JPH1068057A - Galvannealed steel sheet excellent in coating suitability and corrosion resistance after coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the coating suitability and corrosion resistance and corrosion resistance after coating of a galvannealed steel sheet by an inexpensive means to be used as the outer board of a car body. SOLUTION: In a galvannealed steel sheet, by prescribing the coating weight to <=90g/m<2> , the compsn. of plating coating film to 0.10 to 0.40wt.% Al and 7 to 15wt.% Fe, the content of Fe present in the topmost surface layer of the plating to 0.02 to 0.15 by the weight ratio of Fe/Zn and the temper area ratio after temper rolling to >=10%, where the conent of Pb present in the plating coating film is regulated to 0.25×(the Al% in the plating coating)-0.04<=Pb<=0.2, its coating appearance and corrosion resistance after coating can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is used for an automobile body such as an automobile outer panel whose appearance is regarded as important.
The present invention relates to a galvannealed steel sheet. Especially,
In addition to ensuring workability such as powdering properties and flaking properties, which are important as the basic performance of alloyed hot-dip galvanized steel sheets used for automobile bodies, paintability, which is a problem of alloyed hot-dip galvanized steel sheets, More particularly, the present invention relates to an alloyed hot-dip galvanized steel sheet that suppresses the occurrence of coating defects and has excellent corrosion resistance after coating.

[0002]

2. Description of the Related Art Conventionally, alloyed hot-dip galvanized steel sheets have excellent corrosion resistance and weldability after painting. As widely applied, its demand is increasing.

In recent years, in particular, alloyed hot-dip galvanized steel sheets are more advantageous in cost than conventional electroplated steel sheets or organic composite coated steel sheets based on electroplated steel sheets. Not only as a steel plate,
Application to car body outer panels, where paintability and finished appearance after painting are important, is being studied.

However, alloyed hot-dip galvanized steel sheets are susceptible to coating defects (gas pins, crater rings) at the time of electrodeposition coating, which is indispensable in the body coating process, due to the problem of the coating (alloy phase structure). In addition, there is a problem of paintability in which the corrosion resistance after painting is reduced with the progress of local corrosion in the flawed portion.

As a method for solving this problem, for example, as disclosed in Japanese Patent Application Laid-Open No. 3-257192, an upper layer plating having a very high Fe content is conventionally provided on the upper layer of a galvannealed steel sheet. Electrically applied two-layer plating has been applied. However, this method is not preferable because the plating process becomes complicated and the number of processes increases, which causes an increase in cost.

[0006] Conventionally, for the purpose of ensuring sharpness after coating of an alloyed hot-dip galvanized steel sheet, for example, Japanese Patent Application Laid-Open No. 4-285149 has been proposed.
As disclosed in Japanese Unexamined Patent Publication (Kokai) No. H11-209, it is disclosed that the roughness of the base material surface is specified. However, according to the results of the study by the present inventors, the level of the roughness of the steel sheet itself is almost zero. And the roughness in finer plating crystal units is the dominant factor.

[0007] For example, Japanese Patent Application Laid-Open No. Hei 3-211264 discloses a method for reducing the roughness of an alloyed hot-dip coated steel sheet by defining the amount of Al in the coating after Zn plating and the alloying temperature. ing. In this method, since the alloying temperature is low, a large amount of the soft alloy phase, ζ phase, remains on the plating surface layer, so that there is a problem of deterioration in workability such as slidability.

Further, Japanese Patent Laid-Open Publication No. 4-8386 discloses a method for coping with this by defining the temperature of a steel sheet entering a Zn plating tank.
No. 0 discloses this. In such a method of increasing the temperature of the intruding material, an outburst reaction, which is an abnormal Fe diffusion reaction from the base material grain boundary, is likely to occur, and microscopically, the surface roughness may be rather increased, Not preferred.

[0009]

SUMMARY OF THE INVENTION The first object of the present invention is to use a conventional galvannealing process without performing an upper plating process, which causes a cost increase, due to the complexity of the processing process. Accordingly, it is an object of the present invention to provide an inexpensive alloyed hot-dip galvanized steel sheet which is required for use as an outer panel of a vehicle body and has improved paintability, that is, appearance after coating, which is a problem of the galvannealed steel sheet.

