WO2013099320A1

WO2013099320A1 - Multiple coated steel pipe and method for producing same

Info

Publication number: WO2013099320A1
Application number: PCT/JP2012/064245
Authority: WO
Inventors: 菅原　啓司; 星野　俊幸; 泰宏原田; 雅仁金子
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2011-12-26
Filing date: 2012-05-25
Publication date: 2013-07-04
Also published as: IN2014KN01188A; KR20140112031A; CN104024718B; RU2573334C1; JP5187455B1; JP2013223971A; KR101585821B1; CN104024718A

Abstract

Provided is a multiple coated steel pipe that is obtained by covering the outer surface of an anti-corrosion layer, which covers the outer surface of a steel pipe, with a protective layer that has adequate adhesiveness and releasability. Specifically provided are: a multiple coated steel pipe which is obtained by covering the outer surface of a steel pipe that serves as a base with an anti-corrosion layer that is composed of a polyethylene resin layer and with a protective layer that is composed of a polypropylene resin layer and serves as an upper layer of the anti-corrosion layer; and a method for producing the multiple coated steel pipe. The polypropylene resin that forms the protective layer is a copolymerized resin which contains 19-23% by mole of an ethylene component, and has a melt flow rate of 0.53-0.60 g/10 min. The shear viscosity of the polypropylene resin at 280˚C is from 1.7 × 10³ Pa·s to 2.0 × 10³ Pa·s as measured at a shear rate of 10/sec, and is from 5.3 × 10² Pa·s to 6.0 × 10² Pa·s as measured at a shear rate of 100/sec.

Description

Multi-coated steel pipe and manufacturing method thereof

INDUSTRIAL APPLICABILITY The present invention provides a corrosion protection layer and a protection layer suitable for applications such as a gas pipe, a cable protection tube, and a water supply pipe. In particular, a multi-layered steel pipe having a coating layer (covering layer) has a suitable adhesion between the anti-corrosion layer and the protective layer, even if the line speed of a continuous line is increased. The present invention relates to a multi-coated steel pipe having a peel property and excellent field workability, and a method for producing the same. .

A coated steel pipe coated with a polyethylene resin layer (polyethylene resin layer) as an anticorrosion layer on a steel pipe has not only excellent corrosion protection performance but also a wide operating temperature limit and electrical insulation. (Electrical insulating properties) and chemical resistance, it is widely used for gas pipes, water pipes, cable protection pipes and the like. In view of the fact that the steel pipe is often used for local construction, the coated steel pipe is provided with an anticorrosion layer (polyethylene resin layer) from mechanical external force at the time of handling operation (transportation). For the purpose of protection, there is one in which a multilayer coating layer (multi coating layer) is provided on the surface (outer surface) of the anticorrosion layer using a polypropylene resin layer as a protective layer.

As illustrated in FIG. 1, in this multi-coated steel pipe, an outer surface of a steel pipe 1 as a base material is coated with an anticorrosion layer (polyethylene resin layer) 3 via an adhesive layer 2, and The surface (outer surface) of the anticorrosion layer (polyethylene resin layer) 3 is covered with a protective layer (polypropylene resin layer) 4. Moreover, for the purpose of further improving the corrosion resistance, the inner surface side of the steel pipe may be coated depending on the application.

Usually, such a multi-coated steel pipe is manufactured in a continuous line, and the anticorrosive layer and the protective layer are formed by extrusion coating in a multi-coated line as shown in FIG. That is, the steel pipe 1 which is a base material conveyed from the upstream side of the line is preheated to about 40 to 80 ° C. by a steel pipe preheating device (preheating device of steel pipe) 10 and is applied with an adhesive applicator of adhesive layer. 20, an adhesive (not shown) heated to 150 to 200 ° C. is applied to the surface. After that, the steel pipe 1 is conveyed to an anticorrosion layer extrusion coating machine (Extruder for corrosion protection layer) 30, and the anticorrosion layer resin 300 in a molten state (about 200 to 260 ° C.) is extrusion coated on the surface. It is cooled by a cooling device (cooling device of corrosion protection layer) 31 to become an anticorrosion layer 3. Next, the steel pipe 1 is conveyed to a protective layer extrusion coater (Extruder for protection layer) 40, and a protective layer resin 400 in a molten state (about 260 to 270 ° C.) is extrusion coated on the surface of the anticorrosion layer 3 to cool the protective layer. The surface is cooled by the machine 41 to become the protective layer 4.

By the way, when the coated steel pipe is applied to a gas pipe or a cable protection pipe, the pipe ends of the plurality of steel pipes are connected by welding or the like at the time of construction. Here, when connecting the pipe ends of the multi-coated steel pipes having the protective layer (polypropylene resin layer) coated on the surface (outer surface) of the anticorrosion layer (polyethylene resin layer) by welding or the like, As shown in FIG. 3, the anticorrosion layer 3 and the protective layer 4 at the end of the multi-coated steel pipe (region of about 100 mm to 150 mm from the end of the steel pipe) as shown in FIG. 3 for ensuring the ease of repairing the exposed steel part after welding. Is peeled to expose the steel, and only the protective layer 4a is peeled off (region of about 100 mm to 150 mm from the end of the anticorrosive layer). Therefore, the protective layer (polypropylene resin layer) is required to have releasability to the extent that it can be manually peeled off from the anticorrosion layer (polyethylene resin layer) during welding.

On the other hand, if the peelability of the protective layer (polypropylene resin layer) becomes excessively high, the adhesion between the protective layer (polypropylene resin layer) and the anticorrosion layer (polyethylene resin layer) is significantly lowered, which causes various problems. For example, when the adhesiveness is lowered, when the cut 5 is circumferentially formed in the protective layer 4 at the end of the steel pipe in order to peel off the protective layer 4a at the end of the multi-coated steel pipe from the anticorrosive layer 3, protection around the cut 5 is performed. The layer 4 may partially float from the anticorrosion layer 3. As a result, after peeling off the protective layer 4 a from the anticorrosive layer 3, the end 4 ′ of the protective layer 4 b remaining on the anticorrosive layer 3 floats from the anticorrosive layer 3, and a gap 6 is formed between the anticorrosive layer 3 and the protective layer 4. And the appearance also deteriorates. In addition, after connecting the pipe ends of the multi-coated steel pipes by welding or the like, repairs such as wrapping anticorrosion tape around the connection are performed. However, if such a protective layer float remains, Problems also occur when repairs are performed. Further, when the adhesion between the anticorrosion layer and the protective layer is lowered, there is a problem that air may enter between the layers due to some impact, which deteriorates the appearance.

As described above, between the anticorrosive layer and the protective layer, when the protective layer at the pipe end of the multi-coated steel pipe is peeled off from the anticorrosive layer, it is easy to peel off and does not cause the above-described floating. It is necessary to have sex. That is, it is not preferable that the adhesion between the anticorrosive layer and the protective layer is too large or too small. If any polyethylene resin and polypropylene resin are used as the anticorrosion layer and the protective layer, respectively, the resin is fused, and the releasability is remarkably deteriorated or floating is generated.

Therefore, various techniques have been proposed so far in order to ensure the peelability between the polyethylene resin of the anticorrosion layer and the polypropylene resin of the protective layer, with respect to such a multi-coated steel pipe.
For example, in Patent Document 1, the corrosion prevention layer extrusion-coated on the outer surface of a steel pipe is cooled to 25 ° C. or less, and then the protective layer is extrusion-coated thereon to prevent the corrosion-proof layer and the protective layer from being fused. Thus, techniques for ensuring the peelability of the protective layer have been proposed. According to such a technique, it is said that the fusion between the anticorrosion layer and the protective layer can be prevented without impairing the film properties of the anticorrosion layer and the protective layer.

