JP5594244B2 - Α + β type titanium alloy having a low Young's modulus of less than 75 GPa and method for producing the same - Google Patents

Description

  The present invention relates to an α + β-type titanium alloy member having a low Young's modulus and a manufacturing method thereof, which is suitable as a material for parts for automobiles and motorcycles, such as suspension springs and engine valve springs for automobiles and motorcycles, and a frame material for spectacles.

  The Young's modulus of titanium at room temperature is about 100 to 120 GPa in the case of industrial pure titanium, α-type titanium alloy, α + β-type titanium alloy consisting of α-phase and β-phase, and β-type mainly containing β-phase. For titanium alloys, it is about 70-90 GPa. However, even in a β-type titanium alloy, when an α-phase is precipitated by aging heat treatment in the α + β two-phase region, the Young's modulus increases to 100 to 120 GPa as in the case of the α-type titanium alloy and α + β-type titanium alloy. Further, since the desired Young's modulus of titanium differs for each alloy member, a titanium alloy that matches the desired Young's modulus is selected for each member in which titanium is used for the purpose of weight reduction and corrosion resistance.

  In general, suspension springs, engine valve springs, and the like of automobiles or motorcycles function as springs in order to ensure a certain amount of expansion / contraction (displacement) of the entire spring when a constant load is applied to the spring. Adjust the number of turns. In this case, the lower the Young's modulus of the material, the fewer the number of turns, and the lighter the spring can be. Titanium alloys, etc. not only have a specific gravity of about 60% or less compared to steel materials, but also have a Young's modulus of about 50-60%, so the number of turns is greatly reduced and the spring as a whole contributes to weight reduction. There are features. Moreover, it is suitable not only for suspension springs and engine valve springs, but also as parts for which weight reduction and flexibility are required, such as frame materials for eyeglasses and vines for eyeglasses.

  In general, when a low Young's modulus is desired in a titanium material, a β-type titanium alloy is used. Typical β-type titanium alloys include Ti-15V-3Cr-3Sn-3Al, Ti-22V-4Al, Ti-15Mo-5Zr-3Al, Ti-10V-2Fe-3Al, Ti-3Al-8V-6Cr-4Mo. -4Zr, Ti-1.5Al-4.5Fe-6.8Mo of Patent Document 1, and Ti-15Mo-3Al of Patent Document 2. Further, as a titanium alloy having a low Young's modulus, Patent Document 3 containing 10 to 35 mass% Zr and 8 to 14 mass% Cr is disclosed in Patent Document 4 as 13 to 28 atomic% Nb and 0.1 to 10 atoms. One containing at least% Sn is one containing at least 30 to 60 mass% Va group (vanadium group) in Patent Document 5 and one or more of 0.3 to 3 mass% O, N or C in Patent Document 6 , Containing 1.8% or less of Al, Mo equivalent = Mo + 0.67 × V + 0.44 × W + 0.28 × Nb + 0.22 × Ta + 2.9 × Fe + 1.6 × Cr + 1.1 × Ni + 1.4 × Co + 0.77 × The thing containing Mo equivalent which consists of Cu-Al 3-11 is described.

  In addition, there is an alloy that exhibits a low Young's modulus comparable to that of a β-type titanium alloy even in an α + β-type titanium alloy that requires less content of V, Mo, and Nb as β-stabilizing elements. Patent Document 7 contains 4.4 to 5.5 mass% Al, 1.4 to 2.1 mass% Fe, 1.5 to 4.5 mass% Mo, and Si is less than 0.1% as an impurity , C is less than 0.01% high-strength α + β type titanium alloy, and the Young's modulus is 75 GPa or more and less than 100 GPa by water-cooling the hot-rolled wire from a temperature of 810 ° C. or more and 940 ° C. or less. A method for producing an α + β type titanium alloy is described.

Japanese Patent No. 2859102 JP 2004-183058 A JP 2004-353039 A JP 2005-113227 A Japanese Patent No. 3375083 JP 2004-162171 A JP 2007-314834 A

  Ti-15V-3Cr-3Sn-3Al, Ti-22V-4Al, Ti-15Mo-5Zr-3Al, Ti-10V-2Fe-3Al, Ti-15Mo-3Al having low Young's modulus represented by β-type titanium alloy All of them (see Patent Document 1) contain 10% or more of relatively expensive additive elements such as Va group represented by V and Mo, and therefore tend to be high in price and density.

  Further, Ti-1.5Al-4.5Fe-6.8Mo (see Patent Document 2), which is a metastable β-type titanium alloy in which the amount of Mo added is relatively reduced, has a high Mo equivalent of 16% or more. The phase is very stable. In general, it is known that a low Young's modulus appears in an unstable β phase or α ″ martensite phase, and the Young's modulus of this alloy is considered to be equivalent to that of a general β-type titanium alloy.

