JP2002322527A - Al-Zn-Mg BASED ALLOY PLATED STEEL PRODUCT - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Al-Zn-Mg based alloy plated steel product which has excellent corrosion resistance. SOLUTION: On the plane analysis of the cross-section in a plating film on the surface of steel by using an X-ray microanalyzer, an Al-Zn phase in which Al and Zn are precipitated on the same region, and essentially consisting of those elements, a Zn-enriched phase segregated on the grain boundary of the Al-Zn phase, and essentially consisting of Zn, and an Mg-Si phase in which Mg and Si are precipitated on the same region have been confirmed. The Al-Zn phase occupies the volume of >=50% in the plating film, and at least a part of the Mg-Si phase is precipitated on the surface of the plating film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention belongs to the technical field of imparting excellent corrosion resistance to primary steel products made of steel and secondary steel products using these primary steel products in part or in whole. The present invention relates to a primary and / or secondary steel product obtained by hot-dip coating an Al-Zn-Mg-based alloy containing Mg as a main component containing Al and Zn as main components.

[0002]

2. Description of the Related Art A technique for improving the corrosion resistance by applying Zn plating to the surface of a steel base material has been known for a long time and is still widely used at present. Corrosion protection by Zn plating often depends on the sacrificial corrosion protection effect due to elution of Zn in preference to steel, and the service life of steel largely depends on the elution rate of Zn. For this reason, Zn plating alone is not always sufficient to use steel materials for a long period of time, and painting and periodic repairs are often used together.

[0003] As means for improving the corrosion resistance of Zn plating,
Al-Zn alloy plating comprising 25-75% by mass of Al and 0.5% by mass or more of the Al content of Si, with the balance being Zn
No. 17971. Further, Japanese Unexamined Patent Application Publication No. 2000-104153 discloses an Al-Zn-Mg alloy plating in which 1.0 to 5.0% by mass of Mg is added to the Al-Zn alloy plating. This plating is designed mainly to improve the corrosion resistance of the bent portion and the cut end, but when the present inventors conducted a corrosion test using a thin steel plate,
A tendency was observed that the corrosion resistance of the flat part also improved. But,
As a result of the test with an increased number of samples, some samples showed corrosion resistance several times higher than that of Al-Zn alloy plating, while others showed only the same level of corrosion resistance as Al-Zn alloy plating. Was.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide an Al-Zn-Mg-based alloy plated steel product having excellent corrosion resistance.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, first, the structure of plating was examined in detail. As a result, it was found that samples exhibiting excellent corrosion resistance had common features. The present invention has been made based on this finding. (1) When a cross section of the plating film on the steel surface is subjected to surface analysis using an X-ray microanalyzer, Al and Zn precipitate in the same region, and an Al-Zn phase mainly composed of these elements, Zn-rich Zn segregated at the grain boundaries of the Al-Zn phase
Phase and Mg-Si phase in which Mg and Si precipitated in the same region were recognized, Al-Zn phase occupied 50% by volume or more in the plating film, and at least a part of the Mg-Si phase was plated. Al-Zn-Mg alloy plated steel products characterized by being deposited on the film surface.

(2) The Al-Zn-Mg alloy plated steel product according to the above (1), wherein the area of the Mg-Si phase occupying the plating film surface is 70% or less. (3) Al and Zn precipitate in the same region, segregate at the grain boundaries of the Al-Zn phase mainly composed of these elements, and contain Zn mainly composed of Zn.
The Al-Zn-Mg-based alloy-plated steel product according to the above (1) or (2), wherein the phase contains a Zn-Mg-based intermetallic compound.

[0007]

1 and 2 show the results of a cross-sectional analysis of a plating sample embedded in a resin at a magnification of 1000 using an X-ray microanalyzer. In this figure, three types of layers are generally recognized from the secondary electron image (abbreviated as SE in the figure). These layers are ground iron, plating and embedded resin in order from the bottom. Focusing on the plating layer,
The plating of the present invention accounts for a large portion of the total plating
It can be seen that there are (a) phase, (b) phase segregated at the grain boundary of (a) phase, and (c) phase scattered in the plating layer and partially deposited on the plating surface.

