JP2004131761A

JP2004131761A - Method for producing fastener material made of titanium alloy

Info

Publication number: JP2004131761A
Application number: JP2002295052A
Authority: JP
Inventors: Hideaki Fukai; 深井　英明; Atsushi Ogawa; 小川　厚; Kuninori Minagawa; 皆川　邦典
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2002-10-08
Filing date: 2002-10-08
Publication date: 2004-04-30

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently produce a fastener material made of a titanium alloy with a thick diameter of ≥10 mm which has a high strength of ≥1,100 MPa, and has little variation in the strength properties. <P>SOLUTION: The stock for a fastener material consisting of a titanium alloy has chemical components expressed by inequalities (1) and (2): 5≤Mo equivalent=[Mo]+0.67×[V]+1.67×[Cr]+2.86×[Fe]≤15---(1), and 2.5≤Al equivalent=[Al]+0.33×[Sn]+0.17×[Zr]≤7.5---(2); wherein, [ ] denotes the mass ratio of each componential element is subjected to solution treatment, is next subjected to screw cutting by rolling, and is then subjected to aging treatment. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
この発明は、チタン合金製ファスナー材の製造方法、特に、疲労特性に優れ且つ１１００ＭＰａ以上の平均強度を有する直径が１０ｍｍ以上のチタン合金製ファスナー材の製造方法に関するものである。
【０００２】
【従来の技術】
【特許文献１】
米国特許第５１６０５５４号特許明細書
【０００３】
チタン合金は、軽量且つもともと高強度であるため、高比強度の材料が要求される宇宙航空分野においてファスナー材（ボルト等）として広く使用されている。
【０００４】
図５に現状の高強度ファスナー材の製造フローを示すが、中でもα＋β型合金およびβ型チタン合金は、溶体化−時効処理によって更なる高強度化が達成可能であるために、前記ファスナー材向けの素材として使用されたり、適用の検討がなされている。しかしながら、Ｔｉ−６Ａｌ−４Ｖ合金に代表されるα＋β型合金は、溶体化処理後の冷却において、高強度化のために高冷却速度が必要となるため、十分な冷却速度を確保し難い太径（例えば、１０ｍｍ径以上）のファスナー材の場合には、その高強度化の程度に限界がある。
【０００５】
一方、β型合金は、溶体化処理後の冷却速度に対する達成強度レベルの感受性がα＋β型合金に比較してかなり低く、特許文献１に記載されているように高いレベルの高強度化が可能である。しかしながら、時効処理の時間が１０時間を超し、製造性の面で問題があると共に、β相を安定化するためにＭｏをはじめとする重く且つ高価な元素を大量に含有するので、比強度およびコストの面でも、現状不都合がある。
【０００６】
また、ファスナー材の製造においては、ネジ部および頭部の加工が必要である。現状ネジ部の加工には、切削による方法と転造による方法とがある。
【０００７】
切削による加工法は、強度等、素材の材料特性に比較的影響されることなく加工が可能であるが、切削工具による工具疵が残存するため、疲労特性の劣化が大きく、疲労特性が重要視される航空機分野においては、ファスナー材の材質における信頼性に問題が生じる。
【０００８】
一方、転造による加工法は、大きな素材変形を伴う加工となるため、特に、素材の強度が高い場合には、十分にネジ部の形状が成形できないか、あるいはネジ部に割れが生じる等の問題が生じる。また、加工条件によっては、ネジ部で加工硬化によって切欠感受性が高くなって、疲労強度の低下を招く恐れもある。また、ネジ加工での大変形に伴い、特に、１１００ＭＰａを超える高強度材では、ネジ部の底（谷）にシェアマークが発生し、シェアマークに沿った亀裂の進展が起こって、ファスナー材の引張強度や疲労強度の劣化を招く恐れもある。
【０００９】
これに対して、高強度材のネジ加工の際に、焼鈍等の熱処理によってネジ加工部分を軟化させてからネジ加工する方法も行われている。しかしながら、この方法では、ネジを切るために軟化させる領域を制御することが大変困難で、軟化させる領域が少ない場合には、シェアマークの発生が生じ、やはりファスナー材としての引張強度や疲労強度の劣化を招く。一方、軟化させる領域が大きい場合には、ネジ切り後にも硬度の低い部分が残存し、その低硬度の部分に起因してファスナー材として高い強度が安定的に得られないという不都合が生じる。
【００１０】
また、溶体化−時効処理によって高強度化された素材を用いて転造によるネジ加工を行うのではなく、溶体化処理後の比較的軟化した素材を用い、転造によってネジ加工を行うことも考えられる。
【００１１】
この方法をβ型チタン合金に適用した場合には、溶体化処理後のβ型チタン合金は、単相組織であって結晶粒が１００μｍ程度と粗大で、やはり転造の際に特定の結晶に歪が集中してシェアマークが形成されてしまう。
【００１２】
一方、α＋β型チタン合金に適用した場合には、β型チタン合金に比較して結晶粒は微細ではあるが、Ｔｉ−６Ａｌ−４Ｖのような現用材ではＨＣＰの結晶構造を有し、変形し難いα相の体積分率が多いために、大変形の加工が困難であったり、β相の安定性が低いことと転造によって導入された歪とに起因して、時効析出が異常に促進され、この結果、時効後に安定した延性が得られない等の不都合がある。
【００１３】
【発明が解決しようとする課題】
上述のように、従来のα＋β型チタン合金を用いた場合には、高強度化に際して、強度達成レベルの面で、また、従来のβ型チタン合金を用いた場合には、製造性やコストの面でそれぞれ問題があった。また、ファスナー材の製造においても、シェアマークの発生による強度のばらつきや疲労強度の劣化等の問題があった。
【００１４】
従って、この発明の目的は、疲労特性に優れ且つ１１００ＭＰａ以上の平均強度を有する直径が１０ｍｍ以上のチタン合金製ファスナー材の製造方法を提供することにある。
【００１５】
【課題を解決するための手段】
この発明は、上述した問題を解決するためになされたものであり、下記を特徴とするものである。
【００１６】