  Moreover, what contained 10-35 mass% Zr and 8-14 mass% Cr (refer patent document 3), 13-28 atomic% Nb, and 0.1-10 atomic% Sn (refer patent document 4) ), Those containing 30 to 60% by mass of Va group (vanadium group) (see Patent Document 5) can obtain low Young's modulus. However, the titanium alloy itself has a high density because it contains 10% or more of relatively expensive additive elements such as Va group represented by V and Mo, and a large amount of high-density elements. Yes. That is, since the weight density becomes high, there is a concern that the original light weight advantage of the titanium alloy is lost.

  Further, it contains 0.3 to 3% by mass of one or more of O, N, or C and contains 1.8% or less of Al, and Mo equivalent = Mo + 0.67 × V + 0.44 × W + 0.28 × Nb + 0 .22 × Ta + 2.9 × Fe + 1.6 × Cr + 1.1 × Ni + 1.4 × Co + 0.77 × Cu—Al containing Mo equivalent 3 to 11 (see Patent Document 6) is an interstitial element Since some O, N, or C is contained 0.3% or more, there is a concern that the workability is lowered. Further, O, which is an α-stabilizing element, is more likely to solidify and segregate than Al, which is the same α-stabilizing element, so there is a concern about variations in materials when manufacturing a large ingot.

  On the other hand, in the α + β type titanium alloy, the added amounts of V, Mo, and Nb are small, and these are considered to be cheaper than the β type titanium alloy when estimated from the alloy composition. 4.4-5.5 mass% Al, 1.4-2.1 mass% Fe, 1.5-4.5 mass% Mo is contained, Si is less than 0.1% as an impurity, and C is 0.00. Among high-strength α + β type titanium alloys suppressed to less than 01%, those that are water-cooled from a temperature of 850 ° C. or higher and 940 ° C. or lower (see Patent Document 7) have Young's modulus of 75 GPa or more and less than 100 GPa. Low Young's modulus is obtained. However, this is almost the same as a general β-type titanium alloy.

  Therefore, the present invention uses an α + β type titanium alloy having a relatively inexpensive alloy composition, and has an Young's modulus of less than 75 GPa, more preferably an Young's modulus of less than 70 GPa, comparable to a low Young's modulus β type titanium alloy. It aims at providing a titanium alloy member and its manufacturing method.

In order to solve the above problems, the gist of the present invention is as follows.
(1) By mass%, containing 3.5% or more and less than 6.5% Al, 0.6% or more and less than 3.1% Fe, and less than 4.0% Cr, 7.0% Less than V and less than 5.0% Mo, one or more elements are equivalent to Mo equivalent = [% Mo] + 2.9 × [% Fe] + 0.67 × [% V] + 1.1 × [% Ni ] + 1.6 × [% Cr] + 1.6 × [% Mn] + 0.28 × [% Nb] − [% Al], the Mo equivalent is 2.0% or more and less than 8.0%. And an α + β-type titanium alloy consisting of the balance Ti and unavoidable impurities, wherein Si is less than 0.1% and C is less than 0.01% as impurities, The average Mo equivalent of the alloy component in the martensite phase is 3.85% or more and less than 10.00%, and the β phase in the Mo equivalent range or 1-phase or 2-phase total area ratio of the martensite phase 55% or more and less than 99%, Young's modulus and less than 75 GPa alpha + beta type titanium alloy.
(2) The α + β-type titanium according to (1) above, wherein a part of the Fe is replaced with one or more types of Mn of less than 0.25% and Ni of less than 0.15% alloy.
(3) The α + β type titanium alloy according to any one of (1) to (2) above, which is cold worked after solution treatment.
(4) The above-mentioned (1) to (3), characterized by subjecting to a solution heat treatment that cools at a cooling rate of water cooling or higher from a temperature that exceeds β transformation point−100 ° C. and less than the β transformation point, and then performs cold working. The method for producing an α + β type titanium alloy according to any one of (3).

  According to the present invention, an α + β type titanium alloy member having a Young's modulus of less than 75 GPa comparable to a low Young's modulus β type titanium alloy can be provided by using an α + β type titanium alloy having a relatively inexpensive alloy composition. The effect of is immeasurable.

It is a figure which shows the optical microscope observation photograph explaining a pro-eutectoid alpha phase grain, (a) what carried out cold processing of 10% to the sample cooled by water from 900 degreeC, (b) is a beta phase thru | or a martensite. In order to make the identification easier, the sample of (a) was subjected to a decoration heat treatment at 550 ° C. for 4 hours. Here, the pro-eutectoid α phase is an α phase that is precipitated during heat treatment in the α + β two-phase region where there is a solution. The solution treatment is terminated by cooling with water cooling after the heat treatment. The α phase observed at that time is an α phase precipitated during the heat treatment, that is, a pro-eutectoid α phase.

