JP4492959B2 - Heat resistant titanium alloy and engine valve formed thereby - Google Patents

Description

  The present invention relates to an α-β type heat-resistant titanium alloy that is easy to hot work, excellent in high temperature strength, and difficult to peel off oxide scale.

  Titanium alloys are used in various fields because they are lightweight, excellent in corrosion resistance, and easily obtained in high strength. In the field of heat-resistant structural parts used in high-temperature environments such as aircraft and automobile engines, high-temperature strength is particularly required. As such a high temperature titanium alloy, Nearα alloy (Ti-5.5Al-4Sn-4Zr-1Nb-0.5Si-0.06C alloy. The numerical value attached to the element indicates the mass% of the element. The same shall apply hereinafter).

  This Nearα alloy has a durable temperature of about 600 ° C. or less, but recently, a titanium alloy with further improved heat resistance has been proposed. For example, in Japanese Patent No. 3049767 (Patent Document 1), Al: 4.7 to 7.0%, Sn: 3.0 to 5.0%, Zr: 2.5 to 6.0%, Mo: A heat-resistant titanium alloy composed of more than 1.0% to 7.0%, Si: 0.05 to 0.5%, Nb: 3% or less, and the balance substantially consisting of Ti is disclosed in Japanese Patent No. 3303641 (Patent Document 2). Are Al: 5.0-7.0%, Sn: 2.0-5.0%, Zr: 2.0-5.0%, Nb: 0.1-1.5%, Mo: 0.00. 1-2.0%, Si: 0.1-0.6%, B: 0.3-2.5%, Hf: 0.1-3.0% and / or Ta: 0.1-3. A titanium alloy consisting of 0%, balance Ti and inevitable impurities is described.

Although the Nearα alloy and the titanium alloy disclosed in Patent Documents 1 and 2 are excellent in high-temperature strength, they have high deformation resistance at high temperatures, so the workability in hot forging and rolling is poor, Cracks and wrinkles are likely to occur during the process. For such a problem, there is a method of repeatedly performing heating and processing in the production process of the titanium alloy, but a new problem arises that the production cost increases. For this reason, a titanium alloy with improved hot workability has been proposed in Japanese Patent Laid-Open No. 2003-201530 (Patent Document 3). This titanium alloy contains Mo, V, Cr, and Fe in the range of Al: 3 to 7%, C: 0.08 to 0.25%, and the Mo equivalent represented by the following formula is 3.0 to 10%. .
Mo equivalent = [Mo] + [V] /1.5+1.25 [Cr] +2.5 [Fe]
However, [element] indicates the content (mass%) of the element.
Japanese Patent No. 3049767 Japanese Patent No. 3303641 JP 2003-201530 A

However, although the titanium alloy described in Patent Document 3 has good hot workability, there is a problem that sufficient high-temperature strength cannot be obtained. Furthermore, when titanium alloys are exposed to high temperatures for a long time, oxidation of the surface proceeds and oxide scales are formed, but in environments where mechanical loads repeatedly operate at high temperatures, such as engine valves for automobiles, etc. The oxide scale peels off. The peeled oxide scale adversely affects other parts constituting the engine as foreign matter. For this reason, in a conventional engine valve made of a heat-resistant titanium alloy, the surface has to be subjected to an oxidation resistance treatment to suppress the generation of scale. Such a process becomes a factor of high manufacturing cost.
The present invention has been made in view of such a problem, and an object of the present invention is to provide a heat-resistant titanium alloy having good hot workability, excellent high-temperature strength and high-temperature creep characteristics, and excellent resistance to scale peeling at high temperatures. .

The heat-resistant titanium alloy of the present invention is mass% (hereinafter simply referred to as “%”).
Al: 6.0 to 8.0%,
Mo: 1.0-3.0%,
Si: 0.05-0.40%,
C: It contains 0.08 to 0.25%, and the balance consists of Ti and inevitable impurities.

