JP2985768B2 - Surface-treated steel sheet with excellent corrosion resistance after processing - Google Patents

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 to a fuel tank for automobiles and motorcycles, which exhibits high corrosion resistance to fuels such as gasoline and gasohol, and also to a stove and a boiler. The present invention relates to a surface-treated steel sheet suitable for applications such as a kerosene tank, an oil filter that requires strict workability and high corrosion resistance after processing.

[0002]

2. Description of the Related Art Materials for fuel tanks such as automobiles and motorcycles have not only weldability but also general corrosion resistance on the outer side (hereinafter referred to as outer corrosion resistance), and inner side has a fuel corrosion resistance against gasoline and other fuels. Is required. These are collectively called corrosion resistance or corrosion resistance after processing. Conventionally, turn sheets (10 to 25% Sn-Pb alloy plated steel sheets) have been widely used as fuel tank materials. However, Pb in the plating film is harmful to the human body, the plating film is easily dissolved in the alcohol oxide when alcohol-containing fuel is used, and pinholes in the plating film are inevitable.
There is a problem in that Fe that is more base than the plating film is preferentially corroded from the pinholes, resulting in insufficient perforation corrosion resistance. Therefore, alternative materials have been demanded.

In particular, in recent years, gasoline / alcohol mixed fuels (such as M15 containing about 15% methanol, M85 containing about 85% by weight of methanol, etc.) are called gasohol due to emission regulations in consideration of environmental issues. The use of representative 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 having excellent fuel corrosion resistance to the alcohol-containing fuel.

[0004] From this point of view, application of a Zn-Ni alloy electroplated steel sheet to a fuel tank has been conventionally considered in consideration of corrosion resistance after processing and cost. The following publications can be cited as prior art.

JP-A-58-45396 discloses that the Ni content is 5%.
A surface-treated steel plate for a fuel tank is shown, in which a chromate treatment is applied on a Zn-Ni alloy electroplating having a thickness of 0.5 to 20 μm and a thickness of 0.5 to 20 μm.

JP-A-5-106058 discloses that the Ni content is 8
A surface-treated steel sheet for a fuel tank, which is provided with a Zn—Ni alloy plating of 2020 wt% at an adhesion amount of 10-60 g / m ² and subjected to a chromate treatment, is shown.

[0007] These surface-treated steel sheets have very good external corrosion resistance, but their fuel corrosion resistance, especially after processing, is still not sufficient. For example, salt water is mixed in alcohol-containing fuel. Corrosion was likely to occur in very harsh environments such as cases. Further, when the chromate film or the plating film is thickened to improve this, there is a problem that weldability, which is an important performance as a material for a fuel tank, is deteriorated.

As a matter of course, techniques for providing cracks in a plating layer are disclosed in JP-A-5-25679 and JP-A-4-33.
Although these are disclosed in Japanese Patent No. 7099, these are intended to improve the adhesion of the upper plating layer by providing an additional plating layer on the underlying plating layer provided with cracks by the anchor effect. This is intended to improve the impact adhesion resistance when used for an outer panel of an automobile.

Furthermore, Japanese Patent Application Laid-Open No. 62-297490 discloses that a crack is formed in a Ni alloy plating layer.
Thereby, a fine uneven pattern is formed on the surface to realize blackening.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art of Zn-Ni alloy plating + chromate surface-treated steel sheet. The purpose is to develop a technology that can improve the fuel corrosion resistance after processing without impairing the weldability.

