JP2003277905A - HOT DIP Al-Zn BASE ALLOY COATED STEEL SHEET EXCELLENT IN SURFACE APPEARANCE AND BENDING WORKABILITY AND ITS PRODUCING METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip Al-Zn base alloy coated steel sheet excellent in the surface appearance and bending workability, and its producing method. <P>SOLUTION: The coated film formed on the steel sheet surface is made of the hot dip Al-Zn base coated film which has the composition containing 30-70 mass% Al, 0.1-1.0 mass% Si, 0.002-0.20 mass% total of one or more elements from among Cr, V and Zr and satisfying Sr with 0.5-4.0 to A=[Sr/(Si+10(Cr+V+Zr))]×100, and has at least a dendrite zone and a interface alloy layer and a structure having ≤1500 pieces/mm<SP>2</SP>interface alloy layer grains of ≥5 μm long diameter existing on the most upper layer part in this interface alloy layer. In this way, the surface appearance and bending workability are drastically improved. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten Al-Zn alloy plated steel sheet which is widely used in the fields of building materials, home appliances, etc. The present invention relates to an Al-Zn alloy plated steel sheet.

[0002]

2. Description of the Related Art Melting containing 25 to 75% by mass of Al on the surface
A hot-dip Al-Zn alloy-plated steel sheet having an Al-Zn-based alloy coating film is more widely used in the field of building materials, home appliances, etc. because it has better corrosion resistance than ordinary hot-dip galvanized steel sheets.
Further, the hot-dip Al-Zn alloy plated steel sheet is excellent in heat reflectivity and spangle appearance, so that it is often used as a roof or wall material for houses, warehouses, public buildings and the like.

Recently, the corrosion resistance evaluated by the time until the occurrence of red rust in the salt spray test is 3 to 5 times that of ordinary hot-dip galvanized steel sheets and 5% Al-Zn plated steel sheets. In the applications that require, hot-dip galvanized steel sheets and 5% Al-Zn-plated steel sheets that have been conventionally used are replaced by these hot-dip Al-Zn alloy-plated steel sheets.

However, since the coating film of the hot dip Al-Zn alloy plated steel sheet is harder than that of the hot dip galvanized steel sheet or the 5% Al-Zn plated steel sheet, cracks are generated particularly in a portion to which severe bending is applied. There is a problem that it is easy to do. Therefore, it is difficult to form the hot-dip Al-Zn alloy-plated steel sheet into a complex shape with high commercial value and rich in designability, and the hot-dip Al-Zn alloy-plated steel sheet has spangle size and surface gloss Since unevenness is likely to occur, there was a problem when it was used unpainted.

To address such problems, for example, Japanese Patent Publication No. 61
JP-A-28748 discloses that after plating, a steel sheet is heated to a temperature of 93 to 427 ° C., and at this temperature, logt = 7102.4 / T-11.04 (where, t: time (s), T: heating temperature ( K)) is held for a minimum time t and then slowly cooled to room temperature, that is, a so-called overaging treatment is performed to soften the plating and improve workability. However, the method described in Japanese Examined Patent Publication No. 61-28748 has a problem that it takes a long time to perform overaging and in-line production is impossible.

In addition, "Aluzinc Plus: A new continuous
hot-dip 55% Al-Zn protective coating ”(Roc.of G
alvatech 92, p412 (AMSTERDAM))
It is mentioned that the workability may be improved by the combined addition of Sr, V and Cr. However, this report does not clarify the appropriate composition range and the degree of workability improving effect.

Richard Ley also wrote "Theorized Effec
t of Strontium Addition on Al-Si Alloys "(I
nter ZAC 98 Conference, Los Angels, CA USA Septemb
er1998), it has been shown that the workability of 55% Al-Zn plated steel sheet may be improved by adding Sr to the coating film. However, the workability was not improved enough to withstand severe processing.

Further, in JP-A-9-256132, a method of refining the plating film by adding a small amount of Mo, W, Nb, Ta to the plating film or a method of adding a small amount of Cr or V to the plating film Describes a method of suppressing the occurrence of cracks in the alloy layer. However, the method of refining the structure by forming a second phase by adding a small amount of Mo, W, Nb, and Ta has a problem in that appearance defects such as fluctuations in spangle size due to plating conditions are likely to occur. Further, since these metals are difficult to dissolve in the plating bath and have a low addition yield, there is a problem that the amount of dross increases and management of the plating bath requires a lot of labor.

