OA18972A - Screw joint for pipe and manufacturing method for screw joint for pipe - Google Patents

Abstract

There is provided a threaded connection for pipe the tightening torque of which is easy to adjust. A threaded connection for pipe according to the present embodiment includes a pin (3) and a box (4). The pin (3) and the box (4) have contact surfaces (34) and (44) that include thread parts (31) and (41), metal seal parts (32) and (42), and shoulder parts (33) and (43), respectively. The threaded connection for pipe includes a Zn-Ni alloy plating layer (100) on the contact surface (34) or (44) of one of the pin (3) and the box (4), and includes a Cu-Sn-Zn alloy plating layer (110) on the contact surface (34) or (44) of the other one. The threaded connection for pipe further includes a lubricating coating (200) in a liquid state or a semisolid state on at least one of the Zn-Ni alloy plating layer (100) and the Cu-Sn-Zn alloy plating layer (110).

Description

DESCRIPTION

TITLE OF INVENTION

SCREW JOINT FOR PIPE AND MANUFACTURING METHOD FOR SCREW JOINT FOR PIPE

TECHNICAL FIELD

The present invention relates to a threaded connection for pipe and a method for producing a threaded connection for pipe, more specifically relates to a threaded connection for oil country tubular goods and a method for producing a threaded connection for oil country tubular goods.

BACKGROUND ART

For drilling an oil field or a natural gas field, oil country tubular goods are used. The oil country tubular goods are formed by coupling a plurality of steel pipes in proportion to the depth of a well. The Steel pipes are coupled by fastening threaded connections for pipe formed at end portions of the steel pipes. The oil country tubular goods are drawn up and loosened for inspection or the like, and after the inspection, fastened again and reused.

A threaded connection for pipe includes a pin and a box. The pin includes an external thread part and an unthreaded métal contact part that are formed on an outer peripheral surface of a front end portion of a steel pipe. The box includes an internai thread part and an unthreaded métal contact part that are formed on an inner peripheral surface of a front end portion of a steel pipe. Each of the unthreaded métal contact parts includes a métal seal part and a shoulder part. In fastening the steel pipes, the external thread part and the internai thread part are brought into contact with each other, the métal seal parts are brought into contact with each other, and the shoulder parts are brought into contact with each other.

The thread parts and the unthreaded métal contact parts of the pin and the box repeatedly are subjected to strong friction in fastening and loosening the steel pipes. If these régions do not hâve a sufficient durability against friction, galling (unrepairable seizure) occurs when the fastening and loosening are repeated. Therefore, threaded connections for pipe are required to hâve a sufficient durability against friction, namely, an excellent galling résistance.

In conventional practices, to enhance galling résistance, a compound grease, which contains heavy metals, has been used. By applying the compound grease on the surface of a

-2threaded connection for pipe, the galling résistance of the threaded connection for pipe can be improved. However, heavy metals such as Pb, Zn, and Cu contained in the compound grease may hâve an influence on the environment. For this reason, there is a demand for developing a threaded connection for pipe for which no compound grease is used.

There are proposed threaded connections for pipe for which a grease containing no heavy metals (called green dope or yellow dope) is used in place of compound greases. For example, Japanese Patent Application Publication No. 2008-215473 (Patent Literature 1), National Publication of International Patent Application No. 2005-514573 (Patent Literature 2), and Japanese Patent Application Publication No. 2003-074763 (Patent Literature 3) describe threaded connections for pipe that are excellent in galling résistance even when a grease containing no heavy metals is used.

The threaded connection for Steel pipes described in Japanese Patent Application Publication No. 2008-215473 (Patent Literature 1 ) is a threaded connection for Steel pipes that includes a pin and a box each of which has a contact surface including a thread part and an unthreaded métal contact part. A feature of the threaded connection for steel pipes is that the contact surface of at least one of the pin and the box includes a first plating layer made of a CuZn alloy. Patent Literature 1 describes that this feature allows sufficient leakage résistance and galling résistance to exert when a green dope is applied, or even when no dope is applied, and further allows an excellent corrosion résistance to exert, preventing the occurrence of crevice corrosion even when a green dope or a lubricating coating is present on a plating layer.

The tubular member described in National Publication of International Patent Application No. 2005-514573 (Patent Literature 2) includes an internai métal thread and/or an extemal métal thread in at least one of the ends thereof. At least part of a thread ridge ofthe internai thread or extemal thread is covered with an alloy that contains copper and tin. The alloy of copper and tin contains 20% by weight to 80% by weight of copper. Patent Literature 2 describes that this enables the provision of an improved metal-metal seal coupling part dispensing with the use of an environmentally hazardous grease. Furthermore, Patent Literature 2 describes the use of a PTFE-based lubricant or a graphite-based lubricant in coupling tubular members of this type.

The connection for oil country tubular goods described in Japanese Patent Application Publication No. 2003-074763 (Patent Literature 3) is a connection for oil country steel tubular goods that includes a pin part and a coupling, the pin part including an extemal thread and a metal-metal seal part in one end of a steel pipe that contains 9% by mass or more of Cr, the coupling being made of the same material and including, in both ends thereof, box portions each

-3 of which includes an internai thread and a metal-metal seal part. A feature of the connection for oil country tubular goods is that one Cu-Sn alloy layer is disposed on the surfaces of the internai thread and metai-metal seal part of the coupling. Patent Literature 3 describes that this feature makes the sealing ability of the connection better than those of conventional connections even when a green dope is used, and makes it possible to dramatically suppress galling in the connection.

In the techniques disclosed in Patent Literature l to Patent Literature 3 described above, a specified alloy plating layer is formed on a contact surface, whereby galling résistance is enhanced even when a green dope is used. Meanwhile, Japanese Patent Application Publication No. 2008-095019 (Patent Literature 4) and International Application Publication No. W02009/057754 (Patent Literature 5) propose different techniques. Patent Literature 4 and Patent Literature 5 propose greases that substitute for a green dope.

The composite for forming a lubricating coating described in Japanese Patent Application Publication No. 2008-095019 (Patent Literature 4) contains one, or two or more basic oily materials selected from basic sulfonates, basic salicylates, basic phenates, and basic carboxylates. The composite shows a biodegradability (BOD) of 20% or more after 28 days in seawater. Patent Literature 4 describes that the composite shows a galling résistance and an antirust property on a par with those of the case of applying a compound grease, and by virtue of its high biodegradability, the composite can be used in a threaded connection for steel pipes to which no compound grease is applied, without being concemed about environmental pollution even in countries or régions that are strict about environmental régulation.

