.07% or less. However, Si has an action as a deoxidizer, and also has an action of securing the surface layer hardness of the carburized component by suppressing the hardness reduction during the tempering treatment. In order to sufficiently exert such an action, Si is preferably contained in the steel in an amount of 0.01% or more, more preferably 0.02% or more, and further preferably 0.03% or more. Is recommended.
〈Ｍｎ：０．６％以下（０％を含まない）〉
Ｍｎは、鉄やセメンタイト中に固溶し、鋼の変形抵抗を増大させる。またＭｎ量の増大に伴い、縞状の偏析が顕著となり、材質のバラツキが大きくなる結果、冷間鍛造時に割れが発生しやすくなる。そこでＭｎ量は低減することが好ましい。よってＭｎ量の上限を、０．６％と定めた。Ｍｎ量は、好ましくは０．５％以下、より好ましくは０．４％以下である。しかしＭｎは、脱酸剤として作用し、酸化物系介在物量を低減して鋼材の内部品質を高める作用も持つ。このような作用を充分に発揮させるためにＭｎは、好ましくは０．０５％以上、より好ましくは０．１０％以上、さらに好ましくは０．１５％以上の量で鋼中に含まれていることが推奨される。
<Mn: 0.6% or less (excluding 0%)>
Mn dissolves in iron and cementite and increases the deformation resistance of steel. In addition, as the amount of Mn increases, striped segregation becomes prominent and material variation increases. As a result, cracks are likely to occur during cold forging. Therefore, it is preferable to reduce the amount of Mn. Therefore, the upper limit of the amount of Mn is set to 0.6%. The amount of Mn is preferably 0.5% or less, more preferably 0.4% or less. However, Mn acts as a deoxidizing agent, and also has an effect of increasing the internal quality of the steel material by reducing the amount of oxide inclusions. In order to sufficiently exert such an action, Mn is preferably contained in the steel in an amount of 0.05% or more, more preferably 0.10% or more, and further preferably 0.15% or more. Is recommended.
〈Ｐ：０．０３％以下（０％を含まない）〉
Ｐは、鋼中に不可避的に含まれる元素であり、結晶粒界に偏析して部品の衝撃特性を低下させるため、できるだけ低減することが好ましい。そのためＰ量の上限を０．０３％と定めた。Ｐ量は、好ましくは０．０２％以下、より好ましくは０．０１５％以下である。
<P: 0.03% or less (excluding 0%)>
P is an element inevitably contained in the steel, and is preferably reduced as much as possible because it segregates at the grain boundaries and lowers the impact characteristics of the component. Therefore, the upper limit of the P amount is set to 0.03%. The amount of P is preferably 0.02% or less, more preferably 0.015% or less.
〈Ｓ：０．０２％以下（０％を含まない）〉
Ｓは、鋼中に不可避的に含まれる元素であり、Ｔｉと結合して、Ｔｉ硫化物（ＴｉＳ）やＴｉ炭硫化物（Ｔｉ4Ｃ2Ｓ2）を形成し得る。このようにＴｉがＳにより消費されると、結晶粒粗大化防止に有効な微細ＴｉＣ量が減少するため、結晶粒粗大化防止特性が低下する。そこでＳ量は、できるだけ低減することが好ましく、その上限を０．０２％と定めた。Ｓ量は、好ましくは０．０１５％以下であり、さらに好ましくは０．０１２％以下である。
<S: 0.02% or less (excluding 0%)>
S is an element inevitably contained in the steel, and can combine with Ti to form Ti sulfide (TiS) or Ti carbon sulfide (Ti 4 C 2 S 2 ). Thus, when Ti is consumed by S, the amount of fine TiC effective for preventing the coarsening of crystal grains decreases, so that the crystal grain coarsening prevention characteristics are deteriorated. Therefore, the amount of S is preferably reduced as much as possible, and the upper limit is set to 0.02%. The amount of S is preferably 0.015% or less, more preferably 0.012% or less.
〈Ｃｒ：１．２５〜２％〉
Ｃｒは、焼入性を向上させる作用を有するが、他の合金元素（Ｓｉ、Ｍｎ、Ｎｉ、ＭｏおよびＭｎ等）と比べ、変形抵抗を増大させない元素である。そこで鋼の変形抵抗を極力低く抑え、且つ肌焼鋼の焼入性を、ＪＩＳ規格鋼、例えばＳＣＭ４２０Ｈと同程度に保持するために、本発明は、他の合金元素を低減または省略し、且つＣｒを増量したこと、即ちその下限を１．２５％と定めたことを特徴の１つとする。Ｃｒ量は、好ましくは１．３０％以上、より好ましくは１．４０％以上である。しかしＣｒ量があまりにも過剰になると、変形抵抗に悪影響を及ぼし、また焼入性も過剰となる。そこでＣｒ量の上限を２％と定めた。Ｃｒ量は、好ましくは１．８％以下、より好ましくは１．６％以下である。
<Cr: 1.25 to 2%>
Cr has an effect of improving hardenability, but is an element that does not increase deformation resistance compared to other alloy elements (Si, Mn, Ni, Mo, Mn, etc.). Therefore, in order to keep the deformation resistance of the steel as low as possible and to maintain the hardenability of the case-hardened steel at the same level as JIS standard steel, for example, SCM420H, the present invention reduces or omits other alloy elements, and One of the characteristics is that the amount of Cr is increased, that is, the lower limit is set to 1.25%. The amount of Cr is preferably 1.30% or more, more preferably 1.40% or more. However, when the amount of Cr is excessively large, the deformation resistance is adversely affected and the hardenability is excessive. Therefore, the upper limit of Cr content is set to 2%. The amount of Cr is preferably 1.8% or less, more preferably 1.6% or less.
〈Ａｌ：０．１％以下（０％を含まない）〉
Ａｌは、脱酸剤として作用し、酸化物系介在物量を低減して鋼の内部品質を高める元素である。そこでＡｌは、好ましくは０．００４％以上、より好ましくは０．００６％以上、さらに好ましくは０．０１０％以上の量で鋼中に含まれていることが推奨される。特に、Ｔｉを０．０５％以上含有させて冷間鍛造性を一段と改善する場合は、溶鋼中の酸素や窒素の活量を下げてＴｉ系介在物（例えば、ＴｉＮやＴｉＯ2など）の生成を抑制するために、Ａｌを多めに含有させることが好ましい。この場合は、Ａｌ量は、例えば、０．０４％以上、より好ましくは０．０４５％以上とすればよい。しかしＡｌ量が過剰になると、粗大で硬い非金属介在物（Ａｌ2Ｏ3）が生成し、鋼の疲労特性が低下する。そこでＡｌ量の上限を０．１％と定めた。Ａｌ量は、好ましくは０．０７％以下、より好ましくは０．０５％以下である。
<Al: 0.1% or less (excluding 0%)>
Al is an element that acts as a deoxidizer and increases the internal quality of steel by reducing the amount of oxide inclusions. Therefore, it is recommended that Al is contained in the steel in an amount of preferably 0.004% or more, more preferably 0.006% or more, and further preferably 0.010% or more. In particular, when the Ti content is 0.05% or more to further improve the cold forgeability, the activity of oxygen and nitrogen in the molten steel is lowered to produce Ti inclusions (eg, TiN, TiO 2, etc.). In order to suppress this, it is preferable to contain a large amount of Al. In this case, the amount of Al may be, for example, 0.04% or more, more preferably 0.045% or more. However, when the amount of Al is excessive, coarse and hard non-metallic inclusions (Al 2 O 3 ) are generated, and the fatigue characteristics of the steel are deteriorated. Therefore, the upper limit of Al content was set to 0.1%. The amount of Al is preferably 0.07% or less, more preferably 0.05% or less.