Further, in Japanese Patent Laid-Open No. 9-209109, Al:
40-70%, Si: 0.5-1.5%, Zr, Hf,
Hot-dip Zn-Al alloy-plated steel sheet having a Zn-Al alloy-plated coating with a spangle particle size of 0.7 mm or less containing 0.01 to 0.4% each of V and 0.01 to 0.4% of Ti and / or 0.40% of Ti. Is disclosed. The technique disclosed in Japanese Patent Application Laid-Open No. 9-209109 aims to make the spangle size fine by adding Zr, Hf, and V to obtain a hot-dip Zn-Al alloy-plated steel sheet having excellent designability. According to the study by the present inventors, the plated steel sheet manufactured by the technique described in Japanese Patent Application Laid-Open No. 9-209109 has improved designability due to the finer spangle size, but the unevenness of the interface alloy layer increases. Then
Further, there is a problem that the workability tends to decrease rather than increase in the interdendrite region.

[0010]

Generally, the bending workability of a hot-dip Al-Zn alloy-plated steel sheet containing 30 mass% or more of Al in the coating film is 5% when the hot-dip galvanized steel sheet and the Al concentration are low.
Inferior to Al-Zn plated steel sheets, etc., cracks are generated from the Si precipitates precipitated in the Al-Fe-Si-based alloy layer and the plating film generated on the interface side with the steel plate base iron, It is thought that this crack is likely to propagate Si precipitates and an Al-Zn eutectic structure that crystallizes in the thickness direction of the plating film between primary crystal Al (dendritic regions) (interdendrite) that is the main phase. ing. From the above, in the above-mentioned conventional technique, (1) softens the primary crystal Al (dendritic region), relaxes stress concentration on the interdendrite, and makes cracks difficult to propagate. (2) Trace amount of alloying element There has been proposed a method of improving bending workability, which can be roughly classified into two systems, that is, adding to a plating film and refining the structure of the plating film.

However, of the above two methods of improving bending workability, (1) requires equipment for batch processing, resulting in insufficient productivity, and (2), molten Al-Zn alloy. At present, the bending workability of plated steel sheets has not been improved to a sufficiently high level. The present invention advantageously solves the above-mentioned problems of the prior art, can be manufactured in-line with high productivity without performing complicated bath management, and has excellent surface appearance and bending workability Al-Zn alloy plating. It is an object of the present invention to provide a steel sheet and a method for manufacturing the steel sheet.

[0012]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present inventors investigated the influence of the plating film structure on the bending workability of a molten Al—Zn alloy plated steel sheet. As a result, it was found that the coarse convex interface alloy layer particles having a major axis of 5 μm or more present in the uppermost layer of the interface alloy layer deteriorate the bending workability. In order to provide good bending workability, the inventors of the present invention have 1500 particles / mm of coarse convex interface alloy layer particles having a major axis of 5 μm or more.
^We have found that it is necessary to reduce it to ² or less. In addition, the inventors of the present invention have found that an appropriate amount of Cr, V, Zr is contained in the plating film.
Is added together with Sr to cool down to 540 ° C after plating.
Cr, V, Zr by cooling at a cooling rate of 20 ℃ / S or more
Was unevenly distributed in the interface alloy layer, the growth of the convex interface alloy layer particles was prevented or suppressed, and it was found that the number of coarse convex interface alloy layer particles of 5 μm or more could be 1500 particles / mm ² or less.

Further, it has been found that the surface appearance of the plated steel sheet is remarkably improved by containing an appropriate amount of Sr in the plating film. First, an explanation will be given of the results of an experiment which is the basis of the present invention conducted by the present inventors. Various hot-dip Al-Zn alloy-plated steel sheets manufactured by varying the plating bath composition and plating conditions, in accordance with JIS Z 2248, 2t
A bending test was carried out to measure the crack occupying area ratio of 2t bending. The crack occupying area ratio of 2t bending is obtained by taking a 50 times backscattered electron image of the crack in the 2t bent portion using a scanning electron microscope, and image-processing a range of width 1mm and length 5mm sandwiching the bending line. The area occupied by cracks was calculated and used as the area occupied by 2t bending cracks. Further, for the same hot-dip Al-Zn alloy plated steel sheet, the upper layer of the plating film was dissolved with a 10% iodine-ethanol solution to expose the interface alloy layer,
The surface texture of the interfacial alloy layer was examined using a scanning electron microscope.
15 fields of view were imaged at a magnification of 00 times. The size of interfacial alloy layer particles present in the uppermost layer and the number per unit area (frequency of existence) were determined from the obtained micrographs using an image analyzer.