The composite described in International Application Publication No. W02009/057754 (Patent Literature 5) is a composite for forming a lubricating coating on a threaded connection for pipe. The composite contains one or both of a rosin and a calcium fluoride, a metallic soap, a wax, and a basic métal sait of aromatic organic acid. Patent Literature 5 describes that this provides a lubricating coating that contains no hazardous heavy métal such as lead, which imposes a load on the global environment, and this allows the lubricating coating to provide a galling résistance, an gastightness, and an antirust property, and further to secure a high ΔΤ (torque-on-shoulder résistance).

In addition to the techniques disclosed in the above-mentioned Patent Literature 1 to Patent Literature 5, galling résistance can be improved even with a lubricating coating in a solid state. For example, the threaded connection for steel pipes described in Japanese Patent Application Publication No. 2002-327874 (Patent Literature 6) has a feature that two or more

-4layers of lubricating coating in a solid state containing a lubricating powder are formed on at least one of a pin and a box. Patent Literature 6 describes that this enables the obtain of a threaded connection for steel pipes having excellent galling résistance and gastightness, which makes it unnecessary to apply compound grease containing haevy métal powder.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent Application Publication No. 2008-215473

Patent Literature 2: National Publication of International Patent Application No. 2005514573

Patent Literature 3: Japanese Patent Application Publication No. 2003-074763 Patent Literature 4: Japanese Patent Application Publication No. 2008-095019 Patent Literature 5: International Application Publication No. W02009/057754 Patent Literature 6: Japanese Patent Application Publication No. 2002-327874

SUMMARY OF INVENTION

TECHNICAL PROBLEM

In fastenîng Steel pipes, there is a predetermined optimal torque at which the fastening is to be terminated. FIG. 1 is a graph illustrating the relation between number of tums N of a steel pipe and torque T in fastening a threaded connection for pipe having shoulder parts. Referring to FIG. 1, when the threaded connection for pipe is being fastened, at first, the torque T increases in proportion to number of tums N. The rate of increase of the torque T at this point is low. When the fasten proceeds, the shoulder parts corne into contact with each other. The torque T at this point is called a shouldering torque Ts. When the fasten further proceeds after the torque T reaches the shouldering torque Ts, the torque T increases in proportion to number of tums N again. The rate of increase of the torque T at this point is high. At the time when the torque T reaches a given numeric value (tightening torque To), the fastening is completed. When the shouldering torque Ts in the fastening satisfies 0.05To < Ts < 0.8To, métal seal parts interfère with each other at an appropriate interfacial pressure. In this case, the gastightness of the threaded connection for pipe is increased.

When the fastening is further carried on after the torque T reaches the tightening torque To, the torque T becomes excessively high. When the torque T becomes excessively high, parts of a pin and a box suffer plastic deformation. The torque T at this point is called a yield

-5torque Ty. When a différence between the shouldering torque Ts and the yield torque Ty (hereinafter, will be referred to as a torque-on-shoulder résistance ΔΤ) is larger, a wider range of the tightening torque To is allowed. As a resuit, the tightening torque To is easier to adjust. Therefore, the higher the torque-on-shoulder résistance AT is, the more preferable it is.

Using the techniques described in aforementioned Patent Literature l to Patent Literature enables the galling résistance of a threaded connection for pipe to be improved. However, although providing improved galling résistances, the threaded connections for pipe and the composites for forming lubricating coatings described in Patent Literature I to Patent Literature 6 provide low torque-on-shoulder résistances ΔΤ în some cases.

An objective of the present invention is to provide a threaded connection for pipe the tightening torque To of which is easy to adjust, and to provide a method for producing the threaded connection for pipe.

SOLUTION TO PROBLEM

The threaded connection for pipe in the present embodiment includes a pin and a box, a

Zn-Ni alloy plating layer, and a Cu-Sn-Zn alloy plating layer. The pin and the box each hâve a contact surface that includes a thread part, a métal seal part, and a shoulder part. The Zn-Ni alloy plating layer is disposed on a contact surface of one of the pin and the box. The Cu-SnZn alloy plating layer is disposed on a contact surface of the other one of the pin and the box.

The threaded connection for pipe further includes a lubricating coating in a liquid state or a semisolid State on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer.

A method for producing a threaded connection for pipe according to the present embodiment is a method for producing a threaded connection for pipe including a pin and a box.

The pin and the box each hâve a contact surface that includes a thread part, a métal seal part, and a shoulder part. The producing method includes a Zn-Ni alloy plating layer forming step, a CuSn-Zn alloy plating layer forming step, and a coating forming step. In the Zn-Ni alloy plating layer forming step, a Zn-Ni alloy plating layer made of a Zn-Ni alloy is formed on a contact surface of one of the pin and the box. In the Cu-Sn-Zn alloy plating layer forming step, a Cu30 Sn-Zn alloy plating layer made of a Cu-Sn-Zn alloy is formed on a contact surface of the other one of the pin and the box. In the coating forming step, a lubricating coating in a liquid State or a semisolid State is formed on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer.

-6ADVANTAGEOUS EFFECTS OF INVENTION

The threaded connection for pipe according to the present embodiment includes a Zn-Ni alloy plating layer on a contact surface of one of a pin and a box, and includes a Cu-Sn-Zn alloy plating layer on a contact surface of the other one. Therefore, the threaded connection for pipe provides a high torque-on-shoulder résistance AT, and the tightening torque To thereof is easy to adjust.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. I] FIG. I is a graph illustrating the relation between the number of tums N and torque T in a threaded connection for pipe.

[FIG. 2] FIG. 2 is a diagram illustrating the configuration of a threaded connection for pipe according to the present embodiment.

[FIG. 3] FIG. 3 is a cross-sectional view ofthe threaded connection for pipe according to the present embodiment.

[FIG. 4] FIG. 4 is a cross-sectional view of an example of contact surfaces of the threaded connection for pipe (Pin) according to the present embodiment.

