JP6683258B2 - Hot-dip Al-plated steel sheet and method for manufacturing hot-dip Al-plated steel sheet - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a hot-dip Al-based plated steel sheet having excellent chemical conversion treatment property and coating film swelling resistance, and a method for producing the same.

As a plated steel material having excellent corrosion resistance and high-temperature oxidation resistance, Al-based plated steel sheets are widely used in the fields of muffler materials for automobiles and building materials.
However, for Al-based plated steel sheets, although corrosion products stabilize and exhibit excellent corrosion resistance in environments with low chloride ion concentrations and in corrosive environments under dry conditions, chlorination may occur in wet conditions such as in the snow melting salt spraying area. There is a problem that sufficient corrosion resistance cannot be exhibited in an environment in which an object is exposed for a long time. This is because, when exposed to chloride for a long time in a wet state, the elution rate of the plating becomes extremely high, which easily leads to corrosion of the base steel sheet. Further, when an Al-based plated steel sheet is used for coating, there is a problem that the corrosion rate of Al increases due to an alkaline atmosphere under the coating film, causing blistering of the coating film.

Therefore, various techniques have been developed for the purpose of improving the corrosion resistance of the molten Al-based plated steel sheet.
For example, in Patent Document 1, the surface of the steel sheet has an intermetallic compound coating layer containing Al, Fe, Si and having a thickness of 5 μm or less, and the surface of the intermetallic compound coating layer has a weight of Disclosed is a hot-dip aluminized steel sheet having a coating layer in which Si: 2 to 13%, Mg: more than 3% to 15%, and the balance substantially consisting of Al.

In addition, in Patent Document 2, a molten Al-Mg-Si-based plating layer containing Mg: 3 to 10% and Si: 1 to 15% by weight and the balance consisting of Al and unavoidable impurities is provided on a steel sheet surface. a molten Al group plated steel sheet formed in, the plating layer is at least "Al phase", consists "Mg ₂ Si phase", high corrosion resistance with a metal structure diameter is 10μm or less of the "Mg ₂ Si phase" A plated steel sheet is disclosed.

Further, in Patent Document 3, the surface of the steel material contains Mg: 6 to 10 mass%, Si: 3 to 7 mass%, Fe: 0.2 to 2 mass% and Mn: 0.02 to 2 mass% with the balance being A plating layer composed of Al and unavoidable impurities is provided, and the plating layer has an αAl-Mg ₂ Si- (Al-Fe-Si-Mn) pseudo-ternary eutectic structure, and the pseudo-ternary in the plating layer An Al-based plated steel material having an area ratio of eutectic structure of 30% or more is disclosed.
Further, in Patent Document 4, the plating layer on the surface of the steel material contains a massive intermetallic compound composed of one or more IIa group (alkaline earth metal) elements and one or more IVb group elements. However, there is disclosed an aluminum-plated surface-treated steel material having excellent corrosion resistance, characterized in that the major axis of the intermetallic compound exceeds 10 μm and the ratio of the minor axis to the major axis is 0.4 or more.

JP, 2000-239820, A Japanese Patent No. 4199404 Japanese Patent No. 5430022 Japanese Patent No. 5000039

However, the technique of Patent Document 1 has a problem that the Al ₃ Mg ₂ phase is precipitated in the plating layer and the local dissolution of the plating layer starting from this phase proceeds.
Further, the technique of Patent Document 2 has a problem in that a long and thin needle-shaped or plate-shaped Al—Fe compound is deposited in the plating layer, and this is used as a local cathode to locally dissolve the plating layer. It was
Further, in the technique of Patent Document 3, as a result of the incorporation of Mn into the Al-Fe compound into the eutectic structure, it is possible to further improve the corrosion resistance including prevention of local deterioration of the corrosion resistance. However, when a coating film is provided on a hot-dip Al-plated steel sheet, the area under the coating film is an alkaline / low oxygen environment, and the galvanic pair forms with the noble part of the underlying steel sheet where the plating layer is exposed due to flaws. To do. As a result, the base steel sheet is sacrificed and corrosion-protected, but the corrosion rate of the plating layer is extremely increased, and blisters may occur. Therefore, further improvement of the coating film swelling resistance has been desired.
Furthermore, in the technique of Patent Document 4, it is considered that an intermetallic compound having a size larger than the plating thickness may exist during plating, and the intermetallic compound is inevitably exposed on the plating surface. In that case, in the chemical conversion treatment step performed as a pretreatment for coating, the intermetallic compound is actively dissolved in the chemical conversion treatment to form a massive oxide, which inhibits the chemical conversion treatment reaction, resulting in chemical conversion treatment on the plating. There is a problem that it is difficult to form a film uniformly (poor chemical conversion treatment). Then, the deterioration of the chemical conversion processability may lead to a decrease in the adhesiveness of the coating film formed thereafter.

  Therefore, an object of the present invention is to provide a hot-dip Al-based plated steel sheet having excellent chemical conversion treatment property and coating film swelling resistance, and a method for producing the hot-dip Al-based plated steel sheet.

