JPH0324296A - Electrolytic chromated steel sheet for welded can - Google Patents

Abstract

PURPOSE:To improve the weldability of the electrolytic chromated steel sheet by remarkably reducing the carbon content of the base steel sheet in the chromated steel sheet having the double coating films of a metallic chromium layer and a chromium hydroxide layer on its surface. CONSTITUTION:The surface of a mild steel sheet is degreased with alkali, pickled and cleaned. The sheet is then dipped in an aq. soln. contg. CrO3 as the essential component, a fluorine compd. such as NaF and a small amt. of sulfuric acid and electrolyzed with the sheet as a cathode. The sheet is then dipped for >=2sec in an aq. soln. contg. >=25g/l CrO3. The easily soluble Cr hydroxide on the sheet surface is dissolved to deposit a smooth metallic chromium layer on the sheet surface at 45-90mg/m<2> and simultaneously to form a difficulty soluble chromium hydroxide layer at 1-10mg/m<2> as chromium. In this case, the carbon content of the base sheet is reduced to 0.001-0.02%, and the difficult weldability of the electrolytic chromated steel sheet as its defect is drastically improved.

Description

[Detailed description of the invention] [1! ! Industrial application field] This Hatsuro is used for food cans such as beverage cans and food cans, or
This invention relates to an electrolytic chromic acid treated steel sheet for welded cans used in fields related to miscellaneous cans such as 1st cans and fine art cans.

[Conventional technology]

Two metallic chromium layers and a chromium hydrated oxide layer on the surface of the copper plate.
Electrolytic chromic acid treated steel sheet with layer film (hereinafter referred to as TFS-
CT) has been increasingly used in recent years as a substitute for tinplate in the fields of beverage cans, food cans, art cans, 18/cans, etc. This is because TFS-CT is cheaper than tinplate and also has an excellent function as a steel plate for coating. However, this TFS-C
When T 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 ε2 film before welding poses problems in terms of can manufacturing cost, quality, and hygiene. The chromium hydrated oxide film cannot be removed by polishing (2. Weldable TF8-CT and its manufacturing method {2) Various methods have been proposed. For example,
Japanese Patent Publications No. 57-19752, No. 57-36986, No. 61-213398, No. 63-186894, etc. are already known. Special Publication Showa 57-19752 is a steel plate surface {: 3~40m9
/m2 of Jinma chromium layer and 2 to 1 chromium on top of it.
It consists of a non-metallic chromium layer mainly composed of 5 mg/m'' chromium oxide, and is characterized by having a bolus of metallic chromium layer.
If the amount of metallic chromium is simply reduced and the metallic chromium layer is made into a bolus of 12, the corrosion resistance will decrease.

In addition, the heating applied to cure the paint applied to the surface (2) is intentional because the surface of the steel plate protruding from the pores of the metal chromium layer is oxidized; 1) the exposed steel plate surface improves weldability. {: has the disadvantage of not contributing.

Special Publication No. 57-36986 is sulfate ion, nitrate ion,
In an aqueous solution mainly composed of chromic acid, dichromic acid, etc., without intentionally adding C= anions such as chlorine ions, 0.0% is applied to the steel plate surface. Paintability, weldability, workability C: Excellent TF8- This is a CT manufacturing method that reduces the amount of metallic chromium in order to improve weldability and workability, and reduces the corrosion resistance caused by non-metallic materials mainly consisting of chromium permanent oxide. This is compensated for by modifying the chromium layer.However, from the viewpoint of weldability, as in the aforementioned Japanese Patent Publication No. 57-19572, there are many bores in the metallic chromium layer, which are exposed by heating for curing the paint. The surface of the copper plate may be oxidized and weldability may deteriorate.

