Electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish

Abstract

 An electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish, comprising: a steel sheet; a zinc electroplating layer formed on at least one surface of the steel sheet; and a zinc alloy electroplating layer formed on the zinc electroplating layer. The zinc electroplating layer has a center-line mean roughness (Ra) of up to 1.5 µm and a plating weight of from 25 to 150 g/m² per surface of the steel sheet. The zinc alloy electroplating layer comprises zinc and at least one element selected from the group consisting of cobalt, manganese, nickel, iron and chromium. The zinc alloy electroplating layer contains such at least one element in an amount of from 3 to 99 wt.% relative to the zinc alloy electroplating layer, and has a plating weight of from 1 to 20 g/m² per surface of the steel sheet.

Description

REFERENCE TO PATENTS, APPLICATIONS AND PUBLICATIONS PERTINENT TO THE INVENTION

[0001] As far as we know, there is available the following prior art document pertinent to the present invention:
"Plating & Surface Finishing", March 1989, pp. 62-69.

[0002] The contents of the prior art disclosed in the above-mentioned prior art document will be discussed hereafter under the heading of the "BACKGROUND OF THE INVENTION".

FIELD OF THE INVENTION

[0003] The present invention relates to an electrogalvanized steel sheet having two electroplating layers and excellent in antifriction upon press-forming, corrosion resistance and painting finish.

BACKGROUND OF THE INVENTION

[0004] In general, the body of an automobile is exposed to a corrosive environment, and particularly to a severe corrosive environment in a coastal area or a cold area where an automobile tends to come into contact with a substance containing chlorine ions having a violent corrosivity.

[0005] An electrogalvanized steel sheet is conventionally widely used as a steel sheet for an automobile body having an excellent corrosion resistance even in such a severe corrosive environment.

[0006] The conventional electrogalvanized steel sheet has however the following problems:

(1) The zinc electroplating layer of the electrogalvanized steel sheet, having a relatively low hardness, is deformed upon the press-forming of the electrogalvanized steel sheet, thus increasing a contact area between the zinc electroplating layer and the pressing portion of a press. In other words, the electrogalvanized steel sheet has a frictional coefficient higher than that of the other steel sheets such as a cold-rolled steel sheet or a zinc alloy electroplated steel sheet. When press-forming the electrogalvanized steel sheet, therefore, cracks may be produced in the zinc electroplating layer thereof. As is clear from the above description, the conventional electrogalvanized steel sheet is poor in antifriction (hereinafter referred to as the "problem 1"); and

(2) To ensure a sufficient corrosion resistance, the electrogalvanized steel sheet has a relatively large plating weight per surface of the steel sheet, i.e., the zinc electroplating layer of the electrogalvanized steel sheet has a relatively large thickness. When applying an electropainting to the electrogalvanized steel sheet having such a relatively thick zinc electroplating layer to form a painting film on the surface thereof, bubbles tend to occur in the painting film which lead to a defect having a recess or a projection in the painting film. The electrogalvanized steel sheet having been subjected to the electropainting, is further subjected to a finish painting to form a finish painting film on this painting film. The above-mentioned defect having a recess or a projection exerts an adverse effect even on the finish painting film, thus deteriorating the appearance of the painted electrogalvanized steel sheet. As is evident from the above description, the conventional electrogalvanized steel sheet is poor in painting finish (hereinafter referred to as the "problem 2").

[0007] It is a conventional practice, as a means for solving the problem 1, to apply a high-viscosity lubricant oil onto the surface of the electrogalvanized steel sheet prior to press-forming the electrogalvanized steel sheet to improve antifriction of the electrogalvanized steel sheet.

[0008] Application of the high-viscosity lubricant oil onto the surface of the electrogalvanized steel sheet as described above poses however the following problems:

(a) The high-viscosity lubricant oil contaminates the working place; and

(b) It is necessary to remove the high-viscosity lubricant oil applied onto the surface of the electrogalvanized steel sheet prior to applying painting thereto. This removing operation is not however easy. Complete removal of the high-viscosity lubricant oil requires much time and labor.

