JPS6365042A - Ti alloy excellent in crevice corrosion resistance and combining high strength with high ductility and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a Ti alloy excellent in crevice corrosion resistance and combining high strength with high ductility, by subjecting a Ti-alloy ingot containing specific compositional amounts of Al, V, Mo, and Ni, to hot working and heat treatment under specific conditions. CONSTITUTION:The Ti-alloy ingot having a composition which consists of, by weight, 2-5% Al, 5-12% V, 0.5-8% Mo, 0.5-8% Ni, and the balance Ti with inevitable impurities and in which a condition represented by 14%<=1.5 V+Mo+2.4Ni<=21 is satisfied is subjected to hot working at 600-950 deg.C, to solution heat treatment at 700-800 deg.C, and then to aging treatment at 300-600 deg.C. In this way, the Ti alloy excellent in crevice corrosion resistance, causing no cracks in the course of hot working, and combining high strength with high ductility can be obtained. The above alloy is suitable for us as structural parts for aircraft, various members used under corrosive environment of seawater, etc., and the like, requiring the above characteristics.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention has excellent crevice corrosion resistance, high strength and high ductility, and is particularly applicable to aircraft parts that require these characteristics and seawater atmosphere. The present invention relates to a Ti alloy suitable for use as various members exposed to corrosive environments such as the present invention, and a method for producing the same.

[Conventional technology]

Conventionally, in fields where good and well-balanced properties of strength, oxidation resistance, and hot workability are required, such as aircraft jet engines, the ), α+β type Ti alloys represented by the composition of Ti-6%At-4%V, and those with poor hot workability,
A quasi-α-type Ti alloy having a composition of Ti-8%, V-1%V-1%Mo, and whose structure is mostly α phase is used. This is because α-type Ti alloys have poor strength and hot workability, while β-type Ti alloys have poor oxidation resistance.

And Ti-6%A! -4%V alloy and Ti-8%A!
-1CH V-1% MO alloy is hot worked at a temperature of 850°C or higher, particularly 900°C or higher for the former, and 950°C or higher for the latter, and after annealing, it is melted at a high temperature of 950°C or higher. The former Ti alloy is further treated with 500
It is manufactured by aging treatment at a temperature within the range of ~600°C. Note that the latter Ti alloy has a low aging ability, so it is not subjected to aging treatment.

[Problem that the invention seeks to solve]

However, as mentioned above, the conventional α+β type Ti
The hot working temperature of alloys and quasi-α type Ti alloys is 850°C.
Because of the above, for example, when trying to obtain a forged product close to the shape and dimensions of the final product using constant temperature forging, it is necessary to have a forged product with excellent heat resistance and a complex and smooth inner surface that corresponds to the shape of the final product. Rather than requiring expensive molds,
These Ti alloys have poor thermoeconomic efficiency due to not only high hot working temperatures but also high solution treatment temperatures, and moreover, they often generate scale, and when exposed to corrosive environments such as seawater atmosphere. , which has problems such as the tendency to cause crevice corrosion.

[Means for solving problems]

Therefore, from the above-mentioned viewpoints, the inventors of the present invention have developed a method that has high ductility and is capable of hot working at a much lower temperature, and is also capable of solution treatment and aging treatment at a much lower temperature. As a result of conducting research to develop a Ti alloy that can ensure high strength through this aging treatment and also has excellent crevice corrosion resistance, we found that M: 2 to 5%, V:
5-12%, Mo: 0.5-81b, N
i: 0.5-8%, and further contains Zr: 0.5-8%, Cr: 0.1-3%, Fe as necessary.
:0.1-3%, Sn:O, 1-4%.

Contains one or more of the following, and 14%≦1.5×V (%) + Mo (%) + 2.4 X
Ni (%) 521% (hereinafter referred to as conditional expression 1), 14%≦1.5 X v (%) + Mo (%) + 2.4
XN1 (%) + 0.6 x Zr (%) + 1.9
×Cr(%)-1-1,1x Fe(%)≦21chi(
A Ti alloy that satisfies the following conditional expression 22%≦M(%)+Sn(%)≦6chi (hereinafter referred to as conditional expression 3), and has a composition with the remainder consisting of Ti and unavoidable impurities, has a relatively low temperature ( For example, it shows an α+β structure at 700°C), and the ratio of α phase to β phase is close to 1:1, so it exhibits high ductility. Not only can it be easily hot worked at temperatures in the range of 950°C, but it can also be solution treated at temperatures lower than conventional conditions, i.e. in the range of 700 to 800°C. Furthermore, aging treatment can be applied,
In addition, we have found that this aging treatment provides a strength equal to or greater than that of the conventional α+β-type aged Ti alloy, and in addition, it has excellent crevice corrosion resistance. be.

