JP4283698B2 - Precoated steel sheet having excellent end face corrosion resistance and method for producing the same - Google Patents

Description

  The present invention belongs to a technical field related to a precoated steel sheet having excellent end face corrosion resistance and a method for producing the same.

  In order to improve the corrosion resistance of the steel sheet, various types of zinc plating are applied, and further, coating may be performed thereon. However, the end face of the steel plate is inferior in corrosion resistance because iron and zinc-based plating are exposed.

  As methods for improving the corrosion resistance of such end faces, the following methods (the following (1) to (10)) are known.

(1) Those described in JP-A-3-100180:
“Chromate treatment liquid is applied to the surface of an alloyed hot dip galvanized steel sheet or zinc-nickel electroalloy plated steel sheet and dried without washing with water to form a film. The chromate treatment liquid is hexavalent. The composition preferably contains chromium ions, trivalent chromium ions, and phosphate ions, and further contains at least one of silica and an organic polymer resin, and the coating amount is 2 to 500 mg / in terms of the total chromium weight. about m ² is suitable. subsequently on said film, applying a primer coating if necessary, after baking, polyester, baked by applying a coating material such as an acrylic. Thus processability, other such adhesion "We will get a plated steel sheet with excellent endurance corrosion resistance without compromising the performance of."

(2) Those described in Japanese Patent Laid-Open No. 5-146750:
A zinc phosphate coating 2 of 0.2 to 2.0 g / m ^{2 is} formed on one surface of a plated steel sheet 1 having zinc-based plating on both sides as a coating base treatment, and strontium is formed on the zinc phosphate coating 2. A rust-preventing primer film 3 containing 20 to 50% by weight of chrome is formed, and further, a top coat film 4 is formed on the primer film 3 and the other surface is coated with a Cr equivalent amount. What forms the chromate film layer 5 of 20 mg / m < ² > or more.

(3) Those described in JP-A-6-009902:
“Phosphorus pentoxide, orthophosphoric acid, condensed phosphoric acid, metal phosphate, metal condensed phosphate, phosphorous compound (A) selected from heterocondensates of phosphoric acid and mixtures thereof, vanadyl compound, vanadium pentoxide, Vanadium compound (B) selected from vanadate, calcined condensate of vanadic acid, heterocondensate of vanadic acid and a mixture thereof, P ₂ O _{5 in} component A / V ₂ O ₅ in component B Mixing is performed so that the molar ratio is 0.3 to 100. Thereby, a vanadium / phosphate-based anticorrosive pigment having a pH of 5 to 9 in which 1 g is dispersed in water is produced. -The anticorrosive pigment and this vanadium / phosphate anticorrosive pigment are mixed so that the former / latter weight ratio is 97/3 to 40/60 to produce a coating composition. thing.

(4) Those described in JP-A-8-309917:
“The steel sheet thickness t (mm) and the coating weight w (g / m ² ) per side satisfy both t ≦ 1.6, w ≦ 130, and w / t ≧ 60 on both sides of the zinc-based plated steel plate. A coating film containing 25% or more of a rust preventive pigment is formed in the coating film, thereby obtaining a good coated steel sheet having both the end face corrosion resistance and the appearance and corrosion resistance of the processed part. It can be used for a long time. "

(5) Japanese Patent Application Laid-Open No. 9-012931
“A mixture of phosphate rust preventive pigment (A) and calcium ion exchange silica rust preventive pigment (B) in a weight ratio of 95: 5 to 20:80 is added to the coating film in an amount of 5 to 40 (wt. %, Preferably 10 to 30% The component A is preferably modified by wet mixing aluminum dihydrogen triphosphate, aluminum metaphosphate or the like with zinc oxide or magnesium oxide. Component B preferably has a calcium content of 2 to 15%, and the coating resin constituting the target composition has an average molecular weight of 330 to 3750 using bisphenol A and epichlorohydrin as raw materials. A bisphenol A type epoxy resin having an epoxy equivalent of 180 to 3300 is preferable. "

(6) Japanese Patent Application Laid-Open No. 9-38570:
In the method of coating a zinc-based steel sheet, comprising subjecting the zinc-based steel sheet to chemical conversion treatment, then applying an undercoat paint, and then applying a topcoat paint to obtain a coated steel sheet, the undercoat paint is based on 100 parts by weight of the resin. A method for coating a zinc-based steel sheet containing 1 to 100 parts by weight of magnesium oxide.

