JPH10180181A - Baking method for coated steel sheet in continuous coating line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically shorten paint film-baking time and to contrive increase of performance of a paint film and to produce an excellent-quality coated steel sheet with high productivity. SOLUTION: In the case of performing thick film coating of a steel sheet S in a continuous coating line, after application of a paint, the steel sheet S is rapidly heated at 100-170 deg.C within 'critical solvent evaporation rate free from generation of forming' by induction heating. Thereafter, baking treatment is successively applied to the steel sheet S in a hot air baking furnace 4. Preliminary heating time until a curing agent in paint starts reaction is shortened by induction heating 3 performed in the front step of a baking process. Also, evaporation of a solvent is promoted at a short time and abnormal generation of the surface outward appearance such as foaming is prevented. Curing of a paint film is performed from the surface by hot air baking 4 performed thereafter. Accordingly, various performance such as hardness of the paint film, workability and color tone is secured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for baking a coated steel sheet in a continuous coating line.

[0002]

2. Description of the Related Art In recent years, so-called pre-coated steel sheets, which are coated in advance on a coil-shaped or sheet-shaped steel sheet or a plated steel sheet and processed into a product by a user after shipment, have been widely used in the field of home electric appliances and the like. In addition, such coated films of coated steel sheets are also required to have excellent quality characteristics with respect to various properties such as surface appearance, color tone, coating film hardness, workability, and stain resistance of the coating film surface. In the production of the pre-coated steel sheet as described above, the method of hardening and finishing the coating film by hot air baking in a hot air baking furnace, which has been conventionally performed, is itself a technology established as a method of baking a coated steel sheet, The resulting coating film is almost satisfactory in appearance and other various properties. However, the problem with this conventional method is that it is inferior in productivity. The baking process requires a relatively long time (usually about 60 seconds), and the size of the steel sheet (the thickness and size of the steel sheet) and the coating film. Depending on the thickness or the like, the baking time is further lengthened, and thus the productivity is poor.

In a heat transfer mechanism to a coating film by a hot air baking method, heat is transmitted to the coating film from outside the steel sheet. Therefore, if baking is carried out rapidly, the coating film hardens from the coating film surface, and the solvent remains in the coating film, so that the processability is particularly liable to deteriorate. Further, if the blowing speed of the hot air is excessively increased for the purpose of promoting the evaporation of the solvent, the surface of the coating film is cracked and the surface appearance is significantly deteriorated. Therefore, in order to increase the line speed in the hot air baking method, it is necessary to increase the length of the hot air baking furnace for evaporating the solvent, which causes a problem in terms of equipment space and energy consumption.

[0004] In order to solve such a problem of the hot-air baking system, Japanese Patent Application Laid-Open No. 6-114330 proposes a method of performing baking using an induction heating furnace. In this baking method, immediately after coating, while blowing a low-temperature airflow lower than the coating film curing temperature in an induction heating furnace, baking treatment is performed by rapidly heating within the "limit heating temperature at which no abrasion occurs",
Alternatively, after preliminary heating at a temperature lower than the coating film curing temperature, a baking treatment by rapid heating similar to the above is performed.

[0005]

However, Japanese Patent Laid-Open No.
In the baking method disclosed in JP-A-114330, since a sufficient curing time cannot be obtained in a coating film curing temperature range, the curing of the surface portion of the coating film is insufficient compared with a coating film obtained by baking with hot air, and the resulting coating film is hardly cured. The film has poor performance such as hardness and stain resistance. Also, when a coloring pigment is added to the paint, the distribution of the pigment in the coating film changes compared to the conventional hot-air baking method, causing a difference between the color tone and the target color. Needs to be adjusted. Accordingly, an object of the present invention is to solve such problems of the prior art, and in particular, to significantly shorten the baking time of the coating film and improve the performance of the coating film. It is an object of the present invention to provide a printing method which can be manufactured with high flexibility.

[0006]

According to the method of the present invention for solving the above-mentioned problems, when a thick film is applied on a continuous coating line, after coating of the coating material, the critical solvent which does not generate wrinkles due to induction heating after coating is applied. Evaporation rate "(However," Liquid solvent evaporation rate that does not cause cracking ": In thick film coating, after applying the paint, the coating film is heated from room temperature to just before the curing temperature of the curing agent by induction heating. A method of baking coated steel sheets in a continuous coating line, which comprises rapidly heating to a temperature of 100 ° C. or more and 170 ° C. or less within a limit temperature rising rate at which the surface does not generate cracks, and subsequently baking in a hot air baking furnace. is there.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors mainly applied a coated steel sheet which is a typical coated steel sheet, such as a polyester resin-based paint, to a precoated steel sheet for use in home electric appliances and the like, and used a thick film coating. As a result of intensive studies on the printing method, the following findings were obtained. As described above, the method of baking a coated steel sheet in a continuous coating line includes a method of electrically heating the coating film from the inside of the steel sheet as in an induction heating method and a method of baking the coating film from the outside of the steel sheet as in a hot air baking method. There is a heating method, but as a result of the examination, the baking process is shortened by adopting a baking method in which heating by an induction heating furnace is performed before the baking process and heating by a hot air baking furnace is performed after the baking process. It has been found that it can be carried out in a short time and that good coating performance can be obtained.

