JP2002348649A - HOT-DIP Al-Zn ALLOY PLATED STEEL SHEET EXCELLENT IN WORKABILITY, AND MANUFACTURING METHOD THEREFOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip Al-Zn plated steel sheet more excellent in workability than conventional ones, and a manufacturing method therefor. SOLUTION: This steel sheet has an Al-Zn alloy plated layer comprising dendrite parts, inter-dendrite parts between the dendrite parts, and an interfacial alloy layer between those and the steel surface, on the surface of the steel sheet. The Al-Zn alloy plated layer includes, by mass ratio, Al of 25-75%, Si of more than 1% but 5% or less, and Sr in the range of 0.2-2% of the Si content. The Al-Zn alloy plated layer may include one or more of Cr, V, and Zr of 0.01%-2.0% in total. In addition, the interfacial alloy layer has the alloy grains with long diameter of 5 μm or more, preferably of 1,500 pieces/mm<2> or less on the top layer region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Al-Zn alloy coated steel sheet and a method for producing the same, and particularly to 25-75% (mass ratio, hereinafter referred to as "particularly") of Al widely used in the fields of building materials and home appliances. The same unless otherwise specified) and a method for producing the same.

[0002]

2. Description of the Related Art 55% Al-Zn alloy plated steel sheet
A hot-dip Al-Zn alloy-coated steel sheet containing 25 to 75% of Al has excellent corrosion resistance as compared with a normal hot-dip galvanized steel sheet, and is therefore widely used in fields such as building materials and home appliances. However, since this hot-dip Al-Zn alloy-plated steel sheet has a hard plating layer, when forming into building materials and parts for home appliances,
There is a problem that a classic is likely to be generated in a severe processing condition.

[0003] To cope with such a problem, for example, Japanese Patent Publication No. 28748/1986 discloses that a steel sheet is plated after plating.
gt = 7102.4 / T-11.04 (where, t: time (seconds), T: heating temperature (K)) has been proposed to perform overaging treatment under conditions represented by: In addition,
Japanese Patent Application Laid-Open No. 11-343559 discloses a proposal to improve crack resistance by making the agglomerated portion of Zn exist in the interdendrite portion constituting the plating layer in an area ratio of 1.0 to 30% in the cross section of the plating film. It has been done.

Further, the Inter ZAC 98 Conference (Los An
gels, CA USA, September 1998).
Is “Theorized Effects of Strontium Additions On
Al-Si Alloys ”report, in which the addition of Sr was 55
It mentions the possibility of improving the workability of the% Al-Zn plated steel sheet.

[0005]

[Problems to be solved by the invention]
The means described in Japanese Patent Application Laid-Open No. 61-28748 requires a long time, for example, at least 2.5 hours at 200 ° C. due to overaging treatment, and has a problem that productivity is extremely low. In addition, in the proposal described in Japanese Patent Application Laid-Open No. 11-343559, no study was made on the effect of Si on workability,
Also, no consideration is given to improvement in workability due to overaging, and therefore workability is not sufficient.

On the other hand, means for adding Sr to the plating bath is as follows:
Although the Si crystals precipitated in the interdendrite part are spherical and fine, thereby improving the workability of the plated steel sheet, according to the experiments of the present inventors, sufficient workability cannot be ensured yet There are cases. In addition, each of the conventional proposals described above individually deals with the cause of workability deterioration caused by the dendrite portion or the interdendrite portion, and does not pay attention to their interdependence. Therefore, the hot-dip Al-Zn alloy-plated steel sheet does not yet have sufficient workability.

An object of the present invention is to solve the above-mentioned problems relating to a hot-dip Al-Zn alloy-coated steel sheet, and to provide a hot-dip Al-Zn alloy having excellent corrosion resistance and excellent workability as compared with the prior art.
An object of the present invention is to propose an alloy-plated steel sheet and a method for manufacturing the same.

