Screws and method for forming corrosion-resistant coatings on the surfaces of screws

Abstract

To provide a screw that does not require over-tapping on a nut or the like and exhibits sufficient corrosion resistance even in a severe corrosive environment. [Solution] The screw of the present invention is a rod-shaped screw that has, in the axial direction, a tip portion with a thread groove formed therein, a head portion formed with a larger diameter than the tip portion, and a base portion disposed between the tip portion and the head, and is formed using an iron-based metal material, wherein the tip portion, head, and base portion have a first coating layer including a plating layer on the surface of the material, and further have a second coating layer formed by coating on the surface of the first coating layer, the tip portion, head, and base portion all have the same thickness of the first coating layer, and the second coating layer is formed so that a desired relationship is established when the film thickness of the tip portion is T1 (μm), the film thickness of the base portion is T2 (μm), and the film thickness of the head is T3 (μm).

Description

[Technical Field]

[0001] The present invention relates to a thread and a method for forming a corrosion-resistant coating on the surface of the thread. [Background technology]

[0002] Conventionally, methods for improving the corrosion resistance of screws have included chromate treatment in addition to electrolytic zinc plating, etc. Known chromate treatments for screws include bright chromate, colored chromate, and black chromate, and are known to provide higher corrosion resistance to screws than plating alone.

[0003] However, for screws used in severe corrosive environments such as offshore structures, chromate treatment alone is not sufficient, and other surface treatment methods are also being used to improve corrosion resistance. Also, because chromate treatment contains hexavalent chromium, surface treatment methods such as trivalent chromate treatment and non-chromium surface treatment, which do not contain hexavalent chromium, are also being considered.

[0004] For example, Patent Document 1 discloses a non-chromium surface treatment agent containing an alkoxysilane oligomer obtained by hydrolyzing an alkoxysilane and an anti-corrosion pigment such as zinc in an alcohol solution. The non-chromium surface treatment agent of Patent Document 1 forms a siliceous coating on the treated surface by drying a sol obtained by hydrolyzing an alkoxysilane. By coating the surface of the treated object, such as a screw, with a siliceous coating and improving the sacrificial corrosion protection effect of the zinc with the anti-corrosion pigment, the corrosion resistance of components such as screws can be improved. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-264170 Summary of the Invention [Problem to be solved by the invention]

[0006] The non-chromium surface treatment agent of Patent Document 1 is effective as a method for improving the corrosion resistance of screws, but its corrosion resistance is not sufficient when considering screws to be used in severe corrosive environments such as offshore structures.

[0007] One option is to form a thick corrosion-resistant coating on the surface of a thread to ensure sufficient corrosion resistance even in a severe corrosive environment. However, forming a thick corrosion-resistant coating on the surface of a thread increases the nominal diameter of the thread, which creates the problem of requiring additional processing such as over-tapping.

[0008] Over-tapping is a process in which the inner diameter of the nut or other part that tightens the screw is oversized to prevent the problem of nuts or other parts not fitting smoothly onto the screw due to the large outer diameter of the screw resulting from forming a thick coating. Over-tapping increases the cost of producing parts, so even if corrosion resistance is improved, new cost issues arise.

[0009] The present invention has been made in view of the above-mentioned problems, and has as its object to provide a screw that can reduce the labor required for over-tapping on nuts and the like, while still exhibiting sufficient corrosion resistance even in severe corrosive environments.

[0010] The present invention also provides a method for forming a corrosion-resistant coating on the surface of a screw, which exhibits sufficient corrosion resistance even in a severe corrosive environment and can reduce the need for over-tapping on nuts and the like. The present invention aims to provide a method for forming a corrosion-resistant coating on the surface of a screw that can be easily removed. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the screw of the present invention employs the following technical measures. That is, a screw according to one aspect of the present invention is a screw having a tip portion on which a thread groove is formed, a head portion formed with a larger diameter than the tip portion, and a base portion disposed between the tip portion and the head in the axial direction, and formed in a rod shape using an iron-based metal material, wherein the tip portion, the head, and the base portion have a first coating layer including a plating layer on the surface of the material, and further have a second coating layer formed by coating on the surface of the first coating layer, and the first coating layer has a plating layer that coats the surface of the base material, a corrosion-resistant layer that coats the surface of the plating layer, and a coating layer that coats the surface of the corrosion-resistant layer, 2 The coating layer is formed of a nylon-based resin coating containing a copolymer nylon resin, and the tip, head, and base ends all have the same thickness of the first coating layer. The second coating layer is formed so that the relationship between the following equations (1) and (2) holds when the thickness of the tip, base, and head is T1 (μm), T2 (μm), T3 (μm), and the thickness of the first coating layer is T4 (μm). [Number 1] 10 (μm)≦T1 <T2<T3≦150(μm)···(1) [Number 2] 3≦T4≦10(μm) T4≦T1≦30(μm) (2)

[0013] The second coating layer may be formed so that the relationship of the following formula (3) holds between the thickness T1 (μm) of the second coating layer at the tip end and the thickness T2 (μm) of the second coating layer at the base end. [Number 3] 30(μm)≦(T2-T1)≦70(μm)...(3)

[0014] The second coating layer may be formed so that the relationship of the following formula (4) holds between the thickness T2 (μm) of the second coating layer at the base end and the thickness T3 (μm) of the second coating layer at the head. [Number 4] 0<(T3-T2)≦80(μm) (4)

[0015] The thickness T1 of the second coating layer at the tip end may be 10 (μm) or more and 30 (μm) or less, the thickness T2 of the second coating layer at the base end may be 50 (μm) or more and 100 (μm) or less, and the thickness T3 of the second coating layer at the head may be greater than 100 (μm) and 150 (μm) or less.

[0017] The material may be formed of carbon steel for mechanical structures, the plating layer may be formed of a zinc-nickel alloy having a nickel co-deposition rate of 5% by weight or more and 18% by weight or less, the corrosion-resistant layer may be formed of an oxide of trivalent chromium, and the coating layer may be formed of a siliceous coating containing silica.

[0019] A method for forming a corrosion-resistant coating on the surface of a screw according to one embodiment of the present invention is a method for forming a corrosion-resistant coating on the surface of a screw that has, in the axial direction, a tip portion with a thread groove formed thereon, a head portion formed with a larger diameter than the tip portion, and a base portion disposed between the tip portion and the head, and that is formed into a rod shape using an iron-based metal material, the method comprising the steps of: plating the surface of the material with a zinc-nickel alloy to form a plating layer; treating the surface of the plating layer with trivalent chromate to form a trivalent chromium layer; and contacting the surface of the trivalent chromium layer with a non-chromium surface treatment agent solution containing an alkoxysilane oligomer in an alcohol solvent, to coat the surface of the trivalent chromium layer with a coating layer formed of a siliceous film. Thus, a first coating layer is formed in which the plating layer, the trivalent chromium layer, and the coating layer are stacked.A first coating step is performed to form the coating layer, followed by a first baking and drying step, and then a second coating step is performed in which a negatively charged nylon-based resin is sprayed onto the surface of the coating layer after the first baking and drying step to coat a second coating layer on the surface of the coating layer, and after the second coating layer is formed, a second baking and drying step is performed to form the corrosion-resistant coating having the plating layer, the trivalent chromium layer, the coating layer, and the second coating layer, and when the nylon-based resin is sprayed onto the surface of the coating layer, the nylon-based resin is sprayed onto the surface of the coating layer. By varying the voltage applied between the RON-based resin and the surface of the coating layer at each of the tip, base, and head portions, the second coating layer is formed with different film thicknesses at each of the tip, base, and head portions, and when the film thickness of the second coating layer at the tip portion is T1 (μm), the film thickness at the base portion is T2 (μm), the film thickness at the head is T3 (μm), and the film thickness of the first coating layer is T4 (μm), a corrosion-resistant coating is formed on the surface of the screw so that the relationship between the following equations (1) and (2) holds. [Number 1] 10 (μm)≦T1 <T2<T3≦150(μm)···(1) [Number 2] 3≦T4≦10(μm) T4≦T1≦30(μm) (2)

[0020] The second baking and drying may be carried out at a temperature higher than that of the first baking and drying. [Effects of the Invention]

[0021] The screw and the method for forming a corrosion-resistant coating on the surface of the screw of the present invention do not require over-tapping of nuts, etc., and can still exhibit sufficient corrosion resistance even in severe corrosive environments. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a cross-sectional view of the screw of the present embodiment. [Figure 2]FIG. 2 is a diagram comparing the layer structure of the corrosion-resistant coating formed on the surface of the screw of this embodiment at the tip, base, and head portions. [Figure 3] 3A to 3C are process diagrams of a method for forming a corrosion-resistant coating on the surface of a screw according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] The screw of the present invention will be specifically described with reference to FIGS.

[0024] The screws of the present invention are bolts primarily used in conjunction with nuts and the like, and include large sizes with a nominal diameter of 10 mm or greater. Specifically, the screws of the present invention include bolts with a nominal diameter of 2 mm or greater and 70 mm or less, but preferably bolts with a nominal diameter of 14 mm or greater and 50 mm or less. The screws of the present invention also include threaded fasteners with tapered tips, commonly known as wood screws and tapping screws. Furthermore, the screws of the present invention include threaded fasteners known as special screws, such as torque-hole screws, hexagonal socket screws, tamper-resistant screws, flange screws, thumb screws, air-bleed screws, and anti-loosening screws. The screws of this embodiment are hexagonal bolts with a short, hexagonal column-shaped head, but the head shape may also be square, countersunk, round, flat, pan, truss, or countersunk. FIG. 1 is a cross-sectional view of the screw of this embodiment.

[0025] 1, the screw of this embodiment is formed in a long shape along the axial direction (hereinafter referred to as the axial direction) with respect to the axis passing through the center. The screw of this embodiment is divided into three parts in the axial direction: a tip part, a base part, and a head part.

