JPH09324282A - Surface treated steel sheet excellent in corrosion resistance after working - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve fuel corrosion resistance after working by allowing cracks, having respectively specified density and size, to occur in a Zn alloy plating film of a surface treated steel sheet having the Zn alloy electroplating film and a chromate film. SOLUTION: A Zn-X alloy electroplating film, containing at least one kind selected from 3-18%, by weight, Ni, 0.02-3% Co, 25-45% Mn, and 8-20% Cr, is formed on a steel sheet by (5 to 50)g/m<2> coating weight per side. Further, a chromate film is formed on the above film by (10 to 200)mmg/m<2> coating weight expressed in terms of metallic Cr. At this time, cracking is allowed to occur in the Zn-X alloy plating film by means of chemical etching, etc. The density of cracks is regulated to a value in the range of 1000 to 150000 pieces in the region of a visual field of 1×1mm at the surface of the plating. Further, the cracks of <=0.5μm maximum width comprise >=90% and the cracks having a depth not smaller than 80% of the thickness of the plating film comprise >=80%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet having excellent corrosion resistance after processing, particularly a fuel tank for automobiles and motorcycles, which exhibits high corrosion resistance to fuels such as gasoline and gasohol, and also to stoves, boilers and the like. The present invention relates to a surface-treated steel sheet suitable for applications such as kerosene tanks, oil filters requiring high workability and high corrosion resistance.

[0002]

2. Description of the Related Art In addition to weldability, fuel tank materials for automobiles and motorcycles have general corrosion resistance on the outer surface (hereinafter referred to as outer surface corrosion resistance), but inner surface has fuel corrosion resistance to fuel such as gasoline. Is required. These are collectively referred to as corrosion resistance or post-processing corrosion resistance. Conventionally, a turn sheet (10 to 25% Sn-Pb alloy plated steel sheet) has been widely used as a material for a fuel tank. However, Pb in the plating film is harmful to the human body, when the alcohol-containing fuel is used, the plating film is easily dissolved in alcohol oxide, and pinholes in the plating film are unavoidable,
As a result of preferential corrosion of Fe, which is electrically baser than the plating film, from this pinhole, there is a problem that the pitting corrosion resistance becomes insufficient, and alternative materials have been demanded.

In recent years, in particular, a gasoline / alcohol mixed fuel called gasohol (such as M15 containing about 15% methanol and M85 containing about 85% by weight methanol) has been used due to exhaust gas regulations in consideration of environmental problems. The use of typical alcohol-containing fuels is being promoted in some countries. However, since the conventional turn sheet is easily corroded by the alcohol-containing fuel as described above, there is an urgent need to develop a fuel tank material excellent in fuel corrosion resistance against the alcohol-containing fuel.

From this point of view, it has been conventionally studied to apply the Zn-Ni alloy electroplated steel sheet to the fuel tank in consideration of the corrosion resistance after processing and the cost. The following publications can be cited as conventional techniques.

Japanese Unexamined Patent Publication (Kokai) No. 58-45396 discloses a Ni content of 5
A surface-treated steel sheet for a fuel tank, which has been subjected to chromate treatment on Zn-Ni alloy electroplating having a thickness of .about.50 wt% and a thickness of 0.5 to 20 .mu.m, is shown.

Japanese Unexamined Patent Publication No. 5-106058 discloses a Ni content of 8
A surface-treated steel sheet for a fuel tank is shown which is provided with a Zn-Ni alloy plating of -20 wt% at a deposition amount of 10-60 g / m ² and is chromated.

Although these surface-treated steel sheets have very good corrosion resistance on the outer surface, they cannot be said to have sufficient fuel corrosion resistance, especially fuel corrosion resistance after processing. For example, alcohol-containing fuel is mixed with salt water. Corrosion was likely to occur in a very severe environment such as a case. Further, if the chromate film or the plating film is thickened to improve this, there is a problem that the weldability, which is an important performance as a fuel tank material, deteriorates.

