WO2024214328A1 - Molten al-zn-based plated steel sheet and method for manufacturing same - Google Patents

Abstract

A purpose of the present invention is to provide a molten Al-Zn-based plated steel sheet having stably excellent workability and processed part corrosion resistance.　In order to achieve the above purpose, a plating film according to the present invention is characterized by containing 45-65 mass % of Al, 1.0-3.0 mass % of Si, and the balance of Zn and unavoidable impurities, wherein the content of Mg in the unavoidable impurities is less than 0.5 mass % based on the total mass of the plating film.

Description

Hot-dip Al-Zn coated steel sheet and manufacturing method thereof

The present invention relates to hot-dip Al-Zn-plated steel sheets that have stable and excellent workability and corrosion resistance in processed areas, and to a method for manufacturing the same.

Hot-dip Al-Zn-plated steel sheets, typified by 55% Al-Zn, are known to exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial corrosion protection properties of Zn with the high corrosion resistance of Al, as disclosed in Patent Document 1, for example.
For this reason, 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 outdoors for long periods of time, and in the field of civil engineering and construction such as guardrails, wiring and piping, soundproof walls, etc. In particular, the demand for materials with excellent corrosion resistance under harsher 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, and maintenance-free materials, has been increasing in recent years, and thus the demand for hot-dip Al-Zn coated steel sheets has been increasing in recent years.

The coating of hot-dip Al-Zn-plated steel sheets is composed 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-plated steel sheets can achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same coating thickness.

In general, hot-dip Al-Zn plated steel sheet is produced by using a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base steel sheet, and subjecting the base steel sheet to recrystallization annealing and hot-dip plating treatment in an annealing furnace of a continuous hot-dip plating facility.
In addition to the specified concentrations of Al and Zn, Si is usually added to the coating bath to suppress the excessive growth of the interfacial alloy layer that forms at the interface between the base steel (base steel sheet) and the coating. This action of Si makes it possible to control the thickness of the interfacial alloy layer of hot-dip Al-Zn coated steel sheets to about 1 to 5 μm. For a given coating thickness, the thinner the interfacial alloy layer, the thicker the main layer that exhibits high corrosion resistance will be. Therefore, it is known that suppressing the growth of the interfacial alloy layer leads to improved corrosion resistance.

Special Publication No. 46-7161 JP 2004-285387 A Special table 2016-540885 publication

However, it is known that when hot-dip Al-Zn-plated steel sheets are processed, such as by bending, cracks can occur in the plating film in the processed area depending on the degree of processing (degree of processing). In hot-dip Al-Zn-plated steel sheets, the thick interfacial alloy layer mentioned above is the starting point of the cracks, and the dendrite gaps in the plating film become the crack propagation path. Therefore, even when the same degree of processing is performed by bending, the cracks tend to open relatively large compared to hot-dip galvanized steel sheets with the same plating film thickness. Therefore, in applications with a large degree of processing, there are problems such as the appearance being marred by the occurrence of large cracks visible to the naked eye, and the corrosion resistance of the cracked areas where the base steel sheet is exposed is significantly reduced compared to areas without cracks (decreased corrosion resistance of processed areas).

In the manufacture of hot-dip plating, it is generally known that impurities inevitably get mixed into the plating bath, and hot-dip Al-Zn plating is no exception. Impurities that get mixed into the plating film include impurities contained in the plating raw materials, and Fe, Cr, Ni, Cu, Co, W, Mg, Ca, etc. that get mixed in due to elution from the base steel sheet or bath-immersed equipment, and these components are inevitably included in the plating film.
In particular, the production volume of hot-dip Zn-Al-Mg coated steel sheets and hot-dip Al-Zn-Si-Mg coated steel sheets, which have high corrosion resistance, has increased in recent years. As a result of the expanded distribution of Zn raw materials containing high concentrations of Mg, which are produced by recycling dross generated during the production of these steel sheets, Mg is often mixed into the coating bath as an impurity, and ultimately into the coating film.