[0008]

Means for Solving the Problems The present inventors have proposed molten Al-
A detailed study was conducted on the effect of Sr addition on the workability of Zn alloy-plated steel sheets.When the amount of Sr added was at a certain ratio with respect to the content of Si, the spheroidization of Si crystals in the interdendrite portion was ensured. And that the precipitation of Sr in the interdendrite portion is promoted, whereby the Sr concentration in the dendrite portion is reduced, and the hardness can be easily reduced by the overaging treatment.

Further, the present inventors have found that 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 in the plating layer deteriorate the bending workability. . The present inventors have found that in order to provide good bending workability, it is necessary to reduce the number of coarse convex interface alloy layer particles having a long diameter of 5 μm or more to 1500 particles / mm ² or less. I learned. In addition, the present inventors include Cr, V, and Zr in the plating layer by adding an appropriate amount of Cr, V, and Zr together with Sr, so that Cr, V, and Zr are unevenly distributed in the interface alloy layer. It has been found that it is possible to prevent or suppress the number of coarse convex interface alloy layer particles of 5 μm or more to be 1500 particles / mm ² or less.

The present invention has been completed based on the above findings and further studies. That is, the present invention has, on the surface of the steel sheet, a dendrite portion, an interdendritic portion existing between the dendrite portions, and an Al-Zn alloy plating layer composed of an interface alloy layer existing at an interface between the dendrite portion and the steel plate ground iron. , The Al-Zn alloy plating layer contains Al by mass ratio.
25-75%, Si is more than 1% and 5% or less, and Sr is 0.2% of Si content.
A hot-dip Al-Zn alloy-plated steel sheet excellent in workability, characterized in that it is contained in the range of 2% to 2%;
In the present invention, the Sr concentration in the interdendrite portion is preferably 20 to 150 times the average Sr concentration of the Al—Zn alloy plating layer.

Further, in the present invention, the Al—Zn alloy plating layer has a mass ratio of 25 to 75% Al, more than 1% Si and 5% or less.
Contains Sr in the range of 0.2 to 2% of the Si content, and further contains C
One, two or more of r, V and Zr in a total of 0.01 to 2.
0%, and in the present invention, Al-Z
In the n-alloy plating layer, the number of interfacial alloy layer particles having a major axis of 5 μm or more existing in the uppermost layer portion of the interfacial alloy layer is 1500 particles /
It is preferably not more than mm ² .

In the present invention, the hardness of the α-Al phase in the dendrite portion is preferably Hv120 or less.
Further, the present invention provides a method of forming a molten Al-Zn alloy plating layer by immersing a steel sheet in a molten Al-Zn alloy plating bath, and then pulling up the steel sheet from the molten Al-Zn alloy plating bath and cooling it to form a molten Al-Zn alloy plating layer.
In the method for producing an Al-Zn alloy-plated steel sheet, the cooling is performed by raising the temperature from the hot-dip Al-Zn alloy plating bath to 260 ° C.
Molten aluminum with excellent workability, characterized in that the cooling rate until the cooling is 20 ° C / s or more and 100 ° C / s or less
-A method for producing a Zn alloy-plated steel sheet.

Further, in the present invention, after the cooling, the hot-dip Al—Zn alloy plated steel sheet may further have a rolling reduction of 0.5% to 5%.
It is preferable to perform skin pass rolling, and then perform an overaging treatment in a temperature range of 130 to 260 ° C. The molten Al-Zn
When the composition of the Al-Zn alloy plating layer is
25-75%, Si is more than 1% and 5% or less, Sr is 0.2-
It is preferable to adjust the composition to be contained in the range of 2%.

Further, in the present invention, in addition to the composition of the Al-Zn alloy plating layer, one or more of Cr, V and Zr are contained in a mass ratio of 0.01 to 2.0% in total. It is preferable to use a composition.

