JP2023100555A - HOT DIPPED Al-Zn-Si-Mg PLATED STEEL SHEET AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

To provide a hot dipped Al-Zn-Si-Mg plated steel sheet which stably has excellent corrosion resistance.SOLUTION: In order to achieve the above-described purpose, the present invention provides a hot dipped Al-Zn-Si-Mg plated steel sheet which has a plated coating. The plated coating has a composition that contains 45 to 65 mass% of Al, 1.0 to 4.0 mass% of Si and 1.0 to 10.0 mass% of Mg, with the balance being made up of Zn and unavoidable impurities; a Co content in the unavoidable impurities is 0.080 mass% or less relative to total mass of the plated coating; and diffraction intensities of Si and Mg2Si in the plated coating as determined by X-ray diffraction satisfy a relational expression (1): Si (111)/Mg2Si (111)≤0.8. Si (111): Diffraction intensity of (111) plane of Si (interplanar spacing d=0.3135 nm) Mg2Si (111): Diffraction intensity of (111) plane of Mg2Si (interplanar spacing d=0.3668 nm).SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a hot-dip Al-Zn-Si-Mg plated steel sheet having stably excellent corrosion resistance and a method for producing the same.

Hot-dip Al-Zn coated steel sheets, represented by 55% Al-Zn, are known to 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. It is Therefore, due to its excellent corrosion resistance, hot-dip Al-Zn coated 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 guardrails, wiring and piping, and soundproof walls. . In particular, the demand for materials with excellent corrosion resistance and maintenance-free materials is increasing in harsher usage environments such as acid rain due to air pollution, spraying of snow-melting agents to prevent road freezing in snowy areas, and development of coastal areas. Therefore, the demand for hot-dip Al-Zn coated steel sheets is increasing in recent years.

The coating film of the hot-dip Al-Zn coated steel sheet consists of a part (α-Al phase) where Al containing supersaturated Zn has solidified into a dendrite, and a Zn-Al eutectic 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. This characteristic film structure complicates the path of progression of corrosion from the surface, making it difficult for corrosion to progress easily. It is also known that it is possible to achieve superior corrosion resistance.

Attempts have been made to extend the service life of such hot-dip Al-Zn-coated steel sheets, and Mg-added hot-dip Al-Zn-Si-Mg-coated steel sheets have been put to practical use.
As such a hot-dip Al-Zn-Si-Mg plated steel sheet, for example, Patent Document 1 includes an Al-Zn-Si alloy containing Mg in the plating film, and the Al-Zn-Si alloy has a Molten Al-Zn-Si- alloy containing 60% by weight elemental aluminum, 37-46% by weight elemental zinc and 1.2-2.3% by weight Si, the concentration of Mg being 1-5% by weight A Mg-based plated steel sheet is disclosed.
In addition, in Patent Document 2, by including one or more of 2 to 10% Mg and 0.01 to 10% Ca in the plating film, corrosion resistance is improved, and the protective effect after the base steel plate is exposed A hot-dip Al-Zn-Si-Mg coated steel sheet is disclosed for the purpose of increasing the
Furthermore, in Patent Document 3, a coating layer containing Mg: 1 to 15%, Si: 2 to 15%, Zn: 11 to 25% by mass, and the balance being Al and unavoidable impurities is formed. , A molten Al-Zn-Si-Mg system that improves the corrosion resistance of flat plates and end faces by reducing the size of intermetallic compounds such as Mg ₂ Si phase and MgZn ₂ phase in the plating film to 10 μm or less. A plated steel sheet is disclosed.

The hot-dip Al-Zn coated steel sheet described above has a beautiful appearance with a spangle pattern with a white metallic luster. be. Therefore, techniques have been developed to improve the appearance of hot-dip Al-Zn coated steel sheets.
For example, Patent Document 4 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which wrinkle-like irregularities are suppressed by containing 0.01 to 10% Sr in the plated film.
Further, Patent Document 5 also discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which 500 to 3000 ppm of Sr is contained in the plated film to suppress mottling defects.

