JP7097488B2 - Fused Al-Zn-Si-Mg based plated steel sheet - Google Patents

Description

The present invention relates to a molten Al-Zn-Si-Mg-based plated steel sheet that is stable and has excellent corrosion resistance.

It is known that hot-dip Al-Zn-based plated steel sheets represented by 55% Al-Zn-based steel sheets exhibit high corrosion resistance among hot-dip galvanized steel sheets because they have both the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al. Has been done. Therefore, due to its excellent corrosion resistance, hot-dip Al-Zn plated steel sheets are mainly used in the field of building materials such as roofs and walls that are exposed to the outdoors for a long period of time, and in the field of civil engineering and construction such as guard rails, wiring pipes, and soundproof walls. .. In particular, there is an increasing demand for materials with excellent corrosion resistance and maintenance-free materials under harsher usage environments such as acid rain due to air pollution, spraying of snowmelt to prevent road freezing in snowy areas, and coastal area development. Therefore, in recent years, the demand for molten Al-Zn-based plated steel sheets has been increasing.

The plating film of the molten Al-Zn-based plated steel sheet is composed of the portion where Al containing Zn in hypersaturation is solidified into a dendrite (α-Al phase) and the Zn-Al co-crystal structure existing in the dendrite gap (interdendrite). It is characterized by having a structure in which a plurality of α-Al phases are laminated in the film thickness direction of the plating film. Due to such a characteristic film structure, the corrosion progress path from the surface becomes complicated, so that the corrosion does not easily proceed, and the hot-dip Al-Zn-based plated steel sheet becomes a hot-dip galvanized steel sheet having the same plating film thickness. It is also known that better corrosion resistance can be achieved.

Attempts have been made to further extend the life of such hot-dip Al-Zn-based plated steel sheets, and molten Al-Zn-Si-Mg-based plated steel sheets to which Mg has been added have been put into practical use.
As such a molten Al-Zn-Si-Mg-based plated steel sheet, for example, Patent Document 1 contains an Al-Zn-Si alloy containing Mg in a plating film, and the Al-Zn-Si alloy is 45 to An alloy containing 60% by weight elemental aluminum, 37-46% by weight elemental zinc and 1.2-2.3% by weight Si, wherein the Mg concentration is 1-5% by weight, molten Al-Zn-Si-. Mg-based plated steel sheets are disclosed.
Further, in Patent Document 2, the plating film contains at least one of 2 to 10% Mg and 0.01 to 10% Ca to improve corrosion resistance and protect the base steel sheet after it is exposed. A hot-dip Al-Zn-Si-Mg-based plated steel sheet for the purpose of enhancing the above is disclosed.
Further, in Patent Document 3, Mg: 1 to 15%, Si: 2 to 15%, Zn: 11 to 25% are contained in mass%, and the balance forms a coating layer composed of Al and unavoidable impurities. , Molten Al-Zn-Si-Mg system with improved corrosion resistance of flat plate and end face by reducing the size of intermetallic compounds such as Mg ₂ Si phase and Mg Zn ₂ phase present in the plating film to 10 μm or less. Plated steel plates are disclosed.

Since the above-mentioned molten Al-Zn-based plated steel sheet has a beautiful appearance with a spangle pattern of white metallic luster, it is often used without painting, and the actual situation is that there is a strong demand for its appearance. be. Therefore, techniques for improving the appearance of hot-dip Al-Zn-based plated steel sheets have also been developed.
For example, Patent Document 4 discloses a molten Al-Zn-Si-Mg-based plated steel sheet in which wrinkle-like unevenness defects are suppressed by containing 0.01 to 10% Sr in the plating film.
Further, Patent Document 5 also discloses a molten Al-Zn-Si-Mg-based plated steel sheet in which mottled defects are suppressed by containing 500 to 3000 ppm of Sr in the plating film.
Further, Patent Document 6 discloses a molten Al-Zn-Si-Mg-based plated steel sheet in which 0.001 to 1.0% of Sr is contained in the plating film to achieve both surface appearance and corrosion resistance.
Further, Patent Document 7 also describes a molten Al-Zn-Si-Mg-based plated steel sheet in which 0.001 to 1.0% of Sr is contained in the plating film to achieve both surface appearance and corrosion resistance between the flat plate portion and the processed portion. Is disclosed.
Furthermore, Patent Document 8 also discloses a molten Al-Zn-Si-Mg-based plated steel sheet that achieves both surface appearance and corrosion resistance by containing 0.01 to 0.2% Sr in the plating film. ..
Further, Patent Document 9 discloses a molten Al-Zn-Si-Mg-based plated steel sheet having improved corrosion resistance by controlling the Si and Mg concentrations in the plating film at a specific ratio.

