JP2018001547A - Induction heating method for polyolefin-coated steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that, in the case, upon production of a polyolefin resin-coated steel pipe, rapid heating is performed by high frequency induction heating, surface oxidation is made ununiform by the influence of the ruggedness and adsorption water in blast treatment, and severe cathode peeling test performance required for a line pipe has not been satisfied.SOLUTION: A steel pipe is subjected to a step of being homogenized to 100 to 160°C in the first stage, and high frequency heating is performed in the two stage or more to suppress the ununiform oxidation in the surface of the steel. In this way, even in the case it is subjected to heating to a high temperature required for the melting to curing of a powder primer, its deterioration in adhesion can be suppressed, and its cathode peeling resistance can be improved.SELECTED DRAWING: Figure 1

Description

  Polyolefin-coated steel pipes are usually used for the outer surface coating for anticorrosion of line pipes used for energy transportation such as oil and gas. Prior to this polyolefin coating, the steel pipes are induction-heated. The present invention relates to this induction heating method.

  A three-layer coated steel pipe is used when long-term corrosion resistance and weathering resistance during construction are required in an outer-surface polyolefin-coated steel pipe used for oil and gas energy transportation line pipes. The structure is a three-layer structure consisting of an epoxy resin primer layer, an adhesive layer, and a polyolefin resin layer having high anticorrosion properties from the steel pipe side. Polyolefin resins to be coated include polyethylene resins and polypropylene resins. However, since they are inexpensive and highly reliable, in general, polyethylene resins are used, and polypropylene resins are used when high temperature and weather resistance are required. used.

  Since line pipes have a very large accident loss due to leakage, we consider cases in which polyolefin-coated steel pipes have flaws that reach the steel surface before or during burial. Therefore, in the unlikely event that there is a flaw, cathodic protection is performed as a corrosion prevention method that lowers the potential below the natural potential of iron in order to suppress corrosion of the brim. However, since it is difficult to accurately capture the wrinkle generation rate and potential, there are cases where the anticorrosion becomes excessive. In that case, a phenomenon in which peeling of the coating progresses around the collar portion, that is, so-called cathode peeling occurs. If the stripping of the cathode progresses and the exposed area of the steel material becomes large, corrosion will eventually occur. Therefore, the stripping resistance of the cathode is a particularly important item in the anticorrosion performance of the line pipe.

  Polyolefin coating has a three-layer coating structure to suppress cathode peeling, and surface treatment and resin improvement are performed day and night. In order to suppress cathode peeling, primer treatment of steel and primer are important. At present, instead of a thin film primer coated with a liquid epoxy resin, a thick film primer coated with a high performance powder epoxy resin has been used. In order to cure the resin in any epoxy resin, it is essential to heat the steel pipe before or after primer coating by a method such as gas or induction heating.

  However, in the manufacturing method using the induction heating method with a liquid epoxy resin, as shown in Patent Document 1, the heating temperature of the steel pipe is as low as 180 ° C., and the heating is performed after the liquid epoxy resin coating. On the other hand, for thick film coating with powder epoxy resin, it is necessary to heat the steel pipe before coating, and at the same time, the heating temperature of the steel pipe is higher than that of the liquid. This is especially true for powdered epoxy resins, because the higher the melt viscosity, the lower the melt viscosity. Therefore, it is better to perform blasting on the base and paint at a high temperature on steel pipes with fine irregularities of several μm or less on the surface. It depends on the reason why it is obtained.

JP 58-74337 A

  In particular, in the coating using a powder epoxy resin as a primer, the steel heating temperature needs to be 180 ° C. or higher, and if the temperature is further increased, the melt viscosity decreases and the permeability to the surface of the rough steel material increases. Cathode peeling performance is also improved. For this reason, 200 degreeC or more is preferable as steel material temperature at the time of powder epoxy resin coating. However, in high frequency induction heating, a phenomenon occurs in which the performance of the powder primer cannot be exhibited sufficiently even when heated to 200 ° C. or higher. That is, when the blasted steel pipe is heated by high-frequency induction heating, there is a problem that sufficient anti-cathode peeling performance cannot be obtained.

