JP3149129B2 - Hot-dip Zn-Al-Mg-based coated steel sheet with good corrosion resistance and surface appearance and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Zn-Al-Mg plated steel sheet having good corrosion resistance and surface appearance, and a method for producing the same.

[0002]

2. Description of the Related Art Various developmental researches have been conducted on a hot-dip Zn-Al-Mg plated steel sheet using a plating bath containing Zn and an appropriate amount of Al and Mg because of its excellent corrosion resistance. However, there is no commercial success as an industrial product at present.

For example, in US Pat. No. 3,505,043, molten Zn—Al—Mg having excellent corrosion resistance using a hot-dip plating bath composed of 3 to 17% by weight of Al, 1 to 5% by weight of Mg, and the balance of Zn. Since the proposal of plated steel sheets, a proposal has been made to further improve corrosion resistance and productivity by blending various additional elements with the basic bath composition of this type and regulating the production conditions (Japanese Patent Publication No. 64-8702). And Japanese Patent Publication No. Sho 64-11112 and Japanese Patent Laid-Open Publication No. Hei 8-60324.

[0004]

SUMMARY OF THE INVENTION Such molten Zn-A
In the industrial production of 1-Mg plated steel sheets, it is necessary that not only the obtained hot-dip coated steel sheets have excellent corrosion resistance, but also that a strip having good corrosion resistance and surface appearance can be produced with good productivity. That is, it is necessary to be able to stably and continuously produce a hot-dip Zn-Al-Mg plated steel sheet having good corrosion resistance and surface appearance using an in-line annealing type continuous hot-dip plating apparatus.

[0005] In the ternary equilibrium diagram of Zn-Al-Mg, the ternary eutectic point (melting point = 343) at which the melting point is lowest when Al is about 4% by weight and Mg is about 3% by weight.
° C). Therefore, in producing a hot-dip Zn-Al-Mg plated steel sheet based on a ternary alloy of Zn-Al-Mg, at first glance, it is advantageous to have a composition near this ternary eutectic point.

However, when the bath composition near the ternary eutectic point is employed, the structure of the Zn ₁₁ Mg ₂ system (details will be described later, but the Al / Zn / Zn ₁₁ Mg ₂ The ternary eutectic material itself or Zn ₁₁ Mg in which [Al primary crystal] or [Al primary crystal] and [Zn single phase] are mixed.
_The phenomenon of local crystallization of the ₂ phase) occurs. This locally crystallized Zn ₁₁ Mg ₂ -based phase is replaced with another phase (for example, Zn ₂ Mg ₂
(Mg-based phase and details will be described later), and it is more likely to discolor.
Significantly degrade surface appearance. Therefore, the product value as a hot-dip Zn-based plated steel sheet is significantly reduced.

In addition, according to the experience of the present inventors, when the Zn ₁₁ Mg ₂ phase is locally crystallized, it is apparent that the crystallized portion is preferentially corroded. It became. No effective means for solving such a problem has been found so far, including the above-mentioned publications.

Accordingly, an object of the present invention is to solve such a problem and to provide a molten Zn—Al alloy having good corrosion resistance and good surface appearance.
-To provide an Mg-plated steel sheet.

[0009]

According to the present invention, Al:
4.0 to 10.0% by weight, Mg: 1.0 to 4.0% by weight, T
i: 0.002 to 0.1% by weight, B: 0.001 to 0.04
A hot-dip Zn-base coated steel sheet in which a hot-dip coating layer composed of 5% by weight, the balance being Zn and unavoidable impurities is formed on the surface of the steel sheet, and the coating layer is [Al / Zn / Zn ₂ Mg
Ternary eutectic structure], and a metal structure in which [primary crystal Al phase] and [primary crystal Al phase] and [Zn single phase] are mixed in a molten Zn—Al— alloy having good corrosion resistance and surface appearance. M
Provide a g-plated steel sheet.

The metal structure of the plating layer is preferably
Total amount of [primary Al phase] and [ternary eutectic structure of Al / Zn / Zn ₂ Mg]: 80% by volume or more, [Zn single phase]: 15
% By volume (including 0% by volume).

In the hot-dip coated steel sheet having the metallized coating layer, the bath temperature of the plating bath is set to a melting point or higher and lower than 410 ° C., and the cooling rate after plating is controlled to 7 ° C./sec or higher.
Alternatively, it can be manufactured by controlling the bath temperature of the plating bath to 410 ° C. or higher and the cooling rate after plating to 0.5 ° C./sec or higher.

Here, the "ternary eutectic structure of Al / Zn / Zn ₂ Mg" means, for example, an Al phase, a Zn phase and a metal as shown in the electron micrograph of FIG. The compound has a ternary eutectic structure with a Zn ₂ Mg phase, and the Al phase forming this ternary eutectic structure is actually “Al—Zn—Mg” at a high temperature in a ternary system diagram of Al—Zn—Mg. "Phase" (Zn
(Al solid solution containing a small amount of Mg). The Al ″ phase at a high temperature usually appears at room temperature as being separated into a fine Al phase and a fine Zn phase. The Zn phase in the ternary eutectic structure dissolves a small amount of Al,
In some cases, it is a Zn solid solution in which a small amount of Mg is further dissolved. The Zn ₂ Mg phase in the ternary eutectic structure is Zn-M
g in the binary equilibrium diagram for Zn: an intermetallic compound phase present at around 84% by weight. In the present specification, the ternary eutectic structure composed of these three phases is referred to as [ternary eutectic structure of Al / Zn / Zn ₂ Mg].

