JPH02267300A - Electrolytically chromated steel sheet having superior corrosion resistance, weldability and fine surface color tone - Google Patents

Abstract

PURPOSE:To obtain an electrolytically chromated steel sheet for a can having superior corrosion resistance, weldability and a fine surface color tone by forming a metallic Cr layer having fine granular projections on at least one side of a steel sheet, an Sn layer on the Cr layer by tinning and a specified chromate layer on the Sn layer. CONSTITUTION:A metallic Cr layer 2 having >=2X10<5> granular projections 5 per 1mm<2> is formed on at least one side of a steel sheet 1 by 5-200mg/m<2> by cathodic electrolysis with a known electrolytic soln. contg. CrO3. An Sn layer 3 is then formed on the Cr layer 2 by 30-800mg/m<2> by tinning with a known tinning soln. and a Cr oxide hydrate layer or a chromate layer 4 consisting of metallic Cr and Cr oxide hydrate is further formed on the Sn layer 3 by 3-30mg/m<2> (expressed in terms of Cr).

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an electrolytically chromate-treated steel sheet that has excellent corrosion resistance and weldability, and has a good surface color tone, and is suitable as a material for cans.

[Prior Art] On at least one surface of a steel sheet, there is a chromate coating consisting of a metallic chromium layer as a lower layer and a hydrated chromium oxide layer as an upper layer formed on the metallic chromium layer. Electrolytic chromate-treated steel sheets have excellent not only corrosion resistance but also paint adhesion between the chromate film and the paint film formed thereon, and are cheaper than tin-plated steel sheets. Therefore, electrolytic chromate-treated steel sheets are widely used in place of tin-plated steel sheets as materials for cans such as food cans, bale cans, 182 cans, and oil cans.

Cans made of electrolytic chromate-treated steel sheets include adhesive cans (3-piece cans), which consist of a top lid, a bottom lid, and a body whose joints are bonded with an adhesive such as organic resin or special cement.
There are also two-piece cans consisting of a top-shaped can body and a barrel.

However, electrolytic chromate-treated steel sheets are not often used as materials for welded cans, which consist of a top cover, a bottom cover, and a body whose seams are welded by electric resistance welding.The reason is that electrolytic chromate-treated steel sheets This is because the weldability of steel plates is poor.

However, the demand for welded cans has increased in recent years because of their high joint strength. Therefore, in order to use electrolytic chromate-treated steel sheets as materials for welded cans, it is required to improve their weldability.

The causes of poor weldability of electrolytic chromate-treated steel sheets are as follows. That is, both the lower metallic chromium layer and the upper hydrated chromium oxide layer that form the chromate coating are non-thermal conductive, and the upper hydrated chromium oxide layer is electrically non-conductive. It is. Therefore, when welding the joints of overlapping parts of can bodies by electric resistance welding, the hydrated chromium oxide layer as the upper layer becomes an electric resistance layer, so the contact resistance value of the parts to be welded is It gets expensive. The contact resistance value is a standard for evaluating whether excessive current flows locally during welding. In other words, when the contact resistance value is high, the welding current path is narrow, so excessive current tends to flow locally. The contact resistance value of the chromate treated steel plate is 10" to 10' μΩ/m
", which is extremely high compared to other surface-treated steel sheets for welded cans. Therefore, when electrolytic chromate-treated steel sheets are welded by electric resistance welding, the welding current value is low immediately after welding starts, and after a certain period of time has passed, The welding current reaches the specified value.

As a result, at the start of welding, the electrolytic chromate-treated steel sheet locally generates heat, dust is generated, and defects such as blowholes occur in the weld. Therefore, when welding electrolytically chromate-treated steel sheets, conventionally, the chromate coating on the parts to be welded must be removed by polishing or the like, which requires a lot of time and effort.

The following method is known as a means to solve the above-mentioned problem.

