JPH09241812A - Galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet excellent in peeling resistance and sliding characteristic by regulating Fe content so that it is higher in the outermost surface of a plating layer than in the interface between the plating layer and the steel sheet. SOLUTION: Although the conventional galvannealed steel sheet has a phase structure in which Fe concentration is decreased with the approach to the surface of a plating layer, the galvannealed steel sheet of this invention has a phase structure (ζ-phase, δ-phase, and Γ-phase are successively formed) in which Fe concentration is increased with the approach to the surface of a plating layer. Fe content in the outermost surface of the plating layer is higher than Fe content in the interface between the plating layer and the steel sheet. A Zn phase free from diffusion of Fe is present between the ζ-phase and/or δ1 -phase and the steel sheet. A hot dipping layer and an Fe or Fe-Zn alloy plating layer (<=50wt.% Zn content) are present in succession on an Al-Zn plating layer (0.07-0.25wt.% Al content). By this method, the steel sheet, free from surface defects and excellent in external appearance, can be obtained with high efficiency.

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 having excellent peeling resistance and sliding characteristics of a plated layer.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are widely used in automobiles, home electric appliances, etc. because of their excellent corrosion resistance, paintability and weldability after painting. This plated steel sheet is usually
Immediately after hot-dip galvanizing the steel sheet, a heat treatment is performed in an alloying furnace, and Fe in the steel sheet and Zn in the plating layer cause an alloying reaction by diffusion to alloy the entire plating layer with Zn-Fe. The plating layer is used. The alloy plating layer has a Γ phase or Γ ₁ phase [hereinafter, Γ phase (Fe
₃ Zn ₁₀ )], δ ₁ phase (FeZn ₇ ) and ζ phase (FeZn ₁₃ ). Of these, the Γ phase has the highest Fe concentration, followed by δ ₁ , and the ζ phase has Fe
The lowest concentration. As described above, since Fe is supplied from the steel sheet side in the alloying reaction, the Γ phase with the highest Fe concentration is generated at the interface with the steel sheet, and the δ ₁ phase is generated on it, forming the δ ₁ phase on the outermost surface of the plating layer. The basic phase structure is that the ζ phase is generated. That is, the Fe concentration in the plating layer tends to decrease from the steel sheet side toward the surface of the plating layer.

Here, the relationship between the hardness of each alloy phase and the Fe concentration tends to become harder as the Fe concentration increases. That is, since the Γ phase has the highest Fe concentration, it has the property of being hard and brittle,
Therefore, when the alloyed hot-dip galvanized steel sheet is pressed, peeling easily occurs from the Γ-phase interlayer or from the interface between the Γ-phase and the steel sheet. Therefore, in order to suppress the generation of the Γ phase that adversely affects the peeling of the plating layer as much as possible, a method such as controlling the heat treatment is adopted in order to suppress the degree of alloying.

On the other hand, the ζ phase has the lowest Fe concentration and is soft and rich in toughness, so it is advantageous in terms of improving the peel resistance, but because it is soft, it adheres by sliding with the die during press working. Occurs, the frictional resistance increases and the material flow into the mold is hindered, causing material cracking. Therefore, for the purpose of suppressing the formation of the ζ phase that adversely affects the sliding characteristics, a method such as controlling the heat treatment in order to further advance the degree of alloying and change the ζ phase to the δ ₁ phase Can be considered. However, when the degree of alloying is advanced, the Γ phase is thickly formed as described above, which causes a problem that the peeling resistance is deteriorated.

As described above, since the peeling resistance of the plating layer and the sliding property are contradictory to each other, it is extremely difficult to provide a steel sheet excellent in both these properties, and plating conditions and alloying conditions are extremely narrow. The current situation is to operate while controlling the range. However, at the actual operation level, such control itself is extremely difficult. Therefore, in consideration of the shape of the molded part, the mold shape, etc. It is manufactured by substituting one of the materials with emphasis on characteristics. Therefore, there is a strong demand for the provision of a Zn-Fe alloy plated steel sheet that satisfies both characteristics.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object thereof is hot dip galvannealing which is excellent in both peeling resistance and sliding property of a plating layer. To provide steel sheets.

[0007]

The alloyed hot-dip galvanized steel sheet of the present invention, which has solved the above-mentioned problems, has the gist of satisfying the following requirements. Focusing on the Fe content of the plating layer, the steel sheet of the present invention is
The gist is that the Fe content in the outermost surface of the plating layer is higher than the Fe content in the interface between the plating layer and the steel sheet. Preferably, the Fe content in the plating layer gradually increases from the steel sheet side toward the plating layer surface.