[0013] The "primary Al phase" is, for example, a phase which looks like an island with a clear boundary in the ternary eutectic structure as shown in the electron micrograph of FIG. This is derived from the “Al” phase at high temperature in the ternary equilibrium diagram of Al—Zn—Mg (an Al solid solution that dissolves Zn and contains a small amount of Mg).
The Al "phase at this high temperature differs in the amount of Zn and Mg dissolved in solid solution depending on the Al and Mg concentrations in the plating bath.
The l ″ phase usually separates into a fine Al phase and a fine Zn phase at room temperature, but the island-like shape seen at room temperature is
It can be seen that the phase remains in the form of the 1 ″ phase. The phase originating from the Al ″ phase at this high temperature (referred to as an Al primary crystal) and retaining the form in the form of the Al ″ phase is referred to as a book. In the specification, this is referred to as “primary Al phase.” This “primary Al phase” can be clearly distinguished from the Al phase forming the ternary eutectic structure by microscopic observation.

The [Zn single phase] refers to a phase which looks like an island with a clear boundary in the ternary eutectic structure as shown in a typical example in an electron micrograph of FIG. (It looks a little whiter than the primary Al phase), and in fact, a small amount of Al and a small amount of Mg may be dissolved. This [Zn single phase] can be clearly distinguished from the Zn phase forming the ternary eutectic structure by microscopic observation.

In this specification, [Al / Zn / Zn ₂ M
g of the ternary eutectic structure), and a metal structure in which [primary Al phase] or [Zn single phase] is mixed in the base material is abbreviated as “Zn ₂ Mg-based phase”. Sometimes called.
Further, what is called “Zn ₁₁ Mg ₂ -based phase” in this specification is [ternary eutectic structure of Al / Zn / Zn ₁₁ Mg ₂ ].
Metallographic structure of the base itself or in this base [primary crystal Al
Phase) or a mixed metal structure of [primary Al phase] and [Zn single phase]. When the latter Zn ₁₁ Mg ₂ -based phase appears as spots of visible size, the surface appearance is significantly deteriorated and the corrosion resistance is also reduced. The plating layer according to the present invention is characterized in that there is substantially no spot-like Zn ₁₁ Mg ₂ -based phase of a size that is visible.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fused Zn-Al-M according to the present invention
g-plated steel sheet, the composition of the plating layer (substantially corresponding to the composition of the hot-dip bath) is Al: 4.0 to 10.0% by weight, Mg: 1.0 to 4.0% by weight, Ti: 0.002-0.00.
1% by weight, B: 0.001 to 0.045% by weight, the balance being Z
n and unavoidable impurities. Then, the structure of the plating layer was changed to a metal structure in which [primary Al phase] was mixed in the base material of [Al / Zn / Zn ₂ Mg ternary eutectic structure] It is characterized by having a metal structure in which a crystalline Al phase] and a [Zn single phase] are mixed, thereby simultaneously improving corrosion resistance, surface appearance and manufacturability.

The composition of the plating layer is 4.0 to 10.0% by weight of Al, 1.0 to 4.0% by weight of Mg, and the remainder is Zn and unavoidable impurities. / Z
n / Zn ₂ Mg ternary eutectic structure]
The present invention has the advantage that corrosion resistance, surface appearance, and manufacturability can be simultaneously improved by using a metal structure in which [metal phase] is mixed, or a metal structure in which [primary Al phase] and [Zn single phase] are mixed in the substrate. The applicants found out the contents and filed the contents of the earlier Japanese Patent Application No. 8-352467 (filing date: December 13, 1996).
It described in. Even after the filing of this application, the present inventors have determined that the molten Zn-A
We have been conducting research on 1-Mg plated steel sheets.
It has now been newly found that the addition of Ni can further suppress the formation and growth of Zn ₁₁ Mg ₂ -based phases. According to this finding, it is possible to form a plating layer having the above structure even when the control range of the bath temperature and the cooling rate is wider than in the case where no Ti / B is added. In addition, it is possible to more stably produce a hot-dip coated steel sheet having excellent corrosion resistance and surface appearance.

The alloy composition itself of a plating layer obtained by adding appropriate amounts of Ti and B to a hot-dip Zn plating layer is disclosed in, for example,
No. 66666 (fine grain refinement of Zn-Al alloy by adding Ti and B), Japanese Unexamined Patent Publication (Kokai) No. 62-23976 (fine refining of spangles), and Japanese Unexamined Patent Publication (Kokai) No. 2-138451 (coating by impact after painting) Suppression of peeling)
No. 2,748,851 (improvement of elongation and impact value) and the like.
-Al-Mg-based hot-dip coating was not used. Therefore, the effect of Ti / B on microstructural behavior such as Zn ₂ Mg phase formation and Zn ₁₁ Mg ₂ phase suppression was unknown. JP-A-2-274851 discloses that Mg may be contained in an amount of up to 0.2% by weight. Not intended until. According to the findings of the present inventors, the Z
In the n-Al-Mg hot-dip plating, even if the bath temperature and cooling rate are such that a Zn ₁₁ Mg ₂ -based phase is formed, Ti
Size of Zn ₁₁ Mg ₂ system phase by adding an appropriate amount of B is very small, Ti and B were found to be able to stably grow a Zn ₂ Mg-based phase.

As described above, the present invention is based on Japanese Patent Application No.
It differs from the hot-dip Zn-Al-Mg coated steel sheet proposed in -352467 in that the composition of the hot-dip coating layer contains Ti and B in an appropriate amount, but there is no substantial difference in the structure of the coating layer. . That is, as a result of suppressing the generation and growth of the Zn ₁₁ Mg ₂ phase by the application of Ti · B, a metal structure composed of the Zn ₂ Mg phase similar to that of the prior application can be obtained more advantageously. In the obtained metal structure, the addition amount of Ti.B may be very small, and the existence of a phase containing Ti.B has not been identified at present. Therefore, the hot-dip Zn-Al-Mg-plated steel sheet of the present invention contains an appropriate amount of Ti · B in the plating layer. / Zn ₂ Mg
[Ternary eutectic structure], a metal structure in which [primary Al phase] is mixed in the matrix, or [primary Al phase] and [Z
n single phase] will be described as having a mixed metal structure.

