JP7410388B2 - Painted plated steel plate - Google Patents

Description

The present invention relates to a painted plated steel plate.

Pre-coated steel sheets coated with a colored organic film have come to be used for home appliances, building materials, etc. in place of the conventional post-coated steel sheet products that are painted after forming. This pre-coated steel sheet is made by coating a rust-preventing steel sheet or plated steel sheet with a colored organic film, and has the characteristics of having a beautiful appearance, sufficient workability, and good corrosion resistance. have.

Pre-coated steel sheets (hereinafter also referred to as coated steel sheets) are steel sheets that are shipped with the coating desired by the customer for the product formed in advance, which allows the customer to omit painting and related work. At the same time, since equipment for such work is no longer required, its use is expanding in various fields. In early pre-painted steel sheets, chromate chemical conversion treated steel sheets were used as base steel sheets for coating. Later, due to the toxicity issue of hexavalent chromium that could be leached from the chromate chemical conversion coating, a chromate-free type coating was introduced that used a base steel sheet that had been subjected to rust prevention treatment that did not contain hexavalent chromium, instead of the chromate chemical conversion coated steel sheet. Steel plates have attracted attention, and their use has increased particularly in the field of home appliances in recent years. On the other hand, in the field of building materials that require long-term corrosion resistance outdoors, chromate coated steel sheets are mainly used.

With the aim of expanding the application of chromate-free coated steel sheets to the field of building materials, many studies have been made to achieve the high corrosion resistance of chromate chemical conversion treatment with chromate-free treatment. However, a chromate-free treatment that fully satisfies the required performance has not yet been provided.

On the other hand, recently, galvanized steel sheets, which are the base material for chromate-free treatment, have become highly corrosion resistant, and galvanized steel sheets, which have been the mainstream in the past, are now being replaced by zinc alloy-based galvanized steel sheets with additions of aluminum, magnesium, silicon, etc. Therefore, a chromate-free treatment with high corrosion resistance is now required for these zinc alloy plated steel sheets.

Up to now, studies have been conducted on chromate-free post-treatment of zinc alloy coated steel sheets.
For example, Patent Document 1 describes corrosion resistance, detergent resistance such as alkali resistance and solvent resistance, adhesion properties such as film adhesion, paint adhesion and printing adhesion, and water resistance such as humidity discoloration resistance and dew condensation resistance. A chromium-free surface-treated zinc-plated steel sheet has been proposed that has excellent workability and sliding properties.
Patent Document 1 describes, as a technique for providing the above-mentioned steel sheet, an organosilicon compound (C ), a film-forming component (c) containing three components in a specific ratio: a water-based urethane resin (E) having a specific structural unit, and a cationic phenol resin (F) having a specific structure, and titanium hydrogen fluoride. An inhibitor component (d) containing four components in a specific ratio: acid (H), zirconium hydrofluoric acid (I), phosphoric acid compound (J), and vanadium (IV) compound (K), and polyethylene wax (L). ) and an aqueous medium, and by applying and drying an aqueous metal surface treatment agent having a pH of 4 to 6, it is described that a composite film containing each component is formed.

Patent Document 2 proposes a chromium-free coated steel sheet that is not only environmentally friendly but also has excellent end face corrosion resistance.
Patent Document 2 describes the following technology as a technology for providing the above-mentioned steel plate.
A water dispersion in which polyurethane with a glass transition temperature Tg of 0°C or more and polyurethane with a glass transition temperature Tg of -10°C or less are mixed at a predetermined mass ratio on the upper layer of the plating film of the base steel sheet, which is a zinc-based plated steel sheet. A chemical conversion treatment layer is formed using a chemical conversion treatment agent containing polyurethane, silica, a zirconium compound, and a silane coupling agent. Furthermore, an undercoat film layer and a topcoat film layer are formed on top of the chemical conversion treatment layer. As a result, the critical peel strength of the chemical conversion treatment layer becomes as high as 5 mN or more, and the corrosion resistance of the coated steel sheet, particularly the end face corrosion resistance, is significantly improved without adversely affecting the environment.

Patent Document 3 proposes an aqueous base treatment composition for coated steel sheets that forms a base treatment layer that provides excellent under-eaves corrosion resistance while sufficiently ensuring work adhesion without containing chromium.
Patent Document 3 describes, as a technique for providing the above composition, a base treatment containing a specific organosilicon compound, a hexafluorometallic acid, a urethane resin having a specific cationic group, a vanadium compound, and an aqueous medium. It is stated that in the composition, when the cationic group and total amine value of the urethane resin have specific values, a base treatment layer can be obtained that provides excellent under-eaves corrosion resistance while sufficiently ensuring processing adhesion. ing.

Patent Document 4 proposes a method for coating metal surfaces using an aqueous composition for pretreatment or treatment before coating.
Patent Document 4 describes the following technology.
The aqueous composition comprises, in addition to water, (a) at least one hydrolyzable or/and at least partially hydrolyzed silane, (b) at least one metal chelate, (c) at least one An organic film-forming agent and (d) at least one long-chain alcohol as a film-forming aid or/and (e) at least one inorganic compound in particulate form are included.
A clean, pickled, washed and/or pretreated metal surface is contacted with the aqueous composition to form a coating on the metal surface. The coating is subsequently dried, partially or completely tightened and optionally additionally cured. The dried and optionally cured coating has a layer thickness in the range from 0.01 to 10 μm.

Japanese Patent Application Publication No. 2011-106029 JP2011-219832A Japanese Patent Application Publication No. 2014-214315 Special Publication No. 2006-519308

However, none of the techniques disclosed in the patent documents satisfy the need for corrosion resistance in applications of painted plated steel sheets to home appliances, building materials, and the like. In addition, in the technology of both patent documents, corrosion resistance against cracks that occur in the plating layer or coating film due to processed parts (processed parts by bending, press working, etc.) of zinc-based plated steel sheets containing aluminum, magnesium, etc. In particular, no consideration has been given to suppressing white rust on processed parts.
Therefore, there is a demand for a painted steel sheet that satisfies these properties.

Therefore, the present invention is a chromate-free product that does not use harmful hexavalent chromium and has high corrosion resistance while suppressing the occurrence of white rust even if cracks occur in the plating layer or paint film due to bending, press working, etc. The purpose is to provide type painted plated steel sheets.

Means for solving the problem includes the following aspects.

