JPH09217161A - Hot-dip Zn-Al alloy plated steel sheet - Google Patents

Abstract

(57) Abstract: A hot-dip Zn-Al alloy plated steel sheet having a smooth surface and excellent gloss is provided. A hot-dip Zn-Al alloy-plated steel sheet containing 2 wt% or more and 6 wt% or less of Al, wherein the surface layer of the plating layer is a continuous β phase, or the β phase and a eutectic phase, and the lower layer is a eutectic phase. , Β phase is exposed on the surface of the plating layer.

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 alloy plated steel sheet, and more particularly to a hot-dipped Zn-Al alloy plated steel sheet having a smooth surface and excellent gloss.

[0002]

2. Description of the Related Art Hot-dip galvanizing of steel sheets is carried out mainly for the purpose of improving corrosion resistance. The hot-dip galvanized steel sheets are mainly used for automobiles, building materials and home appliances. In particular, since the hot-dip Zn-Al-based alloy plating obtained by adding a large amount of aluminum to the usual hot-dip galvanizing has excellent corrosion resistance, its development has progressed, and in recent years, hot-dip Zn-Al-based alloy plated steel sheet with about 5 wt% aluminum added has been developed. Is widely used.

[0003] This hot-dip Zn-Al alloy plated steel sheet is
Used naked or painted. When used for these purposes, molten Zn- with a smooth surface and excellent gloss
Al-based alloy plated steel sheets are desired.

The conventional coating layer structure of a hot-dip Zn-Al alloy plated steel sheet containing about 5 wt% aluminum is shown in the photograph of the microscopic structure of FIG. 1 (A) and its schematic view in FIG. 2 (A). As described above, the zinc-rich β phase (hereinafter, simply referred to as β phase) 1 exists inside the plating layer, and the eutectic phase 2 exists so as to surround the β layer. In the process of cooling and solidifying the plating layer, the crystal grains of the Zn-Al alloy contract, the grain boundaries are dented, and a hexagonal pattern occurs. The hexagonal pattern not only deteriorates the smoothness of the plated steel sheet, but also becomes noticeable when coated, which impairs the coating appearance. Further, in addition to the poor luster of the eutectic phase existing on the surface of the plated layer, and the presence of a hexagonal pit, the plated steel sheet is inferior in gloss and smoothness to the hot-dip galvanized steel sheet.

The following proposals have been made to reduce or eliminate the tortoiseshell pattern. (1) JP-A-5-125515 and JP-A-6-15
No. 8256 discloses that after plating with a plating bath containing 3 to 10 wt% of Al, 0.01 to 1 wt% of Ti, and the balance Zn and unavoidable impurities, the steel sheet after plating is cooled to obtain Ti. A technique is disclosed in which the spangle of the plating layer is made fine by the effect of making the spangle fine, and the surface of the plating layer is smoothed.

(2) Japanese Patent Laid-Open No. 2-73954 discloses that
Al is 3.0 to 10 wt% and Si is 0.2 to 0.
45%, Mg 0.01 to 1.0 wt%, the balance Zn and 0.02 wt% or less plating is performed by a plating bath consisting of inevitable impurities, and the addition of Si improves the wettability with the steel sheet and the plating layer. A technique is disclosed in which the difference between the freezing point of the eutectic phase and the freezing point of its grain boundaries is reduced to relatively uniformly solidify and thereby reduce the occurrence of the hexagonal pattern.

(3) In JP-A-6-264208, plating is carried out in a Zn-Al alloy plating bath containing 3.5 to 70.0% Al, and a plating layer in a molten state is treated with an aqueous solution of an endothermic compound. Spray the mist, quench the steel plate,
A technique for preventing the formation of a hexagonal pattern by suppressing the growth of crystal grains is disclosed.

Further, the following technique for improving the quality of the plating layer is disclosed, paying attention to the existence state of the β phase of the plating layer.

(4) In JP-A-60-110860, a steel sheet plated with a Zn-Al alloy plating bath containing 7 wt% or less of Al is cooled after plating at a cooling rate of 5 to 40 ° C./sec. Then, by segregating β phase 1 in the vicinity of the plating layer surface as shown in the micrograph of the plating layer structure of FIG. 1 (B) and its schematic view, FIG. 2 (B), corrosion resistance and phosphate Techniques for improving processability are disclosed.

