JP2001303228A - Galvannealed steel sheet excellent in spot weldability, its producing method and evaluating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet excellent in continuous spotting in a spot welding, its producing method and an evaluating method for continuous spotting in the spot welding for steel sheet. SOLUTION: This galvannealed steel sheet is the one, in which the difference between the maximum value and the minimum value in an atomic concentration ratio of Al and Zn on the surface of the plated layer in the range corresponding to the surface in contact with an electrode for spot welding, is <=3.0. The producing method of this steel sheet is performed by allowing to contact with acid solution, alkali solution or both solutions dissolving zinc after performing skinpass rolling in the range of 0.3-1.5% total elongation ratio in division of >=2 times after the alloying treatment. Then, the evaluating method of the spot welding for steel sheet is performed by using the difference between the maximum value and the minimum value in the atomic concentration ratio of Al and Zn on the surface of the plated layer in the range corresponding to the surface in contact with the electrode for spot welding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent continuous spot welding properties, a method for producing the same, and a method for evaluating continuous spot welding properties.

[0002]

2. Description of the Related Art An alloyed hot-dip galvanized steel sheet is immersed in a hot-dip zinc bath to deposit a galvanized layer, and then heated to diffuse iron (Fe) of the base steel sheet into zinc (Zn). And the plating layer is made of a Zn-Fe alloy. The content of Fe in the plating layer is usually selected to be about 7 to 20% by mass. By alloying, the adhesion of the paint applied on it improves, spangles disappear, the hardness of the plating layer increases, so the corrosion resistance after painting is further improved,
The painted surface is beautiful and spot welding is more stable than with Zn only plating. For this reason, a steel plate having excellent press formability is used as a base material, and is widely applied to an outer plate of an automobile body.

[0003] Spot welding is frequently used for assembling a steel plate into an automobile body. In many spot weldings, two or more steel plates are overlapped, the tip of an electrode tip mainly composed of copper is brought into contact with and pressurized on the steel plate, and welding is performed by resistance heating. In this spot welding, a large number of points must be welded continuously when assembling an automobile body.However, if spot welding is performed continuously, the tip of the electrode tip in contact with the steel plate may be worn or deformed. As a result, the flow of the welding current changes, and a nugget of an appropriate shape for maintaining the welding strength cannot be obtained. If so, the welding operation must be interrupted in order to clean or replace the electrode tips. Therefore, in order to increase the productivity in spot welding, it is necessary to increase the number of points that can be continuously welded with a newly replaced electrode tip as much as possible. In the application of an alloyed hot-dip galvanized steel sheet to an automobile body, it is strongly desired that the steel sheet has a large number of continuous spots at which the normal spot welding points can be obtained, that is, excellent continuous spotting properties. However, it has been conventionally pointed out that the alloyed hot-dip galvanized steel sheet has poor spot welding continuous hitting property, and the following guess is made as a cause.

One is that the main component of the plating layer is Zn,
Zn easily forms an alloy with copper (Cu), and the melting point of the alloy is low and brittle.
When n is diffused, the electrode tip is greatly worn and its shape is impaired, so that a normal spot welding point cannot be obtained with a small number of hit points. Another is that aluminum oxide (Al ₂ O ₃ ) exists on the surface of the plating layer, and A
Since l ₂ O ₃ is an insulator, the electrical resistance of the contact portion between the electrode chip and the surface of the steel plate plating increases, the heat generation at that portion increases, and the wear of the electrode chip increases. In the galvanizing bath, in order to enhance the adhesion between the plating layer and the base steel sheet, and to suppress excessive alloying so that hard and brittle Γ ₁ phase (Fe ₅ Zn ₂₁ phase) does not appear. , 0.08-0.2%
About aluminum (Al) is added. This A
1 is also taken in the galvanized layer attached to the steel sheet, Al is more easily oxidized than Zn, and exists on the surface as Al ₂ O ₃ .

