JP3873313B2 - Method for producing high-strength titanium alloy - Google Patents
Method for producing high-strength titanium alloy Download PDFInfo
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- JP3873313B2 JP3873313B2 JP01948996A JP1948996A JP3873313B2 JP 3873313 B2 JP3873313 B2 JP 3873313B2 JP 01948996 A JP01948996 A JP 01948996A JP 1948996 A JP1948996 A JP 1948996A JP 3873313 B2 JP3873313 B2 JP 3873313B2
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Description
【0001】
【発明の属する技術分野】
本発明は航空機をはじめとする輸送関連機器、更には遠心分離機等の軽量高強度が要求される各種機器に使用されるチタン合金の製造方法に関し、更に詳しくは高強度で且つ冷間加工性に優れたβ単相型のチタン合金製造方法に関する。
【0002】
【従来の技術】
チタン合金は軽量でありながら強度が高く、比重で標準化した比強度は金属材料の中で最も高く、耐食性も非常に優れることから、軽量高強度材料として航空宇宙分野を中心に多くの分野で使用されており、今後は遠心分離機等の分野でも需要が増大すると予想される。このチタン合金はα型、α+β型およびβ型に大別されており、β型チタン合金は更に、高温のβ域から急冷されたときにマルテンサイト変態が起こらずにβ単相組織が得られる狭義のβ型チタン合金と、冷却までω相が生じるNearβ型チタン合金とに細分される。本明細書では狭義のβ型チタン合金をβ単相型チタン合金と称してNearβ型チタン合金から区別し、単にβ型というときは両方を意味するものとする。
【0003】
β単相型チタン合金は、前述したようにβ単相域からの急冷により比較的安定なβ相が得られるので、冷間加工性に優れている。また熱処理性に優れ、高温からの急冷を伴う溶体化処理とこれに続く時効処理とにより高強度を得やすいという特徴もある。これに対し、Nearβ型チタン合金は、β単相型チタン合金よりも合金量が少なく、熱間変形抵抗が小さいために、鍛造性に優れており、また強度と破壊靱性のバランスが優れている。
【0004】
いずれにしても、これらのβ型チタン合金は、特に比強度の点で他のチタン合金より有利なため、航空機を中心とする分野において従来の代表的なチタン合金であるTi−6Al−4V合金(α+β型)に代わるものとして好んで使用されている。これまでに開発されたβ単相型チタン合金としてはTi−15V−3Cr−3Sn−3AlやTi−3Al−8V−6Cr−4Mo−4Zr等がある。またNearβ型チタン合金としてはTi−10V−2Fe−3AlやTi−5Al−2Sn−2Zr−4Mo−4Cr等がある。
【0005】
ところで、チタン合金はα+β型、β型を問わず、溶体化処理とこれに続く時効処理とにより、高強度化を図ることができ、その強度はこれらの熱処理条件を調整することにより変化する。ここでβ型、とりわけβ単相型チタン合金は、その熱処理により高強度を得やすいことは前述した通りである。そのため、従来からも熱処理や加工熱処理の改良により、β単相型チタン合金の優れた特性である比強度を更に向上させる試みが多くなされている。またβ単相域からの急冷により比較的安定なβ相が得られ、溶体化処理の状態での冷間加工性が良好なことから、成分組成の改良によりその優れた加工性を更に高める試みもなされている。
【0006】
例えば、鉄と鋼(1987−S1510)〔文献1〕には、冷間圧延と溶体化処理を繰り返して時効処理を行うことにより高強度を得る方法が開示されている。また特開昭61−250138号公報〔文献2〕にはV:8〜25%、Al:0.5〜5%、Cr:1.0%未満、Fe:1.0%以下、Mn:1.0%以下、残部実質的にTiよりなる冷間加工性に優れたβ単相型チタン合金が示され、特開昭61−250139号公報〔文献3〕にはV:5〜15%、Al:0.5〜5%、Cr:1〜3%、Si:1.0%、Mn:1.0%以下、残部実質的にTiよりなる冷間加工性に優れたチタン合金が示されている。更に電気製鋼第61巻第2号〔文献4〕では、代表的なβ単相型チタン合金であるTi−13V−11Cr−3Alにおいてβ安定化元素であるV,Crの各量をそれぞれ3〜16%,3〜11%の範囲で変化させたときの冷間加工性および時効特性が調査されている。