Japanese Patent No. 5020228 Japanese Patent No. 5000039 Japanese Unexamined Patent Publication No. 2002-12959 Japanese Patent No. 3983932 Special Table 2011-514934 International Publication No. 2020/179147 International Publication No. 2020/179148 Japanese Unexamined Patent Publication No. 2020-143370 International Publication No. 2016/140370

However, the technique of containing Mg in the plating film as disclosed in Patent Documents 1 to 3 does not always bring about the improvement of corrosion resistance uniquely.
In the molten Al-Zn-Si-Mg-based plated steel sheets disclosed in Patent Documents 1 to 3, the corrosion resistance is improved only by containing Mg in the plating component. The characteristics of the intermetallic phase were not considered, and it was not possible to uniformly talk about the superiority or inferiority of corrosion resistance. Therefore, even when a molten Al-Zn-Si-Mg-based plated steel sheet is manufactured using the same plating bath composition, the corrosion resistance varies when the corrosion acceleration test is carried out, and the Al-Zn-based plated steel sheet to which Mg is not added is present. There was a problem that it did not necessarily have an advantage over.
Similarly, in order to improve the appearance of the plating, it is not always possible to eliminate the wrinkle-like unevenness defect only by adding Sr to the plating film, and the molten Al-disclosed in Patent Documents 4 to 8 can be eliminated. In some cases, the Zn-Si-Mg-based plated steel sheet could not achieve both corrosion resistance and appearance. In addition, since Mg is an element that easily oxidizes, Mg contained in the plating bath generates an oxide (top dross) near the bath surface, and in the case of hot-dip plating, the bath of the plating bath over time. FeAl-based compounds (bottom dross) containing iron that are unevenly distributed in the middle or bottom may be generated, and these dross adhere to the surface of the plating film and cause convex defects, which impairs the appearance of the surface of the plating film. There was also a fear.

In addition, when the steel sheet is plated with a bath in which Mg is added to the molten Al-Zn-Si bath, Mg ₂ Si phase, Mg Zn ₂ phase, and Si phase are deposited in the plating film in addition to the α-Al phase. It is known. However, the effect of the precipitation amount and abundance ratio of each phase on the corrosion resistance has not been clarified.
In the molten Al-Zn-Si-Mg-based plated steel sheet disclosed in Patent Document 9, the concentration of Si and Mg is controlled at a specific ratio, and the corrosion resistance is improved by eliminating the precipitation of the Si phase in the plating film. However, it cannot always be said that the Si phase can be suppressed, and even if the formation of the Si phase in the plating film can be suppressed, excellent corrosion resistance may not be obtained, which is technically incomplete. Met.

In view of such circumstances, it is an object of the present invention to provide a molten Al-Zn-Si-Mg-based plated steel sheet having stable and excellent corrosion resistance.

As a result of studies to solve the above problems, the present inventors have found the Mg ₂ Si phase, Mg Zn ₂ phase, and Si phase formed in the plating film of the molten Al-Zn-Si-Mg-based plated steel sheet. It was found that the amount of precipitation increases or decreases depending on the balance of each component in the plating film and the formation conditions of the plating film, the abundance ratio changes, and one of the phases may not precipitate depending on the balance of the composition. In addition, the corrosion resistance of the molten Al-Zn-Si-Mg-based plated steel sheet changes depending on the abundance ratio of these phases, and the corrosion resistance is stable especially when there are more MgZn ₂ phases than the Mg ₂ Si phase or Si phase. Investigated to improve.
However, for these Mg ₂ Si phase and Si phase, a general method such as a scanning electron microscope is used to observe the secondary electron image or backscattered electron image of the plating film from the surface or cross section. It is known that it is very difficult to distinguish the difference between the phases. As a method for more detailed analysis, it is possible to obtain microscopic information by observing with a transmission electron microscope, but Mg ₂ Si phase and Si phase that influence macroscopic information such as corrosion resistance and appearance. It was not possible to grasp the abundance ratio of.
Therefore, as a result of further diligent research, the present inventors focused on the X-ray diffraction method and quantified the phase abundance ratio by using the intensity ratio of specific diffraction peaks for the Mg ₂ Si phase and the Si phase. In addition to being able to stably achieve excellent corrosion resistance when the Mg ₂ Si phase and Si phase satisfy a specific abundance ratio in the plating film, it also suppresses the occurrence of dross and has good surface appearance. I found that I could secure it.
Furthermore, the present inventors control the abundance ratio of Mg ₂ Si phase, Si phase, etc. in the molten Al-Zn-Si-Mg-based plated steel sheet, and then control the Sr concentration in the bath to wrinkle. It was also found that a plated steel sheet with excellent surface appearance can be obtained by reliably suppressing the occurrence of irregularities.

