JP5602486B2 - Painted steel sheets, processed products, and flat panel TVs - Google Patents

Description

The present invention relates to a coated steel sheet, a processed product, and a thin TV panel, and more particularly to a coated steel sheet having good corrosion resistance.
The coated steel sheet of the present invention is suitably used as a material for a housing of an AV device or the like typified by a thin TV panel such as a liquid crystal television or a plasma television.

For example, when a coated steel sheet is formed into a TV panel or the like, it is generally subjected to pressing or bending, and it is required that bending workability and appearance after pressing are good. Yes. Usually, in pre-coated steel sheets (painted steel sheets), mainly modified polyester resin or epoxy resin is used for the undercoat on the outer surface side to ensure adhesion to the underlying steel sheet, corrosion resistance, etc. 2-coated steel sheet that imparts stain resistance, design, scratch resistance, ethanol resistance, and hydrochloric acid resistance or alkali resistance barrier properties by using polyester or acrylic coatings for the film It is common to do.
Further, there is a three-coated steel sheet in which an intermediate coating film is formed between the undercoat film and the topcoat film.

  As a thin film of a conventional two-coated steel sheet, the inventors previously added a resin particle in Patent Document 1 to add a resin particle to the film even at a film thickness of 10 μm or less. We proposed a coated steel sheet with excellent properties and appearance after press working, and with sufficient coating film hardness.

JP 2007-269010 JP

On the other hand, in the above-mentioned use of the coated steel sheet having a coating thickness of 10 μm or less, it is desired to ensure corrosion resistance in a more severe environment. Such a coated steel sheet according to Patent Document 1 leaves room for further improvement in the corrosion resistance of the coating.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to advantageously solve the above-described problems, and to provide a coated steel sheet having good corrosion resistance together with its processed product, particularly a thin TV panel.

  In order to achieve the above-mentioned object, the present inventors have found that the wax added to ensure the press workability of this type of coated steel sheet causes deterioration in corrosion resistance, and completed the present invention. It came to do.

The gist of the present invention is as follows.
(1) a zinc-based plating layer formed on both surfaces of the steel sheet;
A chemical conversion film not containing chromium formed on at least one surface of the zinc-based plating layer;
An organic film containing an organic resin, a color pigment, and a wax formed on the chemical film;
Have
The wax has a crystallinity of 80% or less and a content of 0.1% by mass or more and 10% by mass or less.
The zinc-based plating layer is an electrogalvanized layer or a molten Zn-Al plated layer,
A coated steel sheet, wherein the organic resin has a number average molecular weight of 8000 to 10,000, a glass transition temperature Tg of 50 to 70 ° C., and a hydroxyl value of 5 to 60 KOHmg / g .

(2) The coated steel sheet according to (1), wherein the wax is a microcrystalline wax.

(3) The coated steel sheet according to (1) or (2) , wherein the organic film contains carbon black.

(4) When the chemical coating and the organic coating are applied to one surface of a steel sheet, the chemical conversion film is provided on the zinc-based plating layer on the other surface of the steel sheet, and the conductive load on the other surface is 500 g. The coated steel sheet according to any one of the above (1) to (3) , wherein:

(5) The coated steel sheet according to (4) above, wherein the chemical conversion film on the other surface of the steel sheet contains a resin.

(6) A processed product formed by pressing the coated steel sheet according to any one of (1) to (5 ) above.

(7) A thin TV panel, wherein the processed product according to (6) is pressed so that a surface of the coated steel plate having an organic film is a convex surface exposed to the outside.

  According to the present invention, a coated steel sheet having good corrosion resistance can be provided together with the processed product, particularly a thin TV panel.

It is an enlarged photograph of the wax of an organic film. It is an external appearance photograph of the coated steel plate after a corrosion resistance test.

Hereinafter, the present invention will be specifically described.
In the coated steel sheet of the present invention, a zinc-based plating layer is formed on both surfaces of the steel sheet, and a chemical conversion film and an organic film are sequentially formed on at least one surface of the zinc-based plating layer.
Details of each part of the coated steel sheet will be described below.

