JP4555749B2 - Method for improving delayed fracture resistance of high strength bolts - Google Patents
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- JP4555749B2 JP4555749B2 JP2005246973A JP2005246973A JP4555749B2 JP 4555749 B2 JP4555749 B2 JP 4555749B2 JP 2005246973 A JP2005246973 A JP 2005246973A JP 2005246973 A JP2005246973 A JP 2005246973A JP 4555749 B2 JP4555749 B2 JP 4555749B2
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Description
本発明は、土木・建築、自動車や各種産業機械等に広く使用されているボルトに関するものであり、特に強度が１５００ＭＰａ以上で且つ耐遅れ破壊特性に優れた高強度ボルトの耐遅れ破壊特性向上方法に関するものである。 The present invention relates to a bolt widely used in civil engineering / architecture, automobiles, various industrial machines, and the like, and in particular, a method for improving delayed fracture resistance of a high-strength bolt having a strength of 1500 MPa or more and excellent delayed fracture resistance. It is about.
自動車や各種産業機械の軽量化、高性能化あるいは土木・建築構造物の建設費削減のために、高強度ボルトのニーズが高まっている。高強度ボルトは、例えばＪＩＳ Ｇ４１０５で規定されているＳＣＭ４３５やＳＣＭ４４０などの低合金鋼を使い、所定の形状に冷間成形後、焼入れ・焼戻し処理によって製造されている。しかし、引張強さが１２００ＭＰａを超えると遅れ破壊が発生しやすくなるという問題があった。
高強度鋼の耐遅れ破壊特性を向上させる技術として、例えば、特許文献１にはＰ、Ｓ含有量を低減することが有効であり、また、特許文献２にはＳｉ、Ｍｎ含有量を規制するとともに焼入れ処理後、焼戻し工程中で曲げ加工または引き抜き加工を施す方法が開示されている。更に、特許文献３〜６には、合金元素や熱処理時に析出する炭化物に着目した耐遅れ破壊特性向上技術が開示されている。更に、特許文献７、８には、パーライト鋼を伸線加工により強化したボルトが開示されている。これらの技術によって、高強度ボルトの耐遅れ破壊特性は、ある程度向上するものの、抜本的な解決には至っていなかった。
As a technique for improving delayed fracture resistance of high-strength steel, for example,
本発明は、前述のような従来技術の問題点を解決し、強度が１５００ＭＰａ以上の耐遅れ破壊特性に優れた高強度ボルトの耐遅れ破壊特性向上方法を提供することを課題とする。 An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for improving delayed fracture resistance of a high-strength bolt excellent in delayed fracture resistance having a strength of 1500 MPa or more.
本発明者らは、ボルトの高強度化と耐遅れ破壊特性を両立する鋼材成分や組織について詳細に解析した。この結果、高強度ボルトの耐遅れ破壊特性を向上させるためには、析出強化と水素トラップ効果のある合金炭化物を利用することが極めて有効であることを明らかにした。更に、合金炭化物による析出強化と水素トラップ効果を最大限に発揮するための鋼材成分、熱処理条件を明確にした。また、焼入れ・焼戻し処理によって製造した種々の強度レベルの実ボルトを用いて、遅れ破壊特性を詳細に解析した。この結果、ナットを完全に締め付けた場合、ボルトの強度が１５００ＭＰａを超えると、ねじ部よりもボルト首下部で遅れ破壊しやすいことが明らかになった。そこで、ボルト首下部の耐遅れ破壊特性を向上させる手段について種々検討した結果、圧縮残留応力を付与させることが有効なこと、更に圧縮残留応力の付与方法として従来のショットピーニングよりも超音波打撃処理が遅れ破壊特性の向上に対して極めて有効なことを見い出した。
以上の検討結果に基づき、鋼材組成と圧縮残留応力および超音波打撃処理による圧縮残留応力の付与方法を最適に選択すれば、耐遅れ破壊特性の優れた高強度ボルトを実現できると言う結論に達し、本発明をなしたものである。
The present inventors analyzed in detail about the steel material component and structure | tissue which satisfy | fills the high intensity | strength of a bolt, and delayed fracture resistance. As a result, in order to improve the delayed fracture resistance of high-strength bolts, it became clear that the use of alloy carbides with precipitation strengthening and hydrogen trapping effect is extremely effective. Furthermore, the steel material components and heat treatment conditions for maximizing the precipitation strengthening and hydrogen trap effect by alloy carbide were clarified. In addition, the delayed fracture characteristics were analyzed in detail using real bolts of various strength levels manufactured by quenching and tempering. As a result, it was revealed that when the nut was completely tightened, if the bolt strength exceeded 1500 MPa, it was more likely to break later than the threaded portion at the bolt neck. Therefore, as a result of various investigations on means for improving the delayed fracture resistance at the bottom of the bolt neck, it is effective to apply compressive residual stress, and moreover, ultrasonic impact treatment than conventional shot peening is applied as a method of applying compressive residual stress. Has been found to be extremely effective in improving delayed fracture characteristics.
Based on the results of the above studies, the conclusion was reached that a high-strength bolt with excellent delayed fracture resistance could be realized by optimally selecting the steel material composition, compressive residual stress, and compression residual stress application method by ultrasonic impact treatment. The present invention has been made.
