Vibration damping spring

Abstract

The vibration damping spring is formed from a wire having a copper or copper alloy core wire and a steel coating layer covering the outer circumferential surface of the core wire. The wire diameter is 4 mm or less. The core wire is arranged to form the outer circumferential surface of the core wire and includes a boundary layer having a copper or copper alloy parent phase and a plurality of island regions that are regions containing iron and are dispersed in the parent phase. The area ratio of the core wire in a cross section perpendicular to the longitudinal direction of the wire is 20% or more and 90% or less.

Description

[Technical field]

[0001] The present disclosure relates to vibration damping springs. [Background technology]

[0002] Shock absorbers using springs are known. For example, a shock absorber for an automobile has been proposed in which a shock absorber that contributes to vibration attenuation (damping) is arranged between a coil spring and a spring seat (see, for example, Japanese Patent Application Laid-Open No. 2002-178737 (Patent Document 1)). In this shock absorber, vibration damping is mainly provided by the shock absorber. Also, a spring wire is known that has a copper core wire and a stainless steel coating layer that covers the core wire, with the aim of achieving both electrical conductivity and strength (see, for example, Japanese Patent Application Laid-Open No. 59-205105 (Patent Document 2)). In this spring wire, an interdiffusion layer is formed between the core wire and the coating layer, which is said to improve sag resistance and fatigue resistance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2002-178737 A [Patent Document 2] Japanese Patent Application Publication No. 59-205105 Summary of the Invention

[0004] The vibration damping spring according to the present disclosure is formed from a wire having a copper or copper alloy core wire and a steel coating layer covering the outer circumferential surface of the core wire. The wire diameter is 4 mm or less. The core wire is arranged to form the outer circumferential surface of the core wire and includes a boundary layer having a copper or copper alloy parent phase and a plurality of island regions that are regions containing iron and are dispersed in the parent phase. The area ratio of the core wire in a cross section perpendicular to the longitudinal direction of the wire is 20% or more and 90% or less. [Brief description of the drawings]

[0005] [Figure 1]FIG. 1 is a schematic diagram showing the structure of a vibration damping spring. [Diagram 2] FIG. 2 is a schematic diagram showing the structure of the wire. [Diagram 3] FIG. 3 is a schematic cross-sectional view showing a structure in the vicinity of the boundary layer of the wire. [Figure 4] FIG. 4 is a flow chart showing an outline of a method for manufacturing a vibration damping spring. [Diagram 5] FIG. 5 shows the results of EDX analysis near the boundary layer. [Figure 6] FIG. 6 is a diagram showing the results of a line analysis of elements in a direction perpendicular to the interface between the core wire and the coating layer of the sample of the example. [Figure 7] FIG. 7 is a diagram showing the results of a line analysis of elements in a direction perpendicular to the interface between the core wire and the coating layer of the sample of the comparative example. [Figure 8] FIG. 8 is a schematic diagram showing the structure of a test device for a vibration damping test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] [Problem that this disclosure aims to solve] An object of the present disclosure is to provide a vibration-damping spring with excellent vibration-damping properties.

[0007] [Effects of this disclosure] According to the above vibration-damping spring, it is possible to provide a vibration-damping spring with excellent vibration-damping properties.

[0008] [Description of the embodiments of the present disclosure] First, the embodiments of the present disclosure will be listed and described. (1) The wire is formed from a strand having a copper or copper alloy core wire and a steel coating layer covering the outer surface of the core wire. The wire diameter is 4 mm or less. The core wire is arranged to form the outer surface of the core wire and includes a boundary layer having a copper or copper alloy parent phase and a plurality of island regions that are regions containing iron and are dispersed in the parent phase. The area ratio of the core wire in a cross section perpendicular to the longitudinal direction of the strand is 20% or more and 90% or less.

[0009] The automobile shock absorber (vibration damping device) of Patent Document 1, in which vibration damping is mainly provided by the shock absorber, provides excellent vibration damping, but has the problem of difficulty in miniaturization. In contrast, the vibration damping spring of the present disclosure provides excellent vibration damping to the spring itself. This makes it possible to miniaturize the vibration damping device.

