JPS6043499A - Production of steel sheet electroplated with zinc on one surface - Google Patents

Abstract

PURPOSE:To produce a steel sheet which is electroplated with Zn on one surface and has excellent chemical convertability by maintaining preliminarily the non-plating surface at a negative potential in a degreasing liquid contg. silicate and degreasing electrolytically the steel sheet in the stage of forming the plating layer of a Zn metal on one side of the steel sheet. CONSTITUTION:A steel sheet such as a steel sheet for automobiles is passed through the inside of a vessel 2 in which a degreasing liquid contg. 3% sodium orthosilicate is filled to degrease electrolytically said sheet in the stage of electroplating Zn or Zn-Ni alloy, etc. only on one surface of said sheet. Electrode plates are placed in the electrolytic degreasing liquid in this case and the non-plating surface 3 of the steel sheet 1 is placed to face the positive electrode. The non-plating surface of the plate 1 is maintained at a negative polarity and is electrolytically degreased. Or if the polarity of the steel sheet changes according to the movement of the sheet 1, the surface 3 is electrolytically degreased so as to have the negative polarity in the final. Zn or Zn-Ni alloy is thereafter plated on the surface 4 by which the steel plate which has the non-plating surface having excellent phosphatability and paintability and is galvanized on one surface is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a zinc-based single-sided electroplated steel sheet.

In recent years, galvanized steel sheets, i.e.
The use of galvanized steel sheets or steel sheets coated with alloys such as zinc-nickel and zinc-iron is increasing. The most severely corroded part of an automobile body is the box structure on the inside of the automobile. These parts are said to be susceptible to corrosion because they are prone to incomplete chemical conversion treatment or painting, and their structure allows salt, water, and debris to accumulate. In order to strengthen the rust prevention ability of these parts, zinc-plated copper sheets have come to be used frequently in place of cold-rolled steel sheets. In this case, since double-sided plated steel plates have problems with appearance and weldability after painting, steel plates plated on only one side are often used.

It is desirable that the non-plated surface of a single-sided plated steel sheet has the same performance as a cold-rolled steel sheet, and is required to have excellent chemical conversion treatment properties and paintability. On the other hand, zinc-based electroplating on steel plate strips can be performed at high current density and the bath composition can be controlled relatively easily, so that plating is usually performed using an acidic bath such as a sulfuric acid bath or a chloride bath. For this reason, when manufacturing a single-sided plated steel sheet, the non-plated surface is corrosion resistant, but acid-insoluble matter accumulates or impurities precipitate, resulting in staining and deterioration of chemical conversion treatment properties. For example, it is known that steel sheets containing a large amount of phosphorus, such as phosphorous-added steel, have a high dissolution rate in acids.
Single-sided electroplated steel sheets made from this material have a problem in that chemical conversion treatment defects may occur due to contamination on the non-plated surface.

Defective chemical conversion treatment refers to the formation of localized areas where phosphate skin is not formed, generally referred to as sagging, or the formation of uneven coarse crystals, resulting in deterioration of corrosion resistance and adhesion after painting. Or because it causes unevenness in appearance,
This is an important problem in terms of quality.

In order to prevent staining of the non-plated surface and to prevent deterioration of chemical conversion treatment properties, it is necessary to reduce the dissolution of Fe. Measures to achieve this include lowering the pH of the plating bath, lowering the bath temperature, or increasing the line speed, but all of these methods have limitations due to plating conditions or equipment. . The present invention has been developed in view of the above-mentioned current situation.
This was achieved as a result of extensive research aimed at obtaining a zinc-based single-sided electroplated steel sheet with excellent chemical conversion treatment properties on the cold-rolled surface.

According to the present invention, when producing a zinc-based single-sided electroplated steel sheet in an acid bath, the non-plated surface of the steel sheet is electrolytically degreased in a degreasing bath containing silicate with negative polarity before plating, or
Alternatively, if the polarity on the steel plate side of the electrolytic degreasing changes sequentially as the steel plate moves, the above objective can be achieved by making the final polarity of the non-plated surface of the steel plate negative.

The specific effects, appropriate range, etc. of the method according to the present invention will be explained in detail below.

Figure 1 shows a steel plate that has been electrolytically degreased at 10 A/dm2 x I 0 sec in a 70°C degreasing bath containing 3% sodium orthosilicate, and then treated with zinc sulfate as the main component at a pH of 1.8 and a bath temperature of 55°C. This figure shows the relationship between the electrolytic degreasing polarity, the amount dissolved by the plating solution, and the surface appearance when immersed in a zinc plating solution for 1OOsee.

