TUNGSTEN WIRE

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of PCT Application No. PCT/JP2024/011753, filed Mar. 25, 2024 and based upon and claiming the benefit of priority from Japanese Patent Application No. 2023-58943, filed Mar. 31, 2023, the entire contents of all of which are incorporated herein by reference.

FIELD

Embodiments of the present invention relate to a tungsten wire.

BACKGROUND

Conventionally, a stainless steel needle has been generally used as a medical suture needle, and in recent years there has been an increasing need for a thinner medical needle in order to reduce the burden on a patient. However, if the needle becomes thinner, the needle tends to be bent or curved, leading to a problem that the workability lowers in a short time. In order to prevent this, there is a demand for a material having higher strength and rigidity than the stainless steel material. In particular, a rhenium-tungsten alloy (Re—W alloy) becomes a material having high strength and rigidity by strengthening the solid solution of rhenium in tungsten.

In recent years, with the progress of mass production of suture needles, in a case of using a rhenium tungsten wire (ReW wire), a long wire is required in order to automatically and continuously process the ReW wire into a predetermined needle shape. In order to obtain such a small-diameter ReW wire, a sintered boy is first subjected to primary processing such as swaging and drawing (wire drawing) processing, and the resultant is processed until the diameter becomes 0.3 mm or more and 1.2 mm or less (medium wire) and the diameter becomes 0.05 mm or more and 0.3 mm or less (thin wire). Hereinafter, in some cases, a wire with a diameter of 0.3 mm or more and 1.2 mm or less is referred to as a medium wire, and a wire with a diameter of less than 0.3 mm is referred to as a thin wire. Then, a proper amount of thin ReW wire is subjected to additional necessary processes such as a drawing process and heat treatment, so that the wire may have a predetermined diameter. In the course of this thinning process, a crack or a breakage caused from the crack tends to easily occur during the wire drawing process. Due to such a breakage occurring during the wire drawing process for a thin wire, a necessary length cannot be secured in a case of using a multi-stage wire drawing machine that employs multiple dies for processing, leading to a significant decrease in yield. Furthermore, even in a case where a necessary length can be secured, repair and restart, which are required after the wire breakage, increase the number of processing steps.

Conventional measures against wire breakage include a method in which management of a lubricant and conditions for wire drawing processing are strictly controlled. For example, a lubricant to be applied to the surface of a tungsten wire contains graphite (C) powder, and a thickening agent, has a specific weight of 1.0 g/cm³ or more and 1.1 g/cm³ or less, and a change amount in specific weight during processing is 0.05 g/cm³ or less. The wire drawing process is executed by controlling a tungsten wire temperature at 500° C. or higher and 1300° C. or lower, a drawing die temperature at 300° C. or higher and 650° C. or lower, a drawing rate at 10 m/min or higher and 70 m/min or lower, and an area reduction ratio in the final drawing step at 5% or more and 15% or less (see Patent Document 1)

In addition, there is a method in which the number of recrystallizations is controlled by heat treatment during an intermediate step, thereby improving processability. For example, there is a ReW wire (see Patent Document 2) in which a final recrystallization treatment is conducted at a time when a cross-section reduction ratio (area reduction ratio) of a molded body from a sintered body reaches 75% or more and 90% or less, and the number of recrystallization grains in the central portion and surface portion of the molded body is adjusted to 500 to 800 grains/mm². Patent Document 1 discloses a method for suppressing variations in processability by regulating the processing conditions in the wire drawing step. Patent Document 2 discloses a method for giving a predetermined area reduction ratio up to a recrystallization treatment from a sintered body, and controlling the number of crystals by a heat treatment, and this effect is related to processing up to a finished diameter of 1.0 mm.

CITATION LIST

Patent Literature

• Patent Document 1: International Publication No. 2009/066659
• Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. H3-219039
• Patent Document 3: International Publication No. 2023/008430

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically illustrating an example of a sample taken from a ReW wire according to an embodiment.

FIG. 2 is a schematic diagram giving a schematic illustration of crystal orientations.

FIG. 3 is a schematic diagram giving a schematic illustration of a bcc structure.

