JP2002266061A - Galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a galvannealed steel sheet having excellent sliding characteristics at press forming. SOLUTION: The galvannealed steel sheet has a flat part on the surface of an iron-zinc alloy plating layer; a Zn oxide layer of >=10 nm thickness is formed on the surface of the flat part; and P is contained in the oxide layer. The content of P in the above oxide layer is 0.05 to 20 atomic %. The area ratio of the above flat part on the iron-zinc alloy plating layer is 20 to 80%.

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 slidability during press forming.

[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. Alloyed hot-dip galvanized steel sheet for such applications is
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 is less likely to flow 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, in which adhesion is unlikely to occur, 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 め っ き 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 tends 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 that the press formability of the galvannealed alloy itself be improved.

As a method for solving the above problem, Japanese Patent Application Laid-Open
In 53-60332 and JP-A-2-190483, an oxide film mainly composed of ZnO is formed by subjecting a surface of a galvanized 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, or 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 coated steel sheet is inferior in surface reactivity due to the presence of Al oxide, and due to large surface irregularities. I found that. That is, when the prior art is applied to an alloyed hot-dip coated steel sheet, the reactivity of the surface is low, so that a predetermined film is formed on the surface even when performing electrolytic treatment, dipping treatment, coating oxidation treatment, heat treatment, and the like. Is difficult, and the film thickness becomes thin in a portion having low reactivity, that is, a portion having a large amount of Al oxide. 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.

An object of the present invention is to solve the above problems and to provide an alloyed hot-dip coated steel sheet having excellent slidability during press forming.

[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 controlled the thickness of the oxide layer of the flat surface layer existing on the surface of the alloyed hot-dip coated steel sheet. By doing so, it was found that excellent press formability can be obtained stably.

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.

The present invention has been made based on the above findings. The first invention has a flat portion on the surface of an iron-zinc alloy plating layer, and the surface of the flat portion has a thickness of 10 nm or more. Zn
The present invention provides an alloyed hot-dip galvanized steel sheet, wherein an oxide layer is formed and P is contained in the oxide layer.

The second invention provides the galvannealed steel sheet according to the first invention, wherein the content of P in the oxide layer is in the range of 0.05 to 20% in at%.

A third invention provides the galvannealed steel sheet according to the first invention and the second invention, wherein the area ratio of the flat portion on the surface of the iron-zinc alloy plating is 20 to 80%. I do.

[0016]

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 portion of the surface of the alloyed hot-dip galvanized steel sheet is a portion directly contacted by the mold during 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 an 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 a mold. Among them, a Zn oxide layer containing P in the surface layer is effective. Although the cause is not clear, the presence of P in the oxide layer partially forms an oxide film of PO bond, which contributes to the improvement of the slidability as compared with the case of only Zn oxide. It is thought that it is because it is big. Also,
It is considered that this is because the slidability can be easily improved even with a small amount of P as compared with a phosphoric acid-based film represented by P ₂ O ₅ or the like.

A film having no ζ phase on the surface is advantageous in that a thick oxide film can be easily applied as compared with a film in which the ζ phase remains. Although the cause is not clear, the ζ phase has a smaller amount of solid solution of Al in the alloy phase as compared with the δ ₁ phase, so that a large amount of Al oxide layer is formed on the surface layer after the alloying treatment, and It may be difficult to activate the surface before treatment.

In order to form such a Zn oxide layer containing P on the flat portion of the plating surface, the alloyed hot-dip galvanized steel sheet after the alloying treatment is subjected to temper rolling, and further contacted with an acidic solution. After that, it is obtained by, for example, contacting with a sodium phosphate solution to neutralize the solution remaining on the surface, but it is sufficient that the Zn oxide layer containing P is finally formed on the plating surface layer. There is no limit to the method.

At the time of actual press molding, the oxide on the surface layer is worn out and scraped off. Therefore, when the contact area between the mold and the workpiece is large, a sufficiently thick oxide film is required. 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 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 the ζ phase remains, and a film having less than 10% can be regarded as a film in which the ζ phase does not remain. In the case where a portion crushed by pressure adjustment or the like exists on the plating surface, it is difficult to determine the shape based on the shape. Therefore, such a portion is excluded in advance and the area ratio is calculated.

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 exhibiting good slidability can be obtained. Is more effective. This is because, in the 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. In addition, the oxide layer
The content of P needs to be in the range of 0.05 to 20% at%. This is because when the content of P is less than 0.05%, the effect of improving slidability is small, and conversely, when the content exceeds 20%, there is not much change in the effect. On the other hand, the upper limit of the thickness of the oxide layer is not particularly provided, but if it exceeds 200 nm, the reactivity of the surface is extremely reduced, and it becomes difficult to form a chemical conversion treatment film. .

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. Also, X
By performing the same measurement using X-ray photoelectron spectroscopy (XPS), the P concentration profile in the depth direction is obtained, and the value at which the P concentration becomes maximum with respect to the depth corresponding to the thickness of the oxide layer Is defined as the P content of the oxide layer.

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.

For 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 or the like, S, N, B, Cl, Na, Mn, C
Even if a, Mg, Ba, Sr, Si, etc. are taken into the oxide layer, the effects of the present invention are not impaired.

[0030]

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, by immersing in a sulfuric acid solution at 50 ° C. and pH 2.0 and thoroughly washing with water, immersing in disodium hydrogen phosphate at 50 ° C. and pH 10 to obtain a Zn oxide layer containing P on the surface layer of the flat portion Was formed. 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,
The measurement of the flat area ratio, the thickness of the oxide layer, the P content of the oxide layer, and the press formability test were performed. The measurement of the thickness of the oxide layer in the flat portion and the P content of the oxide layer, and the press formability test were performed as follows. (1) Oxide layer thickness and P content measurement Auger electron spectroscopy (AES) was used to measure the content (at%) of each element in the flat part, followed by Ar sputtering to a predetermined depth, followed by AES The content of each element in the plating film was measured, and by 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. 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.

