JP3179401B2 - Hot-dip Zn-Al-Mg plated 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 excellent 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, using a normal continuous hot-dip plating equipment, a hot-dip Z having good corrosion resistance and surface appearance can be obtained.
It is necessary that n-Al-Mg plated steel sheets can be stably and continuously produced. In this specification, a hot-dip Zn-Al-Mg-plated steel strip manufactured by passing a steel strip through a continuous hot-dip plating facility may be referred to as a hot-dip Zn-Al-Mg-plated steel sheet for convenience. That is, the plated steel sheet and the plated steel strip represent the same thing.

[0005] In the ternary equilibrium diagram of Zn-Al-Mg, the ternary eutectic point at which the melting point becomes the lowest is obtained when Al is about 4% by weight and Mg is about 3% by weight (melting point = 343).
° 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 adopted, the Zn ₁₁ Mg ₂ phase, in fact, Al / Zn / Zn ₁₁ Mg ₂ The original eutectic substrate itself or [Al primary crystal] or [Al
A phenomenon occurs in which a Zn ₁₁ Mg ₂ -based phase in which [primary crystal] and [Zn single phase] are mixed is locally crystallized. This locally crystallized Zn ₁₁ Mg ₂ -based phase is more easily discolored than other phases (Zn ₂ Mg-based phase). Significantly worsens. Therefore, the product value as a hot-dip coated 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, this Z
It has also been clarified that the n ₁₁ Mg ₂ -based portion is preferentially corroded.

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% by weight, Mg: 1.0 to 4.0% by weight, balance being Zn and molten Zn-Al- containing inevitable impurities.
A continuous hot-dip Zn-based steel sheet having an Mg plating layer formed on the surface of the steel sheet, wherein the plating layer is [Al / Zn / Zn
₍₂ ) A ternary eutectic structure of Mg in which a [primary Al phase] or a metallic structure in which [primary Al phase] and [Zn single phase] are mixed is a continuous molten Zn with good corrosion resistance and surface appearance. −
Provided is an Al-Mg 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).

The plating layer of the hot-dip Zn—Al—Mg plated steel sheet according to the present invention has substantially no [ternary eutectic structure of Al / Zn / Zn ₁₁ Mg ₂ ], and has a [Al / Zn
/ Zn ₁₁ Mg ₂ ternary eutectic structure]
Phase) or a metal structure in which [primary crystal Al phase] and [Zn single phase] coexist.

The hot-dip coated steel sheet having a plated layer of a metal structure according to the present invention has a bath temperature of a plating bath in a continuous hot-dip plating facility of not less than the melting point and less than 470 ° C. , preferably 450 ° C.
Steel by and controlling the cooling rate after plating to 0.5 ° C. / sec or more or less as to and operatively controlling the cooling rate after plating than 10 ° C. / sec, or a bath temperature of the plating bath 470 ° C. or higher It can be manufactured continuously in the form of a band.

Here, the "ternary eutectic structure of Al / Zn / Zn ₂ Mg" means, for example, 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 the ternary eutectic structure is actually “Al—Zn—Mg” at a high temperature in a ternary equilibrium 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].

[0014] The "primary Al phase" is a phase which appears as an island with a clear boundary in the above-mentioned ternary eutectic structure as shown in a typical example in an 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
in the matrix of g ternary eutectic structure] [primary crystal Al phase] or [primary crystal Al phase] and that of [Zn single phase] is mixed-metal structure is referred to as "Zn ₂ Mg-based phase" Sometimes. Also, in this specification, what is called “Zn ₁₁ Mg ₂ -based phase”
Metallographic structure of the substrate itself [ternary eutectic structure of Al / Zn / Zn ₁₁ Mg ₂ ], or [primary Al phase] or [primary Al phase] and [Zn single phase] mixed in this substrate Represents the metal structure obtained. 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.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fused Zn-Al-M according to the present invention
g-plated steel sheet, the composition of the plating layer is Al: 4.0-4.0.
10% by weight, Mg: 1.0 to 4.0% by weight, with the balance being Zn and unavoidable impurities. Then, the plating layer of the steel sheet has a metallic structure in which [primary Al phase] is mixed in a [Al / Zn / Zn ₂ Mg ternary eutectic] base, or a [primary Al phase] in the base. ] And [Zn single phase] are substantially composed of a mixed metal structure.
Corrosion resistance, surface appearance and manufacturability were simultaneously improved.

