Austenitic stainless steel wire rod, a medical device formed from the wire rod, and a wire drawing method.

Abstract

To provide an austenite stainless steel-made wire drawing material comprising flexure ductility and twisting characteristic and to provide a medical appliance using the material.SOLUTION: There is provided an austenite stainless steel-made wire drawing material having tensile strength of a WPB standard value or higher in JISG 4314. A martensite phase fraction of the material per unit volume is 55% or less in a wire diameter of 0.6-0.7 mm, 66% or less in that diameter of 0.265 mm or more but less than 0.3 mm and 79% or less in the diameter of 0.145 mm or more but 0.16 mm or less. The wire drawing method for the austenite stainless steel wire material includes setting a preliminary step to an area reduction zone near 30% and setting a subsequent step to that zone near 10%.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a drawn wire made of austenitic stainless steel, a medical instrument formed from the wire, and a wire drawing method, and particularly to a high-strength drawn wire having bending ductility and torsional properties.

Background Art

[0002] As materials for medical instruments such as suturing needles and knives, austenitic stainless steel wires such as SUS302 and SUS304 are processed by wire drawing to be work-hardened, thereby providing a material for medical instruments that is difficult to break and rust.

[0003] However, these stainless steel wires have a large degree of hardening with respect to the degree of processing, and as the processing progresses, the hardness and tensile strength increase, and the processing difficulty tends to increase.

[0004] Therefore, when attempting to obtain a wire with a small wire diameter, wire breakage may occur during processing, or even if the desired wire diameter is obtained, the ductility with respect to bending and torsion is insufficient, and aging deterioration associated with use becomes a problem.

[0005] As means for solving problems such as wire breakage due to such wire drawing, techniques such as those in Patent Documents 1 to 5 below, in which the area reduction rate by die processing is devised, are known.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

[0007] However, these methods can lead to wire breakage or degradation due to the work hardening behavior unique to austenitic stainless steel. Therefore, sufficient strength cannot be obtained from drawing austenitic stainless steel, and new methods are still desired.

[0008] In particular, for thin, wire-shaped medical instruments such as suture needles, there is a demand for wire with a diameter of 0.7 mm or less and a tensile strength equivalent to piano wire, exceeding the WPB (Waste Pressure Balance) of JIS G4314. Obtaining such high-strength wire through the wire drawing process of austenitic stainless steel has been extremely difficult.

[0009] In other words, even when the goal is to obtain a medical device made of austenitic stainless steel that is resistant to rust and breakage, forming a linear medical device with a wire diameter of 0.7 mm or less from conventional austenitic stainless steel wire rods presents significant room for improvement in terms of strength, and the thinner the wire diameter, the higher the required strength and ductility. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Therefore, the present invention aims to provide a drawn wire rod made of austenitic stainless steel having bending ductility and torsional properties, a medical device formed from said wire rod, and a wire drawing method. [Means for solving the problem]

[0011] To achieve the above objective, the invention described in claim 1 is a drawn wire rod made of austenitic stainless steel having a tensile strength equal to or greater than the JIS G4314 WPB standard value, wherein the martensite phase fraction per unit volume of the drawn wire rod is 55% or less for wire diameters of 0.6 mm or more and 0.7 mm or less, 66% or less for wire diameters of 0.265 mm or more and less than 0.3 mm, and 79% or less for wire diameters of 0.145 mm or more and 0.16 mm or less. In this way, by suppressing the martensite phase fraction per unit volume to 55% or less, the necessary bending ductility and torsional strength can be obtained even with austenitic stainless steel wire rods with small wire diameters.

[0012] The martensite phase fraction generated by wire drawing is equivalent to the processing-induced martensitic transformation and can be measured as a value per unit volume by neutron diffraction.

[0013] In the region where the wire diameter becomes thinner, the criteria for the martensite phase fraction are relaxed due to changes in the thermal history during wire drawing, allowing for a limit of 66% or less for wire diameters between 0.265 mm and less than 0.3 mm, and 79% or less for wire diameters between 0.145 mm and 0.16 mm.

[0014] Furthermore, the invention described in claim 2 is the invention described in claim 1, wherein the austenitic stainless steel consists of the following main components by weight: C: 0.15 or less, Si: 1.00 or less, Mn: 2.00 or less, P: 0.045 or less, S: 0.03 or less, Ni: 6.00 to 14.00, and Cr: 16.00 to 20.00.

[0015] Austenitic materials with this blending ratio, when cold-worked into spring material, become useful materials for medical devices, and their martensitic transformation behavior due to work hardening is consistent with the spirit of this invention. Standard materials that correspond to this blending ratio include SUS301, SUS302, SUS304, and SUS316.

