JP6078008B2 - Case-hardening steel and method for manufacturing machine structural parts - Google Patents
Case-hardening steel and method for manufacturing machine structural parts Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 51
- 239000010959 steel Substances 0.000 title claims description 51
- 238000004519 manufacturing process Methods 0.000 title claims description 12
- 238000000034 method Methods 0.000 title claims description 10
- 238000005255 carburizing Methods 0.000 claims description 27
- 229910052710 silicon Inorganic materials 0.000 claims description 20
- 229910052804 chromium Inorganic materials 0.000 claims description 15
- 229910052748 manganese Inorganic materials 0.000 claims description 15
- 239000000463 material Substances 0.000 claims description 15
- 238000005496 tempering Methods 0.000 claims description 14
- 238000010791 quenching Methods 0.000 claims description 13
- 230000000171 quenching effect Effects 0.000 claims description 13
- 229910052802 copper Inorganic materials 0.000 claims description 8
- 229910000760 Hardened steel Inorganic materials 0.000 claims description 6
- 238000003754 machining Methods 0.000 claims description 6
- 239000000203 mixture Substances 0.000 claims description 6
- 238000005242 forging Methods 0.000 claims description 4
- 239000000126 substance Substances 0.000 claims description 3
- 239000012535 impurity Substances 0.000 claims description 2
- 238000005452 bending Methods 0.000 description 32
- 230000000052 comparative effect Effects 0.000 description 21
- 230000003247 decreasing effect Effects 0.000 description 21
- 230000000694 effects Effects 0.000 description 15
- 239000010410 layer Substances 0.000 description 13
- 230000002829 reductive effect Effects 0.000 description 11
- 238000009661 fatigue test Methods 0.000 description 10
- 229910001566 austenite Inorganic materials 0.000 description 9
- 230000007423 decrease Effects 0.000 description 9
- 239000002344 surface layer Substances 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 7
- 150000001247 metal acetylides Chemical class 0.000 description 6
- 230000000717 retained effect Effects 0.000 description 6
- 239000013078 crystal Substances 0.000 description 4
- 238000005520 cutting process Methods 0.000 description 4
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 3
- 230000002159 abnormal effect Effects 0.000 description 3
- 229910052799 carbon Inorganic materials 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 230000006872 improvement Effects 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- 230000002411 adverse Effects 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 238000011835 investigation Methods 0.000 description 2
- 238000005461 lubrication Methods 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 230000002093 peripheral effect Effects 0.000 description 2
- 238000005096 rolling process Methods 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 229910000859 α-Fe Inorganic materials 0.000 description 2
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 238000005256 carbonitriding Methods 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
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- 238000011156 evaluation Methods 0.000 description 1
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- 230000020169 heat generation Effects 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 230000000977 initiatory effect Effects 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
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- 229910052759 nickel Inorganic materials 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
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- 238000001556 precipitation Methods 0.000 description 1
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- 229910052720 vanadium Inorganic materials 0.000 description 1
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Description
本発明は、自動車や各種産業機器等の機械構造用部品の素材として供する、肌焼鋼、なかでも高い曲げ疲労強度および面圧疲労強度を有する機械構造用部品の素材として適した肌焼鋼およびそれを用いて作製される機械構造用部品に関するものである。 The present invention provides a case-hardened steel to be used as a material for machine structural parts such as automobiles and various industrial equipment, and especially a case-hardened steel suitable as a material for machine structural parts having high bending fatigue strength and surface fatigue strength. The present invention relates to a machine structural component manufactured using the same.
機械構造用部品、例えば自動車等の駆動伝達部品に用いられている歯車は、近年、省エネルギー化による車体重量の軽量化に伴って、その小型化が要求される一方、エンジンの高出力化により負荷が増大しているため、耐久性の向上が課題とされている。
一般的に、歯車の耐久性は、歯元の曲げ疲労破壊並びに歯面の面圧疲労破壊によって決定されるため、これまで、曲げ疲労強度および耐ピッチング性の向上を目的とし、微量元素の添加による介在物の形態制御や浸炭異常層の発生抑制をはかったり、あるいは、焼戻し軟化抵抗性を付与した、浸炭肌焼鋼が種々提案されている。
In recent years, gears used for mechanical structure parts, for example, drive transmission parts such as automobiles, are required to be reduced in size as the vehicle weight is reduced due to energy saving. Therefore, improvement in durability is an issue.
In general, the durability of gears is determined by the bending fatigue fracture of the tooth root and the surface pressure fatigue fracture of the tooth surface. So far, with the aim of improving bending fatigue strength and pitting resistance, adding trace elements Various carburized case-hardened steels have been proposed that control the form of inclusions and suppress the occurrence of abnormal carburization layers, or impart resistance to temper softening.
例えば、特許文献１には、鋼中のSiを低減すると共に、Mn、Cr、MoおよびNiの量を制御することにより、浸炭熱処理後の表面の粒界酸化層を低減して亀裂の発生を少なくし、また不完全焼入層の生成を抑制することにより、表面硬さの低減を抑えて疲労強度を高め、さらにCaを添加して、亀裂の発生・伝播を助長するMnSの延伸を制御する方法が開示されている。 For example, in Patent Document 1, while reducing Si in steel and controlling the amount of Mn, Cr, Mo, and Ni, the grain boundary oxide layer on the surface after carburizing heat treatment is reduced, and cracks are generated. Reduces surface hardness and increases fatigue strength by reducing the generation of incompletely hardened layers and adding Ca to control MnS stretching that promotes crack initiation and propagation A method is disclosed.
特許文献２には、素材としてSiを0.25〜1.50％添加した鋼材を用いて焼戻し軟化抵抗を高める方法が開示されている。 Patent Document 2 discloses a method of increasing temper softening resistance using a steel material to which Si is added in an amount of 0.25 to 1.50%.
