Threaded connection for pipe or tube and method of producing the threaded connection for pipe or tube

Abstract

An objective of the present invention is to provide a threaded connection for pipe or tube having an excellent anti-misalignment property and an appropriate shouldering torque, and further having excellent corrosion resistance and an excellent adhesiveness of a solid lubricant coating, and to provide a method of producing the threaded connection for pipe or tube. The threaded connection for pipe or tube according to the present embodiment includes a pin (13) and a box (14). The pin (13) and the box (14) have contact surfaces including thread portions (15) and (20) and unthreaded metal contact portions. The threaded connection for pipe or tube comprises an alloy plating layer (21) consisting of a Zn-Ni alloy on the contact surface of at least one of the pin (13) and the box (14), a phosphating layer (22), and a solid lubricant coating (23). These are stacked in an order of the alloy plating layer (21), phospahating layer (22), and the solid lubricant coating (23), from a contact surface side.

Description

Pipe thread joint and method for manufacturing pipe thread joint The present invention relates to a pipe thread joint and a method for manufacturing a pipe thread joint, and more particularly to an oil well pipe thread joint and a method for manufacturing an oil well pipe thread joint. For the extraction of oil fields and natural gas fields, oil well pipes are used. The oil well pipe is formed by connecting a plurality of steel pipes according to the depth of the well. The connection of the steel pipes is performed by screwing together the pipe thread joints formed at the ends of the steel pipes. The oil well pipe is pulled up for inspection or the like, unscrewed, inspected, and then screwed together again for reuse. The pipe thread joint includes a pin and a box. The pin includes a male thread portion and a non-threaded metal contact portion formed on the outer peripheral surface of the tip of the steel pipe. The box includes a female thread portion and a non-threaded metal contact portion formed on the inner peripheral surface of the tip of the steel pipe. The threaded and non-threaded metal contact portions of the pin and the box are repeatedly subjected to strong friction when the steel pipes are screwed together and unscrewed. If these parts do not have sufficient durability against friction, galling (irreparable seizure) occurs when screwing together and unscrewing are repeated. Therefore, the pipe thread joint is required to have sufficient durability against friction, that is, excellent seizure resistance. Conventionally, in order to improve seizure resistance, a compound grease containing heavy metals called dope has been used. By applying the compound grease to the surface of the pipe thread joint, the seizure resistance of the pipe thread joint can be improved. However, heavy metals such as Pb, Zn, and Cu contained in the compound grease may affect the environment. For this reason, the development of a pipe thread joint that does not use compound grease is desired. Japanese Patent Application Laid-Open No. 8-105582 (Patent Document 1), International Publication No. 1996 / 10710 (Patent Document 2), and Japanese Patent Application Laid-Open No. 2008-215473 (Patent Document 3) propose pipe thread joints that are excellent in seizure resistance even without compound grease. The pipe screw joint described in Patent Document 1 has a Cr content of 10% or more. On the contact surface of the box or pin, there is a nitriding treatment layer with a thickness of 1 to 20 μm, an undercoat treatment layer of an iron plating layer with a thickness of 0.5 to 15 μm, and a manganese phosphate-based chemical conversion coating layer with a thickness of 5 to 25 μm, or a manganese phosphate-based chemical conversion coating layer with a thickness of 5 to 25 μm, and further a three-layer coating layer composed of a solid lubricating coating layer with a thickness of 10 to 45 μm is formed. According to Patent Document 1, this can prevent the occurrence of seizure of the screw joint over a long period even when the number of repeated uses of the steel pipe increases. On the contact surface of the box or pin of the pipe screw joint described in Patent Document 2, a phosphoric acid-based chemical conversion coating layer or a nitriding treatment layer and a phosphoric acid-based chemical conversion coating layer are provided, and a resin coating layer in which molybdenum disulfide or tungsten disulfide powder is dispersed and mixed in a resin is formed on the phosphoric acid-based chemical conversion coating layer. Further, the film thickness of the resin coating is not less than the film thickness of the phosphoric acid-based chemical conversion coating layer. According to Patent Document 2, this can prevent the occurrence of seizure of the screw joint over a long period even when the number of repeated uses of the steel pipe increases. The pipe screw joint described in Patent Document 3 is characterized in that at least one of the contact surfaces of the pin and the box has a first plating layer made of a Cu-Zn alloy. According to Patent Document 3, the screw joint has excellent leak resistance and seizure resistance, and further, when a lubricating coating is formed on the plating layer, the crevice corrosion is improved. To suppress the seizure of the pipe screw joint, it is effective to form a plating layer containing a metal with high hardness and melting point. Therefore, conventionally, copper (Cu) plating or Cu alloy plating has been used. The hardness and melting point of Cu are high. Therefore, when Cu is contained in the plating layer, the hardness and melting point of the entire plating layer increase. Therefore, the seizure resistance of the pipe screw joint increases. Japanese Patent Application Laid-Open No. 8-105582 International Publication No. 1996 / 10710 Japanese Patent Application Laid-Open No. 2008-215473 Incidentally, the evaluation of seizure resistance is usually carried out with the axes of the steel pipes to be screwed together aligned. However, when actually screwing a pipe thread joint, the axes of the steel pipes (or the steel pipe and the coupling) to be screwed together may be misaligned. This is called misalignment. When misalignment occurs, the threaded parts and the non-threaded metal contact parts of the pin and the box are subjected to strong shear stress in addition to strong friction. This shear stress is significantly larger compared to the case without misalignment. Therefore, when misalignment occurs, seizure is more likely to occur. Accordingly, a pipe thread joint is required to have a performance of suppressing seizure even when misalignment occurs, that is, misalignment resistance. On the other hand, the above-mentioned non-threaded metal contact part includes a metal seal part and a shoulder part. When screwing a pipe thread joint, the shoulder parts of the pin and the box come into contact with each other. The torque generated at this time is called shoulder ring torque. When screwing a pipe thread joint, after reaching the shoulder ring torque, further screwing is performed until fastening is completed. Thereby, the airtightness of the pipe thread joint is enhanced. When further screwing is performed, the metal constituting at least one of the pin and the box begins to