JP7564133B2 - Hot-dip Al-Zn-Si-Mg-plated steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a hot-dip Al-Zn-Si-Mg-plated steel sheet that has stable and excellent corrosion resistance, and a method for manufacturing the same.

Hot-dip Al-Zn-coated steel sheets, such as 55% Al-Zn, are known to have high corrosion resistance among hot-dip galvanized steel sheets, because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al. For this reason, hot-dip Al-Zn-coated steel sheets are used mainly in the field of building materials such as roofs and walls that are exposed to the outdoors for long periods of time, and in the field of civil engineering and construction such as guardrails, wiring and piping, and soundproof walls, due to their excellent corrosion resistance. In particular, the demand for materials with excellent corrosion resistance and maintenance-free materials in more severe usage environments such as acid rain caused by air pollution, the spraying of de-icing agents to prevent roads from freezing in snowy areas, and coastal area development has been increasing in recent years, and the demand for hot-dip Al-Zn-coated steel sheets has been increasing in recent years.

The coating of hot-dip Al-Zn coated steel sheets consists of areas where Al containing supersaturated Zn has solidified into dendrites (α-Al phase) and a Zn-Al eutectic structure that exists in the gaps between dendrites (interdendrites), and is characterized by a structure in which the α-Al phase is layered in the thickness direction of the coating. This characteristic coating structure makes the corrosion progression path from the surface complex, making it difficult for corrosion to progress easily, and it is also known that hot-dip Al-Zn coated steel sheets can achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same coating thickness.

Attempts have been made to further extend the life of such hot-dip Al-Zn coated steel sheets, and hot-dip Al-Zn-Si-Mg coated steel sheets to which Mg has been added have been put to practical use.
As such a hot-dip Al-Zn-Si-Mg-plated steel sheet, for example, Patent Document 1 discloses a hot-dip Al-Zn-Si-Mg-plated steel sheet that contains an Al-Zn-Si alloy containing Mg in a plating film, the Al-Zn-Si alloy being an alloy containing 45 to 60 wt % of elemental aluminum, 37 to 46 wt % of elemental zinc, and 1.2 to 2.3 wt % of Si, and the concentration of the Mg is 1 to 5 wt %.
Furthermore, Patent Document 2 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which the plating film contains one or more of 2-10% Mg and 0.01-10% Ca to improve corrosion resistance and to enhance protective action after the base steel sheet is exposed.
Furthermore, Patent Document 3 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which a coating layer is formed containing, by mass%, 1-15% Mg, 2-15% Si, 11-25% Zn, with the remainder being Al and unavoidable impurities, and the size of intermetallic compounds such as _Mg2Si phase and _MgZn2 phase present in the plating film is set to 10 μm or less, thereby improving the corrosion resistance of the flat plate and end faces.

The above-mentioned hot-dip Al-Zn coated steel sheet has a beautiful appearance with a spangle pattern of white metallic luster, and therefore is often used without painting, and there are strong demands for its appearance. Therefore, technologies for improving the appearance of hot-dip Al-Zn coated steel sheet have been developed.
For example, Patent Document 4 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which the plated film contains 0.01 to 10% Sr, thereby suppressing wrinkle-like irregularity defects.
Patent Document 5 also discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which mottling defects are suppressed by including 500 to 3000 ppm of Sr in the plated film.

