JP2010229483A - Zn-Al BASED PLATED STEEL SHEET HAVING EXCELLENT CORROSION RESISTANCE, AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Zn-Al based plated steel sheet containing Mg-Si and having notably improved corrosion resistance. <P>SOLUTION: In the Zn-Al based plated steel sheet having excellent corrosion resistance and having a Zn-Al based plating layer having an average composition, by mass%, of Al:25 to 75%, Si:1 to 10% and Mg:0.5 to 6%, further containing one or more of Ti and B in the range of ≤0.1% in total if needed and having the balance consisting of Zn and inevitable impurities, the plating layer includes a Zn rich part, an Al rich part, and an Si rich part, the Zn rich part has a region having ≥4.5 mass% Mg concentration, and the Al rich part and the Si rich part have organization states where a region having ≥4.5 mass% Mg concentration is not observed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention is a Mg-Si-containing Zn-Al-based plated steel sheet, wherein Mg in the plating layer has a unique plating layer structure in which the Mg is mainly distributed in the Zn-rich part instead of the Si-rich part, And a manufacturing method thereof.

  As a plated steel sheet having a good balance between the sacrificial anticorrosive action of the Zn-plated steel sheet and the high corrosion resistance of the plated surface of the Al-plated steel sheet, a Zn-Al-based plated steel sheet containing 25 to 75% Al is known. For example, a molten Zn-55% Al-1.5% Si alloy-plated steel sheet has been put into practical use. In this specification, such a Zn-Al-based plating containing Si is referred to as “Si-containing Zn—Al-based plating”. In recent years, the demand for higher corrosion resistance has increased, and "Mg.Si-containing Zn-Al plating" containing several percent of Mg based on Si-containing Zn-Al plating has been studied (Patent Documents 1 to 4, etc.) For example, a molten Zn-55% Al-1.5% Si-2% Mg alloy-plated steel sheet is disclosed.

JP 2000-104153 A JP 2000-328214 A JP 2002-322527 A JP 2001-115247 A

The Mg · Si-containing Zn—Al-plated steel sheet is improved in the corrosion resistance of the flat portion (plated surface) and the cut end face as compared with the steel containing no Mg. In the plated layer of this type of hot-dip steel sheet, most of Mg is present in the “Si-rich portion” as an Mg—Si intermetallic compound (Mg ₂ Si, etc.). Compounds (such as Zn ₂ Mg) are also found in the “Zn-rich part”. It is considered that Mg in the plating layer forms a highly protective Mg-containing Zn-based corrosion product when the plating layer corrodes, and this mainly improves the corrosion resistance.

  However, even in such Mg · Si-containing Zn—Al-based plated steel sheets, corrosion that causes problems in Si-containing Zn—Al-based plated steel sheets that do not contain Mg may be seen, and the effect of improving corrosion resistance is not always sufficient. It can not be said. Further, when Mg is contained in the hot dipping bath, there is a problem that the surface properties of the plated surface are likely to deteriorate. For these reasons, Mg / Si-containing Zn—Al-based plated steel sheets are rarely used.

According to the study by the inventors, it can be considered that most of Mg is present in the Si-rich portion as a cause of insufficient effect of improving the corrosion resistance of the Mg · Si-containing Zn—Al-based plated steel sheet. In the Zn-Al-based plated steel sheet, the Zn-rich portion in the plating layer tends to be selectively corroded more than the Al-rich portion, so that only a small amount of Mg is distributed in the Zn-rich portion is a protective property. It is not advantageous for the formation of high Mg-containing Zn-based corrosion products. In addition, the Mg ₂ Si phase itself does not have good corrosion resistance, and a large amount of the Mg ₂ Si phase is considered to be a factor that lowers the corrosion resistance of the plating layer.

  In view of such a current situation, the present invention aims to provide a Mg · Si-containing Zn—Al-based plated steel sheet that has better corrosion resistance than conventional ones.

