WO2018181392A1 - HOT-DIPPED Al COATED STEEL SHEET AND METHOD FOR PRODUCING SAME - Google Patents

Abstract

The purpose of the present invention is to provide a hot-dipped Al coated steel sheet which has excellent corrosion resistance after coating and excellent corrosion resistance after processing. In order to solve the above-described problem, a hot-dipped Al coated steel sheet according to the present invention is provided with a coating film that is composed of a coating layer and an interfacial alloy layer that is present at the interface between the coating layer and a base steel sheet; and this hot-dipped Al coated steel sheet is characterized in that the interfacial alloy layer contains Mn and the coating layer contains an Mg₂Si phase that has a length of 5 μm or more.

Description

Fused Al-based plated steel sheet and manufacturing method thereof

The present invention relates to a hot-dip Al-based plated steel sheet having excellent post-painting corrosion resistance and post-processing corrosion 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 and the corrosion resistance after painting.
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.

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

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 deposited in the plating layer, and the local dissolution of the plating layer proceeds using this as a local cathode. It was.
Furthermore, with respect to 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, 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-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 plate is sacrificial and anticorrosive, but the corrosion rate of the plating layer is extremely increased, and blistering may occur. Therefore, the corrosion resistance after the coating film is provided (hereinafter referred to as “corrosion resistance after coating”). ) Was desired to be further improved.

Further, in the molten Al-based plated steel sheet, an alloy layer (interface alloy layer) mainly composed of Al and Fe is usually formed at the interface between the plating layer and the base steel sheet. This interfacial alloy layer is harder than the plating layer that is the upper layer, and becomes the starting point of cracks during processing, leading to a decrease in workability, and the underlying steel sheet is exposed from the generated crack part. , “Corrosion resistance after processing”).
Therefore, in addition to the above-described demand for improving post-coating corrosion resistance, development of a molten Al-based plated steel sheet that has been further improved in post-processing corrosion resistance has been desired.

An object of the present invention is to provide a molten Al-based plated steel sheet having excellent post-coating corrosion resistance and post-processing corrosion 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 of the coating film (the effect of improving the corrosion resistance after painting) 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 the effect of suppressing the corrosion of plating, it is estimated that the effect of improving the corrosion resistance after coating can be obtained. The inventors of the present invention have further conducted diligent research and found that Mg ₂ Si having a large particle diameter (major axis exceeding 5 μm) can be formed during plating by containing a required amount of Mg and Si.
Furthermore, the present inventors can suppress the thickness of the interface alloy layer by including a required amount of Mn in the interface alloy layer existing at the interface between the plating layer and the base steel sheet, and can reduce the thickness of the interface alloy. It has also been found that, as a result of the layer composition being modified to be different from the conventional one, it is possible to improve workability and to realize excellent post-processing corrosion resistance.

The present invention has been made based on the above findings, and the gist thereof is as follows.
1. A molten Al-based plated steel sheet comprising a plating film comprising a plating layer and an interface alloy layer present at the interface between the plating layer and the base steel sheet,
The molten Al-based plated steel sheet, wherein the interface alloy layer contains Mn, and the plating layer has Mg ₂ Si having a major axis of 5 μm or more.

2. 2. The hot-dip Al-based plated steel sheet as described in 1 above, wherein the interface alloy layer further contains Al, Fe and Si.

3. 3. The hot-dip Al-based plated steel sheet as described in 1 or 2 above, wherein the Mn content in the interface alloy layer is 5 to 30% by mass.

4). In plating equipment, Mg: 6-15% by mass, Si: more than 7% by mass and 20% by mass or less, and Mn: more than 0.5% by mass and 2.5% by mass or less, with the balance being Al and inevitable impurities 4. The hot-dip Al-based plated steel sheet according to any one of 1 to 3, wherein the plating layer is formed using

5. 5. The molten Al-based plated steel sheet according to 4 above, wherein the plated layer is formed by allowing the base steel sheet to pass through the plating bath and then cooling at a cooling rate of less than 15 K / s.

