JPH06108187A - Nitrogen-added high strength titanium alloy - Google Patents

Abstract

PURPOSE:To provide a titanium alloy to obtain high ductility while maintaining a high strength by means of a single heat treatment without using complicated heat treatment, to reduce hot deformation resistance, and to give superior hot workability. CONSTITUTION:The nitrogen-added high strength titanium alloy having a composition where N, Al, and O are contained and they satisfy the relations of 0.06<=N<=0.20, by weight%, 2.0<=Al<=6.0, and 8.2<=Al+20XO+40XN<=14.6 and further either or both of V and Mo and one or more elements among Fe, Cr, and Mn are contained and they satisfy the relations of 6.2<=V+Mo+2X(Fe+Cr+Mn)<=12.1 and 1.0<=Mo and which has the balance Ti with inevitable impurities, is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen-added high strength titanium alloy which is excellent in manufacturability during hot forging.

[0002]

2. Description of the Related Art The strength of titanium material is increased by O (oxygen), A
It is made by adding elements such as l and V, and currently there are high strength titanium alloys with various compositions. Titanium alloys are not standardized in Japan, but industrial pure titanium is specified by the O and Fe contents according to JIS. It is known that the higher the content of these, the higher the strength. On the other hand, in the United States, many titanium materials including titanium alloys are standardized in ASTM and the like. However, N (nitrogen) is only defined as an unavoidable impurity. For example, Ti- which is a typical titanium alloy
In 6Al-4V, N is 0.05 wt% or less,
The same is true for most other titanium alloys, such as Ti-5Al-
In the 2.5Sn alloy, N is only slightly specified to be 0.07 wt% or less. As described above, there is no standardized titanium alloy in which N is positively added to increase the strength.

Japanese Patent Laid-Open No. 1-252747 attempts to increase the strength by positively adding N, O, C, etc. to pure titanium. However, the ductility deteriorates even if the absolute value of the strength is not high. It cannot be considered that there is a large advantage in mechanical properties as compared with a typical titanium alloy, Ti-6Al-4V alloy.

JP-A-2-173234 and JP-A-3-27
A high-strength titanium alloy to which alloy elements such as Al, V, Mo, and Fe have been added has been proposed for 42381, but N is considered to be an unavoidable impurity, and its amount is specified by the ASTM standard or the like. It is considered to be less than 05 wt%. Therefore, it is not a titanium alloy in which N is positively added to increase the strength. Further, the characteristics of these alloys are that the deformation resistance is small and the workability (superplastic elongation) is excellent at a temperature of 800 ° C. or lower, and it is considered that the optimum hot working temperature is 800 ° C. or lower. However, unlike superplastic processing, which has a very low strain rate during processing, in hot forging with a high strain rate,
At a processing temperature of 800 ° C. or lower, the titanium material itself has a large elastic modulus and a high repulsive force, resulting in splashes and the like, so that it cannot be said that the productivity is good.

On the other hand, the document Titanium A Te
chnical Guide (ASM INTERNA
TIONAL, Metals Park, 1988 P
As described in 62-P68, as a method for strengthening a titanium alloy, there is a method by heat treatment (solution treatment aging treatment). However, the solution aging treatment requires a heat treatment of 2
There are drawbacks such as that it has to be rotated, and that the strength obtained depends on the cooling rate after solution heat treatment, so that it is difficult to obtain high strength with a large member.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and provides excellent ductility while maintaining high strength by a single heat treatment (annealing) without using a complicated heat treatment. An object of the present invention is to provide a nitrogen-added high-strength titanium alloy that can be processed and has low hot deformation resistance and excellent hot workability.

[0007]

Means and Actions for Solving the Problems
Contains N, Al, and O, and these are in weight%, and satisfy the relationship of 0.06 ≦ N ≦ 0.20 2.0 ≦ Al ≦ 6.0 8.2 ≦ Al + 20 × O + 40 × N ≦ 14.6 In addition, one or more of V and Mo, F
Contains one or more of e, Cr, and Mn, and these are wt%
Then, a nitrogen-added high-strength titanium alloy characterized by satisfying the relation of 6.2 ≦ V + Mo + 2 × (Fe + Cr + Mn) ≦ 12.1 1.0 ≦ Mo, and the balance being Ti and inevitable impurities is provided.

The inventors of the present invention have conducted extensive studies to solve the above-mentioned problems, and as a result, found that N is effective as an element that causes a large increase in strength at room temperature and substantially reduces the deformation resistance during hot working. I found it. N has the effect of greatly increasing the β transformation point, and therefore the upper limit temperature of the α + β region of the N-added titanium alloy increases. Since hot working of α + β type titanium alloys is usually performed in the α + β range, an increase in the upper limit temperature, that is, the β transformation point substantially means an increase in the working temperature, and the material is softened by the temperature increase and the hot deformation resistance is descend.

Based on such findings of the inventors of the present invention, the present invention selects a composition of another alloying element that does not impair the effect of the above N addition while adding the N in a fixed amount and taking the following points into consideration. It has been completed.

