Oil-well metal pipe

Abstract

           An oil-well metal pipe having excellent galling resistance is provided.  An oil-well metal pipe (1) according to the present disclosure includes a pipe main body (10), and the pipe main body (10) includes a pin (40) formed at a first end portion (10A) and having a pin contact surface (400) including an external thread part (41), and includes a box (50) formed at a second end portion (10B) and having a box contact surface (500) including an internal thread part (51).  The oil-well metal pipe (1) according to the present disclosure further includes a Zn-Ni alloy plating layer (100) that is formed on a pin contact surface (400) and / or a box contact surface (500), the Zn-Ni alloy plating layer (100) consisting of 9.0 to 20.0% by mass of Ni, with the balance being Zn and impurities, a precipitate phase includes a g phase, and a crystallite diameter D (nm) of the g phase and a Ni content [Ni] (mass%) satisfy Formula (1).           D£12.3´[Ni]-81.6   (1)

Description

DESCRIPTION TITLE OF INVENTION: OIL-WELL METAL PIPE TECHNICAL FIELD

[0001] The present disclosure relates to an oil-well metal pipe, and more particularly relates to an oil-well metal pipe with a threaded connection formed. BACKGROUND ART

[0002] An oil-well metal pipe is used for drilling in oil fields and natural gas fields (hereinafter, oil fields and natural gas fields are collectively referred to as "oil wells"). Specifically, at the oil well drilling site, in accordance with the depth of the oil well, a plurality of oil-well metal pipes are connected to form an oil country tubular goods connected body as typified by a casing pipe or a tubing pipe. An oil country tubular goods connected body is formed by fastening threaded connections formed at the end portion of oil-well steel pipe to each other. Inspections may in some cases be conducted on oil country tubular goods connected bodies. When conducting an inspection, the oil country tubular goods connected body is lifted up and the threaded connection is loosened. Oil-well metal pipes that are loosened and detached from the oil country tubular goods connected body are then inspected. After the inspection, the threaded connections of oil-well metal pipes are refastened and reused as a part of the oil country tubular goods connected body.

[0003] An oil-well metal pipe includes a pipe main body which includes a first end portion and a second end portion. The pipe main body includes a pin that is formed at the first end portion, and a box that is formed at the second end portion. The pin has a pin contact surface including an external thread part on an outer peripheral surface of the first end portion of the pipe main body. The box has a box contact surface including an internal thread part on an inner peripheral surface of an end portion (second end portion) of the pipe main body that is on the opposite side to the pin. When threaded connections formed at the end portion of oil-well metal pipes are fastened together, the pin contact surface comes in contact with the box contact surface.

[0004] The pin contact surface and the box contact surface repeatedly experience strong friction during fastening and loosening of the oil-well metal pipe. Unless there is sufficient durability with respect to friction at these regions, galling (uncorrectable seizure) will occur during repeated fastening and loosening. Accordingly, an oil-well metal pipe is required to have sufficient durability with respect to friction, that is, to have excellent galling resistance.

[0005] Heretofore, heavy metal-containing compound greases, which are referred to as "dopes", have been used to improve the galling resistance. Application of a compound grease to the pin contact surface and / or the box contact surface can improve the galling resistance of an oil-well metal pipe. However, heavy metals contained in compound greases, such as Pb, Zn, and Cu, may affect the environment. For this reason, the development of an oil-well metal pipe that is excellent in galling resistance without the use of a compound grease is desired.

[0006] In an oil-well metal pipe disclosed in Patent Literature 1 (International Application Publication No. WO2016 / 170031), a Zn-Ni alloy plating layer is formed on a pin contact surface or a box contact surface instead of using a compound grease. The Zn contained in the Zn-Ni alloy plating layer formed on a contact surface of the oil-well metal pipe enhances the corrosion resistance of the oil-well metal pipe by sacrificial protection. In addition, it is disclosed in Patent Literature 1 that the Zn-Ni alloy plating layer is also excellent in a wear resistance characteristic. CITATION LISTPATENT LITERATURE

[0007] Patent Literature 1: International Application Publication No. WO2016 / 170031 SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0008] In recent years, as oil wells become harsher, there is a demand for oil-well metal pipes having further excellent galling resistance.

[0009] An objective of the present disclosure is to provide an oil-well metal pipe having excellent galling resistance. SOLUTION TO PROBLEM

[0010] An oil-well metal pipe according to the present disclosure, including:a pipe main body including a first end portion and a second end portion,whereinthe pipe main body includes:a pin formed at the first end portion; anda box formed at the second end portion,the pin includes:a pin contact surface including an external thread part, andthe box includes:a box contact surface including an internal thread part,the oil-well metal pipe further including:a Zn-Ni alloy plating layer formed on the pin contact surface and / or the box contact surface,wherein the Zn-Ni alloy plating layer consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities,a precipitate phase of the Zn-Ni alloy plating layer includes a γ phase, anda crystallite diameter D of the γ phase and a Ni content [Ni] in the Zn-Ni alloy plating layer satisfy Formula (1):D≤12.3×[Ni]-81.6 (1)where, the crystallite diameter of the γ phase is substituted, in units of nm, for D in Formula (1); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (1). ADVANTAGEOUS EFFECT OF INVENTION

[0011] The oil-well metal pipe according to the present disclosure has excellent galling resistance. BRIEF DESCRIPTION OF DRAWINGS

[0012] [FIG. 1] FIG. 1 is a graph illustrating a relation between a Ni content [Ni] (mass%) in a Zn-Ni alloy plating layer and a Vickers hardness HV (Hv) of the Zn-Ni alloy plating layer.[FIG. 2] FIG. 2 is a graph illustrating a relation among a Ni content [Ni] (mass%) in a Zn-Ni alloy plating layer, a crystallite diameter D (nm) of a γ phase, and a Vickers hardness HV (Hv) of the Zn-Ni alloy plating layer.[FIG. 3] FIG. 3 is a configuration diagram illustrating one example of an oil-well metal pipe according to the present embodiment.[FIG. 4] FIG. 4 is a partial cross-sectional view illustrating a cross section (longitudinal cross section) along a pipe axis direction of a coupling of the oil-well metal pipe illustrated in FIG. 3.[FIG. 5] FIG. 5 is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe illustrated in FIG. 4, that illustrates a portion in the vicinity of a pin of the oil-well metal pipe.[FIG. 6] FIG. 6 is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe illustrated in FIG. 4, that illustrates a portion in the vicinity of a box of the oil-well metal pipe.[FIG. 7] FIG. 7 is a partial cross-sectional view illustrating a cross section (longitudinal cross section) along a pipe axis direction of a coupling of the oil-well metal pipe according to the present embodiment, that is different from FIG. 4.[FIG. 8] FIG. 8 is a configuration diagram illustrating an integral type oil-well metal pipe according to the present embodiment.[FIG. 9] FIG. 9 is an enlarged view of a pin contact surface illustrated in FIG. 5.[FIG. 10] FIG. 10 is an enlarged view of a box contact surface illustrated in FIG. 6.[FIG. 11] FIG. 11 is an enlarged view of a pin contact surface having a different structure from that in FIG. 9.[FIG. 12] FIG. 12 is an enlarged view of a box contact surface having a different structure from that in FIG. 10.[FIG. 13] FIG. 13 is an enlarged view of a pin contact surface having a different structure from that in FIG. 9 and FIG. 11.[FIG. 14] FIG. 14 is an enlarged view of a box contact surface having a different structure from that in FIG. 10 and FIG. 12. DESCRIPTION OF EMBODIMENTS

[0013] The present embodiment is described in detail below with reference to the accompanying drawings. The same reference symbols will be used throughout the drawings to refer to the same or like parts, and description thereof will not be repeated.

[0014] The present inventors conducted detailed investigations regarding means for increasing galling resistance of an oil-well metal pipe. As a result, the present inventors obtained the following findings.

[0015] When oil-well metal pipes are fastened together, metals of contact surfaces slide each other while being in contact with each other under a high pressure. At this point, heat generation may occur due to deformation resistance of the metals. At the same time, when a temperature locally rises to a melting point of the metals or higher, melted metals fuse together, bringing about galling. Therefore, it is considered that, to increase galling resistance of an oil-well metal pipe, a method of forming a plating layer having a low deformation resistance, that is, a high hardness, is effective. Here, a Zn-Ni alloy constituting a Zn-Ni alloy plating layer has high hardness and high melting point. Therefore, there has been proposed a technique for increasing galling resistance of an oil-well metal pipe by forming a Zn-Ni alloy plating layer on a pin contact surface and / or a box contact surface of the oil-well metal pipe.

[0016] Precipitate phases in a Zn-Ni alloy plating layer have been known for changing in accordance with a Ni content. Specifically, a Zn-Ni alloy can include, as its precipitate phase, an eta phase (η phase), a gamma phase (γ phase), and an alpha phase (α phase). More specifically, the η phase of the Zn-Ni alloy is a Zn phase in which Ni is dissolved. The γ phase of the Zn-Ni alloy is a phase of an intermetallic compound (Ni5Zn21). The α phase of the Zn-Ni alloy is a Ni phase in which Zn is dissolved. Here, of the η phase, the γ phase, and the α phase being the precipitate phase of the Zn-Ni alloy, the γ phase has the highest hardness. Therefore, the precipitate phase of the Zn-Ni alloy plating layer preferably includes a γ phase.