The above-mentioned object is that Al: 25 to 75%, Si: 1 to 10%, Mg: 0.5 to 6% in terms of mass composition, and Ti or B, if necessary, for one or two of Ti and B. A plated steel sheet having a total content of 0.1% or less and having a remaining Zn and a Zn—Al-based plating layer that is an inevitable impurity, the plating layer comprising a Zn-rich portion, an Al-rich portion, and a Si-rich portion The Zn-rich part has a region with an Mg concentration of 4.5% by mass or more, and the Al-rich part and the Si-rich part have an excellent corrosion resistance with a structure state in which a region with an Mg concentration of 4.5% by mass or more is not observed. This is achieved by using a Zn-Al-based plated steel sheet. The plating adhesion amount per one side of the steel plate is, for example, 30 to 200 g / m ² .

As a manufacturing method of this plated steel sheet, the first steel consisting of Si: 1 to 12%, Zn: 0 to 10%, the balance Al and unavoidable impurities in mass% so as to satisfy the following plating requirements using the steel sheet as a base material. A first plating layer is formed by hot dip plating, and Al: 3 to 22%, Mg: 1 to 10%, Si: 0 to 2%, Ti: 0 to 0.1%, and B on the mass. : A multi-layer plated steel sheet is produced by forming a second plated layer by applying a second hot dipping comprising 0 to 0.05%, the balance Zn and unavoidable impurities, and then the multi-layer plated steel sheet is 500 to 650 Zn and Mg derived from the second hot dipping are generated by heating to 0 ° C., causing the second plating layer to melt and reacting with the first plating layer to produce a newly melted Zn rich portion in the first plating layer. Is distributed throughout the thickness of the plating layer. Method of cooling later is provided.
[Plating requirements]
In the average composition of the whole plating layer, the first and the second so that the content of Al, Si and Mg is Al: 25 to 75%, Si: 1 to 10%, Mg: 0.5 to 6% by mass%. The bath composition and the plating adhesion amount in the second hot dipping are set.

  In the heating of the multi-layer plated steel sheet, for example, the heating temperature can be set to 550 to 650 ° C., and the heating time can be set to 0 to 5 seconds for soaking.

  According to the present invention, it has become possible to industrially produce an Mg · Si-containing Zn—Al-based plated steel sheet in which Mg in the plating layer is mainly present in the Zn-rich portion, not in the Si-rich portion. The Mg · Si-containing Zn—Al-based plated steel sheet having such a plated layer structure is superior in corrosion resistance as compared with those having a conventional plated layer structure.

The plating layer cross-sectional SEM photograph in the Mg * Si containing Zn-Al type plated steel plate of the example of this invention. The image which shows the EPMA surface analysis result of Zn in the area | region corresponding to FIG. The image which shows the EPMA surface analysis result of Al in the area | region corresponding to FIG. The image which shows the EPMA surface analysis result of Mg in the area | region corresponding to FIG. The image which shows the EPMA surface analysis result of Si in the area | region corresponding to FIG. The image which shows the EPMA surface analysis result of Fe in the area | region corresponding to FIG. The figure which shows the plating layer cross-sectional SEM photograph and EDX analysis result about the conventional Si containing Zn-Al type plated steel plate manufactured by 1 bath plating, and Mg * Si containing Zn-Al type plated steel plate. The cross-sectional photograph of the plating layer after CCT150 cycle.

  Conventionally, a Zn—Al-based plated steel sheet containing about 55% Al is manufactured by immersing a base steel sheet once in an alloy plating bath, as with other general hot-dip plated steel sheets. Since the number of immersions in the plating bath is one, this is referred to as “one-bath plating” in this specification. When Si-containing Zn—Al-based plating is performed by one bath plating, the plating layer exhibits a structure composed of a Zn-rich portion, an Al-rich portion, and a Si-rich portion. In the present specification, among the metal elements contained in the plating layer, the portion with the highest Zn content in mass% is “Zn-rich portion”, the portion with the highest Al content is “Al-rich portion”, and the Si content is The highest part is called “Si rich part”. Also in the case of the Mg · Si-containing Zn—Al-based plated steel sheet, the plating layer is composed of a Zn-rich portion, an Al-rich portion, and a Si-rich portion. The Mg rich part is not formed, and Mg exists mainly as an intermetallic compound in the Si rich part. Even when the diffusion treatment is performed by heating after plating, it is difficult to diffuse Mg concentrated in the Si-rich portion into the plating layer, and a metal structure in which Mg is concentrated in the Zn-rich portion cannot be obtained. .