[0005] As an alloyed hot-dip galvanized steel sheet having improved continuous spot welding properties, for example, Japanese Patent Application Laid-Open No.
Publication No. 2 discloses that the total amount of Al in the plating layer is 0.2 to 0.5% by mass,
The invention of a steel sheet in which the e content is 7 to 12% by mass and the total amount of metallic Al in the plating layer is limited to 0.01 to 0.2 g / m ² is disclosed. JP-A-6-73521 discloses that an oxide film on a plating surface is 50 to 1000 mg / m ² ,
An invention of a steel sheet having a voltage of 1.6 V · sec or less has been proposed. However, the above-mentioned publications do not specifically show the conditions for generating the unoxidized metallic Al amount in these steel sheets, the method of controlling the amount of the surface oxide film, the amount of Al in the film, and the like.

Further, JP-A-10-330902 discloses that the amount of metallic Zn in the outermost layer of an alloyed hot-dip galvanized layer is 50%.
There is disclosed an invention of a steel sheet having excellent spot weldability in which the amount of Al ₂ O ₃ is 15% by mass or less and the amount of Al ₂ O ₃ is 15% by mass or less. In this case, the pH of the surface of the galvannealed steel
The amount of metal Zn and the amount of Al ₂ O ₃ are reduced by contact with an alkaline solution having a pH of 9.0 or more or an acid solution having a pH of 3.0 or less.

[0007] Alloyed hot-dip galvanized steel sheets are required as extremely important properties such as adhesion of paint and resistance to powdering that peels off during processing. Since these points are more important, the improvement of the continuous spotting property of the spot welding of the steel sheet is not always sufficient. Also,
For the evaluation of spot weldability, a test in which welding spots are actually repeated is performed, and this requires a large number of man-hours. For this reason, it may not be easy to carry out many tests, and it cannot be said that an alloyed hot-dip galvanized steel sheet having sufficiently excellent spot weldability has been obtained.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet which has an effect of suppressing wear of a welding electrode during spot welding and which has improved continuous welding properties. It is another object of the present invention to provide an evaluation method capable of judging the continuous spot welding property of a galvannealed steel sheet without actually performing a spot welding test.

[0009]

Means for Solving the Problems The inventors of the present invention have investigated the surface condition of a plating film, which is considered to greatly affect the continuous hitting property of a galvannealed steel sheet, and the appearance of the reaction with the welding electrode. investigated. After repeating the RBI continuously
As a result of observing the state of the surface of the tip of the electrode tip, the electrode tip used for welding a steel sheet having a large tendency to produce welding defects with a small number of spots shows that even if the number of spots is the same, Cu
It was presumed that the -Zn-Fe alloy was generated more unevenly, which caused the unevenness of the tip of the electrode tip to become severe and greatly reduced the number of continuous spots in spot welding. Therefore, first, a steel sheet with large and small electrode tip wear was selected, and the steel sheet surface before temper rolling was examined in detail.

[0010] In the alloyed hot-dip galvanized steel sheet before temper rolling, irregularities having a height difference of about 5 to 10 µm are generated at intervals of about several µm to several hundred µm. This is because the base Fe is diffused into the Zn plating layer by heating after plating to adhere zinc and alloyed, but this alloying does not always proceed uniformly, and the alloying progresses depending on the steel sheet surface portion. It is considered that the degree was different and the thickness of the plating layer was changed accordingly. Such irregularities are alleviated by temper rolling, but local fluctuations in the composition seemed to be inevitable to some extent.

At the time of spot welding, it is estimated that the part where the tip of the electrode tip and the steel plate come into contact with and react to each other has a depth of about 10 to 100 nm from the surface. On the other hand, on the surface of the plating layer, a film mainly composed of an oxide is formed on an extremely surface portion thereof by plating or heating during alloying treatment. Although the thickness of the film is about several tens to several hundreds of nm, the amount of Al in the oxide is:
The value was much higher than the amount of Al contained in the plating bath. Since Al has a greater affinity for oxygen than Zn, Al is selectively oxidized on the surface of the plating layer during the high temperature of the alloying treatment immediately after plating, and Al is formed.
₂ O ₃ , and solid solution A in Zn near the very surface portion
1 concentration decreases relatively largely. Therefore, it is presumed that solid-solution Al diffused from the inside of the plating layer and oxidation proceeded further, resulting in a large increase in the amount of Al element on the very surface portion in the form of oxide.

