JPH10219475A - Zn-mg plated steel sheet excellent in coating adhesion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plated steel sheet whose coating adhesion improves only by contacting treatment with alkali solution and excellent in corrosion resistance. SOLUTION: This plated steel is the one in which a Zn-Mg layer with a Mg concn. of <=0.5wt.% and a Zn-Mg layer with a Mg concn. of 5 to 16wt.% are successively laminated on a base steel respectively as a primary layer and a secondary layer, and having an uppermost layer in which Mg is oxidized by contacting treatment using a silicate-contg. alkali soln. The uppermost layer is composed of Mg compounds with >=100Å thickness contg. Si, O and H. By the Mg compound layer lying on the surface layer, its coating adhesion improves.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Zn--Mg-based steel sheet which exhibits good coating adhesion only by contacting it with an alkali solution containing silicate as a pretreatment for coating.

[0002]

2. Description of the Related Art The adhesion of a coating film of a plated steel sheet is affected by the morphology of the surface of the plated layer, the composition and composition of the surface layer, and the like. In a normal galvanized steel sheet, heat treatment causes Zn to be
To form a Zn—Fe alloy layer, increase the surface roughness of the plating layer, and inactivate the surface, thereby improving coating film adhesion. On the user side, the paint is directly applied after degreasing, but in many cases, a phosphate treatment is performed prior to the coating in order to further improve the coating film adhesion and the post-paint corrosion resistance. When there is no pre-painting equipment on the user side, chromate treatment may be performed as post-treatment after plating. The plated steel sheet subjected to the chromate treatment is ready for painting only by degreasing on the user side.

[0003]

In order to perform phosphating on the user side, facilities for the phosphating are required. In addition, the surface condition (form, composition, etc.) of the delivered plated steel sheet
Is required, the setting of the processing conditions becomes troublesome. In this regard, the plated steel sheet subjected to the chromate treatment is a convenient material for the user side because the pretreatment for painting only requires alkali degreasing.
However, the chromate treatment requires control over chromium elution. The present invention has been devised to solve such a problem. By effectively utilizing the properties of a highly corrosion-resistant Zn-Mg-based plating layer, an alkaline solution can be obtained without requiring a chromate treatment. It is an object of the present invention to provide a plated steel sheet exhibiting excellent coating film adhesion by a pretreatment just to be brought into contact with a steel sheet.

[0004]

Means for Solving the Problems A plated steel sheet according to the present invention comprises:
In order to achieve the object, a Zn-Mg layer having a Mg concentration of 0.5 wt% or less and a Zn-Mg layer having a Mg concentration of 5 to 16 wt%
The layers are sequentially laminated on the base steel as a first layer and a second layer, respectively, and have a Mg compound containing Si, O and H formed by contact treatment using a silicate-containing alkali solution. It is characterized by. The uppermost layer is preferably formed of a Mg compound layer containing Si, O, and H having a thickness of 100 ° or more. Further, at the interface between the plating layer and the base steel, Zn-Fe or Zn-Fe
-When the Mg alloy layer is formed, the occurrence of powdering during press forming, bending and the like is suppressed.

[0005]

The Zn-Mg-based plated steel sheet exhibits much better corrosion resistance than the conventional plated steel sheet. A Zn—Mg-based plated steel sheet is manufactured by sequentially or simultaneously depositing Zn and Mg on an underlying steel and performing an appropriate diffusion treatment, but the physical properties are greatly affected by the layer structure of the plated layer. The present inventors have conducted various investigations and studies on the relationship between the Zn-Mg-based plated steel sheet and physical properties such as corrosion resistance and workability. As a result, a Zn—Mg layer having a Mg concentration of 0.5% by weight or less is formed on the base steel, and a Zn—Mg layer having a high Mg concentration is formed thereon.
It has been found that when forming a layer, a plated steel sheet having very excellent corrosion resistance and good workability can be obtained.

[0006] The present inventors have further investigated and studied this Zn-Mg-based plated steel sheet, and as a result, have found that the Mg concentration is not more than 0.1.
A Zn—Mg layer of 5% by weight or less and a Zn—Mg layer with a Mg concentration of 5 to 16% by weight as a first layer and a second layer, respectively 2
When immersed in a silicate-containing alkaline solution, or sprayed with a silicate-containing alkaline solution, a plated steel sheet having a layered Zn-Mg-based plating layer is chromate-free and exhibits excellent coating adhesion. I found it to be in a state. Simply immersing in a silicate-containing alkali solution or spraying the silicate-containing alkali solution to contact the Z
The reason that the coating adhesion of the n-Mg-based plated steel sheet is improved is as follows.
It is inferred as follows.

