Coated fastener and its manufacturing method

Abstract

One aspect of the present invention provides a fastener having improved surface hardness and corrosion resistance of the material through a thermal surface treatment layer, and a method for manufacturing the same. Another aspect of the present invention provides a coated fastener having improved mechanical properties such as excellent impact resistance and fatigue resistance and high corrosion resistance through the formation of a zinc-aluminum inorganic coating layer, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a coated fastener and a method for producing the same. [Background technology]

[0002] As part of efforts to reduce carbon dioxide, a greenhouse gas, in order to curb global warming, industries related to the development of renewable energy are expanding. In the case of solar power generation, due to the issue of environmental destruction, solar power plants built on land are being relocated to environments vulnerable to corrosion, such as on water or offshore, and ensuring corrosion resistance of the structures that support the solar panels is essential. Furthermore, there is a trend toward relocating the sites of land-based solar power plants to environments vulnerable to corrosion, such as abandoned salt fields and deserts.

[0003] Structures that support solar panels are generally made of steel, and the fasteners used in assembling the structures are also made of carbon steel wire. Such steel materials are exposed to various corrosive environments depending on the location of the solar panels. For structures that support solar panels installed on the seashore, on water, or offshore, where corrosion is likely to occur, corrosion resistance is a key factor determining the durability of the structure, making corrosion resistance extremely important. In structures that support solar panels, most connections between structures are fastened with fasteners. Corrosion of the fasteners can weaken the structural strength and lead to dangerous situations, such as the collapse of the structure. Thus, fasteners are key components that play an important role in extending the lifespan of buildings not only in the renewable energy field but also in the construction field.

[0004] Meanwhile, inorganic coatings are one way to ensure corrosion resistance. These inorganic coatings can prevent damage to the fastener due to friction by coating the surface of the fastener with an inorganic substance, thereby enhancing the corrosion resistance of the fastener. Such inorganic coating layers are typically formed by adding metal powders such as aluminum, zinc, and tin. Currently, various levels of corrosion resistance can be achieved by controlling the inorganic coating layer depending on the usage environment and the required level of corrosion resistance.

[0005] While inorganic coatings have various advantages in terms of corrosion resistance, excessive fastening using power tools, etc., to obtain sufficient fastening force in structures that require high fastening strength often damages the fastener material as well as the inorganic coating layer, resulting in failure to ensure the required corrosion resistance. Summary of the Invention [Problem to be solved by the invention]

[0006] One aspect of the present invention is to provide a fastener, a coated fastener, and a manufacturing method thereof that can ensure sufficient corrosion resistance without damaging the material and coating layer when fastening a structure that requires high fastening strength.

[0007] The object of the present invention is not limited to the above-mentioned content, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the overall content of this specification. [Means for solving the problem]

[0008] A fastener according to one aspect of the present invention includes a thermal surface treatment layer formed on a surface layer portion, the thermal surface treatment layer including a nitride layer, and the nitride layer can have an A value calculated by the following [Relation 1] of 60% or more. [Equation 1]

number

[0009] The average thickness of the nitride layer mentioned above may be 10 μm to 50 μm.

[0010] The thermal surface treatment layer described above may further include an oxide layer.

[0011] The average thickness of the oxide layer described above may be 1 μm to 5 μm.

[0012] The Vickers hardness of the surface layer portion of the fastener may be at least twice as high as the Vickers hardness of the center portion of the fastener.

[0013] The coated fastener according to another aspect of the present invention may further include a zinc-based layer formed on the fastener.

[0014] The coated fastener described above may further include an aluminum-based layer formed on the zinc-based layer.

[0015] The aluminum-based layer may contain 1.0 wt % to 18 wt % of silicon oxide relative to the total weight of the aluminum-based layer.

[0016] The silicon oxide layer contained in the aluminum-based layer may have an average thickness of 1.0 mm or more and 15.0 mm or less.

[0017] The coated fastener described above may further include a functional coating layer.

