JP2001262309A - HOT DIP Zn AND Mg-CONTAINING Al BASE COATED STEEL SHEET EXCELLENT IN RUST-FLOWING RESISTANCE ON EXPOSED PART OF STEEL BASE PART AND LOCAL CORROSION RESISTANCE ON COATED SURFACE, AND ITS PRODUCING METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip Zn and Mg-containing Al base coated steel sheet simultaneously improving a rust-flowing from the exposed part of a steel base part, local corrosion on coated surface and corrosion for long term on the coated surface. SOLUTION: This Zn and Mg-containing Al base coated steel sheet excellent in the rust-flowing resistance on the exposed part of the steel base part and the local part corrosion resistance on the coated surface, has the An and Mg- containing Al base coated layer formed by adding Zn and Mg on the fused Al base coated layer in non-solidified state at least on the one side surface of the steel sheet and forms the solid-solution at the ratio of 5-20 mass% Zn and 0.1-1.0 mass% Mg in the Al phase constituting the Zn and Mg-containing Al base coated layer. In the Al base coated layer, 3-15 mass% Si can be contained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Zn / Mg-containing Al-plated steel sheet having excellent resistance to red rust flow in an exposed portion of a steel substrate and excellent local corrosion resistance of a plated surface, and a method for producing the same. It is.

[0002]

[Prior art] "Al-based steel sheets" represented by Al-coated steel sheets and Al-Si alloy-coated steel sheets have higher corrosion resistance on the plated surface than Zn-plated steel sheets. It is used for applications such as exterior building materials.

However, since the surface of the plated layer of the Al-based plated steel sheet is covered with a strong oxide film, the sacrificial corrosion preventing action of the plated layer usually does not work. For this reason, although the corrosion resistance of the plated surface itself is excellent, red rust due to the corrosion of the steel substrate occurs in the exposed portion of the steel substrate such as the cut end surface. In particular, in a construction in which the end face of a plated steel sheet is superimposed on another plated steel sheet, this red rust may flow out onto the plated steel sheet and significantly impair the appearance.

[0004] A hot-dip Al-Zn alloy coated steel sheet is known as an Al-based coated steel sheet exhibiting a sacrificial anticorrosion action, and has already been put to practical use in building materials and the like. However, this type of alloy-plated steel sheet can prevent red rust flow from the end face, but tends to cause pitting corrosion on the plated surface due to unavoidable segregation of Zn in the plating layer, and has high local corrosion There is a disadvantage that.

[0005] On the other hand, Japanese Patent Application Laid-Open No.
While the plating layer of l-coated steel sheet is in an unsolidified state, Zn
A method is disclosed in which powder is sprayed at a specific flow rate to diffuse Zn into a plating layer. According to this method, a sacrificial corrosion-proof action is imparted to the plating layer of the hot-dip Al-based plated steel sheet, and the problem of local corrosion is also significantly reduced as compared with the above-mentioned alloy-coated steel sheet. However, when the present inventors conducted additional tests in detail, it was found that the sacrificial corrosion prevention effect was sometimes insufficient under some conditions, and it was not always possible to completely prevent the flow of red rust from the end face. Furthermore, local corrosion of the plating surface cannot always be completely prevented.

Accordingly, the present inventors have studied to solve the above-mentioned problem in the invention of Japanese Patent Application Laid-Open No. Hei 7-224367. As a result, it has been found that there is a particularly suitable range for the configuration of the plating layer and the Zn powder spraying conditions, and this was proposed in Japanese Patent Application No. 11-033461.

