JPH07207430A - Zn-mg alloy plated steel sheet excellent in corrosion resistance after coating and corrosion resistance at exposed part - Google Patents

Abstract

PURPOSE:To obtain a plated steel sheet excellent in both corrosion resistance after coating and red rust resistance at the base steel exposed part. CONSTITUTION:The concn. of Mg in a Zn-Mg alloy plating layer disposed on the surface of a steel sheet is continuously or stepwise reduced from the interface between the plating layer and the base steel toward the surface of the plating layer. A plating layer may be formed by laminating plural layers different from each other in the concn. of Mg. The interfacial part having a high concn. of Mg has high corrosion preventing action to the base steel and the surface layer part having a low concn. of Mg maintains its sacrificial corrosion preventing action.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is a Z which is excellent in corrosion resistance after coating and also in corrosion resistance of a cut end surface where a base steel is exposed.
It relates to an n-Mg alloy plated steel sheet.

[0002]

2. Description of the Related Art Conventionally, various surface treatments have been performed to improve the corrosion resistance of steel sheets. A Zn-plated steel sheet, which is a typical surface-treated steel sheet, is mainly manufactured by a hot dipping method, an electroplating method, or the like. In some cases, Zn-plated steel sheets are also manufactured by the vapor deposition plating method. Zn
As the environment in which plated steel sheets are used becomes more severe, even more excellent corrosion resistance is required. In order to satisfy this requirement, various studies have been conducted for each method such as hot dipping and electroplating.

When attempting to improve the corrosion resistance of a Zn-plated steel sheet by the hot dip coating method, it is first thought that the amount of the Zn-plated layer deposited is increased. However, since the upper limit of the adhesion amount is restricted from the viewpoint of manufacturing, there is a limit in improving the corrosion resistance by increasing the adhesion amount. Further, an increase in the amount of adhesion, that is, an increase in the thickness of the plated layer tends to cause defects such as galling and flaking when press-forming the plated steel sheet. If a thick plating layer is to be formed by the electroplating method, it is necessary to slow down the line speed or increase the number of cells, which causes problems in productivity, equipment load, and the like. Therefore, the corrosion resistance is improved by applying Zn-Ni-based Zn alloy plating. However, since the Zn-Ni alloy plating layer is hard and brittle, defects such as cracks and chips are likely to occur in the plating layer during molding. When such a defect occurs in the plating layer, the base steel is exposed through the defect portion, so that the original performance of the plating layer is not exhibited and corrosion starting from the defect portion progresses.

From the above background, it has been attempted to manufacture a Zn-based alloy plated steel sheet having high corrosion resistance by a vapor deposition method. In particular, studies on Zn-Mg alloy plating exhibiting an excellent anticorrosion effect are under way, and it is necessary to form a Zn-Mg alloy plating layer containing 0.5 to 40% by weight of Mg (JP-A-64-17852). Gazette), in order to improve the adhesion and workability of the plated layer, Zn, Ni, Cu, Mg, Al, Fe, between the Zn-Mg alloy plated layer and the base steel,
Interposing an intermediate layer of Co, Ti, etc. (Japanese Patent Laid-Open No. 2-1
No. 41588) is known. As a method for improving the peeling resistance of the coating film after coating, forming a Zn-Mg alloy plating layer mainly composed of Zn-Mg based intermetallic compound and containing no Mg phase is disclosed in JP-A-64-17853. Have been introduced in. In addition, JP-A 64-25990
In the publication, Zn-Ti is formed on the Zn-Mg alloy plating layer.
By providing the alloy plating layer, the corrosion resistance after coating is improved.

[0005]

The Zn-Mg alloy plated steel sheet has much higher corrosion resistance than the pure Zn plated steel sheet. However, it is difficult to achieve both the corrosion resistance after coating and the red rust resistance in the exposed portion of the base steel such as the peeled portion of the plating layer and the cut end surface. As the Mg concentration of the plating layer increases, the corrosion resistance after bare and after coating is improved up to a Mg concentration of 25% by weight. However, as the Mg concentration increases,
Red rust is easily generated on the exposed base steel such as the cut end surface. M
The influence of the g concentration on the occurrence of red rust is considered as follows. In the Zn-Mg alloy-plated steel sheet, the higher the Mg concentration, the more protective corrosion product ZnCl ₂ .4Zn.
A large amount of (OH) ₂ and Zn (OH) ₂ is formed on the steel plate surface. These corrosion products suppress the elution of the plating layer and improve the corrosion resistance. However, since the corrosion product has a high insulating property, it reduces the sacrificial anticorrosive action. As a result, it becomes difficult for the sacrificial anticorrosive current to flow, and red rust is likely to occur on the exposed portion of the steel plate such as the cut end surface.