However, in the technique proposed in Patent Document 1, it is necessary to lengthen the cooling time in order to cool the anticorrosion layer to 25 ° C. or lower. Therefore, restrictions for securing the cooling time, such as lengthening the cooling line at the time of manufacture or slowing the line speed, occur, which is industrially disadvantageous. As described above, the coating of the anticorrosion layer and the coating of the protective layer are usually carried out in one continuous line. However, in recent years, the line speed has tended to increase to improve productivity. It is difficult to ensure wearability.

Patent Document 2 proposes a technique for preventing fusion between both layers by spraying an inorganic powder such as calcium oxide between the anticorrosive layer and the protective layer. . And according to the technology, it is said that a protective steel layer can be obtained in which the protective layer of the joint is easily peeled off during welding, and the protective layer can be prevented from expanding and contracting due to a change in environmental temperature (environmental temperature). Yes. However, the technique proposed in Patent Document 2 is also disadvantageous industrially because it requires equipment for spraying powder and the handling of the powder in the factory is complicated.

Patent Document 3 includes an antioxidant and a nucleating agent, Patent Document 4 includes a lubricant and an inorganic filler, and Patent Document 5 includes an antistatic agent (an anti-oxidant). A technique for adding antistatic agent) to the anticorrosion layer or the protective layer has been proposed. And according to these techniques, it is supposed that various additives (additive agent) will prevent the fusion | melting between resin, and the peelability of a corrosion prevention layer and a protective layer will improve significantly. However, in the techniques proposed in Patent Documents 3 to 5, when the addition amount of various additives is very small, the effect of preventing fusion between resins becomes insufficient when the line speed of the continuous line is increased. On the other hand, if the amount of various additives added is increased, there are disadvantages in that the physical properties of the resin are different from those expected and the cost is increased.

In contrast to these technologies, Patent Document 6 discloses that either one of the anticorrosion layer and the protective layer is made of polyethylene alone, the other layer is made of 20 to 40% by weight (27.3 to 50% by mole) of polyethylene, and 60 to 80% of polypropylene. A technique of making a copolymer or blended resin by blending in weight percent (50 to 72.7 mol%) has been proposed. And according to such a technique, it is said that a coated steel pipe is obtained in which the anticorrosion layer and the protective layer are not fused even if continuous double extrusion coating is performed without using a welding inhibitor.

JP-A 49-130956 JP-A-50-139422 JP-A-10-76601 Japanese Patent Laid-Open No. 10-76602 Japanese Patent Laid-Open No. 10-76603 JP 54-158720 A

However, the technique proposed in Patent Document 6 cannot sufficiently secure the hardness of the protective layer. For the purpose, the protective layer of the multi-coated steel pipe has a higher degree of scratch resistance, so that the resin for the protective layer needs a certain degree of hardness. Therefore, in the multi-coated steel pipe, the anticorrosion layer is usually formed of polyethylene resin and the protective layer is formed of polypropylene resin. As described above, the ethylene component of the resin of the protective layer is 20 to 40% by weight (27.3 to 50%). If the amount is too large, the resin becomes soft, and the protective layer is easily wrinkled and does not perform its function. According to the study by the inventors, the hardness of the protective layer is usually measured by the test method defined in ASTM D2240 (D type) when considering impact shock at transportation and construction sites. It is preferable that it is 70 or more.

Also, with the technique proposed in Patent Document 6, if the line speed of the continuous line is increased during the production of the multi-coated steel pipe, the fusion between the anticorrosion layer and the protective layer cannot be suppressed. As described above, when a multi-coated steel pipe is manufactured, the steel pipe 1 is usually preheated and the protective layer 4 is cooled in a continuous line as shown in FIG. And, the fusibility of the anticorrosion layer 3 and the protective layer 4 depends on the temperature of both layers when the molten protective layer 4 is coated on the outer surface of the anticorrosion layer 3, and the higher these temperatures, the easier the fusion occurs. The lower the temperature, the harder it is to fuse. On the other hand, from the viewpoint of productivity, it is preferable that the line speed is high. When the line speed is increased, the extrusion discharge rate of the resin (the anticorrosion layer resin 300 and the protective layer resin 400) must be increased. As a result, the temperature of the extruded resin is increased. Usually, it becomes higher due to shear heating during extrusion.

Therefore, for example, when the line speed is increased to about 10 m / min or more, when the molten anticorrosion layer resin (polyethylene, melting point: 120 ° C. or higher) 300 is extrusion coated with the anticorrosion layer extrusion coating machine 30, the anticorrosion layer 3 during extrusion is applied. The resin temperature reaches 230 to 280 ° C. In addition, even when the anticorrosion layer resin is cooled by water cooling, which is an industrially rational cooling method, after coating the anticorrosion layer 3, in consideration of the realistic water cooling efficiency of the continuous line, JIS G 3452 ( In a large-diameter pipe having a nominal diameter of 100 A or more as defined in 2010), the surface temperature of the anticorrosion layer 3 at the time of covering the protective layer 4 can be lowered only to about 40 to 70 ° C. Further, when the molten protective layer resin (polypropylene, melting point: 160 ° C. or higher) 400 is extrusion coated with the protective layer extrusion coating machine 40, the temperature of the protective layer resin 400 when coated on the anticorrosion layer 3 is 260 to 290. Also reaches ℃.

As described above, when the line speed is increased, the anticorrosion layer and the protective layer are likely to be high in temperature and are in a state of being very easily fused. However, in the technique proposed in Patent Document 6, no consideration has been given to the case where the line speed is increased. Therefore, the technique proposed in Patent Document 6 has a problem that when the line speed is increased for the purpose of improving productivity, the anticorrosion layer and the protective layer are fused, and good peelability cannot be obtained.
Furthermore, in the technique proposed in Patent Document 6, no consideration is given to the extrusion performance of the protective layer resin and the strength of the protective layer resin when the line speed is increased. When the extrudability decreases, the load on the extruder increases and the extrusion discharge amount of the polypropylene resin cannot be maintained. In addition, when the strength of the protective layer resin is reduced, when an impact is applied after the product is manufactured, the protective layer penetrates the protective layer and damages the anticorrosive layer, which is a key for preventing corrosion of the steel pipe. Therefore, the technique proposed in Patent Document 6 cannot stably provide a high-quality multi-clad steel pipe with high production efficiency.

The present invention advantageously solves the above-described problems of the prior art. That is, the present invention is an anticorrosion layer in a multi-coated steel pipe in which a corrosion prevention layer made of a polyethylene resin layer and a protection layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer are coated on the outside of the steel pipe that is a base material. While significantly improving the peelability of the polyethylene resin layer and the polypropylene resin layer, which is a protective layer, it has moderate adhesiveness that does not cause floating during welding construction between the anticorrosive layer and the protective layer. Multi-coating with scratch resistance and excellent extrudability and weld strength (weld impact value) of the protective layer resin (weld strength in the present invention refers to Izod impact value) It is an object to provide a method of manufacturing a steel pipe and a multi-coated steel pipe.

In particular, the present invention is applicable even when manufactured at a high line speed or a large-diameter tube or pipe (for example, a nominal diameter defined in JIS G 3452 (2010): 100 A or more). An object of the present invention is to provide a multi-coated steel pipe having such excellent characteristics, or a method for producing the same.

In order to solve the above problems, the present inventors have coated a corrosion prevention layer made of a polyethylene resin layer and a protection layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer on the outside of a steel pipe that is a base material. With respect to the multiple coated steel pipes, various factors affecting various properties such as the peelability and adhesion between the anticorrosion layer and the protective layer were studied.
First, in the technique proposed in Patent Document 6, that is, a technique in which a copolymer formed by blending polyethylene and polypropylene is used as a protective layer, multiple coated steel pipes are formed on a continuous line with a high line speed. In consideration of manufacturing, the ethylene component contained in the polypropylene resin was examined.