[0017] Here, if a Ni content in a Zn-Ni alloy consisting of Ni, and with the balance being Zn and impurities, is less than 9.0%, there are cases where a precipitate phase of the Zn-Ni alloy excessively includes the η phase, and the precipitate phase is not made mainly of the γ phase. In addition, if the Ni content in the Zn-Ni alloy is more than 20.0%, there are cases where the precipitate phase of the Zn-Ni alloy excessively includes the α phase, and the precipitate phase is not made mainly of the γ phase. On the other hand, when the Ni content in the Zn-Ni alloy is 9.0 to 20.0%, the precipitate phase stably includes the γ phase, and the precipitate phase is made mainly of the γ phase. Note that, in the present description, the phrase "precipitate phase being made mainly of a γ phase" means that the γ phase accounts for 50% or more of the precipitate phase of the Zn-Ni alloy plating layer in volume ratio. Thus, the Zn-Ni alloy plating layer according to the present embodiment is made to have a chemical composition that consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities. As a result, the hardness of the Zn-Ni alloy plating layer is increased, and a galling resistance of the Zn-Ni alloy plating layer may be thereby increased.

[0018] On the other hand, even in a case of forming a Zn-Ni alloy plating layer that had a chemical composition consisting of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities, and a precipitate phase included a γ phase, there were cases where galling resistance of an oil-well metal pipe cannot be increased sufficiently. Hence, the present inventors focused their attention on the crystal structure of a Zn-Ni alloy plating layer, and conducted investigations regarding increasing the hardness of the Zn-Ni alloy plating layer to increase galling resistance of an oil-well metal pipe.

[0019] FIG. 1 is a graph illustrating a relation between a Ni content (mass%) in a Zn-Ni alloy plating layer and a Vickers hardness HV (Hv) of the Zn-Ni alloy plating layer. FIG. 1 was created using Ni contents (mass%) and Vickers hardnesses HV (Hv) determined by methods which are described later, in examples which are described later. Referring to FIG. 1, it is confirmed that the Vickers hardness is increased with an increase in the Ni content. Specifically, at each of white circles () in FIG. 1, the Vickers hardness HV (Hv) and the Ni content [Ni] (mass%) satisfy the following Formula (2).HV≥40×[Ni]-145 (2)where, the Vickers hardness of the Zn-Ni alloy plating layer is substituted, in units of Hv, for HV in Formula (2); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (2).

[0020] In contrast, each of black circles () in FIG. 1 does not satisfy Formula (2) shown above, unlike the white circles () in FIG 1. That is, there were cases where even Zn-Ni alloy plating layers each having the aforementioned chemical composition failed to increase their Vickers hardnesses sufficiently. Hence, the present inventors conducted detailed investigations regarding factors other than the Ni content that influence the hardness of the Zn-Ni alloy plating layer. As a result, it was revealed that the hardness of the Zn-Ni alloy plating layer is influenced by a crystallite diameter of a γ phase of a precipitate phase in the Zn-Ni alloy plating layer.

[0021] A crystallite diameter means a size of fine crystals that are present as monocrystals in a polycrystal. That is, as the crystallite diameter of the γ phase decreases, the Zn-Ni alloy plating becomes an aggregate of finer monocrystals. The relation among a Ni content in a Zn-Ni alloy plating layer, a crystallite diameter of a γ phase, and a Vickers hardness is described hereunder with respect to a table. Table 1 is a table that is an excerpt of part of the examples which are described later.

[0022] [Table 1] TABLE 1   Test NumberContent of Ni [Ni] (mass%)Vickers Hardness HV (Hv)Crystallite Diameter D (nm)313.655352712.743155812.5420571013.638487 

[0023] Referring to Table 1, as to Test Numbers 3 to 8, their Vickers hardnesses HV increase as their Ni contents [Ni] increase. In contrast, as to Test Number 10 whose crystallite diameter D was large, it is confirmed that its Vickers hardness HV is small while its Ni content [Ni] is higher than Ni contents of Test Numbers 7 and 8 whose crystallite diameters D were small. That is, the present inventors found that it is possible to increase the hardness of the Zn-Ni alloy plating layer to thereby increase the galling resistance of an oil-well metal pipe by decreasing the crystallite diameter D rather than simply increasing the Ni content. This will now be described in more detail using the drawings.

[0024] FIG. 2 is a graph illustrating a relation among a Ni content [Ni] (mass%) in a Zn-Ni alloy plating layer, a crystallite diameter D (nm) of a γ phase, and a Vickers hardness HV (Hv) of the Zn-Ni alloy plating layer. FIG. 2 was created using Ni contents (mass%), crystallite diameters D (nm) determined by methods which are described later, and Vickers hardnesses HV (Hv) determined by methods which are described later, in the examples which are described later. Note that, white circles () in FIG. 2 means examples in which the Ni content and the Vickers hardness satisfy Formula (2). Black circles () in FIG. 2 mean examples in which the Ni content and the Vickers hardness do not satisfy Formula (2).

[0025] Referring to FIG. 2, it is confirmed that the crystallite diameter D has a tendency to increase with an increase in the Ni content [Ni]. Referring to FIG. 2, it is further confirmed that when the crystallite diameter D and the Ni content [Ni] satisfy the following Formula (1), the Vickers hardness HV satisfies the Formula (2), and the hardness of the Zn-Ni alloy plating layer is increased.D≤12.3×[Ni]-81.6 (1)where, the crystallite diameter of the γ phase is substituted, in units of nm, for D in Formula (1); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (1).

[0026] Accordingly, the oil-well metal pipe according to the present embodiment includes a Zn-Ni alloy plating layer that is formed on a pin contact surface and / or a box contact surface, and the Zn-Ni alloy plating layer consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities, a precipitate phase of the Zn-Ni alloy plating layer includes a γ phase, and a crystallite diameter D of the γ phase and the Ni content [Ni] in the Zn-Ni alloy plating layer satisfy Formula (1). As a result, the Vickers hardness of the Zn-Ni alloy plating layer is increased, and galling resistance of the oil-well metal pipe can be thereby increased.

[0027] The details of the reason why the crystallite diameter D of the γ phase in the Zn-Ni alloy plating layer and the Ni content [Ni] in the Zn-Ni alloy plating layer satisfying Formula (1) increase the Vickers hardness of the Zn-Ni alloy plating layer have not been clarified. However, the present inventors surmise that the reason is as follows. As described above, crystallites mean fine crystals that are present as monocrystals. Therefore, by definition, there is no strain typified by lattice defect inside a crystallite. That is, this shows that the smaller the crystallite diameter D, the more strains typified by lattice defects are present between crystallites. Consequently, the total number of strains is increased, and the present inventors surmise that there may be a possibility that the hardness of the Zn-Ni alloy plating layer is thereby increased.

[0028] Note that there can be also a possibility that a mechanism different from the mechanism surmised by the present inventors increases the hardness of the Zn-Ni alloy plating layer to thereby increase the galling resistance of an oil-well metal pipe. However, the fact that the crystallite diameter D of the γ phase in the Zn-Ni alloy plating layer and the Ni content [Ni] in the Zn-Ni alloy plating layer satisfying Formula (1) increase the Vickers hardness of the Zn-Ni alloy plating layer more than simply increasing the Ni content [Ni] has been demonstrated by the examples which are described later.

[0029] The gist of the oil-well metal pipe according to the present embodiment which was completed based on the above findings is as follows.

[0030] [1]An oil-well metal pipe, including:a pipe main body including a first end portion and a second end portion,whereinthe pipe main body includes:a pin formed at the first end portion; anda box formed at the second end portion,the pin includes:a pin contact surface including an external thread part, andthe box includes:a box contact surface including an internal thread part,the oil-well metal pipe further including:a Zn-Ni alloy plating layer formed on the pin contact surface and / or the box contact surface,wherein the Zn-Ni alloy plating layer consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities,a precipitate phase of the Zn-Ni alloy plating layer includes a γ phase, anda crystallite diameter D of the γ phase and a Ni content [Ni] in the Zn-Ni alloy plating layer satisfy Formula (1):D≤12.3×[Ni]-81.6 (1)where, the crystallite diameter of the γ phase is substituted, in units of nm, for D in Formula (1); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (1).

[0031] [2]The oil-well metal pipe according to [1], whereina Vickers hardness HV of the Zn-Ni alloy plating layer and a Ni content [Ni] in the Zn-Ni alloy plating layer satisfy Formula (2).HV≥40×[Ni]-145 (2)where, the Vickers hardness of the Zn-Ni alloy plating layer is substituted, in units of Hv, for HV in Formula (2); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (2).

[0032] [3]The oil-well metal pipe according to [1] or [2], further including:as an upper layer of the Zn-Ni alloy plating layer,one or more types selected from the group consisting of a chemical conversion treatment layer and a lubricant coating.

[0033] Hereunder, the oil-well metal pipe according to the present embodiment is described in detail.

[0034] [Structure of oil-well metal pipe]First, the structure of the oil-well metal pipe according to the present embodiment will be described. The oil-well metal pipe has a well-known structure. The available types of oil-well metal pipe are a T&C type oil-well metal pipe and an integral type oil-well metal pipe. Hereunder, each type of oil-well metal pipe is described in detail.

[0035] [Case where oil-well metal pipe is T&C type]FIG. 3 is a configuration diagram illustrating one example of an oil-well metal pipe 1 according to the present embodiment. FIG. 3 is a configuration diagram illustrating the oil-well metal pipe 1 of a so-called "T&C (threaded and coupled) type". Referring to FIG. 3, the oil-well metal pipe 1 includes a pipe main body 10.

[0036] The pipe main body 10 extends in the pipe axis direction. A cross section perpendicular to the pipe axis direction of the pipe main body 10 is a circular shape. The pipe main body 10 includes a first end portion 10A and a second end portion 10B. The first end portion 10A is an end portion on the opposite side to the second end portion 10B. In the T&C type oil-well metal pipe 1 illustrated in FIG. 3, the pipe main body 10 includes a pin tube body 11 and a coupling 12. The coupling 12 is attached to one end of the pin tube body 11. More specifically, the coupling 12 is fastened by threading to one end of the pin tube body 11.