[0011]

The inventors of the present invention have conducted various studies to solve the above-mentioned problems, and as a result, have found that continuous Zn-X (X = Ni, Co, Mn and Cr) using an acidic bath. Kind of or two or more types, hereinafter referred to as X) In the final stage of the alloy electroplating process, the energization is stopped, immersion is performed for a short time in a plating solution to create a crack, and a Zn-based thin plating is applied thereon, Chromate treatment or Zn-X alloy plating film on which cracks are formed, Zn-based thin plating is applied, this surface is also cracked, and then subjected to chromate treatment to improve the corrosion resistance after processing, especially the resistance to corrosion. It was noticed that the fuel corrosivity was significantly improved. As a result of investigating the cause, the immersion in the acidic plating solution caused cracks in the Zn-X alloy plating film, and the crack density, the maximum width and the depth of the cracks on the plating surface thus formed were within a specific range. Found that fuel corrosion resistance after processing was improved,
The present invention has been completed.

Here, the present invention provides a Zn—X alloy electroplating film having a coating amount on one side of 5 to 50 g / m ² on a steel plate, wherein X is Ni: 3 to 18 wt%, Co: 0.02-3wt
%, Mn: 25 to 45 wt%, and Cr: at least one selected from the group consisting of 8 to 20 wt%. The Zn-X alloy plating film is coated with Zn-based plating at 7 g / m ² or less. above,
Metal Cr equivalent coating amount becomes the surface treated steel sheet having a chromate film of ^{10~200 mg / m 2, Zn-} X alloy plating film having a crack, 1m in the density of the cracks plated surface
The number of areas surrounded by cracks in the field of view of m × 1 mm is in the range of 1000 to 150,000, and the cracks with a maximum width of 0.5 μm or less exist in 90% or more, and the crack depth is Is a surface-treated steel sheet excellent in corrosion resistance after processing, characterized in that 80% or more of the plating film thickness is present.

From another aspect, the present invention provides a Zn—X alloy electroplating film having a coating weight on one side of 5 to 50 g / m ² on a steel plate, wherein X is Ni: 3 to 18wt%, Co: 0.02
-3 wt%, Mn: 25-45 wt%, Cr: at least one selected from the group consisting of 8-20 wt%, and a Zn-based plating of 7 g / m ² or less is applied on the Zn-X alloy plating film, further thereon, a metal Cr equivalent coating amount becomes the surface treated steel sheet having a chromate film of 10 to 200 mg / m ^2, has a crack Zn-X alloy plating film, the density of the cracks in the plated surface Expressed by the number of areas surrounded by cracks in the field of view of 1 mm × 1 mm is a range of 1000 to 150,000,
90% or more of the cracks have a maximum width of 0.5 μm or less, and 80% or more of the cracks have a crack depth of 80% or more of the plating film thickness. This is a surface-treated steel sheet having excellent corrosion resistance after processing, characterized by having been subjected to a heat treatment.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the accompanying drawings is a schematic explanatory view showing a cross-sectional structure of a plating film of a surface-treated steel sheet according to the present invention. In FIG. The coating 2 is provided thereon with a chromate coating 3 via a Zn-based plating film 5, and the plating film 2 is provided with a crack 4. May be provided.

[0015] The Zn- of the plated steel sheet used in the present invention.
The alloy composition of the X alloy plating film is as follows: 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: However, when X includes two or more alloying elements, the second and subsequent elements are preferably Ni:
3 to 18 wt%, Co: 0.02 to 3 wt%, Mn: 25 to 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 may be limited to 5% or less. The X content of the plating film does not mean the X content immediately after the Zn-X alloy electroplating, but means the average value of the X content after cracking of the plating film. In the present specification, this is simply referred to as X content.

When the X content of the entire plating is not more than the above range when each of X alone is added, the corrosion resistance after processing and the fuel corrosion resistance are not sufficient, while the X content is If each of them is higher than the above range, including the case where two or more kinds of X are added, the workability or the external corrosion resistance becomes insufficient. In particular, the second and subsequent elements added in a total amount of 5% or less are added for further improving the external corrosion resistance. When the total amount exceeds 5%, workability is slightly deteriorated.

