JPS59159997A - Manufacture of steel sheet electroplated with ironzinc on one side - Google Patents

Abstract

PURPOSE:To improve the suitability of a surface of a steel sheet to chemical conversion treatment after removing a thin plated film by electrolysis by supplying DC contg. a prescribed percentage of an AC component (ripple current) to carry out Fe-Zn alloy plating on one side of the steel sheet. CONSTITUTION:A steel sheet is fed to a plating vessel, where thick plating is carried out on a surface to be plated by supplying large electric current, and thin Fe-Zn alloy plating is carried out on a surface not to be palted. Electric current for the thin plating is supplied by superposing an AC component on a DC component so as to regulate the ripple percentage represented by a formula (DELTAI/Iavg)X100 (where DELTAI is electric current of only the AC component, and Iavg is average electric current) to 20-300%. The suitability of the surface of the steel sheet to chemical conversion treatment after removing the thin plated film by electrolysis is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a single-sided electroplated steel sheet in which the chemical conversion treatment property of the non-plated surface is improved.

Single-sided electroplated steel sheets are widely used as materials that meet the demands of the automotive steel sheet field. Since plated steel sheets used in automobiles and the like are generally electrically coated and further coated with an intermediate coat and a top coat, a chemical conversion treatment such as phosphate treatment is usually performed as a base treatment for these coats. When carrying out a chemical conversion treatment, it is necessary to have properties that do not inhibit the generation of nuclei of chemical conversion coating crystals. For iron-zinc alloy single-sided electroplated steel sheets, the plating bath usually contains a high concentration of Fe3+.
3+ causes erosion and dissolution, resulting in severe deterioration of chemical conversion properties. This tendency becomes a big problem as high tensile strength steel plates are used as steel plates. As a countermeasure, we proposed a method in which a thin layer of electroplating is applied to the non-plated surface to protect the steel substrate, and after plating, the thin plating is removed by electrolysis using the non-plated surface as an anode (Japanese Patent Application No. 57-207417).
. When manufacturing single-sided electroplated steel sheets, the steel sheets after washing are passed through a coating bath, and the coated side is plated thickly using a large cathode current, and the non-plated side is plated thinly using a small cathode current. In this treatment tank, the plating layer is removed by a small cathode current on the unplated surface and a large anodic current on the non-plated surface to improve chemical conversion properties.

The present invention relates to a direct current (ripple current) cathode current containing an alternating current component for applying thin plating to a non-plated surface in an iron-zinc alloy plating bath.

The AC component may be three-phase, six-phase, half-wave, or full-wave.Also, even if the waveform changes due to the reactance of the conductor between the power source and the electrolytic tank, as long as the ripple rate is appropriate, This falls within the scope of this patent. Conventionally, there were restrictions on the plating current density of the positive waxing current used to apply protective thin plating to non-plated surfaces in a plating bath. That is, when performing thin plating (approximately 1 to 5iii'/m") to protect the unplated steel sheet base in an iron-zinc alloy electroplating tank, and electrolytically removing the thin plating layer with an anodic current in a treatment tank after plating, When the plating current density is within the range of 10 to 40 and 80 or more (A/dm") as shown later in the conventional direct current method without alternating current, thin plating is insufficiently removed and partial film dissolution remains and unevenness occurs. It was occurring. However, in the case of a production line.

Plating current density often varies depending on the number of plating baths used and the strip line speed. Therefore, there was a need for a method that would allow complete electrolytic removal of the thin plating film in the treatment bath over all plating current density ranges and would not leave any residue on the surface after electrolytic removal. That is, the plating current density is 1
Even within the range of 0 to 40 and 80 or more (A/drn"),
As a method for removing thin plating electrolytically, the plating current in the plating bath is superimposed on the DC component, and the ripple rate is adjusted to 20 to 200 lines. The inventors have discovered that the removal is complete and no dissolution remains and there is no unevenness.

The reason why the value of the ripple rate is limited within the range of 20 to 200% in the present invention is as follows.

(2) If the lower limit is less than 20 coefficients, the plating layer superposition efficiency due to current fluctuations will be low, and formation defects may occur due to uneven dissolution when removing a thin plating film.

■The reason why the upper limit was set at 200% is because the ripple rate is higher than 200%, and there is no particular restriction in terms of thin plating film efficiency, or the plating power consumption is excessive.

The present invention will be explained in detail below.

