JPH06248424A - Corrosion resistant zn-mg alloy plated steel sheet excellent in workability - Google Patents

Abstract

PURPOSE:To obtain a plated steel sheet excellent in corrosion resistance and workability by forming a Zn-Mg alloy plated layer in which a concn-gradient is fitted to Mg content. CONSTITUTION:On this plated steel sheet, a Zn-Mg alloy plated layer in which Mg content is continuously reduced in a direction of the depth of the plated layer, and the Mg content in the lowermost part of the plated layer is regulated to <=6.0wt.%. In this way, because of low Mg content on the lowermost part of the plated layer, the difference in ductility in the boundaries with the base steel is reduced, and the peeling of the plated layer caused by stress at the time of forming can be suppressed. Furthermore, the low Mg content in the lowermost part of the plated layer secures the sacrificial corrosion preventing effect characteristic of Zn, and the prevention of corrosion in the base steel can be attained even in a state in which defects are generated on the plated layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plated steel sheet on which a Zn-Mg alloy plating is formed which does not cause defects such as peeling during forming and exhibits high corrosion resistance.

[0002]

2. Description of the Related Art Conventionally, various surface treatments have been performed to improve the corrosion resistance of steel sheets. Among them, the Zn-plated steel sheet is widely used as a material which is economical and has good corrosion resistance. The Zn-plated steel sheet is mainly manufactured by hot dipping, electroplating and the like. Further, in some cases, Zn-plated steel sheets are manufactured by vapor deposition plating. The demand for improved corrosion resistance tends to increase year by year. Along with this, various studies have been conducted in various manufacturing methods 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 zinc-plated layer deposited increases. However, since the upper limit of the adhered amount is limited from the manufacturing aspect, there is a limit in improving the corrosion resistance by increasing the adhered 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 an attempt is made to form a thick plating layer by electroplating, productivity will be significantly impaired. In this respect, in the electroplating method, Zn-Ni-based Zn alloy plating or the like is performed to improve the corrosion resistance. However, since the Zn-Ni alloy plating layer is hard and brittle, defects such as cracks and chips are likely to occur during molding. When such a defect occurs in the plating layer, the underlying 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 proceeds. From such a background, it has been attempted to manufacture a Zn-based alloy plated steel sheet having high corrosion resistance by a vapor deposition method. Among them, Zn-Mg alloy plating exhibits an excellent anticorrosion effect. For example, JP-A-64-
Japanese Patent No. 17852 discloses forming a Zn-Mg alloy plating layer containing 0.5 to 40% by weight of Mg. Further, when an intermediate layer of Zn, Ni, Cu, Mg, Al, Fe, Co, Ti or the like is interposed between the Zn-Mg alloy plating layer and the base steel, the adhesion and workability of the plating layer are improved. This is introduced in Japanese Patent Laid-Open No. 2-141588.

[0004]

The Zn-Mg alloy plated layer has improved corrosion resistance as the Mg content increases, but
On the other hand, the plating layer becomes brittle. In particular, the embrittlement of the plating layer due to the increase of the Mg content becomes remarkable when the Mg content exceeds 6% by weight. As a result, the ductility of the plated layer is significantly reduced, and defects such as cracks and chips are likely to occur in the plated layer during work forming. Further, the plating layer cannot follow the work deformation of the base steel, peeling occurs between the plating layer and the base steel, and defects such as powdering occur. Powdering adversely affects the surface condition of press-formed products. That is, the plating layer separated from the base steel is pressed against the press-formed plated steel sheet to generate surface defects such as point-like scratches. The peeling of the plating layer is Zn
This is because the ductility of the Mg alloy plating layer is insufficient. Therefore, even when a Zn alloy layer having a high ductility is formed between the Zn-Mg alloy plating layer and the base steel, the difference in elongation between the Zn alloy layer and the Zn-Mg alloy plating layer is large, and therefore, still remains. The occurrence of powdering cannot be suppressed.

By the way, in the pure Zn plating layer, even if there are defects such as cracks, corrosion of the base steel can be avoided by the sacrificial anticorrosion action of Zn itself. On the other hand, in a Zn-Mg alloy plating layer having a high Mg content, a strong insulating corrosion product such as Zn (OH) ₂ is formed in the plating layer, and the sacrificial anticorrosive action of Zn is lost. As a result, even if minute cracks occur in the plating layer, corrosion of the base steel occurs and progresses at an early stage. This phenomenon remarkably appears when the Mg content of the Zn-Mg alloy plated layer is 15% by weight or more.

