JPS637938A - High corrosion-resistant surface-treated steel plate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a highly corrosion-resistant surface-treated steel sheet suitable for use in automobile bodies and the like.

[Prior Art] In recent years, steel plates used as automatic JFI 41J bodies are required to have excellent corrosion resistance, and there has been a strong tendency to use surface-treated steel plates with high corrosion resistance in place of the conventionally used cold-rolled steel plates. There is.

The first example of such surface-treated steel sheets is galvanized steel sheets, but with this type of steel sheet, it is necessary to increase the adhesion of zinc 1iY in order to improve corrosion resistance, and along with this, workability and weldability There is a problem of deterioration. To improve this problem, Ni, Fe, Mn, M
Zinc alloy plated steel sheets and multilayer plated steel sheets to which one or more elements such as O, Co, AM, Cr, etc. are added have been researched and developed, and these steel sheets have poor weldability compared to the above-mentioned galvanized steel sheets.

Corrosion resistance can be improved without deteriorating workability. However, when a steel plate is applied to the bag structure or bent part (hemming part) of an automobile body plate, a high degree of corrosion resistance is required on the surface. A problem with multilayer plated steel sheets is that their corrosion resistance is still insufficient. As a steel plate with a high degree of corrosion resistance, rust-preventive coated steel plates with zinc-rich coatings, such as those seen in Japanese Patent Publication No. 45-24230 and Japanese Patent Publication No. 47-6882, have been researched and developed, and representative examples include: is known as Synchrometal.

However, even with this anti-corrosion coated steel sheet, the coating may peel off in the processed parts such as press forming, and the corrosion resistance may deteriorate. is still far from being fully satisfactory.

For this reason, the present inventors have developed a zinc-rich system 2@
From the viewpoint that there is a limit to the performance improvement of anti-rust coated steel sheets with films, we developed a new steel sheet with a thin film (less than about a few thick) that does not use any metal powder such as Zn powder.
JP-A-58-224174, JP-A-60-50179
No., JP-A-60-50180 and JP-A-60-501
It was proposed as No. 81 etc. This steel plate is based on a zinc or zinc alloy plated steel plate, and is coated with a chromate coating.
! 2 and a top layer of organic composite silicate film, which has excellent processability and corrosion resistance.

Furthermore, in some parts of the inner surface of the automobile body (trunk lid, hood, etc.), two or more coats of -L coating are applied to the cationic electrodeposited coating, and the steel sheet according to the above proposal Due to concerns about the adhesion of such multilayer coatings, we improved the above-mentioned steel plate and developed a method for manufacturing a multilayer coating device 1 rust-proof steel plate with excellent multilayer coating adhesion in Japanese Patent Laid-Open No. 1983-1999.
It was proposed as No. 174879.

This invention has an organic polymer coating of 18% by baking at a high temperature of 250 to 350°C. The polymer film is sufficiently cross-linked to ensure excellent paint adhesion for multi-layer coating; if the cross-linking of the polymer film is insufficient, the film will soften and swell due to the alkali generated at the interface during cationic electrodeposition. , the deterioration of paint adhesion was improved by crosslinking by high-temperature baking.

[Problems to be Solved by the Invention] However, subsequent research by Miyo et al., who invented the present invention, revealed that the above-mentioned steel plate has extremely excellent paint adhesion (adhesion of two or more coats of multilayer paint) by baking at a high temperature of 250°C or higher. This assumes a case in which electrodeposited coating is difficult to form even though it is possible to ensure the

If there is a problem with the so-called bare corrosion resistance (unpainted corrosion resistance) and the surface treatment film is damaged, for example, if the iron substrate is subjected to cross-cutting, deep drawing, drawbead processing, etc. It was found that the steel sheet tends to be slightly inferior to steel plates manufactured by No. 58-224174 and the like.