The present inventors, as a result of repeated studies to solve the above problems, conventionally, about Mg ₂ Si in the plating, which was supposed to be the starting point of corrosion, does not miniaturize, but increases its size. It was noted that the effect of improving the swelling resistance of the coating film can be obtained. Although the mechanism has not been clarified, Mg ₂ Si located near the plating surface due to the large grain size dissolves almost simultaneously with the dissolution of the α-Al phase that occurs from the plating surface in a corrosive environment, and Mg and Si are concentrated. This produces eroded corrosion products. Since this corrosion product has the effect of suppressing the corrosion of plating, it is presumed that the effect of improving the coating film swelling resistance can be obtained. Then, the inventors of the present invention have further earnestly studied, and include a required amount of Mg and Si, Ca, Ti, which inhibits the grain size increase of Mg ₂ Si, Ti, molten Al-based plating excluding Mn etc. It has been found that by performing the treatment, Mg ₂ Si having a large grain size (major axis exceeds 10 μm) can be formed during plating.
On the other hand, it was newly found that there is a problem that the chemical conversion treatability deteriorates when the grain size and the plating thickness come close to each other due to the increase in grain size and Mg ₂ Si is exposed on the plating surface. From the above, Mg ₂ Si having a large grain size and being present in the state of being not exposed on the surface is most effective in improving the chemical conversion treatment property of the plating and the swelling resistance of the coating film when the base steel sheet is exposed. We found that it was a good condition.

The present invention has been made based on the above findings, and the gist thereof is as follows.
1. Plating contains 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, and the balance has a composition of Al and inevitable impurities, and the major axis during the plating is more than 10 μm Mg. _A hot-dip Al-based plated steel sheet having ₂ Si, wherein the closest distance between the Mg ₂ Si and the plating surface is 0.5 μm or more.

2. The plating contains 50 to 90% by mass of Al, 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, with the balance being Zn and inevitable impurities. A hot-dip Al-based plated steel sheet, characterized in that it has Mg ₂ Si having a major axis of more than 10 μm and the closest distance between the Mg ₂ Si and the plating surface is 0.5 μm or more.

3. The ratio of the content of Mg with respect to Si (Mg / Si) in the plating is less than 1.1, The hot-dip Al-based plated steel sheet according to 1 or 2 above.

4. In the range of the length 1 mm in the plate width direction in the cross section parallel to the plate thickness direction of the plating, characterized in that there are five or more Mg ₂ Si having a major axis of more than 10 μm, any one of the above 1 to 3. Hot-dip Al-plated steel sheet.

5. The hot-dip Al-based plating according to any one of 1 to 4 above, wherein the area ratio of Mg ₂ Si having a major axis of more than 10 μm is 2% or more in a cross section parallel to the plate thickness direction of the plating. steel sheet.

6. The hot-dip Al-based plated steel sheet according to any one of 1 to 5 above, wherein the Mg ₂ Si has a major axis of 15 μm or more.

7. The hot-dip Al-based plated steel sheet according to any one of 1 to 6 above, wherein the amount of the deposited plating is 35 to 100 g / m ² per side.

8. In a plating facility, a molten Al-based plated steel sheet containing 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, with the balance being Al and unavoidable impurities. Manufacturing method.

9. The plating facility is characterized by using a plating bath containing 50 to 90 mass% of Al, 5 to 20 mass% of Si and 6 mass% to 10 mass% of Mg, and the balance Zn and unavoidable impurities. And a method for producing a molten Al-based plated steel sheet.

10. 9. The method for producing a hot-dip Al-based plated steel sheet according to 7 or 8, wherein the base steel sheet is passed through the plating bath and then cooled at a cooling rate of less than 15 K / s.

  According to the present invention, it is possible to provide a hot-dip Al-based plated steel sheet having excellent chemical conversion treatment properties and coating film swelling resistance, and a method for producing the hot-dip Al-based plated steel sheet.

It is the figure which showed the sample for evaluation of coating film swelling resistance in an Example. It is the figure which showed the cycle of the corrosion promotion test in an Example.

Hereinafter, the present invention will be specifically described.
(Hot-dip Al-plated steel sheet)
The hot-dip Al-plated steel sheet of the present invention is a hot-dip Al-plated steel sheet including a plating layer and an interfacial alloy layer existing at the interface between the plating layer and the base steel sheet. Here, in the present invention, the plating layer and the interface alloy layer are collectively referred to as “plating”.
The plating layer and the interface alloy layer can be observed by using a scanning electron microscope or the like in a cross section parallel to the plate thickness direction of the polished and / or etched hot-dip Al-based plated steel sheet. There are several kinds of cross-section polishing methods and etching methods, but the methods are not particularly limited as long as they are generally used when observing a cross section parallel to the plate thickness direction of the plated steel sheet. In addition, the observation conditions under a scanning electron microscope are, for example, an acceleration voltage of 15 kV and a magnification of 1000 times or more in a backscattered electron image, so that the plating (plating layer and interface alloy layer) can be clearly observed. is there.

And in the present invention, the plating contains 5 to 20% by mass of Si and 6% by mass or more and 10% by mass or less of Mg, and the balance has a composition consisting of Al and unavoidable impurities. It is characterized in that it has Mg ₂ Si having a major axis of more than 10 μm, and the closest distance between the Mg ₂ Si and the plating surface is 0.5 μm or more.
By forming Mg ₂ Si with a large particle size (major axis exceeds 10 μm) during plating and adjusting it so that it is not exposed on the plating surface, chemical conversion treatment and coating resistance when the underlying steel sheet is exposed Any of the film swelling properties can be greatly improved.

