JP3240910B2 - Hot-dip Zn-Al alloy coated steel sheet with smooth surface and excellent gloss - 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 alloy coated steel sheet, and more particularly to a hot-dip Zn-Al alloy coated steel sheet having a smooth surface and excellent gloss.

[0002]

2. Description of the Related Art Hot-dip coating on steel sheets is mainly performed for the purpose of improving corrosion resistance. Hot-dip coated steel sheets are mainly used for automobiles, building materials, and home appliances. In particular, hot-dip Zn-Al-based alloy plating in which a large amount of aluminum is added to normal hot-dip galvanizing has excellent corrosion resistance. Is widely used.

[0003] This hot-dip Zn-Al alloy plated steel sheet is
Used naked or painted. When used for these applications, molten Zn-
An Al-based alloy plated steel sheet is desired.

[0004] The structure of a plating layer of a conventional hot-dip Zn-Al-based alloy-coated steel sheet to which about 5 wt% of aluminum has been added is shown in FIG. As shown in the figure, a zinc-rich β phase (hereinafter, simply referred to as a β phase) 1 exists inside a plating layer, and a eutectic phase 2 exists around the β layer. In the process of cooling and solidifying the plating layer, the crystal grains of the Zn-Al-based alloy shrink, the grain boundaries are depressed, and a turtle pattern is generated. This turtle pattern not only lowers the smoothness of the plated steel sheet, but also becomes noticeable when painted, and has the problem of impairing the appearance of the painted steel sheet. Further, in addition to the poor luster of the eutectic phase existing on the surface of the plating layer, and the presence of a depression in the shape of a turtle pattern, the luster and smoothness of the plated steel sheet are inferior to those of the galvanized steel sheet.

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

(2) JP-A-2-73954 discloses that
Al is 3.0 to 10 wt%, and Si is 0.2 to 0.1 wt% of Al.
Plating is performed in a plating bath composed of 45%, Mg in an amount of 0.01 to 1.0 wt%, and the balance being Zn and 0.02 wt% or less, and the wettability with the steel sheet is improved by adding Si 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 the grain boundary is reduced to relatively uniformly solidify, thereby reducing the occurrence of a turtle pattern.

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

In addition, the following technique for improving the quality of a plating layer is disclosed by paying attention to the existence state of the β phase in the plating layer.

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

[0010]

However, the above (1)
According to the techniques disclosed in (3) to (3), although the turtle pattern becomes less conspicuous because the turtle pattern becomes finer or the depression becomes smaller, the turtle pattern is still present on the surface of the plating layer, and the gloss is reduced. Not improved.

In the technique disclosed in (4), there is no mention of a turtle pattern or luster. When the present inventors investigated, it was found that the plating layer had a turtle pattern and was inferior in smoothness and gloss.

The present invention has been made in view of the above-mentioned circumstances, and has been made of a molten Zn-A having a smooth surface and excellent gloss.
An object is 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 state of the β phase in the plating layer greatly affects the quality of the plated steel sheet, and have studied the state of the β phase in the plating layer and the plating quality. The relationship was deeply studied. As a result, we found that by enriching the β phase in the plating surface layer and exposing the β phase directly to the surface of the plating layer, it is possible to improve the surface gloss of the plating layer and eliminate the crevices pattern and further improve the corrosion resistance. did.

[0014] The present invention is based on this finding, the configuration and its feature is the molten Zn-Al alloy coated steel sheet containing Al or less 2 wt% or more 6 wt%, the plating
That the bare beta phase to the layer surface, beta-phase surface layer are continuous plating layer, the surface lower layer ing from eutectic phase excellent smooth, glossy
It is a hot-dip Zn-Al alloy plated steel sheet.

The reason for limiting the hot-dip Zn—Al-based alloy plated steel sheet of the present invention will be described. FIG. 1 shows the state of the plated layer of the steel sheet of the present invention in comparison with the state of the plated layer according to the prior art.
And FIG.

FIG. 1 is a photograph of a plating layer structure by a scanning electron microscope at a magnification of 600 times. The left photograph is a photograph showing a cross-sectional structure of the plating layer, and the right photograph is a photograph showing a surface structure of the plating layer. . In FIG. 1, (A) and (B) are photomicrographs of the structure of a plating layer of a plated steel sheet found in the prior art, and (C) is a plating in which a β phase is exposed on the plating layer surface.
Micrographs of the layer structure (reference figures), (D) and (E) show micrographs of the structure of the plated layer 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 the entire surface is a eutectic phase and the surface texture of FIG. 1 (E) is the entire β phase, a schematic diagram of the surface texture in FIG. 2 is not shown.

