JP2023152869A - Method for manufacturing polyethylene-coated steel pipe - Google Patents

Abstract

To provide a method for manufacturing a polyethylene-coated steel pipe which is excellent in cathode peeling resistance and suppresses a change of mechanical property.SOLUTION: In a method for manufacturing a polyethylene-coated steel pipe having a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer and a polyethylene layer layered in this order on a steel pipe surface, a steel pipe surface temperature at the time of coating of the powder epoxy resin primer when the powder epoxy resin primer layer is formed is 160-210°C.SELECTED DRAWING: None

Description

The present invention relates to a method for manufacturing polyethylene-coated steel pipes, and in particular, to a method for manufacturing polyethylene-coated steel pipes, which are used for water pipes, cable protection pipes, line pipes, civil engineering applications, etc. The present invention relates to a method for producing a polyethylene-coated steel pipe, which has excellent cathodic peeling resistance and suppresses changes in mechanical properties of the steel pipe during production.

Polyethylene-coated steel pipes, whose surfaces are coated with polyethylene as an anticorrosion layer, have excellent corrosion resistance and are used in various types of piping. Polyethylene-coated steel pipes are increasingly being used for laying on the seabed or buried underground, and in such cases, cathodic protection is often used in combination. Although cathodic protection further improves the corrosion resistance of polyethylene-coated steel pipes, on the other hand, when cathodic protection is implemented, the polyethylene coating tends to peel off from the outer surface of the steel pipe, a problem known as cathodic peeling. It is being

Chromate treatment is known to be effective as a method for suppressing such cathode peeling. For example, Patent Document 1 discloses a resin-coated heavy corrosion-resistant steel material that is a chromate-coated steel material having a chromate layer on the surface of the steel material, and in which an epoxy primer layer, a maleic anhydride-modified polyolefin layer, and a polyolefin layer are sequentially laminated.

Japanese Patent Application Publication No. 2005-35061

However, in recent years, from the perspective of reducing environmental impact, there has been a demand for organic coated steel materials that are not subjected to chromate treatment, that is, are non-chromate treated and have excellent corrosion resistance.

The method described in Patent Document 1 has a problem in that it requires chromate treatment, which has a high environmental impact, when coating a steel pipe. Furthermore, when the mechanical properties of the steel pipe change due to heating during coating, there is a tendency that the strength generally increases and the workability decreases. In recent years, there has been an increase in applications where workability is more important from the viewpoint of on-site workability and stability when used as piping, and there has been a need for technology that can accommodate this.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a polyethylene-coated steel pipe that has excellent cathode peeling resistance and suppresses changes in mechanical properties.

In order to achieve the above object, the present inventors have made extensive studies and found that in a method for manufacturing a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on the surface of the steel pipe, the powder The polyethylene coating has excellent cathodic peeling resistance while suppressing changes in the mechanical properties of the steel pipe by keeping the surface temperature of the steel pipe at 160 to 210°C when painting the powder epoxy resin primer layer. It was discovered that steel pipes can be obtained.

The present invention has the following configuration.
[1] A method for manufacturing a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on the surface of the steel pipe,
A method for producing a polyethylene-coated steel pipe, characterized in that the surface temperature of the steel pipe during coating with the powder epoxy resin primer when forming the powder epoxy resin primer layer is 160 to 210°C.
[2] Before forming the powder epoxy resin primer layer, blasting is performed on the steel pipe surface, and the surface roughness Rz of the steel pipe surface after the blasting is 60 to 100 μm, and the peak count number is 20 or more. The method for producing a polyethylene-coated steel pipe according to [1], characterized by:

According to the present invention, it is possible to provide a polyethylene-coated steel pipe that has excellent cathodic peeling resistance and suppresses changes in mechanical properties.

According to the present invention, a polyethylene-coated steel pipe with excellent cathode peeling resistance can be manufactured by non-chromate treatment, so that the environmental load is reduced. Furthermore, according to the present invention, the mechanical properties of the steel pipe can be changed by using a powdered epoxy resin primer as the material for forming the primer layer, and by setting the surface temperature of the steel pipe to 160 to 210°C when forming the primer layer. A polyethylene-coated steel pipe can be obtained that has excellent cathode peeling resistance while suppressing this. Being able to suppress changes in the mechanical properties of the steel pipe has the advantage of facilitating the design of the steel pipe itself. In other words, changes in the mechanical properties of the steel pipe before and after coating are suppressed, making it easier to design and manage the mechanical properties of the polyethylene-coated steel pipe after coating from the mechanical properties of the steel pipe before coating. , polyethylene-coated steel pipes having desired mechanical properties can be efficiently produced.