Japanese Patent No. 5020228 Japanese Patent No. 5000039 JP-A-2002-12959 Japanese Patent No. 3983932 Japanese Patent Publication No. 2011-514934

However, the technique of incorporating Mg into the plating film as disclosed in Patent Documents 1 to 3 does not necessarily uniquely improve corrosion resistance.
In the hot-dip Al-Zn-Si-Mg-coated steel sheets disclosed in Patent Documents 1 to 3, corrosion resistance is improved only by including Mg in the coating components, but the above four elements (Al, Zn, The influence of ingredients other than Si and Mg) and the characteristics of the metallic phase and intermetallic compound phase that make up the plating film were not considered, and it was not possible to uniformly talk about the superiority or inferiority of corrosion resistance. Therefore, even when a hot-dip Al-Zn-Si-Mg-coated steel sheet is produced using a plating bath composition in which the contents of the above four element components are the same, the corrosion resistance varies when an accelerated corrosion test is performed. There is a problem that it is not necessarily superior to Al-Zn plated steel sheets to which Mg is not added.
Similarly, in terms of improving the appearance of plating, it is not necessarily possible to eliminate wrinkle-like unevenness defects by simply adding Sr to the plating film. For Zn-Si-Mg plated steel sheets, there were cases where both corrosion resistance and appearance could not be achieved. In addition, since Mg is an element that is easily oxidized, the Mg contained in the plating bath generates oxides (top dross) near the bath surface. FeAl-based compounds (bottom dross) containing iron unevenly distributed in the middle or bottom may occur, and these dross adhere to the surface of the plating film, causing convex defects and spoiling the appearance of the plating film surface. There was also fear.
In addition, when a steel plate is plated in a molten Al-Zn-Si bath with Mg added, in addition to the α-Al phase, the Mg ₂ Si phase, MgZn ₂ phase, and Si phase precipitate in the plating film. It is known. However, the effect of the precipitation amount and existence ratio of each phase on corrosion resistance has not been clarified.

In view of such circumstances, an object of the present invention is to provide a hot-dip Al-Zn-Si-Mg plated steel sheet having excellent corrosion resistance in a stable manner, and a method for producing the same.

The inventors of the present invention conducted studies to solve the above problems, and found that the composition of the plating film of the hot-dip Al-Zn-Si-Mg-coated steel sheet can be obtained only by controlling the concentrations of Al, Zn, Si and Mg. Not only that, but it is also important to control the concentration of elements contained as impurities, and among them, it was found that the deterioration of corrosion resistance can be effectively suppressed by appropriately controlling the Co content.
In addition, regarding the Mg ₂ Si phase and Si phase formed in the plating film of the hot-dip Al-Zn-Si-Mg plated steel sheet, the amount of precipitation increases or decreases depending on the balance of each component in the plating film and the conditions for forming the plating film. Depending on the compositional balance, one of the phases may not be precipitated, and the corrosion resistance of the hot-dip Al-Zn-Si-Mg coated steel sheet changes depending on the abundance ratio of these phases. ₂ We have found that the corrosion resistance is stably improved when the Si phase is less than the Si phase or Si phase. However, for these Mg ₂ Si phases and Si phases, general techniques such as scanning electron microscopy are used to observe secondary electron images or backscattered electron images of the plating film from the surface or cross section. It is known that it is very difficult to distinguish between different phases, and it is possible to obtain microscopic information by observation using a transmission electron microscope, but macroscopic information such as corrosion resistance and appearance is known. However, it was not possible to ascertain the existence ratio of Mg ₂ Si and Si phases, which influences the most important information.

Therefore, as a result of further intensive research, the present inventors focused on the X-ray diffraction method and used the intensity ratio of specific diffraction peaks of the Mg ₂ Si phase and the Si phase to quantify the existence ratio of the phases. Furthermore, when the Mg ₂ Si phase and Si phase satisfy a specific existence ratio in the plating film, it has been found that excellent corrosion resistance can be stably achieved.
In addition, the present inventors controlled the Co content and the abundance ratio of the Mg ₂ Si phase and Si phase in the plating film described above, and then controlled the Sr concentration in the plating bath to reduce wrinkles. It has also been found that the occurrence of unevenness defects can be reliably suppressed, and a plated steel sheet with excellent surface appearance can be obtained.

