JP2005126812A - Galvanized steel sheet superior in corrosion resistance, coating applicability and adhesiveness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvanized steel sheet superior in corrosion resistance and coating applicability, by combining a phosphate film with a Cr-free chemical conversion coating as a substitute for chromate sealing hazardous to the environment. <P>SOLUTION: This galvanized steel sheet employs a galvanized steel sheet or a zinc-alloy-plated steel sheet as a substrate, and has the phosphate film and the Cr-free chemical conversion coating formed on the surface of the substrate. In the Cr-free chemical conversion coating, there coexist an oxide or hydroxide of a valve metal and a fluoride of a valve metal. The valve metal includes Ti, Zr, Hf, V, Nb, Ta, Mo and W. The phosphate film is preferably formed so as to cover 50% or more by an area rate of the substrate. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a zinc-based plated steel sheet that does not contain hexavalent Cr that is harmful to the environment, exhibits excellent corrosion resistance, paintability, and adhesion, and is useful as an exterior material, interior material, surface material, vehicle steel sheet, and the like.

  When galvanized steel sheets and zinc alloy plated steel sheets (hereinafter collectively referred to as zinc-based plated steel sheets) are used in a sea salt particle scattering atmosphere or a high-temperature and high-humidity atmosphere, white rust that causes appearance deterioration occurs on the steel sheet surface. The sacrificial anticorrosive action of the layer is also impaired. Generation of white rust can be prevented by chromate treatment, phosphate treatment, and the like. However, in chromate treatment, elution of hexavalent Cr from the formed film is inevitable. Even in the phosphate treatment, the chromate treatment is usually required after the phosphate treatment, so the problem of hexavalent Cr elution is unsolved.

In order to form a chemical conversion film that does not contain hexavalent Cr, which is harmful to the environment, improvement of phosphate treatment has been studied. For example, Patent Document 1 introduces post-treatment of a phosphate film with a treatment liquid containing a hydrazine derivative, silica fine particles, and an acid having an etching action on the metal surface. Patent Document 2 introduces the provision of an organic resin film containing a thiocarbonyl group-containing compound on a zinc-based plated steel sheet via a phosphate treatment film.
JP 2001-207271 A JP 2000-248367 A

  In conventional phosphate coatings, the inorganic phosphate, which is a component of the coating, is hard and not ductile. Therefore, when a zinc-plated steel sheet after chemical conversion treatment is formed, film defects such as cracks and delamination occur in the phosphate coating. Is easy to introduce. The film defect becomes a starting point of corrosion occurrence, and the corrosion resistance improving action of the chemical conversion film is extremely lowered. In order to suppress the occurrence of corrosion due to film defects, even if the phosphate film is sealed with an organic resin containing a thiocarbonyl group-containing compound, sufficient post-coating corrosion resistance cannot be obtained.

  The present invention pays attention to the fact that a Cr-free chemical conversion film coexisting a valve metal oxide or hydroxide and a valve metal fluoride exhibits a self-healing action, and seals a phosphate film with a Cr-free chemical conversion film. Accordingly, an object of the present invention is to provide a zinc-based plated steel sheet in which defects in a phosphate film are repaired with a Cr-free chemical film even after being processed into a product shape, and exhibit excellent corrosion resistance, paintability, and adhesion.

  When galvanized steel sheets are subjected to phosphate treatment, a phosphate film is formed on the surface of the substrate. The phosphate film consists of a large number of precipitated crystals rising from the surface of the substrate. Usually exposed. In the present invention, the exposed surface of the base material is covered with a chemical conversion film in which valve metal oxide or hydroxide and valve metal fluoride coexist by Cr-free chemical conversion treatment after phosphate treatment. The phosphate film is preferably formed on the zinc-based plating layer with a coverage of 50 area% or more, and the chemical conversion film is formed with a film thickness of 10% or less of the protruding height of the phosphate. The valve metal is a metal element whose oxide exhibits high insulation resistance, and examples thereof include Ti, Zr, Hf, V, Nb, Ta, Mo, and W.

