JPH10226862A - Galvannealed steel sheet excellent in press formability and smoothness of plating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably obtain a galvannealed steel sheet excellent in press formability and smoothness of plating film by regulating the contents of Fe and Al in a galvannealing layer of a steel sheet and the coating weight of plating alloy, respectively. SOLUTION: A steel sheet 1 is passed through a hot dip galvanizing bath 2 to undergo hot dip galvanizing on the surface. After regulation of coating weight by a coating weight regulating device 6, the resultant hot dip galvanized steel sheet is heated in an alloying furnace 13, by which the galvanizing layer is alloyed with the steel sheet. By regulating the components of the hot dip galvanizing bath 2 and properly regulating the temp. of the alloying furnace, the resultant galvannealing layer can be provided with a composition consisting of, by weight, 7-15% Fe, 0.35-1.0% Al, and the balance Zn and the coating weight can be regulated to (15 to 100)g/m per side of the steel sheet. By regulating the thickness of a hard and brittle γ-phase to <=1.0μm and also the amount of an μ-phase of pure zinc to <0.1%, the galvannealed steel sheet, excellent in powdering resistance and flaking resistance, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet, and more particularly, to a novel galvannealed steel sheet having excellent press formability and smoothness of a plating film.

[0002]

2. Description of the Related Art A so-called alloyed hot-dip galvanized steel sheet obtained by heat-treating a hot-dip galvanized steel sheet is widely used as a material for automobiles, home appliances and building materials, and has press formability, weldability, corrosion resistance, paintability, etc. Performance is required. In particular, excellent powdering resistance and flaking resistance are required as press moldability. Powdering is a phenomenon in which a plating layer peels off during press forming, and is likely to occur when a hard and brittle Г phase having a high Fe concentration and a large thickness is generated at the interface between the base steel sheet and the plating layer. On the other hand, flaking is a peeling phenomenon of a plating layer caused by "mold seizure" at the time of press working.
When the η phase (pure zinc phase) that has not been alloyed remains tends to occur. When these powdering and flaking occur, a peeled piece adheres to the mold during the press forming of the steel sheet, and the product has a pressing flaw, etc., so the cleaning of the press mold is inevitable and the yield of the pressing process is reduced. There was a problem.

In order to solve the above problem, Japanese Patent Publication No. 3-55
No. 544 discloses that the effective Al amount in a plating bath is 0.10% by weight or less, and the immersion time is 3 seconds, preferably 2 seconds or less, so that the Fe—Al alloy layer formed in the plating bath is thinned. We have proposed a technology that leads to an alloying furnace in a healthy state. As a result, there is no η phase or ζ phase on the plating surface layer, and the Г phase thickness at the interface of the base steel sheet can be reduced to 1 μm or less, and alloying with excellent powdering resistance and flaking resistance is achieved. Galvanized steel sheets can now be manufactured. The composition of the plating layer is F
e: 8 to 12% by weight, Al: 0.05 to 0.25% by weight, with the balance being Zn.

[0004]

However, according to the study of the present inventors, even the alloyed hot-dip galvanized steel sheet disclosed in the above-mentioned Japanese Patent Publication No. 3-55544 requires more severe conditions such as actual automotive applications. In the press working, it became clear that there was a problem in the flaking resistance, and the cause was studied diligently. As a result, the presence of the ζ phase was confirmed in the conventional galvannealed steel sheet by the semi-quantitative X-ray diffraction method described below, and the steel sheet contained 0.05 to 0 Al in the plating layer. Since the content was as low as 0.25% by weight, it was found that the Δ phase was easily formed and the flaking resistance was poor.

Accordingly, as a method of improving the flaking resistance of the conventional alloyed hot-dip galvanized steel sheet, the Al concentration in the plating layer is simply increased by increasing the Al concentration in the plating bath to increase the Al content in the plating layer. However, the Fe—Al alloy layer is formed thicker than necessary at the time of plating. As a result, at the time of alloying treatment after plating, the diffusion of Fe in the alloy layer is suppressed, so it is necessary to alloy at a high temperature or to promote Fe diffusion by alloying for a long time. .

