JP2002363722A - COATED HOT DIP Al-Zr ALLOY PLATED STEEL SHEET HAVING EXCELLENT WORKABILITY AND CORROSION RESISTANCE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coated hot dip Al-Zn alloy plated steel sheet which has excellent workability and corrosion resistance. SOLUTION: The front and back faces of a steel sheet are respectively provided with an Al-Zn alloy plated layer consisting of dendrite parts, an interdendrite part present on the space between the dendrite parts, and a boundary alloy layer present on their boundaries. Either or both of the surface and back faces are provided, as an upper layer of the Al-Zn alloy plated layer, with a chemical conversion treatment layer, or an organic coating film layer via a primer layer as well. The Al-Zn alloy plated layer contains, by mass, 25 to 75% Al, >1 to 5% Si, and Sr in the range of 0.2 to 2% of the Si content, or further contains one or more kinds selected from Cr, V and Zr by 0.01 to 2.0% in total. Further, the concentration of Sr in the interdendrite part is preferably 20 to 150 times the average Sr concentration in the Al-Zn alloy plated layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated molten Al-Zn alloy coated steel sheet, and more particularly to a painted molten Al-Zn alloy containing 25 to 75% by mass of Al widely used in the fields of building materials and home appliances.
It relates to a galvanized steel sheet.

[0002]

2. Description of the Related Art 55% Al-Zn alloy plated steel sheet
Hot-dip Al-Zn coated steel sheets containing 25-75% Al by mass are widely used in the fields of building materials and home appliances as so-called PMC steel sheets (pre-coated metal) by coating their surfaces. However, this type of coated steel sheet has a hard plating layer, so when forming it into building materials and parts for home appliances, cracks occur where the processing conditions are severe, and the coating film peels off from there as a starting point, or the corrosion resistance is poor. There is a problem of deterioration.

In order to cope with such a problem, the plating layer is made as soft as possible. For example, Japanese Patent Publication No.
= 7102.4 / T-11.04 (where, t: time (seconds), T: heating temperature (K)) has been proposed to perform overaging treatment under the conditions shown. In addition, JP-A-11
Japanese Patent No. 343559 discloses a coated steel sheet, in which the agglomeration portion of Zn is present in an interdendritic portion constituting a plating layer in an area ratio of 1.0 to 30% in a cross section of a plating film to thereby improve crack resistance. Suggestions for improvement have been made.

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. Further, in the proposal described in Japanese Patent Application Laid-Open No. 11-343559, no consideration is given to the effect of Si on the workability, and no consideration is given to the improvement of the workability due to overaging, and therefore the 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 can still be ensured. Therefore, the corrosion resistance of the coated steel sheet may not be sufficiently ensured in some cases. In addition, each of the above-mentioned conventional proposals individually deals with a cause of workability deterioration caused by a dendrite portion or an interdendrite portion, and does not pay attention to their interdependence. Therefore, the coated hot-dip Al-Zn alloy-plated steel sheet has not yet had sufficient workability and has insufficient corrosion resistance.

An object of the present invention is to solve the above-mentioned problems relating to a coated hot-dip Al-Zn alloy-coated steel sheet, and to provide a coating film having excellent workability as compared with the conventional one and, if necessary, high workability. An object of the present invention is to propose a coated hot-dip Al-Zn-containing plated steel sheet which is excellent in peeling resistance of a coating film, and thus excellent in corrosion resistance.

[0008]

Means for Solving the Problems The present inventors have proposed a method of coating and melting.
A detailed study was conducted on the effect of Sr addition on the workability of the hot-dip Al-Zn alloy coated steel sheet, which is the base of the Al-Zn alloy coated steel sheet, and the amount of Sr added was at a certain ratio with respect to the Si content At the time, the spheroidization of the Si crystal in the interdendritic portion is reliably performed, and the precipitation of Sr in the interdendritic portion is promoted, whereby the Sr concentration in the dendritic portion decreases and the hardness decrease due to overaging treatment is reduced. It was found that it can be easily achieved.