The present invention was made based on the above findings, and the gist thereof is as follows.
1. A hot-dip Al-Zn-Si-Mg plated steel sheet comprising a plating film,
The plating film has a composition containing 45 to 65% by mass of Al, 1.0 to 4.0% by mass of Si, and 1.0 to 10.0% by mass of Mg, with the balance being Zn and unavoidable impurities,
The Co content in the unavoidable impurities is 0.080% by mass or less with respect to the total mass of the plating film, and the diffraction intensity of Si _and Mg Si in the plating film by an X-ray diffraction method has the following relationship: A hot-dip Al-Zn-Si-Mg plated steel sheet, which satisfies (1).
Si(111)/ _Mg2Si (111)≦0.8 (1)
Si (111): Diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of the (111) plane of Mg ₂ Si (d = 0.3668 nm)

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

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

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

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

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

7. A method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film,
The plating film is formed in a plating bath containing 45 to 65% by mass of Al, 1.0 to 4.0% by mass of Si, and 1.0 to 10.0% by mass of Mg, with the balance being Zn and unavoidable impurities. , comprising a hot-dip plating process in which the base steel plate is immersed,
A method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet, characterized in that the Co content in the inevitable impurities in the plating bath is controlled to 0.080% by mass or less with respect to the total mass of the plating bath. .
8. 8. The method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet according to 7 above, wherein the plating bath further contains Sr: 0.01 to 1.0% by mass.

According to the present invention, it is possible to provide a hot-dip Al-Zn-Si-Mg plated steel sheet that stably has excellent corrosion resistance.

It is a figure for demonstrating the flow of the combined cycle test (JASO-CCT) of a Japanese automobile standard.

(hot-dip Al-Zn-Si-Mg plated steel sheet)
The hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention has a plated film on the surface of the steel sheet. The plating film has a composition containing 45 to 65% by mass of Al, 1.0 to 4.0% by mass of Si, and 1.0 to 10.0% by mass of Mg, with the balance being Zn and unavoidable impurities.

The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, from the viewpoint of the balance between corrosion resistance and operational aspects. This is because when 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 a dendritic solidification structure of the α-Al phase can be obtained. Since the dendrite solidification structure is laminated in the film thickness direction of the plated film, the corrosion progression path becomes complicated, and the corrosion resistance of the plated film itself is improved. In addition, the more the dendrite part of this α-Al phase is laminated, the more complicated the corrosion progression path becomes, and the more difficult it is for the corrosion to easily reach the base steel plate, so the corrosion resistance is improved. % or more. On the other hand, when the Al content in the plating film exceeds 65% by mass, most of the Zn changes into a solid solution structure in α-Al, the dissolution reaction of the α-Al phase cannot be suppressed, and Al-Zn - Corrosion resistance of Si-Mg system plating deteriorates. Therefore, the Al content in the plating film should be 65% by mass or less, preferably 60% by mass or less.

Si in the plating film mainly suppresses the growth of Fe-Al and/or Fe-Al-Si interfacial alloy layers generated at the interface with the base steel sheet, and does not deteriorate the adhesion between the plating film and the steel sheet. added for a purpose. Actually, when a steel sheet is immersed in an Al-Zn-based plating bath containing Si, Fe on the surface of the steel sheet and Al and Si in the bath undergo an alloying reaction, resulting in Fe-Al and/or Fe-Al-Si An intermetallic compound layer is formed at the base steel plate/plating film interface, but at this time, the growth rate of Fe-Al-Si alloys is slower than that of Fe-Al alloys, so the ratio of Fe-Al-Si alloys is The higher it is, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film should 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 does the effect of suppressing the growth of the interfacial alloy layer saturate, but the excessive Si phase in the plating film promotes corrosion. Therefore, the Si content should be 4.0% by mass or less. Furthermore, the Si content in the plating film is preferably 3.0% by mass or less from the viewpoint of suppressing the existence of excessive Si phases. 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% by mass of Mg. By containing Mg in the plated film, the above-mentioned Si can be present in the form of an intermetallic compound of the Mg ₂ Si phase, and promotion of corrosion can be suppressed.
In addition, when the plating film contains Mg, an MgZn ₂ phase, which is an intermetallic compound, is also formed in the plating film, resulting in the effect of further improving the corrosion resistance. When the Mg content in the plating film is less than 1.0% by mass, Mg is used for solid solution in the α-Al phase, which is the main phase, rather than for forming the intermetallic compounds (Mg2Si, MgZn2). Sufficient corrosion resistance cannot be secured. On the other hand, when the Mg content in the plating film increases, the effect of improving the corrosion resistance is saturated, and the workability decreases due to the weakening of the α-Al phase. . Furthermore, the Mg content in the plating film is preferably 5.0% by mass or less from the viewpoint of suppressing dross generation during plating formation and facilitating plating bath management. In terms of the relationship with the Si content, the content of Mg is preferably 3.0% by mass from the viewpoint of easily satisfying the relational expression (1) described later, considering compatibility with dross suppression. Therefore, it is more preferable to set the content of Mg to 3.0 to 5.0% by mass.