[0006] However, alloying at high temperature is not preferable because a thick phase is generated at the interface with the base steel sheet and the powdering resistance is inferior. Since the time must be prolonged or the steel sheet speed must be reduced, the equipment cost increases and the product yield decreases. Furthermore, if the Fe—Al alloy layer is formed thicker than necessary during plating, the alloyed plating layer becomes significantly uneven and the sliding resistance (coefficient of friction) during press molding increases. However, there is also a problem that the metal is deteriorated.

[0007] In view of such circumstances, an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which suppresses the occurrence of powdering and flaking, and is superior in press formability and plating film smoothness. I have.

[0008]

Means for Solving the Problems In order to achieve the above object, the present inventors have focused on making Al more contained than a plating layer currently in practical use, and have solved the above-mentioned problem caused by high Al. I worked hard on my research. The present invention
That is, the steel sheet is immersed in a hot-dip galvanizing bath, and then the steel sheet is removed from the plating bath to form a plating layer on the surface, and then the steel sheet is subjected to a heat alloying treatment. Alloyed hot-dip galvanized steel sheet obtained by Fe: 7 to 15% by weight, Al: 0.3
It has a plating layer having a composition of 5 to 1.0% by weight, the balance being Zn and inevitable impurities, and having a plating layer of 15 to 100 g / m ² per side on at least one side, and having an η phase in the plating layer of 0.1 to 0.1 g / m ^2. It is an alloyed hot-dip galvanized steel sheet having excellent press formability and smoothness of a plating film, characterized in that the thickness thereof is less than 1.0% by weight and less than 1.0 μm at the interface between the base iron and the plating layer.

Further, the present invention is characterized in that Al is not concentrated at the interface between the base iron and the plating layer, and the surface roughness of the plating layer is 1.5 μm or less in center line average roughness (Ra). It is an alloyed hot-dip galvanized steel sheet excellent in press formability and smoothness of a plating film. In this case, the Al-enriched layer at the interface between the iron / plating layer and the Fe-Al alloy layer formed at the interface between the iron / plating layer during plating remains after the alloying process.

Further, the present invention provides an alloy excellent in press formability and plating film smoothness, characterized in that the adhesion amount of the plating layer and the Fe content in the plating layer satisfy the following relational expression. It is also a galvanized steel sheet. When 15 ≦ W <60, [Fe] ≦ 21−W / 7.5 When 60 ≦ W <100, [Fe] ≦ 16-W / 20, where W is the amount of adhesion per side (g / m ^2). [Fe]: Average Fe content in plating layer (% by weight) In the present invention, the alloyed hot-dip galvanized steel sheet is reduced in the amount of η phase and ζ phase in the plating layer, and the iron / plating layer interface is reduced. By reducing the thickness of the Г phase, the thickness of the plating layer itself is also reduced, so that the occurrence of powdering and flaking can be suppressed. In other words, a galvannealed steel sheet excellent in press formability and plating film smoothness has been newly obtained.

[0011]

DETAILED DESCRIPTION OF THE INVENTION First, in the galvannealed steel sheet according to the present invention, the thickness of the 抑制 phase and the amount of the η phase, which are disadvantageous to press formability, must be suppressed. For this reason,
The thickness of the Г phase at the interface between the base iron and the plating layer is 1.0 μm.
If the thickness exceeds 1.0 μm, the powdering resistance is inferior.