FIG. 4 is a schematic diagram giving a schematic illustration of a deformation in a die during a wire drawing process, and a stress acting on central and surface portions.

DETAILED DESCRIPTION

The problem to be solved by the present invention is to provide a tungsten wire that improves breakage during a drawing process, in particular, near a finished size with a diameter of 0.05 mm or more and 0.3 mm or less indicated by a sign φ, by controlling crystal orientations.

In order to solve the above problem, a tungsten wire includes a tungsten alloy containing rhenium, wherein, according to an EBSD analysis on a unit area of 40 μm×40 μm in a portion located within 100 μm concentrically extending from a central axis in a cross section along a wire radial direction perpendicular to a wire drawing direction in a main body portion of the tungsten wire, an area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101> to a measurement field is 90% or more and 99.9% or less on an IPF map (Inverse pole figure map). Note that <101> is parallel to the wire drawing direction.

Hereinafter, referring to the drawings, a tungsten wire according to an embodiment is described. In the description below, a tungsten wire may also be referred to as “ReW wire”. The drawings are schematic ones, and are not intended to limit each component to, for example, a dimensional ratio, etc., illustrated in the drawings.

FIG. 1 illustrates an example of a sample taken from a ReW wire according to the embodiment. In FIG. 1, an axial direction of the sample is defined as an ND (Normal Direction). A cross section perpendicular to the axial direction of the sample is defined as a measurement surface S₀ of the sample. In addition, a horizontal (radial) direction in the cross section perpendicular to the axial direction of the sample is defined as a TD (Transverse Direction). A direction, which is a horizontal (radial) direction in the cross section perpendicular to the axial direction of the sample and is orthogonal to the TD, is defined as an RD (Reference Direction). A sampling position is freely selected. However, in order to arrange for a flow of subsequent steps with good yields and to check variations in the entire wire, it is preferred that sampling be conducted at positions in each ReW wire where the head and tail ends thereof are cut off and n samples be taken from each position (n being 1 or greater). The head and tail ends include portions made under the unstable conditions at the starting and stopping of a wire drawing device, and therefore these end portions are not sampled. The lengths of the portions with instability vary depending on the layout and size of the device. Preferably, the length of the sample taken from the ReW wire is a length (100 mm or more and 150 mm or less) which enables multiple observations of the cross section using a resin filling technique. A ReW wire that has undergone the wire drawing process includes a mixture layer on its surface. The mixture layer contains W, C, and O as constituent elements. A main body portion that remains after removing the mixture layer is used as a sample. The sample is subjected to resin filling and polishing so that the cross section (S₀) perpendicular to the axial direction (ND (Normal Direction)) serves as a measurement surface. Etching is conducted as needed. A surface roughness Ra of the measurement surface is measured by a laser microscope at a magnification of 50 times, and the surface roughness Ra is 0.08 μm or more and 0.12 μm or less. Note that the mixture layer can be removed by surface polishing such as electrolysis.

The measurement surface S₀ illustrated in FIG. 1 is subjected to crystal orientation analysis by an electron backscattered diffraction (EBSD) method. In the EBSD, a crystal specimen is irradiated with an electron beam. The electrons are diffracted and emitted as reflected electrons from the specimen. The diffraction pattern is projected, and the crystal orientation can be measured from the projected pattern. X-ray diffraction (XRD) is a method for measuring an average value of the crystal orientations of multiple crystals. The EBSD, in contrast, allows for the acquisition of information on each crystal grain, and thus the measurement of its crystal orientation. The orientation distribution of crystal grains can then be analyzed from the crystal orientation data. The method may also be called an electron backscattered diffraction pattern (EBSP) method.

For the EBSD analysis, for example, use can be made of a thermal field emission scanning electron microscope (TFE-SEM) JSM-7800F Prime manufactured by JEOL, Ltd., and a DigiView IV slow scan CCD camera, OIM Data Collection ver. 7.3×, and OIM Analysis ver. 8.0 by TSL Solutions.

