CN113039299B

CN113039299B - Titanium alloy wire rod and method for manufacturing titanium alloy wire rod

Info

Publication number: CN113039299B
Application number: CN201980072341.0A
Authority: CN
Inventors: 塚本元气; 国枝知德; 三好辽太郎; 高桥一浩; 山崎达夫
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-11-15
Filing date: 2019-11-14
Publication date: 2022-07-19
Anticipated expiration: 2039-11-14
Also published as: TW202024346A; JPWO2020101008A1; TWI718763B; KR20210043652A; KR102539690B1; WO2020101008A1; JP7024861B2; CN113039299A

Abstract

A titanium alloy wire rod comprising an alpha phase and a beta phase, wherein in a cross section perpendicular to the longitudinal direction, a metallographic structure of an outer peripheral region having a depth of 3% of a wire diameter from a surface toward a center of gravity is an equiaxed structure having alpha grains having an average grain diameter of 10.0 [ mu ] m or less, and in the cross section perpendicular to the longitudinal direction, a metallographic structure of an inner region including the center of gravity at a position of 20% of the wire diameter from the center of gravity toward the surface is a needle-like structure.

Description

Titanium alloy wire rod and method for manufacturing titanium alloy wire rod

Technical Field

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

Background

Titanium is lightweight and has high strength, and therefore, is a material having excellent specific strength and also excellent corrosion resistance, and is used for various applications such as aircraft, chemical equipment, exterior materials for buildings, ornaments, and consumer goods. 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, and are widely used as titanium alloys.

Patent document 1 proposes an α + β type titanium alloy composed of 0.5% or more and less than 1.4% of Fe, 4.4% or more and less than 5.5% of Al, and the balance titanium and impurities, for the purpose of obtaining a titanium alloy having stable fatigue strength with less variation and high hot workability.

Documents of the prior art

Patent literature

Patent document 1: japanese laid-open patent publication No. 7-70676

Disclosure of Invention

Problems to be solved by the invention

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

In view of the above problems, an object of the present invention is to provide a titanium alloy wire rod excellent in fatigue strength and creep strength and a method for producing a titanium alloy wire rod capable of industrially stably producing a titanium alloy wire rod.

Means for solving the problems

The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, focused attention on the characteristics of the needle-like structure and equiaxed structure of the titanium alloy wire rod and the positions where the needle-like structure and equiaxed structure exist. The needle-like structure is excellent in creep characteristics and the equiaxed structure is excellent in fatigue characteristics. Further, by arranging the needle-like structure and the equiaxed structure at predetermined positions, a titanium alloy wire rod capable of achieving both fatigue strength and creep strength at an excellent level was found. Further, the present inventors have found that, as a method for arranging a predetermined needle-like structure and equiaxed structure, heat generated during processing in the production of a titanium alloy wire can be utilized, and further studied it, and as a result, the present invention has been completed.

The gist of the present invention completed based on the above findings is as follows.

[1]

A titanium alloy wire, which is a titanium alloy wire comprising an α phase and a β phase, having the following chemical composition:

in mass percent

Al: 0% to 7.0%,

V: 0% to 6.0%,

Mo: 0% to 7.0%,

Cr: 0% to 7.0%,

Zr: 0% to 5.0%,

Sn: 0% to 3.0%,

Si: 0% to 0.50%, B,

Cu: 0% to 1.8%, a,

Nb: 0% to 1.0%,

Mn: 0% to 1.0%,

Ni: 0% to 1.0%,

S: 0% to 0.20%,

REM: 0% to 0.20%,

Fe: 0% to 2.10%, a,

N: 0% to 0.050%,

O: 0% to 0.250% inclusive,

C: 0% to 0.100%,

The balance is as follows: ti and impurities, and the balance of the Ti and the impurities,

the contents of Al, Mo, V, Nb, Fe, Cr, Ni and Mn satisfy the following formula (1),

the titanium alloy wire has a metallographic structure having an equiaxed structure with alpha grains having an average grain diameter of 10 [ mu ] m or less in a cross section perpendicular to the longitudinal direction, the metallographic structure being a region from the surface to the center of gravity and extending to a depth of 3% of the wire diameter,

in the cross section perpendicular to the longitudinal direction, the metallographic structure of the inner region including the center of gravity at a position from the center of gravity to the surface of 20% of the line diameter is a needle-like 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 the formula (1), [ element symbol ] represents the content (mass%) of the corresponding element symbol, and the element symbol which is not contained is substituted for 0.

[2]

The titanium alloy wire according to [1], which contains, in mass%

Al: 4.5% to 6.5%,

Fe: 0.50% or more and 2.10% or less.

[3]

The titanium alloy wire according to [1], which contains, in mass%

Al: 2.0% to 7.0%,

V: 1.5% or more and 6.0% or less.

[4]

The titanium alloy wire rod according to [1], which contains, in mass%

Al: 5.0% to 7.0%,

Mo: 1.0% to 7.0%,

Zr: 3.0% to 5.0%,

Sn: 1.0% or more and 3.0% or less.

[5]

The titanium alloy wire rod according to any one of [1] to [4], wherein in the cross section perpendicular to the longitudinal direction, an average aspect ratio of the α crystal grains in the outer peripheral region is 1.0 or more and less than 3.0, and an average aspect ratio of the α crystal grains in the inner region is 5.0 or more.

[6]

The titanium alloy wire according to [5], wherein an area of a region including a center of gravity in which an average aspect ratio of the α crystal grains is 5.0 or more in the cross section perpendicular to the longitudinal direction is 40% or more with respect to an area of the cross section.

[7]

The titanium alloy wire rod according to any one of [1] to [6], wherein an average crystal grain diameter of the α crystal grains in the outer peripheral region is 5.0 μm or less.

[8]

The titanium alloy wire rod according to any one of [1] to [7], wherein a wire diameter is 2.0mm or more and 20.0mm or less.

[9]

A method for manufacturing a titanium alloy wire rod, comprising the steps of:

heating the titanium alloy billet to a temperature of not less than (beta transformation point-200) DEG C;

processing the titanium alloy material under the following conditions: the total cross-sectional shrinkage is 90.0% or more, and the average cross-sectional shrinkage per 1 pass of at least 1 or more passes from the final pass is 10.0% or more, and the drawing speed is 5.0m/s or more.

[10]

The method for producing a titanium alloy wire according to item [9], further comprising a step of performing a heat treatment in a temperature range of (β transformation point-300) ° C or higher and (β transformation point-50) ° C or lower.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a titanium alloy wire rod excellent in fatigue strength and creep strength and a method for producing a titanium alloy wire rod capable of industrially stably producing a titanium alloy wire rod can be provided.

Drawings

Fig. 1 is an explanatory view schematically showing an isometric structure.

Fig. 2 is an explanatory view schematically showing a needle-like structure.

Fig. 3 is an oblique sectional view schematically showing a titanium alloy wire according to an embodiment of the present invention.

Fig. 4 is an explanatory view schematically showing a state of determining the major axis and the minor axis.

Fig. 5 (a) to (e) are explanatory views schematically showing the manufacturing process of the titanium alloy wire rod of the present embodiment in this order.

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

< 1. titanium alloy wire rod >

First, a titanium alloy wire rod according to the present embodiment will be described.

(1.1 metallographic structure)

First, a metallic structure of the titanium alloy wire rod according to the present embodiment will be described. The titanium alloy wire rod according to the present embodiment is composed of an α + β type titanium alloy having a chemical composition described later, and has a two-phase structure mainly composed of an α phase at room temperature and a small amount of a β phase in the α phase. Here, the phrase "mainly" as the α phase means that the area ratio of the α phase is 70% or more. The area ratio of the beta phase is about 2 to 30 percent. In the titanium alloy wire rod focused on each embodiment of the present invention, it is difficult to measure the area ratio of the β phase, and the allowable measurement error is ± 5%.

