JPS6097074A - Preparation of double layer coated steel pipe - Google Patents

Abstract

PURPOSE:To prepare a coated steel pipe excellent in properties such as impact resistance and heat resistance, by spraying a powder paint to a preheated steel pipe to perform under coat treatment before spraying a top coat thereto in a cooling zone. CONSTITUTION:The surface of a steel pipe 1 is preheated to about 240-160 deg.C at first and coated with an under coat based on a bisphenol type epoxy resin and a curing agent to form an under coat film with a thickness of 50-300mum. In the next step, the surface of the pipe 1 is cooled to 80-130 deg.C in a cooling zone 4 prior to gelling and curing said film and a polyurethane type paint is further applied so as to form a film with a thickness of 0.3mm. or more. Thus obtained coated steel pipe is excellent in properties such as impact resistance, anti-wear property or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing coated steel pipes that are excellent in impact resistance, abrasion resistance, heat resistance, and corrosion resistance.

As an anti-corrosion coating method for the outer surface of steel pipes, in recent years, powder coatings mainly composed of thermosetting resins, especially epoxy powder coatings, have been applied using electrostatic coating methods to coat steel pipe surfaces with a film thickness of 250 to 500 μm.
Nowadays, a method is being adopted in which paint is applied over a range of 100% and heat cured at the same time. The coated steel pipe obtained by this method is superior in terms of heat resistance and corrosion resistance compared to conventional extrusion-coated steel pipes made of thermoplastic resins such as polyethylene and polyvinyl chloride. However, when handling or transporting coated steel pipes, there is a problem in that the coating film is easily destroyed by impact force among external forces applied to the coated surface.

On the other hand, polyurethane tar-based paint coatings have performance comparable to the aforementioned thermoplastic resin coatings in terms of abrasion resistance and impact resistance, but because the water absorption rate of the coating film is relatively high, the water resistance of the coating film is Since the cathodic peeling resistance is inferior to that of epoxy powder coating, we use a thick coating with a film thickness of 1.0 WJI or more.As a result, the cost required for coating is lower than that of epoxy powder coating. It also has the disadvantage of being expensive.

Therefore, the present invention has been made in view of the above-mentioned current situation, and its purpose is to provide coated steel pipes that are superior to conventional coated steel pipes in terms of impact resistance, abrasion resistance, heat resistance, and corrosion resistance. The aim is to provide a method for efficiently manufacturing.

In other words, in the present invention, the surface of the steel pipe is preheated to about 240 to 160°C, and an undercoat powder coating containing a bisphenol type epoxy resin and a hardening agent as main components is applied to the surface to give a temperature of 50 to 160°C. A base coating film of 300 μm is formed, and this coating film is gelled, but it is cooled before it reaches complete hardening. /Tantalum-based paint with 03 or more films Iv
To provide a method for manufacturing a multi-layer welded steel pipe, which is characterized in that the welded steel pipe is welded in such a manner that

The present invention will be explained in more detail below.

As the epoxy powder coating used as the base coating in the present invention, among bisphenol-type epoxy resins, those that are solid at room temperature and have a softening point of 70 to 125°C can be used. Hardening agents that are solid at room temperature, such as dicyandiamide, acid anhydride, aromatic amine, and dihydrazide, which are commonly used for powder coatings, can be used. These materials may be used alone or as a mixture of two or more. Furthermore, the epoxy resin and curing agent include pigment components such as coloring pigments and extender pigments, curing accelerators such as phenol and imidazole to cure at low temperatures, and improve the fluidity of the coating film. Additives such as flow aids are mixed in advance to achieve this, and the mixture is melted and mixed below the curing temperature, followed by pulverization and classification.
A powder coating material used in the present invention having a particle size of 20 to 100 microns is obtained.

