JPH0892714A - Galvannealed steel sheet excellent in chemical convertibility and cation electrodeposition coating property and its production - Google Patents

Abstract

PURPOSE: To improve the chemical convertibility and electrodeposition coating properties of a steel sheet by specifying the compsn. of a Zn-Fe-Al alloy plating layer to be applied to the steel sheet and the coating weight and a ζ phase on the surface of the plating. CONSTITUTION: The compsn. of a plating layer on a galvannealed steel sheet is composed of, by weight, 7 to 13% Fe, <=0.40% Al, and the balance Zn. A ζphases is made present on the surface of the plating layer by >=1.0% surface coating ratio and the coating weight is regulated to >=20g/m<2> . The plating layer is provided at least one side of the steel sheet. Furthermore, after the steel sheet is galvanized by the same galvanizing bath, preferably, the plated steel sheet is heated to about 480 to 600 deg.C at about <=20 deg.C/sec temp. rising rate and is alloyed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a construction material and an alloyed hot-dip galvanized steel sheet used for automobile bodies and the like, and particularly excellent in chemical conversion treatability and electrodeposition coating property during production of automobile bodies. Alloyed hot-dip galvanized steel sheet.

[0002]

2. Description of the Related Art Generally, galvannealed steel sheets are
After hot-dip galvanizing a steel sheet, it is heated to a temperature not lower than the melting point of Zn in an alloying furnace, and the plating layer is an alloy of Fe and Zn, that is, an alloy phase consisting of Γ, δ ₁ , and ζ phases from the steel sheet side. It is what Since this alloyed hot-dip galvanized steel sheet has both excellent post-painting corrosion resistance and weldability, it has been conventionally used as a building material, and recently, various studies have been conducted to use it as a material for automobile bodies. Has been done.

When an alloyed hot-dip galvanized steel sheet is applied to the outer surface of an automobile body, in addition to corrosion resistance, workability (peeling resistance), chemical conversion treatment as pretreatment for coating, and cationic electrodeposition coating property are important. . If the peel resistance of the alloyed hot-dip galvanization is poor, pressing scratches will occur during press forming. Therefore, a method of improving the peel resistance by controlling the phase constitution of the alloy layer is known, and is disclosed in Japanese Patent Publication No. 3-55544. In the gazette
A technique for suppressing the generation of the phase and the ζ phase is disclosed. Further, if the galvannealed steel sheet has poor chemical conversion treatment, the chemical conversion treatment may be uneven, and if the cationic electrodeposition coating property is poor, crater-like coating defects may occur during electrodeposition coating. Defects that occur during chemical conversion treatment and electrodeposition coating remain in appearance even after overcoating, require repair, and also deteriorate corrosion resistance, reducing the commercial value.

[0004]

Therefore, as a technique for improving the chemical conversion treatment property and the electrodeposition coating property, Japanese Patent Publication No. S58-58-
No. 15554, Zn4 on alloyed hot dip galvanized
A technique for forming a Zn-Fe plated layer of 0% or less is disclosed. However, Z on the galvannealing
In order to form a Zn-Fe plated layer of n40% or less, it is necessary to adhere an upper Zn-Fe plated layer of Zn40% or less by electroplating, which raises a problem of increase in equipment cost and manufacturing cost. It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet having good chemical conversion treatability and cationic electrodeposition coating property, which were difficult to obtain without increasing costs in the prior art, and a method for producing the same.

[0005]

That is, in the present invention, the plating layer composition is: Fe: 7 to 13 wt%, preferably 9 to
11 wt%; Al: 0.40 wt% or less, preferably
0.10 to 0.40 wt%; balance Zn; surface coverage of ζ phase is 1.0% or more, preferably 1
0.0 to 30.0%; an alloyed hot dip galvanized steel sheet having a plating layer having a coating weight of 20 g / m ² or more on at least one surface and having excellent chemical conversion treatment property and cationic electrodeposition coating property.

