JP2985771B2 - Surface-treated steel sheet with excellent corrosion resistance after processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fuel tank for automobiles and motorcycles, which exhibits high corrosion resistance to steel plates having excellent corrosion resistance after processing, especially to gasoline, gasohol and other fuels.
Furthermore, the present invention relates to a surface-treated steel sheet suitable for applications such as kerosene tanks such as stoves and boilers, and oil filters that require severe processing 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 surface (hereinafter referred to as outer corrosion resistance), and the inner surface has fuel corrosion resistance against fuel such as gasoline. 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.

[0006] 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. An object of the present invention is to develop a technology capable of improving fuel corrosion resistance after processing without impairing weldability and without substantially increasing cost.

[0011]

Means for Solving the Problems The present inventors have conducted various studies in order to solve such problems, and as a result, have found that a continuous Zn-X (X = Ni, Co, Mn, Cr) using an acidic bath. Kinds of or two or more kinds, hereinafter referred to as X) When the current is stopped at the final stage of the alloy electroplating step and immersion is performed for a short time in a plating solution, corrosion resistance after processing, particularly fuel corrosion resistance, is remarkably improved. I noticed that. As a result of investigating the cause, cracking occurs in the Zn-X alloy plating layer by immersion in this acidic plating solution, and the crack density on the plating surface thus generated and the maximum width of the crack are within a specific range. The present inventors have found that when the plating film having this crack is coated with a chromate film, the fuel corrosion resistance after processing is improved, and the present invention has been completed.

In the present invention, the alloy composition of the Zn—X alloy of the plating film provided on the uppermost layer 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%
% Is a Zn-X alloy electroplated film containing at least one member selected from the group consisting of: and having a coating weight of 7 g / m ² or less. the less noble Zn-based plating layer has 10 to 50 g / m ^2, Zn system having a chromate film of 10 to 200 mg / m ² further metal Cr equivalent coating weight on the uppermost of Zn-X alloy plating film Made of multi-layer plated surface treated steel sheet, and Zn-X
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 areas surrounded by cracks in a visual field of 1 mm x 1 mm on the plating surface,
A surface-treated steel sheet having excellent corrosion resistance after processing, characterized in that 90% or more of the cracks have a maximum width of 0.5 μm or less.

[0013]

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. An X alloy plating film 2 is provided thereon, and a chromate film 3 is provided thereon, and a crack 4 is provided on the plating film 2. A Zn-based plating film 8 is provided as a lower layer of the uppermost plating film 2.

In the present invention, the alloy composition of the Zn—X alloy plating film of the uppermost layer of the plated steel sheet used is such that X is Ni:
3 to 18 wt%, Co: 0.02 to 3 wt%, Mn: 25 to 45 wt%, Cr:
At least one member selected from the group consisting of 8 to 20% by weight. However, when X includes two or more alloying elements,
Preferably, the second and subsequent elements are, like the first element, Ni: 3 to 18% by weight, Co: 0.02 to 3% by weight, Mn: 25 to 45% by weight.
%, Cr: 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 uppermost plating film means not the X content immediately after the Zn—X alloy electroplating of the uppermost layer, but the average value of the X content after cracking of the uppermost plating film. . In the present specification, this is simply referred to as X content.

If the X content of the uppermost layer is X alone, and if the X content is too low in each case, the outer corrosion resistance and the fuel corrosion resistance after processing are not sufficient. Including the case where two or more kinds of X are added, if each is higher than the above range, the workability or the outer surface corrosion resistance becomes insufficient.

In particular, the second and subsequent elements are added for a total improvement of 5% or less to further improve the external corrosion resistance. When the total amount exceeds 5%, the workability is slightly deteriorated. In the case of Ni alone, its content is preferably 3 to 14%.
Alternatively, it is 9 to 18%, more preferably 10 to 14%, and still more preferably 11 to 13%.

If the coating weight of the uppermost layer exceeds 7 g / m ² , workability and weldability are deteriorated. The plating weight of the uppermost layer is preferably ² to 6 g / m ² . The lower plating film may be any Zn-based plating that is more potential than the above-described uppermost plating film.
Examples include pure Zn plating and Zn-Fe alloy plating.

