JPH0246678B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing an electrolytically chromate-treated steel sheet with excellent adhesion to secondary paint. In recent years, adhesive cans using electrolytic chromate-treated steel sheets (hereinafter referred to as "TFS adhesive cans") have been used for soft drink cans, etc., instead of solder cans using tin-plated steel sheets.
has become widely used. The reason is
For example, when TFS adhesive cans are used for carbonated beverage cans, sufficient seam strength is obtained due to the excellent primary paint adhesion of the electrolytically chromate-treated steel sheet, which prevents leakage from the seam and decrease in vacuum level. This is because it has an excellent property of not causing fatal defects as a can. These TFS adhesive cans can be used for fruit juice cans (so-called hot pack cans), which need to be heated to 90 to 100 degrees Celsius and then immediately filled for sterilization, such as fruit juice cans, and cans that can be heated to around 130 degrees Celsius after filling. It has also come to be used for beverage cans or food cans (so-called retort cans) that require high-temperature heat sterilization in pressurized steam. However, TFS adhesive cans are
Under the harsh conditions of high temperature and high humidity as described above, there was a problem with the adhesion of the secondary paint. That is, TFS adhesive cans are manufactured by painting electrolytic chromate-treated steel plates and then bonding the can body with nylon adhesive. However, when hot pack or retort sterilization is applied to these TFS adhesive cans,
The adhesive strength at the interface between the treated steel sheet and the coating film decreases, and peeling occurs at the seam (can body joint) where the most stress is applied, which tends to cause defects such as leakage of contents and a decrease in the degree of vacuum. This peeling phenomenon is generally defined as secondary paint adhesion, and occurs because water intrudes between the paint film and the substrate under high temperature and high humidity environments, inhibiting the adhesive force at the interface. The faster the penetration rate, the more the adhesive strength deteriorates over a wider area. By the way, electrolytic chromate treated steel sheets generally consist of two layers: a metallic chromium layer with a thickness of about 0.005 to 0.03 μm in the lower layer, and a hydrated chromium oxide layer with a thickness of about 0.01 to 0.04 μm in the upper layer. It has a chromate coating on the surface of the steel plate. A method for forming such an electrolytic chromate film is to cathodically electrolyze a steel sheet in an electrolytic solution containing chromic anhydride as a main ingredient and one or two of sulfates and fluorine compounds added as an auxiliary agent. A one-step method in which a chromium layer and a hydrated chromium oxide layer are formed, and a metallic chromium layer and a hydrated chromium oxide layer are formed on the surface of a steel sheet in the same electrolytic solution as above, and then the water There is a two-step method in which the hydrated chromium oxide is dissolved and removed, and then cathodic electrolysis is performed again in the same electrolytic solution as above to form a hydrated chromium oxide layer. However, with any of these methods, it is difficult to impart secondary paint adhesion to the steel plate that is sufficiently satisfactory as a material for adhesive cans. For this reason, various studies have been conducted on means for solving the above problem, and for example, the following methods have been proposed. (1) Electrolytic chromate-treated steel sheets had a hydrated chromium oxide layer formed using an electrolytic solution containing sulfate, and the adhesion of the secondary paint deteriorated due to the sulfate radicals contained in the film. Use salt-free electrolytes. (2) Since chromic anhydride, which is the main component of the electrolyte, contains sulfate radicals as impurities, the steel plate after the coating is formed should be immersed in water at 60 to 100°C, or
The steel plate is immersed in a hot aqueous solution of a weak alkaline salt or a neutral salt, or cathodic electrolysis is performed using the aqueous solution to remove sulfate groups in the hydrated chromium oxide layer and to modify the coating. According to the above method, although the adhesion of the secondary paint is improved to some extent, it is still insufficient and there is a problem that the product performance is unstable. As a result of conducting research to solve the above-mentioned problems, the present inventors have found that the cause of variation in the adhesion of secondary paint on electrolytically chromate-treated steel sheets is due to the state of precipitation of the metallic chromium layer. It was found that those in which many angular projections were generated had good secondary paint adhesion, and those in which no angular projections were generated on the metal chromium layer were poor in secondary paint adhesion. The state of deposition of this metallic chromium layer is determined by whether electrolysis is performed intermittently or continuously. In a system such as a vertical production line (such as a vertical production line), a large number of angular protrusions are generated, whereas, on the other hand, in a horizontal continuous plating system, for example, in which electricity is applied continuously during a predetermined processing time, they are difficult to occur. In addition, the precipitation of angular protrusions is also affected by the composition of the electrolytic solution; in the case of a CrO ₃ -F binary electrolyte, angular protrusions may not occur at all, or even if they do occur, production will be delayed. The speed will be low. On the other hand, CrO ₃ ^- −SO ₄ ^2- system or CrO ₃ −SO ₄ ^2- −