[0026] As shown in FIG. 1A, the tip portion is a portion where a screw groove is formed in a spiral shape on the outer periphery. The nominal diameter (outer diameter) of the tip is equal to the nominal diameter (outer diameter) of the base end and smaller than the outer diameter of the head. The tip is threaded using a lathe, tapping die, heading, or other means.

[0027] As shown in B of Fig. 1, the base end is a portion generally called a trunk portion or a cylindrical portion, and unlike the tip end which has a thread groove, no thread groove is formed on the outer periphery. In this embodiment, since the tip end is threaded, the nominal diameter of the base end is approximately the same as that of the tip end.

[0028] 1C, the head portion is formed to have a larger diameter than the distal end portion and the proximal end portion. In this embodiment, the head portion has a hexagonal shape.

[0029] FIG. 2 is an enlarged view showing the cross-sectional structure (layer structure) of the surface of each portion of the screw in FIG. 1 (three portions: the tip portion, the base portion, and the head portion).

[0030] As shown in the lower part of Figure 2, a corrosion-resistant coating is formed on the tip, base, and head portions. The corrosion-resistant coating has a first coating layer and a second coating layer that covers the surface of the first coating layer. The first coating layer also has a base material, a plating layer that covers the surface of the base material, a corrosion-resistant layer that covers the surface of the plating layer, and a coating layer that covers the surface of the corrosion-resistant layer.

[0031] Next, the layer structure of the corrosion-resistant film, that is, the first coating layer, the substrate on which the first coating layer is formed, the plating layer, corrosion-resistant layer, and coating layer that constitute the first coating layer, and the second coating layer will be described.

[0032] The substrate on which the first coating layer is formed is made of an iron-based metal such as carbon steel (SC) for mechanical structures or alloy steel for mechanical structures such as chromium-molybdenum steel (SCM). In addition to the above, the iron-based metal constituting the substrate may be soft steel wire, hard steel wire, cold heading carbon steel, or alloy steel for mechanical structures such as chromium-molybdenum steel (SCM). The substrate of this embodiment is made of carbon steel for mechanical structures.

[0033] The first coating layer is a coating formed on the surface of the base material. The first coating layer of the present invention has at least a plating layer. In this embodiment, the first coating layer is formed of three layers: a plating layer, a corrosion-resistant layer, and a coating layer, and these three layers are collectively referred to as the first coating layer. In the first coating layer of this embodiment, a plating layer is formed on the base material, a corrosion-resistant layer is formed on the plating layer, and a coating layer is formed on the corrosion-resistant layer. The thickness of the first coating layer, including all three layers, is set to be 3 μm or more and 10 μm or less. By setting the thickness (total thickness) of the first coating layer to be 3 μm or more and 10 μm or less, it is possible to keep the film thickness of the threaded tip thin, eliminating the need for over-tapping on nuts, etc., and improving the convenience and versatility of the screw.

[0034] The plating layer covers the surface of the metal substrate and is formed of zinc, nickel, tin, copper, cobalt, or an alloy thereof. Specifically, the plating layer can be formed using zinc plating (electrogalvanized), nickel plating, zinc-nickel alloy, zinc-iron alloy, tin-cobalt plating, chrome plating, copper plating, tin plating, or a combination of these. The metal constituting the plating layer is preferably zinc or a zinc alloy, which has a sacrificial corrosion protection effect against iron. The thickness of the plating layer is set to 2 μm or more and 8 μm or less. By setting the thickness of the plating layer to 2 μm or more, the sacrificial corrosion protection effect of zinc can be utilized to provide high corrosion protection against the metal substrate. Furthermore, by setting the thickness of the plating layer to 8 μm or less, over-tapping of nuts and the like is unnecessary, improving the convenience and versatility of the screw. The plating layer in this embodiment is formed of a zinc-nickel alloy. The zinc-nickel alloy plating layer of this embodiment is formed of a zinc-nickel alloy with a nickel co-deposition rate of 5% by weight or more and 18% by weight or less, more preferably 8% by weight or more and 12% by weight or less. By using a zinc-nickel alloy plating layer with a nickel co-deposition rate of 5% by weight or more and 18% by weight or less, more preferably 8% by weight or more and 12% by weight or less, a screw with high hardness and excellent corrosion resistance can be obtained.

[0035] The corrosion-resistant layer coats the surface of the plating layer, thereby improving the corrosion resistance of the base plating layer and, in turn, the corrosion resistance of the base material. A chromate coating (a chemical conversion coating of trivalent chromium, including trivalent white and trivalent black coatings) can be used as the corrosion-resistant layer. The thickness of the corrosion-resistant layer is 0.5 μm or less, which is thinner than the coating layer and plating layer. Preferably, the corrosion-resistant layer is a trivalent chromium chromate coating, which is free from the risk of emitting harmful substances and has good corrosion resistance despite being a thin film having a thickness of 0.5 μm or less. The corrosion-resistant layer of this embodiment is formed of a trivalent chromium chromate coating.

[0036] The coating layer is a layer that coats the surface of the corrosion-resistant layer, thereby further enhancing the corrosion resistance of the corrosion-resistant layer. The coating layer of the present invention coats the surface of the corrosion-resistant layer and is formed from a siliceous coating. The coating layer of this embodiment is formed on the surface of the corrosion-resistant layer by contacting the surface of the corrosion-resistant layer with a surface treatment agent solution (a chromium-free surface treatment solution) in which an alkoxysilane compound is suspended in an alcohol solvent, and then drying to volatilize the alcohol solvent. By coating the surface of the corrosion-resistant layer with a siliceous coating coating layer, it is possible to further enhance the corrosion resistance of the substrate compared to a case in which only a plating layer and a corrosion-resistant layer are present.

[0037] Furthermore, if the surface of the corrosion-resistant layer is coated with a siliceous coating layer as in this embodiment, the second coating layer made of a nylon-based resin, which will be described later, can be formed with good affinity to the first coating layer. In other words, the siliceous coating layer is optimal for forming the second coating layer made of a nylon-based resin with good adhesion.

[0038] The thickness of the coating layer is set to 0.5 μm or more and 2 μm or less. By making the coating layer thicker than 0.5 μm, it is possible to achieve high corrosion resistance for the metal substrate. In addition, by making the coating layer thicker than 2 μm, over-tapping is not required for nuts, etc., which improves the convenience and versatility of the screw.

[0039] The second coating layer is a layer that further enhances the corrosion resistance of the corrosion-resistant layer (first coating layer) by covering the surface of the corrosion-resistant layer (first coating layer). The second coating layer differs from the first coating layer in the film thickness at each location on the thread.

[0040] The thickness of the second coating layer is between 10 μm and 30 μm at its thinnest point, while the thickness of the first coating layer, including the plating layer, corrosion-resistant layer, and coating layer, is a maximum of 10 μm, and the thickness of the second coating layer at its thickest point, the head, is a maximum of 150 μm. In other words, the second coating layer has a sufficiently larger thickness than the first coating layer, providing a thick coating on the thread surface. By providing such a second coating layer, the surface of the base material can be thickly coated, greatly improving the corrosion resistance of the thread.

[0041] The second coating layer of the present invention is formed from a paint of nylon-based resin. The nylon-based resin may be water-based or oil-based. In other words, the paint is applied to the surface of the first coating layer, and the applied paint is heat-treated to evaporate the water or organic solvent that serves as the solvent, and the copolymerized nylon resins fuse together, thereby coating the surface of the first coating layer with the second coating layer. By using the second coating layer in which the solvent evaporates and the resin components fuse together through heat treatment in this way, a thick second coating layer can be coated on the surface of the substrate. In addition, the nylon-based resin Grease is water-repellent, so it also has the effect of preventing saltwater and seawater from penetrating the base material. Therefore, by forming a second coating layer on the surface of the first coating layer, the corrosion resistance of the screw can be greatly improved.

[0042] As mentioned above, applying a second coating layer improves corrosion resistance compared to applying only the first coating layer. Furthermore, when applying a second coating layer, the thicker the layer, the better the corrosion resistance can be expected. Therefore, from the standpoint of corrosion resistance alone, it is preferable to apply the second coating layer as thick as possible. However, applying a thick second coating layer also increases the nominal diameter (outer diameter) of the screw. This requires additional processing, such as overtapping, which can potentially increase the manufacturing cost of the screw.

[0043] Therefore, the screw of the present invention forms a second coating layer with an optimal thickness at each of the tip, base, and head portions, thereby reducing the need for unnecessary processing such as over-tapping, and achieving both convenience as a screw and high corrosion resistance.

[0044] Specifically, in the screw of the present invention, the thickness of the second coating layer is set as follows. "Regarding the relationship in equation (1)" As shown in Figure 2, when the thickness of the second coating layer at the tip is T1 (μm), the thickness of the second coating layer at the base is T2 (μm), and the thickness of the second coating layer at the head is T3 (μm), the screw of the present invention is formed with a second coating layer such that the relationship of the following formula (1) holds. [Number 5] 10 (μm)≦T1 <T2<T3≦150(μm)···(1)

[0045] As shown in Figure 2 and formula (1), if the thickness T1 at the tip end is thinner than the thickness T2 at the base end and the thickness T3 at the head, the thickness of the tip end where the screw is screwed into a nut, etc., can be kept low even when a second coating layer is formed. This reduces the need for extra processing such as over-tapping, and makes it possible to keep the manufacturing cost of the screw low. On the other hand, since the second coating layer is formed thick at the part exposed to the most severe corrosive environment, such as the head, the corrosion resistance of the screw can be greatly improved. In other words, the screw of the present invention provides a second coating layer of an optimal thickness at each part in the axial direction depending on the corrosive environment to which each part is exposed.

