JP2017133052A - Case hardened steel excellent in coarse particle prevention property, fatigue property and machinability during carburization and manufacturing method therefor - Google Patents

Case hardened steel excellent in coarse particle prevention property, fatigue property and machinability during carburization and manufacturing method therefor Download PDF

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JP2017133052A
JP2017133052A JP2016012208A JP2016012208A JP2017133052A JP 2017133052 A JP2017133052 A JP 2017133052A JP 2016012208 A JP2016012208 A JP 2016012208A JP 2016012208 A JP2016012208 A JP 2016012208A JP 2017133052 A JP2017133052 A JP 2017133052A
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JP6766362B2 (en
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慶 宮西
Kei Miyanishi
慶 宮西
聡 志賀
Satoshi Shiga
聡 志賀
根石 豊
Yutaka Neishi
豊 根石
水上 英夫
Hideo Mizukami
英夫 水上
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Abstract

PROBLEM TO BE SOLVED: To provide a case hardened steel excellent in coarse particle prevention property and machinability during carburization, capable of suppressing heat treatment strain due to carburization hardening and providing excellent fatigue property after carburization hardening and a manufacturing method therefor.SOLUTION: There is provided a case hardened steel containing C:0.10 to 0.30%, Si:0.02 to 1.5%, Mn:0.3 to 1.8%, S:over 0.020 to 0.050%, Cr:0.4 to 2.0%, Al:0.005 to 0.05%, Ti:0.06 to 0.20%, Bi:0.0001 to 0.0050% and further one or more kind of Ca:0.0005 to 0.0050%, Mg:0.0003 to 0.0050%, Te:0.0003 to 0.20%, satisfying following formula (1) and excellent in coarse particle prevention property, fatigue property and machinability. 5.0≥Ti/S≥3.0 Formula (1), where Ti is the content of Ti (mass%) and S is the content of S (mass%).SELECTED DRAWING: None

Description

本発明は、浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼およびその製造方法に関する。   The present invention relates to a case hardening steel excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing, and a method for producing the same.

歯車、軸受部品、転動部品、シャフト、等速ジョイント部品などの浸炭部品は、通常、以下に示す方法により製造されている。例えば、JIS G 4052、JIS G 4104、JIS G 4105、JIS G 4106などに規定されている中炭素の機械構造用合金鋼を鍛造し、切削により所定の形状に加工した後、浸炭焼入れを行う。   Carburized parts such as gears, bearing parts, rolling parts, shafts, constant velocity joint parts and the like are usually manufactured by the following method. For example, a medium carbon alloy steel for machine structure specified in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. is forged, processed into a predetermined shape by cutting, and then carburized and quenched.

浸炭部品を製造する際に行う鍛造では、冷間鍛造(転造も含む)又は熱間鍛造が行われている。冷間鍛造は、製品の表面肌および寸法精度が良く、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。このため、近年、熱間鍛造から冷間鍛造へ切り替える傾向が強くなっている。その結果、冷間鍛造後に浸炭焼き入れして製造される浸炭部品が、近年顕著に増加している。   In forging performed when manufacturing a carburized part, cold forging (including rolling) or hot forging is performed. Cold forging has a good surface texture and dimensional accuracy of the product, has a lower production cost than hot forging, and has a good yield. For this reason, in recent years, the tendency to switch from hot forging to cold forging has increased. As a result, carburized parts manufactured by carburizing and quenching after cold forging have increased significantly in recent years.

冷間鍛造後に浸炭焼き入れして製造される浸炭部品の大きな課題として、熱処理歪みの低減が挙げられる。例えば、シャフトが熱処理歪みによって曲がると、シャフトとしての機能が損なわれる。また、歯車や等速ジョイント部品では、熱処理歪みが大きいと、騒音や振動の原因となる。
浸炭部品の熱処理歪みの最大の原因は、浸炭時に発生する粗大粒である。従来、粗大粒を抑制するために、冷間鍛造後、浸炭焼入れの前に、焼鈍が行われていた。しかし、近年、コスト削減の視点から、焼鈍省略の指向が強まっている。そのため、焼鈍を省略しても粗大粒を生じない鋼材が強く求められている。
A major problem of carburized parts manufactured by carburizing and quenching after cold forging is reduction of heat treatment distortion. For example, when the shaft bends due to heat treatment strain, the function as the shaft is impaired. In addition, in gears and constant velocity joint parts, if heat treatment distortion is large, it may cause noise and vibration.
The largest cause of heat treatment distortion of carburized parts is coarse particles generated during carburizing. Conventionally, annealing has been performed after cold forging and before carburizing and quenching in order to suppress coarse grains. However, in recent years, there is an increasing tendency to omit annealing from the viewpoint of cost reduction. Therefore, there is a strong demand for steel materials that do not produce coarse grains even if annealing is omitted.

一方、歯車、軸受部品、転動部品の中でも、高面圧が負荷される軸受部品、転動部品においては、高深度浸炭が行われている。通常、高深度浸炭では、十数時間から数十時間の長時間を要するため、省エネルギーの視点から、浸炭時間の短縮が重要な課題となっている。
浸炭時間を短縮するためには、浸炭温度の高温化が有効である。浸炭温度は、通常の浸炭では930℃程度であるが、高温浸炭では990〜1090℃の温度域で行う。しかし、浸炭時間を短縮するために高温浸炭を行うと、粗大粒が発生し、浸炭部品に必要な転動疲労特性等の疲労特性が十分に得られない場合があった。そのため、高温浸炭を行っても粗大粒が発生しない高温浸炭に適した肌焼鋼が求められている。
On the other hand, among gears, bearing parts, and rolling parts, high-depth carburization is performed in bearing parts and rolling parts to which high surface pressure is applied. Usually, deep carburization requires a long time of several tens of hours to several tens of hours, so shortening the carburizing time is an important issue from the viewpoint of energy saving.
Increasing the carburizing temperature is effective for shortening the carburizing time. The carburizing temperature is about 930 ° C. in normal carburizing, but is performed in a temperature range of 990 to 1090 ° C. in high temperature carburizing. However, when high-temperature carburizing is performed to shorten the carburizing time, coarse grains are generated, and fatigue characteristics such as rolling fatigue characteristics necessary for carburized parts may not be obtained sufficiently. Therefore, there is a need for a case-hardened steel suitable for high-temperature carburizing that does not generate coarse grains even when high-temperature carburizing is performed.

また、高面圧が負荷される歯車、軸受部品、転動部品は、大型部品が多く、通常「棒鋼−熱間鍛造−必要により焼準等の熱処理−切削−浸炭焼入れ−必要により研磨」の工程を経て製造される。浸炭時の粗大粒の発生を抑制するためには、熱間鍛造後の熱間鍛造部材が、浸炭時の粗大粒を抑制できる適正な材質である必要がある。そのためには、棒鋼の素材として、浸炭時の粗大粒を抑制できる適正な材質を用いる必要がある。   In addition, gears, bearing parts, and rolling parts loaded with high surface pressure are often large-sized parts, and are usually "bar steel-hot forging-heat treatment such as normalization if necessary-cutting-carburizing and quenching-polishing if necessary". It is manufactured through a process. In order to suppress the generation of coarse grains during carburizing, the hot forged member after hot forging needs to be an appropriate material that can suppress the coarse grains during carburizing. For this purpose, it is necessary to use an appropriate material that can suppress coarse grains during carburization as a material for the steel bar.

特許文献1には、Ti:0.05〜0.2%、S:0.001〜0.15%を含有し、N:0.0051%未満に制限し、熱間圧延後のAlNの析出量を0.01%以下に制限した浸炭時の粗大粒防止特性と疲労特性に優れた肌焼鋼が開示されている。
特許文献2には、Ti:0.03〜0.30%、S:0.010〜0.10%を含有し、N:0.020%以下に制限し、Ti系硫化物の個数密度を規定した肌焼鋼が開示されている。
Patent Document 1 contains Ti: 0.05 to 0.2%, S: 0.001 to 0.15%, N: limited to less than 0.0051%, and precipitation of AlN after hot rolling. A case-hardened steel excellent in coarse grain prevention characteristics and fatigue characteristics during carburization with an amount limited to 0.01% or less is disclosed.
Patent Document 2 contains Ti: 0.03 to 0.30%, S: 0.010 to 0.10%, N: limited to 0.020% or less, and the number density of Ti-based sulfides. A defined case hardening steel is disclosed.

特許第4448456号公報Japanese Patent No. 4448456 特開2007−31787号公報JP 2007-31787 A

しかしながら、従来の肌焼鋼は、今後のさらなる高温浸炭化のニーズに対応するには、粗大粒防止能力が不足する可能性がある。また、肌焼鋼は、十分な被削性を有している必要がある。また、肌焼鋼の用途として主要な歯車、シャフトにおいては、さらなる疲労強度特性の向上が望まれている。
本発明は、このような事情に鑑みてなされたものであり、浸炭時の粗大粒防止特性および被削性に優れ、浸炭焼き入れによる熱処理歪みを抑制できるとともに、浸炭焼き入れ後に優れた疲労特性が得られる肌焼鋼およびその製造方法を提供することを課題とする。
However, the conventional case-hardened steel may lack the ability to prevent coarse grains to meet future needs for further high-temperature carburization. Moreover, the case-hardened steel needs to have sufficient machinability. Further, in the main gears and shafts for use in case-hardened steel, further improvement in fatigue strength characteristics is desired.
The present invention has been made in view of such circumstances, is excellent in coarse grain prevention characteristics and machinability during carburizing, can suppress heat treatment distortion due to carburizing quenching, and has excellent fatigue characteristics after carburizing quenching It is an object of the present invention to provide a case-hardened steel and a method for producing the same.

[1] 化学組成が質量%で、
C:0.10〜0.30%、
Si:0.02〜1.5%、
Mn:0.3〜1.8%、
S:0.020超〜0.050%、
Cr:0.4〜2.0%、
Al:0.005〜0.05%、
Ti:0.06〜0.20%、
Bi:0.0001〜0.0050%
を含有し、さらに、
Ca:0.0005〜0.0050%、
Mg:0.0003〜0.0050%、
Te:0.0003〜0.20%
の1種または2種以上を含有し、
P:0.050%以下、
N:0.01%以下、
O:0.0025%以下
に制限し、
残部が鉄および不純物であり、
下記式(1)を満たすことを特徴とする浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
5.0≧Ti/S≧3.0 式(1)
(式(1)中のTiは、Tiの含有量(質量%)であり、Sは、Sの含有量(質量%)である。)
[1] The chemical composition is mass%,
C: 0.10 to 0.30%,
Si: 0.02 to 1.5%,
Mn: 0.3 to 1.8%
S: more than 0.020 to 0.050%,
Cr: 0.4 to 2.0%,
Al: 0.005 to 0.05%,
Ti: 0.06-0.20%,
Bi: 0.0001 to 0.0050%
In addition,
Ca: 0.0005 to 0.0050%,
Mg: 0.0003 to 0.0050%,
Te: 0.0003 to 0.20%
Containing one or more of
P: 0.050% or less,
N: 0.01% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
Case hardening steel excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing, characterized by satisfying the following formula (1).
5.0 ≧ Ti / S ≧ 3.0 Formula (1)
(Ti in the formula (1) is the content (mass%) of Ti, and S is the content (mass%) of S.)

[2] 前記化学組成が質量%で、
Mo:0.02〜1.5%、
Ni:0.1〜3.5%、
V:0.02〜0.5%、
B:0.0002〜0.005%
の1種または2種以上を含有する[1]に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
[3] 化学組成が質量%で、
Nb:0.04%未満を含有する[1]または[2]に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
[2] The chemical composition is mass%,
Mo: 0.02 to 1.5%,
Ni: 0.1 to 3.5%
V: 0.02-0.5%
B: 0.0002 to 0.005%
The hardened steel excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing according to [1], containing one or more of the above.
[3] The chemical composition is mass%,
Nb: Case-hardening steel excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburization according to [1] or [2], containing less than 0.04%.

[4] ベイナイトの組織分率が30%以下である[1]〜[3]のいずれかに記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
[5] フェライト結晶粒度番号がJIS G0552で規定されている8〜11番である[1]〜[4]のいずれかに記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
[6] マトリックス中の長手方向断面において、検査基準面積:100平方mm、検査数:16視野、予測を行なう面積:30000平方mmの条件で測定された極値統計によるTi系析出物の最大直径が40μm以下である[1]〜[5]のいずれかに記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
[4] The case hardening steel excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing according to any one of [1] to [3], wherein the bainite has a structure fraction of 30% or less.
[5] Ferrite grain size number is No. 8 to 11 defined in JIS G0552, [1] to [4] is excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing Hardened steel.
[6] Maximum cross-sectional diameter of Ti-based precipitate by extreme value statistics measured under conditions of inspection reference area: 100 square mm, number of inspections: 16 fields of view, prediction area: 30000 square mm, in longitudinal section in matrix The case-hardened steel excellent in coarse grain prevention characteristics, fatigue characteristics, and machinability during carburizing according to any one of [1] to [5], wherein is.