[0016] The present invention can be effectively applied even to such generally circulated SUS materials, and can obtain an austenitic stainless steel wire having a tensile strength equal to or higher than that of piano wire and sufficient ductility without any special composition change.

[0017] The invention according to claim 3 is a medical instrument formed by using the wire drawing material according to claim 1.

[0018] Thus, by using the high-strength wire according to the present invention, it is possible to preferably provide a medical instrument that requires sufficient bending ductility and torsional strength even with a thin wire diameter such as a suture needle.

[0019] The invention according to claim 4 is a method of wire drawing a wire made of austenitic stainless steel, comprising a step of setting a first area reduction rate range with respect to the wire diameter before the wire drawing process, and a step of setting a second area reduction rate range with respect to the wire diameter after the wire drawing process, wherein the first area reduction rate range is set to be 2 times or more that of the second area reduction rate range.

[0020] Thus, by setting the area reduction rate of the wire drawing process in the former stage to be 2 times or more that of the wire drawing process in the latter stage, even when the austenitic stainless steel wire is reduced in area to a wire diameter of 0.7 mm, the processing-induced martensite transformation per unit volume can be maintained at 55% or less.

[0021] Here, the first and second area reduction rate ranges mean the area reduction rates set within a predetermined range. The phrase "2 times or more" means that the difference between the area reduction rate ranges only needs to be 2 times or more, and it may also be 2 times or more of the average value of each area reduction rate range. More preferably, by setting the minimum value of the first area reduction rate range to be 2 times or more that of the maximum value of the second area reduction rate range, a sufficient difference can be ensured between the two.

[0022] The invention according to claim 5 is the invention according to claim 4, wherein the first area reduction rate range is set to be 20% or more.

[0023] Thus, by setting the area reduction rate range in the previous stage to 20% or more, even when the wire diameter is reduced to 0.7 mm, the martensite phase fraction can be maintained at 55% or less, and the required torsional strength can be obtained.

[0024] Further, in the invention according to claim 6, in the invention according to claim 5, the first area reduction rate range is set based on 30%, and the second area reduction rate range is set based on 10%.

[0025] Thus, by setting the area reduction rate in the previous stage based on 30% and the area reduction rate in the subsequent stage based on 10%, it is possible to suitably control the martensite phase fraction while avoiding an excessive increase in the number of dies.

[0026] Here, the 30% reference means that the average value of the area reduction rates constituting the first area reduction rate range may be 30%, the median value may be 30%, or it may be around 30% such as 30% ± 3%. The same applies to the 10% reference.

[0027] Further, in the invention according to claim 7, in the invention according to claim 4, the first area reduction rate range is set in any of the wire diameter ranges having a tensile strength less than the JIS G4314 WPB standard value, and the second area reduction rate range is set in any of the wire diameter ranges having a tensile strength equal to or higher than the JIS G4314 WPB standard value.

[0028] Thus, by allocating the first area reduction rate range and the second area reduction rate range before and after based on the JIS G4314 WPB standard value, it is possible to suitably control the martensite phase fraction while obtaining the tensile strength of the JIS G4314 WPB standard value.

[0029] This is related to the fact that when the diameter is reduced to a tensile strength equal to or higher than the WPB standard value, problems with strength and ductility begin to become prominent. Practically, the reference value for allocation before and after may be increased to up to WPB standard value + 100 to 200 MPa. As a result, the higher area reduction rate range in the first half can be increased, and the number of dies can be reduced.

[0030] Specifically, when obtaining wire diameters of 0.6 mm to 0.7 mm, a first reduction ratio range is set for wire diameters with a tensile strength of less than 2300 MPa, and a second reduction ratio range is set for wire diameters with a tensile strength exceeding 2400 MPa, thereby obtaining a high-strength, high-ductility drawn wire. [Effects of the Invention]

[0031] According to the present invention, austenitic stainless steel drawn wires possessing bending ductility and torsional properties can be obtained. In particular, it is possible to form austenitic stainless steel drawn wires and medical devices with a wire diameter of 0.7 mm or less and a tensile strength equal to or greater than the JIS G4314 WPB standard value. [Brief explanation of the drawing]