1A and 2A, as described above, the β phase exists inside the plating layer, and only the eutectic phase exists in the surface layer of the plating layer. No β phase is observed.

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

1 (D) and 2 (D), the segregation of the β phase in the surface layer of the plating layer is clearer, and the ratio of the β phase increases as approaching the surface. According to the surface texture, the exposed area ratio of the β phase on the plating layer surface is further increased, and the β phase is continuous .

In FIGS. 1E and 2E, 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.

In the plating layer of the plated steel sheet of the present invention, in addition to the β phase segregating in the surface layer, the β phase is exposed on the surface, and the β phase is continuous on the surface of the plating layer. There is a characteristic in that.

The β phase is exposed on the surface of the plating layer,
When the surface layer is a continuous β phase , the surface has excellent gloss and smoothness. The β phase is exposed on the surface, and the surface layer of the plating layer
The reason why the surface β is excellent in gloss and smoothness due to the continuous β phase is not always clear, but the β phase, which is the primary crystal phase, is crystallized on the surface, and the crystal during the formation of the eutectic phase is formed. The shrinkage of grains is alleviated, and the depressions at the grain boundaries are less noticeable, and the β phase having properties closer to zinc is exposed on the surface of the plating layer, and the surface layer of the plating layer is continuous β
In addition to the fact that the surface properties are close to those of zinc plating, the presence of the primary β phase changes the surface state of the eutectic layer that has crystallized later and changes the eutectic phase. It is considered that the decrease in gloss caused by the deterioration was alleviated .

Further, since a continuous eutectic phase is formed in the lower layer, it is excellent in corrosion resistance. Surface layer of plating layer is continuous
In the case of the β phase, the segregation of the β phase into the surface layer portion is further promoted to become a β phase having a high flattening ratio, so that the continuous eutectic layer formed in the lower layer becomes thicker, so that the corrosion resistance is further improved.

Al in the plating layer is 2 wt% or more and 6 wt%.
It is necessary to include the following. If the Al content is less than 2 wt%, the volume fraction of the β phase increases and the eutectic phase cannot be made continuous, and a part of the β phase reaches the steel sheet interface, so that the corrosion resistance decreases.

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 β phase is exposed on the surface. Continuous
It becomes impossible to form a plated layer which is a β phase .

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 in a range not exceeding 1 wt%.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION There is no particular limitation on the steel sheet to be used as the base plate for Zn-Al based alloy plating of the present invention, and a cold-rolled steel sheet manufactured by a conventional method or a hot-rolled steel sheet 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-based alloy-coated steel sheet of the present invention is prepared by using a commonly used hot-dip galvanizing equipment, and a hot-dip Zn—Al-based alloy-plating bath containing 2% to 6% by weight of Al as a hot-dip bath component. By immersion plating on the steel sheet. To adjust the properties of the plating layer, L
Component elements such as a, Ce, Ti, Mg, Sn, Zr, and Pb may be added in a range not exceeding 1 wt%.

At the time of immersion plating, the plating bath temperature was set to 42
Plating is performed by setting the temperature of the steel sheet in the plating bath to 650 ° C. or less at 0 to 500 ° C., ensuring a sufficient immersion time, and rapidly cooling the plated steel sheet by blowing air.

It should be noted that the reason why the plating layer of the present invention is formed by plating with a sufficient immersion time as described above and 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 sheet interface, and the concentration of aluminum decreases near the steel sheet interface and the steel sheet passes through the alloy layer to form a plating layer. As a result of the diffusion of iron into the plating layer, the composition of the plating component 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 portion In addition to the segregation of the β phase, it is considered that the β phase, which is the primary crystal phase, was extremely easily precipitated on the surface of the plating layer.

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

[0035]

Examples are shown below. In hot-dip galvanizing equipment, a low-carbon steel sheet with a sheet thickness of 0.8 mm was used as a plating base sheet.
A plating bath (No. 1 to No. 37) composed of Al and 0.01 wt% of misch metal having the component composition shown in Table 1 and the balance consisting of Zn and unavoidable impurities, and further added a trace amount of Mg or Ti to this plating bath. Plating bath (No. 38,
No. After immersion in (39) to perform alloy plating, the coating weight was adjusted to 140 g / m2 and then cooled to produce a plated steel sheet. At this time, the plating bath immersion time of the steel sheet is reduced by changing the running speed of the steel sheet to change the plating immersion time and the cooling air flow rate blown to the steel sheet after plating.
~ 60 seconds, the cooling rate of the steel sheet after plating is adjusted in the range of 3 ~ 30 ° C / second, and the state of segregation of β phase of the plating layer and the exposed area ratio of β phase to the plating layer surface are different. Manufactured. Table 1 also shows the plating conditions other than the composition of the plating bath components.