FIG. 1 shows the relationship between the obtained area ratio of 2t bending cracks and the frequency of existence of interfacial alloy layer particles having a major axis of 5 μm or more. From FIG. 1, it is understood that when the existence frequency of the coarse interface alloy layer particles of 5 μm or more is 1500 particles / mm ² or less, the 2t bending crack occupying area ratio is 3% or less, and bending workability is remarkably improved. In order to reduce the existence frequency of coarse interfacial alloy layer particles of 5 μm or more to 1500 particles / mm ² or less, the plating film should contain one or more of Cr, V, and Zr in a total amount of 0.002 mass% or more. Can be achieved. Fig. 2 (a) shows an example of the interfacial alloy layer uppermost layer structure observed with a scanning electron microscope (SEM) for a molten Al-Zn alloy plated steel sheet containing Cr: 0.033 mass% in the plating film. . In the plated steel sheet whose structure is shown in FIG. 2 (a), the existence frequency of coarse interface alloy layer particles of 5 μm or more is as small as 1305 particles / mm ^2, and the 2t bending crack occupying area ratio is 1.5%. In Fig. 2 (b),
Regarding the molten Al-Zn alloy plated steel sheet containing Cr: less than 0.002 mass%, an example of the interface alloy layer uppermost layer structure observed by using a scanning electron microscope (SEM) is shown as a comparison. In the plated steel sheet whose structure is shown in FIG. 2 (b), the existence frequency of coarse interface alloy layer particles of 5 μm or more is 8524 particles / mm ² ,
The existence frequency of coarse interface alloy layer particles is high, and the bending workability is deteriorated with the 2t bending crack occupying area ratio being 15.2%.

The present invention has been completed on the basis of the above-mentioned findings by further studies. That is, the gist of the present invention is as follows. (1) A hot-dip Al-Zn alloy-plated steel sheet having a hot-dip Al-Zn alloy-plated film formed on the surface thereof, wherein
Zn-based alloy plating film contains 30-70 mass% Al and 0.1-Si
1.0 mass%, one or more of Cr, V and Zr in total of 0.002 to 0.20 mass% and Sr as A = [Sr / {Si + 10
(Cr + V + Zr)}] × 100 (where Sr, Si, Cr, V,
Zr: the content (mass%) of each element is contained so as to satisfy 0.5 to 4.0, and has at least a dendrite region and an interface alloy layer, and is present in the uppermost layer portion of the interface alloy layer, 1500 particles of interfacial alloy layer with major axis of 5 μm or more
A hot-dip Al-Zn alloy plated steel sheet excellent in surface appearance and bending workability, characterized by having a structure of mm ² or less. (2) In a method for producing a molten Al-Zn alloy-plated steel sheet, which comprises immersing the steel sheet in a molten Al-Zn-based alloy plating bath, then pulling it up and cooling it to form a molten Al-Zn-based alloy plating film on the surface of the steel sheet. The molten Al-Zn-based alloy plating bath, the molten Al-Zn-based alloy plating film is Al 30 ~ 70mass%, Si 0.
1 to 1.0 mass%, one or more of Cr, V, and Zr in total of 0.002 to 0.20 mass%, and Sr is A = [Sr / {Si +
10 (Cr + V + Zr)}] × 100 (where Sr, Si, Cr,
V, Zr: content (mass%) of each element is adjusted to a composition so as to satisfy 0.5 to 4.0, and the cooling rate is 20 ° C. from the plating bath temperature to 540 ° C.
A method for producing a hot-dip Al-Zn alloy-plated steel sheet having excellent surface appearance and bending workability, which is characterized by cooling at s or more. (3) In (2), 250-
A method for producing a hot-dip Al-Zn alloy-plated steel sheet, characterized by performing a residence treatment in a temperature range of 170 ° C.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The hot-dip Al-Zn alloy plated steel sheet of the present invention is a plated steel sheet obtained by forming a hot-dip Al-Zn alloy-plated film on the surface of a steel sheet (plating original plate). Molten Al-Zn
The molten Al—Zn alloy plating film formed on the surface of the system alloy plated steel sheet has a structure including at least a dendrite region and an interface alloy layer on the interface side with the steel sheet base iron. This dendrite region consists of an α phase in which Zn is solid-solved in Al,
It is the main constituent phase of the plating film. The inter-dendrite region existing between the dendrite regions is Al-Zn.
It is composed of eutectic, and depending on the Si content, between the Al-Zn eutectic
In some cases, Si precipitates (Si crystals) may precipitate. Further, the interface alloy layer formed on the interface side with the steel plate base iron of the plating film, Fe-Si-Al-based intermetallic compound containing a small amount of Zn,
FeAl ₄ Si _0.2 (τ _5c ) and FeAl ₃ (θ).

