JP5353754B2 - Metastable β-type titanium alloy having low Young's modulus and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semistable &beta; type titanium alloy having a low Young's modulus of &lt;80 GPa. <P>SOLUTION: The semistable &beta; type titanium alloy having a low Young's modulus of &lt;80 GPa has a composition comprising, by mass%, &ge;3.5 to &lt;7.0% Al, &ge;1.4 to &lt;3.6% Fe and &ge;2.0 to &lt;10.0% Mo, and the balance Ti with inevitable impurities, and in which Mo equivalent in the formula of Mo equivalent=2.9&times;%Fe+%Mo-%Al is &ge;6.0 to &lt;14.0. By performing cooling from a temperature equal to or above a &beta;transformation point at a cooling rate equal to or above that of water cooling, its microstructure is composed of a &beta; phase or two phases of a &beta; phase and an &alpha;'' martensitic phase, thus the semistable &beta; type titanium alloy having a low Young's modulus of &lt;80 GPa is produced. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a metastable β-type titanium alloy having a low Young's modulus and suitable for use as a material for automobile and motorcycle parts such as suspension springs and engine valve springs for automobiles and motorcycles, and frame materials for glasses.

  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. As described above, since the desired Young's modulus of titanium is different for each alloy member, a titanium alloy suitable for 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 smaller the number of windings, 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.

  It is suitable not only for suspension springs and engine valve springs, but also for parts that require weight reduction and flexibility, such as frame materials for glasses and vine materials for glasses.

  In particular, when a low Young's modulus is desired, an α + β type titanium alloy or a β type titanium alloy is used. In the α + β type titanium alloy, the content of V, Mo and Nb which are β stabilizing elements is small, Ti-6Al-4V, Ti-4.5Al-3V-2Mo-2Fe (AMS4899), Patent Document 1 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, C is 0 There is a high-strength α + β-type titanium alloy that is suppressed to less than 0.01%. Patent Document 2 describes a method for producing an α + β type titanium alloy having a Young's modulus of 75 GPa or more and less than 100 GPa by water-cooling a hot-rolled wire from a temperature of 810 ° C. or more and 940 ° C. or less.

  On the other hand, when a lower Young's modulus is desired, 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 3, and Ti-15Mo-3Al of Patent Document 4. Further, as a titanium alloy having a low Young's modulus, Patent Document 5 containing 10 to 35 mass% Zr and 8 to 14 mass% Cr is disclosed in Patent Document 6 as 13 to 28 atomic% Nb and 0.1 to 10 atoms. The one containing 30% by mass of Sn in the patent document 7 contains one or more of 0.3 to 3% by mass of O, N or C in the patent document 8 containing 30 to 60% by mass of the Va group (vanadium group). , 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.

Japanese Patent Laying-Open No. 2005-320618 JP 2007-314834 A Japanese Patent No. 2859102 JP 2004-183058 A JP 2004-353039 A JP 2005-113227 A Japanese Patent No. 3375083 JP 2004-162171 A

  In the α + β type titanium alloy, the addition amount of V, Mo, Nb is small like Ti-6Al-4V and Ti-4.5Al-3V-2Mo-2Fe (AMS4899). Therefore, these are considered to be cheaper than the β-type titanium alloy when estimated from the alloy composition. However, the Young's modulus of these α + β type titanium alloys is about 100 to 120 GPa as described above, which is a very high value compared to the β type titanium alloy.

  Moreover, 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, C is contained. In a high-strength α + β-type titanium alloy suppressed to less than 0.01% (see Patent Document 1), the Young's modulus is 75 GPa or more and less than 100 GPa by water cooling from a temperature of 810 degrees or more and 940 degrees or less (see Patent Document 2). ), A low Young's modulus is obtained as an α + β type titanium alloy. However, this is almost the same as β-type titanium alloy.

  On the other hand, Ti-15V-3Cr-3Sn-3Al, Ti-22V-4Al, Ti-15Mo-5Zr-3Al, Ti-10V-2Fe-3Al, having a low Young's modulus represented by a β-type titanium alloy, Ti-15Mo-3Al (see Patent Document 3) contains 10% or more of relatively expensive additive elements such as Va group represented by V and Mo, and therefore tends to increase in price and density. There is.

  Further, Ti-1.5Al-4.5Fe-6.8Mo (see Patent Document 4), which is a metastable β-type titanium alloy in which the addition amount of Mo 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 a general β-type titanium alloy.

