JPH032391A - Production of zinc plated steel sheet excellent in chemical convertibility and performance after coating - Google Patents

Abstract

PURPOSE:To obtain the plated steel sheet excellent in chemical convertibility and performance after coating by applying light anodic peeling and grinding to the surface of a Zn-plated steel sheet. CONSTITUTION:The surface of a Zn-plated steel sheet is anodically peeled in an electrolyte at pH4-7 at the quantity of electricity of 1-10C/dm<2>, and then the surface of the plating is ground with a nylon-brush roll, etc. The Zn-plated steel sheet is fully dipped, as required, in a chromate soln. before being anodically peeled. In addition, an electrolyte contg. 0.05-2mol/l total phosphoric acid and kept at pH4-7 is preferably used for the peeling.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing zinc-based coated steel sheets, and in particular, zinc-based steel sheets with excellent chemical conversion treatment properties and post-painting performance suitable for use as external rust-preventing steel sheets for automobile bodies. The present invention relates to a post-treatment method for plated steel sheets.

(Conventional description) Zinc-based coated steel sheets can achieve high corrosion resistance without impairing the workability of cold-rolled steel sheets, and can be mass-produced, so they are used in a wide range of fields where cold-rolled steel sheets have traditionally been used, such as automobiles, home appliances, and building materials. Its uses are being expanded.

In recent years, it has been used as a rust prevention measure for automobiles in response to sprayed salt to prevent roads from freezing during the winter in cold regions.

Electric alloy plated steel plates such as Zn-Ni and Zn-Fe and alloyed hot-dip plated steel plates with excellent workability and weldability have been developed, and their application as anti-IGZ plates for automobile bodies is expanding. There are many properties required for rust-preventing steel sheets for automobile bodies, but chemical conversion treatment (phosphate treatment) and post-painting performance are particularly important factors.

However, in zinc-based coated steel sheets, including pure zinc, the phosphate crystals generated during phosphate conversion treatment are hopite (
Zn3(p○-t) 24H20), and this hovite is a phosphoophyllite (Zn
, Fe(PO, ), 4H, O), the alkali resistance is inferior to that of Fe(PO, ), 4H, O), so the post-painting performance of zinc-based plated steel sheets, such as water-resistant adhesion and post-painting corrosion resistance, is unstable. In addition, the hopite crystals tend to be non-uniform and large, which also has a negative effect on the performance after painting.

(Problem to be solved by the invention) Conventionally, in order to solve this problem, for example, Japanese Patent Laid-Open No. 58-1
1795 Publication < Fe on Zn-Ni alloy plating
- Zn alloy plating or JP-A-59-21159
No. 2 discloses a method using 2WJ plating in which Fe--P alloy plating is applied on zinc-based plating. These technologies aim to generate phosphofluorite as a chemical conversion coating similar to cold-rolled steel sheets by applying an Fe-rich upper layer plating on top of the zinc-based plating, thereby improving performance after painting. be. but.

In these methods, the upper Fe-rich plating layer has a higher potential than the lower zinc-based plating layer, so local battery corrosion tends to occur between the upper and lower plating layers, or equipment dedicated to the upper layer plating is required. and the composition of the upper layer plating,
There are problems such as difficulty in controlling the amount of adhesion.

Furthermore, Japanese Patent Application Laid-Open No. 63-45383 discloses a method for improving chemical conversion properties by subjecting M lead-based electroplating to an anodic electrolytic treatment. However, with this method alone, the chemical conversion treatment properties are insufficient, and since the amount of electricity is as large as IOC/dm or more, there are problems with the appearance after the anodic electrolytic treatment, such as darkening and black streaks.

The present invention aims to solve the problems of the prior art described above and to provide a method for producing a zinc-based plated steel sheet with excellent chemical conversion treatment properties and post-painting performance.

(Means for Solving the Problems) The present inventors first studied the relationship between the state of the outermost surface of zinc-based plating involved in chemical conversion reactions and chemical conversion processability, and found that the heterogeneity and size of chemical crystals are oxide film on the surface,
It was found that it depends on the thickness of II (mainly ZnO).

