JPS639038B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a stain-free steel sheet for retort treatment, which has excellent adhesion between a metallic chromium coating and a chromium hydrated oxide coating, and excellent adhesion between a chromium hydrated oxide coating and a paint. The conventional two-part method deposits metallic chromium and the hydrated chromium oxide film separately, so it has the advantage of being able to easily control the amount of each deposited. Because the compositions are different, if a steel plate after chromium plating is directly transferred to an electrolytic chromic acid treatment bath, the tanning aid anions eutectoid in the chromium hydrated oxide generated during chromium plating will be deposited in the coating of the final product. The chromium plating solution that remains or adheres to the steel surface may be brought into the electrolytic chromic acid treatment bath, causing fluctuations in the amount of chromium hydrated oxide film deposited, or re-precipitation of partially eluted auxiliary anions. This may cause inconveniences such as Therefore, it is necessary to thoroughly wash with water after chromium plating and before entering the electrolytic chromic acid treatment bath to prevent this, but washing with water after chrome plating alone will not remove the chromium water generated during chrome plating. It is difficult to remove the chromium plating aid anion in the oxide. In particular, in chromium plating using sulfuric acid or its compounds as an aid, even if electrolytic chromic acid treatment is performed using a chromic acid solution, sulfuric acid groups are eutectoid in the final chromium hydrated oxide. This causes problems in adhesion with the paint film. As a method to overcome this drawback, by performing reverse electrolysis in the solution after chromium plating, it is possible to reduce the chromium hydrated oxide produced during chromium plating and the auxiliary anions therein. We have found that thoroughly washing with water before entering the electrolytic chromic acid treatment bath and then applying electrolytic chromic acid in an aqueous chromic acid solution is extremely effective in improving retort resistance. Gansho 56-62766
(hereinafter referred to as the original invention). However, after repeated research, it was discovered that although the original invention is extremely effective in improving retort treatment resistance, the chromium hydrated oxide film may become non-uniform. As a result of repeated research, it was possible to obtain a uniform chromium hydrated oxide film, and the retort processability was further improved, leading to the present invention. In the present invention, a cold-rolled steel sheet is plated with metallic chromium of 20 to 200 mg/ ^m2 on one side, and the surface of this metallic chromium layer is
This relates to a stain-free steel sheet with a chromium hydrated oxide coating of ~50 mg/m ² . If the metallic chromium layer is less than 20mg/ ^m2 , there will be many microcracks and pinholes, resulting in poor corrosion resistance.
If the ^amount is more than 200 mg/m2, no improvement in corrosion resistance can be expected, so normal chrome-plated steel sheets are used at 20 to 200 mg/ ^m2.
It has a metallic chromium layer. On the other hand, if the amount of hydrated chromium oxide film (hereinafter referred to as Cr ^OX film) is less than 5 mg/m ² , the desired paint adhesion cannot be expected, and if it is more than 50 mg/m ² , the appearance may deteriorate or Cracks may appear in the Cr ^OX film, making it impractical. Furthermore, the amount of Cr ^OX film is within the practical range of 10 to 30 mg/m ² and is most desirable. A stain-free steel plate can only be used as a food can material after being painted, and when used as a food can, adhesion with the coating film is particularly important. In order to obtain a stain-free steel sheet with excellent adhesion to the coating film and resistance to retort treatment after filling, chrome plating is performed using a general-purpose chromium plating bath using an auxiliary agent containing anions such as sulfuric acid or its compounds. The original invention proposes a method in which reverse electrolysis is then carried out at 0.1 to 15 coulombs/dm ² in this bath, and then electrolytic chromic acid treatment is carried out in an aqueous chromic acid solution via a water washing tank. The stain-free steel sheet obtained by this method is extremely effective in improving retort treatment resistance, but
As mentioned above, this treatment alone may cause the Cr ^OX film to become non-uniform. Therefore, in the present invention, a conventionally degreased and pickled steel sheet is chromium plated in a chromium plating bath containing sulfuric acid or its compounds as an auxiliary agent, and then reverse electrolysis is performed in this bath to form a chrome plating layer. generated at the time
After reducing the auxiliary anions such as SO ₄ ^2- that have eutectoided in the Cr ^OX film, electrolytic chromic acid treatment is performed, and the feature is that this treatment is performed under the conditions described below. . In other words, when performing electrolytic chromic acid treatment,
In order to obtain a Cr ^OX film with a uniform electrolytically deposited film structure and good retort treatment resistance, a first electrolytic chromic acid treatment is performed to uniformly modify the Cr ^OX film roughened by reverse electrolysis; The treatment is divided into two stages, including a second electrolytic chromic acid treatment to ensure the amount of Cr ^OX film.
