US20250305096A1

US20250305096A1 - Tungsten wire

Info

Publication number: US20250305096A1
Application number: US18/864,018
Authority: US
Inventors: Kazuhiro Daijo; Naoki Kohyama; Yoshihiro Kodama
Original assignee: Panasonic Intellectual Property Management Co Ltd
Current assignee: Panasonic Intellectual Property Management Co Ltd
Priority date: 2022-05-27
Filing date: 2023-05-17
Publication date: 2025-10-02
Also published as: DE112023002422T5; TW202346613A; CN119137298A; JP2023174257A; TWI864759B; WO2023228833A1

Abstract

A tungsten wire includes tungsten as a major component. Here, 4758×D2−7258.3×D+5275.5≤T≤4758×D2−7258.3×D+6100 is satisfied where T (MPa) is a tensile strength of the tungsten wire, D (mm) is a diameter of the tungsten wire, and a roundness of the tungsten wire is at most 2.0%.

Description

TECHNICAL FIELD

The present invention relates to a tungsten wire.

BACKGROUND ART

Patent Literature (PTL) 1 discloses a tungsten wire having a tensile strength of at least 4800 MPa.

CITATION LIST

Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No. 2020-105548

SUMMARY OF INVENTION

Technical Problem

The workability of the above-described conventional tungsten wire in drawing (wire drawing) using a die deteriorates with an increase in tensile strength and an increase in the amount of additive. For this reason, the die is easily worn, which causes a deterioration in the roundness of the tungsten wire.
In view of the above, an object of the present invention is to provide a tungsten wire having an improved roundness.

Solution to Problem

In accordance with an aspect of the present disclosure, a tungsten wire includes tungsten as a major component, wherein 4758×D²−7258.3×D+5275.5≤T≤4758×D²−7258.3×D+6100 is satisfied where T (MPa) is a tensile strength of the tungsten wire, D (mm) is a diameter of the tungsten wire, and a roundness of the tungsten wire is at most 2.0%.

Advantageous Effects of Invention

According to the present invention, it is possible to produce a tungsten wire having an improved roundness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating an external appearance and a cross section of a tungsten wire according to an embodiment.

FIG. 2 is a diagram for describing a roundness.

FIG. 3 is a flowchart illustrating an example of a manufacturing method of the tungsten wire according to the embodiment.

FIG. 4 is a flowchart illustrating another example of the manufacturing method of the tungsten wire according to the embodiment.

FIG. 5 is a graph illustrating the relationship between diameters and tensile strengths of tungsten wires containing rhenium or cerium.

FIG. 6 is a graph illustrating the relationship among rhenium contents, tensile strengths, and roundnesses of tungsten wires containing rhenium and having a diameter of 50 μm.

FIG. 7 is a graph illustrating the relationship among rhenium contents, tensile strengths, and roundnesses of tungsten wires containing rhenium and having a diameter of 30 μm.

FIG. 8 is a graph illustrating the relationship among cerium contents, tensile strengths, and roundnesses of tungsten wires containing cerium and having a diameter of 30 μm.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a tungsten wire and a metal mesh according to embodiments of the present invention will be described in detail with referenced to the drawings. It should be noted that each of the embodiments described below shows a specific example of the present invention. As such, the numerical values, shapes, materials, structural components, the arrangement and connection of the structural components, steps, the processing order of the steps, and so on, shown in the following embodiments are mere examples, and therefore do not limit the present invention. Among the structural components in the embodiments described below, those not recited in the independent claims will be described as optional structural components.
In addition, each diagram is a schematic diagram and not necessarily strictly illustrated. Accordingly, for example, scale sizes, etc. are not necessarily exactly represented. In each of the diagrams, substantially the same structural components are assigned with the same reference signs, and redundant descriptions will be omitted or simplified.
In addition, a term representing a relationship between the components, a term representing a shape of a component, and a numerical range are used in the present description. Such terms and range are each not representing only a strict meaning of the term or range, but implying that a substantially same range, e.g., a range that includes even a difference as small as few percentages, is connoted in the term or range.