It was also confirmed that a large number of protrusions were generated in the F-based electrolyte. Even if the above electrolytic conditions are satisfied, the formation of angular protrusions is selective to crystal planes and is formed only on the (001) plane, and the condition of the steel sheet to be treated has a large influence on the formation of angular protrusions. give. That is, the metal chromium deposited directly copies the crystal orientation of the steel sheet to be treated, and grows epitaxially in the same direction as the crystal orientation. This is because strong epitaxy is thought to occur in chromium electrodeposited on steel sheets due to similarities such as both having a BCC structure, similar lattice constants, or both being transition elements. Therefore, in the metallic chromium layer of a normal electrolytically chromate-treated steel sheet, crystal grains with the same crystal orientation as the steel sheet grow, but the crystal growth rate differs depending on the orientation of the crystal planes. When the overvoltage is large, such as in chromium electrodeposition, it is said that planes with orientations close to (111) to (011) grow preferentially because the nucleation energy of the (111) plane is the lowest. This trend is also observed in practice. The constraining force of the orientation of the steel plate is strong on the (111) plane, and on the (001) plane.
Since it is the weakest among the planes, the thickness increases uniformly on planes with an orientation close to (111), but the restraining force of the steel plate is weak on the (001) plane, so planes other than the (001) can grow freely to some extent, and as a result, As a result, angular protrusions are generated on the (001) plane. In this way, in order to produce an electrolytically chromate-treated steel sheet with excellent adhesion to the secondary paint, it is necessary to pay attention to the crystal plane distribution of the steel sheet to be treated, but this is nearly impossible on an actual production line. Furthermore, in actual production lines, there are problems such as the number of angular protrusions on the (001) plane not necessarily being constant even if the above electrolytic conditions are satisfied. From the above-mentioned viewpoints, the inventors of the present invention have found that the adhesion of the secondary paint does not deteriorate even when placed in a high-temperature and high-humidity environment, and that there is no problem when used on hot pack cans or retort cans. In order to produce electrolytic chromate-treated steel sheets that do not cause any inconvenience, we have conducted numerous tests and researches. (2) When anodic electrolysis is carried out using a quantity of electricity (of course, this also results in intermittent electrolytic plating), angular protrusions are formed on all crystal orientation planes; Therefore, when electrolytic chromate treatment of a steel plate is performed, cathodic electrolytic treatment is performed intermittently, and if the electrolytic treatment is performed using the steel plate as an anode in at least one of the multiple non-energized intervals during the intermittent cathodic electrolysis, precipitation occurs. A large number of angular protrusions are uniformly formed on all crystal orientation planes of the metallic chromium layer.
We have come to the knowledge shown in (1) and (2) above that it is possible to obtain an electrolytically chromate-treated steel sheet with significantly superior secondary paint adhesion. This invention was made based on the above findings, and involves cathodic electrolytic treatment using a steel plate as a cathode in an electrolytic solution containing at least one of chromic anhydride, chromate, and dichromate as a main component. , by applying an electrolytic chromate coating on the surface of the steel plate, the inner layer of which is metallic chromium and the outer layer of which is hydrated chromium oxide, in one step using a group of a plurality of electrolytic cells, or