In the case of conventional thin plating with a direct current cathode current, a plating bath test was conducted to confirm the relationship between the current density and the residual unevenness caused by electrolytic removal of the plating layer after plating. went. That is, protective thin plating was performed in an iron-zinc alloy electroplating bath, and the higher the plating current density, the higher the iron content of the plating film. By changing the plating current density with a smooth DC current (ripple rate of 3 inches or less), the amount of plating deposited is about 2F? After applying a thin plating of /+mF,
Table 1 shows the results of an experiment in which anodic electrolytic removal was carried out by applying a current of 80 A/dm'' for 2 seconds.

Table 1 (for smooth DC thin plating using conventional technology) shows the results of evaluating the chemical appearance by changing the cathode thin plating current density from 5 to 120 A/dm. After removal, chemical conversion treatment is performed, and the results are judged based on chemical adhesion unevenness, crystal unevenness (coarsening), etc. Allowable lower limit], Δ mark: acceptable, × mark: unacceptable. From these results, the chemical appearance after electrolytic removal of the plating layer where the plating current density is in the range of 10 to 40 and 80 or more (A/drn") It was confirmed that this was not good.

Based on the results in Table 1, the following points were considered. The conventional smooth direct current plating method has produced a plating film with a substantially constant iron content that corresponds to the plating current density and the composition of the iron-zinc alloy plating solution. Thin plated skin B with iron content in a certain range
'A (RI, during the anodic electrolytic treatment, the solution caused residual unevenness. Simply increasing the electrolytic removal time (80A/drn" x 2 seconds) did not lead to improvement of the unevenness, and the plating current density was changed periodically. We thought that increasing the plating current would be effective in removing unevenness, and that it would be possible to completely remove the anode by applying a plating current (hereinafter referred to as ripple current) waveform in which an alternating current component is superimposed on a smooth direct current component. This is because by diversifying the composition of the thin plating film in the direction of the plating thickness, it is possible to create a thin plating film with a composite composition having many different galvanic potentials.

The ripple current referred to here is obtained, for example, from the electric circuit diagram shown in FIG. In the figure, AC three-phase 440v is input from the left end H, and the current of each phase of the three-phase AC is rectified by the three-phase rectifier A through the switch S to become a half-wave current. The current becomes a three-phase half-wave or a superimposed six-phase half-wave current through the three-phase single-phase transformer circuit B, and is rectified by the single-phase rectifier C to become a current containing a DC component. After being washed away, it enters the thin plating bath G where it becomes a 60-phase half-wave ripple current, which is a six-phase half-wave alternating current with a direct current added. Current measurement F measures the average current, and voltage measurement E measures the average voltage.

FIG. 2 shows an example of the measurement result of the current waveform of the six-phase half-wave ripple current obtained by the electric circuit shown in FIG. 1.

The box in the figure indicates the period, and the number of six-phase half-wave AC cycles is 60X2X.
It is the reciprocal of 3 - seconds.

60 Figure 3 is an explanatory diagram of the ripple rate of the six-phase half-wave ripple current. AC in the figure indicates an alternating current component and DC indicates a direct current component. The ripple rate is defined.

Ripple rate - 21 - x 100 (9)) How does the ripple rate of the six-phase half-wave ripple current on the cathode affect its chemical formability when applying protective thin plating to an iron-zinc alloy plating bath? We conducted an experiment to confirm this. The results are shown in Table 2.

Chemical formation of protective thin plating with smooth direct current Experiments were conducted at plating average current densities of 10-40 and 80 or more (A/drn" 10.15.20 as plating average current.
40, 80, 100, 120 (A/dm2) K, protective thin plating was performed while changing the ripple rate, and the respective chemical formation evaluations were determined.

Generally, in a plating tank, the plating surface of the single-sided plating method of iron-zinc metal-containing electroplating is as follows: ■ Iron-zinc-based main component with small amounts of C01Cr, fVIn, Ni, Snx Cu,
It may contain one or more of Ti, Pb, In, Mo, and the like.

■The plating tank may be of vertical type, horizontal type, radial type, cell type, etc.

■An iron-free iron-zinc bath is used when plating the upper layer of a plated surface, and its composition is Fe≧60%.

A zinc-rich zinc-iron bath is used for plating the lower layer of a plated surface, and its composition is Zr≧6.

Both of these are used.

In the experiments shown in Table 2, the iron and zinc alloy plating baths were zinc lynch bath - Fe"50VN (sulfate), Zn"30V/l.
@acid), temperature 50'C, Na25o450 Vl.

Iron dip bath/Fe”50VA (sulfate), Zn”3r/
βQrN,? Salt temperature 55-C, Na2so450V
The amount of protective thin plating deposited was 2 when tested for both cases of A.
? The energization time was adjusted so that it was /m. Three samples of each sample were manufactured and investigated under the same conditions.

After the protective thin plating, the thin plating layer was removed by anodic electrolysis in a separate tank, and the chemical formation was then evaluated.