JP-A No. 61-213398 discloses a steel plate surface (210-4
0 ms/m2 smooth metallic chromium layer and 3~30 mg/m2
This invention relates to TFS-CT for welded cans, which has a permanent chromium oxide layer with a uniform thickness of m2 and has excellent paint corrosion resistance. The manufacturing method is characterized by dissolving a part of the deposited Gold Island chromium layer by anodizing. However, using a known chromic acid bath,
It is extremely difficult to coat the surface of a steel plate with 0 mg/m'' of metallic chromium precipitated in a continuous manner rather than in a bolus form. This results in an increase in the pores of the metal chromium layer and an increase in the exposure rate on the surface of the steel plate.

JP-A No. 63-186894 discloses that 50 to 15
0 mg/m2 metal chromium layer and 5~ as chromium
20 m9/m2 of chromium oxide film and gold J
A part of l4 chromium <<This relates to TF8-CT for welded cans, which is characterized by having two protrusions. This was discovered based on the idea that if the metal 7 ROM layer has protrusions, the contact electrical resistance value (hereinafter referred to as Rc value) will decrease, and as a result, weldability will improve. Including cases where the precipitation form of metallic chromium is different, such as smooth or protruding, it cannot be concluded that there is a correlation between the Re value and weldability ζ. Furthermore, the metallic chromium layer is extremely thick in the protruding parts, and extremely thin in the non-protruding parts, so the iron exposure rate 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.

The conventional technology improves weldability by reducing the amount of metallic chromium or by forming a metallic chromium layer, and at the same time improves chromium permanent oxide (technical idea is to provide corrosion resistance, paint corrosion resistance, and paint adhesion). However, from the viewpoint of oxidation of the surface of the steel sheet during heating, it is thought that sufficient detailed consideration was not necessarily given to factors affecting weldability (1) and (2).

In addition, as mentioned above, all of the conventional technologies have TF8-C
Improvement of the coating structure and coating form of T (2) This is to improve weldability, and carbon g in steel, which has a large effect on weldability,
ut = I wasn't paying attention at all. However, the effect of the amount of carbon in the play is not so noticeable in the case of surface-treated steel sheets with excellent weldability such as furiki. In the case of hard-to-weld, surface-treated steel sheets such as is within the range of
There were cases where the carbon content in the steel exceeded the range in which good weldability could be obtained.

[Problems to be solved by the present invention]

As mentioned above, a number of efforts have been made to put TF8-CT for welded cans into practical use, but it has not been possible to find a material with a wide enough weldable current range to be put to practical use as a material for welded cans. It has not been done.

The present invention is based on determining the carbon content in the play and the optimal film structure of TF8-CT to obtain a sufficiently wide weldable current range. The purpose is to provide steel plates.

[Means to solve the problem]

In order to achieve the above object, the present invention uses a cold-rolled steel plate with a carbon content in the range of 0.001 to 0.0296 (2), and uses a 45% carbon steel plate with a smooth and high coverage rate on the steel plate. ~9
Q mg/m2 gold true chromium layer is applied, and
Dissolving the easily soluble hydrated chromium oxide in the outermost layer};
This forms a uniform female-soluble chromium hydrated oxide layer containing 1 to 10 mg/m2 of chromium. The present invention will be explained in detail below. The manufacturing method of TF8-CT is to precipitate chromium on the steel plate surface using a high concentration chromic acid bath on 100 g/l g of chromium to which auxiliary agents such as fluorine compounds and sulfuric acid have been added. A two-component method in which # electrode electrolysis is performed in a low-concentration chromic acid bath containing an auxiliary agent to mainly form a chromium hydrated oxide film, and a chromic acid 1 method with the addition of auxiliary agents such as fluorine compounds and sulfuric acid.
Two one-component methods are known in which cathodic electrolysis is carried out in a chromic acid bath under 0.009/in to form a gold-containing chromium layer and a chromium permanent oxide layer at the same time. Looking at the relationship between the amount of metallic chromium and the exposure rate of the steel plate surface from the bore of the metallic chromium layer for the metallic chromium layer in which the chromium hydrated oxide was dissolved and removed with a hot alkaline solution, it is found that =, (AJ
Cr03 monofluoride bath fBI Cr03 monofluoride monosulfuric acid bath (CI) A metallic chromium layer with fewer pores can be obtained in the order of CrOa monosulfuric acid bath. Therefore, in the present invention I2, a metal plate with a high coverage rate is applied to the plate. In order to precipitate chromium J, {= is a chromic acid bath to which a fluorine compound and sulfuric acid have been added;
It is more preferable to use a chromic acid bath containing only fluorine compounds. A chromic acid bath to which a fluorine compound and sulfuric acid are added has a slightly inferior coverage, but is not easily affected by impurity ions in the bath, so it has excellent operability.