[0012] The amount of Al on the very surface portion of the oxide was not affected much by the amount of Al in the zinc bath, but seemed to be largely dependent on conditions such as alloying treatment. Although Al ₂ O ₃ is an insulator even though it is a thin film on the very surface, the electric resistance and the heat generated at the contact part when the electrode tips and steel plates are contact-pressed during spot welding, It is considered that this greatly affects the reaction between Cu and Zn. This effect appeared to be much greater than the local variation in composition due to the alloying process described above.

A more detailed examination of the amount of Al and Zn contained in the film formed on the surface reveals that, within the range in which the tip of the electrode tip for spot welding is pressed against the steel plate surface, a local area is obtained. It has been found that different places show different values. In particular, the Al amount shows the maximum value in the concave portion of the plating layer formed by the alloying reaction, and has the minimum value in the convex portion. In the vicinity thereof, the amounts of Al and Zn in a number of concave portions and convex portions are analyzed, and the atomic concentration ratio of Al (mol%) to Zn (mol%) [(Al mol%) / (Z
nmol%)], it was found that the maximum value or the minimum value was almost equal to the maximum value or the minimum value in the portion where the tip of the electrode tip was pressed.

Therefore, within the range of the size of the surface in contact with the electrode tip, the difference between the almost maximum value and the almost minimum value of the atomic concentration ratio of Al and Zn, or the difference between the concave portion and the convex portion, is considered. The difference between the atomic concentration ratios of Al and Zn was determined, and the relationship between the magnitude of the difference and the continuous spotting property of spot welding was compared. As a result, they have found that this value is an extremely good index for judging the quality of spot weldability of a steel sheet.

If the value of the difference between the maximum value and the minimum value of the atomic concentration ratio of Al and Zn is X, a steel sheet having a small X value is excellent in continuous spot welding of spot welding, and a steel sheet having a large X value is continuous. The results show that the hitting property is poor. If this X is determined by surface analysis, the spot weldability of the steel sheet can be determined without actually performing a spot welding test based on the magnitude of the X value. X
The smaller the value is, the better the continuous spotting property of spot welding is, and the better the uniformity of the composition of the plating layer surface is. Therefore, the non-uniform formation of Cu-Zn-Fe alloy which can be used as an electrode for spot welding is suppressed. Was thought to be.

An X value considered as an index of the non-uniformity,
Examining the relationship with the number of continuous spot welding spots, X
It has also been found that when the value exceeds a certain value, the spot weldability rapidly deteriorates when the value exceeds the value, and it can be determined that the weldability is good when the value is less than the value. However, it has been difficult to significantly reduce the X value of the steel sheet even if various conditions are changed within the range of a normal method for producing a galvannealed steel sheet.

As described above, in the alloyed hot-dip galvanized steel sheet, the adhesion of the paint and the powdering resistance during processing are extremely important. These characteristics are the plating conditions of immersion in a zinc bath and the processing conditions of alloying. Or the degree of alloying. Therefore, it is not easy to change these conditions and factors.
Other measures must be taken. From such a viewpoint, attention was paid particularly to the X value, and a method of stably sufficiently lowering the X value was further studied.

[0018] The temper rolling can alleviate the irregularities formed by the alloying treatment as described above. However, the X value did not decrease in the temper rolling, but rather tended to increase.
This is probably because the protruding portions of the plating layer are greatly stretched or crushed by rolling, but the bottoms of the concave portions are little deformed and remain almost as they are. That is, Al
This is because the convex portion having the minimum value of the atomic concentration ratio between Zn and Zn approaches the zinc composition of the plating layer having a smaller amount of Al due to deformation, whereas the concave portion having the maximum value remains as it is.