〈Ｔｉ：０．０７％以下（０％を含まない）〉
Ｔｉは、ピンニング効果により浸炭処理時の結晶粒粗大化を抑制する微細なＴｉＣを形成させるために、鋼中に含有させる必要がある。しかしＴｉ量が過剰になると、鋼の変形抵抗が増大してしまう。そこでＴｉ量の上限を、０．０７％と定めた。Ｔｉ量は、好ましくは０．０６％以下、より好ましくは０．０５％以下、更に好ましくは０．０４５％以下、特に好ましくは０．０４０％以下である。
<Ti: 0.07% or less (excluding 0%)>
Ti needs to be contained in the steel in order to form fine TiC that suppresses the coarsening of crystal grains during carburization due to the pinning effect. However, when the amount of Ti becomes excessive, the deformation resistance of steel increases. Therefore, the upper limit of Ti content is set to 0.07%. The amount of Ti is preferably 0.06% or less, more preferably 0.05% or less, still more preferably 0.045% or less, and particularly preferably 0.040% or less.
〈０．０１≦［Ｔｉ］−３．４２［Ｎ］≦０．０５ ・・・ （１）〉
上記のようにＴｉは、ピンニング効果を発揮する微細なＴｉＣを形成させるために必要な元素であり、ある程度その量を確保する必要がある。しかしＴｉがＮと化合して形成されるＴｉＮ析出物は、結晶粒粗大化防止にほとんど寄与しない。そこで本発明では、Ｔｉ量とＮ量との関係を規定した。具体的にはＮ（原子量：１４．０）は、１質量％あたり、３．４２質量％のＴｉ（原子量：４７．９）と結合して、ＴｉＮを形成し得る。そこで全てのＮがＴｉと結合してＴｉＮが形成されたとしても、適正量の微細ＴｉＣを形成させるために必要なＴｉ量を確保するために、上記式（１）の関係を定めた。良好な結晶粒粗大化防止特性を実現するために［Ｔｉ］−３．４２［Ｎ］は、０．０１以上、好ましくは０．０１５以上、より好ましくは０．０２以上である。しかし［Ｔｉ］−３．４２［Ｎ］の値が大きくなりすぎると、Ｔｉ量および微細なＴｉＣ量が過剰になり、変形抵抗が増大する。そこで［Ｔｉ］−３．４２［Ｎ］は、０．０５以下、好ましくは０．０４以下、より好ましくは０．０３５以下、特に好ましくは０．０３０以下である。
<0.01 ≦ [Ti] −3.42 [N] ≦ 0.05 (1)>
As described above, Ti is an element necessary for forming fine TiC that exhibits a pinning effect, and it is necessary to ensure the amount thereof to some extent. However, TiN precipitates formed by combining Ti with N hardly contribute to prevention of grain coarsening. Therefore, in the present invention, the relationship between the Ti content and the N content is defined. Specifically, N (atomic weight: 14.0) can combine with 3.42 mass% Ti (atomic weight: 47.9) per 1 mass% to form TiN. Therefore, even if all N is combined with Ti to form TiN, the relationship of the above formula (1) is defined in order to ensure the amount of Ti necessary for forming an appropriate amount of fine TiC. [Ti] -3.42 [N] is 0.01 or more, preferably 0.015 or more, more preferably 0.02 or more in order to realize good crystal grain coarsening prevention characteristics. However, when the value of [Ti] -3.42 [N] becomes too large, the amount of Ti and the amount of fine TiC become excessive, and the deformation resistance increases. Therefore, [Ti] -3.42 [N] is 0.05 or less, preferably 0.04 or less, more preferably 0.035 or less, and particularly preferably 0.030 or less.
〈Ｎ：０．００８％以下（０％を含まない）〉
Ｎは、鋼中に不可避的に含まれる元素であり、Ｔｉと結びついてＴｉＮを形成し、その結果、結晶粒粗大化防止に有効な微細ＴｉＣ量を低減させるという悪影響を有する。さらにＮ量が過剰であると、粗大なＴｉＮ析出物が生成して、冷間鍛造時に割れが発生しやすくなり、またＴｉと結びつかないＮは、鉄中に固溶して変形抵抗を著しく増大させることがある。よってＮ量は、できるだけ少ないことが好ましく、その上限を０．００８％と定めた。Ｎ量は、好ましくは０．００６％以下、より好ましくは０．００４％以下である。
<N: 0.008% or less (excluding 0%)>
N is an element that is inevitably contained in steel and forms TiN in combination with Ti. As a result, N has an adverse effect of reducing the amount of fine TiC effective in preventing grain coarsening. Further, if the amount of N is excessive, coarse TiN precipitates are formed, and cracking is likely to occur during cold forging, and N that is not bound to Ti significantly dissolves in iron and greatly increases deformation resistance. There are things to do. Therefore, the N amount is preferably as small as possible, and the upper limit is set to 0.008%. The N amount is preferably 0.006% or less, more preferably 0.004% or less.
〈Ｂ：０．０００５〜０．００５％〉
Ｂは、鋼の変形抵抗を増大させず、微量で鋼の焼入性を大幅に向上させる作用を有する元素である。焼入性向上作用を充分に発揮させるために、Ｂ量の下限を０．０００５％と定めた。Ｂ量は、好ましくは０．０００８％以上、より好ましくは０．００１０％以上である。しかしＢ量が過剰になっても、焼入性向上作用は飽和し、またＢ窒化物が形成され、冷間鍛造時に割れが発生しやすくなる。そこでＢ量の上限を０．００５％と定めた。Ｂ量は、好ましくは０．００２５％以下、より好ましくは０．００２０％以下である。
<B: 0.0005 to 0.005%>
B is an element that does not increase the deformation resistance of the steel and has the effect of greatly improving the hardenability of the steel in a small amount. In order to sufficiently exhibit the hardenability improving effect, the lower limit of the B amount was set to 0.0005%. The amount of B is preferably 0.0008% or more, more preferably 0.0010% or more. However, even if the amount of B becomes excessive, the hardenability improving action is saturated, and B nitride is formed, and cracks are likely to occur during cold forging. Therefore, the upper limit of the B amount is set to 0.005%. The amount of B is preferably 0.0025% or less, more preferably 0.0020% or less.
本発明の肌焼鋼の基本成分組成は上記の通りであり、残部は実質的にＦｅである。但し原料、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 component composition of the case hardening steel of the present invention is as described above, and the balance is substantially Fe. 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.
〈Ｃａ：０．００５％以下〉
本発明の肌焼鋼の基本成分組成は、上記の通りであるが、必要に応じて、鋼中にＣａを含有させても良い。Ｃａは、鋼中で酸化物または硫化物として存在し、鋼の切削性を向上させる作用を有するからである。この作用を充分に発揮させるために、必要に応じて、Ｃａを、好ましくは０．０００５％以上、より好ましくは０．０００７％以上、さらに好ましくは０．００１０％以上の量で含有させることが望ましい。しかしＣａ量が過剰になると、粗大な酸化物が生成して、冷間鍛造時に割れが発生しやすくなる。そこでＣａを含有させる場合、その上限を０．００５％と定めた。含有させる場合のＣａ量は、好ましくは０．００４％以下、より好ましくは０．００３％以下である。
<Ca: 0.005% or less>
Although the basic component composition of the case hardening steel of this invention is as above-mentioned, you may contain Ca in steel as needed. This is because Ca exists as an oxide or sulfide in steel and has an effect of improving the machinability of the steel. In order to fully exhibit this effect, Ca is preferably contained in an amount of 0.0005% or more, more preferably 0.0007% or more, and further preferably 0.0010% or more, as necessary. desirable. However, when the amount of Ca becomes excessive, coarse oxides are generated, and cracks are likely to occur during cold forging. Therefore, when Ca is contained, the upper limit is set to 0.005%. When Ca is contained, the Ca content is preferably 0.004% or less, more preferably 0.003% or less.