This invention was made based on the above findings, and includes At: 2 to 5%, V25 to 129%, Mo: 0.5 to 8%, N
i: 0.5 to 8%, and if necessary, Zr: 0.5 to 8%, Cr: O, 1 to 3%, F
e: O, 1-3%, Sn: 0.1-4%.

contains one or more of the following, and satisfies the above conditional expressions 1.2 and 3,
A Ti alloy having a composition in which the remainder consists of Ti and unavoidable impurities, and a Ti alloy ingot having the above composition, 6
Hot working at temperatures within the range of 00-950°C, 700-950°C
Solution treatment at temperatures within the range of 800 °C, and 300 °C
The present invention is characterized by a method for manufacturing a high-strength, high-ductility Ti alloy with excellent crevice corrosion resistance by sequentially performing aging treatments at temperatures in the range of ~600°C.

Next, the reason why the composition and manufacturing conditions are limited as described above in the Ti alloy and the manufacturing method thereof of the present invention will be explained.

Composition (a) AM The M component has the effect of strengthening the α phase, but if its content is less than 2 T, the strength of the α phase, and by extension the strength of the entire Ti alloy, cannot be maintained at the desired value. On the other hand, if the content exceeds 5 Ti, it is necessary to increase the content of V and MO, which are β stabilizing elements to keep the β transformation point low, and as a result, the heat of the Ti alloy increases. Machinability deteriorates,
Specifically, since deformation resistance increases and a large press is required during forging, the content is set at 2 to 5%.

(b) v The V component has the effect of suppressing the β-transform point particularly low and widening the β-phase stabilization region, and also has the effect of strengthening the β-phase without impairing the ductility of the Ti alloy. If the content is less than 5%, it is not impossible to keep the β transformation point low, and it is difficult to maintain a volume ratio of α phase to β phase of approximately 1:1 at around 700°C. As a result, it becomes impossible to lower the hot working temperature and solution treatment temperature, and on the other hand, if its content exceeds 12%, the hot workability of the Ti alloy deteriorates, and specifically, the deformation resistance increases. Since a large press is required during forging, the content should be reduced from 5 to 5.
It was set at 12%.

(c) M.

In addition to particularly strengthening the β phase, the Mo component also has the effect of suppressing the β transformation point to a low level and widening the stabilization region of the β phase. However, if its content is less than 0.5%, it will not strengthen the β phase. As a result, the strength improvement effect of Ti alloy is low, while its content is 8
Since the ductility of the Ti alloy decreases when the content exceeds 0.5%, the content is set at 0.5% to 8%.

(d) Ni The N1 component has the effect of significantly improving crevice corrosion resistance, but if its content is less than 0.5%, the desired excellent crevice corrosion resistance cannot be secured; Even if the content exceeds 8 g, the effect of further improving the above-mentioned effects will not appear, and instead the hot workability will deteriorate, so the content was set at 0.5 to 8 g. .

(e) Zr, Cr, and Fe These components have the effect of further improving the strength of the Ti alloy, so they are included as necessary when even higher strength is required. The content is Z
r: less than 0.5%, Cr: less than O51%, and Fe
If the content exceeds Zr: 8%, Cr: 3%, and Fe: 3%, the ductility decreases and hot working becomes difficult. Zr: 0.5 to E1%, respectively.

Cr: O, 1-3%, and Fe: 0.1-3%.

(f) The 5n Sn component has the effect of stabilizing the α phase, which is a low-temperature phase, and improving the strength of the Ti alloy.This effect is the same as that of M, so if necessary, a partial substitute component for M may be used. However, if the content is less than 0.1 yen, the strength improvement effect due to replacing a part of M with Sn cannot be obtained, while on the other hand, if the content exceeds 4 yen, the ductility decreases. decreases,
Since hot workability would be impaired, the content was determined to be 0.1 to 4.