[0018] A method for manufacturing a fastener according to yet another aspect of the present invention includes the steps of preparing a fastener; and forming a thermal surface treatment layer by heat-treating the surface of the fastener. The step of forming the thermal surface treatment layer may involve heat treatment at a temperature of 550°C to 590°C in a non-oxidizing atmosphere or a reducing atmosphere.

[0019] The ammonia (NH3) fraction in the non-oxidizing atmosphere or reducing atmosphere described above may be 60% by volume or more and 80% by volume or less.

[0020] A method for manufacturing a coated fastener according to one aspect of the present invention may include, after forming a thermal surface treatment layer, forming a zinc-based layer by immersing the fastener in a coating solution containing zinc, and, after forming the zinc-based layer, forming an aluminum-based layer by immersing the fastener in a coating solution containing aluminum.

[0021] The coating solution containing aluminum may contain 1.0 wt % to 18 wt % of silicon oxide based on the total weight of the coating solution.

[0022] The method for manufacturing the coated fastener described above may further include a cleaning step for removing impurities before the zinc-based layer forming step.

[0023] The method for manufacturing the coated fastener described above may further include a step of forming a functional coating layer. [Effects of the Invention]

[0024] The present invention provides a fastener having improved surface hardness and corrosion resistance through a thermal surface treatment layer, and a method for manufacturing the same.

[0025] In addition, the present invention provides a coated fastener and a manufacturing method thereof that has improved mechanical properties such as excellent impact resistance and fatigue resistance and high corrosion resistance by forming a zinc-aluminum inorganic coating layer. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a photograph showing a cross section of a coated fastener according to an embodiment of the present invention. [Figure 2] 1 shows EBSD image photographs observed on the thermal surface treatment layers of Comparative Examples 5 to 6 and Invention Examples 7 to 9. [Figure 3] 1 shows photographs of the appearance of coated fasteners of Comparative Example 3 and Invention Examples 5 and 6. [Figure 4] 1 shows photographs of the appearance of fasteners of Examples 3 and 4, Reference 1 (STS304), and Reference 2 (Comparative Example 2) fastened to highly corrosion-resistant alloy-plated steel sheets using a fastener fastening tool, and then subjected to a combined corrosion test for 300 cycles. DETAILED DESCRIPTION OF THE INVENTION

[0027] While the present invention will be described in detail below with reference to preferred embodiments thereof, it should be understood that the present invention can be practiced in various other forms and should not be construed as being limited to the embodiments set forth below.

[0028] In this specification, the term "comprises" is used to indicate that other elements may be further included, rather than excluding other elements, unless specifically stated to the contrary.

[0029] In the present specification, unless otherwise specified, the unit of % means % by weight.

[0030] It should be noted that, although not essential, the technical solutions according to each aspect of the present invention can be usefully used in the technical solutions of other aspects, and the compositions and various useful parameters according to each aspect of the present invention can be advantageously combined with other aspects as appropriate.

[0031] As described above, when an inorganic-coated fastener is overtightened with a power tool or the like in order to obtain sufficient fastening force in a structure requiring high fastening force, the fastener material may be damaged as well as the inorganic coating layer, making it difficult to ensure corrosion resistance.

[0032] The inventors of the present invention have discovered that a thermal surface treatment that successively forms nitrides or nitrides and oxides on the surface of a fastener material can improve the surface hardness and corrosion resistance of the material.

[0033] From this perspective, a fastener according to one embodiment of the present invention may include a thermal surface treatment layer formed on a surface layer portion, and the thermal surface treatment layer may include a nitride layer, and the nitride layer may have an A value calculated by the following [Relation 1] of 60% or more. Each component will be described in detail below. [Equation 1]

number

[0034] The fastener according to an embodiment of the present invention may include a thermal surface treatment layer formed on the surface layer.

[0035] The fastener may be manufactured by cold forging a base steel, and the alloy components of the base steel are not particularly limited. For example, the base steel may be cold heading carbon steel or mild steel.

