TWI724674B - Melting Al-Zn-Mg-Si-Sr coated steel sheet and manufacturing method thereof - Google Patents

Abstract

The object of the present invention is to provide a molten Al-Zn-Mg-Si-Sr coated steel sheet having good surface appearance and excellent corrosion resistance of the flat portion and the processed portion. In order to achieve the objective, the present invention is characterized in that the plating layer contains Al: 40% by mass to 70% by mass, Si: 0.6% by mass to 5% by mass, Mg: 0.1% by mass to 10% by mass, and Sr: 0.001% by mass % To 1.0% by mass, with the remainder including Zn and unavoidable impurities, the plating layer includes an interface alloy layer existing at the interface with the base steel sheet, and a main layer existing on the alloy layer, _{In the Mg 2} Si observed in the cross section in the thickness direction of the plating layer, _{the area ratio of Mg 2} Si existing in the thickness range from the surface of the main layer to 50% is 50% or more, _{And the area ratio of Mg 2} Si extending from the surface of the main layer to reaching the interface alloy layer is 50% or less.

Description

Melting Al-Zn-Mg-Si-Sr coated steel sheet and manufacturing method thereof

The present invention relates to a molten Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance of the flat portion and the processed portion, and a manufacturing method thereof.

The molten Al-Zn-based coated steel sheet can take into account the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al, so it also shows high corrosion resistance in the hot-dip galvanized steel sheet. For example, Patent Document 1 discloses a molten Al-Zn-based coated steel sheet containing 25% by mass to 75% by mass of Al in the coating layer. Moreover, because of its excellent corrosion resistance, molten Al-Zn coated steel sheets have been exposed to long-term outdoor building materials such as roofs or walls, guardrails, wiring piping, sound insulation walls and other civil engineering fields. In recent years, Demand is expanding.

The coating layer of the molten Al-Zn-based plated steel sheet includes a main layer and an interface alloy layer existing at the interface between the base steel sheet and the main layer. The main layer mainly includes: the part where Al containing Zn undergoes dendrite solidification ( The dendrite part of the α-Al phase) and the remaining part of the dendrite gap (interdendritic) mainly composed of Zn, and the α-Al phase is laminated in the thickness direction of the coating layer. Into the structure. With this characteristic film structure, the corrosion path from the surface becomes complicated, so it is difficult for corrosion to reach the base steel sheet easily. The molten Al-Zn-based coated steel sheet has the same thickness as the coating layer. Compared with hot-dip galvanized steel sheet, it can achieve superior corrosion resistance.

In addition, there is known a technique aimed at further improving corrosion resistance by including Mg in the plating layer of the molten Al-Zn-based plated steel sheet. As a technology for molten Al-Zn-based coated steel sheet containing Mg (fused Al-Zn-Mg-Si coated steel sheet), for example, Patent Document 2 discloses the following Al-Zn-Mg-Si coated steel sheet , Which contains the Al-Zn-Si alloy containing Mg in the coating layer, the Al-Zn-Si alloy contains 45% to 60% by weight of element aluminum, 37% to 46% by weight of element zinc, and 1.2 An alloy of elemental silicon with a weight% to 2.3 weight %, and the Mg concentration is 1 weight% to 5 weight %.

Among them, the molten Al-Zn coated steel sheet containing Mg disclosed in Citation 2 has the following problem: Although it has excellent corrosion resistance, it is likely to be caused by an oxide layer formed on the surface of the coating layer. Wrinkle-like defects (hereinafter referred to as "wrinkle-like defects") with a length of about several mm to several hundred mm (hereinafter referred to as "wrinkle-like defects") impair the appearance of the coating surface.

Therefore, for example, in Patent Document 3, regarding a molten Al-Zn-Mg-based plated steel sheet, a technique for improving the surface appearance by including Sr in the plating layer is disclosed.

In addition, in Patent Document 4, regarding the molten Al-Zn-Mg-based plated steel sheet, a technique for improving the workability by including Sr in the plating layer is disclosed.

[Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Publication No. 46-7161

Patent Document 2: Japanese Patent No. 5020228

Patent Document 3: Japanese Patent No. 3983932

Patent Document 4: Japanese Patent No. 6368730

The molten Al-Zn-Mg-based plated steel sheets of Patent Document 3 and Patent Document 4 contain Sr in the coating layer, so that the generation of wrinkle-like defects can be suppressed, and the surface appearance can be improved.

However, regarding the molten Al-Zn-Mg coated steel sheet containing Sr in Citation 3 and Citation 4, by containing Sr, _{the content of Mg 2} Si near the surface of the coating layer is reduced. As a result, there is corrosion resistance. The risk of decreased sex.

In addition, in the molten Al-Zn-based plated steel sheets disclosed in Patent Document 2 and Patent Document 3, although Mg ₂ Si formed in the plating layer exhibits the effect of improving the corrosion resistance, the plating is performed during bending. The layer is broken and cracks are generated. As a result, there is a problem that the corrosion resistance of the processed part (corrosion resistance of the processed part) is poor.

In view of the above-mentioned circumstances, the present invention aims to provide a molten Al-Zn-Mg-Si-Sr plated steel sheet with good surface appearance and excellent corrosion resistance of the flat part and the processed part, as well as the flat part and the processed part. A method for manufacturing molten Al-Zn-Mg-Si-Sr coated steel sheet with excellent corrosion resistance.

The inventors of the present invention conducted studies to solve the above-mentioned problems, and as a result, they focused on the improvement effect of corrosion resistance brought about by the inclusion of Mg due to the presence of Mg ₂ Si in the plating layer when the plating layer is corroded. Mg that is preferentially dissolved and dissolved in corrosion products formed on the surface of the plating layer is found to be concentrated. Therefore, Mg ₂ Si present near the surface of the plating layer is more important. _{In addition, further studies have been carried out repeatedly, and as a result, it has been found that the Mg 2} Si contained in the main layer (hereinafter, sometimes referred to as "plating main layer" or "main layer") constituting the plating layer Most of it is concentrated near the surface of the main plating layer. Even when Sr is contained in the plating layer, there is sufficient Mg ₂ Si on the surface of the plating layer. Therefore, it not only suppresses the occurrence of wrinkle defects, but also It can also achieve excellent corrosion resistance.

