JPH0874064A - Organic composite clad steel sheet excellent in corrosion resistance and its production - Google Patents

Abstract

PURPOSE: To inexpensively produce an organic composite clad steel sheet excellent in corrosion resistance by forming a Cr oxide layer on the surface layer of a Zn-Cr allay plated layer with a specified compsn. provided on the surface of the steel sheet and providing a resin layer contg. silica on the upper layer. CONSTITUTION: At least one face of a steel sheet is electroplated with a Zn-Cr allay contg. 5 to 20wt.% Cr by 1 to 30g/m<2> coating weight. Next, this Zn-Cr allay plated layer is subjected to surface oxidation to form a Cr oxide layer in which Cr is oxidized over 0.01 to 0.1μm thickness on the surface layer. Moreover, on the upper layer, a resin layer contg. about 20 to 60 pts.wt. silica and organic high polymer resin such as epoxy resin is formed by 0.2 to 1.5g/m<2> .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite coated steel sheet which is low in manufacturing cost and excellent in corrosion resistance, and particularly suitable for use as a steel sheet for automobile bodies, and a method for producing the same.

[0002]

2. Description of the Related Art In response to the strong social demand for high corrosion resistance of automobile bodies, the use of surface-treated steel sheets obtained by plating cold-rolled steel sheets with zinc or zinc-based alloy for automobile bodies is expanding. As such surface-treated steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, electrogalvanized steel sheet, etc. are known. Corrosion resistance of the bag structure and bending part (hemming part) inside the car body where the coating is not sufficiently spread is low, and further improvement in corrosion resistance is required.

The simplest method of improving the corrosion resistance of a zinc-based plated steel sheet is to increase the coating weight (area weight). However, an increase in the weight per unit area causes a decrease in workability, weldability, and other qualities, and also an increase in cost. Therefore, various attempts have been made to obtain high corrosion resistance with a smaller basis weight by applying a zinc-based alloy plating in which Zn and another metal are alloyed, and as one of them, JP-A-1-191979 and No. 3,120,393, etc., Cr is used as an alloying element in a Zn-based plating layer.
A Zn-Cr alloy-plated steel sheet obtained by adding is disclosed. These Zn-Cr alloy-plated steel sheets have a normal Z
Although it has better corrosion resistance than the n-type plated steel sheet, the corrosion resistance is still insufficient under severe corrosion conditions.

Therefore, in order to further improve the corrosion resistance of these steel sheets, a chromate layer is formed on the zinc-based plating or zinc-based alloy plating layer, and an organic polymer resin layer (hereinafter referred to as resin layer) is formed on the chromate layer. Various organic composite coated steel sheets (hereinafter, referred to as organic coated steel sheets) formed by forming (1) are formed, and are industrially produced and put into practical use. These organic coated steel sheets have a resin layer on the surface to improve the corrosion resistance, and the chromate layer is
It is formed to improve the adhesion of the resin layer. For example,
In JP-A-64-79382, a chromate layer is formed on a Zn-Cr alloy plating layer, and a resin layer is formed on the chromate layer to improve the adhesion between the resin layer and the plating layer. It is disclosed that this provides better corrosion resistance.

However, since the automobile body is formed by press forming, the corrosion resistance of the processed portion of the organic coated steel sheet is deteriorated as compared with the flat plate state. Further, there is a demand for further reduction of the weight per unit area for the purpose of cost reduction, and the appearance of an organic coated steel sheet having a small weight per unit area and excellent in corrosion resistance is desired.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and it has been difficult to achieve both of the above-mentioned problems, that is, the reduction of the coating amount and the excellent corrosion resistance. An object of the present invention is to provide an organic composite-coated steel sheet that is compatible with each other, does not require chromate treatment, and is inexpensive and has excellent corrosion resistance, and a method for producing the same.

