JPH07243056A - Method for uniformizing appearance of chromate treatment for undercoating - Google Patents

Abstract

PURPOSE:To uniformize and stabilize a chromate appearance and a coating appearance even in a high-productivity line without deteriorating the coating performance by a synergistic effect of the optimization of the physical properties of a silica sol-contg. chromate treating soln. and roll coating conditions and the chromate pretreatment by substitution plating of Ni, etc. CONSTITUTION:In the silica sol-contg. chromate treatment by a natural twin-roll method, at least one kind of element among Ni, Fe, Sb and Co is substitution- plated in a normal-temp. bath by 5-100mg/m<2> coating weight as the chromate pretreatment with joint use of brushing. The plating is washed with water, then the bath is controlled to 35-60 deg.C and the linear pressure of the coating roll on a steel sheet to 0.1-2.0kg/mm<2>, the surface is chromate-treated, and the chromate appearance at 30-200mg/m<2> coating weight and the 2C2B coating appearance are uniformized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an undercoating chromate treatment method, particularly a chromate treatment method containing silica sol, for ensuring a uniform coating appearance of a precoated steel sheet on which a zinc-based plated steel sheet is used as an undercoat.

[0002]

2. Description of the Related Art In recent years, there has been a great need for reducing the cost of surface-treated steel sheets in the fields of home appliances and automobiles, and the precoating of postcoats is progressing further, while the demand for higher performance of coating performance is also high. In particular, when pre-coating a steel sheet, the first condition is to have a uniform appearance without coating defects, but in recent years, there has been an increasing demand for a more uniform appearance and high image clarity. Pre-coated steel sheet is usually a primer and a top 2
Coat 2 bake coating (2C2B) is the mainstream, and as a conventional technique for improving the appearance of the coating, there are many techniques obtained by combining the surface smoothing of the base original plate and the primer and top coating specifications. Among these, there is no conventional technology that disclosed the causal relationship between the coating chromate treatment of the coating base and the coating appearance, and it is a chromate treatment technology exclusively from the viewpoint of paint adhesion and corrosion resistance as an intermediate coating between the coating and the steel sheet. No more than disclosure.

For example, Japanese Patent Application Laid-Open No. 50-158535 discloses an example of attempting to make a chromate film insoluble. This discloses a chromate solution of chromic anhydride-phosphoric acid-aqueous polymer compound, and 6-chromium ion in the treatment solution is reduced by 70% or more with a reducing agent such as ethylene glycol. The main purpose of this so-called resin chromate technology is to improve the coating performance such as corrosion resistance and paint adhesion by modifying the film to include a high-polymer resin that is a poorly soluble chromate film that suppresses chromium elution. However, there is no disclosure of the technology relating to the uniformity of the chromate-treated appearance and the coating appearance by 2C2B, which is the gist of the present invention.

Further, as a chromate treatment containing silica sol, Japanese Patent Application No. 2-11109 and Japanese Patent Publication No. 4-2099.
There is a disclosure of Japanese Patent No. The main purpose of this technology is a chromate treatment method with high solubility of chromium and a mixture of granular silica sols with different particle diameters, which is difficult to dissolve and has high corrosion resistance. There is no description of the disclosed technology related to. At present, there is no disclosure of a technique for solving the phenomenon that the appearance pattern of the base chromate treatment extends to the appearance of the top coating in the above-mentioned conventional techniques.

[0005]

Under the high productivity line, chromate treatment and phosphate treatment are often used as the coating base treatment for precoated steel sheets, and in recent years, the chromate coating type is a technique. Is the mainstream of. As a coating method for the chromate treatment, a roll coating method and a gas wiping method have been conventionally used, but the former tends to be frequently used for precoated steel sheets. The chromate treatment by the roll coating method includes a natural method in which the traveling direction of the steel sheet and the roll rotation direction are synchronized and a reverse method in which the roll is rotated in the opposite direction to the steel sheet, but when the chromate treatment liquid has a low viscosity, the former natural method is used. The method is advantageous under high production line.