  The inventors of the present invention have made extensive studies on an α + β type titanium alloy member having a Young's modulus of less than 75 GPa, which is comparable to a low Young's modulus β type titanium alloy, and a manufacturing method thereof. As a result, in the α + β type titanium alloy, which can reduce the amount of expensive additive elements compared to the β type titanium alloy, the content of each element is within a predetermined range, and the β phase or martensite phase obtained by solution heat treatment It has been found that a low Young's modulus can be obtained by controlling the amount of the alloy element and the microstructure within a predetermined range.

  The present invention is described in detail below. % Means mass% unless otherwise specified.

  The material index of the present invention will be described. In the titanium alloy, the α + β type titanium alloy has a higher Young's modulus than the β type titanium alloy that exhibits a low Young's modulus. On the other hand, substitutional solid solution elements such as Fe, Ni, Cr, Mn which are β eutectoid β stabilization elements which stabilize the β phase, and V, Mo which are all solid solution β stabilization elements. Since the amount added can be reduced, it can be manufactured at a lower cost than a β-type titanium alloy. Therefore, in the present invention, in an α + β-type titanium alloy that can be manufactured at a relatively low cost compared to a β-type titanium alloy, the Young's modulus is set to less than 75 GPa, which is comparable to a β-type low Young's modulus titanium alloy. More preferably, the index is less than 70 GPa.

[Al addition amount]
Al is an α-stabilizing element and has an effect of increasing strength by solid solution strengthening. Furthermore, Al is suppressed to 3.5% or more because it suppresses the increase in Young's modulus by suppressing the formation of the ω phase in the β phase. However, if the amount added is increased, the amount of β-stabilizing element needs to be increased, and in order to avoid a decrease in ductility at high temperature and room temperature, the upper limit was made less than 6.5%.

[Fe content]
Fe is a relatively inexpensive β-stabilizing element and has the effect of increasing strength by solid solution strengthening. The lower limit was made 0.6% in order to reduce the amount of the relatively expensive β-stabilizing element added and suppress the cost increase. However, if the amount added is too large, segregation is likely to occur during solidification, and segregation becomes noticeable in large ingots of several hundred kg or more. Therefore, the upper limit was set to 3.1%.

[Contents of Cr, V, Mo]
In order to reduce the Young's modulus, it is necessary to leave more unstable β-phase and martensite phase having a low Young's modulus at room temperature. As described above, when the β phase or the martensite phase is stabilized only at Fe at room temperature, the amount of Fe added becomes larger than necessary, which causes a problem of segregation. Cr, V, and Mo are β-stabilizing elements as described above, and when added, β-phase can remain in a large amount up to room temperature as in the case of Fe. Therefore, in the present invention, one or more of these elements are contained. Cr is an element having a relatively high β-stabilizing ability and can be added in a smaller amount. Furthermore, it has the effect of increasing strength and improving workability. However, since Cr is easily segregated during solidification, the upper limit of addition is set to 4.0%. On the other hand, V is difficult to cause segregation and has a feature that is easy to manufacture. However, since V is expensive as a raw material, there is a problem that the cost increases when the addition amount is increased. Therefore, the upper limit is set to 7.0%. Mo is also a β-stabilizing element like Fe, and has the effect of stabilizing the β phase at room temperature. Since Mo exhibits reverse segregation with Fe during solidification, the material is easily homogenized during melting. However, since Mo is a relatively expensive element, the cost increases as the amount added increases. Furthermore, when Mo is added in a large amount, segregation during solidification becomes remarkable, so the upper limit was made 5.0%.

[Total content of additive elements]
In order to reduce the Young's modulus, as described above, it is necessary to leave a large amount of unstable β phase or martensite phase having a low Young's modulus at room temperature. In general, in titanium, Mo equivalent described in the following formula is used as a β phase stabilization index at room temperature. In order to leave a large amount of the unstable β phase and martensite phase at room temperature, the Mo equivalent needs to be 2.0% or more, so this was made the lower limit. On the other hand, when the Mo equivalent is 8.0% or more, the β transformation point becomes higher than the β transformation point, that is, the β phase remains to room temperature even after cooling from the β single phase. The heat treatment makes the β phase more stable and does not develop a low Young's modulus, so the upper limit was made less than 8%. Further, when the Mo equivalent contained in the titanium alloy is 3.5% to 8.0% and the solution heat treatment temperature before cold working is within the preferred range of the present invention as described later, It was found that the Mo equivalent in the β phase or martensite phase of the microstructure was 3.85% or more and less than 10.00%.
Mo equivalent = [% Mo] + 2.9 × [% Fe] + 0.67 × [% V] + 1.1 × [% Ni] + 1.6 × [% Cr] + 1.6 × [% Mn] +0.28 × [% Nb]-[% Al]

