JP2007119858A - Chrome-free pre-coated steel sheet - Google Patents

Abstract

The present invention provides a precoated steel sheet having corrosion resistance on the side used as a back surface and / or excellent workability, and a method for producing the precoated steel sheet.
Chromium-free base treatments 13 and 16 are provided on both surfaces of a zinc-based plated steel sheet 11a, and a polyester resin and / or an epoxy-modified polyester on the base treatment 13 is provided on one surface which is a front surface. An undercoat coating layer 14 containing a resin based binder as a binder and containing at least one of an ion-exchange oxide rust preventive pigment or a phosphoric acid rust preventive pigment is provided, and one or more layers are provided on the undercoat coat layer. The top coat layer 15 is provided, and the other surface which is the back surface is also provided with a polyester resin and / or an epoxy-modified polyester resin as a binder and an ion-exchange oxide rust preventive pigment or phosphoric acid rust preventive. An undercoat film layer 17 containing at least one kind of pigment is provided, and one or more topcoat film layers 18 are provided above the undercoat film layer.
[Selection] Figure 1

Description

  The present invention relates to a precoated steel sheet used outdoors or semi-outdoors such as an air conditioner outdoor unit, a washing machine or a water heater, a vending machine, a power supply box, etc., and has excellent workability and corrosion resistance and does not contain chromium. It relates to steel plates.

  Pre-coated steel sheets (hereinafter sometimes referred to as PCM) have many uses due to their functionality and are applied to various industrial products. An example of the application is a case of an air conditioner outdoor unit.

  A schematic diagram of a conventional cross-sectional structure of the PCM used in this way is shown in FIG. In FIG. 2, the upper side of the paper is the front surface (design surface), and the lower side of the paper is the back surface (non-design surface). For example, when applied to an outdoor unit of an air conditioner, the front surface appears outside the outdoor unit, and the back surface is arranged inside the outdoor unit. In the conventional PCM, plating film layers 22 and 22 are provided on the front and back surfaces of the steel plate 21. A steel plate in which the plated layers 22 and 22 are provided on the steel 21 is a so-called plated steel plate. Further, chromium-based ground treatments 23 and 23 were formed outside the plating film layers 22 and 22. An undercoat coating layer 24 was provided on the outer side of the chromium-based primer treatment 23 on the front surface side, and an upper coating layer 25 was provided on the outer side thereof. On the other hand, only one service coat layer 29 was provided on the back surface outside the chromium base treatment 23. Here, hexavalent chromium was used for the chromium-based primer treatments 23 and 23 and the undercoat coating layer (primer) 24, and in some cases, the service coat layer 29 also contained hexavalent chromium.

  Under such circumstances, in recent years, PCM is also required to be chromium-free in order to meet environmental regulations. In order to make PCM chrome-free, it is necessary to make chrome-free both of the coating base treatment (conventionally chromate treatment) and the undercoat coating layer (primer, which contains a conventional chromium-based anticorrosive pigment).

  In response to such demands, conventional techniques such as Patent Documents 1 to 8 are disclosed for chromium-free PCM.

  Patent Documents 1 and 2 disclose a technique in which a chromium-free chemical conversion treatment containing tannic acid or the like is performed, and then an undercoat coating layer containing a phosphate-based anticorrosive pigment (or calcium exchangeable silica or the like) is provided. Has been. Patent Documents 3 and 4 disclose a technique for providing an undercoat coating layer containing Ca ion-exchanged silica and a phosphate-based anticorrosive pigment as an antirust pigment. Patent Document 5 discloses a technique for providing an undercoat coating layer containing Zn Mg-modified phosphate as a phosphoric acid anticorrosive pigment.

  Further, Patent Documents 6 and 7 disclose a technique for providing an undercoat film layer containing a phosphoric acid-based anticorrosive pigment and preferably wet silica, and Patent Document 8 discloses a vanadate pigment and a phosphate pigment. A technique for providing an undercoat coating film layer containing silica and silica is disclosed.

  Therefore, the base treatment and undercoat coating layer made chromium-free as described above have been used in the conventional PCM with the configuration shown in FIG.

JP 2001-89868 A Japanese Patent Laid-Open No. 2003-166079 JP 2004-1775096 A JP 2001-212506 A Japanese Patent Laid-Open No. 9-12931 JP-A-5-208166 JP-A-5-317806 JP-A-7-51620