  According to this titanium alloy, an appropriate amount of Al, which is an α-phase strengthening element, is added to ensure excellent strength and creep characteristics at high temperatures, and a predetermined amount of Si and Mo is added to excessively increase high temperature characteristics. Exfoliation of the oxide scale can be suppressed without deteriorating, and by adding an appropriate amount of C, the β transformation point can be increased without impairing the high temperature characteristics, thereby increasing the processing temperature in the α-β region. By raising the temperature, good hot workability can be obtained. For this reason, not only hot workability but also high temperature strength and high temperature creep properties, and further suitable for use as a material for engine valves for aircraft and automobiles that require excellent scale peeling resistance.

In the titanium alloy, in place of a part of Ti, Nb: 0.40% or less, or further, one or more elements of V, Cr, Fe are added within a range where the following Mo equivalent is 4.0% or less. It can be a component to be included.
Mo equivalent = [Mo] + [V] /1.5+1.25 [Cr] +2.5 [Fe]
However, [element] indicates the content (mass%) of the element. The same applies below.

  The heat-resistant titanium alloy of the present invention has Al: 6.0-8.0%, Mo: 1.0-3.0%, Si: 0.05-0.40%, C: 0.08-0.25 %, And the balance consists of Ti and inevitable impurities. Hereinafter, the reason for component limitation will be described.

Al: 6.0 to 8.0%
Al is an α-stabilizing element and is an essential element for obtaining high strength and creep characteristics at high temperatures. If the content is less than 6.0%, this effect is too small. Therefore, the lower limit of the Al content is 6.0%, preferably 6.5%. On the other hand, when Al is added excessively, Ti ₃ Al which is an intermetallic compound is generated when exposed at a high temperature, and the material becomes brittle. Generally, titanium alloys are known to cause embrittlement when exposed to high temperatures when the Al equivalent represented by the following formula is 9.0% or more. Usually, oxygen is about 0 as an inevitable impurity element. Therefore, the upper limit of the amount of Al is 8.0%, preferably 7.% so that the value of [Al] +1/3 [Sn] +1/6 [Zr] is 8.0% or less. 5%.
The Al equivalent is well known to those skilled in the art and is described, for example, in a titanium alloy handbook (Rodney Boyer, Gerhard Welsch, EW Collings, Materials Properties Handbook Titanium Alloys (1994), P77).
Al equivalent = [Al] +1/3 [Sn] +1/6 [Zr] +10 [O]

Mo: 1.0-3.0%
Mo has the effect of densifying the oxide scale, and the adhesion of the scale to be formed is increased. Therefore, the peel resistance of the scale after being exposed to a high temperature is greatly improved. If it is less than 1.0%, such an action is too small. On the other hand, if it exceeds 3.0%, Mo which is a β-stable element becomes excessive, and the high-temperature strength decreases instead. For this reason, the lower limit of the Mo amount is 1.0%, preferably 1.5%, and the upper limit is 3.0%, preferably 2.5%.

Si: 0.05-0.40%
Si has the effect of densifying the oxide scale, improving the peel resistance of the scale, and improving the creep characteristics at high temperatures. If it is less than 0.05%, such action is too small. On the other hand, if it exceeds 0.40%, an intermetallic compound of Ti and Si is precipitated, and the ductility at room temperature is lowered. Therefore, in the present invention, the lower limit of the Si amount is 0.05%, preferably 0.10%, and the upper limit is 0.40%, preferably 0.30%.

C: 0.08 to 0.25%
The final hot working of the titanium alloy needs to be performed in the α-β region below the β transformation point in order to obtain equiaxed crystal grains in order to ensure workability. In order to perform processing at as high a temperature as possible without causing overheating, it is usually heated to about (transformation point −50) ° C. and subjected to hot processing. For this reason, the heating temperature at the time of hot working in the α-β region is determined by the β transformation point, and the higher the β transformation point, the better the hot workability. C has the effect of increasing the β transformation point, and in order to enable processing in the α-β region of 1000 ° C. or higher, the amount of C should be added at least 0.08%, preferably 0.10%. is necessary. As a result, cracks during processing can be greatly reduced, the number of reheating can be reduced, and productivity is improved. On the other hand, if added excessively, a large amount of TiC precipitates and fatigue characteristics are lowered, so the upper limit of the C content is 0.25%, preferably 0.20%. The near α alloy Ti-8Al-1Mo-1V alloy has a transformation point of 1040 ° C. and is usually heated to an α-β region of about 990 ° C. and hot-worked. Since the strength is high, the hot workability is very poor, and cracking occurs as soon as the material temperature drops even a little. For this reason, at present, reheating is repeatedly performed and hot working such as forging is performed.