[Problems to be solved by the invention]

According to the invention proposed in Japanese Patent Application No. Hei 11-033461, the means for stably imparting "sacrificial corrosion protection" and "prevention of localized corrosion of the plated surface" to a hot-dip Al-coated steel sheet has been almost established. . However, the long-term corrosion resistance of the plated surface, which is an advantage of hot-dip
There is a tendency for the steel to deteriorate compared to the system-plated steel sheet, and the degree of the deterioration increases as the amount of Zn added increases. This deterioration of long-term corrosion resistance is caused by the fact that the corrosion products generated by the corrosion of Zn do not function as a rust preventive film, that is, the corrosion products of Zn easily peel off from the surface without protecting the plating layer. It is considered something. The corrosion products of Zn cause "white rust" and impair the appearance of the plated surface. Particularly in a salt-damaged environment such as a coastal area, the corrosion rate is high, which is likely to cause a problem.

The present invention stably imparts "sacrificial corrosion protection performance" and "local corrosion prevention performance of a plated surface" to a hot-dip Al-based steel plate and greatly reduces deterioration of "long-term corrosion resistance of the plated surface". That is, an object of the present invention is to provide an Al-based plated steel sheet having even better overall corrosion resistance.

[0009]

Means for Solving the Problems As a result of detailed research, the inventors have found that the addition of Zn to the hot-dip Al-based plating layer and the addition of an appropriate amount of Mg reduce the deterioration of “long-term corrosion resistance of the plated surface”. We have found that it can be greatly reduced.

That is, in order to achieve the above object, the invention of claim 1 is directed to a hot-dip Al-based plating layer in an unsolidified state.
The steel sheet has a Zn / Mg-containing Al-based plating layer formed by adding Zn and Mg on at least one surface of the steel sheet.
In the Al phase constituting the Al-based plating layer, Zn is 5 to 20% by mass,
Also characterized in that Mg is dissolved in a solid solution at a ratio of 0.1 to 1.0% by mass. Zn-Mg-containing Al-based coated steel sheet with excellent red rust flow resistance at the exposed portion of the steel substrate and localized corrosion resistance of the plated surface. It is.

[0011] The surface structure of the hot-dip Al-based plated steel sheet is usually from the bottom, "steel base material" → "Fe-Al-based alloy layer" → "
Plating layer ". Is produced by the reaction between Fe in the steel sheet substrate (base plate) and Al in the plating bath. The plating layer is mainly composed of a plating metal, and exhibits a metal structure of a single phase of an Al phase or a multiple phase composed of an Al phase and another phase depending on the composition of a plating bath. In the invention of claim 1, this Al
The amount of Zn dissolved in the phase is 5 to 20% by mass with respect to the total amount of elements in the Al phase, and the amount of Mg dissolved in the same phase is the total amount of elements in the Al phase. Means 0.1 to 1.0% by mass with respect to the amount of Further, since the plated steel sheet of claim 1 is "excellent in local corrosion resistance", it is necessary to ensure that Zn or Mg concentrated portions (undissolved phase, segregation, etc.) do not occur in the Al phase. means. Examples of “excellent in local corrosion resistance” include those in which local corrosion is not recognized by a salt spray test described later.

According to a second aspect of the present invention, in the first aspect of the present invention, the hot-dip Al-based plating layer includes a molten aluminum containing 3 to 15% by mass of Si.
This is a point that is determined to be adhered in the l-system plating bath.

According to a third aspect of the present invention, there is provided a method for producing an Al-based plated steel sheet according to the first or second aspect, wherein the steel sheet is immersed in a hot-dip Al-based plating bath, pulled up, and after adjusting the coating weight, the plating layer is formed. While Zn is in an unsolidified state, the Zn-Mg alloy powder is sprayed onto at least one surface of the steel sheet together with a carrier gas.