[7] 化学組成が質量%で、
C:0.10〜0.30%、
Si:0.02〜1.5%、
Mn:0.3〜1.8%、
S:0.020超〜0.050%、
Cr:0.4〜2.0%、
Al:0.005〜0.05%、
Ti:0.06〜0.20%、
Bi:0.0001〜0.0050%
を含有し、さらに、
Ca:0.0005〜0.0050%、
Mg:0.0003〜0.0050%、
Te:0.0003〜0.20%
の1種または2種以上を含有し、
P:0.050%以下、
N:0.01%以下、
O:0.0025%以下
に制限し、
残部が鉄および不純物であり、
下記式(1)を満たす鋼を、1150℃以上の温度で保持時間10分以上加熱して線材または棒鋼に熱間圧延する工程を含む浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼の製造方法。
5.0≧Ti/S≧3.0 式(1)
(式(1)中のTiは、Tiの含有量(質量%)であり、Sは、Sの含有量(質量%)である。)
[7] The chemical composition is mass%,
C: 0.10 to 0.30%,
Si: 0.02 to 1.5%,
Mn: 0.3 to 1.8%
S: more than 0.020 to 0.050%,
Cr: 0.4 to 2.0%,
Al: 0.005 to 0.05%,
Ti: 0.06-0.20%,
Bi: 0.0001 to 0.0050%
In addition,
Ca: 0.0005 to 0.0050%,
Mg: 0.0003 to 0.0050%,
Te: 0.0003 to 0.20%
Containing one or more of
P: 0.050% or less,
N: 0.01% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
The steel satisfying the following formula (1) is heated at a temperature of 1150 ° C. or higher for a holding time of 10 minutes or longer and hot-rolled into a wire or a steel bar. A method for producing excellent case-hardened steel.
5.0 ≧ Ti / S ≧ 3.0 Formula (1)
(Ti in the formula (1) is the content (mass%) of Ti, and S is the content (mass%) of S.)

[8] 前記熱間圧延後に800〜500℃の温度範囲を1℃/秒以下の冷却速度で徐冷し、熱間圧延して冷却した後の鋼のベイナイトの組織分率が30%以下となるようにする[7]に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼の製造方法。
[9] 前記熱間圧延の仕上げ温度を840〜1000℃とし、フェライト結晶粒度番号がJIS G0552で規定されている8〜11番である鋼となるようにする[7]または[8]に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼の製造方法。
[8] After the hot rolling, the temperature range of 800 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./second or less, and the steel bainite has a structure fraction of 30% or less after being hot rolled and cooled. The manufacturing method of the case hardening steel excellent in the coarse grain prevention characteristic at the time of carburizing, fatigue characteristics, and machinability as described in [7].
[9] The finishing temperature of the hot rolling is 840 to 1000 ° C., and the ferrite crystal grain size number is 8 to 11 as defined in JIS G0552 [7] or [8] Of hardened steel with excellent coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing.

本発明の肌焼鋼は、所定の化学組成を有するので、浸炭時の粗大粒防止特性および被削性に優れる。したがって、本発明の肌焼鋼によれば、浸炭焼き入れによる熱処理歪みを抑制できるとともに、浸炭焼き入れ後に優れた疲労特性が得られる。また、本発明の肌焼鋼を浸炭焼入れして製造した浸炭部品は、熱処理歪みが少なく、優れた疲労特性を有する。
本発明の肌焼鋼の製造方法によれば、浸炭時の粗大粒防止特性および被削性に優れ、浸炭焼き入れによる熱処理歪みを抑制でき、浸炭焼き入れ後に優れた疲労特性が得られる本発明の肌焼鋼を製造できる。
Since the case-hardened steel of the present invention has a predetermined chemical composition, it has excellent coarse grain prevention characteristics and machinability during carburizing. Therefore, according to the case hardening steel of this invention, while being able to suppress the heat processing distortion by carburizing quenching, the fatigue characteristic outstanding after carburizing quenching is acquired. Moreover, the carburized part manufactured by carburizing and quenching the case-hardened steel of the present invention has less heat treatment distortion and has excellent fatigue characteristics.
According to the method for producing the case-hardened steel of the present invention, the present invention is excellent in coarse grain prevention characteristics and machinability during carburizing, can suppress heat treatment distortion due to carburizing and quenching, and obtains excellent fatigue characteristics after carburizing and quenching. Can be made.

本発明者らは、上記課題を解決するために、鋭意検討した。その結果、以下に示す(1)〜(5)の知見を得た。
(1)肌焼鋼中のS含有量とTi含有量との関係を適正化(5.0≧Ti/S≧3.0)することで、肌焼鋼の浸炭時に、曲げ疲労特性を低下させる圧延方向および/または鍛伸方向に延伸して粗大化するMnSの生成量を最小限に抑制し、微細なTi系の炭硫化物を生成させる。Ti系の炭硫化物は、粗大粒防止のためのピン止め効果を発現する。さらに、Ca、Mg、Teの1種または2種以上を含有することにより、MnSが圧延方向および/または鍛伸方向に延伸することを抑制する。これらの結果、肌焼鋼を浸炭焼き入れした後に優れた疲労特性が得られる。
In order to solve the above-mentioned problems, the present inventors have intensively studied. As a result, the following findings (1) to (5) were obtained.
(1) By optimizing the relationship between S content and Ti content in case-hardened steel (5.0 ≧ Ti / S ≧ 3.0), the bending fatigue characteristics are reduced during carburizing of case-hardened steel. The amount of MnS that is coarsened by stretching in the rolling direction and / or forging direction is minimized, and fine Ti-based carbon sulfide is generated. Ti-based carbon sulfide exhibits a pinning effect for preventing coarse grains. Furthermore, by containing one or more of Ca, Mg, and Te, MnS is prevented from being stretched in the rolling direction and / or the forging direction. As a result, excellent fatigue properties are obtained after carburizing and quenching the case-hardened steel.

これに対し、従来の技術では、肌焼鋼に含まれるTiとSとのバランスおよびMnSの延伸防止を考慮していなかった。このため、肌焼鋼に、曲げ疲労破壊の起点となる圧延方向および/または鍛伸方向に延伸したMnSが存在している可能性があり、肌焼鋼を浸炭焼き入れした後に十分な疲労特性が得られない場合があった。   On the other hand, in the prior art, the balance between Ti and S contained in the case-hardened steel and the prevention of MnS stretching were not taken into consideration. For this reason, there is a possibility that MnS stretched in the rolling direction and / or forging direction, which is the starting point of bending fatigue fracture, exists in the case hardening steel, and sufficient fatigue characteristics after carburizing and quenching the case hardening steel May not be obtained.

(2)肌焼鋼の浸炭時における結晶粒の粗大化を防止するには、ピン止め粒子としてAlN、NbNを活用するよりも、TiC、TiCSを主体とするTi系析出物を、浸炭時に微細析出させることが有効である。さらに、肌焼鋼中に微量のBiを添加することにより、Ti系析出物の浸炭時の成長・粗大化が抑制され、粗大粒防止特性が一層向上する。 (2) To prevent coarsening of crystal grains during carburizing of case-hardened steel, rather than using AlN and NbN as pinning particles, Ti-based precipitates mainly composed of TiC and TiCS are finer during carburizing. It is effective to deposit. Furthermore, by adding a small amount of Bi to the case hardening steel, growth and coarsening of the Ti-based precipitates during carburization are suppressed, and the coarse grain prevention characteristics are further improved.

肌焼鋼の浸炭時にTi系析出物によるピン止め効果を安定して発揮させるためには、肌焼鋼の製造工程における熱間圧延して冷却した後の鋼材中にTi系析出物を微細析出させておく必要がある。そのためには、熱間圧延時の冷却過程におけるオーステナイトからの拡散変態時に、Ti系析出物を相界面析出させる必要がある。熱間圧延ままの組織にベイナイトが生成すると、Ti系析出物の相界面析出が困難になるため、ベイナイトを実質的に含まない組織とすることが好ましい。   In order to stably exhibit the pinning effect of Ti-based precipitates when carburizing case-hardened steel, Ti-based precipitates are finely precipitated in the steel material after hot rolling and cooling in the case-hardened steel manufacturing process. It is necessary to keep it. For this purpose, it is necessary to precipitate the Ti-based precipitates at the phase interface during the diffusion transformation from austenite in the cooling process during hot rolling. When bainite is generated in the structure as hot-rolled, it becomes difficult to precipitate the phase interface of the Ti-based precipitates. Therefore, it is preferable to make the structure substantially free of bainite.

熱間圧延して冷却した後の鋼材中にTi系析出物を微細析出させるには、熱間圧延の条件を最適化すれば良い。すなわち、熱間圧延における加熱温度を高温にすることで、Ti系析出物を一旦マトリックス中に固溶させる。そして、熱間圧延後にTi系析出物の析出温度域を徐冷する。このことにより、ベイナイトの生成を抑制できるとともに、Ti系析出物を多量に生成させて微細分散させることができる。   In order to finely precipitate Ti-based precipitates in the steel material after being hot-rolled and cooled, the hot-rolling conditions may be optimized. That is, by increasing the heating temperature in hot rolling, the Ti-based precipitate is once dissolved in the matrix. And the precipitation temperature range of a Ti-type precipitate is annealed after hot rolling. As a result, the generation of bainite can be suppressed, and a large amount of Ti-based precipitates can be generated and finely dispersed.

(3)さらに、Ti系析出物と併用して、NbCを主体とするNbの炭窒化物を肌焼鋼の浸炭時に微細析出させることにより、粗大粒防止特性が一層向上する。肌焼鋼の浸炭時にNbの炭窒化物によるピン止め効果を安定して発揮させるためには、肌焼鋼の製造工程における熱間圧延して冷却した後の鋼材中にNbの炭窒化物を微細析出させておく必要がある。そのためには、Nbの炭窒化物もTi系析出物と同様に、熱間圧延時の冷却過程におけるオーステナイトからの拡散変態時に、相界面析出させる必要がある。また、熱間圧延ままの組織にベイナイトが生成すると、Nbの炭窒化物の相界面析出が困難になるため、ベイナイトを実質的に含まない組織とすることが好ましい。 (3) Further, by using together with Ti-based precipitates, Nb carbonitrides mainly composed of NbC are finely precipitated at the time of carburizing the case hardening steel, thereby further improving the coarse grain prevention characteristics. In order to stably exhibit the pinning effect of Nb carbonitride during carburizing of case-hardened steel, Nb carbonitride is added to the steel material after hot rolling and cooling in the case-hardened steel manufacturing process. It is necessary to deposit finely. For this purpose, Nb carbonitrides need to be precipitated at the phase interface during diffusion transformation from austenite in the cooling process during hot rolling, as with Ti-based precipitates. In addition, when bainite is generated in a hot-rolled structure, it becomes difficult to precipitate the phase interface of the Nb carbonitride, and therefore it is preferable to have a structure that does not substantially contain bainite.

熱間圧延して冷却した後の鋼材中にNbの炭窒化物を微細析出させるには、熱間圧延における加熱温度を高温にしてNbの炭窒化物を一旦マトリックス中に固溶させた後、Nbの炭窒化物の析出温度域を徐冷する。このことにより、Nbの炭窒化物を多量に生成させて微細分散させることができる。   In order to finely precipitate Nb carbonitride in the steel material after being hot-rolled and cooled, after heating the heating temperature in hot rolling to a high temperature and once dissolving Nb carbonitride in the matrix, The precipitation temperature range of Nb carbonitride is gradually cooled. As a result, a large amount of Nb carbonitride can be produced and finely dispersed.

(4)熱間圧延して冷却した後の鋼材中に含まれるフェライト結晶粒が過度に微細であると、肌焼鋼の浸炭時に粗大粒が発生しやすくなる。熱間圧延して冷却した後の鋼材中のフェライト結晶粒の粒度は、圧延仕上げ温度を制御することで適正化できる。 (4) If the ferrite crystal grains contained in the steel material after hot rolling and cooling are excessively fine, coarse grains are likely to be generated during carburizing of the case-hardened steel. The grain size of the ferrite crystal grains in the steel material after hot rolling and cooling can be optimized by controlling the rolling finishing temperature.