Moreover, the plating film contains Zn and unavoidable impurities. Among these, the unavoidable impurities contain Fe. This Fe is inevitably included in the plating bath due to elution of the steel sheet and bath equipment into the plating bath, and as a result of being supplied by diffusion from the base steel sheet during the formation of the interfacial alloy layer. to 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, Co, W and the like. These components are due to the elution of the WC-based and Co-Cr-W-based thermal spray coatings applied to the substrate steel plate, stainless steel equipment in the bath, and the equipment in the bath. In addition, it is unavoidably included in the plating film by using the pot or bath equipment used in the production of the plated steel sheet to which these ingredients are intentionally added. will be

The hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention is characterized in that the content of Co in the unavoidable impurities is 0.080% by mass or less with respect to the total mass of the plating film. Since Co contained in the plating film may deteriorate the corrosion resistance of the hot-dip Al-Zn-Si-Mg plated steel sheet, the contents of Al, Zn, Si and Mg in the plating film described above are Deterioration of corrosion resistance can be suppressed by appropriately controlling Co content and suppressing the content of Co as an unavoidable impurity. From the same point of view, the Co content in the unavoidable impurities is preferably 0.020% by mass or less, more preferably 0.001% by mass or less, and 0% by mass with respect to the total mass of the plating film. most preferably.

Note that Co contained in the plating film mainly exists as a solid solution element in α-Al. When Co is contained in a concentration exceeding the solid solubility limit, it may precipitate as an Fe-Co-Al-based compound or an Fe-Co-Al-Si-based compound in the alloy layer formed at the substrate iron-plating interface.
Although the mechanism by which the corrosion resistance deteriorates due to the inclusion of Co as an impurity is uncertain, the mechanism by which the corrosion rate increases due to the decrease in the barrier properties of the α-Al phase in which Co is dissolved, and the Co We presume the mechanism by which the compound containing the functions as a cathode in a corrosive environment and forms a local cell with the surrounding solidified tissue to increase the corrosion rate.

Furthermore, the total content of unavoidable impurities in the plating film is not particularly limited, but if it is contained excessively, it may affect various characteristics of the plated steel sheet, so the total content is 5.0% by mass or less. It is preferable to

In the hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention, the diffraction intensities of Si and Mg ₂ Si in the plated film according to the X-ray diffraction method must satisfy the following relationship (1): .
Si(111)/ _Mg2Si (111)≦0.8 (1)
Si (111): Diffraction intensity of Si (111) plane (d = 0.3135 nm) Mg ₂ Si (111): Diffraction intensity of Mg ₂ Si (111) plane (d = 0.3668 nm)

As described above, in the hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention, the existence ratio of the Mg ₂ Si phase and the Si phase generated in the plating film due to the inclusion of Mg and Si is set to a specific ratio. Control is important. The effects of these factors on corrosion resistance are currently being investigated, and there are many unclear points, but the following mechanism is presumed.

When a hot-dip Al-Zn-Si-Mg coated steel sheet is exposed to a corrosive environment, the above intermetallic compounds dissolve preferentially over the α-Al phase. is an environment rich in In such a Mg-rich environment, it is assumed that the formed corrosion products are difficult to decompose, and as a result, the protective effect of the plating film increases. In addition, since the effect of improving the protective action 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, the existence ratio of the Si phase to the Mg ₂ Si phase is reduced. is considered to be effective.

The existence ratio of Si and Mg ₂ Si in the plating film is determined by using the diffraction peak intensity obtained by the X-ray diffraction method, and the relationship (1): Si (111) / Mg ₂ Si (111) ≤ 0.8. However, if the abundance ratio of Si and Mg ₂ Si in the plating film does not satisfy the relationship (1), that is, Si (111) / Mg ₂ Si (111)>0.8, the above Since the amount of Si phase present in the plating film increases, the Mg-rich environment described above cannot be obtained in the vicinity of the corrosion product, making it difficult to obtain the effect of improving the protective action of the plating film. From the same point of view, 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. Especially preferred.