According to a fourth aspect of the present invention, there is provided the method of the third aspect, wherein the powder is sprayed on the unsolidified plating layer within a temperature range of + 10 ° C. to + 60 ° C. with respect to the solidification starting temperature of the plating layer. It was done. Here, the solidification start temperature of the plating layer is the solidification start temperature in the case where Zn and Mg are not added, that is, the solidification start temperature in the case where the plated metal in an unsolidified state before the addition of Zn and Mg solidifies with the same composition. Say.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a sacrificial corrosion preventing action is produced by dissolving Zn in an Al phase constituting an Al-based plating layer. Further, by dissolving Mg in the same phase, deterioration of the long-term corrosion resistance of the plated surface is suppressed. that time,
Prevent local corrosion of plating surface by dissolving Zn and Mg in the same phase and dispersing them homogeneously, that is, by including Zn and Mg so as not to create concentrated portions of Zn or Mg (segregation etc.) I do.

The Zn / Mg-containing Al-based plated steel sheet of the present invention has a Zn / Mg-containing Al-based plated layer formed by adding Zn and Mg to an unsolidified molten Al-based plated layer. It is necessary. As a method of adding Zn and Mg, for example, a method of spraying a Zn—Mg alloy powder together with a carrier gas as described later is exemplified. By directly adding Zn and Mg to the unsolidified plating layer, the plating layer solidifies significantly faster than usual. Even when the solidification rate is high and Zn or Mg is enriched, the Zn and Mg can be frozen in a homogeneously dispersed state in the Al phase.

In an Al-based plated steel sheet, when Zn is dissolved in the Al phase constituting the plating layer at a concentration of 5% by mass or more, the flow of red rust from the exposed portion of the steel base can be stably prevented. A sacrificial anticorrosion action can be imparted. However, Al
If the Zn concentration in the phase exceeds 20% by mass, the long-term corrosion resistance deteriorates significantly. Also, when such a large amount of Zn is added to the unsolidified plating layer, the plating layer is rapidly cooled due to the large heat removal from the Zn, and the Zn may be frozen in a state where the Zn cannot be sufficiently diffused into the Al phase. There is. In this case, there is a tendency for local corrosion. Therefore, in the present invention, Zn dissolved in the Al phase
Is defined as 5 to 20% by mass.

During the course of various experiments, the inventors made a solid solution of Mg together with Zn in the Al phase constituting the Al-based plating layer, thereby significantly reducing the tendency of long-term corrosion resistance deterioration due to Zn addition. I found that it can be reduced. There are many unclear points about the mechanism by which this effect occurs, but Mg
It is presumed that the addition of Zn causes the corrosion products of Zn to form a strong film having an antirust effect.

As a result of detailed examination, when Mg was dissolved at a concentration of 0.1% by mass or more in the Al phase, a clear effect of preventing deterioration of long-term corrosion resistance was recognized. However, if the Mg concentration exceeds 1.0% by mass, local corrosion occurs. This is Al
This is presumed to be due to the formation of an alloy phase of Zn and Mg (which is considered to be an intermetallic compound) in the phase. Therefore, in the present invention, the concentration of Mg which forms a solid solution in the Al phase is defined as 0.1 to 1.0% by mass.

Zn and Mg in the Al phase must be solid-dissolved and uniformly dispersed without forming a concentrated portion. Such a dispersion form is achieved by “rapid solidification” when Zn and Mg are added to a hot-dip Al-based plating layer in an unsolidified state. In a dispersion method in which a concentrated portion of Zn or Mg is present, corrosion tends to proceed preferentially in the concentrated portion, and so-called localized corrosion is exhibited. When localized corrosion occurs, not only does the appearance deteriorate, but also the corrosion rate of the plating of the parts where Zn or Mg is concentrated increases, the plating layer (whole) disappears early, and red rust occurs from the steel sheet surface Become like

The Zn / Mg-containing Al-based plating layer as defined above exhibits a sufficient sacrificial corrosion prevention action when formed on at least one surface of the steel sheet. That is, one surface of the steel sheet can be the Zn-Mg-containing Al-based plating layer, and the other surface can be a normal hot-dip Al-based plating layer.