[FIG. 5] FIG. 5 is a cross-sectional view of an example of contact surfaces of the threaded connection for pipe (Box) according to the present embodiment.

[FIG. 6] FIG. 6 is a graph for illustrating a torque-on-shoulder résistance (ΔΤ') in examples.

DESCRIPTION OF EMBODIMENTS

The present embodiment will be described below in detail with reference to the accompanying drawings. The same or équivalent éléments will be denoted by the same reference numerals and the description thereof will not be repeated.

The present inventors conducted variety of studies about the relation between surface treatment on a contact surface of a threaded connection for pipe and torque-on-shoulder résistance AT. Consequently, the following fïndings were obtained.

To increase the torque-on-shoulder résistance ΔΤ, it is effective to decrease a shouldering torque Ts or to increase a yield torque Ty. A lubricating coating in a liquid State or a semisolid state changes its shape in accordance with a receiving pressure. Therefore, the lubricating coating in a liquid State or a semisolid State has a low frictional force in comparison with a lubricating coating in a solid state. A low frictional force makes both the shouldering torque Ts

-7and the yield torque Ty decrease. When galling does not occur, yield torque Ty decreases remarkably. As a resuit, torque-on-shoulder résistance ΔΤ decreases.

Meanwhile, as the receiving pressure increases, the lubricating coating in a liquid State or a semisolid state decreases in thickness. In a final phase of the fastening, a high pressure is applied to the lubricating coating. Therefore, the lubricating coating is tom locally, and base metals corne into direct contact with each other. The Chemical compositions of the base metals are normally the same or close to each other. In this case, galling is likely to occur. When galling occurs locally, a load imposed on a threaded connection for pipe rapidly rises with a small number of tums N of the fastening. For this reason, the adjustment of the tightening torque To becomes difficult.

In the case of the lubricating coating in a solid State as in Patent Literature 6, since the deformation amount of the lubricating coating corresponding to the pressure is small, the abovementioned problem hardly occurs. On the other hand, as described above, in the case of the lubricating coating in a liquid state or a semisolid State, the base metals are easily in direct 15 contact with each other during the fastening.

The present inventors found that by disposing specified alloy plating layers on contact surfaces of a pin and a box, torque-on-shoulder résistance ΔΤ increases. Specifically, on a contact surface of one of the pin and the box, a Zn-Ni alloy plating layer made of a Zn-Ni alloy is disposed. On a contact surface of the other one of the pin and the box, a Cu-Sn-Zn alloy 20 plating layer made of a Cu-Sn-Zn alloy is disposed. This disposition allows a yield torque Ty to be kept high.

The reason that the disposition of the aforementioned alloy plating layers allows the yield torque Ty to be kept high is conjectured as follows. Zn-Ni alloy plating and Cu-Sn-Zn alloy plating hâve appropriate coefficients of friction. Therefore, when the Zn-Ni alloy plating layer 25 and the Cu-Sn-Zn alloy plating layer corne into contact with each other, the frictional force therebetween increases, and the yield torque Ty increases. Furthermore, the Zn-Ni alloy plating and the Cu-Sn-Zn alloy plating hâve high hardnesses and fusing points. Therefore, even in a final phase of the fastening, the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer suffer minor damage, and a direct contact between base metals is inhibited. As a resuit, the 30 yield torque Ty is kept high, which facilitâtes the adjustment of the tightening torque To.

The threaded connection for pipe in the present embodiment, which is completed based on the above findings, includes a pin and a box, a Zn-Ni alloy plating layer, and a Cu-Sn-Zn alloy plating layer. The pin and the box each hâve a contact surface that includes a thread part,

-8a métal seal part, and a shoulder part. The Zn-Ni alloy plating layer is disposed on a contact surface of one of the pin and the box. The Cu-Sn-Zn alloy plating layer is disposed on a contact surface of the other one of the pin and the box. The threaded connection for pipe further includes a lubricating coating in a liquid state or a semisolid state on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer.

The threaded connection for pipe according to the present embodiment includes the Zn-Ni alloy plating layer on the contact surface of one of a pin and a box, and includes the Cu-Sn-Zn alloy plating layer on the contact surface of the other one. Therefore, the threaded connection for pipe provides a high torque-on-shoulder résistance ΔΤ, and the tightening torque To thereof is easy to adjust.

It is preferable that the Zn-Ni alloy plating layer mentioned above has a thickness of l to 20 pm, the Cu-Sn-Zn alloy plating layer has a thickness of l to 20 pm, and the lubricating coating has a thickness of 30 to 300 pm.

In this case, the torque-on-shoulder résistance ΔΤ increases more stably.

Preferably, the Zn-Ni alloy mentioned above consists of 85 to 91 mass % ofZn and 9 to 15 mass % of Ni, with the balance being impurities.

Preferably, the Cu-Sn-Zn alloy mentioned above consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

A method for producing the threaded connection for pipe according to the present embodiment is a method for producing a threaded connection for pipe including a pin and a box. The pin and the box each hâve a contact surface that includes a thread part, a métal seal part, and a shoulder part. The producing method includes a Zn-Ni alloy plating layer forming step, a CuSn-Zn alloy plating layer forming step, and a coating forming step. In the Zn-Ni alloy plating layer forming step, a Zn-Ni alloy plating layer made of a Zn-Ni alloy is formed on a contact surface of one of the pin and the box. In the Cu-Sn-Zn alloy plating layer forming step, a CuSn-Zn alloy plating layer made of a Cu-Sn-Zn alloy is formed on a contact surface of the other one of the pin and the box. In the coating forming step, a lubricating coating in a liquid State or a semisolid state is formed on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer.

By the producing method according to the present embodiment, it is possible to produce a threaded connection for pipe that includes a Zn-Ni alloy plating layer on a contact surface of one of a pin and a box, and includes a Cu-Sn-Zn alloy plating layer on a contact surface of the other

-9one. The threaded connection for pipe provides a high torque-on-shoulder résistance ΔΤ, and the tightening torque To thereof is easy to adjust.

Description will be made below about a threaded connection for pipe according to the présent embodiment and a method for producing the threaded connection for pipe.

