Zinc-plated steel plate with excellent blackening resistance and corrosion resistance and manufacturing method therefor

METHOD FOR PRODUCING SAME

[0001]

The present invention relates to a zinc-plated steel sheet having superior blackening resistance and corrosion resistance, and more particularly, to a zinc-plated steel sheet that can be used in home appliance, construction material, civil engineering, mechanical, automotive, furniture and container applications. In addition, the present invention also relates to a method for producing that zinc-plated steel sheet.

[0002]

Zinc-plated steel sheets having superior corrosion resistance are widely used in applications such as home appliances, construction materials or automobiles. In addition, a Zn-Al-Mg-Si-plated steel sheet described in Patent Document 1 is known as a technology for further enhancing the corrosion resistance of zinc-plated steel sheets.

[0003]

On the other hand, these zinc-plated steel sheets are susceptible to the formation of white rust caused by corrosion of zinc, and technologies are known that consist of carrying out chromate treatment or other chemical conversion treatment in order to inhibit this.

Although zinc-plated steel sheets that have undergone chromate treatment are resistant to the formation of white rust, on the other hand, they have the problem of the plated surface becoming discolored to a grayish black color when exposed to the atmosphere for a long period of time. This discoloration phenomenon may hereinafter be referred to as blackening. Blackening occurs particularly prominently in Zn-Al-based alloy-plated steel sheets or Zn-Al-Mg-based alloy-plated steel sheets in which Α1 or Mg has been added to the zinc plating.

[0004]

Patent Documents 2 to 6 describe technologies for improving blackening resistance of zinc-plated steel sheets. Patent Document 2 discloses a technology consisting of treating a zinc-plated steel sheet with a solution in which Ni ions or Co ions are mixed. Patent Document 3 discloses a technology consisting of treating a zinc-plated steel sheet with a chromate treatment solution that contains nitrate ions and has a specific composition. Patent Document 4 discloses a technology consisting of coating an aqueous treatment solution containing 1 g/1 to 100 g/1 of molybdate ion as Mo, 0.2 to 2 of, as the weight ratio of P/Mo, phosphate ions, 0.03 to 0.3 of, as the weight ratio of Co/Mo, cobalt ions and 1 g/1 to 300 g/1 of oxycarboxylic acid onto a zinc-plated steel sheet at 10 g/m² to 120 g/m² as the adhered amount of Mo. Moreover, Patent Document 5 discloses a technology for a plating layer structure having a zinc phosphate treatment layer on a zinc plating layer, and having a portion adhered with 0.1 mg/m² to 500 mg/m² of Ni interposed between the zinc plating layer and the phosphate treatment layer. Patent Document 6 discloses that blackening resistance of a hot-dipped Zn-Al-Mg alloy-plated steel sheet can be improved by forming a phosphate coating on the surface of the hot-dipped Zn-Al-Mg alloy-plated steel sheet and forming a chemical conversion treatment coating obtained by crosslinking a specific aqueous fluorine-containing resin on the phosphate coating.

[0005]

As will be subsequently described, a paint film is used in the present invention that contains flake-like aluminum particles. Patent Document 7 discloses a precoated metal sheet imparted with a metallic appearance by a paint film containing an aluminum pigment, wherein the surface of the aluminum pigment is coated to prevent blackening of the pigment caused by contact between the aluminum pigment and the underlying metal sheet.

[0006]

On the other hand, although unrelated to blackening resistance, Patent Document 8 describes a paint composition containing thin film, flake-like aluminum that is used to impart steel sheets and the like with a plated appearance having a superior aesthetic appearance. Patent Document 9 describes a technology for preventing discoloration of a paint film or other decreases in aesthetic appearance of a coated steel sheet obtained by dispersing aluminum particles in a paint film, by inhibiting elution of aluminum from the paint film in a strongly alkaline environment.

Prior Art Documents

Patent Documents

[0007]

Problems to be Solved by the Invention

[0008]

As was previously described, although Zn-Al-Mg-based alloy-plated steel sheets have recently been developed for use as zinc-plated steel sheets having superior corrosion resistance, these steel sheets in which Α1 and Mg have been added to the zinc plating conversely have the significant problem of blackening. Since Zn-Al-Mg-Si alloy-plated steel sheets having superior corrosion resistance in particular have superior corrosion resistance for a long period of time and are resistance to the formation of white rust, there is a growing desire to use these steel sheets at locations such as exterior panels that are visible from the outside in building material structures or home appliances and the like.

There is also a growing demand for inhibiting blackening in Zn-Al-Mg-Si alloy-plated steel sheets due to the increasing emphasis being placed on the design of architectural structures and home appliances in recent years in particular.

[0009]

According to the technologies described in Patent Documents 2 to 6, the blackening resistance of Zn-Al-Mg-based alloy-plated steel sheets, including Zn-Al-Mg-Si alloy-plated steel sheets, can be improved to a certain degree. However, these technologies are merely technologies for inhibiting blackening of zinc-plated steel sheets during the time they are stored in a warehouse or transported prior to being used by a construction material manufacturer, home appliance manufacturer or automobile manufacturer or other user following the production of the zinc-plated steel sheets by a steel manufacturer. Consequently, even if these technologies for imparting resistance to blackening were applied, there was the problem of the occurrence of blackening after a long period of time had elapsed following assembly of the zinc-plated steel sheets into building material structures, home appliances or automobile parts and use of those products and parts.

[0010]

On the other hand, although the technology described in Patent Document 7 prevents blackening of an aluminum pigment contained in a paint film of a precoated steel sheet, it does not prevent blackening of the surface of the underlying zinc plating of the precoated steel sheet when exposed to the atmosphere for a long period of time. [0011]

With the foregoing in view, an object of the present invention is to provide a Zn-Al-Mg-Si alloy-plated steel sheet that is capable of demonstrating both corrosion resistance and blackening resistance over a long period of time.

Means for Solving the Problems

[0012]

As a result of conducting extensive studies to solve the aforementioned problems, the inventors found that, by providing a coating film containing aluminum (Al) on a plating layer on the surface of a Zn-Al-Mg-Si alloy-plated steel sheet so that the Al in the coating film does not contact the plating layer, and by making the coverage rate of Al, which is defined as the ratio of the area of the portion of the plating layer concealed by the Al in the coating film to the total area of the observed field of view when observing the coating film containing Al from the direction perpendicular to the surface thereof, to be 75% to 100%, corrosion resistance and long-term blackening resistance are ensured.

[0013]

The invention of the present application was completed on the basis of these findings, and the gist of the present invention is as indicated below.

[1] A zinc-plated steel sheet having superior blackening resistance and corrosion resistance, comprising:

a steel sheet,

a Zn-Al-Mg-Si alloy plating layer formed on the surface of the steel sheet, and

a coating film containing Α1 formed on the plating layer;

characterized in that the Α1 contained in the coating film containing Α1 is separated from the plating layer by the presence of an insulating substance, and

the coverage rate of Al, which is defined as the ratio of the area of the portion of the plating layer concealed by the Al in the coating film to the total area of the observed field of view when observing the coating film containing Al from the direction perpendicular to the surface thereof, is 75% to 100%.

[2] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the coating film containing Al is composed of an insulating substance containing flake-like Al particles, and the Al particles are not present within a range of at least 0.5 μπΐ from the interface between the coating film containing Al and the plating layer.

[3] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the average particle diameter of the Al particles is 5 μπΐ to 30 μπΐ and the aspect ratio thereof is 20 or more.

[4] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the coating film containing Al is composed of at least two layers, consisting of an intermediate layer formed with an insulating substance and an Al metal layer, in that order starting from the plating layer side.

[5] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the Al metal layer is composed of an aggregate of flake-like Al particles.

[6] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the insulating substance is a resin.

[7] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the resin is a polyester resin crosslinked with a melamine compound.

[8] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the glass transition temperature Tg of the polyester resin is -20°C to 70°C and the number average molecular weight thereof is 15, 000 to 25,000.

[9] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the thickness of the coating film containing Α1 is 2 μπΐ to 10 μπΐ.

[10] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized by having a clear resin coating film on the coating film containing Α1.

[11] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the thickness of the clear resin coating film is 0.2 μπΐ to 20 μπΐ.

[12] The zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the zinc plating layer contains 0.01% by weight to 60% by weight of Al, 0.001% by weight to 10% by weight of Mg and 0.001% by weight to 2% by weight of Si.