また、特許文献３には、浸炭あるいは浸炭窒化処理時の表面炭素量および窒素の量を特定範囲内に制御することにより、微細な炭化物の生成を促し、表層部の高い硬さを確保して耐ピッチング性を高める方法が開示されている。 Further, in Patent Document 3, by controlling the amount of surface carbon and nitrogen during carburizing or carbonitriding within a specific range, the generation of fine carbides is promoted and high hardness of the surface layer portion is ensured. A method for increasing the pitting resistance is disclosed.
しかしながら、上述した特許文献１〜３に記載の発明はいずれも、以下に述べる問題があった。
まず、特許文献１の記載によれば、Siを低減すると粒界酸化層および不完全焼入れ層が低減するため、歯車の歯元での曲げ疲労による亀裂発生を抑えることはできる。しかしながら、逆に焼戻し軟化抵抗が低下して、破壊の発生が歯元から歯面側に移行する結果、歯面での摩擦熱による焼戻し軟化を抑えることができなくなって表面が軟化するため、ピッチングが発生し易くなることが問題になる。
However, the inventions described in Patent Documents 1 to 3 described above have the following problems.
First, according to the description of Patent Document 1, when Si is reduced, the grain boundary oxide layer and the incompletely hardened layer are reduced, so that the generation of cracks due to bending fatigue at the gear teeth can be suppressed. However, the resistance to temper softening decreases and the occurrence of fracture shifts from the tooth base to the tooth surface. As a result, temper softening due to frictional heat on the tooth surface cannot be suppressed, and the surface softens. It becomes a problem that it becomes easy to generate | occur | produce.
特許文献２では、焼戻し軟化抵抗を上げるために逆にSi等を添加し、一方、粒界酸化の進行を抑制するために浸炭工法を真空浸炭あるいはプラズマ浸炭等に限定しているが、これらの特殊な浸炭手法では、製造コストが嵩むという不利があり、工業的規模での量産化には不適であった。 In Patent Document 2, Si or the like is added to increase the temper softening resistance, while the carburizing method is limited to vacuum carburizing or plasma carburizing to suppress the progress of grain boundary oxidation. The special carburizing method has the disadvantage of increasing the manufacturing cost, and is not suitable for mass production on an industrial scale.
また、特許文献３に記載の技術は、高価な合金であるＶ、Moを多量に添加する必要があり、製造コストの大幅な増加を招いてしまうだけでなく、炭窒化物の析出により、連続鋳造時の割れの発生が懸念されるものであった。 In addition, the technique described in Patent Document 3 requires the addition of a large amount of expensive alloys such as V and Mo, which not only leads to a significant increase in manufacturing cost, but also due to the precipitation of carbonitrides. There was concern about the occurrence of cracks during casting.
そこで、本発明は、比較的安価な生産コストの下に得られる、高い曲げ疲労強度および面圧疲労強度を有する機械構造用部品の素材として適した肌焼鋼およびそれを素材として用いた機械構造用部品について提案することを目的とする。 Accordingly, the present invention provides a case-hardened steel suitable as a material for machine structural parts having high bending fatigue strength and surface fatigue strength obtained at a relatively low production cost, and a machine structure using the same. The purpose is to propose parts for use.
本発明者らは、上記課題を解決するため、浸炭焼入れ・焼戻し後の疲労特性に及ぼす、成分および浸炭層の硬さの影響について鋭意検討を行った。その結果、以下のa）〜e）の事項を見出すに到った。 In order to solve the above-mentioned problems, the present inventors diligently studied the influence of the components and the hardness of the carburized layer on the fatigue characteristics after carburizing and tempering. As a result, the following items a) to e) were found.
a）鋼材中のSi、MnおよびCrを増量して焼戻し軟化抵抗を高めることによって、例えば歯車としたときの接触面での発熱による軟化を抑えれば、歯車駆動時に生じる歯面の亀裂発生を抑制することができる。 a) By increasing the amount of Si, Mn and Cr in the steel material to increase the temper softening resistance, for example, if the softening due to heat generation at the contact surface when a gear is used is suppressed, the cracking of the tooth surface that occurs when the gear is driven will occur. Can be suppressed.
b）曲げ疲労および疲労亀裂の起点となり得る粒界酸化層については、Si、MnおよびCrを所定量以上添加することにより、粒界酸化層の成長方向が深さ方向から表面の密度増加方向に変わる。従って、起点となるような深さ方向に成長した酸化層がなくなるため、曲げ疲労および疲労亀裂の起点となり難くなる。 b) For grain boundary oxide layers that can be the starting point of bending fatigue and fatigue cracks, the growth direction of the grain boundary oxide layer is changed from the depth direction to the surface density increasing direction by adding a predetermined amount of Si, Mn and Cr. change. Therefore, since there is no oxide layer grown in the depth direction to be a starting point, it becomes difficult to be a starting point for bending fatigue and fatigue cracks.
c）上記a）およびb）で述べたとおり、Si、MnおよびCrは、焼戻し軟化抵抗の向上と粒界酸化層の制御に有効であるが、これらの効果を両立させるためには、Si、MnおよびCrについて、その含有量を厳密に制御する必要がある。 c) As described in a) and b) above, Si, Mn, and Cr are effective in improving the temper softening resistance and controlling the grain boundary oxide layer. In order to achieve both of these effects, Si, The contents of Mn and Cr need to be strictly controlled.
d）浸炭後の面疲労特性は、試験時の最大せん断応力の発生深さに相当する、表面から200μm深さの位置の硬さと相関があり、当該位置における硬さをHV730以上とすれば、ピッチング（素材の疲れが主原因で歯面に剥離損傷が発生すること）による破壊を効果的に抑制することができる。 d) The surface fatigue characteristics after carburizing correlate with the hardness at a position 200 μm deep from the surface, which corresponds to the depth of occurrence of the maximum shear stress during the test, and if the hardness at that position is HV730 or higher, It is possible to effectively suppress breakage due to pitting (peeling damage occurs on the tooth surface due to material fatigue).