本発明は以上の知見に基づいてなされたものであって、その要旨とするところは、次の通りである。
（１） 質量％で、
Ｃ ：０．３〜０．６％、
Ｓｉ：０．０５〜２％、
Ｍｎ：０．１〜２％、
Ａｌ：０．００２〜０．１％、
Ｍｏ：１〜３％を含有し、
さらに、
Ｖ ：０．０５〜１％、
Ｔｉ：０．０１〜１％、
Ｎｂ：０．０１〜１％の１種または２種以上を含有し、残部がＦｅおよび不可避不純物からなり、
かつ、前記Ｃの質量％比が下記（Ａ）式を満足する成分を有する鋼材からボルトを成形加工した後、焼入れ温度：９００℃以上、焼戻し温度：５５０〜７００℃の条件で熱処理を行い、その後、前記ボルトに対する超音波振動子の硬度比：１．２以上、超音波振動子の振動数：１０〜６０ｋＨｚ、超音波の出力：５００〜５０００Ｗ、超音波振動子のボルト首下部への押し付け力：１０〜１０００Ｎの条件で、前記ボルトの首下部に超音波振動端子を押付けて打撃処理を施し、前記首下部表層の圧縮残留応力が前記鋼材の引張強さの２０〜９０％であることを特徴とする引張強さが１５００ＭＰａ以上の高強度ボルトの耐遅れ破壊特性向上方法。
０．２≦（０．０６Ｍｏ＋０．１８Ｖ＋０．２５Ｔｉ＋０．１３Ｎｂ）／Ｃ
・・・（Ａ）
（２）前記鋼材がさらに、質量％で、
Ｃｒ：０．１〜２％、
Ｎｉ：０．０５〜１％、
Ｃｕ：０．０５〜１％、
Ｂ ：０．０００３〜０．０１％の１種または２種以上を含有することを特徴とする請求項１に記載の高強度ボルトの耐遅れ破壊特性向上方法。
The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.3-0.6%
Si: 0.05-2%
Mn: 0.1 to 2%,
Al: 0.002 to 0.1%,
Mo: 1 to 3%,
further,
V: 0.05 to 1%
Ti: 0.01 to 1%,
Nb: 0.01 to 1% of 1 type or 2 types or more, with the balance consisting of Fe and inevitable impurities,
And, after forming a bolt from a steel material having a component in which the mass% ratio of C satisfies the following formula (A) , heat treatment is performed under conditions of quenching temperature: 900 ° C. or higher, tempering temperature: 550 to 700 ° C., Thereafter, the hardness ratio of the ultrasonic vibrator to the bolt is 1.2 or more, the vibration frequency of the ultrasonic vibrator is 10 to 60 kHz, the output of the ultrasonic wave is 500 to 5000 W, and the ultrasonic vibrator is pressed to the lower part of the bolt neck. Under the condition of force: 10 to 1000 N, an ultrasonic vibration terminal is pressed against the neck lower part of the bolt to perform a striking treatment, and the compressive residual stress of the neck lower surface layer is 20 to 90% of the tensile strength of the steel material. A method for improving delayed fracture resistance of a high-strength bolt having a tensile strength of 1500 MPa or more.
0.2 ≦ (0.06Mo + 0.18V + 0.25Ti + 0.13Nb) / C
... (A)
(2) The steel material is further mass%,
Cr: 0.1 to 2%,
Ni: 0.05 to 1%,
Cu: 0.05 to 1%,
B: One type or two or more types of 0.0003 to 0.01% are contained, The method for improving delayed fracture resistance of a high-strength bolt according to
本発明によれば、高強度ボルトの首下部に超音波打撃処理を施すことにより、強度が１５００ＭＰａ以上の耐遅れ破壊特性に優れた高強度ボルトの耐遅れ破壊特性向上方法を大幅に向上させる方法を提供することができるなど、産業上有用な著しい効果を奏する。
According to the present invention, a method for greatly improving the delayed fracture resistance improvement method of a high strength bolt excellent in delayed fracture resistance having a strength of 1500 MPa or more by applying an ultrasonic impact treatment to the lower neck portion of the high strength bolt. It is possible to provide a significant effect that is industrially useful.
以下に本発明を実施するための最良の形態について説明する。
まず、本発明の対象とする鋼の成分の限定理由について述べる。
Ｃは、ボルトの強度を確保する上で必須の元素であるが、０．３％未満では所要の強度が得られず、一方０．６％を越えると延性が低下するため、０．３〜０．６％の範囲に制限した。
Ｓｉは、リラクゼーション特性を向上させるとともに固溶体硬化作用によって強度を高める作用がある。０．０５％未満では前記作用が発揮できず、一方、２％を超えても添加量に見合う効果が期待できないため、０．０５〜２％の範囲に制限した。
Ｍｎは、脱酸、脱硫のために必要であるばかりでなく、焼入れ処理時にマルテンサイト組織を得るための焼入性を高めるために有効な元素であるが、０．１％未満では上記の効果が得られず、一方２％を越えて添加しても添加量に見合う効果が得られないため、０．１〜２％の範囲に制限した。
The best mode for carrying out the present invention will be described below.
First, the reasons for limiting the components of the steel that is the subject of the present invention will be described.
C is an essential element for securing the strength of the bolt, but if it is less than 0.3%, the required strength cannot be obtained, while if it exceeds 0.6%, the ductility is lowered. Limited to a range of 0.6%.
