JP4579715B2 - Chemically treated steel sheet with excellent corrosion resistance, coating adhesion, and adhesion - Google Patents

Description

  The present invention does not contain hexavalent chromium harmful to the environment, exhibits excellent corrosion resistance, paintability, coating film adhesion, and adhesion, and is useful as an exterior material, interior material, cover material, vehicle steel plate, etc. About.

  When a galvanized steel sheet, a zinc alloy plated steel sheet, etc. are used in a sea salt particle scattering atmosphere or a high-temperature and high-humidity atmosphere, white rust that causes the appearance deterioration is generated on the surface of the steel sheet, and the sacrificial anticorrosive action of the plating 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 chromium 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 chromium elution has not been solved.

In order to form a chemical conversion film that does not contain hexavalent chromium, which is harmful to the environment, improvements to phosphate treatment are being 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

The chemical conversion treated steel sheet is usually subjected to phosphate treatment on the steel manufacturer side and formed into a product shape on the user side. In the molding process, since conventional inorganic phosphate films are hard and not ductile in conventional phosphate films, film defects such as cracks and peeling are easily introduced. A film defect becomes a starting point of corrosion occurrence, and the corrosion resistance improving action and coating film adhesion of the chemical conversion film are extremely lowered. In order to improve the adhesion of the plating layer / phosphate coating, the method of adjusting the surface roughness of the galvanized steel sheet treated with phosphate (Patent Document 3), the film thickness of the phosphate coating in addition to the surface roughness Although the method (patent document 4) etc. which adjust this are proposed, it has not yet fully satisfied the workability in a user side, and adhesiveness after a process.
Japanese Patent Laid-Open No. 2003-119569 Japanese Patent Laid-Open No. 2002-105668

  The present invention is based on the knowledge obtained as a result of investigating and examining the influence of phosphate-based chemical conversion treatment on the surface of the plating layer by selecting Zn-Al-Mg alloy-plated steel sheet exhibiting high corrosion resistance as the base material. An object of the present invention is to provide a chemical conversion treated steel sheet that is formed by forming a phosphate film in which a phosphate crystal penetrates into a plating layer, thereby suppressing cracking and peeling due to forming and maintaining excellent film adhesion. .

The chemical conversion treated steel sheet of the present invention is based on a molten Zn—Al—Mg alloy plated steel sheet in which the ratio of [Al / Zn / Zn ₂ Mg ternary eutectic structure] to the outermost surface of the plating layer is 60 area% or more. And the plating layer surface of the base material is covered with a phosphate film, a deposited layer, and an organic resin-based chemical conversion film. Deposition layer coating weight: range from 0.05 to 5.0 mg / m ² in Ni, Co, the layer was deposited at least one selected from Fe and / or coating weight: Mn in 0.05~30mg / m ² It is the layer which precipitated. The phosphate film is composed of phosphate crystals having an average particle size of 0.5 to 5.0 μm whose base portion bites into the plating layer and stands up from the plating layer. The chemical conversion film contains an oxide or hydroxide of valve metal and a fluoride of valve metal, and has an interfacial reaction layer formed on a plating layer or a deposited layer exposed between phosphate crystals, and a bubble. An organic resin film in which a metal oxide or hydroxide and a bubble metal fluoride coexist is formed on the interface reaction layer.

  In improving coating film adhesion and film adhesion by phosphate treatment, the proportion of phosphate crystals occupying the outermost surface of the plating layer is 50 area% or more, and the average depth of phosphate crystals that bite into the plating layer The thickness is preferably 0.05 μm or more. Ti, Zr, Hf, V, Nb, Ta, Mo, W, etc. are used for the valve metal of a chemical conversion film. The film thickness of the chemical conversion film is preferably 0.001 μm or more and 10% or less of the film thickness of the phosphate film.

