Coated fastener and manufacturing method for same

Abstract

One aspect of the present invention is to provide a fastener in which the surface hardness and corrosion resistance of a material have been improved through a thermal surface treatment layer, and a manufacturing method for same. Another aspect of the present invention is to provide a coated fastener having improved mechanical properties, such as excellent impact resistance and fatigue resistance, and high corrosion resistance due to the presence of a zinc-aluminum inorganic coating layer, and a manufacturing method for same.

Description

Coated fastener and its manufacturing method

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

[0002] To curb global warming and reduce carbon emissions, industries related to renewable energy generation are developing. In the case of solar power generation, environmental concerns are driving the relocation of solar power plants from land-based to harsh environments, such as water or offshore, making it essential to ensure corrosion resistance for the structures supporting the solar panels. Furthermore, land-based solar power plant sites, such as abandoned salt farms and deserts, are increasingly being relocated to environments less susceptible to corrosion.

[0003] Structures supporting solar panels are typically made of steel, and the fasteners used during assembly are also made of carbon steel wire. This steel is exposed to a variety of corrosive environments depending on the installation location of the solar power system. For solar power structures installed in areas prone to corrosion, such as coastal areas or waterways, corrosion resistance plays a crucial role in determining the durability of the structure, making corrosion resistance crucial. In solar power structures, most connections are made using fasteners, and if these fasteners corrode and reduce structural strength, dangerous situations such as structural collapse can occur. Therefore, fasteners are key components that play a crucial role in extending the lifespan of buildings, not only in the renewable energy sector but also in the construction sector.

[0004] Meanwhile, one way to ensure corrosion resistance is to use an inorganic coating. This coating protects the fastener's surface from friction-induced damage and enhances its corrosion resistance. This inorganic coating layer is typically formed by adding metal powders such as aluminum, zinc, or tin. Currently, varying levels of corrosion resistance can be achieved by controlling the inorganic coating layer depending on the application environment and the required level of corrosion resistance.

[0005] While inorganic coatings have various advantages in terms of corrosion resistance, in cases where excessive fastening is performed using power tools to obtain sufficient fastening force in structures requiring high fastening force, the inorganic coating layer is often damaged along with the fastener material, resulting in failure to secure the required corrosion resistance.

[0006] One aspect of the present invention is to provide a fastener, a coated fastener, and a method for manufacturing the same, which can secure sufficient corrosion resistance without damaging the material and coating layer when fastening a structure requiring high fastening force.

[0007] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall content of this specification.

[0008] A fastener according to one aspect of the present invention includes a heat surface treatment layer formed on a surface portion, wherein the heat surface treatment layer includes a nitride layer, and the nitride layer can have an A value of 60% or more derived by the following [Relational Expression 1].

[0009] [Relationship 1]

[0010] (In the above [Relationship 1], ε is the epsilon phase (ε-Fe) of the nitride layer xN(2≤x≤3)) means the area fraction (%) of the nitride layer, and γ′ means the area fraction (%) of the gamma prime phase (γ′-Fe4N) of the nitride layer.

[0011] The average thickness of the above-described nitride layer may be 10 µm to 50 µm.

[0012] The above-described heat surface treatment layer may additionally include an oxide layer.

[0013] The average thickness of the above-described oxide layer may be 1 µm to 5 µm.

[0014] The Vickers hardness of the surface layer of the fastener described above may be at least twice the Vickers hardness of the center layer of the fastener.

[0015] A coated fastener according to another aspect of the present invention may additionally include a zinc layer formed on the fastener.

[0016] The above-described coated fastener may additionally include an aluminum layer formed on the zinc layer.

[0017] The above-described aluminum layer may contain silicon oxide in an amount of 1.0 wt% or more and 18 wt% or less based on the total weight thereof.

[0018] The average thickness of the silicon oxide layer included in the above-described aluminum layer may be 1.0 mm or more and 15.0 mm or less.

[0019] The above-described coated fastener may additionally include a functional coating layer.

[0020] A method for manufacturing a fastener according to another aspect of the present invention comprises the steps of preparing a fastener; and the step of forming a heat surface treatment layer for heat treating the surface of the fastener; wherein the step of forming the heat surface treatment layer may be heat treating at a temperature of 550°C to 590°C in a non-oxidizing atmosphere or a reducing atmosphere.