In this natural roll method, the problem is that the steel sheet is separated from the coating roll by the surface tension.
A considerable amount of a linear pattern (roping phenomenon) occurs due to the separation and aggregation of the chromate liquid film, which is transferred to the surface of the coating film of 2C2B coating, which lowers the commercial value of the coated steel sheet. That is, the present invention has been proposed from the viewpoint that it is necessary to achieve uniform appearance of the silica sol-containing chromate treatment in order to make the appearance of coating with 2C2B stable and uniform.

[0007]

In order to solve the above problems in the present invention, a detailed study for optimizing the chromate treatment conditions was carried out based on the following technical idea. That is, (1) occurrence of roping during roll coating of the chromate composition results from secondary aggregation of silica sol in the liquid. Silica sol is a necessary and bad component due to the physical properties of the 2C2B coating film, and it is inevitable to exclude it from the system. Therefore, in the silica sol-containing chromate composition, in order to eliminate the roping phenomenon, it is necessary to reduce the viscosity of the treatment liquid to improve the wettability and spreadability on the surface of the steel sheet, and to increase the drying speed of the liquid film. Therefore, there is an appropriate bath temperature (liquid temperature) range for sufficiently exerting this effect. (2) Further, there is an appropriate linear pressure condition for reducing the coating roll in the natural roll method. (3) Further, in order to improve the uniform wettability of the chromate solution on the surface of the original plate, it is necessary to remove the oxide film on the original plate surface and strengthen the adhesion of the formed chromate film. Based on such findings, the present invention has been proposed below.

First, the structure of the chromate treatment method in the present invention will be described. Roll linear pressure is 0.1 to 2.0
In the kg / mm ² natural two-roll coating method, a silica sol-containing chromate solution having a bath temperature of 35 to 60 ° C. is used as a solid film on the surface of a zinc-based plated steel sheet so as to be 30 to 200 mg / m ² in terms of metallic chromium. A chromate treatment method for a coating base, which comprises applying and immediately drying with hot air.

The zinc-based plated steel sheet applied to the present invention may be any one obtained by a known plating method. For example, in the electroplating system, Zn plating and any alloy element of Ni, Cr and Fe are used. Or one or more Zn
A system alloy plated steel sheet may be used. Further, in the electric dispersion plating system, a Zn-based dispersion alloy plated steel sheet obtained by uniformly dispersing and depositing metal oxides such as SiO ₂ , TiO ₂ , ZrO ₂ , and BaCrO ₄ on the basis of Zn-Ni and Zn-Fe is used. You may be taken. Further, hot-dip galvanized steel sheets, Zn-Al alloy plated steel sheets, and galvanized steel sheets obtained by alloying them may be applied in the hot-dip galvanizing system.

However, since at least a surface oxide film is inevitably formed on the surface of these steel sheets, Ni, Fe, and Sb must be removed in accordance with the removal of these surface oxide films in order to uniformly form the chromate film. It is effective to positively form a plating layer of at least one of Co and Co. These chromate pretreatments may have a coating amount as small as 5 to 100 mg / mm ² at most, and are characterized by performing displacement plating while brushing the surface of the steel sheet in the treatment bath.

[0011]

The action limits for the constituent factors of the method of the present invention will be described below. According to the coating chromate treatment method of the present invention, in the surface treatment of a coated steel sheet, in achieving stable and uniform coating appearance, it is intended to achieve uniform silica sol-containing chromate treatment appearance from the following effects. (1) Bath temperature When the bath temperature of the silica-containing chromate bath is less than 35 ° C, the viscosity of the liquid decreases little, the wettability of the liquid film on the surface of the underlying steel sheet is poor, and the immediate dryness of the liquid film is also low, resulting in a roping pattern. It is difficult to obtain a uniform chromate film appearance that eliminates the problem. On the other hand, if the temperature exceeds 60 ° C., the evaporation of water becomes large, making it difficult to control the bath concentration, making it difficult to control the appropriate amount of adhering and the appearance. Therefore,
The suitable bath temperature range is 30 to 60 ° C, preferably 38
~ 45 ° C is good.