From the viewpoint of the metal phase structure in the plating layer, when attention is paid to the interface between the plating layer and the steel sheet, the steel sheet of the present invention has a ζ phase and / or a δ ₁ phase at least at the interface between the plating layer and the steel sheet. (At that time, a Zn phase in which Fe is not diffused may exist between the ζ phase and / or the δ ₁ phase and the steel sheet), while focusing on the outermost surface of the plating layer In this case, the steel sheet of the present invention is
Alternatively, it has a gist in that the δ ₁ phase exists. Specifically, a ζ phase, a δ ₁ phase, and a Γ phase may sequentially exist in the plating layer from the steel sheet side toward the plating layer surface, or a Zn phase in which Fe is not diffused and a ζ phase , Δ ₁ phase, Γ
The phases may be present sequentially.

Focusing on the plating layer, the steel sheet of the present invention is
An Fe or Fe-Zn alloy plating layer (Zn: 50 wt% or less) is present on the hot-dip galvanized layer, or an Al-Zn plating layer (Al: 0.07-
0.25% by weight) on top of the hot dip galvanized layer, F above
The gist is that e or Fe-Zn alloy plated layers are sequentially present. In the following description, "%" means% by weight unless otherwise specified.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, since the conventional plated steel sheet is manufactured by diffusing and supplying Fe from the steel sheet side, the Fe content rate at the surface of the plating layer is the Fe content rate at the interface between the plating layer and the steel sheet. Inevitably, the phase structure is controlled so that desired characteristics can be obtained on the assumption that the plating layer has such a Fe concentration distribution. On the other hand, as a result of intensive studies to solve the above problems, the present inventor reduced the Fe concentration on the steel sheet side as much as possible in order to improve the peeling resistance of the plating layer, while improving the sliding characteristics. On the surface of the plating layer
In view of the finding that it is effective to increase the e concentration as much as possible, it was possible to provide a steel sheet having a novel Fe concentration distribution in which the Fe concentration distribution in the plated layer is completely different from the conventional plated steel sheet.

As described above, the steel sheet of the present invention is completely different in the Fe concentration distribution of the plated layer from the conventional galvannealed steel sheet. When the plated layer of the steel sheet was examined in more detail, it was found to have the following features.

The Fe content of the plating layer The steel sheet of the present invention is completely opposite to the Fe content of the outermost surface of the plating layer in comparison with the Fe content of the interface between the plating layer and the steel sheet. It is different from the conventional steel plate which has the relationship of. Preferably, the Fe content in the plating layer increases sequentially from the steel sheet side toward the plating layer surface. One example is shown in FIG.

In the figure, (a) shows the Fe concentration distribution in the cross section of the plated layer in the steel sheet of the present invention, and (b) shows a schematic diagram of the Fe concentration distribution in the cross section of the plated layer in the conventional plated steel sheet. As is clear from FIG. 1, the conventional galvannealed steel sheet has a phase structure in which the Fe concentration decreases toward the surface of the plating layer (the Γ phase, the δ ₁ phase, and the ζ phase are sequentially generated). On the other hand, the present invention has a phase structure in which the ζ phase, the δ ₁ phase, and the Γ phase are sequentially generated toward the plating layer surface. The Fe concentration distribution and phase configuration shown in FIG. 1 are examples of the present invention. In short, the Fe content on the outermost surface of the plating layer is higher than the Fe content on the interface between the plating layer and the steel sheet. It goes without saying that other phase configurations can be obtained as long as they satisfy the above relationship. This point will be described in detail later.

Metallic Phase Structure in Plating Layer The steel sheet of the present invention is different from the conventional steel sheet in which the Γ phase mainly exists at the interface in that the ζ phase and / or the δ ₁ phase exist at least at the interface between the plating layer and the steel sheet. Is completely different. On the other hand, when viewed from the outermost surface of the plating layer, the steel sheet of the present invention has a Γ phase and / or
Alternatively, the presence of the δ ₁ phase is completely different from the conventional steel sheet in which the ζ layer is mainly present on the outermost surface of the plating layer. Specifically, from the steel plate side to the plating layer surface, the plating layer has a ζ phase,
The δ ₁ phase and the Γ phase sequentially exist. However, this phase structure is representative of the present invention, and Fe of the plating layer is
The concentration is not particularly limited as long as it satisfies the above relationship. Therefore, for example, the outermost surface of the plating layer is not necessarily Γ
It does not have to be a phase, and a phase having a higher Fe content or a lower Fe content than the Γ phase may be generated. Similarly, the interface between the steel sheet and the plating layer does not necessarily have to consist of the ζ phase alone, and may be, for example, a mixed phase of the ζ phase and the δ ₁ phase, or _one in which the δ ₁ phase is directly generated. Belong to the range of. Further, the Fe—Zn alloy phase such as the ζ phase does not necessarily need to directly exist in the interfacial steel sheet between the plating layer and the steel sheet, and the Zn phase in which Fe has not diffused (hereinafter, may be simply referred to as Zn phase). ) May partially remain. That is, the Zn phase, the ζ phase, the δ ₁ phase, and the Γ phase may sequentially exist. In this case, it goes without saying that the alloy phase existing on the Zn phase does not necessarily have this relationship.

