JP7120166B2 - Method for producing hot-dip Al-Zn-based plated steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot-dip Al--Zn plated steel sheet having excellent corrosion resistance after painting.

In general, a hot-dip Al-Zn-based plated steel sheet is produced by using a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base steel sheet, and recrystallization annealing the base steel sheet in an annealing furnace of a continuous hot-dip galvanizing line. , is manufactured by performing a hot-dip plating process. The Al—Zn-based plating layer thus formed comprises an alloy phase existing at the interface with the base steel sheet and an upper layer existing thereon. Furthermore, the upper layer mainly consists of a portion (α-Al phase) where Zn is supersaturated and Al is dendrite-solidified, and the remaining dendrite gap portion (Zn-rich portion), and the dendrite-solidified portion is the plating layer. is laminated in the film thickness direction. This characteristic layer structure of the upper layer complicates the path of progress of corrosion from the surface, making it difficult for corrosion to easily reach the base steel plate. As a result, the hot-dip Al--Zn-coated steel sheet can have better corrosion resistance than the hot-dip galvanized steel sheet having the same coating layer thickness.

In addition to the plating components, unavoidable impurities, and Fe eluted from the steel sheet or equipment in the plating bath, Si is usually added to the plating bath to suppress excessive alloy phase growth. Si is present in the alloy layer in the form of an intermetallic compound, or in the upper layer in the form of an intermetallic compound, a solid solution, or a simple substance. The action of Si suppresses the growth of the alloy layer at the interface of the hot-dip Al--Zn plated steel sheet, and the thickness of the alloy layer is about 1 to 5 μm. If the thickness of the plating layer is the same, the thinner the alloy layer, the thicker the upper layer, which is effective in improving corrosion resistance. In addition, since the alloy layer is harder than the upper layer and acts as a starting point for cracks during processing, suppressing the growth of the alloy layer reduces the occurrence of cracks and improves bending workability. Since the base steel sheet is exposed at the cracked portion and the corrosion resistance is poor, suppressing the growth of the alloy layer and suppressing the crack generation also improves the corrosion resistance at the bent portion.

Demand for hot-dip Al-Zn coated steel sheets with excellent corrosion resistance is increasing mainly in the field of building materials such as roofs and walls exposed to the outdoors for a long period of time, and in recent years, they have also come to be used in the field of automobiles. rice field. Particularly in the automobile field, there is a demand to reduce the weight of automobile bodies, improve fuel efficiency, and reduce _CO2 emissions as part of global warming countermeasures. Therefore, it is strongly desired to reduce the weight by using high-strength steel sheets and to reduce the gauge by improving the corrosion resistance of the steel sheets. However, when the hot-dip Al--Zn plated steel sheet is used in the field of automobiles, especially for exterior panels, the following problems arise.

When a hot-dip Al-Zn-based plated steel sheet is used as an automobile outer panel, the plated steel sheet is provided to automobile manufacturers etc. in a state where it is plated by a continuous hot-dip plating facility, where it is processed into a panel part shape and then chemically treated. It is common to apply general coating for automobiles, including treatment, electrodeposition coating, intermediate coating, and top coating. However, when the paint film of the outer panel using the hot dip Al-Zn plated steel sheet is damaged, the coating layer with the unique phase structure consisting of the above-mentioned α-Al phase and the Zn-rich phase is formed. As a result, preferential dissolution of Zn (selective corrosion of the Zn-rich phase) occurs at the coating film/plating interface starting from the scratch. Then, this preferential dissolution progresses deep into the healthy portion of the coating, causing large swelling of the coating film. As a result, in some cases, sufficient corrosion resistance (hereinafter referred to as post-painting corrosion resistance) due to damage to the paint film after painting cannot be ensured.

