JPH05117791A - High strength and high toughness cold workable titanium alloy - Google Patents

Abstract

PURPOSE:To provide a cold workable alpha,beta-Ti alloy having excellent strength- toughness balance. CONSTITUTION:This Ti alloy is an alpha,beta-Ti alloy consisting of 1. 5-4.0% Al, 1.5-4.0% V, 0.10-10.0% Nb, <=0.30% Fe, <=0.30% O and the balance Ti. At the time of production, heat treatment is carried out at 350-850 deg.C for >=15min after >=20% cold working. This alloy can be finished to a product shape by cold working and a mechanical working process can considerably be shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a titanium alloy used in the fields of chemical industry and energy development or as a structural material for general industrial use, and particularly to a titanium alloy having high strength and high toughness and capable of being cold worked.

[0002]

2. Description of the Related Art Titanium alloys have been conventionally used as aerospace materials because of their high specific strength (strength / specific gravity). However, in recent years, the demand for materials for oil wells that can withstand harsh environments and the motivation for reviewing automotive materials for the purpose of weight reduction have increased, and these energy development fields and general industrial structural materials, etc. The application of the titanium alloy is getting attention.

Titanium alloys are roughly classified into α type and α type.
/ Β type, β type, but high strength and toughness,
The α / β type titanium alloy is most representative of the titanium alloys because of its excellent thermal stability and rich practical data.

Ti-6 is a practical alloy of α / β type titanium alloys.
Al-4V, Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-4Zr-2Mo, Ti
-3Al-2.5V, Ti-8Al-1Mo-1V, etc., but when applied to non-aeronautical / space applications, simply having a high specific strength is not sufficient, and in relation to other required performance, These practical alloys cannot be used as they are, and various improvements have been made.

For example, the applicant has added 0.02 to 0.20% of one or more platinum group elements to the α / β type titanium alloy, or 0.005 to 0.12% of the platinum group element to the α / β type titanium alloy, and 0.05 to 2.0 for one or more of Ni, Co, W and Mo
%, It has been proposed to improve the corrosion resistance of titanium alloys in harsh oil well environments (Japanese Patent Laid-Open No. 63-
114931).

[0006]

However, even if these performances are improved, most α / β type titanium alloys have the drawback of poor cold workability, which results in poor dimensional accuracy. After finishing to a general shape by hot working, there is no choice but to machine into a product shape over a long period of time.
There was a fundamental problem that the cost was high.

However, among the α / β type titanium alloys, Ti-3Al
-2.5V is known to be an exceptionally cold workable alloy. This alloy is standardized to AMS4943 and AMS4944, the basic composition of which is 2.5-3.5% Al,
V is 2.0 to 3.0%, Fe is 0.30% or less, and O is 0.12% or less, which is an alloy mainly used for hydraulic piping of aircraft. With this alloy, the product shape can be finished by cold working, and the machining cost, which is the main factor of the high cost, can be greatly reduced. However, compared with other α / β type titanium alloys, the alloy elements are less, so the strength is low, and the toughness deteriorates even when strengthened by work hardening, and an excellent balance between strength and toughness values cannot be obtained. It could not be said that it has desirable properties for use in the field of energy development and structural materials for general industry.

An object of the present invention is a titanium alloy which can be cold worked and has an excellent balance of strength and toughness.
Specifically, Ti-3, which is a conventional cold-workable α / β alloy
It is to provide a titanium alloy having a strength-toughness value balance superior to that of Al-2.5V.

Further, as a further object of the present invention,
It has cold workability such that cracks do not occur even in cold working of 50% or more, 0.2% proof stress at room temperature is 90.0 kgf / mm ² or more,
In addition, the Charpy impact value at room temperature (25 ° C) is the toughness value.
It is intended to provide a titanium alloy which is 4.0 kg-m / cm ² or more, which can be cold worked, and which has an excellent balance of strength and toughness values.

[0010]

Means for Solving the Problems The inventors of the present invention have made various studies in order to solve such problems, and have obtained the following findings. (a) Strength and toughness after cold working while maintaining cold workability of Ti-3Al-2.5V by adding Nb to the alloy of Al and V composition near Ti-3Al-2.5V We have found that we can improve the balance of values.

(B) 0.02 to 0.20% of at least one platinum group element is added to the α / β type titanium alloy, or 0.005 to 0.12% of the platinum group element and Ni is added to the α / β type titanium alloy. 0.05% to 2.0% in total of one or more of Co, W, and Mo
Even if added, it exhibits excellent corrosion resistance even in an environment containing hydrogen sulfide while maintaining the above-mentioned balance between cold workability and excellent strength / toughness values.