請求項１記載の発明は、下記（１）および（２）式、
５≦Ｍｏ等量＝［Ｍｏ］＋０．６７×［Ｖ］＋１．６７×［Ｃｒ］＋２．８６
×［Ｆｅ］≦１５　　　　　　　　　−−−（１）
２．５≦Ａｌ等量＝［Ａｌ］＋０．３３×［Ｓｎ］＋０．１７×［Ｚｒ］
≦７．５　　　　　　　　　　　　−−−（２）
但し、上記（１）および（２）式において、　［　　］は、各成分元素の質量割合を示す。
で表される化学成分を有するチタン合金からなるファスナー材用素材を溶体化処理し、次いで、転造によるネジ加工を施し、そして、時効処理を施して、疲労特性に優れ且つ１１００ＭＰａ以上の平均強度を有する直径が１０ｍｍ以上の、チタン合金製ファスナー材を製造することに特徴を有するものである。
【００１７】
請求項２記載の発明は、請求項１記載の発明において、ファスナー材用素材が、Ａｌ：４．０〜５．０％、Ｖ：２．５〜３．５％、Ｆｅ：１．５〜２．５％、Ｍｏ：１．５〜２．５％（以上、質量％）を含有することに特徴を有するものである。
【００１８】
請求項３記載の発明は、請求項１または２記載の発明において、ファスナー材用素材のβ変態点をＴβ（℃）としたときに、溶体化処理温度をＴβ−１００（℃）〜Ｔβ（℃）未満として、溶体化処理後の前記ファスナー材用素材のミクロ組織における初析α相の体積分率を、１０〜６０％の範囲内とすることに特徴を有するものである。
【００１９】
請求項４記載の発明は、請求項３記載の発明において、溶体化処理温度を９５０℃以下とすることによって、転造によるネジ加工前のミクロ組織における初析α相および変態β相の平均結晶粒径を共に１０μｍ以下にすることに特徴を有するものである。
【００２０】
請求項５記載の発明は、請求項１から４の何れか１つに記載の発明において、溶体化処理後、ファスナー材用素材を５℃／ｓｅｃ以上の冷却速度で冷却して、ファスナー材用素材の硬度を３５０ＨＶ以下とし、且つ、変態β組織中に析出するα相のアスペクト比を３以下にすることに特徴を有するものである。
【００２１】
請求項６記載の発明は、請求項１から５の何れか１つに記載の発明において、時効処理温度を５００〜５７０℃の範囲内とすることに特徴を有するものである。
【００２２】
【発明の実施の形態】
まず、この発明のチタン合金製ファスナー材の化学成分の効果に関して説明する。
【００２３】
高強度化の観点から、溶体化−時効処理による強度上昇の潜在能力、すなわち時効硬化能を有することがファスナー材用素材の化学成分として必要となる。このため、溶体化処理時において、後の時効処理時にα相を析出させることが可能なように析出相を固溶できる程度のβ相の安定度と、後の時効処理時に工業的に許容される時間内でα相を析出させて、強度上昇が可能な程度のβ相の安定度、さらには転造により導入される歪に起因して、極端に時効析出が促進されないためのβ相の安定度とを兼ね備えることが必要となる。
【００２４】
また、時効処理時にα相が析出可能なα相の潜在能力も必要となる。さらに、溶体化処理後の圧延によるネジ加工時に粗大なα相あるいはβ相が存在すると、シェアマークの発生が促進されるので、それを防ぐために溶体化処理時の粒成長を抑制する必要があり、溶体化処理温度でαおよびβの２相組織であることが必要となる。
【００２５】
このため、α相の安定度とβ相の安定度とを適切に制御する必要がある。α相の安定度が高過ぎ、あるいはβ相の安定度が低過ぎでα単相の場合、逆にα相の安定度が低過ぎ、あるいはβ相の安定度が高過ぎでβ単相の場合には、溶体化処理時に粒成長が進み、その後の圧延でのネジ加工時の大変形でシェアマークが発生してしまう。これら溶体化処理時のα相の固溶、並びに粒成長を抑制するためのピンニング効果、および時効処理時のα相析出の点で、ファスナー材用素材の化学成分が、下記式（１）および（２）を満足する必要がある。
【００２６】
５≦Ｍｏ等量＝［Ｍｏ］＋０．６７×［Ｖ］＋１．６７×［Ｃｒ］＋２．８６
×［Ｆｅ］≦１５　　　　　　　　　−−−（１）
２．５≦Ａｌ等量＝［Ａｌ］＋０．３３×［Ｓｎ］＋０．１７×［Ｚｒ］
≦７．５　　　　　　　　　　　　−−−（２）
但し、上記（１）および（２）式において、　［　　］は、各成分元素の質量
割合を示す。
【００２７】
上記（１）および（２）式の限定理由を説明する。
【００２８】
Ｍｏ等量が５％未満であると、β相が十分なα相の固溶能を持たず適切な時効硬化能を有することができなかったり、ピンニングによるα相の粒成長抑制効果を持たなかったりする。一方、Ｍｏ等量が１５％を超えた場合には、β相が安定過ぎて時効処理でのα相の時効析出に膨大な時間を要したり、溶体化処理時にβ単相となり初析α相によるβ相のピンニングでの粒成長抑制効果を失う。
【００２９】
Ａｌ等量が２．５％未満であると、溶体化処理時にβ単相となりピンニングによるβ相の粒成長抑制効果を失ったり、時効処理時にα相の時効析出が十分になされず高強度化が達成されなかったりする。一方、Ａｌ等量が７．５％を超えると、溶体化処理時にα相を十分に固溶できなく、時効処理時にα相の時効析出が十分になされず高強度化が達成されなかったりする。従って、上記式（１）および（２）を満足するＭｏ等量およびＡｌ等量の兼ね合いが必須となる。
【００３０】
この発明は、上記（１）および（２）式を満足する化学成分を有するチタン合金からなるファスナー材用素材（棒材）を、図１に示すように、溶体化処理し、溶体化処理後の比較的軟化した素材に、転造によるネジ加工を施し、そして、時効処理を施すことによって、疲労特性に優れ且つ１１００ＭＰａ以上の平均強度を有する直径が１０ｍｍ以上のチタン合金製ファスナー材を製造するものである。
【００３１】
この発明において、ファスナー材用素材の化学成分は、それぞれ質量割合でＡｌ：４．０〜５．０％、Ｖ：２．５〜３．５％、Ｆｅ：１．５〜２．５％、Ｍｏ：１．５〜２．５％を含有するチタン合金であることが望ましい。
【００３２】
α＋β型チタン合金において、Ａｌは、α相を安定化させるのに必須の元素であり、また強度上昇作用を有する。しかし、Ａｌ含有量が４．０％未満では強度への十分な寄与がない。一方、Ａｌ含有量が５．０％を超えると延靭性が劣化する。従って、Ａｌ含有量は、４．０〜５．０％の範囲内とする。
【００３３】
Ｖ、ＦｅおよびＭｏは、それぞれβ相を安定化させる元素であると共に、強度上昇作用も有する。しかし、Ｖ含有量が２．５％未満では高強度化への効果が十分ではないこと共に、十分にβ相が安定しない。一方、３．５％を超えるとβ変態点の低下により加工温度領域が狭くなることに加え、高価な金属元素の大量添加による高コスト化を招く。従って、Ｖ含有量は、２．５〜３．５％の範囲内とする。
【００３４】
Ｆｅは、上記作用と同様な作用を有するが、Ｆｅ含有量が１．５％未満では高強度化への効果が十分ではないと共に十分にβ相が安定しない。さらに、Ｆｅは、拡散速度が速く加工性を改善する効果を有するが、Ｆｅ含有量が１．５％未満では、この効果が十分に発揮できない。一方、２．５％を超えるとβ変態点の低下により加工温度領域が狭くなることに加え、偏析による材質の劣化を招く。従って、Ｆｅ含有量は、１．５〜２．５％の範囲ないとする。
【００３５】