The polyethylene-coated steel pipe of the present invention is manufactured by sequentially laminating a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer on the surface of the steel pipe.

Hereinafter, one embodiment of the method for manufacturing a polyethylene-coated steel pipe of the present invention will be described.

The method for producing a polyethylene-coated steel pipe of this embodiment includes the steps of forming a powder epoxy resin primer layer on the surface of the steel pipe, and forming a modified polyethylene resin adhesive layer on the surface of the steel pipe on which the powder epoxy resin primer layer has been formed. a step of forming a polyethylene layer on the surface of the steel pipe on which the modified polyethylene resin adhesive layer has been formed; and a step of cooling the steel pipe after forming the polyethylene layer. In this embodiment, these steps are performed continuously on a manufacturing line (coating line).
In this specification, the process of forming a powder epoxy resin primer layer, the process of forming a modified polyethylene resin adhesive layer, and the process of forming a polyethylene layer are collectively referred to as a coating process.

In the step of forming the powder epoxy resin primer layer, before applying the powder epoxy resin primer, the steel pipe surface is subjected to blasting treatment to clean the steel pipe surface and impart surface roughness. In order to remove salt adhering to the surface of the steel pipe, cleaning with water may be performed before blasting. Furthermore, after the blasting process and before heating the steel pipe, foreign matter such as iron powder may be removed by washing with water, pickling with phosphoric acid, or the like.

The surface (outer surface) of the steel pipe after blasting preferably has a surface roughness Rz of 60 to 100 μm and a peak count of 20 or more. Here, the surface roughness Rz is the ten-point average roughness defined by JIS B0601:1994. Moreover, the peak count number is the number of average lengths of contour curve elements included in the length L of the contour curve defined in JIS B0601:2013, and here the length L=10 mm. When the surface roughness Rz is 60 μm or more, the effect of increasing the adhesion surface area due to the surface roughness and improving the cathode peeling resistance due to the anchoring effect is further enhanced. When the surface roughness Rz is 100 μm or less, wetting becomes sufficient and it becomes easier to suppress the formation of voids between the coating film and the steel pipe surface, and the coating film thickness at the top of the surface roughness becomes smaller. This makes it easier to suppress the effect, and the effect of improving cathode peeling resistance is further enhanced. The surface roughness Rz is more preferably 60 to 90 μm. Further, when the peak count number on the surface of the steel pipe is 20 or more, the effect of improving the cathode peeling resistance due to the increase in the adhesive surface area is further enhanced. The peak count number is more preferably 25 or more. Note that the upper limit of the peak count number is not particularly limited, but is preferably 50 or less from the viewpoint of ensuring wettability of the paint.

Such blasting can be carried out by using steel grit, steel shot, or the like as an abrasive for blasting. By changing the type, particle size, etc. of the abrasive, the surface roughness Rz and peak count number of the steel pipe surface can be changed. The abrasive material is not particularly limited, but for example, steel grit may be used alone, steel shot may be used alone, or a mixture of steel grit and steel shot may be used.

After blasting, the steel pipe is heated. The steel pipe can be heated by induction heating or the like. This heating is performed to melt and harden the powder epoxy resin primer, and the temperature of the steel pipe surface (outer surface of the steel pipe) is 160 to 210°C. If the temperature is less than 160°C, the curing of the powder epoxy resin primer will be insufficient and good adhesion will not be obtained. Furthermore, if the temperature exceeds 210° C., strain aging hardening of the steel pipe occurs, resulting in a material change that increases the strength of the steel pipe. A more preferred range of the temperature is 170-200°C.

In the step of forming a powder epoxy resin primer layer, after the heating, a powder epoxy resin primer is applied to the surface of the steel pipe (outer surface of the steel pipe) to form a powder epoxy resin primer layer. The coating includes electrostatic powder coating.

The powder epoxy resin primer may be selected according to the characteristics of the coating line. Specifically, as the powder epoxy resin primer, a material that hardens at a steel plate surface temperature of 160 to 210° C. is used. Furthermore, it is preferable to use a material that does not completely harden before coating the modified polyethylene resin (second layer coating) when forming the second modified polyethylene resin adhesive layer. Since the powder epoxy resin primer layer is not completely cured before the second layer is applied, the adhesion between the powder epoxy resin primer layer and the second modified polyethylene resin adhesive layer is improved. is further increased, and the possibility of causing poor adhesion can be reduced. Furthermore, it is preferable to use a material that is sufficiently cured (the curing reaction is completed) before cooling is started after the coating process. However, since the curing reaction may proceed due to residual heat even after cooling has started, any material that can be sufficiently hardened by the time cooling is completed can be used.

