JPH0221307B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a method for producing a coated steel sheet that has been subjected to pre-painting treatment to have a corrosion-resistant and weather-resistant colored coating on the surface and a conductive coating on the back surface. The present invention relates to a method of manufacturing an electrically conductive coated steel sheet that can form a conductive coated steel sheet. In recent years, in the fields of home appliances and structural building materials, in order to streamline processes, in-house painting of necessary parts has been abolished, and pre-coating, in which painted steel plates are used to process parts, has become popular. Conventionally, this pre-coating was initially applied to parts such as colored galvanized iron plates for roofs and walls, which had a corrosion-resistant and weather-resistant colored coating on the front side and a double-sided coated steel plate with a rust-preventing service coating on the back side, which could be used as is. However, recently it has been expanded to include parts that undergo electric welding and soldering. However, with the above-mentioned double-sided coated steel sheet, electric welding and soldering cannot be performed because the coating film becomes an obstacle. For this reason, the coated steel plates used in the field of parts that undergo these processes are single-sided coated steel plates that have a corrosion-resistant and weather-resistant colored coating on the front side and are left unpainted on the back side. However, conventional single-sided coated steel sheets were manufactured using a continuous roll-coat coating line that stopped painting the back side and painted only the front side, so the unpainted back side was treated with phosphate and chromic acid as a pre-painting treatment. Sealing, sometimes chromic acid sealing, was the only thing that had been done. For this reason, the corrosion resistance of the back side was poor, with white rust appearing in a salt spray test for about 48 hours, and the product had the disadvantage of rusting during horizontal handling or storage. In addition, for phosphate-treated products, when phosphate crystals are wound into a coil, they may be transferred to the coating on the front side, or they may be built up on a forming roll during roll forming, and the coating may be removed when forming the painted surface. It also had the disadvantage of being damaged. Therefore, in order to eliminate this drawback, the present inventors formed an electrically conductive film made of any one of polyethyleneimine, silica sol-polyethyleneimine, or water-soluble acrylic resin on the unpainted surface of the back side, and electrically welded the back side. We have developed an electrically conductive coated steel sheet that has corrosion resistance without compromising its properties or solderability. As shown in Figure 1, the tensile strength of the electrically conductive coating on electrically coated steel sheets rapidly decreases when the coating thickness exceeds 2.5μ, and when it becomes less than 0.1μ, the corrosion resistance becomes insufficient, so it is usually 0.1μ. It was necessary to make it ~2.5μ. However, this film thickness is within the range when the film is formed directly on the steel plate, and if there is an insulating phosphate film or thick chromate film between the film and the steel sheet base, the upper limit of the film thickness will be thinner than 2.5μ. I had to. For this reason, in order to prevent deterioration of corrosion resistance due to reduction in film thickness, it is usually better not to apply pre-painting treatment to the back side where the electrically conductive film is formed. By the way, as for manufacturing equipment for painted steel sheets, the spray method is generally used for pre-painting treatment, and the roll coating method is used for painting, but it is efficient to use this method of manufacturing equipment for electrically conductive painted steel sheets as well. be. In this case, the formation of the conductive film is carried out at a plate temperature of 40 to 40°C after applying an aqueous solution containing 5 to 50% of each resin.
Since it has to be done by drying at 300°C, it is advantageous in terms of equipment and cost to apply it at the same time as painting the front side, and to dry it at the same time as the drying of the coating film. However, when applying in this way, even if the spray is applied to the back side of the pre-painting pre-treatment zone, the pre-treatment will go around to the back side, and the pre-treatment film will be partially formed on the surface where the conductive film is formed, causing variations in electric weldability. There was a problem. In order to eliminate such variations in electric weldability, the present invention provides a method for producing an electrically conductive coated steel sheet that can be manufactured without a pretreatment liquid permeating the electrically conductive coating surface. The inventors of the present invention have conducted various studies on a method for producing an electrically conductive coated steel sheet that has an electrically conductive film on the back surface and does not have a partial pretreatment film that has been wrapped around the back surface. - It was discovered that each film of polyethyleneimine or water-soluble acrylic resin exhibits remarkable water resistance when dried at 40 to 300°C (plate temperature) and can withstand pre-painting treatment. In order to prevent the pre-painting treatment liquid from going around to the back side, it is necessary to form this film on the back side before the pre-painting treatment.
It was confirmed that either the depth method can prevent the pretreatment film from forming on the back surface. That is, the present invention applies an aqueous solution containing any one of polyethyleneimine, silica sol-polyethyleneimine, or water-soluble acrylic resin to the back side of a steel plate and dries it, and then pre-paints the surface to improve corrosion resistance and weather resistance. The present invention provides a method for manufacturing an electrically conductive coated steel sheet, which is characterized by forming a colored coating film. The present invention will be specifically explained below using Examples. FIG. 2 shows the steps of the manufacturing method of the present invention divided into front and back surfaces. First, the steel plate is washed with hot water. This hot water washing is not particularly necessary if the original plate to be coated, such as a cold-rolled steel plate or a plated steel plate, is clean. After washing with hot water, an aqueous resin solution is applied to the back surface to form a conductive film. In forming this electrically conductive film, it is usually preferable to apply the aqueous solution with the back side facing down using a roll coating method so that the aqueous solution does not go around to the front side. As an aqueous solution, use polyethyleneimine, silica sol-polyethyleneimine, or water-soluble acrylic resin alone for 5 to 50 minutes.
When using an aqueous solution containing 50%, the dry film thickness is 0.1~
A conductive film with a thickness of 2.5μ is obtained. After forming the conductive film in this way, the
Dry at 300℃ (plate temperature) to evaporate moisture. This drying imparts water resistance to the electrically conductive film. After a water-resistant, electrically conductive film is thus formed on the back surface, the surface is subjected to phosphate treatment and chromic acid sealing as conventional pre-painting treatments. Even if these pretreatments are applied to the back side of the steel sheet, no pretreatment film is formed, so that the pretreatment film is formed only on the front side. Therefore, next, a corrosion-resistant and weather-resistant paint is applied only to the surface side on which the pretreatment film has been formed, thereby producing an electrically conductive coated steel sheet. Table 1 shows the conditions for forming a conductive film on a conductive coated steel plate by the above method and the pre-painting treatment conditions. Table 3 shows the manufacturing conditions for the formation of a conductive film and the pre-painting treatment conditions when a conductive coated steel sheet is manufactured by applying a corrosion-resistant and weather-resistant paint afterwards. This is what is shown. The phosphate treatment and chromic acid sealing were sprayed only on the front surface, and were not applied to the back surface. In addition, each sample number was painted with primer paint.
This was done by baking at 210°C for 30 seconds, and then baking the top coat at 230°C for 35 seconds.