<1>
steel plate and
A plating layer containing zinc and provided on one or both sides of the steel plate,
a chemical conversion coating provided on the plating layer provided on one side of the steel plate, or on at least one of the plating layers provided on both sides of the steel plate;
one or more coating films provided on the chemical conversion coating;
has
A carboxylic acid derivative having one or more of a silanol group, a vinyl group, a vinylene group, an allyl group, and a sulfur-containing group is added to either or both of the chemical conversion treatment film and the coating film in contact with the chemical conversion treatment film. , a painted plated steel sheet containing a concentration of 0.01% by mass or more and 40% by mass or less based on the solid content of the chemical conversion treatment film or the solid content of the coating film in contact with the chemical conversion treatment film.
<2>
The coated steel sheet according to <1>, containing the carboxylic acid derivative at a concentration of 0.5% by mass or more and 40% by mass or less based on the solid content of the chemical conversion treatment film or the solid content of the coating film in contact with the chemical conversion treatment film. .
<3>
The painted steel sheet according to <1> or <2>, wherein the plating layer contains aluminum in an amount of 0.5% by mass or more and 60% by mass or less, and the remainder consists of zinc and impurities.
<4>
The painted steel sheet according to <3>, wherein the plating layer further contains magnesium in an amount of 0.5% by mass or more and 15% by mass or less.
<5>
The painted steel sheet according to any one of <1> to <4>, wherein the average thickness of the coating film in contact with the chemical conversion coating is 4 μm or more.
<6>
The coated steel sheet according to any one of <1> to <5>, wherein the coating film in contact with the chemical conversion treatment film contains any one or more of polyester resin, epoxy resin, acrylic resin, and urethane resin.
<7>
According to any one of <1> to <6>, the coating film in contact with the chemical conversion film contains any one or more of vanadate, tungstate, silicate, and phosphate. Painted plated steel plate.
<8>
The painted steel sheet according to any one of <1> to <7>, wherein the chemical conversion treatment film contains at least one of a silane coupling agent and a zirconium compound.
<9>
The painted plated steel sheet according to any one of <1> to <8>, wherein the chemical conversion coating contains any one or more of silica, phosphoric acid and its salts, fluoride, and vanadium compound.
<10>
The painted plated steel sheet according to any one of <1> to <9>, wherein the chemical conversion treatment film contains any one or more of urethane resin, polyester resin, acrylic resin, and epoxy resin.
<11>
The painted steel plate was subjected to bending in the rolling direction at a plate temperature of 20°C with a radius of curvature three times the plate thickness, and then bent under the following conditions (1) in accordance with CCT-JASO M609. ) Regarding the corrosion test piece subjected to 30 cycles of the combined cycle corrosion test,
By observing the longitudinal section of the plating layer in the bending direction,
In an arbitrary width range of 200 μm of the plating layer excluding the crack portion of the plating layer formed in the bent portion of the corrosion test piece,
Among the corrosion products formed between the plating layer and the coating film in contact with the chemical conversion coating, starting from the crack part of the plating layer, the area where the thickness of the corrosion products is 2 μm or more. The total length is
The painted steel plate according to any one of <1> to <10>, which satisfies condition (2).
-Condition (1)-
(A) Salt spray treatment: Spraying 5% sodium chloride aqueous solution at 35°C for 2 hours (B) Drying treatment: Drying treatment at 60°C for 4 hours (C) Wet treatment: 50°C, Wet treatment at 95% RH for 2 hours, provided that (A), (B), and (C) are treated in this order in one cycle.
-Condition (2)-
Total length of the region where the thickness of the corrosion product is 2 μm or more (μm) / 200 (μm) × number of cracks in the plating layer <0.5
<12>
The painted steel plate was subjected to bending in the rolling direction at a plate temperature of 20°C with a radius of curvature three times the plate thickness, and then bent under the following conditions (1) in accordance with CCT-JASO M609. ) Regarding the corrosion test piece subjected to 15 cycles of the combined cycle corrosion test,
By observing the longitudinal section of the plating layer in the bending direction,
In an arbitrary width range of 200 μm of the plating layer excluding the crack portion of the plating layer formed in the bent portion of the corrosion test piece,
Among the corrosion products formed between the plating layer and the coating film in contact with the chemical conversion coating, starting from the crack part of the plating layer, the area where the thickness of the corrosion products is 2 μm or more. The total length is
The painted steel plate according to any one of <1> to <10>, which satisfies condition (2).
-Condition (1)-
(A) Salt spray treatment: Spraying 5% sodium chloride aqueous solution at 35°C for 2 hours (B) Drying treatment: Drying treatment at 60°C for 4 hours (C) Wet treatment: 50°C, Wet treatment at 95% RH for 2 hours, provided that (A), (B), and (C) are treated in this order in one cycle.
-Condition (2)-
Total length of the region where the thickness of the corrosion product is 2 μm or more (μm) / 200 (μm) × number of cracks in the plating layer <0.5

According to the present invention, without using harmful hexavalent chromium, even if cracks occur in the plating layer or paint film due to bending, press working, etc., the present invention is chromate-free and has high corrosion resistance while suppressing the generation of white rust. We can provide type painted plated steel sheets.

FIG. 2 is a schematic diagram for explaining a method of measuring the total length (μm) of a region where the thickness of a corrosion product is 2 μm or more.

An example of the coated steel sheet of the present invention will be described below.
In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower limit and upper limit.
In numerical ranges described in stages, the upper limit or lower limit described in a certain numerical range may be replaced with the upper limit or lower limit of another numerical range described in stages.
In a numerical range, the upper limit value or lower limit value described in a certain numerical range may be replaced with the value shown in the examples.
The term "step" is included in the term not only an independent step but also a step that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved.
A "combination of preferred embodiments" is a more preferred embodiment.

In this specification, "a plating layer containing only zinc as a plating component" is referred to as a "zinc plating layer", and a "plating layer containing aluminum, magnesium, etc. in addition to zinc as a plating component" is referred to as a "zinc alloy plating layer", ""zinc plating layer", "zinc alloy plating layer" and "zinc-containing plating layer" are also collectively referred to as "zinc-based plating layer".
In addition, "steel sheet with a galvanized layer" is referred to as "galvanized steel sheet", "steel sheet with zinc alloy plating layer" is referred to as "zinc alloy plated steel sheet", and "steel sheet with zinc-based plating layer" is referred to as "zinc-based coated steel sheet". Also called.

The painted steel plate of the present invention is
steel plate and
A plating layer containing zinc and provided on one or both sides of the steel plate,
a chemical conversion coating provided on the plating layer provided on one side of the steel plate, or on at least one of the plating layers provided on both sides of the steel plate;
one or more coating films provided on the chemical conversion coating;
has.
The painted steel sheet of the present invention has one of a silanol group, a vinyl group, a vinylene group, an allyl group, and a sulfur-containing group in either or both of the chemical conversion treatment film and the coating film in contact with the chemical conversion treatment film. The carboxylic acid derivative having 1 or more is contained at a concentration of 0.5% by mass or more and 40% by mass or less based on the solid content of the chemical conversion coating or the solid content of the coating film in contact with the chemical conversion coating.

Due to the above structure, the painted steel sheet of the present invention does not use harmful hexavalent chromium, and even if cracks occur in the plating layer or paint film due to bending, press working, etc., it can suppress the generation of white rust. This is a chromate-free coated steel sheet that also has high corrosion resistance.
The paint-plated steel sheet of the present invention was discovered based on the following findings.

In chromate-free coated steel sheets, the interfaces where corrosion occurs are the interface between the plating layer and the chemical conversion coating, the interface between the plating layer and the coating film in contact with the chemical conversion coating, and the interface between the chemical conversion coating and the chemical conversion coating. Either the interface with the coating film that is in contact with However, the chemical conversion coating is small, on the order of tens to hundreds of nanometers, compared to the paint film in contact with the chemical conversion coating, and the interface cannot be clearly identified by cross-sectional observation using a scanning electron microscope. Furthermore, since the thickness of the corrosion products produced is thicker than the thickness of the chemical conversion coating, it is not possible to identify at which interface the corrosion products are formed. Therefore, unless otherwise specified, the area where corrosion occurs is between the plating layer and the paint film in contact with the chemical conversion film (hereinafter, the "paint film in contact with the chemical conversion film" is referred to as the "underlayer paint film"). (also referred to as). In other words, the corrosion products formed at each interface are referred to as the corrosion products formed between the plating layer and the lower coating film.
Note that a chemical conversion film is formed between the plating layer and the lower coating film, but the chemical conversion film is usually on the order of nanometers in thickness. Since the coating film may come into contact with the plating layer, corrosion products may also be formed at the interface between the plating layer and the underlying coating film.

First, the present inventors conducted the following study in order to clarify the reason why white rust is likely to occur when bending, press working, etc. are performed.
Corrosion acceleration tests were conducted on painted plated steel sheets that were subjected to general chromate-free chemical conversion treatment and painting, and painted plated steel sheets that were subjected to chromate chemical conversion treatment and painting, using combined cycle tests on bent painted plated steel sheets. Later cross-sectional observations were made. As a result, the present inventors obtained the following knowledge.
When a general chromate-free chemical conversion treatment is applied, a painted steel sheet (hereinafter referred to as "painted galvanized steel sheet"), which is a galvanized steel sheet with a galvanized layer, and a zinc alloy plating layer containing aluminum, magnesium, etc. There is a difference in the corrosion behavior of processed parts between paint-plated steel sheets (hereinafter referred to as "painted zinc-alloy-plated steel sheets") that use zinc alloy-plated steel sheets as base sheets.

