JP4470515B2 - Surface-treated steel sheet with excellent white rust resistance and appearance - Google Patents

Description

  The present invention relates to a surface-treated steel sheet that is optimal for applications such as automobiles, home appliances, and building materials, and particularly relates to an environment-adaptive surface-treated steel sheet that does not contain heavy metals such as Cr at the time of manufacture and products.

  Conventionally, chromic acid, dichromic acid or salts thereof have been applied to the surface of zinc-plated steel sheets or aluminum-based plated steel sheets for automobiles, home appliances, and building materials in order to improve corrosion resistance (white rust resistance, red rust resistance). Steel sheets treated with a treatment liquid as a main component are widely used. This so-called chromate treatment is an economical treatment which is excellent in the effect of improving the corrosion resistance and can be carried out relatively easily.

  The chromate treatment uses hexavalent Cr, which is a pollution-controlling substance, but since it is industrially performed in a closed system, hexavalent Cr is not normally released to nature in the treatment process. Also, in products, the elution of hexavalent Cr from the chromate film is suppressed to a level that can be almost ignored by means such as organic film sealing. Therefore, the risk of contamination of the environment and human body due to the chromate treatment is actually very low. However, due to recent global environmental problems, the amount of heavy metals contained in products is not to pollute the environment when voluntarily reducing the use of heavy metals, including hexavalent Cr, or when the shredder dust of discarded products is dumped. There is a growing movement to regulate this.

  From such a background, in order to improve the corrosion resistance of the galvanized steel sheet, in particular, to suppress the occurrence of white rust of the galvanized steel, a chemical conversion treatment technique that does not depend on the chromate treatment, so-called chromium-free chemical conversion treatment technique, has attracted attention. As this type of technology, for example, a method of forming a thin film by a method such as dipping, coating, or electrolytic treatment using an inorganic compound, an organic compound, an organic polymer material, or a solution combining these is known. Yes.

  However, the fact is that the gap between the level of corrosion resistance that can be achieved with these known techniques and that obtained with conventional chromate treatment is still large. One of the reasons for staying at such a current level is that all of the conventional knowledge is related to the manufacturing method, and there is almost no knowledge about the structural requirements of the chemical conversion coating having excellent white rust resistance. The knowledge disclosed as product patents related to chromium-free chemical conversion technology is simply a wording device that is actually a manufacturing method patent, or a structural knowledge that can be easily inferred from the manufacturing method. Little is known about the specific structural requirements of the chemical conversion coating that contributes to the above.

Furthermore, when the chemical conversion treatment is performed using a hot dip galvanized steel sheet as a base steel sheet, it is necessary to consider another problem that an interference color tends to occur in a resin chemical conversion film. However, there are no known specific structural requirements of the chromium-free coating that can suppress interference color and maintain excellent appearance as a hot dip galvanized steel sheet while obtaining excellent corrosion resistance.
Accordingly, the object of the present invention is to solve such problems of the prior art, and is a surface-treated steel sheet having a galvanized steel sheet as a base steel sheet and a chromium-free chemical conversion coating film, which has excellent white rust resistance and appearance. It is providing the surface-treated steel plate which has the chemical conversion treatment film structure obtained.

The inventors of the present invention first examined the following principle about the corrosion inhibition principle for inhibiting the corrosion of the galvanized steel sheet.
The initial corrosion of the galvanized steel sheet on which the chemical conversion film is formed proceeds in the following process.
Corrosion factors (oxygen, hydrogen, chlorine ions, moisture, etc.) penetrate into the chemical conversion coating and diffuse to the chemical conversion coating / plating layer interface, and the following redox at the chemical conversion coating / plating layer interface: The reaction dissolves zinc.
(1) Cathode reaction: 2H ₂ O + O ₂ + 4e ⁻ → 4OH ⁻
(2) Anode reaction: 2Zn → 2Zn ²⁺ + 4e ⁻

Therefore, in order to improve the corrosion resistance of galvanized steel sheet, especially white rust resistance, it is essential to suppress the progress of both reactions (1) and (2).
(a) Barrier layer for inhibiting the cathode reaction as a diffusion barrier for corrosion factors
(b) A dense inert layer (slightly soluble layer) formed on the surface of the plating layer using the chemical reaction at the time of chemical conversion coating formation to suppress the anode reaction
It is effective to have a film structure having
(c) The chemical conversion film contains a self-repairing substance that reacts with the defective part (or damaged part) of the above-mentioned inactive layer and repairs it, and traps and makes the Zn ions to be eluted from the part insoluble. A film structure is more effective.

