JP7069519B2 - bolt - Google Patents
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- JP7069519B2 JP7069519B2 JP2018013856A JP2018013856A JP7069519B2 JP 7069519 B2 JP7069519 B2 JP 7069519B2 JP 2018013856 A JP2018013856 A JP 2018013856A JP 2018013856 A JP2018013856 A JP 2018013856A JP 7069519 B2 JP7069519 B2 JP 7069519B2
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Description
本発明は、ボルトに係り、更に詳細には、疲労強度が高い高強度ボルトに関する。 The present invention relates to bolts, and more particularly to high-strength bolts having high fatigue strength.
自動車の軽量化、コンパクト化、高性能化への要求から、ボルトの高強度化が要求されており、引張強さが１２００ＭＰａを超える高炭素鋼のボルトが知られている。 From the demands for weight reduction, compactification, and high performance of automobiles, high strength of bolts is required, and high carbon steel bolts having a tensile strength of more than 1200 MPa are known.
しかし、高炭素鋼の高強度ボルトは、静的応力下において、ある時間経過後、突然脆性的に破壊する、所謂「遅れ破壊」の発生が顕著であり、ボルト強度が高いほど遅れ破壊が発生し易い。 However, high-strength bolts made of high-carbon steel suddenly break brittlely after a certain period of time under static stress, so-called "delayed fracture", and the higher the bolt strength, the more delayed fracture occurs. Easy to do.
上記遅れ破壊は、静的応力下において、ある時間経過後、突然脆性的に破壊する現象であり、材料－環境－応力の相互作用によって生じる一種の環境脆化であり、水素による材質劣化と考えられている。 The above-mentioned delayed fracture is a phenomenon of sudden brittle fracture under static stress after a certain period of time, and is a kind of environmental embrittlement caused by the interaction of material-environment-stress, and is considered to be material deterioration due to hydrogen. Has been done.
特許文献１には、所定の組成の高炭素鋼を用いることで、遅れ破壊を抑制した高強度ボルトが開示されている。 Patent Document 1 discloses a high-strength bolt that suppresses delayed fracture by using a high carbon steel having a predetermined composition.
しかし、高強度ボルトをエンジンなどの周期的に変化する荷重、すなわち繰り返し荷重、あるいは変動荷重がかかるに部材に用いる場合には、引張強さ以下の荷重であっても繰り返して負荷がかかることで破断することがある。 However, when a high-strength bolt is used for a member such as an engine that is subject to a cyclically changing load, that is, a repetitive load or a fluctuating load, the load is repeatedly applied even if the load is less than the tensile strength. May break.
本発明は、このような従来技術の有する課題に鑑みてなされたものであり、その目的とするところは、耐遅れ破壊性を確保しつつ疲労強度を向上させた高炭素鋼の高強度ボルトを提供することにある。 The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a high-strength bolt of high carbon steel having improved fatigue strength while ensuring delayed fracture resistance. To provide.
本発明者は、上記目的を達成すべく鋭意検討を重ねた結果、遅れ破壊を抑制した所定の組成の高強度ボルトにおいては、モリブデン炭化物（Ｍｏ２Ｃ）及び鉄炭化物（Ｆｅ３Ｃ）を含む炭化物の面積率を所定の範囲にすることで、上記目的が達成できることを見出し、本発明を完成するに至った。 As a result of diligent studies to achieve the above object, the present inventor contains molybdenum carbide (Mo 2 C) and iron carbide (Fe 3 C) in a high-strength bolt having a predetermined composition in which delayed fracture is suppressed. We have found that the above object can be achieved by setting the area ratio of carbides within a predetermined range, and have completed the present invention.