  In the present invention, a Mg · Si-containing Zn—Al-based plated steel sheet having a plating layer structure in which Mg mainly exists in the Zn-rich portion is obtained not by one-bath plating but by two-bath plating + diffusion treatment. In the two-bath plating, a first hot-dip plating is performed on the surface of a base material, and a second hot-dip plating is performed on the surface of the second hot-dip plating in another plating bath, thereby forming a plating layer having a multilayer structure. The plating bath used for the first hot dipping is referred to as “first plating bath”, and the plating layer formed thereby is referred to as “first plating layer”. A plating bath used for the second hot dipping is called a “second plating bath”, and a portion of a new plating layer formed thereby is called a “second plating layer”. “%” In the composition of the plating bath or plating layer means “% by mass” unless otherwise specified.

[Base steel sheet]
Various steel plates that have been conventionally used as base materials for hot-dipped steel sheets such as Zn-based plated steel sheets, Al-based plated steel sheets, and Zn-Al-based plated steel sheets can be used as the base steel sheet that serves as the plating base plate. . Various steel types are selected according to the application, and stainless steel can also be applied.

[First hot dipping]
The first hot dipping is “Al-based plating” consisting of Si: 1 to 12% by mass, Zn: 0 to 10%, the balance Al and unavoidable impurities. However, the bath composition and plating adhesion amount in the first hot dipping must be set so that the average composition in the multilayer plating layer composed of the first plating layer and the second plating layer falls within the range described later.

  Si in the first hot dipping has an action of reducing the liquidus temperature of the Al-based plating bath. Moreover, it has the effect | action which suppresses that a brittle Fe-Al-type reaction layer produces | generates thick in an interface with a base-material steel plate, and produces | generates a thin Al-Fe-Si-type reaction layer. However, if the Si content of the plating bath exceeds 12%, the eutectic composition is passed and the liquidus temperature tends to rise. In addition, when such a large amount of Si is contained in the first plating bath, a large amount of Si crystallized phase is formed at the interface between the first plating layer and the second plating layer, and the factor that hinders the homogenization of the composition by the diffusion treatment Can be. For this reason, Si content of the 1st plating bath shall be 12% or less.

  Zn can be contained in the first plating bath as necessary. However, if the Zn content is too large, Zn in the second plating bath is used to adjust the average composition of the entire plating layer to the range described later. It becomes necessary to set the content lower, and the Mg content becomes relatively high, which becomes a factor of reducing the second hot dipping property. For this reason, when Zn is contained in the first plating bath, it is performed in a range of 10% or less.

  The balance of Si and Zn in the first hot dipping is Al and inevitable impurities. Inevitable impurities are allowed to be mixed in the range of 2% or less, and other impurity elements are preferably set to a range of 1% or less in total. The first hot dipping can be performed using the production line of the Al-based plated steel sheet as it is.

[Second hot dipping]
The second hot-dip plating is Al: 3 to 22% by mass, Mg: 1 to 10%, Si: 0 to 2%, Ti: 0 to 0.1%, B: 0 to 0.05%, the balance This is “Zn—Al—Mg-based plating” made of Zn and inevitable impurities. However, the bath composition and the plating adhesion amount in the second hot dipping must be set so that the average composition in the multilayer plating layer composed of the first plating layer and the second plating layer falls within the range described later.

The second hot dipping is mainly responsible for supplying Mg into the plating layer. When trying to supply the target composition of Si and Mg from one hot dipping bath by one bath plating, the formation of Mg ₂ Si phase is inevitable in the solidification process of the plating bath, and the effect of improving corrosion resistance by adding Mg can be fully utilized. Absent. Therefore, in the present invention, a Mg-containing plating layer is newly formed using a second plating bath in which Si is not added or the amount of Si added is kept low. Further, since the first hot dipping is Al-based plating, the second hot dipping also has a role of supplying Zn, which is a constituent component of the target Zn-Al-based plating layer. However, when Mg is contained in the Zn-based plating bath, the plating property is lowered due to the generation of Mg oxide-based dross. In order to suppress this, it is effective to contain Al in the Mg-containing plating bath. Moreover, since Al is one of the main components of the target Zn—Al-based plating layer, there is no problem in adding it to the second plating bath. Therefore, in the present invention, the second hot dipping is Zn—Al—Mg based plating.

  In order to ensure the later-described Mg content in the final plating layer, it is desirable that the Mg content in the second hot dipping bath is 1% or more. You may manage to 2% or more. On the other hand, when the Mg content in the plating bath increases, Mg oxide-based dross tends to occur. For this reason, the Mg content in the second plating bath is regulated to 10% or less, more preferably 8% or less, and may be controlled to 7% or less or 6% or less. Mg in the plating bath has a function of reducing the surface oxide film of the Al-based plating layer formed by the first hot dipping and activating the surface, and the second plating layer is formed on the first plating layer. It is presumed that it is also useful for forming a film with good adhesion without causing defects such as non-plating. Further, it is presumed that the heat treatment described later is also useful for smoothly promoting diffusion at the interface between the first plating layer and the second plating layer.

  When the Al content in the second hot dipping is less than 3%, the effect of suppressing the formation of Mg oxide-based dross becomes insufficient. On the other hand, if the Al content in the plating bath exceeds 22%, the melting point of the plating bath increases and it becomes difficult to obtain a plated surface with good surface properties. The Al content in the second plating bath is more preferably 15% or less.

The second plating bath can further contain one or more of Si, Ti, and B. When Si is contained in the plating bath, the plating layer is prevented from being blackened and the glossiness of the surface is maintained, which is advantageous when the storage period until the diffusion treatment is long. However, when the Si content is increased, the Mg ₂ Si phase is generated, so when Si is contained in the second plating bath, it is performed in a range of 2% or less. Ti, the inclusion of one or two B, formation and growth of Zn ₁₁ Mg ₂ phase, which causes a patchy appearance defects are suppressed. In addition, the degree of freedom in bath temperature and cooling rate for obtaining a molten Zn—Al—Mg-based plating layer with good surface properties is expanded. When one or more of Ti and B are contained, the content of Ti is 0.1% or less and B is 0.05% or less.

  The balance other than the above elements in the second hot dipping is Zn and inevitable impurities. Inevitable impurities are allowed to be mixed in the range of 2% or less, and other impurity elements are preferably set to a range of 1% or less in total. The second hot dipping can be carried out using the production line of the Zn—Al—Mg plated steel sheet as it is.

[Diffusion treatment]
After forming the multi-layered plating layer as described above, diffusion treatment by heating is performed. In the actual plating line, this heat treatment can be carried out by continuously passing through a heating furnace after the second hot dipping. The heating temperature is in the range of 500 to 650 ° C. This temperature range is a temperature range in which a newly melted Zn-rich portion is generated in the first plating layer by melting the second plating layer and reacting with the first plating layer, and the Si-rich portion does not melt. It is a temperature range. In the present invention, a Zn-rich portion melted in the plating layer is generated to make the plating layer structure uniform in the thickness direction. At that time, the Al-rich portion in the multilayer plating layer may be melted. However, when Mg in the molten liquid phase reacts with the unmelted Si-rich part, Mg ₂ Si may be formed in the Si-rich part. Therefore, it is desirable that the heating time be as short as possible. .

Specifically, it is only necessary to ensure a heating time sufficient for Zn and Mg derived from the second hot dipping to be distributed over the entire thickness of the plating layer. If Mg has diffused and reached the vicinity of the Al—Fe—Si reaction layer interposed at the interface between the base steel plate and the plating layer, it can be determined that the diffusion has sufficiently progressed. For example, the heating temperature can be 550 to 650 ° C., and the heating time can be soaking 0 to 5 seconds. Here, “soaking” is the time from when the temperature of the plating layer reaches the target temperature until cooling. For example, “600 ° C., soaking 0 seconds” is a heating pattern that immediately cools when the plating layer temperature reaches 600 ° C. According to the configuration of the first plating layer and the second plating layer, the data of the optimum heating pattern may be obtained in advance by preliminary experiments. Heating atmosphere is not particularly limited, it can be an air atmosphere, a non-oxidizing atmosphere such as N ₂ gas, a reducing atmosphere such as N ₂ -H ₂ gas.

[Final plating layer]
The average composition of the final plating layer (target plating layer) obtained by the diffusion treatment is such that the content of Al, Si, and Mg is Al: 25 to 75%, Si: 1 to 10%, Mg in mass%. : Set to 0.5 to 6%. When adding 1 type or 2 types of Ti and B by 2nd hot dipping, you may contain 1 type or 2 types of Ti and B in the range of a total 0.1% or less. The remaining elements are Zn and inevitable impurities.

  Al is an element that imparts “high corrosion resistance of the plated surface”, and Zn is an element that imparts “sacrificial anticorrosive action”. In consideration of the balance of the characteristics, the Al content is set in the range of 25 to 75% according to the application.

  Mg forms a Zn-based corrosion product having excellent protective properties and plays a role of suppressing the progress of corrosion of the plating layer. For this purpose, the Mg content in the plating layer needs to be 0.5% or more, more preferably 1% or more, and even more preferably 2% or more. However, excessive Mg causes a decrease in the workability of the plating layer, and the amount of Mg added in the second hot dipping increases, so the Mg content in the average composition is set to a range of 6% or less.

  Si has the effect of suppressing the formation of a brittle Fe—Al-based reaction layer generated between the base steel plate and the plating layer as described above, and the effect of suppressing the blackening of the plating layer surface. Or it is further mix | blended with a 2nd plating bath. In the average composition of the entire plating layer, the Si content is 1% or more. However, since a large amount of Si content may cause a large amount of massive precipitates to be produced, which causes a decrease in the workability of the plating layer, the Si content in the average composition is made 10% or less.

  The plating layer having a multilayer structure having the above average composition is composed of a Zn-rich portion, an Al-rich portion, and a Si-rich portion by the above diffusion treatment, and the Zn-rich portion has a region having an Mg concentration of 4.5% by mass or more. In the Al-rich part and the Si-rich part, a structure state in which a region having an Mg concentration of 4.5 mass% or more is not observed is exhibited. The cross-sectional structure is basically a structure in which the Zn-rich portion is filled in a network between the Al-rich portions, and the Si-rich portion is partially present.

  The most different point from the conventional Mg · Si-containing Zn—Al-plated steel sheet by one-bath plating is that, according to the present invention, most of the Mg in the plating layer is distributed in the Zn-rich portion. . Almost no Mg is detected in the Si-rich portion, or the amount is very small. That is, when the Si rich portion is analyzed by microscopic means such as EDX, a region having an Mg concentration of 4.5% by mass or more is not observed. Si is considered to exist almost in the form of metallic Si. On the other hand, the Zn-rich part has a part where the Mg concentration becomes 4.5% by mass or more when analyzed by microscopic means such as EDX. Since Mg is present in a large amount in the Zn-rich portion that is easily corroded in this way, a highly protective Mg-containing Zn-based corrosion product is rapidly formed when corrosion occurs in the Zn-rich portion. It is thought that the corrosion resistance of the plating layer is greatly improved.

It is desirable that the total plating adhesion amount per one side of the steel plate is 30 to 200 g / m ² . If the coating amount is too small, the corrosion resistance (particularly long-term corrosion resistance) of the steel sheet is insufficient, and if it is too large, the workability is reduced.

[Invention Example]
A plain steel cold-rolled steel sheet having a thickness of 0.8 mm (C content: 0.04 mass%) was prepared as a base steel sheet. The base steel plate was used as a plating original plate, and the first hot dip plating was performed using a hot dip Al plating line to obtain a hot dip Al plated steel plate having a coating adhesion amount of 75 g / m ² per side. Next, using this plated steel plate as a base plate, a second hot-dip plating is performed using a hot-dip Zn—Al—Mg-based plating line, and a multi-layer plated steel plate having a second plating layer with a plating adhesion amount of 60 g / m ² per side is obtained. Obtained. The plating bath compositions are as follows.
(First plating bath)
In mass%, Si: 9%, Zn: 0%, balance Al and inevitable impurities (second plating bath)
In mass%, Al: 6%, Mg: 3%, Si: 0.025%, Ti: 0.02%, B: 0.005%, the balance Zn and inevitable impurities In this case, in the average composition of the entire plating layer The contents of Al, Si and Mg are as follows.
(Average composition)
In mass%, Al: 53.2%, Si: 5.0%, Mg: 1.3%

  The multi-layer plated steel sheet was subjected to diffusion treatment by heating in an air atmosphere at a heating pattern of 600 ° C. and soaking for 0 seconds to obtain a Mg · Si-containing Zn—Al-based plated steel sheet.

  While performing SEM observation about the section of this plated steel plate, surface analysis by EPMA was performed. FIG. 1 illustrates an SEM photograph of a cross section of the plating layer. 2 to 6 show the EPMA surface analysis results of Zn, Al, Mg, Si, and Fe for the regions corresponding to FIG. 1, respectively. The position where the element is detected in the surface analysis image is displayed in white. As a result of the surface analysis, the plating layer is composed of a Zn-rich portion, an Al-rich portion, and a Si-rich portion, and it can be seen that Mg is concentrated in the Zn-rich portion. Mg enrichment is not observed in the Si-rich part.

The EDX analysis values (mass%) for the positions 1 to 5 displayed in FIG. 1 are as follows.
Analysis position 1; Zn: 36.0%, Al: 64.0%, Si: 0%, Mg: 0%
Analysis position 2; Zn: 0%, Al: 0%, Si: 100%, Mg: 0%
Analysis position 3; Zn: 91.7%, Al: 2.9%, Si: 0%, Mg: 5.4%
Analysis position 4; Zn: 0%, Al: 0%, Si: 100%, Mg: 0%
Analysis position 5: Zn: 90.5%, Al: 2.8%, Si: 0%, Mg: 6.7%
The Zn-rich portion (analysis positions 3 and 5) has a region with an Mg concentration of 4.5% or more, and the Si-rich portion (analysis locations 2 and 4) is considered to be a metal Si phase.

[Conventional example]
As a plated steel sheet produced by one bath plating, a Si-containing Zn-Al-based plated steel sheet (Zn-55% Al-1.5% Si alloy-plated steel sheet) and a Mg-Si-containing Zn-Al-based plated steel sheet (Zn -55% Al-1.5% Si-2% Mg alloy-plated steel sheet) was prepared. In FIG. 7, the SEM photograph of the plating layer cross section about these is shown. In FIG. 7, the EDX analysis results (mass%) at the positions indicated by alphabets are also shown. In any case, the cross-sectional structure of the plating layer is composed of a Zn-rich portion, an Al-rich portion, and a Si-rich portion. However, in the case of containing Mg, Mg is most abundant in the Si-rich portion, and when X-ray diffraction was performed separately, the presence of the Mg ₂ Si phase was recognized.

