JP2008055774A - Coated steel plate, processed product and panel for thin television - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coated steel plate excellent in brightness stability and the concealability of a foundation color and a foundation flaw even in a thin coating film with a film thickness of 10 μm or below, and also to provide a processed product and a panel for a thin television. <P>SOLUTION: A zinc type plating layer and a chemical forming film containing no chromium are successively formed on both sides of a steel plate 2, and an undercoating film 4 is formed on the chemical forming film provided on one side of the steel plate 2. A topcoating film 5 being an organic resin film containing a scaly substance 6 having light reflecting physical properties is formed on the undercoating film 4. The existing ratio of the scaly substance in the direction parallel to the surface of the steel plate is 30% or above, and the total thickness of the undercoating and topcoating films is set at 10 μm or below. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention sequentially forms a zinc-based plating layer and a chromium-free conversion coating on both surfaces of a steel plate, and forms an undercoat coating and a top coating with a specific coating structure on the conversion coating on one surface of the steel plate. The present invention relates to a coated steel sheet, a processed product, and a thin TV panel, which are formed, have a total film thickness of 10 μm or less, and are excellent in lightness stability, substrate color and substrate hiding property. The coated steel sheet of the present invention can be used as a material for AV equipment typified by a thin TV panel such as a liquid crystal television or a plasma television.

  Usually, pre-coated steel sheets use mainly modified polyester resins and epoxy resins for the undercoat of the outer surface to ensure adhesion and corrosion resistance with the underlying steel sheet, and use polyester resin, acrylic resin, etc. for the outer surface top coat. By doing so, stain resistance, design, scratch resistance, hydrochloric acid resistance, alkali resistance, barrier properties, and the like are mainly imparted.

  In addition, as described in Patent Document 1, a conventional precoated steel sheet generally has an undercoat film thickness of about 5 μm and a topcoat film thickness of 15 μm. Since it takes a lot of time for painting and baking, it is desired to make the coating film thinner from the viewpoint of rationalization of painting work and resource saving.

  By the way, when the coating film is formed by reducing the thickness to 10 μm or less with the conventional coating film composition, it is assumed that the film thickness of the top coating film usually varies about ± 1.5 μm with respect to the target film thickness.

If the coating film is as thin as 10 μm or less in this way, the color and wrinkles of the base of the steel sheet may be partially visible, and if the film thickness fluctuation is about ± 1.5 μm, the lightness (L value) As a result, the surface appearance becomes poor and stable design properties cannot be obtained.
JP-A-4-215873

  An object of the present invention is to sequentially form a zinc-based plating layer and a chromium-free conversion coating on both surfaces of a steel plate, and have an undercoat coating and a specific coating structure on the conversion coating on one surface of the steel plate. An object of the present invention is to provide a coated steel sheet, a processed product, and a thin TV panel that are excellent in lightness stability, base color, and base concealment by forming a thin top coat film and having a total film thickness of 10 μm or less.

  As a result of repeated studies to solve the above-mentioned problems and obtain a coated steel sheet with excellent performance, the present inventors sequentially formed a zinc-based plating layer and a conversion film not containing chromium on both surfaces of the steel sheet. Then, an undercoating film is formed on the chemical conversion film on one surface of the steel sheet, and an overcoating film which is an organic resin film containing a scaly substance having physical properties that reflects light is formed on the undercoating film. In addition, when the ratio of the scale-like substance in the direction parallel to the surface of the steel sheet is 30% or more, and the total film thickness of the undercoat film and the topcoat film is 10 μm or less, the lightness stability and the substrate It was found that a coated steel sheet excellent in both the color and the hiding property of the base sheet can be obtained.

The present invention has been made based on such findings, and the gist thereof is as follows.
(1) A zinc-based plating layer and a conversion coating not containing chromium are sequentially formed on both surfaces of the steel sheet, and an undercoat film is formed on the conversion film on one surface of the steel sheet, and light is applied to the undercoat film. Forming a top coat film that is an organic resin film containing a scaly substance having reflective properties, and the presence ratio of the scaly substance in a direction parallel to the surface of the steel sheet is 30% or more, and an undercoat film A coated steel sheet characterized in that the total film thickness of the top coat film is 10 μm or less.

