WO2018181391A1

WO2018181391A1 - Hot-dipped al coated steel sheet and method for producing hot-dipped al coated steel sheet

Info

Publication number: WO2018181391A1
Application number: PCT/JP2018/012569
Authority: WO
Inventors: 林太佐藤; 俊佑山本; 安藤　聡
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2017-03-31
Filing date: 2018-03-27
Publication date: 2018-10-04
Also published as: JP6683258B2; JPWO2018181391A1

Abstract

Provided is a hot-dipped Al coated steel sheet which exhibits excellent chemical conversion coating properties and excellent coating film bulging resistance. The present invention is characterized in that: the coating film has a composition that contains 5-20% by mass of Si and more than 6% by mass but 10% by mass or less of Mg, with the balance made up of Al and unavoidable impurities; the coating film contains an Mg₂Si phase having a length of more than 10 μm; and the closest distance between the Mg₂Si phase and the coating film surface is 0.5 μm or more.

Description

Fused Al-based plated steel sheet and method for producing molten Al-based plated steel sheet

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

As a plated steel material excellent in corrosion resistance and high-temperature oxidation resistance, Al-based plated steel sheets are widely used in the muffler materials for automobiles and the building materials field.
However, for Al-based plated steel sheets, corrosion products stabilize and exhibit excellent corrosion resistance in environments with low chloride ion concentrations or in corrosive environments under dry conditions. There is a problem that sufficient corrosion resistance cannot be exhibited in an environment where the object is exposed to the object 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 fast and the base steel sheet is easily corroded. Further, when an Al-based plated steel sheet is used after coating, there is a problem in that since the bottom of the coating film becomes an alkaline atmosphere, the corrosion rate of Al increases and blistering of the coating film is caused.

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

Further, Patent Document 2 includes a molten Al-Mg-Si plating layer containing Mg: 3 to 10% and Si: 1 to 15% by weight, with the balance being Al and inevitable impurities. High corrosion resistance having a metal structure in which the plated layer is composed of at least “Al phase” and “Mg ₂ Si phase” and the major axis of “Mg ₂ Si phase” is 10 μm or less. A plated steel sheet is disclosed.

Further, Patent Document 3 contains Mg: 6 to 10% by mass, Si: 3 to 7% by mass, Fe: 0.2 to 2% by mass, and Mn: 0.02 to 2% by mass on the surface of the steel material, and the balance is A plating layer comprising Al and inevitable impurities, the plating layer having an αAl-Mg ₂ Si- (Al-Fe-Si-Mn) pseudo-ternary eutectic structure, and a pseudo-ternary in the plating layer An Al-based plated steel material having an eutectic structure area ratio of 30% or more is disclosed.
Furthermore, Patent Document 4 includes a massive intermetallic compound in which a plating layer on the surface of a steel material is composed of one or more group IIa (alkaline earth metal) elements and one or more group IVb elements. In addition, an aluminum-plated surface-treated steel material excellent in 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 is disclosed.

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 in that an Al ₃ Mg ₂ phase is precipitated in the plating layer, and local dissolution of the plating layer starts from this phase.
In addition, the technique of Patent Document 2 has a problem that a long needle-like or plate-like Al—Fe compound is precipitated in the plating layer, and the local dissolution of the plating layer proceeds using this as a local cathode. It was.
Furthermore, as for the technique of Patent Document 3, as a result of the Al—Fe compound being incorporated into the eutectic structure by the addition of Mn, further improvement in corrosion resistance including prevention of local corrosion resistance deterioration can be achieved. However, when a coating film is provided on a hot-dip Al-based plated steel sheet, the galvanic pair is formed with a more noble part of the potential of the underlying steel sheet where the coating layer is exposed to alkali and low oxygen environment and the plating layer is exposed by wrinkles, etc. To do. As a result, the base steel sheet is sacrificial and corrosion-proof, but the corrosion rate of the plating layer is extremely increased, and blistering may occur. Therefore, further improvement of the coating film swelling resistance has been desired.
Furthermore, regarding the technique of Patent Document 4, it was considered that an intermetallic compound having a size larger than the plating thickness may be present during plating, and the intermetallic compound is necessarily exposed to 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, thereby inhibiting 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 (poisoning property is poor). And deterioration of chemical conversion property may cause the adhesiveness fall of the coating film formed after that.

