JPS58281A - Production of steel pipe coated internally with thermoplastic synthetic resin - Google Patents

Abstract

PURPOSE:To improve the adhesive power of a paint film having long-term corrosion resistance by coating a thermoplastic synthetic resin on the inside surface of a steel pipe heated to a prescribed temp. then cooling the resin paint film under control at specific cooling rates down to the temp. at which solidfication ends. CONSTITUTION:A thermoplastic synthetic resin such as PE is coated on the inside surface of a steel pipe heated to a prescribed temp. In cooling this steel pipe in the next treating stage, it is cooled under control to <=60 deg.C/min cooling rate of the pipe body down to the temp. at which the resin paint film ends solidification. More specifically, this cooling is the forced cooling with air differing from the simple air cooling, and the cooling rate is adequately controlled in a <=60 deg.C/min range by the combination use of a heat insulating furnace or the like. The cooling method employed in this case intends to cool the outside circumference of the pipe so as to cool the entire part of the pipe thereby cooling the paint film coated on the inside surface of the pipe through the pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an inner-coated steel pipe, such as a water supply pipe, in which the inner surface of the pipe is coated with a thermoplastic synthetic resin such as polyethylene.

Powder thermal fusion has been generally known as one of the methods for coating the inner surface of a steel pipe with a thermoplastic synthetic resin. In this fusion method, the tube is heated to about 0 to 310 degrees Celsius, and the heat capacity of the tube is used to melt the powdered thermoplastic synthetic resin to form a resin coating on the inner surface of the tube. However, when forming such a thick coating, a problem arises when coating the inside of the pipe by blowing it into the pipe. What matters is whether the adhesion to the surface is good or not. The adhesion of the coated film is usually evaluated from the initial adhesion, which is the value after the coating is applied, and the secondary adhesion, which is the value obtained by accelerated tests such as salt water immersion after coating. The long-term adhesion performance of the film is judged from the value of the secondary adhesion force.

The resin coating formed on the inner surface of the tube by the powder thermal fusion method described above is usually cooled to @K as the next step, but in the past, this cooling treatment generally A cooling process using water cooling has been adopted due to its time saving and equipment advantages.

However, the inventors of the present invention have found that the secondary adhesion of the coating film to the inner surface of the tube is greatly affected by the cooling conditions that are performed as the next step of the coating film formation process, and in particular, the water cooling process. It has been discovered that when such rapid cooling is performed, this secondary adhesion is significantly reduced. Of course, this decrease in the secondary adhesion of the inner resin coating is a major problem in guaranteeing the long-term corrosion protection performance of steel pipes, and it goes without saying that appropriate measures must be taken to counter this problem. It is said that

The present invention, when forming a resin coating on the inner surface of a steel pipe by such a powder heat fusion method, improves the coating film by controlling the cooling conditions that are carried out after the coating film formation process under appropriate conditions. The purpose is to improve adhesion with long-term corrosion protection.

A feature of the present invention is that the inner surface of a steel pipe heated to a predetermined temperature is coated with a thermoplastic synthetic resin, and when the steel pipe is cooled as the next treatment step, the pipe is heated until the temperature reaches a temperature below which the resin coating completes solidification. The cooling rate of the body is 60℃/! In other words, this air cooling is different from simple air cooling; it is forced air cooling using air, and when used in conjunction with a heat retention furnace, the cooling rate (°C/min) can be increased by 10°C/min.
The point is that it is properly controlled within a range of less than a minute. In this case, it is cold.

The cooling method involves first cooling the outer periphery of the entire tube, and then cooling the paint film attached to the inner surface of the tube via the tube.

The features of the present invention as described above were confirmed based on the experimental results described below. Figure 1 of the attached drawings is a graph showing the influence of the water cooling start temperature on the secondary adhesion of a coating film when polyethylene is used as the resin material. After coating the inner surface of the tube with the film, if water cooling is started when the tube temperature is 710°C or higher as shown in the figure, the secondary adhesion of the coating film will be almost zero6.
3 AV/7011m, but when the tube temperature was allowed to cool to below 10°C and then water-cooled, the secondary adhesion force was the same as the -order adhesion force, l'l'f y, JH/ It was about lo-. At this time, the cooling rate was 7° C./min, and the cooling rate of water cooling was 10″C/min.