Electrolytic chromic acid treatment is performed in separate treatment baths with each bath composition as described below. The main ingredient of the chromic acid treatment aqueous solution used in the present invention is:
Comprised of at least one compound selected from chromate anhydride and dichromate,
Chromates include alkali metal salts of chromic acid,
Alkaline earth metal salts, ammonium salts, etc. are suitable, and as dichromates, alkali metal salts, alkaline earth metal salts, ammonium salts, etc. of dichromic acid are suitable, but there are no particular restrictions on the type of compound. do not have. The composition of the electrolytic chromic acid aqueous solution bath may be the same in the first treatment and the second treatment, or may be different depending on the bath composition, concentration, electrolytic conditions, etc. of the fluoride auxiliary agent. The concentration of the chromic acid aqueous solution that inevitably contains impurity anions is not particularly limited, but is suitably in the range of 10 to 200 g/CrO ₃ . concentration is 10
If it is less than 1 g/g, the electrical resistance of the bath increases, causing problems such as overvoltage of the rectifier and heating of the steel plate, which is not preferable. Also, 200g/
Exceeding this is not desirable because the desired effect cannot be expected to improve much and economic loss due to taking out the bath is large. The bath temperature of such a chromic acid treatment bath is
A temperature of 35 to 60°C is appropriate. If it is lower than 35℃, temperature control becomes difficult due to the rise in cooling water temperature in summer,
If the temperature is higher than ℃, the materials of the electrolytic cell that can withstand the chromic acid aqueous solution are limited. When reverse electrolysis is performed in the same solution after chromium plating, the Cr ^OX film formed during chrome plating is dissolved, and at the same time or after completion of the chromium plating, the anode Cr ^OX film, which is different from the previous cathode Cr ^OX film, becomes metallic chromium. formed on the layer. It has been revealed that the exposure of metallic chromium is less when compared to conventional cathodic coatings due to the formation of this film, but depending on the type of auxiliary agent used for chromium plating, its bath concentration, and electrolytic conditions, it is possible to form the metal chromium anodically. Abnormal precipitation may be observed in some parts of the coated film. When the conventional two-component electrolytic chromic acid treatment was applied to this film, the retort treatment resistance was good, but abnormal precipitation of the Cr ^OX film was sometimes observed. We conducted various studies using fluorine compounds to find ways to avoid this problem, and found that it is effective to first perform the first electrolytic chromic acid treatment using a simple salt fluoride as an auxiliary agent. I understand. During electrolytic chromic acid treatment, H ₂ gas is generated by electrolysis of water at the cathode, and the pH near the cathode increases. Along with this, an all compound with a high degree of polymerization is formed in which a hydroxyl group, a water molecule, etc. are coordinated with a low-valent chromium ion. On the other hand, fluorine ions existing near the cathode become free due to the increase in pH near the cathode.
F ^- ion, which replaces hydroxyl groups and water molecules coordinating low-valent chromium, promotes the reduction reaction to metallic chromium, and cleaves the all bonds of all compounds, reducing its polymerization degree. decreases the
Dissolves Cr ^OX film. Therefore, the formation and dissolution of the Cr ^OX film are repeated, and the Cr ^OX film is formed thin and uniformly. The first electrolytic chromic acid treatment according to the present invention is performed to modify and make the Cr ^OX film uniform when the anode Cr ^OX film formed on metal chromium by reverse electrolysis becomes rough. Precipitation of metallic chromium or Cr ^OX
It is not intended to increase the amount of film. The simple salt-based fluorine compound used in the first electrolytic chromic acid treatment is one that dissolves in water and ionizes nearly 100%, and in an aqueous chromic acid solution, it dissociates into ions and H ⁺ ...
...It is said that F ^- hydrogen bonds are formed, and HF,
_NaHF2 , NaF, _NH4F , _KHF2 , KF, _MgF2 ,
A typical example is LiF ₂ , but there is no need to be particularly limited. The amount of simple salt fluorine compound added as an auxiliary agent is 0.1 to 10 g/F equivalent.
A range of is appropriate. Although the concentration range does not need to be particularly limited, the effects of the present invention cannot be expected to be so great if it is less than 0.1 g/ and more than 10 g/. The electrolytic conditions for the first electrolytic chromic acid treatment are not particularly limited, but generally the current density is 5 to 100 A/dm ²
Good range. If this is less than 5A/ ^dm2
This may cause precipitation abnormalities in the Cr ^OX film.