Here, in the 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 the (111 ) plane (interplanar spacing d = 0.3668 nm).
As a method for measuring Si (111) and Mg ₂ Si (111) by X-ray diffraction, a part of the plating film is mechanically scraped off and powdered, and X-ray diffraction is performed (powder X line diffraction measurement method). For the measurement of diffraction intensity, measure the diffraction peak intensity of Si corresponding to the interplanar spacing d = 0.3135 nm and the diffraction peak intensity of Mg ₂ Si corresponding to the interplanar spacing d = 0.3668 nm, and calculate the ratio of these. can obtain Si (111)/Mg ₂ Si (111).
In addition, the amount of plating film required when performing powder X-ray diffraction measurement (the amount of plating film scraped off) is 0.1 g or more from the viewpoint of measuring Si (111) and Mg ₂ Si (111) with high accuracy. Any amount is sufficient, preferably 0.3 g or more. In addition, when the plating film is cut out, the powder may contain steel sheet components other than the plating film, but these intermetallic compound phases are contained only in the plating film, and the above-mentioned peak strength No effect. Furthermore, the reason why the plating film is powdered and subjected to X-ray diffraction is that when X-ray diffraction is performed on the plating film formed on the plated steel sheet, the correct phase ratio is affected by the plane orientation of the solidification structure of the plating film. This is because it is difficult to calculate.

Further, in the hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention, the concentrations of Al, Zn, Si, Mg and Co as an unavoidable impurity are controlled, and the corrosion resistance is improved more stably. It is preferable that 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 the (111) plane of Si (d = 0.3135 nm) 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. It is most excellent for stabilizing the corrosion resistance to form a film in which no phase exists (the Si(111) diffraction peak intensity is set to zero).
The method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction is as described above.

Here, there is no particular limitation on the method for satisfying the relationship (1) and relationship (2) described above. For example, in order to satisfy the relationship (1) and the relationship (2), by adjusting the balance of the Si content, the Mg content and the Al content in the plating film, Si and Mg ₂ Si The existence ratio (diffraction intensity of Si (111) and Mg ₂ Si (111)) can be controlled. The balance of the Si content, the Mg content, and the Al content in the plating film does not necessarily satisfy the relationship (1) or the relationship (2) if it is set to a constant content ratio. It is necessary to change the content ratio of Mg and Al depending on the content (% by mass).
In addition to adjusting the balance of the Si content, Mg content and Al content in the plating film, by adjusting the conditions during plating film formation (for example, cooling conditions after plating) , the diffraction intensity of Si (111) and Mg ₂ Si (111) can be controlled so as to satisfy the relationship (1) or (2).

Moreover, in the molten Al-Zn-Si-Mg steel sheet of the present invention, the plating film preferably contains 0.01 to 1.0% by mass of Sr. By containing Sr in the plating film, it is possible to more reliably suppress the occurrence of surface defects such as wrinkle-like irregularities, and to realize good surface appearance.
The wrinkle-like defects are wrinkle-like irregularities formed on the surface of the plating film, and are observed as whitish streaks on the surface of the plating film. Such wrinkle-like defects tend to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip plated steel sheet, by including 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, so that the wrinkle-like defects It is possible to suppress the occurrence of

In addition, the plating film has the effect of improving the stability of corrosion products and delaying the progress of corrosion in the same way as Mg described above, so that the total content of Cr, Mn , V, Mo, Ti, Ca, Sb and B are preferably further contained. The reason why the total content of the above components is set to 0.01 to 10% by mass is that a sufficient corrosion retarding effect can be obtained and the effect will not be saturated.

From the viewpoint of satisfying various characteristics, the coating weight of the plating film is preferably 45 to 120 g/m ² per side. When the coating weight 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. This is because, when it is m ² or less, it is possible to achieve excellent corrosion resistance while suppressing the occurrence of plating cracks and the like during processing. From the same point of view, it is more preferable that the coating amount of the plating film is 45 to 100 g/m ² .

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

In addition, the component composition of the plating film can be confirmed by immersing the plating film in hydrochloric acid or the like to dissolve it and confirming the solution by ICP emission spectrometry, atomic absorption spectrometry, etc., as with the content of Co described above. This method is merely an example, and any method may be used as long as it can accurately quantify the component composition of the plating film, and is not particularly limited.

The plating film of the hot-dip Al-Zn-Si-Mg 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 accurately controlled by controlling the composition of the plating bath.