In addition, as disclosed in JP-A-62-27587, a double-layer plated steel sheet in which Ni plating is superposed on Zn-Ni alloy plating has been studied, but There is a problem that the number of manufacturing processes increases and the cost increases.

For the sake of introduction, there are the following conventional techniques from the viewpoint of forming cracks in the plating film, but none of them disclose the corrosion resistance after processing.

That is, in Japanese Unexamined Patent Publication Nos. 5-25679 and 4-337099, crack widths of steel sheets: 0.01 to 0.5
μm, crack density: on a Zn-Ni alloy plating thin underlayer having minute cracks with a crack area fraction of 10 to 60%,
A high-corrosion-resistant surface-treated steel sheet having a Zn-Ni alloy plating layer and excellent in impact resistance and adhesion is disclosed. However, these surface-treated steel sheets are used for exterior exteriors of automobiles,
The present invention relates to a high-corrosion-resistant surface-treated steel sheet with excellent impact resistance adhesion, which relates to a so-called steel sheet that is less likely to cause blistering even when the coating film on the outer surface of the automobile exterior is damaged by stones, scratches, etc. is there.

The upper Zn-Ni alloy plating layer is made to enter even between the cracks of the undercoating layer to improve the impact resistance adhesion of the upper Zn-Ni alloy plating film by the anchor effect. .

Japanese Unexamined Patent Publication (Kokai) No. 62-297490 discloses a steel material having 3 to
A Zn-Ni alloy plating layer containing 15% Ni is formed to a thickness of 0.5 to 2 μm, a Ni alloy plating layer containing 15 to 75% Ni is formed to a thickness of 0.3 to 1.5 μm, and this Ni alloy is formed. After forming fine cracks uniformly on at least the surface of the plating layer, a chromate film is formed on this Ni alloy plating layer in terms of metal Cr of 5
Disclosed is a blackened surface-treated steel material that forms ~ 100 mg / m ² . The fine cracks at this time have a width of 0.1 to 0.4 μm, a length of 1 to 10 μm, and a depth of 0.2 to 1 μm, and account for 60% or more of the area ratio of the entire crack.

The surface-treated steel material has a two-layer structure on the surface of the steel material, a Zn-Ni alloy plating layer having a low Ni content is first formed on the surface of the steel material, and a black color having a high Ni content and fine cracks is formed thereon. By forming the layer, the adhesion of the black layer after processing is improved.

Therefore, in the above example, the Ni content of the upper Zn-Ni plating layer having fine cracks is very high,
Even if chromate treatment is applied to it, high corrosion resistance cannot be obtained even in the flat plate state.

Moreover, the Zn-Ni plating layer has a two-layer structure,
(Lower layer plating thickness ≧ upper layer plating thickness) And, since cracks in the upper layer plating film do not progress to the lower layer plating film, new cracks occur in the lower layer plating layer due to press working, and the underlying steel sheet is exposed. And the corrosion resistance after processing is significantly deteriorated.

[0016]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the prior art of such Zn-Ni alloy plating + chromate surface-treated steel sheets for fuel containing alcohol-containing fuel. It is an object of the present invention to develop a technology capable of improving fuel corrosion resistance after processing without impairing weldability and substantially increasing cost.

[0017]

As a result of various studies to solve the above problems, the present inventors have found that one of continuous Zn-X (X = Ni, Co, Mn, Cr) using an acidic bath is used. Type or two or more types (hereinafter abbreviated as X)) Stopping energization at the final stage of the alloy electroplating process and immersing it in the plating solution for a short time significantly improves the corrosion resistance after processing, especially the fuel corrosion resistance. I realized that I would be done. As a result of investigating the cause, cracks were generated in the Zn-X alloy plating layer by immersion in this acidic plating solution, and the crack density, maximum width of cracks, and crack depth of the plating film surface thus generated were specified. It was found that the fuel corrosion resistance after processing was remarkably improved when it was within the range of (4), and the present invention was completed.