Figure 2 is a graph showing the effect of cooling rate on the secondary adhesion of a coating film using polyethylene as the resin material, and shows that after the formation of the polyethylene coating film, the tube temperature is /10' to 90°C. When the area was cooled at a rate of 60°C/min or more, the secondary adhesion of the coating film was almost zero, but when the cooling rate was controlled to 10°C/min or less, the secondary adhesion was almost zero. The adhesion force was almost the same as the initial adhesion force value. The same thing can be said about coatings made of synthetic resins other than polyethylene; for example, as shown in Figure 3, the secondary adhesion is not as good as that of nylon coatings, and the graph showing the effect of cooling rate shows that It was found that when the secondary adhesion force was controlled at 60° C./min or less, the value of the initial adhesion force could be maintained at a much higher level.

Next, the present invention will be explained with reference to examples.

Example 1 8, G, P (carbon steel pipe for piping)! OA's, s■
After pickling the inner surface of the steel pipe, chemically treating it, and applying a primer coating, the pipe body was heated to a temperature of 2!0 CK and coated with polyethylene powder.

This steel pipe was post-heated at 160°C for 10 minutes, then placed in a furnace with an internal temperature set to 10°C, and slowly cooled to 10°C in 1 minute. An example in which a steel pipe that has been post-heated (110°C for 10 minutes) is left to cool at room temperature (Invention 2), and a steel pipe that has been post-heated (140°C for 10 minutes) under the same conditions as above is removed. Example of cooling to room temperature by blowing 1 mK air (invention 3)
, and an example in which the conventional method not according to the present invention was used to immediately cool down to room temperature by water cooling after post-heating (conventional method), and the /100 peel adhesion force (
-secondary and second-order) were measured. The results are shown in Table IK.

Table 1 (1100 vinyl adhesion of polyethylene coating)
Melting point of polyethylene 128°C Solidification temperature of polyethylene 110-90°C (n5cm constant) Initial adhesion Adhesion after coating Secondary adhesion Crab 3 - Adhesion after 14 days immersion in salt water Example 2 S, G, P Som's 1.1■ After pickling and chemical conversion treatment of the inner surface of the steel pipe, and applying a primer coating, the pipe body is heated,
The tube was heated to 10'' and coated with an adhesive nylon coating.

An example in which this steel pipe was post-heated at 200°C for 3 minutes and then allowed to cool at room temperature (Invention 1), and an example in which the steel pipe was post-heated under the same conditions (-
An example of a steel pipe subjected to heating at 00°C for X minutes) and cooled to room temperature by blowing a small amount of air on the outer surface (invention 2), and a case where a steel pipe is cooled to room temperature by blowing a small amount of air on the outer surface and immediately cooled to room temperature by cooling with water after post-heating by a conventional method not based on the invention. Examples of cooling (conventional method) were carried out,
The tro' beer adhesion (-order and quadratic) in each example was measured. The results are shown in Table 2.

Table S (tto0 peel adhesion of nylon coating film) Melting point of nylon 177°C Solidification temperature of nylon 166° to 140″C By performing controlled cooling according to the present invention in the above deviation example, the secondary adhesion was increased. I can see that it has improved dramatically.

As described above, by performing cooling treatment under the conditions specified by the present invention after coating the inner surface of a steel pipe with a thermoplastic synthetic resin, the synthetic resin can guarantee long-term coating adhesion. It became possible to manufacture internally coated steel pipes.

[Brief explanation of the drawing]

Figure 7 is a graph showing the effect of water cooling start temperature on the secondary adhesion of polyethylene coatings, Figure 2 is a graph showing the effect of cooling rate on the secondary adhesion of polyethylene coatings, and Figure 3 is a graph showing the influence of cooling rate on the secondary adhesion of a nylon coating. Patent applicant Nippon Steel Tube Co., Ltd. Steel Pipe Galvanized Co., Ltd. Inventors Nagakuni Kanhiko Kobayashi Manabu Yoshiyuki Morioka Tadao Kiwami Yukichi Watanabe Yoshiaki Fujiwara Hide Noguchi Daido Hiro Yamamoto - Yoichi Ishiyama 1 Continued on page 0 Inventor Yo Ishiyama Sakura-type Kamiza 559-57 0 Inventor Shigeru Inoue Matsudo-type Higure 832-70 0 Applicant Steel Pipe Zinc Plating Co., Ltd. 2-6 Kayabacho, Nihonbashi, Chuo-ku, Tokyo

Claims (1)

[Claims] (i) The inner surface of a steel pipe heated to a prescribed temperature is coated with a thermoplastic synthetic resin, and when cooling the steel pipe as the next treatment step, the cooling rate of the pipe body is controlled until the temperature drops below the temperature at which the resin coating completes solidification. A method for producing a thermoplastic synthetic resin inner-coated steel pipe, characterized by controlling cooling to a temperature of 10° C./min or less.