(Zinc-based plating layer)
The steel sheet to form a zinc-based plating layer to be the base steel sheet painted steel sheet of the present invention, electric galvanized steel sheet, or A aluminum - zinc alloy plated steel sheet (for example, molten zinc -55 wt% aluminum alloy plated steel sheet, molten Zinc-5 mass% aluminum alloy plated steel sheet ) can be used.

(Chemical conversion film)
The chemical conversion film formed on at least one surface of the zinc-based plating layer does not contain chromium from the viewpoint of environmental protection. The chemical conversion film is formed mainly for improving the adhesion between the plating layer and the organic 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. Silica is preferably contained in the chemical conversion film in an amount of 1 to 20% by mass in terms of improving adhesion and corrosion resistance.
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. Moreover, it is preferable to make phosphoric acid and a phosphoric acid compound contain 0.1-5 mass% in a chemical conversion film at the point which corrosion resistance improves. Furthermore, you may add 1 or more types, such as resin, such as an acrylic resin, an epoxy resin, an amine modified epoxy resin, and a polyester resin, and a silane coupling agent.
The film thickness of the chemical conversion film is preferably 0.1 to 1.5 μm from the viewpoint of adhesion and corrosion resistance. More preferably, it is 1.0 μm or less.

(Organic film)
The organic film formed on the chemical conversion film includes an organic resin, a color pigment, and a wax. The organic resin and the color pigment will be described later. In the present invention, it is important that the crystallinity of the wax is 80% or less. That is, by making the wax crystallinity 80% or less, the corrosion resistance of the organic film can be greatly improved. The following is a detailed description of the process of determining the appropriate crystallinity of the organic film wax.

The present inventors have discovered that voids are formed around the wax contained in the organic film during repeated research on coated steel sheets having good corrosion resistance, and these voids cause the corrosion resistance to deteriorate. I found out.
Conventionally, in order to ensure press workability during oil coating, high-density polyethylene wax having a crystallinity of more than 80% has been used, and as shown in FIG. 1 (a), polyethylene wax is included in the organic film. In some cases, voids existed at the outer edge of the wax.
Therefore, various waxes were examined in order to suppress the formation of the voids, and it was found that the formation of voids can be avoided by using microcrystalline wax. That is, as shown in FIG. 1B, when the microcrystalline wax is included in the organic film instead of the polyethylene wax, the gap between the wax and the film substrate is eliminated.

Next, the present inventors used a coated steel sheet having an organic film with a film thickness of 3 μm as a corrosion resistance test (a process of spraying a coated steel sheet with salt water having a concentration of 5 mass% for 8 hours and resting for 16 hours as one cycle. 5 cycles were carried out).
The coated steel sheet used here was formed by sequentially forming a zinc-based plating layer and a chromium-free conversion film on both surfaces of the steel sheet, and cured with a crosslinking agent on the conversion film on one surface of the steel sheet. An organic film containing a polyester resin and resin particles having an average particle diameter of 3 to 40 μm, a glass transition temperature of 70 to 200 ° C. and higher hardness than the polyester resin is formed.
FIG. 2 (a) shows the test results of a sample containing a high-density polyethylene wax having a crystallinity of more than 80% in the organic film, and FIG. 2 (b) shows a test result of a sample containing microcrystalline wax in the organic film.
The sample containing the microcrystalline wax in the organic film (FIG. 2 (b)) has less white spot-like irregularities (roughness) than the sample containing the conventional polyethylene wax in the organic film (FIG. 2 (a)). I was able to confirm.
This white rough portion is considered to be the appearance of corrosion products. That is, in the sample (FIG. 2 (a)) in which the conventional polyethylene wax is included in the organic film, the presence of the void portion causes the corrosive factor to enter from this void portion, and the corrosion product is leached out from the galvanized layer. It is considered to be a part. On the other hand, in the sample containing the microcrystalline wax in the organic film (FIG. 2 (b)), since there is no void portion, the corrosion factor does not enter, the corrosion product does not leach from the galvanized layer, and there are few rough portions. it is conceivable that.
Thus, it has been found that the use of microcrystalline wax can improve the corrosion resistance of the coated steel sheet.

