JPH0527720B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention is applicable to the production of electrolytic chromic acid treated steel sheets for welded cans used in the fields of food cans such as beverage cans and food cans, and miscellaneous cans such as 18 cans and art cans. Regarding the method. [Prior art] Electrolytic chromic acid treated steel sheets (hereinafter referred to as TFS-CT), which have a two-layer film of a metallic chromium layer and a chromium hydrated oxide layer on the steel sheet surface, are used for beverage cans, food cans, art cans, 18 cans, etc. In recent years, it has been increasingly used as a substitute for tinplate in the field. this is,
TFS-CT is cheaper than tinplate, and
This is because it has an excellent function as a steel plate for painting base. However, when this TFS-CT is used as a material for welded cans, it is necessary to remove the film consisting of metallic chromium and chromium hydrated oxide by mechanical grinding or the like immediately before welding.
Removing the film before welding poses problems in terms of manufacturing cost, quality, and hygiene. Various methods have been proposed for weldable TFS-CT and its manufacturing method without removing the chromium hydrated oxide film by polishing. For example, Tokuko Sho 57-19752, Tokuko Sho 5-
36986, JP-A-61-213398, and JP-A-63-186894 are already known. Tokuko Sho 57-19752, 3 to 40 mg/m ² on the steel plate surface.
2 to 15 as a metallic chromium layer and chromium on top of it
It consists of a non-metallic chromium layer mainly composed of mg/m ² of chromium oxide, and is characterized by making the metallic chromium layer porous. However, if the amount of metallic chromium is simply reduced in this way and the metallic chromium layer is made porous, corrosion resistance decreases. In addition, the heating applied to cure the paint applied to the surface oxidizes the surface of the steel sheet exposed through the pores of the metal chromium layer, so the intentionally exposed surface of the steel sheet does not contribute to improving weldability. There are drawbacks. Special Publication No. 57-36986 is for sulfate ions, nitrate ions,
In an aqueous solution containing chromic acid, dichromic acid, etc. as the main ingredients without intentionally adding anions such as chlorine ions, 0.5 to 30 mg/ ^m2 of metallic chromium and 2 to 50 mg/m2 of chromium are added to the surface of the steel sheet. paintability, weldability, characterized by the production of ^m2 of chromium hydrated oxide,
The present invention relates to a method for manufacturing TFS-CT with excellent processability. This reduces the amount of metallic chromium in order to improve weldability and workability, and compensates for the resulting decrease in corrosion resistance by modifying the non-metallic chromium layer, which is mainly composed of hydrated chromium oxide. . However, from the viewpoint of weldability, the aforementioned
-Like 19572, there are many pores in the metal chromium layer,
Heating to cure the paint may oxidize the exposed surface of the steel plate, reducing weldability. JP-A No. 61-213398 discloses 10 to 40 mg/m ² on the steel plate surface.
The present invention relates to TFS-CT for welded cans, which has a smooth metallic chromium layer and a chromium hydrated oxide layer with a uniform thickness of 3 to 30 mg/m ² and has excellent paint corrosion resistance. The manufacturing method is characterized by dissolving a part of the deposited metal chromium layer by anodizing. However, using a known chromic acid bath, it is extremely difficult to coat the surface of a steel plate continuously, rather than in a porous manner, with 10 to 40 mg/m ² of metallic chromium precipitated, and once precipitated metallic chromium is anodized. The method of dissolving the metal chromium layer instead increases the number of pores in the metal chromium layer and increases the exposure rate of the steel sheet surface. In JP-A-63-186984, 50 to 150mg/
m ² of metallic chromium layer and 5-20 mg/m ² as chromium
TFS for welded cans, which has a chromium oxide film and has protrusions on a part of the metal chromium.
- Concerning CT. This was discovered based on the idea that if the metal chromium layer has protrusions, the contact electrical resistance value (hereinafter referred to as Rc value) will decrease, resulting in improved weldability.
