JP3144352B2 - Spot weldable galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention can improve continuous hitting by suppressing the wear of an electrode tip during spot welding and improving the durability of the electrode tip.
Furthermore, spot welding that can improve continuous spotting by preventing development (spattering) in which molten metal scatters from the steel sheet at the time of continuous spotting and preventing the welding phenomenon (sticking) between the steel sheet and the electrode tip. The present invention relates to an excellent galvannealed steel sheet and a method for producing the same.

[0002]

2. Description of the Related Art As is well known, an automobile body is assembled by performing resistance welding (particularly spot welding) to panels pressed into various shapes and joining them. In this spot welding, it is required to increase the number of continuous hit points as much as possible in order to improve productivity.

[0003] In order to improve the continuous point of spot welding, it is important how to suppress damage to the electrode tip. Furthermore, if a sticking phenomenon occurs during welding, the electrode tips fall off and welding cannot be performed. Therefore, it is necessary to perform surface cleaning of the electrode tips. In addition, when the spatter occurs, the molten metal adheres to the steel plate, so that it is necessary to perform care of the body surface. Therefore, it is important to suppress the sticking phenomenon and the sputtering phenomenon as much as possible in terms of improving productivity.

[0004] In recent years, from the viewpoint of improving rust prevention properties, surface-treated steel sheets, particularly alloyed hot-dip galvanized steel sheets, in which a hot-dip galvanized steel sheet is subjected to a heat diffusion treatment to alloy a zinc-plated layer and a base steel, have been developed. And has been actively applied to panels for automobile bodies. However, this galvannealed steel sheet
Since the melting point of zinc, which is the main component of the plating layer, is low, zinc diffuses into the electrode tip containing copper as the main component during spot welding, resulting in severe wear of the electrode tip and a low number of continuous spots during spot welding. It has been regarded as a problem.

Accordingly, in order to improve the continuous hitting property of a galvannealed steel sheet, for example, Japanese Patent Application Laid-Open No.
Japanese Patent No. 104463 proposes an invention in which a Zn oxide film as a semiconductor is formed on the outermost layer of a galvannealed steel sheet.

On the other hand, in an alloyed hot-dip galvanized steel sheet, 0.08 to 0.17% by weight of Al (hereinafter, referred to as “the present invention”) is added to the hot-dip galvanizing bath in order to improve the adhesion between the coating layer and the base steel sheet. Unless otherwise specified, “%” means “% by weight”), so that the surface of the formed galvannealed layer is generally made of 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 a problem that a brittle alloy layer is formed thickly on the surface of the electrode tip, the electrode tip is significantly worn, and the continuous hitting property is reduced.

From the viewpoint of such weldability, Japanese Patent Laid-Open No.
No. 21750 discloses that the amount of Al ₂ O ₃ is reduced as much as possible by limiting the amount of Al in the galvannealed layer,
The invention aimed at improving the continuous hitting property is disclosed in JP-A-6-73.
Japanese Patent Application Publication No. 521 proposes inventions in which the limitation of the amount of Al is combined with the above-described formation of the Zn oxide film to improve 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.

That is, the invention proposed in Japanese Patent Application Laid-Open No. 59-104463 discloses a method of forming a Zn oxide film as a semiconductor on the surface of an alloyed hot-dip galvanized layer, and using the barrier effect of the Zn oxide film to form a molten alloy. An object of the present invention is to suppress a Cu-Zn alloying reaction between a metal Zn existing on the surface of a galvanized layer and an electrode made of Cu. However, the conductivity of the Zn oxide film varies greatly depending on the state of its existence, and in some cases, the amount of heat generated on the electrode tip surface during spot welding varies greatly. Therefore, the pressurization and energization at the time of spot welding, the Zn oxide film is easily destroyed, and it is not possible to reliably suppress the Cu-Zn alloying reaction described above, which also contributes to the variation in spot weldability. Conceivable.

That is, since the conductivity of the Zn oxide film varies greatly depending on the state of its existence, even if a zinc oxide film is formed on the surface of the alloyed hot-dip galvanized layer, the continuous spotting property changes significantly, and the continuous spotting property is ensured. It is difficult to improve.

