JP2021031759A - Molten Al—Zn alloy plated steel sheet with excellent corrosion resistance in the processed part and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip Al—Zn-based alloy-plated steel sheet and a method for manufacturing the same, which can obtain excellent corrosion resistance of a processed portion even when bent with a high degree of processing.
SOLUTION: The surface of a steel plate contains a plating layer containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass%, and the balance is made of Zn and unavoidable impurities. It is a hot-dip Al—Zn-based alloy plated steel sheet having a content of 15.0% or more, and this plated steel sheet has an average cooling rate of 12 in the temperature range from the plating bath to 450 ° C. in the cooling process after hot-dip galvanizing. It can be obtained by performing a heat treatment under predetermined conditions after the temperature is set to ° C./s or higher.
[Selection diagram] Fig. 1

Description

The present invention relates to a hot-dip Al—Zn-based alloy-plated steel sheet typified by a Zn-55% Al-plated steel sheet and a method for producing the same.

Hot-dip Al-Zn-based alloy-plated steel sheets typified by Zn-55% Al-plated steel sheets have excellent corrosion resistance, but the plated steel sheets are bent because the plated layer is harder than pure galvanized steel sheets. When this is received, there is a drawback that cracks are likely to occur in the plating layer of the processed portion. If the crack resistance during processing is poor in this way, there arises a problem that the corrosion resistance of the processed portion deteriorates. This deterioration of the corrosion resistance of the bent portion occurs in any of the plated steel sheet, the chemical conversion-treated plated steel sheet, and the coated steel sheet on which the plated steel sheet is used as a base, which is a big problem.
As a technique for solving such a problem, Patent Documents 1 to 3 disclose a technique for improving workability by heat-treating a plated steel sheet.

Japanese Unexamined Patent Publication No. 2002-322573 Japanese Unexamined Patent Publication No. 2003-213395 Japanese Unexamined Patent Publication No. 2006-70326

By applying the techniques shown in Patent Documents 1 to 3, the workability of the plated steel sheet can be improved, but the effect is not always sufficient, and the plating layer is a soft pure galvanized steel sheet. In comparison with the above, cracks may still occur depending on the degree of processing of the plated steel sheet, and sufficient corrosion resistance of the processed portion may not be obtained.

Therefore, an object of the present invention is to provide a hot-dip Al—Zn-based alloy-plated steel sheet that solves the above-mentioned problems of the prior art and can obtain excellent corrosion resistance of the processed portion even when bent with a high degree of processing. is there.
Another object of the present invention is to provide a manufacturing method capable of stably manufacturing such a molten Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion.

As a result of repeated studies to solve the above problems, the present inventors set the elongation rate of the plating layer to a predetermined value or more, and more preferably to suppress the hardness of the Al dendrite portion of the plating layer to a predetermined value or less. It was found that the occurrence of cracks is suppressed even when bending is performed with a high degree of processing, and excellent corrosion resistance of the processed portion can be obtained. Further, in order to obtain such a plated steel sheet having excellent corrosion resistance in the processed portion, it is important to increase the average cooling rate in a predetermined temperature range immediately after plating in the manufacturing method of heat-treating the plated steel sheet in which the plated layer is solidified. It turned out.

The present invention has been made based on such findings, and the gist of the present invention is as follows.
[1] The surface of the steel sheet contains a plating layer containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass%, and the balance is Zn and unavoidable impurities.
A hot-dip Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion, characterized in that the elongation of the plating layer defined below is 15.0% or more.
Elongation: In the bending test, when the test piece is bent 180 ° with an inner spacing nt (however, t: plate thickness of plated steel sheet, n: numerical value that can include decimal parts), El (%) specified by the following formula ) Is the elongation of the outer surface of the bent portion, and the elongation El (%) at the minimum n value at which no crack is observed when the outer surface of the bent portion of the test piece is observed with a 10-fold loupe.
El (%) = (πt (n + 2) -πt (n + 1)) / (π (n + 1)) * 100
= 1 / (n + 1) * 100

[2] In the molten Al—Zn alloy plated steel sheet of the above [1], the molten Al—Zn alloy plated steel sheet having excellent corrosion resistance in the processed portion, characterized in that the hardness of the Al dendrite portion of the plating layer is 150 Hv or less. ..
[3] In the molten Al—Zn based alloy plated steel sheet of the above [1] or [2], the molten Al—Zn based alloy having excellent corrosion resistance in the processed portion, characterized in that the thickness of the plating layer is 10 to 30 μm. Plated steel plate.

