【発明の詳細な説明】[Detailed description of the invention]
（目的）
一般にチタン合金は比強度が高く、しかも耐食
性に優れているため宇宙航空機分野を始めとし
て、陸上分野でも各種化学プラントや原子力発電
等の設備において使用量が次第に増加している。
しかしながら、チタン合金は上記のような優れ
た特性をもちながら、難加工材の１つと言われる
程に加工性が悪く、特に熱間加工の可能な温度域
が著しく制限されている。このようなことから近
年チタン合金の加工を容易にする恒温加工が研究
され始めている。（例えば、Titanium and
Titanium Alloys Scientific and
Technological Aspects Vol.1P327 1982）。また
恒温加工時の延性を向上させる手段として予め素
材を微細等軸α晶にし、これによつて超塑性現象
を利用しようとする研究も行われている（例え
ば、Titanium′80Science and Technology
Vol.2P983 1980）。しかしながら、このような超
塑性等を利用する恒温加工の研究開発は必ずしも
十分とは言えず、多くの問題を残しているのが現
状である。恒温加工は高温にて材料を加工する場
合、加工中材料の温度を一定に保つ加工法である
が、実用的には鍛造において恒温加工を行う事が
多く、一般に文献等では恒温鍛造、等温鍛造ある
いはIsothermal Forgingの言葉が使われている。
（構成）
これらの点に鑑み本発明者等は鋭意研究を重ね
た結果、β変態点〜β変態点−150℃の温度域で
恒温加工を行うα＋β型チタン合金の恒温加工用
素材を予めβ変態点以下恒温加工温度＋20℃以上
の温度に加熱保持後急冷することを特徴とする恒
温加工用α＋β型チタン合金の熱処理方法により
α＋β型チタン合金の恒温加工時の延性を大きく
改良することに成功した。恒温加工時の延性は、
通常その温度における引張試験の伸びの値によつ
て評価されるが、本発明ではこの引張りによる伸
びの等性が著しく向上する。
（発明の具体的説明）
本発明はβ変態点〜β変態点−150℃の温度域
で恒温加工を行う恒温加工用素材を予めβ変態点
以下でかつ恒温加工温度＋20℃以上の温度に加熱
保持後急冷するものであるが、これにより素材の
組成は初晶α相と冷却中に生じるα＋βの層状組
織（冷却速度が十分に速い場合はさらにマルテン
サイト）となる。この素材をさらに恒温加工のた
めに、前記恒温加工温度に加熱するとβ相の割合
が増加してくるが、依然として前記恒温加工温度
＋20℃以上に加熱急冷する熱処理によつて現われ
た初晶α相とα＋βの層状組織が残存する。この
ように初晶α相とα＋βの層状組織が残存した状
態で恒温加工を行うと延性が著しく改良されるの
が分かつた。なお、前記熱処理でマルテンサイト
が生じても同様の結果が得られる。
本発明で前記熱処理を恒温加工温度＋20℃以上
としたのは、これ未満の温度であると恒温加工の
ために加熱した時、α＋βの層状組織がほとんど
消えてしまい、延性の改良が殆んど見られなくな
るためである。また、この熱処理の温度が低すぎ
るとβ相が安定化し冷却によつて生じるはずのα
＋βの層状組織やマルテンサイトの生成が抑制さ
れるようになるので、好ましくは恒温加工温度を
β変態点〜β変態点−100℃とし、また恒温加工
温度が本発明の範囲内で比較的低温で行う場合に
は、前記熱処理温度を恒温加工温度＋50℃以上で
行うのが望ましい。例えば、代表的なα＋β型チ
タン合金であるTi−6Al−4V合金では前記熱処
理を900℃以上とすることが推奨される。
また、熱処理温度の上限はβ変態点以下とする
必要があるが、β変態点を超えると初晶α相が消
えてしまい、本発明の加工性を増加させる効果が
失われるからである。この上限温度は適度な初晶
α相を残存させることができるβ変態点−20℃以
下で行うと顕著な効果がある。
前記本発明の熱処理において加熱保持後急冷す
るが、冷却速度が遅すぎると冷却中に生じるα＋
βの層状組織のα晶が粗大化し、本発明の効果が
失われるので、好ましくは10℃／min以上の冷却
速度で行うことが推奨される。これには通常水冷
が用いられる。また加熱保持時間は恒温加工用素
材の寸法等によつて十分な熱処理を受ける時間に
適宜選択される。
本発明が適用されるα＋β型チタン合金の代表
例としてはTi−６％Al−４％Ｖ、Ti−６％Al−
６％Ｖ−２％Snがあげられる。以上のチタン合
金の他、α＋β型チタン合金であれば本発明の適
用を妨げるものは全く存在せず、前記代表例にな
んら制限されるものではない。また本願発明の熱
処理を受ける材料は、上記の如きα＋β型チタン
合金インゴツトをインゴツトブレークダウンした
後の材料が用いられるが、通常β変態点以下で断
面減少率30％以上の加工を受けたα＋β型チタン
合金が使用される。
次に実施例について説明する。
（実施例）
代表的なα＋β型チタン合金であるTi−6Al−
4V合金を用いた実施例を示す。
510φのインゴツトをβ域でインゴツトブレイ
クダウンした後α＋β域で鍛造及び熱処理を行い
６インチのビレツトを製造した。このビレツト
は、均一な等軸α晶組織を有していた。この材料
のβ変態点は、995℃であつた。この６インチビ
レツトに種々の熱処理を行つた。その後10φ×
GL20mmサイズの引張試験片を切り出した。恒温
加工の延性は、引張試験の伸びより評価した。引
張試験の温度は恒温加工温度900℃を想定し、900
℃で行つた。歪速度は、１×10-3sec-1とし、試
験はAr雰囲気中で行つた。
その結果を第１表に示す。
No.１〜３は本発明の熱処理を行つたものであ
る。No.４〜７は比較例であり、No.４は、恒温加工
温度＋20℃以上という条件にはずれる。
No.５は、β変態点を超す熱処理となつており、
本発明からはずれる。
また、No.６は、急冷するという本発明の条件を
みたしておらず、No.７は、本発明の熱処理がなさ
れていない。
No.１〜３の本発明の熱処理を行つたものは、い
じれも300％以上の伸びを示しており、No.４〜７
の本発明の規制からはずれる条件に比べて大きく
延性が改良されたことがわかる。
(Purpose) In general, titanium alloys have high specific strength and excellent corrosion resistance, so their usage is gradually increasing not only in the spacecraft field but also in the terrestrial field as well as in various chemical plants, nuclear power generation facilities, and other equipment. However, although titanium alloy has the above-mentioned excellent properties, its workability is so poor that it is said to be one of the difficult-to-work materials, and in particular, the temperature range in which it can be hot worked is severely limited. For this reason, research has begun in recent years on constant temperature processing to facilitate the processing of titanium alloys. (For example, Titanium and
Titanium Alloys Scientific and
Technological Aspects Vol.1P327 1982). In addition, as a means to improve ductility during constant temperature processing, research is being conducted to make the material into fine equiaxed α crystals in advance and utilize the superplastic phenomenon (for example, Titanium′80Science and Technology