  However, when such a chromium-free PCM is used for the above-described air conditioner outdoor unit, for example, it is found that there is a problem that rust is likely to occur on the inner surface of the outdoor unit, which hardly occurred in the conventional chromium-containing PCM. It was. That is, in an air conditioner outdoor unit installed outdoors, an external corrosive factor such as salt enters the outdoor unit through the vent, and it is described as the inner surface of the casing (PCM is a non-design surface, hereinafter referred to as “back surface”). May stick to it). On the inner surface of the housing, salt and the like are not washed away by rain or the like, and dry salt and the like are concentrated. In this sense, since the corrosive environment is different from the inner surface of the housing and the outer surface of the housing (PCM is a design surface, hereinafter referred to as “front surface”), the occurrence of corrosion (for example, rust) Is also different. As described above, in the conventional chromium-containing PCM, a coating layer called a service coat is usually formed on the back surface after the chromate treatment as a base treatment (this service coat includes Chrome-based rust preventive pigments are often included.) In conventional chromium-containing PCM, the corrosion resistance of the inner surface of the casing (hereinafter sometimes referred to as inner surface corrosion resistance) is a problem with such a back surface structure. Almost never. For this reason, in PCM, the performance of the front surface (design surface) may be more important, and the corrosion resistance of the back surface (internal corrosion resistance) has hardly been studied in the study of chromium-free. However, when an outdoor exposure test of chromium-free PCM was conducted, the inner surface corrosion resistance was inferior to that of chromium-containing PCM even though the performance of the front surface was at a practical level.

  Further, after the PCM cuts out from the coil and creates a blank, it is formed into a desired shape. In PCM, the end of PCM is usually bent so that the end face is not exposed to the outside of the processed product. This bending process is not limited to bending an unprocessed flat plate part, and the part subjected to the drawing process may be further bent. However, the chromium-free base treatment has a problem that the coating layer on the front surface is cracked or peeled off particularly at such a site.

  Therefore, an object of the present invention is to provide a precoated steel sheet having corrosion resistance on the side used as the back surface and / or excellent workability, and a method for producing the precoated steel sheet.

As a result of intensive studies, the inventor has completed the present invention by obtaining and embodying the following idea.
(1) The back side, which is conventionally called a service coat and is provided with a single coating layer, is provided with a coating layer containing a rust-preventive pigment as the lower layer, as well as the front surface, A multi-layer structure having more than one layer is adopted.
(2) The undercoat coating layer on the front surface is constructed as follows.
-As the binder resin, a resin having an excellent balance between hardness and workability (ductility) is used.
-The average particle size of the rust preventive pigment is improved by reducing the average particle size of the chromium-free pigment.
-The content of the rust preventive pigment is suppressed to a predetermined range or less (in this case, if the end face corrosion resistance is inferior, the content of the rust preventive pigment on the back surface may be larger than that on the front surface).
The present invention will be described below. In order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are appended in parentheses, but the present invention is not limited to the illustrated embodiments.

  According to the first aspect of the present invention, there is provided a polyester-based resin and / or an epoxy on one surface which is a front surface of a zinc-based plated steel sheet (11a) having a chromium-free ground treatment (13, 16) on both surfaces. An undercoat film layer (14) containing a modified polyester resin as a binder and containing at least one of an ion-exchange oxide rust preventive pigment or a phosphoric acid rust preventive pigment is provided, and further an upper layer of the undercoat paint layer In addition, one or more top coat layers (15) are provided on the other side, and on the other side which is the back side, a polyester-based resin and / or an epoxy-modified polyester-based resin are used as a binder to prevent the ion-exchange type oxide. An undercoat coating layer (17) containing at least one of a rust pigment or a phosphoric acid-based anticorrosive pigment is provided, and one or more topcoat coating layers (18) are further formed on the undercoat coating layer. It is provided to solve the above problems by providing a precoated steel sheet (10), characterized in.

  Invention of Claim 2 is the precoat steel plate (10) of Claim 1, Comprising: The average molecular weight of resin which is a binder of the said undercoat coating film layer (14) of a front surface is 5000-25000 And the glass transition temperature of this binder resin is 10 degreeC or more and 40 degrees C or less, It is characterized by the above-mentioned.

  Invention of Claim 3 is the precoat steel plate (10) in any one of Claim 1 or 2, Comprising: The average particle diameter of the antirust pigment contained in undercoat film layers (14, 17) is 30 micrometers or less. It is characterized by being.

  Invention of Claim 4 is the precoat steel plate (10) in any one of Claim 1 thru | or 3, Comprising: The rust preventive pigment in the undercoat film layer (14) of the front surface is this undercoat film. It is characterized by being contained in an amount of 15 mass% or more and 50 mass% or less with respect to the adhesion amount of the layer.

Invention of Claim 5 is the precoat steel plate (10) in any one of Claim 1 thru | or 4, Comprising: The adhesion amount of the undercoat film layer (14) of the front surface is 3 g / m < ² > or more and 30 g / m m ² or less.

The invention of claim 6 is the precoated steel sheet (10) according to any one of claims 1 to 5, wherein the coating amount of the undercoat coating layer (17) on the back surface is 3 g / m ² or more and the top coat is applied. The adhesion amount of the film layer (18) is 3 g / m ² or more.

  The invention of claim 7 is the precoated steel sheet (10) according to any one of claims 1 to 6, wherein the chromium-free base treatment (13, 16) is from a silane coupling agent, silica, Ti salt, or Zr salt. It is an organic-inorganic composite treatment having one or more selected substances and a resin as main components.

  Here, the “main component” refers to a component that is contained most in the constituent components. In the present invention, one or a plurality of substances selected from a silane coupling agent, silica, Ti salt, and Zr salt and a resin are used. The total solid content is preferably 70% by mass and more preferably 90% by mass with respect to the total solid content of the ground treatment.

