JP2010053419A - Titanium alloy for heat resistant member having excellent creep resistance and high temperature fatigue strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a titanium alloy for a heat resistant member having sufficient creep resistance higher than that of the conventional heat resistant alloy at 850°C higher than that of the conventional using maximum temperature at a low cost. <P>SOLUTION: Disclosed is a titanium alloy for a heat resistant member having a composition comprising, by mass, 5.0 to <6.5% Al, 0.5 to <5.0% Sn, 4.6 to <6.0% Zr, 0.3 to <0.5% Mo, 0.41 to <0.60% Si and 0.05 to <0.20% O and satisfying Fe+Cr+Ni: <0.07%, and the balance titanium with inevitable impurities. In this way, it has creep resistance higher than that of the conventional titanium alloy and is inexpensive, thus is suitable for use to high temperature applications such as an automotive engine valve, and not only contributes to the high output of an automotive engine, the reduction of its fuel consumption and noise suppression therein, but also can widely obtain its effect by the enlargement of applications to mass-produced products. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a titanium alloy for heat-resistant members that is excellent in creep resistance and high-temperature fatigue strength.

  Conventionally, titanium alloys that are lightweight, high-strength and excellent in heat resistance have been developed for use in aircraft engine parts. For example, Ti-6Al-2Sn-4Zr-2Mo-0.1Si is known as a typical alloy.

  In Patent Document 1, as a titanium alloy having excellent heat resistance, by weight, Al: 5.0 to 7.0%, Sn: 3.0% to 5.0%, Zr: 2.5% to 6 Alloy composed of 0.0%, Mo: 2.0% to 4.0%, Si: 0.05 to 0.80%, C: 0.001 to 0.200%, O: 0.05 to 0.20 Is disclosed.

  Patent Document 2 discloses an alloy of Ti-6% Al-3% Sn-5% Zr-1% Mo-0.25% Si.

  In Patent Document 3, Al: 5.0 to 7.0%, Sn: 2.0 to 5.0%, Zr: 2.0 to 5.0%, Mo: 0.10 to 1.00%, Si: 0.20 to 0.60%, Hf: 0.10 to 1.00%, Nb: 1.50% or less, Ta: 0.50% or less, W: 0.50% or less, Cu: 1 An alloy composed of one or more of 0.000% or less or C: 0.10% or less, the balance Ti and unavoidable impurities is disclosed.

  In Patent Document 4, as an alloy having good creep resistance at 500 ° C. to 600 ° C., Al: 5.5 to 6.5%, Sn: 2.00 to 4.00%, Zr: 3.5 to An alloy consisting of 4.5%, Mo: 0.3-0.5%, Si: 0.35-0.55%, Fe: 0.03% or less, O: up to 0.14% is disclosed. Yes.

JP-A-10-195563 JP-A-48-4320 Japanese Patent No. 2737500 JP-A-63-118035

  As described above, the conventional heat-resistant titanium alloy is assumed to have a use temperature of up to 600 ° C. However, as the application to automotive applications, where the demand for higher performance, lower fuel consumption, and lower cost of the engine is increasing, it is hoped that the characteristics will be improved in accordance with the operating temperature that is expected to reach 800 ° C to 850 ° C. It is rare.

  The inventors diligently investigated and analyzed the cause of breakage of members used in automobiles, and came to recognize the following problems. That is, the breakage of the heat-resistant member is caused by a local increase in load than expected due to creep deformation during use. Therefore, while the conventional countermeasures have been to increase the strength, the inventors have thought that suppressing creep deformation is a more important solution. At the same time, it goes without saying that high temperature fatigue strength is reduced and high costs due to the use of special additive elements are unacceptable.

  However, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, which is a typical heat-resistant titanium alloy, has a problem that its creep resistance is low at a high temperature of 850 ° C.

  Further, the alloys of the invention described in Patent Document 1 and Patent Document 2 contain a large amount of Mo which is a β-stabilizing element like Ti-6Al-2Sn-4Zr-2Mo-0.1Si. Easy to increase the phase and reduce creep resistance.

  The alloy described in Patent Document 3 is expensive to add to Hf, and is difficult to use industrially except for special cases. In addition to Hf, elements such as Nb, Ta, W, and Cu are also β-stabilizing elements, and the addition of these elements further reduces creep resistance. C is an α-stabilizing element. However, since it is an intrusion-type element, atom migration easily occurs at high temperatures, and therefore does not particularly improve creep resistance at a high temperature range of about 850 ° C.

