JPS62149379A - Method for uniformly curing coated film on painted steel sheet of cut sheet by induction heating - Google Patents

Abstract

PURPOSE:To uniformly bake and cure the paints on painted steel sheets of cut sheets by conveying the adjacent steel sheets in proximity to each other in an induction heating furnace in the stage of continuously conducting the painted steel sheets of cut sheets to the induction heating furnace and baking and curing the paints coated on the surface of the steel sheets by induction heating. CONSTITUTION:The painted steel sheets 1 of cut sheets are held in proximity to each other so as to be spaced at <=5mm from each other and are conveyed in such a state in the induction heating furnace 3 in the stage of continuously conducting the steel sheets to the induction heating furnace and baking and curing the paints coated on the surfaces of the steel sheets. As a result, the uniform baking and curing of the paints are made possible.

Description

Detailed Description of the Invention (Industrial Field of Application) This invention is capable of uniformly curing the paint over the entire surface of the sheet when baking and curing the paint applied to the surface of a coated steel sheet by induction heating. This invention relates to a method for uniformly curing a coating film.

Today, prepainted steel sheets, commonly referred to as colored iron sheets, are the most common commercially available sheet materials and are used for many other purposes, including building materials, home appliances, vehicles, steel furniture and utensils.

Its main features include excellent moldability, corrosion resistance, weather resistance, and nonflammability, as well as low cost.

By the way, with the recent introduction of advanced printing technology in the field of pre-coated steel sheets, high-grade decorative steel sheets with excellent designs have come to be manufactured. Demand for steel sheets has been increasing.

However, with the current manufacturing method of prepainted steel sheets, it is quite difficult to produce product sheets with less dust adhesion.

This is because hot air ovens with a large heat capacity are currently commonly used as baking and curing ovens for paints, but because they use a large amount of hot air, it is extremely difficult to remove dust. Regarding this point, various methods have been tried in the past to clean the furnace, but it has not been possible to achieve a sufficiently satisfactory clean.

Recently, induction heating furnaces have been attracting attention as clean bake-hardening furnaces.

(Prior Art) Heating of magnetic materials by induction heating has been known for a long time and has been widely put into practical use for melting, hardening, heating, etc. of metals. This is because (1) heating is possible in a short period of time, (2) control characteristics in heating and heating are excellent, and (3)
This is due to the following characteristics: (4) heating can be performed in a clean atmosphere; and (5) steel plates have excellent heat uniformity in the width direction.

However, most of the applications of induction heating for steel sheets are for continuous steel strips. This is because when a cut plate of magnetic material is passed through an induction heating furnace, the temperature at the leading and trailing ends in the conveyance direction is extremely lower than that at the center, and the entire magnetic material is heated uniformly. This is because I was unable to do so.

On the other hand, regarding the above-mentioned high-grade decorative steel sheets with excellent design,
There are particularly strong requirements for printing finish accuracy, flatness, shape, etc., and in order to meet these requirements, a cut plate production method must be adopted.

(Problems to be Solved by the Invention) As mentioned above, for high-grade decorative steel sheets, it is necessary to adopt a cut plate production method due to the required characteristics, but the high-quality decorative steel sheets of such cut plates! When a clean induction heating method was introduced as a baking hardening method for paint on an a-slope, there remained a problem in that the coating film could not be baked and hardened uniformly.

The present invention advantageously solves the above problems, and proposes a method that can achieve uniform baking hardening of paint even when an induction heating furnace is used as a heating furnace for high-grade decorative steel sheets. purpose.

(Means for solving problems) First, the background to the elucidation of this invention will be explained.

When heating a magnetic material by electromagnetic induction, low frequency heating or high frequency heating is selectively used depending on the purpose of heating. In other words, when the frequency is low, the induced current penetrates deep from the surface of the magnetic material and directly heats the inside of the magnetic material, making it possible to heat the entire body, whereas when the frequency is high, the induced current
The penetration depth becomes shallow, and the magnetic material flows only through the surface layer, resulting in a so-called skin effect in which the surface layer is heated significantly.