[4] The steel sheet is hot-dip galvanized in a plating bath containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass%, and the balance is Zn and unavoidable impurities, and then the plated steel sheet in which the plating layer is solidified is heated. A method for manufacturing a hot-dip Al—Zn-based alloy-plated steel sheet to be treated.
In the cooling process of the plated steel sheet after hot-dip galvanizing, the average cooling rate in the temperature range up to 450 ° C. after leaving the plating bath is set to 12 ° C./s or more.
In the heat treatment, after heating the plated steel sheet to a temperature T (° C.) in a temperature range of 100 to 300 ° C., the cooling rate C (1) specified by the following equation (1) is applied from this temperature T (° C.) to 80 ° C. A method for producing a molten Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion, which comprises cooling at an average cooling rate (° C./hr) of ° C./hr) or less.
C = (T-80) / 10 ... (1)

[5] The method for producing a molten Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion, which is characterized in that the plating bath temperature is 600 ° C. or lower in the production method of the above [4].
[6] In the manufacturing method of the above [4] or [5], the molten Al-Zn system having excellent corrosion resistance of the processed portion, characterized in that the temperature of the steel sheet entering the plating bath is + 20 ° C. or less. Manufacturing method of alloy plated steel sheet.
[7] In any of the above-mentioned production methods [4] to [6], a molten Al—Zn-based alloy having excellent corrosion resistance in a processed portion, characterized in that the Fe concentration in the plating bath is 0.5 mass% or less. Manufacturing method of plated steel sheet.

[8] A chemical conversion-treated steel sheet having an excellent corrosion resistance in a processed portion, which has a chemical conversion-treated film on the surface of the molten Al—Zn-based alloy-plated steel sheet according to any one of the above [1] to [3].
[9] The hot-dip Al—Zn-based alloy-plated steel sheet according to any one of [1] to [3] above is characterized by having a chemical conversion-treated film on the surface and a single-layer or multi-layer coating film on the upper layer. A coated steel sheet with excellent corrosion resistance.
[10] A chemical conversion having excellent corrosion resistance in a processed portion, which is characterized by forming a chemical conversion treatment film on the surface of a molten Al—Zn-based alloy-plated steel sheet obtained by any of the above-mentioned production methods [4] to [7]. Manufacturing method of treated steel sheet.
[11] A chemical conversion-treated film is formed on the surface of the molten Al—Zn-based alloy-plated steel sheet obtained by any of the above-mentioned production methods [4] to [7], and then a single layer or multiple layers are applied to the upper layer. A method for producing a coated steel sheet having excellent corrosion resistance in a processed portion, which is characterized by forming a film.

The hot-dip Al—Zn-based alloy-plated steel sheet of the present invention suppresses the occurrence of cracks in the bent portion even when bent with a high degree of processing, and excellent corrosion resistance in the processed portion can be obtained.
Further, according to the manufacturing method of the present invention, it is possible to stably manufacture such a molten Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in the processed portion.

Explanatory drawing which shows how to obtain the elongation of the outer surface of a bending part by a bending test Explanatory drawing showing process of Japanese Automotive Standards Organization compound cycle test (JASO-CCT) performed in Example

The hot-dip Al—Zn-based alloy-plated steel sheet of the present invention contains a plating layer containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass% on the surface of the steel sheet, and the balance is Zn and unavoidable impurities.
If the Al content in the plating layer is less than 40 mass%, the effect of improving the corrosion resistance by Al cannot be sufficiently obtained. On the other hand, if the Al content exceeds 70 mass%, Zn is insufficient, so that the sacrificial anticorrosion ability of Zn is reduced.
Further, if the Si content in the plating layer is less than 0.6 mass%, the effect of suppressing the formation of the interfacial alloy layer due to the addition of Si cannot be sufficiently obtained, so that the processability deteriorates. On the other hand, when the Si content exceeds 15 mass%, the corrosion resistance also deteriorates.