In addition, the inventors of the present invention focused on that _{although the Mg 2} Si in the main plating layer has a corrosion resistance improvement effect, cracks generated in the interface alloy layer propagate to the surface of the main plating layer during bending. The route reduces the workability, and therefore the desired corrosion resistance of the processed part cannot be obtained. _{It was also discovered that by reducing the amount of Mg 2} Si from the interface alloy layer to the surface of the main plating layer, cracks generated from the interface alloy layer as a starting point during steel sheet processing can be prevented from penetrating the main plating layer and propagating to the main plating layer. Since the surface of the main layer is plated, the corrosion resistance of the processed part can also be improved.

The present invention was completed based on the above knowledge, and its gist is as follows.

1. A molten Al-Zn-Mg-Si-Sr coated steel sheet, characterized in that the coating layer contains Al: 40% by mass to 70% by mass, Si: 0.6% by mass to 5% by mass, and Mg: 0.1% by mass ~10% by mass and Sr: 0.001% by mass to 1.0% by mass, with the remainder including Zn and unavoidable impurities, and the plating layer includes an interface alloy layer present at the interface with the base steel sheet, and master alloy layer on the interface layer, the Mg ₂ Si in the cross-sectional direction of the plate thickness of the coating was observed in, from the inner surface of the main layer to a thickness range of up to 50% of present Mg ₂ The area ratio of Si is 50% or more, and the area ratio _{of Mg 2} Si that continuously exists in the interdendritic portion from the interface alloy layer to the surface of the main layer (hereinafter, sometimes referred to as "stretching") is 50 %the following.

2. The molten Al-Zn-Mg-Si-Sr coated steel sheet as described in the above 1, characterized in that: in the Mg ₂ Si observed in the cross section in the thickness direction of the coating layer, from _{The area ratio of Mg 2} Si existing in the thickness range from the surface of the main layer to 50% is 60% or more, and it extends from the surface of the main layer to the Mg ₂ Si that reaches the interface alloy layer. The area ratio is less than 50%.

3. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to the above 2, characterized in that: in the Mg ₂ Si observed in the cross section in the thickness direction of the coating layer, _{The area ratio of Mg 2} Si existing in the thickness range from the surface of the main layer to 50% is 60% or more, and it extends from the surface of the main layer to the Mg ₂ Si that reaches the interface alloy layer. The area ratio is less than 40%.

4. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to any one of 1 to 3, characterized by The ratio of the area ratio of the Si phase to the total area ratio of _{the Mg 2} Si and Si phase observed in the cross section in the thickness direction of the plating layer is 30% or less.

5. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to any one of 1 to 4, wherein the main layer has an α-Al phase dendrite portion, and the dendrite The average distance between dendrite arms of the crystal part and the thickness of the plating layer satisfy the following formula (1).

t/d≧1.5…(1)

t: thickness of plating layer (μm), d: average distance between dendrite arms (μm)

6. A method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet, characterized in that a coating bath is used, and the coating bath contains Al: 40% by mass to 70% by mass and Si: 0.6% by mass ~5 mass%, Mg: 0.1 mass% to 10 mass%, and Sr: 0.001 mass% to 1.0 mass%, and the remainder contains Zn and inevitable impurities, and the bath temperature is 585°C or less, which is applied to the steel plate At the time of hot-dip coating, the temperature of the steel sheet when entering the coating bath (entering plate temperature) is set to the temperature (coating bath temperature + 20°C) obtained by adding 20°C to the bath temperature of the coating bath.

7. The method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet according to the above 6, characterized in that the entry temperature of the steel sheet is equal to or lower than the bath temperature of the coating bath.

8. The method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet according to 6 or 7, characterized in that: after the steel sheet is subjected to hot-dip coating, an average temperature of 10°C/s or more Cooling rate The steel sheet is cooled until the plate temperature becomes a temperature obtained by subtracting 150°C from the bath temperature of the coating bath (plating bath temperature-150°C).

According to the present invention, it is possible to provide a molten Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance of the flat part and the processed part, and the flat part having good surface appearance And a method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet with excellent corrosion resistance of the processed part.

A-B: interval

L: distance

Fig. 1(a) is a diagram showing the state before and after corrosion of the molten Al-Zn-Mg-based plated steel sheet, and Fig. 1(b) is a diagram showing the state before and after the corrosion of the molten Al-Zn-based plated steel sheet.

Figure 2 (a) is the energy dispersive X-ray spectroscopy (Scanning Electron Microscope-Energy Dispersive X-Ray Analysis, SEM-) of the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention. EDX) to show the state of each element, Figure 2 (b) is about a part of the molten Al-Zn-Mg-Si-Sr plated steel sheet shown in Figure 2 (a), to observe the Mg ₂ Si and A diagram explaining the method of the Si phase.

3 is a method for calculating the area ratio of _{Mg 2} Si present in the thickness range of the surface of the main layer to 50% in the _{Mg 2} Si observed in the cross section in the thickness direction of the plating layer Figure.

FIG. 4 is used for extending from the surface of the Mg ₂ Si is calculated in a cross-sectional direction of the plate thickness of the coating observed to autonomously layer area ratio of Mg ₂ Si process until the interfacial alloy layer to be described in FIG. .

Fig. 5 is a diagram for explaining a method of measuring the distance between dendrite arms.

FIG. 6 is a diagram for explaining the flow of the combined cycle test (Japanese Automobile Standards Organization-Cyclic Corrosion Test (JASO-CCT)) of the Japanese Automobile Standards.

(Molten Al-Zn-Mg-Si-Sr coated steel sheet)

The molten Al-Zn-Mg-Si-Sr coated steel sheet that is the object of the present invention has a coating layer on the surface of the steel sheet, and the coating layer includes: an interface alloy layer existing at the interface with the base steel sheet, and the alloy The main floor above the floor. In addition, the plating layer contains Al: 40% by mass to 70% by mass, Si: 0.6% by mass to 5% by mass, Mg: 0.1% by mass to 10% by mass, and Sr: 0.001% by mass to 1.0% by mass, and has The remainder contains Zn and unavoidable impurities.

In terms of the balance between corrosion resistance and operation, the Al content in the plating layer is set to 40% by mass to 70% by mass, preferably 45% to 65% by mass. If the Al content of the main layer of the plating layer is 40% by mass or more, good corrosion resistance can be ensured. The main layer mainly includes: a part where Zn is supersaturated and Al undergoes dendrite solidification (a-Al phase dendrite part), and a part of the remaining dendrite gaps (interdendritic part), and the dendrite can be realized. The crystal portion is laminated in the thickness direction of the plating layer and has a structure excellent in corrosion resistance. In addition, the more dendrite portions of the α-Al phase are layered, the more complicated the corrosion path becomes, and the more difficult it is for the corrosion to reach the base steel sheet, so the corrosion resistance is improved. From the same viewpoint, the Al content in the plating layer is preferably set to 45% by mass or more. On the other hand, if the Al content in the plating layer exceeds 70% by mass, the content of Zn, which has a sacrificial anticorrosion effect on Fe, decreases, resulting in poor corrosion resistance. 化. Therefore, the Al content in the plating layer is set to 70% by mass or less. In addition, if the Al content in the plating layer is 65% by mass or less, the adhesion amount of the plating decreases, and even when the base steel sheet is easily exposed, it has a sacrificial anticorrosion effect on Fe, and sufficient corrosion resistance is obtained. Therefore, the Al content of the plating main layer is preferably set to 65% by mass or less.