As described above, according to the present invention, cracks of a specific density are generated on the surface of the Zn-X alloy plating film, and the plating film surface having the cracks is directly subjected to the chromate treatment to allow the chromate to enter the cracks. ,
The chromate film is firmly fixed by the anchor effect, the coating area of the chromate film with excellent corrosion resistance can be increased due to cracks, the cracks are generated in advance in the plating film, and the cracks are covered with the chromate film, so that during press working By suppressing the generation of cracks that newly occur to expose the base steel sheet, it is possible to improve the corrosion resistance as a whole. In particular, it is possible to improve the corrosion resistance even after severe drawing. In this respect, the above-mentioned Japanese Patent Publication No. 5-25679 and Japanese Patent Laid-Open No.
It is completely different from the technique disclosed in Japanese Patent Publication No. 337099 in any of the configurations, technical ideas, and uses of the invention. In particular, the present invention provides a surface-treated steel sheet suitable for fuel tanks, kerosene tanks, oil filters, etc. of automobiles and the like that require severe corrosion resistance after forming and is completely different from the prior art. Further, compared with the technique disclosed in Japanese Patent Laid-Open No. 62-297490, the plating film constitution is different, and the purpose and effect of providing cracks are also essentially different.

According to the present invention, a Zn-X alloy electroplating film having a coating amount on one side of 5 to 50 g / m ² is provided on a steel sheet. The alloy composition of X is Ni: 3 to 18 wt%, Co: 0.02-3wt
%, Mn: 25 to 45 wt%, Cr: 8 to 20 wt%, and at least one selected from the group consisting of 10 to 200 mg / m ^{2 in} terms of metal Cr equivalent deposition amount on the Zn-X alloy plating film. Of the surface-treated steel sheet provided with the chromate film, the Zn-X alloy plating film under the chromate film has cracks, and the density of the cracks is surrounded by the cracks in the visual field of 1 mm x 1 mm on the plating surface. 1000 to 150000 expressed as the number of areas
The crack width is within the range of 90% or more, and the crack depth is 80% or more of the plating film thickness. The surface-treated steel sheet has excellent corrosion resistance after processing.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the accompanying drawings is a schematic explanatory view showing a sectional structure of a plating film of a surface-treated steel sheet according to the present invention. A coating 2 is provided, and a chromate coating 3 is provided on the coating 2, and cracks 4 are formed in the plating coating 2.

Zn− of the plated steel sheet used in the present invention
The alloy composition of the X alloy plating film is such that X is Ni: 3 to 18 wt.
%, Co: 0.02 to 3 wt%, Mn: 25 to 45 wt%, Cr: 8 to 20 wt
At least one selected from the group consisting of% is included. However, when X includes two or more kinds of alloying elements, preferably the second and subsequent elements are Ni:
3-18 wt%, Co: 0.02-3 wt%, Mn: 25-45 wt%, Cr:
It may be selected from the group consisting of 8 to 20 wt%, or the second and subsequent elements may be selected from Ni, Co, Mn and Cr and the total amount thereof may be limited to 5% or less.

The X content of the plating film does not mean the X content immediately after electroplating the Zn--X alloy, but means the average value of the X content of the plating film after cracking. In the present specification, this is simply referred to as the X content.

When X alone is added, if the X content of the entire plating film is lower than the above range, the outer surface corrosion resistance and the fuel corrosion resistance after processing are not sufficient, while the X content is low. If X is higher than the above range, including the case where two or more kinds of X are added, the workability or outer surface corrosion resistance becomes insufficient.

In particular, the second and subsequent elements, which are added in a total amount of 5% or less, are added in order to further improve the external surface corrosion resistance, and when the total amount exceeds 5%, a slight deterioration in workability is observed. In the case of Ni alone, its content is preferably 3 to 14
% Or 9 to 18%, more preferably 10 to 14%, and further preferably 11 to 13%.

If the coating weight (amount per one side, the same applies hereinafter) is less than 5 g / m ² , the corrosion resistance after processing is insufficient, while if it is more than 50 g / m ² , the realized performance is It is saturated and uneconomical, and the weldability deteriorates. The coating weight is preferably 7 to 30 g / m ² , more preferably 10 to 25 g / m ^2.
It is.