The η phase in the plating layer is less than 0.1% by weight. If the content is 0.1% by weight or more, mold seizure occurs during press molding, and the flaking resistance is poor. Further, the ζ phase, which is also disadvantageous to press formability, is reduced to such an extent that it is hardly detected even in semi-quantitative X-ray diffraction. For this reason, the Al in the plating layer is reduced by 0.35 to
1.0% by weight and Fe is 7 to 15% by weight. The reason is as follows. (1) Reasons for Al: 0.35 to 1.0% by weight Conventional Fe: 7 to 15% by weight, if Al in the plating layer is less than 0.35% by weight, the ζ phase remains on the plating surface layer When the Al content is less than 0.35% by weight, the content of Fe must exceed 15% in order to avoid the formation of the ζ phase. This is because processing is required. When the alloying treatment is performed at a high temperature or for a long time, the residual of the ζ phase is suppressed, but the formation of the Γ phase is promoted, and the thickness of the Γ phase easily exceeds 1 μm. Thus, in the present invention, the content of Al in the plating layer is set to 0.35% by weight or more, and the Δ phase is suppressed to a level that does not cause a problem without changing the conventional alloying conditions. Therefore, in the present invention, the amount of the ζ phase is not limited. This is because the amount of the ζ phase is not revealed by the potential measurement, and it is usually difficult to grasp quantitatively. In the present invention, it was confirmed by semi-quantitative analysis by the X-ray diffraction method. In other words, FIG. 4 shows that, measured using the intensity of a diffraction line corresponding to the crystal lattice spacing d = 1.26, which is a characteristic peak of the ζ phase, Fe: 7 to 15% by weight and various Al contents 4 shows the Al content in the coating layer of the alloyed hot-dip galvanized steel sheet obtained by alloying a large amount of plating and the X-ray diffraction intensity of the ζ layer. FIG. 4 shows that Al according to the present invention: 0.3
It can be seen that the formation of the ζ phase is suppressed in the alloyed hot-dip galvanized steel sheet of 5% by weight or more as compared with the conventional alloyed hot-dip galvanized steel sheet.

On the other hand, when Al in the plating layer exceeds 1.0% by weight, the Al concentration in the plating bath must be 0.5% by weight or more, and dross (Fe-Al
System, Al-O system, etc.), which undesirably increases, and adheres to the steel sheet during plating to cause dross defects. (2) Reason for Fe: 7 to 15% by weight When Fe is less than 7% by weight, the ζ phase is likely to remain on the plating surface layer, and in extreme cases, a portion where the η phase remains at 0.1% by weight or more is generated. . As a result, the flaking resistance is remarkably deteriorated, which is not preferable. On the other hand, when Fe exceeds 15% by weight, the Δ phase at the interface with the base steel sheet is 1.0 μm
Since it is formed with the above thickness, the powdering resistance deteriorates, which is not preferable.

The components of the plating layer other than Al and Fe are Zn and unavoidable impurities. Next, in the present invention, it is preferable that the adhesion amount of the plating layer is in the range of 15 to 100 g / m ² per one side. From the viewpoint of press formability, the adhesion amount is the smaller is advantageous, it is less than 15 g / m ^2, can not be obtained corrosion resistance is required, whereas, if it exceeds 100 g / m ^2, the thickness of Г phase If the thickness is 1 μm or less and the η phase in the plating layer is less than 0.1% by weight, a long alloying heat treatment is required, which is not practical. The plating layer of the present invention is preferably formed on both surfaces or one surface, and if necessary, may be the plating layer of the present invention described above on only one surface. In that case, the other surface may have a different amount of adhesion, or may have a different plating composition and surface properties according to the present invention described above.

As described above, according to the present invention, it is clear that the formation of the layer ζ is remarkably suppressed in the range where the Al content in the plating layer exceeds 0.35% by weight. King performance has been significantly improved. Therefore, a point to be considered when actually attempting to manufacture a plated steel sheet having 0.35% by weight or more of Al in the above-described plating layer using a hot-dip galvanizing apparatus shown in FIG. This will be described below.