Ｍｏは、上記作用と同様な作用を有するが、Ｍｏ含有量が１．５％未満では高強度化への効果が十分ではないこと共に、十分にβ相が安定せず、一方、２．５％を超えるとβ変態点の低下により加工温度領域が狭くなることに加え、高価な金属元素の大量添加による高コスト化を招く。しかも、その効果が飽和すると共に、β相が安定しすぎて溶体化−時効処理にて十分な高強度化が達成できない。従って、Ｍｏ含有量は、１．５〜２．５％の範囲内とする。
【００３６】
上記の化学成分のファスナー材用素材を用いることによって、溶体化処理後の圧延によるネジ加工時にシェアマークの抑制が可能な程度の微細組織を達成することが可能となると共に、時効処理によって高強度化も可能となる。
【００３７】
次に、この発明の溶体化処理条件とミクロ組織の効果に関して説明する。
【００３８】
まず、ファスナー材用素材となるチタン合金のβ変態点Ｔβ（℃）とした場合に、溶体化処理温度をＴβ−１００（℃）〜Ｔβ（℃）未満の範囲内とすることによって、ファスナー材用素材のミクロ組織における初析α相の体積分率を１０〜６０％の範囲内とすることができる。
【００３９】
このようにすることによって、圧延によるネジ加工時のシェアマークを抑制可能な組織の微細化とネジ加工時の大変形に耐え得る加工性とを達成することができる。
【００４０】
しかし、溶体化時の初析α相の体積分率が１０％未満であると、β相の粒成長を十分に抑制することができず、その結果としてネジ加工時に粗大な変態β相に大変形を加えることになり、シェアマークの発生に繋がる。一方、初析α相の体積分率が６０％を超えると、ネジ加工時にＨＣＰの結晶構造を有して変形能の小さいα相に大変形を加えることになり、やはりシェアマークの発生に繋がる。従って、溶体化処理時のミクロ組織における初析α相の体積分率は、１０〜６０％の範囲内とする必要があり、また、溶体化処理温度は、初析α相の体積分率となる温度範囲を採る必然性がある。
【００４１】
また、初析α相および変態β相の各々の平均結晶粒径を１０μｍ以下とすれば、圧延によるネジ加工時にシェアマークを抑制する効果がある。変形能が高いＢＣＣの結晶構造を有するβ相単相のβ型チタン合金において、圧延によるネジ加工時にシェアマークが発生して引張特性のばらつき等、特性劣化に繋がるのは、単相であるがため平均結晶粒径が５０μｍ以上と大きく、このためにβ相内にシェアマークが形成されることによるためで、これには存在する相の全ての結晶粒径の微細化が必要である。
【００４２】
一方、初析α相および変態β相の平均結晶粒径が１０μｍを超えると、変形時に歪が集中してシェアマークが発生する。従って、歪を分散させてシェアマークの発生を抑制するために各々の相の平均結晶粒径を１０μｍ以下にする必要がある。
【００４３】
前述のように、結晶粒成長促成効果のあるＭｏの添加、成分の限定や溶体化処理温度の限定により初析α相の体積分率を１０〜６０％の範囲内とすることに起因した初析α相によるβ相のまたβ相による初析α相のピンニング効果により各々の相の粒成長は抑制されるが、結晶粒成長には、この他に溶体化処理温度自身の影響もある。つまり、溶体化処理温度が高温であれば、他の結晶粒成長抑制効果があっても、少なからず進行する。このため、初析α相および変態β相の平均結晶粒径を１０μｍにするためには、溶体化処理温度は９５０℃以下とする必要がある。
【００４４】
さらに、変態β相内に析出したα相のアスペクト比を３以下にすることにより、圧延によるネジ加工時の変態β相の変形能を改善することができ、且つ、ボルトとしての引張特性評価の際の特性を改善することができる。
【００４５】
変態β相内の析出α相の形態が針状である場合、初析α相と変態β相との界面以外にも、変態β相中の針状α相同士の界面からもボイドが発生し、その結果、ボイドの密度が上がることによりボイドの連結が促進されて、延性が低下するので望ましい組織ではない。このため、変態β相内に析出したα相のアスペクト比を小さくする必要があり、これには、溶体化処理後の冷却速度を５℃／ｓｅｃ以上とする必要がある。
【００４６】
溶体化処理後の冷却速度が５℃／ｓｅｃ未満である場合には、図２に示すように、β相中に針状のα相の析出が生じる。この発明の化学成分の場合には、β相の安定度が適度に高いため、溶体化処理後の冷却速度が５℃／ｓｅｃ未満の空冷であっても、その後の時効処理において時効硬化能を有する。しかしながら、冷却速度の遅い空冷における冷却過程での析出のため、その後の時効処理によって得られる組織は、針状となる。変態β相内の析出α相の形状が針状の場合には、前記のように延性が低下する。
【００４７】
これに対して、溶体化処理後の冷却速度が５℃／ｓｅｃ以上の場合には、その後の時効処理で図３に示すようなアスペクト比の小さい析出α相を有する変態β相となるため、延性が改善される。このため、溶体化処理後の冷却は、５℃／ｓｅｃ以上である必要がある。
【００４８】
なお、図４に、溶体化処理後の冷却速度とファスナー材と同じ切欠係数を有するノッチ引張試験での強度との関係を示す。図４から明らかなように、冷却速度５℃／ｓｅｃ以上の場合には、高いノッチ引張強度が得られており、ファスナー材としての引張強度も高強度が得られることが分かる。
【００４９】
次に、ファスナー材用素材の材料特性の効果について説明する。
【００５０】
まず、溶体化処理後の硬度が３５０ＨＶ以下であると、圧延でのネジ加工の際の加工性を確保する効果がある。このためには、やはり溶体化処理後の冷却の冷却速度を５℃／ｓｅｃ以上とする必要がある。
【００５１】
溶体化処理後の硬度が３５０ＨＶを超える場合には、硬度が高すぎて圧延でのネジ加工の際の大変形に対応する加工性がなく、割れが発生したり、シェアマークが発生したりし、ファスナー材としての特性劣化に繋がる。このため、溶体化処理後の硬度を３５０ＨＶ以下とする必要がある。
【００５２】
また、ファスナー材用素材の引張延性で伸びが５％以上であると、切欠形状であるネジ部を有するファスナー材において、十分に高い引張強度を達成させることができる。
【００５３】
延性が低い場合には、ファスナー材用素材の強度を高めに設定しても、高強度化に伴う延性低下のため、高い切欠引張強度を得ることができず、切欠形状のネジ部を有するファスナー材において十分な引張強度を得ることができない。このため、ファスナー材用素材の引張延性で伸びが５％以上である必要がある。このためには、時効処理温度を５００℃以上にする必要がある。時効処理の温度が５００℃未満の場合には延性が低くなる。しかし、５７０℃を超えると延性は高くなるものの強度が低下して、ファスナー材として必要な強度レベルが達成できない。
【００５４】
【実施例】
次に、この発明を実施例により、さらに説明する。
【００５５】
（実施例１）
表１に示す化学成分の１２ｍｍφの直径のチタン合金棒材をファスナー素材として、表２に示す条件で溶体化処理を行い、酸化スケール除去のためのピーリングを施し、そして、圧延機を用いた転造によって直径１０ｍｍφのファスナー材を各々１０本ずつ作製した。表１に、溶体化処理後の冷却法、ミクロ組織の初析α相の体積分率、ミクロ組織の平均結晶粒径、硬度（ＨＶ）、および、ネジ底のミクロ組織を光学顕微鏡にて観察したときのシェアマークの有無を調べた結果を合わせて示す。
【００５６】
なお、表１には、この発明で用いているＭｏ等量およびＡｌ等量および各合金のβ変態点（Ｔβ）も合わせて示す。また、表２において、溶体化処理後の冷却が空冷の場合、冷却速度は４℃／ｓｅｃであり、水冷の場合では５２℃／ｓｅｃであった。
【００５７】
【表１】