EMBODIMENT

[Tungsten Wire]

First, a tungsten wire according to an embodiment will be described with reference to FIG. 1 . FIG. 1 is a schematic diagram illustrating an external appearance and a cross section of tungsten wire 1 according to the present embodiment.
As illustrated in FIG. 1 , tungsten wire 1 is wound around winding frame 2 and stored. Winding frame 2 may be referred to as a bobbin, reel, spool, drum, or the like in some instances. Tungsten wire 1 has a total length on the order of kilometers, for example, from 50 km or more to 300 km or less.
Tungsten wire 1 illustrated in FIG. 1 is utilized in the manufacture of tungsten products. For example, tungsten wire 1 is used as the core of a saw wire. Specifically, the saw wire is a fixed-abrasive-grain wire. The saw wire includes tungsten wire 1 as its core and further includes abrasive grains such as diamond particles or cubic boron nitride (CBN) particles. The abrasive grains are made to adhere to the surface of tungsten wire 1. The adhesion may be electrodeposition or adhesion with a resin bond. Alternatively, the saw wire may be a loose-abrasive wire. For example, the saw wire may be tungsten wire 1 itself rather than tungsten wire 1 with abrasive grains.
For example, the saw wire is used to cut a semiconductor ingot such as a silicon (Si) or silicon carbide (SiC) ingot. By slicing the semiconductor ingot with the saw wire, semiconductor wafers can be manufactured. At this time, a smaller diameter of the saw wire allows a smaller cutting margin, reducing losses, and thus it is possible to increase the number of wafers that can be obtained. It should be noted that a cutting object is not limited to a semiconductor ingot and may be glass, concrete, rock crystal, ceramic, or the like.
Tungsten wire 1 contains tungsten (W) as a major component. The term “major component” means that the content (content percentage) of an element is more than 50 wt %. For example, the content of the tungsten in tungsten wire 1 is at least 97 wt %. The content of the tungsten in tungsten wire 1 may be at least 99 wt %, at least 99.9 wt %, or at least 99.99 wt %. Tungsten wire 1 may contain, in addition to additives described later, inevitable impurities which are inevitably mixed therein through the processes of manufacturing.
Tungsten wire 1 contains, for example, rhenium (Re). The content of the rhenium in tungsten wire 1 is, for example, at least 0.1 wt % and at most 3 wt %. Rhenium and tungsten form their alloy (solid solution).
When the content of the rhenium is high, it is possible to increase the tensile strength of tungsten wire 1. On the other hand, when the content of the rhenium is excessively high, it is difficult to render tungsten wire 1 thinner while maintaining a high tensile strength thereof. More specifically, wire breakage is likely to occur, which makes it difficult to perform drawing in long lengths. The workability of tungsten wire 1 can be enhanced by reducing the content of the rhenium and setting the content of the tungsten to at least 97 wt %. In addition, since rhenium is rare and expensive, it is possible to mass-produce in long lengths tungsten wire 1 which is inexpensive, by reducing the content of the rhenium.
It should be noted that the metal used for the tungsten alloy may be osmium (Os), ruthenium (Ru), or iridium (Ir). The content of osmium, ruthenium, or iridium is equivalent to the content of the rhenium, for example. In these cases, the same advantageous effects can be yielded as in the case of a rhenium tungsten alloy. Tungsten wire 1 may be an alloy wire containing an alloy of tungsten and metal of at least two types other than tungsten.
Tungsten wire 1 may contain potassium (K). The content of the potassium in tungsten wire 1 is, for example, at least 0.001 wt % and at most 0.01 wt %. Potassium in tungsten wire 1 is present in the grain boundaries of tungsten. Tungsten wire 1 containing potassium can also yield a tensile strength higher than a general tensile strength of piano wire.
Tungsten wire 1 may contain a rare earth element. The content of the rare earth element in tungsten wire 1 is, for example, at least 0.03 wt % and at most 0.3 wt %. The content of the rare earth element in tungsten wire 1 may be, for example, at least 0.03 wt % and at most 0.09 wt %. The rare earth element is, for example, cerium (Ce), lanthanum (La), yttrium (Y), samarium (Sm), or the like. The rare earth element in tungsten wire 1 is present in the grain boundaries of tungsten. Tungsten wire 1 containing the rare earth element can also yield a tensile strength higher than a general tensile strength of piano wire.
As an example, the diameter of tungsten wire 1 is at most 100 μm. Using tungsten wire 1 having a smaller diameter as the core of a saw wire enables a reduction in the losses of materials being cut. The diameter of tungsten wire 1 may be at most 80 μm, at most 70 μm, at most 60 μm, at most 50 μm, at most 40 μm, at most 30 μm, at most 20 μm, or at most 15 μm. In addition, by a manufacturing method described later, tungsten wire 1 having an ultrafine diameter of at most 13 μm is also produced. The diameter of tungsten wire 1 may be at most 10 μm, at most 8 μm, or at most 7 μm. The diameter of tungsten wire 1 is, for example, at least 5 μm, but is not limited to this example.
As an example, the tensile strength of tungsten wire 1 is at least 4800 MPa. The tensile strength may be at least 4900 MPa, at least 5000 MPa, at least 5200 MPa, or at least 5500 MPa. Furthermore, tungsten wire 1 having an ultra-high tensile strength of at least 5800 MPa is also produced. The tensile strength of tungsten wire 1 may be at least 5900 MPa or at least 6000 MPa. The tensile strength can be measured, for example, based on the tensile test of the Japanese Industrial Standards (JIS H 4460 8).
When the tensile strength is denoted as T (unit: MPa), and the diameter is denoted as D (unit: mm), tungsten wire 1 according to the present embodiment satisfies a predetermined relationship. Specifically, tungsten wire 1 of which the tensile strength commensurate with its diameter is higher than conventional one is produced. A specific relationship between tensile strength T and diameter D will be described later with a specific working example, with reference to FIG. 5 .
In the present embodiment, the roundness of tungsten wire 1 is at most 2.0%. That is, a cross section of tungsten wire 1 (the cross section perpendicular to the axial direction of tungsten wire 1) has a shape sufficiently close to a perfect circle. It should be noted that the difference between the cross section of tungsten wire 1 and a perfect circle is exaggerated in the illustration of FIG. 1 .
Here, the roundness will be described with reference to FIG. 2 . FIG. 2 is a diagram for describing the roundness. The roundness is an index that indicates the magnitude of the deviation of the cross-sectional shape of tungsten wire 1 from a perfect circle. A lower roundness means a smaller deviation, that is, that the cross-sectional shape of tungsten wire 1 is close to the perfect circle. A higher roundness means a larger deviation, that is, that the cross-sectional shape of tungsten wire 1 deviates from the perfect circle.
Specifically, roundness f (unit: %) is given by Formula (1) shown below.
$\begin{matrix} f = (A - B) / ((A + B) / 2) \times 100 & (1) \end{matrix}$
A and B are as shown in FIG. 2 . Specifically, a maximum inscribed circle and a minimum circumscribed circle are first defined for the cross section of tungsten wire 1. The maximum inscribed circle and the minimum circumscribed circle are defined in such a manner as to make them concentric and minimize a distance between them. That is, when the radius of the minimum circumscribed circle is denoted as r(A), and the radius of the maximum inscribed circle is denoted as r(B), r(A)−r(B) is minimized.
In this case, A denotes the diameter of the minimum circumscribed circle and is given by 2×r(A). B denotes the diameter of the maximum inscribed circle and is given by 2×r(B). As shown in Formula (1) above, roundness f represents, in terms of percentage, a proportion of the difference between the diameter of the minimum circumscribed circle and the diameter of the maximum inscribed circle to the average value of the diameter of the minimum circumscribed circle and the diameter of the maximum inscribed circle. When f=0, the minimum circumscribed circle and the maximum inscribed circle coincide with each other, and thus the cross section of tungsten wire 1 is the perfect circle.
According to the present embodiment, it is possible to produce tungsten wire 1 of which the tensile strength commensurate with its diameter is higher than conventional one and that has an improved roundness (i.e., its cross section is close to a perfect circle). For example, when tungsten wire 1 is used as the core of a saw wire, its improved roundness enables a reduction of variations in the thickness of a sliced object (a wafer). In particular, when the roundness exceeds 2% or 3%, the resulting total thickness variation (TTV), which is a variation in the thickness of a wafer, steeply deteriorates.

