JP7460887B2 - Multi-layer plated steel plate - Google Patents

Description

The present invention relates to multi-layer plated steel sheets.

Zn-based plated steel sheets exhibit sacrificial corrosion protection at exposed steel base areas such as cut edges and bent sections, and have excellent corrosion resistance. On the other hand, Al-based plated steel sheets do not exhibit sacrificial corrosion protection like Zn-based plated steel sheets, but have better corrosion resistance of the plated surface than Zn-based plated steel sheets. Therefore, they are suitable for use in areas where high corrosion resistance is required and where Zn-based plating is difficult to apply, such as areas near the coast where there is a large amount of airborne salt. As a plated steel sheet that combines the properties of both Zn-based plated steel sheets and Al-based plated steel sheets, Patent Document 1 discloses a multi-layer plated steel sheet having a first plated layer (lower layer) consisting of Si: 0-12%, Zn: 0-1%, and the balance Al, and a second plated layer (upper layer) consisting of Al: 3-22%, Mg: 0.5-8%, and the balance Zn, on top of the lower layer.

Japanese Patent Application Publication No. 2010-144193

The multilayer plated steel sheet disclosed in Patent Document 1 has the above-mentioned structure, and thus has excellent corrosion resistance compared to conventional Zn-based plated steel sheets and Al-based plated steel sheets. However, when the multilayer plated steel sheet disclosed in Patent Document 1 is used under harsher environmental conditions, such as in coastal areas where sea spray is directly splashed onto the plated steel sheet, or in places where water flows, such as rain gutters, Corrosion may progress and red rust may occur. Therefore, multilayer plated steel sheets with higher corrosion resistance are required.

One aspect of the present invention aims to realize a multi-layer plated steel sheet with high corrosion resistance.

In order to solve the above problems, a multi-layer plated steel sheet according to one embodiment of the present invention has a base steel sheet, a hot-dip Al-based plating layer applied to the surface of the base steel sheet, and a hot-dip Zn-based plating layer applied on the hot-dip Al-based plating layer, the hot-dip Zn-based plating layer has a melting point that is 80°C or more lower than that of the hot-dip Al-based plating layer, the hot-dip Zn-based plating layer has a penetration part whose interface with the hot-dip Al-based plating layer is located closer to the base steel sheet than other regions, the penetration part has a first region at the interface that mainly contains Zn-Al compounds and has an Al concentration of 30% or more by mass, a second region is formed between the first region and the hot-dip Al-based plating layer in which the Al concentration decreases from the hot-dip Al-based plating layer toward the first region, the average thickness of the penetration part in the thickness direction of the multi-layer plated steel sheet is 1 μm or more, and the Al concentration in the second region is 50% or more.

According to one aspect of the present invention, a multilayer plated steel sheet with high corrosion resistance can be realized.

It is an electron micrograph which shows an example of the cross section of the plating layer of the multilayer plating steel plate in one embodiment of this invention. FIG. 2 is a schematic diagram of the multi-layer plated steel sheet shown in FIG. 1 . FIG. 3 is an enlarged view of a region near an intrusion portion shown in FIG. 2 . This figure shows an example of a cross section of a multilayer plated steel plate layer after a corrosion experiment was conducted on the multilayer plated steel plate shown in FIG. 1, and is an electron micrograph of the vicinity of the interface between the lower layer and the upper layer.

One embodiment of the present invention will be described in detail below with reference to the drawings. Note that the following description is provided to facilitate a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. In this application, "A-B" indicates A or more and B or less. In this specification, the concentration of a component is expressed in mass % unless otherwise specified.

〔Definition of terms〕
In the following description, immersing a base steel sheet in a hot-dip Al-based coating bath to form a hot-dip Al-based coating layer (lower layer) on the surface of the base steel sheet may be referred to as a first hot-dip coating treatment, and immersing the steel sheet after the first hot-dip coating in a hot-dip Zn-based coating bath to form a hot-dip Zn-based coating layer (upper layer) on the surface may be referred to as a second hot-dip coating treatment.

The multi-layer plated steel sheet after the second hot-dip plating has a base steel sheet, a hot-dip Al-based plating layer as a lower layer applied to the surface of the base steel sheet, and a hot-dip Zn-based plating layer as an upper layer applied on the hot-dip Al-based plating layer. The lower layer and upper layer are sometimes collectively referred to as the multi-layer plating layer.