On the surface of the zinc-based plating layer 1 subjected to the phosphate treatment, a phosphate film 3 composed of a large number of phosphate crystals 2 is formed (FIG. 1a). The phosphate crystal 2 grows along the direction rising from the surface of the zinc-based plating layer 1 and has a sharp tip. The zinc-based plating layer 1 is exposed between the adjacent phosphate crystals 2.
When the zinc-based plating layer 1 after the formation of the phosphate coating 3 was treated with a Cr-free chemical conversion treatment liquid containing a valve metal fluoride, a part of the phosphate crystal 2 was dissolved and dissolved by the etching action of the treatment liquid. The phosphate component reacts with the exposed surface of the zinc-based plating layer 1 together with the treatment liquid component, and a Cr-free chemical conversion film 4 is generated. Since the exposed surface of the zinc-based plated layer 1 is covered with the Cr-free chemical conversion film 4, the zinc-based plated steel sheet is shielded from the environment by the phosphate film 3 and the Cr-free chemical conversion film 4. The granular reaction product 5 is also deposited on the surface of the phosphate crystal 2 rising from the zinc-based plating layer 1 (FIG. 1b).

  In the Cr-free chemical conversion film 4, valve metal oxide or hydroxide and valve metal fluoride coexist. The oxide or hydroxide of the valve metal exhibits an excellent environmental barrier ability and protects the zinc-based plating layer 1 from a corrosive atmosphere. Valve metal fluoride dissolves when exposed to corrosive atmospheres and self-repairs defects in phosphate coating 3 and chemical conversion coating 4 in the process of reprecipitation as a sparingly soluble oxide or hydroxide. To do.

  Since the Cr-free chemical conversion film 4 is formed on the surface of the zinc-based plating layer 1 exposed between the adjacent phosphate crystals 2, it grows in comparison with the chemical conversion film formed on the untreated zinc-based plating layer 1. Is promoted, and the ability to shut off the environment and the self-healing action are significantly improved. Although the environmental barrier ability and self-healing action are observed in the chemical conversion film 4 having a film thickness of 0.001 μm or more, if the chemical conversion film 4 grows too thick, the effect of the phosphate crystal 2 on the adhesion of the paint or adhesive is impaired. Therefore, it is preferable to control the film thickness of the chemical conversion film 4 to 10% or less of the average height of the phosphate crystal 2 rising from the surface of the zinc-based plating layer 1. The film thickness of the chemical conversion film 4 can be controlled by the concentration of the Cr-free chemical conversion treatment liquid, the processing time, and the like. Since the protrusion height of the phosphate crystal 2 ranges from a fine size of 1 to 2 μm to a normal size of 5 to 20 μm, the film thickness of the chemical conversion film 4 is selected according to the protrusion height.

  Although the chemical conversion film 4 is an inorganic and non-ductive film, it is divided by the phosphate crystal 2, so that a decrease in adhesion to the zinc-based plating layer 1 is also reduced. The granular reaction product 5 deposited on the surface of the phosphate crystal 2 is also a fluoride supply source exhibiting a self-repairing action. Furthermore, since the sharp corners of the hard and non-ductile phosphate crystal 2 disappear due to the etching action of the Cr-free chemical conversion treatment liquid, cracks and peeling are less likely to occur in the phosphate coating 3 during processing such as press molding. . Therefore, the function of the phosphate film 3 effective for improving the adhesion of the coating film formed on the chemical conversion treated steel sheet and the adhesion when laminating the film is maintained.

  As the original plate subjected to the chemical conversion treatment, a zinc-based plated steel plate manufactured by an electroplating method, a hot dipping method, or a vapor deposition method is used. Examples of zinc alloy plating include Zn—Al, Zn—Mg, Zn—Ni, and Zn—Al—Mg. An alloyed galvanized steel sheet that has been subjected to alloying treatment after hot dipping can also be used as a raw sheet for chemical conversion treatment.

  As the phosphating solution, an aqueous solution to which metal ions such as Zn, Mn, Mg, Ca, Ni, Co and the like are added in addition to phosphate ions as required is used. Nitrate ions may be included in the phosphate treatment solution as a treatment accelerator. In the phosphating solution, it is preferable to adjust the phosphate ion concentration to a range of 0.03 to 0.5 mol / l and the metal ion concentration to a range of 0.01 to 0.5 mol / l. When nitrate ions are included, the nitrate ion concentration is adjusted to a range of 0.01 to 1.0 mol / l.