As a result of intensive studies to solve such problems, it has become clear that the cause is due to the characteristics of high-frequency induction heating. That is, in high-frequency induction heating, heat is generated by an induced current flowing through the steel material, but when the frequency is high, a portion having a shallow surface particularly generates heat. This is called the skin effect. For example, when the frequency is 20000 Hz, the penetration depth of the induced current is a very shallow region of 50 μm in general carbon steel. On the other hand, since the surface of the blasted steel material has irregularities of 50 μm or more, the convex portion becomes extremely hot and non-uniform oxidation proceeds on the steel material surface. For this reason, it is thought that adhesiveness with a powder primer coating film falls.
Therefore, we examined in detail how to prevent performance degradation in powder epoxy resin coating due to high-frequency induction heating in the coating line. As a result, the steel pipe is heated in two or more stages, specifically, the first stage is uniformly heated to 100 to 160 ° C., and then the second stage is heated to 200 to 260 ° C. or more, so that the steel material surface is non-uniform. It has been found that oxidation can be suppressed and coating can be performed at a high temperature without reducing the anti-cathode peeling performance of the powder epoxy resin.
Furthermore, by setting the frequency of high-frequency induction heating to 3000 Hz or less, and more desirably 2000 Hz or less, the penetration depth of the induction current is sufficiently ensured with respect to the roughness, and uneven heating of the convex portion generated by the blasting process is reduced. I can do it.

  The polyolefin-coated steel pipe manufactured by the heating method of the present invention does not become uneven in the blast surface by performing high-frequency induction heating in two or more stages. Therefore, when a powder epoxy resin is used as a primer, It becomes possible to heat to a high temperature of 200 ° C. or higher, and good cathode peeling performance can be obtained.

FIG. 1 is a schematic view showing an example of the arrangement of induction heating devices in the method for producing a polyolefin resin-coated steel pipe of the present invention. FIG. 2 is a schematic diagram showing an example of the arrangement of induction heating devices in a general method for producing a polyolefin resin-coated steel pipe. FIG. 3 is an image diagram of a steel pipe surface temperature curve when a plurality of stages (two stages) of heating and soaking of the present invention are performed. FIG. 4 is an image diagram of a steel pipe surface temperature curve when a single (one stage) heat treatment without a general soaking time is performed. FIG. 5 shows an example of the relationship between the frequency of high frequency induction heating and the current penetration depth.

Hereinafter, the polyolefin coating material of the present invention will be described.
There is no restriction | limiting in particular in the steel pipe used for the coating | cover of this invention, It can apply to the steel types used for a line pipe, such as normal steel or high alloy steel. In addition, restrictions on size and thickness are attributed to equipment.

  Next, the epoxy resin primer layer will be described. Powder epoxy resin paints are bisphenol A type epoxy resin and bisphenol F type epoxy resin, or mixed with polyfunctional phenol novolac type epoxy resin or halogenated epoxy resin. , An imidazole compound, a dicyandiamide, and the like. As the inorganic pigment, a pigment such as silica, titanium oxide, wollastonite, mica, talc, kaolin, chromium oxide, zinc borate, and zinc phosphate, metal powder such as zinc and Al, ceramic powder, and the like can be used as appropriate. . For powder epoxy resin paints, brands sold for steel pipe coating by manufacturers such as JOTUN, Arsonsisi, 3M, etc. are used as appropriate. In Japan, it can be obtained from Nippon Paint Co., Ltd. or Kansai Paint Co., Ltd., and the primer layer is suitably 150 to 600 μm in thickness.

  After coating the powder epoxy resin primer layer, a polyolefin resin film is laminated through a modified polyolefin resin adhesive. The modified polyolefin resin adhesive is modified based on a polyolefin resin modified with maleic anhydride, or a copolymer of polyolefin and maleic anhydride, a copolymer of polyolefin, acrylic acid ester and maleic anhydride. Things can be used. The modified polyolefin resin adhesive is supplied as powder or pellets. Examples of the adhesive used for polyethylene supplied in pellets include NE060, NE065, NE080 manufactured by Mitsui Chemicals, Borcoat ME0420 manufactured by BOREALIS, Lucalen G3710E manufactured by Lyondell Basell, and the like. In the case of pellets, the outer surface of the steel pipe after primer application is coated with a resin that has been heated and melted using an adhesive extruder using a T die. For small and medium diameter steel pipes, round dies may be used. As another method, there is a method in which the modified polyolefin resin adhesive is pulverized and powdered, and this powder is applied. By these methods, an adhesive layer of 0.1 to 0.4 mm is formed.

