JPH0774473B2 - Manufacturing method of surface-treated steel sheet with excellent paintability and corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a method for producing a surface-treated steel sheet having excellent coating properties such as chemical conversion treatment property, electrodeposition coating property, paint adhesion property and corrosion resistance.

[Problems to be Solved by Conventional Techniques and Inventions]

In recent years, a problem of car body corrosion has occurred in areas where rock salt is sprayed for the purpose of so-called winter snow melting, such as northern Europe, Canada, and the United States, and as a countermeasure, corrosion-resistant surface-treated steel sheets have been adopted instead of conventional cold-rolled steel sheets. The amount used is also increasing year by year.

Among them, galvannealed steel sheet (abbreviated as GA) is one of them, and is capable of relatively thick plating.

Excellent weldability.

The cost is relatively low.

Due to such advantages, it is used more often than other surface treated steel sheets (vehicle body anticorrosion steel sheets). At the same time, the demand for quality of GA is becoming severe.

That is, when GA was directly used as an anticorrosive steel sheet for automobiles, it was pointed out by the automobile manufacturer that chemical conversion treatment (phosphate treatment), swimming paint finish, and corrosion resistance after painting were poor.

As a countermeasure, for example, as shown in Japanese Patent Publication No. 58-15554, G
There is a method of applying Fe-Zn alloy plating with a Fe% of 40% or more on the surface of A.

Besides, thin Fe or Ni, Fe-P, or Zn on the surface of GA
-There is also a method of plating with an alloy such as Ni.

However, in this way, on the GA surface, Fe, Ni, Fe-Zn, Fe-P, Z
When plating with n-Ni, etc., there were the following problems in the actual production line. For example, when a two-layer plated steel sheet is manufactured by the following process, if the two-layer plated steel sheet is usually subjected to a phosphate treatment before electrodeposition coating such as by an automobile manufacturer, there is often a problem that a white and light colored streak pattern is generated on the surface. It was

Observation of this white-light color streak pattern with a scanning electron microscope revealed that the phosphate crystals were incomplete and the base material appeared to be sparse (sparse) in places.

As a result of investigating measures to solve this problem, it was found that by clarifying the cause of the problem, it can be solved by devising a pretreatment method before the upper layer plating.

[Means for Solving the Problems]

That is, the gist of the present invention is that GA contains one or more of caustic soda and sodium phosphate in an amount of 10 to 150 g /, and further contains a cationic, anionic or nonionic surfactant.
Degreasing treatment is performed in a degreasing aqueous solution containing 1 to 1 g /. Alternatively, it is a method for producing a surface-treated steel sheet excellent in coatability and corrosion resistance by further performing acid treatment as a subsequent treatment in a sulfuric acid aqueous solution having a concentration of 1 to 20 g /, followed by metal plating or content plating or laminated plating of these.

Hereinafter, the present invention will be described in detail.

As a result of investigating the cause of the light white color after the phosphate treatment, which is the first problem, by using an EPMA analyzer, Si element (probably the formation of SiO ₂ ) was found in the light white area compared to the normal area. I found many things. As a result of further research on the cause of this SiO ₂ formation, a degreasing agent such as sodium orthosilicate (Na ₄ S
iO ₄ ).

That is, when GA is immersed in a degreasing solution in which conventionally used sodium orthosilicate is dissolved, SiO ₂ is deposited on the GA surface due to a hydrolysis reaction. The amount of SiO ₂ deposited varies depending on the thickness of the oxide film on the GA surface and the difference in activity, but it is pale white after the phosphate treatment even if degreasing conditions (sodium orthosilicate concentration, temperature, immersion time) are selected. The abnormal part does not disappear completely.

Therefore, as a result of investigating the selection of the degreasing agent, GA is first degreased. For the degreasing treatment, a degreasing liquid containing one or two kinds of sodium hydroxide and sodium phosphate and a surfactant is used. Caustic soda is for alkaline degreasing (vegetable oil) and has a strong saponification effect. Also, sodium phosphate is weakly alkaline and saponified,
It has a small emulsifying and dispersing effect, but when used for degreasing zinc, it has the characteristic of not damaging the substrate. In addition, since it has the effect of softening hard water, it improves the degreasing effect and improves the washing effect. These degreasing liquids are used alone or in a mixture with a surfactant. However, the concentration is 10g /
A thin degreasing aqueous solution of less than swells even if it permeates into the inside of the oil and fat, and the adhesive force between the oil and fat and the plating surface cannot be weakened. On the contrary, if the concentration exceeds 150 g /, the plating surface is likely to be exposed to alkali and should be avoided.

In addition, the surfactant significantly improves the cleaning effect. This is to enhance the permeation, wetting and washing power of the alkali into the metal surface. Further, the removed oil is emulsified, floats in the liquid, and does not adhere to the plating surface again. From the above, the surfactants generally used for degreasing treatment such as anionic surfactant petroleum-based detergent, cationic surfactant pyridine type, nonionic surfactant ester type are used. However, when the concentration of the surfactant in the degreasing aqueous solution is less than 0.1 g /, even if the surfactant has a lipophilic or hydrophilic group, which is a feature of the surfactant, it does not exhibit excellent activity unless its molecular weight is considerably large. The limit is 0.1g /. If the concentration is more than 1 g / g, the effect is not improved, and a problem such as generation of bubbles is often caused. Further, after degreasing, the activator remains on the plating surface, which impairs the upper layer plating.