The hot-dip Al-Zn alloy-plated steel sheet formed on the surface of the hot-dip Al-Zn alloy-plated steel sheet of the present invention has an average Al content of 30 to 70 mass% and a Si content of 0.1 to 1.0 ma.
One or more of ss%, Cr, V and Zr in total
0.002 to 0.20 mass%, Sr is A = [Sr / {Si + 10 (Cr + V
+ Zr)}] × 100 (where Sr, Si, Cr, V, and Zr: the content (mass%) of each element) is 0.5 to 4.0, and the balance is Zn. It is a plating film of.

If the Al content in the plating film is less than 30 mass%, the corrosion resistance is insufficient. On the other hand, if it exceeds 70 mass%, the corrosion resistance of the end face is deteriorated, the plating film is hardened, and the bending workability of the plated steel sheet is significantly deteriorated. Incidentally, it is more preferably 40 to 60 mass%. Si in the plating film is 0.1
If it is less than mass%, the interfacial alloy layer grows thicker than 20% of the plating film thickness, and the bending workability of the plated steel sheet deteriorates. On the other hand, if it exceeds 1.0 mass%, even if Sr is contained in a complex manner, Si precipitates are coarse and a large amount are deposited in the plating film, and bending workability is significantly reduced. The content is preferably 0.4 to 1.0 mass%.

Cr, V, and Zr all make the interface alloy layer particles in the uppermost layer of the interface alloy layer finer and flatten the interface between the interface alloy layer and the upper plating layer to remarkably improve bending workability. Has the effect of causing. Cr (0.045m
Secondary Ion Mass Spectroscopy (hereinafter referred to as SIMS) for hot dip Al-Zn alloy plated steel sheets containing as% (containing as%) or V (containing 0.031mass%).
High-sensitivity element mapping analysis was performed. The result is shown in FIG. FIG. 3A shows the results of high-sensitivity element mapping analysis of Si and Cr in the cross section of the plating film, and FIG.

From FIG. 3, Cr (FIG. 3 (a)) or V (FIG. 3)
It can be seen that (b)) is unevenly distributed in the interface alloy layer in the plating film. The present inventors have confirmed that Zr has the same action as Cr and V. From this,
It is considered that Cr, V, and Zr are deeply involved in the alloying reaction of the plating film and increase the frequency of nucleation of the alloying reaction to make the interface alloy layer particles in the uppermost layer of the interface alloy layer finer. The generation position of the solidification nuclei of the primary crystal Al phase is not always clear, but it is highly likely that it is at the interface between the plating coating upper layer having a high interfacial energy and the interfacial alloy layer. Above all, it is highly possible that the interface is with the convex interface alloy layer particles existing in the uppermost layer of the interface alloy layer. By including any one of Cr, V, and Zr in the plating film, the particles of the convex interface alloy layer are miniaturized, and therefore the frequency of nucleation of primary crystal Al is increased, so that the plating becomes a crack propagation path. The frequency of interdendrites that penetrate the layer is also thought to decrease.