e）浸炭熱処理では、表面の炭素濃度が被処理材の形状の影響を大きく受ける。すなわち、処理材の平坦部分では狙い通りの硬さや組織が得られても、歯先等の角部では浸炭が過剰となり、粗大炭化物の生成に伴う疲労強度の低下が懸念されている。特に、最近用いられるようになってきた真空浸炭では、この傾向がより顕著になっている。これに対し、炭化物の生成抑制効果のある、Si、Cuを適量添加することで、過剰浸炭による疲労強度の低下を抑制することができる。 e) In carburizing heat treatment, the carbon concentration on the surface is greatly influenced by the shape of the material to be treated. That is, even if the intended hardness and structure are obtained at the flat portion of the treated material, carburization is excessive at the corners such as the tooth tips, and there is a concern that the fatigue strength is reduced due to the formation of coarse carbides. In particular, this tendency is more remarkable in vacuum carburizing that has recently been used. In contrast, the addition of appropriate amounts of Si and Cu, which have an effect of suppressing the formation of carbides, can suppress a decrease in fatigue strength due to excessive carburization.
本発明は上記の知見に立脚するものであり、その要旨構成は、次のとおりである。
１．質量％で、
Ｃ：0.15％以上0.25％以下、
Si：0.50％以上0.90％未満、
Mn：0.40％以上1.20％以下、
Ｓ：0.010％以上0.030％以下、
Cu：0.05％以上0.50％以下、
Cr：0.80％以上1.80％以下、
Mo：0.10％以下、
Al：0.020％以上0.060％以下、
Ｎ：0.0060％以上0.0200％以下および
０：0.0015％以下
を、下記（1）式および（2）式を満足する範囲の下に含有し、残部はFeおよび不可避不純物からなる化学組成を有し、浸炭温度900〜1050℃で60〜600min、焼入れ温度800〜900℃で10〜120minでの浸炭焼入れ・120〜250℃で30〜180minでの焼戻し後に得られる表面から200μm深さの位置での硬さがHV730以上であることを特徴とする肌焼鋼。
記
1.7≧［％Si］＋（［％Mn］＋［％Cr］）／3≧1.1 …（1）
0.16≧［％Si］×（［％Cu］／2）≧0.03 …（2）
但し、［ ］は括弧内の元素の含有量（質量％）
The present invention is based on the above findings, and the gist of the present invention is as follows.
1. % By mass
C: 0.15% or more and 0.25% or less,
Si: 0.50% or more and less than 0.90%,
Mn: 0.40% to 1.20%,
S: 0.010% or more and 0.030% or less,
Cu: 0.05% or more and 0.50% or less,
Cr: 0.80% to 1.80%,
Mo: 0.10% or less,
Al: 0.020% or more and 0.060% or less,
N: 0.0060% or more and 0.0200% or less and 0: 0.0015% or less are contained within a range satisfying the following formulas (1) and (2), and the balance has a chemical composition composed of Fe and inevitable impurities, Hardening at a depth of 200 μm from the surface obtained after carburizing and quenching at a carburizing temperature of 900 to 1050 ° C. for 60 to 600 min, quenching temperature of 800 to 900 ° C. for 10 to 120 min and tempering at 120 to 250 ° C. for 30 to 180 min. A case-hardened steel with a thickness of HV730 or higher.
Record
1.7 ≧ [% Si] + ([% Mn] + [% Cr]) / 3 ≧ 1.1 (1)
0.16 ≧ [% Si] × ([% Cu] / 2) ≧ 0.03 (2)
However, [] is the element content in parentheses (mass%)
２．前記１に記載の肌焼鋼を素材とし、該素材に機械加工を行って部品形状とした後、浸炭焼入れ処理を施す機械構造用部品の製造方法。 2. The 1-hardened steel as a material according to, after the part shape by performing the machining on said workpiece, the manufacturing method of parts for facilities to machine structural carburizing quenching treatment.
３．前記機械加工の前に鍛造を施す前記２に記載の機械構造用部品の製造方法。 3. Method for manufacturing a machine structural parts according to the 2 to facilities forging prior to the machining.
本発明によれば、高い曲げ疲労強度および面圧疲労強度を有する機械構造用部品の素材として適した肌焼鋼を提供することができる。すなわち、機械構造用部品として例えば歯車を、本発明鋼を用いて作製した場合に、その歯元の曲げ疲労特性のみならず、歯面の面圧疲労特性にも優れた歯車を量産することが可能になる。 ADVANTAGE OF THE INVENTION According to this invention, the case hardening steel suitable as a raw material of the machine structural component which has high bending fatigue strength and surface pressure fatigue strength can be provided. That is, for example, when a gear is manufactured using the present invention steel as a machine structural component, it is possible to mass-produce gears that are excellent not only in the bending fatigue characteristics of the tooth root but also in the surface pressure fatigue characteristics of the tooth surface. It becomes possible.
以下、本発明を具体的に説明する。
まず、本発明において、鋼の成分組成を上記の範囲に限定した理由について説明する。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。
Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding a component shall mean the mass%.
Ｃ：0.15％以上0.25％以下
Ｃは、浸炭処理後の焼入れにより中心部の硬さを高めるために0.15％以上を必要とするが、含有量が0.25％を超えると、機械構造用部品における芯部の靭性が低下するため、Ｃ量は0.15〜0.25％の範囲に限定した。好ましくは0.17〜0.23％の範囲である。
C: 0.15% or more and 0.25% or less C needs 0.15% or more in order to increase the hardness of the central part by quenching after carburizing treatment, but if the content exceeds 0.25%, the core in machine structural parts Since the toughness of the part decreases, the C content is limited to a range of 0.15 to 0.25%. Preferably it is 0.17 to 0.23% of range.