[0037] FIG. 4 is a partial cross-sectional view illustrating a cross section (longitudinal cross section) that is parallel to the pipe axis direction of the coupling 12 of the oil-well metal pipe 1 illustrated in FIG. 3. Referring to FIG. 3 and FIG. 4, the pipe main body 10 includes a pin 40 and a box 50. The pin 40 is formed at the first end portion 10A of the pipe main body 10. When performing fastening, the pin 40 is inserted into the box 50 of another oil-well metal pipe (not illustrated), and is fastened by threading to the box 50 of the other oil-well metal pipe 1.

[0038] The box 50 is formed at the second end portion 10B of the pipe main body 10. When performing fastening, the pin 40 of another oil-well metal pipe 1 is inserted into the box 50, and the box 50 is fastened by threading to the pin 40 of the other oil-well metal pipe 1.

[0039] [Structure of pin]FIG. 5 is a cross-sectional view of a portion in the vicinity of the pin 40 of the oil-well metal pipe 1 illustrated in FIG. 4, that is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe 1. A dashed line portion in FIG. 5 represents the structure of the box 50 of another oil-well metal pipe in the case of fastening the oil-well metal pipe 1 to another oil-well metal pipe 1. Referring to FIG. 5, the pin 40 includes a pin contact surface 400 on the outer peripheral surface of the first end portion 10A of the pipe main body 10. The pin contact surface 400 contacts the box 50 of the other oil-well metal pipe 1 when performing fastening to the other oil-well metal pipe 1.

[0040] The pin contact surface 400 includes at least an external thread part 41 formed in the outer peripheral surface of the first end portion 10A. The pin contact surface 400 may further include a pin sealing surface 42 and a pin shoulder surface 43. In FIG. 5, on the outer peripheral surface of the first end portion 10A, the pin sealing surface 42 is disposed further on the front end side of the first end portion 10A than the external thread part 41. In other words, the pin sealing surface 42 is disposed between the external thread part 41 and the pin shoulder surface 43. The pin sealing surface 42 is provided in a tapered shape. Specifically, the external diameter of the pin sealing surface 42 gradually decreases from the external thread part 41 toward the pin shoulder surface 43 in the longitudinal direction (pipe axis direction) of the first end portion 10A.

[0041] When performing fastening with another oil-well metal pipe 1, the pin sealing surface 42 contacts a box sealing surface 52 (described later) of the box 50 of the other oil-well metal pipe 1. More specifically, during fastening, when the pin 40 is inserted into the box 50 of the other oil-well metal pipe 1, the pin sealing surface 42 contacts the box sealing surface 52. Subsequently, when the pin 40 is screwed further into the box 50 of the other oil-well metal pipe 1, the pin sealing surface 42 closely contacts the box sealing surface 52. By this means, during fastening, the pin sealing surface 42 closely contacts the box sealing surface 52 to thereby form a seal that is based on metal-to-metal contact. Therefore, the gastightness can be increased in each of the oil-well metal pipes 1 that are fastened to each other.

[0042] In FIG. 5, the pin shoulder surface 43 is disposed at the front end face of the first end portion 10A. In other words, in the pin 40 illustrated in FIG. 5, the external thread part 41, the pin sealing surface 42 and the pin shoulder surface 43 are disposed sequentially in that order from the center of the pipe main body 10 toward the first end portion 10A. During fastening to another oil-well metal pipe 1, the pin shoulder surface 43 opposes and contacts a box shoulder surface 53 (described later) of the box 50 of the other oil-well metal pipe 1. More specifically, during fastening, the pin shoulder surface 43 contacts the box shoulder surface 53 as a result of the pin 40 being inserted into the box 50 of the other oil-well metal pipe 1. By this means, during fastening, a high torque can be obtained. Further, the positional relation between the pin 40 and the box 50 in the fastening state can be stabilized.

[0043] Note that, the pin contact surface 400 of the pin 40 includes at least the external thread part 41. In other words, the pin contact surface 400 may include the external thread part 41, and need not include the pin sealing surface 42 and the pin shoulder surface 43. The pin contact surface 400 may include the external thread part 41 and the pin shoulder surface 43, and need not include the pin sealing surface 42. The pin contact surface 400 may include the external thread part 41 and the pin sealing surface 42, and need not include the pin shoulder surface 43.

[0044] [Structure of box]FIG. 6 is a cross-sectional view of a portion in the vicinity of the box 50 of the oil-well metal pipe 1 illustrated in FIG. 4, that is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe 1. A dashed line portion in FIG. 6 represents the structure of the pin 40 of another oil-well metal pipe 1 in the case of fastening the oil-well metal pipe 1 to another oil-well metal pipe 1. Referring to FIG. 6, the box 50 includes a box contact surface 500 on the inner peripheral surface of the second end portion 10B of the pipe main body 10. When performing fastening to another oil-well metal pipe 1, the box contact surface 500 contacts the pin contact surface 400 of the pin 40 of the other oil-well metal pipe 1 when the pin 40 is screwed into the box 50.

[0045] The box contact surface 500 includes at least an internal thread part 51 formed in the inner peripheral surface of the second end portion 10B. When performing fastening, the internal thread part 51 engages with the external thread part 41 of the pin 40 of the other oil-well metal pipe 1.

[0046] The box contact surface 500 may further include the box sealing surface 52 and the box shoulder surface 53. In FIG. 6, on the inner peripheral surface of the second end portion 10B, the box sealing surface 52 is disposed further on the pipe main body 10 side than the internal thread part 51. In other words, the box sealing surface 52 is disposed between the internal thread part 51 and the box shoulder surface 53. The box sealing surface 52 is provided in a tapered shape. Specifically, the internal diameter of the box sealing surface 52 gradually decreases from the internal thread part 51 toward the box shoulder surface 53 in the longitudinal direction (pipe axis direction) of the second end portion 10B.

[0047] When performing fastening to another oil-well metal pipe 1, the box sealing surface 52 contacts the pin sealing surface 42 of the pin 40 of the other oil-well metal pipe 1. More specifically, during fastening, when the pin 40 of the other oil-well metal pipe 1 is screwed into the box 50, the box sealing surface 52 contacts the pin sealing surface 42, and when the pin 40 is screwed in further, the box sealing surface 52 closely contacts the pin sealing surface 42. By this means, during fastening, the box sealing surface 52 closely contacts the pin sealing surface 42 to thereby form a seal that is based on metal-to-metal contact. Therefore, the gastightness can be increased in each of the oil-well metal pipes 1 that are fastened to each other.

[0048] The box shoulder surface 53 is disposed further on the pipe main body 10 side than the box sealing surface 52. In other words, in the box 50, the box shoulder surface 53, the box sealing surface 52 and the internal thread part 51 are disposed sequentially in that order from the center of the pipe main body 10 toward the front end of the second end portion 10B. When performing fastening to another oil-well metal pipe 1, the box shoulder surface 53 opposes and contacts the pin shoulder surface 43 of the pin 40 of the other oil-well metal pipe 1. More specifically, during fastening, the box shoulder surface 53 contacts the pin shoulder surface 43 as a result of the pin 40 of the other oil-well metal pipe 1 being inserted into the box 50. By this means, during fastening, a high torque can be obtained. Further, the positional relation between the pin 40 and the box 50 in the fastening state can be stabilized.

[0049] The box contact surface 500 includes at least the internal thread part 51. When performing fastening, the internal thread part 51 of the box contact surface 500 of the box 50 contacts the external thread part 41 of the pin contact surface 400 of the pin 40 in a manner such that the internal thread part 51 corresponds with the external thread part 41. The box sealing surface 52 contacts the pin sealing surface 42 in a manner such that the box sealing surface 52 corresponds with the pin sealing surface 42. The box shoulder surface 53 contacts the pin shoulder surface 43 in a manner such that the box shoulder surface 53 corresponds with the pin shoulder surface 43.

[0050] In a case where the pin contact surface 400 includes the external thread part 41 and does not include the pin sealing surface 42 and the pin shoulder surface 43, the box contact surface 500 includes the internal thread part 51 and does not include the box sealing surface 52 and the box shoulder surface 53. In a case where the pin contact surface 400 includes the external thread part 41 and the pin shoulder surface 43 and does not include the pin sealing surface 42, the box contact surface 500 includes the internal thread part 51 and the box shoulder surface 53 and does not include the box sealing surface 52. In a case where the pin contact surface 400 includes the external thread part 41 and the pin sealing surface 42 and does not include the pin shoulder surface 43, the box contact surface 500 includes the internal thread part 51 and the box sealing surface 52 and does not include the box shoulder surface 53.

[0051] The pin contact surface 400 may include a plurality of the external thread parts 41, may include a plurality of the pin sealing surfaces 42, and may include a plurality of the pin shoulder surfaces 43. For example, the pin shoulder surface 43, the pin sealing surface 42, the external thread part 41, the pin sealing surface 42, the pin shoulder surface 43, the pin sealing surface 42 and the external thread part 41 may be disposed in that order on the pin contact surface 400 of the pin 40 from the front end of the first end portion 10A toward the center of the pipe main body 10. In such case, the internal thread part 51, the box sealing surface 52, the box shoulder surface 53, the box sealing surface 52, the internal thread part 51, the box sealing surface 52 and the box shoulder surface 53 are disposed in that order on the box contact surface 500 of the box 50 from the front end of the second end portion 10B toward the center of the pipe main body 10.

[0052] In FIG. 5 and FIG. 6, a so-called "premium joint" is illustrated in which the pin 40 includes the external thread part 41, the pin sealing surface 42 and the pin shoulder surface 43, and the box 50 includes the internal thread part 51, the box sealing surface 52 and the box shoulder surface 53. However, as described above, the pin 40 may include the external thread part 41 and need not include the pin sealing surface 42 and the pin shoulder surface 43. In this case, the box 50 includes the internal thread part 51 and does not include the box sealing surface 52 and the box shoulder surface 53. FIG. 7 is a view illustrating one example of the oil-well metal pipe 1 in which the pin 40 includes the external thread part 41 and does not include the pin sealing surface and the pin shoulder surface, and the box 50 includes the internal thread part 51 and does not include the box sealing surface and the box shoulder surface. The oil-well metal pipe 1 according to the present embodiment may have the structure illustrated in FIG. 7.

