JPH09324252A - Production of zero spangled hot dip galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To refine spangles on the surface of plating and to improve the quality of the appearance of a steel sheet by suitably holding the amt. of residual iron powder on the surface of a steel sheet after cleaning and therefore actuating the iron powder as spangle forming nuclei at the time of plating. SOLUTION: As a plating matrix steel sheet, a steel sheet in which iron powder is adhered to the surface by >=15mg/m<2> is used. The amt. of the residual iron powder adhered to the surface of the steel sheet depends on the constitution of cleaning equipment, but, as for the regulation of the residual amt., the method of controlling the rotational speed of an alkali scrubber brush is preferably used, and without requiring the remarkable reconstruction of the equipment and while the oil content is securely removed, the iron content can be allowed to remain in some degree. Ordinarily, the rotational speed of the scrubber brush is managed so as to regulate the relative speed between the brush and the steel sheet to >=800m/min, but, by reducing the same to about <=600m/min, the amt. of the iron powder to be adhered is regulated to 15mg/m<2> . Furthermore, at the time of the plating, it is preferable that the content of Al in the bath is regulated to/to 0.3%, Pb to <=0.05% and Sb to <=0.02%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zero-spangle hot-dip zinc having excellent appearance quality by making the spangles on the surface of the plating layer fine during the production of hot-dip galvanized steel sheet by applying hot-dip zinc-based plating to the surface of the steel sheet. The present invention relates to a method for producing a system-plated steel sheet.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets are widely used in the fields of automobiles, home appliances, building materials and civil engineering, for the purpose of preventing corrosion of the steel sheets. By the way, as the hot-dip galvanized steel sheet, there is known a regular spangle material having a surface texture in which a flower pattern called "spangle" blooms on the surface of a plated layer due to crystal growth during cooling and solidification. Is a Pb having a spangle growth action (effect of increasing spangle size) in the plating bath.
It is manufactured by adding Sb. That is, when Pb or Sb is added to the hot-dip galvanizing bath, the melting point of the plating alloy is lowered, the time until it solidifies after adhering to the steel sheet becomes longer, and the growth of crystals in the plating layer is promoted.

This regular spangle material has a beautiful flower pattern on the surface and is frequently used by users who like its appearance. The spangle pattern is also useful in that it has a function of visually guaranteeing that zinc-based plating is applied.

On the other hand, in recent years, from the viewpoint of improving the aesthetic appearance, more and more various colors are demanded rather than metallic luster, and in order to meet such demands, the surface of hot-dip galvanized steel sheet may be coated and used. Since the number of coatings is increasing and the corrosion resistance is further improved by applying the coating, when used in an environment that is easily exposed to the atmosphere such as roofs and wall materials and is easily corroded, the coating will be applied from the viewpoint of improving the corrosion resistance. Is often applied.

However, when the regular spangle material described above is used as the coating base, a spangle pattern may be raised to the surface of the coating film, impairing the coating appearance.
Further, it has been pointed out that it is difficult to obtain good coating film adhesion. Therefore, in applications where such problems are disliked, a minimum spangle material in which the spangle is miniaturized or a zero spangle material in which the spangle is almost eliminated is preferred.

The above minimum spangle material and zero spangle material are refrigerants (water or an aqueous solution obtained by adding a chemical solution such as ammonium chloride to water to the surface of the plating layer, or a mixture of zinc powder and air before the plating layer solidifies. Etc.) to increase the solidification rate and suppress spangle growth.

As another method, there is known a method of reducing the amount of spangle growing elements such as Pb and Sb added to the plating bath to make the spangle finer. For example, in Japanese Patent Publication No. 36-20560, the amount of A1 in a hot dip galvanizing bath is 0.2% by weight or less, the amount of Pb is 0.05% or less, the amount of Sb is 0.02% or less, and the amount of Sn is 0.8%. The following zero spangle materials are disclosed.