【table】

[Table] (Note) * indicates iron phosphate treatment.

【table】

[Table] The performance investigation in Table 1 was conducted in the following manner. (1) Salt spray test The test was conducted for 720 hours in accordance with JIS Z 2371, and the area ratio of rust was evaluated using the following criteria. Evaluation points Rust occurrence area ratio (%) 5 0 4 1~10 3 11~30 2 31~70 1 71~99 0 100 (2) Humidity test Conducted for 720 hours in a humidity tester at 50℃ and 98%RH , evaluated using the same criteria as the salt spray test. (3) Weldability test Welding current 5~ using a 35° x 4mmφ electrode
Spot welding was performed under the conditions of 9KA, applied force of 150Kg, and energization time of 10 cycles, and the tensile strength per spot after welding was evaluated using the following criteria. ◎ Peeling at 250Kg or more ○ Peeling at 200-249Kg △ Peeling at 100-199Kg × Peeling at less than 100Kg (4) Solderability test A 2mmφ Sb-Pb solder wire was cut into 5mm pieces. ,
This was placed on a film coated with chloride flux and melted with a soldering iron, and its spread area was evaluated using the following criteria. Evaluation symbol Spread area ○ 6 mmφ or more △ 5 mmφ × Less than 5 mmφ (5) Appearance Pre-painting treatment liquid is attached or unevenly formed on the back side ×, Pre-painting treatment liquid is not attached and pre-treated Those in which no film was formed were evaluated as ○. As is clear from Table 3, according to the present invention, no pretreatment film is formed on the back surface, and there is no deterioration in film performance due to phosphate treatment or chromic acid sealing. As described above, although the manufacturing method of the present invention requires a heating device to form an electrically conductive film, it is possible to obtain an electrically conductive coated steel sheet in which no pretreatment film is present on the unpainted surface. Therefore, the electrically conductive coated steel sheet obtained by the present invention has uniform electrical conductivity on the back surface, and there is no variation in electric weldability.

[Brief explanation of drawings]

Figure 1 shows hot-dip galvanized steel sheets with a thickness of 0.5 mm with a conductive film of silica sol-polyethyleneimine formed on one side, spot welded under constant conditions of a current density of 7 KA, an applied force of 150 kg, and an energizing time of 10 cycles. It is a graph showing the relationship between the conductive film thickness, the welded part tensile strength, and the weld failure rate in the case of the present invention. Second
The figure is a manufacturing process diagram of an embodiment of the present invention.

Claims (1)

[Claims] 1. After applying an aqueous solution containing any one of polyethyleneimine, silica sol-polyethyleneimine, or water-soluble acrylic resin to the back side of the steel plate and drying it, the surface is subjected to pre-painting treatment to make it corrosion resistant.
A method for producing an electrically conductive coated steel sheet, characterized by forming a weather-resistant colored coating film.