The present invention has been made based on the above findings, and the gist thereof is as follows.
1. 1. A molten Al-Zn-Si-Mg-based plated steel sheet with a plating film.
The plating film contains Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass and Mg: 1.0 to 10.0% by mass, and has a composition in which the balance is Zn and unavoidable impurities.
A molten Al-Zn-Si-Mg-based plated steel sheet characterized in that the diffraction intensities of Si and Mg ₂ Si in the plating film by the X-ray diffraction method satisfy the following relationship (1).
Si (111) / Mg ₂ Si (111) ≤ 0.8 ・ ・ ・ (1)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm)

2. 2. The molten Al-Zn-Si-Mg-based plated steel sheet according to 1 above, wherein the diffraction intensity of Si in the plating film by the X-ray diffraction method satisfies the following relationship (2).
Si (111) = 0 ・ ・ ・ (2)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm)

3. 3. The molten Al-Zn-Si-Mg-based plated steel sheet according to 1 or 2, wherein the plated film further contains Sr: 0.01 to 1.0% by mass.

4. The molten Al-Zn-Si-Mg-based plated steel sheet according to any one of 1 to 3, wherein the content of Al in the plating film is 50 to 60% by mass.

5. The molten Al-Zn-Si-Mg-based plated steel sheet according to any one of 1 to 4, wherein the content of Si in the plating film is 1.0 to 3.0% by mass.

6. The molten Al-Zn-Si-Mg-based plated steel sheet according to any one of 1 to 5, wherein the content of Mg in the plating film is 1.0 to 5.0% by mass.

According to the present invention, it is possible to provide a molten Al-Zn-Si-Mg-based plated steel sheet having stable and excellent corrosion resistance.

It is a figure for demonstrating the flow of the Japanese Automotive Standards Organization compound cycle test (JASO-CCT).

The molten Al-Zn-Si-Mg-based plated steel sheet of the present invention has a plating film on the surface of the steel sheet. The plating film contains Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass and Mg: 1.0 to 10.0% by mass, and has a composition in which the balance is Zn and unavoidable impurities.

The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, in view of the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45% by mass, dendrite solidification of Al occurs, and a plating film structure mainly composed of the dendrite solidified structure of the α-Al phase can be obtained. By adopting a structure in which the dendrite solidified structure is laminated in the film thickness direction of the plating film, the corrosion progress path becomes complicated and the corrosion resistance of the plating film itself is improved. Further, the more the dendrite portions of the α-Al phase are laminated, the more complicated the corrosion progress path becomes, and the more difficult it is for corrosion to reach the base steel sheet. Therefore, the corrosion resistance is improved, and the Al content is 50 mass. % Or more is preferable. On the other hand, when the Al content in the plating film exceeds 65% by mass, most of Zn changes to a structure that is solid-dissolved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, so that Al-Zn -The corrosion resistance of Si-Mg plating deteriorates. Therefore, the Al content in the plating film needs to be 65% by mass or less, preferably 60% by mass or less.

Si in the plating film mainly suppresses the growth of Fe-Al-based and / or Fe-Al-Si-based interfacial alloy layers generated at the interface with the underlying steel sheet, and does not deteriorate the adhesion between the plating film and the steel sheet. Added for the purpose. When the steel plate is actually immersed in an Al-Zn-based plating bath containing Si, Fe on the surface of the steel plate and Al or Si in the bath undergo an alloying reaction, and Fe-Al-based and / or Fe-Al-Si-based. The metal-metal compound layer is formed at the base steel plate / plating film interface. At this time, the Fe-Al-Si alloy has a slower growth rate than the Fe-Al alloy, so the ratio of the Fe-Al-Si alloy is high. The higher the value, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film needs to be 1.0% by mass or more. On the other hand, when the Si content in the plating film exceeds 4.0% by mass, not only the growth suppressing effect of the above-mentioned interfacial alloy layer is saturated, but also the presence of an excess Si phase in the plating film promotes corrosion. Therefore, the Si content should be 4.0% or less. Further, the content of Si in the plating film is preferably 3.0% or less from the viewpoint of suppressing the presence of an excessive Si phase. The Si content is preferably 1.0 to 3.0% by mass from the viewpoint of easily satisfying the relational expression (1) described later in relation to the Mg content described later.