The average thickness of each alloy plating phase may vary depending on the manufacturing method described later, but in order to obtain desired characteristics, for example, the ζ phase, δ ₁ phase, Γ phase toward the plating layer surface.
In the case of a steel sheet in which phases are sequentially present, preferable average thickness of each phase is ζ phase: 1 to 4 μm (more preferably 2 to 3 μm), δ
₁ phase: 2 to 8 μm (more preferably 3 to 6 μm), Γ
Phase: 1 to 4 μm (more preferably 2 to 3 μm).

Layer Structure of Plating Layer From the viewpoint of the layer structure of the plating layer of the steel sheet of the present invention,
The greatest feature is that an Fe or Fe-Zn alloy plating layer (Zn: 50% or less, hereinafter sometimes abbreviated as Fe-Zn alloy plating layer) is present on the hot-dip galvanized layer. Have. Alternatively, an Al-Zn plating layer (Al: 0.07 to 0.25) is provided under the hot dip galvanizing layer.
%, Hereinafter may be abbreviated as Al-Zn plating layer)
May be present (that is, on the Al-Zn plated layer,
A hot-dip galvanized layer and the above Fe or Fe-Zn alloy plated layer are sequentially present).

This layer structure will be described in detail below in the process of explaining the method for manufacturing a steel sheet according to the present invention. In order to manufacture the steel sheet of the present invention, it is not a method of diffusing Fe from the steel sheet side to the galvanized layer in a diffused manner as in the conventional galvannealed steel sheet, but conversely, Fe is fed from the galvanized layer side to the steel sheet side. Supply and diffuse. That is, first, a steel sheet is subjected to hot dip galvanizing to obtain a hot dip galvanized layer (lower plating layer). After the lower plating layer is solidified, an Fe or Fe-Zn alloy plating layer is applied to the surface (upper plating layer). Then, Fe is supplied and diffused from the upper plating layer to the lower plating layer by performing heat treatment, and the entire plating layer is formed into a Fe-Zn alloy layer, and the Fe concentration in the plating layer is changed from the steel plate side to the plating layer surface. It can be adjusted to have an increasing tendency toward Hereinafter, each step will be described in detail.

First, the steel sheet is dipped in a hot dip galvanizing bath containing 0.07 to 0.25% Al to carry out hot dip galvanizing (formation of a lower plating layer). When the steel sheet is immersed in the plating bath in this way, Al in the bath preferentially reacts with Fe of the steel sheet, and as a result, an extremely thin Fe-Al alloy layer of about 0.1 μm or less is formed at the interface with the steel sheet. It This layer is Z
It is called a barrier layer because it serves as a barrier when n and Fe diffuse and suppresses the formation of a Zn—Fe alloy layer. In a normal alloying treatment, heat treatment at a high temperature for a long time causes diffusion to proceed through the barrier layer, resulting in promotion of formation of an alloyed plating layer. In contrast, in the present invention,
No heat treatment for alloying is performed at this stage, and after the lower plating layer is solidified, an Fe or Fe—Zn alloy plating layer is subsequently applied (formation of the upper plating layer). Since the barrier layer described above does not exist between the upper plating layer and the lower plating layer generated in this way, if heat treatment is performed at this stage, heating at a lower temperature and for a shorter time than at the interface with the steel sheet will occur. Alloying will proceed rapidly by the treatment.

In hot dip plating performed prior to formation of the upper layer plating, the Al content in the plating bath is 0.07 to
It is necessary to control to 0.25%. Al concentration is 0.07
If it is less than%, the above-mentioned barrier effect due to the formation of the Fe-Al alloy layer cannot be obtained. That is, if the Al concentration is low,
During hot dip coating, a large amount of Zn was found at the interface between the steel plate and the plating layer.
-The Fe alloy layer is generated, and further the diffusion reaction proceeds from the interface between the steel plate and the plated layer by the heat treatment performed after the formation of the upper layer plating, and as a result, the Γ phase is generated at this interface,
The peel resistance will be reduced. As described above, in order to suppress the alloying reaction of Zn-Fe at the steel sheet / hot dip coating interface due to the hot dip galvanizing treatment and the heat treatment after the upper layer plating, Al needs to be 0.07% or more. is there. It is preferably 0.10% or more. However, if the Al concentration exceeds 0.25%, the oxidation reaction will occur violently in the plating bath, and a large amount of dross will adhere to the surface of the plating layer. Sliding property with the mold is deteriorated and the material is cracked. Therefore, the Al concentration is 0.25% or less. It is preferably 0.22% or less.

