JPH0353029B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Fields] Steel pipes and reinforcing bars have traditionally been used without any anti-corrosion treatment, but when they are used in locations where steel corrosion is a problem, appropriate anti-corrosion measures are required. It is. Commonly used anticorrosion methods include cathodic protection, lining with anticorrosion materials, and painting. The present invention relates to coating among these anti-corrosion measures, and in particular, for the purpose of preventing corrosion of steel materials such as steel pipes and reinforcing bars, powdered organic resin is continuously applied to the surface of steel pipes and reinforcing bars from the ends in a coating line. It is about the method. [Prior art] There are methods of painting steel pipes and reinforcing bars by electrocoating liquid paint, roll coater painting, and spray painting, but the painting targeted by the present invention involves cleaning the surface of the object and applying it to a specified area. The present invention relates to a powder coating method in which the powder coating is heated to a high temperature, a powder coating is sprayed, and the coating is melted and hardened by the heat of the object to be coated to form a coating film. Conventionally, when painting steel pipes or reinforcing bars, as shown in Fig. 1, the object to be painted 5 with a cleaned surface is continuously sent to the painting line and heated in sequence in a preheating furnace 1 to a predetermined temperature. The coating machine 2 sprays and deposits the powder coating material, and the coating is applied by melting and hardening the coating material using the heat of the object to be coated. At this time, the preheating temperature of the object to be coated usually differs depending on the type of paint to be coated and the type of curing agent, but due to the characteristics of line coating, the higher the coating temperature is, the faster the coating will cure. This increases production efficiency and is economical. In this sense, the coating temperature has conventionally been determined based on the curing characteristics and heat resistance of the paint. For example, the coating temperature for epoxy resin powder coatings is generally 220 to 250°C. At this time, even if the coating temperature is 230℃, the temperature of the steel surface has risen to approximately 250℃ at the outlet of the preheating furnace, as shown by line A in Figure 2, and special post-heating is required after coating. There is no need to harden the paint film using cooling device 4.
The coating finishes curing near the entrance. As described above, in the past, coating temperature conditions were determined only based on the curing characteristics of the paint, and the influence of the oxide film on the surface of the steel plate formed in the preheating furnace was ignored.
However, with conventional methods in which coating conditions are set based on the coating film curing characteristics and productivity of the paint, it is not possible to expect good adhesion of the coating film inherent to epoxy resin, and bending resistance processing is difficult. Steel pipes and reinforcing bars require high strength and impact resistance, and the adhesion of their coatings has often been a problem. On the other hand, from the viewpoint that coatings applied to steel pipes and reinforcing bars may cause defects such as coating peeling during use, other coated steel sheets as shown in Japanese Patent Application Laid-Open No. 143633/1983 are used. There is also a method in which a chromate treatment, which is a chemical conversion treatment similar to that used in the present invention, is applied in advance, and then an epoxy resin powder coating is applied thereon to maintain its corrosion resistance. [Problems to be solved by the invention] Various properties are required for the coatings applied to steel pipes and reinforcing bars, which are the object of the present invention, and in particular, steel pipes cannot be used buried in the ground. In many cases, reinforcing bars are embedded in concrete, so when they are embedded, soil and aggregate come into contact with the surface of the coating. Therefore, impact resistance of the paint film is required. Of course, these steel pipes and reinforcing bars may be used after being bent after being painted, and the coating film must also have bending resistance. As described above, coatings applied to steel pipes and reinforcing bars are required to have excellent adhesion. The present invention provides a powder coating method with good adhesion and excellent impact resistance that does not require pre-painting treatment. [Means/effects for solving the problem] As a result of studying various methods for improving the impact resistance of the coating film in the above-mentioned powder coating, the present inventor discovered that in the process of preheating the steel material, We discovered through thermal analysis of iron that an extremely thin oxide film forms on the surface of steel. Figure 3 shows the relationship between the amount of oxide film formed on the surface of the steel material and the preheating temperature through thermal analysis.