The Si in the plating layer is to suppress the growth of the interface alloy layer generated at the interface with the base steel sheet, and is added to the plating bath to improve corrosion resistance or workability, and is inevitably included in the main layer. In the case of the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, if Si is contained in the coating bath and the hot-dip coating treatment is performed, the base steel sheet is immersed in the coating bath at the same time, Fe on the surface of the steel sheet undergoes an alloying reaction with Al or Si in the bath to form an alloy containing Fe-Al-based and/or Fe-Al-Si-based compounds. The formation of the Fe-Al-Si-based interface alloy layer can suppress the growth of the interface alloy layer. Furthermore, when the Si content in the plating layer is 0.6% by mass or more, the growth of the interface alloy layer can be sufficiently suppressed. On the other hand, when the Si content of the plating layer exceeds 5% by mass, the workability in the plating layer is reduced, and the Si phase serving as the cathode portion is likely to precipitate. The precipitation of the Si phase can be suppressed by increasing the Mg content and having a certain relationship between the Si content and the Mg content as described later. However, in this case, the manufacturing cost will increase or _{the amount of Mg 2} Si will increase. The resulting decrease in workability also makes the composition management of the plating bath more difficult. Therefore, the Si content in the plating layer is set to 5% by mass or less. Furthermore, considering that the growth of the interfacial alloy layer and the precipitation of the Si phase can be suppressed more reliably, or that it can cope with _{the case where Si is consumed as Mg 2} Si, it is preferable to add The Si content of is set to exceed 2.3% by mass to 3.5% by mass.

The plating layer contains 0.1% to 10% by mass of Mg. When the main layer of the plating layer is corroded, Mg is contained in the corrosion product, the stability of the corrosion product is improved, the progress of the corrosion is delayed, and as a result, the effect of improving the corrosion resistance is obtained. More specifically, Mg present in the main layer of the plating layer is bonded to the Si to generate Mg ₂ Si. As shown in Fig. 1(a), the Mg ₂ Si dissolves in the initial stage when the plated steel sheet is corroded, so Mg is included in the corrosion product. The Mg contained in the corrosion product has the effect of densifying the corrosion product, and can improve the stability of the corrosion product and the barrier property of external corrosion factors. On the other hand, when Mg is not contained in the plating layer, as shown in FIG. 1(b), Mg is not contained in the corrosion product, and the desired corrosion resistance cannot be obtained.

Here, the reason why the Mg content of the plating layer is 0.1% by mass or more is that when the plating layer contains Si in the concentration range, the Mg concentration is set to 0.1% by mass or more. _{, Mg 2} Si can be generated, and the corrosion delay effect can be obtained. From the same viewpoint, the Mg content of the plating layer is preferably 1% by mass or more, more preferably 3% by mass or more. On the other hand, the reason for setting the Mg content of the plating layer to 10% by mass or less is that when the Mg content of the plating layer exceeds 10% by mass, in addition to the saturation of the corrosion resistance improvement effect As a result, the manufacturing cost has risen and the composition management of the plating bath has become difficult. From the same viewpoint, the Mg content of the plating layer is preferably 6% by mass or less.

In addition, by setting the Mg content in the plating layer to 1% by mass or more, the corrosion resistance after coating can also be improved. If the coating layer of the previous molten Al-Zn-plated steel sheet that does not contain Mg is in contact with the atmosphere, a dense and stable Al ₂ O ₃ oxide film is immediately formed around the α-Al phase. The protective effect brought by the film, the solubility of the α-Al phase is very low compared to the solubility of the Zn-rich phase in the dendrite. As a result, when the coated steel sheet using the conventional Al-Zn-based coated steel sheet as the base is damaged in the coating film, selective corrosion of the Zn-rich phase occurs at the coating/plating interface starting from the damaged part, and Going deep into the painted area causes large swelling of the coating film, so the corrosion resistance after painting is poor. Therefore, from the viewpoint of obtaining excellent corrosion resistance after coating, the Mg content in the plating layer is preferably 1% by mass or more, and more preferably 3% by mass or more.

On the other hand, in the case of a coated steel sheet using a molten Al-Zn-based coated steel sheet containing Mg in the coating layer, the Mg ₂ Si phase or Mg-Zn compound precipitated in the dendrite (MgZn ₂ , Mg ₃₂ (Al, Zn) _49, etc.) are eluted in the initial stage of corrosion, and Mg is mixed in the corrosion products. Corrosion products containing Mg are very stable, whereby corrosion is suppressed in the initial stage. Therefore, it is possible to suppress the Zn-rich phase that has become a problem when the Al-Zn-based coated steel sheet is used as a base for the coated steel sheet. Large film expansion caused by selective corrosion. As a result, the molten Al-Zn-based coated steel sheet containing Mg in the coating layer exhibits excellent corrosion resistance after coating. When the Mg in the plating layer is less than 1% by mass, the amount of Mg eluted during corrosion is small, and the corrosion resistance after coating may not improve. Furthermore, when the Mg content in the plating layer exceeds 10% by mass, not only the effect is saturated, but the corrosion of the Mg compound is strongly generated, and the solubility of the entire plating layer is excessively increased. As a result, even if the corrosion is The product is stabilized, and its dissolution rate is also increased. Therefore, a large expansion width may occur, and the corrosion resistance may deteriorate after coating. Therefore, in order to stably obtain excellent corrosion resistance after coating, it is preferable to set the Mg content in the plating layer to 10% by mass or less.

In addition, the plating layer contains 0.001% by mass to 1.0% by mass of Sr. By containing Sr in the coating layer, the occurrence of wrinkle defects can be suppressed, and the surface appearance of the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention can be improved.

The wrinkle-shaped defect is a wrinkle-shaped asperity formed on the surface of the plating layer, and it is observed as whitish streaks on the surface of the plating layer. Such streak defects are easily generated when a large amount of Mg is added to the plating layer. Therefore, in the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, by making the coating layer contain Sr, the surface layer of the coating layer causes Sr to be oxidized more preferentially than Mg , Suppressing the oxidation reaction of Mg, thereby being able to suppress the occurrence of the streak-shaped defects.

