JP6197761B2 - Manufacturing method of cold processed products - Google Patents

Manufacturing method of cold processed products Download PDF

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JP6197761B2
JP6197761B2 JP2014150923A JP2014150923A JP6197761B2 JP 6197761 B2 JP6197761 B2 JP 6197761B2 JP 2014150923 A JP2014150923 A JP 2014150923A JP 2014150923 A JP2014150923 A JP 2014150923A JP 6197761 B2 JP6197761 B2 JP 6197761B2
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岩本 隆
岩本  隆
祐太 今浪
祐太 今浪
清史 上井
清史 上井
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JFE Steel Corp
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本発明は、自動車などの輸送機械や、建設機械およびその他の産業機械などの構成要素となる機械部品、特に浸炭処理、あるいは浸炭窒化処理して使用される機械構造部品に適した、冷間加工品の製造方法に関するものである。   The present invention is a cold working suitable for machine parts used as components of transport machines such as automobiles, construction machines and other industrial machines, particularly machine structural parts used by carburizing or carbonitriding. The present invention relates to a method for manufacturing a product.

自動車のクランクシャフトや歯車には、優れた疲労特性、耐磨耗性および耐ピッチング性が求められる。そのために、クロム鋼、クロム−モリブデン鋼およびニッケル−クロム−モリブデン鋼等を、熱間鍛造や冷間鍛造に代表される塑性加工および切削加工により所定の形状に加工後、肌焼きと呼ばれる浸炭処理や浸炭窒化処理を施すことによって、製造されるのが通例である。   Automotive crankshafts and gears are required to have excellent fatigue characteristics, wear resistance and pitting resistance. Therefore, after carving chromium steel, chromium-molybdenum steel, nickel-chromium-molybdenum steel, etc. into a predetermined shape by plastic working and cutting represented by hot forging and cold forging, carburizing treatment called case hardening Ordinarily, it is manufactured by performing carbonitriding.

ここで、冷間鍛造は、熱間鍛造に比べて製造コストが低く、製品歩留まりも良好であることに加え、製品の表面性状や寸法精度に優れている。そこで、従来は熱間鍛造で製造されていた部品を、冷間鍛造による製造へ切り替えることが指向されている。その結果、近年では、冷間鍛造後に浸炭や浸炭窒化といった表面硬化処理を施して製造される部品の割合が急増している。   Here, cold forging has a lower manufacturing cost than hot forging, a good product yield, and excellent surface properties and dimensional accuracy of the product. Therefore, it has been directed to switch parts that were conventionally manufactured by hot forging to manufacture by cold forging. As a result, in recent years, the proportion of parts manufactured by performing surface hardening treatment such as carburizing and carbonitriding after cold forging has increased rapidly.

上記の浸炭処理や浸炭窒化処理において、焼入れ時に熱処理歪が生じると、例えば歯車の場合には、騒音や振動の原因となるだけでなく、形状が狂うために、研磨等の修正工程が必要となり、またシャフト形状の部品の場合には曲がりを生じ、これを矯正することが必要となるため、いずれも製造コストが上昇する原因となる。
上記熱処理歪の発生は、特に高温で処理される浸炭部品で顕著である。この熱処理歪は、熱処理中にγ粒が局所的に粗大化し、焼入れ性が不安定となるため、マルテンサイト変態時の膨張による応力不均一に起因して生じるとされている。
In the above carburizing and carbonitriding processes, if heat treatment distortion occurs during quenching, for example, in the case of gears, not only causes noise and vibration, but also the shape is out of order, so a correction process such as polishing is required. Further, in the case of a shaft-shaped part, a bend occurs, and it is necessary to correct this, which causes the manufacturing cost to increase.
The occurrence of the heat treatment strain is particularly remarkable in carburized parts processed at high temperatures. This heat treatment strain is said to be caused by stress nonuniformity due to expansion during martensitic transformation because γ grains are locally coarsened during heat treatment and the hardenability becomes unstable.

従来、浸炭処理におけるγ粒の粗大化を防止するために、熱間圧延後の鋼材中のAlNの析出状態や、NbやTiの炭窒化物などの微細析出させる際の析出量や分布等を制御することによって、これら析出物のピン止め効果を活用する手法が、試みられてきた。
例えば、特許文献1および特許文献2には、鋼の熱履歴とAl、Nb、N量を調整して発現する、AlとNb窒化物のピン止め効果によって、γ粒の粗大化を抑制することが提案されている。しかし、AlやNbの窒化物は、熱間圧延や焼鈍などの熱履歴を経過する際に粗大化しやすく、かつ析出物の粗大化に伴って、浸炭処理や浸炭窒化処理の加熱時のγ粒粗大化抑制能が著しく低下するという問題があった。
Conventionally, in order to prevent coarsening of γ grains in carburizing treatment, the precipitation state of AlN in the steel material after hot rolling, the precipitation amount and distribution when fine precipitation such as Nb and Ti carbonitrides, etc. Attempts have been made to take advantage of the pinning effect of these precipitates by controlling.
For example, in Patent Document 1 and Patent Document 2, the coarsening of γ grains is suppressed by the pinning effect of Al and Nb nitride, which is expressed by adjusting the thermal history of steel and the amounts of Al, Nb, and N. Has been proposed. However, Al and Nb nitrides are likely to become coarse when passing through a thermal history such as hot rolling and annealing, and with the coarsening of precipitates, γ grains during heating in carburizing and carbonitriding treatments. There was a problem that the ability to suppress coarsening was significantly reduced.