【0007】
【発明が解決しようとする課題】
しかしながら、熱処理により高強度を得やすいβ単相型チタン合金と言えども、溶体化処理とこれに続く時効処理とによる通常の熱処理の場合は、塑性伸びが得られる最高引張強さは1700MPa が限界であり、更なる高強度化は加工熱処理的な手法により行われてきた。前出の文献1に記載されている方法もその一つであり、通常の熱処理よりも高強度が得られるとしている。
【0008】
この方法は冷間加工と溶体化処理を繰り返すことにより回復組織とした後に時効処理を施し、この時効処理により非常に微細なα相を均質に析出させるものである。しかし、回復という不安定な状態を利用するため、この方法は薄板の形状には適用可能なるも、製品の寸法や形状が大きくなる場合に安定した特性を得ることが難しいという欠点がある。そのため、冷間加工と溶体化処理を繰り返さなくとも従来以上の高強度が得られる合金組成の出現が待望されている。
【0009】
一方、冷間加工性の改善については、溶体化処理後のβ単相状態で冷間加工を施す必要があるため、組織の調整幅が小さく、そのため専ら合金組成の面から改善が進められてきた。前出の文献2〜4ではTi−V−Cr−Al系において検討が加えられているが、いずれの場合も強度に関する検討はなされていない。また実際にその強度を調査しても、冷間加工と溶体化処理の繰り返しなしでは、従来以上の高強度を得ることは困難である。
【0010】
本発明の目的は、冷間加工と溶体化処理を繰り返さずとも従来以上の高強度が容易に得られるβ単相型チタン合金の製造方法を提供することにある。
【0011】
具体的には熱間加工と時効処理とからなる通常熱処理により、熱処理後の室温における引張強さが1800MPa 以上を示すβ単相型チタン合金の製造方法を提供する。
【0012】
【課題を解決するための手段】
高強度を得るために、本発明者らはチタン合金の型としてβ単相型を選択し、更にそのβ単相型チタン合金において簡易なプロセスにより安定して高強度を得るために、熱間加工とこれに続く時効処理を施した状態で高強度が得られる成分構成をTi−V−Cr−Al系において検討した。
【0013】
Ti−V−Cr−Al系はβ安定元素としてVおよびCrを添加し、α安定化元素としてAlを添加したもので、前出の文献2−4に示された合金もこれに属する。しかし、本発明者らの調査によると、これらはCr量およびAl量の少なくとも一方が不足するために高強度化が制限されており、通常の熱処理で1800MPa 以上の高強度を得るためには3.0%以上のCrと3.5%以上のAlを複合添加する必要のあることが判明した。また、熱間加工については、その合金をβ域で加工しながらα+β温度域まで冷却するのが有効なことが判明した。
【0014】
本発明は上記知見に基づきなされたもので、重量比でV:5.0〜15.0%、Cr:3.0〜6.0%、Al:3.5〜6.0%を含み、残部がTiおよび不可避不純物からなるチタン合金にβ域からα+β域において熱間加工を施すと共に、β域における加工度を50%以上とし、α+β域でその熱間加工を終了させた後に300〜550℃のα+β温度域において時効処理を施すことにより、時効処理後の室温における引張強さを1800MPa以上とする高強度チタン合金の製造方法を要旨とする。本発明においては、より高い強度を得るためにCrは3.5%以上が特に望ましい。
【0015】
【発明の実施の形態】
本発明のチタン合金製造方法は、高強度が得られるβ単相型を製造するものであり、特にβ安定化元素としてVおよびCrを添加し、α安定化元素としてAlを添加したTi−V−Cr−Al系において、V:5.0〜15.0%、Cr:3.0〜6.0%、Al:3.5〜6.0%とすることにより前記目的を達成するものである。そのチタン合金における合金成分の添加理由は次の通りである。
【0016】
V:5.0〜15.0%