From FIG. 1, it can be seen that when the electrolytic degreasing polarity is set to (-), the amount of steel plate melting is significantly suppressed. Also, when using an alternating polarity method in which the polarity changes sequentially, the final polarity can be set to (-).
It can be seen that the amount dissolved is small. The stain on the surface of the copper plate has a strong correlation with the amount of acid dissolution, and those with a large amount of acid dissolution had blackish stains on the entire surface and streaky stains in the rolling direction.

Various elements, including C, are concentrated near the surface of the steel plate, and if the dissolved amount is large, it is thought that these become stains as insoluble residues called smut. In fact, when the surface was investigated using EPMA, it was found that there was a lot of C, which is known to be an element that inhibits chemical conversion treatment, especially in the streaky parts of the dirty board.

Furthermore, it is known that when a steel plate is subjected to cathodic electrolysis treatment in an aqueous solution containing a silicate, 5102 or a compound in which -H20 is bonded to it is electrodeposited on the surface. The reason why the non-plated surface of the single-sided electroplated steel sheet has good chemical conversion treatment properties according to the present invention is that, as mentioned above, the presence of a Si compound on the surface protects the Fe surface.
This is thought to be because the generation of dirt is suppressed.

In electrolytic degreasing, the polarity of the steel plate can be set to (=) by direct energization using a conductor roll. Furthermore, in the case of an indirect energization method that does not use a conductor roll, the polarity of the non-plated surface can be set to (-) by changing the polarity of the electrodes on both sides of the strip as shown in FIG. 2a. In addition, in the indirect energization method, depending on the electrode arrangement δ, the polarity of the steel plate may change sequentially as the slip I/slip changes as shown in Fig. 2b.
Since the present invention requires that a Si compound be applied to the surface of the copper plate before entering the plating bath, the electrodes must be set so that the final polarity of at least the non-plated surface is (-). .

Although a non-plated surface with excellent chemical conversion treatability can be obtained by the method described below, if a brushing treatment is performed after plating, the chemical conversion treatability can be further improved. The reason for this is that impurities such as C on the non-plated surface that inhibit chemical conversion treatment are removed by brushing, and the surface is activated.

The present invention will be explained in more detail below.

The steel used in the present invention is not limited in its type or size, and is subjected to pretreatment such as degreasing, pickling, and water washing, followed by single-sided zinc-based electroplating in an acid bath.

The acidic plating bath is a sulfate bath, a chloride bath, or a mixed bath thereof, and is usually used at a pH of 1 to 4 and a bath temperature of 40 to 70°C.

111i lead-based plating includes zinc plating, zinc-nickel ζ zinc-iron, or Cr, MOLGo, etc.
This also includes alloy plating containing other metals such as zinc, and in the present invention, the type and content of metals other than zinc in the plating layer are not limited. Furthermore, the current density,
Plating conditions such as liquid flow rate are also appropriately selected depending on the type of plating and bath composition, and are not prescribed.

Electrolytic degreasing performed prior to plating is normally performed to obtain a clean surface, but the present invention also aims to actively adhere Si oxides to the surface of the steel sheet, so it is not necessary to use a bath. It is essential to contain silicate. Silicates include sodium silicates, i.e., sodium orthosilicate (Na4 SiO4), sodium metasilicate (
Na25i03) or a liquid mixture of various sodium silicates, so-called water glass, etc. are suitable. It is also possible to use potassium or lithium silicates.

The composition of the electrolytic degreasing bath is one that contains the above-mentioned silicic acid compound, and other components such as NaOH1Na20.
03, Na4P207, Na3PO4,
Regardless of the presence of surfactants, chelating agents, etc. and their concentrations, if the concentration of silicate is about 0.5 to 5%,
It is possible to achieve the desired purpose in both degreasing and prevention of non-plated surface staining due to adhesion of Si compounds. The conditions for electrolytic degreasing are not particularly limited other than the polarity mentioned above, but for example, bath temperature 40-80°C, current density 1-5
9.2 under conditions such as OA/dm2 and electrolysis time of 2 to 20 seconds.
~5 mg/m' of Si is deposited, achieving the desired goal. The higher the bath temperature, the greater the amount of Si compound deposited.

Regarding the current density, since the attachment of the 81 compound becomes more reliable at a current density of 2 A/dm2 or more, it is desirable to operate at a current density of 2 A/dm2 or higher. In addition, 2A/dm2
At the above current density, the amount of the s1 compound attached is less influenced by the current density and tends to depend on the electrolysis time. Therefore, in the polar arrangement as shown in Figure 2b, (-
It is more effective to lengthen the cathodic electrolysis time of the steel plate by changing the lengths of the ) and (+) electrodes. Also, the first
Although there are cases where the polarity changes sequentially as shown in D in the figure, the Si compound once attached is not completely removed by anodic electrolysis and a certain amount remains. Therefore, even in such a case, it is effective to adjust the length of the electrodes, including not only the final electrode but also the intermediate electrode, so that the cathode electrolysis takes a longer time.