The generated red rust is very noticeable on a white or light-colored coated steel sheet, and significantly impairs the design of the coated steel sheet. In this respect, the reduction of the commercial value of the coated steel sheet due to the red rust generated on the cut end surface becomes a serious problem. The present invention has been devised in order to solve such a problem, by adjusting the Mg concentration in the thickness direction of the plating layer, excellent corrosion resistance after coating, such as the cut end face exposed base steel It is an object to obtain a Zn-Mg alloy plated steel sheet having excellent corrosion resistance.

[0007]

Means for Solving the Problems The plated steel sheet of the present invention is
In order to achieve the object, a Zn-Mg alloy plating layer in which the Mg concentration continuously or stepwise decreases from the interface between the plating layer and the base steel toward the surface of the plating layer is formed. And The plating layer may be a stack of a plurality of plating layers. In this case, a plurality of Zn-Mg alloy plated layers are formed in which the plated layer in contact with the base steel has a high Mg concentration and the Mg concentration gradually decreases toward the plated layer surface.

[0008]

[Function] Among the plating layers, the portion at the interface with the base steel is the portion that is most likely to corrode since it is in contact with the steel sheets having different potential differences. In the present invention, by increasing the Mg concentration in this portion, corrosion resistance is secured and corrosion under the coating film is suppressed. This inhibition of corrosion also causes the corrosion rate of the plating layer upper layer portion exhibiting a sacrificial anticorrosion action to be delayed, and maintains the sacrificial anticorrosion action for a long period of time. On the contrary, when the Mg concentration on the interface side is insufficient, corrosion under the coating film is promoted, and the coating film swells and the like occurs at an early stage. On the other hand, the Mg concentration is lowered in the upper layer portion of the plating layer. As a result, a highly insulating corrosion product ZnCl ₂ .4Zn (O
Generation of H) ₂ and Zn (OH) ₂ is suppressed, and a large amount of conductive corrosion products such as ZnO are generated. Therefore, the sacrificial anticorrosive action is sufficiently exerted, and the occurrence of red rust on the cut end face and the like is prevented. The Mg concentration on the surface of the plating layer is preferably 8% by weight or less in order to obtain the required sacrificial anticorrosion effect.

Even when the plating layer has a multi-layer structure, the Mg concentration at the interface portion in contact with the base steel is increased to improve the corrosion resistance under the coating film. On the other hand, the Mg concentration in the upper layer is preferably 8
By reducing the content to less than 1% by weight, the sacrificial anticorrosion action prevents the occurrence of red rust on the exposed portion of the base steel. By forming a Zn-Mg alloy plating layer having a gradient in Mg concentration in this way, Mg
Corrosion resistance after coating is improved on the lower layer side (interface side) with high concentration and excellent corrosion resistance. Suppressed. Moreover, since the corrosion under the coating film is suppressed by the plating layer on the interface side, the sacrificial anticorrosive action is maintained for a long period of time.

[0010]

[Example] As a plating original plate, C: 0.02% by weight, S
i: 0.02% by weight, Mn: 0.22% by weight, P: 0.
010 wt%, S: 0.009 wt% and Al: 0.0
An Al-killed steel sheet having a thickness of 0.8 mm and containing 5% by weight and the balance being substantially Fe was used. A 200 mm × 200 mm test piece was cut out from this Al-killed steel plate, and after cleaning the surface by ultrasonic cleaning in an organic solvent, it was set in a vacuum vapor deposition apparatus. After depressurizing the inside of the vacuum deposition apparatus to 5 × 10 ⁻⁵ Torr, the oxide film was removed from the surface of the steel sheet by sputter etching using Ar glow discharge to activate the surface of the steel sheet. When producing a gradual reduction type Zn-Mg alloy plated steel sheet in which the Mg concentration continuously decreases toward the surface of the plating layer, Mg is vapor-deposited on the activated test piece,
Next, Zn was vapor-deposited. Plate temperature during vapor deposition is 100-120
The test piece was heated by the heater so that the temperature became 0 ° C. Further, in order to make the concentration uniform in a plane, the test piece was vapor-deposited while rotating.