The present inventors have also 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 desirable to reduce the number of coarse convex interface alloy layer particles having a major axis of 5 μm or more to 1500 particles / mm ² or less. . In addition, the present inventors prevent or suppress the growth of the convex interfacial alloy layer particles by including an appropriate amount of Cr, V, and Zr together with Sr in the plating film, so that the coarse convex of 5 μm or more is formed. 15 interface alloy layer particles
It has been found that the number can be reduced to 00 / mm ² or less.

The present invention has been completed based on the above findings and further studies. That is, the present invention has an Al-Zn alloy plating layer composed of 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 sheet base iron on the front and rear surfaces of the steel plate. Then, on one or both of the front and back surfaces of the steel sheet, as an upper layer of the Al-Zn alloy plating layer, a chemical conversion treatment layer, or a painted molten Al-Zn alloy having an organic coating layer via a further primer layer. A plated steel sheet, wherein the Al-Zn alloy plating layer has a mass ratio of 25 to 75 Al.
%, More than 1% of Si and 5% or less, and Sr of 0.2 to 2% of Si content
A coated hot-dip Al-Zn alloy-plated steel sheet having excellent workability and corrosion resistance, characterized in that the steel sheet is contained in the range described above.

In the present invention, in particular, in order to reduce the hardness of the α-Al (Zn) phase in the dendrite portion, the Sr concentration in the interdendrite portion of the Al—Zn alloy plating layer is set to:
The average Sr concentration of the Al—Zn alloy plating layer is preferably 20 to 150 times. Thereby, the age hardening of the plating layer after plating is suppressed, and the time of the overaging treatment performed later can be shortened.

In the present invention, the Al—Zn alloy plating layer may further include one or more of Cr, V, and Zr in mass ratio.
It is preferable to contain 0.01 to 2.0% in total of at least seeds,
Further, in the present invention, it is preferable that the number of interface alloy layer particles having a major axis of 5 μm or more and present in the uppermost layer portion of the interface alloy layer of the Al—Zn alloy plating layer is 1500 particles / mm ² or less. In the present invention, the hardness of the α-Al phase in the dendrite portion is preferably Hv120 or less.

Further, according to the present invention, after the steel sheet is immersed in a molten Al-Zn alloy plating bath to form an Al-Zn alloy plating layer on the front and back surfaces, the steel sheet is pulled up from the molten Al-Zn alloy plating bath and cooled. To form a molten Al-Zn alloy plating layer,
After forming a Zn alloy plated steel sheet, a chemical conversion treatment layer or a further primer layer is formed as an upper layer on one or both of the front and back surfaces of the hot-dip Al-Zn alloy plated steel sheet, and then an organic coating is performed. Paint melting to form a film layer
In the method for producing an Al-Zn alloy-plated steel sheet, the molten Al
-Zn alloy plating bath, the composition of Al-Zn plating layer
Al is 25 to 75%, Si is more than 1% and 5% or less, and Sr has a Si content of 0.1%.
Painted molten Al with excellent workability and corrosion resistance, characterized in that the composition is adjusted to be contained in the range of 2 to 2%.
-A method for producing a Zn alloy-plated steel sheet. Further, in the production method of the present invention, in addition to the composition of the Al-Zn alloy plating layer, one or more of Cr, V, and Zr are further contained in a mass ratio of 0.01 to 2.0% in total. Preferably, the cooling is carried out from the hot-dip Al-Zn alloy plating bath and the cooling rate up to 260 ° C is 20 to 100 ° C.
/ S is preferable. Further, in the production method of the present invention, 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 overaging treatment at a temperature range of 130 to 260 ° C. Is preferred.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The coated hot-dip Al-Zn alloy coated steel sheet of the present invention is an Al-Zn alloy coated steel sheet represented by a 55% Al-Zn alloy coated steel sheet.
This is a coated hot-dip Al-Zn alloy plated steel sheet having a chemical conversion treatment layer on at least one side of the hot-dip Al-Zn alloy plated steel sheet having a Zn alloy plated layer, or an organic coating layer via a primer layer.

[0015] The hot-dip Al-Zn alloy-plated steel sheet, which is the base (original sheet) of the coated hot-dip Al-Zn alloy-plated steel sheet of the present invention, comprises:
On the front and back surfaces of the steel sheet, there is provided an Al—Zn alloy plating layer composed of a dendrite part, an interdendrite part existing between the dendrite parts, and an interface alloy layer existing at the interface between the dendrite part and the steel sheet ground iron. Then, the coated hot-dip Al-Zn alloy-plated steel sheet of the present invention is formed on one or both of the front and back surfaces of the steel sheet, as an upper layer of the Al-Zn alloy plating layer, through a chemical conversion treatment layer, or further through a primer layer. Form a coating layer.