Vはβ全率固溶型のβ安定化元素であり、β単相型チタン合金を得るために不可欠なものである。本発明のチタン合金ではCrを3.0%以上添加するが、Vが5.0%未満ではCrが3.0%の場合に安定なβ単相が得られず、強度の低下を招く。逆にVが15.0%を超えるとCrが3.0%の場合でもβ相の安定化度が高くなりすぎ、時効による析出強化の割合が低下するため、目的とする高強度が得られない。従って、Vは5.0〜15.0%とした。
【0017】
Cr:3.0〜6.0%
Crは共折型のβ安定化元素であり、Vと同様にβ単相型合金を得るために必要である。このCrはVよりもβ安定化能が大きい。本発明のチタン合金ではCrを3.0%以上、望ましくは3.5%以上と比較的多量に添加する。Crが少ないと冷間加工と溶体化処理の繰り返しがない場合にβ相の安定が不十分となって高強度が得られない。一方、6.0%を超えるとβ相の安定化が進みすぎるために、時効による析出強化が不十分となり高強度が得られない。
【0018】
Al:3.5〜6.0%
Alはα安定化元素であるが、α相やβ相の固溶強化および時効により析出するα相の微細化による高強度化のために不可欠である。本発明のチタン合金ではAlを3.5%と比較的多量に添加する。これにより冷間加工と溶体化処理の繰り返しなしでも時効処理後に十分な強度が得られる。Alが3.5%未満では固溶強化や時効析出のα相の微細化が十分ではなく、目的とする高強度が得られない。逆に6.0%を超えるとα相の硬さが高くなり、時効処理によりα2 相と呼ばれるTi3 Al金属間化合物が生成するために脆化が生じ、延性低下および強度低下が生じる。
【0019】
本発明のチタン合金製造方法は、β→α+β域での熱間加工とこれに続くα+β域での時効処理とにより目的とする高強度を得ることができ、複雑な繰り返しプロセスを必要とせず、溶体化処理工程さえも省略できる。なお、溶体化処理を行った場合は、80%以上の限界圧縮率の冷間加工も可能である。
【0020】
すなわち本発明のチタン合金製造方法は、高強度化に特に有効で且つ簡易な合金製造法であり、上記チタン合金にβ域において50%以上の熱間加工を施し、α+β域でその熱間加工を終了させた後に300〜550℃のα+β温度域において時効処理を施すものである。本発明のチタン合金製造方法において高強度が得られる理由は次の通りである。
【0021】
β単相型チタン合金において高強度を得るためには、母相のβ相を微細化すると共に、時効処理によりα相を微細に析出させた組織が必要である。しかし、β単相域においてはβ粒の成長が著しいために、その微細化は困難とされてきた。本発明者はβ粒の微細化を鋭意検討した結果、β域で加工を行いながらα+β温度域まで冷却をすることにより、微細β粒組織が生成することを知見した。
【0022】
このときの加工が全てβ域で行われた場合には、粗大なβ粒を基本とする組織となるために延性が大きく低下し、時効処理後も高強度が発現できない状況となる。また、このときのβ域における加工が50%未満の場合には、α+β域における加工度が増大するために細かい組織となり、延性は向上するものの、初析α相が多量に生成することにより、β相の量が減少し、目的とする高強度は得られない。
【0023】
すなわち、初析α相が多量に生成した状態で時効を施すと、β相中に時効α相が細かく析出し、初析αと時効αを含むβ相からなる組織が得られる。このとき強度が高いのは後者の時効α相を含むβ相の領域であり、初析α相の生成にともない、この高強度の領域が減少するために強度が低下するのである。微細なβ粒とするには、全加工度におけるα+β域における加工度は10〜20%が望ましい。
【0024】
熱間加工後の時効はα相を均質且つ微細に析出させる点から、高強度を得るために不可欠である。このとき300℃未満であると、目的とする組織を得るために非常に長い時間が必要となり、工業的価値が低下する。一方、550℃を超えると短時間のうちに粗大なα相が不均質に析出し、目的とする高強度の組織は得られない。望ましくは400〜450℃での比較的長い時間の時効処理である。なお、最適な時効時間は温度により異なり、制限を受けない。
【0025】
【実施例1】
表1に示す各種組成のTi−V−Cr−Al系β単相型チタン合金について、Arアーク溶解で直径60mm、重量1kgの鋳塊を得た。各鋳塊をβ域の1100℃に加熱してβ域での鍛造により直径35mmの丸棒とし、続いて950℃に加熱してβ域で鍛造を開始し、α+βで鍛造を終了することにより直径15mmの丸棒とした。
