JP2015166495A - Case hardening steel excellent in cold forgeability and crystal grain coarsening suppression performance - Google Patents
Case hardening steel excellent in cold forgeability and crystal grain coarsening suppression performance Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 56
- 239000010959 steel Substances 0.000 title claims abstract description 56
- 239000013078 crystal Substances 0.000 title claims abstract description 21
- 230000001629 suppression Effects 0.000 title claims abstract description 8
- 239000002244 precipitate Substances 0.000 claims abstract description 44
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 25
- 229910001562 pearlite Inorganic materials 0.000 claims abstract description 14
- 229910000760 Hardened steel Inorganic materials 0.000 claims description 13
- 239000000203 mixture Substances 0.000 claims description 12
- 239000012535 impurity Substances 0.000 claims description 2
- 229910052742 iron Inorganic materials 0.000 claims description 2
- 238000005096 rolling process Methods 0.000 description 28
- 238000010438 heat treatment Methods 0.000 description 18
- 238000005255 carburizing Methods 0.000 description 13
- 230000000694 effects Effects 0.000 description 12
- 238000010273 cold forging Methods 0.000 description 10
- 238000000034 method Methods 0.000 description 9
- 238000001816 cooling Methods 0.000 description 8
- 239000000463 material Substances 0.000 description 8
- 229910001566 austenite Inorganic materials 0.000 description 7
- 238000004519 manufacturing process Methods 0.000 description 7
- 238000012545 processing Methods 0.000 description 7
- 238000005242 forging Methods 0.000 description 6
- 230000007423 decrease Effects 0.000 description 5
- 229910052799 carbon Inorganic materials 0.000 description 4
- 238000005520 cutting process Methods 0.000 description 4
- 238000002149 energy-dispersive X-ray emission spectroscopy Methods 0.000 description 4
- 238000012360 testing method Methods 0.000 description 4
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 3
- 229910001563 bainite Inorganic materials 0.000 description 3
- 238000009826 distribution Methods 0.000 description 3
- 238000002156 mixing Methods 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 238000010791 quenching Methods 0.000 description 3
- 230000000171 quenching effect Effects 0.000 description 3
- 238000004088 simulation Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
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- 238000009863 impact test Methods 0.000 description 2
- 229910000734 martensite Inorganic materials 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 230000003647 oxidation Effects 0.000 description 2
- 238000007254 oxidation reaction Methods 0.000 description 2
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
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- 238000011156 evaluation Methods 0.000 description 1
- 238000009661 fatigue test Methods 0.000 description 1
- 238000003754 machining Methods 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
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- 238000005482 strain hardening Methods 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
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Abstract
Description
本発明は、建産機や自動車の分野で用いられる機械構造用材料に供する、冷間鍛造性に優れ、かつ浸炭処理を行った場合でも結晶粒の粗大化が抑制される、すなわち加工性に優れ、かつ高い結晶粒粗大化抑制能を有する肌焼鋼と、その製造方法に関するものである。 The present invention is used for machine structural materials used in the field of construction machinery and automobiles, is excellent in cold forgeability, and even when carburizing is performed, coarsening of crystal grains is suppressed, that is, in workability. The present invention relates to a case-hardened steel having excellent and high crystal grain coarsening suppressing ability and a method for producing the same.
棒鋼を冷間成形して製造される、例えば自動車部品等の素材には、高い冷間鍛造性が要求される。そのため、軟化焼鈍を施して炭化物を球状化し、冷間鍛造性を高めることが行われている。また、鋼の成分組成の観点からは、変形抵抗に大きく影響するSiを低減するなどの提案もなされている。更に、Ｂの焼入れ性を有効利用した鋼の提案もある。 High cold forgeability is required for materials such as automobile parts manufactured by cold forming steel bars. For this reason, soft annealing is performed to spheroidize carbides to improve cold forgeability. In addition, from the viewpoint of the composition of steel, proposals have been made to reduce Si, which greatly affects deformation resistance. Further, there is a proposal of steel that effectively uses the hardenability of B.
例えば、特許文献１には、Ｂの焼入れ性向上の効果分だけ他の合金元素を減量することによって、焼ならし工程から硬さを低くし、従来鋼に対して歯切り性を飛躍的に向上させた、浸炭歯車用鋼が提案されている。 For example, in Patent Document 1, by reducing the amount of other alloy elements by the effect of improving the hardenability of B, the hardness is reduced from the normalizing step, and the cutting performance is dramatically improved compared to conventional steel. Improved carburized gear steels have been proposed.
また、特許文献２では、固溶強化元素であるSiおよびMnを低減して焼入れ性をＢで確保する成分系と、製造条件との組み合わせにより、冷間加工性を確保する肌焼鋼が提案されている。 Patent Document 2 proposes a case-hardening steel that secures cold workability by combining the component system that secures hardenability with B by reducing the solid solution strengthening elements Si and Mn and the manufacturing conditions. Has been.
一方で、近年になって、建産機や自動車分野で用いられる部品の製造コストの低減が強く望まれており、鍛造・切削等の機械加工にかかるコストを低減する取り組みがなされている。例えば、切削加工から鍛造加工への変更や、鍛造でも熱間鍛造に代えて、寸法精度が高く、鍛造後の切削コストを低減できる冷間鍛造が指向されている。 On the other hand, in recent years, it has been strongly desired to reduce the manufacturing cost of parts used in the field of industrial machinery and automobiles, and efforts are being made to reduce the cost of machining such as forging and cutting. For example, a change from cutting to forging, or cold forging that has high dimensional accuracy and can reduce cutting costs after forging is directed instead of hot forging in forging.
しかしながら、冷間鍛造後の部品は浸炭処理工程において高温で処理されるため、オーステナイト粒（γ粒）の粗大化が起こりやすいという問題がある。その結果、焼入れ後に熱歪が生じ、部品寸法が変化するため、仕上げ加工や研磨等の余分な工程が必要となり、生産性が著しく阻害され、コスト上昇を招くことになる。 However, since the parts after cold forging are processed at a high temperature in the carburizing process, there is a problem that austenite grains (γ grains) are likely to be coarsened. As a result, heat distortion occurs after quenching and the part dimensions change, so that extra steps such as finishing and polishing are required, productivity is significantly hindered, and costs are increased.