In the plating of the conventional hot-dip Al-based plated steel sheet not containing a specific amount of Mg during the plating, when exposed to the atmosphere, around the α-Al phase, a dense and stable oxide film of Al ₂ O ₃ is formed. Formed immediately. While the solubility of the α-Al phase decreases due to the protective effect of this oxide film formed, needle-like Si and Al-Fe-Si compounds formed in the interdendrite part (the part other than the α-Al phase) Shows a more noble potential than the α-Al phase and locally acts as a cathode site, so that a local battery is formed under the coating film. For this reason, partial selective corrosion of interdendrite starting from the flaw of the coating film occurs at the coating film / plating interface, and corrosion progresses to cause large coating film swelling, resulting in a decrease in coating film swelling resistance. It will be.

Further, in the molten Al-Mg-based plated steel sheet, Mg or Mg ₂ Si present during plating destroys the Al oxide film formed around the α-Al phase described above, and improves the solubility of the α-Al phase. Can be raised. This allows uniform corrosion of the α-Al phase, and suppresses selective corrosion of interdendrite as compared with the case of using a conventional Al-plated steel sheet. However, when a cathode having a large area with respect to the anode exists, for example, when the base steel sheet is exposed due to a flaw penetrating the coating film, swelling of the coating film due to uniform corrosion of the α-Al phase becomes remarkable, and the coating resistance No improvement in film swelling can be expected. The swelling of the coating film is a phenomenon different from the corrosion of the base steel sheet, that is, the occurrence of red rust, and has a great influence on the quality characteristics of the coated plated steel sheet. The main causes of coating swelling are an increase in volume due to corrosion of the Al phase and a decrease in coating adhesion due to elution of the coating pretreatment layer at the Al phase corrosion tip in an alkaline environment.
That is, the former problem can be expected to be improved by reducing the dissolution rate of the α-Al phase, and the latter problem can be expected to be improved by increasing the adhesion amount of the chemical conversion treatment layer. .
When Mg ₂ Si having a large particle size (hereinafter, sometimes referred to as “agglomerated Mg ₂ Si particles”) is present during plating as in the present invention, Mg ₂ Si dissolves almost simultaneously with the α-Al phase. However, a corrosion product in which Mg and Si are concentrated is generated. Since this corrosion product has an effect of suppressing the corrosion of the α-Al phase during plating and the steel sheet, it is possible to realize extremely excellent resistance to swelling of the coating film, which is not available in the prior art.

The effect of improving the coating film swelling resistance of the lumped Mg ₂ Si particles contained in the plating is particularly effective when the particle size is large, specifically, large Mg ₂ Si having a major axis exceeding 10 μm. Therefore, in the present invention, the major axis of Mg ₂ Si in the plating is set to exceed 10 μm, preferably 12 μm or more, and more preferably 15 μm or more.
Here, for the "major axis of Mg ₂ Si", when observing Mg ₂ Si in a cross section parallel to the plate thickness direction of plating using a scanning electron microscope, the longest diameter of each Mg ₂ Si That is. Mg ₂ Si can be judged by performing elemental mapping with SEM-EDX and detecting Mg and Si overlapping. In addition, “having Mg ₂ Si having a major axis of more than 10 μm” means that in a cross section parallel to the plate thickness direction of plating, it is orthogonal to the plate thickness direction when observing a cross section parallel to the plate thickness direction. When a range of a length of 1 mm in the direction of (1) is observed by a scanning electron microscope, one or more particles have a major axis of more than 10 μm, and preferably 5 or more. If the number of Mg ₂ Si having a major axis of more than 10 μm is 5 or more in a range of 1 mm in the plate width direction in a cross section parallel to the plate thickness direction of plating, the coating when a flaw reaching the base steel plate occurs It can be said that there is a sufficient amount of Mg ₂ Si to suppress the film swelling. On the other hand, when the number of Mg ₂ Si is 4 or less, the amount of Mg ₂ Si exposed in the flaw portion may be insufficient and the sufficient effect may not be exhibited. Regarding the "having Mg ₂ Si having a major axis of more than 10 μm", even when observing any cross section (however, excluding the interfacial alloy layer) of the plating in the molten Al-based plated steel sheet of the present invention , The condition can be met.

Further, for Mg ₂ Si contained in the plating, in the cross section parallel to the plate thickness direction of the plating, the area ratio of Mg ₂ Si having the major axis exceeding 10 μm is preferably 2% or more, and 3% or more Is more preferable, and 5% or more is particularly preferable.
As described above, the large particle size Mg ₂ Si suppresses the selective corrosion of interdendrite and contributes to the improvement of the coating film swelling resistance. Therefore, by setting the area ratio of Mg ₂ Si having the major axis of more than 10 μm to 2% or more, more excellent coating film swelling resistance can be realized.
However, if the proportion of Mg ₂ Si having a large particle diameter is too large, cracking of the plating is likely to occur when the steel sheet is bent, and the bending workability of the steel sheet deteriorates, so that the major axis exceeds 10 μm. The upper limit of the area ratio of Mg ₂ Si is preferably about 10%.
Incidentally, the area ratio of Mg ₂ Si in the present invention, for example, a cross section parallel to the plate thickness direction of the plating film of the Al-based plated steel sheet, element mapping by SEM-EDX, Mg and Si in one field of view the area of the portion to be detected (Mg ₂ portions Si is present) overlap, divided by the area ratio in the area of the whole observed plated (%), a method of deriving the image processing is used, Mg ₂ Si is The method is not particularly limited as long as the area ratio of the existing portion can be grasped.