(7) Japanese Patent Application Laid-Open No. 10-280116:
“The shape of the cut end face is set so as to satisfy the formula: t ² / (M · R _S ) ≦ 0.2. In the formula, t is the plate thickness (mm), and M is the Zn weight per one side (g / M ² ), R _S is the shear plane ratio (= shear plane thickness / plate thickness), or the cut end face when cutting with a pair of upper and lower tools in the manufacture of a galvanized surface-treated steel sheet Zinc plating is performed so that the relationship between the ratio of the shearing surface, the plate thickness of the steel sheet, and the basis weight of Zn satisfies the above formula, and cutting is performed. It is possible to universally suppress the rust of the cut surface that occurs from a position where there is no presence. Preferably, the shear surface ratio is 0.5 to 0.9, and the Zn areal weight per side is 5 g / m ² or more.

(8) Those described in JP-A-11-138690:
“In a precoated steel sheet having a coating composed of a lower primer layer and an upper top layer on both the front and back surfaces of a galvanized steel sheet or aluminum plated steel sheet that has been subjected to chemical conversion treatment, the primer layers on both the front and back surfaces are coated. It contains 25-50% by weight of rust preventive pigment based on the dry weight of the film, and the top layer on the surface contains 5-20% by weight of melamine resin as a curing agent with respect to the dry weight of the coating film, and the balance Is made of a polyester resin having a number average molecular weight of 15000 to 25000, and the glass transition temperature of the top layer on the surface is 5 to 20 ° C. ”.

(9) JP-A-2002-194557 describes:
“It relates to steel cutting plates. In this steel cutting plate, a chemical conversion coating is formed directly on the surface of the base steel, in which oxides or hydroxides of valve metals and fluorides exhibit high insulation resistance. It is also possible to use a plated steel plate as the base steel, in which case a chemical conversion coating is formed directly on the cut end surface where there is no plating layer and the base steel is exposed. , Hf, V, Nb, Ta, Mo, W, etc., are used singly or in combination of two or more, in order to make the self-repair action by fluoride effective, O contained in the chemical conversion coating And the concentration ratio F / O of F is preferably adjusted so that the atomic ratio is 1/100 or more. "

(10) Those described in JP-A-2003-003280:
“On the surface, it contains Mg: 0.1 to 10% and Al: 0.2 to 80% in mass%, and the balance has an adhesive layer made of Zn and inevitable impurities, and 0.001 to 0.001 on the end face It has an organic corrosion inhibitor of 10 g / m ^{2, and} the adhesion layer further contains Si: 0.01 to 10% by mass, and the organic corrosion inhibitor is an unsaturated alcohol. Or one or more of saturated linear primary amines, saturated linear secondary amines, saturated linear tertiary amines, thioureas, phosphonic acids, morpholines, and imidazolines. It is preferable. "
Japanese Patent Laid-Open No. 3-100180 Japanese Patent Laid-Open No. 5-146750 JP-A-6-009902 JP-A-8-309917 Japanese Patent Laid-Open No. 9-012931 Japanese Patent Laid-Open No. 9-38570 Japanese Patent Laid-Open No. 10-280116 JP-A-11-138690 JP 2002-194557 A Japanese Patent Laid-Open No. 2003-003280

  As described above, methods for improving the end surface corrosion resistance of steel sheets have been proposed, but there are problems such as environmental problems and implementation difficulties. These problems are described below for each method.