Further, the plating layer further comprises Cr, Ni, Co, Mn, Ca, V, Ti, B, Mo, Sn, Zr, which are known as stable elements of corrosion products in molten Al—Zn alloy plating. One or more selected from Sr, Li, Ag and the like can be contained in a content of less than 1 mass% for each element. When the content of each of these elements is less than 1 mass%, further improvement in corrosion resistance can be expected due to the effect of stabilizing the corrosion product without inhibiting the effect of the present invention.
In the plating layer, the base steel plate component taken into the plating layer by the reaction between the plating bath and the base steel plate during the plating process, and the unavoidable impurities in the plating bath (the base steel plate component dissolved in the plating bath, and the base steel plate component dissolved in the plating bath). It contains unavoidable impurities contained in the ingot used when building the plating bath). Since the base steel plate component is incorporated into the plating layer during the plating process, Fe may be contained in the plating layer by several mass%. Examples of the types of unavoidable impurities in the plating bath include Fe, Mn, P, S, C, Nb, Ti, and B as the base steel sheet components. Examples of impurities in the ingot include Fe, Pb, Sb, Cd, As, Ga, and V. The Fe in the plating layer cannot be quantified separately from the one taken in from the base steel plate and the one in the plating bath. The total content of unavoidable impurities in the plating layer is not particularly limited, but the total amount of unavoidable impurities excluding Fe is 1% by mass or less from the viewpoint of maintaining corrosion resistance and uniform solubility of the plating. Is preferable.

The plating layer of the molten Al—Zn-based alloy plated steel sheet of the present invention is required to have an elongation of 15.0% or more as defined below in order to ensure the corrosion resistance of the bent portion. Further, this elongation is more preferably 20.0% or more.
Here, as shown in FIG. 1, the "elongation" of the plated layer means that in the bending test, the test pieces are separated by an inner spacing nt (however, t: the thickness of the plated steel sheet, n: a numerical value that can include a decimal part). When El (%) specified by the following formula is defined as the elongation of the outer surface (outer plating layer) of the bent portion when bending 180 °, the outer surface of the bent portion of the test piece is 10 times loupe. It is the elongation El (%) at the minimum n value at which no crack is observed when observed in.
El (%) = (πt (n + 2) -πt (n + 1)) / (π (n + 1)) * 100
= 1 / (n + 1) * 100
When the elongation of the plating layer is 15.0% or more, high corrosion resistance of the bent portion can be obtained. Further, if the elongation is 20.0% or more, higher corrosion resistance of the bent portion can be expected.

Further, in the plating layer of the molten Al—Zn alloy plated steel sheet of the present invention, the hardness of the Al dendrite portion is preferably 150 Hv or less. When the hardness of the Al dendrite portion is 150 Hv or less, the elongation of the plating layer is improved and the workability is improved. From this viewpoint, the hardness of the Al dendrite portion is more preferably 130 Hv or less.
Al dendrite can be easily identified by observing the cross section of the plating layer. The hardness of only Al dendrite can be measured by micro Vickers measurement with a load of about 5 gf using a fine measuring indenter. In the present invention, the hardness of five randomly selected Al dendrites is measured for the same sample, and the average value thereof is taken as the hardness of the Al dendrite portion.
The thickness of the plating layer is preferably 10 to 30 μm. If the thickness of the plating layer is less than 10 μm, the corrosion resistance may decrease, while if it exceeds 30 μm, the workability may decrease and the post-processing corrosion resistance may decrease. In the present invention, the thickness of five randomly selected plating layers is measured for the same sample, and the average value thereof is taken as the thickness of the plating layer.

Next, a method for producing the hot-dip Al—Zn-based alloy-plated steel sheet of the present invention will be described.
In the method for producing a hot-dip Al—Zn alloy-plated steel sheet of the present invention, the steel sheet is melted in a plating bath containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass%, and the balance is Zn and unavoidable impurities. After plating, the galvanized steel sheet in which the plated layer is solidified is heat-treated under predetermined conditions.
Here, the reason for limiting the composition of the plating bath is the same as the reason for limiting the composition of the plating layer described above.

A technique for improving workability by heat-treating a hot-dip Al—Zn-based alloy-plated steel sheet (for example, Zn-55% Al-plated steel sheet) to soften the plating layer is used in a relatively low temperature range of about 100 to 300 ° C. Heat treatment is performed. As a result of detailed investigation and examination to find out the conditions that can remarkably enhance the effect of this technology, the present inventors have found that the effect of this technology is the cooling rate after plating, more specifically, the primary crystal Al. It was found that the cooling rate in the temperature range of ~ 450 ° C. from the time when the plating started to solidify to the time when it completely solidified was greatly affected.

