JP2020122199A - Multi-layer plated steel sheet and manufacturing method thereof - Google Patents

Abstract

To provide a multi-layer plated steel sheet capable of showing excellent corrosion resistance, even when subjected to bending; and to provide a manufacturing method thereof.SOLUTION: A multi-layer plated steel sheet has a base material steel sheet, a molten Al-based plating layer applied onto the surface of the base material steel sheet, and containing, in terms of mass%, Si:0-12% and Zn:0-1%, and a molten Zn-based plating layer applied onto the molten Al-based plating layer, and containing, in terms of mass%, Al:0-22% and Mg:0-1%.SELECTED DRAWING: Figure 1

Description

The present invention relates to a multi-layer plated steel sheet and a method for manufacturing the same.

Zinc (Zn)-based plated steel sheets are widely used mainly in the fields of building materials, automobiles, home appliances, etc. by utilizing the sacrificial anticorrosive action of Zn.

Further, the aluminum (Al)-based plated steel sheet is superior in corrosion resistance of the Al-based plated layer on the surface thereof to the Zn-based plated layer formed on the surface of the Zn-based plated steel sheet. On the other hand, in the Al-based plated steel sheet, red rust is likely to occur in exposed portions of the steel base material such as the cut end face and the bent portion in the initial stage when the exposure to the atmospheric environment is started. This is because the Al-based plating layer does not exert a sacrificial anticorrosive action on the steel substrate.

BACKGROUND ART Conventionally, a technique of multi-layer plating in which Al-based plating and Zn-based plating are stacked on a steel sheet is known. For example, in Patent Document 1, Al: 20 to 75%, Si: 0.1 to 5%, the balance is a molten Al-Zn-based plating layer composed of Zn and unavoidable impurities as a base, Disclosed is a hot-dip Zn-Al alloy-plated steel sheet formed by forming a hot-dip Zn-based plating layer having a composition of Al: 0.1 to 10%, the balance Zn and unavoidable impurities. In this hot-dip Zn-Al alloy plated steel sheet, the sacrificial anticorrosive action is enhanced by the upper layer having a higher Zn content than the lower layer, and the rust resistance and the like of the cut end surface is improved.

Further, for example, in Patent Document 2, Si: 1 to 12%, Zn: 0 to 1%, the balance is a molten Al-based plating layer having a composition consisting of Al and inevitable impurities, and Al: 3 to 22 is formed thereon. %, Mg: 0.5 to 8%, the balance Zn, and a multilayer Zn-plated steel sheet on which a hot-dip Zn-based plating layer having a composition of unavoidable impurities is formed.

JP, 2006-219716, A JP, 2010-144193, A

In the method described in Patent Document 1, when performing hot-dip plating for forming the upper layer on the lower layer, diffusion of Zn or the like is likely to occur between the lower layer and the upper layer, and therefore the region where the lower layer and the upper layer are integrated. Can occur over a wide area. This is because the solidus temperature of the lower layer of the molten Zn-Al-based plating layer is relatively low, and the Zn concentration portion (interdendrite portion) is present in the plating layer. In the above case, the overall corrosion resistance of the plating layer may be reduced.

The multi-layer plated steel sheet described in Patent Document 2 combines the excellent corrosion resistance of the lower molten Al-based plated layer and the sacrificial anticorrosive action of the upper molten Zn-based plated layer to expose the steel base material such as the cut end face. It is possible to improve the occurrence of red rust in parts. However, in such a multi-layer plated steel sheet, even if the plating layer is severely cracked due to bending (when a large amount of exposed parts of the steel base material is generated), the corrosion resistance is unlikely to deteriorate. desired.

In view of such a situation, an aspect of the present invention is to provide a multi-layer plated steel sheet that exhibits excellent corrosion resistance even when bent and a method for manufacturing the same.

In order to solve the above problems, a multilayer plated steel sheet according to one aspect of the present invention is a base steel sheet and Si:0 to 12% and Zn:0 in mass% applied to the surface of the base steel sheet. A hot-dip Al-based plating layer containing 1 to 1%, and a hot-dip Zn-based plating layer containing Al: 0 to 22% and Mg: 0 to 1% by mass%, which is applied on the hot-dip Al-based plating layer. It is characterized by having.

Moreover, the manufacturing method of the multilayer coating steel plate in one aspect of the present invention comprises immersing the base steel plate in a molten Al-based plating bath containing Si: 0 to 12% and Zn: 0 to 1% by mass, The first step of forming a molten Al-based plating layer on the surface of the base steel sheet, and the plated steel sheet formed by the first step, in a state where the steel sheet temperature is adjusted to 280 to 570°C, mass% And a second step of applying a molten Zn-based plating layer on the above-mentioned molten Al-based plating layer by immersing in a molten Zn-based plating bath containing Al: 0 to 22% and Mg: 0 to 1%. It is characterized by that.