As the "plating bath" for producing the Zn / Mg-containing Al-plated steel sheet of the present invention, an alloy plating bath of Al and another metal can be used in addition to a pure Al plating bath. When using the latter alloy plating bath, the bath composition is adjusted so that the formed plating layer contains at least 50% by volume or more of the Al phase. As an example of alloy plating bath, Si: 3 ~ 15
A hot-dip Al-based alloy plating bath containing 1% by mass.
At this time, it is desirable that the balance be substantially Al. The content of Fe up to approximately 2% by mass as an impurity is allowed. Si
Advantages of using an addition bath include: i) the melting point of the plating bath is lowered, so that the durability of a pot for putting molten metal is good; and ii) the growth of the Fe-Al alloy layer can be suppressed. Points.

Such a Zn / Mg-containing Al-based steel sheet is
Immerse the steel sheet in a hot-dip Al plating bath and pull it up.After adjusting the coating weight, Zn-Mg
It can be manufactured by a method of spraying the alloy powder together with the carrier gas onto at least one surface of the steel sheet. Although a mixed powder composed of Zn powder and Mg powder can be used instead of the Zn-Mg alloy powder, it is preferable to use a Zn-Mg alloy powder alloyed at a predetermined ratio in advance from the viewpoint of ensuring uniformity. .
When the powder is sprayed, the spray amount is controlled so that Zn and Mg are dissolved at a predetermined concentration in the Al phase.

The sprayed Zn-Mg alloy powder is reacted with the unsolidified plating metal, and Zn and Mg are contained in the Al phase of the plating layer.
In order to uniformly disperse the powder, the timing of spraying the powder, that is, the temperature of the unsolidified plating layer at the time when the powder is sprayed is very important. Considering the addition amount of the powder required in the present invention, by spraying the powder onto the unsolidified plating layer in a temperature range of + 10 ° C to + 60 ° C with respect to the solidification starting temperature of the plating layer, Zn and Mg It has been confirmed that homogeneous dispersion is very easy to realize.

If the temperature of the plating layer at the time of spraying the powder is lower than “solidification starting temperature + 10 ° C.”, solidification starts rapidly due to heat removal from the adhered powder, and Zn or Mg does not sufficiently diffuse into the plating layer. It is very likely that coagulation will be completed. In this case, Zn or Mg forms a concentrated portion, and the plated surface exhibits local corrosion, which is not preferable.

When the temperature of the plating layer at the time of spraying the powder is in the range of “solidification starting temperature + 10 ° C. to + 60 ° C.”, unless the particle diameter of the powder is extremely large, for example, more than twice the thickness of the plating layer. , The added Zn and Mg are Al
A time margin at a high temperature sufficient to sufficiently diffuse into the phase and homogenize is secured, and a high solidification rate that does not cause segregation of Zn or Mg is achieved. In this case, Zn and
Since Mg is frozen in a state of being solid-dissolved in the Al phase and homogeneously dispersed, the plated surface does not exhibit local corrosion, and a preferable one is obtained.

However, at a plating layer temperature exceeding “solidification start temperature + 60 ° C.”, the effect of rapid solidification due to heat removal from the powder is not sufficiently exhibited, and a solidification form similar to that in a normal hot-dip coated steel sheet appears. It will be easier. That is, Zn and Mg diffused in the unsolidified plating layer are liable to cause segregation in the Al phase when solidified, and in this case, the plated surface exhibits local corrosion, which is not preferable. Therefore, in the present invention, it is desirable to spray the powder on the unsolidified plating layer within a temperature range of + 10 ° C. to + 60 ° C. with respect to the solidification starting temperature of the plating layer. In consideration of the influence of fluctuations in operating conditions and the like, it is more preferable to spray the powder onto the unsolidified plating layer in a temperature range of + 10 ° C to + 50 ° C with respect to the solidification starting temperature of the plating layer.

For example, when a plating bath containing 9.5% by mass of Si is used as the hot-dip Al plating bath, since the solidification start temperature of the plating layer is 590 ° C., the temperature of the plating layer at the time of spraying the powder is 600 to 650 ° C. , Preferably in the range of 600 to 640 ° C.