The coated hot-dip Al-Zn alloy-plated steel sheet of the present invention is characterized in that the workability of the hot-dip Al-Zn alloy-plated steel sheet, which is the base sheet, is enhanced. Incidentally, the composition of the organic coating layer itself formed as an upper layer of the Al-Zn alloy plating layer,
There is no limitation as long as the processability is not impaired.
Hereinafter, the improvement of the workability of the Al—Zn alloy plating layer of the hot-dip Al—Zn alloy plated steel sheet as the original sheet will be described in detail.

FIG. 1 shows a molten 55% Al-1.1, which is a typical example of a hot-dip Al-Zn alloy-plated steel sheet serving as a base sheet (base) of the present invention.
5 is a metallographic photograph showing a cross section of a plating layer of a 6% Si—Zn alloy plated steel sheet. As shown here, the molten Al-Zn of the present invention
The plating layer of the alloy plated steel sheet includes a dendrite portion A and an interdendrite portion B existing between the dendrite portions.
And an interfacial alloy layer C existing at the interface with the steel plate base iron 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. The interdendrite part B is composed of α-Al (Zn) constituting the dendrite part as shown in FIG.
The space between the phases is filled, and eutectic Al-Zn and Si crystals are deposited. 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, and is made of an Al-Fe-Si-Zn-based quaternary intermetallic compound.

In the coated hot-dip Al-Zn alloy-coated steel sheet of the present invention, the plating layers formed on the front and back surfaces of the hot-dip Al-Zn alloy-plated steel sheet as the original sheet have 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 the range of 0.2 to 2% of Si content, or one or more of Cr, V, and Zr 0 in total.
It has a composition containing from 0.01 to 2.0%, with the balance being substantially Zn. Therefore, it is possible to avoid the problem that the plating layer becomes soft, cracks are generated during the forming process, and the coating is peeled off from the cracks.

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 painted steel sheet is significantly deteriorated. In addition, it is more preferably 40 to 60% by mass. In addition, Si in the plating layer
However, if it is 1% or less, the interface alloy layer exceeds 10% of the total thickness of the plating layer, and the bending workability of the painted steel sheet deteriorates. 5
%, Si crystals are coarsely and abundantly precipitated in the plating layer, and the bending workability is significantly reduced. Therefore, the content of Si in the plating layer is limited to more than 1% and 5% or less. Incidentally, the content is preferably 1.3 to 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,
The Si content is set to 1.6%, and the Sr / Si (mass% ratio) is 0 to
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.

[0022]

[Table 1]

From Table 1, it can be seen that by making Sr / Si 0.002 or more, that is, making the Sr in the plating layer 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, coarse Sr / Si-based precipitates and defects such as pinholes due to the Sr / Si tend to be generated in the plating layer, thereby deteriorating the workability.

Further, as described above, by including Sr in the Al-Zn alloy plating layer with a certain relationship with the Si content, the Si crystal present in the interdendrite portion can be made spherical, Consequently, peeling of the coating film during the molding process can be avoided. Further, in order to improve the corrosion resistance of the coated plated steel sheet by softening the entire plating layer, it is preferable to also soften the α-Al (Zn) phase in the dendrite portion. For this reason, in the present invention, it is preferable that the Sr concentration in the interdendritic portion of the Al—Zn alloy plating layer be 20 to 150 times the average Sr concentration of the Al—Zn alloy plating layer. By concentrating Sr in the interdendrite portion, the hardness of the dendrite portion, that is, the α-Al (Zn) phase is reduced.