[0021] The fraction of ammonia (NH3) in the non-oxidizing atmosphere or reducing atmosphere described above may be 60% by volume or more and 80% by volume or less.

[0022] A method for manufacturing a coated fastener according to one aspect of the present invention may include a step of forming a zinc layer by immersing the fastener in a coating solution containing zinc after a step of forming a heat surface treatment layer; and may include a step of forming an aluminum layer by immersing the fastener in a coating solution containing aluminum after the above-described step of forming a zinc layer.

[0023] The coating solution containing the aluminum described above may contain silicon oxide in an amount of 1.0 wt% or more and 18 wt% or less based on the total weight thereof.

[0024] The method for manufacturing the above-described coating fastener may additionally include a cleaning step for removing impurities before the zinc layer forming step.

[0025] The method for manufacturing the above-described coating fastener may additionally include a functional coating layer forming step of forming a functional coating layer.

[0026] The present invention can provide a fastener and a method for manufacturing the same in which the surface hardness and corrosion resistance of a material are improved through a heat surface treatment layer.

[0027] In addition, the present invention can provide a coated fastener and a method for manufacturing the same, which have improved mechanical properties such as excellent impact resistance and fatigue resistance and high corrosion resistance by forming a zinc-aluminum inorganic coating layer.

[0028] Fig. 1 is a photograph showing a cross-section of a coating fastener according to one embodiment of the present invention.

[0029] Figure 2 shows EBSD image photographs observed in the thermal surface treatment layers of Comparative Examples 5 to 6 and Invention Examples 7 to 9.

[0030] Figure 3 shows an external photograph of the coated fasteners of Comparative Example 3 and Inventive Examples 5 and 6.

[0031] Figure 4 shows an appearance photograph after performing 300 cycles of a composite corrosion test after fastening the fasteners of Invention Examples 3 and 4 and Reference 1 (STS304) and Reference 2 (Comparative Example 2) to a high-corrosion-resistant alloy-plated steel plate using a fastener fastening tool.

[0032] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0033] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0034] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0035] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0036] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0037] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0038] Although not essential, it should be noted that the technical solutions according to each aspect of the present invention can also be usefully applied to other aspects of the present invention. Furthermore, the compositions and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to achieve beneficial effects.

[0039] As described above, if an inorganic coated fastener is excessively fastened with a power tool or the like to obtain sufficient fastening force in a structure requiring high fastening force, the inorganic coating layer may be damaged along with the fastener material, making it difficult to secure corrosion resistance.

[0040] The inventors of the present invention found that the surface hardness and corrosion resistance of a material can be improved through thermal surface treatment that continuously forms nitrides or nitrides and oxides on the surface of a fastener material.

[0041] From this point of view, a fastener according to one embodiment of the present invention may include a heat surface treatment layer formed on a surface portion, wherein the heat surface treatment layer may include a nitride layer, and the nitride layer may have an A value of 60% or more derived by the following [Relational Expression 1]. Each configuration will be described in detail below.

[0042] [Relationship 1]

[0043] (In the above [Relationship 1], ε is the epsilon phase (ε-Fe) contained in the nitride layer x N(2≤x≤3)) means the area fraction (%) of the gamma prime phase (γ′-Fe4N) contained in the nitride layer, and γ′ means the area fraction (%) of the gamma prime phase (γ′-Fe4N) contained in the nitride layer.

[0044] A fastener according to one embodiment of the present invention may include a heat surface treatment layer formed on a surface portion.

[0045] The above fastener can be manufactured by cold forging a base steel, and the alloy composition of the base steel is not particularly limited, but as an example, the base steel can be carbon steel or mild steel for cold heading.

[0046] Although not necessarily limited thereto, the cold headed carbon steel may be a cold headed carbon steel having an alloy composition according to Examples 1 to 3 below.

[0047] Example 1

[0048] Carbon steel for cold heading, comprising, in 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 unavoidable impurities.

[0049] Example 2

[0050] Carbon steel for cold heading, comprising, in 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 unavoidable impurities.

[0051] Example 3

[0052] Carbon steel for cold heading, comprising, 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 unavoidable impurities.

[0053] Additionally, the above-mentioned mild steel may be mild steel having an alloy composition according to Example 4 below.