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

However, the techniques of incorporating Mg into the plating film as disclosed in Patent Documents 1 to 3 do not necessarily uniquely improve corrosion resistance.
In the hot-dip Al-Zn-Si-Mg-plated steel sheets disclosed in Patent Documents 1 to 3, the corrosion resistance is improved only by including Mg in the plating components, but the effects of components other than the above four elements (Al, Zn, Si, Mg) and the characteristics of the metal phase and intermetallic compound phase that constitute the plating film are not taken into consideration, and it is not possible to uniformly discuss the superiority or inferiority of the corrosion resistance. Therefore, even when hot-dip Al-Zn-Si-Mg-plated steel sheets are manufactured using a plating bath composition with the same contents of the above four element components, there is a problem that the corrosion resistance varies when an accelerated corrosion test is performed, and the steel sheets are not necessarily superior to Al-Zn-plated steel sheets to which Mg is not added.
Similarly, in improving the appearance of the plating, wrinkle-like irregular defects cannot necessarily be eliminated by merely adding Sr to the plating film, and there have been cases where both corrosion resistance and appearance have not been achieved in the hot-dip Al-Zn-Si-Mg-plated steel sheets disclosed in Patent Documents 4 and 5. In addition, since Mg is an element that is easily oxidized, Mg contained in the plating bath may generate oxides (top dross) near the bath surface, and in the case of hot-dip plating, FeAl-based compounds (bottom dross) containing iron that are unevenly distributed in the bath or at the bottom of the plating bath over time may be generated, and these dross may adhere to the surface of the plating film, causing convex defects and impairing the appearance of the plating film surface.
It is also known that when a steel sheet is plated in a molten Al-Zn-Si bath with added Mg, the Mg ₂ Si phase, MgZn ₂ phase, and Si phase precipitate in addition to the α-Al phase in the plated film. However, the effect of the amount and abundance ratio of each phase on corrosion resistance has not been clarified.

In view of the above circumstances, the present invention aims to provide a hot-dip Al-Zn-Si-Mg-plated steel sheet with stable and excellent corrosion resistance, and a manufacturing method thereof.

As a result of investigations aimed at solving the above-mentioned problems, the present inventors have noticed that it is important not only to control the concentrations of Al, Zn, Si, and Mg in the composition of the plating film of a hot-dip Al-Zn-Si-Mg-plated steel sheet, but also to control the concentrations of elements contained as impurities. In particular, they have found that deterioration of corrosion resistance can be effectively suppressed by appropriately controlling the Co content.
In addition, it was found that the amount of precipitation of the Mg ₂ Si and Si phases formed in the plating film of hot-dip Al-Zn-Si-Mg-plated steel sheets increases or decreases, and their ratios change depending on the balance of each component in the plating film and the conditions under which the plating film is formed, and that some phases may not precipitate depending on the balance of the compositions, and that the corrosion resistance of hot-dip Al-Zn-Si-Mg-plated steel sheets changes depending on the ratios of these phases, and that the corrosion resistance is stably improved especially when the Si phase is less than the Mg ₂ Si and Si phases. However, it is known that it is very difficult to distinguish the difference between the Mg ₂ Si and Si phases even if the plating film is observed from the surface or cross section using a general method, such as a scanning electron microscope, to obtain secondary electron images or backscattered electron images, and although it is possible to obtain microscopic information by observation using a transmission electron microscope, it was not possible to grasp the ratios of the Mg ₂ Si and Si phases, which affect macroscopic information such as corrosion resistance and appearance.

As a result of further intensive research, the inventors focused on X-ray diffraction method and discovered that by utilizing the intensity ratio of specific diffraction peaks of the Mg ₂ Si phase and the Si phase, the phase abundance ratio can be quantitatively determined, and further, that when the Mg ₂ Si phase and the Si phase satisfy a specific abundance ratio in the plating film, stable and excellent corrosion resistance can be achieved.
In addition, the inventors have discovered that by controlling the Co content and the ratio of Mg ₂ Si phase to Si phase in the above-mentioned plating film, and then controlling the Sr concentration in the plating bath, it is possible to reliably suppress the occurrence of wrinkle-like uneven defects and obtain a plated steel sheet with excellent surface appearance.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A hot-dip Al-Zn-Si-Mg-based plated steel sheet having a plating film,
The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the remainder being Zn and unavoidable impurities;
A hot-dip Al- _Zn- Si-Mg-plated steel sheet, characterized in that the Co content in the unavoidable impurities is 0.080 mass% or less with respect to the total mass of the plating film, and the diffraction intensities of Si and Mg2Si in the plating film by an X-ray diffraction method satisfy the following relationship (1):
Si (111)／Mg ₂ Si (111)≦0.8...(1)
Si (111): Diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d = 0.3668 nm)

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

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

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

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

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

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

The present invention provides hot-dip Al-Zn-Si-Mg-plated steel sheets that have stable and excellent corrosion resistance.

This is a diagram to explain the flow of the Japanese Automotive Standard Combined Cycle Test (JASO-CCT).

