JPH0754084A - Highly corrosion resistant titanium alloy excellent in cold processibility and weldability - Google Patents

Abstract

PURPOSE:To provide a highly corrosion resistant Ti alloy excellent in cold processibility and weldability. CONSTITUTION:This Ti alloy consists of 1.0 to <3.0% Al, 1.5-4.5% V, 0.1-6.0% Mo, 0.1-1.5% Ni and the balance Ti with inevitable impurities. This Ti alloy has high strength and high corrosion resistance and is also excellent in cold processibility and weldability (strength of its weld zone). Since the weldability is satisfactory, a Ti alloy tube, etc., suitable for use in a corrosive environment or atmosphere in the chemical industry, the field of energy exploitation, etc., can be produced at a low cost without adopting mechanical cutting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly corrosion-resistant titanium alloy used in the chemical industry (piping, etc.), energy development (umbilical tube, oil well pipe for oil development, etc.) and aircraft (hydraulic piping, etc.).

[0002]

2. Description of the Related Art Titanium has the advantages of high specific strength (strength / specific gravity) and excellent corrosion resistance. Conventionally, titanium alloys have been developed in the direction of individually improving these advantages. As methods for improving these, Al, V, Ni
However, in many cases, sufficient consideration was not given to cold workability, and cold workability was often impaired even if strength and corrosion resistance were improved.

Titanium alloys generally have poor cold workability and weldability, and in many cases they must be finished by machining after hot working. However, titanium alloy has poor machinability,
As a result, the product yield is lowered and the cutting time is prolonged, resulting in higher costs.

Among titanium and titanium alloys, alloys that can be cold worked include pure Ti which is α-type titanium, Ti-3Al-2.5V which is α + β-type titanium alloy, and many β-type titanium alloys. To be

Among these, the β-type titanium alloy can be cold worked, but has a large deformation resistance during hot working and is difficult to manufacture by the conventional process. High temperature range (100-4
When it is used at around 00 ° C), there is a problem that it becomes brittle due to the precipitation of α phase and ω phase and lacks thermal stability.

Ti-3Al-2.5V is a typical cold workable α + β type titanium alloy, the basic component of which is Al
2.5-3.5% by weight and V 2.0-3.0% by weight (this alloy is standardized to AMS). Although this alloy can be cold worked like pure Ti, its strength is lower and corrosion resistance is not sufficient compared to many other titanium alloys.
Practical use is narrowed.

Japanese Patent Laid-Open No. 25418/1975 discloses Al, Zr, M
A titanium alloy containing o, V, Cr, Ni and Fe is shown, but it was invented for the purpose of improving strength and fracture toughness, and it has little or no cold workability or weldability. Not considered at all.

As is well known, titanium has an advantage that it is a material excellent in corrosion resistance, but further improvement in corrosion resistance is required for use in a severe corrosive environment, and high strength is also required for some applications. . To meet these requirements, for example, JP-A-61-257448 discloses a titanium alloy to which Al, Ni, Zr, etc. are added, and which has a high corrosion resistance and high strength due to the combined action of Ni and Zr. There is. In addition, JP-A-63-17
Japanese Patent No. 9033 discloses a titanium alloy to which high strength and high corrosion resistance are achieved by adding Al, Ni, Mo and the like. However, these titanium alloys have not been sufficiently examined for cold workability and weldability. In particular, in the former invention, the degree of occurrence of edge cracking when the reduction ratio of cold rolling is 30% is used as a criterion for determining the quality of cold workability. But this
The reduction rate of 30% is a fairly low standard, and even if the reduction rate is higher, a titanium alloy excellent in cold workability is not shown.

Japanese Unexamined Patent Publication No. 3-166350 and Japanese Unexamined Patent Publication No. 3-166350
No. 274238 discloses a β-phase stabilizing element (Fe, Ni, Co, Cr,
It is shown that titanium alloys which increase the β phase ratio by adding Mo, V) and achieve high cold workability and strength improvement by solid solution strengthening. The inventors of the present invention have disclosed in Japanese Patent Laid-Open No. 5-117791.
In the publication, Nb is added to Ti-3Al-2.5V alloy in an amount of 0.1% or more and 10.0% or less, and a titanium alloy having high strength and high toughness while maintaining good cold workability is disclosed. However, in these inventions, the corrosion resistance and weldability have not been clarified.