Highly corrosion-resistant surface-treated steel sheets for automobiles have excellent workability and weldability, as well as l) corrosion resistance in areas where electrodeposited coatings are difficult to form, such as bag structures and hemming areas, that is, a high degree of bare corrosion resistance.

2) 2 coats such as the trunk lead and the inside of the hood (
Paintability (paint adhesion, paint corrosion resistance) for multi-layer coatings (cationic electrodeposition + top coat).

In recent years, the importance of rust prevention for automobile bodies has become even more important.

It is difficult to say that the steel sheet described above has sufficient properties.

Mitsubishi II was created in view of these conventional problems, and provides a highly corrosion-resistant surface-treated steel sheet that has excellent workability and weldability, as well as excellent bare corrosion resistance, paint adhesion to multi-layer coatings, and paint corrosion resistance. That is.

[Means for Solving the Problems] Therefore, the basic feature of the present invention is to have a chromate film on the surface of a galvanized or zinc alloy plated steel sheet, and on the top of the chromate film.

A base resin made by adding at least one basic nitrogen atom and at least two 1% i groups of primary water to the end of an epoxy resin, strontium chromium, lead chromate, zinc chromate, chromic acid A resin composition in which an additive consisting of one or more of barium, calcium chromate, and zinc potassium chromate is blended, and the resin/additive weight ratio is 99/1 to 30/70. The reason is that it has a film structure with a film.

The present invention will be explained in detail below.

The present invention uses a zinc-plated or zinc-alloy plated steel sheet as a starting material, and has a chromate film on its surface and a basic epoxy resin film containing predetermined additives on top of the chromate film.

The zinc-coated steel sheets used as starting materials include galvanized steel sheets, zinc-iron alloy plated steel sheets, zinc-nickel alloy blank steel sheets, zinc-manganese alloy-plated steel sheets, zinc-aluminum alloy plated steel sheets, and zinc-cobalt-plated steel sheets. The plating components of chromium alloy plated steel sheets and any of these steel sheets include old, Fe, Mn, No, and C.

It is possible to use one or more added elements such as Jl and Cr, and it may also be a composite plated steel sheet with two or more layers of the same or different types of plating as described above. For example, Fe-Zn with different Fe contents
It is possible to form a plating film with two or more layers of alloy plating. Among these, zinc-nickel alloy plated steel sheets and zinc-manganese alloy plated steel sheets are particularly preferred from the viewpoint of corrosion resistance, and when these steel sheets are used. , the zinc-nickel alloy plated steel sheet has a nickel content in the plating film of 5 to 20.
It is preferable that the wt% zinc-manganese alloy plated steel sheet has a manganese content in the range of 30 to 851χ.

As a plating method for these galvanized steel sheets, any practicable method among electrolytic method, melting method, vapor phase method, etc. can be adopted. However, the rust-proof steel sheets that are the object of the present invention are mainly used for automobile bodies, and in such applications, it is important not to damage the material of the cold-rolled steel sheets to be plated. Because there is electricity, no heat is generated.

It can be said that air plating is advantageous.

A chromate film is formed on the surface of the above material plated steel sheet by chromate treatment.

This chromate film has a chromium adhesion of 1 to 1000 mg/m' (dry), preferably to to 2
It is appropriate to set it at about 00 mg/rrr' (the above is converted into metal chromium). 27 amount with chrome is 11000ra/
Exceeding rrf will deteriorate workability and weldability, and
If it is less than g/m', the film may become non-uniform, which is undesirable. Also, it is preferable that hexavalent Cr exists in the chromate film. Cr has a repairing effect, so if the steel plate is scratched. From there, it acts to suppress corrosion.

The chromate treatment for such a base film may be performed by any known method such as one-reaction type, coating type, electrolytic type, etc.

The coating-type chromate treatment solution has a partially reduced chromic acid solution as its main component, and if necessary, it is mixed with a water-dispersible or water-soluble organic resin such as acrylic resin and/or a particle size of several hours or more.
It is a 3-6-piece product containing hundreds of tiles of silica (colloidal silica, fumed silica). In this case, the Cr/Cr ratio is 1/1 to 1/
3.