〈Ｎｂ：０．０１５％以下〉
Ｎｂは、Ｔｉと並存した場合、変形抵抗を増大させる作用が大きい。Ｎｂは、Ｔｉと並存しない場合は、変形抵抗に、それほど大きな影響を及ぼさない。しかしＮｂとＴｉとが並存すると、微細析出して、変形抵抗を大きく増大させることがある。そのためＴｉを用いる本発明では、鋼中にＮｂを含有させないことが好ましい。しかしＮｂは、原料から不純物として鋼中に混入することがある。よってＮｂが不純物として含まれる場合も、その量を制限する必要があり、本発明ではＮｂ量の上限を０．０１５％と定めた。Ｎｂ量は、好ましくは０．０１０％以下、より好ましくは０．００８％以下に抑えることが推奨される。
<Nb: 0.015% or less>
Nb has a large effect of increasing deformation resistance when coexisting with Ti. When Nb does not coexist with Ti, Nb does not affect the deformation resistance so much. However, when Nb and Ti coexist, they may precipitate finely and greatly increase the deformation resistance. Therefore, in this invention using Ti, it is preferable not to contain Nb in steel. However, Nb may be mixed into the steel as an impurity from the raw material. Therefore, when Nb is contained as an impurity, it is necessary to limit the amount thereof. In the present invention, the upper limit of the Nb amount is set to 0.015%. It is recommended that the amount of Nb is preferably 0.010% or less, more preferably 0.008% or less.
（イ）Ｔｉ系析出物について
〈直径が０．０１〜０．２μｍであるＴｉＣ析出物の個数が５〜３０個／μｍ2〉
次にＴｉＣなどのＴｉ系析出物について説明する。本発明の肌焼鋼は、結晶粒粗大化を防止するために、適正量の微細ＴｉＣが析出していることを特徴の１つとする。ＴｉＣが、微細で多数析出するほど、結晶粒粗大化を防止するピンニング効果が良好に発揮される。しかしＴｉＣ析出物があまりにも微細であると、ピンニング効果も発揮されず、またその個数を明確に特定することができない。そこで本発明は、直径が０．０１〜０．２μｍであるＴｉＣ析出物（以下、「微細ＴｉＣ」と省略することがある。）に着目し、その個数を適正に調整したことを特徴とする。具体的には結晶粒粗大化を防止するために、微細ＴｉＣの個数下限を５個／μｍ2と定めた。微細ＴｉＣは、好ましくは１０個／μｍ2以上、より好ましくは１５個／μｍ2以上である。しかし微細ＴｉＣの個数が過剰になると、変形抵抗が増大しすぎる。そこで微細ＴｉＣの個数上限を３０個／μｍ2と定めた。微細ＴｉＣは、好ましくは２５個／μｍ2以下、より好ましくは２０個／μｍ2以下である。
(A) Ti-based precipitates <Number of TiC precipitates having a diameter of 0.01 to 0.2 μm is 5 to 30 / μm 2 >
Next, Ti-based precipitates such as TiC will be described. The case-hardened steel of the present invention is characterized in that an appropriate amount of fine TiC is precipitated in order to prevent crystal grain coarsening. The finer and more precipitated TiC, the better the pinning effect that prevents crystal grain coarsening. However, if the TiC precipitate is too fine, the pinning effect is not exhibited, and the number cannot be clearly specified. Therefore, the present invention is characterized by focusing on TiC precipitates having a diameter of 0.01 to 0.2 μm (hereinafter sometimes abbreviated as “fine TiC”) and adjusting the number thereof appropriately. . Specifically, in order to prevent coarsening of crystal grains, the lower limit of the number of fine TiCs was set to 5 / μm 2 . The fine TiC is preferably 10 pieces / μm 2 or more, more preferably 15 pieces / μm 2 or more. However, when the number of fine TiCs becomes excessive, the deformation resistance increases too much. Therefore, the upper limit of the number of fine TiC was set to 30 / μm 2 . The fine TiC is preferably 25 pieces / μm 2 or less, more preferably 20 pieces / μm 2 or less.
上記のように結晶粒粗大化を防止するＴｉＣのピンニング効果は、ＴｉＣが粗大になると充分に発揮することができない。さらに粗大なＴｉＣ析出物が形成されると、その分だけ微細ＴｉＣ量が低減し、結晶粒粗大化防止特性がさらに損なわれる。そこで本発明において、鋼中の微細ＴｉＣを一定量以上確保するために、直径が０．２μｍを超えるＴｉＣ析出物（以下、「粗大ＴｉＣ」と省略することがある。）を、０．２個／μｍ2以下に抑えていることが好ましい。 As described above, the pinning effect of TiC that prevents coarsening of crystal grains cannot be sufficiently exhibited when TiC becomes coarse. When coarser TiC precipitates are formed, the amount of fine TiC is reduced by that amount, and the crystal grain coarsening preventing property is further impaired. Therefore, in the present invention, in order to secure a certain amount or more of fine TiC in the steel, 0.2 TiC precipitates having a diameter exceeding 0.2 μm (hereinafter, sometimes abbreviated as “coarse TiC”). / Μm 2 or less is preferable.
本発明において、鋼中の微細ＴｉＣおよび粗大ＴｉＣの個数は、透過型電子顕微鏡（ＴＥＭ）により測定した値を採用する。具体的には、まず肌焼鋼が円柱状の場合は高さ方向の中央部のＤ／４位置（Ｄ：直径）、肌焼鋼が角形状または板状の場合は長手方向および幅方向の中央部のＤ／４位置（Ｄ：厚み）から、抽出レプリカ法にて観察用サンプルを作製する。このサンプルを用いて、５万倍以上の観察倍率および２．５μｍ以上×３．５μｍ以上の観察視野で、少なくとも４視野を観察し、直径が０．０１〜０．２μｍであるＴｉＣ析出物、および直径が０．２μｍを超えるＴｉＣ析出物の各視野での個数を計測する。そして各視野での単位面積（１μｍ2）あたりの個数を求め、これらの平均値を、微細ＴｉＣおよび粗大ＴｉＣの個数として採用する。 In the present invention, the number of fine TiC and coarse TiC in the steel is a value measured with a transmission electron microscope (TEM). Specifically, first, when the case-hardened steel has a cylindrical shape, the D / 4 position (D: diameter) in the center in the height direction, and when the case-hardened steel has a square shape or plate shape, the longitudinal direction and the width direction. A sample for observation is produced from the D / 4 position (D: thickness) in the center by the extraction replica method. Using this sample, a TiC precipitate having an observation magnification of 50,000 times or more and an observation field of 2.5 μm or more × 3.5 μm or more, at least four fields of view having a diameter of 0.01 to 0.2 μm, The number of TiC precipitates having a diameter exceeding 0.2 μm in each field is measured. Then, the number per unit area (1 μm 2 ) in each visual field is obtained, and the average value thereof is adopted as the number of fine TiC and coarse TiC.