In a cross section perpendicular to the longitudinal direction, a metallographic structure of an outer peripheral region from the surface to the center of gravity to a position 3% of the wire diameter is an equiaxed structure having equiaxed α -grains having an average grain diameter of 10 μm or less, and in the cross section perpendicular to the longitudinal direction, a metallographic structure of an inner region including the center of gravity from the center of gravity to a position 20% of the wire diameter is a needle-like structure having needle-like α -grains.

As shown in fig. 1, in the equiaxed structure of the α + β titanium alloy, an equiaxed α -crystal grain a assembly structure is formed, and a fine β -phase b is present in a grain boundary between α -crystal grains a and in a crystal grain.

The needle-like structure is a metal structure in which titanium in a β phase at a high temperature is cooled to develop a needle-like α phase from a grain boundary. As shown in fig. 2, the acicular structure of the α + β titanium alloy has a structure in which acicular α (as indicated by symbol c in fig. 2) and acicular β (as indicated by symbol e in fig. 2) that have developed into a needle shape from the grain boundary position of the original β crystal grains are layered.

Thus, the equiaxed structure and the needle-like structure can be distinguished by observing the metallographic structure.

The titanium alloy wire rod according to the present embodiment has excellent fatigue strength and creep strength at the same time by arranging the needle-like structure and the equiaxed structure at predetermined positions. Specifically, the titanium alloy has excellent creep characteristics of the needle-like structure and excellent fatigue characteristics of the equiaxed structure. The starting point of fatigue fracture occurs in the vicinity of the surface layer (outer periphery) of the titanium alloy wire rod. Therefore, the present inventors have conceived that a fine equiaxed structure is arranged near the surface layer of the titanium alloy wire rod to improve the fatigue strength, and a needle-like structure having excellent creep strength is arranged near the center of gravity of the titanium alloy wire rod to ensure sufficiently excellent creep strength.

As an index of the fine equiaxed structure in the vicinity of the surface layer, the present inventors have focused attention on the average long diameter ratio and the average crystal grain diameter of α crystal grains in the outer peripheral region of the titanium alloy wire rod, and found that the fatigue strength of the titanium alloy wire rod can be improved by making them within a predetermined range, that is, by forming a region of the fine equiaxed structure (equiaxed structure region) in the outer peripheral region. Further, as an index of the needle-like structure in the inner region including the center of gravity, the present inventors have paid attention to the average aspect ratio of α crystal grains in the region including the center of gravity, and found that the creep strength of the titanium alloy wire rod can be improved by making the average aspect ratio constant or higher, that is, by forming the needle-like structure (needle-like structure region) in the region including the center of gravity. This can improve both the creep strength and the fatigue strength of the titanium alloy wire rod.

The present inventors have also found that a titanium alloy wire having a metallographic structure as described above can be produced by a method for producing a titanium alloy wire according to the present embodiment, which will be described in detail later, and have completed the present invention. The following specifically describes the metallic structure of the titanium alloy wire rod according to the present embodiment.

Fig. 3 is an explanatory view schematically showing 1 example of the titanium alloy wire rod 1 according to the present embodiment. For convenience of explanation, the sizes of the respective regions shown in the drawings are appropriately enlarged and reduced, and are not actual sizes of the respective regions.

The cross-sectional shape of the titanium alloy wire rod according to the present invention may be any shape, and the titanium alloy wire rod 1 according to the present embodiment will be described below as having a circular cross-section in a cross-section perpendicular to the longitudinal direction L. 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 is defined as a region that is cut from the outer peripheral surface 3 to the center of gravity G to a depth d corresponding to 3% of the wire diameter R in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. In some cases, scale or the like may adhere to the outer peripheral surface 3 of the titanium alloy wire rod 1, but the thickness of the deposit is not included in the outer peripheral surface that is the starting point of measurement of the depth d of the outer peripheral region 2.

In the present specification, as shown in fig. 3, the inner region 4 is defined as a region including the center of gravity G at a position of 20% of the line 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. 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 as a so-called "geometric center" defined based on the cross-sectional shape thereof. In the present embodiment, since a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 is circular, the center of gravity G shown in fig. 3 is the center of the circular cross section.

In the present embodiment, since the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 is circular, the wire diameter R can be defined as the diameter of the circular cross section. When the cross section of the titanium alloy wire rod 1 is not circular, for example, elliptical, the linear diameter R may be defined as an average of a major diameter and a minor diameter in an elliptical cross section.

In a cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1, the titanium alloy wire 1 according to the present embodiment has an equiaxial structure having equiaxial α -grains in a metallographic structure of an outer peripheral region 2 extending from the outer peripheral surface 3 of the titanium alloy wire 1 to a depth d corresponding to 3% of the wire diameter R toward the center of gravity G. When the metallographic structure of the outer peripheral region 2 is an equiaxed structure, the ductility of the outer peripheral region 2 of the titanium alloy wire rod 1 is improved, the surface properties are improved, and defects that become starting points of fatigue fracture at the surface are reduced. This can prevent breakage of the titanium alloy wire rod 1 during production, and can improve fatigue characteristics. On the other hand, when the metallographic structure of the outer peripheral region 2 of the titanium alloy wire rod 1 is a needle-like structure, the ductility is reduced, and as a result, the fatigue strength of the titanium alloy wire rod 1 cannot be made excellent.

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 more excellent fatigue strength, the upper limit is preferably 2.5, and more preferably 2.0. When the metallographic structure of the outer peripheral region 2 is a completely equiaxed structure, the average aspect ratio of the α crystal grains is theoretically "1". Therefore, the lower limit of the average aspect ratio of α grains in outer peripheral region 2 is 1.0.

In the present embodiment, the average crystal grain diameter of the α crystal grains in the outer peripheral region is 10.0 μm or less. As a result, the metallographic structure in the outer peripheral region is a fine structure, the surface roughness is reduced with the equiaxial formation of α -grains, and defects at the surface, which are starting points of fatigue fracture, are reduced, resulting in an improvement in the fatigue strength of the titanium alloy wire rod. On the other hand, if 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 rod cannot be made excellent due to, for example, increased surface roughness.

The average crystal grain size of the 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 rod, it is preferably 5.0 μm or less, and more preferably 3.0 μm or less.

The lower limit of the average crystal grain diameter of the α crystal grains in the outer peripheral region may be, for example, 1.0 μm. If the amount is less than this, the production is difficult and 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, the metallographic structure of the inner region 4 including the center of gravity from the center of gravity G of the titanium alloy wire rod 1 to a position of 20% of the wire diameter on the surface exhibits a needle-like structure having needle-like α crystal grains. When the metallographic structure of the inner region 4 is a needle-like structure, the creep strength of the titanium alloy wire rod is improved. On the other hand, if the metallographic structure of the inner region 4 in the titanium alloy wire rod 1 does not sufficiently develop into a needle-like structure, the creep strength of the titanium alloy wire rod 1 is insufficient.

Creep is the following phenomenon: the dislocation introduced into the metallographic structure by the deformation is recovered by diffusion of atoms, and the material is softened and deformed. Therefore, the rate of recovery (diffusion rate of atoms) affects creep. The needle-like structure is considered to have excellent creep strength because the α/β interface formed by the needle-like structure has high uniformity and the diffusion rate of atoms is low. The creep strength can be improved by forming the metallographic structure of the inner region 4 including the center of gravity G of the titanium alloy wire rod 1 into a needle-like structure.

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

In the present embodiment, in the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1, the area ratio of the region including the center of gravity G (the needle-like structure region including the center of gravity G and having needle-like α crystal grains) in which the average aspect ratio of the α crystal grains is 5.0 or more may be 20% or more, for example, with respect to the area of the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. 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% or more, with respect to the area of the cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1.

From the viewpoint of making the metallographic structure an equiaxed structure in the outer peripheral region, the area ratio of a region including the center of gravity G (needle-like structure region including needle-like α crystal grains including the center of gravity G) in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1 in which the average aspect ratio of α crystal grains is 5.0 or more is preferably 90% or less, and more preferably 80% or less, with respect to the area of the cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1.

