JP2004018999A - METHOD FOR MANUFACTURING HOT-DIP Zn-Al-Mg ALLOY PLATED STEEL SHEET - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-dip Zn-Al-Mg alloy plated steel sheet that does not cause a polka-dot pattern and has excellent surface appearance. <P>SOLUTION: This manufacturing method is characterized by cooling the steel sheet coming out from the hot dipping bath after being immersed in the hot dipping bath, when the plated film is solidified, in such a state in which solid particles with sizes of 50 μm or larger capable of sticking to the surface of a molten plated film do not substantially exist, in the temperature range of 350-360°C or lower but the solidification temperature or higher, and by solidifying the hot-dip plated film by cooling. <P>COPYRIGHT: (C)2004,JPO

Description

【００４３】
【表２】

【００４４】
【発明の効果】
以上説明したように、本発明によると、溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板を製造する場合に遭遇する水玉模様発生を抑制しためっき鋼板を作成することが可能であり、また、この安価な手段によれば、溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板の広範囲での普及に大きく寄与できるのであって、本発明は工業的に価値の高い発明である。
【図面の簡単な説明】
【図１】Ｚｎ−Ｍｇ　二元系状態図である。
【図２】本発明に係る方法を実施するための連続溶融めっき設備の概要図である。
【図３】鉄粉散布温度と水玉模様の発生の関係を示した図である。
【図４】鉄粉径と水玉模様との関係を示した図である。
【符号の説明】
１：めっき浴
２：鋼帯
３：ワイビングノズル
４：トップロール
５：冷却設備
６：配管[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a hot-dip Zn-Al-Mg alloy-plated steel sheet, and particularly to a method for manufacturing a hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent surface appearance.
[0002]
[Prior art]
2. Description of the Related Art In recent years, hot-dip galvanized steel sheets that are inexpensive and have excellent corrosion resistance have been widely used in industrial fields such as automobiles, home appliances, civil engineering and construction materials. The hot-dip galvanized steel sheet may be used after being processed and then coated, or may be used without being processed as it is (hereinafter simply referred to as “uncoated use”). For this reason, it is important for the hot-dip galvanized steel sheet not only to have corrosion resistance but also to have a long-term excellent appearance when used without coating in the atmosphere.
[0003]
By the way, various methods for improving the corrosion resistance of hot-dip galvanized steel sheet have also been studied. For example, in US Pat. No. 3,505,043, a percentage by mass (hereinafter, a% expression representing a chemical composition means mass%) A method for producing a hot-dip Zn—Al—Mg-plated steel sheet having excellent corrosion resistance using a hot-dip plating bath made of Al: 3 to 17%, Mg: 1 to 5%, and the balance being Zn has been proposed.
[0004]
More recently, for example, Japanese Patent Application Laid-Open No. Hei 10-226865 discloses a ternary eutectic containing Al / Zn / Zn ₂ Mg containing 4.0 to 10% of Al and 1.0 to 4.0% of Mg. There has been proposed a hot-dip Zn-Al-Mg alloy-plated steel sheet comprising a plated film having a metal structure in which primary Al phases are mixed in a matrix body.
[0005]
However, in the hot-dip Zn-Al-Mg alloy-plated steel sheet, a spot-like pattern appears on the surface layer of the steel sheet, and this pattern may turn black (hereinafter referred to as a "polka dot pattern") when left for a while. This polka dot pattern is a phenomenon peculiar to the hot-dip Zn-Al-Mg alloy-plated steel sheet caused by the Mg ₂ Zn ₁₁ phase in the plating film as described in Japanese Patent Application Laid-Open No. 10-226865.
[0006]
As a method of improving a polka dot pattern, Japanese Patent Laid-Open No. Hei 10-226865 proposes a method in which a cooling rate after plating is set to a predetermined value or more. However, the present inventors have studied and found that a polka dot pattern may still occur even when the cooling rate is increased for the reasons described below.