[0036] Although not necessarily limited thereto, the cold heading carbon steel may be a cold heading carbon steel having an alloy composition according to the following examples 1 to 3. Example 1 A cold heading carbon steel containing, by weight, C: 0.18-0.23%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P: 0.03% or less, S: 0.040% or less, Cu: 0.30% or less, Ni: 0.25% or less, Cr: 0.90-1.20%, B: 5 ppm or less, with the remainder being Fe and other unavoidably contained impurities. Example 2 A cold heading carbon steel containing, by weight, C: 0.17-0.22%, Si: 0.15-0.35%, Mn: 0.70-0.90%, P: 0.03% or less, S: 0.040% or less, Cr: 0.70-0.90%, Ti: 0.01-0.05%, with the remainder being Fe and other unavoidably contained impurities. Example 3 A cold heading carbon steel containing, by weight, C: 0.08-0.13%, Si: 0.10% or less, Mn: 0.30-0.60%, P: 0.03% or less, S: 0.035% or less, Al: 0.04% or less, with the remainder being Fe and other unavoidably contained impurities.

[0037] The mild steel may have an alloy composition according to Example 4 below. Example 4 Mild steel containing, by weight, C: 0.08 to 0.25%, Si: 0.1 to 0.6%, Mn: 0.45 to 1.0%, P: 0.04% or less, S: 0.05% or less, with the balance being Fe and unavoidable impurities.

[0038] However, since various types of cold heading carbon steel and mild steel are used as fastener materials in the technical field to which the present invention pertains, it should be noted that the steel materials that can be used as the base steel used in manufacturing the fastener of the present invention are not limited to the examples given above.

[0039] The thermal surface treatment layer refers to a layer formed by thermally treating the fastener surface, and may include a nitride layer.

[0040] According to one embodiment of the present invention, the average thickness of the nitride layer may be 10 μm to 50 μm, and in another embodiment, the average thickness of the nitride layer may be 20 μm to 30 μm. Thus, the cross section for measuring the average thickness of the nitride layer can be observed using an optical microscope or a scanning electron microscope.

[0041] The nitride layer may include a compound layer and a diffusion layer, and the compound layer may be an epsilon phase (ε-Fe 2-3 N) and gamma prime phase (γ'-FeN), and the diffusion layer 、 Epsilon phase (ε-Fe 2-3 N), gamma prime phase (γ'-FeN), along with α"-Fe 16 It can refer to a layer made of a nitride such as N2.

[0042] In particular, according to one embodiment of the present invention, the nitride layer is formed of the epsilon phase (ε-Fe x N (2≦x≦3) may be more than that of the gamma prime phase (γ'-FeN), and more specifically, the nitride layer may have an A calculated by the following [Relation 1] of 60% or more. [Equation 1]

number

[0043] That is, in one embodiment of the present invention, by setting A to 60% or more, the hardness of the thermal surface treatment layer can be ensured to be high relative to the internal hardness of the base steel. In another embodiment, A may be 70% or more, and in yet another embodiment, it may be 80% or more. On the other hand, the higher A is, the more advantageous it is for achieving the above-mentioned object, so its upper limit is not particularly limited. However, if no gamma prime phase is present, the upper limit of A is 100%, so in one embodiment of the present invention, the upper limit of A can be 100%. In another embodiment, A may be 99% or less, and in yet another embodiment, it may be 90% or less, for the purpose of controlling porosity and whitening in the compound layer.

[0044] That is, in one embodiment of the present invention, by increasing the hardness of the thermal surface treatment layer through the A value to be higher than that of the interior of the base steel, surface damage can be suppressed despite repeated physical impacts from a tool when fastening a fastener.

[0045] More specifically, the Vickers hardness of the surface layer portion of the fastener may be at least twice as high as that of the center portion of the fastener. In this case, the Vickers hardness of the surface layer portion of the fastener may be determined by measuring fine Vickers hardness at three randomly selected points at 0.1 mm intervals at a depth of 10 μm from the surface layer portion of the fastener and averaging the measured values. The Vickers hardness of the center portion of the fastener may refer to the fine Vickers hardness at any one of the points located radially from the surface layer portion of the fastener. In another embodiment, the Vickers hardness of the thermal surface treatment layer may be at least 1.5 times as high as that of the center portion of the fastener.