[0010]

However, the above (1)
In the case of the technology disclosed in (3) to (3), the hexagonal pattern becomes finer or the dents become smaller, so that the hexagonal pattern is less noticeable, but the hexagonal pattern still exists on the surface of the plating layer, and the gloss is Not improved.

Further, the technique disclosed in (4) does not mention the hexagonal pattern or gloss. When the present inventors investigated, the plating layer had a hexagonal pattern and was inferior in smoothness and gloss.

The present invention has been made in consideration of the above circumstances, and is a molten Zn-A having a smooth surface and excellent gloss.
It is an object to provide an l-based alloy plated steel sheet.

[0013]

Means for Solving the Problems The present inventors have paid attention to the fact that the existence state of the β phase of the plating layer has a great influence on the quality of the plated steel sheet, and I studied about the relationship. As a result, we found that by enriching the β phase in the plating surface layer and exposing the β phase directly to the plating layer surface, it is possible to improve the surface gloss of the plating layer and eliminate the hexagonal pattern, and further improve the corrosion resistance. did.

The present invention is based on this finding, and its characteristic constitution is a hot-dip Zn-Al alloy plated steel sheet containing 2 wt% or more and 6 wt% or less of Al, and the surface layer of the plating layer is continuous β. Phase, or a β phase and a eutectic phase, and a lower layer is a eutectic phase, and a β phase is exposed on the surface of the plating layer.

The reasons for limiting the hot-dip Zn-Al alloy-plated steel sheet of the present invention will be described. The state of the plating layer of the steel sheet of the present invention is compared with the state of the plating layer according to the prior art, and FIG.
2 will be described.

FIG. 1 is a photograph of a plating layer structure under a scanning electron microscope at a magnification of 600. The photograph on the left side is a photograph showing the sectional structure of the plating layer, and the photograph on the right side is a photograph showing the surface texture of the plating layer. . 1, (A) and (B) are micrographs of the plating layer structure of the plated steel sheet found in the prior art, and (C) to (E) are micrographs of the plating layer structure of the plated steel sheet of the present invention.

FIGS. 2A to 2E are diagrams schematically showing the β phase and the eutectic phase of the plating layer structure corresponding to FIGS. 1A to 1E, respectively.

In FIGS. 1 and 2, 1 is a β phase, 2 is a eutectic phase, and 3 is a base steel sheet. Also, FIGS. 1A and 1B
Since the surface texture of (1) is the whole eutectic phase and the surface texture of FIG. 1 (E) is the whole β phase, the schematic view of the surface texture of FIG. 2 is not shown for these.

In the case of FIGS. 1A and 2A, as already described, the β phase exists inside the plating layer, and only the eutectic phase exists in the surface layer portion of the plating layer. No β phase is observed.

In the case of FIG. 1 (B) and FIG. 2 (B), β phase is segregated and exists in the surface layer portion of the plating layer. However, according to the surface texture, the β phase is not exposed on the surface of the plating layer. This is probably because a thin eutectic phase film exists near the surface.

In the case of FIGS. 1C and 2C, the segregation of β phase in the surface layer portion of the plating layer is clearer. According to the surface texture, the β phase is exposed on the surface of the plating layer, but the exposed area ratio is low.

In the case of FIGS. 1D and 2D, the segregation of the β phase in the surface layer portion of the plating layer is clearer, and the proportion occupied by the β phase increases toward the surface. According to the surface texture, the exposed area ratio of β phase on the surface of the plating layer is higher.

In the case of FIGS. 1 (E) and 2 (E), the segregation of the β phase in the surface layer of the plating layer is remarkable, and the exposed area ratio of the β phase on the surface of the plating layer is 100% or close to it. ing.

The plated layer of the plated steel sheet of the present invention is characterized in that the β phase is segregated in the surface layer portion and the β phase is exposed on the surface thereof.

Since the β phase is exposed on the surface of the plating layer, the surface has excellent gloss and smoothness. Although the reason why the surface has excellent gloss and smoothness due to the exposure of the β phase, it is not always clear, but the β phase, which is the primary crystal phase, crystallized on the surface, Since the shrinkage of crystal grains is relaxed and the dents at the grain boundaries are less noticeable, and the β phase, which has a property closer to that of zinc, is exposed on the surface of the plating layer, the surface properties become closer to those of zinc plating. In addition to the above, it is considered that the surface state of the eutectic layer crystallized later due to the presence of the β phase of the primary crystal was changed and the decrease in gloss due to the eutectic phase was alleviated. When the exposed area ratio of the β phase is 30% or more, the gloss is more excellent.