The TF'S-CT of the present invention is provided as a material for cans, and is always used after being coated.

As shown in Figure 2, if the heavy equipment is not heated during the baking process, the Rc value, which is an indicator of weldability, will decrease as the amount of metallic chromium decreases.However, assuming paint baking, When heating is performed at 210° C. for 20 minutes, the Rc value decreases in the range of 45 to 90 mg/m 2 of metal chromic acid, and it is presumed that a two-optimal range of weldability exists. The reason for this is that the amount of metallic chromium is 45 mg/
Below m2, as shown in Figure 1, the iron exposure rate increases significantly with the thinning of Kinjo chromium, and this is thought to be due to the formation of iron oxide, which has poor conductivity, on the steel plate surface due to heating. It will be done. In addition, when the amount of gold true chromium is 90 ms/m2 or more, the exposed area of the steel plate decreases, but the pressure applied by the electrode a-ru causes thick metal chromium fi (:- minute cracks (
It is thought that the Rc value becomes large because of the two-dimensional structure.

Figure 3 shows the relationship between the weldable current range during welding of TF8-CT after heating at 210°C for 20 minutes with a metal chromium beast under the condition that the amount of #1g chromium hydrated oxide skin M is 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.

Therefore, the weldable current range is evaluated by a simple method of general Rc measurement.

In the case of TF8-CT, which has a smooth metallic chromium layer,
A very good correlation is observed between the Rc value, the weldable current range, and C2. However, when the precipitation form of the metallic chromium layer is changed, for example, in the case of TFS-CT having two protrusions C in a part of the metallic chromium layer obtained by the method disclosed in JP-A-63-186894, This relationship is not clear. Part C of metallic chromium layer: The thickness of the protruding portion of the metallic chromium layer having protrusions is 3 to 10 times the thickness of the smooth portion.

Therefore, the forge weldability of the steel is hindered in the protruding parts, and iron dioxide is more likely to grow on the surface of the copper plate than in the smooth parts because the iron exposure rate is high. For these reasons, it is thought that the weldable current range of TF8-CT, which has a portion of the gold-plated chromium layer (i.e., a protrusion), is narrow even though the Rc value is quite small.