The present inventors first realize such a film structure, specifically, a barrier layer containing the resin component (above (a)) is formed on the chemical conversion film, and further, a galvanized layer. It was found that a chromium-free surface-treated steel sheet with excellent white rust resistance can be obtained by synergistic effect by coating the surface layer of with a zinc phosphate-based inert coating ((b) above). (Patent Literature 1, Patent Literature 2). In addition, by depositing a zinc phosphate-based compound that has reactivity with Zn ions in the chemical conversion film, repair of the defective part of the inert film and capture / inactivation of Zn ions to be eluted It has also been proposed in the above-mentioned patent document that a chromium-free surface-treated steel sheet with significantly improved white rust resistance can be obtained by enabling ((c) above).
JP 2003-105554 A JP 2003-105555 A

  However, as a result of repeated studies by the present inventors, it has been found that when the above method is applied to a hot dip galvanized steel sheet, although white rust resistance is improved, an interference color that impairs the appearance tends to occur. And as a result of investigating the cause, the interference color is likely to occur mainly due to the surface shape of the hot dip galvanized steel sheet, that is, the electrogalvanized steel sheet having fine irregularities peculiar to electrodeposition crystals and On the other hand, on hot-dip galvanized steel sheets with a metallic luster (smooth surface), the smoothness of the front and back surfaces of the chemical conversion coating was high, and it was found that the interference condition for visible light was easily established. . Therefore, the present inventors paid attention to the above-described zinc phosphate compound, and studied various methods for promoting the refraction and scattering of visible light in the chemical conversion coating by appropriately depositing it in the chemical conversion coating. As a result, the present inventors have found a film configuration of a chemical conversion film that can suppress interference color without sacrificing corrosion resistance. According to this film configuration, it is possible to suppress interference colors while maintaining the glossiness peculiar to a hot dip galvanized steel sheet, and since it is a film configuration obtained without adding a special additive, the cost is reduced. It is also advantageous in that it requires little consideration for other characteristics such as corrosion resistance.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A surface-treated steel sheet having an organic resin, Zn and P on the surface of a hot dip galvanized steel sheet, and having a chemical conversion treatment film not containing Cr, wherein the chemical conversion treatment film contains Zn and P. The amorphous compound is contained in a dispersed state , and the occupation ratio of the amorphous compound in an arbitrary length range of 1 μm or more along the film thickness center line in the film thickness cross section of the chemical conversion coating is 21% or more. A surface-treated steel sheet with excellent white rust resistance and appearance.
[2] In the surface-treated steel sheet according to [1], the chemical conversion treatment film comprises an amorphous compound layer containing Zn, P and O, and an organic resin matrix layer having a thickness of 400 to 800 nm thereon, A surface-treated steel sheet excellent in white rust resistance and appearance, wherein an amorphous compound containing Zn and P is contained in a dispersed state in the organic resin matrix layer.

[3] In the surface-treated steel sheet according to [1] or [2], the molar ratio [Zn] / [P] of Zn and P in the amorphous compound containing Zn and P is 1.4 or less. A surface-treated steel sheet with excellent white rust resistance and appearance.
[4] In the surface-treated steel sheet according to any one of [1] to [3] above, the amorphous compound in an arbitrary length range of 1 μm or more along the film thickness center line in the film thickness section of the chemical conversion film. A surface-treated steel sheet excellent in white rust resistance and appearance, characterized in that the occupation ratio is 30% or more.
[5] A surface-treated steel sheet having excellent white rust resistance and appearance, wherein the organic resin is an epoxy resin in the surface-treated steel sheet according to any one of [1] to [4] .

  According to the present invention, it is possible to provide a surface-treated steel sheet that is excellent in white rust resistance and product appearance and optimal for automobiles, home appliances, and building materials without adding heavy metals such as Cr in the chemical conversion coating. .