First, the molten Zn-Al-M of the present invention
The reason for limiting the amount of each component in the basic composition of the plating layer of the g-plated steel sheet (substantially corresponding to the plating bath) will be described.

The Al in the plating layer serves to improve the corrosion resistance of the plated steel sheet and to suppress dross generation during the production of the plated steel sheet. If the Al content is less than 4.0% by weight, the effect of improving corrosion resistance is not sufficient, and the effect of suppressing the generation of dross of Mg oxide is low. On the other hand, when the Al content is 1
If the content exceeds 0.0% by weight, the growth of the Fe-Al alloy layer becomes remarkable at the interface between the plating layer and the base steel sheet, and the plating adhesion deteriorates. The preferred Al content is 4.0 to 9.0% by weight, the more preferred Al content is 5.0 to 8.5% by weight, and the more preferred Al content is 5.0 to 7.0% by weight.

The Mg in the plating layer has a function of generating a uniform corrosion product on the surface of the plating layer and significantly increasing the corrosion resistance of the plated steel sheet. If the Mg content is less than 1.0%, the effect of uniformly producing such corrosion products is not sufficient, while if the Mg content exceeds 4.0%, the effect of improving the corrosion resistance by Mg is saturated, On the contrary, Mg oxide-based dross is likely to be generated.
4.0%. The preferred Mg content is 1.5-4.0% by weight, the more preferred Mg content is 2.0-3.5% by weight, and the more preferred Mg content is 2.5-3.5% by weight.

The Ti in the plating layer serves to suppress the formation and growth of the Zn ₁₁ Mg ₂ phase. Ti content is 0.0
If it is less than 02% by weight, such effects are not sufficient. On the other hand, if the Ti content exceeds 0.1% by weight, Ti-Al-based precipitates grow in the plating layer, and the plating layer has irregularities (corresponding to what is called spots in the field terms). This is not preferable because it causes damage to the image. Therefore,
The Ti content is 0.002 to 0.1% by weight.

B in the plating layer serves to suppress the formation and growth of the Zn ₁₁ Mg ₂ phase. B content 0.001
If the amount is less than the percentage by weight, such an effect is not sufficient. On the other hand,
If the B content exceeds 0.045% by weight, a Ti-B or Al-B-based precipitate grows in the plating layer, and irregularities (spots) occur in the plating layer, and the appearance is impaired. Not preferred. Therefore, the B content is 0.001.
The content is preferably set to 0.045% by weight. In this range of the B content, Ti-B-based compounds such as TiB ₂
It was found that even if there was, there was no problem of causing irregularities in the plating layer because the amount of the presence was small. Therefore, when B and Ti are added to the bath, a part of them can be added in the form of TiB ₂ ,
This can also suppress the generation and growth of the Zn ₁₁ Mg ₂ -based phase.

In the hot-dip Zn-Al-Mg plated steel sheet, it was found that the crystallization of Zn ₁₁ Mg ₂ deteriorates the surface appearance and the corrosion resistance as described above.
On the other hand, the structure of the plating layer is such that [primary Al phase] or [primary Al phase] and [Zn single phase] are mixed in the matrix of [ternary eutectic structure of Al / Zn / Zn ₂ Mg]. It was found that the metal structure had an extremely good surface appearance and excellent corrosion resistance.

Here, the structure in which the [primary Al phase] is mixed in the matrix of [Al / Zn / Zn ₂ Mg ternary eutectic] refers to the microscopic observation of the cross section of the plating layer as described above. Ternary eutectic structure of Al / Zn / Zn ₂ Mg]
This is a metal structure in which [primary Al phase] precipitated first in the base material. FIG. 1 is a secondary electron image (magnification: 2000) of a cross section of a plating layer showing a typical metal structure.
The basic composition of the plating layer hot-dip coated on the surface of the lower steel base material (the part that looks dark) is 6Al-
3Mg-Zn (Ti / B additive). The structure of the photograph shown in FIG. 1 is depicted on the right side, and the diagram explaining the phases in the structure is shown on the right. As shown in the figure, the ternary eutectic structure of Al / Zn / Zn ₂ Mg [Primary Al phase] is in a mixed state. FIG. 2 shows [Al / Zn / Zn ₂ Mg in FIG.
Ternary eutectic structure] is a secondary electron image (magnification: 10000 times) of an electron microscope in which the substrate is enlarged. As shown in the descriptive diagram on the right, the substrate is composed of Zn (white portion) and Al
It has a ternary eutectic structure composed of (a part that looks dark and granular) and Zn ₂ Mg (the remaining part that looks like a rod).

The microstructure of [Al / Zn / Zn ₂ Mg ternary eutectic] in which [primary Al phase] and [Zn single phase] are mixed in the base material is a microscopic observation of the plating layer cross section. Then, a metal structure in which [primary Al phase] and [Zn single phase] coexist in the [Al / Zn / Zn ₂ Mg ternary eutectic structure] base material. That is, there is no difference from the former metallographic structure except that a small amount of [Zn single phase] is crystallized, and even if a small amount of [Zn single phase] is crystallized, the corrosion resistance and appearance are substantially the same as the former structure. Is excellent.

FIG. 3 is a secondary electron image (magnification: 2000 times) of a cross section of the plating layer showing a typical metal structure thereof.
The basic composition of the plating layer is 6Al-3Mg-Zn.
(Ti / B additive material). As can be seen in FIG.
The point that the [primary Al phase] is mixed in the [Al / Zn / Zn ₂ Mg ternary eutectic structure] matrix is the same as that of FIG. ] (A portion slightly grayer than the primary Al phase).