[Manufacturing Method]

Next, a manufacturing method of tungsten wire 1 according to the present embodiment will be described with reference to FIG. 3 and FIG. 4 . FIG. 3 and FIG. 4 are each a flowchart illustrating an example of the manufacturing method of tungsten wire 1 according to the present embodiment.
As illustrated in FIG. 3 , first, an additive is added to tungsten (S10). For example, only the additive (an element to be doped) or a compound containing the additive (e.g., an oxide or aqueous solution of the additive) is added to tungsten powders in a predetermined amount. The element to be doped is rhenium, potassium or a rare earth element such as cerium or lanthanum. Unnecessary components contained in the compound are removed thereafter by sintering or the like. To the obtained doped tungsten powders, tungsten powders are added in a predetermined proportion to adjust the added amount of the element to be doped. That is, the element to be doped may be added in a content larger than an intended content of element to be doped.
In this manner, after the element to be doped is added to tungsten powders in a small amount, tungsten powders are added to thin the element to be doped (reduce the content of the element to be doped). That is, an amount of the element to be doped that is treated in the addition process of the element to be doped is small, which allows small adding equipment to be used. In addition, a large amount of doped tungsten powders can be obtained by performing the addition process, and thus the number of steps in the addition process can be reduced. Accordingly, it is possible to enhance the productivity of tungsten wire 1.
Next, a tungsten ingot is prepared by pressing and sintering on the aggregate of the obtained doped tungsten powders (S12).
Next, swaging processing is performed on the prepared tungsten ingot (S14). Specifically, the tungsten ingot is forged and compressed from around to be extended, thus being formed into a wire-shaped tungsten wire. Rolling processing may be performed instead of the swaging processing.
For example, by repeatedly performing the swaging processing, the tungsten ingot having a diameter of at least about 15 mm and at most about 25 mm is formed into a tungsten wire having a diameter of at least about 3 mm and at most about 4 mm. Annealing treatment is performed in a midcourse process of the swaging processing to keep workability in subsequent treatments. For example, the annealing treatment is performed at a temperature of at least 2000 degrees Celsius and at most 2400 degrees Celsius within the range where the diameter of the tungsten ingot is at least 8 mm and at most 10 mm. However, when the diameter is less than 8 mm, the annealing treatment is not performed in the swaging process so as to keep the tensile strength through crystal grain refining.
Next, the tungsten wire is heated at 900 degrees Celsius before being subjected to heat drawing (S16). Specifically, the tungsten wire is heated directly with a burner or the like. Heating the tungsten wire forms an oxide layer on the surface of the tungsten wire so that the tungsten wire does not break in the heat drawing that is subsequently performed.
Next, the heat drawing is performed (S18). Specifically, drawing of the tungsten wire, that is, wire drawing (thinning) of the tungsten wire is performed using one or more wire drawing dies while the tungsten wire is heated. The heating temperature is, for example, 1000 degrees Celsius. It should be noted that the higher the heating temperature is, the more the workability of the tungsten wire is enhanced, and the drawing can be performed easily. The heat drawing is repeatedly performed while replacing one of the wire drawing dies with another. A reduction rate in a cross-section area of the tungsten wire made by performing the drawing once with one wire drawing die is, for example, at least 10% and at most 40%. In the heat drawing process, a lubricant made of graphite dispersed in water may be used.
The heat drawing (S18) is repeated until the tungsten wire having a desired diameter is obtained (No in S20). Here, the desired diameter is a diameter when the number of times to perform the drawing is two. For example, the desired diameter is approximately 150 μm.
It should be noted that, in the repetition of the heat drawing, a wire drawing die used in a certain heat drawing has a smaller bore diameter than a wire drawing die used in an immediately previous heat drawing. Furthermore, in the repetition of the heat drawing, the tungsten wire is heated in a certain heat drawing at a heating temperature lower than a heating temperature in an immediately previous heat drawing. That is, the heating temperature is decreased stepwise. The heating temperature in the final heat drawing is, for example, 400 degrees Celsius so as to make the final heat drawing contribute to crystal grain refining.
When the tungsten wire having the desired diameter is obtained, and the number of times to perform the drawing is two (Yes in S20), a drawing at room temperature is performed (S22). It should be noted that, as illustrated in FIG. 4 , electrolytic polishing (S21) may be performed before the drawing at room temperature (S22). In the drawing at room temperature, the tungsten wire is drawn without being heated, so that further crystal grain refining is implemented. In addition, the drawing at room temperature yields the advantageous effects of aligning crystal orientations in a processing axis direction (specifically, a direction parallel to the axis of tungsten wire 1.