[0007]

The present inventors have found that Zn--C
As a result of earnestly studying reduction of the areal weight of the r alloy plated steel sheet and realization of high corrosion resistance, the Cr content of the Zn—Cr alloy plated layer was adjusted, and the surface layer of this plated layer was C.
It is a Cr oxide layer in which r is oxidized, and by controlling the thickness of the Cr oxide layer, it has been found that it is possible to achieve both of the previously unattainable characteristics of reduction of the basis weight and high corrosion resistance, and the present invention is achieved. Came to.

That is, the organic composite-coated steel sheet excellent in corrosion resistance of the present invention is Zn-Cr alloy plating having a Cr content of 5 to 20 wt% on at least one surface of the steel sheet.
The surface layer has a Zn-Cr alloy plating layer, which is a Cr oxide layer in which Cr is oxidized over 0.01 to 0.1 μm, in an amount of 1 to 30 g / m ² , and an upper layer of silica and an organic high layer. Provided is an organic composite-coated steel sheet having excellent corrosion resistance, which has a resin layer having a molecular resin of 0.2 to 1.5 g / m ² .

Further, according to the method for producing an organic composite coated steel sheet having excellent corrosion resistance of the present invention, at least one surface of the steel sheet is coated with C
Zn with an r content of 5 to 20 wt% and a basis weight of 1 to 30 g / m ²
By forming a -Cr alloy plating layer and oxidizing the surface of this Zn-Cr alloy plating layer, a thickness of 0.01-
A Cr oxide layer formed by oxidizing Cr over 0.1 μm is formed, and a resin layer having silica and an organic polymer resin is formed thereon in an amount of 0.2 to 1.5 g / m ^2. A method for manufacturing an organic composite coated steel sheet having excellent corrosion resistance is provided.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The organic composite coated steel sheet having excellent corrosion resistance of the present invention (hereinafter referred to as organic coated steel sheet) and the method for producing the same will be described in detail below.

In the organic coated steel sheet of the present invention, the steel sheet serving as a base sheet is not particularly limited, various cold-rolled steel sheets can be used, and a steel sheet suitable for the organic coated steel sheet can be used as the base sheet. Good. For example, when the organic coated steel sheet of the present invention is used for an automobile body, since press working is performed, a cold-drawn steel sheet for deep drawing (low carbon steel or ultra low carbon steel) may be used.

The present invention provides Zn--C on such a steel sheet.
Although it has an r alloy plating layer, this plating layer has a Cr content of 5 to 20 wt% and a plating adhesion amount (basis weight) of 1 to 3.
At 0 g / m ² , and on the surface layer, a Cr oxide layer was formed by oxidizing the plating surface to convert Cr in the plating into oxide.
It has 01 to 0.1 μm.

The method for forming the Zn-Cr alloy plating layer is not particularly limited, and various known forming methods can be used. For example, the current density is 30 to 150 A / dm ² , and a steel plate (original plate) is used. It can be formed by electroplating under the condition that the relative velocity of the plating solution is 0.5 to 3 m / sec.

In the organic coated steel sheet of the present invention, Zn--C
The Cr content of the r alloy plating layer is 5 to 20 wt%.
If the Cr content is less than 5 wt%, the Cr oxide layer described below will not be sufficient, and corrosion resistance will be significantly deteriorated. If the Cr content exceeds 20 wt%, the Cr oxide layer will be deteriorated. Although it can be sufficiently formed, there is a problem in that the plating adhesion is deteriorated and the so-called powdering in which the plating layer is separated into powder during processing becomes remarkable. In addition, preferably, by setting the Cr content of the Zn-Cr alloy plating layer in the range of 10 to 20 wt%, more preferable results in terms of sufficient formation of the Cr oxide layer, corrosion resistance, powdering resistance, and the like. To get

In the organic coated steel sheet of the present invention, Z
The basis weight of the n-Cr alloy plating layer is 1 to 30 g / m ² . If the basis weight is less than 1 g / m ² , sufficient corrosion resistance cannot be obtained, and the Cr oxide layer will not be sufficient, which causes inconvenience, and the basis weight exceeds 30 g / m ^2. Also, no further improvement effect of corrosion resistance can be obtained, which is economically disadvantageous to the contrary, and further, the conduction resistance of the plating layer increases and the spot weldability deteriorates, and the plating layer becomes too thick, There is also an inconvenience in that the plating causes delamination inside the plated layer. In addition, preferably, Zn-Cr
By setting the basis weight of the alloy plating layer to be in the range of 10 to 25 g / m ² , more preferable results can be obtained in terms of corrosion resistance, cost, spot weldability, powdering resistance and the like.