(2) The coated steel sheet according to the above (1), wherein the existing ratio of the scaly substance in a direction perpendicular to the surface of the steel sheet is 30% or less.

(3) The coated steel sheet according to (1) or (2), wherein the scaly substance is Al or Ni.

(4) The coated steel sheet according to (1), (2) or (3), wherein the scale-like substance is coated with an organic resin.

(5) The coated steel sheet according to any one of (1) to (4) above, wherein the other surface of the steel sheet has a conductive load of 500 g or less.

(6) A processed product formed by pressing the coated steel sheet according to any one of the above (1) to (5) so that the one surface of the coated steel sheet has a convex surface.

(7) Using the coated steel sheet according to any one of the above (1) to (5), and performing press working so that the one surface of the coated steel sheet is a convex surface exposed to the outside. A panel for a flat-screen TV.

  ADVANTAGE OF THE INVENTION According to this invention, it became possible to provide the coated steel plate excellent in the brightness stability, the base color, and the concealment property of the base material, the processed goods manufactured using the coated steel plate, and the panel for thin televisions.

Hereinafter, the configuration of the present invention and the reasons for limitation will be described.
The coated steel sheet of the present invention is formed by sequentially forming a zinc-based plating layer and a conversion film not containing chromium on both surfaces of the steel sheet, and forming an undercoat on the conversion film on one surface of the steel sheet. In addition, forming a top coat film that is an organic resin film containing a scaly substance having a physical property of reflecting light, the proportion of the scaly substance in the direction parallel to the surface of the steel sheet is 30% or more, The coated steel sheet is characterized in that the total film thickness of the undercoat film and the topcoat film is 10 μm or less.

(Zinc-based plating)
Examples of the galvanized steel sheet used as the base steel sheet of the coated steel sheet of the present invention include a hot dip galvanized steel sheet, an electrogalvanized steel sheet, an alloyed hot dip galvanized steel sheet, and an aluminum-zinc alloy plated steel sheet (for example, hot dip zinc-55 mass). % Aluminum alloy-plated steel sheet, hot-dip zinc-5 mass% aluminum alloy-plated steel sheet), iron-zinc alloy-plated steel sheet, nickel-zinc alloy-plated steel sheet, various zinc-based plated steel sheets such as blackened nickel-zinc alloy-plated steel sheet Can be used.

(Chemical conversion film)
A chemical conversion film is formed on both surfaces of a plated steel sheet having a zinc-based plating layer. The chemical conversion film is a chemical conversion film that does not contain chromium from the viewpoint of the environment. This chemical conversion film is formed mainly for improving the adhesion between the plating layer and the undercoat film. Any material that improves adhesion can be used, but it is more preferable to improve not only adhesion but also corrosion resistance. It is preferable to contain silica fine particles from the viewpoint of adhesion and corrosion resistance and to contain phosphoric acid and / or a phosphoric acid compound from the viewpoint of corrosion resistance. As the silica fine particles, either wet silica or dry silica may be used, but it is preferable that silica fine particles having a large effect of improving adhesion, particularly dry silica, be contained. The phosphoric acid or phosphoric acid compound may contain at least one selected from these metal salts and compounds such as orthophosphoric acid, pyrophosphoric acid, and polyphosphoric acid. Furthermore, you may add 1 or more types, such as resin, such as an acrylic resin, and a silane coupling agent.

(Undercoat)
The undercoating film is formed on the chemical conversion film on one surface of the steel sheet and as a lower layer of the topcoating film.

  The undercoating film may be one having a composition conventionally used. For example, it is composed of at least one organic resin selected from an epoxy-modified polyester resin, an epoxy resin, an acrylic resin, and a urethane resin. It is preferable to ensure adhesion to the base steel sheet, corrosion resistance, and the like.