As measurement positions of the EBSD analysis, three points were observed at a magnification of 1000 times within 100 μm (central portion) concentrically from the central axis of the sample and within 50 μm (outer peripheral portion) inside from the outer periphery of the sample. The crystal orientations of the three points were obtained, the area ratios were obtained, and then an average value of the area ratios of the obtained crystal orientations of the three points was obtained. Note that each measurement region was 40 μm×40 μm. The measured portions may partially overlap each other. Alternatively, an average value of area ratios of crystal orientations may be obtained by measuring 16 times a region of 10 μm×10 μm with no overlapping at a magnification of 5000 times, and the average value may be converted into the area ratio of the crystal orientation obtained by once measuring the region of 40 μm×40 μm. Therefore, for example, in a case where the region of 10 μm×10 μm is set as a one-time measurement region, this region may be measured 48 times (16×3) without no overlapping, the area ratio of the crystal orientation may be obtained in each region, and an average value of 48 area ratios is obtained. In addition, in a case of a ReW wire with a small diameter of the sample (for example, a diameter of 30 μm), the area ratios of crystal orientations obtained by measuring crystal orientations of central portions of multiple cross sections may be averaged. Note that the magnification is preferably 1000 times or more. If the magnification is less than 1000 times, there is concern that the spatial resolution lowers and the measurement cannot be performed with sufficient accuracy. Measurement is conducted under such measurement conditions that an electron beam acceleration voltage is 15 kV, an irradiation current is 15 nA, a specimen inclination angle is 70 degrees, and an interval is 200 nm/step. Here, the sample is a main body portion of the ReW wire excluding the mixture layer. The ReW wire of the embodiment may be a wire that does not substantially include a mixture layer on the surface, a wire that substantially includes Re, W and inevitable impurities (for example, oxygen atoms), or a wire that includes a mixture layer on the surface. Even in the case where the ReW wire of the embodiment includes the mixture layer on the surface, at a time when the central portion of the main body portion excluding the mixture layer is subjected to EBSD analysis, an area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101> to the measurement field is 90% or more and 99.9% or less on an IPF map. Note that <101> is parallel to the wire drawing direction.

The IPF map (Inverse pole figure map) is a crystal orientation map that is based on an inverse pole figure. The IPF map can indicate the distribution states of specified crystal orientations, which face in specified specimen directions (ND, TD, RD, etc.), and also the distribution states of orientation ranges. In addition, the area ratio of the specified crystal orientations and orientation ranges can be obtained by image analysis. The IPF map is created according to the above-described EBSD measurement method.

For a crystal orientation, a fundamental vector is used to indicate the direction. A notation constituted by a combination of numeric characters sandwiched by square brackets ([ ]) exclusively indicates only a specific crystal orientation. A notation constituted by a combination of numeric characters sandwiched by angle brackets (< >) represents a specific crystalline orientation and a direction equivalent to the specific crystal orientation. For example, <101> means that and a direction equivalent to are included. In addition, for example, that the dominant orientation is <101> means that the <101> orientations form a largest proportion among all the crystal orientations.

Each of crystal lattices of a metal includes a specific slip plane and a slip direction. From a microscopic point of view, plastic deformation occurs due to slippage of a crystal lattice. Repeated deformation in the same direction as in a wire drawing process consequently gives convergence to a specific slip plane or slip direction. It is known that, in a wire drawing process, a metal having a body-centered cubic lattice (bcc) involves creation of a <110> orientation texture in parallel with the wire drawing direction (which forms the final stable orientation). FIG. 2 generally illustrates [110] and [110] orientations, and FIG. 3 generally illustrates an atomic arrangement in the bcc. As is understood from the drawings, <101> and <110> in the bcc are equivalent to each other.