In such a baking method, first, heating by an induction heating furnace is performed before the baking step, so that the preheating time until the curing agent in the coating material starts to react is shortened and the baking time is shortened. As a result, the evaporation of the solvent is promoted, and the occurrence of abnormalities in the surface appearance such as a side surface is also prevented. Then
The coating film is cured from the surface by heating in a hot air baking oven at the later stage of the baking process, which can secure various properties such as hardness and workability of the coating film, and the color tone can be maintained by the conventional hot air baking method. An equivalent coating is obtained.
Therefore, in the method of the present invention, in baking thick film coating in a continuous coating line, rapid heating is performed by induction heating in the first stage of the baking process performed after application of the paint, and baking in a hot air heating furnace is performed in the second stage of the baking process. Basically,

[0009] Next, the present inventors examined the appropriate heating and heating conditions for induction heating performed before the baking process, and examined the coating film thickness, heating rate and surface speed when using various paints and curing agents. The relation with the appearance was investigated. As a result, regarding the coating thickness, after induction heating in the first stage of the baking process, even in the case of performing hot-air heating in the second stage of the baking process, as described above, when the dry coating thickness is less than approximately 10 μm, No abnormal appearance of the surface such as armpits occurred regardless of the heating rate. However, the coating thickness of pre-coated steel sheets such as home appliances is generally about 10 to 30 μm in consideration of the durability of the coating film. It was found that there was a "critical solvent evaporation rate at which no fire occurred" as shown in FIG.

FIG. 1 shows that a white polyester resin / melamine resin (curing agent) -based coating material is applied to a chromate-treated galvanized steel sheet by a roll coater, and then the temperature is lower than the curing temperature of the curing agent in an induction heating furnace ( Immediately before), and then baked in a hot-air baking furnace, examined whether the appearance of the coating surface deteriorated due to the occurrence of armpits when heated and heated in an induction heating furnace. This is shown by the relationship between the dry coating thickness and the heating rate. According to this, it can be seen that there is an upper limit to the heating and heating rate at which no crack occurs when the dry coating film thickness is in the range of about 10 to 30 μm.

Therefore, in the present invention, when performing thick film coating (dry film thickness: about 10 to 30 μm), after applying the paint, the temperature is raised from room temperature to just before the curing temperature of the curing agent in an induction heating furnace. , The critical heating rate at which the coating surface does not generate cracks is called the "critical solvent evaporation rate at which cracks do not occur"
In the induction heating before the baking step, the steel sheet coated with the paint is rapidly heated within the “limit solvent evaporation rate at which no crack occurs”. The "limit solvent evaporation rate at which no cracking occurs" varies depending on the type of paint, the type and blending amount of the solvent, the thickness of the coating film, and the like, but can be easily grasped by experiments. For example, the polyester resin / melamine resin (curing agent) -based coating material has a “limit solvent evaporation rate at which no cracking occurs” of 55 ° C./sec, and the polyester resin / isocyanate (curing agent) -based coating material does not have a cracking. "Limit solvent evaporation rate" is 70 ° C /
sec. The heating speed of rapid heating by induction heating is 2
The temperature is preferably set to 0 ° C./sec or more. A more preferable heating rate in the case of the polyester resin / isocyanate (curing agent) coating is 35 ° C./sec or more.

Next, FIGS. 2 and 3 show that a white polyester resin / melamine resin (curing agent) -based paint is applied to a chromate-treated galvanized steel sheet by a roll coater, and then “induction heating furnace”. In the case of heating at a temperature within the "limit solvent evaporation rate at which no baking occurs" and then performing baking in a hot-air baking furnace, the relationship between the ultimate temperature (plate temperature) in induction heating, the baking occurrence state, and the coating film hardness is determined. Investigation and the results are shown. According to this, even if the temperature is raised within the “limit solvent evaporation rate at which no crack occurs” by induction heating in the previous stage of the baking process, for example, the curing reaction starting temperature of the melamine resin as a curing agent is about 130 to 150 ° C. The curing reaction initiation temperature of the blocked isocyanate is 14
When the temperature reached by induction heating is higher than 170 ° C., the curing of the coating film considerably proceeds from the inside of the coating film, and the hardness of the coating film becomes insufficient. (See FIG. 3).

