JPH0723545B2 - Method for producing Zn-based electroplated steel sheet having excellent chemical conversion treatability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method for producing a Zn-based electroplated steel sheet, and more particularly to a post-treatment method for a Zn-based electroplated steel sheet with good conversion treatment on the plated surface. is there.

<Prior art and its problems> Zn-based electroplated steel sheets can be used in a wide range of fields where cold-rolled steel sheets have been used, such as automobiles, home appliances, and building materials, because they can achieve high corrosion resistance without degrading the material quality. Is being expanded.

Particularly, in recent years, electro-alloy plated steel sheets such as Zn-Ni and Zn-Fe have been developed for the purpose of improving corrosion resistance, have been actively used as rust-prevented plated steel sheets for automobile bodies, and production has increased remarkably. However, on the Zn-based electroplated steel sheet, crystals of phosphate generated during phosphate chemical conversion treatment
₃ (PO ₄ ) ₂ · 4H ₂ O), this hopeite forms on cold rolled steel.

Since the adhesiveness of the paint is inferior to phosphophyllite (Zn ₂ · Fe (PO ₄ ) ₂ · 4H ₂ O), its use site is mainly on the inside of the vehicle body, and there was a problem in using it on the outside of the vehicle body .

Conventionally, in order to solve this problem, for example, JP-A-56-13
The method by two-layer plating found in 3488 and 58-11795 is used. In this technology, the Zn-based plating is plated with Fe-rich, the corrosion resistance is in the lower layer, and the chemical conversion treatment is in the upper layer. It is done. However, this method (1) increases the cost due to the upper layer plating, (2) it is difficult to control the upper layer plating because the basis weight is small, and (3) the precious metal is plated on the electrically base metal. Therefore, when the upper layer plating amount is increased, the dissolution amount of the lower layer metal is increased and the corrosion resistance is deteriorated.

<Objects of the Invention> The present invention has been made in view of the above circumstances, and is a Zn-based compound having good chemical conversion treatability that can be efficiently subjected to phosphite chemical conversion treatment with good adhesion without increasing manufacturing costs. An object is to provide a method for manufacturing an electroplated steel sheet.

<Problems to be Solved by the Invention> The present inventors investigated the chemical conversion treatability of electric Zn or Zn-Ni alloy plated steel sheets.

The phosphate chemical conversion treatment improves secondary adhesion such as primary adhesion and hot water resistance as a coating base treatment.

Generally, phos produced on cold rolled steel sheet as described above.
Although the adhesion of phophyllite is good, the hopeite formed on the Zn-based electroplated steel sheet is easily dissolved in the alkaline aqueous solution, so the secondary adhesion to hot water is poor. However, when a large number of Zn-based electroplated steel sheets were subjected to a hot water-resistant secondary adhesion test, most of them showed poor adhesion, but some showed good adhesion. A study of the chemical conversion treatability of this Zn-based electroplated steel sheet with good secondary adhesion to hot water showed the same
Even with hopeite, it has been revealed that the phosphate conversion crystals are all fine.

<Means for Solving Problems> Therefore, the present inventors have made various studies on a method for obtaining a plated surface in which the phosphate chemical crystals formed on the Zn-based electroplated steel sheet are always fine and uniform. . As a result, Zn
After electrolytic electroplating, after electrolytic cleaning in a weak acid to neutral aqueous solution, the phosphoric acid compound as P on the plating surface is 0.01 to
The present inventors have found that it is extremely effective to attach 50 mg / m ^{2 and} have completed the present invention. That is, the present invention is characterized by subjecting a steel sheet to Zn-based electroplating, electrolytically cleaning the plated surface in a weak acid to a neutral aqueous solution, and then depositing P on the plated surface in an amount of 0.01 to 50 mg / m ^2. The present invention provides a method for producing a Zn-based electroplated steel sheet.

<Specific Configuration and Action of the Invention> The method for producing a Zn-based electroplated steel sheet having excellent chemical conversion treatability of the present invention will be described in detail below.