In the present invention, the plating layer composition is; Fe:
7 to 13 wt%, preferably 9 to 11 wt%; Al:
0.40 wt% or less, preferably 0.10 to 0.40 w
t%; balance Zn; 0.3% or more, preferably 0.3 to 0.8, of the ζ phase in the plating layer in the X-ray diffraction intensity ratio I (ζ) defined by the following formula [1] It is a galvannealed steel sheet having a coating layer having a coating weight of 20 g / m ² or more on at least one surface and having excellent chemical conversion treatment property and cationic electrodeposition coating property. I (ζ) = ζ / (ζ + δ ₁ + Γ + Γ ₁ ) Equation (1) However, in equation [1], ζ, δ ₁ , Γ, and Γ ₁ are respectively the following interstitial distances (d ) Is the X-ray diffraction intensity measured by the method. ζ; d = 0.2056 nm δ ₁ ; d = 0.213
6 nm Γ; d = 0.915 nm Γ ₁ ; d = 0.131
5 nm

Further, according to the present invention, the Al concentration is 0.145 w.
After galvanizing the steel sheet in a hot dip galvanizing bath of t% or less, preferably 0.120 to 0.145 wt% or less; the temperature of the galvanized steel sheet is raised at a rate of 20 ° C./sec or less, preferably 5 to 20. C./sec; by heating to 480 to 600.degree. C., preferably 490 to 560.degree. C., and alloying, a method for producing an alloyed hot-dip galvanized steel sheet excellent in chemical conversion treatment property and cationic electrodeposition coating property. is there.

[0008]

The present invention will be described in detail below. The inventors of the present invention investigated the relationship between the chemical conversion treatability and electrodeposition coatability of the alloyed hot-dip galvanized steel sheet, and the Fe and Al contents in the plating layer and the phase constitution, and obtained the following findings. The invention was completed.

In the present invention, the Fe content in the plating layer is set to 7 to 13 wt%, because if the Fe content is less than 7 wt%, the Zn-η phase tends to remain and the corrosion resistance after coating deteriorates. If it exceeds%, the uniformity of chemical conversion treatment deteriorates and craters occur during electrodeposition coating (cratering), and the sacrificial corrosion resistance deteriorates, resulting in significant corrosion resistance after coating. More preferably, 9 to 11 wt
%.

The Al content in the plating layer is 0.40 wt%
The reason for the following is that if the Al content exceeds 0.40 wt%, the uniformity of the chemical conversion treatment deteriorates, uneven chemical conversion treatment is likely to occur, and craters are likely to occur during electrodeposition coating. Further, 0.10 wt% or more is required to secure the peel resistance, and therefore, it is preferably 0.10 to 0.40 wt.
%, And more preferably 0.20 to 0.30 wt%.

The amount of the ζ phase is set to 1.0% or more in terms of the surface coverage, because if it is less than 1.0%, uneven treatment is likely to occur during chemical conversion treatment, and at the same time crater defects are likely to occur during electrodeposition coating. This is because On the other hand, if the ζ phase has a surface coverage of more than 30.0%, there is no further improvement in chemical conversion treatment and cratering resistance, but the η phase may remain and corrosion resistance deteriorates, which is not preferable. More preferably 1
It is 0.0 to 30.0%. Here, the surface coverage of the ζ phase means the area ratio in which the ζ phase exists on the galvanized surface,
As described later in Examples, the surface of the plating layer is observed using an electron microscope, and the area ratio of the ζ phase is calculated from the crystal morphology.

Further, the inventors of the present invention have studied the quantitative determination of the ζ phase by the diffraction intensity ratio by the X-ray diffraction method as a simpler method for measuring the ζ phase amount instead of the surface coverage, and obtained the following findings. It was X-ray diffraction method or electrochemical method may be used to quantify the amount of ζ phase, but to measure the amount of ζ phase on the outermost surface, the surface of the steel sheet is observed with an electron microscope. However, it is effective to classify the crystal forms of the δ ₁ phase (granular) and the ζ phase (columnar) in the part where the skin path is not applied, and calculate the respective covered areas. FIG. 1 shows the ζ-phase X-ray diffraction intensity ratio I (ζ) defined by the following formula [1] and the number of craters generated (pieces / cm ² ) according to the knowledge of the inventors of the present invention.
Shows the relationship with.