[Content of Si and C]
If Si and C are contained in large amounts as impurity elements, the room temperature ductility, cold workability, and hot workability may be deteriorated. Si is less than 0.1%, and C is less than 0.01%. If there was, it was found that there was no problem and was set as the upper limit of each. Since Si and C are inevitable to be contained as inevitable impurities, the lower limit value of the substantial content is usually 0.005% or more for Si and 0.0005% or more for C.

[Contents of Ni and Mn]
In the present invention, as necessary, a part of Fe is replaced with one or two of Ni less than 0.15% and Mn less than 0.25%. In this method, a part of Fe is replaced with an inexpensive element having the same function as Fe.

Here, the upper limit of the addition amount of Ni and Mn is set to less than 0.15% and less than 0.25%, respectively, because these elements are added in excess of the above upper limit and are intermetallic compounds that are in an equilibrium phase. This is because (Ti ₂ Ni, TiMn) is generated and fatigue strength and room temperature ductility deteriorate. The total amount of Ni, Mn, and Fe needs to be 0.6% or more and less than 3.1%. If this is less than 0.6%, it is necessary to increase the amount of Cr, V, and Mo, which are the same β-stabilizing elements, in order to obtain a microstructure after cold working described later. This is because the cost increases, and if it is 3.1% or more, segregation during the production of a large ingot becomes remarkable.

[Inevitable impurities]
Typical impurities include O, N, and H. Similarly to 60 types of JIS H 4600 (Ti-6Al-4V), it is preferable that O is 0.2% or less, N is 0.05% or less, and H is 0.015%. Further, in order to improve room temperature ductility and cold workability, it is more preferable that O is 0.15% or less, N is 0.02% or less, and H is 0.01% or less.

[Microstructure]
In a titanium alloy, the Young's modulus is greatly influenced by the structure, and varies greatly depending on the microstructure such as the area ratio of the β phase and the martensite phase. For example, as described in Patent Document 7, even within this patent component, when annealed at 750 ° C. for 1 hour, the area ratio of β phase and martensite phase is about 20%, and Young's modulus is about 115 GPa, which is a value that is not different from a normal α + β type titanium alloy. In order to obtain a low Young's modulus, the above-mentioned Mo equivalent is a β phase of 3.85% or more and 10.00% or less, a martensite phase generated from such β phase by water-cooling or solution-induced transformation after solution treatment, Or it turned out that the twin interface which has a thermoelastic martensitic property is needed. In the microstructure, if the total area ratio of the β phase or the martensite phase within the Mo equivalent range is 55% or more, a low Young's modulus of less than 75 GPa is obtained as the final titanium alloy. 55% was set as the lower limit. On the other hand, if the β phase or martensite single phase is used, the β phase becomes very coarse during the heat treatment, and the fatigue characteristics and ductility are remarkably reduced, so the upper limit was made 99%. In addition, as a crystal other than the β phase and the martensite phase, which are Mo equivalents in the above range, a β phase, a martensite phase, an α phase, and an inevitable impurity phase deviating from the Mo equivalents in the above range are observed. Examples of the inevitable impurity phase include a fine ω phase.

[Measuring method of microstructure]
A method for measuring the β phase or martensite phase in the titanium alloy will be described. The β phase or the martensite phase can be easily identified by an optical micrograph obtained by etching a cross-section embedded polishing sample with a nitric hydrofluoric acid aqueous solution, and further after a heat treatment (decoration heat treatment) at about 500 to 550 ° C. for about 4 hours. When observed, it can be identified more clearly. FIG. 1 shows an example of an optical microscope observation photograph. FIG. 1 (a) shows an example of claim 1 of the present invention, in which a sample cooled by water from 930 ° C. is subjected to 10% cold working, and (b) shows that β-phase or martensite can be more easily identified. The sample of (a) was subjected to a decoration heat treatment at 550 ° C. for 4 hours. In FIG. 1, a nitric hydrofluoric acid aqueous solution having a nitric acid concentration of about 12% and a hydrofluoric acid concentration of about 1.5% is used for etching. In FIG. 1 (a), the stretched crystal grains which are black and have a grain size of about 5 μm indicated by solid arrows are β phases stretched by processing. Furthermore, the crystal grains observed in a needle shape are the martensite phase. In addition, when a decoration heat treatment is performed as shown in FIG. 1B, a fine α phase is precipitated in the β phase or the martensite phase, and the place where the β phase or the martensite phase was before the heat treatment becomes black, It can be distinguished more clearly. From these photographs, the total area ratio occupied by the β phase or martensite phase in the observation measurement visual field was measured using an image analyzer, and the value was defined as the total area ratio of the β phase or martensite phase.

