JP3927167B2 - Molten Mg-containing zinc alloy plated steel sheet with excellent corrosion resistance and blackening resistance - Google Patents

Description

  The present invention relates to a molten Mg-containing zinc alloy-plated steel sheet that maintains a beautiful surface state for a long period of time and is used for an exterior material, an interior material, a surface material, a steel sheet for vehicles, and the like.

  Galvanized steel sheets are frequently used as steel materials having good corrosion resistance, but the steel sheet surface may turn black when exposed to a humid atmosphere, exhaust gas atmosphere, atmosphere in the vicinity of the coast, etc. for a long time. In particular, a plated steel sheet on which an alloy plating layer containing an element such as Al or Mg, which is easier to oxidize than Zn, is likely to turn black. A plated steel sheet that has turned black exhibits a surface with a markedly lowered color tone, which also causes harm to the environment. It is believed that the surface of the plating layer is slowly corroded and black discoloration occurs due to the generation of oxygen-deficient amorphous zinc oxide.

In the chromate-treated steel sheet, the discoloration is suppressed by substituting the plated steel sheet with an aqueous solution containing ions of Fe, Ni, Co, etc. prior to the chemical conversion treatment to precipitate Fe, Ni, Co, etc. on the surface of the plating layer ( Patent Document 1). It is also effective in suppressing black discoloration by thermally decomposing a cobalt salt attached to the steel sheet surface from a Co-containing aqueous solution and precipitating cobalt oxide on the plating layer surface (Patent Document 2). Although some chemical conversion treatment liquids containing ions such as Fe, Ni, and Co have been studied, there is a concern that the environmental barrier ability of the chemical conversion treatment film may be reduced due to the addition of a noble metal to Zn to the chemical conversion treatment liquid. Since the addition amount cannot be increased, long-term blackening resistance cannot be expected.
JP 59-177381 A Japanese Patent Publication No.7-96699

  By the way, the chemical conversion treatment tends to be switched from a chromate treatment having a large environmental load to a Cr-free treatment using a chemical solution such as titanium, zirconium, molybdenum, or phosphate. Even if the conventional black change suppression measure is adopted in the Cr-free treatment, the black discoloration cannot be suppressed to the same extent as the chromate-treated steel plate. Insufficient suppression of black discoloration is considered to be caused by the fact that the Cr-free treatment liquid reacts with the plating layer on the surface of the steel sheet and the oxide film is removed from the surface of the plating layer. In particular, when a galvanized steel sheet containing highly reactive Al or Mg is used for the chemical conversion treatment original sheet, the surface of the plated steel sheet is likely to turn black in a short time.

  As a result of investigation and research to eliminate the black discoloration observed in the Cr-free chemical conversion coating, the present invention shows that the coexistence of molybdenum oxyacid salts, amines, and group 4A metal oxyacid salts in the molten Mg-containing zinc alloy plating layer. A surface condition that is beautiful over a long period of time by forming an organic chemical conversion treatment film containing molybdenum oxyacid salt, amines, and group 4A metal oxyacid salt based on the knowledge found to be effective in suppressing black discoloration An object of the present invention is to provide a molten Mg-containing zinc alloy-plated steel sheet in which is maintained.

The present invention is directed to a molten Mg-containing zinc alloy-plated steel sheet containing Mg: 0.1 to 10% by mass. Chemical conversion coatings dispersed in organic resins such as epoxy and olefin are formed on the surface of the zinc alloy plating layer.
Examples of the soluble molybdenum oxyacid salt include hexavalent molybdenum oxyacid salt, ammonium molybdate, and alkali metal molybdate. As amines, aliphatic amines, alicyclic amines, alkanolamines and the like are used. Oxyacid salts such as Ti, Zr, and Hf are used as the oxyacid salt of Group 4A metal, which is another component.

  When a chemical conversion treatment solution containing hexavalent molybdenum oxyacid salt is applied to a hot dip galvanized steel sheet, a low valence (divalent to tetravalent) molybdenum compound is produced by the reaction between the hexavalent molybdenum oxyacid salt and the zinc alloy plating layer. To do. Molybdenum compounds form a chemical conversion coating on the surface of the zinc plating layer, but when amines are present in the reaction system, black discoloration of the zinc alloy plating layer can be suppressed. The black discoloration suppressing action of amines is presumed as follows.

