JP2001020050A - HOT DIP Zn-Al-Mg PLATED STEEL EXCELLENT IN CORROSION RESISTANCE IN NONCOATED PART AND COATED EDGE PART AND ITS PRODUCTION - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot dip Zn-Al-Mg plated steel sheet requiring corrosion resistance, e.g. for building materials such as roof materials and wall materials and automotive, machine and household appliance products used in a noncoated or coated state and to provide a method for producing it. SOLUTION: This hot dip Zn-Al-Mg plated steel excellent in the corrosion resistance of a noncoated part and a coated edge part has a hot dip plating layer composed of, by weight, 3 to 15% Al and 4 to 10% Mg, and the balance Zn with inevitable impurities, in which, in the plating layer, a single phase of one or two kinds of Zn2Mg and Zn11Mg2 is precipitated by the dimensions of >=0.1 μm grain size. Moreover, as to the method for producing the above plated steel excellent in the stability of a plating bath, plating is executed with a plating bath in which the relation between the Al content and the Mg content in the plating bath is prescribed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to unpainted or painted building materials such as roofing materials and wall materials, automobiles, machines,
Fused Zn-Al-M, which requires corrosion resistance of home appliances
The present invention relates to a g-plated steel sheet and a method for producing the same.

[0002]

2. Description of the Related Art For the purpose of improving the corrosion resistance of hot-dip galvanized steel sheets, attempts to add a metal such as Al or Mg to a coating layer have been known for some time. For example, JP-A-8-
No. 60324, JP-A-56-41359, and JP-A-56-96062.

[0003] Of these, Japanese Patent Application Laid-Open No. Hei 8-60324 discloses that the maximum content of Al in a plating layer is 0.25% by weight,
The content of Mg is as small as 3% by weight at the maximum, and although the effect of improving corrosion resistance is recognized, a significant improvement cannot be expected.

[0004] Also, Japanese Patent Application Laid-Open No. 56-41359 discloses that
Mg content up to 1.5 wt% and Al 1.5 wt%
By setting the ratio of Al / Mg to 0.2 or less, corrosion of the grain boundary of the plating layer is suppressed and reactivity with the phosphating solution is improved. However, when considering corrosion resistance in bare use, the corrosion resistance is inferior to that of a commercially available Zn-5% Al-based plated steel sheet because the content of Al is small, and further improvement is desired.

Japanese Patent Application Laid-Open No. 56-96062 discloses that
When the content of Mg is at most 2.5% by weight, the content of Al is 3% by weight or less, and the portion where molten metal is present has an oxygen concentration of
It is characterized in that plating is performed in a gas atmosphere of m or less. However, even in this technique, the contents of Al and Mg are not sufficient, and when corrosion resistance after painting or after painting is considered, there is no great difference from the existing Zn-5% Al system, and further improvement has been desired.

Japanese Patent Application Laid-Open No. Hei 10-226865 discloses a technique for improving the corrosion resistance by increasing the contents of Al and Mg. This means that 4 to 10% by weight of Al and 1 to
4% by weight, and the alloy phase of the plating layer is Al / Zn / Mg
The ternary eutectic structure is mainly specified to improve the corrosion resistance and appearance. The effect of improving the corrosion resistance largely depends on Mg added to the plating, but the details of the effect are unknown. Further, as the effect of Mg on the appearance, the presence of a Zn ₂ Mg alloy is defined as the existence form of Mg is important. Furthermore, it is defined that the primary crystal Al phase is main as compared with the Zn single phase. However, the form of existence of the Zn ₂ Mg alloy is not described in detail, but merely states that the alloy exists in the ternary eutectic. Therefore, there is nothing suggesting a cause for the corrosion resistance of the Zn ₂ Mg alloy or an optimum existence state.

