JPH09324284A - Surface treated steel sheet excellent in corrosion resistance after working - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the fuel corrosion resistance after working of a steel sheet, in a surface treated steel sheet applied with a galvannealing layer having a specified compsn. and chromating coating film, by generating cracks on the galvannealing layer and incorporating a lubricant into the chromating coating film. SOLUTION: On a steel sheet, Zn-X plating coating film of 5 to 50g/m<2> coating weight per one side contg. at least one kind selected from among 3 to 18% Ni, 0.02 to 3% Co, 25 to 45% Mn and 8 to 20% Cr is formed. The surface is applied with chromating coating film of 10 to 200mg/m<2> by the coating weight expressed in terms of metallic Cr. At this time, cracks are generated on the galvannealing coating film by the application of chemical etching or the like. The number of the cracks is regulated to 1000 to 150000 pieces in the region of the visual field of 1×1mm on the plating surface, the cracks with <=0.5μm maximum width are present by >=90%, and the ones with depth by >=80% of the plating coating thickness are present by >=80%. Furthermore, a lubricant such as water soluble resin or the like is incorporated into the chromating coating film.

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 a kerosene tank, an oil filter that requires extremely severe workability and high corrosion resistance after processing.

[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 to this, as disclosed in Japanese Patent Laid-Open No. 62-27587, a two-layer plated steel sheet in which Ni plating is overlaid 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 purpose of introduction, techniques for forming cracks in the plating layer are disclosed in Japanese Patent Laid-Open Nos. 5-25679 and 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 resistance when used as an outer panel of an automobile.

Further, Japanese Patent Laid-Open No. 62-297490 discloses that a crack is formed in the 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 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.

[0012]

As a result of various investigations to solve such 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 referred to as X) When the electric current is stopped at the final stage of the alloy electroplating process and immersion in the plating solution is carried out for a short time, the corrosion resistance after processing, especially the fuel corrosion resistance is remarkably improved. I realized that. 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, the maximum width of cracks, and the depth of cracks on the surface of the plating film thus generated were The inventors have found that when the cracks are covered with a chromate film having lubricity, the fuel corrosion resistance after processing is significantly improved when the cracks are within a specific range, and the present invention has been completed.

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
90% or more of the cracks have a maximum width of 0.5 μm or less, 80% or more of the crack depth is 80% or more of the plating film thickness, and the chromate film is lubricated. It is a surface-treated steel sheet having excellent corrosion resistance after processing, which is characterized by containing a material.

[0014]

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 synthetic composition of the X alloy plating film is X: Ni: 3-18 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.

The X content of this plating film, if X is added alone, is lower than the above range for each, the outer surface corrosion resistance and the fuel corrosion resistance after processing are not sufficient, while the X content is , X are added in excess of the above ranges, including the case where two or more kinds are added, the workability or the external 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 outer surface corrosion resistance, and when the total amount exceeds 5%, workability is slightly deteriorated. 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 (the 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 high. 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, but by entering the chromate in such cracks, the anchor effect that the chromate film is firmly fixed, and the large surface area covered by the chromate film excellent in corrosion resistance cracks, In 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, whereas cracks are generated in the plating film in advance and the cracks are generated. It is conceivable that, by covering with a chromate film, there are few cracks newly generated during press working and the corrosion resistance is improved as a whole.

In the present invention, the density of cracks is represented by the number of regions surrounded by cracks in the visual field of 1 mm × 1 mm 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 controlled to be 1000 or more and 150,000 or less.
By causing 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.

Further, the maximum width of cracks is 90% or more of 0.5 μm or less. 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 set as the maximum width of the region, and the ratio of those having a width of 0.5 μm or less is determined. If the maximum crack width of 0.5 μm or less is out of the range of 90% or more, the environmental barrier effect of the plating film is impaired, and both the corrosion resistance after processing and the fuel corrosion resistance deteriorate. Preferably, the maximum crack width is 0.
90% or more of particles having a size of 4 μm or less are present.

The depth of cracks 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 cracks present in this range, and comparing this 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 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 external corrosion resistance after processing. And the fuel corrosion resistance is impaired.

Preferably, the crack density is 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-mentioned 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 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 surface-treated plated steel sheet according to the present invention is used as a material for a fuel tank, for example, the plating film corresponding to the inner surface of the tank is preferably immersed in an acidic plating solution as described above. Although the cracks are generated as defined in 1., it is preferable that the plating film on the outer surface side of the tank is treated in the same manner to generate the cracks similarly to the inner surface side. This not only improves the fuel corrosion resistance of the inner surface of the tank, but also significantly improves the corrosion resistance of the outer surface of the tank. 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 defined in the present invention, at least the surface which is used without coating and which is required to have 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 it is used by coating, if the cracks of the plating film are covered with a chromate film and the coating film is coated on the cracks, 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.

