JPH09276786A - Weldable organic composite plated steel sheet excellent in corrosion resistance of cut face - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a weldable org. composite plated steel sheet excellent in corrosion resistance of a cut face. SOLUTION: A Zn plating layer at a coating weight corresponding to coating weight (g/m<2> )/thickness (mm) of steel sheet = 30 to 130, a chromate film at 10-150mg/m<2> total Cr coating weight and an org. film having 0.5-20μm thickness are successively formed on one side or both sides of a steel sheet from the steel sheet side, and the content of the conductive inorg. pigment in the org. coating film is controlled to 1-40vol.%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weldable organic composite plated steel sheet having excellent weldability, corrosion resistance and workability and suitable as a rustproof steel sheet for automobiles.

[0002]

2. Description of the Related Art It is well known that electro-galvanized steel sheets are already widely used as surface-treated steel sheets which can be mass-produced without impairing the corrosion resistance of cold-rolled steel sheets, the corrosion resistance after painting and the workability. In recent years, the use of Zn-plated steel sheets has been attempted as an anti-rust measure for automobiles against scattered rock salt for preventing road freezing in winter in cold regions, and the demand for corrosion resistance in severe corrosive environments tends to increase.

In order to improve the corrosion resistance of these Zn-plated steel sheets, it is known to improve the corrosion resistance by the amount of Zn plating (the amount of adhesion). However, as a method other than increasing the amount of plating, an alloy for suppressing the dissolution of Zn itself is used. Many platings have been proposed. Many of these contain iron group elements such as Fe, Ni and Co as alloy components. These Zn-iron group electroplated steel sheets are characterized by having excellent corrosion resistance after unpainting or after painting, and are industrially produced and put to practical use. However, it is desired to further improve the corrosion resistance.

[0004] In response to this demand, Z is used for automobiles and the like.
Organic composite plated steel sheets have been developed in which an n-based plated steel sheet is subjected to a chromate treatment and an upper layer is coated with an organic film. On the other hand, the need to omit the assembling process has been increasing mainly in the home appliance industry, and the pre-coating of post-coating has been promoted. Also for automotive outer panels, there is a growing need for steel sheets for automobiles, which are pre-coated with electrodeposition coating, or without electrodeposition coating and intermediate coating. When a precoated steel sheet is used for an automobile, its weldability is a problem, but Japanese Patent Application Laid-Open Nos. Sho 62-73938 and 63-270131 disclose the problem.
Japanese Patent Application Laid-Open Publication No. H11-163873 has already disclosed an organic composite plated steel sheet provided with conductivity by containing a conductive pigment.

[0005]

However, the above-mentioned organic composite plated steel sheet is premised on the existence of electrodeposition coating in the automobile manufacturing process, and when the electrodeposition coating or the electrodeposition coating or the intermediate coating is omitted. However, the corrosion resistance of the cut surface where the iron surface is exposed was not considered, and the corrosion resistance of the cut surface did not have sufficient performance. Originally, the purpose of organic composite steel sheets is to increase the corrosion resistance of the general surface, and improvements in coating compositions that mainly form the uppermost organic film, such as the inclusion of a rust preventive pigment in the organic film, have been promoted. . However, the corrosion resistance of the cut surface depends more on the sacrificial corrosion protection ability of the plating layer than on the anticorrosion performance of the organic coating, and a sufficient study of the plating layer is required. The present invention solves the above-mentioned problems and provides an organic composite plated steel sheet having improved corrosion resistance, in particular, cut surface corrosion resistance.

[0006]