In addition, as mentioned above, the Sn component has the effect of improving the strength of the Ti alloy like the M component, so when the Sn component is included, it is necessary to include 2 or more in combination with M. can ensure high strength, while AM+S
If the content of n exceeds 6, the transformation temperature will rise and the hot workability will decrease. Therefore, as shown in conditional expression 3, At (%) + Sn (ch): 2 to 6 %.

(g) Conditional Expression 1.2 There is a difference in β phase stabilizing ability between each alloy component. When the MO acid component is 1, V: 1.5 times, Ni
: 2.4 times, Zr: 0.6 times, Cr: 1.9 times, and Fe: 1.1 times. Therefore, the value obtained by multiplying the content of each component by these ratios is The sum total becomes the β phase stabilizing ability, but if this value is less than T4, the β transformation point is insufficiently lowered, and the hot working temperature and solution treatment temperature are not lowered sufficiently; %, ductility decreases and hot workability deteriorates, so conditional expression 1.2 must be satisfied.

B. Manufacturing conditions (a> Hot working temperature A Ti alloy ingot having the composition described in A above is subjected to hot working such as hot forging, hot rolling, and even hot extrusion. If the hot working temperature is less than 600°C, recrystallization is difficult and the deformation resistance is high. On the other hand, if the hot working temperature exceeds 950°C, coarsening of crystal grains occurs, resulting in undesirable distortion and a constant temperature. In the case of forging, expensive molds are required, so the hot working temperature is set at 600-900.
The temperature was set at 50°C.

In particular, if it is necessary to erase the cast structure, it is preferable to start hot working at a temperature close to 900°C or higher;
A temperature of °C is preferred. This means that the Ti alloy of this invention
α suitable for hot working at temperatures within the range of 650-750℃
This is because a coexistence state occurs in which the ratio of the phase and the β phase is approximately 1:1 in terms of capacity ratio.

(b) Annealing This step is not an essential step, but is performed as necessary when performing cold working as a subsequent step. As for the conditions, it is desirable to maintain the temperature within the range of 650 to 750°C for 0.5 to 2 hours.

(C) Solution treatment temperature Ti alloys that have been hot-worked and further annealed and cold-worked as necessary are subjected to solution treatment, but at a lower temperature than conventional conditions of 700 to 800°C. It is necessary to carry out the test at a temperature within the range of . This means that when the temperature is below 700°C, α
M, which is a phase stabilizing element, is not sufficiently dissolved in the β phase, and therefore, even if an aging treatment is performed after this step, the desired strength cannot be secured. This is because if it exceeds the β transformation point, the amount of pro-eutectoid α phase becomes too small and the structure becomes non-uniform. Note that the solution treatment time is sufficient to uniformly heat the alloy.

(d) Aging treatment temperature If the temperature is less than 300℃, the diffusion rate is slow and β
Since precipitation of the minute α phase in the phase does not occur, age hardening does not occur.On the other hand, if the temperature exceeds 600°C, overaging occurs and the strength decreases, so the temperature is lowered to 30°C.
The temperature was set at 0 to 600°C.

Further, the aging treatment time varies depending on the temperature, but considering economic efficiency, it is preferably 0.5 to 10 hours.

Note that, if necessary, cold working may be performed after annealing, or if annealing is not performed, after solution treatment, that is, before aging treatment.

〔Example〕

Next, the Ti alloy of the present invention and its manufacturing method will be explained using examples.

In a two-stage melting process using a vacuum arc melting furnace, molten metals having the compositions shown in Table 1 were prepared, and the diameter:
After making an ingot with dimensions of 20011φ x length = 500 pieces, it was hot forged at 900°C to a thickness of 50mm x
A slab with dimensions of width: 600 m x length: 500 cm is then hot rolled at 720°C to form a hot rolled plate with a thickness of 3 m. The presence or absence of processing cracks was observed, and then solution treatment was carried out under water cooling conditions after being held at 750°C for 1 hour.
Inventive Ti alloy plates 1 to 17 and conventional Ti alloy plate 1.2 were produced by aging under conditions of holding at 0° C. for 5 hours.