In the molten Al-based plated steel sheet according to an embodiment of the present invention, the plating contains 5 to 20% by mass of Si and 6% by mass or more and 10% by mass or less of Mg, and the balance is composed of Al and unavoidable impurities. Have.
Further, in the molten Al-based plated steel sheet according to another embodiment of the present invention, the plating contains 50 to 90 mass% of Al, 5 to 20 mass% of Si and 6 mass% to 10 mass% of Mg. , The balance being Zn and inevitable impurities.
As described above, the large particle size Mg ₂ Si suppresses the selective corrosion of interdendrite and contributes to the improvement of the coating film swelling resistance. Therefore, in the present invention, while containing a specific amount of Mg and Si, forming a compound with Si, there is a risk of inhibiting the increase in particle size of Mg ₂ Si, Ca, Ti, Mn and the like composition With this, it is possible to form Mg ₂ Si having a large grain size (major axis exceeds 10 μm) during plating.

The content of Al, which is the main component of the plating, is 50% by mass or more, preferably more than 80% by mass, and more preferably more than 85% by mass in view of the balance between corrosion resistance and operation. When the Al content of the plating main layer is 50% by mass or more, dendrite solidification of Al occurs. With this structure, Al is composed of dendrite solidified parts (α-Al phase dendrite parts) and remaining dendrite gap parts (interdendrite parts), and the dendrite parts are laminated in the thickness direction of the plating and have excellent corrosion resistance. Can be secured. In addition, the more the dendrite portion of the α-Al phase is laminated, the more complicated the corrosion progress path becomes, and the more difficult the corrosion is to reach the base steel sheet, the more the corrosion resistance is improved.
On the other hand, considering the contents of other components contained in the plating, the upper limit of the Al content is 90% by mass.

The plating contains 5 to 20 mass% of Si. If the Si content is less than 5% by mass, the above-mentioned large particle size Mg ₂ Si may not be reliably formed. On the other hand, if the Si content exceeds 20%, a brittle FeAl ₃ Si ₂ intermetallic compound is generated in the interface alloy layer described later, and the workability of plating is reduced. Therefore, the content of Si is preferably 5 to 20%, and more preferably 8 to 10% by mass from the viewpoint of achieving both excellent coating film swelling resistance and workability. When the Si content is 8 to 10% by mass, the thickness of the interface alloy layer described later can be controlled to be small.

The plating contains Mg in an amount of more than 6% by mass and 10% by mass or less. Mg contained in the plating, by forming the above-mentioned large particle size Mg ₂ Si, suppresses the selective corrosion of interdendrite, as a result of being able to maintain a uniform dissolution property of the plating, the coating swelling resistance of Contribute to improvement. When the content of Mg is 6% by mass or less, a sufficient amount of Mg ₂ Si having a large particle size cannot be formed, and the Al oxide film capable of suppressing the selective corrosion of interdendrite does not break, so that the resistance is high. No improvement in film swelling can be expected. On the other hand, when the content of Mg exceeds 10% by mass, oxidation of the plating bath becomes remarkable and stable operation becomes difficult. Therefore, from the viewpoint of obtaining excellent coating film swelling resistance and manufacturability of plating, the Mg content is set in the range of more than 6% by mass and 10% by mass or less.

Furthermore, the ratio of the content of Mg to the content of Si in the plating (Mg / Si) is preferably less than 1.1. When the ratio of the content of Mg to the content of Si in the plating is 1.1 or more, there is a possibility that Mg ₂ Si having a large particle size cannot be formed sufficiently.

  Further, the plating may contain Zn in addition to Al, Si and Mg described above. Zn becomes a base solution by making a solid solution in the Al phase and the corrosion resistance of the plating itself is reduced, but on the contrary, the sacrificial anticorrosion action is strengthened. The Zn content in the plating is preferably 0 to 20 mass% (including 0), more preferably 10 mass% or less, and particularly preferably 5 mass% or less. When the Zn content is 5% by mass or more, the plating exhibits a sufficient sacrificial anticorrosion effect on Fe, so when the steel substrate is exposed in an environment with a low chloride ion and a low aluminum corrosion rate. Sufficient corrosion resistance is obtained. On the other hand, when the content of Zn exceeds 20 mass%, the sacrificial anticorrosive action becomes excessive and the dissolution rate of plating becomes extremely high. Therefore, the content of Zn is set in the range of 0 to 20% by mass (including 0) from the viewpoint of both the blistering resistance of the coating film and the corrosion resistance of the plating itself. From the same viewpoint, the content of Zn is more preferably 0 to 5 mass% (including 0).

