JPS61266600A - Production of steel sheet electrogalvanized on one side - Google Patents

Abstract

PURPOSE:To obtain a non-plated surface and plated surface having good surface characteristics by subjecting the surfaces of a steel sheet which is plated on one side to specific times of washing and draining then wetting preliminarily the surface of the steel plate with an electrolyte and subjecting the surface to an anodizing treatment under specific conditions. CONSTITUTION:One side of the steel sheet 1 is electrogalvanized. The steel sheet 1 emitted from a final plating cell 2 is fed via conducting rolls 4 for plating to >=4 stages of washing sprays 8 and squeezing rolls 5, by which the steel sheet 1 surfaces are washed and drained repeatedly >=4 times. The steel sheet is then fed via dam rolls 6 to electrolyte sprays 9 where at least the non-plating surface is preliminarily wetted with the electrolytic pickling soln. Such steel sheet is fed via conducting rolls 7 for electrolytic pickling to an electrolytic pickling tank 3. The sheet is subjected to the anodizing treatment at about >=20A/dm<2> anode current density and about 40-500Coulomb/dm<2> quantity of electricity in said tank. An aq. soln. contg. at least one kind of sulfate and phosphate is used for the electrolytic pickling soln.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing a zinc-based single-sided electroplated steel sheet,
In particular, the present invention relates to a method for producing a zinc-based single-sided electroplated steel sheet using an electrolytic pickling method that produces a non-plated surface and a plated surface with good surface properties.

<Prior art and its problems> In recent years, the use of zinc-based electroplated steel sheets has increased in order to prevent corrosion of automobile bodies caused by antifreeze agents sprayed on roads. In this case, steel plates plated on both sides have problems with weldability and appearance after painting, so steel plates plated on only one side are often used. , it is required to be as good as cold-rolled steel sheets.

However, in the production of zinc-based electroplated steel sheets, acidic baths such as sulfuric acid baths and chloride baths are used as plating baths, so corrosion resistance on the non-plated surfaces cannot be avoided. The accumulation of impurities and the precipitation of impurities often cause stains, streak-like unevenness, and deterioration of chemical conversion properties. Furthermore, plating precipitation on non-plated surfaces due to the plating current passing around is also often a cause of deterioration in chemical conversion treatment properties.

Deterioration of chemical conversion treatment property refers to the fact that uniform phosphorus/salt crystals are not formed, and some areas with no crystals called skelet are formed, or uneven coarse crystals are formed. This is an important problem in terms of quality because it causes deterioration in corrosion resistance and adhesion, and non-uniform appearance. Therefore,
As a countermeasure to this problem, so-called electrolytic pickling is performed, in which after plating, the non-plated material is electrolyzed in an acid or salt aqueous solution using it as an anode.

Taking a Na2SO4 aqueous solution as an example, electrolytic pickling has an excellent effect on cleaning the steel plate surface due to the combination of the chemical effect of H2SO4 generated on the steel plate surface and the physical effect of the 02 gas generated at the same time. JP-A-58-61294, JP-A No. 58-133395
No. 58-181889, etc. disclose this method.

However, in electrolytic pickling of the non-plated side of a single-sided plated steel sheet, over-etching, in which etching of the iron surface progresses abnormally, often occurs. This phenomenon is caused by Zn-
This phenomenon is particularly likely to occur when plating alloys such as Ni, and is accelerated when the material has a high acid dissolution rate (for example, phosphorous-added steel), or when the amount of deposited elements such as Ni, which are more harmful than Fe, exceeds a certain level. This is thought to be due to the fact that the surface is not passivated due to a small amount of free acid in the plating solution adsorbed on the steel sheet surface, making it an active surface, and the activation of local battery action between Ni, etc. and Fe.

When over-etched, the non-plated surface becomes non-uniform and the elemental concentration layer on the steel surface is dissolved, resulting in a significant deterioration in chemical conversion treatment properties. Furthermore, a problem arises in that a large amount of sludge increases in the electrolytic bath.

We previously proposed in Japanese Patent Application No. 59-166993 that as a means to prevent over-etching, the surface of the steel plate should be dried once after washing with water after plating, and then subjected to electrolytic pickling. However, this method requires a dryer to be placed in front of the electrolytic cell for drying, which has the problems of requiring a considerable amount of space and increasing operating costs.