In order to prevent cracking of the plating layer during processing and maintain the sacrificial anticorrosion effect of the plating layer, Zn
-It is necessary to set the Mg content of the Mg alloy plating layer low. However, the reduction of Mg content is
The advantage of n-Mg alloy plating is not fully utilized, and the corrosion resistance in the flat portion of the plated steel sheet is deteriorated. The present invention has been devised to solve such a problem, and by providing a concentration gradient in the thickness direction of the Zn-Mg alloy plating layer with respect to the thickness direction of the Zn-Mg alloy plating layer, Zn- excellent in corrosion resistance is obtained. An object of the present invention is to provide a plated steel sheet which improves the workability and also utilizes the sacrificial anticorrosion effect of Zn without impairing the advantages of the Mg alloy plating layer.

[0007]

Means for Solving the Problems The plated steel sheet of the present invention is
In order to achieve the object, the Mg content continuously decreases in the depth direction of the plating layer, and a Zn-Mg alloy plating layer having a Mg content at the bottom of the plating layer of 6.0 wt% or less is formed. It is characterized by A Zn-Mg alloy plated steel sheet having a concentration gradient in the Mg content is, for example, Z
It is manufactured by performing Mg plating after n plating and then heating to alloy the plated layer. Any of hot dipping, electroplating and vapor deposition plating may be adopted for forming the Zn layer, but vapor deposition plating is preferable in consideration of subsequent formation of the Mg layer. It is thermodynamically impossible to perform Mg plating by an electroplating method using an ordinary aqueous solution. When using the plating method using molten salt, M
Although formation of the g-plated layer is possible in principle, operation at high temperature is required and there is no practical technique. For this reason, the vacuum evaporation method, especially the arc evaporation method, is adopted for forming the Mg layer.

Mg vapor evaporated by resistance heating, electron beam heating, arc heating or the like is induced on the surface of the original plating plate and deposited as a Mg layer. In order to obtain a Mg layer having a uniform property, it is necessary to accurately control the evaporation amount of Mg. However, the sublimable metal Mg is
When vaporizing by resistance heating, electron beam heating, etc., it vaporizes in the solid state in the crucible. Therefore, since the surface area of solid Mg changes and the evaporation amount changes with it, it is difficult to control the evaporation amount accurately. On the other hand, in the arc evaporation method, the Journal of Welding Society, Vol. 54, No. 6 (1
985) As described on pages 352-357,
Since evaporation will occur at the cathode bright spot of the arc discharge,
The evaporation amount can be controlled by adjusting the arc current. In this respect, the arc evaporation method is the most suitable method for forming the Mg layer.

In order to simplify the equipment structure, a Zn vapor deposition device and then a Mg vapor deposition device are arranged in the same vacuum chamber with respect to the running direction of the plating original plate, and the vapor deposition is performed in a state where Zn vapor and Mg vapor are mixed. Apply to. In this case, the vapor ratio of Zn is high on the upstream side and the vapor ratio of Mg is high on the downstream side with respect to the traveling direction of the original plating plate. Therefore, Z in which the Mg content gradually and continuously changed
An n-Mg alloy plating layer is formed on the surface of the original plating plate. As the plating base plate used in the present invention, ordinary steel such as Al-killed steel is substituted, but other various steel plates such as low alloy steel, ferritic stainless steel, austenitic stainless steel, etc. are used. be able to. For example, in the case where stainless steel is used as a plating base plate, the base steel having excellent corrosion resistance works synergistically with the Zn-Mg alloy plating layer, and corrosion resistance comparable to that of high-grade Cr-high Ni high-grade stainless steel is obtained.

The plated steel sheet of the present invention is manufactured, for example, by using the equipment outlined in FIG. Plate 1
0 is rewound from the pay-off reel 11, guided by the deflector rolls 12 and 13, and introduced into the vacuum chamber 20. The plating original plate 10 has its traveling direction changed by the deflector rolls 14 and 15 arranged in the vacuum chamber 20, is sent out from the vacuum tank 20 as the plated steel plate 16, and is then wound around the winding reel 17 via the heating chamber 60. . An inlet side vacuum seal 21 and an outlet side vacuum seal 22 are provided on the inlet side and the outlet side of the vacuum chamber 20, respectively. Vacuum chamber 2 between the inlet vacuum seal 21 and the outlet vacuum seal 22
The inside of 0 is formed by the partition walls 23 to 26 in which slits through which the plating original plate 10 passes are formed.
It is divided into a vapor deposition zone 40 and a Mg vapor deposition zone 50. A plurality of vacuum pumps are connected to the vacuum chamber 20, and the inside pressure is reduced to about 1 × 10 ⁻³ Pa. Although two vacuum pumps 27 and 28 are shown in FIG. 1, other vacuum pumps may be arranged at appropriate places.