It is preferable that the equiaxed structure continuously changes from the equiaxed structure to the needle-like structure between the outer peripheral region 2 formed of the equiaxed structure shown in fig. 1 and the needle-like structure region including the center of gravity G, but there is no problem even if these structures coexist.

The average grain size and the average aspect ratio of α grains in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 can be determined as described below. First, a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 was mirror-polished and then etched with a mixed aqueous solution of hydrofluoric acid and nitric acid. The average crystal grain diameter and average aspect ratio can be determined by observing an optical micrograph of the surface.

The average crystal grain diameter can be measured by a line segment method (according to JIS G0551). In an outer peripheral region 2 of a depth d corresponding to 3% of a wire diameter R from an outer peripheral surface 3 of a titanium alloy wire 1 toward a center of gravity G, 5 line segments are vertically and horizontally drawn on an optical microscope photograph taken at a magnification of, for example, 500 times, and an average crystal grain diameter is calculated for each line segment by using the number of grain boundaries crossing the line segment, and the average crystal grain diameter is obtained by an arithmetic average of 10 average crystal grain diameters in total.

The average aspect ratio can be calculated by: in an outer peripheral region 2 of a titanium alloy wire rod 1 from an outer peripheral surface 3 to a center of gravity G to a depth d corresponding to 3% of a wire diameter and an inner region 4 including the center of gravity G at a position from the center of gravity G to the surface 3 to 20% of the wire diameter R, for example, a long axis and a short axis are measured for any 50 crystal grains in an optical microscope photograph taken at a magnification of 500 times, and the values obtained by dividing the long axis by the short axis are averaged. Here, as shown in fig. 4, "major axis 11" refers to a line segment having the largest length among line segments connecting arbitrary 2 points on the grain boundary 10 (contour) of the α phase, and "minor axis 12" refers to a line segment having the largest length among line segments perpendicular to the major axis 11 and connecting arbitrary 2 points on the grain boundary 10 (contour).

Here, it is considered that the average aspect ratio of α crystal grains is the same value when measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 and when measured in a cross section parallel to the longitudinal direction of the titanium alloy wire rod 1. However, when the measurement is performed in a cross section parallel to the longitudinal direction L of the titanium alloy wire rod 1, it may be difficult to distinguish a structure having elongated α grains elongated by rolling from a needle-like structure having needle-like α grains. Therefore, the value is determined using a value measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. It is considered that the value of the aspect ratio of the α crystal grains differs between the case where the structure having the α crystal grains elongated by rolling is measured in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1 and the case where the structure is measured in a cross section parallel to the longitudinal direction L of the titanium alloy wire rod 1. Specifically, the structure having α grains elongated by rolling is observed as α grains having a large aspect ratio (for example, 5.0 or more) when measured in a cross section parallel to the longitudinal direction L of the titanium alloy wire rod 1, and as α grains having 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. 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, it is possible to distinguish between the α crystal grains elongated by rolling and the needle-like α crystal grains.

When the average grain size and the average aspect ratio of α grains are determined, it is considered that α grains having the same orientation are aligned with a fine needle-like β phase interposed therebetween. EBSD has difficulty detecting fine beta phases and thus EBSD-based analysis may be difficult to perform.

Strictly speaking, the center of gravity G exists as a "point" in a cross section perpendicular to the longitudinal direction L of the titanium alloy wire rod 1. Therefore, when 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 rod 1 is observed, the aspect ratio of the α crystal grains is observed in a region ranging from the center of gravity G to the outer circumferential surface 3 by 20% of the wire diameter R, and the observed aspect ratio is averaged.

The above description explains the metallic structure of the titanium alloy wire rod according to the present embodiment.

(1.2 chemical composition)

Next, the chemical composition of the titanium alloy wire rod according to the present embodiment will be described. The chemical composition of the titanium alloy wire rod 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 at the time of use or at room temperature, and for example, an α + β type titanium alloy having various compositions described in JIS H4600 and JIS H4650 can be used. Alternatively, the element described below may be contained. In the present specification, including the following description, unless otherwise specified, "%" represents the content, and the "%" represents mass%.

Al: 0% or more and 7.0% or less

Aluminum (Al) is an element that is solid-dissolved in the α phase to strengthen the α phase. The α + β type titanium alloy wire rod may not contain Al, but may contain Al in an amount of 2.0% or more, preferably 2.5% or more, in order to obtain the effect. On the other hand, when the content of Al is too large, α may exist depending on the chemical composition₂Phase (Ti)₃Al) precipitates to lower the ductility, and the amount of α phase increases to lower the hot workability, so 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 the thermoformability and heat treatability. The α + β type titanium alloy wire rod may not contain V, but may contain 1.5% or more, preferably 2.0% or more, of V in order to obtain the effect. On the other hand, if the content of V 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 rod may decrease, so the content of V may be 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, improving the hot formability and hot treatability. The α + β type titanium alloy wire rod may not contain Mo, but may contain Mo in an amount of 1.0% or more, preferably 1.5% or more, in order to obtain the effect. 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 titanium alloy wire rod may decrease, so the Mo content may 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, improving thermoformability and heat treatability. The α + β type titanium alloy wire rod may not contain Cr, but may contain Cr in an amount of 2.0% or more, preferably 3.0% or more, in order to obtain the 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 rod may decrease, so the Cr content may be 7.0% or less, preferably 6.0% or less.

Zr: 0% or more and 5.0% or less

Zirconium (Zr) is an element that strengthens both the α phase and the β phase. The α + β type titanium alloy wire rod may not contain Zr, but may contain 1.5% or more, preferably 2.0% or more, in order to obtain this effect. On the other hand, when the Zr content is too large, a promoter α exists depending on the chemical composition₂Phase (Ti)₃Al) and hence the ductility decreases, the Zr content may be 5.0% or less, preferably 4.5% or less.

Sn: 0% or more and 3.0% or less

Tin (Sn) is an element that strengthens both the α phase and the β phase. The α + β type titanium alloy wire rod may not contain Sn, but may contain 1.0% or more, preferably 1.5% or more of Sn in order to obtain the effect. On the other hand, when the content of Sn is too large, there is a promotion of α depending on the chemical composition₂Phase (Ti)₃Al) and hence ductility is reduced, the content of Sn may be 3.0% or less, preferably 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 rod may not contain Si, but may contain Si in an amount of 0.04% or more, preferably 0.07% or more, in order to obtain the effect. On the other hand, if the Si content is too large, the creep strength may be reduced by precipitation of silicide depending on the chemical composition, and therefore the Si content may be 0.50% or less, preferably 0.35% or less.

Cu: 0% or more and 1.8% or less

Copper (Cu) stabilizes the β phase and also dissolves in the α phase to strengthen the α phase. The α + β type titanium alloy wire rod may not contain Cu, but may contain Cu in an amount of 0.4% or more, preferably 0.8% or more, in order to obtain the effect. On the other hand, when the Cu content is too large, Ti is present depending on the chemical composition₂Since the fatigue strength is lowered by Cu deposition, the Cu content may be 1.8% or lessPreferably 1.5% or less.

Nb: 0% or more and 1.0% or less

Niobium (Nb) improves oxidation resistance. The α + β type titanium alloy wire rod may not contain Nb, but may contain 0.1% or more, preferably 0.2% or more of Nb in order to obtain the effect. 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 rod may decrease, so the Nb content may be 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, improving thermoformability and heat treatability. The α + β type titanium alloy wire rod may not contain Mn, but may contain Mn in an amount of 0.1% or more, preferably 0.2% or more, in order to obtain the 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 rod may decrease, so the Mn content may be 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, improving thermoformability and heat treatability. The α + β type titanium alloy wire rod may not contain Ni, but may contain 0.1% or more, preferably 0.2% or more of Ni in order to obtain the effect. 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 rod may decrease, so the Ni content may 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 rod may not contain S, but may contain 0.01% or more, preferably 0.03% or more, of S in order to obtain the effect. On the other hand, when the content of S is too large, the thermoformability may be deteriorated due to the generation of inclusions depending on the chemical composition, and therefore the content of S may be 0.20% or less, preferably 0.10% or less.