  Moreover, the thing containing 10-35 mass% Zr and 8-14 mass% Cr (refer patent document 5), the 13-28 atomic% Nb, the thing containing 0.1-10 atomic% Sn (refer patent document 6) ), Those containing 30 to 60% by mass of Va group (vanadium group) (see Patent Document 7) all contain 10% or more of relatively expensive additive elements such as Va group and Mo represented by V In addition, the titanium alloy itself has a high density because it contains a large amount of high density elements.

  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 8) 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.

  Therefore, the present invention provides a β-type titanium alloy that exhibits a low Young's modulus by using a metastable β-type titanium alloy in which the alloy element is controlled by a formula consisting of Mo equivalents.

  The present inventor has earnestly studied a titanium alloy having a lower Young's modulus than a general β-type titanium alloy by making the Mo equivalent as low as possible in order to make the β phase or α ″ martensite phase unstable. Repeated. As a result, the present inventors have found that in a Ti—Al—Fe—Mo based metastable β-type titanium alloy, a low Young's modulus is expressed by setting the content of each element within a predetermined range.

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 7.0% Al, 1.4% or more and less than 3.6% Fe, 2.0% or more and less than 10.0% Mo, the balance Ti In addition, in the formula of Mo equivalent = 2.9 × [Fe%] + [Mo%] − [Al%], the Mo equivalent is 6.0 or more and 14.0%. A metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa, characterized by being less than 80 GPa.
(2) Containing 3.5% or more and less than 5.0% Al, 1.4% or more and less than 3.6% Fe, 2.0% or more and less than 9.0% Mo, the balance Ti and unavoidable In the formula of Mo equivalent = 2.9 × [Fe%] + [Mo%] − [Al%], the Mo equivalent is 6.0 or more and less than 14.0%. A metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa.
(3) The metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa as described in (1) or (2) above, wherein the Mo equivalent is 7.0 or more and less than 12.0% .
(4) The metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa according to (1) or (2) above, wherein the Mo equivalent is 7.5 or more and less than 11.5% .
(5) The microstructure is composed of a β-phase single phase or two phases of a β-phase and an α ″ martensite phase. The low structure of less than 80 GPa according to any one of (1) to (4) above Metastable β-type titanium alloy with Young's modulus.
(6) The microstructure is composed of a β phase single phase or a β phase and an α ″ martensite phase by cooling from a temperature equal to or higher than the β transformation point at a cooling rate equal to or higher than water cooling. A method for producing a metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa according to any one of 1) to (5).

  According to the present invention, by adjusting the Mo equivalent so as to make the β phase as unstable as possible, a β-type titanium alloy having a Young's modulus lower than that of a general β-type titanium alloy can be provided. unfathomable.

The change in Young's modulus is shown with Mo equivalent on the horizontal axis and Young's modulus on the vertical axis. It is a figure which shows an example of the shape memory characteristic result by a bending test.

  The present invention is described in detail below. Hereinafter, the content of the additive element is indicated by “mass%”.

  The material index of the present invention will be described. Among titanium alloys, β-type titanium alloys mainly composed of β-phase have a lower Young's modulus than α-type titanium alloys and α + β-type titanium alloys. As a method for stabilizing the β phase, a large amount of substitutional solid solution elements such as eutectoid β stabilization elements Fe, Ni, Cr, Mn, and all-solid solution β stabilization elements V, Mo, etc. It has been added to. However, in order to stabilize the β phase or α ″ martensite phase in the Ti material at room temperature, it is necessary to add 6 to 10% or more in Mo equivalent. Since Mo is a relatively expensive element, stabilizing the β phase or the α ″ martensite phase with only Mo results in a very high alloy cost. Therefore, the present inventor used as an index to reduce the Mo equivalent, which is a guideline for the addition amount of the β-phase stabilizing element, and to add Fe, which is a relatively inexpensive element. Moreover, since a low Young's modulus is observed in the unstable β phase or α ″ martensite phase, the element addition amount was set to 6% or more and less than 14% in Mo equivalent. Also, as described above, the Young's modulus is 80 GPa for a general β-type titanium alloy. Therefore, in the present invention, the Young's modulus is set to less than the lower limit level of 80 GPa. Preferably, the element addition amount is more than 7.0% and less than 12.0% in Mo equivalent, and the Young's modulus is less than 75 GPa. More preferably, the element addition amount is 7.5% or more and 11.5% or less in Mo equivalent, and the Young's modulus is less than 70 GPa.