Even if the manufacturing method of the minimum spangle material or zero spangle material as described above is adopted, if the spangle cannot be sufficiently miniaturized, the spangle will appear on the surface of the product and the appearance will be deteriorated. If a steel sheet with spangles exposed is used as a coating base, the coating may become non-uniform or non-painted portions may occur. Therefore, the spangles must be sufficiently miniaturized in order to avoid such problems.

When it is attempted to adjust the spangle size by spraying a refrigerant on the surface of the plating layer before solidification, the installation position of the quenching device for spraying the refrigerant (ie, the rapid solidification position) is
It must be properly set according to the line speed, the plate thickness, the penetration temperature into the plating bath, the coating adhesion amount, etc. If this position is inappropriate, the spangle cannot be made sufficiently fine. However, the conventional quenching device is of a type in which the cooling section is relatively short and quenching is performed in spots, and the installation position is adjusted by the operator while visually observing the spangle size. If the quenching device shifts to the downstream side or the upstream side of the solidification position of the plating layer, the plating layer is not rapidly solidified and the spangle is not sufficiently miniaturized.

Also, there is a limit to the installable position of the quenching device, and even if the installing position is slightly shifted up or down, the quenching effect is remarkably changed, and a satisfactory spangle miniaturization effect cannot be obtained. Further, in the refrigerant injection method as described above, since the nozzles are likely to be clogged, careful maintenance is required to always obtain a stable spangle miniaturization effect.

On the other hand, as described above, the spangle can be miniaturized by suppressing the concentration of the spangle forming elements (Pb, Sb, etc.) in the plating bath, but even if the concentration of these elements is reduced, the spangle can be miniaturized. There is a limit, and in order to achieve sufficient miniaturization, it is necessary to simultaneously use the spraying of a refrigerant on the surface of the plating layer when the plating layer is solidified, as described above.

As a solution to such problems, Japanese Patent Laid-Open No. 61-64863, for example, discloses a method of adding a small amount of Mg to a plating bath to promote miniaturization of spangles, and Japanese Laid-Open Patent Publication No. 7-18399. The publication proposes a method of preventing nozzle clogging by appropriately controlling the composition and pH of an aqueous solution used as a refrigerant. However, even with these methods, spangles cannot be stably and sufficiently miniaturized.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a method capable of stably and reliably obtaining a zero spangle material having a small spangle size. It is a thing.

[0014]

[Means for Solving the Problems] The manufacturing method of the present invention capable of solving the above problems is a method for manufacturing a hot dip galvanized steel sheet in which iron powder is formed on the surface as a base steel sheet.
The characteristic is that the spangle on the plating surface is made finer by using a steel sheet having 5 mg / m ² or more attached thereto and causing the iron powder to act as a spangle producing nucleus. A preferable plating bath used when carrying out the present invention has an Al content of 0.1 to 0.3% by weight and Pb
A molten zinc-based plating bath having a content of 0.05% by weight or less and an Sb content of 0.02% by weight or less can be mentioned. A molten zinc-based plating bath in which the Pb and Sb contents are regulated by such an Al content. By using the plating bath, it is possible to more reliably obtain a zero spangle material having excellent workability and plating adhesion and having a smaller spangle diameter.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a steel sheet produced by using a tandem rolling mill or the like is washed with a washing facility, and then annealed in a reducing atmosphere, preferably in an indirect heating type annealing furnace, and then molten zinc is used. When performing system plating, the amount of iron powder adhering to the surface of the steel sheet after cleaning is set to 15 mg / m ² or more, thereby achieving a finer spangle size, which has been completely recognized in relation to spangle. By increasing the adhering amount of iron powder, which has never happened, it was possible to achieve stable and reliable miniaturization of spangles. Hereinafter, the details of the experiment leading to the present invention will be described in detail.