(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 plating film on the surface of the steel sheet, and the plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities.

The Al content in the plating film is 45 to 65 mass%, preferably 50 to 60 mass%, in consideration of the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45 mass%, dendritic solidification of Al occurs, and a plating film structure mainly composed of α-Al phase dendritic solidification structure can be obtained. The dendritic solidification structure is layered in the thickness direction of the plating film, which complicates the corrosion progression path and improves the corrosion resistance of the plating film itself. In addition, the more the α-Al phase dendrite parts are layered, the more complex the corrosion progression path becomes, making it difficult for corrosion to reach the base steel sheet, and therefore the corrosion resistance improves, so it is preferable to set the Al content to 50 mass% or more. On the other hand, if the Al content in the plating film exceeds 65 mass%, most of the Zn changes to a structure in which it is solid-solved in α-Al, and the dissolution reaction of the α-Al phase cannot be suppressed, and the corrosion resistance of the Al-Zn-Si-Mg plating deteriorates. For this reason, the Al content in the plating film must be 65% by mass or less, and preferably 60% by mass or less.

The Si in the plating film is added mainly to suppress the growth of the Fe-Al and/or Fe-Al-Si interfacial alloy layer formed at the interface with the base steel sheet, and to prevent the deterioration of the adhesion between the plating film and the steel sheet. In fact, when a steel sheet is immersed in an Al-Zn plating bath containing Si, the Fe on the steel sheet surface reacts with the Al and Si in the bath to form an Fe-Al and/or Fe-Al-Si intermetallic compound layer at the interface between the base steel sheet and the plating film. At this time, the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, so the higher the ratio of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film must be 1.0 mass% or more. On the other hand, if the Si content in the plating film exceeds 4.0 mass%, not only will the effect of suppressing the growth of the interfacial alloy layer described above saturate, but the presence of an excess Si phase in the plating film will promote corrosion, so the Si content is set to 4.0 mass% or less. Furthermore, the Si content in the plating film is preferably 3.0 mass% or less from the viewpoint of suppressing the presence of excess Si phase. In addition, from the viewpoint of easily satisfying the relational expression (1) described later in relation to the Mg content described later, it is preferable that the Si content is 1.0 to 3.0 mass%.

The plating film contains 1.0 to 10.0 mass % of Mg. By including Mg in the plating film, the above-mentioned Si can be present in the form of an intermetallic compound of _Mg2Si phase, and the promotion of corrosion can be suppressed.
In addition, when the plating film contains Mg, the _MgZn2 phase, which is an intermetallic compound, is also formed in the plating film, and the effect of further improving corrosion resistance is obtained. When the Mg content in the plating film is less than 1.0 mass%, Mg is used to dissolve in the α-Al phase, which is the main phase, rather than to form the intermetallic compounds (Mg2Si, MgZn2), so sufficient corrosion resistance cannot be ensured. On the other hand, when the Mg content in the plating film is high, the effect of improving corrosion resistance is saturated, and the workability is reduced due to the weakening of the α-Al phase, so the content is 10.0 mass% or less. Furthermore, the Mg content in the plating film is preferably 5.0 mass% or less from the viewpoint of suppressing the generation of dross during plating formation and facilitating plating bath management. In addition, 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 mass%, and in consideration of compatibility with dross suppression, the Mg content is more preferably 3.0 to 5.0 mass%.

The plating film also contains Zn and unavoidable impurities. Among these, the unavoidable impurities include Fe. This Fe is inevitably contained in the plating film as a result of dissolution of the steel sheet or bath-immersed equipment into the plating bath, and as a result of being supplied by diffusion from the base steel sheet during the formation of the interface alloy layer. The Fe content in the plating film is usually about 0.3 to 2.0 mass%.
Other unavoidable impurities include Cr, Ni, Cu, Co, W, etc. These elements are inevitably contained in the plating film due to the fact that they are dissolved in the plating bath from the WC-based or Co-Cr-W-based thermal spray coating applied to the base steel sheet or the bath-immersing equipment made of stainless steel, that they are contained as impurities in the metal ingots that are the raw materials for the plating bath, and that they are produced using pots or bath-immersing equipment that were used in the production of plated steel sheets to which these elements were intentionally added.