(Top coat)
The top coat film is an organic resin film containing a scaly substance having physical properties that reflects light on the undercoat film.

  Examples of the organic resin include polyester resins, epoxy-modified polyester resins, and acrylic resins. Particularly, polyester resins and acrylic resins are mainly used to impart stain resistance, design properties, scratch resistance, barrier properties, and the like. Etc. are preferably used. In order to cure the organic resin, it is preferable to use a cross-linking agent such as a melamine resin, urea, or an isocyanate compound in terms of a balance between processability and chemical resistance. The cross-linking agent is preferably 1 to 50 parts by mass with 100 parts by mass as the total with the organic resin.

  The total film thickness of the undercoat film and the topcoat film is 10 μm or less from the viewpoint of rationalizing the painting work and saving resources.

  The thickness of the undercoat coating film is preferably in the range of 1 to 5 μm. If the film thickness of the undercoat film is less than 1 μm, the corrosion resistance and the adhesion between the chemical conversion film will be insufficient, and if the film thickness exceeds 5 μm, it is disadvantageous from the viewpoint of rationalization of painting work and resource saving. Because it becomes.

  The thickness of the top coat film is preferably in the range of 3 to 7 μm. If the film thickness of the top coat is less than 3 μm, the design and barrier properties will be insufficient, and if the film thickness exceeds 7 μm, it will be disadvantageous from the viewpoint of rationalization of painting work and resource saving. It is.

  The film thickness of the top coat film, the undercoat film, and the organic resin layer described later is obtained by observing the cross section with an optical microscope or an electron microscope, obtaining film thicknesses at any three locations per field of view, and observing at least 5 fields of view. The average value is 15 or more.

  The present inventors have studied to reduce the total film thickness of the undercoat film and the topcoat film to 10 μm or less from the viewpoint of rationalizing the painting operation and saving resources.

  1 (a), (b) and (c) are schematic views showing the relationship between the coating film thickness of the coated steel sheet and the light beam penetration depth, and a1 and a2 in the figure are the light beam penetration depths. Yes, t1 and t2 are the film thickness of the coating film.

  First, in the conventional coated steel sheet 1A as shown in FIG. 1 (a), the film thickness t1 of the coating film 3A formed on the zinc-based plated steel sheet (steel sheet) 2 having a chemical conversion film not containing chromium has a light beam penetration depth. Since it is sufficiently thicker than a1, i.e., t1> a1, the color and wrinkles of the base of the steel plate 2 are not seen through, and even if there is a film thickness variation of about ± 1.5 μm. The brightness (L value) hardly changes, and a stable design property is obtained.

  However, as shown in FIG. 1B, when the film thickness t2 of the coating film 3B of the conventional coated steel sheet 1B is made thinner than the light penetration depth a1, that is, when t2 <a1, the steel sheet When the color and wrinkles of the base 2 are partially seen through, and the film thickness fluctuates by about ± 1.5 μm, the brightness (L value) fluctuates and stable design cannot be obtained.

  For this reason, even if the present inventors make the film thickness t2 of the coating film 3C of the coated steel plate 1C thin as in FIG. 1B, as shown in FIG. If the coating composition is such that the depth a2 stops in the coating film 3C, that is, the coating composition that satisfies t2> a2, the color and wrinkles of the base of the steel plate 2 will not be seen through, Even if there is a film thickness variation of about ± 1.5 μm, there is almost no change in brightness (L value), and it is considered that stable design properties can be obtained. As a result of further intensive studies, as shown in FIG. By forming the scaly substance 6 having the physical property of reflecting light in the top coat film 5, the color and wrinkles of the base of the steel plate 2 are not seen through, and ± 1.5 μm Even if the film thickness fluctuates to a certain extent, brightness (almost no change in L value) and stable design can be obtained. The heading, is to have completed the present invention.