[0007]
Further, according to the publication, a method of improving a polka dot pattern by setting a plating bath temperature to a predetermined temperature or higher without significantly increasing a cooling temperature is also disclosed. However, depending on the operating conditions such as the plating equipment (see FIG. 1 described below) and the line speed, the plating bath temperature should be as low as possible and the plating cooling rate should be as low as possible in order to complete the solidification of the plating film by the time the top roll is reached. In some cases, this method must be used after being enlarged to some extent, and this method cannot always be adopted.
[0008]
Furthermore, Japanese Patent Application Laid-Open No. 2001-107212 discloses that the addition of Ti and B improves the formation of a polka dot pattern. If such a means is employed, the concentration of Ti and B in the bath can be controlled. Is required, and the complexity increases.
[0009]
As described above, hot-dip galvanized steel sheets are widely used today, but in the case of hot-dip Zn-Al-Mg alloy-plated steel sheets, which are considered to be particularly excellent in corrosion resistance, the above-mentioned problems are encountered. Therefore, the implementation is not necessarily widely spread, and further improvement is required.
[0010]
[Problems to be solved by the invention]
Here, an object of the present invention is to solve the problem of the prior art that a polka dot pattern is generated in a hot-dip Zn-Al-Mg alloy-plated steel sheet, and to cover a wider range of a hot-dip Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance. The goal is to develop technologies that can be put to practical use.
[0011]
[Means for Solving the Problems]
The present inventors have observed in detail a Zn-Al-Mg plated steel sheet having a polka dot pattern when air-cooled after hot-dip plating, and solid particles considered to be iron rust are observed near the center of the polka dot pattern. There was a case.
[0012]
Therefore, the present inventors considered that the attachment of the solid particles might be related to the occurrence of a polka dot pattern and conducted an investigation. As a result, it has been found that when solid particles adhere in a temperature range from a polka dot generation temperature to a solidification temperature in a cooling stage after hot-dip plating, a polka dot pattern is generated.
[0013]
Although the polka dot generation mechanism at this time is not necessarily clear, the present inventors assumed the following hypothesis based on the above findings.
That is, the polka dot pattern is attributed to the Zn ₁₁ Mg ₂ phase as described above. Here, the Zn ₁₁ Mg ₂ phase is considered to be a structure precipitated by supercooling of the plating film from the Zn—Mg binary phase diagram of FIG. 1 (note that the numerical value of the temperature described in FIG. In the case of a Zn-Al-Mg ternary system containing Al and containing Al, the temperature differs from the temperature shown in Fig. 1.) Solid particles near the solidification temperature (immediately before solidification) Is considered to cause local overcooling of the plating film, thereby causing a Zn ₁₁ Mg ₂ phase to be precipitated, thereby causing a polka dot pattern.
[0014]
The present invention has been completed by conducting a series of experiments based on the above findings and hypotheses and confirming the effects thereof. The gist of the present invention is as follows.
(1) In a method of manufacturing a hot-dip Zn-Al-Mg alloy-plated steel sheet in which a steel sheet is immersed in a hot-dip plating bath and then pulled up to cool the plated film, The molten Zn-Al is characterized in that cooling in a temperature range from a polka dot initiation temperature of a plating film to a solidification temperature of the plating film is performed so that solid particles having a particle size of 50 μm or more do not adhere to the surface of the hot-dip coated steel sheet. -A method for producing a Mg alloy-plated steel sheet.
[0015]
(2) In a method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet in which a steel sheet is immersed in a hot-dip plating bath and then pulled up to cool the plating film, A molten Zn-Al, characterized in that cooling at least in a temperature range from 360 ° C to 320 ° C at a temperature of the plating film is performed so that solid particles having a particle size of 50 µm or more do not adhere to the surface of the hot-dip coated steel sheet. -A method for producing a Mg alloy-plated steel sheet.