  As the polyolefin resin to be coated on the modified polyolefin adhesive layer, those commercially available for coating a steel pipe can be used. A typical polyolefin is polyethylene, and in addition, polypropylene is used for high temperatures. As the polyethylene resin, a brand used for coating a steel pipe can be used. For example, NOVATEC ER002S manufactured by Japan Polyethylene Co., Ltd., Borcoat HE3450 manufactured by BOREALIS Co., Ltd. Can be used.

  Polypropylene resins generally use copolymers with better low-temperature properties than homopolymers, and include color pigments, filling enhancers, antioxidants, UV absorbers, and hindered amine-based weather resistance as measures against heat resistance and weather resistance. It is preferable that an agent or the like is added, and has both low temperature toughness and oxidation deterioration resistance at high temperature use. When carbon black is used as the color pigment of polypropylene resin, the effect of the antioxidant disappears at high temperatures, and therefore it cannot be used in high performance coated steel pipe products. For this reason, the color of the color pigment is generally white. As the polypropylene resin, for example, TX1843B of Japanese polypropylene in Japan, Borcoat BB108E-1199 manufactured by BOREALIS, and Moplen Coat EP60R / BIANCO manufactured by LyondellBasell can be used overseas.

  The polyolefin resin coating layer covers 2 mm or more in order to suppress wrinkling during handling. The thicker the polyolefin resin coating layer is, the better the weather resistance and anticorrosion properties are.

The method for producing a polyolefin-coated steel pipe according to the present invention will be described below.
The steel pipe is blasted in order to remove the rust and dirt on the surface of the steel pipe and secure the roughness required for bonding after removing the oil adhesion in advance. In general, steel shot grains and grid grains are used as the abrasive for blasting. Further, when a clean surface is required, a ceramic material such as alumina may be used. When dirt such as iron powder adheres to the surface after the blast treatment, treatments such as brushing, suction, and washing with liquid can be performed. For cleaning, a method using an acidic cleaning agent is effective, and the steel material is heated to 30 to 80 ° C. to increase the reactivity. As the acid of the cleaning agent, phosphoric acid is preferable, and for example, Oakite 31, 32, 33, 131 of Chemetall can be used. After washing, wash with water and dry. Further, even if chromate or other chemical conversion treatment is performed, the effect of the present invention is not reduced. However, the effect of the heating method of the present invention is reduced when a protective film is formed on the steel surface by a cleaning agent or chemical conversion treatment.

Next, a method for heating a steel pipe before painting by high frequency induction heating according to the present invention will be described. In the present invention, even when a steel pipe having fine irregularities of about 50 μm by the blast treatment described above is heated at a high temperature of 200 ° C. or higher by high frequency induction heating, uneven oxidation of the steel surface is prevented and the cathode peeling resistance is deteriorated. Can be suppressed.
The surface of steel during blasting is a clean surface, but it is immediately covered with thin oxides and molecular water. It is observed by X-ray photoelectron spectroscopy (XPS / ESCA) that the thickness of the oxide layer at this initial stage is about 5 nm. On the other hand, in high frequency induction heating, only the steel pipe surface is instantaneously heated to a high temperature. In particular, the blasted steel pipe may have roughness, and particularly the convex portion and the bald portion having a small contact area with the steel pipe instantaneously become a high temperature, so that surface layer oxidation due to adhering water occurs violently. Therefore, when the steel material surface is heated suddenly by normal one-step heating, the oxide film grows rapidly with the surface being non-uniform, and the steel material surface is covered with a thick iron oxide layer coating of several tens of nanometers. The wettability and adhesion are greatly reduced. On the other hand, if low-temperature heating is performed in advance to remove crystal water and a thin oxide film of 10 nm or less is uniformly formed on the steel surface in advance under relatively mild oxidation conditions, then the oxidation becomes intense. Even when heated to a high temperature, the rapid growth of the oxide is suppressed, so the influence of adhesion inhibition is reduced.

  For this reason, in the first heating step, heating is performed for the purpose of removing adhering water from the steel material surface and forming a thin uniform oxide film. At this time, if the temperature at the time of soaking of the steel material is adjusted in the range of 100 to 160 ° C., the balance between the removal of adhering water and the oxidation of the steel material is good, and a uniform protective oxide film of about 10 to 20 nm can be formed. Thereafter, a method is proposed in which heating is performed at 200 ° C. or higher at which the powder primer exhibits performance after soaking in the next heating step. Although the basic method of the present invention is two-stage heating, if the process of heating to 100 to 160 ° C. once forming a thin oxide film is included as the essence of the present invention, the subsequent heating process is divided. There is no problem as a multistage process.