The plating film contains 1.0 to 10.0% of Mg. By containing Mg in the plating film, the above-mentioned Si can be present in the form of an intermetallic compound of Mg ₂ Si phase, and the promotion of corrosion can be suppressed.
Further, when Mg is contained in the plating film, MgZn ₂ phase, which is an intermetallic compound, is also formed in the plating film, and the effect of further improving the corrosion resistance can be obtained. When the Mg content in the plating film is less than 1.0% by mass, Mg is used for solid solution to the α-Al phase, which is the main phase, rather than the formation of the intermetallic compound (Mg ₂ Si, MgZn ₂ ). Therefore, sufficient corrosion resistance cannot be ensured. On the other hand, when the Mg content in the plating film is large, the effect of improving the corrosion resistance is saturated and the workability is lowered due to the weakening of the α-Al phase, so the content is set to 10.0% or less. Further, the Mg content in the plating film is preferably 5.0% by mass or less from the viewpoint of suppressing the generation of dross during plating formation and facilitating the management of the plating bath. From the viewpoint of easily satisfying the relational expression (1) described later in relation to the Si content, the Mg content is preferably 3.0% by mass, and compatibility with dross suppression is taken into consideration. Then, it is more preferable to set the Mg content to 3.0 to 5.0% by mass.

In the molten Al-Zn-Si-Mg-based plated steel sheet of the present invention, the diffraction intensities of Si and Mg ₂ Si in the plating film by the X-ray diffraction method need to satisfy the following relationship (1). ..
Si (111) / Mg ₂ Si (111) ≤ 0.8 ・ ・ ・ (1)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm), Mg ₂ Si (111): Diffraction strength of Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm)

As described above, in the present invention, it is important to control the abundance ratio of the Mg ₂ Si phase and the Si phase generated in the plating film due to the inclusion of Mg and Si to a specific ratio. The effects of these on corrosion resistance are currently being investigated and there are many unclear points, but the following mechanisms are presumed.

When the molten Al-Zn-Si-Mg galvanized steel sheet is exposed to a corrosive environment, the above-mentioned intermetallic compound dissolves preferentially over the α-Al phase, and as a result, the vicinity of the corrosive product formed is Mg. Will be a rich environment. It is estimated that in such an Mg-rich environment, the corrosion products formed are less likely to be decomposed, and as a result, the protective effect of the plating film is enhanced. In addition, the effect of improving the protective effect of the plating film is more reliably expressed when Si in the plating film exists as the Mg ₂ Si phase instead of the Si phase, so that the abundance ratio of the Si phase to the Mg ₂ Si phase is lowered. Is considered to be effective.

The abundance ratio of Mg ₂ Si and Si in the plating film has a relationship (1): Si (111) / Mg ₂ Si (111) ≤ 0.8 using the diffraction peak intensity obtained by the X-ray diffractometry. It is necessary to satisfy, but when the abundance ratio of Mg ₂ Si and Si in the plating film does not satisfy the relationship (1), that is, when Si (111) / Mg ₂ Si (111)> 0.8, the plating is performed. Since the amount of Si phase present in the film increases, the above-mentioned environment rich in Mg cannot be obtained in the vicinity of the corrosion product, and it becomes difficult to obtain the effect of improving the protective effect of the plating film. From the same viewpoint, the abundance ratio of Si to Mg ₂ Si (Si (111) / Mg ₂ Si (111)) is preferably 0.5 or less, more preferably 0.3 or less, and 0.2 or less. Is particularly preferable.
Regarding the abundance ratio of Mg ₂ Si and Si in the plating film, the composition of the plating film tentatively satisfies the range of the present invention (Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass and Mg: Even if it contains 1.0 to 10.0% by mass and the balance consists of Zn and unavoidable impurities), if the abundance ratio of Mg ₂ Si and Si does not satisfy the relationship (1), the plating film according to the present invention. The effect of improving the protective effect of the above cannot be sufficiently obtained.

Here, in the above relationship (1), Si (111) is the diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135 nm), and Mg ₂ Si (111) is (111) of Mg ₂ Si. ) Diffraction intensity of the plane (plane spacing d = 0.3668 nm).
As a method of measuring Si (111) and Mg ₂ Si (111) by the X-ray diffraction, a part of the plating film is mechanically scraped off and X-ray diffraction is performed in a powdered state (powder X). It can be calculated by the line diffraction measurement method). For the measurement of the diffraction intensity, the diffraction peak intensity of Si corresponding to the surface spacing d = 0.3135 nm and the diffraction peak intensity of Mg ₂ Si corresponding to the surface spacing d = 0.3668 nm are measured, and these ratios are calculated. Si (111) / Mg ₂ Si (111) can be obtained.
The amount of plating film (amount to scrape off the plating film) required for powder X-ray diffraction measurement is 0.1 g or more from the viewpoint of accurately measuring Si (111) and Mg ₂ Si (111). It may be sufficient, preferably 0.3 g or more. Further, when the plating film is scraped off, steel plate components other than the plating film may be contained in the powder, but these intermetallic compound phases are contained only in the plating film, and the above-mentioned peak strength can be obtained. It has no effect. Further, the reason why the plating film is powdered and X-ray diffraction is performed is that when X-ray diffraction is performed on the plating film formed on the plated steel sheet, it is affected by the plane orientation of the solidification structure of the plating film and has the correct phase ratio. This is because it is difficult to perform the calculation.