In the case of painted galvanized steel sheets, the entire galvanized layer corroded from the cracks that occurred in the bent parts. On the other hand, in painted zinc alloy-plated steel sheets, corrosion progresses between the plating layer and the coating film in contact with the chemical conversion coating, starting from cracks in the plating layer caused by bending. This is because the zinc alloy plating layer also contains aluminum, magnesium, etc., which are more oxidizing than zinc, and has a higher ability to elute plating components or form protective corrosion products than a zinc plating layer. Oxidation progresses due to elution of plating components or formation of corrosion products from cracks into the plating layer through a specific highly oxidizing plating layer that exists between the plating layer and the paint film in contact with the chemical conversion coating. It is assumed that this led to the occurrence of white rust.

On the other hand, for painted steel sheets that have been subjected to chemical conversion treatment and painting containing chromate, the coating layer or coating produced by bending does not depend on the steel sheet having a zinc plating layer, a zinc alloy plating layer containing aluminum, magnesium, etc. Corrosion between the plating layer and the paint film in contact with the chemical conversion coating starting from the cracks in the film did not progress easily, and the occurrence of white rust was small. The reason for this is that in chromate chemical conversion treatment, the chemical conversion coating has a high oxidation ability, and the chemical conversion coating has a high affinity with the metal or its oxide that makes up the plating layer, so the chemical conversion coating can be quickly and firmly formed. It can form a bond with the surface of the plating layer, which is effective for improving the corrosion resistance of processed parts.
In addition, it has a high bonding property with the silica component contained in the chromate chemical conversion treatment solution, so the chemical conversion treatment film is strongly bonded to the plating layer and the paint film, and the bond between the chemical conversion treatment film, the plating layer, and the paint film is improved. It's getting even higher. Therefore, in order to achieve corrosion resistance of processed parts like chromate chemical conversion treatment even with chromate-free chemical conversion treatment, it is effective to introduce a component that has a high bond with both the plating component, the chemical conversion treatment film, and the coating film in contact with it. .

In this way, in both galvanized steel sheets and painted zinc-plated steel sheets whose base sheets are zinc alloy-plated steel sheets, white rust can be suppressed by appropriately controlling the elution of plating components while maintaining the corrosion resistance of the steel sheet. is important. Particularly in painted zinc alloy plated steel sheets, it is important to suppress the progress of corrosion between the plating layer and the coating film in contact with the chemical conversion coating.
To this end, in addition to conventional painted galvanized steel sheets, carboxylic acid derivatives are effective as components that have high bonding properties with plating components such as aluminum and magnesium, and also in painted zinc alloy-plated steel sheets. Also, it has one or more of silanol group, vinyl group, vinylene group, allyl group, and sulfur-containing group as a component that enhances the bonding with the coating film in contact with the chemical conversion coating, so that aluminum, magnesium, etc. It improves the corrosion resistance of processed parts of painted zinc alloy coated steel sheets containing the same amount as chromate chemical conversion treatment, and also improves the corrosion resistance of processed parts of painted galvanized steel sheets compared to conventional chromate-free treatment.

More specifically, it is as follows.

The present inventors focused on the corrosion between the plating layer and the lower coating film in a painted zinc alloy coated steel sheet whose base plate is a zinc alloy coated steel sheet having a zinc alloy plating layer containing aluminum and magnesium, and investigated the corrosion in detail. A survey was conducted. Then, the present inventors obtained the following knowledge.

Using a scanning electron microscope and an energy-dispersive X-ray analyzer, a sample was prepared by bending a coated steel sheet that had been subjected to chromate chemical conversion treatment or chromate-free chemical conversion treatment and subjected to a combined cycle corrosion test. Observation of the state of corrosion between the plating layer and the coating film revealed that the plating layer corroded starting from cracks in the plating layer caused by bending, and corrosion products were deposited on the cracks or the coating film.

Furthermore, in painted plated steel sheets that have been subjected to chromate-free chemical conversion treatment, corrosion progresses from cracks between the plating layer and the underlying coating film.
On the other hand, similar corrosion at the interface was not observed in chromate chemical conversion treatment, and this behavior was thought to be unique to chromate-free chemical conversion treatment.

Therefore, the present inventors investigated the differences between chromate-free chemical conversion treatment and chromate-free chemical conversion treatment in order to achieve interfacial corrosion resistance between the plating layer and the lower coating film, similar to chromate chemical conversion treatment. did. As a result, the present inventors obtained the following findings.

Chromate chemical conversion treatment has a barrier effect that prevents oxygen and water from permeating the surface of the plating layer and steel sheet, a self-healing function when the surface of the chromate chemical conversion coating is scratched and the plating layer and steel sheet are exposed, and in addition, chromate chemical conversion treatment Due to the effect of the silica component in the coating film, which is commonly contained, the plating layer and the chemical conversion coating are strongly bonded, and the chemical conversion coating and the coating film in contact with the chemical conversion coating are strongly bonded, resulting in interfacial corrosion resistance. is increasing.

The present inventors have focused on the interface between the plating layer and the lower coating film, and have conducted intensive studies on means for achieving high bonding even with chromate-free chemical conversion treatment. Then, the present inventors focused on increasing the bonding strength at the interface in order to suppress corrosion at the interface between the plating layer and the lower coating film. Therefore, the present inventors conducted extensive studies on additive substances that enhance interfacial bonding properties. As a result, the following findings were obtained.

The carboxylic acid derivative is effective for galvanized steel sheets and zinc alloy-plated steel sheets containing aluminum and magnesium, and further contains a silanol group, vinyl group, vinylene group, allyl group, or sulfur-containing group in the carboxylic acid derivative molecule. It is especially effective if

The reason why carboxylic acid derivatives increase the interfacial bonding strength with zinc alloy plating layers containing aluminum, magnesium, etc. This is thought to be due to the acid-base reaction and complex formation reaction contributing to the bonding with the surface of the plating layer.

However, the effect was insufficient simply by introducing the carboxylic acid derivative into the chemical conversion coating and the coating film in contact with the chemical conversion coating. In other words, the plating layer and the underlayer coating are bonded not only to the surface of the plating layer, but also to the chemical conversion coating and the underlying coating, thereby firmly bonding the plating layer, the chemical conversion coating, and the underlying coating. This is important for further improving the bonding properties of the interface with the membrane. If the carboxylic acid derivative has a silanol group, vinyl group, vinylene group, allyl group, or sulfur-containing group, these will bond with the chemical conversion film and coating film, and the carboxyl derivative will further bond with the plating layer. As a result, it was thought that the bond between the plating layer and the chemical conversion coating, and between the chemical conversion coating and the coating film in contact with the chemical conversion coating would be strengthened.

Here, in the carboxylic acid derivative having a silanol group, the silanol group bonds with the chemical conversion coating and the resin functional group and inorganic component of the coating film in contact with the chemical conversion coating. Furthermore, the silanol group forms a covalent bond with the hydroxyl group on the surface of the plating layer.

In the carboxylic acid derivative having a vinyl group, vinylene group, allyl group, or sulfur-containing group, the carboxyl group portion contributes to interfacial bonding with the plating layer. The vinyl group, vinylene group, or allyl group forms a stronger bond with the chemical conversion coating and the lower layer coating through a graft reaction, resulting in interfacial bonds between the plating layer and the chemical conversion coating, and between the chemical conversion coating and the lower coating. Increase your power even more. The sulfur-containing group forms a strong bond with the plating layer through an adsorption reaction with the plating layer, a thiourethane reaction with the chemical conversion film and the lower coating film, and an ene-thiol reaction, thereby forming a strong bond between the plating layer and the chemical conversion film. , and further increase the interfacial bonding strength between the chemical conversion coating and the underlying coating.

In addition, these carboxylic acid derivatives often contain alkyl groups in their molecules, but since alkyl groups have an affinity with organic sites in chemical conversion coatings and underlying coatings, they can be used in plating layers. This further increases the interfacial bonding strength between the chemical conversion treatment film and the chemical conversion treatment film and the underlying coating.

Due to such a combined effect, carboxylic acid derivatives having a silanol group, vinyl group, vinylene group, allyl group, or sulfur-containing group have a low equilibrium oxygen partial pressure and are easily oxidized on the surface, such as aluminum and magnesium. The base reaction and complex formation reaction enhance the bond between the plating layer and the chemical conversion coating and the underlying coating, greatly suppressing the occurrence of white rust on the bent part after the combined cycle corrosion test, and improving the bond between the plating layer and the underlying coating. Interfacial corrosion between the two can also be suppressed.

