JPH02111899A - Production of electrolytically chromated steel sheet having excellent weldability - Google Patents

Abstract

PURPOSE:To stably and economically produce the title electrolytically chromated steel sheet having excellent weldability by cathodically electrolyzing a steel sheet in a chromic acid bath added only with a fluorine compd., and dipping the sheet in a soln. contg. a specified amt. of chromic acid. CONSTITUTION:A steel sheet is degreased, pickled, and then cathodically electrolyzed in the chromic acid bath added only with a fluorine compd. or further added, as required, with a small amt. of sulfuric acid. As a result, a metallic chromium layer and a hydrated chromium oxide layer are formed on the sheet. The sheet is successively dipped in a soln. contg. >=25g/l of chromic acid for >=2sec. The easily soluble hydrated chromium oxide is dissolved and removed by the dipping, and a smoothly deposited metallic chromium layer of 45-90mg/m<2> and a uniform hardly soluble hydrated chromium oxide layer of 1-10mg/m<2> as chromium are formed on the sheet surface. By this method, an electrolytically chromated steel sheet having excellent weldability can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to food cans such as beverage cans, 9 food cans, or
181! This invention relates to a method for manufacturing electrolytic chromic acid treated steel sheets for welded cans used in the field of miscellaneous cans such as cans and art cans.

[Conventional technology]

Two metallic chromium layers and a chromium hydrated oxide layer on the steel plate surface.
Electrolytic chromic acid treated steel sheet with no film (hereinafter referred to as “r
(referred to as “S-CT”) is a single-serve beverage can.

In recent years, it has been increasingly used as a substitute for tinplate in the field of art cans, 18J cans, etc. This is TFS-C
This is because T is cheaper than tinplate and has excellent performance as a steel plate for use as a base for painting. However, when this TFS-CT is used as a material for welding cans, metallic chromium is used.

Immediately before welding, it is necessary to remove the film made of hydrated chromium oxide by means such as grinding.

Removing the film before welding has a negative impact on manufacturing costs and quality.
There are hygiene issues. TFS-CT that can be welded without polishing away the chromium aqueous oxide film and its manufacturing method (
Regarding the second, various methods have been proposed. For example, Special Publication No. 57-19752. Special Publication Showa 57-36986. JP-A-61-213398 and JP-A-63-186894 are already known.

Tokuko Sho 57-19752 has 3 to 4011 on the surface of the steel plate!
9/m2 metal chromium layer and chromium on top of it as 2~
It consists of a non-metallic chromium layer mainly composed of 15 mg/m2 of chromium oxide, and is characterized in that the metallic chromium layer is porous. However, if the amount of metallic chromium is simply reduced (and the metallic chromium layer is made porous), the corrosion resistance will decrease.

In addition, the heating applied to cure the paint applied to the surface oxidizes the surface of the steel sheet exposed through the bore 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 sulfate ion, nitrate ion,
In an aqueous solution containing chromic acid, monobichromic acid, etc. as the main component 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. The present invention relates to a method for manufacturing TFS-CT which is characterized by producing aqueous chromium oxide of mg/m2 and has excellent paintability, weldability, and workability. This is to reduce the amount of metallic chromium in order to improve weldability and workability, and to compensate for the resulting decrease in corrosion resistance by modifying the nonmetallic chromium layer, which is mainly composed of aqueous chromium oxide. However, from the viewpoint of weldability, as in the above-mentioned Japanese Patent Publication No. 57-19572, there are many bores in the metal chromium layer, and the exposed steel plate surface is oxidized by heating to cure the paint, reducing weldability. There is a risk.

JP-A No. 61-213398 discloses a steel plate surface (210 to 40
mg/m2 smooth metallic chromium layer and 3-30mg/m2
The present invention relates to a TFS-CT for welded cans that has a uniformly thick chromium hydrated oxide layer and has excellent paint corrosion resistance. The manufacturing method is characterized by dissolving a part of the precipitated metal chromium lfi by anodizing C2. However, using a known chromic acid bath,
It is extremely undesirable for the precipitated chromium metal at mg/m2 to continuously cover the surface of the steel sheet instead of in a porous manner, and the method of dissolving the precipitated chromium metal by anodizing may actually cause the chromium metal layer to spread. increase the bore of
This results in an increase in the exposure rate of the steel plate surface.

JP-A No. 63-186894 discloses that 50 to 150
mg/m2 metallic chromium layer and 5 to 20 mg/m2 as chromium.
A TF for welded cans having a chromium oxide film of p+g/m2 and having protrusions on a part of the metal chromium
This is related to S-CT-2. This was discovered based on the idea that a metallic chromium layer (having two protrusions) would reduce the contact electrical resistance value (hereinafter referred to as Rc value), and as a result, improve weldability. It cannot be said that there is a correlation between the Rc value and weldability, including the case where the precipitation form of chromium is different, such as smooth or uneven.

