JPH04301044A - High toughness titanium alloy capable of cold working - Google Patents

Abstract

PURPOSE:To obtain a Ti alloy well balanced between strength and impact value by properly combining the proportions of components in an alpha plus beta Ti alloy, cold working conditions, and heat treatment conditions. CONSTITUTION:An ingot of an alpha plus beta Ti alloy of Ti-Al-V-Ni type having a composition consisting of, by weight, 2.0-4.0% Al, 1.5-3.0% V, 0.1-1.5% Ni, and the balance Ti is formed into a plate by means of hot forging and hot rolling, finished into a sheet of final thickness by means of cold working, such as cold rolling, at >=20% draft, and successively annealed at 350-800 deg.C for >=15min. By this method, the alpha plus beta Ti alloy having >=70.0kgf/mm<2> 0.2% yield strength at room temp., also having >=5.0kgf/mm<2> Charpy impact value at 25 deg.C room temp. at the time of evaluating toughness value by the Charpy impact test, and excellent in cold workability as well as in strength and toughness can be obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to titanium alloys, particularly α+β type alloys, used in the chemical industry and energy development fields, or as general industrial structural materials.

0002

[Prior Art] Ti alloys currently used as general titanium alloys include industrial pure Ti, α type, α+β type, and
Various types of titanium alloys are known, such as titanium alloys of α+β type and α+β type. α+β type titanium alloys include Ti-6Al-4V,
Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al
-2Sn-4Zr-2Mo, Ti-3Al-2.5V,
Ti-8Al-1Mo-1V is known.

As described above, various alloying elements are known for α+β type titanium alloys, among which the following are examples of Ni added to titanium alloys. a) 0.02 to 0.2% platinum group element or 0.005 to 0.1% platinum group element in α+β type titanium alloy
2% and one or more of Ni, Co, W, and Mo.
A titanium alloy with 0.05 to 2.0% added to improve corrosion resistance in harsh oil well environments (Japanese Patent Application Laid-Open No. 63-114981)
No.). b) An alloy characterized by adding 2% of Co, Fe, Ni, and Cr to improve the superplastic properties of the Ti-6Al-4V alloy (METALLURGICAL TR
ANSACTION A, Vol. 14A, Dec.
1983, pp. 2535-2544).

In addition, Ti, which is an α+β type titanium alloy,
-3Al-2.5V alloy is known as a cold-workable alloy, and its basic composition is Al of 2.5 to 3
．． 5%, and V is 2.0 to 3.0%. This alloy is standardized by AMS, specifically AMS 4943
A has standards as an annealed material. According to this document, after cold working, annealing is prescribed at 593 to 788°C for 15 to 60 minutes. In addition, AMS 4944 has a standard for cold-worked materials, and according to this, after cold working, 370
It is specified that stress relief annealing should be performed at a temperature of 30 minutes or more at a temperature of 0.degree. C. or higher. However, the above two standards do not specify the degree of cold work.

[0005]

[Problems to be Solved by the Invention] Titanium alloys are used as structural materials by taking advantage of their characteristics of high strength and low specific gravity. However, since titanium alloys have low toughness, when used as structural materials, their resistance to cracking becomes weak. Therefore, when using a titanium alloy as a structural material, it is necessary to pay attention not only to improving the strength but also to improving the toughness value. However, to date, it is unclear how to improve toughness while ensuring strength.

[0006] One way to solve this problem is to strengthen the process by cold working, but in the previously known Ti-3Al-2.5V alloy, which is a cold-workable alloy, after cold working, The toughness is significantly reduced, and although the strength target is met, the result is unsatisfactory in terms of toughness. It is herein a general object of the present invention that cold workable
An object of the present invention is to provide a titanium alloy with an excellent balance of strength and toughness.

A more specific object of the present invention is to have a 0.2% proof stress of 70.0 kgf/mm2 or more at room temperature, and a Charpy impact value of 5.0 kgf/mm2 at room temperature (25°C) when the toughness is evaluated by Charpy. An object of the present invention is to provide a titanium alloy having excellent strength and toughness of 0 kgf/mm2 or more.