[0053] [Case where oil-well metal pipe is integral type]The oil-well metal pipe 1 illustrated in FIG. 3, FIG. 4 and FIG. 7 is a so-called "T&C type" oil-well metal pipe 1, in which the pipe main body 10 includes the pin tube body 11 and the coupling 12. However, the oil-well metal pipe 1 according to the present embodiment may be an integral type instead of a T&C type.

[0054] FIG. 8 is a configuration diagram of an integral type oil-well metal pipe 1 according to the present embodiment. Referring to FIG. 8, the integral type oil-well metal pipe 1 includes a pipe main body 10. The pipe main body 10 includes a first end portion 10A and a second end portion 10B. The first end portion 10A is disposed on the opposite side to the second end portion 10B. As described above, in the T&C type oil-well metal pipe 1, the pipe main body 10 includes the pin tube body 11 and the coupling 12. In other words, in the T&C type oil-well metal pipe 1, the pipe main body 10 is constituted by fastening two separate members (the pin tube body 11 and the coupling 12). In contrast, in the integral type oil-well metal pipe 1, the pipe main body 10 is formed in an integral manner.

[0055] With reference to FIG. 8, the pin 40 is formed at the first end portion 10A of the pipe main body 10. When performing fastening, the pin 40 is inserted in and screwed into the box 50 of another integral type oil-well metal pipe 1, and thereby fastened to the box 50 of the other integral type oil-well metal pipe 1. The box 50 is formed at the second end portion 10B of the pipe main body 10. When performing fastening, the pin 40 of another integral type oil-well metal pipe 1 is inserted in and screwed into the box 50, to thereby fasten the box 50 to the pin 40 of the other integral type oil-well metal pipe 1.

[0056] The structure of the pin 40 of the integral type oil-well metal pipe 1 is the same as the structure of the pin 40 of the T&C type oil-well metal pipe 1 illustrated in FIG. 5. Similarly, the structure of the box 50 of the integral type oil-well metal pipe 1 is the same as the structure of the box 50 of the T&C type oil-well metal pipe 1 illustrated in FIG. 6. Note that, in FIG. 8, the pin shoulder surface, the pin sealing surface and the external thread part 41 in the pin 40 are disposed in that order from the front end of the first end portion 10A toward the center of the pipe main body 10. Therefore, the internal thread part 51, the box sealing surface and the box shoulder surface in the box 50 are disposed in that order from the front end of the second end portion 10B toward the center of the pipe main body 10. However, similarly to FIG. 5, it suffices that the pin contact surface 400 of the pin 40 of the integral type oil-well metal pipe 1 includes at least the external thread part 41. Further, similarly to FIG. 6, it suffices that the box contact surface 500 of the box 50 of the integral type oil-well metal pipe 1 includes at least the internal thread part 51.

[0057] In short, the oil-well metal pipe 1 according to the present embodiment may be a T&C type or may be an integral type.

[0058] [Chemical composition of pipe main body]In the oil-well metal pipe 1 according to the present embodiment, the chemical composition of the pipe main body 10 is not particularly limited. The pipe main body 10 may have a chemical composition corresponding to carbon steel, may have a chemical composition corresponding to stainless steel, or may have a chemical composition corresponding to Ni-based alloy. In a case where the pipe main body 10 has the chemical composition corresponding to stainless steel, the stainless steel may be, for example, martensitic stainless steel, ferritic stainless steel, ferritic-martensitic duplex stainless steel, ferritic-austenitic duplex stainless steel, or austenitic stainless steel. Further, the pipe main body 10 may be formed with a plating layer on its surface. For example, the pipe main body 10 may include a base material having a chemical composition corresponding to carbon steel and a plating layer formed on a surface of the base material. In this case, the plating layer may be, for example, a Ni plating layer, a Cu plating layer, or a Cr plating layer.

[0059] [Zn-Ni alloy plating layer]In the oil-well metal pipe 1 according to the present embodiment, a Zn-Ni alloy plating layer is formed on the pin contact surface 400 and / or the box contact surface 500. In other words, the Zn-Ni alloy plating layer may be formed on the pin contact surface 400, and need not be formed on the box contact surface 500. Alternatively, the Zn-Ni alloy plating layer may be formed on the box contact surface 500, and need not be formed on the pin contact surface 400. Further, the Zn-Ni alloy plating layer may be formed on the pin contact surface 400 and on the box contact surface 500.

[0060] In the following description, the structure on the pin contact surface 400 in a case where the Zn-Ni alloy plating layer is formed on the pin contact surface 400, and the structure on the box contact surface 500 in a case where the Zn-Ni alloy plating layer is formed on the box contact surface 500 are described.

[0061] [Structure on pin contact surface in case where Zn-Ni alloy plating layer is formed on pin contact surface]FIG. 9 is a cross-sectional view of the vicinity of the pin contact surface 400 in a case where a Zn-Ni alloy plating layer 100 is formed on the pin contact surface 400. Referring to FIG. 9, the oil-well metal pipe 1 further includes the Zn-Ni alloy plating layer 100 formed on the pin contact surface 400 of the pin 40.

[0062] The Zn-Ni alloy plating layer 100 may be formed on one part of the pin contact surface 400 or may be formed on the entire pin contact surface 400. The interfacial pressure increases, in particular, in the final stage of fastening at the pin sealing surface 42. Therefore, in a case where the Zn-Ni alloy plating layer 100 is partially formed on the pin contact surface 400, the Zn-Ni alloy plating layer 100 is preferably formed on at least the pin sealing surface 42. As mentioned above, the Zn-Ni alloy plating layer 100 may be formed on the entire pin contact surface 400.

[0063] [Structure on box contact surface in case where Zn-Ni alloy plating layer is formed on box contact surface]FIG. 10 is a cross-sectional view of the vicinity of the box contact surface 500 in a case where the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500. Referring to FIG. 10, in this case, the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500. The Zn-Ni alloy plating layer 100 may be formed on one part of the box contact surface 500 or may be formed on the entire box contact surface 500. The interfacial pressure increases, in particular, in the final stage of fastening at the box sealing surface 52. Therefore, in a case where the Zn-Ni alloy plating layer 100 is partially formed on the box contact surface 500, the Zn-Ni alloy plating layer 100 is preferably formed on at least the box sealing surface 52.

[0064] [Composition of Zn-Ni alloy plating layer]As described above, the Zn-Ni alloy plating layer 100 is formed on at least one contact surface among the pin contact surface 400 and the box contact surface 500. Here, the Zn-Ni alloy plating layer 100 is composed of a Zn-Ni alloy. Specifically, the Zn-Ni alloy plating layer 100 according to the present embodiment has a chemical composition that consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities. That is, in the present embodiment, a Ni content [Ni] in the Zn-Ni alloy plating layer 100 is 9.0 to 20.0% by mass.

[0065] With an increase in the Ni content [Ni] in the Zn-Ni alloy plating layer 100, the hardness of the Zn-Ni alloy plating layer 100 is increased, and the galling resistance of the oil-well metal pipe 1 is increased. On the other hand, if the Ni content [Ni] is excessively high, a desired precipitate phase which is described later is not obtained. Accordingly, in the present embodiment, the Ni content [Ni] in the Zn-Ni alloy plating layer 100 is made 9.0 to 20.0% by mass.

[0066] A preferable lower limit of the Ni content [Ni] in the Zn-Ni alloy plating layer 100 is, in mass%, 10.0%, more preferably is 11.0%, and further preferably is 12.0%. A preferable upper limit of the Ni content [Ni] in the Zn-Ni alloy plating layer 100 is, in mass%, 19.0%, more preferably is 18.0%, and further preferably is 17.0%. Here, the term "impurities" in the Zn-Ni alloy plating layer refers to substances other than Zn and Ni that are contained in the Zn-Ni alloy plating layer 100 in a process of producing the oil-well metal pipe 1, and the like, and whose contents are within a range that does not influence the effects of the present embodiment.

[0067] In the present embodiment, the chemical composition of the Zn-Ni alloy plating layer 100 can be measured by the following method. A sample including the Zn-Ni alloy plating layer 100 (sample including a contact surface on which the Zn-Ni alloy plating layer 100 is formed) is taken from the oil-well metal pipe 1. The Zn-Ni alloy plating layer 100 of the obtained sample is dissolved in hydrochloric acid at 10% concentration to obtain a liquid solution. The obtained liquid solution is subjected to elemental analysis by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), and the Ni content (mass%) and Zn content (mass%) in the Zn-Ni alloy plating layer 100 are determined.

[0068] [Thickness of Zn-Ni alloy plating layer]In the present embodiment, the thickness of the Zn-Ni alloy plating layer 100 is not particularly limited. The thickness of the Zn-Ni alloy plating layer 100 is, for example, 1 to 20 μm. If the thickness of the Zn-Ni alloy plating layer 100 is 1 μm or more, the galling resistance can be further improved. Even if the thickness of the Zn-Ni alloy plating layer 100 is more than 20 μm, the aforementioned effects will be saturated. The lower limit of the thickness of the Zn-Ni alloy plating layer 100 is preferably 3 μm, and more preferably is 5 μm. The upper limit of the thickness of the Zn-Ni alloy plating layer 100 is preferably 18 μm, and more preferably is 15 μm.

[0069] The thickness of the Zn-Ni alloy plating layer 100 in the present embodiment can be measured by the following method. The thickness of the Zn-Ni alloy plating layer 100 is measured at an arbitrary four locations on the pin contact surface 400 or the box contact surface 500 on which the Zn-Ni alloy plating layer 100 is formed, using an eddy current phase-type coating thickness gauge PHASCOPE PMP10 manufactured by Helmut Fischer GmbH. The measurement is performed by a method conforming to ISO (International Organization for Standardization) 21968 (2005). The measurement locations are four locations (four locations at 0°, 90°, 180° and 270°) in the pipe circumferential direction of the oil-well metal pipe 1. The arithmetic mean value of the measurement results is taken as the thickness of the Zn-Ni alloy plating layer 100.

[0070] [Precipitate phases in Zn-Ni alloy plating layer]In the present embodiment, a precipitate phase of the Zn-Ni alloy plating layer 100 includes a γ phase. In this regard, a precipitate phase of the Zn-Ni alloy plating layer 100 formed by an electroplating process can include an η phase, γ phase, and α phase. Here, the η phase in the Zn-Ni alloy is a phase of a hexagonal crystalline structure that has a chemical formula Zn and has lattice constants of a = 0.267 nm and c = 0.495 nm. The γ phase in the Zn-Ni alloy is a phase of a cubic crystalline structure that has a chemical formula Ni5Zn21 and has a lattice constant of α = 0.890 nm. The α phase in the Zn-Ni alloy is a phase of a face-centered cubic crystalline structure that has a chemical formula Ni and has a lattice constant of a = 0.352 nm.

[0071] As described above, the Zn-Ni alloy plating layer 100 according to the present embodiment has a chemical composition that consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities. Therefore, the precipitate phase of the Zn-Ni alloy plating layer 100 according to the present embodiment stably includes a γ phase. Note that, although the Zn-Ni alloy plating layer 100 has the aforementioned chemical composition, the precipitate phase can include a η phase and an α phase in addition to the γ phase. Accordingly, the precipitate phase of the Zn-Ni alloy plating layer 100 according to the present embodiment includes at least a γ phase.

[0072] Preferably, the precipitate phase in the Zn-Ni alloy plating layer 100 according to the present embodiment is made mainly of a γ phase. In the present description, the phrase "being made mainly of a γ phase" means that the γ phase accounts for 50% or more of the precipitate phase of the Zn-Ni alloy plating layer 100 in volume ratio. Note that preferably the volume ratio of the γ phase is high. Therefore, the volume ratio of the γ phase may be 60% or more, may be 70% or more, may be 80% or more, or may be 90% or more. Most preferably, the precipitate phase in the Zn-Ni alloy plating layer 100 according to the present embodiment is made of a γ phase, with the balance being impurities. That is, it is most preferable that phases other than the γ phase in the precipitate phase in the Zn-Ni alloy plating layer 100 according to the present embodiment are small enough to be negligible.

[0073] The precipitate phase in the Zn-Ni alloy plating layer 100 in the present embodiment can be identified by the following method. A surface of the Zn-Ni alloy plating layer 100 is subjected to X-ray diffraction measurement under the following measurement conditions. The phase can be identified by comparing obtained measured profiles with values listed on the ASTM Card.- Apparatus: RINT-2500 made by Rigaku Corporation- X-ray tube: Co-Kα line- Scan range: 2θ = 10 to 110°- Scan step: 0.02°

[0074] [Crystallite diameter of γ phase in Zn-Ni alloy plating layer]In the present embodiment, a crystallite diameter D (nm) of the γ phase and a Ni content [Ni] (mass%) in the Zn-Ni alloy plating layer 100 satisfy Formula (1):D≤12.3×[Ni]-81.6 (1)where, the crystallite diameter of the γ phase in the Zn-Ni alloy plating layer 100 is substituted, in units of nm, for D in Formula (1); and the Ni content in the Zn-Ni alloy plating layer 100 is substituted, in units of mass%, for [Ni] in Formula (1).

[0075] Here, crystallites mean fine crystals that are present as monocrystals in a polycrystal. In addition, crystal grains are typically defined with grain boundaries forming orientation differences of 15° or more. Therefore, one crystal grain defined in this manner includes a plurality of crystallites. That is, in the present embodiment, the crystallite diameter D of the γ phase is defined as a diameter of a crystallite, which is a structure even finer than a crystal grain in the γ phase.

[0076] No attention has been paid to the crystallite diameter of the Zn-Ni alloy plating layer 100. However, as a result of detailed investigations conducted by the present inventors, it was revealed that, in the Zn-Ni alloy plating layer 100 having the aforementioned chemical composition, the crystallite diameter D (nm) of the γ phase is increased with an increase in the Ni content [Ni] (mass%). Further, in the Zn-Ni alloy plating layer 100 having the aforementioned chemical composition, the fact that a hardness of the Zn-Ni alloy plating layer 100 is increased when the crystallite diameter D (nm) of the γ phase and the Ni content [Ni] (mass%) in the Zn-Ni alloy plating layer 100 satisfy Formula (1) has been demonstrated by the examples which are described later.

[0077] A preferable upper limit of the crystallite diameter D of the γ phase is 12.3×[Ni]-85.0 nm, more preferably is 12.3×[Ni]-88.0 nm, and further preferably is 12.3×[Ni]-90.0 nm. Note that, in the present embodiment, preferably the crystallite diameter D of the γ phase in the Zn-Ni alloy plating layer 100 is small. A lower limit of the crystallite diameter D of the γ phase may be more than 0 nm, may be 10 nm, or may be 20 nm.

[0078] The crystallite diameter of the γ phase in the Zn-Ni alloy plating layer 100 in the present embodiment can be measured by the following method. A surface of the Zn-Ni alloy plating layer 100 is subjected to X-ray diffraction measurement under the same measurement conditions as in the aforementioned identification of the precipitate phase in the Zn-Ni alloy plating layer 100. The measurement locations are four locations (four locations at 0°, 90°, 180° and 270°) in the pipe circumferential direction of the oil-well metal pipe 1.

[0079] From the measured profiles obtained by the X-ray diffraction measurement, a crystallite diameter is calculated based on a diffraction peak of 2θ = 49.0 to 52.0° and the Scherrer's equation (Formula (I)) described hereunder. An arithmetic mean value of the crystallite diameters (nm) measured at the four locations is defined as a crystallite diameter D (nm) of the γ phase in the Zn-Ni alloy plating layer 100. Note that in the Zn-Ni alloy plating layer 100 having the aforementioned chemical composition, the diffraction peak of 2θ = 49.0 to 52.0° is a diffraction peak on a (411) plane of the γ phase.Crystallite diameter (nm) = (0.9×λ) / (B×cosθ) (I)where, a wavelength of an entering X-ray is substituted, in units of nm, for λ in Formula (I), a full width at half maximum of the diffraction peak is substituted, in units of radian, for B in Formula (I), and a Bragg angle at the diffraction peak is substituted, in units of radian, for θ in Formula (I).

[0080] [Vickers hardness of Zn-Ni alloy plating layer]The Zn-Ni alloy plating layer 100 according to the present embodiment has a chemical composition consisting of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities, the precipitate phase of the Zn-Ni alloy plating layer 100 includes a γ phase, and in the Zn-Ni alloy plating layer 100, the crystallite diameter D (nm) of the γ phase and the Ni content [Ni] (mass%) in the Zn-Ni alloy plating layer 100 satisfy Formula (1). As a result, a Vickers hardness HV (Hv) of the Zn-Ni alloy plating layer 100 according to the present embodiment and the Ni content [Ni] (mass%) in the Zn-Ni alloy plating layer 100 satisfy Formula (2).HV≥40×[Ni]-145 (2)where, the Vickers hardness of the Zn-Ni alloy plating layer 100 is substituted, in units of Hv, for HV in Formula (2); and the Ni content in the Zn-Ni alloy plating layer 100 is substituted, in units of mass%, for [Ni] in Formula (2).

[0081] As mentioned above, the Vickers hardness HV of the Zn-Ni alloy plating layer 100 having the aforementioned chemical composition is increased with an increase in the Ni content [Ni]. On the other hand, in the Zn-Ni alloy plating layer 100 according to the present embodiment, the Ni content [Ni] is not simply increased, but the crystallite diameter D is decreased with respect to the Ni content [Ni]. As a result, the Vickers hardness HV of the Zn-Ni alloy plating layer 100 is further increased than when the Ni content [Ni] is simply increased. As a result, the galling resistance of the oil-well metal pipe 1 can be increased.

[0082] A preferable lower limit of the Vickers hardness HV of the Zn-Ni alloy plating layer 100 is 40×[Ni]-140 Hv and further preferably is 40×[Ni]-135 Hv. Note that the Vickers hardness HV of the Zn-Ni alloy plating layer 100 in the present embodiment is preferably made as high as possible. An upper limit of the Vickers hardness HV of the Zn-Ni alloy plating layer 100 may be, for example, 40×[Ni]+150 Hv.

[0083] The Vickers hardness HV of the Zn-Ni alloy plating layer 100 in the present embodiment can be measured by the following method. An oil-well metal pipe 1 on which a Zn-Ni alloy plating layer 100 is formed is cut in a direction perpendicular to an axial direction of the pipe. On an obtained cut surface, a Vickers hardness test is performed on five freely-selected locations on the Zn-Ni alloy plating layer 100 by a method conforming to JIS Z 2244(2009). A test temperature is set at a normal temperature (25°C), and a test force (F) is set at 0.01 N. The measurement is made by using a microhardness tester Fischerscope HM2000 made by Fischer Instruments K.K. Out of five measurement results obtained, an arithmetic mean value of three measurement results excluding a maximum value and a minimum value is determined to be a Vickers hardness HV (Hv) of the Zn-Ni alloy plating layer 100. That is, the Vickers hardness HV according to the present embodiment means Hv0.001.

[0084] [Other optional structures of oil-well metal pipe 1 of the present embodiment]The oil-well metal pipe 1 of the present embodiment may further include, as an upper layer of the Zn-Ni alloy plating layer 100, one or more types selected from the group consisting of a chemical conversion treatment layer and a lubricant coating. Hereunder, the chemical conversion treatment layer and the lubricant coating will be described.