6). 6. The hot-dip Al-based plated steel sheet as described in 4 or 5 above, wherein the composition of the plating bath satisfies the following relationship.
Equation (1): MIN {Si% × ([Mg 2 Si] mol / [Si] mol), Mg% × ([Mg 2 Si] mol / (2 × [Mg] mol))} / Al%> 0.13
M%: concentration by mass of element M, [M] _mol : molar mass of element M, MIN (a, b): smaller value of a or b

7). 7. The hot-dip Al-based plated steel sheet as described in any one of 1 to 6 above, wherein the thickness of the plating film is 10 to 35 μm.

8). In plating equipment, Mg: 6-15% by mass, Si: more than 7% by mass and 20% by mass or less, and Mn: more than 0.5% by mass and 2.5% by mass or less, with the balance being Al and inevitable impurities A method for producing a hot-dip Al-based plated steel sheet, characterized in that

9. 9. The method for producing a molten Al-based plated steel sheet as described in 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 and a method for producing the molten Al-based plated steel sheet that are excellent in post-coating corrosion resistance and post-processing corrosion resistance.

It is the figure which showed the SEM image and SEM-EDX profile of the cross section of a plating film about the hot-dip Al system plated steel plate which concerns on one Embodiment of this invention. It is the figure which showed the sample for evaluation of the corrosion resistance after coating 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 molten Al-based plated steel sheet of the present invention has a plating film (hereinafter also simply referred to as “plating”) comprising a plating layer and an interface alloy layer present at the interface between the plating layer and the base steel sheet. It is a hot-dip Al-based plated steel sheet.
The plated layer and the interface alloy layer can be observed by using a scanning electron microscope or the like for the cross section 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 the cross section 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, the plated layer and the interface alloy layer can be clearly observed.

In the present invention, the interface alloy layer contains Mn, and the plating layer has Mg ₂ Si having a major axis of 5 μm or more.
When the interfacial alloy layer contains Mn, the potential of the interfacial alloy layer is reduced, and as a result of approaching the potential of the plating layer, dissolution of the plating layer due to different metal contact corrosion is alleviated, and post-coating corrosion resistance can be improved. Further, by incorporating Mn into the interface alloy layer, the thickness of the interface alloy layer can be suppressed, so that the workability can also be improved. Furthermore, when the base steel sheet is exposed by forming large-diameter Mg ₂ Si having a major axis of 5 μm (hereinafter sometimes referred to as “aggregated Mg ₂ Si grains”) in the plating layer. Corrosion resistance after coating can be greatly improved.

The effect of improving the post-coating corrosion resistance of the massive Mg ₂ Si particles contained in the plating layer is particularly effective when the particle size is large, specifically, when the major axis is large Mg ₂ Si exceeding 5 μm. Therefore, in the present invention, the major axis of Mg ₂ Si in the plating layer exceeds 5 μm, preferably 10 μm or more, and more preferably 15 μm or more.
Here, for the above-mentioned “major axis of Mg ₂ Si”, when using a scanning electron microscope to observe Mg ₂ Si in the cross section of the plating layer, out of all Mg ₂ Si present in the observation field, It is the diameter of Mg ₂ Si with the longest diameter. In addition, “having Mg ₂ Si with a major axis exceeding 5 μm” means which observation field of view when a range of 1 mm in the plate width direction is observed with a scanning electron microscope in the plate thickness direction cross section of the plating layer. In the inside, it means a state in which one or more of the major axis exceeds 5 μm. In addition, as for “having Mg ₂ Si whose major axis exceeds 5 μm”, any cross section of the plating (however, excluding the interface alloy layer) was randomly observed in the molten Al-based plated steel sheet of the present invention. Even in this case, the condition can be satisfied.
The number of Mg ₂ Si having a major axis exceeding 5 μm is preferably 5 or more. If the number of Mg ₂ Si whose major axis exceeds 5 μm in the range of 1 mm in the plate width direction in the cross section in the plate thickness direction of the plating layer is 5 or more, the coating swells when wrinkles reaching the base steel plate occur It can be said that there is a sufficient amount of Mg ₂ Si to suppress the above. 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.