Another problem with conventional electrolytic pickling is that when electrolyzed in a pickling bath containing phosphate, the plated surface becomes white.
There were many cases in which stains were unevenly grayish-black and the chemical conversion treatment properties of these areas were significantly deteriorated. This is thought to occur because *phosphoric acid in the bath reacts with the galvanized surface to form a film of insoluble phosphate.

<Object of the Invention> In view of the above-mentioned current situation, the object of the present invention is to provide an electrolytic method that allows both the plated surface and the non-plated surface to be easily treated by chemical conversion without causing over-etching of the non-plated surface or staining of the plated surface. The present invention provides a method for manufacturing a zinc-based single-sided electroplated steel sheet using a pickling method.

<Configuration of the Invention> According to the present invention, after zinc-based electroplating is applied to one side of a steel sheet, the non-plated side is electrolyzed using an aqueous solution containing one or more selected from sulfates and phosphates. When pickling, wash and drain the surface of the steel plate after plating.
The above object can be achieved by a method for producing a zinc-based single-sided electroplated steel sheet, which is characterized in that the steel sheet is wetted in advance with an electrolytic solution and then anodized at an anode current density of 20 A/d+a2 or more.

The electrolytic pickling method in the method of manufacturing a zinc-based single-sided electroplated steel sheet of the present invention will be described in detail below.

Table 1 shows the unplated side of a 0.8 mm thick 5PCD steel plate with 30 g/m2 Zn-Ni alloy plating on one side (
Old material loading amount 80 mg/s2) was converted to Na2 S04 200
g/view, p) + 8.5.4 OA/dm in 55°C bath
The figure shows the water washing conditions after plating and before electrolytic pickling and the state of the non-plated surface after electrolytic pickling when electrolytic pickling was performed for 2 x 2.5 seconds. The water washing time after plating was kept constant at 10 seconds throughout, and the number of water washing and roll squeezing was changed. For washing, use a jet of tap water, and the roll aperture is 80.
It was carried out using a flat roll of m/mφ. According to the results shown in Table 1, what prevents the occurrence of over-etching is not simply the time of washing with water, but repeating washing with water and rolling, especially turning the edges four or more times, to prevent the occurrence of over-etching. Recognize.

Even if the overall water washing time is constant, the number of repetitions of water washing and roll squeezing increases. This is considered to be because 1118 is physically removed. If only water washing is used or if the water washing roll is squeezed three times or less, even if water washing is carried out for the same length of time, the removal of trace amounts of free acid on the surface will be incomplete, and active parts that have not been passivated will remain, causing electrolytic current to flow there. It is thought that this concentrates and causes over-etching.

The number of repetitions of the water washing and roll drawing process varies depending on the thickness and type of the zinc-based electroplated steel sheet to be treated, and must be determined experimentally depending on each case, but it should be repeated at least 4 times. It is necessary. However, repeating the process too many times complicates the manufacturing process of the zinc-based single-sided electroplated steel sheet and leads to increased costs, so it is a strong argument that it is important to limit the process to as few times as possible.

The time for one washing and roll squeezing should be enough time to sufficiently wash the non-plated and plated surfaces of the steel plate with tap water shower, and enough time to squeeze out the water on the surface of the steel plate by washing with water. - As an example, one process of washing with water and squeezing with a roll may be carried out for about 1°5 seconds to 3 seconds.

Note that the over-etched part (Fig. 1b) was observed under a microscope (x
As shown in the photograph of the metal surface structure taken by A. TOOO (Fig. The situation is clearly different from that of the previous section.

Based on the above findings, the present invention is characterized in that after plating, water washing and roll squeezing of the steel plate surface are repeated four or more times, and then the steel plate is pre-wetted with an electrolytic bath and then anodized. The reason for this is particularly to prevent staining of the plating surface that occurs when electrolyzing in a bath containing phosphate. The reason why plating surfaces tend to stain when electrolyzed without first wetting them in an electrolytic bath is thought to be as follows: The metal (M+) phosphate is in the following equilibrium state in an aqueous solution. .

2NH4PO4: M2HPO4+ H3PO4 (1)
3M 2HPO4:2M3PO4+ H3PO4(2
) If the concentration of the bath is low, the dissociation of phosphate will proceed and the amount of free phosphoric acid will increase. If a water film exists on the plating surface, the bath becomes diluted on the plating surface, and the generated H
3 It is thought that PO4 and Zn react to form insoluble phosphates, resulting in staining.

FIG. 2 shows an example of a schematic configuration when the present invention is applied to an actual plating line.