このような事情から、冷間鍛造に適し、しかも浸炭処理において結晶粒粗大化を防止可能な肌焼鋼の提供が強く求められている。このため、浸炭時における肌焼鋼の結晶粒粗大化防止の技術は様々提案されており、Al, Nb, Ti等の元素を添加することによって、AlN, NbC, TiC等の析出物を微細に分散させる技術が汎用されている。 Under these circumstances, there is a strong demand for a case-hardened steel that is suitable for cold forging and that can prevent grain coarsening in carburizing treatment. For this reason, various techniques for preventing grain coarsening of case-hardened steel during carburizing have been proposed. By adding elements such as Al, Nb, and Ti, precipitates such as AlN, NbC, and TiC can be refined. Dispersing technology is widely used.
例えば、特許文献３には、結晶粒の粗大化防止のためにN量を増加させて析出Al窒化物量を増加させることが提案されているが、Ｂを活用するにはＮを低減する必要があり、Ｂ添加鋼では適用できない手法である。
また、特許文献４では、Al, Nb, Ti等の窒化物、炭化物、炭窒化物形成元素の含有量と、各析出物の大きさ、分布密度、ベイナイト組織分率、フェライトバンド評点および圧延条件とを制御することによって、上記した問題の解決を図ることが提案されているが、種々の寸法形状の鋼材を圧延により製造する実操業では、これら多数のパラメーターを制御することは事実上不可能であった。
For example,
In Patent Document 4, the content of nitride, carbide, carbonitride forming elements such as Al, Nb, Ti, etc., the size, distribution density, bainite structure fraction, ferrite band score, and rolling conditions of each precipitate Although it has been proposed to solve the above-mentioned problems by controlling the above, it is practically impossible to control these many parameters in the actual operation of manufacturing steel materials of various dimensions and shapes by rolling. Met.
本発明は、上記の実情に鑑み開発されたものであり、冷間鍛造を行っても良好な鍛造性を示すと共に、浸炭処理のための加熱による結晶粒の粗大化を効果的に抑制することのできる肌焼鋼を、その有利な製造方法に併せて提案することを目的とする。 The present invention has been developed in view of the above circumstances, and exhibits good forgeability even when cold forging is performed, and effectively suppresses coarsening of crystal grains due to heating for carburizing treatment. The purpose of this invention is to propose a case-hardening steel that can be manufactured together with its advantageous manufacturing method.
発明者らは、上記の目的を達成すべく、鋼組成や析出物の分布形態などについて鋭意研究を重ねた結果、鋼の成分組成を特定した上で、フェライトとパーライトの組織分率が80％以上の組織とし、さらにNbを含む直径50nm未満の析出物数とNbを含む直径50nm以上の析出物数とを所定の関係の下に規制することによって、優れた冷間鍛造性および結晶粒粗大化抑制能を兼ね備えた肌焼鋼が得られることを見出した。本発明は、上記の知見に立脚するものである。 In order to achieve the above object, the inventors have conducted extensive research on the steel composition and the distribution form of precipitates, and as a result, after specifying the component composition of the steel, the structure fraction of ferrite and pearlite is 80%. By controlling the number of precipitates with a diameter of less than 50 nm containing Nb and the number of precipitates with a diameter of 50 nm or more containing Nb under a predetermined relationship, excellent cold forgeability and grain coarseness It has been found that a case-hardened steel having an ability to suppress crystallization is obtained. The present invention is based on the above findings.
すなわち、本発明の要旨構成は、以下のとおりである。
（１）Ｃ：0.10〜0.35質量％、
Si：0.01〜0.50質量％、
Mn：0.30〜1.50質量％、
Ｐ：0.02質量％以下、
Ｓ：0.03質量％以下、
Al：0.04〜0.10質量％、
Cr：0.5〜2.5質量％、
Ｂ：0.0005〜0.0050質量％、
Nb：0.015〜0.10質量％、
Ti：0.003質量％以下、
Mo：0.01質量％以下および
Ｎ：0.0080質量％未満
を含有し、残部はFe及び不可避不純物からなる成分組成を有し、フェライトおよびパーライトの分率が80％以上の組織を有し、Nbを含む直径50nm未満の析出物が30個/μm2以上および、Nbを含む直径50nm以上100nm以下の析出物が３個/μm2以下であり、かつNbを含む直径50nm未満の析出物数ｎAおよびNbを含む直径50nm以上100nm以下の析出物数ｎBが、
ｎA−５ｎB＞30
の関係を満たすことを特徴とする冷間鍛造性および結晶粒粗大化抑制能に優れた肌焼鋼。
That is, the gist configuration of the present invention is as follows.
(1) C: 0.10 to 0.35 mass%,
Si: 0.01 to 0.50 mass%,
Mn: 0.30-1.50 mass%,
P: 0.02 mass% or less,
S: 0.03 mass% or less,
Al: 0.04 to 0.10% by mass,
Cr: 0.5 to 2.5% by mass,
B: 0.0005-0.0050 mass%,
Nb: 0.015-0.10 mass%,
Ti: 0.003 mass% or less,
Mo: 0.01% by mass or less and N: less than 0.0080% by mass, the balance having a component composition consisting of Fe and inevitable impurities, having a structure in which the fraction of ferrite and pearlite is 80% or more, and containing Nb The number of precipitates having a diameter of less than 50 nm is 30 / μm 2 or more, and the number of precipitates having a diameter of 50 nm or more and 100 nm or less containing Nb is 3 / μm 2 or less, and the number n A of precipitates having a diameter of less than 50 nm containing Nb and The number n B of Nb-containing precipitates having a diameter of 50 nm to 100 nm is
n A -5n B > 30
A case-hardened steel excellent in cold forgeability and crystal grain coarsening suppression ability, characterized by satisfying the above relationship.
（２）前記成分組成は、更に、
Cu：1.0質量％以下、
Ni：0.50質量％以下および
Ｖ：0.50質量％以下
のうちから選ばれる１種または２種以上を含有する前記（１）に記載の冷間鍛造性および結晶粒粗大化抑制能に優れた肌焼鋼。
(2) The component composition further includes:
Cu: 1.0 mass% or less,
Skin hardening excellent in cold forgeability and crystal grain coarsening suppression ability as described in (1) above containing one or more selected from Ni: 0.50 mass% or less and V: 0.50 mass% or less steel.