As explained above, 1; Weldability of the present invention 1; Excellent TF
The metallic chromium layer of S-CT prevents the steel plate from being directly exposed to air and oxidized during heating, and is smooth, thin, and easily destroyed by the pressure of the electrode roll during welding. High coverage is required. Next, T of the present invention
F8-CTI = 45-9 with smooth and high coverage
The chromium hydrated oxide formed on the metallic chromium layer of 0 mg/m" has excellent uniformity and weldability when electrolyte C is poorly soluble. Immediately after electrolysis, the chromium hydrated oxide layer is coated with an electrolytic solution containing a large amount of auxiliary agent (easily soluble) on the outermost surface, and an electrolytic solution containing a small amount of auxiliary agent underneath. The composition of the easily soluble hydrated chromium oxide in the outermost layer varies depending on electrolytic conditions such as bath composition, bath temperature, and current density, as well as on the stirring state of the liquid. Because it is easy to
Non-uniform precipitation tends to occur in the longitudinal direction. On the other hand, below <<: a certain @soluble chromium hydrated oxide e.g. It is
The electrolytic conditions and the stirring state are almost constant regardless of the electrolytic conditions, and the agitation condition is almost constant regardless of the electrolytic conditions. In addition, if we take a closer look, the difference in the thickness of the hardly soluble chromium hydrated oxide depending on the crystal orientation of the steel observed with an electron microscope is due to the chromium hydration immediately after electrolysis. The uniformity is superior to the difference in thickness depending on the crystal orientation of the oxide.In this way, the chromium hydrated oxide layer immediately after electrolysis has a uniform thickness. On top of that, easily soluble chromium hydrated oxide particles with non-uniform thickness are formed, resulting in an overall non-uniform thickness, which is undesirable as it causes local heat generation during welding. Even with the amount of chromium hydrated oxide, compared to TF8-CT immediately after electrolysis,
After electrolysis, TFS-CT, which is fully immersed in an electrolytic solution to remove easily soluble chromium hydrated oxide, has better bath adhesion, and is better than a simple TFS-CT as disclosed in the conventional technology. It was found that stable weldability could not be obtained by specifying the amount of chromium permanent oxide. In order to obtain poorly soluble chromium hydrated oxide, it is necessary to immerse it in the electrolytic solution for at least 2 seconds or more after electrolysis. As shown in Figure IR44, most of the easily soluble hydrated chromium oxide dissolves within 2 seconds after electrolysis, leaving only the permanent chromium oxide, which is difficult to fd.
In addition to the electrolytic solution, a chromic acid solution of 2 5 g/l or more can be used as the solution for bath-dissolving the easily soluble chromium hydrated oxide. Although a higher temperature is preferable, the easily bathable hydrated chromium oxide can be dissolved even at room temperature.

From the viewpoint of weldability, it is better to have less hydrated chromium oxide, but even if it is immersed in a high-concentration chromic acid bath for a long time, it cannot be reduced to less than 1 ms/m2 as chromium. . Furthermore, if the amount of chromium exceeds 1 oIns/m2, even if the uniformity is good, the a-soluble chromium hydrated oxide film becomes too thick and weldability deteriorates. Therefore, li1
The soluble chromium hydrate oxide rate is defined as 1 to 10 μm/m2.

It is thought that the hardly soluble hydrated chromium oxide film obtained in this way has excellent uniformity, so that the distribution of the bath wetting flow becomes uniform and the weldability is improved.

Optimal film composition and film morphology of TFS-CT. 《Based on the knowledge of Mr.
Electrolytic chromic acid treated steel sheet with excellent weldability, characterized by the formation of a smoothly precipitated metallic chromium layer of ■/m2 and a hardly soluble chromium hydrated oxide layer of 1 to 10 ■/m2 of chromium. Regarding the manufacturing method of
I applied for 440.

The cold-rolled steel sheet used in this application is a steel sheet with a normal chemical composition used for can materials, and the carbon content in the steel is 0.05.
It was 5%.

However, a more detailed study of material-side factors that affect weldability revealed that C, 81, Mn, P, S, Cu, AI! ，
It has become clear that, among certain components such as N, carbon in particular has a large effect on weldability.

It has long been known that carbon mold in steel affects the mechanical properties of the weld heat-affected zone, and that forge weldability deteriorates when carbon content in steel exceeds 0.10%. ．． 1
In the range of less than 0%, no detailed study has been made regarding carbon content in steel and weldability.

Conventionally, tin metal has been used as a material for welded cans, but materials such as tin metal, which retain a high temperature of gold aUa even after heating to bake the paint, have good weldability, and the amount of carbon in the steel can be reduced. There is relatively little correlation between the weldability and weldability. Also, normal T
In the case of F8-CT, because the coating composition is inappropriate, welding cannot be performed even if the carbon concentration in the shell is made low. Conventional technology {:
In the case of the disclosed TPS-CT for welded cans, there is a tendency that the weldable current range becomes wider when the carbon content is reduced, but it still does not have sufficient weldability. On the other hand, the special glaze TFS-
In the case of CT, the influence of carbon in copper is remarkable, and the carbon content has been increased from the conventional range of 0.03 to 0.10%.
0. O O 1-0. It has been found that by lowering the weldability to a range of 0.0296, the weldability is further improved.