On the other hand, when hot-dip Al--Zn plated steel sheets are used in the field of building materials as roofing materials or wall materials for buildings, corrosion resistance after coating is also a problem. When used as a roof material or wall material, the hot-dip plated steel sheet is generally supplied to a construction company or the like in a state in which the undercoat and topcoat are applied, and is used after being sheared to the required size. For this reason, the uncoated end face of the steel sheet is inevitably exposed, and from this point, swelling of the coating film called edge creep may occur. When a hot-dip Al--Zn plated steel sheet is used in the field of building materials, selective corrosion of the Zn-rich phase occurs at the paint film/plating interface starting from the end face of the steel sheet, as in the case of automobile outer panel. As a result, the corrosion resistance after painting was sometimes inferior due to the edge creep that was significantly larger than that of the hot-dip Zn plating.

In order to solve the above problem, for example, Patent Document 1 discloses that Mg or further Sn or the like is added to the plating composition to form Mg compounds such as Mg ₂ Si, MgZn ₂ , Mg ₂ Sn in the plating layer. , discloses a hot-dip Al—Zn-based plated steel sheet that is improved in the generation of red rust from the steel sheet end face.

JP-A-2002-12959

However, although the post-painting corrosion resistance in the case where the hot-dip Al—Zn-coated steel sheet disclosed in Patent Document 1 is coated, was examined, it was found that the problem of post-painting corrosion resistance was still not resolved.

Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for producing a hot-dip Al--Zn plated steel sheet having excellent post-coating corrosion resistance.

In order to solve the above problems, the inventors of the present invention conducted intensive studies and found that a base steel sheet plated using an Al—Zn-based plating bath containing predetermined amounts of Mg and Si was cooled under specific conditions. , and by heating, the MgZn ₂ phase in the plating layer transforms into a more stable Mg ₂ Zn ₁₁ phase. As a result, in the plating layer composed of the α-Al phase, the Zn-rich phase, the Mg-Zn compound, and the Mg ₂ Si phase, the Mg-Zn compound phase is the Mg ₂ Zn ₁₁ phase. You can control the structure. As a result, the present inventors have found that it is possible to manufacture a hot-dip Al--Zn plated steel sheet exhibiting unprecedented excellent corrosion resistance after painting.

The present invention is based on the above findings, and has the following features.
[1] The present invention includes Al, Mg, Si, and Zn in the component composition, and in the component composition, the Al, Mg, and Si are mass%, Al: 25 to 75 mass%, Mg: 1 to A plating treatment step of immersing the base steel plate in a plating bath containing 10 mass% and Si: 1 to 3 mass% ;
A primary cooling step of cooling the steel sheet after the plating treatment step to a primary cooling stop temperature of 250° C. or less;
A heating step of heating the steel plate after the primary cooling step to a heating temperature of 300 to 380 ° C.;
A secondary cooling step of cooling the steel plate after the heating step;
A method for producing a hot-dip Al—Zn-based plated steel sheet, comprising:

[2] In the present invention, the composition of the plating bath further contains one or more selected from the group consisting of Cr, Ni, Ca, Sr, Mn, Ti, B, Sn, In and Bi. preferably.
[3] In the present invention, the average cooling rate of the steel sheet in the primary cooling step is preferably 150°C/s or less.

[4] In the present invention, the average heating rate of the steel sheet in the heating step is preferably 5 to 30° C./s.

[5] In the present invention, the temperature of the plating bath is preferably the solidification start temperature of the plating bath +80° C. or less.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to produce a hot-dip Al--Zn plated steel sheet having excellent post-coating corrosion resistance.

It is the figure which showed the sample for evaluation of corrosion resistance after painting. It is the figure which showed the cycle of the corrosion acceleration test.

A method for manufacturing a hot-dip Al—Zn plated steel sheet according to the present invention includes a plating process, a primary cooling process, a heating process, and a secondary cooling process. Each step will be described below.
<Plating process>
In this plating treatment step, the base steel sheet contains Al: 25 to 75 mass%, Mg: 1 to 10 mass%, and Si: 1 to 3%, and the balance is Zn and unavoidable impurities. It is a step of immersing in

The manufacturing method of the hot-dip Al—Zn-based plated steel sheet is not particularly limited, but for example, it is performed using a continuous hot-dip galvanizing line (CGL). In this method, the base steel sheet is immersed in a plating bath for plating, so both sides of the base steel sheet are plated.