In the pretreatment zone 30, a high frequency heating device 31 and an ion beam etching device 32 are arranged along the running direction of the plating original plate 10. Pretreatment zone 3
The plating base plate 10 sent to 0 is the high frequency heating device 3
After being heated to a predetermined temperature in 1, the irradiation with the ion beam 33 emitted from the ion beam etching device 32,
The surface condition is adjusted to be active for the plated metal. Zn
In the vapor deposition zone 40, the Zn vapor deposition device 41 is provided with the plating base plate 1.
It is arranged so as to face both sides of 0. Zn vapor deposition device 41
Is provided with a guide hood 43 that causes Zn vapor evaporated from a Zn melting tank 42, which is a Zn source, to flow toward the original plating plate 10. The guide hood 43 has an opening 44 wider than the original plating plate 10, and a slit 45 through which the original plating plate 10 passes is formed in the opening 44.

In the Mg vapor deposition zone 50, the Mg vapor deposition apparatus 5 is provided.
1 are arranged so as to face both sides of the original plating plate 10.
The Mg vapor deposition apparatus 51 employs a method of vaporizing Mg by arc heating, and the amount of Mg vaporization, and thus Zn-
The variation rate of the Mg content in the Mg alloy plating layer can be suppressed within ± 5%. The heating chamber 60 is maintained in a nitrogen atmosphere, and the high-frequency heating device 61 is arranged so as to face both sides of the traveling plated steel sheet 16. The high frequency heating device 61 heats the original plating plate 16 to alloy the Zn layer and the Mg layer formed on the surface.

In the equipment configuration shown in FIG. 1, the high frequency heating device 31, the ion beam etching device 32, the Zn
The vapor deposition device 41 and the Mg vapor deposition device 51 are arranged vertically in the vacuum chamber 20. However, the present invention is not limited to this, and an appropriate arrangement state is adopted depending on the space for arranging the equipment, the shape of the equipment, and the like. Further, a plasma etching device may be adopted instead of the ion beam etching device 32. Further, when the plating original plate 10 has a surface state sufficiently active for Zn vapor deposition, the ion beam etching device 31, the plasma etching device, the high frequency heating device 31 and the like can be omitted.

[0014]

The Zn-Mg alloy plating layer formed on the surface of the base steel has a Mg content having a concentration gradient that continuously decreases in the depth direction, and the Mg content at the bottom is 6.0% by weight.
It is regulated below. The ductility of the plated layer is high at the lowermost portion and becomes lower continuously toward the upper layer according to the concentration gradient of Mg. Further, since the lowermost Mg content is low, the sacrificial anticorrosive action peculiar to Zn is not impaired. On the other hand, since the surface layer portion has a high Mg content, it exhibits excellent resistance to corrosion from the surface. When forming a plated steel sheet, peeling that occurs in the plating layer causes a difference in the amount of deformation between the base steel and the plating layer due to the different ductility of the base steel, and a large stress occurs at the interface between the base steel and the plating layer. There is a cause. In this respect, in the plated steel sheet of the present invention,
Since the ductility of the plating layer changes gently according to the concentration gradient of Mg, no large stress is generated during the forming process at the interface between the plating layer and the base steel and inside the plating layer. As a result, peeling that tends to occur in the plating layer is prevented.

