JP2008207091A - Coated steel plate, processed goods and panel for thin-shaped television - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coated steel plate excellent in burring processability compared to a conventional coated steel plate, processed goods and a panel for thin-shaped television. <P>SOLUTION: A zinc based plating layer and a chemical film containing no chromium are successively formed on both surfaces of a steel plate, an under coating film is formed on the chemical film of one surface of the steel plate, a top coating film is formed on the under coating film and a partial defect is formed in the vicinity of the boundary of the under coating film/the chemical film after burring processing, wherein the ratio of the formed defect is 5-60% and the total thickness of the under coating film and the top coating film is ≤10 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention provides a burring process in which a zinc-based plating layer and a conversion coating not containing chromium are sequentially formed, and an undercoat coating and a topcoat coating having a specific coating structure are formed on the conversion coating on one surface of the steel sheet. The present invention relates to a coated steel plate, a processed product, and a thin panel for television that are excellent in performance. 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.

  Normally, pre-coated steel sheets use mainly modified polyester resins and epoxy resins for the undercoat on the outer surface to ensure adhesion to the base steel sheet, corrosion resistance, etc. Mainly imparting stain resistance, design properties, scratch resistance, barrier properties, etc. by using paints.

  In addition, precoated steel sheets are required to have many performances such as high hardness, high workability, contamination resistance, chemical resistance, water resistance, and corrosion resistance. In particular, workability, particularly press workability, is a very important performance for a pre-coated steel sheet that is subjected to press work after painting and baking. The press workability mentioned here means that there is little damage to the coating film in the process of bending, drawing, cutting, etc. when processing from a flat metal plate into various shapes. In processing such as, the greater the degree of elongation and flexibility of the coating film, the better the workability, but in severe press processing such as drawing, not only the elongation and flexibility of the coating film, but also deformation and processing Strength that can withstand the stress of time and scratch resistance are also important.

For the required characteristics of such a pre-coated steel sheet, for example, in Patent Document 1, by applying a coating composition containing a polyester resin, a melamine resin (curing agent), an antirust pigment, organic polymer fine particles, and the like, Coated steel sheets that satisfy the workability, corrosion resistance, adhesion, impact resistance, scratch resistance, and design properties with one coat have been proposed.
Japanese Patent Laid-Open No. 9-111183

  The coated steel sheet described in Patent Document 1 assumes that a chromate-based film containing chromium is used as a chemical conversion film. This is not environmentally preferable, and the chromate film is hard and has poor workability. Therefore, when the burring process as shown in FIGS. 1 (a) and 1 (b) is performed on the coated steel sheet, there is a problem that the coated film of the processed part is peeled off and the subsequent corrosion resistance cannot be maintained. . The burring process is a process of making a pilot hole in a flat plate and stretching and flanging it into a cylindrical shape, and obtaining a threaded portion.

  Therefore, the object of the present invention is to solve the above-described problems of the prior art, and sequentially form a zinc-based plating layer and a conversion coating not containing chromium on both sides of the steel plate, and the conversion coating on one side of the steel plate On top of this, by forming an undercoat film having a specific film structure and an overcoat film that is an organic resin film, a coated steel sheet, a processed product, and a thin TV with excellent burring workability while maintaining the conventional performance Is to provide panels.

  Generally, the undercoat coating film in the coated steel sheet has an effect of adhesion to the base (steel sheet), and the top coat film has an effect of corrosion resistance. When the coated steel sheet is subjected to burring, there is a problem that the coating film is peeled off. A cellophane adhesive tape is applied to the coated steel sheet after burring, and this is peeled off. Further analysis was conducted. As a result, as shown in FIG. 2, it was found that separation occurred between the steel plate and the chemical conversion film. The inventors of the present invention have repeatedly studied to solve the above-mentioned problems and obtain a coated steel sheet having an excellent performance. When the burring process is performed, a defect occurs in the top coat film, and the entire coat film is formed. Since the generated stress cannot be reduced, it has been found that peeling occurs between the steel sheet and the chemical conversion film. As a result of further intensive studies, it is possible to reduce the stress of the entire coating film by actively generating partial defects in the vicinity of the interface between the undercoat coating film / the conversion coating film, and to produce a coated steel sheet having excellent burring workability. It was found that it 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, an undercoat film is formed on the conversion film on one surface of the steel sheet, and an overcoat is formed on the undercoat film. A coating film is formed, a partial defect is generated in the vicinity of the undercoat / chemical conversion film interface after burring, the generation ratio of the defect is 5 to 60%, and the total of the undercoat film and the topcoat film A coated steel sheet having a thickness of 10 μm or less.