(1) Those described in JP-A-3-100180:
Since hexavalent chromium is included, there is a concern about adverse effects on the human body and the environment.
(2) Those described in Japanese Patent Laid-Open No. 5-146750:
Since hexavalent chromium is included, there is a concern about adverse effects on the human body and the environment.
(3) Those described in JP-A-6-009902:
Since it contains vanadium pentoxide and hexavalent chromium, there are concerns about adverse effects on the human body and the environment.
(4) Those described in JP-A-8-309917:
Since the content of the rust preventive agent is large, the cost is high, and the adhesion of the coating film and the appearance are deteriorated.
(5) Japanese Patent Application Laid-Open No. 9-012931
The end face rust prevention effect is insufficient.
(6) Japanese Patent Application Laid-Open No. 9-38570:
Since the pH of magnesium oxide is low, the effect is insufficient (only magnesium oxide has a low pH and the effect is insufficient).
(7) Japanese Patent Application Laid-Open No. 10-280116:
The restrictions on equipment are large, and know-how on cutting work is necessary, which is difficult to implement.
(8) Those described in JP-A-11-138690:
Since the content of the rust preventive agent is large, the cost is high, and the adhesion of the coating film and the appearance are deteriorated.
(9) JP-A-2002-194557 describes:
Since fluoride is used, there are concerns about danger during processing and environmental impact.
(10) Those described in JP-A-2003-003280:
Since it is post-processing, the application range is limited.

  The present invention has been made paying attention to such circumstances, and its purpose is to provide a precoated steel sheet having no environmental problems as in the case of containing hexavalent chromium and having excellent end face corrosion resistance, and a method for producing the same. It is something to be offered.

  In order to achieve the above object, the present inventors have intensively studied, and as a result, completed the present invention. According to the present invention, the above object can be achieved.

The present invention thus completed and capable of achieving the above object relates to a precoated steel sheet, and is a precoated steel sheet according to claims 1 to 7 (precoated steel sheet according to the first to seventh inventions) . Yes, it has the following structure.

That is, the pre-coated steel sheet according to claim 1 is a pre-coated steel sheet having a coating film composed of a lower primer layer and an upper top layer on the surface of the galvanized steel sheet , and there is no coating film on the end surface and the iron and zinc-based steel sheet. In the precoated steel sheet in which the plating is exposed , the primer layer is composed of a resin layer containing a basic substance in which the pH of the saturated aqueous solution is 10.5 or more and the amount dissolved in 1 kg of water at 20 ° C. is 100 g or less. It is the precoat steel plate which is excellent in end surface corrosion resistance [1st invention].

  The precoated steel sheet according to claim 2 is the precoated steel sheet having excellent end face corrosion resistance according to claim 1, wherein the content of the basic substance is 3 to 25% by mass [second invention].

  The precoated steel sheet according to claim 3 is the precoated steel sheet having excellent end face corrosion resistance according to claim 1 or 2, wherein the basic substance has a particle size of 10 μm or less [third invention].

  The precoated steel sheet according to claim 4 is excellent in end face corrosion resistance according to any one of claims 1 to 3, wherein the basic substance is at least one selected from Group IIA oxides and Group IIA hydroxides. A pre-coated steel sheet [fourth invention].

  The precoated steel sheet according to claim 5 is the precoated steel sheet having excellent end face corrosion resistance according to claim 4, wherein the Group IIA oxide and Group IIA hydroxide are magnesium oxide and calcium hydroxide, respectively. .

  The precoated steel sheet according to claim 6 is the precoated steel sheet excellent in end surface corrosion resistance according to any one of claims 1 to 5, wherein silica is contained in the primer layer [Sixth Invention].

  The precoated steel sheet according to claim 7 is the precoated steel sheet having excellent end surface corrosion resistance according to claim 6, wherein the content of the silica is 1 to 50% by mass [seventh invention].

  The precoated steel sheet according to the present invention has no environmental problem as in the case of containing hexavalent chromium, and is excellent in end face corrosion resistance. Therefore, there is no environmental problem as in the case of containing hexavalent chromium as a material (pre-coated steel plate) for applications requiring corrosion resistance, and it can be suitably used.