If it exceeds 100A/ ^dm2 , it is not practical due to the rectifier capacity. Further, the amount of electricity is preferably 0.1 to 50 coulombs/ ^dm2 . If the amount of electricity is less than 0.1 coulombs/ ^dm2 , the intended purpose cannot be achieved, and if the amount of electricity is less than 50 coulombs/dm
If m ² or more, the precipitation of metallic chromium only increases, and the effects of the present invention cannot be expected to be so great. As mentioned above, the Cr ^OX film formed by reverse electrolysis is modified by the first electrolytic chromic acid treatment, but since the purpose is not to secure the amount of Cr ^OX film, it is important to ensure that it is sufficient. No, no. Therefore,
After conducting various studies using fluoride to ensure the amount of Cr ^OX film after the first electrolytic chromic acid treatment, we found that the second electrolytic chromic acid treatment using a complex fluoride as an auxiliary agent is It turned out to be effective. During electrolytic chromic acid treatment, the pH near the cathode increases. The ionization state of the complex ion fluorine differs depending on the pH, and unlike simple salt fluoride, which is nearly 100% ionized and F ^- near the cathode participates in the formation and dissolution of the Cr ^OX all compound. Only a small portion of free fluoride ions are involved. Therefore, more Cr ^OX is produced by dissolution, and the desired amount of Cr ^OX is ultimately secured. In the second electrolytic chromic acid treatment following the first electrolytic chromic acid treatment according to the present invention, after the first electrolytic chromic acid treatment modifies the Cr ^OX film and makes it uniform, the amount of Cr ^OX film is secured. It is given in order to do something. The complex salt-based fluorine compound used in the second electrolytic chromic acid treatment refers to a compound that dissolves in water and undergoes hydrolysis to exist as a complex ion.It also exists as a complex ion in an aqueous chromic acid solution, and contains Na ₂ SiF ₆ , NaBF _Four ,
NH ₄ BF ₄ , K ₂ SiF ₆ , KBF ₄ , Na ₂ TiF ₆ , K ₃ AlF ₆
These are representative examples, but there is no need to be particularly limited. The amount of the complex salt type fluorine compound added as an auxiliary agent is suitably in the range of 0.1 to 10 g/F (F). The concentration range does not need to be particularly limited, but if it is less than 0.1 g/, the effect of the present invention cannot be expected much, and if it exceeds 10 g/, it will be the upper limit of solubility, although it depends on the type of compound. The electrolytic conditions for the second electrolytic chromic acid treatment are not particularly limited as it is sufficient to apply the amount of electricity necessary to ensure the amount of Cr ^OX film, but generally the current density is preferably 5 to 50 A/ ^dm2 . If it is less than 5 A/dm ² , the electrolysis time is too long to be practical, and if it exceeds 50 A/dm ² , abnormal precipitation of the Cr ^OX film may occur. Next, the processing according to the present invention will be specifically explained. After pre-treating a cold-rolled steel plate with a thickness of 0.22 mm by conventional methods such as degreasing and pickling, 250 g of CrO ₃ /,
In a chromium plating bath containing 2.5g/H ₂ SO ₄ ,
Electrolysis was carried out for 1.0 seconds at a bath temperature of 55° C. and a current density of 60 A/dm ² . Figure 1 shows the EPMA line analysis of the Cr ^OX film at this time. Immediately after chromium plating, reverse electrolysis was performed in the same solution at a current density of 15 A/dm ² for 0.2 seconds. Figure 2 shows the EPMA line analysis of the Cr ^OX film at this time. After washing this chrome-plated steel plate thoroughly with water,
In a chromic acid aqueous solution containing 80g of CrO ₃ /, 4g of NaF / (5% by weight of CrO ₃ ) and a bath temperature of 50℃.
A first electrolytic chromic acid treatment was performed at a current density of 50 A/dm ² for 0.2 seconds. Figure 3 shows the EPMA line analysis of the Cr ^OX film at this time. Then,
Contains CrO ₃ 60g/, Na ₂ SiF ₆ 4g/, bath temperature 40
at a current density of 15 A/dm ² in an aqueous solution of chromic acid at
A second electrolytic chromic acid treatment was performed for 1.0 seconds. Figure 4 shows the EPMA line analysis of the Cr ^OX film at this time. In addition, 400x optical microscope photos are shown in the 6th section.