[Method for measuring alloy components in β phase or martensite phase]
Since the β-phase or martensite phase and the α-phase are different from each other in elements that are easily concentrated, they can be easily discriminated by observation with an optical microscope or SEM. For this reason, the amount of elements contained in the β phase or the martensite phase can be easily measured by EDX (Energy Dispersive X-ray Spectrometory) analysis of the crystal grains discriminated as the β phase or the martensite phase by the structure observation. Moreover, if EPMA (Electron Probe Micro Analyzer) is used, the amount of elements contained in a β phase or a martensite phase in a wider range can be measured.

  In the β phase or martensite phase, the components are almost constant within the range of variation, so the alloy components at 10 points were measured and the average value was defined as the average Mo equivalent in the β phase or martensite phase. .

Furthermore, Mo equivalent is represented by the following formula as described above.
[% Mo] + 2.9 × [% Fe] + 0.67 × [% V] + 1.1 × [% Ni] + 1.6 × [% Cr] + 1.6 × [% Mn] + 0.28 × [% Nb]-[% Al]

  Each composition has a different distribution coefficient to the α-phase, β-phase and martensite phase. In addition, Al contributes negatively to the Mo equivalent. That is, depending on the distribution of each component, there may be a negative Mo equivalent. For example, an α phase in which Al is easy to partition may have a negative Mo equivalent. Including such a case, the Mo equivalent contained in the entire alloy is kept constant.

[Measurement method of total area ratio of 1 phase or 2 phases of β phase or martensite phase in which average Mo equivalent of alloy component in β phase or martensite phase is 3.85% or more and less than 10.00%]
When the average Mo equivalent in the β phase or martensite phase measured in [Method for measuring alloy components in β phase or martensite phase] is 3.85% or more and less than 10.00%, the β phase or the martensite phase The total area ratio of one phase or two phases of the β phase or martensite phase in which the average Mo equivalent of the alloy component is 3.85% or more and less than 10.00% is the same as in the [Measurement Method of Microstructure]. It is set as the value of the total area ratio of the measured β phase or martensite phase. On the other hand, when the average Mo equivalent in the β phase or martensite phase measured in the above [Method for measuring alloy components in β phase or martensite phase] is less than 3.85% or 10.00% or more, The total area ratio of one phase or two phases of β phase or martensite phase in which the average Mo equivalent of the alloy component in the martensite phase is 3.85% or more and less than 10.00% is zero.

[Alloy elements in β phase or martensite phase]
As described above, in the titanium alloy, the alloy component contained in the β phase or the martensite phase greatly affects the expression of the low Young's modulus. That is, the Mo equivalent is 3.85% or more and less than 10.00%, and the total area ratio of one phase or two phases of the β phase or martensite phase in the Mo equivalent range is required to be 55% or more. It is said. Here, the reason will be described.

  The β phase, martensite phase, and twin interface to achieve a low Young's modulus can be introduced by solution treatment and subsequent cold working. However, even in the presence of a large amount of β phase or martensite phase, if it is β phase or martensite phase not within the Mo equivalent, it is necessary to obtain a low Young's modulus even if cold working is performed. These structures are not introduced, and a low Young's modulus of less than 75 GPa cannot be obtained.

  In order to obtain a low Young's modulus, it is generally known that the β phase or the martensite phase needs to be in an unstable state at room temperature. The stability of the β phase or martensite phase at room temperature is determined by the content of elements contained in the phase, particularly the β stabilizing elements. If the amount of β-stabilizing element decreases, the martensite phase becomes too large, and if it increases too much, the β-phase becomes stable, and even if it is subjected to the solution heat treatment range of the present invention described later or cold processing, low Young's modulus Cannot be obtained. In the present invention, it has been found that a low Young's modulus can be obtained by adjusting the amount of alloy elements in the β phase or martensite phase to a specific range in the Mo equivalent as described above. Further, it was confirmed that even when such a β phase was transformed into a martensite phase by cold working, the contained Mo equivalent did not change. That is, the Mo equivalent contained in the β phase or the martensite phase does not change before and after cold working. Therefore, if the Mo equivalent contained in the β phase or martensite phase of the final product is 3.85% or more and less than 10.00%, the Mo equivalent contained in the β phase or martensite phase before cold working. Is 3.85% or more and less than 10.00%, and therefore, a low Young's modulus can be realized by subsequent cold working.

