JPH02129331A - Beta-type titanium alloy having excellent cold workability - Google Patents

Abstract

PURPOSE:To obtain the title Ti alloy by adding specific amount of V to an Al-contg. Ti alloy having specific compsn. CONSTITUTION:The beta-type Ti alloy is constituted of, by weight, 15 to 25% V, 2 to 5% Al, 0.5 to 4% Sn, <=0.12% O and the balance Ti. The beta-type Ti alloy has low resistance to deformation as subjected to solution heat treatment, has excellent cold deformability and is provided with high strength after subjected to aging treatment. In the beta-type Ti alloy, structure can not be converted into the single phase of a beta phase even if subjected to solution heat treatment and is converted into the martensitic one in the case of <15% V content; while, in the case of >25%, the single phase of a beta-phase is obtd., but the age hardenability is deteriorated and the material cost is furthermore run up.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention has low deformation resistance in a solution-treated state;
This invention relates to a β-type titanium alloy that has excellent cold deformability and has high strength after aging heat treatment.

(Conventional technology) Titanium alloys have a low specific gravity and high strength, which makes them one of the highest specific strengths (strength/specific gravity) among practical metal materials. It has been advanced. However, recently, the use of titanium alloys has been expanding to general civilian materials such as materials for automobile parts and materials for medical devices. It's starting to look like this.

Titanium alloys generally have poor cold workability, and if cold workability is easy, product manufacturing costs will be low. Pure Ti, which has relatively good cold workability, lacks strength as a processed product and is difficult to apply to parts that require high specific strength.α+β type Ti, one of the most typical titanium alloys, -6Aj
! -4V has high strength as a processed product, but has extremely poor deformability and can only be manufactured by hot processing.
Manufacturing costs increase. For this reason, single-phase β-phase titanium alloys (β-type titanium alloys) with a body-centered cubic crystal structure with good cold workability are attracting attention, such as Ti-3A18.
V 6Cr-4Mo 4Zr, Ti 15V-
β-type titanium alloys such as 3Cr3Affi 3Sn are known.

β-type titanium alloys have good workability when subjected to solution treatment, and can be strengthened by aging treatment after processing to precipitate the α phase, making them desirable properties as materials for precision parts.

However, the previously known β
Mold titanium alloy has good deformability but extremely high deformation resistance, so when performing cold forging, for example, molds such as dies and punches often crack or chip. Because of its high temperature, seizure with rolls and dies is likely to occur even when performing cold rolling or cold wire drawing. One proposal to solve these problems is published in Japanese Patent Application Laid-Open No. 61-2501.
It is disclosed in Publication No. 38. However, the alloy disclosed herein uses only AN as an α-phase stabilizing element, and solid solution hardening due to Al is large in the solution state, and the hardness cannot be said to be sufficiently low. The temperature range for proper aging treatment that can result in high hardness is narrow, making manufacturing difficult.

(Problems to be Solved by the Invention) An object of the present invention is to eliminate the above-mentioned problems in conventional β-type titanium alloys and further improve its cold workability. Specifically, the tensile strength in the solution treated state is 7.
5kgf/+u+” or less (1 (ν hardness 240 or less),
The cold upsetting compression ratio is 80% or more, and a tensile strength of 120 kgf/mm” or more can be obtained by aging within 20 hours.
In addition, the appropriate temperature range for aging treatment is wide, and β is easy to manufacture.
The purpose of the present invention is to provide a single-phase titanium alloy.

(Means for solving the problems) The gist of the present invention is r weight%, V: 15-25%, A2:
2-5%, Sn: 0.5-4%, Oxygen: 0.12
% or less, the balance is Ti and unavoidable impurities, and the β-type titanium alloy has excellent cold workability.

Here, cold workability refers to a property that combines cold deformation resistance and cold deformability.

The alloying elements for changing the titanium alloy from the β phase and obtaining the metastable β phase at room temperature in a cooled state include ■, Mo,
There are Nb, Ta, Cr, Fe, Mn, etc. On the other hand, the properties generally required for β-type titanium alloys are: ■ Easy melting and low segregation; ■ Low specific gravity of added alloying elements; ■ Good hot workability; ■ Cold workability. ■ The solid solution hardening effect of the additive elements is small so that the lightness of Ti is not compromised by the β phase, ■ High strength can be obtained by aging, ■ The alloy itself It is cheap, etc.

In the present invention, (1) was adopted as an alloying element that satisfies the above (1) to (2). The reason is as follows.