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

Co contained in the plating film is mainly present as a solid solution element in α-Al. When Co is contained in a concentration exceeding the solid solubility limit, it may precipitate as Fe-Co-Al compounds or Fe-Co-Al-Si compounds in the alloy layer formed at the base steel/plating interface.
In addition, although the mechanism by which the corrosion resistance deteriorates due to the inclusion of Co as an impurity is uncertain, it is hypothesized that the corrosion rate increases due to a decrease in the barrier properties of the α-Al phase in which Co is dissolved, and that the corrosion rate increases due to compounds containing Co functioning as a cathode in a corrosive environment and forming a local battery with the surrounding solidified structure.

Furthermore, the total content of unavoidable impurities in the plating film is not particularly limited, but if contained in excess, it may affect various properties of the plated steel sheet, so it is preferable that the total content be 5.0 mass% or less.

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, as determined by an X-ray diffraction method, are required to satisfy the following relationship (1):
Si (111)／Mg ₂ Si (111)≦0.8...(1)
Si (111): Diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm), Mg ₂ Si (111): Diffraction intensity of the Mg ₂ Si (111) plane (plane spacing d = 0.3668 nm)

As described above, in the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, it is important to control the ratio of the Mg ₂ Si phase and the Si phase, which are generated in the plating film due to the inclusion of Mg and Si, to a specific ratio. Although the effects of these on corrosion resistance are still under investigation and many points remain unclear, the following mechanism is presumed.

When hot-dip Al-Zn-Si-Mg-plated steel sheets are exposed to a corrosive environment, the above-mentioned intermetallic compounds dissolve preferentially over the α-Al phase, resulting in a Mg-rich environment in the vicinity of the formed corrosion products. It is presumed that in such a Mg-rich environment, the formed corrosion products are less likely to decompose, and as a result, the protective effect of the plating film is enhanced. In addition, the protective effect of the plating film is more reliably expressed when the Si in the plating film exists as the Mg ₂ Si phase rather than the Si phase, so it is considered effective to reduce the ratio of the Si phase to the Mg ₂ Si phase.

The abundance ratio of Si and Mg ₂ Si in the plating film must satisfy the relationship (1): Si (111)/Mg ₂ Si (111)≦0.8 using the diffraction peak intensity obtained by X-ray diffraction. However, when the abundance ratio of Si and Mg ₂ Si in the plating film does not satisfy the relationship (1), that is, when Si (111)/Mg ₂ Si (111)>0.8, the Si phase present in the plating film increases, and the above-mentioned Mg-rich environment cannot be obtained in the vicinity of the corrosion product, and the protective effect of the plating film is difficult to obtain. 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 particularly preferably 0.2 or less.

Here, in the above relationship (1), Si(111) is the diffraction intensity of the Si(111) plane (plane spacing d=0.3135 nm), and Mg ₂ Si(111) is the diffraction intensity of the Mg ₂ Si(111) plane (plane spacing d=0.3668 nm).
The method for measuring Si(111) and _Mg2Si (111) by X-ray diffraction involves mechanically scraping off a portion of the plating film, powdering it, and performing X-ray diffraction (powder X-ray diffraction measurement method). The diffraction intensity is measured by measuring the Si diffraction peak intensity corresponding to the interplanar spacing d = 0.3135 nm and the _Mg2Si diffraction peak intensity corresponding to the interplanar spacing d = 0.3668 nm, and calculating the ratio between these to obtain Si(111)/ _Mg2Si (111).
The amount of the plating film required for carrying out the powder X-ray diffraction measurement (amount of the plating film to be scraped off) is 0.1 g or more, preferably 0.3 g or more, from the viewpoint of accurately measuring Si(111) and _Mg2Si (111). When the plating film is scraped off, steel sheet 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 do not affect the above-mentioned peak intensity. Furthermore, the plating film is powdered and X-ray diffraction is carried out because, when X-ray diffraction is carried out on the plating film formed on the plated steel sheet, it is difficult to calculate the correct phase ratio due to the influence of the plane orientation of the plating film solidification structure.