The electrochemical method is to measure the electrochemical potential of the surface of a steel sheet to show the state of the surface of the steel sheet, but the initial potential is unstable and the reproducibility is poor due to the influence of oxides on these surfaces. Therefore, it is not suitable for quantitative determination of ζ phase. On the other hand, the X-ray diffraction method includes information on the inside of the plating layer as compared with the surface coverage by the above-mentioned electron microscope observation, but the adhesion amount 2
It has been found by the present inventors that a relatively good quantitative property can be obtained with an alloyed hot-dip galvanized steel sheet of 0 g / m ² or more, and the amount of ζ phase by X-ray diffraction, chemical conversion treatability, and cationic electrodeposition coating property. The following limiting values are obtained between and. That is, as is clear from FIG. 1, the value I of the following formula [1] is I
When (ζ) is less than 0.3, uneven treatment is likely to occur during chemical conversion treatment, and craters are likely to occur during electrodeposition coating. Therefore, the amount of ζ phase is 0.3 or more when defined by the following formula [1]. Also, the value I of the following formula [1]
If (ζ) exceeds 0.8, Zn-η may remain and the corrosion resistance after coating may deteriorate. Therefore, the value I (ζ) of the following formula [1] is preferably 0.3 to 0. 8 is good. More preferably, it is 0.4 to 0.8.

I (ζ) = ζ / (ζ + δ ₁ + Γ + Γ ₁ ) Equation [1] However, in the above equation [1], ζ, δ ₁ , Γ, and Γ ₁ are
These are the X-ray diffraction intensities measured at the following interlattice distances (d). ζ; d = 0.2056 nm δ ₁ ; d = 0.213
6 nm Γ; d = 0.915 nm Γ ₁ ; d = 0.131
The basis weight of 5 nm is defined as 20 g / m ² or more for the purpose of corrosion resistance. If it is less than 20 g / m ² , the anticorrosion function of the Zn—Fe alloy layer is lowered. 20 g / m ² or more and 90 g / m ² or less, more preferably 30 or more and 60 g or more
/ M ² or less.

The chemical conversion treatment reaction is known as a reaction in which the surface of the Zn--Fe plating layer is dissolved when it comes into contact with the chemical conversion treatment liquid, and thereafter zinc phosphate is deposited on the surface of the plating layer to form a film. If Al exceeds 0.40 wt% in the plating layer, Al is concentrated on the plating surface in a large amount during alloying, and an oxide film is partially formed to form a Zn-Fe plating layer surface during chemical conversion treatment. Dissolution is suppressed,
It is presumed that the formation of the chemical conversion film is prevented. Further, the ζ phase is electrically baser than the δ ₁ phase, has a high solubility during the chemical conversion treatment, and contributes to the formation of a uniform chemical conversion film. Good chemical conversion treatability is obtained even when the ζ phase having a lower Fe content remains, but it is not suitable because the corrosion resistance deteriorates.

The occurrence of craters during electrodeposition coating is generally related to the uniformity of the electrical resistance of the surface of the electrodeposited object, and if the electrical resistance of the surface of the electrodeposition object is non-uniform, partial discharge occurs, It is considered that the paint coagulated by the heat is hardened, the flow at the time of baking does not occur, and craters are generated, and the nonuniform chemical conversion treatment is one of the causes of craters. Therefore, craters are likely to occur when the amount of Al is large and the amount of ζ phase is small.

In the present invention, as the composition of the galvannealed layer, only the contents of Fe and Al are specified, but other components such as Pb, Cd, Sn, In, Li,
Sb, As, Bi, Mg, La, Ce, Ti, Zr, N
Even if a small amount of i, Co, Cr, Mn, P, S, O, B or the like is added or inevitably mixed, the effect of the present invention is essentially unchanged.