According to the study of the present inventors, the time for immersing the steel sheet in the plating bath is 3 seconds (steel sheet speed: 100 mm / m
in) and increasing the Al concentration in the bath to about 0.16% by weight, a plated steel sheet containing 0.35% by weight or more of Al in the plating layer could be produced. However, Al in the plating bath reacts preferentially with Fe eluted from the steel sheet, and Fe-A
An alloy layer is formed. That is, if the Al concentration in the plating bath is increased to 0.16% by weight, the Fe-Al alloy layer at the steel sheet interface becomes thick. As a result, the diffusion of Fe from the steel sheet in the alloying process after plating is excessively suppressed, and the η phase is 0.1% by weight on the steel sheet surface.
Or alloying at a high temperature or alloying for a long time is forced. Therefore, in order to manufacture the steel sheet according to the present invention, Al in the plating layer is 0.35.
% By weight or more and F generated at the steel sheet interface during plating
The e-Al alloy layer must be thin (Al content in the Fe-Al alloy layer is preferably 0.1 g / m ³ or less).

Therefore, the time of immersion in the plating bath is set to 0.
This object was achieved with about 001 to 0.5 seconds. If the immersion time exceeds 0.5 seconds, the Al in the plating layer is reduced to 0.1.
This is because the two conditions of 35% by weight or more and that the Fe—Al alloy layer at the time of plating is made thinner cannot be satisfied.
On the other hand, if the immersion time is less than 0.001 second, the formation of the Fe-Al alloy layer during plating is excessively suppressed, and the diffusion of Fe from the steel sheet to the plating layer during the alloying process after plating is excessive. Therefore, the formation of the Г phase at the steel sheet interface cannot be suppressed, and the thickness of the Г phase becomes 1.0 μm or more, which is not preferable. According to the study of the present inventors, the Fe—Al alloy layer formed at the time of plating was preferably 0.03 to 0.1 g / m ^{2 in} terms of the amount of Al in the Fe—Al alloy layer. Further, in order to form the above-mentioned proper Fe-Al alloy layer, it is preferable to set the Al concentration in the plating bath to 0.18 to 0.45% by weight.

After the steel sheet is subjected to hot dip galvanizing under the above-described plating conditions and then subjected to alloying treatment under ordinary conditions, the Al content in the plated layer is 0.35% by weight or more, and Phase formation, and reduce the η phase in the plating layer to 0.1%.
Less than 1% by weight.
This enabled the production of an alloyed hot-dip galvanized steel sheet having a thickness of 0 μm or less. In addition, the alloyed hot-dip galvanized steel sheet of the present invention manufactured by such a method has a higher Al concentration in the plating layer than a conventional alloyed hot-dip galvanized steel sheet (Japanese Patent Publication No. 3-55544), and thus the coated galvanized steel sheet after coating is obtained. Corrosion resistance (JI
SC 0024).

In the present invention, the soft η phase remaining on the plating surface layer was determined as a ratio of the η phase to all the plating layers by using a potential measurement method. In other words, ZnSO ₄ · 7
The sample anode is subjected to constant current electrolysis at a current density of 20 mA / dm ² in an electrolytic solution containing 100 g / L of H ₂ O and 200 g / L of NaCl. And
If the measured potential is −1000 mm volt (with respect to the standard electrode) or less, it is determined that the η phase exists (the potential of the base iron is −
400 mm volts (around the standard electrode). Also,
For the hard and brittle Г phase formed at the interface between the base iron and the plating layer, the thickness of the Г phase was measured by observing the cross section of the plating layer with an optical microscope. Further, for discrimination of the alloy phase, the cross section of the plating layer was analyzed by XMA (X-ray microanalysis), and the region where the Fe composition was 18.5 to 31.0% by weight was defined as the Δ phase. The interface between the base steel and the plating layer, but .GAMMA phase and .GAMMA ₁ phase is generated, the identification is very difficult, the .GAMMA phase in the present invention, it alloy layer containing .GAMMA ₁ phase It is.