In the central portion of the ReW wire of the embodiment, an area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101> to the measurement field is 90% or more and 99.9% or less. Note that <101> is parallel to the ND. The orientation difference means a relative difference in orientation of crystal grains. In the central portion, the area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101>, which is parallel to the ND, is preferably 95% or more and 99.9% or less, and more preferably 98% or more and 99.9% or less. In addition, the area ratio accounted for by crystal orientations having an orientation difference of 5 degrees or less from <101> to the measurement field is preferably 88% or more and 98% or less, and more preferably 92% or more and 98% or less. The ReW wire of the embodiment has a bcc structure, and with the progress of the wire drawing process, the convergence to <101> that is parallel to the ND direction is advanced. If the ratio of crystal orientations having an orientation difference of 15 degrees or less from <101> is above 99.9%, and if the ratio of crystal orientations having an orientation difference of 5 degrees or less from <101> is above 98%, a plastic deformation during the thinning process hardly occurs so that cracks would easily occur. In some instances, annealing for recrystallization needs to be performed at a stage of the thinning process with a large diameter. Recrystallization degrades the processability of the ReW wire, while increasing the likelihood of crack occurrence. If the ratio of crystal orientations having an orientation difference of 15 degrees or less from <101> is below 95%, or if the ratio of crystal orientations having an orientation difference of 5 degrees or less from <101> is below 88%, reinforcement by a process for remedying the brittleness of the W material becomes insufficient, leading to easier occurrence of cracks in the wire drawing process for the medium wire and so on.

FIG. 4 illustrates a deformation in a die during a wire drawing process, and a stress acting on a central portion 2 and a surface portion 1 (also referred to as an outer peripheral portion 1). In a ReW wire for a wire drawing process, a plastic deformation progresses due to an ND-directional tensile stress acting on the center, and <101> is a dominant orientation. The outer peripheral portion 1 is deformed due to a shearing force and accordingly increases the ratio of a <227> orientation, while the dominant orientation is <101>.

In the outer peripheral portion of the ReW wire of the embodiment, the area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101>, which is parallel to the ND, to the measurement field is preferably 70% or more and 93% or less, and more preferably 85% or more and 93% or less. In addition, the area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <227> to the measurement field is preferably 20% or less. Note that <227> is parallel to the ND. If the area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101> is below 70%, or the area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <227> is above 20%, there is a possibility that the ReW wire was under a large shearing force, and wire drawing conditions were abnormal (for example, abnormal lubrication). In such cases, cracks easily occur. Further, the large shearing force may have created a difference in residual stress between the inner and the outer portions, which could cause cracks. The upper limit of the area ratio of crystal orientations having an orientation difference of 15 degrees or less from <101>, which is parallel to the ND, is preferably 93% or less, in view of the balance with the inside of the ReW wire for the wire drawing process. In the case of exceeding 93%, there is a possibility that only the outer peripheral portion has been processed. For the outer peripheral portion, the lower limit of the area ratio of crystal orientations having an orientation difference of 15 degrees or less from <227>, which is parallel to the ND, is not particularly limited, but it is preferably 3% or more since the shearing force by the die is exerted. A more preferable lower limit value is 5%.

A grain size is determined from a crystal grain map that is prepared by using the EBSD analysis data. One crystal grain is identified from two or more consecutive measurement points showing a difference in crystal orientation angle of 5 degrees or less, and results are color mapped. Subsequently, for each crystal grain identified in the crystal grain map, the diameter of a circle having an equivalent area (equivalent circle diameter) is calculated and given in a histogram. The average grain size (d_A) is obtained by the following formula, assuming that N_A represents the total number of grains, A_i represents the area ratio of an individual grain, and d_i represents the equivalent circle diameter.

d A = ∑ i = 1 N A   ( A i × d i ) ( Math . 1 )

The ReW wire of the embodiment has an average grain size of 0.15 μm or more and 0.23 μm or less on the crystal grain map of the central portion. A preferable range of the average grain size is 0.17 μm or more and 0.21 μm or less. The maximum grain size is 0.8 μm or more and 2.0 μm or less. A preferable range of the maximum grain size is 1.0 μm or more and 1.8 μm or less. In a case where the average grain size is less than 0.15 μm, there is concern that the drawing force in the wire thinning process increases and cracks easily occur due to the influence of grain boundary strengthening. In a case where the average grain size exceeds 0.23 μm, the reinforcement by processing for remedying the brittleness of the W material becomes insufficient, leading to an increase in the likelihood of crack occurrence during the wire drawing process for the medium wire and so on. In addition, there is concern that the strength in the size of a finished product, such as a medical needle, is insufficient. If the maximum grain size exceeds 2.0 μm, the presence of such grains causes inhomogeneity in texture, and creates differences in strength and deformation capability in minute regions, leading to nonuniformity in internal stress and possible occurrence of cracks. The lower limit of the maximum grain size is not particularly limited, but it is preferably 0.8 μm or more.