FIG. 4 is a secondary electron image (magnification: 2000 times) of the cross section of the plating layer of the metal structure obtained when the cooling rate after the hot-dip plating was increased from that of FIG. The composition of the layers is the same as in FIG. In the structure of FIG. 4, the [primary Al phase] is slightly smaller than that of FIG. 3, and a [Zn single phase] exists in the vicinity thereof.
[Primary Al phase] and [Zn single phase] are [Al / Zn / Zn ₂
Mg ternary eutectic structure].

The proportion of these structures in the entire plating layer is the former, that is, the first precipitate in the matrix of [ternary eutectic structure of Al / Zn / Zn ₂ Mg] [primary crystal Al
[Al / Zn / Zn ₂ M]
g ternary eutectic structure] + [primary crystal Al phase] is 80%
% Or more, preferably 95% by volume or more. Binary eutectic of Zn / Zn ₂ Mg or a small amount of Zn ₂ Mg may be mixed in the remainder.

The latter, ie, [Al / Zn / Z
In a metal structure in which [primary Al phase] is scattered in the base material of [n ₂ Mg ternary eutectic structure] and [Zn single phase] is crystallized, [A
1 / Zn / Zn ₂ Mg ternary eutectic structure] + [primary crystal Al
Phase] is 80% by volume or more and [Zn single phase] is 15% by volume.
% By volume or less. The remainder may be a binary eutectic of Zn / Zn ₂ Mg or a small amount of Zn ₂ Mg.

In both the former and latter structures, Zn ₁₁ Mg
Desirably, substantially no _two phases are present. This Z
In the range of the plating composition according to the present invention, if the n ₁₁ Mg ₂ phase is to continuously produce hot-dip coated steel sheets with a normal continuous hot-dip coating equipment, [Al / Zn / Zn ₁₁ Mg ₂
Ternary eutectic structure] in the substrate
It has been found that it tends to appear as a “spotted” as a phase of the metal structure in which the primary crystal and the Zn single phase are mixed.

FIG. 5 is a photograph showing the surface appearance of a plated steel sheet (No. 2 in Table 3 of Example 3 described later) in which Zn ₁₁ Mg ₂ -based phase appeared in a spot-like manner. As shown in FIG. 5, spots with a radius of about 2 to 7 mm (discolored blue)
Appear in the mother phase. The size of the spots differs depending on the bath temperature and the cooling rate of the hot-dip layer.

FIG. 6 is a secondary electron image (magnification: 2,000 times) of the cross section of the sample obtained by shearing the sample so as to pass through the spots appearing in FIG. As can be seen in FIG.
The structure of the spot portion is such that [Al primary crystal] is mixed in the base material of [ternary eutectic structure of Al / Zn / Zn ₁₁ Mg ₂ ]. In some samples, [Al primary crystal]
And [Zn single phase] may be mixed.

FIG. 7 is a secondary electron image (magnification: 10000 times) of the electron microscope (magnification: 10000 times) obtained by increasing only the substrate portion (portion not including the primary Al crystal) in FIG. A ternary eutectic structure in which Zn ₁₁ Mg ₂ and Al (a part that looks slightly darker and granular) existed, ie, [Al /
Ternary eutectic structure of Zn / Zn ₁₁ Mg ₂ ] clearly appears.

FIG. 8 is a secondary electron image (magnification: 10000 times) of the spot portion appearing as shown in FIG. 5 as viewed from the boundary between the mother phase and the spot phase. The half is the parent phase and the right half is the spot phase. The left half of the matrix portion is similar to that of FIG. 2 [Al / Zn / Zn ₂
Mg ternary eutectic structure], and the right half shows the same [ternary eutectic structure of Al / Zn / Zn ₁₁ Mg ₂ ] shown in FIG. Comparing only the two intermetallic compounds, Z
It can be seen that n ₁₁ Mg ₂ is slightly more corrosive than Zn ₂ Mg.

From these FIGS. 5 to 8, it can be seen that spot-like Zn ₁₁
As for the Mg ₂ phase, [Al primary crystal] or [Al primary crystal] and [Zn single phase] are mixed in a matrix of [Al / Zn / Zn ₁₁ Mg ₂ ternary eutectic structure]. It has a metal structure, and the Zn ₁₁ Mg ₂ phase is Zn ₂ M
In the matrix of the g-phase, ie, [Al / Zn / Zn ₂ M
g ternary eutectic structure] in the matrix of [primary Al phase] or a metal structure in which [primary Al phase] and [Zn single phase] are mixed, as spots of visible size It can be seen that it appears in dots.

FIG. 9 shows a typical example of X-ray diffraction which is the basis for specifying the above metal structure. The peaks marked with a circle in the figure are those of the Zn ₂ Mg intermetallic compound,
The peaks marked are those of the Zn ₁₁ Mg ₂ intermetallic compound.
In each case of X-ray diffraction, a square plating layer sample of 17 mm × 17 mm was sampled, and Cu-
X-ray irradiation was performed under the conditions of a Kα tube, a tube voltage of 150 Kv, and a tube current of 40 mA.

The upper chart of FIG. 9 is No. 10 in Table 4 of Example 5 described later, and the middle and lower charts are those of Table 4.
No. 1 in the middle, and those in the middle and lower are Zn ₁₁
The sample was collected so that the spots of the Mg ₂ -based phase were partially included in the sample area. The percentage of the spot area in the sample area was visually observed.
%, The lower one is about 70%. From these X-ray diffractions, the ternary eutectic structure shown in FIG. 2 is [Al / Zn / Zn
₂ ternary eutectic structure], and the ternary eutectic structure shown in FIG. 7 is [Al / Zn / Zn ₁₁ Mg ₂ ].