Further, the inventions proposed in Japanese Patent Application Laid-Open Nos. Hei 4-21750 and Hei 6-73521 limit the amount of Al in the galvannealed layer. It is trying to eliminate Al ₂ O ₃ as an insulator from the surface layer of the galvannealed layer as much as possible. However, even the inventions according to these proposals have a small and insufficient effect of improving the continuous hitting property.

[0012] Here, an object of the present invention is to suppress the occurrence of spatter and sticking phenomena during spot welding, thereby suppressing the wear of the electrode tip and improving the durability, thereby improving the continuous hitting point. An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which can be improved in spot weldability and a method for producing the same.

[0013]

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 alloyed hot-dip galvanized layer has a large effect on the continuous hitting property.

Further, by performing the acid or alkali treatment in the present invention, after the surface is cleaned, welding is performed.
(Sticking) It has also been found that leaving a small amount of Al ₂ O ₃ in the surface layer is effective for sticking in order to easily cause the phenomenon.

[0015] That is, in order to suppress the electrode tip welded (sticking), it is necessary to leave some extent Al ₂ O ₃ traces, also excessively when Al ₂ O ₃ was remaining, electrode The heat generation on the surface becomes excessive and the Zn oxide layer is destroyed.
_It was found that if ₂ O ₃ was present, the diffusion of metal Zn to the electrode tip was promoted, so that there was a problem that the weldability was significantly deteriorated.

In general, galvannealed steel sheets are
It contains Al in the plating bath, and when the base steel sheet is immersed in the plating bath, an Fe-Al alloy layer is first formed to suppress an excessive reaction between the steel sheet and the molten zinc. This Fe-Al
In the subsequent alloying process by heating,
It is thought to act as a barrier coating for iron that diffuses heat from the steel sheet. Therefore, since the starting point of iron diffusion from the steel sheet is not uniform but dispersed, the alloying treatment is apparently completed and unreacted metal Zn does not appear to exist. On the surface, where the iron diffusion is delayed,
Metal Zn (η layer) or Zn—Fe intermetallic compound with low Fe content (ζ phase) remains.

[0017] In the galvannealed alloying, since it is immersed in a plating bath containing Al, it is present on the surface of the plating layer.
Al changes to Al ₂ O ₃ with the subsequent alloying treatment. Furthermore, due to alloying treatment, it was present inside the plating layer
Since Al is concentrated on the surface,
Al ₂ O ₃ remains considerably.

Therefore, in order to suppress both the heat generation of the electrode chip and the diffusion of the metal Zn to the electrode chip to improve the continuous hitting property and to prevent the occurrence of spatter, the metal Zn existing in the outermost layer of the plating layer is required. Is effective. On the other hand, in order to improve the welding phenomenon, it is effective to leave Al ₂ O ₃ in a trace amount.

Here, both Zn and Al present on the outermost surface of the plating are amphoteric metals, and are treated with an alkali solution or an acid solution.
Can be easily dissolved. Therefore, in order to reduce and control 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 of these treatments is performed after completing the normal alloying treatment. Just do it. The present inventors, based on these findings, further,
After repeated studies, the present invention has been completed.

Here, the gist of the present invention is that Fe
Content: with 8 to 16 wt%, metallic Zn amount in the outermost layer: 40 wt% or less, and, Al ₂ O in the outermost surface layer
The ratio of the amount of _{3 to the} amount of metal Zn is Al ₂ O ₃ /Zn=0.05 to 0.40 by weight.
It is an alloyed hot-dip galvanized steel sheet excellent in spot weldability, characterized by having a plating layer of:

According to a second aspect of the present invention, a base steel sheet is immersed in a hot-dip galvanizing bath, and the obtained hot-dip galvanized steel sheet is heated and alloyed to obtain a Fe content of 8 to 16% by weight. A galvannealed steel sheet having a -Zn alloy plating layer, and then contacting the galvannealed steel sheet with an alkaline solution having a pH of 9.0 or more, and / or
By contacting with an acid solution of 3.0 or less, the amount of metal Zn in the outermost layer of the Fe-Zn alloy plating layer is 40% by weight or less, and the ratio of the amount of Al ₂ O _{3 to the} amount of metal Zn in the outermost layer is:
This is a method for producing an alloyed hot-dip galvanized steel sheet excellent in spot weldability, characterized in that Al ₂ O ₃ /Zn=0.05 to 0.40 by weight ratio.

