JP3144343B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention can improve continuous hitting by suppressing wear of an electrode tip during spot welding and improving durability, and furthermore, at the time of continuous hitting, The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in spot weldability and capable of suppressing occurrence of a phenomenon (spattering) of molten metal during welding, and a method for producing the same.

[0002]

2. Description of the Related Art As is well known, a vehicle body is assembled by performing resistance welding (particularly spot welding) to panels formed into various shapes by press forming.
In this spot welding, since a large number of spot weldings are continuously performed, it is required to increase the number of continuous spots as much as possible in order to improve the productivity.

[0003] In order to improve the continuous hitting property of spot welding, it is very important how to suppress or prevent the deterioration of the surface appearance due to the adhesion of the molten metal due to the wear and scattering of the electrode tip.

[0004] In recent years, from the viewpoint of improving rust prevention, surface-treated steel sheets, particularly alloyed hot-dip galvanized steel sheets obtained by alloying a galvanized layer and a base steel by performing a heat diffusion treatment after hot-dip galvanizing, It is actively applied to automotive body panels. However, this alloyed hot-dip galvanized steel sheet has a lower melting point than that of the base steel sheet, zinc, which is the main component of the galvanized layer, and wear of the electrode tip due to diffusion of zinc into the electrode tip, which is mainly copper. Therefore, it has been regarded as a problem that the continuous hitting property at the time of spot welding is reduced.

[0005] In order to improve the continuous hitting property of an alloyed hot-dip galvanized steel sheet, for example, JP-A-59-104463 discloses that
There has been proposed an invention in which a Zn oxide film, which is a semiconductor, is formed on a surface layer of a galvannealed steel sheet.

On the other hand, in an alloyed hot-dip galvanized steel sheet, in order to improve the adhesion between the coating layer and the base steel sheet, hot-dip galvanized steel sheet is used.
In order to contain about 0.08 to 0.17% by weight of Al in the bath (hereinafter, "%" means "% by weight" unless otherwise specified in this specification), the formed alloy Generally, Al ₂ O
₃ (Alumina) remains. However, since Al ₂ O ₃ is an insulator, heat generation on the electrode tip surface during spot welding becomes excessive. Therefore, there is also a problem that a brittle alloy layer is formed thick on the electrode chip surface, chip wear becomes remarkable, and continuous hitting property is reduced.

From the viewpoint of such weldability, Japanese Patent Laid-Open No.
Japanese Patent No. 21750 discloses an invention in which the amount of Al ₂ O ₃ is reduced by limiting the amount of Al in the alloyed hot-dip galvanized layer to improve the continuous hitting property. Inventions have been proposed in which the combination of the above-mentioned restriction and the formation of the above-mentioned Zn oxide film improves the continuous hitting property.

[0008]

However, even with the inventions according to these proposals, it is not possible to improve the continuous hitting property during spot welding of the galvannealed steel sheet.

In other words, the invention proposed in Japanese Patent Application Laid-Open No. 59-104463 discloses a method of forming a Zn oxide film, which is a semiconductor, on the surface of an alloyed hot-dip galvanized layer. An object of the present invention is to suppress the Cu-Zn reaction between the metal Zn existing on the surface of the zinc plating layer and the electrode tip made of Cu. However, since the conductivity of the Zn oxide film varies greatly depending on the state of existence, the heat value generated on the electrode tip surface during spot welding greatly varies. Therefore, there is a problem that spot weldability varies. In addition, due to the pressurization and energization during spot welding, the Zn oxide film was easily broken,
The inability to reliably suppress the Cu-Zn reaction is also considered to be one factor that causes the spot weldability to vary. That is, oxidation
Since the conductivity of the Zn film varies greatly depending on the state of its existence, even if a zinc oxide film is formed on the surface layer of the galvannealed layer, the continuous hitting property changes significantly, and it is difficult to reliably improve the continuous hitting property.