Si：0.50％以上0.90％未満
Siは、本発明において最も重要な元素である。Siは、歯車等が転動中に到達すると予想される200〜300℃の温度域における軟化抵抗を高めると共に、浸炭表層部の硬さ低下を引き起こす残留オーステナイトの生成を抑制しつつ、焼入れ性を向上させる元素である。また、浸炭時に粗大な炭化物の生成を抑制する効果も有しており、これらの鋼を得るには、少なくとも0.50％の添加が不可欠である。しかしながら、一方でSiはフェライト安定化元素であり、過剰な添加はAc3変態点を上昇させ、通常の焼入れ温度範囲で炭素の含有量の低い芯部でフェライトが出現し易くなり強度の低下を招く。また、過剰な添加は浸炭前の鋼材を硬化させ、切削性を劣化させる不利もある。この点、Si量が0.90％未満であれば、上記のような弊害は生じないので、Si量は0.50％以上0.90％未満の範囲に限定した。好ましくは0.70〜0.90％未満の範囲である。
Si: 0.50% or more and less than 0.90%
Si is the most important element in the present invention. Si enhances the softening resistance in the temperature range of 200 to 300 ° C, which is expected to reach during rolling of gears, etc., and suppresses the formation of retained austenite that causes a decrease in the hardness of the carburized surface layer, while increasing the hardenability. It is an element to improve. It also has the effect of suppressing the formation of coarse carbides during carburization, and at least 0.50% addition is essential to obtain these steels. However, on the other hand, Si is a ferrite stabilizing element, and excessive addition raises the Ac 3 transformation point, and ferrite tends to appear in the core portion having a low carbon content in the normal quenching temperature range, resulting in a decrease in strength. Invite. Excessive addition also has the disadvantage of hardening the steel before carburizing and degrading the machinability. In this respect, if the Si content is less than 0.90%, the above-described adverse effects do not occur. Therefore, the Si content is limited to a range of 0.50% or more and less than 0.90%. Preferably it is 0.70 to less than 0.90% of range.
Mn：0.40％以上1.20％以下
Mnは、焼入性に有効な元素であり、少なくとも0.40％の添加を必要とする。しかしながら、Mnは、浸炭異常層を形成し易く、また過剰な添加は残留オーステナイト量が過多となって硬さの低下を招くため、上限を1.20％とした。好ましくは0.60〜1.00％の範囲である。
Mn: 0.40% to 1.20%
Mn is an element effective for hardenability, and requires addition of at least 0.40%. However, Mn tends to form an abnormal carburization layer, and excessive addition causes an excessive amount of retained austenite and leads to a decrease in hardness, so the upper limit was made 1.20%. Preferably it is 0.60 to 1.00% of range.
Ｓ：0.010％以上0.030％以下
Ｓは、Mnと硫化物を形成し、被削性を向上させる作用を有するので、少なくとも0.010％以上含有させる。一方、過剰な添加は、部品の疲労強度および靭性を低下させるため、上限を0.030％とした。
S: 0.010% or more and 0.030% or less S forms a sulfide with Mn and improves the machinability, so it is contained at least 0.010% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was made 0.030%.
Cu：0.05％以上0.50％以下
Cuは、本発明において重要な効果を有する元素の一つである。特に、炭化物の生成を抑制することで、過剰浸炭による疲労強度低下の抑制に効果がある。また、焼入れ性及び耐食性の向上にも寄与するため、0.05％以上含有させる必要がある。一方、0.50％を超えて添加した場合、素材硬さの上昇を招いて冷間加工性が劣化してしまうため、Cu含有量は0.50％以下の範囲内とする必要がある。好ましくは0.10〜0.30％の範囲である。
Cu: 0.05% or more and 0.50% or less
Cu is one of the elements having an important effect in the present invention. In particular, by suppressing the formation of carbides, there is an effect in suppressing a decrease in fatigue strength due to excessive carburization. Moreover, in order to contribute to the improvement of hardenability and corrosion resistance, it is necessary to contain 0.05% or more. On the other hand, if added over 0.50%, the hardness of the material is increased and the cold workability deteriorates, so the Cu content needs to be in the range of 0.50% or less. Preferably it is 0.10 to 0.30% of range.
Cr：0.80％以上1.80％以下
Crは、焼入性のみならず焼戻し軟化抵抗の向上にも有効な元素であるが、含有量が0.80％に満たないとその添加効果に乏しく、一方1.80％を超えると軟化抵抗を高める効果は飽和し、むしろ浸炭異常層を形成し易くなるため、Cr量は0.80〜1.80％の範囲に限定した。好ましくは0.90〜1.50％の範囲である。
Cr: 0.80% to 1.80%
Cr is an element effective for improving not only hardenability but also temper softening resistance. However, if its content is less than 0.80%, its addition effect is poor, while if it exceeds 1.80%, the effect of increasing softening resistance is The Cr content is limited to the range of 0.80 to 1.80% because it becomes saturated and rather easily forms an abnormal carburized layer. Preferably it is 0.90 to 1.50% of range.
Mo：0.10％以下
Moは、焼入れ性および靭性を向上させると共に、浸炭処理後の結晶粒径を微細化する効果を有するため、好ましくは0.03％以上で添加する。一方、多量に添加すると、製造コストを上昇させるため、0.10％を上限とした。なお、上記効果を発揮させるため、上限値は0.07％とすることが好ましい。
Mo: 0.10% or less
Mo improves the hardenability and toughness and has the effect of refining the crystal grain size after carburizing treatment, so it is preferably added at 0.03% or more. On the other hand, if added in a large amount, the production cost is increased, so the upper limit was made 0.10%. In order to exhibit the above effect, the upper limit value is preferably 0.07%.
Al：0.020％以上0.060％以下
Alは、Ｎと結合してAlNを形成し、オーステナイト結晶粒の微細化に寄与する元素であり、この効果を得るためには0.020％以上の添加を必要とするが、含有量が0.060％を超えると疲労強度に対して有害なAl 203介在物の生成を助長するため、Al量は0.020〜0.060％の範囲に限定した。好ましくは0.020〜0.040％の範囲である。
Al: 0.020% or more and 0.060% or less
Al is an element that combines with N to form AlN and contributes to the refinement of austenite crystal grains. To obtain this effect, 0.020% or more of addition is required, but the content is 0.060%. In order to promote the formation of Al 2 0 3 inclusions that are harmful to fatigue strength, the Al content is limited to a range of 0.020 to 0.060%. Preferably it is 0.020 to 0.040% of range.