According to a preferred embodiment of the present invention, contacting the alloyed hot-dip galvanized steel sheet with an acid solution after contacting the same with an alkali solution suppresses heat generation on the electrode chip surface and reduces sticking and spatter generation. It is desirable to prevent it.

The “outermost layer” in the present invention is:
Among the alloyed hot-dip galvanized layers, metal Zn or Al ₂ O ₃
Means that the surface layer has a depth within a range that reduces the continuous spotting property during spot welding, for example, a surface layer from the plating layer surface to a depth of 500 mm in the depth direction. is there. Specifically, XPS method (X-ray photoelectron spectroscopy)
Is the surface layer within the range analyzed by the above, and is the surface layer region from the outermost surface in the plating film until the metal Zn and metal Fe are stabilized, that is, until reaching the bulk alloy layer.

[0024]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of an alloyed hot-dip galvanized steel sheet excellent in 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 invention comprises: (I) an amount of metal Zn in the outermost layer of the plating layer: 40% or less; and (II) Al in the outermost layer of the plating layer.
₂ O ₃ is characterized by three points: Al ₂ O ₃ / metal Zn content: 0.05 to 0.40, and (III) Fe content of the plating layer: 8 to 16%. Hereinafter, these characteristics will be described separately.

(I) The amount of metal Zn in the outermost layer of the plating layer: 40
% Or less First, the mechanism by which the metal Zn 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 configuration of an alloy layer on the surface of No. 1.

As shown in FIGS. 1 (a) and 1 (b), on the surface of the electrode tip 1 made of copper, β-brass layers 2a, γ- Zn-Fe alloy layer 3 carried from the brass layer 2b and the galvannealed layer
In a three-layer structure.

In general, in a continuous galvanized steel sheet, the tip of the electrode tip (the part in contact with the steel sheet) forms a brass layer due to the diffusion of Zn into Cu,
The lack of this causes an internal defect phenomenon in which the center of the tip of the electrode tip is depressed. When this internal pitting phenomenon occurs, the contact area between the steel plate and the electrode tip decreases, and it becomes impossible to secure an energizing path for forming a nugget.

Here, when metal Zn is present in the outermost layer of the conventional galvannealed galvanized steel sheet, the Zn—Fe alloy layer 3 is formed as shown in FIG. Zn-Fe alloy layer 3a with low Fe content (Fe content: about 10%, hardness 150Hv)
Is formed. Thus, in the alloyed hot-dip galvanized steel sheet in which the metal Zn is present on the outermost layer of the plating layer, the Zn-Fe alloy layer 3a having a low Fe content is first formed on the γ-brass layer 2b, and thereafter, On this Zn-Fe alloy layer 3a, a high Fe content
A Zn-Fe alloy layer 3b (Fe content: about 25%, hardness: about 300 Hv) is formed.

However, in such an electrode tip structure, since the Zn—Fe alloy layer 3a having a low Fe content is soft, the Zn—Fe alloy layer 3b, which is hard and has a high Fe content, is formed continuously. At the time of continuous hitting where welding is performed by applying pressure, it becomes difficult to receive mechanical wear, so it becomes difficult for the Zn-Fe alloy layer to drop out, and it will be in a state where the internal pit phenomenon has occurred,
The continuous hitting property is significantly reduced.

On the other hand, according to the present invention, when metal Zn does not exist in the outermost layer of the galvannealed steel sheet,
As shown in FIG. 1 (b), Zn—Fe is hard and has a high Fe content.
This is a form in which only the alloy layer 3b exists. Therefore, at the time of continuous hitting, the Zn-Fe alloy layer 3 is repeatedly cut off partly because it is susceptible to mechanical wear, and the surface is microscopically exposed. It becomes difficult to secure a stable power supply path,
It is considered that the continuous hitting property is remarkably improved.

Therefore, it is effective to reduce the metal Zn existing 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.