[0009] With regard to the frictional coefficient of an electrogalvanized steel sheet, the "Plating & Surface Finishing", March 1989, pp. 62-69, teaches as follows (hereinafter referred to as the "prior art"):

(i) Application of a conventional anticorrosive oil onto the surface of the zinc electroplating layer having crystals oriented along the 〈0001〉 plane, leads to a relatively large frictional coefficient thereof of 0.19; and

(ii) Application of a conventional anticorrosive oil onto the surface of the zinc electroplating layer having crystals oriented along the 〈10

X〉 plane (where, X is 1, 2, 3 or 4), on the other hand, results in a small frictional coefficient thereof of 0.13.

[0010] Apart from the above-mentioned problems resulting from application of the high-viscosity lubricant oil, the electrogalvanized steel sheet applied with the high-viscosity lubricant oil on the surface thereof has a small frictional coefficient of 0.11. If the orientation of the crystals of the zinc electroplating layer along the 〈10

X〉 plane (where X is 1, 2, 3 or 4) as taught by the prior art can be maintained, an antifriction of the same order as in the application of the high-viscosity lubricant oil would be available by the application of the conventional anticorrosive oil which is easy to remove, onto the surface of the electrogalvanized steel sheet.

[0011] However, the crystal orientation of the zinc electroplating layer of the electrogalvanized steel sheet depends upon electroplating conditions, and among others, upon an electric current density. As a result, it is inevitable to alter the plating conditions in response to the width, for example, of the steel sheet to be electroplated. In the manufacture of the electrogalvanized steel sheet in an industrial scale, it is practically impossible to maintain the orientation of the crystals of the zinc electroplating layer along the 〈10

X〉 plane (where, X is 1, 2, 3 or 4).

[0012] A means to solve the problem 2 has not as yet been proposed.

[0013] Under such circumstances, there is a strong demand for the development of an electrogalvanized steel sheet excellent in antifriction, corrosion resistance and painting finish, but such an electrogalvanized steel sheet has not as yet been proposed.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is therefore to provide an electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish.

[0015] In accordance with one of the features of the present invention, there is provided an electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish, characterized by comprising:
   a steel sheet;
   a zinc electroplating layer formed on at least one surface of said steel sheet, said zinc electroplating layer having a center-line mean roughness (Ra) of up to 1.5 µm and a plating weight within a range of from 25 to 150 g/m² per surface of said steel sheet; and
   a zinc zlloy electroplating layer formed on said zinc electroplating layer, said zinc alloy electroplating layer comprising zinc and at least one element selected from the group consisting of cobalt, manganese, nickel, iron and chromium, said zinc alloy electroplating layer containing said at least one element in an amount within a range of from 3 to 99 wt.% relative to said zinc alloy electroplating layer, and said zinc alloy electroplating layer having a plating weight within a range of from 1 to 20 g/m² per surface of said steel sheet.

[0016] The term "center-line mean roughness" (Ra) as used herein means a value of the surface roughness as expressed by the following formula:


where,

L

: measuring length, and

f(X)

: roughness curve.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Fig. 1 is a graph illustrating, in the case where an electrogalvanized steel sheet having a zinc electroplating layer as a single layer on each of the both surfaces thereof is subjected to an electropainting, the relationship between a defect occurrence ratio and a center-line mean roughness (Ra) of the zinc electroplating layer;

[0018] Fig. 2 is a graph illustrating, for the electrogalvanized steel sheet of the present invention having a zinc electroplating layer formed on the surface of the steel sheet and a zinc-cobalt alloy electroplating layer formed on the zinc electroplating layer, the relationship between a frictional coefficient of the electrogalvanized steel sheet and a cobalt content in the zinc-cobalt alloy electroplating layer; and

[0019] Fig. 3 is a schematic front view illustrating an apparatus for measuring a frictional coefficient.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] From the above-mentioned point of view, extensive studies were carried out to develop an electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish.