undergo plastic deformation. The torque generated at this time is called yield torque. The torque at the completion of fastening (referred to as fastening torque) is set so as to obtain a sufficient seal surface pressure regardless of the magnitude of the thread interference amount. If there is a sufficient difference between the shoulder ring torque and the yield torque, a margin can be created within the range of the fastening torque. As a result, adjustment of the fastening torque becomes easy. Therefore, it is preferable that the shoulder ring torque does not become excessively high. A pipe thread joint is required to have appropriate torque characteristics (shoulder ring torque, yield torque) in addition to the above-mentioned misalignment resistance. On the other hand, after being manufactured, the oil well pipe is transported by a ship or the like and stored for a certain period until it is used. The transportation and storage of the oil well pipe may take a long time. Furthermore, the oil well pipe may be stored outdoors. When stored outdoors for a long time, rust may occur on the threaded joint for the oil well pipe, and the airtightness and seizure resistance of the threaded joint for the oil well pipe may decrease. Therefore, the threaded joint for the oil well pipe is required to have excellent corrosion resistance in addition to the above-described misalignment resistance and appropriate torque characteristics (shouldering torque, yield torque). A solid lubricating film is formed on the threaded portion and the non-threaded metal contact portion for the purpose of improving the seizure resistance. As described above, when the oil well pipe is stored outdoors, the oil well pipe may be repeatedly exposed to high and low temperatures. When repeatedly exposed to high and low temperatures, the adhesion of the solid lubricating film may decrease. If the adhesion of the solid lubricating film decreases, the solid lubricating film peels off when the threads are tightened. If the solid lubricating film peels off, the misalignment resistance of the threaded joint for the pipe decreases and furthermore, the shouldering torque increases. Therefore, the threaded joint for the pipe is required to have high adhesion of the solid lubricating film even when subjected to repeated temperature changes. Although the threaded joints for pipes disclosed in Patent Documents 1 to 3 described above have been studied for seizure resistance, they have not been studied for misalignment resistance. Therefore, even if the seizure resistance is sufficient, the misalignment resistance may be low. Furthermore, the shouldering torque may be too high, or the corrosion resistance and the adhesion of the solid lubricating film may be low. An object of the present invention is to provide a threaded joint for a pipe having excellent misalignment resistance and appropriate shouldering torque, and further having excellent corrosion resistance and excellent adhesion of a solid lubricating film, and a method for manufacturing the same. The threaded joint for a pipe of the present embodiment includes a pin and a box. The pin and the box have a contact surface having a threaded portion and a non-threaded metal contact portion. The threaded joint for a pipe includes an alloy plating layer made of a Zn-Ni alloy, a phosphate coating, and a solid lubricating film on at least one contact surface of the pin and the box. These are laminated in the order of the alloy plating layer, the phosphate coating, and the solid lubricating film from the contact surface side. The manufacturing method of the tubing screw joint of the present embodiment is a manufacturing method of a tubing screw joint including a pin and a box. The pin and the box have contact surfaces having a threaded portion and a non-threaded metal contact portion. The manufacturing method of the present embodiment includes an alloy plating layer forming step, a phosphate coating forming step, and a solid lubricant coating forming step. In the alloy plating layer forming step, an alloy plating layer made of a Zn-Ni alloy is formed on at least one contact surface of the pin and the box. In the phosphate coating forming step, after forming the alloy plating layer, a chemical conversion treatment is performed to form a phosphate coating. In the solid lubricant coating forming step, after forming the phosphate coating, a solid lubricant coating is formed. The tubing screw joint of the present embodiment has excellent misalignment resistance and appropriate shoulder torque, and further has excellent corrosion resistance and excellent adhesion of the solid lubricant coating. FIG. 1 is a schematic diagram of screw tightening of a tubing screw joint when misalignment occurs. FIG. 2 is a diagram showing the relationship between the rotation speed and torque of an oil well tubing screw joint. FIG. 3 is a diagram showing the configuration of the tubing screw joint according to the present embodiment. FIG. 4 is a cross-sectional view of the tubing screw joint according to the present embodiment. FIG. 5 is a cross-sectional view of the contact surface of the tubing screw joint according to the present embodiment. FIG. 6 is a diagram plotting the results of the shoulder torque measurement tests for Test No. 8 and Test No. 9. Hereinafter, the present embodiment will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are denoted by the same reference numerals and their description will not be repeated. The inventors of the present invention have conducted various studies on the relationship between a tubing screw joint and misalignment resistance, shoulder torque, corrosion resistance, and adhesion of a solid lubricant coating. As a result, the following findings were obtained. In a conventional pipe screw joint, even if the seizure resistance is sufficient, the misalignment resistance may be insufficient. Misalignment refers to the situation shown in FIG. 1. Referring to FIG. 1, a coupling 2 is attached to the tip of a steel pipe 1. A pin 3 is formed at the other tip of the steel pipe 1. A coupling 5 is attached to the tip of another steel pipe 4. A box is formed on the inner peripheral surface of the coupling 5. The pin 3 of the steel pipe 1 is inserted into the coupling 5 and tightened with a screw. Thereby, the steel pipe 1 is connected to the steel pipe 4. When tightening the screw, the central axis in the longitudinal direction of the steel pipe 1 and the central axis in the longitudinal direction of the steel pipe 4 may not align and may cross each other. This is called misalignment. If screw tightening is performed in a state where misalignment has occurred, seizure is more likely to occur compared to the case without misalignment. In order to improve the misalignment resistance of the pipe screw joint, it is effective to form a plating layer having high hardness and high melting point on the screw portion and the non-screw metal contact portion (hereinafter referred to as the contact surface). If the hardness of the plating layer is high, the plating layer is less likely to be damaged when tightening and loosening the screws of the pipe screw joint. Furthermore, if the melting point of the plating layer is high, the plating layer is less likely to melt out even when the temperature locally becomes high during screw tightening and loosening of the pipe screw joint. An alloy plating layer made of a Zn-Ni