Here, the internal pit phenomenon will be described as follows. FIGS. 2 (a) and 2 (b) are an explanatory view of the internal pit phenomenon seen in the conventional electrode tip and an explanatory view of the shape of the steel plate contact portion of the electrode tip according to the present invention, respectively. In the figure, the gray part is the contact part with the steel plate.

As shown in FIG. 2 (a), when a soft Zn—Fe alloy layer is formed, the layer itself is soft, so that it is hardly subjected to mechanical wear and does not drop, so that it is macroscopic. The electrode tip state is maintained with a large internal squeeze. The phenomenon in which the contact area between the electrode and the steel sheet, which becomes an energization path at the time of welding and the contact area between the electrode and the steel sheet decreases, is called an internal pit phenomenon. As a result, when the contact area is reduced, the energization path is narrowed, so that a sufficient nugget cannot be formed, and the continuous hitting property is reduced.

On the other hand, as in the present invention, soft Zn-F
In the case where the e-alloy layer is not formed and the hard Zn-Fe alloy layer is formed, as shown in FIG.
Since the alloy layer is hard, it is formed in a microscopic manner by mechanical impact and becomes in a state where it repeatedly loses, and the surface shape of the electrode changes while leaving microscopic irregularities. The large internal pitting phenomenon as shown in FIG. Therefore, the contact area between the steel plate and the electrode is secured. See the gray part in the figure.

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, Zn containing metal Zn in a plating film
When measuring the Fe alloy layer, it is effective to employ an X-ray diffraction method or the like. However, since 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 (X-ray photoelectron spectroscopy) capable of analyzing the outermost layer of the plating layer was used.

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

[0040]

[Table 1]

As a result of this confirmation experiment, in order to obtain good continuous spotting properties, the amount of metal Zn in the outermost layer in the range of 500 ° in the depth direction from the surface of the plating layer is preferably as small as possible. The amount is 40% or less, preferably 30% or less. Metal Zn existing in the outermost layer of the plating layer
If the amount exceeds 40%, a Zn-Fe alloy layer 3a is formed on the surface of the electrode 1 as shown in FIG.

The amount of metallic zinc in the outermost layer of the plating layer is 40%
1B, the Zn-Fe alloy layer 3a is not formed on the surface of the γ-brass layer 2b, as shown in FIG.
Are formed directly. The Zn-Fe alloy layer 3b easily drops microscopically due to the pressure of the electrode tip during spot welding, and microscopically roughens the surface of the electrode tip. To prevent the phenomenon of
A stable energizing path is possible. Therefore, continuous hitting property can be ensured.

(II) Al ₂ O _{3 in the} outermost layer of the plating layer is expressed as Al ₂ O ₃ / metal Zn, and 0.05 to 0.40 Al ₂ O ₃ present in the outermost layer of the plating layer is an insulator. _The portion where ₂ O ₃ exists cannot be a current-carrying path during welding. Therefore, the electric resistance is high and the amount of heat generated on the surface of the electrode tip becomes excessive, and the growth of the γ-brass layer 2b is promoted. In the γ-brass layer 2b, peeling occurs at the time of welding and pressurization. As a result, the tip area of the electrode tip is significantly increased. When the tip area of the electrode tip (the area where the steel sheet and the electrode tip are in contact) increases, the current density during welding decreases, making it difficult to obtain sufficient heat generation to form a nugget, and significantly increasing Is reduced.

Further, in the portion where Al ₂ O ₃ exists in the outermost layer of the plating layer, the amount of heat generated is excessive, so that spatter is likely to occur. If the spatter is remarkably generated, the molten metal adheres to the surface of the steel sheet and impairs the appearance of the surface. Therefore, surface maintenance work is required, and the productivity is reduced. Therefore,
In order to secure the number of continuous hit points and suppress generation of spatter, it is preferable to remove Al ₂ O ₃ (Al oxide) present in the outermost layer of the plating layer to some extent.

