JP7245395B2 - Method for manufacturing three-layer coated metal pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a three-layer coated metal pipe in which an epoxy resin layer, an adhesive layer and a polyolefin resin layer are laminated in this order.

Conventionally, steel pipes used in steel structures used in sea areas and steel pipes used in pipelines laid in harsh environments such as cold regions are required to have corrosion resistance and impact resistance, and steel pipes that meet these requirements. As such, a three-layer coated steel pipe is used in which an epoxy resin layer, an adhesive layer, and a polyolefin resin layer are laminated and coated on the outer peripheral surface, and various methods for manufacturing the same have been proposed.

For example, in Patent Document 1, after coating a steel pipe with an epoxy powder coating, a polyolefin resin and a modified polyolefin resin are co-extruded from respective extruders through a T-die into strips to form an adhesive layer and a polyolefin layer on a rotating steel pipe. , forming a three-layer coating on the surface of the steel pipe.
However, this method has the drawback that it cannot be applied to bent pipes because the layers are formed while the steel pipe is being rotated.

In Patent Document 2, after forming an epoxy primer layer on the outer surface of a steel pipe, a polyolefin molding having a polyolefin adhesive layer and a polyolefin layer is fused to the steel pipe to form a three-layer coating on the steel pipe surface. there is The polyolefin molded article of Patent Document 2 is in the form of a sheet or panel.

In Patent Document 3, first, an incompletely cured fixing layer of a reaction-curing epoxy resin is formed on the outer peripheral surface of a bent metal pipe, and then a thin layer of a hot melt adhesive and a thick film of polyolefin are formed on the fixing layer. A three-layer coating is formed on metal bends by forming a prefabricated multi-layer coating with the layers in contact and then inductively heating the metal bends beneath the coating.
In Patent Document 3, methods for forming a thick polyolefin film layer on a curved pipe include a method of wrapping a polyolefin tape in a bandage, a method of thermal spraying, and a method of temporarily expanding a plurality of short polyolefin pipes. It describes a method of closely fitting by restoring the diameter of the original fitting, and a method of welding and joining the ends of polyolefin curved tubular molded body segments divided into a plurality of segments to integrate them.

In the methods using sheets and tapes described in Patent Documents 2 and 3, it is necessary to pay close attention to avoid inclusion of air bubbles, and it is difficult to control the thickness of the polyolefin resin layer. . Also, the polyolefin resin layer cannot be thickened by thermal spraying.
Furthermore, in the method using a plurality of short pipes or the method using bent tube-shaped molded body segments, the thickness of the joint portion of the short pipes or segments is different from that of other portions, which may cause problems.

Moreover, Patent Document 3 describes that although it is theoretically possible to apply a three-layer coating using powder coating, it is difficult to put into practical use.

The three-layer coated metal pipe is expected to be used under various conditions, and the development of a method capable of producing a three-layer coated metal pipe suitable for the conditions of use with high productivity is desired.

JP 2006-247887 A JP 2017-177458 A JP-A-2004-130669

The present invention has been made in view of the above background art, and an object thereof is to provide a manufacturing method capable of manufacturing a three-layer coated metal pipe with high adhesion strength and high reliability with good productivity. Another object of the present invention is to provide a three-layer coated metal pipe manufactured by such a method.

As a result of intensive studies to solve the above problems, the inventor preheated the metal pipe to a specific temperature before forming the epoxy resin layer, and after the completion of coating the epoxy resin layer and the adhesive layer, Furthermore, by heating (reheating) the metal pipe at a high temperature, that is, by heating in two stages, even when all three layers are formed by powder coating, the adhesion strength is large and the reliability is high. The inventors have found that it is possible to manufacture a three-layer coated metal pipe, and have completed the present invention.

That is, the present invention provides a method for manufacturing a three-layer coated metal pipe in which an epoxy resin layer, an adhesive layer, and a polyolefin resin layer are laminated in this order on the outer peripheral surface of the metal pipe, the epoxy resin layer, the adhesive layer and the A polyolefin resin layer is formed by powder coating, and a method for producing a three-layer coated metal pipe is provided, characterized in that the following steps (1) to (5) are performed in this order.