On the other hand, if the ultimate temperature of the induction heating is less than 100 ° C., hot air baking is performed in a state where the solvent in the paint is insufficiently evaporated. Even if it is performed within the "evaporation rate", abnormal surface appearance such as a side surface occurs during baking with hot air (see FIG. 2). Therefore, in the present invention, 100 ° C.
The condition is that rapid heating is performed to a temperature of 170 ° C. or less, and thereafter, the film is introduced into a hot-air baking furnace and baking is performed. In the induction heating before the baking process, an LFX induction heating furnace that generates a magnetic field in the traveling direction of the steel sheet and a TFX method that generates a magnetic field in the thickness direction of the steel sheet are used in accordance with the type and thickness of the base steel sheet. It is preferable to use an induction heating furnace appropriately selected.

By performing the above-described paint baking,
In the induction heating before the baking process, the coating is on the inside (steel plate side)
The solvent evaporates efficiently because it is heated from the beginning, so that the solvent evaporates in a short time.Furthermore, in the hot air baking subsequent to the baking process, almost no consideration is needed for solvent evaporation, so the rapid rise to the paint curing temperature In addition, sufficient heating is possible, and as a result, a high-speed and short-time baking process can be performed as compared with the conventional hot-air baking method. In addition, better surface appearance and coating performance can be obtained as compared with the conventional induction heating method.

[0015] Further, the method of the present invention is, for example, by installing a hot air baking furnace on the exit side when the existing drying furnace is an induction heating furnace, or by using a hot air baking furnace when the existing drying furnace is a hot air baking furnace. By placing an induction heating furnace on the entrance side,
Each of them can be easily implemented by using the existing facilities, thereby greatly improving the productivity. The method of the present invention on the premise of thick film coating (dry film thickness: about 10 to 30 μm) is a baking method suitable for the top coating of steel sheets on which thick film coating is generally performed. Usually, the coated steel sheet to be coated with such a top coat is firstly a base steel sheet (cold rolled steel sheet,
At least one side of various plated steel sheets) is subjected to a chemical conversion treatment such as chromate treatment, and then the chemical conversion treated surface is subjected to an undercoating and baking treatment to form an undercoating film. It is manufactured by painting and baking the above-mentioned overcoat film on the upper layer. However, it is needless to say that the baking method of the present invention can be applied to the undercoating film and the intermediate coating film of the three-coating material when these are applied in a thick film (dry film thickness: about 10 to 30 μm).

[0016]

Against EXAMPLES substrate steel sheet which has been subjected to coating type chromate treatment to the surface of the galvanized steel sheet galvanized amount per one side 30 g / m ² (thickness 0.6 mm), dry film thickness 5μ
m undercoat paint (KP862 manufactured by Kansai Paint Co., Ltd.)
0) was applied and baked, and then, in the stoving equipment shown in FIG. 4, a top coat was applied using the following top coats A to C to a dry film thickness.・ Paint A Polyester / melamine (curing agent) based paint: molecular weight 18
000, dry coating thickness 20 μm, critical solvent evaporation rate at which no fire occurs 37 ° C./sec. Paint B Polyester / melamine (curing agent) -based paint: molecular weight 12
000, dry film thickness 18 μm, critical solvent evaporation rate at which no cracks are generated 30 ° C./sec. Paint C: polyester / isocyanate (curing agent) -based paint: molecular weight 8000, dry film thickness 18 μm, critical solvent evaporation rate at which no cracks are generated 45 ° C / sec

In FIG. 4, reference numeral 1 denotes a coating roll for coating the front surface of a steel plate, 5 denotes a backup roll, 2 denotes a coating roll for coating the back surface of a steel plate, 3 denotes an induction heating furnace constituting a baking apparatus,
Denotes a hot air baking furnace, and 6 and 7 denote support rolls provided on the entrance side and the exit side of the baking apparatus, respectively. Steel plate S
In the position wound around the backup roll 5, the front side is first painted by the painting roll 1, and then the back side is painted by the painting roll 2 at the exit side of one support roll 6 forming the catenary curve X. Immediately after such coating, it is introduced into the induction heating furnace 3 and heated, and most of the solvent is evaporated. Subsequently, the steel sheet S is introduced into the hot-air baking furnace 4 and is baked to the paint curing temperature.