The Zn-based electroplated steel sheet in the present invention broadly includes a Zn-plated steel sheet and a Zn-based alloy-plated steel sheet mainly containing Zn. For example, Zn-Ni system, Zn-Mn system, Zn-Cr system, Zn-
Fe-based alloy plating, as well as P, Co, Cr, Sn,
One or two of Sb, V, Fe, Ti, Ni, Mn, As, Bi, etc.
Sulfuric acid bath for any Zn-based alloy or composite plating film, such as ones intentionally added or unavoidably mixed
It is plated with an acidic liquid such as a chloride bath or a mixed bath thereof.

In the present invention, the steel sheet plated with such Zn-based electroplating is then electrolytically cleaned in a weak acid to neutral aqueous solution.

The pH after electrolytic cleaning should be 4 or more. If the pH is less than 4, the chemical dissolution of the plating layer will significantly increase, causing plating cracks,
It causes deterioration of corrosion resistance.

As the electrolytic cleaning liquid, any electrolytic cleaning liquid having a pH of 4 or more may be used as long as it is usually used as an electrolytic cleaning liquid, and any component may be contained, but Na, K, Li, Mg,
It is preferable to use, as a main component, at least one of sulfates, oxyacids, and phosphates such as Al, Ca, Be, Ba, Sr, etc., using sulfuric acid, oxyphosphoric acid, or sodium hydroxide solution. It is desirable to adjust the pH. The reason for this is that since the deposition potential of the metal ions is lower than the hydrogen reduction potential, electrolysis of the aqueous solution does not cause deposition of the metal,
Maintaining the metal ion concentration in the bath and maintaining the cathode are easy, and the acid produced on the anode and the alkali produced on the cathode balance in an equivalence relationship, which is very convenient for maintaining the bath pH. Is.

The electrolytic cleaning tank in the present invention is not particularly limited, and a horizontal type, a vertical type, a radial cell or the like is used, and the liquid temperature can be arbitrarily selected from room temperature to 70 ° C.

Further, in the present invention, it is preferable to use a Zn-based electroplated steel sheet for electrolytic cleaning as an anode and perform electrolytic cleaning at a current density of 5 A / dm ² or more. If it is less than 5 A / dm ² , the stirring effect of oxygen gas and the amount of Zn solution will be small, so at the end of plating, the Zn (OH) ₂ film, which is an insulating material covering the plating surface, will not be sufficiently destroyed and the surface will be cleaned. It is not preferable because it lowers the chemical conversion ability.

In the present invention, P is added to the electrolytically cleaned plating surface in an amount of 0.01 to
Attach 50 mg / m ² . If the amount of P attached is less than 0.01 mg / m ² , the effect of improving chemical conversion treatability is small. On the other hand, if it exceeds 50 mg / m ² , the chemical conversion crystals become too fine, and an appropriate chemical conversion film amount cannot be secured, resulting in a slick appearance, and secondary adhesion such as primary adhesion after coating and adhesion to hot water resistance. It is not preferable because it deteriorates the property. As a supply source of P to be attached to the non-plated surface, NaH ₂ PO ₄ , Na ₂ HPO ₄ , KH ₂ PO ₄ , K ₂ HPO ₄ , NaPH ₂ O ₂ , Na ₃ PO ₄ , H
Soluble hypophosphorous acid, phosphorous acid, orthophosphoric acid or their alkali and alkaline earth salts such as PH ₂ O ₂ , H ₂ PHO ₃ , H ₃ PO ₄ , HPO ₃ and H ₄ P ₂ O ₇ Available.

The reason why the chemical conversion treatment property is improved by attaching P to the plated surface is not clear, but it is considered as follows. By uniformly adhering P to the surface of the plated surface, that portion becomes a cathode and promotes the anode reaction, that is, the dissolution of Zn. It is considered that this cathode portion becomes a crystal nucleus of the phosphate film, and the chemical conversion treatability is improved.

By the way, the method of attaching P to the surface of the plating surface of the compound may be any method, but from the viewpoint of adhesion efficiency and workability, the compound is made into an aqueous solution and subjected to a dipping treatment, a spray treatment or a roll coater. Preferably.

<Examples> Next, details of the present invention will be described based on Examples.

In each of the following examples, SPCC with a plate thickness of 0.7 mm is used.
It was plated using a corresponding cold rolled steel sheet.