As a result, when increasing the line speed, the technique proposed in Patent Document 6 has found that the amount of ethylene component contained in the polypropylene resin is too high and the fusion between the anticorrosion layer and the protective layer becomes remarkable. The inventors have found that by reducing the ethylene component contained in the polypropylene resin to a desired amount, the fusion between the anticorrosion layer and the protective layer is greatly suppressed, and appropriate peelability and adhesion can be obtained. Further, it has been found that by reducing the ethylene component contained in the polypropylene resin to a desired amount, the hardness of the protective layer resin is improved and a protective layer having a desired scratch resistance can be obtained.

On the other hand, the present inventors confirmed that the extrudability and weld strength of the protective layer resin are still insufficient only by adjusting the ethylene component contained in the polypropylene resin. In addition, it was also confirmed that moderate peelability and adhesion may not be obtained only by adjusting the ethylene component. As a result of further investigations, by optimizing the melt flow rate of polypropylene resin and the shear viscosity at 280 ° C., the extrudability and weld strength of the protective layer resin have increased dramatically. It was found that the peelability and adhesion between the anticorrosive layer and the protective layer were further improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] A multi-coated steel pipe in which a corrosion prevention layer made of a polyethylene resin layer and a protection layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer are coated on the outside of a steel pipe that is a base material, and the protection layer is formed The polypropylene resin is a copolymerization resin containing 19 to 23 mol% of an ethylene component, the melt flow rate of the polypropylene resin is 0.53 to 0.60 g / 10 min, and the polypropylene resin at 280 ° C. The shear viscosity is 1.7 × 10 ³ to 2.0 × 10 ³ Pa · s when measured at a shear rate of 10 / sec, and 5.3 × 10 ² to 6.5 when measured at a shear rate of 100 / sec. A multi-coated steel pipe characterized by being 0 × 10 ² Pa · s.

[2] The multi-coated steel pipe according to [1], wherein a peel strength between the anticorrosion layer and the protective layer is 0.6 N / 10 cm width or more and 15 N / 10 cm width or less.
[3] When manufacturing a multi-coated steel pipe in which a corrosion prevention layer made of a polyethylene resin layer and a protection layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer are coated on the outside of the steel pipe as a base material,
After coating the outside of the steel pipe with a polyethylene resin, the surface temperature of the polyethylene resin is cooled to 40 ° C. or more and 70 ° C. or less to form the anticorrosion layer,
Next, it is a copolymer resin containing 19 mol% or more and 23 mol% or less of an ethylene component on the surface of the anticorrosion layer, the melt flow rate is 0.53 g / 10 min or more and 0.60 g / 10 min or less, and shearing at 280 ° C. The viscosity is 1.7 × 10 ³ Pa · s or more and 2.0 × 10 ³ Pa · s or less when measured at a shear rate of 10 / sec, and 5.3 × 10 ² when measured at a shear rate of 100 / sec. A polypropylene resin having a Pa · s of 6.0 × 10 ² Pa · s or less is used, and the temperature of the polypropylene resin is 260 ° C. or more and 290 ° C. or less so that the coating thickness is 0.9 mm or more and 1.8 mm or less. The protective layer is formed by coating and cooling at a cooling rate of 153 ° C./sec or more and 450 ° C./sec or less until the surface temperature of the polypropylene resin becomes 170 ° C. or less. A method for producing a multi-coated steel pipe characterized by comprising:

[4] In [3], the above-mentioned polypropylene resin is coated so that the draw ratio when coating the polypropylene resin is in the range of 5 times to 10 times. Steel pipe manufacturing method.

According to the present invention, even when the line speed is increased, the protective layer resin is excellent in extrudability and has an appropriate adhesion and peelability between the anticorrosive layer and the protective layer, and is welded. In some cases, the floating between the anticorrosion layer and the protective layer is also suppressed, and a multi-coated steel pipe having a protective layer with good scratch resistance can be easily and inexpensively manufactured. In addition, since the weld strength of the protective layer resin is also excellent in the present invention, even if an impact is applied to the weld portion of the protective layer (resin junction point in the protective layer extrusion coating machine), particularly at low temperatures, after the product is manufactured. The protective layer does not break from that part. Therefore, according to the present invention, it is possible to stably produce a high quality multi-clad steel pipe with high production efficiency, and there is a remarkable industrial effect.

It is a figure which shows typically the layer structure of a multi-clad steel pipe. It is a figure which shows typically a part of continuous line (multiple coating line) which manufactures a multi-coating steel pipe. It is a figure which shows a mode that the protective layer of a pipe end part is peeled at the time of welding construction of a multiple covering steel pipe. It is a figure which shows a mode that the protective layer in the multi-clad steel pipe of an Example is peeled.

Hereinafter, the present invention will be described in detail.
The multiple-coated steel pipe of the present invention is a multiple-coated steel pipe in which a corrosion prevention layer made of a polyethylene resin layer and a protective layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer are coated on the outside of the steel pipe that is a base material. The polypropylene resin forming the protective layer is a copolymer resin containing 19 to 23 mol% of an ethylene component, has a melt flow rate of 0.53 to 0.60 g / 10 min, and has a shear viscosity at 280 ° C. , When measured at a shear rate of 10 / sec, 1.7 × 10 ³ to 2.0 × 10 ³ Pa · s, and when measured at a shear rate of 100 / sec, 5.3 × 10 ² to 6.0 × 10 ² Pa · s.
The present invention relates to a multi-coated steel pipe in which a corrosion prevention layer made of a polyethylene resin layer and a protection layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer are coated on the outer side of the steel pipe as a base material. From a continuous line to high-speed production, particularly to make the line speed about 10 m / min or more.

And when manufacturing this multi-coating steel pipe, this invention is a copolymer resin which contains 19 mol% or more and 23 mol% or less of ethylene components for the polypropylene resin which forms a protective layer, and melt flow rate is 0.53 g / 10 min or more and 0.60 g / 10 min or less, and the shear viscosity at 280 ° C. is 1.7 × 10 ³ Pa · s or more and 2.0 × 10 ³ Pa · s or less when measured at a shear rate of 10 / sec. Yes, a polypropylene resin having a viscosity of 5.3 × 10 ² Pa · s or more and 6.0 × 10 ² Pa · s or less when measured at a shear rate of 100 / sec.

Further, in the present invention, after the outer surface of the steel pipe as a base material is coated with a polyethylene resin, the surface temperature of the polyethylene resin is cooled to 40 ° C. or more and 70 ° C. or less to form the anticorrosion layer, and then the anticorrosion layer is formed. A polypropylene resin having the above-mentioned predetermined characteristics is used on the surface of the resin, the temperature of the polypropylene resin is 260 ° C. or more and 290 ° C. or less, and the coating thickness is 0.9 mm or more and 1.8 mm or less, and 153 ° C. A multi-coated steel pipe is manufactured by forming the protective layer by cooling the surface temperature of the polypropylene resin to 170 ° C. or less at a cooling rate of / sec or more and 450 ° C./sec or less. .