(1) A metal chromium layer as a lower layer with an adhesion amount of 3 to 40 mg/m2 formed on the surface of a steel plate, and a metal chromium layer formed on the metal chromium layer, as disclosed in Japanese Patent Publication No. 57-19752. chromium plating with excellent weldability, characterized by a hydrated chromium oxide layer as an upper layer with a coating amount of 2 to 15 μ/ml, and a porous metal chromium layer. Steel plate (hereinafter referred to as prior art 1) (2) 10 to 40 u/cm formed on the surface of a steel plate disclosed in JP-A No. 61-213398.
a metallic chromium layer as a lower layer of the adhesion amount of nl, and
Formed on top of the metal chromium layer, 3~30+ag/m
Tin-free for welded cans with excellent corrosion resistance after painting, consisting of a hydrated chromium oxide layer with a uniform thickness of 2, and a smooth metal chromium layer. Steel plate (hereinafter referred to as prior art 2) (3) Metallic chromium as a lower layer formed on the surface of a steel plate with a coating weight of 5 to 200 μ/M, disclosed in JP-A-62-54096. and a hydrated chromium oxide layer with a deposition amount of 3 to 30 μ/m2 formed on the metal chromium layer, and a large number of granular protrusions are formed over the entire surface of the metal chromium layer. Electrolytic chromate-treated steel sheet with excellent weldability, characterized by
Prior Art 3) (Problems to be Solved by the Invention) The above-mentioned Prior Art 1 has the following problems. Namely, in order to improve weldability, it is necessary to reduce the amount of deposited metal chromium layer and As a result of forming the metallic chromium layer in a porous manner, corrosion resistance deteriorates significantly.

Prior art 2 described above has the following problems. That is, in order to improve weldability, the amount of the metallic chromium layer deposited is reduced as in Prior Art 1.

In Prior Art 2, the metallic chromium layer is formed in a smooth shape to keep the thickness of the hydrated chromium oxide layer uniform;
The deterioration of corrosion resistance due to the decrease in the amount of deposited metallic chromium layer
Still big.

According to the above-mentioned prior art 3, the hydrated chromium oxide layer as the upper layer, which is non-conductive, is destroyed by a large number of granular protrusions formed over the entire surface of the metal chromium layer as the lower layer during electric resistance welding. As a result, the contact resistance value of the part to be welded decreases, and weldability improves. and,
Since the amount of deposited metal chromium layer is also at a normal level, corrosion resistance does not deteriorate.

However, prior art 3 has the following problems. That is, due to the large number of fine granular protrusions formed over the entire surface of the metallic chromium layer, reflected light is diffracted and interfered in the metallic chromium layer, resulting in the surface of the electrolytically chromated steel sheet appearing black or brown. Its surface color is significantly impaired. Furthermore, the greater the number of granular protrusions formed on the surface of the metal chromium layer, the more hydrated chromium oxide is likely to precipitate, making it difficult to form a hydrated chromium oxide layer with a predetermined amount of deposit.

Therefore, an object of the present invention is to provide an electrolytically chromate-treated steel sheet that has excellent corrosion resistance and weldability, and has a good surface color tone.

[Means to solve the problem]

From the above-mentioned viewpoints, the present inventors have conducted extensive research in order to develop an electrolytically chromate-treated steel sheet with excellent corrosion resistance, weldability, and surface color. As a result, we obtained the following knowledge.

(1) Tin-plated steel sheets have good weldability, so
Widely used as a material for welded cans. The reason why tin-plated steel sheets have good weldability is as follows. That is,
Tin is soft and has a low melting point. Therefore, when welding the joints of the overlapped parts of the can body by electric resistance welding, the overlapped parts can be brought into contact over a wide range, and the Joule heat generated during welding can melt the tin. Therefore, the contact area of the overlapping portions is further expanded. Furthermore, an excessive welding current does not flow locally in the part to be welded, and the generation of dust due to local heat generation of the steel plate is prevented. Regarding sex-enhancing effects,
Further research revealed that in order for tin to improve weldability, tin does not necessarily need to be present on the entire surface of the steel plate, but only partially on the surface of the steel plate. ,
In addition, if tin plating is applied to a metal chromium layer on which many granular or angular protrusions (hereinafter referred to as granular protrusions) are formed, the granular protrusions will improve weldability, and a small amount of tin will be used. It was found that weldability was significantly improved.

