JP3494134B2 - Hot-dip plating method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot dipping method for metallic materials, and more particularly to a hot dipping method for applying aluminum-zinc (Al-Zn) alloy plating to metallic materials such as steel materials by utilizing flux treatment. Regarding

[0002]

2. Description of the Related Art Steel materials are widely used in structures, but various corrosion inhibitors have been used because they are easily corroded. Above all, hot-dip galvanizing has been used in a wide variety of ways as a relatively economical rust preventive method, from small joint parts such as screws and bolts to large structural members such as H-shaped steel. However, since the galvanized coating is poor in resistance to salt corrosion in the vicinity of the coast and the like, an anticorrosive coating having more excellent corrosion resistance has been demanded.

In such a background, it has been found that hot-dip Al-Zn alloy plating has significantly higher corrosion resistance than hot-dip galvanizing. In particular, it was confirmed that hot-dip Al-Zn alloy plating consisting of 55% Al, 1.5% Si, and the balance Zn is the best in terms of both the corrosion resistance of the plating coating and the sacrificial corrosion resistance for steel materials. In terms of anticorrosion thin steel sheet, it has reached a considerable industrial production now.

The hot-dip coating of thin steel sheets is generally carried out in a continuous hot-dip plating facility having a hot-dip galvanizing bath located on the outlet side of the continuous annealing equipment. In typical continuous hot-dip galvanizing equipment, the steel sheet is first cleaned by heating it in a weakly oxidizing non-oxidizing furnace, and then introduced into a reducing furnace connected to the non-oxidizing furnace to reduce and anneal it in an atmosphere containing hydrogen. Then, it is introduced into the hot dip bath without contact with the atmosphere to perform hot dip plating. The steel sheet is shielded from the atmosphere from the time of cleaning to the time of entering the plating bath, during which degreasing and reduction of oxides are carried out, and the steel sheet enters the hot dip bath under the condition that it is easily wet with the molten metal. Such continuous hot dip coating equipment was developed for zinc plating, but is also used for aluminum plating and Al-Zn alloy plating. That is, the hot dip Al—Zn alloy plating of a thin steel sheet can be operated using continuous hot dip galvanizing equipment for hot dip galvanizing simply by changing the composition of the plating bath and the operating conditions.

On the other hand, hot dip plating other than thin steel sheets, for example,
Continuous hot-dip coating of wires and batch-type hot-dip of steel materials such as structural members and various parts have been performed by immersing the steel material in a molten metal bath in the atmosphere. In this case, even if the steel material is degreased and pickled in advance to clean the surface and remove oxides, oxidation before the invasion of the bath is unavoidable, so a flux called salt called flux is used. In general, a means for melting oxides formed on the surface of a steel material that is inevitably formed and promoting wetting by molten metal has been used.

There are a dry method and a wet method as the processing method using the flux. The dry method is a method of depositing flux as an aqueous solution on a steel material, drying the flux to deposit it on the surface of the steel material, and then dipping the steel material in a molten metal bath to carry out hot dip plating.

In the wet method, a flux having a lower melting temperature than the plating bath temperature is used, and this flux is put on the molten metal bath in the plating tank. The flux melts at the temperature of the molten metal bath and floats due to the difference in specific gravity. Thus, a flux molten layer having an appropriate thickness is formed on the molten metal, and the steel material is allowed to enter the molten metal through this flux molten layer, whereby the steel material is coated with the flux immediately before plating.
In this case, when the steel material is pulled up from the molten metal, it passes through the flux melting layer again, so that it is necessary to remove the flux reattached to the surface of the plated product from the surface of the plated product after plating, which complicates the operation.

Generally, a dry method using an aqueous solution containing zinc chloride and ammonium chloride is adopted for the flux treatment of the hot dip galvanizing, and a fluoride type flux is used for the flux treatment of the hot dip aluminum plating having a high plating bath temperature. A wet method using is often used. However, in the case of hot dip Al-Zn alloy plating, in the dry flux treatment for hot dip galvanizing, Al in the hot metal bath mainly reacts with ammonium chloride (NH ₄ Cl) in the flux to form sublimable aluminum chloride ( AlCl ₃ ) is generated, resulting in frequent non-plating due to deterioration of the flux function due to decomposition of the flux. On the other hand, the wet flux treatment for hot dip aluminum coating is difficult to apply to Al-Zn alloy plating in which the plating bath temperature is considerably lower than that of aluminum.

From this viewpoint, some fluxes for hot dip Al—Zn alloy plating have been proposed so far. For example, JP-A-58-136759 discloses a flux composition for Al-Zn alloy plating, which comprises zinc chloride and at least one chloride, fluoride or silicofluoride of an alkali metal or alkaline earth metal. Is disclosed. Since this flux is used in the dry method, it has excellent operability, but its flux function is not sufficient, and non-plating tends to occur frequently as the Al concentration in the molten metal increases. In particular, in 55% Al-Zn alloy plating with high Al concentration, which has excellent corrosion resistance,
This phenomenon becomes serious.