The possible reasons for this are as follows. The softening of the plating layer by the heat treatment occurs by crystallizing Zn into the Al primary crystals by the heat treatment from the Zn supersaturated Al primary crystals. In this case, if the cooling rate after plating is slow, Zn will crystallize from the Al primary crystals to some extent during cooling, so that the supersaturation degree of the Al primary crystals before the heat treatment will be low, and the effect of the heat treatment applied after that will be effective. It becomes smaller.
Even if Zn is crystallized from the Al primary crystal in the cooling process after plating, the plating layer is slightly softened, but the effect of significantly improving the bending workability cannot be obtained. In order to improve workability, heat treatment in a temperature range of 100 to 300 ° C. is effective, and in order to efficiently crystallize Zn from Al primary crystals in this temperature range, 450 after leaving the plating bath. It is important to increase the degree of supersaturation of Zn in Al primary crystals by quenching in a temperature range up to ° C.

Further, when the cooling rate after plating is slow, the time required for the plating to stay at a high temperature for solidification becomes long, so that the thickness of the interfacial alloy layer formed at the interface between the plating and the base steel sheet becomes thick and the workability deteriorates. ..
Further, when the cooling rate after plating is slow, the density of dendrites becomes coarse, so that the corrosion resistance deteriorates. The reason for this is that when the interdendrite corrodes from the surface of the steel sheet, if the density of the dendrite is coarse, the corrosion path to the steel sheet becomes short, and the corrosion of the steel sheet starts in a short time. ..

Based on the above findings, in the manufacturing method of the present invention, in the cooling process of the plated steel sheet after hot dip galvanizing, the average cooling rate in the temperature range from the exit of the plating bath to 450 ° C. is set to 12 ° C./s or more. To do. Further, this cooling rate is preferably 30 ° C./s or higher, and more preferably 50 ° C./s or higher.
In order to control the average cooling rate after plating to 12 ° C./s or higher, for example, gas cooling by blowing air or nitrogen or mist cooling by blowing mist is performed on the plated steel sheet, and the cooling conditions by these are appropriately controlled.
Since the temperature of the steel sheet immediately after leaving the plating bath is considered to be the same as the temperature of the plating bath, the time from when the steel sheet leaves the plating bath until the temperature of the steel sheet reaches 450 ° C is calculated. The average cooling rate can be obtained by dividing the value of [° C.] by this time.
There is no particular upper limit to the average cooling rate in the temperature range up to 450 ° C after leaving the plating bath, but in general, the practical upper limit is about 100 ° C / s due to restrictions on the cooling capacity of equipment and cooling media. ..

By heat-treating a plated steel sheet that has been cooled under the above cooling conditions and the plating layer has solidified under the following predetermined conditions, the effect of this heat treatment (softening effect of the plating layer) is maximized. It can be demonstrated.
That is, in this heat treatment, after heating the plated steel sheet to a temperature T (° C.) in the temperature range of 100 to 300 ° C., the cooling rate from this temperature T (° C.) to 80 ° C. is defined by the following equation (1). Cool (slowly cool) at an average cooling rate (° C./hr) of C (° C./hr) or lower.
C = (T-80) / 10 ... (1)
Here, this equation (1) is an empirical formula derived as a result of the present inventors examining in detail the influence of the heating of the plating layer and the subsequent cooling conditions on the plating layer based on experiments.

In this heat treatment, when the heating temperature T is less than 100 ° C., the heating time when trying to obtain the effect of the heat treatment becomes so long that it is difficult to carry out in the process, while the heating temperature T exceeds 300 ° C. Since the formation of the interfacial alloy layer between the base steel plate and the plating layer is promoted, the workability is deteriorated and the corrosion resistance of the processed portion is also lowered.
If the average cooling rate from the heating temperature T to 80 ° C. exceeds the cooling rate C (° C./hr) defined by the above equation (1), sufficient workability cannot be ensured.
This average cooling rate can be obtained by determining the time from the heating temperature T to 80 ° C. and dividing the value of [heating temperature T-80 ° C.] by this time.
The method of heat treatment is not particularly limited, and examples thereof include a method in which a plated steel sheet is wound around a coil and then heat-treated offline in a batch-type heating furnace. In this case, the heating temperature T and the average cooling rate from the heating temperature T to 80 ° C. are measured by, for example, a thermocouple attached to the coil.

Under the above manufacturing conditions, a molten Al—Zn-based alloy plated steel sheet having a plating layer elongation of 15.0% or more (preferably 20.0% or more) can be obtained.
Further, since the improvement in the elongation of the plating layer is realized by softening the Al dendrite portion, the above-mentioned production conditions are optimized and the hardness of the Al dendrite portion of the plating layer is made as small as possible (preferably 150 Hv or less). ) Is preferable.