(5)Tiを含有する肌焼鋼を浸炭焼入れして製造した浸炭部品では、Ti系析出物が疲労破壊の起点となるため、疲労特性、特に転動疲労特性が不足しやすくなる。肌焼鋼の化学組成を低N化するとともに、熱間圧延における加熱温度を高温化し、Ti析出物の最大サイズを小さくすることで、疲労特性の改善が可能となる。 (5) In a carburized part manufactured by carburizing and quenching a case-hardened steel containing Ti, the Ti-based precipitate is the starting point of fatigue failure, so that fatigue characteristics, particularly rolling fatigue characteristics, are likely to be insufficient. The fatigue characteristics can be improved by lowering the chemical composition of the case-hardened steel, increasing the heating temperature in hot rolling, and reducing the maximum size of the Ti precipitate.

本発明は、以上の新規な知見に基づいてなされたものである。
以下、本発明の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼およびその製造方法について詳細に説明する。
まず、肌焼鋼の化学組成について説明する。なお、各元素の含有量の「%」は「質量%」を意味する。
The present invention has been made based on the above novel findings.
Hereinafter, the case hardening steel excellent in the coarse grain prevention characteristic at the time of carburizing at the time of this invention, fatigue characteristics, and machinability, and its manufacturing method are demonstrated in detail.
First, the chemical composition of case hardening steel will be described. In addition, “%” of the content of each element means “mass%”.

(C:0.10〜0.30%)
Cは、鋼に必要な強度を与えるのに有効な元素である。C含有量が0.10%未満であると、必要な引張強さを確保できない。C含有量が0.30%を越えると、鋼が硬くなって、冷間加工性が劣化するとともに、浸炭焼き入れ後の芯部靭性が劣化する。したがって、C含有量は、0.10〜0.30%の範囲内にする必要がある。
(C: 0.10 to 0.30%)
C is an element effective for giving the steel the necessary strength. If the C content is less than 0.10%, the required tensile strength cannot be ensured. If the C content exceeds 0.30%, the steel becomes hard, the cold workability deteriorates, and the core toughness after carburizing and quenching deteriorates. Therefore, the C content needs to be in the range of 0.10 to 0.30%.

(Si:0.02〜1.5%)
Siは、鋼の脱酸に有効な元素であるとともに、鋼に必要な強度、焼入れ性を与え、鋼の焼戻し軟化抵抗を向上するのに有効な元素である。Si含有量が0.02%未満であると、上記効果が十分に得られない。一方、Si含有量が1.5%を越えると、鋼の硬さの上昇を招き、冷間鍛造性が劣化する。以上の理由から、Si含有量を0.02〜1.5%の範囲内にする必要がある。
(Si: 0.02-1.5%)
Si is an element effective for deoxidation of steel, and is an element effective for imparting necessary strength and hardenability to the steel and improving the temper softening resistance of the steel. The said effect is not fully acquired as Si content is less than 0.02%. On the other hand, if the Si content exceeds 1.5%, the hardness of the steel is increased, and the cold forgeability deteriorates. For these reasons, the Si content needs to be in the range of 0.02 to 1.5%.

肌焼鋼が冷間加工を受けるものである場合、Si含有量の好適範囲は0.02〜0.3%である。特に、冷間鍛造性を重視する場合は、Si含有量を0.02〜0.15%の範囲にするのが望ましい。また、Siは粒界強度の増加に有効な元素である。さらに、Siは、肌焼鋼が軸受部品、転動部品などの浸炭部品の素材として用いられる場合には、これら浸炭部品の転動疲労過程での組織変化および材質劣化の抑制による高寿命化に有効な元素である。Si添加による高強度化を指向する場合には、Si含有量の好適範囲は0.2〜1.5%である。特に、肌焼鋼が高いレベルの転動疲労強度を有する浸炭部品の素材として用いられる場合には、Si含有量を0.4〜1.5%の範囲にするのが望ましい。   When the case-hardened steel is subjected to cold working, the preferable range of the Si content is 0.02 to 0.3%. In particular, when emphasizing cold forgeability, the Si content is desirably in the range of 0.02 to 0.15%. Si is an element effective for increasing the grain boundary strength. In addition, Si, when case-hardened steel is used as a material for carburized parts such as bearing parts and rolling parts, increases the life of these carburized parts by suppressing structural changes and material deterioration during the rolling fatigue process. It is an effective element. In the case of aiming to increase the strength by adding Si, the preferable range of the Si content is 0.2 to 1.5%. In particular, when the case-hardened steel is used as a material for a carburized part having a high level of rolling fatigue strength, it is desirable that the Si content is in the range of 0.4 to 1.5%.

なお、Si添加による軸受部品、転動部品の転動疲労過程での組織変化および材質劣化の抑制の効果は、浸炭焼き入れした後の組織中の残留オーステナイト量(通称、残留γ量)が30〜40%の時に特に大きい。残留γ量をこの範囲で制御するには、いわゆる浸炭浸窒処理を行うことが有効である。浸炭浸窒処理は、浸炭後の拡散処理の過程で浸窒を行う処理である。浸炭浸窒処理は、表面の窒素濃度が0.2〜0.6%の範囲になる条件が適切である。なお、この場合の浸炭時の炭素ポテンシャルは0.9〜1.3%の範囲とするのが望ましい。   In addition, the effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts and rolling parts due to the addition of Si is that the amount of retained austenite in the structure after carburizing and quenching (commonly called residual γ amount) is 30. Especially large at ~ 40%. In order to control the residual γ amount within this range, it is effective to perform a so-called carburizing and nitriding treatment. The carburizing and nitriding process is a process of performing nitriding in the process of diffusion after carburizing. In the carburizing and nitriding treatment, conditions under which the surface nitrogen concentration is in the range of 0.2 to 0.6% are appropriate. In this case, it is desirable that the carbon potential at the time of carburizing is in the range of 0.9 to 1.3%.

(Mn:0.3〜1.8%)
Mnは、鋼の脱酸に有効な元素であるとともに、鋼に必要な強度、焼入れ性を与えるのに有効な元素である。Mn含有量が0.3%未満では、上記効果が十分に得られない。Mn含有量が1.8%を越えると、その効果は飽和するのみならず、鋼の硬さの上昇を招き、冷間鍛造性が劣化する。そのため、Mn含有量は0.3%〜1.8%の範囲内にする必要がある。Mn含有量の好適範囲は0.5〜1.2%である。なお、鋼の冷間鍛造性を重視する場合には、Mn含有量を0.5〜0.75%の範囲にするのが望ましい。
(Mn: 0.3-1.8%)
Mn is an element effective for deoxidation of steel, and also an element effective for imparting necessary strength and hardenability to the steel. If the Mn content is less than 0.3%, the above effect cannot be obtained sufficiently. If the Mn content exceeds 1.8%, the effect is not only saturated, but also the hardness of the steel is increased, and the cold forgeability deteriorates. Therefore, the Mn content needs to be in the range of 0.3% to 1.8%. The suitable range of Mn content is 0.5 to 1.2%. In addition, when importance is attached to the cold forgeability of steel, it is desirable to make Mn content into the range of 0.5 to 0.75%.

(P:0.050%以下)
Pは、冷間鍛造時の変形抵抗を高め、靭性を劣化させる元素であるため、冷間鍛造性を劣化させる。また、Pは、焼入れ、焼戻し後の部品の結晶粒界を脆化させることによって、疲労強度を劣化させる。したがって、P含有量は、できるだけ低減することが望ましく、0.050%以下に制限する必要がある。P含有量の好適範囲は0.015%以下である。
(P: 0.050% or less)
P is an element that increases deformation resistance during cold forging and degrades toughness, and therefore degrades cold forgeability. Further, P causes fatigue strength to deteriorate by embrittlement of grain boundaries of parts after quenching and tempering. Therefore, it is desirable to reduce the P content as much as possible, and it is necessary to limit it to 0.050% or less. The suitable range of P content is 0.015% or less.

(S:0.020超〜0.050%)
Sは、鋼中でMnSを形成する。MnSは、浸炭部品の曲げ疲労の破壊の起点となりうるため、MnSの生成を防止する必要がある。このため、S含有量の上限を0.050%とし、かつ下記式(1)を満たす範囲とする。S含有量が上記範囲内であると、鋼中のSのうち大部分がTi系炭硫化物として存在するため、浸炭焼き入れ後に優れた疲労特性が得られる。より好ましくは、S含有量は0.035%以下である。また、鋼中のSのうちTi系炭硫化物として存在していない残り一部は、MnSとして存在する。このMnSが被削性を担保するために、S含有量を0.020%超とする必要がある。良好な被削性を得るために、好ましくは、S含有量は0.025%以上である。
(S: more than 0.020 to 0.050%)
S forms MnS in the steel. Since MnS can be a starting point for bending fatigue fracture of carburized parts, it is necessary to prevent the formation of MnS. For this reason, the upper limit of the S content is 0.050%, and the range satisfies the following formula (1). When the S content is within the above range, most of the S in the steel exists as a Ti-based carbon sulfide, so that excellent fatigue characteristics can be obtained after carburizing and quenching. More preferably, the S content is 0.035% or less. Moreover, the remaining part which does not exist as Ti-type carbon sulfide among S in steel exists as MnS. In order for MnS to ensure machinability, the S content needs to be more than 0.020%. In order to obtain good machinability, the S content is preferably 0.025% or more.

5.0≧Ti/S≧3.0 式(1)
(式(1)中のTiは、Tiの含有量(質量%)であり、Sは、Sの含有量(質量%)である。)
5.0 ≧ Ti / S ≧ 3.0 Formula (1)
(Ti in the formula (1) is the content (mass%) of Ti, and S is the content (mass%) of S.)

(Cr:0.4〜2.0%)
Crは、鋼に強度、焼入れ性を与えるのに有効な元素である。さらにCrは、肌焼鋼が軸受部品、転動部品などの浸炭部品の素材として用いられる場合に、浸炭焼き入れした後の残留γ量を増大させるとともに、転動疲労過程での組織変化および材質劣化の抑制による高寿命化に有効な元素である。Cr含有量が0.4%未満ではその効果は不十分である。Cr含有量が2.0%を越えると、鋼の硬さの上昇を招き、冷間鍛造性が劣化する。以上の理由から、Cr含有量を0.4〜2.0%の範囲内にする必要がある。Cr含有量の好適範囲は0.7〜1.6%である。
(Cr: 0.4-2.0%)
Cr is an element effective for imparting strength and hardenability to steel. Further, Cr increases the amount of residual γ after carburizing and quenching when case-hardened steel is used as a material for carburized parts such as bearing parts and rolling parts, and changes in structure and material during rolling fatigue. It is an element that is effective in extending the life by suppressing deterioration. If the Cr content is less than 0.4%, the effect is insufficient. If the Cr content exceeds 2.0%, the hardness of the steel is increased and the cold forgeability deteriorates. For these reasons, the Cr content needs to be in the range of 0.4 to 2.0%. The suitable range of Cr content is 0.7 to 1.6%.

なお、Cr添加による軸受部品、転動部品の転動疲労過程での組織変化および材質劣化の抑制の効果は、浸炭焼き入れした後の組織中の残留γ量が30〜40%の時に特に大きい。残留γ量をこの範囲で制御するには、表面の窒素濃度が0.2〜0.6%の範囲になる条件で、浸炭浸窒処理を行うことが有効である。   The effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts and rolling parts due to the addition of Cr is particularly large when the amount of residual γ in the structure after carburizing and quenching is 30 to 40%. . In order to control the residual γ amount within this range, it is effective to perform the carburizing and nitriding treatment under the condition that the surface nitrogen concentration is in the range of 0.2 to 0.6%.

(Al:0.005〜0.05%)
Alは脱酸剤として添加する。Al含有量が0.005%未満であると、その効果は不十分である。一方、Al含有量が0.05%を越えると、肌焼鋼の製造時に行う熱間圧延における加熱によりAlNが溶体化せずに残存し、Ti(Nbを含有する場合にはTiおよびNb)の析出物の析出サイトとなる。その結果、Ti系析出物(Nbを含有する場合にはTi系析出物およびNbの炭窒化物)の微細分散を阻害し、浸炭時の結晶粒の粗大化を助長する。以上の理由から、Al含有量は0.005〜0.05%の範囲内にする必要がある。Al含有量の好適範囲は0.025〜0.04%である。
(Al: 0.005-0.05%)
Al is added as a deoxidizer. If the Al content is less than 0.005%, the effect is insufficient. On the other hand, when the Al content exceeds 0.05%, AlN remains without solution by heating in hot rolling performed during the production of the case-hardened steel, and Ti (Ti and Nb when Nb is contained) It becomes a precipitation site of the precipitate. As a result, the fine dispersion of Ti-based precipitates (Ti-based precipitates and Nb carbonitrides when Nb is contained) is inhibited, and the coarsening of crystal grains during carburization is promoted. For the above reasons, the Al content needs to be in the range of 0.005 to 0.05%. The preferable range of Al content is 0.025 to 0.04%.