(2) Roll linear pressure The linear pressure of the coating roll with respect to the steel plate of the natural type two-roll method has a suitable range. Roll linear pressure is 0.1kg / m
If it is less than m ² , it is easily affected by the shape of the steel sheet such as the warp of the steel sheet, the contact of the coating roll with the surface of the steel sheet becomes unstable, and the control of the adhesion amount and the appearance of chromate are likely to be nonuniform, which is not a good idea. On the other hand, 2.0 kg / mm ²
If the roll linear pressure exceeds the range, roll defects due to grinding marks and abrasion of the coating roll are likely to be transferred, and it is not a good idea from the viewpoint of nonuniform coating appearance and cost. A preferable roll linear pressure is 0.3 to 1.5 kg / mm ² .

(3) Chromium adhesion amount In the present invention, the adhesion amount of the coating base chromate is 3 in terms of metal Cr in view of ensuring uniformity of coating performance and coating appearance.
Control is required at 0-200 mg / m ² . When the amount of deposited chromium is less than 30 mg / m ² , it is difficult to maintain high corrosion resistance as a coated steel sheet in a severe corrosive environment.
Further, when it exceeds 200 mg / m ² , the separation and aggregation (roping) of the coating roll liquid film from the steel plate surface is out of the above control range, and the viscosity of the liquid film on the steel plate is increased, so that the film is dried quickly. It is difficult to obtain a uniform chromate appearance due to the dripping, which is not a good idea under a high productivity line.
The preferable chromium deposition amount is 50 to 150 mg / m ^2.
Is good.

(4) Displacement Plating as Chromate Pretreatment In the present invention, the purpose of this displacement plating is to further improve the wettability of the chromate solution to the surface of the steel sheet. At least 1 for each element of Ni, Fe, Sb, Co
If it is more than one kind and its adhesion amount is less than 5 mg / m ² , the uniform wettability of the chromate solution to the steel sheet is not sufficient.
On the other hand, if it exceeds 100 mg / m ² , it is not good in terms of productivity including deterioration of coating corrosion resistance and cost. Further, as means for applying the displacement-plated steel sheet, any of known dipping method, spraying method, electroplating method, etc. may be used, and either acidic or alkaline may be used even if the plating solution is at a pH. Even if it exists, it does not hinder the invention of this application. However, in order to perform the displacement plating with high productivity under the productivity line, it is characterized in that the liquid temperature is not particularly required, and the temperature is appropriately adjusted in the normal temperature liquid so as to obtain the required plating deposition amount. It requires the combined use of the brushing of.

[0015]

EXAMPLES The effects of the present invention will be described in detail below with reference to examples. A surface of a zinc-based plated steel sheet having a plate thickness of 0.7 mm and a plate width of 914 mm, which was controlled to have a predetermined coating amount by a known formulation, was alkali degreased-washed with Ni at a line speed of 70 m / min on a coating line. , Fe, Sb and Co are immersed in a displacement plating bath at room temperature (24 ° C.) containing at least one of Fe, Sb and Co, and the surface of the steel sheet is brushed in the bath so that a predetermined amount of plating is deposited, washed with water and dried. Shortly thereafter
The two-roll method of the natural type is controlled so that a predetermined chromium deposition amount is achieved under the chromate treatment conditions of the present invention specified in Tables 1 to 3 of the examples.

After that, it is immediately dried and air-cooled in a hot air drying furnace, and then immediately enters the 2C2B roll coating step.
The primer coating is a high-polymer polyester resin coating (600 PEU / Nippon Paint) containing a rust-preventive pigment, which is controlled to 5 μm as a solid film, baked to a maximum board temperature of 220 ° C. on the furnace outlet side, and water-cooled and dried. A high-molecular polyester resin paint (FLC1500HQ / Nippon Paint) is used as a top coat to apply a solid film having a thickness of 17 μm, followed by baking so that the maximum plate temperature on the furnace outlet side is 230 ° C. and water cooling and drying.