  The reason why the scaly substance 6 contained in the top coat film 5 is limited to the scaly form is that the surface of the base steel sheet 2 can be effectively coated in the top coat film 5. The “scale-like substance” as used herein is one having an average thickness / average particle diameter of less than 0.15. More specifically, those having an average particle diameter of 5 to 50 μm and an average thickness of 0.02 to 1.5 μm are preferable because the coating effect is large. The average particle diameter and the average thickness are obtained by observing with an optical microscope or an electron microscope and calculating an average value for any 10 particles. The average particle diameter is the average value of the maximum diameter and the minimum diameter when the plane portion of the scale-like substance is observed from the direction perpendicular to the plane, and the average value of 10 pieces as the particle diameter of one scale-like substance, To do. The average thickness is an average value of 10 when the average value of the thickness when the scaly substance is measured at two arbitrary positions in the cross-sectional direction is the thickness of one scaly substance.

  In addition, the existence ratio of the scaly substance in a direction parallel to the surface of the steel plate needs to be 30% or more.

  FIG. 3 (a) is a plan view when the top coat film 5 of the coated steel sheet 1E is seen through from the direction perpendicular to the surface of the coated steel sheet, and FIG. 3 (b) is a plan view of FIG. 3 (a). It is sectional drawing on the II line. Here, the existence ratio in the direction parallel to the surface of the steel plate 2 of the scale-like substance 6 present in the top coat film 5 of the coated steel plate 1E is obtained by projecting the scale-like substance 6 on a plane parallel to the surface of the steel plate 2. FIG. 3A shows the ratio of the total length (A + B) of the scaly substance 6 to the predetermined unit width (X) of the top coat film when considered on the line II ((A + B) / X). Moreover, in practice, the predetermined unit width (X) and total length (A + B) are measured by observing the cross section (FIG. 3 (b)) on the II line of the coated steel sheet 1E. The abundance ratio of substance 6 ((A + B) × 100 / X) can be obtained. In addition, the predetermined unit width (X) of the top coat film 5 is a visual field width in a predetermined visual field range when the coated steel sheet 1B is viewed in a section on the line I-I, as shown in FIG. 3 (b). is there. Further, the total length (A + B) of the scale-like substance is the total length occupied by the scale-like substance 6 on the II line, as shown in FIG. As shown in b), the length of the portion located under the scaly substance 6 that exists at different depths in the top coat 5 is not considered. In addition, this time, as an example, the length of the scaly substance 6 was measured by observing the cross section along the line I-I in FIG. Any cross section can be used as long as the cross section is perpendicular to the cross section.

  The method for measuring the predetermined unit width and length of the scaly substance 6 may be any method as long as it can observe the cross section of the coated steel sheet and specify the length of the scaly substance in the direction parallel to the surface of the steel sheet. For example, as shown in FIGS. 4A and 4B, there is a method of measuring by observing a cross-section FIB (Focused Ion Beam). A portion indicated by an arrow in the figure is a scaly substance (here, Al powder), and the proportion of the scaly substance can be obtained by measuring the length of the part. Other measurement methods include a measurement method using a scanning analytical electron microscope (SEM-EDX), an electron beam microanalyzer (EPMA), and the like.

  In addition, the reason why the ratio of the scale-like substance 6 in the direction parallel to the steel plate surface is limited to 30% or more is that the color and wrinkles of the base of the steel plate 2 can be seen through by setting it to 30% or more. In addition, even if there is a film thickness variation of about ± 1.5 μm, there is almost no change in lightness (L value), and a stable design can be obtained.

  Moreover, it is preferable that the presence rate of the scale-like substance 6 in a direction perpendicular to the surface of the steel plate 2 is 30% or less.