  The amines contained in the chemical conversion treatment film form a complex with a part of the hexavalent molybdenum oxyacid salt and suppress a reduction reaction when the molybdenum oxyacid salt reacts with the zinc alloy plating layer. As a result, a complex oxyacid salt of pentavalent and hexavalent molybdenum (so-called “molybdenum blue”) is generated and taken into the chemical conversion film. Molybdenum blue in the film is reoxidized to hexavalent molybdenum oxyacid salt by oxygen that has permeated through the film. Since permeated oxygen is captured by the oxidation reaction of molybdenum blue → hexavalent molybdenum oxyacid salt, the oxidation of Zn and the generation of oxygen-deficient amorphous zinc oxide are reduced, and the blackening resistance is improved. The permeation of permeated oxygen is also effective for improving the corrosion resistance.

  Hexavalent molybdenum oxyacid salt also dissolves in moisture that has permeated the film. When oxygen-deficient amorphous zinc oxide is generated in the film, the eluted hexavalent molybdenum oxyacid salt reacts with amorphous zinc oxide and is converted to a stable zinc compound. This also suppresses black discoloration. Furthermore, the hexavalent molybdenum oxyacid salt forming a complex with amines is converted into hexavalent molybdenum oxyacid salt again through molybdenum blue effective for preventing oxidation of Zn by a similar reaction.

  In other words, the coexistence of amines causes molybdenum to change the valence of hexavalent → pentavalent / hexavalent → hexavalent, so that black discoloration due to oxidation of Zn is suppressed over a long period of time. However, in the hot-dip plated layer containing Mg that is more easily oxidized than Zn, the oxide of Mg is concentrated on the surface layer of the plated layer immediately after formation. The magnesium oxide concentration layer delays the sacrificial anticorrosive action of Zn (corrosion reaction of Zn), and inhibits the reaction between the hexavalent molybdenum oxyacid salt and Zn at the initial stage of applying the chemical conversion treatment solution. As a result, the tendency of the low-valent molybdenum compound to concentrate at the interface is weakened, and an effective effect on the initial white rust resistance cannot be obtained. Of course, Mg ions eluted in the etching reaction are hexavalent molybdenum oxyacids. It binds to the salt and inhibits the complex formation reaction between amines and hexavalent molybdenum oxyacid salt. For this reason, the plating layer containing Mg does not sufficiently exhibit the blackening suppression effect of the hexavalent molybdenum oxyacid salt and is inferior in corrosion resistance.

  The adverse effect resulting from the concentration of magnesium oxide can be suppressed by including a group 4A metal oxyacid salt in the chemical conversion coating. Group 4A metal oxyacid salts such as Ti, Zr, and Hf are sources of Group 4A metal oxyacid ions that react with Mg ions eluted by the etching reaction. The reaction product is concentrated at the interface to improve the initial white rust resistance and suppress the compound formation reaction between Mg ions and hexavalent molybdenum oxygenate ions. Therefore, the chemical conversion treatment plated steel sheet with which the blackening suppression effect of hexavalent molybdenum oxyacid salt is maintained, and which was excellent in both corrosion resistance and blackening resistance is obtained.

In this invention, the plated steel plate which provided the zinc alloy plating layer of Mg: 0.1-10 mass% on the steel plate surface, such as normal steel, a low alloy steel, stainless steel, is used for the original plate. In applications where workability is required, a deep drawing steel plate in which Ti, Nb, etc. are added to ultra low carbon steel is a suitable material.
Although a zinc alloy plating layer is provided on the steel sheet surface according to a conventional method, the zinc alloy plating layer is adjusted in the range of Mg: 0.1 to 10% by mass. Mg is an effective component for improving the corrosion resistance of the galvanized layer. When Mg is added in an amount exceeding 0.1% by mass, the effect of Mg is only saturated when added in excess of 10% by mass. Instead, it becomes an oxide, black dross is generated on the oil surface of the hot dipping bath, and the quality is liable to deteriorate.