On the other hand, when metal Zn and Al ₂ O ₃ are not completely observed on the outermost layer and the outermost surface of the clean plating layer is exposed, a sticking phenomenon in which the electrode tip and the steel sheet are welded to each other is likely to occur. At times, there is a problem that the electrodes are missing and the productivity is reduced. If Al ₂ O ₃ as an insulator is left in the outermost layer to some extent, it can be expected as a barrier effect against welding. Therefore, it is also effective to leave Al ₂ O ₃ in such an amount that the continuous spotting property is not deteriorated at the time of continuous spotting by spot welding. Also, when the sticking phenomenon occurs, the alloy layer at the tip of the electrode tends to adhere to the surface of the steel sheet, and the damage to the tip of the electrode increases.
Continuous hitting property is reduced.

The amount of Al ₂ O _{3 in the} outermost layer of the plating layer was determined by the XP
The S method was used to evaluate the average Al ₂ O ₃ abundance from the outermost surface of the plating to 500 mm in the depth direction. As a result, in the depth direction
Metal Zn content in the outermost layer of plating layer up to 500 Å, Al ₂ O ₃
It is effective for sticking prevention that the abundance ratio of the amount (Al ₂ O ₃ / Zn) is between 0.05 and 0.40. When it is smaller than 0.05, a remarkable sticking phenomenon occurs, and the continuous hitting property decreases. On the other hand, if it is larger than 0.40, the amount of Al ₂ O ₃ becomes relatively large, and the weldability is reduced.
Preferably, it is 0.10 to 0.30.

(III) Fe content of the plating layer: 8 to 16% The amount of metal Zn is greatly affected by the Fe content of the plating layer subjected to the alloying treatment. Fe content in plating layer is 8%
If it is less than 7, the alloying process is not completed up to the outermost layer of the plating layer, and the metal Zn (η layer) remains thick in the outermost layer, and cannot be sufficiently removed even by performing the metal Zn removing process described later. On the other hand, if the Fe content of the plating layer is more than 16%, the amount of metal Zn remaining in the outermost layer of the plating layer is reduced to some extent, but a powdering phenomenon occurs in which the plating layer peels off during press working. Therefore, in the present invention, the Fe content in the plating layer of the galvannealed steel sheet is 8% or more,
Limited to 16% or less. Preferably, it is 9% or more and 13% or less.

In the alloyed hot-dip galvanized steel sheet excellent in spot weldability according to the present invention, the constitution other than the above is
Since it is the same as a normal galvannealed steel sheet, further description is omitted. Next, a method for producing an alloyed hot-dip galvanized steel sheet by reducing the amounts of metal Zn and Al ₂ O ₃ in the outermost layer of the plating layer to the above ranges will be described.

According to the present invention, there is no particular limitation on the stage until the production of an alloyed hot-dip galvanized steel sheet, and it may be manufactured by any suitable means. For example, a cold-rolled steel sheet that has undergone preliminary heat treatment may be used in a hot-dip galvanizing bath. The alloyed hot-dip galvanized steel sheet may be manufactured by immersing, lifting from the plating bath, and performing alloying heat treatment. The basis weight in the hot-dip galvanizing is not particularly limited by the present invention, but is preferably 30 to 70 g / m ² per one side. The alloyed hot-dip galvanized steel sheet thus manufactured is then immersed in an alkali solution and / or an acid solution.

Since Zn and Al are amphoteric metals, acid solutions,
Dissolves in any of the alkaline solutions. Zinc oxide, Al ₂ O ₃
An alkali solution is effective in dissolving an oxide film such as an acid solution, and an acid solution is effective in dissolving metal Zn and metal Al while leaving the oxide film. Instead, any method may be used. Metal Zn in the plating layer the outermost surface layer to 40% or less, the amount of Al ₂ O ₃ Al ₂ O
_It suffices if the ₃ / metal Zn ratio can be reduced to 0.05 to 0.40, respectively.

When an alkaline solution is used, the pH is 9.
0 or more. If the pH is lower than 9.0, zinc oxide and Al ₂ O ₃ cannot be effectively removed. From the same viewpoint, preferably, the pH is 11.0 or more.