As described above, in the organic coated steel sheet of the present invention, the surface of such a Zn--Cr alloy plating layer is oxidized by oxidizing the surface of the Zn--Cr alloy plating, and the Cr in the plating is trivalent. And a Cr oxide layer of hexavalent Cr oxide is formed in a thickness of 0.01 to 0.1 μm. The above Z
The areal weight and Cr content of the n-Cr alloy plating layer are numerical values including this Cr oxide layer. By having this Cr oxide layer, a resin layer containing silica and an organic polymer resin and a Zn—Cr alloy plating layer, which will be described in detail later, are formed without forming a chromate layer (without the need for chromate treatment). It is possible to realize an organic coated steel sheet having excellent corrosion resistance even with a low basis weight, by improving the adhesion to

As described above, in the conventional organic composite steel sheet, as disclosed in JP-A-64-79382, a chromate layer is formed on the plating layer by the chromate treatment, and the chromate layer is formed on the chromate layer. A resin layer is formed. Here, the reason for forming the chromate layer in the invention of the above publication is to improve the adhesion of the resin layer and thereby to improve the corrosion resistance. Various methods have been studied for the reason why the adhesion between the plating layer and the resin layer containing silica is improved by having the chromate layer. For example, in "Iron and Steel vol.77, No.7", when a resin layer containing silica is formed on the chromate layer, the silica component in the resin layer is selectively oriented on the chromate layer side. It is disclosed that the silanol group of silica and the metal component of the chromate layer are strongly bonded to each other to improve the adhesiveness of the resin layer.

Here, since Cr ⁶⁺ has a function of passivating (self-repairing action) the plating surface exposed from the defective portion of the chromate layer, it is also known that the corrosion resistance is improved by having the chromate layer. ing. However, Zn- having no resin layer but only the chromate layer
Since the corrosion resistance of the Cr alloy plated steel sheet is far inferior to that of the steel sheet having the resin layer, the contribution of the chromate layer to the corrosion resistance is mainly due to the improvement of the adhesion of the resin layer.
It is considered to prevent the corrosive component from coming into contact with the plating layer.

As a result of earnest studies on the adhesiveness of such a resin layer, the present inventors have found that the adhesiveness between the chromate layer and the resin layer depends on the Cr oxide in the chromate layer and the silanol group of silica. Was strongly coordinated. As a result of further study on this point, Z
Metal Cr is present in the n-Cr alloy plating layer,
Since Cr on the surface of the plating layer is an oxide, this C
The r oxide may play the role of a chromate layer, and a Cr oxide layer formed by oxidizing Cr in the Zn-Cr alloy plating layer is formed on the surface of the Zn-Cr alloy plating layer in a thickness of 0.01 μm.
By having the above, Zn-C can be formed without forming a chromate layer (that is, without performing chromate treatment).
It has been found that sufficient adhesion between the r alloy plating layer and the resin layer can be obtained.

The Cr oxide layer on the surface of such a Zn--Cr alloy plating layer has a thickness of 0.01 to 0.1 μm.
If the thickness of the Cr oxide layer is less than 0.01 μm, sufficient adhesion between the resin layer and the Zn—Cr alloy plated layer cannot be obtained, and sufficient corrosion resistance cannot be obtained. On the contrary, even if the thickness of the Cr oxide layer exceeds 0.1 μm, the effect of further improving the corrosion resistance (improving the adhesiveness of the resin layer) cannot be obtained, and the film of the Zn—Cr alloy plating layer is not formed. Since the resistance becomes high, the spot weldability and the electrodeposition paintability are poor, which poses a practical problem. Preferably, by setting the thickness of the Cr oxide layer to 0.03 to 0.08 μm,
More favorable results are obtained in terms of adhesion between the Cr oxide layer and the resin layer, spot weldability, and electrodeposition coating property.