Regarding the Sr content in the plating layer, it is required to be 0.001% by mass or more. The reason is to obtain the effect of suppressing the generation of the streak-shaped defect. From the same viewpoint, the Sr content in the plating layer is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and particularly preferably 0.05% by mass or more. On the other hand, the Sr content in the plating layer is required to be 1.0% by mass or less. The reason is that if the content of Sr becomes too large, the effect of suppressing the occurrence of streak defects is saturated, which is disadvantageous in terms of cost. From the same viewpoint, the Sr content in the plating layer is preferably 0.7% by mass or less, more preferably 0.5% by mass or less, and particularly preferably 0.3% by mass or less.

Furthermore, the plating layer includes: the base steel sheet component mixed into the plating layer due to the reaction between the plating bath and the base steel sheet during the plating process, or the ingot used when the plating bath is bathed ( ingot) unavoidable impurities contained in. As a mess The base steel sheet component incorporated in the plating layer may contain Fe in the order of several %. As the types of impurities that are unavoidable in the plating bath, for example, as the base steel sheet component, Fe, Mn, P, S, C, Nb, Ti, B, etc. can be cited. In addition, examples of impurities in the ingot include Fe, Pb, Sb, Cd, As, Ga, and V. In addition, with regard to Fe in the plating layer, it is impossible to distinguish and quantify Fe mixed from the base steel sheet and Fe present in the plating bath. The total content of the unavoidable impurities is not particularly limited, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, the total amount of the unavoidable impurities other than Fe is preferably 1% by mass or less. .

In addition, in the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, the coating layer can also be contained in the Zn-Al based coated steel sheet at a content of less than 1% by mass of each element. The stable element of the corrosion product is known to be at least one or more selected from Cr, Ni, Co, Mn, Ca, V, Ti, B, Mo, Sn, Zr, Li, Ag, and the like. If the content of each of these elements is less than 1% by mass, the effect disclosed in the present invention is not hindered, and the corrosion resistance is further improved by the corrosion product stabilization effect.

Furthermore, regarding the interface alloy layer, it is a layer that exists at the interface with the base steel sheet in the plating layer. As described above, Fe on the surface of the steel sheet and Al or Si in the bath are inevitably formed by the alloying reaction. Fe-Al-based and/or Fe-Al-Si-based compounds. Since the interface alloy layer is hard and brittle, if it grows thickly, it becomes a starting point for cracks during processing, so it is preferably thinned.

Furthermore, in the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, it is characterized in that in the Mg ₂ Si observed in the cross section in the thickness direction of the coating layer, from the main layer surface area ratio to a thickness range of up to 50% of Mg ₂ Si is present in 50% or more, and from the surface of the primary layer extends from an area ratio reaches up to the interface of the alloy layer of Mg ₂ Si Less than 50%.

With this, good surface appearance can be achieved, and the corrosion resistance of the flat plate portion and the processed portion can also be improved.

In the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, in the Mg ₂ Si observed in the cross section in the thickness direction of the coating layer, from the surface of the main layer to 50% _{The area ratio of Mg 2} Si existing in the thickness range up to this point is set to 50% or more. Because the plating layer contains Mg ₂ Si, when the plating layer is corroded, the Mg ₂ Si is preferentially dissolved, and the Mg dissolved in the corrosion products generated on the surface of the plating layer is concentrated, by This can exhibit excellent corrosion resistance. However, when Sr is contained in the plating layer in order to suppress wrinkle-like defects, etc., there is a problem that sufficient Mg ₂ Si does not exist on the surface of the plating layer main layer, and it is difficult to make Mg in the plating layer. The corrosion products are concentrated, thereby reducing the corrosion resistance. Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, _{most of the Mg 2} Si contained in the main plating layer (specifically, 50% or more) Concentrated in the vicinity of the surface of the plating main layer, even when Sr is contained in the plating layer, sufficient Mg ₂ Si can be present on the surface of the main layer. Therefore, a sufficient amount of Mg ₂ Si is dissolved during corrosion. Mg can be concentrated in corrosion products, and as a result, the generation of wrinkle-like defects can be suppressed, and excellent corrosion resistance can also be achieved.

That is, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, _{the area ratio of Mg 2} Si existing in the thickness range from the surface of the main layer to 50% is large (50%). _{% Or more), therefore, most of the Mg 2} Si present on the surface of the main layer is dissolved during corrosion, and the Mg concentration of the corrosion product formed on the surface of the plating layer after corrosion becomes higher. Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the generation of wrinkle-like defects can be suppressed, and excellent corrosion resistance can also be achieved.

On the other hand, the conventional molten Al-Zn-Mg-Si-Sr plated steel sheet contains _{less Mg 2} Si (less than 50%) in the thickness range from the surface of the main layer to 50%. _{, The amount of Mg 2} Si dissolved during corrosion is insufficient, and the Mg concentration of corrosion products on the surface of the plating layer after corrosion is relatively low. Therefore, in the conventional molten Al-Zn-Mg-Si-Sr plated steel sheet, although the generation of wrinkle-like defects can be suppressed, the corrosion resistance is reduced.

Further, the area ratio of Mg ₂ Si is derived (area ratio from an inner surface of the main layer to a thickness range of up to 50% of the Mg ₂ Si is present) may be, for example, using a scanning electron microscope and energy dispersive Type X-ray spectroscopy (SEM-EDX).

For example, as shown in FIG. 2(a), after obtaining the cross-sectional state of the plating layer in the thickness direction, as shown in FIG. 2(b), Mg and Si are respectively mapped (mapping) (Mg is represented by red , Si is represented by blue). After that, in the mapped Mg and Si, the part (the part indicated by purple in FIG. 2(b)) that overlaps at the same position can be set as Mg ₂ Si.