(1) A step of heating the metal pipe to a temperature equal to or higher than the glass transition point (Tg1) of the epoxy resin, which is the raw material of the epoxy resin layer, and equal to or lower than the crosslinking reaction temperature of the epoxy resin. (3) Forming on the outer peripheral surface of the metal tube by electrostatic coating or fluidized immersion method (3) Applying a hot-melt adhesive onto the semi-molten epoxy resin coating film by electrostatic coating or fluidized immersion method. Step of forming a coating film (4) Step of heating the metal tube at a temperature higher than that in step (1) Step (5) Step of forming a coating film of polyolefin resin on the coating film of the hot-melt adhesive

The present invention also provides a method for producing a three-layer coated metal pipe, wherein the step (5) is performed by an electrostatic coating method or a fluidization dipping method, and the following step (6) is performed after the step (5). .

(6) The step of heating the metal pipe

The present invention also provides a method for producing a three-layer coated metal pipe, wherein the following step (7) is performed once or more after step (6).

(7) On the uppermost polyolefin resin coating film on the outer peripheral surface of the metal pipe, a polyolefin resin is further applied by an electrostatic coating method or a fluidized bed method to thicken the polyolefin resin coating film, and then , the step of heating the metal tube

ADVANTAGE OF THE INVENTION According to this invention, the three-layer coated metal pipe with high adhesive strength and high reliability can be manufactured with good productivity.

In the present invention, the preheating temperature of the metal pipe before forming the epoxy resin layer is lower than the deterioration temperature of the epoxy resin, and the epoxy resin layer does not deteriorate. Excellent corrosion resistance.

In the present invention, the hot-melt adhesive is applied immediately after applying the epoxy resin in a semi-molten state. That is, the open time between the application of the first layer and the application of the second layer can be substantially zero. Therefore, in the present invention, not only the productivity is improved, but also the adhesive strength of the adhesive layer is improved, making it possible to manufacture a highly reliable three-layer coated metal pipe.

In the present invention, after the first and second layers are applied, the first layer, preferably the first and second layers are cured in step (4), so more than necessary before the first layer is applied. There is no need to heat the metal tube for the first time. Therefore, in the present invention, deterioration of the epoxy resin, which is the material of the first layer, does not easily occur.
For example, in order to form the polyolefin resin material of the third layer with a sufficient thickness using powder coating, the temperature of the metal pipe must be about 250°C. When the third layer is applied at such a temperature, the temperature of the metal tube must be kept at about 320° C. before the first layer is applied, which causes deterioration of the epoxy resin. For this reason, in Patent Document 3, it is said that the three-layer coating by powder coating is difficult to put into practical use.
In the present invention, since the metal pipe is heated in stages, it is possible to prevent deterioration of the epoxy resin caused by excessive heating of the metal pipe.

In the present invention, since the uppermost polyolefin resin layer is formed by powder coating, there is no problem of generating air bubbles seen in the methods using sheets and tapes as in Patent Documents 2 and 3.

The method of the present invention has a step of heating (reheating) the metal tube before forming the uppermost polyolefin resin layer. Therefore, the thickness of the polyolefin resin layer can be easily controlled as compared with the method of forming the polyolefin resin layer only by preheating before applying the epoxy resin, as in Patent Document 3. In particular, in the present invention, since the polyolefin resin layer can be applied multiple times, it is possible to manufacture a three-layer coated metal pipe having a thick polyolefin resin layer.

In the present invention, at the stage when the coating of the raw material for the first epoxy resin layer and the second adhesive layer is completed (the stage before heating (reheating) the metal tube at a high temperature), the crosslinking reaction of the epoxy resin is not progressing. For this reason, the metal pipe coated with the epoxy resin or coated with the epoxy resin and the hot-melt adhesive is stored, and the third polyolefin resin layer is formed, for example, at another location. can be done separately.

In the present invention, metal pipes can be uniformly heated by induction heating, unlike the mobile heating method disclosed in Patent Document 3. Therefore, in the present invention, the adhesive strength of the entire metal pipe can be made uniform.

In the present invention, since it is not necessary to rotate the metal pipe when forming the coating film, the present invention can also be applied to a bent metal pipe.

1 is a schematic diagram showing the structure of a three-layer coated metal pipe manufactured by the method for manufacturing a three-layer coated metal pipe of the present invention; FIG. FIG. 2 is a schematic diagram showing the structure of a three-layer coated metal pipe manufactured by the method for manufacturing a three-layer coated metal pipe of the present invention (when polyolefin resin layers are formed in multiple steps).

Although the present invention will be described below, the present invention is not limited to the following embodiments, and can be arbitrarily modified and implemented.

