JPS6224505B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to zinc or zinc alloy plated steel sheets (hereinafter referred to as
It is simply called galvanized steel sheet. ) relates to a chemical conversion treated steel sheet whose surface has been subjected to chromate treatment. The double-layer chromate-treated steel sheet of the present invention can be used in fields that require particularly high lubricity, as well as corrosion resistance and excellent performance as a paint base, such as as a base-treated steel sheet for home appliances and building materials. Available. [Prior Art] Chromate-treated steel sheets are often used as chemically treated steel sheets for the purpose of rust prevention of galvanized steel sheets, from the viewpoint of both rust prevention and cost. However, in most cases, its purpose is only to temporarily prevent rust from the time the steel plate is manufactured until it is used by customers, and its corrosion resistance is, for example, 24 to 48 at most in the salt spray test (JISZ-2371).
The level is so low that it will cause rust in about a few hours. Therefore, in severe corrosive environments, it is necessary to take measures to prevent corrosion, such as applying a separate coating with a thickness of 5 μm or more to processed products. However, such methods are not economical, and in recent years, it has been desired to develop chromate-treated steel sheets that require excellent press workability and high corrosion resistance after processing, even though they are simply chemically treated without painting. It became like that. There are two general chemical conversion treatment methods for galvanized steel sheets: chromate treatment and phosphate treatment. ,
Chromate treatment, which is advantageous in terms of cost, is considered effective. Conventional chromate treatments are roughly divided into three types, electrolytic type, reaction type, and coating type, depending on the treatment method, and the chromate film produced has its own characteristics as described below. In other words, the electrolytic chromate treatment method uses an aqueous solution of chromic acid or its salt as a basic bath, and if necessary, performs cathodic electrolysis treatment in a bath to which an appropriate amount of anion such as sulfuric acid is added, thereby forming a gel on the surface of a galvanized steel sheet. This method produces a chromate film consisting of Cr ³⁺ in the form of
Because Cr ⁶⁺ is low and the Cr ⁶⁺ /Cr ³⁺ ratio is small,
Because it is a film that is poorly soluble in water and relatively porous, it has excellent compatibility with paints and exhibits an excellent anchoring effect as a base for painting. However, since the ratio of Cr ⁶⁺ /Cr ³⁺ is small, the self-repairing effect of Cr ⁶⁺ can hardly be expected, so high corrosion resistance cannot be obtained even in the state of an unprocessed plate (flat plate), and in addition, it is not possible to obtain high corrosion resistance even in the state of an unprocessed plate (flat plate). If this happens, there are drawbacks such as further deterioration in corrosion resistance. Similar to the electrolytic chromate treatment bath, the reactive chromate treatment method involves adding appropriate amounts of mineral acids such as sulfuric acid or nitric acid and, if necessary, hydrofluoric acid or phosphoric acid to a bath based on chromic acid. A galvanized steel sheet is immersed ⁱⁿ a treatment bath of
The chromate film that forms on the galvanized surface layer interface is unique in that a thick film can be obtained in a relatively short period of time, but like the electrolytic type, only a film consisting mainly of Cr ³⁺ with little Cr ⁶⁺ can be obtained. Therefore, although the amount of chromate film is high, its corrosion resistance is low. Next, regarding the coating-type chromate treatment method, an aqueous solution using chromic anhydride is used as a basic bath, and a predetermined amount of a treatment solution containing an inorganic polymer such as silica sol is applied to the surface of a galvanized steel sheet and dried. The chromate film obtained by this treatment method contains a relatively large amount of Cr ⁶⁺ , which has a self-repairing effect, so it has the best corrosion resistance among the different types of chromate films mentioned above. Therefore, high corrosion resistance can be expected depending on the amount of chromium deposited. However, since Cr ⁶⁺ is soluble in water, it is easily washed away, and there is a natural limit to the effective deposition amount, including in terms of cost. As mentioned above, galvanized steel sheets that have been subjected to conventional inorganic chromate treatment have recently become more common to undergo complex processing, especially as the market diversifies. High corrosion resistance is particularly required, but as mentioned above, this requirement cannot be met due to the characteristics of each chromate film. For example, in general consumers, chromate-treated steel sheets are often used in the following process. That is, the processing steps include blanking → press molding → welding, assembly → degreasing → painting. The quality problems of the chromate-treated steel sheet in this process are as follows. Falling off of the chromate film at the mold part during press processing. Rust formation from processed parts during the post-degreasing process. After degreasing, rust forms from the fingerprints that adhere when handling the product. Elution of chromate film during degreasing. Recently, resins have been used in many cases as a technology to solve these problems. That is,
The coating structure is a chromate-treated steel sheet consisting of a two-layer coating consisting of a conventional chromate coating and a resin coating. An example of such a case is, for example, the 1977-35620, which is coated with clear resin.