Vol.2P983 1980). However, the research and development of constant temperature processing that utilizes such superplasticity is not necessarily sufficient, and many problems remain. Isothermal processing is a processing method that keeps the temperature of the material constant during processing when processing materials at high temperatures, but in practice, isothermal processing is often performed in forging, and generally in literature, isothermal forging, isothermal forging Alternatively, the term Isothermal Forging is used. (Structure) In view of these points, the inventors of the present invention have conducted intensive research and found that a material for isothermal processing of α+β type titanium alloy, which is subjected to isothermal processing in the temperature range of β transformation point to β transformation point -150°C, is Succeeded in greatly improving the ductility of α+β type titanium alloy during isothermal processing using a heat treatment method for α+β type titanium alloy for isothermal processing, which is characterized by heating and holding at a temperature of 20°C or more above the isothermal processing temperature below the transformation point, and then rapidly cooling it. did. The ductility during constant temperature processing is
Normally, the evaluation is based on the elongation value in a tensile test at that temperature, but in the present invention, the uniformity of elongation due to tension is significantly improved. (Specific Description of the Invention) The present invention involves heating a material for isothermal processing in a temperature range of β transformation point to β transformation point -150°C to a temperature below the β transformation point and above the isothermal processing temperature + 20°C. After holding, the material is rapidly cooled, and as a result, the composition of the material becomes a primary α phase and an α+β layered structure (further martensite if the cooling rate is sufficiently fast) generated during cooling. When this material is further heated to the above-mentioned isothermal processing temperature for isothermal processing, the proportion of β phase increases, but the primary α phase that appears due to the heat treatment of heating and quenching above the above-mentioned isothermal processing temperature + 20 ° C. and a layered structure of α+β remains. It was found that ductility is significantly improved when constant temperature processing is performed in a state where the primary α phase and the α+β layered structure remain. Note that similar results can be obtained even if martensite is generated during the heat treatment. The reason why the heat treatment in the present invention is set to the isothermal processing temperature + 20°C or higher is that if the temperature is lower than this, the α+β layered structure will almost disappear when heated for isothermal processing, and the ductility will hardly be improved. This is because it will not be seen. In addition, if the temperature of this heat treatment is too low, the β phase will be stabilized and the α phase that should be generated by cooling will be