The powder used is desirably adjusted to have a particle diameter of 5 μm or more and twice or less the thickness of the plating layer. If the particle diameter is less than 5 μm, the carrier gas flows along the steel sheet immediately before reaching the plating layer,
The speed of collision with the plating layer is reduced, and it becomes difficult to break the oxide film on the plating layer surface. In this case, the powder particles only adhere to the plating layer and do not diffuse into the plating layer. On the other hand, large particles whose particle diameter exceeds twice the thickness of the plating layer are in a state where about half or more of the particles protrude from the plating layer at the time of collision with the plating layer. It becomes difficult to completely diffuse into the phase.

When Zn.Mg is added to only one side of a steel sheet by using the above-described powder spraying method, it is desirable to spray only gas from a dummy nozzle to the other side of the steel sheet. By equalizing the blowing force applied to both sides of the steel plate, the posture of the steel plate during the passing is stabilized, and the work efficiency and quality can be improved.

[0031]

EXAMPLE A powder spraying apparatus was installed in an existing continuous hot-dip galvanizing line to produce a Zn / Mg-containing Al-based plated steel sheet.
An Al-killed low carbon steel sheet was used as a plating base sheet, and a plating bath composition was Al-9.5% by mass Si-1.8% by mass Fe. Gas wiping nozzle (YG nozzle) for steel sheet that has exited the plating bath
The amount of plating was adjusted to a thickness of 40 μm per one side by using, and a Zn-Mg alloy powder was sprayed together with a carrier gas (N ₂ ) onto only one side of the steel sheet from a powder spray nozzle provided immediately above the gas wiping nozzle. It has been confirmed that the solidification start temperature of the plating layer formed in this bath is 590 ° C., and the powder spray nozzle was set so that the powder was sprayed to a position where the plating layer was in an unsolidified state at 640 ° C.
In addition, only gas (N ₂ ) is sprayed from a dummy nozzle provided at a position opposite to the powder spray nozzle on the opposite surface of the steel plate so that the blowing force applied to both surfaces of the steel plate is equalized, and the posture of the steel plate is stabilized. It has become. Powder has a particle size of 15 ~
The flow rate of the carrier gas was adjusted to 70 μm (average 42 μm), and the flow rate of the carrier gas was adjusted to 30 m / s at the steel sheet position. By changing the composition of the used Zn-Mg alloy powder, and the amount of powder sprayed, Zn in the Al phase constituting the plating layer
Various hot-dip Al-coated steel sheets having a concentration of 0 to 40% by mass and a Mg concentration of 0 to 2.0% by mass were obtained. The temperature of the plating layer was measured with a radiation thermometer.

With respect to the specimens collected from each plated steel sheet,
Red rust resistance of exposed steel substrate, long-term corrosion resistance of plated surface,
And the local corrosion resistance of the plated surface was investigated. The red rust flow resistance of the exposed portion of the steel base was measured by laminating the cut end face of the test piece so that it overlapped the plated surface of another test piece, and exposing it to the coastal industrial zone in Sakai City, Osaka Prefecture for one month. The evaluation was based on the occurrence of red rust flow. The long-term corrosion resistance of the plated surface is determined by performing a salt spray test in accordance with JIS Z 2371 on the plated surface that has been sprayed with powder (or any surface that has not been sprayed with powder). evaluated. The local corrosion resistance of the plated surface was evaluated by observing the sample surface after the above salt spray test with a loupe and by determining whether or not local corrosion such as pitting corrosion occurred. Table 1 shows the results.

[0033]

[Table 1]

In Table 1, in Nos. 1 to 6 and 9 to 14 in which the Zn concentration in the Al phase constituting the plating layer is 5 to 20% by mass, generation of red rust is observed on the cut end surface. No red rust flow occurred, and the sacrificial anticorrosion action by Zn addition was sufficiently exhibited. However, in No. 15 where the Zn concentration in the Al phase was as high as 28% by mass, localized corrosion occurred on the plated surface. This is because Zn was concentrated in a state where it was not sufficiently diffused due to an excessive amount of Zn added, and a Zn-enriched portion was formed.