[0054] Example 4

[0055] 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 remainder being Fe and unavoidable impurities.

[0056] However, since various types of cold headed carbon steel or mild steel are used as fastener materials in the technical field to which the present invention belongs, it should be noted that the steel materials that can be used as the base steel in manufacturing the fastener of the present invention are not limited to the examples described above.

[0057] In addition, the above-mentioned heat surface treatment layer refers to a layer formed by heat surface treatment of the fastener and may include a nitride layer.

[0058] 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. In this way, a cross-section for measuring the average thickness of the nitride layer can be observed using an optical microscope or a scanning electron microscope.

[0059] In addition, the nitride layer may include a compound layer and a diffusion layer, wherein the compound layer is an epsilon phase (ε-Fe 2-3 N) and gamma prime phase (γ′-Fe4N) are mixed, and the diffusion layer refers to a layer in which the epsilon phase (ε-Fe 2-3 N), along with gamma prime phase (γ′-Fe4N), α″-Fe 16 It may refer to a layer composed of nitrides such as N2.

[0060] In particular, according to one example of the present invention, the nitride layer is an epsilon phase (ε-Fe x N(2≤x≤3)) may be greater than the gamma prime phase (γ′-Fe4N), and more specifically, the nitride layer may have A of 60% or more derived by the following [Relationship 1].

[0061] [Relationship 1]

[0062] (In the above [Relationship 1], ε is the epsilon phase (ε-Fe) contained in the nitride layer x N(2≤x≤3)) means the area fraction (%) of the gamma prime phase (γ′-Fe4N) contained in the nitride layer, and γ′ means the area fraction (%) of the gamma prime phase (γ′-Fe4N) contained in the nitride layer.

[0063] That is, in one embodiment of the present invention, by setting A to 60% or more, the hardness of the heat surface treatment layer can be secured higher than the internal hardness of the base steel. As another example, A may be 70% or more, and as another example, it may be 80% or more. On the other hand, since the higher A is, the more advantageous it is for achieving the above-described purpose, the upper limit thereof is not specifically limited, but when no gamma prime phase is included at all, the upper limit of the A value is 100%, and therefore, in one embodiment of the present invention, the upper limit of the A value may be 100%. As another example, for the purpose of controlling the pores and whitening layer of the compound layer, A may be 99% or less, and as another example, it may be 90% or less.

[0064] That is, one embodiment of the present invention can suppress surface damage despite repeated physical impact by a tool when fastening a fastener by making the hardness of the heat surface treatment layer higher than the inside of the base steel through the A value.

[0065] More specifically, the Vickers hardness of the surface portion of the fastener may be at least twice the Vickers hardness of the center portion of the fastener. In this case, the Vickers hardness of the surface portion of the fastener may be an average value obtained by randomly selecting three points at intervals of 0.1 mm at a depth of 10 μm from the surface portion of the fastener, measuring the micro Vickers hardness at the corresponding points, and the Vickers hardness of the center portion of the fastener may refer to the micro Vickers hardness at any one point among points located at the radial center from the surface portion of the fastener. As another example, the Vickers hardness of the heat-treated layer may be at least 1.5 times the Vickers hardness of the center portion of the fastener.

[0066] In addition to the nitride layer, the thermal surface treatment layer according to a non-limiting embodiment may additionally include an oxide layer mainly composed of magnetite (Fe3O4) on top of the nitride layer. This oxide layer, by being positioned on top of the nitride layer, may assist the thermal surface treatment layer to have higher hardness and corrosion resistance. In addition, as another example, the oxide layer may have an average thickness of 1 μm to 5 μm, and in another example, may have a thickness of 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.

[0067] Hereinafter, a coating fastener according to one embodiment of the present invention will be described in detail.

[0068] That is, a fastener according to an example of the present invention may include an inorganic coating layer positioned on the upper portion of the fastener described above to improve corrosion resistance.

[0069] The inventors of the present invention also discovered that by forming a zinc-aluminum inorganic coating layer on the surface of the material of the fastener, a fastener having improved mechanical properties such as impact resistance and fatigue resistance and high corrosion resistance can be provided, which are far superior to those of existing inorganic coatings, and thus the present invention was derived.