The room temperature is a temperature within the range of, for example, at least 0 degrees Celsius and at most 50 degrees Celsius. As an example, the room temperature is 30 degrees Celsius. Specifically, the drawing of tungsten wire is performed using a plurality of wire drawing dies having different bore diameters. In the drawing at room temperature, a liquid lubricant such as a water-soluble liquid lubricant. Since the drawing at room temperature does not involve heating, evaporation of the liquid is restrained. Therefore, it is possible to make the liquid lubricant to exert its function sufficiently. In contrast to heat drawing performed at 600 degrees Celsius or higher, which is a conventional, traditional method of processing a tungsten wire, processing a tungsten wire while the tungsten wire is not heated but cooled with the liquid lubricant restrains dynamic recovery and dynamic recrystallization, contributes to crystal grain refining, and can yield a high tensile strength without breakage.
The processing proportion of the drawing at room temperature is, for example, at least 70%. The processing proportion is given by Formula (2) shown below, using Db denoting the diameter immediately before the drawing at room temperature and Da denoting the diameter immediately after the drawing at room temperature.
$\begin{matrix} Processing proportion = {1 - {(Da / Db)}^{2}} \times 100 & (2) \end{matrix}$
Formula (2) shows that the more the diameter is reduced by the drawing at room temperature, the higher a value of the processing proportion becomes. For example, when diameter Db immediately before the drawing at room temperature is assumed to be constant, diameter Da immediately after the drawing at room temperature decreases with an increase in the processing proportion. As the processing proportion is increased, the degree of thinning of the tungsten wire by the drawing at room temperature increases. That is, a thinner tungsten wire is obtained. The processing proportion of the drawing at room temperature is at least 70% but may be at least 80%, at least 90%, or at least 95%. The diameter immediately after the drawing at room temperature is roughly within the range of, for example, at least 50 μm and at most 120 μm.
Next, low-temperature hot drawing is performed (S24) after the drawing at room temperature. That is, the tungsten wire is subjected to the final drawing while being heated at a low temperature. The temperature at this time is higher than the temperature (the room temperature) of the drawing at room temperature (S22) and lower than the temperature of the heat drawing (S18). Specifically, the temperature of the low-temperature hot drawing is within the range of at least 100 degrees Celsius and at most 300 degrees Celsius. As an example, the temperature is 200 degrees Celsius or 300 degrees Celsius. The diameter after the low-temperature hot drawing is roughly within the range of, for example, at least 20 μm and at most 100 μm.
Finally, the tungsten wire formed by performing the low-temperature hot drawing is subjected to electrolytic polishing (S26) to adjust the diameter. The electrolytic polishing is carried out, for example, as a result of the generation of a potential difference between the tungsten wire and a counter electrode in a state in which the tungsten wire and the counter electrode are bathed into electrolyte such as aqueous sodium hydroxide.
Through the above-described processes, tungsten wire 1 according to the present embodiment is manufactured. Immediately after being manufactured through the above-described processes, tungsten wire 1 has a length of, for example, at least 50 km, which enables industrial use of tungsten wire 1. Tungsten wire 1 is cut to an appropriate length in accordance with its usage mode and can be used in a needle shape or a rod shape.
It should be noted that the processes shown in the manufacturing method of tungsten wire 1 are performed in-line, for example. Specifically, the plurality of wire drawing dies used in step S18 are located in a production line in descending order of bore diameter. In addition, heating equipment such as a burner is located between every adjacent wire drawing dies. In addition, electrolytic polishing equipment may be located between every adjacent wire drawing dies. On a downstream side (post-processing side) of the wire drawing dies used in step S18, one or more wire drawing dies used in step S22 and one or more wire drawing dies used in step S24 are located in descending order of bore diameter, and on a downstream side of a wire drawing die having a smallest bore diameter, electrolytic polishing equipment is located. It should be noted that the processes may be performed individually.
It should be noted that the above-described manufacturing method of tungsten wire 1 is a mere example. The temperature, the diameter, and the like in each process can be adjusted as appropriate.
As described above, in the manufacturing method of tungsten wire 1 according to the present embodiment, the heat drawing is performed at a first temperature being a high temperature, then the drawing at room temperature is performed at a second temperature being the room temperature, and then the low-temperature hot drawing is performed at a third temperature being a low temperature. The third temperature is higher than the second temperature (the room temperature) and lower than the first temperature (the high temperature).
In this manner, tungsten wire 1 is manufactured through a new process, the low-temperature hot drawing (also called low-temperature processing). By performing the low-temperature hot drawing, tungsten wire 1 that has a small diameter, a high tensile strength, and an improved roundness is produced.