A second object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having improved corrosion resistance after painting, which has been similarly demanded for an outer panel of a vehicle body and which is a problem of an alloyed hot-dip galvanized steel sheet. It is to provide at low cost.

[0011]

The present inventors investigated the paintability (electrodeposition) of alloyed hot-dip galvanized steel sheets having various compositions and surface shapes, and found that the following points were found in the paint finish. It has been found that it greatly affects the corrosion resistance after painting.

With respect to the corrosion resistance after coating, the Fe% (degree of alloying) of the entire plating film layer cannot be arranged, and the amount of Fe on the outermost surface of the plating is very important.

Paint finish (gas pin resistance during electrodeposition)
Therefore, it is important to control fine irregularities on the surface of the steel sheet.

On the other hand, the fine irregularities are caused by a local difference in the diffusion rate of Fe in the plating base material, and the microscopic irregularities of the alloy plating can be stably removed without changing the conditions in the alloying process. As another means for suppressing this, by adding Pb to the plating bath, this local difference in Fe diffusion rate can be suppressed.

As described above, by adding Al to secure plating adhesion and adding Pb to suppress unevenness of plating, an oxide layer of Al and Pb is formed on the surface of the plated steel sheet, and Since the amount of Fe on the outermost surface of the plating is reduced, it is difficult to secure the above-mentioned good corrosion resistance after coating. Therefore, there are optimal values for the amounts of Al and Pb.

Further, as a method for ensuring a sufficient amount of Fe in the surface layer, in order to effectively remove the amphoteric metal Al and Pb oxide layers, it is preferable to wash the plating surface with acid or alkali.

Here, the gist of the present invention is as follows. (1) The coating weight is 90 g / m ² or less on the steel sheet, and
The plating film composition has a plating film with an Al content of 0.10 to 0.40 wt% and a Fe content (degree of alloying) of 7 to 15 wt%, and the Fe content in the outermost layer of the plating film is Fe / Zn Weight ratio: 0.02 to 0.15, when temper rolling after plating, the area where the surface of the plating film is crushed by temper rolling (temper area ratio) is 10
% Alloyed hot-dip galvanized steel sheet with excellent coating appearance and corrosion resistance after coating.

(2) The coating weight on the steel sheet is 90 g / m ² or less, and the Al content of the plating film composition is 0.10 to 0.40 wt%.
Fe content (degree of alloying) has a plating film of 7 to 15 wt%, and the amount of Fe present in the outermost layer of the plating film is 0.02 to 0.15 by weight ratio of Fe / Zn. When rolling, the area where the surface of the plating film is crushed by temper rolling (temper area ratio) is 10
%, And the amount of Pb present in the plating film is a film composition satisfying the relationship of the following formula.

0.25 × (Plating film Al%) − 0.04 ≦ Pb ≦ 0.2 (3) After the alloying step, the coating appearance and the corrosion resistance after the coating according to the above (2), characterized by being washed with alkali or acid. Excellent galvannealed steel sheet.

[0020]

As described above, according to the present invention, in order to prevent fine irregularities in order to prevent gas pins as a cause of appearance defects after coating, when the plating film is low Al, it is Pb-free, that is, zero. As a result, the gas pin resistance is improved by crushing the plating surface by temper rolling, and when Al in the plating film is high, Pb is included in the coating to suppress micro-roughness, and Gas pin resistance is improved by crushing the plating surface by temper rolling.

However, when Pb is contained in the plating film in this way, Pb oxides may be generated and the adhesion of the coating film may be deteriorated. In such a case, oxidation of the surface which causes a decrease in corrosion resistance after coating may be caused. The film is removed with an alkali or acid. Next, the reason defined above in the present invention will be described.

Weight per unit area: An alloyed hot-dip galvanized steel sheet forms an alloy plating layer by utilizing heat diffusion from a base material. Therefore, when the weight per unit area is larger than 90 g / m ² , the important performance of the present invention is obtained. Therefore, since the amount of Fe in the outermost layer of the plating cannot be sufficiently secured, the corrosion resistance after coating decreases. Also, up to the plating surface layer,
If the alloyed film is sufficiently formed, the Fe content is high and the brittle Γ phase is formed at the interface between the base material and the plating. If the basis weight is too small, there is a problem of corrosion resistance,
The plating weight is preferably 25 to 70 g / m ² .