According to the present invention, as described above, cracks are generated in the Zn-X alloy plating film at a density in the range of 1000 to 150,000 pieces / mm ² , and chromate treatment is performed on the cracks.
Fuel corrosion resistance after processing is dramatically improved. The reason is not necessarily clear, by entering the chromate into such cracks, the anchor effect that the chromate film is firmly fixed, and the increased surface area covered by the chromate film excellent in corrosion resistance due to cracks,
In the case of a non-crack type Zn-X alloy plated steel sheet, cracks are generated in the plating film during press working and the corrosion resistance is deteriorated by exposing the underlying steel sheet. It is conceivable that, by coating with a chromate film, the number of cracks newly generated during press working is small and the corrosion resistance is improved as a whole.

In the present invention, the crack density is represented by the number of regions surrounded by cracks in a 1 mm × 1 mm field of view on the surface of the plating film. The crack density was measured by randomly taking 30 SEM (scanning electron microscope) photographs of the surface of the plating film of the sample at a magnification of 1000 times, and each was in a randomly set 0.1 mm × 0.1 mm field of view. The number of regions surrounded by cracks (the number of cracks) is counted by image analysis. The average value of the number of cracks obtained from 30 photographs is calculated, and the value multiplied by 100 is taken as the crack density. The “area surrounded by cracks” is an area divided into islands by the cracks 4 as seen in the SEM photograph, as schematically shown in FIG.

According to the present invention, the crack density determined as described above is adjusted to Z of 1000 or more and 150,000 or less.
By generating cracks in the n-X alloy plating film, for example, the corrosion resistance against corrosion by gasoline or gasohol, that is, the fuel corrosion resistance after processing is dramatically improved. If the crack density is higher than 150,000, the number of cracks will be too large and the coating rate of the plating will be too small to deteriorate the fuel corrosion resistance after processing. If the crack density is less than 1000, the effect of improving the fuel corrosion resistance after processing deteriorates. Preferably, the crack density is 10
It is from 00 to 50,000.

The maximum width of cracks is 0.5 μm or less, and 90% or more thereof are present. The maximum crack width is a value obtained by measuring the maximum crack width among the cracks present in the 0.1 mm × 0.1 mm field of view of the above 30 SEM photographs. That is, one field of view is selected for each photograph, and the maximum width of each field of view is defined as the maximum width of the region, and the ratio of those having a width of 0.5 μm or less is determined. The maximum crack width is 0.5 μm or less
If it is out of the range of 90% or more, the environmental barrier effect of the plating film is hindered and both the external corrosion resistance and the fuel corrosion resistance after processing deteriorate. Preferably, the maximum crack width is 0.
90% or more of 4 μm or less is present.

The crack depth is obtained by taking a SEM photograph of the cross section at a magnification of 2000 times in the range of 1 mm in length, measuring the depth of the crack existing in this range, and comparing the depth with the plating thickness. is there. The number of cracks with a depth of 80% or more of the plating thickness shall be 80% or more of the total number of cracks. Within this range, the external corrosion resistance and the fuel corrosion resistance after processing are excellent. If the depth of cracks is shallow, such as less than 80% of the plating thickness, or if the proportion of cracks of 80% or more of the plating thickness is less than 80%, new cracks will occur during processing and the outer surface after processing Corrosion resistance and fuel corrosion resistance are impaired.

The crack density is preferably 1000 to 50,000.
90 pieces with a maximum crack width of 0.4 μm or less.
%, And 80% or more of the plating thickness is 95% or more.

The method for generating the above cracks in the Zn-X alloy plating film is not particularly limited, and a mechanical method by performing plastic working such as bending back or tension after the plating treatment is also possible. Chemical treatment by etching with an alkaline aqueous solution is preferable because it is superior in control of crack density and uniformity of cracks. In order to control the crack density and the like within the above range, for example, immersion conditions, especially time may be adjusted.