Subsequently, the features of the second present invention will be described below. As described above, generally, Al in the plating bath
When the concentration increases, a thicker Fe-Al alloy layer is formed at the interface between the base iron and the plating layer while the steel sheet is immersed in the plating bath. If the Fe-Al alloy layer at the time of plating is formed thicker than necessary, the diffusion of Fe from the steel sheet during the alloying process after plating is extremely suppressed, and as a result, even after the alloying process is completed. , Fe-Al
State in which alloy layer (Al-enriched layer) remains (Al-enriched layer)
Alloyed hot-dip galvanized steel sheet having Such an alloyed hot-dip galvanized steel sheet in which such an Al-enriched layer remains has significantly deteriorated continuous spotting properties when spot welding is performed. Although the mechanism of this deterioration is unknown, it is not preferable that the above-mentioned Al-enriched layer remains at the interface of the galvannealed steel sheet in view of the continuous hitting point during welding.

The presence or absence of the Al-enriched portion is determined by scanning and analyzing an arbitrary cross section of the plated layer after the alloying treatment by EPMA, and determining whether or not there is an Al peak appearing at the interface between the base iron and the plated layer. Identify. That is, FIG. 3 exemplifies a result measured by the above method for a cross section in the rolling direction of the alloyed hot-dip galvanized steel sheet. If there is no Al peak at the interface, it is determined that there is no Al-enriched portion ( FIG.
(A)). FIG. 3B shows the case where there is Al concentration. When the Al concentration in the plating bath is increased, the Fe-Al
The alloy layer is formed thicker than necessary, and makes the diffusion of Fe from the steel sheet uneven during the alloying process after plating. In other words, the Fe-Al alloy layer at the time of plating is thick, the diffusion of Fe in the alloying process is suppressed more than necessary, and compared with the diffusion of Fe from the grain boundaries on the surface of the base iron, The diffusion of Fe becomes extremely slow, causing a large difference in the Fe diffusion rate between the grain boundaries and the grains. As a result, the degree of alloying of the plating layer formed in the grain boundaries and in the grains on the surface of the ground iron becomes non-uniform, and the irregularities of the alloyed plating layer become remarkable.

On the other hand, when plating, the Fe-Al alloy layer is thin,
If properly formed, the difference between the grain boundary of the Fe crystal grains on the surface of the base iron and the amount of Fe diffusion in the grains falls within an allowable range, and a smooth alloyed plating layer is formed. Irregularities of the alloyed plating layer are smoothed to some extent by temper rolling, but if there are extreme irregularities, since they remain after temper rolling, the surface roughness of the alloyed plating layer is increased, The sliding resistance at the time of press molding is increased. As a result, the so-called flaking resistance deteriorates because the alloyed plating layer peels off during sliding deformation.
-The formation of an Al alloy layer is not preferable.

Therefore, in order to suppress the remaining of the Al-enriched layer after the above-mentioned alloying heat treatment process and the excessive unevenness of the alloyed plating layer, the Fe-Al alloy layer at the time of plating must be properly formed. Had to be formed. Specifically, the Fe-Al alloy layer at the time of plating was converted to an Al amount of 0.0
It is 3 to 0.1 g / m ² . This is because, as in the first invention, the Al content in the plating layer is 0.35% by weight or more, and the immersion time in the plating bath is 0.001%.
To 0.5 seconds, and the Al concentration in the plating bath is 0.18 to
It can be achieved by adjusting the content to 0.45% by weight. As a result, no Al-enriched layer remained at the interface of the galvannealed steel sheet, and a smooth alloyed plated layer having a center line average roughness (Ra) of 1.5 μm or less was formed. Therefore, the steel sheet
It simultaneously satisfies the continuous hitting and flaking resistance required by the user.