(Ti:0.06〜0.20%)
Tiは、鋼中で微細なTiC、TiCS、TiなどのTi系炭化物、Ti系炭硫化物を生成させ、これにより浸炭時のγ粒の微細化を図るために添加する。Ti含有量が0.06%未満では、その効果は不十分である。一方、0.20%を超えてTiを含有させると、TiCによる析出硬化が顕著になり、冷間加工性が顕著に劣化するとともに、TiN主体の析出物が顕著となり、浸炭焼き入れ後の転動疲労特性が劣化する。以上の理由から、Ti含有量を0.06〜0.20%の範囲内にする必要がある。Ti含有量の好適範囲は、0.10〜0.15%未満である。
(Ti: 0.06-0.20%)
Ti is added to produce fine Ti-based carbides such as TiC, TiCS, and Ti 4 C 2 S 2 in the steel, and Ti-based carbosulfides, thereby making the γ grains finer during carburization. If the Ti content is less than 0.06%, the effect is insufficient. On the other hand, if Ti is contained in an amount exceeding 0.20%, precipitation hardening due to TiC becomes remarkable, cold workability is remarkably deteriorated, and precipitates mainly composed of TiN become prominent, and the conversion after carburizing and quenching is significant. Dynamic fatigue characteristics deteriorate. For these reasons, the Ti content needs to be in the range of 0.06 to 0.20%. The suitable range of Ti content is 0.10 to less than 0.15%.

なお、本発明の肌焼鋼または肌焼鋼を鍛造してなる鍛造部材を浸炭焼き入れすると、固溶Tiと浸炭時に侵入してくる炭素および窒素とが反応して、浸炭層に微細なTiCおよびTiN(以下、「Ti(C,N)」と記す場合がある。)が多量に析出する。これらのTi(C,N)は、浸炭焼き入れ後に得られる軸受部品、転動部品などの浸炭部品における転動疲労寿命の向上に寄与する。したがって、特に高いレベルの転動疲労寿命を指向する軸受部品、転動部品を製造する場合には、浸炭時の炭素ポテンシャルを0.9〜1.3%の範囲で高めに設定すること、あるいは、いわゆる浸炭浸窒処理を行うことにより、Ti(C,N)の析出を促進することが有効である。浸炭浸窒処理は、上記のように浸炭後の拡散処理の過程で浸窒を行う処理であり、表面の窒素濃度が0.2〜0.6%の範囲になる条件が適切である。   In addition, when carburizing and quenching the case-hardened steel or the case-forged member formed by forging the case-hardened steel of the present invention, solute Ti reacts with carbon and nitrogen that enter during carburizing, and the carburized layer has fine TiC. And TiN (hereinafter sometimes referred to as “Ti (C, N)”) precipitates in large amounts. These Ti (C, N) contributes to the improvement of the rolling fatigue life in carburized parts such as bearing parts and rolling parts obtained after carburizing and quenching. Therefore, when manufacturing bearing parts and rolling parts that are particularly oriented to a high level of rolling fatigue life, the carbon potential during carburization should be set high within a range of 0.9 to 1.3%, or It is effective to promote the precipitation of Ti (C, N) by performing so-called carburizing and nitriding treatment. The carburizing and nitriding treatment is a treatment in which nitriding is performed in the course of the diffusion treatment after carburizing as described above, and conditions under which the surface nitrogen concentration is in the range of 0.2 to 0.6% are appropriate.

(Bi:0.0001〜0.0050%)
Biは、本発明において重要な元素である。鋼中に微量のBiを含有すると、鋼の凝固組織の微細化に伴い、硫化物が微細分散する。さらに、鋼中に微量のBiを添加することにより、結晶粒の粗大化を抑制するTi系析出物等の析出物が浸炭時に成長・粗大化することを抑制できる。上記の効果を得るには、Bi含有量を0.0001%以上とする必要がある。しかし、Bi含有量が0.0050%を超えると、デンドライト組織微細化効果が飽和し、かつ鋼の熱間加工性が劣化し、肌焼鋼の製造時に行う熱間圧延が困難となる。これらのことから、Bi含有量を0.0001%〜0.0050%とする。Bi含有量の好適範囲は、0.0010〜0.0025%である。
(Bi: 0.0001 to 0.0050%)
Bi is an important element in the present invention. If a small amount of Bi is contained in the steel, sulfides are finely dispersed as the solidification structure of the steel is refined. Furthermore, by adding a small amount of Bi to the steel, it is possible to suppress the growth and coarsening of precipitates such as Ti-based precipitates that suppress the coarsening of crystal grains during carburizing. In order to obtain the above effect, the Bi content needs to be 0.0001% or more. However, if the Bi content exceeds 0.0050%, the dendritic structure refinement effect is saturated, the hot workability of the steel is deteriorated, and hot rolling performed during the production of the case hardening steel becomes difficult. For these reasons, the Bi content is set to 0.0001% to 0.0050%. The suitable range of Bi content is 0.0010 to 0.0025%.

(Ca:0.0005〜0.0050%、Mg:0.0003〜0.0050%、Te:0.0003〜0.20%の1種または2種以上)
Ca、Mg、Teは、いずれも圧延時および/または鍛伸時にMnSが延伸するのを抑制し、曲げ疲労強度をさらに向上させる元素である。この効果を確実に得るためには、Ca含有量を0.0005%以上、Mg含有量を0.0003%以上、Te含有量を0.0003%以上のうち1種または2種以上を含有する。しかし、Ca、Mg、Teの各元素の含有量が上記を超えると、曲げ疲労強度を向上させる効果が飽和する上、経済性が損なわれる。そのため、Ca、Mg、Teの1種または2種以上を含有させる場合、Ca含有量を0.0050%以下、Mg含有量を0.0050%以下、Te含有量を0.20%以下とする。Ca、Mg、Teの各元素の好ましい含有量は、Caは0.0010〜0.0020%、Mgは0.0007〜0.0015%、Teは0.0007〜0.15%である。
(Ca: 0.0005 to 0.0050%, Mg: 0.0003 to 0.0050%, Te: one or more of 0.0003 to 0.20%)
Ca, Mg, and Te are elements that suppress the elongation of MnS during rolling and / or forging and further improve the bending fatigue strength. In order to ensure this effect, the Ca content is 0.0005% or more, the Mg content is 0.0003% or more, and the Te content is 0.0003% or more. . However, if the content of each element of Ca, Mg, and Te exceeds the above, the effect of improving the bending fatigue strength is saturated and the economy is impaired. Therefore, when one or more of Ca, Mg, and Te are contained, the Ca content is 0.0050% or less, the Mg content is 0.0050% or less, and the Te content is 0.20% or less. . The preferable content of each element of Ca, Mg, and Te is 0.0010 to 0.0020% for Ca, 0.0007 to 0.0015% for Mg, and 0.0007 to 0.15% for Te.

(N:0.01%以下)
Nは、鋼中のTiと結びつくと、粒制御にほとんど寄与しない粗大なTiNを生成する。TiNは、TiC、TiCS主体のTi系析出物、NbC主体のNbCおよびNbN(以下、「Nb(C,N)」と記す場合がある。)の析出サイトとなり、Ti系析出物およびNbの炭窒化物の微細析出を阻害し、粗大粒の生成を促進する。上記の悪影響は、N含有量が0.01%を超える場合に特に顕著である。以上の理由から、N含有量を0.01%以下にする必要がある。N含有量は0.0051%未満に制限するのが望ましい。
(N: 0.01% or less)
N, when combined with Ti in steel, produces coarse TiN that hardly contributes to grain control. TiN serves as a precipitation site for TiC, TiCS-based Ti-based precipitates, NbC-based NbC and NbN (hereinafter sometimes referred to as “Nb (C, N)”), and Ti-based precipitates and Nb charcoal. It inhibits the fine precipitation of nitride and promotes the formation of coarse grains. The above adverse effect is particularly noticeable when the N content exceeds 0.01%. For these reasons, the N content needs to be 0.01% or less. It is desirable to limit the N content to less than 0.0051%.

(O:0.0025%以下)
本発明の肌焼鋼のような高Ti鋼では、鋼中のOはTi系の酸化物系介在物を形成する。Ti系の酸化物系介在物が鋼中に多量に存在すると、TiCの析出サイトとなり、肌焼鋼の製造時に行う熱間圧延時にTiCが粗大析出し、浸炭時に結晶粒の粗大化を抑制できなくなる。そのため、O含有量はできるだけ低減することが望ましい。以上の理由から、O含有量を0.0025%以下に制限する必要がある。O含有量の好適範囲は0.0020%以下である。なお、軸受部品、転動部品などの浸炭部品においては、酸化物系介在物が転動疲労破壊の起点となるので、肌焼鋼のO含有量が低いほど浸炭部品の転動寿命が向上する。そのため、肌焼鋼が軸受部品、転動部品などの浸炭部品の素材として用いられる場合、O含有量を0.0012%以下に制限するのが望ましい。
(O: 0.0025% or less)
In a high Ti steel such as the case-hardened steel of the present invention, O in the steel forms Ti-based oxide inclusions. When Ti-based oxide inclusions are present in a large amount in the steel, they become TiC precipitation sites, TiC coarsely precipitates during hot rolling during the manufacture of case-hardened steel, and can suppress the grain coarsening during carburizing. Disappear. Therefore, it is desirable to reduce the O content as much as possible. For these reasons, it is necessary to limit the O content to 0.0025% or less. A preferable range of the O content is 0.0020% or less. In carburized parts such as bearing parts and rolling parts, oxide inclusions are the starting point for rolling fatigue failure. Therefore, the lower the O content of case-hardened steel, the longer the rolling life of the carburized parts. . Therefore, when case hardening steel is used as a material for carburized parts such as bearing parts and rolling parts, it is desirable to limit the O content to 0.0012% or less.

本発明の肌焼鋼の化学組成では、さらにMo、Ni、V、Bの1種又は2種以上を含有してもよい。
(Mo:0.02〜1.5%)
Moは、鋼に強度、焼入れ性を与える効果があり、さらに軸受部品、転動部品においては、浸炭後の残留γ量を増大させるとともに、転動疲労過程での組織変化、材質劣化の抑制による高寿命化に有効な元素である。その効果を得るためにはMo含有量を0.02%以上とする必要がある。ただし、Mo含有量が1.5%を越えると、硬さの上昇を招き、切削性、冷間鍛造性が劣化する。以上の理由から、Mo含有量を1.5%以下の範囲内にする必要がある。Mo含有量の好適範囲は0.05〜0.5%である。
In the chemical composition of the case hardening steel of this invention, you may contain 1 type (s) or 2 or more types of Mo, Ni, V, and B further.
(Mo: 0.02-1.5%)
Mo has the effect of imparting strength and hardenability to steel, and in bearing parts and rolling parts, it increases the amount of residual γ after carburizing, and also suppresses structural changes and material deterioration during rolling fatigue. It is an element effective for extending the life. In order to acquire the effect, it is necessary to make Mo content 0.02% or more. However, if the Mo content exceeds 1.5%, the hardness increases, and the machinability and cold forgeability deteriorate. For these reasons, the Mo content needs to be in the range of 1.5% or less. A preferable range of the Mo content is 0.05 to 0.5%.

なお、Mo添加による軸受部品、転動部品の転動疲労過程での組織変化および材質劣化の抑制の効果は、Cr添加による上記効果と同様に、浸炭焼き入れした後の組織中の残留γ量が30〜40%の時に特に大きい。   In addition, the effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts and rolling parts due to the addition of Mo is the same as the above effect due to the addition of Cr, the amount of residual γ in the structure after carburizing and quenching. Is particularly large at 30-40%.

(Ni:0.1〜3.5%)
Niは、鋼に強度、焼入れ性を与える効果がある。その効果を得るためにはNi含有量を0.1%以上とする必要がある。ただし、Ni含有量が3.5%を越えると、硬さの上昇を招き、切削性、冷間鍛造性が劣化する。以上の理由から、Ni含有量を3.5%以下の範囲内にする必要がある。Ni含有量の好適範囲は0.2〜2.0%である。
(Ni: 0.1-3.5%)
Ni has the effect of imparting strength and hardenability to the steel. In order to obtain the effect, the Ni content needs to be 0.1% or more. However, if the Ni content exceeds 3.5%, the hardness is increased, and the machinability and cold forgeability deteriorate. For these reasons, the Ni content needs to be in the range of 3.5% or less. The preferred range for the Ni content is 0.2-2.0%.