また、特許文献3および特許文献4には、Al、Nb、Tiなどの窒化物、炭化物、炭窒化物形成元素の含有量を調整し、かつ各析出物の大きさや分布を圧延条件の規定にて制御する、手法が開示されている。しかし、種々のサイズの圧延を行う、実際のラインでは、安定的に製造できず、依然効果を安定的に発揮するとは言いがたい状況にあった。   In Patent Document 3 and Patent Document 4, the content of nitride, carbide, carbonitride-forming elements such as Al, Nb, and Ti is adjusted, and the size and distribution of each precipitate are defined as rolling conditions. A method for controlling is disclosed. However, in an actual line where various sizes of rolling are performed, it was difficult to manufacture stably, and it was still difficult to say that the effect was stably exhibited.

特許文献5には、鋼中のC、TiおよびMoの含有量を所定の範囲に制御して、フェライト相中に粒径10nm未満のナノ析出物を分散させることによって、浸炭処理時のγ粒の粗大化を防止し、熱処理歪を小さくする手法が提案されている。しかしながら、この技術では、靭性が低いことに加えて、浸炭層以外の強度が十分でなく、シャフトや歯車に適用することが困難という問題があった。   In Patent Document 5, the content of C, Ti, and Mo in steel is controlled within a predetermined range, and nanoprecipitates having a particle size of less than 10 nm are dispersed in the ferrite phase, so that γ grains at the time of carburizing treatment are obtained. There has been proposed a technique for preventing the coarsening of the steel and reducing the heat treatment strain. However, this technique has a problem that in addition to low toughness, the strength other than the carburized layer is not sufficient and it is difficult to apply to a shaft or gear.

特開昭58-45354号公報JP 58-45354 A 特開昭61-261427号公報JP 61-261427 JP 特開平11-50191号公報Japanese Unexamined Patent Publication No. 11-50191 特開平11-335777号公報Japanese Patent Laid-Open No. 11-335777 特開2003-321731号公報Japanese Patent Laid-Open No. 2003-321731

本発明は、上記の問題点に鑑み開発されたものであり、浸炭処理や浸炭窒化処理を受けた際においても部品内部の強度および靭性を確保しつつ、熱処理歪の原因となる結晶粒の粗大化を効果的に抑制することができる、冷間加工品の有利な製造方法について提案することを目的とする。   The present invention has been developed in view of the above-mentioned problems, and the coarseness of crystal grains that cause heat treatment distortion while ensuring the strength and toughness inside the component even when subjected to carburizing treatment or carbonitriding treatment. It is an object of the present invention to propose an advantageous method for producing a cold-worked product that can effectively suppress the formation of the cold-processed product.

さて、発明者らは、上記の目的を達成すべく、冷間鍛造用素材と冷間鍛造方法に関して鋭意研究を重ねた。
その結果、冷間鍛造用素材の成分組成を好適化して、冷間鍛造前の焼鈍を省略しても、優れた寸法精度を有する冷間鍛造を実現し、さらに焼鈍を省略することにより焼鈍時の微細炭窒化物の粗大化を回避することが、浸炭加熱時のγ粒粗大化を抑止し、浸炭焼き入れ後に、従来よりも格段に優れた寸法精度の機械部品を得られることを見出した。さらに、浸炭処理後の旧γ粒の粗大化を阻止することによって、疲労強度についても有利な向上が達成されることの知見も得た。
本発明は、上記の知見に立脚するものである。
Now, in order to achieve the above-mentioned object, the inventors have conducted intensive research on a cold forging material and a cold forging method.
As a result, by optimizing the component composition of the material for cold forging, even if annealing before cold forging is omitted, cold forging with excellent dimensional accuracy is realized, and further annealing is omitted by omitting annealing It has been found that avoiding coarsening of fine carbonitrides in the steel can prevent the coarsening of γ grains during carburizing heating, and obtain machine parts with much superior dimensional accuracy after carburizing and quenching. . Furthermore, the knowledge that advantageous improvement was also achieved in terms of fatigue strength by preventing the coarsening of the prior γ grains after carburizing treatment was also obtained.
The present invention is based on the above findings.

すなわち、本発明の要旨構成は次のとおりである。
1.鋼素材に熱間圧延、次いで冷間鍛造を施した後、浸炭処理を行って冷間加工品を製造するに当たり、
前記冷間鍛造の前あるいは途中段階で行う焼鈍の回数を、下記(1)式を満足させ、
前記浸炭処理を行った後の冷間加工品における、オーステナイト粒径が50μm以下の結晶粒の面積率を80%以上、かつオーステナイト粒径が300μm超えの結晶粒の面積率を10%以下とすることを特徴とする冷間加工品の製造方法。

nA≦3−εc ・・・(1)
(nA<0の場合は、nA=0とする)
εc:冷間鍛造により導入される相当塑性ひずみ(複数回冷間鍛造の場合は総和)の最大値
That is, the gist configuration of the present invention is as follows.
1. In manufacturing a cold-worked product by performing carburizing treatment after hot rolling and then cold forging on the steel material,
The number of annealing performed before or in the middle of the cold forging satisfies the following formula (1):
In the cold-worked product after performing the carburizing treatment, the area ratio of crystal grains having an austenite grain size of 50 μm or less is 80% or more, and the area ratio of crystal grains having an austenite grain size of more than 300 μm is 10% or less. The manufacturing method of the cold work goods characterized by the above-mentioned.
Record
nA ≦ 3-εc (1)
(If nA <0, nA = 0)
εc: Maximum value of equivalent plastic strain (sum in the case of multiple cold forgings) introduced by cold forging