  The reason why voids are less likely to occur when microcrystalline wax is used is considered to be due to the low crystallinity of microcrystalline wax. A wax having a high degree of crystallinity has a large coefficient of thermal expansion in the temperature range near the melting point, and therefore volume shrinkage when cooled is increased. Then, a gap is generated around the wax due to a difference between the shrinkage rate of the wax and the organic resin around the wax, and this gap becomes a starting point of corrosion. Therefore, a wax having a low crystallinity does not generate voids, and therefore, corrosion does not occur starting from the voids, thereby improving the corrosion resistance of the coated steel sheet. Here, when the relationship between the crystallinity of the wax and the corrosion resistance of the coated steel sheet was investigated, as shown in the examples described later, the wax crystallinity of 80% or less is effective in improving the durability. I confirmed that there was.

The microcrystalline wax is a kind of petroleum wax as defined in Japanese Industrial Standard (JIS K2235), and is a solid wax separated at room temperature from a residue obtained by distillation under reduced pressure or from a heavy distillate. . The number of carbon atoms is about 30-60, the molecular weight is about 500-800, and the melting point is generally higher than that of paraffin wax. Further, in terms of composition, there are many isoparaffins and cycloparaffins with small crystals, resulting in microcrystals. The melting point of microcrystalline wax is mainly determined by the difference in crystallinity and is subdivided into five types depending on the melting point.
Examples of the low crystallinity wax include microcrystalline wax, natural wax such as beeswax, montan wax and paraffin wax, and synthetic wax such as low density polyethylene wax, which can be suitably used in the present invention. it can.

  The wax used in the present invention preferably has a melting point of 60 ° C. or higher and 120 ° C. or lower. Because if the melting point is 60 ° C. or higher, there is no stickiness during handling, re-melting due to high temperature environment during transportation, problems of surface whitening, and workability deterioration due to slippage during slitting, while the melting point is 120 ° C. or lower. This is because sufficient lubricity can be obtained during press working.

Moreover, it is suitable that the content rate of a wax is 0.1 to 10 mass%.
This is because if it is 0.1% by mass or more, the press workability of the coated steel sheet can be improved, while if it is 10% by mass or less, the effect is maintained and the cost is not disadvantageous.

(Organic resin)
The organic resin used for the organic film is not particularly limited, and examples thereof include acrylic, polyester, epoxy, fluorine, polyurethane, and the like, but polyester based mainly on the hardness and the bending workability. It is preferable to contain a resin.
The polyester-based resin is a copolymer obtained by heat-reacting a polybasic acid component and a polyhydric alcohol by a known method. As the polybasic acid component, for example, phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, maleic acid, adipic acid, fumaric acid and the like can be used. Examples of the polyhydric alcohol include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, triethylene glycol, Glycerin, pentaerythritol, trimethylolpropane, trimethylolethane and the like can be used. Commercially available polyester resins include “Almatex” (registered trademark) (trade name, manufactured by Mitsui Toatsu Chemical Co., Ltd.), “Alkinol” (trade name, manufactured by Sumitomo Bayer Urethane Co., Ltd.), “Desmophene” "(Registered trademark) (trade name, manufactured by Sumitomo Bayer Urethane Co., Ltd.)," Byron "(registered trademark) (trade name, manufactured by Toyobo Co., Ltd.), and the like.
Further, as a more preferable polyester-based resin, a urethane-modified polyester resin obtained by reacting a polyester-based resin with an aliphatic diisocyanate compound can be used. Specific examples of the aliphatic diisocyanate compound include hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, dicyclohexylmethane-4,4-diisocyanate, and methylcyclohexane diisocyanate.