It has been shown in the above-mentioned Japanese Patent Application Laid-Open No. 10-330902 that the treatment with an acid solution or an alkali solution is effective in improving the continuous spotting property of spot welding, and is effective in greatly reducing the X value. There is. Normally, alloyed hot-dip galvanized steel sheets are subjected to temper rolling at an elongation of about 0.2 to 1.5% to suppress the occurrence of stretcher strain during the processing of the base steel sheet after alloying treatment and to correct the shape such as the flatness of the steel sheet. Is required. Therefore, the treatment with an acid solution or an alkali solution described in the above publication is performed in a state where temper rolling has been performed.

Focusing on this, the temper-rolled steel sheet was temper-rolled once more, and then treated with an acid solution or an alkali solution. As a result, the X value could be greatly reduced. . As a result of investigating the continuous spotting property of the spot welding of this steel sheet, it was confirmed that it was extremely excellent.

In the temper rolling, the surface of the steel sheet is subjected to a large shear deformation several times or more the degree of work indicated by the elongation.
Even in the case of a plated steel sheet, a strong working is applied to the surface of the plating layer. Therefore, even if the total elongation is the same, if the temper rolling is performed twice or three times, a much larger process can be applied to the surface of the plating layer than in one rolling. It is presumed that the strain due to this processing greatly promoted the reaction in the treatment with the acid solution or the alkali solution, and made the surface uniform, thereby greatly reducing the X value.

From the above results, the limits of each condition were further confirmed, and the present invention was completed. The gist of the present invention is as follows. (1) An alloyed hot-dip galvanized steel sheet having a plating layer having an Fe content of 7 to 20% by mass, in a range of a size corresponding to a surface in contact with an electrode tip of spot welding,
The difference between the maximum value (Al / Zn) max and the minimum value (Al / Zn) min of the atomic concentration ratio of Al and Zn on the surface of the plating layer, that is, the value of X represented by the following equation is 3.0 or less. An alloyed hot-dip galvanized steel sheet having excellent continuous spot welding properties by spot welding.

X = (Al / Zn) max− (Al / Zn) min (2) After the alloying treatment, the temper rolling with the total elongation in the range of 0.3 to 1.5% is performed by 2 2. The alloyed molten zinc excellent in continuous spot welding of spot welding according to claim 1, characterized in that, after being divided into multiple times, the solution is brought into contact with an acid solution, an alkali solution or both solutions in which zinc is dissolved. Manufacturing method of plated steel sheet. (3) Continuous spotting property in spot welding of alloyed hot-dip galvanized steel sheet, within the range corresponding to the surface in contact with the electrode tip of spot welding, the atomic concentration ratio of aluminum and zinc on the plating layer surface, A spot weldability evaluation method characterized by making a judgment based on a difference between a maximum value and a minimum value.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The galvannealed steel sheet to which the present invention is applied has a zinc adhesion amount of 20 to 10 with respect to one side of the steel sheet.
It is in a range usually adopted for automobiles having a plating layer of 0 g / m ² and a Fe content of 7 to 20% by mass. This F
The e content is usually manufactured in consideration of the corrosion resistance of the steel sheet and the adhesion of the paint, the flaking resistance to the flakes of the plating layer during processing, the powdering resistance, etc. Is the same as if Further, Ni, Co, Mn, Mg, and other elements for the purpose of improving various performances such as paint adhesion, corrosion resistance, and workability may be added to the plating layer of the steel sheet in small amounts.

The difference between the maximum value and the minimum value of the atomic concentration ratio of Al and Zn on the surface of the plating layer is determined by the area corresponding to the portion where the welding electrode is in contact with the steel sheet, that is, the area within the range of about 10 to 100 mm ² . Can be obtained by The area in contact with the steel plate at the tip of the electrode tip for spot welding depends on the thickness of the steel plate,
For example, since 0.8mm when the electrode tip of the galvannealed steel sheet before and after are the following diameter of about 7mm, the maximum atomic concentration ratio of the surface aspect 7mm angle or within the scope of an area 50 mm ^2,, Al and Zn Measure the value and the minimum value. The atomic concentration of Al on the plating layer surface is Almol%, the atomic concentration of Zn is Znmol%, and the maximum value of the atomic concentration ratio is (Al /
If Zn) max and the minimum value are expressed as (Al / Zn) min, the difference X between the maximum value and the minimum value is expressed as X = (Al / Zn) max- (Al / Zn) min. .