[0046] In addition to the nitride layer, the thermal surface treatment layer according to a non-limiting embodiment may further include an oxide layer mainly composed of magnetite (Fe3O4) on the nitride layer. The oxide layer may be located on the nitride layer to help the thermal surface treatment layer have higher hardness and corrosion resistance. In another embodiment, the oxide layer may have an average thickness of 1 μm to 5 μm, and as another example, 2 μm to 3 μm. The average thickness of the oxide layer may be measured in the same manner as the average thickness of the nitride layer described above.

[0047] A coated fastener according to one embodiment of the present invention will be described in detail below.

[0048] That is, the fastener according to one embodiment of the present invention may include an inorganic coating layer disposed on the fastener to improve corrosion resistance.

[0049] The inventors of the present invention have also discovered that by forming a zinc-aluminum inorganic coating layer on the surface of the fastener material, a fastener can be provided that has significantly improved mechanical properties such as impact resistance and fatigue resistance, as well as high corrosion resistance, which are far superior to conventional inorganic coatings, and have thus devised the present invention.

[0050] More specifically, the inorganic coating layer may refer to a layer coated with a resin containing an inorganic pigment, and the powder used as the inorganic pigment may have at least one of a spherical shape and a plate shape. The plate-shaped powder may be advantageous over a spherical powder in that it can delay corrosion by increasing the migration path of moisture, a corrosion-causing substance that causes corrosion, and thereby extending the time it takes for moisture to reach the fastener substrate. Therefore, although not necessarily limited thereto, the powder used as the inorganic pigment may have a plate-shaped shape.

[0051] Examples of powders used as inorganic pigments in the inorganic coating layer include aluminum, zinc, silicon, tin, calcium, titanium, and other inorganic materials. In particular, a coated fastener according to a non-limiting embodiment of the present invention may further include a zinc-based layer formed on the fastener, and may further include an aluminum-based layer formed on the zinc-based layer. In this case, "zinc-based" and "aluminum-based" may mean that zinc and aluminum are added in the highest weight ratios among the inorganic pigments contained in the resin.

[0052] In particular, the inorganic coating layer of the fastener according to one embodiment of the present invention has a zinc-based layer as an undercoat and an aluminum-based layer as a topcoat, as described above, which allows the color of the coated fastener to be controlled to be brighter and strengthens the bond between the zinc-based layer and the aluminum-based layer.

[0053] The ratio of zinc to aluminum contained in the inorganic coating layer may be 2:1, but is not necessarily limited to this.

[0054] According to one aspect of the present invention, the aluminum-based layer of the coated fastener of the present invention may contain 1.0 wt. % to 18 wt. % silicon oxide based on its total weight. When the aluminum-based layer contains silicon oxide, it can prevent corrosion-causing substances from penetrating the coating layer and increase the migration paths of corrosion-causing substances within the coating layer. As a result, the coated fastener of the present invention can ensure excellent corrosion resistance. To achieve the above object, the aluminum-based layer according to one embodiment of the present invention may contain 1.0 wt. % or more silicon oxide based on its total weight. In another embodiment, it may contain 3.0 wt. % or more silicon oxide.

[0055] On the other hand, if the amount of silicon oxide added is too large, the resin may gel, increasing viscosity and hindering smooth coating. Furthermore, if a solvent is added to adjust viscosity, the corrosion resistance of the coated fastener may be reduced. Therefore, in one embodiment of the present invention, the upper limit of the silicon oxide content in the aluminum-based layer may be 18 wt. %. In another embodiment, the upper limit of the silicon oxide content may be 10 wt. % or 5 wt. %.

[0056] For purposes similar to those described above, according to another aspect of the present invention, the silicon oxide layer included in the aluminum-based layer may have an average thickness of 1.0 mm or more and 15.0 mm or less. In another embodiment, the average thickness of the silicon oxide layer may be 5.0 mm to 15.0 mm or 7.0 mm to 13.0 mm.

[0057] The coated fastener according to one embodiment of the present invention may optionally include a zinc plating layer between the fastener and the inorganic coating layer.