2.前記鋼素材は、質量%で、
C:0.10〜0.35%、
Si:0.01〜0.13%、
Mn:0.30〜0.80%、
P:0.03%以下、
S:0.03%以下、
Al:0.01〜 0.045%、
Cr:0.5〜3.0%、
B:0.0005〜0.0040%、
Nb:0.003〜0.080%および
N:0.0080%以下
を含み、不純物として混入するTiを0.005%以下に抑制し、残部はFe及び不可避的不純物の成分組成を有することを特徴とする前記1に記載の冷間加工品の製造方法。
2. The steel material is mass%,
C: 0.10 to 0.35%,
Si: 0.01 to 0.13%
Mn: 0.30 to 0.80%
P: 0.03% or less,
S: 0.03% or less,
Al: 0.01-0.045%,
Cr: 0.5-3.0%
B: 0.0005-0.0040%,
2. Nb: 0.003-0.080% and N: 0.0080% or less, Ti mixed as impurities is suppressed to 0.005% or less, and the balance has a component composition of Fe and inevitable impurities A manufacturing method for cold processed products.

3.前記鋼素材はさらに、質量%で、
Sb:0.0003〜 0.50%および
Sn:0.0003〜 0.50%
のうちから選んだ1種または2種以上を含有することを特徴とする前記2に記載の冷間加工品の製造方法。
3. The steel material is further mass%,
Sb: 0.0003 to 0.50% and
Sn: 0.0003 to 0.50%
The method for producing a cold-worked product as described in 2 above, which comprises one or more selected from among the above.

本発明によれば、最適な冷間鍛造素材と冷間鍛造方法との組み合わせにより、優れた寸法精度を有する、機械構造部品に最適の冷間加工品を得ることができる。従って、この冷間加工品を機械構造部品に供することによって、輸送機械や、建設機械およびその他の産業機械などの低騒音化、さらには高疲労強度化を図ることができる。   According to the present invention, a combination of an optimal cold forging material and a cold forging method makes it possible to obtain an optimal cold-worked product for a machine structural component having excellent dimensional accuracy. Therefore, by using this cold-worked product for machine structural parts, it is possible to reduce the noise and further increase the fatigue strength of transportation machines, construction machines, and other industrial machines.

焼鈍条件を示す図である。It is a figure which shows annealing conditions. 中間素材の形状および寸法を示す図である。It is a figure which shows the shape and dimension of an intermediate material. 試験片の形状および寸法を示す図である。It is a figure which shows the shape and dimension of a test piece. 浸炭焼入れおよび焼戻しの条件を示す図である。It is a figure which shows the conditions of carburizing quenching and tempering.

以下、本発明を詳しく説明する。
本発明の冷間加工品の製造方法においては、浸炭処理による表面硬化が施すことが必須であるが、例えば機械構造部品として高い寸法精度と疲労強度を獲得するために、浸炭処理後の結晶粒径を微細にすることが重要である。
具体的には、浸炭処理後の冷間加工品において、オーステナイト粒径が50μm以下の結晶粒の面積率を80%以上、かつオーステナイト粒径が300μm超えの結晶粒の面積率を10%以下とする必要がある。好ましくは、オーステナイト粒径が50μm以下の結晶粒の面積率を90%以上、かつオーステナイト粒径が300μm超えの結晶粒の面積率を5%以下とする。
The present invention will be described in detail below.
In the method for producing a cold-worked product of the present invention, it is essential to perform surface hardening by carburizing treatment. For example, in order to obtain high dimensional accuracy and fatigue strength as a machine structural part, crystal grains after carburizing treatment It is important to make the diameter fine.
Specifically, in the cold-worked product after carburizing treatment, the area ratio of crystal grains with an austenite grain size of 50 μm or less is 80% or more, and the area ratio of crystal grains with an austenite grain size of more than 300 μm is 10% or less. There is a need to. Preferably, the area ratio of crystal grains having an austenite grain size of 50 μm or less is 90% or more, and the area ratio of crystal grains having an austenite grain size exceeding 300 μm is 5% or less.

すなわち、オーステナイト粒径が50μm以下の結晶粒の面積率を80%以上とするのは、疲労強度を十分に向上させるためである。さらに、オーステナイト粒径が300μm超えの結晶粒の面積率を10%以下とするのは、浸炭処理後の寸法精度を確保するとともに、疲労強度を十分に向上させるためである。   That is, the reason why the area ratio of crystal grains having an austenite grain size of 50 μm or less is set to 80% or more is to sufficiently improve fatigue strength. Furthermore, the reason why the area ratio of crystal grains having an austenite grain size exceeding 300 μm is 10% or less is to ensure the dimensional accuracy after the carburizing treatment and to sufficiently improve the fatigue strength.