After depositing Zn, the inside of the vapor deposition apparatus is filled with N ₂
Mg was diffused into Zn by heating to 270 to 350 ° C. for 10 seconds with gas to 700 Torr. As a result, a Zn-Mg alloy plated steel sheet having a high Mg concentration at the interface and a continuously decreasing Mg concentration toward the surface was obtained. In this case, the total thickness of the plating layer was 5 μm. Then, after cooling the steel plate to 120 ° C. in a N ₂ gas atmosphere of 700 Torr, the plated steel plate was taken out from the vacuum chamber opened to the atmosphere. In the obtained Zn-Mg alloy plated steel sheet, Mg is segregated on the surface as it is, and the water resistant secondary adhesion of the coating film is poor. Therefore, the plated steel sheet was immersed in 2% hydrochloric acid for 5 seconds to remove the Mg segregation layer of about 0.1 μm. The obtained Zn-Mg alloy plated steel sheet is schematically shown in Fig. 1 (a), and Fig. 1 (b) shows a change in Mg concentration in the plating layer thickness direction.

In the case of producing a multi-layer reduced type plated steel sheet having a multi-layered plating layer in which the Mg concentration varies stepwise, Ar is used.
Mg and Zn were simultaneously vapor-deposited on the plated steel sheet activated by glow discharge, and the vapor deposition rates of Mg and Zn were adjusted for each layer. In this case, heating after vapor deposition was not performed.
Other manufacturing conditions were set in the same manner as in the case of the taper-type Zn-Mg alloy plated steel sheet. 2.5 μm each for a two-layer plating layer and 2 μm for a three-layer plating layer
m, 1 μm and 2 μm. Further, in order to remove the Mg segregation layer on the surface, pickling was performed in the same manner as in the case of the gradually decreasing Zn-Mg alloy plated steel sheet. A Zn-Mg alloy plated steel sheet having a three-layer structure is schematically shown in FIG. 2 (a), and FIG. 2 (b) shows a change in Mg concentration in the thickness direction of the plated layer.

As a comparative example, a plated steel sheet on which the following Zn--Mg alloy plated layer was formed was manufactured. In each of the plated steel sheets, the thickness of the plating layer and pickling were adjusted in the same manner as described above. Uniform Zn-Mg alloy plating layer in which the Mg concentration is evenly distributed from the interface to the surface. Zn and Mg are simultaneously vapor-deposited at a constant ratio. Gradual increase Zn-Mg alloy in which the Mg concentration is continuously increased from the interface to the surface. Plating Layer Mg deposition was performed subsequent to Zn deposition, and then Mg was diffused into Zn by heating at 270 to 350 ° C. for 10 seconds in an N ₂ gas atmosphere of 700 Torr. -Multilayer increasing type Zn-Mg alloy plating layer in which a plurality of layers in which the Mg concentration gradually increases toward the surface are formed. Process similar to the multilayer decreasing type Zn-Mg alloy plating layer.

Chromate treatment was applied to each of the vapor-deposited test pieces as a chemical conversion treatment before coating. Chromate treatment amount is 20-25mg / in terms of chromium weight per unit area
It was set to m ² . After the chromate treatment, a white acrylic resin baking paint was used, and a bar coater was applied so that the thickness of the coating film after drying was 20 μm, followed by baking and drying. The corrosion resistance after coating was determined by applying a cross cut to a coated Zn-Mg alloy-plated steel sheet with a cutter knife, and determining the degree of swelling of the coating film swollen from the cross cut portion by a composite cycle corrosion test. As for the swelling of the coating film, the swelling widths of the four most swollen portions were measured, and the average value was taken as the maximum coating film swelling width. Combined cycle corrosion test is based on JIS Z2371, salt spray for 6 hours → temperature 50 ° C and relative humidity 30%
1 cycle of forced drying of 3 hours → exposure to an atmosphere of temperature 50 ° C and relative humidity of 98% for 14 hours → drying of exposure to an atmosphere of temperature 25 ° C and relative humidity of 65% for 1 hour. By repeating 50 cycles, went.