If the coating weight of the lower layer (amount per one side, the same applies hereinafter) is less than 10 g / m ^{2, the} corrosion resistance of the outer surface after processing is insufficient, and if it is more than 50 g / m ² , The performance is saturated and uneconomical, and the weldability deteriorates. The coating weight of the lower plating film is preferably 12 to 30.
g / m ^2, more preferably 15-25 g / m ^2.

The lower plating film may be provided directly on the surface of the steel sheet, or may be further provided as a pre-plating layer, for example, on a Ni plating layer provided in advance, or may be provided on another plating film. It may be determined appropriately as needed.

According to the present invention, as described above, cracks are generated in the uppermost Zn—X alloy plating film at a density in the range of 1000 to 150,000 / mm ² , and chromate treatment is performed thereon. The subsequent fuel corrosion resistance 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 the case of 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 as a whole is improved.

In the present invention, the crack density is represented by the number of regions surrounded by cracks in a visual field of 1 mm × 1 mm on the plating film surface. This crack density measurement is in a 0.1 mm x 0.1 mm field of view that randomly takes 30 SEM (scanning electron microscope) photographs of the plating film surface of the sample at a magnification of 1000 times and sets each photograph at random. This is performed by counting the number of areas surrounded by cracks (the number of cracks) by image analysis. 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 4.

According to the present invention, cracks are generated on the surface of the Zn—X alloy plating film on the upper layer so that the crack density thus obtained is not less than 1000 and not more than 150,000, so that, for example, The corrosion resistance against gasohol corrosion, that is, the fuel corrosion resistance after processing is dramatically 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. Also,
If the crack density is less than 1,000, the effect of improving the fuel corrosion resistance after processing is hardly obtained. 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. One field of view is selected for each photograph, and the maximum width in each field is defined as the maximum crack width. That is, one field of view is selected for each photograph, and the maximum width of each field is defined as the maximum width of the area.
The ratio of those having a size 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 hindered,
Both the outer corrosion resistance and the fuel corrosion resistance after processing deteriorate. Preferably, 90% or more of the cracks have a maximum width of 0.4 μm or less.

The method for generating the cracks on the surface of the Zn—X alloy plating film as the uppermost layer 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. However, the chemical treatment by etching with an acid or alkali aqueous solution is preferable because it is superior in control of crack density and crack uniformity. 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.

An electroplating of the uppermost Zn-X alloy 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 surface-treated plated steel sheet according to the present invention is used, for example, as a fuel tank material, the plating film corresponding to the inner surface of the tank is preferably immersed in an acidic plating solution as described above. Although cracks are generated as specified, it is preferable to treat the plating film on the outer surface of the tank in the same manner to generate cracks as on the inner surface. 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, this Zn-X alloy electroplated film is used as the uppermost layer, and the underlying layer is electrically lower in potential.
Although a Zn-based plating film is provided, even if a crack is formed during processing by providing such a lower layer and forming the plating into two layers, the cracks generated are separated between the upper plating or the lower plating, The occurrence of cracks over the entire thickness of the plating is suppressed, and the exposure of the ground iron is prevented. In addition, in the case of the present invention, since the base Zn-based plating film is provided in the lower layer, even if the ground iron is exposed, the base base is provided with a potential-base Zn-based material, so that even more excellent external corrosion resistance and fuel resistance are provided. It is corrosive.

In the present invention, after forming each such plating film, a chromate treatment is applied to at least a surface which is used without being painted and needs corrosion resistance after processing, and a chromate film is formed on the plating film. The cracks in the plating film are covered with a chromate film. Even when painted and used, the cracks in the plating film are covered with a chromate film,
If the surface is coated with a paint film, the corrosion resistance of the outer surface is dramatically improved. Therefore, 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.