In a method for producing an electrolytic chromate-treated steel sheet, which is formed by multiple steps using two or more groups of multiple electrolytic cells separated by at least one step of drag-out treatment and water washing treatment, During a cathode electrolytic treatment process using electrolytic solutions of the same composition using multiple electrolytic cells in a group, use of an electrolytic solution having the same composition as the electrolyte of the cathodic electrolysis process in other than the first and final energizing parts. and by subjecting the steel plate to an anodic electrolytic treatment at least once, thereby uniformly forming a large number of angular protrusions on the metal chromium layer, to obtain an electrolytic chromate-treated steel plate with excellent adhesion to a secondary paint. It has the following characteristics. In this method, cathodic electrolysis is carried out intermittently because, in the case of a vertical production line system as shown in Figure 1, electrolysis is not carried out in the conductor roll and sink roll areas. It will be done. In anodic electrolysis, during cathodic electrolysis to deposit metallic chromium, an electrolytic solution with the same composition as the electrolytic solution used in cathodic electrolysis is used, and the poles of the supplied current are temporarily reversed. It is also possible to prepare separately and perform anodic electrolysis during continuous cathodic electrolysis. In this way, it can also be carried out using a horizontal production line system. FIG. 1 schematically shows an example of an electrolytic chromate treatment line for steel strip, with FIG. 1a showing a 2-step system and FIG. 1B showing a 1-step system. In addition, in FIG. 1, 1 is a steel strip, 2 is an electrode, 3 is a conductor roll, and 4 is a sink roll. Adjust the anodic electrolytic treatment position to this first
To explain the processing line as shown in the figure, the 2-step method shown in Fig. 1a is performed using at least one pass from #2 to #7, #10, and #11, and the process shown in Fig. 1b is In the 1-step method, at least one of passes #2 to #7 is used. The anode electrolyte at this time uses an electrolyte having the same component composition as the electrolyte in the cathode electrolytic treatment step. Even if anodic electrolysis is performed before the #1 pass and cathodic electrolysis is performed thereafter, granular protrusions cannot be formed in the metal chromium layer. Even if anodic electrolysis is performed after the final cathodic electrolysis to deposit metallic chromium (after the 2nd step in the 2-step method), it is of course a meaningless process since there is no subsequent metallic chromium deposition step. Electricity during anodization is 5 coulombs/dm ² because metal chromium is oxidized and eluted when anodic electrolysis is performed.
It is best to avoid exceeding this. because,
This is because, although it depends on the anodizing position, if a current of 5 coulombs/dm ² or more is applied, more than half of the metal chromium basis weight will be dissolved and removed, making it unsuitable for an actual production line. Further, even if the amount of current is less than 0.01 coulomb/dm ² , the effect of improving the adhesion of the secondary coating film cannot be obtained. Therefore, the amount of electricity processed is 0.01~
It is best to select within a range of up to 5 coulombs/dm ² . In this way, sufficient improvement effects can be obtained even with anodic electrolysis using a low amount of electricity, so in the production line as shown in Figure 1, it is not necessary to use the entire one pass for anodic electrolysis, and the first A small anodizing electrode may be installed between the passes in the figure for processing. The mechanism by which uniform angular protrusions are generated by anodic electrolysis is unknown, but by anodic electrolysis, a hydrated chromium oxide layer that does not contain any sulfate groups and is hardly soluble in the electrolyte solution is formed. It is presumed that the modification or new generation of chromium and the activation of minute portions of the metallic chromium layer by anodic electrolysis are largely involved in the precipitation of metallic chromium during the next cathodic electrolysis. Next, in order to further clarify the effects of the present invention, the present invention will be specifically explained using Examples and Comparative Examples. Of course, the present invention is not limited to the processing modes of the embodiments. Comparative example A 0.22 mm thick cold rolled steel plate (equivalent to T ₄ ) was electrolytically degreased in 30 g/sodium orthosilicate, washed with water,
After electrolytic pickling in sulfuric acid and water washing, in the first solution below, bath temperature: 45°C, current density: 25A/dm ²
Under the electrolytic conditions of
Total energization time: Perform only cathode electrolysis for 1.8 seconds, and after washing with water, energize as shown in Figure 2b in the second solution below under the electrolytic conditions of bath temperature: 45℃ and current density: 30A/ ^dm2 . Using this method, cathode electrolysis was performed with energization time: 0.3 seconds, non-energization time: 0.3 seconds, and total energization time: 0.9 seconds, followed by washing with water and drying. First liquid CrO ₃ : 175 g/, Na ₂ SiF ₆ : 5 g/, Na ₂ SO ₄ : 0.7 g/. Second liquid _CrO3 : 50g/, _NH4F : 2g/. The electrolytic chromate-treated steel sheet prepared in this way was coated with phenol epoxy paint and baked at a temperature of 210°C and a baking time of 10 minutes, and then placed in a steam retort pot at 130°C. The peeling time was measured and the adhesion of the secondary paint was evaluated.