(Steel pipe)
In the present invention, the type of steel pipe used as a base material for the multiple-coated steel pipe is not particularly limited, and is a conventionally known steel type used for forged welded steel pipes, ERW steel pipes, gas and water pipes, cable protection pipes, and the like. Any steel pipe can be applied. Further, the size of the steel pipe is not particularly limited. Usually, carbon steel pipes for pipes having a nominal diameter of 25A to 500A defined in JIS G 3452 (2010) are used depending on the application. In this invention, the thing of nominal diameter 100A or more and 200A or less is made into object. This is because the multi-coated steel pipe used for the above-mentioned application is a carbon steel pipe for piping normally specified in JIS G 3542 (2010), and a steel pipe having a size smaller than the nominal diameter is coated at high speed. However, in particular, there is no significant problem with respect to the fusion and peeling between the anticorrosion layer and the protective layer. Furthermore, for steel pipes with a size larger than the above nominal diameter, the coating speed cannot usually be increased because of factory equipment. Therefore, the problem in the present invention does not occur.
In addition, a known pickling treatment or blast treatment can be applied to the outer surface of the steel pipe as a base material as a base treatment. Subsequent to the base treatment, a known primer coating for improving the adhesion between the anticorrosion layer and the steel pipe or an adhesive defined in JIS G 3469 (2010) can be applied. Note that the inner surface of the steel pipe may be left as it is, or may be painted before and after the outer surface multiple coating.

(Anti-corrosion layer)
The polyethylene resin used for the anticorrosion layer is a homopolymer of ethylene or ethylene and α-olefin (olefin), for example, propylene, 1-butene, 1-pentene, 1-hexene. , 4-methyl-1-pentene, 1-octene, 1-decene and the like, which is a polymer mainly composed of ethylene. The polyethylene resin preferred in the present invention has a density of 920 to 950 kg / m ³ , more preferably a density of 940 to 950 kg / m ³ , a Vicat softening temperature of 110 to 130 ° C., and a tensile strength (Tensile Yield strength). ): 30 to 50 N / mm ² (MPa), Tensile failure at break (Elongation at break): 500 to 900%, and the thickness of the anticorrosion layer is suitably about 0.6 to 1.5 mm. In addition, as for said density, Vicat softening temperature, tensile strength, and a tensile fracture strain, all shall use the value measured according to prescription | regulation of JISG3469 (2010).
The polyethylene resin layer (anticorrosion layer) is formed on the outside of the steel pipe by melt-extruding these polyethylene resins. This polyethylene resin layer is not only a single layer but also acid-modified polyethylene (Acid modified polyethylene). It is good also as a composite layer which coextruded two layers (the steel pipe side is acid-modified polyethylene).

(Protective layer)
The polypropylene resin used for the protective layer is a polymer obtained by polymerizing monomers ethylene and propylene by a known method such as a Ziegler-Natta catalyst, that is, a block polypropylene (block which is usually referred to industrially). copolymer (polypropylene), which is a mixture of polypropylene, polyethylene, and an ethylene-propylene random copolymer. And in this invention, it is essential that the ethylene component which occupies for the whole polypropylene resin is 19-23 mol% (13.5-16.6 mass%).
The ratio of the ethylene component can be obtained by dissolving a polypropylene resin in a solvent and using a method such as nuclear magnetic resonance spectroscopy of the resin component.

When the proportion of the ethylene component is less than 19 mol% (13.5% by mass), the affinity (adhesion) with the polyethylene resin as the anticorrosion layer is too low, and as described above, Problems such as floating may occur between the protective layer and the like. On the other hand, when the proportion of the ethylene component exceeds 23 mol% (16.6 mass%), it becomes easy to fuse with the polyethylene resin as the anticorrosion layer, and appropriate peelability cannot be obtained. Furthermore, when the proportion of the ethylene component exceeds 23 mol% (16.6% by mass), the hardness of the polypropylene resin becomes low, and the protective layer is easily wrinkled.

In the present invention, it is essential that the melt flow rate of the polypropylene resin of the protective layer is 0.53 to 0.60 g / 10 min. The melt flow rate is a melt mass flow rate (MFR) defined in JIS K6921-2 (2010), and is a value obtained by a method defined in JIS K7210 (1999). When the melt flow rate is less than 0.53 g / 10 min, the extrudability of the polypropylene resin is lowered, so that the load on the extrusion coating machine increases and the extrusion discharge amount of the polypropylene resin cannot be maintained. Such a problem particularly appears when manufacturing at a high line speed, that is, when increasing the coating speed of the resin.

On the other hand, if the melt flow rate is less than 0.53 g / 10 min, the strength of the weld portion of the resin when the polypropylene resin is extruded into a cylindrical shape is lowered. On the other hand, when the melt flow rate exceeds 0.60 g / 10 min, there are problems that the strength of the obtained protective layer is lowered and the extrusion characteristics are lowered.
As described above, in the present invention in which the multi-coated steel pipe is manufactured in a continuous line with a high line speed, the melt flow rate of the polypropylene resin of the protective layer is 0.53 to 0.60 g / It is important to set it to 10 min.

Further, in the present invention, the shear viscosity at 280 ° C. of the polypropylene resin is 1.7 × 10 ³ to 2.0 × 10 ³ Pa · s when measured at a shear rate of 10 / sec, and the shear rate is It is essential that it is 5.3 × 10 ² to 6.0 × 10 ² Pa · s when measured at 100 / sec. The shear viscosity is measured by a capillary rheometer (sometimes called a capillary rheometer) defined in JIS K7199 (1999), and the capillary tube has an inner diameter D and a length L. , L / D = 10/1.

The shear viscosity is less than 1.7 × 10 ³ Pa · s when measured at a shear rate of 10 / sec, or less than 5.3 × 10 ² Pa · s when measured at a shear rate of 100 / sec. In this case, the peel strength (peeling strength) between the anticorrosive layer (polyethylene resin) and the protective layer (polypropylene resin) becomes too high, and problems such as no good peelability between the anticorrosive layer and the protective layer become apparent. To do.
On the other hand, when the shear viscosity exceeds 2.0 × 10 ³ Pa · s when measured at a shear rate of 10 / sec, or exceeds 6.0 × 10 ² Pa · s when measured at a shear rate of 100 / sec. In this case, the strength of the weld portion of the resin when the polypropylene resin is extruded into a cylindrical shape is lowered. In addition, the peel strength (peeling strength) between the anticorrosion layer (polyethylene resin) and the protective layer (polypropylene resin) becomes too low, and floating easily occurs between the anticorrosion layer and the protective layer during welding. It will cause trouble.

In the present invention, the reason for prescribing the case where the shear viscosity at 280 ° C. of the polypropylene resin is measured at two shear rates (10 / sec, 100 / sec) is that when the surface of the anticorrosion layer is extrusion coated. This is because the shear rate of the polypropylene resin (protective layer resin) is taken into consideration. It is extremely difficult to measure the shear rate when extrusion coating a molten polypropylene resin (protective layer resin). From a shear rate of 10 / sec or less depending on the extrusion coating conditions, it is 100 / sec or more. A shear rate is assumed. Therefore, in the present invention, the shear viscosity is defined in two ways: when the shear rate is low (10 / sec) and when the shear rate is high (100 / sec).

In the present invention, by using the polypropylene resin having the ethylene component, the melt flow rate and the shear viscosity adjusted to the desired values as described above as a protective layer, good adhesion and adhesion between the anticorrosion layer and the protective layer, protection Sufficient hardness for the layer (durometer hardness type D defined in ASTM D2240: 70 or more) is ensured. In addition, when the protective layer requires further mechanical strength, low-temperature impact resistance, etc., tensile fracture measured in accordance with the provisions of JIS K7162 (1994) The point stress is 22 to 45 MPa, the tensile elongation at measurement measured in accordance with JIS K7162 (1994) is 600 to 900%, and is measured in accordance with JIS K 7110 (1999). An Izod impact value of 2 to 6 kJ / m ² at −20 ° C. and a density of 905 to 910 kg / m ³ measured in accordance with JIS K7112 (1999) is usually used for the protective layer. Polypropy It can be used as a len resin.