REM: 0% or more and 0.20% or less

By containing both S and a rare earth element (REM), machinability is improved. The α + β type titanium alloy wire rod may not contain REM, but may contain REM in an amount of 0.01% or more, preferably 0.03% or more, in order to obtain the effect. On the other hand, when the content of REM is too large, the hot formability may be deteriorated by the generation of inclusions depending on the chemical composition, and therefore, the content of REM may be 0.20% or less, preferably 0.10% 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) may be mentioned, and these elements may be contained alone in 1 kind or may be contained in combination of 2 or more kinds. When 2 or more rare earth elements are contained, for example, a mixture or a compound of rare earth elements such as a misch metal (misch metal alloy) before separation and purification, and didymium (alloy containing Nd and Pr) can be used. When 2 or more rare earth elements are contained, the REM amount refers to the total amount of all rare earth elements.

Fe: 0% or more and 2.10% or less

Iron (Fe) is an element that strengthens the beta phase. The α + β type titanium alloy wire rod may not contain Fe, and may contain 0.50% or more, preferably 0.70% or more of Fe in order to obtain the effect. On the other hand, if the Fe content is too large, manufacturability may be reduced by segregation of Fe or ductile ductility may be reduced by precipitation of intermetallic compounds (TiFe) depending on the chemical composition, and therefore 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 is solid-dissolved in the α phase to strengthen the α phase. The α + β type titanium alloy wire rod may not contain N, but may contain 0.002% or more, preferably 0.005% or more of N in order to obtain the effect. On the other hand, if the N content is too large, low-density inclusions (TiN) may be formed depending on the chemical composition and may become a starting point of fatigue fracture, and therefore, the N content is 0.050% or less, preferably 0.030% or less.

O: 0% or more and 0.250% or less

Oxygen (O) is an element that is solid-dissolved in the α phase to strengthen the α phase. The α + β type titanium alloy wire rod may not contain O, but may contain 0.050% or more, preferably 0.100% or more of O in order to obtain the effect. On the other hand, when the content of O is too large, the α phase may excessively increase depending on the chemical composition, and the ductility may decrease, so that the content of O is 0.250% or less, preferably 0.200% or less.

C: 0% or more and 0.100% or less

Carbon (C) is solid-soluble in the α phase to strengthen the α phase, and improves machinability by containing S together. The α + β type titanium alloy wire rod may not contain C, but may contain 0.005% or more, preferably 0.010% or more, in order to obtain the effect. On the other hand, when the content of C is too large, carbide may excessively increase depending on the chemical composition, and the hot formability may deteriorate, and therefore, the content of C is 0.100% or less, preferably 0.080% or less.

The balance of the chemical components of the titanium alloy wire rod according to the present embodiment may be titanium (Ti) and impurities. The impurities are components mixed in from raw materials and other reasons in the industrial production of the titanium alloy wire rod, and are components that are allowed within a range that does not adversely affect the titanium alloy wire rod according to the present embodiment.

Examples of the impurities include hydrogen (H), tantalum (Ta), cobalt (Co), tungsten (W), palladium (Pd), boron (B), chlorine (Cl), sodium (Na), magnesium (Mg), and calcium (Ca). When the elements H, Ta, Co, Pd, W, B, Cl, Na, Mg, and Ca are contained as impurities, the contents thereof are, for example, 0.05% or less, respectively, and the total content thereof is 0.10% or less.

Mo equivalent

In the chemical components of the titanium alloy wire rod according to the present embodiment, the contents of Al, Mo, V, Nb, Fe, Cr, Ni, and Mn further satisfy the following formula (1).

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

The Mo equivalent a shown on the right side of the above formula (1) is used to numerically express the degree of stabilization of the β phase by each element (β stabilizing element) Mo, V, Nb, Fe, Cr, Ni, and Mn described in the formula for stabilizing the β phase. In this case, the degree of stabilization of the β phase by the β stabilizing element other than Mo is made relative to the degree of stabilization of the β phase by Mo by a positive coefficient. On the other hand, since Al is an element (α stabilizing element) which is solid-dissolved in the α phase to strengthen the α phase, the correlation coefficient of Al in the above Mo equivalent a is a negative value.

[ range of Mo equivalent A: -4.00 & lt, A & lt, 6.00 & gt

The titanium alloy wire rod according to the present embodiment contains at least any 1 or more elements selected from the group consisting of Mo, V, Nb, Fe, Cr, Ni, and Mn so that the value of Mo equivalent a represented by the above formula (1) is in the range of-4.00 or more and 6.00 or less. When the value of Mo equivalent A is less than-4.00, the amount of beta phase is too small, the formation of a needle-like structure is difficult, and the creep property is not improved. The lower limit of the Mo equivalent A is preferably-3.50, more preferably-3.00. On the other hand, if the value of Mo equivalent a exceeds 6.00, the β phase cannot form a needle-like α phase during cooling, and the inside becomes a β single-phase structure, so that the creep characteristics are not improved. The upper limit of the Mo equivalent A is preferably 5.00, more preferably 4.00.

The titanium alloy wire rod with the chemical composition is an alpha + beta type titanium alloy wire rod with alpha phase and beta phase.

More specifically, the titanium alloy wire rod may contain

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.

Note that the present invention may be applied to any other type of device

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.

The titanium alloy wire rod with the chemical composition is an alpha + beta type titanium alloy wire rod with an alpha phase and a beta phase, and has stable fatigue strength with less deviation and high hot workability. Examples of the titanium alloy wire rod having such a chemical composition include Super-TiX 51AF (Ti-5Al-1Fe, manufactured by Nippon iron Co., Ltd.).

Alternatively, the titanium alloy wire rod may contain

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.

Note that the present invention may be applied to any other type of device

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.

The titanium alloy wire rod with the chemical composition is also a titanium alloy wire rod containing an alpha phase and a beta phase of an alpha + beta type, and has stable fatigue strength with less deviation and high hot workability. Further, examples of the titanium alloy wire rod having such a chemical composition include Ti-3Al-2.5V, Ti-6Al-4V, SSAT-35(Ti-3Al-5V, manufactured by Nippon iron Co., Ltd.).

In addition, the titanium alloy wire rod may contain

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.

Note that the present invention may be applied to any other type of device

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.

The titanium alloy wire rod with the chemical composition is an alpha + beta type titanium alloy wire rod containing an alpha phase and a beta phase, and has particularly excellent creep characteristics. Examples of the titanium alloy wire rod having such a chemical composition include Ti-6Al-2Sn-4Zr-2Mo-0.08Si, and Ti-6Al-2Sn-4Zr-6 Mo.

The chemical composition of the titanium alloy wire rod according to the present embodiment is explained above.

(1.3 diameter, shape)

The wire diameter R of the titanium alloy wire rod 1 according to the present embodiment is not particularly limited, and may be, for example, 2mm to 20 mm. When the wire diameter R of the titanium alloy wire rod 1 is 2mm or more, the needle-like structure having needle-like α crystal grains can be formed in the inner region 4 including the center of gravity G, and the fine equiaxial structure having fine equiaxial α crystal grains can be more reliably formed in the outer peripheral region 2, and both the fatigue strength and the creep strength can be more reliably and simultaneously excellent. Further, by setting the wire diameter R of the titanium alloy wire rod 1 to 20mm or less, it is possible to perform wire drawing at high speed, the central portion of the rod wire is easily processed stably and generates heat, and a needle-like structure is easily obtained in the inner region 4 near the center of gravity. The lower limit of the wire diameter R of the titanium alloy wire rod 1 according to the present embodiment is preferably 3mm, 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 shape, and may be a polygon such as an ellipse or a square, for example, in addition to a circle.