Such an alkaline solution is not limited to a specific type, but may be an alkaline solution that does not remain on the surface of the galvannealed steel sheet through a water washing and drying process immediately after the alkali treatment. preferable. This is because, when applied as an automobile body panel, if an alkali component remains on the surface, a surface defect may be generated in a chemical conversion treatment and a coating process. Preferable examples of such an alkali solution include an aqueous solution of an alkali metal 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. If the pH is higher than 3.0, metal Zn, effectively
This is because metal Al cannot be removed. From the same viewpoint, preferably, the pH is 2.0 or less.

When such an acid solution is used, it is not necessary to limit to a specific kind. However, an oxidizing acid solution such as a nitric acid solution remarkably oxidizes the surface of the galvannealed steel sheet, It is not preferable because a zinc oxide layer and an Al ₂ O ₃ layer are easily formed. When the pH is 3.0 or less, an electroplating solution containing metal ions based on a hydrochloric acid solution or a sulfuric acid solution can be used.

Examples of the alkali treatment method and the acid treatment method include an immersion method and a spray method. Any method can be used as long as it can control and reduce metal Zn and Al ₂ O ₃ within the above-mentioned range.
It is not necessary to limit to a particular method. However, from the viewpoint of productivity, in order to effectively remove the metal Zn, Al ₂ O ₃ amount Al, it is necessary to raise the temperature of the solution or mechanically remove the surface oxide layer, thereby applying surface stress. It is also preferable to use a surface polishing method that increases the etching rate.

In the treatment with an alkali solution or an acid solution in the present invention, the specific conditions are not particularly limited as long as the desired outermost layer is obtained under the above-mentioned conditions. Is preferably 2 to 10 seconds in an alkaline solution. For acid solutions preferably 2-10
Seconds are fine.

Further, in any of the alkali treatment and the acid treatment, in order to prevent surface contamination due to the remaining treatment liquid,
Immediately after the treatment, washing and drying are preferably performed in order to secure the stability of the surface state of the outermost layer.

Further, it is desirable to combine the alkali treatment and the acid treatment, since the spot weldability can be effectively improved. That is, after the alkali treatment, by performing the acid treatment, by alkali treatment, zinc oxide, by temporarily removing the Al ₂ O ₃ layer, a subsequent acid treatment, removal of the Al ₂ O ₃ layer Is slow, it is possible to leave Al ₂ O ₃ to some extent after effectively removing metal Zn.

As described above, the alloyed hot-dip galvanized steel sheet according to the present invention, which is excellent in spot weldability, suppresses the abrasion of the electrode tip during spot weldability and improves the durability. Performance can be improved,
Further, by slightly leaving Al ₂ O ₃ on the outermost layer, the sticking phenomenon can be prevented.

[0060]

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. (Example 1) Cold rolled steel sheets having the chemical composition values shown in Table 2 (thickness: 0.
8 mm) for the test.

[0061]

[Table 2]

Using this cold-rolled steel sheet, hot dip galvanizing and then alloying heat treatment are performed by conventional means, and the Fe content (degree of alloying) of the plated layer is reduced to 7%, 8%, 11%, 13%.
25 alloyed hot-dip galvanized steel sheets were manufactured at seven levels of%, 14%, 16% and 17%.

These samples were subjected to alkali treatment under the treatment conditions shown in Table 3, followed by washing and drying.
After performing the acid treatment under the treatment conditions shown in Table 4, washing with water and drying are performed. Alternatively, after performing the alkali treatment under the treatment conditions shown in Table 3 and washing with water, immediately carry out the acid treatment under the treatment conditions shown in Table 4. It was washed with water and dried. In order to clarify the influence of the amount of metal Zn and the amount of Al ₂ O _{3 on the} outermost layer of the plating, the respective treatment times of the alkali treatment and the acid treatment were changed.

[0064]

[Table 3]

Incidentally, the amount of metal Zn in the outermost layer of the plating layer,
The amount of Al ₂ O ₃ was measured by the XPS method under the measurement conditions in Table 1 described above. In the state where these two samples were superimposed, Table 5
The spot welding was continuously performed up to 5,000 points under the conditions shown in (1) and the nugget diameter was measured at every 50 spots to evaluate the continuous spotting property in spot welding.