即ち、本発明のボルトは、
０．５０質量％以上０．６５質量％以下の炭素（Ｃ）と、
１．５質量％以上２．５質量％以下のケイ素（Ｓｉ）と、
１．０質量％以上２．０質量％以下のクロム（Ｃｒ）と、
０．２質量％以上１．０質量％以下のマンガン（Ｍｎ）と、
１．５質量％以上５．０質量％以下のモリブデン（Ｍｏ）を含有し、
不純物であるリン（Ｐ）と硫黄（Ｓ）との合計含有量が０．０３質量％以下であり、
残部が、鉄（Ｆｅ）である組成を有するボルトである。
そして、金属組織が、マルテンサイト組織を有し、軸の中心線に直交する断面における炭化物の面積率が１％以上１０％以下、フェライトの面積率が１％以下、かつ残留オーステナイトの面積率が３～１４％であり、上記炭化物がモリブデン炭化物（Ｍｏ２Ｃ）及び鉄炭化物（Ｆｅ３Ｃ）を含み、
上記炭化物の面積率の比（Ｆｅ
３
Ｃ／Ｍｏ
２
Ｃ）が、５以上１５以下、かつ、全炭化物に対する、モリブデン炭化物（Ｍｏ
２
Ｃ）の面積率が５．５％以上８．７％以下であることを特徴とする。
That is, the bolt of the present invention is
Carbon (C) of 0.50% by mass or more and 0.65% by mass or less,
With silicon (Si) of 1.5% by mass or more and 2.5% by mass or less,
Chromium (Cr) of 1.0% by mass or more and 2.0% by mass or less,
Manganese (Mn) of 0.2% by mass or more and 1.0% by mass or less,
Contains 1.5% by mass or more and 5.0% by mass or less of molybdenum (Mo),
The total content of the impurities phosphorus (P) and sulfur (S) is 0.03% by mass or less.
The balance is a bolt having a composition of iron (Fe).
The metal structure has a martensite structure, the area ratio of carbides in the cross section orthogonal to the center line of the axis is 1% or more and 10% or less, the area ratio of ferrite is 1% or less, and the area ratio of retained austenite is. 3-14 %, the carbides containing molybdenum carbide (Mo 2 C) and iron carbide (Fe 3 C).
The ratio of the area ratio of the carbide (Fe 3 C / Mo 2 C) is 5 or more and 15 or less, and the area ratio of the molybdenum carbide (Mo 2 C) to the total carbide is 5.5% or more and 8.7% or less. It is characterized by being.
本発明によれば、所定の組成の炭化物の面積率を１％以上１０％以下にしたため、耐遅れ破壊性を確保しつつ疲労強度を向上させた高炭素鋼の高強度ボルトを提供することができる。 According to the present invention, since the area ratio of carbide having a predetermined composition is 1% or more and 10% or less, it is possible to provide a high-strength bolt of high-carbon steel having improved fatigue strength while ensuring delayed fracture resistance. can.
本発明のボルトについて詳細に説明する。
上記ボルトは、０．５０質量％以上０．６５質量％以下の炭素（Ｃ）と、１．５質量％以上２．５質量％以下のケイ素（Ｓｉ）と、１．０質量％以上２．０質量％以下のクロム（Ｃｒ）と、０．２質量％以上１．０質量％以下のマンガン（Ｍｎ）と、１．５質量％以上５．０質量％以下のモリブデン（Ｍｏ）を含有し、不純物であるリン（Ｐ）と硫黄（Ｓ）との合計含有量が０．０３質量％以下であり、残部が、鉄（Ｆｅ）である組成を有し、断面における炭化物の面積率が１％以上１０％以下であり、上記炭化物がモリブデン炭化物（Ｍｏ２Ｃ）及び鉄炭化物（Ｆｅ３Ｃ）を含む。
The bolt of the present invention will be described in detail.
The bolts include carbon (C) of 0.50% by mass or more and 0.65% by mass or less, silicon (Si) of 1.5% by mass or more and 2.5% by mass or less, and 1.0% by mass or more. Contains 0% by mass or less of chromium (Cr), 0.2% by mass or more and 1.0% by mass or less of manganese (Mn), and 1.5% by mass or more and 5.0% by mass or less of molybdenum (Mo). , The total content of the impurities phosphorus (P) and sulfur (S) is 0.03% by mass or less, the balance is iron (Fe), and the area ratio of carbides in the cross section is 1. % Or more and 10% or less, and the carbides include molybdenum carbide (Mo 2 C) and iron carbide (Fe 3 C).
上記組成の高強度ボルトにおいては、焼入れなどの熱処理を行っていないか又は熱処理が不十分であると、金属組織中にクロムやマンガンを含む粗大化した炭化物が多くなり疲労強度が低下する。 In the high-strength bolt having the above composition, if the heat treatment such as quenching is not performed or the heat treatment is insufficient, coarsened carbides containing chromium and manganese increase in the metal structure, and the fatigue strength decreases.