From the viewpoint of such a metal structure, in Tables 3 to 4 in Examples described later and further in FIG.
_A plating layer according to the present invention in which substantially no ₁₁ Mg ₂ -based phase is present is denoted as “Zn ₂ Mg”, and a spot-like spot-like Zn ₁₁ Mg _{2 having} a size visible in the base of the Zn ₂ Mg-based phase. Those in which the system phase appeared are indicated as “Zn ₂ Mg + Zn ₁₁ Mg ₂ ”. The appearance of such a spot-like Zn ₁₁ Mg ₂ phase deteriorates the corrosion resistance and significantly lowers the surface appearance. Therefore, it is desirable that the plating layer according to the present invention be made of a metal structure substantially free of a Zn ₁₁ Mg ₂ phase having a size that can be visually observed, that is, a substantially Zn ₂ Mg phase.

More specifically, the plated layer of the hot-dip Zn—Al—Mg plated steel sheet having the composition in the above-mentioned range according to the present invention has a base material of [ternary eutectic structure of Al / Zn / Zn ₂ Mg] of 50%. In the range of not less than 100% by volume and less than 100% by volume, and [primary Al phase] exists in the range of more than 0% by volume and not more than 50% by volume in the eutectic structure, and in some cases, the [Zn single phase] was present in an amount of 0 to 15% by volume, and when the surface of the plating layer was observed with the naked eye, a Zn ₁₁ Mg ₂ -based phase (Al / Zn / Zn ₁₁ M
phase with matrix of a ternary eutectic structure of g ₂₎ are those which do not exist in the visible size. That is, the metallographic structure of the plating layer is a base material of [ternary eutectic structure of Al / Zn / Zn ₂ Mg]: 5
0 to less than 100% by volume, [primary Al phase]: more than 0 to 50% by volume, and [Zn single phase]: 0 to 15% by volume.

Here, “become substantially” means that the other phases,
Typically, a spot-like Zn ₁₁ Mg ₂ -based phase is not present in an amount that affects the appearance, and a small amount of Zn ₁₁ Mg ₂ -based phase that cannot be distinguished by visual observation exists. However, such a small amount is acceptable because it does not particularly affect the corrosion resistance and surface appearance. That is, if the Zn ₁₁ Mg ₂ phase is present in an amount which is observed in the form of spots with the naked eye, the appearance and corrosion resistance are adversely affected, and thus are outside the scope of the present invention. Also,
A Zn ₂ Mg-based binary eutectic, a Zn ₁₁ Mg ₂ -based binary eutectic, and the like can be allowed to exist in a trace amount that cannot be discriminated by visual observation with the naked eye.

According to the present invention, the molten Zn—Al—
In order to manufacture a Mg (Ti · B) -plated steel sheet, the bath temperature of the hot-dip bath having the above composition and the cooling rate after plating may be typically controlled within the range of the hatched area shown in FIG. all right. This range is wider than the range described in Japanese Patent Application No. 8-352467. This may be considered to be the effect of the addition of Ti.B.

That is, as shown in FIG. 10 and as shown in the examples described below, when the bath temperature is lower than 410 ° C. and the cooling rate is lower than 7 ° C./sec, the above-mentioned Zn ₁₁ Mg ₂
The phases of the system appear as spots, and the object of the present invention cannot be achieved. The appearance of such a Zn ₁₁ Mg ₂ phase itself can be understood to some extent by looking at the equilibrium phase near the ternary eutectic point on the Zn—Al—Mg ternary equilibrium diagram.

However, when the bath temperature is 410 ° C. or higher, the influence of the cooling rate is reduced, and the Zn ₁₁ Mg ₂ phase does not appear even at a slow cooling rate such as 0.5 ° C./sec. It has been found that the metal structure specified in the invention can be obtained. Similarly, even if the bath temperature is lower than 410 ° C, the cooling rate is set to 7 ° C.
It was found that the metal structure specified by the present invention could be obtained when the rate was set to / sec or more. This is a structure state that cannot be expected from the ternary equilibrium diagram of Zn—Al—Mg, and is a phenomenon that cannot be explained in terms of equilibrium theory.

By utilizing this phenomenon, in an in-line annealing type hot-dip plating facility, Al: 4.0 to 10.0% by weight, Mg: 1.0 to 4.0% by weight, Ti: 0.002 to 0.00%.
1% by weight, B: 0.001 to 0.045% by weight, the balance being Z
A hot-dip plating bath consisting of n and unavoidable impurities. The bath temperature of the plating bath is set to a melting point or higher and lower than 410 ° C., and the cooling rate after plating is controlled to 7 ° C./sec or higher. C. or higher and the cooling rate after plating is arbitrary (actually, the lower limit of 0.5 in actual operation is 0.5).
If the surface of the steel sheet is hot-dip coated (at a temperature of at least ° C / sec or more), the molten Zn-Al-Mg (Ti.
B) A plated steel sheet can be manufactured industrially.

The bath composition of Al and Mg was completely matched with the ternary eutectic composition (in the ternary equilibrium diagram, Al = 4% by weight, Mg = 3% by weight, Zn = 93% by weight). It was thought that it would be advantageous if it had the lowest melting point.
Actually, the final solidified portion is closed and the surface state becomes uneven, and the appearance is deteriorated. Therefore, it is better to avoid the complete ternary eutectic composition. Regarding the composition of Al, Zn ₁₁ Mg ₂ is more easily crystallized in the composition on the hypoeutectic side, so that the composition on the hypereutectic side in the above composition range is preferable.

