JP4098205B2 - Heat-resistant Ti alloy with excellent high-temperature strength - Google Patents

Description

  The present invention relates to an α-type Ti alloy and an α + β-type Ti alloy having excellent high-temperature strength.

  As a heat-resistant Ti alloy, it is effective to add an α-stabilizing element to Ti for solid solution strengthening. Al, O, and C are known as α-stabilizing elements. Al, which has a large solid solubility in Ti and a large solid solution strengthening ability, is mainly used.

Examples of such heat-resistant Ti alloys having good high-temperature strength include Ti-5Al-2.5Sn alloy (numerical values indicate mass% , the same applies hereinafter), Ti-8Al-1Mo-1V alloy, Ti-6Al-2Sn- 4Zr-2Mo alloy, Ti-6Al-2Nb-1Ta-0.8Mo alloy, Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si alloy, Ti-5Al-3.5Sn-3Zr-1Nb-0 .3Si alloy (IMI829), Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si alloy (IMI834) and the like have been put into practical use.

In recent years, there has been an increasing demand for high temperature strength, and attempts have been made to add C in combination with Al in order to improve high temperature strength.
However, when a large amount of C is added, coarse TiC precipitates and the fatigue strength is remarkably lowered. Therefore, conventionally, the amount added is 0.1mass% (hereinafter sometimes simply indicated as "%".) Suppressed and if is often below.

  Note that a Ti-4.5Al-4Cr-0.5Fe-0.2C alloy is known as a Ti alloy with a large amount of C added at 0.2%, but this alloy exhibits high strength at room temperature. However, high strength is not exhibited at high temperature (for example, about 760 ° C.).

  This invention is made | formed in view of this problem, and it aims at providing the heat resistant Ti alloy which improved the high temperature strength by adding C to 0.1% or more.

  The present inventor has earnestly studied that the high-temperature strength is inferior even though the Ti-4.5Al-4Cr-0.5Fe-0.2C alloy contains 0.2% of C. Mo equivalent (the amount of additive element converted to the amount of Mo which is a typical β stabilizing element. The calculation method of Mo equivalent will be described later) is as large as 3.35 (%), so-called β-rich Ti alloy It was found that the addition of C does not contribute to the improvement of the high temperature strength because the amount of α phase to be strengthened by C is small at a high temperature of about 760 ° C. In addition, TiC produced by adding a large amount of C can be dissolved in the α phase by performing an appropriate processing heat treatment if there is a sufficient amount of α phase at a high temperature. C exceeding the solid solubility limit of the phase becomes TiC and is finely dispersed in the structure. It was found that finely dispersed TiC does not adversely affect mechanical properties such as tensile strength and fatigue strength, and that C is re-dissolved in the α phase at a high temperature. The present invention has been made based on such findings.

That is, the heat-resistant Ti alloy of the present invention is a Ti alloy composed of an α phase or an α + β phase, the chemical composition is mass%, Al: 6.5 to 9.0%, Mo: 0.5 to 1.5. %, V: 0.6~1.4%, C : 0.1~0.25%, the balance being Ti and impurities, Mo equivalent represented by the following formula ([Mo] eq) is not more -4 C exceeding the solid solubility limit of the α phase is finely dispersed in the structure as TiC.
[Mo] eq = 1.0 [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] -1.0 [Al]
However, [X] shows content mass% of element X.

  The Mo equivalent in the heat-resistant Ti alloy is focused on the amount of α phase at a high temperature, and the amount of α phase at 760 ° C. corresponds to an area ratio of 80% or more, preferably 90% or more.

According to this heat-resistant Ti alloy, since the β-stabilizing element is regulated to a Mo equivalent of −4 or less (preferably −6 or less) under a predetermined chemical composition, the amount of α phase at a high temperature, for example, 760 ° C. 80% by area or more (preferably 90% or more), and C exceeding the solid solubility limit of the α phase at room temperature is finely dispersed in the structure in the form of TiC, so a large amount of 0.1 to 0.25% C added to is dissolved in the α phase at a high temperature, the α phase can be effectively solid-solution strengthened, and an excellent high temperature strength can be obtained.

According to the heat-resistant Ti alloy of the present invention , the β-stabilizing element is regulated to a Mo equivalent of −4 or less under a predetermined chemical composition , and therefore the α phase content at a high temperature at 760 ° C. is 80 area% or more. In addition, C exceeding the solid solubility limit of the α phase at room temperature is finely dispersed in the structure in the form of TiC, so a large amount of C added in a large amount of 0.1 to 0.25% was generated at a high temperature. Since it is effectively dissolved in the α phase, an excellent high temperature strength can be obtained.