【0026】
得られた各種成分組成の丸棒を長さ150mmに切断し、圧縮試験片を採取した後の各残材に対して、α+β域の400℃に1週間保持後に空冷する時効処理を施した。時効処理後の各材より直径4mm、平行部の長さ20mmの引張試験片を各2本採取し、0.2%耐力、引張強さ及び伸びを調査した。2回目の鍛造でのβ域における加工度、α+β域における鍛造終了温度、及び試験結果を表1に示す。
【0027】
【表1】
【0028】
Vを5.0〜15.0%、Crを3.0〜6.0%、Alを3.5〜6.0%添加した本発明のチタン合金製造方法(No. 2〜10)は、冷間加工と溶体化処理の繰り返しを行っていないにもかかわらず、時効処理後に1800MPa 以上の引張強さを示し、最高で2000MPa に達する。また、このときの伸びはいずれも2.0%以上を示す。溶体化処理後の限界圧縮率についてはいずれも80%以上を示す。従来のチタン合金においてはこのような強度レベルは冷間加工と溶体化処理の繰り返しなしでは達成できなかったものであり、本発明のチタン合金製造方法が如何に優れているかが明らかである。
【0029】
ちなみに、本発明のチタン合金製造方法に属しないNo. 1はVが少ないため、時効処理後の強度が目標値に達していない。Vが過剰添加されたNo. 11,12も強度が目標未達である。No. 13,15,17はCrが3.0%未満のため、強度が不足している。Crが過剰添加されたNo. 14,16,18は時効処理後の強度が目標値に達していない。No. 19,21,23はAlが3.5%未満のために強度が不足している。Alが過剰添加されたNo. 20,22,24は時効処理の強度が低い上にNo. 22,24については延性も著しく劣る。
【0030】
【実施例2】
実施例1と同様にTi−10V−4Cr−4Al(wt%)の鍛造材について、1100℃の加熱によりβ鍛造を行って直径40mmの丸棒を得た後に、条件を変化させた2回目の鍛造により直径15mmの丸棒を得た。各鍛造材に実施例1と同じ条件で直接時効を施し、引張試験片を採取して機械的性質を調査した。2回目の鍛造条件および調査結果を表2に示す。なお、本合金のβ変態点は800℃であり、2回目の鍛造(40mm→15mm)における全体の加工度は約86%である。
【0031】
No. 1はβ域で加工を終えたために、時効処理後の強度が目標値に達していない。No. 4,5はβ域における加工度が50%未満のために、やはり強度が目標未達である。No. 6はα+β域での加工のため、No. 1と同様に強度が著しく低い。これらに対し、β域で加工度50%以上の鍛造を行い、且つその鍛造をα+β域で終了したNo. 2,3は、冷間加工と溶体化処理の繰り返しを行っていないにもかかわらず、1800MPa 以上の引張強さを示す。また、これらの伸びはいずれも2.0%以上である。
【0032】
【表2】
【0034】
【発明の効果】
以上に説明した通り、本発明のチタン合金の製造方法は冷間加工と溶体化処理を繰り返さずとも、熱間圧延とこれに続く時効処理の簡略な工程により、高強度化を図ることができるので、その繰り返しで問題となる強度の不安定が回避され、製品の形状や寸法にかかわらず安定して高強度を得ることができる。そして溶体化処理を行った場合は、80%以上の限界圧縮率の冷間加工も可能である。従って、本発明はチタン合金の用途拡大等に大きく貢献する。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a method for producing a titanium alloy used for various equipment requiring light weight and high strength such as an aircraft and other transportation-related equipment, and more specifically, high strength and cold workability. The present invention relates to a method for producing a β single-phase type titanium alloy excellent in the above.