FIG. 6 shows an example of the interdendrite portion.
The ratio [Sr] i / [Sr] a between the Sr concentration [Sr] i and the average Sr concentration [Sr] a of the Al-Zn alloy plating layer, and the micro-Vickers hardness of the α-Al (Zn) phase in the dendrite part Hv _0.0025
Shows the relationship with From FIG. 6, it can be seen 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 precipitates Si-based precipitates, which may cause defects such as pinholes presumed to be caused by the precipitates. Therefore, in the present invention, [Sr] i / [Sr] a is 20 or more and 150
It is preferable to set the following. Note that [Sr] i / [Sr] a
In order to concentrate Sr in the interdendrite portion, the Sr in the plating bath should be within a predetermined range with respect to the Si content, and then, as described later, after the steel sheet is pulled out of the plating bath 20 to 100 ° C / s until the temperature reaches 260 ° C
This can be achieved by cooling at

Further, in the present invention, the composition of the Al—Zn alloy plating layer is added to
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, and Zr are deeply involved in the alloying reaction of the plating layer and increase the frequency of nucleation of the alloying reaction, thereby minimizing the particles of the interface alloy layer at the uppermost layer of the interface alloy layer. Conceivable. 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, and thus the nucleation frequency of primary Al increases,
Therefore, it is considered that the frequency of the interdendrite which penetrates the plating layer, which becomes the propagation path of the crack, also decreases.

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%.

In the present invention, the uppermost layer of the interface alloy layer
The interfacial alloy layer particles having a major axis of 5 μm or more
Pieces / mm^TwoIt is preferable to set the following. Molten Al-Zn alloy
Top layer of interfacial alloy layer in plating layer formed on plated steel sheet
The interface alloy layer particles are dispersed in the portion. This interface alloy
Layer particles are Fe-Al-Si based intermetallic compound, FeAl_FourSi
_0.2(Τ _5c) Is a polygonal particle in which Zn is dissolved
Of these, the convex and coarse interface alloy layer particles form the primary Al phase.
It is likely to be a coagulation nucleus. This convex and coarse interface
By reducing the frequency of gold layer particles,
The frequency of nucleation decreases, and the
-Dendrites are also less frequently present,
It is considered that the workability is improved. Interface of 5μm or more
The presence frequency of gold layer particles is 1500 pieces / mm^TwoMore than
Then, bending workability deteriorates. Therefore, in the present invention,
A field with a major axis of 5 μm or more, which exists in the uppermost layer of the plane alloy layer
Frequency of surface alloy layer particles is 1500 / mm^TwoThe following
Is preferred. Presence of interfacial alloy phase particles with a major axis of 5 μm or more
1500 pieces / mm^TwoIn order to achieve the following, Al-Zn alloy
In addition to the above composition, the composition of the
In terms of ratio, one or more of Cr, V, and Zr are used in a total of 0.
It is preferably contained in the range of 01 to 2.0%.

Further, by containing one or more of Cr, V, and Zr, the particles of the interfacial alloy layer existing in the uppermost layer of the interfacial alloy layer can be miniaturized, and the α-Al ( Z
n) There is also an effect that it is possible to shorten the time of the overaging treatment which is the softening treatment of the phase. 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 Vickers hardness. α-Al (Zn)
When the hardness of the phase is Hv120 or less, the entire plating layer is softened, and the bendability is significantly improved. If the hardness of the α-Al (Zn) phase in the dendrite portion exceeds Hv120 in terms of micro Vickers hardness, remarkable improvement in bending workability cannot be obtained. In order to soften the α-Al (Zn) phase in the dendrite portion, [Sr] i / [Sr] a is not less than 20 and not more than 150, and an overaging treatment is performed as described later. This is also possible. In this case, the hardness of the dendrite portion can be set to Hv100 or less, which is more preferable.

Next, the method for producing the coated hot-dip Al-Zn 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 gold plating layer. . Here, in the present invention, the composition of the molten Al-Zn alloy plating bath is such that the average composition of the Al-Zn alloy plating layer is 25 to 75% by mass, Al is more than 1% and 5% or less, and Sr is Si. Of content
In a range of 0.2 to 2%, or further, one or more of Cr, V, and Zr are contained in a total of 0.01 to 2.0%, and the balance is substantially the same as that of Zn. ,adjust. In practice, the composition of the hot-dip Al-Zn alloy plating bath is adjusted to a mass ratio of 25-75% Al, more than 1% and 5% or less, and Sr in the range of 0.2-2% of the Si content, or even Cr , V, and Zr, the composition preferably contains a total of 0.01 to 2.0%.