Furthermore, this carboxylic acid derivative having one or more of a silanol group, a vinyl group, a vinylene group, an allyl group, and a sulfur-containing group is also effective for galvanized layers, and can further improve the corrosion resistance of processed parts.

Based on the above findings, the paint-plated steel sheet of the present invention, with the above structure, can prevent white rust even if cracks occur in the plating layer or paint film due to bending, press working, etc., without using harmful hexavalent chromium. It has been discovered that a chromate-free coated steel sheet can be produced that suppresses corrosion and also has high corrosion resistance.

Hereinafter, the painted steel plate of the present invention will be explained in detail.

<Steel plate>
The steel plate is a steel plate on which a plating layer is formed. The steel plate is not particularly limited. Examples of steel sheets include ultra-low C type (ferrite-based structure), Al-k type (ferrite containing pearlite), two-phase structure (for example, ferrite containing martensite, and ferrite containing bainite). Any type of steel sheet may be used, such as a strain-induced transformation type (structure containing retained austenite in ferrite), a microcrystalline type (structure containing ferrite mainly), etc.

<Zinc-based plating layer>
The zinc-based plating layer (plating layer containing zinc) is a plating layer that contains impurities as well as zinc. Specifically, examples of the zinc-based plating layer include a zinc plating layer, a zinc-aluminum-magnesium plating layer, a zinc-aluminum-magnesium-silicon plating layer, a zinc-aluminum plating layer, a zinc-aluminum-silicon plating layer, etc. It will be done.

The zinc-based plating layer contains small amounts of cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc. as dissimilar metal elements or impurities. A plating layer may also be mentioned.

In particular, from the viewpoint of corrosion resistance, the zinc-based plating layer is preferably a plating layer containing aluminum in addition to zinc, or a plating layer containing aluminum and magnesium. In other words, it is preferable to use a zinc alloy-plated steel sheet as the original sheet because it provides better corrosion resistance than a galvanized steel sheet.
Specifically, the zinc-based plating layer preferably contains aluminum in an amount of 0.5% by mass or more and 60% by mass or less, with the balance being zinc and impurities, and contains aluminum in an amount of 0.5% by mass or more and 60% by mass. Hereinafter, a plating layer further containing magnesium in an amount of 0.5% by mass or more and 15% by mass or less, with the remainder being zinc and impurities is more preferable.
Examples of the zinc alloy plating layer containing any of zinc, aluminum, and magnesium include a zinc-aluminum-magnesium plating layer and a zinc-aluminum-magnesium-silicon plating layer. %Mg plating layer, Zn-11%Al-3%Mg-0.2%Si plating layer, Zn-55%Al-2%Mg-1.6%Si plating layer, trace amounts of Ni and Cr in these plating layers. There are various types of plating layers containing , Ti, and the like.

The method for forming the zinc-based plating layer is not particularly limited, and any known method such as electroplating, hot-dip plating, vapor deposition plating, dispersion plating, vacuum plating, etc. may be used.

The amount of zinc-based plating layer deposited on one side of the steel sheet is not particularly limited, but is preferably 15 g·m ^-2 or more and 140 g·m ^-2 or less. More preferably, it is 30 g·m ^-2 or more and 90 g·m ^-2 or less.
If the amount of the zinc-based plating layer is less than 15 g·m ⁻² , the amount of adhesion is too small, resulting in unplated areas, and the anticorrosion effect of the plating may not be exhibited. Further, when the amount of the zinc-based plating layer is more than 140 g·m ⁻² , corrosion resistance is high, but the phenomenon of the plating turning black is unlikely to occur.

<Carboxylic acid derivative>
As the carboxylic acid derivative, a carboxylic acid derivative having one or more of a silanol group, a vinyl group, a vinylene group, an allyl group, and a sulfur-containing group is applied.

Examples of carboxylic acid derivatives include carboxylic acids and salts thereof. Examples of carboxylic acid derivatives include acid anhydrides, halogenated acyl compounds, ester compounds, amide compounds, and nitrile compounds, which are compounds in which the hydroxyl group of the carboxyl group in the molecule is substituted with another heteroatom. The carboxylic acid derivative can form a complex with metals (zinc, aluminum, magnesium, etc.) and their oxides, and contributes to improving the interfacial bonding force between the plating layer and the underlying coating film.

In a carboxylic acid derivative having a silanol group, the silanol group is generated by hydrolysis of the alkoxy group or halogen bonded to the silyl group of the silane coupling agent. Therefore, carboxylic acid derivatives having a silanol group also include carboxylic acid derivatives having a silane to which an alkoxy group or halogen is bonded before hydrolysis.
Carboxylic acid derivatives having silanol groups link molecules together by polymerizing with reactive groups in the chemical conversion coating or in the lower coating film in contact with the chemical conversion coating, or by forming siloxane bonds in which silanol groups self-condense with each other. . In addition, the carboxylic acid derivative having a silanol group is bonded to the metal of the plating layer and the steel sheet, as well as to the hydroxyl group of its oxide through dehydration condensation. Therefore, it contributes to improving the interfacial bonding force between the plating layer and the lower coating film.

The carboxylic acid derivative having a silanol group is not particularly limited, but specific examples include (3-triethoxysilyl)propylsuccinic anhydride, (3-trimethoxysilyl)propylsuccinic anhydride, and N-(trimethoxysilyl)propylsuccinic anhydride. silylpropyl) ethylenediamine triacetate trisodium salt and the like. Also included are substances that have both a silanol group and a carboxylic acid derivative group in one molecule by reacting a silane coupling agent with a carboxylic acid salt.

Carboxylic acid derivatives having a vinyl group, vinylene group, or allyl group are not particularly limited, but specific examples include angelic acid anhydride, allylsuccinic anhydride, 3-acetamidophthalic anhydride, 2-butene-1 -ylsuccinic anhydride, crotonic anhydride, cis-4-cyclohexene-1,2-dicarboxylic anhydride, 1-cyclohexene-1,2-dicarboxylic anhydride, 2-dodecen-1-ylsuccinic anhydride Itaconic anhydride, 2-octenylsuccinic anhydride, oleic anhydride, (2,7-octadien-1-yl)succinic anhydride, methyl fumaraldehyde, methyl crotonate, 3-methoxyacrylic acid Examples include methyl, 1-(carbamoylmethyl)cyclohexaneacetic acid, succinimide, and methyl succinimate.

In the carboxylic acid derivative having a sulfur-containing group, examples of the sulfur-containing group include the following groups.
1) A group having sulfide (-S-) (for example, a thiophene ring group, a dithiine ring group, a thiazolyl group, or a fused ring group thereof)
2) Group having disulfide (-SS-) 3) Thiol group (-SH)
4) Alkylthio group (-SR:R=alkyl group)
5) Arylthio group (-SAr: Ar=aryl group)
6) Thiocarboxylic acid groups (thioacetic acid, thiopropionic acid, thioglycolic acid, etc.), thiocarboxylic acid alkyl ester groups, thiocarboxylic acid salts)

The carboxylic acid derivative having a sulfur-containing group is not particularly limited, but specific examples include 5,6-dihydro-1,4-dithiine-2,3-dicarboxylic acid anhydride, (acetylthio)acetic acid, 3-(acetylthio) ) propionic acid, S-(N,N-dimethylthiocarbamoyl)thioglycolic acid, methylenebis(thioglycolic acid), 3-(methylthio)propionic acid, 2,2'-thiodiglycolic acid, S-allyl-L- Cysteine, ammonium thioglycolate, sodium thioglycolate, 5,5'-dithiobis(2-nitrobenzoic acid), 4,4'-dithiodibutyric acid, ethyl (methylthio)acetate, ethyl 3-(methylthio)propionate, Methyl 3-[(2-methoxy-2-oxoethyl)thio]propionate, methyl (methylthio)acetate, methyl 3-methylthiophene-2-carboxylate, methyl 3-(methylthio)propionate, methylthiomethyl acetate, 5- Examples include methyl-2-thiophenecarboxylic acid, 3-(methylthio)propionic acid, and 3-(methylthio)propyl acetate.