(2) The coated steel sheet according to (1), wherein the undercoat coating film has a pigment content of 5 to 60% by mass.

(3) The coated steel sheet according to (2), wherein the pigment is one or more selected from TiO ₂ , C black, Mg-treated tripolyphosphate Al, Ca-treated tripolyphosphate Al and Mg-exchanged SiO ₂ .

(4) The coated steel sheet according to any one of (1), (2), and (3) above, wherein the other surface of the steel sheet has a conductive load of 500 g or less.

(5) A processed product formed by pressing the coated steel sheet according to any one of (1) to (4) so that the one surface of the coated steel sheet has a convex surface.

(6) Using the coated steel sheet according to any one of the above (1) to (4), and performing press working so that the one surface of the coated steel sheet becomes a convex surface exposed to the outside. A panel for a flat-screen TV.

  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 plate, and an undercoat coating is formed on the conversion coating on one surface of the steel plate, In addition, a top coat film is formed, and a partial defect for reducing internal stress is generated in the vicinity of the interface between the undercoat film / chemical conversion film after burring, and the generation ratio of the defect is 5 to 60%. In addition, it is possible to provide a coated steel sheet, a processed product, and a thin TV panel excellent in burring workability, wherein the total film thickness of the undercoat film and the topcoat film is 10 μm or less.

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 chromium-free conversion film on both surfaces of the steel sheet, and forming an undercoat on the conversion film on one side of the steel sheet, the undercoat film A top coat film is formed on the top, a partial defect is generated in the vicinity of the interface between the undercoat film / chemical conversion film after burring, and the generation ratio of the defect is 5 to 60%, and the undercoat film And the total thickness of the top coat 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 film is a chemical film containing no 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. In addition, since the film thickness of the chemical film tends to be disadvantageous for adhesion and corrosion resistance when the film thickness is thin, it tends to be disadvantageous for electromagnetic shielding properties when the film thickness is thick, so the range is 0.02 to 1.0 μm. It is preferable that
By having the chemical conversion film as described above, it becomes possible to have the same degree of corrosion resistance and adhesion as a conventional chromate film.

(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. It is necessary that a partial defect for reducing internal stress is generated in the vicinity of the undercoat / chemical conversion film interface after the burring process, and the generation ratio of the defect is 5 to 60%. As shown in FIG. 2, the inventors of the present invention are concerned with the problem that when a burring process is performed on a coated steel sheet, the coating film on the processed part is peeled off. Focusing on the fact that they are doing, we have studied earnestly about solutions to problems. As a result, defects are generated during the burring process by actively generating defects in the vicinity of the undercoat / chemical conversion film interface during burring and controlling the ratio of the generated defects to be 5 to 60%. It has been found that due to partial defects in the coating film / chemical conversion film, the stress applied to the coating film during burring can be reduced and peeling of the coating film can be suppressed.

  Here, FIG. 3 is the figure which observed the coating-film / chemical-coating film interface of the part which carried out the burring process of the coated steel plate 1 by this invention from the cross section. The generation ratio of defects 5 generated in the vicinity of the interface 4 of the undercoat coating film 2 / the conversion coating film 3 is the ratio of the partial defect generation length (a + b + c) to the length L of the burring portion ((a + b + c) / L × 100%), and this ratio ((a + b + c) / L × 100%) needs to be in the range of 5 to 60%. If it is less than 5%, the stress generated in the coating film during burring cannot be sufficiently reduced, so that the coating film may be peeled off. If it exceeds 60%, the defect length is too long and the adhesiveness deteriorates. It is.