（３）前記組織はフェライトの分率が70％以上である前記（１）または（２）に記載の冷間鍛造性および結晶粒粗大化抑制能に優れた肌焼鋼。 (3) The case-hardened steel excellent in cold forgeability and crystal grain coarsening suppressing ability according to (1) or (2), wherein the structure has a ferrite fraction of 70% or more.
本発明によれば、冷間鍛造性に優れる、しかも浸炭処理のための加熱による結晶粒の粗大化を効果的に抑制した肌焼鋼を提供することができるため、工業上非常に有用である。 According to the present invention, it is possible to provide a case-hardened steel that is excellent in cold forgeability and that effectively suppresses coarsening of crystal grains due to heating for carburizing treatment, which is very useful industrially. .
以下、本発明の肌焼鋼について具体的に説明する。
まず、本発明において、鋼の成分組成を上記の範囲に限定した理由について、成分毎に詳しく説明する。
Ｃ：0.10〜0.35質量％
浸炭熱処理後の焼入れにより中心部の硬度を高めるために、0.10質量％以上のＣを必要とするが、含有量が0.35質量％を超えると、鋼素材芯部の靭性が低下するため、Ｃ量は0.10〜0.35質量％の範囲に限定した。好ましくは、0.3質量％以下の範囲である。
Hereinafter, the case-hardened steel of the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described in detail for each component.
C: 0.10 to 0.35 mass%
In order to increase the hardness of the central part by quenching after carburizing heat treatment, 0.10% by mass or more of C is required, but if the content exceeds 0.35% by mass, the toughness of the steel material core part decreases, so the amount of C Is limited to the range of 0.10 to 0.35 mass%. Preferably, it is 0.3 mass% or less.
Si：0.01〜0.50質量％
Siは、脱酸剤として必要であり、少なくとも0.01質量％以上の添加が必要である。しかしながら、Siは浸炭表層で優先的に酸化し、粒界酸化を促進する元素である。また、フェライトを固溶強化するため、変形抵抗を高めて冷間鍛造性を劣化させることから、上限を0.50質量％とする。好ましくは0.03〜0.35質量％である。
Si: 0.01 to 0.50 mass%
Si is necessary as a deoxidizing agent, and at least 0.01% by mass or more must be added. However, Si is an element that preferentially oxidizes in the carburized surface layer and promotes grain boundary oxidation. Further, in order to strengthen the solid solution of ferrite, the deformation resistance is increased and the cold forgeability is deteriorated, so the upper limit is made 0.50% by mass. Preferably it is 0.03-0.35 mass%.
Mn：0.30〜1.50質量％
Mnは、焼入性の向上に有効な元素であり、少なくとも0.30質量％の添加を必要とする。しかし、Mnは粒界酸化を引き起こしやすく、また過剰な添加は残留オーステナイトを増加させ、表面硬さの低下を招くことから、上限を1.50質量％とした。好ましくは0.60〜1.40質量％の範囲である。
Mn: 0.30-1.50 mass%
Mn is an element effective for improving hardenability, and requires addition of at least 0.30% by mass. However, Mn tends to cause grain boundary oxidation, and excessive addition increases residual austenite, leading to a decrease in surface hardness, so the upper limit was made 1.50% by mass. Preferably it is the range of 0.60-1.40 mass%.
Ｐ：0.02質量％以下
Ｐは、結晶粒界に偏析し、靭性を低下させるため、その混入は低いほど望ましいが、0.02質量％までは許容される。好ましくは、0.018質量％以下である。
P: 0.02% by mass or less P is segregated at the grain boundary and lowers the toughness. Therefore, the lower the content, the better, but 0.02% by mass is acceptable. Preferably, it is 0.018 mass% or less.
Ｓ：0.03質量％以下
Ｓは、硫化物系介在物として存在し、被削性の向上に有効な元素であり、0.005質量％以上で添加することが好ましい。しかしながら、過剰な添加は疲労強度や靱性の低下を招くため、上限を0.03質量％とした。好ましくは、0.025質量％以下である。
S: 0.03 mass% or less S is an element that exists as a sulfide inclusion and is effective for improving machinability, and is preferably added in an amount of 0.005 mass% or more. However, excessive addition causes a decrease in fatigue strength and toughness, so the upper limit was made 0.03% by mass. Preferably, it is 0.025 mass% or less.
Al：0.04〜0.10質量％
Alは、鋼中のＮをAlNとして固定することによって、Ｂの焼入れ性効果を得るための重要な元素である。この効果を得るためには、少なくとも0.04質量％の添加が必要である。しかしながら、含有量が0.10質量％を超えると、疲労強度に対して有害なA1203介在物の生成を助長するため、Al量は0.04〜0.10質量％の範囲に限定した。好ましくは0.045〜0.09質量％である。
Al: 0.04-0.10 mass%
Al is an important element for obtaining the hardenability effect of B by fixing N in steel as AlN. In order to obtain this effect, it is necessary to add at least 0.04% by mass. However, if the content exceeds 0.10 mass%, in order to facilitate the generation of
Cr：0.5〜2.5質量％
Crは、焼入性のみならず、焼戻し軟化抵抗の向上に寄与し、さらには炭化物の球状化促進にも有用な元素であるが、含有量が0.5質量％に満たないと、その添加効果に乏しく、一方、2.5質量％を超えると、浸炭部での残留オーステナイトの生成を促進し、疲労強度に悪影響を与える場合がある。よって、Cr量は0.5〜2.5質量％の範囲に限定した。好ましくは0.6〜2.0質量％の範囲である。
Cr: 0.5-2.5% by mass
Cr is an element that contributes not only to hardenability but also to improving resistance to temper softening, and also useful for promoting the spheroidization of carbides. On the other hand, if it exceeds 2.5% by mass, the formation of retained austenite in the carburized part is promoted, and the fatigue strength may be adversely affected. Therefore, the Cr content is limited to the range of 0.5 to 2.5 mass%. Preferably it is the range of 0.6-2.0 mass%.