上記の組織とするには、冷間鍛造を行う際に必須である焼鈍の回数を制限することが肝要である。この点について、以下に詳述する。
さて、鋼素材には、冷間鍛造処理後に浸炭処理を施して冷間加工品とするが、この浸炭処理後に疲労強度が劣化する場合が散見された。
そこで、発明者らは、この点についても検討を重ねた結果、疲労強度の劣化が生じた場合は、浸炭処理後に結晶粒が粗大化していることが判明した。さらに、この原因について調査したところ、この結晶粒の粗大化は冷間鍛造時における焼鈍の回数と強い相関があることが判明した。
In order to obtain the above-described structure, it is important to limit the number of times of annealing, which is essential when performing cold forging. This point will be described in detail below.
Now, the steel material is subjected to a carburizing process after the cold forging process to obtain a cold-worked product. However, there are some cases where the fatigue strength deteriorates after the carburizing process.
Thus, as a result of repeated studies on this point, the inventors have found that when fatigue strength deteriorates, the crystal grains are coarsened after the carburizing treatment. Furthermore, when the cause was investigated, it became clear that the coarsening of this crystal grain had a strong correlation with the frequency | count of annealing at the time of cold forging.

冷間鍛造は、複数回にわたって行うのが通例であり、その複数回の冷間鍛造の間に鋼材の軟化を目的として適宜焼鈍を挟んで行っている。この焼鈍の回数nAが下記(1)式を満たさなくなると、結晶粒が粗大化して疲労強度が劣化することを突き止めたのである。
すなわち、浸炭時の結晶粒の粗大化は、Al窒化物やNb炭窒化物の微細分散により抑制を可能としているが、焼鈍を複数回行うと、Al窒化物やNb炭窒化物が粗大化してしまい、結果として、浸炭時の結晶粒粗大化抑制能を失ってしまうため、焼鈍は下記(1)式を満足する回数に制限する必要がある。より好ましくは、下記(2)式を満足する回数である。

nA≦3−εc ・・・(1)
nA≦2.5−εc ・・・(2)
(nA≦0の場合は、nA=0とする)
Cold forging is usually performed a plurality of times, and annealing is appropriately performed between the plurality of cold forgings for the purpose of softening the steel material. When the number of times of annealing nA does not satisfy the following formula (1), it has been found that the crystal grains become coarse and the fatigue strength deteriorates.
In other words, coarsening of crystal grains during carburization can be suppressed by fine dispersion of Al nitride and Nb carbonitride, but if annealing is performed multiple times, Al nitride and Nb carbonitride are coarsened. Consequently, as a result, the ability to suppress coarsening of crystal grains during carburization is lost, and thus annealing needs to be limited to the number of times that satisfies the following expression (1). More preferably, the number of times satisfies the following expression (2).
Record
nA ≦ 3-εc (1)
nA ≦ 2.5−εc (2)
(If nA ≦ 0, nA = 0)

ここで、焼鈍の回数を、冷間鍛造により導入される相当塑性ひずみに応じて規定する理由は以下の通りである。すなわち、相当塑性ひずみが大きい程、焼鈍後の組織中のフェライトが微細化しやすく、この微細化によって、浸炭加熱中の逆変態オーステナイトの核生成サイトが増加し、浸炭初期のオーステナイトが微細化する。極度に微細化したオーステナイトは、異常粒成長を起こしやすい。よって、相当塑性ひずみが大きくなる程、焼鈍回数を減らし、浸炭初期のオーステナイトの微細化を抑制することによって、浸炭処理中の異常粒成長を防止できる。   Here, the reason why the number of annealing is specified according to the equivalent plastic strain introduced by cold forging is as follows. That is, as the equivalent plastic strain is larger, the ferrite in the structure after annealing is more easily refined. By this refinement, the number of nucleation sites of reverse transformed austenite during carburizing heating increases, and austenite at the initial stage of carburizing becomes refined. Extremely refined austenite tends to cause abnormal grain growth. Therefore, abnormal grain growth during the carburizing process can be prevented by decreasing the number of annealing and suppressing the refinement of austenite at the initial stage of carburizing as the equivalent plastic strain increases.

この規定に従って冷間鍛造を施し、そして浸炭処理して得られる冷間加工品は、オーステナイト粒径が50μm以下の結晶粒の面積率は80%以上、かつオーステナイト粒径が300μm超えの結晶粒の面積率は10%以下となる。   The cold-worked product obtained by performing cold forging according to this rule and carburizing treatment has an area ratio of crystal grains with an austenite grain size of 50 μm or less of 80% or more and an austenite grain size of 300 μm or more. The area ratio is 10% or less.

前記εcは、冷間鍛造により導入される相当塑性ひずみ(複数回冷間鍛造の場合は総和)の最大値であり、通常行われているFEM解析を用いた、鍛造条件設定時のシミュレーションにて求めることができる。   The εc is the maximum value of the equivalent plastic strain introduced by cold forging (the sum in the case of multiple cold forgings), and is a simulation when setting forging conditions using the usual FEM analysis. Can be sought.

なお、冷間鍛造時における焼鈍条件については特に制限はなく、従来公知の条件で行えばよい。好ましい焼鈍条件は、760〜780℃程度の温度域で10〜300min程度である。なお、本発明において、冷間鍛造条件については特に制限はなく、従来公知の条件で行えばよい。   In addition, there is no restriction | limiting in particular about the annealing conditions at the time of cold forging, What is necessary is just to carry out on conventionally well-known conditions. A preferable annealing condition is about 10 to 300 min in a temperature range of about 760 to 780 ° C. In the present invention, the cold forging conditions are not particularly limited, and may be performed under conventionally known conditions.