Further, the Mg content in the lowermost portion is 6.0.
The sacrificial anticorrosive action of Zn is ensured because the content of Zn is regulated to not more than weight%. Therefore, even if the base steel is exposed by performing processing under severe conditions such that the plating layer is cracked, the sacrificial anticorrosive action of Zn suppresses the early corrosion of the base steel. On the other hand, the Mg content of the surface layer portion is preferably 6% by weight or more from the viewpoint of ensuring the corrosion resistance of the flat portion. When alloying a plated steel sheet on which a Zn layer and a Mg layer are formed, the optimum Zn
-To form the Mg alloy plating layer, the alloying temperature is set to 1
It is preferable to maintain the temperature in the range of 40 to 350 ° C. 14
Heating below 0 ° C. does not cause an alloying reaction between the Zn layer and the Mg layer. On the other hand, heating above 350 ° C. promotes the alloying reaction between the base steel and the Zn layer, and causes formation of a Zn—Fe-based brittle alloy layer at the interface. Such a brittle alloy layer causes powdering when forming a plated steel sheet.

[0016]

Example A steel plate having a composition shown in Table 1 and a plate thickness of 0.5 mm was used as a plating original plate. After pickling and degreasing the original plating plate, Zn-
A Mg alloy plated steel sheet was manufactured.

[Table 1]

After depressurizing the vacuum chamber 20 to 1 × 10 ⁻³ Pa, the vacuum plating plate 20 is passed at a plate passing speed of 30 m / min.
And heated to 150 ° C. by the high frequency heating device 31. Next, the plating original plate 10 was subjected to ion beam etching under the conditions shown in Table 2. At this time, since the source gas of the ion beam is flown, the degree of vacuum in the pretreatment zone 30 is 0.
It went down to 05Pa.

[Table 2]

An average film thickness of 0.67 is formed on the surface of the plating original plate 10 whose surface has been activated by ion beam etching.
A Zn layer having a thickness of μm and a Mg layer having an average thickness of 0.1 μm were formed. The film thicknesses of the Zn layer and the Mg layer could be controlled by controlling the evaporation amounts of Zn and Mg and adjusting the stripping speed and surface temperature of the original plating plate. Table 3 shows the plating conditions at this time.

[Table 3]

Zn-M formed on the obtained plated steel sheet
The g-alloy plated layer was investigated by Auger electron spectroscopy (AES). As is clear from Table 2 showing the investigation results, it was confirmed that the Mg content was highest in the surface layer portion of the plating layer and had a concentration gradient that decreased in the depth direction.
The Mg content was 38.5% by weight in the surface layer and 2.
It was 0% by weight. On the other hand, the Zn content is 6 in the surface layer.
It was 1.5% by weight and 98.0% by weight at the bottom. Zn
-A test piece was cut out from the plated steel sheet on which the Mg alloy plating layer was formed, and subjected to a salt spray test according to JIS Z2371. The time until the area of red rust generated on the surface of the test piece reached 5% of the total surface area of the test piece was measured, and corrosion resistance was evaluated as 5% red rust generation time. As is clear from FIG. 3 showing the investigation result, the plated steel sheet according to the present invention is
The corrosion resistance was far superior to that of the conventional Zn-plated steel sheet. Further, it can be seen that the corrosion resistance is improved as the Mg content in the upper portion of the plating layer is increased.

The Zn-Mg alloy-plated steel sheet was subjected to a draw bead test and a cycle corrosion test of a scratched test piece to investigate the workability and the corrosion resistance after working. In addition, in order to investigate the influence of the Mg concentration of the Zn-Mg alloy plated layer, a Zn-Mg alloy plated steel sheet having a lowermost Mg concentration of more than 6.0 wt% was used as a comparative example. In the draw bead test,
A test piece cut out from a Zn-Mg alloy-plated steel plate was placed in a mold equipped with a bead having a height of 4 mm and a radius of 0.5 mm, and a pulling force of 500 kgf and a pulling speed of 200 m /
The test piece was pulled out of the mold in minutes. The amount of the plating layer peeled from the base steel after drawing was measured and used as the amount of powdering. In the cycle corrosion test, a test piece in which a scratch penetrating the plating layer was attached with a cutter knife was used. Corrosion tests were performed on the scratched test pieces in the cycle shown in Table 4 to examine whether or not red rust was generated in the scratched portions. Table 5 shows the results of the draw bead test and the cycle corrosion test.

[Table 4]

[Table 5]

As is clear from Table 5, the Zn-Mg alloy-plated steel sheets of Test Nos. 1 to 3 according to the present invention have no cracks during bending and exhibit excellent formability. Further, the plating layer was not peeled off from the base steel, and the amount of powdering generated was extremely small. In addition, since the Mg content was low at the bottom of the plating layer, no early corrosion starting from the scratched portion attached to the plating layer was detected. It is speculated that this is because the sacrificial anticorrosive action of Zn is maintained.

