Manufacturing method of polyethylene coated steel pipe

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

[Technical field]

[0001] The present invention relates to a method for producing a polyethylene-coated steel pipe, and in particular to a method for producing a polyethylene-coated steel pipe which is used for water pipes, cable protection pipes, line pipes, civil engineering applications, etc. and which has a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer on its surface, has excellent cathodic peeling resistance, and suppresses changes in the mechanical properties of the steel pipe during production. [Background technology]

[0002] Polyethylene-coated steel pipes, in which the surface of the pipe is coated with polyethylene as a corrosion-resistant layer, have excellent corrosion resistance and are used for various piping. Polyethylene-coated steel pipes are increasingly being laid on the seabed or buried underground, and in such cases, cathodic protection is often used in combination. Cathodic protection further increases the corrosion resistance of polyethylene-coated steel pipes, but on the other hand, when cathodic protection is performed, there is a problem that the polyethylene coating is easily peeled off from the outer surface of the steel pipe, and this problem is known as cathodic peeling.

[0003] It is known that chromate treatment is an effective method for suppressing such cathodic peeling. For example, Patent Document 1 discloses a chromate-coated steel material having a chromate layer on the surface of the steel material, which is a resin-coated heavy-duty corrosion-resistant steel material in which an epoxy primer layer, a maleic anhydride-modified polyolefin layer, and a polyolefin layer are laminated in this order. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-35061 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in recent years, from the viewpoint of reducing the environmental load, there has been a demand for organic coated steel materials that are not chromated, i.e., that are non-chromate treated and have excellent corrosion resistance.

[0006] The method described in Patent Document 1 has a problem in that a chromate treatment, which has a high environmental impact, is required when coating a steel pipe. In addition, when the mechanical properties of a steel pipe change due to heating during coating, there is a tendency that the strength increases and the workability decreases. In recent years, the number of applications in which workability is more important has increased from the viewpoints of on-site workability and stability during use as piping, and a technology that can meet this demand has been demanded.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a method for producing a polyethylene-coated steel pipe which has excellent resistance to cathodic peeling and is suppressed in changes in mechanical properties. [Means for solving the problem]

[0008] The inventors have conducted intensive research to achieve the above-mentioned object and have discovered that in a manufacturing method for polyethylene-coated steel pipes 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 a steel pipe, by setting the steel pipe surface temperature during the powder epoxy resin primer coating to form the powder epoxy resin primer layer at 160 to 210°C, a polyethylene-coated steel pipe having excellent cathodic peeling resistance can be obtained while suppressing changes in the mechanical properties of the steel pipe.

[0009] The present invention has the following configuration. [1] A method for manufacturing polyethylene-coated steel pipes 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 a steel pipe, A method for producing a polyethylene-coated steel pipe, characterized in that the surface temperature of the steel pipe during the powder epoxy resin primer coating when forming the powder epoxy resin primer layer is 160 to 210°C. [2] A method for producing a polyethylene-coated steel pipe according to [1], characterized in that a blast treatment is performed on the steel pipe surface before forming a powder epoxy resin primer layer, and the surface roughness Rz of the steel pipe surface after the blast treatment is 60 to 100 μm and the peak count number is 20 or more. Effect of the Invention

[0010] According to the present invention, it is possible to provide a polyethylene-coated steel pipe which has excellent resistance to cathodic peeling and is suppressed in changes in mechanical properties.

[0011] According to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance can be manufactured by non-chromate treatment, and thus the environmental load is reduced. Furthermore, according to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance can be obtained while suppressing changes in the mechanical properties of the steel pipe by using a powder epoxy resin primer as a material for forming the primer layer and setting the steel pipe surface temperature at 160 to 210°C when forming the primer layer. Since the change in the mechanical properties of the steel pipe can be suppressed, there is an advantage that the design of the steel pipe itself becomes easier. That is, since the change in the mechanical properties of the steel pipe before and after coating is suppressed, it becomes easier to design and manage the mechanical properties of the polyethylene-coated steel pipe after coating based on the mechanical properties of the steel pipe before coating, and a polyethylene-coated steel pipe with the desired mechanical properties can be efficiently manufactured. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The polyethylene-coated steel pipe of the present invention is produced by successively laminating a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer on the surface of a steel pipe.