[0002]
[Prior art]
Titanium alloy is lightweight but has high strength, and the specific strength standardized by specific gravity is the highest among metal materials and has excellent corrosion resistance. Therefore, it is used as a lightweight high-strength material in many fields, mainly in the aerospace field. In the future, demand is expected to increase in the field of centrifuges and the like. This titanium alloy is roughly classified into α-type, α + β-type and β-type, and β-type titanium alloys can obtain a β single-phase structure without martensitic transformation when quenched from a high temperature β region. It is subdivided into a narrow β-type titanium alloy and a Near β-type titanium alloy in which an ω phase is generated until cooling. In the present specification, a β-type titanium alloy in a narrow sense is referred to as a β single-phase type titanium alloy and is distinguished from a Near β-type titanium alloy, and the term “β-type” means both.
[0003]
As described above, the β single phase type titanium alloy is excellent in cold workability because a relatively stable β phase is obtained by rapid cooling from the β single phase region. Moreover, it has the characteristics that it is excellent in heat treatment property and high strength is easily obtained by solution treatment with rapid cooling from a high temperature and subsequent aging treatment. On the other hand, Near β type titanium alloy has a smaller amount of alloy than β single phase type titanium alloy and has low hot deformation resistance, so it has excellent forgeability and excellent balance between strength and fracture toughness. .
[0004]
In any case, these β-type titanium alloys are more advantageous than other titanium alloys particularly in terms of specific strength. Therefore, Ti-6Al-4V alloy, which is a typical titanium alloy in the field of aircraft, is a typical example. It is preferably used as an alternative to (α + β type). Examples of the β single-phase titanium alloy developed so far include Ti-15V-3Cr-3Sn-3Al and Ti-3Al-8V-6Cr-4Mo-4Zr. Moreover, there are Ti-10V-2Fe-3Al and Ti-5Al-2Sn-2Zr-4Mo-4Cr as Near β type titanium alloys.
[0005]
By the way, regardless of α type, β type and β type, titanium alloys can be strengthened by solution treatment and subsequent aging treatment, and the strength changes by adjusting these heat treatment conditions. Here, as described above, β-type, particularly β-single-phase titanium alloy, can easily obtain high strength by heat treatment. Therefore, many attempts have been made to further improve the specific strength, which is an excellent characteristic of β single-phase titanium alloys, by improving heat treatment and thermomechanical treatment. In addition, a relatively stable β phase is obtained by rapid cooling from the β single-phase region, and since the cold workability in the solution treatment state is good, an attempt to further improve its excellent workability by improving the component composition It has also been made.
[0006]
For example, iron and steel (1987-S1510) [Document 1] discloses a method of obtaining high strength by repeating cold rolling and solution treatment and performing an aging treatment. JP-A-61-250138 [Document 2] describes V: 8 to 25%, Al: 0.5 to 5%, Cr: less than 1.0%, Fe: 1.0% or less, Mn: 1 A β single-phase titanium alloy excellent in cold workability consisting essentially of Ti with the balance being substantially equal to or less than Ti is shown, and Japanese Patent Laid-Open No. 61-250139 [Document 3] describes V: 5 to 15%, Al: 0.5-5%, Cr: 1-3%, Si: 1.0%, Mn: 1.0% or less, the balance being a titanium alloy having excellent cold workability substantially consisting of Ti is shown. ing. Furthermore, in Electric Steel Making Vol. 61, No. 2 [Reference 4], the amounts of V and Cr, which are β-stabilizing elements, in Ti-13V-11Cr-3Al, which is a typical β single-phase titanium alloy, are 3 to 3, respectively. Cold workability and aging characteristics when changed in the range of 16% and 3 to 11% have been investigated.
[0007]
[Problems to be solved by the invention]
However, although it is a β single phase type titanium alloy that easily obtains high strength by heat treatment, in the case of normal heat treatment by solution treatment and subsequent aging treatment, the maximum tensile strength at which plastic elongation can be obtained is limited to 1700 MPa. Therefore, further strengthening has been performed by a thermomechanical method. The method described in the above-mentioned document 1 is one of them, and it is said that higher strength can be obtained than ordinary heat treatment.