[0021] As a result, the following findings were obtained: An electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish is available by the following steps:

(1) forming a zinc electroplating layer on at least one surface of a steel sheet;

(2) limiting a center-line mean roughness (Ra) of the zinc electroplating layer to up to 1.5 µm;

(3) limiting a plating weight of the zinc electroplating layer within a range of from 25 to 150 g/m² per surface of the steel sheet;

(4) forming a zinc alloy electroplating layer comprising zinc and at least one element selected from the group consisting of cobalt, manganese, nickel, iron and chromium, on the zinc electroplating layer;

(5) limiting a content of the above-mentioned at least one element in the zinc alloy electroplating layer within a range of from 3 to 99 wt.% relative to the zinc alloy electroplating layer; and

(6) limiting a plating weight of the zinc alloy electroplating layer within a range of from 1 to 20 g/m² per surface of the steel sheet.

[0022] The present invention was made on the basis of the above-mentioned findings. The electrogalvanized steel sheet of the present invention having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish is described below with reference to the drawings.

[0023] The electrogalvanized steel sheet of the present invention excellent in antifriction, corrosion resistance and painting finish comprises a steel sheet, a zinc electroplating layer formed on at least one surface of the steel sheet and a zinc alloy electroplating layer formed on the zinc electroplating layer.

[0024] The zinc electroplating layer has a function of imparting an excellent corrosion resistance to the electrogalvanized steel sheet.

[0025] The center-line mean roughness (Ra) of the zinc electroplating layer exerts an important effect on painting finish of the electrogalvanized steel sheet. The effect of the center-line mean roughness (Ra) of the zinc electroplating layer exerting on painting finish of the electrogalvanized steel sheet is described below with reference to Fig. 1.

[0026] Fig. 1 is a graph illustrating, in the case where an electrogalvanized steel sheet having a zinc electroplating layer as a single layer on each of the both surfaces thereof is subjected to an electropainting, the relationship between an occurrence ratio of bubbles and a center-line mean roughness (Ra) of the zinc electroplating layer. More particularly, each of a plurality of steel sheets for automobile hood having a center-line mean roughness (Ra) of 0.8 µm was electrogalvanized to form a zinc electroplating layer having a plating weight of 60 g/m² per surface of the steel sheet on each of the both surfaces of each steel sheet. Then, each of these steel sheets, having the zinc electroplating layer formed on the both surfaces thereof, was subjected to a conventional electropainting to form a painting film on the zinc electroplating layer, thereby preparing a plurality of automobile hoods. Then, the relationship between a ratio of the number of automobile hoods having bubbles in the painting films thereof to the total number of the thus prepared automobile hoods, i.e., an occurrence ratio of bubbles in the painting film, on the one hand, and a center-line mean roughness (Ra) of the zinc plating layer, on the other hand, was investigated. In Fig. 1, the ordinate represents the occurrence ratio of bubbles in the painting film, and the abscissa represents the center-line mean roughness (Ra) of the zinc electroplating layer.

[0027] As is clear from Fig. 1, when the center-line mean roughness (Ra) of the zinc electroplating layer is up to 1.5 µm, the occurrence ratio of bubbles in the painting film is 0%. When the center-line mean roughness (Ra) is over 1.5 µm, on the other hand, the occurrence ratio of bubbles in the painting film sharply increases, and a center-line mean roughness (Ra) of 2.5 µm results in an occurrence ratio of bubbles in the painting film of even 10%.