[Threaded Connection for Pipe]

A threaded connection for pipe includes a pin and a box. FIG. 2 is a diagram illustrating the configuration of a threaded connection for pipe according to the présent embodiment. Referring to FIG. 2, the threaded connection for pipe includes a Steel pipe l and a coupling 2. At the both ends of the Steel pipe l, a pin 3 is formed that includes an external thread part on its outer surface. At the both ends of the coupling 2, a box 4 is formed that includes an internai thread part on its inner surface. By fastening the pin 3 and the box 4, the coupling 2 is attached to an end of the Steel pipe l. Besides, there is an integral-type threaded connection for oil country tubular goods, which does not include a coupling 2 but includes a pin 3 provided at one end of the Steel pipe l and a box 4 provided at the other end. The threaded connection for pipe according to the présent embodiment is available for both of coupling-type threaded connections for pipe and integral-type threaded connections for pipe.

The pin 3 and the box 4 each hâve a contact surface that includes a thread part, a métal seal part, and a shoulder part. FIG. 3 is a cross-sectional view of the threaded connection for pipe according to the présent embodiment. Referring to FIG. 3, the pin 3 includes an external thread part j l, a métal seal part 32, and a shoulder part 33. The box 4 includes an internai thread part 41, a métal seal part 42, and a shoulder part 43. The parts that corne into contact with each other when the pin 3 and the box 4 are fastened are referred to as contact surfaces 34 and 44. Specifically, when the pin 3 and the box 4 are fastened, the thread parts (the external thread part 31 and the internai thread part 4i ) corne into contact with each other, the métal seal parts (the métal seal parts 32 and 42) corne into contact with each other, and the shoulder parts (the shoulder parts 33 and 43) corne into contact with each other. In other words, the contact surfaces 34 and 44 include the thread parts 3 l and 41, the meta! seal parts 32 and 42, and the shoulder parts 33 and 43, respectively.

FIG. 4 and FIG. 5 is a cross-sectional view of an example of the contact surfaces 34 and 44 of the threaded connection for pipe according to the présent embodiment. The alloy plating layers of the threaded connection for pipe according to the présent embodiment differs on the contact surface of the pin 3 and on the contact surface of the box 4. In FIG. 4, the threaded connection for pipe includes a Zn-Ni alloy plating layer I00 on the contact surface 34 ofthe pin

- ιο3. In FIG. 5, the threaded connection for pipe includes a Cu-Sn-Zn alloy plating layer 110 on the contact surface 44 of the box 4. The threaded connection for pipe further includes lubricating coatings 200 on both of the Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110.

The disposition of the alloy plating layers according to the présent embodiment is not limited to FIG. 4 and FIG. 5. The Cu-Sn-Zn alloy plating layer 110 may be disposed on the contact surface 34 of the pin 3, and the Zn-Ni alloy plating layer 100 may be disposed on the contact surface 44 of the box 4.

Normally, métal plating is applied to threaded connection for pipe for the purpose of improving galling résistance or corrosion résistance. In this case, the threaded connection for pipe has a plating layer only on either the contact surface of the pin 3 or the contact surface of the box 4. Even with plating layers on both contact surfaces of the pin 3 and the box 4, the threaded connection for pipe has the plating layers of the same Chemical composition on the contact surfaces of the pin 3 and the box 4. However, the threaded connection for pipe according to the présent embodiment includes alloy plating layers of different Chemical compositions on the contact surface of the pin 3 and on the contact surface of the box 4. Furthermore, each of the alloy plating layer of the threaded connection for pipe according to the présent embodiment has spécifie Chemical composition.

[Zn-Ni Alloy Plating Layer]

The Zn-Ni alloy plating layer 100 is disposed on a contact surface of one of the pin 3 and the box 4. The Zn-Ni alloy plating layer 100 is made of a Zn-Ni alloy. The Zn-Ni alloy consists of Zn and Ni, with the balance being impurities. Preferably, the Zn-Ni alloy consists of 85 to 91 mass % of Zn and 9 to 15 mass % of Ni, with the balance being impurities.

The Chemical composition of the Zn-Ni alloy plating layer 100 is analyzed by the following method. Arbitrary 5 measurement points of the Zn-Ni alloy plating layer 100 are measured by using a handheld fluorescent X-ray analyzer (DELTA Premium DP-2000) manufactured by Olympus Innovex Co., Ltd. The measurement conditions are as follows. Xray beam diameter: 3 mm, inspection time: 120 seconds, measurement mode: Bl, analysis software: Alloy Plus. The arithmetic average of the measurement results of the 5 measurement points is taken as the Chemical composition of the Zn - Ni alloy plating layer 100.

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, I to 20 pm. When the thickness of the Zn-Ni alloy plating layer 100 is l pm or more, the torque-on-shoulder résistance ΔΤ can

- 11 be increased stably. When the thickness of the Zn-Ni alloy plating layer 100 becomes more than 20 pm, the above effect is however saturated.

The thickness of the Zn-Ni alloy plating layer 100 is measured by the following method. One of the contact surfaces 34 and 44, on which the Zn-Ni alloy plating layer 100 is formed, is brought into contact with a probe of a film thickness gauge of a phase-sensitive eddy-current type that is compilant with International Organization for Standardization (ISO) 21968 (2005). Measurement is made on a phase différence between a high-frequency magnetic field on an input side of the probe and an eddy current on the Zn-Nt alloy plating layer 100 that is excited by the high-frequency magnetic field. The phase différence is converted into the thickness of the ZnNi alloy plating layer 100.

The threaded connection for pipe may include the Zn-Ni alloy plating layer 100 partially on one of the contact surface 34 or 44 of the pin 3 and the box 4. The threaded connection for pipe may include the Zn-Ni alloy plating layer 100 entirely on one of the contact surface 34 or 44 of the pin 3 and the box 4. The métal seal parts 32 and 42 are subjected to a high interfacial pressure in particular in a final phase of the fastening, Therefore, in the case of disposing the Zn-Ni alloy plating layer 100 partially on one of the contact surfaces 34 and 44, it is préférable to dispose the Zn-Ni alloy plating layer 100 on one of the métal seal parts 32 and 42. That is, the threaded connection for pipe may include the Zn-Ni alloy plating layer 100 only on the métal seal part 32 or 42 on the contact surface 34 or 44 of one of the pin 3 and the box 4. Meanwhile, when the Zn-Ni alloy plating layer 100 is disposed entirely on one of the contact surfaces 34 and 44, the production efficiency of the threaded connection for pipe is increased.