The heat-resistant Ti alloy according to the embodiment will be described.
This heat-resistant Ti alloy is Al: 6.5-9.0%, Mo: 0.5-1.5%, V: 0.6-1.4%, C: 0.1-0.25%, It consists of the remainder Ti and impurities, and the Mo equivalent calculated by the following formula is adjusted within a range of −4 or less. The following formula is well known to those skilled in the art and is disclosed in, for example, Japanese Patent No. 3365190.
[Mo] eq = 1.0 [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] -1.0 [Al]
However, [X] shows content mass% of element X.

The Mo equivalent indirectly defines the amount of α phase at a high temperature. When the Mo equivalent is larger than −4, the amount of β phase becomes dominant even at 760 ° C., and the α amount is less than 80 area%. Thus, the α phase becomes insufficient. The Mo equivalent is preferably -5 or less, and more preferably -6 or less. By setting the Mo equivalent to −6 or less, the α phase at 760 ° C. is 90% or more, and the high-temperature strength is further improved.

C , like Al, dissolves in the α phase and has the effect of solid solution strengthening of the α phase, increasing not only the room temperature strength but also the high temperature strength and increasing the transformation point (the temperature at which the α phase disappears). . If it is less than 0.1%, the effect of increasing the high-temperature strength and transformation point is small. Coarse TiC which is not melt | dissolves comes to remain, and a mechanical property deteriorates. Therefore, in the present invention, the lower limit of the C amount is 0.10%, preferably 0.12%, and the upper limit is 0.25%, preferably 0.20%.

Al is an α-stabilizing element, strengthens the α-phase by solid solution, improves the high-temperature strength, and raises the transformation point. If it is less than 6.5%, such an action is too small. On the other hand, Ti ₃ Al exceeding 9.0% is generated, and the toughness is lowered. For this reason, the lower limit of the Al amount is 6.5%, preferably 7.0%, and the upper limit is 9.0%, preferably 8.5%.

  Mo is a β-stabilizing element, and is added to slightly strengthen the β phase and form a two-phase structure of α + β. If it is less than 0.5%, such an action is too small. On the other hand, if it exceeds 1.5%, the β phase is strengthened and the high-temperature strength is lowered. For this reason, the lower limit of Mo is 0.5%, and the upper limit is 1.5%.

  V is also a β-stabilizing element, and is added to slightly strengthen the β phase and form a two-phase structure. If it is less than 0.6%, such action is too small. On the other hand, if it exceeds 1.4%, the β phase is strengthened and the high-temperature strength is lowered. Therefore, the lower limit of V is 0.6%, and the upper limit is 1.4%.

The Ti alloy of the present invention is the Ti—Al—Mo—V—C alloy . For reference, a Ti alloy before C addition (base alloy) in which the Mo equivalent is −4 or less is given as follows: 6Al-2Sn-4Zr-2Mo, Ti-5Al-3.5Sn-3Zr-1Nb-0.3Si, Ti-5Al-2.5Sn, Ti-8Al-1Mo-1V, Ti-5.8Al-4Sn-3. Examples include 5Zr-0.7Nb-0.5Mo-0.35Si, Ti-6Al-2Nb-1Ta-0.8Mo, Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si, and the like. . Note that the Mo equivalent of the Ti alloy after adding C so that the C amount is 0.1 to 0.25% using these base alloys is precisely (Mo equivalent of the base alloy) × ( 100-added C%) / 100, but the C content is at most 0.25%, so it can be considered that it is substantially the same as the Mo equivalent of the base alloy.

Next, a manufacturing example of the Ti alloy according to the embodiment will be described.
First, after melting a Ti alloy having a predetermined component and heating the ingot obtained by casting at 1000 to 1200 ° C., hot roughing such as rolling and forging is performed at a rolling reduction of 70 to 80%. Next, as a heat treatment, rolling at a rolling reduction of about 60 to 80% in a temperature range of (sheathing temperature−150) ° C. to (sheathing temperature−50) ° C. (depending on the component system, but approximately 850 to 1090 ° C.). Apply hot working such as forging. Thereby, coarse TiC produced at the time of casting can be dissolved again in the α phase. C once re-dissolved in the α-phase does not precipitate as coarse TiC even by subsequent cooling, and is finely dispersed and precipitated in the structure. When used at a high temperature, the finely dispersed TiC is decomposed and the C is dissolved in the α phase.

  After hot working, annealing is generally performed for the purpose of removing stress, recovering the work structure, and recrystallization. This annealing can stabilize the structure and dimensions, improve the mechanical properties, and the like. Annealing is typically maintained for about 1 hr to 10 hr at 600 to 800 ° C., for example. Depending on the type of the base alloy, a solution treatment for holding at 900 to 1200 ° C. for 5 minutes to 120 minutes after hot working is performed, and after quenching, an aging treatment or the annealing is performed.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limitedly interpreted by this Example.