つまり、上記粗大化した炭化物は、母相よりも硬く、かつ粒界に片状に存在することから、粗大化した炭化物が応力集中部となり、炭化物の近傍の母相内や、炭化物と母相との境界で微細な剥離が生じ易い。 That is, since the coarsened carbide is harder than the matrix and exists in a fragmentary manner at the grain boundary, the coarsened carbide becomes a stress concentration portion and becomes a stress concentration portion in the matrix in the vicinity of the carbide or in the carbide and the matrix. Fine peeling is likely to occur at the boundary with.
そして、微細な剥離が生じると引張強さ以下の荷重であっても繰り返して負荷されることで、上記微細な剥離を起点として次第に亀裂が広がって破断に至る。 Then, when fine peeling occurs, even if the load is equal to or less than the tensile strength, the load is repeatedly applied, so that the crack gradually spreads from the fine peeling as a starting point and leads to breakage.
本発明のボルトは、所望の熱処理を行って、クロムやマンガンを含む粗大化した炭化物を固溶させた後、再度炭化物を析出させる。そして、上記析出した炭化物が、モリブデン炭化物（Ｍｏ２Ｃ）及び鉄炭化物（Ｆｅ３Ｃ）を含み、金属組織中の上記炭化物の面積率が１％以上１０％以下であるため、耐遅れ破壊性を確保しつつ疲労強度を向上させることができる。 The bolt of the present invention undergoes a desired heat treatment to dissolve the coarsened carbides containing chromium and manganese, and then precipitates the carbides again. The precipitated carbide contains molybdenum carbide (Mo 2 C) and iron carbide (Fe 3 C), and the area ratio of the carbide in the metal structure is 1% or more and 10% or less, so that it has a delayed fracture resistance. It is possible to improve the fatigue strength while ensuring the above.
上記炭化物の面積率が１％未満では靱性が低く脆いため疲労強度が低下する。
また、上記炭化物の面積率が１０％を超えると炭化物の析出量が多く、母相に固溶する炭素量が少なく、焼入れ焼き戻しの熱処理を行っても所望の強度が得られない。また、炭化物の面積率が１０％を超えると切欠き効果が増大して疲労強度が低下する。
If the area ratio of the carbide is less than 1%, the toughness is low and the carbide is brittle, so that the fatigue strength is lowered.
Further, when the area ratio of the carbide exceeds 10%, the precipitation amount of the carbide is large, the amount of carbon dissolved in the matrix is small, and the desired strength cannot be obtained even if the heat treatment of quenching and tempering is performed. Further, when the area ratio of the carbide exceeds 10%, the notch effect is increased and the fatigue strength is lowered.
上記モリブデン炭化物（Ｍｏ２Ｃ）の平均直径は１μｍ以下であることが好ましい。炭化物が充分小さいことで亀裂の発生を防止できる。 The average diameter of the molybdenum carbide (Mo 2 C) is preferably 1 μm or less. The occurrence of cracks can be prevented if the carbide is sufficiently small.
上記ボルトは、炭化物の鉄炭化物（Ｆｅ３Ｃ）とモリブデン炭化物（Ｍｏ２Ｃ）との組成比（Ｆｅ３Ｃ／Ｍｏ２Ｃ）が９以上１２未満であることが好ましい。
上記炭化物は、焼き入れによって形成されたマルテンサイト組織に焼き戻し処理を施した際に析出する鉄炭化物(Ｆｅ３Ｃ) またはモリブデン炭化物（Ｍｏ２Ｃ）である。炭化物の鉄炭化物（Ｆｅ３Ｃ）とモリブデン炭化物（Ｍｏ２Ｃ）との組成比（Ｆｅ３Ｃ／Ｍｏ２Ｃ）が９以上１２未満であることで、靱性を有し衝撃に強いボルトとなる。
The bolt preferably has a composition ratio (Fe 3 C / Mo 2 C) of carbides of iron carbide (Fe 3 C) and molybdenum carbide (Mo 2 C) of 9 or more and less than 12.
The carbides are iron carbides (Fe 3 C) or molybdenum carbides (Mo 2 C) that precipitate when the martensite structure formed by quenching is tempered. When the composition ratio (Fe 3 C / Mo 2 C) of the carbide iron carbide (Fe 3 C) and molybdenum carbide (Mo 2 C) is 9 or more and less than 12, the bolt has toughness and is strong against impact. ..