[Cu-Sn-Zn Alloy Plating Layer]

The Cu-Sn-Zn alloy plating layer 110 is disposed on a contact surface of one of the pin 3 and the box 4 on which the Zn-Ni alloy plating layer 100 is not formed. The Cu-Sn-Zn alloy plating layer 110 is made of a Cu-Sn-Zn alloy. The Cu-Sn-Zn alloy consists of Cu, Sn, and Zn, with the balance being impurities. Preferably, the Cu-Sn-Zn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities. The Chemical composition of the Cu-Sn-Zn alloy plating layer 110 is analyzed by the same method as the Zn-Ni alloy plating layer 100 mentioned above.

The thickness of the Cu-Sn-Zn alloy plating layer 110 is not particularly limited. The thickness of the Cu-Sn-Zn alloy plating layer 110 is, for example, l to 20 pm. When the thickness of the Cu-Sn-Zn alloy plating layer 110 is l pm or more, the torque-on-shoulder résistance AT can be increased stably. When the thickness of the Cu-Sn-Zn alloy plating layer

- 12 110 becomes more than 20 pm, the above effect is however saturated. The thickness of the CuSn-Zn alloy plating layer 110 is measured by the same method as that for the Zn-Ni alloy plating layer 100 mentîoned above.

As with the Zn-Ni alloy plating layer 100, the Cu-Sn-Zn alloy plating layer 110 may be disposed partially or entirely on one of the contact surfaces 34 and 44. That is, the threaded connection for pipe may include the Cu-Sn-Zn alloy plating layer 110 partially on one of the contact surface 34 or 44 of the pin 3 and the box 4 on which the Zn-Ni alloy plating layer 100 is not formed. The threaded connection for pipe may include the Cu-Sn-Zn alloy plating layer 110 entirely on one of the contact surface 34 or 44 of the pin 3 and the box 4 on which the Zn-Ni alloy plating layer 100 is not formed. In the case of disposing the Cu-Sn-Zn alloy plating layer 110 partially on one of the contact surfaces 34 and 44, it is préférable to dispose the Cu-Sn-Zn alloy plating layer 110 on one of the métal seal parts 32 and 42. That is, the threaded connection for pipe may include the Cu-Sn-Zn alloy plating layer 110 only on one ofthe métal seal parts 32 or 42 on the contact surface 34 or 44 of the pin 3 and the box 4, on which the Zn-Ni alloy plating layer 100 is not formed. Meanwhîle, when the Cu-Sn-Zn alloy plating layer 110 is disposed entirely on one of the contact surfaces 34 and 44, the production efficiency of the threaded connection for pipe is increased.

The Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110 hâve high hardnesses and fusing points. Therefore, even when fastening and loosening are repeated, damage to the Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110 is inhibited. Consequently, the yield torque Ty is kept high even when fastening and loosening are repeated.

Furthermore, zinc (Zn) contained in the Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110 is a base métal in comparison with iron (Fe), which is the principal component of the Steel pipe. Therefore, the Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110 hâve the effect of sacrificial protection, increasing the corrosion résistance of the threaded connection for pipe.

[Lubricating coating]

The threaded connection for pipe includes a lubricating coating 200 on at least one ofthe Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110. As shown in FIG. 4 and FIG. 5, the threaded connection for pipe may include the lubricating coating 200 in a liquid state or a semisolid state both on the Zn-Ni alloy plating layer 100 and on the Cu-Sn-Zn alloy plating layer 110. The threaded connection for pipe may include the lubricating coating 200 in

- 13 a liquid State or a semisolid state only on the Zn-Ni alloy plating layer 100. The threaded connection for pipe may include the lubricating coating 200 in a liquid state or a semisolid state only on the Cu-Sn-Zn alloy plating layer 110.

The lubricating coating 200 is in a liquid state or a semisolid state. Here, the liquid state refers to a state of substance, which has a constant volume, but does not hâve a certain shape. When the lubricating coating 200 is in a liquid state, the lubricating coating 200 can change its shape freely and flow on the contact surface 34 and 44. The semisolid state refers to a state in which the lubricating coating 200 can flow on the contact surfaces 34 and 44 under an external load (pressure, heat, etc.) while changing its shape freely, as with the liquid state. Examples in the liquid state or the semisolid state include high-viscosity substances such as grease.

The lubricating coating 200 contains a well-known lubricant. Examples of the lubricant include SEAL-GUARD (TM) ECF (TM) from JET-LUBE Inc. Other examples of the lubricant include a lubricant that contains a rosin, a metallic soap, a wax, and a lubricant powder. The lubricant powder is, for example, an earthy graphite. The Chemical compositions of the lubricating coating 200 disposed on the Zn-Ni alloy plating layer 100 and the lubricating coating 200 disposed on the Cu-Sn-Zn alloy plating layer 110 may be the same or different from each other.

The thickness of the lubricating coating 200 is not particularly limited. The thickness of the lubricating coating 200 is, for example, 30 to 300 pm. When the thickness ofthe lubricating coating 200 is 30 pm or more, the effect of decreasing the shouldering torque Ts is further increased. When the thickness of the lubricating coating 200 becomes more than 300 pm, the above effect is however saturated because a surplus of the lubricating coating 200 is removed from the contact surfaces 34 and 44 in the fastening.

The thickness of the lubricating coating 200 is measured by the following method. An arbitrary measurement point (area: 5 mm * 20 mm) of the métal seal portion 32 or 42 ofthe threaded connection for pipe is wiped off with absorbent cotton impregnated with éthanol. The amount of application of the lubricant is calculated from the différence between the weight ofthe absorbent cotton before wiping and the weight of the absorbent cotton after wiping. From the amount of application of the lubricant, the density of the lubricant, and the measurement point area, the average coating thickness of the lubricating coating 200 is calculated.

Unlike a lubricating coating in a solid state, the lubricating coating 200 in a liquid state or a semisolid state changes its shape under pressure. Therefore, the lubricating coating 200 generates a low frictional force. As a resuit, the shouldering torque Ts is decreased.