According to one aspect of the present invention, it is possible to provide a multilayer plated steel sheet that exhibits excellent corrosion resistance even when subjected to bending, and a method for manufacturing the same.

It is an electron micrograph which shows an example of the plating layer cross section of the multilayer plating steel plate in one Embodiment of this invention. (A) is an example of the photograph which shows the cross section of the bending part of the plated steel plate which hot-dip Zn system plating of the composition containing 0.5 mass% Mg was given to the base steel plate, (b) is 3 mass% 3 is an example of a photograph showing a cross section of a bent portion of a hot-dip Zn-plated steel sheet in which a hot-dip Zn-plated layer having a composition containing Mg is applied to a base steel sheet. (A) is an example of an electron micrograph showing an enlarged cross-section of a bent portion of a plated steel sheet in which a hot-dip Zn-based plating having a composition containing 0.5% by mass of Mg is applied to a base steel sheet. ) Is an example of an electron micrograph showing an enlarged cross section of a bent portion of a hot-dip Zn-plated steel sheet in which a hot-dip Zn-plated layer having a composition containing 3% by mass of Mg is applied to a base steel sheet. It is a figure which shows typically an example of the state of the cross section of the bending part in a multilayer plating steel plate, (a) shows the example in which an upper layer is cracked, (b) shows the example in which both an upper layer and a lower layer are cracked, respectively. (A) is an electron micrograph showing an example of a cross section of a bent portion in a multi-layer plated steel sheet (Mg content of the upper layer is 1.0% by mass) of an example of the present invention, and (b) is a comparison. It is an electron micrograph which shows an example of the cross section of the bending part in the multilayered steel plate of an example (Mg content of an upper layer is 3.0 mass %).

A multi-layer plated steel sheet according to an embodiment of the present invention will be described below. Note that the following description is for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Further, in the present application, “A to B” indicates that it is A or more and B or less. Unless otherwise specified, the description of “%” regarding the chemical composition means “mass %”.

<Schematic explanation of findings of the invention>
In the multi-layer plated steel sheet having the molten Al-Zn system plated layer as the lower layer as described in Patent Document 1 above, when the steel layer is exposed due to cracks in the plated layer due to bending or the like, the lower layer is sacrificed and corrosion-protected. Produce an effect. Therefore, the problem that the corrosion resistance is deteriorated due to the crack caused by bending or the like is unlikely to occur.

On the other hand, as described in the above-mentioned Patent Document 2, in a multi-layer plated steel sheet having a molten Al-based plating layer containing Al as a main component in the lower layer (hereinafter, referred to as multi-layer plated steel sheet L2), bending work is performed. If the steel base is exposed due to the above reasons, the corrosion resistance may decrease. It is desired that this type of double-layer plated steel sheet is such that the corrosion resistance does not easily deteriorate even when subjected to bending or the like.

The present inventors intend to produce a multi-layer plated steel sheet having excellent corrosion resistance by having a molten Al-based plated layer as a lower layer, and a multi-layer plated steel sheet having high corrosion resistance even when bent. Aiming to achieve

In the multi-layer plated steel sheet L2 described in Patent Document 2 described above, the hot-dip Zn-based plating layer as the upper layer contains 0.5 to 8 mass% of Mg. In the technique described in Patent Document 2, when forming the upper layer, the oxide film on the surface of the lower molten Al-based plating layer can be reduced by the action of Mg, whereby the lower layer (molten Al-based plating layer) A hot-dip Zn-based plating layer can be suitably formed on it.

When such a multi-layer plated steel sheet L2 is subjected to bending, a severe crack may be caused such that the steel base is exposed. The present inventors conducted a detailed investigation on such cracks and obtained the following findings.

That is, a ternary eutectic phase containing MgZn ₂ is generated in the upper layer (hot-dip Zn-based plating layer) of the multi-layer plated steel sheet L2. This MgZn ₂ is a relatively hard substance and hardens the upper layer. In the bent portion of the multi-layer plated steel sheet L2, it was found that the upper layer of the crack occurs preferentially at the portion where ternary phase containing MgZn ₂ is formed.

(Verification experiment)
An experiment was carried out to verify the difference caused by the Mg content in the cracks that occur in the upper layer when subjected to bending. The results will be described below with reference to FIGS. 2 and 3. In this verification experiment, 4T bending (180° bending with four plates having the same thickness as the test piece sandwiched) was performed as bending. Further, in order to prevent the composition of the hot-dip Zn-based plating layer from fluctuating as much as possible, a single-layer hot-dip Zn-based plating was applied to the base steel sheet instead of the double-layer plating, and an experiment was conducted.