[0016]
(3) The chemical composition of the above (1) or (2), wherein the chemical composition of the plating film is Mg: 1.0% by mass to 10% by mass, Al: 1.0% by mass to 20% by mass, and Zn: the balance. A method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet according to the above.
[0017]
(4) The method for producing a hot-dip Zn-Al-Mg alloy plated steel sheet according to any one of the above (1) to (3), wherein the cooling of the plating film is performed by blowing gas.
The "temperature of the plating film" mentioned above is considered to be substantially equal to the plate temperature in practice, so that there is no problem in controlling the temperature in the production line even at the plate temperature.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, an embodiment of the manufacturing method according to the present invention will be described more specifically, and the effects of the embodiment will be described.
[0019]
FIG. 2 is a schematic schematic explanatory view of a plating facility for performing the method according to the present invention.
In the figure, a reduced steel sheet is immersed in a hot-dip plating bath 1 previously adjusted to a predetermined composition via a snout, passed through the bath via a sink roll, and then pulled up from the bath 1. A gas wiping nozzle 3 is provided above the plating bath 1 for adjusting the amount of plating applied. When the plated steel sheet that has exited the hot-dip plating bath, that is, the hot-dip coated steel sheet, passes through the cooling chamber 5, the plating film is cooled by the cooling gas supplied from the pipe 6.
[0020]
The cooled hot-dip coated steel sheet is taken out of the system via the top roll 4.
According to the present invention, means for preventing solid particles such as iron rust and other foreign matter in the piping from adhering to the surface of the plating film still in a molten state during cooling in the cooling chamber 5 which is preferably maintained at a positive pressure. Take the cooling. In the present invention, it is important that solid particles such as dust and dust do not adhere within a predetermined temperature range in the cooling and solidifying process of the plating film.
[0021]
Here, the temperature range in which such solid particles are cooled without substantially attaching them is a range from the polka dot forming temperature to the solidification temperature.
Here, the solidification temperature varies depending on the composition of Zn—Al—Mg in the plating. For example, in the case of Zn—Al—Mg plating containing 3 to 6% of Al and 2 to 4% of Mg, the solidification is The temperature is on the order of 320-330 ° C.
[0022]
On the other hand, the polka dot generation start temperature changes depending on, for example, the solid particle diameter and plate thickness to be attached. As will be described later, as the plate thickness is smaller and the solid particle diameter is larger, the polka dot generation start temperature is higher, that is, a polka dot pattern is generated even if it is attached at a relatively high temperature. According to the aforementioned hypothesis, the larger the diameter of the solid particles, the greater the heat removal effect. This is considered to be because if the thickness is small, the heat capacity of the entire steel sheet is small, so that the effect of the heat removal effect due to the attachment of the solid particles is relatively large and a local supercooling state is likely to occur.
[0023]
Specifically, as a result of the study of the present inventors, in the case of iron powder having a particle diameter of less than 50 μm, a polka dot pattern is generated even if the powder adheres to the plating film at 335 ° C., which is assumed to be just before the solidification temperature of the plating film. Did not. On the other hand, in the case of iron powder having a particle diameter of 1 mm, the polka dot initiation temperature was about 350 to 360 ° C., depending on the plate thickness. That is, when large solid particles adhere from the temperature to the solidification temperature (320 to 330 ° C.), a polka dot pattern is generated. Although various conditions were examined, it was rare that the polka dot initiation temperature exceeded 360 ° C. Therefore, in actual production, it is considered preferable that solid particles having a particle diameter of 50 μm or more are not adhered in a temperature region where the plating surface temperature is from 360 ° C. to 320 ° C.
[0024]
According to the present invention, solid particles are substantially absent, preferably gas cooling. The meaning of this is that there is no restriction on cooling as long as polka dots are not generated.Specifically, since the size of particles attached to polka dots is also involved, particles from the cooling gas It is preferable that particles having a diameter of 100 μm or more, preferably particles having a diameter of 50 μm or more, are substantially removed.