[0010] Specifically, the vibration damping spring of the present disclosure is formed from a wire having a core wire and a coating layer. In a wire having a copper or copper alloy core wire and a steel coating layer, as disclosed in the above Patent Document 2, a diffusion layer is generally formed near the interface between the core wire and the coating layer. For example, a diffusion layer is formed in which iron constituting the coating layer is diffused so as to form the outer peripheral surface of the core wire. In an equilibrium state, the iron constituting the coating layer is hardly dissolved in the copper constituting the core wire. However, by strong processing such as wire drawing, the iron diffuses from the coating layer to the core wire, forming a diffusion layer. This diffusion layer contributes to improving the adhesion between the core wire and the coating layer. As a result, it is considered that the spring wire of Patent Document 2 has improved sag resistance and fatigue resistance.

[0011] On the other hand, in the vibration-damping spring of the present disclosure, the core wire includes a boundary layer arranged to form the outer circumferential surface of the core wire (interface with the coating layer). This boundary layer has a mother phase made of copper or copper alloy, and a plurality of island regions that are regions containing iron and are dispersed in the mother phase. That is, in the boundary layer, the mother phase mainly composed of copper and the phase containing iron are in a separated state. This reduces the adhesion between the core wire and the coating layer. In addition, the yield stress of the steel coating layer is greater than the yield stress of the copper or copper alloy core wire. As a result, when vibrations that load a stress that exceeds the yield stress of the core wire and is less than the yield stress of the coating layer are transmitted to the vibration-damping spring of the present disclosure, the vibration energy is absorbed by repeated plastic deformation in the core wire, and the elasticity of the vibration-damping spring is maintained by the coating layer. At this time, due to the low adhesion between the core wire and the coating layer, repeated plastic deformation in the core wire and maintenance of elasticity by the coating layer occur independently. In particular, by setting the wire diameter to 4 mm or less and the area ratio of the core wire in a cross section perpendicular to the longitudinal direction of the wire to 20% to 90%, excellent vibration damping properties can be obtained. In this manner, the vibration damping spring of the present disclosure is a vibration damping spring with excellent vibration damping properties. By imparting excellent vibration damping properties to the spring itself, the vibration damping spring of the present disclosure can contribute to miniaturization of vibration damping devices.

[0012] (2) In the above (1), the diameter of the island region may be 0.5 μm or less. This configuration makes it possible to more reliably obtain high vibration damping properties. Here, the diameter of the island region means the diameter of a circle (equivalent circle diameter) corresponding to the area of ​​the island region in a cross section along the longitudinal direction of the wire. The area of ​​the island region can be measured, for example, by cutting the wire in a cross section along the longitudinal direction and analyzing the cross section by EDX (Energy-Dispersive X-ray spectroscopy) to determine the area of ​​the region containing iron.

[0013] (3) In the above (1) or (2), the thickness of the boundary layer may be 5 μm or less. This configuration makes it possible to more reliably obtain high vibration damping properties. The thickness of the boundary layer can be measured, for example, by confirming the presence of the island regions by EDX in the same manner as in measuring the diameter of the island regions, and by regarding the thickness of the region where the island regions exist as the thickness of the boundary layer.

[0014] (4) In any of the above (1) to (3), the coating layer may be made of stainless steel. From the viewpoints of yield stress, strength, corrosion resistance, etc., stainless steel is suitable as a material for the coating layer.

[0015] (5) In the above (4), the coating layer may be made of austenitic stainless steel. Austenitic stainless steel, which has high corrosion resistance, is particularly suitable as a material for the coating layer.

[0016] [Details of the embodiment of the present disclosure] Next, an embodiment of a vibration damping spring according to the present disclosure will be described below with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0017] Fig. 1 is a schematic diagram showing the structure of a vibration-damping spring. Fig. 2 is a schematic diagram showing the structure of a wire forming the vibration-damping spring. Fig. 3 is a schematic cross-sectional view showing the structure near the boundary layer of the wire. With reference to Fig. 1, the vibration-damping spring 1 in this embodiment is formed from a wire 10. More specifically, the vibration-damping spring 1 is a coil spring (compression coil spring) in which the wire 10 is processed into a spiral shape.