In the manufacturing method of the present invention, the bath temperature of the plating bath is set to 600 ° C. or lower, and the temperature of the steel sheet entering the plating bath is set to [plating] in order to suppress the formation of the interfacial alloy layer between the base steel plate and the plating layer and the spangle size. Bath temperature + 20 ° C or less] is preferable. Here, it is necessary to suppress the spangle size because when the spangle size is large, the Zn concentration in the central part of the Al primary crystal becomes too low, and Zn is not sufficiently crystallized even by the subsequent heat treatment, so that the plating is sufficient. This is because the layer does not soften.
Further, the spangle size can be suppressed by suppressing the Fe concentration in the plating bath to a low level, and the effect of the present invention can be easily obtained for the above-mentioned reason. Therefore, the Fe concentration in the plating bath is set to 0.5 maaa% or less. It is preferable to do so. Since Fe in the plating bath is mainly an impurity derived from the steel sheet, for example, by setting the bath temperature to a low temperature and the entry plate temperature to a low level to reduce the reactivity between the plating bath and the steel sheet, the Fe in the plating bath is lowered. The concentration can be controlled.

The hot-dip Al—Zn-based plated steel sheet of the present invention can be obtained as a chemical conversion-treated steel sheet by forming a chemical conversion-treated film on the surface thereof. This chemical conversion treatment film is obtained by, for example, a chromate treatment or a chromium-free chemical conversion treatment in which a chromate treatment liquid or a chromium-free chemical conversion treatment liquid is applied to a plated steel sheet and dried at 80 to 300 ° C. (steel plate temperature) without washing with water. It is possible to form. These chemical conversion treatment coatings may be single-layered or multi-layered, and in the case of multiple layers, a plurality of chemical conversion treatments may be sequentially performed.

Further, the molten Al—Zn-based plated steel sheet of the present invention can be made into a coated steel sheet by forming a chemical conversion treatment film on the surface thereof and further forming a single-layer or multi-layer coating film on the upper layer thereof. The base chemical conversion coating is as described above. As the multi-layer coating film, for example, an undercoat coating film and a topcoat coating film having different components of the main resin are formed. Examples of the coating film forming method include roll coater coating, curtain flow coating, and spray coating. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying by means such as hot air drying, infrared heating, and induction heating.

In a continuous hot-dip galvanizing facility equipped with a hot-dip Al-Zn-based plating bath, a cold-rolled steel sheet (plate thickness 0.8 mm) was hot-dip plated to produce a hot-dip Al-Zn-based plated steel sheet. In this manufacturing process, the plated steel sheet discharged from the plating bath was cooled by air blowing after adjusting the amount of plating adhesion by gas wiping, and the average cooling rate up to 450 ° C. was controlled by adjusting the air pressure sprayed at this time. The plated steel sheet was wound around a coil, and the coil was heat-treated by an offline batch heating device.

The molten Al—Zn-based galvanized steel sheet thus produced was subjected to a urethane resin-based chemical conversion film containing _{ZrO 2} , SiO _{2, and phosphoric acid as a rust preventive in a laboratory, and subjected to subsequent tests.} The amount of the chemical conversion film adhered was 1 g / m ² .
Further, the sample of the molten Al-Zn-based plated steel sheet to which the above-mentioned chemical conversion film was applied was coated with an epoxy resin-based primer in a laboratory and dried to form a primer layer having a thickness of 5 μm, and then a melamine-cured polyester. A topcoat coating material of the system was applied and dried to form a topcoat coating film having a thickness of 15 μm, and a sample of a coated steel sheet was produced.

Regarding the manufactured molten Al-Zn-based plated steel sheet, the composition of the plating layer (elongation of the plating layer, hardness and thickness of the Al dendrite portion) was measured by the method described above, and the above chemical conversion treatment material and coating material were used. , The corrosion resistance of the bent part was evaluated as follows. The results are shown in Table 1 together with the manufacturing conditions for the molten Al—Zn-based plated steel sheet.
(1) Evaluation of corrosion resistance of the bent part of the chemical conversion treatment material After each sample of the chemical conversion treatment material is subjected to 180 ° bending (3T bending) by sandwiching three plates of the same thickness inside, the bending part A Japanese Automotive Standards Organization compound cycle test (JASO-CCT) was performed on the outer surface. As shown in FIG. 2, JASO-CCT is a test in which salt spraying, drying and wetting under specific conditions are one cycle.
For the bent portion of each sample, the number of cycles until red rust occurred was measured and evaluated according to the following criteria.
⊚: Number of red rust occurrence cycles ≥ 400 cycles ○: 300 cycles ≤ Number of red rust occurrence cycles <400 cycles ×: Number of red rust occurrence cycles <300 cycles