The C of the surface of the Zn--Cr alloy plating layer
The thickness of the r-oxide layer can be measured by GDS analysis. FIG. 1 shows an organic composite steel sheet in which a Zn-Cr alloy plating layer is formed on the surface of the steel sheet, a Cr oxide layer is formed on the surface thereof, and a resin layer containing silica and an organic polymer resin is further formed on the upper layer. An example of a GDS chart is shown. FIG.
In the example shown in, the thick solid line is Zn and the chain line is C.
r, two-dot chain line is O, thin solid line is carbon, and dotted line is Fe
Are shown respectively. From the charts of Cr and Zn and the chart of Fe, it can be seen that the region indicated by 1 is the Zn-Cr alloy plating layer. In the illustrated example, Zn-Cr
The thickness of the alloy plating layer is 2 μm. A region having a large oxygen content in the Zn-Cr alloy plated layer, that is, a region indicated by 2 is a Cr oxide layer. Further, the region indicated by 3 is found to be a resin layer from the carbon content. A large amount of oxygen also exists in the region indicated by 3, but this is oxygen contained in the organic polymer resin and silica. The layer thickness of the Cr oxide layer can be obtained from the ratio of the region where oxygen is unevenly distributed to the plating layer in this chart. More specifically, the half-value position (or 0 at the start of rising Zn and the end of oxygen)
It is possible to obtain the oxide layer up to the position (from the start of the process) as the oxide layer, and the plating layer from the start of zinc rise to the start of Fe rise from the time ratio of both and the plating layer thickness. Further, the oxide layer may be formed up to a position where the oxygen value becomes 0, instead of the half value position of the falling edge of oxygen.

The method for forming the Cr oxide layer is not particularly limited, and after the Zn-Cr alloy plating layer is formed, it is immersed in an acidic solution such as hydrochloric acid, acetic acid or nitric acid, or the Zn-Cr alloy plating layer is formed. After that, a method of directly immersing it in an acidic plating solution without electricity, an electrochemical oxidation method, and a formed Z
a method of exposing the surface of the n-Cr alloy plating layer to an oxidizing atmosphere such as air, or a method of spraying the acidic solution,
The surface of the formed Zn—Cr alloy plating layer may be oxidized to form a Cr oxide layer on the outermost surface of the plating layer.

In the organic coated steel sheet of the present invention, a Zn--Cr alloy plating layer having a Cr oxide layer on the surface is formed on the upper layer.
A resin layer containing silica and an organic polymer resin (hereinafter referred to as an organic resin) is formed.

The organic resin used is not particularly limited,
Various epoxy resins, acrylic resins, urethane resins, etc.
All of various organic resins used for known organic coated steel sheets can be used. In particular, epichlorohydrin-bisphenol A type epoxy resins and modified resins thereof having an average molecular weight of 2000 or more are preferably used. An epoxy resin having a number average molecular weight of 2000 or more (modified epoxy resin) has a sufficient resin length after curing, and has a good network structure, so that it sufficiently fulfills the function of a silica binder described later. By using, it is possible to obtain good corrosion resistance and paint adhesion. As such an epoxy resin, JP-A-2-
No. 258335, a modified epoxy resin obtained by adding a dialkanolamine to a urethane-modified epoxy resin, and 100 parts by weight of an epichlorohydrin-bisphenol A type epoxy resin disclosed in JP-A 2-29973. The isocyanate compound to 10
˜100 parts by weight of reacted epoxy equivalent 1000˜50
A modified epoxy resin obtained by using a urethane modified epoxy resin of No. 00 and adding a dialkanolamine to 1 equivalent of the epoxy group of the urethane modified epoxy resin is preferably exemplified.