  The alloy of the invention described in Patent Document 4 is an alloy developed for the purpose of increasing the creep resistance of about 500 ° C. to 600 ° C. assuming aircraft use, and the purpose is clearly different from that of the present invention. ing. Accordingly, the creep resistance at 850 ° C. and the high temperature fatigue strength are not sufficient.

In addition, materials based on intermetallic compounds such as TiAl and Ti ₃ Al are not practical because they have excellent creep resistance but are extremely expensive.

  Therefore, the present invention advantageously solves the above-described problems and provides a titanium alloy for a heat-resistant member having excellent creep resistance and excellent high-temperature fatigue strength at 850 ° C. at a low cost.

  As a result of diligent studies to achieve the above-mentioned object, the present inventors have studied by adjusting additive elements in order to improve the creep resistance at 850 ° C. As a result, the creep resistance and high temperature exceeding existing alloys are improved. The present inventors have found a titanium alloy having fatigue strength and low cost.

The gist of the present invention is as follows.
(1) By mass%, Al: 5.0% or more and less than 6.5%, Sn: 0.5% or more and less than 5.0%, Zr: 4.6% or more and less than 6.0%, Mo: 0.00. 3% or more and less than 0.5%, Si: 0.41% or more and less than 0.60%, O: 0.05% or more and less than 0.20%, Fe + Cr + Ni: less than 0.07%, and remaining titanium and inevitable impurities Titanium alloy for heat-resistant members with excellent creep resistance and high-temperature fatigue strength.
(2) The titanium alloy for a heat-resistant member excellent in creep resistance and high-temperature fatigue strength according to claim 1, wherein Nb in an amount of 0.1% to less than 1.0% is added.

  The heat-resistant titanium alloy of the present invention has creep resistance higher than that of conventional titanium alloys and is inexpensive, so it is suitable for use in high-temperature applications such as automotive engine valves. In addition to contributing to lower fuel consumption and lower noise, it is possible to obtain a wide range of effects by expanding its application to mass-produced products, so industrial effects are immeasurable.

  The present invention will be described in detail below.

  As an index of the creep resistance of the titanium alloy of the present invention, the heat resistant titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si material, which has a proven record in applications such as automotive engine valves, is one index. The goal was to exceed the creep resistance at ℃. Specifically, in the creep resistance evaluation method under the test conditions described below, the target was a creep deformation amount of 2% or less. In addition, the elongation at room temperature was 5% or more from the viewpoint of toughness and the like. Some components were evaluated for high temperature fatigue strength.

  Here, the creep resistance evaluation method in the present invention will be described.

  As a method for evaluating creep resistance, a cantilever type test at a high temperature was adopted. Place the weight on the free end of the round bar test piece held horizontally so that the action point of the weight matches, and keep the distance from the fixed end of the test piece holding part to the free end of the test piece, that is, the action point of the weight. The amount of creep deformation was evaluated from the amount of deformation of the test piece after being kept at 850 ° C. in an air atmosphere for 24 hours. The amount of creep deformation was determined by measuring the distance H by which the free end of the test piece after the test was displaced from the central axis of the original round bar test piece before the test, and expressing H / L as a percentage.

  In the first aspect of the present invention, the component ranges of Al, Sn, Zr, Mo, Si, O, and C and the component range of Fe + Cr + Ni are defined to achieve the above-described index.

Al is an element with high solid solution strengthening ability of the α phase, and the high temperature strength and creep strength increase as the amount added increases. In order to obtain a creep resistance higher than that of the comparative material at 850 ° C., addition of 5.0% or more is necessary. However, if Al is added in an amount of 6.5% or more, a brittle α ₂ phase is generated, which increases the processing cost. Therefore, the Al component range is set to 5.0% or more and less than 6.5%.

Sn has an effect of strengthening both the α phase and the β phase, and is an effective element in improving the strength of the α + β two-phase alloy. In order to obtain a creep resistance higher than that of the comparative material at 850 ° C., addition of 0.5% or more is necessary. However, when 5.0% or more is added, an α ₂ phase is generated and embrittles. Therefore, the Sn component range is set to 0.5% or more and less than 5.0%. In the case where Sn segregation may occur, it is more preferably 0.5% or more and less than 3.0% in order to reliably suppress the formation of the α ₂ phase.

  Zr is an element effective for strengthening both the α phase and the β phase. Moreover, when it adds simultaneously with Si, there exists an effect which improves creep resistance. If added more than 6.0%, the creep resistance at 850 ° C. decreases, so the upper limit was made 6.0%. The lower limit was set to 4.6% necessary for obtaining creep resistance at 850 ° C.