[0028] As described above, bubbles are produced in the painting film when the center-line mean roughness (Ra) of the zinc electroplating layer is over 1.5 µm. The reason of this is estimated to be as follows:

(a) A hydrogen gas produced during the electropainting remains on the surface of the zinc electroplating layer under the effect of the increased center-line mean roughness (Ra) of the zinc electroplating layer. This hydrogen gas is confined in the painting film and expands during the process of baking of the painting film, thus forming bubbles in the painting film; and

(b) Piror to applying the electropainting to the electrogalvanized steel sheet, a phosphate film is formed on the surface thereof to improve paint adhesion. During the baking of the painting film after electropainting, water is eliminated from phosphate crystals of the phosphate film. Water thus eliminated is confined in the painting film and vaporized to form bubbles in the painting film.

[0029] As is clear from the above description, the production of bubbles in the painting film during the electropainting of the electrogalvanized steel sheet having a zinc electroplating layer as a single layer on each of the both surfaces thereof, is prevented by limiting the center-line mean roughness (Ra) of the zinc electroplating layer to up to 1.5 µm.

[0030] In the electrogalvanized steel sheet of the present invention, a zinc alloy electroplating layer is formed on the zinc electroplating layer formed on the surface of the steel sheet. Since this zinc alloy electroplating layer has a plating weight within a range of from 1 to 20 g/m² per surface of the steel sheet as described later, i.e., the zinc alloy electroplating layer has a small average thickness, the surface roughness of the zinc electroplating layer exerts an important effect on the surface roughness of the zinc alloy electroplating layer formed thereon. Therefore, the above-mentioned relationship, in the case where the electrogalvanized steel sheet having a zinc electroplating layer as a single layer on each of the both surfaces thereof is subjected to the electropainting, between the occurrence ratio of bubbles in the painting film and the center-line mean roughness (Ra) of the zinc electroplating layer applies also to the zinc electroplating layer on which the zinc alloy electroplating layer has been formed, in the electrogalvanized steel sheet of the present invention.

[0031] Therefore, the center-line mean roughness (Ra) of the zinc electroplating layer of the electrogalvanized steel sheet of the present invention should be limited to up to 1.5 µm.

[0032] In order to limit the center-line mean roughness (Ra) of the zinc electroplating layer to be formed on the surface of the steel sheet to up to 1.5 µm, it suffices to appropriately alter the electroplating conditions for forming the zinc electroplating layer. The plating weight of the zinc electroplating layer is described later. Since, when the zinc electroplating layer has a relatively small thickness, the surface roughness of the steel sheet exerts an effect on the surface roughness of the zinc electroplating layer formed thereon, the center-line mean roughness (Ra) of the steel sheet should be limited to up to 1.5 µm by grinding the surface thereof.

[0033] The plating weight of the zinc electroplating layer exerts an important effect on corrosion resistance and antifriction of the electrogalvanized steel sheet. With a plating weight of the zinc electroplating layer of under 25 g/m² per surface of the steel sheet, an excellent corrosion resistance cannot be imparted to the electrogalvanized steel sheet. With a plating weight of the zinc electroplating layer of over 150 g/m² per surface of the steel sheet, on the other hand, zinc crystals of the zinc electroplating layer become coarser. Such coarsening of zinc crystals poses the following problems: It is impossible to form a uniform zinc alloy electroplating layer having a relatively small thickness as described later on the zinc electroplating layer; in other words, it is impossible to cover the entire surface of the zinc electroplating layer with the zinc alloy electroplating layer and part of the surface of the zinc electroplating layer is exposed, thus making it impossible for the zinc alloy electroplating layer to fully display the function thereof described later of imparting an excellent antifriction to the electrogalvanized steel sheet. The plating weight of the zinc electroplating layer should therefore be limited within a range of from 25 to 150 g/m² per surface of the steel sheet.

[0034] The zinc alloy electroplating layer formed on the zinc electroplating layer has a function of imparting an excellent antifriction to the electrogalvanized steel sheet.

[0035] The zinc alloy electroplating layer comprises zinc and at least one element selected from the group consisting of cobalt, manganese, nickel, iron and chromium.