The plating bath in the present invention has a component composition containing Al: 25 to 75 mass%, Mg: 1 to 10 mass%, Si: 1 to 3%, and the balance being Zn and unavoidable impurities. Then, by adjusting the plating bath composition to the above range, it contains Al: 25 to 75 mass%, Mg: 1 to 10 mass%, and Si: 1 to 3%, and the balance is Zn and unavoidable impurities. A hot-dip Al—Zn-based plated steel sheet having a coating layer having a composition can be obtained. In addition, the Al content, Mg content, Si content, and Zn content in the plating bath are approximately the same as the Al content, Mg content, Si content, and Zn content in the resulting plating layer. Become. Therefore, by controlling the component contents in the plating bath, it becomes possible to control the composition of the plating layer to a target composition.

Hereinafter, each component of the plating bath composition of the method for producing a hot-dip Al—Zn plated steel sheet according to the present invention will be described.

<<Al content>>
The Al content in the plating bath is 25 to 75 mass%, and is preferably 45 to 65 mass%, more preferably 50 to 60 mass%, in terms of the balance between improved corrosion resistance and ease of operation. is more preferred.

By setting the Al content in the plating bath to 25 mass % or more, a plating layer having an Al content of 25 mass % or more can be obtained. As a result, dendritic solidification of Al occurs in the upper layer existing on the alloy layer existing at the interface with the base steel sheet. Therefore, the upper layer is mainly composed of a portion where Zn is supersaturated and Al is dendrite-solidified, and the remaining dendrite gap portion, and the dendrite-solidified portion is laminated in the film thickness direction of the plating layer, resulting in a structure excellent in corrosion resistance. Prepare. In order to stably obtain such a plating phase structure, it is preferable to set the Al content in the plating bath to 45 mass % or more. On the other hand, when the Al content in the plating bath exceeds 75 mass %, the amount of Zn having a sacrificial anticorrosion action against Fe decreases in the resulting plating layer. As a result, corrosion resistance deteriorates when the steel substrate is exposed. In general, the smaller the coating weight, the easier it is for the steel substrate to be exposed. Therefore, in order to obtain sufficient corrosion resistance even with a small coating weight, the Al content in the plating bath should be 75 mass% or less. is preferred. Further, in the Al--Zn hot-dip plating, as the Al content increases, the temperature of the plating bath (hereinafter referred to as "bath temperature") rises, which raises concerns about operational problems. However, if the Al content is within the above range, the bath temperature is appropriate and there is no problem. Further, when the Al content in the plating bath is less than 25 mass%, the Al solidification form of the present invention is different. More specifically, when the Al content is within the range of the present invention, after Al dendrites are precipitated, compounds of Mg and Zn are precipitated in the dendrites (interdendrites). On the other hand, when the Al content is less than 25 mass%, the Al dendritic structure is difficult to form, which is different from the solidified form of Al of the present invention. This tendency appears more remarkably as the Al content in the plating bath is lower.

<<Mg content>>
The Mg content in the plating bath is 1 to 10 mass%, preferably 2 to 10 mass%, more preferably 2 to 5 mass%. If the Mg content is within the above range, Mg will be included in the corrosion products when the resulting plating layer corrodes. As a result, the stability of corrosion products is improved, and progress of corrosion is delayed, resulting in an effect of improving corrosion resistance.

Here, the reason why the Mg content in the plating bath is set to 1 mass % or more is that when the Mg content is set to 1 mass % or more, the corrosion retarding effect of Mg can be obtained in the obtained plating layer. On the other hand, the reason why the Mg content is set to 10 mass % or less is that the effect is not saturated, an increase in manufacturing cost is suppressed, and the composition of the obtained plating layer can be easily controlled. Further, Mg preferably forms an Mg—Zn compound such as MgZn ₂ and Mg ₂ Zn ₁₁ in the upper layer of the resulting plating layer. Mg in the upper layer of the plating layer, in addition to forming Mg ₂ Si by bonding with Si, dissolves in the Al phase or bonds with Zn to form Mg—Zn compounds such as MgZn ₂ and Mg ₂ Zn ₁₁ . It exists only by forming The Mg—Zn compound in the plating layer preferentially dissolves during corrosion to form stable corrosion products containing Mg and Zn. Therefore, the Mg--Zn compound in the coating layer has the effect of reducing the corrosion rate of the hot-dip Al--Zn plated steel sheet.