<Multi-layer plated steel sheet>
Fig. 1 is an electron microscope photograph showing an example of a cross section of a plating layer of a multi-layer plated steel sheet 10 according to one embodiment of the present invention. The electron microscope photograph shown in Fig. 1 is an electron microscope photograph of a multi-layer plated steel sheet in which the composition of the lower layer, which will be described later, is Al-9%Si and the composition of the upper layer is Zn-6%Al-3%Mg. Fig. 2 is a schematic diagram of the multi-layer plated steel sheet 10 shown in Fig. 1. Fig. 3 is an enlarged view of a region near an intrusion portion 20 shown in Fig. 2.

As shown in Figures 1 to 3, the multi-layer plated steel sheet 10 in this embodiment has a base steel sheet 1, a lower layer 2 formed on the surface of the base steel sheet 1, an upper layer 3 formed on the surface of the lower layer 2, and a diffusion layer 30 (second region). The base steel sheet 1, the lower layer 2, the upper layer 3, and the diffusion layer 30 are described in detail below.

[Base material steel plate]
As the base steel plate 1 serving as a plating original plate, various steel plates that are generally used as base materials for Zn-based plated steel sheets and Al-based plated steel plates can be used.

〔Underlayer〕
In this specification, the term "underlayer" refers to a layer that is present in the multi-layer plating layer after the first hot-dip plating treatment (hot-dip Al-based plating treatment) and the second hot-dip plating treatment (hot-dip Zn-based plating treatment) are performed, and that originates from the hot-dip Al-based plating layer formed by the first hot-dip plating treatment.

The lower layer 2 exhibits the excellent corrosion resistance characteristic of a hot-dip Al-based plating layer and plays the role of long-term corrosion resistance of the steel sheet surface. The component composition of the lower layer 2 (molten Al-based plating bath composition during the first hot-dip plating process) includes Si: 0 to 20% and Zn: 0 to 1% in mass %. The remainder may be Al. Further, the remainder may include various additive elements (for example, elements selected from Zn, Mg, Ti, B, Fe, Cr, Sr, alkaline earth elements, Sc, Y, and lanthanide elements). The remainder may contain unavoidable impurities.

Si in the lower layer 2 has the effect of lowering the liquidus temperature of the Al-based plating bath. However, if the Si content of the plating bath exceeds 20% by mass, it tends to exceed the eutectic composition and enter a region where the liquidus temperature increases. Furthermore, when such a large amount of Si is contained, a large amount of Si crystallized phase is formed at the interface between the lower layer 2 and the upper layer 3 described below, and the adhesion between the lower layer 2 and the upper layer 3 tends to deteriorate. In this case, cracks may occur between the lower layer 2 and the upper layer 3 due to the bending process, which causes the sacrificial anticorrosion effect of the Zn in the upper layer 3 to not be sufficiently exerted. Therefore, Si is either not added (0%) or is contained within a range of 20% by mass or less.

If the content of Zn in the lower layer 2 exceeds 1% by mass, the excellent corrosion resistance characteristic of a hot-dip Al-based plating layer will not be exhibited, and this will cause a decrease in the corrosion resistance of the lower layer 2. Furthermore, if the Zn content exceeds 1% by mass, when the second hot-dip plating process is performed, the plating layer formed in the first hot-dip plating process and the plating metal contained in the second hot-dip plating bath may The reaction is promoted, and as a result, distinct lower layer 2 and upper layer 3 are not formed over the entire plating layer, and portions that become a single layer plating layer are likely to be formed. Such a single layer portion may lose the excellent corrosion resistance characteristic of the hot-dip Al-based plating layer.

[Upper layer]
In this specification, the "upper layer" refers to the Zn-based plating formed by the second hot-dip plating process, which is present in the multilayer plating layer after the first hot-dip plating process and the second hot-dip plating process. It means a layer derived from a layer. The upper layer 3 is a Zn-based plating layer that optionally contains Al, and mainly plays the role of sacrificial corrosion protection for the base steel plate (steel base). Moreover, it is preferable that the upper layer 3 contains Mg. The upper layer 3 plays the role of protecting the plated surface by forming a Zn-based corrosion product containing Al or Mg, and protecting the exposed portion of the steel substrate by the Zn-based corrosion product containing Mg.