[0036] The content of the above-mentioned at least one element selected from that group in the zinc alloy electroplating layer exerts an important effect on antifriction and chipping resistance of the electrogalvanized steel sheet. The effect of the content of the above-mentioned at least one element selected from that group in the zinc alloy electroplating layer exerting on antifriction of the electrogalvanized steel sheet, is described below with reference to Fig. 2.

[0037] Fig. 2 is a graph illustrating, for the electrogalvanized steel sheet of the present invention having a zinc electroplating layer formed on the surface of the steel sheet and a zinc-cobalt alloy electroplating layer formed on the zinc electroplating layer, the relationship between a frictional coefficient of the electrogalvanized steel sheet and a cobalt content in the zinc-cobalt alloy electroplating layer. More specifically, for an electrogalvanized steel sheet having a zinc electroplating layer, with a plating weight of 60 g/m² per surface of the steel sheet, formed on one surface of the steel sheet and a zinc-cobalt alloy electroplating layer, with a plating weight of 5 g/m² per surface of the steel sheet, formed on the zinc electroplating layer, the relationship between a frictional coefficient of the electrogalvanized steel sheet and a cobalt content in the zinc-cobalt alloy electroplating layer was investigated. In Fig. 2, the ordinate represents the frictional coefficient of the electrogalvanized steel sheet, and the abscissa represents the cobalt content in the zinc-cobalt alloy electroplating layer.

[0038] As is clear from Fig. 2, when the cobalt content in the zinc-cobalt alloy electroplating layer is under 3 wt.% relative to the zinc-cobalt alloy electroplating layer, the electrogalvanized steel sheet has a relatively large frictional coefficient of at least 0.2, so that the electrogalvanized steel sheet having such a relatively large frictional coefficient is poor in antifriction.

[0039] As described above, the frictional coefficient of the electrogalvanized steel sheet increases when the cobalt content in the zinc-cobalt alloy electroplating layer is under 3 wt.% relative to the zinc-cobalt alloy electroplating layer. The reason of this is estimated to be as follows: It is impossible to sufficiently increase hardness of the zinc-cobalt alloy electroplating layer with such a low cobalt content.

[0040] The content of manganese, nickel, iron or chromium in the zinc alloy electroplating layer also exerts an important effect on the frictional coefficient of the electrogalvanized steel sheet as in the above-mentioned cobalt content in the zinc alloy electroplating layer.

[0041] The zinc alloy electroplating layer should therefore contain at least one element selected from the group consisting of cobalt, manganese, nickel, iron and chromium in an amount of at least 3 wt.% relative to the zinc alloy electroplating layer.

[0042] When the content of the above-mentioned at least one element selected from that group in the zinc alloy electroplating layer is over 99 wt.% relative to the zinc alloy electroplating layer, on the other hand, phosphate crystals of the phosphate film formed on the zinc alloy electroplating layer prior to the electropainting, become coarser, thus resulting in a lower chipping resistance of the electrogalvanized steel sheet. Therefore, the content of the above-mentioned at least one element selected from that group should be limited within a range of from 3 to 99 wt.% relative to the zinc alloy electroplating layer.

[0043] The plating weight of the zinc alloy electroplating layer exerts an important effect on antifriction of the electrogalvanized steel sheet and the manufacturing cost thereof. When the plating weight of the zinc alloy electroplating layer is under 1 g/m² per surface of the steel sheet, the zinc alloy electroplating layer has only a low covering ratio over the entire surface of the zinc electroplating layer, so that most part of the surface of the zinc electroplating layer is exposed, thus making it impossible to impart an excellent antifriction to the electrogalvanized steel sheet. Even when the plating weight of the zinc alloy electroplating layer is over 20 g/m², on the other hand, the effect of imparting an excellent antifriction to the electrogalvanized steel sheet cannot further be improved, and in addition, the manufacturing cost of the electrogalvanized steel sheet becomes uneconomically higher. The plating weight of the zinc alloy electroplating layer should therefore be limited within a range of from 1 to 20 g/m² per surface of the steel sheet.