In addition to Mg, Al, Si, Sb, Pb, etc. may be included in the galvanized layer as necessary.
Al also improves the adhesion between the underlying steel / plating layer, and also exhibits the effect of suppressing a large amount of dross generated due to the formation of Fe-Zn intermetallic compounds in the hot dipping bath. Such an effect is seen when Al is added at 0.1 mass% or more. However, a zinc alloy plating layer containing an excessive amount of Al exceeding 20% by mass is liable to deteriorate the plating quality, and the hot dip plating bath necessary for forming the plating layer also requires a large amount of Al. Cause.

Si is an effective component for improving the corrosion resistance of the zinc alloy plating layer, and suppresses the Fe-Zn alloying reaction and Fe-Al alloying reaction at the base steel / plating layer interface, thereby improving the workability of the zinc-based plated steel sheet. Improve. Such an effect becomes remarkable at 0.005 mass% or more, but excessive Si content exceeding 2 mass% causes a large amount of dross generation and quality degradation.
Sb and Pb are components added when a zinc alloy plating layer having a spangle pattern on the surface of the zinc alloy plating layer is required, but the effect is saturated even if it is added in excess of 1% by mass.
In addition, impurities such as Ti, Ni, and B may be mixed in the hot dipping bath, and in any case, by controlling to 0.1% by mass or less, adverse effects due to the impurities are suppressed.

The chemical conversion treatment liquid has a basic composition in which molybdenum oxyacid salt, amines, and 4A group metal oxyacid salt are blended in an organic resin.
As the organic resin, one or more selected from urethane, acrylic, epoxy, and olefin can be used. Among them, a water-soluble or water-dispersible material obtained by a reaction between an organic polyisocyanate compound and a polyol compound can be used. Urethane resins, particularly self-emulsifying urethane resins are preferred.
Examples of organic polyisocyanate compounds include aliphatic diisocyanates such as phenylene diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, and naphthalene diisocyanate, alicyclic diisocyanates such as cyclohexane diisocyanate, isophorone diisocyanate, norbornane diisocyanate, xylylene diisocyanate, and tetramethylxylylene diisocyanate. is there.

Examples of the polyol compound include polyolefin polyols such as polyester polyol, polyether polyol, polycarbonate polyol, polyacetal polyol, polyacrylate polyol, and polybutadiene.
Examples of the soluble molybdenum oxyacid salt having oxidizing properties include hexavalent molybdenum oxyacid salts such as molybdic acid, ammonium molybdate, and alkali metal molybdate. Of these, molybdic acid and ammonium molybdate exhibit good corrosion resistance and blackening resistance. The compounding amount of the hexavalent molybdenum oxyacid salt with respect to the chemical conversion treatment liquid is preferably adjusted so that the molybdenum oxyacid salt contained in the chemical conversion treatment film is 0.1 to 100 mg / m ^{2 in} terms of metal Mo.

Examples of amines to be blended in the chemical conversion solution with molybdenum oxyacid salt include aliphatic amines such as dimethylamine, trimethylamine, diethylamine and triethylamine, alicyclic amines such as cyclohexylamine and hexamethylenediamine, triethanolamine and isopropanolamine. Alkanolamines such as diethylethanolamine can be used for convenience. Of these, the combination of trimethylamine, isopropanolamine and molybdenum oxyacid salt gives good blackening resistance. The compounding amount of the amines in the chemical conversion treatment liquid is preferably determined so that the molar ratio with respect to Mo contained in the chemical conversion treatment film is 0.1 to 5.
Ti, Zr, Hf and the like can be used for the 4A group metal, and examples of the oxyacid salt of the 4A group metal include a hydrogenate, an ammonium salt, an alkali metal salt, and an alkaline earth metal salt. Since the organic acid salt and carbonate of the group 4A metal also become an oxyacid salt during the film formation process, they can be added to the chemical conversion treatment liquid as a source of the oxyacid salt of the group 4A metal. Peroxides have a similar effect.