The present invention is based on the weight% Al: 1.5.
~ 4.0%, V: 1.5 to 4.0%, Nb: 0.10 to 10.0%, Fe:
It is a titanium alloy having a high strength and a high toughness and being cold workable, which is composed of 0.30% or less, O: 0.30% or less, the balance Ti and inevitable impurities. The present invention, from another aspect, Al: 1.5 to 4.
0%, V: 1.5 to 4.0%, Nb: 0.10 to 10.0%, Fe: 0.30
% Or less, O: 0.30% or less, and platinum group element: 0.02 to 0.
It is a high-strength, high-toughness, cold-workable titanium alloy containing 20% and the balance Ti and unavoidable impurities.

The present invention provides, from another aspect, Al: 1.5-
4.0%, V: 1.5 to 4.0%, Nb: 0.1 to 10.0%, Fe: 0.3
0% or less, O: 0.30% or less, and platinum group element: 0.005
To 0.12% and at least one of Ni, Co, W and Mo in a total amount of 0.05 to 2.0%, and a high strength, high toughness and cold workable titanium alloy composed of the balance Ti and inevitable impurities. is there. Regarding the manufacturing conditions of the alloy according to the present invention, after 20% or more cold working is performed to achieve the above target values of strength and toughness, the temperature is set to 350 to 850 ° C for 15 minutes.
It is desirable to anneal for more than a minute.

[0014]

The reason for limiting the conditions in the present invention will be described below. (1) Al and V Al are α-phase stabilizing elements and the most commonly used additive elements in titanium alloys. On the other hand, V is a β-phase stabilizing element. These elements are added to the titanium alloy for the purpose of solid solution strengthening.

First, for strengthening, it is necessary to add Al of 1.5% or more and V of 1.5% or more. However, if the Al concentration exceeds 4.0%, the solid solution strengthening becomes remarkable and the cold workability deteriorates, so cracking occurs during cold working. Also V
If the content of Al exceeds 4.0%, the strength is remarkably increased, but the toughness is greatly reduced with this, and it becomes impossible to satisfy the target values of the strength and toughness values.

(2) Nb Nb is the most important element of the present invention, and has the effect of solid solution strengthening without deteriorating cold workability and enhancing the balance of strength and toughness values of titanium alloys. If the concentration of Nb is less than 0.10%, there is no strengthening effect, and if it exceeds 10.0%, the strength increases, but the toughness decreases, and the above target value cannot be satisfied.

(3) Fe, O Fe, O are elements added to improve the strength, and are usually
Fe is 0.30% or less and O is 0.30% or less. However, addition of these elements up to near the upper limit causes an increase in strength and promotes a decrease in toughness.
It is recommended that 0.20% or less and O be 0.20% or less.

Other unavoidable impurities are C,
H, N, Y, etc. are contained, and these are usually allowed to be included in the following range. C: 0.10% or less, H: 0.012
5% or less, N: 0.05% or less, Y: 0.005% or less (4) Platinum group elements (Pd, Ru, Rh, Os, Ir and Pt) These elements are all highly concentrated at high temperatures found in oil well environments. H ₂ S or CO ₂ is an effective component that has been required to prevent general corrosion in an environment containing Cl ₂ and Cl ions, and therefore one or more is added, but the effect is
When composite addition with Ni, Co, W and Mo is not made, it becomes clear when the total amount is 0.02% or more, and the larger the addition amount, the better the result obtained. However, even if added in a total amount of more than 0.20%, the above-mentioned effect will be saturated, resulting in costly mischief, so the total amount of platinum group elements added is
It was set at 0.02 to 0.20%.

On the other hand, when composite addition with any one or more of Ni, Co, W and Mo is made, the required addition amount of the platinum group element becomes smaller and the economical efficiency is further improved. That is, in this case, the effect of adding the platinum group element becomes clear by adding 0.005% or more. However, even if added in excess of 0.12%, the cost increase does not show the corresponding improvement in properties. Therefore, when complex addition with Ni, Co, W and Mo is made, platinum group elements are added. The total amount added is 0.005 ~
It was set at 0.12%.

(5) Ni, Co, W and Mo These components can be added in a complex amount to one or more platinum group elements, so that high concentrations of H ₂ S and CO ₂ at high temperature can be obtained.
And very effectively improve the corrosion resistance in an environment containing Cl ions, it has the same action as the platinum group element, but if the added amount is less than 0.05%, the desired effect cannot be obtained for the above action On the other hand, even if added over 2.0%, the corrosion resistance improving effect is saturated, so the total amount of one or more of Ni, Co, W and Mo added was set to 0.05 to 2.0%.