The titanium alloy of the present invention contains the above basic components, and the balance is composed of Ti and unavoidable impurities, but can further contain elements that improve the characteristics within a range that does not impair the characteristics of the alloy of the present invention. For example, instead of a part of Ti, Nb is 0.40% or less and at least one of V, Cr, and Fe is Mo equivalent ([Mo] + [V] /1.5+1.25 [Cr] +2.5 [ Fe]) can contain 4.0% or less, and can have the following composition (remainder Ti and inevitable impurities).
(1) Basic component, Nb
(2) One or more of the basic components V, Cr, Fe
(3) One or more of the basic components, Nb, and V, Cr, Fe

Nb: 0.40% or less It has been found that Nb has an action of suppressing the dropping of the oxide scale after exposure to high temperature, and improving the high temperature strength and creep characteristics at high temperature. The reason for this effect is not clear, but the addition of Nb suppresses the diffusion of oxygen in the oxide scale formed on the surface, and as a result, the growth of the scale is suppressed, so that the scale peeling is suppressed. In addition, since it is possible to suppress the diffusion of oxygen to the base material, it is presumed that creep dislocation does not easily occur and the creep characteristics are improved. Addition of 0.1% or more is preferable in order to effectively exhibit such action. On the other hand, since Nb is a very expensive element, the upper limit of the amount of Nb is set to 0.4%, preferably 0.3%.

One or more of V, Cr, and Fe: Mo equivalent is 4.0% or less. V, Cr, and Fe are β-stabilizing elements that reduce high temperature strength and β transformation point, but improve hot workability, Since cracks during hot working are reduced, the yield can be improved. As a result of investigating an appropriate amount of β-stabilizing element to improve hot workability while maintaining high high-temperature strength and transformation point to some extent, when Mo equivalent exceeds 4.0%, strength and transformation point at high temperature Was found to be significantly reduced. For this reason, the above element is added in Mo equivalent of 4.0% or less, preferably 3.5% or less, more preferably 3.25% or less. Thereby, both high temperature strength and better hot workability can be achieved.

Next, the general manufacturing method of the titanium alloy of this invention is demonstrated. First, a titanium alloy having a predetermined component is melted to obtain an ingot. After heating this ingot to 1100 to 1200 ° C., hot working such as rolling and forging is performed at a rolling reduction of 60% or more. Thereafter, it is heated from 1000 ° C. to 1100 ° C., and subjected to hot working such as rolling and forging at a reduction ratio of 60% or more in the α + β region, and processed into an intermediate shape such as a bar or wire. 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 improve the mechanical properties. Annealing is typically performed by holding at, for example, 600 ° C. to 800 ° C. for about 1 to 10 hours. After this annealing, finish processing is performed and, if necessary, solution treatment is performed at 900 to 1200 ° C. for 5 to 120 minutes, followed by aging treatment or annealing under the same conditions as the above annealing, and the desired machine To develop a characteristic.
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.

  Titanium alloys having various chemical components shown in Table 1 were melted by an arc melting method to obtain an ingot (120 g) having a diameter of 50 mm and a length of 15 mm. Each ingot was heated to 1200 ° C. and hot forged into a round bar of φ20 mm. Then, after heating to 1000 ° C., forging to φ10. Thereafter, the solution was once heated and held in the β region (held at 1200 ° C. for 5 minutes) to perform solution treatment, and then air-cooled to room temperature. Thereafter, annealing was performed at 800 ° C. for 2 hours, followed by air cooling.

The titanium alloy samples obtained as described above were examined for mechanical properties and scale peelability in the following manner. The survey results are also shown in Table 1.
(1) High temperature tensile strength: A high temperature tensile test was performed at 800 ° C., and the tensile strength at the same temperature was measured.
(2) Creep test: Temperature: 760 ° C., stress: 28 MPa, time: 100 hr. The creep elongation was measured.
(3) Scale peelability: After the sample surface is mirror-polished, it is subjected to atmospheric annealing at 750 ° C. for 50 hours and 100 hours, and the presence or absence of peeling of the surface scale is visually observed. (Indicated by “◎”), partly peeled after 100 hr annealing, but not peeled after 50 hr annealing (indicated by “◯” in the table), not acceptable when peeling occurred after 50 hr annealing ( It was evaluated as “x” in the table).