As shown in the figure. As a comparison, after chromium plating, it was immersed in the same solution for 5 seconds and then immersed for 1.0 seconds at a current density of 15 A/dm ² in a chromic acid bath containing 60 g of CrO _{3 and} 4 g of Na ₂ SiF 6 at a bath temperature of 40°C. Electrolytic chromic acid treatment was applied. Figure 5 shows line analysis of the Cr ^OX film at this time using EPMA, and Figure 7 shows an optical micrograph at 400x magnification. Looking at the results of line analysis, we can see that the products treated according to the present invention shown in Figures 3 and 4 are those after chrome plating (Figure 1) and those after reverse electrolysis (Figure 2). , after conventional chromic acid treatment (No. 5
It can be seen that there is less abnormal precipitation of Cr ^OX and the film is more uniform than in Figure). Furthermore, looking at the micrographs, it is clear that the Cr ^OX film formed by the present invention (Fig. 6) is homogeneous and even, but it is clear that the Cr OX film formed by the conventional electrolytic chromic acid treatment (Fig. 7) is uniform. It can be seen that many non-uniform irregularities (looking like black spots) are generated in the Cr ^OX film. Table 1 below shows the evaluation of the unevenness of the Cr ^OX film and the results of the T-peel test as described below for the above-mentioned inventive examples and comparative examples.

[Table] (Measurement of Cr ^OX amount) The prepared sample was immersed in a 300 g/caustic soda solution at 100°C for 10 minutes, and the Cr amount was quantified using a calibration curve based on the difference between the measured values before and after. The amount of Cr ^OX was measured using the fluorescent X-ray method (Xray40KV60m
A. Monitored for 60 seconds). (EPMA measurement) The Cr ^OX film was fixed with a carbon film, and then metallic chromium was dissolved with nital from the base steel side, and then placed on a clean platinum foil, and line analysis was performed using EPMA. Accelerating voltage: 20KV Electron beam intensity: 0.02μA Beam diameter: 2μ Next, the present invention will be specifically explained with reference to examples. Example 1 A cold-rolled steel plate with a thickness of 0.22 mm was electrolytically degreased in a 5% fomethazone solution at 80°C and at a current density of 15 A/dm ² for 10 seconds, then washed with water, and then degreased in 10% sulfuric acid at room temperature for 5 seconds. After immersion and washing with water, the main treatment was performed under the following conditions. Chromium plating treatment Bath composition: 250 g of CrO ₃ / 2.5 g of H ₂ SO ₄ / Bath temperature: 55° C. Electrolytic conditions: 50 A/dm ² × 1.5 seconds After the above chromium plating, a reverse electrolytic treatment was performed in the same solution under the following conditions. Reverse electrolysis conditions: 15A/dm ² ×0.2 seconds After this, the sample was washed with water and immediately subjected to electrolytic chromic acid treatment under the following conditions. Electrolytic chromic acid 1st treatment Bath composition CrO ₃ 80g / NaF 4g / Bath temperature 50℃ Electrolytic conditions 50A/dm ² × 0.2 seconds Electrolytic chromic acid 2nd treatment Bath composition CrO ₃ 60g / Na ₂ SiF ₆ 4g / Bath temperature 40℃ Electrolysis conditions: 15 A/dm ² ×1.0 seconds Immediately after the second electrolytic chromic acid treatment, samples were prepared by washing with water, further washing with hot water, and drying. Example 2 The same treatment as in Example 1 was performed except that electrolytic chromic acid treatment was performed under the following conditions. Electrolytic chromic acid 1st treatment Bath composition CrO ₃ 50g / HF 4g / Bath temperature 40℃ Electrolytic conditions 30A/dm ² × 0.2 seconds Electrolytic chromic acid 2nd treatment Bath composition CrO ₃ 60g / HBF ₄ 3g / Bath temperature 40℃ Electrolytic conditions 15 A/dm ² ×1.0 seconds Example 3 The same treatment as in Example 1 was performed except that electrolytic chromic acid treatment was performed under the following conditions. Electrolytic chromic acid 1st treatment Bath composition CrO ₃ 50g/ Na ₂ Cr ₂ O ₇ 10g/ NH ₄ F 5g/ Bath temperature 40℃ Electrolytic conditions 15A/dm ² × 0.2 seconds Electrolytic chromic acid 2nd treatment Bath composition CrO ₃ 60g/ 5g of Na ₂ AlF ₆ / Bath temperature 40℃ Electrolysis conditions 15A/dm ² × 1.0 seconds Comparative example 1 After the above chromium plating, immerse in the same solution for 5 seconds,
The same treatment as in Example 1 was performed except that the electrolytic chromic acid treatment was performed under the following conditions. Electrolytic chromic acid treatment Bath composition: 60 g of CrO ₃ / 4 g of Na ₂ SiF ₆ / Bath temperature: 40°C Electrolytic conditions: 15 A/dm ² × 1.0 seconds Comparative example 2 After the chromium plating described above, immerse in the same solution for 5 seconds,
The same treatment as in Example 1 was performed except that the electrolytic chromic acid treatment was performed under the following conditions. Electrolytic chromic acid 1st treatment Bath composition CrO ₃ 80g / NaF 4g / Bath temperature 50℃ Electrolytic conditions 50A/dm ² × 0.2 seconds Electrolytic chromic acid 2nd treatment Bath composition CrO ₃ 60g / Na ₂ SIF ₆ 4g / Bath temperature 40℃ Electrolysis conditions: 15 A/dm ² ×1.0 seconds A T-peel test was conducted on the stain-free steel sheet obtained as described above. 50±5mg/d on one side of each of the above stain-free steel plates
After painting the sizing with ^m2 and baking at 190℃ for 10 minutes,
Apply the same amount of gold paint to the remaining opposite side, 210
Baking was carried out for 10 minutes at ℃. After this, a nylon adhesive synthetic resin tape (thickness 100 μm, width 5
mm) and hot press at 190℃, 1Kg/
A plurality of test pieces were created by adhesion by pressing at cm ² for 30 seconds. These test pieces immediately after adhesion were subjected to a T-peel test using an Instron tensile tester at a constant tensile speed of 200 mm/min to measure the peel strength. Furthermore, using another test piece, 15 g of sucrose/
Contains sugar acid solution adjusted to PH3.3 with citric acid.