Ｂ：0.0005〜0.0050質量％
Ｂは、本発明において最も重要な元素である。Ｂは、焼入れ熱処理時にオーステナイト粒界に偏析することにより焼入れ性を高め、素材の硬度上昇に寄与する。この効果により、他の強化元素を削減でき、その結果、変形抵抗の低下による冷間鍛造性の向上が得られる。この効果を発揮するためには、少なくとも0.0005質量％以上の添加が必要である。一方、過剰な添加は、靭性や鍛造性などの低下を招くことから、上限を0.0050質量％とした。好ましいＢ含有量の上限は、0.0030質量％である。
B: 0.0005 to 0.0050 mass%
B is the most important element in the present invention. B segregates at the austenite grain boundaries during the quenching heat treatment, thereby improving the hardenability and contributing to an increase in the hardness of the material. By this effect, other strengthening elements can be reduced, and as a result, an improvement in cold forgeability due to a decrease in deformation resistance can be obtained. In order to exhibit this effect, it is necessary to add at least 0.0005% by mass or more. On the other hand, excessive addition causes a decrease in toughness, forgeability, etc., so the upper limit was made 0.0050% by mass. A preferable upper limit of the B content is 0.0030% by mass.
Nb：0.015〜0.10質量％
Nbは、鋼中でNbCを形成し、浸炭熱処理時のオーステナイト粒径の粗粒化をピン止め効果により抑制する。この効果を十分に得るためには、少なくとも0.015質量％以上の添加が必要である。一方、0.10質量％を超えて添加すると、粗大なNbCの析出による粗粒化抑制能の低下や疲労強度の劣化を招く、おそれがあるため、0.10質量％以下とする。好ましくは、0.022〜0.060質量％である。
Nb: 0.015-0.10 mass%
Nb forms NbC in the steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment by the pinning effect. In order to obtain this effect sufficiently, it is necessary to add at least 0.015% by mass or more. On the other hand, if added in excess of 0.10% by mass, there is a risk of reducing the coarsening suppression ability and deterioration of fatigue strength due to coarse precipitation of NbC, so the content is made 0.10% by mass or less. Preferably, it is 0.022-0.060 mass%.
Ti：0.003質量％以下
Tiは、鋼中への混入を極力回避することが好ましい成分である。Tiは、Ｎと結合し、粗大なTiNを形成しやすく、かように形成されたTiNは（Nb,Ti）(C,N)として複合析出し、粗粒化抑制に有効な微細NbCを減少させるため、上限を0.003質量％とする。
Ti: 0.003 mass% or less
Ti is a component that preferably avoids mixing into steel as much as possible. Ti binds to N and easily forms coarse TiN. TiN formed in this way is precipitated as (Nb, Ti) (C, N) and reduces fine NbC, which is effective for suppressing coarsening. Therefore, the upper limit is made 0.003% by mass.
Mo：0.01質量％以下
Moは、フェライトを固溶強化し冷間鍛造時の変形抵抗を高めてしまうため、混入を極力回避することが好ましい。そこで、上限を0.01質量％とする。
Mo: 0.01% by mass or less
Since Mo enhances the solid solution strengthening of ferrite and increases the deformation resistance during cold forging, it is preferable to avoid mixing as much as possible. Therefore, the upper limit is set to 0.01% by mass.
Ｎ：0.008質量％未満
Ｎは、鋼中への混入を極力回避することが好ましい成分である。従って、Ｎは、Ｂの焼入れ性を確保することと、TiNの形成を抑制するために、0.008質量％未満とした。好まし6質量％以下である。
N: Less than 0.008% by mass N is a component that preferably avoids mixing into steel as much as possible. Therefore, N is made less than 0.008% by mass in order to secure the hardenability of B and to suppress the formation of TiN. It is preferably 6% by mass or less.
また、本発明では、焼入性を高めるために上記成分に、更に、Cu：1.0質量％以下、Ni：0.50質量％以下およびＶ：0.50質量％以下のうちから選ばれる１種または２種以上を含有することができる。
Cuは、焼き入れ性の向上に有効な元素であり、好ましくは0.1質量％以上で添加するが、多量の添加は鋼材の表面性状の劣化や合金コストの増加を招くため、上限を1.0質量％とした。好ましくは0.6質量％以下である。
Moreover, in this invention, in order to improve hardenability, 1 type or 2 types or more chosen from Cu: 1.0 mass% or less, Ni: 0.50 mass% or less, and V: 0.50 mass% or less are further added. Can be contained.
Cu is an element effective for improving hardenability, and is preferably added in an amount of 0.1% by mass or more. However, addition of a large amount leads to deterioration of the surface properties of steel materials and an increase in alloy costs, so the upper limit is 1.0% by mass. It was. Preferably it is 0.6 mass% or less.
NiおよびＶは、焼入れ性や靭性の向上に有効な元素であり、好ましくはそれぞれ0.1質量％以上および0.02質量％以上であるが、高価であることから上限をそれぞれ0.50質量％とした。 Ni and V are effective elements for improving hardenability and toughness, and are preferably 0.1% by mass or more and 0.02% by mass or more, respectively. However, since they are expensive, the upper limit is set to 0.50% by mass, respectively.