Therefore, we presumed that the chemical crystals would become uniform and fine if the a-based coating layer was thinned to a certain extent, and after intensive study on this method, we found that we performed mild electrolytic stripping in a neutral to weakly acidic electrolyte solution, and then It has been found that the best industrial method is to carry out the grinding process. Through these processes, uniform and fine crystals are obtained, and the microscopic irregularities of the chemical crystal and the quasi-microscopic irregularities of the plated surface due to grinding work together,
It has been found that the performance after painting is also significantly improved. In addition, by immersing a zinc-based plated steel sheet in a chromate solution to form a small amount of chromate film, and then performing the electrolytic stripping and grinding treatments described above, it is possible to further improve post-painting performance, especially post-painting corrosion resistance. Understood. Normally, even if a fine chemical conversion film is formed on zinc-based plating, there are microscopic gaps between the chemical crystals, and the plating is exposed in these areas. However, if a trace amount of chromate film is formed in advance and then subjected to electrolytic peeling and grinding treatment, a trace amount of the chromate film can remain unevenly. Since the coating remains covered with the chromate film, there is no exposure of the plating even on a microscopic level, so it is thought that the corrosion resistance after painting will be significantly improved. The present invention is based on these findings, and its gist is as follows.

(1) The plated surface of a zinc-based plated steel sheet is subjected to electrolytic stripping in an electrolytic solution with a pH of 4 to 7 at an amount of electricity of 1 to IOc/drrl', and then the surface of the plated surface is ground. A method for manufacturing zinc-based coated steel sheets with excellent chemical conversion treatment properties and performance after coating.

(2) Immerse a zinc-based plated steel sheet in a chromate solution, and then coat the plated surface with an amount of electricity of 1 to I in an electrolytic solution with a pH of 4 to 7.
A method for producing a zinc-based coated steel sheet with excellent chemical conversion treatment properties and post-painting performance, which comprises performing anodic electrolytic stripping of OC/drn' and subsequently grinding the surface of the plated surface.

(3) When performing electrolytic stripping, the total phosphorus m concentration is 0.05~
2-1: Contains /L7/1, pH is 4-7 (7)
A method for producing a zinc-based coated steel sheet having excellent chemical conversion treatment properties and post-painting performance as described in (1) and (2), characterized in that an electrolytic solution is used.

It is.

(Function) The zinc-based plated steel sheet targeted by the present invention refers to Zn plating or alloy plating or composite plating containing Zn as a main component. The plating method is not particularly limited, but since the material is a steel plate, electroplating and hot-dip plating, which are suitable for high-speed mass production, are advantageous. Specifically, electrogalvanizing, Zn-Ni
, Zn-Fe, Zn-Cr, Zn-N1-C,o, Zn
-Ni-Fe, Zn-Ni-Cr, Zn-Ni-Fe-
Alloy electroplating such as Cr, SiO□, Ti
O□, Al1,0. Oxide fine particles such as BaCr
0. Composite electroplating containing poorly soluble chromic acid compounds and polymer compounds such as, or hot-dip galvanizing 2
Alloyed hot-dip galvanizing, hot-dip Zn-A (l-based alloy plating, etc.).

The amount of plating deposited and the plating composition are not particularly limited. By applying the zinc-based plating to a steel sheet and then subjecting it to the treatment of the present invention, a zinc-based plated steel sheet with excellent chemical conversion treatment properties and post-painting performance can be obtained. In this case, it is efficient to perform the treatment of the present invention continuously after applying zinc-based plating, but if this is not possible due to the equipment, it is effective to perform the treatment of the present invention continuously after zinc-based plating. There is no problem even if it is omitted, and the effect of the present invention will not change even if it is done in this way.