  Mo is a β-stabilized substitutional element and functions to improve hot workability. In order to express this effect, the lower limit was made 0.3% or more. However, at 850 ° C., if the β phase is excessively present, the creep resistance decreases, so the upper limit was made less than 0.5%.

  Si is an element that improves creep resistance. However, a large amount of addition tends to embrittle the titanium alloy due to an increase or coarsening of intermetallic compounds formed with Ti and Zr. Therefore, the addition is 0.41% or more and less than 0.60%.

O is an element that strengthens the α phase. In order to express the effect, 0.05% or more of O is necessary. However, brittle to facilitate the formation of alpha ₂ phase when the O is added 0.20% or more. For this reason, it was set as 0.05% or more and less than 0.20% addition. In the case where Al segregation may occur, the amount is more preferably 0.05% or more and less than 0.14% in order to surely suppress the formation of the α ₂ phase.

  Fe, Cr, and Ni are all β-stabilized substitutional elements. When the β phase is excessively present, creep resistance is lowered, and as a result of investigating the content of these elements that do not adversely affect the creep resistance, Fe + Cr + Ni is less than 0.07%, preferably less than 0.05%. Therefore, this was prescribed.

  In the present invention described in claim 2, in addition to the alloy of the present invention described in claim 1, Nb of 0.1% or more and less than 1.0% is added. Nb is a β-stabilizing element and contributes to solid solution strengthening of the β phase, and also has an effect of suppressing oxidation when exposed to a high temperature of 800 ° C. or higher. If the addition is less than 0.1%, the effect is not sufficient. If the addition is 1.0% or more, the creep resistance decreases due to an increase in β phase, so the addition was made less than 1.0%.

  A typical production process of the titanium alloy of the present invention is as follows. By using a melting method in which sponge titanium or an alloy material is used as a raw material, arc melting or electron beam melting in a vacuum, and casting into a water-cooled copper mold, mixing of impurities is suppressed to obtain an ingot of the titanium alloy component of the present invention. Here, O can be added at the time of dissolution by using, for example, titanium oxide or titanium sponge having a high oxygen concentration. This ingot is heated to 1100 to 1250 ° C., forged into a cylindrical shape with a diameter of 100 mm, then reheated to 1100 to 1250 ° C., and hot rolled to have a cross-sectional square of about 15 to 50 mm square or a diameter of about 15 to 50 mm Can be processed into a rod with a circular cross section. As the final heat treatment, the solution is kept at a temperature above the β transformation point of 1050 to 1130 ° C. for 5 to 60 minutes for solid solution of precipitates, and then air-cooled, and further 500 to 850 ° C. and 30 minutes to 4 Time, air cooling, preferably 750 ° C. to 830 ° C., 45 minutes to 90 minutes aging treatment, with a cross-sectional optical microscope structure, needle-like shape having a width of 10 μm or less in old β grains having a particle size of 100 to 800 μm The α phase is precipitated. This is because the detailed mechanism is not clear, but not only the creep resistance but also the high temperature fatigue strength can be maintained at a high level. If the solution treatment temperature is lower than 1050 ° C., the solid solution is insufficient and the microstructure becomes inhomogeneous and the characteristics are deteriorated. If the temperature is 1130 ° C. or higher, the yield deteriorates due to oxidation. If the aging temperature is lower than 500 ° C. or shorter than the above range, the effect of stabilizing the structure due to aging is small, and the characteristics change greatly during use at high temperatures, which is not preferable. On the other hand, when the aging temperature is higher than 850 ° C. or longer than the above range, the oxide scale layer becomes thick, which is not preferable because the product yield, manufacturability is deteriorated, or the mechanical characteristics are lowered. The final heat treatment may be performed after hot-forming and / or cutting the rod material into a shape close to the final product shape.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
Titanium alloys having the components shown in Table 1 were produced by the arc melting method, and each was made into an ingot of about 1 kg. A 15 mm square bar obtained by forging each of these ingots was used as a raw material. The creep resistance test piece was subjected to air-cooling heat treatment (solution treatment) at 1090 ° C. for 10 minutes, and further subjected to air-cooling heat treatment (aging treatment) at 800 ° C. for 1 hour, and then the test piece was collected. The tensile test was performed at room temperature using a round bar test piece having a parallel portion φ6.25 mm. Room temperature ductility was based on the fact that it was practically 5% or more. The creep resistance test was carried out by placing a 0.5 ± 0.1 kg heat-resistant Ni alloy weight on the free end of a 5 mm diameter round bar test piece held horizontally, at 850 ° C., in the atmosphere. The amount of deformation H after holding for 24 hours in the atmosphere was measured. The deformation amount H is a distance from the center of the free end of the test piece after the test to the center axis of the original round bar test piece before the test. The effective test piece length L from the fixed end to the free end excluding the grip portion of the test piece was 45 mm. The creep resistance was expressed as a percentage of the effective test piece length L and the deformation amount H (H / L). Table 1 shows the 850 ° C. creep resistance test results and the elongation values in the room temperature tensile test. Numerical values that fall outside the scope of the present invention are underlined.