When the molten Zn—Al—Mg alloy-plated steel sheet is subjected to a phosphate treatment with a phosphoric acid treatment solution containing Ni, Co, Fe, Mn, etc., phosphate crystals are generated in a state of being bitten into the surface layer of the plating layer. Therefore, the phosphate film on the plating layer exhibits much better adhesion than the phosphate film obtained when the conventional zinc-based plated steel sheet is subjected to phosphate treatment, and is sound even after forming. Maintained in a state.
It is presumed that the phosphate crystal bites into the plating layer due to the following mechanism, and it occurs for the first time by a combination of a hot-dip Zn-Al-Mg alloy-plated steel sheet and a phosphating solution containing Ni, Co, Fe, Mn, etc. This is a phenomenon (FIG. 1). In the following description, Ni is used as an example of the substitutional precipitation element. However, even when a phosphating solution containing Co, Fe and / or Mn is used, phosphate crystals that have penetrated into the plating layer are generated.

In the molten Zn—Al—Mg alloy-plated steel sheet, most of the plating layer has a microstructure of [Al / Zn / Zn ₂ Mg ternary eutectic structure]. When the plating layer is phosphate-treated with a Ni-containing treatment solution, a substitutional precipitation reaction of Ni occurs preferentially in the Zn ₂ Mg phase having the lowest surface potential in the ternary eutectic structure. When a noble metal Ni is deposited on the surface of the Zn ₂ Mg phase, Zn ₂ Mg phase, the corrosion current flows between the deposition Ni and Zn phase, Zn phase is preferentially dissolved. The dissolution of the Zn phase proceeds in the depth direction because an Al phase that is difficult to etch exists in the plating layer. Since phosphate crystals are precipitated as the Zn phase dissolves, the phosphate crystals are deeply rooted where the Zn phase is deeply etched.

  On the other hand, when an electrogalvanized steel sheet or a hot dip galvanized steel sheet in which the plated layer has a substantially single structure is subjected to phosphate treatment with a Ni-containing treatment solution, Ni is substituted and deposited on almost the entire surface of the plated layer. The plating layer without precipitated Ni is dissolved in the depth direction and the horizontal direction, and phosphate crystals are precipitated. However, since dissolution along the depth direction is difficult to proceed, the precipitated phosphate crystal slightly bites into the plating layer, resulting in a crystal having a relatively large grain size grown in the horizontal direction.

  The phosphate crystal 2 grows in a direction to stand up from the plating layer 1 in a state of being deeply rooted in the plating layer 1. Between the phosphate crystals 2, the plating layer 1 is exposed or the Ni precipitation layer 3 is formed. When the plating layer 1 after the phosphate treatment is treated with a chromium-free chemical conversion treatment solution containing a fluoride of valve metal, a part of the phosphate crystal 2 is dissolved by the etching action of the treatment solution, and the dissolved phosphate component Reacts with the exposed surface of the plating layer 1 together with the treatment liquid component to form an interface reaction layer 4a, and an organic resin film 4b is formed on the interface reaction layer 4a (FIG. 2). The chemical conversion film 4 composed of the interface reaction layer 4a and the organic resin film 4b has a self-repairing action and blocks the exposed surface of the plating layer 1 from the environment. The granular reaction product 5 is also deposited on the surface of the phosphate crystal 2 rising from the plating layer 1.

  In the interfacial reaction layer 4a and the organic resin film 4b of the chromium-free chemical conversion film 4, valve metal oxide or hydroxide and valve metal fluoride coexist. The oxide or hydroxide of valve metal exhibits an excellent environmental barrier ability and protects the plating layer 1 from a corrosive atmosphere. Valve metal fluoride dissolves when exposed to a corrosive atmosphere, and self-repairs the defective portions of the phosphate coating and chemical conversion coating 4 in the process of reprecipitation as a sparingly soluble oxide or hydroxide. .

  Since the chromium-free chemical conversion film 4 is formed on the surface of the plating layer 1 exposed between the adjacent phosphate crystals 2, it is compared with the chemical conversion film formed on the untreated plating layer 1 or the Ni precipitation layer 3. Growth is promoted, and the ability to block the environment and the self-repairing action are greatly improved. Environmental barrier ability and self-healing action can be seen in the chemical conversion film 4 with a film thickness of 0.001 μm or more. However, if the interface reaction layer 4a grows too thick, both the phosphate crystal 2 and the interface reaction layer 4a are hard, so during processing Therefore, it is preferable to control the thickness of the interfacial reaction layer 4a to 10% or less of the average height of the phosphate crystals 2 rising from the surface of the plating layer 1. The thickness of the interface reaction layer 4a can be controlled by the concentration of the chromium-free chemical conversion treatment liquid, the treatment 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 thickness of the interface reaction layer 4a is selected according to the protrusion height.