以上、本発明の成分組成範囲について説明したが、本発明は、成分組成を上記の範囲に調整し、さらに鋼組織を制御するとともに、析出物の分布形態を規定することが重要である。具体的には、
（ｉ）鋼組織の80％以上がフェライト＋パーライトの組織であること、
（ii）Nbを含む直径50nm未満の析出物が30個/μm2以上、Nbを含む直径50nm以上100nm以下の析出物が３個/μm2以下であること、そして
（iii）Nbを含む直径50nm未満の析出物数ｎAおよびNbを含む直径50nm以上の析出物数ｎBがｎA−５ｎB＞30の関係を満たすこと、を同時に満足することが肝要である。
As described above, the component composition range of the present invention has been described. However, in the present invention, it is important to adjust the component composition to the above range, further control the steel structure, and define the distribution form of precipitates. In particular,
(I) 80% or more of the steel structure is a ferrite + pearlite structure,
(Ii) Nb-containing precipitates with a diameter of less than 50 nm are 30 pieces / μm 2 or more, Nb-containing precipitates with a diameter of 50 nm to 100 nm are 3 pieces / μm 2 or less, and (iii) Nb-containing diameters it is the diameter 50nm or more precipitates number n B including the number of precipitates n a and Nb less than 50nm satisfy the relationship n a -
ここに、上記した肌焼鋼の組織並びに析出物の形態に関して、浸炭処理時に粗粒化が発生する温度を調査した結果について、図１に示す。すなわち、フェライトおよびパーライトの組織分率、ｎAおよびｎBを種々に変化して肌焼鋼を作製し、後述する実施例における浸炭シュミレーションに従って加熱、冷却し、旧オーステナイト粒径を観察し、該粒径が粒度番号５以上となる加熱温度を粗粒化温度として求めた。得られた粗粒化温度と、フェライト＋パーライトの組織分率、ｎAおよびｎBとの関係に整理して図１に示す。
図１に示すように、上記した条件（ｉ）、（ii）および（iii）を共に満足して初めて粗粒化温度960℃以上とすることができる。
なお、粗粒化温度の評価において960℃を境界としたのは、一般的な浸炭温度が930〜960℃程度であり、結晶粒粗大化抑制能を厳しく評価するためである。
以下に、上記した条件（ｉ）、（ii）および（iii）につき、その限定理由を述べる。
FIG. 1 shows the results of investigating the temperature at which coarsening occurs during the carburizing process with respect to the structure of the case-hardened steel and the form of precipitates. That is, the case fractions of ferrite and pearlite, n A and n B were variously changed to produce a case-hardened steel, heated and cooled according to the carburization simulation in the examples described later, and the prior austenite grain size was observed, The heating temperature at which the particle size was 5 or more was determined as the coarsening temperature. FIG. 1 shows the relationship between the obtained coarse graining temperature and the ferrite + pearlite structure fraction, n A and n B.
As shown in FIG. 1, the coarsening temperature of 960 ° C. or higher can be achieved only when the above conditions (i), (ii), and (iii) are all satisfied.
In the evaluation of the coarsening temperature, 960 ° C. was used as the boundary because the general carburizing temperature is about 930 to 960 ° C., and the crystal grain coarsening suppressing ability is strictly evaluated.
The reasons for limitation will be described below for the above conditions (i), (ii) and (iii).
（ｉ）フェライトおよびパーライトの組織分率が80％以上
冷間鍛造に供する肌焼鋼の組織は、ベイナイトやマルテンサイトを多く含んでいる場合、冷間鍛造時の変形抵抗が増すことや、その後の浸炭時に結晶粒の粗粒化を生じやすくなることから、フェライトおよびパーライトからなる組織を主体とする必要がある。このため、フェライトおよびパーライトの組織分率を80％以上とする。フェライトおよびパーライトの組織分率が100％であってもよい。さらに好ましくは、冷間鍛造時の変形抵抗をより減少させるため、フェライトの組織分率を70％以上とする。
尚、ここでの組織分率は、面積分率である。
(I) The structure fraction of ferrite and pearlite is 80% or more. When the structure of the case-hardened steel subjected to cold forging contains a large amount of bainite and martensite, the deformation resistance during cold forging increases, Therefore, it is necessary to mainly use a structure composed of ferrite and pearlite. For this reason, the structural fraction of ferrite and pearlite is 80% or more. The structure fraction of ferrite and pearlite may be 100%. More preferably, the ferrite structure fraction is 70% or more in order to further reduce the deformation resistance during cold forging.
The tissue fraction here is an area fraction.
（ii）Nbを含む直径50nm未満の析出物が30個/μm2以上、Nbを含む直径50nm以上100nm以下の析出物が３個/μm2以下
Nbを含む析出物は、微細なものほど結晶粒の粗大化を抑制するピンニング効果が強い。しかしながら、ＮとAlが結合したAlNを核として析出するAlN・Nb(C,N)は粗大であり、結晶粒の成長を抑制する効果はない。このような析出物のうち、粗大な析出物は鋼材の加工性を低下させるため、できるだけ微細なNbCを生成させることが好ましい。そこで、
（iii）Nbを含む析出物で直径50nm未満のものの個数ｎAを、単位面積μm2当たり30個以上、Nbを含む析出物で直径50nm以上100nm以下のものの個数ｎBを、単位面積μm2当たり３個以下と規定し、さらにｎA−５ｎB＞30を満たすことが重要である。
(Ii) Nb-containing precipitates with a diameter of less than 50 nm are 30 pieces / μm 2 or more, and Nb-containing precipitates with a diameter of 50 nm to 100 nm are 3 pieces / μm 2 or less.
The finer the precipitate containing Nb, the stronger the pinning effect that suppresses the coarsening of crystal grains. However, AlN · Nb (C, N) that precipitates with AlN bonded with N and Al as a nucleus is coarse and has no effect of suppressing the growth of crystal grains. Among such precipitates, coarse precipitates reduce the workability of the steel material, so it is preferable to generate NbC as fine as possible. there,
(Iii) The number n A of Nb-containing precipitates having a diameter of less than 50 nm is 30 or more per unit area μm 2 and the number n B of Nb-containing precipitates having a diameter of 50 to 100 nm is the unit area μm 2. It is important that the number is 3 or less per unit and that n A -5 n B > 30 is satisfied.
なお、Nbを含み結晶粒の成長抑制に効果の高い析出物はNbCであり、NbCを上記したように微細に分散させるためには、製造工程中、第１段の熱間加工である鋼片圧延時の加熱、第２段の熱間加工である棒鋼圧延時の加熱、同棒鋼の加工完了温度が重要であり、これらの工程における処理条件を以下のとおりにすることが必要になる。 Note that the precipitate containing Nb and highly effective in suppressing the growth of crystal grains is NbC. In order to finely disperse NbC as described above, a steel slab that is the first stage of hot working during the manufacturing process. The heating at the time of rolling, the heating at the time of rolling the steel bar which is the second stage of hot working, and the processing completion temperature of the steel bar are important, and it is necessary to set the processing conditions in these steps as follows.