[0070] More specifically, the inorganic coating layer may refer to a layer coated with a resin to which an inorganic pigment has been added, and the shape of the powder used as the inorganic pigment may be at least one of a spherical shape and a plate shape. The plate-shaped powder described above may be advantageous over the spherical powder in that it can delay corrosion by increasing the movement path of moisture, which is a corrosion-causing substance, and extending the time it takes for moisture, etc. to reach the fastener substrate. Therefore, although not necessarily limited thereto, the shape of the powder used as the inorganic pigment may be a plate shape.

[0071] The types of powders used as inorganic pigments of the above-described inorganic coating layer may include inorganic materials such as aluminum, zinc, silicon, tin, calcium, and titanium. In particular, a coating fastener according to a non-limiting example of the present invention may additionally include a zinc layer formed on the above-described fastener, and may additionally include an aluminum layer formed on the zinc layer. In this case, the zinc-based and aluminum-based may mean that among the inorganic pigments included in the resin, zinc and aluminum are added at the highest weight ratio.

[0072] In particular, the inorganic coating layer of the fastener according to an example of the present invention can control the color of the coated fastener to be brighter and strengthen the bond between the zinc layer and the aluminum layer by forming a zinc layer as a lower layer and an aluminum layer as a upper layer, as described above.

[0073] Additionally, the ratio of zinc and aluminum included in the inorganic coating layer may be 2 to 1, but is not necessarily limited thereto.

[0074] According to one aspect of the present invention, the aluminum layer of the coated fastener of the present invention may contain silicon oxide in an amount of 1.0 wt% or more and 18 wt% or less based on the total weight thereof. When the aluminum layer contains silicon oxide, penetration of corrosion-causing substances into the coating layer can be prevented, and the migration path of corrosion-causing substances within the coating layer can be increased. As a result, the coated fastener of the present invention can secure excellent corrosion resistance. In order to achieve the above-described object, the aluminum layer according to one example of the present invention may contain silicon oxide in an amount of 1.0 wt% or more based on the total weight thereof. In another example, it may contain silicon oxide in an amount of 3.0 wt% or more.

[0075] On the other hand, if the amount of silicon oxide added is excessively large, the resin may gel and the viscosity may increase, which may not only prevent the coating from being performed smoothly, but also, if a solvent is added to control the viscosity and then coated, 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 included in the aluminum layer may be 18 wt%. In another embodiment, the upper limit of the silicon oxide may be 10 wt% or 5 wt%.

[0076] For purposes similar to those described above, according to another aspect of the present invention, the average thickness of the silicon oxide layer included in the aluminum layer may be 1.0 mm or more and 15.0 mm or less. As another example, 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.

[0077] A coated fastener according to an example of the present invention may optionally include a zinc plating layer between the fastener described above and the inorganic coating layer described above.

[0078] As an example of another aspect of the present invention, the coated fastener of the present invention may additionally include a functional coating layer positioned on top of the inorganic coating layer. This functional coating layer is intended to improve the adhesion between the coating layers when the inorganic coating layer is composed of multiple layers, and to eliminate pores in the coating layer that may be generated due to the addition of inorganic pigments. In this case, it may be helpful in preventing corrosion because the path through which a corrosion-causing substance moves to the fastener can be increased. Although not necessarily limited thereto, the functional coating layer may include an epoxy-based sealing agent or an inorganic silicone resin sealing agent mainly composed of an alkoxysilane compound.

[0079] The fastener of the present invention described above can effectively absorb repeated physical impacts by a tool when fastening the fastener by maintaining high surface hardness through a heat surface treatment layer, thereby suppressing surface damage.

[0080] In addition, according to one aspect of the present invention, the inorganic coating layer including the zinc layer and the aluminum layer can suppress contact between water, oxygen, ions, etc., which cause corrosion reactions, and the fastener material, and can increase the electrical resistance of the coated fastener, thereby preventing corrosion reactions between the anode and the cathode in the coated fastener. In addition, according to a non-limiting example of the present invention, the coated fastener of the present invention promotes passivation of the fastener material using a rust-preventive additive, etc., added to the inorganic coating layer, thereby suppressing corrosion of the fastener, and at the same time, securing excellent corrosion resistance of the coated fastener through selective dissolution of zinc and aluminum inorganic substances.