Further, the following can be found by comparing the phases (a) to (c) with the characteristic X-ray image. That is, (1) Al and Zn are detected in a region corresponding to the (a) phase, and other elements are hardly detected. Furthermore, when the contents of Al and Zn in this phase were quantified using an X-ray microanalyzer, the total of the two was at least 50% by mass or more, and usually at least 80% by mass. From these facts, the (a) phase contains Al and Zn in a total amount of 50% by mass or more, preferably contains 80% by mass or more, and A mainly containing these metals.
It is an l-Zn phase. The upper limit of the content of Al and Zn in the (a) phase is not particularly limited.

(2) In a region corresponding to the (b) phase, Mg is slightly recognized, but most is Zn. Then, as a result of quantification using an X-ray microanalyzer, the content of Zn in this phase is at least 50% by mass or more, usually 8% or more.
It was 0% by mass or more. From these facts, the (b) phase has a Zn content of 5%.
This Zn-rich phase mainly contains Zn at 0% by mass or more, preferably at 80% by mass or more. Incidentally, Z in the phase (b)
The upper limit of the n content is not particularly specified. Under the conventional plating conditions, this Zn-rich phase was not formed as a clear phase at the grain boundary of the Al-Zn phase, whereas in the present invention, this Zn-rich phase was observed in FIG. As described above, the present invention is characterized in that it does not exist simply at the grain boundaries but exists as a clear phase. Although the width of the Zn-rich phase is not limited, it is usually present in the order of 1 to 2 μm, and even if it is generalized, it can be present in a width of at least 0.3 μm or more.

(3) The region corresponding to the (c) phase is the Mg-Si phase since Mg and Si are simultaneously detected. The feature of the plating of the present invention is that a part of the Mg-Si phase is exposed on the plating surface. As will be described later, although the clear reason is unknown, the formation of the (b) phase as an independent phase allows Mg-
It is considered that a part of the Si phase is formed by being exposed on the plating surface. Exposing part of the Mg-Si phase to the plating surface has the effect of improving corrosion resistance.

The relationship between the cross-sectional structure of the plating layer and the corrosion resistance will be described with reference to FIGS. Although the ratio of Al-Zn phase in the entire plating layer is high, this is an essential condition for developing excellent corrosion resistance.
Volume% or more is required. If it is less than this, the effect of increasing corrosion resistance by adding Mg becomes small. The upper limit is not particularly specified, but is usually about 90 to 95% by volume.

The Zn-rich phase forms along the grain boundaries of the Al-Zn phase, but this does not itself have a great effect on the corrosion resistance. However, by the formation of this phase, Mg
-Si phase is easily segregated on the plating surface. From the results of analysis by wide-angle X-ray diffraction, the Mg-Si phase was found to be an intermetallic compound Mg ₂ Si
Could be identified. This Mg ₂ Si is a compound that is easily eluted in water and can keep the plating surface alkaline, so that the corrosion products are stably maintained as a dense basic oxide film, so that the corrosion resistance of the plating is improved. like this
In order to utilize the function of Mg-Si phase most effectively, Mg-Si
It is necessary that at least a part of the phase is segregated on the plating surface, and for that purpose, it is important that the Zn-rich phase be formed along the grain boundaries of the Al-Zn alloy phase. Thus, Mg-S
The segregation of at least a part of the i-phase on the plating surface improves the corrosion resistance.

Next, the relationship between the structure of the plating surface and the corrosion resistance will be described. 3 and 4 show the element distribution on the surface of the plating sample of FIGS. 1 and 2 using an X-ray microanalyzer (8
(Times 00). From FIG. 2, it can be seen that, on the plating surface, the phase segregated at the grain boundaries of the Al-Zn phase is Mg-
This indicates that the Si phase is dominant. As described above, the Mg-Si phase has a function of stabilizing the basic film by being appropriately eluted, but when the elution becomes severe, the plating surface becomes strongly alkaline and promotes the elution of Zn and Al. Become like If the area ratio of the Mg-Si phase in the entire plating surface exceeds 70%, such adverse effects become dominant, and the corrosion resistance is reduced. Therefore, the area ratio of the Mg-Si phase in the entire plating surface is preferably 70% or less. Although the lower limit is not particularly specified, it is preferable that at least 2 to 3% is present, and usually 5% or less.
About 10%.