Al content: The Al content in the plating film is important for improving the adhesion of the plating, and in the present invention, it is also necessary to ensure the workability as the basic performance of the galvannealed steel sheet. In addition, addition of Al is inevitable, and it is preferable that Al can be incorporated into the plating film as much as possible. Therefore, when the Al content is less than 0.10 wt%, sufficient plating adhesion cannot be secured, and there is a problem with workability.

On the other hand, the amount of Al in the plating film suppresses the diffusion of Fe from the base material during the heat alloying treatment, thereby delaying the alloying. Therefore, when the amount of Al is larger than 0.50 wt%, usually, when the amount of Al is larger than 0.40 wt%, the diffusion of Fe to the outermost surface of the plating is delayed, and the outermost layer of the plating in the present invention is
It is difficult to secure a uniform and sufficient Fe content, which is not preferable.

Further, when the amount of Al increases, an outburst reaction, which is abnormal diffusion of Fe from the base metal grain boundary, tends to occur, so that microscopic unevenness of the plating layer increases, which is another important factor in the present invention. Gas pinning, which is a coating film defect at the time of electrodeposition coating, which is a performance, is likely to occur, which is a problem.

Degree of alloying: Degree of alloying means the workability of plating
(Adhesion) is greatly affected, and the workability is improved as the degree of alloying is lower. On the other hand, when the degree of alloying is higher, plating peeling tends to occur, which is not preferable. Further, Fe% of the outermost plating surface, which is an important finding in the present invention, is also affected by the plating film layer, and if the degree of alloying is low, it is not preferable because a sufficient amount of Fe on the outermost plating surface cannot be secured. Therefore,
In the present invention, the range of the degree of alloying is 7 to 15 wt%.
It was specified. Preferably 8 to 13% is optimal.

Outermost layer Fe / Zn weight ratio: In an alloyed hot-dip galvanized steel sheet utilizing the diffusion of Fe from the base material, a Fe concentration gradient is formed in the depth direction of the coating film layer. On the other hand, in an alloyed hot-dip galvanized steel sheet, corrosion resistance after scratching after painting may be deteriorated.

In general, when a chemical conversion coating is formed in the pretreatment step of coating, in a galvannealed steel sheet, a portion where the amount of Fe in the surface layer of the coating is locally insufficient tends to exist, and the resistance to corrosion is increased. A chemical conversion film having deteriorated alkalinity is easily formed. Furthermore, if a healthy chemical conversion film and a deteriorated chemical conversion film are mixed, mainly at the scratched part of the coating film, corrosion concentrates on the deteriorated conversion crystal, the damage of the chemical conversion film increases, and the corrosion resistance after painting increases. Cause a decrease.

Therefore, it is important to suppress the degree of alloying in the plating film and to reduce the amount of Fe present on the outermost surface of the plating as much as possible without lowering the workability of the plating. Here, the "outermost layer" is a surface layer portion that affects the formation of the chemical conversion film, and is generally a region having a depth of 100 mm or less from the surface. Specifically, "XPS" used in the examples described later is also used. Refers to the area analyzed by

Therefore, in the present invention, the effect can be expected when the amount of Fe present in the outermost layer is 0.02 to 0.15 in terms of Fe / Zn weight ratio. If less than 0.02,
Because the amount of Fe on the surface layer is insufficient and a uniform amount of Fe cannot be secured,
A sound chemical conversion film is not formed, and a sufficiently good corrosion resistance after coating cannot be obtained. When it is larger than 0.15, it is possible to obtain a sound chemical conversion crystal, but since the degree of alloying of the plating film is too high, the possibility of the plating film is a problem. In order to stably secure corrosion resistance and workability after coating, the Fe content is preferably 0.05 to 0.10. In addition,
The degree of alloying and the outermost Fe / Zn ratio of the plating film can be adjusted, for example, by changing the alloying treatment conditions.

Temper area ratio: Generally, in order to adjust the surface roughness after plating and to improve the mechanical characteristics,
After alloying, temper rolling is performed by a skin pass.
When the skin pass is not performed, the alloyed hot-dip galvanized steel sheet has large irregularities for each alloying crystal of the plating, and has a microscopically large irregular surface.