By the way, as for φ steel plate <0.27mm to 1.2mm
In the case of a thin steel plate whose main thickness is Since they cancel each other out, energy loss is large. In addition, a few μm to several tens of μm of paint applied to the surface of a steel plate,
In order to heat the steel plate to 200 to 300°C and bake harden it in a short time of about 10 to 60 seconds, it is not necessarily necessary to heat the entire steel plate, but rather a heating method that mainly involves heating the surface layer is better.

For this reason, it is desirable to use high frequency waves at the 10 KHz level for induction heating of painted steel sheets.

However, heating of the surface layer is affected by heat radiation loss from the surface and heat diffusion loss inside the steel plate, and in the case of cut plates in particular, the edges perpendicular to the magnetic flux are strongly affected by the magnetic flux distribution. The plate temperature at the leading and rear ends of the plate was extremely low compared to the center of the plate, and as a result, the paint applied to the surface of the cut-coated steel plate (hereinafter referred to as the heated plate) could not be baked and hardened uniformly. It is.

In order to solve the above problem and make it possible to uniformly bake and harden the heated plate using the induction heating method, the inventors conducted extensive research and discovered the following facts.

In other words, (1) When a heated plate, which is a single magnetic substance, is conveyed by a conveyor or the like in an induction coil, the magnetic field generated between the induction coil and the heated plate is parallel to the direction of movement of the heated plate. A closed circuit is created, but this electromagnetic field is parallel to the surface at the center of the heated plate, but is inclined at a certain angle of entry at the leading and trailing ends. (2) Therefore, the magnetic flux The induced current flowing in the width direction of the plate perpendicular to the edge is smaller at the edges than at the center, and various types of scorching losses are also added, so the plate temperature does not rise and the heated plate cannot be uniformly heated. In order to uniformly heat the plates to be heated, it is sufficient to reduce the inclination of the magnetic flux at the ends or to make them parallel, and for this purpose, the front and rear ends of adjacent plates to be heated should be brought close to each other. They found that it is effective to place them on a conveyor.

This invention is based on the above knowledge.

That is, in this invention, the cut and coated steel plates are continuously introduced into an induction heating furnace, and when the paint applied to the surface of the steel plates is baked and hardened by induction heating, the adjacent steel plates are brought close to each other and the inside of the induction heating furnace is heated. This is a method for uniformly curing the coating of coated cut steel plates by induction heating, and the distance between adjacent coated cut steel plates is preferably in the range of 5 mm or less.

(Function) When a heated plate, which is a thin steel plate with a thickness of 0.27 to 1.2 mm, is placed in a high frequency magnetic field of, for example, 10 KHz, theoretically the penetration depth of the induced current is Below one point,
The thickness is approximately 0.2 mm, and current flows only through the very surface layer. Therefore, if there is any disturbance in the magnetic flux distribution, a uniform distribution of current cannot be obtained.

In other words, at the front and rear ends of the heated plate, the magnetic flux enters at a certain angle at an angle, so the magnetic flux density is smaller than at the center, resulting in a smaller current.
The plate temperature does not rise.

By the way, the length of the lower plate temperature part at this end is
Approximately 100 to 2, regardless of the thickness and length of the heated plate
00 mm, which corresponds to the gradient part of the magnetic flux.

Therefore, if the plates to be heated are placed close to each other, disturbances in magnetic flux at the ends are eliminated, and the plate temperature at the ends quickly recovers. In this case, it is preferable that the heated plates be in contact with each other in order to reduce the influence of disturbances in the magnetic flux distribution. On the other hand, in order to prevent thermal distortion of the plates to be heated and prevention of overlapping due to swinging of the plates during transportation, it is preferable that the plates be separated by some distance, but if the plate spacing exceeds 5 mm, the magnetic flux at the edge Since the effect of improving the disorder is significantly reduced, it is desirable that the distance between the plates be 5 mm or less.