In the case of using Ni alone, the content is preferably 3 to 14% or 9 to 18%, more preferably 10 to 14%, and further preferably 11 to 13%. Also, the coating weight
(The amount per one side, the same applies hereinafter) is less than 5 g / m ² ,
If the corrosion resistance after processing is insufficient, on the other hand, if it is more than 50 g / m ² , the achieved performance 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 .

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 / mm ² , and a chromate treatment is performed thereon.
Fuel corrosion resistance after processing is dramatically improved. Although the reason is not necessarily clear, the incorporation of chromate into such cracks increases the anchor effect of the chromate film being firmly fixed and the surface area covered by the chromate film having excellent corrosion resistance to the cracks. In non-cracked Zn-X alloy plated steel sheets, cracks occur in the plating film during press working, and the corrosion resistance is deteriorated by exposing the base steel sheet.On the other hand, cracks are generated in the plating film in advance, and the cracks are generated. By covering with a chromate film, it is conceivable that cracks newly generated at the time of press working are small and the corrosion resistance after working as a whole is improved.

In the present invention, the crack density is measured for the Zn—X alloy plating film on the original plate before the upper thin plating is applied, and the area surrounded by the cracks in the visual field of 1 mm × 1 mm on the plating film surface is measured. It is represented by the number of This crack density was measured using a 1000x magnification SEM on the surface of the plated film of the sample.
(Scanning electron microscope) Thirty photos are taken at random, and the number of crack-enclosed areas (number of cracks) in a 0.1 mm x 0.1 mm field of view randomly set for each photo is counted by image analysis. It is done by doing. One field of view is measured for one photograph. The average value of the number of cracks obtained from 30 photographs is calculated, and the value multiplied by 100 is defined as the crack density. The “region surrounded by cracks” is, as schematically shown in FIG. 2, a region which is seen in an SEM photograph and is divided into islands by cracks.

According to the present invention, Z is determined so that the crack density determined in this way is not less than 1000 and not more than 150,000.
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 remarkably improved. If the crack density is more than 150,000, the number of cracks is too large, and the plating coverage becomes too small, so that the fuel corrosion resistance after processing deteriorates. If the crack density is less than 1,000, the effect of improving the fuel corrosion resistance after processing is not sufficient. Preferably, the crack density is 1000 to 50,000.

It is assumed that 90% or more of the cracks have a maximum width of 0.5 μm or less. The maximum width of the crack is a value obtained by measuring the largest crack width among the cracks existing in the visual field of 0.1 mm × 0.1 mm of the above 30 SEM photographs. That is, 1 for each photo
The individual field of view is selected, and the maximum width of each field is defined as the maximum width of the area, and the ratio of those having a maximum width of 0.5 μm or less is determined. If the maximum crack width is 0.5 μm or less and the ratio is out of the range of 90% or more, the environmental barrier effect of the plating film is impaired, and both the outer corrosion resistance after processing and the fuel corrosion resistance deteriorate. Preferably a maximum crack width of 0.4
90% or more of the particles having a size of μm or less exist.

The crack depth was obtained by taking an SEM photograph of a cross section at a magnification of 2000 times within a range of 1 mm in length, measuring the crack depth in this range, and comparing this with the plating thickness. The number of cracks with a depth of 80% or more of the plating thickness
When 80% or more of the total number of cracks in this visual field is present, the outer surface corrosion resistance and the fuel corrosion resistance after processing are excellent. If the crack depth is shallow, such as less than 80% of the plating thickness, or if the proportion of cracks that are 80% or more of the plating thickness is less than 80%, cracks will be newly generated during processing,
Corrosion resistance and fuel corrosion resistance after processing are impaired.

Preferably, the crack density is 1000 to 50,000
90 cracks with a maximum crack width of 0.4 μm or less
% Or more, and 80% or more of the plating thickness is 95% or more.

The method for generating the cracks on the surface of 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 acid or alkali aqueous solution is preferable because it is excellent in control of crack density and uniformity of cracks. In order to control the crack density or the like within the above range, for example, the immersion conditions, particularly the time, may be adjusted.