[Indicator of additive element content]
In order to lower the Young's modulus, it is necessary to stabilize the β phase or α ″ martensite phase having a low Young's modulus at room temperature. On the other hand, if the amount of β-stabilizing element added is too large, the alloy cost increases, segregation during solidification of the additive element, and the β phase becomes too stable, resulting in an increase in Young's modulus. Appropriate amounts of elements need to be added. In the present invention, Al, Fe, and Mo are added as additive elements, and the amount added is adjusted by the Mo equivalent shown below.
Mo equivalent = 2.9 × [Fe%] + [Mo%] − [Al%]

[Indicator of Mo equivalent]
In the titanium alloy, the β phase can be left by cooling from the β transformation point to a temperature higher than water cooling from the high temperature, or an α ″ martensite phase or an α ′ martensite phase can be generated. However, when the α ′ martensite phase is generated, the Young's modulus increases, which is not preferable. Since the α ′ martensite phase is generated when the Mo equivalent is decreased, the lower limit of the Mo equivalent is set to 6.0% or more in order not to generate the α ′ martensite phase. However, if the Mo equivalent is 14.0% or more, the total amount of additive elements becomes too large, and segregation of solidification during melting and alloy costs increase. In addition, the Young's modulus increases because the β phase is too stabilized. Therefore, in the present invention, the upper limit of Mo equivalent is set to 14.0%. Since a low Young's modulus can be obtained by destabilizing the β phase, it is preferably 7.0 or more and less than 12.0%. Further, in order to obtain a low Young's modulus, the content is preferably 7.5 or more and less than 11.5%.

[Al addition amount]
Al is an α stabilizing element, and it is necessary to reduce the addition amount as much as possible in order to stabilize the β phase. However, Al suppresses the generation of the ω phase in the β phase and suppresses the increase in Young's modulus by stabilizing the α ″ martensite phase that tends to exhibit a low Young's modulus. It was above. However, when the addition amount is increased, the addition amount of the β-stabilizing element is also increased, so the upper limit was set to 7.0%. Furthermore, the upper limit is preferably set to 5.0% in order to minimize the addition amount of the β-stabilizing element and reduce the cost.

[Fe addition amount]
On the other hand, Fe is a β-stabilized substitutional solid solution element, and the degree of stabilization of the β phase or α ″ phase at room temperature increases according to the amount added. In order to reduce the additive element of Mo which is a relatively expensive additive element as much as possible, addition of 1.4% or more is necessary. However, since it is easy to segregate at the time of solidification, the effect becomes remarkable when the addition amount is increased. Therefore, the upper limit of the addition amount is set to 3.6%.

[Mo addition amount]
As mentioned above, in order to stabilize the β phase or α ″ martensite phase at room temperature only with Mo, it is necessary to add 6 to 10% or more, and the addition amount is increased by adding Al. I have to let it. In the present invention, Fe is used as a β-stabilizing element. However, as described above, segregation is remarkable in Fe, and it is difficult to stabilize the β-phase only with Fe. On the other hand, since Mo exhibits reverse segregation with Fe, it is easy to homogenize the material during melting. In order to stabilize the β phase or α ″ martensite phase even at room temperature in the Al and Fe component ranges, it is necessary to add 2.0% or more of Mo. 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 10.0%. Preferably, the upper limit is set to 9.0% in order to obtain a low Young's modulus more stably or to prevent a cost increase.

[Microstructure]
The titanium alloy of the present invention is preferably composed of a single phase of β phase or two phases of β phase and α ″ martensite phase. Thereby, combined with the composition of the titanium alloy, a metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa can be obtained. The identification of the microstructure can be performed by optical microscopy and X-ray diffraction.

[Production method of titanium alloy]
The titanium alloy of the present invention contains the composition of the titanium alloy and is cooled at a cooling rate equal to or higher than water cooling from a temperature equal to or higher than the β transformation point, so that the microstructure is β-phase single phase or β-phase and α ′. 'Two phases of martensite phase. The β transformation point temperature can be determined using suggested thermal analysis or the like.

[Other characteristics]
[Two-way shape memory characteristics]
It is known that β-type titanium alloys such as Ti—Nb and Ti—Mo alloys have shape memory characteristics utilizing a β-phase work-induced martensitic transformation when heated partially. The titanium alloy of the present invention also has this shape memory characteristic when the Mo equivalent is 6.5 to 9.4. Furthermore, it has a two-way shape memory characteristic that causes a shape change in the same direction as the deformation direction by applying heat exceeding the shape memory characteristic.

  Titanium alloys having the components shown in Table 1 were arc-melted to prepare about 100 g ingots, which were heated to 900-930 ° C. and hot forged into plate materials having a thickness of about 3 mm. Table 1 shows the structural structure and Young's modulus when the material is air-cooled at 950 ° C. or 970 ° C. for 30 minutes and then cooled with water. In this heat treatment condition, No. Any of the titanium alloys 1 to 14 is water-cooled from the β transformation point temperature or higher. In Table 1, numerical values that deviate from the scope of the present invention are underlined.