  The method for measuring the defect generation rate is not limited as long as it is a method that can identify partial defects in the vicinity of the coating film / chemical conversion film. For example, as shown in FIG. 4, a scanning analytical electron microscope (SEM-EDX) The method of measuring by is mentioned. As shown in C in FIG. 4, the black part is a defect generated in the vicinity of the interface of the undercoat / chemical conversion film, and the generation ratio of the defect can be obtained by measuring the length of the part. it can. Other measurement methods include measurement with an electron beam microanalyzer (EPMA).

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

The undercoat coating film preferably has a pigment content of 5 to 60% by mass. If it is less than 5% by mass, the undercoating film is tough, so the stress applied to the entire coating film cannot be sufficiently reduced when burring is performed, and the top coating film may be destroyed. This is because if the content is more than% by mass, the undercoat coating film is destroyed and the coating film is peeled off. Furthermore, the pigment is preferably one or more selected from TiO ₂ , C black, Mg-treated tripolyphosphate Al, Ca-treated tripolyphosphate Al and Mg-exchanged SiO ₂ . The types of these pigments may be selected from rust preventive pigments and colored pigments according to required characteristics.

(Top coat)
The top coat film is an organic resin film formed on the undercoat film. Examples of the organic resin film include polyester resins, epoxy-modified polyester resins, acrylic resins, and the like. In particular, polyester resins and acrylic resins are mainly used for imparting stain resistance, design properties, scratch resistance, barrier properties, and the like. It is preferable to use a resin or the like.

  The total film thickness of the undercoat film and the topcoat film needs to be 10 μm or less from the viewpoint of rationalizing the painting operation and saving resources.

  In addition, it is preferable that the film thickness of an undercoat coating film is the range of 1-5 micrometers. This is because 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 rationalizing the painting work and saving resources. Because it becomes.

  Moreover, it is preferable that the film thickness of a top coat film is the range of 3-7 micrometers. This is because if the film thickness of the top coat film 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 coating 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.

  In addition, when the coating film is as thin as 10 μm or less, the color and wrinkles of the base of the steel sheet may be partially seen through, and when the film thickness variation is about ± 1.5 μm, the brightness ( (L value) may change, so that it is preferable to contain 6 to 12% by mass of scaly Al in order to obtain stable design properties. Although it may be applied when the coating film is thick like the conventional coated steel plate, the effect as described above, especially when the total film thickness of the undercoat film and the topcoat film is as thin as 3 to 10 μm Is preferable in that it is remarkable.

  The reason why the scale shape is preferable as the Al shape to be contained in the top coat film is that the surface of the steel sheet can be effectively coated. The size of the “scale-like Al” here means specifically those having an average particle diameter of 5 to 50 μm and an average thickness of 0.02 to 0.7 μm. 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 an average value of 10 when the average value of the maximum diameter and the minimum diameter when the flat portion of the scale-like Al is observed from the direction perpendicular to the plane is the particle diameter of one Al. The average thickness is an average value of 10 pieces when the average value of the thicknesses when the scaly Al is measured at two arbitrary positions in the cross-sectional direction is taken as the thickness of one Al.

  Moreover, the reason why 6 to 12% by mass is preferable as the content of the flaky Al is that the surface of the steel plate cannot be sufficiently covered with the flaky Al in the top coat when the content is less than 6% by mass, which is stable. This is because the design property cannot be obtained. If the amount exceeds 12% by mass, scaly Al is formed by overlapping multiple layers in the top coating film, which may make the top coating film itself brittle and increase the cost. Because it invites.

  Furthermore, it is preferable that the scaly Al is coated with an organic resin in terms of improving adhesion with the 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 scaly Al.

The scaly Al preferably has a water surface diffusion area of 10000 to 50000 cm ² / g, more preferably 20000 to 40000 cm ² / g. As used herein, the “water surface diffusion area” specifically refers to JIS K 5906: 1998, where a sample is washed with petroleum spirit or acetone and dried to form a powder, and the powder is It means the area (cm ² / g) when sprayed on top and uniformly coated with Al powder. Since this water surface diffusion area is proportional to the area (surface area) that covers (hides) the surface of the steel sheet, the larger the water surface diffusion area, the larger the surface area that covers the surface of the steel sheet.