  Further, as described above, the α + β type titanium alloy of the present invention is subjected to cold working after solution treatment.

[Indicator of Mo equivalent of β phase or martensite phase]
In the α + β type titanium alloy, the β phase can be left or the α ″ martensite phase or the α ′ martensite phase can be generated by cooling from the α + β two-phase high temperature region at a rate higher than that of water cooling. However, when the Mo equivalent in the above-mentioned crystal is lowered, the martensite phase is stabilized and a low Young's modulus cannot be obtained. In order not to stabilize the martensite phase, it is necessary to set the lower limit of the Mo equivalent to 3.85% or more. Preferably, it is 4.00% or more. On the other hand, if the Mo equivalent in the β phase is set to 10.00% or more, the β phase becomes too stable and the Young's modulus increases. Therefore, in the present invention, the upper limit is set to 10.00%. Since a low Young's modulus can be obtained by making the β phase unstable, it is preferably less than 9.00%. Further, when the Mo equivalent contained in the titanium alloy according to the present invention is 3.5% to 8.0% as described above, the present invention can be achieved by applying the manufacturing method according to the present invention. It was found that the Mo equivalent in the β phase or martensite phase of the microstructure of the titanium alloy was 3.85% or more and less than 10.00%.

[Production method of titanium alloy]
[Solution heat treatment: Temperature]
The titanium alloy of the present invention contains the composition of the titanium alloy and is cooled at a cooling rate of water cooling or higher from a temperature of β transformation point −100 ° C. or higher and lower than β transformation point in the solution heat treatment step. . In the present invention, the amount of elements contained in the β phase or the martensite phase and the area ratio of those phases within the range are defined as described above. In the alloy of the present invention, when heat treatment is performed at a temperature lower than β transformation point−100 ° C., first, within this component range, Mo equivalent in β phase becomes 10.00% or more, and second, β phase or martensite. The area ratio of the phase is less than 55%. According to the first result, the total area ratio of one or two phases of β phase or martensite phase in the Mo equivalent range of the present invention becomes zero. As a result of these first and second events, the Young's modulus becomes 75 GPa or higher even if cold working described later is performed, so the lower limit of the solution heat treatment temperature is set to β transformation point −100 ° C. or higher. The β transformation point is preferably −80 ° C. or higher. On the other hand, if heat treatment is performed at a temperature exceeding the β transformation point, the metal structure may coarsen as described above to deteriorate fatigue characteristics and ductility, and secondly, the Mo equivalent in the β phase is 3. It may be less than 85%, and third, the area ratio of the β phase or the martensite phase is 90% or more. According to the second result, the total area ratio of one phase or two phases of β phase or martensite phase in the Mo equivalent range of the present invention may be zero. Furthermore, even if a β phase or a martensite phase having the Mo equivalent within the scope of the present invention is formed, there is a concern that the total area ratio of one or two of them exceeds 99%. Therefore, in the present invention, the upper limit of the heat treatment temperature is less than the β transformation point.

  Note that the heat treatment time may be a short time of about 1 minute depending on the shape of the material to be heat-treated and the heat capacity of the furnace as long as the temperature of the material to be heat-treated is maintained.

  In addition, in summary, the components contained in the titanium alloy are within the scope of the present invention, and in the solution heat treatment step, cooling is performed at a cooling rate of water cooling or higher from a temperature of β transformation point −100 ° C. or higher and lower than the β transformation point. Thus, the average Mo equivalent of the alloy component in the β phase or martensite phase of the microstructure is 3.85% or more and less than 10.00%, and one phase or one of the β phase or martensite phase in the Mo equivalent range. An α + β type titanium alloy having a total area ratio of two phases of 55% or more and less than 99% can be obtained.

[Solution heat treatment: cooling rate]
In the present invention, after the heat treatment at the above temperature, the cooling rate is set to water cooling or higher. When the cooling rate is air cooling, a fine α phase is precipitated in the β phase grains during cooling, and the area ratio of the β phase or the martensite phase is greatly reduced. In addition, since the β-stabilizing element diffuses into the β phase, the Mo equivalent in the β phase increases. Furthermore, since the area ratio of the β phase is lowered, the Young's modulus is very high. On the other hand, when cooled at a cooling rate higher than water cooling, α phase does not precipitate in the inner grains of β phase and martensite phase, and unstable residual β phase or martensite phase can remain in a large amount at room temperature, and 75 GPa Since the Young's modulus of less than can be obtained, the cooling rate was set to water cooling or higher.

  The solution treatment performed here generates an unstable β phase in order to develop a low Young's modulus, and further generates a work-induced martensite phase by subsequent cold working, or thermoelastic martensite. The main purpose is to introduce a twin interface having special characteristics.