[13] A method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized by coating a zinc plating layer on the surface of the steel sheet with a coating material containing flake-like Al particles and an insulating substance in a solvent, the viscosity thereof under conditions of a shear velocity of 1 s^_1 as measured with a rotational viscometer at 25° being 150 mPa-s to 1500 mPa-s, and the viscosity thereof at a shear velocity of 10,000 s^_1 as measured with a rotational viscometer at 25°C being 50 mPa-s to 150 mPa-s, followed by heating the steel sheet to a peak metal temperature of 180°C to 230°C at a heating rate of 5°C/s to 70°C/s in an induction heating furnace to form the coating film containing Α1.

[14] The method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the coating material is prepared by mixing flake-like Α1 particles with 100 parts by weight of an aqueous emulsion-type polyester resin solid and 10 parts by weight to 30 parts by weight of a melamine compound solid as a crosslinking agent.

[15] The method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that the viscosity of the coating material is adjusted using a viscosity modifier.

[16] The method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that 0.2 parts by weight to 10 parts by weight of a surfactant composed mainly of a urethane-modified polyether based on 100 parts by weight of a dispersion of the aqueous emulsion-type polyester resin is used for the viscosity modifier.

[17] The method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized in that Α1 particles having an average particle diameter of 5 μπΐ to 30 μΐη and aspect ratio of 20 or more are used for the Α1 particles.

[18] A method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance described in, characterized by:

(a) forming an intermediate layer of an insulating substance on a zinc plating layer on the surface of a steel sheet followed by forming an Α1 metal layer thereon by a plating method, or

(b) coating a zinc plating layer on the surface of a steel sheet with a liquid material for forming an intermediate layer of an insulating substance, spraying flake-like Α1 particles onto the liquid material, and then allowing the liquid material to solidify to form an intermediate layer of an insulating substance and an Α1 metal layer thereon.

[19] The method for producing the zinc-plated steel sheet having superior blackening resistance and corrosion resistance of, characterized in that the plating method is vacuum deposition plating.

[0014]

According to the present invention, a novel Ζη-Α1-Mg-Si-based zinc-plated steel sheet can be provided that is provided with long-term blackening resistance in addition to superior corrosion resistance inherently possessed by Zn-Al-Mg-Si-based zinc-plated steel sheets.

[0015]

The following provides an explanation of embodiments of the present invention.

The zinc-plated steel sheet of the present invention is composed by being provided with a steel sheet, a Ζη-Al-Mg-Si alloy plating layer formed on the surface of the steel sheet, and a coating film containing Α1 formed on the Zn-Al-Mg-Si alloy plating layer.

There are no particular limitations on the steel sheet, and a hot-rolled steel sheet, cold-rolled steel sheet or other ordinary steel sheet can be used. There are also no particular limitations on the type of steel, and for example, Al-killed steel, ultra-low carbon steel to which has been added Ti or Nb and the like, or high tensile steel to which has been added thereto elements such as Ρ, Si or Μη, can be used.

[0017]

The Zn-Al-Mg-Si alloy plating layer is a plating layer formed on the surface of the steel sheet. This plating layer is a plating layer composed of 0.1% by weight to 60% by weight of Al, 0.001% by weight to 10% by weight of Mg and 0.001% by weight to 2% by weight of Si, with the remainder consisting of Ζη and incidental impurities.

[0018]

If the Al content of the Zn-Al-Mg-Si alloy plating layer is less than 0.01% by weight, the effect of improving corrosion resistance of the plated steel sheet attributable to the addition of Al is not demonstrated, while if the content exceeds 60% by weight, the effect of improving corrosion resistance ends up being saturated.

The Al content is preferably 1% by weight to 60% by weight and more preferably 5% by weight to 60% by weight. [0019]

If the Mg content of the Zn-Al-Mg-Si alloy plating layer is less than 0.001% by weight, the effect of improving corrosion resistance of the plated steel sheet attributable to the addition of Mg is not demonstrated, while if the content exceeds 10% by weight, the Mg is not completely melted in the plating bath and rises to the surface of the bath in the form of an oxide (typically referred to as dross), thereby resulting in the risk of a poor appearance or the formation of portions that are not plated (typically referred to as non-plated portions) caused by adherence of oxides to the plating surface layer when zinc plating is carried out in this plating bath. The Mg content is preferably 1% by weight to 5% by weight and more preferably 1% by weight to 4% by weight.

[0020]

If the Si content of the Zn-Al-Mg-Si alloy plating layer is less than 0.001% by weight, the effect of improving corrosion resistance is not demonstrated. In addition, if the Si content is less than 0.001% by weight, there is increased susceptibility to the formation of oxides containing Ζη, Mg or Α1 (typically referred to as dross). On the other hand, if the Si content exceeds 2% by weight, the Si is not completely melted in the plating bath and rises to the surface of the bath in the form of an oxide (typically referred to as dross), thereby resulting in the risk of a poor appearance or the formation of portions that are not plated (typically referred to as non-plated portions) caused by adherence of oxides to the plating surface layer when zinc plating is carried out in this plating bath. Dross may be slightly formed even at a Si content of about 1% by weight depending on the case. The Si content is preferably 0.01% by weight to 1% by weight and more preferably 0.01% by weight to 0.5% by weight.

[0021]

The adhered amount of the Zn-Al-Mg-Si alloy plating layer on one side of the steel sheet is preferably 10 g/m² or more from the viewpoint of corrosion resistance, and preferably 350 g/m² or less from the viewpoint of processability.

[0022]

The zinc-plated steel sheet of the present invention having superior blackening resistance and corrosion resistance is provided with a coating film containing Α1 on the Zn-Al-Mg-Si alloy plating layer. This coating film is important in terms of improving blackening resistance.

Plated steel sheets having a Zn-Al-Mg-Si alloy plating layer to which has been added Α1 and Mg are known to be susceptible to the occurrence of blackening. The main cause of blackening is oxidation of the plating layer surface. Simply covering the Zn-Al-Mg-Si alloy plating layer with a coating film composed mainly of a film forming material such as a resin is not sufficient for preventing permeation of oxygen in the air and is therefore ineffective for preventing blackening. In the present invention, blackening can be effectively prevented by causing Α1 to be present in the coating film that covers the plating layer. Since Α1 forms a stable Α1 oxide in the surface layer in the atmosphere, it is an extremely stable metal. Consequently, by providing an Al-containing coating film on the surface of the Zn-Al-Mg-Si alloy-plated steel sheet, the Zn-Al-Mg-Si alloy plating layer becomes resistant to blackening over a long period of time. In addition, blocking of the pathway by which oxygen permeates the coating film by Α1 present in the coating film also greatly contributes to prevention of blackening of the plating layer.

[0024]

It is important that the Α1 in the coating film that covers the plating layer conceal the surface of the plating layer as much as possible from the viewpoint of blocking the permeation pathway of oxygen. Consequently, in the present invention, the coverage rate of Al, which is defined as the ratio of the area of the portion of the plating layer concealed by the Al in the coating film to the total area of the observed field of view when observing the coating film containing Al from the direction perpendicular to the surface thereof, is required to be 75% to 100%. Al coverage rate is preferably as high as possible, and is therefore preferably 85% or more and more preferably 95% or more, for example.

Since Α1 surface oxides are substances that are nobler than the Ζη contained in the plating layer of the Zn-Al-Mg-Si alloy-plated steel sheet, they easily cause contact corrosion between different metals in a state in which they are in contact with each other. Therefore, in the present invention, the Α1 in the coating film and the plating layer are required to be separated by an insulating substance in order to prevent contact between the Α1 in the coating film and the surface of the plating layer. The interval between the Α1 in the coating film and the plating layer is required to be 0.5 μπΐ or more and is preferably 1.0 μΐη or more. If the interval is less than 0.5 μπΐ, insulating effects are not obtained.

If the interval exceeds 3.0 μπΐ, not only do insulating effects become saturated, but it becomes difficult to form such a large interval per se. The interval is most preferably of the order of 0.5 μπΐ to 1.5 μπΐ.

[0026]

Flake-like Α1 particles can be used for the Α1 in the coating film that covers the plating layer. Flake-like Α1 particles are preferable since they are able to easily form a coating film containing Α1 by coating the plating layer with a coating material in which they are dispersed. Flake-like Α1 particles having an average particle diameter of 5 μπΐ to 30 μπΐ and aspect ratio (ratio of average particle diameter/thickness) of 20 or more can be used for the flake-like Α1 particles. If the average particle diameter is less than 5 μπΐ, the Α1 coverage rate is more likely to be less than 75% thereby diminishing the effect of concealing the plating layer.