FIG. 2( a) is an example of a photograph showing a cross section of a bent portion of a plated steel sheet in which a hot-dip Zn-based plating having a composition containing 0.5% by mass of Mg is applied to a base steel sheet. As shown in (a) of FIG. 2, in the hot-dip Zn-based plated layer having a Mg content of 0.5% by mass, large cracks and relatively small cracks were scattered. Further, it was observed that cracks formed on the outer surface of the hot-dip Zn-based plating layer progressed toward the base steel sheet to form cracks.

FIG. 2B is an example of a photograph showing a cross section of a bent portion of a hot-dip Zn-plated steel sheet in which hot-dip Zn-based plating having a composition containing 3% by mass of Mg is applied to a base steel sheet. As shown in (b) of FIG. 2, the hot-dip Zn-based plating layer of this example was severely cracked, and the shape of the crack caused the hot-dip Zn-based plating layer to break, which caused a crack. The situation was observed.

Next, the cross section of the bent portion was enlarged to confirm the material structure. FIG. 3A is an example of an electron micrograph showing an enlarged cross section of a bent portion of a plated steel sheet in which a hot-dip Zn-based plating having a composition containing 0.5 mass% of Mg is applied to a base steel sheet. .. FIG. 3( b) is an example of an electron micrograph showing an enlarged cross section of a bent portion of a hot-dip Zn-plated steel sheet in which hot-dip Zn-based plating having a composition containing 3% by mass of Mg is applied to a base steel sheet. Is.

As shown in FIG. 3A, in the hot-dip Zn-based plating layer 10 having a Mg content of 0.5% by mass, a Zn phase 11 and a primary crystal Al phase 12 are present as a matrix, and a ZnZn 3 containing MgZn ₂ is included. The existence ratio of the original eutectic phase 13 is low. The Zn phase 11 is a light gray area, and the primary crystal Al phase 12 is a relatively dark gray area. The ternary eutectic phase 13 is a portion having a fine striped structure structure. Although some cracks were observed on the outer surface of the hot-dip Zn-based plating layer 10 of this example, no large cracks that would expose the surface of the base steel sheet were generated.

On the other hand, as shown in FIG. 3B, the hot-dip Zn-based plating layer 20 of this example is mainly composed of a primary crystal Al phase 21 and a ternary eutectic phase 22 containing MgZn _2. The proportion of the ternary eutectic phase 22 present is high. It was also observed that the fracture of the ternary eutectic phase 22 caused a large crack exposing the surface of the base steel sheet. Then, it was observed that the surface of the base steel sheet became rough under the influence of the rupture of the ternary eutectic phase 22 at the location where the crack occurred.

(Regarding the mode of cracking in multi-layer plated steel sheet)
Next, an aspect of cracks that occur in the bent portion of the multi-layer plated steel sheet having the lower layer and the upper layer formed on the steel sheet will be described with reference to FIG. FIG. 4 is a diagram schematically showing an example of a cross section of a bent portion in a double-layer plated steel sheet, where (a) is an example in which the upper layer is broken, and (b) is an example in which both the upper layer and the lower layer are broken. Showing. Here, for the sake of clarity, a multi-layer plated steel sheet in which the base steel sheet 31, the lower layer 32, and the upper layer 33 are formed with clear boundaries in this order will be described as an example.

In the case of a multi-layer plated steel sheet, there may be a case where only the upper layer 33 is broken ((a) in FIG. 4) and a case where both the upper layer 33 and the lower layer 32 are broken ((b) in FIG. 4) when bending is performed. When the lower layer 32 is a hot-dip Al-based plating layer, when the state shown in FIG. 1B is reached, the steel base (the surface of the base steel plate 31) is exposed and the hot-dip Al-based plating layer is applied to the steel base. On the other hand, since the sacrificial anticorrosive action is not exerted, the corrosion resistance may decrease.

From the results of the above verification experiments, for example, in the above-mentioned multilayer plated steel sheet L2, a large amount of ternary eutectic phase containing MgZn ₂ was generated in the upper layer, and when subjected to bending, the ternary eutectic phase was formed. It is considered that a phenomenon such as breakage occurs in the phase portion and that cracks may occur in both the upper layer and the lower layer due to the phenomenon. This will be described in more detail below.

In general, the ternary eutectic phase containing MgZn ₂ is not formed as a thermodynamically stable phase when a molten Zn-based plating bath containing Al and Mg is solidified to form an upper layer, but rather is formed as MgZn by supercooling. It is considered that the nuclei of ₂ are generated as a metastable phase due to the high nucleation rate. When the Mg concentration contained in the upper layer is high, the existing region of the ternary eutectic phase in the upper layer becomes wide. For example, in the multi-layer plated steel sheet L2, when the Mg concentration contained in the upper layer is 3% by mass, a microstructure mainly composed of a ternary eutectic phase and a primary crystal Al phase is formed ((b of FIG. 3 described above). See)).