Thus, the molten Zn-Al according to the present invention
-The characteristic of the Mg-plated steel sheet is that it has a specific metal structure. First, the basic plating composition of the plated steel sheet will be described.

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 it exceeds 0% by weight, F at the interface between the plating layer and the base steel sheet
The growth of the e-Al alloy layer becomes remarkable, 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.

In such a ternary composition of Zn—Al—Mg containing Zn in an amount of Al and Mg, if the Zn ₁₁ Mg ₂ phase is crystallized, the surface appearance is deteriorated and the corrosion resistance is lowered as described above. It turned out to be worse. On the other hand, the structure of the plating layer was changed to [ternary eutectic structure of Al / Zn / Zn ₂ Mg]
It was found that the surface texture was extremely good and the corrosion resistance was excellent when the metal structure was composed of [primary Al phase] or [primary Al phase] and [Zn single phase] mixed in the base material.

Here, the microstructure of [Al / Zn / Zn ₂ Mg ternary eutectic] in which the [primary Al phase] is mixed in the base material is defined as [ Al
/ Was initially precipitated material mixture of Zn / Zn ₂ Mg ternary eutectic structure] [primary crystal Al phase] is a metal structure coexist. FIG. 1 shows a typical example of the metal structure.

FIG. 1 is an electron microscope secondary electron image (magnification: 2000 times) of a section of a plating layer showing a typical metal structure obtained according to the present invention. The composition of the plating layer formed by hot-dip plating on the surface of 6Al-3Mg-Zn (Al almost 6% by weight, M
g about 3% by weight, with the balance being Zn). A picture depicting the structure of the photograph in Fig. 1 and explaining the phases in the structure is shown on the right.
As shown in the figure, an independent island-like [primary Al phase] is mixed in the [Al / Zn / Zn ₂ Mg ternary eutectic structure] matrix.

FIG. 2 shows [Al / Zn / Zn] in FIG.
_{2 is} a photograph of a secondary electron image (magnification: 10000 ×) of an electron microscope in which a base part of [ternary eutectic structure of 2Mg] is enlarged. As shown in the descriptive diagram on the right, this base is composed of Zn (stripe). A ternary eutectic structure composed of Al (a part that looks somewhat dark and granular) and Zn ₂ Mg (a part existing between Zn stripes and other than Al) Have.

The microstructure of [Al / Zn / Zn ₂ Mg ternary eutectic] in which [primary Al phase] and [Zn single phase] are mixed in the substrate is microscopically observed on 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 shows a typical example of the metal structure.

FIG. 3 is a secondary electron image (magnification: 2000 times) of a cross section of a plating layer showing a typical metallographic structure obtained according to the present invention.
-3Mg-Zn (Al approximately 6% by weight, Mg approximately 3% by weight, balance Zn). As can be seen in FIG.
/ Zn / Zn ₂ Mg ternary eutectic structure] in which independent island-shaped [primary Al phases] are mixed in the matrix, as in FIG. [Zn single phase]
(A part slightly grayish than the primary Al phase) is present.

FIG. 4 is a photograph (magnification: 2000) of an electron microscope secondary electron image of a cross section of a plating layer of a metal structure obtained when the cooling rate after hot-dip plating is increased from that of FIG.
And the composition of the plating layer is the same as that of FIG. In the structure of FIG. 4, the primary crystal Al phase is higher than that of FIG.
Is slightly smaller and [Zn single phase] is present in the vicinity thereof, but [primary Al phase] and [Zn single phase] are [Al / Zn single phase].
/ Ternary eutectic structure of Zn ₂ Mg] in the base material.

The proportion of these structures in the whole plating layer is the former, that is, the ternary eutectic structure of [Al / Zn / Zn ₂ Mg], which was first deposited in the substrate [primary crystal Al
[Al / Zn / Zn ₂ M]
g of ternary eutectic structure] + [the total amount of primary crystal Al phase] is 80 volume% or more, preferably 90 volume% or more, more preferably 95 volume% or more, Zn / Zn ₂ Mg binary co Crystals or a small amount of Zn ₂ Mg may be mixed.

