JP3934372B2 - High strength and low Young's modulus β-type Ti alloy and method for producing the same - Google Patents

Description

【００２８】
これらの結果から、次のように考察できる。即ち、溶体化温度を５５０℃、５００℃としたものでは、時効が進んでいるので強度（０．２％耐力）は高くなっているが、ヤング率も１１５ＧＰａ程度と高くなっている。また溶体化温度をＴβよりも高い条件（８００℃，８５０℃）で処理したものでは、冷間加工率をかなり高くしないと強度を高くできず、また冷間加工率が低いもの（０〜２０％）では０．２％耐力がさほど高くなっていない。
【００２９】
これらに対して、溶体化温度をＴβ未満〜６００℃の温度範囲（即ち、７８０〜６００℃）とすると共に、加工率：３０％以上で冷間加工したものでは、９００ＭPa以上の強度（０．２％耐力）と１０５ＧＰａ以下のヤング率が確保できていることが分かる。
【００３０】
実施例２
Ｔｉ−１５Ｖ−３Ｃｒ−３Ｓｎ−３Ａｌよりなるβ型Ｔｉ合金（Ｔβ：７６０℃）を、真空アーク溶解後、鍛造、熱間圧延して厚さ：５ｍｍの板材とした。この板材を供試材として用い、下記表２に示す製造工程で製造した板材の引張り試験を実施し、０．２％耐力およびヤング率を測定した。その結果を、下記表２に併記する。
【００３１】
【表２】

【００３２】
この結果から、次のように考察できる。即ち、本発明で規定する条件を満足する製造工程（Ｎｏ．１〜４）によって得られたＴｉ合金では、８９ＧＰａ以下のヤング率と９３０ＭＰａ以上の強度（０．２％耐力）を確保していることが分かる。これに対して、本発明で規定する条件を外れる製造工程（Ｎｏ．５〜８）によって得られたＴｉ合金では、強度（０．２％耐力）が若干高い傾向を示すものの、ヤング率がいずれも１０５ＧＰａよりも高くなっていることが分かる。
【００３３】
実施例３
Ｔｉ−１５Ｍｏ−５Ｚｒ−３Ａｌよりなるβ型Ｔｉ合金（Ｔβ：７８５℃）を、真空アーク溶解後鍛造、熱間圧延して直径：９．５ｍｍφの線材とした。この線材を供試材として用い、下記表３に示す条件で溶体化処理後、各種加工率で冷間加工を行なった。得られた各線材において、α相分率をＸ線回折によって求めると共に、前述した方法で（長径）／（短径）比を測定した。また、前記実施例１、２と同様にして、０．２％耐力およびヤング率を測定した。その結果を、一括して下記表３に示すが、本発明で規定する要件を満足するもの（Ｎｏ．６〜９，１２〜１４）のものでは、α相が適度に析出した加工組織を有しており、９１５ＭＰａ以上の０．２％耐力と共に８５ＧＰａ以下のヤング率が確保できていることが分かる。
【００３４】
【表３】