The present inventors used a continuous hot-dip galvanizing line (CGL) in which rolling is performed by using a tandem rolling mill and then indirect heating type annealing is performed in a reducing atmosphere, and various conditions are changed. The change in spangle size when a zero spangle material was manufactured was investigated. In this case, when performing post-annealing by rolling with a tandem rolling mill, a cleaning facility is provided to prevent oil components adhering to the surface of the rolled steel sheet from being brought into the annealing furnace, and a method of cleaning the steel sheet surface with an alkali cleaning solution. It was adopted.

The inventors of the present invention have studied various factors that seem to affect the spangle of the hot-dip galvanized layer from various angles. As a result, we found that if the amount of iron powder adhering to the surface of the steel sheet after cleaning is properly controlled, spangles can be remarkably miniaturized even when the normal air-cooling method is adopted without using a special rapid solidification method. . That is, in the conventional example, as a means for refining spangles, cooling conditions when the plating layer solidifies (rapid cooling solidification conditions)
Although the composition control of plating bath and plating has been adopted,
According to the present invention, it is possible to obtain a completely new finding that the spangle can be made finer by controlling the surface properties of the original plating plate, especially the amount of iron powder adhering to the surface.

When a hot dip galvanized steel sheet is produced by CGL, a steel sheet rolled by a tandem cold rolling mill is usually used as a plating original sheet. In tandem cold rolling,
Rolling is performed using rolling oil, but due to severe friction between the rolling roll and the steel sheet, separation of iron powder from the steel sheet surface occurs,
This again undergoes roll reduction and adheres to the steel sheet surface. Rolling oil adheres to the surface of the steel sheet as described above, and when performing hot dip galvanizing with CGL having an indirect heating type annealing furnace, when oil is brought into the annealing furnace, it is carbonized. Since the surface quality is significantly deteriorated, oil is removed through the inlet side cleaning equipment before being sent to the annealing furnace.

At this time, conventionally, it is considered that the iron powder adhering to the surface of the steel sheet is also removed at the same time, and the iron powder remaining on the surface of the steel sheet is peeled off and accumulated by rolls in the annealing furnace to cause a flaw. It is thought that it is desirable to reduce the amount as much as possible because it may cause the generation or cause problems such as being brought into the plating bath and causing a large amount of dross. In fact, it is used for ordinary hot-dip galvanizing. The amount of iron powder adhering to the surface of the base steel sheet is considered to be about 10 mg / m ² or less.

Furthermore, it has never been studied so far about how the iron powder remaining without being removed at this time affects the surface properties of the hot dip galvanized coating. However, according to the results of various studies conducted by the present inventors, as the amount of iron powder adhered in the rolling step and remaining without being removed in the washing step increases, the spangle is surely miniaturized, and the remaining It has been found that if the iron powder amount is set to 15 mg / m ² or more, spangles can be miniaturized to the extent that they cannot be visually discerned even when a normal air cooling method is used after plating. The upper limit of the amount of iron powder is not particularly limited, but the spangle refining effect is almost saturated at about 25 mg / m ² , and if the amount of iron powder adhered is too large, the smoothness of the plating layer surface is rather impaired. It is desirable to control the amount of iron powder adhered to 25 mg / m ² or less.

The mechanism by which spangles are made finer depending on the amount of iron powder adhering to the surface of the steel sheet is not necessarily clarified, but the iron powder adhering to the surface of the steel sheet serves as a solidification nucleus during solidification of the plating layer. It is thought that the number of spangles increases and each spangle is miniaturized.

The amount of iron powder remaining on the surface of the steel sheet after cleaning is determined by the structure of the cleaning equipment, that is, whether or not each equipment such as alkali dip, alkali scrubber, electrolytic degreasing, hot water scrubber, and high pressure cleaning is installed and the cleaning conditions, for example. It depends on complicated factors such as the number of stages of scrubber brush, material, rotation speed, electrolytic degreasing time, electrolytic density, and composition and temperature of alkaline cleaning liquid. Among them, the method of controlling the rotation speed of the alkali scrubber brush can control so that the adhered iron content can be left to some extent while reliably removing the oil content without requiring a major facility modification. That is, the conventional scrubber brush is operated so that the relative speed of the brush and the steel plate is 800 m / min or more, but if this is reduced to about 600 m / min or less, the iron powder adhesion amount Can be about 15 mg / m ² or more (see FIG. 2 below). Also, not only the relative speed between the scrubber brush and the steel plate, but also by changing the number of stages of the scrubber brush, the material, etc., the oil content can be reliably removed and the adhered iron powder can remain to some extent. It is also possible to adopt the method of.