The ReW wire of the embodiment has a ratio of average grain sizes of the central portion to average grain sizes of the outer peripheral portion of greater than 1.00 and less than 1.20 on the crystal grain map for the central portion and the outer peripheral portion. Here, in a case where the average grain size of the central portion is d1 and the average grain size of the outer peripheral portion is d2, the ratio is expressed by d1/d2. A more preferable range of d1/d2 is 1.02 or more and 1.15 or less. If the ratio is 1.20 or more, there is a possibility that only the outer peripheral portion was processed, or a large shearing force was applied, leading to easier crack occurrence in the wire thinning process. If this ratio is 1.00 or less, there is a possibility that only the outer peripheral portion has been recrystallized by the heating in the processing steps up to the medium wire. In this case, a difference in deformation capability occurs between the inner portion and the outer portion, leading to nonuniformity in internal stress and causing crack occurrence in the wire thinning process.

The ReW wire of the embodiment contains Re in an amount of 11 wt % or more and 30 wt % or less. In a case where the Re content is less than 11 wt %, the strength decreases, and, for example, if the ReW wire is used as a medical suture needle, a predetermined shape cannot be maintained, and the accuracy in surgery lowers. In addition, if the Re content exceeds 30 wt %, a deformation stress becomes too large, and the wire thinning process becomes difficult. The Re amount is a value obtained from analysis by inductively coupled plasma optical emission spectrometry (ICP-OES).

Next, a method for manufacturing the ReW wire according to the embodiment is described. Although the manufacturing method is not particularly limited, the following method, for instance, may be adopted.

A W powder and a Re powder are mixed such that the mixture has a Re content of 11 wt % and 30 wt % or less. The mixing method is not particularly limited, but a method of mixing the powders in a slurry form using water or an alcohol-based solution is particularly preferred since this method can provide a powder with good dispersiveness. It is assumed that the Re powder to be mixed has, for example, an average particle size of less than 8 μm. The W powder is a pure W powder excluding inevitable impurities, or a doped W powder containing K in an amount determined in view of the yields up to wire materials. It is assumed that the W powder has, for example, an average particle size of less than 16 μm.

Subsequently, the mixture powder is put into a predetermined mold and press-molded. The pressure employed here is preferably 150 MPa or greater. For the sake of easier handling, the molded object may be subjected to preliminary sintering at 1200° C. to 1400° C. in a hydrogen furnace. The obtained molded object is sintered in a hydrogen atmosphere, or in an inert gas atmosphere of argon or the like. or under vacuum. The sintering temperature is preferably 2500° C. or above. If the sintering temperature is less than 2500° C., Re atoms and W atoms do not sufficiently diffuse during sintering. The upper limit of the sintering temperature is 3400° C. (or equal to or lower than the melting point of W of 3422° C.). If the upper limit of the sintering temperature exceeds the melting point of W (3422° C.), the molded object cannot maintain its shape, and would turn into a defect. The relative density after the sintering is preferably 90% or more. With the sintered object having a relative density of 90% or more, it is possible to reduce the occurrence of cracking, chipping, breaking, etc., in the later swaging process (SW process).

The molding step and the sintering step may be simultaneously carried out through hot pressing in a hydrogen atmosphere, or in an inert gas atmosphere of argon, etc., or under vacuum. The pressure in the pressing is preferably 100 MPa or greater, and the heating temperature is preferably 1700° C. to 2825° C. This hot pressing method can provide a dense sintered object even at a relatively low temperature.