また、上記炭化物が、モリブデン炭化物（Ｍｏ２Ｃ）を面積率で７．８％以上９％以下含むことが好ましい。
モリブデン炭化物（Ｍｏ２Ｃ）の析出量が面積率で７．８％以上９％以下であるボルトは、焼き入れ処理前に存在するクロムやマンガンを含む粗大炭化物を焼き入れ加熱時に充分に固溶する。その結果、適正に焼き入れ処理が行われマルテンサイト組織を有するため引張強度および疲労強度が向上する。
Further, it is preferable that the carbide contains molybdenum carbide (Mo 2 C) in an area ratio of 7.8% or more and 9% or less.
For bolts in which the precipitation amount of molybdenum carbide (Mo 2 C) is 7.8% or more and 9% or less in terms of area ratio, coarse carbides containing chromium and manganese existing before quenching are sufficiently dissolved during quenching and heating. do. As a result, the quenching treatment is appropriately performed and the martensite structure is obtained, so that the tensile strength and the fatigue strength are improved.
上記ボルトは、軸部断面の全金属組織に対するフェライトの面積率が１％以下であり、かつ残留オーステナイトの面積率が５％以上１２％以下であることが好ましい。残留オーステナイトの面積率が上記範囲であることで疲労強度と引張強度とが向上する。 It is preferable that the bolt has an area ratio of ferrite of 1% or less with respect to the total metal structure of the cross section of the shaft portion, and an area ratio of retained austenite of 5% or more and 12% or less. When the area ratio of the retained austenite is within the above range, the fatigue strength and the tensile strength are improved.
上記残留オーステナイトは、焼入れした鋼、すなわち、炭化物が固溶したマルテンサイト組織の中に残っているオーステナイト組織である。
そして、上記マルテンサイト組織は、同じ化学組成の組織の中で最も高い硬度を示す組織であるが脆く割れが生じ易い組織である。
The retained austenite is a hardened steel, that is, an austenite structure remaining in the martensite structure in which carbides are dissolved.
The martensite structure is a structure having the highest hardness among the structures having the same chemical composition, but is brittle and easily cracked.
上記残留オーステナイトの面積率が１２％以下であることで、引張強度が向上する。
また、残留オーステナイトの面積率が５％以上であることで、高い硬度を示す組織であるマルテンサイト面積率が減少し、亀裂先端への応力集中を緩和し進展が抑制されて疲労強度が向上すると共に、熱処理時の割れや使用中の割れが抑制される。
When the area ratio of the retained austenite is 12% or less, the tensile strength is improved.
Further, when the area ratio of the retained austenite is 5% or more, the martensite area ratio, which is a structure showing high hardness, is reduced, the stress concentration on the crack tip is relaxed, the growth is suppressed, and the fatigue strength is improved. At the same time, cracking during heat treatment and cracking during use are suppressed.
また、フェライト組織はマルテンサイト組織に対して硬度が低く、面積率が１％を超えると、引張強度および疲労強度が低下する。また、オーステナイト組織と比べて固溶できる炭素量が少ないため、フェライト組織の面積率が１％を超えると、炭化物が粗大化して疲労強度が低下する。 Further, the ferrite structure has a lower hardness than the martensite structure, and when the area ratio exceeds 1%, the tensile strength and the fatigue strength decrease. Further, since the amount of carbon that can be solid-solved is smaller than that of the austenite structure, if the area ratio of the ferrite structure exceeds 1%, the carbides become coarse and the fatigue strength decreases.
上記ボルトは、引張強度が１５００ＭＰａ以上１８００ＭＰａ以下であることが好ましい。引張強度が上記範囲であれば、例えば、複リンク機構を有するレシプロエンジンのロアリンクに好適に使用できる。 The bolt preferably has a tensile strength of 1500 MPa or more and 1800 MPa or less. As long as the tensile strength is within the above range, it can be suitably used, for example, for the lower link of a reciprocating engine having a double link mechanism.
次に、ボルトの組成について説明する。 Next, the composition of the bolt will be described.