[0018] 1 and 2, the wire 10 forming the vibration damping spring 1 includes a core wire 20 and a coating layer 30. The core wire 20 is made of copper (pure copper; copper content is 99.9% by mass or more) or a copper alloy. As the copper alloy, for example, C1020, C5191, etc., specified in JIS H 3250 can be used. In this embodiment, the core wire 20 is made of copper (pure copper with a copper content of 99.9% by mass or more).

[0019] In this embodiment, the cross-sectional shape of the core wire 20 perpendicular to the longitudinal direction Y of the wires 10 (the direction along the arrow in FIG. 2) is circular. Note that the cross-sectional shape is not limited to a circle, and may be, for example, an ellipse or a rectangle (including a shape in which the parts corresponding to the vertices are arc-shaped). The core wire 20 has an outer circumferential surface 21. In this embodiment, the outer circumferential surface 21 has a cylindrical surface shape.

[0020] The coating layer 30 covers the outer peripheral surface 21 of the core wire 20. In this embodiment, the coating layer 30 has a hollow cylindrical shape. The coating layer 30 has an inner peripheral surface 31 and an outer peripheral surface 32. The inner peripheral surface 31 and the outer peripheral surface 32 each have a cylindrical surface shape. The inner peripheral surface 31 is in contact with the outer peripheral surface 21 of the core wire 20 over the entire circumference. The outer peripheral surface 32 is the outer peripheral surface 11 of the wire 10. The coating layer 30 is made of steel. As the steel, various steels such as stainless steel, spring steel, carbon steel, and mild steel, which are specified in the JIS standard, can be used. As the stainless steel, for example, austenitic stainless steels such as JIS standard SUS304 and SUS316 can be used. The coating layer 30 may be made of stainless steel (for example, austenitic stainless steel). In this embodiment, the coating layer 30 is made of SUS304, which is an austenitic stainless steel.

[0021] 2, the wire diameter D of the wire 10 is 4 mm or less. Here, the wire diameter D means the diameter of the cross section perpendicular to the longitudinal direction Y of the wire 10 when the cross section is circular. When the cross section is other than circular, the wire diameter D means the diameter of a circle having an area corresponding to the cross section (equivalent circle diameter). The area ratio of the core wire 20 in the cross section perpendicular to the longitudinal direction Y of the wire 10 is 20% or more and 90% or less.

[0022] Referring to FIG. 3, the core wire 20 includes a boundary layer 22. The boundary layer 22 is arranged to form an outer peripheral surface 21 of the core wire 20. The boundary layer 22 has a parent phase 23 and a plurality of island regions 24 dispersed in the parent phase 23. The parent phase 23 is made of copper or a copper alloy. In this embodiment, the parent phase 23 is made of pure copper. The island regions 24 are regions containing iron. In this embodiment, the island regions 24 contain iron and chromium, which are elements contained in SUS304, which is the material of the coating layer 30. The diameter of the island regions 24 is 0.5 μm or less. The diameter of the island regions 24 becomes smaller with increasing distance from the outer peripheral surface 21 of the core wire 20 (the interface with the coating layer 30). The thickness t of the boundary layer 22, which is a region in which the island regions 24 exist, is 5 μm or less.

[0023] In the vibration-damping spring 1 of this embodiment, the boundary layer 22 of the core wire 20 has a mother phase 23 made of pure copper and island regions 24 that are regions containing iron and are dispersed in the mother phase 23. That is, in the boundary layer 22, the mother phase 23 mainly composed of copper and the island regions 24 that are phases containing iron are in a separated state. This reduces the adhesion between the core wire 20 and the coating layer 30. As a result, when the vibration-damping spring 1 expands and contracts, repeated plastic deformation of the core wire 20 made of pure copper and maintenance of elasticity by the coating layer 30 made of SUS304 occur independently. In this way, the vibration-damping spring 1 has excellent vibration-damping properties. Furthermore, excellent vibration-damping properties are obtained by having the wire diameter D of the wire 10 be 4 mm or less and the area ratio of the core wire 20 in the cross section perpendicular to the longitudinal direction of the wire 10 be 20% or more and 90% or less. Furthermore, by having the wire diameter D of the wire 10 be 4 mm or less, the vibration-damping spring 1 can be easily miniaturized. In this way, the vibration-damping spring 1 of this embodiment is a compact vibration-damping spring with excellent vibration-damping properties. The vibration-damping spring 1 can contribute to miniaturization of vibration-damping devices by imparting excellent vibration-damping properties to the spring itself. A miniaturized vibration-damping device can be used, for example, as a vibration-damping device for reducing the vibration of a structure when a door is opened and closed.