【００５８】
【表２】

【００５９】
その後、表３に示す条件で時効処理を施した。表３には、ファスナー材の平均引張強度の他、強度のばらつきを確認するためファスナー材の引張強度の標準偏差、および、溶体化処理後、ネジ加工を行わず、そのまま時効処理を施した棒材からＡＳＴＭ　Ｅ８に従って引張試験片を採取して、棒材の引張特性（強度および伸び）を調べた結果を合わせて示す。
【００６０】
【表３】

【００６１】
また、各ファスナー材の疲労特性調査のため、目標強度レベルより高い１２４０ＭＰａの約３５％強に当たる４５０ＭＰａを最大応力とし、その１０％に当たる４５ＭＰａを最小応力としたファスナー材の疲労試験を行い、破断回数を調べた。この結果を表３に合わせて示す。なお、この疲労特性試験では、１０００００回を最大繰り返し数とし、破断に至らない場合、試験を中止した。さらに、表３には、析出α相のアスペクト比も合わせて示す。
【００６２】
まず、この発明では、「溶体化処理→転造によるネジ加工→時効処理」というネジの製造工程を採るが、溶体化処理温度が各合金のＴβ以上の符号Ｂ３、Ｄ１およびＥ２は、溶体化処理時に初析α相の体積分率が１０％未満（何れも、０％）であったので、β相の著しい粒成長が生じ、その結晶粒径は、１０μｍを著しく超えていた。この結果、粗大な結晶粒が転造の際に大変形を受けるため、シェアマークが発生した。
【００６３】
溶体化処理温度がＴβ−１００℃より低い符号Ａ２の場合には、ＨＣＰの結晶構造を有し、変形能の低い初析αの体積分率が６０％を超え、変形能の低い初析α相の体積分率が大きいことに起因してやはりシェアマークが発生した。
【００６４】
各相の体積分率を制御するための溶体化処理温度がこの発明の範囲内にあっても符号Ａ１のように、溶体化処理温度自体が高温且つ合金成分がこの発明の範囲外の場合には、各相の結晶粒が粗大であったり、あるいは、符号Ｂ４のように溶体化処理後の冷却の冷却速度が５℃／ｓｅｃ未満の空冷であって、溶体化処理後の硬度が３５０ＨＶを超える高硬度の場合には、やはりシェアマークが発生した。
【００６５】
表３から明らかなように、シェアマークが発生した場合には、たとえ溶体化時効処理後の棒材での強度が１１００ＭＰａを超えるものであっても、ファスナー材としての強度は１１００ＭＰａを超えず、またシェアマークに起因して強度のばらつきも標準偏差で１００ＭＰａ超えと大きかった。さらには、破断までの繰り返し数が５００００回未満と、ファスナー材としての疲労特性も悪かった。
【００６６】
また、符号Ａ１１、Ａ１２は、シェアマークの発生以外に、歪時効が異常に促進された結果、引張試験での標準偏差が大きかった。
【００６７】
次に、化学成分、および溶体化処理温度がこの発明の範囲内である符号Ｂ２は、溶体化処理後のネジ加工素材における初析α相の体積分率および各相の結晶粒径共にこの発明の範囲内にあり、シェアマークの発生はないものの、溶体化処理後の冷速の冷却速度が５℃／ｓｅｃ未満の空冷であったので、時効処理後の析出α相は針状となっており、そのアスペクト比は３を著しく超えていた。このため、この針状α相界面でも引張試験の際にボイドが発生して、強度の割には延性が低く、シェアマークの発生がないためファスナー材としての強度のばらつきは小さいものの、切欠を有するファスナー材としての引張試験では十分な強度が得られなかった。また、破断までの繰り返し数が５００００回未満と、ファスナー材としての疲労特性も悪かった。
【００６８】
化学成分がこの発明の範囲外の符号Ｄ１は、極めてβ相が安定なため、溶体化処理後の冷却が空冷であっても析出α相が針状にならず、析出α相のアスペクト比はこの発明の範囲内にあるが、前述の説明の通りシェアマークが発生してしまうため、ファスナー材としての引張強度および疲労特性は悪かった。
【００６９】
最後に時効処理温度については、温度が低いと得られる棒材としての強度は高くなるが、その分、延性は低下する。逆に、温度が高ければ得られる棒材としての延性は高くなるが、強度は低下する。時効処理の温度がこの発明の範囲より低い符号Ｃ１２、Ｄ１２、Ｅ１２およびＥ２２は、何れも、棒材としての伸びが５％未満であり、たとえ符号Ｃ１２およびＥ１２のように他の条件を満足していてもファスナー材としての強度が低下し、その標準偏差も１００ＭＰａ以上とばらつきも大きく、さらに破断までの繰り返し数が５００００回未満と、ファスナー材としての疲労特性も悪かった。
【００７０】
この発明範囲を満足する符号Ｂ１１、Ｂ１２、Ｂ５１、Ｃ１１、Ｅ１１およびＥ１３は、何れも、ファスナー材としての強度は１１００ＭＰａを超え、その標準偏差も１００ＭＰａ未満とばらつきも小さく、さらに破断までの繰り返し数が１０００００回を超えて、ファスナー材としての疲労特性も良好であった。中でも符号Ｂ１１、Ｂ１２およびＢ５１は、ファスナー材としての強度の標準偏差が１００ＭＰａ未満でばらつきが小さく、破断までの繰り返し数が１０００００回を超えてファスナー材としての疲労特性もが良好であるばかりか、ファスナー材としての引張強度は１２４０ＭＰａを超え、さらに高強度であった。
【００７１】
上記の結果からも明らかなように、ファスナー材用素材の化学成分、溶体化処理条件、溶体化処理後のミクロ組織、ＨＶ硬度および時効処理条件がこの発明の範囲内であるファスナー材は、１１００ＭＰａを超えるファスナー材としての引張強度を有し、また、その強度レベルは棒材からの強度低下も小さく、さらにファスナー材としての引張強度の標準偏差も１００ＭＰａ以下とばらつきも小さく、しかも、疲労特性も良好であった。
【００７２】
（実施例２）
溶体化処理後、直ちに時効処理を行い、その後圧延によりネジ加工を行ったファスナー材（符号Ｂ０１）の引張特性を、同一条件にて溶体化処理の後、ネジ加工を行い、その後、時効処理を行った実施例１のファスナー材（符号Ｂ１１）の特性と共に表４に示す。なお、溶体化時効後にネジ加工を行ったファスナー材は１０本であり、その形状は、実施例１と同じものである。
【００７３】
【表４】

