JP2669004B2 - Β-type titanium alloy with excellent cold workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is a β type which has a low deformation resistance in a solution-treated state, an excellent cold deformability, and a high strength after aging heat treatment. It relates to a titanium alloy.

(Prior Art) Titanium alloy has a small specific gravity and high strength, and therefore has an extremely high specific strength (strength / specific gravity) among practical metal materials. Therefore, it has been developed and put into practical use mainly as an aircraft material. I came. However, recently, materials for automobile parts,
The use of titanium alloys is expanding to general-purpose materials such as materials for medical devices, and accordingly, improvement of properties of titanium alloys and cost reduction have been strongly demanded.

Titanium alloys generally have poor cold workability. If cold working 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-6Al-4V, one of the most typical titanium alloys
Has a high strength as a processed product, but its deformability is extremely poor and it can be manufactured only by hot working, resulting in a high manufacturing cost. Therefore, a β-phase single-phase titanium alloy (β-type titanium alloy) having a body-centered cubic crystal structure with good cold workability has been attracting attention, and for example, Ti-3Al-8V-6Cr-4Mo.
Β-type titanium alloys such as -4Zr and Ti-15V-3Cr-3Al-3Sn are known. The β-type titanium alloy has good workability in the solution-treated state, and can be strengthened by subjecting it to an aging treatment and precipitating an α phase after the processing, and has a desirable property as a precision component material.

However, the above-mentioned β-type titanium alloys known so far have a good deformability but a very high deformation resistance, so that, for example, when performing cold forging, a die,
Dies such as punches are often cracked or chipped. Further, since the deformation resistance is high, seizure with a roll or a die easily occurs even when cold rolling or cold drawing is performed. One proposal for solving such a problem is disclosed in Japanese Patent Laid-Open No. 61-250138. However, the alloy disclosed herein uses only Al as the α-phase stabilizing element, and has a large solid solution hardening due to Al in a solutionized state, and cannot be said to have sufficiently low hardness. In addition, the temperature range of the appropriate aging treatment that may result in high hardness after aging is narrow, and manufacturing is difficult.

(Problems to be Solved by the Invention) An object of the present invention is to eliminate the above-mentioned problems in the conventional β-type titanium alloy and further improve the cold workability thereof. Specifically, in the state of solution treatment, the tensile strength
75kgf / mm ² or less (Hv hardness is 240 or less), cold upsetting compressibility is 80% or more, a tensile strength of 120kgf / mm ² or more can be obtained within 20 hours of aging treatment, and proper aging treatment It is an object of the present invention to provide a β-phase single-phase titanium alloy having a wide temperature range and easy production.

(Means for Solving Problems) The gist of the present invention is “V: 15 to 25% by weight, Al: 2.5 to 5% by weight”.
%, Sn: 0.5 to 4%, oxygen: 0.12% or less, and a balance of Ti and unavoidable impurities and excellent cold workability.

Here, the cold workability refers to a property combining cold deformation resistance and cold deformability.

V, Mo, N are alloy elements for obtaining a metastable β phase at room temperature in a state where a titanium alloy is rapidly cooled from the β phase.
b, Ta, Cr, Fe, Mn, etc. On the other hand, the properties generally required for β-type titanium alloys are that they are easy to melt and segregate little, the specific gravity of the additive alloy elements is small, the hot workability is good, and the cold workability is excellent. The small amount of β phase is obtained so that the solid solution hardening effect of the additive element is small and the lightness of Ti is not impaired, high strength is obtained by aging, and the alloy itself is inexpensive.

In the present invention, V is employed as an alloy element satisfying the above conditions. The reason is as follows. Mo has a high specific gravity and melting point, Nb and Ta are expensive elements, and if they are not added in large amounts, they do not form a β phase. Cr and Fe have a remarkable effect of solution hardening, and excessively harden the alloy in a solution state. Next, in order to harden the aged α phase, Al
Is effective, but since the effect of solid solution hardening during solution treatment is large, excessive addition increases the processing load during cold working, so part of it is replaced with Sn, which has a small solid solution hardening effect, The workability was not impaired. Although Sn has a small solution hardening effect, it is useful for hardening the α phase during aging treatment, and contributes to the improvement of the stability and hardness of age hardening. Next, paying particular attention to oxygen in the impurities, an allowable upper limit value thereof was set in order to improve cold workability.