Si has an effect of improving relaxation properties and increasing strength by a solid solution hardening effect. If the content is less than 0.05%, the above-described effect cannot be exhibited. On the other hand, if the content exceeds 2%, an effect commensurate with the amount of addition cannot be expected. Therefore, the content is limited to a range of 0.05 to 2%.
Mn is not only necessary for deoxidation and desulfurization, but is also an effective element for enhancing the hardenability for obtaining a martensite structure during the quenching treatment. On the other hand, even if added over 2%, an effect commensurate with the amount added cannot be obtained, so the content was limited to the range of 0.1 to 2%.
Ａｌは、脱酸および熱処理時においてＡｌＮを形成することによりオーステナイト粒の粗大化を防止する効果がある。また、Ｂを添加する場合、Ｎを固定し焼入性および耐水素脆化特性の向上に有効な固溶Ｂを確保する効果も有している。０．００２％未満では上記の効果が発揮されず、０．１％を越えても効果が飽和するため０．００２〜０．１％の範囲に限定した。
Ｍｏ、Ｖ、Ｔｉ、Ｎｂは、いずれも焼戻し処理時に微細な合金炭化物として析出し、ボルトの高強度化に対して極めて有効な元素である。また、合金炭化物は水素をトラップさせる効果もあるため、耐遅れ破壊特性を向上させる作用も有している。
Ｍｏが１％未満、Ｖが０．０５％未満、Ｔｉが０．０１％未満、Ｎｂが０．０１％未満では上記の効果が十分に発揮できず、一方、それぞれＭｏが３％、Ｖが１％、Ｔｉが１％、Ｎｂが１％を超えて添加しても上記効果が飽和するため、Ｍｏは１〜３％、Ｖは０．０５〜１％、Ｔｉは０．０１〜１％、Ｎｂは０．０１〜１％の範囲に限定した。また、Ｍｏを含有し、且つＶ、Ｔｉ、Ｎｂの１種または２種以上を含有させることが必要である。この理由は、Ｍｏの合金炭化物だけでは析出強化能と水素トラップ能が弱いためである。Ｖ、Ｔｉ、Ｎｂの複合合金炭化物にすることにより、析出強化能と水素トラップ能が向上する。更に、Ｃとの質量％比が下記（Ａ）式を満足するようにＭｏ、Ｖ、Ｔｉ、Ｎｂを添加することが重要である。
０．２≦（０．０６Ｍｏ＋０．１８Ｖ＋０．２５Ｔｉ＋０．１３Ｎｂ）／Ｃ
・・・（Ａ）
Ｃとの質量％比が０．２未満では、析出する合金炭化物の体積分率が低すぎて、析出強化能と水素トラップ能が十分でなく、本発明で目的とする耐遅れ破壊特性に優れた高強度ボルトを実現させることが困難なため、下限を０．２に限定した。
Al has the effect of preventing austenite grains from coarsening by forming AlN during deoxidation and heat treatment. Moreover, when adding B, it has the effect of fixing N and ensuring the solid solution B effective in improving hardenability and hydrogen embrittlement resistance. If the content is less than 0.002%, the above effect is not exhibited. If the content exceeds 0.1%, the effect is saturated, so the content is limited to the range of 0.002 to 0.1%.
Mo, V, Ti, and Nb all precipitate as fine alloy carbides during tempering, and are extremely effective elements for increasing the strength of bolts. Further, since the alloy carbide has an effect of trapping hydrogen, it also has an action of improving delayed fracture resistance.
When Mo is less than 1%, V is less than 0.05%, Ti is less than 0.01%, and Nb is less than 0.01%, the above effects cannot be sufficiently exhibited. Even if 1%, Ti is added to 1%, and Nb exceeds 1%, the above effect is saturated, so that Mo is 1 to 3%, V is 0.05 to 1%, Ti is 0.01 to 1% , Nb was limited to a range of 0.01 to 1%. Moreover, it is necessary to contain Mo and to include one or more of V, Ti, and Nb. This is because the precipitation strengthening ability and hydrogen trapping ability are weak only with Mo alloy carbides. By using a composite alloy carbide of V, Ti, and Nb, precipitation strengthening ability and hydrogen trapping ability are improved. Furthermore, it is important to add Mo, V, Ti, and Nb so that the mass% ratio with C satisfies the following formula (A).
0.2 ≦ (0.06Mo + 0.18V + 0.25Ti + 0.13Nb) / C
... (A)
If the mass% ratio with C is less than 0.2, the volume fraction of the alloy carbide to be precipitated is too low, the precipitation strengthening ability and the hydrogen trapping ability are not sufficient, and the excellent delayed fracture resistance intended in the present invention is excellent. Since it is difficult to realize a high strength bolt, the lower limit was limited to 0.2.