Third, the relationship between a small amount of Mg contained in the Zn-rich phase and the corrosion resistance described briefly in the description of FIGS. 1 and 2 will be described. From the results of analysis by the wide-angle X-ray diffraction method, Mg in the Zn-rich phase was found to be Mg-Zn based intermetallic compound (MgZn ₂ ,
Mg ₂ Zn ₁₁ ) was found to have formed. Here, the Zn-rich phase is the Zn-rich phase described with reference to FIGS. These Mg-Zn based intermetallic compounds are not as strong as the Mg-Si phase, but can contribute to the improvement of the corrosion resistance of the plating, and have the effect of reducing the dissolution rate of the Zn-rich phase in particular.
Therefore, in order to impart further corrosion resistance to plating, it is preferable that these intermetallic compounds are present.
The content of these Zn-Mg-based intermetallic compounds in the Zn-rich phase is not particularly limited, but is usually about 0.5 to 10% by volume.

Finally, a method for manufacturing a plated product of the present invention will be described. The present invention has been disclosed previously by the applicant.
As a result of exploring conditions for realizing excellent corrosion resistance on the basis of Al-Zn-Mg-based plating, they have found that excellent corrosion resistance can be achieved when the above-described (claim) structure is formed. The basic conditions for realizing such an organization are described below.

The plating of the present invention uses a quaternary alloy bath composed of Al, Zn, Mg and Si. Although the ratio of these elements is not particularly specified, Al: 25% by mass or more, Mg: 10
% Or less, preferably 0.5% by mass or more with respect to Si: Al, and the balance being Zn. In particular, when the added amount of Mg exceeds 10% by mass, the plating becomes brittle and cracks easily occur on the plating surface.

The bath is heated above the melting point, preferably above the melting point.
Set to the high temperature side of about 40-50 ° C, and immerse the steel material for plating. When plating is performed in the air, the bath surface may be maintained in an inert gas atmosphere by nitrogen blowing or the like in order to suppress the generation of top dross. Also, the immersion time is preferably determined appropriately according to the shape and size of the steel material,
As a rough guide, it takes about 3 seconds for thin steel sheets, about 30 seconds for wires, and about 5 to 10 minutes for shaped steel and steel pipes.

After pulling the steel material out of the plating bath, it is cooled as rapidly as possible at a rate exceeding 40 ° C./sec. Rapid cooling is essential to form the above-described structure in the plating layer. The steel products of the present invention include not only primary steel products made of steel, but also secondary steel products.

Here, the primary steel product is, for example, a steel material having a flat plate, a corrugated sheet, a band, a wire, a rod, a cross section of a circular shape,
Square, elongation, pipe arched, arched steel pipes, H-shaped, H-shaped lip, I-shaped, T-shaped, Z-shaped
Shaped steel with shape, lip Z shape, equilateral angle shape, unequal angle shape, unequal thickness angle shape, spherical flat shape, groove shape, lip groove shape, hat shape, U shape, H shape, Z shape and straight line Steel sheet pile, steel pipe sheet pile, expanded metal, deck plate,
T-shape and L-shape steel pipe joints, bolts, nuts, washers, etc.

A secondary steel product is a steel product formed by using part or all of these primary steel products. Examples thereof include a welding wire mesh, a diamond wire mesh, a crimp wire mesh, a turtle wire mesh, a woven wire mesh, Japanese basket, parallel wire strand, wire rope, herb wire, stranded wire, deformed wire rope, formwork, clamp, clamp, temporary scaffold,
Grating, yo wall, segment, siding,
Panels, baskets, fences, suspension bridges, cable-stayed bridges, steel bridges, power transmission towers, radio towers, monitoring towers, steel columns for supporting railway electric overhead lines, beams for supporting railway electrical overhead lines, steel houses, lighting,
Utility poles, road signposts, fluid transport piping, culvert drains, guardrails, steel sidewalks, dams, etc.