(2) The tin plating film has a white color tone.

Therefore, if the tin plating layer exists on the granular protrusions formed on the surface of the metal chromium layer, the surface color tone of the electrolytically chromated steel sheet will not be impaired.

(3) If a tin coating layer is formed on the metal chromium layer formed on the surface of the steel plate, it is possible to eliminate the process when melting the tin coating layer or baking the paint film formed on the top layer. During application, the metallic chromium layer acts as a barrier to prevent alloying of the tin with the steel plate. Therefore, the amount of attached tin plating layer can be reduced.

The present invention was made based on the above-mentioned findings, and the present invention has been made based on the above-mentioned findings.
A metal chromium layer as a lower layer with a deposition amount of 00 mg/Il'f, and a metal chromium layer formed on the metal chromium layer with a coating amount of 30 to 80 mg/Il'f.
A tin plating layer as an intermediate layer with a coating amount of 0/A and a thickness of 3 to 30 m in terms of chromium formed on the tin plating layer.
chromium) 11 as an upper layer with a coating amount of 2×10′ pieces/trm on the metallic chromium layer.
It is characterized by the fact that granular protrusions are formed at a density higher than ``.

Next, the present invention will be explained with reference to the drawings. 1st
The figure is a schematic cross-sectional view of an electrolytically chromate-treated steel sheet of the present invention. As shown in FIG. 1, the electrolytic chromate treatment i4 of the present invention is formed on at least one surface of the 'w4 plate l as an underlying layer of metallic chromium Tf! I2, a tin plating layer 3 as an intermediate layer formed on the metal chromium N2, and a chromate layer 4 as an upper layer formed on the tin plating layer 3. A large number of granular protrusions 5 are formed on the surface.

Because of the large number of granular protrusions 5 formed on the surface of the metal chromium layer 2, the tin plating layer 3 and the chromate layer 4
It is also formed in a shape along the protrusion 5.

The amount of deposited metal chromium layer 2 as the lower layer is from 5 to 200
It should be limited within the range of mg/m2. If the adhesion amount of the metal chromium layer 2 is less than 5 u/rrf, the desired corrosion resistance cannot be obtained, and the tin plating layer 3 formed on the metal chromium layer 2 may not be baked during the melting process or the paint film may be baked. During processing, the barrier effect of the metal chromium layer 2 is small, and the amount of tin alloyed is large. On the other hand, even if the amount of deposited metallic chromium 512 exceeds 200 .mu./M, no particular improvement in corrosion resistance is obtained, and on the contrary, weldability deteriorates. The preferred range of the amount of deposited metal chromium layer 2 is from 50 to 150 mg/rd.

In this invention, granular protrusions 5 are formed on the surface of the metal chromium layer 2.
The reason for forming the granular protrusions 5 is to improve weldability through a synergistic effect with the tin plating layer 3 formed on the metal chromium layer 2.
The density of the granular protrusions 5 should be limited to 2×10
If the number is less than 5 pieces/m'', the desired effect described above cannot be obtained.

On the surface of the metal chromium layer of conventional electrolytic chromate treated steel sheets, where no tin plating layer is formed on the metal chromium layer,
When forming granular protrusions to improve weldability, it is necessary that the granular protrusions are uniformly distributed on all crystal orientation planes of the steel sheet. However, in the present invention, since the tin plating layer 3 having an effect of improving weldability is formed on the metal chromium layer 2, the granular protrusions 5 having the above-mentioned density are not necessarily formed on all crystal orientation planes of the steel sheet 1. It is not necessary to form them in a uniform distribution, and they may be formed in a concentrated manner on a specific crystal plane.