Further, in order to obtain a desired alloyed steel sheet, the upper plating layer is an Fe or Fe-Zn alloy plating layer (Z
n: 50% or less). this house,
Zn concentration in the Fe-Zn alloy plating layer exceeds 50% (that is, Fe concentration in the upper plating layer is 50% or less).
In order to control the Fe concentration of the entire plating layer by the heat treatment after the formation of the upper plating layer so as to satisfy the above relationship, it is necessary to increase the plating adhesion amount of the upper layer inevitably. Cause rise. From such a viewpoint, Zn: 5
It is 0% or less (preferably 40% or less).

The coating amount of the upper layer is appropriately adjusted depending on the coating amount of the final product and the molten zinc coating amount of the lower layer. Further, an electroplating method is usually used for performing the upper layer plating, but in addition, a displacement plating method or the like can be appropriately adopted.

As described above, according to the present invention, it is possible to promote the diffusion of Fe from the upper plating layer to the lower plating layer while suppressing the diffusion of Fe from the steel sheet side to the lower plating layer.
It is necessary to appropriately select appropriate heat treatment conditions (heating temperature, heating time, etc.). Among them, the heating temperature is usually 300 to 700 ° C., though it varies depending on the heating time.
It is preferable that Generally, when the heating temperature becomes much lower than the melting point of Zn (about 420 ° C.), Z in the plating layer
Since n is not melted and diffused between solids, diffusion of Fe is suppressed and the progress of the alloying reaction can be easily controlled, but there is a problem that productivity is reduced. Therefore, it is preferable that the lower limit value is at least 300 ° C. On the other hand, when the heating temperature is increased, Fe diffuses from the steel sheet side beyond the barrier layer, so the upper limit is preferably 700 ° C. or less. More preferably, it is 350 to 650 ° C.
In this way, the heating temperature is appropriately adjusted in relation to the heating time. For example, when heating at a low temperature of 420 ° C. or lower, the heating time is long (for example, 1 to 3 hours), and The heat treatment is not limited to the inside of the plating line, and it is effective to perform batch heating in a coil shape. The heating method is not particularly limited, and a commonly used method such as gas heating or high frequency induction heating may be appropriately adopted.

The steel sheet used in the present invention is not particularly limited as long as it is a steel sheet normally used as an alloyed hot-dip galvanized steel sheet.
F (Interstitial Free) steel or the like is used. In a normal plated steel sheet, Zn-Fe alloying reaction (so-called outburst reaction) occurs preferentially at the active steel grain boundaries, and the Γ phase easily develops. However, in IF steel, the reaction with the steel sheet is suppressed. Therefore, it is useful in that the Γ phase is difficult to generate.

The present invention will be described in detail below based on examples.
However, the following examples do not limit the present invention,
All modifications and alterations without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

[0025]

[Examples] Under various conditions shown in Table 1, a steel sheet containing Cr: 0.002% and Ti: 0.05% as main components was hot-dipped in a hot-dip galvanizing line, and then electroplated. Then, Fe or Fe-Zn alloy was applied thereto, and then heat treatment (heating at 450 ° C. for 5 to 60 seconds) was performed to manufacture steel sheets No. 1 to 6.

Separately from this method, the above steel sheet is melted.
Conventional alloy by plating and then alloying treatment
Three types of hot-dip galvanized steel sheets were produced (No. 7-
9). Of these, No. 7 is press formability (press sliding
Of the ζ phase on the surface of the plating layer.
Suppress generation as much as possible δ₁ Phase and then Γ phase
It was done. In addition, No. 8 has a special plating peeling resistance.
It is a material that emphasizes
Suppress generation of Γ phase as much as possible₁ Phase, then ζ
A phase has been formed. Furthermore, No. 9 is press molding
The manufacturing conditions so that it has both plating resistance and plating peeling resistance.
Strictly controlled, the entire plating layer is δ ₁ From the phase
It becomes.