Generally, the following is the procedure for anodic electrolysis in a treatment tank.

■The pH of the anodic electrolytic bath does not matter whether it is acidic, neutral, or alkaline, but the risk of dissolving the plated surface and the accumulation of Fe5Zn ions in the removed thin plating film may cause problems.
Approximately 6 to 10 baths are commonly used.

■Thin plating removal efficiency hardly changes depending on the anodic electrolysis current density. At low current densities, the scale of the anode removal equipment becomes too large, while at high current densities, the unit power consumption during anodic electrolysis worsens.

The anodic electrolysis conditions in the experiments shown in Table 2 were as follows: The plating bath was 1 mol Na.
The pH was adjusted to 8 using a 2SO4 solution, the temperature was set to 50°C, and the electrolytic current density was set to 80 A/dm' and the current flow time was set to 2 seconds. In addition, for the phosphate treatment after anodic electrolysis, the material was first treated with a surface treatment agent and then treated with a chemical conversion treatment agent. As the surface treatment agent, a 20-second spray of Percolene ZT manufactured by Nippon Parkerizing Co., Ltd. was used, and as a chemical conversion treatment agent, a 20-second immersion of Hontelite +3030 manufactured by Nippon Parkerizing Co., Ltd. was used.

According to the experimental results in Table 2, the method for determining chemical conversion evaluation is that when chemical conversion treatment is performed after thin plating is removed by anodic electrolysis, and at that time, chemical conversion deposition unevenness occurs in two or more places, the chemical conversion crystal grains have a large diameter and are coarse grains. It was determined that the chemical conversion treatment properties were poor when the coating adhered sparsely. The chemical formation evaluation was rated ◎○△× in order from good to bad.

According to Table 2, when the plating average current density is 10 A//drn, when the ripple rate is 15, the evaluation is not so good with a Δ mark, but when the ripple rate is 20% and 100%, The chemical evaluation is marked with ○ and ◎, indicating that the chemical evaluation is improved compared to smooth DC.

■When the plating average current density is 15 A/dm', when the ripple rate is 10%, it is bad with an X mark, but when it is 20% and 50%, the chemical formation evaluation is improved with an ○ mark and a ◎ mark.

■When the plating average current density is 20A/drr]'', when the ripple rate is 18%, it is marked △, which is not very good, but at 25%,
When it is 45%, the chemical evaluation is improved with a circle mark.

■Ripple rate is 1 at plating average current density of 401Vdm2
When there are 7 candies, it is marked Δ, which is not so good, but 24% and 65%.
Then, the chemical evaluation is improved by marking ○ or ◎.

■Ripple rate is 1 at plating average current density of 80A/dm.
When it is 0%, it is marked Δ, which is not very good, but 35th section, 80%
Then, the chemical evaluation is improved by marking ○ or ◎.

■At a plating average current density of 100 A/dm2, when the ripple rate is 15, it is marked Δ, which is not very good, but when the ripple rate is 15, it is not very good.
For section 50, the chemical evaluation fd is improved with ○ and ◎ marks.

■Plating average current density 120 A/dm' i-j:
l) At a pull rate of 10%, it is marked with an X, which is not good, but at a pull rate of 30%, it is marked with an O, and the chemical conversion evaluation is improved.

Therefore, even in the direct current plating current density range of 10-40.80 or more (A/d m2), which has poor chemical conversion treatment properties on non-plated surfaces (Table 1), when applying ripple current, the ripple rate is 20.
It has been found that if it is 200%, the chemical conversion treatment property of the non-plated surface after removing the thin plating is significantly improved.

As described above, in the production of single-sided electroplated steel sheets, the present invention can improve the chemical conversion properties of the non-plated surface after electrolytic treatment, and therefore enables high-quality, high-efficiency production of single-sided electroplated steel sheets.

[Brief explanation of the drawing]

Figure 1 is an electric circuit diagram for obtaining half-wave six-phase ripple current, Figure 2
The figure is a measurement diagram of an example of a specific waveform of a half-wave hole current (ripple rate 40%, ripple rate 75ch, ripple rate 200%), and Fig. 3 is an explanatory diagram of the ripple rate. Figure 1 Figure 3 When closed Figure 2

Claims (1)

[Claims] (1) In the production of single-sided electroplated steel sheets, when carrying out thin protective plating on the non-plated surface in a Fe-ZH alloy electroplating tank, an AC component is superimposed on a DC component as a plating current to reduce the ripple rate to 20%. A method for producing an iron-zinc single-sided electroplated steel sheet, the method comprising: adjusting the iron-zinc single-sided electroplated steel sheet to 200%.