(Function) The function and effect of the alloy components in the β-type titanium alloy of the present invention and the reasons for limiting the respective contents will be described below. In addition, the content of all alloy components is represented by wt%.

(A) V: 15 to 25% V forms a solid solution in a titanium alloy base material to stabilize the β phase, forms a β phase single phase structure at room temperature, and improves cold workability. However, when the V content is less than 15%, the β phase single phase cannot be obtained even if the solution treatment is performed, and the martensite structure is formed. When it is more than 25%, the β phase becomes a single phase, but the time required for the aging treatment with poor age hardening becomes long. On the other hand, if V is added excessively, the specific gravity increases, the raw material cost increases, and it is not economical.

As described above, as the β-phase stabilizing element, there are Mo, Ta, Nb, Cr, Fe, Mn, etc. in addition to V. Among them, β is inexpensive and has low strength in the solution state. The elements constituting the single-phase alloy are limited to Mo and V. In an alloy containing Cr, Fe, and Mn, the tensile strength becomes 75 kgf / mm ² (Hv 240 or more) in a solution state, and the deformation resistance increases. Of V and Mo, Mo has a high melting point and is difficult to dissolve, so segregation is likely to occur.
Alloys containing Ni also have poor hot workability. After all, V is the most preferable β-phase stabilizing element in practice, and its content is set to 15 to 25% for the above reason.

(B) Al: 2.5 to 5% The titanium alloy of the present invention is a metastable β phase simple substance in the solution-treated state, and when this is aged, the α phase is precipitated and strength is increased. is there. In order to obtain high strength by aging precipitation of the α phase, not only dispersion strengthening of the α phase but also strengthening of the precipitated α phase itself is effective. The most effective alloying element for solid solution strengthening of α-titanium is Al. The addition of Al also 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 Al does not appear remarkably when the content is less than 2.5%. On the other hand, if the Al content exceeds 5%, the strength (hardness) in the solution heat treated state increases and the cold workability deteriorates. That is, the proper content of Al is 2.5 to 5%.

(C) Sn: 0.5-4% Sn has the effect of widening the appropriate temperature range for aging treatment because it promotes and stabilizes the aging precipitation of α phase and suppresses the formation of ω phase, and the strength after aging treatment is strong. Higher. further,
While Al hardens the alloy base while Sn hardens the base hardly, it is effective to reduce Al and replace it with Sn to reduce deformation resistance. like this
The effect of Sn is poor at 0.5% or less, and if it exceeds 4%, an increase in hardness of the base material cannot be avoided. Therefore, the Sn content is 0.
5 to 4%.

In addition to the above alloy components, the balance is substantially Ti. Substantially Ti means that it is accompanied by impurities that are inevitably contained when industrially manufactured. However, in the alloy of the present invention, oxygen in impurities is suppressed as described below.

(D) Oxygen: 0.12% or less Oxygen is an α-phase stabilizing element. If it is present in a large amount, it inhibits β-phase from becoming a single phase, hardens the substrate, and deteriorates cold workability. That is, the deformation resistance is increased, the deformability is reduced,
This causes cracks in cold workability. 0.12%
If it is less than 0.1%, the adverse effect is small, so the content is set to 0.12% or less.

Although Fe is a β-phase stabilizing element, it is harmful because it increases the hardness after solution treatment. 0.3% or less, preferably as small as possible.

Hereinafter, the characteristics of the titanium alloy of the present invention will be specifically described with reference to examples.

(Example 1) Using a vacuum melting furnace, a titanium alloy having the composition shown in Table 1 was melted and cast into a 140 mmφ ingot. This ingot is subjected to hot forging and hot rolling under ordinary conditions to obtain a diameter of 20 mm, and then subjected to a solution treatment (β transus + 20 ° C).
The sample was prepared by heating it at 30 ° C. for 30 minutes and then cooling it with water.

With respect to the above-mentioned test materials, hardness and tensile strength after solution treatment were measured, and a compression test was performed in order to evaluate deformability. The hardness was measured with an Hv hardness meter. Further, in the compression test, a test piece having a height of 14 mmφ × 21 mm shown in Table 1 was cut out by cutting and compressed using a smooth compression plate, and the deformability and the deformation resistance were measured.