The various steel plates thus obtained were subjected to an actual press test for automobiles, and the state of plating peeling and the presence or absence of cracks in the material were evaluated. This actual press test was conducted under more severe conditions than usual press conditions. That is, the plate thickness reduction rate of the material under the normal press is about 10% at the maximum, whereas in the present embodiment, the plate thickness reduction rate is 20%.
The mold and wrinkle holding force have been set to be more rigorous. The evaluation criteria for plating peeling resistance and material cracking are:
It is as follows.

[Peeling resistance to peeling] ○: Almost no peeling △: Peeling is considerable ×: Peeling is large [Cracks in material] ○: No cracking △: Cracking is considerable ×: Cracking is severe The results are also shown in Table 1. In the table, * 1 means a material with a focus on press moldability, * 2 means a material with a focus on plating peeling resistance, and * 3 means a material with a focus on press moldability and plating peeling resistance.

[0029]

[Table 1]

As shown in Table 1, in the press formability-oriented material of No. 7, since there is no soft ζ phase on the plating layer surface,
It can be seen that the peeling is remarkable and that the plating surface is considerably cracked even though it is in the δ ₁ phase. Also N
In the material with emphasis on the strippability of o.8, since the Γ phase is not generated at the interface between the steel plate and the plating layer, the peeling hardly occurs, but the plating surface is in the ζ phase, so that the crack becomes severe. Furthermore, in the No. 9 compatible material of press formability and plating releasability, since the Γ phase is not generated but the ζ phase is not present,
Peeling was considerably generated, and a large amount of cracks were generated on the plated surface even though it was in the δ ₁ phase. Thus, it is understood that the conventional galvannealed steel sheet cannot satisfy both press formability and plating peeling resistance.

On the other hand, N satisfying the requirements of the present invention
o.1 to 4 consist of the phase composition and the Fe concentration distribution shown in FIG. 1, and since the Γ phase is not generated and the ζ phase is present at the interface between the steel sheet and the plating layer, the plating peeling hardly occurs. Further, since the Γ phase having a higher Fe concentration than the δ ₁ phase is generated on the surface of the plating layer, the material is not cracked. Therefore, according to the present invention, it is possible to obtain a Zn-Fe alloy-plated steel sheet excellent in both the peeling resistance of the plating layer and the press formability.

On the other hand, No. 5 is a comparative example in which the Al concentration in the plating bath is low, and a Γ phase is generated in the plating layer, causing a large amount of peeling. In addition, No. 6 is a comparative example in which the Al concentration in the plating bath is high, and dross adheres to reduce slidability and cause severe cracking. From these results, the use of the steel sheet of the present invention makes it possible to apply it to parts having a complicated shape in which peeling and cracking easily occur, and it is expected that the application will be expanded.

[0033]

Since the steel sheet of the present invention is constituted as described above, it is highly excellent in both peeling resistance and sliding characteristics,
It is possible to efficiently obtain a steel sheet having a good appearance in that it has no surface defects.

[Brief description of drawings]

FIG. 1 is a schematic diagram comparing the Fe concentration distributions in cross sections of plated layers.

Claims (9)

[Claims] 1. The Fe content on the outermost surface of the plating layer is
An alloyed hot-dip galvanized steel sheet, characterized in that the Fe content is higher than the Fe content at the interface between the plated layer and the steel sheet. 2. The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the Fe content in the plated layer gradually increases from the steel sheet side toward the surface of the plated layer. 3. The galvannealed steel sheet, wherein the metal phase in the plating layer is such that at least an ζ phase and / or a δ ₁ phase is present at the interface between the plating layer and the steel sheet. 4. F between the ζ phase and / or the δ ₁ phase and the steel sheet
An alloyed hot-dip galvanized steel sheet, characterized in that there is a Zn phase in which e has not diffused. 5. The metal phase on the outermost surface of the plating layer is Γ
A galvannealed steel sheet characterized by the presence of a phase and / or a δ ₁ phase. 6. The plating layer is formed on the surface of the plating layer from the steel sheet side.
Toward ζ phase, δ ₁ Phase and Γ phase exist sequentially
An alloyed hot-dip galvanized steel sheet characterized by the following. 7. The plating layer comprises a Zn phase, a ζ phase and a δ ₁ phase in which Fe is not diffused from the steel sheet side toward the surface of the plating layer.
Hot-dip galvanized steel sheet characterized in that phases and Γ phases sequentially exist. 8. Fe or F on the hot dip galvanized layer
An alloyed hot dip galvanized steel sheet having an e-Zn alloy plating layer (Zn: 50% by weight or less). 9. An Al—Zn plating layer (Al: 0.07 to
0.25% by weight), and a hot dip galvanized layer and an Fe or Fe-Zn alloy plated layer (Zn: 50% by weight or less) are sequentially present on the galvannealed steel sheet.