The plating solution 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 alloy plating bath adjusted to the above composition and cooled, the cooling rate between the time when the steel sheet is pulled up from the plating bath and reaches 260 ° C is set to 20 ° C / s or more. , 100 ° C./s or less. If the cooling rate up to 260 ° C. is less than 20 ° C., it is difficult to make the particles in the uppermost layer of the interface alloy layer finer. In addition, 260 ℃
If the cooling temperature up to 100 ° C / s exceeds 100 ° C / s, Sr cannot be concentrated in the interdendrites, and [Sr] i /
[Sr] a cannot be set to 20 or more.

In the present invention, preferably, after the above-mentioned process is performed to form a hot-dip Al-Zn alloy-plated steel sheet, the α-Al (Zn) phase in the dendrite portion is further softened by reducing the entire plating layer. It is preferable for softening to significantly improve bending workability. As means for softening the α-Al (Zn) phase,
For example, a steel sheet after plating is represented by logt = 7102.4 / T-11.04 (where t = time (seconds) and T = heating temperature (K)) described in JP-B-61-28748. Method of overaging under conditions, or JP-A-4-41
No. 567, a method of subjecting a steel sheet to a shot blast treatment after plating and holding the steel sheet at 150 to 270 ° C. for 10 minutes or less is preferable, but the method described below is more preferably performed quickly and extremely effectively. This is advantageous because the α-Al (Zn) phase can be softened.

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 and 5%
It is preferable to set the following. 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 skin pass rolling, an overaging treatment is performed. Thereby, precipitation of Zn supersaturated in the α-Al (Zn) phase is intended. 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 the overaging treatment after the skin pass rolling, the time required for the 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. The time required for the overaging treatment can be further reduced by including one or more of Cr, V, and Zr in the plating layer in a total amount of 0.01 to 2.0%.

As the means for skin pass rolling and overaging treatment, those usually used in the treatment of steel sheets may be used. In the present invention, a hot-dip Al-Zn alloy-plated steel sheet having a soft hot-dip Al-Zn alloy-plated layer produced in this way, a chemical conversion treatment layer as an upper layer of the hot-dip Al-Zn alloy-plated layer, and directly on the Preferably, an organic coating layer is formed, or a chemical conversion treatment layer and a primer layer are further formed thereon, and then the organic coating layer is formed to obtain a coated hot-dip Al-Zn alloy plated steel sheet. The organic coating layer is made of molten Al
-It is preferable to form as an upper layer of the molten Al-Zn alloy plating layer on one or both of the front and back surfaces of the Zn alloy-plated steel sheet.

For the formation of the chemical conversion treatment layer, the primer layer and the organic coating layer, it is sufficient to use a conventional coated steel sheet or a layer which is employed for producing PCM. For the formation of the chemical conversion treatment layer, a usual chromate treatment, phosphate treatment or the like can be used, and for the formation of the primer layer, an rust preventive pigment (for example, epoxy resin, polyester, modified polyester, modified epoxy resin, etc.) It can be obtained by applying a mixture (primer) of a mixture of zinc chromate, strontium chromate, barium chromate, and the like, and a curing agent (melanin, isocyanate resin, etc.). Also,
The organic coating layer is formed by applying and baking an appropriate amount of a top coat such as a polyester paint, a fluorine resin paint, an acrylic resin paint, a vinyl chloride vinyl paint, or a silicone paint, which are generally known. Obtainable. In addition, it is also possible to appropriately add a coloring pigment to the primer, or to add various coloring pigments or extender pigments to the overcoating paint to form a coating film having high workability. The coating thickness and application method (spray coating, roll coating, brushing, etc.) of these paints are also the same
It is enough to adopt it.

In the formation of the above-mentioned chemical conversion treatment layer, primer layer and organic coating layer, the baking (drying) conditions are the conditions necessary for the overaging treatment (130 to 260 ° C., 30 seconds or more).
In such a case, it is possible to skip the overaging treatment and continuously proceed to the coating step after skin pass rolling. Hereinafter, the present invention will be described in more detail based on examples.