[0044] Now, the electrogalvanized steel sheet of the present invention, having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish, is described further in detail by means of examples while comparing with examples for comparison.

EXAMPLES

[0045] Each of cold-rolled steel sheets having a thickness of 0.7 mm and a center-line mean roughness (Ra) within a range of from 0.8 to 1.0 µm was subjected to a conventional degreasing treatment and a conventional pickling treatment to remove rust from the both surfaces thereof. Then, the steel sheet from the both surfaces of which rust was thus removed, was subjected to an electroplating under the conditions shown in Table 1 to form a zinc electroplating layer on each of the both surfaces of the steel sheet.

[0046] Then, the steel sheet having the zinc electroplating layer formed on each of the both surfaces thereof was subjected to another electroplating under other conditions also shown in Table 1 to form a zinc alloy electroplating layer on the zinc electroplating layer. Thus, samples of the electrogalvanized steel sheet within the scope of the present invention (hereinafter referred to as the "samples of the invention") Nos. 1 to 66 were prepared.

[0047] For each of the samples of the invention Nos. 1 to 66, the plating weight per surface of the steel sheet and the center-line mean roughness (Ra) of the zinc electropainting layer, as well as elements other than zinc and the contents thereof in the zinc alloy electroplating layer, and the plating weight per surface of the steel sheet of the zinc alloy electroplating layer are also shown in Table 1.

[0048] For comparison purposes, each of steel sheets identical with those in the samples of the invention Nos. 1 to 66 was subjected to a conventional degreasing treatment and a conventional pickling treatment to remove rust from the both surfaces thereof. Then, the steel sheet from the both surfaces of which rust was thus removed, was subjected to an electroplating under the conditions shown in Table 2 to form a zinc electroplating layer on each of the both surfaces of the steel sheet.

[0049] Then, the steel sheet having the zinc electroplating layer formed on each of the both surfaces thereof was subjected to another electroplating under other conditions also shown in Table 2 to form a zinc alloy electroplating layer on the zinc electroplating layer. Thus, samples of the electrogalvanized steel sheet outside the scope of the present invention (hereinafter referred to as the "samples for comparison") Nos. 1 to 10 were prepared. Each of the samples for comparison Nos. 1 and 2 had only a zinc electroplating layer as a single layer.

[0050] The plating weight per surface of the steel sheet and the center-line mean roughness (Ra) of the zinc electroplating layer for each of the samples for comparison Nos. 1 to 10, as well as elements other than zinc and the contents thereof in the zinc alloy electroplating layer, and the plating weight per surface of the steel sheet of the zinc alloy electroplating layer for each of the samples for comparison Nos. 3 to 10 are also shown in Table 2.

[0051] Then, for each of the thus prepared samples of the invention Nos. 1 to 66 and the samples for comparison Nos. 1 to 10, antifriction, corrosion resistance and painting finish were investigated by means of performance tests as described below. The results of these tests are also shown in Tables 1 and 2.

(1) Antifriction test:

[0052] A mineral oil type anticorrosive oil for a steel sheet (product name: NOX RUST 530F40) made by Parker Industries, Inc. was applied onto one surface of each sample. The frictional coefficient of the sample onto the one surface of which the anticorrosive oil was applied, was measured with the use of an apparatus as shown in Fig. 3, thereby evaluating antifriction of the sample on the basis of the thus measured frictional coefficient.

