JP6459704B2 - Steel for cold forging parts - Google Patents

Steel for cold forging parts Download PDF

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JP6459704B2
JP6459704B2 JP2015065055A JP2015065055A JP6459704B2 JP 6459704 B2 JP6459704 B2 JP 6459704B2 JP 2015065055 A JP2015065055 A JP 2015065055A JP 2015065055 A JP2015065055 A JP 2015065055A JP 6459704 B2 JP6459704 B2 JP 6459704B2
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steel
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cold forging
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JP2015212414A (en
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徹志 千田
徹志 千田
根石 豊
豊 根石
直樹 松井
直樹 松井
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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本発明は、冷間鍛造部品用鋼に関するものである。   The present invention relates to steel for cold forged parts.

冷間鍛造(転造も含む。)は、製品の表面肌や寸法精度が良く、熱間鍛造に比べて製造コストが低く、歩留まりも良好であるため、ボルト、ギア部品、シャフトをはじめとする多くの分野に適用されている。これらの機械部品等の素材には、従来、JIS G 4051、JIS G 4052、JIS G 4053などの機械構造用炭素鋼鋼材、構造用鋼材、機械構造用合金鋼鋼材が用いられる。冷間鍛造部品用鋼は、例えば、熱間圧延によって製造される棒鋼などである。   Cold forging (including rolling) has better surface texture and dimensional accuracy of the product, lower manufacturing costs than hot forging, and better yield, so bolts, gear parts, shafts, etc. It is applied in many fields. Conventionally, carbon steel materials for mechanical structures such as JIS G 4051, JIS G 4052, and JIS G 4053, structural steel materials, and alloy steel materials for mechanical structures are used as materials for these machine parts and the like. The steel for cold forged parts is, for example, a bar steel manufactured by hot rolling.

従来の中炭素の炭素鋼や合金鋼は、圧延ままの硬度が高く、延性が不足しているため、ボルト等の部品に成形する際に割れを生じたり、冷間鍛造工具が著しく消耗してコスト高となったりする等の問題がある。そのため、冷間鍛造を行う前に、焼鈍や球状化処理を行って冷間鍛造性を確保し、冷間鍛造後に、焼入れ、焼戻しなどの調質熱処理を施して、部品の機械特性を向上させている。これらの熱処理のうち、特に、冷間鍛造前に行う焼鈍や球状化処理については、多大な製造コストを要するため、省略を可能とする素材及びプロセスが求められている。   Conventional medium carbon carbon steel and alloy steel have high hardness as rolled and lack of ductility, so cracking occurs when forming into parts such as bolts, and cold forging tools are significantly consumed. There are problems such as high costs. Therefore, before cold forging, annealing and spheroidizing treatment is performed to ensure cold forgeability, and after cold forging, tempering heat treatment such as quenching and tempering is performed to improve the mechanical properties of the parts. ing. Among these heat treatments, in particular, annealing and spheroidization performed before cold forging require a large production cost, and therefore materials and processes that can be omitted are required.

そこで、焼鈍や球状化処理を省略するために、C、Cr、Mo等の合金元素量を低減して、熱間圧延ままでの硬度の低下及び延性の向上を図り、焼入れ性の低下を微量のBの添加によって補う、いわゆる低炭素ボロン鋼が提案されている(例えば、下記の特許文献1〜3を参照。)。Bが鋼中の固溶Nと結合してBNが生成すると、Bの持つ焼入れ性向上効果が失われてしまうため、かかる低炭素ボロン鋼では、Tiを添加し、TiNを析出させて鋼中のNを固定し、BNの生成を抑制することが必要とされる。   Therefore, in order to omit the annealing and spheroidizing treatment, the amount of alloying elements such as C, Cr, and Mo is reduced, the hardness and ductility are improved as hot rolled, and the hardenability is reduced by a small amount. So-called low-carbon boron steel that is supplemented by the addition of B has been proposed (see, for example, Patent Documents 1 to 3 below). When B is combined with solute N in the steel to produce BN, the effect of improving the hardenability of B is lost. Therefore, in such low carbon boron steel, Ti is added to precipitate TiN in the steel. It is necessary to fix N of N and suppress the generation of BN.

一方、低炭素ボロン鋼は、高強度を得るために低温での焼戻しを行うことが必要になり、耐遅れ破壊特性が低下するという問題がある。低炭素ボロン鋼を高強度部品に適用するため、鋼中の不純物を低減し、合金元素の添加量を制御することによって、冷間鍛造性と耐遅れ破壊特性とを両立させた鋼が提案されている(例えば、下記の特許文献4、5を参照。)。耐遅れ破壊特性は、これらの技術によって、ある程度向上するものの、部品の形状や使用環境によっては遅れ破壊の発生が懸念される。   On the other hand, low carbon boron steel needs to be tempered at a low temperature in order to obtain high strength, and there is a problem that delayed fracture resistance is lowered. In order to apply low-carbon boron steel to high-strength parts, steel with both cold forgeability and delayed fracture resistance has been proposed by reducing impurities in the steel and controlling the amount of alloying elements added. (For example, see Patent Documents 4 and 5 below.) Although the delayed fracture resistance is improved to some extent by these techniques, there is a concern that delayed fracture may occur depending on the shape of the component and the usage environment.

特開平5−339676号公報JP-A-5-339676 特公平5−63524号公報Japanese Examined Patent Publication No. 5-63524 特開昭61−253347号公報JP-A-61-253347 特開平8−60245号公報JP-A-8-60245 特開平11−92868号公報JP-A-11-92868

本発明は、冷間鍛造前の焼鈍や球状化焼鈍の省略が可能であり、冷間鍛造性に優れ、かつ、焼入れ、焼戻しなどの調質熱処理によって高強度化した機械部品等の耐遅れ破壊特性を確保することができる、冷間鍛造部品用鋼を提供するものである。   The present invention can omit the annealing and spheroidizing annealing before cold forging, has excellent cold forging properties, and is resistant to delayed fracture of machine parts and the like that have been strengthened by tempering heat treatment such as quenching and tempering. The present invention provides a steel for cold forged parts that can ensure the characteristics.