On the other hand, in JP-A-10-306357 and JP-A-10-265926, it is attempted to obtain a desired metal structure by further adding Ti or B to a plating bath, or in a cooling process after plating. By spraying water or aqueous solution, Zn ₁₁ Mg
Techniques such as aggressive precipitation of ₂ have been disclosed. As described above, the Zn-Al-Mg alloy plating has a problem that the deposited alloy structure is complicated and greatly varies depending on the manufacturing conditions, and accordingly, extremely important characteristics such as corrosion resistance and appearance of the plated steel material are greatly changed. Having. However, particularly for the Mg-based alloy phase, it merely specifies the type of the intermetallic compound, and does not mention the form of its existence.

In consideration of a specific use environment, in many cases, a plated steel sheet is bent or extruded to cause damage to the plated layer to the ground iron, or the cut end surface of the coating material is often exposed. Although it occurs, it is desired to provide a plated steel material exhibiting the best corrosion resistance even in such an environment. However, the existing technology stipulates that the Mg-based alloy exists in the ternary eutectic system, so that Mg is present in a finely mixed state with Al and Zn. Conceivable. However, when considering the characteristics of each element and the interaction of the corrosion reaction, it is doubtful whether such a fine existence state is optimal or not, but it is not mentioned at all.

[0009]

As described above, a significant improvement in the corrosion resistance of Zn-based plated steel sheets requires A
Although the addition of l or Mg is effective, despite the fact that the state of the presence of Mg is important, the prior art merely specifies that the Mg is distributed in the ternary eutectic, and is desirable because it has more excellent corrosion resistance. The form of the shape is not clear.
Therefore, an object of the present invention is to clarify the structure of a plated steel material having the highest corrosion resistance in a Zn-Al-Mg-based plated layer, and to provide a plated steel material and a manufacturing method thereof.

[0010]

The gist of the present invention is to provide Al:
3 to 15% by weight, Mg: 4 to 10% by weight, the balance has a hot-dip layer made of Zn and unavoidable impurities, and one or two kinds of Zn ₂ Mg and Zn ₁₁ Mg _{2 in} the plated layer Is a hot-dip Zn-Al-Mg-plated steel material excellent in corrosion resistance of the unpainted part and the painted end face part, wherein the single phase is precipitated with a particle size of 0.5 μm or more. Further, in a hot-dip plating method comprising 3 to 15% by weight of Al, 4 to 10% by weight of Mg, and the balance of Zn and unavoidable impurities, the content of Al is set to Al% by weight = Mg
The above-mentioned Zn, which is excellent in plating bath stability, characterized in that plating is performed in a plating bath having a higher Al side than the relationship between Al and Mg defined by weight% x 1.83-3.33.
-A method for producing an Al-Mg plated steel material.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Zn-Al-Mg plating of the present invention
In the steel material, the composition of the plating layer is Al: 3 to 15% by weight, M
g: 4 to 10% by weight, balance being Zn and unavoidable impurities
Consists of In particular, the present inventors have studied the existence state of Mg and the processing.
Examining the relationship between the corrosion resistance of the unpainted part and the corrosion resistance of the painted end face
As a result, Mg became Zn_TwoMg or Zn₁₁Mg_TwoOne kind of
Or two species are finely distributed in the ternary eutectic
But with a certain size of Zn _TwoMg and Zn₁₁Mg_Twoof
It is better to form a single phase and to exist independently in the plating layer.
The present invention was found. The ZnMg alloy
Gold single phase has a particle size of 0.5μm or more
As a result, it was found that the effect of improving the corrosion resistance was particularly great. So
The mechanism of the development of corrosion resistance has been completely clarified.
Although it is not, it is presumed to be roughly the following mechanism. That is,
This Zn-Mg alloy is an electrochemically active metal
It is an alloy of Zn and Mg.
Shows active properties. However, at the same time, Mg ions
It is known to highly inactivate Zn corrosion products.
You. Therefore, in a corrosive environment, the Zn-Mg alloy phase has priority.
Due to Mg ions eluted quickly
Deactivates the corrosion products of Zn and prevents the subsequent corrosion reaction
Stop. An alloy having such properties, Z
When present in a ternary eutectic of n / Al / ZnMg, Z
Active metals other than n-Mg alloy, such as Zn alone and Al
Since there is a single substance, in a corrosive environment, Mg ion
And the effect of Mg is weakened.
It is thought to be. On the other hand, Zn-Mg alloy is a single phase
If present, the relative concentration of Mg is 1: 2 or 1:
Corrosion corrosion of Zn eluted to 5 at the same time
The effect of stabilizing the object can be sufficiently exhibited. One
In other words, if the ground iron layer is exposed
Even if the cathodic reaction is large and the corrosion reaction is activated,
Stable corrosion product phase to suppress corrosion reaction
You. Single phase of this Zn-Mg alloy exists in the plating layer
But it is unavoidable that other phases exist
Is acceptable. That is, Zn phase, Al phase, Zn / A
1 / ZnMg ternary eutectic phase.