alloy is formed on the contact surface. The hardness and melting point of the Zn-Ni alloy are high. Therefore, the misalignment resistance of the pipe screw joint can be improved. In the case of a pure metal, zinc (Zn) has a lower hardness and melting point compared to copper (Cu) that has been conventionally used for plating. However, in the case of a Zn-Ni alloy, the plating layer has sufficient high hardness and high melting point, and the misalignment resistance can be improved. If a Zn-Ni alloy is used, the corrosion resistance of the pipe screw joint can be further improved. Zinc (Zn) is a base metal compared to iron (Fe), nickel (Ni), and chromium (Cr). Therefore, if a plating layer containing zinc (Zn) is formed on the contact surface, the plating layer is preferentially corroded (sacrificial corrosion) rather than the steel material. Thereby, the corrosion resistance of the pipe screw joint is enhanced. On the other hand, in order to enhance lubricity, a solid lubricating film is usually formed on the contact surface of a pipe screw joint. The solid lubricating film may have reduced adhesion when repeatedly exposed to high and low temperatures. The solid lubricating film with reduced adhesion peels off when the pipe screw joint is tightened and loosened. In particular, when tightening is performed in a situation where misalignment occurs, the solid lubricating film is likely to peel off. If the solid lubricating film peels off, the lubricity of the pipe screw joint decreases. The plating layer generally has a flat surface. Therefore, when a solid lubricating film is formed on the plating layer, the adhesion of the solid lubricating film is likely to decrease. Therefore, a phosphate film is formed on the Zn-Ni alloy plating layer, and a solid lubricating film is formed thereon. Thereby, the adhesion of the solid lubricating film is enhanced. The phosphate film has irregularities on its surface. When a solid lubricating film is formed on a surface having irregularities, the adhesion is enhanced due to the so-called anchor effect. If the adhesion of the solid lubricating film is high, peeling is suppressed even when repeatedly exposed to high and low temperatures. Conventional Cu plating and Cu alloy plating such as Cu-Sn-Zn alloy have Cu as the main component. In this case, a phosphate film cannot be formed on these platings. Therefore, when Cu plating or Cu-Sn-Zn alloy plating is used, it is necessary to directly form a solid lubricating film on the plating layer. In this case, the adhesion of the solid lubricating film is low. By forming an alloy plating made of Zn-Ni alloy and a phosphate film, the adhesion of the solid lubricating film is enhanced. If the adhesion of the solid lubricating film is enhanced, peeling is suppressed. If peeling of the solid lubricating film is suppressed, high lubricity is maintained during tightening and loosening. Therefore, the misalignment resistance of the pipe screw joint is enhanced. If the high lubricity of the tubing screw joint is maintained, the shoulder-ringing torque during screw tightening will further decrease. FIG. 2 is a diagram showing the relationship between the rotational speed and torque of the tubing screw joint. Referring to FIG. 2, when the pin and the box are screwed together, the shoulder portions of the pin and the box come into contact with each other. The torque generated at this time is called the shoulder-ringing torque. When tightening the tubing screw joint, after reaching the shoulder-ringing torque, further screw tightening is performed until the fastening is completed. As a result, the airtightness of the tubing screw joint is enhanced. The torque at the end of fastening (fastening torque) is set so as to obtain sufficient sealing surface pressure regardless of the amount of screw interference. If there is a sufficient difference between the shoulder-ringing torque and the yield torque, there will be a margin in the range of the fastening torque. As a result, the adjustment of the fastening torque becomes easier. Therefore, it is preferable that the shoulder-ringing torque does not become excessively high. If the adhesion of the solid lubricant film is sufficiently high, the shoulder-ringing torque will remain low even when screw tightening and unscrewing are repeated. That is, even after repeated use, the adjustment of the fastening torque is easy. Based on the above findings, the tubing screw joint of the present embodiment includes a pin and a box. The pin and the box have a contact surface having a threaded portion and a non-threaded metal contact portion. The tubing screw joint includes an alloy plating layer made of Zn-Ni alloy, a phosphate coating, and a solid lubricant film on at least one contact surface of the pin and the box. These are laminated in the order of the alloy plating layer, the phosphate coating, and the solid lubricant film from the contact surface side. The tubing screw joint of the present embodiment includes an alloy plating layer made of Zn-Ni alloy. Therefore, it is excellent in misalignment resistance and corrosion resistance. The tubing screw joint of the present embodiment further includes a phosphate coating under the solid lubricant film, so the adhesion of the solid lubricant film to the tubing screw joint is high. Therefore, it has high misalignment resistance and an appropriate shoulder-ringing torque. The phosphate coating is preferably a manganese phosphate coating. In this case, the adhesion of the solid lubricant film is further enhanced. The thickness of the alloy plating layer is preferably 1 to 20 μm, the thickness of the phosphate coating is preferably 5 to 20 μm, and the thickness of the solid lubricant film is preferably 5 to 50 μm. The manufacturing method of the pipe screw joint of the present embodiment is a manufacturing method of a pipe screw joint including a pin and a box. The pin and the box have contact surfaces having a threaded portion and a non-threaded metal contact portion. The manufacturing method of the present embodiment includes an alloy plating layer forming step, a phosphate coating forming step, and a solid lubricant coating forming step. In the alloy plating layer forming step, an alloy plating layer made of a Zn-Ni alloy is formed on at least one contact surface of the pin and the box. In the phosphate coating forming step, after forming the alloy plating layer, a chemical conversion treatment is performed to form a phosphate coating. In the solid lubricant coating forming step, after forming the phosphate coating, a solid lubricant coating is formed. Hereinafter, the pipe screw joint and its manufacturing method according to the present embodiment will be described in detail. [Pipe screw joint] The pipe screw joint includes a pin and a box. FIG. 3 is a diagram showing the configuration of the pipe screw joint according to the present embodiment. Referring to FIG. 3, the pipe screw joint includes a steel pipe 11 and a coupling 12. At both ends of the steel pipe 11, pins 13 having male threaded portions on the outer surface are formed. At both ends of the coupling 12, boxes 14 having female threaded portions on the inner surface are formed. By screwing the pin 13 and the box 14 together, the coupling 12 is attached to the end of the steel pipe 11. On the other hand, there is also an integral type oil well pipe screw joint in which one end of the steel pipe 11 is used as