First, the invention of the first Yodo uses the plating surface as an anode to generate an electric fi+ of 1 to 10 C/drr in an electrolytic solution with a pi of 4 to 7.
The method is characterized in that f anodic electrolytic stripping is performed and then the surface of the plated surface is ground. In the present invention, chemical conversion treatment properties can be improved by electrolytic stripping treatment using the plated surface as an anode. Electrolytic reduction treatment using the plated surface as a cathode does not sufficiently remove the zinc oxide film that inhibits chemical conversion properties, and if gold dust ions are present as impurities, they precipitate, making it even more difficult to remove the zinc oxide film. Regarding the pH of the electrolytic solution, which cannot improve chemical conversion treatment properties,
If the pH is less than 4, the plating layer is likely to be chemically dissolved, resulting in cracking of the plating layer or blackening of the plating surface, resulting in poor performance after painting. The electrolyte also becomes unstable.As for the amount of electricity, I
If it is less than C/dm, the effect of improving chemical conversion treatment properties will be insufficient, and if it exceeds IOC/drri, there is a concern that black streaks or black spots will occur on the plating surface, and the amount of dissolution of the plating layer itself will increase, which is disadvantageous.

There are no particular restrictions on the current density, and it only depends on the amount of electricity, threading conditions, and equipment conditions, but from the point of productivity
．． More than 1A/dm! 100A/drr considering power loss due to increase in electrolysis voltage and maintaining a good appearance?
The following are preferred. Following this mild anodic electrolytic peeling treatment, a grinding treatment is performed.

There are no particular limitations on the method of grinding, and for example, brushing with a nylon brush roll (manufactured by Hotani Co., Ltd.) or a Scotch Bride roll (manufactured by Sumitomo 3M Co., Ltd.) is possible; It is preferable to use abrasive grains such as alumina (AQzoa) and silicon carbide (SiC), which are used in boat fishing. In addition, the particle size of the abrasive grains is #100 to #500 in the case of a nylon brush roll (manufactured by Hotani Co., Ltd.). In the case of Scotch Bride Roll (manufactured by Sumitomo 3M Ltd.), those of #150 to #1000 are preferred.

Regarding the grinding process conditions, for example, when grinding is performed by rolling down the roll while keeping the rotation speed of a nylon brush roll or Scotch Bride roll constant, the motor load current generated by pressing the roll against the plating surface is 1 to 1. To 50A! l! ! Preferably. If it is less than IA, the grinding process is too much, and if it exceeds 50A, the grinding marks on the plated surface become too deep, and there is a concern that it will adversely affect the image clarity and corrosion resistance after painting. The number of rolls and reduction amount used in the grinding process should be 30% or more of the plated surface, preferably
It is preferable that adjustment is made so that 50% or more is ground randomly. When the grinding area ratio is less than 30%, the grinding treatment has no effect and does not contribute to corrosion resistance after painting. This grinding treatment is effective when performed while washing with water, and is more effective and preferable when performed during washing with water after peeling off the anode WIN.

The second invention is to immerse a zinc-based plated steel sheet in a chromate solution, and then perform anodic electrolytic stripping on the plated surface in an electrolytic solution with a p++ of 4 to 7 with an amount of electricity of 1 to IOC/drrr, and then remove the plated surface. It is characterized by grinding the surface. The difference between this and the first invention is that the material is immersed in a chromate solution before the anodic electrolytic stripping and grinding treatment. The chromate solution may be one used for ordinary electrolytic chromate or coating chromate containing Cr as a main component.The amount of chromate film is preferably 1 to 10 μ/a or more in terms of total Cr amount, and it is suitable for anodic electrolytic peeling and grinding. Painter corrosion resistance can be significantly improved by adjusting the treatment conditions to reduce the remaining area of the chromate film to 50% or less.If it is less than 1 inch/bo, most of it will be removed by anodic electrolytic stripping and grinding treatment. , it is almost impossible to expect an improvement in the corrosion resistance of the paint machine due to residual Cr.Also, if it exceeds 10 Cr/Po, the CrfA degree in the electrolyte will increase, and the electrolytic stripping effect will decrease and the electrolyte will become unstable. This is not desirable because it becomes

The electrolytic solution used for anodic electrolytic stripping is Na.