In the component composition of the upper layer 3 (the composition of the hot-dip Zn-based plating bath used in the second hot-dip plating process), the Al concentration is less than 22% by mass. The remainder may be Zn. In addition, the remainder may contain various additive elements (for example, elements selected from Al, Si, Mg, Ti, B, Fe, Cr, Sr, alkaline earth elements, Sc, Y, and lanthanide elements). good. The remainder may contain unavoidable impurities.

Here, as shown from the phase diagram of the Al-Si system corresponding to the plating layer components of the lower layer 2, the lowest melting point in the lower layer 2 (that is, the smallest difference in melting point with the upper layer) is Si: 12.1%, and its melting point is 577°C. As will be described later, when the concentration of Al contained in the upper layer 3 exceeds 22%, the plating layer becomes a single layer, and the melting point at this time is about 495°C. Therefore, in the multilayer plated steel sheet 10 according to the present embodiment, the upper layer 3 has a melting point lower than that of the lower layer 2 by 80°C or more, so that the lower layer 2 is melted in a large amount when performing the second hot-dip plating treatment. This can be prevented.

The upper layer 3 preferably contains 0-8% Mg. If necessary, the upper layer 3 may further contain one or more of Ti: 0.1% or less, B: 0.05% or less, and Si: 2% or less.

The Al in the upper layer 3 has the effect of improving the corrosion resistance of the coating layer. In addition, the inclusion of Al in the coating bath also has the effect of suppressing the generation of Mg oxide-based dross. If the Al content exceeds 22%, the melting point of the coating bath becomes high, and when the second hot-dip coating process is performed, the reaction with the underlying coating layer formed in the first hot-dip coating process proceeds excessively, making it easy for parts to become a single-layer coating layer locally. In addition, the sacrificial corrosion protection effect on the base steel sheet 1 and the lower layer 2 decreases. It is more preferable that the Al content be 15% or less.

The Mg in the upper layer 3 stably maintains the corrosion products that form on the surface of the plating layer as protective corrosion products, significantly enhancing the corrosion resistance of the plating layer. In addition, on exposed areas of the steel base, such as cut ends, Mg-containing Zn-based corrosion products formed by sacrificial corrosion protection accumulate to form a protective film, which acts to protect the exposed areas of the steel base.

In addition, Mg present in the plating bath has the effect of activating the surface of the Al-based plating layer formed by the first hot-dip plating process, improving wettability with the second hot-dip plating bath, preventing the occurrence of spot-like plating defects in the upper layer 3, and contributing to improving adhesion with the lower layer 2. The above activation effect is thought to be manifested by the Mg in the second hot-dip plating bath reducing the surface oxide film of the underlying Al-based plating layer. If Mg exceeds 8%, Mg-based oxide dross is more likely to occur in the plating bath.

The upper layer 3 may further contain one or more of Ti, B, and Si as plating metal components. When one or both of Ti and B are contained in the plating bath, the formation and growth of the _Zn11Mg2 phase, which causes spotted appearance defects, is suppressed. When Si is contained, blackening of the plating layer is prevented and the surface gloss is maintained. When one or more of these components are contained, the ranges are Ti: _0.1 % or less, B: 0.05% or less, and Si: 2% or less.

Fe is permitted as an unavoidable impurity in the upper layer 3 at a concentration of 2% or less, and other impurity elements are preferably contained at a total concentration of 1% or less.

Although various crystallized phases are observed in the upper layer 3, the composition of elements constituting the upper layer 3 approximately reflects the plating bath composition in the second hot-dip plating process. By having such an upper layer plating composition, the sacrificial corrosion protection effect of the upper layer 3 effectively acts on the lower layer 2 and the base steel plate, and the exposed parts of the steel substrate such as cut edges are exposed to stable corrosion products containing Zn and Mg. covered with a film. This film suppresses oxygen reduction reactions on the surface of the steel base, thereby protecting the exposed steel base over a long period of time. Even if the corrosion (dissolution) of the upper layer 3 progresses and the lower layer 2 is exposed, and the sacrificial anticorrosion effect decreases, the protection of the exposed steel substrate is maintained by the corrosion products containing Zn and Mg. .