Further, since a continuous eutectic phase is formed in the lower layer, the corrosion resistance is excellent. The exposed area ratio of β phase is 50%
In the above case, segregation of the β phase to the surface layer portion is further promoted to form a β phase having a high flatness, and the continuous eutectic layer formed in the lower layer becomes thicker, so that the corrosion resistance is further improved.

2 wt% or more and 6 wt% or more of Al in the plating layer
Must include: When Al is less than 2 wt%, the volume fraction of the β phase increases, the eutectic phase cannot be continuous, and part of the β phase reaches the steel sheet interface, so that the corrosion resistance decreases.

Further, when Al exceeds 6 wt%, the β phase disappears, the β phase segregates in the surface layer portion of the plating layer, which is a feature of the present invention, and the plating layer in which the β phase is exposed cannot be formed. .

In order to adjust the properties of the plating layer,
In some cases, component elements such as La, Ce, Ti, Mg, Sn, Zr and Pb are added. The Zn-Al-based alloy plating of the present invention includes those in which the component elements for the above purpose are added within a range not exceeding 1 wt%.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION There is no particular limitation on the steel sheet that serves as a plating base sheet for Zn-Al alloy plating of the present invention, and cold-rolled steel sheet manufactured by a conventional method or hot-rolled steel sheet that has been pickled and descaled is used. be able to. It may be a steel sheet subjected to continuous annealing and temper rolling after cold rolling.

The hot-dip Zn-Al alloy plated steel sheet of the present invention utilizes a hot-dip galvanizing facility which is commonly used, and a hot-dip Zn-Al alloy plating bath containing 2 wt% to 6 wt% of Al as a hot-dip bath component. The steel sheet is subjected to dip plating to manufacture. In order to adjust the properties of the plating layer, L
Component elements such as a, Ce, Ti, Mg, Sn, Zr and Pb may be added within a range not exceeding 1 wt%.

During immersion plating, the plating bath temperature is set to 42
The temperature of the steel sheet invading the plating bath is set to 0 to 500 ° C., the temperature of the steel sheet is set to 650 ° C. or less, plating is performed with sufficient immersion time, and the plated steel sheet is blown with air to be rapidly cooled.

The reason why the plating layer having the constitution of the present invention is formed by plating which secures sufficient immersion time as described above and by rapidly cooling the plated steel sheet by air blowing is not always clear. However, due to sufficient immersion time in the plating bath, an extremely thin alloy layer is formed at the steel plate interface, and the concentration of aluminum in the vicinity of the steel plate interface is reduced and the alloy layer passes through the alloy layer and the steel plate forms the plating layer. As a result of the diffusion of iron into the interior, the composition of the plating components becomes uneven in the thickness direction of the plating layer in the molten state, and when the plating layer is rapidly cooled and solidified from this state, the plating layer surface layer part In addition to the segregation of the β phase, it is considered that the β phase, which is the primary crystal phase, is extremely likely to precipitate on the surface of the plating layer.

After cooling, if necessary, shape correction by a leveler or skin pass, surface adjustment, chromate treatment, and oiling may be applied.

[0035]

Examples are shown below. Using a hot dip galvanizing facility, a low carbon steel plate with a plate thickness of 0.8 mm as a plating original plate,
A plating bath (No. 1 to No. 37) consisting of Al having the component composition shown in Table 1, 0.01 wt% of misch metal, and the balance Zn and unavoidable impurities, and a trace amount of Mg or Ti added to this plating bath. Plating bath (No. 38,
No. After being immersed in 39) for alloy plating, the coating weight was adjusted to 140 g / m ² and then cooled to produce a plated steel sheet. At that time, the running time of the steel sheet is changed to 1 by changing the running speed of the steel sheet to change the plating immersion time and the flow rate of cooling air blown onto the plated steel sheet.
For 60 seconds, the cooling rate of the steel sheet after plating is adjusted in the range of 3 to 30 ° C./second, and the segregation state of β phase of the plating layer, the plated steel sheet having different exposed area ratio of β phase to the surface of the plating layer Manufactured. Table 1 also describes plating conditions other than the composition of the plating bath components.

[0036]

[Table 1]

For the obtained plated steel sheet, the plating layer structure, the β phase segregation rate of the plating layer, the exposed area rate of the β layer on the surface of the plating layer, the surface gloss, the surface smoothness, and the corrosion resistance were investigated.