The adhesion amount of the tin plating layer 3 as an intermediate layer formed on the metal chromium 712 should be limited within the range of 30 to 800 .mu./i. When melting the tin plating layer 3,
Alternatively, during processing, a part of the tin penetrates into the steel plate 1 through the bore of the metal chromium N2, causing baking to the paint film.
Alloying is unavoidable, therefore the tin plating layer 3
If the amount is less than 30 mg/m2, the desired effect of improving weldability cannot be obtained, considering the above-mentioned amount of alloyed tin. On the other hand, even if the amount of the tin plating layer 3 exceeds 800 g/nf, no particular improvement in weldability can be obtained, and since tin is an expensive metal, it is uneconomical.

The thickness of the granular projections 5 formed on the surface of the metal chromium layer 2 is about 3 to 10 times the thickness of the flat part of the metal chromium layer 2. Therefore, when melting the tin plating layer 3 or
During the baking process on the paint film, it becomes alloyed and disappears.
The tin on the metal chromium layer 2 has granular protrusions 5 than its flat parts.
There are fewer.

As a result, tin can remain at least on the granular protrusions 5 during the above-described melting treatment or baking treatment.

As mentioned above, in order to improve weldability, it is sufficient that tin exists partially, so the amount of tin plating layer 3 adhered is small enough that tin is present only on the granular protrusions 5. In this case, as mentioned above, the density of the granular protrusions 5 needs to be 2 x 10' pieces/m2 or more, and if the density is less than 2 x 10'pieces/m'', the granular protrusions 5
Even if tin is present thereon, the effect of improving weldability is low because the number of granular protrusions 5 is small.

By forming the granular projections 5 on the surface of the metal chromium layer 2 and forming the tin plating layer 3 thereon, the following effects can be obtained. That is, as described above, when a large number of granular protrusions 5 are formed on the surface of the metal chromium N2, reflected light is diffracted and interfered in the metal chromium layer 2, resulting in a change in the surface color tone of the electrolytic chromate-treated steel sheet. Although it is significantly impaired, the white tin plating layer 3 formed on the metal chromium 112 improves the surface color tone.

Furthermore, in a conventional electrolytic chromate-treated steel sheet in which a chromate film consisting of a metal chromium layer as a lower layer and a hydrated chromium oxide layer as an upper layer is formed, when granular protrusions are formed on the surface of the metal chromium layer, hydrated chromium It has a relatively large amount of oxides and easily adheres to it.

For this reason, it has not been easy to form a predetermined amount of hydrated chromium oxide layer, but if the tin plating layer 3 is formed on the metal chromium layer 2 as in the present invention, the tin plating layer 3 can be A predetermined amount of a hydrated chromium oxide layer or chromate layer 4 can be easily formed thereon.

Furthermore, the tin plating layer 3 is formed in an uneven shape along the granular projections 5 of metal chromium N2 as the lower layer. Therefore, the uneven tin plating layer 3 can improve the corrosion resistance after painting and the corrosion resistance on the paint film. The density of the granular protrusions 5 on the metal chromium layer 2 is 2×10′ pieces/
wm'', the unevenness formed on the tin plating layer 3 decreases and the above-mentioned effect cannot be obtained. Therefore, from this point of view as well, the density of the granular protrusions 5 should be 2X10'pieces/m'' or more. Requires.

The chromate layer 4 as an upper layer formed on the tin plating layer 3 is composed of a hydrated chromium oxide layer, or a metallic chromium layer and a hydrated chromium oxide layer on the metallic chromium layer. The amount of the chromate layer 4 to be deposited should be limited to a range of 3 to 30 cm/bo in terms of chromium. If the amount of deposited chromate layer 4 is less than 3 mg/m2 in terms of chromium,
Corrosion resistance and paintability deteriorate. On the other hand, chromate layer 4
If the amount of adhesion exceeds 30 μ/rrr in terms of chromium, weldability deteriorates. A preferable range of the amount of the chromate layer 4 deposited is 5 to 15 mg/n (in terms of chromium).