  The invention of claim 8 is a method for producing the precoated steel sheet (10) according to claim 7, wherein an alkaline aqueous solution having a pH of 13 or more or a pH of 1.5 to 4.0 is formed on both surfaces of the zinc-based plated steel sheet (11a). After contacting with an acidic aqueous solution of the above, it is washed with water and dried, and thereafter an organic-inorganic composite treatment mainly comprising one or more substances selected from a silane coupling agent, silica, Ti salt and Zr salt and a resin. Next, the above-mentioned problems are solved by providing a method for producing a pre-coated steel sheet in which an undercoat coating film layer (14, 17) is formed and a top coating film layer (15, 18) is further formed.

  The invention according to claim 9 is a method for producing the precoated steel sheet (10) according to claim 7, wherein an aqueous solution containing Ni, Co, Fe is brought into contact with both surfaces of the galvanized steel sheet (11a). Then, it is washed with water and dried, and then subjected to an organic-inorganic composite treatment mainly comprising one or more substances selected from a silane coupling agent, silica, Ti salt and Zr salt and a resin, and then an undercoat The said subject is solved by providing the manufacturing method of the precoat steel plate which forms a film layer (14, 17) and forms a top coat film layer (15, 18) further.

Invention of Claim 10 is a manufacturing method of the precoat steel plate (10) of Claim 9, Comprising: By making it contact with the aqueous solution containing Ni, Co, Fe, the surface of a galvanized steel plate (11a) wherein Ni, Co, and be deposited in 2 mg / ^{m 2} or more 10 mg / ^{m 2} or less of Fe to.

  The invention described in claim 11 is a processed product processed into a casing shape using the precoated steel sheet (10) according to any one of claims 1 to 7, wherein the front surface of the precoated steel sheet is The outer surface of the housing is processed with the back surface facing the inner surface.

  According to the present invention, a pre-coated steel sheet having excellent non-design surface corrosion resistance and excellent workability can be obtained, which is useful for home appliances such as air conditioner outdoor units and washing machines that are used outdoors. .

  Such an operation and a gain of the present invention will be clarified from the best mode for carrying out the invention described below.

  The best mode of the present invention and the reasons for limitation will be described below.

  FIG. 1 is a view schematically showing a cross section of a precoated steel sheet 10 according to one embodiment of the present invention. In FIG. 1, the upper side of the paper is the front surface, and the lower side of the paper is the back surface. The precoated steel plate 10 includes a steel plate 11 and galvanized layers 12 and 12 on both the front and back surfaces of the steel plate 11. The steel plate 11 and the galvanized layers 12 and 12 form a so-called galvanized steel plate 11a. Then, a chromium-free undercoating 13, an undercoat coating layer 14, and a topcoat coating layer 15 are laminated in this order from the front surface side zinc plating layer 12 toward the front surface side. Here, the undercoat coating film layer 14 contains a binder and a rust preventive pigment.

  On the other hand, on the back surface side galvanized layer 12, a chromium-free undercoat treatment 16, an undercoat coating layer 17, and a topcoat coating layer 18 are laminated in this order toward the back surface. Here, the undercoat coating film layer 17 contains a binder and a rust preventive pigment.

Next, each layer will be described.
<Galvanized steel sheet>
As the zinc-based plated steel sheet 11a to be the base steel sheet, a hot dip galvanized steel sheet, an alloyed hot dip galvanized steel sheet, an electrogalvanized steel sheet, an electrogalvanized steel sheet, a molten Zn-A1 alloy plated steel sheet (for example, molten Zn-5 mass) Various zinc-based plated steel sheets such as% A1 alloy-plated steel sheet and hot-dip Zn-55 mass% A1 alloy-plated steel sheet). The plating type and adhesion amount may be determined according to the required corrosion resistance. However, galvanizing with good ductility is effective from the viewpoint of workability, which is the subject of the present invention, and hot dip galvanizing (usually about 0.3 to 0.4% by mass of Al) steel sheet or zinc A plated steel plate or the like is preferably used.

  In addition, it is desirable that the workability of the steel plate itself is good for applications that require severe processing such as drawing. For example, it is desirable to use a steel plate adjusted to a high EL value and a high r value using extremely low carbon steel.

<Ground treatment>
The ground treatment 16 is a chromium-free treatment. For this purpose, for example, a conventionally used zinc phosphate treatment or the like can be used, but one or more substances selected from silane coupling agents, silica, Ti salts, and Zr salts and resins are mainly used. The thing by the organic inorganic composite process as a component is especially preferable. Examples of such organic-inorganic composite chromium-free treatment include those using EC-2000 manufactured by Nippon Paint Co., Ltd. As a preferable composition of the organic-inorganic composite treatment, silane coupling agent 20-60% by mass, silica 10-60% by mass, Ti salt 1-30% by mass, Zr salt 1-30% by mass, and resin 1-50% by mass. is there. The coating weight, from the viewpoint of adhesion and corrosion resistance of the coating layer is preferably 10 mg / ^{m 2} or more, usually 30 mg / ^{m 2} about. On the other hand, if it is too much, the adhesiveness is lowered, so 200 mg / m ² or less is preferable.