[0015]

FIG. 1 is a metallographic photograph showing a cross section of a plated layer of a 55% Al-1.6% Si-Zn alloy plated steel sheet, which is a typical example to which the present invention is applied. As shown here, the plating layer of the hot-dip Al-Zn alloy plated steel sheet of the present invention is:
It comprises a dendrite portion A, an interdendrite portion B existing between the dendrite portions, and an interface alloy layer C existing at the interface with the steel plate D. Among these, the dendrite portion A is made of an α-Al (Zn) phase in which Zn is dissolved in Al as a solid solution, and forms a main constituent phase of the plating layer. As shown in FIG. 1, the interdendritic portion B fills the space between the α-Al (Zn) phases constituting the dendritic portion, and is a eutectic Al-Zn and Si crystal precipitated. The interface alloy layer C is
A thin connecting portion existing at the interface between the steel sheet (base iron) D and the plating layer is made of an Al-Fe-Si-Zn quaternary intermetallic compound.

The plating layer formed on the surface of the hot-dip Al-Zn alloy-coated steel sheet of the present invention has an average composition (including the above A to C).
Contains 25 to 75% of Al, more than 1% and less than 5% of Si and Sr in a range of 0.2 to 2% of Si content by mass ratio, or one or more of Cr, V, Zr or 2 or more types in total
0.01 to 2.0%, with the balance being substantially Zn.

If the Al content in the plating layer is less than 25% by mass, the corrosion resistance is insufficient. On the other hand, if it exceeds 75%, the end face corrosion resistance is deteriorated, the plating layer is hardened, and the bending workability of the plated steel sheet is significantly deteriorated. Also, the plating layer
If the content of Si is 1% or less, the interface alloy layer exceeds 10% of the total thickness of the plating layer, and the bending workability of the plated steel sheet deteriorates. 5%
If the content exceeds the range, Si crystals are coarsely and largely precipitated in the plating layer, and the bending workability is significantly reduced. For this reason,
The content of Si in the plating layer was limited to more than 1% and 5% or less. In addition,
Preferably it is 1.3-2.0%.

Further, Sr is contained in the plating layer, and Si is contained in the plating layer.
By containing 0.2% or more of the content, the angular Si crystals in the plating layer can be changed into spherical fine Si crystals, and cracks are effectively generated from the interdendrites during bending. Can be prevented.
On the other hand, if the Sr content in the plating layer exceeds 2% of the Si content, coarse Sr / Si-based precipitates precipitate in the plating layer, causing defects such as pinholes presumably caused by this. This tends to increase the workability. For this reason, Sr was limited to the range of 0.2 to 2% of the Si content.

In the average composition of the Al—Zn alloy plating layer,
Si content is 1.5% and Sr / Si (mass% ratio) is 0-
The molten Al-Zn alloy plated steel sheet changed between 0.03 is subjected to constant current electrolysis in 1% salicylic acid-4% methyl salicylate-10% potassium iodide aqueous solution to dissolve and remove only the upper layer of the plating layer. The state where the insoluble Si crystal was left as a residue on the interface alloy layer was observed using a scanning electron microscope.
FIGS. 2 to 5 show the obtained micrographs of typical scanning electron microscope structures. Table 1 summarizes the relationship between the Sr / Si ratio in the plating layer and the Si crystal shape.

[0020]

[Table 1]

From Table 1, by setting Sr / Si to 0.002 or more, that is, Sr in the plating layer to 0.2% or more of the Si content, the angular Si crystal as shown by the arrows in FIGS. It can be seen that it changes into a spherical fine Si crystal as shown in FIGS. Thereby, it is possible to effectively prevent the crack from being generated from the interdendrite portion when the steel sheet undergoes bending deformation. However, in the plating layer
When the Sr content is large, there is a tendency that coarse Sr / Si-based precipitates and defects such as pinholes are generated in the plating layer, thereby deteriorating the workability.

Further, in the present invention, it is preferable that the Sr concentration in the interdendritic portion of the Al—Zn alloy plating layer is 20 to 150 times the average Sr concentration of the Al—Zn alloy plating layer. As described above, by concentrating Sr in the interdendritic portion, the dendrite portion, that is, α-Al (Z
n) The phase hardness decreases. FIG. 6 shows the ratio [Sr] i / [Sr] a between the Sr concentration [Sr] i in the interdendritic portion and the average Sr concentration [Sr] a in the Al—Zn alloy plating layer and the α-Al in the dendritic portion. The relation with the micro-Vickers hardness Hv of the (Zn) phase is _0.0025 .