For Mg ₂ Si contained in the plating layer, in the cross section in the plate thickness direction of the plating layer, the area ratio of Mg ₂ Si having a major axis exceeding 5 μm is preferably 2% or more, and 3% or more. More preferably, it is more preferably 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 corrosion resistance after coating. Therefore, by setting the area ratio of Mg ₂ Si having a major axis exceeding 5 μm to 2% or more, better post-coating corrosion resistance can be realized.
However, if the proportion of Mg ₂ Si with a large particle size is too large, cracking of the plating when the steel plate is bent tends to occur, and the bending workability of the steel plate is deteriorated, so the major axis exceeds 5 μm. The upper limit of the area ratio of Mg ₂ Si is preferably about 10%.
Note that the area ratio of Mg ₂ Si in the present invention is, for example, a portion where a cross section of a plating film of an Al-based plated steel sheet is mapped by SEM-EDX, and Mg and Si are detected in a single field of view ( A method of deriving the area ratio (%) by dividing the area of the Mg ₂ Si area by the area of the plating (observation field of view) by image processing is used, but the area ratio of the area where Mg ₂ Si exists is The method is not particularly limited as long as it can be grasped.

Further, Mg ₂ Si having a major axis of 5 μm or more formed in the plating layer preferably has a closest distance from the plating layer surface of 0.5 μm or more. When the large particle size Mg ₂ Si is exposed on the outermost surface of the plating, it becomes a starting point for local corrosion in the chemical conversion treatment process performed as a pretreatment for coating, and also reduces the corrosion resistance or coating adhesion after coating. Because.
Here, for the closest distance between Mg ₂ Si having a major axis of 5 μm or more and the surface of the plating layer, the cross section of the molten Al-based plated steel sheet was observed using a scanning electron microscope, and the major axis in the observation field was 5 μm. The distance between the Mg ₂ Si and the plating layer surface closest to the above is the distance. In the present invention, it is preferable that the closest distance between Mg ₂ Si having a major axis of 5 μm or more and the plating layer surface is 0.5 μm or more, regardless of which part of the plating layer is measured.

The interface alloy layer of the hot-dip Al-based plated steel sheet of the present invention contains Mn as described above, and the content is preferably 5 to 30% by mass. This is because more excellent post-painting corrosion resistance and post-processing corrosion resistance can be realized.
In addition, the interface alloy layer further contains Al, Fe, and Si, and the concentrations thereof are Al: 30 to 90 mass%, Fe: 5 to 70 mass%, and Si: 0 to 10 mass%, respectively. Preferably there is. The interfacial alloy layer further contains Al, Fe, and Si in the above concentration range, thereby including Fe ₂ Al ₅ , Fe ₄ Al _13, and α-Al (Fe, Mn) Si as crystal components. Fe ₂ Al ₅ , Fe ₄ Al ₁₃ and α-Al (Fe, Mn) Si have a three-layer structure ((base steel plate) / Fe ₂ Al ₅ / Fe ₄ Al ₁₃ / α-Al (Fe, Mn) Si / (plating layer) structure) is formed, and the most basic α-Al (Fe, Mn) Si is located immediately below the plating layer. As a result, the galvanic corrosion of the plating layer can be slowed down, and excellent post-coating corrosion resistance and post-processing corrosion resistance can be realized.

Here, FIG. 1 shows an example of an SEM image and a SEM-EDX profile of the cross section of the plating film for a hot-dip Al-based plated steel sheet according to an embodiment of the present invention. As can be seen from FIG. 1, the plating film of the Al-based plated steel sheet has a Mg ₂ Si phase having a major axis of 5 μm or more, and Mn is contained in the interface alloy layer. It can also be seen that Mn is not substantially present in the plating layer and is localized in the interface alloy layer.

Further, the hot-dip Al-based plated steel sheet of the present invention contains Mg: 6 to 15% by mass, Si: more than 7% by mass and 20% by mass or less, and Mn: more than 0.5% by mass and 2.5% by mass or less in the plating equipment. And the said plating layer and the said interface alloy layer can be formed by using the plating bath which remainder consists of Al and an unavoidable impurity.
This is because Mg ₂ Si having a major axis of 5 μm or more can be more reliably formed in the plating layer obtained by the above method, and Mn can be more reliably taken into the interface alloy layer.
In addition, the composition of the plating layer of the hot-dip Al-based plated steel sheet of the present invention is almost the same as the composition of the plating bath. Therefore, the composition of the plating layer can be controlled with high accuracy by controlling the plating bath composition. Moreover, the control of the composition of the interface alloy layer formed by the reaction between the plating bath and the steel plate can also be performed with high accuracy by controlling the plating bath composition.