As described above, unavoidable impurities in the plating film may cause deterioration of the properties of the hot-dip plated steel sheet, such as the appearance, corrosion resistance, and workability, and the presence or absence of the effect of impurities often depends on the composition of the plating film and the impurity concentration. In other words, even impurities of the same composition may be harmful to the properties of the plated steel sheet in some cases and harmless in other cases. For this reason, the effect of impurities on the properties of each hot-dip plated steel sheet is investigated, and technologies for controlling the impurity concentration have been developed to stably obtain the required properties.
For example, Patent Document 2 discloses a hot-dip galvanized steel sheet having excellent appearance, which contains, in mass %, 0.10 to 0.6% Al, 0.03 to 0.3% Bi, and the balance being Zn and unavoidable impurities, with the contents of each of Pb, Sn, and Cd as the unavoidable impurities being controlled to 0.002%.
Furthermore, Patent Document 3 discloses a hot-dip Zn-Al-Mg plated steel sheet having excellent corrosion resistance, which contains 4.4 to 5.6% Al, 0.3 to 0.56% Mg, with the balance being Zn and unavoidable impurities, and in which the content of Ni as one of the unavoidable impurities is controlled so as not to be contained.

However, the techniques disclosed in Patent Documents 2 and 3 focus on improving corrosion resistance, and do not fully consider the effects of unavoidable impurities on formability and corrosion resistance of worked parts in Mg-free hot-dip Zn-Al-plated steel sheets or hot-dip Al-Zn-plated steel sheets with a high Al concentration. Thus, there has been a demand for the development of a technique that can more reliably and stably achieve excellent formability and corrosion resistance of worked parts.
In addition, in the manufacture of hot-dip galvanized steel sheets, not just Al-Zn-galvanized steel sheets, most impurity control is limited to concentration control, and no technology has been established to control the shape, such as size. There has been a demand for the development of technology that can achieve more stable and excellent workability and corrosion resistance in processed areas.

In view of these circumstances, the present invention aims to provide a hot-dip Al-Zn-plated steel sheet that reliably and stably has excellent workability and corrosion resistance in processed areas, and a manufacturing method thereof.

As a result of investigations conducted by the inventors to solve the above problems, they have noticed that for hot-dip Al-Zn plated steel sheets, it is important not only to control the concentrations of Al, Zn, and Si in the composition of the hot-dip Al-Zn plated film, but also to control the concentrations of elements contained as impurities. In particular, they have found that by properly controlling the Mg content, deterioration of corrosion resistance can be effectively suppressed, and further that by properly controlling the size of Mg-Zn compounds present as impurities in the plated film, deterioration of workability and corrosion resistance of processed parts can be more effectively suppressed.

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-plated steel sheet having a plating film,
The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 3.0 mass% Si, and the remainder being Zn and unavoidable impurities,
A hot-dip Al-Zn plated steel sheet, characterized in that the Mg content in the unavoidable impurities is less than 0.5 mass% with respect to a total mass of the plating film.

2. The hot-dip Al-Zn-plated steel sheet according to 1 above, characterized in that the plating film contains Mg-Zn-based compounds, and the major axis of the Mg-Zn-based compounds is less than 10 μm.

3. The hot-dip Al-Zn-plated steel sheet according to 1 above, characterized in that the plating film does not contain any Mg-Zn-based compounds.

4. The hot-dip Al-Zn plated steel sheet according to any one of 1 to 3 above, wherein the plating film further contains 0.01 to 3.0 mass% in total of one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi.
5. The hot-dip Al-Zn plated steel sheet according to any one of 1 to 4 above, characterized in that the diffraction intensity of MgZn ₂ in the plated film by X-ray diffraction method satisfies the following relationship (1):
MgZn ₂ (100)=0...(1)
MgZn ₂ (100): Diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm)

6. A method for producing a hot-dip Al-Zn-plated steel sheet having a plating film, comprising the steps of:
The formation of the plating film includes a step of forming the plating film on a base steel sheet using a plating bath having a composition containing 45 to 65 mass% Al and 1.0 to 3.0 mass% Si, with the balance being Zn and unavoidable impurities;
A method for producing a hot-dip Al-Zn plated steel sheet, comprising controlling a Mg content in unavoidable impurities in the plating bath to less than 0.5 mass% based on a total mass of the plating bath.
7. The method for producing a hot-dip Al-Zn coated steel sheet according to 6 above, characterized in that the coating bath further contains 0.01 to 3.0 mass % in total of one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi.

The present invention provides a hot-dip Al-Zn-plated steel sheet and its manufacturing method that has reliable and stable excellent workability and corrosion resistance in the processed area.