[Relationship Between Tensile Strength and Diameter]

Next, the relationship between tensile strengths and diameters of tungsten wires 1 according to the present embodiment will be described with reference to FIG. 5 .
FIG. 5 is a graph illustrating the relationship between diameters and tensile strengths of tungsten wires 1 containing rhenium or cerium. In FIG. 5 , the horizontal axis represents the diameter (unit: μm) of tungsten wire 1, and the vertical axis represents the tensile strength (unit: MPa) of tungsten wire 1.
The inventors of the present invention manufactured a plurality of samples of tungsten wire 1 based on the above-described manufacturing method. The content of the rhenium (Re content) was adjusted by adjusting an amount of rhenium powders added to tungsten powders. The diameter was adjusted with wire drawing dies having different bore diameters. The tensile strengths were actually measured values obtained by measuring tensile strengths of manufactured tungsten wires 1. The tensile strengths were measured, for example, based on the tensile test of the Japanese Industrial Standards (JIS H 4460 8). It should be noted that tungsten wires 1 having the same Re content, the same diameter, and different tensile strengths were successfully obtained by adjusting the heating temperature and/or the processing proportion in the wire drawing processing.
Specific numerical values of Re contents, diameters, and tensile strengths of the plurality of samples of tungsten wire 1 illustrated in FIG. 5 are shown in Table 1 and Table 2.
Table 1 is a table showing the diameters and the tensile strengths of tungsten wires 1 containing rhenium (Re) in predetermined amounts.