The details of the present invention and the reasons for limitation will be described below.
The surface-treated steel sheet of the present invention has a chemical conversion treatment film containing an organic resin, Zn and P on the surface of a hot dip galvanized steel sheet, and not containing Cr, and the chemical conversion treatment film contains Zn and P. A crystalline compound is contained in a dispersed state.
In order to clarify the relationship between the white rust resistance of a chromium-free surface-treated steel sheet obtained by systematically changing the components of the chemical conversion coating and the coating structure, the present inventors have developed a focused ion beam equipped with a microsampling mechanism. These coating structures were thoroughly investigated using a processing device, a transmission electron microscope, an energy dispersive X-ray analyzer, etc., and white rust resistance evaluated by the salt spray test (JIS-Z-2371) Matched with gender. As a result, the chemical conversion treatment solution containing organic resin and phosphoric acid and Zn are chemically reacted to form a chemical conversion treatment film containing organic resin, Zn and P on the plating surface, and this film contains Zn and P. Excellent white rust resistance is obtained when an amorphous compound is deposited, and when the hot dip galvanized steel sheet is used as a base steel sheet by appropriately dispersing this amorphous compound in the film. It has been found that interference colors that tend to occur are suppressed. The reason why such excellent white rust resistance and appearance can be obtained is that the amorphous compound deposited in a dispersed state in the chemical conversion coating is (1) repair of the chemical conversion coating in a corrosive environment, (2) visible It is presumed that this is because of the two effects of promoting light refraction and scattering. For this reason, the present invention is a chemical conversion film containing an organic resin, Zn and P and not containing Cr on the surface of a hot dip galvanized steel sheet, and containing an amorphous compound containing Zn and P in a dispersed state. The formed chemical conversion film is formed. In addition, it can be confirmed by their electron diffraction patterns that the above-described compound existing (precipitated) in a dispersed state in the film and the compound layer formed on the plated surface described later are amorphous.

  Further, among the film structures of the chemical conversion coating as described above, the chemical conversion coating is formed of an amorphous compound layer containing P, Zn, and O formed on the plating surface, and a thickness of 400 to 400 nm. When it has a film structure (pseudo two-layer film structure) composed of an 800 nm organic resin matrix layer, and the organic resin matrix layer contains an amorphous compound containing Zn and P in a dispersed state In particular, it was found that excellent white rust resistance was obtained.

  In the case where the organic resin matrix layer has a wavelength of visible light, that is, the film thickness of about 400 to 800 nm, an interference color is likely to occur in principle. However, even with such a film thickness, the amorphous compound remains in the organic resin matrix layer. In the dispersed state, the generation of interference colors is suppressed by the scattering and refraction effects as described above, and it has been found that an excellent appearance can be maintained. Further, good white rust resistance can be secured within this film thickness range. Therefore, in the present invention, the chemical conversion treatment film comprises an amorphous compound layer containing Zn, P and O and an organic resin matrix layer having a thickness of 400 to 800 nm thereon, and the above-mentioned organic resin matrix layer includes It is preferable that an amorphous compound containing Zn and P is contained in a dispersed state.

Further, when the molar ratio [Zn] / [P] of Zn and P of the amorphous compound existing in a dispersed state in the chemical conversion coating is 1.4 or less, the effect of improving white rust resistance is particularly remarkable. It turns out that there is. The reason why the white rust resistance improving effect is particularly remarkable when satisfying such conditions is not necessarily clear, but the amorphous compound is composed of primary zinc phosphate (Zn (H ₂ PO ₄ ) _2. Since it has a composition close to H ₂ O) or dibasic zinc phosphate (ZnHPO ₄ .2H ₂ O), it reacts with Zn ions to be eluted from the defective portion of the chemical conversion coating and is hardly soluble. It is presumed to fulfill a self-repair function. For the above reasons, the molar ratio [Zn] / [P] of Zn and P in the amorphous compound is desirably 1.4 or less.

Furthermore, when the said amorphous compound contains the silane compound, the said molar ratio [Zn] / [P] will be less than 1.0, and it turned out that remarkably excellent white rust resistance is obtained. The reason for this is not necessarily clear, but the self-repairing function is achieved because the amorphous compound has a composition close to that of highly water-soluble primary zinc phosphate (Zn (H ₂ PO ₄ ) ₂ .H ₂ O). It is estimated that this is because it is significantly increased. It should be confirmed that the amorphous compound contains a silane compound by using X-ray absorption fine structure method, transmission electron microscopy, energy dispersive X-ray analysis, and electron energy loss spectroscopy in combination. Can do. For the above reasons, the amorphous compound preferably contains a silane compound.