Schematic diagrams of a three-layer coated metal pipe 1 manufactured according to the present invention are shown in FIGS. 1 and 2. FIG. In the present invention, the three-layer coated metal pipe 1 is manufactured by laminating the epoxy resin layer 11, the adhesive layer 12, and the polyolefin resin layer 13 on the outer peripheral surface 10S of the metal pipe 10 in this order. In the present invention, the epoxy resin layer 11, the adhesive layer 12, and the polyolefin resin layer 13 are all formed by powder coating.

In the present invention, the uppermost polyolefin resin layer 13 of the three-layer coated metal pipe 1 can be formed in multiple steps. For example, FIG. 2 shows an example in which the polyolefin resin layer 13A and the polyolefin resin layer 13B are formed in two steps.

Examples of the metal pipe 10 in the present invention include an electric resistance welded steel pipe, a butt welded steel pipe, a seamless steel pipe, a spiral steel pipe, a UOE steel pipe, and the like, but are not limited to these.

Further, for example, the method of rotating a metal pipe to form a coating film as in Patent Document 1 cannot be applied to a bent pipe. Therefore, the feature of the present invention that all layers are formed by powder coating is more likely to be utilized when the metal pipe 10 is a bent pipe. However, the case where the metal pipe 10 is a straight pipe does not fall outside the scope of the present invention.

In the present invention, since the epoxy resin layer 11, the adhesive layer 12, and the polyolefin resin layer 13 are all formed by powder coating, problems such as inclusion of air bubbles are less likely to occur, and the uniformity of each layer can be easily ensured. .

The epoxy resin layer 11 is formed using powdered epoxy resin as a raw material. As the epoxy resin, a commercially available powdered epoxy resin can be appropriately used. By providing the epoxy resin layer, the corrosion resistance of the metal pipe can be enhanced.

Examples of the epoxy resin used as the raw material of the epoxy resin layer 11 include bisphenol A type epoxy resin, bisphenol F type epoxy resin, o-cresol novolac type epoxy resin, and the like.

The average thickness of the epoxy resin layer 11 is preferably 50 μm or more, more preferably 70 μm or more, and particularly preferably 100 μm or more. Also, it is preferably 1000 μm or less, more preferably 500 μm or less, and particularly preferably 300 μm or less.
When it is within the above range, the corrosion resistance is sufficiently exhibited, and the cost can be easily suppressed.

The polyolefin resin layer 13 is provided for the purpose of improving impact resistance. The polyolefin resin layer 13 is formed using polyolefin resin as a raw material.

The polyolefin resin is not particularly limited, and examples thereof include low-density polyethylene resin, medium-density polyethylene resin, high-density polyethylene resin, polypropylene resin, polybutene resin, polyethylene-propylene copolymer resin, and the like.
These may be used individually by 1 type, and may use 2 or more types together.

The average thickness of the polyolefin resin layer 13 is preferably 0.1 mm or more, more preferably 0.3 mm or more, and particularly preferably 0.5 mm or more. Further, it is preferably 10 mm or less, more preferably 7 mm or less, and particularly preferably 5 mm or less.
Sufficient impact resistance can be exhibited as it is more than the said minimum. It is advantageous in cost in it being below the said upper limit.

The average thickness of the polyolefin resin layer 13 is usually appropriately selected within the range described above, depending on the required protective properties. In the present invention, the polyolefin resin layer 13 can be easily formed with a uniform thickness, and the thickness can be easily controlled. Furthermore, in the present invention, it is possible to form the polyolefin resin layer multiple times. The present invention is suitable for producing a three-layer coated metal pipe 1 having a polyolefin resin layer 13 with a thickness near the above upper limit (that is, with a large thickness).

Adhesive layer 12 is provided to improve adhesion between epoxy resin layer 11 and polyolefin resin layer 13 . The adhesive layer 12 is formed using a hot-melt adhesive as a raw material. Hot melt adhesives have the property of adhering well to both epoxy resins and polyolefin resins.

The hot-melt adhesive is not particularly limited as long as it has the above properties, but it may be modified polyolefin resin obtained by modifying polyolefin resin with an acid anhydride such as maleic anhydride, a copolymer of olefin and other monomers, etc. Examples of the latter include ethylene-maleic anhydride copolymers, ethylene-maleic anhydride-acrylic ester copolymers, ethylene-acrylic acid copolymers (EAA), ethylene-methacrylic acid copolymers (EMAA), Examples include ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid ester copolymer, and the like.
These may be used individually by 1 type, and may use 2 or more types together.