In addition, when applying the said organic-inorganic composite process as a base treatment, it is preferable to perform what is called a surface adjustment process on the surface of a zinc plating layer prior to this base treatment. As the surface conditioning treatment, there is a method of contacting an acidic aqueous solution of pH 1.5 to 4.0 or an alkaline aqueous solution of pH 13 or more, more preferably a method of contacting an aqueous solution containing Ni, Co, Fe with the surface of the galvanized layer. Can be mentioned. When using chromium-free PCM and high processing conditions as in the present invention, a treatment of forming Ni, Co, and Fe at 2 mg / m ² or more and 10 mg / m ² or less in terms of metal is preferable.

<Undercoat layer>
After the above-mentioned coating ground treatment is formed on the front and back surfaces of the steel sheet, undercoat coating layers 14 and 17 are provided on both sides. The undercoat coating layers 14 and 17 may be coating layers formed from the same paint on the front surface / back surface, but may not be the same as long as they are within the scope of the present invention. For example, as described later, the content of the anticorrosive pigment may be changed on the front and back surfaces.

  As the binder resin contained in the undercoat coating layers 14 and 17, a polyester resin or an epoxy-modified polyester resin obtained by modifying this with an epoxy resin can be used. Among these, those having an average molecular weight of 5000 to 25000 and a glass transition temperature (hereinafter sometimes referred to as “Tg”) of 10 ° C. or more and 40 ° C. or less are preferable. If the average molecular weight is too small, the processability tends to decrease, and if it is too large, the viscosity of the coating material for forming such a coating layer increases and the productivity decreases. If the Tg is too low, the coating layer may be too soft. If it is too high, the viscosity of the coating tends to increase, and the workability may be lowered.

  A cross-linking agent such as melamine, benzoguanamine, isocyanate, epoxy, or phenol resin can be added to the paint for forming the undercoat coating layers 14 and 17. In particular, a melamine resin is preferred when forming a polyester resin coating layer having excellent processability. In addition, when emphasizing processability, isocyanate or the like can also be used.

  The rust preventive pigment contained in the undercoat coating layers 14 and 17 is chromium-free, and specifically, an ion exchange oxide and / or a phosphoric acid rust preventive pigment can be used. Examples of ion-exchange oxides include silica, montmorillonite, vermiculite, saponite and the like ion-exchanged with Ca, Mg, Sr, Ba, and Zn. Examples of phosphoric acid-based rust preventive pigments include Zn phosphate, Ca phosphate, Al phosphate, Al metaphosphate, Mg metaphosphate, trihydrogen phosphate trihydrogen, and Mg triphosphate. By containing these anticorrosive pigments in the undercoat coating layers 14 and 17, it is considered that the active ingredient in the pigment is eluted in a corrosive environment and a protective film against corrosion is formed.

  The undercoat film layer 14 on the front surface preferably contains 15% by mass or more and 50% by mass or less of a rust preventive pigment based on the total adhesion mass of the undercoat film layers 14 and 17. If the content of the rust preventive pigment is too small, the corrosion resistance tends to be adversely affected. The total adhesion mass of the undercoat coating layers 14 and 17 is, for example, a decrease in mass when the coating layer is peeled off, and in addition to the mass of the binder resin, the rust prevention contained in the coating layer. This is the total mass of various components such as pigments, extender pigments and colored pigments. The total amount of the rust preventive pigment, extender pigment and colored pigment is preferably 70% by mass or less.

  As the rust preventive pigment to be contained in the undercoat coating layer 14 on the front surface, it is preferable to use those having an average particle size of 30 μm or less. More preferably, it is 20 micrometers or less, More preferably, it is 10 micrometers or less. If the particle size is too large, the bending workability tends to be inferior. This is considered to be because it tends to be a starting point of cracks when subjected to processing. In the PCM of the present invention, by using a rust preventive pigment having a small particle diameter, it is possible to withstand severe processing such as bending after drawing. In addition, the use of a rust preventive pigment having a small particle diameter has an effect that it is advantageous when high gloss or high definition is required after top coating.

  Here, the particle size can be measured by any method. Among these, the scattering method using laser light is particularly preferable.

  In addition, regarding these performances, it is considered that the particle size of extender pigments and colored pigments also affects, but for example, titania, colloidal silica, gas phase silica, etc. used as extender pigments, or carbon black used as a color pigment, Generally, it is a fine particle, and its influence is small.

The adhesion amount of the undercoat coating layer 14 on the front surface is preferably 3 g / m ² or more and 30 g / m ² or less as a total mass. More preferably, it is 5 g / m ² or more. Moreover, preferably 10 g / m ² or less is more preferable. This is because if the amount of the undercoat coating layer 14 is too small, it is disadvantageous for corrosion resistance, while if too much, it is disadvantageous for workability (particularly, workability after drawing).