<Corrosion resistance test>
Corrosion resistance test pieces were cut out from the plated steel sheets of the present invention examples and the conventional examples and subjected to a combined cycle corrosion test (CCT). The dimension of the test piece is 75 mm × 150 mm × plate thickness. Here, in order to compare the corrosion resistance of the plated surface, the cut end surface and the entire back surface were covered with an insulating tape. The CCT conditions were based on the neutral salt spray cycle test of JIS H8502. “Salt spray (5% NaCl aqueous solution, 35 ° C.) 2 h → Dry (60 ° C., 25% RH) 4 h → Wet (50 ° C., 98% RH) 2h "is one cycle. After 150 cycles, the corrosion product was removed with a 10% aqueous ammonium chloride solution at a liquid temperature of 70 ° C., and the weight loss from the corrosion was determined from the difference in mass from before the test.
As a result, the corrosion weight loss was as follows.
Si-containing Zn-Al plated steel sheet by one bath plating; 31 g / m ²
Mg / Si-containing Zn-Al plated steel sheet by one bath plating; 23 g / m ²
Mg · Si-containing Zn—Al-based plated steel sheet of the present invention example: 12 g / m ²

FIG. 8 illustrates a cross-sectional photograph of a typical plating layer after the CCT 150 cycle.
In the example of the present invention, the corrosion resistance of the plated surface was remarkably improved as compared with the conventional Mg · Si-containing Zn—Al-based plated steel sheet by one bath plating.

  In addition, a plated steel sheet having a multi-layer structure in which the bath composition of the first hot dipping and the second hot dipping is variously changed within the range according to the present invention by a separate pilot line (Ti, B, Si are not added to the second plating bath) In addition, various Mg-Si-containing Zn-Al-plated steel sheets corresponding to the present invention are produced by carrying out diffusion treatment under the conditions according to the present invention, and the above corrosion resistance test is carried out on these. Yes. As a result, in each case, a remarkable corrosion resistance improvement effect was confirmed as compared with the conventional example.

Claims (5)

  A plated steel sheet having an average composition of mass%, Al: 25 to 75%, Si: 1 to 10%, Mg: 0.5 to 6%, remaining Zn and Zn—Al based plating layer which is an unavoidable impurity, The plating layer is composed of a Zn-rich portion, an Al-rich portion, and an Si-rich portion. The Zn-rich portion includes a region having an Mg concentration of 4.5% by mass or more, and the Al-rich portion and the Si-rich portion have an Mg concentration of 4 A Zn-Al-based plated steel sheet excellent in corrosion resistance that exhibits a microstructure in which a region of 0.5 mass% or more is not observed.   The average composition of the Zn—Al-based plating layer is Al: 25 to 75%, Si: 1 to 10%, Mg: 0.5 to 5% in mass%, and one or two of Ti and B are added together. The Zn-Al-based plated steel sheet according to claim 1, which is contained in a range of 0.1% or less and is the balance Zn and inevitable impurities. Zn-Al-based plated steel sheet according to claim 1 or 2 coating weight per steel sheet one side is 30 to 200 g / m ^2. First plating is performed by using a steel plate as a base material and applying the first hot dipping which consists of Si: 1 to 12%, Zn: 0 to 10%, the balance Al and inevitable impurities in mass% so as to satisfy the following plating requirements. A layer is formed, and Al: 3 to 22%, Mg: 1 to 10%, Si: 0 to 2%, Ti: 0 to 0.1%, B: 0 to 0.05%, A multi-layer plated steel sheet is manufactured by forming a second plating layer by applying a second hot-dip plating composed of the remaining Zn and inevitable impurities, and then heating the multi-layer plated steel sheet to 500 to 650 ° C. By melting the layer and reacting with the first plating layer, a newly melted Zn-rich portion is generated in the first plating layer, and Zn and Mg derived from the second hot-dip plating are distributed throughout the thickness of the plating layer. Zn with excellent corrosion resistance after cooling Method of manufacturing the Al-based plated steel sheet.
[Plating requirements]
In the average composition of the whole plating layer, the first and the second so that the content of Al, Si and Mg is Al: 25 to 75%, Si: 1 to 10%, Mg: 0.5 to 6% by mass%. The bath composition and the plating adhesion amount in the second hot dipping are set.   5. The method for producing a Zn—Al-based plated steel sheet according to claim 4, wherein the heating of the multi-layer plated steel sheet is performed at a heating temperature of 550 to 650 ° C. and a heating time of 0 to 5 seconds for soaking.