The present inventors have found that such fine irregularities of each plating crystal grain become a starting point of current concentration at the time of electrodeposition coating, and as a result, a discharge phenomenon occurs at the time of electrodeposition coating, which is likely to cause gas pins. I found something.

Therefore, in order to reduce the microscopic unevenness of each plated crystal grain generated in the galvannealed steel sheet, the area ratio of the plated surface crushed by the skin pass roll, the area of the surface crushed in a visual field of 0.5 × 0.5 mm, is reduced. Ratio (hereinafter referred to as temper area ratio) is measured by image analysis. If the temper area ratio is less than 10%, a poor electrodeposition finish due to unevenness of the plating crystal occurs.
It is effective to crush the above. Preferably, this temper area ratio is 20% or more. The upper limit of the temper area ratio is not particularly limited, but is generally 60% or less.

By the way, regarding the coating defect at the time of electrodeposition,
It depends not only on the irregularities of each plating crystal grain, but also on the convex defects along the base material crystal grains that depend on the outburst reaction. Such an outburst reaction occurs in the plating film.
It is generally known that the dependence on the amount of Al is large, and becomes significant as the amount of Al increases. In order to suppress such an Al-dependent outburst reaction, by lowering the amount of Al and lowering the intruding material temperature, it is possible to suppress the reaction between the plating base material and Al in the plating bath during hot dipping. Although effective, such a method is difficult to stably suppress the outburst reaction.

In order to stably secure workability, it is effective to increase the amount of Al as much as possible. However, if Al is added excessively, an outburst reaction easily occurs as described above, and large irregularities are generated. This will create the surface shape. It is also effective to add Pb to the plating bath to suppress the surface irregularities. Therefore, the addition of Pb is a prerequisite in the present invention, and is the amount of plating film Al in the case of no Pb addition.
By containing 0.10 to 0.40 wt% or more, specifically, 0.30 wt% or more of Al, stable workability can be secured.

Although the effect of this Pb is unknown in detail, it reacts with the plating base material during immersion in the plating bath and forms Fe-Al
It is presumed that the presence of Pb in the alloy phase promotes the diffusion of Fe from within the base material crystal grains and suppresses the outburst in order to disperse the Fe diffusion starting point in the subsequent alloying process. .

However, as the amount of Al increases significantly, the amount of Pb required for the effect of suppressing the outburst reaction increases, and the lower limit of the amount of added Pb is defined by the following equation.

0.25 × (plating film Al%) − 0.04 ≦ Pb The upper limit of the amount of added Pb is 0.2 wt%, and if it is larger than this, a spangle pattern which occurs during the solidification process during plating remains, and Even after the chemical conversion treatment, the spangle pattern of the plating remains, which is not preferable. Further, since Pb that cannot be dissolved is left at the plating / base metal interface, it is not preferable because plating adhesion such as powdering property and flaking property decreases. Further, as will be described later, when the addition of Pb increases, the surface oxide film layer becomes thicker, and the Fe content on the outermost surface of the plating relatively decreases, so that corrosion resistance after painting also becomes a problem.

However, in this case, the upper limit of the amount of Al is set to 0.
If it is more than 60%, it is difficult to suppress fine irregularities, and excessive Pb is added, which is not preferable.

As described above, when Al and Pb are added for the purpose of ensuring workability and finishing the electrodeposition coating,
Al and Pb, which are easily oxidizable metals, easily form an oxide film on the surface layer of a galvannealed steel sheet.
As a result, an oxide film layer is formed on the plating surface layer,
There is a problem that the amount of Fe on the outermost surface of the plating required to obtain corrosion resistance after painting is reduced.

Therefore, it is necessary to remove the oxide layer and secure a sufficient amount of Fe in the outermost layer of the plating in order to secure the corrosion resistance after coating. As a method of removing the oxide layer, By immersing in an acidic solution or an alkaline solution, the oxide film layer of the amphoteric metal elements Al and Pb can be easily removed.

As a treatment method at that time, for example, the Pb and Al oxide layers on almost the surface can be removed by immersing in an acid solution for 10 seconds in a sulfuric acid solution having a pH of 1.5 and a temperature of 60 ° C. Also,
Alkaline solutions can be almost completely removed by immersing them in a sodium hydroxide solution having a pH of 13.0 and a temperature of 70 ° C. for 5 seconds.