【００７４】
表４から明らかなように、ファスナー材用素材の化学成分、溶体化処理条件および時効処理条件がこの発明の範囲内であっても、シェアマークが発生して、ファスナー材としての引張強度は、目標の１１００ＭＰａに到達せず、さらにはファスナー材としての引張強度の標準偏差は１００ＭＰａ以上とばらつきも大きく、ファスナー材として良好な特性が得られなかった。
【００７５】
【発明の効果】
以上説明したように、この発明によれば、１１００ＭＰａ以上の高強度でかつ強度特性のばらつきの少ない１０ｍｍ径以上の太径のチタン合金製ファスナー材が効率良く製造可能となるとった工業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】この発明のチタン合金製高強度ファスナー材の製造のフロー図である。
【図２】溶体化処理後の冷却速度が２℃／ｓｅｃ（空冷）での溶体化時効処理材のミクロ組織を示す電子顕微鏡写真である。
【図３】溶体化処理後の冷却速度が２０℃／ｓｅｃ（水冷）での溶体化時効処理材のミクロ組織を示す電子顕微鏡写真である。
【図４】溶体化処理後の冷却速度とファスナー材と同じ切欠係数を有するノッチ引張試験での強度との関係を示すグラフである。
【図５】現状のチタン合金製高強度ファスナー材の製造のフロー図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a titanium alloy fastener material, and more particularly to a method for manufacturing a titanium alloy fastener material having excellent fatigue characteristics and an average strength of 1100 MPa or more and a diameter of 10 mm or more.
[0002]
[Prior art]
[Patent Document 1]
U.S. Pat. No. 5,160,554
[0003]
Titanium alloys are lightweight and originally high in strength, and thus are widely used as fastener materials (bolts and the like) in the field of aerospace where high specific strength materials are required.
[0004]
FIG. 5 shows the current production flow of the high-strength fastener material. Among them, α + β-type alloys and β-type titanium alloys are particularly suitable for the above-mentioned fastener materials because further strengthening can be achieved by solution-aging treatment. It has been used as a material for or has been studied for application. However, α + β type alloys typified by Ti-6Al-4V alloy require a high cooling rate for high strength in cooling after solution treatment, so that a large diameter for which it is difficult to secure a sufficient cooling rate is required. In the case of a fastener material having a diameter of, for example, 10 mm or more, there is a limit to the degree of strength enhancement.
[0005]
On the other hand, the β-type alloy has much lower sensitivity of the achieved strength level to the cooling rate after the solution treatment than the α + β-type alloy, and as described in Patent Document 1, a high level of strength can be obtained. is there. However, the aging treatment time exceeds 10 hours, and there is a problem in terms of manufacturability. In addition, since a large amount of heavy and expensive elements such as Mo for stabilizing the β phase is contained, the specific strength is low. There are also disadvantages in terms of cost and current situation.
[0006]
Further, in manufacturing the fastener material, it is necessary to process a screw portion and a head portion. Currently, there are a method of cutting and a method of rolling to process a thread portion.
[0007]
The machining method by cutting enables machining without being relatively affected by the material properties of the material such as strength. However, since tool flaws due to the cutting tool remain, the fatigue properties are greatly deteriorated, and the fatigue properties are regarded as important. In the aircraft field, there is a problem in the reliability of fastener materials.
[0008]
On the other hand, since the rolling method involves a large deformation of the material, particularly when the strength of the material is high, it is not possible to form the thread part sufficiently or crack the thread part. Problems arise. Further, depending on the processing conditions, notch sensitivity may be increased due to work hardening in the threaded portion, which may cause a reduction in fatigue strength. Further, with the large deformation in the screw processing, especially in the case of a high-strength material exceeding 1100 MPa, a shear mark is generated at the bottom (valley) of the screw portion, a crack is developed along the shear mark, and the fastener material is formed. There is a possibility that the tensile strength or the fatigue strength may be deteriorated.
[0009]
On the other hand, when screwing a high-strength material, a method of softening the threaded portion by heat treatment such as annealing and then screwing is also used. However, in this method, it is very difficult to control a region to be softened for cutting a screw. If the region to be softened is small, a shear mark is generated, and the tensile strength and the fatigue strength of the fastener material are also reduced. It causes deterioration. On the other hand, when the region to be softened is large, a portion having low hardness remains even after the thread cutting, and there is a disadvantage that high strength as a fastener material cannot be stably obtained due to the low hardness portion.
[0010]
Also, instead of performing threading by rolling using a material that has been strengthened by solution-aging treatment, it is also possible to perform threading by rolling using a relatively softened material after solution treatment. Conceivable.
[0011]
When this method is applied to a β-type titanium alloy, the β-type titanium alloy after the solution treatment has a single-phase structure and a coarse crystal grain of about 100 μm. The distortion concentrates and a share mark is formed.
[0012]
On the other hand, when applied to an α + β type titanium alloy, the crystal grains are finer than those of the β type titanium alloy, but the active material such as Ti-6Al-4V has the crystal structure of HCP and is deformed. The large volume fraction of the difficult α phase makes it difficult to process large deformations, and abnormally accelerates aging precipitation due to the low stability of the β phase and the strain introduced by rolling. As a result, there is an inconvenience that stable ductility cannot be obtained after aging.
[0013]
[Problems to be solved by the invention]
As described above, when the conventional α + β type titanium alloy is used, when the strength is increased, the strength achievement level is required. When the conventional β type titanium alloy is used, the productivity and cost are reduced. There was a problem in terms of each. Also, in the production of fastener materials, there have been problems such as variations in strength due to generation of shear marks and deterioration of fatigue strength.
[0014]
Accordingly, an object of the present invention is to provide a method of manufacturing a titanium alloy fastener material having a diameter of 10 mm or more and having excellent fatigue properties and an average strength of 1100 MPa or more.
[0015]
[Means for Solving the Problems]
The present invention has been made to solve the above-mentioned problem, and has the following features.
[0016]
The invention according to claim 1 is based on the following formulas (1) and (2):
5 ≦ Mo equivalent = [Mo] + 0.67 × [V] + 1.67 × [Cr] +2.86
× [Fe] ≦ 15 ----- (1)
2.5 ≦ Al equivalent weight = [Al] + 0.33 × [Sn] + 0.17 × [Zr]
≤7.5 --- (2)
However, in the above formulas (1) and (2), [] indicates the mass ratio of each component element.
A solution for a fastener material made of a titanium alloy having a chemical component represented by the following formula is subjected to solution treatment, then subjected to threading by rolling, and then subjected to aging treatment to provide excellent fatigue properties and an average strength of 1100 MPa or more. The method is characterized in that a titanium alloy fastener material having a diameter of 10 mm or more and having the following is produced.
[0017]
According to a second aspect of the present invention, in the first aspect of the invention, the material for the fastener material is Al: 4.0 to 5.0%, V: 2.5 to 3.5%, and Fe: 1.5 to 5.0%. It is characterized in that it contains 2.5% and Mo: 1.5 to 2.5% (more than mass%).
[0018]
In the invention according to claim 3, in the invention according to claim 1 or 2, when the β transformation point of the fastener material is Tβ (° C.), the solution treatment temperature is Tβ-100 (° C.) to Tβ ( ° C), the volume fraction of the pro-eutectoid α phase in the microstructure of the material for fastener material after the solution treatment is within the range of 10 to 60%.
[0019]
According to a fourth aspect of the present invention, in the third aspect, the solution treatment temperature is 950 ° C. or less, so that the average crystal of the primary α phase and the transformed β phase in the microstructure before thread forming by rolling. The feature is that both the particle diameters are set to 10 μm or less.