If the average particle diameter exceeds 30 μπΐ, a portion of the Α1 particles end up being outside the coating film as a result of being excessively large, thereby resulting in the formation of an irregular surface and the risk of a poor appearance. If the aspect ratio is less than 20, the Α1 coverage rate is more likely to be less than 75%.

Although there are no particular limitations thereon, the upper limit of the aspect ratio is preferably less than 300. Α1 particles having an aspect ratio of greater than 300 are difficult to produce and difficult to acquire.

[0027]

Average particle diameter is obtained by measuring the major axis and minor axis of a single arbitrary aluminum particle, defining the average of the sum thereof as the average particle diameter of a single Α1 particle, or in other words, defining in the manner of [average particle diameter of single Α1 particle] = [(major axis + minor axis)]/2, and using the average of measuring 100 arbitrary Α1 particles as the average particle diameter. The average particle diameters of individual Α1 particles can be measured by magnifying the Α1 particles with an optical microscope or electron microscope. In addition, average particle diameter can also be determined by determining cumulative weight distribution using a sieve or a laser diffraction-type particle size distribution analyzer based on the principle of laser diffraction. Average particle diameter may also be determined based on cumulative weight distribution, by calculating the particle diameter at 50% in the cumulative weight (typically referred to as average particle diameter D50). In the present invention, the average particle diameter of 100 particles as measured with a microscope or the particle diameter at 50% in the cumulative weight can be used as average particle diameter.

[0028]

In addition, the average thickness of Α1 particles required to determine aspect ratio in the present invention can be the Α1 particle average thickness defined as the average thickness of 100 arbitrary Α1 particles determined by measuring the thickness of an arbitrary cross-section of an arbitrary Α1 particle by observing with an optical microscope or electron microscope (typically, the dimension in the direction perpendicular to the plane in which the aforementioned major axis and minor axis are measured).

[0029]

In this manner, aspect ratio is defined as [aspect ratio] = [Α1 particle average particle diameter measured as described above]/[Al particle average thickness measured as described above].

[0030]

The Al-containing coating film on the plating layer can be formed with a coating material containing flake-like Al particles, for example, and in a coating film formed in this manner, the Al particles are oriented in a direction parallel or nearly parallel to the underlying plating layer, are dispersed in a continuous phase formed by an insulating substance in the form of a film forming component in the coating material, and are gathered in the upper portion of the coating film. Only the continuous phase is present in the lower portion of the coating film, and Al particles are separated from the surface of the plating layer by forming the previously described interval.

[0031]

The coating film containing Al may be composed of at least two layers consisting of an intermediate layer formed with an insulating substance and an Al metal layer in that order starting from the plating layer side.

[0032]

The Al metal layer in this case can be formed in the form of an aggregate of flake-like Al particles on the intermediate layer formed using an insulating substance on the plating layer, or may be formed as a continuous Al layer by plating. In the case of an aggregate of flake-like Al particles, the previously described flake-like Al particles can be used. In contrast to aggregates of flake-like Al particles being present with the particles dispersed in a continuous resin phase in the coating film formed by a coating material containing flake-like Α1 particles as previously explained (although resin is present between adjacent particles and contact is possible among particles at the upper portion of the coating film having a higher particle concentration, there is extremely little contact, if any, among particles at the lower portion of the coating film having a lower particle concentration), aggregates of flake-like Α1 particles are formed by using the resin of the intermediate layer as an adhesive or binder, and are fixed in position by the resin that fills the intervals therebetween while making mutual contact. As a result thereof, such aggregates of flake-like Α1 particles can be included in the "A1 metal layer" in the present invention.

[0033]

In the case where the coating film containing Α1 is composed of at least two layers consisting of an insulating substance intermediate layer and an Α1 metal layer, the thickness of the intermediate layer may be 0.5 μπΐ or more and the thickness of the Α1 metal layer may be 1.5 μΐη to 9.5 μΐη. If the thickness of the intermediate layer is less than 0.5 μπΐ, insulating effects are not obtained and corrosion resistance is inferior. The thickness of the intermediate layer is more preferably 0.5 μΐη to 3 μΐη and even more preferably 0.5 μπΐ to 1.5 μπΐ. If the thickness exceeds 3 μπΐ, further coating is not required since insulating effects are saturated. If the thickness of the Α1 metal layer is less than 1.5 μπΐ, concealment of the lower plating layer is inadequate, while if the thickness exceeds 9.5 μπΐ, there is the risk of processability becoming inferior. The thickness of the Α1 metal layer is preferably 2.5 μπΐ to 9.5 μπΐ and more preferably 3.5 μπΐ to 9.5 μπΐ.

[0034]

The Α1 material in the coating film that contains Α1 may be composed of pure Α1 or an Α1 alloy composed mainly of Α1. Commonly known Α1 alloys can be used for the Α1 alloy.

[0035]

A resin is preferable for the insulating substance in the coating film containing Α1. The insulating substance fulfills the role of preventing corrosion of the plating layer attributable to contact between different metals, i.e., between Α1 in the coating film and the plating layer, and the insulating substance having an insulation resistance of 10 Ω or more is preferable.

[0036]

In the case where the coating film containing Α1 is formed with a resin having flake-like Α1 particles dispersed therein, the resin is preferably a polyester resin crosslinked with a melamine compound. This polyester resin preferably has a glass transition temperature Tg of -20°C to 70°C and a number average molecular weight of 15,000 to 25,000. If the glass transition temperature Tg is lower than -20°C, there is the risk of a decrease in the adhesion of processed portions of the coating film layer containing Α1. If the glass transition temperature Tg is higher than 70°C, there is the risk of a decrease in the processability of the coating film layer containing Α1 and the formation of cracks in the coating film layer during processing. Tg is more preferably 0°C to 50°C. The glass transition temperature Tg can be determined by measuring the coating film resin with a differential scanning calorimeter abbreviated as DSC or a thermomechanical analyzer abbreviated as ΤΜΑ. In the case where the number average molecular weight of the polyester resin is less than 15,000, there is the risk of a decrease in processability of the coating film and the formation of cracks in the coating film during processing. If the number average molecular weight exceeds 25,000, there is the risk of the occurrence of streak-like coating defects commonly referred to as ribbing during coating, or defects such as decreases in coverage rate caused by difficulty in uniformly dispersing the Α1 particles, due to excessively high viscosity when in the form of a coating liquid.

Number average molecular weight can be measured by a commonly known method such as gel permeation chromatography abbreviated as GPC.

[0037]

There are no particular limitations on the resin of the intermediate layer in the case where the coating film containing Α1 is composed of at least two layers consisting of an intermediate layer and an Α1 metal layer thereon, and a commonly known resin can be used.

Examples of resins that can be used include polyester resin, epoxy resin, urethane resin, acrylic resin and melamine resin. However, since there are many cases in which zinc-plated steel sheets are used after forming and processing, polyester resin or urethane resin having superior processability is more preferable. Epoxy resin is also preferable since it demonstrates superior adhesion with metal. When an intermediate layer is formed by applying a coating liquid obtained by dissolving such a resin in a solvent or emulsifying and dispersing it in water or a solvent, workability during production is improved thereby making this more effective. In addition, if a curing agent such as melamine or isocyanate is added to these resins to obtain a thermosetting resin, adhesion between the Α1 metal layer and Zn-Al-Mg-Si alloy plating layer is enhanced, thereby making this more preferable. If the resin is the same as that of the coating film layer containing Al, adhesion with the coating film layer containing Al is superior, thereby making this more preferable.

[0038]

In the case where the coating film containing Α1 is composed of an intermediate layer and an Α1 metal layer thereon, a layer separate from these can also be present. For example, in the case of forming the Α1 metal layer by a plating method, a layer can be provided between the intermediate layer and Α1 metal layer that is effective for enhancing adhesion of plating to the intermediate layer.