  When the phosphate ion concentration is less than 0.03 mol / l, phosphate crystals are not sufficiently precipitated in a short time treatment. Conversely, when the concentration exceeds 0.5 mol / l, the stability of the phosphate treatment solution decreases. Sludge is likely to occur. Metal ion concentration: 0.01 to 0.5 mol / l is a total value of various metal ions. If it is less than 0.01 mol / l, phosphate crystals cannot be sufficiently precipitated in a short time treatment. On the other hand, if it exceeds 0.5 mol / l, the stability of the phosphating solution is lowered. The reaction promoting effect by nitrate ions is observed at 0.01 mol / l or more, but if an excessive amount of nitrate ions exceeding 1.0 mol / l is included, the surface of the zinc-based plating layer is inactivated by oxidation action, On the contrary, the reactivity decreases.

  When phosphating a zinc-based plated steel sheet on which a plating layer containing Al is formed, there is a tendency for Al eluted from the plating layer to inhibit the phosphate reaction. Adverse effects can be suppressed by adding fluoride to the phosphating solution. Fluoride includes sodium fluoride, potassium fluoride, sodium hydrogen fluoride and the like, and the effect of adding fluoride becomes remarkable when the free fluorine ion concentration is 30 ppm or more. In order to enable continuous operation, it is preferable that a certain amount of fluoride is continuously added to the phosphating solution to maintain the fluorine ion concentration at 30 ppm or more.

  Phosphate treatment is preferably carried out at a liquid temperature in the range of 40 to 80 ° C. When the liquid temperature does not reach 40 ° C., phosphate crystals are not sufficiently precipitated by short-time treatment. On the other hand, when the liquid temperature exceeds 80 ° C., the stability of the phosphating solution decreases, and sludge generation and water evaporation increase, making it difficult to manage the concentration in continuous operation. As long as a phosphating solution having a liquid temperature of 40 to 80 ° C. is used, the necessary phosphate film is formed in about 2 to 6 seconds for spraying and about 3 to 9 seconds for immersion. The Even if the treatment time is set long, there is no problem because the precipitation of phosphate becomes saturated and the appearance does not change.

The phosphate crystal 2 grows from the precipitation starting point on the surface of the zinc-based plating layer 1, and the precipitation reaction stops when the entire surface is covered with the phosphate crystal 2 and reaches a saturated state. The coverage and adhesion amount of the phosphate coating 3 can be controlled by adjusting the contact time with the phosphating solution and the growth rate of the crystal, and can also be controlled by increasing the precipitation starting point and thus adjusting the crystal size. For example, when the contact time is shortened or the treatment liquid temperature is lowered and the crystal growth rate is slowed even at the same contact time, the coverage rate and the adhesion amount of the phosphate coating 3 are reduced. Moreover, since the precipitation starting point can be increased by increasing the concentration of the treatment solution used for the surface conditioning treatment prior to the phosphate treatment or by raising the temperature of the phosphate treatment solution, the phosphate crystal 2 becomes finer, resulting in phosphoric acid. The adhesion amount of the salt film 3 is reduced.
The phosphate film 3 is preferably formed with a coverage of 50 area% or more with respect to the surface of the zinc-based plating layer 1. The coverage of the phosphate film 3 is affected by the film thickness and the size of the phosphate crystal 2, but by providing the phosphate film 3 at 50 area% or more, sufficient coating film adhesion and post-coating corrosion resistance can be obtained. can get.

After forming the phosphate coating 3, the galvanized steel sheet is treated with a Cr-free chemical conversion treatment solution. The Cr-free chemical conversion treatment solution itself can use the treatment solution introduced by the present inventors in Patent No. 3305703, and contains a soluble halide or oxyacid salt as a source of valve metal. The valve metal includes one or more metals selected from Ti, Zr, Hf, V, Nb, Ta, Mo, and W. For example, although a fluoride of Ti is effective as a Ti source and an F source, a soluble fluoride such as (NH ₄ ) F may be added separately to the Cr-free chemical conversion treatment solution. The same applies to valve metals other than Ti, but Ti will be described below as an example.