  Conventionally, based on the recognition that hydrogen ions are corrosive ions, a method of exchanging hydrogen ions with cations such as calcium by an ion exchange action has been adopted. However, as a result of investigations to obtain further corrosion resistance, the present inventors have found that excellent rust prevention is exhibited by insolubilizing eluted zinc ions in the form of zinc hydroxide. The present invention has been made based on such findings. Details will be described below.

  On the end face of the galvanized steel sheet, steel and zinc are exposed, and there is concern about their corrosion. In practice, galvanization acts as a sacrificial anode and elutes preferentially. As in the following reaction, the eluted zinc ions lower the pH by a hydrolysis reaction.

Zn → Zn ²⁺ + 2e ^-
Zn ²⁺ + H ₂ O = ZnOH ⁺ + H ⁺

In a state where the pH is lowered, the dissolution of zinc continuously occurs, and the eluted zinc becomes white rust, and the film bulge is generated from the end face. In order to prevent this, it is necessary to suppress elution of zinc. It is considered that the elution of zinc can be suppressed by suppressing the decrease (acidification) of the pH. If a basic substance is added to the coating film, acidification by zinc ions is suppressed, and the zinc ions are insolubilized as zinc hydroxide, basic zinc chloride or the like. When the zinc ion is zinc hydroxide, the reaction formula shows the following. Note that in this scheme, the left side of OH ^- is OH produced by the dissolution of the basic substance in the coating film ^- can be.

^{ZnOH + + H + + 2OH -} → Zn (OH) 2 + H 2 O

  Since the eluted zinc is insolubilized on the spot, the dissolution of zinc is suppressed and the corrosion resistance is improved.

  Since zinc hydroxide has the most stable pH range of about 8.5 to 10.5, a basic substance that exhibits a pH of at least 10.5 in a saturated solution is effective. There is no particular upper limit. When the basic substance is a mixture, the pH of the mixture is obtained. What is necessary is just to specify pH as a mixture = basic substance.

  In addition, when the basic substance is dissolved in 1 kg of water at 20 ° C: more than 100 g, the solubility is too high, causing the film to swell and poor corrosion resistance. Dissolved amount: Must be 100 g or less.

  The present invention has been made on the basis of such knowledge, and is a precoated steel sheet having the above-described configuration.

  As can be seen from the above, the precoated steel sheet according to the present invention is excellent in end face corrosion resistance. That is, although zinc elution occurs once, acidification by zinc ions generated by this zinc elution is suppressed, and zinc ions are insolubilized as zinc hydroxide, basic zinc chloride, etc. Therefore, it is excellent in end surface corrosion resistance.

  Moreover, the precoated steel sheet according to the present invention can be obtained without using a chromium compound, and does not contain hexavalent chromium. Therefore, there is no environmental problem as in the case of containing hexavalent chromium. Since it can be obtained without using fluoride, there is no problem of danger during processing operations and influence on the environment as in the case of using fluoride. Since it can be easily manufactured without restrictions on equipment, it is easy to implement. Furthermore, it is not essential to contain a rust inhibitor, and even when a rust inhibitor is added, there is no need to increase the amount of the rust inhibitor added. Does not cause deterioration of film adhesion and appearance. The application range is not limited because of post-processing as in the method described in JP-A-2003-003280.

  The basic substance is an oxide or hydroxide of an element of Group IA (Group 1A), Group IIA (Group 2A), Group IIIA (Group 3A). In the present invention, the basic substance used may be any of the above oxides and hydroxides, but the oxides and hydroxides of Group 1A elements have too high solubility, so that the adhesion of the resin deteriorates, Oxides and hydroxides of Group 3A elements are lower in pH when they are dissolved in water and become saturated aqueous solutions than Group 2A element oxides and hydroxides. 2A group element oxides and hydroxides are preferred [fourth invention]. An example of the oxide of the 2A group element is magnesium oxide, and an example of the hydroxide of the 2A group element is calcium hydroxide.