When the Zn-X alloy electroplating is performed in an acidic bath (eg, a sulfate bath), the acidic plating solution can be used for etching. That is, in the electroplating process in which a 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 a plating solution in a non-energized state. By doing so, the plating surface can be etched and cracks can be generated. As a result, the post-plating etching is performed using the conventional plating equipment and plating solution without using a separate processing tank or acid or alkali aqueous solution prepared for etching, and necessary cracks are generated on the plating surface. Thus, the surface-treated steel sheet of the present invention can be manufactured efficiently without reducing the cost and increasing the number of steps. Of course, the immersion treatment of the plating solution can be performed in an immersion tank provided separately from the plating bath.

When the Zn-X alloy electroplated steel sheet according to the present invention is used, for example, as a material for a fuel tank, the plating film corresponding to the inner surface of the tank is preferably immersed in an acidic plating solution as described above. Causes cracks as defined in the present invention, but it is preferable to treat the plating film on the outer surface side of the tank in the same manner to generate cracks as on the inner surface side. Thereby, in addition to the improvement of the fuel corrosion resistance of the tank inner surface, the corrosion resistance of the tank outer surface is also significantly improved. Also, as a practical matter, performing an etching treatment by immersion in an acidic plating solution or the like on only one side of a plated steel sheet requires a step of sealing or the like, and the operation becomes complicated. It is also advantageous from above.

In the present invention, a Zn-based plating (also referred to as Zn-based thin plating) film is further provided on the Zn-X alloy electroplating film.
Although an alloy composition different from that of the X alloy plating may be used, the same composition is more advantageous in the process. Examples of another type of Zn-based plating include pure Zn plating and Zn-Fe plating. The amount of adhesion is preferably 5 g / m ² or less in terms of cost. Even when cracks occur during processing by providing such a Zn-based thin plating film and forming the plating into two layers, the generated cracks are divided between the upper plating and the lower plating, so that the cracks over the entire plating thickness are formed. Is suppressed, and the exposure of the iron base is prevented. For this reason, it shows excellent outer surface corrosion resistance and fuel corrosion resistance.

In another embodiment of the present invention, cracks are also provided in the Zn-based plating film. However, the method for generating cracks in such Zn-based thin plating is not particularly limited.
As in the method of generating cracks in the underlying Zn-X plating film, etching in a plating solution is desirable. The density and width of the cracks at this time are not particularly limited, but it is preferable that 90% or more of the cracks have a size of 1,000 to 150,000 and a maximum width of 0.5 μm or less based on the same criteria as described above. The crack depth is preferably 80% or more of the plating thickness.
It is better to exist.

After forming each of the plating films defined in the present invention, at least a surface which is used without being painted and needs corrosion resistance after processing is subjected to a chromate treatment to form a chromate film on the plating film. Then, the cracks in the plating film are covered with a chromate film. Even in the case of coating applications, if the cracks in 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, and the outer surface may be subjected to a chromate treatment.

In the present invention, the chromate film is made of metal Cr.
It is formed so that the conversion amount becomes 10 to 200 mg / m ² . If the amount is less than 10 mg / m ² , the corrosion resistance after processing is not sufficiently exhibited, while if it exceeds 200 mg / m ² , weldability such as seam weldability deteriorates. The preferable coating amount of the chromate film is 50 to 180 mg / m ² in terms of metal Cr.

The chromate film may be any of a coating type, an electrolytic type and a reactive type. If a large amount of hygroscopic Cr ⁶⁺ is contained in the chromate film, the moisture in the fuel is adsorbed and fixed on the surface of the chromate film, and the portion may be partially corroded. Therefore, Cr in the chromate film
It is preferable that the proportion of ⁶⁺ is as small as possible. In that sense, it is desirable that the amount of Cr ^{6+ be} 5% or less of the total amount of Cr.