The content of the carboxylic acid derivative having one or more of a silanol group, a vinyl group, a vinylene group, an allyl group, and a sulfur-containing group is 0.01 mass based on the solid content of the chemical conversion coating or the solid content of the lower layer coating. % or more and 40% by mass or less (preferably 0.5% by mass or more and 40% by mass or less).
If the content of the carboxylic acid derivative is less than 0.01% by mass, the interfacial bonding between the plating layer and the lower coating film may not be sufficiently improved.
If the content of the carboxylic acid derivative exceeds 40% by mass, the content of the carboxylic acid derivative is too high and the ratio of other components becomes low, which may make it difficult to obtain sufficient performance other than corrosion resistance.
More preferably, the content of the carboxylic acid derivative is 1% by mass or more and 30% by mass or less in the chemical conversion coating, and 0.5% by mass or more and 20% by mass or less in the lower layer coating. More preferably, the content of the carboxylic acid derivative is 1% by mass or more and 20% by mass or less in the chemical conversion coating, and 0.5% by mass or more and 10% by mass or less in the lower layer coating.

<Chemical conversion film (hereinafter also referred to as "film")>
A chemical conversion coating is formed by chemical conversion treatment after removing impurities such as oil and surface oxides adhering to the surface of a plated steel sheet through a degreasing process and a cleaning process.

The chemical conversion coating may contain one or more selected from resins, silane coupling agents, zirconium compounds, silica, phosphoric acid and its salts, fluorides, and vanadium compounds. Including these substances further improves the film formability after application of the chemical conversion treatment solution, the barrier properties (denseness) of the film against corrosion factors such as moisture and corrosive ions, and the adhesion of the film to the plated surface. Contributes to raising the level of corrosion resistance.
In particular, when a chemical conversion film contains one or more of a silane coupling agent and a zirconium compound, it forms a crosslinked structure in the film and further strengthens the bond with the plating surface, which improves the adhesion and barrier properties of the film. You can increase your sexuality.
Additionally, if the chemical conversion coating contains any one or more of silica, phosphoric acid and its salts, fluoride, and vanadium compounds, it may act as an inhibitor and form a precipitated film or a passive film on the plating or steel surface. Therefore, corrosion resistance can be improved.

[resin]
The resin is not particularly limited, and for example, known organic resins such as polyester resin, polyurethane resin, epoxy resin, phenol resin, acrylic resin, and polyolefin resin can be used. In order to further improve the adhesion with the plated steel sheet, it is preferable to use at least one resin that has a forced moiety or polar functional group in its molecular chain (polyester resin, urethane resin, epoxy resin, acrylic resin, etc.). . The resins may be used alone or in combination of two or more.

The content of the resin is preferably 0% by mass or more and 85% by mass or less based on the solid content of the film. More preferably, the content is 0% by mass or more and 60% by mass or less, and more preferably 1% by mass or more and 40% by mass or less. If the resin content exceeds 85% by mass, the proportions of other film constituents may decrease, and the performance required for the film other than corrosion resistance may deteriorate.

[Silane coupling agent]
As the silane coupling agent, it is a compound other than the above-mentioned carboxylic acid derivative having a silanol group, and various silane compounds can be used.
Examples of the silane coupling agent include γ-(2-aminoethyl)aminopropyltrimethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, and γ-(2-aminoethyl)aminopropyltriethoxysilane. , γ-(2-aminoethyl)aminopropylmethyldiethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ- Methacryloxypropyltriethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(N-vinylbenzylaminoethyl) -γ-aminopropylmethyldimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltriethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropylmethyldiethoxy Silane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane , γ-mercaptopropylmethyldimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropylmethyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, vinyltriacetoxysilane, γ-Chloropropyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, γ-chloropropyltriethoxysilane, γ-chloropropylmethyldiethoxysilane, hexamethyldisilazane, γ-anilinopropyltrimethoxysilane, γ-aniline Linopropylmethyldimethoxysilane, γ-anilinopropyltriethoxysilane, γ-anilinopropylmethyldiethoxysilane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, octadecyldimethyl [3 -(trimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldimethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(triethoxysilyl)propyl]ammonium chloride, octadecyldimethyl[3-(methyldiethoxy) Examples include silyl)propyl]ammonium chloride, γ-chloropropylmethyldimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, methyltrichlorosilane, dimethyldichlorosilane, and trimethylchlorosilane.
Among these, a silane coupling agent having a glycidyl ether group (for example, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, etc. having a glycidyl ether group) is used as the silane coupling agent. This particularly improves the processing adhesion of the lower coating film. Furthermore, use of a triethoxy type silane coupling agent can improve the storage stability of the surface treatment agent. This is thought to be because triethoxysilane is relatively stable in an aqueous solution and has a slow polymerization rate.
One type of silane coupling agent may be used, or two or more types may be used in combination.

[Zirconium compound]
Examples of the zirconium compound include, but are not limited to, zirconium normal propylate, zirconium normal butyrate, zirconium tetraacetylacetonate, zirconium monoacetylacetonate, zirconium bisacetylacetonate, zirconium monoethylacetoacetate, and zirconium acetylacetonate. Examples include bisethyl acetoacetate, zirconium acetate, zirconium monostearate, zirconium carbonate, ammonium zirconium carbonate, potassium zirconium carbonate, and sodium zirconium carbonate.
The zirconium compounds may be used alone or in combination of two or more.

Note that the zirconium carbonate compound undergoes a crosslinking reaction with the resin to form a film having a crosslinked structure of zirconium and resin. Further, when a zirconium carbonate compound is applied and dried, carbonate ions are volatilized, and the remaining zirconium compounds are bonded to each other via oxygen, resulting in a high molecular weight. In this process, the -Zr-OH group forms a Zr-OM bond (M: metal element in the plating layer) with the surface of the plating layer.

[Total content of silane coupling agent and zirconium compound]
The content of the silane coupling agent and zirconyl salt in the film is preferably 5% by mass or more and 80% by mass or less. More preferably, it is 20% by mass or more and 70% by mass. If the content is less than 5% by mass, the effect of improving adhesion to the base material and corrosion resistance may not be obtained, and if the content exceeds 80% by mass, processability may be reduced.

[silica]
Silica is a general term for silica that has a fine particle size and can stably maintain a water-dispersed state when dispersed in water. Silica is effective in improving the corrosion resistance of painted steel sheets and in improving the adhesion of the underlying coating film.
There are no particular limitations on the silica, but for example, fine silica particles such as colloidal silica and fumed silica having a primary particle diameter of 5 to 50 nm are preferable. Examples of silica include "Snowtex N", "Snowtex C", "Snowtex UP", "Snowtex PS" (all manufactured by Nissan Chemical Industries), and "Adelite AT-20Q" (manufactured by Asahi Denka Kogyo). Commercially available silica gel such as silica gel, powdered silica such as Aerosil #300 (manufactured by Nippon Aerosil), etc. can be used. Silica may be selected as appropriate depending on the required performance.
Silica may be used alone or in combination of two or more.

The content of silica is preferably 0% by mass or more and 30% by mass or less based on the solid content of the film. More preferably, it is 1% by mass or more and 20% by mass or less. If the silica content is more than 30% by mass, the film may become brittle and the processing followability during forming of the painted plated steel sheet may be reduced.

[Phosphoric acid and its salts]
Examples of phosphoric acid and its salts include phosphoric acids and their salts such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, and tetraphosphoric acid; ammonium salts such as triammonium phosphate and diammonium hydrogen phosphate; Phosphonic acids and their salts such as aminotri(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid); Organic phosphoric acids such as phytic acid and salts thereof. Note that examples of salts of phosphoric acid other than ammonium salts include metal salts with Na, Mg, Al, K, Ca, Mn, Ni, Zn, Fe, and the like.
Phosphoric acid and its salts may be used alone or in combination of two or more.