The effect of suppressing the interference color varies depending on the degree of dispersion of the amorphous compound deposited in the chemical conversion film, and this degree of dispersion is determined by the occupancy ratio of the amorphous compound at the center of the film thickness cross section. Can be represented. Occupancy rate here is a value evaluated from a transmission electron microscope image of the chemical conversion film cross section, and as shown in FIG. 3, the film thickness center line of the film thickness cross section (film thickness direction cross section) of the chemical conversion film. The total length ratio occupied by the amorphous compound in an arbitrary length range of 1 μm or more along (the line connecting the center points in the film thickness direction) is shown (the length of the thick line passing through the amorphous compound in FIG. 3) The ratio of the total length to the measurement length L). Visible light is refracted / scattered by an amorphous compound as the occupancy increases, so the disturbance in the phase of visible light reflected on the front and back of the chemical conversion coating increases accordingly, producing interference colors. Is suppressed. As a result of comparing the degree of interference color and the occupancy rate of the amorphous compound, it was found that a particularly remarkable suppression effect was obtained when the occupancy rate was 30% or more. For the above reasons, it is preferable that the occupancy ratio of the amorphous compound at the central portion of the film thickness cross section of the chemical conversion coating is 30% or more.

Epoxy resins are suitable for the organic resin that is a component of the barrier layer of the chemical conversion coating. The present inventors evaluated the barrier properties of each organic resin by comparing the corrosion resistance of surface-treated steel sheets prepared by applying and baking only various organic resins to galvanized steel sheets, and as a result, various organic resins Among them, the epoxy resin has the best barrier properties.
The hot dip galvanized steel sheet used as the base steel sheet of the surface-treated steel sheet of the present invention is not only a normal hot dip galvanized steel sheet, but also a Zn—Ni, Zn—Fe, Zn—Cr, Zn—Mn, Zn—Co. Zn-Co-Cr-based, Zn-Cr-Ni-based, Zn-Cr-Fe-based, Zn-5% Al-based, Zn-55% Al-based, Zn-Mg-based, Zn-Al-Mg-based, etc. Various alloy-type hot dip galvanized steel sheets containing Zn, and zinc-based composite hot dip galvanized steel sheets in which metal oxides are dispersed in these plating films are included.

The surface-treated steel sheet of the present invention contains an organic resin (for example, one or more of a water-dispersible resin and a water-soluble resin) and phosphoric acid, and further melts a specific chemical conversion treatment liquid containing a silane compound as necessary. It can obtain by apply | coating to the surface of a galvanized steel plate, and drying. In the amorphous compound layer containing Zn and P and the amorphous compound layer containing Zn, P and O contained in a dispersed state in the chemical conversion film, phosphoric acid in the treatment solution reacts with zinc in the base galvanizing. Zinc is eluted by a weakly acidic treatment liquid containing an appropriate amount of phosphoric acid, and reacts with phosphoric acid in the treatment liquid to produce an amorphous compound. For this reason, it is preferable to perform a chemical conversion treatment using the processing liquid whose ratio of phosphoric acid is 10-50 mass parts (ratio of solid content) with respect to 100 mass parts of organic resin, and about pH 2-4. When the ratio of phosphoric acid in the treatment liquid is less than 10 parts by mass with respect to 100 parts by mass of the organic resin, a sufficient amount of amorphous compound is not generated, whereas when it exceeds 50 parts by mass, the coating becomes non-uniform, May cause uneven appearance. Moreover, adjusting the pH of the treatment liquid to 2 to 4 is more likely to occur when the zinc elution rate is lower in the acidic to neutral region, but re-dissolution of the generated amorphous compound is also likely to occur. As a result, the amount of the amorphous compound remaining in the film increases in the pH range of 2 to 4. The optimum conditions for the treatment liquid vary somewhat depending on other components contained in the treatment liquid, but may be adjusted as appropriate within the above range.
The surface-treated steel sheet of the present invention can be obtained by adhering the above treatment liquid to a plated steel sheet by a method such as coating, dipping, spraying or the like usually performed, and baking and drying at an optimum temperature according to the resin component.