本発明者らは、冷間鍛造性を確保し、高強度かつ耐遅れ破壊特性に優れた冷間鍛造部品が得られるように、冷間鍛造部品用鋼の合金元素について検討を行った。その結果、熱間圧延ままでの冷間鍛造性を確保するために析出強化元素の添加を制限した場合、Pの粒界偏析を抑制することが特に重要であるという知見を得た。そして、コスト制約のためにPの含有を許容させるには、鋼中のPの粒界偏析を抑制するNdの添加が有効であることを見出した。   The inventors of the present invention have studied the alloy elements of steel for cold forged parts so as to obtain a cold forged part that ensures cold forgeability and has high strength and excellent delayed fracture resistance. As a result, it has been found that when the addition of precipitation strengthening elements is restricted in order to ensure the cold forgeability in hot rolling, it is particularly important to suppress P grain boundary segregation. And it was found that the addition of Nd that suppresses the grain boundary segregation of P in steel is effective to allow the inclusion of P due to cost constraints.

本発明は、このような知見に基づいてなされたものであり、その要旨とするところは、以下の通りである。   This invention is made | formed based on such knowledge, The place made into the summary is as follows.

[1]質量%で、
C :0.22〜0.40%、
Si:0.01〜0.15%、
Mn:0.1〜1.5%、
Cr:0.1〜1.5%、
Al:0.01〜0.10%、
Ti:0.01〜0.10%、
B :0.0003〜0.0060%、
Nd:0.001〜0.050%、
N :0.0010〜0.010%
を含有し、残部が、鉄及び不純物からなり、
不純物におけるP及びSが、
P :0.015%以下、
S :0.015%以下
である、冷間鍛造部品用鋼。
[2]Mn及びPの含有量[質量%]が、下記(式1)を満たす、上記[1]に記載の冷間鍛造部品用鋼。
Mn×P≦1.5×10−2 ・・・(式1)
[3]質量%で、
Nb:0.10%以下、
V :0.30%以下、
Mo:0.30%以下
のうち、1種又は2種以上を更に含有する、上記[1]又は[2]に記載の冷間鍛造部品用鋼。
[4]質量%で、
Ca:0.005%以下、
Mg:0.005%以下、
Zr:0.005%以下
のうち、1種又は2種以上を更に含有する、上記[1]〜[3]の何れか1つに記載の冷間鍛造部品用鋼。
[5]鋼のビッカース硬さHと、C、Si、Mn、Cr、Ti、Mo、Nbの含有量[質量%]とが、下記(式2)を満たす、上記[1]〜[4]の何れか1つに記載の冷間鍛造部品用鋼。
<273.5C+39.1Si+54.7Mn+30.4Cr+708Ti+136.7Mo+599Nb+20 ・・・ (式2)
なお、前記冷間鍛造部品用鋼が、Mo又はNbの少なくとも何れか一方を含有しない場合、上記式2において、該当する元素の含有量としてゼロを代入する。
[6]内部組織がフェライト分率40%以上のフェライト・パーライト組織である、上記[1]〜[5]の何れか1つに記載の冷間鍛造部品用鋼。
[1] By mass%
C: 0.22 to 0.40%,
Si: 0.01 to 0.15%,
Mn: 0.1 to 1.5%
Cr: 0.1 to 1.5%
Al: 0.01 to 0.10%,
Ti: 0.01-0.10%,
B: 0.0003 to 0.0060%,
Nd: 0.001 to 0.050%
N: 0.0010 to 0.010%
The balance consists of iron and impurities,
P and S in the impurity are
P: 0.015% or less,
S: Steel for cold forging parts that is 0.015% or less.
[2] The steel for cold forged parts as described in [1] above, wherein the content [% by mass] of Mn and P satisfies the following (formula 1).
Mn × P ≦ 1.5 × 10 −2 (Formula 1)
[3] By mass%,
Nb: 0.10% or less,
V: 0.30% or less,
Mo: Steel for cold forged parts as described in [1] or [2] above, further containing one or more of 0.30% or less.
[4] By mass%,
Ca: 0.005% or less,
Mg: 0.005% or less,
Zr: The steel for cold forged parts as described in any one of [1] to [3], further containing one or more of 0.005% or less.
[5] and the Vickers hardness H V of the steel, C, Si, Mn, Cr , Ti, Mo, the content of Nb [wt%] and is, satisfies the following equation (2), the above-mentioned [1] - [4 ] The steel for cold forging components as described in any one of.
H V <273.5C + 39.1Si + 54.7Mn + 30.4Cr + 708Ti + 136.7Mo + 599Nb + 20 (Formula 2)
In addition, when the said steel for cold forging components does not contain at least any one of Mo or Nb, in said Formula 2, zero is substituted as content of an applicable element.
[6] The steel for cold forged parts as described in any one of [1] to [5] above, wherein the internal structure is a ferrite pearlite structure having a ferrite fraction of 40% or more.

以上説明したように本発明によれば、冷間鍛造性を確保し、かつ高強度化させる、焼入れ、焼戻しなどの調質熱処理を施した冷間鍛造部品の耐遅れ破壊特性の低下を抑制し得る、冷間鍛造部品用鋼の提供が可能になり、産業上の貢献が極めて顕著である。   As described above, according to the present invention, the cold forgeability is ensured and the strength is increased, and the deterioration of the delayed fracture resistance of cold forged parts subjected to tempering heat treatment such as quenching and tempering is suppressed. Thus, it is possible to provide steel for cold forging parts, and the industrial contribution is extremely remarkable.

耐遅れ破壊特性の評価に用いた試験片を説明する図である。It is a figure explaining the test piece used for evaluation of delayed fracture resistance.

以下に、本発明の好適な実施の形態について詳細に説明する。   Hereinafter, preferred embodiments of the present invention will be described in detail.

(冷間鍛造部品用鋼について)
本発明の実施形態に係る冷間鍛造部品用鋼は、冷間鍛造によって製造される機械部品等の素材として用いられる。以下では、かかる冷間鍛造部品用鋼について、詳細に説明する。
(About steel for cold forging parts)
The steel for cold forging parts according to the embodiment of the present invention is used as a material for machine parts and the like manufactured by cold forging. Below, this steel for cold forging components is demonstrated in detail.