In addition, 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 from the viewpoint of more stably improving corrosion resistance, it is preferable that the diffraction intensity of Si in the plated film, as determined by an X-ray diffraction method, satisfies the following relationship (2):
Si (111)=0...(2)
Si (111): Diffraction intensity of the Si (111) plane (planar spacing d = 0.3135 nm) It is generally known that in the dissolution reaction of an Al alloy in an aqueous solution, the presence of a Si phase as a cathode site promotes the dissolution of the surrounding α-Al phase. Therefore, reducing the amount of Si phase is also effective in terms of suppressing the dissolution of the α-Al phase. Among these, a coating without a Si phase, as shown in relationship (2) (making the diffraction peak intensity of the Si (111) zero) is the most excellent in terms of stabilizing corrosion resistance.
The method for measuring the diffraction peak intensity of the Si (111) plane by X-ray diffraction is as described above.

Here, the method for satisfying the above-mentioned relationship (1) or (2) is not particularly limited. For example, in order to satisfy the relationship (1) or (2), the abundance ratio of Si and Mg ₂ Si (diffraction intensity of Si(111) and Mg ₂ Si(111)) can be controlled by adjusting the balance of the Si content, Mg content, and Al content in the plating film. The balance of the Si content, Mg content, and Al content in the plating film does not necessarily satisfy the relationship (1) or (2) by setting them to a constant content ratio, and it is necessary to change the content ratio of Mg and Al depending on the Si content (mass%), for example.
In addition to adjusting the balance of the Si content, Mg content, and Al content in the plating film, the diffraction intensities of Si(111) and _Mg2Si (111) can be controlled so as to satisfy relationship (1) or relationship (2) by adjusting the conditions during plating film formation (e.g., cooling conditions after plating).

In the hot-dip Al-Zn-Si-Mg steel sheet of the present invention, the plating film preferably contains 0.01 to 1.0 mass% Sr. By containing Sr in the plating film, the occurrence of surface defects such as wrinkle-like irregularities can be more reliably suppressed, and good surface appearance can be achieved.
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 a wrinkle-like defect is likely to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip plated steel sheet, by adding Sr to the plating film, Sr is oxidized preferentially over Mg in the surface layer of the plating film, and the oxidation reaction of Mg is suppressed, thereby making it possible to suppress the occurrence of the wrinkle-like defect.

The plating film preferably further contains one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Sb, and B in a total amount of 0.01 to 10 mass%, because these elements, like Mg, improve the stability of corrosion products and retard the progression of corrosion. The reason why the total content of the above-mentioned elements is set at 0.01 to 10 mass% is that a sufficient corrosion retardation effect can be obtained and the effect does not saturate.

From the viewpoint of satisfying various characteristics, the coating weight of the plating film is preferably 45 to 120 g/ ^m2 per side. When the coating weight of the plating film is 45 g/ ^m2 or more, sufficient corrosion resistance is obtained for applications requiring long-term corrosion resistance, such as building materials, while when the coating weight of the plating film is 120 g/m2 or ^less , excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From the same viewpoint, the coating weight of the plating film is more preferably 45 to 100 g/ ^m2 .

The coating weight of the plating film can be calculated, for example, by dissolving and peeling off a specific area of the plating film in a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the difference in the weight of the steel sheet before and after peeling. To determine the coating weight per side using this method, the plating surface of the non-target side is sealed with tape so as not to be exposed, and then the dissolution described above is carried out.

The composition of the plating film can be confirmed by immersing the plating film in hydrochloric acid or the like to dissolve it, and then subjecting the solution to ICP emission spectrometry, atomic absorption spectrometry, or the like, just like the Co content described above. This method is merely one example, and any method that can accurately quantify the composition of the plating film may be used, 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 a composition that is almost equivalent to that of the plating bath as a whole. Therefore, the composition of the plating film can be controlled with high precision by controlling the plating bath composition.

The base steel sheet constituting the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention is not particularly limited, and cold-rolled steel sheet, hot-rolled steel sheet, etc. can be used as appropriate depending on the required performance and standards.

Furthermore, there is no particular limitation on the method for obtaining the base steel sheet. For example, in the case of the hot-rolled steel sheet, one that has been subjected to a hot rolling process and a pickling process can be used, and in the case of the cold-rolled steel sheet, it can be manufactured by adding a cold rolling process. Furthermore, in order to obtain the properties of the steel sheet, it is also possible to undergo a recrystallization annealing process, etc., before the hot-dip plating process.