If the total amount of one or more of Cr, V, and Zr in the plating film is less than 0.002 mass%, the above effect is not observed. On the other hand, Cr, V,
If the total content of one or more Zr exceeds 0.2 mass%, it is necessary to add a large amount of these elements to the plating bath, which causes a large amount of dross, and It causes surface defects such as dross adhesion and non-plating. Therefore, the total amount of one or more of Cr, V, and Zr in the plating film is limited to the range of 0.002 to 0.20 mass%. The content is preferably 0.01 to 0.10 mass%.

Further, in the present invention, Sr
= [Sr / {Si + 10 (Cr + V + Zr)}] × 100 is 0.5 to 4.
It is contained so as to satisfy 0. Sr in the plating film is an element that, in addition to the effect of suppressing the coarse growth of Si precipitates in the plating film, prevents the occurrence of surface defects by concentrating on the surface of the plating film and improving the surface appearance. is there. In addition, Sr also has the effect of reducing spangle unevenness. Further, in the present invention, one or more of Cr, V, and Zr are indispensably contained, and since the acicular coarsening of the Si precipitate becomes remarkable due to the inclusion of Cr, V, and Zr, Si precipitation is remarkable. The inclusion of Sr that suppresses the coarse growth of the material becomes important.

With respect to the hot dip Al-Zn alloy plated steel sheet containing Sr (0.012 mass% content) in the plating film, high sensitive element mapping analysis of the cross section of the plating film was performed using SIMS. The result is shown in FIG. From FIG. 4, it can be seen that Sr is strongly concentrated in the Si precipitates, the interface between the interface alloy layer and the plating coating upper layer, and the surface of the plating coating. This is Sr
It is considered that this is because the interfacial energy of is lower than that of Al and Si, and it is presumed that the presence of this surface concentrated layer suppresses the surface defects.

Amount of Sr in plating film is small and A is 0.5
If it is less than Sr, Sr is consumed for surface concentration of Si, and the above-mentioned effect cannot be expected. On the other hand, if Sr is excessive in the plating film and A exceeds 4.0, coarse precipitates of Sr / Si system will be distributed in the plating film, and the bending workability of the plated steel sheet will be deteriorated. Further, in the present invention, in order to improve the corrosion resistance within the range in which the workability is not significantly deteriorated, the plating film may contain 2 mass% or less of Mg.

The plated coating of the hot dip Al-Zn alloy plated steel sheet of the present invention has at least a dendrite region and an interfacial alloy layer, and the major axis of the interfacial alloy layer is 5 μm or more. Interface alloy layer particles of 1500 / mm ²
It has the following organization. The uppermost layer of the interface alloy layer in the plating film formed on the molten Al-Zn alloy plated steel sheet is
As shown in FIG. 2, the interfacial alloy layer particles are dispersed. The interfacial alloy layer particles consist of Fe-Al-Si intermetallic compound, FeAl ₄ S.
i _0.2 (τ _5c ), of which the convex and coarse interfacial alloy layer particles are highly likely to be the solidification nuclei of the primary Al phase. By reducing the presence frequency of this convex and coarse interface alloy layer particles, the nucleation frequency of the primary crystal Al phase is reduced, and it is considered that the presence frequency of interdendrite, which is a crack propagation path, is also reduced. This improves bending workability. 5
When the existence frequency of the interfacial alloy layer particles having a size of μm or more exceeds 1500 particles / mm ² , the bending workability deteriorates as shown in FIG. Therefore, in the present invention, the existence frequency of the interfacial alloy layer particles having a major axis of 5 μm or more existing in the uppermost layer of the interfacial alloy layer is limited to 1500 particles / mm ² or less.

Next, a method for manufacturing the hot-dip Al—Zn alloy plated steel sheet of the present invention will be described. In the present invention, the steel plate to be the plating original plate is a steel plate manufactured by a usual method, for example,
Hot rolled sheet such as low carbon aluminum killed steel sheet and ultra low carbon steel sheet,
The cold-rolled sheet can be appropriately selected and used according to the application. The steel plate, which is a plating original plate, is preferably subjected to a heat treatment that also serves as recrystallization, such as electrolytic degreasing and pickling, and then immersed in a plating bath to form a plating film on the surface.