A further object of the present invention is to provide a titanium alloy with an excellent balance of strength and toughness by optimally combining the component proportions of the titanium alloy, cold working conditions, and heat treatment conditions. It is.

[0009]

[Means for Solving the Problems] The present inventors have conducted various studies in order to solve the above problems, and have obtained the following knowledge. (a) Ti-, which is known as a cold-workable alloy
When a 3Al-2.5V alloy is subjected to cold working, its strength is improved by cold working, but when the strength is strengthened by increasing the degree of cold working, the toughness decreases as the strength increases. (b) Evaluating the toughness of the above titanium alloy,
As a result of observing the fracture mode, it was found that fine cracks occur in front of the main crack that propagates during the fracture, and these cracks connect and develop into cracks. Furthermore, the location where fine cracks occur is at the interface between the α and β phases. Therefore, if it is possible to increase the resistance to fracture cracking, it is possible to improve the toughness of α+β type titanium alloys. That is, by strengthening the interface between the α phase and the β phase, it is possible to increase the resistance to the generation of minute cracks, and it is possible to improve the toughness.

(c) On the other hand, Ni is known as a so-called β-stabilizing element that dissolves solidly in the β phase of titanium alloys. In order to strengthen the interface between the α phase and β phase, Ni
It is effective to strengthen the interface between the α phase and the β phase by adding a small amount of and performing appropriate heat treatment if necessary, thereby increasing the crack propagation resistance, that is, the toughness value. (d) In addition, when cold working is performed for the purpose of strengthening work, if the heat treatment conditions after cold working are appropriately controlled, there is no decrease in toughness and the above-mentioned targets for strength and toughness can be achieved.

[0011] Here, in the present invention, Al is 2% by weight.
．． 0 to 4.0%, V 1.5 to 3.0%, Ni
It is a titanium alloy with a high toughness value and good cold workability, containing 0.1 to 1.5% of Ti and the remainder consisting of Ti and unavoidable impurities. In addition, regarding the manufacturing conditions of the alloy according to the present invention, in order to achieve the above-mentioned goal,
After 0% or more processing, 350 ℃ or more 800 ℃
It is desirable to anneal at the following temperature for 15 minutes or more.

0012

[Operation] Next, reasons for limiting the alloy composition and suitable processing conditions in the present invention will be described. (1) Al, V Al is an α-phase stabilizing element in titanium alloys and is the most commonly used additive element. On the other hand, V is a β-phase stabilizing element. These elements are added to titanium alloys for the purpose of solid solution strengthening. First, in order to strengthen, Al is 2.0
% or more, and it is necessary to add V in an amount of 1.5% or more. However, when the Al concentration exceeds 4.0%, solid solution strengthening becomes significant and cold workability decreases, resulting in cracking during cold working. Further, when V is contained in an amount exceeding 3.0%, the strength increases significantly, but the toughness decreases accordingly, making it impossible to satisfy the above-mentioned target values for strength and toughness.

(2) Ni If the concentration of Ni is less than 0.1%, the effect of improving the toughness value will not be exhibited. Also, if the content exceeds 1.5%, Ti2N
Since the intermetallic compound of i is precipitated and causes embrittlement, it becomes impossible to improve the toughness value. Other unavoidable impurities include Fe, C, H, O, N, Y, etc., and these are normally allowed to be included within the following ranges. Fe: 0.3% or less, C: 0.10% or less, H: 0.
0125% or less, O: 0.20% or less, N: 0.0
5% or less, Y: 0.005% or less It is desirable that the manufacturing conditions for the titanium alloy according to the present invention be within the following range.

[0014] That is, 20% or more of the material is cold-processed, and then it is processed at a temperature of 350°C or more and 800°C or less.
Add heat treatment by holding for 5 minutes or more and then cooling in air. Among the above, the working degree in cold working is given by the following formula. R=100×(Ao−Af)/AoR:
Degree of cold working (%) Ao: Cross-sectional area before working Af: Cross-sectional area of product after working Note that if the degree of cold working is less than 20%, the strength target will not be achieved. Furthermore, if the heat treatment temperature is less than 350 °C, the effects of cold working will remain and the toughness target will not be achieved;
At temperatures above 800°C, complete recrystallization occurs and the strength target cannot be achieved. Furthermore, the holding time during heat treatment is 15
If it is less than 1 minute, the effect of heat treatment will not be exhibited and the toughness target will not be achieved. For economic reasons, it is appropriate to limit the heat treatment time to about 8 hours.