No. 57-108292, in which a composite resin made by adding silica sol and a silane coupling agent to an aqueous emulsion resin is coated, or a composite made by adding chromium salt and metal powder to an aqueous emulsion resin. Special Publication No. 58-5262, which coats prepolymer resin and forms a film by irradiating it with ultraviolet rays and electron beams.
and so on. These technologies are characterized by improved anti-fingerprint adhesion properties of chromate-treated steel sheets, improved corrosion resistance from those areas, and superior performance as a paint base.In this respect, conventional chromate-treated steel sheets It can be said that some of the remaining quality problems have been resolved. However, as mentioned above, in response to the diversification of the market, advanced press forming processing is required in general customer processing processes, but in this case, the elongation of the upper resin film is sufficient compared to the processing elongation of the base steel plate. It is often not possible to follow the process, and the resin film sometimes breaks during processing, and sometimes loses its adhesion to the substrate and peels off, resulting in a significant loss of commercial value. In addition, due to the frictional heat generated during press processing, these aqueous emulsion resin films with a low degree of polymerization and low Tg generally have thermoplastic characteristics, so they tend to soften and swell, so the selection of press oil is particularly important. There is also the disadvantage that the resin film that is required or falls off from the substrate depending on the degree of processing accumulates on press equipment such as dies and punches, causing damage such as deterioration of workability and working environment. Furthermore, the corrosion resistance of parts where the resin film whitens or peels off during press working may drop to the same level as conventional chromate-treated steel sheets, and the deterioration of corrosion resistance of processed parts due to lack of lubricity may not necessarily be considered. It's not something to be satisfied with. [Objective of the Invention] The object of the present invention is to eliminate the various drawbacks of conventional chromate-treated steel sheets and chromate-treated steel sheets consisting of a two-layer coating combined with a water-based emulsion resin coating, as described above, and further, Chromate treatment provides high lubricity, maintains high corrosion resistance even after processing, and exhibits excellent performance as a paint base, as it satisfies high quality demands such as the elimination of painting. The point is to provide steel plates. [Structure of the Invention] In forming a chromate film on the surface of zinc or galvanized steel sheet, the present invention provides a first layer film in which the ratio of Cr ⁶⁺ /Cr ³⁺ is 3/10 to 30/10, and It has a chromate film with a chromium adhesion amount of 30 to 150 mg/m ² (one side), and a second layer composite chromate film with a water soluble content of 0.3 to 1.5% on top.
A powdered chromium compound of Sr, Ca, Zn, and Pb with a particle size of 5 to 20 μm and fixed acid groups.
A thermosetting pigment made by urethane-modifying an epoxy resin with an amine at the polymer end as a binder, and a rust-preventive pigment consisting of 3 to 25 wt% of one or two kinds and 1 to 6 wt% of composite aluminum phosphate. Using 40 to 70 wt% of resin, 1 to 15 wt% of polyolefin wax as a lubricant,
Molybdenum disulfide 0.3-10wt%, silicone resin
This is a two-layer chromate-treated steel sheet characterized by forming a composite chromate film containing 0.01 to 1.5 wt% in a range of 1 to 10 g/m ² , and its main points are as follows. The first point is that the chromate film of the first layer has high corrosion resistance by suppressing swelling and elution with water, and also improves the adhesion between the base plated surface and the composite chromate film of the upper second layer. The key to improving this is to set an appropriate ratio of Cr ⁶⁺ /Cr ³⁺ in the chromate film. Secondly, in the second composite chromate film on top of the second layer, we improved the water swelling resistance of the anticorrosion pigment to water, and furthermore, in order to provide excellent adhesion as a paint base, we added urethane-modified The point is that a thermosetting resin is used as a binder for the rust-preventing pigment. The third point is the most important point in the configuration of the present invention, which is to provide sufficient elongation to the composite chromate film during press processing, prevent damage to the film due to processing, and to prevent damage to the film after processing. The lubricant is used to maintain the product appearance and high corrosion resistance. In the first layer chromate film, Cr ⁶⁺ /