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

As a result of repeated studies to solve the above-mentioned problems, the present inventors have increased the size of Mg ₂ Si during plating, which has been conventionally considered as a starting point of corrosion, rather than miniaturization. It was noticed that the effect of improving the swelling resistance of the coating film can be obtained. The mechanism is not clear, but Mg ₂ Si, which is located near the plating surface with a large particle 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. Producing corrosive corrosion products. Since this corrosion product has an effect of suppressing corrosion of plating, it is presumed that an effect of improving the swelling resistance of the coating film can be obtained. Then, the present inventors conducted further diligent research, including molten aluminum-based plating excluding Ca, Ti, Mn and the like that contain the required amount of Mg and Si and inhibit the increase in the particle size of Mg ₂ Si. It has been found that Mg ₂ Si having a large particle diameter (longer diameter exceeds 10 μm) can be formed during the plating.
On the other hand, it has been newly discovered that there is a problem that the chemical conversion treatment performance is lowered when Mg ₂ Si is exposed to the plating surface as the particle size and plating thickness approach due to the increase in particle size. From the above, the presence of Mg ₂ Si in the plating with a large particle size and not exposed to the surface is the most effective in improving the chemical conversion treatment property of the plating and the swelling resistance of the coating film when the underlying steel plate is exposed. I 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. The plating contains 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, the balance is composed of Al and inevitable impurities, and the major axis is over 10 μm during the plating. _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 more than 6% by mass and 10% by mass or less of Mg, and the balance is composed of Zn and inevitable impurities. A molten Al-based plated steel sheet having Mg ₂ Si having a major axis exceeding 10 μm and a closest distance between the Mg ₂ Si and the plating surface of 0.5 μm or more.

3. The molten Al-based plated steel sheet according to 1 or 2, wherein a ratio of Mg content to Mg (Mg / Si) in the plating is less than 1.1.

4). 4. The method according to any one of 1 to 3, wherein there are 5 or more Mg ₂ Sis having a major axis exceeding 10 μm in a range of 1 mm in the plate width direction in a cross section parallel to the plate thickness direction of the plating. Fused Al-based plated steel sheet.

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

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

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

8). In a plating facility, a hot-dip Al-based plated steel sheet characterized by using a plating bath containing 5-20% by mass of Si and 6% by mass to 10% by mass of Mg, with the balance being Al and inevitable impurities. Manufacturing method.

9. The plating equipment uses a plating bath containing 50 to 90 mass% Al, 5 to 20 mass% Si, and more than 6 mass% to 10 mass% Mg, with the balance being Zn and inevitable impurities. A method for producing a molten Al-based plated steel sheet.

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

According to the present invention, it is possible to provide a molten Al-based plated steel sheet excellent in chemical conversion treatment property and paint film swelling resistance, and a method for producing the molten Al-based plated steel sheet.

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

Hereinafter, the present invention will be specifically described.
(Fused Al-based plated steel sheet)
The hot-dip Al-based plated steel sheet of the present invention is a hot-dip Al-based plated steel sheet that includes a plating layer and an interface alloy layer that exists at the interface between the plating layer and the underlying 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 thickness direction of the polished and / or etched molten Al-based plated steel sheet. There are several types of methods for polishing and etching the cross section, but there is no particular limitation as long as it is a method generally used for observing a cross section parallel to the thickness direction of the plated steel sheet. Moreover, if the observation conditions with a scanning electron microscope are, for example, an acceleration voltage of 15 kV and a magnification of 1000 times or more in a reflected electron image, plating (plating layer and interface alloy layer) can be clearly observed. is there.

In the present invention, the plating contains 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, and the balance is composed of Al and unavoidable impurities. It has Mg ₂ Si whose major axis exceeds 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 exceeding 10 μm) during plating and adjusting it so that it is not exposed on the plating surface, it is possible to achieve chemical conversion treatment and resistance to coating when the underlying steel sheet is exposed. Any of the film swellability can be greatly improved.

In the conventional hot-dip Al-based plated steel plate that does not contain a specific amount of Mg during the plating, a dense and stable Al ₂ O ₃ oxide film is formed around the α-Al phase when exposed to the atmosphere. Immediately formed. The protective action of the formed oxide film reduces the solubility of the α-Al phase, while the needle-like Si and Al-Fe-Si compounds formed in the interdendrite part (parts other than the α-Al phase) All show a noble potential than the α-Al phase and locally act as cathode sites, so that a local battery is formed under the coating film. For this reason, partial selective corrosion of the interdendrite starting from the buttocks of the coating film occurs at the coating film / plating interface, and the corrosion progresses to cause large coating swelling, resulting in a decrease in coating swelling resistance. It will be.