まず、本発明の実施形態に係る冷間鍛造部品用鋼の化学成分について、説明する。本実施形態に係る冷間鍛造部品用鋼は、質量%で、C:0.22〜0.40%、Si:0.01〜0.15%、Mn:0.1〜1.5%、Cr:0.1〜1.5%、Al:0.01〜0.10%、Ti:0.01〜0.10%、B:0.0003〜0.0060%、Nd:0.001〜0.050%、N:0.0010〜0.010%、を含有し、残部が、鉄及び不純物からなり、不純物におけるP及びSが、P:0.015%以下、S:0.015%以下である。   First, chemical components of the steel for cold forged parts according to the embodiment of the present invention will be described. The steel for cold forged parts according to the present embodiment is mass%, C: 0.22 to 0.40%, Si: 0.01 to 0.15%, Mn: 0.1 to 1.5%, Cr: 0.1-1.5%, Al: 0.01-0.10%, Ti: 0.01-0.10%, B: 0.0003-0.0060%, Nd: 0.001- 0.055%, N: 0.0010 to 0.010%, the balance is made of iron and impurities, and P and S in the impurities are P: 0.015% or less, S: 0.015% It is as follows.

[C:0.22〜0.40%]
Cは、鋼の強度を向上させる元素である。本実施形態では、冷間鍛造部品の強度を高めるため、C量を0.22%以上とする。C量は、好ましくは0.25%以上である。一方、C量が0.40%を超えると、冷間鍛造部品用鋼の冷間鍛造性が低下し、冷間鍛造部品の延性、靱性が低下し、遅れ破壊特性も劣化する傾向があるので、C量は、0.40%以下とする。C量は、好ましくは0.35%以下であり、より好ましくは0.30%以下である。
[C: 0.22 to 0.40%]
C is an element that improves the strength of steel. In this embodiment, in order to increase the strength of the cold forged part, the C amount is set to 0.22% or more. The amount of C is preferably 0.25% or more. On the other hand, if the amount of C exceeds 0.40%, the cold forgeability of the steel for cold forged parts decreases, the ductility and toughness of the cold forged parts decrease, and the delayed fracture characteristics tend to deteriorate. , C amount is 0.40% or less. The amount of C is preferably 0.35% or less, and more preferably 0.30% or less.

[Si:0.01〜0.15%]
Siは、鋼の脱酸に有効であり、また、鋼の強度、焼入れ性及び焼戻し軟化抵抗を向上させる有用な元素である。本実施形態では、かかる効果を得るために、Si量を0.01%以上とする。一方、本発明では、Si量が0.15%を超えると、硬さの上昇によって冷間鍛造性が劣化するため、Si量を0.15%以下とする。Si量は、好ましくは0.10%以下であり、より好ましくは0.07%以下である。
[Si: 0.01 to 0.15%]
Si is effective for deoxidizing steel and is a useful element for improving the strength, hardenability and temper softening resistance of steel. In this embodiment, in order to obtain such an effect, the Si amount is set to 0.01% or more. On the other hand, in the present invention, if the Si amount exceeds 0.15%, the cold forgeability deteriorates due to the increase in hardness, so the Si amount is set to 0.15% or less. The amount of Si is preferably 0.10% or less, and more preferably 0.07% or less.

[Mn:0.1〜1.5%]
Mnは、鋼の脱酸に有効であり、また、鋼の強度及び焼入れ性を向上させる有用な元素である。本実施形態では、かかる効果を得るためにMn量を0.10%以上とする。好ましくはMn量を0.40%以上とする。一方、本発明では、Mn量が1.50%を超えると、硬さの上昇によって冷間鍛造性が劣化するため、上限を1.50%以下とする。好ましくはMn量の上限を0.70%とする。
[Mn: 0.1 to 1.5%]
Mn is effective for deoxidation of steel and is a useful element for improving the strength and hardenability of steel. In the present embodiment, the Mn content is 0.10% or more in order to obtain this effect. Preferably, the amount of Mn is 0.40% or more. On the other hand, in the present invention, if the amount of Mn exceeds 1.50%, the cold forgeability deteriorates due to the increase in hardness, so the upper limit is made 1.50% or less. Preferably, the upper limit of the amount of Mn is 0.70%.

[Cr:0.1〜1.5%]
Crは、鋼の強度、焼入れ性及び焼戻し軟化抵抗を向上させる有用な元素である。本実施形態では、かかる効果を得るためにCr量を0.1%以上とする。Cr量は、好ましくは0.2%以上である。一方、1.5%を超えるCrを添加すると、硬さの上昇によって冷間鍛造性が劣化するため、Cr量は、1.5%以下とする。Cr量は、好ましくは0.9%以下である。
[Cr: 0.1 to 1.5%]
Cr is a useful element that improves the strength, hardenability and temper softening resistance of steel. In the present embodiment, the Cr amount is 0.1% or more in order to obtain such an effect. The amount of Cr is preferably 0.2% or more. On the other hand, when Cr exceeding 1.5% is added, cold forgeability deteriorates due to an increase in hardness, so the Cr content is 1.5% or less. The amount of Cr is preferably 0.9% or less.

[Al:0.01〜0.10%]
Alは、鋼の脱酸に有効であり、微細なAlやAlNを形成する元素であり、結晶粒の粗大化の抑制にも有効である。本実施形態では、かかる効果を得るために、0.01%以上のAlを添加する。一方、0.10%を超えるAlを添加しても効果が飽和するので、Al量は、0.10%以下とする。
[Al: 0.01 to 0.10%]
Al is effective in deoxidizing steel, is an element that forms fine Al 2 O 3 and AlN, and is also effective in suppressing coarsening of crystal grains. In this embodiment, in order to obtain such an effect, 0.01% or more of Al is added. On the other hand, since the effect is saturated even if Al exceeding 0.10% is added, the amount of Al is made 0.10% or less.

[Ti:0.01〜0.10%]
Tiは、鋼中の固溶Nを固定する元素であり、本実施形態では、BNの生成を抑制して、Bによる焼入れ性向上効果を得るために、0.01%以上のTiを添加する。また、Tiは、微細なTi(CN)やTiCを形成し、結晶粒の粗大化の抑制にも有効な元素である。結晶粒が粗大化すると、粒界面積が減少し、粒界へのPの偏析が助長され、冷間鍛造部品の靱性や耐遅れ破壊特性が劣化することがある。冷間鍛造部品の結晶粒の粗大化を抑制するために、0.02%以上のTiを添加することが好ましい。一方、Ti量が0.10%を超えると、硬さの上昇によって冷間鍛造性が劣化するので、Ti量は、0.10%以下とする。Ti量は、好ましくは、0.05%以下である。また、鋼中の固溶Nを固定するためには、質量%で、N量の3.4倍以上のTi量を添加することが好ましい。
[Ti: 0.01 to 0.10%]
Ti is an element that fixes solute N in steel. In the present embodiment, 0.01% or more of Ti is added in order to suppress the formation of BN and obtain the effect of improving the hardenability by B. . Ti is an element that forms fine Ti (CN) or TiC and is effective in suppressing the coarsening of crystal grains. When the crystal grains become coarse, the grain boundary area decreases, and segregation of P to the grain boundary is promoted, and the toughness and delayed fracture resistance of the cold forged part may be deteriorated. In order to suppress the coarsening of the crystal grains of the cold forged part, it is preferable to add 0.02% or more of Ti. On the other hand, if the Ti content exceeds 0.10%, the cold forgeability deteriorates due to the increase in hardness, so the Ti content is 0.10% or less. The amount of Ti is preferably 0.05% or less. Moreover, in order to fix the solid solution N in steel, it is preferable to add Ti amount 3.4% or more of N amount by mass%.