【００３５】
【発明の効果】
本発明は以上の様に構成されており、時効による高強度化に伴うヤング率の上昇の問題を解消し、低ヤング率を維持したまま高強度化を達成することのできるβ型Ｔｉ合金が実現できた。
【図面の簡単な説明】
【図１】各溶体化温度における冷間加工率と０．２％耐力の関係を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a β-type Ti alloy having a fine metal structure and capable of exhibiting both high strength and low Young's modulus, and a useful method for producing such a Ti alloy.
[0002]
[Prior art]
The β-type Ti alloy can be easily cold worked, and exhibits high strength by performing an aging treatment after the solution treatment to precipitate the α phase. Because of these characteristics, β-type Ti alloys are widely used as materials for high-strength bolts and springs, and the demand thereof is expected to increase in the future.
[0003]
Examples of β-type Ti alloys currently in practical use include Ti-13V-11Cr-3Al, Ti-15V-3Cr-3Sn-3Al, Ti-15Mo-5Zr-3Al, Ti-3Al-8V-6Cr-4Mo— 4Zr or the like is a typical example. In order to strengthen these β-type Ti alloys, after hot working, the solution is heated (heated up) to a β-phase temperature range, followed by a solution treatment, and then an aging treatment of about 25% α-phase in the β-phase. The method of precipitating is adopted.
[0004]
Further, as a means for further enhancing the strengthening of the β-type Ti alloy, after heating to the β-phase temperature range and performing a solution treatment, cold working is performed to introduce dislocations inside the crystal, and then A method of precipitating a fine α phase by aging treatment has also been proposed [for example, “Iron and Steel” Vol. 73, No. 12 (1992)].
[0005]
However, the method as described above has a problem that the Young's modulus also increases at the same time although the strength can be increased. That is, one of the characteristics of titanium alloys (particularly, the β phase) is that they have a low Young's modulus, and particularly when applied as a material for springs, a high strength and a low Young's modulus are required. However, it has been difficult to achieve both high strength and low Young's modulus with the conventional strengthening method.
[0006]
[Problems to be solved by the invention]
The present invention has been made under these circumstances, and its purpose is to solve the problem of an increase in Young's modulus associated with an increase in strength due to aging, and to achieve an increase in strength while maintaining a low Young's modulus. It is an object of the present invention to provide a β-type Ti alloy that can be manufactured and a useful method for producing such a β-type Ti alloy.
[0007]
[Means for Solving the Problems]
The method for producing a β-type Ti alloy of the present invention that can achieve the above-mentioned object is to produce a β-type Ti alloy that is used in the cold working state by sequentially performing hot working, solution treatment and cold working. In summary, the solution temperature is at least a temperature range of less than the β transformation point to 600 ° C. and cold working is performed at a working rate of 30% or more to produce a β-type Ti alloy having a Young's modulus of 105 GPa or less. It is what has.
[0008]
In the above method, (1) the hot working finishing temperature or (2) the temperature from the heating of the hot working to the end of the working may be set to a temperature range of less than the β transformation point to 600 ° C. By adding, the effect of this invention can be improved more.
[0009]
Furthermore, the purpose of the above is to sequentially perform hot working and cold working to produce a β-type Ti alloy that is used in the cold working state. (1) Hot working finishing temperature or (2) Hot working The temperature from heating to the end of processing is set to a temperature range of less than the β transformation point to 600 ° C., and cold processing is performed at a processing rate of 30% or more to produce a β-type Ti alloy having a Young's modulus of 105 GPa or less. Can also be achieved.
[0010]
On the other hand, the β-type Ti alloy of the present invention that can achieve the above object is a β-type Ti alloy that is used as it is cold-worked without being subjected to an aging treatment, and is 20% by volume or less in the β-phase matrix. The α-phase is precipitated in a processed structure and has a gist in that the Young's modulus is 105 GPa or less. The “processed structure” is a cross section parallel to the processing direction (in the case of a plate material, a cross section parallel to the processing direction and perpendicular to the plate surface), and the (major axis) / (minor axis) of β particles is Means an organization of 1.4 or higher.
[0011]
The titanium alloy of the present invention is premised on “used as cold worked”, which means that it is used without aging treatment. The aging treatment is a heat treatment for precipitating the α phase in the β phase matrix by maintaining it at a temperature usually exceeding 450 ° C. for about 1 hour or longer. Therefore, heat that does not accompany precipitation of α-phase, such as heating the titanium surface instantaneously to color it by oxidation, or inevitably heating at 300 to 350 ° C. for about 10 minutes during plating. The history does not correspond to the aging treatment in the present invention, and performing such heat treatment is included in “used as cold worked”.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In a conventional β-type Ti alloy material processing method, a solution treatment performed prior to cold working is heated to a high β single-phase temperature range (in the present invention, this heating temperature is referred to as “solution treatment”). It is common sense that this is done by calling "temperature". This is because when the solution treatment is performed by heating to the (α + β) two-phase temperature range (that is, the temperature range below the β transformation point), the ductility decreases due to the presence of the pro-eutectoid α phase, and cold working becomes difficult. Because it was thought to be.
[0013]
However, according to what the present inventors have confirmed through experiments, in the material that has been subjected to solution treatment by heating to the (α + β) two-phase temperature range, Although the strength is slightly higher and the ductility is lowered, it is found that there is almost no difference in cold workability, and that the same strong work as before can be performed. In particular, it is clear that there is almost no difference in cold workability when the solution treatment is performed at a solution treatment temperature within a temperature range of less than the β transformation point (hereinafter sometimes abbreviated as “Tβ”) to 600 ° C. It became.
[0014]
According to a further study by the present inventors, when a β-type Ti alloy is heated to a high β temperature range and subjected to a solution treatment, the crystal grains are likely to be coarsened, and the crystal grains are once coarsened. As a result, it was also found that the strength could not be increased unless a strong reduction was performed in the cold working. However, as specifically shown in the examples below, the solution treatment is performed at a solution temperature of less than Tβ to 600 ° C., and then cold working is performed with a small amount of pro-eutectoid α phase mixed therein. As a result, it was found that the microstructure became extremely uniform and fine, and the physical properties were further improved.
[0015]
The reason why such a phenomenon occurs can be considered as follows. That is, by setting the solution temperature to less than Tβ lower than the β phase temperature range to 600 ° C., during the subsequent cold working, the interface with the pro-eutectoid α phase present in a small amount in the β phase is also distorted. It is considered that dislocations are uniformly introduced into the entire crystal, resulting in a uniform and fine microstructure, and high strength can be obtained without significantly degrading the ductility. The solution treatment may be performed twice or more in some cases. In this case, the effect of the present invention can be obtained by setting the solution temperature to the above temperature range at least during the final solution treatment. Demonstrated.