The content of phosphoric acid and its salts is preferably 0% by mass or more and 20% by mass or less based on the solid content of the film. More preferably, it is 1% by mass or more and 10% by mass or less.
When the content of phosphoric acid and its salts exceeds 20% by mass, the coating becomes brittle, and the processing followability of the coating when forming a painted plated steel sheet may decrease.

[Fluoride]
Examples of the fluoride include zirconium ammonium fluoride, ammonium silicofluoride, titanium ammonium fluoride, sodium fluoride, potassium fluoride, calcium fluoride, lithium fluoride, titanium hydrofluoric acid, and zirconium hydrofluoride.
The fluorides may be used alone or in combination of two or more.

The content of fluoride is preferably 0% by mass or more and 20% by mass or less based on the solid content of the film. More preferably, it is 1% by mass or more and 10% by mass or less. If the fluoride content is more than 20% by mass, the coating may become brittle, and the processing followability of the coating when forming a painted plated steel sheet may be reduced.

[Vanadium compound]
Examples of vanadium compounds include vanadium compounds obtained by reducing pentavalent vanadium compounds such as vanadium pentoxide, metavanadate, ammonium metavanadate, sodium metavanadate, and vanadium oxytrichloride to divalent to divalent to tetravalent with a reducing agent; vanadium trioxide; , vanadium dioxide, vanadium oxysulfate, vanadium oxyoxalate, vanadium oxyacetylacetonate, vanadium acetylacetonate, vanadium trichloride, phosphovanadomolybdic acid, vanadium sulfate, vanadium dichloride, vanadium oxide, etc. with an oxidation number of 4 to 2. Examples include vanadium compounds.
Vanadium compounds may be used alone or in combination of two or more.

The content of the vanadium compound is preferably 0% by mass or more and 20% by mass or less based on the solid content of the film. More preferably, it is 1% by mass or more and 10% by mass or less. When the content of the vanadium compound exceeds 20% by mass, the film becomes brittle, and the processing followability of the film when forming a painted plated steel sheet may be reduced.

[Method for forming chemical conversion treatment agent]
The method for producing the chemical conversion treatment agent is not particularly limited, but includes, for example, a method in which each film-forming component is mixed, stirred with a disper, and dissolved or dispersed. In order to improve the solubility or dispersibility of each film-forming component, a known hydrophilic solvent or the like may be added as necessary. An acid, alkali, etc. may be added to the chemical conversion treatment agent for pH adjustment within a range that does not impair its performance.

To form a chemical conversion coating, a chemical conversion treatment agent is applied to a plated steel sheet, and the coating film is heated and dried.
The method of applying the chemical conversion treatment agent is not particularly limited, and generally known coating methods such as roll coating, air spray, airless spray, and dipping can be used.
The heat drying temperature is preferably 50 to 250°C. If the temperature is less than 50° C., the evaporation rate of water is slow and sufficient film-forming properties cannot be obtained, so that the anti-corrosion ability may be insufficient. If the temperature exceeds 250°C, the alkyl moiety of the organic silane coupling agent may undergo denaturation due to thermal decomposition or the like, resulting in a decrease in adhesion and corrosion resistance. The heating temperature is more preferably 70 to 160°C.
The heating drying method is not particularly limited, and examples thereof include hot air, induction heating, near-infrared rays, direct flame, and the like alone or in combination. For example, when hot air drying is used, the heating drying time is preferably 1 second to 5 minutes.

[Amount of chemical conversion film deposited per side of galvanized steel sheet]
The amount of the chemical conversion film deposited on one side of the plated steel sheet is preferably 10 to 1000 mg/m ² in terms of solid content. If it is less than 10 mg/m ² , sufficient processing adhesion and corrosion resistance will not be ensured, and if it exceeds 1000 mg/m ² , processing adhesion may deteriorate.
The amount of chemical conversion coating applied per side of the plated steel sheet is more preferably 20 to 800 mg/m ² , most preferably 50 to 600 mg/m ² .

<Coating film>
A coating film is one or more layers formed on a chemical conversion coating. One to three coating layers are often provided on each side of a painted steel plate. When the paint film is a multilayer of two or more layers, the paint film in contact with the chemical conversion film (lower layer paint film) is sometimes called the primer paint film, and is used to ensure adhesion and corrosion resistance between the paint film and the chemical conversion film. is often aimed at. On the other hand, the upper coating film is often intended to provide design properties, barrier properties, and other surface functionality through coloring. In addition, unless otherwise specified, the coating film shown here refers to the coating film in contact with the chemical conversion coating.

The coating film contains, for example, a resin. Preferably, the coating film contains a pigment. In addition to these components, the coating film may also contain additives such as a leveling agent, an antifoaming agent, a coloring agent, a viscosity modifier, and an ultraviolet absorber. The coating liquid for forming the coating film is preferably obtained by dispersing or dissolving each of the above components in a solvent.

[resin]
The resin is not particularly limited. Examples include polyester resin, polyester resin, acrylic resin, epoxy resin, urethane resin, and fluororesin.
Examples of the resin include resins obtained by crosslinking these resins with a butylated melamine resin, a methylated melamine resin, a butylmethyl mixed melamine resin, a urea resin, an isocyanate resin, or a crosslinking agent component of a mixed system thereof.
Examples of the resin include electron beam curable resins and ultraviolet curable resins.
Among these, as the binder resin, any one or more of polyester resin, epoxy resin, acrylic resin, and urethane resin is preferable.
These binder resins may be used alone or in combination of two or more.

The content of the resin is preferably 20 to 90% by mass based on the solid content of the coating film. When the content of the resin is less than 20% by mass, there are few components that serve as a matrix, so cracks are likely to occur during processing, etc., and corrosion resistance may be affected. Furthermore, if the content of the pigment exceeds 90% by mass, the corrosion resistance and adhesion of the coating film may not be ensured because the content of the antirust pigment is small. From the viewpoint of processability, corrosion resistance, etc., the resin content is more preferably 30 to 80% by mass.

[Pigment]
Pigments are broadly classified into rust-preventing pigments, coloring pigments, and extender pigments.
Examples of antirust pigments include vanadates, tungstates, silicates, phosphates, and the like.
Coloring pigments include known inorganic and organic coloring pigments. Examples of inorganic coloring pigments include titanium oxide, zinc oxide, zirconium oxide, calcium carbonate, barium sulfate, alumina, kaolin clay, carbon black, iron oxide, and the like. Examples of organic coloring pigments include Hansa yellow, pyrazolone orange, phthalocyanine, and azo pigments.
Examples of extender pigments include talc, clay, silica, mica, alumina, calcium carbonate, barium sulfate, and the like.
Among these, anticorrosion pigments are preferred as pigments from the viewpoint of improving corrosion resistance. That is, it is preferable that the lower coating film contains one or more of vanadate, tungstate, silicate, and phosphate.

Here, vanadate salts include calcium vanadate, magnesium vanadate, ammonium metavanadate, potassium vanadate, sodium vanadate, ammonium vanadate, phosphorus vanadate, vanadium oxide, and the like.
Examples of the tungstate salt include sodium tungstate, calcium tungstate, ammonium tungstate, lithium tungstate, and magnesium tungstate.
Examples of silicates include sodium silicate, potassium silicate, lithium silicate, calcium ion exchange silica, and the like.
Phosphates include sodium dihydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium tripolyphosphate, aluminum tripolyphosphate, magnesium tripolyphosphate, sodium dihydrogen phosphate monohydrate, and phosphorus. Examples include sodium acid dihydrogen dihydrate, calcium hypophosphite, and the like.

The pigment content is preferably 5 to 70% by mass based on the solid content of the coating film. If the pigment content is less than 5% by mass, the rigidity and cohesive force of the coating film will decrease, resulting in peeling of the coating film when the coating surface rubs against the mold during press working of painted plated steel sheets. It may become easier to cause (paint film scratching). Moreover, when the content of pigment exceeds 70% by mass, processability may decrease. From the viewpoint of the balance between corrosion resistance, chemical resistance, and processability, the pigment content is more preferably 15 to 70% by mass, and even more preferably 20 to 50% by mass.