すなわち、以下に、本発明に従う肌焼鋼の具体的な製造方法について説明する。
本発明に係る肌焼鋼は、まず、1160℃以上の温度に加熱して第１段の熱間加工を施し、次いで1000℃以下に加熱して行う第２段の熱間加工を、900℃以下の温度にて終了し、その後0.1〜1.0℃／sの速度にて冷却する。
より好ましくは、上述した成分組成になる溶鋼から鋳片を作製し、該鋳片を、一旦、1160℃以上の温度に10分以上加熱して1160℃〜850℃の温度範囲にて減面率40％以上の圧延を行い、750℃以上の温度範囲にて圧延を終了（ここまでの圧延工程を鋼片圧延）し、次いで950℃〜800℃の温度範囲に加熱後、950℃〜800℃の温度範囲にて減面率40％以上の圧延を行い、820℃〜720℃の温度以上の温度範囲にて圧延を終了（ここまでの圧延工程を棒鋼圧延）し、その後0.1〜1.0℃／sの速度にて冷却する。
ここで、減面率とは、上記した指定の温度域において１パス以上の圧延を行った際の、累積減面率を意味する。
That is, the specific manufacturing method of the case hardening steel according to this invention is demonstrated below.
The case-hardened steel according to the present invention is first heated to a temperature of 1160 ° C. or higher, subjected to the first stage of hot working, and then heated to 1000 ° C. or lower to perform the second stage of hot working. Finish at the following temperature, and then cool at a rate of 0.1 to 1.0 ° C./s.
More preferably, a slab is produced from molten steel having the above-described component composition, and the slab is once heated to a temperature of 1160 ° C. or higher for 10 minutes or more, and the area reduction rate in a temperature range of 1160 ° C. to 850 ° C. Roll at 40% or more, finish rolling at a temperature range of 750 ° C or higher (the rolling process up to this point is steel slab rolling), then heat to a temperature range of 950 ° C to 800 ° C, then 950 ° C to 800 ° C Is rolled at a temperature reduction rate of 40% or more in the temperature range, and finished at a temperature range of 820 ° C. to 720 ° C. or more (the rolling process up to this point is steel bar rolling), and then 0.1 to 1.0 ° C. / Cool at the speed of s.
Here, the area reduction rate means a cumulative area reduction rate when rolling for one pass or more in the specified temperature range described above.
以下、各処理条件を上記のように限定した理由について説明する。
第１段の熱間加工（鋼片圧延）における加熱温度：1160℃以上
本発明では鋼片圧延に際し、析出物を十分に固溶させ、熱間加工時およびその後の冷却過程で微細に分散させる。その際、加熱温度が1160℃未満では、析出物を十分に固溶させることができない。すると、熱間加工後に粗大な析出物が残存または生成し、浸炭時にγ粒の粗大化を抑制することができなくなる。そのため、鋼片圧延加熱温度は1160℃以上に規定した。
より好ましくは、結晶粒を微細化し、後述の第２段の熱間加工（棒鋼圧延）後のフェライト分率を上げるため、1160℃以上にて10分以上加熱し、1160℃〜850℃の温度範囲にて減面率40％以上の圧延を行い、γ域にて加工することによってフェライトの核生成サイトを増加させた上で、加工組織が残らないように750℃以上の温度範囲にて圧延を終了することである。
Hereinafter, the reason why each processing condition is limited as described above will be described.
Heating temperature in the first stage of hot working (steel slab rolling): 1160 ° C. or higher In the present invention, the precipitate is sufficiently dissolved in the slab rolling, and finely dispersed during the hot working and the subsequent cooling process. . At that time, if the heating temperature is less than 1160 ° C., the precipitate cannot be sufficiently dissolved. Then, coarse precipitates remain or are generated after hot working, and the coarsening of γ grains cannot be suppressed during carburizing. Therefore, the steel slab rolling heating temperature was specified to be 1160 ° C or higher.
More preferably, in order to refine the crystal grains and increase the ferrite fraction after the second stage hot working (rolling of steel bars) described later, heat at 1160 ° C. or more for 10 minutes or more, and a temperature of 1160 ° C. to 850 ° C. Rolling at a reduction rate of 40% or more in the range, increasing the nucleation site of ferrite by processing in the γ region, and rolling at a temperature range of 750 ° C or higher so that no processed structure remains. Is to end.
第２段の熱間加工（棒鋼圧延）における加熱温度：1000℃以下
上記の加熱により固溶したNbCを微細に析出させ、浸炭時の粗粒化抑制効果を向上させるために、1000℃以下の温度範囲に加熱することが肝要である。なお、加熱温度は、後述する仕上げ温度を900℃以下にできる温度以上とすればよい。
また、本発明では、圧延後の組織とNbCの状態を制御するために、熱間加工の仕上げ温度を900℃以下とする。すなわち、仕上げ温度が900℃より高いと、冷間鍛造時の変形抵抗が増すことや、NbCの成長による結晶粒の粗大化等が生じてしまう。フェライトの分率を上げて冷間鍛造時の変形抵抗を減少させるために、第２段の熱間加工（棒鋼圧延）につき、950℃〜800℃の温度範囲に加熱後、950℃〜800℃の温度範囲にて減面率40％以上の圧延を行い、820℃〜720℃の温度範囲にて圧延を終了することが、より好ましい。このことにより、組織を微細化して圧延後のフェライト分率を増加させることができる。
Heating temperature in the second stage of hot working (steel rolling): 1000 ° C or less In order to finely precipitate the NbC dissolved by the above heating and improve the effect of suppressing coarsening during carburization, the temperature is 1000 ° C or less. It is important to heat to a temperature range. In addition, what is necessary is just to make heating temperature more than the temperature which can make the finishing temperature mentioned later 900 degrees C or less.
In the present invention, in order to control the structure after rolling and the state of NbC, the finishing temperature for hot working is set to 900 ° C. or lower. That is, when the finishing temperature is higher than 900 ° C., deformation resistance during cold forging increases, and crystal grains become coarse due to NbC growth. In order to increase the ferrite fraction and reduce deformation resistance during cold forging, the second stage of hot working (steel rolling) is heated to a temperature range of 950 ° C to 800 ° C and then 950 ° C to 800 ° C. It is more preferable to perform rolling with a reduction in area of 40% or more in this temperature range and finish the rolling in the temperature range of 820 ° C to 720 ° C. Thereby, the structure can be refined and the ferrite fraction after rolling can be increased.