According to the present invention, there is provided an electro-Zn plating layer or a hot-dip Zn plating layer having excellent sacrificial anti-corrosion ability when a cut surface is corroded, and a chromate film and an organic film are appropriately formed. The present invention provides an organic composite plated steel sheet in which the corrosion resistance, especially the corrosion resistance of the cut surface, is dramatically improved by forming the layer in the upper layer. The gist of the present invention is (1) coating amount [g / m ² ] / plate thickness [mm] = 30 to 1 on one side or both sides of the steel plate in order from the steel plate side.
Zn plating layer having an adhesion amount corresponding to 30, a chromate film having a total Cr adhesion amount of 10 to 150 mg / m ^{2, and} a film thickness of 0.
A weldable type organic composite plated steel sheet, wherein an organic coating having a thickness of 5 to 20 μm is formed, and the ratio of the conductive inorganic pigment contained in the organic coating is 1 to 40 vol%. (2) The Zn plating layer is made of electric Zn plating or hot-dip Z
The weldable type organic coated steel sheet according to (1) above, which is n-plated. (3) The weldable type organic composite plated steel sheet according to (1), wherein the chromate film is a poorly soluble chromate film having a water-soluble content of 30% or less.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a base plating layer will be described. To enhance the corrosion resistance of the cut portion, electric Zn plating or hot-dip Zn plating is effective. Regarding the corrosion resistance of the general surface, various Zn-based alloy platings have been developed, but in such Zn-based alloy plating, preferential dissolution of Zn occurs at the time of corrosion, and the iron surface is exposed like a cut surface. In some cases, sacrificial corrosion protection is weak. In that regard, the electric Zn plating and the hot-dip Zn plating have a high sacrificial anticorrosion ability against steel because of a single phase of Zn, which is advantageous. The hot-dip Zn plating may contain a trace amount of Al, Pb, Cd, Sb, Ti, Mg, or the like. The adhesion amount of Zn plating is within a range that satisfies the adhesion amount of plating [g / m ² ] / plate thickness of steel plate [mm] = 30 to 130. If it is less than 30, sufficient corrosion resistance of the cut surface cannot be obtained. Further, even if it exceeds 130, its effect is saturated, so the upper limit is set to 130 in consideration of economy.

The chromate layer applied on the plating layer is
It has the effect of ensuring adhesion with the organic coating. Since the Zn plating layer has good reactivity with an acidic treatment solution composed of Cr ⁶⁺ and / or Cr ^3+, any of conventionally known coating-type chromate treatment, reactive-type chromate treatment, and electrolytic-type chromate treatment can be applied. The amount of the chromate film deposited is 10 to 150 mg / m ² as the total Cr amount. 10m
If g / m ² , the adhesion of the organic coating is insufficient and the corrosion resistance deteriorates. If it exceeds 150 mg / m ² , the weldability deteriorates, which is not preferable in practice. A more preferred range is 20 to 100 mg / m ² as the total amount of Cr.

The water-soluble content of the chromate film is 30%.
The following is preferable, and 20% or less is preferable. 30% soluble in water
If it exceeds the range, since a large amount of soluble Cr ⁶⁺ is contained, the chromium elution resistance or water swelling resistance deteriorates, the secondary adhesion of the upper coating film decreases, or high corrosion resistance performance as an organic composite plated steel sheet is obtained. Hateful. An organic film is applied to the upper layer of the chromate film. The thickness of the organic coating is 0.5-20μ
m. If it is less than 0.5 μm, the corrosion resistance is not sufficient,
If it exceeds 20 μm, the press workability may deteriorate, and the upper limit is set to 20 μm. Preferably, it is 2 to 10 μm.

The organic coating may be either solvent type or water-soluble type, and for example, epoxy resin, alkyd resin, oil-free polyester resin, acrylic resin, acrylic ethylene resin, melamine resin, urethane resin, phenol resin, polycarbonate resin, etc. can be used. . If necessary, various additives such as a rust preventive pigment, a curing agent, a coloring pigment, or a lubricant that further improves press workability may be added.
The coating method may be any known method such as roll coating, spray coating, curtain flow coating and the like. The conductive pigment is intended to make the coating film conductive, and includes SUS powder, ferrosilicon, iron phosphide, Zn.
Powder, Ni powder, conductive carbon, one or two
It is possible to use a combination of two or more species.

The average particle size is 1 to 15 μm, preferably 4 to 12 μm. If the average particle size is less than 1 μm, it is not preferable because secondary aggregation occurs in the coating material and the particles are coarsened to cause generation of particles, and the processing strength of the coating film is lowered. If the average particle size exceeds 15 μm, the appearance of the coating film becomes rough and has a rough surface, and the appearance of unevenness is likely to occur between the mold scratches and the processed and non-processed parts of the pressed product, which is not preferable in terms of quality.

The compounding ratio of the conductive pigment is an important factor in quality. The mixing ratio in the present invention is preferably 1 to 40 vol%. If the compounding ratio is less than 1 vol%, the contact resistance of the coating film is high and the conductive function is not sufficient, so that the appropriate current range is narrow, and there are difficulties in spot weldability such as poor continuous spotting due to adhesion of the organic film burning residue to the electrode. is there. If the content exceeds 40 vol%, the function as a resin binder is exceeded, which hinders uniform coating techniques such as roll coating. Even if the coating is solidified, it has poor adhesion and deteriorates corrosion resistance.