the pin 13 and the other end is used as the box 14 without using the coupling 12. The pipe screw joint of the present embodiment can be used for both the coupling type and the integral type pipe screw joints. The pin and the box have contact surfaces with threaded portions and non-threaded metal contact portions. FIG. 4 is a cross-sectional view of the pipe screw joint according to the present embodiment. Referring to FIG. 4, the pin 13 includes a male threaded portion 15 and a non-threaded metal contact portion. The non-threaded metal contact portion is formed at the tip of the pin 13 and includes a metal seal portion 16 and a shoulder portion 17. The portion that comes into contact when the pin 13 and the box 14 are screwed together is referred to as the contact surface. Specifically, when the pin 13 and the box 14 are screwed together, the shoulder portions (shoulder portions 17 and 18), the metal seal portions (metal seal portions 16 and 19), and the threaded portions (male threaded portion 15 and female threaded portion 20) come into contact with each other. That is, the contact surface includes the shoulder portion, the metal seal portion, and the threaded portion. FIG. 5 is a cross-sectional view of the contact surface of the pipe screw joint according to the present embodiment. Referring to FIG. 5, the pipe screw joint includes an alloy plating layer 21, a phosphate coating 22, and a solid lubricant coating 23 on at least one contact surface of the pin 13 and the box 14. These are laminated in the order of the alloy plating layer 21, the phosphate coating 22, and the solid lubricant coating 23 from the contact surface side. [Alloy plating layer 21] An alloy plating layer 21 made of a Zn-Ni alloy is formed on at least one contact surface of the pin and the box. The hardness and melting point of the alloy plating layer 21 are high. Therefore, the misalignment resistance of the pipe screw joint is enhanced. Zn contained in the alloy plating layer 21 is a base metal. Therefore, the corrosion resistance of the pipe screw joint is enhanced. The Zn-Ni alloy contains Zn and Ni, and the balance consists of impurities. The impurities are, for example, Fe, S, O, C, etc. The preferred Zn content of the Zn-Ni alloy is 85% by mass, and more preferably 90% by mass. The preferred Ni content of the Zn-Ni alloy is 10 to 15% by mass. The alloy plating layer 21 has a large Zn content. Therefore, the effect of sacrificial corrosion protection is large. The Zn and Ni contents in the Zn-Ni alloy are measured by the following method. The measurement of the Zn and Ni contents is performed, for example, using a handheld fluorescent X-ray analyzer (Olympus DP2000 (trade name DELTA Premium)). Any four locations (at arbitrary 0°, 90°, 180°, and 270° locations in the circumferential direction of the pipe) on the surface of the metal seal portion plated with the Zn-Ni alloy are subjected to compositional analysis. The measured contents of Zn and Ni are determined according to the measurement mode of the alloy. The thickness of the alloy plating layer 21 is preferably 1 to 20 μm. If the thickness of the alloy plating layer 21 is 1 μm or more, the misalignment resistance and corrosion resistance of the pipe screw joint can be stably enhanced. If the thickness of the Zn-Ni alloy plating layer 21 is 20 μm or less, the adhesion of the plating is stabilized. Therefore, the thickness of the alloy plating layer is preferably 1 to 20 μm. The thickness of the alloy plating layer 21 is measured by the following method. A probe of an overcurrent phase type film thickness measuring instrument compliant with ISO (International Organization for Standardization) 21968 (2005) is brought into contact with the contact surface on which the alloy plating layer 21 is formed. The phase difference between the high-frequency magnetic field on the input side of the probe and the overcurrent on the alloy plating layer 21 excited thereby is measured. This phase difference is converted into the thickness of the alloy plating layer 21. In the film thickness measurement at the screw joint, any four locations (at arbitrary 0°, 90°, 180°, and 270° locations in the circumferential direction of the pipe) of the metal seal portion are measured. [Phosphate coating 22] A phosphate coating 22 is formed on the alloy plating layer 21. The phosphate coating 22 has irregularities on its surface. Since the solid lubricant coating 23 enters and cures in these irregularities, the adhesion of the solid lubricant coating 23 is enhanced. Since the adhesion of the solid lubricant coating 23 is high, the durability of the solid lubricant coating is improved. Therefore, even when the pipe screw joint is repeatedly used, peeling of the solid lubricant coating 23 is suppressed. Furthermore, the phosphate coating 22 is porous. Therefore, even when the solid lubricant coating is worn out due to repeated use of the pipe screw joint, a part of the solid lubricant coating 23 or wear powder remains in the pores of the phosphate coating 22. Therefore, even when repeatedly used, the pipe screw joint maintains high lubricity. The phosphate coating 22 contains metal phosphate crystals, and the balance is impurities. The type of metal phosphate is not particularly limited. The metal phosphate is, for example, zinc phosphate, iron phosphate, manganese phosphate, and calcium phosphate. Crystals of these metal phosphates form a film, and the phosphate coating 22 is formed. The phosphate coating 22 is preferably a manganese phosphate coating or a zinc phosphate coating. Particularly preferred is a manganese phosphate coating from the viewpoint of porosity. When manganese phosphate is used for the phosphate coating 22, the adhesion of the solid lubricating coating 23 is enhanced as compared with the case where other metal phosphates are used. The thickness of the phosphate coating 22 is preferably 5 to 20 μm. If the thickness of the phosphate coating 22 is 5 μm or more, the adhesion of the solid lubricating coating 23 and appropriate shoulder torque can be stably obtained. On the other hand, if the thickness of the phosphate coating 22 exceeds 20 μm, the above effects are saturated. Further, if the thickness of the phosphate coating 22 is 20 μm or less, coarsening of the phosphate crystals can be suppressed, and the alloy plating layer 21 made of Zn-Ni alloy can be sufficiently coated. Therefore, the thickness of the phosphate coating 22 is preferably 5 to 20 μm. The thickness of the phosphate coating 22 is measured in the same manner as the above-described alloy plating layer 21. [Solid lubricating coating 23] A solid lubricating coating 23 is formed on the phosphate coating 22. The solid lubricating coating 23 enhances the lubricity of the pipe thread joint. Well-known solid lubricating coatings 23 can be used. The solid lubricating coating 23 contains, for example, lubricating particles and a binder. The solid lubricating coating 23 may contain a solvent and other components as necessary. The lubricating particles reduce the friction coefficient of the surface of the solid lubricating coating 23. The lubricating particles are not particularly limited as long as they are particles having lubricity. The lubricating particles are, for example, graphite, MoS2 (molybdenum disulfide), WS2 (tungsten disulfide), BN (boron nitride), PTFE (polytetrafluoroethylene), CF x (graphite fluoride), CaCO3 (calcium carbonate), or