The evaluation criteria were as follows: The number of continuous hits until the nugget formation was no longer possible was measured. Indicated. In addition, less than 3000 points are indicated by crosses.

Further, these samples were simultaneously evaluated for a sputtered state and a sticking state. As the evaluation criteria, the ratio of occurrence of spatter and sticking was counted, and the incidence of spatter and sticking were measured. Both the spatter generation rate and the sticking generation rate are indicated by ○ as target values of less than 5%, and are indicated by ◎ marks as excellent values of less than 3%. Further, 5% 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 cylindrical drawing was evaluated. The processing conditions are summarized in Table 6.
The evaluation criteria of the powdering property were as follows. A tape was attached to the side wall of the overhang portion formed by performing the cylindrical drawing, and then peeled off, and the peeling amount was measured by a gravimetric method. As evaluation criteria at that time, a mark of 20 mg / piece or less as a target value per sample was indicated by a circle, and a mark of 10 mg / piece or less as an excellent value was indicated by a mark. In addition, more than 20 mg / unit was indicated by a cross.

[0069]

[Table 5]

Table 7 summarizes the test results relating to the continuous hitting property, spatter generation rate, sticking generation rate and powdering. From Nos. 1 to 21, the degree of alloying is 16% by weight or less in order to secure good powdering properties.

Further, it can be seen that with the surface metal Zn content of the present invention, good spot weldability (continuous spotting property) can be ensured. On the other hand, it can also be seen that the Al ₂ O ₃ amount has an appropriate range in terms of the metal Zn ratio. Even if the continuous hitting property can be ensured, if the Al ₂ O ₃ / metal Zn ratio is large, the spatter generation rate increases, and the target weldability cannot be secured.
Also, it is found that if the Al ₂ O ₃ / metal Zn ratio is too small, the sticking occurrence rate cannot achieve the target value even if the continuous hitting property can be ensured (No. 16).

From the above results, regarding the continuous hitting property,
The amount of metal Zn in the outermost layer of the plating layer has a great effect, and even if continuous spotting properties can be ensured, if an appropriate amount of Al ₂ O ₃ cannot be ensured, it is not effective against sputtering and sticking. Therefore, it can be seen that securing the amount of metal Zn and the amount of Al ₂ O _{3 in the} outermost layer of the plating layer in the present invention is very effective for spot weldability required for automotive body panels.

[0073]

[Table 7]

(Example 2) Next, in order to clarify the conditions of the alkali treatment and the acid treatment, the influence of the pH of each treatment solution was examined.

In this example, a cold rolled steel sheet having a chemical composition shown in Table 2 and having a thickness of 0.8 mm was used as the base material, and the basis weight per hot-dip galvanized film was 43 g / m ^2. hand,
An alloyed plating layer having a Fe content of 10% was formed in the plating layer.

For the alkali treatment, an aqueous solution of potassium hydroxide was used under the treatment conditions shown in Table 8, and for the acid treatment, treatment solutions adjusted to various pHs using hydrochloric acid under the treatment conditions shown in Table 9 were used.
The treatment time was 5 seconds immersion, water washing, and drying. However, when examining the effect of the alkali treatment solution, the acid treatment pH after the alkali treatment was 1.5 and the immersion time was 5 seconds. When examining the effect of the acid treatment solution, the alkali treatment pH before the acid treatment was 12.
5, immersion time = 5 seconds. A continuous spot test was performed under the same conditions as in Example 1.

[0077]

[Table 8]

The results of continuous hitting are shown by graphs in FIGS. FIG. 3 shows the effect of the pH of the alkaline treatment liquid on the continuous dot property, and FIG. 4 shows the effect of the pH of the acid treatment liquid on the continuous dot property.

From the graph shown in FIG.
Is 9.0 or more, and preferably 11.0 or more. On the other hand, from the graph shown in FIG.
It can be seen that it is 3.0 or less, preferably 2.0 or less.

Example 3 Further, the superiority of the combination of the alkali treatment and the acid treatment was examined. The base material is based on a cold-rolled steel sheet having a thickness of 0.8 mm having the chemical composition values shown in Table 2, and has a basis weight of 43 g / m ² per one side of the alloyed hot-dip galvanized film. An alloyed plating layer having a content of 10% was formed.