Next, the features of the third aspect of the present invention will be described. Even when the composition of Fe is 15% by weight or less, when the amount of the plating layer attached is large, the alloyed hot-dip galvanized steel sheet having poor powdering resistance may be obtained.
That is, when the amount of adhesion is large, alloying up to the plating layer requires a high-temperature or long-time alloying treatment.
The phase will grow to a thickness of 1.0 μm or more. This is not preferable because the powdering resistance is deteriorated. Therefore, the effect of the average Fe in the plating layer on the powdering resistance
The relationship between the content and the amount of adhesion was investigated. As a result, it was found that the amount of adhesion increased and the powdering resistance deteriorated even with the same Fe content, and the following empirical formula was obtained. That is, if the average Fe content and the amount of adhesion satisfy the following relational expression, the deterioration of the powdering resistance could be prevented. In other words, the desired powdering resistance can be obtained by adjusting the Fe content according to the amount of adhesion, and the product manufactured under such conditions is the third invention.

When 15 ≦ W <60, [Fe] ≦ 21−W / 7.5 (1) When 60 ≦ W <100, [Fe] ≦ 16−W / 20 (2) : Amount per one surface (g / m ² ) [Fe]: Average Fe content in plating layer (% by weight) The conditions for using the means for adjusting the amount of plating and the conditions for alloying are as follows.

Plating bath temperature: 450 to 520 ° C. Steel strip running speed: 60 to 200 m / min (the upper limit of the speed is not particularly limited, but can be determined according to the capacity of the alloying furnace) : Although not particularly limited, alloying temperature using a gas wiping device (using nitrogen gas): 450 to 600 ° C (preferably 470 to 600 ° C)
(520 ° C.) Alloying time: 5 to 30 seconds (preferably, 10 to 20 seconds) By the way, in the conventional continuous galvanizing equipment shown in FIG. 2, the immersion length of the steel sheet 1 in the plating bath 2 is used. Immersion time is about 2-3 seconds at the minimum. In order to produce the first to third galvannealed steel sheets according to the present invention, the immersion time in the plating bath is 0.00
1 to 0.5 second or less is more preferable, and another type of plating apparatus in which the immersion time of the steel sheet 1 in the plating bath is short is required.
Therefore, the inventor has proposed, for example, Japanese Patent Laid-Open No.
No. 3,356, JP-A-63-303045 discloses a hot-dip galvanized steel sheet manufacturing apparatus having no sink roll 3, that is, an aerial pot method.

[0027]

EXAMPLE A steel plate 1 to be plated had a thickness of 0.8 mm.
An ultra-low carbon steel sheet having a width of 1000 mm was selected and subjected to plating and alloying on both sides under the following conditions in a continuous hot-dip galvanizing line of an aerial pot type shown in FIG. (Plating conditions) Plating bath temperature: 470 ° C Steel strip running speed: 150 m / min Immersion time of the steel strip in the plating bath: 0.08 seconds Temperature of the steel strip: 480 to 485 ° C (Alloying conditions) Heating temperature: 470-550 ° C. Holding time at this temperature: 10 seconds As a comparative example, a conventional hot-dip plating apparatus shown in FIG. The same steel sheet was processed for ~ 3 seconds. Both sides were plated, and in the subsequent treatment, both were adjusted in the amount of adhesion with a gas wiping device 6, and then subjected to alloying treatment in an alloying heating furnace to obtain an alloyed hot-dip galvanized steel sheet shown in Table 1. . Then, samples were taken from arbitrary positions of these alloyed hot-dip galvanized steel sheets, and a test for evaluating powdering resistance, flaking resistance, and continuous weldability was performed.

The evaluation criteria in each test are as follows. <Powdering resistance> A cellophane tape was stuck on the compression bending side of the alloyed hot-dip galvanized steel sheet, a 90 ° bending-back test was performed, the cellophane tape was peeled off, and the peeled Zn adhered to the cellophane tape was measured by fluorescent X-rays. Then, it was evaluated by the count number of Zn.

Excellent ：: 1000 cps (number of peeled Zn counts / second) or less Good Δ: 1000 to 3000 cps Normal Δ: 3000 to 5000 cps Poor ×: 5000 cps or more <Flaking resistance> Bead-type drawing shown in FIG. 5 Test (pressing load: 5 kgf / cm ² , drawing speed: 500 m
m / min), a cellophane tape was stuck on the sliding portion of the alloyed hot-dip galvanized steel sheet, and then the cellophane tape was peeled off, and the amount of peeled Zn adhered to the cellophane tape was measured in the same manner as in the evaluation of the powdering resistance. evaluated.