FIG. 6 shows that when [Sr] i / [Sr] a is 20 or more, the hardness Hv _0.0025 starts to decrease, and when it is 150 or more, the effect is saturated. When [Sr] i / [Sr] a is 20 or more, the Zn content in the α-Al (Zn) phase starts to decrease, and the hardness decreases to 120 or less at a micro Vickers hardness Hv _0.0025. Conceivable. [Sr] i / [Sr] a is
It is not clear why the hardness of the dendrite portion is high when it is less than 20, but it is considered that one of the reasons is that Sr precipitates extremely finely in the dendrite. Further, as described above, the addition of excessive Sr causes coarse Sr /
This also causes Si-based precipitates and thereby defects such as pinholes. Therefore, in the present invention, [Sr] i
/ [Sr] a is preferably 20 or more and 150 or less.
[Sr] i / [Sr] a is set to 20 to 150 and Sr is enriched in the interdendrite part. Thus, it can be achieved by cooling the steel sheet from the plating bath to 260 ° C. at a temperature of 20 ° C./s or more and 100 ° C./s or less.

Further, in the present invention, the composition of the Al—Zn alloy plating layer is added to the above-mentioned respective compositions, and the composition is further added with a mass ratio of C
One, two or more of r, V and Zr in a total of 0.01 to 2.
It is preferable to contain 0%. Cr, V, and Zr all have the effect of reducing the size of the interface alloy layer particles in the uppermost layer of the interface alloy layer, flattening the interface between the interface alloy layer and the upper plating layer, and significantly improving bending workability. Have. Cr, V, Zr
It is considered that the alloy is deeply involved in the alloying reaction of the plating layer and increases the nucleation frequency of the alloying reaction, thereby miniaturizing the interface alloy layer particles in the uppermost layer of the interface alloy layer. Although the generation position of the solidification nucleus of the primary Al phase is not always clear, it is highly likely that the solidification nucleus is at the interface between the upper layer of the plating layer having high interface energy and the interface alloy layer. In particular, it is highly likely that the interface is with the convex interfacial alloy layer particles existing in the uppermost layer of the interfacial alloy layer.
By containing any of Cr, V, and Zr in the plating layer, the particles of the convex interface alloy layer become finer,
It is considered that the frequency of nucleation of primary Al increases and, as a result, the frequency of the presence of interdendrite penetrating the plating layer, which serves as a crack propagation path, is thought to decrease.

If the total amount of one or more of Cr, V and Zr in the plating layer is less than 0.01% by mass, the above-mentioned effects cannot be obtained. On the other hand, Cr, V, Zr
If the total amount of one or more of these elements is to exceed 2.0%, it is necessary to add a large amount of these elements to the plating bath, causing a large amount of dross, and causing dross on the steel sheet. It causes surface defects such as adhesion and non-plating. Therefore, the total amount of one or more of Cr, V, and Zr in the plating layer is limited to the range of 0.01 to 2.0% by mass. Incidentally, the content is preferably 0.01 to 0.5%.

Further, according to the present invention, the particles of the interface alloy layer having a major axis of 5 μm or more, which are present in the uppermost layer of the interface alloy layer, are removed by 1500 times.
The number is preferably not more than the number of pieces / mm ² . Interfacial alloy layer particles are dispersed in the uppermost layer of the interfacial alloy layer in the plating layer formed on the hot-dip Al-Zn alloy plated steel sheet. This interface alloy layer is made of Fe-Al-Si based intermetallic compound, FeAl ₄ Si _0.2 (τ _5c ).
These are polygonal particles in which Zn is dissolved in a trace amount, and among them, convex and coarse interface alloy layer particles are highly likely to be solidification nuclei of the primary Al phase. It is considered that by reducing the frequency of the presence of the convex and coarse interface alloy layer particles, the frequency of nucleation of the primary Al phase is reduced, and the frequency of the presence of interdendrite, which is a crack propagation path, is also considered to be reduced. Thereby, bending workability is improved. When the frequency of the interface alloy layer particles having a particle size of 5 μm or more exceeds 1500 particles / mm ² , the bending workability deteriorates. For this reason, in the present invention, it is preferable that the presence frequency of the interface alloy layer particles having a major axis of 5 μm or more in the uppermost layer portion of the interface alloy layer be 1500 particles / mm ² or less. The presence frequency of interface alloy layer particles having a major axis of 5 μm or more was determined to be 1500 particles / mm ²
In order to make the following, the composition of the Al-Zn alloy plating layer, in addition to each of the above-mentioned composition, further, by mass ratio, one or more of Cr, V, Zr 0.01 to 2.0 in total. %.