[0039]

An additive such as a pigment, aggregate or rust preventive can be added, as needed, to the resin that composes the film coating containing Α1. The addition of a pigment or aggregate is more preferable since, in addition to enhancing the strength of the coating film, it also enhances adhesion between Α1 and the Zn-Al-Mg-Si alloy plating layer. In addition, the addition of a rust preventive is more preferable since it improves corrosion resistance of the Zn-Al-Mg-Si alloy plating layer. The amount of additive added may be suitably determined so as not to be disadvantageous for the coating film of the present invention.

[0040]

The thickness of the coating film containing Α1 is preferably within the range of 2 μπΐ to 10 μπΐ. In the case where the film thickness is less than 2 μπΐ, there is the risk of the coverage rate being less than 75% as a result of being unable to completely conceal the plating layer. If the film thickness exceeds 10 μπΐ, there is the risk of processability becoming inferior. Film thickness can be measured by observing a cross-section with an optical microscope or electron microscope.

[0041]

As was previously explained, a portion in which Α1 particles dispersed in a continuous resin phase are not present (portion consisting of resin only), or an intermediate layer composed of resin independent from the Α1 metal layer, is located on the side of the coating film containing Α1 that contacts the plating layer. The thickness thereof is as was previously explained.

[0042]

In the zinc-plated steel sheet of the present invention, a clear resin coating film can be provided on the coating film containing Α1. In the case where the coating film containing Α1 has an Α1 metal layer in particular, the clear resin coating film is able to improve the fingerprint resistance thereof. The clear resin coating film also has the effect of smoothing the surface of the plated steel sheet by filling in surface irregularities on the surface of the coating film containing Α1.

[0043]

There are no particular limitations on the type of clear resin, and a commonly known resin can be used, examples of which include polyester resin, epoxy resin, urethane resin, acrylic resin and melamine resin.

However, since there are many cases in which zinc-plated steel sheets are used after forming and processing, polyester resin or urethane resin having superior processability is more preferable. Epoxy resin is also preferable since it demonstrates superior adhesion with metal (in this case, Α1 exposed on the surface of the coating film containing Al). If a curing agent such as melamine or isocyanate is added to the clear resin to obtain a thermosetting resin, hardness of the coating film is improved resulting in superior scratch resistance, thereby making this more preferable.

[0044]

A rust preventive can also be added to the clear resin as necessary. The addition of a rust preventive is preferable since it improves corrosion resistance of the coating film containing Al. Moreover, the addition of pigment or aggregate enhances the strength of the clear resin coating film and enhances the adhesion with the coating film containing Al, thereby making this more preferable.

[0045]

The thickness of the clear resin coating film layer is preferably within the range of 0.2 μπΐ to 20 μπΐ. If the thickness is less than 0.2 μπΐ, the effect on fingerprint resistance may diminish, while if the thickness exceeds 20 μπΐ, there is the possibility of the occurrence of a coating film defect typically referred to as boiling when a solvent-dissolved type or emulsiondispersed type of resin coating liquid is applied followed by drying and curing.

[0046]

In the case of using Al particles in the coating film containing Al, there are cases in which the Al particles may slightly diffuse and disperse into the clear resin coating film thereabove depending on the production method. In this case, the portion to which the Al particles have diffused is regarded as the coating film containing Al, and the thickness extending to that portion (distance from the surface of the plating layer) can be taken to be the thickness of the coating film containing Al. Thus, in this case, the thickness of the clear resin coating film refers to the thickness of the portion where Al particles are not present (portion consisting of resin only).

[0047]

In the present invention, the thicknesses of the plating layer, coating film containing Al, insulating resin intermediate layer, Al metal layer and clear resin coating film as well as the interval between the Al in the coating film and the plating layer and the like can be determined by embedding the zinc-plated steel sheet produced in resin, and observing a cross-section exposed by grinding in the direction of thickness with an electron microscope or the like.

In the zinc-plated steel sheet of the present invention, a known chemical conversion treatment may be carried out on the surface of the Zn-Al-Mg-Si alloy plating layer. Examples of applicable chemical conversion treatment include chromate treatment, phosphoric acid-based treatment, silica-based treatment, Mo-based treatment, Co-based treatment, Ni-based treatment and Zr-based treatment. In an embodiment in which an Α1 metal layer is located on the surface of the coating film containing Α1 and a clear resin coating film is further provided thereon, the aforementioned chemical conversion treatment can also be carried out on the Α1 metal layer.

[0049]

The following provides an explanation of a method for producing the zinc-plated steel sheet of the present invention.

[0050]

The zinc-plated steel sheet of the present invention in which the coating film containing Α1 is composed of an insulating substance that contains flake-like Α1 particles, and Α1 particles are not present within a range of at least 0.5 μπΐ from the interface between the coating film containing Α1 and the aforementioned plating layer, can be produced by coating the zinc plating layer on the surface of the steel sheet with a coating material that contains flake-like Α1 particles and an insulating substance in a solvent, has a viscosity under conditions of a shear velocity of 1 s^_1 as measured with a rotational viscometer at 25°C of 150 mPa-s to 1500 mPa-s, and has a viscosity at a shear velocity of 10,000 s^_1 as measured with a rotational viscometer at 25°C of 50 mPa-s to 150 mPa-s, followed by heating the steel sheet to a peak metal temperature of 180°C to 230°C at a heating rate of 5°C/s to 70°C/s in an induction heating furnace to form the aforementioned coating film containing Α1.

[0051]

The inventors found that, when a solution is prepared in which flake-like Α1 particles are dispersed in a solution of a resin, which is an insulating substance, (which is also referred to as a "coating liquid" or "coating material" hereinafter) and the viscosity thereof is adjusted to specific conditions, the viscosity thereof is controlled in a step of drying and baking of the coating liquid applied to the plated steel sheet serving as the base material and the flake-like Α1 particles rise to the upper layer portion of the coating film formed due to convection of the coating liquid, while a lower layer portion appears that does not contain Α1 particles. Namely, the inventors found that, by adjusting the viscosity of a coating liquid containing flake-like Α1 particles and an insulating substance in the form of a resin or emulsified resin in a solvent as measured with a rotational viscometer at 25°C to 150 mPa-s to 1500 mPa-s under conditions of a shear velocity of Is^-1 and to 50 mPa-s to 150 mPa-s under conditions of a shear velocity of 10,000 s^_1, followed by coating the coating liquid directly onto a zinc plating layer and heating to a peak metal temperature of 180°C to 230°C at a heating rate of 5°C/s to 70°C/s in an induction heating furnace to form a coating film, the Α1 particles rise to the upper layer portion of the coating film in the drying and baking step following the start of heating, and as a result of the coating film ultimately being cured while in that state, Α1 particles are not present within a range of at least 0.5 μπΐ from the interface between the coating film and the plating layer, thereby resulting in the formation of a coating film having a unique configuration in which Α1 particles are only present in the upper layer portion.

[0052]

In this case, an aqueous emulsion-type polyester resin can be preferably used for the resin serving as the insulating substance. Moreover, adhesion with the Ζη-Α1-Mg-Si alloy plating layer can be enhanced by using, as a crosslinking agent, a melamine compound to obtain a thermosetting resin. Examples of aqueous emulsion-type polyester resins include members of the Vylonal® series manufactured by Toyobo Co., Ltd. Examples of melamine compounds serving as crosslinking agents include members of the Cymel® Series manufactured by Cytec Industries Inc. In general, 10 parts by weight to 30 parts by weight of melamine crosslinking agent in terms of the solid content thereof can be used to 100 parts by weight of aqueous emulsion-type polyester resin in terms of the solid content thereof. The amount of melamine crosslinking agent used is more preferably 10 parts by weight to 20 parts by weight based on 100 parts by weight of aqueous emulsion-type polyester resin in terms of the solid content thereof.

[0053]

Preparation of the coating material can be carried out by adding an aqueous emulsion-type polyester resin, a melamine crosslinking agent and flake-like Α1 particles (as previously explained) to an aqueous solvent such as water or alcohol followed by stirring. The blending ratio of the aqueous emulsion-type polyester resin and melamine crosslinking agent is as previously described.

The incorporated amount of flake-like Α1 particles may be determined according to the properties of the particles used and the film thickness of the coating film formed.

In particular, the incorporated amount of Α1 particles increases relatively as film thickness increases (the thickness of the portion where Α1 particles are present becomes relatively larger than the interval between the Α1 in the coating film and the plating layer). On the other hand, the interval between the Α1 in the coating film and the plating layer varies depending on the viscosity conditions and drying/baking conditions of the coating material. Thus, the amount of flake-like Α1 particles incorporated in the coating material may be determined experimentally in consideration of these requirements. However, coating materials prepared in this manner do not normally satisfy the aforementioned viscosity conditions. A viscosity modifier can be used so that the viscosity of the coating material satisfies the aforementioned conditions.