  In Table 1, no. 1-16 are examples of the present invention, No. No. 28 is a conventionally used heat-resistant titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si, No. 28 No. 29 is a heat-resistant titanium alloy described in Patent Document 4, No. 29. 17 to 27 are comparative examples in which any of the components is outside the scope of the present invention.

  No. The inventive examples 1 to 16 all had good 850 ° C. creep resistance and room temperature ductility. The 850 ° C. creep resistance is No. 28 heat-resistant titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si material, and No. 28 Good values were achieved compared to 29 heat-resistant titanium alloy.

  Comparative Example No. No. 17 has an Al content outside the lower limit. No. 19 has a Sn content outside the lower limit. No. 21 has a Zr content outside the lower limit. No. 22 has a Zr content outside the upper limit. In No. 23, the Mo content exceeded the upper limit. No. 24 has a Si content outside the lower limit. No. 26 has an Nb content exceeding the upper limit. In No. 27, the Fe + Cr + Ni content exceeded the upper limit, and in all cases, the 850 ° C. creep property was poor. Comparative Example No. In No. 18, Al was outside the upper limit. No. 20 has Sn exceeding the upper limit. In No. 25, O exceeded the upper limit, and the ductility at room temperature was poor in all cases.

(Example 2)
Further, a 200 kg ingot was produced by a vacuum arc melting method, and a φ15 mm round bar material was obtained by forging and hot rolling. After performing the same solution treatment and aging treatment as in Example 1, a round bar test piece having a parallel part φ8 mm was prepared, and a rotating bending fatigue test was performed at 850 ° C., 3000 rpm, in an air atmosphere. Table 2 shows the test components and 10 ⁷ times fatigue strength in the 850 ° C. rotating bending fatigue test. In addition, numerical values outside the scope of the present invention are underlined.

In Table 2, no. Reference numeral 1 denotes an example of the present invention. No. 2 is a conventionally used heat-resistant titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si, but its fatigue strength of 10 ⁷ times in the 850 ° C. rotating bending fatigue test is less than 100 MPa. No. 3 is a heat-resistant titanium alloy described in Patent Document 4, but the fatigue strength at 850 ° C. is 130 MPa. The alloy of the present invention not only has excellent creep resistance, but also has high fatigue strength that surpasses any existing alloy.

(Example 3)
Further, using the φ15 mm round bar material of the present invention component used in Example 2, (1) After solution treatment at 1000 ° C. to 1120 ° C., after air cooling, 800 ° C., 1 hour, air cooling heat treatment (aging treatment) ), And (2) a bar material that was subjected to a heat treatment (aging treatment) at 800 ° C. for 1 hour after solution cooling at 1090 ° C. and after furnace cooling. The same creep resistance test as in Example 1 was performed. Table 3 shows the test results. In addition, numerical values outside the scope of the present invention are underlined.

  In Table 3, no. In Nos. 1 and 2, the solution treatment temperature is outside the lower limit of the range of the present invention, and the creep resistance is poor.

Claims (2)

  In mass%, Al: 5.0% or more and less than 6.5%, Sn: 0.5% or more and less than 5.0%, Zr: 4.6% or more and less than 6.0%, Mo: 0.3% or more Creep resistance comprising less than 0.5%, Si: 0.41% or more and less than 0.60%, O: 0.05% or more and less than 0.20%, Fe + Cr + Ni: less than 0.07% and the balance titanium and inevitable impurities And titanium alloy for heat-resistant members with excellent high-temperature fatigue strength.   The titanium alloy for heat-resistant members excellent in creep resistance and high-temperature fatigue strength according to claim 1, wherein Nb in an amount of 0.1% or more and less than 1.0% is added.