[0012] In order to form this Zn-Mg alloy phase alone, it is necessary to optimize the composition of Zn-Al-Mg. The present inventors have conducted intensive studies and found that the Mg concentration in the bath was 4-1 to 1.
If the weight ratio of Al is between 0% by weight and the weight percentage of Al is 1.83 times the weight percentage of Mg and 3.33 is added, if Al on the Al-rich side and the balance are Zn and unavoidable impurities, It has been found that a single phase of Zn ₂ Mg or Zn ₁₁ Mg ₂ is precipitated as a primary crystal in the plating layer and has a grain size of 0.5 μm or more. The size of the alloy phase also depends on the cooling rate from the molten state after plating, and it has been found that the slower the cooling rate, the larger the alloy phase grows and grows.

When the content of Mg is 4% by weight or less, a single phase of Zn—Mg cannot be obtained even if the above relational expression is satisfied, and the effect of improving corrosion resistance cannot be expected. In addition, when Mg is 10
%, The Mg content becomes higher.
g ₂ Zn ₃ and MgZn are deposited, which not only deteriorates the corrosion resistance, but also causes black spots in the form of a buffer film when the deposits appear on the plating surface, and the product is markedly remarkable. You lose value. Therefore, Mg is 4
10 to 10% by weight, more preferably 4 to 6% by weight.

Although the Zn-Mg alloy phase is preferably Zn ₂ Mg or Zn ₁₁ Mg ₂ from the viewpoint of corrosion resistance, which phase is formed depends on the plating bath temperature and cooling after plating. Determining the phase is not industrial because it is determined by a number of operating factors, such as the speed, the temperature at which the plate enters the plating bath, and the cooling method after plating. However, it is desirable to precipitate a Zn ₂ Mg phase having a higher Mg content.

The higher the Al content, the higher the effect of improving corrosion resistance.
If the content is less than 3% by weight, the lower limit is set. Also, 3
When the content is less than 4% by weight, the stability of the plating bath containing 4% by weight or more of Mg cannot be maintained. This means that since Mg is an easily oxidizable metal, in the state of a molten metal at a high temperature, an oxide is formed on the surface to exhibit a defect of selectively leaving the system. Although this oxide is called dross, it has an adverse effect of being caught in the plating layer and deteriorating the plating quality, so it is important to keep the amount of dross low. The upper limit is 15% by weight. Fifteen
Even if the content exceeds 10% by weight, the effect of improving the corrosion resistance is saturated, and the melting point of the bath increases as the content of Al increases, so that a large amount of heating energy is required, which is not economical. The range of Al is more preferably in the range of 5 to 8% by weight.