In the present invention, the top coat applied on the base coat of the above-mentioned epoxy powder paint includes 10 to 100 parts by weight of a bituminous substance such as coal tar pitch in 100 parts by weight of a resin whose main component is polyol. A base resin that has been mixed and dispersed in advance and a curing agent such as an isocyanate compound such as tolylene diisocyanate or a polyisocyanate are blended just before spraying so that the ratio of the base resin and curing agent is constant. An example of the painting process is the first one, which is preferably a polyurethane tar-based paint that has the property of hardening in an extremely short time after it is applied to the surface to be painted by air spraying immediately. As shown diagrammatically in the figure. The steel pipe 1 is heated to a predetermined temperature by a high-frequency induction coil 2, and is heated through a base coating booth 3 with epoxy powder paint, a cooling zone 4 such as water cooling or air cooling, and a top coat coating booth 5 with polyurethane tar paint (top coat). 1 conveyance ski roll 6,
It is conveyed by the second conveyance skier roll 7 or the like.

The above-mentioned polyurethane cool-based paint is applied to the steel pipe on which the base coat has been formed, and then conveyed forward by conveying skinny rolls as shown in Figure 1, so that the top coat coat is applied to the steel pipe.
The gelation of the topcoat must be completed before contacting the skie roll 7. If the top coat comes into contact with the second ski roll 7 before this gelation is completed, it is not preferable because the coat is still in a liquid state and will cause adhesion to the roll and peeling of the coat from the surface of the steel pipe.

Therefore, the top coat used in the present invention 5-. 7
There is a certain required value for the gelling time of % r, which is determined by the distance between the top coat coating position 5 and the second skie roller 7 and the conveyance speed of the steel pipe.

For example, in the case of a large-diameter steel pipe with a diameter of 20 inches or more, if the steel pipe transport speed is 1 m/- and the distance between the top coat coating position 5 and the second ski roll 7 is 1.5 m, the top coat will gel. The time must be 1.5 minutes or less.

In order to gel the topcoat paint in a very short time like this, it is sufficient to heat the main ingredient of the topcoat paint with a heater, or to heat the surface of the steel pipe to be coated, whichever method is used. However, since the steel pipe is heated to a temperature between 240℃ and 160℃ during the base coating before the topcoat is applied, it is energy-saving to use this residual heat to harden the topcoat. It is advantageous from the following points.

Therefore, when applying the top coat, the surface temperature of the steel pipe is 80 to 1
If the temperature is within the range of 30°C, coated steel pipes can be manufactured that have excellent coated surface appearance, corrosion resistance, and impact resistance. If the top coat is applied when the surface temperature of the steel pipe is below 80°C, the surface appearance of the top coat will be good, but it is not preferable because the interlayer adhesion with the base coat will deteriorate. do not have. Furthermore, if the surface temperature of the steel pipe exceeds 130°C, the top coat will gel immediately after being applied to the surface of the steel pipe, making it difficult to form a smooth and uniform coating. A film thickness of less than LM is not preferable because impact resistance and anticorrosion properties decrease.

The film thickness of the powder coating used for the base coating of the present invention may be in the range of 50 to 300μ, but preferably 1
It is in the range of 00 to 200μ. This is because if the thickness of the base coating film is less than 50 μm, the anticorrosion properties will decrease, and if it exceeds 300 μm, no improvement in impact resistance will be observed.

More importantly, the topcoat must be applied before the basecoat gels and fully cures. This is the first stage shown in Figure 1 after the base paint is applied.
This is because if the gelation of the coating film does not occur before it comes into contact with the 0-Rule 6, the undercoat coating will peel off and the corrosion resistance will deteriorate.

Furthermore, in order to improve interlayer adhesion with the topcoat paint film, that is, to prevent delamination, it is necessary to prevent unreacted functional groups from remaining in the undercoat film and the functional groups in the topcoat film. Since chemical bonds are formed between the two, it is best to complete the topcoat application before the reactive groups in the basecoat film are almost completely consumed.

If the thickness of the topcoat paint is 0.3 IllJ or more, it will be effective in terms of impact resistance and abrasion resistance, but if the film thickness is 2N or more, the applied paint will sag. 1. Preferably, from the viewpoint of the increase in the cost of coating materials, etc.
A range of 5 to 0.8 degrees is good. In addition, 2wm depending on the purpose
If a film thickness of 1.5n or less is required, apply a topcoat to a film thickness of 1.5u or less, cool with water, and harden the topcoat again. It is best to form it by coating it on a coated film, and this does not cause any problems.

Under the conditions described above, the method of the present invention is carried out through the steps shown in the following examples. Steel pipe 1 to induction coil 2
After preheating to a predetermined temperature in the coating booth 3, a powder coating is sprayed to a thickness of 50 to 300 μm using an electrostatic coating gun installed in the coating booth 3 to form a base coating film. Next, the surface temperature of the steel pipe is cooled to a range of 80 to 130°C in the cooling zone 4, and a top coat is sprayed using an airless sprayer placed in the coating booth 5 to achieve a film thickness of O63. It should be within the range of ~1.5+u. The top coat applied to the surface of the steel pipe immediately gels through a curing reaction and then completely hardens, so it is manufactured with sufficient hardness that it will not be scratched even when it comes in contact with the conveyor rolls installed on the line. Ru.

The present invention will be specifically described below with reference to Examples and Comparative Examples.

[Example 1] Bisphenol type epoxy resin (softening point 100°C) 10
0 parts by weight, 5 parts by weight of dicyandiamide, 0.3 parts by weight of 2-undecylimidazole, 40 parts by weight of red iron, a flow control agent, etc. were heated and melted in an extruder, mixed, crushed and classified, and an undercoat with an average particle size of 40 μm was applied. I obtained the paint for use. Next, as a top coat paint, West German T, 1. B che
After heating a steel pipe with a diameter of 508 mm using MJE's polyurethane cool paint (trade name Protegor) to 180°C, the powder paint for the undercoat was electrostatically applied to a film thickness of 150μ, and the tube passed through an air cooling zone. At this time, the surface temperature of the steel pipe was cooled to 100 to 110°C, and the above-mentioned top coating was spray-coated to a film thickness of 1.0+U, and allowed to cool at room temperature to obtain a multi-layer coated steel pipe.

[Comparative Example 1] The coating film thickness of the top coat polyurethane tar paint is 0.15
A painted steel pipe was obtained in the same manner as in Example 1, except for the fin.

[Comparative Example 2] A coated steel pipe was obtained in the same manner as in Example 1, except that the film thickness of the epoxy powder coating for undercoating was 20 μm.

[Comparative Example 3] A coated copper pipe was obtained in the same manner as in Example 1, except that the top coat was applied at a surface temperature of 50° C. after the undercoat was applied.

[Comparative Example 4] A coated steel pipe was obtained in the same manner as in Example 1, except that the top coat was applied after the surface temperature of the steel pipe coated with the undercoat reached 140°C.

[Comparative Example 5] A coated steel pipe was obtained in the same manner as in Example 1, except that after the undercoat film was completely cured, it was once cooled to room wetness, and the surface temperature was raised to 100°C again before the topcoat was applied. Ta.

The following performance tests were conducted on the coated plates obtained in the above Examples and Comparative Examples. The results are shown in Table 1.

C Impact Resistance Test) A section 100 m wide and 300 m long cut out from a painted steel pipe was evaluated using a Gardner impact tester according to the method of ASTMG]4.

(Cathode peeling resistance test) Using a section cut from a painted steel pipe with a width of 150 mm and a length of 150 mm, the initial hole was 5 mm, 3 mm NaCl water, and a potential of -1,5.
■After testing at 23℃ for 60 days, the peeling distance of the coating film from the initial hole was evaluated. After being immersed in warm water for 60 days, it was taken out, a 2-inch width cross cut was made with a knife on the painted surface, and the peeling rate of the paint film was evaluated using Sellotape.

As is clear from the test results shown in Table 1, the multilayer steel sheet manufactured by the method of the present invention has excellent performance in impact resistance, cathode peeling resistance, and hot water resistance, and has a painted surface appearance. It turns out that it is excellent.

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[Brief explanation of the drawing]

FIG. 1 is a diagrammatic front view of an apparatus used for manufacturing multi-layer coated steel pipes. Explanation of symbols

Claims (1)

[Claims] The surface of the steel pipe is preheated to about 240 to 160°C, and an undercoat powder coating containing bisphenol-type epoxy resin and a curing agent as main components is applied to the surface to form a base coating film with a film thickness of 50 to 300 μm. Although this coating film is gelled, it is cooled before it reaches complete hardening, and when the surface temperature of the steel pipe reaches 80 to 130°C, a polyurethane tar-based paint is applied on the base coating film to a film thickness of 0.3 ffa or more. 1. A method for manufacturing a multi-layer coated steel pipe, characterized by coating the pipe in the following manner.