  Since the chemical conversion film 4 is mainly composed of a ductile organic resin film 4b, it maintains a healthy film state even when subjected to a certain degree of processing. In addition, since the chemical conversion film 4 is divided by the phosphate crystals 2, a decrease in adhesion to the 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 chromium-free chemical conversion treatment liquid, cracks and peeling are less likely to occur in the phosphate coating 3 during processing such as press molding. .

The hot-dip Zn—Al—Mg alloy-plated steel sheet used for the chemical conversion raw plate contains Al: 2.5-15 mass%, Mg: 2.0-4.0 mass%, and the balance is substantially Zn alloy plating. Manufactured by hot dipping using a bath. The composition of the alloy plating bath is such that a plating layer mainly composed of a microstructure of [Al / Zn / Zn ₂ Mg ternary eutectic structure] effective on the formation of phosphate crystals in the required form is formed on the surface of the steel sheet. To a predetermined composition. When [Al / Zn / Zn ₂ Mg ternary eutectic structure] is 60 area% or more (preferably 80 area% or more), the reactivity with the phosphating solution increases.

When the metal structure of the plating layer is [Al / Zn / Zn ₂ Mg ternary eutectic structure], Ti, B, Ti are used to suppress the formation and growth of the Zn ₁₁ Mg ₂ phase which adversely affects the appearance and corrosion resistance. It is beneficial to add a B alloy or a Ti, B containing compound to the plating bath. The addition amount to the plating bath is preferably determined in the range of Ti: 0.001 to 0.1% by mass and B: 0.001 to 0.045% by mass. Excessive addition of Ti and B is a cause for the growth of precipitates in the plating layer, which may inhibit the phosphate treatment.
Further, in order to improve plating adhesion during processing, Si having an action of suppressing the growth of the Al—Fe alloy layer at the interface between the plating layer and the base steel is added in the range of 0.005 to 0.5 mass%. You can also

  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 plating layer is inactivated by the oxidation action and reacts instead. Sex is reduced.

A phosphoric acid treatment solution is known in which metal ions such as Zn, Mn, Mg, Ca, Ni, Co, and Fe are added as needed in addition to phosphate ions. In the present invention, Ni, Co, and Fe are known. A phosphating solution containing at least one metal ion of Mn is used. Ni, Co, Fe, and Mn are preferentially replaced with the Zn ₂ Mg phase having the lowest surface potential in the [Al / Zn / Zn ₂ Mg ternary eutectic structure] of the molten Zn—Al—Mg alloy-plated steel sheet. precipitated, Ni is a noble metal, Co, Fe or Mn compound Zn phase in the vicinity by deposition to Zn ₂ Mg phase is preferentially dissolved. The dissolution of the Zn phase proceeds in the depth direction, and a phosphate crystal is precipitated with the dissolution of the Zn phase, so that the phosphate crystal has a deep root. The precipitated phosphate crystals form a fine and dense film reflecting the microstructure of [Al / Zn / Zn ₂ Mg ternary eutectic structure].

The phosphating solution preferably contains 0.001 to 0.1 mol / l Ni, Co, Fe, Mn. With 0.001 mol / l or more of Ni, Co, Fe, and Mn, dissolution of the Zn phase proceeds sufficiently, and phosphate crystals with deep roots and good adhesion are formed. However, when an excessive amount of Ni, Co, Fe, or Mn exceeding 0.1 mol / l is contained, the amount of substitutional precipitation of a noble metal or compound becomes excessive, and there is a concern that the corrosion resistance of the phosphate film is lowered.
Since the molten Zn-Al-Mg alloy-plated steel sheet is phosphate-treated, there is concern about the inhibition of the phosphate reaction by Al eluted from the plating layer, but elution by adding fluoride to the phosphate treatment solution The adverse effect of Al can be suppressed. 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. Further, when a new solution is constantly supplied to the surface of the plating layer to suppress an increase in pH at the interface during precipitation of phosphate crystals, the reactivity of the treatment solution is improved, and deeply rooted phosphate crystals are generated.