[0081] In addition, the aluminum layer described above can prevent corrosion-causing substances from penetrating the coating layer by containing silicon oxide at a certain level or higher, and can increase the migration path of corrosion-causing substances within the coating layer. As a result, the fastener of the present invention can secure excellent corrosion resistance.

[0082] More specifically, a coated fastener according to an example of the present invention may not exhibit red rust after 150 cycles of a cyclic corrosion test (CCT; ISO 14993). As another example, the coated fastener may not exhibit red rust after 200 cycles of a cyclic corrosion test, and as another example, may not exhibit red rust after 250 cycles of a cyclic corrosion test.

[0083] Meanwhile, Fig. 1 is a photograph showing a cross-section of a coated fastener according to one embodiment of the present invention. Referring to Fig. 1, it can be seen that the coated fastener includes a fastener and an inorganic coating layer formed on the upper portion of the fastener, and that a heat-treated surface layer is formed on the surface of the fastener. Furthermore, it can be seen that the inorganic coating layer includes a zinc layer and an aluminum layer formed on the upper portion of the zinc layer.

[0084] Hereinafter, a method for manufacturing a fastener according to one embodiment of the present invention will be described. However, the following method for manufacturing a fastener is merely exemplary, and the fastener of the present invention does not necessarily have to be manufactured using this method. In other words, it should be noted that any manufacturing method that satisfies the claims of the present invention can be used to implement each embodiment of the present invention without any problems.

[0085] A method for manufacturing a fastener according to one aspect of the present invention may include a step of preparing a fastener; and a step of forming a heat surface treatment layer for heat treating the surface of the fastener. Each step is described in detail below.

[0086] First, a method for manufacturing a fastener according to one aspect of the present invention can prepare a fastener. For example, such a fastener can be obtained by cold forging a base steel. However, since this method for manufacturing a fastener from a base steel is well-known to those skilled in the art, it is not described in detail in this specification. Since the alloy composition of the base steel has been previously described, a detailed description thereof will be omitted below.

[0087] Thereafter, the surface of the fastener can be heat-treated to form a heat-treated surface layer. By heat-treating the surface of the fastener in this manner, a heat-treated surface layer with higher hardness compared to the interior of the fastener can be secured, as described above. As a result, one example of the present invention can reduce surface damage even when subjected to repeated physical impacts from a tool during fastening.

[0088] According to one embodiment of the present invention, the heat surface treatment layer can be formed by heat treatment at a temperature of 550°C or higher in a non-oxidizing atmosphere or a reducing atmosphere. This is to form an appropriate crystal phase within the heat surface treatment layer to improve the surface hardness of the fastener compared to the center. More specifically, the nitride layer within the heat surface treatment layer may have an A value of 60% or higher derived from the following [Relational Formula 1]. Since this has been described above with respect to the fastener, a detailed description thereof will be omitted. As another example, the heat treatment may be performed at a temperature of 570°C or higher or a temperature of 580°C or higher.

[0089] [Relationship 1]

[0090] (In the above [Relationship 1], ε is the epsilon phase (ε-Fe) of the nitride layer x N(2≤x≤3)) means the area fraction (%) of the nitride layer, and γ′ means the area fraction (%) of the gamma prime phase (γ′-Fe4N) of the nitride layer.

[0091] On the other hand, if the temperature during the heat treatment becomes excessively high, the tissue may become cementitized, resulting in increased surface brittleness and increased possibility of defects. Therefore, it is preferable that the temperature during the heat treatment be 590°C or lower. As another example, the upper limit of the temperature during the heat treatment may be 585°C.

[0092] In addition, as a non-limiting example of the present invention, the fraction of ammonia (NH3) in the non-oxidizing atmosphere or reducing atmosphere may be 60% by volume or more and 80% by volume or less. That is, in one example of the present invention, by making the fraction of ammonia (NH3) 60% by volume or more, the epsilon phase can be formed at an appropriate area fraction. On the other hand, if the fraction of ammonia in the atmosphere is excessively high, a problem of easily forming a brittle cementite structure may occur, so in one embodiment of the present invention, the upper limit of the fraction of ammonia (NH3) may be 80% by volume. In addition, although not necessarily limited thereto, the non-oxidizing atmosphere or reducing atmosphere may contain 5% to 20% by volume of carbon dioxide, 15% or less of oxygen, and the remainder being nitrogen.