◎: Excellent 〇: Good △: Slightly poor ×: Poor (No problem with ◎, △) <Continuous weldability> Two spots of galvannealed steel sheets are continuously stacked by a spot welding machine. Then, the evaluation was made by the number of hit points when the average nugget diameter became 4√t (t: plate thickness). [Electrode conditions] Electrode: CF type Electrode tip angle: 90 ° Electrode material: Cu-Cr [Welding conditions] Welding current: 9.2 kAamperating force: 190 kgf Energizing time: 10 cycles ◎: Excellent 〇: Good △: Slightly poor ×: Poor (◎, Δ: no problem) The results of the above test are shown in Table 1 together with the results of Comparative Examples. From Table 1, it is clear that all of the galvannealed steel sheets according to the present invention are more excellent in powdering resistance, flaking resistance, and weldability than the comparative examples.

[0031]

[Table 1]

[0032]

As described above, according to the present invention, a galvannealed steel sheet excellent in press formability and plating film smoothness can be stably provided.
Therefore, it can be expected that the material, such as a material in the fields of automobiles, home appliances, and building materials, can withstand severer molding processing than before.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a plating apparatus for producing a galvannealed steel sheet according to the present invention.

FIG. 2 is a longitudinal sectional view of a conventional plating apparatus.

3A and 3B are diagrams showing analysis results of a plating layer cross-sectional direction using EPMA, where FIG. 3A is an example of the present invention and FIG. 3B is a comparative example.

FIG. 4 is a diagram showing the relationship between the amount of Al in a plating layer and the X-ray diffraction intensity (d = 1.26) of the ζ phase present in the plating layer.

FIG. 5 is a diagram illustrating a method for evaluating a bead-type anti-flaking property of a galvannealed steel sheet.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Steel plate (plated steel plate) 2 Hot-dip galvanizing bath 3 Sink roll 4 Snout 5 Support roll 6 Adhesion amount adjustment device (gas wiping device in this embodiment) 7 Deflector roll 8 Guide roll 9 Sealing device 10 Pot 11 Punch 12 Dice 13 Alloying furnace 14 Auxiliary bath 15 Plating bath supply unit 16 Dross recovery unit

Claims (3)

[Claims] 1. A galvannealed steel sheet obtained by immersing a steel sheet in a hot-dip galvanizing bath, removing the steel sheet from the bath to form a plating layer on the surface, and subsequently subjecting the steel sheet to a heat alloying treatment. A plated steel sheet, comprising: Fe: 7 to 15% by weight, Al: 0.35 to 1.0% by weight, the balance being Zn and unavoidable impurities, and 15 to 100 g / m ² per side. Press-formability and plating characterized in that at least one surface of the plating layer is formed, the η phase in the plating layer is less than 0.1% by weight, and the thickness of the Г phase at the interface between the base iron and the plating layer is 1.0 μm or less. Alloyed hot-dip galvanized steel sheet with excellent coating smoothness. 2. The method according to claim 1, wherein Al is not concentrated at the interface between the base iron and the plating layer, and the surface roughness of the plating layer is center line average roughness (Ra).
2. The galvannealed steel sheet according to claim 1, wherein the galvannealed steel sheet is excellent in press formability and smoothness of a plating film. 3. The press formability and the smoothness of a plating film according to claim 1 or 2, wherein the adhesion amount of the plating layer and the Fe content in the plating layer satisfy the following relational expression. Excellent alloyed hot-dip galvanized steel sheet. When 15 ≦ W <60, [Fe] ≦ 21−W / 7.5 When 60 ≦ W <100, [Fe] ≦ 16-W / 20, where W is the amount of adhesion per side (g / m ^2). [Fe]: average Fe content in plating layer (% by weight)