[0046] Furthermore, the thickness T2 of the second coating layer at the base end is thicker than the thickness T1 of the tip end but thinner than the thickness T3 of the head end. In other words, since the base end is not threaded and overtapping is not required, it is preferable to make the thickness T2 of the second coating layer at the base end thicker. Furthermore, the base end is located inside the screw hole, which is closed by the head when the screw is tightened, and the corrosive environment at the base end is less severe than that at the head end. Therefore, even if the thickness T2 of the second coating layer at the base end is thinner than the thickness T3 of the head end, the corrosion resistance of the screw is not significantly reduced. However, although the corrosive environment at the base end is not as severe as that at the head end, it is still more severe than that at the tip end. Therefore, when considering the corrosive environment, the second coating layer at the base end needs to be thinner than the head end but thicker than the tip end. Meanwhile, the thinner the thickness of the second coating layer, the lower the manufacturing cost. Therefore, by making the thickness T2 of the second coating layer at the base end thinner than the thickness T3 of the head end, the manufacturing cost of the screw can be reduced.

[0047] Taking all of the above points into consideration, in order to reduce the manufacturing costs involved in over-tapping and coating while still being able to exhibit sufficient corrosion resistance in a severe corrosive environment, the screw of the present invention is formed so that the film thickness T2 of the second coating layer at the base end is thicker than the film thickness T1 at the tip end and thinner than the film thickness T3 at the head.

[0048] "Regarding the relationship in equation (2)" Regarding the film thickness T1 of the second coating layer at the tip, when the film thickness of the first coating layer is T4 (μm), it is preferable that the film thickness T1 be formed so that the relationship of the following formula (2) holds in addition to the relationship of the above-mentioned formula (1). [Number 6] 3≦T4≦10(μm) T4≦T1≦30(μm) (2)

[0049] If the thickness T4 of the first coating layer is set to 3 μm or more and 10 μm or less, the first coating layer will have a sufficient thickness to provide good corrosion resistance. Furthermore, since the second coating layer at the tip does not become too thick (it can be kept thin), the need for extra processing such as over-tapping can be reduced. Therefore, a thread formed so that the relationship of Equation (2) holds in addition to the relationship of Equation (1) can achieve both better corrosion resistance and reduced manufacturing costs for the thread by reducing the need for over-tapping, compared to a thread that does not satisfy the relationship of Equation (2).

[0050] "Regarding the relationship in equation (3)" The thread is preferably formed so that the relationship of the following formula (3) holds between the film thickness T1 of the second coating layer at the distal end and the film thickness T2 of the second coating layer at the proximal end. [Number 7] 30(μm)≦(T2-T1)≦70(μm)...(3)

[0051] When the difference in thickness between the second coating layer at the distal end (T1) and the second coating layer at the proximal end (T2) is 30 μm or more, the second coating layer at the proximal end can effectively prevent saltwater (seawater) from penetrating the gap between the distal end and the nut. In other words, a certain amount of gap is created between the distal end and the nut to allow the screw to rotate. If the second coating layer at the proximal end (T2) is 30 μm or more thicker than the distal end, a difference in thickness between the second coating layer at the proximal end and the distal end occurs. This gap is then covered by the second coating layer at the proximal end, which has a sufficient thickness. This prevents saltwater from penetrating the gap, further improving the corrosion resistance of both the distal and proximal ends. Furthermore, if the thickness of the second coating layer at the proximal end (T2) is 70 μm or less compared to the thickness of the second coating layer at the distal end (T1), the thickness of the second coating layer at the proximal end will not be too thick, thereby reducing the manufacturing cost of the screw.

[0052] Therefore, a thread formed so that the relationship of formula (3) is established in addition to the relationships of formulas (1) and (2) can achieve even better corrosion resistance not only at the tip end but also at the base end, compared to a thread that does not satisfy the relationship of formula (3), and it can also achieve further reductions in the manufacturing costs of the thread.

[0053] "Regarding the relationship in equation (4)" The second coating layer may be formed so that the relationship of the following formula (4) holds between the thickness T2 (μm) of the second coating layer at the base end and the thickness T3 (μm) of the second coating layer at the head. [Number 8] 0<(T3-T2)≦80(μm) (4)

[0054] A thinner second coating layer can reduce the manufacturing cost of the screw. However, the head of the screw is exposed to a more corrosive environment than the base end. In other words, by making the thickness T3 (μm) of the second coating layer at the head thicker than the thickness T2 (μm) of the second coating layer at the base end, as shown in formula (4), it is possible to improve the corrosion resistance of the screw while minimizing increases in manufacturing costs.

[0055] Therefore, a screw formed so that the relationship of formula (4) is satisfied in addition to the relationships of formulas (1) to (3) can achieve even better corrosion resistance and can reduce the manufacturing costs of the screw compared to a screw that does not satisfy the relationship of formula (4).

[0056] "Method for forming a corrosion-resistant coating on the surface of a screw of the present invention" The method of the present invention for forming a corrosion-resistant coating on the surface of a screw (hereinafter referred to as the method for forming a corrosion-resistant coating) involves forming a corrosion-resistant coating on the surface of a screw that has, in the axial direction, a tip portion with a thread groove formed therein, a head portion formed with a larger diameter than the tip portion, and a base portion disposed between the tip portion and the head, and that is formed into a rod-like shape using an iron-based metal material.

[0057] Specifically, the corrosion-resistant coating of the present invention is formed through a "pretreatment process," "plating process," "corrosion-resistant layer formation process," "pre-drying process," "first coating process," "primary drying process," "second coating process," "secondary drying process," and "cooling process."

[0058] The "pretreatment process" consists of a degreasing process to remove processing oils and other substances adhering to the surface of the ferrous metal substrate, and a process to apply acids such as hydrochloric acid or nitric acid to the surface of the ferrous metal substrate to enable active electrode reactions. After the pretreatment process, the ferrous metal substrate is plated in the plating process.

[0059] The "plating process" is a process of plating the surface of an iron-based metal substrate that has undergone a pretreatment process. In the plating process of this embodiment, zinc-nickel alloy plating is performed to form a zinc alloy plating layer. Specifically, zinc-nickel alloy plating is performed by immersing the workpiece in an ammonium chloride plating bath containing 20 g / L to 50 g / L of zinc ions and 10 g / L to 40 g / L of nickel ions for 20 to 90 minutes to form an alloy layer with a nickel co-deposition rate of 5% to 18% (zinc co-deposition rate of 82 to 95%). It is preferable to add an organic acid-based leveling agent or brightener to the plating bath as appropriate. After the plating process, the screw (more precisely, the semi-finished screw) is washed with water and then sent to the corrosion-resistant layer formation process.

[0060] In this embodiment, zinc-nickel alloy plating is used as the plating process, but if a plating other than zinc-nickel alloy plating is used in the plating process, the liquid composition of the plating bath and the treatment temperature can be changed as appropriate.

[0061] The "corrosion-resistant layer forming process" is a process of forming a corrosion-resistant layer on the surface of the plating layer by performing chemical conversion treatment or painting. The corrosion-resistant layer forming process in this embodiment is a trivalent chromate treatment that forms a trivalent chromium layer on the surface of the plating layer. Specifically, the trivalent chromate treatment is performed by immersing a semi-finished screw (a screw that has undergone the plating process) in a chemical conversion solution containing chromium chloride (CrCl), nitrates such as sodium nitrate, and complexing agents such as citric acid and malonic acid. Note that the chemical conversion solution may also contain cobalt salts, colloidal silica, etc. After the corrosion-resistant layer forming process, the screw (more precisely, the semi-finished screw) is washed with water and then sent to a preliminary drying process.

[0062] It is also possible to promote the formation of a trivalent chromium layer by performing a preliminary primer treatment before the trivalent chromate treatment. Although a film may be formed by the primer treatment, this film is extremely thin and does not significantly affect the film thickness relationship for each part of the thread described above.

[0063] In addition, in this embodiment, trivalent chromate treatment is mentioned as the corrosion-resistant layer forming process, but as long as trivalent chromate is used, in other words, as long as the chromate treatment does not contain hexavalent chromium (non-hexavalent chromium chromate treatment), the treatment may be carried out in a bath other than a chloride bath, and stabilizers and complexing agents contained in the bath may be other than the above-mentioned nitrates, complex citric acid, and malonic acid. In this case, the liquid composition of the chemical conversion solution, the treatment time, and the treatment temperature can be appropriately changed.

[0064] The "pre-drying step" is a step of drying the screw (more precisely, the semi-finished screw) on which the corrosion-resistant layer has been formed in the corrosion-resistant layer forming step. In the case where the above-mentioned zinc-nickel alloy plating layer is formed and trivalent chromate treatment is performed, drying is performed at a temperature of 60°C to 100°C.

[0065] The "first coating step" is a step of applying a coating layer to a screw (semi-finished screw) that has been dried in the preliminary drying step. In the first coating step of this embodiment, a surface treatment agent containing an alkoxysilane compound in an alcohol solvent is brought into contact with the surface of the corrosion-resistant layer, and the alcohol solvent is evaporated by drying, thereby forming a coating layer on the surface of the corrosion-resistant layer. The screw (semi-finished screw) on which the coating layer has been formed in the first coating step is sent to the primary drying step.

[0066] The "primary drying step" is a step of drying the surface of the screw on which the coating layer has been formed in the first coating step, and hardening the formed coating layer. When the surface treatment agent solution containing the above-mentioned alkoxysilane compound is used in the first coating step, drying is carried out at a drying temperature of 80°C to 120°C, and the alcohol solvent is volatilized to harden the coating layer.

[0067] The drying temperature in the primary drying step is set to 80°C to 120°C in order to volatilize the alcohol used as a solvent in the surface treatment agent used in the first coating step and promote the gelation of the alkoxysilane compound. In the first coating layer dried in the primary drying step, the surface of the corrosion-resistant layer (plating layer) can be covered with a coating layer of the gelled alkoxysilane compound, thereby improving the corrosion resistance of the plating layer.