また、浸炭処理条件についても特に制限はなく、従来公知の条件で行えばよい。一般的な浸炭処理としては、浸炭ガス雰囲気中、例えばRXガス(CO/CO2)の雰囲気中にて900〜960℃および1〜20hで浸炭を施したのち、この浸炭温度から、あるいは840〜880℃にて10〜120min保持後焼入れし、ついで120〜250℃にて30〜300minで焼き戻しを行うことが推奨される。 Moreover, there is no restriction | limiting in particular about carburizing process conditions, What is necessary is just to carry out on conventionally well-known conditions. As a general carburizing process, after carburizing in a carburizing gas atmosphere, for example, in an RX gas (CO / CO 2 ) atmosphere at 900 to 960 ° C. and 1 to 20 hours, from this carburizing temperature, or from 840 to It is recommended to hold for 10 to 120 minutes at 880 ° C. and then quench and then temper at 120 to 250 ° C. for 30 to 300 minutes.

次に、上記した鋼素材の成分組成について、各成分の好適範囲と限定理由を詳しく説明する。なお、成分に関する「%」表示は、特に断らない限り「質量%」を意味するものとする。
まず、基本成分としては、C:0.10〜0.35%、Si:0.01〜0.13%、Mn:0.30〜0.80%、P:0.03%以下、S:0.03%以下、Al:0.01〜 0.045%、Cr:0.5〜3.0%、B:0.0005〜0.0040%、Nb:0.003〜0.080%およびN:0.0080%以下を含み、不純物として混入するTiを0.005%以下に抑制し、残部はFe及び不可避的不純物であることが好ましい。
Next, with regard to the above-described component composition of the steel material, the preferred range and reason for limitation of each component will be described in detail. In addition, unless otherwise indicated, the "%" display regarding a component shall mean "mass%".
First, as basic components, C: 0.10 to 0.35%, Si: 0.01 to 0.13%, Mn: 0.30 to 0.80%, P: 0.03% or less, S: 0.03% or less, Al: 0.01 to 0.045%, Cr: 0.5 -3.0%, B: 0.0005-0.0040%, Nb: 0.003-0.080% and N: 0.0080% or less, Ti mixed as impurities is suppressed to 0.005% or less, and the balance may be Fe and inevitable impurities preferable.

C:0.10〜0.35%
Cは、冷間鍛造品に施す浸炭処理後の焼入れによって、鍛造品中心部において十分な硬度を得るために、0.10%以上とすることが好ましい。一方、Cの含有量が0.35%を超えると、冷間鍛造素材の硬度上昇にともない、上記した(1)式を満足する条件での冷間鍛造が困難となり、さらには浸炭焼き入れ後の中心部の靱性が劣化するため、C量は0.10〜 0.35%の範囲とするのが好ましい。なお、靭性および冷間鍛造性の面から、より好ましくは0.25%以下、さらに好ましくは0.20%以下である。
C: 0.10 to 0.35%
C is preferably set to 0.10% or more in order to obtain sufficient hardness at the center of the forged product by quenching after carburizing treatment applied to the cold forged product. On the other hand, if the C content exceeds 0.35%, cold forging under the conditions satisfying the above-mentioned formula (1) becomes difficult as the hardness of the cold forging material increases, and further, the center after carburizing and quenching is difficult. Since the toughness of the part deteriorates, the C content is preferably in the range of 0.10 to 0.35%. In view of toughness and cold forgeability, it is more preferably 0.25% or less, and still more preferably 0.20% or less.

Si:0.01〜0.13%
Siは、脱酸剤として有用であり、少なくとも0.01%の添加が好ましい。しかしながら、Siは浸炭表層で優先的に酸化し、粒界酸化を促すだけでなく、フェライトを固溶強化し変形抵抗を高めて冷間鍛造性を劣化させるため、上限を0.13%とするのが好ましい。より好ましくは0.02〜0.10%、さらに好ましくは0.02〜0.09%の範囲である。
Si: 0.01-0.13%
Si is useful as a deoxidizer and is preferably added in an amount of at least 0.01%. However, Si not only preferentially oxidizes at the carburized surface layer and promotes grain boundary oxidation, but also enhances solid solution strengthening of ferrite to increase deformation resistance and deteriorate cold forgeability, so the upper limit is made 0.13%. preferable. More preferably, it is 0.02 to 0.10%, and still more preferably 0.02 to 0.09%.

Mn:0.30〜0.80%
Mnは、焼入性の向上に有効な元素で有り、0.30%以上の添加により効果を発揮することになる。しかし、Mnの過剰な添加は、固溶強化による変形抵抗の上昇を招いて冷間鍛造性を劣化させるため、上限を0.80%とすることが好ましい。より好ましくは0.60%以下、さらに好ましくは0.55%以下である。
Mn: 0.30 to 0.80%
Mn is an element effective for improving hardenability, and the effect is exhibited by addition of 0.30% or more. However, excessive addition of Mn causes an increase in deformation resistance due to solid solution strengthening and deteriorates cold forgeability, so the upper limit is preferably made 0.80%. More preferably, it is 0.60% or less, More preferably, it is 0.55% or less.

P:0.03%以下
Pは、結晶粒界に偏析して靭性を低下させるため、その混入は低いほど望ましいが、0.03%までは許容される。好ましくは0.025%以下である。また、下限については特に限定せずとも問題はないが、不要な低P化は精錬時間の増長や精錬コストの上昇を招くため、好ましくは0.010%以上とするのがよい。より好ましくは0.013%以上である。
P: 0.03% or less P is segregated at the grain boundaries to lower toughness. Therefore, the lower the mixing, the better, but 0.03% is acceptable. Preferably it is 0.025% or less. There is no problem even if the lower limit is not particularly limited, but unnecessary lowering of P leads to an increase in the refining time and an increase in the refining cost. Therefore, the lower limit is preferably 0.010% or more. More preferably, it is 0.013% or more.