  As described above, the undercoat coating layer 17 on the back surface side may or may not be a coating layer formed from the same paint as the front surface. From the viewpoint of cost and work, the coating materials for forming the undercoat coating layers 14 and 17 on the front surface side and the back surface side are exactly the same, or at least the binder resin system (including the crosslinking agent) is the same. It is preferable that the particle size and content of the rust preventive pigment and other pigments are different.

On the other hand, the back surface does not significantly affect the (bending) processing of the front surface. Therefore, when workability is prioritized and the amount of adhesion of the undercoat layer 14 on the front surface and the content of the anticorrosive pigment are suppressed, and the corrosion resistance of the steel plate end surface is also required, the undercoat film on the back surface The adhesion amount of the layer 17 and the content of the rust preventive pigment can be supplemented by increasing them from the front surface. On the back surface, the adhesion amount of the undercoat coating layer 17 is preferably 5 g / m ² or more from the viewpoint of the corrosion resistance of the back surface, and the adhesion amount may be increased if necessary for performance. However, 30 g / m ² or less is preferable from the viewpoint of reducing coating film defects in a normal manufacturing process. The anticorrosive pigment in the undercoat coating layer 17 may be contained in an amount of about 50% by mass.

  In addition, the undercoat coating layers 14 and 17 may contain extender pigments, colored pigments, and the like in addition to the binder resin and the rust preventive pigment. Usually, the undercoat coating layers 14 and 17 often contain extender pigments and colored pigments such as fine silica particles.

<Top coat layer>
After providing the above-described undercoat coating layers 14 and 17, topcoat coating layers 15 and 18 are provided on the outer surfaces of the undercoat coating layers 14 and 17. Although the top coat layers 15 and 18 are not particularly limited, in order to provide corrosion resistance and workability, the same polyester resin or epoxy-modified polyester resin as the resin used for the undercoat coating layers 14 and 17 is used. Is preferred. The crosslinking agent is the same, but a melamine resin is preferable from the viewpoints of cost, weather resistance, and coating layer hardness. In addition, isocyanate can be used when emphasizing processability. Further, an epoxy phenol resin may be used.

  The top coat layers 15 and 18 may contain extender pigments such as colored pigments and titania for improving the design. Moreover, you may contain a rust preventive pigment.

The back coating layer 18 is essential for ensuring the corrosion resistance of the back surface, and the adhesion amount is preferably 3 g / m ² or more, more preferably 5 g / m ² or more. Further, the upper limit is not particularly limited from the viewpoint of performance. From the viewpoint of a balance between cost increase and performance saturation, about 30 g / m ² is preferable. In order to suppress the pressure mark phenomenon, it is preferable to use the same resin as the paint for the top coat film layer 15 on the front surface, that is, the same paint. However, in order to distinguish the front and back surfaces, the appearance may be different from that of the front surface (by changing the color pigment). From the viewpoint of ensuring designability and corrosion resistance, the amount of adhesion of the top coat layers 15 and 18 is preferably 3 g / m ² or more, more preferably 5 g / m ² or more.

<Method for forming coating layer>
The coating layer may be formed by a normal method, using a paint containing a resin component and a rust-preventing pigment as a predetermined binder, and applying the paint to the steel sheet by a roll coat method, spray method, etc., and baking at a predetermined temperature. That's fine. For example, the paint may be solvent-based or water-based. Further, from the viewpoint of the performance and paintability of the coated steel sheet, the paint may contain a color pigment, an extender pigment, a crosslinking catalyst, a surfactant, and the like.

  The baking temperature is, for example, a polyester resin having a melamine resin preferably used in the present invention as a cross-linking agent. In the case of the undercoat coating layers 14 and 17, the highest material temperature (PMT) is 200 ° C. to 250 ° C. and the baking time. Is 30 seconds to 90 seconds, and in the case of the top coat layers 15 and 18, a PMT of 220 ° C. to 300 ° C. and a baking time of 45 seconds to 120 seconds are preferable.

<Processed products>
The precoated steel sheet 10 of the present invention is suitable for use in products processed with the front surface facing the housing outer surface and the back surface facing the housing inner surface, and particularly suitable for products used outdoors. Used for. Examples of such applications include an outdoor unit of an air conditioner, a washing machine, a water heater, a vending machine, and a power supply box.

  Next, examples of the precoated steel sheet according to the present invention will be described together with comparative examples.

NPC200 manufactured by Nippon Paint Co., Ltd. was sprayed for 3 seconds as a surface adjustment treatment on both surfaces of a hot dip galvanized steel sheet (plate thickness was 0.5 mm, plating coverage was 60 g / m ² per side). At this time, the treatment liquid temperature was 60 ° C., and the sprayed steel sheet was immediately washed with water and then dried. NPC200 is an alkaline treatment liquid containing Co, and the amount of Co deposited on the galvanized surface after the surface conditioning treatment was about 3 mg / m ² .