[0010]

Titanium alloys are expected to be used as structural materials for corrosive environments by taking advantage of the characteristics of high corrosion resistance and high specific strength. However, it is hard to say that many titanium alloys have sufficient corrosion resistance to withstand use in harsh corrosive environments. Further, when a titanium alloy is used as a structural material because it is generally poor in cold workability and weldability, it is necessary to manufacture a product by machining from a state of hot finish. Therefore, the product yield is lowered and the cost is increased.

The present invention provides a highly corrosion resistant titanium alloy excellent in cold workability and weldability in order to improve the product yield when manufacturing structural members from a titanium alloy and reduce the production cost. It was made for the purpose.

The specific goals are as follows.

The cold workability should be Ti-3Al-2.5V alloy or more in the cold workability test described later.

The 0.2% proof stress of the welded portion at room temperature showing weldability is 600 MPa or more, and the 0.2% proof stress of the base material portion at room temperature is also 600 MPa or more.

The fracture strength of the weld at room temperature is 90% or more of that of the base metal.

From the practical aspect of application to structural materials, a lower limit is set for the elongation indicating weldability, and the tensile elongation of the welded portion at room temperature is 5.0% or more. Is also 10.0% or more.

The corrosion weight loss is determined by the corrosion resistance test described later.
0.5mm / year or less.

[0018]

[Means for Solving the Problems] The present inventors have examined the effects of various additive elements on cold workability, strength, weldability and corrosion resistance, and have made it possible to improve the corrosion resistance and cold workability, strength and weldability. We have developed a balanced titanium alloy.

The gist of the present invention lies in the following titanium alloys.

% By weight, Al: 1.0% to less than 3.0%, V:
1.5-4.5%, Mo: 0.1-6.0% and Ni: 0.1-1.5
%, The balance is Ti and unavoidable impurities, and is a highly corrosion resistant titanium alloy with excellent cold workability and weldability.

[0021]

The titanium alloy of the present invention improves the corrosion resistance, cold workability, and weldability by adding Al, V, and Mo in several% to Ti and further adding Ni.

The reason for limiting the chemical composition as described above will be explained. Hereinafter,% means% by weight.

Al: 1.0% to less than 3.0% Al is an α-phase stabilizing element. Although it is contained for the purpose of solid solution strengthening and corrosion resistance improvement, in order to obtain the target strength and corrosion resistance, Al
Must be contained at 1.0% or more. On the other hand, if the Al content is
If it exceeds 3.0%, the cold workability deteriorates, so that the target cold workability of Ti-3Al-2.5V alloy or more cannot be achieved. Therefore, the range of Al content is set to 1.0% to less than 3.0%. The preferred range is 1.5 to 2.5%.

V: 1.5 to 4.5% V is a β-phase stabilizing element. Although it is contained for the purpose of solid solution strengthening, it is necessary to contain 1.5% or more to obtain the target strength. On the other hand, if the V content exceeds 4.5%, the cold workability and corrosion resistance deteriorate. Therefore, the range of V content is 1.5-
It was set to 4.5%.

The preferred range is 2.5-3.5%.
Further, V is an element that deteriorates weldability like Mo, but the degree thereof is smaller than that of Mo, and there is no problem if it is within the above range.

Mo: 0.1-6.0% Mo contributes to the improvement of cold workability, strength and corrosion resistance, but if the Mo content is less than 0.1%, its effect is small. However, Mo is a refractory metal, and if it is contained in a large amount, segregation may occur, elongation may decrease, and weldability may deteriorate.
% Should be kept below. Therefore, the range of the Mo content is set to 0.1% to 6.0%. The preferred range is 0.8 to 3.0%.

Although V and Mo may be added as simple metals, they are Al-V master alloy, Al-Mo master alloy or Al-Mo-.
You may add in the form of V mother alloy.