When Zn is formed in the lower layer and Mg is formed in the upper layer by a vapor deposition method and heated, mutual diffusion of Mg and Zn in the upper layer forms a Zn ₂ Mg intermetallic compound. The upper layer has a structure in which Zn ₂ Mg intermetallic compound and Zn are mixed in the subsequent cooling process. The surface layer is covered with a Mg compound layer containing O and H having a thickness of several tens of mm by oxidation of Mg. Unlike the case of vapor-deposited Zn plating, the surface morphology of the plated steel sheet has no hexagonal crystal peculiar to Zn, and the crystal becomes amorphous. In addition, the interdiffusion between Zn and Mg results in a surface morphology with severe irregularities. It is considered that such a surface morphology provides a good foothold of the paint compared with the vapor-deposited Zn plating layer and improves the adhesion.

When the surface of the Zn—Mg-based plating layer is immersed in a silicate-containing alkaline solution, the surface of the plating layer is washed, and part of Mg in the second layer moves to the surface layer. The transferred Mg becomes a new M containing O and H in the alkaline solution.
g Compound layer is formed. Unlike the case of the Mg compound layer before immersion in the alkaline solution, the generated Mg compound layer completely covers the plating surface layer and has a large film thickness. Further, Si is transferred from the alkaline solution onto the M, and M containing Si
Since a g, O, H compound layer is generated, it exhibits a barrier effect on the Zn—Mg plating layer. Moreover, Mg
Since the passivation region is caused by oxides or hydroxides on the alkali side, a strong film is formed by alkali degreasing, and the barrier effect after coating is increased.

The Mg, O, H compound layer containing Si, which is stacked on the Mg compound layer containing O, H, which has grown thick, has a high chemical affinity for the coating components. From these facts, it is inferred that the surface of the plating layer immersed in the silicate-containing alkali solution has excellent coating film adhesion. It is inferred that even when the silicate-containing alkali solution is sprayed, the coating film adhesion on the surface of the plating layer is improved for the same reason. Furthermore, the coating film swelling and corrosion under the coating film are suppressed by the Mg, O, and H compound layers containing Si. Zn-Mg plated steel sheet treated with silicate-containing alkali solution,
Even in the processed part, good coating adhesion is obtained, which is the same as that of the unprocessed part. For example, when a sliding process such as drawing of a cylindrical cup is applied to a Zn—Mg-based plated steel sheet, a surface Mg compound layer is removed, and an active Zn ₂ Mg-based metal surface is exposed.
However, the exposed metal surface is immediately oxidized into an Mg compound layer. Therefore, corrosion of the second layer immediately below is prevented. In this Mg compound layer, a Mg, O, H compound layer containing Si is newly generated by a contact treatment with an alkali solution. As a result, excellent coating film adhesion is exhibited as in the unprocessed portion. On the other hand, in pickling or the like in which the surface portion of the plating layer is chemically removed, Mg is preferentially eluted into the acid solution, and the plating layer surface becomes Zn-rich. Therefore, there is no growth of the Mg compound layer which is effective for improving the adhesion of the coating film, and no good secondary adhesion is exhibited.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A Zn-Mg plated steel sheet according to the present invention comprises a Zn-Mg layer having a Mg concentration of 0.5% by weight or less,
A Zn—Mg-based plating layer having a two-layer structure in which Zn—Mg layers having a concentration of 5 to 16% by weight are sequentially laminated on the base steel as a first layer and a second layer, respectively. After depositing Zn for the first layer, Mg is deposited and Zn and Mg are interdiffused to form a first layer having a low Mg concentration and a second layer having a high Mg concentration. The first layer has a low Mg concentration of 0.5% by weight, is soft and rich in ductility, and acts as an intermediate layer that absorbs the difference in the amount of deformation between the steel sheet that deforms during forming and the second layer that hardly deforms. As a result, the occurrence of powdering during press forming, bending forming, and the like is suppressed. Mg concentration is 0.
If it exceeds 5% by weight, the material hardens and the effect of improving workability is weakened.