The pH is preferably 1.5 to 4.0 (more preferably 2 to 3), and the Cr/Ct ratio is determined by using general organic transition agents (e.g. sugars, alcohols, etc.) or non-alkaline reducing agents. and adjust to the specified ratio. Further, as the coating type chromate treatment, any method such as a roll coater method, a dipping method, a spray method, etc. may be used. In the coating type chromate treatment, a film is obtained by drying without washing with water after the chromate treatment. The reason for drying without washing with water is that Cr is removed by the usual washing with water, and the Or/Cr ratio is maintained stably as it is, and the basicity formed at the top is The Evogishi resin film suppresses excessive outflow of Cr in a corrosive environment, effectively maintains the passivation effect over a long period of time, and provides a highly corrosion-resistant material.

On the other hand, in electrolytic chromate treatment, the cathode 1 is subjected to decomposition treatment in a bath containing chromic anhydride and one or more of 7 ions such as sulfuric acid, phosphoric acid fluoride, or halogen oxygen acid, and then washed with water and dried. Comparing the chromate films produced by two treatment methods of 0 or more to form a film, the coating-type chromate has superior corrosion resistance compared to the electrolytic chromate because it contains more hexavalent chromium in the film. Furthermore, when heat treated as described below, the skin 11!2 becomes dense and strong, resulting in better corrosion resistance compared to electrolytic chromate.On the other hand, electrolytic chromate can be used with or without heat treatment. It has the advantage of a high degree of completeness of the film, and it also has the advantage of making it easy to control the amount of film deposited.In terms of corrosion resistance, coated chromate is the most desirable, and single-sided treated steel sheets are used for automotive rust-preventing steel sheets. In many cases, from this point of view, coating type and electrolytic type are preferable.

A basic epoxy resin coating is formed on the chromate coating.

This resin film is made by adding at least one basic nitrogen atom and at least two or more primary hydroxyl groups to the terminal end of an epoxy resin, and adding it to a base resin dissolved in an organic solvent.
This film is formed by heating and drying a resin composition treatment liquid containing one or two specified chromium compounds as additives.

The epoxy resin is preferably one based on a condensate obtained by condensing bisphenol A and epichlorohydrin. Examples of the epoxy resin include epoxidized oil,
There are epoxy resins such as polybutadiene that consist only of aliphatic structures or alicyclic structures, but in order to obtain excellent corrosion resistance, it is preferable to use epoxy resins mainly composed of the above condensates. For example, Epicote 828. +001,1004.100? , 1009.
1010 (all manufactured by Shell Kagaku Co., Ltd. 5J), etc. can be used. This epoxy resin desirably has an average molecular weight of 1,500 or more, especially when curing at low temperatures is required. Incidentally, the above Epicoat can be used alone or in a mixture of different types. In order to introduce a basic nitrogen atom and a primary hydroxyl group into an epoxy resin, for example, a method can be adopted in which alkanolamine alkylalkanolamine is added to the oxirane group of the epoxy resin. Examples of these amines include monoethanolamine, jetanolamine, dimethylaminoethanol, monopropanolamine, dibrobanolamine, and jetanolamine, and these amines are used alone or in combination.

Alternatively, the epoxy resin may be partially modified with other compounds. However, in this case, it is necessary to contain an average of 2 moles or more of primary hydroxyl groups in one molecule of the epoxy resin.

Methods for partial modification of epoxy resins include: (1) Esterification with monocarboxylic acids (monocarboxylic acids include saturated or unsaturated fatty acids such as coconut oil fatty acids, soybean oil fatty acids, and castor oil fatty acids, acetic acid, and propionic acid). (2) Modification with aliphatic or aromatic amines (aliphatic or aromatic amines include monomethylamine, dimethylamine, Aliphatic amines such as monoethylamine, diethylamine, isopropylamine, aromatic amines such as aniline, etc.) (3) Modification with oxyacids (oxyacids include lactic acid, γ-oxypropionic acid, etc.).