The resin layer of the organic coated steel sheet according to the present invention is obtained by further dispersing silica in such an organic resin. The present inventors dramatically improve the corrosion resistance of the organic coated steel sheet of the present invention by adding silica to the resin layer. As described above, by containing silica in the resin layer, the adhesion between the Cr-Zn plating layer having the Cr oxide layer on the surface and the resin layer is improved, and the effect of improving the corrosion resistance by having the resin layer. Can be expressed reliably. Further, as disclosed in JP-A-4-314872, by containing silica in the resin layer,
It is considered that the Zn-based corrosive product generated when the steel sheet is exposed to a corrosive environment is stably held by the silanol groups present on the surface of silica, and thereby high corrosion resistance is exhibited.

The content of silica in the resin layer is not particularly limited, but is 10 parts by weight to 100 parts by weight with respect to 100 parts by weight of the resin.
A range of parts by weight (dry weight ratio), particularly 20 to 60 parts by weight is preferable. By containing 10 parts by weight or more of silica with respect to 100 parts by weight of the resin, the effect of improving the adhesion between the plating layer and the resin layer and the effect of retaining the Zn-based corrosive product can be stably and In addition, by adjusting the content of silica to 100 parts by weight or less, the compatibility with the organic resin (composition) is sufficiently adjusted to adjust the coating material forming the resin layer, The workability at the time of application can be improved.

The usable silica is not particularly limited, and various known organic solvent-dispersible silicas such as vapor phase silica obtained by heating silicon tetrachloride at a high temperature, hydrophobic vapor phase silica having an alkyl group, Any of various known silicas such as ordinary hydrophilic gas phase silica and liquid phase silica can be preferably used. Here, the average particle size of silica is preferably in the range of 0.05 to 2 μm, and particularly preferably in the range of 0.3 to 1.5 μm. By setting the average particle diameter of silica to be 0.05 μm or more, the dispersed state of silica in the resin layer can be preferably made uniform, and the spot weldability of the organic coated steel sheet of the present invention can be improved. . In addition, by setting the average particle diameter of silica to 2 μm or less, it is possible to reliably prevent the silica from protruding from the resin layer, and to significantly increase the electric resistance between the electrode and the steel plate in spot welding, which results in welding sparks. It is possible to prevent damage to the electrodes due to the occurrence and also obtain good spot weldability.

In the organic coated steel sheet of the present invention, the adhesion amount of such a resin layer is 0.2 to 1.5 g / m ² . If the adhesion amount is less than 0.2 g / m ² , practically sufficient corrosion resistance cannot be secured, and if the amount of the resin layer exceeds 1.5 g / m ² , the film resistance becomes too high and spot weldability Will decrease. When the organic coated steel sheet of the present invention is exposed to a corrosive environment in a bare state, it is preferable that the adhesion amount is 0.3 g / m ² or more. If it is ² g / m ² or more, good corrosion resistance can be obtained. Preferably, by setting the amount of adhesion to 0.5 to 1.3 g / m ² , suitable results are obtained in terms of compatibility of spot weldability with corrosion resistance.

In the resin layer of the organic coated steel sheet according to the present invention, a lubricant, a curing agent (crosslinking agent), a pigment, a rust preventive agent and a dispersion agent may be added to the resin layer of the present invention, as required. Various additives that are added to known organic coated steel sheets, such as stabilizers, may be contained.

The method for forming such a resin layer is not particularly limited, but as an example, the organic resin is dissolved and mixed in an appropriate solvent according to the organic resin to be used, and further silica or various kinds of various kinds are further added. A coating composition was prepared by using additives, and the coating composition was coated on the above-mentioned Cr-Zn plated layer by a known means such as a roll coater or a spray coater so that the dry weight was 0.2 to 1.5 g / m ² . It may be applied so that the maximum plate temperature is about 100 to 180 ° C. and baked. When silica is added to the coating composition, the silica is butanol,
An organic solvent-dispersed silica sol obtained by dispersing in an appropriate organic solvent such as xylene, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol mono-n-propyl ether is preferably used.