a combination thereof. Preferably, graphite, graphite fluoride, MoS2, and PTFE are used. When the solid lubricating coating 23 is 100% by mass, the preferred content of the lubricating particles is 5 to 40% by mass. The binder binds the lubricating particles in the solid lubricating film 23. As the binder, an organic resin, an inorganic resin, or a mixture thereof can be used. When using an organic resin, a thermosetting resin or a thermoplastic resin can be used. Examples of the thermosetting resin include an epoxy resin, a polyimide resin, a polycarbodiimide resin, a polyethersulfone resin, a polyetheretherketone resin, a phenol resin, a furan resin, a urea resin, and an acrylic resin. Examples of the thermoplastic resin include a polyamideimide resin, a polyethylene resin, a polypropylene resin, a polystyrene resin, and an ethylene vinyl acetate resin. When the solid lubricating film 23 is 100% by mass, the preferable content of the binder is 60 to 95% by mass. When using an inorganic resin, polymetaxane can be used. Polymetaxane refers to a polymer compound in which the repetition of metal-oxygen bonds forms the main chain skeleton. Preferably, polytitanoxane (Ti-O) and polysiloxane (Si-O) are used. These inorganic resins are obtained by hydrolyzing and condensing a metal alkoxide. The alkoxy group of the metal alkoxide is, for example, a lower alkoxy group such as a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, an isobutoxy group, a butoxy group, and a tert-butoxy group. When it is necessary to dissolve or disperse the lubricating particles and the binder, a solvent is used. The solvent is not particularly limited as long as it can disperse or dissolve the components contained in the solid lubricating film 23. As the solvent, an organic solvent or water can be used. Examples of the organic solvent include toluene and isopropyl alcohol. The solid lubricating film 23 can contain other components as necessary. Examples of the other components include a rust preventive, a corrosion inhibitor, a surfactant, a wax, a friction modifier, and a pigment. The content of each of the lubricating particles, the binder, the solvent, and the other components is appropriately set. The thickness of the solid lubricating film 23 is preferably 5 to 50 μm. If the thickness of the solid lubricating film 23 is 5 μm or more, high lubricity can be stably obtained. On the other hand, if the thickness of the solid lubricating film 23 is 50 μm or less, the adhesion of the solid lubricating film 23 is stabilized. Further, if the thickness of the solid lubricating film 23 is 50 μm or less, the screw tolerance (clearance) of the sliding surface becomes wider, so the surface pressure during sliding becomes lower. Therefore, it is possible to suppress the fastening torque from becoming excessively high. Therefore, the thickness of the solid lubricating film 23 is preferably 5 to 50 μm. The thickness of the solid lubricating film layer 23 is measured by the following method. Apply the solid lubricating film on a flat plate under the same conditions as when applying the solid lubricating film layer 23 to the pipe screw joint. Among the coating conditions of the pipe screw joint and the flat plate, make the conditions such as the distance between the coating object and the nozzle tip, the injection pressure, the viscosity of the composition, and the rotation speed of the coating object the same. To make the viscosity of the composition the same, make the temperature of the tank, piping, and nozzle outlet the same as that of the pipe screw joint and the flat plate. Calculate the coating amount of the composition per unit time from the difference between the weight of the flat plate before applying the composition and the weight of the flat plate after applying the composition. Solidify the composition on the flat plate to form the solid lubricating film layer 23. Measure the film thickness of the solid lubricating film layer 23 using a film thickness gauge. Calculate the weight of the solid lubricating film layer 23 from the difference between the weight of the flat plate before applying the composition and the weight of the flat plate after forming the solid lubricating film layer 23. Calculate the density of the solid lubricating film layer 23 from the film thickness and weight of the solid lubricating film layer 23. Next, calculate the coating target area of the pipe screw joint from the screw shape and size (inner diameter, wall thickness, etc.). The coating target area corresponds to the area when the threaded formation surface with irregularities is developed into a plane. Calculate the average film thickness of the solid lubricating film layer 23 for the pipe screw joint from the coating time of the composition on the pipe screw joint, the coating target area, and the density of the solid lubricating film layer 23. [Base material of pipe screw joint] The composition of the base material of the pipe screw joint is not particularly limited. The base material is, for example, carbon steel, stainless steel, alloy steel, etc. Among alloy steels, high-alloy steels such as duplex stainless steel and Ni alloys containing alloying elements such as Cr, Ni, and Mo have high corrosion resistance. Therefore, if these high-alloy steels are used as the base material, excellent corrosion resistance can be obtained in a corrosive environment containing hydrogen sulfide, carbon dioxide, etc. [Manufacturing method] The manufacturing method of the pipe screw joint according to this embodiment includes an alloy plating layer forming step, a phosphate coating forming step, and a solid lubricant coating forming step. Each step is carried out in the order of the alloy plating layer forming step, the phosphate coating forming step, and the solid lubricant coating forming step. The manufacturing method includes a pretreatment step before the alloy plating layer forming step as required. [Alloy plating layer forming step] In the alloy plating layer forming step, an alloy plating layer 21 made of Zn-Ni alloy is formed on at least one contact surface of the pin 13 and the box 14. The alloy plating layer 21 is formed by electroplating. Electroplating is performed by immersing at least one contact surface of the pin 13 and the box 14 in a plating bath containing zinc ions and nickel ions and applying an electric current. A commercially available plating bath can be used. The plating bath preferably contains zinc ions: 1 to 100 g / L and nickel ions: 1 to 50 g / L. The conditions of electroplating can be set as appropriate. The conditions of electroplating are, for example, plating bath pH: 1 to 10, plating bath temperature: 10 to 60 °C, current density: 1 to 100 A / dm 2 , and treatment time: 0.1 to 30 minutes. [Phosphate coating forming step] After the alloy plating layer forming step, a phosphate film forming step is carried out. The phosphate film forming step can be carried out by a well-known phosphate chemical conversion treatment. For example, a bath in which the above-mentioned metal phosphate is dissolved in water is prepared. The prepared bath is heated, and the pin 13 or the box 14 on which the alloy plating layer 21 is formed is immersed therein for a certain period of time. Thereby, a phosphate film 22 is formed on the alloy plating layer 21. The bath used in the phosphate film forming step preferably contains 50 to 200 g / L of metal phosphate. The