Here, regarding the obtained Mg ₂ Si, as shown in FIG. 3, _{the area ratio (A%) of Mg 2} Si with respect to the entire area of the main layer of the plating layer was measured. Thereafter, the main layer was divided into two halves in the thickness direction, and the area ratio _{of Mg 2} Si existing in the thickness range from the surface to 50% relative to the entire area of the main layer was measured (B %). Further, with respect to the Mg ₂ Si was calculated area within the thickness range of up to customize the surface layer of the entire main layer of the plated layer area ratio of Mg ₂ Si is (A%) to 50% in terms of the present The ratio ((B%)/(A%)×100%) of the rate (B%) can be obtained in the Mg ₂ Si present in the observation field of view. The thickness of the surface of the autonomous layer is within the range of 50%. The proportion of the area occupied by the existing Mg _{2 Si (X%).}

In addition, in the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, the area ratio _{of Mg 2 Si extending from the surface of the main layer to reaching the interface alloy layer is set to} Below 50%, the corrosion resistance of the processed part can also be greatly improved. _{Although the Mg 2} Si contained in the main plating layer has the effect of improving the corrosion resistance, as described above, it becomes the propagation path of cracks generated in the interface alloy layer and reduces the workability. -In the Zn-Mg-Si-Sr plated steel sheet, sufficient workability and corrosion resistance of the processed part cannot be obtained. Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, by reducing _{the amount of Mg 2} Si from the interface alloy layer to the surface of the main plating layer (the Mg ₂ The area ratio of Si is set to 50% or less). When the steel sheet is processed, even if cracks occur in the hard interface alloy layer, most of the cracks only propagate in the vicinity of the interface alloy layer and do not reach the entire surface. The surface of the main layer of the plating layer. As a result, good workability and corrosion resistance after processing can be ensured.

On the other hand, in the previous molten Al-Zn-Mg-based plated steel sheet, when cracks occur from the interface alloy layer as a starting point, most of these cracks reach the surface of the main plating layer Therefore, sufficient corrosion resistance of the processed part cannot be obtained.

Further, with respect to the area ratio ₂ Si Mg (independent from the surface layer extends to the area ratio of Mg ₂ Si in the alloy layer until reaching the interface) is derived, for example, can also be used by using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX) is performed.

As described above, after acquiring the cross-sectional state of the plating layer in the thickness direction (FIG. 2(a)), Mg and Si are respectively mapped (FIG. 2(b)). After that, in the mapped Mg and Si, the overlapping portions at the same position can be referred to as Mg ₂ Si (FIG. 2(b)).

Here, regarding the obtained Mg ₂ Si, as shown in FIG. 4, _{the area ratio (A%) of Mg 2} Si with respect to the entire area of the main layer of the plating layer was measured. Thereafter, the Mg ₂ Si particles measured in field of view as in the present, independent from the surface layer extends to the Mg ₂ Si particles (particles represented by an arrow in FIG. 4) until reaching an interface with respect to the alloy plating layer The area ratio (C%) of the entire main layer of the cladding layer. Furthermore, by calculating _{the area ratio (A%) of Mg 2} Si of the entire main layer of the plating layer, the surface of the autonomous layer of the entire main layer of the plating layer extends to reach the interface alloy The ratio ((C%)/(A%)×100%) of the area ratio (C%) _{of Mg 2} Si up to the layer _{can be obtained from the Mg 2} Si particles present in the observation field. The area ratio (Y%) _{of Mg 2} Si extending from the surface of the layer to reaching the interface alloy layer.

Furthermore, regarding the Mg ₂ Si "extends from the surface of the autonomous layer to the interface alloy layer" in the present invention, it is not only that the Mg ₂ Si extends from the surface of the autonomous layer to reach (contact) the The distance between the lower end of the Mg ₂ Si and the upper end of the interface alloy layer is very small, and the distance between the lower end of the Mg 2 Si and the upper end of the interface alloy layer is very small. _{When the distance between the lower end of the Mg 2} Si and the upper end of the interface alloy layer is 1.0 μm or less when viewed in the cross section of ). The reason is that in this case, when cracks are generated starting from the interface alloy layer, most of these cracks also reach the surface of the main plating layer.

_{Regarding the Mg 2} Si observed in the cross section in the thickness direction of the plating layer, from the viewpoint of achieving more excellent corrosion resistance, it is preferably a thickness from the surface of the main layer to 50% _{The area ratio of Mg 2} Si existing in the range is 60% or more.

_{In addition, with regard to the Mg 2} Si observed in the cross section in the thickness direction of the plating layer, from the viewpoint of achieving more excellent corrosion resistance of the processed part, it is preferably from the surface of the main layer _{The area ratio of Mg 2} Si extending to the interface alloy layer is 40% or less.

_{Furthermore, with regard to the Mg 2} Si observed in the cross section in the thickness direction of the plating layer, from the viewpoint of achieving more excellent corrosion resistance and corrosion resistance of the processed part, it is more preferable that: the ratio of the area of the inner surface layer to a thickness in the range up to 50% of Mg ₂ Si is present in 60% or more, and extends from the main surface layer to reach the interface area until the Mg ₂ Si alloy layer ratio It is less than 40%.

Here, the area ratio of the cross section in the thickness direction of the plated coating of observed Mg ₂ Si, the randomly selected cross-section will be plated coating 10 places the observed Mg ₂ Si The area ratio is averaged.

Furthermore, since the plating layer contains Si as a constituent, as described above, depending on the composition of Si and Mg in the plating layer, it may be formed in the plating layer. Si phase. However, from the viewpoint of further improving corrosion resistance and workability, it is preferable to suppress the formation of the Si phase as much as possible.

In particular, it has been found in the present invention that _{the content ratio of Mg 2} Si that improves corrosion resistance and the Si phase that becomes a cathode site when the plating layer is corroded and deteriorates corrosion resistance is important. That is, the essence of the present invention is that even if _{the absolute amount of Mg 2} Si that improves corrosion resistance is large, if the amount of Si phase that deteriorates corrosion resistance is large, good corrosion resistance cannot be ensured. Therefore, the ratio is controlled to Below a certain value.

Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the measurement by the method shown below is relative to the Mg observed in the cross section in the thickness direction of the coating layer. ₂ The total area ratio of Si and Si phase is the area ratio of Si phase observed in the cross section in the thickness direction of the plating layer (area ratio of Si phase/ _{total area of Mg 2} Si and Si phase Rate) is preferably 30% or less, more preferably 10% or less.

In addition, the method of deriving the area ratio of the Si phase can be performed by using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX) _{as in the case of the Mg 2 Si, for example.}

As described above, after acquiring the cross-sectional state of the plating layer in the thickness direction (FIG. 2(a)), Mg and Si are respectively mapped (FIG. 2(b)). After that, in the mapped Mg and Si, the portion indicated by blue in FIG. 2(b) where Si does not exist in the position where Si is present can be regarded as the Si phase. The area ratio (D%) of the Si phase can be calculated from the ratio of the total area of the blue part to the area of the plating layer in the observed field of view. Furthermore, the area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer relative to the total area ratio of the _{Mg 2 Si and Si phases (area ratio of the Si phase/Mg 2} Si and The total area ratio of the Si phase) can be calculated as (D%/(A%+D%)×100%).