The number average molecular weight of the organic resin used in the organic coating 8 000-10000, glass transition temperature Tg of 5 0 to 70 ° C., it is necessary a hydroxyl value of 5 ~60KOHmg / g.
If the number average molecular weight of 8 000 or higher, there is no cross-linking between molecular weight of the organic film is too short, since the crosslinking density is too large, or the bending workability deteriorates to decrease elongation, organic film strength This is because the film of the deformed deformation portion does not easily peel off. On the other hand, if the number average molecular weight is 10,000 or less, a sufficient crosslinking density is obtained. As a result, the barrier property is sufficient and the film does not swell.
If the glass transition temperature Tg of 5 0 ° C. or higher, does not decrease toughness of organic coating on a sufficient press formability can be obtained, the organic film hardness, also properties such as adhesion of the organic film after processing On the other hand, if the glass transition temperature Tg is 70 ° C. or lower, sufficient bending workability can be obtained.
Furthermore, if the hydroxyl value of the polyester resin is 5 KOHmg / g or more, a sufficient film hardness is obtained because the crosslinking reaction is sufficient. On the other hand, if the hydroxyl value is 60 KOHmg / g or less, sufficient processing is achieved. This is because the sex is obtained.

  Moreover, the organic film may be added with high hardness resin particles in the film. The resin particles are, for example, acrylic beads and nylon beads, and it is preferable to use resin particles having an average particle diameter of 3 to 40 μm, a glass transition temperature of 70 to 200 ° C., and higher hardness than the polyester resin. . The press workability can be improved while ensuring the bending workability by containing high-hardness resin particles having an average particle diameter of 3 to 40 μm and a glass transition temperature of 70 to 200 ° C. The reason why the preferable ranges of the average particle diameter, glass transition temperature and hardness of the resin particles are defined as described above is as follows.

  The resin particles act as a lubricant or an agent that has an effect of suppressing contact between the mold and the chemical conversion film as the base, and improve press workability. If the average particle diameter of the resin particles is 3 μm or more, the effect as a lubricant or the contact suppressing effect between the mold and the chemical conversion film is sufficient, and the press workability improving effect can be exhibited. The film itself does not peel from the coating film, the sliding resistance does not increase, and press workability can be ensured. For this reason, the average particle diameter of the resin particles is preferably 3 to 40 μm. Further, if the glass transition temperature of the resin particles is 70 ° C. or higher, the hardness of the resin particles will not be insufficient. On the other hand, if it is 120 ° C. or lower, the resin particles themselves will not work as sliding resistance. Also has excellent press workability.

  The “average particle diameter of the resin particles” herein refers to the cross section of the organic film observed with an optical microscope in at least three fields of view, and the average diameter of the maximum diameter of each resin particle and the diameter orthogonal thereto is calculated in each field of view. And it means the average value of these calculated average diameters.

  In addition, when resin particles are added, the hardness is higher than that of an organic resin (for example, a polyester resin) used for the organic film. By making the resin particles have a high hardness, it is possible to prevent the resin particles from resisting the mold and causing the mold surface to come into direct contact with the chemical conversion film or the galvanized layer during press processing. This is because the above effect cannot be obtained when the hardness of the resin particles is equal to or softer than that of the organic resin.

The “hardness” of the resin particles and organic resin here can be evaluated by the Tg. When Tg is high, the hardness is high.
Further, if the hardness of the resin particles is excessively higher than the hardness of the organic resin, stress is applied to the interface between the resin particles in the organic film and the organic resin (for example, polyester resin) used in the organic film during the molding process. Since the resin particles are likely to concentrate and be easily detached from the organic film, the Tg difference between the resin particles and the organic resin used for the organic film is preferably in the range of 20 to 130 ° C.

  In order to express a particularly excellent press workability improving effect, the content of the resin particles is preferably 5 to 20% by mass in the organic film.

  Examples of the resin species of the resin particles include acrylic resin and nylon resin. In particular, it is preferable to use a nylon resin as the resin particle because it can be easily applied by roll coating.