By applying the chromate treatment of the method of the present invention thus produced to the base, the chromate appearance is made uniform and the coating appearance can be stably and dramatically improved without lowering the coating performance of 2C2B. It is clearly understood that this is an epoch-making chromate treatment method which is not available in the prior art. The coating performance by the coating chromate treatment of the present invention thus obtained is shown in Tables 1 to 3.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

(Effect of temperature rise of bath) First, as a chromate bath temperature effect of the present invention, regarding the coating performance including the appearance of chromate treatment and the appearance of coating, Example No. 1 was used. 1-No. 7
And the comparative example No. 8 to No. 9 shows. As is clear from this, the effect of improving the uniformity of the coating appearance according to the present invention is clear, and by controlling the chromate treatment bath within the appropriate bath temperature range, the chromate appearance is dramatically improved, which results in a uniform 2C2B coating appearance. It can be seen that the improvement is in proportion to the improvement in sex. Although the mechanism of the bath temperature effect for improving the appearance has not been sufficiently clarified, the surface tension of the chromate solution due to the temperature rise does not change much or, on the contrary, tends to rise slightly. It is thought that it is not due to. Rather, the viscosity (kinematic viscosity) of the treatment liquid decreases and the drying speed of the liquid film increases, which is considered as follows. The effect of decreasing the viscosity (kinematic viscosity) of the treatment liquid is that when the chromate liquid film on the surface of the coating roll comes into contact with the steel plate and then leaves the steel plate, the liquid film separates. It is presumed that this is due to the synergistic effect of suppressing the roping of the liquid film by increasing the stickiness of the liquid film and increasing the immediate drying property of the separated liquid film by increasing the temperature.

(Effect of linear pressure of coating roll) In the present invention, it is necessary to control the linear pressure of the coating roll in order to make the chromate appearance uniform. 3 and No. 10-No. 13 and the comparative example No. 14-No. Shown in 15. As is clear from this, in the uniform coating of chromate by the natural type two rolls, it is necessary to control the linear pressure of the coating roll, and by controlling to the proper linear pressure of the invention of the present application, it was stable even under a high productivity line. It can be seen that a uniform appearance is obtained. If the linear pressure deviates from the appropriate range, it depends on the shape of the steel plate, or liquid runs out to hinder the smooth rotation of the coating roll, making it difficult to stably secure the uniform chromate appearance.

(Effect of Chromium Adhesion Amount) In the present invention, the appropriate chromium adhesion amount range of the chromate film is required to achieve both the uniform coating appearance and the improvement of the coating performance especially under severe corrosive environment. Although it is a factor, no. 3 and No. 16-No. Shown in 22
In addition, the comparative example No. 23-No. 24. As is clear from this, when the amount of coating adheres to the proper range of the present invention, the chromate appearance as well as the overall coating performance including the coating appearance is deteriorated, and the commercial value is greatly impaired.

(Effect of Displacement Plating as Pretreatment for Chromate) Regarding the effect of improving the uniformity of the chromate film by the displacement plating of the present invention, Example No. 3 and No. 25
~ No. No. 37 shows the effect of the plating adhesion amount and the plating element system, as a comparative example of the plating adhesion amount. 31-No.
32. As is apparent from this, when the amount of the appropriate plating adhesion amount range referred to in the present invention is deviated, not only the chromate appearance but also the overall coating performance including the coating appearance is deteriorated, and the commercial value is greatly impaired.

