WO2020101008A1 - Titanium alloy wire rod and method for manufacturing titanium alloy wire rod - Google Patents

Abstract

A titanium alloy wire rod including an α phase and a β phase, the metallographic structure in an outer circumferential region extending from the surface toward the center of gravity to a depth of 3% of the wire diameter being an isometric structure having α crystal grains in which the average crystal grain size thereof is 10.0 µm or less in a cross section perpendicular to the longitudinal direction, and the metallographic structure in an internal region including the center of gravity and extending to a position at 20% of the wire diameter from the center of gravity to the surface being an acicular structure in a cross section perpendicular to the longitudinal direction.

Description

Titanium alloy wire and method for manufacturing titanium alloy wire

The present invention relates to a titanium alloy wire and a method for manufacturing the titanium alloy wire.

Titanium is a material that is lightweight and has high strength, so it has excellent specific strength and corrosion resistance, and is used in various applications such as aircraft, chemical plants, exterior materials for buildings, ornaments, and consumer products. In particular, α + β type titanium alloys such as Ti-6Al-4V, Ti-6Al-6V-2Sn and Ti-6Al-2Sn-4Zr-2Mo have excellent mechanical properties such as specific strength, ductility, toughness and heat resistance. It has been widely used among titanium alloys.

Patent Document 1 discloses Fe of 0.5% or more and less than 1.4%, 4.4% or more of 5 for the purpose of obtaining a titanium alloy having stable fatigue strength with little variation and high hot workability. An α + β type titanium alloy has been proposed which comprises less than 0.5% Al, the balance titanium, and impurities.

JP-A-7-70676

High-strength titanium alloy wire rods such as Ti-6Al-4V and Ti-5Al-1Fe used for aircraft fasteners (bolts, nuts, etc.) and automobile valves, etc. require even more excellent fatigue strength and creep strength. Therefore, further improvement is required.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a titanium alloy wire excellent in fatigue strength and creep strength and titanium that can be manufactured industrially in a stable manner. An object is to provide a method for manufacturing an alloy wire.

As a result of intensive studies to solve the above problems, the present inventors have paid attention to the characteristics of the needle-like structure and the equiaxed structure of the titanium alloy wire and the location thereof. The acicular structure has excellent creep properties, and the equiaxed structure has excellent fatigue properties. Then, by arranging the needle-like structure and the equiaxed structure at predetermined positions, a titanium alloy wire rod that simultaneously achieves both excellent fatigue strength and creep strength at an excellent level was found. Further, as a method of arranging a predetermined needle-like structure and equiaxed structure, it was found that the processing heat generated during the production of a titanium alloy wire can be utilized, and as a result of further investigation, the present invention was achieved.

The gist of the present invention completed based on the above findings is as follows.
[1]
A titanium alloy wire rod containing an α phase and a β phase,
In mass%,
Al: 0% or more and 7.0% or less,
V: 0% or more and 6.0% or less,
Mo: 0% or more and 7.0% or less,
Cr: 0% or more and 7.0% or less,
Zr: 0% or more and 5.0% or less,
Sn: 0% or more and 3.0% or less,
Si: 0% or more and 0.50% or less,
Cu: 0% or more and 1.8% or less,
Nb: 0% or more and 1.0% or less,
Mn: 0% or more and 1.0% or less,
Ni: 0% or more and 1.0% or less,
S: 0% or more and 0.20% or less,
REM: 0% or more and 0.20% or less,
Fe: 0% to 2.10%,
N: 0% to 0.050%,
O: 0% to 0.250%,
C: 0% or more and 0.100% or less,
The balance: Ti and impurities,
Content of Al, Mo, V, Nb, Fe, Cr, Ni and Mn has a chemical composition satisfying the following formula (1),
In a cross section perpendicular to the longitudinal direction, the metal structure in the outer peripheral region from the surface toward the center of gravity to a depth of 3% of the wire diameter is an equiaxed structure having α crystal grains with an average crystal grain size of 10 μm or less. ,
A titanium alloy wire rod, wherein, in a cross section perpendicular to the longitudinal direction, the metal structure in the inner region including the center of gravity from the center of gravity to the position of 20% of the wire diameter is a needle-shaped structure.
−4.00 ≦ [Mo] +0.67 [V] +0.28 [Nb] +2.9 [Fe] +1.6 [Cr] +1.1 [Ni] +1.6 [Mn] − [Al] ≦ 6 .00 ・ ・ ・ (1)
In addition, in Formula (1), the notation of [elemental symbol] represents the content (mass%) of the corresponding element symbol, and 0 is substituted for the element symbol that does not contain.
[2]
In mass%,
Al: 4.5% or more and 6.5% or less,
Fe: 0.50% or more and 2.10% or less,
The titanium alloy wire rod according to [1], including:
[3]
In mass%,
Al: 2.0% or more and 7.0% or less,
V: 1.5% or more and 6.0% or less,
The titanium alloy wire rod according to [1], including:
[4]
In mass%,
Al: 5.0% or more and 7.0% or less,
Mo: 1.0% or more and 7.0% or less,
Zr: 3.0% or more and 5.0% or less,
Sn: 1.0% or more and 3.0% or less,
The titanium alloy wire rod according to [1], including:
[5]
In a cross section perpendicular to the longitudinal direction, the average aspect ratio of α crystal grains in the outer peripheral region is 1.0 or more and less than 3.0, and the average aspect ratio of α crystal grains in the inner region is 5.0 or more. The titanium alloy wire according to any one of [1] to [4].
[6]
[5], wherein in a cross section perpendicular to the longitudinal direction, the area of the region including the center of gravity in which the average aspect ratio of α crystal grains is 5.0 or more is 40% or more with respect to the area of the cross section. Titanium alloy wire rod.
[7]
The titanium alloy wire according to any one of [1] to [6], wherein the α crystal grains in the outer peripheral region have an average crystal grain size of 5.0 μm or less.
[8]
The titanium alloy wire according to any one of [1] to [7], which has a wire diameter of 2.0 mm or more and 20.0 mm or less.
[9]
A step of heating the titanium alloy material to a temperature of (β transformation point −200) ° C. or higher;
The titanium alloy material has a total area reduction rate of 90.0% or more, and an average area reduction rate of 1% or more per pass in at least one or more passes from the end, and a wire drawing speed. With a processing speed of 5.0 m / s or more,
And a method for manufacturing a titanium alloy wire rod.
[10]
The method for producing a titanium alloy wire according to [9], further comprising a heat treatment in a temperature range of (β transformation point −300) ° C. or higher and (β transformation point −50) ° C. or lower.

According to the present invention, it is possible to provide a titanium alloy wire having excellent fatigue strength and creep strength and a method for manufacturing a titanium alloy wire capable of industrially and stably manufacturing a titanium alloy wire.

It is an explanatory view showing typically an equiaxed organization. It is explanatory drawing which showed the acicular tissue typically. 1 is a perspective sectional view schematically showing a titanium alloy wire rod according to an embodiment of the present invention. It is explanatory drawing which shows typically the state which determines a long axis and a short axis. (A)-(e) is explanatory drawing which shows the process in which the titanium alloy wire of this embodiment is manufactured typically in order.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
<1.
Titanium alloy wire rod>
First, the titanium alloy wire rod according to the present embodiment will be described.

(1.1 Metal structure)
First, the metallographic structure of the titanium alloy wire according to the present embodiment will be described. The titanium alloy wire according to the present embodiment is made of an α + β type titanium alloy having a chemical composition described later, and has a two-phase structure in which the α phase is the main component and a small amount of the β phase exists in the α phase at room temperature. Here, the α-phase being “mainly” means that the area ratio of the α-phase is 70% or more. The area ratio of the β phase is about 2% to 30%. In the titanium alloy wire rods of interest in each embodiment of the present invention, it is difficult to measure the β-phase area ratio, and the allowable measurement error is ± 5%.
In the titanium alloy wire rod according to the present embodiment, in the cross section perpendicular to the longitudinal direction, the metal structure in the outer peripheral region from the surface to the center of gravity up to the position of the wire diameter of 3% has an equiaxed α with an average crystal grain size of 10 μm or less. In the cross-section perpendicular to the longitudinal direction, which has an equiaxed structure having crystal grains, the metal structure in the internal region including the center of gravity from the center of gravity to the position of 20% of the wire diameter toward the surface has a needle-like α It is an acicular structure with crystal grains.

As shown in FIG. 1, the equiaxed structure of the α + β type titanium alloy has a texture of equiaxed α crystal grains a, and a fine β phase exists in the grain boundaries between the α crystal grains a and in the grains. b exists.
The acicular structure is an α-phase metallic structure that develops in acicular form from the grain boundaries due to the cooling of titanium that was in the β phase at high temperature. As shown in FIG. 2, in the needle-like structure of α + β-type titanium alloy, needle-like α (indicated by symbol c in FIG. 2) and needle-like β (shown in FIG. (Indicated by reference sign e) is a layered structure.
Thus, by observing the metallographic structure, it is possible to distinguish the equiaxed structure from the needle-shaped structure.