[0010] Japanese Patent Application Laid-Open Nos. Hei 4-21750 and Hei 6-73
The invention proposed by Japanese Patent No. 521 limits the amount of Al in the galvannealed layer, and does not completely eliminate Al ₂ O ₃ as an insulator from the surface layer of the galvannealed layer. . Therefore, the heat generated in the surface layer of the electrode chip becomes excessive due to Al ₂ O ₃ remaining on the surface layer of the alloyed hot-dip galvanized layer, thereby promoting the destruction of the Zn oxide layer and the diffusion of metal Zn into the electrode chip. Therefore, even the inventions according to these proposals have a small and insufficient effect of improving the continuous hitting property.

[0011] It is an object of the present invention to improve the durability by suppressing the wear of the electrode tip during spot welding, thereby improving the continuous hitting point and preventing the occurrence of scattering. An object of the present invention is to provide a galvannealed steel sheet having excellent spot weldability and a method for producing the same.

[0012]

The present inventors Means for Solving the Problems] is a major factor affecting the continuous dotting property, Zn oxide and Al ₂ O
We thought that there might be other factors other than ₃ , and we studied diligently. As a result, the present inventors have found that the amount of metal Zn remaining in the outermost layer of the galvannealed layer has a great effect on the continuous hitting property.

In general, in alloyed hot-dip galvanizing, the Al content in the plating bath is controlled to be relatively high near the upper limit, and first, the base steel sheet reacts preferentially with the Al in the bath, so that the base metal reacts preferentially. An Fe-Al alloy layer is formed on the surface of the material steel sheet, and an excessive reaction between the steel sheet and the molten zinc is prevented. It is considered that since the formed Fe-Al alloy layer remains even after the alloying treatment by heating, good plating adhesion can be ensured.

[0014] In the alloying process, the Fe-Al alloy layer acts as a barrier film of iron thermally diffusing from the steel sheet, and the starting point of iron diffusion from the steel sheet is considered to be not uniform but dispersed. Therefore, it seems that the alloying process is completed and there is no unreacted metal Zn,
In a portion of the outermost layer of the plating layer where the diffusion of iron is delayed, generally, metal Zn (η phase) remains.

[0015] In the galvannealing, a plating bath containing Al is used.
Changes to Al ₂ O ₃ by a subsequent alloying treatment. Also, since Al inside the plating layer is concentrated on the surface by the alloying treatment, the outermost layer of the plating layer generally contains Al ₂ O ₃
There are also quite a few.

Therefore, in order to suppress both the heat generation on the electrode chip surface and the diffusion of the metal Zn to the electrode chip to improve the continuous hitting property, the metal existing on the outermost layer of the plating layer is required.
It is effective to reduce and remove Zn (η phase) and Al ₂ O ₃ .

Here, the metal Z existing in the outermost layer of the plating layer
In order to reduce and remove both n and Al ₂ O ₃ , both Zn and Al are amphoteric metals and can be easily dissolved by an alkali solution or an acid solution. Therefore, in order to reduce the metal Zn and Al ₂ O ₃ present in the outermost layer of the plating layer, an alkali treatment, an acid treatment, or a combination thereof may be performed after the normal alloying treatment is completed.

If an acid treatment is performed after the alkali treatment as a combined treatment of the alkali treatment and the acid treatment, Al ₂ O ₃ and Zn oxide are dissolved by the alkali treatment, and the metal Zn is dissolved by the acid treatment to appropriately oxidize. Zn can be formed, and heat generation in the electrode chip can be further suppressed, which is more effective. The present inventors have further studied based on these findings and completed the present invention.

Here, the gist of the present invention is that Fe
Content: 8-16% and metal Zn in the outermost layer
This is an alloyed hot-dip galvanized steel sheet having excellent spot weldability, characterized by having a plating layer having an amount of 50% or less and an Al ₂ O ₃ amount of 15% or less.