(V:0.02〜0.5%)
Vは、鋼に強度、焼入れ性を与える効果がある。その効果を得るためにはV含有量を0.02%以上とする必要がある。ただし、V含有量が0.5%を越えると、硬さの上昇を招き、切削性、冷間鍛造性が劣化する。以上の理由から、V含有量を0.5%以下の範囲内にする必要がある。V含有量の好適範囲は0.15〜0.2%である。
(V: 0.02-0.5%)
V has the effect of imparting strength and hardenability to the steel. In order to acquire the effect, it is necessary to make V content 0.02% or more. However, if the V content exceeds 0.5%, the hardness increases, and the machinability and cold forgeability deteriorate. For these reasons, it is necessary to keep the V content within the range of 0.5% or less. The suitable range of V content is 0.15-0.2%.

(B:0.0002〜0.005%)
Bは、鋼に強度、焼入れ性を与えるのに有効な元素である。また、Bは、棒鋼・線材圧延において、圧延後の冷却過程でボロン鉄炭化物を生成することにより、フェライトの成長速度を増加させ、圧延ままで軟質化を促進する効果がある。さらにBは、浸炭材の粒界強度を向上させて、浸炭部品としての疲労強度・衝撃強度を向上させる効果も有する。それらの効果を得るためには、B含有量を0.0002%以上とする必要がある。しかしながら、B含有量が0.005%を超えると、上記の効果は飽和し、かえって衝撃強度劣化等の悪影響が懸念される。したがって、B含有量を0.005%以下の範囲内にする必要がある。B含有量の好適範囲は0.0010〜0.003%である。
(B: 0.0002 to 0.005%)
B is an element effective for imparting strength and hardenability to steel. Further, B has the effect of increasing the growth rate of ferrite and promoting softening as it is rolled by forming boron iron carbide in the cooling process after rolling in the steel bar / wire rolling. Further, B has an effect of improving the grain boundary strength of the carburized material and improving the fatigue strength and impact strength as the carburized component. In order to obtain these effects, the B content needs to be 0.0002% or more. However, if the B content exceeds 0.005%, the above effect is saturated, and there is a concern about adverse effects such as deterioration of impact strength. Therefore, the B content needs to be in the range of 0.005% or less. A preferable range of the B content is 0.0010 to 0.003%.

本発明の肌焼鋼の化学組成では、さらにNbを含有してもよい。
(Nb:0.04%未満)
Nbは、浸炭時に鋼中のC、Nと結びついてNb(C,N)を形成し、結晶粒の粗大化抑制に有効な元素である。Nbを添加することにより、Ti系析出物による粗大粒防止効果が一層有効になる。これは、Ti系析出物にNbが固溶し、Ti系析出物の粗大化を抑制するためである。Nbを含有することによる上記効果は、Nb含有量を増加させることに伴って増大するものの、0.03%未満、あるいは0.02%未満、さらには0.01%未満といった微量添加においても、Nbを含有しない場合に比較して、粗大粒防止特性は顕著に向上する。
The chemical composition of the case hardening steel of the present invention may further contain Nb.
(Nb: less than 0.04%)
Nb is an element effective in suppressing coarsening of crystal grains by forming Nb (C, N) in combination with C and N in steel during carburizing. By adding Nb, the effect of preventing coarse grains due to the Ti-based precipitates becomes more effective. This is because Nb dissolves in the Ti-based precipitate and suppresses the coarsening of the Ti-based precipitate. Although the above-mentioned effect by containing Nb increases as the Nb content is increased, it is less than 0.03%, or less than 0.02%, and even in a small amount such as less than 0.01%, Compared with the case where Nb is not contained, the coarse grain prevention property is remarkably improved.

しかし、Nb添加は、切削性や冷間鍛造性の劣化、浸炭特性の劣化を引き起こす。特に、Nbの含有量が0.04%以上であると、素材の硬さが硬くなって切削性、冷間鍛造性が劣化するとともに、圧延素材を熱間圧延する際の加熱によりNbの炭窒化物を固溶させにくくなる。以上の理由から、Nb含有量を0.04%未満にする必要がある。切削性、冷間鍛造性等の加工性を重視する場合、Nb含有量の好適範囲は0.03%未満である。また、加工性に加えて、浸炭性を重視する場合、Nb含有量の好適範囲は0.02%未満である。さらに、特別に浸炭性を重視する場合、Nb含有量の好適範囲は0.01%未満である。   However, the addition of Nb causes deterioration of machinability, cold forgeability, and carburization characteristics. In particular, when the Nb content is 0.04% or more, the hardness of the material becomes hard and the machinability and cold forgeability deteriorate, and the Nb charcoal is heated by heating when the rolled material is hot-rolled. It becomes difficult to dissolve nitrides. For the above reasons, the Nb content needs to be less than 0.04%. When emphasizing workability such as machinability and cold forgeability, the preferred range of Nb content is less than 0.03%. In addition to workability, when emphasizing carburization, the preferred range of Nb content is less than 0.02%. Furthermore, when carburizing properties are particularly important, the preferred range for the Nb content is less than 0.01%.

また、粗大粒防止特性と加工性との両立を図るために、Nb含有量は、Ti含有量に応じて調整することが推奨される。具体的には、Nb含有量とTi含有量との合計含有量(Ti+Nb)の好適範囲は、0.07〜0.17%未満である。特に、肌焼鋼が高温浸炭されるものや、冷間鍛造されるものである場合、Nb含有量とTi含有量との合計含有量の望ましい範囲は、0.091%超〜0.17%未満である。   In order to achieve both the coarse grain prevention characteristics and the workability, it is recommended that the Nb content be adjusted according to the Ti content. Specifically, the suitable range of the total content (Ti + Nb) of Nb content and Ti content is 0.07 to less than 0.17%. In particular, when the case-hardened steel is high-temperature carburized or cold-forged, the desirable range of the total content of Nb content and Ti content is more than 0.091% to 0.17%. Is less than.

(ベイナイトの組織分率:30%以下)
本発明の肌焼鋼は、ベイナイトの組織分率が30%以下であることが好ましい。肌焼鋼にベイナイト組織が混入していると、浸炭時に粗大粒が発生する原因となる。また、肌焼鋼中のベイナイト組織は、冷間加工性改善の視点からも少ないことが望ましい。肌焼鋼中のベイナイト組織による悪影響は、ベイナイトの組織分率が30%を超えると特に顕著になる。以上の理由から、ベイナイトの組織分率を30%以下に制限することが好ましい。肌焼鋼が高温浸炭されるものである場合など、浸炭時の粗大粒防止に対して浸炭条件が厳しい場合、ベイナイトの組織分率の好適範囲は20%以下である。また、肌焼鋼が冷間鍛造されるものである場合など、浸炭時の粗大粒防止に対してさらに浸炭条件が厳しい場合、ベイナイトの組織分率の好適範囲は10%以下である。
(Bainite structure fraction: 30% or less)
The case-hardened steel of the present invention preferably has a bainite structural fraction of 30% or less. When the bainite structure is mixed in the case-hardened steel, coarse grains are generated during carburizing. Further, it is desirable that the bainite structure in the case hardening steel is small from the viewpoint of improving cold workability. The adverse effect of the bainite structure in the case-hardened steel becomes particularly remarkable when the bainite structure fraction exceeds 30%. For the above reasons, it is preferable to limit the bainite structure fraction to 30% or less. When carburizing conditions are severe for preventing coarse grains during carburizing, such as when case-hardened steel is subjected to high-temperature carburizing, the preferable range of the bainite structure fraction is 20% or less. Moreover, when carburizing conditions are further severe with respect to prevention of coarse grains during carburization, such as when the case-hardened steel is cold-forged, the preferred range of the bainite structure fraction is 10% or less.

(フェライト結晶粒度:8〜11番)
本発明の肌焼鋼は、フェライト結晶粒度番号がJIS G0552で規定されている8〜11番であることが好ましい。肌焼鋼のフェライト粒が過度に微細であると、浸炭時にオーステナイト粒が過度に微細化する。オーステナイト粒が過度に微細になると、粗大粒が生成しやすくなる。特に、フェライト結晶粒度がJIS G0552で規定されている11番を超えると、その傾向が顕著になる。また、オーステナイト結晶粒度がJIS G0551で規定されている11番を超えて過度に微細になると、特開平2003−34843公報の鋼材と同様に、焼入れ性の劣化による強度不足等の弊害を生じる。一方、フェライト結晶粒度番号がJIS G0552で規定されている8番未満であると、フェライト結晶粒度が粗粒であるため、延性が劣化し、冷間鍛造性が劣化する。以上の理由から、フェライト結晶粒度番号をJIS G0552で規定されている8〜11番の範囲内にすることが好ましい。
(Ferrite grain size: No. 8-11)
The case-hardened steel of the present invention preferably has a ferrite crystal grain size number of 8 to 11 as defined in JIS G0552. When the ferrite grains of the case-hardened steel are excessively fine, the austenite grains are excessively refined during carburizing. When austenite grains become excessively fine, coarse grains tend to be generated. In particular, when the ferrite crystal grain size exceeds No. 11 defined in JIS G0552, the tendency becomes remarkable. Further, when the austenite grain size exceeds # 11 specified in JIS G0551, it becomes adversely affected such as insufficient strength due to deterioration of hardenability as in the steel material disclosed in Japanese Patent Laid-Open No. 2003-34843. On the other hand, if the ferrite crystal grain size number is less than No. 8 defined in JIS G0552, the ferrite crystal grain size is coarse, so the ductility deteriorates and the cold forgeability deteriorates. For the above reasons, it is preferable to set the ferrite grain size number within the range of 8 to 11 defined in JIS G0552.

(Ti系析出物の最大直径:40μm以下)
本発明の肌焼鋼は、マトリックス中の長手方向断面において、検査基準面積:100平方mm、検査数:16視野、予測を行なう面積:30000平方mmの条件で測定された極値統計によるTi系析出物の最大直径が40μm以下であることが好ましい。
本発明で対象とする浸炭部品の要求特性の一つとして、転動疲労特性や面疲労強度のような接触疲労強度の向上が挙げられる。肌焼鋼中に粗大なTi系析出物が存在すると、これを浸炭焼入れして製造した浸炭部品における接触疲労破壊の起点となり、疲労特性が劣化する。
(Maximum diameter of Ti-based precipitate: 40 μm or less)
The case-hardened steel of the present invention is a Ti-based material based on extreme value statistics measured under the conditions of inspection standard area: 100 square mm, number of inspections: 16 fields of view, prediction area: 30000 square mm in the longitudinal section in the matrix. It is preferable that the maximum diameter of the precipitate is 40 μm or less.
One of the required characteristics of the carburized parts targeted in the present invention is an improvement in contact fatigue strength such as rolling fatigue characteristics and surface fatigue strength. If coarse Ti-based precipitates are present in the case-hardened steel, it becomes a starting point for contact fatigue failure in a carburized part manufactured by carburizing and quenching, and fatigue characteristics deteriorate.

極値統計により、検査基準面積:100平方mm、検査数16視野、予測を行なう面積:30000平方mmの条件で測定した時のTi系析出物の最大直径が40μmを超えると、特に、接触疲労特性に及ぼすTi系析出物の悪影響が顕著になる。以上の理由から、上記条件で測定された極値統計によるTi系析出物の最大直径を40μm以下とすることが好ましく、30μm以下とすることがより好ましい。   According to extreme value statistics, when the maximum diameter of the Ti-based precipitate exceeds 40 μm when measured under the conditions of inspection standard area: 100 square mm, inspection number of 16 fields, prediction area: 30000 square mm, particularly contact fatigue The adverse effect of Ti-based precipitates on the properties becomes significant. For the above reasons, the maximum diameter of the Ti-based precipitates according to the extreme value statistics measured under the above conditions is preferably 40 μm or less, and more preferably 30 μm or less.

極値統計による析出物の最大直径の測定・予測方法は、1993年3月8日養賢堂発行の「金属疲労 微小欠陥と介在物の影響」233頁〜239頁に記載の方法による。なお、本発明で用いているのは、二次元的検査により一定面積内(予測を行なう面積:30000平方mm)で観察される最大析出物を推定するという二次元的検査方法である。詳細な測定手順は、実施例欄で述べる。   The method for measuring and predicting the maximum diameter of precipitates by extreme value statistics is based on the method described in pages 233 to 239 of “Effects of Metal Fatigue Microdefects and Inclusions” published on March 8, 1993 by Yokendo. In addition, what is used in the present invention is a two-dimensional inspection method in which the maximum precipitate observed within a certain area (predicted area: 30000 square mm) is estimated by a two-dimensional inspection. Detailed measurement procedures are described in the Examples section.