  The content of the basic substance is preferably 3 to 25% by mass [second invention]. When the amount is less than 3% by mass, the effect of improving the end face corrosion resistance is low. When the amount is more than 25% by mass, irregularities are formed on the surface and the appearance is deteriorated.

  The particle size of the basic substance is not particularly limited, and neither a lower limit nor an upper limit is particularly defined, but is preferably 10 μm or less [third invention]. If it is larger than 10 μm, unevenness is observed in the appearance after coating, which is not preferable.

  The composition of the primer layer (primer film) according to the present invention contains a basic substance and a resin, and in the simplest case, the basic substance and the resin, or the basic substance and the resin and unavoidable impurities. However, the present invention is not limited to this. For example, silica may be added as necessary.

  Silica has a rust preventive action. The rust prevention mechanism of silica is that silica elutes in an alkaline environment and reprecipitates in the corroded area to block the corrosive environment. By using it together with a basic substance, effective silica elution can be promoted. it can. Therefore, it is desirable that the primer layer also contains silica [sixth invention]. Silica has a higher rust prevention effect as the particle size is smaller. A particle size of about 2 to 100 nm is a preferred range. The silica content is preferably 1 to 50% by mass [seventh invention]. If the amount is less than 1% by mass, the rust preventive effect of silica is hardly recognized, and if it exceeds 50% by mass, the resin becomes too small and the corrosion resistance decreases.

  The thickness of the primer layer may be appropriately determined as necessary, but is preferably 10 to 20 μm. It is because it is excellent in corrosion resistance and a moldability in it being this range.

  A top coat is performed on the primer layer to form a top layer. The thickness of the top layer is preferably 15 to 35 μm, which is a range excellent in corrosion resistance and moldability. As the component of the top layer, one containing about 20% by mass of titanium oxide in the resin is usually used. The component titanium oxide is a filler (resin saving).

  The pigments in the primer layer (lower layer) and top layer (upper layer) can be analyzed by XRD and EPMA. After specifying the element, the content is measured by fluorescent X-ray analysis or the like. The pigment is a basic substance, or a basic substance and other additives (for example, titanium oxide). When the primer layer and the top layer are formed, the measurement can be performed by the above method. However, as the preparation of the analysis sample, it is necessary to peel off or collect the coating film with a blade or the like or to prepare a cross-sectional sample.

  Measurement of the pH of a saturated aqueous solution of a basic substance is carried out by saturating the basic substance with water to produce a saturated aqueous solution of the basic substance, and measuring the pH of this aqueous solution. At this time, the amount of water is not limited, for example, about 50 ml.

  The primer layer according to the present invention can be obtained by mixing and dispersing a basic substance in a solvent-based primer (undercoat), coating and drying. A base treatment film is usually formed between the primer layer (film) and the galvanized steel sheet. As the base treatment agent for forming this base treatment film, silica-based (silica, silane coupling agent, etc.) Commercial processing agents can be used. The undercoat film is preferably about 0.5 μm. Since the base treatment film is intended for adhesion, the effect is saturated even if it is thickened.

  As the solvent-based primer, water-based resins, polyester-based resins, and polyethylene-based resins can be used, but polyester-based resins are preferable. This is because, in the case of the water-based resin, there is a possibility that the basic substance is too dissolved and the coating film is peeled off. In the case of using this water-based resin, it is preferable to use a basic substance that is hardly soluble when applied and dried.

A galvanized steel sheet, after the surface treatment, by the pH of the saturated aqueous solution was applied to a resin containing a basic substance dissolution amount is less than 100g for 20 ° C. water 1kg with at 10.5 or more drying, further overcoat When the coating is performed , a precoated steel sheet according to the present invention having a base treatment film is obtained. Therefore, there is no environmental problem as in the case of containing hexavalent chromium, and a precoated steel sheet having excellent end face corrosion resistance and excellent coating film adhesion can be obtained. When the basic substance is one or more of Group IIA oxides and Group IIA hydroxides having a saturated aqueous solution having a pH of 10.5 or more and a dissolved amount in 1 kg of water at 20 ° C. of 50 g or less, it is obtained. precoated steel sheet to be the that Do excellent in end face corrosion resistance at a higher level.