In another preferred embodiment of the present invention, in order to further enhance 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 the 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 be degraded and the operation may be adversely affected. 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 low water absorption is better than wet process silica (colloidal silica or silica sol). Even when the chromate film contains silica, the metal of the chromate film
The adhesion amount in terms of Cr may be the same as described above.

[0034]

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

(Examples 1 and 2) Preparation of surface-treated steel sheet sample A 0.8 mm thick cold-rolled steel sheet equivalent to JIS SPCE was subjected to Zn-X alloy electroplating in a sulfate bath on both sides under the following conditions. Using the plating bath as it is, the plating film on both surfaces was etched by immersing the produced plated steel sheet in an acidic plating solution without electricity, and cracks were introduced into the Zn-X alloy plating film. The crack density, maximum crack width, and 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 large maximum crack width was required, biaxial tension was applied to the plated steel sheet after etching. The crack density, maximum crack width, and crack depth of the etched plating film were determined from SEM photographs as described above.

[Zn-X alloy electroplating conditions] Plating bath composition (both base and thin plating): 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 conditions: Bath temperature 45 to 50 ° C Current density 50 to 100 A / dm ^Two- fluid flow rate 0.06 to 1.40 m / s Cracks in the Zn-Y (Y = Ni, Co, M
(n, Cr) -based alloy electroplating was performed by the same operation (Example 1).

In Example 2, a thin plating of this Zn-Y alloy was provided, and after the energization was stopped, the coating was immersed in the same plating solution to provide cracks. A coating type chromate solution having the following composition is applied by a roll coater to the plated surface of the steel sheet provided with the Zn-X alloy electroplated film and the Zn-Y alloy electroplated film, and baked at 150 to 300 ° C. To form a chromate film, thereby preparing a sample of the surface-treated steel sheet of the present invention.

As silica, dry silica (trade name Aerosil 200) having an average primary particle diameter of 7 nm was used. In some tests, wet-process silica (trade name: Snowtex O) having an average primary particle diameter of 10 nm was also used.

[Composition of the chromate treatment liquid] Cr ³⁺ : 30 g / L Cr ⁶⁺ : 2 g / L SiO ₂ : 70 g / L Fuel corrosion resistance of the surface-treated steel sheet produced in this way to gasoline and alcohol-containing fuel , External corrosion resistance, and weldability were tested by the following methods. The test results are summarized in Tables 1 and 2.

FIG. 3 is a graph showing a comparison with the prior art regarding the fuel corrosion resistance. In the drawing, Table 1 is used as an example of the present invention.
No. 1 was used, and a comparative example in which no crack was provided at this time is shown as "no crack". The fuel corrosion resistance is more than three times improved depending on the presence or absence of the crack. In addition,
The turn sheet (Sn / Pb: 0.10, adhesion amount: 45 g / m ² ) showed significant corrosion of the wall from the shoulder of the punch, and the corrosion was particularly likely to occur in the part where the plating was damaged due to processing.

Test Method [Fuel Corrosion Resistance] A blank (press punched test piece) of a surface-treated steel sheet was cylindrically drawn 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 closed, and the fuel corrosion resistance was evaluated based on the following criteria based on the maximum erosion depth (Pm) of 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: 50 mm (diameter , Shoulder r = 5mm) Die diameter: 51mm (diameter, shoulder r = 5mm) Blank holder pressure: 10KN Draw height: 30mm Surface roughness: # 1200 Polished every time Molding without lubricant (degreasing before molding) Degreasing condition 2 % Lid sole immersion (Liquid temperature 53 ℃) 3 minutes → Pure water immersion (
Room temperature) 1.5 minutes → Dry (165 ° C) 8 minutes → Room temperature 20 minutes →
Drying (165 ° C) for 15 minutes (Note) Aggressive methanol
Is 95% anhydrous methanol, 0.1% NaCl + 0.08% Na ₂ SO ₄
+ 5% aqueous solution containing 10% formic acid.