Figure 5 shows the relationship between the amount of carbon in steel and the weldable current range.When the amount of poorly soluble chromium hydrated oxide is constant, the weldable current range narrows as the carbon content increases. Recognize.

In addition, the figure shows TF8 for welded cans manufactured using conventional technology.
Although the weldable current range of -CT is shown, it can be seen that good weldability as in the present invention cannot be obtained just by reducing the amount of carbon*1 in the single ζ2 steel.

From the above results, the steel plate surface (=45 with smooth and good coverage)
Kanejima chromium layer of ~90 mg/fn2 and a of l~10 mg/m2 as chromium! In TF8-CT with a soluble chromium hydrated oxide layer, carbon 1 in the
It can be seen that TFS-CT defined in the range of t'0.001 to 0.02% has good weldability. As described above, by combining the optimal film structure of TFS-CT and steel with limited carbon solder, it is possible to provide an electrolytically chromic acid treated steel sheet with excellent high-speed weldability.

〔Example〕

The contents of the present invention will be explained in detail below using Examples and Comparative Examples.

Using hot-rolled steel sheets with different carbon numbers in the steel, sheet polishing, 0.
A cold-rolled steel plate of 22 m and tensile strength 'I'-4 was produced under conditions I2 shown in Table I. This cold-rolled steel sheet was subjected to normal degreasing and pickling, and then cathodically electrolyzed in a chromic acid bath to form a smooth metallic chromium layer with a surface C of 45 to 90 mg/m2.
And TFS-CT with 1 to 10 mg/m2 of refractory chromium hydrated oxide l1 as chromium! I built a li. At the same time, using cold-rolled steel sheets with different amounts of carbon in the play,
Comparative Examples 1-3 according to representative examples of the prior art
: Two TFS-CTs were manufactured.

The coating amount, weldability, corrosion resistance, etc. of these TF8-CTs are shown in Table 2. As is clear from Table 2, a steel plate with a carbon content of 0.001 to 0.0296 in copper is used as a substrate, and 4
The TFS-CT of the present invention has a smooth metal chromium layer of 5 to 90 ma/m2 and a hardly soluble chromium hydrated oxide layer of 1 to 10 mg/m2 of chromium. It has a weldable AIT flow range and has remarkable weldability 1: Excellent.

Comparative Example 1 was manufactured according to the manufacturing method disclosed in the examples of JP-A No. 61-213398. Comparative Example 3 was manufactured according to the manufacturing method of Tokuichi Sho 63-261440. All of these use ordinary cold-rolled steel sheets, and the carbon density in the steel is 0. It was 0.596. Comparative Example 2 was manufactured according to the example of JP-A No. 61-213398, but the carbon content in the steel was 0.0
The substrate is a cold-rolled steel plate with a low density of 1%. As can be seen from Comparative Example 2, it is clear that TFS-CT with good weldability cannot be obtained by simply lowering the carbon content in the steel.Also, in terms of corrosion resistance, Example The TFS-CT shown in 1 to 5 is the TF8-C shown in Comparative Examples 1 to 2.
Better than T.

Next (=, the evaluation method for the evaluation items in Table 2 showing examples and comparative examples.

{1) Iron exposure rate from the pores of the metal chromium layer at 95℃ 7
．． 5 8 - TFS-CT is immersed in NJIOH M solution for 5 minutes to completely dissolve and remove the outermost layer of chromium hydrated oxide. Next, leave 30ml of the sample. −
Read the current immediately before passivation when anodic polarization is performed at a polarization rate of 125 mV/min in NaHzPO4 solution. The iron exposure rate was determined from the calibration curve of the exposure rate and passivation current.

(2) Weldable current range After baking TFS-CT at 210°C for 20 minutes, use an experimental plate welding machine at a frequency of 60 Hz, a width of 0.4 mm, a speed of 5 m/min, and a pressure of 50 kgK. After welding was performed, the joint condition of the welded part was evaluated. Increase the welding current in increments of 25, and set the welding upper limit to the current at which splat begins to occur, then lower the welding current, and set the current just before the welding strength becomes lower than the breaking strength of the base metal. As the welding lower limit, this difference in current was defined as the weldable current range.