Then, following the surface conditioning treatment, chromium free base treatment was performed on both surfaces of the hot dip galvanized steel sheet using EC2000 manufactured by Nippon Paint Co., Ltd. The treatment method is a method in which a treatment liquid is applied by a bar coating method and then dried. The adhesion amount of the base treatment was about 70 mg / m ^{2 on} both sides.

  Subsequent to the base treatment, an undercoat coating layer was formed on each of the front surface and the back surface. The undercoat layer is composed of a commercially available polyester resin having physical properties shown in Table 1 after baking, a commercially available methylated melamine and a butylated melamine, and a polyester: methylated melamine: butylated melamine of 80: 16: 4 (parts by mass). ) And 0.5 parts by mass of paratoluenesulfonic acid as a curing catalyst was added to 100 parts by mass of the resin solid content.

  And using the undercoat which mix | blended the antirust pigment and extender pigment shown in Table 2 in the solvent, this was apply | coated to the steel plate by the bar-coat method. It was formed by baking in a hot air oven for 50 seconds under conditions such that the maximum reached temperature of the steel sheet was 220 ° C.

After forming the undercoat coating film layer, the top coating film layer was formed on each of the front surface and the back surface with a solvent-based paint having the following composition.
(1) Polyester resin C 50 parts by mass (2) Methylated melamine crosslinking agent 10 parts by mass (3) Extender pigment (titania) 31.5 parts by mass (4) Colored pigment (carbon black) 0.5 parts by mass (5) Polyethylene wax 8 parts by mass (solid content)
This paint was applied to the steel plate by the bar coating method, and was baked in a hot air oven for 60 seconds under the condition that the maximum reached temperature of the steel plate was 235 ° C.

  In addition, as a conventional example, a precoated steel sheet containing chromium was also prototyped. This is because, after chromate treatment on both surfaces of the above hot-dip galvanized steel sheet, strontium chromate was added as a rust preventive pigment instead of the rust preventive pigment shown in Table 2 on each of the front and back surfaces. An undercoating film layer was formed using the undercoating paint, and an overcoating film layer was formed on the front surface using the above-described overcoating paint.

  The PCM formed as described above can be used for the corrosion resistance of the inner surface and the processability of the front surface by changing the particle size of the anti-corrosion pigment, the blending amount, the coating amount of the undercoat coating layer, the presence or absence of the overcoating of the back surface, etc. The method was evaluated as follows.

<Evaluation of corrosion resistance>
(Corrosion resistance of cut and end parts)
Evaluation of corrosion resistance was performed by a cyclic corrosion test on a portion where a cross cut was made on the test surface with a cutter knife and a cut end portion (cut so as not to generate burrs on the test surface side). Cyclic corrosion test was conducted after 30 hours of salt spray (35 ° C) 2 hours followed by drying (60 ° C) for 4 hours and then wet (50 ° C, humidity 100%) for 2 hours. It was evaluated with. The evaluation criteria are as follows.
(Cross cut part)
◎… Maximum swollen width of coating layer is 1 mm or less ○… Maximum swollen width of coating layer is more than 1 mm and 3 mm or less ×… Maximum swelling width of coating layer is more than 3 mm (edge)
◎… Maximum swollen width of paint film layer is 2 mm or less ○… Maximum swollen width of paint film layer is more than 2 mm and less than 5 mm ×… Maximum swollen width of paint layer is more than 5 mm

(Internal corrosion resistance)
The inner surface corrosion resistance was determined by spraying a 0.1% NaCl aqueous solution onto the test surface (back surface) of a test piece cut into a 100 mm square, adhering about 100 mg as a salt amount, and drying it at room temperature. After maintaining for 120 hours at 50 ° C. and a relative humidity of 85%, the occurrence of white rust and red rust on the back surface was investigated. The evaluation criteria are as follows.
◎… No rust (red rust) occurrence ○… Small point-like rust (red rust or white rust diameter is about 0.5mm or less) occurs, the score is 10 points or less ×… Large rust (red rust or white rust The diameter is larger than 0.5 mm).

<Processability>
(Workability by bending)
The workability was evaluated by the following two bending processes with the test surface (front surface) being the outer surface.
(1) The test piece cooled to 0 ° C. is bent for 2 t.
(2) The test piece is bent 0 t at room temperature (about 25 ° C.).
After bending, the state of the bent portion was visually observed and evaluated according to the following evaluation criteria.
◎… No crack ○… Within 10 point-like cracks (per 25 mm width)
Δ: Over 10 point-like cracks (per 25 mm width)

(Processability after drawing)
After cylindrical drawing (drawing condition: blank diameter is 100 mmφ, punch diameter is 50 mmφ, die diameter is 52.4 mmφ, and wrinkle pressing force (BHF) is 1.5 ton) so that the test surface (front surface) becomes the outer surface The cylindrical diaphragm side face was cut out, and this was subjected to 2t bending at a room temperature (about 25 ° C.) and perpendicular to the drawing direction. The condition of the bent portion was evaluated based on the same criteria as the above workability.