Further, in the molten Al-Zn-Si-Mg-based plated steel sheet of the present invention, the diffraction intensity of Si in the plating film by the X-ray diffraction method is as follows because the corrosion resistance can be improved more stably. It is preferable to satisfy the relationship (2).
Si (111) = 0 ・ ・ ・ (2)
Si (111): Diffractive intensity of Si (111) plane (plane spacing d = 0.3135 nm) Generally, in the dissolution reaction of Al alloy in an aqueous solution, the Si phase is present as a cathode site, so that the surrounding area is Since it is known to promote the dissolution of the α-Al phase, reducing the Si phase is also effective from the viewpoint of suppressing the dissolution of the α-Al phase, and among them, Si as described in the relationship (2). A phase-free film (setting the diffraction peak intensity of Si (111) to zero) is the most excellent for stabilizing corrosion resistance.
The method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction is as described above.

Here, the method for satisfying the above-mentioned relationship (1) and relationship (2) is not particularly limited. For example, in order to satisfy the relationship (1) and the relationship (2), the balance between the Si content, the Mg content and the Al content in the plating film is adjusted to obtain Mg ₂ Si and Si. The abundance ratio (diffraction intensity of Mg ₂ Si (111) and Si (111)) can be controlled. It should be noted that the balance between the Si content, the Mg content and the Al content in the plating film does not necessarily satisfy the relationship (1) and the relationship (2) if the content ratio is set to a certain level. For example, it is necessary to change the content ratio of Mg and Al depending on the content of Si (% by mass).
In addition to adjusting the balance between the Si content, Mg content, and Al content in the plating film, the conditions for forming the plating film (for example, cooling conditions after plating) can be adjusted. , The diffraction intensity of Mg ₂ Si (111) and Si (111) can be controlled so as to satisfy the relationship (1) and the relationship (2).

The molten Al-Zn-Si-Mg-based plated steel sheet of the present invention contains Zn and unavoidable impurities.
Of these, the unavoidable impurity contains Fe. This Fe is inevitably contained in the plating bath due to elution of the steel sheet and the equipment in the bath, and is supplied by diffusion from the base steel sheet at the time of forming the interfacial alloy layer, and as a result, it is inevitably contained in the plating film. Will be included. The Fe content in the plating film is usually about 0.3 to 2.0% by mass. Other unavoidable impurities include Cr, Ni, Cu and the like. The total content of the unavoidable impurities is not particularly limited, but if it is excessively contained, it may affect various characteristics of the plated steel sheet, so that the total content is preferably 5.0% by mass or less.

Further, in the molten Al-Zn-Si-Mg-based steel sheet of the present invention, it is preferable that the plating film contains 0.01 to 1.0% by mass of Sr. When the plating film contains Sr, it is possible to more reliably suppress the occurrence of surface defects such as wrinkle-like uneven defects, and it is possible to realize good surface appearance.
The wrinkle-like defect is a wrinkle-like uneven defect formed on the surface of the plating film, and is observed as a whitish streak on the surface of the plating film. Such wrinkle-like defects are likely to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip plated steel sheet, by containing Sr in the plating film, Sr is preferentially oxidized over Mg in the surface layer of the plating film, and the oxidation reaction of Mg is suppressed, thereby causing the wrinkle-like defect. It is possible to suppress the occurrence of.

In the molten Al-Zn-Si-Mg-based steel sheet of the present invention, the abundance ratios of Si and Mg ₂ Si in the above-mentioned plating film satisfy the relationship (1), and the plating film has a mass of 0.01 to 1.0 mass. It is preferable to contain% Sr. Thereby, the effect of improving the surface appearance by the above-mentioned Sr can be further enjoyed. Although the cause of this is not clear, it is presumed that when the amount of Si in the plating film increases, the oxidation of the plating surface layer is difficult to be suppressed in the first place, which affects the effect of improving the appearance when Sr is added. .. When the Sr content in the plating film is less than 0.01% by mass, it is difficult to obtain the effect of suppressing the occurrence of the wrinkle-like defects described above, and the Sr content in the plating film exceeds 1.0% by mass. The Sr content in the plating film should be 0.01 to 1.0% by mass because Sr is excessively incorporated into the interfacial alloy layer and may affect the plating adhesion more than the appearance improving effect. Is preferable.