[Coating film thickness]
The thickness of the coating film is preferably 4 μm or more (for example, 4 to 50 μm). The film thickness of a coating film, when the coating film is multilayered, means the total film thickness of the multilayers.
However, when the coating film is multilayer, the lower coating film (the coating film in contact with the chemical conversion coating) preferably has a thickness of 4 to 15 μm. If the film thickness is less than 4 μm, sufficient corrosion resistance and chemical resistance may not be obtained. On the other hand, if the film thickness exceeds 15 μm, workability may deteriorate. From the viewpoint of a good balance between corrosion resistance, chemical resistance, and processability, the thickness of the lower coating film is more preferably in the range of 4 to 10 μm.
Further, the thickness of the upper coating film is preferably 5 to 30 μm than the lower coating film. If the film thickness is less than 5 μm, there is a concern that not only the chemical resistance and corrosion resistance will deteriorate, but also the color hiding property will deteriorate, and a sufficient design property may not be obtained. Moreover, when the film thickness exceeds 30 μm, workability deteriorates. The thickness of the upper coating film is more preferably 10 to 25 μm.
In addition, when the upper layer coating film is multilayer, the film thickness of the upper layer coating film means the film thickness of each coating film.

[Method for forming coating film]
The coating film is formed by applying a coating liquid, then drying and curing the coating film.
There are no particular restrictions on the method for applying the coating, and examples include dipping, curtain flow, roll coating, bar coating, electrostatic, brush coating, T-die, laminating, and spraying. Can be mentioned. Further examples include a wet-on-wet coating method and a multilayer simultaneous coating method.
Any method can be used for heating and curing, such as hot air, near-infrared rays, far-infrared rays, high-frequency induction heating, and a heating method using a combination of these.

<Characteristics of painted steel sheet>
It is preferable that the painted steel plate has the following properties (corrosion resistance properties).
After bending a painted plated steel plate at a plate temperature of 20°C in the rolling direction of the steel plate with a radius of curvature that is three times the plate thickness, the following conditions (1) in accordance with CCT-JASO M609 are applied. Regarding the corrosion test pieces subjected to the combined cycle corrosion test shown in 30 cycles or 15 cycles,
By observing the longitudinal section of the plating layer in the bending direction,
In the arbitrary width range of 200 μm of the plating layer excluding the cracked part of the plating layer formed in the bent part of the corrosion test piece,
Among the corrosion products formed between the plating layer and the coating film in contact with the chemical conversion coating, starting from the crack part of the plating layer, the length of the area where the thickness of the corrosion products is 2 μm or more. A characteristic whose total satisfies condition (2).

-Condition (1)-
(A) Salt spray treatment: Spraying 5% sodium chloride aqueous solution at 35°C for 2 hours (B) Drying treatment: Drying treatment at 60°C for 4 hours (C) Wet treatment: 50°C, Wet treatment at 95% RH for 2 hours, provided that (A), (B), and (C) are treated in this order in one cycle.

-Condition (2)-
Total length of the region where the thickness of the corrosion product is 2 μm or more (μm) / 200 (μm) × number of cracks in the plating layer < 0.5

Paint-plated steel sheets that meet this corrosion resistance property can suppress interfacial corrosion between the plating layer and the underlying coating film, even if cracks occur in the plating layer during bending, press working, etc. , the occurrence of white rust can be easily suppressed.

Here, "an arbitrary width of 200 μm of the plating layer excluding the crack part of the plating layer" refers to a 200 μm wide region of the plating layer starting from the upper edge of the crack part of any plating layer that serves as a reference (Fig. 1 middle, a+b=200 μm).
In addition, the total length (μm) of the region where the thickness of corrosion products is 2 μm or more is the distance between the plating layer and the lower coating film in “any width of 200 μm of the plating layer excluding the cracked part of the plating layer”. The total length of the region in which corrosion products with a thickness of 2 μm or more are formed ((see c+d+e=total length in FIG. 1) is shown.
In addition, corrosion products that form between the plating layer and the coating film that is in contact with the chemical conversion coating may not form continuously from the crack area, but may form in pieces, or the thickness may vary depending on the area. In this case, the "total length (μm) of the region where the thickness of corrosion products is 2 μm or more" in condition (2) is the sum of only the length of the region of corrosive organisms whose thickness is 2 μm or more. do.
Furthermore, corrosion products formed between the plating layer and the paint film in contact with the chemical conversion coating are the corrosion behavior normally seen in painted zinc-based alloy plated steel sheets, but as corrosion progresses, the painted zinc Similar to galvanized steel sheets, the entire plating layer also corrodes. In other words, the "total length (μm) of the region where the thickness of the corrosion product is 2 μm or more" in condition (2) is the amount of corrosive organisms that are generated directly above the region where the plating layer is not completely corroded. It shows.
Note that other conditions for corrosion resistance properties are as shown in Examples described later.
Here, in FIG. 1, A indicates the upper edge of a crack in an arbitrary plating layer that serves as a reference, and B indicates an upper edge of a crack in a plating layer adjacent to the crack in an arbitrary plating layer that serves as a reference. .
Further, 10 is a steel plate, 12 is a plating layer, 14 is a chemical conversion coating, 16A is a lower coating film, 16B is a coating film on the lower coating film (upper coating film), and 18 is a corrosive organism.

Hereinafter, the present invention will be explained in more detail with reference to Examples. However, these examples do not limit the present invention.

(1) Zinc-based plated steel plate A zinc-based plated steel plate having zinc-based plating layers on both sides as shown in Table 1 was prepared. A mild steel plate with a thickness of 0.5 mm was used as the zinc-based plated steel plate. The surface of the zinc-plated steel sheet was subjected to alkaline degreasing treatment, washed with water and dried before use.

(2) Formation of chemical conversion film The chemical conversion treatment agents for forming the chemical conversion film are the carboxylic acid derivatives shown in Tables 2 to 5 and 15 to 16, the resins shown in Table 6, and the silanes shown in Table 7. Coupling agents, zirconium compounds shown in Table 8, silica shown in Table 9, phosphoric acid and its salts shown in Table 10, fluorides shown in Table 11, vanadium compounds shown in Table 12 in the amounts shown in Table 17 (dry It was prepared by blending the membrane concentration = mass % based on the solid content of the chemical conversion treatment membrane and stirring using a disperser.
Next, apply a chemical conversion treatment agent to both sides of the zinc-based plated steel sheet prepared in (1) above using a roll coater so that the coating amount is 100 mg/m ² per side, and dry at a steel sheet temperature of 100°C. A chemical conversion film was formed.
The content of each component (dry film concentration) in the chemical conversion film in Table 17 is mass % based on the solid content of the film.

(3) Coating film The coating liquid for forming the coating film contains the carboxylic acid derivatives shown in Tables 2 to 5 and 15 to 16, the resins shown in Table 13, and the pigments shown in Table 14, as shown in Table 17. It was prepared by blending in the indicated amount (dry film concentration = mass % based on the solid content of the coating film) and stirring using a disperser.
Next, apply the coating solution to both sides of the zinc-based plated steel sheet having the chemical conversion coating prepared in (2) above using a roll coater so as to have a predetermined film thickness, and dry at a steel sheet temperature of 220 ° C. A coating film was formed.
In addition, if the coating film is two layers, further coat FLC100 paint manufactured by Nippon Paint Coatings Co., Ltd. with a roll coater on the coating film in contact with the chemical conversion coating (lower coating film) to a thickness of 15 μm, and then apply the coating at 230°C. A coating film was formed by drying at the temperature reached by the substrate.
The content of each component in the coating film (dry film concentration) in Table 17 is mass % based on the solid content of the coating film.

(4) Evaluation method and evaluation criteria A test plate was taken from the painted steel plate obtained in (3) above, and the test plate was evaluated using the evaluation method and evaluation criteria shown below.