[0054]

A phenomenon in which a liquid such as a coating liquid demonstrates high viscosity in the region of low shear velocities and low viscosity in the region of high shear velocities is typically referred to as the shear thinning effect. A highly concentrated liquid containing granular fine particles is typically used as a dense dispersion system in order to impart a shear thinning effect to a liquid. It is publicly said that as a result of using a dense dispersion system, the inter-particle distance between fine particles added to the coating liquid becomes shorter, and attraction between particles increases, resulting in the appearance of the shear thinning effect. However, differing from granular fine particles, inter-particle distance is unlikely to become shorter even if flake-like metal particles having a comparatively high specific gravity are added at a high concentration as in the present invention. In addition, due to the high specific gravity, if the amount added is increased in an attempt to further reduce inter-particle distance, there is increased susceptibility to the occurrence of particle settling, thereby making this unsuitable.

[0055]

A technique is known for controlling the viscosity of the same liquid at different shear velocities by adding a specific additive. Additives typically referred to as rheology control agents are used in this case. In the case of adding such a rheology control agent to a coating liquid, the rheology control agent reacts slightly with the resin in the coating liquid and causes it to form a network phase in the coating liquid.

However, the aforementioned required coating liquid viscosity conditions cannot be satisfied by adding an ordinary rheology control agent to the coating liquid used in the present invention.

[0056]

In the present invention, a specific viscosity modifier can be used so that the viscosity of the coating liquid satisfies the aforementioned conditions. The viscosity modifier used in the invention is a type of substance differing from ordinary rheology control agents in that it does not react with the resin in the coating liquid and the molecular terminal chains thereof are bound to the resin in the coating liquid with a weak bonding force such that the chains are adsorbed thereto.

Examples of this substance include a surfactant mainly composed of a urethane-modified polyether, and for example, "SN-Thickener 629N" manufactured by San Nopco Ltd. may be referred to. Since the amount of viscosity modifier added in the present invention varies according to such factors as the type of resin or type of solvent used in the coating liquid, the amount added may be suitably selected as necessary. More specifically, in the present invention, it is necessary to form the coating film containing Α1 so that Α1 particles are not present within a range of at least 0.5 μπΐ from the interface between the coating film and the plating layer, and the amount of viscosity modifier used is determined so as to satisfy this condition. In general, the viscosity modifier can be used in an amount of 0.2 parts by weight to 10 parts by weight based on 100 parts by weight of a dispersion of the aqueous emulsion-type polyester resin. If the amount used is less than 0.2 parts by weight, it becomes difficult to obtain the required coating film viscosity conditions, while if the amount used exceeds 10 parts by weight, there is the risk of gelling of the aqueous emulsion-type polyester resin.

The preferable added amount of the viscosity modifier is 0.2 parts by weight to 1.0 part by weight based on 100 parts by weight of a dispersion of the aqueous emulsion-type polyester resin.

[0057]

As a result of forming a coating film by coating a coating liquid for which viscosity has been adjusted directly onto a zinc plating layer followed by heating to a peak metal temperature of 180°C to 230°C at a heating rate of 5°C/s to 70°C/s and completing curing of the resin in the coating liquid, a coating film is formed in which Α1 particles are not present within a range of at least 0.5 μΐη from the interface between the coating film and the plating layer, thereby resulting in Α1 particles only being present in the upper layer portion. Heating is required to be carried out in an induction heating furnace. The reason for this is that induction heating is advantageous for obtaining a coating film having a configuration in which the distribution of Α1 particles is concentrated on one side (farthest side from plating layer) by inhibiting the settling of Α1 particles to the plating layer surface that have risen to the upper portion of the coating liquid on the plating layer due to Marangoni convection as will be subsequently described, by the flow of solvent that evaporates from the plating layer side to the coating liquid surface side as a result of heating from the side of the base material of plated steel sheet by induction heating. If the heating rate is less than 5°C/s, it becomes difficult to generate Marangoni convection due to the slow heating rate, or Α1 particles do not easily float upward due to a slow convection velocity, and there is the risk of the presence of the Α1 particles within a range of 0.5 μπΐ from the interface between the coating film and the plating layer, resulting in inferior corrosion resistance. In addition, if the heating rate exceeds 70°C/s, the coating material ends up curing while in a boiled state in the solvent drying step, due to the excessively rapid heating rate, thereby resulting in the risk of the formation of coating defects (typically referred to as boiling) in which remnants of air bubbles caused by boiling remain in the coating film. In addition, since heating time becomes short if the heating rate is excessively rapid, there is increased likelihood of the coating film surface layer being slightly uncured, thereby causing peeling of the coating film. If the peak metal temperature is lower than 180°C, the coating film is not completely cured resulting in the risk of the coating film surface not being dried, and if the coating film surface layer is uncured (undried), this can cause peeling of the coating film. If the peak metal temperature exceeds 230°C, the coating material ends up curing while in a boiled state in the solvent drying step, thereby resulting in the risk of the formation of coating defects (typically referred to as boiling) in which the remnants of air bubbles caused by boiling remain in the coating film. Moreover, the coating film becomes hard due to progression of curing of the coating film caused by baking at a high temperature, thereby increasing susceptibility to the formation of cracks or peeling of the coating film caused by processing. Since zinc-plate steel sheets are typically used after processing, processed portions where cracking or peeling occurs in the coating film due to processing are likely to become a source of corrosion.

[0058]

Although the detailed mechanism of the formation of the aforementioned coating film having a unique configuration has not been elucidated, the inventors have presumed it to be as indicated below. When a coating liquid is coated onto the plating layer surface of a base material of plated steel sheet at, in general, room temperature or a slightly higher temperature (such as a temperature of the order of 30°C or 40°C) and heating is started for the purpose of drying and baking, regions of different temperatures soon appear in the coating liquid, Marangoni convection occurs and the coating liquid begins to flow. During the initial stage of this coating liquid flow, namely when under conditions in which shear velocity is comparatively high, Α1 particles rise to the upper portion of the coating liquid on the plating layer because the viscosity of the coating liquid is low, and after the drying and baking step proceeds, convection subsides and shear velocity decreases, a film is formed in which Α1 particles are not contained in the lower portion since the Α1 particles no longer settle due to the high viscosity of the coating liquid.

[0059]

In this manner, in the present invention, by using a coating liquid for which viscosity has been adjusted so as to satisfy the aforementioned specific conditions, the movement of Α1 particles by Marangoni convection, which is generated in a process in which a coating liquid coated onto a plating surface is heated for drying and baking, is controlled, thereby causing the Ai particles to be concentrated in the upper portion of the coating liquid during the initial stage of heating. As a result of continuing to heat further while maintaining this state, the solvent of the coating liquid is vaporized and the resin is cured resulting in the formation of a coating film in which Al particles are not present within a range of at least 0.5 μπΐ from the interface with the plating layer.

[0060]

The coating film having the unique configuration of the present invention in which Al particles are not present within a range of at least 0.5 μπΐ from the interface with the plating layer is realized by heat treatment at a high temperature range providing a peak metal temperature of 180°C to 230°C. On the other hand, in the present invention, although it may seem strange at first glance, the use of a coating liquid for forming such a coating film is required that satisfies a viscosity condition defined by measured values (at different shear velocities) at 25°C. However, the inventors found that, by using a coating material that satisfies this viscosity condition and forming a coating film under the aforementioned drying and baking conditions, a coating film is obtained in which Α1 particles are actually not present within a range of at least 0.5 μΐη from the interface with the plating layer (see the Examples).

[0061]

The zinc-plated steel sheet according to the present invention, in which a coating film containing Α1 is composed of at least two layers consisting of an intermediate layer formed with an insulating substance and an Α1 metal layer in that order starting from the plating layer side, can be produced by:

(a) forming an intermediate layer of an insulating substance on a zinc plating layer on the surface of a steel sheet followed by forming an Α1 metal layer thereon by a plating method, or

(b) coating a zinc plating layer on the surface of a steel sheet with a liquid material for forming an intermediate layer of an insulating substance, spraying flake-like Α1 particles onto the liquid material, and then allowing the liquid material to solidify to form an intermediate layer of an insulating substance and an Α1 metal layer thereon.