冷却速度：0.1〜1.0℃／s
鋼組織は、熱間加工（圧延）終了後の冷却速度に依存して変化する。冷却速度が大き過ぎる場合に生成される、ベイナイトやマルテンサイトといった組織は、冷間加工（鍛造）時の変形抵抗を著しく増大させ、また、これらの組織が鋼中に多く存在すると浸炭時に結晶粒を粗大化させてしまう。このため、棒鋼圧延終了後の冷却速度は0.1〜1.0℃／sとする。
Cooling rate: 0.1-1.0 ° C / s
The steel structure changes depending on the cooling rate after the end of hot working (rolling). Microstructures such as bainite and martensite, which are generated when the cooling rate is too high, remarkably increase the deformation resistance during cold working (forging). Will be coarsened. For this reason, the cooling rate after completion | finish of steel bar rolling shall be 0.1-1.0 degrees C / s.
以下、実施例に従って、本発明の構成および作用効果をより具体的に説明する。しかし、本発明は下記の実施例によって制限を受けるものではなく、本発明の趣旨に適合し得る範囲内にて適宜変更することも可能であり、これらは何れも本発明の技術的範囲に含まれる。 Hereinafter, according to an Example, the structure and effect of this invention are demonstrated more concretely. However, the present invention is not limited by the following examples, and can be appropriately changed within the scope that can meet the gist of the present invention, and these are all included in the technical scope of the present invention. It is.
表１に示す成分組成の鋼を溶製し、これを加熱(鋼片加熱)して170mm角断面の中間素材へ熱間圧延して室温まで空冷し、さらに再加熱(棒鋼圧延時加熱)した後、熱間圧延後空冷して直径50mmの棒鋼とした。一部の鋼種には強制冷却を施した。表２には、鋼片圧延時加熱温度、同1160℃〜850℃における減面率、棒鋼圧延時加熱温度、同950℃〜800℃における減面率、同仕上げ温度、冷却速度、フェライトの組織分率、フェライトおよびパーライトの組織分率、Nbを含有する析出物の数密度および機械的特性を示す。得られた棒鋼について鋼組織を光学顕微鏡にて400倍の倍率で鋼毎に10視野観察しフェライトとパーライトが占める面積分率を組織分率として求めた。その後、冷間鍛造性試験片を作製した。 Steel with the component composition shown in Table 1 was melted, heated (steel slab heating), hot rolled to an intermediate material with a 170 mm square cross section, air cooled to room temperature, and reheated (heated during bar rolling). Thereafter, it was hot-rolled and air-cooled to obtain a steel bar having a diameter of 50 mm. Some steel grades were subjected to forced cooling. Table 2 shows the heating temperature at the time of steel slab rolling, the area reduction rate at 1160 ° C to 850 ° C, the heating temperature at the time of steel bar rolling, the area reduction rate at 950 ° C to 800 ° C, the finishing temperature, the cooling rate, and the ferrite structure The fraction, the structure fraction of ferrite and pearlite, the number density of precipitates containing Nb and the mechanical properties are shown. With respect to the obtained steel bar, the steel structure was observed with an optical microscope at a magnification of 400 times for 10 fields per steel, and the area fraction occupied by ferrite and pearlite was determined as the structure fraction. Then, the cold forgeability test piece was produced.
ここで、冷間鍛造性は、限界据え込み率および変形抵抗の２項目にて評価した。 すなわち、棒鋼の表面から直径の1／4の深さ位置（1／4Ｄ位置）から、直径：10mmおよび高さ：15mmの試験片を採取し、300ｔプレス機を用いて、60％据え込み時の圧縮荷重を測定し、日本塑性加工学会が提唱している端面拘束圧縮による変形抵抗測定方法を用いて変形抵抗を求めた。 Here, the cold forgeability was evaluated by two items, the limit upsetting rate and the deformation resistance. In other words, a specimen with a diameter of 10 mm and a height of 15 mm was taken from a depth position (1 / 4D position) of 1/4 of the diameter from the surface of the steel bar, and installed at 60% using a 300-ton press. The compressive load was measured, and the deformation resistance was determined using the deformation resistance measurement method proposed by the Japan Society for Technology of Plasticity by end face constrained compression.
限界据え込み率は、変形抵抗を測定した方法で圧縮加工を行い、端部に割れが入ったときの据え込み率を限界据え込み率とした。変形抵抗値が899MPa以下および限界据え込み率が74％以上であれば、冷間鍛造性は良好であるといえる。 The limit upsetting rate was defined as the upsetting rate when compression processing was performed by a method of measuring deformation resistance and a crack occurred at the end. If the deformation resistance value is 899 MPa or less and the limit upsetting rate is 74% or more, it can be said that the cold forgeability is good.
次に、上記棒鋼から、８mmφ×12mmの据え込み試験片を作製し、圧下率70％の据え込み加工を行った後、浸炭シミュレーションを行った。浸炭シミュレーションの条件は、930〜980℃の種々の温度に３時間加熱後、水冷する条件である。その後、試験片を、軸を含む面で軸方向に切断後、切断面を研磨・エッチングし、旧オーステナイト粒径を観察して粗粒が発生する温度（粗粒化温度）を求めた。浸炭処理は通常、900〜970℃の温度域で行われるため、粗粒化温度が960℃未満のものは、結晶粒の粗大化抑止効果に劣ると判定した。なお、粒度番号５より大きい粗粒があった場合に、粗大化したと判定した。 Next, an upsetting test piece of 8 mmφ × 12 mm was prepared from the steel bar, and after upsetting with a rolling reduction of 70%, carburization simulation was performed. The conditions for the carburizing simulation are conditions of heating to various temperatures of 930 to 980 ° C. for 3 hours and then cooling with water. Thereafter, the test piece was cut in the axial direction along the plane including the shaft, and then the cut surface was polished and etched, and the temperature at which coarse grains were generated (the coarsening temperature) was determined by observing the prior austenite grain size. Since the carburizing process is normally performed in a temperature range of 900 to 970 ° C, it was determined that the coarsening temperature of less than 960 ° C was inferior in the effect of suppressing the coarsening of crystal grains. In addition, when there was a coarse particle larger than the particle size number 5, it was determined that the particle was coarsened.