  Each measurement condition and test condition will be described below. The cross-section embedded polishing material was observed after etching at room temperature using a nitric hydrofluoric acid aqueous solution (nitric acid concentration: about 12%, hydrofluoric acid concentration: about 1.5%). The constituent phases were identified by X-ray diffraction. The Young's modulus was measured by a resonance method after cutting a test piece from a plate material.

  Next, a method for measuring the shape memory characteristics in two directions will be described. FIG. 2 shows a schematic diagram of a method for evaluating shape memory characteristics in two directions. In the present invention, a bending test was conducted in order to examine whether or not the shape memory characteristics are in two directions. After cutting out the plate-like test piece, it was bent into a U shape so that the bending diameter was 5 mm or 10 mm. Then, after hold | maintaining in a heating furnace for 5 minutes for every 50 degreeC to 100-500 degreeC, the shape memory characteristic of two directions was evaluated by measuring the bending angle of a bending test piece at each temperature. Each sample had the smallest angle at 250 ° C., and the angle at that time was defined as θ1. In addition, each sample had the largest angle at 500 ° C., and the angle at that time was defined as θ2.

  From Table 1, No. of the Example which is an alloy component of this invention. 1-9 and no. Nos. 15 to 18 have a β-phase single phase or two phases of β-phase and α ″ martensite phase, and the Young's modulus is a sufficiently low value of less than 80 GPa.

  On the other hand, no. No. 10 has a β single phase, but has a high Mo equivalent of 14.7, a stable β phase, and a high Young's modulus of 81 GPa.

  Moreover, No. of the comparative example. 11 or No. No. 12 has a low Mo equivalent of 5.3 or 3.9, so the structural structure is two phases of α ′ martensite phase and α ″ martensite phase or α ′ martensite single phase, and has a high Young's modulus. It can be seen that the Young's modulus is as high as 83 GPa or 92 GPa.

  Moreover, No. of the comparative example. 13 or No. No. 14 has a Mo equivalent of 6.2 or 7.9 and is within the scope of the present invention. In No. 13, the Young's modulus is as high as 97 GPa because the Al content and the Mo content are low and the structural structure is an α ′ phase single phase. On the other hand, no. No. 14 is composed of two phases, a β phase and an α ″ martensite phase, but the ω phase is not sufficiently suppressed because the amount of Al added is as small as 3%, and the Young's modulus is as high as 87 GPa.

  From Table 2, the Examples, No. 2, 3, and 4 are bent into a U shape and then subjected to heat treatment, whereby the bending angle becomes gentle and shows shape memory characteristics. Further, by holding at a higher temperature, the angle of bending becomes steep, and shape memory characteristics in the same direction as the bending direction are exhibited.

  On the other hand, no. 10 or No. No. 11 shows no change in the bending angle even after heat treatment after bending, and does not show shape memory characteristics in two directions.

  The β-type titanium alloy of the present invention has a very low Young's modulus because it has a smaller amount of additive elements such as V and Mo, which are more expensive additive elements than conventional β-type titanium alloys, and therefore has a very low Young's modulus. It is suitable for use as a material for automobile or motorcycle parts such as suspension springs and engine valve springs, and as a frame material for eyeglasses, and contributes to weight reduction of these parts.

Claims (6)

  Contains 3.5% or more and less than 7.0% Al, 1.4% or more and less than 3.6% Fe, 2.0% or more and less than 10.0% Mo, and the balance is Ti and inevitable In the formula of Mo equivalent = 2.9 × [Fe%] + [Mo%] − [Al%], the Mo equivalent is not less than 6.0 and less than 14.0%. A metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa.   Contains 3.5% or more and less than 5.0% Al, 1.4% or more and less than 3.6% Fe, 2.0% or more and less than 9.0% Mo, the balance Ti and unavoidable In the formula of Mo equivalent = 2.9 × [Fe%] + [Mo%] − [Al%], the Mo equivalent is not less than 6.0 and less than 14.0%. A metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa.   The metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa according to claim 1 or 2, wherein the Mo equivalent is 7.0 or more and less than 12.0%.   The metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa according to claim 1 or 2, wherein the Mo equivalent is 7.5 or more and less than 11.5%.   The low Young's modulus of less than 80 GPa according to any one of claims 1 to 4, characterized in that the microstructure consists of a single phase of β phase or two phases of β phase and α '' martensite phase. A metastable β-type titanium alloy.   The production of a metastable β-type titanium alloy having a low Young's modulus of less than 80 GPa according to any one of claims 1 to 5, wherein the cooling is performed at a cooling rate equal to or higher than water cooling from a temperature equal to or higher than a β transformation point. Method.

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