<<Si content>>
The Si content in the plating bath is 1 to 3 mass%, preferably 1.5 to 3 mass%, more preferably 2 to 3 mass%. When the Si content is within the above range, the resulting plated steel sheet has improved corrosion resistance or workability.

Si is added to the plating bath for the purpose of suppressing the growth of the interfacial alloy layer formed at the interface with the base steel sheet and improving corrosion resistance or workability, and then contained in the plating layer. Specifically, when a hot-dip Al—Zn-based plated steel sheet is produced by performing a plating treatment in a plating bath containing Si, the steel sheet is immersed in the plating bath at the same time as Fe on the surface of the steel sheet and the content of Fe in the plating bath. Al or Si undergoes an alloying reaction to form Fe--Al and/or Fe--Al--Si compounds. The formation of this Fe--Al--Si interfacial alloy layer suppresses the growth of the interfacial alloy layer. By setting the Si content in the plating bath to 1 mass % or more, it is possible to sufficiently suppress the growth of the interfacial alloy layer. On the other hand, when the Si content in the plating bath exceeds 3 mass%, not only is the effect of suppressing the growth of the interfacial alloy layer saturated, but also the amount of Mg ₂ Si formed increases, resulting in relatively high corrosion resistance. This reduces the amount of Mg--Zn compounds formed.

The rest of the plating bath other than the above is Zn and unavoidable impurities. The above are the basic components of the plating bath in the present invention. In addition to the above basic components, the composition of the plating bath may further contain the following optional components, if necessary.

The component composition of the plating bath further contains 0.01 to 1 mass% of one or more selected from the group consisting of Cr, Ni, Ca, Sr, Mn, Ti, B, Sn, In, and Bi. is preferred. Further, in the present invention, the hot-dip Al-Zn plated steel sheet is a general term for plated steel sheets having a coating layer containing predetermined amounts of Al, Mg and Zn, and the hot-dip Al-Zn plated steel sheet is within the scope of the present invention. is not particularly limited as long as it satisfies Examples of hot-dip Al-Zn plated steel sheets include Al-Zn-Si-Mg plated steel sheets, Al-Zn-Si-Mg-Cr plated steel sheets, and Al-Zn-Si-Mg-Ni plated steel sheets. .

Although the lower limit of the bath temperature is not particularly limited, it is preferably +40° C. with respect to the solidification start temperature, which is the solidification point of the plating bath. If the bath temperature is less than +40°C with respect to the solidification start temperature, the plating bath may solidify when the bath temperature drops due to disturbance during operation, causing problems in continuous hot-dip galvanizing equipment. .

On the other hand, the upper limit of the bath temperature is not particularly limited either. However, if the bath temperature is excessively increased, abnormal growth of the interfacial alloy layer may cause deterioration in the workability of the plated steel sheet, generation of fumes from the plating bath, or increase in production cost. Unless there is a particular reason, the upper limit of the bath temperature is preferably +80°C with respect to the solidification initiation temperature. Therefore, unless there is a particular reason, the optimum bath temperature range is +40°C or more and +80°C or less with respect to the solidification start temperature.

Moreover, the type of the base steel plate used in the present invention is not particularly limited. For example, pickled and descaled hot-rolled steel sheets or steel strips, or cold-rolled steel sheets or steel strips obtained by cold-rolling them may be mentioned. Also, the type of composition of the base steel sheet is not particularly limited. For example, as the base steel plate, various steel plates such as low carbon steel, ultra-low carbon steel, IF steel, or high tensile strength steel to which various alloying elements are added are used.