  Etching agents, silane coupling agents, inorganic compounds, lubricants, color pigments, dyes, and the like are added to the chemical conversion solution as necessary. As the etching agent, phosphoric acid, phosphate, fluoride or the like is used, and the adhesion of the chemical conversion film is enhanced by activating the substrate surface. As the silane coupling agent, a compound having a functional group such as an amino group, an epoxy group, a vinyl group, a mercapto group, an acryloyl group, or an alkoxy group is used to enhance the adhesion of the chemical conversion film to the substrate. Inorganic compounds (oxides, phosphates, etc.) such as Mg, Ca, Sr, Ti, Zr, Hf, V, W, Mn, B, Si, and Sn have the effect of densifying the film and improving water resistance. Present. Fluorine-based, polyethylene-based, styrene-based organic waxes, inorganic disulfides such as molybdenum disulfide and talc are brought into the film from the chemical conversion solution, improving the lubricity of the chemical conversion film and, in turn, the workability of the chemical conversion steel sheet. Let It is also possible to add a predetermined color tone to the chemical conversion film by blending an inorganic pigment, an organic pigment, an organic dye or the like.

  A chemical conversion coating film is formed on the surface of the steel sheet by applying the chemical conversion liquid prepared to a predetermined composition to the zinc-based plated steel sheet by a roll coating method, a spin coating method, a spraying method, or the like and drying. The coating amount of the chemical conversion solution is preferably adjusted so that the dry film thickness of the chemical conversion coating is 0.1 to 5 μm. Sufficient environmental barrier properties (oxygen barrier properties) are exhibited at a dry film thickness of 0.1 μm or more, and blackening resistance and corrosion resistance are ensured. The effect of improving blackening resistance and corrosion resistance is improved as the chemical conversion treatment film becomes thicker. However, saturation at a film thickness of 5 μm and further increase in the film thickness increase the processing cost. The applied chemical conversion treatment liquid can be dried at room temperature, but considering continuous operation, it is preferable to maintain the temperature at 50 ° C. or higher to shorten the drying time. However, when dried at a high temperature exceeding 200 ° C., the organic components contained in the film are thermally decomposed, and there is a concern that quality such as corrosion resistance may be deteriorated.

The chemical conversion treatment film produced from the chemical conversion treatment liquid described above contains molybdenum oxyacid salt, amines, and oxyacid salt of group 4A metal, but the content of molybdenum oxyacid salt is 0.1 to 100 mg / m ² , The molar ratio of amines to Mo is preferably adjusted to 0.1 to 300 mg / m ² in terms of the group 4A metal equivalent of the oxyacid salt content of the group 0.1 to 5, 4A metal.
The black discoloration suppressing action of molybdenum oxyacid salt becomes significant at 0.1 mg / m ² , and black discoloration over a long period is effectively suppressed. However, when an excessive amount of molybdenum oxyacid salt exceeding 100 mg / m ² is contained, the discoloration suppressing action is saturated and the soluble component contained in the chemical conversion film increases, so that water resistance, corrosion resistance, etc. are reduced. There is a fear.

By including amines in a molar ratio of 0.1 or more with respect to Mo, reduction of the reduction ability of the reduction of molybdenum oxyacid salt is prevented, and the black discoloration suppressing effect is well maintained over a long period of time. However, if an excessive amount of amines exceeding 5 molar ratio is included, the ability to suppress the reduction of molybdenum oxyacid salt becomes stronger, and the formation of molybdenum pentavalent and hexavalent composite oxides is also suppressed. Cannot be maintained.
The oxyacid salt of Group 4A metal has a remarkable effect of suppressing an adverse effect caused by the concentration of magnesium oxide at 0.1 mg / m ² or more, but an excessive amount of Group 4A metal exceeding 300 mg / m ² is effective. If an oxyacid salt is included, the chemical conversion coating becomes porous, and the workability and corrosion resistance are likely to be reduced.

  Two types of molten Mg-containing zinc alloy-plated steel sheets A and B are used as the chemical conversion treatment original plate, and the chemical conversion treatment liquid is obtained by adding urethane oxyacid salt, amines, 4A group metal oxyacid salt to urethane resin: 200 g / l. By applying and drying, an organic chemical conversion film was formed on the surface of the original plate. The film thickness of the chemical conversion treatment film was variously changed depending on the coating amount of the aqueous resin liquid. Table 1 shows the composition of the chemical conversion treatment solution used, and Table 2 shows the film thickness and composition of the chemical conversion coating formed on the surface of the plated steel sheet.