[0032] [Figure 1] This is a cross-sectional view illustrating the concept of wire drawing according to the present invention. [Figure 2] This is a comparison diagram showing the results of comparing the present invention with a conventional example. [Figure 3] This is a graph plotting the results from Figure 2. [Figure 4] This figure shows the processing conditions and results for Example 1. [Figure 5] This figure shows the processing conditions and results for Example 2. [Figure 6] This figure shows the processing conditions and results for Comparative Example 1. [Figure 7] This figure shows the processing conditions and results for Comparative Example 2. [Figure 8] This figure shows the processing conditions and results for Comparative Example 3. [Figure 9] This figure shows the results of the bending and twisting tests in Examples 1 and 2 and Comparative Examples 1, 2, and 3. [Figure 10] This figure shows the processing conditions and results for Example 3. [Figure 11] This figure shows the processing conditions and results for Example 4. [Figure 12] This figure shows the processing conditions and results for Comparative Example 4. [Figure 13]This figure shows the processing conditions and results for Comparative Example 5. [Figure 14] This figure shows the processing conditions and results for Example 5. [Figure 15] This figure shows the processing conditions and results for Example 6. [Figure 16] This figure shows the processing conditions and results for Comparative Example 6. [Figure 17] This figure shows the processing conditions and results for Comparative Example 7. [Modes for carrying out the invention]

[0033] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0034] Figure 1 is a cross-sectional view illustrating the concept of wire drawing according to the present invention. As shown in Figure (a), the starting material is an austenitic stainless steel wire rod with a diameter of 2.0 mm. This is drawn into a wire rod with a diameter of 1.396 mm as shown in Figure (b) by multi-stage wire drawing using a die with a reduction ratio of 30%, and then drawn into a wire rod with a diameter of 1.073 mm as shown in Figure (c) by multi-stage wire drawing using a die with a reduction ratio of 20%, and then drawn into a wire rod with a diameter of 0.600 mm as shown in Figure (d) by multi-stage wire drawing using a die with a reduction ratio of 10%.

[0035] As shown in Figure (a), in the initial stage, the cross-section of the wire 10 has a relatively uniform austenite structure. However, as shown in Figures (b) to (d), when the wire diameter is reduced, it changes to a core-shell structure consisting of a core C and a shell S. As work hardening progresses, work-induced martensitic transformation occurs from the core C to the shell S, that is, the martensite phase fraction increases.

[0036] This work hardening behavior, unique to austenitic stainless steel, improves the rigidity of the inherently soft stainless steel wire, making it suitable for use in medical devices such as sutures.

[0037] However, we discovered that if the martensite phase fraction becomes too high as the wire diameter decreases, it can lead to wire breakage, which can be a factor in determining the strength limit of austenitic materials. As a result of diligent research, we have devised a method for reducing the surface area that can suppress the martensite phase fraction.

[0038] Figure 2 is a comparison diagram showing the results of comparing the present invention with a conventional example. Figure (a) shows the wire drawing method according to the present invention, and Figure (b) shows a conventional example. Each figure shows the relationship between the reduction in surface area and the martensite phase fraction at each reduction in surface area when processing austenitic SUS302 and SUS304 using the same wire drawing process as in Figure 1.

[0039] As shown in Figure (a), by setting the reduction ratio to 30% in the earlier stage when the wire diameter is large and to 10% in the later stage when the wire diameter is small, the martensite phase fraction can be suppressed to 55% or less, which is necessary to obtain strength equivalent to piano wire. In the figure, the region indicated by the symbol RRH is the reduction ratio range where the reduction ratio is set to a higher level, and the region indicated by the symbol RRL is the reduction ratio range where the reduction ratio is set to a lower level.

[0040] In contrast, as shown in the conventional example in Figure (b), if the reduction ratio is set to a low 11% for all wire diameters, the martensite phase fraction rises to nearly 60% at around 1.0 mm wire diameter, and further exceeds 80% at the final stage of 0.6 mm wire diameter. Values ​​marked with an asterisk (*) in the figure indicate a martensite phase fraction exceeding 55%, resulting in a metal structure that does not provide sufficient bending ductility and torsional strength.

[0041] Figure 3 is a graph plotting the results from Figure 2. The dashed line in the figure indicates the reference value TH, which is the 55% martensite phase fraction used as the standard for achieving strength in the present invention. The solid line represents the present invention, and the dotted line represents the conventional example. As shown in the figure, variable reduction ratio processing, which shifts from a higher reduction ratio to a lower reduction ratio, is useful for suppressing the martensite phase fraction, and it is possible to obtain high-strength, small-diameter drawn wires that could not be obtained with conventional fixed reduction ratio processing.