[0093] After the step of forming a heat-treated surface layer, the method for manufacturing a fastener according to one aspect of the present invention may optionally additionally include a zinc plating step of zinc plating the heat-treated fastener. As described above, this is to improve corrosion resistance through the zinc plating layer. This zinc plating step may be performed through hot dip galvanizing or electrolytic galvanizing.

[0094] Hereinafter, a method for manufacturing a coated fastener according to one aspect of the present invention will be described in detail.

[0095] First, although it is not a mandatory 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 a fastener on which a heat surface treatment layer is formed as described above.

[0096] This cleaning step is intended to remove impurities from the fastener surface and increase surface roughness prior to the subsequent formation of an inorganic coating layer, thereby enhancing the adhesion of the coating layer. As a non-limiting example, the cleaning may be performed chemically or physically.

[0097] Next, a method for manufacturing a coated fastener according to an example of the present invention may include a zinc layer forming step of immersing the above-described heat-surface-treated fastener into a zinc-added coating solution; and an aluminum layer forming step of immersing the fastener into an aluminum-added coating solution. As a result, an inorganic coating layer including the above-described zinc layer and aluminum layer can be formed.

[0098] More specifically, the zinc and aluminum may be added to the coating solution in the form of powder or flakes. Furthermore, as a non-limiting example, the coating solution may include a resin and a solvent in addition to zinc and aluminum. The resin may serve to impart chemical and physical properties to the coating layer, fix the zinc and aluminum powders, and block corrosion-causing substances. Furthermore, the solvent, as a volatile liquid, may serve to dissolve and uniformly mix the substances added to the coating solution. Furthermore, the coating solution described above may additionally contain substances that can be easily added by those skilled in the art for additional functions, such as dispersants, anti-settling agents, storage stabilizers, anti-foaming agents, and rust inhibitors.

[0099] In addition, in the zinc layer forming step and the aluminum layer forming step, a centrifuge may be used to appropriately control the thickness thereof. However, since a means for appropriately controlling the thickness can be easily adopted by a person skilled in the art, the present invention is not necessarily limited thereto. In addition, according to a non-limiting embodiment of the present invention, the thickness of the zinc layer and the aluminum layer may be appropriately 10 μm to 20 μm and 5 μm to 30 μm, respectively, for the purpose of improving the fastening properties of the fastener and exhibiting sufficient corrosion resistance.

[0100] Next, as a method for hardening the zinc and aluminum layers, a hot air gun can be used. However, the above does not exclude other means that a person skilled in the art can employ to achieve the hardening purpose.

[0101] In addition, as described above, according to the method for manufacturing a coating fastener according to an example of the present invention, the coating solution to which the aluminum powder is added may contain silicon oxide in an amount of 1.0 wt% or more and 18 wt% or less based on the total weight of the coating solution. As this has also been described above, a detailed description thereof will be omitted.

[0102] Meanwhile, according to another aspect of the present invention, the method for manufacturing the coated fastener of the present invention may additionally include a functional coating layer forming step for forming a functional coating layer for the purpose of further improving the corrosion resistance of the wire.

[0103] Hereinafter, a coated fastener and its manufacturing method according to one aspect of the present invention will be described in more detail through specific examples. It should be noted that the following examples are intended solely to facilitate understanding of the present invention and are not intended to define the scope of the invention. The scope of the invention can be determined by the matters set forth in the claims and matters reasonably inferred therefrom.

[0104] First, a fastener was manufactured using a base iron having the alloy composition of Table 1. Thereafter, the surface of the fastener was heat-treated under the conditions of Table 2 below to form a heat-treated layer. At this time, the average thickness of the nitride layer in the heat-treated layer is shown in Table 2 below. Next, the fastener was cut so as to pass through the radial center of the fastener, and the obtained cross-section was observed using EBSD. Image analysis was performed on the obtained EBSD images to measure the area fraction of each crystal phase in the nitride layer, and the results are shown in Table 2 below. Fig. 2 shows EBSD images observed in Comparative Examples 5 and 6, and Inventive Examples 7 to 9. Table 3 shows the Vickers hardness of the heat-treated layer of the fastener and the Vickers hardness of the center, along with the above-mentioned area fractions. Then, the heat-treated fastener was chemically cleaned to remove surface contaminants such as oil, and then immersed in a zinc-added coating solution and cured to form a zinc layer. Next, the fastener on which the zinc layer was formed was immersed in an aluminum-added coating solution and cured to form an aluminum layer. At this time, 3 wt% of silicon oxide was also added to the aluminum-added coating solution. For the zinc layer and the aluminum layer, the thicknesses were adjusted to 10 μm and 5 μm, respectively, using a centrifuge, and curing was performed using a hot air gun. After curing, the average thickness of the silicon oxide layer formed on the aluminum layer was measured, and the thickness was 10 μm. In order to confirm the corrosion resistance of the coated fastener thus obtained, a cyclic corrosion test (CCT; ISO 14993) was performed, and the results are shown in Figures 3 and 4 below.