1 is an enlarged schematic view of a cross section of a hot-dip Al-Zn plated steel sheet according to an embodiment of the present invention. FIG.

(Hot-dip Al-Zn coated steel sheet)
As shown in FIG. 1 , the hot-dip Al-Zn plated steel sheet of the present invention has a plating film 20 on the surface of a base steel sheet 10 .
The plating film 20 has a composition containing 45 to 65 mass % Al, 1.0 to 3.0 mass % Si, and the remainder 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 has a structure in which it 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 improving the corrosion resistance. Therefore, it is preferable that the Al content in the plating film is 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 hot-dip Al-Zn 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.

Also, 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 that forms 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, an alloying reaction occurs between the Fe on the steel sheet surface and the Al and Si in the bath, and an Fe-Al and/or Fe-Al-Si intermetallic compound layer forms 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 3.0 mass%, not only will the aforementioned effect of inhibiting the growth of the interfacial alloy layer saturate, but the presence of excess Si phase in the plating film will also reduce workability, so the Si content should be 3.0% or less.

The plating film contains Zn and unavoidable impurities. Among these, the unavoidable impurities contain 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, Mg, Ca, etc. These elements are inevitably contained in the plating film due to the fact that they are dissolved in the plating bath from the base steel sheet, the bath equipment made of stainless steel, or the WC-based or Co-Cr-W-based thermal spray coating applied to the bath equipment, 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 equipment that were used in the production of plated steel sheets to which these elements were intentionally added.

The hot-dip Al-Zn-plated steel sheet of the present invention is characterized in that the Mg content in the unavoidable impurities is less than 0.5 mass% with respect to the total mass of the plating film. Since Mg contained in the plating film may deteriorate the workability and corrosion resistance of the processed part of the hot-dip Al-Zn-plated steel sheet, the deterioration of the workability and corrosion resistance of the processed part can be suppressed by appropriately controlling the Al, Zn, and Si contents in the plating film described above and further suppressing the Mg content as an unavoidable impurity. From the same viewpoint, the Mg content in the unavoidable impurities is preferably 0.3 mass% or less with respect to the total mass of the plating film, and more preferably 0.1 mass% or less. Thus, the lower the Mg content in the plating film, the better the corrosion resistance of the hot-dip Al-Zn-plated steel sheet of the present invention is, so there is no particular lower limit. However, since it is technically difficult to make the Mg content in the plating film exactly 0.000% by mass, the lower limit of the Mg content in the plating film is essentially about 0.001% by mass.

When Mg is contained in the inevitable impurities, Mg-Zn compounds may be contained as impurities in the coating of the hot-dip Al-Zn plated steel sheet. In most cases, the Mg-Zn compounds are _MgZn2 , but they are not particularly limited and may include other binary intermetallic compounds such as _Mg2Zn11 , and ternary intermetallic _compounds such as _Mg21 (Al,Zn) ₁₇ and _Mg32 (Al,Zn) ₄₉ .
From the viewpoint of realizing better workability and corrosion resistance of the worked portion, it is preferable that the plating film does not contain these Mg-Zn based compounds.

The presence of Mg-Zn compounds in the plating film can be confirmed, for example, by using a scanning electron microscope to observe the plating film from the surface or cross section using secondary electron images or backscattered electron images, and analyzing it using energy dispersive X-ray spectroscopy (EDS). For example, 5 to 10 locations on a 100 μm plating cross section can be arbitrarily selected, and observation and element mapping analysis can be performed at an accelerating voltage of 5 kv or less, and further point analysis can be performed on the areas where Mg is detected to confirm the composition of Mg-Zn inclusions. This method is merely one example, and any method that can confirm the presence of Mg-Zn compounds can be used, and is not particularly limited.