[0013] Hereinafter, one embodiment of the method for producing a polyethylene-coated steel pipe of the present invention will be described.

[0014] The method for producing a polyethylene-coated steel pipe of this embodiment includes a step of forming a powder epoxy resin primer layer on a steel pipe surface, a step of forming a modified polyethylene resin adhesive layer on the steel pipe surface on which the powder epoxy resin primer layer has been formed, a step of forming a polyethylene layer on the steel pipe surface on which the modified polyethylene resin adhesive layer has been formed, and a step of cooling the steel pipe after the polyethylene layer has been formed. In this embodiment, these steps are performed continuously in a production line (coating line). In this specification, the process of forming the powder epoxy resin primer layer, the process of forming the modified polyethylene resin adhesive layer, and the process of forming the polyethylene layer are collectively referred to as the coating process.

[0015] In the step of forming the powder epoxy resin primer layer, the surface of the steel pipe is subjected to a blast treatment before the powder epoxy resin primer coating to clean the surface of the steel pipe and impart surface roughness. In order to remove salt adhering to the surface of the steel pipe, washing with water may be performed before the blast treatment. Furthermore, after the blast treatment, foreign matter such as iron powder may be removed by washing with water or pickling with phosphoric acid before heating the steel pipe.

[0016] The surface (outer surface) of the steel pipe after the blasting treatment 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 in JIS B0601:1994. The peak count is the number of the average length of the contour curve elements included in the length L of the contour curve defined in JIS B0601:2013, where the length L is 10 mm. When the surface roughness Rz is 60 μm or more, the effect of improving cathodic peeling resistance due to the increase in the adhesive surface area caused by the surface roughness and the anchoring effect is further enhanced. When the surface roughness Rz is 100 μm or less, wetting is sufficient, making it easier to prevent the occurrence of voids between the coating film and the steel pipe surface, and also easier to prevent the coating film thickness at the peaks of the surface roughness from becoming smaller, making it easier to improve the effect of improving cathodic peeling resistance. The surface roughness Rz is more preferably 60 to 90 μm. Furthermore, when the peak count number on the steel pipe surface is 20 or more, the effect of improving cathodic peeling resistance due to the increase in the adhesive surface area is further enhanced. The peak count number is more preferably 25 or more. There is no particular restriction on the upper limit of the peak count number, but it is preferably 50 or less from the viewpoint of ensuring the wettability of the paint.

[0017] Such blasting can be carried out by using steel grit, steel shot, etc. as an abrasive for blasting. By changing the type, particle size, etc. of the abrasive, the surface roughness Rz and the peak count number of the steel pipe surface can be changed. The abrasive 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.

[0018] After the blasting treatment, 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 set to 160 to 210°C. If the temperature is less than 160°C, the powder epoxy resin primer will not harden sufficiently and good adhesion will not be obtained. If the temperature exceeds 210°C, strain aging hardening of the steel pipe will occur, causing a material change that increases the strength of the steel pipe. The temperature is more preferably in the range of 170 to 200°C.

[0019] In the step of forming a powder epoxy resin primer layer, after the heating, the powder epoxy resin primer is applied to the surface (outer surface) of the steel pipe to form a powder epoxy resin primer layer. As the application, electrostatic powder coating can be used.

[0020] The powder epoxy resin primer may be selected according to the characteristics of the coating line. Specifically, the powder epoxy resin primer is a material that hardens at a steel plate surface temperature of 160 to 210°C. Furthermore, a material that does not completely harden before the modified polyethylene resin is applied (second layer coating) when forming the second layer of modified polyethylene resin adhesive layer is preferable. By not completely hardening the powder epoxy resin primer layer before the second layer coating, the adhesion between the powder epoxy resin primer layer and the second layer of modified polyethylene resin adhesive layer is further improved, and the risk of causing poor adhesion can be reduced. Furthermore, a material that is sufficiently hardened (hardening reaction is completed) before cooling is started after the coating process is preferable. However, since the hardening reaction may proceed due to residual heat even after cooling is started, any material that is sufficiently hardened before cooling is completed can be used.