The effect of Al on bath stability is significant.
The bath stability is ensured if Al is contained in the above relational expression with respect to Mg. The principle of its operation has not been clarified, but it is presumed to be roughly as follows.
That is, the Mg oxide (dross) is of a volume shrinkage type, and even if dross is generated, the oxide itself does not exhibit a barrier effect against oxygen and the oxidation reaction is continued. On the other hand, Al forms a passivation film of a volume expansion type and blocks oxygen permeation. Therefore, once Al dross covers the plating bath surface, further dross generation is suppressed. As described above, it has been found that the dross generated from a molten alloy composed of metals having different properties is strongly affected by the properties of the metal having a higher content.
Was found to form a stable dross when the content of Mg was at least a certain value with respect to Mg.

The steel material to be used is not particularly limited.
There are steel wires. Although the material of the steel sheet is not particularly limited,
There are killed steel, low C steel, high tensile steel and the like. Examples of the plating manufacturing method include a method of applying wet plating by pre-plating Ni or the like, a flux method, a Sendzimir method, and the like. For the purpose of changing the appearance after plating, spraying water spray, air-water spray, spraying sodium phosphate aqueous solution or Zn powder, further spraying Zn phosphate powder, Mg hydrogen phosphate powder or an aqueous solution thereof good.

[0018]

EXAMPLES Preparation of plated steel sheets was performed using a Sendzimer-type continuous hot-dip Zn plating equipment. The treated steel sheet is A
It is a 1-killed steel plate with a thickness of 1.2 mm. The treated steel sheet was first treated in a reducing furnace having a furnace temperature of 700 ° C. and a dew point of −40 ° C. in a 10% H ₂ —N ₂ atmosphere, then cooled, and immersed in a hot-dip plating bath of various plating compositions at a furnace temperature of 460 ° C. Immersion time is 4
After plating, the amount of deposition was controlled by N ₂ gas wiping after plating. A sample for corrosion test evaluation was prepared from the obtained steel sheet, and the following corrosion resistance test was performed.

The unpainted corrosion resistance test was 150 mm × 70
The plated steel sheet cut into mm was subjected to an Erichsen process with an extrusion height of 8 mm, and the end face was tape-sealed. Then, a salt spray test according to JIS Z2371 was performed to evaluate the corrosion loss of the plated layer after the test for 500 hours. The loss of corrosion of the Erichsen-processed part was 10 mg / m ² or less.
mg / m ² ○, 31-50 mg / m ² △, 51 mg / m ²
x was defined as m ² or more, and acceptable if it was ○ or more.

On the other hand, the corrosion resistance after coating is as follows:
A chromate treatment with an adhesion amount of 50 mg / m ² was applied, and a melamine alkyd paint was applied with a 10 μm bar coater and baked at 180 ° C. for 20 minutes. Cut one side of the long side of this sample with a shear to expose the end face,
A combined environmental cycle test was performed. Cycle conditions are: salt spray test 2 hours → dry 1 hour → wet (60 ° C 90RH)
Three hours were defined as one cycle, and the blister width from the end face after 21 days was evaluated. The blister width is 2mm or less ◎, 2-4m
m was ○, 4 to 6 mm was Δ, 6 mm or more was X, and 以上 or more was passed. The evaluation of the appearance was evaluated as ◎ when the uniform color tone and no unevenness were observed, and as x when there was a spot, a plated pattern, and an uneven color tone.

The analysis of the alloy phase in the plating layer was performed by embedding and polishing in the cross-sectional direction of the plating layer of the plated steel material, and the distribution state and size of the alloy phase were observed using EPMA. Further, X-ray diffraction measurement was performed to identify a Zn-Mg alloy phase.

(Example 1) The composition of the plating bath was Zn-3Al.
-4Mg, and the plating adhesion amount is 100 mg / m ² on one side.
Was adjusted. The Zn ₁₁ Mg ₂ phase was dispersed with a particle size of about 0.5 μm in the obtained plating layer. Furthermore, a Zn single phase and a ternary eutectic phase were also observed. Regarding the corrosion resistance, both the unpainted and the painted were evaluated as ○, and the appearance was also good.