炭素（Ｃ）の含有量は、０．５０質量％以上０．６５質量％以下である。
炭素の含有量が０．５０質量％未満であると、十分な焼き戻し軟化抵抗が得られず、焼き戻しによって軟化するため、高温での焼き戻しができず、耐遅れ破壊性が低下する。
また、炭素の含有量が０．６５質量％を超えると、水素を集積するセメンタイトの量が著しく増加するため、耐遅れ破壊性が優れたものとならない。
The carbon (C) content is 0.50% by mass or more and 0.65% by mass or less.
If the carbon content is less than 0.50% by mass, sufficient tempering softening resistance cannot be obtained and the material is softened by tempering, so that tempering at a high temperature cannot be performed and the delayed fracture resistance is lowered.
Further, when the carbon content exceeds 0.65% by mass, the amount of cementite that accumulates hydrogen increases remarkably, so that the delayed fracture resistance is not excellent.
ケイ素（Ｓｉ）の含有量は、１．５質量％以上２．５質量％以下である。
ケイ素の含有量が１．５質量％未満であると引張強度が低下し、また、十分な焼き戻し軟化抵抗が得られず、高温での焼き戻しを実施できないため、耐遅れ破壊性が低下する。 ケイ素の含有量が２．５質量％を超えると、鍛造性が著しく悪化するため、ボルトを所定の形状に成形することが困難になる。
The content of silicon (Si) is 1.5% by mass or more and 2.5% by mass or less.
If the silicon content is less than 1.5% by mass, the tensile strength is lowered, sufficient tempering and softening resistance cannot be obtained, and tempering at a high temperature cannot be performed, so that the delayed fracture resistance is lowered. .. If the silicon content exceeds 2.5% by mass, the forgeability is significantly deteriorated, and it becomes difficult to form the bolt into a predetermined shape.
クロム（Ｃｒ）の含有量は、１．０質量％以上２．０質量％以下である。
クロムの含有量が１．０質量％未満であると、十分な焼き戻し軟化抵抗が得られず、高温での焼き戻しが実施できないため、耐遅れ破壊性が低下する。
また、クロムの含有量が２．０を超えると鋼材の冷間鍛造性が低下する。
The content of chromium (Cr) is 1.0% by mass or more and 2.0% by mass or less.
If the chromium content is less than 1.0% by mass, sufficient tempering and softening resistance cannot be obtained, and tempering at a high temperature cannot be performed, so that the delayed fracture resistance is lowered.
Further, when the chromium content exceeds 2.0, the cold forging property of the steel material deteriorates.
マンガン（Ｍｎ）の含有量は、０．２質量％以上１．０質量％以下である。
マンガンは焼入れ性向上元素であり、０．２質量％以上含むことで引張強度が向上する。
また、マンガンの含有量が１．０質量％を超えると、結晶粒界への偏析が促進されることによって、粒界強度が低下し耐遅れ破壊性が低下する。
The content of manganese (Mn) is 0.2% by mass or more and 1.0% by mass or less.
Manganese is an element for improving hardenability, and when it is contained in an amount of 0.2% by mass or more, the tensile strength is improved.
Further, when the manganese content exceeds 1.0% by mass, segregation to the grain boundaries is promoted, so that the grain boundary strength is lowered and the delayed fracture resistance is lowered.
モリブデン（Ｍｏ）の含有量は、１．５質量％以上５．０質量％以下である。
モリブデンの含有量が１．５質量％未満であると、水素のトラップサイトとなるモリブデン系炭化物の生成量が十分なものとならないため、水素脆化を抑制できず耐遅れ破壊性が低下する。
また、モリブデンは、マルテンサイト組織を得る焼入れ性を高め、焼戻し処理の際に軟化抵抗増加させ、高強度を得るために有効な元素であるが、５．０質量％超えて含有しても増加による効果が得られない。
The content of molybdenum (Mo) is 1.5% by mass or more and 5.0% by mass or less.
If the molybdenum content is less than 1.5% by mass, the amount of molybdenum-based carbide that becomes a hydrogen trap site is not sufficient, so that hydrogen embrittlement cannot be suppressed and the delayed fracture resistance is lowered.
Molybdenum is an element that is effective for enhancing hardenability to obtain a martensite structure, increasing softening resistance during tempering, and obtaining high strength, but it increases even if it is contained in excess of 5.0% by mass. The effect of is not obtained.