In the above-mentioned texture structure, the ternary eutectic phase exists continuously from the interface between the upper layer and the lower layer to the outer surface of the upper layer, and the ternary eutectic phase and the lower layer may be in contact with each other. In this case, some bonding force is generated between the ternary eutectic phase and the lower layer. Therefore, the lower layer is easily affected by the breakage occurring in the upper layer. In such a state, when the hard ternary eutectic phase in the upper layer is instantaneously broken by bending the double-layer plated steel sheet L2 in such a state, the lower layer is cracked together with the upper layer and the steel base is We have found that the phenomenon of exposure is likely to occur.

Conventionally, in a hot-dip Zn-based plating layer containing Al and Mg, a ternary eutectic phase is finely formed by the addition of Mg, and the corrosion resistance is improved by the action of Mg relatively uniformly dispersed in the plating layer. .. In such technical common sense, the inventors of the present invention, based on the above findings, allow the action of Mg to be suppressed and reduce the Mg content of the upper layer to 1% by mass or less, thereby forming the lower layer. It has been found that cracks can be made less likely to occur.

That is, in the multi-layer plated steel sheet according to one aspect of the present invention, the Mg content of the upper hot-dip Zn-based plated layer is 1 mass% or less, and the proportion of the hard ternary eutectic phase in the upper layer is small. In this case, the upper layer contains a primary crystal Al phase and a primary crystal Zn phase that are relatively softer than the ternary eutectic phase, and cracks are unlikely to occur in the upper layer due to bending. When such an upper layer is bent, for example, elongation and rupture occur near the surface of the upper layer (a portion where relatively high stress is generated), which causes a phenomenon that a local crack occurs. Can happen. As the crack progresses toward the lower layer, cracks in the upper layer occur. The progress of such cracks may stop at the interface between the upper and lower layers. Therefore, even if cracks occur in the upper layer, it is difficult for the phenomenon that the upper layer and the lower layer both crack due to the cracks in the upper layer. Therefore, it is possible to prevent the steel base material from being exposed.

In the multi-layer plated steel sheet according to one aspect of the present invention, the molten Al-based plated layer having a high Al concentration is the lower layer. Even if the upper layer is cracked, the lower layer is unlikely to be cracked, so that the corrosion of the steel base can be effectively prevented by the lower layer. Therefore, the multi-layer plated steel sheet according to one aspect of the present invention exhibits excellent corrosion resistance even when subjected to bending.

〔Definition of terms〕
In the following description, immersing the base steel sheet in a hot dip Al plating bath to form a hot Al plating layer on the surface of the base steel sheet may be referred to as first hot dip plating. Then, immersing the steel sheet after the first hot dip plating in a hot dip Zn plating bath to form a hot dip Zn plating layer on the surface may be referred to as second hot dipping.

The multi-layer plated steel sheet after the second hot dip coating has a base steel sheet, a molten Al-based plating layer which is a lower layer applied to the surface of the base steel sheet, and a hot-dip Al-based plating layer. And a hot-dip Zn-based plating layer which is an applied upper layer. The lower layer and the upper layer may be collectively referred to as a multilayer plating layer. The multi-layer plated steel sheet may have an intermediate layer between the lower layer and the upper layer.

<Multi-layer plated steel sheet>
The multi-layer plated steel sheet according to the embodiment of the present invention will be described below. FIG. 1 is an electron micrograph showing an example of a cross-section of a plated layer of a multi-layer plated steel sheet 10 according to an embodiment of the present invention.

As shown in FIG. 1, the multi-layer plated steel sheet 10 includes a base steel sheet 1, a lower layer 2 formed on the surface of the base steel sheet 1, an alloy layer 3 formed at the interface between the base steel sheet 1 and the lower layer 2, and , The upper layer 4 formed on the surface of the lower layer 2. Here, the lower layer 2 is a hot-dip Al-based plating layer containing 9% by mass of Si, and the upper layer 4 is a hot-dip Zn-based plating layer containing 1.0% by mass of Al and 1.0% by mass of Mg.

The multi-layer plated steel sheet 10 has a clear multi-layer plating structure in which the interface can be observed between the lower layer 2 and the upper layer 4. The base steel sheet and various layers will be described in detail below.

[Base steel sheet]
As the base steel plate that serves as a plating base plate, various steel plates generally used as a base material for Zn-based plated steel plates and Al-based plated steel plates are applicable.