TABLE 1

			Tensile
	Re content	Diameter	strength
No.	[wt %]	[μm]	[MPa]

1	1	18	5230
2	1	27	5153
3	1	32	5360
4	1	32	5692
5	1	32	5799
6	1	32	5822
7	1	40	5589
8	1	41	5550
9	1	42	5480
10	1	42	5432
11	1	44	5230
12	1	51	4970
13	1	52	5094
14	1	53	5109
15	1	54	5343
16	1	82	4729
17	1	50	5033
18	1	40	5380
19	0.3	35	5105
20	0.3	35	5051
21	0.3	37	5006
22	0.3	37	5012
23	0.3	38	5021
24	0.3	39	5100
25	0.5	18	5231
26	0.5	32	5822
27	0.5	35	5086
28	0.5	37	5096
29	0.5	38	5004
30	0.5	39	5088
31	0.5	41	5054
32	0.5	42	5075
33	0.5	47	5083

Table 2 is a table showing the tensile strengths of tungsten wires 1 having diameters of 50 μm and 30 μm for each Re content.

	TABLE 2

	Tensile strength for each Re content [MPa]

Diameter	1	0.3	0.5	0.1	3	5
[μm]	wt %	wt %	wt %	wt %	wt %	wt %

50	5590	5460	5570	5240	5600	5690
30	5780	5650	5770	5420	5800	5890

As illustrated in FIG. 5 , there is a negative correlation between the diameters and the tensile strengths. That is, the smaller the diameter is, the higher the tensile strength is, and the larger the diameter is, the lower the tensile strength is.
When the diameter is denoted as D [mm], tungsten wires 1 according to the present embodiment satisfy the range given by Formula (3) shown below.
$\begin{matrix} 4758 \times D^{2} - 7258.3 \times D + 5275.5 \leq T \leq 4758 \times D^{2} - 7258.3 \times D + 6100 & (3) \end{matrix}$
The quadratic function on the left-hand side of the inequality of Formula (3) is illustrated with a lower broken line in FIG. 5 . The lower broken line in FIG. 5 is determined by polynomial approximation using a polynomial based on a plurality of samples that have the smallest tensile strength for the diameters out of the samples in Table 1. The quadratic function on the right-hand side of the inequality of Formula (3) is illustrated with an upper broken line in FIG. 5 . The upper broken line in FIG. 5 is determined by translating the polynomial such that all the samples shown in Table 1 and Table 2 fall between the two broken lines.
FIG. 5 also illustrates the relationship between diameters and the tensile strengths of tungsten wires 1 containing cerium. Specific numerical values are as shown in Table 3. Table 3 is a table showing the diameters and the tensile strengths of tungsten wires 1 containing cerium (Ce) in predetermined amounts.

TABLE 3

Ce content	Diameter	Tensile strength
[wt %]	[μm]	[MPa]

0.3	42	5558
0.3	34	5594
0.3	35	5743

As illustrated in FIG. 5 and shown in Table 3, tungsten wires 1 containing cerium also satisfy the relationship given by Formula (3) above. Here, tungsten wires 1 containing cerium are described as an example, and the same result is obtained from tungsten wire 1 containing another rare earth element having characteristics similar to cerium, such as lanthanum (La).

[Tensile Strength and Roundness]

Next, the relationship between tensile strengths and roundnesses of tungsten wires 1 will be described.

FIG. 6 is a graph illustrating the relationship among Re contents, tensile strengths, and roundnesses of tungsten wires containing rhenium and having a diameter of 50 μm. Table 4 is a table showing the tensile strengths and the roundnesses of tungsten wires containing rhenium in predetermined amounts and having a diameter of 50 μm.

TABLE 4

Re content	Tensile strength	Roundness
[wt %]	[MPa]	[%]

0.1	5240	0.4
0.3	5460	0.5
0.5	5570	0.5
1.0	5590	0.9
3.0	5600	1.8
5.0	5690	2.7

As illustrated in FIG. 6 and shown in Table 4, a tendency was observed for the tensile strength to steeply rise from 5240 MPa to 5570 MPa with an increase in the Re content from 0.1 wt % to 0.5 wt % and then gently rise close to 5690 MPa with an increase in the Re content from 1.0 wt % to 5.0 wt %. In addition, it is found that the roundness increased from 0.4% to 2.7% at a substantially constant rate with an increase in the Re content from 0.1 wt % to 5 wt %. The roundness was at most 2.0% within the range where the Re content was at most 3.0 wt %, while the roundness was greater than 2.0% when the Re content was 5.0 wt %.
Tungsten wires 1 according to the present embodiment having a diameter of 50 μm successfully combined tensile strengths at least 5200 MPa and roundnesses at most 2.0%. In addition, within the range where the Re content was at least 0.5 wt % and at most 1.0 wt %, tensile strengths at least 5500 MPa and roundnesses at most 1.0% were successfully yielded. That is, high tensile strengths and more improved roundnesses were successfully combined.
FIG. 7 is a graph illustrating the relationship among Re contents, tensile strengths, and roundnesses of tungsten wires containing rhenium and having a diameter of 30 μm. Table 5 is a table showing the tensile strengths and the roundnesses of tungsten wires containing rhenium in predetermined amounts and having a diameter of 30 μm.