Another aspect of the present invention is to provide an alloyed hot-dip galvanized steel sheet having a Fe content of 8 to 16% by immersing a base steel sheet in a hot-dip galvanizing bath and then performing an alloying treatment by heating. An alloy with excellent spot weldability, characterized in that after forming the layer, the metal Zn in the outermost layer of the galvannealed layer is reduced to 50% or less and Al ₂ O ₃ to 15% or less, respectively. This is a method for producing a galvannealed steel sheet.

In the method for producing an alloyed hot-dip galvanized steel sheet having excellent spot weldability according to the present invention, the metal Zn
And reducing the Al ₂ O ₃ by simply contacting the outermost layer with an alkaline solution having a pH of 9.0 or more and / or by contacting an acid solution having a pH of 3.0 or less with a simple and easy method. Effective and desirable.

In the method for producing an alloyed hot-dip galvanized steel sheet having excellent spot weldability according to the present invention, contacting with an alkali solution and then with an acid solution further suppresses heat generation on the surface of the electrode tip. It is desirable to prevent the occurrence of scattering.

The “outermost layer” in the present invention is:
Among the alloyed hot-dip galvanized layers, metal Zn or Al ₂ O ₃
Means a range in which the continuous hitting property during spot welding is reduced by the presence of, for example, a range of 500 mm from the surface of the plating layer in the depth direction.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of an alloyed hot-dip galvanized steel sheet having excellent spot weldability and a method for producing the same according to the present invention will be described in detail.

The alloyed hot-dip galvanized steel sheet having excellent spot weldability according to the present embodiment comprises (I) an amount of metal Zn in the outermost layer of the plating layer: 50% or less, and (II) an Fe content of the plating layer: 8 to 16%.
%, (III) Al ₂ O ₃ content in the outermost surface of the plating layer: 15% or less. Hereinafter, these characteristics will be described separately.

I. Metal Zn content in outermost layer of plating layer: 50%
First, the mechanism by which the metal Zn present in the outermost layer of the galvannealed steel sheet reduces the continuous hitting property will be described.

FIGS. 1 (a) and 1 (b) show an electrode tip made of a Cu alloy after continuous spotting in a case where a continuous spot test for performing continuous spot welding was performed on a galvannealed steel sheet. FIG. 3 is an explanatory view schematically showing a state of formation of an alloy layer on the surface of No. 1.

As shown in FIG. 1A, a β-brass layer 2a, a γ-brass layer 2b formed by diffusion of Zn into the electrode chip 1 and an alloy It has a three-layer structure of the Zn—Fe alloy layer 3 brought in from the hot-dip galvanized layer.

In the continuous spot test, the Zn—Fe alloy layer 3 has an extremely high conductivity as compared with the brass layer 2, so that it is difficult for the Zn—Fe alloy layer 3 to become an energizing path during spot welding. Therefore, electricity flows through the brass layer 2, so that Zn
It is effective not to form the Fe alloy layer 3 as much as possible in order to improve the continuous hitting property.

On the other hand, the Zn—Fe alloy layer 3 is generally very hard and brittle.
As shown in (b), it is mechanically dropped by pressurization,
The γ-brass layer 2b tends to be exposed on the surface of the electrode tip 1. Therefore, the growth of the Zn—Fe alloy layer 3 is suppressed,
In addition, it is effective to improve the continuous hitting point by making it mechanically worn and easily dropped.

At this time, if metal Zn is present in the outermost layer of the galvannealed steel sheet, as shown in FIG. 1 (a), γ is generated at the beginning of the continuous striking point where the Zn—Fe alloy layer 3 is formed. −
On the surface of the brass layer 2b, a Zn-Fe alloy layer 3a having a low Fe content (Fe
Content: about 10%, Hardness: about 150Hv 5g) is formed. Thus, in the alloyed hot-dip galvanized steel sheet in which metal Zn is present on the outermost layer of the plating layer, the Zn-Fe alloy layer 3a having a low Fe content is formed on the γ-brass layer 2b, and the Zn-Fe alloy layer On 3a,
Normal high Fe content Zn-Fe alloy layer 3b (Fe content: about 25
%, Hardness: about 300Hv 5g) is formed.