Here, the area ratio of the Si phase with respect to the total area of the _{Mg 2} Si and Si phase observed in the cross section in the thickness direction of the plating layer is a random selection of the plating layer The area ratio of the Si phase observed in the cross section of 10 locations is averaged.

In addition, the area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer (the area ratio of the Si phase in the observation field: D%) is preferably 10% or less, more preferably 3% the following.

Here, the area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer is the area of the Si phase observed in the cross section of 10 randomly selected locations of the plating layer The rate is averaged.

Furthermore, from the viewpoint of further improving the initial corrosion resistance of the plated steel sheet, the area ratio of the Si phase observed on the surface of the plating layer (the area ratio of the Si phase in the observation field) is preferably 1 % Or less, more preferably 0.5% or less. Here, the method for deriving the area ratio of the Si phase on the surface of the plating layer is the same as in the case of observing the cross section, and can be performed by using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX). The method of obtaining the area ratio can be based on the cross-sectional observation method, and it can be an average of the area ratios of the Si phases observed on the surface of 10 randomly selected locations of the plating layer.

Furthermore, from the viewpoint of further improving the initial corrosion resistance, the thickness of the plating layer is relative to the total area _{of the Mg 2 Si and Si phases observed on the surface of the plating layer.} The area ratio of the Si phase observed in the cross section in the direction (the area of the Si phase/ _{the total area of the Mg 2} Si and the Si phase) is preferably 20% or less, more preferably 10% or less. The actual observation method and the calculation method of the area ratio are based on the cross-sectional observation method described above.

Furthermore, when observing the main layer of the plating layer or the interface alloy layer by a scanning electron microscope, the observation is performed after polishing and/or etching the cross section or surface of the plating layer. There are several methods of polishing or etching the cross section or surface, and there are no particular limitations as long as it is a method used when observing the cross section or surface of the plating layer. In addition, if the observation conditions with a scanning electron microscope are, for example, an acceleration voltage of 5kV to 20kV, and a magnification of about 500 to 5000 times in the secondary electron beam image or the reflected electron image, the observation can be clearly observed. Section of the plating layer. In addition, in the case of mapping using EDX, the previous area ratio can also be obtained by analyzing at the same magnification.

In addition, the main layer of the plating layer has an α-Al phase dendrite portion, and the average interdendritic arm distance of the dendrite portion and the thickness of the plating layer preferably satisfy the following formula (1).

t/d≧1.5…(1)

t: thickness of plating layer (μm), d: average distance between dendrite arms (μm)

By satisfying the formula (1), the arms of the dendrite portion containing the α-Al phase can be relatively reduced, and the path between the dendrites that are preferentially corroded can be ensured long, thereby further improving the corrosion resistance .

In addition, the distance between the dendrite arms of the dendrite portion refers to the center distance between adjacent dendrite arms (dendrite arm pitch). In the present invention, for example, as shown in FIG. 5, a scanning electron microscope (SEM) or the like is used to magnify and observe the surface of the main layer of the plating layer (for example, observation at 200 times), in a randomly selected field of view, as follows The interval of the second widest dendrite arm (secondary dendrite arm) is measured. Select the part where three or more secondary dendrite arms are arranged (in FIG. 5, select three in the A-B interval), and measure the distance along the direction in which the arms are arranged (distance L in FIG. 5). After that, the measured distance was divided by the number of dendrite arms (L/3 in FIG. 5) to calculate the distance between dendrite arms. Regarding the distance between the dendrite arms, three or more locations were measured in one field of view, and the average of the distance between the dendrite arms was calculated, and the calculated average value was used as the average distance between the dendrite arms.

Furthermore, from the viewpoint of achieving both processability and corrosion resistance at a high level, the thickness of the plating layer is preferably 10 μm to 30 μm, and more preferably 20 μm to 25 μm. The reason is that when the plating layer is 10 μm or more, sufficient corrosion resistance can be ensured, and when the plating layer is 30 μm or less, sufficient workability can be ensured.

Furthermore, the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention can also be made into a surface-treated steel sheet that further includes a chemical conversion treatment film and/or a coating film on its surface.

(Method for manufacturing molten Al-Zn-Mg-Si-Sr coated steel sheet)

Next, the manufacturing method of the molten Al-Zn-Mg-Si-Sr plated steel sheet of this invention is demonstrated.

The manufacturing method of the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention is characterized by using a coating bath containing Al: 40% by mass to 70% by mass, Si: 0.6 mass% to 5 mass%, Mg: 0.1 mass% to 10 mass%, and Sr: 0.001 mass% to 1.0 mass%, and the remainder contains Zn and inevitable impurities, and the bath temperature is 585°C or less, When hot-dip coating is applied to the steel sheet, the temperature of the steel sheet when entering the coating bath (entry plate temperature) is set to the temperature obtained by adding 20°C to the bath temperature of the coating bath (plating bath temperature + 20°C )the following.

The molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the manufacturing method has good surface appearance, and the flat plate portion and the processed portion are also excellent in corrosion resistance.

In the manufacturing method of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, although not particularly limited, from the viewpoint of manufacturing efficiency and quality stability, continuous hot-dip coating equipment is generally used.

In addition, the kind of base steel sheet used in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention is not particularly limited. For example, hot-rolled steel sheets or steel strips that have been pickled and derusted, or cold-rolled steel sheets or steel strips obtained by cold-rolling these can be used.

In addition, the conditions of the pretreatment step and the annealing step are not particularly limited, and any method can be adopted.

In the method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, the coating bath contains Al: 40% by mass to 70% by mass, Si: 0.6% by mass to 5% by mass, and Mg: 0.1% by mass to 10% by mass and Sr: 0.001% by mass to 1.0% by mass, and the balance includes Zn and unavoidable impurities.

Thereby, the molten Al-Zn-Mg-Si-Sr plated steel sheet of the desired composition can be obtained. In addition, the type, content, and function of each element contained in the coating bath are described in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention.

In addition, the molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the production method of the present invention is substantially the same as the composition of the coating bath as a whole. Therefore, the control of the composition of the main layer can be performed accurately by controlling the composition of the plating bath.

In addition, in the manufacturing method of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the content of Mg and Si in the coating bath preferably satisfies the following formula (2).

M _Mg /(M _Si -0.6)≧1.0…(2)

M _Mg : Mg content (mass%), M _Si : Si content (mass%)

By making the contents of Mg and Si in the plating bath satisfy the relational expression, the formed plating layer can suppress the generation of Si phase (for example, as observed in the cross section in the thickness direction of the plating layer The area ratio of the Si phase becomes 10% or less, and the area ratio of the Si phase observed on the surface of the plating layer becomes 1% or less), and the workability and corrosion resistance are further improved.