The reason why the surface diffusion area of the scaly Al is preferably 10000 to 50000 cm ² / g is that if it is less than 10000 cm ² / g, the area covering the surface of the steel sheet becomes small, and This is because there is a tendency that it cannot be sufficiently concealed, and when it exceeds 50000 cm ² / g, scaly Al is formed by overlapping multiple layers in the top coat, making the top coat itself brittle. Because there is a tendency to.

  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, the other surface of the steel sheet also has the above-mentioned chemical conversion film not containing chromium, so that 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. In particular, it is suitable for a member that is threaded by burring. 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 used for the undercoat film after the above-described chemical conversion treatment is performed on both surfaces of the zinc-based plated steel sheet to be coated. Apply the paint on one side or, if necessary, the coating for the organic resin layer on the other side and heat it to form an undercoat. Then, apply only one side of the steel sheet to the top coat. It is manufactured by applying and heating the paint.

  What is necessary is just to apply | coat and heat the coating material which mixed and stirred scaly Al to the coating material for top coat films.

  The method for 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 a top coat film and an undercoat film obtained by crosslinking and curing the resin. The heating conditions are 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 a top coat film and a bottom coat 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, it is preferable to apply the above-described coating for the organic resin layer to the back 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.

(Invention Examples 1-10 and Comparative Examples 1-2)
As galvanized steel sheets for coating, electrogalvanized steel sheets with a thickness of 0.5 mm 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 (iV) were performed to prepare a coated steel sheet.

(I) A chemical conversion treatment liquid was applied to the front surface and heated so that the ultimate plate temperature reached 100 ° C. after 20 seconds of heating to form a front surface chemical conversion film having the composition shown in Table 3.
(Ii) Next, after applying a chemical conversion treatment liquid on the back surface to form a conversion film of the back surface having the composition shown in Table 3, an undercoat having the pigment content shown in Table 1 is applied to the front surface. Then, after 30 seconds of heating, a heat treatment was performed to reach a final plate temperature of 210 ° C., and an undercoat film (3 μm) on the front side shown in Table 4 was formed.
(Iii) Thereafter, a top coating film coating composition having the composition shown in Table 1 as a top coating film is applied to the front surface so that the film thickness becomes 2 μm or 6 μm, and then, on the back surface, if necessary, Table 5 After applying an organic resin paint with an anti-corrosive pigment added so that it has the following composition, heat treatment is performed to reach a reached plate temperature of 230 ° C. 50 seconds after the start of heating, and the top surface shown in Tables 1 and 2 is overcoated. The organic resin layer of the coating film and the back surface was formed.
(IV) Then, using an Erichsen extrusion device, the test steel plate was subjected to burring, which will be described later, and a cross section near the coating film / chemical conversion film interface in the range of 50 μm in the vicinity of the end surface of the burring portion was observed to generate defects. The rate was measured by SEM-EDX.

  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 2 μm) obtained as described above. The evaluation method of the test conducted in this example is shown below.

<Evaluation of the front side>
(1) Film adhesion after burring process The film adhesion after burring process was performed by observing the peeled state of the coated film after affixing an adhesive tape to each coated steel sheet that had been subjected to burring, Evaluation was performed according to the following evaluation criteria. In the burring process, a 5.5 mmφ pilot hole was drilled, the front side was turned upward, and a 10 mm high cylinder was extruded from below with a 13 mmφ punch as shown in FIG.
○: Slight peeling only at the burring rising edge m ×: Continuous peeling

(2) Corrosion resistance after burring processing Corrosion resistance after burring processing is applied to each coated steel sheet subjected to burring processing as described above for the SST test (salt spray test: JIS Z 2371-2000), and the peeling state of the coating film is observed. Evaluation was performed according to the following evaluation criteria.
○: Only peeling that does not reach the rising portion m from the hole end surface n side occurs ×: Peeling that occurs from the hole end surface n side to the rising portion m occurs