In the present invention, the plating bath in which the steel sheet is immersed is
Use a molten Al-Zn alloy plating bath. In the molten Al-Zn alloy plating bath, the formed molten Al-Zn alloy plating film is Al.
30 to 70 mass%, Si 0.1 to 1.0 mass%, the total amount of one or more of Cr, V, and Zr is 0.002 to 0.20 mass
%, Sr, A = [Sr / {Si + 10 (Cr + V + Zr)}] × 10
The composition of the plating bath is adjusted so that 0 satisfies 0.5 to 4.0. There is no problem even if the plating bath contains Mg.

In the present invention, the temperature of the plating bath (bath temperature) and the plate temperature when the steel plate enters the plating bath (penetration plate temperature) are
The temperature is preferably 600 ° C or lower. Molten Al-Zn described above
After immersing the steel plate in the system alloy plating bath, the steel plate is molten Al
It is pulled out from the -Zn alloy plating bath and cooled, and a molten Al-Zn alloy plating film is formed on the surface of the steel sheet. For cooling, the cooling rate from the plating bath temperature to 540 ° C is 20 ° C / s
That is all. If the cooling rate up to 540 ° C is less than 20 ° C / s, the interfacial alloy layer particles grow coarsely and the bending workability deteriorates. To make the cooling rate up to 540 ℃ 20 ℃ / s or more,
There are methods such as gas cooling and mist cooling, and any of them can be suitably used. It is not necessary to particularly limit the cooling conditions at 540 ° C. or lower, and there is no problem with ordinary cooling methods such as air cooling. The plated steel sheet is wound into a coil after cooling.

It should be noted that, after the plating film is formed, the residence treatment may be performed during the cooling process before winding into a coil.
The residence treatment is performed in the temperature range of 170 to 250 ° C during cooling down to 60
s or less, or 10 ~ in the temperature range of 170 ℃ ~ 250 ℃
This is a process of holding (holding) for a short time of 60 seconds. This staying treatment further improves bending workability. The holding for a short time may be performed by, for example, installing an oven immediately before winding the coil and heating it in a temperature range of 170 to 250 ° C. for 60 seconds or less.

As the production equipment of the above-mentioned molten Al—Zn alloy plated steel sheet of the present invention, a continuous annealing line, a plating bath, a cooling equipment and a coil winding equipment which have been conventionally used may be used. The molten Al-Zn alloy plated steel sheet having the plating layer thus produced is subjected to a chemical conversion treatment to form a chemical conversion treatment layer, or a primer treatment is further applied thereon to form a primer layer, on which organic coating is applied. By applying a treatment to form an organic coating layer, a coated hot-dip Al-Zn alloy plated steel sheet can be obtained. As these means, those used for producing ordinary coated steel plates and PCM may be used.

That is, as the chemical conversion treatment, usual chromate treatment, phosphate treatment, etc. can be used, and the primer layer is made of epoxy resin, polyester, modified polyester, modified epoxy resin or the like, if necessary, with a rust preventive pigment (for example, It can be obtained by applying a mixture (primer) of zinc chromate, strontium chromate, barium chromate, etc.) and a curing agent (melamine, isocyanate resin, etc.). As the organic coating layer, a generally known top coat paint such as polyester paint, fluororesin paint, acrylic resin paint, vinyl chloride vinyl paint, silicone paint is applied in an appropriate amount.
It can be obtained by baking. It is also possible to add a color pigment appropriately to the primer, or to add various color pigments or extender pigments to the topcoat paint to obtain a coating film having high processability. In addition, the coating thickness and coating method (spray coating, roll coating, brush coating, etc.) of these paints are enough to be those used in ordinary PCM.

The baking (drying) conditions in the above chemical conversion treatment, primer treatment, and organic coating treatment preferably satisfy the conditions (130 to 260 ° C., 30 seconds or more) necessary for overaging treatment. After the skin pass rolling, the coating process can be continuously performed. Further, the Al—Zn alloy plated steel sheet having the plated layer thus produced may be lubricated to form a lubricated coating layer. Known resins, polyester resins, acrylic resins, acrylic-styrene resins, urethane resins and the like can be used as the resin constituting the lubricating coating layer, and as the lubricant, a polyolefin resin, a fluororesin is used. In addition to silicone resins, fatty acids such as stearic acid and olefinic acid and their esters can be used. Chromic acid-based rust preventives (zinc chromate, strontium chromate, barium chromate, etc.), phosphate-based rust preventives, molybdic acid-based rust preventives and borate-based rust preventives A non-chromic acid type rust preventive such as a rust preventive can also be blended.