[Outer surface corrosion resistance]
Except that the drawing height was changed to 25 mm, after cylindrical drawing was performed under the same conditions as the above fuel corrosion resistance test, the edges were sealed, and the outer surface was subjected to SST (salt spray test) according to JIS Z2371. For 2000 hours. Outer surface corrosion resistance after processing is SST 20
It was evaluated by the maximum pit depth (Pm) after 00 hours.

◎: Pm <0.1 mm ○: 0.1 mm ≦ Pm <0.4 mm △: 0.4 mm ≦ Pm <0.8 mm ×: 0.8 mm ≦ Pm [Weldability] After conducting a continuous seam welding test under the following conditions for 100 m , And a cross-sectional microscopic observation of the welded portion was performed and evaluated according to the following criteria.

Seam welding conditions Pressure: 300 kgf Energizing time: 3 cycles Resting time: 2 cycles Current: 13,000A Speed: 2.5 m / min Weldability evaluation standard ○: Good welding ×: Blow hole present

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

The surface-treated steel sheet according to the present invention, when used in the manufacture of a fuel tank, exhibits high fuel corrosion resistance not only to gasoline but also to alcohol-containing fuels such as gasohol and the like. It can be manufactured efficiently and inexpensively using the X alloy electroplating apparatus as it is, and is excellent in safety because it does not contain Pb harmful to the human body.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram of a surface crack of a plating film.

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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Eiji Kajiyama 3, Kaji-shi, Kashima-shi, Ibaraki Sumitomo Metal Industries, Ltd. Inside Kashima Works (72) Inventor Yoshihiro Kawanishi 3, Kaji-shi, Kashima-shi, Ibaraki Sumitomo Metal Industries Kajima Works, Ltd. (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 28/00

Claims (2)

(57) [Claims] 1. Zn on a steel sheet having a single-sided adhesion amount of 5 to 50 g / m ² .
-X alloy electroplating film is provided, and its alloy composition is X
However, Ni: 3 to 18 wt%, Co: 0.02 to 3 wt%, Mn: 25 to 45 wt%
%, Cr: at least one selected from the group consisting of 8 to 20 wt%
Seed-containing Zn-X alloy plating film is coated with Zn-based plating of 7 g / m ² or less, and a chromate film with a coating weight of 10 to 200 mg / m ^{2 in} terms of metal Cr is further provided on the surface. Made of treated steel sheet, Zn-X alloy plating film has cracks,
The density of this crack is 1000 to 1500 expressed as the number of areas surrounded by cracks in a 1 mm × 1 mm field of view of the plating surface.
Range of 00 and the maximum width of this crack is 0.5 μm
A surface-treated steel sheet having excellent corrosion resistance after processing, characterized in that at least 90% of the following are present and at least 80% of cracks have a depth of at least 80% of the thickness of the plating film. ^2. Zn having a single-sided adhesion amount of 5 to 50 g / m ² on a steel plate.
-X alloy electroplating film is provided, and its alloy composition is X
However, Ni: 3 to 18 wt%, Co: 0.02 to 3 wt%, Mn: 25 to 45 wt%
%, Cr: at least one selected from the group consisting of 8 to 20 wt%
Seed-containing Zn-X alloy plating film is coated with Zn-based plating of 7 g / m ² or less, and a chromate film with a coating weight of 10 to 200 mg / m ^{2 in} terms of metal Cr is further provided on the surface. Made of treated steel sheet, Zn-X alloy plating film has cracks,
The density of this crack is 1000 to 1500 expressed as the number of areas surrounded by cracks in a 1 mm × 1 mm field of view of the plating surface.
Range of 00 and the maximum width of this crack is 0.5 μm
90% or more of the following are present, crack depth is 80% or more of the plating film thickness is 80% or more, and Zn
A surface-treated steel sheet with excellent corrosion resistance after processing, characterized in that the surface of the system plating film is also cracked.