Furthermore, when the plating film contains an Mg-Zn based compound, it is preferable that the major axis of the Mg-Zn based compound is small.
Since the Mg-Zn compounds present in the plating film are hard and brittle, they may become the starting point of cracks when subjected to severe bending or stretching, which may cause deterioration of workability and corrosion resistance of the processed part. In particular, when coarse Mg-Zn compounds are present in the plating film, the workability and corrosion resistance of the processed part of the hot-dip Al-Zn-plated steel sheet may be significantly reduced. Therefore, in order to obtain a hot-dip Al-Zn-plated steel sheet having better workability and corrosion resistance of the processed part, it is effective to control the size of the Mg-Zn compounds contained as impurities in the plating film to be small, and specifically, it is preferable that the major axis of the Mg-Zn compounds is less than 10 μm. From the same viewpoint, it is preferable that the major axis of the Mg-Zn compounds is as small as possible.
The major axis of the Mg-Zn-based compound can be measured, for example, by observing a cross-section of the plating film using a scanning electron microscope to obtain a backscattered electron image, confirming that the compound is an Mg-Zn-based compound using EDS, and then observing a backscattered electron image with an enlarged observation field that includes the Mg-Zn-based compound.
In the present invention, the long diameter of the Mg-Zn based compounds is defined as the average of the top five long diameters of all the Mg-Zn based compounds observed in a continuous cross section of the plating film having a length of 5 mm in a direction parallel to the surface of the base steel sheet.

In addition, in the hot-dip Al-Zn plated steel sheet of the present invention, it is preferable that the diffraction intensity of MgZn ₂ in the plated film by an X-ray diffraction method satisfies the following relationship (1).
MgZn ₂ (100)=0...(1)
MgZn ₂ (100): Diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm),

As described above, in order to stabilize the workability and corrosion resistance of the processed part of the hot-dip Al-Zn-plated steel sheet, it is important to suppress the content of Mg as an inevitable impurity and to control the formation of Mg-Zn-based compounds in the plating film as little as possible. In particular, by minimizing the amount of MgZn ₂ , which is thermodynamically stable and easily formed in the hot-dip Al-Zn-plated film (by making the diffraction intensity of MgZn ₂ (100) zero), the workability and corrosion resistance of the processed part can be further stabilized.

Here, in the above-mentioned relationship (1), MgZn ₂ (100) is the diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d=0.4510 nm).
The method for measuring MgZn ₂ (100) by X-ray diffraction can be calculated by mechanically scraping off a part of the plating film, powdering it, and then subjecting it to X-ray diffraction (powder X-ray diffraction measurement method). The diffraction intensity can be measured by measuring the diffraction peak intensity of MgZn ₂ corresponding to the interplanar spacing d = 0.4510 nm.

In addition, the amount of the plating film required for performing 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 measuring MgZn ₂ (100) with high accuracy. In addition, 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 performed because, 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 plating film solidification structure, making it difficult to evaluate the amount of substance present from the diffraction intensity of a specific interplanar spacing.

Here, the method for satisfying the above-mentioned relationship (1) is not particularly limited. For example, the amount of MgZn2 and Mg2Zn11 (diffraction intensity of MgZn2(100) and Mg2Zn11(321)) can be controlled to be low by controlling the Mg content in the plating film to be low and reducing the ratio of the Mg content to the Zn _content (for _example , Mg _{/ Zn} to _{be 0.008} or less, preferably 0.006 or less).
Furthermore, other than the method of controlling the Mg content in the plating film, the Mg content in the plating film can be controlled to a specific value and then the conditions for forming the plating film (e.g., cooling conditions after plating) can be adjusted to satisfy the above relationship (1).

Furthermore, it is preferable that the Mg content relative to the Fe content (Mg/Fe) of the unavoidable impurities in the plating film is 1.0 or less. This reduces the amount of Mg dissolved in the Fe-Al and/or Fe-Al-Si interface alloy layers, suppressing the increase in hardness of the interface alloy layers due to solid solution strengthening, thereby achieving better workability and corrosion resistance of the processed parts. From the same perspective, it is more preferable that the Mg/Fe ratio is 0.6 or less.

The plating film may further contain 0.01 to 3.0 mass% in total of one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi. These elements have the effect of improving the stability of the corrosion products when the plating film corrodes, thereby delaying the progress of corrosion, and the effect of stabilizing the spangle size on the plating surface, thereby improving the surface appearance.

Moreover, 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, and 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 derived, for example, by a method in which a specific area of the plating film is dissolved and peeled off with a mixed solution of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the coating weight is calculated from 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 above-mentioned dissolution is carried out.
Similarly to the above-mentioned plating weight, the composition of the plating film can be confirmed by immersing the plating film in a hydrochloric acid solution or the like to dissolve it, and subjecting the solution to ICP emission spectrometry, atomic absorption spectrometry, etc. 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 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.