As described above, the plating bath contains 6 to 15% by mass of Mg. Mg contained in the plating bath is mainly distributed to the plating layer in the solidification process, and can form the above-described large particle size Mg ₂ Si, which contributes to improvement of corrosion resistance after coating. Here, when the Mg content is less than 6 mass, a sufficient amount of Mg ₂ Si with a large particle size cannot be formed, and the destruction of the Al oxide film that can suppress the selective corrosion of the interdendrite does not occur. The improvement in corrosion resistance after painting cannot be expected. On the other hand, when the Mg content exceeds 15% by mass, the plating bath is remarkably oxidized and stable operation becomes difficult. Therefore, from the viewpoint of obtaining excellent post-coating corrosion resistance and plating layer manufacturability, the Mg content is in the range of 6 to 15%. From the same viewpoint, the Mg content is preferably 7 to 10% by mass.

Further, the plating bath contains more than 7 mass% and not more than 20 mass% of Si. When the Si content is 7% by mass or less, there is a possibility that Mg ₂ Si having the large particle diameter described above cannot be reliably formed when the plating layer is solidified. On the other hand, when the Si content exceeds 20%, the FeAl ₃ Si ₂ intermetallic compound to be reduced is generated in the interface alloy layer described later, so that the workability of the plated layer and the post-working corrosion resistance are lowered. Therefore, from the viewpoint of achieving both excellent post-coating corrosion resistance and post-processing corrosion resistance, the Si content is more than 7 mass% and not more than 20 mass%, preferably 7.5 to 15 mass%, preferably 8 to 10 mass%. It is more preferable to set it as the mass%.

Furthermore, the composition of the plating bath preferably satisfies the following formula (1).
Equation (1): MIN {Si% × ([Mg 2 Si] mol / [Si] mol), Mg% × ([Mg 2 Si] mol / (2 × [Mg] mol))} / Al%> 0.13
Here, M% represents the mass% concentration of the element M in the plating bath, and [M] _mol represents the molar mass of the element M in the plating bath. MIN (a, b) indicates a smaller value of a and b.
Al-Mg ₂ Si eutectic point of pseudo-binary system of the plating layer is in the _{86.1% Al-13.9% Mg 2} Si by mass%, a large particle diameter by the Mg ₂ Si excess than this Mg ₂ Si can be deposited in the plating layer. However, since Al is also consumed during the formation of the interface alloy layer, the bath composition for obtaining the eutectic plating layer is approximately 88.5% Al-11.5% Mg ₂ Si. Mg ₂ Si% / Al% at this time is 0.13 (= 11.5 / 88.5), and Mg ₂ Si in the bath is larger than this, so that Mg ₂ Si having a large particle size is formed in the plating layer. Can be deposited. The calculated maximum Mg ₂ Si% formed by Mg and Si in the plating layer is determined by the number of moles of Mg and the number of moles of Si. The number of moles of Mg exceeds twice the number of moles of Si. In this case, since Mg is excessive, Si% × ([Mg ₂ Si] _mol / [Si] _mol ). Similarly, when twice the number of moles of Si is less than the number of moles of Mg, Si becomes excessive, so the calculated maximum Mg ₂ Si% formed of Mg and Si in the plating layer is Mg%. × ([Mg ₂ Si] _mol / (2 × [Mg] _mol )).
From the above, considering the case where either Mg or Si is excessive, the calculated Mg ₂ Si% is MIN {Si% × ([Mg ₂ Si] _mol / [Si] _mol ), Mg % × ([Mg ₂ Si] _mol / (2 × [Mg] _mol ))}. From these, it is preferable that the composition of the plating bath satisfies the above formula (1), and the formula (2): MIN {Si% × ([Mg ₂ Si] _mol / [Si] _mol ), Mg% It is more preferable that x ([Mg ₂ Si] _mol / (2 × [Mg] _mol ))} / Al%> 0.15 is satisfied.

Furthermore, the plating bath may contain 0.01 to 1% by mass of Fe. Fe is an element contained in the plating bath as a result of the Fe dissolved from the underlying steel plate being mixed into the plating bath when the plating layer is formed. About the upper limit of the content, it is 1 mass% from the relationship of the saturated dissolution amount of Fe in a plating bath.