さらに、上記棒鋼について、圧延方向と平行に、10mm角×55mm長さ、10Ｒ２mmＵノッチ衝撃試験片を作製した。この試験片について930℃、７時間、カーボンポテンシャル1.1質量％の条件で浸炭を実施後、60℃で油冷し、170℃、２時間の焼戻し処理を施した後、JISZ 2242で規定されるシャルピー衝撃試験を試験回数６回にて行い、６回中における最小の吸収エネルギーを評価した。 Further, a 10 mm square x 55 mm long, 10R2 mm U notch impact test piece was produced in parallel with the rolling direction for the above bar steel. This test piece was carburized at 930 ° C for 7 hours under a carbon potential of 1.1% by mass, then oil-cooled at 60 ° C, tempered at 170 ° C for 2 hours, and then Charpy as defined in JISZ 2242. The impact test was conducted 6 times, and the minimum absorbed energy in 6 times was evaluated.
また、回転曲げ疲労試験は、930℃、７時間、カーボンポテンシャル1.1質量％の条件で浸炭したサンプルを用い回転数3500rpmで実施し、107回の耐疲労強度にて評価した。 Further, the rotating bending fatigue test was carried out at a rotation speed of 3500 rpm using a sample carburized under conditions of 930 ° C., 7 hours, carbon potential of 1.1 mass%, and evaluated by fatigue resistance strength of 10 7 times.
ここで、Ｎｂ析出物の観察は、圧延したままの棒鋼からサンプル採取し、透過型電子顕微鏡（ＴＥＭ）およびエネルギー分散型Ｘ線分析（ＥＤＸ）によって行った。直径：50nm未満のNb含有析出物の数密度は、抽出レプリカ法により試料を作製し、10万倍の倍率で、鋼毎に20視野観察し、EDXにてNb含有析出物と検出されたものについて画像処理により円相当径ならびにその密度を算出することで求めた。この際、直径が５nm未満の析出物は正確に計測するのが困難なため、直径５〜50nmの析出物を計測した。
また、直径：50〜100nmのNb析出物の数密度は抽出レプリカ法により試料を作製し、10万倍の倍率で、鋼毎に20視野観察し、EDXにてNb含有析出物と検出されたものについて画像処理により円相当径ならびにその密度を算出することで求めた。
Here, the Nb precipitates were observed from a rolled steel bar and sampled by a transmission electron microscope (TEM) and energy dispersive X-ray analysis (EDX). Diameter: The number density of Nb-containing precipitates with a diameter of less than 50 nm was prepared by the extraction replica method, observed at 20 magnifications for each steel at a magnification of 100,000, and images of what were detected as Nb-containing precipitates by EDX. The circle equivalent diameter and its density were calculated by processing. At this time, since it is difficult to accurately measure a precipitate having a diameter of less than 5 nm, a precipitate having a diameter of 5 to 50 nm was measured.
The number density of Nb precipitates with a diameter of 50 to 100 nm is a sample prepared by the extraction replica method, observed at 20 magnifications per steel at a magnification of 100,000, and detected as Nb-containing precipitates by EDX. The circle equivalent diameter and its density were calculated by image processing.
表２に示したとおり、本発明に従う発明例はいずれも、結晶粒粗大化抑制能についてはいうまでもなく、冷間鍛造性並びにその他の機械的性質も良好であることが分かる。 As shown in Table 2, it can be seen that all of the inventive examples according to the present invention have good cold forgeability and other mechanical properties, not to mention the ability to suppress coarsening of crystal grains.
Claims (3)
Si：0.01〜0.50質量％、
Mn：0.30〜1.50質量％、
Ｐ：0.02質量％以下、
Ｓ：0.03質量％以下、
Al：0.04〜0.10質量％、
Cr：0.5〜2.5質量％、
Ｂ：0.0005〜0.0050質量％、
Nb：0.015〜0.10質量％、
Ti：0.003質量％以下、
Mo：0.01質量％以下および
Ｎ：0.0080質量％未満
を含有し、残部はFe及び不可避不純物からなる成分組成を有し、フェライトおよびパーライトの分率が80％以上の組織を有し、Nbを含む直径50nm未満の析出物が30個/μm2以上および、Nbを含む直径50nm以上100nm以下の析出物が３個/μm2以下であり、かつNbを含む直径50nm未満の析出物数ｎAおよびNbを含む直径50nm以上100nm以下の析出物数ｎBが、
ｎA−５ｎB＞30
の関係を満たすことを特徴とする冷間鍛造性および結晶粒粗大化抑制能に優れた肌焼鋼。 C: 0.10 to 0.35 mass%,
Si: 0.01 to 0.50 mass%,
Mn: 0.30-1.50 mass%,
P: 0.02 mass% or less,
S: 0.03 mass% or less,
Al: 0.04 to 0.10% by mass,
Cr: 0.5 to 2.5% by mass,
B: 0.0005-0.0050 mass%,
Nb: 0.015-0.10 mass%,
Ti: 0.003 mass% or less,
Mo: 0.01% by mass or less and N: less than 0.0080% by mass, the balance having a component composition consisting of Fe and inevitable impurities, having a structure in which the fraction of ferrite and pearlite is 80% or more, and containing Nb The number of precipitates having a diameter of less than 50 nm is 30 / μm 2 or more, and the number of precipitates having a diameter of 50 nm or more and 100 nm or less containing Nb is 3 / μm 2 or less, and the number n A of precipitates having a diameter of less than 50 nm containing Nb and The number n B of Nb-containing precipitates having a diameter of 50 nm to 100 nm is
n A -5n B > 30
A case-hardened steel excellent in cold forgeability and crystal grain coarsening suppression ability, characterized by satisfying the above relationship.
Cu：1.0質量％以下、
Ni：0.50質量％以下および
Ｖ：0.50質量％以下
のうちから選ばれる１種または２種以上を含有する請求項1に記載の冷間鍛造性および結晶粒粗大化抑制能に優れた肌焼鋼。 The component composition further includes:
Cu: 1.0 mass% or less,
The case hardening steel excellent in cold forgeability and crystal grain coarsening suppression ability according to claim 1, comprising one or more selected from Ni: 0.50 mass% or less and V: 0.50 mass% or less .
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