This shows that oxidation progresses rapidly at temperatures above 220°C. From this result, it is clear that when painted at the conventional coating temperature of 220°C to 250°C, the temperature of the steel material surface in the heating furnace was 220°C, and an oxide film was formed on the surface. If the preheating method is an electromagnetic induction method, it is considered that due to its characteristics, the electromagnetic density of the steel surface layer becomes high, the steel surface temperature further increases, and an oxide film is likely to be formed. The present inventor has discovered that if an oxide film formed in such a heating furnace exists in the lower layer of the paint film, it will oxidize due to bending or impact, even if the paint film itself has strength, as shown in Figure 4. It has been experimentally confirmed that the destruction of the film results in the destruction of the upper coating film, which adversely affects impact resistance. Here, 1 in the figure is a case where an oxide film 10 exists between the surface of the steel material 6 and the coating film 7, and the peeling coating film 8 under the punch 9 of the impact tester
2 shows the cross section of the coating film when no oxide film is present. Therefore, when applying powder coating to the surface of a steel material, the present inventor preheated the material at a temperature at which no oxide film was substantially formed on the surface of the steel material, that is, at a temperature at which the oxide film did not become thick enough to affect destruction. By doing so, the above problems were solved. That is, in the present invention, when applying powder coating to the surface of a steel material, after the surface of the steel material is cleaned, preheating is performed in a temperature range that does not substantially form an oxide film on the surface of the steel material, and then powder coating is immediately applied. Application of a powder coating with excellent impact resistance to the surface of a steel material, characterized by applying a body paint to form a coating film, and then applying post-heating to finish the curing reaction of the coating film. The present invention provides a method. As shown in FIG. 1, the present invention heats a steel material in the atmosphere using a preheating device 1, but maintains a temperature range within which the oxide film formed on the surface of the steel material does not reach a thickness that would affect paint film damage, for example. In the example of the steel material in FIG. 2, rapid heating is performed to bring the surface temperature of the steel material within the range of 180 to 220°C. Then, powder coating is immediately applied by the coating machine 2 before the oxide film grows. The applied powder coating melts due to the retained heat of the steel material and forms a coating film. Since the retained heat of the above-mentioned steel material is not sufficient for curing the coating film, the steel material is subsequently transported to the post-heating furnace 3, and the temperature at which the curing reaction of the coating film is completed in the furnace, e.g., 160 to 240 Heat and maintain within the temperature range of °C. The thermal history of the surface of the steel material in the above-mentioned painting line by the method of the present invention is as shown by line B in FIG.
That is, D is the preheating furnace outlet temperature, E is the coating temperature,
F is the post-heating furnace inlet temperature, G is the post-heating furnace outlet temperature,
H indicates the cooling device inlet temperature. In this way, the temperature at the outlet of the preheating furnace should be 220℃ or less, and the temperature should be 160℃ to 240℃ after painting.
The coating film is sufficiently cured by performing post-heating in the temperature range of °C. [Example] Next, the present invention will be explained in more detail with reference to Examples. (1) First, on a painting line with a line speed of 8 m/min, a corrugated reinforcing bar with a diameter of 22 Φ mm and a length of 5 m was subjected to grid blasting to remove oxides on the steel surface and clean it. 160, 180, 200, 220, 240,
The temperature was controlled to 260°C. Using a coating machine located approximately 60 cm away from the furnace outlet, epoxy resin powder paint of Nitto Denko Co., Ltd.'s Nito Powder No. 550 was applied, and the film thickness was adjusted to 200 μm. At this time, preheating temperature 160-220