Further, since the plating film can improve the stability of the corrosion product and delay the progress of corrosion in the same manner as the above-mentioned Mg, Cr and Mn in total of 0.01 to 10% by mass. , V, Mo, Ti, Ca, Ni, Co, Sb and B are preferably selected from one or more. The total content of the above-mentioned components is set to 0.01 to 10% by mass because a sufficient corrosion delay effect can be obtained and the effect is not saturated.

The amount of the plating film adhered is preferably 45 to 120 g / m ² per side from the viewpoint of satisfying various characteristics. When the adhesion amount of the plating film is 45 g / m ² or more, sufficient corrosion resistance can be obtained even for applications that require long-term corrosion resistance such as building materials, and the adhesion amount of the plating film is 120 g / m / m. This is because when m ² or less, excellent corrosion resistance can be realized while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the amount of the plating film adhered is more preferably 45 to 100 g / m ² .

The amount of the plating film adhered is calculated from, for example, the difference in the weight of the steel sheet before and after peeling by dissolving and peeling the plating film of a specific area with a mixed solution of hydrochloric acid and hexamethylenetetramine shown in JIS H 0401: 2013. Can be derived. In order to determine the amount of plating adhesion per one surface by this method, it can be determined by sealing with tape so that the plating surface of the non-target surface is not exposed, and then performing the above-mentioned dissolution.

Further, the component composition of the plating film can be confirmed, for example, by immersing the plating film in hydrochloric acid or the like to dissolve the plating film, and confirming the solution by ICP emission spectroscopic analysis, atomic absorption spectroscopy or the like. This method is merely an example, and any method can be used as long as the component composition of the plating film can be accurately quantified, and the method is not particularly limited.

The plating film of the molten Al-Zn-Si-Mg-based plated steel sheet obtained by the present invention has almost the same composition as the plating bath as a whole. Therefore, the composition of the plating film can be controlled accurately by controlling the composition of the plating bath.

The base steel sheet constituting the molten Al-Zn-Si-Mg-based plated steel sheet of the present invention is not particularly limited, and a cold-rolled steel sheet, a hot-rolled steel sheet, or the like may be appropriately used according to the required performance and specifications. Can be used.

Further, the method for obtaining the base steel plate is not particularly limited. For example, in the case of the hot-rolled steel sheet, a steel sheet that has undergone a hot-rolling step and a pickling step can be used, and in the case of the cold-rolled steel sheet, it can be manufactured by further adding a cold-rolling step. Further, in order to obtain the characteristics of the steel sheet, it is possible to go through a recrystallization annealing step or the like before the hot-dip plating step.

The method for producing the molten Al-Zn-Si-Mg-based plated steel sheet of the present invention is not particularly limited. For example, it can be manufactured by washing, heating, and immersing the base steel sheet in a plating bath in a continuous hot-dip plating facility. In the heating step of the steel sheet, recrystallization baking is performed to control the structure of the base steel sheet itself, and in order to prevent oxidation of the steel sheet and reduce a trace amount of oxide film existing on the surface, a nitrogen-hydrogen atmosphere, etc. It is effective to heat in the reducing atmosphere of.

Further, as for the plating bath used when manufacturing the molten Al-Zn-Si-Mg-based plated steel sheet of the present invention, as described above, the composition of the plating film is almost the same as the composition of the plating bath as a whole. Therefore, it is possible to use one containing Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass and Mg: 1.0 to 10.0% by mass, and the balance having a composition of Zn, Fe and unavoidable impurities. ..

Further, the bath temperature of the plating bath is not particularly limited, but is preferably in the temperature range of (melting point + 20 ° C.) to 650 ° C.
The lower limit of the bath temperature is set to a melting point of + 20 ° C., because the bath temperature must be equal to or higher than the freezing point in order to perform the hot-dip plating treatment. This is to prevent coagulation due to a local decrease in bath temperature. On the other hand, the upper limit of the bath temperature is set to 650 ° C. If the temperature exceeds 650 ° C, rapid cooling of the plating film becomes difficult, and the interfacial alloy layer formed between the plating film and the steel sheet may become thick. Because.

Further, the temperature of the base steel plate that penetrates into the plating bath (penetration plate temperature) is not particularly limited, but from the viewpoint of ensuring the plating characteristics and preventing the change in the bath temperature in the continuous hot-dip plating operation, the plating bath is described. It is preferable to control the temperature within ± 20 ° C.

Further, the immersion time of the steel sheet in the plating bath is 0.5 seconds or more. This is because if it takes less than 0.5 seconds, a sufficient plating film may not be formed on the surface of the base steel sheet. The upper limit of the dipping time is not particularly limited, but it is preferably within 8 seconds because the interfacial alloy layer formed between the plating film and the steel sheet may become thicker if the dipping time is lengthened.