[0042] Based on the above findings, the control factors for the martensite phase fraction that were verified will be described as examples and comparative examples. In the following description, the parameters of the examples and comparative examples will be the number of dies through which the wire passes, the resulting wire diameter, the reduction ratio of each die, the resulting tensile strength, the upper limit of the WPB standard of JIS G4314 used as the basis for the strength target, and the relationship between these parameters and the resulting martensite phase fraction.

[0043] Figure 4 shows the processing conditions and results for Example 1. As shown in the "Reduced Surface Area" column in the figure, the reduced surface area was set to around 30% for dies 1 to 4, and to around 10% for dies 5 to 9. As a result, even when the diameter was reduced to 0.6 mm, the martensite phase fraction was kept below 55%.

[0044] As can be deduced from the relationship with Example 2 described later, as shown by the symbol RRH in the figure, it is important to set a higher reduction ratio range for wire diameters with a tensile strength of less than 2300 MPa, and as shown by the symbol RRL in the figure, to set a lower reduction ratio range for wire diameters with a tensile strength exceeding 2400 MPa, in order to control the martensite phase fraction. Note that the reduction ratio set in the tensile strength range of 2300 MPa to 2400 MPa does not have as much effect on the control of the martensite phase fraction as in the ranges shown by symbols RRH and RRL, but it is desirable to set it to a value equivalent to or between the two.

[0045] Figure 5 shows the processing conditions and results for Example 2. This figure shows an example where the number of dies in Example 1 was reduced from 9 to 8 to shorten the process. As shown by the reduction ratio in die 5 in the figure, it can be seen that the martensite phase fraction is suitably controlled even when the reduction ratio in the tensile strength range of 2300 MPa to 2400 MPa is increased to 19%.

[0046] Figure 6 shows the processing conditions and results for Comparative Example 1. As shown in the region labeled RRH in the figure, if the reduction ratio is set to 19% for wire diameters with a tensile strength of less than 2300 MPa, even if the reduction ratio is reduced to 10% for wire diameters with a tensile strength exceeding 2400 MPa, the martensite phase fraction will exceed 55% when the diameter is reduced to nearly 0.7 mm. Therefore, in order to obtain sufficient strength for wire diameters of 0.7 mm or less, it is desirable to set the reduction ratio in the preceding step to 20% or more.

[0047] Figure 7 shows the processing conditions and results for Comparative Example 2. As indicated by the symbols RRL in the figure, in the conventional method in which the reduction ratio is fixed at a low value from start to finish, the martensite phase fraction when the diameter is reduced to 0.7 mm significantly exceeds 55%.

[0048] Figure 8 shows the processing conditions and results for Comparative Example 3. This figure also shows that the conventional method, in which the reduction ratio is fixed at a low value, does not produce wires of 0.7 mm or less with sufficient strength.

[0049] Figure 9 shows the results of the bending and twisting tests in Examples 1 and 2 and Comparative Examples 1, 2, and 3. The bending test shown in the figure is the result when a 180-degree aging treatment was performed after wire drawing, followed by a 90-degree bending test. The twisting test shown in the figure is the result when a 180-degree aging treatment was performed after wire drawing, and the torsional fracture angle was confirmed.

[0050] As shown in the figure, Examples 1 and 2 meet the acceptance criteria for bending and twisting tests, and it can be seen that sufficient bending ductility and torsional strength are obtained.

[0051] On the other hand, in Comparative Example 1, the bending test met the acceptance criteria, but the torsion test did not, indicating insufficient torsional strength. Furthermore, in Comparative Examples 2 and 3, both bending ductility and torsional strength were found to be insufficient.

[0052] Figure 10 shows the processing conditions and results for Example 3. This figure shows an example of obtaining a wire diameter of 0.265 mm. By changing the reduction ratio from around 30% (indicated by the symbol RRH) to around 10% (indicated by the symbol RRL), a drawn wire that passed both the bending and twisting tests was obtained, even with a wire diameter of 0.265 mm.

[0053] Figure 11 shows the processing conditions and results for Example 4. This figure shows an example where the number of dies was increased from 8 in Example 3 to 9, and, as with Example 3, a drawn wire that passed the bending test and twisting test was obtained.

[0054] Figure 12 shows the processing conditions and results for Comparative Example 4. This figure shows an example where the diameter was reduced to 0.257 mm using the conventional method with a fixed reduction ratio of 10%. However, the drawn wire obtained under these processing conditions failed to pass the bending and twisting tests.

[0055] Figure 13 shows the processing conditions and results for Comparative Example 5. This figure shows an example where the diameter was reduced to 0.268 mm using the conventional method with a fixed reduction ratio of 16%. However, the drawn wire obtained under these processing conditions failed to pass the bending and twisting tests.