  From Table 1, Sample Nos. 1 to 17 of the inventive example have a high-temperature strength at 800 ° C. of about 250 MPa or more, an elongation after creep test of 2% or less, and extremely excellent creep characteristics. I understand. Further, there is no peeling of the scale at 750 ° C. and 50 hours. On the other hand, none of the sample Nos. 21 to 29 of the comparative example has high temperature strength, creep property at high temperature, and scale resistance.

  An alloy having a base alloy of Ti-7Al-2Mo-0.2Si-0.2Nb and having various C contents shown in Table 2 is melted by an arc melting method, and an ingot having a diameter of 50 mm and a length of 15 mm ( 120 g) was obtained. Each ingot was heated to 1200 ° C. and forged into a round bar of φ24 mm. Then, it heats to the processing upper limit temperature of ((beta) transformation point-50) (degreeC), and reheats suitably so that hot processing may be performed in the processing temperature range of ((beta) transformation point-50) (degreeC) -900 (degreeC) alpha-beta range. However, hot forging was performed up to φ14 in the α-β region, and the number of heating times and the surface properties after forging were observed to examine the occurrence of cracks. The results are shown in Table 2. Excellent with no cracks (indicated by “◎” in the table), good with small cracks (indicated by “O” in the table), and not frequently cracked (indicated by “x” in the table) ).

After forging, the solution was heated and held in the β region (held at 1200 ° C. for 5 minutes), subjected to a solution treatment, and air-cooled to room temperature. Thereafter, annealing was performed at 800 ° C. for 2 hours, followed by air cooling. A fatigue test piece having a parallel part diameter of 6.0 mm was collected from the obtained sample, and subjected to an Ono-type rotary bending fatigue test under the conditions of a stress of 400 MPa and a rotational speed of 3600 rpm, and the number of revolutions until fracture was examined. However, the number of rotations was up to 1.0 × 10 ⁷ times. The test results are also shown in Table 2.

From Table 2, the sample No. 1 (Comparative Example) with substantially no C added is not much different from a normal heat-resistant titanium alloy with a processing upper limit temperature of 975 ° C., and heating is required twice to maintain the processing temperature range. Met. In addition, the forged sample was frequently cracked. On the other hand, Nos. 32-35 added with C amount of 0.11% or more have a working upper limit temperature of 1030 ° C. or higher, can be hot-worked at a higher temperature than ordinary titanium alloys, and the number of heating is also 1 I'm done with times. Looking at the sample after forging, sample No. 32 with C: 0.11% showed slightly small cracks, but the sample with 0.15% or more of C added had no cracks.
In addition, regarding fatigue characteristics, the sample numbers 32 and 33 (invention examples) with 0.11% and 0.15% of C addition within the scope of the invention were tested when the rotation speed was 1.0 × 10 ⁷ times or more. The piece did not break and showed very good fatigue properties. Sample No. 34 (invention example) with a C addition amount of 0.23% also had a good fatigue characteristic with a rotation speed of up to 7.8 × 10 ⁶ times. On the other hand, Sample No. 35 (Comparative Example) with a C addition amount of 0.30% was broken at a rotational speed of 7.6 × 10 ⁴ times, and the fatigue characteristics deteriorated rapidly.

Claims (4)

mass%
Al: 6.0 to 8.0%,
Mo: 1.0-3.0%,
Si: 0.05-0.40%,
C: 0.08 to 0.25%
A heat-resistant titanium alloy comprising the balance of Ti and inevitable impurities.   Furthermore, the heat-resistant titanium alloy according to claim 1 containing Nb: 0.40% or less. The heat-resistant titanium alloy according to claim 1 or 2, further comprising one or more elements of V, Cr, and Fe in a range where the following Mo equivalent is 4.0% or less.
Mo equivalent = [Mo] + [V] /1.5+1.25 [Cr] +2.5 [Fe]
However, [element] indicates the content (mass%) of the element. An engine valve formed of the titanium alloy according to any one of claims 1 to 3.