[Cold processing]
The present invention is characterized in that a low Young's modulus of less than 75 GPa can be obtained by performing cold working after solution treatment. This is because, as described above, the Young's modulus is increased by introducing a work-induced martensitic transformation of the unstable β phase and a twin interface having thermoelastic martensitic characteristics in the β phase or martensite phase. This is because it becomes lower. Even in the alloy component of the present invention, the Young's modulus of less than 75 GPa can be obtained by cold working after solution treatment by setting the Mo equivalent in the β phase or martensite phase within the range of the present invention. I can do it. More preferably, the Young's modulus is less than 70 GPa. As for cold working, for example, cold working at a working rate of 0.5% or more in wire drawing (in the case of wire drawing, the ratio of the difference in cross-sectional area before and after processing divided by the area before processing). Even if it is applied, it contributes to a decrease in Young's modulus. That is, when the Young's modulus of this sample after the solution treatment and the Young's modulus of the sample further subjected to cold working were compared, it was confirmed that the latter decreased by 5 GPa to 20 GPa. Thus, it is one of the technical features of the present invention that the Young's modulus is lowered by transforming the unstable β phase into a martensite phase by cold working. For example, in Patent Document 3, a titanium alloy containing Zr, Cr is subjected to a solution treatment to obtain a low elastic modulus, that is, a low Young's modulus. A technique for increasing the strength while maintaining the Young's modulus is disclosed. This is because the β phase is stable because it contains a large amount of Cr, and it can be presumed that the low Young's modulus was not exhibited even after cold working.

  Examples of the cold working include, besides wire drawing, swaging, drawing, extrusion, rolling, pressing, bending, rolling, and the like.

  Titanium alloy ingots were produced from the titanium alloys having the components shown in Table 1 by a vacuum arc melting (VAR) method, and round bars produced by hot forging and hot rolling were used as materials.

  A round bar having the components shown in Table 1 and hot-forged and hot-rolled was heated at a temperature within the range of the present invention for 1 hour to be solution-treated, and then water-cooled. If the heating temperature before water cooling exceeds Tβ (β transformation point) −100 ° C. listed in Table 1 and is less than the β transformation point, it is within the scope of the present invention. For cold working after the heat treatment, the round bar was drawn at a working rate of about 3 to 20%. Mo equivalent in β phase or martensite phase before cold working, Mo equivalent in β phase or martensite phase after cold working, total area ratio of β phase and martensite phase, within the Mo equivalent range of the present invention Table 2 shows the total area ratio of certain β phases and martensite phases and Young's modulus after cold working. The Mo equivalent in the β phase or martensite phase was unchanged before and after cold working. Below, each measurement condition and test condition are demonstrated. As for the area ratio of the β phase or martensite phase, a nitric hydrofluoric acid aqueous solution (nitric acid concentration is about 12%, hydrofluoric acid concentration is about 1.5%) is used for the embedded polishing sample of the cross section of the test piece before and after the cold working. The film was observed after etching at room temperature and measured by image analysis. The Young's modulus was measured using data measured by attaching a strain gauge using ASTM E8M subsize (diameter of parallel part: 6.25 mm, length: 25 mm). The Young's modulus was measured by approximating the stress up to half of the proof stress by linear approximation.

  No. in Table 2 A-1 to A-12 are composed of the alloy component system of the present invention, and both the heat treatment temperature and the cooling rate are within the scope of the present invention. On the other hand, no. A-13 and 14 are within the scope of the present invention in both the heat treatment temperature and the cooling rate, but the component systems are different. In Tables 1 and 2, numerical values outside the scope of the present invention are underlined.

  From Tables 1 and 2, No. of the example which is the alloy component range of Claim 1 is shown. In A-1 to A-10, the Mo equivalent in the β phase or the martensite phase is within the range of the present invention, and the total area ratio of the β phase and the martensite phase is 55% or more. For this reason, the Young's modulus after cold working also shows a sufficiently low value of less than 75 GPa.

  From Tables 1 and 2, Example No. which is the alloy component range of claim 2 is used. In A-11 to A-12, the Mo equivalent in the β phase or the martensite phase is within the range of the present invention, and the total area ratio of the β phase and the martensite phase is 55% or more. For this reason, the Young's modulus after cold working also shows a sufficiently low value of less than 75 GPa.

  On the other hand, from Tables 1 and 2, the comparative example No. A-13 has no added Cr, V and Mo. Therefore, after heat treatment at a temperature within the range of the present invention, when water-cooled, even if the Mo equivalent in the β phase or martensite phase is within the range of the present invention, the total area ratio of the β phase and the martensite phase is as small as 49%. Therefore, the Young's modulus after cold working is as high as 80 GPa.