Mo has a high specific gravity and melting point, Nb and Ta are expensive elements and will not form a β phase unless added in large quantities, Cr and Fe have a significant solid solution hardening effect and will not cause the alloy to excessively harden in the solution state. Although i is effective for hardening the alpha phase precipitated by aging, adding too much of it will cause hardening during cold working. Since the processing load would be high, a part of it was replaced with Sn, which has a small solid solution hardening effect, so as not to impair cold workability. Although Sn has a small solid solution hardening effect, it is useful for hardening the α phase during aging treatment and contributes to improving the stability and hardness of age hardening. In order to improve cold workability, the allowable upper limit was determined.

(Function) Below, the function and effect of the alloy components in the β-type titanium alloy of the present invention and the reason for limiting the content of each will be explained.

In addition, all the contents of alloy components are expressed in weight %.

(a) V: 15 to 25% (2) stabilizes the β phase by forming a solid solution in the titanium alloy base, forming a single β phase structure at room temperature, and improving cold workability.

However, if the content of (1) is less than 15%, even if solution treatment is performed, it will not be possible to form a single β phase, resulting in a martensitic structure. When the amount is more than 25%, the β-phase becomes a single phase, but the age hardenability is poor and the time required for aging treatment becomes longer. Moreover, if (2) is added excessively, the specific gravity will increase and the raw material cost will also increase, making it uneconomical.

As mentioned above, in addition to (2), β-phase stabilizing elements include Mo5Ta, Nb, Cr, Fe, Mn, etc. Among these, the β-phase single element is inexpensive and has low strength in the solution state. The elements forming the phase alloy are limited to MO and (2).

For alloys containing Cr, Fe, and Mn, the tensile strength in the solution state is 75 kgf/mm” or more (Hv240 or more).
As a result, deformation resistance increases. Among V and Mo, Mo has a high melting point and is difficult to dissolve, so segregation tends to occur, and M
Alloys containing o also have poor hot workability.As a result, the preferred β-phase stabilizing element in practical terms is acid, and for the reasons mentioned above, the content (2) is set at 15 to 25%.

(blA12-5% The titanium alloy of the present invention is a metastable β phase alone in the solution state, and when it is aged, the α phase precipitates and an increase in strength is obtained.α phase In order to obtain high strength through aging precipitation, it is effective not only to dispersion strengthen the α phase but also to strengthen the precipitated α phase itself.The most effective alloying element for solid solution strengthening of α titanium is A2. Furthermore, the addition of IiV has the effect of suppressing the precipitation of the ω phase, which makes the alloy brittle, and promoting the precipitation of the α phase.

The above-mentioned effect of Al1 is not noticeable when its content is less than 2%. On the other hand, AI! If the content exceeds 5%, the strength (hardness) in the solution treatment state increases and cold workability decreases. That is, the appropriate content of 1 is 2 to 5%.

(C) Sn: 0.5-4% Sn promotes and stabilizes the aging precipitation of the α phase and suppresses the formation of the ω phase, so it has the effect of widening the appropriate temperature range for aging treatment, and has the effect of widening the appropriate temperature range for aging treatment. increase the strength of moreover,
While Al1 solid-solution strengthens the alloy matrix, Sn does not harden the matrix very much, so reducing A2 and replacing it with Sn is effective in reducing deformation resistance.

Such an effect of Sn is poor at 0.5% or less;
%, an increase in the hardness of the substrate is unavoidable. Therefore,
The content of Sn is 0.5 to 4%.

In addition to the above alloy components, the remainder is substantially Ti.

The term "substantially Ti" means that impurities are inevitably included when industrially produced. However, in the alloy of the present invention, oxygen in the impurity is particularly suppressed as described below.

(d) Oxygen: 0.12% or less Oxygen is an α-phase stabilizing element, and if present in a large amount, it inhibits β-phase single phase formation, hardens the base material, and deteriorates cold workability. That is, it increases deformation resistance, reduces deformability, and causes cracks to occur during cold working. 0.1
If it is 2% or less, the negative effect is small, so 0.12
% or less.

Although Fe is a β-phase stabilizing element, it is harmful because it increases the hardness after solution treatment. It is desirable that the content be as low as possible, 0.3% or less.

Hereinafter, the characteristics of the titanium alloy of the present invention will be specifically explained using examples.

(Example 1) Using a vacuum melting furnace, a titanium alloy having the composition shown in Table 1 was melted and cast into an ingot of 140III1φ. This ingot was hot-forged and hot-rolled under normal conditions to a diameter of 20IIImφ, and then solution-treated (β
A sample material was prepared by heating and holding at a temperature of transus +20°C for 30 minutes and then cooling with water.

