JP2985769B2 - Surface treated steel sheet with excellent weldability and corrosion resistance after processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a surface-treated steel sheet having excellent weldability and post-process corrosion resistance, particularly a fuel tank for automobiles and motorcycles, and a stove which exhibits high corrosion resistance to fuels such as gasoline and gasohol. The present invention relates to a surface-treated steel sheet suitable for applications such as kerosene tanks for boilers and the like, and oil filters that require extremely high corrosion resistance in extremely severe 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.

In addition, as disclosed in Japanese Patent Application Laid-Open No. 62-27587, double-layer plated steel sheets in which Ni plating is superimposed on Zn-Ni alloy plating have been studied. There was a problem that the number of processes increased and the cost increased.

For the introduction, cracking techniques in the plating layer are disclosed in JP-A-5-25679 and JP-A-4-337099. Further, a plating layer is further provided to improve the adhesion of the upper plating layer by the anchor effect. This is intended to improve the impact adhesion resistance when used for an outer panel of an automobile.

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.

[0011]

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.

[0012]

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 or two or more, hereinafter referred to as X) When the current is stopped in the final stage of the alloy electroplating process, immersion is performed for a short time in the plating solution, and a chromate film is provided thereon, without impairing the weldability at all. It was noticed that the corrosion resistance after processing, especially the fuel corrosion resistance, was improved. As a result of investigating the cause, they found that the immersion in the acidic plating solution caused cracks in the Zn—X alloy plating layer and improved fuel corrosion resistance after processing, and completed the present invention.

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 sheet, wherein X is Ni: 3 to 18 wt%, : 0.02 to 3 wt%,
Mn: at least one selected from the group consisting of 25 to 45 wt% and Cr: 8 to 20 wt%. On this Zn-X alloy plating film, 10 to 200 mg / m ² in terms of metal Cr equivalent deposition amount It consists of a surface-treated steel sheet with a chromate film formed thereon, and the Zn-X alloy plating film has cracks, and the density of the cracks is represented by 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 excellent in weldability and corrosion resistance after processing, characterized in that the number is less than 1,000.

[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 with a chromate coating 3 thereon, and the plating coating 2 is provided with a crack 4.

[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 means not the X content immediately after the Zn—X alloy electroplating, but the average value of the X content after the occurrence of cracks on the plating surface. In the present specification, this is simply referred to as X content.

When the X content of this plating film is individually lower than the above-mentioned ranges when X alone is added, both the outer corrosion resistance and the fuel corrosion resistance after processing are not sufficient, while the X content is 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 added in a total amount of 5% or less are added for further improvement of the outer surface corrosion resistance. In the case of Ni alone, the content is preferably 3 to 14
% Or 9-18%, more preferably 10-14%, even more preferably 11-13%.

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 poor. 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, when a crack is generated in the Zn-X alloy plating film as described above and a chromate treatment is performed thereon, the fuel corrosion resistance after processing is remarkably improved. Although the reason is not always clear, the chromate film penetrates into such cracks, so that the chromate film is firmly fixed, and the non-crack type Zn-X alloy plated steel sheet has a plating film during press working. Cracks occur and the corrosion resistance deteriorates due to the underlying steel sheet being exposed.On the other hand, cracks are generated in the plating film in advance, and the cracks are newly formed during press working by covering the cracks with the chromate film. It is conceivable that 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 visual field of 1 mm × 1 mm on the plating surface. This crack density was measured by randomly taking 30 SEM (scanning electron microscope) photographs of the plating surface of the sample at a magnification of 1000 times, and setting the cracks in a 0.1 mm × 0.1 mm field of view set at random for each photograph. The number of cracks (the number of cracks) surrounded by 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 defined as the crack density. “Area surrounded by cracks” refers to cracks 4 seen in an SEM photograph as schematically shown in FIG.
Means an area divided into islands by

According to the present invention, by generating cracks having a crack density of less than 1000 thus obtained, weldability can be improved, and, for example, corrosion resistance against corrosion by gasoline or gasohol, ie, processing Subsequent fuel corrosion resistance is dramatically improved. When the crack density is increased, the cracks are too large, and the plating coverage is too small, and the weldability is deteriorated.