  Examples of the present invention and comparative examples will be described below. The present invention is not limited to this embodiment, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which are within the technical scope of the present invention. include.

Plating adhesion amount per side: A hot-dip plated steel sheet (plate thickness: 0.6 mm) on which a hot-dip Zn plating layer of 60 g / m ² was formed was cut into a size of 130 mm × 60 mm to expose the base steel on the cut end face.

  The cut galvanized steel sheet (thickness: 0.6 mm) was degreased. In this degreasing treatment, FC-364S manufactured by Nihon Parkerizing Co., Ltd. was used as a degreasing agent, and after being immersed in a degreasing agent-containing solution having a concentration of 3% by weight and a temperature of 60 ° C. for 15 seconds, this was washed with water and dried. Was degreased.

After applying the surface treatment agent (aqueous polyolefin: 100 parts + colloidal silica ST-XS (manufactured by Nissan Chemical): 30 parts + silane coupling agent: 10 parts) to the surface of the defatted galvanized steel sheet Then, this was dried in a hot air drying furnace to form a base treatment film having an adhesion amount of 200 mg / m ² . The ultimate plate temperature during drying was 150 ° C.

  Next, the anticorrosive pigment of P641 primer paint (resin: polyester resin) manufactured by Nippon Paint as an undercoat is added to the surface of the galvanized steel sheet (the surface of the surface treatment film) on which the surface treatment film is formed as shown in Table 1. What was changed into the agent was applied with a bar coater, and baked in a hot air drying oven under the condition that the maximum temperature reached 210 ° C. to form a primer layer (undercoating). At this time, the thickness of the primer layer was adjusted to 15 μm.

  Furthermore, on the primer layer (undercoating), as an overcoating, Nippon Paint FL100HQ (polyester resin) is applied with a bar coater and baked in a hot air drying oven at a final plate temperature of 210 ° C. A layer (top coat) was formed. At this time, the thickness of the top layer was adjusted to 15 μm. Next, the top layer formed was cut into 120 mm × 50 mm to expose the base steel on the cut end face.

  As described above, a precoated steel sheet was obtained in which a base treatment film was formed on the surface of the galvanized steel sheet (excluding the cut end face), and a coating layer composed of a primer layer (lower layer) and a top layer (upper layer) was formed on the surface. In addition, although the coating film which consists of this primer layer and top layer is formed and exists on the surface (except for a cutting end surface) of a galvanized steel plate, it is not formed on the cutting end surface and does not exist.

  The precoated steel sheet thus obtained was subjected to film component analysis, film thickness measurement for film thickness confirmation, and end face corrosion resistance test. Further, for the same pigment as the pigment (basic substance or the like) used in the production of the precoated steel sheet, the pH of the saturated aqueous solution of the pigment and the particle size of the pigment were measured. These analyzes, tests, and measurements were performed by the following methods.

  <Analysis of membrane components> Elements in the membrane were analyzed by XRD and EPMA. At this time, XRD was measured from the coating surface, and EPMA was measured from the coating cross section. After identifying the elements in the film by this analysis, the content was measured by fluorescent X-ray analysis. At this time, fluorescent X-ray analysis was measured from the surface.

  <Measurement of pH of Saturated Aqueous Pigment> The pigment was saturated to about 50 ml of water to prepare a saturated aqueous solution of the pigment, and the pH of the aqueous solution was measured.

  <Measurement of particle diameter of pigment> SEM (scanning electron microscope) observation of the cross section of the coating film was performed.

  <Measurement of film thickness> The film thickness was measured by SEM observation of the cross section of the coating film.