In the present embodiment described above, in the cross section perpendicular to the longitudinal direction L of the titanium alloy wire 1, the metallographic structure passing through the outer peripheral region 2 is a fine equiaxed structure having equiaxed α -grains with an average grain diameter of 10 μm or less, and the metallographic structure of the inner region 4 including the center of gravity G is a needle-like structure having needle-like α -grains, so that the fatigue strength and creep strength of the titanium alloy wire are excellent 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, the titanium alloy wire according to the present embodiment can be used for any application, but can be suitably used for fasteners (fasteners) such as bolts and nuts, valves, and the like. The titanium alloy wire according to the present embodiment can be suitably used as a fastener or a valve material for transportation equipment such as an aircraft, an automobile, or the like.

The titanium alloy wire rod according to the present embodiment described above can be produced by any method, but for example, it can also be produced by the method for producing a titanium alloy wire rod according to the present embodiment described below.

< 2. method for producing titanium alloy wire rod

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 the steps of: a step (heating step) of heating the titanium alloy material to a temperature of not less than (beta transformation point-200) DEG C; and a step (machining step) of machining the α + β type titanium alloy material under the following conditions: the total cross-sectional shrinkage is 90.0% or more, and the average cross-sectional shrinkage per 1 pass of at least 1 or more passes from the final pass is 10% or more, and the drawing speed is 5m/s or more. The respective steps will be explained below.

(2.1 preparation of titanium alloy ingot)

First, a titanium alloy material is prepared before the above steps.

As the titanium alloy material, a material having the above chemical composition can be used, and a material produced by a known method can be used. For example, a titanium alloy ingot can be obtained by making an ingot from titanium sponge by a vacuum arc melting method, and hot forging it at a temperature of a β single phase region. The titanium alloy ingot may be subjected to pretreatment such as cleaning treatment and pickling, if necessary.

The wire diameter of the titanium alloy material may be appropriately selected according to the cross-sectional shrinkage rate predetermined in the machining step and the wire diameter of the titanium alloy wire rod predetermined.

(2.2 heating Process)

In this step, the titanium alloy ingot is heated to a temperature of not less than (beta. transformation point-200) ° c. This promotes a reduction in deformation resistance, facilitates maintenance of the temperature near the center of gravity of the titanium alloy material at or above the β -transus point in the subsequent processing step, and promotes development of the needle-like structure near the center of gravity of the titanium alloy material. As a result, in the processing step described later, the average aspect ratio of α crystal grains in the vicinity of the center of gravity (inner region) can be set to 5.0 or more. On the other hand, when the heating temperature in this step is lower than (β transformation point-200) ° c, the deformation resistance becomes too large, or it becomes difficult to maintain the temperature in the vicinity of the center of gravity of the titanium alloy material at β transformation point or higher in the later-described working step, and thus the needle-like structure cannot be sufficiently developed in the vicinity of the center of gravity of the titanium alloy material, 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, and more preferably (β transformation point-125) ° c or higher. The upper limit of the heating temperature in this step is not particularly limited, but from the viewpoint of a reduction in yield due to scale formation, the heating temperature is preferably (β transformation point +100) ° c or less, and more preferably (β transformation point +50) ° c or less.

In the present specification, the "β transformation point" refers to a temperature at which β transformation is completed when the titanium alloy is heated. The titanium alloy wire rod according to the present embodiment or the titanium alloy material as a raw material thereof is an α + β two-phase region in which an α phase and a β phase exist at room temperature or in a use environment, and the temperature at which β transformation starts is equal to or lower than the room temperature or the use environment.

The beta transus temperature T can be obtained from a phase diagram. The Phase diagram can be obtained, for example, by the CALPHAD (Computer Coupling of Phase diagnostics and Thermochemistry) method, for which, for example, the comprehensive thermodynamic computing system Thermo-Calc and the specified database (TI3) from Thermo-Calc Software AB can be used.

(2.3 working procedure)

This step is a so-called wire drawing step of drawing the titanium alloy billet by successively passing through a plurality of rolling passes.

The working process is performed by continuous rolling instead of reversible rolling. Continuous rolling is a method in which a rolled material is continuously passed through a plurality of rolling passes arranged in series, and is rolled in sequence in one direction at each rolling pass. By manufacturing a titanium alloy wire rod by using continuous rolling, a titanium alloy billet can be processed under the following conditions: the total cross-sectional shrinkage is 90.0% or more, and the average cross-sectional shrinkage per 1 pass of at least 1 or more passes from the final pass is 10% or more, and the drawing speed is 5m/s or more.

Here, a process of manufacturing the titanium alloy wire rod according to the present embodiment through a machining process will be described with reference to the drawings (a drawing showing a cross section perpendicular to the longitudinal direction). Fig. 5 (a) to (e) schematically show the manufacturing process of the titanium alloy wire rod of the present embodiment in this order.

First, in the heating step, the microstructure is changed to an α + β structure having a β phase as a main phase or a β single phase by heating to a temperature of (β transformation point-200) ° c or higher. Here, as shown in fig. 5 (a), a case of a β single-phase structure composed of only β crystal grains 20 will be described. Then, in the initial stage of the processing, as shown in fig. 5 (b), when the phase changes from the β phase to the α phase with a decrease in temperature, needle-shaped α crystal grains 21 are formed, and an acicular structure composed of the α phase and the β phase is formed. The needle-like structure is a structure in which needle-like α and needle-like β are layered and arranged as a needle.

Next, in the middle stage of the machining step, the needle-like α crystal grains 21 are divided by applying machining, and further, the equiaxed α crystal grains 22 are formed by grain growth, as shown in fig. 5 (c). In the middle stage of the working process, the drawing speed (strain speed) is also low, and the heat generation during working is low, so that the temperature near the center of gravity does not exceed the β transformation point (the temperature is as high as the β single phase region). Therefore, an equiaxed α + β type equiaxed structure in which equiaxed α crystal grains 22 are mixed with equiaxed β crystal grains is formed.

Subsequently, in the latter stage of the working process, the drawing speed increases, and the temperature rises to a temperature equal to or higher than the β -transus point near the center of gravity due to heat generated by the working. As a result, as shown in fig. 5 (d), the α phase changes to the β phase in the inner region including the center of gravity, and a β single-phase structure composed only of β crystal grains 23 is formed.

In general, titanium alloys have a large deformation resistance, and generate a large amount of heat during rolling or wire drawing. In particular, in the latter stage of the working process, since the average cross-sectional shrinkage and the drawing speed become large, the heat generation during the pass becomes large. In the internal region of the titanium alloy material, for example, near the center of gravity, the heat dissipation is smaller than the heat generated by the working, and therefore the temperature in this region rises to the β -transformation point or higher.

On the other hand, in the outer peripheral region, heat can be sufficiently dissipated from the outer peripheral surface even in the latter stage of the working process, and the refining and equiaxial formation of the metallographic structure are performed by working at a relatively low temperature. Thereby, the α crystal grains 24 in the outer peripheral region become fine equiaxed grains having an average grain diameter of 10 μm or less. Further, as described above, the metallographic structure in the outer peripheral region is sufficiently refined and equiaxed, whereby the occurrence of defects on the outer peripheral surface can be suppressed, and the occurrence of problems such as breakage during production can be suppressed.

Since the vicinity of the center of gravity of the titanium alloy material is also cooled at the end of the working step, as shown in fig. 5 (e), as the temperature decreases, a needle-like α crystal grains 25 are generated at the time of phase transformation from the β phase to the α phase, and a needle-like structure is formed in the inner region including the center of gravity. Thus, the titanium alloy wire of the present embodiment can be produced, in a cross section perpendicular to the longitudinal direction, with a fine equiaxed structure 24 as the metallographic structure in the outer peripheral region and a needle-like structure 25 as the metallographic structure in the inner region.

In the machining step, the titanium alloy wire rod of the present embodiment can be produced by a process including machining a titanium alloy material under the following conditions: the total cross-sectional shrinkage is 90.0% or more, and the average cross-sectional shrinkage per 1 pass of at least 1 or more passes from the final pass is 10% or more, and the drawing speed is 5m/s or more. That is, in a cross section perpendicular to the longitudinal direction L, the metallographic structure of the outer peripheral region 2 cut from the surface 3 to the center of gravity G to the depth d corresponding to 3% of the wire diameter is an equiaxed structure having equiaxed α -grains having an average grain diameter of 10 μm or less, and the metallographic structure of the inner region 4 including the center of gravity G at a position cut from the center of gravity G to the surface 3 to 20% of the wire diameter is a needle-like structure having needle-like α -grains. In a cross section perpendicular to the longitudinal direction L, the average aspect ratio of 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 α crystal grains in the inner region 4 is 5.0 or more.