If the crack density is too high, the weldability will be poor. In particular, for applications where seam weldability is important, such as a gasoline tank, a crack density of less than 1,000 is desirable. The preferred range for this crack density is 300
More than 1000 pieces.

The method for generating the 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 aqueous alkali solution is preferable because it is superior 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 separately provided in the plating bath.

After forming the Zn-X alloy plating film defined in the present invention, 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. Is formed, and the cracks in the plating film are covered with a chromate film. Even in the case of coating and using, if the cracks of the plating film are coated with a chromate film and the coating film is coated thereon, the outer surface corrosion resistance is remarkably improved, and the outer surface side 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.

This 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 regard 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 amount of metal Cr equivalent may be the same as described above.

[0031]

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-X alloy electroplating with a sulfate bath on both sides under the following conditions, and this plating bath was used as it was. By immersing the plated steel sheet in an acidic plating solution without electricity, the plating films on both surfaces were etched to introduce cracks in the Zn-X alloy plating film. The crack density was adjusted by changing the immersion time in the plating solution. The crack density of the etched plating surface is as described above.
Determined from SEM photographs.

[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 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 Both surfaces were etched to generate cracks in the plating film. -X alloy-coated steel sheet is coated on both sides with a coating type chromate solution of the following composition using a roll coater,
A chromate film was formed by baking at 0 ° C. to prepare 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⁺ : 5 g / L Cr ⁶⁺ : 0.2 g / L SiO ₂ : 70 g / L Fuel corrosion resistance of the surface-treated steel sheet thus produced 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.

Test Method [Fuel Corrosion Resistance] A blank 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 sealed in the cup, and the vessel was sealed. 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 Bunch diameter: 50 mm (diameter, shoulder r) Die diameter: 51mm (diameter, shoulder r = 5mm) Blank holder pressure: 10KN Drawing height: 30mm Surface roughness: # 1200 Polished every time Molding without lubricant (degreasing before molding) Degreasing condition 2% lidsol immersion (Liquid temperature 53 ° C) 3 minutes → immersion in pure water
(Normal temperature) 1.5 minutes → Dry (165 ° C) 8 minutes → Leave at normal temperature 20 minutes → Dry (165 ° C) 15 minutes Composition of fuel test solution Gasoline: Regular gasoline 95%, Ni% NaCl aqueous solution 5% Gasohol M15: Regular gasoline 84 % Aggressive methanol 15% Distilled water 1% (Note) Aggressive methanol
Is 95% anhydrous methanol, 0.1% NaCl + 0.08% Na ₂ SO ₄
A mixed solution with 5% of an aqueous solution containing + 3% 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 Δ: 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: 200 kgf Energizing time: 3 cycles Resting time: 2 cycles Current: 13,000 A Speed: 2.5 m / min Weldability evaluation standard ◎: Good welding (nugget diameter at 50 m portion of continuous seam welding ≧ ○: good weld (nugget diameter at continuous seam welding 50m part <3mm) △: blow hole present ×: unwelded part

[0040]

[Table 1]

[0041]

The surface-treated steel sheet according to the present invention, when used in the manufacture of a fuel tank, is not only excellent in weldability but also has a high resistance not only to gasoline but also to alcohol-containing fuels such as gasohol. It exhibits fuel corrosiveness, can be manufactured efficiently and inexpensively using a conventional Zn-X alloy electroplating apparatus as it is, and has excellent 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.

──────────────────────────────────────────────────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. 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 the alloy composition is X,
Ni: 3 to 18 wt%, Co: 0.02 to 3 wt%, Mn: 25 to 45 wt%,
Cr: A surface treatment containing at least one selected from the group consisting of 8 to 20 wt%, and forming a chromate film of 10 to 200 mg / m ² in terms of a metal Cr equivalent amount on this Zn-X alloy plating film. It is made of a steel plate, and the Zn-X alloy plating film has cracks, and the density of the cracks is expressed as the number of areas surrounded by cracks in a visual field of 1 mm × 1 mm on the plating surface.
A surface-treated steel sheet having excellent weldability and corrosion resistance after processing, characterized in that the number is less than the number of pieces.