JPH07106611B2 - Method for producing organic composite coated steel sheet having excellent corrosion resistance and weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention mainly relates to a method for producing a highly corrosion-resistant organic composite coated steel sheet, which is used by press forming as an automobile body.

<Prior Art> In recent years, high corrosion resistance is required for automobile bodies, and surface-treated steel sheets obtained by subjecting conventional cold-rolled steel sheets to Zn plating or Zn-based alloy plating are being applied.

Furthermore, in areas where corrosion is more severe, a high degree of bare corrosion resistance is required for the inner plate bag structure and the bending part where the coating after press / body assembly is not sufficiently spread, and as a response to this, for example, Japanese Patent Publication No. 60-33192. Gazette and JP-A-58-224
As disclosed in Japanese Patent No. 174, etc., an organic composite coated steel sheet in which a chromate and an organic coating are applied on a Zn-based plated steel sheet has been developed.

Japanese Patent Publication No. 60-33192 discloses a composite excellent in corrosion resistance by forming a composite organic silicate resin film composed of a water-based resin and silica sol having a particle size of 7 to 100 μm on a Zn-based plated steel sheet in a film thickness of 0.01 to 4 μm. Although it is advocated that a coated steel sheet can be obtained, according to the study by the present inventors, when such a huge silica sol is used as compared with the film thickness, the silica sol is not retained in the coating film and cannot be carried out. is there.

JP-A-58-224174 discloses a resin and a particle size of several to several tens nm.
The purpose is to apply a coating composition containing a water-dispersed silica sol on a zinc-plated steel sheet to develop high corrosion resistance, but when using an existing silica sol, the water content in the coating film during chemical conversion treatment Invades, and chromium in the lower chromate elutes to pollute the environment, causes peeling of the coating film at the time of degreasing with alkali, which leads to deterioration of corrosion resistance, moisture enters under the coating film when exposed to a corrosive environment However, due to the high alkali in the cathode part, the bond at the coating film-chromate interface is broken, the adhesion after coating is poor, the corrosion resistance is often inferior, the presence of uniformly dispersed silica causes The problem is that thermal decomposition is hindered and weldability is significantly deteriorated.

Among these, items (1) to (4) are phenomena caused by poor adhesion between the coating film and the chromate interface. This is because the silanol groups on the surface of the silica particles are bonded to the hydroxyl groups in the organic resin, which hinders the bonding between the organic resin and the chromate layer and reduces the adhesion between the coating film and the chromate.

In order to secure the adhesive force between the coating film and the chromate, a method of increasing the number of hydroxyl groups in the organic resin is effective. However, although the adhesion between the coating film and the chromate is imparted by this method, most of the silanol groups on the surface of the silica particles are bonded to the organic resin, so that the silanol groups stably retain the corrosion products during corrosion. Becomes impossible,
The problem described above is derived.

Further, as disclosed in JP-A-63-35798 and JP-A-64-65179, a method of adding dry fumed silica to an organic solvent may be mentioned. However, when fumed silica is used, good corrosion resistance and weldability are obtained, but the viscosity in the paint increases significantly, which hinders control of the coating amount during coating and often causes aggregation during formulation. There is a problem that coating becomes impossible.

The applicant of the present application, in Japanese Patent Application No. 1-268478, has proposed an organic composite coated steel sheet using silica sol dispersed in an organic solvent in which the total amount of alkali metal is regulated, and the organic composite coated steel sheet has good corrosion resistance, It has weldability and adhesion after painting. However, when an organic solvent was used as the paint solvent, special consideration was required for the circulation pollution by the volatile solvent, the health maintenance of the worker, and the prevention of flame generation due to ignition.

<Problems to be Solved by the Invention> The present invention has been made to solve the above-mentioned problems.
Maintains good compatibility between water-dispersed silica sol and water-based organic resin in a paint using water as a solvent, and regulates the dispersion state of the silica sol in the paint, resulting in an organic material with excellent corrosion resistance, weldability and adhesion after coating. It is intended to provide a method for manufacturing a coated steel sheet.