Regarding the above sample materials, the hardness and tensile strength after solution treatment were measured, and a compression test was conducted to evaluate the deformability. The hardness was measured using an ILV hardness meter. In addition, the compression test was performed on the 14mmφ×211111
A specimen of the same height was cut out, compressed using a smooth compression plate, and the deformability and deformation resistance were measured.

Table 1 shows the tensile strength, hardness, and cold compressibility limit (all average values of the measured values of 5 test pieces) of the test materials.
Note that the cold compressibility limit is the limit height that can be compressed without cracking when test specimen A in Table 1 is compressed (the first
From H) at the top of the table, 100 X (H, -H) /H,
(%).

Table 1 also lists the measured values of tensile strength and elongation after aging treatment. For 0-aging treatment, the aging temperature at which the tensile strength reaches the maximum is around 475°C for all β-type titanium alloys. Therefore, except for Nα14, the temperature was set to 475°C for 20 hours. (Since Nα14 is an α+β type titanium alloy, the heat treatment was performed by heating at 750°C → furnace cooling.) As shown in Table 1, the titanium alloy according to the present invention (floor 1
~7) All have a tensile strength of 75 kgf after solution treatment.
/mm'' or less, and the hardness is Hv240 or less, making it an extremely low-strength titanium alloy. Therefore, the deformability in the solution treatment state is extremely good, with all of them being 80% or more at the critical compressibility.

Among the comparative alloys (Nα8 to 11), N with a low content of ■
α8 is too strong because it does not form a β single phase structure, and AI
! , , Sn, or N with too high content of oxygen (0) (
19,1O111 also has a tensile strength of 80kgf/WIIlz
or more (hardness of 25011v or more), and the limit compression ratio has not reached 80%.

The comparative alloy Nα12 has too much ■ content,
In this case, the strength in the solution state is low and the critical compressibility is 8
It becomes 0% or more. However, this alloy requires an appropriate age hardening time of more than 50 hours to provide the final product with a strength of about 120 kgf/mm'', which is not practical.

Conventional Example No. 13 corresponds to the alloy disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 61-250138.

This generally has higher hardness and tensile strength during solution treatment than the examples of the present invention.

Figure 1 shows the inventive alloy (NIl12.7) and the comparative alloy (NIl12.7).
kll) and the conventional alloy (Nα14.15).

According to FIG. 1, the deformation resistance of the alloy of the present invention is the same as that of the conventional material Ti-6Al-4V (14) and Ti-15V3.
It is significantly smaller than Cr-3Al-3Sn (NCL15), indicating good cold workability. The large deformation resistance and low deformability of the comparative alloy (Nα11) is mainly due to the high oxygen content of 0.30%.

(Example 2) The age hardening characteristics of the present invention Shakinso 5 shown in Table 1 were investigated. As a comparison material, a conventional side stone 13 (Tr-22V-4Aj) which does not contain Sn and has the same level of other impurities was used.

The solution treatment conditions are (β transus +20°C)X3
0 minutes → water cooling.

FIG. 2 shows the relationship between aging treatment temperature, tensile strength, and elongation. The zero aging time was kept constant for 20 hours.

Ti 20V 4Aj2- of the invention alloy (Nα5)
Compared to Ti-22V-4A 1 alloy, ISn alloy has
Ti-
It has high strength exceeding 22V-4A2. In addition, it can be seen that the aging temperature range in which high strength is exhibited is wide, and that the material has stable aging characteristics. Further, the ductility is also equal to or higher than that of Ti-22V-4Al alloy.

C) Effect of the invention) The β-type titanium alloy of the present invention is similar to the currently used alloy (Ti-
22V-4Al, Ti-15V 3Cr 3Al
3Sn) has superior cold workability in a solution-treated state. Therefore, the life of the mold during cold forging is extended, and the occurrence of seizure with rolls and dies during cold rolling and wire drawing is reduced.
The alloy of the present invention, which greatly contributes to reducing the cost of manufacturing titanium parts, can be widely used in the field of daily necessities such as automobile valve train parts, aerospace parts, and eyeglass frames.

[Brief explanation of the drawing]

FIG. 1 is a deformation resistance curve obtained by a compression test for the sample materials used in the examples. FIG. 2 is a diagram showing the age hardening characteristics of the alloy of the present invention and a conventional β-type titanium alloy.

Claims (1)

[Claims] In weight%, V: 15-25%, Al: 2-5%, Sn:
A β-type titanium alloy with excellent cold workability, consisting of 0.5 to 4%, oxygen: 0.12% or less, and the remainder Ti and unavoidable impurities.