Further, the base steel sheet constituting the hot-dip Al-Zn-Si-Mg-coated steel sheet of the present invention is not particularly limited, and cold-rolled steel sheets, hot-rolled steel sheets, etc. are appropriately selected according to the required performance and standards. can be used.

Furthermore, the method for obtaining the base steel sheet is not particularly limited. For example, in the case of the hot-rolled steel sheet, a hot-rolled steel sheet and a pickling process may be used, and in the case of the cold-rolled steel sheet, a cold-rolling process may be further added. Furthermore, in order to obtain the characteristics of the steel sheet, it is also possible to undergo a recrystallization annealing process or the like before the hot dip plating process.

(Manufacturing method of hot-dip Al-Zn-Si-Mg coated steel sheet)
A method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention is a method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film, wherein the plating film is formed by Al: A hot-dip plating process in which the base steel sheet is immersed in a plating bath containing 45-65% by mass, 1.0-4.0% by mass of Si, 1.0-10.0% by mass of Mg, and the balance being Zn and unavoidable impurities. Have a process.
The hot-dip plating process is not particularly limited, except for the conditions of the plating bath, which will be described later. For example, it can be produced by washing, heating, and immersing the base steel sheet in a plating bath in a continuous hot-dip plating facility. In the steel sheet heating process, recrystallization annealing or the like is performed to control the structure of the base steel sheet itself, and a nitrogen-hydrogen atmosphere or the like is applied to prevent oxidation of the steel sheet and reduce a small amount of oxide film existing on the surface. Heating in a reducing atmosphere is effective.

Further, with respect to the plating bath used in the hot-dip plating process, as described above, the composition of the plating film as a whole is almost the same as the composition of the plating bath, so Al: 45 to 65% by mass, Si: A composition containing 1.0 to 4.0% by mass and 1.0 to 10.0% by mass of Mg, with the balance being Zn and unavoidable impurities can be used.

Then, in the method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention, the Co content in the inevitable impurities of the plating bath is controlled to 0.080% by mass or less with respect to the total mass of the plating bath. characterized by As described above, Co contained in the plating film may deteriorate the corrosion resistance of the hot-dip Al-Zn-Si-Mg plated steel sheet. Deterioration of corrosion resistance can be suppressed by appropriately controlling the content and further suppressing the content of Co as an unavoidable impurity.
In addition, the content of Co as an unavoidable impurity in the plating bath should be controlled to 0.080% by mass or less with respect to the total mass of the plating bath, and more preferably 0.020% by mass or less. It is more preferable to make it 0.001% by mass or less. If the Co content in the plating bath is 0.080% by mass or less, the produced hot-dip Al-Zn-Si-Mg plated steel sheet can have sufficiently excellent corrosion resistance. Further, when the Co content in the plating bath is 0.020% by mass or less, more excellent corrosion resistance can be obtained, and when it is 0.001% by mass or less, particularly excellent corrosion resistance can be obtained.
Thus, the lower the Co content in the plating bath, the better the corrosion resistance of the hot-dip Al-Zn-Si-Mg plated steel sheet. The Co content of is most preferably 0%.

Here, the means for reducing the Co content in the plating bath is not particularly limited.
For example, it is effective to suppress the elution of a Co--Cr--W-based thermal spray coating applied to equipment in a bath into the plating bath, so it is preferable to apply a thermal spray coating that does not contain Co.

Moreover, as another means for reducing the Co content in the plating bath, it is preferable to use a metal ingot with a low content of Co among impurities as a raw material for the plating bath.
Furthermore, it is also effective not to use the pots and equipment in the bath used for manufacturing coated steel sheets to which Co is intentionally added for manufacturing hot-dip Al-Zn-Si-Mg coated steel sheets. This is because it is possible to suppress the dissolution of Co-containing metal lumps adhering to the pot and the equipment in the bath and mixing into the plating bath.

Furthermore, although the bath temperature of the plating bath is not particularly limited, it is preferably in the temperature range of (melting point +20°C) to 650°C.
The reason why the lower limit of the bath temperature is set to the melting point +20°C is that the bath temperature must be higher than the freezing point in order to perform the hot-dip plating process. This is to prevent coagulation due to a local temperature drop in the bath. On the other hand, the upper limit of the bath temperature is set to 650°C because if it 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. It's for.

In addition, the temperature of the substrate steel sheet that enters the plating bath (immersion plate temperature) is not particularly limited. It is preferable to control the temperature within ±20°C.