[0036]

[Table 1]

[0037] For the obtained plated steel sheet, plating layer structure, polarized flat rate of β phase in the plating layer, the exposed area ratio of the β layer of the plated layer surface, surface gloss, surface smoothness was examined corrosion resistance.

The exposed area ratio of the β phase on the surface of the plating layer was measured by enlarging the plating layer surface of the test piece by 600 times, observing the surface structure, and analyzing the image.

The plating layer structure is prepared by buffing the section of the plating layer to a mirror finish, etching it with a 1% nital etching solution, observing the segregation state of the β phase with a microscope, and examining the plating layer surface. In consideration of the results of microscopic observation of the exposed state of the β-phase, evaluations were made according to the following A to E according to the results.

A: As shown in FIG. 1 (A), the β phase was observed inside the plating layer and no segregation was observed in the surface layer.

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

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

[0043] D: As seen in FIG. 1 (D), a clearer segregation of β-phase in the surface layer of the plating layer, much cage increasing the ratio occupied by the β phase gets closer to the surface, of the further plating layer The β phase exposed on the surface is continuous, and the exposed area ratio of the β phase is 60% or more and less than 95%.

E: As can be seen from FIG. 1E, segregation of the β phase in the surface layer of the plating layer is remarkable, the β phase exposed on the surface of the plating layer is continuous, and the β phase is exposed. Those with an area ratio of 95% or more.

Regarding the segregation state of the β phase in the section of the plating layer, the ratio (a / a) of the dimension (a) of the β phase crystal in the direction parallel to the surface of the plating layer and the dimension (b) in the thickness direction of the plating layer.
The evaluation was made based on the β-phase flatness obtained from b). In the case of the plating layer structure having the evaluation E, the β phase was a substantially continuous layer, and the flatness could not be measured.

The surface gloss was determined by measuring the gloss of the test piece at an incident angle of 20 in accordance with the gloss measurement method specified in JIS-Z8741.
° and measured in Comparative Example No. corresponding to the conventional method. With reference to the gloss value of 1, the case where the gloss value was set to 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 performing a salt spray test specified in JIS-Z2371 and evaluating the time until the occurrence of red rust.

Table 2 shows the results of the investigation.

[0050]

[Table 2]

In the case of No. 1 where the Al concentration of the plating layer was within the range of the present invention. 1 to No. 33, no. 38, no. When compared for the 39, beta layer is exposed on the surface of the plating layer, because
The present invention example in which the surface layer of the coating layer is a continuous β phase is a comparative example.
Also, as compared with the reference examples , the gloss is excellent, the surface roughness is small, and the surface smoothness is excellent. Further, it is β Aihen flat rate 3.1 or more, the thickness of the lower continuous eutectic phase is thicker, corrosion resistance salt spray test more excellent corrosion resistance becomes over 600 hours.

In the case of No. 3 where the Al concentration of the plating layer was below the range of the present invention. 34, no. 35, the body volume fraction of β phase is increased, it becomes to exist β phase to reach the interface with the steel sheet, the corrosion resistance is inferior.

When the Al concentration of the plating layer exceeds the range of the present invention, 36, no. In No. 37, since no β phase is formed on the surface of the plating layer, gloss and surface smoothness are poor.

[0054]

The hot-dip Zn-Al alloy-plated steel sheet of the present invention has excellent surface gloss and smoothness in addition to corrosion resistance, so that it exhibits a good appearance when used naked, and has an excellent painted appearance when used painted. A coated steel sheet can be obtained.

[Brief description of the 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 plating layer structure in the micrograph shown in FIG.

[Explanation of symbols]

 1 β phase 2 Eutectic phase 3 Steel plate

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-110860 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. A hot-dip Zn—Al-based alloy-coated steel sheet containing 2 wt% or more and 6 wt% or less of Al, wherein a surface of a plating layer is provided.
Phase is exposed to the surface , and the surface layer of the plating layer is continuous β
Phase, the surface of the lower layer is characterized by comprising the eutectic phase smoothing
Hot-dip Zn-Al-based alloy plated steel sheet with excellent luster .