以上が本発明の対象とする鋼の基本成分であるが、本発明においては、更にこの鋼材に、Ｃｒ：０．１〜２％、Ｎｉ：０．０５〜１％、Ｃｕ：０．０５〜１％、Ｂ ：０．０００３〜０．０１％、の１種または２種以上を含有せしめることができる。
Ｃｒは、焼入性の向上および焼戻し処理時の軟化抵抗を増加させるために有効な元素であるが、０．１％未満ではその効果が十分に発揮できず、一方２％を超えて添加しても効果が飽和するため、０．１〜２％の範囲に限定した。
Ｎｉは、高強度化に伴って劣化する延性を向上させるとともに熱処理時の焼入性を向上させて引張強さを増加させるために添加されるが、０．０５％未満ではその効果が少なく、一方１％を越えても添加量にみあう効果が発揮できないため、０．０５〜１％の範囲に制限した。
Ｃｕは、焼入れ処理時の焼入性を向上させる効果を有しているが、０．０５％未満ではその効果が不十分であり、１％を超えて添加しても効果が飽和するために、０．０５〜１％の範囲に限定した。
Ｂは、耐遅れ破壊特性を向上させる効果があり、更にオーステナイト粒界に偏析することにより焼入性を著しく高める効果も有しているが、Ｂが０．０００３％未満では前記の効果が発揮されず、０．０１％を超えても効果が飽和するため０．０００３〜０．０１％に制限した。
Ｐ、Ｓについては特に制限しないものの、高強度ボルトの耐遅れ破壊特性を向上させる観点から、それぞれ０．０１５％以下が好ましい範囲である。また、ＮはＡｌ、Ｖ、Ｎｂ、Ｔｉの炭窒化物を生成することによりオーステナイト粒の細粒化効果があるが、０．０１５％を越えると延性が低下するため、０．００２〜０．０１５％が好ましい範囲である。
The above are the basic components of the steel that is the subject of the present invention. In the present invention, Cr: 0.1 to 2%, Ni: 0.05 to 1%, Cu: 0.05 to One or two or more of 1% and B: 0.0003 to 0.01% can be contained.
Cr is an effective element for improving the hardenability and increasing the softening resistance during the tempering treatment. However, if it is less than 0.1%, the effect cannot be sufficiently exerted, while adding over 2%. However, since the effect is saturated, it is limited to the range of 0.1 to 2%.
Ni is added to improve ductility which deteriorates with increasing strength and to improve the hardenability during heat treatment and increase the tensile strength, but less than 0.05% has little effect, On the other hand, even if it exceeds 1%, the effect of matching the added amount cannot be exhibited, so the content was limited to 0.05 to 1%.
Cu has the effect of improving the hardenability during the quenching process, but the effect is insufficient if it is less than 0.05%, and the effect is saturated even if added over 1%. , Limited to the range of 0.05 to 1%.
B has an effect of improving the delayed fracture resistance, and also has an effect of remarkably improving hardenability by segregating at the austenite grain boundary. However, when B is less than 0.0003%, the above effect is exhibited. Not exceeding 0.01%, the effect is saturated, so the content is limited to 0.0003 to 0.01%.
P and S are not particularly limited, but from the viewpoint of improving the delayed fracture resistance of high-strength bolts, 0.015% or less is a preferable range. Further, N has an effect of refining austenite grains by forming carbonitrides of Al, V, Nb, and Ti. However, if it exceeds 0.015%, the ductility is lowered. 015% is a preferred range.
次に、ボルト首下部の圧縮残留応力の限定理由について説明する。下記に説明する超音波打撃処理による圧縮残留応力がボルト強度の２０％未満では、耐遅れ破壊特性の向上効果が少ないために、圧縮残留応力の下限をボルト強度の２０％に制限した。一方、ボルト強度の９０％を超えるような圧縮残留応力を付与しても上記の効果が飽和するため、上限をボルト強度の９０％に限定した。耐遅れ破壊特性の向上と超音波打撃処理のコストの観点で、好ましいボルト首下部の圧縮残留応力の範囲は、ボルト引張強さの３０〜７０％である。なお、本発明の残留応力はＸ線法で測定したものである。
本発明の高強度ボルトは、焼入れ・焼戻し処理によって所定の強度を得るものであり、焼戻しマルテンサイトが主体の組織である。その他の組織として、フェライト、ベイナイト、パーライトの１種または２種以上を面積率で１０％以下を含有しても良い。フェライト、ベイナイト、パーライトの面積率は、ボルト中心部において２ｍｍ2以上の視野を光学顕微鏡（５００倍）で観察することによって、測定できる。
Next, the reason for limiting the compressive residual stress at the bottom of the bolt neck will be described. When the compressive residual stress by the ultrasonic impact treatment described below is less than 20% of the bolt strength, the effect of improving the delayed fracture resistance is small, so the lower limit of the compressive residual stress is limited to 20% of the bolt strength. On the other hand, since the above effect is saturated even when compressive residual stress exceeding 90% of the bolt strength is applied, the upper limit is limited to 90% of the bolt strength. From the viewpoint of improving delayed fracture resistance and the cost of ultrasonic impact treatment, a preferable range of compressive residual stress under the bolt neck is 30 to 70% of the bolt tensile strength. The residual stress of the present invention is measured by the X-ray method.
The high-strength bolt of the present invention obtains a predetermined strength by quenching and tempering treatment, and is mainly composed of tempered martensite. As other structures, one or more of ferrite, bainite, and pearlite may be contained in an area ratio of 10% or less. The area ratio of ferrite, bainite, and pearlite can be measured by observing a field of view of 2 mm 2 or more with an optical microscope (500 times) at the center of the bolt.
