JPS6013078A - Double-chromated steel sheet - Google Patents

Abstract

PURPOSE:To provide superior corrosion resistance, press workability and spot weldability to a zinc or zinc alloy plated steel sheet by forming a chromate film having specified conditions on each surface of the plated steel sheet and a composite chromate film having specified conditions on said chromate film. CONSTITUTION:A chromate film having 3/10-3/1 ratio of Cr<6+>/Cr<3+> and contg. 30-150mg/m<2> Cr is formed as the 1st layer on each surface of a zinc or zinc alloy plated steel sheet. A composite chromate film contg. 10-80wt% powdered Cr compound of <=1mum particle size in a thermosetting resin as a binder is then formed as the 2nd layer on said chromate film by 0.5-5g/m<2>. The acid group of the Cr compound is fixed beforehand with an alkali metal or an alkaline earth metal. The thermosetting resin is prepd. by modifying epoxy resin having amines at polymer terminals with urethane.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface-treated steel sheet in which the surface of a galvanized or zinc alloy coated steel sheet (hereinafter simply referred to as a galvanized steel sheet) is subjected to chromate treatment.

[Industrial application field]

The surface-treated steel sheet of the present invention is used, for example, as a steel sheet for home appliances and automobiles, which requires excellent corrosion resistance, spot weldability, and finish coatability.

[Prior art]

Chromate-treated steel sheets are often used as chemically treated steel sheets for the purpose of rust-preventing galvanized steel sheets.

However, most of the purposes are to temporarily prevent rust from the time the steel plate is manufactured until it is used by the customer.
Its corrosion resistance can be determined by, for example, the salt spray test (JIS Z-237
1), the corrosion resistance was only at a low level, with rust forming in about 24 to 48 hours at most.

Therefore, under severe corrosive environments, processed products
It is necessary to prevent corrosion by applying a coating with a thickness of μm or more. However, this method is unfavorable from an economic point of view, and in recent years there has been a desire to develop chromate-treated steel sheets that have excellent corrosion resistance by simply applying chemical conversion treatment without painting. .

Generally, chemical conversion treatment methods are broadly divided into chromate treatment,
There are two types of phosphate treatment, but chromate treatment is considered to be more effective in terms of high corrosion resistance and workability.

Conventional chromate treatments are broadly classified into three types based on their treatment methods: electrolytic, reactive, and coating, and the chromate films produced each have their own characteristics as described below. In other words, the electrolytic chromate method uses an aqueous solution prepared by dissolving chromic acid or its salt as a basic bath, adds an appropriate amount of sulfuric acid, etc., and performs cathodic electrolysis to form a gel-like Cr on the surface of a base galvanized steel sheet. This method produces a chromate film, and this chromate film is cr6+/cr3+
Because it is possible to reduce the ratio of

However, if the ratio of Cr /Cr is too small, the rust prevention properties will be reduced. In the two-reaction chromate method, a galvanized steel sheet is immersed in a treatment solution made by adding appropriate amounts of mineral acids such as sulfuric acid and nitric acid and, if necessary, hydrofluoric acid and phosphoric acid to the basic chromic acid mentioned above. This is a method in which a chromate film is reduced and precipitated on the galvanized surface through a chemical reaction at the galvanized surface.

The chromate film obtained by this method is relatively thick, but like the electrolytic type, it is a film mainly composed of cr3+ with little cr6+, so it is poorly soluble in water and has an excellent anchoring effect as a paint base. It has poor action and corrosion resistance is not high considering the high amount of chromium deposited.

Next, regarding the coating type chromate method, a predetermined amount of a treatment solution made by adding an inorganic polymer such as silica sol to a basic bath of an aqueous solution using chromic anhydride is applied onto a galvanized steel sheet and dried. Since the chromate film obtained by this treatment method contains a large amount of Cr, which has a self-repairing effect, it has the best rust prevention ability among the three types, including the different types of chromate films mentioned above, and the amount of chromium deposited is Shows high corrosion resistance according to

Shikajinaka 1pcr has the property of being soluble in water.