With respect to the coated steel sheet thus obtained, the surface appearance, coating film hardness,
Workability, stain resistance and color tone were evaluated. The results are shown in Table 1 together with the conditions for baking the top coat. (1) Surface Appearance The coating film surface was visually observed, and the surface appearance was evaluated based on the following criteria. ：: No abnormality in surface appearance such as armpit on coating surface (pass) ×: Abnormality in surface appearance such as armpit on coating surface (fail)

(2) Coating Film Hardness The hardness of the coating film surface was measured by a pencil hardness test method according to JIS K5400. The measured coating film hardness is evaluated as follows. Pass: The coating film hardness is equal to or higher than the “conventional example” using the same paint. Fail: The coating film hardness is lower than “conventional example” using the same paint. (3) Workability JIS K 5400 The processability of the coating film was measured by the bending test method according to the above, and evaluated as follows. ：: Workability is equal to or higher than “conventional example” using the same paint (pass) ×: Workability is inferior to “conventional example” using the same paint (fail)

(4) Stain resistance Corresponding to each test material, those coated with the same paint and dry film thickness were baked by a conventional hot air baking method (a baking method using only a hot air baking furnace). Create “hot air baking material”
Each test material and the surface of the coating film of the “hot-air baking material” were painted with red magic ink (manufactured by Uchida Yoko Co., Ltd.), and after 24 hours, a solution of petroleum benzene / ethanol = 50/50 was added to absorbent cotton. After wiping the magic, ΔE (= geometric mean of [L, a, b values after test]-[L, a, b values before test]) was measured, and ΔE of the test material was ΔE ± 2
The stain resistance of the coating film surface was evaluated according to the following criteria depending on whether or not it was within the range. ：: ΔE of the test material is in the range of ΔE ± 2 of the hot-air baked material (pass) ×: ΔE of the test material is out of the range of ΔE ± 2 of the hot-air baked material (fail)

(5) Color Tones corresponding to each test material were coated with the same paint and the same dry film thickness, and were baked by a conventional hot air baking method (a baking method using only a hot air baking furnace). "Baked material"
Color difference ΔE of coating surface between each test material and this “hot-air baking material”
(= [L, a, b values of coating film surface of test material of this example]-
[Square average of L, a, b values of coating surface of “hot-air baking material”] was measured, and the color tone was evaluated based on the following criteria. ：: color difference within ΔE ± 1 (pass) ×: color difference outside ΔE ± 1 (fail)

According to Table 1, No. 1 of the present invention was used. 1 to
No. 9 can perform coating baking in a shorter time than the hot air baking method, and has the same characteristics as the conventional material obtained by the hot air baking method. On the other hand, in Comparative Example No. 10-1
The coated steel sheet No. 4 can perform coating baking in a shorter time than the hot air baking method, but is inferior in any of surface appearance, coating film hardness, workability, stain resistance and color tone.

[0023]

[Table 1]

[0024]

As described above, according to the method for baking a coated steel sheet of the present invention, baking treatment can be performed in a short time, and surface appearance, coating film hardness, workability, stain resistance, color tone, etc. The various performances are not inferior to those obtained by the conventional hot air baking method, and therefore, high quality, high performance coiled coated steel sheets can be stably manufactured with high productivity, which is extremely useful in industry. It is an invention.

[Brief description of the drawings]

FIG. 1 shows whether the appearance of the coating film surface is deteriorated due to the occurrence of armpits when the coated steel sheet is heated and heated to just before the curing temperature of the curing agent in an induction heating furnace, by measuring the thickness of the dried coating film, the heating rate and the heating rate. Graph shown by the relationship

[Fig. 2] Degradation of coating film surface appearance due to the occurrence of armpits when the coated steel sheet is heated and heated in an induction heating furnace within the "limit solvent evaporation rate at which no armpits occur" and subsequently baked in a hot air baking oven Graph showing the presence / absence of heat in relation to the temperature reached in the induction heating furnace and the induction heating time

[Fig. 3] The coated steel sheet is heated in an induction heating furnace at a temperature within the "limit solvent evaporation rate at which no cracks are generated", and subsequently subjected to a baking treatment in a hot air baking furnace to determine whether or not the coating film hardness has deteriorated. Graph showing the relationship between the ultimate temperature in the heating furnace and the induction heating time

FIG. 4 is an explanatory view schematically showing a configuration of a coating facility used in the embodiment.

[Explanation of symbols]

1, 2, coating roll, 3 induction heating furnace, 4 hot air baking furnace, 5 backup roll, 6, 7 support roll, S steel plate

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Kenji Araki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Honko Co., Ltd.

Claims (1)

[Claims] When performing a thick film coating on a continuous coating line, after applying the coating material, the coating is heated by induction heating to obtain a "limit solvent evaporation rate that does not cause cracking" (however, a "limit solvent evaporation rate that does not cause cracking": In applying the film, after applying the coating material, the temperature is raised from room temperature to just before the curing temperature of the curing agent by induction heating, and the temperature rise is limited to 100 ° C. or more and 170 ° C. A method for baking a coated steel sheet in a continuous coating line, comprising rapidly heating to the following temperature and subsequently performing baking treatment in a hot-air baking furnace.