図１は、本発明の高強度ボルトの耐遅れ破壊特性向上方法の実施形態を例示する図である。
図１において、１は首下部、２は超音波振動端子を示す。
図１に示すように、焼入れ・焼戻し処理を行った高強度ボルトの首下部１に超音波振動端子２を押付けて、図１の矢印の方向に超音波打撃処理を施して、高強度ボルトの首下部１の表層に前記ボルトを構成する鋼材の引張強さの２０〜９０％の高圧縮残留応力を付与することによって、高強度ボルトの耐遅れ破壊特性を著しく向上させることができる。
本発明は、前記ボルトを成形加工した後、焼入れ温度：９００℃以上、焼戻し温度：５５０〜７００℃の条件で熱処理を行い、その後、前記ボルトに対する超音波振動子の硬度比：１．２以上、超音波振動子の振動数：１０〜６０ｋＨｚ、超音波の出力：５００〜５０００Ｗ、超音波振動子のボルト首下部への押し付け力：１０〜１０００Ｎの条件で、前記ボルトの首下部に超音波振動端子を押付けて打撃処理を施すことを特徴とする。
FIG. 1 is a diagram illustrating an embodiment of a method for improving delayed fracture resistance of a high-strength bolt of the present invention.
In FIG. 1, 1 is a neck lower part, 2 shows an ultrasonic vibration terminal.
As shown in FIG. 1, the ultrasonic vibration terminal 2 is pressed against the
In the present invention, after forming the bolt, heat treatment is performed under the conditions of quenching temperature: 900 ° C. or higher and tempering temperature: 550 to 700 ° C., and then the hardness ratio of the ultrasonic vibrator to the bolt: 1.2 or higher. The ultrasonic vibrator has a vibration frequency of 10 to 60 kHz, an ultrasonic output of 500 to 5000 W, and a pressing force of the ultrasonic vibrator to the lower neck of the bolt: 10 to 1000 N. A striking process is performed by pressing the vibration terminal.
以下に、本発明の製造方法の限定理由について説明する。
焼入れ処理の加熱温度が９００℃未満では、未溶解の炭化物や合金炭化物が多すぎて、析出強化能が低下するため、焼入れ温度の下限を９００℃に限定した。上限は特に限定しないものの、１１００℃を超えるとオーステナイト粒が粗大化して耐遅れ破壊特性が低下するため、好ましい加熱温度の上限は１１００℃である。なお、焼入れは水冷または油冷を行い、マルテンサイト組織にするものである。焼戻し処理は、焼戻し温度が５５０℃未満では合金炭化物の析出が不十分なため、合金炭化物による析出強化と耐遅れ破壊特性の向上が期待できない。一方、焼戻し温度が７００℃を超えると合金炭化物の粗大化が起き、合金炭化物の析出強化能と水素トラップ能が低下する。以上の理由で、焼戻し温度範囲を５５０〜７００℃に制限した。
Below, the reason for limitation of the manufacturing method of this invention is demonstrated.
When the heating temperature of the quenching treatment is less than 900 ° C., there are too many undissolved carbides and alloy carbides and the precipitation strengthening ability is lowered, so the lower limit of the quenching temperature is limited to 900 ° C. The upper limit is not particularly limited, but if it exceeds 1100 ° C., the austenite grains become coarse and delayed fracture resistance decreases, so the preferable upper limit of the heating temperature is 1100 ° C. In addition, quenching performs water cooling or oil cooling to make a martensite structure. In the tempering treatment, when the tempering temperature is lower than 550 ° C., the precipitation of alloy carbide is insufficient, and therefore precipitation strengthening and delayed fracture resistance cannot be expected to be improved by the alloy carbide. On the other hand, when the tempering temperature exceeds 700 ° C., the alloy carbide coarsens, and the precipitation strengthening ability and hydrogen trapping ability of the alloy carbide are reduced. For the above reasons, the tempering temperature range was limited to 550 to 700 ° C.
次に、焼入れ・焼戻し処理後の超音波打撃処理の条件について説明する。
超音波振動子の硬度がボルトの硬度の１．２倍未満では、超音波打撃処理によるボルト首下部への圧縮残留応力を効率的に付与することが困難であるため、ボルトに対する超音波振動子の硬度比を１．２以上に限定した。なお、超音波振動子の先端の曲率半径は特に限定しないものの、ボルト首下部の曲率半径（首下部丸み）よりも大きい場合は効率的に圧縮残留応力を付与することが出来ないため、超音波振動子の先端半径は首下部の曲率半径と同等以下にすることが好ましい条件である。超音波振動子の振動数が１０ｋＨｚ未満では、効率的に圧縮残留応力を付与することができないため、下限を１０ｋＨｚに限定した。一方、６０ｋＨｚを超える振動数で超音波打撃処理を行っても圧縮残留応力の導入効果が飽和するため、振動数の上限を６０ｋＨｚに制限した。振動数の好ましい範囲は、２０〜４０ｋＨｚである。超音波の出力が５００Ｗ未満では、所定の圧縮残留応力を付与させるための超音波打撃処理時間が長くなり経済的でないため、下限を５００Ｗに限定した。超音波出力が５０００Ｗを超えても効果が飽和するため、５０００Ｗを上限にした。超音波振動子のボルト首下部への押し付け力が１０Ｎ未満では、効率的に圧縮残留応力を付与することができず経済的でないため、下限を１０Ｎに制限した。一方、押し付け力が１０００Ｎを超えて超音波打撃処理を行っても効果が飽和するため、上限を１０００Ｎに制限した。
Next, the conditions of the ultrasonic impact treatment after the quenching / tempering treatment will be described.