  In addition, the said plating contains the base steel plate component taken in during plating by reaction of a plating bath and a base steel plate during a plating process, and an unavoidable impurity in a plating bath. As a base steel plate component taken in during plating, Fe is contained in the range of several% to several tens%. Examples of the unavoidable impurities in the plating bath include Fe, Cr, Cu, Mo, Ni, Zr, and the like. Regarding the Fe in the plating, it cannot be quantified by distinguishing between the one taken in from the base steel sheet and the one in the plating bath. The total content of unavoidable impurities is not particularly limited, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, the total amount of unavoidable impurities excluding Fe is preferably 1% by mass or less.

Further, the plating, apart from the above-mentioned unavoidable impurities, within a range that does not impair the effects of the present invention, from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr and B. It is also possible to contain one or more elements selected (hereinafter, may be referred to as “arbitrary content element”).
However, from the viewpoint of more reliably obtaining Mg ₂ Si having a large particle size, it is preferable that these optional elements are not included in the plating. These elements react with Al, Fe or Si to form an intermetallic compound, which serves as a nucleation site, and thus may inhibit the formation of Mg ₂ Si having a large particle size.

Furthermore, the ratio of the content of the optional element to the content of Mg in the plating (content of the optional element / content of Mg) is preferably 0.1 or less, and 0.01 or less. More preferably, 0 is particularly preferable. When the ratio of the content of the optional contained element to the content of Mg in the plating exceeds 0.2, the amount of the reaction product with Si increases, and it is not possible to sufficiently form Mg ₂ Si having a large particle size. There is a risk.

Further, the hot-dip Al-based plated steel sheet of the present invention includes an interface alloy layer existing at the interface between the plated layer and the base steel sheet during the plating.
The main components of the interface alloy layer are Al ₁₃ Fe ₄ and Al ₅ Fe ₂ , which are more brittle than the plating layer, and therefore are preferably thinned from the viewpoint of workability. Specifically, the thickness of the interface alloy layer is preferably 10 μm or less, and more preferably 5 μm or less. Regarding the thickness of the plating layer and the interface alloy layer, a cross section parallel to the plate thickness direction of the molten Al-based plated steel sheet is observed using a scanning electron microscope, and the average thickness of the interface alloy layer in the observation field of view. It can be grasped by calculating. The number of observation visual fields may be one, but may be an average of thicknesses obtained from a plurality of observation visual fields (for example, five visual fields).
The thickness of the interface alloy layer can be controlled by adjusting the Si content in the plating bath.

Mg ₂ Si having a major axis of more than 10 μm formed during the plating must have a closest distance to the surface of the plating of at least 0.5 μm. When the large particle size Mg ₂ Si is exposed on the surface of the plating, Mg ₂ Si forms an oxide after dissolution in the chemical conversion treatment step carried out as a pretreatment for coating, and the chemical conversion reaction does not occur in that portion, and the chemical conversion film is formed. Is not formed.
Here, the shortest distance between the major axis is greater than 10 [mu] m Mg ₂ Si and before plating the surface, by observing the cross section parallel to the thickness direction of the molten Al-based plated steel sheet with a scanning electron microscope, It is the distance between the Mg ₂ Si having a major axis of more than 10 μm and the surface of the plating closest to each other in the observation visual field. In addition, in the present invention, it is shown that the closest distance between Mg ₂ Si having a major axis of more than 10 μm and the surface of the plating is 0.5 μm or more in any part of the plating.

Further, the coating weight of the molten Al-based plated steel sheet of the present invention is preferably 35 to 150 g / m ² per side. As described above, in order to maximize the effect of the present invention, it is most desirable that the large particle size Mg ₂ Si is not exposed on the surface. In order to prevent Mg ₂ Si having a major axis of more than 10 μm from being exposed on the plating surface, a deposition amount of 35 g / m ² or more is preferable. If it is 35 g / m ² or more, excellent corrosion resistance is obtained, and if it is 150 g / m ² or less, excellent workability is obtained. Further, from the viewpoint of obtaining more excellent corrosion resistance and workability, it is preferable that the coating weight is 45 to 110 g / m ^2, and more preferably 45 to 80 g / m ² . Furthermore, the lower limit of the coating weight is more preferably 50 g / m ² or more in consideration of the fact that the molten Al-based plated steel sheet of the present invention forms Mg ₂ Si having a large grain size.

The coating amount of the molten Al-based plated steel sheet of the present invention is determined by the weight method by immersing the molten Al-based plated steel sheet in 35% hydrochloric acid containing an inhibitor component for preventing the dissolution of the base steel sheet to dissolve the plating. Then, from the result of the composition analysis of the solution dissolved with hydrochloric acid, the amount of deposited plating excluding Fe is calculated. The content (mass%) of Si, Mg, Al and other unavoidable impurities is calculated by removing Fe from the result of the composition analysis of the solution dissolved with hydrochloric acid. By this method, a value close to the composition of the plating bath can be obtained. Most of Fe is taken from the base steel sheet and varies depending on the plating conditions and plating thickness, and when it is calculated by including it in the plating content (mass%), the contents of Si, Mg, Al, etc. also vary. The content does not include Fe. Further, Si produces a residue containing SiO ₂ as a main component in a hydrochloric acid solution. When quantifying Si in the plating, in addition to the analysis of the components of Si in the hydrochloric acid solution, the residue filtered off is alkali-melted, and the acid-dissolved product is subjected to ICP-AES analysis.