And the said plating bath contains Mn more than 0.5 mass% and 2.5 mass% or less. Mn is dissolved in α-AlFeSi, which is a compound contained in the interface alloy layer and the plating layer, to form α-Al (Fe, Mn) Si. Since α-AlFeSi exhibits a higher potential than Fe and Al, it functions as a local cathode during the corrosion of the plating layer, and the corrosion of the plating layer is accelerated by increasing its volume fraction. On the other hand, α-Al (Fe, Mn) Si in which Mn is dissolved is known to show a significantly lower potential than α-AlFeSi. A part of Mn is dissolved in the α-Al phase, and the potential of α-Al in which Mn is dissolved becomes noble. That is, the anode during corrosion of the plating layer becomes noble by solid solution of Mn. Therefore, by adding Mn to the Al-based plating having the interface alloy layer, the potential difference between the anode and the cathode during the corrosion is reduced, and the corrosion rate is reduced.
The Mn content in the plating bath is more than 0.5% by mass and not more than 2.5% by mass, preferably 0.5 to 2.0% by mass, more preferably 0.8 to 1.2% by mass. When the Mn content is 0.5% by mass or less, there is a possibility that sufficient workability and work corrosion resistance cannot be obtained because the amount of Mn taken into the interface alloy layer is small. The upper limit of the Mn content is 2.5% by mass because of the relationship of the saturated dissolution amount of Mn in the plating bath.

In the molten Al-based plated steel sheet of the present invention, the ratio of the contents of Mg and Mn in the plating bath is important from the viewpoint of achieving both high levels of post-coating corrosion resistance and post-processing corrosion resistance. Specifically, the ratio of the Mn content (mass%) to the Mg content (mass%) in the plating bath (Mn content / Mg content) is preferably 0.003 to 0.3. 0.03-0.3 is more preferable, and 0.1-0.3 is particularly preferable. If the ratio of the content of Mn to the content of Mg in the plating bath is less than 0.003, the amount of Mn taken into the interface alloy layer is not sufficient, and there is a possibility that sufficient corrosion resistance after processing may not be obtained. On the other hand, if the ratio of the content of Mn to the content of Mg in the plating bath exceeds 0.3, the formation of Mg ₂ Si having a large particle size cannot be sufficiently formed, and the corrosion resistance after coating may be reduced. .

Further, the plating bath contains Al in addition to the above-mentioned Mg, Si and Mn. The content of Al, which is the main component of the plating bath, is preferably 50% by mass or more, more preferably more than 75% by mass, and still more preferably more than 80% by mass from the balance between corrosion resistance and operation.

The film thickness of the plated film of the hot-dip Al-based plated steel sheet of the present invention is preferably 10 to 35 μm per side. This is because excellent corrosion resistance can be obtained when the thickness of the plating film is 10 μm or more, and excellent workability can be obtained when the thickness is 35 μm or less. From the viewpoint of obtaining better corrosion resistance and workability, the thickness of the plating film is preferably 12 to 30 μm, more preferably 14 to 25 μm. Furthermore, the film thickness of the plating film is more preferably 15 μm or more considering that the molten Al-based plated steel sheet of the present invention forms Mg ₂ Si having a large particle size.

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.

Further, the plating bath is selected from Ca, Sr, V, Cr, Mo, Ti, Ni, Co, Sb, Zr, and B, as long as the effects of the present invention are not impaired, apart from the inevitable impurities described above. It is also possible to contain one or two or more kinds of elements (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 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-plated steel sheet according to the present invention includes: Mg: 6 to 15% by mass; Si: more than 7% by mass and 20% by mass or less; and Mn: more than 0.5% by mass and 2.5% by mass or less in a plating facility. It is characterized by using a plating bath containing Al and inevitable impurities.
With such a production method, it is possible to produce a hot-dip Al-based plated steel sheet that has normal corrosion resistance and is excellent in post-coating corrosion resistance and post-processing corrosion resistance.