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 showing the treatment conditions. In these treatments, the amount of metal Zn and the amount of Al ₂ O ₃ in the outermost layer of the plating layer were changed by changing the immersion time.

As described above, in Example 1 with respect to a sample subjected to one or both of the alkali treatment and the acid treatment,
In the same manner as in the above, the continuous hitting property, the spatter generation rate, and the sticking generation rate were investigated.

[0083]

[Table 10]

[0084]

[Table 11]

Table 12 shows the results. From Table 12, stable surface metal Zn amount, in order to ensure the surface alumina amount, alkali treatment, only pickling treatment, and when alkali treatment after pickling treatment, may be insufficient. It turns out that pickling after the alkali treatment is very effective.

[0086]

[Table 12]

[0087]

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. According to the alloyed hot-dip galvanized steel sheet according to the present invention, the continuous hitting property during spot welding can be drastically improved, and the spatter phenomenon and the sticking phenomenon can also be improved.

Further, the galvannealed steel sheet according to the present invention can be subjected to an extremely simple step of performing one or both of an alkali treatment and an acid treatment after the galvannealed galvanization, and particularly to the acid treatment after the alkali treatment. By performing the treatment, the treatment time can be shortened, and a galvannealed steel sheet having a stable outermost surface state can be produced extremely efficiently.

Therefore, according to the present invention, in the automobile body assembling step, the frequency of maintenance of the electrode tips can be reduced by increasing the number of continuous hit points at the time of welding, and the frequency of occurrence of the spatter phenomenon can be reduced. Since surface maintenance can be eliminated and the frequency of occurrence of the sticking phenomenon can be reduced, welding trouble 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 a figure which shows typically the formation state of the alloy layer on the surface of a chip | tip.

FIGS. 2 (a) and 2 (b) are an explanatory view of the internal pit phenomenon observed in the conventional electrode tip and an explanatory view of the shape of the steel plate contact portion of the electrode tip according to the present invention, respectively. is there.

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

FIG. 4 is a graph showing the effect of the pH of the acid treatment liquid on the number of continuous shots in another example of the present invention.

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

Claims (3)

(57) [Claims] (1) Fe content: 8 to 16% by weight, metal Zn content in the outermost layer: 40% by weight or less, and
The ratio of the amount of Al ₂ O _{3 to the} amount of metal Zn in the outermost layer is
An alloyed hot-dip galvanized steel sheet having excellent spot weldability, characterized by having a plating layer in which ₂ O ₃ /Zn=0.05 to 0.40. 2. A base steel sheet is immersed in a hot-dip galvanizing bath.
The obtained hot-dip galvanized steel sheet is heated and subjected to alloying treatment to obtain an alloyed hot-dip galvanized steel sheet having a Fe-Zn alloy plating layer having an Fe content of 8 to 16% by weight. By contacting the galvanized steel sheet with an alkaline solution having a pH of 9.0 or more and / or by contacting it with an acid solution having a pH of 3.0 or less, the amount of metal Zn in the outermost layer of the Fe-Zn alloy plating layer: 40 Weight% or less, and the ratio of the amount of Al ₂ O _{3 to the} amount of metal Zn in the outermost layer is Al ₂ O ₃ / Zn
= 0.05 to 0.40, a method for producing an alloyed hot-dip galvanized steel sheet having excellent spot weldability. 3. A base steel sheet is immersed in a hot-dip galvanizing bath.
The obtained hot-dip galvanized steel sheet is heated and alloyed to form an alloyed hot-dip galvanized steel sheet having an Fe-Zn alloy coating layer having an Fe content of 8 to 16% by weight. The steel sheet is brought into contact with an alkaline solution having a pH of 9.0 or more and then with an acid solution having a pH of 3.0 or less, whereby metal Zn in the outermost layer of the Fe-Zn alloy plating layer is formed.
Amount: not more than 40% by weight, and the ratio of the amount of Al ₂ O _{3 to the} amount of metal Zn in the outermost layer is Al ₂ O ₃ /Zn=0.05 to 0.40 by weight ratio. An excellent method for producing galvannealed steel sheets.