Ｎ：0.0060％以上0.0200％以下
Ｎは、Alと結合してAlNを形成し、オーステナイト結晶粒の微細化に寄与する元素である。従って、適正添加量はAlとの量的バランスで決まるが、その効果を発揮するためには0.0060％以上の添加が必要である。しかし、過剰に添加すると凝固時の鋼塊に気泡が発生したり、鍛造性の劣化を招くため、上限を0.0200％とする。好ましくは0.0100〜0.0150％の範囲である。
N: 0.0060% or more and 0.0200% or less N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains. Accordingly, the appropriate addition amount is determined by a quantitative balance with Al, but 0.0060% or more of addition is necessary to exert the effect. However, if added excessively, bubbles are generated in the steel ingot at the time of solidification or the forgeability is deteriorated, so the upper limit is made 0.0200%. Preferably it is 0.0100 to 0.0150% of range.
Ｏ：0.0015％以下
Ｏは、鋼中において酸化物系介在物として存在し、疲労強度を損なう元素である。従って、含有量は低いほど望ましいが、0.0015％までは許容される。
O: 0.0015% or less O is an element that exists as an oxide inclusion in steel and impairs fatigue strength. Therefore, the lower the content, the better, but 0.0015% is acceptable.
なお、被削性を向上させるために必要に応じて、Pb、SeおよびCa等の快削元素を含有させてもよい。また、Ｐは、結晶粒界に偏析し、浸炭層および芯部の靭性を低下させるので、その混入は低いほど望ましいが、0.020％までは許容される。 In order to improve machinability, free cutting elements such as Pb, Se, and Ca may be included as necessary. Further, P segregates at the grain boundaries and lowers the toughness of the carburized layer and the core. Therefore, the lower the content, the better, but 0.020% is acceptable.
以上、本発明の基本成分の適正組成範囲について説明したが、本発明では、各々の元素が単に上記の範囲を満足するだけでは不十分であり、Si、Mn、CrおよびCuについては、次式（1）および（2）の関係を満足させることが重要である。
1.7≧［％Si］＋（［％Mn］＋［％Cr］）／3≧1.1 …（1）
0.16≧［％Si］×（［％Cu］／2）≧0.03 …（2）
上式（1）は、焼入性および焼戻し軟化抵抗性に影響を与える因子を示し、その値が1.1未満では焼入性および焼戻し軟化抵抗性の改善効果に乏しい。一方、上式（1）が1.7を超えると、芯部硬さの増加によって加工性が劣化するだけでなく、浸炭表層部のMs点が低下し、残留オーステナイト量が過多となって表層硬さの低下を招くことになる。
As described above, the appropriate composition range of the basic component of the present invention has been described. However, in the present invention, it is not sufficient that each element simply satisfies the above range. For Si, Mn, Cr and Cu, the following formulas are used. It is important to satisfy the relationship of (1) and (2).
1.7 ≧ [% Si] + ([% Mn] + [% Cr]) / 3 ≧ 1.1 (1)
0.16 ≧ [% Si] × ([% Cu] / 2) ≧ 0.03 (2)
The above formula (1) indicates factors affecting the hardenability and temper softening resistance. If the value is less than 1.1, the effect of improving the hardenability and temper softening resistance is poor. On the other hand, if the above formula (1) exceeds 1.7, not only will the workability deteriorate due to the increase in core hardness, but the Ms point of the carburized surface layer will decrease, resulting in an excessive amount of retained austenite and surface hardness. Will be reduced.
上式（2）は、浸炭性に影響を与える因子を示し、0.03未満では炭化物生成の抑制効果に乏しく、疲労強度に悪影響を与える場合がある。一方0.16を超えると、上記の改善効果が飽和するだけでなく、表面性状や加工性の劣化を招く。 The above equation (2) indicates a factor that affects carburization, and if it is less than 0.03, the effect of suppressing the formation of carbide is poor, and fatigue strength may be adversely affected. On the other hand, if it exceeds 0.16, not only the above-mentioned improvement effect is saturated, but also surface properties and workability are deteriorated.
さらに、上掲した元素の規定式に加え、浸炭後の鋼材表面から200μm深さの位置における硬さをHV730以上とすることも重要である。歯車においてピッチング発生に繋がる歯面および歯元の初期亀裂は、ローラーピッチング疲労試験時の最大せん断応力発生深さに相当する、表面から200μm深さの位置の硬さが高いほど発生しにくい。初期亀裂の発生を抑制し、長寿命を得るため、表層部から200μm位置における硬さをHV730以上に限定した。なお、本発明で硬さ（HV）は全て荷重300gfで求めた値として規定する。 Furthermore, in addition to the elemental formulas listed above, it is also important to set the hardness at a position 200 μm deep from the steel surface after carburization to HV730 or higher. The tooth surface and the initial crack of the tooth root that lead to the occurrence of pitching in the gear are less likely to occur as the hardness at a position 200 μm deep from the surface corresponding to the maximum shear stress generation depth in the roller pitching fatigue test is higher. In order to suppress the occurrence of initial cracks and obtain a long life, the hardness at the 200 μm position from the surface layer was limited to HV730 or more. In the present invention, the hardness (HV) is defined as a value obtained with a load of 300 gf.
本発明に係る肌焼鋼から機械構造用部品を作製する際の製造条件については、特に制限は無いが、好適な製造条件は次の通りである。
前記した好適成分組成からなる鋼素材を溶解鋳造してビレットとし、熱間圧延後、機械構造用部品としての予備成形を行う。次に、機械加工、あるいは鍛造後に機械加工を行い機械構造用部品形状とした後、浸炭焼入れ処理を施し、必要に応じて更に歯面に研磨加工を施して最終製品とする。
なお、浸炭焼入れ処理は、浸炭温度900〜1050℃で60〜600min、焼入れ温度800〜900℃で10〜120minとし、焼戻しは120〜250℃で30〜180minの範囲とする。
Although there is no restriction | limiting in particular about the manufacturing conditions at the time of producing the machine structural component from the case hardening steel based on this invention, The suitable manufacturing conditions are as follows.