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 the method for producing a hot-dip Al-plated steel sheet according to the present invention includes Mg: 6 to 15% by mass, Si: more than 7% by mass and not more than 20% by mass, and Mn: more than 0.5% by mass and not more than 2.5% by mass. And the balance is made of Al and inevitable impurities. The plating bath may contain about 0.01 to 1% by mass of Fe.
In addition, inevitable impurities and optional contained elements are the same as those described in 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 (intrusion plate temperature) is not particularly limited, but from the viewpoint of ensuring 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 a sufficient plating layer 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 between the plating layer and the 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 in the manufacturing method of the hot dip galvanized steel sheet of this invention, it is preferable to cool the steel sheet after the hot dip plating that has passed through the plating bath at a cooling rate of less than 15 K / s.
By performing a gentle cooling process of less than 15 K / s after performing the hot dipping using the above-described plating bath, Mg ₂ Si having a larger major axis exceeding 5 μm can be formed during plating. 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, the solidification of the plating will be slow, resulting in a dripping pattern on the plating surface, resulting in a noticeable deterioration in appearance and a decrease in chemical conversion treatment. It is preferable to do.
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 1-24)
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. The composition of the plating bath was changed to various conditions at a line speed of 200 mpm and an immersion time of 2 seconds to produce a molten Al-based plated steel sheet for each sample.
In addition, about the composition of the plating bath, about 2 g was extracted from the plating bath used for the production of the sample, and the bath composition was confirmed by chemical analysis. Table 1 shows the composition of the plating bath of each sample. Note that the balance of the plating bath is Al and inevitable impurities.
The cooling rate of cooling with nitrogen gas after immersion in the plating bath is shown in Table 1.
Moreover, about the film thickness of the said plating film, it was set as the average value of 10 points | pieces when the distance from a base steel plate to the plating surface was measured using the electromagnetic induction type film thickness meter at arbitrary 10 points | pieces of each sample. The thickness of the plating film obtained by this method includes the thickness of the interface alloy layer. Table 1 shows the thickness of the plating film of each sample.
Furthermore, for the composition of the interfacial alloy layer, from the molten Al-based plated steel sheet of each sample, any three sections were cut by shearing, and the average of semi-quantitative analysis values measured by EDX at any five points of the interfacial alloy layer Values were used. Table 1 shows the composition of the interfacial alloy layer of each sample.
Further, in the cross section cut out by the shearing process, the cross section in the plate thickness direction of the plating layer was observed in a range of 1 mm in the plate width direction with a scanning electron microscope (SEM), and the major axis of Mg ₂ Si in the plating layer was measured. Table 1 shows the major axis of Mg ₂ Si of each sample.

(Evaluation)
The following evaluation was performed about each obtained sample.
(1) Evaluation of post-coating corrosion resistance Each sample of molten Al-based plated steel sheet was sheared to a size of 80 mm x 70 mm, and then subjected to zinc phosphate treatment as a chemical conversion treatment in the same manner as automotive exterior coating treatment. Wearing paint was applied. 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. 2, 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 plated with a cutter knife and plated steel sheet A sample with a crosscut scratch having a length of 60 mm and a central angle of 60 ° was used as a sample for evaluation of corrosion resistance after coating up to a depth reaching the base steel plate.
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 coating swelling width was measured, and the corrosion resistance after coating was evaluated according to the following criteria. The evaluation results are shown in Table 1.
A: Maximum paint film swell width ≦ 1.0 mm
A: 1.0 mm <maximum paint film swelling width ≦ 1.5 mm
Δ: 1.5 mm <maximum paint film swelling width ≦ 2.0 mm
×: Maximum film swelling width> 2.0 mm

(2) Corrosion resistance evaluation after bending For each unpainted sample of molten Al-based plated steel sheet, four sheets of the same thickness are sandwiched inside and subjected to 180 ° bending (4T bending), then the outside of the bending In addition, a salt spray test based on JIS Z2371-2000 was conducted. The time until red rust occurred in each sample was measured and evaluated according to the following criteria. The evaluation results are shown in Table 1.
○: Red rust occurrence time ≥ 4000 hours △: 3500 hours ≤ red rust occurrence time <4000 hours ×: Red rust occurrence time <3500 hours

(3) Evaluation of bending back workability Each uncoated sample of molten Al-based plated steel sheet was sheared to a size of 30 mm x 230 mm, and then a draw bead mold (round bead: convex R4 mm-shoulder R0.5 mm, material: SKD11 ) Pulling was performed under the conditions of a holding load of 500 kg and a drawing speed of 200 mm / min. After processing, the surface of the bead side was observed with a scanning electron microscope (SEM), and after measuring the maximum value of any 10 crack widths in two fields of view in a field of view of 500 × 240 μm × 320 μm, an average was calculated. The average value of the maximum crack width was evaluated according to the following criteria. In addition, evaluation shows that it is excellent in a bending back workability, so that the maximum crack width is small. The evaluation results are shown in Table 1.
○: Maximum crack width ≦ 20μm
Δ: 20 μm <maximum crack width ≦ 25 μm
×: Maximum crack width> 25 μm