[0053] The apparatus for measuring the frictional coefficient of the sample comprised, as shown in Fig. 3, a rack 2; a supporting stand 5, provided vertically movably on the rack 2 along a plurality of guide rods 12 and 13 attached vertically to the rack 2, and having a plurality of rollers 6 on the upper end thereof; a supporting stand driving mechanism (not shown) for vertically moving the supporting stand 5; a first load cell 8, provided between the supporting stand 5 and the rack 2, for measuring the force applied to the supporting stand 5; a pressing block 4 fitted to a frame 3 fixed to the rack 2 so as to project toward the supporting stand 5; a horizontally movable sliding table 7 mounted on the rollers 6 of the supporting stand 5 between the supporting stand 5 and the pressing block 4; a sliding table driving mechanism (not shown), provided on another rack 11, for horizontally moving the sliding table 7; and a second load cell 9, provided between an operating rod 10 connected to the sliding table driving mechanism and one end of the sliding table 7, for measuring the force applied to the sliding table 7.

[0054] By operating the supporting stand driving mechanism, the supporting stand 5 was moved upward to lift up the sliding table 7 on the upper surface of which a sample 1 was placed. Thus, the upper surface of the sample 1 was pressed against the lower end of the pressing block 4, and the force N in the arrow A direction was measured by means of the first load cell 8. Then, by operating the sliding table driving mechanism, the sliding table 7 was horizontally moved in the arrow B direction, together with the sample 1 placed on the upper surface thereof, and the force F applied to the sliding table 7 was measured by means of the second load cell 9 at the moment when the sliding table 7 reached the moving speed of 1 m/minute.

[0055] The ratio of the force F to the force N, i.e., the ratio F/N was determined, and the thus determined value was used as the frictional coefficient.

(2) Corrosion resistance test:

[0056] Each of the samples having a width of 70 mm and a length of 150 mm was subjected to a dipping type phosphating for a steel sheet for automobile in a phosphating solution (product name: PBL3080) made by Nihon perkerizing Co., Ltd., to form a phosphate film on the surface of the sample. Then, the sample was subjected to a cation type electropainting with the use of a paint (product name: ELECRON 9400) made by Kansai Paint Co., Ltd., to form a painting film having a thickness of 20 µm on the phosphate film. Then, a notch was provided on the thus formed painting film. A salt spray test was carried out on the sample having the thus notched painting film. More specifically, the sample was exposed to the open air for a period of one year, during which salt water having a sodium chloride content of 5 wt.% was sprayed over the sample at a rate of twice a week. Then, the maximum blister width of the painting film was measured on one side of the notch on the sample after the salt spray test, and corrosion resistance was evaluated by means of the thus measured maximum blister width of the painting film. The criteria for evaluation were as follows:

A:

a maximum blister width of under 1 mm;

B:

a maximum blister width within a range of from 1 mm to under 2 mm;

C:

a maximum blister width within a range of from 2 mm to under 2.5 mm; and

D:

a maximum blister width of at least 2.5 mm.

(3) Painting finish test:

[0057] First, 100 sheets of each sample having a width of 70 mm and a length of 150 mm were prepared. As in the case of the corrosion resistance test, each of the thus prepared samples was subjected to a dipping type phosphating for a steel sheet for automobile in a phosphating solution (product name: PB 3080) made by Nihon Perkerizing Co., Ltd., to form a phosphate film on the surface of the sample. Then, the sample was subjected to a cation type electropainting with the use of a paint (product name: ELECRON 9400) made by Kansai Paint Co., Ltd., to form a painting film having a thickness of 20 µm on the phosphate film. Then, the number of samples having a defect in the thus formed painting film caused by bubbles was counted to determine the ratio of the number of such defective samples to the 100 samples, and painting finish was evaluated on the basis of the thus determined value, i.e., the defect occurrence ratio. The criteria for evaluation were as follows:

A:

a defect occurrence ratio of 0%;

B:

a defect occurrence ratio within a range of from 1 to 5%; and

C:

a defect occurrence ratio of over 5%.

[0058] As is clear from Table 1, all the samples of the invention Nos. 1 to 66 had a frictional coefficient of up to 0.17, and were therefore excellent in antifriction.