Next, a method for producing an electrolytically chromate-treated steel sheet of the present invention will be described. After degreasing and pickling the steel plate 1, a known method consisting of at least one of chromic anhydride and chromate as a main agent and at least one of sulfuric acid, sulfate and fluoride as an auxiliary agent is applied. A metal chromium layer 2 as a lower layer is formed on at least one surface of the steel plate 1 with an adhesion amount of 5 to 200 mg/m 2 by cathodic electrolyzing the W4 temporary 1 using an electrolytic solution. During this cathodic electrolytic treatment, for example, the cathodic electrolytic treatment may be performed intermittently, or
During the cathodic electrolytic treatment, the steel plate is subjected to an anodic electrolytic treatment at least once, or the degreased and pickled steel plate is treated with chromic anhydride, chromate and dichromium prior to the cathodic electrolytic treatment. Granular protrusions 5 having a density of 2×10' particles/column or more are formed on the metal chromium layer 2 by means such as anodic electrolytic treatment using an electrolytic solution containing at least one of the acid salts.

Next, tin plating is applied to the metal chromium layer 2 using a known plating solution such as a stannous sulfate solution (ferrostane solution), a stannous chloride solution (halogen solution), or an alkaline tin plating solution, and the metal chromium layer 2 is coated with 30 to 30% tin. A tin plating layer 3 is formed as an intermediate layer with a coating weight of 800 .mu./r+1. At this time, the adhesion of tin plating becomes poor due to chromium oxide present on the surface of the metal chromium layer. Therefore, when forming the metallic chromium layer, the generation of chromium oxides may be suppressed by, for example, increasing the temperature of the plating solution, or after the formation of the metallic chromium layer, the anode may be immersed in an electrolytic solution for a relatively long period of time. It is necessary to perform electrolytic treatment to reduce chromium oxide as much as possible.

Since the plating current during tin plating is more likely to concentrate on the granular protrusions than on the flat parts of the metal chromium layer, the current density is higher on the granular protrusions than on the flat parts. Therefore, the amount of tin deposited is greater on the granular projections than on the flat portion, and even if the overall amount of adhesion is small, tin can always be deposited on the granular projections.

For example, at least one of chromic anhydride, chromate, and dichromate is applied to the steel plate 1 on which the tin-coated layer 3 as an intermediate layer is formed on the metal chromium layer 2 as described above. The tin plating layer 3 is subjected to cathodic electrolytic treatment using a known electrolytic solution as a main component or a known electrolytic solution in which an auxiliary agent consisting of at least one of sulfuric acid, sulfate, and fluoride is added to the main component. On top of that, 3 to 3 in terms of chromium.
0! A chromate layer 4 is formed as an upper layer of IIg/rrl. In this way, the electrolytically chromate-treated steel sheet of the present invention is obtained.

Next, the present invention will be explained in more detail using examples and comparing with comparative examples.

〔Example〕

A cold-rolled steel plate with a thickness of 0.22 am is degreased and then pickled using a normal method, and then the degreased and pickled cold-rolled steel plate is subjected to cathodic electrolytic treatment using the electrolyte shown in (1) below. is applied, and during cathodic electrolytic treatment, (A) shown in (2) below is applied.
) to (C) to form granules on at least one surface of the cold-rolled steel sheet with a coating amount within the range of the present invention and a density within the range of the present invention. A metal chromium layer was formed as a lower layer, which had protrusions.

Next, using the electrolytic solution shown in (3) below, the steel plate on which the metal chromium layer as a lower layer is formed is tin-plated, and the metal chromium layer has a coating amount within the range of the present invention.
After forming a tin plating layer as an intermediate layer, the following (
4) Perform cathodic electrolytic treatment using any of the electrolytes (A) to (C) shown in (A) to (C) and current density to form an upper layer on the tin plating layer having a coating amount within the range of the present invention. A chromate layer was formed. Thus, the specimen of the present invention (hereinafter referred to as the specimen of the present invention) Nα1 having a metallic chromium layer as a lower layer, a tin plating layer as an intermediate layer, and a chromate layer as an upper layer within the scope of the present invention shown in Table 1 ~
7 was prepared.

(Electrolyte component composition for forming 11 metal chromium layer Crys: 180 g/f NazSiPth: 5 g/l1NalSO
a: 1 g/'Temperature: 45'C (2) Projection forming means (A) Anodic electrolytic treatment is performed during the cathodic electrolytic treatment.