[0021] Such a powder epoxy resin primer can be selected, for example, by determining the cure time of a candidate powder epoxy resin primer at a predetermined temperature (e.g., 180°C) according to CSA Standards (CSA: Canadian Standards Association, CSA Z245.20 Series 18, Section 12.1) and taking into consideration the relationship between this cure time and the characteristics of the coating line (time required for the coating process (line speed)). Such a powder epoxy resin primer may be used with the cure time adjusted by a known method, or may be selected from commercially available products.

[0022] As described above, in the present invention, a powder epoxy resin primer is used as the primer. In general, in addition to the powder epoxy resin primer, for example, a liquid epoxy resin primer is known as a primer. 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. It is known that cathodic peeling is caused by oxygen that permeates the resin layer being reduced to hydroxide ions on the steel surface. In the present invention, a powder epoxy resin primer is used as the primer, so that a thick primer layer can be formed, and therefore oxygen permeation through the resin layer can be suppressed, and the cathodic peeling resistance can be further improved. Furthermore, in the present invention, a material that can be hardened at low temperature is used as the powder epoxy resin primer, so that changes in the mechanical properties of the steel pipe due to coating (particularly, high strength due to the thermal history during coating) can also be avoided.

[0023] 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 the modified polyethylene resin adhesive layer, the modified polyethylene resin adhesive is applied to the steel pipe surface on which the powder epoxy resin primer layer has been formed by a method such as powder coating or extrusion coating, thereby forming the modified polyethylene resin adhesive layer. In the step of forming the polyethylene layer, the polyethylene layer can be formed by extrusion coating the heated and melted polyethylene resin onto the surface of the steel pipe on which the modified polyethylene resin adhesive layer has been formed. Thereafter, the steel pipe on which the polyethylene layer has been formed is cooled (water-cooled).

[0024] The above process produces a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are laminated in this order on the surface of the steel pipe. The polyethylene-coated steel pipe has suppressed changes in mechanical properties. Specifically, the rate of change in 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 steel pipe before coating (MPa)) / yield strength of steel pipe before coating (MPa)] is less than 5%.

[0025] When mechanical property changes occur during the thermal history of the coating process and the strength of the steel pipe after coating increases, the workability of the steel pipe decreases. For this reason, mechanical testing may be performed after coating, but since it may be difficult to perform mechanical testing after coating due to process reasons, a polyethylene-coated steel pipe that does not change mechanical properties due to the thermal history of the coating process, i.e., in which the change in mechanical properties is suppressed, is required. The polyethylene-coated steel pipe of the present invention has an excellent effect of suppressing the change in mechanical properties, and is particularly suitable for applications in which the above-mentioned slight change in mechanical properties is problematic. An example of such an application is a gas pipe. EXAMPLES

[0026] The steel pipe used was a carbon steel pipe of API SPECIFICATION 5L, X65 with an outer diameter of 610 mm, a pipe thickness of 17.6 mm, and a length of 12 m. Before coating (before painting with powder epoxy resin primer), tensile test pieces were 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 with different particle sizes as an abrasive to clean the steel pipe surface and impart a surface roughness of Rz 52 to 95 μm and a peak count of 18 to 35. The steel pipe surface (outer surface) was then heated to 132 to 265 ° C. by induction heating. The heated outer surface of the steel pipe was painted with a powder epoxy resin primer (PIPECLAD 2000 LAT-Slow GEL manufactured by Valspar) to a thickness of about 300 μm by electrostatic powder coating. The cure time of the powder epoxy resin primer at 180°C (based on CSA Z245.20 Series 18, section 12.1) was 130 seconds. After that, a modified polyethylene resin adhesive layer and a polyethylene layer were successively formed. The modified polyethylene resin adhesive layer was formed by electrostatic powder coating with a modified polyethylene resin adhesive (ME0433 manufactured by Borealis) to a thickness of about 200 μm. The polyethylene layer was formed by wrapping polyethylene (HE3450 manufactured by Borouge) in a sheet shape using an extruder and a T-die, with a total thickness of 3.2 mm. In this example, the time required for the coating process (the time from coating of the powder epoxy resin primer to the start of cooling) was 130 seconds. After coating, water cooling was performed to produce a polyethylene-coated steel pipe.