<Example 1> (1) Zn-Ni bath composition _{NiSO 4 · 6H 2 O 250g /} ZnSO 4 · 7H 2 O 130g / Na 2 SO 4 40g / (2) pH 1.6 (3) bath temperature 60 ° C. (4) Current density 50A / dm ² (5) Plating time 15sec (6) After galvanizing Zn-Ni alloy on cold-rolled steel sheet under the plating condition of coating weight 20g / m ² .
Immediately after washing with water, the plated surface was electrolytically washed under the conditions shown in Table 1, followed by washing with water and drying. The polarities in the table are the polarities of the steel sheet to be treated. Then Na
After immersing H ₂ PO ₄ in water or an aqueous solution obtained by dissolving it in warm water for 10 seconds, it was squeezed by a roll in a wet state and dried.

The amount of P attached on the plated surface after the above treatment was analyzed by an atomic absorption method and the appearance was visually determined. The appearance evaluation of the plating surface is as follows. The results are also shown in Table 1.

Appearance of plated surface: ○ -Uniform and no unevenness △ -Some unevenness × -There is unevenness on the entire surface After this, a phosphate treatment liquid Granodine SD manufactured by Nippon Paint Co., Ltd.
Chemical conversion treatment was performed using 2000 (Di _P method). The chemical conversion film was evaluated visually and at the same time, 5 scanning spots were randomly observed with a scanning electron microscope to evaluate the uniformity, the density of crystals, etc., and the gravimetric method (using ammonium chromate solution).
The amount of coating film was determined by. The results are also shown in Table 1. The evaluation criteria of the chemical conversion film at this time are as follows. Appearance of chemical conversion film: ◎ …… The appearance is uniform and uniform, and the crystals are uniform and dense in all 5 places. ○ …… The appearance is almost uniform and uniform, but one crystal is slightly uneven. Almost uniform with no unevenness, but the crystals at 3 points are slightly uneven. △ ... The appearance is partially uneven, and the crystals at 3 points are not uniform, and mixed grains of coarse particles and fine particles. There is unevenness on the entire surface, the crystals are non-uniform in all 5 places, and a mixture of coarse particles and fine particles. The hot water secondary adhesion test is performed after chemical conversion treatment by cation electrodeposition coating (Power Top U-30, made by Nippon Paint). ) 20μm, intermediate coat 30μm, top coat 40μm (both amino alkyd paint, manufactured by Nippon Paint Co., Ltd.), 90μm in total, 3 coats are applied, and after immersing in pure water at a liquid temperature of 50 ° C for 240 hours, knife on the coating surface 2mm square base that reaches the base with 100 scratches
Individual pieces were attached and the cellophane tape was peeled off twice, and the residual rate (%) of the coating film was evaluated. The results are also shown in Table 1.

<Example 2> (1) Zn bath composition _{ZnSO 4 · 7H 2 O 400g /} Na 2 SO 4 35g / (2) pH 2 (3) bath temperature 55 ° C. (4) current density 40A / dm ² (5) Plating Time 20sec (6) Electrolytic Zn plating of cold-rolled steel sheet under plating conditions with a coating weight of 20 g / m ² , immediately followed by washing with water, electrolytically washing the plated surface under the conditions shown in Table 2, then washing with water and drying. did.

Then, after immersing NaH ₂ PO ₄ in water or an aqueous solution obtained by dissolving it in warm water for 5 seconds, it was squeezed by a roll in a wet state and dried. The test method and evaluation method for the amount of P deposited on the plated surface after the above treatment, the appearance, the chemical conversion film, and the hot water secondary adhesion are the same as in Example 1. The results are shown in Table 2.

<Effects of the Invention> As described in detail above, according to the present invention, the appearance after the chemical conversion treatment is excellent, a dense and fine phosphate crystal is stably obtained, and the adhesion of the coating material of the Zn-based electroplated steel sheet is obtained. Is further improved, the lifespan of various products until rusting increases, and there are great benefits.

Claims (1)

[Claims] 1. A steel sheet is subjected to Zn-based electroplating, and then the plated surface is electrolytically cleaned with a weak acid to a neutral aqueous solution, and thereafter, a compound of a phosphoric acid compound as P is added on the plated surface in an amount of 0.01 to 50 mg / m 2.
^{(2) A} method for producing a Zn-based electroplated steel sheet having excellent chemical conversion treatability, which is characterized by being adhered.