[0025]
As a method of cooling in the actual plating line, it is considered that gas such as compressed air or nitrogen is often blown through piping in the factory.In this case, however, dust, dust, rust and even ambient It is preferable to remove incoming sea salt particles and the like.
[0026]
As described above (or as shown in the examples), since the size of particles affects the generation of polka dots, a filter capable of removing particles having a particle size of 50 μm or more is provided in the pipe. It is desirable to take such measures. When a filter is used, a filter cloth filter is preferable from the viewpoint of suppressing the pressure loss of air.
[0027]
The cooling rate at this time is not particularly limited in terms of the polka dot pattern, and varies depending on the plate thickness and the like, but is 2 to 20 ° C./s under normal gas cooling conditions. When the hardness of the film becomes a problem from the viewpoint of workability, 2 to 10 ° C./s is preferable.
[0028]
Next, in a preferred embodiment of the present invention, the chemical composition of the plating film is defined, and the reason will be described more specifically. In this specification, “%” indicating a chemical composition is “% by mass” unless otherwise specified.
[0029]
Chemical composition of plating film;
Since the Al and Mg contents of the plating film are almost the same as the respective contents in the plating bath, the chemical composition of the plating bath and the plating film will be described simultaneously below.
[0030]
Al: Originally, the amount of Mg dissolved in molten Zn is limited to 0.1%. However, if an appropriate amount of Al is contained in molten Zn, Al forms an oxide film on the molten Zn surface and prevents oxidation of Mg. Thus, the amount of Mg dissolved in the molten Zn can be increased.
[0031]
Mg is contained in the Zn plating bath at 1.0% or more, and in order to enhance the corrosion resistance of the plating film, Al is contained in the plating bath at 1.0% or more. Since the Al content of the plating film is substantially the same as the Al concentration of the plating bath, the Al content of the plating film is set to 1.0% or more. Further, even if more than 20% of Al is added, the effect of dissolving Mg cannot be enhanced, so the upper limit is 20%. Preferably it is 15% or less.
[0032]
Mg: Mg has an effect of improving the corrosion resistance of the plating film, and is contained in an amount of 1.0% or more to obtain the effect. Desirably, it is 2.0% or more. If Mg is excessively contained in the plating bath, a large amount of dross (oxide) is generated on the plating bath surface, which impairs the operability of hot-dip plating. If the Mg content exceeds 10%, it hardly dissolves in the bath, so the upper limit of the Mg content in the plating bath is 10% or less. Therefore, the Mg content of the plating film is set to 10% or less. Desirably, it is 8.0% or less.
[0033]
In the study of the present inventors, in the region of Al: about 3 to 11% and Mg of about 2 to 5%, when the same solid particles are adhered at the same temperature, the composition ratio of Al and Mg is low. As the polka dot pattern is more likely to be generated or the diameter of the generated polka dot pattern becomes larger and more conspicuous, it has been found that it is important to suppress this. Therefore, the method of the present invention is particularly effective even in a Zn-Al-Mg alloy plating in a region where the composition ratio of Al and Mg is relatively small (for example, about 3 to 6% of Al and about 2 to 3% of Mg). It is considered a way.
[0034]
In the preferred embodiment of the present invention, the balance of the hot-dip coating is Zn, but it is within the scope of the present invention to appropriately mix other elements as long as the effects of the present invention are exhibited.
[0035]
Next, the operation and effect of the present invention will be described more specifically with reference to examples.
[0036]
【Example】
(Example 1)
When a steel sheet having the shape and dimensions shown in Table 1 is immersed in a plating bath having the composition shown in Table 2, pulled up, and allowed to cool, the temperature of the steel sheet surface, that is, the temperature of the plating film is measured with a radiation thermometer. When the temperature reached a predetermined temperature, iron powder having a particle diameter of 1 mm (referred to as the major axis of the particle; the same applies hereinafter) was sprayed on the surface of the steel sheet.