The surface of the galvannealed steel sheet is several μm
There are irregularities at intervals of about m to several hundreds of μm. Usually, the top of the convex portion shows the minimum value of the atomic concentration ratio of Al and Zn, and the bottom of the concave portion shows the maximum value of this atomic concentration ratio. Since the minimum value or the maximum value is substantially equal to the minimum value or the maximum value in the vicinity thereof, the atomic concentration ratio of Al and Zn at the top of the convex portion and the bottom of the concave portion can be determined within the above-mentioned area range. May be measured at a plurality of points, and the value at the top of the convex portion or the value at the bottom of the concave portion may be averaged to obtain a minimum value (Al / Zn) min or a maximum value (Al / Zn) max. The method of analyzing the surface composition is that the composition at a depth of about 10 nm from the surface can be measured, and a few μm
A method capable of quantification within a circle with a diameter of about 10 μm for detecting the atomic concentration ratio of Al and Zn at the bottom of the recess or at the top of the projection for irregularities with an interval of about several hundred μm from It does not specifically limit, if it is. Auger electron spectroscopy, which is one of the surface analysis methods at present, is preferable from the viewpoint of simplicity of measurement and quantitative accuracy.

When performing surface analysis, as-received samples must be cleaned of surface contamination, but it is not preferable to change the surface composition for the purpose of cleaning, for example, sputtering treatment with ion irradiation. .
In the method of the present invention, the relative concentrations of Al and Zn are measured, and the influence of some contamination can be eliminated. When the spot weldability is evaluated using the above-mentioned X value, it is understood that the smaller the value of X, the better the continuous hot spotting property of the galvannealed steel sheet. As a criterion for the determination, as a result of comparing the X value of some steel sheets with the number of continuous hit points at which the nugget shape is deteriorated and welding is incomplete, when the X value exceeds 7.0, the continuous hit number is extremely poor. It was clear that if the number was 7.0 or less, a sufficient number of continuous hit points could be obtained. Therefore, for example, as a criterion for determining the spot weldability, a steel sheet having an X value of 7.0 or less may be good, and a steel sheet having an X value exceeding 7.0 may be bad. In addition, the X value
If it is 4.5 or less, extremely excellent continuous hitting property is exhibited.

In the steel sheet of the present invention, the above-mentioned X value is 3.0 or less. For example, when aligning the welding conditions and comparing the spot weldability of steel sheets, for steel sheets with an X value of less than 7.0, the number of continuous spots is less than 3,000 points, but the X value exceeds 3.0.
For steel plates up to 7.0, 3000-5000 welding points are possible. On the other hand, when the steel sheet has an X value of 3.0 or less, the number of welding points exceeds 5000. The method for producing the galvannealed steel sheet having an X value of 3.0 or less is not particularly limited. However, it can be easily manufactured by the following method. That is, in the alloyed steel sheet, the temper rolling in which the total elongation is in the range of 0.3 to 1.5% is performed twice or more, and then the steel sheet is brought into contact with an acid or alkali solution in which zinc is dissolved. By such a manufacturing method, an alloyed hot-dip coated steel sheet having excellent spot weldability with an X value of 3.0 or less can be obtained irrespective of plating conditions, alloying treatment conditions, plating layer compositions, and the like.
The elongation percentage of the temper rolling is 0.3% or more in total.
%, The effect of the temper rolling of suppressing the stretcher strain cannot be sufficiently obtained, and the X value cannot be sufficiently reduced even when the film is brought into contact with an acid or alkali solution thereafter. Further, temper rolling exceeding 1.5% is not preferable because the workability of the steel sheet is reduced. Temper rolling is performed twice or more because the X value is greatly reduced by the subsequent contact with an acid or alkali solution.
This is because it can be reduced to 3.0 or less. This is presumably because even if the total elongation of the temper rolling is within a small range, a large strain is given to the plating layer surface.