A steel material having the above-mentioned preferred component composition is melt cast to form a billet, and after hot rolling, preforming as a machine structural part is performed. Next, after machining or forging, machining is performed to obtain a machine structural part shape, and then carburizing and quenching is performed, and if necessary, the tooth surface is further ground to obtain a final product.
The carburizing and quenching treatment is performed at a carburizing temperature of 900 to 1050 ° C. for 60 to 600 min, a quenching temperature of 800 to 900 ° C. for 10 to 120 min, and tempering at 120 to 250 ° C. for 30 to 180 min.
表１に示す化学組成の鋼を溶製し、連続鋳造によりブルームとした。次いで、ビレット圧延を経て、さらに棒鋼圧延により50mmφの棒鋼とした。かくして得られた棒鋼に、1200℃で60minの加熱処理を施し、1100℃にて熱間鍛造を行って36mmφとし、その後１℃／sで室温まで冷却して丸棒鋼とし、さらに得られた丸棒鋼に対し、925℃で60minの焼準処理を実施した。 Steel having the chemical composition shown in Table 1 was melted and made into bloom by continuous casting. Next, billet rolling was performed, and the steel bar was further rolled into a 50 mmφ steel bar. The steel bar thus obtained was subjected to heat treatment at 1200 ° C. for 60 minutes, hot forged at 1100 ° C. to 36 mmφ, and then cooled to room temperature at 1 ° C./s to obtain a round bar steel. The steel bar was subjected to normalizing treatment at 925 ° C. for 60 minutes.
次に、焼準処理後の丸棒鋼から、小野式回転曲げ疲労試験片、ローラーピッチング疲労試験片、20mφの丸棒を採取した。表１のNo.１〜31鋼の各試験片に対して、図１に示す条件の、浸炭焼入れ・焼戻しを施した後、回転曲げ疲労試験、ローラーピッチング疲労試験および20mmφの丸棒の表面から200μm深さの位置での硬さ調査を実施した。なお、No.32及び33鋼については、真空浸炭焼入れ・焼戻し後に同様の調査を実施した。また、熱間鍛造材を20mm厚に切断したものを試験材として、被削性を外周旋削試験により評価した。以下に、それぞれの調査内容について詳細に説明する。 Next, Ono type rotating bending fatigue test piece, roller pitting fatigue test piece, and 20 mφ round bar were collected from the round bar steel after the normalizing treatment. Each test piece of No. 1-31 steel in Table 1 is subjected to carburizing quenching and tempering under the conditions shown in FIG. 1 and then from the surface of a rotating bending fatigue test, a roller pitting fatigue test and a 20 mmφ round bar. A hardness survey at a depth of 200 μm was conducted. For No. 32 and 33 steels, the same investigation was conducted after vacuum carburizing and tempering. Moreover, the machinability was evaluated by a peripheral turning test using a hot forged material cut to a thickness of 20 mm as a test material. The details of each survey are described below.
回転曲げ疲労特性
直径36mmの丸棒鋼から、図２に示す寸法および形状の平行部直径８mmの試験片を採取し、平行部にこれと直角方向の深さ２mmの切欠き（切欠き係数α：1.56）を全周に付与した回転曲げ疲労試験片を作製した。得られた試験片に対して、浸炭焼入れ・焼戻し処理を行った後、小野式回転曲げ疲労試験機を用いて、回転数：3000rpmで回転曲げ疲労試験を実施し、107回を疲労限度として、回転曲げ疲労強度を測定した。
Rotating Bending Fatigue Properties From a round steel bar with a diameter of 36 mm, a test piece with a diameter of 8 mm in parallel with the dimensions and shape shown in Fig. 2 was taken, and a notch with a depth of 2 mm perpendicular to this was cut into the parallel part (notch coefficient α: A rotating bending fatigue test piece having 1.56) applied to the entire circumference was prepared. After carburizing and tempering the obtained test piece, using the Ono type rotating bending fatigue tester, a rotating bending fatigue test was carried out at a rotation speed of 3000 rpm, with 10 7 times as the fatigue limit. The rotational bending fatigue strength was measured.
ローラーピッチング疲労特性
ローラーピッチング疲労試験機を使用して、80℃のミッションオイルを潤滑に用い、すべり率：40％、回転数：1500rpmにてローラーピッチング疲労試験を行った。その際、107回を疲労限度として評価した。
Roller pitching fatigue characteristics Using a roller pitching fatigue tester, 80 ° C mission oil was used for lubrication, and a roller pitching fatigue test was performed at a slip rate of 40% and a rotation speed of 1500 rpm. At that time, 10 7 times were evaluated as fatigue limit.
硬さ調査
発明鋼および比較鋼の20mmφの丸棒を用いて、浸炭焼入れ・焼戻し処理後に、切断し、表面から200μm深さ位置の硬さを、ビッカース硬さ計により測定した。また、焼戻し軟化抵抗性の評価のため、その後、さらに250℃で２時間の焼戻しを行った後に、同位置での硬さを測定した。
Hardness investigation Using a 20 mmφ round bar of the invented steel and comparative steel, the steel was cut after carburizing and tempering, and the hardness at a depth of 200 μm from the surface was measured with a Vickers hardness meter. Further , for evaluation of temper softening resistance, after further tempering at 250 ° C. for 2 hours, the hardness at the same position was measured.