[0085] [Chemical conversion treatment layer]The oil-well metal pipe 1 of the present embodiment may further include a chemical conversion treatment layer, as an upper layer of the Zn-Ni alloy plating layer 100. Referring to FIG. 11, in a case where the Zn-Ni alloy plating layer 100 is formed on the pin contact surface 400, a chemical conversion treatment layer 110 may be formed on the Zn-Ni alloy plating layer 100.

[0086] The chemical conversion treatment layer 110 is not particularly limited, and a well-known chemical conversion treatment layer can be used. The chemical conversion treatment layer 110, for example, may be an oxalate chemical conversion treatment layer, may be a phosphate chemical conversion treatment layer, may be a borate chemical conversion treatment layer, or may be a chromate coating. In a case where the chemical conversion treatment layer 110 is a chromate coating, preferably the chromate coating does not contain hexavalent chromium.

[0087] In some cases the oil-well metal pipe 1 will be stored outdoors for a long period of time until actually being used at the oil well drilling site. In a case where the oil-well metal pipe 1 is exposed to the atmosphere for a long period of time outdoors, the chemical conversion treatment layer 110 enhances the corrosion resistance of the contact surface 400 or 500, and can suppress the occurrence of rust (white rust) at the contact surface 400 or 500. The film thickness of the chemical conversion treatment layer 110 is not particularly limited. The film thickness of the chemical conversion treatment layer 110 is, for example, within the range of 10 to 200 nm.

[0088] [Lubricant coating]The oil-well metal pipe 1 of the present embodiment may further include a lubricant coating as an upper layer of the Zn-Ni alloy plating layer 100. Referring to FIG. 12, in a case where the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500, the lubricant coating 120 may be formed on the Zn-Ni alloy plating layer 100.

[0089] Referring to FIG. 13, in a case where the Zn-Ni alloy plating layer 100 is formed on the pin contact surface 400, a lubricant coating 120 may be formed on the chemical conversion treatment layer 110 formed on the Zn-Ni alloy plating layer 100. Further, referring to FIG. 14, in a case where the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500, the lubricant coating 120 may be formed on the Zn-Ni alloy plating layer 100. That is, referring to FIG. 12 to FIG. 14, the lubricant coating 120 may be formed directly on the Zn-Ni alloy plating layer 100 or may be formed on the chemical conversion treatment layer 110 formed on the Zn-Ni alloy plating layer 100.

[0090] Note that the arrangement of the chemical conversion treatment layer 110 and the lubricant coating 120 is not limited to the arrangements illustrated in FIG. 11 and FIG. 14. In other words, the chemical conversion treatment layer 110 may be formed on the pin contact surface 400 on which the Zn-Ni alloy plating layer 100 is not formed, may be formed on the box contact surface 500 on which the Zn-Ni alloy plating layer 100 is not formed, may be formed on the Zn-Ni alloy plating layer 100 formed on the pin contact surface 400, or may be formed on the Zn-Ni alloy plating layer 100 formed on the box contact surface 500. The lubricant coating 120 may be formed on the pin contact surface 400 on which the Zn-Ni alloy plating layer 100 is not formed, may be formed on the box contact surface 500 on which the Zn-Ni alloy plating layer 100 is not formed, may be formed on the Zn-Ni alloy plating layer 100 formed on the pin contact surface 400, may be formed on the Zn-Ni alloy plating layer 100 formed on the box contact surface 500, may be formed on the chemical conversion treatment layer 110 formed on the pin contact surface 400, may be formed on the chemical conversion treatment layer 110 formed on the box contact surface 500, may be formed on the chemical conversion treatment layer 110 formed on the Zn-Ni alloy plating layer 100 formed on the pin contact surface 400, or may be formed on the chemical conversion treatment layer 110 formed on the Zn-Ni alloy plating layer 100 formed on the box contact surface 500.

[0091] The lubricant coating may be solid, or may be in a semi-solid state or a liquid state. A commercially available lubricant can be used as the lubricant coating. The lubricant coating contains, for example, lubricating particles and a binder. As necessary, the lubricant coating may contain a solvent and other components. The lubricating particles are not particularly limited as long as they are particles having lubricity. The lubricating particles are, for example, one or more types selected from the group consisting of particles of graphite, MoS2 (molybdenum disulfide), WS2 (tungsten disulfide), BN (boron nitride), PTFE (polytetrafluoroethylene), CFx (graphite fluoride), and CaCO3 (calcium carbonate).

[0092] The binder, for example, is one or two types selected from the group consisting of an organic binder and an inorganic binder. The organic binder is, for example, one or two types selected from the group consisting of a thermosetting resin and a thermoplastic resin. The thermosetting resin, for example, is one or more types selected from the group consisting of polyethylene resin, polyimide resin and polyamide-imide resin. The inorganic binder, for example, is one or two types selected from the group consisting of compounds containing alkoxysilane and siloxane bonds. An example of a commercially available lubricant is SEAL-GUARD ECF (trade name) manufactured by JET-LUBE LLC. Other examples of the lubricant coating include a lubricant coating containing rosin, metallic soap, wax or a lubricant powder.

[0093] [Method for producing oil-well metal pipe 1]A method for producing the oil-well metal pipe 1 according to the present embodiment is described hereunder. Note that, as long as the oil-well metal pipe 1 of the present embodiment has the structure described above, a method for producing the oil-well metal pipe 1 is not limited to the following production method. However, the production method described hereunder is one favorable example for producing the oil-well metal pipe 1 of the present embodiment.

[0094] The method for producing the oil-well metal pipe 1 includes a preparation process (S1) of preparing a hollow shell in which the pin 40 or the box 50 is formed, and a Zn-Ni alloy plating layer formation process (S2). Hereunder, each process of the method for producing the oil-well metal pipe 1 of the present embodiment is described in detail.

[0095] [Preparation process (S1)]In the preparation process (S1), a hollow shell in which the pin 40 or the box 50 is formed is prepared. In the present description, the phrase "hollow shell in which the pin or the box is formed" means either of the pipe main body 10 and the pin tube body 11 in a T&C type oil-well metal pipe 1 and the pipe main body 10 in the integral type oil-well metal pipe 1.

[0096] The hollow shell in which the pin 40 or the box 50 is formed is produced, for example, by the following method. A starting material is produced using molten steel. Specifically, a cast piece (a slab, bloom or billet) is produced by a continuous casting process using the molten steel. An ingot may also be produced by an ingot-making process using the molten steel. As necessary, the slab, bloom or ingot may be subjected to blooming to produce a billet. The starting material (a slab, bloom or billet) is produced by the above described process. The prepared starting material is subjected to hot working to produce a hollow shell. The hot working method may be piercing-rolling by means of the Mannesmann process, or may be a hot-extrusion process. The hollow shell after hot working is subjected to well-known quenching and well-known tempering to adjust the strength of the hollow shell. A hollow shell is produced by the above process. Note that, in a case where the oil-well metal pipe 1 is the T&C type, a hollow shell for the coupling 12 is also prepared. The method for producing the hollow shell for the coupling 12 is the same as the method for producing the hollow shell that is described above.

[0097] In a case where the oil-well metal pipe 1 is the T&C type, threading is performed with respect to the outer surface of both end portions of the hollow shell for the pin tube body 11, to form the pin 40 that includes the pin contact surface 400. By means of the above process, a hollow shell (the pin tube body 11) in which the pin 40 is formed is prepared in a case where the oil-well metal pipe 1 is the T&C type. Note that, in a case where the oil-well metal pipe 1 is the T&C type, the coupling 12 may also be prepared. Specifically, threading is performed with respect to the inner surface of both end portions of the hollow shell for the coupling 12, to form the box 50 that includes the box contact surface 500. The coupling 12 is produced by the above process.

[0098] In a case where the oil-well metal pipe 1 is the integral type, threading is performed on the outer surface of the first end portion 10A of the hollow shell to form the pin 40 that includes the pin contact surface 400. In addition, threading is performed with respect to the inner surface of the second end portion 10B of the hollow shell to form the box 50 that includes the box contact surface 500. By means of the above process, in a case where the oil-well metal pipe 1 is the integral type, a hollow shell (pipe main body 10) in which the pin 40 and the box 50 are formed is prepared.

[0099] [Other optional processes]The preparation process (S1) of the present embodiment may further include at least one of a grinding process and an Ni strike plating process.

[0100] In the case of performing a grinding process in the preparation process (S1) according to the present embodiment, for example, a sandblasting treatment, and finishing by machine grinding are performed in the grinding process. The sandblasting treatment is a treatment in which a blast material (abrasive) is mixed with compressed air and the mixture is propelled onto the contact surface. Examples of the blast material include spherical shot material and angular grid material. The surface roughness of the contact surface can be increased by the sandblasting treatment. The sandblasting treatment can be carried out by a well-known method. For example, air is compressed by a compressor, and the blast material is mixed with the compressed air. The blast material may be composed of, for example, stainless steel, aluminum, ceramic material, or alumina. The sandblasting treatment conditions such as the propelling speed are not particularly limited, and can be appropriately adjusted according to well-known conditions.

[0101] In the Ni strike plating process, an Ni strike plating layer is formed on the surface of the hollow shell. The Ni strike plating layer is an extremely thin undercoat plating layer, and increases the adhesion of the Zn-Ni alloy plating layer 100. Note that, the plating bath to be used in the Ni strike plating process is not particularly limited, and a well-known bath can be used. Further, the conditions for forming the Ni strike plating layer are not particularly limited, and can be appropriately adjusted and set.

[0102] Note that, in a case where a Ni strike plating process is performed, a Ni strike plating layer is formed on the contact surface 400 or 500 of the pipe main body 10. That is, in a case where the Ni strike plating layer is formed, the Ni strike plating layer is included in the pipe main body 10.