The exposed area ratio of the β phase on the surface of the plating layer was measured by image analysis by observing the surface texture of the plating layer of the test piece magnified 600 times.

The plating layer structure was obtained by buffing the cross section of the plating layer to finish it into a mirror surface, etching it with a 1% nital corrosive solution, and then observing the segregation state of the β phase under a microscope. Considering the microscopic observation result of the exposed state of the β phase, the following AE was evaluated according to the result.

A: As shown in FIG. 1 (A), β phase was observed inside the plating layer, and segregation was not observed in the surface layer portion.

B: As shown in FIG. 1B, the β phase is segregated in the surface layer portion of the plating layer, but is not exposed on the surface.

C: As seen in FIG. 1 (C), the β phase is segregated in the surface layer portion of the plating layer and is also exposed on the surface, and the exposed area ratio is less than 60%.

D: As shown in FIG. 1 (D), the segregation of the β phase in the surface layer portion of the plating layer is clearer, and the ratio of the β phase increases toward the surface. The exposed area ratio of β phase on the surface of the layer is 60% or more 95
Less than%.

E: As shown in FIG. 1 (E), segregation of β phase in the surface layer portion of the plating layer is remarkable, and the exposed area ratio of β phase on the surface of the plating layer is 95% or more.

Regarding the β-phase segregation state of the cross section of the plating layer, the ratio of the dimension (a) of the β phase crystal in the direction parallel to the surface of the plating layer to the dimension (b) in the thickness direction of the plating layer (a /
It was evaluated by the β phase flatness obtained from b). In the case where the plating layer structure was evaluated E, the β phase was an almost continuous layer, and the flatness could not be measured, so that value is not shown in Table 2.

The surface gloss is determined by measuring the gloss of the test piece at an incident angle of 20 in accordance with the gloss measurement method defined in JIS-Z8741.
Measured in °, the comparative example No. corresponding to the conventional method. Based on the gloss value of 1, the case where this gloss value was 1 was evaluated by relative comparison.

The surface smoothness was evaluated by measuring the surface roughness Rmax with a stylus type surface roughness meter.

The corrosion resistance was evaluated by the salt spray test specified in JIS-Z2371 and the time until the occurrence of red rust.

The results of the investigation are shown in Table 2.

[0050]

[Table 2]

No. 1 having an Al concentration in the plating layer within the range of the present invention. 1 to No. 33, no. 38, no. In comparison with No. 39, the example of the present invention in which the β layer is exposed on the surface of the plating layer has excellent gloss and a small surface roughness and excellent surface smoothness as compared with the comparative example. The exposed area ratio of β phase is 3
When it is 0% or more, the gloss is more excellent. Further, when the exposed area ratio of the β phase is 50% or more, the β phase segregation ratio is 2.5 or more, and the thickness of the continuous eutectic phase in the lower layer becomes thicker, so that the corrosion resistance by the salt spray test is 600 or less. More than the time, more excellent corrosion resistance.

No. 3 in which the Al concentration of the plating layer is below the range of the present invention. 34, no. In No. 35, the deposition fraction of the β phase increases, and the β phase reaching the interface with the steel sheet exists, so that the corrosion resistance is poor.

No. 3 in which the Al concentration of the plating layer exceeds the range of the present invention. 36, no. In No. 37, the β phase is not formed on the surface of the plating layer, and thus gloss and surface smoothness are poor.

[0054]

EFFECTS OF THE INVENTION The hot-dip Zn-Al alloy-plated steel sheet of the present invention has excellent surface gloss and smoothness in addition to corrosion resistance. It is possible to obtain a coated steel sheet.

[Brief description of drawings]

FIG. 1 is a micrograph instead of a drawing showing a cross-sectional structure and a surface structure of a plating layer.

FIG. 2 is a schematic diagram for explaining a plated layer structure of the micrograph shown in FIG.

[Explanation of symbols] 1 β phase 2 eutectic phase 3 steel sheet

Claims (1)

[Claims] 1. A hot-dip Zn-Al alloy-plated steel sheet containing Al in an amount of 2 wt% or more and 6 wt% or less, wherein the surface of the plating layer is a continuous β phase, or the β phase and a eutectic phase, and the lower layer is a eutectic phase. And a β phase is exposed on the surface of the plated layer, a hot dip Zn-Al alloy plated steel sheet.