When a molten Zn—Al—Mg alloy-plated steel sheet is treated with a phosphoric acid treatment solution heated to 40 ° C. or higher, phosphate crystal nuclei are easily formed on the surface of the plating layer because of high reactivity. As the number of crystal nuclei is increased, the crystal starting point increases and fine phosphate crystals are precipitated.
Bringing the phosphoric acid treatment solution into contact with the plating layer while stirring is also an effective method for precipitating fine phosphate crystals. By stirring, a new solution is always supplied to the surface of the plating layer, the increase in pH due to dissolution of the components of the plating layer is suppressed, and the reactivity is maintained at a high level. Fine phosphate crystals are also deposited by supplying a phosphating solution to the surface of the plating layer by spraying.

  The temperature and stirring strength of the phosphoric acid treatment solution are adjusted so that the average particle diameter of the phosphate crystals is 0.5 to 5.0 μm and the average bite depth is 0.05 μm or more. The biting depth is obtained by dissolving the phosphate crystals only with diammonium chromate aqueous solution, then observing the traces of phosphate crystals on the surface using a scanning laser microscope, and measuring the depth of the traces. Desired. When the biting depth is 0.05 μm or more, the adhesion of the phosphate crystals is sufficient and the phosphatized steel sheet does not fall off during the forming process. However, if the biting depth exceeds 5.0 μm, the amount of dissolution of the plating becomes too large, and the stability of the phosphating solution is also lowered.

  In order to ensure adhesion and corrosion resistance after coating, a phosphate crystal having an average particle size of 0.5 μm or more is preferable. However, if the average particle size exceeds 5.0 μm, the phosphate crystal is affected by stress during molding. There is concern about cohesive failure. The crystal grain size can also be adjusted by adding reaction inhibitors such as organic acids and polysaccharides to the phosphating solution.

The amount of Ni, Co, Fe and Mn precipitated during phosphating is controlled in the range of 0.05 to 5.0 mg / m ² for Ni, Co and Fe and 0.05 to 30 mg / m ² for Mn. Yes. Reactivity is improved at a precipitation amount of 0.05 mg / m ² or more, and a deep-root phosphate crystal is likely to be precipitated. However, if the amount of precipitation exceeds 5.0 mg / m ² for Ni, Co, and Fe and the amount of precipitation exceeds 30 mg / m ² for Mn, an excessive amount of noble metal or Mn compound will remain, so phosphoric acid. There is concern about a decrease in the corrosion resistance of the salt film.
The phosphate film is preferably formed so that the coverage of the outermost surface of the plating layer is 50 area% or more. 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 film, the molten Zn—Al—Mg alloy plated 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 separately added 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 Cr-free chemical conversion treatment solution contains a resin component that forms a network of a chemical conversion film in addition to a Ti source that becomes titanium oxide or titanium hydroxide and titanium fluoride, and if necessary, an organic acid or metal phosphoric acid having a chelating action You may mix | blend salt or composite phosphate, various pigments, and a lubricant.
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.

  Components that form the resin matrix of the conversion coating include olefins such as urethane, epoxy, polyethylene, polypropylene, and ethylene-acrylic acid copolymers, styrenes such as polystyrene, polyesters, and copolymers or modifications thereof. Organic resins such as products and acrylics are added to the chemical conversion treatment liquid.

  As the urethane resin, a water-soluble or water-dispersible urethane resin obtained by reacting an organic polyisocyanate compound and a polyol compound, particularly a self-emulsifying urethane resin is preferable. 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. Can be mentioned. On the other hand, examples of the polyol compound include polyester polyols, polyether polyols, polycarbonate polyols, polyacetal polyols, polyacrylate polyols, polyester amide polyols, and polybutadiene-based polyolefin polyols.