According to the present invention in order to achieve the above object <Means for Solving the Problem>, the Zn-plated steel sheet or a Zn-based alloy plated steel sheet on the coating weight per unit area is 10 to 40 g / m ^2, Cr in the chromate The ratio of ⁶⁺ is 70% or less with respect to the total chromium amount, and chromate treatment is performed so that the chromate adhesion amount becomes 5 to 500 mg / m ^{2 in} terms of chromium, and the average particle size of the secondary particles is A resin having a number average molecular weight of 2000 or more and a hydroxyl value of 50 or more based on a water-dispersed silica sol having a specific surface area of 0.05 to 3.0 μm and a specific surface area of 50 to 400 m ² / g and a water-dispersed or water-soluble organic resin. The coating composition containing the composition and having a silica sol ratio of 10 to 100 parts by weight (dry weight) with respect to 100 parts by weight (dry weight) of the resin composition has a dry weight of 0.2 to 3.0 g / m ²
Provided is a method for producing an organic composite-coated steel sheet excellent in corrosion resistance and weldability, which is characterized by applying.

The present invention will be described in more detail below.

A Zn-plated steel sheet or a Zn-based alloy-plated steel sheet is used as the starting material of the present invention.

The types of plating applied to steel sheets include pure Zn plating and Zn-Ni, which have been used to improve corrosion resistance.
Binary alloy plating such as alloy plating (Ni content 8 to 16 wt%), Zn-Fe alloy plating (Fe content ratio 5 to 30 wt%), Zn-Ni
-Cr alloy plating, Zn-Co-Cr alloy plating, and other ternary alloy plating can be used, and Zn-Co-Cr-Al ₂ O ₃
You may use composite dispersion plating such as. These platings are mainly carried out by an electroplating method, but other methods (for example, vapor deposition plating, hot dipping, etc.) may be used as long as they have a smooth surface suitable for applying the thin film coating material of the present invention.

These platings are performed to impart corrosion resistance to the steel sheet, and the basis weight is in the range of 10 to 40 g / m ² . If the basis weight is less than 10 g / m ² , the corrosion resistance may be insufficient, and if it is more than 40 g / m ² , there is no further significant improvement in the corrosion resistance, and the weldability and press workability may be impaired. is there.

On these Zn or Zn-based alloy plating layers, a chromate treatment is firstly performed to impart high corrosion resistance and secondly to impart adhesion to the upper coating film.

Chromate adhesion amount is 5 to 500 mg / m ^{2 in} terms of chromium,
The preferred range is 10 to 150 mg / m ² . The amount of chromium deposited
If it is less than 5 mg / m ² , a chromate uncoated portion is generated, which is not preferable from the viewpoint of corrosion resistance and coating adhesion, and if it exceeds 500 mg / m ² , there is no further significant improvement in corrosion resistance, and the insulation film resistance Markedly increases, impairing weldability and electrodeposition paintability. In the range of 10 to 150 mg / m ² , stable and good performance can be obtained in all aspects of corrosion resistance, adhesion, weldability, and electrodeposition paintability.

Examples of the chromate treatment method include a coating type chromate method using a roll coater and a roll squeeze method, an electrolytic chromate method, and a reactive type chromate method, but any method may be used.

The chromate treatment liquid contains a water-soluble chromium compound as a main component, to which an appropriate amount of phosphate radicals, anions such as fluorine ions, metal ions such as Zn, Ni and Co, and organic substances such as starch and methanol are added as necessary. . Further, silica sol can be added for the purpose of improving the corrosion resistance.

In addition, the ratio of Cr ⁶⁺ in chromate is 70% of the total chromium amount.
% Or less. This is because if the Cr ⁶⁺ content exceeds 70%, the chromium elution resistance during alkaline degreasing deteriorates.