The composition for chemical conversion treatment for preparing the surface-treated steel sheets of the inventive examples and comparative examples was prepared by blending a water-dispersible epoxy resin, phosphoric acid, and a silane compound in the ratios shown in Table 1, and then a paint stirrer (homo It was prepared by stirring for a predetermined time using a mixer. As the galvanized steel sheet to be subjected to chemical conversion treatment, a hot dip galvanized steel sheet having a coating adhesion amount of 60 g / m ² was used. After this hot-dip galvanized steel sheet is degreased with alkali, washed with water and dried, a predetermined amount of the above chemical conversion composition is applied to the surface with a bar coater, and left for 5 seconds or 10 minutes, and then an induction heating furnace is used. Using the hot plate furnace, the ultimate plate temperature of 140 ° C. or the ultimate plate temperature of 60 ° C. was used for heat drying, and five types of surface-treated steel sheets described in Table 1 were prepared. The adhesion amount of the chemical conversion film was adjusted to 1.0 g / m ² depending on the solid content (heating residue) of the chemical conversion composition and the application conditions (such as the bar coater count).

The white rust resistance of the obtained surface-treated steel sheet was evaluated by performing a salt spray test (JIS-Z-2371) on the steel sheet and evaluating the time until the white rust generation area ratio reached 5%. The evaluation criteria are as follows.
◎: 72 hours or more ○: 48 hours or more, less than 72 hours Δ: 24 hours or more, less than 48 hours ×: less than 24 hours Further, the appearance was evaluated from the degree of interference color visually. The evaluation criteria are as follows.
◎: Uniform bright appearance ○: Minor interference color that can be ignored in appearance ×: Interference color that cannot be ignored in appearance

  A cross-sectional sample for observation with a transmission electron microscope was prepared using a microsampling mechanism attached to a focused ion beam processing apparatus (FIB) FB2000A manufactured by Hitachi, Ltd. Prior to the FIB processing, the surface of the surface-treated steel sheet for preparing the cross-section sample is protected on the surface of the specimen from a Ga ion beam irradiation during sample processing. Flash vapor deposition was performed. The surface of the cross-section sample cut-out portion of the test material introduced into the FIB processing apparatus is further coated with a C protective film to a thickness of about 500 nm using the chemical vapor deposition (CVD) mechanism of the FIB apparatus, The sample was cut out. The cross-sectional sample taken out using the microsampling mechanism was fixed to a straight portion of a Mo meniscus special mesh using the CVD mechanism, and then finished to a film thickness of about 100 nm suitable for transmission electron microscope observation.

For the compositional analysis of the cross-sectional sample thus prepared, a transmission electron microscope CM20FEG manufactured by Philips equipped with a field emission electron gun and a product made by EDAX equipped with a Super-UTW type detector attached thereto are provided. An energy dispersive X-ray analyzer, Phoenix, was used. The acceleration voltage during microscopic image observation and elemental analysis was 200 kV.
Regarding the examples of the present invention, the compound precipitated in a dispersed state in the chemical conversion coating and the compound layer deposited on the plating surface were confirmed to be amorphous by their electron diffraction patterns. The composition of these amorphous compounds was determined by performing background removal, peak separation, and quantitative correction calculation from the spectrum data collected from these positions. At that time, if the background processing is performed automatically, the net intensity (net intensity) in the light element region may not be calculated properly. Therefore, manually specify the energy position where the peak does not appear and connect it manually. Treated (removed) by ground method. In the peak separation, characteristic X-rays such as Mo caused by Mo mesh used as a support for the cross-sectional sample and Ga caused by FIB processing were also considered. All quantitative calculations were performed using K-series characteristic X-rays of each element contained in the chemical conversion coating. The correction calculation was performed by thin film approximation, and a Zaluzec model was used as a correction factor (so-called K _AB factor). The molar ratio of Zn and P in the amorphous compounds listed in Table 1 was determined as the ratio of the atomic concentrations of Zn and P thus determined. Moreover, the occupation ratio (FIG. 3) of the amorphous compound in the film thickness cross-sectional center part of the organic resin matrix layer was calculated | required from the transmission electron microscope image of the chemical conversion treatment film cross section.