  FIG. 3C is an enlarged view of a portion of the top coat film 5 in FIG. Here, the ratio of the scale-like substance 6 in the top coat film 5 is the ratio of the average length in the direction perpendicular to the surface of the steel plate 2 of the scale-like substance 6 to the film thickness (T) of the top coat film 5. Yes, the average length in the direction perpendicular to the surface of the base steel sheet 2 is any 5 extending in the direction perpendicular to the base steel sheet 2 with respect to the film thickness of the top coat film 5 when viewed in the cross section of the coated steel sheet 1E. The average length of the total length occupied by the scaly substance 6 on the straight line of the book. Specifically, as shown in FIG. 3 (c), the thickness of the top coat film 5 is T, the total length of the scaly substance 6 on the II-II line is C, and the scaly form on the III-III line. The total length of the substance 6 is D, the total length of the scaly substance 6 on the IV-IV line is E + F, the scaly substance 6 does not exist on the V-V line, and the scaly form on the VI-VI line Since the length of the substance 6 is G, the existence ratio (%) of the scale-like substance 6 in the direction perpendicular to the surface of the steel plate 2 at this time is (C / T + D / T + E / T + F) / T + G / T) × 100/5. The five straight lines extending in the direction perpendicular to the base steel plate 2 for determining the average length of the scale-like substance 6 may be anywhere in the cross-section, so as to increase the accuracy of the average length of the scale-like substance 6. Alternatively, five or more straight lines may be used. In addition, this time, as an example, by observing the cross section along the line I-I in FIG. 3A, the scale-like substance 6 was measured to obtain the existence ratio, but in reality, it was perpendicular to the surface of the base steel plate 2. Any cross section may be used as long as it is a cross section.

  Further, as a method for measuring the average length of the scale-like substance 6 and the film thickness of the top coat film 5, the cross-sectional FIB observation is performed in the same manner as the method for measuring the length of the scale-like substance in the direction parallel to the surface of the steel sheet. Any method can be used as long as the film thickness of the top coat film of the coated steel plate and the length of the scaly substance in the direction perpendicular to the surface of the steel plate can be specified. Furthermore, the reason why the ratio of the scale-like substance 6 in the direction perpendicular to the steel sheet surface is limited to 30% or less is that if the scale-like substance 6 in the top coat 5 is 30% or less, the surface of the base steel sheet 2 is removed. It is because it can cover more effectively and a more stable design property is obtained.

  In order to obtain the above-described ratio of the scale-like substance 6, the content of the scale-like substance 6 is preferably 6 to 12% by mass. If it is less than 6% by mass, the scaly substance 6 in the top coat film 5 cannot sufficiently cover the surface of the base steel plate 2, and stable design properties tend to be not obtained. If it exceeds%, the scaly substance 6 is formed by overlapping several layers in the top coat film 5, making the top coat film itself brittle and increasing the cost.

Further, the scaly substance 6 preferably has a water surface diffusion area of 10,000 to 50,000 cm ² / g, more preferably 20,000 to 40,000 cm ² / g. As used herein, the “water surface diffusion area” specifically refers to JIS K 5906: 1998, where a sample is washed and dried with petroleum-based spirit or acetone to form a powder, and the powder is It means the area (cm ² / g) when the sample powder is uniformly spread and coated on top. Since this water surface diffusion area is proportional to the area (surface area) covering (hiding) the surface of the base steel plate 2, it means that the larger the water surface diffusion area, the larger the surface area covering the surface of the base steel plate 2. .

The reason for limiting the water surface diffusion area of the scaly substance 6 to 10000-50000 cm ² / g is that if it is less than 10000 cm ² / g, the area covering the surface of the base steel plate 2 becomes small, and the color of the base surface of the steel plate 2 This is because it tends not to be able to sufficiently conceal and haze, and when it exceeds 50000 cm ² / g, the scaly substance 6 is formed in layers in the top coating film 5 and is overcoated. This is because the coating film itself tends to be brittle.

  Furthermore, the scaly substance 6 having the physical property of reflecting light is preferably a metal, and more preferably Al or Ni. This is because when other materials are used, the reflectance of light is low and stable design properties cannot be obtained sufficiently.