[0041]

EXAMPLES By mass ratio, C: 0.045%, Si: 0.01%, Mn:
0.17%, S: 0.005%, Al: 0.019%, low-carbon aluminum-killed steel consisting of the balance of Fe and unavoidable impurities is processed according to a conventional method to form a cold-rolled steel sheet, which is melted by a continuous hot-dip plating facility. Plating was performed to obtain a hot-dip Al-Zn alloy plated steel sheet. 99.9% Zn ingot and 99.99% Al ingot were used as the mother alloy in the plating bath, and 15% Si-
Al alloy, 10% Sr-Al alloy, 10% Cr-Al alloy, 2% V-Zn
The composition of the plating bath was adjusted using the alloy and the 5% Zr-Al alloy so as to have the plating layer composition shown in Table 3.

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 2, and then over-aged in a continuous annealing furnace or a batch type annealing furnace under the conditions shown in Table 2. Was given.

A test piece was sampled from the obtained hot-dip Al-Zn alloy plated steel sheet, and the characteristics of the plated layer were investigated. The characteristics of the plating layer include the hardness of the dendrite part of the plating layer, the average composition of the plating layer, and the Sr of the interdendritic part.
Concentration, state of dendrite Si crystal, interface alloy layer ratio,
The size and number of the interface alloy layer particles were investigated. The hardness Hv _0.0025 of the dendrite portion (α-Al phase) was measured from the cross section of the plating layer with a load of 24.5 mN (2.5 gf) using a micro Vickers hardness meter.

The composition of the plating layer was determined by analyzing the cross section of the plating layer at each of ten points by EPMA, and determining the average value as the average concentration of the plating layer 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 was measured at each of five points on the cross section of the plating layer using a scanning electron microscope, and the average value was defined as the interface alloy layer thickness.
(Plating layer thickness) × 100 (%). Further, the size and the number of the interface alloy layer particles present in the uppermost layer in the plating layer were determined as follows. From the obtained plated steel sheet, samples were taken from five places, the upper layer of the plating layer was dissolved with a 10% iodine-ethanol solution, the interface alloy layer was exposed, and the surface structure of the interface alloy layer was examined using a scanning electron microscope. And observed at a magnification of 2000 times. Each sample was imaged in 15 visual fields, and the average value of the size and number of interfacial alloy layer particles was obtained for each visual field using an image analyzer from the obtained micrograph, and the average of those average values was used as the value of each steel sheet. . And
Among the interface alloy layer particles, the presence frequency (pieces / mm ² ) was calculated for the interface alloy layer particles having a major axis of 5 μm or more in size.

The results obtained are summarized in Tables 2 and 3.

[0047]

[Table 2]

[0048]

[Table 3]

The hot-dip Al-Zn alloy plated steel sheet thus obtained was subjected to a chromate treatment (or a phosphate treatment) under the conditions shown in Table 4 to form a chemical conversion treatment layer, and then a direct or primer layer was formed. Apply a high-molecular polyester-based paint or fluororesin paint to a thickness of 20 to 30 μm to form an organic coating layer, and bake and dry at 180 to 220 ° C for 5 to 10 minutes to obtain a coated molten Al-Zn alloy It was a plated steel sheet (product).

The obtained product was evaluated for workability by a bending test. Judgment of workability after painting
The product was cut into a test piece having a size of 60 mm in the rolling direction and 20 mm in the width direction to form a test piece. A 2t bending test and an Erichsen 5 mm extrusion test were performed, and after the test, the crack state was observed. The 2t bending test was performed at a bending radius of 1t (2t bending) in accordance with the bending test according to JIS Z 2248. The Erichsen 5 mm extrusion test was performed in accordance with JIS Z 2247. For the observation of the state of the crack, the crack occurrence rate was determined by visual observation using a microscope.

Further, the test piece subjected to the 2t bending test and the Erichsen 5 mm extrusion test described above was further subjected to a combined cycle test (CCT), and the corrosion resistance after processing was evaluated from the degree of corrosion of the processed portion. Combined cycle test (CCT)
Is SST (5% NaCl salt spray: 35 ° C; 4 hours) + dry (2 hours) + HCT (constant temperature: 49 ° C, 95% RH; 4 hours)
Was used as one cycle, and 200 cycles were performed. After the test,
Visually measure the area of white rust generated in the processed part of the test piece,
The ratio of the area of white rust occurrence to the entire area of the processed portion and the white rust occurrence area ratio were calculated, and the corrosion resistance after processing was evaluated based on the white rust occurrence area rate.