(Curing agent)
In addition, for the purpose of imparting stain resistance, abrasion resistance, etc., it is possible to add a curing agent component to make the organic film into a baked cured film. As the curing agent component, an isocyanate compound and / or an amino resin can be used. Moreover, you may mix and use these 2 or more types. As the isocyanate compound, an isocyanate compound obtained by a general production method can be used. Among them, phenol, cresol, aromatic secondary amine, tertiary alcohol, which can be used as a one-component paint, can be used. Polyisocyanate compounds blocked with a blocking agent such as lactam or oxime are preferred.
More preferred polyisocyanate compounds include non-yellowing hexamethylene diisocyanate (hereinafter abbreviated as HDI) and derivatives thereof, tolylene diisocyanate (hereinafter abbreviated as TDI) and derivatives thereof, 4,4′-diphenylmethane diisocyanate (hereinafter referred to as “diisocyanate”). , XDI) and derivatives thereof, isophorone diisocyanate (hereinafter abbreviated as IPDI) and derivatives thereof, trimethylhexamethylene diisocyanate (hereinafter abbreviated as TMDI) and derivatives thereof, hydrogenated TDI and derivatives thereof, hydrogenated MDI and derivatives thereof And hydrogenated XDI and its derivatives. Furthermore, "Sumijoule" (registered trademark) (trade name, manufactured by Sumitomo Bayer Urethane Co., Ltd.), "Desmodule" (registered trademark) (trade name, manufactured by Sumitomo Bayer Urethane Co., Ltd.), "Coronate" (registered trademark) ) (Trade name, manufactured by Nippon Polyurethane Co., Ltd.) can also be used.

As the amino resin that is a curing agent, resins obtained by reacting urea, benzoguanamine, melamine and the like with formaldehyde, and those obtained by alkyl etherification with alcohols such as methanol and butanol can be used. Specific examples include methylated urea resins, n-butylated benzoguanamine resins, and iso-butylated melamine resins. In addition, “Cymel” (trade name, manufactured by Mitsui Ciamid Co., Ltd.), “Uban” (registered trademark) (trade name, manufactured by Mitsui Toatsu Chemicals), “Sumimar” (registered trademark) (trade name, Sumitomo) A commercially available amino resin such as “Chemical Industry Co., Ltd.” and “Melan” (registered trademark) (trade name, manufactured by Hitachi Chemical Co., Ltd.) can also be used.
When curing using a curing agent, the blending ratio (weight ratio of solid content) of the organic resin and the curing agent is organic resin / curing agent: 99/1 to 60/40, desirably 95/5 to 75/25. It is preferable to do this. It is preferable that the compounding quantity of a hardening | curing agent shall be 0.75-20 mass% in the ratio in organic membrane | film | coat solid content. If the amount of the curing agent is 0.75% by mass or more, the organic film hardness is sufficient, while if it is 20% by mass or less, the organic film is not cracked during bending.

(Pigment)
The organic film can be blended with a rust preventive pigment and a rust preventive agent depending on the purpose and application. Anti-corrosive pigments include zinc phosphate, zinc phosphite, aluminum phosphate, aluminum phosphite, molybdate, phosphate molybdate, vanadic acid / phosphoric acid mixed pigment, silica, and calcium silicate. Generally known rust preventive pigments and rust preventives such as adsorbed type silica can be used.

The organic coating preferably contains 5 to 15% by mass of carbon black as a color pigment in the coating. This is because the subsequent painting process can be omitted, and the base color and base cover can be concealed. The reason specified in the above range is that if the amount is 5% by mass or more, the pigment will not be too little, and the hiding property of the base color and the base layer will be sufficient. This is because the pigment does not increase too much and the organic film does not become brittle.
In addition, color pigments such as titanium dioxide, iron oxide, phthalocyanine, and quinacridone may be used in combination.