<Primary cooling process>
This primary cooling step is a step of cooling the steel sheet after the hot-dip Al—Zn-based plating treatment to a primary cooling stop temperature of 250° C. or lower. In the present invention, the MgZn ₂ phase is transformed into the Mg ₂ Zn ₁₁ phase by combining the primary cooling step and the heating step, as will be described later in the heating step which is the next step. In order for this phase transformation to occur, the plating layer must be completely solidified and the _MgZn2 phase must be crystallized before the heating process. For that purpose, it is important to set the primary cooling stop temperature sufficiently low. Specifically, it is necessary to cool the steel sheet before the heating process to a cooling stop temperature of 250° C. or lower to completely solidify the plating layer. For the above reasons, the primary cooling stop temperature is set to 250° C. or lower. Although the lower limit of the primary cooling stop temperature is not specified, it is preferably higher from the viewpoint of energy costs in the subsequent heating process. For example, if the primary cooling stop temperature is 200° C. or higher, the heat treatment can be performed without any particular problem.

Further, in the conventional method for manufacturing a hot-dip galvanized steel sheet, the coating layer is formed in a non-equilibrium state after the base steel sheet is immersed in the coating bath until the coating layer is formed. As a result, the conventional method for producing a hot-dip galvanized steel sheet provides a coating layer mainly composed of the MgZn ₂ phase. Furthermore, in the conventional method of manufacturing a hot-dip galvanized steel sheet, a coating layer mainly composed of the Mg ₂ Zn ₁₁ phase can be obtained even if the base steel sheet is immersed in the coating bath and then subjected to soaking or slow cooling with the cooling stopped. It turned out not to be. However, in the present invention, the MgZn ₂ phase can be transformed into the Mg ₂ Zn ₁₁ phase by combining the next heating step and the primary cooling step. As a result, a plated steel sheet having a plated layer mainly composed of Mg ₂ Zn ₁₁ phase can be produced.

The average cooling rate in the primary cooling process should be within the range in which the plating layer is completely solidified. For example, from the viewpoint of productivity improvement, the average cooling rate in the primary cooling step is preferably 10° C./s or more, more preferably 30° C./s or more. If the cooling rate in the primary cooling step is too high, the plating layer will be extremely supercooled. Therefore, even when the surface temperature of the plated steel sheet is lowered to about 380 ° C. or lower, which is the final solidification point (Zn-rich phase: solidification point of Al/Zn eutectic), part of the plated layer is in a molten state. there is a possibility. In addition, considering the performance of the manufacturing equipment, the manufacturing equipment may be overloaded. From these points, the average cooling rate in the primary cooling step is preferably 150° C./s or less, more preferably 50° C./s or less. The average cooling rate in this specification refers to the average cooling rate in the range from the temperature of the plating bath to the cooling stop temperature, and is obtained by ((bath temperature) - (primary cooling stop temperature)) / (cooling time). can be done.

<Heating process>
The main heating step is a step of heating the steel plate after the primary cooling step to a heating temperature of 300 to 380°C.

The inventors of the present invention conducted various experiments focusing on the solidification structure of the plating, particularly the Mg—Zn compound, and found that after cooling to a predetermined temperature, the steel plate after the Al—Zn-based plating treatment containing the MgZn _two -phase. was found to transform the MgZn ₂ phase into the Mg ₂ Zn ₁₁ phase by heat-treating in a specific temperature range. Although the mechanism of phase transformation from the MgZn ₂ phase to the Mg ₂ Zn ₁₁ phase by heat treatment is not clear, the present inventors believe as follows. That is, it is presumed that Mg diffused from the MgZn ₂ phase to the Zn phase adjacent to the MgZn ₂ phase, resulting in a solid-phase transformation to the Mg ₂ Zn ₁₁ phase, which is the most thermodynamically stable phase.

The heating temperature in this heating step must be 300° C. or higher. If the heating temperature is less than 300° C., the phase transformation from the MgZn ₂ phase to the Mg ₂ Zn ₁₁ phase takes time, and the Mg ₂ Zn ₁₁ phase is not sufficiently formed. Moreover, when the heating temperature is increased, the phase transformation is promoted more. However, if the heating temperature exceeds 380° C., the Zn-rich phase (Al/Zn eutectic) in the coating layer melts, resulting in a decrease in manufacturability. Therefore, the heating temperature should be in the range of 300 to 380.degree.