TABLE 5

Re content	Tensile strength	Roundness
[wt %]	[MPa]	[%]

0.1	5420	0.3
0.3	5650	0.4
0.5	5770	0.6
1.0	5780	1.0
3.0	5800	1.9
5.0	5890	3.2

As illustrated in FIG. 7 and shown in Table 5, a tendency was observed for the tensile strength to steeply rise from 5420 MPa to 5770 MPa with an increase in the Re content from 0.1 wt % to 0.5 wt % and then gently rise to 5890 MPa with an increase in the Re content from 1.0 wt % to 5.0 wt %. This tendency was similar to the tendency when the diameter was 50 μm, and the reduction in the diameter made the tensile strengths higher compared with those when the diameter was 50 μm.
In addition, it is found that the roundness increased from 0.3% to 3.2% at a substantially constant rate with an increase in the Re content from 0.1 wt % to 5 wt %. The roundness was at most 2.0% within the range where the Re content was at most 3.0 wt %, while the roundness was greater than 2.0% when the Re content was 5.0 wt %. This tendency was similar to the tendency when the diameter was 50 μm, and a rate of the increase in the roundness was higher than that when the diameter was 50 μm because the reduction in the diameter made the tensile strengths higher.
Tungsten wires 1 according to the present embodiment having a diameter of 30 μm successfully combined tensile strengths at least 5400 MPa and roundnesses at most 2.0%. In addition, within the range where the Re content was at least 0.5 wt % and at most 1.0 wt %, tensile strengths at least 5750 MPa and roundnesses at most 1.0% were successfully yielded. That is, high tensile strengths and more improved roundnesses were successfully combined.
As seen from the above, it is found that the roundnesses were improved more than conventional ones within the range where the Re content was at least 0.1 wt % and at most 3.0 wt %. In addition, within the range where the Re content was at least 0.1 wt % and at most 3.0 wt %, higher tensile strengths were successfully yielded. Within the range where the Re content was at least 0.5 wt % and at most 1.0 wt %, the higher tensile strengths and the more improved roundnesses were successfully combined. It should be noted that the tensile strengths can be increased more by reducing the diameters.

FIG. 8 is a graph illustrating the relationship among Ce contents, tensile strengths, and roundnesses of tungsten wires 1 containing cerium and having a diameter of 30 μm. Table 6 is a table showing the tensile strengths and the roundnesses of tungsten wires containing cerium (Ce) in predetermined amounts and having a diameter of 30 μm.

TABLE 6

Ce content	Tensile strength	Roundness
[wt %]	[MPa]	[%]

0.02	5400	0.4
0.04	5520	0.5
0.05	5570	0.7
0.09	5730	1.0
0.30	5900	1.9
0.50	5920	3.1

As illustrated in FIG. 8 and shown in Table 6, a tendency was observed for the tensile strength to steeply rise from 5400 MPa to 5730 MPa with an increase in the content of cerium (Ce content) from 0.02 wt % to 0.09 wt % and then gently rise to 5920 MPa with an increase in the Ce content from 0.09 wt % to 0.50 wt %. When the Ce content was at least 0.03 wt %, the tensile strength was at least 5500 MPa. In particular, when the Ce content was at least 0.30 wt %, a tensile strength as high as at least 5900 MPa was yielded.
In addition, it is found that the roundness increased from 0.4% to 3.1% at a substantially constant rate with an increase in the Ce content from 0.02 wt % to 0.5 wt %. The roundness was at most 2.0% within the range where the Ce content was at most 0.30 wt %, while the roundness was greater than 2.0% when the Ce content was 0.50 wt %. In addition, it is found that, within the range where the Re content was at least 0.03 wt % and at most 0.09 wt %, the roundnesses were at most 1.0%, and thus the roundnesses were improved.

[Effects Etc.]