Since the formation of +β layered structure and martensite is suppressed, it is preferable that the isothermal processing temperature is between the β transformation point and the β transformation point -100°C, and that the isothermal processing temperature is relatively low within the range of the present invention. In the case of carrying out the heat treatment, it is desirable that the heat treatment is carried out at a constant temperature processing temperature +50°C or higher. For example, for Ti-6Al-4V alloy, which is a typical α+β type titanium alloy, it is recommended that the heat treatment be performed at 900°C or higher. Further, the upper limit of the heat treatment temperature needs to be equal to or lower than the β transformation point; however, if the β transformation temperature is exceeded, the primary α phase disappears, and the effect of increasing workability of the present invention is lost. A remarkable effect can be obtained if the upper limit temperature is set to below -20° C., the β-transformation point, which allows a suitable amount of the primary α phase to remain. In the heat treatment of the present invention, rapid cooling is performed after heating and holding, but if the cooling rate is too slow, α +
Since the α crystals in the β layered structure become coarse and the effects of the present invention are lost, it is recommended that cooling be carried out preferably at a cooling rate of 10° C./min or higher. Water cooling is usually used for this. Further, the heating holding time is appropriately selected depending on the dimensions of the material for constant-temperature processing, etc., so that the material undergoes sufficient heat treatment. Typical examples of α+β type titanium alloys to which the present invention is applied include Ti-6%Al-4%V, Ti-6%Al-
6%V-2%Sn is mentioned. In addition to the above-mentioned titanium alloys, there is nothing that prevents the application of the present invention to α+β type titanium alloys, and the present invention is not limited to the above-mentioned representative examples. Further, the material to be subjected to the heat treatment of the present invention is a material obtained by ingot breakdown of the α+β type titanium alloy ingot as described above, but usually α+β titanium alloy ingots processed at a temperature below the β transformation point with a reduction in area of 30% or more are used. type titanium alloy is used. Next, an example will be described. (Example) Ti-6Al-, a typical α+β type titanium alloy
An example using a 4V alloy will be shown. A 510φ ingot was broken down in the β region, and then forged and heat treated in the α+β region to produce a 6-inch billet. This billet had a uniform equiaxed α crystal structure. The β transformation point of this material was 995°C. This 6-inch billet was subjected to various heat treatments. Then 10φ×
A tensile test piece of GL20mm size was cut out. The ductility of constant temperature processing was evaluated from the elongation of the tensile test. The temperature for the tensile test was 900°C, assuming a constant temperature processing temperature of 900°C.
I did it at ℃. The strain rate was 1×10 −3 sec −1 and the test was conducted in an Ar atmosphere. The results are shown in Table 1. Nos. 1 to 3 were subjected to the heat treatment of the present invention. Nos. 4 to 7 are comparative examples, and No. 4 deviates from the condition of constant temperature processing temperature +20°C or more. No. 5 is heat treated to exceed the β transformation point,
This is outside the scope of the present invention. Further, No. 6 did not meet the condition of the present invention of rapid cooling, and No. 7 was not subjected to the heat treatment of the present invention. Nos. 1 to 3, which were subjected to the heat treatment of the present invention, showed an elongation of more than 300% in terms of tampering, and Nos. 4 to 7.
It can be seen that the ductility was greatly improved compared to the conditions deviating from the regulations of the present invention.
【表】【table】
【表】【table】