Since the Zn-Fe alloy layer 3a having a low Fe content is flexible, the Zn-Fe alloy layer 3b having a high Fe content formed at the subsequent continuous striking becomes difficult to be mechanically dropped. For this reason, the Zn—Fe alloy layer 3 that hardly becomes an energizing path during welding is uniformly present over the surface of the electrode tip 1, and it is difficult to secure an energizing path in a continuous spot test, and the continuous spot property is significantly reduced. . Therefore, it is necessary to reduce the metal Zn (η phase) present in the outermost layer of the galvannealed steel sheet and not to form the Zn-Fe alloy layer 3a having a low Fe content in order to improve the continuous hitting property. It is valid.

Next, the effect of the amount of metal Zn in the outermost layer of the plating layer on continuous spotting during spot welding was examined by the following method. Generally, an X-ray diffraction method or the like is effective for measuring the η phase. However, it is difficult to measure the amount of metal Zn in the outermost layer of the plating layer by the X-ray diffraction method.
An XPS method capable of analyzing the outermost layer of the plating layer was used.

That is, 50 mm from the plating layer surface in the depth direction.
For each sample in which the amount of metal Zn was changed in the range of 0 °, the continuous hitting property was evaluated. Here, the measurement conditions of the XPS method are summarized in Table 1. In addition, in the method of quantifying metal Zn, the amount of metal Zn was calculated from the peak area of the metal Zn, with the abundance being weight%. In addition, in order to measure the thickness of the metal Zn, Ar ion sputtering was performed, from 0 to 40 seconds (up to 500 ° in the depth direction), and metal Zn at the time of each sputtering when measuring at a pitch of 5 seconds.
The abundance was integrated and the average value of the abundance (average metal Zn abundance) was used.

[0035]

[Table 1]

As a result of this confirmation experiment, it is preferable that the amount of metal Zn in the range of 500 ° in the depth direction from the surface of the plating layer be as small as possible in order to obtain good continuous spotting property. It is 50% or less, preferably 30% or less. If the amount of metal Zn in the outermost layer of the plating layer exceeds 50%, the Zn-Fe alloy layer 3a is formed on the surface of the electrode chip 1, and it is not possible to secure sufficient continuous spotting properties.

If the amount of metallic Zn in the outermost layer of the plating layer is 50% or less, the Zn—Fe alloy layer 3a is not formed on the surface of the γ-brass layer 2b as shown in FIG. -The Fe alloy layer 3b is directly formed. The Zn-Fe alloy layer 3b is easily worn and peeled off by the pressure of the electrode tip 1 during spot welding, so that the γ-brass layer 2b is exposed on the surface of the electrode tip 1. Therefore, continuous hitting property is ensured.

II. Fe content (degree of alloying) of the plating layer: 8
1616% The amount of metal Zn is also greatly affected by the Fe content of the alloyed plating layer. If the Fe content of the plating layer is less than 8%, the alloying treatment is not completed up to the surface layer of the plating layer, metal Zn (η phase) remains thick on the surface layer, and it is sufficient even if metal Zn removal treatment described later is performed. Metal Zn cannot be removed. On the other hand, when the Fe content of the plating layer is more than 16%, the amount of metal Zn remaining on the surface of the plating layer is reduced to some extent, but a powdering phenomenon in which the plating layer peels off during press working occurs. Therefore, in the present invention, the Fe content of the plating layer of the galvannealed steel sheet is limited to 8% or more and 16% or less. From the same viewpoint, it is more preferably 9% or more and 13% or less.

III. Al ₂ O ₃ content in the outermost layer of the plating layer: 15% or less Since Al ₂ O ₃ present in the outermost layer of the plating layer is an insulator,
In the portion where Al ₂ O ₃ is present, it cannot be a current-carrying path during welding. For this reason, the heat generation on the surface of the electrode chip becomes excessive due to high resistance, which is not preferable because the growth of the Zn—Fe alloy layer 3a is promoted. In addition, in the portion where Al ₂ O ₃ is present in the outermost layer of the plating layer, the calorific value is excessive, so that spatter occurs between the steel plate and the electrode tip, causing scattering. When this scattering occurs, the molten metal adheres to the surface of the steel sheet, which may impair the surface appearance, necessitating a surface care operation, and lowering productivity. Therefore, it is also effective to remove Al ₂ O ₃ (Al oxide) existing in the outermost layer of the plating layer in order to secure the number of continuous dots.