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, and a binary eutectic of Zn / Zn ₂ Mg or a small amount of Zn ₂ Mg may be mixed.

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, the n ₁₁ Mg ₂ -based phase has a ternary eutectic structure of [Al / Zn / Zn ₁₁ Mg _{2] in} an attempt to continuously produce a plated steel sheet with a normal continuous hot-dip coating equipment. ] In the matrix of [Al primary crystal] or a phase of a metal structure in which [Al primary crystal] and [Zn single phase] are mixed.

FIG. 5 is a photograph showing the surface appearance of a plated steel sheet (No. 13 in Table 3 of Example 3 described later) in which a Zn ₁₁ Mg ₂ 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 sample obtained by shearing the sample so as to pass through the spots appearing in FIG. 5 and viewing the cross section. 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: 10,000 times) obtained by increasing the magnification of only the base portion (portion not containing 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 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 in FIG. 9 is No. 3 in Table 3 of Example 3 described later, and the middle and lower charts are those in Table 3 in Example 3.
No.14, the middle and lower ones are Zn ₁₁ Mg
_The sample was collected in such a manner that the spots of the _two phases were partially contained in the sample area. The percentage of the spot area in the sampled 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 M
g 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 metallographic structure, in Tables 3 to 11 of Examples described later and in FIG.
The plating layer according to the present invention in which the n ₁₁ Mg ₂ -based phase is substantially absent is denoted as “Zn ₂ Mg”, and a spot-like Zn ₁₁ Mg having a size visible in the matrix of the Zn ₂ Mg-based phase. Those in which _two- system phases have 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 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, 0% by volume of island-like [primary Al phase]
In the range of more than 50% by volume and less than 50% by volume. In some cases, 0 to 15% by volume of island-like [Zn single phase] was present. When the surface of the plating layer was observed with the naked eye, A spot-like Zn ₁₁ Mg ₂ phase (Al / Zn /
The phase having a base material of a ternary eutectic structure of Zn ₁₁ Mg ₂ ) does not exist in a visually recognizable 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 phase
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—
It has been found that in order to manufacture the Mg-plated steel sheet, the bath temperature of the hot-dip bath having the above composition and the cooling rate after the plating may be typically controlled within the range of the hatched area shown in FIG.

That is, as shown in FIG. 10 and as shown in the examples described below, when the bath temperature is lower than 470 ° C. and the cooling rate is lower than 10 ° C./sec, the Zn ₁₁ Mg
_{The two} phases 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 exceeds 470 ° C.,
It was found that the influence of the cooling rate was reduced, the Zn ₁₁ Mg ₂ -based phase did not appear, and the metal structure specified in the present invention was obtained. Similarly, even when the bath temperature is lower than 470 ° C. , more preferably 450 ° C. or lower, if the cooling rate is 10 ° C./sec or more, more preferably 12 ° C./high, the metal structure specified in the present invention can be obtained. I understand. 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 equipment, 4.0 to 10% by weight of Al, 1.0 to 4.0% by weight of Mg, and the balance consisting of Zn and unavoidable impurities. A plating bath having a bath temperature of not less than melting point and less than 470 ° C., preferably not more than 450 ° C., and a cooling rate after plating controlled at not less than 10 ° C./sec, preferably not less than 12 ° C./sec. If the surface of the steel sheet is hot-dip, or if the bath temperature of the plating bath is 470 ° C or higher and the cooling rate after plating is optional (at least 0.5 ° C / sec which is the lower limit in actual operation) When hot-dip galvanized steel is used, a hot-dip Zn-Al-Mg-plated steel sheet having a corrosion-resistant and surface-appearing surface having a metallized plating layer according to the present invention can be industrially manufactured.

The ternary eutectic composition (in the ternary equilibrium diagram, Al = 4% by weight, Mg = 3% by weight, Zn =
(93% by weight) was thought to be advantageous because the melting point was the lowest, but it was thought that it would be advantageous, but in reality, the final solidified portion would be closed and the surface condition would be uneven, resulting in poor appearance. The perfect ternary eutectic composition should be avoided. Regarding the composition of Al, the composition on the hypoeutectic side is more Zn ₁₁ M
Since g ₂ is easily crystallized, it is preferable to set the composition on the hypereutectic side in the above composition range.