In addition, in the upper layer 3, it is preferable that the amount of Zn-based plating is 10 g/ ^m2 or more. If the upper layer 3 is too thin, plating defects in the upper layer 3 increase. Also, the sacrificial anticorrosion effect of the upper layer 3 and the protective effect of the corrosion products may not be fully exhibited. However, since an excessively thick layer is uneconomical, it is preferable that the amount of the Zn-based plating is, for example, 300 g/ ^m2 or less.

As shown in FIG. 3, the upper layer 3 has an intrusion portion 20 whose interface with the lower layer 2 is located deeper into the lower layer 2 than in other regions. In other words, the upper layer 3 has the intrusion portion 20 whose interface with the lower layer 2 is located closer to the base steel plate 1 than other regions. The intrusion part 20 has a first region 21 at the interface with the lower layer 2, which mainly contains a Zn--Al compound and has an Al concentration of 30% or more by mass. That is, the first region 21 is formed at the interface between the intrusion portion 20 and the lower layer 2, and is a region that mainly contains a Zn--Al compound and has an Al concentration of 30% or more by mass.

The average thickness of the intrusion portion 20 is 1 μm or more. In this specification, the "average thickness of the intrusion part" refers to the area of the intrusion part 20 over a width of 1000 μm measured by using a cross-sectional photograph of the multilayer plated steel plate 10 cut along a plane parallel to the thickness direction. It means the value calculated by dividing by 1000 μm. When the average thickness of the intrusion portion 20 is 1 μm or more, the first region 21 can be easily formed when performing the second hot-dip plating treatment. In addition, if the intrusion part 20 reaches the base steel plate 1, it will cause a decrease in corrosion resistance, so the average thickness of the intrusion part 20 should be (thickness of the lower layer 2) - (thickness of the intrusion part 20) ≧5 μm. It is preferable that there be. As a result, the multilayer plated steel sheet 10 can exhibit high corrosion resistance even in applications with severe corrosive environments.

[Diffusion layer]
The diffusion layer 30 is a region (layer) formed between the penetration portion 20 (more specifically, the first region 21 of the penetration portion 20) and the lower layer 2. Here, the reference direction is a direction perpendicular to a tangent drawn to a curve derived from the shape of the outer edge of the penetration portion 20 appearing in a cross section when the multi-layer plated steel sheet 10 is cut along a plane parallel to the thickness direction of the multi-layer plated steel sheet 10. The Al concentration of the diffusion layer 30 decreases from the lower layer 2 toward the first region 21 along the reference direction. The Al concentration in the diffusion layer 30 is 50% or more by mass%.

Table 1 shows the composition of the components at points A to G shown in Figure 1. Each concentration is shown in mass %.

As shown in Table 1, the multilayer plated steel sheet shown in FIG. This region includes points E to G, and includes a diffusion layer 30 in which the concentration of Al is 50% or more, and the concentration of Al decreases from the lower layer 2 toward the first region 21.

As described above, the multi-layer plated steel sheet 10 in this embodiment includes the first region 21 and the diffusion layer 30. Here, it is known that when the Al concentration is about 20% or more, the corrosion resistance tends to improve as the Al concentration increases. As described above, the first region 21 and the diffusion layer 30 have an Al concentration of 30% or more, and therefore the corrosion resistance can be improved.

The inventors also discovered that by having a diffusion layer 30 with an Al concentration of 50% or more, the corrosion inhibition effect is particularly enhanced, and as a result, it is possible to inhibit corrosion from reaching the lower layer 2.

Furthermore, it is known that Al plating is more prone to localized corrosion than Zn plating, and when corrosion reaches the base steel plate 1, galvanic corrosion occurs between Fe and Al, which accelerates corrosion. . In contrast, the multilayer plated steel sheet 10 of this embodiment includes a diffusion layer 30. In other words, Zn diffuses and invades the lower layer 2 side. As a result, corrosion occurs uniformly, and corrosion can be prevented from reaching the base steel plate 1, so that corrosion resistance can be further improved.

In addition, in the diffusion layer 30, the concentrations of Zn and Al change gradually, making it difficult for local batteries of Zn and Al to form.