When the steel sheet is passed through an annealing furnace or the like after being washed, the steel sheet is not subjected to the frictional force of the rolling, and the iron powder is peeled off to the extent that it affects the size of the spangle.・ Do not fall out.

The iron powder adhering to the surface of the steel sheet in the rolling process has a flat shape, and the diameter in the spreading direction is distributed in the range of approximately 5 to 100 μm. In this range, the size of the iron powder is plated. Has almost no effect on the size of the layer surface spangle,
Further, the adhered iron powder has a flat shape and does not affect other quality characteristics of the hot-dip galvanized product at all.

As described above, in the present invention, when the amount of iron powder adhering to the surface of the steel sheet after washing is set to 15 mg / m ² or more, when the ordinary air cooling method is adopted without using the rapid solidification method etc. However, spangles can be sufficiently miniaturized, but depending on the degree of required quality, before the solidification of the plating layer, a cooling medium is sprayed on the surface of the plating layer under appropriate conditions to perform rapid solidification, and spangles are formed. Further miniaturization is also effective.

The present invention is characterized in that the amount of iron powder adhering to the surface of the steel sheet is regulated as described above, and the composition of the plating bath is not particularly limited, but a particularly preferred plating bath for obtaining a zero spangle material. The composition is Al: 0.
1 to 0.3 wt%, Pb concentration: 0.05 wt% or less, S
b: 0.02% by weight or less, balance: a plating bath consisting essentially of Zn. By using a plating bath having such a component composition, it is possible to more reliably obtain a high-quality zero spangle material in which the spangle is further miniaturized. Can be obtained.

That is, Al in the plating bath is an effective component for improving the workability of the hot-dip zinc-based plating layer and forms an extremely thin Al-Fe-based alloy layer on the surface of the base steel sheet to form a base steel sheet. It also exerts the effect of suppressing alloying of the plating layer. In order to effectively exert such an action of Al, it is preferable that the Al content in the plating bath is 0.1% by weight or more, and if it is less than this, cracks or the like may occur in the plating layer when processing the plated steel sheet. It tends to occur. However, the workability improving effect of Al is saturated at about 0.3% by weight, and if it is contained in an excessively large amount, the affinity of the plated material with the steel sheet is reduced and the part not partially plated (imperfect There is a possibility that a plated portion) is likely to be formed, or that a large amount of dross made of an Al-based oxide is generated in the plating bath and adheres to the plating surface to hinder the surface smoothness of the plated steel sheet.

Further, Pb and Sb are both elements having a spangle growth action as described above, and the Pb content exceeds 0.05% by weight, or the Sb content is 0.0%.
If it exceeds 2% by weight, spangle growth is likely to occur due to the spangle growth action, and in particular, coarsening of spangle tends to occur when air-cooled after plating. The spangle size varies depending on the amount of iron powder adhering to the surface of the steel sheet, but when a plating bath containing Pb or Sb in excess of the above amount is used, it is subjected to a rapid solidification treatment to sufficiently miniaturize the spangle. Is required,
It becomes difficult to effectively use the object of the present invention. Pb and Sb
A more preferable upper limit value of is 0.02% by weight or less and 0.01% by weight or less, respectively. The remaining component of the plating bath is Z
n and unavoidable impurities, but if necessary, M
It is also possible to contain a small amount of g, Mn, Ni, Ti and the like.