As for the bath temperature, if the temperature is too high, the adhesion of the plating will be reduced. Therefore, as shown in the examples below, the upper limit of the bath temperature is set to 550 ° C. in the bath composition of the present invention. It is good to hot-dip plating.

As described above, in the range of the bath composition specified in the present invention, the bath temperature and the cooling rate after plating are affected by the formation and disappearance behavior of Zn ₁₁ Mg ₂ and Zn ₂ Mg as ternary eutectic. Although it has a significant effect, the reason is not clear at present, but it is considered as follows.

Since the number of Zn ₁₁ Mg ₂ crystallized decreases as the bath temperature is increased and disappears at 470 ° C. or higher, the bath temperature is considered to be directly related to the formation of Zn ₁₁ Mg ₂ phase nuclei. However, although the reason cannot be determined, it is speculated that the physical properties of the reaction layer (alloy layer) between the plating bath and the steel sheet may have an effect. This is because the alloy layer is considered to be the main solidification start position of the plating layer.

As the cooling rate after plating increases, a spot-like phase of Zn ₁₁ Mg ₂ , ie, [Al /
[Ternary eutectic structure of Zn / Zn ₁₁ Mg ₂ ]
The size of a primary phase or a spot-like phase in which [Al primary phase] and [Zn single phase] are mixed gradually becomes so small that visual observation becomes difficult. Then, at a cooling rate of 10 ° C./sec or more, the size is reduced until it becomes impossible to determine visually. That is, it is considered that the growth of the Zn ₁₁ Mg ₂ phase is prevented because the cooling rate is increased.

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

[0051]

【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 = 4.0-9.2% by weight, Mg = 0-5.2
Weight%, balance = Zn Plating bath temperature: 455 ° C Immersion time: 3 seconds Cooling rate after plating: 3 ° C / second or 12 ° C by air cooling
/ Sec (Cooling rate is average value from plating bath temperature to plating layer solidification temperature)

Under the above conditions, a hot-dip Zn-Al-Mg coated steel sheet was manufactured. At that time, the amount of oxide (dross) generated on the bath surface was observed, and a corrosion resistance test of the obtained hot-dip coated steel sheet was performed. . Corrosion resistance is SST (JIS-Z-237)
(Salt spray test in accordance with No. 1) for 800 hours, and was evaluated by the weight loss (g / m ² ) after corrosion. 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.

[0054]

[Table 1]

From the results shown in Table 1, it can be seen that the corrosion resistance is rapidly improved when the Mg content is 1% or more, but that the corrosion resistance is saturated even when added over 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. At a cooling rate of 3 ° C./sec, Zn ₁₁ Mg ₂ is crystallized, and this portion is preferentially corroded.

[Example 2] Relationship between composition (particularly Al content) on corrosion resistance and adhesion.

Treatment equipment: Continuous hot-dip galvanizing line of Sendzimer type (test 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, balance = Zn Plating bath temperature: 460 ° C Immersion time: 3 seconds Cooling rate after plating: 12 ° C / second by air cooling (cooling rate is plating Average value from bath temperature to plating layer solidification temperature)

A hot-dip Zn-Al-Mg coated steel sheet was manufactured under the above conditions, and a corrosion resistance test and an adhesion test of the obtained hot-dip coated steel sheet were performed. Corrosion resistance was the same as in Example 1 using SST
The adhesion was evaluated by the weight loss (g / m ² ) after 800 hours, and the adhesion was evaluated by bending the test piece in close contact. 5
% Or more was evaluated as x. The results are shown in Table 2.

[0059]

[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). Further, even when the Mg content is constant, the SST corrosion weight loss decreases as the Al content increases, and the corrosion resistance is improved. This means that the corrosion resistance is improved even if the amount of [primary Al phase] in the metal structure according to the present invention is relatively increased.

Example 3 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 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: 580 ° C , Dew point: -30 ° C Plating bath composition: Al = 4.8-9.6% by weight, Mg = 1.1-3.9
Wt%, balance = Zn Plating bath temperature: 390-535 ° C Immersion time: within 8 seconds Cooling rate after plating: 3-11 ° C / sec by air cooling (cooling rate is the average from plating bath temperature to plating layer solidification temperature) value)

Under the above conditions, first, Zn-6.2% Al-3.0%
With the Mg bath composition, hot-dip steel sheets were manufactured by changing the yoke bath temperature and the cooling rate after plating, and the microstructure and surface appearance of the coating layers of the resulting coated steel sheets were examined.
Table 3 shows the results.