There is a limit to the amount of chromium deposited in order to use water-soluble paints and maintain excellent paint adhesion. Conventionally, inorganic chromate treatment as described above is often used alone, and in this case, it is difficult to expect high corrosion resistance from the characteristics of the film. Furthermore, generally, chromate-treated steel sheets are often used by consumers in the following process. That is, the processing steps include blanking - press forming - welding 9 assembly - degreasing - painting.

The quality problems of chromate-treated steel sheets in this process are: ■ After degreasing, rust develops from fingerprints that adhere during handling of the product, ■ Falling off of the chromate film due to mold galling during press processing, ■ Chromate during degreasing This includes elution of the film.

Therefore, as a technique to solve these problems, there is a recent example of using clear resin. For example, special public relations
As shown in No. 5620, it consists of two layers of chemical conversion coatings, with a chromate coating formed in the lower layer and a water-soluble resin formed in the upper layer.

However, in this chemical conversion coating, the upper water-soluble resin coating has a low molecular weight as a resin structure and a glass transition point (
TV) is low, and furthermore, the functional groups in the resin are hydrophilic, so during severe press working, there is poor adhesion to the base metal, and the resin film in the processed area tends to peel off easily.

Furthermore, since this resin film is easily swollen by water, its barrier effect on preventing elution of the underlying chromate film is small.

Therefore, although the effect of preventing the adhesion of fingerprints is excellent, the effect of using a water-soluble resin in improving corrosion resistance cannot necessarily be said to be sufficient. Similar examples are proposed in Japanese Patent Publication No. 49-36100 and Japanese Patent Publication No. 50-18455. However, all of these have in common that the functional groups in the upper resin film are hydrophilic, and for this reason, it is difficult to expect high corrosion resistance and press workability. Rather, these are considered to be effective only as paint bases that utilize functional groups.

On the other hand, as an improvement technique for these, a composite steel plate that is a coating base that solves the drawbacks of the above-mentioned water-soluble resin by strengthening its composition and has high corrosion resistance was developed in Japanese Patent Publication No. 57-10.
No. 8292 has been proposed.

In other words, we use a water-dispersible emulsion resin instead of the conventional water-soluble one as the resin that forms the upper layer film, basically polymerize the resin, and then add a chelate compound and an inorganic polymer compound such as silica sol. The crosslinking density was increased and the resin film was made into a polymer resin.This resin film was used as the upper layer, and a reactive chromate film was provided as the lower layer.
Due to the synergistic effect of the two-layer coating, corrosion resistance is dramatically improved.

However, in order to obtain excellent corrosion resistance with this composite steel plate, there is a drawback that the spot weldability must be sacrificed, which is undesirable because it complicates the welding and assembly work in the product processing process at the customer mentioned above. . Therefore, a high degree of corrosion resistance cannot be expected if the thickness of the composite resin film does not inhibit spot weldability.

Furthermore, because the resin film has a high cross-linking density, it has excellent adhesion to the substrate, but during severe press processing, the resin film has the disadvantage of chipping in the processing elongation, mold galling at the processed part, or peeling off the film. .

[Purpose of the invention]

The present invention eliminates the various drawbacks of conventional chromate-treated steel sheets and chromate-treated steel sheets using resins as described above, and is also capable of satisfying high quality demands such as omitting painting. The purpose is to provide a double-layer chromate-treated steel sheet that has poor corrosion resistance, press workability, and poor weldability.

[Structure of the invention]

The gist of the present invention is that in forming a chromate film on the surface of a galvanized steel sheet, Cr/Cr is used as the first layer film.
The ratio of 3/10-3/l and the amount of chromium deposited is 30
A chromate film of ~1507 nW/m' (one side) is formed, and a second film is formed on top of the thermosetting resin, which is made by urethane-modifying an epoxy resin with an amine at the polymer end, as a binder, and an alkali metal or alkaline earth 2, characterized in that a composite chromate film containing 10 to 80% by weight of a powdered chromium compound having a particle size of 1 μm or less and fixing acid groups of similar metals is formed in the range of 05 to 52/I. This is a layered chromate-treated steel sheet, and its contents are described below.