The film thickness of the organic film is preferably 10 μm or less. This is because if the thickness is 10 μm or less, the amount of the organic film that is detached from the cut end face during press working does not increase and does not adhere to the product. Furthermore, the film thickness is more preferably 3 to 7 μm. If the thickness is 3 μm or more, the film thickness of the organic film becomes uniform, and when a color pigment is included, the color tone is stable, the base color and the base layer are sufficiently concealed, and in addition, the corrosion resistance is preferable.
In addition, the film thickness of the said organic film measured the film thickness of arbitrary three places per field using an optical microscope or an electron microscope, and averaged the film thickness measured in 15 or more places in total in at least 5 fields of view. Value.

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. preferable.
In such a case, by having the above-mentioned chemical conversion film not containing chromium on the other surface of the steel sheet, the steel sheet has corrosion resistance and adhesion similar to those of 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 preferable 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.

  Furthermore, in the case where only the chemical conversion film not containing chromium is formed on the other surface of the steel sheet, and the application having a high requirement for corrosion resistance, the chemical conversion film not containing chromium formed on the other surface of the steel sheet is used. However, it is preferable to contain 1 or more types chosen from an epoxy resin, an amine modified epoxy resin, and a polyester resin. Moreover, when these resins are contained, it is preferable to cure using a melamine resin or the like as a curing agent. The content of the resin in the chemical film is preferably 30 to 80% by mass, and when a curing agent is used, the content of the curing agent in the chemical film is preferably 1 to 30% by mass. Moreover, when the resin is contained in the chemical film, the other components of the chemical film are 1 to 20% by mass of phosphoric acid and / or a phosphoric acid compound and 1 to 20% by mass of the chemical film in order to improve corrosion resistance. It is preferable to use inorganic additives such as manganese carbonate, aluminum carbonate, and zinc carbonate.

  Moreover, an organic resin layer can be formed on the chemical conversion film as necessary for applications that require a particularly high degree of 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 as a rust preventive pigment in order to obtain further excellent corrosion resistance. Moreover, it is preferable that content of the antirust pigment with respect to an organic resin layer shall be 0.1-50 mass%. This is because by making it within this range, the corrosion resistance can be improved without deteriorating the conductivity of the organic resin layer.

Furthermore, it is advantageous to include a heat dissipating pigment in the organic resin layer for applications that require a high degree of heat dissipation. Examples of the organic resin species for exhibiting heat dissipation include epoxy resin, acrylic resin, urethane resin, phenol resin, urea resin, and polyester resin. The heat dissipating pigment is not particularly limited, and a known pigment may be used. In order to increase the thermal emissivity, carbon black, aniline black, polymethylene dye, trisazo dye amine salt, cyanine dye Or the metal complex, iron oxide, silicon oxide, and magnesium silicate are illustrated. These heat dissipating pigments may be added singly or in combination of two or more.
Furthermore, desired heat dissipation can be obtained by making the thermal emissivity of the organic resin layer containing the said heat dissipation pigment into the range of 0.3-0.8. Here, the thermal emissivity is a relative expression of the spectral reflectance (R) of the surface in the wavelength region of 4.5 to 25 μm, assuming that the black body radiation at the same temperature is 1 (100%). It is an integration. The specific amount of the heat dissipating pigment added to the organic resin layer is preferably 0.1 to 20% by mass. This is because when the addition amount is 0.1% by mass or more, a sufficient heat dissipation effect can be obtained, and when the addition amount is 20% by mass or less, the corrosion resistance of the organic resin layer is prevented from being deteriorated.
Moreover, it is preferable that the film thickness of an organic resin layer shall be 0.1-1.0 micrometer.

  The coated steel sheet of the present invention is suitable for a processed product that is used after being subjected to bending and pressing. Furthermore, it is suitable for members used in applications such as electronic devices and home appliances that are pressed so that the surface provided with the organic film becomes a convex surface and further require electromagnetic shielding properties. For example, it is suitable for use in a back panel of a thin TV such as a plasma display panel or a liquid crystal television.