Preparation of Surface-treated Steel Sheet Sample A 0.8 mm thick cold-rolled steel sheet equivalent to JIS SPCE was subjected to Zn-based electroplating using a ZnSO ₄ aqueous solution to which iron sulfate was added as a lower layer as necessary as a plating solution under the following conditions. Next, Zn-X alloy electroplating using a sulfate bath is applied on both sides, and the resulting plated steel sheet is immersed in an acidic plating solution without electricity by using the plating bath as it is, thereby plating both sides. The film was etched to introduce cracks in the Zn-X alloy plating film. The crack density and the maximum crack width were adjusted by changing the immersion time in the plating solution. The crack density and the maximum width of the crack on the etched plating surface were obtained from the SEM photograph as described above.

[Zn-based electroplating conditions] Plating bath composition: Fe (FeSO ₄ ) 0.02 to 1.1 mol / L (for Zn-Fe alloy) Zn (ZnSO ₄ ) 0.4 to 0.8 mol / L Na (Na ₂ SO ₄₎ ) 1 mol / L pH 1.5 ~ 2.0 (adjusted by sulfuric acid) Plating condition: Bath temperature 45 ~ 50 ℃ Current density 50 ~ 100 A / dm ² solution flow rate 0.06 ~ 1.40 m / s [Zn-X alloy electroplating condition] plating bath composition: X (sulfate) 0.02 ~1.1 mol / L Zn ( ZnSO 4) 0.4 ~0.8 mol / L Na (Na 2 SO 4) 1 mol / L pH 1.5~2.0 ( adjusted with sulfuric acid) plating conditions: Bath temperature 45 to 50 ° C Current density 50 to 100 A / dm ² solution flow rate 0.06 to 1.40 m / s Both sides of a two-layer Zn-X alloy plated steel sheet that has both surfaces etched and cracked in the plating film Apply the coating type chromate liquid of the composition with a roll coater and apply 150
By baking at ~ 300 ° C to form a chromate film, a sample of the surface-treated steel sheet of the present invention was prepared.

As the 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 chromate treatment liquid] Cr ³⁺ : 50 g / L Cr ⁶⁺ : 2 g / L SiO ₂ : 180 g / L Fuel corrosion resistance of the surface-treated steel sheet produced in this way to gasoline and fuel containing alcohol , External corrosion resistance, and weldability were tested by the following methods. The test results are summarized in Table 1.

FIG. 3 is a graph showing a comparison of the fuel corrosion resistance of gasoline and gasohol after processing with the prior art. In the figure, No. 1 of Table 1 is used as an example of the present invention, and a comparative example in which no crack is provided at this time is shown as "no crack". Can be

The turn sheet (Sn / Pb: 0.10, adhesion amount)
In the case of 45g / m ² ), the corrosion of the wall from the shoulder of the punch was large, and the corrosion was particularly easy to progress in the part where the plating film was damaged due to the 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 condition 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 each 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. The corrosion resistance after processing was evaluated by the maximum pit 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 [Weldability] After performing 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]

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 (1)

(57) [Claims] 1. The alloy composition of a Zn—X alloy of a plating film provided on an uppermost layer is as follows: X is Ni: 3 to 18% by weight, Co: 0.02 to 3% by weight.
%, Mn: 25 to 45 wt%, and Cr: 8 to 20 wt%. This is a Zn-X alloy electroplated film having a coating weight of 7 g / m ² or less, including at least one selected from the group consisting of: As the lower layer of the upper plating, the potential is lower than that of the top plating.
It has a Zn-based plating layer of 10 to 50 g / m ² , and furthermore, the uppermost layer Zn-X
10 to 200 mg / m in metal Cr equivalent weight on the alloy plating film
² , consisting of a Zn-based multi-layer plated surface-treated steel sheet provided with a chromate film. The uppermost Zn-X alloy plating film has cracks, and the density of the cracks is 1 mm on the plating surface.
It is a range of 1000 to 150,000 as represented by the number of regions surrounded by cracks in a field of view of 1 mm, and 90% or more of the cracks have a maximum width of 0.5 μm or less, Surface-treated steel sheet with excellent corrosion resistance after processing.