In the hot-dip Al-Mg plated steel sheet, Mg or Mg ₂ Si present during plating destroys the Al oxide film formed around the α-Al phase described above, thereby increasing the solubility of the α-Al phase. Can be raised. As a result, the α-Al phase can be uniformly corroded, and the selective corrosion of the interdendrite can be suppressed as compared with the case where a conventional Al-based plated steel sheet is used. However, when a cathode having a large area with respect to the anode exists, for example, when the underlying steel sheet is exposed by a flaw penetrating the coating film, the swelling of the coating film due to the uniform corrosion of the α-Al phase becomes significant, and the coating resistance is reduced. The improvement of film swellability is not 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 greatly affects the quality characteristics of the coated plated steel sheet. The main causes of the swelling of the coating film are an increase in volume due to the corrosion of the Al phase and a decrease in the adhesion of the coating film due to the elution of the coating surface treatment layer in an alkaline environment at the tip of the Al phase corrosion.
In other words, 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 layer. .
When Mg ₂ Si having a large particle size (hereinafter, sometimes referred to as “bulky Mg ₂ Si grains”) is present during plating as in the present invention, Mg ₂ Si dissolves almost simultaneously with the α-Al phase. As a result, corrosion products enriched with Mg and Si are produced. Since this corrosion product has the effect of suppressing the corrosion of the α-Al phase and the steel sheet during plating, it is possible to realize extremely excellent film swell resistance not found in the prior art.

The effect of improving the swelling resistance of the massive Mg ₂ Si particles contained in the plating is particularly effective when the particle diameter is large, specifically, when the major axis is large Mg ₂ Si exceeding 10 μm. Therefore, in the present invention, the major axis of Mg ₂ Si in the plating is more than 10 μm, preferably 12 μm or more, more preferably 15 μm or more.
Here, for the “major axis of Mg ₂ Si”, the longest diameter of each Mg ₂ Si when observing Mg ₂ Si in a cross section parallel to the plate thickness direction of the plating using a scanning electron microscope is used. That is. Mg ₂ Si can be judged by performing element mapping with SEM-EDX and detecting Mg and Si overlapping. In addition, “having Mg ₂ Si with a major axis exceeding 10 μm” means that in a cross section parallel to the plate thickness direction of the plating, the plate width direction (when cross section parallel to the plate thickness direction is observed, it is orthogonal to the plate thickness direction). When the range of 1 mm in length) is observed with a scanning electron microscope, it means that there is one or more having a major axis exceeding 10 μm, and preferably 5 or more. If the number of Mg ₂ Si with a major axis exceeding 10 μm is 5 or more in the range of 1 mm in the plate width direction in the cross section parallel to the plate thickness direction, the coating when wrinkles that reach the underlying steel plate occur It can be said that there is a sufficient amount of Mg ₂ Si to suppress film swelling. On the other hand, Mg ₂ Si which is exposed to certain When flaw portion is 4 or less of the Mg ₂ Si might not exert sufficient effect insufficient. In addition, as for “having Mg ₂ Si having a major axis exceeding 10 μm”, even if any cross section (except for the interface alloy layer) of the plating in the hot-dip Al-based plated steel sheet of the present invention is observed The condition can be satisfied.