The amount of Al ₂ O _{3 in the} outermost layer of the plating layer was determined by the XP
It was evaluated by the S method based on the average amount of Al ₂ O ₃ present up to 500 mm in the depth direction from the plating layer surface. As a result, depth 500
If the amount of Al ₂ O ₃ in the outermost layer of the plating layer up to Å is more than 15%, the amount of heat generated on the surface of the electrode chip is large, the continuous hitting property is reduced, and spatter is frequently generated and scattering is likely to occur. Therefore, in the present invention, the Al ₂ O ₃ content in the outermost layer of the plating layer is limited to 15% or less. From the same viewpoint, the content is preferably 10% or less.

The other components of the galvannealed steel sheet having excellent spot weldability according to the present embodiment are the same as those of a normal galvannealed steel sheet, so that further description is omitted. Next, metal Zn in the outermost layer of the plating layer
A method for producing an alloyed hot-dip galvanized steel sheet by reducing the amount and Al ₂ O ₃ amount to the above ranges will be described.

Since Zn and Al are both amphoteric metals,
Dissolves in both acid and alkaline solutions. Zn oxide,
An alkali solution is effective for dissolving an oxide film such as Al ₂ O _3, and an acid solution is effective for dissolving metal Zn and metal Al while leaving the oxide film. Instead, any method may be used. The amount of metal Zn in the outermost layer of the plating layer may be reduced to 50% or less, and the amount of Al ₂ O ₃ may be reduced to 15% or less.

When an alkaline solution is used, its pH is 9.0
That is all. When the pH is lower than 9.0, metal Zn,
This is because Al ₂ O ₃ cannot be removed. From the same viewpoint, the pH is preferably 11.0 or more.

It is not necessary to limit the type of such an alkaline solution to a specific type, but it may be an alkaline solution that does not remain on the surface of the galvannealed steel sheet through a washing and drying process immediately after the alkali treatment. preferable. Such an alkali solution is preferably an alkali metal hydroxide solution such as sodium hydroxide and potassium hydroxide.

On the other hand, when an acid solution is used, its pH is 3.0
It is as follows. When the pH is higher than 3.0, metal Zn,
This is because Al ₂ O ₃ cannot be removed. From the same viewpoint, the pH is preferably 2.0 or less.

Such an acid solution need not be limited to a specific type, but an oxidizing acid solution such as a nitric acid solution oxidizes the surface of the galvannealed steel sheet and causes excessive oxidation.
It is not preferable because a Zn layer is easily formed. The pH is 3.
Below 0, it is also possible to use an electroplating solution containing a metal ion based on a hydrochloric acid solution or a sulfuric acid solution.

As the alkali treatment method and the acid treatment method, there are an immersion method, a spray method, and the like.
Any method can be used as long as it can reduce ₂ O _3, and there is no need to limit to a specific method. However, in order to effectively remove metal Zn and Al ₂ O ₃ from the viewpoint of productivity, it is necessary to consider the effect of increasing the solution temperature and the effect of mechanical removal by applying stress. It is preferable to perform a spray treatment.

Further, in any of the alkali treatment and the acid treatment, in order to prevent surface contamination due to the remaining treatment liquid,
It is preferable to perform washing and drying immediately after the treatment in order to secure the stability of the surface state.

Further, by combining the alkali treatment and the acid treatment, the spot weldability can be further improved, which is desirable. That is, by performing the acid treatment after performing the alkali treatment, it is possible to remove the metal Zn after once removing Zn oxide and Al ₂ O ₃ by the alkali treatment, the metal Zn removal efficiency is dramatically improved, Processing time can be reduced. In addition, since acid treatment is performed lastly, a Zn oxide layer can be formed on the surface of the plating layer to a certain degree in terms of the effect on weldability, which is desirable.