The method for producing a titanium alloy according to the present invention comprises:
In order to achieve the target values of strength and toughness, it is desirable to carry out the treatment under the following treatment conditions. I.e. cold
After processing 20% or more, heat treatment is carried out by holding in the temperature range of 350 to 800 ° C for 15 minutes or more and then air cooling. In addition,
The processing degree specified here is calculated by the following formula.

R = 100 × (A _o −A _f ) / A _o R: Workability (%) A _o : Cross-sectional area of material before processing A _f : Cross-section of product after processing Note that the workability in cold is If it is less than 20%, the strength target may not be achieved. On the other hand, if the heat treatment temperature is less than 350 ° C, the residual stress due to cold working is not removed, and the target of the toughness value cannot be achieved. On the other hand, if the temperature exceeds 800 ° C, the strength will be completely softened and the target of strength will not be achieved.

Further, if the holding time during the heat treatment is less than 15 minutes, the effect of the heat treatment may not be exhibited, the residual stress may not be removed, and the toughness may not be improved. On the other hand, the upper limit of the heat treatment time is not particularly limited, but considering economic reasons, it is desirable to set the upper limit to about 8 hours.

[0024]

Examples (Example 1) For the purpose of investigating the effect of added components on mechanical properties, titanium alloys having the components shown in Table 1 were melted and the mechanical properties were investigated. Impurities in this experiment were C = 0.05%, H = 0.
It was about 005%, N = 0.008%, and Y = 0.001%. As the experimental material, an ingot (outer diameter φ100 mm × height 200 mm) was prepared by skull melting under an argon atmosphere. The ingot was heated to 1100 ° C., forged to a width of 50 mm × thickness of 30 mm, reheated to 900 ° C., and hot-rolled to a width of 50 mm × thickness of 7 mm. The material after hot rolling was subjected to heat treatment at 850 ° C. for 1 hour to be completely annealed.

Next, after machining to a thickness of 6 mm for the purpose of removing scale generated by hot working, the thickness of 3 mm
Up to 50% cold rolling. Depending on the added component system, cracking may occur in this cold rolling. Those that had cracks of 1 mm or more from the edge of the product after cold rolling were excluded from the evaluation targets of mechanical properties because they had poor cold workability. In the evaluation of the cold rolling property, those having cracks were marked with "x", and those without cracks were marked with "○".

The material after cold rolling was heated to 550 ° C., held for 30 minutes, and then air-cooled for heat treatment. A plate-shaped test piece having a wall thickness of 3 mm, a width of 6.25 mm, and a gauge length of 25 mm parallel to the rolling longitudinal direction was taken from the heat-treated material, and the tensile properties were investigated at room temperature in accordance with ASTM. For the purpose of evaluating the toughness value, a JIS No. 1/4 size Charpy impact test piece (V notch) having a width of 2.5 mm in the rolling longitudinal direction was sampled and tested at 25 ° C. In this case, the sampling direction of the Charpy impact test piece was parallel to the rolling direction, and the crack propagation direction was perpendicular to the rolling direction.

Table 1 also shows the test results. The evaluation of the test results was conducted by paying attention to the 0.2% proof stress and the Charpy impact value.
0.2% proof stress is 90.0 kgf / mm ² or more, and Charpy value is 4.
When 0 kg-m / cm ² or more was achieved, it was evaluated as ○. From the results in Table 1, it can be seen that the components within the range specified by the present invention have sufficient cold workability and that the targets of strength and toughness are achieved.

(Example 2) Ti-3.2Al-2.6V-5.0Nb-0.10Fe-
Each element shown in Table 2 was added to the compounding component of 0.11 O as a basic component system. The production conditions of the material were the same as those of Example 1, and a notched four-point bending test piece was sampled from the heat-treated product in parallel with the material rolling direction, and a stress corrosion cracking test was performed.
The test conditions were as follows.

That is, the additional stress is the same as the 0.2% proof stress, and the test environment is 10 atmH ₂ S + 10 atmCO ₂ + 25% NaCl + 1 g.
The temperature was 232 ° C at / 1S. Under these conditions, the test piece was immersed in an autoclave for 6 months, and after taking out the test piece, the notch bottom was magnified 50 times and observed to examine the occurrence of cracks.