Note that there is also a modification method using dicarboxylic acids (e.g. adipic acid, sepathic acid, etc.), but this method causes the epoxy resin to have too much polymer concentration than necessary, and furthermore, it is difficult to control the molecular weight distribution at a constant level. This method is inappropriate for obtaining the coating of the present invention because it is difficult to control and no improvement in corrosion resistance is observed.

The base of the epoxy resin, which is the basic resin of the above-mentioned skin forming composition, can be neutralized with a low molecular acid and used as a water-dispersed or water-soluble composition.
This also applies to skin +19 materials for H steel plates, which require low-temperature drying below 50°C, especially extremely low-temperature drying below 170°C. Therefore, for basic resins, the above-mentioned neutralization process is not necessary. Instead, it is used as a composition dissolved in an organic solvent. In other words, with wood dispersion or water-soluble compositions, it is not possible to obtain a sufficiently strong film under low-temperature drying conditions, and salts are formed in the film with the resistant compounds required for water solubilization. It is easy to draw moisture into and under the film. As the type of organic solvent, one or a mixture of two or more organic solvents commonly used in the paint industry can be used, but for this purpose, it is preferable to avoid alcoholic solvents with high boiling points. Examples include ethylene glycol or diethylene glycol, monoalkyl ethers, and alcohols having a primary hydroxyl group of C or more. Such solvents inhibit the curing reaction of the film. Recommended solvents include hydrocarbon, ketone, ester, and ether solvents.
In addition, alcohols having a low molecular weight of C or less, or alcohols having secondary or tertiary hydroxyl groups are also suitable.The purpose of providing the resin composition film as described above in the present invention is as follows. Can be done. In other words, in order to obtain a high degree of corrosion resistance and adhesion of two or more coats of multilayer coatings, we adopted an epoxy resin as the base and expected to obtain high adhesion to the substrate and cationic electrodeposition as well as high corrosion resistance. (By making the polarity of the Sumata resin basic, deterioration of the resin structure due to alkali generated at the interface during cationic electrodeposition is eliminated. To explain this in detail, first, bisphenol A is added to the base resin. By using an epoxy resin consisting of a condensation reaction between epoxy and epichlorohydrin, excellent adhesion with cationic electrodeposition paints, which are commonly used for automobile and lj body rust prevention, can be expected. by introducing a primary nitrogen atom and a primary hydroxyl group.

(1) It prevents the film from being destroyed by the alkali generated during cationic electrodeposition, and stabilizes the adhesion between the base chromate and the cationic electrodeposited film. By introducing hydroxyl groups, a film with a sufficiently dense structure can be obtained. If the amount is less than 2 moles, a sufficiently stable film cannot be obtained.

In addition, known chromium-based, non-chromium-based antirust pigments, extender pigment 1 color pigments, and the like can be blended into the resin composition film.

Further, as an additive, a monohydric alcohol having 1 to 5 carbon atoms is reacted with part or all of a methylol compound obtained by reacting formaldehyde with one or more selected from incyanate compounds, melamine, urea, and benzoguanamine. The resin composition of 0 or more, which may be used in combination with an alkyl etherified amine resin, etc., is coated to a predetermined thickness by a method such as roll squeezing, roll coater 1, or air knife, and then heated at room temperature to 250°C (preferably room temperature). ~170
If hot air drying is carried out to a plate temperature of 10°F (°C), a dry film can be obtained within several seconds to several minutes. The method of drying the film is not particularly limited to hot air.The steel sheet of the present invention has the great feature that it can be obtained by such low-temperature drying.