Furthermore, the immersion time of the base steel sheet in the plating bath is preferably 0.5 seconds or longer. This is because if the time is 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 immersion time is not particularly limited, but if the immersion time is long, the interfacial alloy layer formed between the plating film and the steel sheet may become thicker, so it is more preferably 8 seconds or less.

The hot-dip Al-Zn-Si-Mg plated steel sheet can also have a coating film formed 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 can be appropriately selected according to the required performance. For example, forming methods such as roll coater coating, curtain flow coating, and spray coating can be used. After applying the coating material containing the organic resin, it is possible to form a coating film by heating and drying by means of hot air drying, infrared heating, induction heating, or the like.

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

(Samples 1-55)
A cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method was used as the base steel sheet. Samples 1-55 were made.
Regarding the composition of the plating bath used in the production of the hot dip plated steel sheet, the composition of the plating bath was adjusted to Al: 5 to 75% by mass and Si: 0.0 to 4.5 so that the composition of the plating film of each sample shown in Table 1 was obtained. % by mass, Mg: 0 to 10% by mass, Co: 0.000 to 0.120% by mass. The bath temperature of the plating bath is 450°C for Al: 5% by mass, 480°C for Al: 15% by mass, 590°C for Al: 30 to 60% by mass, and more than 60% by mass for Al. In the case of , the temperature was set to 630° C., and the temperature of the base steel sheet in which the plating penetrated was controlled to be the same temperature as the plating bath temperature. Furthermore, in the case of Al: 30 to 60% by mass, plating was performed under the condition that the sheet temperature was cooled to a temperature range of 520 to 500°C in 3 seconds.
In addition, the amount of plating film deposited was 85±5 g/m ² per side for samples 1 to 52, 50±5 g/m ² per side for sample 53, 100±5 g/m 2 per side for sample 54, and 100±5 g/m ² per side for sample 55. It was controlled to be 125±5 g/m ² per side.

(evaluation)
Each sample of the hot-dip plated steel sheet obtained as described above was evaluated as follows. Table 1 shows the evaluation results.

(1) Plating film (composition, adhesion amount, X-ray diffraction intensity)
For each sample after plating, a 100 mm diameter piece was punched out, and after sealing the non-measurement surface with tape, the plating was dissolved and peeled off with a mixed solution of hydrochloric acid and hexamethylenetetramine specified in JIS H 0401:2013. From the difference, the adhesion amount of the plating film was calculated. Table 1 shows the adhesion amount of the plating film obtained as a result of the calculation.
After that, the stripping liquid was filtered, and the filtrate and the solid content were analyzed. Specifically, components other than insoluble Si were quantified by subjecting the filtrate to ICP emission spectroscopic analysis.
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 subjecting the solution to ICP emission spectroscopic analysis. The Si concentration in the plating film was obtained by adding the insoluble Si concentration obtained by solid content analysis to the soluble Si concentration obtained by filtrate analysis. Table 1 shows the composition of the plating film obtained as a result of the calculation.
In addition, for each sample, after shearing to a size of 100 mm × 100 mm, the plating film on the surface to be evaluated was mechanically scraped until the underlying steel plate appeared, and the obtained powder was mixed well, then 0.3 g was taken out, X-rays used: Cu-Kα (wavelength = 1.54178 Å), removal of Kβ rays: Ni filter, tube voltage: 40 kV, tube current: 30 mA, Scanning speed: 4°/min, sampling interval: 0.020°, divergence slit: 2/3°, solar slit: 5°, detector: high-speed one-dimensional detector (D/teX Ultra) qualitative analysis was performed. The intensity obtained by subtracting the base intensity from each peak intensity is defined as _each diffraction intensity (cps). 0.3668 nm) was measured. Table 1 shows the measurement results.