Further, according to the method of the present invention shown below,
The alloyed hot-dip galvanized steel sheet of the present invention can be manufactured. Al concentration in hot dip galvanizing bath 0.120
˜0.145 wt%, the steel plate is dipped for a predetermined dipping time, plated with a coating adhesion amount of 20 g / m ² or more, and introduced into an alloying furnace. In the alloying treatment, in order to obtain a desired ζ phase, the temperature rising rate is 20 ° C./sec or less, preferably 5 to 20 ° C. /
sec is 480 to 600 ° C., preferably 490 to
The alloyed hot-dip galvanized steel sheet of the present invention is obtained by performing the treatment at 560 ° C for a predetermined time.

In general, Al is added to the plating bath in order to prevent deterioration of the peeling resistance of the plating layer. However, in the present invention, the Al concentration in the hot dip galvanizing bath is 0.145 wt% or less. If the Al content exceeds 0.145 wt%, a Fe—Al (—Zn) compound layer called an Al-enriched layer is thickly formed at the steel sheet-plating layer interface during hot dip galvanization, and Fe in the steel sheet during alloying is formed. This hinders the diffusion of Pd into the plated layer, making alloying difficult. Further, when the Al concentration is less than 0.120 wt%, the Al-rich layer may not be sufficiently formed during hot dip galvanizing, and Fe diffuses from the steel plate plating into the plating layer to form a Γ phase and thus plating. Since the adhesiveness deteriorates, the Al addition amount is preferably 0.120 to 0.145 wt%.

In order to obtain the ζ phase on the plated surface of the plated steel sheet of the present invention, it is necessary to carry out an alloying treatment for a long time at a low temperature region during alloying. That is, when the coating amount is 20 g / m ² or more, it is difficult to obtain a desired ζ phase on the plating surface when the temperature rise temperature exceeds 20 ° C./sec, and the temperature rise rate during alloying is 20 ° C./sec. It was set to sec or less. Further, when the heating rate is extremely slow, the heating time is long and the productivity is lost. Therefore, the heating rate is preferably 5 to 20 ° C./sec. Further, when the alloying temperature exceeds 600 ° C., Γ,
A large amount of Γ ₁ phase is formed, resulting in poor peeling resistance. Also, 4
If it is less than 80 ° C., alloying becomes slow and productivity is lowered.
Therefore, the alloying temperature is set to 480 to 600 ° C.
The temperature is preferably 490 to 560 ° C.

[0021]

The present invention will be described below with reference to examples, but the present invention is not limited thereto. (Examples 1 to 8 and Comparative Examples 1 to 7) A normal Sendzimir type continuous hot-dip galvanizing line with a plate thickness of 0.7 mm.
Hot dip galvanizing Ti-Nb type ultra low carbon steel sheet of
Subsequently, alloying treatment was performed. A in hot dip galvanizing bath
Table 1 shows the 1 concentration, the alloying conditions, the coating weight after the alloying treatment, the Fe content and Al content in the plating layer, and the surface coverage of the ζ phase.

The measurement of the ζ phase in the plated layer, the chemical conversion treatment property, the cratering resistance and the corrosion resistance were determined by the following tests. (1) Measurement of ζ phase in plating layer a) Measurement of surface coverage of ζ phase The surface coverage of the ζ phase was set to 3,500 times using a scanning electron microscope, and one sample (10 mm square) In 5), five locations were randomly selected except for the skin pass lowering part, and photographs (65 × 95 mm) were taken. The result of the above-mentioned photography in Example 7 is shown in FIG. Where ζ phase and δ ₁
The phases are shown in Figure 2. Next, on the photomicrograph shown in FIG. 2, seven straight lines are drawn in the lateral direction at intervals of 10 mm, and these lines cross the ζ phase shown in FIG. 2 within the total length (95 mm × 7 lines) of the line. The length of the portion is measured, and the surface coverage is calculated by the following formula [2].