(Manufacturing method of hot-dip Al-Zn-Si-Mg-plated steel sheet)
The 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 provided with a plating film, and the formation of the plating film includes a hot-dip galvanizing treatment step of immersing a base steel sheet in a plating bath having a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities.
The hot-dip galvanizing process is not particularly limited except for the conditions of the galvanizing bath described below. For example, the steel sheet can be produced by cleaning, heating, and immersing the base steel sheet in a galvanizing bath in a continuous hot-dip galvanizing 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 heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation of the steel sheet and reducing a small amount of oxide film present on the surface.

As for the plating bath used in the hot-dip plating process, as described above, the composition of the plating film as a whole will be approximately the same as the composition of the plating bath, so a plating bath containing 45-65 mass% Al, 1.0-4.0 mass% Si, 1.0-10.0 mass% Mg, with the balance being Zn and unavoidable impurities can be used.

The method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet according to the present invention is characterized in that the Co content in the inevitable impurities of the coating bath is controlled to 0.080 mass% or less with respect to the total mass of the coating bath. Since Co contained in the coating film may deteriorate the corrosion resistance of the hot-dip Al-Zn-Si-Mg-plated steel sheet as described above, the Al, Zn, Si and Mg contents in the coating bath are appropriately controlled, and further the Co content as an inevitable impurity is reduced, thereby preventing deterioration of the corrosion resistance.
The content of Co as an unavoidable impurity in the coating bath needs to be controlled to 0.080 mass% or less, more preferably 0.020 mass% or less, and even more preferably 0.001 mass% or less, based on the total mass of the coating bath. If the Co content in the coating bath is 0.080 mass% or less, the produced hot-dip Al-Zn-Si-Mg-plated steel sheet can have sufficiently excellent corrosion resistance. If the Co content in the coating bath is 0.020 mass% or less, the produced hot-dip Al-Zn-Si-Mg-plated steel sheet can have even better corrosion resistance, and if the Co content in the coating bath is 0.001 mass% or less, the produced hot-dip Al-Zn-Si-Mg-plated steel sheet can have particularly excellent corrosion resistance.
As described above, the lower the Co content in the coating bath, the better the corrosion resistance of the hot-dip Al-Zn-Si-Mg-coated steel sheet. Therefore, there is no particular lower limit for the Co content, and it is most preferable that the Co content in the coating bath is 0%.

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

As another means for reducing the Co content in the plating bath, it is preferable to use a metal block having a low Co content among impurities as a raw material for the plating bath.
Furthermore, it is also effective not to use pots or bath-immersed equipment used in the production of plated steel sheets to which Co is intentionally added in the production of hot-dip Al-Zn-Si-Mg-plated steel sheets, because this can prevent metal lumps containing Co adhering to the pots or bath-immersed equipment from dissolving and being mixed into the plating bath.

Furthermore, the temperature of the plating bath is not particularly limited, but is preferably in the 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 needs to be equal to or higher than the solidification point in order to perform hot-dip plating, and by setting the temperature to the melting point + 20° C., solidification due to a local drop in the bath temperature of the plating bath is prevented. On the other hand, the reason why the upper limit of the bath temperature is set to 650° C. is that if the bath temperature exceeds 650° C., it becomes difficult to rapidly cool the plating film, and there is a risk that the interfacial alloy layer formed between the plating film and the steel sheet will become thick.

There are also no particular limitations on the temperature of the base steel sheet immersed in the coating bath (immersion sheet temperature), but it is preferable to control it to within ±20°C of the coating bath temperature in order to ensure the coating characteristics in continuous hot-dip coating operations and to prevent changes in the bath temperature.

Furthermore, it is preferable that the immersion time of the base steel sheet in the plating bath is 0.5 seconds or more. This is because if it is less than 0.5 seconds, there is a risk that a sufficient plating film will not be formed on the surface of the base steel sheet. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer being formed between the plating film and the steel sheet, it is more preferable to keep it within 8 seconds.

In addition, depending on the required performance, hot-dip Al-Zn-Si-Mg plated steel sheets can also have a coating formed directly or via an intermediate layer on top of the plated film.