[0015]

Example 1 In this example, titanium alloys having the composition ratios shown in Table 1 were melted and their mechanical properties were investigated. Each sample material weighs 1 kg.
An ingot (outer diameter 50 mm x height 110 mm) was prepared by skull melting in an argon atmosphere. After heating the ingot to 1100℃, it is 50mm wide x 30mm thick.
After processing by forging to 900° C., it was reheated to 900° C. and hot rolled to a width of 50 mm and a thickness of 7 mm. After hot rolling, the material was machined to a thickness of 6mm to remove scale generated during hot working, and then rolled to a thickness of 3mm.
50% cold rolling was applied until

[0016] Depending on the added component system, cracks may occur during this cold working. In this case, products in which cracks of 1 mm or more were generated from the edges of the product after cold working were considered to have poor cold workability and were excluded from the evaluation of mechanical properties. The material after cold rolling was heated to 550°C, held for 30 minutes, and air cooled. A plate-shaped specimen with a wall thickness of 3 mm in the longitudinal direction of the rolling direction, a width of 6.25 mm, and a gage distance of 25 mm was taken from the heat-treated material, and its tensile properties were investigated at room temperature in accordance with ASTM. In addition, for the purpose of evaluating the toughness value, a width of 2.5 mm in the longitudinal direction of rolling JIS No. 4 1/4
A Charpy impact test piece (V-notch) of the same size was taken and tested at 25°C. In this case, the direction in which the Charpy impact test specimens were collected was parallel to the rolling direction, and the direction of crack propagation was perpendicular to the rolling direction.

Table 1 shows the test results. The test results were evaluated by focusing on 0.2% proof stress and Charpy impact value.
A rating of ○ was given when the 0.2% proof stress was 70.0 kgf/mm2 or more and the Charpy value was 5.0 kgf/mm2 or more. From the results in Table 1, it can be seen that the targets for strength and toughness values were achieved with the components within the range defined by the present invention.

[0018]

[Example 2] In this example, Ti-3.2 Al-2.6
150 kg of V-0.6Ni alloy was melted in a vacuum arc, and the resulting ingot (φ300 mm) was
After heating to ℃, it was forged into a width of 50 mm and a thickness of 30 mm. After further heating to 900℃, it is hot rolled to a width of 50mm.
Finished with a thickness of 12mm. 10m thicker than the rolled material
A cold-rolled material of m was sampled and the degree of cold working was varied up to 70% to examine the influence of the degree of cold working. The heat treatment after cold working was annealing at 500°C for 30 minutes. Table 2 shows processing and treatment conditions. The method for investigating mechanical properties after heat treatment was the same as in Example 1. However, the thickness of the tensile test piece was 2 mm.

The results of the investigation on mechanical properties are also shown in Table 2. The evaluation method was the same as in Example 1. From this, it can be seen that when processing is carried out under the above-mentioned desirable manufacturing conditions, the strength and toughness each satisfy the target values. Further, the heat treatment conditions were examined on a material that had been 50% cold worked to a thickness of 5 mm. Table 3 shows the conditions studied. The method for evaluating the results was the same as in Example 1. The results are also shown in Table 3. This shows that the targets for strength and toughness are met when heat treated under the above-mentioned desirable manufacturing conditions.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

Effects of the Invention According to the present invention, a titanium alloy with high strength and high toughness that can be cold-worked and used as a structural material and a method for producing the same are provided, and the field of use of titanium alloys, especially α+β type titanium alloys, is expanded. greatly enlarged.

Claims (1)

[Claims] Claim 1: Al in weight% is 2.0 to 4.0
%, V 1.5 to 3.0%, Ni 0.1 to 1
．． 5%, the balance consisting of Ti and unavoidable impurities, a titanium alloy with high toughness values and good cold workability.