[0024] Next, an example of a method for manufacturing the vibration-damping spring 1 of this embodiment will be described. FIG. 4 is a flow chart showing an outline of the method for manufacturing the vibration-damping spring. Referring to FIG. 4, in the method for manufacturing the vibration-damping spring 1 of this embodiment, a cladding step is first carried out as step S10. In this step S10, a steel pipe having a hollow cylindrical shape to become the coating layer 30 and a metal bar made of copper or a copper alloy to become the core wire 20 are first prepared. In this embodiment, a pipe made of JIS standard SUS304, which is an austenitic stainless steel, and a metal bar made of oxygen-free copper (pure Cu) are prepared. Then, by inserting the metal bar into the pipe, a cladding material in which a copper metal bar is inserted into a steel tube is obtained.

[0025] Next, a first wiredrawing step is performed as step S20. In this step S20, the clad material produced in step S10 is subjected to a wiredrawing process. The wiredrawing process is performed by passing the clad material through a through hole formed in a die. The wiredrawing process may be performed in one process using one die, or in multiple processes using multiple dies. The area reduction rate, which is the difference between the diameter of the clad material before the wiredrawing process and the diameter of the clad material after the wiredrawing process divided by the diameter of the clad material before the wiredrawing process, is set to be 50% or more. As a result, a clad wire including a core wire 20 and a coating layer 30 is obtained.

[0026] Next, a solution treatment is performed as step S30. In step S30, the clad wire obtained in step S20 is subjected to a solution treatment. Specifically, the clad wire obtained in step S20 is subjected to a heat treatment in which it is heated to a temperature range of 900°C to 1100°C, held for 5 seconds to 20 minutes, and then quenched. As a result, in the metal structure of the stainless steel that forms the coating layer 30, the crystal grains that have been elongated by the wire drawing in step S20 are recrystallized, and the martensite structure that has been created by the wire drawing in step S20 disappears. As a result, the coating layer 30 that has been work-hardened in step S20 is softened, and it becomes possible to perform wire drawing again. The heating temperature in the solution treatment is set to the A temperature of the stainless steel that constitutes the coating layer. C3 It is preferable to set the temperature at or above this point.

[0027] Next, a second wiredrawing step is performed as step S40. In this step S40, the clad wire that has been solution-treated in step S30 is drawn. The wiredrawing may be performed in one step using one die, as in step S20, or in multiple steps using multiple dies. This results in a wire 10 including a core wire 20 and a coating layer 30. The area reduction rate, which is the difference between the diameter of the clad wire before the wiredrawing and the diameter of the clad wire (wire 10) after the wiredrawing divided by the diameter of the clad wire before the wiredrawing, is set to 50% or more. By performing the wiredrawing with an area reduction rate of 50% or more, a diffusion layer is formed in the area of ​​the core wire 20 close to the outer circumferential surface 21, as shown in Figs. 2 and 3, which is a layer in which iron (Fe) and chromium (Cr) contained in the stainless steel that forms the coating layer 30 diffuse and penetrate. In the diffusion layer, Fe and Cr are present in such a way that the concentration increases closer to the outer circumferential surface 21.

[0028] Next, a spring processing step is performed in step S50, in which the wire 10 obtained in step S40 is processed into a spiral shape as shown in FIG. 1, thereby forming the wire into a spring shape.

[0029] Next, an annealing step is carried out as step S60. In this step S60, an annealing process is carried out on the wire 10 formed into the shape of a spring in step S50. Specifically, the wire 10 formed into the shape of a spring is heated to a temperature of 400°C or higher, thereby reducing the strain in the wire 10 that occurred in step S50, and in the diffusion layer formed in step S40, a matrix phase 23 mainly composed of copper (Cu) and island regions 24 that are phases containing Fe and Cr are separated as phases to form a boundary layer 22. Through the above steps, the vibration damping spring 1 of this embodiment is completed.