If the ratio of Cr ³⁺ is less than 3/10, the self-healing effect of Cr ⁶⁺ will be poor, and the corrosion resistance after press working will be significantly impaired, which is not very preferable. In addition, if the ratio exceeds 30/10, although it is advantageous for corrosion resistance, the increase in Cr ⁶⁺ leads to a decrease in the water swelling resistance of the chromate film, resulting in a decrease in the resistance to the second layer of the upper composite chromate film. This is disadvantageous in that it impairs adhesion and thus impairs paint adhesion during topcoating. From the above points, the first layer chromate film
The ratio of Cr ⁶⁺ /Cr ³⁺ is preferably 3/10 to 30/10, preferably 5/10 to 25/10. In addition, the amount of chromium deposited on the first layer chromate film (one side) is 30mg/
If it is less than m ² , it is difficult to expect high performance in terms of corrosion resistance after press working. In addition, if it exceeds 150 mg/m ² , there is no problem in terms of showing high corrosion resistance due to a substantial increase in water-soluble Cr ⁶⁺ , but the adhesion with the upper second layer composite chromate film or its use as a base for painting may be affected. It becomes difficult to maintain excellent top coatability. Therefore, the preferred range of chromium deposition is 40 to 120 mg/m ² on one side. In order to obtain the appropriate ratio of Cr ⁶⁺ /Cr ³⁺ in the first layer chromate film, one or a combination of the above-mentioned conventional chromate treatment methods may be used, but corrosion resistance and versatility to the line,
From the economic point of view, coating type chromate treatment is preferred. Next, in the second layer of the composite chromate film, the resin used as the binder for the anti-rust pigment is a bisphenol A type epoxy resin (a ₁ ) with a fatty acid dibasic acid (a ₂ ) and a primary hydroxyalkyl group. A prepolymer [A] having a primary hydroxyl group at the terminal obtained by reacting a secondary amine (a ₃ ) with a polyisocyanate compound (b ₁ ) and an isocyanate protecting agent (b ₂ ) are reacted. The ratio of the number of primary hydroxyl groups contained in A/the number of free isocyanate groups contained in [B] is from 10/1 to the partially protected isocyanate compound [B].
It is a thermosetting urethane-modified epoxy resin made by reacting so that the ratio is 1/1. This resin has the structural formula: [In the formula, -R is -H or -CH ₃ , -A is 〓C
(CH ₃ ) ₂ , n is 0 or an integer from 1 to 14] In order to impart film elongation properties during processing, the resin is chain-extended with a dicarboxylic acid and its It is obtained by adding a secondary amine having a primary hydroxyl group at the end. Partial protection of a polyisocyanate compound is added to this obtained resin, and the binder effect of the rust-preventing pigment is exerted. As the epoxy resin component (a ₁ ) in the present invention, an epoxy resin represented by the above structural formula may be used.
YD-011, YD-014, YD-017, YD-128
and Epicron 4050 manufactured by Dainippon Ink Chemical Industries, Ltd.
Commercially available products such as Examples of fatty acid dibasic acids (a ₂ ) include polymethylene dicarboxylic acids (adipic acid, azelaic acid, etc.), aromatic dicarboxylic acids (phthalic anhydride,
phthalic acid, etc.), and one or more of these can be used, but polymethylene carboxylic acid is particularly effective as a binder for the rust-preventing pigment. Azelaic acid is preferred for improving the elongation of the film. Furthermore, the secondary amine (a ₃ ) has the role of a catalyst that allows the reaction between a ₁ and a ₂ , that is, the addition reaction of glycidyl groups and carboxyl groups, to proceed rapidly at low temperatures. This is important in that it introduces a primary hydroxyl group into the resin, facilitates the crosslinking reaction during film formation, and suppresses the water swelling resistance of Cr ⁶⁺ from the composite chromate film. Furthermore, the nitrogen introduced into the skeleton acts effectively as a catalyst, promoting the curing reaction. Examples of such secondary amines include dialkanolamines (diethanolamine, dipropanolamine, dibutanolamine, etc.) and alkylalkanolamines (ethylethanolamine, etc.), and one or more of these can be used. Particularly preferred is dialkanolamine,
Among them, diethanolamine is most suitable. The prepolymer [A] in the present invention is obtained by reacting the epoxy resin (a ₁ ) with a dicarboxylic acid (a ₂ ) and a secondary amine (a ₃ ), but in this case, the anticorrosive pigment From the binder effect, [g equivalent of glycidyl group in (a ₁ ) - g equivalent of carboxyl group in (a ₂ )]/[total weight (g) of (a ₁ ) + (a ₂ ) + (a ₃ ) ] value is 7×10 ^-4 g equivalent/g or less,
Preferably 4×10 ⁻⁴ to 0.3×10 ⁻⁴ g equivalent/g and [g equivalent of carboxyl group of (a ₂ ) + g equivalent of amino group of (a ₃ )]/[glycidyl of (a ₁ )] The ratio of g equivalent of groups] is 4/5 to 6/5, preferably 9/10 to
It is necessary to set it within a range of 23/20.