[0068] By carrying out the above-mentioned "pretreatment step" to "primary drying step", a first coating layer, which is made up of a plating layer, a corrosion-resistant layer, and a coating layer stacked on the surface of the iron-based metal base, is formed. In this way, a second coating layer is formed in the second coating step on the surface of the coating layer that has been subjected to the first coating step (first baking and drying), in other words, on the surface of the first coating layer.

[0069] "Second coating process" In the second coating step, a second coating layer made of nylon-based resin is applied by electrostatic coating. In this embodiment, a negative potential is applied to the surface of the screw (semi-finished screw) using a grounded spray gun, and nylon-based resin is sprayed onto the surface of the coating layer to form a second coating layer of a desired thickness.

[0070] The nylon resin that forms the second coating layer is formed by applying an anti-rust paint containing copolymer nylon resin. In the secondary drying process (second baking drying) that follows the second coating process, the applied anti-rust paint is heat-treated to evaporate the solvent, and the copolymer nylon resins fuse together, forming the second coating layer on the surface of the first coating layer. When forming the second coating layer, it is necessary to change the film thickness at the tip, base, and head. The film thickness of the second coating layer can be changed by varying the potential applied to the negatively charged nylon resin (in other words, the voltage applied between the nylon resin and the surface of the coating layer) at the tip, base, and head relative to the surface of the earthed screw (semi-finished screw).

[0071] Specifically, by applying a nylon resin to the surface of the coating layer at the tip at -10 kV to -30 kV, a second coating layer is formed on the surface of the coating layer at the tip to a thickness of 10 μm or more and 30 μm or less. Apply to the following thickness.

[0072] Furthermore, a nylon resin is applied to the surface of the coating layer at the base end at −50 kV to −70 kV, and a second coating layer is applied to the surface of the coating layer at the base end to a thickness of 50 μm or more and 100 μm or less.

[0073] Furthermore, a nylon resin is applied to the surface of the coating layer of the head at −80 kV to −100 kV, and a second coating layer is applied to the surface of the coating layer of the head to a thickness of more than 100 μm and not more than 150 μm. The screws (semi-finished screw products) that have undergone the second coating process described above are subjected to a secondary drying process.

[0074] "Secondary drying process" The secondary drying process (second baking drying) is a process in which the second coating layer formed on the surface of the screw is heated at a temperature of 200 to 240°C for 10 to 60 minutes to harden the coated second coating layer. In the secondary drying process, the solvent (mainly water) evaporates from the water-based anti-rust paint coated on the surface of the first coating layer, and the copolymer nylon resins fuse together due to the heat to form the second coating layer.

[0075] The drying temperature in the secondary drying step is set to 200°C to 240°C, which is higher than the drying temperature in the primary drying step (80°C to 120°C). By performing the second baking drying (secondary drying step) at a higher temperature than the first baking drying (primary drying step), it becomes possible to fuse the copolymer nylon resin applied in the second coating step with heat, making it possible to form a dense second coating layer on the surface of the first coating layer, and making it possible to improve the corrosion resistance of the corrosion-resistant coating.

[0076] "Cooling process" The screw (semi-finished screw) after the second coating layer has finished hardening in the secondary drying step is cooled in the cooling step. A corrosion-resistant coating can be formed on the surface of the screw by performing the above-mentioned "pretreatment process," "plating process," "corrosion-resistant layer forming process," "pre-drying process," "first coating process," "primary drying process," "second coating process," "secondary drying process," and "cooling process" in that order. [Example]

[0077] Next, the effects of the screw and the method of forming a corrosion-resistant coating on the surface of the screw of the present invention will be described in more detail using examples and comparative examples. In the examples and comparative examples, a first coating layer and a second coating layer were formed on a bolt (in the examples and comparative examples, examples using bolts are given) made of carbon steel for mechanical structures and having a nominal diameter of 36 mm. First, the procedure and processing conditions for forming the first coating layer will be described.

[0078] "Pretreatment process" The pretreatment process includes a first treatment for degreasing and a second treatment for pickling. First, as a primary treatment, a pretreatment process (primary) was carried out on the surface of the bolt using caustic soda to remove any oil or grease adhering to the surface. Next, as a secondary treatment, a pretreatment process (secondary) using hydrochloric acid was carried out to remove the oxide film on the surface and to activate the electrode reaction.

[0079] "Plating process" The plating process involves immersing the pretreated bolts in an ammonium chloride plating bath (35°C) containing 40g / L of zinc chloride and 30g / L of nickel chloride for 15 to 25 minutes at a current density of 1A / dm 2 A plating layer with a nickel co-deposition rate of 10% (zinc co-deposition rate of 90%) was formed.

[0080] In the corrosion-resistant layer formation process, the plated bolts were immersed in a trivalent chromate treatment solution containing 130 mg / L of chromium (trivalent chromium) and 300 mg / L of cobalt for 10 to 60 seconds to form a chromate coating (corrosion-resistant layer).

[0081] "Pre-drying process" In the preliminary drying step, the bolts on which the chromate coating had been formed in the corrosion-resistant layer forming step were dried with hot air at a temperature of 100°C.

[0082] "First coating process" In the first coating step, the bolts that had been dried in the preliminary drying step were coated with a coating layer using a surface treatment agent solution containing an alkoxysilane compound and zinc rust inhibitor pigment in an alcohol solvent. The coating was performed by immersing the bolts in the surface treatment agent solution for 10 to 60 seconds.

[0083] "Primary drying process" The bolts coated with the coating layer in the first coating process were then dried in the primary drying process, where the solvent on the surface was evaporated at a drying temperature of 80°C to 100°C for 20 minutes. As the alcohol solvent evaporated in the primary drying process, the coating layer gelled on the surface of the corrosion-resistant layer, and the first coating layer was applied to the surface of the corrosion-resistant layer.

[0084] "Second coating process" For bolts that had been dried in the primary drying process, an anti-rust paint containing copolymer nylon resin was supplied to a spray gun, and electrostatic painting was performed by applying a negative potential to the bolt relative to the grounded spray gun, forming a second coating layer on the surface of the bolt after primary drying. The applied voltage was changed depending on the film thickness, changing the film thickness of the applied second coating layer.

[0085] Next, differences in processing conditions between the example and the comparative example will be described. In the examples and comparative examples, the thickness of the coating layer (thickness T4 of the first coating layer) was changed within a range of 6 μm to 12 μm by appropriately changing the time of immersion in the surface treatment agent solution in the first coating step. Also, by changing the voltage applied between the bolt and the spray gun within a range of 0 kV to -120 kV in the second coating step, the thickness of the second coating layer was changed within a range of 5 μm to 50 μm at the tip end (thickness T1), 45 μm to 105 μm at the base end (thickness T2), and 0 μm to 180 μm at the head (thickness T3).

[0086] The evaluation results of the examples and comparative examples are shown in Table 1. [Table 1]

[0087] The evaluation methods and evaluation criteria listed in Table 1 are as follows.

[0088] "Film thickness" For the above-mentioned bolts, the film thickness was measured after plating and after the formation of the second coating layer. The film thickness after plating was measured using fluorescent X-ray analysis. The film thickness of the corrosion-resistant layer and coating layer was measured by cutting the bolt and observing the cross section with an optical microscope or electron microscope. The film thickness of the corrosion-resistant layer and coating layer can also be determined by converting the weight increase during film formation into film thickness. The film thickness of the second coating layer was measured using an eddy current non-contact film thickness meter.

[0089] "Corrosion resistance" Corrosion resistance was evaluated by a salt spray test (SST). The salt spray test conforms to JIS Z 2371, in which a 5% aqueous solution of sodium chloride with a pH of 7 (35°C) was sprayed onto the bolt after the formation of the corrosion-resistant coating, and the time until the appearance of white rust and the time until the appearance of red rust were measured. A sample that took more than 600 hours to appear white rust and more than 4000 hours to appear red rust was rated "Good," while a sample that took 600 hours or less to appear white rust or 4000 hours or less to appear red rust was rated "Poor."

[0090] "Overtapping" The overtapping property was evaluated by randomly selecting five bolts from the examples and comparative examples and threading them into nuts conforming to the "M36" standard in JIS B 1181. The bolts were checked for the occurrence of seizure (the phenomenon in which the bolt gets caught in the nut and cannot be turned, known as galling) midway through the thread groove. With conventional bolts, seizure generally occurs in all bolts that are not overtapped. Therefore, if no seizure occurred in even one of the five bolts (if the nut could be threaded without overtapping), the bolt was evaluated as having suppressed seizure and was given a rating of "Good." Furthermore, if seizure occurred in all five bolts (if the nut could not be threaded on any of the bolts), as in the conventional case, the bolt was evaluated as "Poor."

[0091] "cost" The cost evaluation was based on a bolt with a 50μm thick second coating layer at the tip, base, and front part of the head, and was evaluated as "Good" if the manufacturing cost was lower than this standard, and "Poor" if the manufacturing cost was the same or equivalent. Note that the reference bolt has a thick 50μm coating at the tip, so over-tapping is required at the tip. In other words, the cost to manufacture this reference bolt is the total cost, including the cost of coating formation and the cost of over-tapping.

[0092] Next, the examples and comparative examples will be described in detail. "Example 1" In Example 1, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 10 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0093] In Example 1, since the film thickness T1 = 10 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "Good." Furthermore, since the film thickness T1 = 10 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "Good." Furthermore, since the film thickness T1 = 10 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base ends is 60 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is marked "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head end is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is marked "Good."

[0094] The evaluation of "corrosion resistance" for Example 1 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 1 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 1 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 1, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0095] "Example 2" In Example 2, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0096] In Example 2, since the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base ends is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is marked "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head end is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is marked "Good."