本発明の肌焼鋼中では、Ｃｒを増量し、且つＭｎを減量したため、ＭｎによるＳのトラップ量が低減しており、Ｔｉ系炭硫化物（ＴｉＳまたはＴｉ4Ｃ2Ｓ2）が形成されやすくなっている。Ｔｉ系炭硫化物が生成すると、微細ＴｉＣ量が低減するため、本発明の肌焼鋼中において、Ｔｉ系炭硫化物は存在しないことが好ましい。ここで本発明において「Ｔｉ系炭硫化物が存在しない」とは、肌焼鋼が円柱状の場合は高さ方向の中央部のＤ／４位置（Ｄ：直径）、肌焼鋼が角形状または板状の場合は長手方向および幅方向の中央部のＤ／４位置（Ｄ：厚み）から、４００倍の観察倍率および２０μｍ×２０μｍの１視野を観察して、該視野でＴｉ系炭硫化物を見つけることができない状態をいう。 In the case-hardened steel of the present invention, since Cr is increased and Mn is reduced, the amount of trapped S by Mn is reduced, and Ti-based carbon sulfide (TiS or Ti 4 C 2 S 2 ) is formed. It has become easier. When Ti-based carbon sulfide is generated, the amount of fine TiC is reduced. Therefore, it is preferable that no Ti-based carbon sulfide exists in the case-hardened steel of the present invention. Here, in the present invention, “Ti-based carbon sulfide is not present” means that the case-hardened steel has a columnar shape, the D / 4 position (D: diameter) in the center in the height direction, and the case-hardened steel has a square shape. Alternatively, in the case of a plate shape, an observation magnification of 400 times and one visual field of 20 μm × 20 μm are observed from the D / 4 position (D: thickness) in the center in the longitudinal direction and the width direction, and Ti-based carbon sulfide is observed in the visual field. A state where things cannot be found.
Ｔｉ系炭硫化物は、光学顕微鏡で観察すると、その形状はＪＩＳ Ｇ ０５５５「鋼の非金属介在物の顕微鏡試験方法」に規定されているグループＢ系介在物と同一であり、且つ薄いピンク色を呈しているため、容易に他の介在物と区別することができる。なおＴｉＮも、同じように薄いピンク色で観察されるが、形状はグループＤ系介在物に分類されるため、Ｔｉ系炭硫化物と区別することができる。しかしＴｉＮは、圧延等で加工が加わった際に砕けて、グループＢ系介在物のように見えることがあり、顕微鏡観察だけではＴｉＮとＴｉ系炭硫化物との区別が紛らわしい場合がある。区別が困難である析出物が存在する場合、それを、エネルギー分散型Ｘ線分析装置（ＥＤＸ）を備えた走査型電子顕微鏡（ＳＥＭ）により、ＴｉＮまたはＴｉ系炭硫化物のいずれであるか判定することができる。 When the Ti-carbon sulfide is observed with an optical microscope, its shape is the same as that of the Group B inclusion specified in JIS G 0555 “Microscopic test method for non-metallic inclusions in steel”, and light pink Therefore, it can be easily distinguished from other inclusions. TiN is also observed in the same light pink color, but the shape is classified as a group D inclusion, so that it can be distinguished from a Ti carbon sulfide. However, TiN may be crushed when processed by rolling or the like and may look like a group B inclusion, and the distinction between TiN and Ti carbon sulfide may be misleading only by microscopic observation. When there is a precipitate that is difficult to distinguish, it is determined whether it is TiN or Ti-based carbon sulfide by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX). can do.
〈鋼中に存在するＴｉＮ析出物の最大直径が３０μｍ以下〉
本発明の肌焼鋼において、鋼中に存在するＴｉＮ析出物の最大直径は３０μｍ以下であることが好ましい。ＴｉＮ析出物の最大直径は、より好ましくは２５μｍ以下、さらに好ましくは２０μｍ以下である。最大直径が３０μｍを超えるＴｉＮ析出物が存在すると、冷間鍛造時に割れが発生しやすくなるからである。本発明において「ＴｉＮ析出物の最大直径」は、以下に記載する方法で観察された値を採用する。まず肌焼鋼が円柱状の場合は高さ方向の中央部のＤ／４位置（Ｄ：直径）、肌焼鋼が角形状または板状の場合は長手方向および幅方向の中央部のＤ／４位置（Ｄ：厚み）から、１０ｍｍ×１０ｍｍの面積を有する観察用サンプルを採取する。この観察用サンプルの全面積から、ＴｉＮ析出物を、大きい順に２０個抽出し、それぞれの長径および短径を測定し、その面積（＝長径×短径）を算出する。そしてこの面積に相当する円の直径を各ＴｉＮ析出物の直径とし、これら２０個の直径の平均値を、本発明における「ＴｉＮ析出物の最大直径」として採用する。
<Maximum diameter of TiN precipitates present in steel is 30 μm or less>
In the case-hardened steel of the present invention, the maximum diameter of TiN precipitates present in the steel is preferably 30 μm or less. The maximum diameter of the TiN precipitate is more preferably 25 μm or less, and further preferably 20 μm or less. This is because if TiN precipitates having a maximum diameter exceeding 30 μm are present, cracks are likely to occur during cold forging. In the present invention, the “maximum diameter of TiN precipitate” employs a value observed by the method described below. First, when the case-hardened steel has a columnar shape, the D / 4 position (D: diameter) of the central portion in the height direction, and when the case-hardened steel has a square shape or a plate shape, the D / An observation sample having an area of 10 mm × 10 mm is taken from 4 positions (D: thickness). From the total area of this observation sample, 20 TiN precipitates are extracted in descending order, the major axis and minor axis thereof are measured, and the area (= major axis × minor axis) is calculated. The diameter of the circle corresponding to this area is taken as the diameter of each TiN precipitate, and the average value of these 20 diameters is adopted as the “maximum diameter of TiN precipitate” in the present invention.
（ウ）製造条件について
次に本発明の肌焼鋼の製造方法について説明する。まず本発明の化学成分組成の要件を満たすように鋼材を溶製し、これを、好ましくは２００℃／ｈｒ以上の冷却速度で鋳造する。本発明の化学成分組成の要件を満たし、且つ鋳造時の冷却速度が２００℃／ｈｒ以上であれば、ＴｉＮ析出物の最大直径を３０μｍ以下に抑えることができる。鋳造時の冷却速度は、より好ましくは２３０℃／ｈｒ以上、さらに好ましくは２５０℃／ｈｒ以上である。なお冷却速度が大きすぎると、割れが生じ鋼材の品質に悪影響を及ぼすことがあるため、鋳造時の冷却速度を、好ましくは５００℃／ｈｒ以下、より好ましくは４００℃／ｈｒ以下、さらに好ましくは３５０℃／ｈｒ以下に設定することが推奨される。
(C) Manufacturing conditions Next, the manufacturing method of the case hardening steel of this invention is demonstrated. First, a steel material is melted so as to satisfy the requirements of the chemical component composition of the present invention, and this is preferably cast at a cooling rate of 200 ° C./hr or more. If the requirements of the chemical component composition of the present invention are satisfied and the cooling rate during casting is 200 ° C./hr or more, the maximum diameter of the TiN precipitate can be suppressed to 30 μm or less. The cooling rate during casting is more preferably 230 ° C./hr or more, and further preferably 250 ° C./hr or more. If the cooling rate is too large, cracks may occur and adversely affect the quality of the steel material. Therefore, the cooling rate during casting is preferably 500 ° C./hr or less, more preferably 400 ° C./hr or less, more preferably It is recommended to set it to 350 ° C./hr or less.