Further, by performing the same measurement using X-ray photoelectron spectroscopy (XPS), a P concentration profile in the depth direction is obtained, and the P concentration profile in the depth direction corresponding to the thickness of the oxide layer is determined.
The value at which the P concentration became the maximum was defined as the P content of the oxide layer.

(2) 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 friction coefficient measurement sample 1 collected from a test material is fixed to a sample table 2, and 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. The first for measuring
The load cell 7 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. As a lubricating oil, Noxlast 550HN manufactured by Nippon Parkerizing Co., Ltd. was applied to the surface of Sample 1 for 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, and the lower part of both ends in the sliding direction is a curved surface with a curvature of 4.5 mmR. It has a plane with a length of 3 mm. The shape of the bead 6 shown in Fig. 3 has a width of 10mm, the length of the sample in the sliding direction is 69mm, the lower part of both ends in the sliding direction is a curved surface with a curvature of 4.5mmR, and the lower surface of the bead on which the sample is pressed is 10mm in width, 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 coefficient of friction μ between the test material and the bead is
Formula: Calculated as μ = F / N.

Table 1 shows the test results.

[0039]

【table 1】

As shown in Table 1, the oxide film thickness of the surface layer and P
Content, when the flat part area ratio of the surface layer is within the range of the present invention (Examples 7 to 17 of the present invention), ζ / δ value, ζ phase area ratio is high,
Obviously, even in the case of a film in which the ζ phase remains on the surface layer, the friction coefficient under condition 1 is all very low, and the oxide film thickness
When the thickness was as thick as 20 nm or more (Examples 10 to 17 of the present invention), the coefficient of friction under Condition 2 was also a low value, and further excellent sliding characteristics were exhibited. On the other hand, in Comparative Examples (Comparative Examples 1 to 3) in which the oxide film thickness of the surface layer was out of the range of the present invention, all the friction coefficients showed high values, and the sliding characteristics were lowered. On the other hand, even when the oxide film thickness of the surface layer is included in the range of the present invention, when the flat area ratio is out of the range of the present invention (Examples 1 to 4), the friction coefficient of the condition 1 is slightly reduced. However, the friction coefficient under Condition 2 did not decrease at all, and there was no effect of improving the sliding characteristics. In addition, when the P content of the oxide layer was out of the range of the present invention (Examples 5 and 6 of the present invention), although the slidability was improved, the improvement effect was small as compared with Examples 7 to 17 of the present invention. .

(Example 2) On a cold-rolled steel sheet having a thickness of 0.8 mm,
An alloyed hot-dip galvanized film was formed by a conventional method, and further temper rolling was performed. At this time, by changing the alloying conditions,
By forming a film having no し な い phase on the surface and changing the rolling load in the temper rolling, the flat area ratio on the surface was changed. Subsequently, by immersing in a sulfuric acid solution at 50 ° C. and pH 2.0 and thoroughly washing with water, immersing in disodium hydrogen phosphate at 50 ° C. and pH 10 to obtain a Zn oxide layer containing P on the surface layer of the flat portion Was formed. 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, with respect to the test material prepared by the above method, the Fe content in the plating film was determined in the same manner as in Example 1.
The ζ / δ value, the ζ phase area ratio, the flat portion area ratio, the thickness of the oxide layer, the P content of the oxide layer, and the press formability test were performed.

Table 2 shows the test results.

[0044]

[Table 2]

As shown in Table 2, the ζ / δ value, the ζ phase area ratio was low, and the film did not contain the ζ phase, and the oxide film thickness and P content of the surface layer and the flat area ratio of the surface layer were low. In the case of being within the range of the present invention (Examples 7 to 17 of the present invention), the friction coefficients of Condition 1 were all very low values, and the oxide film thickness was 20 nm.
When the thickness was as thick as the above (Examples 10 to 17 of the present invention), the coefficient of friction under Condition 2 was also a low value, indicating even better sliding characteristics.
On the other hand, in Comparative Examples (Comparative Examples 1 to 3) in which the oxide film thickness of the surface layer was out of the range of the present invention, all the friction coefficients showed high values, and the sliding characteristics were lowered. On the other hand, even when the oxide film thickness of the surface layer is included in the range of the present invention, when the flat area ratio is out of the range of the present invention (Examples 1 to 4), the friction coefficient of the condition 1 is slightly reduced. However, the friction coefficient under Condition 2 did not decrease at all, and there was no effect of improving the sliding characteristics. When the P content of the oxide layer is out of the range of the present invention (Examples 5 and 6 of the present invention), although the slidability is improved, the present invention is
The improvement effect was small compared to 7-17.

[0046]

The alloyed hot-dip galvanized steel sheet of the present invention comprises:
Regardless of the presence or absence of the ζ phase in the plating layer, the sliding resistance during press forming is small, and excellent press formability is obtained stably.

[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.

[Explanation of Signs] 1 Sample for friction coefficient 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 in reference (reference) 4K027 AA05 AA22 AB28 AB36 AB42 AC87

Claims (3)

[Claims] Claims: 1. A flat portion is formed on the surface of an iron-zinc alloy plating layer, and a Zn oxide layer having a thickness of 10 nm or more is formed on a surface layer of the flat portion, and the oxide layer contains P. An alloyed hot-dip galvanized steel sheet. 2. The content of P in the oxide layer is 0.05% at%.
2. The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the content is in the range of 20% to 20%. 3. The area ratio of the flat portion on an iron-zinc alloy plating surface is 20 to 80%.
And the galvannealed steel sheet according to claim 2.