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 a major axis exceeding 10 μm is preferably 2% or more, and 3% or more It is more preferable that it is 5% or more.
As described above, Mg ₂ Si having a large particle size suppresses selective corrosion of the interdendrite and contributes to improvement of the coating film swelling resistance. Therefore, by setting the area ratio of Mg ₂ Si having a major axis exceeding 10 μm to 2% or more, it is possible to realize more excellent coating swelling resistance.
However, Mg ₂ Si with a large particle size tends to cause plating cracking when the steel sheet is bent when the ratio is too large, and deteriorates the bending workability of the steel sheet, so that the major axis exceeds 10 μm. The upper limit of the area ratio of Mg ₂ Si is preferably about 10%.
The area ratio of Mg ₂ Si in the present invention is, for example, elemental mapping of the cross section parallel to the plate thickness direction of the plating film of the Al-based steel plate by SEM-EDX, and 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 hot-dip Al-based plated steel sheet according to one embodiment of the present invention, the plating contains 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, with the balance being Al and inevitable impurities. Have
Further, in the hot-dip Al-based plated steel sheet according to another embodiment of the present invention, the plating contains 50 to 90% by mass of Al, 5 to 20% by mass of Si, and more than 6% by mass and 10% by mass or less of Mg. The balance is composed of Zn and inevitable impurities.
As described above, Mg ₂ Si having a large particle size suppresses selective corrosion of the interdendrite and contributes to improvement of the coating film swelling resistance. Therefore, in the present invention, a specific amount of Mg and Si is contained, and a composition is formed excluding Ca, Ti, Mn, etc. that may form a compound with Si and hinder the increase in the particle size of Mg ₂ Si. Thus, Mg ₂ Si having a large particle size (major axis exceeding 10 μm) can be formed 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, more preferably more than 85% by mass, from the balance between corrosion resistance and operation. If the Al content of the plating main layer is 50% by mass or more, Al dendrite solidification occurs. As a result, it has a structure with excellent corrosion resistance in which Al is dendritic solidified part (α-Al phase dendrite part) and the remaining dendrite gap part (interdendrite part), and the dendrite part is laminated in the film thickness direction of plating. Can be secured. Further, the more the dendrite portion of the α-Al phase is laminated, the more complicated the corrosion progressing path becomes, and the corrosion becomes difficult to reach the base steel plate, so that the corrosion resistance is improved.
On the other hand, in consideration of the content of other components contained in the plating, the upper limit of the Al content is 90% by mass.

The plating contains 5 to 20% by mass of Si. When the Si content is less than 5% by mass, the above-described large particle size Mg ₂ Si may not be reliably formed. On the other hand, when the Si content exceeds 20%, a brittle FeAl ₃ Si ₂ intermetallic compound is formed in the interface alloy layer described later, so that the workability of plating is lowered. Therefore, from the viewpoint of achieving both excellent coating film swell resistance and workability, the Si content is preferably 5 to 20%, more preferably 8 to 10% by mass. 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 of more than 6% by mass and 10% by mass or less. Mg contained in the plating can prevent selective corrosion of the interdendrite by forming the above-mentioned large particle size Mg ₂ Si, and maintain uniform dissolution characteristics of the plating. Contributes to improvement. When the Mg content is 6% by mass or less, a sufficient amount of large-diameter Mg ₂ Si cannot be formed, and the Al oxide film that can suppress selective corrosion of the interdendrite does not break down. The improvement of the film swelling cannot be expected. On the other hand, when the Mg content exceeds 10% by mass, the plating bath is remarkably oxidized and stable operation becomes difficult. Therefore, from the viewpoint of obtaining excellent coating film swelling resistance and plating manufacturability, the Mg content is set in the range of more than 6 mass% and not more than 10 mass%.

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

The plating can also contain Zn in addition to the above-described Al, Si, and Mg. When Zn dissolves in the Al phase, the potential is reduced and the corrosion resistance of the plating itself is reduced, but conversely the sacrificial anticorrosive action is enhanced. The Zn content in the plating is preferably 0 to 20% by mass (including 0), more preferably 10% by mass or less, and particularly preferably 5% by mass or less. By making the Zn content 5% by mass or more, the plating exhibits a sufficient sacrificial anti-corrosion action against Fe, so when the steel substrate is exposed in an environment where the chloride ion is low and the aluminum corrosion rate is low. Corrosion resistance is sufficiently obtained. On the other hand, if the Zn content exceeds 20% by mass, the sacrificial anticorrosive action becomes excessive and the dissolution rate of the plating becomes extremely large. Therefore, from the viewpoint of achieving both the swelling resistance of the coating film and the corrosion resistance of the plating itself, the Zn content is in the range of 0 to 20% by mass (including 0). From the same viewpoint, the Zn content is more preferably 0 to 5% by mass (including 0).

The plating includes a base steel plate component taken into the plating by a reaction between the plating bath and the base steel plate during the plating process, and inevitable impurities in the plating bath. As the base steel plate component taken in during plating, Fe is contained in the order of several% to several tens%. Examples of the inevitable impurities in the plating bath include Fe, Cr, Cu, Mo, Ni, and Zr. About Fe in plating, what is taken in from a base steel plate and what is in a plating bath cannot be distinguished and quantified. The total content of inevitable impurities is not particularly limited, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of plating, the total amount of inevitable impurities excluding Fe is preferably 1% by mass or less.