Ni: 0.1 to 1.5% Ni is an element that contributes to the improvement of strength and corrosion resistance even in a small amount. If the Ni content is less than 0.1%, the effect is small. On the other hand, if it exceeds 1.5%, Ti ₂ Ni, which is an embrittlement phase, is formed, and cold workability is significantly deteriorated. For this reason, the Ni content range was 0.1 to 1.5%. 0.2-0.8% preferred
Is the range.

Others: Inevitable impurities include C, H,
It refers to O, N, Fe, Y and the like, but there is no particular problem as long as it is a normal content level.

O also has a large degree of solid solution strengthening, and even a small amount contributes to the improvement of strength, but on the other hand, it deteriorates cold workability.
Therefore, the O content is preferably 0.2% or less.

With the above chemical composition, it is possible to obtain a titanium alloy having the above-mentioned target strength, elongation, cold workability, weldability and corrosion resistance.

The alloy of the present invention can be applied to a structural material for corrosive environment due to these characteristics, and particularly as a material for umbilical tube which is required to have weldability, strength, cold workability and corrosion resistance. It is suitable. An umbilical tube is a control pipe that connects an oil field on the sea floor and a platform on the sea, and an electric wire for controlling hydraulic pressure is passed through the pipe.

[0033]

EXAMPLE Alloys having the compositions shown in Tables 1 and 2 were melted by a vacuum melting method to obtain a cylindrical ingot having a diameter of 120 mm and a length of 300 mm. Alloy Nos. 1 to 11 in Table 1 are examples of the present invention, Alloys Nos. 12 to 20 in Table 2 are comparative examples having compositions outside the scope of the present invention, and Alloys Nos. 21 and 22 in Table 2 are existing cold workable Pure Ti, Ti-3Al
-2.5V alloy.

[0034]

[Table 1]

[0035]

[Table 2]

After heating these ingots to 1100 ° C., they are forged at 850 ° C. or higher, and the thickness is 60 mm × width 70 mm × length
It has a 680 mm square shape. After that, hot rolling (900 ℃ heating, 7
(Up to 50 ° C) was applied to form a plate material having a thickness of 15 mm and a width of 70 mm, and then annealed at 750 ° C.

In the evaluation of hot workability, the hot workability was poor when the crack was caused by the above-mentioned hot forging and hot rolling, and "Good" when the crack was not caused.

From each of the test materials thus obtained, a cold rolling test piece (thickness 12 mm × width 35 mm × length 200 mm), a cold upsetting test piece (φ6 mm × length 9 mm) and hydrochloric acid A corrosion test piece (thickness 10 mm x width 20 mm x length 20 mm) was cut out and subjected to each test of cold workability and corrosion resistance.

Further, TIG welding (in Ar atmosphere, one pass on each side of both sides, a welding rod was also cut out from the sheet material after hot rolling, to a test material (thickness 4 mm × width 70 mm) cut out from the sheet material after hot rolling. Test piece), a plate tensile test piece having dimensions of parallel part: thickness 2 mm x width 6.25 mm x length 32 mm was cut out from the welded part and the base metal part, and the tensile strength was measured.

The alloys of the present invention having the compositions shown in Table 3 were melted and hot worked by the same method as described above, and the cold working test and the corrosion resistance test were carried out. Furthermore, in order to check the thermal stability of the base material, a test piece (parallel part size: thickness 2 mm x width 6.25 mm x length 32 mm) cut out from the sheet material after hot rolling was used, and the room temperature and high temperature were used. The tensile test was carried out.

[0041]

[Table 3]

Next, the detailed contents of each test will be described.

[Cold Rolling Test] Thickness 12 mm × width 35 mm × length
The 200 mm test piece is reduced by 0.5 mm per pass with a roll, and the limit cold working is performed when edge cracks with a length of 3 mm or more from the plate edge occur at intervals of 5 cm or less along the plate side surface. Was calculated according to the following formula (1).

[0044]

[Equation 1]

[Cold upsetting processing test] φ6 mm × length 9
The above-mentioned test piece of mm is subjected to cold upsetting by a processing master tester at a strain rate of 1.0 × 10 ^-2 sec ^-1 ,
The workability at which cracks occurred was defined as the limit cold workability. In addition,
This upsetting test is done three times for each alloy,
The average value was used as the limit cold workability and calculated according to the following equation (2).