By conveying the heated plate continuously into the induction heating furnace at regular intervals in this way, the temperature at each point on the surface of the heated plate is maintained within ±10°C with respect to the center. In this way, uniform hardening of the coating film on the heated plate is achieved.

(Example) Plate thickness: 0.6 mm, width 2 300 mm, length: 500 mm
A primer made of epoxy resin is applied to a thickness of 5 μm on the surface of a hot-dip galvanized steel sheet that has been subjected to chemical conversion treatment with zinc phosphate, and after baking and hardening it in a hot air oven at 230°C, it is further layered on top of the primer. A heated plate was prepared by applying a top coat made of acrylic modified polyester resin to a thickness of 15 μm.

Next, as shown in FIGS. 1 and 2, the heated plate 1
were placed on a conveyor with plate spacing of 0, 5, and 10 mm, respectively, and conveyed at a conveyor speed of 2 m/min through an induction heating furnace 3 with an effective coil length of 1000 mm, so that the temperature at the center of the plate was The coating film was cured under conditions of 235°C. In the figure, number 4 is an induction sheet feeder, 6 is a paint coater, 7 is a cooling device, 8 is a drying device, and 9 is an exit sheet piler.

During such heat treatment, the central part and edge part (A-H
Table 1 shows the results of measuring the plate temperature at point) and investigating the temperature difference between the center and points A to H. The plate temperature measurement positions are as shown in Fig. 3.

In addition, Table 2 shows the results of investigating the coating film properties of each plate temperature measurement part.
Shown below.

For comparison, a similar investigation was conducted in the case where the heated plate was treated alone, and the obtained results are also shown in Table 1.2.

As is clear from Tables 1 and 2, by bringing the heated plates close together and leading them into the induction heating furnace according to the present invention, the temperature drop at the front and rear ends of the slope was significantly reduced, and the properties of the coating film were improved. In particular, when the distance between the plates was set to 5 mm or less, a particularly immersive effect was obtained.

(Effect of the invention) The effects of this invention can be summarized as follows.

(1) Although it was thought that the conventional induction heating method could not uniformly heat cut and coated steel plates, according to this invention, uniform heating can be achieved by continuously transporting the heated plate at regular intervals through an induction furnace. , making it possible to use it as a clean heating furnace.

(2) Induction heating furnaces can easily create various temperature rising patterns and can perform rapid heating, so they can eliminate wrinkles and voids during the conventional baking and curing process for thick-film solvent-based paints, powder paints, and sol-type paints. It can be expected that even paints that are prone to film defects such as these can be easily cured and of stable quality.

(3) Conventionally, in the production of powder-coated steel sheets, until the powder coating is melted, a hot air stove with a large heat capacity cannot be used due to the danger of paint powder detaching due to wind speed, and static heat sources such as far infrared rays cannot be used. Alternatively, a heating method using a combination thereof has been used, but induction heating can solve this problem and can therefore be used as an advantageous heating furnace as a baking hardening method for powder coatings.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the induction heating furnace and the conveyance state of the heated plate. Figure 2 is a schematic diagram of a heated steel plate production line equipped with an induction heating furnace. Figure 3 is temperature measurement of the heated plate. FIG. 3 is a plan view showing points. 1... Heated plate 2... Conveyor 3
...High frequency induction heating furnace 4...Induction coil 5...
Inlet sheet feeder 6...Paint coater (including printing machine) 7...Water cooling device 8...Drying device 9...Outlet sheet piler

Claims (1)

[Claims] 1. Cut and coated steel plates are successively introduced into an induction heating furnace, and when the paint applied to the surface of the steel plates is baked and hardened by induction heating, the adjacent steel plates are placed close together and induction heated. A method for uniformly curing a coating film on a cut coated steel sheet by induction heating, which is characterized by transporting the sheet through a furnace. 2. The interval between the cut and painted steel sheets introduced into the induction heating furnace is 5.
The method according to claim 1, wherein the size is less than mm.