Including cases where the precipitation form of metallic chromium is different, such as smooth or protruding, it cannot be said that there is a correlation between the Rc value and weldability. Furthermore, the metallic chromium layer is extremely thick in the protruding parts, and extremely thin in the non-protruding parts, so iron exposure is high in the thin metallic chromium parts. Although a method for manufacturing TFS-CT for welded cans has been disclosed, the fact that it has not yet been put into practical use suggests that there are various problems as a material for welded cans. Conventional technology improves weldability by reducing metallic chromium, and at the same time improves corrosion resistance, paint corrosion resistance, and paint adhesion by modifying chromium hydrated oxide, so factors that affect weldability It is thought that no detailed consideration was given to this. In addition, weldability should be evaluated based on the width of the weldable current range, which is the difference between the current at which splashes begin to appear and the appearance deteriorates, and the current at which sufficient joint strength begins to be obtained. but,
All of the weldability evaluation methods described in the examples of the prior art, except for JP-A-63-186894, differ from this evaluation method and may not necessarily evaluate weldability correctly. [Problems to be Solved by the Invention] As mentioned above, various efforts have been made to put TFS-CT for welded cans into practical use, but nothing satisfactory has been achieved. The reason for this is that the TFS-CT disclosed in the prior art did not have a sufficiently wide weldable current range to be put to practical use as a material for welding cans. The object of the present invention is to determine the optimal coating structure for obtaining a sufficiently wide weldable current range, and to provide a stable and economical manufacturing method thereof. [Means for Solving the Problems] In order to achieve the above object, the present invention forms a smooth metal chromium layer with a high coverage of 45 to 90 mg/m ² on a steel plate, and then 1 as chromium by dissolving soluble chromium hydrate oxide
It forms a uniform, hardly soluble chromium hydrated oxide layer of ~10 mg/m ² . The present invention will be explained in detail below. The manufacturing method for TFS-CT is to deposit metallic chromium on the steel plate surface using a high concentration chromic acid bath containing 100 g or more of chromic acid containing auxiliary agents such as fluorine compounds and sulfuric acid, and then use a low concentration chromium bath containing the same auxiliary agents. The two-component method involves cathodic electrolysis in a chromic acid bath to mainly form a chromium hydrated oxide film, and the cathodic electrolysis in a chromic acid bath containing up to 100 g of chromic acid with auxiliary agents such as fluorine compounds and sulfuric acid added. Two one-component methods are known in which a metallic chromium layer and a chromium hydrated oxide layer are formed simultaneously. Looking at the relationship between the amount of metal chromium and the rate of exposure of the metal chromium from the pores on the surface of the steel sheet from the pores of the metal chromium layer in which the hydrated chromium oxide was dissolved and removed using a hot alkaline solution, as shown in Figure 1, (A) CrO ₃ - fluoride bath (B) CrO ₃ - fluoride - sulfuric acid bath
(C) A metallic chromium layer with fewer pores is obtained in the order of CrO ₃ -sulfuric acid bath. Therefore, in the present invention, in order to deposit a metallic chromium layer with a high coverage on a steel plate, a chromic acid bath containing a fluorine compound and sulfuric acid, more preferably a chromic acid bath containing only a fluorine compound, is used. Use is preferred. Although the chromic acid bath containing fluorine compounds and sulfuric acid has a slightly inferior coverage, it is not easily affected by impurity ions in the bath.
It has excellent operability. The TFS-CT of the present invention is provided as a material for welded cans, and is always used after being painted. As shown in Figure 2, if the paint is not heated during the baking process, the Rc value, which is an indicator of the quality of weldability, will be:
It becomes lower as the amount of metallic chromium decreases. However, when heating is performed at 210℃ for 20 minutes assuming paint baking, the Rc value decreases in the range of 45 to 90mg/ ^m2 of metallic chromium, and it is estimated that there is an optimal range for weldability. Ru. The reason for this is that when the amount of metallic chromium is below 45mg/ ^m2 , as shown in Figure 1, the iron exposure rate increases significantly as the metallic chromium decreases, and iron oxide with poor conductivity is formed on the steel plate surface due to heating. This is thought to be for the purpose of In addition, when the amount of metallic chromium is 90 mg/m2 ^or more, the exposed area of the steel plate decreases, but it is thought that the Rc value increases because the thick metallic chromium layer is less likely to cause minute cracks due to the pressure applied by the electrode roll. It will be done. Figure 3 shows the relationship between the amount of metallic chromium and the weldable current range during welding of TFS-CT after heating at 210℃ for 20 minutes, with the amount of hardly soluble chromium hydrated oxide film being constant. This range is determined by setting the current at which splash begins to occur during welding as the welding upper limit, and the welding current at which sufficient joint strength begins to be obtained as the welding lower limit. This indicates that the wider the welding range, the better the weldability. As can be seen from FIGS. 2 and 3, there is a correlation between the width of the weldable current range and the Rc value. However, when the precipitation form of the amount of metallic chromium is changed, for example, the Rc value of TFS-CT with protrusions on a part of the metallic chromium layer obtained by the method disclosed in JP-A-63-186894 is Although it is quite small, the weldable current range is narrow. The reason for this is not clear, but TFS with a smooth metallic chromium layer
Only in -CT, a good correlation is observed between the Rc value and the weldable current range. As explained above, the present invention has excellent weldability.