Furthermore, the hot-dip Al-plated steel sheet of the present invention can further be provided with a chemical conversion coating on its surface.
The type of the chemical conversion coating is not particularly limited, and chromate-free chemical conversion treatment, chromate-containing chemical conversion treatment, zinc phosphate-containing chemical conversion treatment, zirconium oxide-based chemical conversion treatment and the like can be used. Further, it is preferable to contain silica fine particles from the viewpoint of adhesion and corrosion resistance, and to contain phosphoric acid and / or a phosphoric acid compound from the viewpoint of corrosion resistance. As the silica fine particles, either wet silica or dry silica may be used, but it is more preferable to contain silica fine particles having a large effect of improving adhesion, particularly dry silica. Examples of the phosphoric acid and the phosphoric acid compound include those containing one or more selected from orthophosphoric acid, pyrophosphoric acid, polyphosphoric acid, and metal salts and compounds thereof.

Furthermore, molten Al-based plated steel sheet of the present invention, the surface or on the chemical conversion coating, may further comprise a coating.
The paint used for forming the coating film is not particularly limited. For example, polyester resin, amino resin, epoxy resin, acrylic resin, urethane resin, fluororesin or the like can be used. The method of applying the coating material may be, for example, a roll coater, a bar coater, a spray, a curtain flow, electrodeposition, or the like, and is not limited to a specific coating method.

  The base steel sheet used for the molten Al-based plated steel sheet of the present invention is not particularly limited, and it can be used not only for steel sheets similar to those used for ordinary molten Al-based plated steel sheets but also for high-strength steel sheets and the like. . For example, a hot-rolled steel sheet or steel strip pickled and descaled, or a cold-rolled steel sheet or steel strip obtained by cold rolling them can be used.

(Method for manufacturing hot-dip Al-plated steel sheet)
Next, a method of manufacturing the hot-dip Al-plated steel sheet of the present invention will be described.
The method for producing a molten Al-based plated steel sheet according to the present invention is, in plating equipment, 5 to 20% by mass of Si and 6% by mass or more and 10% by mass or less of Mg, Al as a main component, and Zn contained as necessary. And a plating bath composed of unavoidable impurities.
By such a manufacturing method, it is possible to manufacture a hot-dip Al-based plated steel sheet which has ordinary corrosion resistance and is also excellent in chemical conversion treatment and coating film swelling resistance.

  The method for producing the hot-dip Al-based plated steel sheet according to the present invention is not particularly limited, but a method for producing the hot-dip Al-plated steel sheet in a continuous hot-dip galvanizing facility is usually adopted. In this method, the base steel sheet is immersed in the plating bath to perform the plating treatment, so that the plating is applied to both sides of the steel sheet.

The type of the base steel sheet used for the hot-dip Al-based plated steel sheet of the present invention is not particularly limited. For example, a hot-rolled steel sheet or steel strip pickled and descaled, or a cold-rolled steel sheet or steel strip obtained by cold rolling them can be used.
The conditions of the pretreatment step and the annealing step are not particularly limited, and any method can be adopted.

The hot rolling step may be carried out by an ordinary method of winding after slab heating, rough rolling, and finish rolling. Further, the heating temperature, finish rolling temperature, etc. are not particularly specified, and the ordinary temperature can be used.
The pickling step performed after the hot rolling may be carried out by a commonly used method, and examples thereof include washing with hydrochloric acid or sulfuric acid.
The cold rolling step performed after the pickling is not particularly limited, but can be performed at a reduction rate of 30 to 90%, for example. If the rolling reduction is 30% or more, the mechanical properties are not deteriorated, and if it is 90% or less, the rolling cost is not increased.
Regarding the recrystallization annealing step, for example, after performing a cleaning treatment such as degreasing, using an annealing furnace, a heat treatment of heating the steel sheet to a predetermined temperature in a heating zone in the first stage is performed, and a predetermined heat treatment in a soaking zone in the second stage. Can be applied. It is preferable to carry out the treatment under temperature conditions that have the required mechanical properties. In addition, the atmosphere in the annealing furnace activates the surface layer of the steel sheet before the plating treatment, so that annealing is performed on Fe in a reducing atmosphere. The type of reducing gas is not particularly limited, but it is preferable to use a reducing gas atmosphere which is already generally used.

The plating bath used in one embodiment of the method for producing a molten Al-based plated steel sheet according to the present invention contains 5 to 20% by mass of Si and 6% by mass or more and 10% by mass or less of Mg, and the balance is Al and unavoidable. Consist of impurities.
Further, the plating bath used in another embodiment of the method for producing a molten Al-based plated steel sheet of the present invention is 50 to 90 mass% Al, 5 to 20 mass% Si and 6 mass% to 10 mass% or less. It contains Mg, and the balance consists of Zn and inevitable impurities.
The composition of plating of the molten Al-based plated steel sheet of the present invention can be accurately performed by controlling the composition of the plating bath.
The unavoidable impurities and the optional contained elements are the same as the description of the plating on the hot-dip Al-plated steel sheet of the present invention.