  It is preferable that the scaly substance 6 is coated with an organic resin in terms of improving adhesion with an organic resin constituting the top coat film and improving barrier properties (hydrochloric acid resistance and alkali resistance). When the thickness of the resin coating is 0.02 μm or more, an excellent barrier property is obtained, and when the thickness is 0.08 μm or less, a sufficient effect is obtained. Therefore, the resin coating preferably has a thickness of 0.02 to 0.08 μm. Moreover, an acrylic resin is mentioned as organic resin which coat | covers the scale-like substance 6 surface.

  The present invention may be applied when the coating film is thick as in the case of a conventional coated steel sheet, but particularly when the total film thickness of the undercoat film and the topcoat film is as thin as 3 to 10 μm. This is preferable in that the effect is remarkable.

  In addition, when the coated steel sheet of the present invention is used as, for example, a panel for a thin TV, the back surface of the coated steel sheet that becomes the inner surface of the pressed panel may have conductivity due to the necessity for welding, electromagnetic shielding, and the like. Necessary.

In such a case, by forming the chemical conversion treatment film not containing chromium described above on the other surface of the steel sheet, it has the same degree of corrosion resistance and adhesion as the conventional chromate film, and also has excellent conductivity. Specifically, the conductive load is preferably 500 g or less from the viewpoint of electromagnetic shielding properties. More preferably, it is 300 g or less. The conductive load is the minimum load with a surface resistance of 10 ^-4 Ω or less.

  For applications where the degree of demand for corrosion resistance is not so high, the other surface can be formed only as a chemical conversion film containing no chromium, and can be provided as a coated steel sheet having particularly excellent electromagnetic shielding properties.

  Moreover, for applications where the degree of demand for corrosion resistance is high, it is preferable that the other surface is provided with an organic resin layer on the chemical conversion film to improve the corrosion resistance. As an organic resin seed | species of an organic resin layer, an epoxy resin and a polyester resin are preferable. The organic resin layer preferably contains Ca ion exchange silica in order to obtain further excellent corrosion resistance.

  If the film thickness of the organic resin layer is less than 0.1 μm, it is disadvantageous for corrosion resistance, and if it exceeds 1 μm, it is disadvantageous for electromagnetic wave shielding properties, so 0.1 to 1 μm is preferable.

  The above-mentioned coated steel sheet is a member used in applications such as electronic devices and home appliances that are subjected to any one or more pressing processes of deep drawing, overhanging, and bending, and further require electromagnetic shielding properties. It is suitable for. For example, when used for a back panel of a thin TV such as a plasma display panel or a liquid crystal television, excellent electromagnetic shielding properties are exhibited even with a large panel.

  Next, the manufacturing method of the coated steel sheet according to the present invention will be described. The coated steel sheet according to the present invention is subjected to the above-described chemical conversion treatment on both surfaces of the zinc-based plated steel sheet that is the steel sheet to be coated. It is manufactured by applying and heating on both surfaces as necessary to form an undercoat coating film, and then applying and heating an overcoating paint mixed with a scale-like substance and stirring on one surface of the steel sheet.

  The method of applying the top coat and the undercoat paint is not particularly limited, but it is preferably applied by roll coater coating. After the coating is applied, heat treatment is performed by a heating means such as hot air drying, infrared heating, induction heating, etc. to obtain an undercoat film and a topcoat film obtained by crosslinking and curing the resin. The heating condition is a temperature of 170 to 250 ° C. (final plate temperature), and it is preferable to carry out a treatment for 20 to 90 seconds, thereby forming an undercoating film and an overcoating film to produce a coated steel sheet.

  Here, when the heating temperature is less than 170 ° C., the crosslinking reaction does not proceed sufficiently, so that sufficient coating film performance cannot be obtained. On the other hand, when the heating temperature exceeds 250 ° C., the coating film is deteriorated by heat, the design property is lowered, and further, it is not preferable from the viewpoint of rationalization of coating work and resource saving. In addition, if the treatment time is less than 20 seconds, the crosslinking reaction does not proceed sufficiently, so that sufficient coating film performance cannot be obtained. On the other hand, if the processing time exceeds 90 seconds, it is disadvantageous in terms of manufacturing cost. In the coated steel sheet of the present invention, for the purpose of further improving the corrosion resistance of the back surface of the coated steel sheet (the other surface), it is preferable to apply the above-mentioned paint for the organic resin layer to the other surface of the steel sheet in the same manner.