The frequency in the above-mentioned high frequency induction heating is set to 3000 Hz or less, more preferably 2000 Hz or less. It is known that the eddy current due to high frequency induction heating increases as it approaches the surface of the steel material, and decreases exponentially as it goes inside. This is the so-called skin effect. The depth from the surface at the point where the eddy current has decreased to 0.368 times the current density at the surface is called the current penetration depth σ, and it is known that the following (Equation 1) holds.
σ (cm) = 5.03 × 10 ³ × √ (ρ / μf) (1)
ρ (μΩ · cm): Electric resistivity
μ: relative magnetic permeability f (Hz): FIG. 5 shows an example in which the influence of frequency and frequency is calculated, for example, ρ is 20 μΩ · cm with a steel material at room temperature, and the relative magnetic permeability of carbon steel is 1000. However, in actual heating, ρ increases with temperature.

  As is clear from the example of FIG. 5, in high frequency heating, for example, at a frequency of 10,000 Hz, only a very thin region with a penetration depth σ of 70 μm is heated. For this reason, when the steel material has unevenness of 50 μm or more by blasting like a painted steel pipe, the influence of heating only the surface layer is further increased. Therefore, the current penetration depth σ needs to be sufficiently large with respect to the roughness. Therefore, in the present invention, the frequency used to exclude the influence of roughness is set to 3000 Hz or less. The calculated value of σ at 3000 Hz is 130 μm, which is more than twice the roughness. On the other hand, if the frequency is too low, the efficiency of high-frequency heating is reduced, so 200 Hz or more is practical.

  FIG. 1 shows a process conceptual diagram from heating to cooling of a polyolefin-coated steel pipe in the present invention. Moreover, the assumed temperature curve of the steel pipe surface obtained by this process is shown in FIG. In the covering process of the steel pipe 1 in FIG. 1, the steel pipe is uniformly heated to 100 to 160 ° C. using the first stage induction heating coil 2, and the steel pipe is 200 to 260 using the second stage induction heating coil 3. Heat uniformly to ° C. Thereafter, the powder epoxy resin is coated using the powder epoxy resin primer coating device 4, and then the adhesive pellet resin is wound into a film by an adhesive coating device 5 including an extruder and a T die. Further, similarly, the polyolefin resin pellets are wound by a polyolefin resin coating device 6 comprising an extruder and a T die, and then water cooled in a water cooling zone 7 to produce the polyolefin resin coated steel pipe of the present invention.

  As a comparison, FIG. 2 shows a process conceptual diagram from heating to cooling of a normal polyolefin-coated steel pipe. Moreover, the assumed temperature curve of the steel pipe surface obtained by this process is shown in FIG. In the coating process of the steel pipe 1 in FIG. 2, the steel pipe is uniformly heated to 200 to 260 ° C. using the induction heating coil 8. Thereafter, the powder epoxy resin is coated using the powder epoxy resin primer coating device 4, and then wound into a film by the adhesive application device 5. Similarly, a general polyolefin resin-coated steel pipe is manufactured by winding with a polyolefin resin coating device 6 and then water-cooling in a water-cooling zone 7. In this case, a steel pipe having fine irregularities due to blasting is rapidly produced by high-frequency induction heating. Since the high-temperature heating at 200 ° C. or higher is performed, the performance of the powder primer cannot be sufficiently exhibited, and the coating is inferior in the resistance to cathodic peeling.

[Production Methods of Examples and Comparative Examples]
Hereinafter, the manufacturing method of an Example and a comparative example at the time of performing predetermined induction heating using polyethylene as polyolefin of this invention is shown.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to TGD-100 grid blasting made by IKK to remove the rust. Thereafter, dirt and iron powder were removed using OAKITE 31 as the surface cleaning treatment liquid for the steel pipe, and the remaining liquid was washed with water and then air blow dried.
Next, a 200 KW induction heating coil 2 and a 100 KW induction heating coil 3 were arranged in the transport line as an induction heating device for the coating line. The distance between the leading end of the induction heating coil 2 and the leading end of the induction heating coil 3 is adjusted to 1.0 m in Examples 1 to 8 and Comparative Examples 1 to 5 of the present invention, and the line conveying speed of the steel pipe Was set to 1.5 m / min for intermittent heating for 30 seconds or more. On the other hand, in Comparative Examples 6-8, since the one-step heating different from this invention is performed, the distance between two coils was set to 0.
As frequencies of the two coils, 2000 Hz was used in Examples 1 to 6, 200 Hz in Example 7, and 3000 Hz in Example 8 as the scope of the present invention. In Comparative Examples 4 to 5, heating was performed at 5000 Hz and 10000 Hz, which are different from the induction heating frequency of the present invention.