In addition to the inevitable impurities described above, the plating is within the range in which the effects of the present invention are not impaired, from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr and B. It is also possible to contain one or two or more elements selected (hereinafter sometimes referred to as “optionally contained elements”).
However, from the viewpoint of more reliably obtaining a large particle size Mg ₂ Si, it is preferable that these optional elements are not included in the plating. Since these elements react with Al, Fe, or Si to form intermetallic compounds and become nucleation sites, the formation of Mg ₂ Si having a large particle size may be hindered.

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

Moreover, the hot-dip Al-based plated steel sheet of the present invention includes an interface alloy layer that exists at the interface between the plating 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 fragile than the plating layer, and are preferably thin 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. In addition, about the thickness of the said plating layer or the said interface alloy layer, the cross section parallel to the plate | board thickness direction of a hot-dip Al type plated steel plate is observed using a scanning electron microscope, The average thickness of the interface alloy layer in an observation visual field Can be obtained by calculating. Although the number of observation visual fields may be one, it 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 needs to have a closest distance of 0.5 μm or more from the surface of the plating. When Mg ₂ Si having a large particle size is exposed on the surface of the plating, Mg ₂ Si forms an oxide after dissolution in the chemical conversion treatment process performed as a pre-coating treatment, and the chemical conversion reaction does not occur in that portion. Is not formed.
Here, for the closest distance between Mg ₂ Si having a major axis exceeding 10 μm and the surface of the pre-plating, a cross section parallel to the thickness direction of the molten Al-based plated steel sheet is observed using a scanning electron microscope, In the observation field, the distance between the closest part of Mg ₂ Si having a major axis exceeding 10 μm and the plating surface is assumed. In the present invention, it indicates that the closest distance between Mg ₂ Si having a major axis exceeding 10 μm and the surface of the plating is 0.5 μm or more, regardless of which part of the plating is measured.

Further, the coating amount 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 effects of the present invention, it is most desirable that the large particle size Mg ₂ Si is not exposed on the surface. In order not to expose Mg ₂ Si having a major axis exceeding 10 μm on the plating surface, an adhesion 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 better corrosion resistance and workability, the plating adhesion amount is preferably 45 to 110 g / m ^2, and more preferably 45 to 80 g / m ² . Furthermore, the lower limit of the coating adhesion amount 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 a large particle size Mg ₂ Si.

The plating adhesion amount of the molten Al-based plated steel sheet of the present invention is determined by a gravimetric method by immersing the molten Al-based plated steel sheet in 35% hydrochloric acid containing an inhibitor component that prevents dissolution of the underlying steel sheet and dissolving the plating. And the plating adhesion amount except Fe is calculated from the result of the composition analysis of the solution dissolved with hydrochloric acid. The content (mass%) of Si, Mg, Al and other inevitable impurities is calculated by removing Fe from the result of 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 in from the base steel plate and varies depending on the plating conditions and plating thickness. When calculating by adding it to the plating content (mass%), the content of Si, Mg, Al, etc. will also vary. The content excluding Fe. Moreover, Si produces a residue mainly composed of SiO ₂ in a hydrochloric acid solution. When quantifying Si in plating, in addition to component analysis of Si in hydrochloric acid solution, ICP-AES analysis is performed on the residue obtained by alkali melting and further acid-dissolving the residue.

Furthermore, the hot-dip Al-based plated steel sheet of the present invention can further include a chemical conversion film on the surface thereof.
The type of the chemical conversion film 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. Moreover, it is preferable to contain a silica fine particle from the point of adhesiveness and corrosion resistance, and to contain phosphoric acid and / or a phosphoric acid compound from a point of corrosion resistance. As the silica fine particles, either wet silica or dry silica may be used, but it is more preferable that silica fine particles having a large effect of improving adhesion, particularly dry silica, be contained. Examples of the phosphoric acid and the phosphoric acid compound include those containing at least one selected from orthophosphoric acid, pyrophosphoric acid, polyphosphoric acid, and metal salts and compounds thereof.

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

In addition, it does not specifically limit about the base steel plate used for the hot-dip Al type plated steel sheet of the present invention, and it can be used not only for the same steel plate as the steel plate used for the normal hot-dip Al type plated steel plate but also for a high-strength steel plate and the like. . For example, a hot-rolled steel plate or steel strip that has been pickled and descaled, or a cold-rolled steel plate or steel strip obtained by cold rolling them can be used.