The average heating rate (° C./s) in the main heating step is preferably 5 to 30° C./s, more preferably 10 to 15° C./s. When the average heating rate is within the above range, the phase transformation rate from the MgZn ₂ phase to the Mg ₂ Zn ₁₁ phase increases, making it easier to obtain a Mg ₂ Zn ₁₁ -based coating layer. In the present invention, the average temperature increase rate refers to the average temperature increase rate from the cooling stop temperature to the heating temperature, and is obtained by ((heating temperature) - (cooling stop temperature)) / (heating time). can be done.

<Secondary cooling process>
The secondary cooling step is a step of cooling the steel plate after the heating step. The secondary cooling stop temperature is not particularly limited. For example, in the secondary cooling step of the present invention, the temperature of the steel plate immediately after the heating step may be cooled to, for example, room temperature (20° C. to 30° C.) by standing to cool. The secondary cooling rate is not particularly limited. For example, from the viewpoint of productivity, the secondary cooling rate is preferably 10° C./s or more. In addition, considering the performance of manufacturing equipment, etc., the secondary cooling rate is preferably 150° C./s or less.

Both the primary cooling stop temperature and the heating temperature are the surface temperatures of the plated steel sheet. The heating rate, primary cooling rate, and secondary cooling rate are determined based on the surface temperature of the plated steel sheet.

Next, the characteristics of the phase structure of the coating layer of the hot-dip Al—Zn-coated steel sheet produced by the present invention (hereinafter also referred to as coating phase structure or simply phase structure) will be described.

A coating layer of a hot-dip Al—Zn plated steel sheet obtained by a conventional manufacturing method is composed of an α-Al phase, a Zn-rich phase, an Mg—Zn compound, and an Mg ₂ Si phase. However, the Mg--Zn compound phase of the hot-dip Al--Zn plated steel sheets that have been proposed so far has been mainly MgZn ₂ -phase. On the other hand, the hot-dip Al--Zn plated steel sheet produced by the present invention is characterized in that the Mg--Zn compound phase is mainly composed of the Mg ₂ Zn ₁₁ phase. The present inventors have succeeded in manufacturing a hot-dip Al-Zn coated steel sheet having excellent corrosion resistance, especially corrosion resistance after painting, by crystallizing the Mg ₂ Zn ₁₁ phase, which has been crystallized locally, over the entire coating layer. found that it is possible.

By the manufacturing method according to the present invention described above, a hot-dip Al—Zn plated steel sheet having a coating layer having an X-ray intensity ratio of MgZn ₂ /Mg ₂ Zn ₁₁ of 0.2 or less can be obtained. The ratio of the MgZn ₂ phase and the Mg ₂ Zn ₁₁ phase can be confirmed using X-ray diffraction. The X-ray intensity ratio of MgZn ₂ /Mg ₂ Zn ₁₁ is preferably 0.2 or less. By controlling the X-ray intensity within the above range, it is possible to improve the corrosion resistance of the hot-dip Al--Zn plated steel sheet, especially the corrosion resistance after coating. In order to stably improve the corrosion resistance, the X-ray intensity ratio of MgZn ₂ /Mg ₂ Zn ₁₁ is more preferably 0.1 or less. Further, when the X-ray intensity ratio of MgZn ₂ /Mg ₂ Zn ₁₁ is within the above range, a phase transformation from the MgZn ₂ phase to the Mg ₂ Zn ₁₁ phase occurs, and the entire plating layer, for example, the interdendritic gaps (interdendrites) It is considered that a structure filled with Mg ₂ Zn ₁₁ is formed.

Furthermore, it is preferable that the coating weight of the coating layer of the hot-dip Al—Zn-based plated steel sheet produced by the present invention is 35 to 150 g/m ² per side. If the adhesion amount is 35 g/m ² or more, excellent corrosion resistance can be obtained. Further, when the adhesion amount is 150 g/m ² or less, excellent workability can be obtained. From the viewpoint of obtaining better corrosion resistance and workability, the adhesion amount is preferably 40 to 110 g/m ² , more preferably 40 to 80 g/m ² .