Such a powder epoxy resin primer may, for example, meet the CSA Z245.20 Series of CSA standards (CSA: Canadian Standards Association) at a predetermined temperature (for example, 180° C.) for a candidate powder epoxy resin primer. The Cure Time (curing time) in Section 12.1 of Section 18 is determined, and selection can be made based on the relationship between this Cure Time and the characteristics of the coating line (the time required for the coating process (line speed)). Such a powder epoxy resin primer may be one whose cure time is adjusted by a known method, or may be selected from commercially available products.

As described above, in the present invention, a powder epoxy resin primer is used as the primer. Generally, as a primer, in addition to a powder epoxy resin primer, for example, a liquid epoxy resin primer is known. When a liquid epoxy resin primer is used, the thickness of the primer layer is about 50 μm, whereas in the present invention, a powder epoxy resin primer is used to form a primer layer with a thickness of about 150 μm to 500 μm. can do. It is known that cathodic peeling is caused by oxygen passing through the resin layer being reduced to hydroxide ions on the surface of the steel material, and the hydroxide ions. In the present invention, by using a powdered epoxy resin primer as the primer, a thick primer layer can be formed, so oxygen permeation through the resin layer can be suppressed, and cathode peeling resistance can be further improved. can. Furthermore, in the present invention, by using a material that can be cured at low temperatures as the powder epoxy resin primer, changes in mechanical properties of the steel pipe due to coating (in particular, increase in strength due to thermal history during coating) can be achieved. can also be avoided.

After the powder epoxy resin primer layer is formed, a modified polyethylene resin adhesive layer and a polyethylene layer are successively formed. In the step of forming a modified polyethylene resin adhesive layer, a modified polyethylene resin adhesive is applied to the surface of the steel pipe on which the powder epoxy resin primer layer has been formed by a method such as powder coating or extrusion coating. A resin adhesive layer can be formed.
Further, in the step of forming the polyethylene layer, the polyethylene layer can be formed by extrusion coating the surface of the steel pipe on which the modified polyethylene resin adhesive layer is formed with a heated and melted polyethylene resin. After that, the steel pipe after forming the polyethylene layer is cooled (water-cooled).

Through the above steps, a polyethylene-coated steel pipe is obtained in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on the surface of the steel pipe. The polyethylene-coated steel pipe is one in which changes in mechanical properties are suppressed. Specifically, the rate of change in the yield strength of the steel pipe before and after the coating process (before and after coating) [100 x (yield strength of polyethylene-coated steel pipe (MPa) - yield strength of the steel pipe before coating (MPa)) )/yield strength (MPa) of the steel pipe before coating] is less than 5%.

When mechanical properties change due to the thermal history of the coating process and the strength of the steel pipe increases after coating, the workability of the steel pipe decreases. For this reason, mechanical tests are sometimes carried out after coating, but due to the process, it is sometimes difficult to carry out mechanical tests after coating. In other words, a polyethylene-coated steel pipe with suppressed changes in mechanical properties is required. The polyethylene-coated steel pipe of the present invention has an excellent effect of suppressing changes in mechanical properties, and is particularly suitable for applications where slight changes in mechanical properties as described above are a problem. Examples of such uses include gas pipes.

As the steel pipe, an API SPECIFICATION 5L, X65 carbon steel pipe with an outer diameter of 610 mm, a pipe thickness of 17.6 mm, and a length of 12 m was used. Before coating (before applying the powder epoxy resin primer), a tensile test piece was taken from the end of the steel pipe to confirm changes in mechanical properties. The outer surface of the steel pipe was blasted using steel grit of different particle sizes as an abrasive to clean the surface of the steel pipe and give it a surface roughness Rz of 52 to 95 μm and a peak count of 18 to 35. Thereafter, the surface (outer surface) of the steel pipe was heated to 132 to 265° C. by induction heating. A powdered epoxy resin primer (PIPECLAD 2000 LAT-Slow GEL manufactured by Valspar) was applied to a thickness of approximately 300 μm on the outer surface of the heated steel pipe by electrostatic powder coating. The cure time (based on CSA Z245.20 Series 18 Section 12.1) of the powder epoxy resin primer at 180° C. was 130 seconds. Thereafter, a modified polyethylene resin adhesive layer and a polyethylene layer were successively formed. The modified polyethylene resin adhesive layer was coated with a modified polyethylene resin adhesive (ME0433 manufactured by Borealis) to a thickness of about 200 μm by electrostatic powder coating. The polyethylene layer was formed by winding polyethylene (HE3450 manufactured by Borouge) into a sheet using an extruder and a T-die to give a total film thickness of 3.2 mm. In this example, the time required for the coating process (time from application of powdered epoxy resin primer to start of cooling) was 130 seconds. After coating, water cooling was performed to produce a polyethylene-coated steel pipe.