These resins, lubricants and rust preventive pigments are blended according to the purpose and applied to a hot dip Al-Zn alloy plated steel sheet. The mixing ratio of the lubricant to the resin is preferably 0.5 to 5% in terms of mass ratio from the viewpoint of imparting proper lubricity.
Further, the mixing ratio of the rust preventive pigment is 0.2 with respect to the resin in mass ratio.
It is good to be up to 5%. Furthermore, the thickness of the lubricating coating layer is 0.5
It is recommended to set it to ~ 10 μm. This is because if the thickness of the lubricating coating layer is too thin, the corrosion resistance is poor, while if it is too thick, the workability of the lubricating coating layer itself deteriorates.

[0034]

[Example] Mass%, C: 0.043%, Si: 0.01%, Mn:
Steel sheet containing 0.14%, P: 0.015%, S: 0.006%, Al: 0.022% (low carbon Al-killed steel sheet, sheet thickness: 0.5 mm)
After continuous heat treatment (maximum temperature reached: 780 ° C) that doubles as recrystallization annealing using continuous annealing equipment,
It was immersed for 1 second in a molten Al-Zn alloy plating bath having a bath temperature of 600 ° C or less and a bath temperature of 600 ° C or less to form a molten Al-Zn alloy plating film on the surface of the steel sheet. Then, it was pulled up from the plating bath and adjusted by gas wiping so that the amount of adhesion on both sides was 200 g / m ² . Then, with the cooling equipment, the plate temperature is 540
It cooled at the average cooling rate shown in Table 1 until it became (degreeC). The cooling rate was adjusted by measuring the plate temperature with a radiation thermometer attached to the cooling device.

It should be noted that some of the steel plates had a temperature of 540 ° C during cooling.
To 230 ° C. were cooled at an average cooling rate of 30 ° C./s, and then up to 180 ° C. were slowly cooled at an average cooling rate of about 5 ° C./s. Steel plates not subjected to retention treatment are from 540 ℃
It was cooled to 50 ° C at 30 ° C / s. After cooling, it was smoothed with a tension leveler and wound into a coil to obtain a molten Al-Zn alloy plated steel sheet.

For adjusting the composition of the molten Al-Zn alloy plating bath, 99.99% Zn, 99.99% Al, 15% Si-Al mother alloy, 10% Sr-Al alloy, 10% Cr-Al alloy, 2% % V-Zn alloy and 5% Zr-Zn alloy were appropriately used, and the bath composition was adjusted so that the plating film composition (average) shown in Table 1 was obtained. With respect to the obtained molten Al—Zn alloy plated steel sheet, the composition of the plating film, the size and number of particles of the interface alloy layer present in the uppermost layer, the surface property, and the bending workability were investigated.

The composition of the plating film was measured by dissolving the plating film and using ICP emission spectroscopy. Further, the size and number of the interfacial alloy layer particles present in the interfacial alloy layer uppermost layer portion in the plating film, for the obtained plated steel sheet,
Samples were taken from each of 10 locations, and the upper layer of the plating film was dissolved with a 10% iodine-ethanol solution to expose the interface alloy layer. Then, the surface structure of the interface alloy layer using a scanning electron microscope, each sample each 15 field of view at a magnification of 2000 times, of the interface alloy layer particles in each field using an image analyzer from the resulting structure photograph The size and the number were obtained, and the average value of each visual field was used as the value of each steel plate. From these measured values, the existence frequency of the interfacial alloy layer particles having a major axis of 5 μm or more among the interfacial alloy layer particles was calculated.

The surface appearance of the obtained plated steel sheet was visually observed to evaluate the spangle size unevenness.
◎ when there is no unevenness in spangle size, ○ when hardly observed, △ when slightly observed,
When clearly recognized, it was evaluated as x. In addition, JIS B 0
The surface roughness was measured according to the regulations of 601 and Ra (μm
) And Rv (μm) were determined. The measurement area is 15 × 15m
m ²

Regarding the obtained plated steel sheet, JIS
A 2t bending test was carried out in accordance with the regulations of Z 2248, the crack occupying area ratio of the 2t bending was measured, and bending workability was evaluated. A crack of the 2t bent part was photographed with a scanning electron microscope at a magnification of 50 times using a scanning electron microscope, and the width between the bending lines was 1mm.
The area of 5 mm in length was image-processed to calculate the 2t bending crack occupied area ratio.