The metallic chromium layer of TFS-CT needs to be smooth, thin, and have a high coverage so that it prevents oxidation of the exposed steel plate surface during heating and is easily destroyed by the pressure of the electrode roll during welding. Next, in the TFS-CT manufacturing method of the present invention, the chromium hydrated oxide formed on the smooth metal chromium layer with a high coverage of 45 to 90 mg/ ^m2 is a chromium hydrated oxide that is poorly soluble in the electrolyte. Because it has excellent uniformity and excellent weldability, it requires dipping after the electrode.
Immediately after electrolysis, the chromium hydrated oxide layer consists of a layer on the outermost surface that is easily soluble in an electrolytic solution with a high content of auxiliary agents, and a layer below that that is hardly soluble in an electrolytic solution with a low content of auxiliary agents. has been done. The amount of easily soluble hydrated chromium oxide in the outermost layer fluctuates depending on the electrolytic conditions such as bath composition, bath temperature, and current density, as well as the stirring state of the liquid. Easily causes non-uniform precipitation. On the other hand, the amount of poorly soluble chromium hydrated oxide underneath is almost constant regardless of electrolytic conditions or stirring conditions, and even more conveniently, even if the amount of electricity is increased, it remains constant. Because it does not form in a thickness greater than , it has excellent uniformity in the width and length directions. In addition, from a more detailed perspective, the difference in thickness of poorly soluble hydrated chromium oxide depending on the crystal orientation of steel observed with an electron microscope is the difference in thickness depending on the crystal orientation of hydrated chromium oxide immediately after electrolysis. It has superior uniformity compared to . In this way, the chromium hydrated oxide layer immediately after electrolysis forms a readily soluble chromium hydrated oxide layer with a non-uniform thickness on a slightly soluble chromium hydrated oxide layer with a uniform thickness. This is not preferable because it results in a non-uniform thickness as a whole and causes local heat generation during welding. Therefore, even with the same amount of chromium hydrated oxide, TFS-CT immediately after electrolysis
TFS-CT, which is sufficiently immersed in an electrolytic solution after electrolysis to dissolve and remove easily soluble chromium hydrated oxide, has significantly better weldability than the conventional technology. It was found that stable weldability could not be obtained by specifying the amount of chromium hydrated oxide. In order to obtain a hardly soluble chromium hydrated oxide, it is necessary to immerse the material in the electrolytic solution for at least 2 seconds or more after electrolysis. As shown in FIG. 4, most of the easily soluble hydrated chromium oxide dissolves within 2 seconds after the end of electrolysis, leaving only the slightly soluble hydrated chromium oxide. In addition to the electrolytic solution, a chromic acid solution containing 25 g or more can be used as the solution for dissolving the easily soluble hydrated chromium oxide. Although a higher temperature is preferable, easily soluble chromium hydrated oxide can be dissolved even at room temperature. Methods other than immersion can be considered to remove easily soluble chromium hydrated oxides, but the dissolution method using anodic electrolysis dissolves the underlying metallic chromium layer and improves the uniform coverage of metallic chromium. It is not appropriate because it lowers the From the viewpoint of weldability, it is better to have a smaller amount of poorly soluble chromium hydrated oxide, but it cannot be lowered to less than 1 mg/m ² of chromium. Even if the electrolysis conditions are changed or the electrolysis is carried out for a long time, the amount will not exceed 10 mg/m ² . Therefore, the amount of poorly soluble chromium hydrated oxide is defined as 1 to 10 mg/m ² . As described above, since the hardly soluble chromium hydrated oxide film obtained by the manufacturing method of the present invention has excellent uniformity, it is thought that the welding current distribution becomes uniform and weldability is improved. [Examples] The contents of the present invention will be specifically explained below using Examples and Comparative Examples. After subjecting a 0.22 mm thick cold-rolled steel plate to normal degreasing and pickling, TFS-CTs shown in Examples 1 to 4 were manufactured by the manufacturing method of the present invention. At the same time, according to representative examples of the prior art, as shown in Comparative Examples 1 to 4
TFS-CT was manufactured. Table 1 shows the coating amount, weldability, corrosion resistance, etc. of these TFS-CTs. As is clear from Table 1, the amount of metallic chromium is 45 to 90
mg/m ² , the amount of hardly soluble chromium hydrated oxide is in the range of 1 to 10 mg/m ² as chromium, and the TFS-CTs of Examples 1 to 4 were produced according to the production method of the present invention.