(Method for manufacturing molten Al-based plated steel sheet)
Next, the manufacturing method of the hot-dip Al type plated steel sheet of this invention is demonstrated.
The method for producing a hot-dip Al-based plated steel sheet according to the present invention comprises 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, Al as a main component, and Zn contained as necessary. And a plating bath comprising inevitable impurities.
By such a production method, a hot-dip Al-based plated steel sheet having normal corrosion resistance and excellent chemical conversion treatment properties and coating film swelling resistance can be produced.

Although there is no particular limitation on the method for manufacturing a hot-dip Al-based plated steel sheet of the present invention, a method of manufacturing in a continuous hot-dip plating facility is usually employed. In this method, since the base steel sheet is immersed in a plating bath and plating is performed, plating is performed on both surfaces of the steel sheet.

There are no particular limitations on the type of base steel sheet used in the hot-dip Al-based plated steel sheet of the present invention. For example, a hot-rolled steel plate or steel strip that has been pickled and descaled, or a cold-rolled steel plate or steel strip obtained by cold rolling them can be used.
Moreover, it does not specifically limit about the conditions of the said pre-processing process and annealing process, Arbitrary methods are employable.

What is necessary is just to implement about the said hot rolling process by the normal method which winds up through slab heating, rough rolling, and finish rolling. Further, the heating temperature, finish rolling temperature, etc. are not particularly specified, and can be carried out at ordinary temperatures.
The pickling step performed after the hot rolling may be performed by a commonly used method, and examples thereof include cleaning using hydrochloric acid or sulfuric acid.
The cold rolling process performed after the pickling is not particularly limited, but can be performed at a rolling reduction of 30 to 90%, for example. If the rolling reduction is 30% or more, the mechanical properties do not deteriorate, while if it is 90% or less, the rolling cost does not increase.
For the recrystallization annealing step, for example, after cleaning by degreasing and the like, using an annealing furnace, heat treatment is performed to heat the steel sheet to a predetermined temperature in the preceding heating zone, and predetermined heat treatment is performed in the latter soaking zone. Can be applied. It is preferred to process at temperature conditions that have the required mechanical properties. Moreover, in order to activate the surface layer of the steel plate before a plating process, the atmosphere in an annealing furnace anneals with Fe in a reducing atmosphere. In addition, although the kind of reducing gas is not specifically limited, It is preferable to use the reducing gas atmosphere already generally used.

The plating bath used in one embodiment of the method for producing a hot-dip Al-based plated steel sheet of the present invention contains 5 to 20% by mass of Si and 6% by mass to 10% by mass of Mg, with the balance being Al and inevitable Consists of impurities.
The plating bath used in another embodiment of the method for producing a hot-dip Al-based plated steel sheet of the present invention is 50 to 90% by mass of Al, 5 to 20% by mass of Si, and more than 6% by mass and 10% by mass or less. It contains Mg and the balance consists of Zn and inevitable impurities.
The composition of the plated Al-plated steel sheet of the present invention can be accurately performed by controlling the plating bath composition.
The inevitable impurities and optional contained elements are the same as those described in the description of the plating with the hot-dip Al-based 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 was set to the solidification start temperature + 20 ° C. In order to perform the hot dipping process, the bath temperature was 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 component due to the local decrease in bath temperature. On the other hand, the upper limit of the bath temperature is set to 700 ° C. When the bath temperature exceeds 700 ° C., rapid cooling of the plating becomes difficult, and the main component is Al—Fe formed at the interface with the plated steel plate. This is because the thickness of the interface alloy layer is increased.

In addition, 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 securing plating characteristics and preventing changes in bath temperature in the continuous hot-dip plating operation, It is preferable to control the temperature within ± 20 ° C.

Furthermore, the immersion time of the base steel sheet in the plating bath is preferably 0.5 seconds or more. When the immersion time is less than 0.5 seconds, there is a possibility that sufficient plating cannot be formed on the surface of the base steel plate. On the other hand, the upper limit of the immersion time is not particularly limited, but if the immersion time is increased, the thickness of the Al-Fe alloy layer formed at the interface with the plated steel sheet may be increased. Preferably there is.