リンと硫黄との合計含有量は、０．０３質量％以下であり、０．０２質量％以下であることがより好ましい。
リンと硫黄との合計含有量が、０．０３質量％を超えると、粒界偏析が促進され、粒界結合力が小さくなって粒界強度が低下するため、耐遅れ破壊性が低下する。
The total content of phosphorus and sulfur is 0.03% by mass or less, more preferably 0.02% by mass or less.
When the total content of phosphorus and sulfur exceeds 0.03% by mass, grain boundary segregation is promoted, the grain boundary bonding force is reduced, and the grain boundary strength is lowered, so that the delayed fracture resistance is lowered.
次にボルトの製造方法について説明する。
従来の焼入れ焼き戻し処理を行う高強度ボルトは、その細長い形状から軸の中心まで熱が伝わり易いため、従来は焼入れ時間を短くしていた。しかし、粗大な炭化物を形成する可能性のある組成の場合は、所望の温度での加熱時間を充分長くした焼入れを行うことで、優れた疲労特性を有する本発明のボルトを作製できる。
Next, a method for manufacturing bolts will be described.
Conventional high-strength bolts that undergo quenching and tempering have a short quenching time because heat is easily transferred from their elongated shape to the center of the shaft. However, in the case of a composition having a possibility of forming coarse carbides, the bolt of the present invention having excellent fatigue characteristics can be produced by quenching with a sufficiently long heating time at a desired temperature.
具体的には、ボルトの太さなどにもよるが、例えば、ねじ呼び径が１４ｍｍ以下のボルトでは、９２０℃～９４０℃の温度範囲で３０～５０分間加熱した後、１２０℃以下まで冷却して焼入れし、５８０℃～６３０℃の温度範囲焼き戻すことで作製できる。 Specifically, although it depends on the thickness of the bolt, for example, for a bolt having a nominal screw diameter of 14 mm or less, it is heated in a temperature range of 920 ° C to 940 ° C for 30 to 50 minutes and then cooled to 120 ° C or less. It can be produced by quenching and quenching in a temperature range of 580 ° C to 630 ° C.
以下、本発明を実施例により詳細に説明するが、本発明は下記実施例に限定されるものではない。 Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples.
下記表１に示す組成の高強度ボルト用炭素鋼に対して、冷間鍛造、ねじ転造を行った後、下記表２に示す条件で熱処理を行い、ねじ呼び径が１１ｍｍの高強度ボルトを得た。 After cold forging and thread rolling on carbon steel for high-strength bolts having the composition shown in Table 1 below, heat treatment is performed under the conditions shown in Table 2 below to obtain high-strength bolts with a nominal screw diameter of 11 mm. Obtained.
実施例１～１１及び比較例１～５のボルトを以下の方法で評価した。
評価結果を表３に示す。
The bolts of Examples 1 to 11 and Comparative Examples 1 to 5 were evaluated by the following methods.
The evaluation results are shown in Table 3.
＜金属組織の観察＞
ボルトの軸を軸の中心線に直交する面で切断して、その断面の金属組織を走査型電子顕微鏡（Ｓｃａｎｎｉｎｇ Ｅｌｅｃｔｒｏｎ Ｍｉｃｒｏｓｃｏｐｅ：ＳＥＭ）で観察し、炭化物の組成を電子プローブマイクロアナライザー （Ｅｌｅｃｔｒｏｎ Ｐｒｏｂｅ Ｍｉｃｒｏ Ａｎａｌｙｚｅｒ； ＥＰＭＡ）で測定した。ＳＥＭおよびＥＰＭＡの測定結果から画像解析を行い、炭化物の面積率、残留オーステナイト及びフェライトの面積率を算出した。また、モリブデン炭化物（Ｍｏ２Ｃ）の平均直径についても画像解析を行うことで算出した。
<Observation of metallographic structure>
The axis of the bolt is cut at a plane orthogonal to the center line of the axis, the metallographic structure of the cross section is observed with a scanning electron microscope (SEM), and the composition of the carbide is observed with an electron probe microanalyzer (Electron Probe Microscope). Measured by Analyzer; EPMA). Image analysis was performed from the measurement results of SEM and EPMA, and the area ratio of carbides and the area ratio of retained austenite and ferrite were calculated. The average diameter of molybdenum carbide (Mo 2 C) was also calculated by performing image analysis.