  The temperature of the plating bath is preferably in the range of (solidification start temperature + 20 ° C.) to 700 ° C. The lower limit of the bath temperature is set to the solidification start temperature + 20 ° C. In order to perform the hot dip plating treatment, the bath temperature is set to be equal to or higher than the freezing point of the plating raw material and the solidification start temperature + 20 ° C. This is to prevent local coagulation of the composition components due to local lowering of the bath temperature. On the other hand, the upper limit of the bath temperature is 700 ° C., when the bath temperature exceeds 700 ° C., rapid cooling of the plating becomes difficult, and Al-Fe as the main component formed at the interface with the steel sheet for plating is used. This is because the thickness of the interface alloy layer is increased.

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

  Further, the immersion time of the base steel sheet in the plating bath is preferably 0.5 seconds or more. If the immersion time is less than 0.5 seconds, sufficient plating may not be formed on the surface of the base steel sheet. On the other hand, the upper limit of the immersion time is not particularly limited, but if the immersion time is lengthened, the thickness of the Al-Fe alloy layer formed at the interface with the steel sheet for plating may become thicker, so that it is about 5 seconds. Preferably there is.

  The conditions for immersing the base steel sheet in the plating bath are not particularly limited. For example, the line speed of about 150 to 230 mpm can be used for plating mild steel thin products, and the line speed of about 40 mpm can be used for plating thick products. Can be about 5 to 7 m.

Then, in the method for producing a hot-dip Al-plated steel sheet according to the present invention, it is preferable to cool the steel sheet after hot-dip coating at a cooling rate of 15 K / s.
By performing hot dipping using the above-mentioned plating bath and then performing a gentle cooling treatment of less than 15 K / s, it is possible to form Mg ₂ Si having a large major axis of more than 10 μm during plating. Further, it becomes possible to reduce the thickness of the interface alloy layer formed at the interface with the plated steel sheet.
On the other hand, if the cooling rate is less than 5 K / s, the solidification of the plating is slow, so that a dripping pattern may occur on the plating surface, which may cause remarkable deterioration of appearance and deterioration of chemical conversion treatment. It is preferably s or more.
From the same viewpoint, the cooling rate is particularly preferably 8 to 12 K / s.

  Further, in the method for producing a hot-dip Al-plated steel sheet according to the present invention, it is preferable to use nitrogen gas cooling for the cooling treatment. The reason why the nitrogen gas cooling is adopted is that it is economical because it does not require an extremely high cooling rate as described above and does not require large-scale cooling equipment.

In the method for producing a hot-dip Al-plated steel sheet of the present invention, the hot-dip Al-plated steel sheet is not particularly limited except for the plating bath and the cooling conditions after the hot-dip plating, and the hot-dip Al-plated steel sheet can be produced by an ordinary method.
For example, a chemical conversion treatment film may be provided on the surface of the hot-dip Al-plated steel sheet (chemical conversion treatment step), or a coating film may be provided in a separate coating facility (coating film formation step).

Next, examples of the present invention will be described.
(Samples 3-5, 7-9, 11-14, 16-24, 27-30 )
For all of the hot-dip Al-plated steel sheets used as samples, cold-rolled steel sheet with a thickness of 0.8 mm produced by a conventional method was used as the base steel sheet, and the bath temperature of the plating bath was set to 660 ° C and the infiltration temperature was set to 660 by the laboratory hot dipping equipment The composition of the plating bath was changed to various conditions at a temperature of 200 ° C., a line speed of 200 mpm, and a dipping time of 2 seconds, and the molten Al-based plated steel sheet of each sample was manufactured under the cooling rate conditions after plating shown in Table 1.
Further, all samples were punched out into 50 mmΦ, immersed in a 35% hydrochloric acid aqueous solution to dissolve the plating on the base steel sheet, and then weighed before and after the immersion. Furthermore, the plating composition was confirmed by quantifying the composition of the solution by ICP emission spectroscopy. The amount of plating adhered was calculated by removing Fe from the above weight change before and after dipping in hydrochloric acid and the plating composition. The plating composition was also calculated with the balance excluding Fe as 100 mass%. Table 1 shows the coating weight and the plating composition of each sample.

Further, arbitrary 3 cross sections were cut out by shearing from the sample molten Al-based plated steel sheet, and the plating was observed by a scanning electron microscope (SEM). A cross section parallel to the plate thickness direction of plating was observed in the range of 1 mm in the plate width direction, and the major axis and number of the bulk Mg ₂ Si during plating and the distance from the plating surface to the bulk Mg ₂ Si were measured.
Note that the area ratio of Mg ₂ Si, derived for each sample, in observation using the SEM (backscattered electron image), the ratio of the area of the major axis occupied an area of plating is greater than 10 [mu] m Mg ₂ Si (percent) Then, the average of 5 visual fields was defined as the area ratio (%) of each sample.