[0020]
According to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, after the solution treatment, the fastener material is cooled at a cooling rate of 5 ° C./sec or more. It is characterized in that the hardness of the material is 350 HV or less, and the aspect ratio of the α phase precipitated in the transformed β structure is 3 or less.
[0021]
The invention according to claim 6 is characterized in that, in the invention according to any one of claims 1 to 5, the aging treatment temperature is in the range of 500 to 570 ° C.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the effects of the chemical components of the titanium alloy fastener material of the present invention will be described.
[0023]
From the viewpoint of increasing the strength, it is necessary that the material for the fastener material has a potential for increasing the strength by solution-aging treatment, that is, having an age hardening ability. Therefore, during the solution treatment, the stability of the β phase is such that the precipitated phase can be solid-dissolved so that the α phase can be precipitated during the subsequent aging treatment, and is industrially acceptable during the subsequent aging treatment. Precipitation of the α phase within a certain time, the stability of the β phase to such an extent that the strength can be increased, and furthermore, due to the strain introduced by rolling, the β phase of the It is necessary to have both stability and stability.
[0024]
Further, the potential of the α phase capable of precipitating the α phase during the aging treatment is also required. Furthermore, if a coarse α phase or β phase is present during threading by rolling after solution treatment, the generation of shear marks is promoted, and it is necessary to suppress grain growth during solution treatment in order to prevent it. It is necessary to have a two-phase structure of α and β at the solution treatment temperature.
[0025]
Therefore, it is necessary to appropriately control the stability of the α phase and the stability of the β phase. If the stability of the α phase is too high or the stability of the β phase is too low and the phase is α single phase, the stability of the α phase is too low or the stability of the β phase is too high and β phase is too high. In such a case, grain growth proceeds during the solution treatment, and a shear mark is generated due to a large deformation at the time of threading in the subsequent rolling. In terms of the solid solution of the α phase during the solution treatment, the pinning effect for suppressing the grain growth, and the α phase precipitation during the aging treatment, the chemical components of the fastener material are expressed by the following formula (1) and It is necessary to satisfy (2).
[0026]
5 ≦ Mo equivalent = [Mo] + 0.67 × [V] + 1.67 × [Cr] +2.86
× [Fe] ≦ 15 ----- (1)
2.5 ≦ Al equivalent weight = [Al] + 0.33 × [Sn] + 0.17 × [Zr]
≤7.5 --- (2)
However, in the above formulas (1) and (2), [] represents the mass of each component element.
Indicates the ratio.
[0027]
The reasons for limiting the expressions (1) and (2) will be described.
[0028]
If the Mo equivalent is less than 5%, the β phase does not have a sufficient solid solution ability of the α phase, cannot have an appropriate age hardening ability, or does not have an effect of suppressing grain growth of the α phase by pinning. Or On the other hand, when the Mo equivalent exceeds 15%, the β phase is too stable and requires an enormous amount of time for the aging precipitation of the α phase in the aging treatment, or the β phase becomes a single β phase during the solution treatment and the primary α The effect of suppressing the grain growth in the β phase pinning by the phase is lost.
[0029]
If the Al equivalent weight is less than 2.5%, it becomes a β single phase during the solution treatment and loses the effect of suppressing the grain growth of the β phase due to pinning, and the aging precipitation of the α phase is not sufficiently performed during the aging treatment to increase the strength. May not be achieved. On the other hand, when the Al equivalent amount exceeds 7.5%, the α phase cannot be sufficiently dissolved in the solution treatment, and the aging precipitation of the α phase cannot be sufficiently performed during the aging treatment, so that high strength cannot be achieved. . Therefore, it is essential to balance the Mo equivalent and the Al equivalent that satisfy the above equations (1) and (2).
[0030]
According to the present invention, as shown in FIG. 1, a fastener material (bar) made of a titanium alloy having a chemical component satisfying the above formulas (1) and (2) is subjected to a solution treatment, and after the solution treatment. The relatively softened material is subjected to threading by rolling and then subjected to aging to produce a titanium alloy fastener material having a diameter of 10 mm or more, which has excellent fatigue characteristics and an average strength of 1100 MPa or more. Things.
[0031]
In the present invention, the chemical components of the fastener material are Al: 4.0 to 5.0%, V: 2.5 to 3.5%, Fe: 1.5 to 2.5%, by mass ratio, respectively. Mo: A titanium alloy containing 1.5 to 2.5% is desirable.
[0032]
In the α + β type titanium alloy, Al is an element essential for stabilizing the α phase, and has an effect of increasing the strength. However, if the Al content is less than 4.0%, there is no sufficient contribution to strength. On the other hand, if the Al content exceeds 5.0%, the ductility deteriorates. Therefore, the Al content is in the range of 4.0 to 5.0%.
[0033]
V, Fe and Mo are elements that stabilize the β phase, respectively, and also have a strength increasing effect. However, when the V content is less than 2.5%, the effect of increasing the strength is not sufficient, and the β phase is not sufficiently stabilized. On the other hand, when the content exceeds 3.5%, the processing temperature region is narrowed due to the decrease in the β transformation point, and the cost is increased by adding a large amount of expensive metal elements. Therefore, the V content is in the range of 2.5 to 3.5%.
[0034]
Fe has the same function as the above, but if the Fe content is less than 1.5%, the effect of increasing the strength is not sufficient, and the β phase is not sufficiently stabilized. Further, Fe has the effect of increasing the diffusion speed and improving the workability, but if the Fe content is less than 1.5%, this effect cannot be sufficiently exerted. On the other hand, if it exceeds 2.5%, the processing temperature range becomes narrow due to a decrease in the β transformation point, and the material is deteriorated due to segregation. Therefore, it is assumed that the Fe content is not in the range of 1.5 to 2.5%.
[0035]
Mo has the same action as the above, but if the Mo content is less than 1.5%, the effect of increasing the strength is not sufficient, and the β phase is not sufficiently stabilized. %, The processing temperature range is narrowed due to a decrease in the β transformation point, and the cost is increased by adding a large amount of expensive metal elements. In addition, the effect is saturated, and the β phase is too stable, so that a sufficiently high strength cannot be achieved by solution-aging treatment. Therefore, the Mo content is in the range of 1.5 to 2.5%.
[0036]
By using the fastener material of the above chemical components, it is possible to achieve a fine structure capable of suppressing shear marks during screw processing by rolling after solution treatment, and achieve high strength by aging treatment. It becomes possible.
[0037]
Next, the solution treatment conditions and the effect of the microstructure of the present invention will be described.
[0038]
First, when the β transformation point Tβ (° C.) of the titanium alloy used as the material for the fastener material is set, the solution treatment temperature is set in the range of Tβ−100 (° C.) to less than Tβ (° C.). The volume fraction of the pro-eutectoid α phase in the microstructure of the raw material can be in the range of 10 to 60%.
[0039]
