JPH07238383A - Method for uniformizing appearance of chromating for base to be coated - Google Patents

Abstract

PURPOSE:To uniformize chromated appearance and coated appearance even under a high productivity line without lowering coating performance by adopting proper physical characteristics of a silica sol-contg. chromating soln. and proper conditions in roll coating. CONSTITUTION:In chromating with a silica sol-contg. chromating soln. by a natural two-roll method, chromated appearance with 30-200mg/m<2> coating weight of chromate and 2C2B coated appearance are made uniform by controlling the bath temp. to 35-60 deg.C and the linear pressure of coating rolls on a steel sheet to 0.1-2.0kg/mm<2>. By this chromating technique, chromated appearance is made uniform and this uniform appearance considerably improves the uniformity of 2C2B coated appearance without lowering coating performance (corrosion resistance and adhesion of a coating material).

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, in the case of precoating steel sheets, 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 coating 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, many techniques are obtained by smoothing the surface of the base original plate and adjusting the combination of the 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 so-called resin chromate technology aims to improve the resistance to corrosion and paint adhesion by containing a polymer resin in the coating to modify it into a poorly soluble chromate coating that suppresses chromium elution. However, there is no disclosure of the technology relating to the uniformity of the appearance of chromate treatment and the appearance of coating 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 undercoat chromate treatment extends to the appearance of 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 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, the viscosity of the treatment liquid is lowered to smoothly separate and transfer the liquid film on the steel plate surface and the liquid film on the coating roll surface without stringing. In addition to making it possible, it is necessary to increase the drying speed of the liquid film, and there is an appropriate bath temperature (liquid temperature) range for sufficiently exerting this effect. (2) Further, based on the finding that the coating roll in the natural roll method has an appropriate rolling reduction condition, the present invention has been proposed as follows.

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 alloy elements of Ni, Cr and Fe are used. Z consisting of any one or more
An n-based 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. Furthermore, in the hot-dip galvanizing system, hot-dip galvanized steel sheets, Zn-Al alloy plated steel sheets, and galvanized steel sheets obtained by alloying them may be applied.

[0010]

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 There is an appropriate range for the linear pressure of the coating roll with respect to the steel sheet of the natural type two-roll method. 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 deposition amount In the present invention, the coating base chromate deposition amount is 3 in terms of metal Cr in view of ensuring coating performance and uniformity of 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.

[0013]

EXAMPLES The effects of the present invention will be described in detail below with reference to examples. The 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 subjected to alkaline degreasing-water washing-drying at a line speed of 70 m / min on a coating line. After that, a natural two-roll method is used to control the amount of chromium deposition to be a predetermined amount under the chromate treatment conditions of the present invention specified in Tables 1 and 2 of the examples. After that, it is immediately dried in a hot air drying oven and air-cooled, and then immediately enters the 2C2B roll coating process. Primer paint is a high molecular polyester resin paint (600 PEU / Nippon Paint) containing a rust preventive pigment as a solid film of 5μ
m, controlled so that the maximum plate temperature on the furnace outlet side is 220 ° C., and water-cooled and dried, and then a high-polymer polyester resin paint (FLC1500HQ / Nippon Paint) is applied as a top coat in a solid film of 17 μm. It is baked so that the maximum plate temperature on the furnace outlet side is 230 ° C, and is water-cooled and dried.

By applying the chromate treatment of the method of the present invention thus produced to the base, the chromate appearance can be 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 and 2. First,
As for the chromate bath temperature effect of the present invention, the coating performance including the chromate treatment appearance and the coating appearance was evaluated in Example N.
o. 1-No. 7 and the comparative example No. 8 to N
o. 9 shows.

[0015]

[Table 1]

[0016]

[Table 2]

(Effect of increasing temperature of bath) As is clear from this, the effect of improving the uniformity of the coating appearance according to the present invention is clear, and the chromate appearance is dramatically improved by controlling the chromate treatment bath within the appropriate bath temperature range. Improved to 2C2
It can be seen that the improvement in the uniformity of the B coating appearance is in proportion to the improvement. 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. As the effect of decreasing the viscosity (kinematic viscosity) of the treatment liquid,
Separation of the liquid film occurs 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, but at this time, the viscosity is reduced, and the stringiness due to the viscosity of the liquid film is reduced. It is presumed that this is due to the synergistic effect of suppressing the roping of the liquid film and increasing the quick-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 suitable chromium adhesion amount range of the chromate film is required to achieve both the uniform coating appearance and the improvement of the coating performance particularly 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.

(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%) CF: alloyed hot-dip Zn plating (Fe; 8-11%)

* 2. 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)

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

* 5. Coating corrosion resistance Salt spray test (JIS Z-2371) for 960 hours, coating film swelling from the end face of steel plate cut ◎: Very slight, ◯: Small blisters scattered, Δ: Slightly continuous small blisters, ×: Continuous blisters

* 6. Adhesion of paint Primary adhesion: Repeated 1T adhesion bending and repeating the same processed part 3
Peel off the cellophane tape. Secondary adhesion: Immersed in pure water, boiled and left for 2 hours and then left for 24 hours, and then evaluated in the same manner as the primary adhesion. ⊚: No peeling of coating film, ◯: 5% or less of peeling, Δ: 10% or less of peeling, ×: More than 10% of peeling

[0025]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, the chromate bath temperature, the linear pressure of the coating roll, and the linear pressure of the coating roll are used as a method for applying the silica sol-containing chromate treatment in the natural two-roll method to a steel sheet under a highly productive line. It has been difficult in the past by specifying the three areas of chromium adhesion within the appropriate range. 2
This is proposed here as an epoch-making chromate treatment technique capable of achieving uniformization of the chromate film that affects the coating appearance of C2B.

Claims (3)

[Claims] 1. A method for uniform appearance of chromate treatment for a coating base, characterized in that in the silica sol-containing coating chromate treatment by natural type two-roll coating, the treatment liquid has a liquid temperature of 35 to 60 ° C. 2. The uniform appearance treatment method for chromate treatment for a coating base according to claim 1, wherein the linear pressure of the coating roll on the surface of the steel sheet is 0.1 to 2.0 kg / mm ² . 3. The uniform appearance treatment method for chromate treatment for a coating base according to claim 1 or 2, 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.