次に、本発明の肌焼鋼の製造方法について詳細に説明する。
まず、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、鋳造することにより上記の化学組成の鋳片とし、必要に応じて分塊圧延工程を経て、線材または棒鋼に熱間圧延する圧延素材とする。本実施形態では、鋳片のサイズ、凝固時の冷却速度、分塊圧延条件については、特に限定するものではなく、本発明の要件を満足すればいずれの条件でも良い。
次に、上記の化学組成を有する圧延素材を、以下に示す方法により、線材または棒鋼に熱間圧延し、熱間圧延して冷却した後に得られた鋼材である本実施形態の肌焼鋼を得る。
Next, the manufacturing method of the case hardening steel of this invention is demonstrated in detail.
First, steel is melted by a normal method such as a converter, an electric furnace, etc., the components are adjusted, and cast into a slab of the above chemical composition by casting, and if necessary, through a block rolling process, a wire or A rolling material that is hot-rolled into a steel bar. In the present embodiment, the size of the slab, the cooling rate during solidification, and the ingot rolling conditions are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied.
Next, the rolled material having the above chemical composition is hot rolled into a wire or a steel bar by the method shown below, and the case-hardened steel according to the present embodiment, which is a steel obtained after being hot-rolled and cooled, is obtained. obtain.

(加熱温度、保持時間)
本実施形態では、上記の化学組成を有する圧延素材を、1150℃以上の温度で保持時間10分以上加熱して線材または棒鋼に熱間圧延する。熱間圧延における加熱温度が1150℃以上で保持時間が10分以上であると、Ti系析出物、AlN(Nbを含有する場合には、Ti系析出物、Nbの析出物、AlN)をマトリックス中に十分に固溶させることができ、浸炭時の粗大粒防止特性に優れる。
(Heating temperature, holding time)
In this embodiment, the rolling raw material having the above chemical composition is heated to a temperature of 1150 ° C. or higher for a holding time of 10 minutes or longer and hot-rolled to a wire or a steel bar. When the heating temperature in hot rolling is 1150 ° C. or higher and the holding time is 10 minutes or longer, a Ti-based precipitate, AlN (when Nb is contained, a Ti-based precipitate, a Nb precipitate, AlN) is matrixed It can be dissolved sufficiently in the inside, and has excellent properties for preventing coarse grains during carburizing.

これに対し、熱間圧延における加熱温度が1150℃未満である、および/または保持時間が10分未満であると、Ti系析出物、AlN(Nbを含有する場合には、Ti系析出物、Nbの析出物、AlN)をマトリックス中に十分に固溶させることができない。その結果、熱間圧延して冷却した後の鋼材に、Ti系析出物(Nbを含有する場合には、Ti系析出物およびNbの析出物)を微細析出させることができず、熱間圧延して冷却した後の鋼材は、粗大なTi系析出物およびAlN(Nbを含有する場合には、粗大なTi系析出物、Nbの析出物、AlN)が存在するものとなる。したがって、熱間圧延して冷却した後の鋼材は、浸炭時における粗大粒の発生を抑制できない。そのため、熱間圧延するに際して、1150℃以上の温度で保持時間10分以上加熱することが必要である。熱間圧延における加熱条件の好適範囲は1180℃以上の温度で保持時間10分以上である。   On the other hand, when the heating temperature in hot rolling is less than 1150 ° C. and / or the holding time is less than 10 minutes, Ti-based precipitates, AlN (when containing Nb, Ti-based precipitates, Nb precipitates (AlN) cannot be sufficiently dissolved in the matrix. As a result, Ti-based precipitates (Ti-based precipitates and Nb precipitates in the case of containing Nb) cannot be finely precipitated in the steel material that has been hot-rolled and cooled. After cooling, the steel material has coarse Ti-based precipitates and AlN (in the case of containing Nb, coarse Ti-based precipitates, Nb precipitates, and AlN). Therefore, the steel material after being hot-rolled and cooled cannot suppress the generation of coarse grains during carburization. Therefore, when hot rolling, it is necessary to heat at a temperature of 1150 ° C. or higher for a holding time of 10 minutes or longer. A suitable range of heating conditions in the hot rolling is a temperature of 1180 ° C. or higher and a holding time of 10 minutes or longer.

(仕上げ温度)
本実施形態では、熱間圧延の仕上げ温度を840〜1000℃とすることが好ましい。熱間圧延の仕上げ温度を上記範囲とすることにより、フェライト結晶粒度番号がJIS G0552で規定されている8〜11番である鋼が得られる。仕上げ温度が840℃未満であると、フェライト結晶粒度が過度に微細になりすぎて、浸炭時に粗大粒が発生しやすくなる。一方、仕上げ温度が1000℃を超えると、フェライト結晶粒度が粗粒となり、熱間圧延して冷却した後の鋼材の硬さが硬くなって、冷間鍛造性が劣化する。以上の理由から、熱間圧延の仕上げ温度を840〜1000℃とすることが好ましく、920〜1000℃の範囲がより望ましい。熱間圧延の仕上げ温度は、肌焼鋼が冷間鍛造されるものであって、冷間鍛造後、浸炭焼入れの前に、焼鈍を行わない場合には、840〜920℃の範囲であることがより好ましい。
(Finishing temperature)
In this embodiment, it is preferable that the finishing temperature of hot rolling shall be 840-1000 degreeC. By setting the hot rolling finishing temperature within the above range, steel having a ferrite grain size number of 8 to 11 as defined in JIS G0552 can be obtained. When the finishing temperature is less than 840 ° C., the ferrite crystal grain size becomes excessively fine, and coarse grains are likely to be generated during carburizing. On the other hand, when the finishing temperature exceeds 1000 ° C., the ferrite crystal grain size becomes coarse, the hardness of the steel material after hot rolling and cooling becomes hard, and the cold forgeability deteriorates. For the above reasons, it is preferable that the hot rolling finish temperature is 840 to 1000 ° C., and a range of 920 to 1000 ° C. is more desirable. The finish temperature of hot rolling is a range of 840 to 920 ° C. when the case-hardened steel is cold-forged and not annealed after cold forging and before carburizing and quenching. Is more preferable.

(冷却速度)
本実施形態では、熱間圧延後に800〜500℃の温度範囲を1℃/秒以下の冷却速度で徐冷することが好ましい。熱間圧延後に上記の冷却条件で冷却することにより、Ti系析出物の析出温度域の通過時間を十分に確保でき、微細なTi系析出物の分散が促進されるとともに、ベイナイトの組織分率を抑制できる。その結果、ベイナイトの組織分率が30%以下であり、より一層、浸炭時の粗大粒防止特性に優れる鋼が得られる。上記温度範囲での冷却速度が1℃/秒を越えると、ベイナイトの組織分率が大きくなる。また、上記温度範囲での冷却速度が大きいと、熱間圧延して冷却した後の鋼材の硬さが上昇し、冷間鍛造性が劣化する。このため、上記温度範囲での冷却速度はできるだけ小さくするのが望ましい。上記温度範囲での冷却速度の好適範囲は0.7℃/秒以下である。
(Cooling rate)
In this embodiment, it is preferable to gradually cool the temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./second or less after hot rolling. By cooling under the above cooling conditions after hot rolling, it is possible to sufficiently secure the passage time of the Ti-based precipitates in the precipitation temperature range, promote the dispersion of fine Ti-based precipitates, and promote the structure fraction of bainite. Can be suppressed. As a result, the bainite has a structure fraction of 30% or less, and a steel that is further excellent in coarse grain prevention characteristics during carburization can be obtained. When the cooling rate in the above temperature range exceeds 1 ° C./second, the bainite structure fraction increases. Moreover, when the cooling rate in the said temperature range is large, the hardness of the steel materials after hot-rolling and cooling will raise, and cold forgeability will deteriorate. For this reason, it is desirable to make the cooling rate in the above temperature range as small as possible. A preferable range of the cooling rate in the above temperature range is 0.7 ° C./second or less.

なお、冷却速度を小さくする方法としては、例えば、熱間圧延ラインの後方に保温カバーまたは熱源付き保温カバーを設置し、保温カバーにより熱間圧延後の鋼材の徐冷を行う方法が挙げられる。   In addition, as a method of reducing a cooling rate, the method of installing the heat insulation cover or the heat insulation cover with a heat source in the back of a hot rolling line, and performing the slow cooling of the steel material after hot rolling with a heat insulation cover is mentioned, for example.

本実施形態では、熱間圧延して冷却した後に得られた鋼材((線材または棒鋼):肌焼鋼)に、必要に応じて球状化焼鈍を行ってもよい。   In this embodiment, the steel material obtained after hot rolling and cooling ((wire material or bar steel): case-hardened steel) may be subjected to spheroidizing annealing as necessary.

本実施形態の肌焼鋼を用いて浸炭部品を製造する場合、熱間鍛造してから浸炭焼入れを行ってもよいし、冷間鍛造してから浸炭焼入れを行ってもよい。   When manufacturing carburized parts using the case-hardened steel of this embodiment, carburizing and quenching may be performed after hot forging, or carburizing and quenching may be performed after cold forging.

肌焼鋼に熱間鍛造してから浸炭焼入れを行って浸炭部品を製造する場合、例えば「肌焼鋼(線材または棒鋼)−熱間鍛造−必要により焼準(焼きならし)等の熱処理−切削−浸炭焼入れ−必要により研磨」の工程を経て製造する方法が挙げられる。
具体的には、例えば、熱間鍛造は、1150℃以上の加熱温度で行うことができる。
また、浸炭焼き入れの際の条件は特に限定されない。例えば、浸炭温度を950℃〜1090℃の温度域とする高温浸炭を行うことができる。また、浸炭部品における転動疲労寿命を向上させるために、浸炭時の炭素ポテンシャルを0.9〜1.3%の範囲で高めに設定してもよい。また、浸炭後の拡散処理の過程で浸窒を行う浸炭浸窒処理を行ってもよい。浸炭浸窒処理は、浸炭部品における転動疲労寿命を向上させるために、表面の窒素濃度が0.2〜0.6%の範囲になる条件が適切である。
When carburized parts are manufactured by carburizing and quenching after hot forging of case-hardened steel, for example, “Skin-hardened steel (wire rod or bar)-hot forging-heat treatment such as normalization (normalizing) if necessary- The method of manufacturing through the process of "cutting-carburizing quenching-polishing if necessary" is mentioned.
Specifically, for example, hot forging can be performed at a heating temperature of 1150 ° C. or higher.
Moreover, the conditions at the time of carburizing and quenching are not particularly limited. For example, high temperature carburization can be performed with the carburizing temperature in the temperature range of 950 ° C. to 1090 ° C. Moreover, in order to improve the rolling fatigue life in the carburized part, the carbon potential at the time of carburizing may be set higher within a range of 0.9 to 1.3%. Moreover, you may perform the carburizing nitriding process which performs nitriding in the process of the diffusion process after carburizing. In the carburizing and nitriding treatment, in order to improve the rolling fatigue life of the carburized component, the condition that the surface nitrogen concentration is in the range of 0.2 to 0.6% is appropriate.

本実施形態の肌焼鋼は、被削性に優れる。また、本実施形態の肌焼鋼は、浸炭時の粗大粒防止特性に優れるので、浸炭焼き入れによる熱処理歪みを抑制できるとともに、浸炭焼き入れ後に優れた疲労特性を有する浸炭部品が得られる。このため、例えば、本実施形態の肌焼鋼を鍛造した後に高温浸炭を行うことにより、浸炭時間を短縮することができる。また、従来、熱処理歪みによる寸法精度の劣化のために、熱間鍛造から冷間鍛造への切り換えられなかった浸炭部品においても、冷間鍛造への切り替えが可能となる。また、従来、冷間鍛造後に行っていた、熱処理歪みを抑制するための焼鈍を省略できる。
以上のように、本発明による産業上の効果は極めて顕著である。
The case hardening steel of this embodiment is excellent in machinability. Moreover, since the case hardening steel of this embodiment is excellent in the coarse grain prevention characteristic at the time of carburizing, while being able to suppress the heat processing distortion by carburizing quenching, the carburized component which has the outstanding fatigue characteristic after carburizing quenching is obtained. For this reason, for example, carburizing time can be shortened by performing high temperature carburizing after forging the case hardening steel of this embodiment. In addition, even carburized parts that have not been switched from hot forging to cold forging due to deterioration of dimensional accuracy due to heat treatment strain can be switched to cold forging. Moreover, the annealing for suppressing heat-treatment distortion conventionally performed after cold forging can be omitted.
As described above, the industrial effects of the present invention are extremely remarkable.