As described above, according to the alloyed hot-dip galvanized steel sheet of the present embodiment having excellent spot weldability, the durability of the electrode tip during spot welding is suppressed and the durability is improved. Performance can be improved,
Further, the occurrence of scattering can be prevented.

[0051]

Next, the effect of the galvannealed steel sheet having excellent spot weldability according to the present invention will be described with reference to experimental data.

(First Example) Cold rolled steel sheets (sheet thickness: 0.8 mm) having the chemical component values shown in Table 2 were subjected to the test.

[0053]

[Table 2]

Based on the cold-rolled steel sheet, the plating layer Fe
The content (degree of alloying) should be 7%, 8%, 11%, 13%, 14%.
19 alloyed hot-dip galvanized steel sheets were manufactured at seven levels of 17%, 16% and 17%.

These samples were subjected to alkali treatment under the treatment conditions shown in Table 3 and then to water washing, or
After performing the acid treatment under the treatment conditions shown in Table 3, the substrate was washed with water, or after performing the alkali treatment under the treatment conditions shown in Table 3 and then washed with water, immediately subjected to the acid treatment under the treatment conditions shown in Table 4 and washed with water. The amount of metal Zn in the outermost layer of the plating layer, Al ₂ O
_In order to clarify the effect of the _three quantities, the treatment time of each of the alkali treatment and the acid treatment was changed.

[0056]

[Table 3]

[0057]

[Table 4]

The amount of metal Zn in the surface layer of the plating layer, Al ₂
The O ₃ amount was measured by the XPS method showing measurement conditions according to Table 1 described above. In a state where two of these samples were superimposed, spot welding was continuously performed up to 5000 points under the conditions shown in Table 5, and the nugget diameter was measured at every 100 points to evaluate the continuous spotting property in spot welding.

The evaluation criteria were as follows: the number of continuous hits until the nugget formation could not be measured was indicated by a circle as a target value indicating that the number of continuous hits could be secured to 3,000 or more, and the excellent value was evaluated to be able to secure 5,000 hits or more. Indicated by Also,
Less than 3000 RBIs are indicated by crosses.

Further, with respect to these samples, the phenomenon of occurrence of scattering was simultaneously evaluated. The evaluation criterion was to measure the rate of occurrence of spatter by counting the ratio of spatter (spatter), and
Is less than the target value, indicated by ○, and less than 1% is indicated by ◎, which is an excellent value. Further, 3% or more is indicated by a mark x.

Further, in order to evaluate the workability required for an automobile body panel, the powdering property by a cylindrical drawing process was evaluated. The processing conditions are summarized in Table 6. The evaluation criteria for powdering properties are as follows: tape is applied to the side wall of the overhang formed by performing cylindrical drawing, and then peeled off.
The peeling amount was measured by a gravimetric method. As the evaluation criteria at that time, a mark of 20 mg / piece or less per sample as a target value was indicated by a circle, and a value of 10 mg / piece or less per sample was indicated by a circle as an excellent value. In addition, more than 20 mg / unit was indicated by a mark x.

[0062]

[Table 5]

Table 7 summarizes the test results on the number of continuous hits, the rate of scattering, and the powdering. In Table 7, Sample Nos. 1 to 11 are samples of the present invention,
Sample Nos. 12 to 19 are comparative samples.

[0064]

[Table 7]

In each of Sample Nos. 1 to 11, if the number of continuous hits, the sputtering rate and the powdering exceeded the target values, and the Fe content of the plating layer was 8 to 16%, the continuous It can be seen that an alloyed hot-dip galvanized steel sheet excellent in spot weldability, suitable for use in automobile body panels, with improved hitting properties and prevention of scattering was obtained. Also, from Table 7, if the amount of metal Zn and the amount of Al ₂ O _{3 in the} surface layer of the plating layer are within the range of the present invention, spot weldability (continuous spotting property)
It can be seen that can be improved.