[B:0.0003〜0.0060%]
Bは、微量で鋼の焼入れ性を顕著に向上させる有用な元素である。本実施形態では、冷間鍛造部品の強度を高めるため、0.0003%以上のBを添加する。B量は、好ましくは、0.0010%以上である。一方、B量が0.0060%を超えると効果は飽和するので、B量を0.0060%以下とする。B量は、好ましくは、0.0030%以下である。
[B: 0.0003 to 0.0060%]
B is a useful element that significantly improves the hardenability of the steel in a small amount. In the present embodiment, 0.0003% or more of B is added to increase the strength of the cold forged part. The amount of B is preferably 0.0010% or more. On the other hand, since the effect is saturated when the B amount exceeds 0.0060%, the B amount is set to 0.0060% or less. The amount of B is preferably 0.0030% or less.

[Nd:0.001〜0.050%]
Ndは、本実施形態では最も重要な元素であり、Pを含む化合物を形成し、粒界に偏析するPを減少させる。熱間圧延ままの冷間鍛造部品用鋼の靱性及び冷間鍛造性を向上させるとともに、焼入れ、焼戻し後の冷間鍛造部品の結晶粒界を強化させ、耐遅れ破壊特性を向上させるために、本実施形態では、0.001%以上のNdを添加する。Nd量は、好ましくは0.005%以上である。一方、0.050%を超えてNdを添加すると効果が飽和するため、Nd量は0.050%以下とする。Nd量は、好ましくは0.030%以下である。
[Nd: 0.001 to 0.050%]
Nd is the most important element in the present embodiment, forms a compound containing P, and reduces P segregated at the grain boundaries. In order to improve the toughness and cold forgeability of steel for cold forged parts as hot-rolled, strengthen the grain boundaries of cold forged parts after quenching and tempering, and improve delayed fracture resistance, In this embodiment, 0.001% or more of Nd is added. The amount of Nd is preferably 0.005% or more. On the other hand, if Nd is added in excess of 0.050%, the effect is saturated, so the Nd content is 0.050% or less. The amount of Nd is preferably 0.030% or less.

[N:0.0010〜0.010%]
Nは、AlNを形成することにより結晶粒の粗大化を抑制する効果があるため、0.0010%以上含有させる必要がある。一方、Nは、BNを生成してB添加による焼入れ性の向上効果を損なうため、N量を0.010%以下とする。N量は、好ましくは0.0050%以下である。
[N: 0.0010 to 0.010%]
N has an effect of suppressing the coarsening of crystal grains by forming AlN, so it is necessary to contain 0.0010% or more. On the other hand, N generates BN and impairs the effect of improving the hardenability by adding B, so the N amount is set to 0.010% or less. The amount of N is preferably 0.0050% or less.

本実施形態に係る冷間鍛造部品用鋼の化学組成の残部は、Fe及び不純物からなる。ここで、不純物とは、鋼材を工業的に製造する際に、原料としての鉱石、スクラップ、又は製造環境などから混入されるものであって、本実施形態の冷間鍛造部品用鋼の効果に悪影響を与えないで許容されるものを意味する。   The balance of the chemical composition of the steel for cold forged parts according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from the ore as a raw material, scrap, or the manufacturing environment when the steel material is industrially produced, and the effect of the steel for the cold forged parts of the present embodiment. It means what is acceptable without adverse effects.

[P:0.015%以下]
Pは、不純物として含有する。Pは、靱性を劣化させ、冷間鍛造時の変形抵抗を高める元素であり、含有量が0.015%を超えると冷間鍛造性が劣化するため、P量を0.015%以下に制限する。また、Pは、焼入れ、焼戻し後の冷間鍛造部品の結晶粒界を脆化させ、耐遅れ破壊特性を損なうことがあるので、P量を0.010%以下に制限することが好ましい。
[P: 0.015% or less]
P is contained as an impurity. P is an element that deteriorates toughness and increases deformation resistance during cold forging. If the content exceeds 0.015%, cold forgeability deteriorates, so the P content is limited to 0.015% or less. To do. Moreover, since P may cause the grain boundaries of the cold forged parts after quenching and tempering to become brittle and impair delayed fracture resistance, it is preferable to limit the P content to 0.010% or less.

[S:0.015%以下]
Pは、不純物として含有する。Sは、MnSを形成する元素であり、含有量が0.015%を超えると冷間鍛造時に割れを生じ、冷間鍛造性が劣化するため、S量を0.015%以下に制限する。S量は、好ましくは0.010%以下である。
[S: 0.015% or less]
P is contained as an impurity. S is an element that forms MnS, and if the content exceeds 0.015%, cracking occurs during cold forging and cold forgeability deteriorates, so the S amount is limited to 0.015% or less. The amount of S is preferably 0.010% or less.

本実施形態に係る冷間鍛造部品用鋼は、化学成分として、更に、Nb、V、Moの1種又は2種以上を含有してもよい。   The steel for cold forged parts according to this embodiment may further contain one or more of Nb, V, and Mo as chemical components.

[Nb:0.10%以下]
Nbは、微細な炭化物、窒化物、炭窒化物を形成し、特に、Tiとともに添加すると安定な(Nb、Ti)(CN)を形成し、結晶粒の粗大化を抑制する有用な元素である。かかる効果を得るには、0.003%以上のNbを含有させることが好ましい。Nbの含有量は、より好ましくは0.005%以上である。一方、0.10%を超えてNbを含有させても効果は飽和し、また、硬さの上昇によって冷間鍛造性が劣化することがあるため、Nb量は、0.10%以下とすることが好ましい。Nb量は、より好ましくは、0.030%以下である。
[Nb: 0.10% or less]
Nb is a useful element that forms fine carbides, nitrides, carbonitrides, and in particular, forms stable (Nb, Ti) (CN) when added with Ti and suppresses coarsening of crystal grains. . In order to obtain such an effect, 0.003% or more of Nb is preferably contained. The Nb content is more preferably 0.005% or more. On the other hand, even if Nb is contained in excess of 0.10%, the effect is saturated, and the cold forgeability may deteriorate due to the increase in hardness, so the Nb content is 0.10% or less. It is preferable. The amount of Nb is more preferably 0.030% or less.