(3) Contact electrical resistance (Re) After baking TFS-CT at 210° C. for 20 minutes, Rc was measured using a hand-made contact electrical resistance measuring device. The measurement method is as follows: Two copper disc electrodes are in contact with each other: Two stacked sample plates are sandwiched between them.
The RC value was determined by rotating the wiping plate at a circumferential speed of 5 m/min while pressurizing the electrodes with a force of 50k9, passing a 5A DC current between the electrodes, and measuring the voltage between the electrodes. .

(4) Approximately 60% of epoxy/phenol paint was applied to filamentous corrosion TF8-CT.
After applying ms/dm2 and baking at 210°C for 10 minutes, a cross cut was made and a length of 5 m was extended using an Erichsen tester. Sample with 3% NaCI! After dipping, the sample was placed on a stand and left in an atmosphere with a temperature of 45°C and humidity of 85% for 10 days, and the state of rust formation was observed. (5) Salt water spray test After heating TFS-CT at 210°C for 10 minutes, it is placed in a salt water spray tester. The state of rust generation was evaluated when a 3% NaC/solution at 35° C. was sprayed for 1 hour.

(6) Corrosion under the paint film A width of 10 pm was applied to the painted board obtained by the same method as the thread corrosion test.
, put a cross beetle to a depth of 15 μm, and add citric acid 1.
5 96, food 4 1. 5 38 to a corrosive liquid consisting of 96
It was immersed at ℃ for 2 weeks and the corrosion state of the cut portion was evaluated.

〔Effect of the invention〕

The TF8-CT of the present invention has excellent coverage of the metal chromium layer and uniformity of the chromium hydrated oxide film, and has a low carbon content in the film, so it has low-temperature bonding properties (= excellent, very good). {
Because it has excellent weldability, it can be used in a wide range of applications as a material for welded cans that is cheaper than tinplate (2) It has extremely large industrial effects.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the iron exposure rate from the pores of the metallic chromium layer and the metallic chromium enemy after the -fi+1 chromium permanent oxide film of TF8-CT is dissolved and removed with a heated alkaline solution. Figure 2 is a diagram showing the relationship between the contact electrical resistance (Re) value and the amount of gold-plated chromium before and after heating at 210°C for 20 minutes.
The figure shows a constant amount of poorly soluble chromium hydrated oxide.
A diagram showing the weldable current range after heating at 210°C for 20 minutes with two CT+s and the relationship between metal chrome plugs,'! J4 diagram is
Figure 5 shows an example of the dissolution rate of a hydrated chromium oxide film into an electrolytic solution after electrolysis. FIG. 3 is a diagram showing the relationship between the weldable current range and the carbon content in steel after heating for 30 minutes. Patent author Toyo Kohan Co., Ltd. Agent Patent attorney Tadashi Kobayashi 11th son 4〉41 chrome amount (゛V5ノ?j〉nθ57u4 amount (4■ 2〈!”　fig. 4 Inosuzakima (paddy

Claims (1)

[Claims] After degreasing and pickling, the steel plate is subjected to cathodic electrolysis in a chromic acid bath containing a fluorine compound or a fluorine compound and a small amount of sulfuric acid, and then immersed in a solution containing 25 g/l or more of chromic acid for at least 2 seconds. By dissolving easily soluble chromium hydrated oxide, 45 to 90 mg/m^ is applied to the steel plate surface.
2, the smoothly deposited metallic chromium layer and chromium,
A steel sheet treated with electrolytic chromic acid in which a hardly soluble chromium hydrated oxide layer of 1 to 10 mg/m^2 is formed, characterized by reducing the amount of carbon in the steel sheet to a range of 0.001 to 0.02%. Electrolytic chromic acid treated steel sheet for welded cans.