[0021]

Next, the plated steel product of the present invention is described below.
A detailed description will be given based on an embodiment. In this example, a test apparatus capable of maintaining a nitrogen atmosphere throughout a series of plating operations was used. (Example 1) A plating bath containing Al, Mg, and Si at an arbitrary concentration and a balance consisting of Zn and a small amount of unavoidable impurities was prepared and incorporated in the above-described test apparatus, and the plating bath surface was maintained in a nitrogen atmosphere. Was.

Next, a cold-rolled steel sheet (aluminum-killed) of 150 × 70 × 0.8 mm was degreased using a commercially available silicate-based alkaline degreasing agent until water repellency disappeared. This was immersed in a 10% hydrochloric acid aqueous solution (60 ° C.) for about 2 minutes, washed with water, and dried. This steel plate was mounted on the plating test equipment described above, and 10% H
After heating to 800 ° C. in a 2-N ₂ atmosphere and holding it for 10 seconds, it was transferred to a nitrogen atmosphere and immersed in the plating bath when the surface of the steel sheet dropped to the same temperature as the plating bath. After immersion for 3 seconds, it was taken out of the plating bath and cooled at an arbitrary cooling rate in a nitrogen atmosphere.

Using a X-ray microanalyzer, a part of the plated steel sheet was examined for the surface distribution of Al, Zn, Mg and Si on the plated cross section. Further, the plated steel sheet produced in this manner was cut into a size of 100 × 50 mm, and a salt spray test specified in JIS C 0023 was performed for 4 weeks to measure corrosion weight loss. Table 1 shows the results together with the structural characteristics of the plating cross section.

From Table 1, it can be seen that the plating layers are Al-Zn phase, Zn-rich phase and Mg
It has been found that good corrosion resistance can be obtained when the following three conditions are simultaneously satisfied. That is, (a) Al-Zn phase occupies 50% by volume or more in the plating film, (b) Zn-rich Zn phase segregates along the grain boundary of Al-Zn phase,
(c) The case where at least a part of the Mg-Si phase is precipitated on the plating film surface.

The criterion for determining the quality of corrosion resistance is 1.0 g / m
^Assuming that ² , when at least one of the above conditions was not satisfied, the corrosion weight loss exceeded 1.0 g / m ² , and a decrease in corrosion resistance was observed.

[0026]

[Table 1]

(Embodiment 2) In this embodiment, (A bath) Al: 5
A plating bath containing 3% by mass, 3% by mass of Mg, and 0.8% by mass of Si, with the balance being Zn and trace amounts of unavoidable impurities;
Bath) A plating bath was prepared which contained 53% by mass of Al, 5% by mass of Mg, and 10% by mass of Si, and the balance was composed of Zn and a small amount of unavoidable impurities. Using these plating baths, the plating rate was fixed at 45 ° C./sec , and a plated steel sheet was produced in the same manner as in Example 1 above.

The cross section of these coated steel sheets was analyzed using an X-ray microanalyzer, and Al and Zn were deposited in the same region, and an Al-Zn-based phase mainly composed of these elements was added to the plating film by 50 volume. % Of the Al-Zn-based phase segregates at the grain boundary of the Al-Zn-based phase, there is a Zn-rich Zn phase mainly composed of Mg, and part of the Mg-Si phase in which Mg and Si are precipitated in the same region. It was confirmed that it was deposited on the plating film surface.

The surface of the plated steel sheet produced in this way is
The pieces were cut into a size of 100 × 50 mm, and subjected to a salt spray test specified in JISC0023 for 4 weeks to measure the corrosion loss.
As a result, while the sample is corrosion weight loss with A bath was 0.6 g / m ^2, those using B bath was 1.5 g / m ^2. On the other hand, when the element distribution on the surface of these samples was analyzed using an X-ray microanalyzer, the Mg-Si phase was precipitated in all samples, but in the former, the deposition area of the Mg-Si phase was about 80% vs. 40%
Accounted for the degree.