First, in the first layer chromate film, in order to suppress swelling and elution with water to impart high corrosion resistance and to improve the adhesion of the composite chromate film ♂ of the second base plating layer, Cr/Cr in the chromate film is The point is that an appropriate ratio is set for Cr.

Next, in the second layer composite chromate film on top of the chromate film, a urethane-modified thermosetting resin was added to provide even higher corrosion resistance and press workability, as well as excellent adhesion as a coating base. It is used as a binder for chromium compounds, and is the most important technology in the construction of the present invention.

Cr”/Cr3” (7) in the first layer chromate film
If the ratio is less than 3/lO, the self-healing effect of c, 6+ is poor, and the corrosion resistance after pressing is significantly inhibited.If the specific force exceeds 13/1, the corrosion resistance is further improved, but excessive Cr This is also disadvantageous from a cost perspective, as it reduces the swelling resistance of the chromate film to water, and this also leads to a decrease in the adhesion between the base plating and the upper second layer composite chromate film.

From the above, when the composite chromate film is provided in the upper layer, the Cr6yCr3+ ratio of the first layer chromate film in the lower layer is 3.
/10-3/1, preferably 5/10 to 2/1.

Furthermore, if the amount of chromium deposited is less than 30πy/rr1, it is difficult to expect high performance in terms of corrosion resistance ζ after press working, while if it exceeds 150πy/I, there is no problem in obtaining high corrosion resistance, but the This is undesirable because it impedes the adhesion of the two-layer composite chromate film or the top coat. Good talent is 5
o~120~/Bakayoi.

The method for obtaining an appropriate ratio of cr6+/cr3+ in the first layer chromate film may be one of the above-mentioned conventional chromate treatment methods or a combination of two or more, but from the viewpoint of corrosion resistance and economic efficiency, A coating type chromate treatment is preferable.

Note that a reducing agent such as hydrogen peroxide or alcohol may be used to generate Cr in the coating type chromate treatment.

Next, in forming the second layer of composite chromate film, the resin used as a binder for the chromium compound has the structural formula [wherein -R is -H or -CH3, -A is C (CH3
) II-CI-b, -0- or -8- and n are 0 or an integer from 1 to 14. ] An epoxy resin component (a,) consisting of 50 to 100% by weight of the epoxy resin represented by and 50 to 0% by weight of other epoxy resins, a dicarboxylic acid (a2), and a secondary compound having at least one primary hydroxyl group. The product (A) obtained by reacting the class amine (a,), the polyisocyanate compound (bt), and the inocyanate protecting agent (b2), the number of inocyanate groups in (bt)/the active hydrogen in (bt) The product (13) obtained by reacting so that the ratio of the number of groups is 7/1 to 7/6 to the number of primary hydroxyl groups contained in the product (A)/product CB). It is a thermosetting resin obtained by reacting so that the ratio of the number of active incyanato groups contained is 20/1 to 1/1.

This resin has the structural formula: [In the formula, -■ is -H or -CH, - is >C (CI
I3) 2 and n are 0 or an integer of 1 to 14. ] In order to impart film elongation to the general epoxy resin shown in the figure above, the resin is chain-extended with a dicarboxylic acid, and a secondary amine having a primary hydroxyl group is added to the end of the resin. can get.

Furthermore, partial protection of a polyisocyanate compound is added to this obtained resin, so that the binder effect of the chromium compound is exerted.

In the present invention, as the epoxy resin component (,,), an epoxy resin represented by the above structural formula may be used, for example, the product name [Epoto YD-
017J [same YD-014J r same YD-01
1J "YD-128J, manufactured by Dainippon Ink and Chemicals Co., Ltd. [
Commercially available products such as Epiclon 4050J are listed, and the system is constructed by replacing these alone or up to 50% by weight with other epoxy resins (eg, polyalkylene glycol diglycidyl ether, etc.).

When the above substitution rate exceeds 50% by weight, the coating properties of the chromium compound deteriorate, resulting in poor corrosion resistance, water resistance, and adhesion.

Chemical resistance, hardenability, etc. decrease.

In the present invention, the dicarboxylic acids (a2) include 1, for example, polymethylene dicarboxylic acids (adipic acid, azelaic acid, cepatic acid, etc.), aromatic dicarboxylic acids (phthalic anhydride, phthalic acid, isophthalic acid, terephthalic acid, etc.), Examples include dimer acid, polybutadienecarboxylic acid, polyester dicarboxylic acid, etc., and one or more of these may be used. In particular, polymethylenedicarboxylic acid is excellent in terms of coverage and press workability of the chromium compound, and azelaic acid is the most preferred among them.

In the present invention, the secondary amine (a3) has the role of a catalyst that allows the reaction between the aI component and the 32 components, that is, the addition reaction between the glycidyl group and the carboxyl group, to proceed rapidly at a low temperature. The -NH group is added to the glycidyl group to introduce a primary hydroxyl group into the resin, which is important in that it facilitates the crosslinking reaction during film formation and suppresses the hot water extraction resistance of Cr6+ from the composite chromate film. It is. Further, since the nitrogen introduced into the skeleton acts effectively as a catalyst, the curing reaction is promoted.

Examples of such secondary amines include dialkanolamines (jetanolamine, dipropanolamine,
(e.g., jetanolamine), alkylalkanolamines (e.g., ethylethanolamine), and one or more of these may be used. Particularly, dialkanolamines are preferred, and among them, jetanolamine is most suitable.

The product (A) in the present invention is the epoxy resin component (
al), dicarboxylic acid (al), and secondary amine (a,)
,! : can be obtained by reacting, but in this case,
From the viewpoint of coverage of the chromium compound.

[1 equivalent of glycidyl group in (a,) - 2 equivalents of carboxyl group in (al)) / ((a+) + (al) + Ca
5) total type bone (y)] value is 7×1017 equivalent force or less, preferably 4X], 0' to 0.3XIO-'g equivalent/y
and 2 equivalents of carboxyl group of C(,2) + (a3)
The ratio of (V equivalent of the amine group in (a,)) / (7 equivalents of the glycidyl group in (a,
It is necessary to set the value within a range of 23/20.

If the above value exceeds 7×10 → equivalent weight/liter, the coating properties of the chromium compound will deteriorate and sufficient workability will not be obtained.

In addition, if the above ratio is less than 475, sufficient thermosetting properties cannot be obtained, and if it exceeds 615, unreacted dicarboxylic acid or secondary amine remains, impairing the water resistance and chemical resistance of the composite chromate film. inhibit.

The product CB) according to the invention comprises a polyisocyanate compound (b,) and an isocyanate protecting agent (b2) (b).
,) is obtained by reacting so that the ratio of the number of isocyanate groups in (b2) to the number of active hydrogen groups in (b2) is 7/1 to 7/6. There is a mixture of protected fully blocked forms, partially blocked forms where part of the incyanato groups are unprotected, and unblocked forms where all of the incyanato groups are not protected.

In such product CB), the whole block functions as a crosslinking agent when forming a film from a thermosetting resin.

Among the partially blocked bodies, those having one active incyanato group and one or more blocked isocyanate groups are:
Those that are added to the product (A) and have a role as a crosslinking functional group during film formation, and have two or more active isocyanate groups and one or more blocked isocyanate groups are Person C] @Extend the chain to increase the molecular weight, and
It has the role of adding a crosslinkable functional group.

The unblocked product has the role of extending the chain of the product (A) to increase its molecular weight and introducing a urethane bond into the main chain.

The thermosetting resin formed as described above exhibits excellent performance in water swelling resistance, press workability, and top coat adhesion.1 In the second layer composite chromate film of the present invention, the binder effect of the resin By making the chromium compound less soluble and improving water repellency.