Further, by comparing sample Nos. 2, 3, 4, and 5 with sample Nos. 6 and 7, only the alkali treatment and only the acid treatment are performed to reduce the metal Zn and Al ₂ O _3. When compared to the case,
Zn metal by performing an acid treatment after the alkali treatment, by performing the reduction of the Al ₂ O _3, metallic Zn in efficiently plated layer outermost layer as compared with the processing time, it can reduce the Al ₂ O ₃ Understand.

On the other hand, in sample No. 12, the plating layer
Since the Fe content was lower than the lower limit of the range of the present invention, both the amount of metal Zn and the amount of Al ₂ O ₃ could not be within the range of the present invention, and the continuous hitting property was reduced.

Sample No. 13, Sample No. 14, Sample No. 17, Sample No. 1
In No. 8, since the alkali treatment or the acid treatment was not performed after the galvannealing, neither the metal Zn amount nor the Al ₂ O ₃ amount could be within the range of the present invention, and the continuous hitting property was reduced. .

In sample No. 15, although the alkali treatment was performed after the galvannealing, the amount of metallic Zn could not be within the range of the present invention because the treatment time was short.
In sample No. 16, although the acid treatment was performed after the galvannealing, the treatment time was short, so that the amount of Al ₂ O ₃ could not be within the range of the present invention, and the continuous hitting property was reduced. .

In Sample No. 19, since the Fe content of the plating layer exceeded the upper limit of the range of the present invention, powdering occurred, and it was found that the powder was not suitable for use in automobile body panels.

(Second Embodiment) In order to clarify the treatment conditions of the alkali treatment and the acid treatment, the effect of the pH of each treatment solution was examined. As the base material in this example, a cold-rolled steel sheet having a chemical component value shown in Table 2 and a thickness of 0.8 mm was used, and the basis weight was 43 g / m ² ,
A plating layer having a Fe content of 10% was formed.

For alkali treatment, potassium hydroxide was used under the treatment conditions shown in Table 8, and for acid treatment, hydrochloric acid was used under the treatment conditions shown in Table 9, and treatment solutions adjusted to various pHs were used. The treatment time was 5 seconds each for immersion and drying. When examining the effect of the alkali treatment solution, the acid treatment pH after alkali treatment was 1.5,
The immersion time was 5 seconds, and the effect of the acid treatment solution was examined to determine the alkali treatment pH before acid treatment = 12.5 and the immersion time = 5se.
c), and thereafter, a continuous spot test was performed under the same conditions as in the first example.

[0073]

[Table 8]

[0074]

[Table 9]

FIG. 2 and FIG. 3 are graphs showing the results of the continuous hitting test. FIG. 2 shows the effect of the pH of the alkaline treatment liquid on the number of continuous dots, and FIG. 3 shows the effect of the pH of the acid treatment liquid on the number of continuous dots.

From the graph shown in FIG.
It can be seen that the pH is 9 or higher, preferably 11 or higher, while the graph shown in FIG. 3 indicates that the pH during acid treatment is 3 or lower, preferably 2 or lower.

(Third Embodiment) Further, the superiority of the treatment combining the alkali treatment and the acid treatment was examined. The base material has a thickness of 0.8 mm with the chemical composition values shown in Table 2.
The plated layer having a basis weight of 43 g / m ² and an Fe content in the outermost layer of the plated layer of 10% was formed on the basis of the cold-rolled steel sheet.

For such an alloyed hot-dip galvanized steel sheet, the alkali treatment shown in Table 10 and the treatment conditions are shown in Table 10.
One or both of the acid treatments (sulfuric acid-based electroplating solution) indicating the treatment conditions were performed. By changing the immersion time during these treatments, the metal Zn on the plating layer surface layer is changed.
And the amount of Al ₂ O ₃ were varied. As described above, for a sample that has been subjected to one or both of the alkali treatment and the acid treatment,
In the same manner as in the first example, continuous hitting property and scattering rate were investigated.