[0059] The samples of the invention Nos. 16 and 17 showed a maximum blister width within a range of from 1 mm to under 2 mm in the corrosion resistance test, and were therefore excellent in corrosion resistance. All the samples of the invention Nos. 1 to 15 and 18 to 66, except for the samples of the invention Nos. 16 and 17, had a maximum blister width of under 1 mm in the corrosion resistance test, and were therefore particularly excellent in corrosion resistance. Each of the samples of the invention Nos. 16 and 17 was slightly inferior in corrosion resistance to each of the samples of the invention Nos. 1 to 15 and 18 to 66 because the plating weight of the zinc electroplating layer of each of the samples of the invention Nos. 16 and 17 was smaller than the plating weight of the zinc electroplating layer of each of the samples of the invention Nos. 1 to 15 and 18 to 66.

[0060] In addition, all the samples of the invention Nos. 1 to 66 showed a defect occurrence ratio of 0% in the painting finish test, thus having an excellent painting finish.

[0061] As is evident from the above description, all the samples of the invention Nos. 1 to 66 were excellent in antifriction, corrosion resistance and painting finish.

[0062] As is clear from Table 2, in contrast, none of the samples for comparison Nos. 1 to 10 satisfied simultaneously the following three favorable merits possessed by each of the samples of the invention Nos. 1 to 66:

(i) A frictional coefficient of up to 0.17 in the antifriction test;

(ii) a maximum blister width of under 2 mm in the corrosion resistance test; and

(iii) a defect occurrence ratio of 0% in the painting finish test.

[0063] Among others, the samples for comparison Nos. 1 to 3, 5 to 7 and 9 to 10, except for the samples for comparison Nos. 4 and 8 had a large frictional coefficient of at least 0.3.

[0064] Furthermore, a high-viscosity lubricant oil (product name: FERROCOTE 61-MAL-HCL-1) made by Nippon Quaker Chemical Co., Ltd. was applied onto the zinc electroplating layer as the single layer of the sample for comparison No. 2 having a large frictional coefficient, and an antifriction test as described above was effected on the sample for comparison No. 2 applied with the high-viscosity lubricant oil on the zinc electroplating layer thereof. The above-mentioned sample for comparison No. 2 had a frictional coefficient of 0.11. This revealed that the samples of the invention Nos. 1 to 66 applied with the easily removable anticorrosive oil had substantially the same antifriction as that of the sample for comparison No. 2 applied with the high-viscosity lubricant oil which is very difficult to remove.

[0065] According to the present invention, as described above in detail, it is possible to provide an electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish, thus providing industrially useful effects.

Claims

1. An electrogalvanized steel sheet having two electroplating layers and excellent in antifriction, corrosion resistance and painting finish, characterized by comprising:

   a steel sheet;

   a zinc electroplating layer formed on at least one surface of said steel sheet, said zinc electroplating layer having a center-line mean roughness (Ra) of up to 1.5 µm and a plating weight within a range of from 25 to 150 g/m² per surface of said steel sheet; and

   a zinc alloy electroplating layer formed on said zinc electroplating layer, said zinc alloy electroplating layer comprising zinc and at least one element selected from the group consisting of cobalt, manganese, nickel, iron and chromium, said zinc alloy electroplating layer containing said at least one element in an amount within a range of from 3 to 99 wt.% relative to said zinc alloy electroplating layer, and said zinc alloy electroplating layer having a plating weight within a range of from 1 to 20 g/m² per surface of said steel sheet.

2. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc and cobalt.

3. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc and manganese.

4. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc and nickel.

5. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc and iron.

6. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc and chromium.

7. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, iron and nickel,

8. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, iron and cobalt.

9. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, iron and chromium.

10. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, nickel and cobalt.

11. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, nickel and chromium.

12. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, cobalt and chromium.

13. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, manganese and chromium.

14. An electrogalvanized steel sheet as claimed in Claim 1, wherein:

   said zinc alloy electroplating layer comprises zinc, nickel, chromium and cobalt.

Drawing