Table 1 shows the tensile strength, hardness, and cold limit compressibility (average of the measured values of five test pieces) of the test materials. The cold limit compressibility is 100 x (H ₀ from the limit height (H in Table 1B) at which the test piece a in Table 1 was compressed without cracking when compressed. −H) / H ₀ (%).

Table 1 also shows the measured values of tensile strength and elongation after aging treatment. Since the aging temperature at which the tensile strength is maximized is around 475 ° C. for all β-type titanium alloys, the aging treatment was performed at 475 ° C. × 20 hours except for No. 14.
(No. 14 is an α + β type titanium alloy, so it is heated at 750 ℃ →
The furnace heat treatment was used. ) As shown in Table 1, the titanium alloy (N
o.1 to 7) have a tensile strength of 75 kgf / m after solution treatment.
It is a titanium alloy with m ² or less and hardness of Hv 240 or less, which is extremely low strength. Therefore, the deformability in the solution-treated state is 80% or more at the critical compression rate, which is extremely good.

No. 8 with low V content among comparative alloys (Nos. 8-11)
Has a too high strength because it does not become a β single phase structure, Al, Sn,
Alternatively, Nos. 9, 10, and 11 having too high an oxygen (O) content have a tensile strength of 80 kgf / mm ² or more (hardness of 250 Hv or more), and the critical compressibility does not reach 80%.

No. 12 of the comparative alloy has too much V,
In this case, the strength in the solution state is low, and the critical compressibility is 80.
% Or more. However, this alloy is not practical because the appropriate age hardening time for providing the strength of about 120 kgf / mm ² required for the final product is 50 hours or more.

The conventional example No. 13 corresponds to the alloy disclosed in the above-mentioned JP-A-61-250138. The hardness and the tensile strength at the time of solution treatment are generally higher than those of the examples of the present invention.

FIG. 1 shows the alloys of the present invention (Nos. 2 and 7) and the comparative alloy (No. 1).
It is a deformation resistance curve of 1) and the conventional alloy (No. 14, 15).

According to FIG. 1, the deformation resistance of the alloy according to the present invention is the same as that of the conventional materials Ti-6Al-4V (No. 14) and Ti-15V-3Cr-3Al-3Sn.
It is significantly smaller than (No.15), indicating that the cold workability is good. The large deformation resistance and low deformability of the comparative alloy (No. 11) are mainly due to the high oxygen content of 0.30%.

Example 2 The age hardening characteristics of the alloy No. 5 of the present invention shown in Table 1 were examined. As a comparative material, a conventional example No. 13 (Ti-22V-4Al) containing no Sn and having the same level of other impurities was used. The solution treatment conditions were (β-transus + 20 ° C.) × 30 minutes → water cooling.

FIG. 2 shows the relationship between the aging temperature and the tensile strength and elongation. The aging time was fixed at 20 hours.

The Ti-20V-4Al-1Sn alloy of the present invention alloy (No. 5) is Ti
Compared to the -22V-4Al alloy, the strength is low when it is solution-treated,
Due to the aging treatment, the strength becomes higher than that of Ti-22V-4Al. Moreover, it can be seen that the aging temperature range showing high strength is wide and the aging characteristics are stable. Also, the ductility is equal to or higher than that of the Ti-22V-4Al alloy.

(Effects of the Invention) The β-type titanium alloy of the present invention is an alloy of the same kind (Ti-
(22V-4Al, Ti-15V-3Cr-3Al-3Sn) have better cold workability in the solution state. Therefore, the life of the core die during cold forging is extended, and the rolls during cold rolling or wire drawing and the reduction of baking with the die are greatly contributed to the cost reduction of the titanium component manufacturing. INDUSTRIAL APPLICABILITY The alloy of the present invention can be widely used in the field of daily commodities such as automobile valve parts, aerospace parts, and eyeglass frames.

[Brief description of the drawings]

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

Claims (1)

(57) [Claims] 1. By weight%, V: 15-25%, Al: 2.5-5%, S
A β-type titanium alloy excellent in cold workability, consisting of n: 0.5 to 4%, oxygen: 0.12% or less, the balance Ti and unavoidable impurities.