  <End surface corrosion resistance test> The salt spray test (temperature 35 ° C., 5% NaCl solution) defined in JIS-K-5400, 7.8 is carried out for 400 hours, and the case where the surface white rust generation area is less than 10% The case of 10% or more and less than 40% was evaluated according to the criteria of ◯ (good), the case of 40% or more and less than 70% was evaluated as △ (possible), and the case of 70% or more was evaluated based on × (defective). In addition, (double-circle), (circle), and (triangle | delta) was set as the pass, and x was set as the rejection (end surface corrosion resistance is inadequate).

  Table 1 shows the test results. In the case of Nos. 21 to 22 (comparative examples), the pigment (basic substance) contained in the primer layer has a low pH of the saturated aqueous solution, so that the rust prevention effect is insufficient. That is, although this basic substance has a characteristic that the dissolved amount in 1 kg of water at 20 ° C. is 100 g or less, the pH of the saturated aqueous solution is 10.2 and not 10.5 or more, so the rust prevention effect is insufficient. And end surface corrosion resistance is x (defect).

  In the case of No. 23 (Comparative Example), the pigment (basic substance) contained in the primer layer has an excessively high solubility, so that the rust prevention effect is insufficient. That is, this basic substance has a characteristic that the pH of a saturated aqueous solution is 10.5 or more, but the dissolution amount in 1 kg of water at 20 ° C. is 1090, and it is not 100 g or less. And end surface corrosion resistance is x (defect).

  On the other hand, in the case of No. 1 to 20 (examples of the present invention), the basic substance contained in the primer layer has a low pH and a low solubility of the saturated aqueous solution, so that the rust prevention effect is sufficient. It is in. In other words, this basic substance has the characteristics that the saturated aqueous solution has a pH of 10.5 or more and the dissolution amount in 1 kg of water at 20 ° C. is 100 g or less. Corrosion resistance is ◎ (excellent or extremely good), ○ (good) or △ (good). Most of these are ◎ (excellent or very good) or ◯ (good).

  However, in the case of No. 18, since the content of the basic substance was as high as 40% by mass, the appearance was uneven. In the case of No. 19, since the particle size of the basic substance contained was as large as 20 μm, the appearance was uneven. In the case of No. 20, since the content of the basic substance was as low as 1% by mass, the corrosion resistance of the end face was slightly lowered and it was Δ (good).

Since the precoated steel sheet according to the present invention has no environmental problems as in the case of containing hexavalent chromium and has excellent end face corrosion resistance, it contains hexavalent chromium as a material (precoated steel sheet) for applications that require corrosion resistance. There is no environmental problem as in the case of doing so, and it can be suitably used.

Claims (7)

A pre-coated steel sheet having a coating film composed of a lower primer layer and an upper top layer on the surface of the galvanized steel sheet , wherein the end surface has no coating film and iron and zinc-based plating are exposed , A precoated steel sheet having excellent end face corrosion resistance, wherein the primer layer is composed of a resin layer having a saturated aqueous solution having a pH of 10.5 or more and a basic substance having a solubility in 1 kg of water at 20 ° C of 100 g or less.   The precoated steel sheet excellent in end face corrosion resistance according to claim 1, wherein the content of the basic substance is 3 to 25% by mass.   The precoated steel sheet having excellent end face corrosion resistance according to claim 1 or 2, wherein the basic substance has a particle size of 10 µm or less.   The precoated steel sheet having excellent end face corrosion resistance according to any one of claims 1 to 3, wherein the basic substance is at least one selected from Group IIA oxides and Group IIA hydroxides.   The precoated steel sheet excellent in end face corrosion resistance according to claim 4, wherein the Group IIA oxide and Group IIA hydroxide are magnesium oxide and calcium hydroxide, respectively.   The precoated steel sheet having excellent end face corrosion resistance according to any one of claims 1 to 5, wherein the primer layer contains silica.   The precoated steel sheet having excellent end surface corrosion resistance according to claim 6, wherein the silica content is 1 to 50 mass%.