The wire drawing speed in at least 1 or more passes from the final pass described above is much higher than the wire drawing speed (about 0.2 to 2.0 m/s) used in the conventional production of titanium alloy wire rods. The present inventors have found that by using this drawing speed and the above average cross-sectional shrinkage ratio, large heat generation occurs during processing, and the microstructure of the titanium alloy wire rod according to the present embodiment described above can be obtained.

As described above, in the present embodiment, the average cross-sectional shrinkage rate per 1 pass of at least 1 or more passes from the final pass is 10% or more. This can generate sufficient heat generation during at least 1 or more passes from the final pass. On the other hand, if the average cross-sectional shrinkage is less than 10%, sufficient heat generation during 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 does not sufficiently develop.

The average cross-sectional shrinkage rate per 1 pass of at least 1 or more passes from the final pass may be 10% or more, but in order to generate a larger heat generation during processing, a β single-phase structure is formed, and a needle-like structure is formed during subsequent cooling, preferably 15% or more, and more preferably 20% or more. The upper limit of the average cross-sectional shrinkage per 1 pass of at least 1 or more passes from the final pass is not particularly limited, but from the viewpoint of load on equipment, the average cross-sectional shrinkage is preferably 45% or less, and more preferably 35% or less.

The wire drawing speed is 5m/s or more in at least 1 or more passes from the final pass. As a result, the amount of heat radiation can be reduced in at least 1 or more passes from the final pass, and heat generated by the heat generation during the working is accumulated in the internal region 4 including the center of gravity G, so that the temperature of the internal region 4 can be sufficiently increased. On the other hand, if the drawing speed is less than 5m/s in at least 1 or more passes from the final pass, the amount of heat radiation increases, and as a result, heat generated by heat generation due to processing cannot be accumulated in the inner region 4 including the center of gravity G, and the temperature of the inner region 4 cannot be sufficiently increased. Therefore, the β single-phase structure is not formed, and the needle-like structure is hardly formed at the time of subsequent cooling.

The drawing speed may be 5m/s or more in at least 1 or more passes from the final pass, but in order to sufficiently develop the β phase, a needle-like structure is formed at the time of subsequent cooling, preferably 10m/s or more, and more preferably 20m/s or more. The upper limit of the drawing speed is not particularly limited in at least 1 or more passes from the final pass, but the drawing speed is preferably 75m/s or less, more preferably 50m/s or less, from the viewpoint of handling stability and load on equipment.

The total cross-sectional shrinkage of the titanium alloy billet processed in this step is 90% or more. As a result, the metallographic structure of the outer peripheral region 2 is equiaxial and finer as described above. On the other hand, when the total area shrinkage of the titanium alloy material is less than 90%, the equiaxial structure and refinement of the metallographic structure in the outer peripheral region 2 are insufficient. Alternatively, even when the metallographic structure of the outer peripheral region 2 is equiaxial, the α crystal grains cannot be sufficiently refined, and are crystal grains having a large particle diameter.

The total cross-sectional shrinkage may be 90% or more, but is preferably 95% or more, and more preferably 99% or more, in order to more reliably equiaxe and refine the metallographic structure of the outer peripheral region 2.

The cross-sectional shrinkage rate per 1 pass is a reduction rate of the area after the 1 pass to the cross-sectional area before the 1 pass, and the total cross-sectional shrinkage rate is a reduction rate of the cross-sectional area after the processing in the present step to the cross-sectional area of the titanium alloy material before the processing.

The hole pattern of the roll used in this step is not particularly limited as long as the drawing speed and the cross-sectional shrinkage rate can be achieved, and a known hole pattern, for example, a true circle, an oval, or a square, may be used.

The number of passes (number of passes) through the roller in this step is not particularly limited, and may be 5 or more, so that this step can be performed. In order to achieve a cross-sectional shrinkage of 90% or more, it is preferable to perform 10 or more passes.

Through the above steps, the titanium alloy wire rod according to the present embodiment can be industrially stably produced. The obtained titanium alloy wire rod may be subjected to the following heat treatment/post-treatment as required.

(2.4 Heat treatment Process)

The titanium alloy ingot (titanium alloy wire rod) obtained by the above steps may be further subjected to a heat treatment (annealing treatment) in a temperature range of not less than (β transformation point-300) ° c and not more than (β transformation point-50) ° c. This eliminates the strain generated in the above-described working step, and further improves the fatigue strength of the obtained titanium alloy wire rod.

The temperature of the heat treatment in the treatment is (beta transformation point-300) DEG C or higher. This makes it possible to sufficiently remove the strain generated in the machining step. The temperature of the heat treatment is preferably (beta transus point-250) ° c or higher, more preferably (beta transus point-200) ° c or higher.

The temperature of the heat treatment in the present treatment is not more than (. beta. -transformation point-50) ° C. This prevents the occurrence of a structure in which equiaxed structures and needle-like structures coexist (bimodal) in the outer peripheral region 2, thereby preventing the fatigue characteristics from being degraded. The temperature of the heat treatment is preferably (β transformation point-100) ° c or lower.

The time of the heat treatment is not particularly limited, and may be appropriately selected, and may be, for example, 1 minute to 120 minutes, preferably 2 minutes to 60 minutes.

The atmosphere during the heat treatment is not particularly limited, and may be air, vacuum, or inert gas (such as argon). In particular, if the atmosphere is an atmosphere that promotes chemical reactions such as oxidation, the subsequent descaling can be used to cope with the reaction.

(2.5 post-treatment)

Examples of the post-treatment include descaling by acid pickling or cutting, cleaning, and the like, and can be used as needed.

The method for manufacturing the titanium alloy wire rod according to the present embodiment is explained above.

Examples

The following examples are given to specifically explain the embodiments of the present invention. It should be noted that the following embodiments are only 1 example of the present invention, and the present invention is not limited to the following examples.

1. Production of titanium alloy wire

First, ingots having chemical compositions shown in table 1 were produced by a vacuum arc melting method, and hot forged at a temperature in the β single phase region, thereby obtaining titanium round bars having predetermined diameters (wire diameters of 22mm to 180mm) having compositions of alloy species a to O. In each titanium round bar, components other than the compositions described in table 1 were titanium and impurities. In addition, the alloy types a to M are all α + β type titanium alloys that form a two-phase structure having an α phase and a β phase at room temperature and in a use environment. The alloy type N is an α + β type titanium alloy in which a β phase is hardly present at room temperature, and the alloy type O is a metastable β type titanium alloy in which the martensite transformation start temperature is not higher than room temperature.

The alloy species A to M are examples satisfying the composition ranges defined in claim 1.

Alloy types a to a4 are examples satisfying the composition ranges defined in claim 2.

Alloy types B to B5 are examples satisfying the composition ranges defined in claim 3.

Alloy types C to C9 are examples satisfying the composition range defined in claim 4.

[ Table 1]

Next, each of the obtained titanium round bars was heated (heating step), and wire drawing was performed using a roller (processing step). Further, a heat treatment step (heat treatment step) is performed as necessary. The heat treatment was performed for 10 minutes in an atmosphere of 100% argon. Thus, the titanium alloy wire rods according to the respective examples were obtained. The heating temperature (DEG C) in the heating step, the average cross-sectional shrinkage (%) per 1 pass of at least 1 or more passes from the final pass in the processing step, the drawing speed (m/s), the total cross-sectional shrinkage (%) in the processing step, the presence or absence of the heat treatment step, and the heat treatment temperature (DEG C) are shown in tables 2, 3, and 4.