S:0.03%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るが、過剰な添加は冷間鍛造性の低下を招くため、上限を0.03%とするのが好ましい。また、下限については特に限定しないが、特に優れた被削性を確保するために、0.010%以上としてもよい。被削性の観点から、より好ましくは0.012%以上である。
S: 0.03% or less S exists as a sulfide inclusion and is an effective element for improving machinability, but excessive addition causes a decrease in cold forgeability, so the upper limit is made 0.03%. Is preferred. The lower limit is not particularly limited, but may be 0.010% or more in order to ensure particularly excellent machinability. From the viewpoint of machinability, it is more preferably 0.012% or more.

Al:0.01〜 0.045%
Alは、過剰に添加すると鋼中のNをAlNとして固定して、Bの焼入性効果を発現させてしまう。浸炭処理後の部品強度を安定化させるためには、Bの焼入性効果を発現させないことが重要であり、そのためにAl量の上限は0.045%とする。一方、脱酸に有効な元素でもあるため、下限を0.01%とする。好ましくは0.01〜 0.040%、さらに好ましくは0.015〜0.035%の範囲である。
Al: 0.01-0.045%
When Al is added excessively, N in the steel is fixed as AlN, and the hardenability effect of B is expressed. In order to stabilize the strength of the parts after the carburizing treatment, it is important not to exhibit the hardenability effect of B. For this reason, the upper limit of the Al amount is 0.045%. On the other hand, since it is also an element effective for deoxidation, the lower limit is made 0.01%. Preferably it is 0.01 to 0.040%, More preferably, it is 0.015 to 0.035% of range.

Cr:0.5〜3.0%
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。しかしながら、含有量が0.5%に満たないと、その添加効果に乏しく、一方3.0%を超えると、過剰浸炭や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。よって、Cr量は0.5〜3.0%の範囲とするのが好ましい。より好ましくは0.7〜2.5%、さらに好ましくは1.0〜1.8%の範囲である。
Cr: 0.5-3.0%
Cr contributes not only to hardenability but also to improvement of temper softening resistance, and is also an element useful for promoting spheroidization of carbides. However, if the content is less than 0.5%, the effect of addition is poor. On the other hand, if it exceeds 3.0%, excessive carburization and residual austenite are promoted, and fatigue strength is adversely affected. Therefore, the Cr content is preferably in the range of 0.5 to 3.0%. More preferably, it is 0.7 to 2.5%, and still more preferably 1.0 to 1.8%.

B:0.0005〜 0.0040%
Bは、鋼中でNと結合することで、固溶Nを低減させる効果があり、そのため、固溶Nによる冷間鍛造時の動的ひずみ時効を低減することが可能であり、鍛造時の変形抵抗を下げることに寄与する。このためには、0.0005%以上のB添加が望ましいが、一方でB量が0.0040%を超えると、変形抵抗低減効果は飽和し、むしろ靱性の低下を招きさらに添加したBが固溶状態となると、焼き入れ性が著しく不安定化することから、B量は0.0005〜0.0040%の範囲に限定することが好ましい。より好ましくは0.0005〜 0.0030%の範囲である。
B: 0.0005-0.0040%
B has the effect of reducing the solid solution N by combining with N in the steel. Therefore, it is possible to reduce the dynamic strain aging at the time of cold forging by the solid solution N. Contributes to lowering deformation resistance. For this purpose, B addition of 0.0005% or more is desirable. On the other hand, when the amount of B exceeds 0.0040%, the effect of reducing deformation resistance is saturated, but rather, the added B is in a solid solution state due to a decrease in toughness. Further, since the hardenability becomes extremely unstable, the B content is preferably limited to the range of 0.0005 to 0.0040%. More preferably, it is 0.0005 to 0.0030% of range.

Nb:0.003〜 0.080%
Nbは、鋼中でNbCを形成し、浸炭処理時のオーステナイト粒の粗粒化をピン止めにより抑制する効果がある。この効果を得るためには、少なくとも0.003%の添加が必要であるが、0.080%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。このためNb量は0.003〜0.080%の範囲にすることが好ましい。より好ましくは0.010〜0.060%、さらに好ましくは0.015〜0.045%の範囲である。
Nb: 0.003-0.080%
Nb has the effect of forming NbC in steel and suppressing the coarsening of austenite grains during carburizing by pinning. In order to obtain this effect, addition of at least 0.003% is necessary. However, if added over 0.080%, there is a possibility that deterioration of coarsening suppression ability and deterioration of fatigue strength due to precipitation of coarse NbC may be caused. . For this reason, it is preferable to make Nb amount into the range of 0.003-0.080%. More preferably, it is 0.010 to 0.060%, and still more preferably 0.015 to 0.045%.

N:0.0080%以下
Nは、鋼中に固溶し、冷間鍛造時に動的ひずみ時効を生じ、変形抵抗を増大させてしまうため、混入を極力回避することが好ましい成分である。そのため、N量は0.0080%以下とすることが好ましい。より好ましくは0.0070%以下、さらに好ましくは0.0065%以下である。
N: 0.0080% or less N is a preferred component to avoid mixing as much as possible because N dissolves in steel and causes dynamic strain aging during cold forging and increases deformation resistance. Therefore, the N content is preferably 0.0080% or less. More preferably, it is 0.0070% or less, More preferably, it is 0.0065% or less.