〔Underlayer〕
In the present specification, the “lower layer” refers to a hot-dip Al-based plating layer formed by the first hot-dip plating, which is present in the multilayer hot-dip plating layer after the first hot-dip plating and the second hot-dip plating. The derived layer (a part excluding the intermediate layer described below).

This lower layer exerts excellent corrosion resistance peculiar to the molten Al-based plating layer and bears long-term corrosion resistance of the steel sheet surface. The composition of the lower layer (the composition of the molten Al-based plating bath in the first hot dip plating) contains Si: 0 to 12% and Zn: 0 to 1% by mass. The balance may be Al. Further, the balance may contain various additive elements. The balance may contain unavoidable impurities.

Si in the lower layer has a function of reducing the liquidus temperature of the Al-based plating bath. However, when the Si content of the plating bath exceeds 12% by mass, the eutectic composition is exceeded and conversely, the liquidus temperature tends to increase. In addition, 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 and the upper layer described later, and the adhesion between the lower layer and the upper layer is likely to decrease. In this case, the bending may cause a crack between the lower layer and the upper layer, which causes the sacrificial anticorrosive action of Zn in the upper layer to not be sufficiently exhibited. Therefore, Si is not added (0%) or is contained in the range of 12 mass% or less.

When the content of Zn in the lower layer exceeds 1% by mass, the excellent corrosion resistance peculiar to the molten Al-based plating layer is not exhibited, which causes the lower corrosion resistance of the lower layer. In addition, when the Zn content is high as described above, the reaction between the lower layer and the plating metal of the second hot dip is promoted when the second hot dip plating is performed, and as a result, the clear lower layer is formed over the entire plated layer. Moreover, the upper layer is not formed, and the portion to be the single-layer plated layer is easily formed. Such a single layer portion may lose the excellent corrosion resistance peculiar to the molten Al-based plating layer.

It is preferable that Fe is mixed as an unavoidable impurity in the lower layer in a range of 2% or less, and the total amount of other impurity elements is 1% or less.

The lower layer has an Al content of 75 mass% or more. The lower layer may have an Al content of 80% by mass or more, 85% by mass or more, and 90% by mass or more. As the content of Al in the lower layer is larger, the corrosion resistance specific to the molten Al-based plating layer is improved.

Usually, an Al-Fe alloy layer is formed between the lower layer and the base steel sheet. The Al-Fe based alloy layer is a layer mainly containing an Al-Fe based metal compound. The Al-Fe based alloy layer may be an Al-Fe-Si based alloy layer.

Further, an intermediate layer may be interposed between the lower layer and the upper layer, or the lower layer and the upper layer may be in direct contact with each other. The intermediate layer has a structural structure composed of a fine Zn phase and a fine Al phase, and may include a crystallized phase of Si. When there is a portion where the lower layer and the upper layer are in direct contact with each other, in the lower layer in the vicinity of the interface, a gradient composition region generated by diffusion of the Zn component derived from the second hot dip plating is provided. Is preferred. Such a gradient composition is advantageous for improving the mutual adhesion between the lower layer and the upper layer.

The base steel sheet must be in contact with the lower layer on the entire surface to which the plating is attached. That is, it is important that the single layer portion as described above does not exist. Further, it is desirable that the inside of the lower layer in the vicinity of the base material/lower layer interface has a Zn concentration of 0 to 1%. Even in the case where the above-mentioned graded composition region is provided, the corrosion resistance is likely to decrease unless the Zn concentration in the region on the base material side is maintained low.

[Upper layer]
In the present specification, the "upper layer" is derived from the Zn-based plating layer formed by the second hot dip plating, which is present in the multilayer plating layer after the first hot dip plating and the second hot dip plating. Refers to the layer (portion excluding the intermediate layer). This upper layer is a Zn-based plating layer that optionally contains Al and Mg. The upper layer mainly has a sacrificial anticorrosive action, a protective action on the plated surface due to the formation of Zn-based corrosion products containing Al and Mg, and a protective action due to the Zn-based corrosion products containing Mg.

The composition of the upper layer (the composition of the hot-dip Zn-based plating bath used in the second hot dip plating) contains Al: 0 to 22% and Mg: 0 to 1% by mass. The balance may be Zn. Further, the balance may contain various additive elements. The balance may contain inevitable impurities.

The upper layer may further contain one or more of Ti: 0.1% or less, B: 0.05% or less, and Si: 2% or less, if necessary.

Al in the upper layer has a function of improving the corrosion resistance of the upper layer. On the other hand, when the Al content exceeds 22%, the melting point of the hot-dip Zn-based plating bath becomes high, and the reaction with the underlying plating layer formed by the first hot-dip plating is excessive when the second hot-dip plating is performed. It is easy for a portion to proceed to the point of becoming a single-layer plating layer locally. Further, the sacrificial anticorrosive action on the base steel sheet and the lower layer is reduced. The Al content is more preferably 15% or less.

Mg in the upper layer has a function of stably maintaining the corrosion product generated on the surface of the plating layer as a protective corrosion product, and remarkably enhancing the corrosion resistance of the plating layer. In addition, the Mg-containing Zn-based corrosion product generated by the sacrificial anticorrosion action is deposited on the exposed steel base material portion such as the cut end surface to form a protective film, which exhibits an effect of protecting the exposed steel base material portion.

Further, since Mg existing in the plating bath has an action of activating the surface of the Al-based plating layer formed by the first hot dip plating, it improves wettability with the second hot dip plating bath, It contributes to preventing the occurrence of dot-like plating defects in the upper layer and improving the adhesion with the lower layer. It is considered that the above-described activation action is exhibited by the reduction of the surface oxide film of the Al-based plating layer which is the base by Mg in the second plating bath.

On the other hand, as described above, when the Mg content exceeds 1%, the ratio of MgZn ₂ in the plating phase structure (the ratio of the ternary eutectic phase containing MgZn ₂ ) increases, and the upper layer is hardened and processed. There are more cracks due to. In addition, cracks in the lower layer are more likely to occur along with cracks in the upper layer. Due to the cracks in the lower layer, the corrosion resistance of the multi-layer plated steel sheet may decrease. Therefore, Mg in the plating bath in the second hot dip plating is regulated to 0 to 1%. Mg may not be added to the plating bath in the second hot dip plating.

As the plating metal component in the upper layer, one or more of Ti, B and Si can be further contained. When one or two kinds of Ti and B are contained in the plating bath, generation/growth of Zn ₁₁ Mg ₂ phase, which causes a spot-like appearance defect, is suppressed. When Si is contained, the blackening of the plating layer is prevented and the glossiness of the surface is maintained. When one or more of these components are contained, Ti: 0.1% or less, B: 0.05% or less, Si: 2% or less.

It is preferable that Fe is allowed to be mixed as an unavoidable impurity in the upper layer in a range of 2% or less, and the total content of other impurity elements is 1% or less.

Although various crystallized phases are observed in the upper layer, the component composition of the elements constituting the upper layer almost reflects the plating bath composition in the second hot dip plating.

Further, the multi-layer plated steel sheet according to one aspect of the present invention, by having the composition of the upper layer as described above, the sacrificial anticorrosive action of the upper layer effectively acts on the lower layer and the base steel sheet, and exposed steel base such as a cut end surface. The part is covered with a film of a stable corrosion product containing Zn and Mg. This film suppresses the oxygen reduction reaction on the surface of the steel base material at the cut end surface, so that the exposed portion of the steel base material is protected for a long period of time. Even when the upper layer is corroded (dissolved) to expose the lower layer and the sacrificial anticorrosive action is lowered, the corrosion product containing Zn and Mg maintains the protective property of the exposed portion of the steel base material.

In addition, in the upper layer, it is desirable that the amount of Zn-based plating adhered is 10 g/m ² or more. If it is too thin, plating defects in the upper layer will increase. In addition, the sacrificial anticorrosive action and the protective action due to corrosion products may not be sufficiently exerted. However, if it is excessively thick, it becomes uneconomical. Therefore, for example, it is preferable to set it in a range of 300 g/m ² or less.

(Production method)
In the multi-layer plated steel sheet according to one aspect of the present invention, the surface of the base steel sheet is subjected to the first hot dip coating (hot dip Al-based plating) (first step), and then the second hot dip coating (hot dip Zn is applied). It can be manufactured by performing (system plating) (second step). Specifically, the first hot dip coating may be applied on the continuous hot dip coating line to obtain an intermediate product, and the second hot dip plating may be applied to the intermediate hot product on the continuous hot dip plating line. Alternatively, the first hot dip plating bath and the second hot dip plating bath may be installed in series in one continuous line to finish the multi-layer plated steel sheet in one pass.

In order to form a multi-layer plating layer having excellent adhesion between the lower layer and the upper layer, the plating bath composition (described above) in the first hot dip plating and the second hot dip plating is important. Further, the temperature of the steel sheet of the intermediate product when the intermediate product that has undergone the first hot dip coating is subjected to the second hot dip plating becomes important. As a result of various studies, it is effective to immerse the plated steel sheet of the intermediate product in the second hot dip plating bath in a state where the steel sheet temperature is adjusted to 280 to 570° C. (preheated). The steel plate temperature is more preferably adjusted to 400 to 570°C. In the present manufacturing method, since the lower layer is the hot-dip Al-based plating layer, it can be used for the second hot-dip plating with the steel sheet temperature increased.

If the steel sheet temperature is too low, gaps (holes) are likely to occur at the lower layer/upper layer interface. In addition, dot-like plating defects are likely to be formed in the upper layer. If the steel sheet temperature is too high, diffusion at the lower/upper layer interface proceeds, and a portion that will become a single-layer plated layer is 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.

Further, in order to enhance the adhesion between the lower layer and the upper layer, it is preferable to perform the second hot dip plating under the following conditions. For example, (i) the reaction in the second hot dip plating is promoted by increasing the temperature of at least one of the steel plate temperature before immersion in the second plating bath and the bath temperature of the second plating bath. (Ii) Heating the steel sheet before being immersed in the second plating bath is performed in the atmosphere. (Iii) Prolong the immersion time of the steel sheet in the second plating bath. (Iv) Ultrasonic waves are applied to the second plating bath or the steel sheet before being immersed in the second plating bath to enhance the reactivity in the second hot dip plating.

In the first hot dip plating, it is desirable that the amount of Al-based plating adhered is 10 g/m ² or more. If it is thinner than this, unless the condition of the second hot dip plating is controlled very strictly, Zn derived from the second hot dip plating is likely to diffuse up to the vicinity of the base material of the lower layer, resulting in deterioration of corrosion resistance. In addition, a region serving as a single-layer plating layer is likely to occur. The upper limit of the amount of Al-based plating deposited is not particularly limited, but can be set to, for example, 300 g/m ² or less.

In addition, in the second hot dip plating, it is desirable that the Zn-based plating adhesion amount be 10 g/m ² or more. If it is too thin, plating defects in the upper layer will increase. In addition, the sacrificial anticorrosive action and the protective action due to corrosion products may not be sufficiently exerted. However, if it is excessively thick, it becomes uneconomical. Therefore, for example, it is preferable to set it in a range of 300 g/m ² or less.

After plating, a surface treatment such as chemical conversion treatment can be applied if necessary.

A cold-rolled ordinary steel sheet (C content: about 0.04% by mass) with a plate thickness of 0.8 mm is used as a plating base sheet (base steel sheet) in various plating bath compositions using a continuous hot dip pilot line. The hot-dip coating was applied and cooled to room temperature to obtain an intermediate product.

Next, using the pilot line again, the second hot-dip plating (Zn-based plating) was performed with various plating bath compositions in a state where the steel sheet of each intermediate product was preheated to 250°C to 550°C in the atmosphere. The plating composition, the preheating temperature of the steel sheet of the intermediate product, and the coating weight are shown in Table 1.

The bath temperature of the first hot dip plating bath was No. 1 to 13: 600° C., No. 14-19: 650°C, No. 20 to 32: 600°C. The bath temperature of the second hot dip plating bath was No. 1 to 7: 450° C., No. 8 to 13: 500° C., No. 14-18: 500° C., No. 19:550° C., No. 20-25: 400° C., No. 26-29: 500° C., No. 30 to 32: 400°C. The immersion time in the bath in the second hot dip plating is 1 to 3 seconds. The amount of plating adhered was controlled by gas wiping.

The following investigations were conducted on the obtained plated steel sheet.

(1) Number of cracks in machined part A strip-shaped test piece was cut out from a plated steel plate, and the cross section of the one subjected to the above-mentioned 4t bending was observed, and the number of cracks in the plated layer within a range of 1 mm including the bending apex was counted. It was

(2) Red rust resistance during exposure of processed part A strip-shaped test piece was cut out from a plated steel sheet, subjected to the above-mentioned 4t bending work, and then exposed outdoors in a seaside industrial area in Sakai City, Osaka Prefecture. Regarding the test piece after 9 months of exposure, the occurrence of red rust in the 4t bent portion was visually observed and evaluated according to the following criteria.
○: Almost no red rust was generated (good)
Δ: Light discoloration due to red rust is seen, which may cause a problem depending on the application (somewhat poor)
X: A large amount of red rust is generated, and it is noticeable (defective).

(3) Corrosion resistance of processed part A strip-shaped test piece was cut out from a plated steel sheet, subjected to the above-mentioned 4t bending work, and then subjected to a cycle corrosion test (CCT). The CCT conditions are based on JIS H8502 neutral salt spray cycle test, and "salt spray (5% salt water, 35°C) 2h → dry (60°C, 25%RH) 4h → wet (50°C, 98%RH) ) 2h" is one cycle. The corrosion resistance of the processed portion was evaluated by the number of cycles at the time when the occurrence of red rust was observed in the 4t bent portion. A product having a corrosion resistance of 4 or more was judged to be acceptable.

Rating 0: Red rust occurs by 250 cycles Rating 0.5: Red rust occurs at 251 to 300 cycles Rating 1: Red rust occurs at 301 to 350 cycles Rating 1.5: Red rust occurs at 351 to 400 cycles Rating 2: 401 to 450 cycles Red rust occurred at 2.5: 451-500 cycles red rust occurred at 3: 501-550 cycles red rust occurred 3.5: 551-600 cycles occurred at red rust score 4: 601-650 cycles red rust occurred 4. 5: Red rust occurred in 651 to 700 cycles.

The results are shown in Table 1.

Each of the examples of the present invention has a plating layer having an appropriate cross-sectional structure composed of a multi-layer plating layer, and has excellent adhesion between the lower layer and the upper layer. Further, the occurrence of cracks in the processed portion is suppressed and the processed portion has excellent corrosion resistance. The cracks counted in the number of cracks in the machined part in the examples of the present invention occurred only in the upper layer and did not occur in the lower layer (the steel base was not exposed). On the other hand, in the cracks counted in the number of cracks in the processed part in the comparative example, both the upper layer and the lower layer were cracked, and the steel base was exposed.

FIG. 5A shows No. 1 of the present invention. 4 is an electron micrograph showing an example of a cross section of a bent portion of the multilayer plated steel sheet of No. 4. As shown in (a) of FIG. 5, in this double-layer plated steel sheet, a lower layer 42 and an upper layer 43 are formed in this order on the surface of a base steel sheet 41, and cracks generated in the upper layer 43 in the bending portion are It can be seen that the relationship with the lower layer 42 is small. In other words, in this double-layer plated steel sheet, even if the upper layer 43 is cracked, the lower layer 42 is less likely to crack and the steel base is less exposed.

In Comparative Examples Nos. 5 to 7, many cracks were generated in the processed part due to the large content of Mg in the upper layer, and the corrosion resistance of the processed part was poor. In addition, red rust occurred in a relatively shorter time than the invention examples. The above also applies to Comparative Examples Nos. 12, 13, 17, 18, 23 to 25, 28, and 29.

5(b) is a comparative example No. 12 is an electron micrograph showing an example of a cross section of a bent portion of the multilayer plated steel sheet of No. 12. As shown in FIG. 5( b ), in this multi-layer plated steel sheet, a lower layer 52 and an upper layer 53 are formed in this order on the surface of a base steel sheet 51. However, in the bent portion, the upper layer 43 and the lower layer 42 were both cracked, and many places where the surface of the base steel plate 51 was exposed occurred. In other words, in this multi-layer plated steel sheet, cracks are likely to occur in the lower layer 42 at the locations where the upper layer 43 is cracked, and therefore the steel base is easily exposed.

In Comparative Example No. 19, the interface between the lower layer and the upper layer was not clear, and a uniform two-layer structure could not be obtained. The reason for this is as follows. Since the Al content in the composition of the second hot dip plating bath is high, the bath temperature of the second hot dip plating bath is relatively high (550° C.), and the reaction between the lower layer and the hot metal of the second hot dip plating bath. The sex is relatively high. As a result, it is considered that intense atomic diffusion occurred between the lower layer and the upper layer.

In Comparative Examples Nos. 24 and 25, the number of cracks due to bending was larger than that in Comparative Example No. 23, because the amount of plating deposited on the upper layer was relatively large. On the other hand, since the sacrificial anticorrosive action is increased, the red rust resistance and the corrosion resistance are slightly higher.

In Comparative Examples Nos. 30 to 32, the steel sheet preheating temperature was as low as 250° C., and therefore the upper layer was not uniformly formed.

The present invention is not limited to the above-described embodiments, various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the above description are also included in the present invention. It is included in the technical scope of the invention.

Claims (3)

Base steel sheet,
A hot-dip Al-based plating layer that is applied to the surface of the base steel sheet and contains Si: 0 to 12% and Zn: 0 to 1% by mass%;
A hot-dip Zn-based plating layer containing Al: 0 to 22% and Mg: 0 to 1% in mass%, which is applied on the hot-dip Al-based plating layer;
A multi-layer plated steel sheet having: The multi-layer plated steel sheet according to claim 1, wherein the molten Al-based plated layer contains 75% by mass or more of Al. First, a base steel sheet is immersed in a molten Al-based plating bath containing Si: 0 to 12% and Zn: 0 to 1% in mass% to form a molten Al-based plating layer on the surface of the base steel sheet. Steps,
The galvanized steel sheet formed by the first step is a hot-dip Zn-based plating containing Al:0 to 22% and Mg:0 to 1% in mass% with the steel plate temperature adjusted to 280 to 570°C. A second step of immersing in a bath to apply a hot-dip Zn-based plating layer on the hot-dip Al-based plating layer;