Provides high corrosion resistance that can withstand severe corrosive environments.

Next, as the chromium compound constituting the composite chromate film, it is important to have a chromium compound that is hardly soluble in water and has a low degree of dissociation in order to achieve high corrosion resistance. As for Li.

Na, K, AJ, S as alkaline earth metal
r.

One or more of Ca, Zn, and Ba may be used, and alkaline earth metals are preferred, with Sr being most suitable.

Furthermore, when covering a powdered chromium compound with the thermosetting resin, it is necessary to pay attention to the particle size and blending ratio. Among these, it is important to select an appropriate particle size in order to maintain the composite chromate film at a high level of various performances such as high corrosion resistance, press workability, top coat adhesion, and uniformity of finished appearance.

In this case, if the particles are too atomized, it becomes difficult to efficiently exhibit the adhesion of the composite chromate coating to the underlying plating surface and the self-healing action of cr6+, and a long corrosion-resistant life cannot be expected. On the other hand, if the grains are made too coarse, the surface smoothness of the composite chromate film will be lost, and the finished appearance will be unreliable. This greatly reduces product value, such as causing mold galling during press processing.

Thus, the particle size of the chromium compound is preferably 0.1 to 1 μm. In addition, the blending ratio of the chromium compound to the thermosetting resin is determined by the sustainability of the self-repairing action of Cr necessary for high corrosion resistance, and the adhesion of the composite chromate film to the underlying plating surface after press working. From the viewpoint of top coat adhesion 1
0 to 80% is good, and 20 to 60% is good for skillfulness. Since the composite chromate film is made of a chromium compound coated with a resin, its electrical resistance is relatively high, and depending on the amount of the coating, spot weldability may be impaired. Therefore, in order to maintain spot weldability without sacrificing corrosion resistance, it is necessary to
The adhesion amount range of f is good, preferably 0.8 to 3.5 f/
d is good.

According to the present invention, which has a two-layer chromate film structure as described above, the drawbacks of conventional chromate-treated steel sheets can be overcome, and various improvements such as corrosion resistance, top coat adhesion, and press workability can be achieved without impeding spot weldability. Performance can be dramatically improved.

The galvanized steel sheet used in the present invention has a plating layer made of zinc or a zinc-based alloy, and the galvanized steel sheet used in the present invention is made of zinc or a zinc-based alloy.
, Co, Fe, and Cr.

The method of obtaining these galvanized layers may be electroplating, hot-dip plating or vapor deposition plating.
Either single layer or multilayer plating may be used.

〔Example〕

The present invention will be specifically explained below by giving examples, comparative examples, and conventional examples.

As shown in the table below, the Cr/Cr ratio and amount of chromium deposited in various different first layer chromate films, the type of organic resin, type and blending ratio of chromium compound, and predetermined amount of deposited in the second layer composite chromate film. Example N of the present invention consisting of
Various performance tests of corrosion resistance, press workability, continuous spot weldability, and top coat adhesion were conducted for α1 to Nα31.

The results are not shown in comparison with reference examples Nα32 to Nα41 and conventional comparative examples Nα42 to 47.

The method for obtaining the first layer chromate film was as follows.

D; Processing liquid CrOa /H2S 04 =1510.2
(f/z).

Electrolytic chromate treatment at a bath temperature of 45°C and a cathode current density of 10 A/dJ.

(2) Treatment liquid Bt3367/Nippon Parkerizing Co., Ltd. JpH2.0 bath fm40'c, reactive chromate treatment obtained by washing with water after spray treatment.

T; Treatment liquid CrO3/SiO2 sol = 10/20 C
f/l,] However, if necessary, add a predetermined amount of cr with H2O2.
Get a 3+. Spray-on chromate treatment in which the bath temperature is 40°C and gas wiping is performed after spraying to obtain a predetermined amount of chromium deposit.