As shown in FIG. 2, following the plating line of the final plating tank 2 of the multi-stage plating line, a water washing spray 8 and a squeezing roll 5 are provided in multiple stages via a conductor roll 4 for plating. After that, an electrolytic solution spray 9 for spouting an electrolytic pickling solution is provided through a dumb roll 6, and a conductor roll 7 for electrolytic pickling is installed.
The electrolytic pickling tank 3 is disposed on the continuous plating line via the electrolytic pickling tank 3.

The plating tank 2 is provided with a plating solution receiver 10, the washing spray 8 and the squeezing roll 5 are provided with washing water receivers 11, and the electrolytic pickling tank 3 is provided with an electrolytic pickling solution receiver 12, respectively.

In the method of the present invention, it is important not only to perform water-washing roll squeezing but also to repeatedly perform water-washing roll squeezing.
By arranging sprays 8, 8, etc. and squeezing rolls 5, 5, etc. in multiple stages, it is possible to effectively remove residual free acid on the surface after plating and obtain a normal electrolytic surface even in a narrow space. ．． Further, the electrolyte spray 9 may be provided after the conductor roll 7 for electrolytic pickling, but it improves the conductivity between the conductor roll 7 for electrolytic pickling and the steel plate 1 and prevents the occurrence of arc spots. From above.

Preferably, it is located before the electrolytic pickling conductor roll 7.

After being plated in the final plating tank 2, the steel plate 1 passes through a water washing spray 8 and a squeezing roll 5, and undergoes a washing process and a draining process using a roll squeezing process four or more times alternately. Further, after the washing and draining steps, the steel plate 1 is preliminarily wetted with an electrolytic pickling solution at least on the non-plated surface by an electrolytic solution spray 9.

Further, in the present invention, the electrolytic treatment must be performed at a current density of 20 A/dm2 or higher. This is because if it is less than this, the cleaning effect on the non-plated surface will be lowered, and uneven treatment will often occur after electrolysis. The reason for this is thought to be that the oxide film is not completely destroyed or that the stirring force at the interface is reduced due to the generation of 02 gas.

In addition, the amount of electricity for electrolysis depends on the amount of dirt on the non-plated surface and the amount of Zn, etc. attached, but it is approximately 40 to 500 Cron/dm2.
A good uniform surface can be obtained.

The electrolytic pickling solution is an aqueous solution containing at least one of sulfate and phosphate. In addition, the metal ions of these salts are preferably Na, K, Li, Mg, An, etc. The reason for this is that the precipitation potential of the above metal ions is higher than the hydrogen reduction potential, so metal precipitation does not occur in aqueous solution electrolysis. Therefore, it is easy to maintain the metal ion concentration in the bath and maintain the cathode, and the acid generated on the anode and the alkali generated on the cathode are balanced in equal proportions, making it easy to maintain the bath pH. This is because it is very suitable. Note that it is also possible to use NH4 salt.

Furthermore, the reason why sulfate and phosphate were selected is that they do not generate toxic gas when subjected to aqueous electrolysis and have a strong cleaning power on the Fe surface. Chlorides, fluorides, nitrates, etc. may generate toxic gases such as halogen gas and NOx, and carbonates, borates, organic acid salts, etc. have poor acid dissolving power.
Undesirable.

However, it is permissible to add these to the bath for purposes such as conduction aids or pH adjustment.

Zinc-based electroplating in the present invention refers to Zn and Zn-
Previously, alloy plating such as Zn-Fe, and furthermore, Cr
It includes those to which other elements such as Go and Go are added, and is plated with an acidic solution in a sulfuric acid bath, a chloride bath, or a mixed bath thereof. The electrolytic pickling tank may be a horizontal type, a vertical type, or a radial cell, and the liquid temperature can be arbitrarily selected from room temperature to 70°C. The bath temperature is preferably approximately 50° C. or higher, although it cannot be guaranteed because the cleaning efficiency of the non-plated surface varies depending on the type of chemical used and the electrical type conductivity varies.

Practical example The present invention will be specifically described below with reference to Examples.

A phosphorous-added steel plate (P content: 0.1%) having a thickness of 0.71 was coated with a Zn-Ni alloy on one side under the following conditions using a horizontal plating bath.