In JP-A-3-162557, the compounding ratio of zinc chloride and ammonium chloride is 10 to 30: 1 by weight.
Which is a flux composition for Al-Zn alloy plating. This flux is also used in the dry method.Although it is possible to perform relatively good plating for thin products, non-plating tends to occur as the plating temperature rises, and the plating temperature rises to 55% Al-Zn. In alloy plating, non-plating is likely to occur except for thin products.

Japanese Unexamined Patent Publication (Kokai) No. 4-293761 discloses a flux composition for Al alloy plating, which comprises four components of chlorides of zinc, lithium, sodium and potassium. This flux is used by the wet method, and among the above four components, expensive lithium chloride is the main component (40-60 mol%)
Therefore, the cost becomes high. In addition, the effect of suppressing non-plating is not sufficient for thick products, and since the wet method is used, removal of the flux reattached to the plated products becomes complicated.

Japanese Unexamined Patent Publication (Kokai) No. 4-323356 discloses a flux composition for Al-Zn alloy plating comprising an Al-containing alkali metal fluoride (eg, cryolite) and an alkaline earth metal chloride. . This flux is also used in the wet method, especially 55
Although it has been proposed as a suitable material for plating% Al-Zn, it is likely to cause a flux hanging phenomenon (a phenomenon in which flux solidifies to form a shelf and causes a cavity between the molten metal and the flux). There is a problem. Further, since this flux contains a fluoride, there is a problem that the flux re-attached and solidified when the steel material is pulled up from the molten metal cannot be easily removed by a means such as washing with water due to the presence of the fluoride. Therefore, the plating appearance becomes poor.

As described above, all of the conventional fluxes proposed for hot dip Al-Zn alloy plating contain chloride, and the flux function sufficient for hot dip Al-Zn alloy plating (the surface of the metal material to be plated is To improve wettability),
It has been considered necessary to have a flux material containing chloride.

In addition to the above-mentioned method, as described in Japanese Patent Publication No. 61-201767, a two-step plating method has also been proposed in which a steel material is previously galvanized and then hot-dip Al-Zn alloy plating is performed. However, since it requires two plating steps, it is unavoidable that it is disadvantageous in terms of manufacturing cost.

In addition, molten Al-
Even if Zn alloy plating is performed, in the conventional method, there is no preheating step before plating or even if it is insufficient, the immersion time in the molten metal is generally 20 seconds or more, usually 30 to 180 seconds. The length is required, and especially in the case of 45 to 60% Al, the plating bath temperature is high, which may cause the growth of the brittle intermetallic compound layer at the metal material-plating interface during the immersion in the molten metal. It occurs remarkably and the workability of the plated product is significantly reduced.

[0016]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described above, with respect to the flux treatment for molten Al-Zn alloy plating having a high Al content of 45% or more, the dry method has insufficient flux function and easily causes non-plating, In the wet method, the flux is expensive, the phenomenon of hanging the flux easily occurs, it is difficult to remove the flux that has re-deposited after plating,
There was a problem that the plating appearance deteriorated, and neither was satisfactory.

Further, in any of the methods, since the immersion time of the plating bath is long, the above-mentioned intermetallic compound layer is likely to grow and the workability of the plated product tends to be deteriorated. In contrast, hot dip Al-Zn alloy plating, especially 45
The situation was extremely insufficient for industrially carrying out ~ 60% Al-Zn alloy plating.

The present invention provides a hot dip plating method suitable for hot dip Al-Zn alloy plating in which the problems of the prior art are solved, and in particular, Al having an Al content of 45 to 60% is provided.
An object of the present invention is to provide a practical hot-dip plating method capable of forming a plated product having excellent workability in a Zn alloy plating.

[0019]

Means for Solving the Problems The present applicant first dips a metal material in a molten flux bath having a melting point higher by 5 ° C. or more than the plating bath temperature, and then in the molten metal plating bath for hot dipping. A method has been proposed (Japanese Patent Laid-Open No. 10-140310). The melting point of the molten flux material is 5 ℃ from the plating bath temperature
By setting it higher than the above, the flux attached to the metal material and brought into the plating bath solidifies in the plating bath and floats on the surface of the plating bath, so it can be easily removed by skimming, etc. Adhesion can be avoided.

This method has a high Al content of 45% or more.
-A molten flux that can exhibit a sufficient flux function for Zn alloy plating is used, but since it is fluxed before plating as in the dry method, it reattached to the plated product that was associated with the wet method. There is an advantage that the flux removal process is unnecessary. Further, by immersing in a molten flux bath at a temperature higher than that of the molten plating bath immediately before hot dip coating, preheating of the metal material is also achieved at the same time, and the dipping time in the hot molten Al-Zn alloy plating bath is shortened, It is possible to avoid deterioration of workability due to growth of the intermetallic compound layer.