(3) Lightness stability Lightness stability is measured using a spectroscopic color difference meter (“SQ2000” manufactured by Nippon Denshoku Industries Co., Ltd.) for the difference in color tone (L ^* ) between two and 6 μm thick coated steel sheets. And measured according to the following evaluation criteria.
○: △ L ^* ≦ 1
△: 1 <△ L ^* ≦ 2
×: ΔL ^* > 2

(4) Ground surface concealing property The ground surface concealing property produced the coated steel plate by carrying out the above-mentioned processing process after scratching the front surface of the zinc-plated steel plate 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

(5) Hydrochloric acid resistance
After immersing a test piece with the back and end surfaces sealed in a 5 mass% HCl aqueous solution at 20 ° C. for 24 hours, a cellophane adhesive tape made by Nichiban Co., Ltd. was applied, and this was peeled off, and the coating film remaining area ratio Was visually 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>
(6) Conductivity Using a low resistance measurement 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 300 g or less △: Average load at 10 point measurement is more than 300 g and less than 700 g ×: Average load at 10 point measurement is more than 700 g 960g or less

Table 6 shows the evaluation results of the above tests.
According to this, it turns out that the coated steel plates of Examples 1 to 10 all have excellent burring workability and post-burring corrosion resistance, and also have lightness stability and bare ground concealment. Further, the back surface is excellent in conductivity. Furthermore, it can be seen that sufficient performance is obtained even if the heat treatment is performed in a short time, which is very suitable for high-speed operation during production.

  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 plate, and an undercoat coating is formed on the conversion coating on one surface of the steel plate, In addition, a top coat film is formed, and a partial defect for reducing internal stress is generated in the vicinity of the interface between the undercoat film / chemical conversion film after burring, and the generation ratio of the defect is 5 to 60%. In addition, it is possible to provide a coated steel sheet, a processed product, and a thin TV panel excellent in burring workability, wherein the total film thickness of the undercoat film and the topcoat film is 10 μm or less.

The processed part of the conventional coated steel plate which performed the burring process is shown typically, (a) is a perspective view, (b) is II sectional drawing of (a). It is the figure which showed the state of the peeling part of the conventional coated steel plate which performed the burring process, Comprising: (a) is the perspective view which showed typically the peeling state, (b) is the peeling surface in (a) It is a figure which shows the result of having carried out X-ray analysis of (X surface and Y surface). It is sectional drawing which showed typically the cross section of the coating-film / chemical-coating interface vicinity of the process part of the coated steel plate which carried out the burring process. It is the cross-sectional photograph which observed the process part of the coated steel plate which performed the burring process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Coated steel plate 2 Undercoat film 3 Chemical conversion film 4 Undercoat film / chemical conversion film interface 5 Defect 6 Top coat film 7 Plated steel sheet a, b, c Defect generation length L Length of burring process part m Rising part of burring process n Burring hole end face

Claims (6)

  A zinc-based plating layer and a conversion coating not containing chromium are sequentially formed on both surfaces of the steel plate, an undercoat coating is formed on the conversion coating on one surface of the steel plate, and an overcoat coating is formed on the undercoat coating. And forming partial defects in the vicinity of the undercoat / chemical conversion film interface after burring, the generation ratio of the defects being 5 to 60%, and the total film thickness of the undercoat and topcoat Painted steel sheet characterized by being 10 μm or less.   The coated steel sheet according to claim 1, wherein the undercoat coating film has a pigment content of 5 to 60 mass%. 3. The coated steel sheet according to claim ₂ , wherein the pigment is one or more selected from TiO ₂ , C black, Mg-treated tripolyphosphate Al, Ca-treated tripolyphosphate Al, and Mg-exchanged SiO _2. .   The coated steel sheet according to claim 1, 2 or 3, wherein the other surface of the steel sheet has a conductive load of 500 g or less.   The processed product formed by performing press work using the coated steel plate of any one of Claims 1-4 so that said one surface of this coated steel plate may become a convex surface.   A panel for a thin television set, which is formed by pressing the coated steel sheet according to any one of claims 1 to 4 so that the one surface of the coated steel sheet is a convex surface exposed to the outside.