The component composition in the plating layer can be confirmed, for example, by immersing the plating layer in hydrochloric acid or the like to dissolve it, and then performing ICP emission spectrometry or atomic absorption analysis on the solution. This method is merely an example, and any method may be used as long as it can accurately quantify the component composition of the plating layer, and is not particularly limited.

In order to obtain a plating layer mainly composed of Mg ₂ Zn ₁₁ as described above, the plating bath has a specific composition ratio and under specific conditions, a cooling step and a heating step are performed as described above. It is necessary to apply In addition, since the coating layer mainly composed of Mg ₂ Zn ₁₁ is more stable than the conventional MgZn ₂ -phase coating layer, it is possible to provide a hot-dip Al—Zn plated steel sheet with excellent corrosion resistance after coating. Therefore, by the production method according to the present invention described above, a hot-dip Al—Zn plated steel sheet having excellent corrosion resistance after painting can be obtained.

Next, the present invention will be specifically described based on examples. The present invention is not limited to the following examples.

(Manufacturing method of hot-dip Al—Zn-based plated steel sheets of samples 1 to 14)
For all hot-dip Al--Zn plated steel sheets to be samples, a cold-rolled steel sheet having a thickness of 0.8 mm manufactured by a conventional method was used as a base steel sheet. Then, the base steel plate was subjected to hot-dip plating by continuous hot-dip plating equipment under the condition that the coating weight was 50 g/m ² per side, that is, 100 g/m ² on both sides. In addition, for some of the hot-dip Al—Zn-based plated steel sheets, primary cooling was performed by the continuous hot-dip plating equipment after the hot-dip plating treatment to the primary cooling stop temperature: the temperature shown in Table 1. Next, the primarily cooled hot-dip plated steel sheet was heated to the heating temperature shown in Table 1, and then subjected to secondary cooling. Table 1 shows the conditions of plating bath composition, plating bath temperature, primary cooling rate, primary cooling stop temperature, heating rate, heating temperature, and secondary cooling rate.

(Evaluation method)
The composition ratio of the coating layer of the hot-dip Al—Zn-based plated steel sheet obtained as described above and the X-ray intensity ratio of MgZn ₂ /Mg ₂ Zn ₁₁ were measured. In addition, the corrosion resistance after coating the hot-dip Al--Zn plated steel sheet (hereinafter referred to as post-coating corrosion resistance) was evaluated. A detailed measurement method is shown below.

(1) Composition evaluation of the plating layer The hot-dip Al-Zn-based plated steel sheet, which is a sample, is punched to 100 mm Φ, immersed in hydrochloric acid to dissolve the plating layer, and then quantified by ICP emission spectrometry. was confirmed. Table 1 shows the composition of each sample.

(2) Evaluation of X-ray intensity ratio of MgZn ₂ /Mg ₂ Zn ₁₁ The plating layer of the hot-dip Al—Zn-based plated steel sheet was subjected to X-ray diffraction measurement under the following conditions, and the peak of MgZn ₂ (2θ = 19.6 °) intensity was divided by the peak intensity of Mg ₂ Zn ₁₁ (around 2θ = 14.6°) to calculate the intensity ratio. Table 1 shows the results.
[X-ray diffraction measurement conditions]
Tube: CuKα ray, tube voltage: 40 kV, tube current: 50 mA

(3) Evaluation of Corrosion Resistance after Painting After shearing the hot-dip Al-Zn-coated steel sheet as a sample into a size of 90 mm × 70 mm, as with the coating treatment for automobile outer plates, chemical conversion treatment was performed under the conditions shown below. After acid zinc treatment, electrodeposition coating, intermediate coating, and top coating were applied.