Furthermore, the base steel sheet constituting the hot-dip Al-Zn-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, the method for obtaining the base steel sheet is not particularly limited. 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 further adding a cold rolling process. Furthermore, in order to obtain the properties of the steel sheet, it is also possible to pass through a recrystallization annealing process or the like before the hot-dip plating process.

As shown in FIG. 1 , the hot-dip Al-Zn plated steel sheet of the present invention has a plating film 20 formed on a base steel sheet 10, but an intermediate layer or a coating film can also be further formed on the plating film, if necessary.
The type of the coating film and the method for forming the coating film are not particularly limited and can be appropriately selected depending on the required performance. For example, the coating film can be formed by a method such as roll coater coating, curtain flow coating, spray coating, etc. After coating the coating material containing an organic resin, the coating film can be formed by heating and drying the coating film by means of hot air drying, infrared heating, induction heating, etc.
The intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip Al-Zn-plated steel sheet and the coating film. Examples include a chemical conversion coating film and a primer such as an adhesive layer. The chemical conversion coating film can be formed, for example, by a chromate treatment or a chromium-free chemical conversion treatment in which a chromate treatment liquid or a chromium-free chemical conversion coating liquid is applied, and then dried at a steel sheet temperature of 80 to 300°C without rinsing with water. These chemical conversion coating films may be single-layered or multi-layered, and in the case of multi-layered films, multiple chemical conversion treatments may be performed sequentially.

(Manufacturing method of hot-dip Al-Zn coated steel sheet)
The method for producing a hot-dip Al-Zn plated steel sheet of the present invention is a method for producing a hot-dip Al-Zn plated steel sheet provided with a plating film.

The manufacturing method of the hot-dip Al-Zn-plated steel sheet of the present invention includes a step of forming the plating film on the base steel sheet using a plating bath having a composition containing 45-65 mass% Al and 1.0-3.0 mass% Si, with the remainder being Zn and unavoidable impurities.

The step of forming the plating film is not particularly limited except for the plating bath conditions described below.
For example, the steel sheet can be produced by cleaning, heating, and immersing the base steel sheet in a coating 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.

The plating bath used in the process of forming the plating film contains 45-65 mass% Al and 1.0-3.0 mass% Si, with the remainder consisting of Zn and unavoidable impurities. As mentioned above, this is because the composition of the plating film as a whole is almost the same as the composition of the plating bath.

The method for producing a hot-dip Al-Zn plated steel sheet of the present invention is characterized in that the Mg content in the unavoidable impurities in the coating bath is controlled to less than 0.5 mass % with respect to the total mass of the coating bath.
As described above, Mg contained in the plating film may deteriorate the workability and corrosion resistance of worked portions of the hot-dip Al-Zn plated steel sheet. Therefore, by appropriately controlling the contents of Al, Zn, and Si in the plating bath and further reducing the content of Mg as an unavoidable impurity, deterioration of the workability and corrosion resistance of worked portions can be suppressed.

　Furthermore, the content of Mg as an unavoidable impurity in the plating bath must be controlled to 0.5 mass% or less, preferably 0.2 mass% or less, based on the total mass of the plating bath. If the Mg content in the plating bath is less than 0.5 mass%, the produced hot-dip Al-Zn-plated steel sheet can have sufficiently excellent workability and corrosion resistance of the processed part, and if it is 0.2 mass% or less, even better workability and corrosion resistance of the processed part can be realized. Thus, the lower the Mg content in the plating bath, the better the corrosion resistance of the hot-dip Al-Zn-plated steel sheet, so there is no particular lower limit for the Mg content. However, since it is technically difficult to make the Mg content in the plating bath completely 0.000 mass%, the lower limit of the Mg content in the plating is substantially about 0.001 mass%.

The means for reducing the Mg content in the coating bath is not particularly limited. For example, it is effective not to intentionally add Mg to the coating bath, or not to use pots or bath-immersed equipment used in the manufacture of coated steel sheets to which Mg is intentionally added, such as Zn-Al-Mg-based coated steel sheets or Al-Zn-Si-Mg-based coated steel sheets, in the manufacture of hot-dip Al-Zn-based coated steel sheets. This is because it is possible to prevent Mg-containing metal lumps adhering to the pots or bath-immersed equipment from dissolving and being mixed into the coating bath.
As another means for reducing the Mg content in the plating bath, it is preferable to use a metal block having a low Mg content among impurities as a raw material for the plating bath.