The Ti source, K _n TiF ₆ (K: an alkali metal or alkaline earth metal, n: 1 or _{2), K 2 [TiO (} COO) 2], (NH 4) 2 TiF 6, TiCl 4, TiOSO 4 , Ti (SO ₄ ) ₂ , Ti (OH) _{4 and the} like. In the Ti source, the blending ratio of each component is selected so that a chemical conversion film composed of oxide or hydroxide and fluoride having a predetermined composition is formed when the chemical conversion treatment liquid is applied and then dried and baked.

  In order to stably maintain the Ti source as ions in the chemical conversion solution, it is preferable to add an organic acid having a chelating action. In the case of adding an organic acid, metal ions are chelated to stabilize the chemical conversion solution, so that the organic acid / metal ion molar ratio is set to 0.02 or more. Examples of the organic acid include tartaric acid, tannic acid, citric acid, succinic acid, malonic acid, lactic acid, acetic acid and the like. Among them, polyphenols such as tartaric acid and other oxycarboxylic acids and tannic acid stabilize the treatment liquid and also complement the self-healing action of fluoride, which is also effective in improving coating film adhesion. It is.

In order to include a soluble or poorly soluble metal phosphate or composite phosphate as an optional component in the chemical conversion film, orthophosphates and polyphosphates of various metals may be added.
Soluble metal phosphate or composite phosphate elutes from the chemical conversion film, elutes to the film defect, reacts with the zinc-based plating layer 1 and precipitates insoluble phosphate, thereby self-repairing action of titanium fluoride To complement. Further, since the atmosphere is slightly acidified when the soluble phosphate is dissociated, the hydrolysis of titanium fluoride, and hence the generation of hardly soluble titanium oxide or hydroxide, is promoted. Metals that produce soluble phosphates or composite phosphates include alkali metals, alkaline earth metals, Mn, etc., various metal phosphates or various metal salts and chemical conversion treatment as phosphoric acid, polyphosphoric acid, phosphate Added to the liquid.

The hardly soluble metal phosphate or composite phosphate is dispersed in the chemical conversion film to eliminate film defects and improve the film strength. There are Al, Ti, Zr, Hf, Zn, etc. as metals that form poorly soluble phosphates or composite phosphates. Various metal phosphates or various metal salts and phosphoric acid, polyphosphoric acid, and phosphate Added to the treatment solution.
Of the zinc alloy plated steel sheets, plated steel sheets with an Al-containing plating layer are prone to black discoloration. In this case, one or more metal salts selected from Fe, Co, and Ni are coated. It is possible to prevent black discoloration by making it exist in the surface. Further, when a large crack is generated in the plating layer due to strict processing or the like, the self-repairing action of fluoride and phosphate may be insufficient. In this case, the presence of a large amount of Mo, W soluble hexavalent oxyacid salt in the film causes the same action as hexavalent chromium to repair cracks in the plating layer and improve the corrosion resistance.

When a phosphate treatment liquid to which oxides, hydroxides, phosphates, fluorides, carbonates, organic acid salts such as Zn, Mn, Mg, and Ca are added is used, Zn, Mn, Mg is added from the treatment liquid. , Ca and the like are incorporated into the phosphate crystal 2. Whereas the phosphate crystal 2 formed with the phosphate treatment liquid not containing metal components such as Mn, Mg, Ca is zinc phosphate alone [Zn ₃ (PO ₄ ) · 4H ₂ O]. The phosphate crystal 2 incorporating Mn, Mg, Ca, etc. becomes a composite phosphate having excellent adhesion and water resistance to the zinc-based plating layer 1, and forms a phosphate film 3 effective in improving corrosion resistance. .