Table 5 shows the obtained results.

[0053]

[Table 4]

[0054]

[Table 5]

As shown here, in the example of the present invention, the crack occurrence rate was 5% or less in both the bent portion and the Erichsen extruded portion, and the white rust occurrence area rate of these test pieces was 5% or less. It was determined that both the post-processability and the post-process corrosion resistance were sufficient. On the other hand, in the comparative examples outside the scope of the present invention, the crack occurrence rate is 6% or more, and the white rust occurrence rate is 31% or more. The necessary parts could not withstand long-term use.

[0056]

As described above, the present invention relates to plating of a molten Al-Zn alloy-plated steel sheet which becomes an original sheet by containing an appropriate amount of Sr according to the Si content in the plating layer of the Al-Zn alloy plating. Since the layer is softened, the peeling resistance applied thereon is improved, and the workability and corrosion resistance of the coated steel sheet are improved. Further, when the Sr concentration in the interdendrite portion is Al-Zn
When the average Sr concentration of the alloy plating layer is 20 to 150 times the average Sr concentration, the above characteristics can be further improved. This makes the coated hot-dip Al-Zn alloy-plated steel sheet of the present invention very suitable as a pre-coated steel sheet for building materials and home appliances.

[Brief description of the drawings]

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

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

FIG. 3 is an enlarged scanning electron microscope structure photograph of FIG. 2;

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

5 is an enlarged scanning electron microscope structure photograph 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 2/26 C23C 2/26 28/00 28/00 C (72) Inventor Hideo Takamura Chuo, Chiba, Chiba 1025 Hamano-cho, Tokyo F-Terminator in Steel Products Co., Ltd. F-term (reference) 4D075 AE03 BB87X CA13 CA33 DA06 DB05 DB07 DC01 DC10 DC18 EA07 EA41 EB15 EB16 EB22 EB32 EB33 EB35 EB38 EB43 EB45 4F100 AA22B ABB ABB ABB ABB ABB ABB ABB BA07 BA10A BA10C EJ68B GB07 GB48 JB02 JL01 YY00B 4K027 AA05 AA22 AB02 AB28 AB33 AB44 AB48 AC32 AC72 AC82 AC87 AE03 4K044 AA02 AB02 BA01 BA02 BA10 BA19 BB03 BB09 BC02 BC03 BC05 CA11 CA16 CA64 CA67

Claims (5)

[Claims] 1. A steel sheet having a dendrite portion, an interdendrite portion existing between the dendrite portions, and an Al—Zn alloy plating layer comprising an interface alloy layer existing at the interface between the dendrite portion and the steel plate ground iron on the front and rear surfaces of the steel sheet. , One or both of the front and back of the steel sheet, as an upper layer of the Al-Zn alloy plating layer, a chemical conversion treatment layer, or a coating molten Al-Zn alloy plating having an organic coating layer further through a primer layer A steel sheet, wherein the Al-Zn alloy plating layer
25-75%, Si is more than 1% and 5% or less, and Sr is 0.2% of Si content.
A coated hot-dip Al-Zn alloy-plated steel sheet having excellent workability and corrosion resistance, characterized in that it is contained in the range of up to 2%. 2. The Al—Zn alloy plating layer according to claim 1, wherein the Sr concentration in the interdendrite portion is 20 to 150 times the average Sr concentration of the Al—Zn alloy plating layer.
2. A coated hot-dip Al-Zn alloy-plated steel sheet according to item 1. 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.
3. The composition according to claim 1, wherein the content is from 01 to 2.0%.
2. A coated hot-dip Al-Zn alloy-plated steel sheet according to item 1. 4. The method according to claim 3, wherein the number of interfacial alloy layer particles having a major axis of 5 μm or more and present in the uppermost layer of the interfacial alloy layer of the Al—Zn alloy plating layer is 1500 particles / mm ² or less.
2. A coated hot-dip Al-Zn alloy-plated steel sheet according to item 1. 5. A method according to claim 1, wherein the hardness of the α-Al phase of the dendrite portion is Hv120 or less.
The coated hot-dip Al-Zn alloy-plated steel sheet according to any one of the above.