Values measured by fluorescent X-ray were used for the Mo amount and 4A group metal amount in the chemical conversion coating. For amines, ESCA is used to analyze 5 points in the depth direction from the surface of the chemical conversion coating to the interface with the galvanized layer, and N (amine) that forms an amine from the N1 peak is separated at each measurement point. The ratio of atomic% of N (amine) and atomic% of Mo: N (amine) / Mo was calculated at each measurement point, and the average value of the calculation results was used as the molar ratio of amines to Mo in the film. .
A: coating weight 60 g / m ^2, a thickness of 0.8mm molten Zn-2 wt% Mg alloy plated steel sheet B: coating weight 60 g / m ^2, melting Zn-6 mass% Al-3 mass thickness 0.8mm % Mg alloy plated steel sheet

A test piece was cut out from each plated steel sheet on which the chemical conversion coating was formed and subjected to a corrosion test and a blackening test.
[Corrosion test]
The end face of the test piece was sealed and sprayed with a 5% NaCl aqueous solution at 35 ° C. in accordance with JIS Z2371. After spraying salt water for a predetermined time, the surface of the test piece was observed. The area ratio of white rust generated on the surface of the test piece is measured, and the white rust generation area ratio is 5% or less, ◎, 5-10% is ◯, 10-30% is △, 30-50% is ▲, 50 The corrosion resistance of the flat part was evaluated with x as% or more.

[Black discoloration test]
After leaving the test piece in a humid atmosphere at a temperature of 50 ° C. and a humidity of 98% RH for a predetermined time, the brightness of the test piece was measured and compared with the brightness before the test. The blackness resistance was evaluated with ◎ for the lightness difference ΔL: less than 1, ◯ for 1-2, Δ for 2-5, ▲ for 5-10, and x for 10 or more.
As can be seen from the investigation results in Table 3, Test Nos. 1 to 5 provided with a chemical conversion treatment film in which molybdenum oxyacid salt and amines coexist showed excellent characteristics in both corrosion resistance and blackening resistance. On the other hand, Test No. 6 containing no molybdenum oxyacid salt was inferior in corrosion resistance, and the surface turned black in a relatively short time. Test number 7 with no addition of amines, although containing molybdenum oxyacid salt, has slightly improved corrosion resistance but is inferior in blackening resistance, and test number 8 not containing group 4A metal oxyacid salt is corrosion resistance and resistance. None of the black denaturation was sufficient.

  As described above, by providing an organic chemical conversion coating on the surface of a molten Mg-containing zinc alloy-plated steel sheet in the presence of molybdenum oxyacid salts, amines, and group 4A metal oxyacid salts, excellent corrosion resistance, Black denaturation is imparted. Therefore, it is a material used in a wide range of fields, such as exterior materials, interior materials, cover materials, and steel plates for vehicles, combined with a Cr-free coating that maintains a beautiful surface state for a long period of time and has a low environmental impact. Is provided.

Claims (5)

  Mg: An organic chemical conversion coating film based on a molten Mg-containing zinc alloy-plated steel sheet containing 0.1 to 10% by mass, in which soluble molybdenum oxyacid salts, amines, and 4A group metal oxyacid salts are dispersed. A molten Mg-containing zinc alloy-plated steel sheet excellent in corrosion resistance and blackening resistance, characterized in that is formed on the substrate surface.   The molten Mg-containing zinc alloy plated steel sheet according to claim 1, wherein the organic chemical conversion coating film contains one or more organic resins selected from urethane, acrylic, epoxy, and olefin.   The molten Mg-containing zinc alloy plated steel sheet according to claim 1, wherein one or more of hexavalent molybdenum oxyacid salt, ammonium molybdate, and alkali metal molybdate are soluble molybdenum oxyacid salts.   The molten Mg-containing zinc alloy plated steel sheet according to claim 1, wherein the amine is one or more selected from aliphatic amine, alicyclic amine, and alkanolamine.   The molten Mg-containing zinc alloy plated steel sheet according to claim 1, wherein the group 4A metal is one or more of Ti, Zr, and Hf.