＜疲労強度試験＞
実施例１～１１及び比較例１～５のボルトの疲労強度を、室温（２５℃）、大気雰囲気にて、軸方向疲労試験機に取り付け、繰返し引張荷重を作用させ疲労試験を行い、ステアケース法にて引張疲労強度（ＭＰａ）を求め、必要疲労強度に対する安全率を算出した。（ＪＩＳＢ１０８１に基づく試験）
○：安全率１．３以上
▲：安全率１．１以上
×：安全率１．１未満
<Fatigue strength test>
The fatigue strength of the bolts of Examples 1 to 11 and Comparative Examples 1 to 5 was attached to an axial fatigue tester at room temperature (25 ° C.) and an atmospheric atmosphere, and a fatigue test was performed by repeatedly applying a tensile load to perform a fatigue test. The tensile fatigue strength (MPa) was obtained by the method, and the safety factor for the required fatigue strength was calculated. (Test based on JISB1081)
○: Safety factor 1.3 or more ▲: Safety factor 1.1 or more ×: Safety factor less than 1.1
＜引張強度の測定＞
ボルトの引張強度（ＭＰａ）を、ＪＩＳ Ｂ１０５１（２００９）にしたがって測定した。
<Measurement of tensile strength>
The tensile strength (MPa) of the bolt was measured according to JIS B1051 (2009).
上記結果より、炭化物の面積率が１％以上１０％を満たす実施例１～１１は、疲労強度が優れることがわかる。
また、炭化物の面積率の比（Ｆｅ３Ｃ／Ｍｏ２Ｃ）が、９以上１２未満を満たす実施例１～３、実施例５，６は、さらに疲労強度が向上することがわかる。
From the above results, it can be seen that Examples 1 to 11 having an area ratio of carbides of 1% or more and 10% are excellent in fatigue strength.
Further, it can be seen that in Examples 1 to 3 and Examples 5 and 6 in which the ratio of the area ratio of carbides (Fe 3 C / Mo 2 C) is 9 or more and less than 12, the fatigue strength is further improved.
Claims (5)
１．５質量％以上２．５質量％以下のケイ素（Ｓｉ）と、
１．０質量％以上２．０質量％以下のクロム（Ｃｒ）と、
０．２質量％以上１．０質量％以下のマンガン（Ｍｎ）と、
１．５質量％以上５．０質量％以下のモリブデン（Ｍｏ）を含有し、
不純物であるリン（Ｐ）と硫黄（Ｓ）との合計含有量が０．０３質量％以下であり、
残部が、鉄（Ｆｅ）である組成を有し、
金属組織が、マルテンサイト組織を有し、
軸の中心線に直交する断面における炭化物の面積率が１％以上１０％以下、フェライトの面積率が１％以下、かつ残留オーステナイトの面積率が３～１４％であり、
上記炭化物がモリブデン炭化物（Ｍｏ２Ｃ）及び鉄炭化物（Ｆｅ３Ｃ）を含み、
上記炭化物の面積率の比（Ｆｅ ３ Ｃ／Ｍｏ ２ Ｃ）が、５以上１５以下、
かつ、
全炭化物に対する、モリブデン炭化物（Ｍｏ ２ Ｃ）の面積率が５．５％以上８．７％以下であることを特徴とするボルト。 Carbon (C) of 0.50% by mass or more and 0.65% by mass or less,
With silicon (Si) of 1.5% by mass or more and 2.5% by mass or less,
Chromium (Cr) of 1.0% by mass or more and 2.0% by mass or less,
Manganese (Mn) of 0.2% by mass or more and 1.0% by mass or less,
Contains 1.5% by mass or more and 5.0% by mass or less of molybdenum (Mo),
The total content of the impurities phosphorus (P) and sulfur (S) is 0.03% by mass or less.
The balance has a composition of iron (Fe) and
The metallographic structure has a martensite structure and
The area ratio of carbides in the cross section orthogonal to the center line of the axis is 1% or more and 10% or less, the area ratio of ferrite is 1% or less, and the area ratio of retained austenite is 3 to 14%.
The carbides include molybdenum carbides (Mo 2 C) and iron carbides (Fe 3 C).
The ratio of the area ratio of the carbides (Fe 3 C / Mo 2 C) is 5 or more and 15 or less.
And,
A bolt characterized in that the area ratio of molybdenum carbide (Mo 2 C) to total carbide is 5.5% or more and 8.7% or less .
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