As described above, tungsten wire 1 according to Embodiment includes tungsten as a major component, wherein 4758×D²−7258.3×D+5275.5≤T≤4758×D²−7258.3×D+6100 is satisfied where T (MPa) is a tensile strength of the tungsten wire, D (mm) is a diameter of tungsten wire 1, and a roundness of the tungsten wire is at most 2.0%.
With this configuration, it is possible to produce tungsten wire 1 of which the tensile strength commensurate with its diameter is higher than conventional one and that has an improved roundness (i.e., its cross section is close to a perfect circle).
For example, it is possible that tungsten wire 1 further includes rhenium, wherein a content of the rhenium in tungsten wire 1 is at least 0.1 wt % and at most 3 wt %.
With this configuration, it is possible to produce tungsten wire 1 having an improved roundness and a higher tensile strength.
For example, it is possible that tungsten wire 1 further includes a rare earth element, wherein a content of the rare earth element in tungsten wire 1 is at least 0.03 wt % and at most 0.3 wt %.
With this configuration, it is possible to produce tungsten wire 1 having an improved roundness and a higher tensile strength.
For example, it is possible that tungsten wire 1, wherein the content of the rare earth element in tungsten wire 1 is at least 0.03 wt % and at most 0.09 wt %, and the roundness of tungsten wire 1 is at most 1.0%.
With this configuration, it is possible to produce tungsten wire 1 having a more improved roundness and a higher tensile strength.
For example, it is possible that a tensile strength of tungsten wire 1 is at least 5800 MPa.
With this configuration, in the case where tungsten wire 1 is used as the core of the saw wire, the saw wire can be stretched tightly, and thus it is possible to restrain the saw wire from vibrating when the saw wire cuts an ingot. By restraining the saw wire from vibrating, a cutting margin of the ingot can be reduced, and thus losses can be reduced.
For example, it is possible that a content of the tungsten in tungsten wire 1 is at least 97 wt %.
For example, it is possible that a diameter of tungsten wire 1 is at most 100 μm.
With this configuration, in the case where tungsten wire 1 is used in, for example, a saw wire to slice an ingot, its small diameter enables a cutting margin to be reduced, and thus it is possible to increase the number of wafers that can be obtained.
For example, it is possible that tungsten wire 1 is used as a core of a saw wire.
With this configuration, in the case where tungsten wire 1 having an improved roundness is used to slice, for example, an ingot, it is possible to reduce variations in the thicknesses of wafers. That is, it is possible to manufacture wafers of high quality.

(Others)

Although the tungsten wire according to the present invention has been described thus far based on the above-described embodiments, the present invention is not limited to the above-described embodiments.
For example, tungsten wire 1 may contain at least two types of elements out of rhenium, potassium, and the rare earth elements.
In addition, for example, tungsten wire 1 may be used as other than the core of a saw wire. For example, tungsten wire 1 may be used as a warp yarn or a weft yarn of a mesh. More specifically, a tungsten mesh is manufactured by performing weaving processing using tungsten wire 1. The tungsten mesh is used for screen printing mesh, cut-resistant clothing, and the like.
Alternatively, tungsten wire 1 may be used as a single wire of a twisted wire. More specifically, a twisted wire is manufactured by performing twisting processing using tungsten wire 1. Twisted wires are used for ropes, catheters, or the like.
Furthermore, tungsten wire 1 may be used for a non-woven fabric, a twisted yarn with an organic fiber such as nylon, or a knitted fabric. For example, after cutting processing is performed on tungsten wire 1 to be less than or equal to a predetermined length, non-woven processing may be performed to manufacture a non-woven fabric.
By improving the roundness of tungsten wire 1, stresses set up in tungsten wire 1 during processing or in use are likely to be even. That is, a high stress is restrained from being locally set up in tungsten wire 1, and thus it is possible to restrain breakage or the like from occurring.
It should be noted that the present invention also includes other forms in which various modifications apparent to those skilled in the art are applied to the embodiments or forms in which structural components and functions in the embodiments are arbitrarily combined within the scope of the present disclosure.

REFERENCE SIGNS LIST

- 1 tungsten wire

Claims

1. A tungsten wire comprising

tungsten as a major component, wherein

4758 \times D^{2} - 7258.3 \times D + 5275.5 \leq T \leq 4758 \times D^{2} - 7258.3 \times D + 6100

is satisfied

where T (MPa) is a tensile strength of the tungsten wire,

D (mm) is a diameter of the tungsten wire, and

a roundness of the tungsten wire is at most 2.0%.

2. The tungsten wire according to claim 1, further comprising

rhenium, wherein

a content of the rhenium in the tungsten wire is at least 0.1 wt % and at most 3 wt %.

3. The tungsten wire according to claim 1, further comprising

a rare earth element, wherein

a content of the rare earth element in the tungsten wire is at least 0.03 wt % and at most 0.3 wt %.

4. The tungsten wire according to claim 3, wherein

the content of the rare earth element in the tungsten wire is at least 0.03 wt % and at most 0.09 wt %, and

the roundness of the tungsten wire is at most 1.0%.

5. The tungsten wire according to claim 1, wherein

a tensile strength of the tungsten wire is at least 5800 MPa.

6. The tungsten wire according to claim 1, wherein

a content of the tungsten in the tungsten wire is at least 97 wt %.

7. The tungsten wire according to claim 1, wherein

a diameter of the tungsten wire is at most 100 μm.

8. The tungsten wire according to claim 1, wherein

the tungsten wire is used as a core of a saw wire.