本発明の肌焼鋼の製造方法において、分塊圧延の加熱温度およびその後の圧延の加熱温度は、微細ＴｉＣの個数に大きく影響するため重要である。まず分塊圧延の加熱温度は１２００℃以上にする必要がある。この加熱温度が低すぎると、Ｔｉ系炭硫化物や粗大ＴｉＣが形成されて、微細ＴｉＣの個数を充分に確保できないおそれがあるからである。分塊圧延の加熱温度は、好ましくは１２２０℃以上、より好ましくは１２５０℃以上である。一方、分塊圧延の加熱温度は、好ましくは１４００℃以下、より好ましくは１３５０℃以下、さらに好ましくは１３２０℃以下に抑えることが推奨される。分塊圧延の加熱温度が高すぎると、鋼片表面の酸化が激しくなり、表面品質の低下および歩留まりの低下を招くからである。 In the method for producing the case-hardened steel of the present invention, the heating temperature of the partial rolling and the heating temperature of the subsequent rolling are important because they greatly affect the number of fine TiCs. First, the heating temperature of the partial rolling needs to be 1200 ° C. or higher. This is because if the heating temperature is too low, Ti-based carbon sulfide and coarse TiC are formed, and the number of fine TiCs may not be sufficiently secured. The heating temperature of the block rolling is preferably 1220 ° C or higher, more preferably 1250 ° C or higher. On the other hand, it is recommended that the heating temperature of the block rolling is preferably 1400 ° C. or lower, more preferably 1350 ° C. or lower, and further preferably 1320 ° C. or lower. This is because if the heating temperature of the block rolling is too high, the surface of the steel slab is oxidized violently, resulting in a decrease in surface quality and a decrease in yield.
分塊圧延後の圧延、例えば棒鋼圧延の加熱温度は８３０℃以上にする必要がある。この加熱温度が低すぎると、微細ＴｉＣが過剰に生成し、肌焼鋼の変形抵抗が増大しすぎるおそれがあるからである。この加熱温度は、好ましくは８５０℃以上、より好ましくは８７０℃以上である。一方、分塊圧延後の圧延加熱温度は９５０℃以下に抑える必要がある。この加熱温度が高すぎると、Ｔｉ系炭硫化物や粗大ＴｉＣが形成されて、微細ＴｉＣの個数を充分に確保できないおそれがあるからである。この加熱温度は、好ましくは９３０℃以下、より好ましくは９００℃以下である。 The heating temperature for the rolling after the block rolling, for example, the steel bar rolling, needs to be 830 ° C. or higher. This is because if the heating temperature is too low, fine TiC is excessively generated, and the deformation resistance of the case-hardened steel may be excessively increased. This heating temperature is preferably 850 ° C. or higher, more preferably 870 ° C. or higher. On the other hand, the rolling heating temperature after the partial rolling needs to be suppressed to 950 ° C. or lower. This is because if the heating temperature is too high, Ti-based carbon sulfide and coarse TiC are formed, and the number of fine TiCs may not be sufficiently secured. This heating temperature is preferably 930 ° C. or lower, more preferably 900 ° C. or lower.
以下、実施例を挙げて本発明をより具体的に説明するが、本発明は以下の実施例によって制限を受けるものではなく、上記・下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。 EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
下記表２に示す化学成分組成の鋼材を、真空誘導溶製炉によって溶製した。なお下記表２に示す組成Ｎｏ．Ｄ、Ｈ、Ｊ、Ｌ、ＭおよびＮが、本発明の化学成分の要件を満たすものである。また組成Ｎｏ．Ａ１は、従来鋼のＳＣＭ４２０Ｈの組成に対応する。こうして得られた溶鋼を、鋳型のサイズを変えて、２３０℃／ｈｒ、８０℃／ｈｒおよび４０℃／ｈｒの冷却速度で鋳造した。次に実機の分塊圧延を模擬して、断面が１５５×１５５ｍｍの鋼塊に鍛造した。このときの分塊圧延の加熱温度を、１１５０℃、１２２０℃および１３００℃に調整した。鋼塊を直径３０ｍｍに棒鋼圧延することにより、肌焼鋼（直径３０ｍｍの棒鋼）を製造した。このときの棒鋼圧延加熱温度を、８００℃、９２５℃、１０５０℃および１１５０℃に調整した。各肌焼鋼の具体的な鋳造時の冷却速度、分塊圧延加熱温度および棒鋼圧延加熱温度を、下記表３に示す。 Steel materials having the chemical composition shown in Table 2 below were melted in a vacuum induction melting furnace. In addition, the composition No. shown in Table 2 below. D, H, J, L, M, and N satisfy the requirements of the chemical component of the present invention. The composition No. A1 corresponds to the composition of conventional steel SCM420H. The molten steel thus obtained was cast at a cooling rate of 230 ° C./hr, 80 ° C./hr, and 40 ° C./hr while changing the mold size. Next, it was forged into a steel ingot having a cross section of 155 × 155 mm by simulating the ingot rolling of the actual machine. The heating temperature of the partial rolling at this time was adjusted to 1150 ° C, 1220 ° C, and 1300 ° C. Case-hardened steel (steel with a diameter of 30 mm) was produced by rolling the steel ingot to a diameter of 30 mm. The steel bar rolling heating temperature at this time was adjusted to 800 ° C, 925 ° C, 1050 ° C, and 1150 ° C. Table 3 below shows specific cooling rates at the time of casting of each case-hardened steel, the partial rolling heating temperature, and the bar rolling heating temperature.
上記のようにして得られた肌焼鋼中において、直径が０．０１〜０．２μｍであるＴｉＣ析出物（微細ＴｉＣ）および直径が０．２μｍを超えるＴｉＣ析出物（粗大ＴｉＣ）の個数を、肌焼鋼（棒鋼）の高さ方向の中央部のＤ／４（３０ｍｍ／４＝７．５ｍｍ）位置から、抽出レプリカ法にて観察用サンプルを作製し、このサンプルを用いて、５万倍の観察倍率および２．９μｍ×３．６μｍの観察視野で４視野を観察して測定した。また肌焼鋼の中央部のＤ／４位置から、４００倍の観察倍率および２０μｍ×２０μｍの１視野を観察して、Ｔｉ系炭硫化物の有無を判定した。なお実施例における肌焼鋼中には、Ｔｉ系炭硫化物と区別することが困難なＴｉＮは存在しなかった。これらの結果を下記表３に示す。 In the case-hardened steel obtained as described above, the number of TiC precipitates (fine TiC) having a diameter of 0.01 to 0.2 μm and TiC precipitates (coarse TiC) having a diameter exceeding 0.2 μm From the D / 4 (30 mm / 4 = 7.5 mm) position of the center part in the height direction of the case-hardened steel (bar steel), an observation sample is prepared by the extraction replica method, and 50,000 using this sample Measurement was performed by observing 4 visual fields with a double observation magnification and an observation visual field of 2.9 μm × 3.6 μm. Further, from the D / 4 position in the center of the case-hardened steel, the observation magnification of 400 times and one visual field of 20 μm × 20 μm were observed to determine the presence or absence of Ti-based carbon sulfide. In the case-hardened steel in the examples, there was no TiN that was difficult to distinguish from Ti-based carbon sulfide. These results are shown in Table 3 below.
また肌焼鋼の中央部のＤ／４位置から、１０ｍｍ×１０ｍｍの面積を有する観察用サンプルを採取し、ＴｉＮ析出物の最大直径を測定した。さらにＴｉＮ析出物の個数も、１０ｍｍ×１０ｍｍの観察用サンプルから、ランダムに１ｍｍ×１ｍｍの視野を２０視野選択し、これらを光学顕微鏡により観察倍率１００倍で観察し、各視野の単位面積（１ｍｍ2）あたりの個数を計測し、これらの平均値を各肌焼鋼のＴｉＮの個数として算出した。これらの結果を下記表３に示す。 Moreover, the sample for observation which has an area of 10 mm x 10 mm was extract | collected from D / 4 position of the center part of case hardening steel, and the maximum diameter of the TiN deposit was measured. In addition, the number of TiN precipitates was also selected from 10 mm × 10 mm observation samples, randomly selecting 20 fields of 1 mm × 1 mm, and these were observed with an optical microscope at an observation magnification of 100 ×, and the unit area of each field (1 mm 2 ) The number per unit was measured, and the average value was calculated as the number of TiN in each case-hardened steel. These results are shown in Table 3 below.