The self-emulsifying urethane resin is produced by introducing a hydrophilic component such as a carboxylic acid-containing compound into the molecule. In the self-emulsifying urethane resin, the ionicity becomes an anion, a cation or a nonion depending on the introduced hydrophilic component. For example, when polyethylene glycol, isocyanate or the like is introduced into the polymer skeleton, it becomes nonionic, when a hydroxyl group is introduced, it becomes anionic, and when a sulfonic acid (salt) group, carboxyl (salt) group or the like is introduced, it becomes cationic.
In a system to which a chemical conversion treatment solution containing phosphate, an oxycarboxylic acid or a silane coupling agent is added, anionic or cationic urethane resin may cause gelation due to aggregation reaction of resin particles, but it is nonionic. Such a phenomenon does not occur in the urethane resin. In this respect, the use of a nonionic urethane resin is preferred, but an anionic or cationic urethane resin can be used without causing gelation by setting the use ratio of the nonionic urethane resin to half or more.

  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 is eluted from the chemical conversion film and eluted in the film defect, and reacts with the plating layer to precipitate insoluble phosphate, thereby complementing the self-healing action of titanium fluoride. . 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.

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. The phosphate crystal 2 formed by the phosphate treatment liquid not containing metal components such as Zn, Mn, Mg, Ca is zinc phosphate alone [Zn ₃ (PO ₄ ) · 4H ₂ O]. On the other hand, phosphate crystals incorporating Zn, Mn, Mg, Ca, etc. become complex phosphates with excellent adhesion and water resistance to the plating layer, and form a phosphate film 3 effective in improving corrosion resistance. .

  After forming the phosphate film, the molten Zn—Al—Mg alloy plated steel sheet is treated with a chromium-free chemical conversion treatment solution. The chromium-free chemical conversion treatment liquid is based on a treatment liquid containing a valve metal source compound composed of a halide or an oxyacid salt and a resin for forming a resin matrix, and if necessary, an organic acid having an ion stabilizing action or titanium fluoride. Contains phosphate that complements the self-healing action of the chemicals and improves corrosion resistance.

You may mix | blend the component used as a pigment and a lubrication agent with a chemical conversion liquid.
Examples of the pigment include inorganic pigments such as titanium oxide, carbon black, calcium carbide, alumina, and zinc oxide, and organic pigments such as a phthalocyanine pigment. When adding a pigment, a compounding quantity is selected in the range of 3-30 mass%. The coloring effect due to the blending of the pigment is observed at 3% by mass or more. However, when an excessive amount of the pigment exceeding 30% by mass is blended, the chemical conversion film becomes porous and the corrosion resistance decreases. You may mix | blend the component used as a pigment and a lubrication agent with a chemical conversion liquid.

  In order to include a wax effective for improving lubricity in the chemical conversion film, organic waxes such as fluorine, polyethylene, and styrene, inorganic lubricants such as silica, molybdenum disulfide, and talc can be added. The low melting point organic wax is considered to bleed on the surface when the film is dried and to exhibit lubricity. High melting point organic waxes and inorganic lubricants are present in a dispersed state in the film, but it is considered that lubricity is manifested by exposure to the film surface in an island-like distribution in the outermost layer of the treated film.

As described above, based on treatment liquid containing Ti source compound consisting of halide or oxyacid salt and resin for resin matrix formation, self-healing of organic acid or titanium fluoride with ion stabilizing action as required The phosphate which supplements an effect | action and has the effect | action which improves corrosion resistance is mix | blended.
When the prepared treatment liquid is applied to a phosphate-treated hot-dip Zn—Al—Mg alloy-plated steel sheet, an interfacial reaction layer 4 a that reacts with fluorine ions and the plating layer 1 or Ti is present between the phosphate crystals 2. It is formed preferentially on the exposed surface of the plating layer 1 (FIG. 2). The interfacial reaction layer 4a is a layer that contains a sufficient amount of titanium fluoride having a self-repairing effect because growth is promoted as compared with the case where it is deposited on the plating layer 1 without the phosphate crystal 2. The thickness of the interface reaction layer 4a can be adjusted by the concentration of the valve metal source contained in the chemical conversion solution. Also, the interface reaction layer 4a grows thick by increasing the reactivity by keeping the chemical conversion treatment solution or the original plate warm.