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. Therefore, evaluation of appearance in Table 3, immediately after the surface of the 24-hour after after plating plating was visually observed and the Zn ₁₁ Mg
Evaluation was made based on the presence or absence of spots where the _two phases were crystallized. The spots where these spots were visually observed were non-uniform, and those where these spots were not visually observed were uniform.

[0066]

[Table 3]

From the results shown in Table 3, when the bath temperature is lower than 470 ° C., the cooling rate is low (less than 10 ° C./sec).
It can be seen that a Zn ₁₁ Mg ₂ phase appears and the appearance becomes non-uniform. On the other hand, even if the bath temperature is lower than 470 ° C., when the cooling rate is increased (at 10 ° C./sec or more), substantially [primary Al phase] and [ternary eutectic of Al / Zn / Zn ₂ Mg] are obtained. And a uniform appearance. Similarly, when the bath temperature is 470 ° C. or higher, even if the cooling rate is low, the [primary crystal Al phase] and the [ternary eutectic structure of Al / Zn / Zn ₂ Mg] are similarly formed, and a uniform appearance is exhibited. Become like

Next, the bath composition was changed to Zn-4.3% Al-1.2% M
g, Zn-4.3% Al-2.6% Mg or Zn-4.3% Al-
Except that 3.8% Mg was used, the bath temperature and cooling rate were changed in the same way to produce hot-dip coated steel sheets, and the microstructure and surface appearance of the coating layers of the obtained coated steel sheets were examined in the same manner as in the previous example.
The results are shown in Tables 4, 5, and 6, respectively.

[0069]

[Table 4]

[0070]

[Table 5]

[0071]

[Table 6] From the results in Tables 4 to 6, it can be seen that the same results as in Table 3 were obtained.

Further, the bath composition was changed to Zn-6.2% Al-1.5% M
g or Zn-6.2% Al-3.8% Mg, except that the bath temperature and cooling rate were changed to produce hot-dip coated steel sheets, and the structure and surface appearance of the coating layer of the obtained coated steel sheets were the same as in the previous example. The results are shown in Tables 7 and 8, respectively.

[0073]

[Table 7]

[0074]

[Table 8]

From the results in Tables 7 and 8, it can be seen that the same results as in Table 3 were obtained.

Further, the bath composition was changed to Zn-9.6% Al-1.1% M
g, Zn-9.6% Al-3.0% Mg or Zn-9.6% Al-
Except that 3.9% Mg was used, a hot-dip coated steel sheet was manufactured by changing the bath temperature and cooling rate in the same manner, and the microstructure and surface appearance of the coating layer of the obtained coated steel sheet were examined in the same manner as in the previous example.
The results are shown in Tables 9, 10 and 11, respectively.

[0077]

[Table 9]

[0078]

[Table 10]

[0079]

[Table 11]

From the results in Tables 9 to 11, it can be seen that the same results as in Table 3 were obtained. And these Table 3
Table 11 summarizes the results, when the bath temperature and the cooling rate in the hatched area as shown in FIG. 10 are employed, the bath composition according to the present invention substantially achieves [primary Al phase] and [Al / Zn / Z
n ₂ Mg ternary eutectic structure] or a metal structure with a small amount of [Zn single phase] added thereto, resulting in the melting of the plated layer having excellent corrosion resistance and surface appearance. A Zn-Al-Mg plated steel sheet can be obtained.

Example 4 Relationship between bath temperature and cooling rate on plating adhesion.

Processing equipment: Continuous hot-dip galvanizing line of NOF type (testing machine) Treated steel sheet: Cold rolled steel sheet of weakly deoxidized steel (thickness: 0.8 mm) Maximum temperature of reduction furnace reached: 780 ° C, dew point: -25 ° C Plating bath composition: Al = 4.5-9.5% by weight, Mg = 1.5-3.9
Weight%, balance = Zn Plating bath temperature: 400 to 590 ° C Immersion time: 3 seconds Cooling rate after plating: 3 ° C / second or 12 ° C by air cooling
/ Sec (cooling rate is the average value from plating bath temperature to plating layer solidification temperature)

Under the above conditions, a hot-dip coated steel sheet was manufactured.
The plating adhesion of the obtained plated steel sheet was examined, and the results are shown in Table 12. Evaluation of plating adhesion was performed in the same manner as in Example 2.