The mechanical properties were investigated by the same method as in Example 1. The results are shown in Table 2. From this result, Al, V, Nb, Fe, and O within the scope of the present invention are further included, and a platinum group element,
Alternatively, it can be seen that the alloy of the present invention containing an appropriate amount of platinum group element and Ni, Co, W, Mo and the like has high strength and toughness and can secure good stress corrosion cracking resistance.

(Example 3) Ti-3.2Al-2.6V-5.0Nb-0.10Fe-
Using a 0.11O alloy, a vacuum arc melted 150 kg ingot (outer diameter φ300 mm) was heated to 1150 ° C and then forged to a width
50mm x thickness 30mm. After further heating to 900 ° C, it was hot-rolled to a width of 50 mm and a thickness of 12 mm. A cold rolled material with a thickness of 10 mm was sampled from the material after rolling, the cold working degree was changed to 70%, and the effect of the cold working degree was investigated. The cold workability investigated is shown in Table 3.

All the heat treatment conditions after cold rolling are 500 ° C. ×
30 minutes. The test piece collecting method and the evaluation method for evaluating the mechanical properties are the same as in Example 1. However, the thickness of the tensile test piece was 2 mm. The evaluation results of mechanical properties are also shown in Table 3. Table 4 shows the conventional alloy Ti-3.2Al-
The mechanical properties of the 2.6V-0.10Fe-0.11O alloy processed under the same conditions as in this example are shown.

In FIG. 1, based on the data in Tables 3 and 4,
The results of comparing the strength / toughness balance of the alloy of the present invention and the conventional alloy are shown. The numbers in the figure represent the cold workability. Figure 1
From this, it is understood that the strength / toughness value balance of the alloy of the present invention is superior to that of the conventional alloy. Further, the alloy of the present invention satisfies the target of strength / toughness values when cold working is carried out at a working ratio of 20% or more. On the other hand, with conventional alloys,
It can be seen that the target values of strength and toughness cannot be satisfied even if the cold workability is adjusted.

Example 4 A heat treatment condition was examined for a material cold-worked by 50% to a thickness of 5 mm. Table 5 shows the heat treatment conditions examined. The alloy composition, the manufacturing history up to cold working, and the evaluation method are the same as in Example 3. The evaluation results of mechanical properties are also shown in Table 5.

Table 6 shows that the conventional alloy Ti-3.2Al-
The mechanical properties of the 2.6V-0.10Fe-0.11O alloy when heat-treated under the same conditions as in this example are shown. FIG. 2 shows the results of comparing the strength / toughness value balances of the alloy of the present invention and the conventional alloy based on the data of Table 5 and Table 6. The white circles show the results in Table 5,
The black circles represent the results in Table 6.

From FIG. 2, it is understood that the strength / toughness value balance of the alloy of the present invention is superior to that of the conventional alloy. Also,
The alloy of the present invention satisfies the target of strength / toughness values when heat-treated in the temperature range of 350 to 800 ° C. and the time of 15 minutes or more. On the other hand, it can be seen that conventional alloys cannot satisfy the strength / toughness targets regardless of heat treatment conditions.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

[Table 5]

[0042]

[Table 6]

[0043]

According to the present invention, a cold workable titanium alloy used as a structural material having high strength and high toughness is provided, and a structural material made of a titanium alloy can be finished by cold working. Become. As a result, the machining cost for finishing the product shape, which is the main factor of cost increase, which was an obstacle to practical use when using titanium alloys in non-aviation and space fields, can be significantly reduced. Development field,
It is considered that titanium alloying of structural materials for general industry will be greatly promoted in the future.

[Brief description of drawings]

FIG. 1 is a comparison diagram of the strength / toughness value balance between the alloy of the present invention and the conventional alloy when the cold workability is changed.

FIG. 2 is a comparison diagram of the strength / toughness value balance between the alloy of the present invention and the conventional alloy when the heat treatment conditions are changed.

Claims (3)

[Claims] 1. Al: 1.5-4.0% and V: 1.% by weight.
5 to 4.0%, Nb: 0.10 to 10.0%, Fe: 0.30% or less, O:
A high-strength, high-toughness, cold-workable titanium alloy, characterized by comprising 0.30% or less and the balance Ti and inevitable impurities. 2. A platinum group element: 0.02% by weight.
The high-strength, high-toughness cold-workable titanium alloy according to claim 1, wherein the content is 0.20%. 3. Platinum group element: 0.005 by weight%
To 0.12% and at least one of Ni, Co, W, and Mo in a total amount of 0.05 to 2.0%, and high strength and high toughness and cold workable titanium according to claim 1. alloy.