If the hardness of the ultrasonic vibrator is less than 1.2 times the hardness of the bolt, it is difficult to efficiently apply the compressive residual stress to the lower part of the bolt neck by ultrasonic striking treatment. The hardness ratio was limited to 1.2 or more. The radius of curvature of the tip of the ultrasonic transducer is not particularly limited, but if it is larger than the radius of curvature of the bottom of the bolt neck (roundness of the bottom of the neck), compressive residual stress cannot be applied efficiently, so ultrasonic waves It is a preferable condition that the tip radius of the vibrator is equal to or less than the curvature radius of the lower neck. If the frequency of the ultrasonic vibrator is less than 10 kHz, the compressive residual stress cannot be applied efficiently, so the lower limit is limited to 10 kHz. On the other hand, since the effect of introducing the compressive residual stress is saturated even if the ultrasonic impact treatment is performed at a frequency exceeding 60 kHz, the upper limit of the frequency is limited to 60 kHz. A preferable range of the frequency is 20 to 40 kHz. If the output of the ultrasonic wave is less than 500 W, the ultrasonic striking treatment time for applying a predetermined compressive residual stress becomes long and not economical, so the lower limit is limited to 500 W. Even if the ultrasonic output exceeds 5000 W, the effect is saturated, so 5000 W was made the upper limit. If the pressing force of the ultrasonic vibrator to the lower part of the bolt neck is less than 10N, it is not economical because compressive residual stress cannot be efficiently applied, so the lower limit is limited to 10N. On the other hand, since the effect is saturated even if the pressing force exceeds 1000 N and the ultrasonic impact treatment is performed, the upper limit is limited to 1000 N.
超音波打撃処理による圧縮残留応力付与は、ショットピーニングによる圧縮残留応力付与よりも、耐遅れ破壊特性が優れている。この理由は、
１）超音波打撃処理による圧縮残留応力はショットピーニングよりも高い
２）超音波打撃処理による圧縮残留応力はショットピーニングよりも鋼材内部まで付与されている
３）超音波打撃処理の部位は塑性変形されており、耐遅れ破壊特性が向上する
４）超音波打撃処理による表面粗さがショットピーニングよりも小さい
ことに起因すると推定される。
The application of compressive residual stress by ultrasonic impact treatment is superior in delayed fracture resistance to the application of compressive residual stress by shot peening. The reason is
1) The compressive residual stress by ultrasonic hitting treatment is higher than that of shot peening 2) The compressive residual stress by ultrasonic hitting treatment is applied to the inside of the steel material rather than shot peening 3) The site of ultrasonic hitting treatment is plastically deformed 4) The delayed fracture resistance is improved. 4) It is estimated that the surface roughness due to the ultrasonic hitting process is smaller than that of shot peening.
以下、実施例により本発明の効果をさらに具体的に説明する。
表１に示す化学成分の鋼材を用いて、図１に示す形状のＭ１０の六角ボルトを成形した。その後、焼入れ・焼戻し処理を行った。ミクロ組織は、いずれも焼戻しマルテンサイトが面積率で９５〜１００％であり、残部はフェライト、ベイナイト、パーライトの１種または２種以上であった。焼入れ・焼戻し処理後に、ボルトに超音波打撃処理を施した。ボルトの熱処理条件、超音波打撃処理条件を表２に示す。また、ボルトの引張強さ、首下部の残留応力を表２に併せて示す。遅れ破壊試験は、同一の条件で製造したボルトをそれぞれ１００本の大気暴露試験を行い、遅れ破壊の破断比率（％）で評価した。なお、大気暴露試験におけるボルトの締め付け荷重はボルト破断荷重の９０％であり、大気暴露期間は２年間で評価した。大気暴露試験の破断比率（％）も表２に示した。表２の試験Ｎｏ.１〜２５が本発明例で、試験Ｎｏ.２６〜４７が比較例である。同表に見られるように本発明例は、いずれもボルトの引張強さが１５００ＭＰａ以上であるとともにボルト首下部に高い圧縮残留応力が導入されている。この結果、遅れ破壊の破断比率が全て０％であり、耐遅れ破壊特性に優れた高強度ボルトが実現されている。
Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
Using steel materials having chemical components shown in Table 1, M10 hexagon bolts having the shape shown in FIG. 1 were formed. Thereafter, quenching and tempering were performed. As for the microstructure, tempered martensite was 95-100% in area ratio in all cases, and the balance was one or more of ferrite, bainite and pearlite. After the quenching and tempering treatment, the bolt was subjected to ultrasonic hitting treatment. Table 2 shows the heat treatment conditions of the bolts and the ultrasonic hitting conditions. Table 2 also shows the tensile strength of the bolt and the residual stress at the bottom of the neck. In the delayed fracture test, 100 bolts manufactured under the same conditions were each subjected to an atmospheric exposure test, and the fracture rate (%) of delayed fracture was evaluated. The bolt tightening load in the air exposure test was 90% of the bolt breaking load, and the air exposure period was evaluated for 2 years. Table 2 also shows the fracture ratio (%) in the air exposure test. Test Nos. 1 to 25 in Table 2 are examples of the present invention, and Test Nos. 26 to 47 are comparative examples. As can be seen from the table, in all of the examples of the present invention, the tensile strength of the bolt is 1500 MPa or more and a high compressive residual stress is introduced to the lower part of the bolt neck. As a result, all fracture rates of delayed fracture are 0%, and a high-strength bolt excellent in delayed fracture resistance is realized.