[0103] [Zn-Ni alloy plating layer formation process (S2)]In the Zn-Ni alloy plating layer formation process (S2), the Zn-Ni alloy plating layer 100 is formed by electroplating on the pin contact surface 400 of the hollow shell in which the pin 40 is formed and / or on the box contact surface 500 of the hollow shell in which the box 50 is formed, after the preparation process (S1).

[0104] In the Zn-Ni alloy plating layer formation process (S2), the Zn-Ni alloy plating layer 100 is formed using a plating bath containing zinc ions and nickel ions. The counter anions to the zinc ions and nickel ions are not particularly limited. For example, chloride ions or sulfate ions may be used as counter anions. That is, in the Zn-Ni alloy plating layer formation process (S2) according to the present embodiment, a chloride bath or a sulfate bath may be used as the plating bath.

[0105] Hereunder, preferable plating conditions will be described. A plating solution contains zinc ion and nickel ion, and a total amount TM of metal ions is preferably made 0.500 to 1.500 mol / L. Further, in the plating solution, a ratio of the nickel ion in the metal ions (content of nickel ion (g / L) / total content of zinc ion and nickel ion (g / L)) is preferably made 30.0 to 55.0%. Preferably, a temperature T of the plating solution is made 40°C or more, and a pH of the plating solution is made 1 to 7. Preferably, a flow rate FR of the plating solution is made 0.2 to 1.5 m / s, and a current density ASD of the plating solution is made 2 to 15 A / dm2. Further preferably, FnA = (10×T+400×FR+700×TM) / ASD, which is given with the temperature T (°C) of the plating solution, the total amount TM (mol / L) of the metal ions in the plating solution, the flow rate FR (m / s) of the plating solution, and the current density ASD (A / dm2) of the plating solution is 165 or more. When the temperature T (°C) of the plating solution, the total amount TM (mol / L) of the metal ions, the flow rate FR (m / s), and the current density ASD (A / dm2) satisfy the aforementioned range, and when FnA satisfies 165 or more, the crystallite diameter D of the γ phase and the Ni content [Ni] in the Zn-Ni alloy plating layer 100 can stably satisfy Formula (1).

[0106] In the Zn-Ni alloy plating layer 100 having the aforementioned chemical composition, the crystallite diameter D of the γ phase is influenced by various factors. Specifically, the temperature T of the plating solution has an influence on a reactivity of a reaction in electroplating. If the temperature T of the plating solution is increased, the reactivity in the electroplating can be increased, and crystallites can be refined. In addition, the total amount TM of the metal ions in the plating solution and the flow rate FR of the plating solution influence an amount of supply of metal ions. Further, the current density ASD of the plating solution influences a power to deposit the supplied metal ions in a form of metals. Therefore, in a Zn-Ni alloy plating layer formation process (S2) according to the present embodiment, the crystallites in the γ phase is refined by adjusting a balance among the temperature T of the plating solution, the total amount TM of the metal ions in the plating solution, the flow rate FR of the plating solution, and the current density ASD of the plating solution. As a result, the crystallite diameter D of the γ phase and the Ni content [Ni] in the Zn-Ni alloy plating layer 100 stably satisfy Formula (1).

[0107] In the case of forming the Zn-Ni alloy plating layer 100 on the pin contact surface 400, the pin contact surface 400 is immersed in the aforementioned plating bath and electroplating is performed. On the other hand, in the case of forming the Zn-Ni alloy plating layer 100 on the box contact surface 500, the box contact surface 500 is immersed in the aforementioned plating bath and electroplating is performed.

[0108] The oil-well metal pipe 1 of the present embodiment having the structure described above is produced by the production processes that are described above. Note that, the aforementioned production processes are one example of production processes for producing the oil-well metal pipe 1 according to the present embodiment, and a method for producing the oil-well metal pipe 1 according to the present embodiment is not limited to the production method described above.

[0109] [Other optional processes]In the method for producing the oil-well metal pipe 1 according to the present embodiment, at least one process among the following chemical conversion treatment process and film formation process may also be performed. These processes are optional processes. Accordingly, these processes need not be performed.

[0110] [Chemical conversion treatment process]In the production method of the present embodiment, as necessary, a chemical conversion treatment process may be performed. That is, the chemical conversion treatment process is an optional process. In the case of performing a chemical conversion treatment process, the chemical conversion treatment layer 110 is formed on the Zn-Ni alloy plating layer 100. In the chemical conversion treatment process, it suffices to perform a well-known chemical conversion treatment. The chemical conversion treatment, for example, may be an oxalate chemical conversion treatment, may be a phosphate chemical conversion treatment, or may be a borate chemical conversion treatment. For example, in the case of performing a phosphate chemical conversion treatment, a chemical conversion treatment that uses zinc phosphate may be performed, a chemical conversion treatment that uses manganese phosphate may be performed, or a chemical conversion treatment that uses calcium zinc phosphate may be performed.

[0111] Specifically, in the case of performing a zinc phosphate chemical conversion treatment, as a treatment solution, for example, a chemical conversion treatment solution containing 1 to 150 g / L of phosphate ions, 3 to 70 g / L of zinc ions, 1 to 100 g / L of nitrate ions, and 0 to 30 g / L of nickel ions can be used. In this case, the temperature of the chemical conversion treatment solution is for example, 20 to 100°C. The chemical conversion treatment layer 110 can be formed by appropriately setting well-known conditions and performing a chemical conversion treatment in this way.

[0112] [Film formation process]As necessary, the production method of the present embodiment may include performance of a film formation process. In other words, the film formation process is an optional process. In the film formation process, a lubricant coating is formed on the Zn-Ni alloy plating layer 100 and / or on the chemical conversion treatment layer 110 and / or on a contact surface (pin contact surface 400 or box contact surface 500) on which the Zn-Ni alloy plating layer 100 is not formed.

[0113] In the film formation process, a lubricant or a composition containing the components of the aforementioned lubricant coating is applied. By this means a lubricant coating is formed. The application method is not particularly limited. Examples of the application method include spray coating, brushing, and immersion. When adopting spray coating as the application method, the composition or lubricant may be heated and then sprayed in a state in which the flowability has been increased. The composition or lubricant is then dried to form a lubricant coating.

[0114] The oil-well metal pipe 1 of the present embodiment are described more specifically hereunder by way of examples. The conditions adopted in the following examples are one example of conditions which are employed for confirming the workability and advantageous effects of the oil-well metal pipe 1 of the present embodiment. Accordingly, the oil-well metal pipe 1 of the present embodiment is not limited to this one example of the conditions. EXAMPLES

[0115] [Plating layer formation process]On commercial cold-rolled steel sheets, Zn-Ni alloy plating layers were formed under different conditions. For a plating solution, a commercial chloride bath for Zn-Ni alloy electroplating ((trade name) DAIN ZINALLOY N2-PL, made by Daiwa Fine Chemicals Co., Ltd.) was used. Specifically, electroplating was performed with various plating conditions within ranges of metal components in the plating solution including total amount of metal ions: 0.500 to 0.900 mol / L, ratio of Ni in the plating solution (content of nickel ion (g / L) / total content of zinc ion and nickel ion (g / L)): 30.0 to 55.0%, and plating solution temperature: 35 to 50°C. Supporting electrolyte and other addition agents were used within ranges of a normal condition of the plating solution used. During the electroplating, the plating solution was circulated by a pump. A circulation amount (flow rate of the plating solution) was changed within a range of 0.2 to 1.2 m / s in terms of a linear velocity of the plating solution. A current density was changed within a range of 4 to 8 A / dm2. In every test number, the thickness of the Zn-Ni alloy plating layer was 7 to 9 μm.

[0116] More specifically, the temperature T of the plating solution of Test Number 1 was 35°C, the temperature T of the plating solution of Test Numbers 2 to 12 was 40°C, and the temperature T of the plating solution of Test Numbers 13 to 23 was 50°C. The total amount of the metal ions in the plating solution of Test Number 1 was 0.600 mol / L, and the total amount of the metal ions in the plating solution of Test Numbers 2 to 23 was 0.891 mol / L. The ratio of Ni in the plating solution of Test Number 1 was 49.4%, and the ratio of Ni in the plating solution of Test Numbers 2 to 23 was 35.7%.

[0117] The flow rate FR of the plating solution of Test Numbers 2, 6, 10, 13, and 17 was 0.2 m / s, the flow rate FR of the plating solution of Test Numbers 1, 3, 7, 14, 18, and 21 was 0.5 m / s, the flow rate FR of the plating solution of Test Numbers 4, 8, 11, 15, 19, and 22 was 0.8 m / s, and the flow rate FR of the plating solution of Test Numbers 5, 9, 12, 16, 20, and 23 was 2.2 m / s. The current density ASD of Test Numbers 2 to 5 and 13 to 16 was 4 A / dm2, the current density ASD of Test Numbers 1, 6 to 9, and 17 to 20 was 6 A / dm2, and the current density ASD of Test Numbers 10 to 12 and 21 to 23 was 8 A / dm2.

[0118] FnA determined from the conditions described above was 162 in Test Number 1, 276 in Test Number 2, 306 in Test Number 3, 336 in Test Number 4, 376 in Test Number 5, 184 in Test Number 6, 204 in Test Number 7, 224 in Test Number 8, 251 in Test Number 9, 138 in Test Number 10, 168 in Test Number 11, 188 in Test Number 12, 301 in Test Number 13, 331 in Test Number 14, 361 in Test Number 15, 401 in Test Number 16, 201 in Test Number 17, 221 in Test Number 18, 241 in Test Number 19, 267 in Test Number 20, 165 in Test Number 21, 180 in Test Number 22, and 200 in Test Number 23.