Those containing phosphates, sulfates, and chlorides such as K, Li, Mgt A Q + Ca, etc. as main components are preferable. Among these, an electrolytic solution containing phosphate as a main component and using phosphoric acid or sodium hydroxide solution as a pH adjuster is most effective in improving chemical conversion treatment properties. In this case, the total phosphorus m:a degree is preferably 0.1 to 2 mol/L.

If it is less than 0.1 mol/L, the effect of improving chemical conversion treatment properties will be insufficient, and if it exceeds 2 mol/L, there will be concerns about precipitation. In addition, strong electrolyte salts such as sodium sulfate or ammonium sulfate may be added to increase the conductivity of the electrolytic solution.
In order to obtain a sufficient effect of improving chemical conversion treatment properties, it is desirable that the concentration of anions other than phosphate ions is 1/2 equivalent or less of the total phosphoric acid concentration. The temperature of the electrolytic solution is not particularly limited, but can be selected from room temperature to 60°C.

The anodic electrolytic stripping may be carried out by any known method such as a horizontal method, a vertical method, or a radial cell method. Horizontal grinding is the most suitable method.

The present invention particularly relates to the plating surface of a single-sided zinc-based plated steel sheet.

Can be applied to one or both sides of a double-sided zinc-based plated steel sheet, but for automobile bodies where the inner rust-preventing side of the car body is an organic composite plating consisting of zinc-based plating/chromate/organic coating, and the outer rust-preventing side is zinc-based plating. It is extremely effective as a post-treatment for zinc-based plating on the exterior rust-preventing side of double-sided anti-rust steel plates. This is because zinc-based plating is applied to both sides of the steel sheet, and then a chromate/organic film is applied to only one side, so the other plated surface is susceptible to chemical conversion treatment due to contamination with chromate solution and baking of the organic film due to oxidation. This is because the condition is even worse than normal. Further, contamination with chromate solution can also be used in the second method of the present invention.

(Example) Examples of the present invention will be described below.

(Example 1) Zn-Ni alloy plating was applied to a cold-rolled steel sheet of 0.8 rrtnl'J , the plating solution flow rate was 100 m/min.

The adhesion amount was 20 g/nr, the Ni composition was 9 to 13%, and the current density was 50 A/dbo. After washing with water and drying, anodic electrolytic peeling and grinding treatment (with two grinding rolls) were performed while washing with water, and finally washing with water and drying was performed.

One part was plated, washed with water, dried, immersed in a chromate solution, washed with water, subjected to anodic electrolytic stripping, and then subjected to a grinding process while washing with water. The grinding area ratio of the thus obtained test piece was visually evaluated using a scanning electron microscope, and the chemical conversion treatment property and paint removal performance were evaluated.

Table 1 summarizes the post-treatment conditions and evaluation results after plating. The evaluation criteria are as follows.

A. Chemical conversion treatment Immersion type phosphate treatment (treatment time: 2 minutes) was performed, and the phosphate film was evaluated.

Dense (10μm or less), no skidding, 20 dense (15μm or less), no skidding: 0 slightly rough (25μm or less), no skidding: Δ rough (over 25μm), with skidding: XB,
After painting performance chemical treatment, cationic electrodeposition coating 20. um (Power Top 0100. manufactured by Nippon Paint), intermediate coat, top coat each 30 μm
(Both are melamine alkyd paints manufactured by Kansai Paint)
A three-coat coating was applied and the following tests were conducted.

(1) Water resistant adhesion After being immersed in distilled water at 40° C. for 240 hours, a 21-kilometer rotist test was carried out and the coating film survival rate (%) was evaluated.

100%: ◎ 95% or more: 0 90% or more: Δ Less than 90%: × (2) After painting, make cross-cut scratches that reach the corrosion-resistant base material, perform the following cyclic corrosion test for 50 cycles, and check the bulge width of the cross-cut portion. was evaluated.