[0058] In another embodiment, the coated fastener of the present invention may further include a functional coating layer positioned on the inorganic coating layer. When the inorganic coating layer is composed of multiple layers, the functional coating layer improves adhesion between the coating layers and eliminates pores that may occur in the coating layer due to the addition of inorganic pigments. This increases the number of paths through which corrosion-causing substances can migrate into the fastener, thereby helping to prevent corrosion. Examples of the functional coating layer include, but are not limited to, an epoxy-based sealant or an inorganic silicone resin sealant primarily composed of an alkoxysilane compound.

[0059] The fastener of the present invention described above maintains high surface hardness through the thermal surface treatment layer, thereby effectively absorbing repeated physical impacts from a tool when fastening the fastener and suppressing surface damage.

[0060] According to one aspect of the present invention, an inorganic coating layer including a zinc-based layer and an aluminum-based layer can prevent the fastener material from coming into contact with water, oxygen, ions, etc., which induce corrosion reactions, and can increase the electrical resistance of the coated fastener, thereby preventing anodic and cathodic corrosion reactions in the coated fastener. Furthermore, according to a non-limiting embodiment of the present invention, the coated fastener of the present invention can inhibit corrosion of the fastener by promoting passivation of the fastener material using an anti-rust additive, etc., added to the inorganic coating layer, while at the same time ensuring good corrosion resistance of the coated fastener through selective dissolution of the zinc and aluminum inorganic substances.

[0061] Furthermore, the aluminum-based layer contains a certain level of silicon oxide, which prevents corrosion-causing substances from penetrating the coating layer and increases the migration paths of corrosion-causing substances within the coating layer, thereby ensuring excellent corrosion resistance for the fastener of the present invention.

[0062] More specifically, a coated fastener according to one embodiment of the present invention may not develop red rust after 150 cycles of a cyclic corrosion test (CCT, ISO 14993). In another embodiment, the coated fastener may not develop red rust after 200 cycles of a cyclic corrosion test, and as another example, may not develop red rust after 250 cycles of a cyclic corrosion test.

[0063] Figure 1 is a photograph showing a cross section of a coated fastener according to one embodiment of the present invention. From Figure 1, it can be seen that the coated fastener includes a zipper and an inorganic coating layer formed on the zipper, and that a thermal surface treatment layer is formed on the surface of the zipper. It can also be seen that the inorganic coating layer includes a zinc-based layer and an aluminum-based layer formed on the zinc-based layer.

[0064] A method for manufacturing a fastener according to one embodiment of the present invention will be described below. However, the method for manufacturing a fastener described below is merely an example, and the fastener of the present invention does not necessarily have to be manufactured by this manufacturing method. In other words, it should be noted that any manufacturing method can be used to realize each embodiment of the present invention as long as it satisfies the scope of the claims of the present invention.

[0065] A method for manufacturing a fastener according to one aspect of the present invention may include the steps of preparing a fastener and forming a thermal surface treatment layer by heat-treating the surface of the fastener. Each step will be described in detail below.

[0066] Preferentially, a method for manufacturing a fastener according to one aspect of the present invention can prepare a fastener. In one embodiment, such a fastener is obtained by cold forging a base steel. However, since this method for manufacturing a fastener from a base steel is common knowledge among ordinary skilled artisans, it will not be described separately in this specification. The alloy composition of the base steel has been described above, so it will not be described below.

[0067] The surface of the fastener can then be heat-treated to form a thermal surface treatment layer. When the surface of the fastener is heat-treated in this manner, a thermal surface treatment layer with high hardness can be secured relative to the interior of the fastener, as described above. As a result, one embodiment of the present invention can reduce surface damage despite repeated physical impacts from a tool during fastening.