From the same viewpoint, M _Mg /(M _Si -0.6) is more preferably 2.0 or more, and still more preferably 3.0 or more.

In the method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, the bath temperature of the coating bath is 585°C or less, preferably 580°C or less. _{With this bath temperature setting, the amount of Mg 2} Si that is large from the surface of the main layer of the plating layer to the interface alloy layer can be reduced. In addition, it also has the effect of suppressing the growth of the interface alloy layer. The reason why the low bath temperature effectively works is that the time for the steel sheet to _{stay in the high temperature region where the Mg 2} Si and the interface alloy layer grows can be shortened. In the case where the bath temperature of the coating bath exceeds 585°C, even if the steel sheet temperature when entering the coating bath is rationalized, it reaches the interface alloy layer from the surface of the main layer The amount of Mg ₂ Si that has been so large has also increased, and since the interface alloy layer grows thickly, the desired workability and corrosion resistance of the processed part cannot be obtained.

In addition, in the manufacturing method of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the temperature of the steel sheet when entering the coating bath (entering plate temperature) is set as the bath from the coating bath The temperature obtained by adding 20°C (plating bath temperature + 20°C) or less. _{Thereby, it is possible to reduce the amount of Mg 2} Si that is large from the surface of the main layer of the plating layer to the interface alloy layer. _{The reason why the large amount of Mg 2} Si can be reduced by lowering the entering plate temperature is the same as the effect when lowering the bath temperature. The reason is that the steel sheet can _{stay in the high temperature region where Mg 2} Si and the interface alloy layer grows. Time becomes short time.

From the same point of view, the entry temperature of the steel sheet is preferably a temperature (plating bath temperature + 10°C) obtained from the bath temperature of the coating bath plus 10°C or less, and more preferably the coating temperature Below the bath temperature of the covering bath.

Furthermore, in the method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention, it is preferable that the steel sheet is subjected to hot-dip coating, and then the steel sheet is heated at an average cooling rate of 10°C/s or more. The steel plate is cooled until the plate temperature becomes the temperature obtained by subtracting 150°C from the bath temperature of the coating bath (coating bath temperature-150°C). _{Regarding the Mg 2} Si formed in the plating layer, it is known that it is easy to change from the bath temperature of the plating bath to the temperature obtained by subtracting 150°C from the bath temperature of the plating bath (plating bath temperature -150 ℃). By increasing the cooling rate in this temperature range to an average of 10℃/sec or more, the growth of Mg ₂ Si can be suppressed and the arrival from the main layer surface of the plating layer can be reduced. The amount _{of Mg 2} Si is as large as the interface alloy layer. Furthermore, by increasing the cooling rate of the steel sheet after the hot-dip coating, the growth of the interface alloy layer can also be suppressed, and as a result, the excellent corrosion resistance of the processed part can be realized.

In addition, the said average cooling rate is calculated|required by calculating the time until the steel sheet reaches the temperature obtained by subtracting 150°C from the coating bath temperature and dividing 150°C by the time.

In addition, by increasing the cooling rate in the temperature range to an average of 10° C./sec or more, _{the amount of Mg 2} Si present in the vicinity of the surface of the main plating layer can be increased to 50% or more of the whole. The reason is as follows. The interface alloy layer is formed by a solid-liquid reaction (a reaction between Al in the bath and the steel sheet) in the plating bath, and Si is also mixed into the interface alloy layer during the reaction. Therefore, the Si concentration near the interface alloy layer when solidification occurs between dendrites becomes lower than the average Si concentration of the main plating layer. Regarding _{the distribution of Mg 2} Si, the surface side of the main plating layer is compared with the interface alloy layer side. Become more. On the other hand, in the cooling process after plating, the plated steel sheet is cooled from the surface, and therefore, interdendritic solidification occurs from the surface. The lower the temperature, the lower _{the solubility of Mg 2} Si in Zn, and at the solid-liquid interface, Mg and Si are discharged on the liquid side and concentrated in the interdendritic part of the liquid. The closer to the equilibrium state, that is, the slower the cooling rate, the more significant this phenomenon is. Therefore, when the cooling rate is slow, _{the distribution of Mg 2} Si tends to exist in the vicinity of the interface alloy layer. That is, by increasing the cooling rate (average 10° C./sec or more), _{the distribution of the Mg 2} Si can suppress the presence of deviation in the vicinity of the interface alloy layer and maintain the original Mg ₂ Si distribution. _{That is, the distribution of Mg 2} Si in the plating layer can be ensured in a state where there is more on the surface side of the main layer than the vicinity of the interface alloy layer.

From the same point of view, the cooling of the hot-dip galvanized steel sheet is more preferably performed at an average cooling rate of 20°C/sec or more, more preferably performed at an average cooling rate of 30°C/sec or more, and particularly preferably Perform at an average cooling rate of 40°C/sec or more.

Furthermore, in the manufacturing method of the present invention, there are no particular restrictions on the bath temperature and the entering plate temperature during the hot-dip coating, and the conditions other than the cooling conditions after the hot-dip coating. The molten metal can be manufactured according to common methods. Al-Zn-Mg-Si-Sr coated steel sheet.

The molten Al-Zn-Mg-Si-Sr coated steel sheet obtained by the manufacturing method of the present invention can also form a chemical conversion treatment film (chemical conversion treatment step) on its surface, or form a coating in another coating equipment. Film (coating film forming step).

Furthermore, the chemical conversion treatment film may be formed by, for example, a chromate treatment or a chromate-free chemical conversion treatment, and the treatment is to coat a chromate treatment liquid or a chromate-free chemical conversion treatment liquid. , Without water washing, the steel plate temperature becomes 80 ℃ ~ 300 ℃ drying treatment. These chemical conversion treatment films may be a single layer or multiple layers. In the case of multiple layers, it is sufficient to perform a plurality of chemical conversion treatments in sequence.

Moreover, about the said coating film, the formation method, such as roll coater coating, curtain coating, and spray coating, can be mentioned. After coating the paint containing organic resin, it can be heated and dried by hot air drying, infrared heating, induction heating and other methods to form a coating film.