[ Table 2]

[ Table 3]

[ Table 4]

2. Analysis/evaluation

The titanium alloy wire rods according to the respective examples were analyzed and evaluated in the following items.

2.1 Observation of metallographic Structure (microstructure)

With respect to the titanium alloy wire rods according to the respective examples, the cross section perpendicular to the longitudinal direction was observed as follows, and whether the metallographic structure of each region of the cross section was an equiaxed structure or a needle-like structure was examined. The average crystal grain diameter and average aspect ratio of the α crystal grains were measured and calculated, and the area ratio of the region having an average aspect ratio of the α crystal grains of 5.0 or more to the cross section was determined. First, the titanium alloy wires according to the respective examples were mirror-polished in 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 average aspect ratio were measured by observing an optical micrograph of the surface. The average crystal grain size was measured by a line segment method in accordance with JIS G0551. Specifically, 5 line segments were drawn in each of the horizontal and vertical directions on an optical microscope photograph taken at a magnification of 500 times, and the average crystal grain diameter was calculated for each line segment by using the number of grain boundaries crossing the line segment, and the arithmetic average of the average crystal grain diameters of 10 lines in total was obtained. The average aspect ratio is calculated as follows: in an optical microscope photograph taken at a magnification of 500 times, the major axis and the minor axis were measured for any 50 crystal grains, and the values obtained by dividing the major axis by the minor axis were arithmetically averaged. Here, the "major axis" refers to a segment having the largest length among segments connecting 2 arbitrary points on the grain boundary (contour) of the α phase, and the "minor axis" refers to a segment having the largest length among segments orthogonal to the major axis and connecting 2 arbitrary points on the grain boundary (contour).

2.2 fatigue Strength

For fatigue strength, according to JIS Z2274: 1978 the rotary bending fatigue test is carried out until 10⁷The maximum stress at which the steel is not broken is taken as the fatigue strength.

2.3 creep strength

For creep strength, according to JIS Z2271: 2010 creep test was performed. Specifically, when a creep test was performed at 400 ℃ for 100 hours, the creep strength was determined as the minimum stress at which 0.2% strain was achieved.

2.4 evaluation

In the same alloy species, in order to compare with titanium alloy wire rods obtained by a manufacturing method corresponding to a conventional manufacturing method, in the examples of alloy species a to O shown in table 2 (all comparative examples), the average reduction (%) in cross section per 1 pass of at least 1 or more passes from the final pass in the processing step was 16%, but the wire drawing speed (m/s) was 2.0m/s (less than 5 m/s). The metallographic structures of the outer peripheral region and the inner region of the titanium alloy wires according to the examples shown in table 2 were equiaxed structures.

On the other hand, in invention examples 1 to 31 of alloy types A to M shown in Table 3, the average reduction (%) in cross section per 1 pass among at least 1 or more passes from the final pass in the working step was 16% and the drawing speed (M/s) was 25M/s. In the titanium alloy wires of invention examples 1 to 31 shown in table 3, the metallographic structure in the outer peripheral region was an equiaxed structure in which an equiaxed α phase was the parent phase and a fine β phase was present in the grain boundaries and grains, and the metallographic structure in the inner region was an acicular structure in which an acicular α phase and a β phase were layered.

In comparative examples 1 and 2 of the alloy type N, O shown in table 3, the average reduction (%) in cross section per 1 pass among at least 1 or more passes from the final pass in the working step was 16%, and the drawing speed (m/s) was 25 m/s. However, the Mo equivalent (Moeq) of comparative example 1 was less than-4.0. In comparative example 1, the metallographic structure in the outer peripheral region was an equiaxed structure of an α single phase in which an α phase composed of equiaxed α crystal grains was a matrix phase and a β phase was hardly present (a very small amount of β phase was present), and the metallographic structure in the inner region was a structure in which an α phase having α crystal grains with a relatively small major diameter was a matrix phase and a β phase was hardly present (a very small amount of β phase was present). More specifically, the internal region of comparative example 1 had a structure in which an equiaxed β phase was finely dispersed in a bulk α phase.

In addition, comparative example 2 had a Mo equivalent (Moeq) of more than 6.0. In comparative example 2, both the metallographic structure in the outer peripheral region and the metallographic structure in the inner region were an equiaxed structure of a β single phase including equiaxed β crystal grains.

In table 3, the metallographic structure of the inner region of comparative examples 1 and 2 and the metallographic structure of the outer region of comparative example 2 were different from the equiaxed structure of the present invention, and therefore were distinguished by adding "﹡".

In tables 2 and 3, the examples of alloy types a to O were compared and evaluated for fatigue strength. The following grades A to C were used for evaluation based on the fatigue strengths of the examples of alloy types A to O shown in Table 2. The evaluation A, B, which is equal to or greater than the standard fatigue strength, was regarded as passed.

A: the fatigue strength is improved by more than 10MPa compared with the standard fatigue strength.

B: the variation from the reference fatigue strength is within the range of-10 MPa or more and less than 10 MPa.

C: the fatigue strength of the steel sheet is reduced by more than 10MPa and 20MPa or less from the reference fatigue strength.

In tables 2 and 3, the creep strength (creep stress) was compared and evaluated for the examples of alloy types a to O. The following grades A to C were used for evaluation based on the creep strength of the examples of alloy types A to O shown in Table 2. Then, the evaluation A, B, which is a case where the creep strength was improved compared with the standard creep strength, was regarded as passed.

A: the standard creep strength is improved by more than 20 MPa.

B: the standard creep strength is improved by more than 10MPa and less than 20 MPa.

C: the standard creep strength ratio fluctuates within a range of-10 MPa or more and less than 10 MPa.

For alloy types a to O shown in table 1, the metallographic structure of the outer peripheral region, the average aspect ratio of α crystal grains, the average crystal grain diameter, and the metallographic structure of the inner region, the average aspect ratio of α crystal grains, the area ratio of the needle-like structure region, and the fatigue strength and creep strength as evaluation criteria in the examples of titanium alloy wire rods obtained by the manufacturing method corresponding to the conventional manufacturing method are shown in table 2. Table 3 shows the metallographic structure of the outer peripheral region, the average aspect ratio of α crystal grains, the average crystal grain diameter and the metallographic structure of the inner region, the average aspect ratio of α crystal grains, the area ratio of the needle-like structure region, the fatigue strength and the evaluation result as the evaluation targets, and the creep strength and the evaluation result as the evaluation targets of invention examples 1 to 31 (alloy types a to M) and comparative examples 1 and 2 (alloy type N, O).

In the invention examples 1 to 31, the fatigue strength was A, B, which was equal to or higher than the standard fatigue strength. In addition, in invention examples 1 to 31, any of the creep strengths evaluated as A, B was improved as compared with the reference creep strength.

On the other hand, the creep strength of comparative examples 1 and 2 was not sufficiently improved.

Next, in table 4, the alloy type A, B, C was compared and evaluated for fatigue strength and creep strength. The heating step and the processing step of invention examples 32 to 54 satisfy the present invention, and the metallographic structure of the outer peripheral region of the titanium alloy wire rods of invention examples 32 to 54 was an equiaxed structure in which an equiaxed α phase was the parent phase and a fine β phase was present in the grain boundaries and grains, and the metallographic structure of the inner region was a needle-like structure in which a needle-like α phase and a needle-like β phase were layered.

On the other hand, in comparative examples 3 to 10, any one of the heating step and the processing step is outside the range of the present invention, and any one of the metallographic structure of the outer peripheral region, the average aspect ratio of the α crystal grains, the average crystal grain diameter of the α crystal grains, or the metallographic structure of the inner region, and the average aspect ratio of the α crystal grains of the titanium alloy wire rods of comparative examples 3 to 10 is outside the range of the present invention.

The wire diameters of invention examples 32 to 54 were 1.5mm to 22.0 mm. The invention examples 32 to 50, 52 and 53 satisfy the wire diameter of 2.0mm to 20.0mm defined in claim 8.