The titanium alloy wire according to the present embodiment has excellent fatigue strength and creep strength at the same time by arranging the needle-like structure and the equiaxial structure at predetermined positions. More specifically, in the titanium alloy, the acicular structure has excellent creep properties and the equiaxial structure has excellent fatigue properties. The starting point of fatigue fracture occurs near the surface layer (outer periphery) of the titanium alloy wire. Therefore, the present inventors arranged a fine equiaxed structure near the surface layer of the titanium alloy wire to improve fatigue strength, and arranged a needle-shaped structure excellent in creep strength near the center of gravity of the titanium alloy wire. Then, it was remembered to secure the creep strength as a sufficiently excellent one.

Then, the present inventors, as an index of the fine equiaxed structure near the surface layer, pay attention to the average aspect ratio and the average crystal grain size of α crystal grains in the outer peripheral region of the titanium alloy wire, and these are within a predetermined range. It has been found that the fatigue strength of the titanium alloy wire rod is improved by the existence of a certain one, that is, by forming a fine equiaxed structure region (equixed structure region) in the outer peripheral region. Furthermore, the present inventors have focused on the average aspect ratio of the α crystal grains in the region including the center of gravity as an index of the needle-like structure in the inner region including the center of gravity, and by having a value of a certain value or more, that is, the center of gravity, It was found that the creep strength of the titanium alloy wire rod is improved by forming a needle-shaped structure (a needle-shaped structure region) in the region including the. As a result, it has become possible to improve the creep strength and fatigue strength of the titanium alloy wire at the same time.

Further, the present inventors have found that the titanium alloy wire rod having the metal structure as described above can be manufactured by the method for manufacturing a titanium alloy wire rod according to the present embodiment, which will be described in detail later, and have arrived at the present invention. The metallographic structure of the titanium alloy wire according to this embodiment will be specifically described below.

FIG. 3 is an explanatory view schematically showing an example of the titanium alloy wire rod 1 according to the present embodiment. It should be noted that the dimensions of each region shown in the drawing are appropriately enlarged or reduced for ease of explanation, and do not show the actual size of each region.

Further, the titanium alloy wire rod according to the present invention may have any cross-sectional shape, but hereinafter, the titanium alloy wire rod 1 according to the present embodiment has a circular cross section in a cross section perpendicular to the longitudinal direction L. As described below. The cross section in the drawing is a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1.

In the present specification, as shown in FIG. 3, the outer peripheral region 2 corresponds to 3% of the wire diameter R from the outer peripheral surface 3 toward the center of gravity G in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. It is defined as a region up to the depth d. In some cases, oxide scale or the like may be attached to the outer peripheral surface 3 of the titanium alloy wire rod 1. However, the thickness of such an attached substance is used as a measurement starting point of the depth d of the outer peripheral region 2. Not included on the outer peripheral surface of.

Further, in the present specification, as shown in FIG. 3, the inner region 4 is 20% of the wire diameter R from the center of gravity G toward the outer peripheral surface 3 in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. It is defined as a region including the center of gravity G up to the position of, in the present specification, the center of gravity G in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 is defined based on the cross-sectional shape, so-called " It is defined as the "geometric center". In the present embodiment, the section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 is a circle, so the center of gravity G shown in FIG. 3 is the center of the circular section.

Furthermore, in the present embodiment, the wire diameter R can be defined as the diameter of the circular cross section because the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 forms a circle. When the titanium alloy wire rod 1 does not have a circular cross section, for example, when it has an elliptical shape, the wire diameter R can be defined as the average value of the major axis and the minor axis in the elliptical cross section.

The titanium alloy wire rod 1 according to this embodiment corresponds to 3% of the wire diameter R from the outer peripheral surface 3 of the titanium alloy wire rod 1 toward the center of gravity G in the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. The metal structure in the outer peripheral region 2 up to the depth d exhibits an equiaxed structure having equiaxed α crystal grains. When the metal structure in the outer peripheral region 2 is an equiaxed structure, the ductility of the titanium alloy wire rod 1 in the outer peripheral region 2 is improved, the surface properties are improved, and there are few defects that cause fatigue fracture on the surface. Become. As a result, it is possible to prevent breakage during the production of the titanium alloy wire rod 1 and improve fatigue characteristics. On the other hand, when the metallographic structure in the outer peripheral region 2 of the titanium alloy wire 1 has a needle-shaped structure, the ductility decreases, and as a result, the fatigue strength of the titanium alloy wire 1 cannot be improved.

The average aspect ratio of the α crystal grains in the outer peripheral region 2 may be 1.0 or more and less than 3.0, but in order to obtain even more excellent fatigue strength, the preferable upper limit is 2.5, and more preferably It is 2.0. The average aspect ratio of the α crystal grains is theoretically “1” when the metal structure in the outer peripheral region 2 has a perfect equiaxed structure. Therefore, the lower limit of the average aspect ratio of the α crystal grains in the outer peripheral region 2 is 1.0.

Further, in the present embodiment, the average crystal grain size of α crystal grains in the outer peripheral region is 10.0 μm or less. As a result, the metal structure in the outer peripheral region becomes finer, the surface roughness is reduced in combination with the equiaxing of α crystal grains, and the defects as the starting point of fatigue fracture on the surface are reduced, resulting in the fatigue strength of the titanium alloy wire rod. Is improved. On the other hand, when the average crystal grain size of the α crystal grains in the outer peripheral region exceeds 10.0 μm, the fatigue strength of the titanium alloy wire cannot be made excellent due to an increase in the surface roughness.

The average crystal grain size of α crystal grains in the outer peripheral region may be 10.0 μm or less, but in order to further improve the fatigue strength of the titanium alloy wire, it is preferably 5.0 μm or less, more preferably 3.0 μm. It is below.
The lower limit of the average crystal grain size of α crystal grains in the outer peripheral region may be 1.0 μm, for example. If it is less than that, it is difficult to manufacture, and there is a risk that the cost may increase.

Next, in the present embodiment, in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1, an internal region including the center of gravity G of the titanium alloy wire rod 1 toward the surface and up to a position of 20% of the wire diameter. The metal structure of No. 4 has a needle-shaped structure having needle-shaped α crystal grains. When the metal structure in the inner region 4 is a needle-shaped structure, the creep strength of the titanium alloy wire rod is improved. On the other hand, when the metal structure of the inner region 4 of the titanium alloy wire 1 is not sufficiently developed as a needle-shaped structure, the creep strength of the titanium alloy wire 1 will not be sufficient.
Creep is a phenomenon in which dislocations introduced into a metal structure due to deformation are recovered by diffusion of atoms, so that the material is softened and deformation proceeds. Therefore, the speed of recovery (atomic diffusion speed) affects creep. It is said that the α / β interface formed by the needle-like structure has high conformity and the diffusion rate of atoms is slow, so that the needle-like structure has excellent creep strength. The creep strength can be improved by making the metallic structure in the inner region 4 including the center of gravity G of the titanium alloy wire 1 into a needle-shaped structure.

The average aspect ratio of the α crystal grains in the internal region 4 including the center of gravity G of the titanium alloy wire rod 1 from the center of gravity G to the position of 20% of the wire diameter toward the surface may be 5.0 or more, but the creep strength In order to further improve the above, it is preferably 6.0 or more, more preferably 7.0 or more. The upper limit of the average aspect ratio of the α crystal grains in the inner region 4 including the center of gravity G is not particularly limited, but can be set to 20.0 or less based on actual results.

Further, in the present embodiment, a region including the center of gravity G in which the average aspect ratio of α crystal grains is 5.0 or more in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1 (the needle-shaped region including the center of gravity G The area ratio of the acicular structure region having α crystal grains) can be, for example, 20% or more with respect to the area of the cross section of the titanium alloy wire rod 1 perpendicular to the longitudinal direction L. From the viewpoint of further improving the creep strength, the area ratio of the needle-like structure region is preferably 40% or more, and more preferably 50% with respect to the area of the cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1. % Or more.

From the viewpoint of equiaxed metal structure in the outer peripheral region, a region including the center of gravity G in which the average aspect ratio of α crystal grains is 5.0 or more in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1 ( The area ratio of the acicular structure region having acicular α crystal grains including the center of gravity G) is preferably 90% or less, more preferably the area ratio of the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. It is 80% or less.
It should be noted that, between the outer peripheral region 2 made of the equiaxed tissue and the needle-shaped tissue region including the center of gravity G shown in FIG. 1, it is desirable that the equiaxial tissue continuously changes to the needle-shaped tissue. It may be a mixed organization.