As described above, based on treatment liquid containing Ti source compound consisting of halide and oxyacid salt, organic acid with ion stabilizing action and self-healing action of titanium fluoride are complemented as needed to improve corrosion resistance The phosphate which has the effect | action which makes it mix | blend.
When the prepared treatment liquid is applied to a phosphate-treated zinc-based plated steel sheet, a zinc-based coating in which a Cr-free chemical conversion film 4 reacted with fluorine ions and zinc-based plating layer 1 or Ti is present between phosphate crystals 2 Preferentially formed on the exposed surface of the plating layer 1. The Cr-free chemical conversion film 4 is a film containing a sufficient amount of titanium fluoride having a self-repairing effect because growth is promoted as compared with the case where it deposits on the zinc-based plating layer 1 without the phosphate crystal 2.
The film thickness of the chemical conversion film 4 can be controlled by the concentration of the chemical conversion solution. Moreover, when apply | coating by a roll coat system, the film thickness of the interface reaction layer 4 can also be controlled by adjusting the wet amount of a roll surface with the roughness of a roll surface, hardness, the peripheral speed at the time of application | coating, etc.
Since the produced Cr-free chemical conversion film 4 is divided by the phosphate crystal 2, it exhibits an action of dispersing stress applied during processing or the like, and adhesion to the zinc-based plating layer 1 does not decrease. Moreover, since most of the phosphate crystals 2 protruding from the zinc-based plating layer 1 are not covered with the chemical conversion film 4, the adhesion of the coating film or adhesive layer provided thereon, the corrosion resistance after coating, etc. are remarkably high. To improve.

Zinc-based plated steel sheet coated with zinc-based plating layer 1 with phosphate crystal 2 and Cr-free conversion coating 4 has a high environmental barrier because the coating defects are self-repaired with Cr-free conversion coating 4, and the coating adheres well. And post-painting corrosion resistance are significantly improved.
The coating amount of the Cr-free chemical conversion treatment liquid is preferably adjusted so that the amount of titanium adhered is 0.1 mg / m ² or more in order to ensure sufficient corrosion resistance.
When the formed chemical conversion film is subjected to elemental analysis by fluorescent X-ray, ESCA, etc., the O and F concentrations contained in the chemical conversion film are measured. When the relationship between the concentration ratio F / O (atomic ratio) calculated from the measured values and the corrosion resistance was investigated, the occurrence of corrosion starting from the film defects was significantly observed at a concentration ratio F / O (atomic ratio) of 1/100 or more. Decreased. This is presumably due to the fact that a sufficient amount of titanium fluoride having a self-repairing action is contained in the chemical conversion film.

The Cr-free chemical conversion film 4 preferably has a thickness of about 5 to 300 nm. The Cr-free chemical conversion film 4 exhibits a sufficient environmental barrier ability at a film thickness of 5 nm or more and a good anticorrosion ability at a film thickness of 10 nm or more. However, when it grows to a thick film exceeding 300 nm, cracks are likely to occur due to stress applied during molding. On the contrary, the corrosion resistance is lowered. The thickness of the Cr-free chemical conversion film 4 can be measured by elemental analysis in the depth direction by AES, TEM observation, or the like.
Although the Cr-free chemical conversion treatment solution applied to the zinc-based plated steel sheet can be dried at room temperature, it is preferable to keep the temperature at 80 ° C. or higher to shorten the drying time in consideration of continuous operation. However, at a drying temperature exceeding 300 ° C., when the Cr-free chemical conversion film 4 is a thick film, cohesive failure may occur and the corrosion resistance may be impaired.

  After forming the chemical conversion film, an organic film having further excellent corrosion resistance can be formed. As this type of film, for example, urethane resin, epoxy resin, polyethylene, polypropylene, olefin resin such as ethylene-acrylic acid copolymer, styrene resin such as polystyrene, polyester, or a copolymer or modified product thereof, When a resin film such as an acrylic resin is provided on the chemical film with a film thickness of 0.1 to 5 μm, high corrosion resistance surpassing that of the chromate film can be obtained. Alternatively, by providing a resin film having excellent conductivity on the chemical conversion film, lubricity is improved and weldability is also imparted. This type of resin film can be formed by, for example, applying an organic resin emulsion by electrostatic atomization.