It is also preferable that the thickness of the lower layer 2 is 5 μm or more. This prevents the penetration portion 20 from reaching the base steel sheet 1, and allows the multi-layer plated steel sheet 10 to exhibit high corrosion resistance even in applications with severe corrosive environments.

Furthermore, if the thickness of the upper layer 3 is made too thin, unplated surfaces are likely to occur. Since the diffusion layer 30 is not formed in the region where unplating occurs, a sufficient effect of improving corrosion resistance cannot be obtained. Therefore, the thickness of the upper layer 3 is preferably 3 μm or more.

In addition, in the multi-layer plated steel sheet 10 of this embodiment, the upper layer 3 has a melting point that is 80°C or more lower than that of the lower layer 2. As a result, when the second hot-dip plating process is performed, Al dissolves and the penetration portion 20 can be formed.

Note that the average thickness of the diffusion layer 30 in the direction along the reference direction is preferably 0.1 μm or more. Thereby, the progress of corrosion can be further suppressed.

Figure 4 shows an example of a cross-section of the multi-layer plated steel sheet layer after a corrosion experiment was conducted on the multi-layer plated steel sheet shown in Figure 1, and is an electron microscope photograph of the vicinity of the interface between the lower layer 2 and the upper layer 3. As shown in Figure 4, when the multi-layer plated steel sheet 10 of this embodiment is corroded, it can be seen that the corroded area 40 is formed on the upper layer 3 side relative to the diffusion layer 30. In other words, it can be seen that the progress of corrosion is suppressed by the diffusion layer 30.

(Production method)
The multi-layer plated steel sheet in one embodiment of the present invention includes a first plating step, a crack forming step, and a second plating step.

In the first plating step, the base steel plate is immersed in a hot-dip Al plating bath containing Si: 0-20% and Zn: 0-1% in mass%, and a lower layer 2 (molten Al This is the process of forming a plating layer).

The crack forming step is a step of forming cracks on the surface of the lower layer 2 formed in the first plating step.
The method for forming the cracks is not particularly limited, but for example, the cracks can be formed by performing a skin pass on the surface of the lower layer 2 formed in the first plating step (more specifically, the oxide film formed on the surface of the lower layer 2). Specifically, the cracks can be formed by performing a skin pass under the condition of a rolling reduction of 0.4%.

Furthermore, by heating and cooling the lower layer 2 formed in the first plating step, cracks may be formed using the difference in thermal expansion between the oxide film and the base material.

In the second plating step, the lower layer 2 in which cracks were formed in the crack forming step is immersed in a molten Zn-based plating bath containing 22% Al by mass %, and an upper layer 3 (molten Zn-based plating) is applied to the surface of the lower layer 2. This is the process of forming a layer).

In the second plating step, since a crack is formed on the surface of the lower layer 2 in the crack forming step, the intrusion portion 20 can be formed using the crack as a starting point. Furthermore, by forming a crack, the formation area of the intrusion portion 20 can be made larger than in the case where no crack is formed, so the contact area between the lower layer 2 and the molten Zn-based plating bath is increased. can do. As a result, Al existing in the lower layer 2 is easily dissolved into the molten Zn-based plating bath, and the diffusion layer 30 can be formed.

In addition, in order to form a multilayer plating layer with excellent adhesion between the lower layer 2 and the upper layer 3, the plating bath composition in the first hot-dip plating process and the second hot-dip plating process is important. Furthermore, the temperature of the steel plate of the intermediate product is important when the intermediate product that has been subjected to the first hot-dip plating process is subjected to the second hot-dip plating process. It is effective to immerse the intermediate plated steel plate in the second hot-dip plating bath with the inlet temperature adjusted to 360° C. or higher. If the steel plate temperature is too low, gaps (pores) are likely to occur at the interface between the lower layer 2 and the upper layer 3. Furthermore, if the steel sheet temperature is too high, diffusion at the interface between the lower layer 2 and the upper layer 3 will proceed, and a portion that will become a single plating layer will be likely to be formed. In some cases, the underlying molten Al-based plating layer may be remelted and the entire plating layer may become a single layer plating layer.