[0062]

A commonly known insulating substance can be used for the insulating substance of the intermediate layer both in the case of producing the zinc-plate steel sheet according to the aforementioned (a) and in the case of producing according to the aforementioned (b), and examples of insulating substances that can be used include polyester resin, epoxy resin, urethane resin, acrylic resin and melamine resin. Since there are many cases in which zinc-plate steel sheets are used after forming and processing, a polyester resin or urethane resin having superior processability is more preferable.

Epoxy resin is also preferable due to its superior adhesion to metal. When the intermediate layer is formed by applying a coating liquid obtained by dissolving such a resin in a solvent or emulsifying and dispersing it in water or a solvent, workability during production is improved thereby making this more effective. In addition, if a curing agent such as melamine or isocyanate is added to these resins to obtain a thermosetting resin, adhesion between the Α1 metal layer and Zn-Al-Mg-Si alloy plating layer is enhanced, thereby making this more preferable.

[0063]

The intermediate layer of the insulating resin can be easily formed on the Zn-Al-Mg-Si alloy plating layer, by using a coating material of the type in which the insulating resin is dissolved in a solvent such as paint thinner or of the emulsion type in which it is dispersed in water, and after applying the coating material to the plating layer, drying and baking it, or by melting the insulating resin at a temperature lower than the melting point of Ζη, and coating the Zn-Al-Mg-Si alloy plating layer therewith, for example.

[0064]

In the case of forming the Α1 metal layer using the method described in (a) above, the Α1 metal layer can be formed on a preliminarily formed intermediate layer of an insulating resin by a commonly known plating method such as vacuum deposition plating, electroplating or ηοηelectrolytic plating. Among these plating methods, vacuum deposition plating is used preferably. Hot dip galvanization is not suitable since the plating layer ends up melting as a result of the melting temperature of Α1 being higher than the melting temperature of the Ζη which is the main component of Zn-Al-Mg-Si alloy plating. [0065]

In the case of forming the Α1 metal layer using the method described in (b) above, the Α1 metal layer can be formed in the form of an aggregate of flake-like Α1 particles by applying a coating material for forming the intermediate layer (the same coating material as that previously explained for the method of (a)) to the zinc plating layer, spraying flake-like Α1 particles onto the applied coating material, and then solidifying the coating material of the intermediate layer by heating.

Here, Α1 particles can be fixedly adhered on the insulating coating film layer by using the resin of the intermediate layer as an adhesive or binder.

[0066]

The blending ratio of the resin and crosslinking agent, heating conditions for drying and baking the coating film and the like are the same as those explained with respect to the method for forming a coating film with a coating liquid having Α1 particles dispersed therein both in the case of using the method described in the aforementioned (a) and the method described in the aforementioned (b).

[0067]

In the case of forming a clear resin coating film on the coating film containing Al, it can be formed in such a way that a coating liquid of the type in which an ordinary clear resin as previously exemplified is dissolved in a solvent such as paint thinner, or that of the emulsion type in which the resin is dispersed in water, is coated onto the coating film containing Al followed by drying and baking, or a clear resin is melted at a temperature lower than the melting of Ζη, and the resultant melt is coated onto the coating film containing Al.

[0068]

In the present invention, there are no particular limitations on the method used to apply the coating material when forming the coating film containing Al and the clear resin coating film, and a method ordinarily used in the coating of steel sheets can be used. For example, a coating method using a roll coater or curtain coater can be used preferably. A method ordinarily used in the coating of steel sheets can also be used for drying and baking the coating material.

[0069]

The following provides a more detailed explanation of the present invention through examples thereof. It goes without saying that the present invention is not limited to the following examples.

[0070]

(1. Formation of Plating Layer)

A cold-rolled steel sheet having a thickness of 1 mm was subjected to hot-dip plating by immersing for 3 seconds in a Zn-Al-Mg-Si plating bath at 450°C to which various types of metal had been added, and a Zn-Al-Mg-Si plating layer was formed on the steel sheet by adjusting the plated amount to 90 g/m² per side by Ν2 wiping.

[0071]

(2. Formation of Coating Film Containing Al Particles)

Al powder was dispersed in a mixed solution of aqueous emulsion-type high molecular weight polyester and melamine resin, and pure water and an additive containing urethane-modified polyether (SN-Thickener 629Ν) manufactured by San Nopco Ltd. were suitably added to the resulting dispersion to prepare a coating liquid for which viscosity during a shear velocity of 1 s^_1 (low shear viscosity) and viscosity during a shear velocity of 10, 000 s^_1 (high shear velocity) were adjusted at 30°C.

Vylonal® MD-1480 (number average molecular weight:

15, 000, Tg: 20°C) , Vylonal® MD-1220 (number average molecular weight: 15, 000, Tg: 67°C) , Vylonal® MD-1100 (number average molecular weight: 20, 000, Tg: 40°C) , Vylonal® MD-1985 (number average molecular weight:

25, 000, Tg: -20°C) , Vylonal® MD-1335 (number average molecular weight: 8,000, Tg: 4°C) and Vylonal® MD-1500 (number average molecular weight: 8,000, Tg: 77°C) manufactured by Toyobo Co., Ltd. were used for the aqueous emulsion-type high molecular weight polyester.

Cymel® 303 manufactured by Mitsui Cytec Ltd. was used for the melamine resin. "Sap 561PS" manufactured by Showa Aluminum Powder Κ.Κ. (average particle diameter: 16 μπΐ, aspect ratio: 20 or more), "Sap 2173SW" manufactured by Showa Aluminum Powder Κ.Κ. (average particle diameter: 6 μπΐ, aspect ratio: 20 or more), "Sap 720N" manufactured by Showa Aluminum Powder Κ.Κ. (average particle diameter: 30 μπΐ, aspect ratio: 2 0 or more) and "Aluminum Powder" reagent manufactured by Kanto Chemical Co., Inc., which were sized for particle diameter using sieves of different mesh sizes followed by extraction of only fine particles (average particle diameter: 20 μπΐ, aspect ratio: less than 20), were used for the Α1 particles.

The polyester resin and melamine resin were blended at a weight ratio of polyester resin solid fraction to melamine resin solid faction of 100:20, and the Α1 particles were added so that the weight ratio of polyester resin to Α1 particles was 100:15. The coating liquid was coated onto the plating layer of the steel sheet with a curtain coater, and the resin was cured by heating in an induction heating furnace to a prescribed peak metal temperature at a heating rate of 50°C/s followed by cooling with water to form a coating film containing Α1. Subsequently, the total thickness of the coating film, the thickness of the portion of the coating film where Α1 particles are not present (portion consisting of resin only), and the Α1 coverage rate were measured. The film thickness was measured by embedding the zinc-plated steel sheet in resin, observing an arbitrary longitudinal cross-section exposed by grinding at a magnification of 500Χ using a scanning electron microscope (SEM), and averaging the measured values obtained from five arbitrary sections. The Α1 coverage rate was determined as the area ratio at which Α1 is detected in all fields of view when an arbitrary plane of the coating film was analyzed by ΕΜΡΑ and Α1 was subjected to elemental mapping in a field of view magnified at 100Χ. This method is hereinafter referred to as the "particle dispersion method".

In addition, samples were also prepared in the particle dispersion method while changing the amount of Α1 particles added and added amount of melamine resin as

necessary.

[0072]

(3. Formation of Coating Film Containing Α1 Metal Layer by Plating)

Vylon® 29CS manufactured by Toyobo Co., Ltd., which is an amorphous polyester resin that is dissolved in an organic solvent consisting of a mixture of cyclohexanone and Solvesso, and Vylonal® MD-1220 manufactured by Toyobo Co., which is a water-dispersed high molecular weight polyester, were used for resin coating liquids. The insulating layer serving as an intermediate layer produced using Vylon® 29CS is hereinafter referred to as the "solvent type", while the insulating layer produced using Vylonal® MD-1220 is hereinafter referred to as the "water-dispersed type". A melamine resin sold under the tradename Cymel® 303 manufactured by Mitsui Cytec Industries Inc. was added, as a curing agent, to these resin coating liquids so that weight ratio of the polyester resin solid fraction to the melamine resin solid fraction was 100:20. Moreover, with respect to the "solvent type" coating liquid, Catalyst ΤΜ600, which is an acidic catalyst manufactured by Mitsui Cytec Industries Inc., was added at 0.5% by weight to a mixed solution of the polyester resin and the melamine resin.