(Evaluation of chemical conversion processability)
Each sample of the hot-dip Al-plated steel sheet was sheared to a size of 80 mm × 70 mm, and then zinc phosphate treatment as chemical conversion treatment was performed under the following conditions in the same manner as the coating treatment for automobile outer panels.
-Zinc phosphate treatment: Degreasing agent made by Nippon Parkerizing Co., Ltd .: FC-E2001, surface conditioner: PL-X, and chemical conversion treatment agent: PB-AX35 (temperature: 35 ° C), free fluorine of chemical conversion treatment liquid The chemical conversion treatment was performed under the conditions of a concentration of 200 mass ppm and a dipping time of the chemical conversion treatment liquid of 120 seconds.
The chemical conversion film was observed with a scanning electron microscope (SEM), and the zinc phosphate treatability was evaluated according to the following criteria. Table 1 shows the evaluation results.
○: Uniform (no scale (no chemical conversion film, undercoating appears))
×: There is a scale

(Evaluation of film swell resistance)
Each sample of the hot-dip Al-plated steel sheet was sheared to a size of 80 mm × 70 mm, treated with zinc phosphate as a chemical conversion treatment, and then subjected to electrodeposition coating in the same manner as the coating treatment for automobile outer panels. Here, the zinc phosphate treatment and electrodeposition coating were performed under the following conditions.
-Zinc phosphate treatment: Degreasing agent made by Nippon Parkerizing Co., Ltd .: FC-E2001, surface conditioner: PL-X, and chemical conversion treatment agent: PB-AX35 (temperature: 35 ° C), free fluorine of chemical conversion treatment liquid The chemical conversion treatment was performed under the conditions of a concentration of 200 mass ppm and a dipping time of the chemical conversion treatment liquid of 120 seconds.
-Electrodeposition coating: An electrodeposition coating made by Kansai Paint Co., Ltd .: GT-100 was used to perform electrodeposition coating so that the film thickness was 15 μm.
After chemical conversion treatment and electrodeposition coating, as shown in Fig. 1, 7.5 mm of the end of the evaluation surface and the non-evaluation surface (rear surface) were sealed with tape, and then the center of the evaluation surface was coated with a knife to plate the steel plate. A sample to which a cross-cut flaw having a length of 60 mm and a central angle of 60 ° was added up to a depth reaching the base steel sheet was used as a sample for evaluation of coating film swelling resistance.
A corrosion acceleration test was performed in the cycle shown in FIG. 2 using the above evaluation sample. After starting the corrosion acceleration test from wetting for 60 cycles, the swelling width of the coating film at the part where the swelling of the coating film from the scratch is the maximum (maximum coating swelling width: maximum on one side with the scratched part in the center) The coating film swelling width) was measured, and the coating film swelling resistance was evaluated according to the following criteria. Table 1 shows the evaluation results.
○: Maximum coating swelling width ≤ 2.0 mm
X: Maximum coating film swelling width> 2.0 mm

  It can be seen from Table 1 that each sample of the present invention example shows excellent results in terms of both chemical conversion treatment property and coating film swelling resistance, as compared with each sample of the comparative example.

According to the present invention, it is possible to provide a hot-dip Al-based plated steel sheet having excellent chemical conversion treatment properties and coating film swelling resistance, and a method for producing the hot-dip Al-based plated steel sheet.

Claims (9)

The plating contains 80% by mass or more of Al, 5 to 20% by mass of Si and 6% by mass or more and 10% by mass or less of Mg, and the balance has a composition of inevitable impurities. Has a Mg ₂ Si content of more than 10 μm, and the closest distance between the Mg ₂ Si and the plating surface is 0.5 μm or more. Plating contains more than 80 wt% and 90 wt% or less Al, 5 to 20 wt% Si and 6 wt% to 10 wt% or less Mg, the balance has a composition of Zn and inevitable impurities, A hot-dip Al-based plated steel sheet having Mg ₂ Si having a major axis of more than 10 μm in the plating and having a closest distance between the Mg ₂ Si and the plating surface of 0.5 μm or more.   The molten Al-based plated steel sheet according to claim 1 or 2, wherein the ratio of the content of Mg to Si in the plating (Mg / Si) is less than 1.1. The Mg ₂ Si having a major axis of more than 10 μm is 5 or more in a range of a length of 1 mm in a plate width direction in a cross section parallel to the plate thickness direction of the plating, any one of claims 1 to 3. Hot-dip Al-plated steel sheet according to. The area ratio of Mg ₂ Si having a major axis of more than 10 μm in a cross section parallel to the plate thickness direction of the plating is 2% or more, melting according to any one of claims 1 to 4. Al-based plated steel sheet. The hot-dip Al-based plated steel sheet according to claim 1, wherein the Mg ₂ Si has a major axis of 15 μm or more. The hot-dip Al-based plated steel sheet according to any one of claims 1 to 6, characterized in that the coating weight is 35 to 100 g / m ² per side. In the plating equipment, using 80% by mass or more of Al, 5 to 20% by mass of Si and 6% by mass or more of 10% by mass or less of Mg, and using a plating bath having the balance of unavoidable impurities, and in the plating bath, After passing through the base steel sheet, a step of cooling at a cooling rate of less than 15 K / s is provided,
Manufacture of hot-dip Al-based plated steel sheet characterized by forming a plating layer having a closest contact distance of 0.5 μm or more between Mg ₂ Si existing in plating and the plating surface with an adhesion amount of 35 g / m ² or more Method. In the plating facility, a plating bath containing 80% by mass to 90% by mass of Al, 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg with the balance Zn and unavoidable impurities is used. , A step of cooling the plating bath at a cooling rate of less than 15 K / s after passing a base steel sheet,
Manufacture of hot-dip Al-based plated steel sheet characterized by forming a plating layer having a closest contact distance of 0.5 μm or more between Mg ₂ Si existing in plating and the plating surface with an adhesion amount of 35 g / m ² or more Method.

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