Since the interface reaction layer 4a is divided by the phosphate crystal 2, the stress applied during processing is dispersed, so that the adhesion to the plating layer 1 is not lowered. Moreover, most of the phosphate crystals 2 protruding from the plating layer 1 are not covered with the interface reaction layer 4a, and the organic resin film 4b in which Ti fluoride, oxide, hydroxide and phosphate are dispersed is formed. Since it is formed on the interface reaction layer 4a, the corrosion resistance after coating, the coating film adhesion, and the adhesion to the adhesive are significantly improved.
A Cr-free chemical conversion treatment solution is applied to a chemical conversion treatment original plate by roll coating, spin coating, spraying, etc., and dried without washing with water, so that a coating with excellent coating adhesion and corrosion resistance after coating is plated. It is formed on the surface layer of the layer. 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.

  It is preferable that the interface reaction layer and the organic resin film have thicknesses of about 5 to 300 nm and 0.1 to 3 μm, respectively. The interfacial reaction layer expresses sufficient environmental barrier ability at a film thickness of 5 nm or more and good anticorrosion ability at 10 nm or more, but when it grows to a thick film exceeding 300 nm, cracks are likely to occur due to stress applied during molding processing, On the contrary, it reduces the corrosion resistance. The arrival of the corrosive component to the interface reaction layer is remarkably suppressed by a chemical conversion film having a thickness of 0.1 μm or more. However, a chemical conversion film having a film thickness exceeding 3 μm not only saturates the effect of improving corrosion resistance but also deteriorates weldability. Note that the thickness of the interface reaction layer and the organic resin film can be measured by elemental analysis in the depth direction by AES, TEM observation, and the like.

The Cr-free chemical conversion treatment liquid applied to the molten Zn—Al—Mg alloy-plated steel sheet can be dried at room temperature, but considering continuous operation, it is preferable to keep the temperature at 80 ° C. or higher to shorten the drying time. However, when the drying temperature exceeds 250 ° C., the organic component contained in the chemical conversion coating is thermally decomposed, and the properties imparted with the organic component 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.

As an original plate, Al: 6.0% by mass, Mg: 3.1% by mass, Zn: the remaining alloy plating layer formed with an adhesion amount per side of 75 g / m ² : Plate thickness: 0.7 mm Molten Zn— An Al—Mg alloy plated steel sheet was used. When the alloy plating layer was observed with a microscope, fine [Al / Zn / Zn ₂ Mg ternary eutectic structure] was observed at a ratio of 75 area% or more.

  After each galvanized steel sheet is phosphated using a phosphating solution (Table 1), the chrome-free conversion coating solution (Table 2) is applied and dried to seal the phosphate coating with a chrome-free conversion coating. did. Table 3 shows combinations of phosphate treatment and chromium-free chemical conversion treatment, and Table 4 shows phosphate coatings and chromium-free chemical conversion coatings formed on the surface of galvanized steel sheets. In Table 4, the depth of penetration of the phosphate film was determined by observing traces of phosphate crystals with a scanning laser microscope after dissolving and removing only phosphate crystals with an aqueous diammonium chromate solution. The average depth of the trace is shown.

A test piece was cut out from the obtained chemical conversion treated steel sheet 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 was evaluated as corrosion resistance of a processed part, where 占有 is less than 5%, ５〜 is 5-10%, △ is 10-30%, ▲ is 30-50%, and x is 50% or more.

[Corrosion test after painting]
The chemical conversion treated steel sheet 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 hot-dip Zn-Al-Mg alloy-plated steel sheet that was chromium-free conversion treated after phosphate treatment was excellent in all of the flat part corrosion resistance, processed part corrosion resistance, and post-coating corrosion resistance. The characteristics are shown. In particular, when a phosphate coating with a high coating rate and a chromium-free chemical coating are combined, the corrosion resistance of the processed parts is remarkably improved, and the self-healing action of the chromium-free chemical coating is expressed. .