- 14Furthermore, unlike a lubricating coating in a solid State, the lubricating coating 200 in a liquid State or a semisolid State is not abraded even when fastening and loosening are repeated. In the fastening, the lubricating coating 200 in a liquid State or a semisolid State changes its shape and moves to a place where interfacial pressure is low. Then, in loosening, the lubricating coating 200 spreads out on the entire contact surface again. Therefore, the shouldering torque Ts of the threaded connection for pipe according to the present embodiment is kept low even when fastening and loosening are repeated.

Further, unlike a lubricating coating in a solid State, the lubricating coating 200 in a liquid State or a semisolid State partially breaks or moves to a place where interfacial pressure is low when the interfacial pressure is high (for example, in the fastening). Therefore, the Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110 are contact each other from the beginning of the fastening. As a resuit, friction is increased without causing galling, and the yield torque Ty increased.

[Base métal of Threaded Connection for Pipe]

The Chemical composition of a base métal of the threaded connection for pipe is not particularly limited. Examples of the base métal include carbon steels, stainless steels, and alloy steels. Of the alloy steels, high alloy steels such as Ni alloys and duplex stainless steels containing alloying éléments such as Cr, Ni, and Mo hâve high anticorrosion properties. Therefore, when these high alloy steels are used as the base métal, the corrosion résistance of the threaded connection for pipe is increased.

[Producing Method]

The method for producing a threaded connection for pipe according to the present embodiment is a method for producing the threaded connection for pipe mentioned before. The producing method includes a Zn-Ni alloy plating layer 100 forming step, a Cu-Sn-Zn alloy plating layer 110 forming step, and a coating forming step.

[Zn-Ni Alloy Plating Layer Forming Step]

In the Zn-Ni alloy plating layer 100 forming step, the Zn-Ni alloy plating layer 100 made of a Zn-Ni alloy is formed on a contact surface of one of the pin 3 and the box 4. The Zn-Ni alloy plating layer 100 is formed by electrolytic plating. The electrolytîc plating is performed in such a manner that the contact surface of one of the pin 3 and the box 4 is immersed in a plating bath containing zinc ions and nickel ions, and energized. The plating bath preferably contains zinc ion: l to I00 g/L and nickel ion: I to 50 g/L. The conditions for the electrolytic plating can be set as appropriate. The conditions for the electrolytic plating are, for example, a

- 15platîng bath pH: l to 10, a plating bath température: 10 to 60°C, a current density: l to 100

A/dnr, and a time period of the treatment: 0.1 to 30 minutes.

[Cu-Sn-Zn Alloy Plating Layer Forming Step]

In the Cu-Sn-Zn alloy plating layer 110 forming step, the Cu-Sn-Zn alloy plating layer 110 made of a Cu-Sn-Zn alloy is formed on a contact surface of one of the pin 3 and the box 4 on which the Zn-Ni alloy plating layer 100 is not formed. The Cu-Sn-Zn alloy plating layer 110 is formed by electrolytic plating. The electrolytic plating is performed in such a manner that the contact surface of one of the pin 3 and the box 4 on which the Zn-Ni alloy plating layer 100 is not formed is immersed in a plating bath containing copper ion, tin ion, and zinc ion, and energized. The plating bath preferably contains copper ion: l to 50 g/L, tin ion: l to 50 g/L, and zinc ion: l to 50 g/L. The conditions for the electrolytic plating can be set as appropriate. The conditions for the electrolytic plating are, for example, a plating bath pH: l to 14, a plating bath température: 10 to 60°C, a current density: l to 100 A/dm², and a time period of the treatment: 0.1 to 40 minutes.

[Coating Forming Step]

After the alloy plating layers mentioned above are formed on the contact surfaces of the pin 3 and the box 4, the coating forming step is performed. In the coating forming step, the lubricating coating 200 in a liquid State or a semisolid state is formed on at least one of the Zn-Ni alloy plating layer 100 and the Cu-Sn-Zn alloy plating layer 110.

The lubricating coating 200 is formed by applying the lubricant mentioned above on the alloy plating layers mentioned above. The method for the application is not particularly limited. Examples of the method for the application include spray application, brush application, and immersion. In the case of adopting the spray application, the lubricant may be sprayed while being heated and increased in fluidity. The lubricating coating 200 may be formed on part of the contact surfaces but is preferably formed on the entire contact surfaces uniformly. The coating forming step may be performed on one or both of the pin 3 or the box 4 including the Zn-Ni alloy plating layer 100, and the pin 3 or the box 4 including the Cu-Sn-Zn alloy plating layer 110.

[Preconditioning Treatment Step]

The producing method may include a preconditioning treatment step before the Zn-Ni alloy plating layer 100 forming step and the Cu-Sn-Zn alloy plating layer 110 forming step, as necessary. Examples of the preconditioning treatment step include pickling and alkaline degreasing. In the preconditioning treatment step, oil content and the like adhered to the • Ιόcontact surface is removed. The preconditioning treatment step may further include grinding work such as sandblast and mechanical grinding finishing. Only one of these preconditioning treatments may be performed, or more than one of these preconditioning treatments may be performed in combination.

EXAMPLES

Hereinafter, examples will be described. In the examples, the contact surface of a pin will be referred to as a pin surface, and the contact surface of a box will be referred to as a box surface. In addition, the Symbol % in the examples means mass percent.

A Steel pipe used in the present examples was VAM21HT (R) from Nippon Steel &

Sumitomo Métal Corporation. The steel pipe had an outer diameter: 127.00 mm (5 inches) and a wall thickness: 9.19 mm (0.362 inches). The type of the steel pipe was l3Cr steel. The Chemical composition of the steel pipe was C: 0.19%, Si: 0.25%, 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.

A pin surface and a box surface of each test number were subjected to the mechanical grinding finishing (at a surface roughness Ra: 3 pm). Subsequently, the preconditioning treatment shown in Table 1 was performed, the plating layers and the lubricating coating shown in Table 1 were formed, whereby the pin and the box of each test number were prepared.