Thus, Mg-S occupying the entire plating surface
It was found that the corrosion resistance was further improved by setting the area ratio of the i-phase to 70% or less. (Example 3) In this example, (A bath) Al: 53% by mass, Mg:
A plating bath containing 3% by mass and 0.8% by mass of Si with the balance being Zn and trace amounts of unavoidable impurities, and (C bath) Al: 63% by mass
%, Mg: 0.2% by mass and Si: 0.9% by mass, with the balance being Zn
And a plating bath comprising a trace amount of inevitable impurities.
Using these baths, 45 ° C. The cooling rate after plating
/ Sec , and a plated steel sheet was produced in the same manner as in Example 1 above.

The surfaces of these plated steel sheets were subjected to surface analysis using an X-ray microanalyzer. In each case, precipitation of Mg-Si phase was recognized along the grain boundaries of Al-Zn phase, and the area ratio of the entire surface was It was less than 70%. In addition, from analysis of the cross-section by an X-ray microanalyzer, both, Al and Zn are precipitated in the same region, and an Al-Zn-based phase mainly containing these elements occupies 50% by volume or more in the plating film, There is a Zn-rich Zn phase mainly segregated at the grain boundaries of the Al-Zn-based phase, and part of the Mg-Si phase in which Mg and Si are precipitated in the same region is deposited on the plating film surface. I confirmed that. Of the former, Mg was found to be present in the Zn-rich Zn phase.As a result of composition analysis using an X-ray microanalyzer, MgZn
_Although identified as the intermetallic compound of No. ₂ , Mg was not found in the Zn-rich phase in the latter.

The surface of the plated steel sheet produced in this way is
The pieces were cut into a size of 100 × 50 mm, and subjected to a salt spray test specified in JISC0023 for 4 weeks to measure the corrosion loss.
As a result, the sample using the bath A had a corrosion loss of 0.6 g / m ² , whereas the sample using the bath C had a corrosion loss of 1.2 g / m ² , indicating a decrease in corrosion resistance. Thus, Zn-M
It was found that the corrosion resistance was improved by mixing the g-based intermetallic compound.

[0033]

The present invention is an Al-Zn-Mg-based alloy-plated steel product containing Mg and Si, and by using this technique, it is possible to provide an unprecedented extremely high corrosion-resistant steel product. .

[Brief description of the drawings]

FIG. 1 is a SEM image of a cross section of a plating of an example and Al and S
It is an EPMA image of i.

FIG. 2 shows EP of Zn, Mg, and Fe in a plating cross section of an example.
It is a MA image.

FIG. 3 is a SEM image of a plating surface and Al and S of an example.
It is an EPMA image of i.

FIG. 4 shows EP of Zn, Mg, and Fe on a plating surface of an example.
It is a MA image.

[Explanation of symbols]

 a ... Al-Zn phase b ... Zn-rich phase c ... Mg-Si phase

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K027 AA02 AA03 AA05 AA06 AA07 AA08 AA12 AA15 AB05 AB32 AC72 AE03 AE36

Claims (3)

[Claims] When a cross-section of a plating film on a steel material is subjected to surface analysis using an X-ray microanalyzer, when a cross-section is analyzed,
And Zn precipitate in the same region, and Al
-Zn phase, Zn-rich Zn phase mainly segregated at the grain boundaries of the Al-Zn phase, and Mg-Si phase in which Mg and Si are precipitated in the same region, and the Al-Zn phase Occupies 50% by volume or more in the plating film, and at least a part of the Mg-Si phase is precipitated on the plating film surface. 2. The Al according to claim 1, wherein the area occupied by the Mg-Si phase on the plating film surface is 70% or less.
-Zn-Mg alloy plated steel products. 3. Al and Zn are precipitated in the same region, segregated at the grain boundaries of an Al—Zn phase mainly composed of these elements, and a Zn-rich Zn phase mainly composed of Zn becomes a Zn—Mg based intermetallic compound. The Al-Zn-Mg-based alloy-plated steel product according to claim 1 or 2, further comprising: