JP2002302753A - Galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet having excellent sliding characteristics at press forming and also excellent chemical treatment properties. SOLUTION: In the galvannealed steel sheet; an iron-zinc alloy plating layer is provided to at least one side of a steel sheet; the surface of the above plating layer has a flat zone; an oxide layer of >=10 nm thickness is formed on the flat zone; and the ratio (atomic %) between Zn and Al in the surface layer of the above flat zone is made to 2.0-8.0. The area ratio of the above flat zone in the surface of the above iron-zinc alloy plating is made to 20-80%.

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 slidability during press forming and excellent chemical conversion treatment.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets have better weldability and paintability than galvanized steel sheets.
It is widely used in a wide range of fields, mainly for automotive body applications. Galvannealed steel sheets for such applications are:
It is press-formed and used. However, the galvannealed steel sheet has a drawback that press formability is inferior to that of a cold-rolled steel sheet. This is because the sliding resistance of the alloyed hot-dip coated steel sheet in the press die is larger than that of the cold-rolled steel sheet. That is, the galvannealed steel sheet hardly flows into the press die in a portion where the sliding resistance between the die and the bead is large, and the steel sheet is easily broken.

[0003] An alloyed hot-dip galvanized steel sheet is subjected to a heat treatment after galvanizing the steel sheet, thereby causing an alloying reaction in which Fe in the steel sheet and Zn in the plating layer diffuse.
This is the one in which an Fe-Zn alloy phase is formed. This Fe-Zn alloy phase is usually a film composed of a Γ phase, a δ ₁ phase, and a ζ phase,
As the concentration decreases, that is, in the order of す な わ ち phase → δ ₁ phase → Γ phase, the hardness and melting point tend to decrease. For this reason, from the viewpoint of slidability, a coating having a high hardness, a high melting point and a high Fe concentration that is unlikely to cause adhesion is effective. It is manufactured with a high average Fe concentration.

[0004] However, a film having a high Fe concentration has a problem that a hard and brittle や す く phase is easily formed at the interface between the plating and the steel sheet, and the phenomenon of peeling from the interface during processing, that is, so-called powdering is apt to occur. For this reason, as shown in JP-A-1-319661, in order to achieve both slidability and powdering resistance, a hard Fe-Zn alloy is coated on the second layer by a method such as electroplating to form an upper layer. Is given.

[0005] As a method for improving the press formability when using a galvanized steel sheet, a method of applying a high-viscosity lubricating oil is widely used. However, in this method, there are problems such as the occurrence of coating defects due to poor degreasing in the coating process due to the high viscosity of the lubricating oil, and the unstable press performance due to running out of oil during pressing. Therefore, there is a strong demand for improving the press formability of the galvannealed alloy itself.

As a method for solving the above problem, Japanese Patent Application Laid-Open
No. 53-60332 and JP-A-2-190483 describe that an oxide film mainly composed of ZnO is formed by subjecting the surface of a zinc-based plated steel sheet to electrolytic treatment, immersion treatment, coating oxidation treatment, or heat treatment. It discloses a technique for improving weldability or workability.

JP-A-4-88196 discloses that a galvanized steel sheet is immersed in an aqueous solution containing 5 to 60 g / l of sodium phosphate and having a pH of 2 to 6 or subjected to electrolytic treatment on the surface of a galvanized steel sheet. A technique for forming an oxide film mainly composed of P oxide by applying the aqueous solution to improve press formability and chemical conversion treatment is disclosed.

[0008] Japanese Patent Application Laid-Open No. 3-91093 discloses that the surface of a galvanized steel sheet is subjected to electrolytic treatment, immersion treatment, coating treatment, coating oxidation treatment, or heat treatment to produce Ni oxide, thereby improving the press formability. A technique for improving chemical conversion treatment is disclosed.

[0009]

However, when the above prior art is applied to a galvannealed steel sheet,
The effect of improving press formability cannot be obtained stably. The present inventors have conducted a detailed study on the cause, and as a result, the alloyed hot-dip galvanized steel sheet has poor surface reactivity due to the presence of Al oxide, and has large surface irregularities. I found something. That is, when the prior art is applied to an alloyed hot-dip galvanized steel sheet, since the surface has low reactivity, even when performing electrolytic treatment, dipping treatment, coating oxidation treatment, heat treatment, etc., a predetermined film is formed on the surface. It is difficult and less reactive,
In portions where the amount of Al oxide is large, the film thickness becomes thin. Also, due to the large irregularities on the surface, the direct contact with the press mold during press molding is the convex part of the surface, but the sliding resistance at the contact part between the thin part of the convex part and the mold is high. And the effect of improving press formability cannot be sufficiently obtained.

[0010] It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet which solves the above-mentioned problems, has excellent slidability during press forming, and is also excellent in chemical conversion treatment.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that the thickness of the oxide layer on the surface layer of the flat portion existing on the surface of the galvannealed steel sheet and It has been found that, by optimizing the oxide shape, excellent press formability can be obtained stably and also excellent in chemical conversion treatment.

The flat part on the surface of the galvannealed steel sheet exists as a convex part as compared with the surrounding area. The flat portion mainly contacts the press mold during the press forming, so that if the sliding resistance in the flat portion is reduced, the press formability can be stably improved. In order to reduce the sliding resistance in this flat part, it is effective to prevent adhesion between the plating layer and the mold. For this purpose, it is necessary to form a hard and high-melting coating on the surface of the plating layer. Is valid. As a result of studying from this viewpoint, it was found that it is effective to control the thickness of the oxide layer on the surface layer of the flat portion.

On the other hand, such an oxide layer suppresses the formation of a chemical conversion treatment film. Therefore, when a thick oxide layer is formed to ensure press formability, the oxide treatment is performed as a pretreatment before the chemical conversion treatment. It has a long time to remove and activate the layer, which is problematic in practice. However, even when a thick oxide film is formed on the surface layer of the flat portion, if the oxide layer has irregularities, the concave portion having a small oxide film thickness serves as a nucleus for the formation of the chemical conversion treatment film, so that the chemical conversion treatment is also excellent.

As described above, the press formability is improved by the effect of the convex portion of the oxide layer, and the galvannealed steel sheet having no adverse effect on the chemical conversion property is obtained by the effect of the concave portion of the oxide layer. be able to. Further, as an index of the unevenness of the oxide layer, a Zn / Al concentration ratio on the plating surface can be used, and it has been found that an appropriate value exists for this value.

The present invention has been made based on the above findings, and the first invention has an iron-zinc alloy plating layer on at least one surface of a steel sheet and a flat portion on the surface of the plating layer. An oxide layer having a thickness of 10 nm or more is formed on the flat portion, and the Zn / Al ratio (at%) in the flat portion surface layer is 2.0.
As described above, there is provided an alloyed hot-dip galvanized steel sheet characterized by being 8.0 or less.

According to a second invention, there is provided the galvannealed steel sheet according to the first invention, wherein the area ratio of the flat portion on the surface of the iron-zinc alloy plating is 20 to 80%.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In the production of an alloyed hot-dip galvanized steel sheet, the steel sheet is subjected to hot-dip galvanizing and then further heated and alloyed. Irregularities exist on the surface of the galvannealed steel sheet due to the difference in reactivity at the plating interface. However, after the alloying treatment, temper rolling is usually performed to secure the material, and the contact with the roll at the time of the temper rolling causes the plating surface to be smoothed and unevenness is reduced. Therefore, at the time of press molding, the force required for the mold to crush the projections on the plating surface is reduced, and the sliding characteristics can be improved.

Since the flat part of the surface of the alloyed hot-dip galvanized steel sheet is in direct contact with the mold at the time of press forming, the presence of a hard and high-melting substance that prevents adhesion to the mold is present. It is important for improving the slidability. In this regard, a δ ₁ single-phase film containing no ζ phase in the surface layer is effective for improving the slidability, but in order for the surface layer to be completely δ ₁ phase, the Fe An alloying treatment must be performed to increase the concentration. As a result, a thick, hard and brittle Γ phase is formed at the interface between the plating and the steel sheet, and there is a problem that powdering is likely to occur during press forming. On the other hand, if an alloying treatment is performed to reduce the thickness of the Γ phase in order to prevent powdering, the ζ phase remains on the surface layer, and there is a problem that the slidability is poor.

From this viewpoint, although the Fe concentration and the Al concentration of the plating film of the galvannealed steel sheet used in the present invention are not particularly specified, the plating layer is mainly δ
_An ideal structure is composed of _one phase and further contains a ζ phase.

On the other hand, the presence of the oxide layer on the surface layer is effective in improving the sliding characteristics even in a film in which the ζ phase remains, since the oxide layer prevents adhesion to the mold. Furthermore,
If ζ phase plating film surface is present, since the reactivity of the surface is increased, the surface as compared with the case of [delta] ₁ single phase, it is possible to generate effectively the oxide layer on the flat portion.

At the time of actual press forming, the oxide on the surface layer is worn away and scraped off. Therefore, when the contact area between the mold and the workpiece is large, a sufficiently thick oxide film must be present. However, an oxide layer is formed on the plating surface by heating during the alloying process, and when partially flattened by a method such as temper rolling, although partially destroyed,
Since most remains, the reactivity of the surface is not sufficient, and it is difficult to obtain a predetermined oxide film thickness by the subsequent oxidation treatment. Therefore, by removing the oxide film remaining on the surface layer, the surface can be activated and a sufficiently thick oxide layer can be provided by the subsequent oxidation treatment, so that good slidability can be obtained.

Here, whether or not the film has the ζ phase remaining on the surface layer can be determined from X-ray diffraction or a photograph taken of an SEM image of the plating surface. That is, from among the X-ray diffraction peaks on the plating surface, d = 1.900Å (ζ
Phase) and the ratio of peak intensity for d = 1.990Å (δ phase) minus the background value, respectively (ζ
If / δ) is 0.2 or more, it can be regarded as a film in which the Δ phase remains, and if it is less than 0.2, it can be regarded as a film in which the Δ phase does not remain.
In addition, the ratio (area ratio) of the ζ phase to the entire photograph is defined as the ζ phase where the shape is columnar from the SEM image of the plating surface.
A film having 10% or more can be regarded as a film in which a ζ phase remains, and a film having less than 10% can be regarded as a film in which a ζ phase does not remain. When a portion crushed by pressure adjustment or the like is present on the plating surface, it is difficult to judge the shape based on the shape. Therefore, such a portion is excluded in advance and the area ratio is calculated.

The oxide layer in the present invention includes Zn, Fe, Al
And one or more oxides and / or hydroxides of other metal elements.

By making the thickness of the oxide layer in the flat portion of the plating surface layer 10 nm or more, an alloyed hot-dip galvanized steel sheet showing good slidability can be obtained, but the thickness of the oxide layer is 20 nm or more. Is more effective. This is because in a press forming process in which the contact area between the mold and the workpiece increases, even if the surface oxide layer is worn, the oxide layer remains and does not cause a decrease in slidability.

Further, upon conducting a detailed investigation on the oxide layer, the present inventors have found that the surface shape of the oxide layer has irregularities. That is, fine gaps exist in the Zn-based oxide layer, and a thin Al-based oxide layer remains on the gaps. The alloyed hot-dip galvanized steel sheet having such an oxide layer is It has been revealed that it is excellent in processing property and advantageous. This mechanism can be considered as follows.

Usually, if a uniform oxide layer is formed on the plating surface layer, the reaction between the chemical conversion treatment solution and Zn is inhibited, and a uniform chemical conversion treatment film cannot be formed. Pretreatment to remove as much as possible has a long time. On the other hand, if there is a concave portion having a thin oxide film as described above, the oxide layer can be removed from this concave portion under normal conditions, and a chemical conversion treatment film is formed starting from this portion. Can be. In addition, since the oxide layer having such irregularities has an effect of retaining the press oil in the concave portions, it is considered that the effect of improving the press formability is also obtained.

Further investigations from the above viewpoints have revealed that the Zn / Al concentration ratio on the plating surface has a close relationship with the unevenness of the oxide on the surface layer. That is, if the Zn-based oxide layer is uniformly formed on the surface and the number of concave portions is small, the Al-based oxide layer present in the concave portions cannot be detected, and Zn / A
l The concentration ratio increases. On the other hand, when an oxide layer having many concave portions is formed, many Al-based oxide layers are detected on the surface, and the Zn / Al concentration ratio decreases. Therefore, various alloyed hot-dip galvanized steel sheets were investigated in detail, and in order to satisfy press formability and chemical conversion treatment properties, the Zn / A
l It was found that the concentration ratio (at%) needs to be in the range of 2.0 or more and 8.0 or less. This is because if the Zn / Al concentration ratio is less than 2.0, the oxide layer is sparse and no improvement effect on press formability is observed, and if it exceeds 8.0, a dense oxide layer is formed, which adversely affects the chemical conversion treatment property. Is exerted.

On the other hand, even if a thick oxide layer is formed, a chemical conversion treatment film can be formed uniformly by the oxide layer having such irregularities. Even if a certain oxide is formed, the reactivity of the surface is extremely reduced and it becomes difficult to form a chemical conversion treatment film.

The oxide layer having such irregularities can be formed, for example, by immersing the galvanized steel sheet after the alloying treatment in an acidic solution and controlling the drying time until washing with water. It is only necessary that an oxide layer having irregularities such that the Zn / Al concentration ratio on the plating surface falls within the range specified in the present invention is formed, and the method is not limited.

The thickness of the oxide layer on the surface of the flat part is Ar
It can be determined by Auger electron spectroscopy (AES) in combination with ion sputtering. In this method, after sputtering to a predetermined thickness, the composition at the depth can be obtained by correcting the relative sensitivity factor from the spectral intensity of each element to be measured. The O content due to oxides or hydroxides reaches a maximum at a certain depth (which may be the outermost layer), and then decreases and becomes constant.
At a position where the O content is deeper than the maximum value, the depth at which the sum of the maximum value and the constant value is 1/2 is defined as the oxide thickness. At the same time, the Zn and Al contents were also analyzed in the depth direction, and the concentration in the surface layer (at%), that is, the concentration at the stage of 0 second sputtering time, was measured. Is calculated, the Zn / Al concentration ratio is obtained.

Here, the area ratio of the flat portion on the plating surface is desirably 20 to 80%. If it is less than 20%, the contact area with the mold in the portion (concave portion) excluding the flat portion becomes large, and the area ratio of the flat portion in which the oxide thickness can be reliably controlled in the area actually in contact with the mold. , The effect of improving press formability is reduced. In addition, the portion excluding the flat portion has a role of holding press oil at the time of press molding. Therefore, when the area ratio of the portion excluding the flat portion is less than 20% (when the area ratio of the flat portion exceeds 80%), oil shortage tends to occur during press molding, and the effect of improving press moldability is reduced.

The flat portion of the plating surface can be easily identified by observing the surface with an optical microscope or a scanning electron microscope. The area ratio of the flat portion on the plating surface can be determined by image analysis of the micrograph.

In producing the alloyed hot-dip galvanized steel sheet according to the present invention, it is necessary that Al is added to the plating bath, but the additional element components other than Al are not particularly limited. That is, in addition to Al, Pb, Sb, Si, Sn, Mg,
Even if Mn, Ni, Ti, Li, Cu, etc. are contained or added, the effects of the present invention are not impaired.

Further, since impurities are contained in the treatment liquid used for the oxidation treatment, etc., P, S, N, B, Cl, Na, M
Even if n, Ca, Mg, Ba, Sr, Si and the like are taken into the oxide layer, the effects of the present invention are not impaired.

[0035]

Next, the present invention will be described in more detail with reference to examples. (Example 1) An alloyed hot-dip galvanized film was formed on a cold-rolled steel sheet having a thickness of 0.8 mm by a conventional method, followed by temper rolling. At this time, the ζ phase ratio of the surface layer was changed by changing the alloying conditions, and the flat area ratio on the surface was changed by changing the rolling load of the temper rolling. Subsequently, the substrate was immersed in a sulfuric acid solution having a pH of 2.0 at 50 ° C., allowed to stand for a while, and then washed with water to form an oxide layer on the surface layer of the flat portion. At this time, the standing time was changed variously, and the unevenness ratio of the oxide layer (= Zn / A
l concentration ratio) and the oxide film thickness were adjusted. Before the above treatment, the substrate was immersed in an aqueous solution of sodium hydroxide having a pH of 12 to remove an oxide layer generated by heating during the alloying treatment.

Next, for the test material produced by the above method, the Fe content in the plating film, the ζ / δ value, the ζ phase area ratio,
Flat area ratio, press formability test, chemical conversion treatment evaluation,
The thickness of the oxide layer and the Zn / Al concentration ratio on the plating surface were measured. Press formability test, chemical conversion treatment evaluation,
In addition, the thickness of the oxide layer and the Zn / Al concentration ratio on the plating surface were measured as follows.

(1) Evaluation Test of Press Formability (Test for Measuring Friction Coefficient) In order to evaluate the press formability, the friction coefficient of each test material was measured as follows. FIG. 1 is a schematic front view showing a friction coefficient measuring device. As shown in the figure, a sample 1 for friction coefficient measurement collected from a test material is fixed to a sample table 2,
The sample table 2 is fixed to an upper surface of a horizontally movable slide table 3. On the lower surface of the slide table 3, a vertically movable slide table support 5 having a roller 4 in contact with the slide table 3 is provided. By pushing this up, a load N on the sample 1 for friction coefficient measurement by the bead 6 is measured. Is mounted on the slide table support 5. A second load cell 8 for measuring a sliding resistance force F for moving the slide table 3 in the horizontal direction with the pressing force applied is attached to one end of the slide table 3. In addition,
As a lubricating oil, Noxlast 550HN manufactured by Nippon Parkerizing Co., Ltd. was applied to the surface of Sample 1 to perform a test.

FIGS. 2 and 3 are schematic perspective views showing the shapes and dimensions of the beads used. The lower surface of the bead 6 slides while being pressed against the surface of the sample 1. The shape of the bead 6 shown in Fig. 2 is 10 mm wide, the length in the sliding direction of the sample is 12 mm, the lower part of both ends in the sliding direction is a curved surface with a curvature of 4.5 mmR, the lower surface of the bead on which the sample is pressed is 10 mm in width, the sliding direction It has a plane with a length of 3 mm. The shape of the bead 6 shown in Fig. 3 is 10 mm wide, the length of the sample in the sliding direction is 69 mm, the lower part of both ends in the sliding direction is a curved surface with a curvature of 4.5 mmR, the lower surface of the bead against which the sample is pressed is 10 mm in width, the sliding direction It has a plane with a length of 60 mm.

The friction coefficient measurement test was performed under the following two conditions. [Condition 1] Using the bead shown in Fig. 2, pressing force N: 400
kgf, sample withdrawal speed (horizontal movement speed of slide table 3): 100 cm / min. [Condition 2] Using the bead shown in Fig. 3, pressing force N: 400
kgf, sample withdrawal speed (horizontal movement speed of slide table 3): 20 cm / min. The friction coefficient μ between the test material and the bead was calculated by the formula: μ = F / N.

It should be noted that, based on the friction coefficient measurement results, condition 1
Coefficient of friction is 50% of Fe-Zn alloy electroplating with a Fe concentration of 50%.
= Less than 0.140) and a coefficient of friction under condition 2 at a level close to that of a two-layer plated material (μ = less than 0.200) was evaluated as ○, and a sample showing a higher friction coefficient than that was evaluated as ×.

(2) Chemical conversion test Each specimen was treated under normal conditions with an immersion type zinc phosphate treatment solution (PBL3080 manufactured by Nippon Parkerizing Co., Ltd.) for use as a base material for automobile coating, and a zinc phosphate film was formed on the surface. Formed. The crystalline state of the zinc phosphate film thus formed was observed with a scanning electron microscope (SEM). A sample in which the film was uniformly formed was evaluated as good, and a film in which the film was confirmed to be inhomogeneous was evaluated as x. It was determined.

(3) Thickness of oxide layer and Zn / Al on plating surface
Measurement of concentration ratio The content (at%) of each element in the flat portion was measured by Auger electron spectroscopy (AES), and subsequently, Ar sputtering was performed to a predetermined depth, and the content of each element in the plating film was measured by AES. By measuring and repeating this, the composition distribution of each element in the depth direction was measured. The O content due to oxides and hydroxides reaches a maximum at a certain depth, then decreases and becomes constant. The depth at which the O content was half of the sum of the maximum value and the constant value at a position deeper than the maximum value was defined as the oxide thickness. Similarly, the results of measuring the contents of Zn and Al in the depth direction show that Z at a sputtering time of 0 second.
Using Zn and Al concentration (at%), Zn on plating surface
/ Al concentration ratio was calculated. Note that, as a preliminary treatment, Ar sputtering was performed for 30 seconds to remove the contamination layer on the surface of the test material.

The test results are shown in Table 1 and FIGS.

[0044]

【table 1】

As shown in Table 1, when the Zn / Al concentration ratio of the plating surface, the oxide film thickness of the flat portion, and the area ratio of the flat portion were within the range of the present invention (Examples 1 to 14 of the present invention), / δ value, ζ phase area ratio is high, and even in a film where ζ phase remains clearly on the surface layer, the coefficient of friction is as low as a two-layer plated material, showing good sliding characteristics, It was good. In contrast, when the flat portion is not formed (Comparative Example 1), when the oxide layer thickness on the flat portion surface is out of the range of the present invention (Comparative Example 2), the oxide film thickness of the flat portion, and Even if the flat area ratio is included in the range of the present invention, Zn / Al
When the concentration ratio is out of the range of the present invention (Comparative Examples 3 to 14)
Was inferior in either sliding characteristics or chemical conversion treatment.

(Example 2) An alloyed hot-dip galvanized film was formed on a cold-rolled steel sheet having a thickness of 0.8 mm by a conventional method, and temper rolling was further performed. At this time, a film having no 条件 phase was formed on the surface by changing the alloying conditions, and the flat portion area ratio on the surface was changed by changing the rolling load of the temper rolling. Subsequently, the substrate was immersed in a sulfuric acid solution having a pH of 2.0 at 50 ° C., allowed to stand for a while, and then washed with water to form an oxide layer on the surface layer of the flat portion. At this time, the unevenness ratio of the oxide layer (= Zn / Al concentration ratio on the plating surface) and the oxide film thickness were adjusted by variously changing the standing time. Before the above treatment, the substrate was immersed in an aqueous solution of sodium hydroxide having a pH of 12 to remove an oxide layer generated by heating during the alloying treatment.

Next, for the test material produced by the above method, the Fe content in the plating film was determined in the same manner as in Example 1.
ζ / δ value, ζ phase area ratio, flat portion area ratio, press formability test, evaluation of chemical conversion treatment, thickness of oxide layer, and Zn / Al concentration ratio of plating surface were measured.

The test results are shown in Table 2 and FIGS.

[0049]

[Table 2]

As shown in Table 2, when the Zn / Al concentration ratio of the plating surface, the oxide film thickness of the flat portion, and the area ratio of the flat portion are within the range of the present invention (Examples 1 to 14 of the present invention), / δ value, ζ phase area ratio is low, even if the film does not contain 二 phase on the surface, the coefficient of friction is as low as a two-layer plated material, showing good sliding characteristics, It was good. On the contrary,
When the flat portion is not formed (Comparative Example 1), when the oxide layer thickness on the flat portion surface is out of the range of the present invention (Comparative Example 2), the oxide film thickness of the flat portion and the flat portion area ratio are reduced. Even when included in the range of the present invention, when the Zn / Al concentration ratio of the plating surface deviates from the range of the present invention (Comparative Examples 3 to 14), either of the sliding characteristics and the chemical conversion treatment was inferior. .

[0051]

The alloyed hot-dip galvanized steel sheet of the present invention comprises:
Irrespective of the presence or absence of the ζ phase in the plating layer, the sliding resistance at the time of press molding is small, excellent press moldability can be obtained stably, and a uniform chemical conversion coating can be formed.

[Brief description of the drawings]

FIG. 1 is a schematic front view showing a friction coefficient measuring device.

FIG. 2 is a schematic perspective view showing a bead shape and dimensions in FIG.

FIG. 3 is a schematic perspective view showing a bead shape and dimensions in FIG. 1.

FIG. 4 is a view showing the relationship between the Zn / Al concentration ratio of the plating surface shown in Example 1 and the friction coefficient under Condition 1.

FIG. 5 is a view showing the relationship between the Zn / Al concentration ratio of the plating surface shown in Example 1 and the friction coefficient under Condition 2.

FIG. 6 is a view showing the relationship between the Zn / Al concentration ratio on the plating surface and the friction coefficient under condition 1 shown in Example 2.

FIG. 7 is a view showing the relationship between the Zn / Al concentration ratio of the plating surface shown in Example 2 and the friction coefficient under Condition 2.

[Explanation of symbols]

 1 Sample for coefficient of friction measurement 2 Sample table 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 First load cell 8 Second load cell 9 Rail N Pressing load F Sliding resistance P Pulling load

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Junichi Inagaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside the Nippon Kokan Co., Ltd. (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan (72) Inventor Masayasu Nagoshi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Nihon Kokan Co., Ltd. (72) Inventor Kaoru Sato 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe F term (reference) 4K027 AA05 AA22 AB02 AB03 AB14 AB22 AB26 AB36 AB37 AB44 AC82 AC87 AE03 AE23 AE24 4K044 AA02 AB02 BA10 BA12 BB03 BC01 BC03 CA11 CA16 CA53

Claims (2)

[Claims] Claims: 1. An iron-zinc alloy plating layer is provided on at least one side of a steel sheet, and has a flat portion on the surface of the plating layer, and an oxide layer having a thickness of 10 nm or more is formed on the flat portion.
And Zn / Al ratio (at%) in the flat layer surface layer is 2.0 or more,
An alloyed hot-dip galvanized steel sheet characterized by being 8.0 or less. 2. The area ratio of the flat portion on an iron-zinc alloy plating surface is 20 to 80%.
2. A galvannealed steel sheet according to claim 1.