Next, the results obtained by the above evaluation method will be described.
Example 1
Table 3 shows the evaluation results when various rust preventive pigments shown in Table 2 are applied to the undercoat coating layer. Here, as shown in Table 3, only the type of pigment contained in the undercoat coating layer was changed in each of the trial numbers (1-1) to (1-15).

  As can be seen from Table 3, it can be seen that even if any rust preventive pigment is included in the undercoat coating layer, the corrosion resistance is good on both the front and back sides. On the other hand, as shown in the trial number (1-16), when no rust preventive pigment is applied, it can be seen that even if a top coat layer is provided, the corrosion resistance is not good on both the front and back surfaces. Thus, when applying chromium-free anticorrosive pigment, regardless of which type of anticorrosive pigment is applied, when using PCM for an air conditioner outdoor unit, etc. It turns out that it should be used. Further, as shown in the conventional example (1-17), the chromium-containing rust preventive pigment has good corrosion resistance regardless of the presence or absence of the top coat layer. As a result, it has been clarified that the corrosion resistance needs to be improved from the viewpoint different from the case of using the chromium-containing rust-preventive pigment when making chromium-free.

  Therefore, this invention can provide the chromium free PCM excellent in corrosion resistance provided with corrosion resistance also on the side used as a back surface.

(Example 2)
Table 4 shows the evaluation results obtained by changing the type of the resin binder contained in the undercoat coating film layer shown in Table 1. In the trial numbers (2-1) to (2-6), the type of the resin binder is changed to A to F.

  According to Table 4, the inner surface corrosion resistance was not different depending on the type of the resin binder. On the other hand, the processability on the front surface tended to be better with a resin binder having a high average molecular weight and a high glass transition temperature.

(Example 3)
Table 5 shows the results of evaluation by changing the particle size of the rust preventive pigment contained in the undercoat coating film layer. Here, in trial number (3-1)-trial number (3-5), it evaluated by changing the particle size of the rust preventive pigment contained in the undercoat coating film layer.

  As can be seen from Table 5, the difference in the particle size of the rust preventive pigment does not affect the internal corrosion resistance. On the other hand, when looking at the workability of the front surface, the workability may be poor at a particle size of 35 μm, and conversely, good workability is shown at 2 μm of a small particle size. Accordingly, it is considered that the smaller the particle size, the better the workability.

Example 4
Table 6 shows the results of evaluating the content of the antirust pigment contained in the undercoat coating layer. Here, in the trial numbers (4-1) to (4-6), the content of the rust preventive pigment contained in the undercoat coating film layer is changed.

  As can be seen from Table 6, the processability of the front surface slightly decreases when the content of the anticorrosive pigment in the undercoat coating layer is the highest as seen in the trial number (4-6). In some cases, the trial number (4-3) having the lowest content showed excellent workability. Thereby, it is thought that the one where the content rate of the antirust pigment in a front surface is low exists in the tendency for workability to be good.

(Example 5)
Table 7 shows the results of evaluation by changing the adhesion amount while paying attention to the adhesion amount of the undercoat coating layer and the top coating layer. In the trial numbers (5-1) to (5-5), the types of resin binders contained in the undercoat coating film layer are shown in Table 2. When 1 is used, the adhesion amount of the undercoat coating layer is changed. Test numbers (5-6) to (5-10) are Nos. Shown in Table 2 for the types of resin binders contained in the undercoat coating film layer. When 14 is used, the adhesion amount of the undercoat coating layer is changed. In the trial numbers (5-11) and (5-12), the resin binder of the undercoat coating film layer is No. 1, respectively. 1 and no. 14 and the total adhesion amount of the undercoat coating layer and the top coating layer on the back surface is 9 (g / m ² ). The lower two rows of Table 7 show a trial number (5-13) and a trial number (5-14) as comparative examples. In these two comparative examples, the top coating film layer is not provided on the back surface, and the adhesion amount of the undercoating coating layer on the back surface is 9 (g / m ² ).

  In Table 7, comparing the results of trial number (5-1) to trial number (5-5) with the result of trial number (5-13), in both cases, the undercoat film layer was protected. Although the rust pigment is contained, it can be seen that the internal corrosion resistance is good and bad depending on the presence or absence of the top coat layer. On the other hand, as explained in Example 1, it is known that the inner surface corrosion resistance is not good when the rust preventive pigment is not contained in the undercoat film layer even if the topcoat film layer is provided. Therefore, from the above results, it was possible to provide an excellent inner surface corrosion resistance by providing an undercoat coating layer containing a rust preventive pigment on the back surface as in the PCM of the present invention, and further providing an overcoat coating layer thereon. PCM can be used. This also appears in the comparison between the results of the trial numbers (5-6) to (5-10) and the results of the trial numbers (5-14). Further, from the comparison between the trial number (5-11) and the trial number (5-13) and the comparison between the trial number (5-12) and the trial number (5-14), the top coat layer and the undercoat layer It can be seen that even if the total adhesion amount with the film layer is the same, excellent internal corrosion resistance cannot be obtained only with the top coat layer.