The thickness of the protective layer is not particularly limited, but is about 0.8 to 2.8 mm from the viewpoint of economy and protection of the anticorrosion layer from wrinkles during transportation and construction. Is preferred.
When the thickness of the protective layer is less than 0.8 mm, it is difficult to protect the anticorrosion layer from wrinkles during transportation and construction. On the other hand, when the thickness of the protective layer exceeds 2.8 mm, heat storage of the polypropylene resin coated on the surface of the anticorrosive layer is controlled even if the temperature of the polypropylene resin (protective layer resin) during extrusion is controlled within the scope of the present invention described later. Since the amount is too large, fusion with the anticorrosion layer (polyethylene resin layer) is likely to occur. More preferably, it is the range of 0.9 mm or more and 1.8 mm or less.

The polypropylene resin as described above is appropriately prepared by a conventionally known method in which ethylene and propylene are used as the main monomers and produced by performing addition polymerization using a Ziegler-Natta catalyst or a single-site catalyst. However, since polypropylene resin manufacturers have prepared a wide variety of resins having various physical properties, those satisfying the physical properties of the present invention can be appropriately used.

In addition, in the polyethylene resin (anticorrosion layer) or the polypropylene resin (protective layer) used as the coating layer of the present invention, an antioxidant for preventing oxidative degradation / light degradation of the resin (light degradation) ( The purpose of the present invention is to add additives such as antioxidants, ultraviolet absorbers, coloring agents such as pigments, and nucleating agents for improving moldability. You may mix | blend within the range which does not impair.

The peel strength (peel strength) between the anticorrosive layer (polyethylene resin) and the protective layer (polypropylene resin) is preferably 0.6 N / 10 cm width or more and 15 N / 10 cm width or less, particularly 0.6 N / 10 cm width to 10 N / 10 cm width is more preferable. The peel strength is a peel strength measured based on a peel strength test described later. If the peel strength is less than 0.6 N / 10 cm width, the protective layer may be peeled off just by making a slit in the protective layer in order to remove the protective layer from the anticorrosive layer, or only the protective layer at the end of the steel pipe When peeling off, problems such as the end of the remaining protective layer floating from the anticorrosive layer are likely to occur. On the other hand, when the peel strength exceeds 15 N / 10 cm width, as described above, it is difficult to peel off the anticorrosion layer and the protective layer during on-site welding, and problems such as poor workability are likely to occur. Become. For the above reasons, the peel strength between the anticorrosion layer and the protective layer is preferably 0.6 N / 10 cm width or more and 15 N / 10 cm width or less.

Next, a method for producing the multi-coated steel pipe of the present invention will be described.

(Formation of anticorrosion layer and protective layer)
In the present invention, the outer surface of the steel pipe is coated with a polyethylene resin to form an anticorrosion layer, and the protective layer is formed on the surface of the anticorrosion layer by coating with the polypropylene resin having predetermined characteristics as described above.
When the anticorrosion layer and the protective layer are formed, they are formed by extrusion coating on a continuous line, for example, a multiple coating line as shown in FIG. In the multiple coating line of FIG. 2, the steel pipe 1 as a base material is conveyed from the upstream side of the line, the steel pipe 1 is preheated by the steel pipe preheating device 10 as necessary, and heated to a predetermined temperature by the adhesive application device 20. An adhesive (not shown) is applied to the surface of the steel pipe 1. Thereafter, the anticorrosive layer extrusion coating machine 30 is extrusion coated with the molten anticorrosion layer resin 300 on the surface of the steel pipe 1 and cooled by the anticorrosion layer cooler 31 to form the anticorrosion layer 3. Then, the protective layer resin 400 in a molten state is extrusion coated on the surface of the anticorrosion layer 3 by the protective layer extrusion coater 40, and the surface is cooled by the protective layer cooler 41 to form the protective layer 4.
The multi-coated steel pipe of the present invention is preferably manufactured from a continuous line from the viewpoint of production efficiency, and the line speed is preferably about 10 to 40 m / min.

By applying such a continuous line, high-speed production of multiple coated steel pipes becomes possible. From the viewpoint of production efficiency, the line speed (conveying speed of the steel pipe 1) is preferably about 10 m / min or more. However, if the line speed is increased too much, a coating layer having a predetermined thickness may not be formed. Therefore, the line speed is preferably 40 m / min or less. More preferably, it is 25 m / min or less.
First, shot blast treatment (absolute blast cleaning), pickling treatment (hydrochloric acid aqueous solution (hydrochloric acid aqueous solution or aqueous solution of sulfuric acid) solution), temperature: room temperature to 90 ° C.) and the like, and then, as shown in FIG. 2, the steel pipe 1 is removed by a steel pipe preheating device 10 such as a high-frequency induction heater as required. Pre-heated and applied with a coating applicator of adhesive 20, rubber, asphalt (a Phalt), heated coating adhesive mainly comprising a resin (Figure shown) on the surface of the steel tube 1.

The preheating temperature of the steel pipe is preferably 40 to 80 ° C. from the viewpoint of ensuring the applicability and adhesion of the adhesive applied to the upper layer. The pressure-sensitive adhesive is mainly composed of rubber, asphalt, resin, etc. stipulated in JIS G3469 (2010). The thickness of the pressure-sensitive adhesive layer is 0.1 to from the viewpoint of economic and uniform application properties. It is preferably about 1 mm.

Subsequently, the polyethylene resin (anticorrosion layer resin 300) in a molten state (about 230 ° C. or more and about 280 ° C. or less) is extruded into a cylindrical shape from the crosshead die of the anticorrosion layer extrusion coating machine 30 to coat the outer surface of the pressure-sensitive adhesive layer. Then, the surface temperature of the polyethylene resin is immediately cooled to 40 ° C. or more and 70 ° C. or less by the anticorrosion layer cooler 31 to form the anticorrosion layer 3. In the present invention in which the line speed is increased to about 10 m / min or more, the cooling by the anticorrosion layer cooler 31 is preferably water cooling with high cooling efficiency.

After forming the anticorrosion layer 3, the polypropylene resin (protective layer resin 400) having a temperature of 260 ° C. or more and 290 ° C. or less is applied to the surface of the anticorrosion layer 3 having a surface temperature of 40 ° C. or more and 70 ° C. or less. Extruded in a cylindrical shape from the crosshead die of the coating machine 40, coated on the outer surface of the anticorrosion layer 3, and the surface temperature of the polypropylene resin was 170 ° C at a cooling rate of 153 ° C / sec or more and 450 ° C / sec or less by the protective layer cooler 41. It cools until it becomes below, solidifies a polypropylene resin, and forms the protective layer 4. FIG. From the viewpoint of cooling efficiency, the cooling by the protective layer cooler 41 is also preferably water cooling.

When the surface temperature of the anticorrosion layer 3 after being cooled by the anticorrosion layer cooler 31 (surface temperature of the anticorrosion layer when coating the polypropylene resin (protective layer resin 400)) exceeds 70 ° C., other conditions are set in the present invention. Even when the specified conditions are satisfied, a problem of fusion between the anticorrosive layer 3 and the protective layer 4 occurs. On the other hand, when the outer surface of the pressure-sensitive adhesive layer is coated with a molten polyethylene resin (anticorrosion layer resin 300) and cooled by water cooling in a cooling zone as shown in FIG. 2, the surface temperature of the polyethylene resin is the length of the cooling zone. However, when a steel pipe of the above-mentioned size having the anti-corrosion layer 3 having the above-described thickness is cooled by a normal line facility, it does not become less than 40 ° C. at a high-speed line speed of about 10 m / min or more. .