GA degreased with such a degreasing aqueous solution removes the contamination of oils and fats and becomes a surface with good activity. However, when degreasing in a sodium phosphate-based degreasing agent solution, the solution temperature for degreasing treatment,
Depending on various conditions such as the treatment time, the phosphate coating may cause light-colored unevenness (difference in gray-black color). This is because a small amount of zinc phosphate is generated on the surface, so in order to remove this uneven appearance, it is preferable to further perform pickling treatment in a sulfuric acid solution after degreasing. In the pickling treatment, if the concentration is less than 1%, the cleaning effect on the GA surface is low, and if it is 20 g / g or more, cracks on the GA surface and the GA plating layer are etched, resulting in deterioration of workability.

GA that has undergone such a pretreatment step is subjected to metal plating of Fe, Ni or the like, alloy plating of Fe-P, Zn-Fe, Zn-Ni or the like, or laminated electroplating thereof. The surface-treated steel sheet thus obtained is free from the problem of light and white streaky streaks, and has excellent paintability and corrosion resistance.

[Effects of Examples and Invention]

Example-1 A sample of an alloyed hot-dip galvanized steel sheet (GA) was plated with an ordinary continuous hot-dip galvanizing line, and then an alloyed steel sheet (a plate thickness was 0.8 mm and a coating weight was 40 g / m on the front side). ^2, the back side is 40 g / m ^2, the plating layer composition Zn: 89%, Fe: 11 %) of NaOH
Alternatively, a degreasing aqueous solution containing Na ₃ PO ₄ and a cationic surfactant concentration of 1 g / was used for immersion for 10 seconds at a temperature of 50 ° C. to 60 ° C., followed by brushing while washing with water.

For further pickling, H ₂ SO ₄ concentration: 1% (10 g /) 2
It was immersed in a 0 ° C to 25 ° C solution for 5 seconds and then washed with water.

Furthermore, Zn-Fe (80%) Zn-P (1-2
%) And Zn-Ni (11%), but Zn-Fe (80%) alloy plating is shown as a representative example. The plating bath composition adopted, the plating conditions, and the obtained plating properties are as follows.

Plating bath composition Plating conditions ZnSO ₄・ 7H ₂ O 16g / Current density 80A / cm ² FeSO ₄・ 7H ₂ O 430g / Plating time 2.8 seconds H ₂ SO ₄ 11g / Temperature 50 ° C pH 1 Plating property Adhesion amount 3.1 to 3.7g / m ² alloy composition Zn-Fe (77 to 85%) The appearance after plating was visually observed and the quality was judged as follows.

◎ No abnormality (uniform surface) ○ Mild flow pattern generation △ Mild linear pattern generation (black and white unevenness generation) × Linear pattern generation (black and white unevenness sharpness) Next, the same specimen was treated with a phosphate coating (for Parker Rising) The full dip drug PB3080) was applied as follows.

Degreasing: 40 ° C x 2 minutes, surface conditioning: 1.4 g /, room temperature x 2 minutes, chemical conversion: 43 ° C x 2 minutes, TA23, FAO.8, coating weight 2.5 g / m ² . The chemical conversion treatment film was judged according to the following standard (visual observation, but SEM observation is also performed if necessary).

◎ Good (dense disc-like crystals) ○ Almost good (dense but some needle-like crystals) △ White part of linear pattern (no crystals exist) × Large number of white parts of linear pattern

Next, ED coating was performed to observe the surface appearance after ED.

The ED conditions are as follows.

Electrodeposition paint Powertop U-80 gray made by Nippon Paint Electrodeposition condition 220V x 3 minutes 28 ° C gap 150mm Pole ratio: contrast 1: 3 (sample) Coating thickness 20-25μ Baking conditions 180 ° C x 3 minutes ED coating surface Judgment criteria ◎ Surface flatness ○ Several micro recesses are seen / 70 × 150mm △ 5-10 recesses / 70 × 150mm × 50 recesses / 70 × 150mm When Zn-Fe (~ 80%) alloy plating is applied Table 1 shows a summary of the test results.

The effect of the present invention is remarkable.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Makoto Suenaga 1-1-1 Edamitsu, Hachimanto-ku, Kitakyushu City, Fukuoka Prefecture Inside the Yawata Works, 1-1-1 Nippon Steel Corporation (72) Fumihiko Suga, Hachimanhigashi, Kitakyushu City, Fukuoka Prefecture 1-1-1 Kueda Mitsue, 1-1, Nippon Steel Co., Ltd., Yawata Works (56) References Japanese Patent Publication Sho 58-15554 (JP, B2) Kase Takanori "Latest Plating Technology" (Sho 58-11-30) Industry Book Co. P. 65-76

Claims (2)

[Claims] 1. An alloyed hot-dip galvanized steel sheet containing 10-150 g / of one or two kinds of caustic soda and sodium phosphate, and 0.1-1 g / of a cationic or anionic or nonionic surfactant. A method for producing a surface-treated steel sheet excellent in paintability and corrosion resistance, which comprises performing a degreasing treatment in a degreasing aqueous solution containing a metal, and then performing a corrosion-resistant metal plating, a corrosion-resistant alloy plating, or a laminated plating of these on the surface. 2. An alloyed hot-dip galvanized steel sheet containing 10 to 150 g / each of one or two kinds of caustic soda and sodium phosphate,
Further, a cationic, anionic or nonionic surfactant is degreased in a degreasing aqueous solution containing 0.1 to 1 g /, followed by pickling in a sulfuric acid solution having a concentration of 1 to 20 g /, and then the surface is corrosion resistant. A method for producing a surface-treated steel sheet excellent in paintability and corrosion resistance, which comprises performing metal plating, corrosion-resistant alloy plating, or laminated plating thereof.