被削性試験
36mmφの丸棒鋼（熱間鍛造材）を用いて、外周旋削試験を行った。該試験にはＰ20種工具を用いて、切込み：２mm、切削速度：200mm／min、送り：0.25mm／revおよび潤滑無の条件にて切削を行って切削時間：900ｓの段階での工具逃げ面磨耗幅を、実体顕微鏡にて測定して評価した。
Machinability test
A peripheral turning test was performed using 36 mmφ round bar steel (hot forged material). In this test, a P20 type tool was used, cutting was performed at a cutting depth of 2 mm, cutting speed: 200 mm / min, feed: 0.25 mm / rev, and no lubrication. The wear width was measured and evaluated with a stereomicroscope.
表２に上記した各調査の結果を示す。本発明鋼（No.１〜13）は、回転曲げ疲労強度が465MPa以上、面圧疲労強度が2650MPa以上であり、表面から200μm位置での硬さはHV730以上が得られ、比較鋼No.14〜33より優れていた。 Table 2 shows the results of each survey described above. The steel of the present invention (Nos. 1 to 13) has a rotational bending fatigue strength of 465 MPa or more, a surface fatigue strength of 2650 MPa or more, and a hardness at a position of 200 μm from the surface is HV730 or more. Better than ~ 33.
すなわち、比較鋼No.14は、Ｃ含有量が本発明範囲より低いために、内部硬さが低くなりすぎて回転曲げ疲労強度が低下した。
比較鋼No.15は、Ｃ含有量が本発明範囲より高いために、芯部の靭性が低下し、回転曲げ疲労強度が低下した。
比較鋼No.16は、Si含有量及び成分範囲規定式（1）（［％Si］＋（［％Mn］＋［％Cr］）/3）の値が本発明の範囲よりも低いために、芯部硬度及び耐焼戻し軟化抵抗が低下し、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.17は、Si含有量及び成分範囲規定式（1）（［％Si］＋（［％Mn］＋［％Cr］）/3）の値が本発明の範囲よりも高いために、焼入れ性が高くなり、被削性が低下した。
比較鋼No.18は、Mn含有量及び成分範囲規定式（1）（［％Si］＋（［％Mn］＋［％Cr］）/3）が本発明の範囲より低いために、芯部硬度及び耐焼戻し軟化抵抗が低下し、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.19は、Mn含有量が本発明の範囲より高いために、浸炭表層部のMs点が低下し、残留オーステナイト量が増加するために、表面から200μm深さ部での硬さが低くなり、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.20は、Ｓ含有量が本発明範囲より低いために、MnSの生成量が乏しく、被削性が低下した。
比較鋼No.21は、Ｓ含有量が本発明範囲より高いために、疲労破壊の起点となるMnSの生成量が多くなり、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.22は、Cu含有量が本発明の範囲より低いために、芯部硬度が低下し、回転曲げ疲労強度が低下した。
比較鋼No.23は、Cu含有量及び成分範囲に係る上記式（2）（［％Si］×（［％Cu］/2）の値が本発明範囲よりも高い結果、焼入れ性が高くなり、被削性が低下した。
比較鋼No.24は、Cr含有量及び成分範囲規定式（1）（［％Si］＋（［％Mn］＋［％Cr］）/3）が本発明の範囲より低いために、芯部硬度及び耐焼戻し軟化抵抗が低下し、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.25は、Cr含有量が本発明の範囲より高いために、浸炭表層部のMs点が低下し、残留オーステナイト量が増加する。よって、表層から200μm部での硬さが低くなり、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.26は、Al含有量が本発明範囲より高いために、Al203介在物の生成量が多くなり、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.27は、Ｎ添加量が本発明範囲よりも低いために、AlN生成量が少なくなり、オーステナイト結晶粒が粗大化してしまうため、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.28は、Ｎ添加量が本発明範囲よりも高いために、熱間鍛造時に割れを生じてしまい、疲労試験が出来なかった。
比較鋼No.29は、Ｏ含有量が本発明範囲より高いために、疲労破壊の起点となる酸化物系介在物の生成量が多くなり、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.30は、本発明成分範囲内であるが、成分範囲に係る上記規定式（1）（［％Si］＋（［％Mn］＋［％Cr］）/3）の値が1.1未満のため、耐焼戻し軟化抵抗が低下し、面圧疲労強度が低下した。
比較鋼No.31は、本発明成分範囲内であるが、成分範囲に係る上記規定式（1）（［％Si］＋（［％Mn］＋［％Cr］）/3）の値が1.7を超えているため、焼入れ性が高くなって被削性が低下した。また、浸炭表層部のMs点が低下し、残留オーステナイト量が増加して表面からから200μm深さ部分での硬さが低くなり、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.32は、本発明成分範囲内であるが、成分範囲に係る上記規定式（2）（［％Si］×（［％Cu］/2）の値が0.03未満と低い結果、過剰浸炭により粗大な炭化物が生成し、回転曲げ疲労強度と面圧疲労強度が低下した。
比較鋼No.33も同様に、本発明成分範囲内であるが、成分範囲に係る上記規定式2）（［％Si］×（［％Cu］/2）の値が0.16を超えているため、焼入性が高くなり、被削性が低下した。
That is, since the comparative steel No. 14 had a C content lower than the range of the present invention, the internal hardness became too low and the rotational bending fatigue strength was lowered.
Since the comparative steel No. 15 had a C content higher than the range of the present invention, the toughness of the core portion was lowered and the rotary bending fatigue strength was lowered.
In Comparative Steel No. 16, the Si content and the component range defining formula (1) ([% Si] + ([% Mn] + [% Cr]) / 3) are lower than the range of the present invention. The core hardness and resistance to tempering softening decreased, and the rotational bending fatigue strength and the surface pressure fatigue strength decreased.
Comparative steel No. 17 has a Si content and a component range defining formula (1) ([% Si] + ([% Mn] + [% Cr]) / 3) because the value is higher than the range of the present invention. , Hardenability increased and machinability decreased.
Comparative steel No. 18 has a core part because the Mn content and the component range defining formula (1) ([% Si] + ([% Mn] + [% Cr]) / 3) are lower than the range of the present invention. Hardness and resistance to tempering softening decreased, and rotational bending fatigue strength and surface fatigue strength decreased.