(2) Corrosion resistance evaluation For each sample of the obtained hot-dip plated steel sheet, after shearing to a size of 120 mm × 120 mm, a range of 10 mm from each edge of the evaluation target surface, and the end surface of the sample and the non-evaluation surface were sealed with tape. A 100 mm×100 mm size exposed surface for evaluation was used as a sample for evaluation. In addition, three identical evaluation samples were produced.
Accelerated corrosion tests were performed on the three samples for evaluation produced as described above with the cycle shown in FIG. The accelerated corrosion test was started from wet and after 300 cycles, the corrosion weight loss of each sample was measured by the method described in JIS Z 2383 and ISO8407 and evaluated according to the following criteria. Table 1 shows the evaluation results.
◎: Corrosion weight loss of all 3 samples is 45 g/m ² or less ○: Corrosion weight loss of all 3 samples is 70 g/m ² or less ×: Corrosion weight loss of 1 or more samples exceeds 70 g/m ²

(3) Surface Appearance For each sample of the obtained hot-dip plated steel sheet, the surface of the plated film was visually observed.
The observation results were evaluated according to the following criteria. Table 1 shows the evaluation results.
◎: No wrinkle-like defects were observed ○: Wrinkle-like defects were observed only in the range of 50 mm from the edge ×: Wrinkle-like defects were observed outside the range of 50 mm from the edge

(4) Workability Each sample of the obtained hot-dip plated steel sheet was sheared into a size of 70 mm x 150 mm, and then subjected to 180° bending (8T bending) with eight sheets of the same thickness sandwiched inside. Cellotape (registered trademark) was strongly adhered to the outer surface of the bent portion after bending, and then 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 tape used were visually observed, and workability was evaluated according to the following criteria. Table 1 shows the evaluation results.
○: Both cracks and peeling are not observed in the plating film △: There are cracks in the plating film, but no peeling is observed ×: Both cracks and peeling are observed in the plating film

(5) Bath stability At the time of manufacturing each sample of hot-dip plated steel sheet, the state of the bath surface of the plating bath is visually checked, bath surface without oxide). Evaluation was performed according to the following criteria, and the evaluation results are shown in Table 1.
〇: Same as molten Al-Zn plating bath (55 mass% Al-balance Zn-1.6 mass% bath) △: For molten Al-Zn plating bath (55 mass% Al-balance Zn-1.6 mass% bath) There are many white oxides compared to ×: Formation of black oxides is observed in the plating bath

From the results in Table 1, it can be seen that the samples of the present invention are superior to the samples of the comparative examples in all of the corrosion resistance, surface appearance, workability and bath stability in a well-balanced manner.

ADVANTAGE OF THE INVENTION According to this invention, the hot-dip Al-Zn-Si-Mg system plated steel plate which has excellent corrosion resistance stably, and its manufacturing method can be provided.

Claims (8)

A hot-dip Al-Zn-Si-Mg plated steel sheet comprising a plating film,
The plating film has a composition containing 45 to 65% by mass of Al, 1.0 to 4.0% by mass of Si, and 1.0 to 10.0% by mass of Mg, with the balance being Zn and unavoidable impurities,
Co content in the unavoidable impurities is 0.080% by mass or less with respect to the total mass of the plating film,
A hot-dip Al-Zn-Si-Mg plated steel sheet, wherein diffraction intensities of Si and Mg ₂ Si in the plated film by an X-ray diffraction method satisfy the following relationship (1).
Si(111)/ _Mg2Si (111)≦0.8 (1)
Si (111): Diffraction intensity of the (111) plane of Si (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of the (111) plane of Mg ₂ Si (d = 0.3668 nm) 2. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1, wherein the diffraction intensity of Si in the plated film by an X-ray diffraction method satisfies the following relationship (2).
Si (111) = 0 (2)
Si (111): Diffraction intensity of the (111) plane of Si (d = 0.3135 nm) The hot dip Al-Zn-Si-Mg plated steel sheet according to claim 1 or 2, wherein the plated film further contains Sr: 0.01 to 1.0% by mass. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 3, wherein the Al content in the plated film is 50 to 60% by mass. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 4, wherein the Si content in the plated film is 1.0 to 3.0% by mass. The hot dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 5, characterized in that the content of Mg in the plated film is 1.0 to 5.0% by mass. A method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film,
The plating film is formed in a plating bath containing 45 to 65% by mass of Al, 1.0 to 4.0% by mass of Si, and 1.0 to 10.0% by mass of Mg, with the balance being Zn and unavoidable impurities. , comprising a hot-dip plating process in which the base steel plate is immersed,
A method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet, characterized in that the Co content in the inevitable impurities in the plating bath is controlled to 0.080% by mass or less with respect to the total mass of the plating bath. . 8. The method for producing a hot dip Al-Zn-Si-Mg plated steel sheet according to claim 7, wherein the plating bath further contains Sr: 0.01 to 1.0% by mass.

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