As for the bath temperature, if the bath temperature is too high, the plating adhesion will decrease. Therefore, in the bath composition of the present invention, the upper limit of the bath temperature is set to 550 ° C., and the hot-dip plating is preferably performed at a bath temperature lower than this.

The effects of the composition, structure, and plating conditions of the plating layer on the corrosion resistance, adhesion, and surface appearance of the hot-dip Zn—Al—Mg plated steel sheet will be specifically described below with reference to examples.

[0050]

【Example】

[Example 1] Relationship between composition (particularly Mg content) on corrosion resistance and manufacturability.

Processing equipment: Sendzimer-type continuous hot-dip galvanizing line (testing machine) Treated steel sheet: Medium-carbon steel hot-rolled steel sheet (thickness: 3.2 mm) Maximum temperature of reduction furnace reached 600 ° C, dew point: -40 ° C Plating bath composition: Al = 4.0 to 9.2 wt%, Mg = 0 to 5.2 wt%, Ti: 0.003 to 0.041 wt%, B: 0.002 to 0.02, balance = Zn Plating bath temperature: 430 ° C Immersion time: 3 seconds After plating Cooling rate: 3 ° C / sec by air cooling (cooling rate is the average value from plating bath temperature to plating layer solidification temperature)

Under the above conditions, the molten Zn—Al—Mg (Ti
-B) A plated steel sheet was manufactured, and the amount of oxide (dross) generated on the bath surface at that time was observed, and a corrosion resistance test of the obtained hot-dip coated steel sheet was performed. Corrosion resistance is SST (JI
(Salt spray test according to S-Z-2371) was carried out for 800 hours, and the weight loss (g / m ² ) was evaluated. Further, the amount of dross generated was visually evaluated as ×, those with a relatively large amount as Δ, and those with a small amount as ◎. The results are shown in Table 1.

[0053]

[Table 1]

From the results shown in Table 1, it can be seen that the corrosion resistance is sharply improved when the amount of Mg is 1% or more, but that the corrosion resistance is saturated even when the content exceeds 4%. Also, 4%
It can be seen that when the amount of Mg exceeds the above, the oxide (dross) on the bath surface increases even if Al is contained. In addition, Ti ・ B
With no addition, Zn ₁₁ Mg ₂ is crystallized at a cooling rate of 3 ° C./sec, and this portion is preferentially corroded.

[Example 2] The relationship of the composition (particularly the amount of Al) on the corrosion resistance and adhesion.

Processing equipment: Continuous hot-dip galvanizing line of Sendzimer type (testing machine) Treated steel sheet: Hot-rolled steel sheet of medium carbon steel (thickness: 1.6 mm) Maximum temperature of reduction furnace reached: 600 ° C, dew point: -40 ° C Plating bath composition: Al = 0.15 to 13.0% by weight, Mg = 3.0% by weight, Ti = 0.05% by weight, B = 0.025% by weight, balance = Zn Plating bath temperature: 440 ° C Immersion time: 3 seconds Cooling rate after plating: 4 ° C / sec by air cooling (cooling rate is the average value from plating bath temperature to plating layer solidification temperature)

Under the above conditions, the molten Zn—Al—Mg (Ti
・ B) A plated steel sheet was manufactured, and a corrosion resistance test and an adhesion test were performed on the obtained hot-dip coated steel sheet. Corrosion resistance was evaluated by corrosion loss (g / m ² ) after 800 hours by SST as in Example 1. Adhesion was measured by bending the test piece tightly. Less than 5% was rated as Δ, and 5% or more of peeling was rated as ×. The results are shown in Table 2.

[0058]

[Table 2]

As can be seen from the results in Table 2, when the Al content is 4.0% or more, the corrosion resistance becomes excellent, but when it exceeds 10%, poor adhesion occurs. This is the alloy layer (Fe-A
1 alloy layer).

[Example 3] Relationship between composition (particularly Ti and B contents) on corrosion resistance and adhesion.

Processing equipment: Sendzimer-type continuous hot-dip galvanizing line (testing machine) Treated steel sheet: Hot rolled steel sheet of weakly deoxidized steel (in-line pickling), thickness: 2.3 mm Maximum temperature of reduction furnace reached: 580 ° C , Dew point: −30 ° C. Plating bath composition: Al = 6.2% by weight Mg = 3.0% by weight, Ti = 0 to 0.135% by weight B = 0 to 0.081% by weight, balance = Zn Plating bath temperature: 450 ° C. Immersion time: 4 seconds Cooling rate after plating: 4 ° C / sec by air cooling (cooling rate is the average value from plating bath temperature to plating layer solidification temperature)

Under the above conditions, the molten Zn—Al—Mg (Ti
・ B) A plated steel sheet was manufactured, and the structure and surface appearance of the plated layer of the obtained plated steel sheet were examined. The results are shown in Table 3.

In the display of the plating layer structure in Table 3, [Zn
The ₂ Mg] as that displayed, the metal structure specified by the present invention, namely [Al / Zn / Zn in the matrix of ₂ Mg ternary eutectic structure] [primary crystal Al phase] or [primary crystal Al phase] And a [Zn single phase] mixed metal structure,
Actually, [primary Al phase] and [Al / Zn / Zn ₂ Mg
Ternary eutectic structure] and the Zn single phase is 15% by volume or less.