The Mg concentration of the second layer is required to be 5% by weight or more in order to form a Mg compound layer containing Si, O and H which is effective for coating film adhesion. However, Mg exceeding 16% by weight
At the concentration, since the second layer is a phase in which Zn ₂ Mg and Mg are mixed, discoloration easily occurs due to metallic Mg, and the appearance deteriorates. In addition, the second layer becomes too hard, and the workability also deteriorates. The Mg concentration of the first layer and the second layer can be controlled by the evaporation amount and the diffusion conditions. For example, a total adhesion amount of 30 g
In the case of / m ² , 28.2 g / m ² of Zn is deposited on the first layer, and 1.8 g / m ² of Mg is deposited on the second layer, followed by heat diffusion. The diffusion conditions are determined by the temperature at the time of vapor deposition.
If the temperature of the steel sheet at the time of vapor deposition is 300 ° C., diffusion occurs within a few seconds. On the other hand, when the steel sheet temperature is about 240 ° C., Z
Although n and Mg diffuse to some extent, post-heating is required to completely diffuse. When heating at the same temperature, heating for 20 to 30 seconds is required. In the case of heating for a long time, complete diffusion can be achieved by heating at 170 ° C. for 180 minutes. Due to the heat diffusion, the surface of the plating layer becomes a form with severe irregularities, and the foothold of the paint is improved.

A Zn—Fe alloy layer or a Zn—Mg layer having a thickness of 0.5 μm or less is provided at the interface between the base steel and the Zn—Mg plating layer.
When forming the -Fe-Mg alloy layer, the adhesion of the plating layer is enhanced, and the effect of suppressing the occurrence of powdering is further increased. However, the Zn—Fe alloy layer or the Zn—Fe—
If the thickness of the Mg alloy layer exceeds 0.5 μm, powdering is more likely to occur. Generation and thickness adjustment of such a Zn—Fe alloy layer or a ZnFe—Mg alloy layer can be controlled by heat treatment conditions after vapor deposition plating. Degreasing before painting is performed by immersing a Zn-Mg-based plated steel sheet in a silicic acid-containing alkali solution, or applying a Zn-M
It is performed by spraying g-plated steel sheet. As the silicate to be added to the alkaline solution, sodium orthosilicate, sodium metasilicate and the like are used alone or in combination. A surfactant may be added to the alkaline solution.

An Mg compound layer containing O and H is formed by dipping in an alkali solution or spraying with an alkali solution, and a Mg compound layer containing Si derived from silicate is further formed thereon.
An O, H compound layer is formed. At this time, if an alkali solution such as sodium hydroxide or sodium phosphate containing no silicate is used, the effect of improving the coating film adhesion is small,
Sufficient secondary adhesion and water resistance cannot be obtained. The contact treatment with the silicate-containing alkali solution is performed by using M containing Si, O, and H.
The processing conditions are set so that the g compound layer has a thickness of 100 ° or more. When the thickness of the Mg compound layer containing Si, O, and H is less than 100 °, the effect of improving the coating film adhesion is small. In addition, when the medium is sprayed or dipped using a medium having a pH of 11.5 or more or a strong alkaline degreasing solution in which Mg is in a passive region, a Mg compound layer containing sufficient Si, O, and H can be obtained.

[0014]

EXAMPLE A Ti-added steel sheet having a thickness of 0.8 mm was used as an original plate for plating. The plating plate was reduced and heated in an N ₂ -50% H ₂ gas atmosphere to remove an oxide film on the surface, and then introduced into a vacuum chamber. In the vacuum chamber, N ₂ gas having a dew point of −60 ° C. was introduced while evacuating with a vacuum pump, and the degree of vacuum was maintained at 5 × 10 ⁻² Torr. Zn in a vacuum chamber
→ Evaporation was performed in the order of Mg to obtain the samples shown in the following table. At this time, the steel sheet temperature at the time of vapor deposition was set to 240 ° C.

[0015]

The plated layer after the heat treatment had the layer structure shown in Table 2.

[0017]

Each sample was immersed in an alkaline solution under the conditions shown in Table 3 as a pretreatment for coating, washed with water, and dried.

[0019]

Next, a solvent type acrylic paint was applied at a thickness of 30 μm, baked and dried. The primary adhesion, secondary adhesion of boiling water, and corrosion resistance of the formed coating film were investigated.
For comparison, the amount of adhesion on one side was 30 g / m ² , and the Fe content was 12
The same coating was applied to a weight percent galvannealed steel sheet. The primary adhesion was determined by applying a checkerboard-shaped cross cut (1 mm square, 100 pieces) to each of the coated samples, and then attaching and peeling off an adhesive tape to determine the amount of the coating film remaining on the steel sheet surface. ◎: No peeling of the coating film was detected, ○: 80% or more of the remaining coating film, 30% or more: 8
Those with less than 0% were evaluated as Δ, and those with less than 30% were evaluated as x.