The average crystal grain size and the average aspect ratio of the α crystal grains in the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 can be obtained as follows. First, a cross section of the titanium alloy wire rod 1 perpendicular to the longitudinal direction L is mirror-polished and then etched with a mixed aqueous solution of hydrofluoric acid and nitric acid. The average crystal grain size and the average aspect ratio can be measured by observing an optical micrograph of the surface.
The average crystal grain size can be measured by the line segment method (based on JIS G 0551). In the outer peripheral region 2 from the outer peripheral surface 3 of the titanium alloy wire 1 toward the center of gravity G to the depth d corresponding to 3% of the wire diameter R, for example, 5 times in the vertical and horizontal directions with respect to an optical microscope photograph taken at a magnification of 500 times. Line segments are drawn line by line, the average grain size is calculated using the number of grain boundaries that cross the line segment, and the average grain size is calculated from the arithmetic average value of the average grain sizes of 10 lines in total.
The average aspect ratio is 20 from the outer peripheral surface 2 of the titanium alloy wire rod 1 toward the center of gravity G to the depth d corresponding to a wire diameter of 3%, and from the center of gravity G to the surface 3 of the wire diameter R of 20. In the internal region 4 including the center of gravity G up to the position of%, the major axis and the minor axis are measured for 50 arbitrary crystal grains with respect to an optical microscope photograph taken at a magnification of, for example, 500 times, and the major axis is measured. Can be calculated as the average of the values divided by the short axis. Here, as shown in FIG. 4, the “major axis 11” refers to a line segment that connects two arbitrary points on the α phase grain boundary 10 (contour) and has the maximum length. , "Short axis 12" means a line segment which is orthogonal to the long axis 11 and which connects any two points on the grain boundary 10 (contour) and has the maximum length.

Here, the average aspect ratio of the α crystal grains is measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1 and in a cross section parallel to the longitudinal direction of the titanium alloy wire 1. Then, it is considered that the values will be similar. However, when measured in a cross section parallel to the longitudinal direction L of the titanium alloy wire rod 1, it becomes difficult to distinguish between a structure having elongated α crystal grains extended by rolling and a needle structure having acicular α crystal grains. there is a possibility. Therefore, it is determined by a value measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1.
In addition, in the structure having α crystal grains elongated by rolling, it is measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1 and in a cross section parallel to the longitudinal direction L of the titanium alloy wire 1. It is considered that the value of the aspect ratio of the α crystal grain is different from that of the case. Specifically, when the structure having α crystal grains elongated by rolling is measured in a cross section parallel to the longitudinal direction L of the titanium alloy wire rod 1, the aspect ratio is large (for example, 5.0 or more). While α crystal grains are observed, the α crystal has a small aspect ratio (for example, about 1.0 to 3.0) when measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. Grains are observed. Therefore, by measuring the average aspect ratio of the α crystal grains in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1, whether the α crystal grains are elongated by rolling or are needle-shaped α crystal grains. Can be distinguished.
Further, when obtaining the average crystal grain size and the average aspect ratio of α crystal grains, it is considered that α crystal grains having the same orientation are arranged with a thin needle-like β phase interposed therebetween. Since it is difficult for EBSD to detect the thin β phase, it may be difficult for EBSD analysis.

Strictly speaking, the center of gravity G exists as a “point” in the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. Therefore, when observing the average aspect ratio of the α crystal grains in the inner region 4 including the center of gravity G of the titanium alloy wire 1, the region α up to 20% of the wire diameter R from the center of gravity G toward the outer peripheral surface 3 is α. It can be calculated by observing the aspect ratio of the crystal grains and averaging the observed aspect ratios.
The metallographic structure of the titanium alloy wire rod according to the present embodiment has been described above.

(1.2 Chemical composition)
Next, the chemical composition of the titanium alloy wire according to the present embodiment will be described. The chemical composition of the titanium alloy wire according to the present embodiment is not particularly limited as long as it can form a two-phase structure having an α phase and a β phase in a temperature environment during use or at room temperature. For example, JIS H 4600 or An α + β type titanium alloy having various compositions described in JIS H 4650 can be adopted. Alternatively, it is possible to contain the elements described below. In addition, in the present specification including the following description, unless otherwise specified, when the content is represented by “%”, the “%” indicates mass%.

Al: 0% or more and 7.0% or less Aluminum (Al) is an element that forms a solid solution in the α phase and strengthens the α phase. The α + β-type titanium alloy wire may not contain Al, but may contain 2.0% or more, preferably 2.5% or more Al in order to obtain this effect. On the other hand, if the content of Al is too large, the α ₂ phase (Ti ₃ Al) may precipitate depending on the chemical composition to reduce the ductility, and the amount of the α phase may increase to improve the hot workability. Since it may decrease, the Al content may be 7.0% or less, preferably 6.5% or less.

V: 0% or more and 6.0% or less Vanadium (V) stabilizes the β phase and improves hot formability and heat treatment property. The α + β type titanium alloy wire does not need to contain V, but in order to obtain this effect, V may be contained in an amount of 1.5% or more, preferably 2.0% or more. On the other hand, if the V content is too large, the volume fraction of the β phase may increase depending on the chemical composition, and the strength of the α + β type titanium alloy wire may decrease. Therefore, the V content is 6.0% or less. , Preferably 5.5% or less.

Mo: 0% or more and 7.0% or less Molybdenum (Mo) also stabilizes the β phase and improves hot formability and heat treatment property. The α + β-type titanium alloy wire may not contain Mo, but in order to obtain this effect, it may contain 1.0% or more, preferably 1.5% or more Mo. On the other hand, if the Mo content is too large, the volume fraction of the β phase may increase depending on the chemical composition and the strength of the α + β type titanium alloy wire may decrease, so the Mo content should be 7.0% or less. , Preferably 6.0% or less.

Cr: 0% or more and 7.0% or less Chromium (Cr) also stabilizes the β phase and improves hot formability and heat treatability. The α + β type titanium alloy wire does not have to contain Cr, but may have 2.0% or more, preferably 3.0% or more Cr in order to obtain this effect. On the other hand, if the Cr content is too large, the volume fraction of the β phase may increase depending on the chemical composition, and the strength of the α + β type titanium alloy wire may decrease. Therefore, the Cr content is 7.0% or less. , Preferably 6.0% or less.

Zr: 0% to 5.0% Zirconium (Zr) is an element that simultaneously strengthens the α phase and the β phase. The α + β type titanium alloy wire does not have to contain Zr, but in order to obtain this effect, it may contain 1.5% or more, preferably 2.0% or more Zr. On the other hand, if the Zr content is too large, the precipitation of the α ₂ phase (Ti ₃ Al) may be promoted and the ductility may be lowered depending on the chemical composition. Therefore, the Zr content is 5.0% or less, It may be 4.5% or less.

Sn: 0% or more and 3.0% or less Tin (Sn) is an element that simultaneously strengthens the α phase and the β phase. The α + β-type titanium alloy wire does not have to contain Sn, but in order to obtain this effect, it may contain 1.0% or more, preferably 1.5% or more Sn. On the other hand, if the Sn content is too large, the precipitation of the α ₂ phase (Ti ₃ Al) may be promoted and the ductility may be lowered depending on the chemical composition, so the Sn content is 3.0% or less, It may be 2.5% or less.

Si: 0% or more and 0.50% or less Silicon (Si) improves heat resistance. The α + β type titanium alloy wire may not contain Si, but may contain 0.04% or more, preferably 0.07% or more Si in order to obtain this effect. On the other hand, if the Si content is too large, the creep strength may decrease due to the precipitation of silicide depending on the chemical composition. Therefore, the Si content is 0.50% or less, preferably 0.35% or less. May be

Cu: 0% or more and 1.8% or less Copper (Cu) stabilizes the β phase and also forms a solid solution in the α phase to strengthen the α phase. The α + β type titanium alloy wire does not have to contain Cu, but in order to obtain this effect, it may contain 0.4% or more, preferably 0.8% or more of Cu. On the other hand, if the Cu content is too large, the fatigue strength may decrease due to the precipitation of Ti ₂ Cu depending on the chemical composition. Therefore, the Cu content is 1.8% or less, preferably 1.5%. It may be as follows.

Nb: 0% or more and 1.0% or less Niobium (Nb) improves oxidation resistance. The α + β type titanium alloy wire does not have to contain Nb, but in order to obtain this effect, it may contain 0.1% or more, preferably 0.2% or more Nb. On the other hand, if the Nb content is too large, the volume fraction of the β phase may increase depending on the chemical composition and the strength of the α + β type titanium alloy wire may decrease, so the Nb content is 1.0% or less. , Preferably 0.8% or less.