Further, by containing one or more of Cr, V, and Zr, the particles of the interfacial alloy layer present in the uppermost layer of the interfacial alloy layer can be miniaturized and the α-Al of the dendrite portion can be reduced.
There is also an effect that the time for the overaging treatment, which is the softening treatment of the (Zn) phase, can be shortened. The reason why the overaging time is shortened and the age hardening is delayed by the inclusion of one or more of Cr, V and Zr is not clear, but one of the reasons is that α-Al (Zn) It is presumed that the precipitation of Zn in the phase was promoted.

In the present invention, it is preferable that the hardness of the α-Al (Zn) phase in the dendrite portion in the plating layer is Hv120 or less in terms of micro-micro Vickers hardness. α-
By setting the hardness of the Al (Zn) phase to Hv120 or less, the entire plating layer is softened, and the bending workability is remarkably improved.
If the hardness of the α-Al (Zn) phase in the dendrite portion exceeds Hv120 in terms of micro-micro Vickers hardness, remarkable improvement in bending workability cannot be obtained. In addition, α of the dendrite part
-To soften the Al (Zn) phase, [Sr] i / [Sr] a
In addition to being not less than 20 and not more than 150, as described below,
It is also possible by performing overage processing. In this case, the hardness of the dendrite portion can be set to Hv100 or less, which is more preferable.

Next, a method for producing a hot-dip Al-Zn alloy plated steel sheet of the present invention will be described. As the steel sheet used in the present invention, any steel sheet manufactured by a usual method, for example, a low carbon aluminum killed steel sheet or an ultra low carbon steel sheet can be suitably used.
In the present invention, these steel sheets are immersed in a hot-dip Al-Zn alloy plating bath, hot dip plating is performed, and the steel sheets are pulled up from the hot-dip Al-Zn alloy plating bath and cooled to form a hot-dip Al-Zn alloy plating layer. Here, in the present invention, the composition of the hot-dip Al-Zn alloy plating bath and the average composition of the Al-Zn alloy plating layer are represented by mass ratio.
Al is 25 to 75%, Si is more than 1% and 5% or less, and Sr has a Si content of 0.1%.
In a range of 2 to 2%, or one or more of Cr, V, and Zr are further contained in a total amount of 0.01 to 2.0%, and the balance is substantially the same as that of Zn. It is preferable to adjust. Furthermore, the composition of the molten Al-Zn alloy plating bath is as follows: Al is 25-75% by mass, Si is more than 1% and 5% or less,
Sr is in the range of 0.2 to 2% of the Si content, or
One, two or more of r, V, and Zr are 0.01 to 2.0 in total.
%.

The plating bath temperature is preferably not lower than the liquidus temperature and not higher than 50 ° C. Further, in the present invention, when the steel sheet is pulled up from the molten Al-Zn plating bath adjusted to the above-described composition and cooled, the cooling rate between pulling up from the plating bath and reaching 260 ° C. is 20 ° C./s or more.
It is preferable to be 100 ° C./s or less. If the cooling rate up to 260 ° C. is less than 20 ° C./s, it is difficult to reduce the size of the interface alloy layer particles in the uppermost layer of the interface alloy layer. On the other hand, if the cooling rate to 260 ° C. exceeds 100 ° C./s, Sr cannot be concentrated in the interdendrite portion, and [S
[r] i / [Sr] a cannot be set to 20 or more.