The results obtained are shown in Table 1.

[0041]

[Table 1]

From Table 1, in all the examples of the present invention, Ra is 0.
90 μm or less, R _V is 9.00 μm or less, it has an excellent surface appearance without defects such as spangle size unevenness and dross adhesion on the plating surface, and the bending workability is 2% and the bending crack occupation area ratio is 3% or less. You can see that

[0043]

According to the present invention, a hot-dip Al-Zn alloy plated steel sheet having excellent surface appearance and bending workability can be easily manufactured without the need for complicated bath management, and can be produced in-line with high productivity and at low cost. It can be manufactured and has a remarkable effect in industry. The present invention also has an effect that production can be performed using conventional continuous plating equipment without requiring special equipment.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the occupied area ratio of 2t bending cracks and the frequency of existence of interfacial alloy layer particles having a major axis of 5 μm or more.

FIG. 2 is a scanning electron microscope structure photograph showing the structure of the interface alloy layer surface of the plating film. (A) is the case where Cr is contained in the plating film, and (b) is the case where Cr is not contained in the plating film.

FIG. 3 is an element distribution diagram showing an element mapping analysis result of a plating film cross section by SIMS. (A) is Si and Cr,
(B) shows the distribution of Si and V.

FIG. 4 is an element distribution diagram showing the result of element mapping analysis of the cross section of the plating film by SIMS, showing the distribution of Si and Sr.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 2/40 C23C 2/40 (72) Inventor Norihiko Nakamura 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made by Kawasaki Iron Co., Ltd. Technical Research Institute (72) Inventor Yoichi Toyama 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Product Research Institute Co., Ltd. F-term (reference) 4K027 AA05 AA22 AB02 AB14 AB28 AB32 AB44 AB48 AC72 AC73 AE02 AE03 AE12 AE27

Claims (3)

[Claims] 1. A hot-dip Al-Zn alloy-plated steel sheet having a hot-dip Al-Zn alloy-plated film formed on the surface thereof, wherein the hot-dip Al-Zn alloy-plated film has an Al content of 30 to 70 mass%, Si
0.1 to 1.0 mass%, one or two of Cr, V, and Zr
0.002 to 0.20mass% in total for seeds and above, Sr below 0.5 for A
To 4.0, the composition has at least a dendrite region, and an interfacial alloy layer, and 1500 interfacial alloy layer particles having a major axis of 5 μm or more are present in the uppermost layer of the interfacial alloy layer. A hot-dip Al-Zn alloy plated steel sheet excellent in surface appearance and bending workability, which has a structure of ² or less. Note A = [Sr / {Si + 10 (Cr + V + Zr)}] × 100 where Sr, Si, Cr, V: content of each element (mass%) 2. A method for producing a molten Al—Zn alloy plated steel sheet, which comprises immersing the steel sheet in a molten Al—Zn alloy plating bath, then pulling it up and cooling it to form a molten Al—Zn alloy alloy coating on the surface of the steel sheet. In the molten Al-Zn-based alloy plating bath, the molten Al-Zn-based alloy plating film is Al 30 ~ 70mass.
%, Si is 0.1 to 1.0 mass%, one or more of Cr, V and Zr is 0.002 to 0.20 mass% in total, and Sr is the following A
Is adjusted so as to satisfy a composition of 0.5 to 4.0, and the cooling is such that the cooling rate from the plating bath temperature to 540 ° C is 20 ° C / s or more. And a method for producing a hot dip Al-Zn alloy plated steel sheet having excellent bending workability. Note A = [Sr / {Si + 10 (Cr + V + Zr)}] × 100 where Sr, Si, Cr, V: content of each element (mass%) 3. The method for producing a hot-dip Al—Zn alloy plated steel sheet according to claim 2, characterized in that, following the cooling, a residence treatment is performed in a temperature range of 250 to 170 ° C.