[0097] The evaluation of "corrosion resistance" for Example 2 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 2 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 2 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 2, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0098] "Example 3" In Example 3, the thickness T4 of the first coating layer including the plating layer, the corrosion-resistant layer, and the coating layer is 8 μm. The thickness T1 of the second coating layer at the tip is 30 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0099] In Example 3, since the film thickness T1 = 30 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 30 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 30 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 40 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0100] The evaluation of "corrosion resistance" for Example 3 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 3 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 3 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 3, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0101] Example 4 In Example 4, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 50 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0102] In Example 4, since the film thickness T1 = 20 μm, the film thickness T2 = 50 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 50 μm, the film thickness difference between the tip and base end is 30 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 50 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 70 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0103] The evaluation of "corrosion resistance" for Example 4 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 4 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 4 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 4, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0104] "Example 5" In Example 5, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 90 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0105] In Example 5, since the film thickness T1=20 μm, the film thickness T2=90 μm, and the film thickness T3=120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." In addition, since the film thickness T1=20 μm and the film thickness T4=8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." ) is evaluated as "Good." Furthermore, since film thickness T1 = 20 μm and film thickness T2 = 90 μm, the difference in film thickness between the tip and base end is 70 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is evaluated as "Good." Furthermore, since film thickness T2 = 90 μm and film thickness T3 = 120 μm, the difference in film thickness between the base end and head is 30 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is evaluated as "Good."

[0106] The evaluation of "corrosion resistance" for Example 5 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 5 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 5 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 5, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0107] "Example 6" In Example 6, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 100 μm.

[0108] In Example 6, since the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 100 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 100 μm, the film thickness difference between the base end and the head is 30 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0109] The evaluation of "corrosion resistance" for Example 6 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 6 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 6 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 6, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0110] "Example 7" In Example 7, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 150 μm.

[0111] In Example 7, since the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 150 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 150 μm, the film thickness difference between the base end and the head is 80 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0112] The evaluation of "corrosion resistance" for Example 7 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 7 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 7 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 7, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0113] "Example 8" In Example 8, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 3 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0114] In Example 8, since the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 3 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0115] The evaluation of "corrosion resistance" for Example 8 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 8 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 8 was also "good" because the manufacturing cost was lower than the standard. That is, in Example 8, the evaluation results for "corrosion resistance," "evaluation of overtapping properties," and "cost" are all "good," and therefore the overall evaluation is also "good."

[0116] "Example 9" In Example 9, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 10 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0117] In Example 9, since the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 10 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0118] The evaluation of "corrosion resistance" for Example 9 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Example 9 was also "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional examples. Furthermore, the evaluation of "cost" for Example 9 was also "good" because the manufacturing cost was lower than the standard. That is, Example 9 is a test for "corrosion resistance," "evaluation of overtapping property," and "cost." Since all the evaluation results are "Good", the overall evaluation is also "Good".

[0119] "Comparative Example 1" In Comparative Example 1, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 50 μm, the thickness T2 of the second coating layer at the base is 50 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0120] In Comparative Example 1, the film thickness T1 = 50 μm, the film thickness T2 = 50 μm, and the film thickness T3 = 120 μm, so formula (1) is not satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "X." Furthermore, the film thickness T1 = 50 μm and the film thickness T4 = 8 μm, so formula (2) is not satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "X." Furthermore, the film thickness T1 = 50 μm and the film thickness T2 = 50 μm, so the difference in film thickness between the tip and base ends is 0 μm, so formula (3) is not satisfied, and the evaluation item for "Formula (3)" in Table 1 is marked "X." Furthermore, the film thickness T2 = 50 μm and the film thickness T3 = 120 μm, so the difference in film thickness between the base end and the head end is 70 μm, so formula (4) is satisfied, and the evaluation item for "Formula (4)" in Table 1 is marked "Good."

[0121] The evaluation of "corrosion resistance" for Comparative Example 1 was "Good" because the time until white rust appeared exceeded 600 hours and the time until white rust appeared exceeded 4000 hours. The evaluation of "evaluation of overtapping property" for Comparative Example 1 was "Poor" because the occurrence of the seizure phenomenon was the same as in the conventional case. Furthermore, the evaluation of "cost" for Comparative Example 1 was "Poor" because the manufacturing cost was higher than the standard. That is, in Comparative Example 1, the evaluation results for "evaluation of over-tapping property" and "cost" were "x", and therefore the overall evaluation was also "x".

[0122] "Comparative Example 2" In Comparative Example 2, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 100 μm, and the thickness T3 of the second coating layer at the head is 100 μm.

[0123] In Comparative Example 2, since the film thickness T1 = 20 μm, the film thickness T2 = 100 μm, and the film thickness T3 = 100 μm, formula (1) is not satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "X." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "O." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 100 μm, the film thickness difference between the tip and base ends is 80 μm, which does not satisfy formula (3), and the evaluation item for "Formula (3)" in Table 1 is marked "X." Furthermore, since the film thickness T2 = 100 μm and the film thickness T3 = 100 μm, the film thickness difference between the base end and the head end is 0 μm, which does not satisfy formula (4), and the evaluation item for "Formula (4)" in Table 1 is marked "X."

[0124] The "corrosion resistance" evaluation for Comparative Example 2 was "Good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4,000 hours. The "overtapping property" evaluation for Comparative Example 2 was also "Good" because the occurrence of the seizure phenomenon was suppressed compared to conventional bolts. Furthermore, the "cost" evaluation for Comparative Example 2 was "Poor" because the manufacturing costs were higher than the standard. That is, in Comparative Example 2, the evaluation result for "cost" was "x", and therefore the overall evaluation was also "x".

[0125] "Comparative Example 3" In Comparative Example 3, the thickness T4 of the first coating layer including the plating layer, the corrosion-resistant layer, and the coating layer is 8 μm, the thickness T1 of the second coating layer at the tip portion is 20 μm, and the thickness T2 of the second coating layer at the base end portion is 20 μm. The thickness T3 of the second coating layer on the head is 110 μm.

[0126] In Comparative Example 3, the film thickness T1 = 20 μm, the film thickness T2 = 20 μm, and the film thickness T3 = 110 μm, so formula (1) is not satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "X." Furthermore, the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, so formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "O." Furthermore, the film thickness T1 = 20 μm and the film thickness T2 = 20 μm, so the difference in film thickness between the tip and base ends is 0 μm, so formula (3) is not satisfied, and the evaluation item for "Formula (3)" in Table 1 is marked "X." Furthermore, the film thickness T2 = 20 μm and the film thickness T3 = 110 μm, so the difference in film thickness between the base end and the head end is 90 μm, so formula (4) is not satisfied, and the evaluation item for "Formula (4)" in Table 1 is marked "X."

[0127] The "corrosion resistance" evaluation for Comparative Example 3 was "x" because the time until white rust appeared exceeded 600 hours, but the time until red rust appeared was 4000 hours or less. The "overtapping property" evaluation for Comparative Example 3 was "good" because the occurrence of the seizure phenomenon was suppressed compared to conventional bolts. Furthermore, the "cost" evaluation for Comparative Example 3 was "good" because the manufacturing cost was lower than the standard. That is, in Comparative Example 3, the evaluation result for "corrosion resistance" was "x", and therefore the overall evaluation was also "x".

[0128] "Comparative Example 4" In Comparative Example 4, the thickness T4 of the first coating layer including the plating layer, the corrosion-resistant layer, and the coating layer is 8 μm, the thickness T1 of the second coating layer at the tip portion is 0 μm, the thickness T2 of the second coating layer at the base end is 0 μm, and the thickness T3 of the second coating layer at the head portion is 0 μm. In other words, in Comparative Example 4, the second coating layer is not formed.

[0129] In Comparative Example 4, since the film thickness T1 = 0 μm, the film thickness T2 = 0 μm, and the film thickness T3 = 0 μm, formula (1) is not satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "X." Furthermore, since the film thickness T1 = 0 μm and the film thickness T4 = 8 μm, formula (2) is not satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "X." Furthermore, since the film thickness T1 = 0 μm and the film thickness T2 = 0 μm, the film thickness difference between the tip and base end is 0 μm, and formula (3) is not satisfied, and the evaluation item for "Formula (3)" in Table 1 is marked "X." Furthermore, since the film thickness T2 = 0 μm and the film thickness T3 = 0 μm, the film thickness difference between the base end and the head is 0 μm, and formula (4) is not satisfied, and the evaluation item for "Formula (4)" in Table 1 is marked "X."

[0130] The "corrosion resistance" evaluation of Comparative Example 4 was "×" because the time until white rust appeared was 600 hours or less and the time until red rust appeared was 4000 hours or less. The "overtapping property" evaluation of Comparative Example 4 was also "〇" because the occurrence of the seizure phenomenon was suppressed more than with conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 4 was also "〇" because the manufacturing cost was lower than the standard. That is, in Comparative Example 4, the evaluation result for "corrosion resistance" was "x", and therefore the overall evaluation was also "x".

[0131] "Comparative Example 5" In Comparative Example 5, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 2 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0132] In Comparative Example 5, the film thickness T1 is 20 μm, the film thickness T2 is 70 μm, and the film thickness T3 is 120 μm. Therefore, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "Good." Furthermore, since film thickness T1 = 20 μm and film thickness T4 = 2 μm, formula (2) is not satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "Poor." Furthermore, since film thickness T1 = 20 μm and film thickness T2 = 70 μm, the difference in film thickness between the tip and base end is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is marked "Good." Furthermore, since film thickness T2 = 70 μm and film thickness T3 = 120 μm, the difference in film thickness between the base end and head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is marked "Good."

[0133] The "corrosion resistance" evaluation of Comparative Example 5 was "×" because the time until white rust appeared exceeded 600 hours, but the time until red rust appeared was 4000 hours or less. The "overtapping property" evaluation of Comparative Example 5 was "○" because the occurrence of the seizure phenomenon was suppressed compared to conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 5 was "○" because the manufacturing cost was lower than the standard. That is, in Comparative Example 5, the evaluation result for "corrosion resistance" was "x", and therefore the overall evaluation was also "x".