The result of analyzing the cross-sectional structure of the chemical conversion coating on the surface-treated steel sheet will be described. Table 1 also shows part of the analysis results, white rust resistance, and appearance evaluation results.
The chemical conversion film of Comparative Example 1 did not contain P, and an amorphous-like precipitated compound was not observed except for large silane compound aggregates. This film had a certain degree of white rust resistance, but had almost no interference color suppression effect.
The chemical conversion film of Comparative Example 2 did not contain Zn or P, and no amorphous precipitated compound was observed. This film was completely inadequate in white rust resistance and had no interference color suppressing effect.

On the other hand, in addition to organic resin, Zn and P were contained in the chemical conversion film of Invention Example 1, and an amorphous compound containing Zn and P was precipitated in a dispersed state. The amorphous compound had a [Zn] / [P] molar ratio of 1.4, but the occupation ratio was less than 30%. Further, an amorphous compound layer containing P, Zn and O was observed on the surface layer of the Zn plating. Both the white rust resistance and the appearance of this film were relatively good.
In the chemical conversion film of Invention Example 2, Zn and P were contained in addition to the organic resin, and an amorphous compound containing Zn, P and Si (silane compound) was precipitated in a dispersed state. The amorphous compound had a [Zn] / [P] molar ratio of less than 1.0 and an occupancy of over 30%. For this reason, this film was very good in both white rust resistance and appearance. In FIG. 1, the electron micrograph (bright field image) of the cross section of the chemical conversion film of this invention example 2 is shown. In this photograph, all indicated by arrows are amorphous compounds. Also in this photograph, an amorphous compound layer containing P, Zn and O is observed on the surface layer of Zn plating (note that the bubble contrast of the surface layer of Zn plating in the center of the photograph is damage caused by electron beam irradiation. ).

  In addition to the organic resin, Zn and P are contained in the chemical conversion film of Invention Example 3, and an amorphous compound having the same composition and molar ratio as that of Invention Example 2 is precipitated in a dispersed state. It was. The white rust resistance of this film was excellent, and as can be seen from comparison with Comparative Examples 1 and 2, an interference color suppressing effect was also observed. However, since this film had an occupancy ratio of less than 30% of the amorphous compound, a slight interference color that was not a problem in appearance was recognized. In FIG. 2, the electron micrograph (bright field image) of the cross section of the chemical conversion film of this invention example 3 is shown. In this photograph, all indicated by arrows are amorphous compounds. Also in this photograph, an amorphous compound layer containing P, Zn and O is observed on the surface layer of the Zn plating.

Electron microscope enlarged photograph of cross section of chemical conversion film of Invention Example 2 in Examples Electron microscope enlarged photograph of cross section of chemical conversion film of Invention Example 3 in Examples Explanatory drawing showing how to determine the occupancy rate of amorphous compound at the center of the film thickness cross section of the chemical conversion coating

Claims (5)

A surface-treated steel sheet having a chemical conversion treatment film containing an organic resin, Zn and P on the surface of a hot dip galvanized steel sheet, and not containing Cr, and an amorphous compound containing Zn and P in the chemical conversion treatment film Is contained in a dispersed state , and the occupation ratio of the amorphous compound in an arbitrary length range of 1 μm or more along the film thickness center line in the film thickness cross section of the chemical conversion coating is 21% or more A surface-treated steel sheet with excellent white rust resistance and appearance.   The chemical conversion treatment film is composed of an amorphous compound layer containing Zn, P and O and an organic resin matrix layer having a thickness of 400 to 800 nm thereon, and the organic resin matrix layer contains Zn and P. The surface-treated steel sheet having excellent white rust resistance and appearance according to claim 1, wherein the crystalline compound is contained in a dispersed state.   The white rust resistance and appearance according to claim 1 or 2, wherein the molar ratio [Zn] / [P] of Zn and P in the amorphous compound containing Zn and P is 1.4 or less. Excellent surface-treated steel sheet. Any of claims 1-3 in which the occupancy of the amorphous compound in any length range 1μm or more along the thickness center line in the thickness cross section of the chemical conversion film is characterized in that 30% or more A surface-treated steel sheet with excellent white rust resistance and excellent appearance. The surface-treated steel sheet having excellent white rust resistance and appearance according to any one of claims 1 to 4 , wherein the organic resin is an epoxy resin.

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