Ti:0.005%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。すなわち、Tiは、Nと結合して粗大なTiNを形成しやすく、またNbとの同時添加は粗大析出物をより生じやすくし、疲労強度の低下を招くことから、その混入は極力低減することが好ましいが、0.005%以下であれば許容される。好ましくは0.003%以下である。
Ti: 0.005% or less
Ti is a component that preferably avoids mixing into steel as much as possible. That is, Ti is likely to bond with N to form coarse TiN, and simultaneous addition with Nb makes it easier to produce coarse precipitates, leading to a decrease in fatigue strength. However, 0.005% or less is acceptable. Preferably it is 0.003% or less.

以上、基本成分について説明したが、本発明では、その他にも必要に応じて、以下の元素を適宜含有させることができる。
Sb:0.0003〜0.50%
Sbは、鋼材表面の脱炭を抑制し、表面硬度の低下を防止するために有効な元素である。ただし、過乗な添加は冷間鍛造性を劣化させることから、Sbは0.0003〜0.50%の範囲で添加することが好ましい。より好ましくは0.0010〜 0.050%、さらに好ましくは0.0015〜0.035%の範囲である。
The basic components have been described above. However, in the present invention, the following elements can be appropriately contained as needed.
Sb: 0.0003 to 0.50%
Sb is an effective element for suppressing the decarburization of the steel surface and preventing the decrease in surface hardness. However, excessive addition degrades the cold forgeability, so Sb is preferably added in the range of 0.0003 to 0.50%. More preferably, it is 0.0010-0.050%, More preferably, it is 0.0015-0.035% of range.

Sn:0.0003〜0.50%
Snは、鋼材表面の耐食性を向上させる上で有効な元素である。ただし、過剰な添加は冷間鍛造性を劣化させることから、Snは0.0003〜0.50%の範囲で含有させることが好ましい。より好ましくは0.0010〜0.050%、さらに好ましくは0.0015〜0.035%の範囲である。
Sn: 0.0003 to 0.50%
Sn is an element effective in improving the corrosion resistance of the steel material surface. However, since excessive addition deteriorates cold forgeability, Sn is preferably contained in a range of 0.0003 to 0.50%. More preferably, it is 0.0010 to 0.050%, and still more preferably 0.0015 to 0.035%.

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。なお、本発明は以下の実施例による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おいて以下の実施例から適宜変更することも可能であり、そ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. Note that the present invention is not limited by the following examples, and can be appropriately modified from the following examples within a range that can be adapted to the gist of the present invention. Included in the scope.

表1に示す成分を有しかつ、種々の直径(表2に示す素材径)を有する丸棒鋼を素材として、1回あたりの断面減少率を0〜40%とする冷間前方押し出し成形(冷間鍛造)の繰返しにより14mmφの中間素材を得た。この中間素材を得るための繰返しの冷間鍛造の前、または途中にて焼鈍を施した。この回数をnAとする。なお、焼鈍は図1に示す条件で実施した。次いで、前記中間素材を冷間密閉鍛造により図2に示す寸法形状とし、さらに切削加工により図3に示す寸法形状とした。
その際、図2の形状への冷間鍛造工程における最大の荷重を測定した。また、冷間前方押し出し成形と冷間密閉鍛造とにより導入される相当塑性ひずみεcは、素材の断面積A0(=(素材径/2)2×π)と冷間密閉鍛造後の断面積A(=(8.5/2)2×π)とから、以下の(3)式により算出した。
εc=−ln(A/A0)…(3)
Cold forward extruding (cold reduction) with a cross-section reduction rate of 0 to 40% per round using round bar steel having the components shown in Table 1 and various diameters (material diameters shown in Table 2) 14mmφ intermediate material was obtained by repeating the forging process. Annealing was performed before or during repeated cold forging to obtain this intermediate material. This number is nA. The annealing was performed under the conditions shown in FIG. Next, the intermediate material was made into the dimensional shape shown in FIG. 2 by cold sealed forging, and further made into the dimensional shape shown in FIG. 3 by cutting.
At that time, the maximum load in the cold forging step to the shape of FIG. 2 was measured. The equivalent plastic strain εc introduced by cold forward extrusion and cold hermetic forging is the cross-sectional area A 0 (= (material diameter / 2) 2 × π) of the material and the cross-sectional area after cold hermetic forging. From A (= (8.5 / 2) 2 × π), it was calculated by the following equation (3).
εc = −ln (A / A 0 ) (3)

得られた冷間加工材を図4に示す条件にて、浸炭、焼入れおよび焼戻しを行った。このようにして得た図3に示す形状の試験片について、その平行部中央(8mmφ部)のC断面(押し出し鍛造方向と直交する断面)を採取し、8mmφ部の断面内の浸炭部から400倍で無作為に10視野のミクロ組織を撮影し、撮影した組織について旧オーステナイト粒径分布を測定し、径が50μm以下の結晶粒の面積率および同300μm超の結晶粒の面積率を、それぞれ定量化した。また、8mmφ部を平行部の長さ方向に2mm間隔で5箇所、各箇所につき30°間隔で6箇所、合計30箇所の直径を測定し、その値の標準偏差を求めた。さらに、当該試験片を小野式回転曲げ疲労試験に供して、疲労特性を調査した。各実施例条件および特性調査結果を表2に示す。   The obtained cold-worked material was carburized, quenched and tempered under the conditions shown in FIG. For the test piece having the shape shown in FIG. 3 obtained in this way, the C cross section (cross section orthogonal to the extrusion forging direction) of the center of the parallel portion (8 mmφ portion) is sampled and 400 from the carburized portion in the cross section of the 8 mmφ portion. The microstructure of 10 fields of view was photographed at random, the prior austenite grain size distribution was measured for the photographed tissue, and the area ratio of crystal grains having a diameter of 50 μm or less and the area ratio of crystal grains exceeding 300 μm were respectively determined. Quantified. In addition, the diameter of the 8 mmφ portion was measured at 5 locations at intervals of 2 mm in the length direction of the parallel portion and 6 locations at intervals of 30 ° for each location, for a total of 30 locations, and the standard deviation of the values was determined. Furthermore, the said test piece was used for the Ono type | formula rotation bending fatigue test, and the fatigue characteristic was investigated. Table 2 shows the conditions of each example and the results of characteristic investigation.