これに対して、比較例であるＮｏ.〜２６〜３２は、いずれも鋼の化学成分が不適切な例である。即ち、Ｎｏ.２６はＭｏを含有していないために熱処理後の強度が低い例である。Ｎｏ.２７、２８、３２は、いずれもＭｏを含有しているものの含有量が低いために目的とする１５００ＭＰａ以上の高強度ボルトが実現できなかった例である。また、比較例であるＮｏ.２９、３１は、いずれもＶ、Ｔｉ、Ｎｂの１種または２種以上を含有していないため、大気暴露試験で遅れ破壊が発生した例である。更に、Ｎｏ.３０はＣ含有量が低すぎるために目的とするボルトの高強度化が実現できなかった例である。
比較例であるＮｏ.３３〜３６は、いずれも焼入れ・焼戻し処理の熱処理条件が不適切な例である。Ｎｏ.３３および３４は、いずれも焼入れ温度が低すぎるために未溶解の炭化物、合金炭化物が多量に存在し、焼戻し後の強度が低かった例である。Ｎｏ.３５、３６はいずれも焼戻し温度が不適切な例であり、Ｎｏ.３５は焼戻し温度が高すぎて強度が低下し、Ｎｏ.３６は焼戻し温度が低すぎるために合金炭化物が析出せず目的とする強度が得られなかった例である。
On the other hand, No.-26-32 which is a comparative example are all examples in which the chemical composition of steel is inappropriate. That is, No. 26 is an example of low strength after heat treatment because it does not contain Mo. Nos. 27, 28, and 32 are examples in which the high strength bolt of 1500 MPa or more which was the target could not be realized because the content of Mo was low but the content was low. Further, Nos. 29 and 31, which are comparative examples, are examples in which delayed fracture occurred in an atmospheric exposure test because none of them contained one or more of V, Ti, and Nb. Further, No. 30 is an example in which the intended strength of the bolt could not be increased because the C content was too low.
Nos. 33 to 36 as comparative examples are examples in which the heat treatment conditions for quenching and tempering treatment are inappropriate. Nos. 33 and 34 are examples in which the quenching temperature is too low, so that a large amount of undissolved carbide and alloy carbide exist and the strength after tempering is low. Nos. 35 and 36 are examples in which the tempering temperature is inappropriate, No. 35 is too high in tempering temperature and the strength is lowered, and No. 36 is too low in tempering temperature so that no alloy carbide precipitates. This is an example in which the intended strength was not obtained.
比較例であるＮｏ.３７、３９、４１は、いずれも焼入れ・焼戻し処理ままのボルトの例である。首下部の残留応力が高い圧縮残留応力になっていないために、遅れ破壊の破断比率が高い例である。
比較例であるＮｏ.３８、４０、４２、４５〜４７は、いずれも超音波打撃処理の条件が不適切な例である。即ち、Ｎｏ.３８はボルトに対する超音波振動子の硬度比が低いために、Ｎｏ.４０および４７はいずれも超音波振動子の振動数が低いために、Ｎｏ.４２および４６はいずれも超音波振動子のボルト首下部への押し付け力が低すぎるために、更にＮｏ.４５は超音波出力が低すぎるために、いずれもボルト首下部の残留応力が高い圧縮残留応力状態になっていない。この結果、暴露試験で遅れ破壊の破断比率が高く、遅れ破壊を防止できなかった例である。
比較例であるＮｏ.４３、４４は、いずれも従来のショットピーニング処理で首下部の残留応力を圧縮残留応力に変化させた例である。ショットピーニング処理では首下部に効率的に高い圧縮残留応力を付与することが困難であるため、大気暴露試験では遅れ破壊が発生した例である。
Nos. 38, 40, 42, 45 to 47, which are comparative examples, are examples in which the conditions of the ultrasonic hitting process are inappropriate. That is, since No. 38 has a low hardness ratio of the ultrasonic vibrator to the bolt, No. 40 and 47 both have the low frequency of the ultrasonic vibrator. Since the pressing force of the vibrator on the lower part of the bolt neck is too low, and No. 45 is too low in ultrasonic output, none of them is in a compressive residual stress state where the residual stress at the lower part of the bolt neck is high. As a result, the fracture rate of delayed fracture was high in the exposure test, and this was an example in which delayed fracture could not be prevented.
Nos. 43 and 44, which are comparative examples, are examples in which the residual stress at the bottom of the neck is changed to compressive residual stress by the conventional shot peening process. In shot peening, it is difficult to efficiently apply high compressive residual stress to the lower neck, so this is an example of delayed fracture in the atmospheric exposure test.