Mn: 0% or more and 1.0% or less Manganese (Mn) also stabilizes the β phase and improves hot formability and heat treatability. The α + β type titanium alloy wire may not contain Mn, but may contain 0.1% or more, preferably 0.2% or more of Mn in order to obtain this effect. On the other hand, if the Mn content is too large, the volume fraction of the β phase may increase depending on the chemical composition and the strength of the α + β type titanium alloy wire may decrease, so the Mn content is 1.0% or less. , Preferably 0.8% or less.

Ni: 0% or more and 1.0% or less Nickel (Ni) also stabilizes the β phase and improves hot formability and heat treatability. The α + β type titanium alloy wire does not have to contain Ni, but in order to obtain this effect, it may contain 0.1% or more, preferably 0.2% or more Ni. On the other hand, if the Ni content is too large, the volume fraction of the β phase may increase depending on the chemical composition, and the strength of the α + β type titanium alloy wire may decrease, so the Ni content should be 1.0% or less. , Preferably 0.8% or less.

S: 0% or more and 0.20% or less Sulfur (S) improves machinability. The α + β-type titanium alloy wire may not contain S, but in order to obtain this effect, it may contain 0.01% or more, preferably 0.03% or more S. On the other hand, if the S content is too large, the hot formability may be deteriorated due to the formation of inclusions depending on the chemical composition. Therefore, the S content is 0.20% or less, preferably 0.10. % Or less.

REM: 0% or more and 0.20% or less A rare earth element (REM) is contained together with S to improve machinability. The α + β type titanium alloy wire does not have to contain REM, but in order to obtain this effect, it may contain 0.01% or more, preferably 0.03% or more of REM. On the other hand, if the REM content is too high, the hot formability may be deteriorated due to the formation of inclusions depending on the chemical composition. Therefore, the REM content is 0.20% or less, preferably 0.10% or less. % Or less.

Here, as REM, specifically, scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm). , Europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). One of them can be contained alone, or two or more thereof can be contained in combination. When two or more kinds of rare earth elements are contained, for example, a mixed rare earth element (Misch metal) before separation and purification, or a mixture or compound of rare earth elements such as didymium alloy (alloy made of Nd and Pr) may be used. .. When two or more kinds of rare earth elements are contained, the REM amount means the total amount of all rare earth elements.

Fe: 0% to 2.10% Iron (Fe) is an element that strengthens the β phase. The α + β type titanium alloy wire does not have to contain Fe, but in order to obtain this effect, it may contain 0.50% or more, preferably 0.70% or more Fe. On the other hand, if the Fe content is too large, the manufacturability may decrease due to the segregation of Fe or the intermetallic compound (TiFe) may precipitate and the toughness and ductility may decrease depending on the chemical composition. The Fe content may be 2.10% or less, preferably 1.50% or less.

N: 0% or more and 0.050% or less Nitrogen (N) is an element that forms a solid solution in the α phase and strengthens the α phase. The α + β type titanium alloy wire does not have to contain N, but in order to obtain this effect, it may contain 0.002% or more, preferably 0.005% or more N. On the other hand, if the content of N is too large, low density inclusions (TiN) may be generated depending on the chemical composition and may be the starting point of fatigue fracture. Therefore, the content of N is 0.050% or less, preferably May be 0.030% or less.

O: 0% or more and 0.250% or less Oxygen (O) is an element that forms a solid solution in the α phase and strengthens the α phase. The α + β-type titanium alloy wire may not contain O, but may contain 0.050% or more, preferably 0.100% or more O in order to obtain this effect. On the other hand, if the O content is too large, the α phase may excessively increase and the ductility may decrease depending on the chemical composition. Therefore, the O content is 0.250% or less, preferably 0.200%. It may be as follows.

C: 0% or more and 0.100% or less Carbon (C) strengthens the α phase by forming a solid solution in the α phase, and improves the machinability by being contained together with S. The α + β type titanium alloy wire does not have to contain C, but in order to obtain this effect, it may contain 0.005% or more, preferably 0.010% or more C. On the other hand, if the content of C is too large, the carbides may excessively increase depending on the chemical composition and the hot formability may decrease. Therefore, the content of C is 0.100% or less, preferably 0. It may be 080% or less.

The balance of the chemical components of the titanium alloy wire according to the present embodiment may be titanium (Ti) and impurities. Impurities mean components that are mixed in due to raw materials and other factors when the titanium alloy wire is industrially manufactured, and are allowed within a range that does not adversely affect the titanium alloy wire according to the present embodiment. .
Examples of such impurities include hydrogen (H), tantalum (Ta), cobalt (Co), tungsten (W), palladium (Pd), boron (B), chlorine (Cl), sodium (Na), magnesium (Mg). ), Calcium (Ca), and the like. When these H, Ta, Co, Pd, W, B, Cl, Na, Mg, and Ca are contained as impurities, their contents are, for example, 0.05% or less, respectively, and are 0.10% or less in total. is there.

Mo equivalent In the chemical composition of the titanium alloy wire according to the present embodiment, the contents of Al, Mo, V, Nb, Fe, Cr, Ni and Mn further satisfy the following formula (1).
−4.00 ≦ [Mo] +0.67 [V] +0.28 [Nb] +2.9 [Fe] +1.6 [Cr] +1.1 [Ni] +1.6 [Mn] − [Al] ≦ 6 .00 ・ ・ ・ (1)
In addition, in Formula (1), the notation of [elemental symbol] represents the content (mass%) of the corresponding element symbol, and 0 is substituted for the element symbol that does not contain.

A = [Mo] +0.67 [V] +0.28 [Nb] +2.9 [Fe] +1.6 [Cr] +1.1 [Ni] +1.6 [Mn]-[Al]

Here, the Mo equivalent A represented by the right side of the above formula (1) is each element (β stabilizing element) Mo, V, Nb, Fe, Cr, Ni that stabilizes the β phase described in the formula. , Mn is used to quantify the degree of stabilization of the β phase. At this time, the degree of stabilization of the β phase by the β stabilizing elements other than Mo is made relative by a positive coefficient with reference to the degree of stabilization of the β phase by Mo. On the other hand, since Al is an element that forms a solid solution in the α phase and strengthens the α phase (α stabilizing element), in the above Mo equivalent A, the coefficient relating to Al has a negative value.

[Mo equivalent A range: -4.00 ≤ A ≤ 6.00]
The titanium alloy wire according to the present embodiment has Mo, V, Nb, Fe, so that the value of Mo equivalent A represented by the above formula (1) is within the range of -4.00 to 6.00. It contains at least one or more elements selected from the group consisting of Cr, Ni, and Mn. When the value of the Mo equivalent A is less than -4.00, the β phase becomes too small and it is difficult to form a needle-like structure, and the creep characteristics are not improved. The lower limit of the Mo equivalent A is preferably −3.50, and more preferably −3.00. On the other hand, when the value of Mo equivalent A exceeds 6.00, a needle-like α phase is not formed from the β phase during cooling, the inside becomes a β single phase structure, and the creep property is not improved. The upper limit of Mo equivalent A is preferably 5.00, more preferably 4.00.
The titanium alloy wire having such a chemical composition is an α + β type titanium alloy wire having an α phase and a β phase.

More specifically, the titanium alloy wire rod is
Al: 4.5% or more and 6.5% or less, preferably 4.8% or more, or 6.2% or less,
Fe: 0.50% or more and 2.10% or less, preferably 0.70% or more, or 1.50% or less,
May be included.
In addition,
N: 0% or more and 0.050% or less, preferably 0.002% or more, or 0.030% or less,
O: 0% or more and 0.250% or less, preferably 0.100% or more, or 0.200% or less,
C: 0% or more and 0.100% or less, preferably 0.001% or more, or 0.080% or less,
May be

The titanium alloy wire with such a chemical composition becomes an α + β type titanium alloy wire having α phase and β phase, and has stable fatigue strength with little variation and high hot workability. Examples of titanium alloy wire rods having such a chemical composition include Super-TiX 51AF (Ti-5Al-1Fe, manufactured by Nippon Steel Corporation).

Alternatively, the titanium alloy wire rod is
Al: 2.0% or more and 7.0% or less, preferably 2.5% or more, or 6.5% or less,
V: 1.5% or more and 6.0% or less, preferably 2.0% or more, or 5.5% or less,
May be included.
In addition,
Fe: 0% or more and 0.50% or less, preferably 0.03% or more, or 0.30% or less,
N: 0% or more and 0.050% or less, preferably 0.002% or more, or 0.030% or less,
O: 0% or more and 0.250% or less, preferably 0.100% or more, or 0.200% or less,
May be

The titanium alloy wire with such a chemical composition also becomes an α + β type titanium alloy wire containing α phase and β phase, and has stable fatigue strength with little variation and high hot workability. Examples of the titanium alloy wire having such a chemical composition include Ti-3Al-2.5V, Ti-6Al-4V, SSAT-35 (Ti-3Al-5V, manufactured by Nippon Steel Corporation).