Further, in the present invention, preferably, after the above-mentioned process is performed, the α-Al (Zn) phase in the dendrite portion is further softened after the hot-dip Al-Zn alloy-plated steel sheet is formed. It is preferable for softening to significantly improve bending workability. As a means for softening the α-Al (Zn) phase, for example, a method described in JP-B-61-28748, in which a steel sheet is subjected to logt = 7102.4 / T-11.04 (where t: time (second) T: Heating temperature (K)), a method of overaging under the conditions represented by
No. 41657, a method of performing shot blast treatment on a steel sheet after plating and holding the steel sheet at 150 to 270 ° C. for 10 minutes or less is preferable.More preferably, the method described below is used.
Advantageously, the α-Al (Zn) phase can be softened quickly and very effectively.

This method is a method of subjecting a hot-dip Al-Zn alloy-plated steel sheet manufactured by the above-described method using hot dip plating to skin pass rolling and overaging.
An appropriate amount of dislocation is introduced into the α-Al (Zn) phase by skin pass rolling. The rolling reduction of skin pass rolling is 0.5% or more, 5
% Is preferable. By performing the skin pass rolling, the processing time of the overaging treatment in the next step is reduced. On the other hand, if the rolling reduction of skin pass rolling is less than 0.5%, the amount of dislocations introduced is insufficient and the above-mentioned effects cannot be expected. On the other hand, even if the rolling reduction exceeds 5%, the effect of shortening the overaging treatment time is saturated, and cracks may occur in the plating layer.

After the skin pass rolling, an overaging treatment is performed. As a result, Zn which is supersaturated in the α-Al (Zn) phase
To precipitate. The temperature of the overaging treatment is suitably in the range of 130 to 260 ° C. When the overaging temperature is as low as less than 130 ° C., P. On the contrary, when the overaging temperature is too high, exceeding 260 ° C., due to the formation of zones, the overhardening temperature is too high to harden due to the formation of Al _2.45 Zn (hexagonal R ₃ m), and the workability is rather deteriorated. The most preferred overaging temperature is 170 to 230 ° C. In the overaging treatment, it is preferable to keep the above-mentioned temperature for 30 seconds to 1 hour.

According to the method of performing overaging treatment after skin pass rolling, the time required for overaging can be greatly reduced as compared with the conventional means even if the rolling time is added. In the present invention, the cooling rate of the overaging treatment is not particularly specified,
In the case of furnace cooling in which a temperature range of 260 ° C. is cooled over 30 seconds or more, it is not particularly necessary to maintain the temperature for a fixed time. In addition, this overaging process can shorten the time when [Sr] i / [Sr] a is in the range of 20 to 150. As described above, in this range, the hardness of the α-Al (Zn) phase is reduced in advance.

As the means for skin pass rolling and overaging treatment, those usually used in the treatment of steel sheets may be used. Hereinafter, embodiments of the present invention will be further clarified based on examples.

[0036]

EXAMPLES By mass ratio, C: 0.045%, Si: 0.01%, Mn:
Low-carbon aluminum-killed steel consisting of 0.17%, S: 0.005%, Al: 0.019%, balance Fe and unavoidable impurities is processed according to a conventional method to form a cold-rolled steel sheet, which is averaged as shown in Table 2 by a continuous hot-dip plating equipment. The composition was subjected to Al-Zn alloy plating to obtain a hot-dip Al-Zn alloy plated steel sheet (product). 99.9% Zn ingot and 99.99% Al ingot were used as the base alloy in the plating bath, and these were used for 15% Si-Al alloy, 10% Sr-Al alloy,
The composition was adjusted using a 10% Cr-Al alloy, a 2% V-Zn alloy, and a 5% Zr-Al alloy so that the plating layer composition shown in Table 2 was obtained.