In addition, the metal chromium layer is extremely thick in the protruding parts, and extremely thin in the areas without protrusions, so in the thin parts of the metal chromium, the iron exposure rate becomes stone.

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,
At the same time, the technical idea is to provide corrosion resistance, paint corrosion resistance, and paint adhesion by modifying chromium hydrated oxide, so it is thought that no detailed study was conducted on factors that affect weldability. The evaluation should be made based on the difference between the current at which splashes begin to appear and the appearance deteriorates, and the current at which the welding starts to occur with sufficient joint strength. However, the weldability evaluation method described in the examples of the prior art is disclosed in JP-A-63
Unlike this evaluation method, all except -186894 may not necessarily evaluate weldability correctly.

[Problems to be solved by the present 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 TF disclosed in the prior art
This is because S-CT 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 to solve the problem]

In order to achieve the above object (2), the present invention forms a smooth metal chromium layer with a high coverage of 45 to 90 mg/m2 on a steel plate, and then dissolves easily soluble chromium hydrated oxide in the outermost layer. (From the second point, 1 to 10 μ/m2 as chromium)
This forms a uniform, slightly soluble chromium hydrated oxide layer.

The present invention will be explained in detail below.

The manufacturing method for TFS-CT is to precipitate metallic chromium on the surface of a steel sheet using a highly concentrated chromic acid bath containing 100 g/1 or more of chromic acid to which auxiliary agents such as fluorine compounds and sulfuric acid have been added, and then to precipitate metallic chromium on the surface of the steel sheet. A two-liquid method in which cathodic electrolysis is performed in a concentrated chromic acid bath to mainly form a chromium hydrated oxide film, and a chromic acid 100 with auxiliary agents such as fluorine compound 6F and acid added.
Two one-component methods are known in which μ'Ah electrolysis is carried out in a chromic acid bath of n/l or less, and a metallic chromium layer and a chromium hydrated oxide layer are simultaneously formed. A metallic chromium layer (there are two) in which hydrated chromium oxide is dissolved and removed using a hot alkaline solution, and the relationship between the metallic chromium layer and the exposure rate of the steel plate surface from the pores of the metallic chromium layer is as shown in Figure 1. , person IC
r0a-fluoride bath mountain] Cr0a-fluoride-sulfuric acid bath (
C) A metallic chromium layer with fewer bores is obtained in the order of Cr03-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 is used.

More preferably, it is preferable to use a chromic acid bath to which only a fluorine compound is added. A chromic acid bath to which a fluorine compound and sulfuric acid have been added has a slightly inferior rice coverage, but is superior in operability because it is less susceptible to the effects of impurity ions in the bath.

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 (2), if the coating is not heated during the coating process, the Re value, which is an indicator of the quality of weldability, will decrease as the chromium value decreases due to gold. ,
When heating at 210℃ for 20 minutes assuming 6-pack baking,
In a range where the amount of metallic chromium is 45 to 90 mg/m2,
It is estimated that the Rc value becomes small and that an optimum range exists for weldability. The reason for this is metal chrome! , ii is 45
A: g/m" or less, as shown in Figure 1, the iron exposure rate increases markedly as the lithium trichrome decreases, and this is thought to be due to the formation of iron oxide with poor conductivity on the surface of the R4 plate due to heating. .Also, metal chromium tyoga 90III9
/m2 or more, the exposed area of 4 steel strips decreases, but
It is thought that the ReC directivity increases because it is difficult for the thick metallic chromium layer to cause -41 damage due to the extreme roll.

Figure 3 shows the amount of metallic chromium and the T after heating at 210°C for 20 minutes when the amount of soluble chromium hydrated oxide film is constant.
This shows the relationship between the weldable current range during welding of F S -CT, and this range is defined as the upper limit of the welding current at 'tK, where the welding starts to occur during welding, and the welding current at which sufficient joint strength begins to be obtained. This was determined as the lower limit. The wider the welding range, the better the weldability.

Welding b] Noryu'l There is a correlation between the width of the At range and the Rc value as shown in FIGS. 2 and 3. However, when the precipitation form of the metallic chromium layer is changed.

For example, a TF having a protrusion on a part of the metallic chromium layer obtained by the method disclosed in Japanese Patent Application Laid-Open No. 63-186894.
Although the Re value of S-CT is quite small, the weldable current range is narrow. Although the reason for this is not clear, only TFS-CT with a smooth metal chromium layer has a good correlation between the Rc value and the weldable current range.

As explained above, the weldability of the present invention (- soaked TFS
-The CT gold [114 chromium layer must be smooth, thin, and have a high coverage to prevent oxidation of the exposed steel plate surface during heating and to be easily destroyed by the pressure of the 7i pole roll during welding. do.