[0030] In the manufacturing method of the vibration-damping spring 1 of this embodiment, the heating temperature of the annealing treatment in step S60 is set to a higher temperature (for example, 400°C or higher) than when the purpose is only to reduce distortion. As a result, after a diffusion layer (a layer in which Fe and Cr are diffused into the copper forming the core wire 20) is formed in step S40, the mother phase 23 mainly composed of Cu and the island regions 24 which are phases containing Fe and Cr are separated in the diffusion layer by the annealing treatment at high temperature in step S60, and a boundary layer 22 is formed. As a result, the adhesion between the core wire 20 and the coating layer 30 is reduced. In this way, according to the manufacturing method of the vibration-damping spring 1 of this embodiment, a vibration-damping spring 1 having excellent vibration-damping properties can be manufactured. EXAMPLES

[0031] The wire forming the vibration damping spring of the present disclosure was fabricated, and the state of formation of the island region (boundary layer) was confirmed. In addition, the vibration damping spring of the present disclosure was fabricated, and an experiment was conducted to confirm the improvement of vibration damping properties. The procedure of the experiment is as follows.

[0032] (1) Formation of island regions (boundary layers) The wire 10 was prepared by carrying out steps S10 to S40 of the manufacturing method described in the above embodiment. Then, assuming the annealing step of step S60, the obtained wire 10 was subjected to a heat treatment of heating at 380°C to 420°C to prepare a sample. A sample (as-drawn sample) was also prepared without step S60. Then, the state of existence of each element near the boundary between the core wire 20 and the coating layer 30 of the obtained sample was analyzed by EDX. FIG. 5 shows the result of EDX analysis near the boundary layer. FIG. 5 shows the analysis results of samples in which the heat treatment temperatures were 380°C, 400°C, and 420°C assuming the annealing step of step S60, and the as-drawn sample. For the samples at each heat treatment temperature, the analysis results of Cu, Fe, and Cr by K line and dark field images (ADF images; annular dark field images) are shown.

[0033] 5, when the heat treatment corresponding to step S60 is not performed or when the heat treatment temperature is 380° C., which is lower than 400° C., no island regions 24 are observed. In contrast, when the heat treatment temperature is 400° C. or higher, 400° C. or 420° C., it is found that a large number of island regions 24, in which Fe and Cr are present but no Cu is present, are formed and dispersed in a region of core wire 20 close to outer circumferential surface 21 of core wire 20 (interface between core wire 20 and coating layer 30).

[0034] FIG. 6 is a diagram showing the results of element line analysis in a direction perpendicular to the interface between the core wire 20 and the coating layer 30 of a sample of an embodiment in which the temperature of the heat treatment corresponding to step S60 is 400°C. FIG. 7 is a diagram showing the results of element line analysis in a direction perpendicular to the interface between the core wire 20 and the coating layer 30 of a sample of a comparative example in which the heat treatment corresponding to step S60 is omitted. In FIG. 6 and FIG. 7, the positive region on the horizontal axis corresponds to the coating layer 30, and the negative region corresponds to the core wire 20. With reference to FIG. 6 and FIG. 7, it can be seen that the content of each element near the interface (the outer peripheral surface 21 of the core wire 20) changes sharply in the sample of the embodiment in which the island region 24 is formed, compared to the sample of the comparative example in which the island region 24 is not observed. That is, by performing heat treatment at 400°C or more, the mother phase 23 mainly composed of Cu and the island region 24, which is a phase containing Fe, Cr and Ni, are separated, and the thickness of the diffusion layer that is thought to contribute to the adhesion between the core wire 20 and the coating layer 30 is reduced.

[0035] (2) Vibration control test In the manufacturing method described in the above embodiment, samples of vibration-damping springs were produced in which the area ratio of the core wire in the cross section perpendicular to the longitudinal direction of the wire was changed, and samples of vibration-damping springs in which the presence or absence of an island region was changed by adjusting the heat treatment temperature in step S60. For comparison, samples were also produced in which a pure copper wire and a SUS304 wire were prepared and processed into a spring shape. The wire diameter of the wire forming the vibration-damping spring was 2 mm, the outer diameter of the vibration-damping spring was 20.0 mm, the spring constant was 3 N / mm, and the free height of the spring was 30 mm. The spring constant was adjusted by the number of effective turns of the spring. These samples were then subjected to a vibration-damping test, and the loss factor and the range of motion of the vibration-damping spring were calculated.