Next, the mechanical properties (at room temperature) of the various plate materials obtained as a result were measured, and the
A test piece of mm x 3'ItjIL was cut out, a hole of 5.5 tsxφ was made in the center of this test piece, and the surface was wet-polished with emme 9-$400, and two sheets were cut out with a sealant material in between. Combine the test pieces and use as a washer Thickness: 2III
Using M5 Teflon and tightening it with a Ti M5 screw, the test piece set in this way was heated with 10% NaC1.
A crevice corrosion test was conducted under the conditions of immersing the specimen in an aqueous solution, heating it to 130° C., and holding it for 100 hours. After the test, the area of crevice corrosion on the surface of the test piece was measured and its ratio was calculated. These results are shown in Table 1.

〔Effect of the invention〕

As is clear from the results shown in Table 1, the present invention Ti
All alloy plates 1 to 17 have high ductility without cracking even when hot worked at an extremely low temperature of 720°C, whereas conventional Ti alloy plates 1.2 have was too low, resulting in lack of toughness,
Cracks cannot be avoided.

In this way, the Ti alloy of the present invention, compared to the conventional Ti alloy, has
Since hot working at extremely low temperatures is possible, mold casting can be performed using relatively inexpensive molds, and as a result, grain growth can be suppressed, so This makes it possible to form a fine structure.

In addition, in the Ti alloy of the present invention, cracks do not occur during hot working, so it is possible to form a hot worked material close to the final product size without the need for additional cutting for finishing. It does not require cold working.

Furthermore, from the results shown in Table 1, none of the Ti alloy plates 1 to 17 of the present invention have high strength equal to or higher than that of the conventional Ti alloy plate 1.2. It is clear that it has excellent crevice corrosion resistance.

As mentioned above, according to the method of the present invention, it is possible to produce a Ti alloy having high strength and high ductility with excellent crevice corrosion resistance, and therefore it can be used for structural parts of aircraft where these properties are required. As a result, it exhibits excellent performance over a long period of time when used in corrosive environments such as K and other corrosive environments.

Claims (5)

[Claims] (1) Contains Al: 2-5%, V: 5-12%, Mo: 0.5-8%, Ni: 0.5-8%,
and 14%≦1.5×V(%)+Mo(%)+2.4×Ni
A high-strength, high-ductility Ti alloy with excellent crevice corrosion resistance, which satisfies the following condition: (%)≦21%, with the remainder consisting of Ti and unavoidable impurities (weight percent). (2) Contains Al: 2-5%, V: 5-12%, Mo: 0.5-8%, Ni: 0.5-8%,
Furthermore, it contains one or more of the following: Zr: 0.5-8%, Cr: 0.1-3%, Fe: 0.1-3%, and 14%≦
1.5×V(%)+Mo(%)+2.4×Ni(%)+
0.6 x Zr (%) + 1.9 x Cr (%) + 1.1 x F
A high-strength, high-ductility Ti alloy with excellent crevice corrosion resistance, which satisfies the condition of e (%)≦21%, and has a composition (weight %) with the remainder consisting of Ti and unavoidable impurities. (3) Contains Al: 2-5%, V: 5-12%, Mo: 0.5-8%, Ni: 0.5-8%,
Further, contains Sn: 0.1 to 4%, and 14%≦1.5×V (%) + Mo (%) + 2.4×Ni
(%) ≦ 21%, 2% ≦ Al (%) + Sn (%) 6%, and has a composition (weight %) with the remainder consisting of Ti and unavoidable impurities. High strength, high ductility Ti alloy with excellent corrosion resistance. (4) Contains Al: 2 to 5%, V: 5 to 12%, Mo: 0.5 to 8%, Ni: 0.5 to 8%,
Furthermore, one or more of Zr: 0.5-8%, Cr: 0.1-3%, Fe: 0.1-3%, and Sn: 0.1-4%. Contains and 14%≦1.5×V (%) + Mo (%) + 2.4×Ni
(%) + 0.6 x Zr (%) + 1.9 x Cr (%) + 1
．． 1×Fe(%)≦21%, 2%≦Al(%)+Sn(
A high-strength, high-ductility Ti alloy with excellent crevice corrosion resistance, which satisfies the following conditions: (5) A Ti alloy ingot containing 2 to 5% of Ai, 5 to 12% of V, 0.5 to 8% of Mo, and 0.5 to 8% of Ni (weight %) of 600 to 600%. 950
After hot working at a temperature within the range of 700-800℃
solution treatment at a temperature within the range of 300 to 600
A high-strength, high-ductility Ti alloy with excellent crevice corrosion resistance, which is characterized by being subjected to aging treatment at a temperature within the range of °C.