P-2; Saitata. The following resin was used as a representative type of organic resin to obtain the second layer of composite chromate film.

p-i; 784 parts of epoxy resin (Epotote YD-017/manufactured by Tobu Kasei) (same parts by weight as below)
After dissolving in 8.4 parts and 196 parts of celloacetate, 25.1 parts of azelaic acid and 13.3 parts of jetanolamine were added and reacted for 4 and a half hours at 140°C under N2 gas. To 165.5 parts of 245 parts of methyl ethyl ketone, 224 parts of -\xamethylene diisocyanate and 151 parts of ε-caprolactam were added to 375 parts of celloacetate and reacted at 100°C for 3 hours to obtain 75 parts of incyanate. Kijiroll 26
．． After adding 3 parts and reacting at 100°C for 3 hours, 19.4 parts of methyl ethyl ketone was added, and 160 parts of a thermosetting resin was added to 25 parts as a chromium compound.

, Epototo YD-017392 parts to Kijiroll 247
After dissolving in 124 parts of celloacetate, 1 part of cepacic acid
35 parts and 6.7 parts of jetanolamine were added and reacted for 8 hours at 140°C under N2 gas to 290 parts.

Separately, 14 parts of incyanate obtained by adding 224 parts of hexamethylene diisocyanate and 151 parts of ε-caprolactam to 375 parts of celloacetate and reacting at 100°C for 3 hours were added. A resin made by adding 160 parts to 25 parts of a thermosetting resin containing 95 parts of isobutyl ketone as a chromium compound.

The material was an electroplated steel sheet, the plating system was zinc or Zn-11Ni-IFe, and a single layer plating with a coating weight of 2 (1/n?) was used as a representative.

From the table below, the teeth 1 to 31 of the implementation side of the present invention have much better corrosion resistance than the conventional comparative examples 42 to 471, and also have excellent spot weldability and excellent performance as a paint base. I know what you're doing.

〔Effect of the invention〕

As described above, the two-layer chromate-treated steel sheet of the present invention eliminates the drawbacks of conventional chromate-treated steel sheets, and it is also possible to freely obtain the color tone of the treated appearance using coloring pigments as necessary. This is a high-performance double-layer chromate-treated steel sheet that also has the following characteristics.

*1 Corrosion resistance evaluation method (1) Test piece square tube press processing → Weak alkaline degreasing → Water washing → Drying 40Wx
40L x 30H (ms FC4336 (made by Nippon Barker Lining) I620 oil (Nippon Craft Oil) 60℃ x 120
Second spray treatment aperture ratio 2.4 (2) Corrosion test *3 Salt spray test (JISZ-2371) Rust development evaluation after 500 hours ◎White rust area ratio: 5% or less Red rust area ratio: 0%0
5-10% 0% △ I 10-30% 5% or less
m) %$620 oil application/day oil application/Japanese work ratio 2.4) Evaluation of peeling status (mold galling) (film removal property of processed parts) ◎ No occurrence ◎ No film removal at all ○ Very unlikely Occurrence ○ Very slight film removal Δ Partial occurrence Δ Partial film removal × Full surface occurrence × Full film removal Continuous spora 1 - Weldability 5 Welding current Appropriate current value x -1-o o (KA) Electrode diameter 45 mm φ Electrode lowering blade 200 Ky Current application time 10 cycles/50 T-1y.

◎ 5000 consecutive points or more ○ ' 4000 # △ # 3000 # ○ Very slight peeling Δ Partial peeling × 50% or more peeling

Claims (1)

[Claims] When forming a chrome-1 film on the surface of a zinc or zinc alloy coated steel sheet, Cr/Cr is used as the first layer film.
The ratio of 3/10 to 3/1 and the amount of chromium deposited is 30 to
150η/? It has a chromate film (on one side), and a second layer film on top of the chromate film, with a thermosetting resin made by urethane modification of an epoxy resin with an amine at the polymer end as a binder, and an alkali metal or alkaline earth It is characterized by forming a composite chromate film containing 10 to 80% by weight of a powdered chromium compound with a particle size of 1 μm or less whose acid groups are fixed with metal, in a range of 0.5 to 52/I. Double layer chromate treated steel plate.