  On the other hand, in tests No. 19 and 20 in which a chromium-free chemical conversion film was provided without phosphating, and tests No. 13 and 14 that did not contain Ti (valve metal), the flat portion corrosion resistance and post-coating corrosion resistance were inferior. The longer the salt spray, the more the white rust was generated in the flat and processed parts. Even in Test Nos. 15 and 16 in which a chromium-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 tests No. 17 and 18 with the conventional chemical conversion treatment that seals the phosphate treatment with chromate treatment, white rust is generated in the processed part even by a short salt spray, and the flat part corrosion resistance and corrosion resistance after painting are sufficient. It was not.

Furthermore, a PET film having a film thickness of 300 μm was bonded to a molten Zn—Al—Mg alloy-plated steel sheet provided with a phosphate film and a chromium-free chemical conversion film using an epoxy adhesive.
The obtained laminated steel sheet was subjected to a tensile test after a draw bead sliding deformation test (load: 200 kgf) to measure the peel strength of the film. When the measurement results were arranged by the penetration depth of the phosphate crystals with respect to the plating layer, when the average penetration depth of the phosphate crystals was 0.05 μm or more, a high peel strength exceeding 80 N / 20 mm was shown (FIG. 3). . Regarding the relationship between the crystal size of the phosphate crystals and the peel strength, it was confirmed that the PET film was bonded with a high peel strength by setting the average particle size of the phosphate crystals to 5.0 μm or less ( FIG. 4). From the results of FIGS. 3 and 4, it can be understood that it is effective to set the average grain size of phosphate crystals in the range of 0.5 to 5.0 μm and the average bite depth to 0.05 μm or more.
Even when a melamine alkyd resin coating is used and a coating film having a film thickness of 25 μm is provided on the chemically treated steel sheet after processing, the average particle diameter is similarly 0.5 to 5.0 μm, and the average biting depth is 0.05 μm or more. Thus, a coating film having excellent adhesion was formed.

  The same original plate as in Example 1 was subjected to phosphate treatment using the No. 1 treatment solution shown in Table 1. In this example, the average height of the phosphate crystals in the range of 0.5 to 5 μm is controlled by controlling the surface conditioning conditions prior to the phosphate treatment, the temperature of the phosphate treatment solution, and the treatment time. An altered phosphate film was formed. For the chromium-free chemical conversion treatment, a No. 3 treatment solution shown in Table 2 was used, and a chemical conversion film having a changed thickness of the interfacial reaction layer was formed by adjusting the coating amount and roll coating conditions.

  A PET film was laminated with an epoxy adhesive on a test piece sampled from the steel sheet after chemical conversion treatment, and the peel strength of the film was measured by a tensile test after a draw bead sliding deformation test (load: 200 kgf). As can be seen from the measurement results in FIG. 5, the peel strength tends to decrease as the interface reaction layer becomes thicker at any phosphate crystal height. And when the thickness of the interface reaction layer was controlled to 10% or less with respect to the average height of the phosphate crystals rising from the plating layer, it was found that good peel strength can be maintained.

A plurality of Zn—Al—Mg alloy-plated steel sheets (plate thickness: constant 0.8 mm) having a constant adhesion amount per side of 90 g / m ² and different coating layer compositions were prepared. When the alloy plating layer was observed with a microscope, a fine [Al / Zn / Zn ₂ Mg ternary eutectic structure] was observed at a ratio of 80 area% or more.
By subjecting each plated steel sheet to phosphate treatment at a liquid temperature of 60 ° C. and a contact time of 5 seconds using the phosphate treatment liquid of Table 6, the No. 4 chemical conversion treatment liquid of Table 2 is applied and dried. The phosphate coating was sealed with a chromium-free chemical conversion coating. The chromium-free chemical conversion film had a total valve metal deposition amount of 30 mg / m ² and a film thickness of 0.2 μm.

Table 7 shows the composition of the plating layer and the combination of the phosphate film formed on the surface of the plating layer.
In Table 7, the amount of substitutional metal precipitation was determined by the following procedure. A test piece of 10 cm × 10 cm cut out from the plated steel sheet after the phosphate treatment was immersed in an aqueous solution of ammonium dichromate (ammonium dichromate: 20 g / l, concentrated ammonium: 480 g / l) at room temperature for 15 minutes. After the salt crystals were dissolved, the test piece was immersed in a 10% aqueous HCl solution at room temperature to dissolve the entire plating layer, and the amount of displacement metal deposited was calculated from the ICP analysis result of the solution. Further, the penetration depth of the phosphate crystals was measured by the same method as in Example 1.