Furthermore, the titanium alloy wire rod
Al: 5.0% or more and 7.0% or less, preferably 5.5% or more, or 6.5% or less,
Mo: 1.0% or more and 7.0% or less, preferably 1.8% or more, or 6.5% or less,
Zr: 3.0% or more and 5.0% or less, preferably 3.6% or more, or 4.4% or less,
Sn: 1.0% or more and 3.0% or less, preferably 1.75% or more, or 2.25% or less may be included.
In addition,
Si: 0% or more and 0.50% or less, preferably 0.06% or more, or 0.10% or less,
Fe: 0% or more and 0.50% or less, preferably 0.03% or more, or 0.10% or less,
N: 0% or more and 0.050% or less, preferably 0.002% or more, or 0.030% or less,
O: 0% or more and 0.250% or less, preferably 0.100% or more, or 0.200% or less,
May be

The titanium alloy wire rod having such a chemical composition becomes an α + β type titanium alloy wire rod containing an α phase and a β phase, and is particularly excellent in creep characteristics. Examples of the titanium alloy wire having such a chemical composition include Ti-6Al-2Sn-4Zr-2Mo-0.08Si and Ti-6Al-2Sn-4Zr-6Mo.
The chemical composition of the titanium alloy wire rod according to the present embodiment has been described above.

(1.3 wire diameter, shape)
The wire diameter R of the titanium alloy wire rod 1 according to the present embodiment is not particularly limited, but can be, for example, 2 mm or more and 20 mm or less. By setting the wire diameter R of the titanium alloy wire rod 2 to 2 mm or more, a needle-shaped structure having needle-shaped α-grain crystals is formed in the inner region 4 including the center of gravity G, and a fine equiaxed α is formed in the outer peripheral region 2. A fine equiaxed structure having crystal grains can be formed more reliably, and fatigue strength and creep strength can be made more reliable at the same time. Further, by setting the wire diameter R of the titanium alloy wire rod 1 to 20 mm or less, it becomes possible to perform wire drawing at a high speed, and the central portion of the bar wire easily and stably generates heat during processing, and the inner area 4 near the center of gravity is formed. A needle-shaped tissue is easily obtained. The lower limit of the wire diameter R of the titanium alloy wire rod 1 according to the present embodiment is preferably 3 mm, and the upper limit of the wire diameter R is preferably 15 mm.

The shape (cross-sectional shape) of the titanium alloy wire rod according to the present embodiment is not limited to the illustrated mode, and may be a polygonal shape such as an ellipse or a square other than a circle.

In the embodiment described above, in the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1, the metal structure in the outer peripheral region 2 has fine equiaxed α crystal grains having an average crystal grain size of 10 μm or less. Since the metal structure in the inner region 4 including the center of gravity G is an equiaxed structure and has a needle-shaped structure having needle-shaped α crystal grains, the titanium alloy wire rod has excellent fatigue strength and creep strength at the same time. ..

The titanium alloy wire rod according to the present embodiment described above has excellent creep strength and fatigue strength in addition to excellent characteristics, corrosion resistance, specific strength and the like derived from the α + β type titanium alloy. Therefore, although the titanium alloy wire according to the present embodiment may be used for any purpose, it can be preferably used for fasteners (fixtures) such as bolts and nuts, valves and the like. The titanium alloy wire according to the present embodiment can be particularly preferably used as a fastener or valve material for transportation equipment such as aircraft and automobiles.
The titanium alloy wire according to the present embodiment described above may be manufactured by any method, but may be manufactured by, for example, the method for manufacturing a titanium alloy wire according to the present embodiment described below.

<2. Titanium Alloy Wire Manufacturing Method>
Next, a method for manufacturing the titanium alloy wire rod according to the present embodiment will be described.
The method for manufacturing a titanium alloy wire according to the present embodiment includes a step of heating a titanium alloy material to a temperature of (β transformation point −200) ° C. or more (heating step), and an α + β type titanium alloy material with a total area reduction ratio of 90% or more, and in at least one or more passes from the last, a step (processing step) in which the average area reduction rate per pass is 10% or more and the wire drawing speed is 5 m / s or more, Have. Hereinafter, each step will be described.

(2.1 Preparation of titanium alloy material)
First, a titanium alloy material is prepared prior to the above steps.
As the titanium alloy material, one having the above-mentioned chemical composition can be used, and one manufactured by a known method can be used. For example, the titanium alloy material can be obtained by producing an ingot from titanium sponge by a vacuum arc melting method and hot forging the ingot at a temperature in the β single phase region. The titanium alloy material may be subjected to pretreatment such as cleaning treatment and pickling, if necessary.

Also, the wire diameter of the titanium alloy material can be appropriately selected according to the area reduction rate planned in the processing step and the wire diameter of the titanium alloy wire material planned.

(2.2 heating process)
In this step, the titanium alloy material is heated to a temperature of (β transformation point −200) ° C. or higher. As a result, it becomes easier to maintain the temperature near the center of gravity of the titanium alloy material at the β transformation point or more in the reduction of deformation resistance and the processing step described later, and it is possible to promote the development of the acicular structure near the center of gravity of the titanium alloy material. .. As a result, the average aspect ratio of α crystal grains in the vicinity of the center of gravity (internal region) can be set to 5.0 or more in the processing step described later. On the other hand, if the heating temperature in this step is less than (β transformation point −200) ° C., the deformation resistance becomes too large, or the temperature near the center of gravity of the titanium alloy material becomes higher than the β transformation point in the processing step described later. As a result, the needle-like structure cannot be sufficiently developed in the vicinity of the center of gravity of the titanium alloy material in some cases, and as a result, the average aspect ratio of α crystal grains in the vicinity of the center of gravity (inner region) cannot be sufficiently increased.

The heating temperature in this step may be (β transformation point −200) ° C. or higher, but from the viewpoint of deformation resistance, it is preferably (β transformation point −150) ° C. or higher, more preferably (β transformation point −125). ℃ or above. The upper limit of the heating temperature in this step is not particularly limited, but the heating temperature is preferably (β transformation point + 100) ° C. or less, more preferably (β transformation point + 50) ° C. or less, from the viewpoint of yield reduction due to scale formation. is there.

In the present specification, the “β transformation point” means the end temperature of β transformation when the titanium alloy is heated. The titanium alloy wire according to the present embodiment and the titanium alloy material that is a raw material thereof are in the α + β two-phase region in which an α phase and a β phase exist at room temperature and a use environment, and the starting temperature of β transformation is at these room temperature and It is below the temperature of the operating environment.
The β transformation temperature T can be obtained from the phase diagram. The phase diagram can be obtained by, for example, the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method. TI3) can be used.

(2.3 Machining process)
This step is a so-called wire drawing step in which the titanium alloy material is drawn by sequentially passing through a plurality of rolling passes.

This processing step is performed by tandem rolling instead of reverse rolling. The tandem rolling is a method in which a rolled material is continuously passed through a plurality of rolling passes arranged in series, and rolling is sequentially performed in one direction in each rolling pass. By producing a titanium alloy wire using tandem rolling, the titanium alloy material has a total area reduction rate of 90% or more, and an average area reduction rate per pass of at least one pass from the last. It is possible to process at 10% or more and a wire drawing speed of 5 m / s or more.

Here, the process in which the titanium alloy wire rod according to the present embodiment is manufactured by the working process will be described with reference to the drawings (a diagram showing a cross section perpendicular to the longitudinal direction). 5 (a) to 5 (e) schematically show, in sequence, the process in which the titanium alloy wire rod according to the present embodiment is manufactured.

First, in the above heating step, the metal structure becomes an α + β structure or a β single phase with the β phase as the main phase by being heated to a temperature of (β transformation point −200) ° C. or higher. Here, as shown in FIG. 5A, a case of a β single-phase structure including only β crystal grains 20 will be described. Then, in the initial stage of processing, as shown in FIG. 5 (b), needle-shaped α crystal grains 21 are formed during the transformation from β phase to α phase due to the temperature decrease, and are composed of α phase and β phase. A needle-like tissue is formed. The needle-like tissue is a tissue in which needle-like α and needle-like β developed in a needle shape are arranged in layers.

Next, in the middle of the processing step, the needle-shaped α crystal grains 21 are divided by being processed, and further, due to grain growth, as shown in FIG. 5C, equiaxed α crystal grains 21 are formed. 22 is formed. In the middle of the working process, the wire drawing speed (strain speed) is still small and the working heat is small, so that the temperature near the center of gravity does not exceed the β transformation point (becomes high in the β single phase region). Therefore, an α + β type equiaxed structure in which the equiaxed α crystal grains 22 and the equiaxed β crystal grains are mixed is formed.