For those at ℃, 1 to 4 minutes in a 250℃ atmosphere
After heating, the temperature of the steel material before cooling after each holding time is
The temperature was set at 170-210°C to completely cure the coating film.
For those with preheating temperatures of 240 and 260°C, the paint film was cured by simply cooling it after painting. The impact resistance of the coating film was examined using an impact tester in which a 1.8Kg weight was applied to the coated reinforcing bars prepared as described above and a hemispherical surface with a diameter of 16mm was applied to the tip.
At this time, the impact value was set in the range of 0.2 to 0.6 kg-m. The results are shown in Table-1. As can be seen from this result, test Nos. 2, 3,
No. 4 (invention example) obtained an impact resistance value of 0.6 kg-m or more. However, good impact resistance values could not be obtained in Test Nos. 5 and 6 (conventional examples). Also, test No.1
(Comparative example) had a good impact resistance value, but the temperature of the steel material was low in the painting line, and the curing speed of the paint was slow.
Since the paint film came into contact with the rolls when it was still soft, the paint film was picked up on the rolls, etc., and the paint film was partially scratched, so the coating conditions were not optimal for such a continuous coating line. (2) Next, on a painting line with a line speed of 4 m/min, steel pipes with a diameter of 50 Φ mm and a length of 4 m are subjected to steel shot blasting to remove oxides on the surface of the steel material, clean it, and then prepare for direct energization. Heated in a heating furnace, 160, 180, 200, 220, 240,
The temperature was controlled to 260°C. Using a coating machine located approximately 1 m away from the furnace outlet, epoxy resin powder paint of Evacrat No. 3500 manufactured by Kansai Paint Co., Ltd. was applied, and the film thickness was adjusted to 100 μm. At this time, the preheating temperature is 160
~220℃ in 250℃ atmosphere 0.5
After heating for ~2.5min, the temperature of the steel material before cooling is 170.
The temperature was raised to ~210°C to completely cure the coating film. For those with preheating temperatures of 240 and 260°C, the paint film was cured by simply cooling it after painting. Using a 1.8Kg weight, paint the reinforcing bars made as above.
The impact resistance of the coating film was examined using an impact resistance tester with a hemispherical surface with a diameter of 16 mm at the tip. At this time, the impact value was set in the range of 0.4 to 0.8 kg-m. The results are shown in Table-2. As can be seen from this result, test Nos. 8, 9,
No. 10 (invention example) obtained an impact resistance value of 0.8 kg-m or more. However, good impact resistance values could not be obtained in Test Nos. 11 and 12 (conventional examples). Also, test No. 7
(Comparative example) had a good impact resistance value, but the coating film curing speed was slow in the painting line, and the coating came into contact with the roll when it was still soft, so the coating film picked up on the roll, etc., and the coating film was damaged. Since some scratches were formed, the coating conditions were not optimal for such a continuous coating line.

【table】

〔Effect of the invention〕

As is clear from the above examples, according to the present invention, a powder coating film with excellent impact resistance can be applied to the surface of steel materials using relatively simple means and without major modification of existing coating lines. It becomes possible to obtain
The industrial effects are extremely significant.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a powder coating line. FIG. 2 is a diagram showing the steel material surface temperature history in the coating line according to the conventional method and the method of the present invention. Third
The figure shows the oxidation rate when iron is heated in the atmosphere. FIGS. 4 1 and 2 are explanatory diagrams showing the state of the coating film when an impact resistance test was conducted on the surface of the coated steel material. 1...Preheating furnace, 2...Painting machine, 3...Post-heating furnace, 4...Cooling device, 5...Object to be coated, 6...
Steel material, 7... Paint film, 8... Peeling coating film, 9... Punch.

Claims (1)

[Claims] 1. When applying powder coating to the surface of a steel material, after cleaning the surface of the steel material, preheating is performed in a temperature range that does not substantially form an oxide film on the surface of the steel material, and then immediately applying powder coating. 1. A method for applying a powder coating material having excellent impact resistance to a steel surface, the method comprising: forming a coating film using a powder coating, and then heating the coating film to terminate the curing reaction of the coating film.