In addition, there are no particular limitations on the conditions for immersing the base steel sheet in the plating bath. For example, when performing plating on mild steel thin materials, it can be performed at a line speed of about 150 to 230 mpm, and when plating on thick materials, it can be performed at a line speed of about 40 mpm. Can be about 5 to 7 m.

And it is preferable that the manufacturing method of the hot dip galvanized steel sheet of this invention cools the steel plate after the said hot dip at a cooling rate of 15 K / s.
After performing the hot-dip plating using the above-described plating bath, Mg ₂ Si having a large major axis exceeding 10 μm can be formed during plating by performing a gentle cooling treatment of less than 15 K / s. Furthermore, it is possible to reduce the thickness of the interface alloy layer formed at the interface with the plated steel plate.
On the other hand, if the cooling rate is less than 5 K / s, since the solidification of the plating is slow, a dripping pattern may be formed on the plating surface, which may cause a noticeable deterioration in appearance and a decrease in chemical conversion property. It is preferable to set it as s or more.
From the same viewpoint, the cooling rate is particularly preferably 8 to 12 K / s.

Moreover, in the method for producing a hot-dip Al-based plated steel sheet according to the present invention, it is preferable to use nitrogen gas cooling for the cooling treatment. The reason for adopting the nitrogen gas cooling is that it is not necessary to extremely increase the cooling rate as described above, and because it does not require a large-scale cooling facility, it is excellent in economic efficiency.

In addition, in the manufacturing method of the hot-dip Al type plated steel sheet of this invention, it does not specifically limit except the cooling conditions after a plating bath and hot-dip plating, A hot-dip Al type plated steel plate can be manufactured in accordance with a conventional method.
For example, a chemical conversion treatment film can be provided on the surface of the molten Al-based plated steel sheet (chemical conversion treatment step), or a coating film can be separately provided in a coating facility (coating film formation step).

Next, examples of the present invention will be described.
(Samples 1-30)
For all the hot-dip Al-based plated steel sheets used as samples, cold-rolled steel sheets with a thickness of 0.8 mm manufactured in a conventional manner are used as the base steel sheet, and the bath temperature of the plating bath is 660 ° C and the infiltration temperature is 660 ° C using a 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. Under the conditions of the cooling rate after plating shown in Table 1, the molten Al-based plated steel sheet of each sample was manufactured.
Further, all the samples were punched out to 50 mmφ, immersed in a 35% hydrochloric acid aqueous solution to dissolve the plating on the base steel sheet, and then the weight before and after the immersion was measured. Further, the plating composition was confirmed by quantifying the composition of the solution by ICP emission spectroscopic analysis. The plating adhesion amount was calculated by removing Fe from the weight change before and after immersion in hydrochloric acid and the plating composition. The plating composition was also calculated with the remainder excluding Fe as 100% by mass. Table 1 shows the plating adhesion amount and the plating composition of each sample.

In addition, arbitrary three cross sections were cut out from a sample of a molten Al-based plated steel sheet by shearing, and plating was observed with a scanning electron microscope (SEM). A cross section parallel to the plate thickness direction of the plating was observed in a range of 1 mm in the plate width direction, and the major axis and the number of massive Mg ₂ Si during plating and the distance from the plating surface to the massive 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) The average of the five fields of view was defined as the area ratio (%) of each sample.

(Evaluation of chemical conversion treatment)
Each sample of the molten Al-plated steel sheet was sheared to a size of 80 mm × 70 mm, and then subjected to zinc phosphate treatment as a chemical conversion treatment under the following conditions in the same manner as the coating treatment for automobile outer plates.
・ Zinc phosphate treatment: Defluorinating agent manufactured by Nihon Parkerizing Co., Ltd .: FC-E2001, surface conditioning agent: PL-X, and chemical conversion treatment agent: PB-AX35 (temperature: 35 ° C) Chemical conversion treatment was performed under the conditions of a concentration of 200 ppm by mass and an immersion time of the chemical conversion solution of 120 seconds.
The chemical conversion film was observed with a scanning electron microscope (SEM), and the zinc phosphate processability was evaluated according to the following criteria. The evaluation results are shown in Table 1.
○: Uniform (There is no scale (the part where there is no chemical conversion film and the base plating appears))
×: There is a scale