[Example]

(Sample 1~Sample 25)

Use a cold-rolled steel plate with a thickness of 0.5 mm manufactured by a common method as a base For the steel sheet, the molten Al-Zn-based coated steel sheet of sample 1 to sample 25 was manufactured in a continuous hot-dip coating equipment. In addition, the composition of the plating bath used in the manufacture is approximately the same as the composition of the plating layer of each sample shown in Table 1. The bath temperature of the plating bath, the entry temperature of the steel sheet, and the self-coating Table 1 shows the cooling rate up to the temperature obtained by subtracting 150°C from the bath temperature of the bath.

After that, with respect to each sample of the obtained molten Al-Zn-based plated steel sheet, a cross-sectional observation was performed at a random location by energy dispersive X-ray spectroscopy (SEM-EDX) using a scanning electron microscope.

In addition, for each sample, each condition of the formed plating layer and each manufacturing condition of the plating were measured or calculated, and shown in Table 1.

(Evaluation)

For each sample of the molten Al-Zn-based plated steel sheet obtained as described above, the following evaluations were performed. The evaluation results are shown in Table 1.

(1) Surface appearance

For each sample of the molten Al-Zn-based coated steel sheet, the surface of the coating layer (both surfaces of each sample) was observed visually in an observation field of steel sheet width of about 1000 mm to 1600 mm × 1000 mm length.

In addition, the observation results were evaluated according to the following criteria.

○: No wrinkle-like defects are observed on either the front surface or the back surface

×: A wrinkle-like defect is observed in at least one of the front surface and the back surface

(2) Corrosion resistance evaluation (corrosion resistance of the flat part)

For each sample of the molten Al-Zn series coated steel sheet, the Japanese automobile standard Compound cycle test (JASO-CCT). Regarding JASO-CCT, it is a test in which salt water spraying, drying, and wetting are set as one cycle as shown in Fig. 6 under specific conditions.

The number of cycles until the occurrence of red rust was measured for each sample, and evaluated in accordance with the following criteria.

○: The number of cycles of red rust generation≧400 cycles

△: 300 cycles≦red rust generation cycles <400 cycles

×: Red rust production cycle number <300 cycles

(3) Evaluation of the corrosion resistance of the bent part (corrosion resistance of the processed part)

For each sample of the molten Al-Zn-Mg-Si-Sr plated steel sheet, 3 sheets of the same thickness were clamped on the inner side and subjected to a 180° bending process (3T bending), and then the Japanese automobile standard was performed on the outer side of the bend The combined cycle test (JASO-CCT). Regarding JASO-CCT, it is a test in which salt water spraying, drying, and wetting are set as one cycle under specific conditions as shown in Figure 6.

The number of cycles until the occurrence of red rust was measured for the processed part of each sample, and evaluated in accordance with the following criteria.

○: Number of cycles of red rust generation≧400 cycles

△: 300 cycles≦red rust generation cycle number <400 cycles

×: Red rust production cycle number <300 cycles

[Table 1]

From Table 1, it can be seen that each sample of the present invention example has a well-balanced and excellent surface appearance, corrosion resistance, and corrosion resistance of a processed part compared with each sample of the comparative example.

[Industrial availability]

According to the present invention, it is possible to provide a fused Al-Zn-Mg-Si-Sr plated steel sheet with good surface appearance and excellent corrosion resistance of the flat part and the processed part, and the flat part with good surface appearance And a method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet with excellent corrosion resistance of the processed part.

Claims (7)

A molten Al-Zn-Mg-Si-Sr coated steel sheet, characterized in that the coating layer contains Al: 40% to 70% by mass, Si: 0.6% to 5% by mass, and Mg: 0.1% to 10% by mass Mass% and Sr: 0.001% to 1.0% by mass, with the remainder including Zn and inevitable impurities, and the plating layer includes an interface alloy layer present at the interface with the base steel sheet, and master alloy layer at the interface layer, Mg ₂ Si in the cross-sectional direction of the plate thickness of the coating was observed in, from the inner surface of the main layer to a thickness range of up to 50% of Mg ₂ Si is present _{The area ratio of Mg 2} Si is 50% or more, and the area ratio of Mg 2 Si extending from the surface of the main layer to the interface alloy layer is 50% or less. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to claim 1, wherein in the Mg ₂ Si observed in the cross section in the thickness direction of the coating layer, from the surface of the main layer area proportion up to 50% of a thickness range of Mg ₂ Si is present in 60% or more, and extends from the main surface layer to reach the interface area until the ratio of Mg ₂ Si alloy layer is 50% the following. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to claim 1 or claim 2, wherein in the Mg ₂ Si observed in the cross section in the thickness direction of the coating layer, from the ₂ Mg Si area ratio of the inner surface of the main layer to a thickness range of up to 50% is present 60% or more, and extends from the main surface layer to reach the area of Mg ₂ Si alloy layer until the interface The ratio is less than 40%. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to claim 1 or claim 2, wherein the Si phase observed in the cross section in the thickness direction of the coating layer is relative to the The ratio of the area ratio of the Si phase, which is the total of the area ratios of _{the Mg 2} Si and Si phases observed in the cross section in the thickness direction of the plating layer, is 30% or less. The molten Al-Zn-Mg-Si-Sr coated steel sheet according to claim 1 or claim 2, wherein the main layer has a dendrite portion of an α-Al phase, and the average dendrite arm of the dendrite portion The distance and the thickness of the plating layer satisfy the following formula (1); t/d≧1.5...(1) t: thickness of the plating layer (μm), d: average distance between dendrite arms (μm). A method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet is characterized in that: a coating bath is used, and the coating bath contains Al: 40% by mass to 70% by mass, and Si: 0.6% by mass to 5 Mass%, Mg: 0.1% to 10% by mass and Sr: 0.001% to 1.0% by mass, and the remainder contains Zn and unavoidable impurities, and the bath temperature is 585°C or less. Hot-dip coating is applied to the steel sheet During coating, the temperature of the steel sheet when entering the coating bath (entering plate temperature) is set below the bath temperature of the coating bath, and the formed coating layer includes an interface alloy layer present at the interface with the base steel sheet, And the main layer existing on the interface alloy layer, in the Mg ₂ Si observed in the cross section in the thickness direction of the plating layer, the thickness range from the surface of the main layer to 50% area ratio of Mg ₂ Si is present in 50% or more, and extends from the main surface layer to reach the interface area until the ratio of Mg ₂ Si alloy layer is 50% or less. The method for manufacturing a molten Al-Zn-Mg-Si-Sr coated steel sheet according to claim 6, wherein after the steel sheet is subjected to hot-dip coating, the steel sheet is cooled at an average cooling rate of 10°C/s or more Until the plate temperature becomes the temperature obtained by subtracting 150°C from the bath temperature of the plating bath (plating bath temperature-150°C).

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