- 17 [Table l]

TABLE l

TEST NUMBER.		PRECONDITIONING TREATMENT	PLATING LAYER (THICKNESS)	LUBRICATING COATING (THICKNESS)
!	PIN SURFACE	-	Zn-Ni ALLOY PLATING LAYER (4.9 μπι)	LUBRICATING COATING J (I20 μιη)
BOX SURFACE	-	Cu-Sn-Zn ALLOY PLATING LAYER (8.0 μπι)	LUBRICATING COATING J (120 μιτι)
2	PIN SURFACE	-	Zn-Ni ALLOY PLATING LAYER (4.9 μπι)	LUBRICATING COATING S (80 μιπ)
BOX SURFACE	-	Cu-Sn-Zn ALLOY PLATING LAYER (8.0 μπι)	LUBRICATING COATING S (80 μπι)
3	PIN SURFACE	-	Cu-Sn-Zn ALLOY PLATING LAYER (8.0 μιπ)	LUBRICATING COATING S (80 μιτι)
BOX SURFACE	-	Zn-Ni ALLOY PLATING LAYER (5.0 μπι)	LUBRICATING COATING S (80 μπι)
4	PIN SURFACE	SANDBLAST		LUBRICATING COATING D (120 μπι)
BOX SURFACE	-	Cu PLATING LAYER (7.0 μιτι)	LUBRICATING COATING D (120 μιτι)
5	PIN SURFACE	SANDBLAST	-	LUBRICATING COATING J ( 120 μπι)
BOX SURFACE	•	Cu PLATING LAYER (7.0 μτη)	LUBRICATING COATING J (120 μτη)
6	PIN SURFACE	SANDBLAST	-	LUBRICATING COATING J ( 120 μιπ)
BOX SURFACE	-	Cu-Sn-Zn ALLOY PLATING LAYER (8.0 uni)	LUBRICATING COATING J (120 μιπ)
7	PIN SURFACE	SANDBLAST	-	LUBRICATING COATING S (80 μπι)
BOX SURFACE	-	Zn-Ni ALLOY PLATING LAYER (5.0 μιτι)	LUBRICATING COATING S (80 μπι)
8	PIN SURFACE	-	Zn-Ni ALLOY PLATING LAYER (4.9 μιτι)	LUBRICATING COATING S (80 μπι)
BOX SURFACE	-	Zn-Ni ALLOY PLATING LAYER (5.0 μιτι)	LUBRICATING COATING S (80 μιη)

[Preconditioning Treatment Step]

The pin surface of test number 4 to test number 7 was subjected to sandblast as the preconditioning treatment. The surface roughness of each pin after the sandblast was an arithmetic average roughness Ra of l .0 pm and a maximum height roughness Rz of 5.2 pm, the arithmetic average roughness Ra and the maximum height roughness Rz being specifïed in JIS B0601:2013.

[Plating Layer Forming Step]

On the pin surface and the box surface of each test number, the plating layers were formed. The formation of the plating layers was performed by electrolytic plating. The

- 18detailed producing conditions for the plating layers of each test number were those shown in Table 2. In the case of forming the Zn-Ni alloy plating layer as the plating layer (for the pin surfaces of test number l, test number 2, and test number 8, and the box surfaces of test number 3, test number 7, and test number 8), use was made of trade name: DAIN Zinalloy N-PL from

Daiwa Fine Chemicals Co., Ltd. as the plating bath, and an electrolytic bath was initially madeup. In the case of forming the Cu-Sn-Zn alloy plating layer as plating layer (for the pin surface of test number 3, and the box surfaces of test number l, test number 2, and test number 6), use was made of a plating bath from NIHON KAGAKU SANGYO CO., LTD. as the plating bath. In the case of forming the Cu plating layer as the plating layer (for the box surfaces of test number 4 and test number 5), use was made of a copper cyanide bath containing copper ion: 20 to 50 g/L and free cyanide: 7 to 20 g/L as the plating bath.

- 19[Table 2]

TABLE2

TEST NUMBER		PLATING LAYER	PLATING BATH PH	PLATING BATH TEMPERATURE (°C)	CURRENT DENSITY (AAInr)	TREATMENT TIME (min)	PLATING LAYER COMPOSITION
l	PIN SURFACE	Zn-Ni ALLOY PLATING LAYER	6.5	25	2	18	Zn: 85.3%, Ni: 14.7%
BOX SURFACE	Cu-Sn-Zn ALLOY PLATING LAYER	I4	45	2	40	Cu: about 63%, Sn: about 30%. Zn: about 7%
2	PIN SURFACE	Zn-Ni ALLOY PLATING LAYER	6.5	25	2	18	Zn:89.5%, Ni: 10.5%
BOX SURFACE	Cu-Sn-Zn ALLOY PLATING LAYER	14	45	2	40	Cu: about 63%, Sn: about 30%. Zn: about 7%
3	PIN SURFACE	Cu-Sn-Zn ALLOY PLATING LAYER	I4	45	2	40	Cu: about 63%, Sn: about 30%, Zn: about 7%
BOX SURFACE	Zn-Ni ALLOY PLATING LAYER	6.5	25	2	18	Zn:85.3%, Ni: 14.7%
4	PIN SURFACE	-	-	-	-	-	-
BOX SURFACE	Cu PLATING LAYER	I4	60	3	20	Cu:l00%
5	PIN SURFACE	-	-	-	-	•	-
BOX SURFACE	Cu PLATING LAYER	I4	60	3	20	Cu:100%
6	PIN SURFACE	-	-	-	*		-
BOX SURFACE	Cu-Sn-Zn ALLOY PLATING LAYER	14	45	2	40	Cu: about 63%. Sn: about 30%, Zn: about 7%
7	PIN SURFACE	-		-	-	-
BOX SURFACE	Zn-Ni ALLOY PLATING LAYER	6.5	25	2	I8	Zn:85.3%, Ni: 14.7%
8	PIN SURFACE	Zn-Ni ALLOY PLATING LAYER	6.5	25	2	I8	Zn:85.3%, Ni: 14.7%
BOX SURFACE	Zn-Ni ALLOY PLATING LAYER	6.5	25	2	I8	Zn:90.3%, Ni:9.7%

[Coating Fonning Step]

-20To the pin surface and the box surface of each test number, lubricant was applied at a normal température, whereby the lubricating coating was formed. For a lubricating coating J and a lubricating coating D, lubricant was applied by brush application. For a lubricating coating S, lubricant was diluted with minera! spirit, and spray application was performed, whereby the lubricating coating S was formed. For the formation of the lubricating coatings, the following lubricants were used.