The secondary adhesion of boiling water was evaluated by immersing each sample in boiling water for 2 hours, and then evaluating the remaining ratio of the coating film by a similar cross-cut → tape peeling test. The water resistance was evaluated by observing the appearance of each sample after immersing it in warm water at 40 ° C. for 240 hours. Was evaluated.
Tables 4 to 6 show the inspection results of Samples 1, 4, and 5, respectively.

[0022]

[0023]

[0024]

As can be seen from Table 4, in the case of Zn-Mg plated steel sheet, the primary adhesion is improved when alkali degreasing is performed as a pretreatment for coating. In the case of contact treatment with an alkali solution containing a silicate such as sodium orthosilicate or sodium metasilicate, the secondary adhesion after boiling water immersion is greater than that in the case of treatment with an alkali solution containing no silicate. Has been improved. In addition, in the case of contact treatment with an alkali solution containing no silicate, slight changes occur in appearance such as slight peeling in the boiling water secondary adhesion test and swelling of the coating film in the water resistance test. Was. On the other hand, as shown in Table 5, in the case of the vapor-deposited Zn-plated steel sheet containing no Mg, the peeling of the coating film occurred in the boiling water secondary adhesion test when any of the degreasing agents was used. However, the water resistance was good when treated with any of the degreasing agents. Further, as shown in Table 6, the alloyed hot-dip galvanized steel sheet had good boiling water secondary adhesion and water resistance regardless of the type of the degreasing agent.

The reason why the adhesion of the coating film varies depending on the type of the plating layer and the degreasing agent is considered as follows.
The Zn-Mg layer as the second layer in the Zn-Mg-based plated steel sheet according to the present invention has a higher activity than Zn due to the inclusion of Mg, and thus is easily dissolved by moisture penetrating under the coating film. Therefore, it is necessary to deactivate the plating layer surface, but when degreasing with an alkali solution containing no silicate such as sodium hydroxide, the deactivation is insufficient,
Water resistance is inferior to that of evaporated Zn plating containing no Mg. In this regard, when degreasing is performed with an alkali solution containing silicate, the deactivation sufficiently proceeds due to the formation of a Mg, O, H compound layer containing Si, and as a result, the water resistance is improved.

Auger analysis was performed on the sample 1 before and after the treatment using the alkali solution containing silicate to determine the concentration distribution of the elements in the depth direction. Figure 1 shows the element concentration distribution before degreasing.
FIG. 2 shows the element concentration distribution after degreasing. As is clear from a comparison between FIG. 1 and FIG. 2, Mg and O showed high values and Zn decreased after alkali degreasing. This indicates that Mg on the surface of the plating layer is oxidized by alkali degreasing, and a Mg compound layer containing O and H is growing. Furthermore, since Si is high in the surface layer portion after alkali degreasing, it can be seen that a Mg, O, H compound layer containing Si is formed in the surface layer.

The Mg compound layer grown on the plating layer surface is
From the result of investigation of the bonding state of Mg and O by ESCA, M
g of oxide or hydroxide. But including Si
Is it possible to elucidate the bonding state of the outermost compound layer?
Was. Depth and thickness of generated Mg oxide or Mg hydroxide
After investigating the relationship with the fat time, as shown in Table 7,
As the processing time becomes longer, the film thickness increases, and the degreasing time is 1 minute
About 200Å (SiO _Two (Converted). Only
However, even if the degreasing time is longer,
It has become slow.

[0029]

The thickness of the Mg compound before and after alkali degreasing was calculated for each sample of the Zn—Mg plated steel sheets used in the experiment. The results shown in Table 8 were obtained. From Table 8, it can be seen that the thickness of the Mg compound layer is M
It can be seen that it depends on the amount of g, and the greater the amount of Mg, the thicker it becomes.

[0031]

Further, the coated plated steel sheet was subjected to a salt spray test (JIS Z2371) to examine corrosion resistance such as swelling resistance and red rust resistance. In the paint film swelling resistance test, a 150 mm × 70 mm sample cut from each coated steel sheet was cross cut, and after 240 hours, a tape peeling test was performed.の も の, less than 3 mm, 3 mm or more, 5 m
Those with less than m were evaluated as Δ, and those with 5 mm or more were evaluated as x. In the red rust resistance test, the red rust resistance was examined based on the red rust occurrence area ratio of the cross cut portion, and ◎ indicates that no red rust was detected in the cross cut portion and ○ indicates that the red rust occurrence area ratio was less than 20%. , 20% or more and less than 50%
% Or more was evaluated as x.