[0016]
Also, the hot working performed prior to the solution treatment as described above is performed at a temperature less than Tβ to 600 ° C., so that the microstructure becomes more uniform and fine, and exhibits excellent strength and ductility. I understand. That is, when the hot working performed prior to the solution treatment is performed in a temperature range of less than Tβ to 600 ° C. and the solution treatment is also performed in a temperature range of less than Tβ to 600 ° C., hot working is performed. The pro-eutectoid α phase is generated in a small amount in the process, and the pro-eutectoid α phase is also generated in the solution treatment step, so that a solution treatment material in which these small amounts of pro-eutect α phase are distributed more uniformly is obtained. It is considered that a more uniform and fine microstructure is formed by the subsequent cold working process. In addition, the material treated under these conditions suppresses crystal grain growth, resulting in very small crystal grains (β grains). This also has a favorable effect on uniform refinement of the microstructure. it is conceivable that.
[0017]
As described above, the above-mentioned effect was obtained by performing the hot working performed prior to the solution treatment in a temperature range of less than Tβ to 600 ° C. This effect is the finishing temperature in the hot working. It has been found that even if the temperature is within the above temperature range, it can be exhibited. The reason why the effect is exhibited only by setting the finishing temperature of hot working only in the above temperature range is that strain is applied below Tβ, and this strain promotes precipitation of α phase, and β phase It is considered that the structure has a small amount of α phase precipitated therein.
[0018]
Furthermore, according to the study by the present inventors, even when the solution treatment is not performed (that is, when hot working → cold working is used), (1) hot working Finishing temperature, or (2) Hot working is carried out at a temperature range from less than Tβ to 600 ° C. from the heating to the end of the hot working, and then cold working is carried out at a working rate of 30% or more. Thus, a β-type Ti alloy having high strength and low Young's modulus could be obtained. The reason why such an effect was obtained is that, as described above, precipitation of α phase was promoted by processing strain, and a small amount of α phase was precipitated in β phase even without solution treatment below Tβ to 600 ° C. This is thought to be due to the microstructure.
[0019]
Incidentally, the solution treatment adopted when carrying out the present invention may be arbitrarily set according to the type of Ti alloy within the temperature range of less than Tβ to 600 ° C. of the β-type Ti alloy, but the preferable upper limit is Tβ-20 ° C, and a preferred lower limit is Tβ-100 ° C. Even when the temperature during hot working (the hot working finishing temperature or the temperature from the hot working heating to the working end) is set to a temperature range of less than Tβ to 600 ° C, the preferable upper limit is Tβ-20 ° C. The preferred lower limit is Tβ-100 ° C.
[0020]
In the method of the present invention, cold working is finally performed with a working rate of 30% or more. However, if this working rate is less than 30%, sufficient strength cannot be obtained. This processing rate may be increased according to the required material strength, and usually about 50 to 95% is adopted, but breakage occurs as the solution treatment temperature (or temperature during hot processing) decreases. Therefore, the processing rate may be set according to the temperature. It should be noted that the cold working is performed subsequent to the solution treatment, and may be performed a plurality of times along with it. However, as in the case of the solution treatment, at least the final cold working is performed. As long as the processing rate is 30% or more.
[0021]
In any case, according to the method of the present invention, the microstructure can be made very fine, and a β-type Ti alloy having high strength and high ductility can be obtained. As described above, in the case of β-type Ti alloy, if the aging treatment is performed, the α phase is precipitated and the Young's modulus is increased accordingly. However, in the method of the present invention, the strength is not increased by the aging treatment. Therefore, the parent phase is almost β-phase, and the Young's modulus can be maintained at a low state of 105 GPa or less.
[0022]
The β-type Ti alloy obtained by each of the above methods is used as it is cold-worked without being subjected to an aging treatment. Specifically, the β-type Ti alloy is less than 20% by volume (fraction) in the β-phase matrix. It exhibits a processed structure in which an α phase is precipitated, and has a Young's modulus of 105 GPa or less. That is, when the fraction of the α phase in the β phase matrix exceeds 20%, a Young's modulus of 105 GPa or less cannot be achieved. The α phase fraction is preferably 15% by volume or less, and more preferably 10% or less. However, if the α phase fraction is too small, it will be difficult to achieve high strength, so it is preferable to secure at least about 2% by volume.
[0023]
The above-mentioned “machined structure” is a section parallel to the machining direction as described above (in the case of a plate material, a section parallel to the machining direction and perpendicular to the plate surface), and the ratio of (major axis) / (minor axis) of β grains The ratio of (major axis) / (minor axis) in the case of a plate material is obtained from a structure photograph in the center in the width direction and around 1/4 t in the plate thickness direction. In this case, it is obtained as an average value when 10 β particles are arbitrarily selected from the structure photograph of the center and the vicinity of the center of the surface, and each (major axis) / (minor axis) ratio is measured.
[0024]
Examples of β-type Ti alloys that can be used in the present invention include Ti-13V-11Cr-3Al, Ti-15V-3Cr-3Sn-3Al, Ti-15Mo-5Zr-3Al, and Ti-3Al-8V-6Cr. -4Mo-4Zr and the like can be mentioned as typical ones, but Ti-8Mo-2Fe-3Al, Ti-11.5V-6Zr-4.5Sn, Ti-10V-2Fe-3Al, Ti-5Al-2Sn- 4Zr-4Mo-2Cr-1Fe, Ti-15Mo-3Al-2.7Nb-0.25Si, etc. [each numerical value means the content (mass%) of each element] can also be used.
[0025]
Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not of a nature that limits the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are all within the technical scope of the present invention. Is included.
[0026]
【Example】
Example 1
A β-type Ti alloy (Tβ: 785 ° C.) made of Ti-15Mo-5Zr-3Al was forged after vacuum arc melting and hot-rolled to obtain a wire having a diameter of 9.5 mmφ. Using this wire as a test material, after heat treatment under the temperature conditions (solution temperature) shown in Table 1 below, cold working is performed at various working rates, and the obtained wire is subjected to a tensile test. The 0.2% proof stress and Young's modulus were measured. The results are shown in Table 1 below. Also, based on this result, FIG. 1 shows the relationship between the cold working rate and the 0.2% proof stress at each solution temperature. In addition, the said processing rate (cold processing rate) is the value calculated | required by the following (1) formula.
Cold working rate = [1- (Cross sectional area after machining / Cross sectional area before machining)] × 100 (%) (1)
[0027]
[Table 1]

[0028]
From these results, it can be considered as follows. That is, when the solution temperature is 550 ° C. and 500 ° C., the aging is advanced and the strength (0.2% proof stress) is high, but the Young's modulus is also high at about 115 GPa. Further, in the case where the solution treatment temperature is higher than Tβ (800 ° C., 850 ° C.), the strength cannot be increased unless the cold work rate is considerably increased, and the cold work rate is low (0 to 20). %), The 0.2% yield strength is not so high.
[0029]
On the other hand, when the solution treatment temperature is set to a temperature range of less than Tβ to 600 ° C. (that is, 780 to 600 ° C.) and cold worked at a processing rate of 30% or more, a strength of 900 MPa or more (0. 2% proof stress) and Young's modulus of 105 GPa or less can be secured.
[0030]
Example 2
A β-type Ti alloy (Tβ: 760 ° C.) made of Ti-15V-3Cr-3Sn-3Al was forged and hot-rolled after vacuum arc melting to obtain a plate having a thickness of 5 mm. Using this plate material as a test material, a tensile test was performed on the plate material manufactured in the manufacturing process shown in Table 2 below, and 0.2% proof stress and Young's modulus were measured. The results are also shown in Table 2 below.
[0031]
[Table 2]

[0032]
From this result, it can be considered as follows. That is, in the Ti alloy obtained by the manufacturing process (Nos. 1 to 4) satisfying the conditions specified in the present invention, a Young's modulus of 89 GPa or less and a strength of 930 MPa (0.2% yield strength) are ensured. I understand that. On the other hand, in the Ti alloy obtained by the manufacturing process (Nos. 5 to 8) that deviates from the conditions specified in the present invention, although the strength (0.2% proof stress) tends to be slightly high, the Young's modulus is It can also be seen that it is higher than 105 GPa.
[0033]
Example 3
A β-type Ti alloy (Tβ: 785 ° C.) made of Ti-15Mo-5Zr-3Al was forged after vacuum arc melting and hot-rolled to obtain a wire having a diameter of 9.5 mmφ. Using this wire as a test material, after the solution treatment under the conditions shown in Table 3 below, cold working was performed at various working rates. In each of the obtained wires, the α phase fraction was determined by X-ray diffraction, and the (major axis) / (minor axis) ratio was measured by the method described above. Further, in the same manner as in Examples 1 and 2, 0.2% proof stress and Young's modulus were measured. The results are collectively shown in Table 3 below, and those satisfying the requirements defined in the present invention (Nos. 6-9, 12-14) have a processed structure in which the α phase is appropriately precipitated. It can be seen that a Young's modulus of 85 GPa or less can be secured with a 0.2% proof stress of 915 MPa or more.
[0034]
[Table 3]

[0035]
【The invention's effect】
The present invention is configured as described above, and a β-type Ti alloy that solves the problem of an increase in Young's modulus associated with an increase in strength due to aging and can achieve an increase in strength while maintaining a low Young's modulus. Realized.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between cold working rate and 0.2% proof stress at each solution temperature.

Claims (5)

  In order to produce a β-type Ti alloy that is subjected to hot working, solution treatment and cold work in order, and used in the cold working state, at least the solution treatment temperature is set to a temperature range below the β transformation point to 600 ° C. A method for producing a β-type Ti alloy having a high strength and a low Young's modulus, characterized by producing a β-type Ti alloy having a Young's modulus of 105 GPa or less by performing cold working at a working rate of 30% or more.   The manufacturing method according to claim 1, wherein (1) the hot working finishing temperature or (2) the temperature from the hot working to the end of the working is in a temperature range of less than the β transformation point to 600 ° C.   In order to manufacture β-type Ti alloy that is used in the cold working by sequentially performing hot working and cold working, (1) hot working finishing temperature, or (2) from hot working heating to the end of working And a cold working is performed at a working rate of 30% or higher to produce a β-type Ti alloy having a Young's modulus of 105 GPa or lower. A method for producing a β-type Ti alloy having high strength and low Young's modulus. It is a β-type Ti alloy that is used as it is cold-worked without being subjected to aging treatment, and an α-phase of 20% by volume or less is precipitated in the β-phase matrix, and a cross section parallel to the working direction (of the plate material) In this case, the (major axis) / (minor axis) of the β particles in the cross section parallel to the processing direction and perpendicular to the plate surface exhibit a processed structure of 1.4 or more , and the Young's modulus is 105 GPa or less. A high-strength and low Young's modulus β-type Ti alloy characterized by being. The β-type Ti alloy according to claim 4, wherein 2 to 20% by volume of an α phase is precipitated in the β phase matrix.

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