The red rust resistance on the cut end face was examined as follows. The coated Zn-Mg alloy coated steel sheet was subjected to a high temperature wet test at a relative humidity of 98% and a temperature of 50 ° C, and the cut end face was observed with the naked eye from the direction perpendicular to the steel sheet. And
The time until red rust was observed on the coating film on the cut end face was measured. As is clear from Table 1 showing the results of comparative examination of the corrosion resistance after coating of each Zn-Mg alloy-plated steel sheet and the red rust resistance of the exposed portion of the base steel, the taper-type Z according to the present invention is shown.
In the n-Mg alloy plated steel sheet, the coating film swelling width is small, and the red rust generation time on the cut end surface is long. From this, it is understood that the corrosion resistance after coating is high and the red rust resistance of the exposed portion of the base steel is also excellent. On the other hand, the uniform type Z of the comparative example
In the n-Mg alloy plated steel sheet, although the coating film swelling width is small, the red rust generation time on the cut end surface is short, and the red rust resistance of the exposed portion of the base steel is poor. In addition, gradually increasing Zn-M
The g-alloy-plated steel sheet has a large swelling width of the coating film and is inferior in corrosion resistance after coating, and is also inferior in red rust resistance because red rust generation time on the cut end surface is short. In this respect, the taper-type Zn-Mg alloy plated steel sheet according to the present invention is excellent in both the corrosion resistance after coating and the red rust resistance of the exposed portion of the base steel.

[0016]

[Table 1]

Table 2 shows the results of an examination of the corrosion resistance after coating and the corrosion resistance of the exposed portion of the base steel of the multi-layer reduced type Zn-Mg alloy plated steel sheet having a two-layer or three-layer structure according to the present invention. Table 3 shows the results of an examination of the corrosion resistance after coating and the corrosion resistance of the exposed portion of the base steel for the multilayer increased Zn-Mg alloy plated steel sheet having a two-layer or three-layer structure. Table 2 and Table 3
In each case, the order of each layer is expressed from the side closer to the interface in contact with the base steel. It is shown in Table 2 that the multilayer reduced Zn-Mg alloy plated steel sheet according to the present invention has a small bulging width of the coating film and a long red rust generation time on the cut end surface. From this, the multilayer reduced Zn-M according to the present invention is
It can be seen that the g-alloy plated steel sheet is excellent in both the corrosion resistance after coating and the red rust resistance of the exposed portion of the base steel. On the other hand, in the multilayer increased Zn-Mg alloy plated steel sheet, as shown in Table 3, the swelling width of the coating film is large and the corrosion resistance after coating is poor, and the red rust occurrence time on the cut end surface is short, so the red rust resistance is improved. Was also inferior.

[0018]

[Table 2]

[0019]

[Table 3]

[0020]

As described above, in the present invention, the Zn--Mg alloy plating layer formed on the surface of the steel sheet is
The Mg concentration is high at the interface in contact with the base steel, and the concentration is gradually or gradually decreased toward the surface of the plating layer. By thus forming the gradient of the Mg concentration, a plated steel sheet excellent in both corrosion resistance after coating and red rust resistance of the exposed portion of the base steel can be obtained. Therefore, the swelling of the coating film due to corrosion after coating is suppressed, and red rust does not occur on the exposed base steel such as the cut end surface and the plating layer defect portion, and a product that maintains a good appearance for a long time can be obtained. .

[Brief description of drawings]

FIG. 1 shows a Zn—Mg alloy plating layer (a) in which the Mg concentration is continuously changed, and Mg in the thickness direction of the plating layer.
Concentration distribution (b)

FIG. 2 shows a Zn—Mg alloy plating layer (a) in which the Mg concentration is changed stepwise and Mg in the thickness direction of the plating layer.
Concentration distribution (b)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Minoru Saito 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd.

Claims (2)

[Claims] 1. A plated steel sheet having a Zn—Mg alloy plating layer in which the Mg concentration is continuously or stepwise reduced from the interface between the plating layer and the base steel toward the surface of the plating layer. 2. A plated steel sheet in which a plurality of Zn—Mg alloy plating layers having a high Mg concentration in a plating layer in contact with a base steel and a decreasing Mg concentration toward the surface of the plating layer are laminated.