The average thickness of the adhesive layer 12 is preferably 50 μm or more, more preferably 70 μm or more, and particularly preferably 100 μm or more. Moreover, it is preferably 400 μm or less, more preferably 300 μm or less, and particularly preferably 200 μm or less.
Within the above range, sufficient adhesiveness can be exhibited, and costs can be easily suppressed.

In the present invention, the three-layer coated metal pipe 1 is manufactured by performing the following steps (1) to (5) in this order. The following step (6), step (7), and other steps can also be performed as necessary.

<Step (1)>
In step (1), the metal tube 10 is heated to a temperature above the glass transition point (Tg1) of the epoxy resin that is the raw material of the epoxy resin layer 11 and below the crosslinking reaction temperature of the epoxy resin.

Step (1) is a step of preheating the metal tube 10 in order to form the epoxy resin layer 11 as the first layer and the adhesive layer 12 as the second layer on the metal tube 10 .
The heating temperature of the metal tube 10 in step (1) is equal to or higher than the glass transition point (Tg1) of the epoxy resin and equal to or lower than the crosslinking reaction temperature of the epoxy resin. That is, the temperature is such that when the epoxy resin is applied to the metal pipe 10 in the next step (2), the epoxy resin is in a semi-molten state.

In step (1), the metal tube 10 is preheated to such a temperature range, and in step (3) described later, a hot-melt adhesive is applied onto the semi-molten epoxy resin coating film. This improves the adhesion between the first layer (epoxy resin layer 11) and the second layer (adhesive layer 12).

If the application of epoxy resin (step (2)) and the application of hot-melt adhesive (step (3)) were performed without performing step (1), electrostatic repulsion would cause the second layer (adhesive layer 12) may peel off.
Further, as described in Patent Document 3, in anticipation of the process of applying the polyolefin resin, if the metal tube 10 is heated at a high temperature of about 320° C. before the epoxy resin is applied, the epoxy resin deteriorates. However, the epoxy resin does not deteriorate at the heating temperature of step (1) of the present invention.
That is, in the present invention, by performing step (1), problems such as peeling of the first layer (epoxy resin layer 11) and the second layer (adhesive layer 12) are less likely to occur.

Note that "below the cross-linking reaction temperature of the epoxy resin" refers to the temperature range in which the epoxy resin is not completely cured by cross-linking and remains in a semi-molten state, and the cross-linking reaction partially starts. However, if the temperature is such that it maintains a semi-molten state, it corresponds to "below the cross-linking reaction temperature of the epoxy resin".

The heating temperature suitable for step (1) varies depending on the type of epoxy resin or the like used, but the heating temperature in step (1) is preferably 60° C. or higher, more preferably 70° C. or higher, and 80° C. or higher. is particularly preferred. Also, it is preferably 140° C. or lower, more preferably 120° C. or lower, and particularly preferably 100° C. or lower.

In the step (1), the specific method for heating the metal tube 10 to a predetermined temperature is not particularly limited, and examples include a method of heating by placing it in a furnace, a method of induction heating, a method of using a gas burner, and the like. be done.
From the viewpoint of uniformity, the method using induction heating is particularly desirable.

<Step (2)>
In step (2), an epoxy resin coating is formed on the outer peripheral surface 10S of the metal tube 10 by an electrostatic coating method or a fluidized bed coating method.
Step (2) is desirably performed immediately after the temperature of the metal tube 10 reaches the above temperature range in step (1).

In step (2), the epoxy resin applied to the metal tube 10 heated to the temperature range is in a semi-molten state.

In the step (2), an epoxy resin coating is formed by an electrostatic coating method or a fluidized bed coating method. It is not desirable to apply the thermal spray method to step (2), because it significantly raises the temperature of the metal pipe, which, for example, exposes the drawbacks of the conventional method.

<Step (3)>
In the step (3), a coating film of a hot-melt adhesive is formed on the semi-molten epoxy resin coating film by an electrostatic coating method or a fluidization dipping method.

From the standpoint of productivity and the quality of the three-layer coated metal pipe 1, it is desirable to carry out the step (3) immediately after the step (2) is completed.