In terms of long-term corrosion resistance, the original long-term corrosion resistance level obtained when Zn was not added was 780 hours as the time of red rust generation by salt spray (No. 7). As the Zn concentration in the Al phase increases, the long-term corrosion resistance deteriorates (Nos. 7, 9, 1).
In contrast, the Zn concentration was 17% by mass and deteriorated in 670 hours (No. 12). The effect of the addition of Mg on the red rust generation time due to salt spray for Nos. 5, 6, 12, and 13 where the Zn concentration in the Al phase is 17 mass% is common.
0% (no addition) of No. 12 was 670 hours, and 0.02% by mass of No. 13
But there is not much improvement at 690 hours. However, in the case of No. 5 with 0.3% by mass of Mg and No. 6 with 0.7% by mass of Mg, respectively, the time was 760 hours, and it can be seen that a remarkable improvement was observed. The Zn concentration in the Al phase was relatively low at 7% by mass.
Similar effects by adding Mg are observed in Examples 9 and 10. As described above, it was demonstrated that the long-term corrosion resistance of the plated surface deteriorated by the addition of Zn was significantly improved when Mg was added in an amount of 0.1% by mass or more.

On the other hand, in Nos. 11 and 14, in which the Mg concentration in the Al phase was as high as 2.5% by mass, although the effect of improving long-term corrosion resistance was large, local corrosion occurred on the plated surface. This is Zn and Mg
This is probably due to the formation of an alloy phase.

Invention examples in which the Zn concentration in the Al layer is 5 to 20% by mass and the Mg concentration is 0.1 to 1.0% by mass (Nos. 1 to 6)
No red rust flow from the cut end surface, the long-term corrosion resistance deterioration of the plated surface was remarkably reduced, and no localized corrosion was observed on the plated surface.

[0038]

According to the present invention, the Zn-Mg-containing Al-based plated steel sheet has a problem of red rust flow from the exposed portion of the steel substrate, a problem of local corrosion of the plated surface, and a long term of the plated surface caused by Zn addition. The problem of deterioration of corrosion resistance was solved at the same time. Among these, the problem was already solved in the Zn-containing Al-based plated steel sheet proposed earlier, but the problem was not solved. The present invention has made it possible to provide an Al-based plated steel sheet which is more excellent in overall corrosion resistance by solving these problems at the same time.

Claims (4)

[Claims] The present invention relates to an unsolidified hot-dip Al-based plating layer containing Zn.
And a Zn-Mg-containing Al-based plating layer formed by adding Mg on at least one surface of the steel sheet, wherein the Zn-Mg-containing Al
In the Al phase that constitutes the system-based plating layer, Zn is solid-dissolved at a ratio of 5 to 20% by mass and Mg is a solid solution at a ratio of 0.1 to 1.0% by mass. Excellent local corrosion
Al-plated steel sheet containing Zn and Mg. 2. The hot-dip Al-based plating layer has a Si content of 3 to 15% by mass.
The Zn-Mg-containing Al-plated steel sheet according to claim 1, which is adhered in a hot-dip Al-containing plating bath containing. 3. A steel sheet is immersed in a hot-dip Al-based plating bath, pulled up, and after adjusting the coating weight, while the plating layer is in an unsolidified state, Zn-Mg alloy powder is added together with a carrier gas to at least one surface of the steel sheet. 3. The method according to claim 1, wherein
The method for producing a Zn / Mg-containing Al-based plated steel sheet according to item 1. 4. A temperature of + 10 ° C. to the solidification start temperature of the plating layer.
The production method according to claim 3, wherein the powder is sprayed on the unsolidified plating layer within a temperature range of + 60 ° C.