[0046]

[Equation 2]

[Hydrochloric acid corrosion test] Thickness 10 mm x width 20 mm x length
The 20 mm test piece was immersed in a 2 wt% HCl boiling solution for 20 hours to measure the corrosion weight loss. The corrosion weight loss was converted into mm / year to evaluate the corrosion resistance.

[Tensile test] Parallel part dimension cut out from the base metal part and the welded part: 0.5% strain / min up to 0.2% proof stress, after proof stress on the above test piece of thickness 2 mm x width 6.25 mm x length 32 mm Is 15%
Weldability was evaluated by applying a tensile rate of strain / min to rupture and measuring the strength. This tensile test was performed at room temperature to compare strength and elongation.

[High temperature tensile test] Dimension of parallel part: thickness 2 mm x
Using the test piece of width 6.25 mm × length 32 mm, at room temperature and
It was carried out at 400 ° C.

The results of the above evaluation tests are shown in Table 4, Table 5 and Table 6.

[0051]

[Table 4]

[0052]

[Table 5 (1)]

[0053]

[Table 5 (2)]

[0054]

[Table 6 (1)]

[0055]

[Table 6 (2)]

The following are clear from the results of Tables 4-6.

According to the results of Alloy Nos. 1 to 3 and Nos. 12 and 13, Al greatly contributes to the strength improvement by solid solution strengthening, but the effect is small when the content is small (in Alloy No. 12, , Al is 0.5% and the strength is low). Further, excessive Al content deteriorates cold workability (Alloy No. 13 also has 4.
0% and poor cold workability).

According to the results of alloys Nos. 4 and 5 and Nos. 14 and 15, V also contributes to the strength improvement by solid solution strengthening. However, if the content is small, the effect is small (in Alloy No. 14, V is 0.5% and the strength is low). Also, excessive V content deteriorates cold workability (alloy No. 15 has the same problem.
It is 6.0%, and the cold workability is lower than that of Ti-3Al-2.5V alloy).

According to the results of Alloys Nos. 6 and 7 and Nos. 16 and 17, Mo contributes to the improvement of cold workability, strength and corrosion resistance. However, according to the results of Alloy No. 16, when the Mo content is less than 0.1%, the contribution to the improvement of the above-mentioned three types of properties is small. Further, excessive Mo content causes a decrease in weldability, which is not preferable (alloy No. 17
Then, Mo is 8.0% and the growth is low).

According to the results of Alloys No. 8, 9 and No. 18, although oxygen greatly contributes to the improvement of strength, it deteriorates cold workability and weldability. However, there is no problem with the normal amount (0.2% or less).

According to the results of Alloys Nos. 10 and 11 and Nos. 19 and 20, Ni greatly contributes to the improvement of strength and corrosion resistance.
If its content is too small, its contribution is small (for example,
Alloy No. 19 has Ni: 0.05%). Further, excessive Ni content significantly deteriorates cold workability, and even hot working is difficult (for example, alloy No. 20 has Ni: 2.5%). However, within the range defined by the present invention, without impairing the cold workability,
It has the effect of improving strength and corrosion resistance.

Existing pure Ti and Ti-3Al-of alloy Nos. 21 and 22
The 2.5V alloy has lower strength and poorer corrosion resistance than the alloy of the present invention.

Alloys Nos. 23 to 25 all exhibit the same good cold workability and corrosion resistance as the base metal part of the alloy of the present invention. The tensile elongations at 400 ° C. all show 20% or more, and the titanium alloy of the present invention is thermally stable.

[0064]

The alloy of the present invention is a high strength and high corrosion resistance titanium alloy excellent in cold workability and weldability (welding strength). Since it has good weldability, it is possible to manufacture, at low cost, titanium alloy tube materials suitable for use in corrosive environment atmospheres in fields such as chemical industry, energy development, and general industry, without using mechanical cutting. .

Claims (1)

[Claims] 1. By weight%, Al: 1.0% to less than 3.0%, V:
1.5-4.5%, Mo: 0.1-6.0% and Ni: 0.1-1.5
%, The balance is Ti and unavoidable impurities, and is a highly corrosion resistant titanium alloy with excellent cold workability and weldability.