Next, in the latter stage of the working process, the wire drawing speed increases, and due to heat generated during working, the temperature rises above the β transformation point near the center of gravity. As a result, as shown in FIG. 5D, in the internal region including the center of gravity, the α phase is transformed into the β phase, and the β single-phase structure including only the β crystal grains 23 is formed.
In general, titanium alloys have large deformation resistance and relatively large heat generation during processing in the rolling process and wire drawing process. In particular, in the latter half of the working process, the average area reduction rate and the wire drawing speed are relatively large, and thus the working heat generation during passing through the rolling pass is large. In the inner region of the titanium alloy material, for example, in the vicinity of the center of gravity, the heat removal is small with respect to the heat generated by processing, and therefore the temperature in that region rises above the β transformation point.

On the other hand, in the outer peripheral region, sufficient heat can be removed from the outer peripheral surface even in the latter stage of the processing step, and the metal structure is refined and equiaxed by being processed at a relatively low temperature. As a result, the α crystal grains 24 in the outer peripheral region become fine equiaxed grains having an average crystal grain size of 10 μm or less. Further, by sufficiently miniaturizing and equiaxing the metal structure in the outer peripheral region as described above, the occurrence of defects on the outer peripheral surface is suppressed, and the occurrence of defects such as breakage during manufacturing is suppressed.

Then, when the processing step is completed, the titanium alloy material is cooled to near the center of gravity, so as shown in FIG. 5 (e), as the temperature lowers, acicular α crystals at the time of transformation from β phase to α phase Grains 25 are generated and needle-like tissue is formed in the internal region including the center of gravity. Thus, in the cross section perpendicular to the longitudinal direction, the metal alloy structure in the outer peripheral region is the fine equiaxed structure 24, and the metal structure in the inner region is the needle-shaped structure 25, the titanium alloy wire of the present embodiment is manufactured. It

In the processing step, the titanium alloy material is made to have a total area reduction rate of 90% or more, an average area reduction rate of 10% or more per pass in at least one or more passes from the end, and wire drawing. By including the step of processing at a speed of 5 m / s or more, the titanium alloy wire rod of the present embodiment is manufactured. That is, in the cross section perpendicular to the longitudinal direction L, the metal structure in the outer peripheral region 2 from the surface 3 toward the center of gravity G to the depth d corresponding to the wire diameter 3% has an average crystal grain size of 10 μm or less, etc. A needle having an equiaxed structure having axial α crystal grains, and the metal structure in the inner region 4 including the center of gravity G from the center of gravity G to the position of 20% of the wire diameter toward the surface 3 has needle-shaped α crystal grains. It becomes a textured structure. Further, in the cross section perpendicular to the longitudinal direction L, the average aspect ratio of the α crystal grains in the outer peripheral region 2 is 1.0 or more and less than 3.0, and the average aspect ratio of the α crystal grains in the inner region 4 is 5. It becomes 0 or more.

Note that the drawing speed in at least one or more passes from the last described above is much higher than the drawing speed (about 0.2 to 2.0 m / s) adopted in the production of conventional titanium alloy wire rods. The inventors of the present invention intentionally adopt such a wire drawing speed together with the above-mentioned average area reduction rate to generate a large amount of processing heat and obtain the metal structure of the titanium alloy wire rod according to the present embodiment described above. I found it possible.

As described above, in this embodiment, the average area reduction rate per pass is at least 10% in at least one pass from the end. As a result, at least one or more passes from the end can generate sufficient processing heat. On the other hand, if the average surface reduction rate is less than 10%, sufficient heat generation due to processing cannot be generated, the temperature of the internal region 4 including the center of gravity G cannot be sufficiently increased, and the β phase Is not fully developed.

In at least one or more passes from the last, the average surface reduction rate per pass may be 10% or more, but a larger processing heat is generated to form a β single-phase structure, and a needle-like structure is formed during subsequent cooling. In order to form, it is preferably 15% or more, more preferably 20% or more. Further, the upper limit of the average surface reduction rate per pass is not particularly limited at least in the last one or more passes, but from the viewpoint of the load on the facility, the average surface reduction rate is preferably 45% or less, more preferably Is 35% or less.

Draw wire speed is 5m / s or more in at least one pass from the last. As a result, the heat removal amount can be reduced in at least one pass from the last, and the heat generated by the processing heat is accumulated in the inner region 4 including the center of gravity G. As a result, the temperature of the inner region 4 becomes sufficiently high. can do. On the other hand, when the wire drawing speed is less than 5 m / s in at least one pass from the last, the amount of heat removal becomes large, and as a result, the heat generated by the processing heat is accumulated in the internal region 4 including the center of gravity G. Therefore, the temperature of the inner region 4 cannot be raised sufficiently. Therefore, the β-single-phase structure is not formed, and it becomes difficult to form a needle-like structure during the subsequent cooling.

At least from the last to one or more passes, the wire drawing speed may be 5 m / s or more, but it is preferably 10 m / s in order to sufficiently develop the β phase and form a needle-like structure during subsequent cooling. Or more, More preferably, it is 20 m / s or more. The upper limit of the wire drawing speed is not particularly limited in at least one pass from the last, but the wire drawing speed is preferably 75 m / s or less, and more preferably from the viewpoint of operation stability and load on equipment. Is 50 m / s or less.

Also, the total area reduction rate of the titanium alloy material processed in this process is 90% or more. Thereby, as described above, the metallographic structure in the outer peripheral region 2 is made equiaxed and refined. On the other hand, when the total area reduction rate of the titanium alloy material is less than 90%, the equiaxed and fine grain structure of the metal structure in the outer peripheral region 2 becomes insufficient. Alternatively, even if the metal structure in the outer peripheral region 2 is equiaxed, the α crystal grains do not become sufficiently fine and have a large grain size.

The total area reduction rate may be 90% or more, but is preferably 95% or more, and more preferably 99% or more in order to more surely equiaxeize and refine the metal structure in the outer peripheral region 2. ..

Note that the area reduction rate per pass refers to the reduction rate of the area after the one pass with respect to the cross-sectional area before the one pass, and the total area reduction rate refers to the cutting of the titanium alloy material before processing in this process. The reduction rate of the cross-sectional area after processing with respect to the area.

Further, the caliber shape of the roll used in this step is not particularly limited as long as the above-mentioned drawing speed and area reduction ratio can be achieved, and a known caliber shape can be used, for example, a perfect circle, an ellipse, A square shape or the like can be used.

Further, the number of times the roll is passed in this step (the number of passes) is not particularly limited, and may be 5 times or more so that this step can be carried out. In addition, it is preferable to perform 10 or more passes in order to achieve a surface reduction rate of 90% or more.

By the above steps, the titanium alloy wire according to the present embodiment as described above can be manufactured industrially and stably. The titanium alloy wire thus obtained may be subjected to the following heat treatment and post-treatment, if necessary.

(2.4 Heat treatment process)
The titanium alloy material (titanium alloy wire rod) obtained by each of the above steps is further subjected to heat treatment (annealing treatment) in a temperature range of (β transformation point −300) ° C. or higher and (β transformation point −50) ° C. or lower. Good. Thereby, the strain generated in the above-mentioned processing step can be removed, and the fatigue strength of the obtained titanium alloy wire rod can be further improved.

In this treatment, the temperature of heat treatment is (β transformation point -300) ° C or higher. Thereby, the strain generated in the processing step can be sufficiently removed. The temperature of the heat treatment is preferably (β transformation point −250) ° C. or higher, more preferably (β transformation point −200) ° C. or higher.

Also, in this treatment, the temperature of heat treatment is (β transformation point −50) ° C. or lower. As a result, it is possible to prevent the fatigue characteristics from being deteriorated due to a mixed (bimodal) structure of an equiaxed structure and a needle-shaped structure in the outer peripheral region 2. The heat treatment temperature is preferably (β transformation point −100) ° C. or lower.

The heat treatment time is not particularly limited and can be appropriately selected. For example, it can be 1 minute or more and 120 minutes or less, preferably 2 minutes or more, or 60 minutes or less.

Also, the atmosphere during the heat treatment is not particularly limited, and may be the atmosphere, vacuum, or an inert gas (argon, etc.). In particular, if the atmosphere is not an atmosphere that promotes a chemical reaction such as oxidation, then it is possible to deal with it by descaling.

(2.5 Post-processing)
Examples of the post-treatment include pickling and removal of oxide scale by cutting, washing treatment, and the like, and they can be appropriately applied as necessary.
The method for manufacturing the titanium alloy wire rod according to the present embodiment has been described above.