Furthermore, in the cooling process after plating, it is preferable to cool the plate temperature from 520 ° C to 500 ° C over 3 seconds or more. The precipitation start temperature of the simple substance Si phase and Mg ₂ Si in the plating film is 500 to 490 ° C for the simple substance Si phase and 520 to 500 ° C for Mg ₂ Si. Therefore, by increasing the residence time in the temperature range of 520 to 500 ° C where only Mg ₂ Si precipitates, the precipitation of Mg ₂ Si is promoted and the precipitation of the simple substance Si phase is suppressed. ) Is easier to satisfy.

The molten Al-Zn-Si-Mg-based plated steel sheet can form a coating film on the plating film directly or via an intermediate layer, depending on the required performance.

The method for forming the coating film is not particularly limited and may be appropriately selected depending on the required performance. For example, a forming method such as roll coater coating, curtain flow coating, spray coating and the like can be mentioned. After painting a paint containing an organic resin, it is possible to heat and dry it by means such as hot air drying, infrared heating, and induction heating to form a coating film.

Further, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film.

(Samples 1-44)
By using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as a base steel sheet and annealing and plating with a hot-dip plating simulator manufactured by Resuka Co., Ltd., the hot-rolled steel sheet under the conditions shown in Table 1 can be obtained. Samples 1-44 were prepared.
Regarding the composition of the plating bath used for manufacturing the hot-dip plated steel sheet, the composition of the plating bath was Al: 30 to 75% by mass and Si: 0.5 to 4.5 so as to be the composition of the plating film of each sample shown in Table 1. It was varied in the range of% by mass, Mg: 0 to 10% by mass, and Sr: 0.00 to 0.15% by mass. The bath temperature of the plating bath is 590 ° C when Al: 30 to 60% by mass and 630 ° C when Al: more than 60% by mass, and the plating penetration plate temperature of the base steel sheet is the same as the plating bath temperature. It was controlled to be. Furthermore, the plating treatment was carried out under the condition that the plate temperature was cooled to a temperature range of 520 to 500 ° C. in 3 seconds.
The amount of the plating film adhered was controlled to be 85 ± 5 g / m ² per side for the samples 1 to 41 and 51 to 125 g / m ² per side for the samples 42 to 44.

(evaluation)
The following evaluations were performed on each sample of the hot-dip galvanized steel sheet obtained as described above. The evaluation results are shown in Table 1.

(1) Composition of plating film (adhesion amount, composition, X-ray diffraction intensity)
For each sample after plating, punch 100 mmφ, seal the non-measurement surface with tape, then dissolve and peel the plating with a mixed solution of hydrochloric acid and hexamethylenetetramine shown in JIS H 0401: 2013, and the mass of the sample before and after peeling. From the difference, the amount of adhesion of the plating film was calculated. Table 1 shows the amount of adhesion of the plating film obtained as a result of the calculation.
Then, the stripping solution was filtered, and the filtrate and solid content were analyzed respectively. Specifically, components other than insoluble Si were quantified by ICP emission spectroscopic analysis of the filtrate.
The solid content was dried and incinerated in a heating furnace at 650 ° C., and then melted by adding sodium carbonate and sodium tetraborate. Furthermore, insoluble Si was quantified by dissolving the melt with hydrochloric acid and ICP emission spectroscopic analysis of the solution. The Si concentration in the plating film is the sum of the soluble Si concentration obtained by the filtrate analysis and the insoluble Si concentration obtained by the solid content analysis. Table 1 shows the composition of the plating film obtained as a result of the calculation.
Furthermore, for each sample, after shearing to a size of 100 mm × 100 mm, the plating film on the evaluation symmetric plane is mechanically scraped until the underlying steel plate appears, the obtained powder is mixed well, and then 0.3 g is taken out and X is taken out. Using a line diffraction line device (“SmartLab” manufactured by Rigaku Co., Ltd.), X-ray used: Cu-Kα (wavelength = 1.54178Å), Kβ ray removal: Ni filter, tube voltage: 40kV, tube current: 30mA, scanning・ Speed: 4 ° / min, sampling interval: 0.020 °, divergence slit: 2/3 °, solar slit: 5 °, detector: high-speed one-dimensional detector (D / teX Ultra), under the conditions of the above powder A qualitative analysis was performed. The intensity obtained by subtracting the base intensity from each peak intensity is defined as each diffraction intensity (cps), and the diffraction intensity of the (111) plane (plane spacing d = 0.3668 nm) of Mg ₂ Si and the (111) plane (plane spacing) of Si. The diffraction intensity of d = 0.3135 nm) was measured. The measurement results are shown in Table 1.