Further, the material indicated as [Zn ₂ Mg + Zn ₁₁ Mg ₂ ] has a size such that a spot-like Zn ₁₁ Mg ₂ -based phase can be visually judged in the [Zn ₂ Mg] -based structure. It has appeared. This spot-like Zn ₁₁ Mg ₂ phase is, as described in the text, [Al / Zn / Zn ₁₁ Mg 2
In the matrix of the _second three-way eutectic structure] [Al primary crystal] or [A
1 primary crystal] and [Zn single phase].
This Zn ₁₁ Mg ₂ phase has a glossy pattern than its surroundings, so it has a prominent pattern, and this part is
If left undisturbed for about 4 hours, it becomes more noticeable because it is oxidized earlier than the other parts and changes color to light brown. In the appearance evaluation display in Table 3, those with spots (existing) indicate those where the Zn ₁₁ Mg ₂ -based spots were observed immediately after plating and 24 hours after plating. , Spots [absent] are those in which these spots were not seen. In addition, bumps refer to those in which unevenness is generated in the plating layer due to a precipitate that has grown coarsely in the plating layer.

[0065]

[Table 3]

From the results shown in Table 3, the addition of Ti.B
It can be seen that Zn ₁₁ Mg ₂ -based spots are less likely to be crystallized, and that good surface properties are obtained. In particular, such an effect is small when B alone is used, and the effect of the composite addition of Ti and B appears. However, when the amount of Ti.B exceeds the range specified in the present invention, irregularities occur and the surface properties are deteriorated.

Example 4 The composition of the bath was as follows (1) to (5): (1) Al = 4.0% by weight Mg = 1.2% by weight, Ti = 0 to 0.135% by weight B = 0 0.02% by weight, balance = Zn (2) Al = 4.2% by weight Mg = 3.2% by weight, Ti = 0 to 0.135% by weight B = 0 to 0.081% by weight, balance = Zn (3) Al = 6.2% by weight Mg = 1.1% by weight, Ti = 0 to 0.135% by weight B = 0 to 0.081% by weight, balance = Zn (4) Al = 6.1% by weight Mg = 3.9% by weight, Ti = 0 to 0.135% by weight B = 0 to 0.081% by weight Example 3 was repeated except that the balance was Zn (5) Al = 9.5% by weight Mg = 3.8% by weight, Ti = 0 to 0.135% by weight B = 0 to 0.081% by weight, and the balance = Zn. As a result, when the amounts of Al and Mg were changed as shown in (1) to (5), the plating layer structure and appearance evaluation were exactly the same as those of each of the amounts of Ti and B shown in Table 3. Was obtained. That is, the effect of the addition of Ti and B depends on Al and M specified in the present invention.
It was found that the effect was exerted regardless of the amount of Al and Mg in the addition range of g.

Example 5 The relationship between the bath temperature and the cooling rate exerted on the structure, and the relationship between the structure and the surface appearance.

Processing equipment: Sendzimer-type continuous hot-dip coating line (testing machine) Treated steel sheet: Hot rolled steel sheet of weakly deoxidized steel (in-line pickling),
Sheet thickness: 2.3mm Maximum temperature of reduction furnace reached: 580 ° C, dew point: -30 ° C Plating bath composition: Al = 6.2% by weight, Mg = 3.0% by weight, Ti = 0 or 0.030% by weight, B = 0 or 0.015 Weight%, balance = Zn Plating bath temperature: 390-500 ° C Immersion time: within 5 seconds Cooling rate after plating: 0.5-10 ° C / sec by air cooling (cooling rate from plating bath temperature to plating layer solidification temperature) Average)

Under the above conditions, a hot-dip coated steel sheet was manufactured by changing the plating bath temperature and the cooling rate after the plating, and the structure and surface appearance of the coating layer of the obtained coated steel sheet were examined. The results are shown in Table 4. Indicated. The indication of the plating layer structure and the presence or absence of spots in the appearance evaluation in Table 4 are the same as those described in Table 3.

[0071]

[Table 4]

[0072] From Table 4 results in comparison with the comparative example of Ti-B no additives, those Ti-B addition it can be seen that does not appear patches of Zn ₁₁ Mg ₂ system even at low bath temperature & low cooling rate.
That is, the plating composition according to the present invention is subjected to hot-dip plating at a bath temperature and a cooling rate in a hatched area shown in FIG.
Substantially [primary crystal Al phase] and [ternary eutectic structure of Al / Zn / Zn ₂ Mg] can be obtained, and a product having a Zn ₁₁ Mg ₂ -based uniform appearance without spots can be obtained. On the other hand, in the case where no Ti · B is added, the above-mentioned Japanese Patent Application No. 8-3524 is used.
As described in No. 67, Zn ₁₁ Mg ₂ -based spots appear unless the bath temperature is preferably 470 ° C. or higher or the cooling rate is 10 ° C./sec or higher when the bath temperature is lower than 470 ° C.

[0073]

As described above, according to the present invention,
We can provide a hot-dip Zn-Al-Mg-based coated steel sheet with excellent corrosion resistance and surface appearance, and an advantageous manufacturing method for it. Applications can be expanded. In particular,
As a result of the suppression of the formation of Zn ₁₁ Mg ₂ -based spots due to the addition of Ti and B, the production of a hot-dip Zn-based coated steel sheet of this type becomes much easier than the invention of Japanese Patent Application No. 8-352467. Was.

[Brief description of the drawings]

FIG. 1 is a photograph of a secondary electron image of an electron microscope showing a metal structure of a cross section of a plating layer of a hot-dip Zn—Al—Mg plated steel sheet according to the present invention and an explanatory diagram thereof.

2] [Al / Zn / Zn ₂ of the metal structure shown in FIG.
FIG. 3 is a photograph of a secondary electron image of an electron microscope and an explanatory diagram thereof, in which a base portion made of [ternary eutectic structure of Mg] is enlarged.

FIG. 3 is a photograph of a secondary electron image of an electron microscope showing a metal structure (the same structure as that of FIG. 1 except for including a Zn single phase) in a cross section of a plating layer of a hot-dip Zn—Al—Mg plated steel sheet according to the present invention. FIG.