以下に、本発明を実施例により、具体的に説明する。
表1に示す組成を有する転炉溶製鋼を連続鋳造して鋳片とし、必要に応じて分塊圧延工程を経て、162mm角の圧延素材とした。
続いて、圧延素材を、表2に示す加熱温度で保持時間を10分以上として加熱し、表2に示す熱間圧延の仕上げ温度で熱間圧延し、熱間圧延後に800〜500℃の温度範囲を表2に示す冷却速度で冷却し、直径24〜30mmの棒鋼を製造した。
Hereinafter, the present invention will be described specifically by way of examples.
Converter molten steel having the composition shown in Table 1 was continuously cast to form a slab, and, if necessary, a batch rolling process was performed to obtain a 162 mm square rolled material.
Subsequently, the rolling material is heated at a heating temperature shown in Table 2 with a holding time of 10 minutes or more, hot-rolled at the finishing temperature of hot rolling shown in Table 2, and a temperature of 800 to 500 ° C. after hot rolling. The range was cooled at a cooling rate shown in Table 2 to produce a steel bar having a diameter of 24 to 30 mm.

Figure 2017133052
Figure 2017133052

Figure 2017133052
Figure 2017133052

熱間圧延して冷却した後の各棒鋼(肌焼鋼)について、ミクロ組織の観察を行い、以下に示す方法により、ベイナイトの組織分率を測定した。
各棒鋼(肌焼鋼)について、JIS G0552の規定にしたがって、フェライト結晶粒度の測定を行ない、粒度番号を調べた。
For each steel bar (skin-hardened steel) after being hot-rolled and cooled, the microstructure was observed and the structure fraction of bainite was measured by the method described below.
For each steel bar (skin-hardened steel), the ferrite crystal grain size was measured in accordance with JIS G0552, and the grain size number was examined.

また、各棒鋼(肌焼鋼)について、以下に示す方法により、極値統計によるTi系析出物の最大直径を調べた。
さらに、各棒鋼(肌焼鋼)について、冷間加工性の指標として、以下に示す方法により、ビッカース硬さを測定した。
それらの結果を表2に示す。
Moreover, the maximum diameter of the Ti-type precipitate by extreme value statistics was investigated by the method shown below about each bar (hardened steel).
Furthermore, Vickers hardness was measured by the method shown below as an index of cold workability for each steel bar (skin-hardened steel).
The results are shown in Table 2.

また、被削性の指標として、各棒鋼を21mm長さで切断し、下記の切削条件でドリル寿命試験を行い、累積穴深さ1000mmを達成する最大周速度が45m/min以上のものを「○」、45m/min未満のものを「×」と評価した。その結果を表2に示す。
「切削条件」
ドリル直径;3mm、ドリル材質;高速度鋼(ハイス)、ドリル周速;45mm/min、ドリル送り;0.25mm/rev、切削油剤;水溶性切削油
In addition, as an index of machinability, each steel bar was cut at a length of 21 mm, a drill life test was performed under the following cutting conditions, and the maximum peripheral speed for achieving a cumulative hole depth of 1000 mm was 45 m / min or more. “Good”, less than 45 m / min was evaluated as “x”. The results are shown in Table 2.
"Cutting conditions"
Drill diameter: 3 mm, drill material: high-speed steel (high speed), drill peripheral speed: 45 mm / min, drill feed: 0.25 mm / rev, cutting fluid: water-soluble cutting oil

「ベイナイトの組織分率」
各棒鋼(肌焼鋼)を、軸方向に対して垂直な方向で切断(横断)してサンプルを採取した。得られたサンプルを樹脂に埋め込んだ後、上記切断された面(観察面)を研磨した。研磨後の観察面に対してナイタール腐食を実施してミクロ組織を観察し、ミクロ組織中のベイナイト組織を特定した。さらに、観察面において、ベイナイト組織の面積率を求め、ベイナイトの組織分率(%)とした。
"Bainite structure fraction"
Each bar (hardened steel) was cut (crossed) in a direction perpendicular to the axial direction, and a sample was taken. After the obtained sample was embedded in resin, the cut surface (observation surface) was polished. Nital corrosion was performed on the observation surface after polishing to observe the microstructure, and the bainite structure in the microstructure was identified. Furthermore, on the observation surface, the area ratio of the bainite structure was determined and used as the bainite structure fraction (%).

「Ti系析出物の最大直径」
極値統計法によるTi系析出物の最大直径の予測は、次の方法で行なった。析出物がTi系であるか否かは、光学顕微鏡におけるコントラストの違いからを判別した。コントラストの違いによる判別法の妥当性は、あらかじめエネルギー分散型X線分光分析装置付き走査型電子顕微鏡にて確認した。
各棒鋼(肌焼鋼)から試験片を採取し、棒鋼の長手方向断面において検査基準面積100平方mm(10mm×10mmの領域)の領域をあらかじめ16視野分準備した。そして各検査基準面積100平方mmにおけるTi系析出物の最大析出物を検出し、これを光学顕微鏡にて1000倍で写真撮影した。各検査基準面積100平方mmの16視野について、16回繰り返し行なった(つまり検査回数16視野)。得られた写真から各検査基準面積における最大析出物の直径を計測した。析出物が楕円形である場合は、長径と短径の相乗平均を求め、その析出物の直径とした。得られた最大析出物の直径の16個のデータを、養賢堂発行「金属疲労 微小欠陥と介在物の影響」233頁〜239頁に記載の方法により、極値確率用紙にプロットし、最大析出物分布直線(最大析出物直径と極値統計基準化変数の一次関数)を求め、最大析出物分布直線を外挿することにより、予測を行なう面積:30000平方mmにおける最大析出物の直径を予測した。
“Maximum diameter of Ti-based precipitates”
The prediction of the maximum diameter of the Ti-based precipitate by the extreme value statistical method was performed by the following method. Whether or not the precipitate is Ti-based was determined from the difference in contrast in the optical microscope. The validity of the discrimination method based on the difference in contrast was confirmed in advance with a scanning electron microscope equipped with an energy dispersive X-ray spectrometer.
Test pieces were collected from each steel bar (skin-hardened steel), and an area having an inspection reference area of 100 square mm (10 mm × 10 mm region) was prepared for 16 fields of view in the longitudinal section of the steel bar. And the largest deposit of Ti system deposit in each inspection standard area 100 square mm was detected, and this was photographed 1000 times with the optical microscope. For 16 visual fields of each inspection standard area of 100 square mm, the test was repeated 16 times (that is, 16 visual fields were inspected). The diameter of the largest deposit in each inspection reference area was measured from the obtained photograph. When the precipitate was elliptical, the geometric mean of the major axis and the minor axis was determined and used as the diameter of the precipitate. 16 data of the diameters of the obtained maximum precipitates are plotted on the extreme probability sheet by the method described in pages 233 to 239 of “Effects of Metal Fatigue Micro Defects and Inclusions” published by Yokendo. Precipitate distribution line (maximum precipitate diameter and linear function of extreme value statistical normalization variable) is determined, and extrapolation of the maximum precipitate distribution line is used to predict the area of the maximum precipitate at 30000 square mm. Predicted.

「ビッカース硬さ(HV)」
圧延後の各棒鋼(肌焼鋼)を、軸方向に対して垂直な方向で切断(横断)してサンプルを採取した。得られたサンプルを樹脂に埋め込んだ後、上記切断された面(観察面)を研磨した。研磨後の観察面に対して表面から直径の1/4の深さの部位について、JIS Z 2244(2009)における「ビッカース硬さ試験−試験方法」に準拠して、荷重10kgでビッカース硬度を合計5回測定し、その平均値をビッカース硬さとした。
"Vickers hardness (HV)"
Each rolled steel bar (case-hardened steel) was cut (crossed) in a direction perpendicular to the axial direction, and a sample was taken. After the obtained sample was embedded in resin, the cut surface (observation surface) was polished. Based on the “Vickers hardness test-test method” in JIS Z 2244 (2009), the Vickers hardness is summed at a load of 10 kg for a portion having a depth of ¼ of the diameter from the surface relative to the observation surface after polishing. The measurement was made 5 times, and the average value was defined as Vickers hardness.

各棒鋼(肌焼鋼)について、球状化焼鈍を行った後、据え込み試験片を作成し、圧下率50%の据え込みを行った後、以下に示す条件で浸炭シミュレーションを行った。
浸炭シミュレーションは、加熱温度を1000℃、1050℃、1100℃の3種類とし、いずれの加熱温度の場合も5時間加熱した後水冷した。浸炭シミュレーションの後の各試験片の切断面を研磨してから腐食し、旧オーステナイト粒径を観察して粗粒発生温度(結晶粒粗大化温度)を求めた。旧オーステナイト粒度の測定は、JIS G 0551に準じて行い、400倍で10視野程度観察し、粒度番号5番以下の粗粒が1つでも存在すれば粗粒発生と判定した。
そして、粗大粒発生温度が、1100℃超のものは結晶粒粗大化特性が良好であると判定し、1100℃以下のものは結晶粒粗大化特性に劣ると判定した。表2に粗粒発生温度を示す。
For each steel bar (skin-hardened steel), after performing spheroidizing annealing, an upsetting test piece was prepared, and after upsetting at a rolling reduction of 50%, carburization simulation was performed under the following conditions.
In the carburizing simulation, three heating temperatures of 1000 ° C., 1050 ° C., and 1100 ° C. were used, and at any heating temperature, after heating for 5 hours, water cooling was performed. The cut surface of each test piece after carburizing simulation was polished and then corroded, and the prior austenite grain size was observed to determine the coarse grain generation temperature (crystal grain coarsening temperature). The prior austenite grain size was measured in accordance with JIS G 0551, observed at 400 magnifications for about 10 fields of view, and if any coarse grain having a grain size number of 5 or less was present, it was determined that coarse grains were generated.
Then, it was determined that the coarse grain generation temperature was higher than 1100 ° C., and the crystal grain coarsening characteristics were good, and those having a coarse grain generation temperature of 1100 ° C. or lower were judged to be inferior in crystal grain coarsening characteristics. Table 2 shows the coarse particle generation temperature.

次に、各棒鋼(肌焼鋼)に圧下率50%で冷間鍛造を行なって、直径12.2mmの円柱状の転動疲労試験片と平行部の直径が9mmの小野式回転曲げ試験片(R1.14の切欠付き)を作製した。小野式回転曲げ試験片については、試験部となる平行部のみを圧延方向に垂直に採取し、両端の掴み部については別の鋼材で制作したものを摩擦圧接にて接合し作製した。
作製した転動疲労試験片および小野式回転曲げ試験片について、1050℃で5時間、炭素ポテンシャル0.8%の条件で浸炭を行なった。焼入れ油の温度は130℃、焼戻しは180℃で2時間行った。
Next, each bar steel (skin-hardened steel) was cold forged at a reduction rate of 50%, and a cylindrical rolling fatigue test piece having a diameter of 12.2 mm and an Ono rotary bending test piece having a parallel part diameter of 9 mm. (With a notch of R1.14) was produced. For the Ono-type rotating bending test piece, only the parallel part as the test part was sampled perpendicularly to the rolling direction, and the grip parts at both ends were produced by welding with other steel materials by friction welding.
The produced rolling fatigue test piece and Ono type rotary bending test piece were carburized at 1050 ° C. for 5 hours under the condition of a carbon potential of 0.8%. The temperature of the quenching oil was 130 ° C., and tempering was performed at 180 ° C. for 2 hours.

得られた各浸炭焼入れ材について、以下に示す方法により、浸炭層のγ(オーステナイト)粒度番号を調査した。
浸炭焼入れ焼戻し後の小野式回転曲げの平行部を、軸方向に対して垂直な方向で切断(横断)してサンプルを採取した。得られたサンプルを樹脂に埋め込んだ後、上記切断された面(観察面)を研磨した。研磨後の観察面に対してオーステナイト粒を現出する腐食を行い、JIS G0551の規定にしたがって、表面から200μm深さの位置を中心とした視野で、オーステナイト粒度を測定し、粒度番号を調べた。
About each obtained carburizing quenching material, (gamma) (austenite) particle size number of the carburized layer was investigated by the method shown below.
Samples were collected by cutting (crossing) the parallel part of the Ono rotary bending after carburizing and tempering in a direction perpendicular to the axial direction. After the obtained sample was embedded in resin, the cut surface (observation surface) was polished. Corrosion that reveals austenite grains was performed on the observation surface after polishing, and the austenite grain size was measured in a visual field centered at a position 200 μm deep from the surface in accordance with JIS G0551, and the grain number was examined. .