[0013]

EXAMPLES Examples of the present invention are shown in Table 1. A low carbon steel plate having a plate thickness of 0.8 mm was subjected to Zn plating by Sendzimer hot dip plating or electroplating, and a chromate layer and an organic coating layer shown in Tables 1 to 3 were sequentially applied. The corrosion resistance of the cut portion was evaluated by the following method. Salt water spray (JIS Z2371) 1 hour → dry (60
C.) 3.5 hours → drying (50 ° C., RH 95%) 3.5 hours The above cycle was set as one cycle, and the presence or absence of red rust on the end face after 20 cycles was evaluated. The results of evaluation of the corrosion resistance of the cut surface are shown in Tables 1 to 3 (Nos. 1 to 112), and it can be seen that the product of the present invention exhibits excellent cut surface corrosion resistance. In contrast, Table 3
Nos. 113, 114, and 116 in the comparative example are plating adhesion amounts,
Corrosion resistance of the cut surface is insufficient due to insufficient chromate adhesion amount or organic film thickness. No. 119 is an example in which there is too much conductive pigment in the organic coating, and the binder force as the organic coating is insufficient, so that the corrosion resistance of the cut surface deteriorates.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

[Table 3]

The weldability was evaluated by a continuous weldability test by spot welding. An appropriate welding current range was determined, and the number of critical continuous welding points at a predetermined welding current value obtained from the result was determined. The appropriate current range was determined by the following procedure. Plate set: 2-piece set of the same type of surface-treated steel sheet shown in Examples Welding electrode: T-16D, Obara Co., Ltd. (Material code: DH
OM) Pressure between electrodes: 200 kgf Welding pattern: Start of pressurization → Hold for 0.5 seconds → Apply predetermined current (0.2 seconds) → Release of pressurization Proper current range measurement: Nugget diameter when welding with the welding pattern of Measure the upper and lower limit current values with the minimum current value that can secure 4 mm or more as the lower limit current value and the minimum current value that causes strong welding between the test piece and the electrode as the upper limit current value.

The limit number of continuous welding spots is the upper limit of the number of continuous welding spots that can secure the necessary nugget diameter, and was determined by the following procedure. Plate set: 2-piece set of the same type of surface-treated steel sheet shown in Examples Welding electrode: T-16D, Obara Co., Ltd. (Material code: DH
OM) Pressure between electrodes: 200 kgf Welding pattern: Start of pressurization → Hold for 0.5 seconds → Apply predetermined current (0.2 seconds) → Release of pressurization Welding current value: Intermediate value of proper welding current range previously obtained ( (Lower limit current value + Upper limit current value) / 2 Limit continuous welding dot number: Under the welding conditions of to, the welding speed is set to 1 point / 3 seconds, and the maximum number of continuous welding points where the diameter of the nugget formed on the test piece is not less than 4 mm. Measure. The weldability was evaluated as ◯ when the number of continuous hit points measured was 500 or more, and x when less than 500. The evaluation results are shown in Table 1
As shown in Table 3 (Nos. 1 to 112), it can be seen that the product of the present invention exhibits excellent weldability. On the other hand, Comparative Example 1 in Table 3
Nos. 15 and 117 have too much chromate adhesion and the thickness of the organic coating, and No. 118 cannot secure sufficient weldability because the conductive pigment in the organic coating is too small.

[0019]

As described above, the organic composite plated steel sheet of the present invention has excellent corrosion resistance of the cut surface and can be used as a steel sheet for automobiles even when the electrodeposition coating is omitted.

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Claims (3)

[Claims] 1. A coating amount [g / m ² ] / plate thickness [mm] = on one side or both sides of the steel sheet in order from the steel sheet side.
A Zn plating layer having an adhesion amount corresponding to 30 to 130, a chromate film having a total Cr adhesion amount of 10 to 150 mg / m ² ,
An organic coating having a film thickness of 0.5 to 20 μm is formed, and the ratio of the conductive inorganic pigment contained in the organic coating is 1 to 40 vol%.
It is a weldable type organic composite plated steel sheet. 2. The weldable type organic galvanized steel sheet according to claim 1, wherein the Zn plating layer is electric Zn plating or hot dip Zn plating. 3. The weldable organic composite plated steel sheet according to claim 1, wherein the chromate film is a poorly soluble chromate film having a water-soluble content of 30% or less.