The elongation percentage per rolling when rolling is performed two or more times in a plurality of times is not particularly limited. However, in order to sufficiently process the surface of the plating layer,
It is desirable that the content be at least 0.1% or more. Although it is effective to divide the number into a plurality of times, the effect is saturated and wasted even if the number of times is increased.

After the temper rolling two or more times, a treatment for contacting with an acid solution or an alkali solution is performed, whereby the X value is greatly reduced. Both Zn and Al are amphoteric metals,
Dissolves in both acid and alkaline solutions. Therefore, this treatment dissolves the plating layer surface and also removes debris such as oxides formed by temper rolling, but the distortion of the plating layer introduced by temper rolling accelerates this dissolution and reduces Promotes

Here, the treatment with an acid solution or an alkali solution is not particularly limited as long as the very surface layer of the plating layer can be dissolved in a short time. For example, in the case of an acid solution, an oxidizing solution such as a nitric acid solution is used. A solution of an acid having an undesired property causes an excessive formation of an oxidized layer, and a pH of 3.0 or less, preferably a hydrochloric acid solution or a sulfuric acid solution of 2.0 or less, or a metal capable of dissolving Zn or Al, such as zinc sulfate. It may be an acidic solution of a salt. If the pH is excessively low, the plating layer is greatly dissolved, which is not preferable. It is preferable to stop the pH to about 1.0. As the alkaline liquid, for example, an aqueous solution of sodium hydroxide, potassium hydroxide or the like having a pH of 9.0 or more, preferably 10.0 or more is used.

These acid solutions or alkali solutions have a temperature of 20
The treatment is performed by dipping or spraying at about 80 ° C. for about 2 to 10 seconds. In the treatment with an acid solution or an alkali solution, it is preferable to immediately wash and dry the mixture immediately after the treatment in order to prevent contamination due to the residual treatment solution. Note that either one of an acid solution and an alkali solution may be performed, or both may be performed in combination, so that the X value can be further reduced.

[0033]

EXAMPLE A 0.8 mm thick cold-rolled steel sheet having the chemical composition shown in Table 1 was used as a base material. In a hot-dip coated steel sheet production line, the Al concentration in the plating bath was set at 0.1 to 0.2%, and the amount of zinc adhering. From 30 to
The alloying after the plating was changed in the range of 90 g / m ² , and the alloying after the plating was adjusted by changing the heating temperature and time to prepare an alloyed hot-dip coated steel sheet.

[0034]

[Table 1] Since temper rolling can be performed only once on the production line, additional rolling was performed on another line in the case of multiple passes.
In this case, when the elongation rate of each pass is the same, for example, when temper rolling at an elongation rate of 0.6% is performed in three passes,
0.2% rolling was performed three times. An aqueous solution of zinc sulfate (pH: 2.0) was used for the acid solution, and an aqueous solution of sodium hydroxide (pH: 10.0) was used for the alkaline solution.

In the analysis of the steel sheet surface, test pieces were randomly sampled at three locations near the center and both ends in the width direction of the coil, degreased with an organic solvent, and then placed on one side of each test piece.
Auger electron spectroscopy within a 5 mm square plane
The concave and convex portions of the plating surface at each point are 10 points each, and the area of one measurement point is a square of approximately 10 μm on a side,
The atomic concentration of Al and Zn was measured. From the measurement results, the value of X represented by the formula was obtained for each test piece, and the average was taken as the X value of the steel sheet.

Two steel sheets whose X values were determined were superposed, and 5000 spots of spot welding were continuously performed under the following conditions. Thereafter, the two steel sheets were peeled off, the nuggets were observed at every 100 hit points, and the number of hit points until a healthy nugget could not be formed was taken as the continuous hit number of the steel sheet.