FIG. 4 shows the metallographic structure of the cross section of the plating layer of the hot-dip Zn—Al—Mg plated steel sheet according to the present invention (the structure is the same as that of FIG. 1 except that a Zn single phase is included. FIG. 2 is a photograph of a secondary electron image of an electron microscope showing a (small tissue) and an explanatory diagram thereof.

FIG. 5 is a photograph showing the metallographic structure of a hot-dip Zn-Al-Mg plated steel sheet in which spot-like Zn ₁₁ Mg ₂ -based phases of visible size are scattered.

6 is a secondary electron image photograph (magnification: 2000 times) of an electron microscope showing a metal structure of a cross section obtained by cutting a spot portion of FIG. 5;

FIG. 7 is a secondary electron image photograph (magnification: 10 magnifications) showing a metal structure obtained by enlarging and copying a ternary eutectic portion in the structure of FIG.
000 times).

8 is a secondary electron image photograph (magnification: 10,000 times) of an electron microscope showing a metal structure at a boundary portion of a spot in FIG. 5, in which a left half is a base portion of a Zn ₂ Mg phase and a right half is a spot portion. This is the base portion of the Zn ₁₁ Mg ₂ phase.

FIG. 9 is an X-ray diffraction diagram obtained by measuring a sample of 17 mm × 17 mm from the plated steel sheets of No. 10 and No. 1 in Table 4 of Example 5 and measuring the X-ray diffraction. .
10 and those in the middle and lower stages are the Zn
_The sample was collected in such a manner that spots of ₁₁ Mg ₂ -based phase were partially included in the sample area.

FIG. 10 is a diagram showing a range of advantageous production conditions for the hot-dip Zn—Al—Mg plated steel sheet of the present invention.

Continuation of the front page (72) Inventor Toshiharu Tachibana Taka 5th Ishizu Nishimachi, Sakai City, Osaka Pref. Nisshin Steel Co., Ltd. Technical Research Institute (56) References JP-A-3-281766 (JP, A) JP-A-2-274851 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40 C22C 18/04 EPAT (QUESTEL)

Claims (8)

(57) [Claims] 1. Al: 4.0 to 10.0% by weight, Mg:
1.0 to 4.0% by weight, Ti: 0.002 to 0.1% by weight,
B: 0.001 to 0.045% by weight, the balance being a hot-dip Zn-base coated steel sheet in which a hot-dip coating layer composed of Zn and unavoidable impurities is formed on the surface of the steel sheet,
A hot-dip Zn-Al-Mg-based coated steel sheet having good corrosion resistance and surface appearance having a metal structure in which [primary Al phase] is mixed in a base material of [ternary eutectic structure of Al / Zn / Zn ₂ Mg]. 2. Al: 4.0 to 10.0% by weight, Mg:
1.0 to 4.0% by weight, Ti: 0.002 to 0.1% by weight,
B: 0.001 to 0.045% by weight, the balance being a hot-dip Zn-base coated steel sheet in which a hot-dip coating layer composed of Zn and unavoidable impurities is formed on the surface of the steel sheet,
Good melting of the corrosion resistance and surface appearance in the matrix of [Al / Zn / Zn ₂ Mg ternary eutectic structure] and [primary crystal Al phase] is [Zn single phase] having the metal structure mixed Zn-Al- M
g-based plated steel sheet. 3. The metal structure of the plating layer is [primary Al phase].
And the total amount of [Al / Zn / Zn ₂ Mg ternary eutectic structure]: not less than 80% by volume and [Zn single phase]: not more than 15% by volume (including 0% by volume). The described hot-dip Zn-Al-Mg-based plated steel sheet. 4. The metallographic structure of the plating layer is [Al / Zn /
[Al ternary eutectic structure of Zn ₁₁ Mg ₂ ]
Or Zn in which [Al primary crystal] and [Zn single phase] are mixed.
_{11. The} method according to claim 1, which is substantially free of a Mg ₂ -based phase.
4. The hot-dip Zn-Al-Mg-based plated steel sheet according to 2 or 3. 5. Al: 4.0 to 10.0% by weight, Mg:
1.0 to 4.0% by weight, Ti: 0.002 to 0.1% by weight,
B: In the method for producing a hot-dip Zn—Al—Mg-based coated steel sheet composed of 0.001 to 0.045% by weight, with the balance being Zn and unavoidable impurities, the bath temperature of the plating bath was set to 41 ° C. or higher.
A method for producing a hot-dip Zn-Al-Mg-based coated steel sheet having good corrosion resistance and surface appearance, wherein the temperature is set to less than 0 ° C and the cooling rate after plating is controlled to 7 ° C / second or more. 6. Al: 4.0 to 10.0% by weight, Mg:
1.0 to 4.0% by weight, Ti: 0.002 to 0.1% by weight,
B: In a method for producing a hot-dip Zn—Al—Mg-based coated steel sheet composed of 0.001 to 0.045% by weight, the balance being Zn and unavoidable impurities, the bath temperature of the plating bath is set to 410 ° C. or higher, and cooling after plating is performed. Melting Z with good corrosion resistance and surface appearance characterized by controlling the speed to 0.5 ° C./sec or more
A method for producing an n-Al-Mg-based plated steel sheet. 7. The method according to claim 7, wherein the plating layer of the plated steel sheet is [Al / Zn
/ Zn ₂ into a green body in the ternary eutectic structure] in Mg [primary crystal Al
7. The molten Zn- according to claim 5, wherein the molten Zn-
A method for producing an Al-Mg-based plated steel sheet. 8. The method for producing a hot-dip Zn—Al—Mg-based steel sheet according to claim 5, wherein Ti and B are added to the plating bath at least in part in the form of TiB _2. .