[0036] FIG. 8 is a schematic diagram showing the structure of a test device for vibration damping tests. Referring to FIG. 8, the vibration damping test device 90 includes a vibration table 91, a lower plate 93, an upper plate 94, and a weight 95. An impedance head 91A is installed on the vibration table 91. A contact tip 91B is installed on the tip of the impedance head 91A. The contact tip 91B is in contact with the lower plate 93. The vibration damping spring 1 is disposed between the lower plate 93 and the upper plate 94 so as to contact the lower plate 93 and the upper plate 94. A weight 95 is placed on the upper plate 94. The load and acceleration were measured by the impedance head 91A, and the loss factor and the width of the movable range of the spring having vibration damping properties were calculated from the relationship between the vibration frequency and the vibration transmissibility. The weight of the weight was determined so that the sprung load was about 100 g. The vibration frequency was set to about 50 Hz. The test results are shown in Table 1.

[0037] [Table 1] In Table 1, Samples A, B, and C are examples in which the heat treatment temperature in step S60 is set to 400°C, resulting in the presence of island regions, and the area ratio of the core wire also satisfies the conditions for a vibration-damping spring of the present disclosure. Samples D and E are comparative examples in which the area ratio of the core wire does not satisfy the conditions for a vibration-damping spring of the present disclosure. Samples F and G are comparative examples formed from a pure copper wire and a SUS304 wire, respectively. Samples H, I, and J are comparative examples in which the heat treatment temperature in step S60 is set to 350°C, resulting in no island regions.

[0038] Referring to Table 1, samples A to C corresponding to the vibration-damping springs of the examples of the present disclosure have both a high loss factor and a wide range of motion. In contrast, sample D, in which the area ratio of the core wire is smaller than the range that satisfies the conditions for the vibration-damping spring of the present disclosure, has a significantly smaller range of motion of the spring with vibration-damping properties than samples A to C. Sample E, in which the area ratio of the wire is larger than the range that satisfies the conditions for the vibration-damping spring of the present disclosure, has an extremely small loss factor and does not exhibit vibration-damping properties. Furthermore, as in samples H, I, and J, in which the area ratio of the core wire is within the range that satisfies the conditions for the vibration-damping spring of the present disclosure, and in which there are no island regions, the loss factor is smaller than samples A to C.

[0039] The above experimental results confirm that the vibration-damping spring of the present disclosure can provide a vibration-damping spring with excellent vibration-damping properties.

[0040] In the above embodiment and examples, a coil spring has been described as an example of the vibration damping spring of the present disclosure, but the vibration damping spring of the present disclosure is not limited to this. The vibration damping spring of the present disclosure may be a spring of another structure, such as a torsion bar or a spiral spring.

[0041] It should be understood that the embodiments and examples disclosed herein are illustrative in all respects and are not limiting in any respect. The scope of the present invention is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0042] 1 vibration damping spring, 10 wire, 11 outer surface, 20 core wire, 21 outer surface, 22 boundary layer, 23 parent phase, 24 island region, 30 coating layer, 31 inner surface, 32 outer surface, 90 vibration damping test device, 91 vibration table, 91A impedance head, 91B contact tip, 93 lower plate, 94 upper plate, 95 weight, D wire diameter, Y longitudinal direction, t thickness.

Claims

1. A copper or copper alloy core wire; and a steel coating layer covering the outer circumferential surface of the core wire. The wire diameter is 4 mm or less, the core wire includes a boundary layer that is arranged to form the outer circumferential surface of the core wire and has a mother phase made of copper or a copper alloy and a plurality of island regions that are regions containing iron and are dispersed in the mother phase; A vibration damping spring, wherein an area ratio of the core wire in a cross section perpendicular to the longitudinal direction of the wire is 20% or more and 90% or less.

2. 2. The vibration damping spring according to claim 1, wherein the island region has a diameter of 0.5 μm or less.

3. 2. The vibration damping spring according to claim 1, wherein the boundary layer has a thickness of 5 μm or less.

4. 4. The vibration damping spring according to claim 1, wherein the coating layer is made of stainless steel.

5. 5. The vibration damping spring according to claim 4, wherein said coating layer is made of austenitic stainless steel.

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