Although the organic composite-coated steel sheet and the method for producing the same according to the present invention have been described above, the present invention is not limited to the above-mentioned examples, and various improvements and modifications can be made without departing from the gist of the present invention. Of course it is good.

[0032]

EXAMPLES The present invention will be described in more detail with reference to specific examples of the present invention. [Example] Electroplating was performed on a low carbon cold-rolled steel sheet having a plate thickness of 0.8 mm under conditions of a current density of 30 to 150 A / dm ² and a relative velocity of a plating solution to the steel sheet of 0.5 to 3 m / sec. Zn-
A Cr alloy plating layer was formed. Next, the steel sheet on which the plating layer was formed was left in a plating solution for a suitable time without being energized to oxidize the surface of the Zn—Cr alloy plating layer to form a Cr oxide layer. For such plated steel sheets,
Dissolve the following organic resin in an organic solvent (solvent prepared by mixing ethylene glycol monobutyl ether and petroleum naphtha (volume ratio of 1: 1)), and add organic solvent-dispersed silica sol (solvent is ethylene glycol monobutyl ether). The dry weight of the coating composition prepared by mixing was adjusted to 0.
Apply ² to 1.5g / m ² and heat up at 4 ℃ / se
c, Immediate water cooling after baking at the ultimate plate temperature of 160 ℃
After drying, an (organic polymer) resin layer was formed, and various organic coated steel sheets were produced. Adhesion amount of Zn-Cr alloy plating layer The Cr content, the thickness of the Cr oxide layer, and the adhesion amount of the resin layer are shown in Table 1 (Nos. 1 to 7) below. The average particle diameter of silica is 1.0 μm, and the content of silica in the resin layer is 40 parts by weight with respect to 100 parts by weight (dry weight) of the organic resin. Further, the details of the organic resin indicated by the resin type in Table 1 are as follows.

Epoxy system Urethane-modified epoxy resin obtained by amine-modifying terminal epoxy groups of epichlorohydrin-bisphenol A type epoxy resin. Acrylic series, number average molecular weight 1000, modified with carboxylic acid
00 acrylic emulsion resin. Urethane series, number average molecular weight 9000, modified by carboxylic acid
0 urethane resin.

Sample No. 57-66, and No. 91 to 104 are examples in which the Cr oxide layer was not formed on the surface of the plating layer, the chromate layer was formed by the chromate treatment, and the chromate layer was not formed.
The chromate treatment conditions are as follows. Further, the chromate adhesion amount is shown in Table 1. Chromate treatment conditions A coating type chromate treatment liquid with a Cr ⁶⁺ / total Cr ratio of 50% was used with a roll coater to obtain a chromate amount of 50 mg / m ² (C
The chromate layer was formed by coating so as to have a r-value) and baking at a maximum reaching temperature of 130 ° C.

Regarding various types of such organic coated steel sheets,
The following various tests were conducted. <Plate corrosion resistance evaluation> 5% NaCl aqueous solution spray (35 ° C)
For 4 hours, dry (60 ° C) for 2 hours, and leave in a moist environment (50 ° C) for 2 hours as one cycle for a combined cycle corrosion test, and evaluate the flat plate corrosion resistance by the amount of decrease in plate thickness after 400 cycles. did. The evaluation is as follows. ◎: Thickness reduction of less than 0.1 mm ○: Reduction of thickness of 0.1 mm to less than 0.3 mm △: Reduction of thickness of 0.3 mm to less than 0.5 mm ×: Reduction of thickness of 0.5 mm or more