(Examples 2 to 4) The composition of the plating bath was 4 Mg.
%, And the Al content was varied from 6 to 15% by weight. The amount of adhesion was the same as in Example 1. The Zn ₂ Mg phase was dispersed in the obtained plating layer with a particle size of about 1.0 to ₂ μm. Furthermore, a Zn single phase and a ternary eutectic phase were also observed. The corrosion resistance was rated ◎ for both unpainted and painted, and the appearance was good.

(Examples 5 to 13) The plating bath composition was changed to Al:
It was the same as Example 1 except that it was changed at 6 to 15% by weight and Mg at 5 to 10% by weight. Z in the obtained plating layer
The n ₂ Mg phase was dispersed with a particle size of about 0.5 to 4 μm.
Further, an Al single phase and a ternary eutectic phase were also observed. The corrosion resistance was rated ◎ for both unpainted and painted, and the appearance was good.

Comparative Example 1 The composition of the plating bath was Zn-2Al
-4Mg, and the amount of adhesion was the same as in Example 1. The Zn ₁₁ Mg ₂ phase was dispersed with a particle size of about 0.1 μm in the obtained plating layer. Furthermore, a Zn single phase and a ternary eutectic phase were also observed. Regarding the corrosion resistance, unpainted was evaluated as △, and painted was evaluated as ×, the appearance was spotted, and the evaluation was ×.

(Comparative Examples 2-3) The composition of the plating bath was Al: 4
-13% by weight, Mg: 3-9% by weight
Was set to a value smaller than the weight% of Mg × 1.83-3.33. A Zn ₂ Mg phase or a Zn ₁₁ Mg ₂ phase was dispersed in the obtained plating layer with a particle size of about 0.1 to 0.3 μm. Further, an Al single phase and a ternary eutectic phase were also observed. Regarding the corrosion resistance, unpainted was evaluated as △, and painted was evaluated as ×, the appearance was spotted, and the evaluation was ×.

(Comparative Examples 4 to 6) The composition of the plating bath was Al: 4
-7% by weight, Mg: 3% by weight, but the Al weight% was smaller than the Mg weight% x 1.83-3.33. The resulting plating layer contains a Zn ₂ Mg phase or Z
The n ₁₁ Mg ₂ phase was dispersed with a particle size of about 0.1 μm. Further, an Al single phase and a ternary eutectic phase were also observed. Corrosion resistance is
The unpainted specimens were evaluated as △ to ×, the painted specimens were evaluated as ×, the appearance was spotted, and the evaluation was ×.

[0028]

[Table 1]

[0029]

The Zn-Al-Mg of the present invention described above
According to the plated steel material, it has excellent corrosion resistance in an unpainted processed portion and corrosion resistance after painting.

 ──────────────────────────────────────────────────の Continued on the front page (72) Yoshihiro Suemune 1 Kimitsu, Kimitsu, Nippon Steel Corporation Kimitsu Works (72) Inventor Kazumi Nishimura 20-1 Shintomi, Futtsu Nippon Steel Corporation 4K027 AA02 AA22 AB05 AB09 AB32 AC04 AC82 AE03 AE21

Claims (2)

[Claims] 1. Al: 3 to 15% by weight, Mg: 4 to 10%
% By weight, the balance having a hot-dip plating layer composed of Zn and unavoidable impurities, and Zn ₂ Mg, Zn
₁₁ Fused Zn-Al with excellent corrosion resistance of unpainted parts and painted end faces, characterized in that one or two single phases of Mg ₂ are precipitated with a particle size of 0.5 μm or more.
-Mg-plated steel. 2. Al: 3 to 15% by weight, Mg: 4 to 10%
Wt%, the balance being Zn and unavoidable impurities, in the hot-dip plating method, the content of Al is Al wt% = M
g% by weight × 1.83−3.33 Al and Mg
2. The method for producing a Zn-Al-Mg plated steel material according to claim 1, wherein the plating is performed in a plating bath having a higher Al side than the relationship described above.