[0008]
In this method, a cold treatment and solution treatment are repeated to obtain a recovery structure, and then an aging treatment is performed, and by this aging treatment, a very fine α phase is uniformly precipitated. However, this method can be applied to the shape of a thin plate because it uses an unstable state of recovery, but there is a drawback that it is difficult to obtain stable characteristics when the size and shape of a product increase. Therefore, the appearance of an alloy composition that can obtain higher strength than conventional without repeating cold working and solution treatment is awaited.
[0009]
On the other hand, for improving cold workability, it is necessary to perform cold working in the β single-phase state after solution treatment, so the adjustment range of the structure is small, and therefore, improvement has been promoted exclusively from the aspect of alloy composition. It was. In the above-mentioned documents 2 to 4, the Ti—V—Cr—Al system has been studied, but in any case, the strength has not been studied. Even if the strength is actually investigated, it is difficult to obtain higher strength than before without repeating cold working and solution treatment.
[0010]
An object of the present invention is to provide a method for producing a β single-phase titanium alloy that can easily obtain higher strength than conventional without repeating cold working and solution treatment.
[0011]
Specifically, the present invention provides a method for producing a β single-phase titanium alloy having a tensile strength at room temperature after heat treatment of 1800 MPa or more by a normal heat treatment comprising hot working and aging treatment.
[0012]
[Means for Solving the Problems]
In order to obtain high strength, the present inventors selected a β single-phase type as the type of titanium alloy, and in order to obtain high strength stably by a simple process in the β single-phase type titanium alloy, In the Ti-V-Cr-Al system, a component structure that can obtain high strength in a state where the processing and the subsequent aging treatment were performed was studied.
[0013]
In the Ti-V-Cr-Al system, V and Cr are added as β-stable elements, and Al is added as an α-stabilizing element, and the alloys shown in the above documents 2-4 also belong to this. However, according to the investigation by the inventors, at least one of the Cr content and the Al content is insufficient, so that the increase in strength is limited. In order to obtain a high strength of 1800 MPa or more by ordinary heat treatment, 3 It has been found that it is necessary to add 0.0% or more of Cr and 3.5% or more of Al in combination. As for hot working, it has been found that it is effective to cool the alloy to the α + β temperature range while processing the alloy in the β range.
[0014]
The present invention has been made based on the above findings, and includes V: 5.0 to 15.0% by weight, Cr: 3.0 to 6.0%, Al: 3.5 to 6.0%, together subjected to hot working in the beta range to a titanium alloy balance of Ti and inevitable impurities alpha + beta range, the processing degree of beta region is 50% or more, the after finishing the hot working in alpha + beta range 300 to 550 The gist is a method for producing a high-strength titanium alloy in which the tensile strength at room temperature after aging treatment is 1800 MPa or more by performing aging treatment in an α + β temperature range of ° C. In the present invention, the Cr content is particularly preferably 3.5% or more in order to obtain higher strength.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The titanium alloy production method of the present invention is to produce a β single-phase type in which high strength is obtained, and in particular, Ti-V in which V and Cr are added as a β-stabilizing element and Al is added as an α-stabilizing element. -In the Cr-Al system, V: 5.0 to 15.0%, Cr: 3.0 to 6.0%, Al: 3.5 to 6.0% to achieve the object. is there. The reason for adding the alloy component in the titanium alloy is as follows.
[0016]
V: 5.0 to 15.0%
V is a β-stabilized β-stabilizing element and is indispensable for obtaining a β single-phase titanium alloy. In the titanium alloy of the present invention, Cr is added in an amount of 3.0% or more. However, if V is less than 5.0%, a stable β single phase cannot be obtained when Cr is 3.0%, leading to a decrease in strength. Conversely, if V exceeds 15.0%, even when Cr is 3.0%, the degree of stabilization of the β phase becomes too high, and the rate of precipitation strengthening due to aging decreases, so the desired high strength can be obtained. Absent. Therefore, V is set to 5.0 to 15.0%.