[0023]

[Equation 1]

A method of calculating the surface coverage of the ζ phase will be specifically described by using the electron micrograph of the above-mentioned Example 7. As shown in FIG. 3, seven straight lines were drawn on the micrograph of FIG. 2 at intervals of 10 mm (L1 to L7). For each of L1 to L7, the length (line segment) where these straight lines cross the ζ phase
Was measured, the total was obtained, and the surface coverage of the ζ phase was calculated from the lengths of these line segments according to the above formula [2]. Figure 3
The length of each line segment is shown inside.

[0025]

[Equation 2]

From the above, the surface coverage of the ζ phase of Example 7 is 9.9 (%). The surface coverage of the ζ phase was similarly obtained for the other examples and comparative examples.

B) Measurement of X-ray diffraction intensity ratio X-ray diffraction intensity at the interstitial distance shown below, ζ,
δ ₁ , Γ and Γ ₁ are calculated as in Examples 1 to 8 and Comparative Examples 1 to 1.
The plated layer of the plated steel sheet of No. 7 was measured, and the X-ray diffraction intensity ratio I (ζ) was obtained from the following formula [1], and used as an index of the ζ phase amount. I (ζ) = ζ / (ζ + δ ₁ + Γ + Γ ₁ ) Equation [1] In the following inter-lattice distance (d), ζ in the equation [1],
The X-ray diffraction intensities of δ ₁ , Γ and Γ ₁ were measured. ζ; d = 0.2056 nm δ ₁ ; d = 0.21
36 nm Γ; d = 0.915 nm Γ ₁ ; d = 0.13
15 nm

(2) Chemical conversion treatment test Phosphate treatment liquid Granodine SD manufactured by Nippon Paint Co., Ltd.
Using 5000, the steel sheet sheared to 75 × 150 mm was subjected to a chemical conversion treatment, and the superiority or inferiority of the uniformity of the chemical conversion treatment film was visually evaluated. The evaluation is shown as follows. ○: Chemical conversion coating is uniform ×: Chemical conversion coating is not uniform

(3) Cratering resistance A steel sheet subjected to chemical conversion treatment is electrodeposited using the electrodeposition paint U-80 (manufactured by Nippon Paint Co., Ltd.) under the following conditions, and five arbitrarily selected points are selected. The number of craters generated on the coated surface within 1 cm ² was visually counted and averaged to obtain the crater generation density (pieces / cm ² ). The results are shown by the X-ray diffraction intensity ratio (I
(Ζ)) and ζ-phase surface coverage are shown in FIG. 1. Electrodeposition voltage: 300V Energizing method: 0V to 300V at boosting speed of 30V / sec
After raising the voltage to, hold it at the electrodeposition voltage. Electrodeposition time: 180 seconds

(4) Corrosion resistance On the surface of a steel plate panel (75 × 150 mm) after electrodeposition coating,
Insert a cross-cut scratch that reaches the base of the steel plate with a cutter knife, and perform a salt spray test (JIS Z2371).
After 0 days, the panel was taken out and the swelling width (blister width (mm) on one side) of the coating film at the crosscut scratched portion was measured and evaluated. Table 1 shows the above results.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

EFFECTS OF THE INVENTION As the surface-treated steel sheet for automobiles, the use of alloyed hot-dip galvanized steel sheet, which has excellent corrosion resistance after painting, is increasing. Defective coatings are likely to occur during cationic electrodeposition coating. According to the present invention, it is possible to provide an alloyed hot-dip galvanized steel sheet that prevents the occurrence of defects during the above chemical conversion treatment and electrodeposition coating, and it is possible to improve productivity in addition to improving corrosion resistance and appearance. .

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an X-ray diffraction intensity ratio I (ζ) of a ζ phase, a crater generated during electrodeposition coating, and a relationship of a ζ phase surface coverage.

FIG. 2 is an electron microscope and a drawing-substitute photograph showing a metal structure.

FIG. 3 is the same electron micrograph as FIG. 2, and is a drawing-substituting photograph showing a metal structure for explaining a method for measuring the surface coverage of a ζ phase using the same.