[V:0.30%以下]
Vは、微細な炭化物、窒化物を形成し、結晶粒の微細化に有効な元素である。かかる効果を得るには、0.01%以上のVを含有させることが好ましい。V量は、より好ましくは、0.10%以上である。一方、0.30%を超えてVを含有させても効果は飽和し、また、硬さの上昇によって冷間鍛造性が劣化することがあるため、V量は、0.30%以下とすることが好ましい。V量は、より好ましくは、0.20%以下である。
[V: 0.30% or less]
V is an element that forms fine carbides and nitrides and is effective in making crystal grains finer. In order to obtain such an effect, it is preferable to contain 0.01% or more of V. The amount of V is more preferably 0.10% or more. On the other hand, even if V exceeds 0.30%, the effect is saturated, and the cold forgeability may deteriorate due to the increase in hardness, so the V amount is 0.30% or less. It is preferable. The amount of V is more preferably 0.20% or less.

[Mo:0.30%以下]
Moは、鋼の強度及び焼入れ性を向上させる元素である。特に、MoはBの焼入れ性向上効果を非常に高める効果を有しており、かかる効果を得るには、0.01%以上のMoを含有させることが好ましい。一方、0.30%を超えてMoを含有させると、硬さの上昇によって冷間鍛造性が劣化することがあるため、Mo量は、0.30%以下とすることが好ましい。Mo量は、より好ましくは、0.15%以下である。
[Mo: 0.30% or less]
Mo is an element that improves the strength and hardenability of steel. In particular, Mo has an effect of greatly enhancing the hardenability improvement effect of B. In order to obtain such an effect, it is preferable to contain 0.01% or more of Mo. On the other hand, if the Mo content exceeds 0.30%, the cold forgeability may deteriorate due to the increase in hardness, so the Mo amount is preferably 0.30% or less. The amount of Mo is more preferably 0.15% or less.

本実施形態に係る冷間鍛造部品用鋼は、化学成分として、更に、Ca、Mg、Zrの1種又は2種以上を含有してもよい。   The steel for cold forged parts according to the present embodiment may further contain one or more of Ca, Mg, and Zr as chemical components.

[Ca:0.005%以下]
Caは脱酸元素であるほか、硫化物の形態制御に有効な元素である。MnSの圧延方向への伸長化を防止する形態制御の効果があり、加工性や靭性の劣化を改善する。かかる効果を得るには、0.0002%以上のCaを含有させることが好ましい。一方、0.005%を超えて含有させると粗大な介在物を生成し、かえって靭性を低下させるため、Caを含有する場合には、Ca量は0.005%以下とする必要がある。
[Ca: 0.005% or less]
In addition to being a deoxidizing element, Ca is an element effective for controlling the form of sulfide. There is an effect of form control that prevents the elongation of MnS in the rolling direction, and the deterioration of workability and toughness is improved. In order to obtain such an effect, 0.0002% or more of Ca is preferably contained. On the other hand, if the content exceeds 0.005%, coarse inclusions are generated and the toughness is lowered. Therefore, when Ca is contained, the Ca content needs to be 0.005% or less.

[Mg:0.005%以下]
MgはCaと同様に硫化物の形態制御に有効な元素である。かかる効果を得るには、0.0002%以上のMgを含有させることが好ましい。一方、0.005%を超えて含有させても効果が飽和するため、Mgを含有する場合には、Mg量は0.005%以下とする必要がある。
[Mg: 0.005% or less]
Mg, like Ca, is an element that is effective in controlling the morphology of sulfides. In order to obtain such an effect, it is preferable to contain 0.0002% or more of Mg. On the other hand, even if the content exceeds 0.005%, the effect is saturated. Therefore, when Mg is contained, the amount of Mg needs to be 0.005% or less.

[Zr:0.005%以下]
ZrはCaと同様に硫化物の形態制御に有効な元素である。かかる効果を得るには、0.0002%以上のZrを含有させることが好ましい。一方、0.005%を超えて含有させても効果が飽和するため、Zrを含有する場合には、Zr量は0.005%以下とする必要がある。
[Zr: 0.005% or less]
Zr is an element effective for controlling the morphology of sulfides, like Ca. In order to obtain such an effect, it is preferable to contain 0.0002% or more of Zr. On the other hand, since the effect is saturated even if contained over 0.005%, when Zr is contained, the amount of Zr needs to be 0.005% or less.

以上、本実施形態に係る冷間鍛造部品用鋼の化学成分について、具体的に説明した。   Heretofore, the chemical components of the steel for cold forged parts according to the present embodiment have been specifically described.

ここで、Mnは、Pの偏析を促進する傾向にあり、鋼中のP量だけでなくMn量も考慮することで、Pの偏析を更に抑制することができる。冷間鍛造部品の靱性や耐遅れ破壊特性を向上させるには、Mn及びPの含有量(単位:質量%)が、下記(式1)を満たすことが好ましい。   Here, Mn tends to promote the segregation of P, and the segregation of P can be further suppressed by considering not only the amount of P in the steel but also the amount of Mn. In order to improve the toughness and delayed fracture resistance of the cold forged parts, it is preferable that the contents of Mn and P (unit: mass%) satisfy the following (formula 1).

Mn×P ≦ 1.5×10−2 ・・・(式1) Mn × P ≦ 1.5 × 10 −2 (Formula 1)

冷間鍛造部品の耐遅れ破壊特性を更に向上させるには、Pの偏析を抑制することが好ましく、Mn×Pの値が小さいほど好ましい。本発明者らの検討の結果、冷間鍛造部品の耐遅れ破壊特性を向上させるには、下記(式3)を満たすことがより好ましいことがわかった。   In order to further improve the delayed fracture resistance of the cold forged parts, it is preferable to suppress the segregation of P, and the smaller the value of Mn × P, the better. As a result of the study by the present inventors, it was found that it is more preferable to satisfy the following (Equation 3) in order to improve the delayed fracture resistance of the cold forged part.