1) Strength higher than Ti-6Al-4V alloy can be obtained after annealing heat treatment. To this end, since the strength is insufficient when N is added alone, other strengthening elements such as Al, O,
V, Mo, Fe, Cr, Mn are added together.

2) Extreme deterioration of ductility due to increase in strength does not occur. Therefore, not only the α-stabilizing element (N, Al, O) but also an appropriate amount of the β-stabilizing element (V, Mo, Fe, Cr, so as to form a mixed structure of α-phase and β-phase)
Mn) is added. Further, this makes the crystal grains finer and improves the ductility. 3) The β transformation point must be 900 ° C or higher. For this,
Consider the balance between α and β stabilizing elements.

When N is added, if a high melting point compound such as TiN is used, undissolved residue may occur during melting. Therefore, it is preferable to use low melting point chromium ferronitride or manganese nitride as a dissolution raw material for N addition. Hereinafter, the present invention will be described in detail. First, the reasons for limiting the amount of each element added will be described.

N is a very effective element for increasing the strength and increasing the β transformation point, but if added excessively, it causes an extreme decrease in ductility and toughness. And the amount of N is 0.06% by weight
If the amount is less than that, the effect will not be exhibited, and the decrease in ductility and toughness will increase if the amount exceeds 0.20% by weight. Therefore, the N content is specified to be 0.06% by weight or more and 0.20% by weight or less.

Al, like N, is an α-stabilizing element and is an element effective for increasing strength and increasing β transformation point, and a large increase in strength can be obtained by adding it together with N and O. Furthermore, it also plays an important role in the formation of α + β two-phase mixed tissue. However, if the amount of Al is less than 2.0% by weight, high strength cannot be obtained, and if it exceeds 6.0% by weight, ductility and toughness are significantly deteriorated. Therefore, the amount of Al is specified to be 2.0% by weight or more and 6.0% by weight or less.

Further, by collecting various experimental results on the relationship between the amount of α-stabilizing element and the strength / ductility, Al
The amount of α stabilizing element represented by + 20 × O + 40 × N (α
If s) is less than 8.2, the strength is not sufficiently high and 1
It was found that if it exceeds 4.6, the decrease in ductility becomes remarkable. Therefore, the value of Al + 20 × O + 40 × N is limited to 8.2 or more and 14.6 or less.

To obtain a fine α + β two-phase structure, β
It is necessary to add an appropriate amount of stabilizing element. In this case, N
In order to effectively achieve the high strength by the addition, at least one of β and all solid solution type V and Mo and at least one of eutectoid Fe, Cr and Mn are used as β stabilizing elements. It is necessary to add Since the eutectoid β-stabilizing element has a large effect of stabilizing the β phase having good workability, it plays an important role in preventing deterioration of ductility and toughness when strengthened. However, eutectoid elements cannot easily be added in large amounts because they tend to form brittle intermetallic compounds with Ti. For this reason, it is necessary to stabilize the β-phase by adding V and Mo, which hardly form a brittle intermetallic compound with Ti, in combination. Summarizing the effects of such β-stabilizing element addition from the experimental results, V + Mo + 2 × (Fe + Cr +
It was found that when the value of Mn) is less than 6.2, the ductility deteriorates, and when it is more than 12.1, the β transformation point is lowered too much. Therefore, V + Mo + 2 × (Fe + C
The value of r + Mn) is specified to be 6.2 or more and 12.1 or less.

Mo is an element which has a great effect on the refinement of the α + β two-phase structure. Therefore, in order to obtain sufficient ductility while maintaining high strength, it is effective to add Mo to refine the microstructure of the alloy. However, if the amount of Mo is less than 1.0% by weight, the effect cannot be substantially obtained. Therefore, the Mo amount is specified to be 1.0% by weight or more.

It is considered that a high melting point compound such as TiN is used for adding N to the titanium alloy. However, when such a high melting point compound is added, the compound remains unmelted and remains as an inclusion in the titanium alloy. there's a possibility that. To reduce this possibility, it is conceivable to use a lower melting nitrogen compound. At the same time, Fe, Cr, M
It is extremely efficient if n and the like can be added. Considering these points, it is preferable to use chromium ferronitride or manganese nitride as the raw material for adding nitrogen. The composition of the nitrogen compounds actually used was 34.9 wt% Fe and 5 respectively.
7.7 wt% Cr, 7.2 wt% N and 93.4 wt%
Mn is 6.6 wt% N, but the effect is not lost even if the composition changes a little.

As described above, according to the present invention, N, which is very effective for strengthening, is positively added, and the α-stabilizing element amount and the β-stabilizing element amount are optimized to obtain (1 ) High strength and high ductility, and (2) β transformation point is 900 ° C.
As described above, the titanium alloy which is easily hot forged is provided.

Therefore, high strength and sufficient ductility can be obtained by a single annealing heat treatment without using a complicated heat treatment. Further, during hot forging, the β transformation point is 900 ° C. or higher, so that hot working can be performed at high temperature, and the low deformation stress and high ductility provide excellent manufacturability.