The sintered object obtained from this sintering step is subjected to a first swaging process (SW process). The first SW process is preferably carried out at a heating temperature of 1300° C. to 1600° C. It is preferred that one heat treatment (one heating) give a ratio of reduction of the cross-sectional area (area reduction ratio) in a range of 5% to 15%. After the first SW process, a heat treatment is performed to control crystal orientations. Since the sintered object after the first SW process is yet to have its true density, strains in the sintered object tend to become nonuniform. Thus, inhomogeneity removal by a heat treatment is conducted. In one example, the heat treatment may employ a direct electrical heating method in a hydrogen atmosphere. In the case of the direct electrical heating, a flowing current preferably has a value of 14 A/mm² to 17 A/mm². If the current value falls below 14 A/mm², strain removal in the first SW process becomes insufficient. In addition, if the current value exceeds 17 A/mm², coarse recrystallization occurs in the outer peripheral portion of the sintered object in the cross section due to nonuniform stains, which would easily cause inhomogeneity in texture. Thus, it becomes difficult to control the crystal orientations.

After the first SW process and the heat treatment, a rolling process (RM (rolling machine) process) is performed. The RM process is preferably carried out at a heating temperature of 1200° C. to 1600° C. The area reduction ratio with one heating is preferably 40% to 75%. As a rolling machine, a two-way to 4-way rolling machine, a die roll rolling machine, or the like may be used. With the RM process, the manufacturing efficiency can be greatly enhanced.

The sintered object (ReW rod) after the RM process is subjected to a second SW process. The second SW process is preferably carried out at a heating temperature of 1200° C. to 1500° C. The area reduction ratio with one heating is preferably 5% to 20%.

The ReW rod after the second SW process is then subjected to a recrystallization treatment. The recrystallization treatment is preferably conducted, for example, by using a high-frequency heater at a processing temperature in a range of 1900° C. to 2100° C., in a hydrogen atmosphere, or in an inert gas atmosphere of argon, etc., or under vacuum. If the heat treatment temperature falls below 1900° C., the recrystallization treatment is not fully carried out, which would easily result in coexistence of the processed texture and the recrystallized texture. If the heat treatment temperature exceeds 2100° C., coarse recrystallization occurs, which would easily cause inhomogeneity in texture. The crystal orientations can be controlled by conducting the recrystallization treatment at a temperature in the range of 1900° C. to 2100° C.

The ReW rod after the recrystallization treatment is subjected to a third SW process. The third SW process is preferably carried out at a heating temperature of 1200° C. to 1500° C. The area reduction ratio with one heating is preferably 10% to 30%. The third SW process is continued until the ReW rod has a drawable diameter (preferably, a diameter of 2 mm to 4 mm).

The ReW rod after the third SW process is subjected to a first wire drawing process until the ReW rod has a diameter of 0.3 mm or more and 1.2 mm or less. The first wire drawing process is preferably carried out in a temperature range of 600° C. to 1100° C. In the ReW wire of the present invention, it is necessary to form a sufficient oxide film on the tungsten surface in order for a carbon-based lubricant to sufficiently penetrate into the surface in the wire drawing step, and it is desirable to perform an annealing treatment multiple times at 900° C. to 1200° C. in an oxidizing atmosphere while the diameters are in a range of 0.3 mm to 1.2 mm. If the temperature falls below 900° C., the oxide film is not sufficiently formed, the lubrication effect is insufficient, and the wire is easily broken. On the other hand, if the temperature exceeds 1200° C., the strength of the ReW wire itself decreases and the wire is broken during wire drawing, and therefore the above temperature range is preferable.

The ReW wire after the first wire drawing process is subjected to a second wire drawing process until the ReW wire has a diameter of 0.05 mm or more and 0.3 mm or less. The second wire drawing process is preferably carried out at 500° C. to 800° C. The process-enabling temperature varies depending on the wire diameter, and becomes higher as the diameter increases. If the processing temperature is lower than the process-enabling temperature, cracks, breakage, etc. frequently occur. If the processing temperature is higher than the process-enabling temperature, occurrence of seizure between the ReW wire and the die is expected, or a decrease in deformation resistance of the ReW wire is expected, and there occurs a diameter variation (thinning) after the wire drawing due to the drawing force. The area reduction ratio is preferably 15% to 35%. If the area reduction ratio is less than 15%, a difference in constitution between the inner and outer portions, a residual stress, etc., occur during the processing, and cracks would occur. If the area reduction ratio is more than 35%, the drawing force excessively increases, which would greatly vary the diameter after the wire drawing and cause a breakage. The drawing rate is determined according to the balance between the capability of the heating device, the distance from the device to the die, and the area reduction ratio. A polishing process may additionally be performed during the wire drawing process. The polishing process may adopt a method of, for example, conducting electrochemical polishing (electrolytic polishing) in an aqueous solution that contains solium hydroxide at a concentration of 7 wt % to 15 wt %. It may also be possible to add a heat treatment for mitigating strains (without causing recrystallization). By the wire drawing process, a ReW wire having a diameter of 0.05 mm to 0.3 mm is obtained.