[0134] "Comparative Example 6" In Comparative Example 6, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 12 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0135] In Comparative Example 6, since the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 12 μm, formula (2) is not satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Poor." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base ends is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head end is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0136] The "corrosion resistance" evaluation of Comparative Example 6 was "Good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4,000 hours. The "overtapping property" evaluation of Comparative Example 6 was also evaluated as "Poor" because the occurrence of the seizure phenomenon was the same as that of conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 6 was evaluated as "Poor" because the manufacturing costs were higher than the standard. That is, in Comparative Example 6, the evaluation results for "evaluation of over-tapping property" and "cost" were "x", and therefore the overall evaluation was also "x".

[0137] "Comparative Example 7" In Comparative Example 7, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 5 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0138] In Comparative Example 7, the film thickness T1 = 5 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, so formula (1) is not satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "x". In addition, the film thickness T1 = 5 μm, the film thickness T4 = 8 μm, so formula (2) is not satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "x". Furthermore, the film thickness T1 = 5 μm, the film thickness T2 = 70 μm, so the film thickness difference between the tip and base ends is 65 μm, which satisfies formula (3), and the evaluation item for "Formula ( Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the difference in film thickness between the base end and the head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0139] The "corrosion resistance" evaluation of Comparative Example 7 was "×" because the time until white rust appeared exceeded 600 hours, but the time until red rust appeared was 4000 hours or less. The "overtapping property" evaluation of Comparative Example 7 was "〇" because the occurrence of the seizure phenomenon was suppressed more than with conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 7 was "〇" because the manufacturing cost was lower than the standard. That is, in Comparative Example 7, the evaluation result for "corrosion resistance" was "x", and therefore the overall evaluation was also "x".

[0140] "Comparative Example 8" In Comparative Example 8, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 35 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0141] In Comparative Example 8, since the film thickness T1 = 35 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 35 μm and the film thickness T4 = 8 μm, formula (2) is not satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Poor." Furthermore, since the film thickness T1 = 35 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 35 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Good." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0142] The "corrosion resistance" evaluation of Comparative Example 8 was "Good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4,000 hours. The "overtapping property" evaluation of Comparative Example 8 was also evaluated as "Poor" because the occurrence of the seizure phenomenon was the same as that of conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 8 was evaluated as "Poor" because the manufacturing costs were higher than the standard. That is, in Comparative Example 8, the evaluation results for "evaluation of over-tapping property" and "cost" were "x", and therefore the overall evaluation was also "x".

[0143] "Comparative Example 9" In Comparative Example 9, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 45 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0144] In Comparative Example 9, since the film thickness T1 = 20 μm, the film thickness T2 = 45 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 45 μm and the film thickness T2 = 70 μm, the film thickness difference between the tip and base end is 25 μm, which does not satisfy formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Poor." Furthermore, since the film thickness T2 = 70 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 50 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0145] The "corrosion resistance" evaluation of Comparative Example 9 was "×" because the time until white rust appeared exceeded 600 hours, but the time until red rust appeared was 4000 hours or less. The "overtapping property" evaluation of Comparative Example 9 was "○" because the occurrence of the seizure phenomenon was suppressed more than with conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 9 was "○" because the manufacturing cost was lower than the standard. That is, in Comparative Example 9, the evaluation result for "corrosion resistance" was "x", and therefore the overall evaluation was also "x".

[0146] "Comparative Example 10" In Comparative Example 10, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 105 μm, and the thickness T3 of the second coating layer at the head is 120 μm.

[0147] In Comparative Example 10, since the film thickness T1 = 20 μm, the film thickness T2 = 105 μm, and the film thickness T3 = 120 μm, formula (1) is satisfied, and the evaluation item for "Formula (1)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is "Good." Furthermore, since the film thickness T1 = 20 μm and the film thickness T2 = 105 μm, the film thickness difference between the tip and base end is 85 μm, which does not satisfy formula (3), and the evaluation item for "Formula (3)" in Table 1 is "Poor." Furthermore, since the film thickness T2 = 105 μm and the film thickness T3 = 120 μm, the film thickness difference between the base end and the head is 15 μm, which satisfies formula (4), and the evaluation item for "Formula (4)" in Table 1 is "Good."

[0148] The "corrosion resistance" evaluation of Comparative Example 10 was "good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4,000 hours. The "overtapping property" evaluation of Comparative Example 10 was also "good" because the occurrence of the seizure phenomenon was suppressed more than with conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 10 was "poor" because the manufacturing costs were higher than the standard. That is, since the evaluation result for "cost" of Comparative Example 10 is "x", the overall evaluation is also "x".

[0149] "Comparative Example 11" In Comparative Example 11, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 70 μm.

[0150] In Comparative Example 11, the film thickness T1 = 20 μm, the film thickness T2 = 70 μm, and the film thickness T3 = 70 μm, so formula (1) is not satisfied, and the evaluation item for "Formula (1)" in Table 1 is marked "X." Furthermore, the film thickness T1 = 20 μm and the film thickness T4 = 8 μm, so formula (2) is satisfied, and the evaluation item for "Formula (2)" in Table 1 is marked "O." Furthermore, the film thickness T1 = 20 μm and the film thickness T2 = 70 μm, so the difference in film thickness between the tip and base ends is 50 μm, which satisfies formula (3), and the evaluation item for "Formula (3)" in Table 1 is marked "O." Furthermore, the film thickness T2 = 70 μm and the film thickness T3 = 70 μm, so the difference in film thickness between the base end and the head end is 0 μm, which does not satisfy formula (4), and the evaluation item for "Formula (4)" in Table 1 is marked "X."

[0151] The evaluation of "corrosion resistance" for Comparative Example 11 was "×" because the time until white rust appeared exceeded 600 hours, but the time until red rust appeared was 4000 hours or less. Also, the evaluation of "evaluation of over-tapping property" for Comparative Example 11 was "◯" because the occurrence of the seizure phenomenon was suppressed compared to conventional bolts. The evaluation of "cost" was given a "good" rating, as the manufacturing costs were lower than the standard. That is, in Comparative Example 11, the evaluation result for "corrosion resistance" was "x", and therefore the overall evaluation was also "x".

[0152] "Comparative Example 12" In Comparative Example 12, the thickness T4 of the first coating layer including the plating layer, corrosion-resistant layer, and coating layer is 8 μm, the thickness T1 of the second coating layer at the tip is 20 μm, the thickness T2 of the second coating layer at the base is 70 μm, and the thickness T3 of the second coating layer at the head is 180 μm.

[0153] In Comparative Example 12, the film thickness T1 was 20 μm, the film thickness T2 was 70 μm, and the film thickness T3 was 180 μm, so the formula (1) was not satisfied, and the evaluation item for "Formula (1)" in Table 1 was marked "Good." Furthermore, the film thickness T1 was 20 μm and the film thickness T4 was 8 μm, so the formula (2) was satisfied, and the evaluation item for "Formula (2)" in Table 1 was marked "Good." Furthermore, the film thickness T1 was 20 μm and the film thickness T2 was 70 μm, so the difference in film thickness between the tip and base end was 50 μm, which satisfied the formula (3), and the evaluation item for "Formula (3)" in Table 1 was marked "Good." Furthermore, the film thickness T2 was 70 μm and the film thickness T3 was 180 μm, so the difference in film thickness between the base end and head was 110 μm, which did not satisfy the formula (4), and the evaluation item for "Formula (4)" in Table 1 was marked "Poor."

[0154] The "corrosion resistance" evaluation of Comparative Example 12 was "Good" because the time until white rust appeared exceeded 600 hours and the time until red rust appeared exceeded 4,000 hours. The "overtapping property" evaluation of Comparative Example 12 was also "Good" because the occurrence of the seizure phenomenon was suppressed compared to conventional bolts. Furthermore, the "cost" evaluation of Comparative Example 12 was "Poor" because the manufacturing costs were higher than the standard. That is, in Comparative Example 12, the evaluation result for "cost" was "x", and therefore the overall evaluation was also "x".

[0155] "Comparison Results Between Examples and Comparative Examples" The following can be concluded from the experimental results of the above-mentioned Examples and Comparative Examples. "Regarding formula (1)"

[0156] First, Comparative Examples 1 to 4, 7, and 12 will be described as experimental conditions that do not satisfy formula (1). In Comparative Example 1, the thickness T1 of the second coating layer at the tip end is 50 μm, the same as the thickness T2 of the second coating layer at the base end, and thus formula (1) is not satisfied. Because the thickness T1 of the second coating layer at the tip end is thick, the overtapping property and cost are evaluated as ×. In Comparative Example 2, the thickness T2 of the second coating layer at the base end is 100 μm, the same as the thickness T3 of the second coating layer at the head, and thus formula (1) is not satisfied. Because the thickness T2 of the second coating layer at the base end is thick, the overtapping property is evaluated as ○, but the cost is evaluated as ×. In Comparative Example 3, the thickness T2 of the second coating layer at the base end is 20 μm, the same as the thickness T1 of the second coating layer at the tip end, and thus is thin. Therefore, the second coating layer at the base end is too thin, and therefore the overtapping property is evaluated as ○, but the corrosion resistance is evaluated as ×. In Comparative Example 4, the second coating layer is not formed on the tip end, base end, and head, and although the overtapping property and cost are evaluated as good, the corrosion resistance is evaluated as ×. In Comparative Example 7, the thickness T1 of the second coating layer at the tip is 5 μm, which is thin. Therefore, the second coating layer at the tip is too thin, and the evaluation of corrosion resistance is ×. In Comparative Example 12, the thickness T3 of the second coating layer at the head is 180 μm, which is thick. Therefore, the second coating layer at the head is too thick, and the evaluation of cost is ×.