  The two induction heating coils 2 and the induction heating coil 3 were temperature controlled by controlling the outputs. In the embodiment of the present invention, the uniform temperature after heating of the induction heating coil 2 is adjusted to 100 to 160 ° C., and the temperature after heating with the induction heating coil 3 is 200 to 260 ° C. . In Comparative Example 1, the heating temperature of the induction heating coil 3 was insufficient and the powder was not sufficiently melted and cured. In Comparative Example 2, the heating temperature in the induction heating coil 2 is 100 ° C. or less, and the removal of adhering water is insufficient. In Comparative Example 3, the heating temperature in the induction heating coil 2 is too high. did. Comparative Examples 4 and 5 are cases where the frequency of induction heating is high and deviates from the scope of the present invention. Comparative Examples 6 to 8 are cases in which the distance between the induction heating coil 2 and the induction heating coil 3 is zero, and no intermittent heating treatment is provided.

  The powdered epoxy resin primer (226N 8G manufactured by 3M) is applied to the heated steel pipe with the aim of 200 μm, and then pellets of Lucalen G3710E manufactured by Lyondell Basell are used as the polyethylene adhesive. Using a T-die, a sheet-like semi-molten state was formed and wound and coated. The polyethylene coating was then coated by wrapping a Lupolen 4552D pellet made by Lyondell Basell into a sheet-like semi-molten state using an extruder and a T-die. The adhesive film thickness was adjusted to 0.2 mm, and the polyethylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polyolefin resin-coated steel pipe.

[Cathode peeling test]
The manufactured polyolefin-coated steel pipe was 150 mm in the length direction and divided into 8 in the circumferential direction to prepare test pieces. After drilling a hole in the coating at the center of the test piece according to the method shown in Annex H of ISO 21809, a test cell is set up with 3% of the prepared standard and 3% saline electrolyte inside After satisfying the above, the whole was placed in an oven at 80 ° C. to control the temperature, and the steel material exposed portion was subjected to cathodic protection of −1.45 V against the silver-silver chloride electrode. After the test was conducted for 28 days, the polyolefin coating was removed, the primer was cut with a cutter in 8 directions centered on the hole, and the cathode peeled portion that was easily peeled by exposing the primer from the pothole portion was exposed. The peel diameter was measured in four directions and averaged, and the value obtained by calculating the peel distance from the initial hole was determined to be 15 mm or less.

Table 1 shows the test results in Examples, and Table 2 shows the test results in Comparative Examples.
When the same coating material is used, the cathode peeling distance is determined by the base treatment of the steel material, the heating temperature of the primer, and the film thickness. In this test, even when these factors were prepared, the performance of cathode peeling test was improved in Example 1 and Examples 4 to 6 of the two-stage heating as compared with Comparative Example 7 of the one-stage heating. Moreover, when the heating temperature of the 1st step | paragraph is changed, 100-160 degreeC of Examples 4-6 is favorable, and it turns out that cathode peeling becomes large in the comparative examples 2 and 3 which remove | deviate from the temperature range of this invention. Further, when the frequency is high and the induced current penetration depth is shallow as in Comparative Examples 4 and 5, the influence of roughness becomes large and the standard performance cannot be satisfied.

  As is clear from the results of Table 1 and Table 2 above, the method of performing the induction heating of the present invention intermittently in two or more stages and adjusting the heating temperature range can adversely affect the high frequency induction heating of the steel material. Since it can be reduced, it is possible to produce a polyolefin-coated steel tube having high performance against the required cathode peeling.

DESCRIPTION OF SYMBOLS 1 Steel pipe 2 Induction heating coil 3 Induction heating coil 4 Powder epoxy resin primer coating device 5 Adhesive coating device 6 Polyolefin coating device 7 Water cooling 8 General induction heating coil

Claims (1)

  When the steel pipe is induction-heated before coating to coat the polyolefin with the steel pipe, induction heating is intermittently performed in two or more stages, the first stage heating temperature is 100 to 160 ° C., and the second and subsequent stage heating temperatures are 200. A method for induction heating of a polyolefin-coated steel pipe, which is -260 ° C and the frequency of induction heating is 3000 Hz or less.