[0068] According to one embodiment of the present invention, the thermal surface treatment layer may be formed by heat treatment at a temperature of 550°C or higher in a non-oxidizing or reducing atmosphere. This is to form an appropriate crystalline phase in the thermal surface treatment layer, thereby increasing the surface hardness of the fastener relative to the center. More specifically, the nitride layer in the thermal surface treatment layer may have an A value of 60% or higher, calculated using the following [Relation 1]. This has been discussed above in relation to the fastener, so a detailed description is omitted. In another embodiment, the heat treatment may be performed at a temperature of 570°C or higher or 580°C or higher. [Equation 1]

number

[0069] On the other hand, if the temperature during the heat treatment is too high, the structure may turn into cementite, which may increase the surface brittleness and the possibility of defects occurring, so the temperature during the heat treatment is preferably 590°C or less. In another embodiment, the upper limit of the temperature during the heat treatment may be 585°C.

[0070] In addition, as a non-limiting embodiment of the present invention, the ammonia (NH3) fraction in the non-oxidizing atmosphere or reducing atmosphere may be 60 vol% or more and 80 vol% or less. That is, in one embodiment of the present invention, the ammonia (NH3) fraction is set to 60 vol% or more, thereby allowing the epsilon phase to be formed at an appropriate area fraction. On the other hand, if the ammonia fraction in the atmosphere is excessively high, a problem may occur in which a highly brittle cementite structure is easily formed. Therefore, in one embodiment of the present invention, the upper limit of the ammonia (NH3) fraction may be set to 80 vol%. And, although not necessarily limited thereto, the non-oxidizing atmosphere or reducing atmosphere may contain 5 vol% to 20 vol% carbon dioxide, 15 vol% or less oxygen, and the remainder nitrogen.

[0071] After the step of forming the thermal surface treatment layer, the method for manufacturing a fastener according to one aspect of the present invention may optionally further include a galvanizing step of galvanizing the thermally surface-treated fastener. This is to improve corrosion resistance through the galvanized layer, as described above. This galvanizing step may be performed by hot-dip galvanizing or electrogalvanizing.

[0072] A method for producing a coated fastener according to one aspect of the present invention will now be described in detail.

[0073] First, although it is not a necessary step, the method for manufacturing a coated fastener according to another aspect of the present invention may include a cleaning step for removing impurities from the fastener having the thermal surface treatment layer formed thereon, as described above.

[0074] The purpose of this cleaning step is to remove impurities from the surface of the fastener and increase the surface roughness before forming the subsequent inorganic coating layer, thereby improving the adhesion of the coating layer. As a non-limiting example, the cleaning can be performed chemically or physically.

[0075] Next, a method for manufacturing a coated fastener according to one embodiment of the present invention can include a step of forming a zinc-based layer by immersing the thermally surface-treated fastener in a coating solution containing zinc, and a step of forming an aluminum-based layer by immersing the fastener in a coating solution containing aluminum, thereby forming an inorganic coating layer including the zinc-based layer and the aluminum-based layer.

[0076] More specifically, the zinc and aluminum may be added to the coating solution in the form of powder or flakes. According to a non-limiting embodiment, the coating solution may include a resin and a solvent in addition to the zinc and aluminum. The resin may impart chemical and physical properties to the coating layer, immobilize the zinc and aluminum powder, and block corrosion-causing substances. Additionally, the solvent is a volatile liquid that dissolves and uniformly mixes materials added to the coating agent. Additionally, the coating solution may further contain materials that can be easily added by ordinary skilled artisans for additional functions, such as dispersants, suspending agents, storage stabilizers, antifoaming agents, and rust inhibitors.

[0077] In addition, in the steps of forming the zinc-based layer and the aluminum-based layer, a centrifuge can be used to appropriately adjust the thickness, but the means for appropriately controlling the thickness can be easily adopted by ordinary engineers, and is not necessarily limited to this. According to one non-limiting embodiment of the present invention, the thicknesses of the zinc-based layer and the aluminum-based layer may be appropriately 10 μm to 20 μm and 5 μm to 30 μm, respectively, for the purpose of improving the fastening performance of the fastener and exhibiting sufficient corrosion resistance.

[0078] Next, one method for hardening the zinc-based layer and the aluminum-based layer is to use a hot air blower, but the above is not intended to exclude other means that a person skilled in the art may employ to achieve the hardening objective.