Therefore, in the present invention, the anticorrosion layer 3 is formed by cooling the surface temperature of the polyethylene resin (anticorrosion layer resin 300) coated on the outside of the steel pipe to 40 ° C. or more and 70 ° C. or less. When cooling the polyethylene resin coated on the outside of the steel pipe (anticorrosion layer resin 300), for example, the cooling in the anticorrosion layer cooler 31 is water-cooled, and the amount of water distributed on the surface of the polyethylene resin (anticorrosion layer resin 300) is By adjusting, the surface temperature of the polyethylene resin (anticorrosion layer resin 300) can be cooled to a desired range (40 ° C. or more and 70 ° C. or less).

In addition, when the surface of the anticorrosion layer 3 having a surface temperature of 40 ° C. or higher and 70 ° C. or lower is melt-extruded by the protective layer extrusion coating machine 40 with the above-described polypropylene resin having the predetermined characteristics (protective layer resin 400), the polypropylene When the temperature of the resin (protective layer resin 400) exceeds 290 ° C., the problem of fusion between the anticorrosive layer and the protective layer occurs even when other conditions satisfy the conditions specified in the present invention. On the contrary, if it is less than 260 ° C., the problem of floating occurs. Therefore, in the present invention, when the surface of the anticorrosion layer 3 having a surface temperature of 40 ° C. or higher and 70 ° C. or lower is coated with the polypropylene resin (protective layer resin 400), the temperature of the polypropylene resin (protective layer resin 400) is 260 ° C. Above 290 ° C.

In the above, the coating thickness of the polypropylene resin (protective layer resin 400) is 0.8 mm or more and 2.8 mm or less. That is, by setting the thickness of the protective layer 4 to 0.8 mm or more and 2.8 mm or less, it is possible to secure the scratch resistance of the protective layer and to suppress the fusion between the protective layer 4 and the anticorrosion layer 3. . More preferably, it is 0.9 mm or more and 1.8 mm or less.

When the protective layer 4 is formed by cooling the polypropylene resin (protective layer resin 400) with the protective layer cooler 41 after coating the outer surface of the anticorrosive layer 3 with the polypropylene resin (protective layer resin 400), the polypropylene resin (protective layer resin) 400) When the cooling rate of the surface temperature exceeds 450 ° C./sec, even if other conditions satisfy the conditions specified in the present invention, a problem of floating between the anticorrosive layer and the protective layer occurs. Therefore, if the temperature is less than 153 ° C./sec, the problem of fusion occurs.

The reason why the problem of floating or fusion occurs as described above is that when the molten protective layer resin (polypropylene resin) is coated on the anticorrosive layer (polyethylene resin layer), the temperature of the interface between them quickly cools down. This is because the fusion between the two layers is less likely to occur and the separation is easy, and conversely, if the decrease in the interface temperature is slow, the fusion between the two layers is more likely to occur and the separation is difficult. Is done. Moreover, it is estimated that the above-mentioned float and fusion | bonding can be suppressed by controlling the cooling rate of the interface between a corrosion prevention layer (polyethylene resin layer) and protective layer resin (polypropylene resin). However, in actual production, it is very difficult to control the cooling rate at the interface between the anticorrosion layer (polyethylene resin layer) and the protective layer resin (polypropylene resin).

Therefore, the present inventors tried to suppress floating and fusion by controlling the cooling rate of the surface temperature of the protective layer resin (polypropylene resin) instead of the cooling rate of the interface. As a result, if the thickness of the protective layer resin (polypropylene resin) is within the range specified in the present invention (0.8 mm or more and 2.8 mm or less), the cooling rate is set at the surface temperature of the protective layer resin (polypropylene resin). It has been found that cooling at a temperature is extremely effective in suppressing floating and fusion. For the above reasons, in the present invention, the outer surface of the anticorrosion layer is coated with a polypropylene resin at 260 ° C. or more and 290 ° C. or less, and then the surface temperature of the polypropylene resin is cooled at a cooling rate of 153 ° C./sec or more and 450 ° C./sec or less. .

Further, when the surface temperature of the polypropylene resin is cooled at a cooling rate of 153 ° C./sec or more and 450 ° C./sec or less, if the cooling stop temperature is more than 170 ° C. as the surface temperature of the polypropylene resin, the protective layer resin (polypropylene resin) There arises a problem of fusion between the resin and the anticorrosion layer resin (polyethylene resin). Therefore, in the present invention, the surface temperature of the polypropylene resin is cooled to 170 ° C. or less at a cooling rate of 153 ° C./sec or more and 450 ° C./sec or less to solidify the polypropylene resin, thereby forming the protective layer 4.

Even if the anticorrosion layer and the protective layer as an upper layer thereof are coated under the above-described conditions, even when the line speed is increased, the protective layer resin has excellent extrudability, and the anticorrosion layer and the protective layer It has moderate adhesion and peelability, suppresses floating between the anticorrosion layer and the protective layer during welding, and also has a protective layer with good anti-wetting and excellent weld strength. Multiple coated steel pipes can be manufactured easily and inexpensively. In addition, by covering the anticorrosion layer and the protective layer as an upper layer under the above-described conditions, the peel strength between the anticorrosion layer and the protective layer is 0.6 N / 10 cm width to 15 N / 10 cm width, or 1 It is possible to set the width between 0.0 N / 10 cm width and 10 N / 10 cm width.

If the peel strength (peel strength) between the anticorrosive layer (polyethylene resin) and the protective layer (polypropylene resin) is less than 0.6 N / 10 cm width, a slit is formed in the protective layer in order to remove the protective layer from the anticorrosive layer. When the protective layer is peeled off just by inserting, or when only the protective layer at the end of the steel pipe is peeled off, problems such as the end of the remaining protective layer floating from the anticorrosive layer easily occur. On the other hand, when the peel strength exceeds 15 N / 10 cm width, as described above, it is difficult to peel off the anticorrosion layer and the protective layer during on-site welding, and problems such as poor workability are likely to occur. Become.

For the above reasons, the peel strength (peel strength) between the anticorrosive layer (polyethylene resin) and the protective layer (polypropylene resin) is preferably 0.6 N / 10 cm width or more and 15 N / 10 cm width or less. It is more preferably 1.0 N / 10 cm width or more and 10 N / 10 cm width or less. According to the present invention, a multi-coated steel pipe having a peel strength between the anticorrosion layer and the protective layer of 0.6 N / 10 cm width or more and 15 N / 10 cm width or less can be obtained, thus eliminating the problems such as workability described above. It is extremely effective in doing so. In addition, the said peel strength is the peel strength measured based on the peel strength test mentioned later.

In producing the multi-coated steel pipe in the above-mentioned scope of the present invention, the polypropylene resin is coated to make the peel strength between the anticorrosive layer and the protective layer 1.0 N / 10 cm width or more and 10 N / 10 cm width or less. It is preferable to coat the polypropylene resin so that the draw ratio (length after coating formation / length during extrusion) is in the range of 5 to 10 times.

Usually, the protective layer is extruded in a cylindrical shape in a molten state from the extruder, and is coated on the outer surface of the anticorrosive layer on the outer surface of the steel pipe while being stretched in the steel pipe traveling direction (steel pipe axis direction), and is cooled and solidified in that state. The draw ratio indicates how much the protective layer resin discharged from the extruder is stretched and coated on the outer surface of the steel pipe, and is obtained by dividing the “length after coating formation” by the “length during extrusion”. .

Here, “extruded length” is assumed that the extruded molten resin was a cylindrical body, the discharge weight per hour of the molten resin discharged from the extruder, the density of the resin, and the die discharge of the extruder The length of the extruded cylinder per hour is calculated from the cross-sectional area of the resin on the surface. Further, the “length after coating formation” corresponds to the traveling speed of the steel pipe per hour (coating speed, that is, line speed).