[0079]

[Table 10]

Table 12 summarizes the test results on the number of continuous hits and the sputtering rate. In Table 12, sample No. 1
-Sample No. 9 is a sample of the present invention, and Sample No. 10-Sample No.
No. 12 is a sample of a comparative example. The evaluation criteria are the same as in the first embodiment.

[0081]

[Table 12]

Samples No. 1 to No. 9 all had the number of continuous spots and the rate of occurrence of scattering exceeding the target values, and had improved continuous spottability. It can be seen that an excellent galvannealed steel sheet was obtained.

Further, in order to stably reduce the amount of metal Zn and the amount of Al ₂ O ₃ , when only alkali treatment is performed (sample No. 6, sample No. 7), when only acid treatment is performed (sample no. .8, Sample No.
9) When alkali treatment is performed after acid treatment (Sample No. 4,
Sample No. 5) is insufficient. In contrast,
When acid treatment is performed after alkali treatment (Sample No. 1 to Sample N
o.3) proves to be very effective.

[0084]

As described above in detail, according to the present invention, there is provided an alloyed hot-dip galvanized steel sheet having good spot weldability within a range in which practical workability does not pose a problem, for example, for an automobile body panel. It became possible to do. ADVANTAGE OF THE INVENTION According to the alloyed hot-dip galvanized steel sheet which concerns on this invention, the continuous spotting property at the time of spot welding can be improved remarkably, and the phenomena which generate | occur | produce can also be improved.

The galvannealed steel sheet according to the present invention can be performed in a very simple step of performing one or both of an alkali treatment and an acid treatment after the galvannealed steel sheet. By performing the acid treatment, the treatment time can be shortened and the production can be performed extremely efficiently.

Therefore, according to the present invention, in the automobile body assembling process, by increasing the number of continuous spots at the time of welding, the frequency of maintenance of the electrode tips can be reduced, and the frequency of occurrence of the scatter phenomenon can be reduced. Troubles can be avoided and productivity can be dramatically improved. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 (a) and FIG. 1 (b) show a Cu alloy electrode after continuous spotting in a case where a continuous spot test for performing continuous spot welding was performed on an alloyed hot-dip galvanized steel sheet. It is explanatory drawing which shows typically the formation state of the alloy layer on the surface of a chip | tip.

FIG. 2 is a graph showing the effect of the pH of an alkaline treatment liquid on the number of continuous shots in a second example.

FIG. 3 is a graph showing the effect of the pH of an acid treatment liquid on the number of continuous shots in a second example.

[Description of sign]

 1 electrode tip 2a β-brass layer 2b γ-brass layer 3a Zn-Fe alloy layer (low Fe content) 3b Zn-Fe alloy layer (high Fe content)

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (4)

(57) [Claims] 1. A Fe content: Metal Zn amount in the outermost layer with 8 to 16 wt%: 50 wt% or less, Al ₂ O ₃ amount: 15
An alloyed hot-dip galvanized steel sheet having excellent spot weldability, characterized by having a coating layer of not more than% by weight. 2. After immersing the base material steel sheet in a hot-dip galvanizing bath, it is heated and alloyed to form an alloyed hot-dip galvanized layer having an Fe content of 8 to 16% by weight. 50% by weight of metal Zn in the outermost layer of the galvannealed layer
A method for producing an alloyed hot-dip galvanized steel sheet having excellent spot weldability, characterized in that Al ₂ O ₃ is reduced to 15% by weight or less, respectively. _3. The reduction of the metal Zn and the Al ₂ O ₃ is as follows:
3. The spot weldability according to claim 2, wherein the outermost layer is formed by contacting the outermost layer with an alkaline solution having a pH of 9.0 or more and / or by contacting the outermost layer with an acid solution having a pH of 3.0 or less. Manufacturing method of galvannealed steel sheet. 4. The method for producing a galvannealed steel sheet having excellent spot weldability according to claim 3, wherein the outermost layer is brought into contact with the acid solution after being brought into contact with the alkaline solution.