Next, in the TFS-CT manufacturing method of the present invention, the hydrated chromium oxide formed on the smooth metal chromium layer with a coverage of 45 to 90 mg/m2 is a chromium hydrate that is poorly soluble in the electrolyte. The chromium hydrated oxide layer has excellent uniformity (-) and excellent weldability, so dipping is required after electrolysis. It consists of a substance that is easily soluble in the electrolytic solution, and a substance that is poorly soluble in the electrolytic solution that has a low content of auxiliary substances.The outermost layer of easily soluble chromium hydrated oxide is Since the amount of chromium produced tends to fluctuate depending on the electrolytic conditions such as current density and the stirring state of the liquid, uneven precipitation tends to occur in the width and length directions of the steel sheet.On the other hand, the poorly soluble chromium below The generation of hydrated oxides is almost constant regardless of the electrolytic conditions or stirring conditions, and it is even more convenient that even if the electric product is increased, it will not be generated beyond a certain thickness, so , uniformity in the longitudinal direction (excellent).Moreover, from a more detailed perspective, the difference in the thickness of poorly soluble chromium hydrated oxide for each crystal orientation of steel observed with an electron microscope is due to the difference in thickness of chromium hydrate immediately after electrolysis It has excellent uniformity compared to the difference in thickness depending on the crystal orientation of hydrated oxide.In this way, the chromium hydrated oxide layer immediately after electrolysis has a uniform thickness. On top of the,
Since the easily soluble chromium hydrated oxide layer is formed with a non-uniform thickness, the thickness becomes non-uniform as a whole, which is undesirable because it causes local heat generation during beach contact. Therefore, even with the same amount of chromium hydrated oxide, TFS-C immediately after electrolysis
'1' F8-CT, which is sufficiently immersed in electrolytic solution after electrolysis to dissolve and remove easily soluble chromium hydrated oxide, has significantly better weldability than T. It was found that stable weldability could not be obtained by simply specifying the amount of chromium hydrated oxide as described above.In order to obtain hardly soluble chromium hydrated oxide, it is necessary to
It is necessary to immerse the electrolyte solution for more than two seconds. As shown in Figure 4, easily soluble chromium hydrated oxide is
Most of it dissolves within 2 seconds, leaving only barely soluble chromium hydrated oxide. As the solution for dissolving the easily soluble chromium hydrated oxide, a chromic acid solution having an electrolytic solution weight of 1:25 g/1 or more can be used. Although a higher temperature is preferable, easily soluble chromium hydrated oxide can be dissolved even at room temperature.

Methods for removing easily soluble chromium hydrated oxide include 1. I can think of other methods other than dipping, but what about using an anode? The dissolution method using F solution involves dissolving the underlying metallic chromium layer,
It is not suitable because it reduces the uniform coverage of Kanayama chromium.

The amount of poorly soluble chromium hydrated oxide is better to be small from the viewpoint of weldability, but it cannot be lowered to less than 1 g/m2B (-) as chromium. m2.Therefore, the amount of resolvable chromium hydrated oxide is 1 = 10 m2.
g/m2.

As described above, the hardly soluble chromium hydrated oxide film obtained by the manufacturing method of the present invention collapses uniformly, so it is thought that the distribution of welding current becomes uniform and weldability is improved.

[Examples] - Hereinafter, the content of the present invention will be specifically explained using Examples and Comparative Examples.

0.22. ml 11 thick cold rolled steel plate C2 After normal degreasing and pickling, the manufacturing method of the present invention (from 2, Examples 1 to 5)
A TFS-CT with C=indicated was manufactured. At the same time, according to the representative examples of the prior art, Comparative Examples 1 to 4 (two shown TFS-
CT was manufactured. These TFS-CT skins l1lIi
As is clear from Table 1, which shows t, weldability, corrosion resistance, etc.
2. The amount of poorly soluble chromium hydrated oxide is 1 to 10 as chromium.
The TPS-CTs of Examples 1 to 5, which were in the range of mg/m2 and produced according to the manufacturing method of the present invention, had a low iron exposure rate in the bore of the aluminum layer and a low iron exposure rate in the chromium hydrated oxide layer. Due to its excellent uniformity C2, 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 manufactured according to the manufacturing method disclosed in the Examples of Japanese Patent Publication No. 57-36986. Comparative example 3 is JP-A-61
-213398 according to the manufacturing method disclosed in Example. Comparative example 4 is JP-A-63-1868
It was manufactured according to the manufacturing method disclosed in Example No. 94, and a part of the metal chromium 1- is protruding.

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 the easily soluble chromium hydrated oxide was not dissolved and removed by immersion after electrolysis, so it had a sufficient welding range. I haven't.

Next (2) Evaluation methods for the evaluation items in Table 1 showing Examples and Comparative Examples (Two are described below.