  The above description is merely an example of the embodiment of the present invention, and various modifications can be made within the scope of the claims.

  Examples of the present invention will be described.

(Examples 1-10 and Comparative Examples 1-2)
Galvanized steel sheets with a thickness of 0.5mm each (plating type code: EG), galvannealed steel sheets (Fe content: 10 mass%, plating type code: GA), hot dip zinc Plated steel sheet (plating type code: GI), hot-dip Zn-Al plated steel sheet (Al content: 4.5 mass%, plating type code: GF), blackened electrogalvanized-nickel alloy plated steel sheet (Ni content: 12 mass%, Plating type code: EZNB) and hot-dip Zn-Al plated steel sheet (Al content: 55 mass%, plating type code: GL) were prepared. Table 1 shows the coating amount of the plated steel sheet. In addition, the plating adhesion amount and the plating composition of one surface (front surface) and the other surface (back surface) of the steel plate were the same. After the prepared plated steel sheet was degreased, the following processing steps (I) to (III) were performed to prepare a coated steel sheet.

(I) The chemical conversion treatment liquid was applied to the front surface, and heated to reach the ultimate plate temperature of 100 ° C. after 20 seconds of heating to form the chemical conversion coating on the front surface shown in Table 3.
(II) Next, after applying the chemical conversion treatment liquid to the back surface, the undercoat paint is applied to the front surface, and after 30 seconds of heating, the final plate temperature reaches 210 ° C., and the back of the composition shown in Table 3 The surface chemical conversion film and the undercoat film (3 μm) on the front side shown in Table 4 were formed.
(III) Then, after applying the top coating composition having the composition shown in Table 1 as the top coating film on the front surface so that the film thickness becomes 3 μm or 6 μm, the composition of Table 5 is applied to the back surface as necessary. After applying the organic resin paint with the anti-rust pigment added, heat treatment is performed to reach a reached plate temperature of 230 ° C. 50 seconds after the start of heating, and the top coat film shown in Tables 1 and 2 An organic resin layer on the back surface was formed.

  Tables 1 and 2 show the structures of the front surface and the back surface of the prepared coated steel sheet, the chemical conversion coating, the undercoat coating, the top coating and the organic resin layer.

  Various tests were performed on the coated steel plate (the film thickness of the top coat film was 3 μm) obtained as described above. The evaluation method of the test conducted in this example is shown below.

<Evaluation of the front side>
(1) Lightness stability Lightness stability is determined by measuring the difference in color tone (L ^* ) from a coated steel plate having a top coat film having the same composition and a film thickness of 6 μm. It measured using SQ2000 "Nippon Denshoku Industries Co., Ltd.", and evaluated according to the following evaluation criteria.
○: △ L ^* ≦ 1
△: 1 <△ L ^* ≦ 2
×: ΔL ^* > 2

(2) Substrate concealing property The subsurface concealing property produced the coated steel plate by performing the above-mentioned processing process after scratching the front side of the zinc-plated steel sheet for coating with a metal pen at the tip. The front side of the coated steel sheet was visually observed and evaluated according to the following evaluation criteria.
○: I do not know the scratch Δ: Slightly understand the scratch ×: Clearly understand the scratch

(3) Hydrochloric acid resistance
After immersing the test piece with the back and end surfaces sealed in an aqueous 5% by mass HCl solution at 20 ° C for 24 hours, the cellophane adhesive tape made by Nichiban Co., Ltd. was applied, and the remaining area ratio of the coating film was peeled off. evaluated.
○: No peeling of coating film △: Less than 100% coating area remaining ratio 50% or more ×: Less than 50% coating film remaining area ratio