The method for forming the coating film is not particularly limited and can be appropriately selected depending on the required performance. Examples include roll coater coating, curtain flow coating, spray coating, etc. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying it using means such as hot air drying, infrared heating, and induction heating.

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 to 55)
A cold-rolled steel sheet having a thickness of 0.8 mm produced by a conventional method was used as a base steel sheet, and annealing and plating were performed using a hot-dip plating simulator manufactured by Rhesca Corporation to produce hot-dip plated steel sheet samples 1 to 55 under the conditions shown in Table 1.
The composition of the plating bath used in the production of hot-dip galvanized steel sheets was varied in the range of Al: 5-75 mass%, Si: 0.0-4.5 mass%, Mg: 0-10 mass%, and Co: 0.000-0.120 mass% so as to obtain the composition of the plating film of each sample shown in Table 1. The bath temperature of the plating bath was controlled to 450°C for Al: 5 mass%, 480°C for Al: 15 mass%, 590°C for Al: 30-60 mass%, and 630°C for Al: over 60 mass%, so that the plating immersion sheet temperature of the base steel sheet was the same as the plating bath temperature. Furthermore, in the case of Al: 30-60 mass%, the plating process was performed under the condition that the sheet temperature was cooled to a temperature range of 520-500°C in 3 seconds.
In addition, the coating weight of the plating film was controlled to be 85±5 g/ ^m2 per side for samples 1 to 52, 50±5 g/ ^m2 per side for sample 53, 100±5 g/ ^m2 per side for sample 54, and 125±5 g/ ^m2 per side for sample 55.

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

(1) Plating film (composition, coating weight, X-ray diffraction intensity)
After plating, a 100 mm diameter cut was made from each sample, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013. The adhesion weight of the plating film was calculated from the difference in mass of the sample before and after removal. The calculated adhesion weights of the plating film obtained are shown in Table 1.
The stripper solution was then filtered, and the filtrate and solid content were analyzed. Specifically, the filtrate was analyzed by ICP atomic emission spectrometry to quantify the components other than insoluble Si.
The solids were dried and incinerated in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was dissolved in hydrochloric acid, and the solution was analyzed by ICP emission spectroscopy to quantify the insoluble silicon. The silicon concentration in the plating film was calculated by adding the soluble silicon concentration obtained by filtrate analysis to the insoluble silicon concentration obtained by solid content analysis. The composition of the plating film obtained as a result of the calculation is shown in Table 1.
In addition, each sample was sheared to a size of 100 mm x 100 mm, and the plating film on the surface to be evaluated was mechanically scraped off until the base steel sheet was revealed. The obtained powder was thoroughly mixed, and 0.3 g was taken out and qualitative analysis of the above powder was performed using an X-ray diffraction device (Rigaku Corporation, "SmartLab") under the following conditions: X-ray used: Cu-Kα (wavelength = 1.54178 Å), Kβ ray removal: 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). The diffraction intensity (cps) was calculated by subtracting the base intensity from each peak intensity, and the diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm) and the diffraction intensity of the _Mg2Si (111) plane (plane spacing d = 0.3668 nm) were measured. The measurement results are shown in Table 1.

(2) Evaluation of corrosion resistance Each sample of the obtained hot-dip galvanized steel sheet was sheared to a size of 120 mm × 120 mm, and then a range of 10 mm from each edge of the surface to be evaluated, as well as the end faces and the surface not to be evaluated, were sealed with tape to expose the surface to be evaluated in a size of 100 mm × 100 mm, which was used as a sample for evaluation. Three identical samples for evaluation were prepared.
Accelerated corrosion tests were carried out on the three evaluation samples prepared as described above, with the cycle shown in Figure 1. The accelerated corrosion tests were started from wetting and continued for 300 cycles, after which the corrosion weight loss of each sample was measured by the methods described in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. The evaluation results are shown in Table 1.
◎: The corrosion loss of all three samples is 45g/m2 or less ^. ○: The corrosion loss of all three samples is 70g/m2 ^or less. ×: The corrosion loss of one or more samples is more than 70g/ ^m2.