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., rapid cooling of the plating film becomes difficult, and there is a risk that the interfacial alloy layer formed between the plating film and the steel sheet becomes thick.

The temperature of the base steel sheet immersed in the coating bath (immersion sheet temperature) is not particularly limited, but it is preferable to control it to within ±20°C of the coating bath temperature in order to ensure the coating characteristics in the continuous hot-dip coating operation 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, in the manufacturing method of the hot-dip Al-Zn-plated steel sheet of the present invention, in addition to the above-mentioned plating film formation process and the heating/cooling process after plating film formation, it is possible to appropriately carry out processes that are used in the manufacture of ordinary plated steel sheets.

[Samples 1 to 32]
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 32 under the conditions shown in Table 1.
The composition of the plating bath used in producing the hot-dip plated steel sheets was changed in various ranges within the ranges of Al: 0.2 to 70 mass%, Si: 0.0 to 3.2 mass%, B: 0.00 to 0.02 mass%, Ca: 0.0 to 1.0 mass%, Ti: 0.0 to 0.1 mass%, V: 0.1 to 0.1 mass%, Cr: 0.0 to 0.2 mass%, Mn: 0.0 to 0.1 mass%, Sr: 0.0 to 0.1 mass%, Mo: 0.0 to 0.1 mass%, In: 0.0 to 0.5 mass%, Sn: 0.0 to 0.5 mass%, Sb: 0.0 to 0.1 mass%, Ce: 0.0 to 1.0 mass%, and Bi: 0.00 to 0.05 mass% so as to obtain the composition of the plating film of each sample shown in Table 1. The bath temperature of the coating bath was controlled to be 460°C when Al was 0.2-5% by mass, 600°C when Al was 35-55% by mass, and 660°C when Al was over 60% by mass, so that the temperature of the base steel sheet entering the coating was the same as the coating bath temperature. Furthermore, when Al was 35-65% by mass, the coating process was carried out 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 29, 50±5 g/ ^m2 per side for sample 30, 100±5 g/ ^m2 per side for sample 31, and 125±5 g/ ^m2 per side for sample 32.

[evaluation]
The following evaluations were carried out on each of the obtained hot-dip plated steel sheet samples. The evaluation results are shown in Table 1.

(1) Plating film (composition, coating amount, Mg-based compounds)
A 100 mm diameter sample was punched out from each of the prepared plated steel sheets, the non-measurement surfaces were sealed with tape, and the plating was then dissolved and stripped off using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401: 2013, and the adhesion weight of the plating film was calculated from the difference in mass of the sample before and after stripping. 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.
Furthermore, after each sample was sheared to a size of 15 mm x 15 mm, the steel sheet was embedded in a conductive resin so that the cross section of the steel sheet could be observed, and mechanical polishing was performed. Using a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss), a backscattered electron image of a width of 100 μm was continuously taken at an accelerating voltage of 3 kv for a continuous cross section of an arbitrarily selected plating film having a length of 5 mm in a direction parallel to the surface of the base steel sheet. Furthermore, in the same device, an energy dispersive X-ray spectrometer (Ultim Extreme manufactured by Oxford Instruments) was used to perform element mapping analysis (Al, Zn, Si, Fe, and Mg) of each cross section at an accelerating voltage of 3 kv. For the parts where high Mg intensity was detected in this analysis, point analysis was performed using the same spectrometer at an accelerating voltage of 3 kv, and the substance was identified from the presence or absence of Mg-based compounds and the semi-quantitative values of the obtained components. The major axis of all Mg-based compounds confirmed in the observation field was measured, and the average of the top five values was taken as the major axis. The calculated major axis is shown in Table 1.
Furthermore, after shearing each sample to a size of 100 mm x 100 mm, the plating film on the evaluation target surface was mechanically scraped off until the base steel sheet was revealed, and the obtained powder was thoroughly mixed, and 0.3 g was taken out and qualitative analysis of the scraped powder was performed using an X-ray diffraction apparatus (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 intensity obtained by subtracting the base intensity from each peak intensity was taken as each diffraction intensity (kcps), and the diffraction intensity (kcps) of the (100) plane of MgZn ₂ (planar spacing d = 0.4510 nm) was measured. The measurement results of the diffraction intensity are shown in Table 1.