(2) Evaluation of corrosion resistance of the bent part of the coating material After each sample of the coating material is subjected to 180 ° bending (3T bending) by sandwiching three plates of the same thickness inside, the outer surface of the bending part The same Japanese Automotive Standards Organization compound cycle test (JASO-CCT) as in (1) above was carried out.
For the bent portion of each sample, the number of cycles until red rust occurred was measured and evaluated according to the following criteria.
⊚: Number of red rust occurrence cycles ≧ 300 cycles ○: 200 cycles ≤ Number of red rust occurrence cycles <300 cycles ×: Number of red rust occurrence cycles <200 cycles

Claims (11)

The surface of the steel sheet contains a plating layer containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass%, and the balance is Zn and unavoidable impurities.
A hot-dip Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion, characterized in that the elongation of the plating layer defined below is 15.0% or more.
Elongation: In the bending test, when the test piece is bent 180 ° with an inner spacing nt (however, t: plate thickness of plated steel sheet, n: numerical value that can include decimal parts), El (%) specified by the following formula ) Is the elongation of the outer surface of the bent portion, and the elongation El (%) at the minimum n value at which no crack is observed when the outer surface of the bent portion of the test piece is observed with a 10-fold loupe.
El (%) = (πt (n + 2) -πt (n + 1)) / (π (n + 1)) * 100
= 1 / (n + 1) * 100 The molten Al—Zn alloy plated steel sheet having excellent corrosion resistance in the processed portion according to claim 1, wherein the hardness of the Al dendrite portion of the plating layer is 150 Hv or less. The hot-dip Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in the processed portion according to claim 1 or 2, wherein the thickness of the plated layer is 10 to 30 μm. The steel sheet is hot-dip galvanized in a plating bath containing Al: 40 to 70 mass% and Si: 0.6 to 15 mass%, and the balance is Zn and unavoidable impurities, and then the plated steel sheet in which the plating layer is solidified is heat-treated. A method for manufacturing Al-Zn alloy plated steel sheets.
In the cooling process of the plated steel sheet after hot-dip galvanizing, the average cooling rate in the temperature range up to 450 ° C. after leaving the plating bath is set to 12 ° C./s or more.
In the heat treatment, the plated steel sheet is heated to a temperature T (° C.) in a temperature range of 100 to 300 ° C., and then the cooling rate C (1) specified by the following equation (1) is applied from this temperature T (° C.) to 80 ° C. A method for producing a molten Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion, which comprises cooling at an average cooling rate (° C./hr) of ° C./hr) or less.
C = (T-80) / 10 ... (1) The method for producing a molten Al—Zn-based alloy-plated steel sheet having excellent corrosion resistance in a processed portion according to claim 4, wherein the plating bath temperature is 600 ° C. or lower. The method for producing a molten Al—Zn-based alloy plated steel sheet having excellent corrosion resistance in a processed portion according to claim 4 or 5, wherein the temperature of the steel sheet entering the plating bath is + 20 ° C. or less. The method for producing a molten Al—Zn-based alloy plated steel sheet having excellent corrosion resistance in a processed portion according to any one of claims 4 to 6, wherein the Fe concentration in the plating bath is 0.5 mass% or less. A chemical conversion-treated steel sheet having an excellent corrosion resistance in a processed portion, which has a chemical conversion-treated film on the surface of the molten Al—Zn-based alloy-plated steel sheet according to any one of claims 1 to 3. Excellent corrosion resistance of the processed portion, which comprises having a chemical conversion treatment film on the surface of the molten Al—Zn-based alloy plated steel sheet according to any one of claims 1 to 3 and having a single-layer or multi-layer coating film on the upper layer thereof. Painted steel plate. A method for producing a chemical conversion-treated steel sheet having excellent corrosion resistance in a processed portion, which comprises forming a chemical conversion-treated film on the surface of the molten Al—Zn-based alloy-plated steel sheet obtained by the production method according to any one of claims 4 to 7. A chemical conversion-treated film is formed on the surface of the molten Al—Zn-based alloy-plated steel sheet obtained by the production method according to any one of claims 4 to 7, and then a single-layer or multi-layer coating film is formed on the upper layer thereof. A method for manufacturing a coated steel sheet with excellent corrosion resistance in the processed part.

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