If the above value exceeds 7×10 ⁻⁴ g equivalent/g, the binder effect of the rust-preventing pigment decreases and sufficient processability cannot be obtained. Also, if the above ratio is less than 4/5,
It is difficult to obtain sufficient thermosetting properties, and if the ratio exceeds 6/5, unreacted dicarboxylic acid or secondary amine will remain, impairing the water resistance and chemical resistance of the composite chromate film. Furthermore, the partially protected isocyanate compound [B] of the present invention is obtained by combining the polyisocyanate compound (b ₁ ) and the isocyanate protecting agent (b ₂ ) in a ratio of the number of isocyanate groups in (b ₁ )/the number of active hydrogen groups in (b ₂ ). The ratio is 7/1~
7/6, but this system usually includes a fully blocked form in which all isocyanate groups are protected, a partially blocked form in which the remaining isocyanate groups are partially protected, and a partially blocked form in which all isocyanate groups are protected. Unblocked products with all isocyanate groups left unprotected are present. In the partially protected isocyanate compound [B], the entire block functions as a crosslinking agent when forming a film from a thermosetting resin. Also, among the partially blocked bodies, those having one active isocyanate group and one or more blocked isocyanate groups are added to the epoxy prepolymer [A] and serve as a crosslinkable functional group during film formation. However, those having two or more active isocyanate groups and one or more blocked isocyanate groups have the role of chain-extending [A] to increase the molecular weight and adding a crosslinkable functional group. On the other hand, the unblocked form has the role of extending the chain of [A] to increase the molecular weight and introducing a urethane bond into the main chain. When the ratio of the number of primary hydroxyl groups contained in the prepolymer [A]/the number of free (active) isocyanate groups contained in the partially protected isocyanate compound [B] is less than 1/1, the binder effect of the rust preventive pigment is is disadvantageous in that it cannot be used effectively, and if it exceeds 10/1, the crosslinking density of the resin becomes excessive,
Since the composite chromate film lacks processing elongation and causes mold galling and whitening during press working, it is preferably 7/1 to 3/1. When the thermosetting urethane-modified epoxy resin prepared as described above is used as a binder for the anti-corrosion pigment, if it is less than 40 wt%, the binder effect is insufficient, so the water swelling resistance of the composite chromate film decreases, and the necessary As a result, the rust-preventing pigment is leached out, leading to a significant decrease in corrosion resistance, and furthermore, during press working, the processed part is accompanied by whitening and peeling phenomena, causing significant damage. On the other hand, if it exceeds 70wt%, the scratch resistance of the composite chromate film decreases,
Preferably, it is 45 to 65 wt%, since it causes scratches on the appearance during handling and impairs the commercial value. Next, among the anti-rust pigments mentioned above, chromium compounds are
If it is less than 3wt%, it is difficult to maintain the corrosion resistance of the pressed part at a high level. Moreover, when it exceeds 20wt%, the lubricity of the composite chromate film decreases,
This may lead to mold galling, whitening or peeling of the press processing part,
Preferably from 7 to 7, as it greatly reduces product value.
17wt% is good. On the other hand, composite aluminum phosphate is effective in improving the creep resistance of the composite chromate film, but when it is less than 1wt%, the effect is halved, and when it exceeds 6wt%, the processing elongation of the composite chromate film is reduced. Preferably 2.
~4wt% is good. The total amount of the above-mentioned anticorrosion pigments is 4 to 25 wt%, preferably 9 to 21 wt%. Furthermore, if the water-soluble content of the above-mentioned rust-preventing pigment exceeds 1.5%, the water swelling resistance of the composite chromate film will decrease, blisters will occur frequently in the film, and the overall corrosion resistance will decrease.