Mn×P ≦ 1.0×10−2 ・・・(式3) Mn × P ≦ 1.0 × 10 −2 (Formula 3)

また、熱間圧延ままの冷間鍛造部品用鋼の硬さが上昇すると、冷間鍛造性が劣化する。そこで、硬さを上昇させるC、Si、Mn、Cr、Ti、及び、Moの含有量(単位:質量%)と、ビッカース硬さHと、が、下記(式2)を満たすと、冷間鍛造性がより良好になる。 Moreover, when the hardness of the steel for cold forged parts as hot-rolled rises, cold forgeability will deteriorate. Therefore, C to increase the hardness, Si, Mn, Cr, Ti and the content of Mo: (unit weight%), and Vickers hardness H V, but satisfies the following equation (2), cold The forgeability becomes better.

<273.5C+39.1Si+54.7Mn+30.4Cr
+708Ti+136.7Mo+599Nb+20 ・・・(式2)
H V <273.5C + 39.1Si + 54.7Mn + 30.4Cr
+ 708Ti + 136.7Mo + 599Nb + 20 (Formula 2)

ここで、本実施形態に係る冷間鍛造用部品鋼が、Mo又はNbの少なくとも何れか一方を含有しない場合、上記式2において、該当する元素の含有量としてゼロを代入すればよい。   Here, when the component steel for cold forging according to the present embodiment does not contain at least one of Mo and Nb, zero may be substituted as the content of the corresponding element in the above formula 2.

また、上記式2に適用される熱間圧延ままの冷間鍛造部品用鋼のビッカース硬さHは、JIS Z 2244に準拠した方法を用いて測定すればよい。 Further, Vickers hardness H V of the hot-rolled cold forging steel which is applied to the equation 2 may be measured using a method based on JIS Z 2244.

圧延鋼材の内部組織は、優れた冷間鍛造性を得るために、フェライト分率が40%以上であるフェライト・パーライト組織であることが好ましい。ここで、フェライトとは、パーライト組織に含まれるラメラセメンタイト間のフェライト相は含まれない。フェライト・パーライト組織とは、フェライトとパーライトの混合組織が面積率で全体の95%以上の組織であることを意味する。残部は、ベイナイト又はマルテンサイトである。   The internal structure of the rolled steel material is preferably a ferrite pearlite structure having a ferrite fraction of 40% or more in order to obtain excellent cold forgeability. Here, the ferrite does not include a ferrite phase between lamellar cementites contained in the pearlite structure. The ferrite-pearlite structure means that the mixed structure of ferrite and pearlite is a structure with an area ratio of 95% or more of the whole. The balance is bainite or martensite.

(冷間鍛造部品用鋼の製造方法について)
次に、本実施形態に係る冷間鍛造部品用鋼に用いられる鋼材の製造方法について説明する。
本実施形態に係る鋼の製造方法では、転炉、電気炉等の通常の方法によって鋼を溶製し、成分調整を行い、鋳造工程、必要に応じて分塊圧延工程を経て、圧延素材とする。得られた圧延素材を常法で熱間圧延することで、冷間鍛造用鋼を製造することができる。以下は、本発明の冷間鍛造用鋼の好ましい製造条件である。
(About the manufacturing method of steel for cold forging parts)
Next, the manufacturing method of the steel materials used for the steel for cold forging components which concerns on this embodiment is demonstrated.
In the steel manufacturing method according to the present embodiment, the steel is melted by a normal method such as a converter or an electric furnace, the components are adjusted, the casting process, and if necessary, the bulk rolling process, To do. The steel for cold forging can be manufactured by hot rolling the obtained rolling material by a conventional method. The following are preferred production conditions for the cold forging steel of the present invention.

分塊圧延工程の前に、鋳片を1200〜1350℃程度の温度に数時間保定する均熱拡散処理を行うと、偏析が軽減され、実部品での遅れ破壊特性が更に向上する。また、均熱拡散処理を行うと、鋳造工程で生じた粗大な析出物が鋼中に固溶し、次工程以降では、生成する析出物が微細になり、冷間鍛造部品用鋼及び冷間鍛造部品の特性が更に向上する。   If a soaking diffusion treatment is performed in which the slab is held at a temperature of about 1200 to 1350 ° C. for several hours before the batch rolling process, segregation is reduced and delayed fracture characteristics in actual parts are further improved. In addition, when soaking and diffusion treatment is performed, coarse precipitates generated in the casting process are dissolved in the steel, and in the subsequent steps, the generated precipitates become fine, and the steel for cold forging parts and cold The characteristics of forged parts are further improved.

また、熱間圧延を行う際には、圧延素材を1050℃以上の温度に加熱することが好ましい。加熱温度を1050℃以上にすると、TiC、Ti(CN)などの合金炭化物が鋼中に固溶し、熱間圧延後にTiC、Ti(CN)などの微細な粒子を析出させることができる。合金炭化物を固溶させる観点からは、加熱温度はできるだけ高温にすることが望ましい。加熱温度の好適範囲は、1150℃以上である。   Moreover, when performing hot rolling, it is preferable to heat a rolling raw material to the temperature of 1050 degreeC or more. When the heating temperature is 1050 ° C. or higher, alloy carbides such as TiC and Ti (CN) are dissolved in the steel, and fine particles such as TiC and Ti (CN) can be precipitated after hot rolling. From the viewpoint of dissolving the alloy carbide, it is desirable that the heating temperature be as high as possible. A preferable range of the heating temperature is 1150 ° C. or higher.

次に、加熱した圧延素材を、線材又は棒鋼の形状に熱間圧延する。熱間圧延の仕上温度は、フェライト変態が開始する温度以上、900℃以下とすることが好ましい。熱間圧延後は、600℃以下の温度まで、2℃/s以下の冷却速度で徐冷することが好ましい。冷却速度が大きいと圧延材の硬さが上昇し、冷間鍛造性が劣化することがあるため、冷却速度は遅い方が望ましく、冷却速度の好適範囲は、1℃/s以下である。また、熱間圧延後、低い温度域、例えば500℃以下まで、2℃/s以下の冷却速度で徐冷すると、冷間鍛造部品用鋼が軟質化し、冷間鍛造性を更に向上させることができる。   Next, the heated rolling material is hot-rolled into the shape of a wire or a steel bar. The finishing temperature of the hot rolling is preferably not less than the temperature at which ferrite transformation starts and not more than 900 ° C. After hot rolling, it is preferable to gradually cool to a temperature of 600 ° C. or lower at a cooling rate of 2 ° C./s or lower. When the cooling rate is large, the hardness of the rolled material is increased and the cold forgeability may be deteriorated. Therefore, it is desirable that the cooling rate is low, and the preferable range of the cooling rate is 1 ° C./s or less. In addition, after hot rolling, if the steel is gradually cooled at a cooling rate of 2 ° C./s or less to a low temperature range, for example, 500 ° C. or less, the steel for cold forged parts becomes soft and further improves the cold forgeability. it can.