(Evaluation of film swell resistance)
Each sample of the molten Al-based plated steel sheet was sheared to a size of 80 mm × 70 mm, and then subjected to zinc phosphate treatment as a chemical conversion treatment, similarly to the coating treatment for automobile outer plates, followed by electrodeposition coating. Here, the zinc phosphate treatment and electrodeposition coating were performed under the following conditions.
・ Zinc phosphate treatment: Defluorinating agent manufactured by Nihon Parkerizing Co., Ltd .: FC-E2001, surface conditioning agent: PL-X, and chemical conversion treatment agent: PB-AX35 (temperature: 35 ° C) Chemical conversion treatment was performed under the conditions of a concentration of 200 ppm by mass and an immersion time of the chemical conversion solution of 120 seconds.
Electrodeposition coating: Electrodeposition coating made by Kansai Paint Co., Ltd .: Using GT-100, electrodeposition coating was applied so that the film thickness was 15 μm.
After chemical conversion treatment and electrodeposition coating, as shown in Fig. 1, the end of the evaluation surface 7.5mm and the non-evaluation surface (back surface) are sealed with tape, and then the center of the evaluation surface is coated with a cutter knife. A sample to which a crosscut scratch having a length of 60 mm and a central angle of 60 ° was added up to a depth reaching the base steel plate was used as a sample for evaluation of coating film swelling resistance.
Using the sample for evaluation, a corrosion acceleration test was performed in the cycle shown in FIG. After starting the corrosion acceleration test from wet and after 60 cycles, the film bulge width of the part where the film bulge from the scratches is the maximum (maximum film bulge width: maximum on one side with the wound centered) The film swelling width was measured, and the film swelling resistance was evaluated according to the following criteria. The evaluation results are shown in Table 1.
○: Maximum swollen width ≦ 2.0mm
×: Maximum film swelling width> 2.0 mm

From Table 1, it can be seen that each sample of the present invention shows superior results in both chemical conversion treatment properties and coating film swell resistance as compared with each sample of the comparative example.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the fusion | melting Al type plated steel plate excellent in chemical conversion treatment property and coating-film swelling resistance, and this fusion | melting Al type plated steel plate can be provided.

Claims

The plating contains 5 to 20 mass% Si and 6 mass% to 10 mass% Mg, and the balance is composed of Al and inevitable impurities, and the major axis exceeds 10 μm during the plating. A hot-dip Al-based plated steel sheet comprising Mg 2 Si, wherein the closest distance between the Mg 2 Si and the plating surface is 0.5 μm or more.
The plating contains 50 to 90% by mass of Al, 5 to 20% by mass of Si, and more than 6% by mass and 10% by mass or less of Mg, and the balance is composed of Zn and inevitable impurities. A molten Al-based plated steel sheet having Mg 2 Si having a major axis exceeding 10 μm and a closest distance between the Mg 2 Si and the plating surface of 0.5 μm or more.
The ratio of Mg content to Mg (Mg / Si) in the plating is less than 1.1, the hot-dip Al-based plated steel sheet according to claim 1 or 2.
4. The method according to any one of 1 to 3, wherein there are 5 or more Mg 2 Sis having a major axis exceeding 10 μm in a range of 1 mm in the plate width direction in a cross section parallel to the plate thickness direction of the plating. Fused Al-based plated steel sheet.
The melt according to any one of claims 1 to 4, wherein an area ratio of Mg 2 Si having a major axis exceeding 10 µm in a cross section parallel to the plate thickness direction of the plating is 2% or more. Al-plated steel sheet.
The hot-dip galvannealed steel sheet according to any one of claims 1 to 5, wherein the major axis of the Mg 2 Si is 15 µm or more.
The hot-dip galvannealed steel sheet according to any one of claims 1 to 6, wherein the amount of the plating is 35 to 100 g / m 2 per side.
In a plating facility, a hot-dip Al-based plated steel sheet characterized by using a plating bath containing 5-20% by mass of Si and 6% by mass to 10% by mass of Mg, with the balance being Al and inevitable impurities. Manufacturing method.
The plating equipment uses a plating bath containing 50 to 90 mass% Al, 5 to 20 mass% Si, and more than 6 mass% to 10 mass% Mg, with the balance being Zn and inevitable impurities. A method for producing a molten Al-based plated steel sheet.
The method for producing a hot-dip Al-plated steel sheet according to claim 8 or 9, wherein after the base steel sheet is passed through the plating bath, cooling is performed at a cooling rate of less than 15 K / s.