Cycle corrosion test salt water spray 'IS (JIS Z 2371): 2
Time humidity [(60℃, R1195%): 2 hours drying
(60°C, 8840%): 4-hour measurement value 3■ or less: ◎ 5m or less: 0 7nn or less: Δ More than 7+m: × In Table 1, Comparative Example 1-27 has (-) polarity, and anodic electrolysis Can be chemically treated as it does not peel off.

Performance after painting is poor. Comparative Example 1-28 has a component concentration of the electrolytic solution, Comparative Example 1-29.30 has a pH of the electrolytic solution, and Comparative Example 1-31.32 has an amount of electricity during electrolysis that is out of the appropriate range, so the chemical composition Processability.

Performance after painting is insufficient. Comparative Examples 1-33 and 34 were not subjected to grinding treatment, and therefore had poor chemical conversion treatment properties and poor performance after painting. In contrast to these comparative examples, inventive examples 1-1 to
No. 26 has good chemical conversion treatment properties and performance after painting, and the example immersed in a chromate solution has particularly good corrosion resistance after painting.

(Example 2) Various zinc-plated steel sheets were subjected to the treatment of the present invention,
The same evaluation as in Example 1 was performed. Table 2 summarizes the results. Present invention examples 2-1 to 9.13 and comparative example 2-
Nos. 17 to 25.29 are electrolytic zinc-based plating, and the adhesion amount is 20 g/rn'.

Present invention examples 2-10 to 12, 14 to 16 and comparative example 2-26
-28.30-32 are hot-dip zinc plating, and the coating weight is 45 g/rd. In Comparative Example 2-19.27, the polarity was (-) and the anodic electrolytic peeling was not performed, so that the chemical conversion treatment property and the performance after painting were poor. Comparative example 2-17.18
, 20, 21°, 24.25, and 26.28 have insufficient chemical conversion treatment properties and post-painting performance because the electrolytic solution or electrolytic conditions for anodic electrolytic stripping are out of the appropriate range. Comparative example 2-2
2.23 is not subjected to grinding treatment, so it is chemical conversion treatable, @
The performance after installation is poor. Comparative Examples 2-29 to 2-32 were not subjected to both anodic electrolytic peeling and grinding treatment, and thus had poor chemical conversion treatment properties and post-painting performance. For these comparative examples,
Examples 2 to 1 to 16 of the present invention have good chemical conversion treatment properties and performance after painting.

(Effects of the Invention) As described above, according to the present invention, a zinc-based plated steel sheet with excellent chemical conversion treatment properties and post-painting performance can be obtained by mild anodic electrolytic peeling and grinding treatment, and is particularly useful for preventing rust on the exterior of automobile bodies. This method is suitable as a post-treatment method for zinc-plated surfaces.

Procedural amendment Month//day of 1999

Claims (3)

[Claims] (1) The plated surface of a zinc-based plated steel sheet is subjected to anodic electrolytic stripping in an electrolytic solution with a pH of 4 to 7 at an amount of electricity of 1 to 10 C/dm^2, and then the surface of the plated surface is ground. A method for producing galvanized steel sheets with excellent chemical conversion treatment properties and post-painting performance. (2) Immerse a zinc-based plated steel sheet in a chromate solution, and then place the plated surface in an electrolytic solution with a pH of 4 to 7 with an amount of electricity of 1 to 1.
A method for producing a zinc-based coated steel sheet with excellent chemical conversion treatment properties and post-painting performance, which comprises performing anodic electrolytic stripping at 10C/dm^2 and subsequently grinding the surface of the plated surface. (3) When performing electrolytic stripping, the total phosphoric acid concentration is 0.05~
2 mol/1 and uses an electrolytic solution having a pH of 4 to 7. The method for producing a zinc-based coated steel sheet with excellent chemical conversion treatment property and post-painting performance according to claims 1 and 2. .