When the draw ratio exceeds 10 times, the peel strength between the anticorrosion layer and the protective layer tends to be low, and when the draw ratio is significantly greater than 10 times, problems such as floating may occur. If it is less than 5 times, problems such as sagging may occur in the coating. In addition, as long as a normal extruder is used, when the steel pipe specified in the present invention is coated at the above speed (about 10 m / min or more), the relationship between the resin discharge amount and the line speed (coating speed), The draw ratio is not less than 5 times.

As described above, when the steel pipe serving as the base material has a large diameter or a high line speed, the surface of the anticorrosion layer when the protective layer is coated tends to be in a high temperature state. Therefore, when the coating condition of the polypropylene resin as the protective layer resin or the resin is outside the scope of the present invention, the anticorrosion layer and the protective layer are extremely fused, and appropriate peelability and adhesion between the anticorrosive layer and the protective layer. It is not possible to obtain a multi-coated steel pipe having the properties. On the other hand, according to the present invention in which a predetermined polypropylene resin is used as the protective layer resin and the coating conditions of the resin are defined, the surface of the anticorrosion layer when the protective layer is coated becomes a high temperature state of 70 ° C. as described above. Even if it exists, the fusion | melting of a corrosion prevention layer and a protective layer is controlled very effectively. Therefore, according to the present invention, it is possible to obtain a multi-coated steel pipe having moderate peelability and adhesion between the anticorrosive layer and the protective layer.

In addition, according to the present invention, a protective layer having a sufficient hardness capable of ensuring a desired scratch resistance by using a predetermined polypropylene resin as the protective layer resin, and having excellent weld strength of the polypropylene resin A multi-coated steel pipe with a protective layer. Furthermore, since the predetermined polypropylene resin used in the present invention exhibits excellent extrudability even when the line speed is increased, according to the present invention, a high-quality multi-coated steel pipe can be made highly efficient and stable. Production is possible.

It should be noted that the present invention can also be applied when the line speed is reduced. Although it is disadvantageous from the viewpoint of productivity, it is assumed that the line speed is made slower than 10 m / min for some reason. Thus, if the line speed is reduced, the surface temperature of the anticorrosion layer at the time of covering the protective layer decreases, and the temperatures of the molten anticorrosion layer resin and the protective layer resin also decrease. However, even in such a case, in the present invention, the water-cooling distance is shortened, the amount of water is reduced, or the temperature setting of the extrusion coating machine is changed, so that the molten anticorrosion layer resin and the protective layer The temperature of the resin can be the same as that at the high line speed. That is, although it has been extremely difficult to cope with an increase in line speed in the prior art, the present invention can be applied regardless of whether the line speed is high or low.

The SGP steel pipe with the nominal diameter shown in Table 1 specified in JIS G 3452 (2010) is used as the base material, and the outer surface of the base material is subjected to blasting and then multiple coatings in a continuous line as shown in FIG. A steel pipe was manufactured.
The steel pipe after blasting is preheated to 50 ° C., and after applying an adhesive specified in JIS G3469 (2010) (adhesive layer thickness: 0.3 mm), density: 950 kg / m ³ , Vicat softening temperature : 121 ° C., tensile strength: 41 N / mm ² (41 MPa), tensile fracture strain: 600% commercially available high density polyethylene resin, or density: 920 kg / m ³ , Vicat softening temperature: 110 ° C., tensile strength: 20 N / mm ² (20 MPa), tensile fracture strain: 1000% commercially available low-density polyethylene resin is melted in an extrusion coating machine, and is cylindrically formed on the outer surface of the adhesive from the crosshead die. The steel tube was extruded and coated with a corrosion protection layer. After coating, the obtained polyethylene layer (anticorrosion layer) was immediately cooled with water to obtain an anticorrosion layer having a thickness shown in Table 1.
The above-mentioned density, Vicat softening temperature, tensile strength, and tensile fracture strain are all measured in accordance with JIS G 3469 (2010).

Next, a commercially available polypropylene resin having the physical properties shown in Table 1 was melted by an extrusion coating machine, and the outer surface of the polyethylene layer (anticorrosion layer) was extrusion coated from the crosshead die under the conditions shown in Table 1. The polypropylene resin layer (protective layer) was immediately water-cooled to obtain a protective layer having a thickness shown in Table 1 to obtain a multi-coated steel pipe. In addition, the thickness of the anticorrosion layer and the protective layer shown in Table 1 is a value in a range (maximum value and minimum value) measured at eight locations in the circumferential direction with respect to a cross section perpendicular to the tube axis direction of the multi-coated steel pipe. .

Further, the melt flow rate of the polypropylene resin shown in Table 1 is a melt mass flow rate (MFR) defined in JIS K6921-2 (2010), and was measured by a method defined in JIS K7210 (1999). The shear viscosity at 280 ° C. was measured with a capillary rheometer (sometimes called a capillary-type rheometer) specified in JIS K7199 (1999). This is a value measured under the condition of 10/1. The density was measured by the method prescribed in JIS K7112 (1999). In addition, although these values are the values measured for the resin before coating the steel pipe, they were the same as the values measured by cutting out the polypropylene resin from the protective layer portion of the manufactured multi-coated steel pipe.

With respect to the multi-coated steel pipe obtained as described above, the adhesion and peelability of the anticorrosion layer (polyethylene resin) and the protective layer (polypropylene resin), resistance to loss of adhesion, and the hardness of the protective layer (polypropylene resin). The weld strength was evaluated. Each evaluation method is as follows.

(I) Adhesiveness and peelability Various multi-coated steel pipes obtained were cut to a length of 50 cm, slits (cuts) were put in the protective layer at a position 20 cm from both pipe ends, and further, In the protective layer having a length of 10 cm sandwiched between the two slits, a slit was formed in the tube axis direction so that only the protective layer having a length of 10 cm sandwiched between the two slits could be peeled off. Next, as shown in FIG. 4, the 180 ° peel strength was measured by peeling off the protective layer having a length of 10 cm in the 180 ° direction at a rate of 50 mm / min. At that time, the maximum 180 ° peel strength measured was 0.6 N / 10 cm width to 10 N / 10 cm width and the adhesion and peelability were very good (◎), and the peel strength exceeded 10 N / 10 cm width. Adhesiveness and releasability are good for those having a width of 15 N / 10 cm or less (O), and other than this (less than 0.6 N / 10 cm width or more than 15 N / 10 cm width) are poor adhesion and releasability (×). did.

(Ii) Float resistance As shown in FIG. 4, after peeling off the 10 cm protective layer described above, the slit portion of the protective layer remaining on the steel pipe was visually observed, and no float was observed. Good floating resistance (◯) and those in which floating was observed were defined as poor floating resistance (×).

(Iii) Hardness of protective layer (polypropylene resin) The hardness of the protective layer is measured in accordance with ASTM D2240 (D type) according to the test method defined by Durometer hardness type D, 70 or more And those that were less than 70 were rejected (x).

(Iv) Weld strength of protective layer (polypropylene resin) Impact was applied to the weld portion of the protective layer of the obtained steel pipe (before the protective layer was peeled off) in the manner of a DuPont impact tester. A 1 kg weight having a steel ball of 25 mmφ as a hitting core was freely dropped from 500 mm above the impact surface, and the weld was impacted. After repeating this 100 times, whether or not the steel was exposed was confirmed with a holiday detector (pinhole testing machine) of 12 kV. The weld strength of the protective layer was good when the steel was not exposed. (◯), the case where the steel pipe exposure was detected was regarded as poor weld strength (x) of the protective layer.
These evaluation results are shown in Table 2.