[0079] In addition, as described above, in accordance with the method for manufacturing a coated fastener according to one embodiment of the present invention, the coating solution containing the aluminum powder can contain 1.0 wt % to 18 wt % of silicon oxide based on the total weight of the coating solution. This has also been described above, so further description is omitted.

[0080] Meanwhile, according to another aspect of the present invention, the method for manufacturing a coated fastener of the present invention may further include a step of forming a functional coating layer to further improve the corrosion resistance of the wire. [Example]

[0081] The following detailed description of a coated fastener and a method for manufacturing the same according to one aspect of the present invention will be given through specific examples. It should be noted that the following examples are provided for the purpose of understanding the present invention and are not intended to define the scope of the present invention. The scope of the present invention can be determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0082] First, fasteners were manufactured using base steel having the alloying composition shown in Table 1. The surface of the fastener was then heat-treated under the conditions shown in Table 2 below to form a thermal surface-treated layer. The average thickness of the nitride layer in the thermal surface-treated layer is shown in Table 2 below. Next, the fastener was cut through the radial center of the fastener, and the resulting cross section was observed using EBSD. Image analysis was performed on the obtained EBSD photographs to measure the area fraction of each crystalline phase in the nitride layer, which is shown in Table 2 below. Figure 2 shows EBSD images observed in Comparative Examples 5 and 6 and Invention Examples 7 to 9. Table 3 shows the Vickers hardness of the thermal surface-treated layer and the Vickers hardness of the center of the fastener, along with the area fractions. The heat-treated fasteners were then chemically cleaned to remove surface contaminants such as oil, and then immersed in a zinc-added coating solution and cured to form a zinc-based layer. The fasteners with the zinc-based layer formed thereon were then immersed in an aluminum-added coating solution and cured to form an aluminum-based layer. The aluminum-containing coating solution was also supplemented with 3 wt. % silicon oxide. The zinc-based and aluminum-based layers were centrifuge-processed to a thickness of 10 μm and 5 μm, respectively, and curing was performed using a hot air blower. After curing, the average thickness of the silicon oxide layer formed on top of the aluminum-based layer was measured and found to be 10 μm. A cyclic corrosion test (CCT; ISO 14993) was performed to confirm the corrosion resistance of the resulting coated fastener, and the results are shown in Figures 3 and 4.

[0083] The nitride layer and its thickness can be measured by cutting the thermally surface-treated fastener and observing the structure of the thermally surface-treated layer on a cross-section obtained using an optical microscope. Specifically, the thermally surface-treated fastener is mounted so that it can be observed from the surface in the depth direction, polished, and etched with an etching solution, and the structure can be observed using an optical microscope. The average thickness of the nitride layer can be determined by measuring the nitride layer thickness at three arbitrary points on the cross-section, and then calculating the average value.

[0084] In addition, the cross section of the inorganic coating layer was observed with a scanning electron microscope (SEM), and then the thickness of the silicon oxide layer was measured by area analysis using an energy dispersive X-ray spectrometer (EDS). As with the thickness of the nitride layer, the average thickness of the silicon oxide layer can be determined by selecting any three points on the cross section where silicon oxide is detected, measuring the thickness of the silicon oxide layer at each point, and then calculating the average value.

[0085] [Table 1]

[0086] [Table 2]

[0087] [Table 3]

[0088] In Tables 2 and 3 above, in Comparative Examples 1, 3 and 5 in which the surface of the fastener was not heat treated and no thermal surface treatment layer was formed, the hardness of the surface and center of the fastener was the same.

[0089] Furthermore, in Comparative Examples 2, 4, and 6, in which the fastener surface was heat-treated but did not satisfy the detailed conditions in the thermal surface treatment stage proposed by the present invention, the A value derived from [Relationship 1] of the present invention was less than 60%, and the Vickers hardness of the surface of the fastener was less than twice the Vickers hardness of the center of the fastener.