In inventive examples 32 to 48 and inventive examples 51 to 54, evaluations were made using grades A to C in the same manner as described above, with respect to the fatigue strength and creep strength in the example of the alloy type A in Table 2, with respect to the fatigue strength and creep strength in the example of the alloy type B in Table 2, with respect to the inventive example 49, and with respect to the fatigue strength and creep strength in the example of the alloy type C in Table 2, with respect to the inventive example 50.

As shown in Table 4, the titanium alloy wire rods according to invention examples 32 to 54 have both excellent fatigue strength and creep strength. In particular, the titanium alloy wire rods according to invention examples 32 to 54 obtained good results in terms of creep strength as compared with the reference comparative examples. On the other hand, the titanium alloy wires according to comparative examples 3 to 10 cannot have both excellent fatigue strength and creep strength.

In comparative example 3, since the total cross-sectional shrinkage was less than 90.0%, the outer peripheral region was a structure in which no equiaxial transformation was completed (not equiaxial transformation) in which a small amount of fine β phase existed in α phase in which the length-diameter ratio of α crystal grains and the crystal grain diameter increased to some extent. In comparative example 3, since the drawing rate was less than 5.0m/s and the heat generation during working was small, the internal region had an equiaxed structure in which a β phase, which is a matrix phase composed of equiaxed α crystal grains, was finely dispersed in an α phase.

In comparative example 4, since the average cross-sectional shrinkage rate was less than 10.0% in at least 1 or more passes from the final pass, and the work heat generation was small, both the inner region and the outer peripheral region had an equiaxed structure in which an α phase composed of equiaxed α crystal grains was the matrix and a small amount of β phase was finely dispersed in the α phase.

In comparative example 5, since the total cross-sectional shrinkage was less than 90.0%, the outer peripheral region was a structure in which fine β phases were present in a small amount in α phases having a somewhat increased aspect ratio of α grains and were not equiaxed (not equiaxed), and the inner region was a needle-like structure in which needle-like α phases and β phases were layered.

In comparative example 6, since the drawing rate was less than 5.0m/s and the heat generation during working was small, both the inner region and the outer peripheral region had an equiaxed structure in which an α phase composed of equiaxed α crystal grains was the matrix phase and a small amount of β phase was finely dispersed in the α phase.

In comparative example 7, since the total cross-sectional shrinkage was less than 90.0%, the outer peripheral region had an equiaxed structure in which an α phase composed of coarse equiaxed α crystal grains was used as a matrix phase and a small amount of β phase was dispersed in the α phase, and the inner region had a needle-like structure in which the needle-like α phase and β phase were layered.

In comparative example 8, since the heating temperature was too low, both the inner region and the outer peripheral region had an equiaxed structure in which an α phase composed of equiaxed α crystal grains was a matrix phase and a small amount of β phase was finely dispersed in the α phase.

In comparative example 9, since the total cross-sectional shrinkage was less than 90.0%, the outer peripheral region was a structure in which no equiaxial transformation was completed (not equiaxial transformation) in which a small amount of fine β phase was present in the α phase in which the length to diameter ratio of α crystal grains and the crystal grain diameter increased to some extent, and the inner region was an acicular structure in which acicular α phase and β phase were layered.

In comparative example 10, since the total cross-sectional shrinkage was less than 90.0%, the outer peripheral region was a structure in which fine β phases were present in a small amount in α phases having a somewhat increased aspect ratio and were not equiaxed (not equiaxed), and the inner region was a needle-like structure in which needle-like α phases and β phases were layered.

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

The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited to these examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive various modifications and alterations within the scope of the technical idea described in the claims, and it is needless to say that the variations and alterations also fall within the technical scope of the present invention.

Description of the reference numerals

alpha crystal grain

beta phase of b

c needle-like form of alpha

e acicular beta

1 titanium alloy wire

L longitudinal direction

2 peripheral region

3 outer peripheral surface

4 inner region

G center of gravity

R wire diameter

d corresponds to a depth of 3%

11 major axis

Grain boundary of 10 alpha phase

12 minor axis

20 beta crystal grain

21 acicular alpha crystal grains

22 equiaxed alpha grains

23 beta crystal grain

24 fine alpha crystal grain with equiaxed (fine equiaxed structure)

25 needle-shaped alpha grains (needle-shaped structure)

Claims

1. A titanium alloy wire, which is a titanium alloy wire comprising an α phase and a β phase, having the following chemical composition:

in mass percent

Al: 0% to 7.0%,

V: 0% to 6.0%,

Mo: 0% to 7.0%,

Cr: 0% to 7.0%,

Zr: 0% to 5.0%,

Sn: 0% to 3.0%,

Si: 0% to 0.50%, B,

Cu: 0% to 1.8%,

Nb: 0% to 1.0%,

Mn: 0% to 1.0%,

Ni: 0% to 1.0%,

S: 0% to 0.20%,

REM: 0% to 0.20%,

Fe: 0% to 2.10% inclusive,

N: 0% to 0.050%,

O: 0% to 0.250% inclusive,

C: 0% to 0.100%,

in the cross section perpendicular to the longitudinal direction, the metallographic structure of the inner region including the center of gravity at a position from the center of gravity to the surface up to 20% of the line diameter is a needle-like structure,

in the formula (1), the [ element symbol ] represents the content of the corresponding element symbol in mass%, and the element symbol which is not included is substituted into 0;

wherein, in the cross section perpendicular to the longitudinal direction, the average aspect ratio of the α crystal grains in the outer peripheral region is 1.0 or more and less than 3.0, and the average aspect ratio of the α crystal grains in the inner region is 5.0 or more.

2. The titanium alloy wire rod according to claim 1, containing, in mass%)

Al: 4.5% to 6.5%,

Fe: 0.50% or more and 2.10% or less.

3. The titanium alloy wire rod according to claim 1, containing, in mass%)

Al: 2.0% to 7.0%,

V: 1.5% or more and 6.0% or less.

4. The titanium alloy wire rod according to claim 1, containing, in mass%)

Al: 5.0% to 7.0%,

Mo: 1.0% to 7.0%,

Zr: 3.0% to 5.0%,

Sn: 1.0% or more and 3.0% or less.

5. The titanium alloy wire according to claim 1, wherein in the cross section perpendicular to the longitudinal direction, an area of a region including a center of gravity in which an average aspect ratio of α grains is 5.0 or more is 40% or more with respect to an area of the cross section.

6. The titanium alloy wire according to claim 1, wherein an average crystal grain diameter of the alpha crystal grains in the outer peripheral region is 5.0 μm or less.

7. The titanium alloy wire according to claim 1, wherein the wire diameter is 2.0mm or more and 20.0mm or less.

8. A method for manufacturing a titanium alloy wire rod, comprising the steps of:

processing the titanium alloy material under the following conditions: a total cross-sectional shrinkage of 90.0% or more, an average cross-sectional shrinkage per 1 pass of at least 1 or more passes from the final pass of 10.0% or more, and a drawing speed of 5.0m/s or more;

the titanium alloy billet has the following chemical composition:

in mass percent

Al: 0% to 7.0%,

V: 0% to 6.0%,

Mo: 0% to 7.0%,

Cr: 0% to 7.0%,

Zr: 0% to 5.0%,

Sn: 0% to 3.0%,

Si: 0% to 0.50%, B,

Cu: 0% to 1.8%, a,

Nb: 0% to 1.0%,

Mn: 0% to 1.0%,

Ni: 0% to 1.0%,

S: 0% to 0.20%,

REM: 0% to 0.20%, a,

Fe: 0% to 2.10%, a,

N: 0% to 0.050% inclusive,

O: 0% to 0.250% inclusive,

C: 0% to 0.100%,

And the balance: ti and impurities;

in the formula (1), [ symbol of element ] represents the content of the corresponding symbol of element in mass%, and the symbol of element not included is substituted for 0.

9. The method of manufacturing a titanium alloy wire according to claim 8, further comprising a step of heat-treating the titanium alloy wire obtained in the step of working in a temperature range of (β transformation point-300) ° C or higher and (β transformation point-50) ° C or lower.