Examples of the present invention will be described.
As galvanized steel sheets for coating, electrogalvanized steel sheets (plating type code: EG) each having a thickness of 0.5 mm, galvannealed steel sheets (Fe content: 10 mass%, plating type code: GA), molten Galvanized steel sheet (plating type code: GI), hot-dip Zn-Al-plated steel sheet (Al content: 4.5 mass%, plating type code: GF) and hot-dip Zn-Al-plated steel sheet (Al content: 55 mass%, Plating type symbol: GL) was prepared. Table 1 shows the coating amount (g / m ² ) per one side 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 degreasing the zinc-based plated steel sheet, the following processing steps (i) and (ii) were performed to prepare a coated steel sheet as a sample. Table 1 shows the coating amount (g / m ² ) and film thickness (μm) of the plated steel sheet.
(I) A chemical conversion treatment agent for forming a chemical conversion film shown in Table 1 (the composition is shown in Table 3) is applied on the front side with a bar coater and dried, and then the chemical conversion chemical shown in Table 2 is provided on the back side. A chemical conversion treatment agent for forming a film (composition is shown in Table 3) is applied, and then heat treatment is performed so that the ultimate plate temperature becomes 100 ° C. after 10 seconds of heating. A chemical conversion film was formed on the surface.
(Ii) Thereafter, the coating for organic film containing the base resin, curing agent, coloring pigment, and wax shown in Table 1 was applied so as to have the dry film thickness shown in Table 1, and reached 20 seconds after the start of heating. Heat treatment was performed so that the plate temperature was 190 ° C., and an organic film on the front side was formed.
The glass transition temperature Tg in Table 1 was measured based on (JIS K 71214.2 (2) [Heat flux differential scanning calorimetry] HMMM is hexamethoxymethylated melamine, and HDI is hexagonal. Methylene diisocyanate.

<Evaluation of the front side>
(1) Hardness Hardness is determined by using an indentation depth (μm) when a test load of 3.0 mN is applied using a regular square pyramidal indenter, and an ultra-fine indentation hardness tester (Fischerscope (registered trademark) HM2000). ) And calculated from the indentation depth.

(2) Bending workability (visual evaluation)
Bending workability is based on JIS Z2248-1996, bending the coated steel sheet into a test piece cut into a size of 60 mm in length and 30 mm in width and bending the coated steel sheet to 180 ° at room temperature. The top of the part was visually observed. Evaluation was performed according to the following criteria.
○: No cracking of the organic film is confirmed ×: Cracking of the organic film is confirmed

(3) Bending workability (Evaluation by ERV)
Evaluation by ERV measured ERV by the ERV method about the sample which performed 0T bending in said (2). ERV (enamel rating value) is a test piece obtained by cutting a coated steel sheet to a size of 60 mm in length and 30 mm in width, and the coated steel sheet is subjected to close bending at room temperature (0T bending according to JIS Z 2248-1996). And the degree of coverage of the colored single film at the top of the contact bending portion is evaluated by the ERV method. Here, the measurement of ERV obtained by the ERV method is performed at a temperature of 20 in a state where the top surface of the bent portion is sealed with an end face with a frame seal tape exposing 10 mm × 10 mm and the contact area with the electrolyte is constant. It is the value measured with the enameler which uses -25 degreeC and 1 mass% salt solution as electrolyte solution. Evaluation was performed according to the following criteria.
◎: Less than 2 mA ○: 2 mA or more, less than 5 mA Δ: 5 mA or more, less than 10 mA ×: 10 mA or more

(4) Substrate concealment The concealment concealment is made by coating the front surface of the plated steel sheet before the formation of the conversion coating with a metal pen at the tip, and then performing the above-described processing steps to produce each coated steel sheet. The front side was visually observed. Evaluation was performed according to the following criteria.
○: Scratches cannot be confirmed △: Scratches can be confirmed

(5) Resistance to press wrinkles Side walls after each coated steel sheet is formed into a cylindrical cup with a blank diameter of 67 mmφ, a punch diameter of 33 mmφ, a molding speed of 300 mm / s, and a wall surface temperature of 80 ° C. so that the front surface becomes convex. The organic film was visually evaluated for damage according to the following criteria.
◎: No damage occurred ○: Some damage was observed ×: Many damage occurred