has a low iron exposure rate through the pores of the metal chromium layer and excellent uniformity of the chromium hydrated oxide layer, so it has a sufficiently wide weldable current range and has excellent weldability and corrosion resistance. Comparative Example 1 was manufactured according to the manufacturing method disclosed in the Examples of Japanese Patent Publication No. 57-19752. Comparative Example 2 was produced according to the production method disclosed in the Examples of Japanese Patent Publication No. 57-36986. Comparative Example 3 was manufactured according to the manufacturing method disclosed in the examples of JP-A No. 61-213398. Comparative Example 4 was manufactured according to the manufacturing method disclosed in the example of JP-A-63-186894, and a portion of the metal chromium layer was protruded. All of these seem to have no practical problems in terms of corrosion resistance, except for Comparative Example 2, but do not have a sufficient welding range. Comparative example 5
was manufactured using the same bath composition and electrolytic conditions as Example 2, but did not have a sufficient welding range because easily soluble chromium hydrated oxide was not dissolved and removed by immersion after electrolysis. . Next, the evaluation method for the evaluation items in Table 1 showing Examples and Comparative Examples will be described. (1) Iron exposure rate from the pores of the metallic chromium layer
Soak for a minute to completely dissolve and remove the outermost layer of chromium hydrate. Subsequently, the sample was sealed with tape, leaving 30 mmφ of the sample, and the current immediately before passivation was read when the sample was anodically polarized in a 1M-NaH ₂ PO ₄ solution at a polarization rate of 125 mV/min. From the calibration curve of iron exposure rate and passivation current obtained separately,
The iron exposure rate was determined. (2) Weldable current range After baking the TFS-CT at 210℃ for 20 minutes, weld it using an experimental plate welding machine at a frequency of 60Hz, a lap width of 0.4mm, a speed of 5m/min, and a pressure of 50kg. After that, the joint condition of the welded part was evaluated. Increase the welding current in 25A increments, and set the current at which splash begins to occur as the welding upper limit, and lower the welding current, and set the welding lower limit as the current just before the welding strength becomes lower than the breaking strength of the base metal. This difference in current was defined as the weldable current range. (3) Contact electrical resistance (Rc) After baking TFS-CT at 20°C for 20 minutes, Rc was measured using a hand-made contact electrical resistance measuring device. The measurement method is
Two stacked sample plates are sandwiched between the contact areas of two copper disc electrodes, and while the electrodes are being pressurized with 50 kg, the sample plate is rotated at a circumferential speed of 5 m/min. A DC current was applied to measure the voltage between the electrodes, and the Rc value was determined. (4) Filiform corrosion Approximately 60 mg/dm ² of epoxy/phenol paint is applied to TFS-CT, baked at 210℃ for 10 minutes, a cross cut is inserted, and an Erichsen tester is used to extend it 5 mm. After immersing the sample in 3% NaCl, stand it on a sample stand and keep it at a temperature of 45℃ and a humidity of 85℃.
% atmosphere for 10 days, and the state of rust formation was observed. (5) Salt water spray test After heating TFS-CT at 210℃ for 10 minutes, place it in a liquid water spray tester. The state of rust generation was evaluated when a 3% NaCl solution at 35°C was sprayed for 1 hour. (6) Corrosion under the paint film On a painted board obtained using the same method as the filamentous corrosion test,
Insert a cross cut with a width of 10 μm and a depth of 15 μm,
38 in a corrosive liquid consisting of 1.5% citric acid and 1.5% salt.
It was immersed at ℃ for 2 weeks and the corrosion state of the cut portion was evaluated.

【table】

[Table] Notes: Ratings ◎ Excellent ○ Excellent △ Average × Poor [Effects of the invention] The TFS produced by the production method of the present invention has excellent coverage of metallic chromium and has a chromium hydrated oxide film. Because it has excellent uniformity and excellent weldability, it can be used in a wide range of applications as a material for welded cans, which is cheaper than tinplate, and has an extremely large industrial effect.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the iron exposure rate from the pores of the metallic chromium layer and the amount of metallic chromium after the chromium hydrated oxide film on the surface of TFS-CT is dissolved and removed with a heated alkaline solution. The figure shows the relationship between the contact electrical resistance (Rc) value and the amount of metallic chromium before and after heating at 210℃ for 20 minutes. in
FIG. 4 is a diagram showing the relationship between the weldable current range and the amount of metallic chromium after heating for 20 minutes, and FIG. 4 is a diagram showing an example of the rate of dissolution of the chromium hydrated oxide film into the electrolytic solution after electrolysis.

Claims (1)

[Claims] 1 After degreasing and pickling, the steel plate is subjected to cathodic electrolysis in a chromic acid bath containing only fluorine compounds, and then immersed in a solution containing 25 g/or more of chromic acid for at least 2 seconds to remove easily soluble chromic acid. By dissolving chromium hydrated oxide, a smooth metallic chromium layer of 45 to 90 mg/m ² is precipitated on the steel sheet surface, and a hardly soluble chromium hydrated oxide layer of 1 to 10 mg/m ² of chromium. A method for manufacturing an electrolytically chromic acid treated steel sheet with excellent weldability, characterized by forming a chromic acid-treated steel sheet.