On the other hand, in the Zn-Mg alloy plated steel sheet of Test No. 4 in which the Mg content was 7.4% by weight at the bottom of the plating layer and Test No. 5 in which the Mg content was 16.3% by weight, powdering occurred. The amount is large, and it can be seen that the plating layer is separated from the base steel during the forming process. Moreover, in the comparative example of test number 5 in which the Mg content in the lowermost part of the plating layer was high, the sacrificial anticorrosive action of Zn could not be expected, and red rust occurred on the surface of the test piece within a short time. From this comparison, it was confirmed that the Zn-Mg alloy plated steel sheet according to the present invention is unlikely to cause cracks in the plated layer even after being subjected to forming processing. Further, since the Mg content is low at the bottom of the plating layer, the sacrificial anticorrosion action of Zn is secured, and even if defects such as scratches occur in the plating layer, corrosion of the base steel does not proceed.

[0023]

As described above, according to the present invention, the Mg content of the Zn-Mg alloy plating layer formed on the surface of the base steel is provided with a concentration gradient such that the Mg content increases from the lower part to the surface part. , Mg content at the bottom is 6.0% by weight
It is regulated below. Due to this concentration gradient of Mg content, a plated steel sheet having high corrosion resistance and excellent formability can be obtained. In this Zn-Mg alloy-plated steel sheet, even if a defect occurs in the plating layer during forming, the sacrificial anticorrosion action of Zn is maintained, and therefore the corrosion of the base steel does not proceed starting from the defective portion. . Therefore, it is used as a high-quality plated steel sheet that maintains excellent corrosion resistance for a long period of time.

[Brief description of drawings]

FIG. 1 shows an example of equipment configuration for producing a Zn—Mg alloy plated steel sheet according to the present invention.

FIG. 2 Concentration gradient of Zn-Mg alloy plating layer formed on the surface of base steel

FIG. 3 Effect of Mg concentration of Zn-Mg alloy plated layer on corrosion resistance

[Explanation of symbols]

10: original plating plate 16: plated steel plate 20: vacuum chamber 40: Zn vapor deposition zone 41: Zn vapor deposition device
50: Mg vapor deposition zone 51: Mg vapor deposition apparatus 60: Heating zone for alloying

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] July 26, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction content]

In the pretreatment zone 30, a high frequency heating device 31 and an ion beam etching device 32 are arranged along the running direction of the plating original plate 10. Pretreatment zone 3
The plating base plate 10 sent to 0 is the high frequency heating device 3
After being heated to a predetermined temperature in 1, the irradiation with the ion beam 33 emitted from the ion beam etching device 32,
The surface condition is adjusted to be active for the plated metal. Zn
In the vapor deposition zone 40, the Zn vapor deposition device 41 is provided with the plating base plate 1.
It is arranged so as to face both sides of 0. Zn vapor deposition device 41
Is provided with a guide hood 43 that causes Zn vapor evaporated from a Zn melting tank 42, which is a Zn source, to flow toward the original plating plate 10. The guide hood 43 has an opening 44 wider than the original plating plate 10, and slits 45 and 46 through which the original plating plate 10 passes are formed in the opening 44.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0022

[Correction method] Change

[Correction content]

On the other hand, in the Zn-Mg alloy plated steel sheet of Test No. 4 in which the Mg content was 7.4% by weight at the bottom of the plating layer and Test No. 5 in which the Mg content was 16.3% by weight, powdering It can be seen that the generated amount is large and the plating layer is peeled from the base steel during the forming process. Moreover, in the comparative example of test number 5 in which the Mg content in the lowermost part of the plating layer was high, the sacrificial anticorrosive action of Zn could not be expected, and red rust occurred on the surface of the test piece within a short time. From this contrast,
It has been confirmed that the Zn-Mg alloy-plated steel sheet according to the present invention is unlikely to have cracks in the plating layer even after being subjected to forming. Further, since the Mg content is low at the bottom of the plating layer, the sacrificial anticorrosion action of Zn is secured, and even if defects such as scratches occur in the plating layer, corrosion of the base steel does not proceed.

[Procedure 3]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

Claims (1)

[Claims] 1. The Mg content continuously decreases in the depth direction of the plating layer, and the Mg content in the lowermost portion is 6.0% by weight.
A plated steel sheet on which a Zn-Mg alloy plated layer described below is formed.