By doing so, it is possible to achieve a finer structure capable of suppressing a shear mark at the time of screw processing by rolling and a workability capable of withstanding large deformation at the time of screw processing.
[0040]
However, if the volume fraction of the pro-eutectoid α phase at the time of solution treatment is less than 10%, the grain growth of the β phase cannot be sufficiently suppressed, and as a result, a large transformed β phase becomes large during threading. Deformation is added, leading to the generation of a share mark. On the other hand, when the volume fraction of the pro-eutectoid α phase exceeds 60%, large deformation is applied to the α phase having a small deformability due to the HCP crystal structure during screw processing, which also leads to the generation of a shear mark. . Therefore, the volume fraction of the pro-eutectoid α phase in the microstructure at the time of solution treatment needs to be within the range of 10 to 60%, and the solution treatment temperature depends on the volume fraction of the pro-eutectoid α phase. It is necessary to adopt a certain temperature range.
[0041]
Further, when the average crystal grain size of each of the primary α phase and the transformed β phase is 10 μm or less, there is an effect of suppressing a shear mark at the time of threading by rolling. In the β-phase single-phase β-type titanium alloy having a high deformability BCC crystal structure, it is the single phase that leads to property deterioration such as the generation of shear marks due to the formation of shear marks during threading by rolling and variations in tensile properties. Therefore, the average crystal grain size is as large as 50 μm or more, and this is due to the formation of a shear mark in the β phase, and this requires miniaturization of the crystal grain size of all existing phases.
[0042]
On the other hand, when the average crystal grain size of the pro-eutectoid α phase and the transformed β phase exceeds 10 μm, strain is concentrated at the time of deformation and a shear mark is generated. Therefore, in order to disperse the strain and suppress the generation of the shear mark, the average crystal grain size of each phase needs to be 10 μm or less.
[0043]
As described above, the addition of Mo having a crystal grain growth promoting effect, the limitation of the components, and the limitation of the solution treatment temperature limit the volume fraction of the primary α phase to 10 to 60%. Although the grain growth of each phase is suppressed by the pinning effect of the β phase by the precipitated α phase and the α phase by the β phase, the crystal grain growth is also affected by the solution treatment temperature itself. In other words, if the solution treatment temperature is high, even if there is another crystal grain growth suppressing effect, it proceeds to a considerable extent. For this reason, the solution treatment temperature must be 950 ° C. or lower in order to make the average crystal grain size of the proeutectoid α phase and the transformed β phase 10 μm.
[0044]
Furthermore, by setting the aspect ratio of the α phase precipitated in the transformed β phase to 3 or less, the deformability of the transformed β phase during screw processing by rolling can be improved, and the evaluation of tensile properties as a bolt can be performed. Characteristics can be improved.
[0045]
When the form of the precipitated α phase in the transformed β phase is acicular, voids are generated not only at the interface between the pro-eutectoid α phase and the transformed β phase but also at the interface between the acicular α phases in the transformed β phase. As a result, the increase in the density of the voids promotes the connection of the voids and decreases the ductility, which is not a desirable structure. For this reason, it is necessary to reduce the aspect ratio of the α phase precipitated in the transformed β phase, and it is necessary to set the cooling rate after the solution treatment to 5 ° C./sec or more.
[0046]
When the cooling rate after the solution treatment is less than 5 ° C./sec, needle-like α phase precipitates in β phase as shown in FIG. In the case of the chemical component of the present invention, the stability of the β phase is moderately high, so that even if the cooling rate after the solution treatment is air cooling of less than 5 ° C./sec, the age hardening ability is reduced in the subsequent aging treatment. Have. However, due to precipitation in the cooling process in air cooling with a slow cooling rate, the structure obtained by the subsequent aging treatment becomes acicular. When the shape of the precipitated α phase in the transformed β phase is acicular, the ductility is reduced as described above.
[0047]
On the other hand, when the cooling rate after the solution treatment is 5 ° C./sec or more, a transformed β phase having a precipitated α phase having a small aspect ratio as shown in FIG. The ductility is improved. Therefore, cooling after the solution treatment needs to be 5 ° C./sec or more.
[0048]
FIG. 4 shows the relationship between the cooling rate after the solution treatment and the strength in a notch tensile test having the same notch coefficient as that of the fastener material. As is clear from FIG. 4, when the cooling rate is 5 ° C./sec or higher, a high notch tensile strength is obtained, and a high tensile strength as a fastener material is obtained.
[0049]
Next, the effect of the material characteristics of the fastener material will be described.
[0050]
First, when the hardness after the solution treatment is 350 HV or less, there is an effect of securing workability in threading in rolling. For this purpose, the cooling rate after the solution treatment must be 5 ° C./sec or more.
[0051]
If the hardness after the solution treatment exceeds 350 HV, the hardness is too high, and there is no workability corresponding to a large deformation at the time of screw processing in rolling, so that cracks are generated or a shear mark is generated. This leads to deterioration of characteristics as a fastener material. Therefore, the hardness after the solution treatment needs to be 350 HV or less.
[0052]
When the elongation of the material for fastener material is 5% or more in tensile ductility, a sufficiently high tensile strength can be achieved in the fastener material having a notched thread portion.
[0053]
When the ductility is low, even if the strength of the material for the fastener material is set to a high value, a high notch tensile strength cannot be obtained due to a decrease in ductility due to the high strength, and a fastener having a notch-shaped screw portion. A sufficient tensile strength cannot be obtained in the material. For this reason, it is necessary that the elongation in the tensile ductility of the fastener material is 5% or more. For this purpose, the aging treatment temperature needs to be 500 ° C. or higher. When the temperature of the aging treatment is lower than 500 ° C., the ductility becomes low. However, when the temperature exceeds 570 ° C., the ductility is increased, but the strength is reduced, and the strength level required for the fastener material cannot be achieved.
[0054]
【Example】
Next, the present invention will be further described with reference to examples.
[0055]
(Example 1)
As a fastener material, a titanium alloy rod having a diameter of 12 mm having a chemical composition shown in Table 1 was subjected to a solution treatment under the conditions shown in Table 2, subjected to peeling to remove oxide scale, and then rolled using a rolling mill. 10 fasteners each having a diameter of 10 mm were produced by molding. In Table 1, the cooling method after the solution treatment, the volume fraction of the pro-eutectoid α phase of the microstructure, the average crystal grain size of the microstructure, the hardness (HV), and the microstructure of the screw bottom are observed with an optical microscope. The result of examining the presence / absence of the share mark at the time of performing is also shown.
[0056]
Table 1 also shows the Mo equivalent and Al equivalent used in the present invention and the β transformation point (Tβ) of each alloy. In Table 2, when the cooling after the solution treatment was air cooling, the cooling rate was 4 ° C / sec, and when the cooling was water cooling, it was 52 ° C / sec.
[0057]
[Table 1]