[0017]
Cr: 3.0-6.0%
Cr is a co-folding β-stabilizing element, and is necessary to obtain a β single-phase type alloy like V. This Cr has a larger β stabilizing ability than V. In the titanium alloy of the present invention, Cr is added in a relatively large amount of 3.0% or more, preferably 3.5% or more. If the amount of Cr is small, the stability of the β phase becomes insufficient and high strength cannot be obtained when there is no repetition of cold working and solution treatment. On the other hand, if it exceeds 6.0%, the stabilization of the β phase proceeds too much, so that precipitation strengthening due to aging becomes insufficient and high strength cannot be obtained.
[0018]
Al: 3.5-6.0%
Al is an α-stabilizing element, but is indispensable for increasing the strength by solid solution strengthening of the α-phase and β-phase and by refinement of the α-phase precipitated by aging. In the titanium alloy of the present invention, Al is added in a relatively large amount of 3.5%. Thereby, sufficient strength can be obtained after the aging treatment without repeating cold working and solution treatment. If Al is less than 3.5%, the solid solution strengthening and the aging precipitation α phase are not sufficiently refined, and the desired high strength cannot be obtained. On the other hand, if it exceeds 6.0%, the hardness of the α phase increases, and Ti 3 Al intermetallic compound called α 2 phase is formed by the aging treatment, so that embrittlement occurs and ductility and strength decrease.
[0019]
The titanium alloy production method of the present invention can obtain the desired high strength by hot working in the β → α + β region and the subsequent aging treatment in the α + β region, and does not require a complicated repeated process. Even the solution treatment step can be omitted. In addition, when the solution treatment is performed, cold working with a limit compression rate of 80% or more is possible.
[0020]
That is, the titanium alloy manufacturing method of the present invention is a particularly effective and simple alloy manufacturing method for increasing the strength. The titanium alloy is subjected to hot working of 50% or more in the β region and the hot working in the α + β region. Aging treatment is performed in the α + β temperature range of 300 to 550 ° C. The reason why high strength is obtained in the titanium alloy manufacturing method of the present invention is as follows.
[0021]
In order to obtain high strength in a β single phase type titanium alloy, a structure in which the β phase of the matrix phase is refined and the α phase is finely precipitated by aging treatment is required. However, since the growth of β grains is remarkable in the β single phase region, it has been difficult to reduce the size. As a result of intensive studies on the refinement of β grains, the present inventor has found that a fine β grain structure is formed by cooling to the α + β temperature range while processing in the β range.
[0022]
When all the processes at this time are performed in the β region, the structure is based on coarse β grains, so that the ductility is greatly reduced, and high strength cannot be developed even after aging treatment. In addition, when the processing in the β region at this time is less than 50%, the degree of processing in the α + β region is increased, resulting in a fine structure, and the ductility is improved, but a large amount of pro-eutectoid α phase is generated. The amount of β phase decreases, and the desired high strength cannot be obtained.
[0023]
That is, when aging is performed in a state where a large amount of pro-eutectoid α phase is formed, the aging α-phase is finely precipitated in the β phase, and a structure comprising β phase including pro-eutect α and aging α is obtained. At this time, the strength is high in the β-phase region including the latter aging α-phase, and as the pro-eutectoid α-phase is formed, the high-intensity region is reduced and the strength is lowered. In order to obtain fine β grains, the processing degree in the α + β region in the total processing degree is desirably 10 to 20%.
[0024]
Aging after hot working is indispensable for obtaining high strength from the point that the α phase is precipitated homogeneously and finely. At this time, if it is lower than 300 ° C., a very long time is required to obtain the target structure, and the industrial value is lowered. On the other hand, when the temperature exceeds 550 ° C., a coarse α phase precipitates inhomogeneously within a short time, and the intended high-strength structure cannot be obtained. Desirable is an aging treatment at 400 to 450 ° C. for a relatively long time. The optimum aging time varies depending on the temperature and is not limited.
[0025]
[Example 1]
With respect to Ti—V—Cr—Al-based β single-phase titanium alloys having various compositions shown in Table 1, ingots having a diameter of 60 mm and a weight of 1 kg were obtained by Ar arc melting. By heating each ingot to 1100 ° C in the β region and forging in the β region to form a round bar with a diameter of 35 mm, then heating to 950 ° C to start forging in the β region and end forging at α + β A round bar having a diameter of 15 mm was used.