A plating bath with adjusted components (bath temperature: 590 to 61)
5 ° C.), immersed for 1 second, pulled up, and then cooled at the cooling rate shown in Table 2 to obtain a hot-dip Al-Zn alloy plated steel sheet. Further, a part of the obtained hot-dip Al-Zn alloy-plated steel sheet was further subjected to skin pass rolling at a rolling reduction shown in Table 3, and then overaged in a continuous annealing furnace or a batch type annealing furnace under the conditions shown in Table 3. Was given.

The workability of the obtained product (hot-dip Al-Zn alloy plated steel sheet) was judged by a bending test. Specifically, a plated steel sheet is cut into a size of 60 mm in the rolling direction and 20 mm in the width direction to form a test piece, and a bending test is performed with a bending radius of 1 t (2 t bending) in accordance with a bending test according to JIS Z 2248. This was performed by measuring the crack area ratio of the bent portion.

The crack area ratio was calculated by multiplying the bent portion by 50:
A doubled reflected electron beam image was taken and the width was 220 mm (for the bending test piece)
The area of the crack that appears in that area is calculated by image analysis by scanning a 50 mm section (section of 1 mm in the bending test piece) across the bending line over 4.4 mm), and the crack area with respect to the total area of the bent part Rate. The hardness of the dendrite part (α-Al (Zn) phase) of the plating layer, the average composition of the plating layer, the Sr concentration of the interdendrite part, the state of the Si crystal of the dendrite part, the interface alloy layer ratio, the interface alloy layer The size and number of particles were also investigated.

Hardness of dendrite part (α-Al (Zn) phase)
For Hv _0.0025, use a micro Vickers hardness tester
It was measured from the cross section of the plating layer as 24.5 mN (2.5 gf). The hardness of the dendrite portion (α-Al (Zn) phase) after skin pass rolling and overaging treatment was also measured at the same time. The composition of the plating layer was determined by analyzing the cross-section of the plating layer at each of ten locations by EPMA, and determining the average value as the average plating layer concentration of each component. Further, the Sr concentration of the interdendrite portion was determined by performing the Sr concentration of each of the 10 interdendrite portions in the cross section of the plating layer by EPMA and calculating the average value. Further, the interdendrite portion was observed with a scanning electron microscope, and the state of the Si crystal was observed.

The interface alloy layer ratio is measured at each of five points on the cross section of the plating layer using a scanning electron microscope, and the average value is defined as the interface alloy layer thickness.
(Plating layer thickness) × 100 (%). The size and the number of interfacial alloy layer particles present in the uppermost layer in the plating layer were determined by taking samples from five places on the resulting plated steel sheet and using a 10% iodine-ethanol solution to coat the upper layer of the plating layer. After melting, the interface alloy layer was exposed, the surface structure of the interface alloy layer was imaged using a scanning electron microscope at a magnification of 2000 times for each sample in 15 visual fields, and an image analyzer was used from the obtained structure photograph using a photograph of the structure. The average value was determined for each sample, and the average of the average values was used as the value of each steel sheet. Then, among the interface alloy layer particles, the existence frequency was calculated for the interface alloy layer particles having a major axis of 5 μm or more in size.

The workability was determined based on the crack area ratio. ◎: 2% or less, 、 ２: more than 2%, 5% or less, ○: 5%, 9% or less: Δ, 9 % If it exceeds%. The obtained results are shown in Tables 2 and 3.

[0043]

[Table 2]

[0044]

[Table 3]

As shown in these figures, in the examples of the present invention, the crack area ratio in the bending test was as low as 9% or less, and the workability was excellent. On the other hand, in Comparative Examples outside the range of the present invention, the crack area ratio in the bending test was as high as more than 9%, and the bending workability was reduced. Further, by performing skin pass rolling and overaging treatment according to the present invention, extremely excellent workability is exhibited. If the conditions of skin pass rolling and overaging treatment are out of the preferred range of the present invention, the effect of improving workability is reduced or the effect of improving is lost.

[0046]

According to the present invention, since an appropriate amount of Sr is contained in the plating layer of a hot-dip Al-Zn alloy plated steel sheet in accordance with the Si content, the workability is better than in the past, and Suitable as a material for products and building materials. In particular, those that have been overaged according to the present invention not only exhibit extremely excellent workability, but also have excellent productivity.