以上、本実施形態に係る冷間鍛造部品用鋼の製造方法について説明した。   In the above, the manufacturing method of the steel for cold forging components which concerns on this embodiment was demonstrated.

以下に、実施例を示しながら、本発明に係る冷間鍛造部品用鋼について、具体的に説明する。なお、以下に示す実施例は、あくまでも本発明に係る冷間鍛造部品用鋼の一例にすぎず、本発明に係る冷間鍛造部品用鋼が下記の例に限定されるものではない。   Hereinafter, the steel for cold forged parts according to the present invention will be specifically described with reference to examples. In addition, the Example shown below is only an example of the steel for cold forging components which concerns on this invention to the last, and the steel for cold forging components which concerns on this invention is not limited to the following example.

本実施例では、表1に示す成分組成を有する鋼を溶製した。ここで、表1中の下線は、本発明の成分組成から外れていることを示す。また、表中のP及びSは、不純物として検出されたP及びSの含有量を示し、意図的に添加したものではない。   In this example, steel having the component composition shown in Table 1 was melted. Here, the underline in Table 1 indicates that the composition is out of the composition of the present invention. P and S in the table indicate the contents of P and S detected as impurities, and are not intentionally added.




溶製した鋼を表2に示した条件で熱間圧延して、φ20mmの丸棒(冷間鍛造用鋼)を製造し、その後に表2に示した条件で焼入れ、焼戻しを行った。焼入れ、焼戻しを行う前の、熱間圧延ままの冷間鍛造用鋼の硬さと冷間鍛造性は、得られた丸棒を用いて評価し、引張強さ及び耐遅れ破壊特性は、丸棒に焼入れ、焼戻しを施して評価した。   The molten steel was hot-rolled under the conditions shown in Table 2 to produce a φ20 mm round bar (steel for cold forging), and thereafter quenched and tempered under the conditions shown in Table 2. Before quenching and tempering, the hardness and cold forgeability of hot forged cold forging steel were evaluated using the obtained round bar, and the tensile strength and delayed fracture resistance were The steel was quenched and tempered for evaluation.

冷間鍛造用鋼の熱間圧延ままのビッカース硬さHは、JIS Z 2244に準拠して測定した。測定箇所は、φ20mmの丸棒のL断面(圧延方向に平行な断面)が現れるように切断、研磨し、直径の1/4の位置にて、H10(98.07N)を測定した。 Vickers hardness H V of as-hot-rolled cold forging steel was measured in accordance with JIS Z 2244. Measurement points, L section (parallel to the rolling direction cross-section) cut into appears a round bar of 20 mm in diameter, polished at 1/4 position of the diameter was measured H V 10 (98.07N).

冷間鍛造用鋼は、熱間圧延ままで組織観察を実施した。表2に示した熱間圧延後のフェライト分率は、鏡面研磨後、ナイタール液でエッチングを行い、光学顕微鏡で上記の硬さ測定位置に相当する領域の5視野を500倍で観察して写真を撮影し、それを画像解析して求めた。なお、いずれの鋼も、フェライトとパーライトの混合組織の面積率は、95%以上であった。   The structure of the steel for cold forging was observed while being hot rolled. Ferrite fraction after hot rolling shown in Table 2 is mirror-polished, etched with a nital solution, and observed with an optical microscope at 5 times the area corresponding to the above hardness measurement position at a magnification of 500. Was obtained by image analysis. In all the steels, the area ratio of the mixed structure of ferrite and pearlite was 95% or more.

冷間鍛造性は、冷間据込み試験で評価した。冷間据込み試験は、φ20mmの丸棒(焼入れ前)から直径14mm、高さ21mmで切欠き付き(深さ:0.8mm、角度:30°、R:0.12mm)の試験片を採取して行った。端面を拘束し、冷間で圧縮率が65%及び70%の圧縮試験を行った。表2の冷間鍛造性の欄には、き裂が発生した水準を記載しており、いずれもき裂が発生しなかった場合は、「>70」を示した。   Cold forgeability was evaluated by a cold upsetting test. In the cold upsetting test, a test piece with a diameter of 14 mm, a height of 21 mm, and a notch (depth: 0.8 mm, angle: 30 °, R: 0.12 mm) is taken from a round bar of φ20 mm (before quenching). I went there. The end face was constrained, and a compression test was performed at a compression rate of 65% and 70% in the cold. In the column of cold forgeability in Table 2, the level at which cracks occurred is described, and when no cracks occurred, “> 70” was indicated.

焼入れ、焼戻し後の引張強さは、引張試験で評価し、耐遅れ破壊特性は、水素チャージを行いながら、定荷重を負荷した際に破断しない最大の負荷応力と引張強さとの比(遅れ破壊強度比)で評価した。水素チャージは、pH3.0の希硫酸中で、電流密度1.0mA/cmの陰極電解条件で行った。引張試験は、JIS Z 2241に準拠して、径が6mmの2号の丸棒引張試験片を採取して、実施した。また、遅れ破壊試験特性は、図1に示す環状切欠き付き遅れ破壊試験片を用いて評価した。 Tensile strength after quenching and tempering is evaluated by a tensile test. Delayed fracture resistance is the ratio between the maximum load stress that does not break when a constant load is applied with hydrogen charging and the tensile strength (delayed fracture (Strength ratio). The hydrogen charge was performed in dilute sulfuric acid having a pH of 3.0 under cathodic electrolysis conditions with a current density of 1.0 mA / cm 2 . In accordance with JIS Z 2241, the tensile test was carried out by collecting a No. 2 round bar tensile test piece having a diameter of 6 mm. The delayed fracture test characteristics were evaluated using a delayed fracture test piece with an annular notch shown in FIG.

得られた結果を、表2に示す。   The results obtained are shown in Table 2.