肌焼鋼の冷間鍛造性を調べるために、まず得られた肌焼鋼（直径３０ｍｍの棒鋼）を球状化焼鈍（軟化焼鈍）して軟質化させた後、旋盤切削して、直径１５ｍｍ×高さ２２．５ｍｍである円柱状の試験片を作製した。なお球状化焼鈍は、７６０℃で５時間保持した後、１０℃／ｈｒで徐冷し、温度が６８０℃に達した後、室温まで空冷することにより行った。 In order to investigate the cold forgeability of the case-hardened steel, first, the obtained case-hardened steel (bar steel having a diameter of 30 mm) was softened by spheroidizing annealing (softening annealing), and then lathe was cut to obtain a diameter of 15 mm × A cylindrical test piece having a height of 22.5 mm was produced. The spheroidizing annealing was performed by holding at 760 ° C. for 5 hours and then gradually cooling at 10 ° C./hr. After the temperature reached 680 ° C., air cooling to room temperature was performed.
冷間鍛造性の指標として、まず圧縮加工における変形抵抗を測定した。具体的には、上記試験片をプレスで加工率７０％まで圧縮し、加工率が１０％、２０％、３０％、４０％および５０％に達したときの変形抵抗を計測し、これらの平均値を各試験片の変形抵抗として求めた。同様の測定を合計３回行い、各試験片の変形抵抗の平均値を、肌焼鋼の変形抵抗の値として採用した。なおこの加工試験は、端面拘束で行った。結果を下記表３に示す。参考のために、各肌焼鋼のビッカース硬さを測定し、その結果も表３に示す。変形抵抗の基準は、従来鋼であるＳＣＭ４２０Ｈの組成（組成Ｎｏ．Ａ１）を有する鋼Ｎｏ．９の値（６５１ＭＰａ）とし、これよりも変形抵抗が１０％以上低減したもの、即ち５８６ＭＰａ以下の変形抵抗を有するものを、変形抵抗が良好であると判定した。 As an index of cold forgeability, first, deformation resistance in compression processing was measured. Specifically, the test specimen was compressed to a processing rate of 70% with a press, the deformation resistance when the processing rate reached 10%, 20%, 30%, 40% and 50% was measured, and the average of these was measured. The value was determined as the deformation resistance of each test piece. The same measurement was performed three times in total, and the average value of the deformation resistance of each test piece was adopted as the value of the deformation resistance of the case hardening steel. This processing test was performed with end face restraint. The results are shown in Table 3 below. For reference, the Vickers hardness of each case-hardened steel was measured, and the results are also shown in Table 3. The standard of deformation resistance is steel No. having the composition (composition No. A1) of SCM420H which is a conventional steel. A value of 9 (651 MPa) was determined, and a sample having a deformation resistance reduced by 10% or more, that is, a sample having a deformation resistance of 586 MPa or less was determined to have good deformation resistance.
さらに冷間鍛造性の指標として、肌焼鋼の変形能（割れ限界加工率）を測定した。具体的には、上記試験片を、加工率５０％まで圧縮加工したのち、段階的に加工率２．５％ずつ圧縮を加え、割れが発生するまでの加工率を求めた。同様の測定を合計８回行い、各試験片の割れ限界加工率の平均値を、肌焼鋼の割れ限界加工率の値として採用した。なおこの加工試験も、端面拘束で行った。結果を下記表３に示す。割れ限界加工率が７０％以上であるものを、変形能が良好であると判定した。 Furthermore, the deformability (crack limit processing rate) of case hardening steel was measured as an index of cold forgeability. Specifically, after compressing the test piece to a processing rate of 50%, the processing rate was gradually increased by 2.5%, and the processing rate until cracking occurred was determined. The same measurement was performed 8 times in total, and the average value of the crack limit processing rate of each specimen was adopted as the value of the crack limit processing rate of the case-hardened steel. This processing test was also performed with end face restraint. The results are shown in Table 3 below. Those having a crack limit processing rate of 70% or more were determined to have good deformability.
結晶粒粗大化防止特性の指標として、以下のようにして、結晶粒粗大化温度を測定した。まず上記試験片をプレスで加工率７０％まで圧縮したものを、浸炭温度Ｔ（℃）を、９００℃から２５℃ずつ上昇させて（Ｔ＝９００℃、９２５℃、９５０℃、９７５℃および１０００℃）、各浸炭温度で浸炭処理を行った。浸炭処理後、圧縮試験片のＤ／４位置のオーステナイト結晶粒を観察し、ＪＩＳ Ｇ ０５５１「鋼のオーステナイト結晶粒度試験方法」に規定されている粒度番号が５番未満である粗い結晶粒が観察された浸炭温度を、結晶粒粗大化温度とした。なお具体的な観察位置を詳細に説明すると、直径１５ｍｍ×高さ２２．５ｍｍの試験片を加工率７０％まで圧縮すると、直径１８．５ｍｍ×高さ６．７５ｍｍの形状に変形するので、圧縮後試験片において高さ方向の中央部（６．７５／２≒３．３８ｍｍ）のＤ／４位置（１８．５／４≒４．６３ｍｍ）を観察した。同様の測定を合計３回行い、各試験片の結晶粒粗大化温度の平均値を、肌焼鋼の結晶粒粗大化温度の値として採用した。結果を下記表３に示す。結晶粒粗大化温度の基準は、従来鋼であるＳＣＭ４２０Ｈの組成（組成Ｎｏ．Ａ１）を有する鋼Ｎｏ．９の値（９５０℃）とし、この温度以上のものを、結晶粒粗大化防止特性が良好であると判定した。 As an index of the crystal grain coarsening prevention characteristic, the crystal grain coarsening temperature was measured as follows. First, the test piece was compressed to a processing rate of 70% with a press, and the carburizing temperature T (° C.) was increased from 900 ° C. by 25 ° C. (T = 900 ° C., 925 ° C., 950 ° C., 975 ° C. and 1000 ° C.). ℃), carburizing treatment was performed at each carburizing temperature. After the carburizing treatment, the austenite crystal grains at the D / 4 position of the compression test piece are observed, and coarse crystal grains having a grain size number specified in JIS G 0551 “Austenite grain size test method for steel” of less than 5 are observed. The carburized temperature thus obtained was defined as the crystal grain coarsening temperature. The specific observation position will be described in detail. When a test piece having a diameter of 15 mm and a height of 22.5 mm is compressed to a processing rate of 70%, it is deformed into a shape having a diameter of 18.5 mm and a height of 6.75 mm. The D / 4 position (18.5 / 4≈4.63 mm) of the central portion (6.75 / 2≈3.38 mm) in the height direction was observed on the rear test piece. The same measurement was performed three times in total, and the average value of the grain coarsening temperature of each test piece was adopted as the value of the grain coarsening temperature of the case-hardened steel. The results are shown in Table 3 below. The standard of the grain coarsening temperature is steel No. having the composition (composition No. A1) of SCM420H, which is a conventional steel. A value of 9 (950 ° C.) was determined, and those above this temperature were judged to have good crystal grain coarsening prevention characteristics.