[0033]

As can be seen from the investigation results in Table 9, in Sample 1 having the Zn-Mg based two-layer structure according to the present invention and having the Mg compound layer on the surface, the primary adhesion and the secondary Good properties were exhibited in both adhesion and corrosion resistance. In Sample 2 having a small Mg content, although the primary adhesion of the coating film was good, slight peeling was detected in the adhesion test after immersion in boiling water. The Mg compound layer formed by alkali degreasing of Sample 2 had a thickness of 110 ° and was a thin layer as compared with Sample 1. Thus, Zn-
It is guessed that the surface morphology and the thickness of the Mg compound layer after alkali degreasing are different depending on the Mg content also in the Mg-based plated steel sheet, which affects the adhesion.

As shown in FIGS. 3 and 4, the unevenness on the surface of the plating layer increases as the Mg content increases, as compared with FIGS. Further, as can be seen from the thickness of the Mg compound layer after alkali degreasing shown in Table 8,
As the content increases, the thickness of the Mg compound layer after alkali degreasing increases, and the coating film adhesion is improved. However, Sample 3 having too much Mg content is inferior in water resistance because metallic Mg is present in the second layer even after alkali degreasing. On the other hand, in the case of the vapor-deposited Zn-plated steel sheet containing no Mg (Sample 4), the coating film swelled due to immersion in boiling water, and the secondary adhesion was poor. In the tape peeling test after the salt spray test, the amount of peeling was large, and the corrosion resistance was poor. In addition, in the alloyed hot-dip galvanized steel sheet (sample 5) not subjected to the chromate treatment, the coating film adhesion was good in both the primary and the secondary, and the tape peeling amount after the salt spray test was small. However, the corrosion resistance at the cross cut portion was inferior to Sample 1 (Example of the present invention). Cr adhesion amount 3
The alloyed hot-dip galvanized steel sheet (sample 6) that had been subjected to a chromate treatment of 0 mg / m ² showed good results in both coating film adhesion and corrosion resistance.

[0036]

As described above, as described above, the Zn-
Mg-based plated steel sheet is made of Zn-
Mg layer as the second layer, the surface of which contains Si, O, and H containing M
Since the g compound layer is formed, it is used as a plated steel sheet having excellent coating film adhesion. Further, since the Zn-Mg layer having a Mg concentration of 0.5% by weight or less is formed in the first layer, the workability is good and the high corrosion resistance peculiar to the Zn-Mg-based plating layer is maintained. Thus, the Z of the present invention
An n-Mg-based plated steel sheet is used as a coated steel sheet having good adhesion and corrosion resistance by a chromate-free pretreatment only by a contact treatment with an alkali solution.

[Brief description of the drawings]

FIG. 1 shows the element concentration distribution of sample 1 on which a Zn—Mg-based plating layer was formed with a Zn deposition amount of 28.2 g / m ² and a Mg adhesion amount of 1.8 g / m ² before alkali degreasing.

FIG. 2 Element concentration distribution of sample 1 after alkaline degreasing

FIG. 3 is a photograph of the metal structure of the zinc-Mg based plating layer of Sample 1 observed by SEM.

FIG. 4 is a photograph of the metal structure of the zinc-Mg based plating layer of Sample 2 similarly observed by SEM.

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[Procedure amendment]

[Submission date] December 1, 1997

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Figure 3

[Correction method] Change

[Correction contents]

FIG. 3

[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Fig. 4

[Correction method] Change

[Correction contents]

FIG. 4

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Minoru Saito 5th Ishizu Nishimachi, Sakai-shi, Osaka Nisshin Steel Co., Ltd.

Claims (3)

[Claims] 1. Zn-Mg having an Mg concentration of 0.5% by weight or less.
Layer and a Zn-Mg layer having a Mg concentration of 5 to 16% by weight are sequentially laminated on the base steel as a first layer and a second layer, respectively.
S formed by contact treatment with silicate-containing alkali solution
A Zn-Mg-based plated steel sheet having excellent adhesion to a coating film having a Mg compound containing i, O and H as an uppermost layer. 2. An S layer having an uppermost layer having a thickness of 100 ° or more.
The Zn-Mg-based plated steel sheet according to claim 1, comprising a Mg compound layer containing i, O and H. 3. A thickness of 0.5 at the interface between the plating layer and the base steel.
The Zn-Mg based steel sheet having excellent coating film adhesion according to claim 1 or 2, wherein a Zn-Fe or Zn-Fe-Mg alloy layer having a thickness of at most μm is formed.