[Brief description of the drawings]

FIG. 1 shows a typical example of a 55% Al—
It is a metallographic photograph which shows the structure of the cross section of the plating layer of a 1.6% Si-Zn alloy plating steel plate.

FIG. 2 is a scanning electron micrograph of a Si crystal residue in an interdendrite portion after dissolving an upper layer of a plating layer when Sr / Si = 0.

FIG. 3 is an enlarged electron micrograph of FIG.

FIG. 4 is a scanning electron micrograph of a Si crystal residue in an interdendrite portion after dissolving an upper layer of a plating layer when Sr / Si = 0.002.

FIG. 5 is an enlarged electron micrograph of FIG.

FIG. 6 shows a Si concentration [Sr] in an interdendrite portion.
i and the ratio of the average Sr concentration [Sr] a of the Al-Zn alloy plating layer [S
r] i / [Sr] a and Vickers hardness Hv of dendrite part
Is a graph showing the relationship between _0.025.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 2/28 C23C 2/28 // B21B 1/22 B21B 1/22 H (72) Inventor Chiaki Kato Chiba 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Kawasaki Steel Engineering Co., Ltd. (72) Hideo Takamura 1025 Hamano-cho, Chuo-ku, Chiba-shi, Chiba Prefecture BD09 CB01 4K027 AA05 AA22 AB02 AB05 AB33 AB44 AB48 AC72 AC87 AE02 AE03 AE12 AE22

Claims (7)

[Claims] 1. A steel sheet comprising a dendrite portion, an interdendrite portion existing between the dendrite portions, and an interface alloy layer existing at an interface between the dendrite portion and the steel plate ground iron.
It has an Al-Zn alloy plating layer, and the Al-Zn alloy plating layer has a mass ratio of 25 to 75% Al, more than 1% Si and 5% or less, and Sr in a range of 0.2 to 2% of Si content. A hot-dip Al-Zn alloy-plated steel sheet having excellent workability, characterized by being contained. 2. Sr in the interdendrite portion
The hot-dip Al-Zn alloy plated steel sheet according to claim 1, wherein the concentration is 20 to 150 times the average Sr concentration of the Al-Zn alloy plated layer. 3. The Al-Zn alloy plating layer further includes one or more of Cr, V, and Zr in a mass ratio of 0.2 or more.
The hot-dip Al-Zn alloy-plated steel sheet according to claim 1 or 2, wherein the steel sheet contains 01 to 2.0%. 4. An uppermost layer of the interface alloy layer,
The hot-dip Al-Zn alloy coated steel sheet having excellent workability according to claim 3, wherein the number of interface alloy layer particles having a major diameter of 5 µm or more is 1500 particles / mm ² or less. 5. A method according to claim 1, wherein the hardness of the α-Al phase of the dendrite portion is Hv120 or less.
A hot-dip Al-Zn alloy-plated steel sheet excellent in workability according to any one of the above. 6. A hot-dip Al-Zn alloy plating layer, wherein the steel sheet is immersed in a hot-dip Al-Zn alloy plating bath, and then the hot-dip steel sheet is pulled up from the hot-dip Al-Zn alloy plating bath and cooled to form a hot-dip Al-Zn alloy plating layer.
In the method for producing a Zn alloy-plated steel sheet, the cooling is performed by:
A hot-dip Al-Zn alloy coated steel sheet having excellent workability, wherein the hot-dip Al-Zn alloy plating steel sheet is cooled at a cooling rate of up to 260 ° C from 20 ° C / s to 100 ° C. Manufacturing method. 7. The hot-dip Al—Zn alloy-plated steel sheet is further subjected to skin pass rolling at a rolling reduction of 0.5% to 5%, and then to an overaging treatment in a temperature range of 130 to 260 ° C. A molten Al-Zn having excellent workability according to claim 6.
Manufacturing method of alloy plated steel sheet.