In step (3), a coating film of hot-melt adhesive is formed by an electrostatic coating method or a fluidization dipping method. As in the case of step (2), thermal spraying significantly raises the temperature of the metal tube, so it is not desirable to apply it to step (3).

Since the hot melt adhesive is used to bond the epoxy resin and the polyolefin resin, it is not necessary to thicken the adhesive layer 12 formed by the hot melt adhesive. Therefore, from the viewpoint of productivity, step (3) is desirably performed by an electrostatic coating method.

When steps (1) to (3) are performed, the metal pipe 10 is coated with an uncured epoxy resin coating and a hot-melt adhesive coating.
In the present invention, the metal pipe 10 can be stored in a state in which the processes up to step (3) have been performed. For example, the process of step (4) transfer can be performed after transferring to another location.

<Step (4)>
In step (4), the metal tube 10 is heated to a higher temperature than in step (1).

The step (4) is a step of heating the metal tube 10 in order to cure the epoxy resin coating film formed in the step (2).
The heating in step (4) also serves as preheating for powder coating of a polyolefin resin coating film with a sufficient thickness in the next step (5).

In order to fulfill the two roles described above, the heating temperature in step (4) must be higher than the heating temperature in step (1).
On the other hand, in order to prevent deterioration of the epoxy resin, the heating temperature in step (4) should not be set too high.

The heating temperature in step (4) is preferably 200° C. or higher, more preferably 210° C. or higher, and particularly preferably 220° C. or higher, although it depends on the type of epoxy resin used. Also, it is preferably 300° C. or lower, more preferably 290° C. or lower, and particularly preferably 280° C. or lower.
If the heating temperature is lower than the above lower limit, the epoxy resin may not sufficiently cure. Moreover, if the heating temperature is higher than the above upper limit, the deterioration of the epoxy resin may progress.

In the step (4), there is no particular limitation on the specific method for heating the metal tube 10 to a predetermined temperature, and examples include a method of putting it in a furnace and heating it, a method of induction heating, a method of using a gas burner, and the like. be done.
From the viewpoint of uniformity, the method using induction heating is particularly desirable.

<Step (5)>
In step (5), a polyolefin resin coating is formed on the hot-melt adhesive coating.
Step (5) is desirably performed immediately after the temperature of the metal tube 10 reaches the above temperature range in step (4).

In the step (5), the method for forming the coating film of the polyolefin resin is not particularly limited as long as it is a method by powder coating. For example, an electrostatic coating method, a fluidization dipping method, a thermal spraying method, and the like can be exemplified.

From the viewpoints of the uniformity of the thickness of the polyolefin resin layer 13 and the cost, the electrostatic coating method and the fluidization dipping method are preferable, and the fluidization dipping method is particularly preferable.
Moreover, there is a demand for forming the polyolefin resin layer 13 thickly depending on the required protective properties. Since the thermal spraying method is not suitable for forming a thick coating film, the electrostatic coating method and the fluidized bed coating method are desirable when it is desired to form a thick polyolefin resin layer 13 .

<Step (6)>
Step (6) is performed after step (5) if necessary. Step (6) is a step of heating the metal tube 10 .
Step (6) is required when step (5) is performed by electrostatic coating or fluidized bed dipping.

The step (6) requires heating of the metal tube 10 when the step (5) is performed by the electrostatic coating method or the fluidized bed dipping method.
On the other hand, when the step (5) is performed by thermal spraying, the temperature of the metal tube 10 rises sufficiently, so the step (6) is not necessary.

The heating temperature in step (6) is preferably 200° C. or higher, particularly preferably 220° C. or higher, although it depends on the type of polyolefin resin used. Moreover, 300 degrees C or less is preferable and 260 degrees C or less is especially preferable.

In step (6), the specific method for heating the metal tube 10 to a predetermined temperature is not particularly limited, and includes a method of heating by placing it in a furnace, a method of induction heating, and a method of passing steam through the inside of the metal tube. and the like.
A method using induction heating is particularly desirable from the viewpoint of uniformity and productivity.

<Step (7)>
The step (7) is performed once or more after the step (6), if necessary, when the step (5) is performed by an electrostatic coating method or a fluidized bed method. In the step (7), a polyolefin resin is further applied on the uppermost polyolefin resin coating film of the outer peripheral surface 10S of the metal tube 10 by an electrostatic coating method or a fluidization dipping method to form a thick polyolefin resin coating film. It is a step of heating the metal pipe 10 after that.