When baked at a high temperature exceeding 250°C, the corrosion resistance deteriorates as in the case of JP-A-60-174879 mentioned above. This is due to the volatilization of water contained in the chromate film components and the hydroxyl groups (-Cr-O
H) It is presumed that this is due to the rapid progress of the dehydration condensation reaction between the two, which leads to the progress of destruction of the chromate film due to the occurrence of cracks in the chromate film, and the progress of reduction of Cr, which reduces the passivation effect of Cr. be done.

The present invention contains one or more predetermined chromium compounds as an additive in the resin composition film, thereby providing excellent corrosion resistance. In a corrosive environment, Cr is directly eluted from the chromium compound in the film, and this exerts an immobilization effect over a long period of time, improving corrosion resistance. 99/1 to 30/70, preferably 90/10 to 50
It is blended in the range of 150.

Here, the blending amount of the additive described in 1- is basic resin/additive:
If it is less than 99/1, no improvement in corrosion resistance can be expected due to the combination, while if it is more than 30/70 or 1-, the adhesion of the two coats will decrease.

As the chromium compound, each powder of strontium chromate, lead chromate, zinc chromate, barium chromate, chromium-ugly calcium, and zinc calcium chromate can be used, and one or more of these powders can be used in the base resin. disperse. Chromium compounds other than these have problems such as poor compatibility with the base resin, or poor adhesion of two-coat paints because they contain a large amount of soluble 6% Cr, although they have an anticorrosion effect. Therefore, the additives used in the present invention are as described above! Limited to Class J chromium compounds.

Note that the present invention does not preclude the use of other known additives, antirust pigments, etc. in addition to the chromium compound as the additive component described above.

1-The resin composition film as described above is applied on the chromate film at a weight of 0.2 to 3.9 g/~2. Preferably 0.5-2.
It is desirable to form the skin with a coating amount of 5 g/m2.
If the coating weight is less than 0.2 g/m, sufficient corrosion resistance cannot be obtained, while if it exceeds 3.9 g/m, weldability (
In particular, continuous multi-point welding performance) is reduced, and 0.2
A range of 3.9 g/m2 is suitable for use as a highly corrosion-resistant surface-treated steel sheet for automobiles.

Furthermore, although cationic electrodeposition coating is applied to automobile bodies,
Wet electrical resistance of chromate film + resin composition film is 20
If the resistance exceeds 0Ω/C112, there is a problem that the cation weight coating film will not be formed properly.For this reason, in the steel sheet of the present invention, which is mainly used for automobile bodies, the hot water side resistance of the chromate film + resin composition film is set to 200Ω. It is preferable to form both films so as to suppress the value to /cta2 or less.

The present invention is directed to the skin as described below. It includes a steel plate having two structures on both sides or one side.

Examples of the embodiments of the steel sheet of the present invention include the following.

(1) One side...Plated film - Chromate film - Resin composition film One side...Fe side (2) One side...No 2 plated film - Chromate film - Resin composition film One side... Plated film (3) Both sides...Plating film - Chromate film 1 - Resin composition film [Example] As a steel plate for the inner surface of an automobile body, the present invention material was used for different plating components and film adhesion as shown in Table 1. An adhesion test and a corrosion resistance test were conducted. Similar tests were also conducted on each steel plate shown in Table 2 as comparative materials.

The plating components of each steel plate are as follows. For each steel plate in the table with a chromate film and a basic epoxy resin film, the plated steel plate is degreased with alkaline, washed with water, dried, and then coated with a coating-type chromate treatment solution. An electrolytic chromate film was formed by coating with a roll coater or dipping in an electrolytic chromate treatment bath, and after drying, a basic epoxy resin liquid was coated as a second layer with a roll coater. After dry blasting, it was heat treated and air cooled.

Xl-Zn alloy electroplating...Ni content 12 XF
e-Zn alloy electroplating number/Fe content 25z)in
-Zn alloy electroplating: Mn content: 6 oz. In addition, coating type chromate treatment, electrolytic chromate treatment,
The details of the q-type ebo-goshi resin liquid are as follows.