[0021]

[Examples] Various titanium alloys shown in Table 1 were melted in a non-consumable argon arc furnace, forged in the β region (heating at 1100 ° C), and then hot rolled to a thickness of 10 mm in the α + β region (heating at 850 ° C). did. This rolled plate was annealed under the condition of heating at 720 ° C., holding for 1 hour, and then cooling in air, and used as a test material for testing the effect of additional elements on strength, ductility, and hot workability. did. In Table 1, alloy numbers 1 to 14 are examples included in the scope of the present invention, and alloy numbers 15 to 24 are comparative examples outside the range. Also, alloy number 2,
3,5,7,9,9,14,21,22,24 used ferro-chromium nitride or manganese nitride as a melting raw material. Note that αs in Table 1 indicates Al + 20 × O + 40 × N, and β
s represents V + Mo + 2 × (Fe + Cr + Mn).

[0022]

[Table 1] These test materials were subjected to a room temperature tensile test to evaluate the mechanical properties at room temperature, and to further understand their β transformation point and hot workability.

The room temperature tensile test was conducted with a parallel portion having a diameter of 6.25 m.
m and a gauge length of 25 mm were used.
The hot workability was investigated by measuring the diameter of the parallel part of 6.0 mm and the length of 32.
mm Tensile test piece was used to heat at 875 ° C. in vacuum and hot tensile test was performed at a strain rate of 10 s ⁻¹ to reduce the area (RA).
It was evaluated by. The β-transformation point of the titanium alloy was measured by collecting a round rod-shaped (diameter 3 mm) electrical resistance test piece from the rolled plate and determining the temperature at which the β-single phase occurs from the change in electrical resistance during continuous heating. The results are shown in Table 2. Alloy number 2
In 0 and 24, the β transformation point was 900 ° C. or lower, and thus hot workability was not measured.

[0024]

[Table 2]

As is clear from this table, alloy numbers 1 to 1
In Example 14 it was confirmed that the increase in strength was achieved by the addition of N without deterioration in ductility, and there was no deterioration in hot workability.

On the other hand, alloy No. 15 is Ti-6Al
-4V alloy, which is lower in strength and inferior in hot workability to the invention examples of the present application. Alloy No. 16 is Ti-6Al
-4V alloy is an example in which 0.10% by weight of N is added.
Although the strength increase due to the addition is recognized, the ductility (El, R) is increased because the microstructure is coarse due to the lack of the amount of β-stabilizing element (βs) and the addition of Mo.
It can be seen that A) and hot workability are deteriorated. Alloy Nos. 17 and 18 are examples in which the amount of N and the amount of α-stabilizing element (αs) are insufficient. In both cases, the amount of N is 0.05% by weight or less, so the strength is low. , The β transformation point is lower than 900 ° C. Alloy No. 19 is a case where the amount of N or the amount of αs is too large. Since the amount of N is 0.25% by weight, the ductility is remarkably reduced. Alloy Nos. 20 and 21 show the case where the Al amount is insufficient and the case where the Al amount is too large, respectively, and in the case of No. 20 having an Al amount of 1.5 wt%, the strength is low and the β transformation point is 900 ° C. or less. On the other hand, with No. 21 having an Al content of 7.0% by weight, it is confirmed that the strength is high but the ductility is low. Alloy No. 22 is an example in which the amount of Mo was insufficient, and when Mo was 0.5% by weight, the microstructure was not refined and the ductility was low. Numbers 23 and 24 respectively show the case where the βs amount is insufficient and the case where it is too large. It is confirmed that No. 23 having βs of 6.0 does not have sufficient ductility, and No. 24 having βs of 14.0 has a β transformation point lower than 900 ° C.

FIG. 1 shows tensile strength (TS) and elongation (El).
It shows the effect of the amount of N on. As is clear from this figure, it was confirmed that excellent strength and ductility can be obtained at the same time when the amount of N is 0.06% by weight or more and 0.20% by weight or less.

[0028]

EFFECTS OF THE INVENTION According to the present invention, the strength is increased by adding N and the amounts of α-stabilizing element and β-stabilizing element are optimized. It is possible to obtain high ductility while maintaining
A nitrogen-added high-strength titanium alloy having low hot deformation resistance and excellent hot workability is provided.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between N content and strength (TS) and ductility (El) for Examples and Comparative Examples.

Front page continuation (72) Inventor Chiaki Ouchi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. Containing N, Al, and O, and these are wt%.
Then, 0.06 ≦ N ≦ 0.20 2.0 ≦ Al ≦ 6.0 8.2 ≦ Al + 20 × O + 40 × N ≦ 14.6, and at least one of V and Mo, One or more of Fe, Cr, and Mn are contained, and the content of these is, by weight%, 6.2 ≦ V + Mo + 2 × (Fe + Cr + Mn) ≦ 12.1 1.0 ≦ Mo, and the balance is Ti and unavoidable impurities. A nitrogen-added high-strength titanium alloy, characterized in that 2. The nitrogen-added high-strength titanium alloy according to claim 1, wherein the N is added by using chromium ferronitride or manganese nitride as a melting raw material.