A composite coating composed of silica sol and an organic resin composition is applied on the chromate coating. In the steel sheet of the present invention, the silica sol in the composite coating is one which exhibits high corrosion resistance because it has a function of stably retaining Zn-based corrosion products generated when the silanol groups on the surface are exposed to a corrosive environment. . However, since it is impossible to apply silica sol alone to a steel plate, an organic resin as a binder is essential. As the organic resin, a water-soluble or water-dispersible polymer organic resin is used. In the present invention, a water-dispersed silica sol is used as the silica sol, but when the water-dispersed silica sol is mixed with an organic resin composition using an organic solvent because water molecules are adsorbed on the silica particle surface in a hydrated form, immediately It will precipitate and gel,
This is because it is difficult to use as a coating composition. Examples of the water-soluble or water-dispersible polymer organic resin include epoxy, urethane, acrylic, polyester, alkyd copolymer and the like.

The number average molecular weight Mn of such a resin composition must be 2000 or more, and the hydroxyl value H (KOH mg / resin g) must be H ≧ 50.

When the number average molecular weight is less than 2000, the resin length becomes short, and the resin does not have a knitted structure, so that it does not function as a binder of silica sol, and corrosion resistance and paint adhesion are significantly impaired. Further, the compatibility with silica sol in the paint is also deteriorated. The upper limit of the number average molecular weight is not particularly limited, but if it is more than 100,000, the resistance of the coating increases and the electrodeposition coatability and spot weldability may be impaired. Therefore, it is preferably 100,000 or less.

The hydroxyl group in the organic resin is a functional group that contributes to the adhesiveness, but as a result of the research conducted by the present inventors, if this amount is less than 50, a strong adhesiveness with the underlying chromate layer is secured. In some cases, good adhesion cannot be obtained between the steel sheet after cationic electrodeposition coating and the electrodeposition coating film.

For this reason, chromium in chromate is eluted during alkaline degreasing, after electrodeposition coating and top coating on the upper layer,
When exposed to a humid environment, the adhesion between the chromate-organic composite film or the electrodeposition coating-organic composite film becomes poor,
There may be a problem that the coating film peels off. Therefore, the hydroxyl value H of the organic resin must be 50 or more. Further, the upper limit of the hydroxyl value H is not particularly defined, but if this value becomes high, the compatibility with silica sol when made into a paint may be lost, and the paint may aggregate or gel.

Next, the reason for limiting the silica sol will be described in detail.

The silica sol of the steel sheet of the present invention has an average particle size of 0.05 to 3.
A water-dispersed silica sol having a particle size of 0 μm and a specific surface area of 50 to 400 m ² / g is used. In addition to this, fumed silica is known as silica. Fumed silica is a silica agglomerated fine powder that has a siloxane bond between particles and a silanol group on the particle surface, but when this is incorporated into a paint,
When mechanical shearing force is applied to the paint, the viscosity of the paint rapidly rises and it cannot be applied industrially stably. As the amount added in an organic solvent increases, the amount increases because of thickening, precipitation and gelation. Is limited. Therefore, a water-dispersed silica sol that is stable in the paint is used.

The specific surface area of silica particles is 50 to 400 m ² / g. If the specific surface area is less than 50 m ² / g, the amount of free silanol groups on the silica surface, where the corrosion products are stably retained, becomes small, resulting in poor corrosion resistance. Therefore, by defining the specific surface area to be 50 m ² / g or more and securing the free silanol groups on the silica surface, it is possible to exhibit good corrosion resistance.

Further, if the specific surface area of silica is more than 400 m ² / g, the number of silanol groups increases more than necessary and gelation easily occurs. When the specific surface area of silica is less than 50 m ² / g, the amount of free silanol groups on the surface of silica is very small, resulting in poor corrosion resistance.