If it is less than 0.3 wt%, water resistance and self-healing action by Cr ⁶⁺ will be insufficient, and high corrosion resistance will be difficult to obtain, so a range of 0.5 to 1% is preferable. In addition, in order to provide high rust prevention and press workability, it is necessary to consider the particle size of the rust preventive pigment, but if it exceeds 20 μm, the appearance smoothness of the composite chromate film will decrease, so press products When the particle size is less than 3 μm, the adhesion of the composite chromate film to the substrate decreases, resulting in whitening and peeling of the processed area during press forming. The thickness is preferably 5 to 15 μm because it is easy to form. Next, among lubricants, polyolefin wax may be made of low molecular weight polyethylene, polypropylene, etc. with a molecular weight of about 1500 to 3000, and
Special silicone resins such as molybdenum disulfide with a particle size of 10 μm or less and a purity of 99.8% or more, and alkylsilanes such as polyoxyalkylsilanes having polyalkyl groups are used. In this case, graphite, silicone oil, etc. are used if only to provide lubricity to the coating film, but graphite is black and difficult to obtain a desired color, while silicone oil is too slippery and cannot be used as a topcoat. It is not appropriate because it tends to cause a decrease in compatibility (repellency, adhesion). Thus, the lubricant used in the present invention is used within the following range. If the low molecular weight polyethylene wax is less than 1wt%, the lubricating effect will be small, and if it exceeds 15wt%, the surface gloss, blocking resistance, and overcoatability of the composite chromate film will deteriorate, so the preferred range is 4 to 8wt%. It is. Molybdenum disulfide is 0.3wt
If it is less than 10wt%, there is almost no lubricating effect, and if it exceeds 10wt%, corrosion resistance will decrease, so the preferred range is
It is 0.5-7wt%. Also, silicone resin is 0.01wt.
If it is less than 1.5wt%, the lubricating effect will be small, and if it exceeds 1.5wt%, the top coatability will be greatly reduced, so it is preferable.
It is 0.03-1.0wt%. From the above, the total amount of lubricant is 5 to 15 wt%, preferably 4 to 10 wt%. In addition to these, coloring pigments such as titanium oxide, red iron oxide, cyanine blue, carbon volak, etc., and extender pigments such as clay, barium sulfate, calcium carbonate, silica, etc. may be used in combination, if necessary. In addition, the composite chromate film constructed in this manner can be obtained by applying and baking by flow coating or roll coating, but if the amount of coating is less than 1 g/m ² , the treated appearance may be transparent. , and cannot maintain high lubricity and high corrosion resistance for processed parts. Moreover, if it exceeds 10 g/m ² , the spot weldability will deteriorate, so it is preferably 2 to 5 g/m ² . The double-layer chromate-treated steel sheet obtained by the method of the present invention as described above is a value-added steel sheet that can be applied to applications that require particularly high lubricity and high corrosion resistance in pressed parts, as well as excellent performance as a paint base. Highly chromate treated steel sheet. The galvanized steel sheets used in the method of the present invention include electrogalvanized steel sheets, electrogalvanized steel sheets,
It may be either a hot-dip galvanized steel sheet or a hot-dip zinc alloy-plated steel sheet. In addition, the baking plate temperature of the second layer composite chromate film is the final plate temperature.
It is sufficient to maintain the temperature within the range of 200 to 240°C, and hot air, infrared rays, etc. may be used as the baking method. [Embodiments of the Invention] Examples of the present invention will be described below. Examples of the present invention are shown in No. 1 to No. 60 in Table 1, and comparative examples are shown in No. 61 to No. 93 in Table 1. Further, those obtained by the conventional method are shown in Nos. 94 to 96 of Table 1.

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〔Effect of the invention〕

As is clear from the results shown in Table 1, the method of the present invention has far superior performance in press workability, corrosion resistance, and as a base for painting, compared to the chromate-treated steel sheet produced by the conventional method. It can be provided as a highly corrosion-resistant chromate-treated steel sheet with excellent lubricity that satisfies the requirements.

Claims (1)

[Claims] 1. When forming a chromate film on the surface of a zinc or zinc alloy plated steel sheet, the first layer is
It has a chromate film with a Cr ⁶⁺ /Cr ³⁺ ratio of 3/10 to 30/10 and a chromium deposition amount of 30 to 150 mg/m ² on one side, and a second layer on top of the chromate film.
6wt% composite aluminum phosphate, and Sr, Ca,
A chromium compound whose acid groups are fixed with Zn and Pb
A powdered anti-corrosion pigment containing 3 to 20 wt% of one or two types, and a water-soluble content of 0.3 to 1.5% and a particle size of 5 to 20 μm, and 1 to 15 wt% of polyolefin wax as its binder. Molybdenum disulfide 0.3
~10wt% of a lubricant, a silicone resin of 0.01 to 1.5wt%, and a mixture of 40 to 70wt% of a thermosetting resin obtained by modifying urethane to an epoxy resin with an amine at the polymer end. A two-layer chromate-treated steel sheet having excellent corrosion resistance and lubricity, characterized by forming a chromate film in the range of 1 to 10 g/m ² .