<Evaluation of corrosion resistance after processing> Oil-free steel plate test pieces were drawn under the following pressing conditions using an Erichsen cup drawing machine, and a salt spray test (JIS
(According to Z-2371) was performed, and the corrosion resistance after processing was evaluated by the time required until the occurrence of red rust. The evaluation is performed every 500 hours. Press conditions Wrinkle holding pressure: 1 ton Punch diameter: 30 mmφ Blank system: 70 mmφ Drawing ratio: 2.12 Drawing speed: 500 mm / sec

<Evaluation of electrodeposition coating property> POWER TOP U-60
0 (manufactured by Nippon Paint Co., Ltd.), treated at 100 V and 28 ° C. for 180 seconds for electrodeposition coating, and then at 170 ° C. for 2 seconds.
After baking for 0 minutes, the appearance was visually observed to evaluate the electrodeposition coatability. The evaluation is as follows. ◎: The number of generated gas pin holes is 0 / cm ² ○: The number of generated gas pin holes is 1 to 5 / cm ² △: The number of generated gas pin holes is 6 to 15 / cm ² ×: Gas pin 16 holes / cm ² or more

<Evaluation of water-resistant secondary adhesion> Electrodeposition coating with a thickness of 20 μm was carried out in the same manner as in the evaluation of electrodeposition coating property, and a top coat (Rugabake White Kansai Paint Co., Ltd.) was coated with 35 μm.
After coating with a thickness of m, dipping this in warm water (pure water) at 40 ° C for 10 days, use a cutter to make 100 squares in a 2 mm square and peel the tape to make it water resistant at a peeling rate. The next adhesion was evaluated. The evaluation is as follows. ◎: Peeling rate less than 5% ◯: Peeling rate 5% or more and less than 15% △: Peeling rate 15% or more and less than 25% ×: Peeling rate 25% or more

<Spot Weldability> Al ₂ O with 6 mmφ Tip
Welding tip made of _3- dispersed copper alloy, pressure 100kg
f, continuous welding was performed at a welding current of 9 kA and a welding time of 10 Hz, and spot weldability was evaluated by the number of continuous welding points until the nugget system fell below the standard system. The evaluation is in 500-dot increments.

The above results are shown in Table 1 below.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

[0045]

[Table 5]

[0046]

[Table 6]

[0047]

[Table 7]

[0048]

As described in detail above, the organic composite-coated steel sheet according to the present invention can reduce the manufacturing cost because the chromate treatment step can be omitted, and has excellent corrosion resistance even if the coating amount is small. It can be suitably used for a wide range of applications where similar quality characteristics are required, including automobile bodies, and its industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a GDS measurement chart of an example of an organic composite coated steel sheet of the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI technical display location C25D 11/38 303 305 (72) Inventor Kyoko Hamahara 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Steel Research and Development Headquarters Steel Research Laboratory (72) Inventor Ryoichi Mukai 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd. Steel Research and Development Headquarters Steel Research Laboratory (72) Invention Kazuo Mochizuki, Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Kawasaki Steel Co., Ltd.

Claims (2)

[Claims] 1. A Zn-Cr alloy plating having a Cr content of 5 to 20 wt% on at least one surface of a steel sheet, the surface layer of which is Cr oxidized for 0.01 to 0.1 .mu.m. 1 Zn-Cr alloy plating layer which is a Cr oxide layer
Has a to 30 g / m ^2, thereon, an organic composite coated steel sheet excellent in corrosion resistance characterized by having 0.2 to 1.5 g / m ² and resin layer having silica and organic polymer resins. ^2. A Zn—Cr alloy plating layer having a Cr content of 5 to 20 wt% and a basis weight of 1 to 30 g / m ² is formed on at least one surface of a steel sheet, and the surface of the Zn—Cr alloy plating layer is formed. By oxidizing, the thickness of 0.01-0.1 μm
Forming a Cr oxide layer formed by oxidizing Cr over
A method for producing an organic composite-coated steel sheet excellent in corrosion resistance, characterized in that a resin layer having silica and an organic polymer resin is formed thereon in an amount of 0.2 to 1.5 g / m ² .