The flux specifically described in this publication is a composition containing at least one chloride as a flux component, like the conventional Al-Zn alloy plating flux. Although molten flux containing chloride has a high flux effect, it produces chlorine gas with strong corrosiveness at high temperatures near the molten 55% Al-Zn alloy plating bath, so high-grade stainless steel such as SUS 304 and SUS 316. Corrosion of the container even if it is used. for that reason,
After about 1 week of plating operation, the flux function significantly deteriorates due to the components eluted from the container, and non-plating frequently occurs. Eventually, the container is punctured, so long-term operation is not possible and practical And the problem remained.

Further, since the melting temperature of the flux is high,
It has been found that the flux bath temperature becomes considerably higher than the plating bath temperature, the preheating temperature is too high, and the growth of the intermetallic compound layer lowers the plating adhesion and the workability in some cases.

The inventors of the present invention have studied in order to solve the above problems by using a flux material which does not corrode a high-grade stainless steel container in the above-mentioned flux treatment with the molten flux bath, and as a result, one or more fluorine Even if the flux does not contain chloride, it has a sufficiently high flux function equivalent to that of chloride-containing flux. Even if this flux is housed in a stainless steel container and operated for a long period of time, the flux function will deteriorate and Corrosion of the container does not occur, the melting point of the flux is lower than the plating bath temperature, and even if a small amount of flux brought into the plating bath melts in the plating bath, the appearance of the plating will not change as long as it is washed with water after plating. The present invention has been achieved by finding that the deterioration does not occur so much.

Here, the present invention is a hot dip plating method in which a metallic material is dipped in a molten flux bath and then dipped in a molten metal plating bath to carry out hot dip plating.
An aluminum-zinc alloy containing 60% by mass of Al and 0.5 to 2% by mass of Si, wherein the flux is composed of one or more kinds of metal fluorides, and has a composition substantially free of chloride, Flux bath temperature is -50 ° C to + of plating bath temperature
It is a hot-dip plating method characterized by being in a temperature range of 100 ° C.

The composition containing substantially no chloride means that the content of chloride is 5% by mass or less. This molten flux preferably has a composition mainly containing aluminum fluoride and potassium fluoride, and is used by being contained in a stainless steel container.

The hot dip Al-Zn alloy plating method of the present invention is excellent in the operability of continuous operation and can produce a plated product having improved workability without causing non-plating. Since the bath temperature of the flux may be lower than the plating bath temperature, this includes both cases where the melting point of the flux is lower than the plating bath temperature and higher than the plating bath temperature. When the flux melting point is lower than the plating bath temperature, the flux brought into the plating bath melts in the plating bath, so that the ease of flux removal achieved in JP-A-10-140310 is somewhat sacrificed. However, since the amount of flux brought in is much smaller than the amount of flux in the plating bath by the wet method, the amount of redeposition of flux on the plating film is also significantly smaller than that by the wet method, and washing with water is not necessary for parts that do not emphasize appearance. In addition, for parts that place importance on appearance, the redeposition flux can be easily removed by simply adding water washing, and a plating appearance that can be sufficiently used in practice can be obtained.

In the method of the present invention, the flux bath temperature can be lowered as compared with the method described in JP-A-10-140310, so that the growth of the intermetallic compound layer is more effectively suppressed and the plating adhesion is improved. Be improved. As for the life of the flux bath, the lower the bath temperature, the more advantageous.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The metal material which can be subjected to Al-Zn alloy hot dip plating by the method of the present invention is not particularly limited, but typical examples are steel materials (e.g., steel wire, shaped steel, steel pipe, steel fittings, for example, , Bolts, nuts, screws, etc.). For example, as a building material for roofs and outer walls, not only in areas with strong salt damage and corrosion such as coastal areas, but also in other areas, high corrosion resistance molten Al-Zn alloy plated steel sheet, especially molten 55% Al-Zn
Alloy-plated steel sheets have come to be used, but if the same plating is applied to the small-sized joining members that join the steel sheets, the corrosion resistance of this joining member can be secured and at the same time, the dissimilar metal material at the joining site can be used. There is also an effect that the plating material can be prevented from being dissolved by the local battery action that occurs when the two come into contact with each other, and the durability of the plating is improved. The hot-dip plating method of the present invention can be applied to the Al—Zn alloy plating of such a small-sized joining member or a large-sized member such as a die steel. In addition to ordinary carbon steel, it can be applied to various metal materials such as alloy steel, Ni alloy, and ferritic stainless steel.