(1) Iron from the bore of metal chromium j Zeng]! &Rate 95
+ in 7.5 N-NaOH solution at °C: TFS-C,T
was soaked for 5 minutes to completely dissolve and remove the outermost layer of chromium hydrate. Subsequently, 30 mm of the sample was anodically polarized in the remaining sample v -/l/L/, IM-Na)fzPO4 solution at a polarization rate of 125 mV/1 nin, and the current immediately before passivation was read. The iron exposure rate was determined from a calibration curve of the iron exposure rate and passivation current, which were determined separately.

(2) Weldable current range After baking TFS-eT at 210°C for 20 minutes, use a cutting board welding machine for actual testing at a frequency of 60I (z, lap width 0.4 mm).

After welding was performed under the conditions of speed W 5 m/min, pressing force 50, and 9 degrees, the joint state of the welded portion was evaluated. Welding current 25
The welding magnetic current is increased in λ increments, and the welding current is set at 1d at which splash begins to occur, and the welding magnetic current is lowered to the welding current just before the welding strength becomes lower than the breaking strength of the base metal. As the lower limit, this difference in current was set as the range of current that can be connected to one volume.

(3) Contact electrical resistance (Re) After testing the TFS-CT at 210° C. for 20 minutes, Re was measured using a hand-made contact electrical resistance measuring device. The measurement method is to sandwich two sample plates fixed at the contact site of two copper disc electrodes, and move the sample plates by rotating at a circumferential speed of 51 minutes while applying pressure between the electrodes with 50:9 blood. At the same time, a DC current of 5 A was passed between the electrodes, and the voltage between the electric bridges was measured to determine the Rc value.

(4) Approximately 60% epoxy/phenol paint is applied to filamentous corrosion TFS-CT.
mg/dm” and baked at 210°C for 10 minutes,
Insert a cross cut and test 5-false with an Erichsen tester. After immersing the sample in 3% NaC7:, stand it on a sample stand and place it in a 3"A room at a temperature of 45°C and a humidity of 85%.
On day 0, the specimen was allowed to stand and the state of rust formation was observed.

(5) Salt water spray wiping The TFS-CT was heated at 210° C. for 10 minutes, and then placed in a salt water spray λ1 tester. 3% NaCJ solution at 35°C
The state of rust formation was evaluated when exposed to a lagoon for hours.

(6) Coated plate C1 width 10 μm obtained by the same method as the filamentous corrosion test on paint film
, make a cross cut with a depth of 15 μm, and add 1.5 μm of citric acid.
% and 1.5% common salt for 2 weeks at 38°C, and the corrosion state of the cut portion was evaluated. [Effects of the Invention] It has excellent coating properties, excellent uniformity of the chromium hydrated oxide film, and extremely excellent weldability, making it possible to apply it to a wide range of applications as a material for welded cans, which is cheaper than tinplate, and has become an industrial material. The above effect is extremely large.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the iron exposure rate from the bore of the metal chromium layer and the metal chromium penetration after the chromium hydrated oxide film on the surface layer of TFS-CT is dissolved and removed with a heated alkaline solution. Figure 2 shows the relationship between the contact electrical resistance (Rc) value and metal chromium rats before and after heating at 210°C for 20 minutes. A diagram showing the relationship between the weldable current range and the metal chromium grave after heating at ℃ for 20 minutes, and Figure 4 is a diagram showing an example of the rate of dissolution of the chromium hydrated oxide film into the electrolytic solution after electrolysis. be. 21 Kenta Mound 2 Rula θ Walking

Claims (2)

[Claims] (1) After degreasing and pickling the steel plate, and cathodic electrolyzing in a chromic acid bath containing only fluorine compounds, continue for 2 seconds or more.
By immersing it in a solution containing 25 g/l or more of chromic acid to dissolve easily soluble chromium hydrated oxide,
A smooth metallic chromium layer of 45-90 mg/m^2 deposited on the surface of the steel plate and 1-10 mg/m^2 as chromium.
A method for producing an electrolytically chromic acid-treated steel sheet with excellent weldability, characterized by forming a hardly soluble chromium hydrated oxide layer. (2) After degreasing and pickling the steel plate, cathodic electrolysis was performed in a chromic acid bath containing a fluorine compound and a small amount of sulfuric acid, and then
The easily soluble hydrated chromium oxide was immersed in a solution containing 25 g/l or more of chromic acid for at least 2 seconds to dissolve it, resulting in a smooth deposit of 45 to 90 mg/m^2 on the surface of the steel sheet. Metallic chromium layer and 1 to 10 mg of chromium/
A method for producing an electrolytically chromic acid-treated steel sheet with excellent weldability, characterized by forming an insoluble chromium hydrated oxide layer of m^2.