  From Tables 1 and 2, the comparative example No. In A-14, the amount of additive element of Al is outside the scope of the present invention. Therefore, when heat treatment and water cooling are performed at a temperature within the range of the present invention, the Young's modulus is as high as about 90 GPa even if the Mo equivalent in the β phase or martensite phase and the total area ratio are within the range of the present invention.

  In Table 3, No. 1 in Table 1, which is an alloy component of the present invention. 1 and no. 5 shows a case where wire cooling is performed at a working rate of about 3 to 20% as cold working after water cooling from various temperatures. If the heating temperature before water cooling exceeds Tβ (β transformation point) −100 ° C. listed in Table 1 and is less than the β transformation point, it is within the scope of the present invention. No. in Table 3 A-16 to 18 are No. 1 in Table 1. 1 and the heat treatment temperature and cooling rate are within the scope of the present invention. On the other hand, no. A-15 and A-19 are also No. 1 in Table 1. However, the heat treatment temperatures are 800 ° C. and 1000 ° C., respectively, which are out of the scope of the present invention. In Table 3, No. A-21 and A-22 are No. in Table 1. 5 is an alloy component, and both the heat treatment temperature and the cooling rate are within the scope of the present invention. On the other hand, A-20 in Table 3 is No. in Table 1. However, the heat treatment temperature is as low as 800 ° C. In Table 3, values outside the scope of the present invention are underlined.

  From Table 3, No. of the example which carried out the water cooling after heat processing at the temperature of (beta) transformation point-100 degreeC or more and less than (beta) transformation point. A-16 to A-18, A-21 and 22 are the sum of the β phase and the martensite phase in which the Mo equivalent in the β phase or the martensite phase is within the scope of the present invention. The area ratio is 55% or more, and the Young's modulus after cold working is a sufficiently low value of less than 75 GPa.

  On the other hand, No. of the comparative example of Table 3. In A-15 and A-20, the heat treatment temperature is as low as 800 ° C., and the Mo equivalent in the β phase or the martensite phase is as high as 10.00% or more. The total area ratio of the β phase and the martensite phase in the Mo equivalent range of the present invention is zero, and the Young's modulus after cold working is as high as 90 GPa or more.

  Further, in the comparative example of Table 3, No. A-19 has an extremely high heat treatment temperature of 1000 ° C., which is higher than the β transformation point. Therefore, the Mo equivalent in the β phase or the martensite phase is as low as 3.84, and the total area ratio of the β phase and the martensite phase in the Mo equivalent range of the present invention is zero. The Young's modulus is also very high at 93 GPa.

  The α + β-type titanium alloy of the present invention has a small amount of additive elements such as V and Mo, which are expensive additive elements, and a very low Young's modulus compared to conventional β-type titanium alloys that can obtain a low Young's modulus. Therefore, it is suitable for use as a component material for automobiles or motorcycles, such as suspension springs and engine valve springs for automobiles or motorcycles, and as a frame material for spectacles, and contributes to weight reduction of these component materials.

Claims (4)

  It contains 3.5% or more and less than 6.5% Al, 0.6% or more and less than 3.1% Fe, and less than 4.0% Cr, less than 7.0% V. One element or more of Mo less than 5.0% is equivalent to Mo equivalent = [% Mo] + 2.9 × [% Fe] + 0.67 × [% V] + 1.1 × [% Ni] +1. In the formula consisting of 6 × [% Cr] + 1.6 × [% Mn] + 0.28 × [% Nb] − [% Al], contained so that the Mo equivalent is 2.0% or more and less than 8.0% And an α + β type titanium alloy comprising Si as an impurity less than 0.1% and C being less than 0.01%, the balance being Ti and inevitable impurities, and having a microstructure β phase or martensite phase The average Mo equivalent of the alloy component is 3.85% or more and less than 10.00%, and the β-phase or 1-phase or 2-phase total area ratio of the onsite phase 55% or more and less than 99%, Young's modulus and less than 75 GPa alpha + beta type titanium alloy.   The α + β type titanium alloy according to claim 1, wherein a part of the Fe is replaced with one or more of Mn of less than 0.25% and Ni of less than 0.15%.   The α + β type titanium alloy according to any one of claims 1 and 2, wherein the α + β type titanium alloy is subjected to cold working after the solution treatment.   Any one of claims 1 to 3, wherein the solution is subjected to a solution heat treatment that is cooled at a cooling rate that is higher than or equal to water cooling from a temperature that exceeds the β transformation point -100 ° C and less than the β transformation point. 2. A method for producing an α + β type titanium alloy according to item 1.