(1) Bath composition NiSO48H20250ginZn
SO47H20130gin Na2 S04 40 gin (2) Bath PH
1,13 (3) Bath temperature: 60°C (4) Current density: 50 A7 dl + 2 (5) Plating time: 15 sec (6) Plating weight: 20 g/m2 Next, water washing and flat roll squeezing are performed the number of times shown in Table 2, and further electrolyte solution is applied. After wetting the steel plate, the non-plated surface was electrolytically pickled under the conditions shown in Table 2, followed by washing with water and drying.

Thereafter, the presence or absence of stains and unevenness on the non-plated surface was visually determined, and as indicators of surface cleanliness, the whiteness was measured using a color difference meter and the amount of Ni deposited was analyzed using chemical analysis. Also, visually compare the presence or absence of dirt and unevenness on the plating surface.
I judged it.

After this, a phosphate treatment solution (Bonderai) #3030 manufactured by Nippon Barker Rising was used.

Chemical conversion treatment was performed using the deep method. The chemical conversion coating was evaluated visually and by scanning electron microscopy in terms of uniformity, crystal density, presence of scratches, etc., and the amount of chemical conversion coating was also measured for the non-plated surface. These results are shown in Table 2.

The non-plated surface shown in Comparative Example 5, which was not electrolytically treated after plating, had low whiteness due to dirt, a large amount of 1 deposited, and poor chemical conversion treatment properties.

However, the non-plated surface of the electrolytically treated surface shown in the example of the present invention has no over-etching, is uniform, has increased whiteness, and has improved surface cleanliness as shown by a significant reduction in the amount of Ni deposited. It also has excellent chemical conversion treatment properties. Also,
The plated surface is clean and has good chemical conversion treatment properties. On the other hand, in Comparative Examples 1 to 3, in which the plate was washed with water after plating and the number of roll drawings was 3 times, over-etching occurred unevenly even though the plate was not plated, and the chemical conversion treatment properties were also poor. Moreover, the amount of old material deposited was also larger than that of the examples of the present invention. The reason why there is a large amount of residual Ni acid despite over-etching is thought to be that the electrolytic current is locally concentrated in active areas. In addition, Comparative Example 1.2 without pre-wetting with electrolyte,
Gray-white precipitate-like stains adhered to the plated surface, and chemical conversion treatment properties also deteriorated.

In Comparative Example 4, which had a low current density, the cleanliness of the non-plated surface was insufficient and the chemical conversion treatment properties were also poor.

<Efficacy of Invention Song Dynasty> Wood Invention 7J Law 1. According to J'', one side of the steel plate, which has been coated with zinc-based electroplating, is coated with sulfate, and 5, . seeds or t
Using an aqueous solution containing Yo2 Insert-1, use electrolytic acid pickling. ]−
Roughly wet at least the plated surface of #1 Jji.1.
f), anode electrode 2 OA/dn+2 -1
And Yang 1 Gokugen, ri゛・jyu). Non-uniform), an uneven over-purple color appears on the electrographic image. Somehow, it's effective.
, removes dirt from the surface, and cleans it.

Prevents plating surface stains 1. l::: 1=-1) 1, 11=Producing a zinc-based single-sided electroplated steel sheet that has excellent chemical conversion treatment properties on both the plating surfaces22 is 1/9

[Brief explanation of the drawing]

Figures 1a and 1b are photographs of the metal surface structure taken with a metallurgical microscope (x1000), Figure 2 U Book/Invention 11,
- A schematic diagram showing an example of manufacturing equipment for 1m electroplated steel sheets based on zinc-based pieces. Explanation of the simple explanation 1...Sweat care JIji λ1・Rin
Pu, 2... the most; hard winter -,,y
t4fi, r<...military. Deacidification tank, 4...metal, kikawa-1゛,...ge' ivy 11 le, 5...diaphragm D, le, 6...♂'1. x I: Tor, 7... = 'rTi +l? i
1/N washing: I-tanctor, meth, water washing: (Sore, 9...electrolyte x, near'l, ,
10...me), acid wash bag (1), i+, =...washing water receiver G1.12...city; deacidifying wash bag: (-1・pa
j′

Claims (1)

[Claims] (1) After applying zinc-based electroplating to one side of a steel sheet, when electrolytically pickling the non-plated side using an aqueous solution containing one or more selected from sulfates and phosphates, Zinc-based single-sided electroplating characterized by repeating water washing and draining of the steel plate surface four or more times after plating, then pre-wetting the steel plate with an electrolytic solution, and then anodizing at an anode current density of 20 A/dm^2 or more. Method of manufacturing steel plates.