No.1〜41は、65%以下の圧縮では割れが発生せず、引張強さは1200MPa以上であり、遅れ破壊強度比は0.50以上であり、冷間鍛造性、引張強さ及び耐遅れ破壊特性の全てが優れている。また、(式1)を満足すると耐遅れ破壊特性がより良好になり、(式2)を満足する、又は内部組織のフェライト・パーライト組織が40%以上だと冷間鍛造性がより向上している。一方、No.42〜54は、成分の何れか本発明の範囲外の比較例であり、冷間鍛造性、引張強さ及び耐遅れ破壊特性の何れかが劣っている。   No. No. 1-41, no cracking occurs at compression of 65% or less, tensile strength is 1200 MPa or more, delayed fracture strength ratio is 0.50 or more, cold forgeability, tensile strength and delayed fracture resistance All of the properties are excellent. Moreover, when satisfying (Equation 1), the delayed fracture resistance becomes better, and when satisfying (Equation 2) or when the ferrite / pearlite structure of the internal structure is 40% or more, the cold forgeability is further improved. Yes. On the other hand, no. Nos. 42 to 54 are comparative examples outside the scope of the present invention, and any of the components is inferior in cold forgeability, tensile strength and delayed fracture resistance.

No.43、44、46、49、51は、それぞれ、C、Si、Mn、Cr、Tiが多く、冷間鍛造性が低下している。例No.42、45,48は、それぞれ、C、Mn、Crが少なく、引張強さが低下している。   No. 43, 44, 46, 49 and 51 are rich in C, Si, Mn, Cr and Ti, respectively, and cold forgeability is lowered. Example No. 42, 45, and 48 have less C, Mn, and Cr, respectively, and the tensile strength is reduced.

No.47は、Pの含有量が多く、Mn%×P%も大きくなり、冷間鍛造性及び耐遅れ破壊特性が低下している。No.50はTi量が少なく、No.52はBを含有していないため、B添加による焼入れ性の向上の効果が不十分になり、引張強さが低下している。   No. No. 47 has a large P content and a large Mn% × P%, and cold forgeability and delayed fracture resistance are degraded. No. No. 50 has a small amount of Ti. Since 52 does not contain B, the effect of improving hardenability by adding B is insufficient, and the tensile strength is reduced.

No.53、54は、Ndが含有されておらず、冷間鍛造性及び耐遅れ破壊特性が低下している。   No. 53 and 54 do not contain Nd, and cold forgeability and delayed fracture resistance are degraded.

以上、本発明の好適な実施形態について詳細に説明したが、本発明はかかる例に限定されない。本発明の属する技術の分野における通常の知識を有する者であれば、特許請求の範囲に記載された技術的思想の範疇内において、各種の変更例または修正例に想到し得ることは明らかであり、これらについても、当然に本発明の技術的範囲に属するものと了解される。
As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to this example. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

Claims (6)

質量%で、
C :0.22〜0.40%、
Si:0.01〜0.15%、
Mn:0.1〜1.5%、
Cr:0.1〜1.5%、
Al:0.01〜0.10%、
Ti:0.01〜0.10%、
B :0.0003〜0.0060%、
Nd:0.001〜0.050%、
N :0.0010〜0.010%
を含有し、残部が、鉄及び不純物からなり、
不純物におけるP及びSが、
P :0.015%以下
S :0.015%以下
である、冷間鍛造部品用鋼。
% By mass
C: 0.22 to 0.40%,
Si: 0.01 to 0.15%,
Mn: 0.1 to 1.5%
Cr: 0.1 to 1.5%
Al: 0.01 to 0.10%,
Ti: 0.01-0.10%,
B: 0.0003 to 0.0060%,
Nd: 0.001 to 0.050%
N: 0.0010 to 0.010%
The balance consists of iron and impurities,
P and S in the impurity are
P: 0.015% or less S: Steel for cold forging parts, which is 0.015% or less.
Mn及びPの含有量[質量%]が、下記(式1)を満たす、請求項1に記載の冷間鍛造部品用鋼。

Mn×P ≦ 1.5×10−2 ・・・(式1)
The steel for cold forged parts according to claim 1, wherein the content [% by mass] of Mn and P satisfies the following (formula 1).

Mn × P ≦ 1.5 × 10 −2 (Formula 1)
質量%で、
Nb:0.10%以下、
V:0.30%以下、
Mo:0.30%以下
のうち、1種又は2種以上を更に含有する、請求項1又は2に記載の冷間鍛造部品用鋼。
% By mass
Nb: 0.10% or less,
V: 0.30% or less,
The steel for cold forged parts according to claim 1 or 2, further comprising one or more of Mo: 0.30% or less.
質量%で、
Ca:0.005%以下、
Mg:0.005%以下、
Zr:0.005%以下
のうち、1種又は2種以上を更に含有する、請求項1〜3の何れか1項に記載の冷間鍛造部品用鋼。
% By mass
Ca: 0.005% or less,
Mg: 0.005% or less,
The steel for cold forged parts according to any one of claims 1 to 3, further comprising one or more of Zr: 0.005% or less.
鋼のビッカース硬さHと、C、Si、Mn、Cr、Ti、Mo、Nbの含有量[質量%]とが、下記(式2)を満たす、請求項1〜4の何れか1項に記載の冷間鍛造部品用鋼。

<273.5C+39.1Si+54.7Mn+30.4Cr
+708Ti+136.7Mo+599Nb+20 ・・・(式2)

なお、前記冷間鍛造部品用鋼が、Mo又はNbの少なくとも何れか一方を含有しない場合、上記式2において、該当する元素の含有量としてゼロを代入する。
A Vickers hardness H V of the steel, C, Si, Mn, Cr , Ti, Mo, the content of Nb [wt%] and is, satisfies the following equation (2), any one of claims 1 to 4 Steel for cold forging parts as described in 1.

H V <273.5C + 39.1Si + 54.7Mn + 30.4Cr
+ 708Ti + 136.7Mo + 599Nb + 20 (Formula 2)

In addition, when the said steel for cold forging components does not contain at least any one of Mo or Nb, in said Formula 2, zero is substituted as content of an applicable element.
内部組織が、フェライト分率40%以上のフェライト・パーライト組織である、請求項1〜5の何れか1項に記載の冷間鍛造部品用鋼。
The steel for cold forged parts according to any one of claims 1 to 5, wherein the internal structure is a ferrite pearlite structure having a ferrite fraction of 40% or more.
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