(2) Corrosion resistance evaluation After shearing each sample of the obtained hot-dip galvanized steel sheet to a size of 120 mm × 120 mm, the range of 10 mm from each edge of the evaluation target surface, and the end face of the sample and the evaluation non-target surface are sealed with tape. The surface to be evaluated was exposed to a size of 100 mm × 100 mm and used as an evaluation sample. Three identical evaluation samples were prepared.
Corrosion acceleration tests were carried out in the cycle shown in FIG. 1 for all three evaluation samples prepared as described above. The corrosion acceleration test was started from wetting and continued until after 300 cycles, and then the corrosion weight loss of each sample was measured by the method described in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. The evaluation results are shown in Table 1.
⊚: Corrosion weight loss of all 3 samples is 45 g / m ² or less ○: Corrosion weight loss of 3 samples is 70 g / m ² or less ×: Corrosion weight loss of 1 sample or more exceeds 70 g / m ²

(3) Surface appearance The surface of the plated film was visually observed for each sample of the obtained hot-dip galvanized steel sheet.
Then, the observation results were evaluated according to the following criteria. The evaluation results are shown in Table 1.
⊚: No wrinkle-like defect was observed ○: Wrinkle-like defect was observed only in the range of 50 mm from the edge ×: Wrinkle-like defect was observed in the range other than 50 mm from the edge.

(4) Workability After shearing each sample of the obtained hot-dip galvanized steel sheet to a size of 70 mm × 150 mm, eight plates of the same thickness were sandwiched inside and subjected to 180 ° bending (8T bending). After the cellophane tape was strongly attached to the outer surface of the bent portion after bending, it was peeled off. The surface condition of the plating film on the outer surface of the bent portion and the presence or absence of adhesion (peeling) of the plating film on the surface of the used tape were visually observed, and the workability was evaluated according to the following criteria. The evaluation results are shown in Table 1.
〇: Both cracks and peeling are not observed in the plating film △: There are cracks in the plating film, but peeling is not observed ×: Both cracks and peeling are observed in the plating film

(5) Bath stability When manufacturing each sample of hot-dip galvanized steel sheet, the condition of the bath surface of the plating bath is visually confirmed, and the bath surface of the plating bath (containing Mg) used when manufacturing the hot-dip Al-Zn-based plated steel sheet. The bath surface without oxide) was compared. The evaluation is performed according to the following criteria, and the evaluation results are shown in Table 1.
〇: Similar to the molten Al-Zn-based plating bath (55% by mass Al-remaining Zn-1.6% by mass bath) △: For the molten Al-Zn-based plating bath (55% by mass Al-remaining Zn-1.6% by mass) Compared to more white oxide ×: Formation of black oxide is observed in the plating bath

From the results in Table 1, it can be seen that each sample of the example of the present invention is superior to each sample of the comparative example in terms of corrosion resistance, surface appearance, processability, and bath stability in a well-balanced manner.

According to the present invention, it is possible to provide a molten Al-Zn-Si-Mg-based plated steel sheet having stable and excellent corrosion resistance.

Claims (5)

A molten Al-Zn-Si-Mg-based plated steel sheet with a plating film.
The plating film contains Al: 45 to 65% by mass, Si: 1.0 to 4.0% by mass and Mg: 1.0 to 10.0% by mass, and has a composition in which the balance is Zn and unavoidable impurities.
The following relationship is the diffraction intensity of Si and Mg ₂ Si in the plating film measured by the X-ray diffractometry in a state where a part of the plating film is mechanically machined into powder until the underlying steel plate appears. A molten Al-Zn-Si-Mg-based plated steel sheet, which is characterized by satisfying (1).
Si (111) / Mg ₂ Si (111) ≤ 0.8 ・ ・ ・ (1)
Si (111): Diffraction intensity of Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm) The molten Al-Zn-Si-Mg-based plated steel sheet according to claim 1, wherein the diffraction intensity of the Si in the plating film by the X-ray diffraction method satisfies the following relationship (2).
Si (111) = 0 ・ ・ ・ (2) The molten Al-Zn-Si-Mg-based plated steel sheet according to claim 1 or 2 , wherein the Al content in the plating film is 50 to 60% by mass. The molten Al-Zn-Si-Mg-based plated steel sheet according to any one of claims 1 to 3 , wherein the content of Si in the plating film is 1.0 to 3.0% by mass. The molten Al-Zn-Si-Mg-based plated steel sheet according to any one of claims 1 to 4 , wherein the content of Mg in the plating film is 1.0 to 5.0% by mass.

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