JP2003055749A - BETA Ti ALLOY WITH HIGH STRENGTH AND LOW YOUNG'S MODULUS, AND ITS MANUFACTURING METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a beta Ti alloy in which the problem of increase in Young' s modulus attendant on the increase of strength by aging can be solved and the increase of strength can be attained while maintaining Young's modulus, and also to provide a useful method for manufacturing such beta Ti alloy. SOLUTION: In successively applying hot working, solution heat treatment and cold working to manufacture the beta Ti alloy used in an as-cold-worked state, at least solution heat treatment temperature is regulated so that it is between a value below the β-transformation point and 600 deg.C and also cold working is carried out at >=30% draft. By this method, the beta Ti alloy having <=105 GPa Young's modulus can be manufactured.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a β-type Ti alloy having a fine metal structure and capable of exhibiting both properties of high strength and low Young's modulus, and useful for producing such a Ti alloy. It is about the method.

[0002]

2. Description of the Related Art A β-type Ti alloy can be easily cold worked and exhibits high strength by precipitating an α phase by aging treatment after solution treatment. Due to these characteristics, β-type Ti alloy is widely used as a material for high-strength bolts and springs, and it is expected that the demand for β-type Ti alloys will further increase in the future.

Examples of β-type Ti alloys currently in practical use include Ti-13V-11Cr-3Al and Ti-1.
5V-3Cr-3Sn-3Al, Ti-15Mo-5Z
r-3Al, Ti-3Al-8V-6Cr-4Mo-4
Zr etc. are mentioned as a typical thing. Then, in order to strengthen these β-type Ti alloys, after hot working, heating (heating) to the β-phase temperature range is performed and solution treatment is performed, and thereafter, by aging treatment, approximately 25% of α-phase is included in the β-phase. Is adopted.

As a means for further enhancing the strength of β-type Ti alloy, after heating to the β-phase temperature range for solution treatment, cold working is performed to introduce dislocations inside the crystal. Then, a method of precipitating a fine α-phase by aging treatment has also been proposed [eg, "Iron and steel" Vo
l.73, No.12 (1992)].

However, the above-mentioned method has a problem that although the strength can be increased, the Young's modulus is also increased at the same time. That is, one of the characteristics of titanium alloys (especially the β phase) is that they have a low Young's modulus, and in particular when they are used as spring materials, they are required to have high strength and a low Young's modulus. However, it has been difficult to achieve both high strength and low Young's modulus by the conventional strengthening method.

[0006]

The present invention has been made under these circumstances, and its purpose is to solve the problem of the increase in Young's modulus due to the increase in strength due to aging and maintain a low Young's modulus. Β that can achieve high strength
Type Ti alloy, and a useful method for producing such a β type Ti alloy.

[0007]

The method for producing a β-type Ti alloy of the present invention which has achieved the above-mentioned object is that hot working, solution heat treatment and cold working are sequentially performed, and the cold working is used as it is. In producing a β-type Ti alloy to be prepared, at least the solution temperature is set to a temperature range from below the β transformation point to 600 ° C., cold working is performed at a working rate of 30% or more, and Young's modulus is β of 105 GPa or less. The point is to manufacture a type Ti alloy.

In the above method, (1) the hot working finish temperature or (2) the temperature from the heating of the hot working to the end of the working may be set within the temperature range from below the β transformation point to 600 ° C., By adding such a configuration, the effect of the present invention can be further improved.

[0009] Further, the above-mentioned object is (1) hot working finishing temperature or (2) hot working in order to manufacture a β-type Ti alloy which is used as it is by carrying out hot working and cold working in sequence. The temperature from the heating of the hot working to the end of the working is set to a temperature range from below the β transformation point to 600 ° C., cold working is performed at a working rate of 30% or more, and the Young's modulus is 105 GPa.
It can also be achieved by producing the following β-type Ti alloy.

On the other hand, the β-type Ti alloy of the present invention which has achieved the above-mentioned object is a β-type Ti alloy which is used as it is in the cold working without aging treatment. It has a worked structure in which α% or less of the volume% is precipitated, and has a gist in that the Young's modulus is 105 GPa or less. In addition, the above-mentioned "processing structure" means
In the cross section parallel to the processing direction (in the case of a plate material, the cross section parallel to the processing direction and perpendicular to the plate surface), (major axis) /
It means a structure having a (minor axis) of 1.4 or more.

The titanium alloy of the present invention is premised on being "used in the cold working state", which means that it is used without aging treatment. The above-mentioned aging treatment is a heat treatment for precipitating the α phase in the β phase matrix by keeping the temperature at over 450 ° C. for about 1 hour or more. Therefore, heat that does not accompany precipitation of α-phase, such as instantaneous heating of the titanium surface to give a burning color by oxidation, or inevitably being heated 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 a heat treatment is included in “used as cold working”.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION A β-type T that has been practically used in the past
In the processing method of the i alloy material, the 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 heating temperature”). It is common sense to implement. This is (α + β) 2
It is believed that if the solution treatment is performed by heating to the 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, making cold working difficult.

However, according to the experiments confirmed by the present inventors, in the case of the material which has been subjected to the solution treatment by heating to the (α + β) 2 phase temperature range, the material is subjected to the solution treatment by heating to the β phase temperature range. Although it has slightly higher strength and lower ductility than the above materials, it has been found that there is almost no difference in cold workability, and that it is possible to carry out the same strong work as in the past. In particular, when the solution heat treatment is performed in a temperature range of less than β transformation point (hereinafter, may be abbreviated as “Tβ”) to 600 ° C., there is almost no difference in cold workability. It became.

According to further investigations by the present inventors, when the β-type Ti alloy is heated to the β-temperature range of high temperature and subjected to the solution treatment, coarsening of the crystal grains is apt to occur, and the crystal grains are once formed. It has also been found that once coarsened, the strength cannot be increased unless strong reduction is performed in cold working. However, as will be specifically shown in Examples described later,
If the solution treatment is carried out within the temperature range from below Tβ to 600 ° C. and then cold working is performed with a small amount of pro-eutectoid α phase mixed, the microstructure will be extremely uniform and fine. It was discovered that the physical properties were further improved.

The reason why such a phenomenon occurs can be considered as follows. That is, by setting the solution temperature to a temperature lower than Tβ which is lower than the β-phase temperature range to 600 ° C., strain at the interface with the pro-eutectoid α-phase, which is present in a small amount in the β-phase, during the subsequent cold working. It is considered that dislocations are uniformly introduced into the entire crystal to form a uniform and fine microstructure, and high strength can be obtained without significantly deteriorating ductility.
In some cases, the solution treatment may be performed twice or more. In this case, at least in the final solution treatment, by setting the solution temperature to the above temperature range,
The effect of the present invention is exhibited.

Also, the hot working performed before the solution treatment as described above is performed at a temperature lower than Tβ to 600 ° C. so that the microstructure becomes more uniform and fine, and excellent strength and ductility are obtained. I also knew that it would work. That is, when the hot working performed prior to the solution treatment is performed in the temperature range of less than Tβ to 600 ° C. and the solution treatment is also performed in the temperature range of less than Tβ to 600 ° C., the hot working is performed. Even in the process
Since the phase is generated and the pro-eutectoid α phase is also generated in the solution treatment step, a solution-treated material in which a small amount of these pro-eutectoid α phase is more evenly distributed is obtained, and the cold working step thereafter. It is considered that a more uniform and finer microstructure is obtained. Further, in the material treated under such conditions, the crystal grain growth is suppressed and the crystal grains (β particles) become very small. Therefore, this also has a favorable effect on the uniform micronization of the microstructure. it is conceivable that.

As described above, the above effects were obtained by carrying out the hot working carried out prior to the solution treatment in the temperature range of less than Tβ to 600 ° C. These effects are hot working. It was found that even if the finishing temperature in the above was set to be in the above range. The reason why the effect is exhibited only by setting only the finishing temperature of hot working within 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.

Further, according to the studies made by the present inventors, even when the solution heat treatment is not performed (that is, when hot working → cold working is used as it is), (1) heat treatment is performed. The finishing temperature of the hot working, or (2) the temperature from the heating of the hot working to the end of the hot working is performed within the temperature range of less than Tβ to 600 ° C., and then the cold working is performed with the working ratio of 30% or more. By doing so, it was possible to obtain a β-type Ti alloy having high strength and low Young's modulus. The reason why such an effect was obtained is probably that, similarly to the above, the precipitation of the α phase was promoted by the processing strain, and a small amount of the α phase was precipitated in the β phase even without the solution treatment of less than Tβ to 600 ° C. This is probably because it has a microstructure.

The solution treatment used when carrying out the present invention may be arbitrarily set in accordance with the type of Ti alloy within a temperature range of less than Tβ of β-type Ti alloy to 600 ° C. A preferable upper limit is Tβ-20 ° C and a preferable lower limit is Tβ-100 ° C. Even when the temperature during hot working (the hot working finish temperature or the temperature from the heating of hot working to the end of working) is in the temperature range of less than Tβ to 600 ° C, the preferable upper limit is Tβ-20 ° C. And
A preferable lower limit is Tβ-100 ° C.

In the method of the present invention, the processing rate is finally 30%.
Although cold working is performed as described above, if the 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 as the solution treatment temperature (or temperature during hot working) becomes lower, fracture occurs. Is likely to occur, the processing rate may be set according to the temperature. Note that cold working is performed subsequent to the solution treatment, and may be performed a plurality of times accordingly, but at least during the final cold working as in the case of the solution treatment above. The processing rate of 30% or more.

In any case, according to the method of the present invention, the microstructure can be made extremely 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, but the method of the present invention does not enhance the strength by the aging treatment. Therefore, the matrix phase becomes almost the β phase, which makes it possible to maintain the Young's modulus in a low state of 105 GPa or less.

The β-type Ti alloy obtained by each of the above methods is used as it is in cold working without aging treatment.
It has a processed structure in which α phase of not more than volume% (fraction) is deposited, and has a Young's modulus of not more than 105 GPa. That is, if the fraction of the α phase in the β phase matrix exceeds 20%, the Young's modulus of 105 GPa or less cannot be achieved. The α phase fraction is preferably 15% by volume or less, more preferably 10% or less. However, if the α phase fraction becomes too small, it becomes difficult to exhibit high strength, so it is preferable to secure at least about 2% by volume.

As described above, the "working structure" is a cross section parallel to the working direction (in the case of a plate material, a cross section parallel to the working direction and perpendicular to the plate surface), (major axis) / (short diameter) of β grains. This means a structure with a (diameter) ratio of 1.4 or more.
The ratio of (minor axis) was determined from the micrograph of a structure near 1 / 4t in the thickness direction at the center in the width direction in the case of a plate material, and from the structure photograph near the center of the surface and the center of the surface in the case of a wire rod. It can be obtained as an average value when the (major axis) / (minor axis) ratio of each piece is arbitrarily selected and measured.

The β-type Ti alloy that can be used in the present invention includes the above-mentioned Ti-13V-11Cr-3A.
1, Ti-15V-3Cr-3Sn-3Al, Ti-1
5Mo-5Zr-3Al, Ti-3Al-8V-6Cr
-4Mo-4Zr and the like are mentioned as typical ones, but other Ti-8Mo-2Fe-3Al, Ti-1
1.5V-6Zr-4.5Sn, Ti-10V-2Fe
-3Al, Ti-5Al-2Sn-4Zr-4Mo-2
Cr-1Fe, Ti-15Mo-3Al-2.7Nb-
0.25Si etc. [each numerical value means the content (mass%) of each element] can also be used.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and the following. It is included in the technical scope.

[0026]

Example 1 A β-type Ti alloy (Tβ: 785 ° C.) made of Ti-15Mo-5Zr-3Al was vacuum arc melted, forged and hot rolled into a wire having a diameter of 9.5 mmφ. Using this wire rod as a test material, it was heated under the temperature conditions (solution heat treatment temperature) shown in Table 1 below to undergo solution treatment, and then cold working was performed at various processing rates, and the obtained wire rod was subjected to a tensile test. It carried out and 0.2% yield strength and Young's modulus were measured. The result is
The results are shown in Table 1 below. Further, based on these results, FIG. 1 shows the relationship between the cold workability and the 0.2% proof stress at each solution temperature. The working rate (cold working rate) is a value calculated by the following equation (1). Cold working rate = [1- (cross-sectional area after working / cross-sectional area before working)] x 100 (%) (1)

[0027]

[Table 1]

From these results, it can be considered as follows. That is, when the solution temperature is 550 ° C. and 500 ° C., the strength (0.2% proof stress) is high because the aging is advanced, but the Young's modulus is also high at about 115 GPa. In addition, the solution temperature is higher than Tβ (80
0%, 850 ° C), the strength cannot be increased unless the cold working rate is considerably high, and 0.2% proof stress is very high for the cold working rate (0-20%). is not.

On the other hand, the solutionizing temperature is less than Tβ.
With the temperature range of 600 ° C. (that is, 780 to 600 ° C.) and cold working at a working rate of 30% or more,
Strength over 900MPa (0.2% yield strength) and 105GPa
It can be seen that the following Young's modulus is secured.

Example 2 β-type T made of Ti-15V-3Cr-3Sn-3Al
The i alloy (Tβ: 760 ° C.) was melted in a vacuum arc, forged and hot rolled into a plate material 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]

From this result, the following can be considered. That is, a manufacturing process (No.
It can be seen that the Ti alloys obtained according to 1 to 4) secure a Young's modulus of 89 GPa or less and a strength (0.2% proof stress) of 930 MPa or more. On the other hand, in the Ti alloy obtained by the manufacturing process (No. 5 to 8) out of the conditions specified in the present invention, the strength (0.2% proof stress) tends to be slightly higher, but the Young's modulus is Also 105
It can be seen that it is higher than GPa.

Example 3 A β-type Ti alloy (Tβ: 785 ° C.) made of Ti-15Mo-5Zr-3Al was vacuum arc melted, forged and hot rolled into a wire having a diameter of 9.5 mmφ. Using this wire as a test material, after solution treatment under the conditions shown in Table 3 below,
Cold working was performed at various processing 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, 0.2% proof stress and Young's modulus were measured in the same manner as in Examples 1 and 2. The results are collectively shown in Table 3 below, which satisfy the requirements specified in the present invention (No. 6 to
It can be seen that in Nos. 9, 12 to 14), the α phase has a worked structure in which it is appropriately precipitated, and a Young's modulus of 85 GPa or less can be secured together with a 0.2% proof stress of 915 MPa or more.

[0034]

[Table 3]

[0035]

EFFECT OF THE INVENTION The present invention is constituted as described above, and it is possible to solve the problem of Young's modulus increase due to high strength due to aging, and to achieve high strength while maintaining a low Young's modulus. A β-type Ti alloy was realized.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between cold workability and 0.2% proof stress at each solution heat treatment temperature.

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Claims (4)

[Claims] 1. A hot-working process, a solution heat treatment and a cold-working process are sequentially performed to produce a β-type Ti alloy to be used in the cold working state. Manufacture of β-type Ti alloy having high strength and low Young's modulus, which is characterized by producing a β-type Ti alloy having a Young's modulus of 105 GPa or less by carrying out cold working at a working rate of 30% or more while keeping the temperature range. Method. 2. The production according to claim 1, wherein (1) the hot working finish temperature or (2) the temperature from the heating of the hot working to the end of the working is within a temperature range from less than β transformation point to 600 ° C. Method. 3. Hot-working and cold-working are sequentially performed to produce a β-type Ti alloy to be used as-is in cold-working, (1) hot-working finishing temperature, or (2) hot-working The temperature from heating to the end of processing is less than β transformation point to 60
Along with the temperature range of 0 ° C, the cold working rate is 30
% Or more, Young's modulus is 105 GPa or less β type T
A method for producing a β-type Ti alloy having high strength and low Young's modulus, which comprises producing an i alloy. 4. A β-type Ti alloy which is used as cold-worked without being subjected to an aging treatment, and which has a worked structure in which 20% by volume or less of α-phase is precipitated in a β-phase matrix. , Young's modulus is 105 GPa or less, high strength and low Young's modulus β-type Ti alloy.