(3) Surface Appearance For each sample of the obtained hot-dip plated steel sheet, the surface of the plating film was visually observed.
The observation results were evaluated according to the following criteria. The evaluation results are shown in Table 1.
◎: No wrinkle defects were observed. ○: Wrinkle defects were observed only within the range of 50 mm from the edge. ×: Wrinkle 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 to a size of 70 mm x 150 mm, and then eight sheets of the same thickness were sandwiched inside and bent 180° (8T bend). Cellophane tape (registered trademark) was firmly attached to the outer surface of the bent part after bending, and then peeled off. The surface condition of the plating film on the outer surface of the bent part and the presence or absence of adhesion (peeling) of the plating film on the surface of the tape used were visually observed, and the workability was evaluated according to the following criteria. The evaluation results are shown in Table 1.
◯: Neither cracks nor peeling are 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 During the production of each sample of hot-dip galvanized steel sheet, the state of the bath surface of the coating bath was visually confirmed and compared with the bath surface of the coating bath used in producing hot-dip Al-Zn-coated steel sheet (bath surface free of Mg-containing oxides). Evaluation was performed according to the following criteria, and the evaluation results are shown in Table 1.
◯: The same level as the hot-dip Al-Zn coating bath (55% Al by mass, balance Zn-1.6% by mass bath) △: There is more white oxide than in the hot-dip Al-Zn coating bath (55% Al by mass, balance Zn-1.6% by mass bath) ×: Formation of black oxide is observed in the coating bath

The results in Table 1 show that the samples of the present invention are well-balanced and superior in terms of corrosion resistance, surface appearance, workability, and bath stability compared to the samples of the comparative examples.

The present invention provides a hot-dip Al-Zn-Si-Mg-plated steel sheet with stable and excellent corrosion resistance, and a method for producing the same.

Claims (8)

A hot-dip Al-Zn-Si-Mg-based plated steel sheet having a plating film,
The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the remainder being Zn and unavoidable impurities;
The Co content in the unavoidable impurities is 0.080 mass% or less with respect to the total mass of the plating film,
A hot-dip Al-Zn-Si-Mg-plated steel sheet, characterized in that the diffraction intensities of Si and Mg ₂ Si in the plating film, as determined by an X-ray diffraction method, satisfy the following relationship (1):
Si (111)／Mg ₂ Si (111)≦0.8...(1)
Si (111): Diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d = 0.3668 nm) 2. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1, characterized in that a diffraction intensity of Si in the plated film, as determined by an X-ray diffraction method, satisfies the following relationship (2):
Si (111)=0...(2)
Si (111): Diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm) The hot-dip Al-Zn-Si-Mg-plated steel sheet according to claim 1 or 2, characterized in that the plating film further contains Sr: 0.01 to 1.0 mass%. The hot-dip Al-Zn-Si-Mg-plated steel sheet according to any one of claims 1 to 3, characterized in that the Al content in the plating film is 50 to 60 mass %. The hot-dip Al-Zn-Si-Mg-plated steel sheet according to any one of claims 1 to 4, characterized in that the Si content in the plating film is 1.0 to 3.0 mass%. The hot-dip Al-Zn-Si-Mg-plated steel sheet according to any one of claims 1 to 5, characterized in that the Mg content in the plating film is 1.0 to 5.0 mass%. A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet having a plating film, comprising the steps of:
The formation of the plating film includes a hot-dip plating process in which a base steel sheet is immersed in a plating bath having a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities;
A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet, characterized in that the Co content in unavoidable impurities in the plating bath is controlled to 0.080 mass% or less, based on the total mass of the plating bath, so that the diffraction intensities of Si and Mg2Si in the plating film, as measured by an X-ray diffraction method, satisfy the following relationship (1 ₎ :
Si (111)／Mg ₂ Si (111)≦0.8...(1)
Si (111): Diffraction intensity of the Si (111) plane (plane spacing d = 0.3135 nm),
Mg ₂ Si (111): Diffraction intensity of the (111) plane of Mg ₂ Si (plane spacing d = 0.3668 nm) The method for producing hot-dip Al-Zn-Si-Mg-plated steel sheet according to claim 7, characterized in that the plating bath further contains Sr: 0.01 to 1.0 mass%.

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