(6) Solvent resistance The ethanol rubbing test was performed 10 times with a load of 4.9 N (0.5 kgf) on the surface of the organic film of each coated steel sheet, and the fluctuation range of the L value (lightness) before and after the ethanol rubbing test. (ΔL) was measured and evaluated according to the following criteria.
◎: ΔL is 1 or less ○: ΔL exceeds 1 and 2 or less ×: ΔL exceeds 2

(7) Corrosion resistance From each of the above samples, a test piece (size: 100 × 50 mm) was cut out, the end and back of the test piece were tape-sealed, and then sprayed with 5% by weight of salt water at 35 ° C. for 8 hours. The process of resting for 16 hours was defined as 1 cycle, and the change in the appearance of the organic film surface after 5 cycles was evaluated. Evaluation was performed according to the following criteria.
◎: No change on the surface of the organic film ○: There is a little rust on the surface of the organic film ×: There are many rust on the surface of the organic film

<Evaluation of the back surface>
(8) Conductivity The surface resistance value of the back surface of each sample was measured using a low resistance measuring device (Loresta GP: manufactured by Mitsubishi Chemical Corporation: ESP probe). 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 the surface resistance is 10 ⁻⁴ Ω or less ◎: Average load of 10 point measurement is 300 g or less ○: Average load of 10 point measurement is over 300 g to 500 g or less Δ: Average load of 10 point measurement is over 500 g 700 g or less x: Average load of 10-point measurement is more than 700 g and 960 g or less

(9) Heat dissipation About each sample, heat dissipation was measured on the conditions shown below.
Measuring device: Far-infrared spectroradiometer JIR-E500, manufactured by JEOL Ltd., measuring temperature: 100 ° C., measuring wavelength: 4.5 to 25 μm, resolution: 8 cm ⁻¹ , integration count: 100 times, other conditions: JIS Based on R1801: 2002 After that, the emissivity was calculated from the measurement results and evaluated according to the following criteria.
◎: 0.5 or more, less than 0.8 ○: 0.3 or more, less than 0.5 Δ: 0.1 or more, less than 0.3 ×: less than 0.1

Table 4 shows the evaluation results of the above tests.
According to this, the coated steel sheets of the examples all have excellent hardness, base concealing property, bending workability, press resistance, solvent resistance, corrosion resistance, and conductivity. In particular, it can be seen that as compared with the comparative example, both the bending excellent workability.

  According to the present invention, a coated steel sheet having good corrosion resistance can be provided together with the processed product, particularly a thin TV panel.

Claims (7)

A zinc-based plating layer formed on both sides of the steel plate;
A chemical conversion film not containing chromium formed on at least one surface of the zinc-based plating layer;
An organic film containing an organic resin, a color pigment, and a wax formed on the chemical film;
Have
The wax has a crystallinity of 80% or less and a content of 0.1% by mass or more and 10% by mass or less.
The zinc-based plating layer is an electrogalvanized layer or a molten Zn-Al plated layer,
A coated steel sheet, wherein the organic resin has a number average molecular weight of 8000 to 10,000, a glass transition temperature Tg of 50 to 70 ° C., and a hydroxyl value of 5 to 60 KOHmg / g .   The coated steel sheet according to claim 1, wherein the wax is a microcrystalline wax. Coated steel sheet according to claim 1 or 2, wherein the organic coating is characterized in that it comprises carbon black. When the chemical conversion film and the organic film are applied to one surface of the steel sheet, the chemical conversion film is provided on the zinc-based plating layer on the other surface of the steel sheet, and the conductive load on the other surface is 500 g or less. The coated steel sheet according to any one of claims 1 to 3 , wherein the steel sheet is coated. The coated steel sheet according to claim 4 , wherein the chemical conversion film on the other surface of the steel sheet contains a resin. The processed product formed by giving the press work to the coated steel plate as described in any one of Claims 1-5 . The processed product according to claim 6 , wherein the surface of the coated steel sheet having an organic film is pressed so as to be a convex surface exposed to the outside.