  Arc melting is performed on the melting raw material in which C is added to the base alloy having the chemical components shown in Table 1 so that the C addition amount (C amount of the Ti alloy after C addition) is 0.12% and 0.20%, The molten metal was cast to obtain an ingot having a diameter of 50 mm and a thickness of 15 mm. The ingot was heated to 1200 ° C. and rough-rolled to a thickness of 10 mm, and then heated to 850 ° C. and hot-rolled to 4 mm. In addition, the sieving temperature of a C addition Ti alloy sample is about 1000-1140 degreeC.

  A test piece having a length of 10 mm and a width of 10 mm was taken from the obtained rolled plate (plate thickness: 4 mm), and heated at 760 ° C. for 2 hours and then water-cooled in order to measure the amount of α phase at 760 ° C. Using this test piece, the structure was observed with an optical microscope, and the α amount (area%) in the field of view of 0.2 mm × 0.5 mm was determined using image software using the photographed tissue photograph. The results are shown in Table 2. Table 2 also shows the Mo equivalent of the base alloy.

  Further, the obtained rolled material was subjected to a solution treatment for air cooling after being held at 1200 ° C. for 5 minutes, and subsequently annealing for air cooling after being held at 760 ° C. for 2 hours was performed. Tensile test specimens were collected from the rolled sheet after annealing in the rolling direction, and subjected to a high-temperature tensile test at 760 ° C. to measure the tensile strength (TS). Similarly, the tensile strength (TS) was also measured for the base alloy. Taking the difference from these tensile strengths, the amount of increase in tensile strength due to the addition of C was determined. The results are also shown in Table 2.

  Based on Table 2, the relationship between the Mo equivalent, the amount of α phase and the amount of increase in tensile strength (TS) is shown in FIG. 1 and FIG. From Table 2 and FIGS. 1 and 2, when the Mo equivalent is −4 or less, the amount of α phase at 760 ° C. is 80% or more, and even if the amount of added C is the same, the Mo equivalent is −4. In the following, it can be seen that the amount of increase in tensile strength is significant in a Ti alloy having an α phase content of 80% or more.

  Examples No. 12 and No. 1 base alloys shown in Table 1 above were used to melt Ti alloys having various C addition amounts (C amounts of Ti alloys after addition of C) as shown in Table 3. A Ti alloy plate having a thickness of 4 mm was manufactured in the same process as in No. 1. Further, a tensile test piece was collected from this Ti alloy plate in the same manner as in Example 1, and the tensile strength at 760 ° C. was measured. The obtained results are also shown in Table 3.

  FIG. 3 shows the relationship between the amount of C added and the tensile strength (TS) at 760 ° C. based on Table 3. From Table 3 and FIG. 3, in the C-added Ti alloy using the No. 1 base alloy (Mo equivalent = 3.4) in Table 1, even when C is added, almost no improvement in high-temperature strength is observed. On the other hand, in the C-added Ti alloy using the base alloy No. 12 in Table 1 (Mo equivalent = −6.3), the high-temperature strength increased as the amount of C added increased. It can be seen that No. 24 of 21 and 0.20% C increased the tensile strength by about 30 MPa. In addition, in Nos. 22 to 24 of the C-added alloy, the Young's modulus was 130 GPa or more.

It is a graph which shows the relationship between the Mo equivalent in Example 1, and the raise amount of tensile strength (TS). It is a graph which shows the relationship between the quantity of (alpha) phase in Example 1, and the raise amount of tensile strength (TS). It is a figure which shows the relationship between C addition amount in Example 2, and the tensile strength (TS) in 760 degreeC.

Claims (4)

Ti alloy composed of α phase or α + β phase, the chemical composition is mass%, Al: 6.5-9.0%, Mo: 0.5-1.5%, V: 0.6-1. 4%, C: 0.1 to 0.25%, the balance is Ti and impurities, and the Mo equivalent ([Mo] eq) represented by the following formula is -4 or less and exceeds the solid solubility limit of the α phase. Is a heat-resistant Ti alloy with excellent high temperature strength finely dispersed in the structure as TiC.
[Mo] eq = 1.0 [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] -1.0 [Al]
However, [X] shows content mass% of element X.   The heat resistant Ti alloy according to claim 1, wherein the Mo equivalent is -6 or less. The heat-resistant Ti alloy excellent in high-temperature strength according to claim 1 or 2, wherein the area ratio of the α phase at 760 ° C is 80% or more. The heat-resistant Ti alloy according to any one of claims 1 to 3, wherein the area ratio of the α phase at 760 ° C is 90% or more.

Images