[0043]

EXAMPLES The effects of the present invention were confirmed by experimentally producing various alloyed hot-dip galvanized steel sheets based on a base metal having a steel composition shown in Table 1 (sheet thickness = 0.8 mm).

First, the composition of the coating film of the galvannealed steel sheet was determined by dissolving the plating film and measuring the amount of each element (Zn, Fe, Al, Pb) by atomic absorption measurement of this solution. Weight and Fewt% (degree of alloying) in film, Alwt
% And Pbwt%.

The amount of Fe on the outermost surface of the plating film was measured by ion sputtering (about several tens of seconds) for 30 seconds in order to remove surface contamination.
A) For a sample with improved surface cleanliness, only depth information of a few A can be measured, and XPS suitable for the analysis of the outermost surface is used to convert the energy spectrum of metal Zn and metal Fe into weight%. After that, it was finally converted to the Fe / Zn weight ratio.

[0046]

[Table 1]

Here, evaluation items of various test materials are described. Workability (Powderability ) The vertical wall after the cylindrical drawing test was peeled off with a tape, and the plating peeling amount (powdering amount) per piece was measured by a gravimetric method. The criterion at that time is determined based on the characteristics required for the steel sheet for the vehicle body. The powdering amount is as follows. If it is 20 mg / piece or less, there is no problem. If possible, less than 10 mg / piece. Good.

[0048] (Evaluation criteria) Plating peeling amount: ○: less than 10 mg / piece Δ: 10 to 20 mg / piece ×: more than 20 mg / piece.

After the coating was subjected to a chemical treatment under the standard recommended conditions of a corrosion-resistant chemical solution after coating (PB-3080: manufactured by Nippon Parker Co., Ltd.), a sample subjected to electrodeposition coating under the following electrodeposition conditions was applied to a Gravelo test machine. After the surface of the coated plate was scratched and subjected to a salt spray test (SST) for 240 hours, the coated film peeling area after tape peeling of the entire coated plate was measured by image analysis. As the peeling measurement area at that time, 50 × 50 mm
In the field of view, there is no problem if the coating film peeling area ratio is 5% or less. If possible, the goal was 2% or less.

(Electrodeposition conditions) Electrodeposition liquid: Power Top U-80 Energization: 200 V Film thickness: 18 μm (Grabero conditions) Temperature: Room temperature Stone: Basalt 100 g (diameter = 5-8 mm) Air pressure: 1.5 K / cm .

After a chemical conversion treatment under the standard recommended conditions of a coating chemical conversion solution (PB-3080: manufactured by Nippon Parker Co., Ltd.), the sample which was subjected to electrodeposition coating under the following electrodeposition conditions showed a coating defect of 50%. The case where the number of gas pins generated in a range of × 50 mm was 3 or more was defined as gas pin generation, and the electrodeposition voltage at which gas pins were generated was investigated as gas pin generation voltage. There is no problem if the gas pin generation voltage ≥ 250V as a criterion at that time. If possible, the gas pin generation voltage ≧ 280V was determined to be good.

(Electrodeposition conditions) Electrodeposition liquid: Power Top U-80 Energization: from 180 to 300 V, 10 V pitch Film thickness: 20 μ (adjustment of energization time) Hereinafter, the effects of the present invention will be specifically described.

First, Table 2 shows the results when Pb was not contained in the plating film. As is clear from the results shown in the table, the basis weight is 90 g / m ² as in Example No. 22.
It turns out that it becomes difficult to ensure workability when the value exceeds.

Also, regarding the degree of alloying, the powdering property is lower than in Examples Nos. 1 and 13, so the upper limit is 15 wt%.
From Examples Nos. 3 and 19, the corrosion resistance after painting deteriorates, indicating that the lower limit of the degree of alloying is 7.0 wt%.
Also, the upper limit of the amount of Al in the coating was 0.40 wt% from Examples Nos. 4 and 17.
From Example No. 14, it can be seen that the lower limit is 0.10 wt%.

Further, regarding the Fe / Zn weight ratio of the outermost layer, the upper limit value from Example No. 1 was 0.15.
o.3, 5, 16, and 17 indicate that the lower limit is 0.02.
In addition, the amount of Fe present in the outermost layer of the plating does not always match the degree of alloying, which is the amount of Fe in the plating film, and cannot be estimated by the degree of alloying to improve corrosion resistance after painting. It turns out to be an important factor. The temper area ratio affects the paintability, and the lower limit is from Examples Nos. 12 and 17.
It turns out that it is 10%.