[0090] On the other hand, in the case of Examples 1 to 9, which satisfy all of the conditions set forth in the present invention, it can be confirmed that the Vickers hardness of the surface portion of the fastener is at least twice as high as the Vickers hardness of the center portion of the fastener. This means that the present invention makes the hardness of the surface portion higher than that of the center portion, thereby making it possible to suppress surface damage despite repeated physical impacts from a tool when fastening the fastener.

[0091] Figure 3 shows photographs of the appearance of the coated fasteners of Comparative Example 3 and Invention Examples 5 and 6. From these photographs, it can be seen that the higher the A value derived from [Relationship 1], which is the relational expression for the ratio of the crystalline phase in the nitride layer, the better the corrosion resistance. That is, unlike Comparative Example 3, Invention Examples 5 and 6 showed almost no red rust even after 150 cycles of the above-mentioned combined corrosion test.

[0092] Figure 4 shows photographs of the appearance of fasteners from Examples 3 and 4, Reference 1 (STS304), and Reference 2 (Comparative Example 2) after fastening them to a highly corrosion-resistant alloy-plated steel sheet with a fastener fastening tool and then conducting a combined corrosion test for 300 cycles. It can be seen from this photograph that Examples 3 and 4, which have a thermal surface treatment layer to control the ratio of the epsilon phase in the nitride layer to a certain level, have superior corrosion resistance compared to References 1 and 2.

Claims

1. A thermal surface treatment layer is formed on the surface layer portion, the thermal surface treatment layer includes a nitride layer; The nitride layer has an A value calculated by the following [Relational Formula 1] of 60% or more. [Relationship 1] [Equation 1] (In the above [Relation 1], ε represents the epsilon phase (ε-Fe ｘ N (2≦x≦3)), and γ' is the gamma prime phase (γ'-Fe ４ N) means the area fraction (%).

2. The fastener of claim 1, wherein the nitride layer has an average thickness of 10 μm to 50 μm.

3. The fastener of claim 1 , wherein the thermal surface treatment layer further comprises an oxide layer.

4. The fastener of claim 3, wherein the average thickness of the oxide layer is between 1 μm and 5 μm.

5. 2. The fastener according to claim 1, wherein the Vickers hardness of the surface layer portion of said fastener is at least twice as high as the Vickers hardness of the center portion of said fastener.

6. The fastener of claim 1 further comprising a zinc-based layer formed on said fastener.

7. The fastener of claim 6 further comprising an aluminum-based layer formed on said zinc-based layer.

8. 8. The fastener according to claim 7, wherein said aluminum-based layer contains 1.0% by weight or more and 18% by weight or less of silicon oxide based on the total weight of said aluminum-based layer.

9. 9. The fastener according to claim 8, wherein the silicon oxide layer contained in said aluminum-based layer has an average thickness of 1.0 mm or more and 15.0 mm or less.

10. 10. The fastener of claim 6, further comprising a functional coating layer.

11. providing a fastener; and forming a thermal surface treatment layer by heat-treating the surface of the fastener; The step of forming the thermal surface treatment layer is a heat treatment at a temperature of 550°C to 590°C in a non-oxidizing atmosphere or a reducing atmosphere.

12. Ammonia (NH ３ 12. The method for producing a fastener according to claim 11, wherein the ratio of the above-mentioned component (a) to the total weight of the fastener is 60% by volume or more and 80% by volume or less.

13. The method for manufacturing a fastener according to claim 11, further comprising, after the step of forming the thermal surface treatment layer, a step of forming a zinc-based layer by immersing the fastener in a coating solution containing zinc.

14. The method for manufacturing a fastener according to claim 13, further comprising, after the step of forming the zinc-based layer, a step of forming an aluminum-based layer by immersing the fastener in a coating solution containing aluminum.

15. The method for manufacturing a fastener according to claim 14, wherein the coating solution to which aluminum is added contains 1.0 wt % to 18 wt % of silicon oxide based on the total weight of the coating solution.

16. The method of claim 13 further comprising a cleaning step to remove impurities prior to the step of forming the zinc-based layer.

17. The method for manufacturing a fastener according to any one of claims 13 to 16, further comprising the step of forming a functional coating layer.

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