[0027] (Tensile test) Test pieces of the full wall thickness were taken from the steel pipe before coating and the steel pipe after coating (polyethylene-coated steel pipe) so that the longitudinal direction of the steel pipe and the test piece were aligned. Five tensile tests were then conducted in accordance with API SPECIFICATION 5L to determine the average value of the yield strength, which is prone to change due to strain-age hardening. Steel pipes with a yield strength change rate (increase rate) of less than 5% after coating were compared to steel pipes before coating were judged to have suppressed changes in mechanical properties. A change rate of less than 5% has the advantage of being easier to design in manufacturing and increasing yields. The test results are shown in Table 1.

[0028] (Cathodic Disbonding 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, exposing the outer surface of the steel pipe. An acrylic cylinder with an inner diameter of 70 mm was placed vertically on the polyethylene layer with the artificial defect as the center and fixed to the polyethylene layer with a sealant, and the inside of the cylinder was filled with a 3 mass% NaCl aqueous solution to create a cell. Platinum was used as the counter electrode, and the potential of the steel pipe at the artificial defect was maintained at -1.5 V against the silver-silver chloride electrode using a potentiostat. The test piece was left stationary in a thermostatic chamber at 80 ° C. and the potential was maintained for 28 days. Next, after recovering the test piece, the acrylic cylinder was removed, and the polyethylene coating was removed from around the artificial defect using a chisel and a cutter. The remaining primer layer was cut in eight directions with a cutter centered on the artificial defect. The cutter was inserted from the artificial defect under the primer layer, and the part where it was easily peeled off and the steel surface was exposed was the peeled part. The length from the end of the artificial defect to the tip of the peeled portion in each of the eight directions was measured, and the average value was taken as the cathodic peeling distance (mm). The smaller the value of this cathodic peeling distance, the better, and "15 mm or less" was considered to be a pass. Polyethylene-coated steel pipes that pass the test are suitable for use in laying on the seabed or burying underground. Table 1 shows the test results.

[0029] [Table 1]

[0030] In the invention examples No. 1 to 7, the rate of change in the yield strength of the steel pipe satisfies the prescribed condition of less than 5%, suppresses the change in the mechanical properties of the steel pipe, and satisfies the prescribed condition of 15 mm or less for the cathodic peeling distance, showing excellent cathodic peeling resistance. No. 8 to 14 are comparative examples. In No. 8 and No. 9, the steel pipe surface temperature when the powder epoxy resin primer is applied is low, the powder epoxy resin primer is insufficiently cured, good adhesion is not obtained, and the cathodic peeling distance exceeds 15 mm. In No. 10 to No. 14, the steel pipe surface temperature when the powder epoxy resin primer is applied is high, and the rate of change in the yield strength of the steel pipe is 5% or more. In No. 11, 12, and 14, the steel pipe surface temperature when the powder epoxy resin primer is applied is even higher, the powder epoxy resin primer is exposed to too much heat, and the resin deteriorates, and the cathodic peeling distance exceeds 15 mm.

[0031] Thus, according to the present invention, a polyethylene-coated steel pipe having excellent cathodic peeling resistance and suppressed changes in mechanical properties can be obtained. According to the present invention, a polyethylene-coated steel pipe having excellent cathodic peeling resistance can be produced by non-chromate treatment, thereby reducing the environmental load. Furthermore, the polyethylene-coated steel pipe produced according to the present invention suppresses changes in the mechanical properties of the steel pipe before and after coating, so it can be suitably used in applications where slight changes in mechanical properties before and after coating are problematic.

Claims

[Claim 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, Before forming the powder epoxy resin primer layer, a blast treatment is performed on the surface of the steel pipe, and the surface roughness Rz of the steel pipe surface after the blast treatment is 60 to 100 μm and the peak count number is 20 to 35, A method for producing a polyethylene-coated steel pipe, characterized in that the surface temperature of the steel pipe after the blast treatment during the powder epoxy resin primer coating for forming the powder epoxy resin primer layer is set to 160 to 210°C.

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