When the Zn-X alloy electroplating is carried out in an acidic bath (eg sulfate bath), this acidic plating solution can also be used for etching. That is, in the electroplating treatment in which the steel sheet is energized in an acidic bath to perform Zn-X alloy plating, as described above, the energization is stopped at the final stage of plating, and the steel sheet is immersed in the plating solution in the non-energized state. By doing so, the plating surface can be etched and cracks can be generated. As a result, etching is performed after plating using the conventional plating equipment and plating solution without using a separate treatment tank or acid or alkaline aqueous solution prepared for etching, and the necessary cracks are generated on the plating surface. Therefore, the surface-treated steel sheet of the present invention can be efficiently manufactured without increasing the number of steps, suppressing the cost. Of course, the immersion treatment of the plating solution can also be carried out in an immersion tank attached separately from the plating bath.

When the Zn-X alloy electroplated steel sheet according to the present invention is used as a fuel tank material, for example, the plating film corresponding to the inner surface of the tank is preferably dipped in an acidic plating solution as described above. Although cracks are generated as specified in the present invention, it is preferable that the plating film on the outer surface side of the tank is treated in the same manner to generate cracks similarly to the inner surface side. As a result, in addition to improving the fuel corrosion resistance on the inner surface of the tank, the corrosion resistance on the outer surface of the tank is also significantly improved. In addition, as a practical problem, performing the etching treatment by dipping in an acidic plating solution etc. on only one side of the plated steel sheet requires a step such as sealing and complicates the operation. It is also advantageous from above.

After forming the Zn-X alloy plating film specified in the present invention, at least the surface which is used unpainted and which requires corrosion resistance after processing is subjected to chromate treatment to form a chromate film on the plating film. Form and cover the cracks in the plating film with a chromate film. Even when used by coating, if the cracks of the plating film are covered with a chromate film and then the coating film is covered with the coating film, the corrosion resistance on the outer surface is dramatically improved. Therefore, the outer surface side may be chromated.

The chromate film in the present invention is made of metallic Cr.
It is formed so that the calculated adhesion amount is 10 to 200 mg / m ² . If this adhesion amount is less than 10 mg / m ² , the corrosion resistance after processing will not be sufficiently exhibited, while if it exceeds 200 mg / m ² , weldability such as seam weldability will deteriorate. The preferable amount of the chromate film deposited is 50 to 180 mg / m ² in terms of metal Cr.

This chromate film may be of coating type, electrolytic type or reactive type. When a large amount of hygroscopic Cr ⁶⁺ is contained in the chromate film, water in the fuel is adsorbed and fixed on the surface of the chromate film, so that the part may be partially corroded. Therefore, Cr in the chromate film
The proportion of ⁶⁺ is preferably as small as possible, and in that sense, it is desirable that the Cr ⁶⁺ content be 5% or less of the total Cr content.

In another preferred embodiment of the present invention, in order to further improve the corrosion resistance of the chromate film, silica is contained in the film in a SiO ₂ / Cr weight ratio of 1.0 to 10.0. If this weight ratio is less than 1.0, the effect of improving the corrosion resistance of the chromate film is insufficient, and if it exceeds 10.0, the stability of the chromate solution may deteriorate and the operation may be adversely affected. It may deteriorate. Preferably, the SiO ₂ / Cr weight ratio is from 1.5 to 9.5.

With respect to the silica species used in the present invention, dry process silica (gas phase silica or fumed silica) having less water absorption is better than wet process silica (colloidal silica or silica sol). Even if the chromate film contains silica, the metal of the chromate film
The adhesion amount in terms of Cr may be the same as above.

[0040]

The present invention will be described below in detail with reference to examples.

(Example 1) Preparation of surface-treated steel sheet sample A cold rolled steel sheet having a thickness of 0.8 mm and equivalent to JIS SPCE was subjected to Zn-X alloy electroplating in a sulfate bath under the following conditions, and this plating bath was used. Using the above as it was, the generated plated steel sheet was immersed in an acidic plating solution without electricity and the plating films on both surfaces were etched to introduce cracks into the Zn-X alloy plating film. The crack density, the maximum crack width, and the crack depth were adjusted by changing the immersion time in the plating solution. When a Zn-X alloy plating film having a low crack density and a maximum crack width of 0.5 μm or less and a small proportion of cracks is required, the plated steel sheet was biaxially stretched after etching. The crack density of the plated surface that has been etched, the maximum width of cracks,
The crack depth was obtained from the SEM photograph as described above.