According to the present invention, a lubricant is added to the chromate film in order to further improve workability and corrosion resistance after working. The type of this lubricant is not particularly limited,
Any water-soluble resin that can be easily mixed uniformly with chromic acid may be used. For example, acrylic resin, epoxy resin, amine resin, etc. may be mentioned. The ratio of this resin and metallic Cr is preferably 0.5 to 1.5 of resin / Cr.

The chromate film is preferably of a coating type so that it can be coated even if a lubricant resin is mixed.
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, it is preferable that the ratio of Cr ^{6+ in} the chromate film is as small as possible, and in that sense, the Cr ⁶⁺ amount is preferably 5% or less of the total Cr amount.

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 (vapor phase silica or fumed silica), which has low 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.

[0035]

The present invention will be described below in detail with reference to examples. Preparation of surface-treated steel sheet sample A cold-rolled steel sheet with a thickness of 0.8 mm, which is equivalent to JIS SPCE, was subjected to Zn-X alloy electroplating in a sulfate bath under the following conditions on both sides, and the plating bath was used as it was to produce The plated coatings on both surfaces were etched by immersing the coated steel sheet in an acidic plating solution without applying electricity, and cracks were introduced into the Zn-X alloy plated coating. The crack density, the maximum crack width, and the crack depth were adjusted by changing the immersion time in the plating solution. The crack density, the maximum width of cracks, and the crack depth of the plated surface subjected to the etching treatment were 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 velocity 0.06 to 1.40 m / s Both surfaces are etched to cause cracks in the plating film Zn -Apply a coating type chromate solution having the following composition on both sides of an X alloy plated steel sheet with a roll coater,
A sample of the surface-treated steel sheet of the present invention was prepared by baking at 0 ° 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. Amine-based, acrylic-based, and epoxy-based resins were used as lubricants, Nippon Paint P304M2 was used as the acrylic resin, and Nagase Kasei Denacast was used as the epoxy resin.

[Composition of chromate treatment liquid] Cr ³⁺ : 50 g / L Cr ⁶⁺ : 2 g / L SiO ₂ : 140 g / L Lubricant: Resin / Cr = 1.0 Gasoline and surface treated steel sheet thus prepared The fuel corrosion resistance to alcohol-containing fuel, the outer surface corrosion resistance, and the weldability were tested by the following methods. The test results are summarized in Table 1.

Regarding the fuel corrosion resistance after processing with respect to gasoline, a comparison with the prior art is shown in a graph in FIG. In the figure, Example No. 1 in Table 1 is used as an example of the present invention, and a conventional example in which no crack is provided at this time is shown as no crack, but the presence or absence of a crack improves the fuel corrosion resistance by about 3 times or more. Can be seen.

In the graph, turn sheet (Sn / Pb: 0.
10, the adhesion was 45 g / m ² ) but the corrosion resistance was quite poor, but in the turn sheet, the corrosion from the punch shoulder to the wall was large, and the corrosion was easy to proceed in the part where the plating film was 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. After encapsulating and sealing the container, the fuel corrosion resistance was evaluated according to the following criteria based on the maximum erosion depth (Pm) on the inner surface on the 180th day.

◎: 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) Bonch diameter: 50 mm (diameter , Shoulder r = 5mm) Die diameter: 51mm (Diameter, Shoulder r = 5mm) Blank holder pressure: 10KN Drawing height: 30mm Surface roughness: # 1200 Polishing every time Molding without lubricant (Degreasing before molding) Degreasing condition 2 % Lidosol immersion (liquid temperature 53 ℃) for 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 was applied to the outer surface after cylindrical draw forming under the same conditions as the above fuel corrosion resistance test except that the overhang height was changed to 25 mm, then the edge was sealed. For 2000 hours. Corrosion resistance after processing is SST 2000
It was evaluated by the maximum erosion depth (Pm) after time.

◎: 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 100 m of continuous seam welding test 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 Rest time: 2 cycles Current: 13,000A Speed: 2.5 m / min Weldability evaluation standard ○: Welding good △: Blowhole exists ×: Unwelded part

[0046]

[Table 1]

[0047]

EFFECTS OF THE INVENTION The surface-treated steel sheet according to the present invention has excellent workability, and when used in the production of a fuel tank, has high fuel corrosion resistance not only for gasoline but also for alcohol-containing fuels such as gasohol. It exhibits excellent properties, can be manufactured efficiently and inexpensively using the conventional Zn-X alloy electroplating apparatus 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 cross-sectional configuration diagram 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.

 ─────────────────────────────────────────────────── ─── 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, 80% or more of the cracks have a depth of 80% or more of the plating film thickness, and the chromate film contains a lubricant. A surface-treated steel sheet having excellent corrosion resistance after processing, which is characterized by