(2) Workability Evaluation Each sample of the obtained hot-dip galvanized steel sheet was sheared to a size of 70 mm x 150 mm, and n sheets (n = 8, 10, 12, 14, 16) of the same thickness were sandwiched inside, and each was subjected to 180° bending (nT bending) so as to obtain a 150 mm apex. Cellophane tape (registered trademark) was firmly attached to the outer surface of the bent part after bending, and then peeled off. The outer surface (apex part) of the bent part was observed with a scanning electron microscope (ULTRA55 manufactured by Carl Zeiss) at an accelerating voltage of 5 kv to confirm the occurrence of cracks, and the workability was evaluated according to the following criteria. The evaluation results are shown in Table 1.
◯: No large cracks with an opening width of 20 μm or more were observed. ×: Large cracks with an opening width of 20 μm or more were observed.

(3) Evaluation of corrosion resistance of processed parts Each sample of the obtained hot-dip plated steel sheet was sheared to a size of 70 mm x 150 mm, and then each end face was sealed with tape. 14 or 16 sheets of the same thickness were sandwiched inside, and a 14T bend or 16T bend was performed to obtain a vertex of 150 mm.
The samples prepared as described above were subjected to the Japanese Automotive Standard Combined Cyclic Test (JASO-CCT). The accelerated corrosion test was started from wet condition and continued for 60 cycles. After that, the appearance of the outer surface (vertex) of the bent part of each sample was visually inspected and judged according to the following criteria. The inspection results and the judgment results are shown in Table 1.
○: No red rust or white rust was observed on the sample that was bent at 16T. ×: Red rust or white rust was observed on the sample that was bent at 16T.

The results in Table 1 show that the samples of the present invention have a good balance of workability and corrosion resistance of the processed parts compared to the samples of the comparative examples.

The present invention provides a hot-dip Al-Zn-plated steel sheet and its manufacturing method that has reliable and stable excellent workability and corrosion resistance in the processed area.

10 Base steel sheet 20 Plating film 21 Main layer 22 Interface alloy layer 211 Dendrite 212 Interdendrite

Claims (7)

A hot-dip Al-Zn-plated steel sheet having a plating film,
The plating film has a composition containing 45 to 65 mass% Al, 1.0 to 3.0 mass% Si, and the remainder being Zn and unavoidable impurities,
A hot-dip Al-Zn plated steel sheet, characterized in that the Mg content in the unavoidable impurities is less than 0.5 mass% with respect to a total mass of the plating film. The hot-dip Al-Zn-plated steel sheet according to claim 1, characterized in that the plating film contains Mg-Zn-based compounds, and the major axis of the Mg-Zn-based compounds is less than 10 μm. The hot-dip Al-Zn-plated steel sheet according to claim 1, characterized in that the plating film does not contain any Mg-Zn-based compounds. The hot-dip Al-Zn plated steel sheet according to any one of claims 1 to 3, characterized in that the plated film further contains one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce and Bi in a total amount of 0.01 to 3.0 mass%. The hot-dip Al-Zn plated steel sheet according to any one of claims 1 to 4, characterized in that a diffraction intensity of MgZn ₂ in the plated film by an X-ray diffraction method satisfies the following relationship (1):
MgZn ₂ (100)=0...(1)
MgZn ₂ (100): Diffraction intensity of the (100) plane of MgZn ₂ (plane spacing d = 0.4510 nm) A method for producing a hot-dip Al-Zn-plated steel sheet having a plating film, comprising:
The formation of the plating film includes a step of forming the plating film on a base steel sheet using a plating bath having a composition containing 45 to 65 mass% Al and 1.0 to 3.0 mass% Si, with the balance being Zn and unavoidable impurities;
A method for producing a hot-dip Al-Zn plated steel sheet, comprising controlling a Mg content in unavoidable impurities in the plating bath to less than 0.5 mass% based on a total mass of the plating bath. 7. The method for producing a hot-dip Al-Zn coated steel sheet according to claim 6, characterized in that the coating bath further contains 0.01 to 3.0 mass % in total of one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi.

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