各浸炭焼入れ材について、点接触型転動疲労試験機(ヘルツ最大接触応力5884MPa)を用いて転動疲労特性を評価した。転動疲労特性の疲労寿命の尺度として「試験結果をワイブル確率紙にプロットして得られる累積破損確率10%における疲労破壊までの応力繰り返し数」として定義されるL10寿命を用いた。転動疲労寿命は、比較鋼であるNo.16のL10寿命を1とした時の各材料のL10寿命の相対値を示した。   About each carburizing hardening material, the rolling fatigue characteristic was evaluated using the point contact type | mold rolling fatigue test machine (Hertz maximum contact stress 5884MPa). L10 life defined as “the number of stress repetitions until fatigue failure at a cumulative failure probability of 10% obtained by plotting test results on Weibull probability paper” was used as a measure of fatigue life of rolling fatigue characteristics. The rolling fatigue life is No. which is a comparative steel. The relative value of the L10 life of each material when the L10 life of 16 is taken as 1 is shown.

各浸炭焼入れ材について、小野式回転曲げ疲労試験装置を用いて曲げ疲労強度を評価した。回転曲げ疲労強度については550MPaの応力で10000000回耐久したものを「○」と評価し、破断したものを「×」と評価した。
これらの結果をまとめて表2に示す。
About each carburizing hardening material, bending fatigue strength was evaluated using the Ono type | formula rotation bending fatigue testing apparatus. With respect to the rotational bending fatigue strength, one that was endured 10000000 times with a stress of 550 MPa was evaluated as “◯”, and one that was broken was evaluated as “x”.
These results are summarized in Table 2.

表2に示すように、本発明鋼は、被削性(ドリル寿命試験)が良好であり、結晶粒粗大化温度は1100℃超であり、1050℃浸炭焼入れ材のγ粒度が整細粒であり、転動疲労寿命および回転曲げ疲労試験の結果も良好であった。   As shown in Table 2, the steel of the present invention has good machinability (drill life test), the grain coarsening temperature is over 1100 ° C, and the γ grain size of the 1050 ° C carburized quenching material is fine grain. The results of rolling fatigue life and rotating bending fatigue test were also good.

一方、比較例11は、MnSを圧延方向に延伸するのを抑制するCa,Mg,Teを含まないため、MnSが延伸して回転曲げ疲労強度が低下した。
また、比較鋼であるNo.12、13は、Biを含まないため、結晶粒粗大化温度が本発明鋼と比べて低かった。
また、比較鋼であるNo.14は、Bi含有量が、本発明で規定する上限を超えているため、熱間圧延時に生じたと推定される初期き裂が存在しており、転動疲労寿命および回転曲げ疲労試験の結果が本発明鋼と比べて劣っていた。
On the other hand, since Comparative Example 11 does not contain Ca, Mg, or Te that suppresses stretching of MnS in the rolling direction, MnS was stretched and rotational bending fatigue strength was reduced.
Moreover, No. which is a comparative steel. Since 12 and 13 do not contain Bi, the crystal grain coarsening temperature was lower than that of the steel of the present invention.
Moreover, No. which is a comparative steel. No. 14, since the Bi content exceeds the upper limit specified in the present invention, there is an initial crack presumed to have occurred during hot rolling, and the results of rolling fatigue life and rotational bending fatigue test are It was inferior to the present invention steel.

比較鋼であるNo.15は、S含有量が少なく、式(1)を満たさないため、MnSを起点とした疲労破壊が発生し、被削性を得るために必要なMnS量が不十分であり、被削性が悪かった。
比較鋼であるNo.16、17は、Ti含有量が少なく、式(1)を満たさないため、MnSを起点とした疲労破壊が発生し、転動疲労寿命および回転曲げ疲労試験の結果が本発明鋼と比べて劣っていた。また、No.16、17では、Ti系硫化物の多量生成に伴う粗大化防止に有効なTi系炭窒化物の析出物が十分に得られず、結晶粒粗大化温度が本発明鋼と比べて低かった。
No. which is a comparative steel. No. 15 has a small S content and does not satisfy the formula (1). Therefore, fatigue failure occurs starting from MnS, and the amount of MnS necessary for obtaining machinability is insufficient, and the machinability is low. It was bad.
No. which is a comparative steel. 16 and 17 have a low Ti content and do not satisfy the formula (1), so fatigue failure occurs starting from MnS, and the results of the rolling fatigue life and the rotating bending fatigue test are inferior to the steel of the present invention. It was. No. In Nos. 16 and 17, Ti-based carbonitride precipitates effective for preventing coarsening due to the large amount of Ti-based sulfides were not sufficiently obtained, and the crystal grain coarsening temperature was lower than that of the steel of the present invention.

比較鋼であるNo.18は、式(1)を満たさないため、粗大なMnSが生成したことに加え、N含有量が多く、粗大なTiNが生成したことにより、MnSやTiNを起点とした疲労破壊が発生し、転動疲労寿命および回転曲げ疲労試験の結果が本発明鋼と比べて劣っていた。さらに、No.18は、粗大なTiNの生成により、粗大粒防止に有効な微細なTi系炭窒化物の析出物が減少したため、粗大粒発生温度が本発明鋼と比べて劣っていた。   No. which is a comparative steel. 18 does not satisfy the formula (1), and in addition to the generation of coarse MnS, the N content is large and the formation of coarse TiN causes fatigue failure starting from MnS and TiN. The results of the rolling fatigue life and the rotating bending fatigue test were inferior to the steel of the present invention. Furthermore, no. No. 18 had a coarse grain generation temperature inferior to that of the steel of the present invention because fine Ti-based carbonitride precipitates effective in preventing coarse grains were reduced due to the production of coarse TiN.

Claims (9)

化学組成が質量%で、
C:0.10〜0.30%、
Si:0.02〜1.5%、
Mn:0.3〜1.8%、
S:0.020超〜0.050%、
Cr:0.4〜2.0%、
Al:0.005〜0.05%、
Ti:0.06〜0.20%、
Bi:0.0001〜0.0050%
を含有し、さらに、
Ca:0.0005〜0.0050%、
Mg:0.0003〜0.0050%、
Te:0.0003〜0.20%
の1種または2種以上を含有し、
P:0.050%以下、
N:0.01%以下、
O:0.0025%以下
に制限し、
残部が鉄および不純物であり、
下記式(1)を満たすことを特徴とする浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
5.0≧Ti/S≧3.0 式(1)
(式(1)中のTiは、Tiの含有量(質量%)であり、Sは、Sの含有量(質量%)である。)
Chemical composition is mass%,
C: 0.10 to 0.30%,
Si: 0.02 to 1.5%,
Mn: 0.3 to 1.8%
S: more than 0.020 to 0.050%,
Cr: 0.4 to 2.0%,
Al: 0.005 to 0.05%,
Ti: 0.06-0.20%,
Bi: 0.0001 to 0.0050%
In addition,
Ca: 0.0005 to 0.0050%,
Mg: 0.0003 to 0.0050%,
Te: 0.0003 to 0.20%
Containing one or more of
P: 0.050% or less,
N: 0.01% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
Case hardening steel excellent in coarse grain prevention characteristics, fatigue characteristics and machinability during carburizing, characterized by satisfying the following formula (1).
5.0 ≧ Ti / S ≧ 3.0 Formula (1)
(Ti in the formula (1) is the content (mass%) of Ti, and S is the content (mass%) of S.)
前記化学組成が質量%で、
Mo:0.02〜1.5%、
Ni:0.1〜3.5%、
V:0.02〜0.5%、
B:0.0002〜0.005%
の1種または2種以上を含有する請求項1に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
The chemical composition is mass%,
Mo: 0.02 to 1.5%,
Ni: 0.1 to 3.5%
V: 0.02-0.5%
B: 0.0002 to 0.005%
The case hardening steel excellent in the coarse grain prevention characteristic at the time of carburizing, fatigue characteristics, and machinability of Claim 1 containing 1 type (s) or 2 or more types.
化学組成が質量%で、
Nb:0.04%未満を含有する請求項1または請求項2に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。
Chemical composition is mass%,
The case-hardening steel excellent in coarse grain prevention characteristics, fatigue characteristics, and machinability during carburizing according to claim 1 or 2, wherein Nb: less than 0.04%.
ベイナイトの組織分率が30%以下である請求項1〜請求項3のいずれか一項に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。   4. The case hardening steel having excellent coarse grain prevention characteristics, fatigue characteristics and machinability during carburization according to any one of claims 1 to 3, wherein the bainite has a structure fraction of 30% or less. フェライト結晶粒度番号がJIS G0552で規定されている8〜11番である請求項1〜請求項4のいずれか一項に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。   The ferrite grain size number is No. 8-11 defined in JIS G0552, and is excellent in coarse grain prevention characteristics, fatigue characteristics, and machinability during carburizing according to any one of claims 1 to 4. Case-hardened steel. マトリックス中の長手方向断面において、検査基準面積:100平方mm、検査数:16視野、予測を行なう面積:30000平方mmの条件で測定された極値統計によるTi系析出物の最大直径が40μm以下である請求項1〜請求項5のいずれか一項に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼。   In the longitudinal section in the matrix, the maximum diameter of Ti-based precipitates is 40 μm or less based on extreme value statistics measured under the conditions of inspection standard area: 100 square mm, number of inspections: 16 fields of view, prediction area: 30000 square mm The case-hardened steel excellent in coarse grain prevention characteristics, fatigue characteristics, and machinability during carburizing according to any one of claims 1 to 5. 化学組成が質量%で、
C:0.10〜0.30%、
Si:0.02〜1.5%、
Mn:0.3〜1.8%、
S:0.020超〜0.050%、
Cr:0.4〜2.0%、
Al:0.005〜0.05%、
Ti:0.06〜0.20%、
Bi:0.0001〜0.0050%
を含有し、さらに、
Ca:0.0005〜0.0050%、
Mg:0.0003〜0.0050%、
Te:0.0003〜0.20%
の1種または2種以上を含有し、
P:0.050%以下、
N:0.01%以下、
O:0.0025%以下
に制限し、
残部が鉄および不純物であり、
下記式(1)を満たす鋼を、1150℃以上の温度で保持時間10分以上加熱して線材または棒鋼に熱間圧延する工程を含む浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼の製造方法。
5.0≧Ti/S≧3.0 式(1)
(式(1)中のTiは、Tiの含有量(質量%)であり、Sは、Sの含有量(質量%)である。)
Chemical composition is mass%,
C: 0.10 to 0.30%,
Si: 0.02 to 1.5%,
Mn: 0.3 to 1.8%
S: more than 0.020 to 0.050%,
Cr: 0.4 to 2.0%,
Al: 0.005 to 0.05%,
Ti: 0.06-0.20%,
Bi: 0.0001 to 0.0050%
In addition,
Ca: 0.0005 to 0.0050%,
Mg: 0.0003 to 0.0050%,
Te: 0.0003 to 0.20%
Containing one or more of
P: 0.050% or less,
N: 0.01% or less,
O: limited to 0.0025% or less,
The balance is iron and impurities,
The steel satisfying the following formula (1) is heated at a temperature of 1150 ° C. or higher for a holding time of 10 minutes or longer and hot-rolled into a wire or a steel bar. A method for producing excellent case-hardened steel.
5.0 ≧ Ti / S ≧ 3.0 Formula (1)
(Ti in the formula (1) is the content (mass%) of Ti, and S is the content (mass%) of S.)
前記熱間圧延後に800〜500℃の温度範囲を1℃/秒以下の冷却速度で徐冷し、熱間圧延して冷却した後の鋼のベイナイトの組織分率が30%以下となるようにする請求項7に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼の製造方法。   After the hot rolling, the temperature range of 800 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./second or less, and the steel bainite has a structure fraction of 30% or less after being hot rolled and cooled. The manufacturing method of the case hardening steel excellent in the coarse grain prevention characteristic at the time of carburizing, fatigue characteristics, and machinability of Claim 7. 前記熱間圧延の仕上げ温度を840〜1000℃とし、フェライト結晶粒度番号がJIS G0552で規定されている8〜11番である鋼となるようにする請求項7または請求項8に記載の浸炭時の粗大粒防止特性と疲労特性と被削性に優れた肌焼鋼の製造方法。   The carburizing time according to claim 7 or 8, wherein a finishing temperature of the hot rolling is set to 840 to 1000 ° C, and the steel has a ferrite crystal grain size number of 8 to 11 defined in JIS G0552. A method for producing case-hardened steel with excellent coarse grain prevention properties, fatigue properties and machinability.
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