１ 首下部
２ 超音波振動端子
1 Lower neck 2 Ultrasonic vibration terminal
Claims (2)
Ｃ ：０．３〜０．６％、
Ｓｉ：０．０５〜２％、
Ｍｎ：０．１〜２％、
Ａｌ：０．００２〜０．１％、
Ｍｏ：１〜３％を含有し、
さらに、
Ｖ ：０．０５〜１％、
Ｔｉ：０．０１〜１％、
Ｎｂ：０．０１〜１％の１種または２種以上を含有し、残部がＦｅおよび不可避不純物からなり、
かつ、前記Ｃの質量％比が下記（Ａ）式を満足する成分を有する鋼材からボルトを成形加工した後、焼入れ温度：９００℃以上、焼戻し温度：５５０〜７００℃の条件で熱処理を行い、その後、前記ボルトに対する超音波振動子の硬度比：１．２以上、超音波振動子の振動数：１０〜６０ｋＨｚ、超音波の出力：５００〜５０００Ｗ、超音波振動子のボルト首下部への押し付け力：１０〜１０００Ｎの条件で、前記ボルトの首下部に超音波振動端子を押付けて打撃処理を施し、前記首下部表層の圧縮残留応力が前記鋼材の引張強さの２０〜９０％であることを特徴とする引張強さが１５００ＭＰａ以上の高強度ボルトの耐遅れ破壊特性向上方法。
０．２≦（０．０６Ｍｏ＋０．１８Ｖ＋０．２５Ｔｉ＋０．１３Ｎｂ）／Ｃ
・・・（Ａ） % By mass
C: 0.3-0.6%
Si: 0.05-2%
Mn: 0.1 to 2%,
Al: 0.002 to 0.1%,
Mo: 1 to 3%,
further,
V: 0.05 to 1%
Ti: 0.01 to 1%,
Nb: 0.01 to 1% of 1 type or 2 types or more, with the balance consisting of Fe and inevitable impurities,
And, after forming a bolt from a steel material having a component in which the mass% ratio of C satisfies the following formula (A) , heat treatment is performed under conditions of quenching temperature: 900 ° C. or higher, tempering temperature: 550 to 700 ° C., Thereafter, the hardness ratio of the ultrasonic vibrator to the bolt is 1.2 or more, the vibration frequency of the ultrasonic vibrator is 10 to 60 kHz, the output of the ultrasonic wave is 500 to 5000 W, and the ultrasonic vibrator is pressed to the lower part of the bolt neck. Under the condition of force: 10 to 1000 N, an ultrasonic vibration terminal is pressed against the neck lower part of the bolt to perform a striking treatment, and the compressive residual stress of the neck lower surface layer is 20 to 90% of the tensile strength of the steel material. A method for improving delayed fracture resistance of a high-strength bolt having a tensile strength of 1500 MPa or more.
0.2 ≦ (0.06Mo + 0.18V + 0.25Ti + 0.13Nb) / C
... (A)
Ｃｒ：０．１〜２％、
Ｎｉ：０．０５〜１％、
Ｃｕ：０．０５〜１％、
Ｂ ：０．０００３〜０．０１％の１種または２種以上を含有することを特徴とする請求項１に記載の高強度ボルトの耐遅れ破壊特性向上方法。 The steel material is further in mass%,
Cr: 0.1 to 2%,
Ni: 0.05 to 1%,
Cu: 0.05 to 1%,
B: One type or two or more types of 0.0003 to 0.01% are contained, The method for improving delayed fracture resistance of a high-strength bolt according to claim 1.
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| JP5251632B2 (en) * | 2008-05-13 | 2013-07-31 | 新日鐵住金株式会社 | High strength steel material with excellent delayed fracture resistance, high strength bolt and manufacturing method thereof |
| JP6458981B2 (en) * | 2014-08-29 | 2019-01-30 | 日産自動車株式会社 | High strength bolt |
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| CN111655883A (en) * | 2018-01-30 | 2020-09-11 | 日产自动车株式会社 | Bolt |
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| DE102019217369A1 (en) * | 2019-11-11 | 2021-05-12 | Robert Bosch Gmbh | Slow-transforming steel alloy, process for the production of the slow-transforming steel alloy and hydrogen storage with a component made from the slow-transforming steel alloy |
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| JPH062033A (en) * | 1992-06-17 | 1994-01-11 | Kobe Steel Ltd | Production of high strength bolt steel |
| JPH07292434A (en) * | 1994-04-22 | 1995-11-07 | Nippon Steel Corp | High strength steel for machine structural use excellent in delayed fracture resistance and hydrogen infiltration resistance and its production |
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| JP2001032044A (en) * | 1999-07-26 | 2001-02-06 | Nippon Steel Corp | Steel for high strength bolt and production of high strength bolt |
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| JP2004131744A (en) * | 2002-10-08 | 2004-04-30 | Nippon Steel Corp | Method for improving fatigue strength of lap welded joint |
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- 2005-08-29 JP JP2005246973A patent/JP4555749B2/en not_active Expired - Fee Related
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|---|---|---|---|---|
| JPH062033A (en) * | 1992-06-17 | 1994-01-11 | Kobe Steel Ltd | Production of high strength bolt steel |
| JPH07292434A (en) * | 1994-04-22 | 1995-11-07 | Nippon Steel Corp | High strength steel for machine structural use excellent in delayed fracture resistance and hydrogen infiltration resistance and its production |
| JPH08225845A (en) * | 1995-02-20 | 1996-09-03 | Daido Steel Co Ltd | Production of high strength bolt excellent in delayed fracture resistance |
| JP2001032044A (en) * | 1999-07-26 | 2001-02-06 | Nippon Steel Corp | Steel for high strength bolt and production of high strength bolt |
| JP2002276637A (en) * | 2001-03-22 | 2002-09-25 | Nippon Steel Corp | High tensile bolt excellent in delay breaking resistance characteristic and its steel product |
| JP2004084010A (en) * | 2002-08-27 | 2004-03-18 | Kobe Steel Ltd | High strength steel having excellent cold workability and delayed fracture resistance, and high strength steel component |
| JP2004131744A (en) * | 2002-10-08 | 2004-04-30 | Nippon Steel Corp | Method for improving fatigue strength of lap welded joint |
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| JP2006131990A (en) | 2006-05-25 |
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