[0119] By the method described above, the Zn-Ni alloy plating layer was formed on the cold-rolled steel sheet of each test number. The formed Zn-Ni alloy plating layer was subjected to a chemical composition measurement test, a precipitate phase identification test, a γ-phase crystallite diameter measurement test, and a Vickers hardness test. In Test Numbers 1, 3, and 18, a galling resistance evaluation test was further conducted using oil-well metal pipes on which corresponding Zn-Ni alloy plating layers were formed.

[0120] [Chemical composition measurement test]The chemical composition measurement test was conducted on the Zn-Ni alloy plating layer of each test number. The cold-rolled steel sheets with the Zn-Ni alloy plating layers formed thereon were immersed in 10% hydrochloric acid at 25°C for 1 to 3 minutes, and their Zn-Ni alloy plating layers were dissolved. The obtained solution was analyzed with a high-frequency inductively coupled plasma (ICP) emission spectroscopy, ICPS-7510 made by Shimadzu Corporation. The Ni content (mass%) in the Zn-Ni alloy plating layer of each test number is shown in Table 2 as "[Ni] (mass%)".

[0121] Further, from the Ni content [Ni] in the obtained Zn-Ni alloy plating layer, a right side of Formula (1): F1 (=12.3×[Ni]-81.6) and a right side of Formula (2): F2 (=40×[Ni]-145) were determined. The obtained F1 and F2 are shown in Table 2. To convert between analysis values obtained by the ICP emission spectroscopy and analysis values obtained by a fluorescent X-ray analyzer, a calibration curve for the fluorescent X-ray analyzer is created using a standard Zn-Ni alloy plating steel sheet of which a Ni content is determined by the above method using the ICP emission spectroscopy. By performing a fluorescent X-ray analysis for a Ni content in a target material using this calibration curve, it is possible to perform an analysis equivalent to an analysis using the ICP emission spectroscopy.

[0122] [Table 2] TABLE 2     Test Number[Ni] (mass%)F1F2D(nm)HV(Hv)113.483391116386213.07837556-313.68639952553413.68639953448514.59743557446613.07837563392712.77536355431812.57235557420913.078375584091013.686399873841112.26834360-1212.57235561-1314.597435754791414.8100447775791515.5109475855721616.0115495946051714.293423764591814.293423675261914.597435745242014.8100447765112113.989411614712214.293423674652314.39442764460 

[0123] [Precipitate phase identification test]The precipitate phase identification test was conducted on the Zn-Ni alloy plating layer of each test number. Surfaces of the Zn-Ni alloy plating layers were subjected to X-ray diffraction measurement under the following measurement conditions. Their phases were identified by comparing obtained measured profiles with values listed on the ASTM Card. In every test number, a Zn-Ni alloy in the Zn-Ni alloy plating layer has a precipitate phase that was made mainly of a γ phase.- Apparatus: RINT-2500 made by Rigaku Corporation- X-ray tube: Co-Kα line- Scan range: 2θ = 10 to 110°- Scan step: 0.02°

[0124] [γ-phase crystallite diameter measurement test]The γ-phase crystallite diameter measurement test was conducted on the Zn-Ni alloy plating layer of each test number. The X-ray diffraction measurement was performed on four freely-selected locations on each of the surfaces of the Zn-Ni alloy plating layers. Conditions of the X-ray diffraction measurement were made the same as those of the aforementioned precipitate phase identification test. From the obtained measured profiles, crystallite diameters were calculated based on the aforementioned Scherrer's equation with respect to diffraction peaks of 2θ = 49.0 to 52.0°. An arithmetic mean value of measurement results at the four locations was determined to be a crystallite diameter D (nm) of a γ phase in each of the Zn-Ni alloy plating layers. The crystallite diameter D (nm) of the γ phase in the Zn-Ni alloy plating layer of each test number is shown as "D (nm)" in Table 2.

[0125] [Vickers hardness test]The Vickers hardness test was performed on the Zn-Ni alloy plating layer of each test number excluding Test Numbers 2, 11, and 12. The cold-rolled steel sheet of each test number was cut perpendicular to a surface of the cold-rolled steel sheet on which the Zn-Ni alloy plating layer was formed. Their Vickers hardness test was conducted by a method conforming to JIS Z2244(2009) on five freely-selected locations on each of exposed cross sections of the Zn-Ni alloy plating layers. The measurement was made by using a microhardness tester Fischer scope HM2000 made by Fischer Instruments K.K. A test temperature was set at a normal temperature (25°C), and a test force (F) was set at 0.01 N. Out of obtained five measurement results, an arithmetic mean value of three measurement results excluding a maximum value and a minimum value was determined to be a Vickers hardness HV (Hv) in each of the Zn-Ni alloy plating layers. The obtained Vickers hardness HV of each test number is shown in the "HV (Hv)" column of Table 2.

[0126] [Galling resistance evaluation test]In Test Numbers 1, 3, and 18, a galling resistance evaluation test was conducted using oil-well metal pipes on which corresponding Zn-Ni alloy plating layers were formed. Specifically, pins and boxes that were made by forming VAM21 (R) being threaded connections (a pin and a box) on hollow shells having an outer diameter of 177.8 mm and a wall thickness of 11.51 mm, and being of a steel grade SM13CRS-110. The Zn-Ni alloy plating layer corresponding to each of Test Numbers 1, 3, and 18 was formed on the aforementioned pins and boxes.

[0127] Galling resistance evaluation was conducted by repeated make-break test conforming to ISO13679 (2011). Fastening and loosening of each pin and box were repeated at a room temperature (about 25°C) with a fastening torque of 25460 N⋅m. Every repetition of the fastening and the loosening, surfaces of their pin contact surfaces and box contact surfaces were visually observed. When galling was observed on a pin sealing surface and / or a box sealing surface, the test was terminated. When galling was observed on an external thread part and / or an internal thread part, and the galling was such minor that the thread parts could be treated for recovery by filing or the like, the galling flaw was repaired, and then the test was continued. Note that a maximum repetition of the fastening and the loosening was set to ten.

[0128] By performing the repeated make-break test under the conditions described above, a repetition until which neither unrecoverable galling on the external thread part and / or the internal thread part nor galling on the pin sealing surface and / or the box sealing surface was confirmed was defined as a maximum fastening repetition. As a result of the galling resistance evaluation test, as to Test Number 1, the repetition of successful fastening without occurrence of a galling was four. As to each of Test Number 3 and Test Number 18, the repetition of successful fastening without occurrence of a galling was ten.

[0129] [Evaluation results]Referring to Table 2, as to Test Numbers 2 to 9 and 11 to 23, the crystallite diameters D of their γ phases were F1 or less. That is, the crystallite diameters D (nm) of their γ phases and the Ni contents [Ni] (mass%) in their Zn-Ni alloy plating layers satisfied Formula (1). Consequently, as to Test Numbers 3 to 9 and 13 to 23, in which their Vickers hardnesses HV were measured, their Vickers hardnesses HV were F2 or more. That is, the Vickers hardnesses HV (Hv) of their Zn-Ni alloy plating layers and the Ni contents [Ni] (mass%) in their Zn-Ni alloy plating layers satisfied Formula (2).

[0130] In contrast, as to Test Numbers 1 and 10, the crystallite diameters D of their γ phase were more than F1. That is, the crystallite diameters D (nm) of their γ phases and the Ni contents [Ni] (mass%) in their Zn-Ni alloy plating layers did not satisfy Formula (1). Consequently, their Vickers hardnesses HV were less than F2. That is, the Vickers hardnesses HV (Hv) of their Zn-Ni alloy plating layers and the Ni contents [Ni] (mass%) in their Zn-Ni alloy plating layers did not satisfy Formula (2).

[0131] An embodiment of the present disclosure has been described above. However, the foregoing embodiment is merely an example for implementing the present disclosure. Accordingly, the present disclosure is not limited to the above embodiment, and the above embodiment can be appropriately modified within a range which does not deviate from the gist of the present disclosure. REFERENCE SIGNS LIST

[0132] 1Oil-well metal pipe10Pipe Main Body10AFirst End Portion10BSecond End Portion40Pin41External Thread Part50Box51Internal Thread part100Zn-Ni alloy Plating Layer110Chemical Conversion Treatment Layer120Lubricant Coating400Pin Contact Surface500Box Contact Surface

Claims

1. An oil-well metal pipe, comprising:a pipe main body including a first end portion and a second end portion,whereinthe pipe main body includes:a pin formed at the first end portion; anda box formed at the second end portion,the pin includes:a pin contact surface including an external thread part, andthe box includes:a box contact surface including an internal thread part,the oil-well metal pipe further comprising:a Zn-Ni alloy plating layer formed on the pin contact surface and / or the box contact surface,wherein the Zn-Ni alloy plating layer consists of 9.0 to 20.0% by mass of Ni, and with the balance being Zn and impurities,a precipitate phase of the Zn-Ni alloy plating layer includes a γ phase, anda crystallite diameter D of the γ phase and a Ni content [Ni] in the Zn-Ni alloy plating layer satisfy Formula (1):D≤12.3×[Ni]-81.6 (1)where, the crystallite diameter of the γ phase is substituted, in units of nm, for D in Formula (1); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (1).

2. The oil-well metal pipe according to claim 1, whereina Vickers hardness HV of the Zn-Ni alloy plating layer and a Ni content [Ni] in the Zn-Ni alloy plating layer satisfy Formula (2):HV≥40×[Ni]-145 (2)where, the Vickers hardness of the Zn-Ni alloy plating layer is substituted, in units of Hv, for HV in Formula (2); and the Ni content in the Zn-Ni alloy plating layer is substituted, in units of mass%, for [Ni] in Formula (2).

3. The oil-well metal pipe according to claim 1 or claim 2, further comprising:as an upper layer of the Zn-Ni alloy plating layer,one or more types selected from the group consisting of a chemical conversion treatment layer and a lubricant coating. 

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