(Note) * 1. Plating system EZ: Electric zinc plating ZN: Electric Zn-Ni alloy plating (Ni; 11.5%) EC: Electric Zn-Cr-Ni alloy plating (Cr; 10)
%, Ni; 2%) EF : Electrical Zn-Fe alloy plating (Fe; 15%) ZNS: Electrical _{Zn-Ni-SiO 2 (Ni} ; 12%, S
iO _2; 3%) ZNT: Electrical _{Zn-Ni-TiO 2 (Ni} ; 12%, T
iO _2; 3%) ZNZ: Electrical _{Zn-Ni-ZrO 2 (Ni} ; 12%, Z
rO _2; 3%) ZNB: Electrical _{Zn-Ni-BaSO 4 (Ni} ; 12%,
BaSO _4; 3%) ZFS: Electrical _{Zn-Fe-SiO 2 (Fe} ; 10%, S
iO ₂ ; 3%) GZ: hot-dip Zn plating GA: hot-dip Zn-Al plating (Al; 5%) GF: alloyed hot-dip Zn plating (Fe; 8-11%)

* 2. Displacement plating method of chromate pretreatment N: Ni plating, F: Fe plating, S: Sb plating,
C: Co plating * 3. Coating chromate treatment Treatment liquid: Silica sol-containing reduced chromate liquid is used (manufactured by Nippon Parkerizing) Chromium adhesion amount: Adhesion amount control by roll peripheral speed ratio, measurement by fluorescent X-ray analysis Coating method: Natural type two rolls (chrome resistance Rubber roll for acid / made by Kinyosha)

* 4. Uniformity of chromate film appearance (visual appearance) ◎: uniform, ◯: slightly roping, △: partial roping, ×: full surface roping or unevenness * 5. Uniformity of coating appearance (visual appearance) ◎: Uniform, ◯: Slight flow unevenness, △: Partial flow unevenness, ×: Entire linear flow or spot unevenness

* 6. Coating corrosion resistance Salt spray test (JIS Z-2371) for 960 hours, coating film swelling from the end face of the steel plate cut ◎: Very slight, ◯: Small blisters scattered, Δ: Slightly continuous small blisters, ×: Continuous blisters * 7. Adhesion of paint Primary adhesion: Repeated 1T adhesion bending and repeating the same processed part 3
Peel off the cellophane tape. Secondary adhesiveness: Immersed in pure water, boiled and left for 2 hours and then for 24 hours, and then evaluated in the same manner as the primary adhesiveness. ⊚: No peeling of coating film, ◯: 5% or less of peeling, Δ: 10% or less of peeling, ×: More than 10% of peeling

[0029]

As described above, according to the present invention, as a method of applying the chromate treatment containing silica sol by the natural type two-roll method to the steel sheet under the line of high productivity, the displacement plating treatment of the specific element system such as Ni is performed. By setting the chromate bath temperature, coating roll linear pressure, and chromium deposition amount within the appropriate ranges, it was possible to achieve a uniform chromate film that affects the appearance of 2C2B coating, which was previously difficult. It is proposed here as a technical chromate treatment technology.

Claims (4)

[Claims] 1. In a chromate treatment for silica sol-containing coating by natural type two-roll coating, Ni, Fe, Sb, Co after alkali degreasing as pretreatment of chromate is performed.
In an aqueous solution at room temperature containing at least one or more of each of the ions, displacement plating is performed while brushing the surface of the steel sheet, and after the surface is washed with water, the temperature of the solution is controlled to 35 to 60 ° C., and chromate treatment is performed. Method for uniform appearance of chromate treatment for paint base. 2. In a silica sol-containing chromate treatment by a natural two-roll coating, Ni, Fe, Sb, Co after alkali degreasing as a pretreatment for chromate is performed.
In a room temperature aqueous solution containing at least one or more of each ion, the steel plate surface is subjected to displacement plating while being brushed, washed with water, and the linear pressure of the coating roll on the steel plate surface is 0.1 to 2.
A method for uniformly treating the appearance of chromate treatment for a coating base, which comprises controlling the chromate treatment at 0 kg / mm ² . 3. A pretreatment of chromate, after alkaline degreasing, to form an element containing at least one of Ni, Fe, Sb and Co in an amount of 5 to 100 mg / m ^2. A chromate treatment uniform appearance treatment method according to 1 or 2. 4. The uniform appearance treatment method for chromate treatment according to claim 1, wherein the amount of chromium deposited on the surface of the steel sheet is 30 to 200 mg / m ² in terms of total Cr.