[Zn-X alloy electroplating conditions] Plating bath composition: X (sulfate) 0.02 to 1.1 mol / L Zn (ZnSO ₄ ) 0.4 to 0.8 mol / L Na (Na ₂ SO ₄ ) 1 mol / L pH 1.5 to 2.0 (adjusted with sulfuric acid) Plating condition: Bath temperature 45 to 50 ° C Current density 50 to 100 A / dm ² Liquid flow rate 0.06 to 1.40 m / s Both sides were etched to generate cracks on the plating film surface. On both sides of the Zn-X alloy plated steel sheet, a coating type chromate solution having the following composition is coated by a roll coater,
A sample of the surface-treated steel sheet of the present invention was produced by baking at ~ 300 ° C to form a chromate film.

As the silica, dry process silica (trade name Aerosil 200) having an average primary particle diameter of 7 nm was used. Wet process silica (trade name Snowtex O) having an average primary particle size of 10 nm was also used in some tests.

[Composition of chromate treatment liquid] Cr ³⁺ : 50 g / L Cr ⁶⁺ : 2 g / L SiO ₂ : 170 g / L Fuel corrosion resistance of the surface-treated steel sheet thus produced to gasoline and alcohol-containing fuel , Outer surface corrosion resistance, and weldability were tested by the following methods. The test results are summarized in Table 1.

Regarding fuel corrosion resistance, a comparison with the prior art is shown in the graph of FIG. In the figure, Table 1 shows examples of the present invention.
Example No. 1 of No. 1 is used, and the conventional example in which no crack is provided at this time is shown as having no crack. However, depending on the presence or absence of the crack, the fuel corrosion resistance is improved about three times or more.

In the graph, the turn sheet (Sn / Pb: 0.
10, the adhesion amount of 45 g / m ² ) is quite poor in fuel corrosion resistance,
It is speculated that the turn sheet has a large amount of corrosion from the punch shoulder to the wall, and the corrosion is likely to proceed in the portion where the plating film is damaged by the processing.

Test Method [Fuel Corrosion Resistance] A blank of the surface-treated steel sheet (press punching test piece) was cylindrically formed under the following conditions to prepare a cup, and 30 ml of gasoline (gasohol) having the following composition was placed in the cup. The container was sealed, the container was sealed, and the fuel corrosion resistance was evaluated according to the following criteria by the maximum erosion depth (Pm) on the inner surface on the 180th day (n = 2).

◎: Pm <0.1 mm ○: 0.1 mm ≤ Pm <0.2 mm △: 0.2 mm ≤ Pm <0.5 mm ×: 0.5 mm ≤ Pm Cylindrical drawing conditions Blank diameter: 100 mm (diameter) Bunch diameter : 50mm (Diameter, Shoulder r = 5mm) Die diameter: 51mm (Diameter, Shoulder r = 5mm) Blank holder pressure: 10KN Drawing height: 30mm Surface roughness: # 1200 Grinding without lubricant (degreasing before molding) ) Degreasing conditions 2% Lidosol immersion (liquid temperature 53 ℃) 3 minutes → pure water immersion (
Normal temperature) 1.5 minutes → Drying (165 ℃) 8 minutes → Normal temperature 20 minutes →
Dry (165 ° C) 15 minutes (Note) Aggressive methanol
Is 95% anhydrous methanol and 0.1% NaCl + 0.08% Na ₂ SO ₄
+ 10% formic acid mixed with 5% aqueous solution.

[External Corrosion Resistance] A blank of the surface-treated steel sheet was
SST (salt spray test) according to JIS Z2371 on the outer surface after cylindrical draw forming under the same conditions as the above fuel corrosion resistance test except that the drawing height was changed to 25 mm, then sealing the edge part. For 2000 hours. The corrosion resistance after processing was evaluated by the maximum erosion depth (Pm) after 2000 hours of SST.