A test piece was cut out from the plated steel sheet after the chromium-free treatment and subjected to the same various corrosion tests as in Example 1.
As can be seen from the corrosion test results in Table 8, when the Zn-Al-Mg alloy-plated steel sheet of various compositions is subjected to the phosphate treatment according to the present invention and then subjected to chromium-free chemical conversion treatment, the flat part corrosion resistance and the processed part corrosion resistance are obtained. Thus, it is confirmed that the coated steel sheet exhibits excellent characteristics in both corrosion resistance after coating.

  As explained above, after forming a phosphate crystal that has a base bite into the plating layer and rising from the plating layer by phosphating, it is sealed with a chromium-free chemical conversion film consisting of an interface reaction layer and an organic resin film. In some cases, a chemically treated steel sheet is obtained in which the phosphate coating maintains a good adhesion state even after processing. In addition, since an organic resin-based chemical conversion film is provided, workability is also good. The coating layer and laminate film are provided through the acid salt film protruding from the chemical conversion coating that penetrates into the plating layer, so the coating film adhesion and film adhesion are excellent, and the original high corrosion resistance of the molten Zn-Al-Mg alloy plating Is also utilized. Therefore, it is used as an exterior material, an interior material, a cover material, a steel plate for vehicles, and the like as a steel material suitable for use in painting and laminating by processing into a product shape on the user side.

Chart showing that the form of phosphate crystals precipitated on the plating layer surface by phosphating differs between hot-dip Zn-Al-Mg alloy-plated steel sheet, electrogalvanized steel sheet, and hot-dip galvanized steel sheet Schematic diagram showing the surface state of a hot-dip Zn-Al-Mg alloy-plated steel sheet that has been subjected to chromium-free chemical conversion after phosphate treatment Graph showing the effect of the average penetration depth of phosphate crystals on the plating layer on the peel strength of laminated films Graph showing the effect of the average grain size of phosphate crystals on the peel strength of laminated films 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: Plating layer 2: Phosphate crystal 3: Ni precipitation layer 4: Chromium-free chemical conversion film 4a: Interfacial reaction layer 4b: Organic resin film 5: Granular reaction product

Claims (5)

The base material is a molten Zn—Al—Mg alloy-plated steel sheet in which the ratio of [Al / Zn / Zn ₂ Mg ternary eutectic structure] to the outermost surface of the plating layer is 60 area% or more,
It contains at least one selected from Ni, Co, Fe, and Mn, the total adhesion amount of Ni, Co, and Fe is in the range of 0.05 to 5.0 mg / m ² , and the adhesion amount of Mn is 0.05 to deposit is in the range of 30 mg / m ^2, average particle diameter: phosphate coating consisting of phosphate crystals of 0.5 to 5.0 .mu.m, a fluoride of an oxide or hydroxide and a valve metal valve metal is The plating layer surface is covered with the coexisting chemical film,
The phosphate crystal bites into the plating layer and stands up from the plating layer, and the chemical conversion film is organic through an interfacial reaction layer formed at the interface between the plating layer and the precipitation layer exposed between the phosphate crystals. It characterized in that it is a resin film, corrosion resistance, coating adhesion, excellent chemical conversion treatment steel sheet adhesion.   The chemical conversion treatment steel plate of Claim 1 with which 50 area% or more of the plating layer outermost surface is covered with the phosphate crystal.   The chemical conversion treatment steel plate according to claim 1 or 2, wherein the phosphate crystal has bitten into the plating layer at an average depth of 0.05 µm or more.   The chemical conversion treatment steel plate of Claim 1 which uses 1 type, or 2 or more types of metals chosen from Ti, Zr, Hf, V, Nb, Ta, Mo, and W for a valve metal.   The chemical conversion treatment steel sheet according to any one of claims 1 to 4, wherein the chemical conversion film has a thickness of 0.001 µm or more and 10% or less of the thickness of the phosphate film.

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