conditions of the phosphate chemical conversion treatment can be set as appropriate. The conditions of the phosphate chemical conversion treatment are, for example, bath temperature: 45 to 90 °C and treatment time: 2 to 10 minutes. [Solid lubricating film forming step] After the phosphate film forming step, a solid lubricating film forming step is carried out. In the solid lubricating film forming step, first, a composition for a solid lubricating film (hereinafter also referred to as a composition) is prepared. The composition is formed by mixing the above-mentioned lubricating particles and a binder. The composition may further contain the above-mentioned solvent and other components. The obtained composition is applied onto the phosphate film 22. The method of application is not particularly limited. For example, the composition using a solvent is sprayed onto the phosphate film 22 using a spray gun. Thereby, the composition is uniformly applied onto the phosphate film 22. The pin 13 or the box 14 onto which the composition is applied is heated and dried. The heating and drying can be carried out using a commercially available hot air drying device or the like. Thereby, the composition is cured, and a solid lubricating film 23 is formed on the phosphate film 22. The conditions of the heating and drying can be set as appropriate in consideration of the boiling points and melting points of the respective components contained in the composition. For a composition that does not use a solvent, the hot melt method can be used. In the hot melt method, the composition is heated to a fluid state. The composition in the fluid state is sprayed, for example, using a spray gun having a temperature holding function. Thereby, the composition is uniformly applied onto the phosphate film 22. The heating temperature of the composition can be set as appropriate in consideration of the melting points and softening temperatures of the above-mentioned binder and other components. The pin 13 or the box 14 onto which the composition is applied is cooled by air cooling or the like. Thereby, the composition is cured, and a solid lubricating film 23 is formed on the phosphate film 22. [Pretreatment step] The above manufacturing process may include a pretreatment process before the alloy plating layer formation process, if necessary. The pretreatment process is, for example, pickling and alkaline degreasing. In the pretreatment process, oil and the like adhering to the contact surface are washed. The pretreatment process may further include grinding such as mechanical grinding finish. Hereinafter, examples will be described. In the examples, the contact surface of the pin is referred to as the pin surface, and the contact surface of the box is referred to as the box surface. Also, % in the examples means mass %. In this example, a threaded steel pipe called VAM21 (registered trademark) manufactured by Nippon Steel & Sumitomo Metal Corporation was used. VAM21 (registered trademark) is a threaded pipe joint for a pipe with an outer diameter of 24.448 cm (9 - 5 / 8 inches) and a wall thickness of 1.199 cm (0.472 inches). The steel type was carbon steel or high Cr steel. The composition of the carbon steel was C: 0.21%, Si: 0.25%, Mn: 1.1%, P: 0.02%, S: 0.01%, Cu: 0.04%, Ni: 0.06%, Cr: 0.17%, Mo: 0.04%, and the balance: Fe and impurities. The composition of the high Cr steel was C: 0.18%, Si: 0.23%, Mn: 0.8%, P: 0.02%, S: 0.01%, Cu: 0.04%, Ni: 0.1%, Cr: 13%, Mo: 0.04%, and the balance: Fe and impurities. Mechanical grinding finish (surface roughness 3 μm) was performed on the pin surface and the box surface of each test number. Thereafter, the plating layer or coating shown in Table 1 was formed to prepare the pins and boxes of each test number. The formation method of each plating layer or coating is as follows. [Test Number 1] In Test Number 1, the following treatment was performed using carbon steel. On the box surface, Zn - Ni alloy plating manufactured by Daiwa Kasei Co., Ltd. was applied by electroplating to form an alloy plating layer with a thickness of 8 μm. The conditions of electroplating were plating bath pH: 6.5, plating bath temperature: 25°C, current density: 2 A / dm 2and the processing time was 18 minutes. The composition of the alloy plating layer was Zn: 85% and Ni: 15%. The surface of the box on which the alloy plating layer was formed was immersed in a manganese phosphate conversion treatment solution at 80 to 95 °C for 10 minutes to form a manganese phosphate coating (surface roughness 10 μm) with a thickness of 12 μm. A composition for forming a solid lubricating coating was applied to the surface of the box on which the manganese phosphate coating was formed. The composition for forming a solid lubricating coating contained a polyamide-imide resin, pure water, dimethyl sulfoxide (DMSO), and PTFE particles. After the composition for forming a solid lubricating coating was spray-coated on the surface of the box, heat drying was performed to form a solid lubricating coating. As the heat drying, preliminary drying (at 85 °C for 10 minutes) and drying (at 280 °C for 30 minutes) were carried out. The average film thickness of the obtained solid lubricating coating was 30 μm. The pin surface was immersed in a zinc phosphate conversion treatment bath at 75 to 85 °C for 10 minutes to form a zinc phosphate coating (surface roughness 5 μm) with a thickness of 8 μm. Further, a composition for forming an ultraviolet-curable resin coating was applied onto the obtained zinc phosphate coating. The composition for forming an ultraviolet-curable resin coating contained an acrylic resin-based ultraviolet-curable resin, aluminum phosphite, and polyethylene wax in a ratio of 1:0.05:0.01. Aluminum phosphite is a rust inhibitor, and polyethylene wax is a lubricant. The applied composition for forming an ultraviolet-curable resin coating was irradiated with ultraviolet rays to form an ultraviolet-curable resin coating with a thickness of 25 μm. The obtained ultraviolet-curable resin coating was colorless and transparent. The conditions for ultraviolet irradiation were as follows. UV lamp: air-cooled mercury lamp UV lamp output: 4 kW Ultraviolet wavelength: 260 nm [Test No. 2] In Test No. 2, the following treatment was carried out using high-Cr steel. For the pin surface and the box surface, an alloy plating layer, a manganese phosphate coating, and a solid lubricating coating were formed in the same manner as the box surface of Test No. 1. The thicknesses of the obtained alloy plating layer and each coating were the same as those of the box surface of Test No. 1. The composition of the obtained alloy plating layer was the same as that of the box surface of Test No. 1. [Test No. 3] In Test No. 3, the following treatment was carried out using carbon steel. On the pin surface, a zinc phosphate coating and an ultraviolet-curable resin coating were formed in the same manner as the pin surface in Test No. 1. The thickness of each formed coating was the same as that in Test No. 1. On the box surface, a manganese phosphate coating and a solid lubricant coating were formed in the same manner as in Test No. 1. The thickness of each obtained coating was the same as that of the box surface in Test No. 1. However, an alloy plating layer was not formed on the box surface of Test No. 3. [Test No. 4] In Test No. 4, the following treatment was carried out