(Cross cut white rust width)
After tape-sealing the end face of the test plate (50 x 100 mm size) and making a cross cut on the steel plate with a cutter knife to create scratches on the steel plate base, 60 cycles of a combined cycle corrosion test in accordance with CCT-JASO M609 were conducted. did. The maximum white rust width of the plating was measured from the cross-cut of the test plate after the test, and evaluated using the following evaluation criteria.
Rating 5: White rust width is less than 2 mm Rating 4: White rust width is 2 mm or more and less than 4 mm Rating 3: White rust width is 4 mm or more and less than 6 mm Rating 2: White rust width is 6 mm or more and less than 8 mm Rating 1: White rust width is 8 mm that's all

(White rust on Eriksen processing part)
After tape-sealing the end face of the test plate (50 x 100 mm size), a 6 mm Erichsen extrusion was performed at the center of the test plate, and a combined cycle corrosion test in accordance with CCT-JASO M609 was conducted for 60 cycles. After the test, the percentage of plating white rust in the circular part extruded by Erichsen processing of the test plate was measured and evaluated according to the following evaluation criteria.
Score 5: Area percentage of white rust is less than 20% Score 4: Area percentage of white rust is 20% or more and less than 30% Score 3: Area percentage of white rust is 30% or more and less than 40% Score 2: Area percentage of white rust is 40% or more and less than 50% Rating 1: Area ratio of white rust is 50% or more

(Corrosion resistance property (value of conditional expression (2)))
At a plate temperature of 20°C, the test plate is sandwiched between 6 laminated steel plates of the same thickness in the rolling direction (L direction) of the steel plate, and the radius of curvature is 3 times the plate thickness. Bending processing was applied.
Next, after sealing the edges of the bent test plate, it was fixed so that the bent surface faced vertically upward. Then, in this state, the test plate was subjected to a combined cycle corrosion test according to CCT-JASO M609 for 30 cycles or 15 cycles under the condition (1) described above.
Next, the test plate was cut along the vertical direction of the bending direction of the plating layer (that is, the direction along the bending direction of the plating layer and the thickness direction of the plating layer).
Next, the longitudinal cross section of the cut test plate (that is, the cross section cut along the bending direction of the plating layer and the direction along the thickness direction of the plating layer) was examined using a scanning electron microscope and energy dispersive X-ray analysis. Elemental analysis was conducted to identify corrosion products as well as the plating layer, chemical conversion coating, and paint film.
Next, in an arbitrary width range of 200 μm of the plating layer excluding the cracked portion of the plating layer, the total length of the region where the thickness of the corrosion product was 2 μm or more was measured. Then, evaluate the left side of the condition (2) described above (total length of the area where the thickness of corrosion products is 2 μm or more (μm)/200 (μm) x number of cracks in the plating layer) using the following criteria. did.
Score 5: The left side of condition (2) is less than 0.2 Score 4: The left side of condition (2) is 0.2 or more and less than 0.3 Score 3: The left side of condition (2) is 0.3 or more and less than 0.4 Score 2: The left side of condition (2) is 0.4 or more and less than 0.5 Score 1: The left side of condition (2) is 0.5 or more

The results of 30 cycles of the combined cycle corrosion test shown in condition (1) are listed in the column of "Condition (2) < 0.5 (30 cycles)" in Table 17.
Furthermore, the results of 15 cycles of the combined cycle corrosion test shown in condition (1) are listed in the column of "Condition (2) < 0.5 (15 cycles)" in Table 17.

From the above results, compared to the comparative example, this example suppresses the occurrence of white rust even if cracks occur in the plating layer or paint film due to bending, press working, etc., without using harmful hexavalent chromium. However, it can be seen that it also has high corrosion resistance.

Claims (12)

steel plate and
A plating layer containing zinc and provided on one or both sides of the steel plate,
a chemical conversion coating provided on the plating layer provided on one side of the steel plate, or on at least one of the plating layers provided on both sides of the steel plate;
one or more coating films provided on the chemical conversion coating;
has
A carboxylic acid derivative having one or more of a silanol group, a vinyl group, a vinylene group, and an allyl group is added to either or both of the chemical conversion treatment film and the coating film in contact with the chemical conversion treatment film. A painted plated steel sheet containing a concentration of 0.01% by mass or more and 40% by mass or less based on the solid content of the treated film or the solid content of the coating film in contact with the chemical conversion treatment film. The coated steel sheet according to claim 1, wherein the carboxylic acid derivative is contained in a concentration of 0.5% by mass or more and 40% by mass or less based on the solid content of the chemical conversion treatment film or the solid content of the coating film in contact with the chemical conversion treatment film. . The painted steel sheet according to claim 1 or 2, wherein the plating layer contains aluminum in an amount of 0.5% by mass or more and 60% by mass or less, and the remainder consists of zinc and impurities. The painted steel sheet according to claim 3, wherein the plating layer further contains magnesium in an amount of 0.5% by mass or more and 15% by mass or less. The painted steel sheet according to any one of claims 1 to 4, wherein the average thickness of the coating film in contact with the chemical conversion coating is 4 μm or more. The coated steel sheet according to any one of claims 1 to 5, wherein the coating film in contact with the chemical conversion treatment film contains one or more of polyester resin, epoxy resin, acrylic resin, and urethane resin. According to any one of claims 1 to 6, the coating film in contact with the chemical conversion film contains one or more of vanadate, tungstate, silicate, and phosphate. Painted plated steel plate. The painted plated steel sheet according to any one of claims 1 to 7, wherein the chemical conversion coating contains at least one of a silane coupling agent and a zirconium compound. The painted plated steel sheet according to any one of claims 1 to 8, wherein the chemical conversion coating contains one or more of silica, phosphoric acid and its salts, fluoride, and vanadium compound. The painted plated steel sheet according to any one of claims 1 to 9, wherein the chemical conversion coating contains one or more of urethane resin, polyester resin, acrylic resin, and epoxy resin. The painted steel plate was subjected to bending in the rolling direction at a plate temperature of 20°C with a radius of curvature three times the plate thickness, and then bent under the following conditions (1) in accordance with CCT-JASO M609. ) Regarding the corrosion test piece subjected to 30 cycles of the combined cycle corrosion test,
By observing the longitudinal section of the plating layer in the bending direction,
In an arbitrary width range of 200 μm of the plating layer excluding the crack portion of the plating layer formed in the bent portion of the corrosion test piece,
Among the corrosion products formed between the plating layer and the coating film in contact with the chemical conversion coating, starting from the crack part of the plating layer, the area where the thickness of the corrosion products is 2 μm or more. The total length is
The painted steel sheet according to any one of claims 1 to 10, which satisfies condition (2).
-Condition (1)-
(A) Salt spray treatment: Spraying 5% sodium chloride aqueous solution at 35°C for 2 hours (B) Drying treatment: Drying treatment at 60°C for 4 hours (C) Wet treatment: 50°C, Wet treatment at 95% RH for 2 hours, provided that (A), (B), and (C) are treated in this order in one cycle.
-Condition (2)-
Total length of the region where the thickness of the corrosion product is 2 μm or more (μm) / 200 (μm) × number of cracks in the plating layer <0.5 The painted steel plate was subjected to bending in the rolling direction at a plate temperature of 20°C with a radius of curvature three times the plate thickness, and then bent under the following conditions (1) in accordance with CCT-JASO M609. ) Regarding the corrosion test piece subjected to 15 cycles of the combined cycle corrosion test,
By observing the longitudinal section of the plating layer in the bending direction,
In an arbitrary width range of 200 μm of the plating layer excluding the crack portion of the plating layer formed in the bent portion of the corrosion test piece,
Among the corrosion products formed between the plating layer and the coating film in contact with the chemical conversion coating, starting from the crack part of the plating layer, the area where the thickness of the corrosion products is 2 μm or more. The total length is
The painted steel sheet according to any one of claims 1 to 10, which satisfies condition (2).
-Condition (1)-
(A) Salt spray treatment: Spraying 5% sodium chloride aqueous solution at 35°C for 2 hours (B) Drying treatment: Drying treatment at 60°C for 4 hours (C) Wet treatment: 50°C, Wet treatment at 95% RH for 2 hours, provided that (A), (B), and (C) are treated in this order in one cycle.
-Condition (2)-
Total length of the region where the thickness of the corrosion product is 2 μm or more (μm) / 200 (μm) × number of cracks in the plating layer <0.5