Cathodic electrolytic current density: 30 A/dm" Anodic electrolytic current density: 2 A/dmffi (B) Prior to the cathodic electrolytic treatment, the pickling treatment is omitted and the anodic electrolytic treatment is performed.

Cathodic electrolytic current density: 100 A/dmz Anodic electrolytic current density: 10 A/dm" (C) Cathodic electrolytic treatment is performed intermittently.

Cathode electrolysis current density: 30 A/dm” (3) Electrolyte composition for forming tin plating layer: 5nSOa
: 50 g/1 Phenolsulfonic acid: 10 g/l Ethoxylated naphtholsulfonic acid: 5 g/l Temperature: 50°C Relative flow rate: 300 m/min (4) Electrolyte and current density for formation of chromate layer ( A) Electrolyte: CrOs 50 g/1 Current density:
10A/dm' (B) Electrolyte: NazCrzOt ・2 F+! 0
30 g/1 Current density: 20 A/dm” (C) Electrolyte: CrOs 20 g/I!.

Naz5040.2 g/1 Current density: L OA /da+"Next, for comparison, the granular protrusions of the tin or metallic chromium layer are outside the scope of the present invention, as shown in Table 1. test specimen (hereinafter referred to as comparative specimen) kl~
5 was prepared. Comparative specimen Nα1 was prepared as follows. A cold-rolled steel plate with a thickness of 0.22mm is degreased and then pickled using a conventional method.Then, the degreased and pickled cold-rolled steel plate is cathode-coated using the electrolyte shown in (1) above. Electrolytic treatment is performed, and during cathodic electrolytic treatment, the above-mentioned (2)
) A metal that is subjected to a protrusion forming treatment by the means shown in (A) and has granular protrusions on at least one surface of the cold rolled steel sheet with an adhesion amount within the range of the present invention and a density within the range of the present invention. Only a chromium layer and a hydrated chromium oxide layer were formed. In comparative specimen N11l, a tin plating layer as an intermediate layer was not formed on the metal chromium layer.

Comparative specimen Nα2 was prepared as follows.

The cold-rolled steel sheet is subjected to cathodic electrolytic treatment using the electrolyte shown in the above (1), and during the cathodic electrolytic treatment, a protrusion formation treatment is performed by the means shown in (C) of the above (2). Then, on at least one surface of the cold rolled steel sheet, a metallic chromium layer was formed as a lower layer having granular protrusions.Next, a metallic chromium layer was formed using the electrolytic solution shown in (3) above. A steel plate is tin-plated to form a tin-plated layer as an intermediate layer on the metal chromium layer, and then cathodic electrolytic treatment is performed using the electrolyte shown in (C) of (4) above, A chromate layer was formed as an upper layer on the tin plating layer. In comparative specimen 8112, the density of granular protrusions is low and outside the range of the present invention.

Comparative specimen No. 3 was subjected to the protrusion formation treatment described in (2) (2) above.
It was prepared by the same method as Comparative Specimen 2 except that it was carried out by the means shown in B). In the comparative sample No. 3, the amount of tin plating layer deposited was so small that it was out of the range of the present invention.

Comparative specimens Nos. 4 and 5 were subjected to cathodic electrolytic treatment for forming a metal chromium layer without protrusion formation treatment at 30A.
It was prepared in the same manner as comparative specimen No. 112, except that the cathodic electrolytic treatment was carried out continuously at a current density of /cl It has not been.

Surface color tone,
Corrosion resistance and weldability after painting were evaluated as described below. The evaluation results are also shown in Table 1.

(1) Surface color tone The surface color tone of each of the present invention specimen and comparative specimen was visually examined and evaluated. The evaluation criteria are as follows.

O: The surface of the specimen does not appear black or brown, and the surface color tone is good, even after applying gloss coating to the surface of the specimen as it is.

×: Even when the specimen is used as it is, the surface appears black or brown, and the surface color tone is poor.