A catalyst was not added to the "water-dispersed type" coating liquid.

[0073]

The resin coating liquid was coated onto the previously prepared plating layer with a bar coater followed by drying and curing under conditions of a peak metal temperature of 200°C in an air-heating furnace and cooling with water to form an insulating layer on the plating layer.

[0074]

Next, an Α1 metal layer was formed by deposition plating Α1 onto the insulating layer with a vacuum deposition plating apparatus. This method is hereinafter referred to as the "deposition method".

[0075]

Film thickness and coverage rate were measured as previously described for the coating films containing an Α1 metal layer obtained by plating that were formed in this manner.

[0076]

(4. Formation of Coating Film Containing Α1 Metal Layer Composed of Α1 Particles)

Flake-like Α1 particles were sprayed onto an intermediate layer in the form of an insulating layer formed in the manner described above to form an Α1 metal film. The Α1 metal layer composed of Α1 particles was produced by coating a coating liquid for the insulating layer produced in the manner described above onto the plating layer with a bar coater, followed by uniformly spraying the Α1 particles on the coating film prior to drying with a sieve, and then drying and curing the coating film in an air-heating furnace under the conditions providing a prescribed peak metal temperature.

In the present examples, particles obtained by drying "Sap 561PS" aluminum paste manufactured by Showa Aluminum Powder Κ.Κ. to form particles (average particle diameter:

16 μΐη) were used for the Α1 particles. This method is hereinafter referred to as "particle spraying".

[0077]

(5. Formation of Clear Resin Coating Film)

A resin coating liquid prepared as the coating liquid for an intermediate layer in the form of an insulating layer explained in the formation of a coating film containing an Α1 metal layer by plating was coated onto the Α1 metal layer with a bar coater, followed by drying and curing in an air-heating furnace under the conditions providing a peak metal temperature of 230°C and then cooling with water to form a clear resin coating film layer.

[0078]

Zinc-plated steel sheets were produced in the manner described above. The details of the zinc-plated steel sheets produced are shown in Tables 1 to 4.

[0079]

[Table 1]

1

[0080]

[Table 2]

Table 2

[0081]

[Table 3]

Table 3

The following evaluation tests were carried out on the zinc-plated steel sheets produced. Furthermore, in each of the tests, tests were carried out using the side having the coating film containing Α1 for the evaluated side .

[0084]

(I. Processability Test)

A cupping test was carried out under conditions of an indentation depth of 8 mm according to the method described in the "Cupping Test" of JIS Κ 5600-5-2. The test was carried out under conditions such that the evaluated side was outside the cup, and a test typically referred to as a tape peeling test was carried out in which tape is affixed to and then peeled from a processed portion after testing.

[0085]

Following completion of the test, damage to the surface of the portion from which the tape was peeled was observed visually, and the case in which there was no damage whatsoever was evaluated as A, the case in which peeling of the Α1 metal layer at the processed portion was less than 20% in terms of the area ratio was evaluated as Β, the case in which peeling of the Α1 metal layer at the processed portion was 20% to 50% in terms of the area ratio was evaluated as C, and the case in which peeling of the Α1 metal layer at the processed portion exceeded 50% in terms of the area ratio was evaluated as D.

[0086]

(II. Corrosion Resistance Test)

The produced zinc-plated steel sheets were cut to a size of 70 mm wide χ 150 mm long, a cut extending to the steel sheet substrate was provided on the evaluated side, and the end surfaces of the cross-sections on all four sides were sealed with tape to produce corrosion resistance test samples. A salt water spraying test was carried out according to the method described in section 9.1 of JIS Κ 5400. The salt water was sprayed so as to contact the evaluated side. The duration of the test was 240 hours.

[0087]

Following completion of the test, the maximum amount of swelling on one side of the cuts was measured, and the case in which the amount of swelling was 3 mm or less was evaluated as A, the case in which the amount of swelling was more than 3 mm to 4 mm or less was evaluated as ΑΒ, the case in which the amount of swelling was more than 4 mm to 5 mm or less was evaluated as Β, the case in which the amount of swelling was more than 5 mm to 10 mm or less was evaluated as C, and the case in which the amount of swelling exceeded 10 mm was evaluated as D.

[0088]

(III. Blackening Resistance Test)

The produced zinc-plated steel sheets were cut to a size of 70 mm wide χ 150 mm long. The cut sheets were subjected to an exposure test in which the steel sheets were exposed for 6 months on a coastline in Kimitsu City, Chiba Prefecture, Japan, the color tone of the steel sheets before and after the exposure test was measured with a spectrophotometer, and the L* value representing lightness of the CIE color system (L*a*b* color system) was measured. Those sheets for which ΛΐΑ≤5 based on the equation AL* = [L* value before testing] - [L* value after testing] were evaluated as A, those sheets for which 5<AL*<10 were evaluated as Β, those sheets for which 10<AL*<15 were evaluated as C, and those sheets for which 15<AL*<20 were evaluated as D.

[0089]

(IV. Fingerprint Resistance)

After adhering a fingerprint to the side to be evaluated by pressing with the index finger, the case in which the fingerprint did not adhere at all was evaluated as A, the case in which the fingerprint was adhered but was able to be removed by wiping with a cloth was evaluated as Β, the case in which the fingerprint was adhered but the remaining fingerprint was difficult to visually confirm after wiping with a cloth was evaluated as C, and the case in which the fingerprint was unable to be removed at all even after wiping with a cloth was evaluated as D.

(V. Appearance Test)

The side to be evaluated was observed visually and evaluated for the presence or absence of visual defects.

[0090]

The following provides a detailed description of the evaluation results (see Tables 5 to 8). Evaluation criteria for the examples of the invention and comparative examples were such that they were evaluated as comparative examples without exception if either of corrosion resistance or blackening resistance was evaluated as D. On the other hand, those that were evaluated as C for only one of the evaluation parameters while others were evaluated as Β or better were considered to be examples of the present invention.

Moreover, those that were evaluated as D or C for fingerprint resistance, but were evaluated as Β for both corrosion resistance and blackening resistance were considered to be examples of the present invention.

[0091]

[Table 5]

Table 5

[0092]

[Table 6]

Table 6

[0093]

[Table 7]

Table 7

[0094]

[Table 8]

Table 8

Tables 5 and 6 indicate evaluation results for zinc-plated steel sheets produced according to the aforementioned "particle dispersion method". The inventions of the present application of Nos. 1)-1 to 1)-24 and Nos. 2)-1 to 2)-11 demonstrated superior evaluation results for each of the parameters of processability, corrosion resistance, blackening resistance and fingerprint resistance.

[0096]

The examples of the present invention of Nos. 1)-1 to 1)-4 tended to have decreased corrosion resistance if the region of the coating film lower portion in which Α1 was not present was 0.5 μπΐ. In addition, in the comparative example of No. 1)-26, since the region of the coating film lower portion in which Α1 was not present was 0 μπΐ, or in other words, the Α1 material and the plating layer were in contact with each other, corrosion resistance was inferior, thereby making this unsuitable.

[0097]

The example of the present invention of No. 1)-6 demonstrated a low Α1 coverage rate of 75% since the thickness of the Α1 metal coating film layer was thin at 2 μπΐ, and blackening resistance tended to decrease. In addition, the comparative example of No. 1)-27 demonstrated inferior blackening resistance since the thickness of the Α1 metal coating film layer was even thinner and the coverage rate was less than 75%, thereby making this unsuitable. The example of the present invention of No. 1)-8 demonstrated decreased processability since the thickness of the Α1 metal coating film layer was excessively thick at 20 μπΐ.

[0098]

In the example of the present invention of No. 1)-16, appearance was slightly poor due to the presence of dross attributable to oxides resulting from oxidation of part of Mg, which was not melted in the plating bath, adhering to the plating layer since the amount of Mg added in the hot-dip galvanized layer was high at 10% by weight. Visual defects caused by dross tend to be disapproved from the viewpoint of design properties at locations that are visibly conspicuous (such as exterior panels of home appliances or buildings). However, since visual defects caused by dross are merely the result of oxides adhering to the plating layer, steel sheets having such visual defects can still be used as plated steel sheet products provided there are no problems in terms of their performance.