using carbon steel. On the pin surface, a zinc phosphate coating and an ultraviolet-curable resin coating were formed in the same manner as the pin surface in Test No. 1. The thickness of each formed coating was the same as that in Test No. 1. On the box surface, a Cu-Sn-Zn alloy plating layer was formed. The plating bath used was a plating bath manufactured by Nippon Chemical Industry Co., Ltd. The Cu-Sn-Zn alloy plating layer was formed by electroplating. The conditions for electroplating were: plating bath pH: 14, plating bath temperature: 45 °C, current density: 2 A / dm 2 and treatment time: 40 minutes. The thickness of the Cu-Sn-Zn alloy plating layer was 8 μm. The composition of the Cu-Sn-Zn alloy plating layer was Cu: about 63%, Sn: about 30%, Zn: about 7%. Subsequently, a solid lubricant coating was formed in the same manner as in Test No. 1. The thickness of the obtained solid lubricant coating was the same as that in Test No. 1. However, since a phosphate coating could not be formed on the box surface of Test No. 4, no phosphate coating was formed. [Test No. 5] In Test No. 5, the following treatment was carried out using carbon steel. On the pin surface, a zinc phosphate coating and an ultraviolet-curable resin coating were formed in the same manner as the pin surface in Test No. 1. The thickness of each formed coating was the same as that in Test No. 1. On the box surface, after forming an alloy plating layer in the same manner as the box surface in Test No. 1, without forming a manganese phosphate coating, the same solid lubricant coating as the box surface in Test No. 1 was formed. The thickness of the obtained alloy plating layer and solid lubricant coating was the same as that of the box surface in Test No. 1. The composition of the obtained alloy plating layer was the same as that of the box surface in Test No. 1. [Test No. 6] In Test No. 6, the following treatments were carried out using high-Cr steel. On the pin surface, an alloy plating layer was formed in the same manner as the box surface in Test No. 1. After that, without forming a manganese phosphate coating, a solid lubricating coating was formed in the same manner as the box surface in Test No. 1. The thicknesses of the obtained alloy plating layer and solid lubricating coating were the same as those of the box surface in Test No. 1. The composition of the obtained alloy plating layer was the same as that of the box surface in Test No. 1. On the box surface, after forming a nitrided layer by nitriding treatment (low-temperature salt bath nitriding method) instead of the alloy plating layer on the box surface in Test No. 1, a manganese phosphate coating and a solid lubricating coating were formed in the same manner as the box surface in Test No. 1. The thickness of the obtained nitrided layer was 2 μm, and the thicknesses of each coating were the same as those of the box surface in Test No. 1. [Test No. 7] In Test No. 7, the following treatments were carried out using carbon steel. On the pin surface, a zinc phosphate coating and an ultraviolet-curable resin coating were formed in the same manner as the pin surface in Test No. 1. The thicknesses of each formed coating were the same as those in Test No. 1. On the box surface, Fe plating was applied by electroplating to form an Fe plating layer with a thickness of 1.5 μm. The composition of the Fe plating bath was 250 g / l FeSO4·7H2O, 42 g / l FeCl2·4H2O, and 20 g / l NH4Cl. The conditions of electroplating were: current density: 10 A / dm 2 , and treatment time: 60 seconds. On the Fe plating layer, a manganese phosphate coating and a solid lubricating coating were formed in the same manner as the box surface in Test No. 1. The composition of each obtained coating was the same as that of the box surface in Test No. 1. [Misalignment Resistance Evaluation Test] Using the pins and boxes of Test No. 1 to Test No. 7, misalignment - accompanied screwing and unscrewing were repeated to evaluate the misalignment resistance. The crossing angle θ of the misalignment was 5°. Screwing and unscrewing were repeated up to 10 times. The screwing and unscrewing speed was 10 rpm, and the tightening torque was 42.8 kN·m. Each time the screwing and unscrewing were performed once, the pin surface and the box surface were visually observed. By visual observation, the occurrence status of seizure was confirmed. When the seizure was minor and recoverable, the seizure flaw was repaired and the test was continued. The number of times that screwing and unscrewing could be performed without causing irrecoverable seizure was measured. The results are shown in Table 2. [Salt spray test] The salt spray test was carried out on the box surfaces of Test No. 1 to Test No. 7. The salt spray test was carried out based on the method described in JIS Z2371 (2000). The size of the test piece was 70 mm × 150 mm, and the thickness was 1 mm. The time when red rust occurred on the surface of the test piece of each test number was measured by visual observation. The results are shown in Table 2. [Protector attachment and detachment test] Using the boxes of Test No. 1 to Test No. 7, the adhesion of the solid lubricating film was evaluated. Specifically, a protector was attached to the box of each test number and stored at - 40°C for 24 hours, followed by storage at 70°C for 24 hours. Furthermore, the detachment of the protector was repeated 3 times at room temperature (20°C). Then, by visual observation, the presence or absence of peeling of the solid lubricating film was confirmed. Furthermore, the peeling area of the solid lubricating film was determined by image processing. Specifically, after the test was completed, the box surface was photographed and the image was binarized. The areas of the portions corresponding to the peeling locations were summed up to obtain the peeling area. The results are shown in Table 2. [Evaluation results] Referring to Table 1 and Table 2, the pipe screw joints of Test No. 1 and Test No. 2 had an alloy plating layer, a phosphate coating, and a solid lubricant coating on at least one contact surface of the pin and the box. Therefore, even when screw tightening and loosening with misalignment were repeated 10 times, seizure did not occur, indicating excellent misalignment resistance. The pipe screw joints of Test No. 1 and Test No. 2 had an alloy plating layer made of Zn-Ni alloy. Therefore, in the salt spray test, no rust occurred even after 2000 hours of testing, indicating excellent corrosion resistance. The pipe screw joints of Test No. 1 and Test No. 2 had a phosphate coating. Therefore, even after the protector attachment / detachment test, the solid lubricant coating did not peel off, indicating excellent adhesion of the solid lubricant coating. On the other hand, the box surface of Test No. 3 did not form an alloy plating layer. Therefore, when screw tightening and loosening with misalignment were performed, seizure occurred at the second time, and the misalignment resistance was low. Furthermore, in the salt spray test, rust occurred after 100 hours, and the corrosion resistance was low. After the protector attachment / detachment test, peeling of the solid lubricant coating was confirmed, and the adhesion of the solid lubricant coating was low. A Cu-Sn-Zn alloy was used instead of the Zn-Ni alloy for the alloy plating layer formed on the box surface of Test No. 4. Furthermore, in Test No. 4, a