・1'8 cloth type chromate treatment condition 31 Needle Cr/Cr=2/3, pH=2.5(K
A chromate treatment solution having a solid content of 20 g/A was coated with a roll coater at room temperature and then dried.

Fist electrolytic chromate treatment conditions CrO1: 50g/l, H2SO4: 0.5g
/l.

A current density of 4.9A/ds" with a bath temperature of 50℃
, cathodic electrolytic treatment was carried out for an electrolysis time of 2.0 seconds, followed by water washing and drying.

-Resin composition The base resin and curing agent prepared as follows were mixed in the proportions shown in Table 3 to prepare a resin composition.

0 Base resin (I) Epicor in a reaction apparatus equipped with a reflux condenser, a stirrer, a thermometer, and a nitrogen gas blowing device) 1004 (Exbo resin manufactured by Shell Chemical Co., Ltd.: molecular weight approximately 1800) 10
00g, pelargonic acid (reagent) 57g, xylene 80g
was added, and the reaction was allowed to proceed at 170° C. until the acid value of the reactant became approximately O. Thereafter, xylene was removed under reduced pressure to obtain a reaction intermediate [A].

(II) I! 1880 g of Epicoat 1009 (epoxy resin manufactured by Shell Chemical Co., Ltd.: molecular weight 3750) was placed in a reaction apparatus equipped with a stirring device, a reflux condenser, a thermometer, and a liquid dropping device.
(0.5 mol) and a mixed solvent of methyl-isobutylketone/xylene = l/l (ffiffi ratio) 1000
After adding g, the mixture was stirred and heated to uniformly dissolve the mixture under the boiling point of the solvent. After that, it was cooled to 70°C, and 70 g of di(n-propanol)amine was collected in a liquid dropping device.
It took several minutes to drip. During this time, the reaction temperature was maintained at 70° C. After the completion of 0 dropwise addition, the reaction temperature was maintained at 120° C. for 2 hours to complete the reaction. The obtained reaction product is referred to as resin A. The active ingredient of Resin A is 66%.

(m) Weigh out 1850 g of the reaction intermediate [A] obtained in (I) and 1000 g of xylene into the same reactor as in (II) above, heat it to 100°C, and add the jetanol fractioned into the liquid dropping device. 85 g of amine and 30 g of monoethanolamine were added dropwise over 30 minutes.

Thereafter, the temperature was maintained at 120°C for 2 hours to complete the reaction. The obtained reaction product is referred to as resin B. The active ingredient of Resin B was 63%.

In addition, in the adhesion test, the sample material after phosphoric acid treatment was electrocoated with cationic electrodeposition paint N0.9210 manufactured by Kansai Paint Co., Ltd. at a thickness of 20 pHQ, and then Amirac N manufactured by Kansai Paint Co., Ltd.
o, 002 was spray-painted at the end of 40, and the primary fabric adhesion and 2
Next, we tested the stability. The primary adhesion test was carried out by cutting 100 goblets at jam intervals on the coating surface of each sample material and attaching adhesive tape to these goblets. After painting, each sample material was immersed in warm water (pure water) at 40°C for 240 hours and then taken out.
Within 0 minutes, carve a 1-layer interval in the same way as in 1-,
This was done by applying an adhesive tape to the rows and then peeling it off.

7' 1/ In addition, the corrosion resistance test was conducted as a cycle test with the above as one cycle.
Evaluation was performed using the prescribed cycles shown in Table and Table 2.

Regarding corrosion resistance after unpainted processing, bead shape tip angle θ
O0 tip RO65 Bead height 5m layer sample size 25m5+X 300 images - pull
Removal speed 20hm/sin press
The test material processed in the drawbead test with a setting force of 500 kg was tested in 50 cycles.

In addition, regarding the wearability after coating, a cross cut was made after electrodeposition coating, a test was conducted for 100 cycles, the maximum blistering width was measured, and half of the blistering width was evaluated. The evaluation criteria for each test result are as follows.