[0157] In contrast to the above-mentioned Comparative Examples 1 to 4, 7, and 12, Examples 1 to 9 are All of these satisfy formula (1), and the evaluations of corrosion resistance, over-tapping ability, and cost are all good. This shows that by forming the film thickness of the tip, base, and head so as to satisfy formula (1), the evaluations of corrosion resistance, over-tapping ability, and cost can all be good. "Regarding formula (2)"

[0158] First, Comparative Examples 1 and 4 to 8 will be described as experimental conditions that do not satisfy formula (2). In Comparative Example 1, the thickness T4 of the first coating layer is 8 μm, while the thickness T1 of the second coating layer at the tip is as thick as 50 μm. In Comparative Example 1, the thickness of the second coating layer at the tip is too thick compared to the thickness T4 of the first coating layer, so formula (2) is not satisfied, and the overtapping property and cost are evaluated as ×. In Comparative Example 4, the second coating layer is not formed on the tip, base end, and head, so formula (2) is not satisfied. In Comparative Example 4, the overtapping property and cost are evaluated as good, but the corrosion resistance is evaluated as ×. In Comparative Example 5, the thickness T1 of the second coating layer at the tip is 20 μm, but the thickness T4 of the first coating layer is thin at 5 μm. Because the thickness T4 of the first coating layer is thin at 5 μm, the overtapping property is evaluated as good, but the corrosion resistance is evaluated as ×. In Comparative Example 6, the thickness T1 of the second coating layer at the tip is 20 μm, but the thickness T4 of the first coating layer is thick at 12 μm. Because the thickness T4 of the first coating layer is as thick as 12 μm, the corrosion resistance is evaluated as ◯, but the overtapping property and cost are evaluated as ×. In Comparative Example 7, the second coating layer is not formed on the tip, base, and head, and the overtapping property and cost are evaluated as good, but the corrosion resistance is evaluated as ×. In Comparative Example 7, the thickness T4 of the first coating layer is 8 μm, but the thickness T1 of the second coating layer at the tip is thin at 5 μm. Because the thickness T1 of the second coating layer at the tip is thin at 5 μm, the overtapping property and cost are evaluated as ◯, but the corrosion resistance is evaluated as ×. In Comparative Example 8, the thickness T4 of the first coating layer is 8 μm, but the thickness T1 of the second coating layer at the tip is thick at 35 μm. Because the thickness T1 of the second coating layer at the tip is thick at 35 μm, the corrosion resistance is evaluated as ◯, but the overtapping property and cost are evaluated as ×.

[0159] In contrast to the above-mentioned Comparative Examples 1 and 4 to 8, Examples 1 to 9 all satisfy formula (2) and are evaluated as being good in terms of corrosion resistance, over-tapping ability, and cost. This shows that by forming the thicknesses of the second coating layer and the first coating layer at the tip portion so as to satisfy formula (2), it is possible to evaluate the corrosion resistance, over-tapping ability, and cost all as being good. "Regarding formula (3)"

[0160] First, Comparative Examples 1 to 4, 9, and 10 will be described as experimental conditions that do not satisfy formula (3).

[0161] In Comparative Example 1, the thickness T1 of the second coating layer at the distal end and the thickness T2 of the base end are both 50 μm, and the thickness of the second coating layer at the distal end and the base end are the same, so formula (3) is not satisfied. Because the thickness T1 of the second coating layer at the distal end is thick, the overtapping property and cost are evaluated as ×. In Comparative Example 2, the thickness T1 of the second coating layer at the distal end is 20 μm, but the thickness T2 at the base end is thick at 100 μm, and the thickness difference is large at 80 μm, so formula (3) is not satisfied. Because the thickness T1 of the distal end is thin at 20 μm, the overtapping property is evaluated as good, but the thickness T2 at the base end is thick, so the cost is evaluated as ×. In Comparative Example 3, the thickness T1 of the second coating layer at the distal end and the thickness T2 at the base end are both 20 μm, so formula (3) is not satisfied. Although the thickness T1 of the tip of the second coating layer is 20 μm, and the evaluation of overtapping property and cost is good, the evaluation of corrosion resistance is poor because the thickness T2 of the base end is thin at 20 μm. In Comparative Example 4, the thickness of the second coating layer at the tip, base end, and head is all 0 μm, and no second coating layer is formed. Since no second coating layer is formed at the tip, The overtapping property was rated as good, and since there was no cost involved in forming the second coating layer, the cost evaluation result was good, but the corrosion resistance evaluation was poor. In Comparative Example 9, the thickness T1 of the second coating layer at the tip portion was 20 μm, but the thickness T2 of the base end was thin at 45 μm. Since the thickness T1 of the second coating layer at the tip portion was thin at 20 μm, the overtapping property evaluation result was good, but since the thickness T2 of the second coating layer at the base end was also thin, the corrosion resistance evaluation was poor. In Comparative Example 10, the thickness T1 of the second coating layer at the tip portion was 20 μm, but the thickness T2 of the second coating layer at the base end was thick at 105 μm. Since the thickness T1 of the second coating layer at the tip portion was 20 μm, the overtapping property was rated as good, and since the thickness T2 of the second coating layer at the base end was thick at 105 μm, the corrosion resistance evaluation result was also good, but since a thick second coating layer had to be formed at the base end, the cost evaluation was poor.

[0162] In comparison with the above-mentioned Comparative Examples 1 to 4, 9, and 10, Examples 1 to 9 all satisfy formula (3) and are evaluated as being good in terms of corrosion resistance, over-tapping ability, and cost. This shows that by forming the film thickness of the second coating layer at the distal end and proximal end so as to satisfy formula (3), it is possible to evaluate the corrosion resistance, over-tapping ability, and cost all as being good. "Regarding equation (4)"

[0163] First, Comparative Examples 2 to 4, 11, and 12 will be described as experimental conditions that do not satisfy formula (4).

[0164] In Comparative Example 2, the thickness T3 of the second coating layer at the head is 100 μm, but the thickness T2 at the base end is also thick at 100 μm, resulting in a thickness difference of 0 μm, and therefore formula (4) is not satisfied. Since the thickness T2 of the second coating layer at the base end is thick at 100 μm, the corrosion resistance is evaluated as ◯, but the cost is evaluated as × due to the cost of film formation. In Comparative Example 3, the thickness T3 of the second coating layer at the head is 110 μm, but the thickness T2 at the base end is thin at 20 μm. Since the thickness T2 of the second coating layer at the base end is thin at 20 μm, the cost is evaluated as ◯, but the corrosion resistance is evaluated as × because the second coating layer at the base end is thin. In Comparative Example 4, a second coating layer is not formed on the tip end, base end, and head, and therefore formula (4) is not satisfied. Since the second coating layer is not formed on any of the tip, base, or head, the evaluation results for cost and overtapping performance are ◯. However, the evaluation result for corrosion resistance is × because the second coating layer on the head, which is exposed to a severe corrosion-resistant environment, is thin. In Comparative Example 11, the film thickness of the second coating layer on both the base and head is 70 μm, and the film thickness difference between the base and head is 0 μm, so formula (4) is not satisfied. Since the film thickness T3 of the head is thin at 70 μm, the evaluation results for cost and overtapping performance are ◯. However, the corrosion resistance of the head, which is exposed to a severe corrosion-resistant environment, is low and the evaluation result for corrosion resistance is ×. In Comparative Example 12, the film thickness T2 of the second coating layer on the base end is 70 μm, but the film thickness T3 of the second coating layer on the head is thick at 180 μm, so formula (4) is not satisfied. Although the evaluation result for corrosion resistance is good because the film thickness T3 of the second coating layer on the head is thick, the evaluation result for cost is × due to the cost of film formation.

[0165] In contrast to the above-mentioned Comparative Examples 2 to 4, 11, and 12, Examples 1 to 9 all satisfy formula (4) and are evaluated as being good in terms of corrosion resistance, over-tapping property, and cost. This shows that by forming the film thickness of the second coating layer on the base end and head so as to satisfy formula (4), it is possible to evaluate the corrosion resistance, over-tapping property, and cost all as being good. "Reclaim"

[0166] A preferred embodiment of the present invention provides a screw as described in the following items. (Item 1) The screw has a tip end portion on which a screw groove is formed, a head portion formed with a larger diameter than the tip end portion, and a base end portion disposed between the tip end portion and the head portion in the axial direction, and is made of an iron-based metal material. A screw formed in a rod shape, wherein the tip portion, the head portion, and the base portion have a first coating layer including a plating layer on the surface of the material, and further have a second coating layer formed by coating on the surface of the first coating layer, the tip portion, the head portion, and the base portion all have the same thickness of the first coating layer, and the second coating layer is formed so that the relationship of the following formula (1) holds when the film thickness of the tip portion is T1 (μm), the film thickness of the base portion is T2 (μm), and the film thickness of the head is T3 (μm). [Number 9] 10 (μm)≦T1 <T2<T3≦150(μm)···(1)

[0167] The screw according to this item 1 can reduce the manufacturing costs involved in over-tapping and coating, while still exhibiting sufficient corrosion resistance in a severe corrosive environment. (Item 2)

[0168] Item 2. The screw according to item 1, which is formed so that the relationship of the following formula (2) holds when the film thickness of the first coating layer is T4 (μm). [Number 10] 3≦T4≦10(μm) T4≦T1≦30(μm) (2)

[0169] According to the screw according to item 2, a screw formed so that the relationship of formula (2) is established in addition to the relationship of formula (1) can achieve even better corrosion resistance and can reduce the manufacturing cost of the screw compared to a screw that does not satisfy the relationship of formula (2). (Item 3)

[0170] The screw according to item 1 or 2, wherein the screw is formed so that the relationship of the following formula (3) holds between a film thickness T1 (μm) of the second coating layer at the tip end and a film thickness T2 (μm) of the second coating layer at the base end. [Number 11] 30(μm)≦(T2-T1)≦70(μm)...(3)

[0171] According to the screw according to item 3, a screw formed so that the relationship of formula (3) holds in addition to the relationships of formulas (1) and (2) can achieve even better corrosion resistance and can reduce the manufacturing cost of the screw compared to a screw that does not satisfy the relationship of formula (3). (Item 4)

[0172] The screw according to any one of items 1 to 3, wherein the screw is formed so that the relationship of the following formula (4) holds between a film thickness T2 (μm) of the second coating layer at the base end and a film thickness T3 (μm) of the second coating layer at the head. [Number 12] 0<(T3-T2)≦80(μm) (4)

[0173] According to the screw according to item 4, a screw formed so that the relationship of formula (4) is satisfied in addition to the relationships of formulas (1) to (3) can achieve even better corrosion resistance and can reduce the manufacturing cost of the screw compared to a screw that does not satisfy the relationship of formula (4). (Item 5)

[0174] 5. The screw according to any one of items 1 to 4, wherein a thickness T1 of the second coating layer at the tip end portion is 10 μm or more and 30 μm or less, a thickness T2 of the second coating layer at the base end portion is 50 μm or more and 100 μm or less, and a thickness T3 of the second coating layer at the head portion is greater than 100 μm and 150 μm or less.