[0026]
The obtained round bars of various component compositions were cut to a length of 150 mm, and each remaining material after the compression test piece was collected was subjected to an aging treatment in which it was air-cooled after being held at 400 ° C. in the α + β region for 1 week. Two tensile test pieces each having a diameter of 4 mm and a parallel part length of 20 mm were taken from each material after the aging treatment, and 0.2% proof stress, tensile strength and elongation were investigated. Table 1 shows the degree of processing in the β region in the second forging, the forging end temperature in the α + β region, and the test results.
[0027]
[Table 1]
[0028]
The titanium alloy production method (No. 2 to 10) of the present invention in which V is added to 5.0 to 15.0%, Cr is added to 3.0 to 6.0%, and Al is added to 3.5 to 6.0%. Despite not repeating cold working and solution treatment, it shows a tensile strength of 1800 MPa or more after the aging treatment, reaching 2000 MPa at the maximum. Further, the elongation at this time is 2.0% or more. The critical compressibility after solution treatment is 80% or more. In conventional titanium alloys, such a strength level cannot be achieved without repeated cold working and solution treatment, and it is clear how excellent the titanium alloy production method of the present invention is.
[0029]
Incidentally, No. 1 which does not belong to the titanium alloy production method of the present invention has a small V, so the strength after aging treatment does not reach the target value. The strengths of Nos. 11 and 12 to which V was excessively added did not reach the target. Nos. 13, 15, and 17 have insufficient strength because Cr is less than 3.0%. In Nos. 14, 16, and 18 in which Cr was excessively added, the strength after aging treatment did not reach the target value. No. 19, 21, 23 have insufficient strength because Al is less than 3.5%. Nos. 20, 22, and 24 to which Al is excessively added have low aging treatment strength, and Nos. 22 and 24 have extremely poor ductility.
[0030]
[Example 2]
For the forged material of Ti-10V-4Cr-4Al (wt%) as in Example 1, β forging was performed by heating at 1100 ° C. to obtain a round bar with a diameter of 40 mm, and then the conditions were changed for the second time A round bar having a diameter of 15 mm was obtained by forging. Each forged material was directly aged under the same conditions as in Example 1, and tensile test specimens were collected to investigate the mechanical properties. Table 2 shows the second forging conditions and investigation results. The β transformation point of this alloy is 800 ° C., and the overall workability in the second forging (40 mm → 15 mm) is about 86%.
[0031]
No. 1 finished processing in the β region, so the strength after aging treatment did not reach the target value. Nos. 4 and 5 are still below the target strength because the degree of processing in the β region is less than 50%. Since No. 6 is processed in the α + β region, the strength is extremely low as in No. 1. On the other hand, No. 2 and 3 forging with a workability of 50% or more in the β region and finishing the forging in the α + β region were not repeated cold working and solution treatment. It shows a tensile strength of 1800 MPa or more. Moreover, all of these elongations are 2.0% or more.
[0032]
[Table 2]
[0034]
【The invention's effect】
As described above, the titanium alloy production method of the present invention can achieve high strength by a simple process of hot rolling and subsequent aging treatment without repeating cold working and solution treatment. Therefore, the instability of the strength which becomes a problem by the repetition is avoided, and high strength can be stably obtained regardless of the shape and dimensions of the product. When solution treatment is performed, cold working with a limit compression ratio of 80% or more is possible. Therefore, the present invention greatly contributes to expansion of applications of titanium alloys.
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US10094003B2 (en) | 2015-01-12 | 2018-10-09 | Ati Properties Llc | Titanium alloy |
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US10502252B2 (en) | 2015-11-23 | 2019-12-10 | Ati Properties Llc | Processing of alpha-beta titanium alloys |
DE102021213902A1 (en) * | 2020-12-11 | 2022-06-15 | Kabushiki Kaisha Toyota Jidoshokki | Non-magnetic element and method of making the non-magnetic element |
KR102544467B1 (en) * | 2022-10-05 | 2023-06-20 | 한밭대학교 산학협력단 | Chromium-added titanium alloy having stress corrosion cracking and manufacturing method thereof |
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