(2) Corrosion resistance after painting The surface of each of the present invention specimen and comparative specimen was
After painting with epoxyphenol paint in an amount of 1/m2 and baking, cross-shaped scratches are made from the coating surface to the steel plate surface using a sharp cutter knife. It was extruded from the back side with a width of 5 am from the longitudinal center of the flaw. Each of the present invention specimen and comparative specimen in such a state was immersed for 4 days in an aqueous solution containing 1.5 wt, χ common salt and 1.5 wt, citric acid at a temperature of 38°C. The corrosion width at the bottom of the cross shape was measured and evaluated as follows.

O: Maximum corrosion width: 0.5 mm or less O: Maximum corrosion width: 20.5 to 2 mm or less Δ: Maximum corrosion width: 1.5 to less than 2 mm (3) Weldability of the present invention specimen and comparative specimen The weldable current when each is welded using a Sudronink welding machine while gradually increasing the current value, from the lowest weldable current value to the highest current value before welding becomes impossible due to the occurrence of splash. The value range, ie, the range of the dial scale for controlling the welding current value, was evaluated as follows.

◎: Scale range 4 or more ○: Scale range 2.5 or more, less than 4 Δ: Scale range 0.5 or more, less than 2.5 As is clear from Table 1, a tin plating layer is formed on the metal chromium layer. In comparison specimen Nαl, which was not prepared, even though granular protrusions were formed on the metallic chromium layer, both the corrosion resistance and weldability after painting were poor, and the surface color tone was poor due to the granular protrusions. there were. Even though a tin layer and a tin layer are formed on the metal chromium layer, the density of granular protrusions on the metal chromium layer is outside the range of the present invention and is low.
In No. 2, the weldability was poor.

Even though a tin plating layer was formed on the metal chromium layer, in comparative specimen No. 3, the amount of tin plating was too small to fall outside the scope of the present invention, and both the corrosion resistance and weldability after painting were poor. Moreover, the surface color tone was poor due to the granular projections. Even though a tin plating layer is formed on the metal chromium layer, comparative specimens Nα4 and 5, in which no granular protrusions are formed on the metal chromium layer, are almost as good as the present invention specimens No. 6 and 7. Although the same amount of tin plating was formed, the weldability was poor due to the small amount of residual tin.

On the other hand, all of the present invention specimens NIILI to 7 had good surface color tone, post-painting corrosion resistance, and weldability.

As mentioned above, the electrolytic chromate treated steel sheet of this invention has
In addition to its weldability, it has excellent corrosion resistance and surface color, so it can be widely used not only as a material for welded cans, but also as a material for adhesive cans, two-piece cans, etc.

〔Effect of the invention〕

As detailed above, according to the electrolytic chromate-treated steel sheet of the present invention, a small amount of tin is produced due to the synergistic effect of the granular protrusions formed on the metal chromium layer and the tin plating layer formed on the metal chromium layer. Industrially useful effects such as improved weldability, excellent corrosion resistance, and good surface color are brought about.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of an electrolytically chromate-treated steel sheet of the present invention. In the drawings, l... steel plate,
2...Metal chromium layer, 3...Tin plating layer,
4... Chromate layer, 5... Granular protrusions.

Claims (2)

[Claims] (1) 5 formed on at least one surface of the steel plate
A metal chromium layer as a lower layer with a deposition amount of ~200 mg/m^2 and a metal chromium layer formed on the metal chromium layer of 30~8
A tin plating layer as an intermediate layer with a coating amount of 00 mg/m^2 and a tin plating layer formed on the tin plating layer with a coating amount of 3.0 mg/m^2 in terms of chromium.
It consists of a chromate layer as an upper layer with a deposition amount of ~30 mg/m^2, and the metal chromium layer has 2 x 10^5 pieces/
An electrolytic chromate-treated steel sheet with excellent corrosion resistance, weldability, and surface color tone, characterized by having granular protrusions formed at a density of mm^2 or more. (2) The electrolytically chromate-treated steel sheet according to claim 1, wherein the chromate layer is composed of a hydrated chromium oxide layer, or a metallic chromium layer and a hydrated chromium oxide on the metallic chromium layer.