Further, the particle size of a general silica sol is 0.05 μm or less, and it is in a state of being uniformly dispersed in a solvent. However, according to the study by the present inventors, when such a uniformly dispersed silica sol is used, spot welding results in accelerated wear of the welding electrode tip and a reduction in the electrode area, resulting in a decrease in the welding current density. It was found that the spot weldability deteriorates, the nugget is not formed, and during spot welding, a spark is generated due to a small number of current-carrying paths, which promotes damage to the electrode and the spot weldability is significantly deteriorated. Therefore, in order to secure good weldability, it is necessary to prevent uniform dispersion, and it is necessary to secondary-aggregate the silica sol primary particles in the coating material. That is,
The primary particles are agglomerated to form secondary particles having an average particle size of 0.05 μm or more to ensure good weldability. Further, if the average particle size of the secondary particles exceeds 3.0 μm, the uniform coating property of the coating film is impaired, which is not preferable. Examples of means for secondary agglomeration of particles include a method of reducing the amount of Na ₂ O normally contained in silica sol to 0.05% by weight or less. When Na ₂ O is contained in the silica sol, Na ⁺ is coordinated on the surface of the silica particle as a counter ion, and an electric double layer having a hydrated layer is formed around it, so that the silica particles repel each other. Maintains a colloidal state and is uniformly dispersed, but the amount of Na ₂ O contained in the silica sol is 0.05.
By reducing the amount to less than 1% by weight, the silica surface will not be charged
It is intended to form a multi-layer so as to prevent it from having an electric charge and to cause primary particles to secondary aggregate.

The dry weight ratio of silica sol to resin composition in the coating composition is 10 to 100 parts by weight of silica sol to 100 parts by weight of resin. If the amount of silica sol is less than 10 parts by weight, the number of silanol groups that stably retain the corrosive component is small and good corrosion resistance cannot be obtained, and if it exceeds 100 parts by weight, compatibility with the resin composition is obtained. This is because it becomes difficult to form a paint and apply it to the steel sheet.

If necessary, a silane coupling agent may be added to the coating composition of the present invention. Thereby, a bridge is formed between the base resin and the silica sol, which is effective in stably fixing the silica sol. As such a silane coupling agent, vinyl silane, methacryloxy silane, epoxy silane, amino silane, mercapto silane, etc. are known. Among them, amino silane has poor compatibility with the resin composition used in the present invention. Therefore, it is not suitable. Further, the silane coupling agent binds to the silanol groups on the surface of the silica sol, which contributes to the corrosion resistance, and eliminates them, so addition of more than necessary amount is harmful for maintaining good corrosion resistance.

Silane coupling agent 2 to 100 parts by weight of silica sol
Addition of more than 0 parts by weight is not preferable from this viewpoint.

As a method for applying the coating composition of the present invention, which is formulated as described above, onto the chromate film, methods such as a roll coater method and an air knife method which are generally used in the industry are used. The coating amount of the coating composition of the steel sheet of the present invention is 0.2 to 3.0 g / m ^{2 in} terms of dry weight. If it is less than 0.2 g / m ² , the corrosion resistance will be poor, and if it exceeds 3.0 g / m ² , the film resistance will increase and the spot weldability and electrodeposition coatability will deteriorate.

The organic composite film has a maximum temperature of 100 ℃ on the surface of the steel sheet.
It is preferable to bake at above 200 °. This is because at temperatures below 100 ° C, the solvent does not dry sufficiently and the solvent remains in the coating film, impairing corrosion resistance.If it exceeds 200 ° C, yield elongation occurs in the steel sheet and stretcher strain occurs during press working. This is because there is a problem with the material.

The method of the present invention may be applied to both sides of a steel plate or only to one side depending on the use of the steel plate.

<Example> Hereinafter, the curing of the present invention will be specifically described based on Examples.

(Example 1) Predetermined Zn shown in Table 4 on a low carbon cold-rolled steel plate having a plate thickness of 0.7 mm.
The surface of the alloy-coated sample was coated with a roll coater to the coating amount shown in Table 4, and baked at the maximum plate temperature of 130 ° C. The organic composite film shown in Table 3 was coated with a roll coater. Apply and reach maximum plate temperature of 160
After baking at 0 ° C., it was immediately water-cooled and dried and used for the test. Table 1 shows the details of the base resin of the paint, and Table 2 shows the details of the silica sol to be added.

The details of each performance evaluation method are as follows.