Examples

A W powder having an average particle size of 15 μm and a Re powder having an average particle size of 5 μm were mixed in a slurry form using an alcohol-based solution, so that the Re content (%) was the content (%) of Examples 1 to 8 shown in Table 1. The obtained mixed powder was press-molded to obtain a molded body. Next, the molded body was subjected to temporary sintering treatment in a hydrogen atmosphere at 1300° C., and then sintered in a hydrogen atmosphere at 3000° C. to obtain a sintered body. The sintered body was subjected to a first swaging process at 1500° C., with an area reduction ratio, which is a reduction ratio of the cross-sectional area, being 10%. After the first swaging process, a heat treatment by electric heating was conducted to perform a nonuniform treatment. Note that sintered bodies were also produced from mixed powders having the Re contents of 10% and 31%, and were designated as Examples 9 and 10.

Next, a rolling process was conducted at 1500° C., with an area reduction ratio being 50%, and then a second swaging process was conducted at 1300° C., with an area reduction ratio being 10%. The ReW rod after the second swaging process was subjected to a recrystallization treatment at 2000° C. in a hydrogen atmosphere. The recrystallized ReW rod was subjected to a third swaging process at 1300° C., with an area reduction ratio being 25%, and was drawn until the diameter becomes 2 mm. A further wire drawing process was conducted to manufacture a ReW wire of φ0.3 mm. During the wire drawing process, at a time when the diameters were φ1 mm and φ0.4 mm, an annealing heat treatment was performed at temperatures shown in Table 1 in an oxidizing atmosphere, and ReW wires of Examples 1 to 8 were obtained. In addition, this wire drawing process was performed at a processing temperature of 600° C. in Examples 4 to 8, and at a processing temperature of 900° C. in Examples 1 to 3. More specifically, in Examples 4 to 8, the temperature at which the wire is drawn from 2 mm to 1 mm in diameter was 600° C., the annealing heat treatment was performed at a time point of the diameter of 1 mm, the temperature at which the wire is drawn from 1 mm to 0.4 mm in diameter was 600° C., the annealing heat treatment was performed at a time point of the diameter of 0.4 mm, and the temperature at which the wire is drawn from 0.4 mm to 0.3 mm in diameter was 600° C. The same applies to Examples 1 to 3, except that the processing temperature is 900° C.

ReW wires shown in Table 1 were produced according to the above-described processing methods and processing conditions. The produced ReW wire had a mixture layer on the surface. With respect to the Re content, Example 9 with 10% did not reach a target tensile strength, and Example 10 with 31% could not be processed in the upstream process, and these could not be tested. Table 1 shows the Re content (wt %); the annealing temperature (° C.) at the diameter of 1 mm; the annealing temperature (° C.) at the diameter of 0.4 mm; the area ratio (%) occupied by crystal orientations having an orientation different of 15 degrees or less from <101> that is parallel to the ND (drawing) direction in the central portion; the area ratio (%) occupied by crystal orientations having an orientation difference of 5 degrees or less from <101> that is parallel to the ND (drawing) direction in the central portion; the area ratio (%) occupied by crystal orientations having an orientation difference of 15 degrees or less from <101> that is parallel to the ND (drawing) direction in the outer peripheral portion; the area ratio (%) occupied by crystal orientations having an orientation difference of 15 degrees or less from <227> that is parallel to the ND (drawing) direction in the outer peripheral portion; the average grain size (μm) on the crystal grain map of the central portion; the maximum grain size (μm) on the crystal grain map of the central portion; and the ratio of the average grain size of the central portion to the average grain size of the outer peripheral portion (central portion/outer peripheral portion). In the ReW wires of Examples 2 to 8, the area ratio (%) occupied by crystal orientations having an orientation difference of 15 degrees or less from <101> that is parallel to the ND (drawing) direction in the central portion satisfies 90% or more and 99.9% or less.