[0056] From the results of Examples 3 and 4 and Comparative Examples 4 and 5 described above, it can be seen that for wire diameters less than 0.3 mm, controlling the martensite phase fraction per unit volume to 66% or less is necessary to form drawn wires with sufficient strength and ductility in both bending and torsion.

[0057] Furthermore, while the standard for the martensite phase fraction in diameters of 0.6 mm to 0.7 mm was 55% or less, it was found that in the region of thinner wire diameters, the standard for the martensite phase fraction could be relaxed to 66% due to changes in the thermal history during the surface reduction process. In Comparative Examples 4 and 5, values ​​exceeding this standard for the martensite phase fraction are indicated with an asterisk (*).

[0058] Figure 14 shows the processing conditions and results for Example 5. This figure shows an example of obtaining a wire diameter of 0.145 mm. By changing the reduction ratio from around 30% (indicated by the symbol RRH) to around 10% (indicated by the symbol RRL), a drawn wire that passed both the bending and twisting tests was obtained, even with a wire diameter of 0.145 mm.

[0059] Figure 15 shows the processing conditions and results for Example 6. This figure shows an example where the number of dies was increased from 8 in Example 5 to 9, and, as with Example 5, a drawn wire that passed the bending test and twisting test was obtained.

[0060] Figure 16 shows the processing conditions and results for Comparative Example 6. This figure shows an example where the diameter was reduced to 0.155 mm using the conventional method with a fixed reduction ratio of 10%. However, the drawn wire obtained under these processing conditions failed to pass the bending and twisting tests.

[0061] Figure 17 shows the processing conditions and results for Comparative Example 7. This figure shows an example where the diameter was reduced to 0.140 mm using the conventional method with a fixed reduction ratio of 16%. However, the drawn wire obtained under these processing conditions failed to pass the bending and twisting tests.

[0062] From the results of Examples 5 and 6 and Comparative Examples 6 and 7 described above, it can be seen that for wire diameters of 0.160 mm or less, controlling the martensite phase fraction per unit volume to 79% or less is necessary to form drawn wires with sufficient strength and ductility in both bending and twisting.

[0063] Furthermore, these examples show that in regions where the wire diameter becomes thinner, the standard for the martensite phase fraction can be relaxed to 79% due to changes in the thermal history during the surface reduction process. In Comparative Examples 6 and 7, values ​​exceeding this standard for the martensite phase fraction are indicated with an asterisk (*). [Explanation of Symbols]

[0064] 10...Wire rod C...Core part S...Shell part

Claims

1. A wire rod made of austenitic stainless steel is drawn to one of the following diameter ranges, and in each diameter range, it has a tensile strength and martensitic phase fraction that are equal to or greater than the upper limit of the JIS G4314 WPB standard, The martensite phase fraction per unit volume of the drawn wire is, Is the wire diameter 0.6 mm or more and 0.7 mm or less, and is the percentage between 46% and 55%? The wire diameter is 0.265 mm or more and less than 0.3 mm, and the percentage is 57% or more and 66% or less, or The wire diameter is between 0.145 mm and 0.16 mm, and the percentage is between 70% and 79%. Austenitic stainless steel drawn wire.

2. The austenitic stainless steel is a drawn wire according to claim 1, comprising, by weight percent, C: 0.15 or less, Si: 1.00 or less, Mn: 2.00 or less, P: 0.045 or less, S: 0.03 or less, Ni: 6.00 to 14.00, and Cr: 16.00 to 20.

00.

3. A medical device formed using the drawn wire material described in claim 1.

4. A method for producing a drawn wire rod according to claim 1 or 2 by drawing a wire rod made of austenitic stainless steel, A step of setting the wire diameter in the wire drawing stage to a first reduction ratio range, The process comprises setting the wire diameter after the wire drawing process to a second reduction ratio range, A wire drawing method in which the first area reduction range is set to twice or more the second area reduction range.

5. The wire drawing method according to claim 4, wherein the first area reduction range is set to 20% or more.

6. The wire drawing method according to claim 5, wherein the first reduction ratio range is set to 30% as a reference and the second reduction ratio range is set to 10% as a reference.

7. The first reduction ratio range is set in any of the wire diameter ranges where the tensile strength is less than the upper limit of the JIS G4314 WPB standard. The wire drawing method according to claim 4, wherein the second area reduction range is set in any of the wire diameter ranges where the tensile strength is equal to or greater than the upper limit of the JIS G4314 WPB standard.

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