[0099]

In the example of the present invention of No. 1)-21, appearance was slightly poor due to the presence of dross attributable to oxides resulting from oxidation of part of Si, which was not melted in the plating bath, adhering to the plating layer since the amount of Si added in the hot-dip galvanized layer was high at 2% by weight. In addition, appearance was also slightly poor due to dross in the example of the present invention of No. 1)-20 in which the amount of Si added was 1% by weight. Visual defects caused by dross tend to be disapproved from the viewpoint of design properties at locations that are visibly conspicuous (such as the exterior panels of home appliances or buildings).

However, since visual defects caused by dross are merely the result of oxides adhering to the plating layer, steel sheets having such visual defects can still be used as plated steel sheet products provided there are no problems in terms of their performance.

[0100]

The comparative example of No. 1)-25 was unsuitable due to inferior corrosion resistance since Α1 and Mg were not contained in the zinc plating layer.

[0101]

The comparative example of No. 1)-30 demonstrated inferior corrosion resistance since the heating rate when baking the coating liquid to form the Α1 metal coating film layer was slow at less than 5°C/s, thereby making it difficult for Α1 particles to rise and causing Α1 particles to be present within a range of 0.5 μπΐ from the interface between the coating film and plating layer.

The comparative example of No. 1)-31 demonstrated the occurrence of the coating defect referred to as boiling, since in this example, the coating material ends up curing while boiling in the solvent drying step and the remnants of air bubbles caused by boiling remain in the coating liquid, due to the heating rate when baking the coating film to form the Α1 metal coating film layer of higher than 70°C/s. Moreover, since heating time is short due to the slow heating rate, the coating film surface layer becomes slightly uncured, causing the fingers to slightly stick to the coating film surface layer (feel sticky) when touched. The uncured coating film surface layer causes the coating film to peel, thereby making this example unsuitable. The coating film surface layer of comparative example of No. 1)-33 was slightly uncured since the peak metal temperature when baking the coating liquid to form the Α1 metal coating film layer was below 180°C, thereby causing fingers to slightly stick to the coating film surface layer (feel sticky) when touched.

The uncured coating film surface layer causes the coating film to peel, thereby making this example unsuitable. In the comparative example of No. 1)-35, the coating defect referred to as boiling was occurred since the coating material ended up being cured while boiling in the solvent drying step and the remnants of air bubbles caused by boiling remain in the coating liquid, due to the peak metal temperature when baking the coating liquid to form the Α1 metal coating film layer of above 230°C.

Moreover, since the coating material was baked at such a high temperature, the coating film became hard as curing of the coating film proceeded, thereby resulting in the formation of cracks or peeling in the coating film when subjected to bending processing at 180 degrees. Since zinc-plated steel sheets are typically used after processing, there is the risk the occurrence of corrosion at processed portions where the coating film has become cracked or peeled due to processing, which significantly decreases product value, thereby making this example unsuitable.

[0102]

In the comparative example of No. 2)-12, the aspect ratio of the Α1 particles was less than 20 and Α1 coverage rate was also low, thereby resulting in inferior blackening resistance and making this example unsuitable. [0103]

Since Vylonal® MD-1335 having a low number average molecular weight (number average molecular weight: 8,000, Tg: 4°C) was used in the example of the present invention of No. 2)-4 and Vylonal® MD-1500 having a high Tg (number average molecular weight: 8,000, Tg: 77°C) was used in the example of the present invention of No. 2)-5, processability tended to decrease in these examples.

Since Vylonal® MD-1220 having a somewhat high Tg (number average molecular weight: 15,000, Tg: 67°C) was used in the example of the present invention of No. 2)-1 and Vylonal® MD-1985 having a somewhat high number average molecular weight and somewhat low Tg (number average molecular weight: 25,000, Tg: -20°C) was used in the example of the present invention of No. 2)-3, processability tended to decrease in these examples.

[0104]

In the example of the present invention of No. 2)-11, a portion of the Α1 particles ended up being outside the coating film since the average particle diameter of the Α1 particles was large at 30 μπΐ, thereby resulting in the slight formation of surface irregularities. Despite this, since surface irregularities are merely the result of Α1 particles present in the coating film being outside the coating film, steel sheets having such surface irregularities can still be used as plated steel sheet products provided there are no problems in terms of their performance.

[0105]

Table 7 indicates evaluation results for zinc-plated steel sheets produced according to the aforementioned "deposition method". As shown in Table 7, examples of the present invention of Nos. 3)-1 to 3)-25 demonstrated superior evaluation results for each of the parameters of processability, corrosion resistance, blackening resistance and fingerprint resistance.

[0106]

The examples of the present invention of Nos. 3)-1 to 3)-13 and Nos. 3)-15 to 3)-25 demonstrated superior processability since the insulating layer (intermediate layer) consisted of resin.

[0107]

Although the example of the present invention of No. 3)-14, in which the thickness of the insulating layer (intermediate layer) exceeded 1.5 μπΐ, demonstrated somewhat inferior processability in comparison with other examples of the present invention in which the insulating coating film layer was 1.5 μπΐ or less, it demonstrated favorable results for parameters other than processability.

[0108]

Although the example of the present invention of No. 3)-20, in which a clear resin coating film was not formed on the Α1 metal coating film layer, demonstrated inferior fingerprint resistance than other examples of the invention having a clear resin coating film, it demonstrated superior results for evaluation parameters other than fingerprint resistance.

[0109]

In the comparative example of No. 3)-21, in which the thickness of the clear resin coating film was 0.2 μπΐ, the evaluation result for fingerprint resistance was at a lower limit level of C.

[0110]

In the example of the present invention of No. 3)-24, although all of the evaluation results were favorable, since the clear resin coating film was comparatively thick at greater than 25 μπΐ, coating defects referred to as boiling tended to occur in the coating film during the course of drying and curing after the application of the clear resin coating film.

[0111]

In the comparative example of No. 3)-8, in which the Si content in the hot-dip galvanized layer was less than 0.001% by weight, corrosion resistance tended to be somewhat inferior to other examples of the invention in which Si was added at 0.001% by weight or more, and appearance defects of the plating attributable to dross occurred extensively, thereby making this example unsuitable. In the invention of the present application of No. 3)-10, in which the Si content in the hot-dip galvanized layer was 2% by weight, slight dross formation was observed. However, since the dross formation was only slight, the level of quality of this example is considered to not present a problem in terms of actual use .

[0112]

In the example of the present invention of No. 3)-11, in which the Si content exceeded 2% by weight, although the appearance of the plating tended to be inferior, due to the formation of dross, to other examples of the invention, all other evaluation parameters were superior.

[0113]

In the invention of the present application of No.

3)-24, the coating defect referred to as boiling occurred since the thickness of the clear film coating was thick at 25 μΐη. Boiling refers to a coating defect in which remnants of the solvent boiled in the step for drying and baking the coating film remain in the form of craters, and occurs easily when the coating film is thick.

Consequently, it is preferable that boiling not occur since products having poor appearance tend to be disapproved from the viewpoint of design properties at locations that are visibly conspicuous (such as the exterior panels of home appliances or buildings),.

However, although the appearance of such products is unattractive, they can still be used without problems provided there are no problems in terms of their performance.

[0114]

On the other hand, the comparative example of No.

3)-26 demonstrated inferior blackening resistance since an Α1 metal coating film layer was not formed on the Ζη-Al-Mg-Si alloy plating layer. The comparative example of No. 3)-27 was unsuitable due to inferior corrosion resistance since it was not provided with an insulating coating film layer composed of an insulating substance between the hot-dip galvanized layer and the Α1 metal layer.

[0115]

Table 8 indicates results of evaluation tests for zinc-plated steel sheets produced according to the aforementioned "particle spraying method". Examples of the present invention of Nos. 4)-1 to 4)-19 and Nos. 4)-21 to 4)-24 were superior for each of processability, corrosion resistance, blackening resistance and fingerprint resistance.

[0116]

According to the present invention, a Zn-Al-Mg-Si-based zinc-plated steel sheet can be provided that realizes both superior corrosion resistance and blackening resistance for a long period of time. As a result, a Zn-Al-Mg-Si-based zinc-plated steel sheet that does not require coating, is inexpensive and superior in corrosion resistance can be applied as an exterior panel of construction materials and home appliances, and users of zinc-plated steel sheets can omit some of the production steps and reduce the production cost of their products. Thus, the present invention can be said to be an invention that demonstrates an extremely high level of industrial value.