Samples for measurement were taken from the ReW of each example, with both ends of the ReW wire being cut off as discussed above, and the EBSD analysis was conducted by the method as described above, so that the area ratio accounted for by the crystal orientations and the crystal grain size were obtained. After the sample was taken, a 5 kg portion was used as a wire and drawn to a diameter of 0.20 mm. The number of times of breaking of the ReW wire until the wire was completed at the diameter of 0.20 mm was evaluated. The number of times of breaking is the total number of wire breaks that occurred in the drawing process in which the wire of 5 kg in total is finished to the diameter of 0.20 mm. For example, in a case where the wire is broken and divided into two, the number of times of breaking is set to one. In a case where the wire is broken and divided into five, the number of times of breaking is set to four. As can be understood from Table 1, the ReW wire according to the embodiment successfully suppressed wire breakage, and can meet a demand for an increase in length in a case of using the ReW wire for a medical suture needle.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention. In addition, the above-described embodiments can be implemented in combination with each other.

Hereinafter, the inventions of the embodiments will be additionally described.

(1). A tungsten wire comprising a tungsten alloy containing rhenium, wherein, according to an EBSD analysis on a unit area of 40 μm×40 μm in a central portion located within 100 μm concentrically extending from a central axis in a cross section along a wire radial direction perpendicular to a wire drawing direction in a main body portion of the tungsten wire, an area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101>, which is parallel to the wire drawing direction, to a measurement field is 90% or more and 99.9% or less on an IPF map.

(2). The tungsten wire according to (1), wherein, on the IPF map, an area ratio accounted for by crystal orientations having an orientation difference of 5 degrees or less from the <101>, which is parallel to the wire drawing direction, to the measurement field is 88% or more and 98% or less.

(3). The tungsten wire according to any one of (1) and (2), wherein, according to the EBSD analysis on the unit area of 40 μm×40 μm in an outer peripheral portion located within 50 μm from an outer periphery of the main body portion of the tungsten wire, an area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <101>, which is parallel to the wire drawing direction, to the measurement field is 70% or more and 93% or less on the IPF map.

(4). The tungsten wire according to any one of (1) to (3), wherein, on the IPF map of the outer peripheral portion of the main body portion of the tungsten wire, an area ratio accounted for by crystal orientations having an orientation difference of 15 degrees or less from <227>, which is parallel to the wire drawing direction, to the measurement field is 3% or more and 20% or less.

(5). The tungsten wire according to any one of (1) to (4), wherein, on a crystal grain map of the central portion of the main body portion of the tungsten wire, an average grain size is 0.15 μm or more and 0.23 μm or less.

(6). The tungsten wire according to any one of (1) to (5), wherein, on a crystal grain map of the central portion of the main body portion of the tungsten wire, a maximum grain size is 0.8 μm or more and 2.0 μm or less.

(7). The tungsten wire according to any one of (1) to (6), wherein, on a crystal grain map of the central portion of the main body portion of the tungsten wire and the outer peripheral portion of the main body portion of the tungsten wire, a ratio of an average grain size of the central portion to an average grain size of the outer peripheral portion is greater than 1.00 and is less than 1.20.

(8). The tungsten wire according to any one of (1) to (7), wherein the tungsten alloy contains rhenium in an amount of 11 wt % or more and 30 wt % or less.

(9). The tungsten wire according to any one of (1) to (8), wherein a diameter of the tungsten wire is 0.05 mm or more and 0.3 mm or less.

REFERENCE SIGNS LIST

S₀ . . . Cross section (measurement surface) perpendicular to an axial direction in an embodiment, ND . . . Normal Direction: Normal-to-cross-section direction (axial direction), TD . . . Transverse Direction: Horizontal-to-cross-section direction (radial direction), RD . . . Reference Direction: Horizontal-to-cross-section direction orthogonal to TD, 1 . . . Outer peripheral portion, 2 . . . Central portion.