[0105] The above nitride layer and the thickness of the nitride layer can be measured by observing the structure of the heat-surface-treated layer using an optical microscope on a cross-section obtained after cutting the heat-surface-treated fastener. Specifically, the heat-surface-treated fastener is mounted so that observation can be made from the surface to the depth, and after polishing, etching with an etchant can be performed to observe the structure using an optical microscope. In addition, the average value of the thickness of the nitride layer can be obtained by arbitrarily setting three points on the cross-section, measuring the thickness of the nitride layer at each point, and then calculating the average value thereof.

[0106] In addition, the cross-section of the inorganic coating layer was observed using a scanning electron microscope (SEM), and the thickness of the silicon oxide layer was measured through surface analysis using an energy-dispersive X-ray spectrometer (EDS). The average thickness of the silicon oxide layer, similar to the thickness of the nitride layer, can be obtained by arbitrarily setting three points in 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 of these points.

[0107] Classification Alloy composition (weight %) Steel grade CSiMnPSCuNiCrBTiAlA0.430.250.800.020.03-----0.02B0.190.200.800.020.02--0.80.00150.02-C0.350.200.750.0250.020.20.21.05---

[0108] ClassificationSteel gradeSurface treatment stageAverage thickness of nitride layer (㎛)A valueTemperature (℃Time (min)Gas fraction (volume %)NH3: N2: CO2Comparative example 1A---00Comparative example 2A5203000: 100: 0555.3Inventive example 1A55030050: 40: 101562.5Inventive example 2A58030060: 30: 102379.8Inventive example 3A55030070: 20: 103093.6Inventive example 4A58030070: 20: 102599.1Comparative example 3B---00Comparative example 4B5203000: 100: 0555.3Inventive example 5B55030070 : 20 : 103093.6 Invention example 6B58030070 : 20 : 102599.1 Comparative example 5C---50 Comparative example 6C5503000 : 100 : 01555.3 Invention example 7C58030050 : 40 : 102362.5 Invention example 8C55030060 : 30 : 103079.8 Invention example 9C58030070 : 20 : 102593.6

[0109] Hardness (Hv) Surface Depth (Substrate) Surface / Deep Comparative Example 12502501 Comparative Example 24752501.9 Invention Example 15302502.1 Invention Example 25912502.4 Invention Example 36122502.4 Invention Example 46102502.4 Comparative Example 33253251 Comparative Example 45533251.7 Invention Example 56723252.1 Invention Example 66723252.1 Comparative Example 53203201 Comparative Example 65983201.9 Invention Example 76353202 Invention Example 86873202.1 Invention Example 97463202.3

[0110] In Tables 2 and 3 above, in Comparative Examples 1, 3 and 5, where the surface of the fastener was not heat-treated and thus no heat surface treatment layer was formed, the hardness of the surface and center of the fastener was the same.

[0111] In addition, heat treatment of the fastener surface was performed, but since the detailed conditions in the heat surface treatment step proposed in the present invention were not satisfied, in the case of Comparative Examples 2, 4 and 6 in which the A value derived by [Relational Expression 1] of the present invention was less than 60%, the Vickers hardness of the surface portion of the fastener was less than twice the Vickers hardness of the center portion of the fastener.

[0112] On the other hand, in the case of invention examples 1 to 9 that satisfy all the conditions presented in the present invention, it can be confirmed that the Vickers hardness of the surface portion of the fastener is at least twice that of the Vickers hardness of the center portion of the fastener. Accordingly, the present invention can suppress surface damage despite repeated physical impacts by a tool during fastening of the fastener by making the hardness of the surface portion higher than that of the center portion.