In the first hot-dip galvanizing treatment, it is desirable to set the Al-based coating weight to 10 g/ ^m2 or more. If it is thinner than this, unless the conditions of the second hot-dip galvanizing treatment are controlled quite strictly, Zn derived from the second hot-dip galvanizing treatment will diffuse to the vicinity of the base material of the lower layer 2, which will tend to reduce corrosion resistance. In addition, a region that becomes a single-layer coating layer is likely to occur. The upper limit of the Al-based coating weight is not particularly specified, but it can be set to, for example, a range of 300 g/m2 or ^less .

In the second hot-dip galvanizing treatment, the Zn-based coating weight is preferably 10 g/ ^m2 or more. If the coating is too thin, the coating defects in the upper layer 3 increase. Also, the sacrificial anticorrosion effect and the protective effect of the corrosion products may not be fully exerted. However, if the coating is too thick, it becomes uneconomical, so it is preferable to set the coating weight to, for example, 300 g/ ^m2 or less.

After the second plating step, surface treatments such as chemical conversion treatments can be performed as necessary.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in different embodiments.

An embodiment of the present invention will be described below. In this embodiment 1, multi-layer plated steel sheets Nos. 1 to 10, 25, and 26 were prepared as inventive examples, and multi-layer plated steel sheets Nos. 11 to 20 , 23, and 24 were prepared as comparative examples. The manufacturing conditions of each multi-layer plated steel sheet are shown in Table 2. The "inlet temperature" shown in Table 2 is the temperature of the steel sheet immersed in the second hot-dip plating bath in the second plating step. Also, "SKP" shown in Table 2 means skin pass. In the multi-layer plated steel sheets of the embodiment and comparative example, the upper layer 3 had a melting point 80°C or more lower than the lower layer 2.

For the multi-layer plated steel sheets Nos. 1 to 24 , skin pass was performed at a rolling reduction of 0.4%.

The produced No. Corrosion resistance tests were conducted on the multilayer plated steel plates Nos. 1 to 24 using the following method. (1) Spraying with salt water containing 5% NaCl for 2 hours, (2) Drying for 4 hours under the conditions of 60°C and 30% RH, and (3) Drying for 2 hours under the conditions of 50°C and 98% RH. The holding time was defined as one cycle, and the number of cycles until red rust occurred was determined.

As shown in Table 2, the multi-layer plated steel sheets No. 1 to No. 3 have the same composition as the multi-layer plated steel sheets No. 11 to No. 13, respectively. The multi-layer plated steel sheets No. 4 and No. 5 have the same composition as the multi-layer plated steel sheet No. 14. The multi-layer plated steel sheets No. 6 and No. 7 have the same composition as the multi-layer plated steel sheet No. 15. The multi-layer plated steel sheets No. 8 to No. 10 have the same composition as the multi-layer plated steel sheets No. 18 to No. 20, respectively. The multi-layer plated steel sheets No. 21 and No. 22 have the same composition as the multi-layer plated steel sheets No. 23 and No. 24, respectively.

As shown in Table 2, No. 1 to 10, No. 21 and no. No. 22 multi-layer plated steel sheets each have the same composition. Compared to multi-layer plated steel sheets Nos. 11 to 15, 18 to 20, 23 and 24 , the number of cycles until red rust occurred was greater. That is, the multilayer plated steel sheet of the example of the present invention had higher corrosion resistance than the multilayer plated steel sheet of the comparative example. This is No. 1 to 10, No. 21 and no. This is thought to be because the multilayer plated steel sheet No. 22 was skin-passed to generate cracks in the lower layer 2, and the average thickness of the intrusion portion 20 was 1 μm or more, forming a diffusion layer.

No. Since the multilayer plated steel plates Nos. 11 to 16 and 18 to 20 were not skin-passed during manufacture, the intervals between cracks formed in the oxide film of the lower layer 2 became too large. The intrusion portion 20 was not sufficiently formed. Therefore, No. 1 having the same composition. The corrosion resistance was inferior to the multilayer plated steel sheets Nos. 1 to 6 and 8 to 10.

No. In No. 17 multilayer plated steel sheet, the inlet temperature was too low at 300° C. in the second plating step, so the diffusion layer 30 was not formed. Therefore, No. 1 with the same composition. The corrosion resistance was inferior to No. 16 multilayer plated steel sheet.