<Evaluation of the back surface>
(4) Conductivity Using a low resistance measuring device (Loresta GP: manufactured by Mitsubishi Chemical Corporation: ESP probe), the surface resistance value of the back surface of the coated plate was measured. At that time, the load applied to the probe tip was increased at 20 g / s, and the load value when the surface resistance was 10 ⁻⁴ Ω or less was evaluated as follows.
Load value when surface resistance is 10 ^-4 Ω or less ☆: Average load at 10 point measurement is 200 g or less ◎: Average load at 10 point measurement is more than 200 g and less than 300 g ○: Average load at 10 point measurement is more than 300 g 500g or less △: Average load of 10 point measurement is over 500g and 700g or less ×: Average load of 10 point measurement is over 700g and 960g or less

Table 6 shows the evaluation results of the above tests.
According to this, the coated steel plates of Examples 1 to 10 all have excellent brightness stability and ground surface concealing properties. Moreover, even if it heat-processes for a short time, sufficient performance is acquired and it turns out that it is very suitable for the high-speed operation at the time of manufacture.

According to the present invention, a conversion coating not containing a zinc-based plating layer and chromium is sequentially formed on both surfaces of a steel sheet, and an undercoat film is formed on the conversion film on one surface of the steel sheet, and the undercoat film is formed on the undercoat film. A scale-like substance having a physical property of reflecting light is formed, forming an overcoat film that is an organic resin film, and the ratio of the scale-like substance in a direction parallel to the surface of the steel sheet is 30% or more, In addition, by producing a coated steel sheet characterized in that the total film thickness of the undercoat film and the topcoat film is 10 μm or less, the coated steel sheet, processed product, and It has become possible to provide panels for flat-screen TVs.

(a), (b) and (c) are sectional views of a coated steel sheet for explaining the relationship with the light penetration depth when the film thickness of the coated steel sheet is increased. It is sectional drawing of the coated steel plate according to this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic of the coated steel plate according to this invention, (a) is a top view when the coated steel plate is seen through from the upper part in a predetermined visual field range, (b) is on the II line in (a). (C) is the figure which expanded the part of the top coat film in (b). (a) is a coated steel sheet according to the present invention, (b) is an example of a cross-sectional FIB (Focused Ion Beam) observation photograph of a comparative coated steel sheet, and (a) is the total proportion of Al powder with respect to a predetermined unit width. When the length is 84.6% and the average length is 2.6% with respect to the film thickness of the top coat film, (b) shows that the Al powder is present at a total length of 16.4% for the specified unit width, and the film thickness of the top coat film. The average length is 0.6%.

Explanation of symbols

1A, 1B, 1C, 1D, 1E Painted steel plate 2 Zinc-based plated steel plate 3A, 3B, 3C, 3D, 3E with a chemical conversion film containing no chromium 4 Undercoat film 5 Topcoat film 6 Scale-like substance 7 Organic resin

Claims (7)

  A zinc-based plating layer and a conversion film not containing chromium are sequentially formed on both surfaces of the steel sheet, and an undercoat film is formed on the conversion film on one surface of the steel sheet, and the physical properties of reflecting light on the undercoat film Forming a top coat film that is an organic resin film containing a scaly substance having a content of the scaly substance in a direction parallel to the surface of the steel sheet of 30% or more, and an undercoat film and a top coat A coated steel sheet characterized by a total film thickness of 10 μm or less.   The coated steel sheet according to claim 1, wherein a ratio of the scaly substance in the direction perpendicular to the surface of the steel sheet is 30% or less.   The coated steel sheet according to claim 1 or 2, wherein the scaly substance is Al or Ni. The coated steel sheet according to claim 1, 2 or 3, wherein the scaly substance is coated with an organic resin.   5. The coated steel sheet according to claim 1, wherein the other surface of the steel sheet has a conductive load of 500 g or less.   A processed product formed by pressing the coated steel sheet according to any one of claims 1 to 5 so that the one surface of the coated steel sheet becomes a convex surface.   A thin panel for a television set formed by pressing the coated steel sheet according to any one of claims 1 to 5 so that the one surface of the coated steel sheet is a convex surface exposed to the outside.