phosphate coating could not be formed. Therefore, when screw tightening and loosening with misalignment were performed, slight seizure occurred at the sixth time, and irreparable seizure occurred at the seventh time. Therefore, the misalignment resistance was low. Furthermore, in the salt spray test, rust occurred after 750 hours, and the corrosion resistance was low. After the protector attachment / detachment test, the solid lubricant coating peeled off at an area ratio exceeding 10%. Therefore, the adhesion of the solid lubricant coating was low. The box surface of Test No. 5 did not form a phosphate coating. Therefore, when screw tightening and loosening with misalignment were performed, slight seizure occurred at the sixth time, and irreparable seizure occurred at the seventh time. Therefore, the misalignment resistance was low. Furthermore, after the protector attachment / detachment test, the solid lubricant coating peeled off at an area ratio exceeding 10%. Therefore, the adhesion of the solid lubricant coating was low. For the box surface of Test No. 6, a nitride layer was formed instead of the Zn-Ni alloy plating layer. Therefore, when screw tightening and loosening with misalignment were performed, seizure occurred at the fourth time, and the misalignment resistance was low. Furthermore, in the salt spray test, rust occurred after 500 hours (pitting), and the corrosion resistance was low. For the box surface of Test No. 7, an Fe layer was formed instead of the Zn-Ni alloy plating layer. Therefore, when screw tightening and loosening with misalignment were performed, seizure occurred at the third time, and the misalignment resistance was low. Furthermore, in the salt spray test, rust occurred after 150 hours, and the corrosion resistance was low. [Shouldering Torque Measurement Test] The change in shouldering torque with respect to the presence or absence of a phosphate coating was measured. First, pins and boxes of high Cr steel VAM21 (registered trademark) were prepared. The high Cr steel used was a steel type (SM13CrS-110) that was particularly prone to seizure among high Cr steels. The size of the VAM21 (registered trademark) used was an outer diameter of 17.78 cm (7 inches) and a wall thickness of 1.151 cm (0.453 inches). Using this VAM21 (registered trademark), as shown in Table 3, the following treatments were performed on the pins and boxes. For the pin surfaces of Test No. 8 and Test No. 9, an alloy plating layer was formed in the same manner as in Test No. 1. The thickness of the obtained alloy plating layer was the same as that in Test No. 1. Subsequently, chromate treatment with trivalent chromate was carried out. For the box surface of Test No. 8, an alloy plating layer was formed in the same manner as in Test No. 1. The thickness of the obtained alloy plating layer was the same as that in Test No. 1. Subsequently, it was immersed in a zinc phosphate chemical conversion treatment bath at 75 to 85 °C for 10 minutes to form a zinc phosphate coating (surface roughness 8 μm) with a thickness of 8 μm. Further, a composition for forming a solid lubricating coating was applied to the box surface on which the zinc phosphate coating was formed. The composition for forming a solid lubricating coating contained an epoxy resin (22% by mass), PTFE particles (9% by mass), a solvent (total 18% by mass), water (40% by mass), and other additives (including pigments) (11% by mass). After the composition for forming a solid lubricating coating was spray-coated, it was heated and dried to form a solid lubricating coating. The average film thickness of the obtained solid lubricating coating was 30 μm. For the box surface of Test No. 9, an alloy plating layer and a solid lubricating coating were formed in the same manner as in Test No. 8. The thicknesses of the obtained alloy plating layer and solid lubricating coating were the same as those in Test No. 8. However, in Test No. 9, a phosphate coating was not formed. Using the pins and boxes of Test No. 8 and Test No. 9, screw tightening was performed, and the number of turns and torque were measured. The measured number of turns and torque were plotted to obtain the shoulder-ringing torque. Screw tightening and screw loosening were repeated 7 times, and the shoulder-ringing torque was obtained each time. From the obtained shoulder-ringing torque, the ratio (ShT%) of the shoulder-ringing torque to the target fastening torque was calculated. The target fastening torque was set to a constant value. The results are shown in Table 4. In Test No. 9, since non-repairable seizure occurred during the 5th screw tightening and screw loosening, subsequent tests were not conducted. Referring to Table 3 and Table 4, the pipe screw joint of Test No. 8 had a Zn-Ni alloy plating layer, a phosphate coating, and a solid lubricating coating on the box surface. Therefore, even when screw tightening and screw loosening were repeated 10 times, the ShT% was generally 70% or less, and the shoulder-ringing torque was stably low. Fig. 6 is a diagram plotting the results of the shoulder-ringing torque measurement tests for Test No. 8 and Test No. 9. In Test No. 8, the ShT% was stably low even when screw tightening and screw loosening were repeated. On the other hand, the box surface of the pipe screw joint of Test No. 9 did not form a phosphate coating. Therefore, when tightening and loosening the screw, the ShT% exceeded 100% and the shoulder torque was high. In Test No. 9, furthermore, irreparable seizure occurred during the fifth screw tightening and loosening. In Test No. 9, since the phosphate coating was not formed, the adhesion of the solid lubricant coating was low. Therefore, it can be presumed that the solid lubricant coating peeled off during the test and the lubricity of the contact surface decreased. The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the gist thereof. 3, 13 Pin 14 Box 15 Male screw part 16, 19 Metal seal part 17, 18 Shoulder part 20 Female screw part 21 Alloy plating layer 22 Phosphate coating 23 Solid lubricant coating

Claims

1. A pipe screw joint comprising a pin and a box, wherein the pin and the box have a contact surface having a threaded portion and a non-threaded metal contact portion, and on at least one of the contact surfaces of the pin and the box, there is an alloy plating layer made of a Zn-Ni alloy, a phosphate coating on the alloy plating layer, and a solid lubricating coating on the phosphate coating.

2. The pipe screw joint according to claim 1, wherein the phosphate coating is a manganese phosphate coating.

3. The pipe screw joint according to claim 1 or claim 2, wherein the thickness of the alloy plating layer is 1 to 20 μm, the thickness of the phosphate coating is 5 to 20 μm, and the thickness of the solid lubricating coating is 5 to 50 μm.

4. A method for manufacturing a pipe screw joint comprising a pin and a box, wherein the pin and the box have a contact surface having a threaded portion and a non-threaded metal contact portion, and the method includes a step of forming an alloy plating layer made of a Zn-Ni alloy on at least one of the contact surfaces of the pin and the box, a step of performing chemical conversion treatment to form a phosphate coating after forming the alloy plating layer, and a step of forming a solid lubricating coating after forming the phosphate coating.

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