[0029]

EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and is not limited to the following examples. It is, of course, possible to modify and implement the present invention, and all of them are included in the technical scope of the present invention.

Example A hot-dip galvanized steel sheet was produced under the following conditions using a continuous hot-dip galvanizing line (CGL). (Conditions) Type of steel plate to be treated: Tandem cold rolled steel plate Plate thickness: 1.0 mm Cleaning on the inlet side Cleaning equipment: Alkaline dip → Alkali scrubber → Alkaline electrolytic degreasing → Hot water scrubber → Hot water rinse Alkaline dip: Immersion time 2 sec Alkali scrubber : Brush material (nylon), number of steps (2 steps / face), brush / steel plate relative speed (500 to 100)
Electrolytic degreasing: time (2 sec), current density (15 A / dm)
² ) Cleaning liquid: Liquid component (NaOH-based alkaline cleaning liquid), liquid temperature 7
0 ℃ annealing furnace: all radiant tube indirect heating furnace (vertical type) Atmosphere Gas: N ₂ -H ₂ mixture gas (H _2: 5 to 25%) peak metal temperature: 700 to 800 ° C. hot-dip galvanized plating bath composition : Al: 0.25% by weight, Pb: 0.01
% By weight, balance: Substantially Zn Plating bath temperature: 460 ° C. Bath penetration plate temperature: 460 ° C. Coating weight: 140 g / m ²

In carrying out the above method, the amount of iron powder remaining on the steel plate after cleaning was changed by changing the relative speed of the alkali scrubber brush / steel plate during cleaning on the inlet side. The amount of iron powder was quantified by ICP analysis (inductively coupled high frequency plasma spectroscopic analysis) after the cellophane tape was attached to the surface of the steel sheet on the outlet side of the cleaning equipment and peeled off, and the iron powder attached to the tape was dissolved in hydrochloric acid. The spangle size was determined by image analysis of the surface of the plating layer to determine the equivalent circle diameter. If the circle-equivalent diameter is 0.5 mm or less, it is difficult to visually identify spangles, and it can be evaluated as a very excellent zero spangle material.

The results are as shown in FIG. 1, and the amount of iron powder adhered and remaining on the surface of the steel sheet was 15 mg / m ² or more and the hot dip galvanizing treatment was performed on the steel sheet without performing quenching treatment after the plating. It can be seen that spangles can be miniaturized to the extent that they cannot be visually identified. Further, FIG. 2 is a graph showing the relationship between the relative speed of the alkali scrubber brush and the steel plate and the amount of iron powder adhered remaining after cleaning. As is clear from this figure, under the above experimental conditions. Then, if the above-mentioned relative speed is suppressed to about 600 m / min or less, the amount of remaining iron powder adhered can be set to about 15 mg / m ² or more.

[0033]

EFFECTS OF THE INVENTION The present invention is configured as described above, and by regulating the amount of iron powder remaining on the surface of a steel sheet after cleaning, it is possible to stabilize spangle without performing quenching treatment after plating. Can be miniaturized, has excellent surface properties,
Further, it is possible to provide a hot-dip galvanized steel sheet that gives a coated steel sheet with excellent surface properties by the subsequent coating treatment.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of iron powder adhering to the surface of a steel plate after cleaning and the spangle size after plating.

FIG. 2 is a graph summarizing the relationship between the relative speed of an alkali scrubber brush and a steel plate and the amount of iron powder adhered after cleaning.

Claims (2)

[Claims] 1. A method for producing a hot-dip galvanized steel sheet, wherein the base steel sheet contains 15 mg / m ^{2 of} iron powder on the surface.
A method for producing a zero spangle hot dip galvanized steel sheet, characterized in that the spangle on the plating surface is made finer by using the above-mentioned adhered steel sheet. 2. An Al content of 0.1 to 0.3% by weight, P
b content is 0.05% by weight or less, Sb content is 0.02
The method according to claim 1, wherein a hot-dip galvanizing bath having a weight% or less is used.