なお上記の結晶粒粗大化温度の測定において、浸炭温度Ｔ＝９００〜９５０℃までのものは、ガス浸炭炉（浸炭ガス：ＲＸガス＋プロパンガス、表面炭素濃度：０．８質量％狙い）で、所定の浸炭温度Ｔ（℃）で１８０分（浸炭：１８０分）、次いで８６０℃で６０分保持した後、油冷（油温：６０℃）した。また浸炭温度Ｔ＝９７５および１０００℃のものは、真空浸炭炉（浸炭ガス：アセチレンガス、表面炭素濃度：０．８質量％狙い）で、所定の浸炭温度Ｔ（℃）で１６０分（浸炭：５０分、拡散：１１０分）、次いで８６０℃で６０分保持した後、油冷（油温：６０℃）した。 In the measurement of the crystal grain coarsening temperature, the one with a carburizing temperature T = 900 to 950 ° C. is a gas carburizing furnace (carburizing gas: RX gas + propane gas, surface carbon concentration: aiming at 0.8 mass%). After maintaining at a predetermined carburizing temperature T (° C.) for 180 minutes (carburizing: 180 minutes) and then at 860 ° C. for 60 minutes, oil cooling (oil temperature: 60 ° C.) was performed. Also, the ones with carburizing temperatures T = 975 and 1000 ° C. are used in a vacuum carburizing furnace (carburizing gas: acetylene gas, surface carbon concentration: 0.8% by mass) for 160 minutes at a predetermined carburizing temperature T (° C.) (carburizing: 50 minutes, diffusion: 110 minutes), and then held at 860 ° C. for 60 minutes, followed by oil cooling (oil temperature: 60 ° C.).
表２および表３の結果から、本発明の化学成分の要件およびＴｉＣ析出物の個数の要件を満たす鋼Ｎｏ．１〜４（組成Ｎｏ．Ｄ）、鋼Ｎｏ．１６（組成Ｎｏ．Ｈ）、鋼Ｎｏ．１８（組成Ｎｏ．Ｊ）、鋼Ｎｏ．２０（組成Ｎｏ．Ｌ）、鋼Ｎｏ．２１（組成Ｎｏ．Ｍ）、鋼Ｎｏ．２２（組成Ｎｏ．Ｎ）は、良好な変形抵抗（５８６ＭＰａ以下）および結晶粒粗大化温度（９５０℃以上）を有し、本発明の肌焼鋼は、変形抵抗および結晶粒粗大化防止特性の両方に優れていることが分かる。特に鋼Ｎｏ．２１（組成Ｎｏ．Ｍ）と鋼Ｎｏ．２２（組成Ｎｏ．Ｎ）は、Ｔｉを０．０５％以上含有しているが、Ｔｉと併せてＡｌを多めに含有しているため、微細ＴｉＣ量は過剰に生成せず、変形抵抗が小さくなり、冷間鍛造性に優れている。 From the results shown in Tables 2 and 3, the steel No. 1 satisfying the requirements of the chemical composition and the number of TiC precipitates of the present invention was obtained. 1-4 (composition No. D), steel No. 1 16 (composition No. H), steel No. 18 (composition No. J), steel No. 18 20 (composition No. L), steel No. 21 (Composition No. M), Steel No. 22 (Composition No. N) has good deformation resistance (586 MPa or less) and grain coarsening temperature (950 ° C. or more), and the case-hardened steel of the present invention has deformation resistance and grain coarsening prevention properties. It turns out that it is excellent in both. In particular, steel no. 21 (Composition No. M) and Steel No. 22 (Composition No. N) contains 0.05% or more of Ti, but contains a large amount of Al together with Ti, so an excessive amount of fine TiC is not generated and deformation resistance is small. It is excellent in cold forgeability.
さらにＴｉＮ析出物の最大直径が３０μｍ以下である鋼Ｎｏ．１、４、１６、１８、２０、２１および２２は、この要件を満たさない鋼Ｎｏ．２および３に比べて、割れ限界加工率が向上しており、ＴｉＮ析出物の要件を満たす本発明の好ましい肌焼鋼は、さらに冷間鍛造時に割れにくくなっていることが分かる。なお鋼Ｎｏ．２および３は、鋳造時の冷却速度が低いため、ＴｉＮ析出物の最大直径が大きくなっている。 Further, the steel No. 1 in which the maximum diameter of the TiN precipitate is 30 μm or less. 1, 4, 16, 18, 20, 21, and 22 are steel Nos. That do not satisfy this requirement. It can be seen that the crack limit working rate is improved as compared with 2 and 3, and the preferred case-hardened steel of the present invention that satisfies the requirements for TiN precipitates is further less susceptible to cracking during cold forging. Steel No. 2 and 3 have a large maximum diameter of TiN precipitates because the cooling rate during casting is low.
一方、鋼Ｎｏ．５〜１５、１７、１９および２３は、本発明の要件を満たさないものであり、変形抵抗および結晶粒粗大化防止特性のいずれか、またはその両方が不充分であるものである。
詳しくは鋼Ｎｏ．５（組成Ｎｏ．Ｄ）は、分塊圧延の加熱温度が低いため、微細ＴｉＣの個数が不充分であり、結晶粒粗大化防止特性が劣っている。
鋼Ｎｏ．６および７（組成Ｎｏ．Ｄ）は、棒鋼圧延の加熱温度が高いため、微細ＴｉＣの個数が不充分であり、結晶粒粗大化防止特性が劣っている。
鋼Ｎｏ．８（組成Ｎｏ．Ｄ）は、棒鋼圧延の加熱温度が低いため、微細ＴｉＣが過剰に生成し、変形抵抗が増大している。
On the other hand, Steel No. Nos. 5 to 15, 17, 19, and 23 do not satisfy the requirements of the present invention, and either or both of the deformation resistance and the crystal grain coarsening preventing property are insufficient.
For details, see Steel No. 5 (Composition No. D) has a low number of fine TiCs due to the low heating temperature of the block rolling, and has poor crystal grain coarsening prevention properties.
Steel No. 6 and 7 (composition No. D) have a high number of fine TiCs due to the high heating temperature of steel bar rolling, and have poor crystal grain coarsening prevention properties.
Steel No. In No. 8 (composition No. D), since the heating temperature of the steel bar rolling is low, fine TiC is excessively generated and the deformation resistance is increased.
鋼Ｎｏ．９（組成Ｎｏ．Ａ１）は、従来鋼のＳＣＭ４２０Ｈの組成に対応し、ＳｉおよびＭｎが過剰であるため、変形抵抗が増大している。
鋼Ｎｏ．１０（組成Ｎｏ．Ａ２）は、従来鋼のＳＣＭ４２０ＨにＮｂを添加したものに対応し、Ｓｉ、ＭｎおよびＮｂが過剰であるため、変形抵抗が増大している。
鋼Ｎｏ．１１（組成Ｎｏ．Ｂ）は、Ｍｎ量が過剰であるため、変形抵抗が増大している。
鋼Ｎｏ．１２（組成Ｎｏ．Ｃ）は、ＴｉとＮｂとが並存し、且つそれらの量が過剰であり、微細ＴｉＣ量が過剰に生成して、変形抵抗が増大している。
Steel No. No. 9 (composition No. A1) corresponds to the composition of SCM420H of conventional steel, and since Si and Mn are excessive, deformation resistance is increased.
Steel No. No. 10 (Composition No. A2) corresponds to the conventional steel SCM420H with Nb added, and Si, Mn and Nb are excessive, so the deformation resistance is increased.
Steel No. 11 (Composition No. B) has an excessive amount of Mn, and thus has an increased deformation resistance.