The embodiments of the present invention will be specifically described below with reference to examples. The following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

1.
Manufacture of Titanium Alloy Wire Rod First, an ingot having the chemical composition shown in Table 1 was prepared by a vacuum arc melting method, and was hot forged at a temperature in the β single phase region to obtain a predetermined composition of alloy types A to O. A round titanium rod having a diameter (wire diameter of 22 mm to 180 mm) was obtained. In addition, in each titanium round bar, components other than the composition shown in Table 1 are titanium and impurities. Further, all of the alloy types A to M are α + β type titanium alloys that form a two-phase structure having an α phase and a β phase at room temperature or a use environment. Further, the alloy type N is an α + β type titanium alloy in which the β phase hardly exists at room temperature, and the alloy type O is a metastable β type titanium alloy having a martensite transformation start temperature of room temperature or lower.
The alloy types A to M are examples satisfying the composition range defined in claim 1.
Alloy types A to A4 are examples satisfying the component range defined in claim 2.
Alloy types B to B5 are examples satisfying the component range defined in claim 3.
Alloy types C to C9 are examples satisfying the component range defined in claim 4.

Next, each titanium round bar obtained was heated (heating process) and wire drawing was performed using a roll (processing process). Further, a heat treatment step was performed if necessary (heat treatment step). The heat treatment was performed for 10 minutes in an atmosphere of 100% argon. Thereby, the titanium alloy wire rod according to each example was obtained. Heating temperature (° C) in the heating step, average area reduction rate (%) per pass in at least one or more passes from the last in the processing step, wire drawing speed (m / s), total area reduction rate in the processing step (%), Presence / absence of heat treatment step, and heat treatment temperature (° C.) are shown in Tables 2, 3 and 4.

2.
Analysis / Evaluation The titanium alloy wire rod according to each example was analyzed and evaluated for the following items.

2.1 Observation of metallographic structure (microstructure) Regarding the titanium alloy wire according to each example, a cross section perpendicular to the longitudinal direction was observed as follows, and the metallographic structure was an equiaxed structure and a needle in each region of the cross section. It was investigated whether it was a tissue. Further, the average crystal grain size and the average aspect ratio of the α crystal grains were measured and calculated, and the area ratio of the region in which the average aspect ratio of the α crystal grains was 5.0 or more to the cross section was obtained. First, the titanium alloy wire according to each example was mirror-polished on a cross section perpendicular to the longitudinal direction, and then etched with a mixed solution of hydrofluoric acid and nitric acid. The average crystal grain size and the average aspect ratio were measured by observing an optical micrograph of the surface. The average crystal grain size was measured by the line segment method according to JIS G 0551. Specifically, an optical micrograph taken at a magnification of 500 times is divided into five vertical and horizontal line segments, and the average grain size is calculated for each line segment using the number of grain boundaries that cross the line segment. Was calculated from the arithmetic average value of the average crystal grain size of 10 in total. The average aspect ratio is the arithmetic average of the values obtained by dividing the major axis by the minor axis by measuring the major axis and minor axis for 50 arbitrary crystal grains in an optical micrograph taken at a magnification of 500 times. Calculated. Here, the "major axis" refers to the line segment that connects two arbitrary points on the grain boundary (contour) of the α phase and has the maximum length, and the "minor axis" means the length. Of the line segments orthogonal to the axis and connecting any two points on the grain boundary (outline), the one having the maximum length is meant.

2.2 Fatigue strength Regarding the fatigue strength, the maximum stress in the case where the rotating bending fatigue test was performed according to JIS Z 2274: 1978 and the specimen was not broken up to 10 ⁷ times was defined as the fatigue strength.

2.3 Creep Strength Regarding the creep strength, a creep test was conducted according to JIS Z 2271: 2010. Specifically, when the creep test was performed for 100 hours in an environment of 400 ° C., the minimum stress reaching 0.2% strain was defined as the creep strength.

2.4 Evaluation In order to make a comparison with a titanium alloy wire obtained by a manufacturing method corresponding to a conventional manufacturing method for the same alloy type, in the examples of alloy types A to O shown in Table 2 (all comparative examples), The average area reduction rate (%) per pass in at least one or more passes from the last in the working process is 16%, but the wire drawing speed (m / s) is 2.0 m / s (5 m / s). Less than). In the titanium alloy wire rod according to the example shown in Table 2, the metal structure has an equiaxed structure in both the outer peripheral region and the inner region.
On the other hand, in invention examples 1 to 31 of alloy types A to M shown in Table 3, the average area reduction rate (%) per pass in at least one or more passes from the last in the working process was 16%, and The speed (m / s) is 25 m / s. In the titanium alloy wire rods of Inventive Examples 1 to 31 shown in Table 3, the metal structure in the outer peripheral region is an equiaxial structure in which the equiaxed α phase is the parent phase and the fine β phase is present in the grain boundaries or grains. The metallic structure in the inner region was a needle-shaped structure in which needle-like α phase and β phase were arranged in layers.
In Comparative Examples 1 and 2 of alloy types N and O shown in Table 3, the average area reduction rate (%) per pass in at least one pass from the last in the working process was 16%, and The speed (m / s) is 25 m / s. However, in Comparative Example 1, the Mo equivalent (Moeq) is smaller than -4.0. In Comparative Example 1, the metal structure in the outer peripheral region has an α phase composed of equiaxed α crystal grains as a mother phase, and a β phase hardly exists (a very small amount of β phase exists). Therefore, the metal structure of the inner region was a structure in which the α phase having α crystal grains with a relatively small aspect ratio was the parent phase, and the β phase was hardly present (the β phase was present in a very small amount). More specifically, the internal region of Comparative Example 1 has a structure in which equiaxed β phase is finely dispersed in block-shaped α phase.
In Comparative Example 2, the Mo equivalent (Moeq) is larger than 6.0. In Comparative Example 2, both the metal structure of the outer peripheral region and the metal structure of the inner region became a β single-phase equiaxed structure composed of equiaxed β crystal grains.
In Table 3, since the metal structures of the inner regions of Comparative Examples 1 and 2 and the metal structure of the outer region of Comparative Example 2 are different from the equiaxed structure of the present invention, they are distinguished by adding " ^* ". did.

In Tables 2 and 3, the fatigue strengths of the alloy types A to O were compared and evaluated. Based on the fatigue strength of the examples of alloy types A to O shown in Table 2, the following grades A to C were used for evaluation. Then, when the fatigue strength was equal to or higher than the standard fatigue strength, that is, the evaluation of A and B was passed.

A: Improved by 10 MPa or more as compared with the standard fatigue strength.
B: There was a variation in the range of -10 MPa to less than 10 MPa as compared with the standard fatigue strength.
C: More than 10 MPa and 20 MPa or less compared to the standard fatigue strength.

Also, in Tables 2 and 3, the creep strengths (creep stresses) of the alloy types A to O were compared and evaluated. Based on the creep strength of the examples of alloy types A to O shown in Table 2, evaluation was made in the following grades A to C. Then, when the creep strength was improved compared with the standard creep strength, that is, the evaluations of A and B were passed.

A: Improved by 20 MPa or more as compared with the standard creep strength.
B: Improved by 10 MPa or more and less than 20 MPa as compared with the standard creep strength.
C: There was variation in the range of -10 MPa or more and less than 10 MPa as compared with the standard creep strength.

Regarding the alloy types A to O shown in Table 1, in the example of the titanium alloy wire obtained by the manufacturing method corresponding to the conventional manufacturing method, the metal structure in the outer peripheral region, the average aspect ratio of α crystal grains, the average crystal grain size, and Table 2 shows the metal structure in the internal region, the average aspect ratio of α crystal grains, the area ratio of the acicular structure region, and the fatigue strength and creep strength which are the criteria for evaluation. Further, the metal structures in the outer peripheral regions of Invention Examples 1 to 31 (alloy species A to M) and Comparative Examples 1 and 2 (alloy species N and O), average aspect ratio of α crystal grains, average crystal grain size, and internal Table 3 shows the metallographic structure in the region, the average aspect ratio of α crystal grains, the area ratio of the acicular structure region, the fatigue strength to be evaluated and the evaluation result, and the creep strength to be evaluated and the evaluation result.
In Invention Examples 1 to 31, the evaluation of fatigue strength was either A or B, which was equal to or higher than the standard fatigue strength. Further, in Inventive Examples 1 to 31, the evaluation of creep strength was either A or B, which was improved compared with the standard creep strength.
On the other hand, in Comparative Examples 1 and 2, the improvement in creep strength was not sufficient.

Next, in Table 4, the fatigue strength and creep strength of the alloy types A, B and C were compared and evaluated. Inventive Examples 32 to 54 satisfy the present invention in the heating step and the processing step, and in the titanium alloy wire rods of Inventive Examples 32 to 54, the metal structure in the outer peripheral region has the equiaxed α phase as the parent phase and its grain boundaries It became an equiaxed structure in which fine β phase was present in the grains and the metallic structure in the inner region became a needle-shaped structure in which needle-like α phase and β phase were arranged in layers.
On the other hand, in Comparative Examples 3 to 10, either the heating step or the processing step is outside the scope of the present invention, and the titanium alloy wire rods of Comparative Examples 3 to 10 have a metal structure in the outer peripheral region and an average aspect ratio of α crystal grains. The average crystal grain size of the α crystal grains, the metal structure of the internal region, or the average aspect ratio of the α crystal grains was outside the scope of the present invention.
The wire diameters of Inventive Examples 32 to 54 were 1.5 mm to 22.0 mm. Invention Examples 32 to 50, 52 and 53 are examples satisfying the wire diameter of 2.0 mm to 20.0 mm defined in claim 8.