Two types of galvanized steel sheets A and B were prepared as original sheets.
(A) Hot-dip galvanized steel sheet having a thickness of 0.8 mm on which an alloy plating layer of Zn-6 mass% Al-3 mass% Mg is formed at a coating adhesion amount of 60 g / m ² per side.
(B) An electroplated steel sheet having a thickness of 0.8 mm on which a Zn plating layer is formed with a coating weight of 20 g / m ² per side.

  Phosphate treatment is carried out by applying a Cr-free chemical conversion treatment solution (Table 2) after drying each plated steel sheet using a phosphate treatment solution (Table 1) mainly composed of Zn-Mn. The film was sealed with a Cr-free chemical film. Table 3 shows the combinations of phosphate treatment and Cr-free chemical conversion treatment, and Table 4 shows the phosphate coating and Cr-free chemical conversion coating formed on the surface of the galvanized steel sheet.

Test pieces were cut out from the zinc-based plated steel sheet after the Cr-free treatment and subjected to various corrosion tests.
[Flat corrosion test]
The end face of the test piece was sealed, and a 5% NaCl aqueous solution at 35 ° C. was sprayed on the test piece surface in accordance with JIS Z2371. After spraying salt water for 72 hours or 240 hours, the surface of the test piece was observed to investigate the occurrence of white rust. The area occupancy ratio of white rust on the surface of the test piece is less than 5 area%, ◎, 5-10 area% is ◯, 10-30 area% is △, 30-50 area% is ▲, and 50 area% or more is x. The flat part corrosion resistance was evaluated.

[Processed part corrosion test]
The test piece subjected to 180-degree bending work that partially damaged the phosphate coating was subjected to the same salt spray as the flat part corrosion test for 24 hours and 120 hours, and the area occupancy rate of white rust generated in the processed part Asked. The area occupancy rate was evaluated as corrosion resistance of a processed part, where を is less than 5%, ５〜 is 5 to 10%, △ is 10 to 30%, ▲ is 30 to 50% and x is 50% or more.

[Corrosion test after painting]
The galvanized steel sheet subjected to chemical conversion treatment was coated with melamine alkyd to form a coating film having a thickness of 30 μm. After the crosscut was put into the coating film and subjected to salt water spraying for 1000 hours, a tape peeling test was performed in which an adhesive tape was applied to the crosscut portion and peeled off instantaneously, and the width at which the coating film was peeled was measured as the corrosion width. Corrosion resistance after coating was evaluated with ◎ when the corrosion width was less than 2 mm, ◯ when it was less than 5 mm, Δ when it was less than 10 mm, and x when the corrosion width was more than 10 mm.

  As can be seen from the corrosion test results in Table 5, the zinc-based plated steel sheet subjected to the Cr-free conversion treatment after the phosphate treatment exhibited excellent properties in all of the flat portion corrosion resistance, the processed portion corrosion resistance, and the post-painting corrosion resistance. In particular, in the tests Nos. 2A to 4B in which a high-coverage phosphate film and a Cr-free chemical film are combined, the corrosion resistance of the processed part is remarkably improved, and the self-repairing action of the Cr-free chemical film is exhibited. It is redeemed that

  On the other hand, the test Nos. 7A and 7B in which the Cr-free chemical conversion film 4 is provided without phosphating and the tests No. 8A and 8B that do not contain Ti (valve metal) are inferior in flat portion corrosion resistance and post-coating corrosion resistance. The longer the salt spray, the more the white rust was generated in the flat and processed parts. Even in Test Nos. 9A and 9B in which a Cr-free chemical conversion film containing no valve metal fluoride was formed, a large amount of white rust was generated on the flat portion when salt spray was applied for a long time. In the tests Nos. 10A and 10B, which have been subjected to the conventional chemical conversion treatment that seals the phosphate treatment with chromate treatment, white rust is generated in the processed part even with a short salt spray, and the flat part corrosion resistance and post-coating corrosion resistance are also sufficient. It was not.