The degree of effectiveness in preventing stains on non-plated surfaces caused by Si compounds deposited by electrolysis varies depending on the corrosion resistance of the base steel, the pH 1 temperature of the plating bath, etc. Therefore, materials that are easily etched, such as the phosphorus-added steel mentioned above, are
(i) It is necessary to control the electrolytic degreasing time or bath temperature depending on the steel type and plating solution conditions, such as increasing the amount of arsenic deposits.

As mentioned above, by brushing after plating, the chemical conversion treatment property of the non-plated surface can be further improved, and it is practical to use a brush roll for this method.

Although some brushes do not contain abrasive grains in their fibers, in the present invention, from the standpoint of removing dirt and activating the surface, it is desirable to use a brush that has abrasive power and contains abrasive grains. The abrasive grains are composed of alumina, silicon carbide, etc., and the grinding power varies depending on the particle size and the diameter of the fiber containing the abrasive grains, but these are not intentionally specified.

Hereinafter, the present invention will be explained by giving preferred examples and comparative examples.

Table 1 shows phosphorus-added 35K, which causes stains on non-plated surfaces.
G class high tensile strength steel plate (thickness 0.7 Ilm, CO,053
%, PO, 075%, MO638%, AI 0.04
%) is used as a material and subjected to single-sided plating of zinc or Zn-Ni alloy, the occurrence of staining on the non-plated surface and chemical conversion treatment properties are shown. Electrolytic degreasing, plating, etc. were performed under the following conditions.

(1) Treatment process Plating → (Brushing) - Chemical conversion treatment (2) Immersion degreasing Alkaline degreasing liquid (3% metazole) 70℃, 1 osec (3) Electrolytic degreasing■ Degreasing bath 3.5% sodium orthosilicate, 70℃■ Electrification Method 1) Direct energization: Connect a power source directly to the steel plate 2) Indirect energization: Set electrodes connected to a power source on both sides of the steel plate ■ Electrolysis conditions 8 A/dm2 X l 2 sec, polarity is shown in Table 1 ( If the polarity changes, set the total electrolysis time to 12 seconds).

(4) Pickling 5% HCI, room temperature, 1Osec [5) Plating ■ Zn plating l) Bath composition: ZnSO4400gin, NaSO43
0g/text 2) Plating conditions: pH 2.0, bath temperature 60°C, current density BOA/dm2. Fabric weight 140g/if (one side) ■ Zn-Ni alloy plating 1) Bath composition: NiSO4-16H20300g/text, Z
nSO4・7H20140g/41, Na250440
g/view 2) Plating conditions: pH 1.8, bath temperature 55°C, current density 50A/dm2. Fabric weight: 30 g/nf (single side) (6) Brushing thread thickness: 0.3 m/m, polishing with a brush containing alumina abrasive grains of particle size #1000 (7) Chemical treatment EI Honpariki Rising Co., Ltd. Bt# 3312
(Zinc phosphate treatment solution) Bath temperature: 60°C, spray treatment From Table 1, which summarizes these results, the non-coated surface of the single-sided plated steel sheet according to the present invention hardly generates stains due to corrosion resistance, and has good chemical conversion treatment properties. It is clear that it is superior.

[Brief explanation of drawings]

Figure 1 shows the polarity of electrolytic degreasing of a steel plate and the dissolution of a copper plate by a plating solution? Graph showing the relationship between il- and surface appearance, 2nd
Figures a and 2b are diagrams showing examples of electrode arrangement when a steel plate strip is electrolytically degreased. Description of row-J- 1...String, 2...Electrolytic degreasing tank, 3...Non-plated surface, 4...Plated surface

Claims (2)

[Claims] (1) When producing a zinc-based single-sided electroplated steel sheet in an acid bath, a single-sided method characterized by electrolytically degreasing the non-plated side of the steel sheet in a degreasing bath containing silicate with at least the final polarity negative before plating. Method of manufacturing plated steel sheets. (2) When manufacturing zinc-based single-sided electroplated steel sheets in an acid bath, electrolytically degrease the non-plated side of the steel sheet in a degreasing bath containing silicate with at least the final polarity negative before plating. A method for producing a single-sided plated steel sheet, which comprises brushing the plated surface.