Inventive Examples 32 to 48 and Inventive Examples 51 to 54 are based on the fatigue strength and creep strength in the example of alloy type A in Table 2, and Inventive Example 49 is the fatigue strength and creep strength in the example of alloy type B in Table 2. Inventive Example 50 was evaluated in the grades A to C in the same manner as above, based on the fatigue strength and creep strength in the example of alloy type C in Table 2 based on the above.

As shown in Table 4, the titanium alloy wires according to Inventive Examples 32 to 54 were excellent in fatigue strength and creep strength at the same time. In particular, the titanium alloy wire rods according to Inventive Examples 32 to 54 had good creep strength compared to the reference comparative example. On the other hand, the titanium alloy wire rods according to Comparative Examples 3 to 10 could not have excellent fatigue strength and creep strength at the same time.

In Comparative Example 3, since the total area reduction rate was less than 90.0%, in the outer peripheral region, a small amount of fine β phase was present in the α phase in which the aspect ratio of α crystal grains and the crystal grain size were increased to some extent. , The organization has not been completed in equiaxed (unequalized). Further, in Comparative Example 3, the wire drawing speed was less than 5.0 m / s, and the heat generation during processing was small. Therefore, in the internal region, β was included in the α phase having the α phase composed of equiaxed α crystal grains as the parent phase. It became an equiaxed structure in which the phases were finely dispersed.
In Comparative Example 4, since the average surface reduction rate in at least one or more passes from the end was less than 10.0% and the heat generation during processing was small, the α phase composed of equiaxed α crystal grains was formed in both the inner region and the outer peripheral region. Was the mother phase, and a small amount of the β phase was finely dispersed in the α phase to form an equiaxed structure.
In Comparative Example 5, since the total area reduction rate was less than 90.0%, the outer peripheral region was equiaxed with a small amount of fine β phase in the α phase in which the aspect ratio of α crystal grains was increased to some extent. Was not completed (non-equiaxial), and the internal region was a needle-shaped structure in which needle-like α-phase and β-phase were arranged in layers.
In Comparative Example 6, since the wire drawing speed was less than 5.0 m / s and the heat generation during processing was small, the α phase composed of equiaxed α crystal grains was used as the mother phase in both the inner region and the outer peripheral region, and It became an equiaxed structure in which a small amount of β phase was finely dispersed.
In Comparative Example 7, since the total area reduction rate was less than 90.0%, the outer peripheral region had the α phase composed of coarse equiaxed α crystal grains as the mother phase, and a small amount of β phase dispersed in the α phase. The inner region became a needle-like structure in which needle-like α phase and β phase were arranged in layers.
In Comparative Example 8, since the heating temperature was too low, the α phase formed of equiaxed α crystal grains was used as the mother phase in both the inner region and the outer peripheral region, and a small amount of β phase was finely dispersed in the α phase to form an equiaxed structure. became.
In Comparative Example 9, since the total area reduction ratio was less than 90.0%, the outer peripheral region contained a small amount of fine β phase in the α phase in which the aspect ratio of α crystal grains and the crystal grain size were increased to some extent. The tissue was not equiaxed (unequiaxed), and the inner region was a needle-shaped tissue in which needle-like α phase and β phase were arranged in layers.
In Comparative Example 10, since the total area reduction rate was less than 90.0%, the outer peripheral region had a small amount of fine β phase in the α phase with an aspect ratio increased to a certain extent, and the equiaxing was completed. The internal region was a needle-like structure in which needle-like α-phase and β-phase were arranged in layers.

In particular, the titanium alloy wire rods according to Inventive Examples 32, 33, 36, 39 to 41, and 45 to 52 in which the area ratio of the needle-shaped tissue region including the center of gravity exceeded 40% were excellent in creep strength. Further, the titanium alloy wire rods according to Inventive Examples 32 to 35, 39, 40, 42 to 44, 47 to 50, 53 and 54 in which the average grain size of α crystal grains in the outer peripheral region is 5.0 μm or less have fatigue strength. Was excellent.

The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various alterations or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

a α crystal grain b β phase c needle-like α
e Needle β
1 Titanium alloy wire L Longitudinal direction 2 Outer peripheral region 3 Outer peripheral surface 4 Internal region G Center of gravity R Depth corresponding to wire diameter d 3% 11 Long axis 10 Grain boundary of α phase 12 Short axis 20 β Crystal grain 21 Needle-like α Crystal grain 22 Equiaxed α crystal grain 23 β Crystal grain 24 Equiaxial fine α crystal grain (fine equiaxed structure)
25 Needle-shaped α crystal grains (acicular structure)

Claims (10)

1. A titanium alloy wire rod containing an α phase and a β phase,
   In mass%,
   Al: 0% or more and 7.0% or less,
   V: 0% or more and 6.0% or less,
   Mo: 0% or more and 7.0% or less,
   Cr: 0% or more and 7.0% or less,
   Zr: 0% or more and 5.0% or less,
   Sn: 0% or more and 3.0% or less,
   Si: 0% or more and 0.50% or less,
   Cu: 0% or more and 1.8% or less,
   Nb: 0% or more and 1.0% or less,
   Mn: 0% or more and 1.0% or less,
   Ni: 0% or more and 1.0% or less,
   S: 0% or more and 0.20% or less,
   REM: 0% or more and 0.20% or less,
   Fe: 0% to 2.10%,
   N: 0% to 0.050%,
   O: 0% to 0.250%,
   C: 0% or more and 0.100% or less,
   The balance: Ti and impurities,
   Content of Al, Mo, V, Nb, Fe, Cr, Ni and Mn has a chemical composition satisfying the following formula (1),
   In a cross section perpendicular to the longitudinal direction, the metal structure in the outer peripheral region from the surface toward the center of gravity to a depth of 3% of the wire diameter is an equiaxed structure having α crystal grains with an average crystal grain size of 10 μm or less. ,
   In a cross section perpendicular to the longitudinal direction, the metal structure in the inner region including the center of gravity from the center of gravity to the position of 20% of the wire diameter toward the surface is a needle-shaped structure
   Titanium alloy wire rod.
   −4.00 ≦ [Mo] +0.67 [V] +0.28 [Nb] +2.9 [Fe] +1.6 [Cr] +1.1 [Ni] +1.6 [Mn] − [Al] ≦ 6 .00 ・ ・ ・ (1)
   In addition, in Formula (1), the notation of [elemental symbol] represents the content (mass%) of the corresponding element symbol, and 0 is substituted for the element symbol that does not contain.
2. In mass%,
   Al: 4.5% or more and 6.5% or less,
   Fe: 0.50% or more and 2.10% or less,
   The titanium alloy wire rod according to claim 1, comprising:
3. In mass%,
   Al: 2.0% or more and 7.0% or less,
   V: 1.5% or more and 6.0% or less,
   The titanium alloy wire rod according to claim 1, comprising:
4. In mass%,
   Al: 5.0% or more and 7.0% or less,
   Mo: 1.0% or more and 7.0% or less,
   Zr: 3.0% or more and 5.0% or less,
   Sn: 1.0% or more and 3.0% or less,
   The titanium alloy wire rod according to claim 1, comprising:
5. In a cross section perpendicular to the longitudinal direction, the average aspect ratio of α crystal grains in the outer peripheral region is 1.0 or more and less than 3.0, and the average aspect ratio of α crystal grains in the inner region is 5.0 or more. The titanium alloy wire rod according to any one of claims 1 to 4, which is
6. The area of a region including the center of gravity in which the average aspect ratio of α crystal grains is 5.0 or more in a cross section perpendicular to the longitudinal direction is 40% or more with respect to the area of the cross section. Titanium alloy wire rod.
7. The titanium alloy wire rod according to any one of claims 1 to 6, wherein the α crystal grains in the outer peripheral region have an average crystal grain size of 5.0 μm or less.
8. The titanium alloy wire rod according to any one of claims 1 to 7, having a wire diameter of 2.0 mm or more and 20.0 mm or less.
9. A step of heating the titanium alloy material to a temperature of (β transformation point −200) ° C. or higher;
   The titanium alloy material has a total area reduction rate of 90.0% or more, and an average area reduction rate of 1% or more per pass in at least one or more passes from the end, and a wire drawing speed. With a processing speed of 5.0 m / s or more,
   And a method for manufacturing a titanium alloy wire rod.
10. The method for producing a titanium alloy wire according to claim 9, further comprising a heat treatment in a temperature range of (β transformation point −300) ° C. or higher and (β transformation point −50) ° C. or lower.
    

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