[0056]

[Table 2]

Next, the effect of adding Pb and the effect of removing the surface oxide will be described. Laboratory scale plating was performed in various hot-dip galvanizing baths with different amounts of Al and Pb. that time,
The same test base metal as in Table 1 was subjected to the test, and after annealing, immediately subjected to plating and alloying treatment, was subjected to the test. With respect to these samples, the surface was crushed by a skin pass in a laboratory so that the elongation was 1%, and the temper area ratio was examined. In this example, in each case, the plating weight is
50 g / m ² , the degree of alloying was 10%.

These alloyed hot-dip galvanized steel sheets were evaluated in the same manner as described above. Partly, as a method of removing an oxide film, acid and alkali washing were performed under the following conditions. Acid cleaning: liquid = 10% hydrochloric acid, temperature = 60 ° C, immersion time =
5 seconds alkaline cleaning: liquid = 5% sodium hydroxide, temperature = 70
° C, immersion time = 5 seconds The relationship between the amount of Al and the amount of Pb is also described in Table 3.

The results are shown in Table 3. From the results shown in Table 3, when the Al content is low as in Example No. 1, sufficient powdering properties may not be ensured in some cases. It is necessary to increase the amount of Al.

On the other hand, from Examples Nos. 32 to 38, it is difficult to secure a sufficient amount of Fe in the surface layer with an excessive amount of Al, and the corrosion resistance after coating is lowered and the temper area ratio cannot be ensured. It turns out that it becomes difficult. Further, as the Al content increases, the temper area ratio also increases, indicating that the lower limit of Pb within the scope of the present invention is required.

On the other hand, when Pb is added in excess of 0.20%,
It can be seen that the Fe content of the surface layer is reduced, and there is a possibility that corrosion resistance after coating within the range of the present invention may not be obtained, and that the powdering property is reduced.

Further, by removing the oxide film on the surface layer with an acid or alkali solution, the amount of Fe in the outermost layer can be easily increased. For example, as shown in Examples Nos. 23 and 24, It can be seen that an optimum alloyed hot-dip galvanized steel sheet can be provided in all required performances.

[0063]

[Table 3]

[0064]

As described in detail above, the galvannealed steel sheet according to the present invention ensures excellent workability required as a rust-preventive steel sheet for an automobile body, and has excellent corrosion resistance after painting and appearance. Therefore, there is no need to take a complicated process of applying an upper layer plating having a high iron content to an upper layer conventionally used. Therefore, as a rust-proof steel sheet for the outer surface of the car body, in the conventional hot-dip galvanizing line,
Provision of an alloyed hot-dip galvanized steel sheet that is easy and cost-effective can be expected.

Claims (3)

[Claims] 1. A steel sheet having a coating weight of 90 g / m ² or less and a coating film composition having an Al content of 0.10 to 0.40 wt%,
Having a plating film whose amount (degree of alloying) is 7 to 15 wt%,
Also, the amount of Fe present in the outermost layer of the plating film is 0.02 to 0.15 in Fe / Zn weight ratio, and the area where the surface of the plating film is crushed by the temper rolling during temper rolling after plating ( Temper area ratio) is 10
% Alloyed hot-dip galvanized steel sheet with excellent coating appearance and corrosion resistance after coating. 2. A steel sheet having a coating weight of 90 g / m ² or less and a coating film composition having an Al content of 0.10 to 0.40 wt%,
Having a plating film whose amount (degree of alloying) is 7 to 15 wt%,
Also, the amount of Fe present in the outermost layer of the plating film is 0.02 to 0.15 in Fe / Zn weight ratio, and the area where the surface of the plating film is crushed by temper rolling during temper rolling after plating ( Temper area ratio) is 10
%, And the amount of Pb present in the plating film is a film composition satisfying the relationship of the following formula. 0.25 × (Plating film Al%) − 0.04 ≦ Pb ≦ 0.2 3. After the alloying treatment step, an alkali or
The galvannealed steel sheet according to claim 2, wherein the steel sheet is pickled and has excellent coating appearance and corrosion resistance after coating.