繰り返し冷間鍛造時の焼鈍回数を初めとする実施条件が、本発明の規定範囲をすべて満足する場合は、良好な寸法精度、疲労強度を有する冷間加工品が得られている。なお、鋼素材の組成範囲が本発明の好適範囲を満足する場合(No.1〜6、8および10〜12)は、組成が好適範囲外となる鋼Gを用いたNo.13,14の、εcの値が同等の場合同士と比較すると、図2の形状への冷間鍛造に際しての変形抵抗が低く、実際の成形に際して鍛造金型寿命の向上が可能となることがわかる。
これに対して、焼鈍回数が規定回数を超えるNo.7および9は、浸炭焼入れ時に結晶粒の粗大化が発生し、最終形状の寸法精度および疲労強度が劣化する結果となった。
When the implementation conditions including the number of annealing in repeated cold forging satisfy all the specified range of the present invention, a cold-worked product having good dimensional accuracy and fatigue strength is obtained. When the composition range of the steel material satisfies the preferred range of the present invention (No. 1-6, 8 and 10-12), No. 1 using steel G whose composition falls outside the preferred range. Compared with cases where the values of εc of 13 and 14 are equivalent, it can be seen that the deformation resistance at the time of cold forging into the shape of FIG. 2 is low, and the life of the forging die can be improved in actual forming. .
On the other hand, Nos. 7 and 9 in which the number of annealing times exceeded the specified number of times resulted in coarsening of crystal grains during carburizing and quenching, resulting in deterioration of dimensional accuracy and fatigue strength of the final shape.

Claims (2)

質量%で、
C:0.10〜0.35%、
Si:0.01〜0.13%、
Mn:0.30〜0.80%、
P:0.03%以下、
S:0.03%以下、
Al:0.01〜0.045%、
Cr:0.5〜3.0%、
B:0.0005〜0.0040%、
Nb:0.003〜0.080%および
N:0.0080%以下
を含み、不純物として混入するTiを0.005%以下に抑制し、残部はFe及び不可避的不純物の成分組成からなる、鋼素材に熱間圧延、次いで冷間鍛造を施した後、浸炭処理を行って冷間加工品を製造するに当たり、
前記冷間鍛造による導入される相当塑性ひずみεcを1.0以上2.2以下とし、
前記冷間鍛造の前あるいは途中段階で行う焼鈍の回数を、下記(1)式を満足させ、
前記浸炭処理は、浸炭温度を950℃以上960℃以下とし、
前記浸炭処理を行った後の冷間加工品における、オーステナイト粒径が50μm以下の結晶粒の面積率を80%以上、かつオーステナイト粒径が300μm超えの結晶粒の面積率を10%以下とすることを特徴とする冷間加工品の製造方法。

nA≦3−εc・・・(1)
(nA<0の場合は、nA=0とする)
εc:冷間鍛造により導入される相当塑性ひずみ(複数回冷間鍛造の場合は総和)の最大値
% By mass
C: 0.10 to 0.35%,
Si: 0.01 to 0.13%
Mn: 0.30 to 0.80%
P: 0.03% or less,
S: 0.03% or less,
Al: 0.01-0.045%,
Cr: 0.5-3.0%
B: 0.0005-0.0040%,
Nb: 0.003-0.080% and N: 0.0080% or less, Ti mixed as impurities is suppressed to 0.005% or less, the balance is composed of Fe and inevitable impurities, the steel material is hot rolled, then cooled After cold forging, carburizing is performed to manufacture cold-worked products.
The equivalent plastic strain εc introduced by the cold forging is 1.0 to 2.2 ,
The number of annealing performed before or in the middle of the cold forging satisfies the following formula (1):
The carburizing treatment is performed at a carburizing temperature of 950 ° C. or more and 960 ° C. or less ,
In the cold-worked product after performing the carburizing treatment, the area ratio of crystal grains having an austenite grain size of 50 μm or less is 80% or more, and the area ratio of crystal grains having an austenite grain size of more than 300 μm is 10% or less. The manufacturing method of the cold work goods characterized by the above-mentioned.
Record
nA ≦ 3-εc (1)
(If nA <0, nA = 0)
εc: Maximum value of equivalent plastic strain (sum in the case of multiple cold forgings) introduced by cold forging
前記鋼素材はさらに、質量%で、
Sb:0.0003〜 0.50%および
Sn:0.0003〜 0.50%
のうちから選んだ1種または2種を含有することを特徴とする請求項1に記載の冷間加工品の製造方法。
The steel material is further mass%,
Sb: 0.0003 to 0.50% and
Sn: 0.0003 to 0.50%
The method for producing a cold-worked product according to claim 1, comprising one or two selected from among them.
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