[0113] Fig. 3 shows the external appearance photographs of the coated fasteners of Comparative Example 3 and Inventive Examples 5 and 6. Looking at these, it can be confirmed that the higher the A value derived by [Relational Expression 1], which is a relational expression for the ratio of crystal phases in the nitride layer, the better the corrosion resistance. That is, unlike Comparative Example 3, it can be seen that in the case of Inventive Examples 5 and 6, almost no red rust occurred even when the above-mentioned composite corrosion test was performed for 150 cycles.

[0114] Figure 4 shows the external appearance photographs of fasteners of Invention Examples 3 and 4 and Reference 1 (STS304) and Reference 2 (Comparative Example 2) fastened to a high-corrosion-resistant alloy-plated steel plate using a fastener fastening tool, and then subjected to 300 cycles of a composite corrosion test. As can be seen from this, Invention Examples 3 and 4, in which a thermal surface treatment layer is formed and the epsilon phase ratio of the nitride layer is controlled to a certain level, have superior corrosion resistance compared to References 1 and 2.

Claims

1. Includes a heat surface treatment layer formed on the surface, The above thermal surface treatment layer includes a nitride layer, A fastener in which the above nitride layer has an A value of 60% or more derived by the following [Relational Formula 1]. [Relationship 1] (In the above [Relational Expression 1], ε is the epsilon phase (ε-Fe) of the nitride layer. x N(2≤x≤3)) means the area fraction (%) of the gamma prime phase (γ′-Fe4N) of the nitride layer, and γ′ means the area fraction (%) of the gamma prime phase (γ′-Fe4N) of the nitride layer.

2. In paragraph 1, A fastener wherein the average thickness of the nitride layer is 10 ㎛ to 50 ㎛.

3. In paragraph 1, A fastener wherein the above heat-treated surface layer additionally includes an oxide layer.

4. In paragraph 3, A fastener wherein the average thickness of the oxide layer is 1 ㎛ to 5 ㎛.

5. In paragraph 1, A fastener wherein the Vickers hardness of the surface layer of the fastener is at least twice that of the Vickers hardness of the center layer of the fastener.

6. In paragraph 1, A coated fastener further comprising a zinc layer formed on the fastener.

7. In paragraph 6, A coated fastener additionally comprising an aluminum layer formed on the zinc layer.

8. In paragraph 7, A coating fastener wherein the aluminum layer contains silicon oxide in an amount of 1.0 wt% to 18 wt% based on the total weight of the aluminum layer.

9. In paragraph 8, A coated fastener wherein the average thickness of the silicon oxide layer included in the above aluminum layer is 1.0 mm or more and 15.0 mm or less.

10. In at least one of paragraphs 6 to 9, A coated fastener additionally comprising a functional coating layer.

11. Step of preparing fasteners; and A step of forming a heat surface treatment layer for heat treating the surface of the fastener; A method for manufacturing a fastener, wherein the above heat surface treatment layer forming step is performed by heat treating at a temperature of 550°C to 590°C in a non-oxidizing atmosphere or a reducing atmosphere.

12. In paragraph 11, A method for manufacturing a fastener, wherein the fraction of ammonia (NH3) in the non-oxidizing atmosphere or reducing atmosphere is 60% by volume or more and 80% by volume or less.

13. In paragraph 11, A method for manufacturing a coated fastener, comprising: a zinc layer forming step of immersing the fastener in a coating solution containing zinc after the above thermal surface treatment layer forming step; 14. In paragraph 13, A method for manufacturing a coated fastener, comprising: an aluminum layer forming step of immersing the fastener in a coating solution containing aluminum after the zinc layer forming step.

15. In paragraph 14, A method for manufacturing a coating fastener, wherein the coating solution to which the aluminum is added contains silicon oxide in an amount of 1.0 wt% or more and 18 wt% or less based on the total weight of the coating solution.

16. In paragraph 13, A method for manufacturing a coated fastener, the method further comprising a cleaning step for removing impurities prior to the zinc layer forming step.

17. In at least one of paragraphs 13 to 16, A method for manufacturing a coated fastener, the method further comprising a functional coating layer forming step of forming a functional coating layer.

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