[0084]

[Table 12]

From the results shown in Table 12, it can be seen that when the bath temperature exceeds 550 ° C., the plating adhesion deteriorates in the bath composition range of the present invention regardless of the cooling rate.

[0086]

As described above, according to the present invention,
A hot-dip Zn-Al-Mg coated steel sheet with excellent corrosion resistance and surface appearance and an advantageous production method can be provided, and because of its excellent corrosion resistance, it can be used in new fields that cannot be achieved with conventional hot-dip Zn-based steel sheets. Can be expanded.

[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 No. 3 and No. 14 in Table 3 of Example 3, and measuring the X-ray diffraction pattern. .3
And the middle and lower ones are the Zn ₁₁
The sample was collected so that the spots of the 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 front page (72) Inventor Toshiharu Tachibana Taka 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. Technical Research Institute (56) References JP-A-3-281766 (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% by weight, Mg: 1.0
A continuous hot-dip Zn-base coated steel sheet having a hot-dip Zn-Al-Mg plated layer composed of Zn and unavoidable impurities formed on the surface of the steel sheet, the balance being [Al / Zn / Zn ₂ Mg ternary eutectic structure good continuous hot-dip Zn-Al-Mg plated steel sheet corrosion resistance and surface appearance having a metal structure in the matrix is [primary crystal Al phase] were mixed]. 2. Al: 4.0 to 10% by weight, Mg: 1.0.
A continuous hot-dip Zn-base coated steel sheet having a hot-dip Zn-Al-Mg plated layer composed of Zn and unavoidable impurities formed on the surface of the steel sheet, the balance being [Al / Zn / good continuous melt of corrosion resistance and surface appearance having a metal structure in the matrix and [primary crystal Al phase] is [Zn single phase] were mixed Zn ₂ Mg ternary eutectic structure] Zn-
Al-Mg 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). communication with the described
Continued hot-dip Zn-Al-Mg plated steel sheet. 4. The metallographic structure of the plating layer is [Al / Zn /
The ternary eutectic structure of Zn ₁₁ Mg ₂ ] itself or a Zn ₁₁ Mg ₂ -based phase in which [Al primary crystal] or [Al primary crystal] and [Zn single phase] are mixed in the substrate. 4. The continuous molten Zn-A according to claim 1, 2, or 3, which is not included.
1-Mg plated steel sheet. 5. Al: 4.0 to 10% by weight, Mg: 1.0
In a method for producing a continuous hot-dip Zn-Al-Mg coated steel sheet comprising up to 4.0% by weight, with the balance being Zn and unavoidable impurities, the bath temperature of the plating bath is set to the melting point or higher and lower than 470 ° C , and the cooling rate after plating is set to Continuous melting Z excellent in corrosion resistance and surface appearance characterized by being controlled at 10 ° C./sec or more
Manufacturing method of n-Al-Mg plated steel sheet. 6. The bath temperature of the plating bath is not lower than the melting point and not higher than 450 ° C.
The method for producing a continuous hot-dip Zn-Al-Mg plated steel sheet according to claim 5, wherein the cooling rate after plating is 10 ° C / sec or more. 7. Al: 4.0 to 10% by weight, Mg: 1.0
In a method for producing a continuous hot-dip Zn-Al-Mg-plated steel sheet comprising up to 4.0% by weight, the balance being Zn and unavoidable impurities, the bath temperature of the plating bath is set to 470 ° C. or higher and the cooling rate after plating is set to 0.5. ℃ / sec or more, and the plating layer
1 / Zn / Zn ₂ Mg ternary eutectic structure]
Crystal phase) or a mixture of [primary crystal phase] and [Zn single phase].
A method for producing a continuous hot-dip Zn-Al-Mg-plated steel sheet having excellent corrosion resistance and surface appearance, characterized by having an existing metal structure . 8. The method according to claim 8, 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
Continuous melt Z according to claim 5 or 6 having a phase] or [metallographic primary crystal Al phase] and the [Zn single phase] mixed
Manufacturing method of n-Al-Mg plated steel sheet.