Another embodiment of the present invention will be described below. In this embodiment 2, multi-layer plated steel sheets No. 25 to 28 were produced as inventive examples, and multi-layer plated steel sheets No. 29 and 30 were produced as comparative examples. The manufacturing conditions for each multi-layer plated steel sheet are shown in Table 2. In producing the multi-layer plated steel sheets No. 25 to 30, all skin pass was performed under the same conditions as above.

A corrosion resistance test was performed on the prepared multi-layer plated steel sheets No. 25 to 30 by the following method. A flowing water path with a width of 3 mm was formed on the multi-layer plated steel sheet. Next, simulated rainwater with 5 ppm chloride ions ^Cl- , ₁₀ ppm nitrate ions ^NO3- , ²⁰ ppm sulfate ions _SO42- , and a pH of 5 was dropped into the flowing water path at 6 ml/min, and the simulated water was constantly flowing through the flowing water path. Then, the number of days until the area of the flowing water path where red rust occurred reached 5% or more was determined and evaluated as follows. In this corrosion resistance test, if the number of days until the area of the flowing water path where red rust occurred reached 5% or more was 70 days or more (i.e., the evaluation value was 4 or 5), it was evaluated as good.
Evaluation value 5: no red rust occurred for 100 days or more Evaluation value 4: 70 to 99 days for red rust occurrence Evaluation value 3: 40 to 69 days for red rust occurrence Evaluation value 2: 20 to 39 days for red rust occurrence Evaluation value 1: less than 20 days for red rust occurrence As shown in Table 3, the multi-layer plated steel sheets No. 25 to 28 had a corrosion resistance test in which the number of days until the area where red rust occurred reached 5% or more was 70 days or more. That is, the multi-layer plated steel sheets No. 25 to 28 had high corrosion resistance. This is thought to be because the average thickness of the penetration portion 20 of the multi-layer plated steel sheets No. 25 to 28 was 1 μm or more, and a diffusion layer was formed.

On the other hand, No. The multilayer plated steel sheet No. 29 had poor corrosion resistance because the coating amount of the lower layer 2 was as thin as 3 μm.

In addition, in the case of the multi-layer plated steel sheet No. 30, the thickness of the upper layer 3 was as thin as 2 μm, so there were areas where the upper layer 3 was not plated (in other words, there were areas that were not plated). As a result, the penetration portion 20 and the diffusion layer 30 were not sufficiently formed, and the corrosion resistance was poor.

1 Base steel plate 2 Lower layer (hot-dip Al-based plating layer)
3 Upper layer (molten Zn-based plating layer)
10 Multilayer plated steel plate 20 Intrusion part 21 First region 30 Diffusion layer (second region)

Claims (3)

A multi-layer plated steel sheet,
base steel plate,
a hot-dip Al-based plating layer applied to the surface of the base steel plate;
a molten Zn-based plating layer applied on the molten Al-based plating layer,
The molten Al-based plating layer contains Si: 0 to 20%, Zn: 0 to 1%, and various additional elements in mass %, and the remainder consists of Al and inevitable impurities,
The hot-dip Zn-based plating layer contains more than 0% and less than 22% Al and various additive elements in mass %, and the remainder consists of Zn and inevitable impurities,
The amount of adhesion on one side of the molten Al-based plating layer is 10 g/m ² or more and 300 g/m ² or less,
The hot-dip Zn-based plating layer has an adhesion amount of 10 g/m ² or more and 300 g/m ² or less on one side, and a thickness of 3 μm or more on one side,
The molten Zn-based plating layer has a melting point that is 80° C. or more lower than the molten Al-based plating layer,
The hot-dip Zn-based plating layer has an intrusion portion in which an interface with the hot-dip Al-based plating layer is located closer to the base steel plate than other regions,
The intrusion portion has a first region at the interface that mainly contains a Zn-Al compound and has an Al concentration of 30% or more in mass %,
A second region is formed between the first region and the molten Al-based plating layer, and the concentration of Al decreases from the molten Al-based plating layer toward the first region,
The average thickness of the intrusion part in the thickness direction of the multilayer plated steel plate is 1 μm or more,
A multilayer plated steel sheet, wherein the concentration of Al in the second region is 50% or more. The multilayer plated steel sheet according to claim 1, wherein the average thickness of the second region is 0.1 μm or more. The multi-layer plated steel sheet according to claim 1 or 2, wherein the hot-dip Zn-based plating layer contains 8% or less Mg by mass.

Images