Further, the specific surface area in Table ₂ is a value by the BEF method using N ₂ gas, the average particle size in Table 2 is the average particle size by centrifugal sedimentation particle size distribution measurement, and the resin composition and silica in Table 3 are mixed. The amount is dry weight%, and the Ni content of Zn-Ni undercoat in Table 4 is
12 wt%, plating A shows Ni: 12 wt%, Cr: 1 wt%, residual Zn, and plating B shows Co: 1 wt%, Cr: 0.8 wt%, Al ₂ O ₃ : 1 wt%, residual Zn, respectively.

As is clear from Table 4, all the examples of the present invention are organic composite coated steel sheets having excellent corrosion resistance, weldability and the like.

◎ Corrosion resistance Salt spray 4 hours 5% NaCl solution 35 ° C Dry 2 hours 60 ° C Wet 2 hours 95% RH 50 ° C A combined cycle corrosion test was performed as one cycle, and the number of cycles in which red rust was generated was evaluated.

◎ Water resistant secondary adhesion Phosphate treatment of sample (PB L3020, Nippon Parker Co., Ltd.)
Then, a cationic electrodeposition coating (Power Top U-600, manufactured by Nippon Paint Co., Ltd.) is applied to 20 μm and baked at 170 ° C. for 20 minutes, and then a top coating is applied (Ruger Bakeho).
Wight, Kansai Paint Co., Ltd.) applied 35 μm, 140
After baking at ℃ × 30 minutes, soak in pure water at 40 ℃ for 10 days,
Using a cutter, put a 2 mm square 10 × 10 square mesh,
Water resistance is measured by measuring the residual rate of the coating film after tape peeling.
The next adhesion was evaluated.

◎ Chromium elution resistance Using a phosphate treatment liquid PB L3020 manufactured by Nippon Parker Co., Ltd., treatment is performed through four steps of degreasing, washing with water, surface conditioning, and chemical conversion. Was used to calculate the elution amount.

◎ Electrodeposition coatability Power Top U-100 (manufactured by Nippon Paint Co., Ltd.) is energized at a voltage of 100V, a bath temperature of 28 ° C for 180 seconds, and then baked at 170 ° C for 20 minutes to be applied on the electrodeposition coating film. The number of generated gas pinholes was measured and evaluated.

○: 0 to 6 pcs / cm ² △: 7 to 10 pcs / cm ² ×: 10 pcs / cm ^{2 over} ◎ Weldability Welding force is 200 kgf, current is 9 kA, welding is done by using copper / chromium alloy welding tip with 6 mmφ tip Perform continuous welding at 10Hz for
Weldability was evaluated by the number of hit points at which no nugget was formed.

○: 3000 points or more △: 1000 to 3000 points ×: Less than 1000 points <Effect of the Invention> Since the present invention is configured as described above, the organic composite-coated steel sheet according to the production method of the present invention has excellent weldability,
It also has good corrosion resistance and paint adhesion, and can be widely used for automobile bodies and other applications.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hajime Hajime No. 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (56) Reference JP-A-58-98172 (JP, A) JP-A-2 -54779 (JP, A) JP 62-7539 (JP, A) JP 63-143270 (JP, A)

Claims (1)

[Claims] 1. Chromate in chromate on a Zn-plated steel sheet or a Zn-based alloy-plated steel sheet having a coating weight of 10 to 40 g / m ^2.
The ratio of ⁶⁺ is 70% or less with respect to the total chromium amount, and chromate treatment is performed so that the chromate adhesion amount becomes 5 to 500 mg / m ^{2 in} terms of chromium, and the average particle size of the secondary particles is A resin having a number average molecular weight of 2000 or more and a hydroxyl value of 50 or more based on a water-dispersed silica sol having a specific surface area of 0.05 to 3.0 μm and a specific surface area of 50 to 400 m ² / g and a water-dispersed or water-soluble organic resin. The coating composition containing the composition and having a silica sol ratio of 10 to 100 parts by weight (dry weight) with respect to 100 parts by weight (dry weight) of the resin composition has a dry weight of 0.2 to 3.0 g / m ²
A method for producing an organic composite-coated steel sheet having excellent corrosion resistance and weldability, which is characterized by coating.