(4) Corrosion resistance evaluation of painted parts For each uncoated sample of molten Al-based plated steel sheet, the same as (1) Post-coating corrosion resistance evaluation for the sample after the above (3) bending back workability evaluation test The chemical conversion treatment and electrodeposition coating were applied. Then, after sealing the non-evaluation surface (back surface) with tape, cut the 60mm length to the depth where it reaches the base steel plate of the plated steel plate with a cutter knife in the center of the evaluation surface, and corrosion resistance of the coated parts A sample for evaluation was used.
A corrosion acceleration test was carried out in the cycle shown in FIG. After the corrosion acceleration test is started from wet and after 30 cycles, the film bulge width of the part where the film bulge from the scratch is the maximum (maximum film bulge width: maximum on one side with the scratch in the center) The film swelling width was measured, and the corrosion resistance after coating was evaluated according to the following criteria. The evaluation results are shown in Table 1.
A: Maximum paint film swell width ≦ 2.0 mm
○: 2.0 mm <maximum paint film swelling width ≦ 4.0 mm
Δ: 4.0 mm ≤ red rust occurrence time ≤ 5.0 mm
×: Maximum film swelling width> 5.0 mm

From Table 1, it was found that each sample of the present invention example was excellent in a well-balanced manner in terms of post-coating corrosion resistance, post-bending corrosion resistance, bend back workability, and painted part corrosion resistance. On the other hand, about each sample of the comparative example, it turned out that there exists a problem in any evaluation item (it becomes x).

According to the present invention, it is possible to provide a molten Al-based plated steel sheet and a method for producing the molten Al-based plated steel sheet, which are excellent in post-coating corrosion resistance and post-processing corrosion resistance.

1 Plating layer (part that is not Mg ₂ Si)
2 Mg ₂ Si
3 Interfacial alloy layer

Claims (9)

A molten Al-based plated steel sheet comprising a plating film comprising a plating layer and an interface alloy layer present at the interface between the plating layer and the base steel sheet,
The molten Al-based plated steel sheet, wherein the interface alloy layer contains Mn, and the plating layer has Mg ₂ Si having a major axis of 5 μm or more. The hot-dip Al-based plated steel sheet according to claim 1, wherein the interface alloy layer further contains Al, Fe, and Si. The molten Al-based plated steel sheet according to claim 1 or 2, wherein the Mn content in the interface alloy layer is 5 to 30% by mass. In plating equipment, Mg: 6-15% by mass, Si: more than 7% by mass and 20% by mass or less, and Mn: more than 0.5% by mass and 2.5% by mass or less, with the balance being Al and inevitable impurities The hot-dip Al-based plated steel sheet according to any one of claims 1 to 3, wherein the plated layer is formed by using a metal. 5. The molten Al-based plated steel sheet according to claim 4, wherein the plated layer is formed by passing the base steel sheet through the plating bath and then cooling at a cooling rate of less than 15 K / s. . The hot-dip Al-based plated steel sheet according to claim 4 or 5, wherein the composition of the plating bath satisfies the following relationship.
Equation (1): MIN {Si% × ([Mg 2 Si] mol / [Si] mol), Mg% × ([Mg 2 Si] mol / (2 × [Mg] mol))} / Al%> 0.13
M%: concentration by mass of element M, [M] _mol : molar mass of element M, MIN (a, b): smaller value of a or b The hot-dip galvanized steel sheet according to any one of claims 1 to 6, wherein the thickness of the plating film is 10 to 35 µm. In plating equipment, Mg: 6-15% by mass, Si: more than 7% by mass and 20% by mass or less, and Mn: more than 0.5% by mass and 2.5% by mass or less, with the balance being Al and inevitable impurities A method for producing a hot-dip Al-based plated steel sheet, characterized in that The method for producing a hot-dip Al-based plated steel sheet according to claim 8, 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.

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