[Explanation of symbols]

[delta] _1-phase portion produced generated ζ phase portion [delta] ₁ galvanized steel sheet surface ζ galvanized steel sheet

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Chiaki Kato 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Steel Research Laboratory, Kawasaki Steel Co., Ltd. (72) Hideo Takamura Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba No. 1 Kawasaki Steel Co., Ltd. Steel Research Laboratory (72) Inventor Kazuo Mochizuki No. 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd. Steel Research Laboratory

Claims (6)

[Claims] 1. The plating layer composition is Fe: 7 to 13 wt%,
Al: 0.40 wt% or less, balance Zn, a ζ phase on the plating surface is 1.0% or more in the surface coverage, and a plating layer with a plating adhesion amount of 20 g / m ² or more is provided on at least one surface of the chemical conversion treatability and Alloyed hot-dip galvanized steel sheet with excellent cationic electrodeposition coating properties. 2. The composition of the plating layer is Fe: 9 to 11 wt%,
Al: 0.10 to 0.40 wt%, balance Zn
Ζ phase on the plating surface is 10.0 to 30.0 in terms of surface coverage
%, A galvannealed steel sheet having a plating layer having a coating weight of 20 g / m ² or more on at least one surface and having excellent chemical conversion treatment properties and cationic electrodeposition coating properties. 3. The plating layer composition is Fe: 7 to 13 wt%,
Al: 0.40 wt% or less, balance Zn, and X-ray diffraction intensity ratio I defined by the following formula [1] on the plating surface
(Ζ) has a ζ phase of 0.3 or more, and the coating weight is 2
An alloyed hot-dip galvanized steel sheet having a coating layer of 0 g / m ² or more on at least one surface and having excellent chemical conversion treatment property and cationic electrodeposition coating property. I (ζ) = ζ / (ζ + δ ₁ + Γ + Γ ₁ ) Equation [1] (However, in equation [1], ζ, δ ₁ , Γ, and Γ ₁ are respectively the following interstitial distances ( represents the X-ray diffraction intensity measured in d) ζ; d = 0.2056 nm δ ₁ ; d = 0.213
6 nm Γ; d = 0.915 nm Γ ₁ ; d = 0.131
5 nm) 4. The composition of the plating layer is Fe: 9 to 11 wt%,
Al: 0.10 to 0.40 wt%, balance Zn
A plating layer having an X-ray diffraction intensity ratio I (ζ) defined by the following formula [1] of 0.3 to 0.8 and a coating amount of 20 g / m ² or more was formed on the plating surface. A galvannealed steel sheet having excellent chemical conversion treatment property and cationic electrodeposition coating property on at least one surface. I (ζ) = ζ / (ζ + δ ₁ + Γ + Γ ₁ ) Equation [1] (However, in equation [1], ζ, δ ₁ , Γ, and Γ ₁ are respectively the following interstitial distances ( represents the X-ray diffraction intensity measured in d) ζ; d = 0.2056 nm δ ₁ ; d = 0.213
6 nm Γ; d = 0.915 nm Γ ₁ ; d = 0.131
5 nm) 5. A hot dip galvanizing bath having an Al concentration of 0.145 wt% or less is applied to the steel sheet, and then the galvanized steel sheet is heated at a rate of 20 ° C./sec or less to 480 to 600.
A method for producing an alloyed hot-dip galvanized steel sheet excellent in chemical conversion treatment property and cationic electrodeposition coating property, which comprises heating to ℃ and alloying. 6. A hot-dip galvanizing bath having an Al concentration of 0.120 to 0.145 wt% or less. After hot-dip galvanizing the steel sheet, the galvanized steel sheet is heated at a temperature rising rate of 5 to 20 ° C./sec.
A method for producing an alloyed hot-dip galvanized steel sheet excellent in chemical conversion treatment property and cationic electrodeposition coating property, which comprises heating to 90 to 560 ° C. and alloying.