When performing the step (7), the polyolefin resin layer 13 is formed by coating multiple times. For example, FIG. 2 is a schematic diagram when step (7) is performed once. Of the polyolefin resin layer 13, the portion derived from the polyolefin resin coating film formed in step (5) is 13A, 13B is the portion derived from the polyolefin resin coating formed in 7).

By performing the step (7), the polyolefin resin layer 13 is thickened by the amount of 13B, so it becomes easy to manufacture the three-layer coated metal pipe 1 having the thick polyolefin resin layer 13, and the demand for such a three-layer coated metal pipe 1 is increased. can respond to

The preferred range of heating temperature in step (7) is the same as the preferred range of heating temperature in step (6) described above.

In step (6), the specific method for heating the metal tube 10 to a predetermined temperature is not particularly limited, and includes a method of heating by placing it in a furnace, a method of induction heating, and a method of passing steam through the inside of the metal tube. and the like.

Step (7) may be performed two or more times. That is, in FIG. 2, 13C, 13D, . . . (not shown) may exist on 13B.

In the present invention, there are multiple steps of heating the metal tube 10 (step (1), step (4), step (6), step (7)). The heating method in each step may be the same or different. At least step (4), step (6) and step (7) are preferably the same, since they are usually carried out in succession.
Further, in the present invention, in the step of heating the metal tube 10, it is desirable to uniformly heat the metal tube 10 by induction heating.

The three-layer coated metal pipe manufactured by the method of the present invention has high adhesion strength and high reliability, and the thickness of the polyolefin resin layer, which is the outermost layer, can be easily controlled and can be made thicker. , gas pipes, water pipes, cable protection pipes, etc.

1 Three-layer coated metal pipe 10 Metal pipe 10S Outer peripheral surface 11 Epoxy resin layer 12 Adhesive layer 13 Polyolefin resin layer 13A Polyolefin resin layer 13B Polyolefin resin layer

Claims (8)

A method for manufacturing a three-layer coated metal pipe in which an epoxy resin layer, an adhesive layer, and a polyolefin resin layer are laminated in this order on the outer peripheral surface of a metal pipe, wherein the epoxy resin layer, the adhesive layer, and the polyolefin resin layer are powdered. A method for manufacturing a three-layer coated metal pipe, which is formed by body coating, characterized by performing the following steps (1) to (5) in this order.
(1) A step of heating the metal pipe to a temperature equal to or higher than the glass transition point (Tg1) of the epoxy resin, which is the raw material of the epoxy resin layer, and equal to or lower than the crosslinking reaction temperature of the epoxy resin. (3) Forming on the outer peripheral surface of the metal tube by electrostatic coating or fluidized immersion method (3) Applying a hot-melt adhesive onto the semi-molten epoxy resin coating film by electrostatic coating or fluidized immersion method. Step of forming a coating film (4) Step of heating the metal tube at a temperature higher than that in step (1) Step (5) Step of forming a coating film of polyolefin resin on the coating film of the hot-melt adhesive 2. The method of claim 1, wherein step (3) is performed immediately after step (2) is completed. 3. The method for producing a three-layer coated metal pipe according to claim 1, wherein in the step of heating the metal pipe, the metal pipe is uniformly heated by induction heating. 4. The method for producing a three-layer coated metal pipe according to claim 1, wherein the heating temperature in step (1) is 60[deg.] C. or more and 140[deg.] C. or less. 5. The method for producing a three-layer coated metal pipe according to claim 1, wherein the heating temperature in step (4) is 200[deg.] C. or more and 300[deg.] C. or less. The three-layer coated metal pipe according to any one of claims 1 to 5, wherein the step (5) is performed by an electrostatic coating method or a fluidized bed dipping method, and the following step (6) is performed after the step (5). manufacturing method.
(6) a step of heating the metal pipe; 7. The method for producing a three-layer coated metal pipe according to claim 6, wherein the following step (7) is performed once or more after step (6).
(7) On the uppermost polyolefin resin coating film on the outer peripheral surface of the metal pipe, a polyolefin resin is further applied by an electrostatic coating method or a fluidized bed method to thicken the polyolefin resin coating film, and then , the step of heating the metal tube 8. The method for manufacturing a three-layer coated metal pipe according to any one of claims 1 to 7, wherein the polyolefin resin layer has an average thickness of 0.1 mm or more and 10 mm or less.

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