(1) Corrosion resistance after unpainted processing O: No red rust O+: Red rust less than 5z 0: /l 51 or more and less than 10$0-:
II 10! 7720% //Δ: tt
2Q% tt5Q% ttX: tt
5Q // (2) Erosion resistance after painting @: 7 ri/vA0.51111 Not i0
+': 〃 0.5m or less 1. Omm/10:
// 1.0 // 2. hm tlo
: // 2. Ott 3. OBm/
/Δ: // 3,0 // 5. Q3
IIttX: tt5. Q ” (3) Two-coat adhesion - @: Peeling area OX O+: tt Less than 5X 0
: /l 5X or more 10% /10-
: // 10% // 20
% //Δ: // 2Q%
// 50$ ttX:
tt 5Q% //(*1)
Solid content added to 100 parts of total solid content of cationic base resin and additives (*2) See Table 3 (*3) See Table 4 As can be seen from the above examples, the steel sheet of the present invention .

As the top layer, a film with alkali resistance is obtained using basic epoxy resin as the base resin, and since the resin is a low-temperature drying type, the chromate film itself does not deteriorate or reduce CRB. Good corrosion resistance can be ensured.

Regarding corrosion resistance after painting, steel plates coated with conventional organic composite silicate that are baked at a low temperature (150℃) tend to have alkali blisters due to the poor alkali resistance of the film, and those that are baked at a high temperature (260℃) In the example of the present invention, the corrosion resistance of the chromate is deteriorated, so the corrosion from the cut part progresses from the side direction, and blisters are slightly likely to occur.In contrast, in the example of the present invention, the alkali resistance of the film is improved, and Since the chromate film maintains good corrosion resistance, good corrosion resistance after painting is obtained. The width of the blisters on Synchro Metal is due to the occurrence of red rust.

Regarding corrosion resistance after processing, conventional organic composite silicates of the low-temperature baking type (150°C) do not have sufficient crosslinking of the skin+1'2 and also have poor alkali resistance, so if the film is partially damaged during processing. The coating deteriorates due to alkaline degreasing during phosphate treatment, resulting in poor corrosion resistance. Furthermore, even high-temperature types have poor corrosion resistance due to deterioration of the corrosion resistance of chromate and damage to the film. On the other hand, in the examples of the present invention, the strength and alkali resistance of the film are improved, and the corrosion resistance of chromate is maintained, so that the film exhibits good corrosion resistance even after processing.

[Effects of the Invention] According to the present invention described below, by forming a strong resin film with excellent alkali resistance on the top layer,
It is possible to obtain superior paint adhesion and corrosion resistance.

In particular, the steel sheet of the present invention has a drying temperature of the resin composition film at a low temperature (
250° C. or lower), there is no deterioration of the chromate film or reduction of Cr due to high temperature baking unlike in conventional steel sheets, and good corrosion resistance due to the chromate film itself is maintained.

In addition, since the steel sheet of the present invention can be manufactured by drying at low temperatures, it is possible to improve productivity and reduce energy consumption, and at the same time, the steel sheet can be manufactured at a temperature of 170°C or lower, preferably 15°C.
By setting the drying temperature (plate temperature) to 0° C. or lower, it is possible to manufacture a highly corrosion-resistant surface-treated steel sheet made from a so-called BH steel sheet having bake hardenability.

Claims (1)

[Claims] A chromate film is formed on the surface of a zinc-plated or zinc alloy-plated steel sheet, and on the top of the chromate film, at least one basic nitrogen atom and at least two primary nitrogen atoms are added at the end of an epoxy resin. An additive consisting of one or more of strontium chromate, lead chromate, zinc chromate, barium chromate, calcium chromate, and zinc potassium chromate is blended into the base resin to which a hydroxyl group is added, A highly corrosion-resistant surface-treated steel sheet comprising a resin composition film having a base resin/additive weight ratio of 99/1 to 30/70.