The peeling time in this case was expressed as the average peeling time for each level of 20 samples of one level (one current density) placed in a retort pot. Separately, the distribution of angular protrusions was observed using transmission electron micrographs of the metallic chromium layer, and the amount of Cr in the hydrated chromium oxide layer was also measured. The results thus obtained are shown in Table 1 together with the results obtained in other Examples. A transmission electron micrograph of the metallic chromium layer is shown in FIG. 3a. Examples 1 to 4 Under the same processing conditions and procedure as in the comparative example, the first liquid was used during the first non-energizing period of intermittent cathodic electrolysis in the first liquid, and the electrolysis rate was 0.05 A/dm ² ( Example 1),
Anodic electrolysis was performed once for a treatment time of 0.3 seconds at a current density of 0.1 A/dm ² (Example 2), 0.2 A/dm ² (Example 3), and 0.3 A/dm ² (Example 4). An electrolytic chromate treated steel sheet was manufactured. A secondary paint adhesion test similar to that of the comparative example was conducted on each of the electrolytic chromate-treated steel sheets obtained in this way, and the average peeling time was investigated, as well as the amount of Cr in the hydrated chromium oxide layer. measure,
It is shown in Table 1. A transmission electron micrograph of the metallic chromium layer of the electrolytically chromate-treated steel sheet obtained in Example 4 is shown in FIG. 3b. Examples 5 to 8 Under the same processing conditions and procedure as in the comparative example, the first solution was used during the second non-energized period of intermittent cathodic electrolysis in the first solution, and the electrolysis rate was 0.05 A/dm ² ( Example 5),
Anodic electrolysis was performed once for a treatment time of 0.3 seconds at a current density of 0.1 A/dm ² (Example 6), 0.2 A/dm ² (Example 7), and 0.3 A/dm ² (Example 8). An electrolytic chromate treated steel sheet was manufactured. Characteristic evaluations similar to those in Examples 1 to 4 were performed on these electrolytic chromate-treated steel sheets, and the results are also shown in Table 1. Examples 9 to 12 Under the same treatment conditions and procedure as in the comparative example, during the third non-energized period of intermittent cathodic electrolysis in the first liquid,
Using the first liquid, 0.05A/dm ² (Example 9),
0.1A/ ^dm2 (Example 10), 0.2A/ ^dm2 (Example
11), anodic electrolysis was performed once at a current density of 0.3 A/dm ² (Example 12) for a treatment time of 0.3 seconds to produce an electrolytic chromate-treated steel sheet. The electrolytic chromate-treated steel sheets thus obtained were also subjected to characteristic evaluations similar to those in Examples 1 to 4, and the results are also listed in Table 1. Furthermore, a transmission electron micrograph of the metallic chromium layer of the electrolytically chromate-treated steel sheet obtained in Example 12 is shown as FIG. 3c.

[Table] From the results shown in Table 1 and Figure 3, it is clear that in each of Examples 1 to 12, the angular protrusions were formed on the underlying crystal plane, compared to the comparative example in which no anodic electrolytic treatment was performed. It can be seen that the occurrence is uniform without any dependence, and it is also clear that a remarkable improvement effect has been obtained in the adhesion of the secondary paint. As described above, according to the present invention, it is possible to easily produce electrolytic chromate-treated steel sheets that exhibit excellent secondary paint adhesion even in high temperature and high humidity environments, and TFS adhesive This brings about useful effects such as further expanding the uses of cans.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an example of an electrolytic chromate-treated steel sheet manufacturing line, with FIG. 1a showing a two-step system and FIG. 1b showing a one-step system. Figures 2a and b are schematic diagrams showing the current supply method in the electrolytically chromate-treated steel sheet production line, and Figure 3 is a transmission micrograph (magnification: 11,000x) of the metallic chromium layer of the electrolytically chromate-treated steel sheet. 3b shows that obtained in Example 4, and FIG. 3c shows that obtained in Example 12. In the drawings, 1... steel strip, 2... electrode, 3... conductor roll, 4... sink roll.

Claims (1)

[Claims] 1. A steel plate is subjected to cathodic electrolysis treatment in an electrolytic solution containing at least one of chromic anhydride, chromate, and dichromate as a main component, and the surface of the steel plate is coated with , an electrolytic chromate coating consisting of an inner layer of metallic chromium and an outer layer of hydrated chromium oxide is formed in one step using a group of a plurality of electrolytic cells, or
In a method for producing an electrolytic chromate-treated steel sheet, which is formed by multiple steps using two or more groups of multiple electrolytic cells separated by at least one step of drag-out treatment and water washing treatment, During a cathode electrolytic treatment process using electrolytic solutions of the same composition using multiple electrolytic cells in a group, use of an electrolytic solution having the same composition as the electrolyte of the cathodic electrolysis process in other than the first and final energizing parts. and an electrolytic chromate-treated steel sheet with excellent secondary paint adhesion, characterized in that the steel sheet is anodically electrolyzed at least once, thereby uniformly forming a large number of angular protrusions on the metal chromium layer. manufacturing method.