Comparative Steel No. 19 has a Mn content higher than the range of the present invention, so the Ms point of the carburized surface layer portion decreases and the retained austenite amount increases. The rotational bending fatigue strength and the contact pressure fatigue strength decreased.
In Comparative Steel No. 20, since the S content was lower than the range of the present invention, the amount of MnS produced was poor and the machinability was lowered.
In Comparative Steel No. 21, since the S content was higher than the range of the present invention, the amount of MnS generated as a starting point for fatigue fracture increased, and the rotary bending fatigue strength and the surface pressure fatigue strength decreased.
In Comparative Steel No. 22, the Cu content was lower than the range of the present invention, so that the core hardness decreased and the rotating bending fatigue strength decreased.
Comparative steel No. 23 has higher hardenability as a result of the value of the above formula (2) ([% Si] × ([% Cu] / 2) relating to the Cu content and the component range being higher than the range of the present invention. , Machinability decreased.
Comparative steel No. 24 has a Cr content and a component range defining formula (1) ([% Si] + ([% Mn] + [% Cr]) / 3) lower than the range of the present invention. Hardness and resistance to tempering softening decreased, and rotational bending fatigue strength and surface fatigue strength decreased.
Since the comparative steel No. 25 has a Cr content higher than the range of the present invention, the Ms point of the carburized surface layer portion is lowered and the retained austenite amount is increased. Therefore, the hardness at the 200 μm portion from the surface layer was lowered, and the rotational bending fatigue strength and the contact pressure fatigue strength were reduced.
In Comparative Steel No. 26, since the Al content was higher than the range of the present invention, the amount of Al 2 0 3 inclusions increased, and the rotary bending fatigue strength and the surface pressure fatigue strength were reduced.
In Comparative Steel No. 27, since the N addition amount is lower than the range of the present invention, the amount of AlN generated is reduced and the austenite crystal grains are coarsened, so that the rotary bending fatigue strength and the surface pressure fatigue strength are reduced.
In Comparative Steel No. 28, since the N addition amount was higher than the range of the present invention, cracking occurred during hot forging, and a fatigue test could not be performed.
In comparative steel No. 29, since the O content was higher than the range of the present invention, the amount of oxide inclusions that became the starting point of fatigue fracture increased, and the rotary bending fatigue strength and the surface pressure fatigue strength decreased.
Comparative steel No. 30 is within the component range of the present invention, but the value of the above defined formula (1) ([% Si] + ([% Mn] + [% Cr]) / 3) relating to the component range is 1.1. Therefore, the resistance to tempering softening decreased and the surface pressure fatigue strength decreased.
Comparative steel No. 31 is within the component range of the present invention, but the value of the above formula (1) ([% Si] + ([% Mn] + [% Cr]) / 3) relating to the component range is 1.7. Therefore, the hardenability increased and the machinability decreased. In addition, the Ms point of the carburized surface layer portion decreased, the amount of retained austenite increased, the hardness at a depth of 200 μm from the surface decreased, and the rotational bending fatigue strength and surface pressure fatigue strength decreased.
Comparative steel No. 32 is within the component range of the present invention, but the value of the above defined formula (2) ([% Si] × ([% Cu] / 2) related to the component range is less than 0.03, resulting in an excessive amount. Coarse carbides formed by carburization, and rotational bending fatigue strength and surface pressure fatigue strength decreased.
Similarly, Comparative Steel No. 33 is within the component range of the present invention, but the value of the above defined formula 2) ([% Si] × ([% Cu] / 2) related to the component range exceeds 0.16. , Hardenability increased and machinability decreased.
Claims (3)
Ｃ：0.15％以上0.25％以下、
Si：0.50％以上0.90％未満、
Mn：0.40％以上1.20％以下、
Ｓ：0.010％以上0.030％以下、
Cu：0.05％以上0.50％以下、
Cr：0.80％以上1.80％以下、
Mo：0.10％以下、
Al：0.020％以上0.060％以下、
Ｎ：0.0060％以上0.0200％以下および
０：0.0015％以下
を、下記（1）式および（2）式を満足する範囲の下に含有し、残部はFeおよび不可避不純物からなる化学組成を有し、浸炭温度900〜1050℃で60〜600min、焼入れ温度800〜900℃で10〜120minでの浸炭焼入れ・120〜250℃で30〜180minでの焼戻し後に得られる表面から200μm深さの位置での硬さがHV730以上であることを特徴とする肌焼鋼。
記
1.7≧［％Si］＋（［％Mn］＋［％Cr］）／3≧1.1 …（1）
0.16≧［％Si］×（［％Cu］／2）≧0.03 …（2）
但し、［ ］は括弧内の元素の含有量（質量％） % By mass
C: 0.15% or more and 0.25% or less,
Si: 0.50% or more and less than 0.90%,
Mn: 0.40% to 1.20%,
S: 0.010% or more and 0.030% or less,
Cu: 0.05% or more and 0.50% or less,
Cr: 0.80% to 1.80%,
Mo: 0.10% or less,
Al: 0.020% or more and 0.060% or less,
N: 0.0060% or more and 0.0200% or less and 0: 0.0015% or less are contained within a range satisfying the following formulas (1) and (2), and the balance has a chemical composition composed of Fe and inevitable impurities, Hardening at a depth of 200 μm from the surface obtained after carburizing and quenching at a carburizing temperature of 900 to 1050 ° C. for 60 to 600 min, quenching temperature of 800 to 900 ° C. for 10 to 120 min and tempering at 120 to 250 ° C. for 30 to 180 min. A case-hardened steel with a thickness of HV730 or higher.
Record
1.7 ≧ [% Si] + ([% Mn] + [% Cr]) / 3 ≧ 1.1 (1)
0.16 ≧ [% Si] × ([% Cu] / 2) ≧ 0.03 (2)
However, [] is the element content in parentheses (mass%)
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