  Further, regarding the front surface, in the trial number (5-1) to the trial number (5-5), the workability after drawing is deteriorated in the trial number (5-5). The trial number (5-5) is the one with the largest amount of adhesion of the undercoat coating layer on the front surface. This tendency is also seen in the trial numbers (5-6) to (5-10), and the workability is improved by not increasing the adhesion amount of the undercoat layer on the front surface more than necessary. I understand that.

  Even if the evaluation result of the above Examples 1-5 is seen, it turns out that PCM of this invention is equipped with the outstanding surface corrosion resistance and front surface workability.

  Although the present invention has been described with reference to the most practical and preferred embodiments at the present time, the invention is limited to the embodiments disclosed herein. However, the present invention can be modified as appropriate without departing from the spirit or concept of the invention that can be read from the claims and the entire specification, and a precoated steel sheet and a method for manufacturing the same with such changes are also included in the technical scope of the present invention. Must be understood.

It is a figure which shows typically the cross-sectional structure of the precoat steel plate of this invention concerning one embodiment. It is a figure which shows typically the cross-sectional structure of the conventional precoat steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Precoat steel plate 11 Steel plate 11a Galvanized steel plate 12 Zinc plating layer 13 Base treatment 14 Undercoat coating layer 15 Topcoat coating layer 16 Basecoat 17 Undercoat coating layer 18 Topcoat coating layer

Claims (11)

Of zinc-based plated steel sheet with chrome-free ground treatment on both sides,
One surface, which is the front surface, contains a polyester resin and / or an epoxy-modified polyester resin as a binder, and contains at least one of an ion-exchange oxide rust preventive pigment or a phosphate rust preventive pigment. An undercoat coating layer is provided, and further, one or more topcoat coating layers are provided on the undercoat coating layer,
On the other surface, which is the back surface, a primer coating containing a polyester resin and / or an epoxy-modified polyester resin as a binder and containing at least one of an ion-exchange oxide rust preventive pigment or a phosphate rust preventive pigment A precoated steel sheet, wherein a film layer is provided, and one or more topcoat film layers are further provided above the undercoat film layer.   2. The precoated steel sheet according to claim 1, wherein an average molecular weight of a resin which is a binder of the undercoat coating layer on the front surface is 5000 to 25000 and a glass transition temperature of the binder resin is 10 ° C. or more. A precoated steel sheet having a temperature of 40 ° C. or lower.   3. The precoated steel sheet according to claim 1, wherein the rust preventive pigment contained in the undercoat coating layer has an average particle size of 30 μm or less. 4.   It is a precoat steel plate in any one of Claims 1 thru | or 3, Comprising: The antirust pigment in the said undercoat film layer of the said front surface is 15 mass% with respect to the adhesion amount of this undercoat film layer. A precoated steel sheet, which is contained in an amount of 50% by mass or less. The precoated steel sheet according to any one of claims 1 to 4, wherein an adhesion amount of the undercoat coating layer on the front surface is 3 g / m ² or more and 30 g / m ² or less. steel sheet. The precoated steel sheet according to any one of claims 1 to 5, wherein the adhesion amount of the undercoat coating layer on the back surface is 3 g / m ² or more and the adhesion amount of the top coating layer is 3 g / m ^2. A precoated steel sheet characterized by the above.   The precoated steel sheet according to any one of claims 1 to 6, wherein the chromium-free base treatment is mainly composed of one or more substances selected from a silane coupling agent, silica, Ti salt, and Zr salt and a resin. A precoated steel sheet characterized by being an organic-inorganic composite treatment.   It is a method of manufacturing the precoat steel plate of Claim 7, Comprising: After contacting the alkaline aqueous solution of pH13 or more, or the acidic aqueous solution of pH1.5-4.0 on both surfaces of a zinc-based plated steel plate, it is washed with water and dried. Thereafter, an organic-inorganic composite treatment mainly comprising one or more substances selected from a silane coupling agent, silica, Ti salt and Zr salt and a resin is performed, and then an undercoat coating layer is formed. The manufacturing method of the precoat steel plate which forms a top coat film layer.   A method for producing a precoated steel sheet according to claim 7, wherein an aqueous solution containing Ni, Co, Fe is brought into contact with both surfaces of a zinc-based plated steel sheet, followed by washing with water and drying, and then a silane coupling agent. , Applying an organic-inorganic composite treatment mainly composed of one or more substances selected from silica, Ti salt and Zr salt and a resin, then forming an undercoat coating layer, and further forming an overcoat coating layer Manufacturing method of precoated steel sheet. The method for producing a pre-coated steel sheet according to claim 9, wherein Ni, Co, Fe is added to the surface of the galvanized steel sheet at a concentration of 2 mg / m ² or more and 10 mg / m ² by contacting with an aqueous solution containing Ni, Co, Fe. The manufacturing method of the precoat steel plate characterized by making it adhere at ² or less.   A processed product processed into a casing shape using the precoated steel sheet according to any one of claims 1 to 7, wherein the front surface of the precoated steel sheet is on the outer surface side of the casing and the back surface is on the inner surface side. Processed product characterized by being processed toward.

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