◎: Pm <0.1 mm ○: 0.1 mm ≤ Pm <0.4 mm △: 0.4 mm ≤ Pm <0.8 mm ×: 0.8 mm ≤ Pm [Welding property] After performing a continuous seam welding test for 100 m under the following conditions: The cross section of the welded portion was observed microscopically and evaluated according to the following criteria.

Seam welding conditions Pressurization force: 300 kgf Energization time: 3 cycles Pause time: 2 cycles Current: 13,000A Speed: 2.5 m / min Weldability evaluation standard ○: Welding good △: Blowhole present ×: Unwelded part .

(Example 2) In this example, in order to show that the corrosion resistance after working is remarkably improved by providing cracks, the post-working corrosion resistance test of the surface-treated steel sheet having the plating film and the chromate film shown in Table 2 was conducted. The results are shown in Fig. 4. In the case of the examples of the present invention, both the maximum width and the depth of the cracks were within the scope of the present invention. In the example of the present invention, almost no erosion was observed even after 2000 hours of SST (salt spray test), but in the conventional example
Erosion with a depth of 0.8 mm and 0.6 mm was also seen in the comparative example.

(Example 3) In this example, the effect of the crack depth on the corrosion resistance after working is examined. As shown in Table 3, the crack depth is less than 80% of the plating thickness. 0 to 70% of crack depth is 80% or more
The effect on the external surface corrosion resistance is considered. As a result, when this ratio is less than 80%, the corrosion resistance of evaluation “Δ” or “x” is observed, which is a practical problem. Therefore, the effect of improving the external corrosion resistance is practically the case where the crack depth of 80% or more of the plating thickness is 80% or more.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

[0057]

EFFECTS OF THE INVENTION The surface-treated steel sheet according to the present invention exhibits high fuel corrosion resistance not only for gasoline but also for alcohol-containing fuels such as gasohol when used in the production of fuel tanks, and has a high fuel corrosion resistance. It can be manufactured efficiently and cheaply using the X alloy electroplating equipment as it is, and is excellent in safety because it does not contain Pb which is harmful to the human body.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a cross-sectional structure of a plating film of a surface-treated steel sheet according to the present invention.

FIG. 2 is a schematic diagram of cracks on the surface of a plating film.

FIG. 3 is a graph showing the results of examples of the fuel corrosion resistance after working of the surface-treated steel sheet according to the present invention and the conventional surface-treated steel sheet.

FIG. 4 is a graph showing test results of post-processing corrosion resistance of surface-treated steel sheets.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Eiji Kajiyama, 3 Hikari, Kashima City, Ibaraki Prefecture Sumitomo Metal Industries, Ltd. Inside Kashima Steel Works (72) Inventor, Yoshihiro Kawanishi 3 Hikari, Kashima City, Ibaraki Sumitomo Metal Industries, Ltd. Kashima Steel Works

Claims (1)

[Claims] Claim: What is claimed is: 1. Zn coated on a steel sheet, having a deposition amount on one side of 5 to 50 g / m ² .
-X alloy electroplating film is provided, and the alloy composition is X
However, Ni: 3-18 wt%, Co: 0.02-3 wt%, Mn: 25-45 wt
%, Cr: at least 1 selected from the group consisting of 8 to 20 wt%
Including a seed, and a metal on the Zn-X alloy plating film.
Zn-X, which is the lower layer of the chromate film, is made of a surface-treated steel sheet with a chromate film of 10 to ²⁰⁰ mg / m ² in terms of Cr
The alloy plating film has cracks, and the density of the cracks is in the range of 1000 to 150,000 in terms of the number of regions surrounded by cracks in the visual field of 1 mm x 1 mm on the plating surface,
Moreover, 90% or more of the cracks have a maximum width of 0.5 μm or less, and the depth of the cracks depends on the plating film thickness.
80% or more of which is 80% or more,
Surface-treated steel sheet with excellent corrosion resistance after processing.