As shown in Table 2, the examples of the present invention had good adhesion and peelability, float resistance, hardness of the protective layer (polypropylene resin), and weld strength of the protective layer (polypropylene resin). On the other hand, none of the comparative examples had good adhesion and peelability. Further, among the comparative examples, the steel pipe No. In Nos. 8 and 9, since the ethylene component of the polypropylene resin exceeded 23 mol%, the hardness of the protective layer (polypropylene resin) was insufficient.

As shown in FIG. 2, an SGP steel pipe having a nominal diameter shown in Tables 3-1 to 3-3 defined in JIS G 3452 (2010) is used as a base material, and the outer surface of the base material is blasted. With such a continuous line, multiple coated steel pipes were manufactured at the line speeds shown in Tables 3-1 to 3-3.
The steel pipe after the blast treatment is preheated to 50 ° C., and after applying an adhesive specified in JIS G3469 (2010) (adhesive layer thickness: 0.3 mm), density: 950 kg / m ³ , Vicat softening temperature : 121 ° C, tensile strength: 41 N / mm ² (41 MPa), tensile fracture strain: 600% commercially available high-density polyethylene resin is melted by an extrusion coating machine, and cylindrical on the outer surface of the adhesive from the crosshead die The steel tube was coated with a corrosion protection layer. After coating, the obtained polyethylene layer (anticorrosion layer) was immediately cooled with water to cool the surface temperature of the polyethylene layer to the temperatures shown in Tables 3-1 to 3-3. Tables 3-1 to 3-3 An anticorrosion layer having the thickness shown was obtained.

The above-mentioned density, Vicat softening temperature, tensile strength, and tensile fracture strain are all measured in accordance with JIS G 3469 (2010). The surface temperature of the polyethylene layer after water cooling (surface temperature immediately before contacting with the polypropylene resin) was measured with a contact thermometer.

Next, commercially available polypropylene resins having the physical properties shown in Table 3-1 to Table 3-3 were melted by an extrusion coating machine, and the outer surface of the polyethylene layer (corrosion protection layer) was crossed from the crosshead die to Table 3-1 to Table 3-3. The polypropylene resin having the temperature shown in 3-3 was extrusion coated at the draw ratios shown in Tables 3-1 to 3-3. After coating, the obtained polypropylene resin layer was immediately cooled with water at the cooling rate shown in Table 1 (the cooling rate of the surface of the polypropylene resin layer), and cooled until the surface temperature of the polypropylene resin layer became 170 ° C. or less. A protective layer having a thickness shown in 3-1 to Table 3-3 was obtained to obtain a multi-coated steel pipe.

Here, the melt flow rate of the polypropylene resin shown in Table 3-1 to Table 3-3 is the melt mass flow rate (MFR) defined in JIS K6921-2 (2010), and is defined in JIS K7210 (1999). It was measured by the method. The shear viscosity at 280 ° C. was measured with a capillary rheometer (sometimes called a capillary-type rheometer) specified in JIS K7199 (1999). This is a value measured under the condition of 10/1. The density was measured by the method prescribed in JIS K7112 (1999). In addition, although these values are values measured for the resin before coating the steel pipe, they were the same as the values measured by cutting out the polypropylene resin from the protective layer portion of the manufactured multi-coated steel pipe.

The cooling rate of the surface of the polypropylene resin layer (protective layer) shown in 3-1 to Table 3-3 is determined from the temperature at the time of coating (the temperature of the polypropylene resin immediately before contacting the anticorrosive layer). The cooling rate until the temperature reaches 170 ° C. The cooling rate and the surface temperature of the polypropylene resin layer during cooling were determined by measuring with a contact thermometer.

Further, the draw ratios of the polypropylene resins shown in Tables 3-1 to 3-3 are values obtained by the above method. The thickness of the anticorrosion layer and the protective layer shown in Table 3-1 to Table 3-3 is a range (maximum value and (Minimum value).

For the multi-coated steel pipe obtained as described above, the adhesion and peelability of the anticorrosion layer (polyethylene resin) and the protective layer (polypropylene resin), the float resistance, and the hardness and weld strength of the protective layer (polypropylene resin). evaluated. Each evaluation method was evaluated in the same manner as in the examples.
These evaluation results are shown in Table 4.

As shown in Table 4, the examples of the present invention had good adhesion and peelability, float resistance, hardness of the protective layer (polypropylene resin), and weld strength of the protective layer (polypropylene resin). On the other hand, none of the comparative examples had good adhesion and peelability. Further, among the comparative examples, the steel pipe No. In Nos. 28 and 29, since the ethylene component of the polypropylene resin exceeded 23 mol%, the hardness of the protective layer (polypropylene resin) was insufficient.

DESCRIPTION OF SYMBOLS 1 ... Steel pipe 2 ... Adhesive layer 3 ... Anticorrosion layer (polyethylene resin layer)
4 ... Protective layer (polypropylene resin layer)
5 ... Cut (slit)
6 ... Clearance (floating)
DESCRIPTION OF SYMBOLS 10 ... Steel pipe preheating apparatus 20 ... Adhesive coating apparatus 30 ... Anticorrosion layer extrusion coating machine 31 ... Anticorrosion layer cooling machine 40 ... Protection layer extrusion coating machine 41 ... Protection layer cooling machine 300 ... Anticorrosion layer resin (polyethylene resin)
400 ... Protective layer resin (polypropylene resin)

Claims

A multi-coated steel pipe in which a corrosion prevention layer made of a polyethylene resin layer and a protection layer made of a polypropylene resin layer as an upper layer of the corrosion prevention layer are coated on the outside of a steel pipe as a base material, and the polypropylene resin forming the protection layer Is a copolymer resin containing 19 to 23 mol% of an ethylene component, the melt flow rate of the polypropylene resin is 0.53 to 0.60 g / 10 min, and the shear viscosity of the polypropylene resin at 280 ° C. is a shear rate of 10 1.7 × 10 3 to 2.0 × 10 3 Pa · s when measured at / sec, and 5.3 × 10 2 to 6.0 × 10 2 Pa · s when measured at a shear rate of 100 / sec. Is a multi-coated steel pipe.
The multi-coated steel pipe according to claim 1, wherein a peel strength between the anticorrosion layer and the protective layer is 0.6 N / 10 cm width or more and 15 N / 10 cm width or less.
When manufacturing a multi-coated steel pipe coated with a protective layer made of a polypropylene resin layer as an upper layer of the anticorrosion layer and a corrosion prevention layer made of a polyethylene resin layer on the outside of the steel pipe as a base material,
After coating the outside of the steel pipe with a polyethylene resin, the surface temperature of the polyethylene resin is cooled to 40 ° C. or more and 70 ° C. or less to form the anticorrosion layer,
Next, it is a copolymer resin containing 19 mol% or more and 23 mol% or less of an ethylene component on the surface of the anticorrosion layer, the melt flow rate is 0.53 g / 10 min or more and 0.60 g / 10 min or less, and shearing at 280 ° C. The viscosity is 1.7 × 10 3 Pa · s or more and 2.0 × 10 3 Pa · s or less when measured at a shear rate of 10 / sec, and 5.3 × 10 2 when measured at a shear rate of 100 / sec. A polypropylene resin having a Pa · s of 6.0 × 10 2 Pa · s or less is used, and the temperature of the polypropylene resin is 260 ° C. or more and 290 ° C. or less so that the coating thickness is 0.9 mm or more and 1.8 mm or less. The protective layer is formed by coating and cooling at a cooling rate of 153 ° C./sec or more and 450 ° C./sec or less until the surface temperature of the polypropylene resin becomes 170 ° C. or less. A method for manufacturing a multi-coated steel pipe.
The method for producing a multi-coated steel pipe according to claim 3, wherein the polypropylene resin is coated so that a draw ratio when coating the polypropylene resin is in a range of 5 to 10 times.