The synergistic action of the phosphate coating and the Cr-free chemical conversion coating is also apparent from FIG. 2 which shows that the area occupancy of white rust changes according to the salt spray time. FIG. 2 is a graph showing the corrosion resistance after using various types of chemical conversion treatment using electrogalvanized steel sheet B as the original plate, but a large amount of white rust is generated early in the phosphate treatment, and Cr-free chemical conversion treatment is performed. In the combined treatment of phosphate, chromate and chromate, white rust increased as the salt spray time increased. On the other hand, when the Cr-free chemical conversion treatment was performed after the phosphate treatment, the generation of white rust was suppressed even after 300 hours of salt water spraying.
Furthermore, even when the PET film was laminated using an epoxy adhesive, it was confirmed that the peel strength of the bonded PET film was high and the phosphate film was maintained in a healthy state.

  The zinc-based plated steel sheet A of Example 1 was used as a base plate, and was subjected to phosphate treatment with the No. 1 treatment solution shown in Table 1. In this example, phosphate crystals whose average height was changed in the range of 1 to 15 μm by adjusting the surface adjustment prior to the phosphate treatment, the liquid temperature during the phosphate treatment, the treatment time, etc. Was generated. In the chromium-free treatment, a treatment solution of No. 2 was used, and a chromium-free chemical conversion film having a changed film thickness was formed on the phosphate film by changing the dilution rate, roll coating conditions, and the like.

  A test piece was cut out from the produced chemical conversion treated steel sheet, and the film adhesion was investigated by the following test. After laminating a PET film on a test piece using an epoxy adhesive, the peel strength was measured by a tensile test in which the tip of the film was forcibly peeled after a draw bead sliding deformation test with a load of 200 kgf. The test results are shown in FIG. 3, and it is observed that the peel strength tends to decrease as the chemical conversion film becomes thicker at any phosphate crystal height, and the average height of the standing phosphate crystals is When the chemical conversion film was formed with a thickness of 10% or less, good peel strength could be maintained.

  As described above, when sealing a phosphate film with a Cr-free conversion film having a self-repairing effect, a conversion film is formed on the surface of the exposed galvanized layer between the phosphate crystals, A granular reaction product is also deposited on the surface of the acid crystal. The chemical conversion film formed between the phosphate crystals grows thicker than the Cr-free chemical conversion film formed on the surface not subjected to the phosphate treatment. Therefore, a sufficient amount of valve metal fluoride necessary for self-repairing film defects that occur during press working, etc. is secured, and a chemically treated galvanized steel sheet that exhibits excellent corrosion resistance, paintability, and adhesion over a long period of time. And is used as an exterior material, interior material, cover material, vehicle steel plate, etc. that have little impact on the environment.

Schematic diagram explaining the process of forming a surface chemical conversion coating on galvanized steel sheet Graph showing that the generation of white rust can be greatly suppressed by the combination of phosphate treatment and Cr-free chemical conversion treatment A graph showing the effect of the ratio between the height of phosphate crystals and the film thickness of the conversion coating on adhesion

Explanation of symbols

1: Zinc-based plating layer 2: Phosphate crystal 3: Phosphate film 4: Chemical conversion film 5: Granular reaction product

Claims (5)

  Zinc-plated steel sheet or zinc alloy-plated steel sheet is used as the base material, and the zinc-based plating layer exposed between the deposited crystals of the phosphate film formed on the surface of the base material is covered with a chemical conversion film, and oxidation of valve metal is performed on the chemical conversion film. Zinc-based galvanized steel sheet with excellent corrosion resistance, paintability and adhesiveness, characterized by the presence of metal or hydroxide and valve metal fluoride.   The galvanized steel sheet according to claim 1, wherein 50% by area or more of the substrate surface is coated with a phosphate film.   The galvanized steel sheet according to claim 1 or 2, wherein one or more metals selected from Zn, Mn, Mg, Ca, Ni and Co are contained in the precipitated crystals of the phosphate coating.   The galvanized steel sheet according to claim 1, wherein one or more metals selected from Ti, Zr, Hf, V, Nb, Ta, Mo, and W are used for the valve metal.   The zinc-based plated steel sheet according to claim 1, wherein the chemical conversion film is formed with a film thickness of 10% or less with respect to the average height of the phosphate crystals rising from the zinc-based plating layer.

Images