[0175] According to the screw according to item 5, by forming the film thickness of the second coating layer at the tip, base, and head within the above-mentioned range, it is possible to obtain a screw that can achieve both good corrosion resistance and low manufacturing costs. (Item 6)

[0176] The screw according to any one of items 1 to 5, wherein the first coating layer has a plating layer that coats the surface of the base material, a corrosion-resistant layer that coats the surface of the plating layer, and a coating layer that coats the surface of the corrosion-resistant layer. According to the screw related to this item 6, a first coating layer having excellent corrosion resistance can be formed, so that a screw having both good corrosion resistance and low manufacturing costs can be obtained. (Item 7)

[0177] The material is made of carbon steel for mechanical structures, the plating layer is formed of a zinc-nickel alloy having a nickel co-deposition rate of 5% by weight or more and 18% by weight or less, the corrosion-resistant layer is formed of an oxide of trivalent chromium, 7. The screw according to any one of claims 1 to 6, wherein the coating layer is formed of a siliceous coating containing silica.

[0178] According to the screw according to this item 7, the compositions of the material, plating layer, corrosion-resistant layer, and coating layer are specified, so it is possible to reliably obtain a screw that can achieve both good corrosion resistance and low manufacturing costs. (Item 8)

[0179] 8. The screw according to any one of items 1 to 7, wherein the second coating layer is formed of a nylon-based resin coating containing a copolymer nylon resin.

[0180] According to the screw according to item 8, it is possible to form a resin coating with a thickness of 150 μm as the second coating layer.

[0181] A preferred embodiment of the present invention provides a method for forming a corrosion-resistant coating on the surface of a screw, as described in the following paragraphs. (Item 9)

[0182] A method for forming a corrosion-resistant coating on the surface of a screw that has, in the axial direction, a tip portion with a thread groove formed thereon, a head portion formed with a larger diameter than the tip portion, and a base portion disposed between the tip portion and the head, and that is formed into a rod shape using an iron-based metal material, the method comprising the steps of: plating the surface of the material with a zinc-nickel alloy to form a plating layer; treating the surface of the plating layer with trivalent chromate to form a trivalent chromium layer; contacting the surface of the trivalent chromium layer with a non-chromium surface treatment agent solution containing an alkoxysilane oligomer in an alcohol solvent to coat the surface of the trivalent chromium layer with a coating layer formed of a siliceous coating; and, after the coating layer is formed, performing a first baking and drying. a second coating step of spraying a negatively charged nylon resin onto the surface of the coating layer after the first baking and drying to coat the surface of the coating layer with a second coating layer, and after forming the second coating layer, a second baking and drying step is performed to form the corrosion-resistant coating having the plating layer, the trivalent chromium layer, the coating layer, and the second coating layer; and when spraying the nylon resin onto the surface of the coating layer, the voltage applied between the nylon resin and the surface of the coating layer is changed for each of the tip end, the base end, and the head, thereby forming a corrosion-resistant coating on the surface of the screw on which the second coating layer is formed with different film thicknesses at each of the tip end, the base end, and the head.

[0183] According to the method for forming a corrosion-resistant coating on a surface according to item 9, it is possible to form a corrosion-resistant coating on the surface of a screw that has both good corrosion resistance and low manufacturing costs. (Item 10)

[0184] 10. The method for forming a corrosion-resistant coating on the surface of a screw according to item 9, wherein the second baking and drying is performed at a temperature higher than that of the first baking and drying.

[0185] According to the method for forming a corrosion-resistant coating on a surface according to item 10, it is possible to melt the resin that forms the second coating layer, and to form a dense corrosion-resistant coating that has excellent corrosion resistance.

[0186] It should be noted that the embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. In particular, in the embodiments disclosed herein, matters not explicitly disclosed, such as operating conditions, operating conditions, various parameters, dimensions, weights, volumes of components, etc., do not deviate from the scope of ordinary practice by a person skilled in the art, and values ​​that can be easily assumed by a person skilled in the art are used.

Claims

1. A screw having a tip end portion on which a screw groove is formed, a head portion formed with a larger diameter than the tip end portion, and a base end portion disposed between the tip end portion and the head portion in the axial direction, and formed in a rod shape using an iron-based metal material, the tip end portion, the head portion, and the base end portion have a first coating layer including a plating layer on a surface of the material, and further have a second coating layer formed by coating on the surface of the first coating layer, the first coating layer includes a plating layer that coats a surface of a base material, a corrosion-resistant layer that coats the surface of the plating layer, and a coating layer that coats the surface of the corrosion-resistant layer; the second coating layer is formed of a nylon-based resin coating film containing a copolymer nylon resin, the tip end, the head end, and the base end all have the same thickness of the first coating layer; The second coating layer has a thickness of T １ (μm), the film thickness of the base end is T ２ (μm), the thickness of the head is T ３ (μm), and the thickness of the first coating layer is T ４ (μm), the screw is formed so that the relationships of the following formulas (1) and (2) hold: [Mathematical formula 1] 10(μm)≦T １ <T ２ <T ３ ≦150(μm)・・・(1) [Equation 2] 3≦T ４ ≦10(μm) T ４ ≦T １ ≦30(μm)・・・(2)

2. The thickness T of the second coating layer at the tip １ (μm), and the thickness T of the second coating layer at the base end ２ 2. The screw according to claim 1, which is formed so that the relationship of the following formula (3) holds between the thickness (μm) and the thickness (μm). [Equation 3] 30(μm)≦(T ２ -T １ )≦70(μm)・・・(3)

3. The thickness T of the second coating layer at the base end ２ (μm), and the thickness T of the second coating layer at the head ３ 3. The screw according to claim 2, which is formed so that the relationship of the following formula (4) holds between the thickness (μm) and the thickness (μm). [Equation 4] 0<(T ３ -T ２ )≦80(μm)・・・(4)

4. The thickness T of the second coating layer at the tip １ is 10 (μm) or more and 30 (μm) or less, The thickness T of the second coating layer at the base end ２ is 50 (μm) or more and 100 (μm) or less, The thickness T of the second coating layer on the head ３ The screw according to claim 1, wherein the thickness is greater than 100 (μm) and less than 150 (μm).

5. The material is made of carbon steel for mechanical structures, the plating layer is formed of a zinc-nickel alloy having a nickel co-deposit ratio of 5% by weight or more and 18% by weight or less, the corrosion-resistant layer is formed of an oxide of trivalent chromium, The screw according to claim 1, wherein the coating layer is formed of a siliceous coating containing silica.

6. A method for forming a corrosion-resistant coating on the surface of a screw that has, in the axial direction, a tip portion with a screw groove formed thereon, a head portion formed with a larger diameter than the tip portion, and a base portion disposed between the tip portion and the head, and that is formed into a rod shape using an iron-based metal material, A zinc-nickel alloy plating is performed on the surface of the material to form a plating layer, a trivalent chromate treatment is performed on the surface of the plating layer to form a trivalent chromium layer; a first coating step is performed in which a non-chromium surface treatment agent solution containing an alkoxysilane oligomer in an alcohol solvent is brought into contact with the surface of the trivalent chromium layer to coat the surface of the trivalent chromium layer with a coating layer formed of a siliceous coating, thereby forming a first coating layer in which the plating layer, the trivalent chromium layer, and the coating layer are stacked; After forming the coating layer, a first baking and drying process is performed, and a second coating process is performed in which a negatively charged nylon resin is sprayed onto the surface of the coating layer after the first baking and drying process to coat a second coating layer on the surface of the coating layer; After forming the second coating layer, a second baking and drying is performed to form the corrosion-resistant coating having the plating layer, the trivalent chromium layer, the coating layer, and the second coating layer; When the nylon-based resin is sprayed onto the surface of the coating layer, the voltage applied between the nylon-based resin and the surface of the coating layer is changed at each of the tip portion, the base portion, and the head portion, thereby forming the second coating layer having a different film thickness at each of the tip portion, the base portion, and the head portion, Regarding the second coating layer, the thickness of the tip portion is T １ (μm), the film thickness of the base end is T ２ (μm), the thickness of the head is T ３ (μm), and the thickness of the first coating layer is T ４ (μm), a method for forming a corrosion-resistant coating on the surface of a screw so that the relationship between the following formulas (1) and (2) holds. [Equation 1] 10(μm)≦T １ <T ２ <T ３ ≦150(μm)・・・(1) [Equation 2] 3≦T ４ ≦10(μm) T ４ ≦T １ ≦30(μm)・・・(2)

7. 7. The method for forming a corrosion-resistant coating on the surface of a screw according to claim 6, wherein the second baking and drying is carried out at a temperature higher than that of the first baking and drying.

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