(Tensile test)
Full-thickness test pieces were taken from the steel pipe before coating and from the steel pipe after coating (polyethylene-coated steel pipe), respectively, so that the longitudinal direction of the steel pipe and the test piece coincided. Then, a tensile test in accordance with API SPECIFICATION 5L was conducted with 5 tests, and the average value of the yield strength, which tends to change due to strain age hardening, was determined. When the yield strength of the steel pipe after coating was compared with that of the steel pipe before coating, it was determined that the change in mechanical properties was suppressed if the rate of change (rate of increase) was less than 5%. When the rate of change is less than 5%, there is an advantage in manufacturing that design is easy and yield can be increased. The test results are shown in Table 1.

(Cathode peeling test)
A test piece measuring 100 mm x 100 mm x total thickness was taken from the polyethylene-coated steel pipe manufactured by the above method. A circular artificial defect with a diameter of 6 mm was formed in the center of the test piece to expose the outer surface of the steel pipe. An acrylic cylinder with an inner diameter of 70 mmφ centered on the artificial defect was placed vertically on the polyethylene layer, fixed to the polyethylene layer with a sealant, and the inside of the cylinder was filled with a 3% by mass NaCl aqueous solution to create a cell. Platinum was used as a counter electrode, and the potential of the steel tube in the artificial defect area was maintained at −1.5 V with respect to the silver-silver chloride electrode using a potentiostat. The test piece was left as it was in a constant temperature bath at 80°C, and the potential was maintained for 28 days. After collecting the test piece, the acrylic cylinder was then removed and the polyethylene coating was removed from around the artificial defect using a chisel and a cutter. Incisions were made in the remaining primer layer in 8 directions with a cutter centered around the artificial defect. A cutter was inserted under the primer layer from the artificial defect, and the part where it was easily peeled off and the steel surface was exposed was designated as the peeled part. The lengths from the edge of the artificial defect part to the tip of the peeled part in eight directions were measured, and the average value was taken as the cathode peeling distance (mm). The smaller the value of this cathode peeling distance, the better it was, and "15 mm or less" was considered acceptable. Polyethylene-coated steel pipes that pass the test are suitable for installation on the seabed or buried underground. Table 1 shows the test results.

Invention example No. 1 to 7, the rate of change in the yield strength of the steel pipe satisfies the specified 5% or less, changes in the mechanical properties of the steel pipe are suppressed, the cathode peeling distance satisfies the specified 15 mm or less, and excellent cathodic peeling resistance is achieved. It shows. No. 8 to 14 are comparative examples. No. In Nos. 8 and 9, the surface temperature of the steel pipe during coating with the powder epoxy resin primer was low, the curing of the powder epoxy resin primer was insufficient, good adhesion was not obtained, and the cathode peeling distance was more than 15 mm. No. In Nos. 10 to 14, the surface temperature of the steel pipe during coating with the powder epoxy resin primer is high, and the rate of change in yield strength of the steel pipe is 5% or more. No. In Nos. 11, 12, and 14, the surface temperature of the steel pipe during coating with the powder epoxy resin primer was high, too much heat was applied to the powder epoxy resin primer, causing the resin to deteriorate, and the cathode peeling distance was more than 15 mm.

As described above, according to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance and suppressed changes in mechanical properties can be obtained. According to the present invention, a polyethylene-coated steel pipe with excellent cathode peeling resistance can be manufactured by non-chromate treatment, so that the environmental load is reduced. Furthermore, the polyethylene-coated steel pipe manufactured by the present invention suppresses changes in the mechanical properties of the steel pipe before and after coating, making it suitable for applications where slight changes in mechanical properties before and after coating are a problem. Applicable.

Claims (2)

In a method for manufacturing a polyethylene-coated steel pipe, in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on the surface of the steel pipe,
A method for producing a polyethylene-coated steel pipe, characterized in that the surface temperature of the steel pipe during coating with the powder epoxy resin primer when forming the powder epoxy resin primer layer is 160 to 210°C. Before forming the powder epoxy resin primer layer, the surface of the steel pipe is blasted, and the surface roughness Rz of the steel pipe surface after the blasting is 60 to 100 μm, and the peak count is 20 or more. The method for manufacturing a polyethylene-coated steel pipe according to claim 1.