Pressing force: 1961 N Squeeze time: 17 cycles Energizing time: 8 cycles Holding time: 5 cycles Welding current: 9.5 kA Tip tip shape: diameter 5 mm (CF type) The zinc adhesion amount of the plating, the Fe content, the total elongation percentage of the temper rolling, the number of times, the presence or absence of the treatment with an acid solution or an alkali solution, the X value, and the results of the evaluation of the number of continuous shots are shown. Here, the number of continuous hit points was evaluated as x for less than 3000 points, as 、 for 3000 to 5000 points, and as ◎ for a sound nugget obtained at 5000 points.

[0038]

[Table 2] As is evident from the results, in the alloyed hot-dip galvanized steel sheets in the generally manufactured ranges of Test Nos. 1 to 6, the value of X was 4.0 or more, and the continuous spot welding shown in the weldability evaluation was performed. The number of hit points was up to about 4000 points. When treated with an alkali solution or an acid solution, X can be reduced to a value of less than 4.0, and a number of continuous welding points close to 5000 can be obtained. These are steel sheets each of which has been subjected to temper rolling once, but the temper rolling of the present invention has been performed multiple times,
Then, for a steel sheet treated with an alkali solution or an acid solution, X
Is less than 3.0, and the number of continuous welding points exceeds 5,000.

The value of the difference X between the maximum value and the minimum value of the atomic concentration ratio of Al and Zn is large when the number of continuous dots is small, and when the value is small, the number of continuous dots is large. It turns out to be an excellent indicator.

[0040]

The alloyed hot-dip galvanized steel sheet of the present invention comprises:
The continuous spotting property of spot welding is particularly excellent. In addition, its manufacturing method is not affected by plating conditions or alloying treatment conditions that govern corrosion resistance, paint adhesion, powdering resistance during processing, etc., which are important for galvannealed steel sheets. Since the weldability can be improved, the contribution to the industry in which such steel sheets are frequently used, such as automobile assembly, is extremely large. Further, the method for evaluating the continuous spotting property of spot welding using the X value indicating the difference between the maximum value and the minimum value of the atomic concentration ratio of aluminum and zinc on the surface of the plating layer is evaluated in a short time with a small number of steel sheet samples. It can be used effectively for quality control of steel sheets, selection of manufacturing conditions, and examination of improvement of its properties.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroshi Takebayashi 4-5-33 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Within Sumitomo Metal Industries, Ltd. (72) Inventor Tamotsu Toki 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture No. 33 Sumitomo Metal Industries, Ltd. F term (reference) 4K027 AA05 AA22 AB02 AB05 AB06 AB13 AB28 AB43 AB44 AC73 AC82 AC87 AE27 AE33

Claims (3)

[Claims] An alloyed hot-dip galvanized steel sheet having a plating layer having an Fe content of 7 to 20% by mass, wherein the plating layer has a size corresponding to a surface in contact with the electrode tip of spot welding. The maximum value (Al / Zn) max and the minimum value (Al / Z) of the atomic concentration ratio of aluminum and zinc on the surface
n) The alloyed hot-dip galvanized steel sheet excellent in continuous spot welding of spot welding, wherein the difference from min, that is, the value of X represented by the following formula is 3.0 or less. X = (Al / Zn) max-(Al / Zn) min ... 2. The total elongation after alloying treatment is 0.3 to 1.5%.
2. The spot welding continuation according to claim 1, wherein after the temper rolling in the range of 2 or more is performed in two or more times, the solution is brought into contact with an acid solution or an alkali solution or both solutions in which zinc is dissolved. A method for manufacturing alloyed hot-dip galvanized steel sheets with excellent hitting properties. 3. The continuous doting property in spot welding of an alloyed hot-dip galvanized steel sheet is defined by the atomic concentration ratio of aluminum and zinc on the surface of a plating layer within a range corresponding to the surface in contact with the electrode tip of spot welding. A spot weldability evaluation method characterized by making a judgment based on a difference between a maximum value and a minimum value.