[0058]
[Table 2]

[0059]
Thereafter, aging treatment was performed under the conditions shown in Table 3. Table 3 shows, in addition to the average tensile strength of the fastener material, the standard deviation of the tensile strength of the fastener material in order to confirm the variation in strength, and a rod that was subjected to aging treatment without performing screw processing after solution treatment. Tensile test pieces were sampled from the material according to ASTM E8, and the results of examining the tensile properties (strength and elongation) of the bar were also shown.
[0060]
[Table 3]

[0061]
In order to investigate the fatigue characteristics of each fastener material, a fatigue test was performed on the fastener material in which 450 MPa, which is about 35% of 1240 MPa higher than the target strength level, was the maximum stress, and 45 MPa, which was 10%, was the minimum stress. Was investigated. The results are shown in Table 3. In this fatigue property test, the maximum number of repetitions was set to 100,000 times, and the test was stopped when no fracture was caused. Table 3 also shows the aspect ratio of the precipitated α phase.
[0062]
First, in the present invention, a screw manufacturing process of “solution treatment → rolling threading → aging treatment” is employed. Symbols B3, D1 and E2 whose solution treatment temperature is equal to or higher than Tβ of each alloy are referred to as solution treatment. At the time of the treatment, the volume fraction of the pro-eutectoid α phase was less than 10% (all 0%), so that remarkable grain growth of the β phase occurred, and the crystal grain size significantly exceeded 10 μm. As a result, since the coarse crystal grains undergo large deformation during rolling, a shear mark is generated.
[0063]
When the solution treatment temperature is A2, which is lower than Tβ-100 ° C., the volume fraction of proeutectoid α having an HCP crystal structure and low deformability exceeds 60%, and proeutectoid α having low deformability is low. Share marks also occurred due to the large volume fraction of the phase.
[0064]
Even when the solution treatment temperature for controlling the volume fraction of each phase is within the range of the present invention, as indicated by reference numeral A1, when the solution treatment temperature itself is high and the alloy component is out of the range of the present invention. Means that the crystal grains of each phase are coarse or that the cooling rate after the solution treatment is air cooling at less than 5 ° C./sec as indicated by reference numeral B4, and the hardness after the solution treatment is 350 HV. When the hardness was higher than that, a share mark was also generated.
[0065]
As is clear from Table 3, when the shear mark occurs, even if the strength of the rod after the solution aging treatment exceeds 1100 MPa, the strength as a fastener material does not exceed 1100 MPa, Further, the variation in strength due to the shear mark was as large as a standard deviation exceeding 100 MPa. Furthermore, when the number of repetitions until breaking was less than 50,000 times, the fatigue properties of the fastener material were poor.
[0066]
In addition, as for symbols A11 and A12, the standard deviation in the tensile test was large as a result of abnormal promotion of strain aging in addition to the occurrence of shear marks.
[0067]
Next, the symbol B2 in which the chemical component and the solution treatment temperature are within the range of the present invention indicates that the volume fraction of the pro-eutectoid α phase and the crystal grain size of each phase in the threaded material after the solution treatment are both the same. Although no shear mark is generated, the cooling rate after the solution treatment was air cooling of less than 5 ° C./sec, so that the precipitated α phase after the aging treatment became acicular. And its aspect ratio was significantly more than 3. For this reason, voids are also generated during the tensile test at the needle-like α-phase interface, and the ductility is low in spite of the strength, and there is no shear mark. Sufficient strength was not obtained in a tensile test as a fastener material. Further, when the number of repetitions until breaking was less than 50,000 times, the fatigue properties of the fastener material were poor.
[0068]
The symbol D1 whose chemical component is out of the range of the present invention indicates that the β phase is extremely stable, so that even if the cooling after the solution treatment is air cooling, the precipitated α phase does not become acicular, and the aspect ratio of the precipitated α phase is Although within the scope of the present invention, as described above, a shear mark is generated, so that the tensile strength and fatigue properties of the fastener material were poor.
[0069]
Finally, as for the aging treatment temperature, the lower the temperature, the higher the strength of the obtained bar, but the lower the ductility. Conversely, when the temperature is high, the ductility of the obtained bar is increased, but the strength is reduced. The symbols C12, D12, E12 and E22 whose aging treatment temperature is lower than the range of the present invention all have an elongation of less than 5% as a bar and satisfy other conditions such as symbols C12 and E12. However, the strength as a fastener material was reduced, the standard deviation thereof was large, that is, 100 MPa or more, and the number of repetitions until breaking was less than 50,000 times, and the fatigue properties of the fastener material were poor.
[0070]
The symbols B11, B12, B51, C11, E11 and E13 satisfying the scope of the present invention all have a strength as a fastener material of more than 1100 MPa, a standard deviation of less than 100 MPa, and a small variation. Exceeded 100,000 times, and the fatigue properties of the fastener material were also good. Above all, the symbols B11, B12 and B51 show that the standard deviation of the strength as the fastener material is less than 100 MPa, the variation is small, and the number of repetitions until break exceeds 100,000 times, and the fatigue property as the fastener material is not only good, The tensile strength as the fastener material exceeded 1240 MPa, and was even higher.
[0071]
As is clear from the above results, the fastener material whose chemical components, solution treatment conditions, microstructure after solution treatment, HV hardness and aging conditions of the fastener material are within the range of the present invention is 1100 MPa. It has a tensile strength as a fastener material exceeding that, and its strength level is small in strength reduction from the bar material, the standard deviation of the tensile strength as the fastener material is also less than 100 MPa and the dispersion is small, and the fatigue characteristics are also low. It was good.
[0072]
(Example 2)
Immediately after the solution treatment, the aging treatment was performed, and then the tensile properties of the fastener material (symbol B01), which had been threaded by rolling, were subjected to the solution treatment under the same conditions, the screw processing was performed, and then the aging treatment was performed. Table 4 shows the characteristics of the fastener material (symbol B11) of Example 1 performed. The number of fastener materials subjected to the screw processing after the solution aging is 10, and the shape is the same as that of the first embodiment.
[0073]
[Table 4]

[0074]
As is clear from Table 4, even when the chemical composition of the material for the fastener material, the solution treatment condition and the aging condition are within the scope of the present invention, a shear mark is generated and the tensile strength as the fastener material is as follows: The target 1100 MPa was not reached, and the standard deviation of the tensile strength of the fastener material was 100 MPa or more, and the dispersion was large, and good characteristics as the fastener material could not be obtained.
[0075]
【The invention's effect】
As described above, according to the present invention, it is possible to efficiently manufacture a large-diameter titanium alloy fastener material having a diameter of 10 mm or more with high strength of 1100 MPa or more and little variation in strength characteristics, which is industrially useful. The effect is brought.
[Brief description of the drawings]
FIG. 1 is a flow chart of manufacturing a titanium alloy high-strength fastener material of the present invention.
FIG. 2 is an electron micrograph showing a microstructure of a solution-aged material at a cooling rate of 2 ° C./sec (air cooling) after solution treatment.
FIG. 3 is an electron micrograph showing a microstructure of a solution-aged material at a cooling rate of 20 ° C./sec (water cooling) after solution treatment.
FIG. 4 is a graph showing the relationship between the cooling rate after solution treatment and the strength in a notch tensile test having the same notch coefficient as that of a fastener material.
FIG. 5 is a flow chart of manufacturing a current titanium alloy high-strength fastener material.

Claims

Equations (1) and (2) below,
5 ≦ Mo equivalent = [Mo] + 0.67 × [V] + 1.67 × [Cr] +2.86
× [Fe] ≦ 15 ----- (1)
2.5 ≦ Al equivalent weight = [Al] + 0.33 × [Sn] + 0.17 × [Zr]
≤7.5 --- (2)
However, in the above formulas (1) and (2), [] indicates the mass ratio of each component element.
A material for a fastener material made of a titanium alloy having a chemical component represented by the following formula: a solution treatment, then a thread forming by rolling, and an aging treatment. A method for producing a titanium alloy fastener material having a diameter of 10 mm or more having the above average strength.

The fastener material,
Al: 4.0 to 5.0%,
V: 2.5-3.5%,
Fe: 1.5 to 2.5%,
Mo: 1.5 to 2.5% (or more, mass%)
The method for producing a fastener material made of a titanium alloy according to claim 1, comprising:

When the β transformation point of the material for the fastener material is Tβ (° C.), the solution treatment temperature is set in a range of Tβ−100 (° C.) to less than Tβ (° C.), and the material for the fastener material after the solution treatment is treated. The method for producing a titanium alloy fastener material according to claim 1 or 2, wherein the volume fraction of the pro-eutectoid α phase in the microstructure of the material is in the range of 10 to 60%.

The solution treatment temperature is set to 950 ° C. or lower, so that the average crystal grain size of both the primary α phase and the transformed β phase in the microstructure before thread forming by rolling is set to 10 μm or less. 3. The method for producing a titanium alloy fastener material according to item 3.

After the solution treatment, the fastener material is cooled at a cooling rate of 5 ° C./sec or more so that the hardness of the fastener material is 350 HV or less, and the aspect ratio of the α phase precipitated in the transformed β structure. The method for producing a titanium alloy fastener material according to any one of claims 1 to 4, wherein the value is set to 3 or less.

The method for producing a titanium alloy fastener material according to any one of claims 1 to 5, wherein the aging treatment temperature is in a range of 500 to 570 ° C.