- Lubricating coating J Corporation	Trade name: SEAL-GUARD (TM) ECF (TM) from JET-LUBE
- Lubricating coating S	Rosin: 10% Ca stearate: 15% Wax: 10% Earthy graphite: 5% Basic Ca sulfonate: the balance
- Lubricating coating D Bulletin 5A2	Compound grease specified in American Petroleum Institute (API)

The lubricating coating J was what is called a yellow dope, which is classified as Yellow according to the guideline of Harmonised Offshore Chemical Notification Format (HOCNF). The lubricating coating S was a grease-like lubricating coating containing no heavy metals. For the formation of the lubricating coating S, the aforementioned lubricant was diluted with minerai spirit and used. The lubricating coating D was a compound grease containing heavy metals such as lead (Pb). Coating conditions were obtained in advance so that the thickness of each lubricating coating had the thickness shown in Table l, and a lubricating coating was formed under the coating conditions.

[Torque-On-Shoulder Résistance Measurement Test]

Using the pin and the box of each test number, torque-on-shoulder résistance was measured. Specifically, the fastening was performed at a tightening speed of 10 rpm and with a tightening torque of 42.8 kN-m. In the fastening, torque was measured, and a torque chart was created as shown in FIG. 6. N in FIG. 6 dénotés number of tums, T in FIG. 6 dénotés torque. Ts in FIG. 6 dénotés shouldering torque. MTV in FIG. 6 dénotés a torque value where a line segment L intersects with the torque chart. The line segment L is a straight line that has the same inclination as that of a linear région in the torque chart after shouldering and that is larger in number of tums than the linear région by 0.2%. Normal!y, to measure torque-on-shoulder résistance, yield torque (Ty) is used. However, in the present example, the yield torque (a

-2l boundary between the linear région and a nonlinear région in the torque chart after shouldering) was unclear. Thus, using the line segment L, MTV was defined. The différence between MTV and Ts was determined as a torque-on-shoulder résistance (ΔΤ'). The torque-on-shoulder résistance (ΔΤ') was determined in the form of relative value using test number 4 as a reference ( 100%). The results of the measurement are shown in Table 3. Test number 4 is a typical combination of surface treatments for a threaded connection for pipe in the case of using a compound grease.

[Table 3]

TEST NUMBER	ΔΓ (%)
l	I28
2	140
3	135
4	100 (REFERENCE)
5	74
6	91
7	88
8	90

[Resuit Of Evaluation]

Referring Table l and Table 3, the threaded connections for pipe of test number l to test number 3 each included the Zn-Ni alloy plating layer on a contact surface of one of the pin and the box, and included a Cu-Sn-Zn alloy plating layer on a contact surface of the other one. The threaded connections for pipe of test number l to test number 3 each included the lubricating coating on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer. Therefore, their torque-on-shoulder résistances were more than 100%.

Comparing test number 2 and test number 3, when the alloy plating layers of the pin surface and the box surface are exchanged with each other, résultant torque-on-shoulder résistances remained high. Therefore, it is conjectured that exchanging the kinds of the alloy plating layers has no great influence on their effect.

The threaded connections for pipe of test number 4 and test number 5 included neither Zn-Ni alloy plating layer nor Cu-Sn-Zn alloy plating layer on both of their pin surfaces and box surfaces. Therefore, résultant torque-on-shoulder résistances were as low as 100% or less.

The threaded connection for pipe of test number 6 included the Cu-Sn-Zn alloy plating layer on its box surface but included no plating layer on its pin surface. Therefore, a résultant torque-on-shoulder résistance was as low as 91%.

- 22 The threaded connection for pipe of test number 7 included the Zn-Ni alloy plating layer on its box surface but included no plating layer on its pin surface. Therefore, a résultant torqueon-shoulder résistance was as low as 88%.

The threaded connection for pipe of test number 8 included the Zn-Ni alloy plating layers on both of its pin surface and box surface but included no Cu-Sn-Zn alloy plating layer. Therefore, a résultant torque-on-shoulder résistance was as low as 90%.

As seen from the above, the embodiment according to the present invention has been described. However, the aforementioned embodiment is merely an example for practicing the present invention. Therefore, the present invention is not limited to the previously-mentioned embodiment, and the previously-mentioned embodiment can be modified and practiced as appropriate without departing from the scope of the present invention.

REFERENCE SIGNS LIST pin box

31,41 thread part

32, 42 métal seal part

33, 43 shoulder part

100 Zn-Ni alloy plating layer

110 Cu-Sn-Zn alloy plating layer

200 lubricating coating

Claims (5)

1. A threaded connection for pipe comprising:

a pin and a box each of which has a contact surface including a thread part, a métal seal part, and a shoulder part;

a Zn-Ni alloy plating layer on the contact surface of one of the pin and the box, the Zn-Ni alloy plating layer being made of a Zn-Ni alloy;

a Cu-Sn-Zn alloy plating layer on the contact surface of another of the pin and the box, the Cu-Sn-Zn alloy plating layer being made of a Cu-Sn-Zn alloy; and a lubricating coating on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer, the lubricating coating being in a liquid state or a semisolid State.

2. The threaded connection for pipe according to claim l, wherein the Zn-Ni alloy plating layer has a thickness of l to 20 pm, the Cu-Sn-Zn alloy plating layer has a thickness of l to 20 pm, and the lubricating coating has a thickness of 30 to 300 pm.

3. The threaded connection for pipe according to claim l or 2, wherein the Zn-Ni alloy consists of 85 to 91 mass % of Zn and 9 to 15 mass % of Ni, with the balance being impurities.

4. The threaded connection for pipe according to any one of claims l to 3, wherein the Cu-Sn-Zn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

5. A method for producing a threaded connection for pipe comprising a pin and a box each of which has a contact surface including a thread part, a métal seal part, and a shoulder part, the method comprising:

a step of forming a Zn-Ni alloy plating layer on the contact surface of one of the pin and the box, the Zn-Ni alloy plating layer being made of a Zn-Ni alloy;

a step of forming a Cu-Sn-Zn alloy plating layer on the contact surface of another of the pin and the box, the Cu-Sn-Zn alloy plating layer being made of a Cu-Sn-Zn alloy; and

- 24a step of forming a lubricating coating on at least one of the Zn-Ni alloy plating layer and the Cu-Sn-Zn alloy plating layer, the lubricating coating being in a liquid state or a semisolid state.