(i) Zinc phosphate treatment: Degreasing agent: FC-E2001, surface conditioning agent: PL-X, and chemical conversion agent: PB-AX35M (temperature: 35 ° C.) manufactured by Nihon Parkerizing Co., Ltd. The chemical conversion treatment was performed under the conditions of a free fluorine concentration of 200 ppm and a chemical conversion treatment agent immersion time of 120 seconds.
(ii) Electrodeposition coating: Electrodeposition coating was performed using an electrodeposition paint: GT-100 manufactured by Kansai Paint Co., Ltd. so that the film thickness was 15 μm.
(iii) Intermediate coating: Intermediate coating: TP-65-P manufactured by Kansai Paint Co., Ltd. was used, and spray coating was applied to a film thickness of 30 μm.
(iv) Top coating: Top coating: Neo6000 manufactured by Kansai Paint Co., Ltd. was used to apply spray coating to a film thickness of 30 μm.

After that, 5 mm of the edge of the evaluation surface of the hot-dip Al—Zn plated steel sheet and the non-evaluation surface (rear surface) of the plated steel sheet were sealed with a tape. After that, as shown in FIG. 1, a cross-cut scratch with a length of 60 mm and a central angle of 90° was added to the center of the evaluation surface with a cutter knife to a depth that reached the base iron of the plated steel sheet. was used as a sample for evaluation.

An accelerated corrosion test was carried out using the evaluation samples according to the cycle shown in FIG. The accelerated corrosion test was started from a wet state, and after 90 cycles, the width of the paint film swelling (maximum paint film swelling width) at the part where the paint film swelling from the scratch was the maximum was measured, and the corrosion resistance after painting was evaluated as follows. was evaluated according to the criteria of Table 1 shows the evaluation results.
○: maximum coating film swelling width ≤ 2.0 mm
×: maximum coating film swelling width > 2.0 mm

上記表１より、本発明例のサンプルでは、比較例のサンプルとは異なり、最大塗膜膨れ幅が２．０ｍｍ以下であった。以上のことから、本発明に係る製造方法により、塗装後耐食性に優れた溶融Ａｌ－Ｚｎ系めっき鋼板が得られたことがわかる。

As shown in Table 1 above, the samples of the present invention had a maximum coating film swelling width of 2.0 mm or less, unlike the samples of the comparative examples. From the above, it can be seen that a hot-dip Al—Zn plated steel sheet having excellent post-coating corrosion resistance was obtained by the manufacturing method according to the present invention.

INDUSTRIAL APPLICABILITY The hot-dip Al--Zn plated steel sheet of the present invention is excellent in corrosion resistance after painting and can be applied in a wide range of fields such as automobiles, home appliances and building materials. By using the hot-dip Al—Zn-based plated steel sheet of the present invention as a high-strength steel sheet, it becomes possible to achieve both weight reduction and excellent corrosion resistance in the automobile field. Also, in the field of building materials, the life of buildings can be extended by using them as roofing materials or wall materials.

Claims (5)

Al, Mg, Si, and Zn are included in the component composition, and in the component composition, the Al, Mg, and Si are mass%, Al: 25 to 75 mass%, Mg: 1 to 10 mass%, and Si: 1 A plating treatment step of immersing the base steel plate in a plating bath of ~ 3 mass%;
A primary cooling step of cooling the steel sheet after the plating treatment step to a primary cooling stop temperature of 250° C. or less;
A heating step of heating the steel plate after the primary cooling step to a heating temperature of more than 300 ° C. and 380 ° C.;
A secondary cooling step of cooling the steel plate after the heating step;
A method for producing a hot-dip Al—Zn-based plated steel sheet, comprising: 2. The plating bath according to claim 1, wherein the composition of said plating bath further contains one or more selected from the group consisting of Cr, Ni, Ca, Sr, Mn, Ti, B, Sn, In and Bi. A method for producing a hot-dip Al-Zn-based plated steel sheet. 3. The method for producing a hot-dip Al--Zn plated steel sheet according to claim 1, wherein an average cooling rate of said steel sheet in said primary cooling step is 150° C./s or less. The method for producing a hot-dip Al—Zn plated steel sheet according to any one of claims 1 to 3, wherein the steel sheet has an average heating rate of 5 to 30°C/s in the heating step. The method for producing a hot-dip Al—Zn plated steel sheet according to any one of claims 1 to 4, wherein the temperature of the plating bath is 80°C or lower than the solidification start temperature of the plating bath.

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