The peel strength after immersion at 70°C for 7 days was measured under the same conditions as above. All tests were conducted using five test pieces of each type. The test results are shown in Table 2 below, and the results are expressed as average values. The T-peel test value immediately after adhesion is used to judge the adhesion of the coating after painting. Furthermore, when performing retort processing such as high-temperature sterilization, if there is a large amount of Cr ^OX dissolved in the water that has penetrated the paint film, the adhesive strength between the paint film and Cr ^OX will decrease.
Retort processing resistance deteriorates. The T-peel test value after 7 days of immersion at 70°C is used for this evaluation. In addition, from the test results in Table 2, the stain-free steel sheets according to the present invention (Examples 1 to 3) have no unevenness in the Cr ^OX film and have excellent retort treatment resistance, but in the conventional examples 1 and 2, It is clear that unevenness occurs in the Cr ^OX film and the retort treatment resistance is also poor. As is clear from the above explanation, according to the present invention, a homogeneous stain-free steel sheet with a Cr ^OX film having good adhesion to paint and retort treatment resistance can be obtained.
This can be used for coffee cans and food cans for retort processing, as well as gallon cans and miscellaneous cans as long as they are adhesive cans. Further, the paint used for painting is not limited to a paint called sizing or gold, but any paint made of an organic solvent may be used, and the adhesive is not limited to only a silon type.

【table】 [Brief explanation of the drawing]

Figure 1 shows EPMA of Cr ^OX film after chrome plating.
Line analysis diagram, Figure 2 is an EPMA line analysis diagram of the Cr ^OX film after reverse electrolysis, Figure 3 is an EPMA line analysis diagram of the Cr ^OX film after the first electrolytic chromic acid treatment, and Figure 4 is an EPMA line analysis diagram of the Cr OX film after the first electrolytic chromic acid treatment. EPMA line analysis diagram of Cr ^OX film after 2 treatments, 5th
The figure shows the Cr ^OX film after conventional electrolytic chromic acid treatment.
EPMA line analysis diagrams, Figures 6 and 7
The figure is a 400x optical micrograph of the surface structure of a metal, FIG. 6 is a micrograph of the same sample as in FIG. 4, and micrographs of the same sample as in FIGS. 7 and 5.

Claims (1)

[Claims] 1 Using a chromium plating bath and an electrolytic chromic acid treatment bath separated from it by a water washing bath, a metallic chromium coating is formed on a steel plate, and a coating consisting mainly of chromium hydrated oxide formed over this is coated on a steel plate. In order to produce a stain-free steel sheet with chromium plating, the steel sheet is chromium plated in a chromium plating bath and then reverse electrolyzed in the chromium plating bath to remove the chromium hydrated oxide film formed during chromium plating. The auxiliary anions eutectoided in the chromium hydrated oxide film were eluted, and then thoroughly washed with water in a water washing bath. The first treatment is performed with electrolytic chromic acid in an aqueous chromic acid solution containing at least one type of compound as the main agent and a simple salt fluorine compound as an auxiliary agent, and then treated with chromic acid anhydride, chromate, and dichromate. A method for producing a stain-free steel sheet, which comprises performing a second electrolytic chromic acid treatment in an aqueous chromic acid solution using at least one selected compound as a main agent and a complex salt-based fluorine compound as an auxiliary agent.