Steel No. In No. 12 (composition No. C), Ti and Nb coexist, and the amounts thereof are excessive, the amount of fine TiC is excessively generated, and the deformation resistance is increased.
鋼Ｎｏ．１３（組成Ｎｏ．Ｅ）は、［Ｔｉ］−３．４２［Ｎ］が０．００３であり、ＴｉＣを形成するためのＴｉ量が少ないため、微細ＴｉＣが充分に生成せず、結晶粒粗大化防止特性が劣っている。
鋼Ｎｏ．１４（組成Ｎｏ．Ｆ）および鋼Ｎｏ．１５（組成Ｎｏ．Ｇ）は、Ｔｉ量が過剰であるために、微細ＴｉＣ量が過剰に生成し、変形抵抗が増大している。
鋼Ｎｏ．１７（組成Ｎｏ．Ｉ）は、［Ｔｉ］−３．４２［Ｎ］が０．００２であり、ＴｉＣを形成するためのＴｉ量が少ないため、微細ＴｉＣが充分に生成せず、結晶粒粗大化防止特性が劣っている。さらにＮ量が過剰であるため、Ｎが鉄中に固溶し、変形抵抗が増大している。
鋼Ｎｏ．１９（組成Ｎｏ．Ｋ）は、Ｓ量が過剰であり、Ｔｉ系炭硫化物が形成された結果、微細ＴｉＣが充分に生成せず、結晶粒粗大化防止特性が劣っている。またＳ量が過剰であるため、変形能（割れ限界加工率）も劣っている。
鋼Ｎｏ．２３（組成Ｎｏ．Ｏ）は、Ｔｉ量が過剰であり、しかもＡｌ量が比較的少ないため、［Ｔｉ］−３．４２［Ｎ］が０．１０３となり、微細ＴｉＣ量が過剰に生成して変形抵抗が増大している。
Steel No. 13 (Composition No. E) has [Ti] -3.42 [N] of 0.003, and the amount of Ti for forming TiC is small, so that fine TiC is not sufficiently generated, and the grain size is large. The anti-oxidation property is inferior.
Steel No. 14 (Composition No. F) and Steel No. 15 (Composition No. G) has an excessive amount of Ti, so an excessive amount of fine TiC is generated, and deformation resistance is increased.
Steel No. 17 (Composition No. I) has [Ti] -3.42 [N] of 0.002, and since the amount of Ti for forming TiC is small, fine TiC is not sufficiently generated, and the grain size is large. The anti-oxidation property is inferior. Furthermore, since the amount of N is excessive, N is dissolved in iron and the deformation resistance is increased.
Steel No. 19 (Composition No. K) has an excessive amount of S, and as a result of the formation of Ti-carbon sulfide, fine TiC is not sufficiently generated, and the crystal grain coarsening preventing property is inferior. Moreover, since the amount of S is excessive, the deformability (crack limit processing rate) is also inferior.
Steel No. 23 (Composition No. O) has an excessive amount of Ti and a relatively small amount of Al, so [Ti] -3.42 [N] is 0.103, and an excessive amount of fine TiC is generated. Deformation resistance is increasing.
Claims (5)
Ｓｉ：０．１％以下（０％を含まない）、
Ｍｎ：０．６％以下（０％を含まない）、
Ｐ ：０．０３％以下（０％を含まない）、
Ｓ ：０．０２％以下（０％を含まない）、
Ｃｒ：１．２５〜２％、
Ａｌ：０．１％以下（０％を含まない）、
Ｔｉ：０．０７％以下（０％を含まない）、
Ｂ ：０．０００５〜０．００５％、および
Ｎ ：０．００８％以下（０％を含まない）
を含有し、且つ下記式（１）：
０．０１≦［Ｔｉ］−３．４２［Ｎ］≦０．０５ ・・・ （１）
〔式中、［Ｔｉ］および［Ｎ］は、それぞれ鋼中のＴｉおよびＮ含有量（質量％）を表す。〕
を満たし、残部がＦｅおよび不可避不純物からなり、
直径が０．０１〜０．２μｍであるＴｉＣ析出物の個数が５〜３０個／μｍ2であることを特徴とする肌焼鋼。 C: 0.1 to 0.3% (meaning mass%, the same shall apply hereinafter),
Si: 0.1% or less (excluding 0%),
Mn: 0.6% or less (excluding 0%),
P: 0.03% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Cr: 1.25 to 2%,
Al: 0.1% or less (excluding 0%),
Ti: 0.07% or less (excluding 0%),
B: 0.0005 to 0.005%, and N: 0.008% or less (excluding 0%)
And the following formula (1):
0.01 ≦ [Ti] −3.42 [N] ≦ 0.05 (1)
[In formula, [Ti] and [N] represent Ti and N content (mass%) in steel, respectively. ]
And the balance consists of Fe and inevitable impurities,
A case-hardened steel, wherein the number of TiC precipitates having a diameter of 0.01 to 0.2 μm is 5 to 30 μm 2 .
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| JP5649838B2 (en) * | 2009-03-27 | 2015-01-07 | Ｊｆｅスチール株式会社 | Case-hardened steel and method for producing the same |
| JP5385656B2 (en) * | 2009-03-27 | 2014-01-08 | 株式会社神戸製鋼所 | Case-hardened steel with excellent maximum grain reduction characteristics |
| JP5432105B2 (en) * | 2010-09-28 | 2014-03-05 | 株式会社神戸製鋼所 | Case-hardened steel and method for producing the same |
| JP5458048B2 (en) * | 2011-03-29 | 2014-04-02 | 株式会社神戸製鋼所 | Case-hardened steel, its manufacturing method, and machine structural parts using case-hardened steel |
| JP5796406B2 (en) * | 2011-04-28 | 2015-10-21 | Ｊｆｅスチール株式会社 | Method for producing case-hardened steel excellent in cold forgeability and ability to suppress grain coarsening |
| JP2013227676A (en) * | 2013-04-12 | 2013-11-07 | Nsk Ltd | Bearing device for supporting wheel |
| JP6182489B2 (en) * | 2014-03-27 | 2017-08-16 | 株式会社神戸製鋼所 | Case-hardened steel that has excellent cold forgeability and can suppress abnormal grain generation during carburizing. |
| JP6683072B2 (en) * | 2016-09-01 | 2020-04-15 | 日本製鉄株式会社 | Steel for carburizing, carburized steel parts and method for manufacturing carburized steel parts |
| JP7135484B2 (en) * | 2018-06-18 | 2022-09-13 | 日本製鉄株式会社 | Carburizing steel and parts |
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| JPH0765140B2 (en) * | 1986-10-20 | 1995-07-12 | 大同特殊鋼株式会社 | Case hardening steel for cold forging |
| JP3490293B2 (en) * | 1997-07-23 | 2004-01-26 | 新日本製鐵株式会社 | Cold forging steel excellent in crystal grain coarsening prevention property and delayed fracture resistance, and its manufacturing method |
| JP3738003B2 (en) * | 2002-12-04 | 2006-01-25 | 新日本製鐵株式会社 | Steel for case hardening excellent in cold workability and properties of preventing coarse grains during carburizing and method for producing the same |
| JP4369250B2 (en) * | 2004-01-13 | 2009-11-18 | 株式会社神戸製鋼所 | High temperature carburizing steel and method for producing the same |
| JP4847681B2 (en) * | 2004-02-06 | 2011-12-28 | 株式会社神戸製鋼所 | Ti-containing case-hardened steel |
| JP2006161143A (en) * | 2004-12-10 | 2006-06-22 | Kobe Steel Ltd | Carburizing steel having excellent rolling fatigue life and high temperature carburizing property |
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