[0037]
The polka dots on the plated surface of the sample thus prepared were visually evaluated. The criterion for the presence or absence of the polka dot pattern is often acceptable if it is within 5 mm in diameter.
[0038]
：: No polka dot pattern is generated. △: There is a case where no polka dot pattern is generated. X: A polka dot pattern is generated.
[0039]
FIG. 3 shows the relationship between the iron powder spraying temperature and the polka dot pattern. In addition, a sample in which iron powder was not sprayed was also prepared, but no polka dot pattern was generated. From these results, it was confirmed that the occurrence of a polka dot pattern is related to the attachment of solid particles, and that the temperature at which the particles adhere is affected.
[0040]
(Example 2)
In this example, the size of the solid particles was investigated by the following method.
When the steel sheet shown in Table 1 was immersed in the plating bath shown in Table 2, pulled up and allowed to cool, the temperature of the steel sheet surface was measured with a radiation thermometer. When the temperature reached 335 ° C., iron powder having a predetermined particle size was removed. Sprayed on steel plate surface. The samples thus prepared were evaluated for the presence or absence of polka dots based on the above-mentioned 〜 to ×.
[0041]
The result is shown in FIG. From this figure, it was found that when the particle size of the solid particles was small, a polka dot pattern was hardly generated.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
【The invention's effect】
As described above, according to the present invention, it is possible to produce a plated steel sheet that suppresses the occurrence of a polka dot pattern encountered when producing a hot-dip Zn-Al-Mg alloy-plated steel sheet, and this inexpensive means. According to this, the present invention can greatly contribute to widespread use of hot-dip Zn-Al-Mg alloy plated steel sheets, and the present invention is an industrially valuable invention.
[Brief description of the drawings]
FIG. 1 is a Zn—Mg binary phase diagram.
FIG. 2 is a schematic diagram of a continuous hot-dip plating facility for performing a method according to the present invention.
FIG. 3 is a diagram showing the relationship between iron powder spraying temperature and the occurrence of polka dots.
FIG. 4 is a diagram showing the relationship between iron powder diameter and polka dots.
[Explanation of symbols]
1: Plating bath 2: Steel strip 3: Wiving nozzle 4: Top roll 5: Cooling equipment 6: Piping

Claims (4)

In a method for manufacturing a hot-dip Zn-Al-Mg alloy-coated steel sheet in which a steel sheet is immersed in a hot-dip plating bath and then pulled up to cool the plated film, when cooling the plated film pulled out of the hot-dip bath, A molten Zn-Al-Mg alloy, characterized in that cooling in a temperature range from a polka dot initiation temperature to a solidification temperature of a plating film is performed so that solid particles having a particle size of 50 µm or more do not adhere to the surface of the hot-dip coated steel sheet. Manufacturing method of plated steel sheet. In a method for manufacturing a hot-dip Zn-Al-Mg alloy-coated steel sheet in which a steel sheet is dipped in a hot-dip plating bath and then pulled up to cool the plating film, when the plating film pulled out of the hot-dip bath is cooled, A molten Zn—Al—Mg alloy, characterized in that cooling at least in the temperature range of 360 ° C. to 320 ° C. is performed so that solid particles having a particle size of 50 μm or more do not adhere to the surface of the hot-dip coated steel sheet. Manufacturing method of plated steel sheet. 3. The molten Zn— according to claim 1, wherein the chemical composition of the plating film is Mg: 1.0% by mass to 10% by mass, Al: 1.0% by mass to 20% by mass, and Zn: Δ balance. A method for producing an Al-Mg alloy plated steel sheet. The method for producing a hot-dip Zn-Al-Mg alloy-plated steel sheet according to any one of claims 1 to 3, wherein the cooling of the plating film is performed by blowing gas.

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