The metal material to be plated is preferably subjected to a usual pretreatment before being immersed in the molten flux bath in the flux bath according to the present invention. For example, when the metal material is steel, pretreatment includes a degreasing process using a warm aqueous solution of sodium orthosilicate, caustic alkali, sodium carbonate, etc., a degreasing process using an organic solvent, and a pickling process using an aqueous solution of an acid such as hydrochloric acid or sulfuric acid. At least one step is included.

The plating metal in the present invention contains 45 to 60%.
It is an Al-Zn alloy containing Al and 0.5 to 2% Si. The plating bath contains Al, Zn, Si, and trace amounts of Mg, Pb, Sn, Fe, S
Elements such as b, Mn, Ni, Cr, Ti, V, Sr, B and Be may be contained.

Since this hot-dip Al-Zn alloy plating has much better corrosion resistance than hot-dip galvanizing and has a beautiful spangle pattern, it is not only used as a coating base material, but also unpainted or clear-painted. As a plated steel sheet that can be used as a product, the amount of it is being used for building materials and outer panels of home appliances is increasing. However, non-plating often occurs in the conventional dry process flux treatment, and even in the wet process flux treatment, there are problems such as deterioration of the plating appearance due to flux removal treatment after plating and flux hanging phenomenon, and the plating bath temperature is high. In addition, the workability was deteriorated due to the growth of the intermetallic compound layer at the material-plating interface.

In the present invention, first, the metal material to be plated, which has been subjected to the appropriate pretreatment described above, if necessary, is immersed in a molten flux bath at a temperature close to the plating bath temperature. By this immersion treatment in the molten flux bath, the surface of the metal material is activated by the action of the flux to improve the wettability, and a flux film is formed on the metal surface when pulled out from the molten flux bath.

Next, the metal material having the flux film is immediately immersed in the molten metal plating bath. The flux film acts to protect the metal material from oxidation until it is immersed in the plating bath, and when immersed in the plating bath, the flux film peels off from the surface of the metal material and floats on the molten metal. The amount of flux floating on the molten metal in the plating bath is small, and either if the surface of the bath is skimmed (if the flux has solidified), or if the fluid is given to the surface of the bath (if the flux has melted). ), Flux reattachment to the plating surface during pulling is minimized, parts that do not emphasize appearance need not be washed with water, and parts that emphasize appearance can be easily removed by adding water washing. Therefore, although the molten flux is used as in the wet method, the flux removal treatment after plating is not necessary.

While the metal material is immersed in the molten flux bath, the metal material is heated to a considerably high temperature even if it is immersed for a short time. That is, the immersion in the molten flux bath also serves as preheating. Therefore, the immersion time in the next hot dip plating bath can be shortened, and the growth of the brittle intermetallic compound layer caused by the dipping in the Al-Zn alloy hot dip bath can be significantly suppressed, and the plating film can be processed. The deterioration of sex is prevented.

The flux composed of one or more kinds of metal fluorides used in the present invention has the same strong flux as the conventional product even if it does not substantially contain chloride unlike the conventional flux. Indicates a function. As a result, non-plating is likely to occur when the flux treatment is performed by the dry method.
Even in the case of Al-Zn alloy hot dip galvanizing containing Al in an amount of not less than 10%, the wettability of the surface of the metal material to be plated is sufficiently secured by the flux processing for a short time with good operability, and non-plating can be prevented. .

The flux containing substantially no chloride is SUS3, which is a general corrosion-resistant storage container, even when the temperature is raised to around 600 ° C., which is close to the operating bath temperature of the 55% Al-Zn plating bath.
Since the corrosion loss of highly corrosion-resistant stainless steel sheets such as 04 and SUS316 is extremely small, it can be used for a long time without degrading the flux, and the life of the container is semi-permanent. Therefore, it is possible to perform a long-term hot dip plating operation without generating non-plating by utilizing the prior flux treatment with the hot melt flux. However, it is acceptable for the molten flux to contain chloride in an amount of 5% by mass or less. This is because the corrosiveness does not significantly increase in that range. But,
The chloride concentration in the flux is preferably 2% by mass or less, and most preferably 0%.

In the hot dip plating method of the present invention, a flux bath for melting the flux is installed in addition to the hot dip metal bath. As described above, it is desirable that this flux tank is manufactured from a highly corrosion-resistant stainless steel plate such as SUS304 or SUS316, which is a general material for a corrosion-resistant container. In the present invention, the molten flux material significantly reduces the corrosion weight loss of such a stainless steel sheet.

Into this flux tank, a flux material composed of one or more kinds of metal fluorides whose composition is adjusted so as to have a melting point capable of raising to a temperature near the bath temperature of the molten metal plating bath, and heated. Melt to prepare a molten flux bath. The temperature of the molten flux bath is in the range of -50 ° C to + 100 ° C with respect to the hot dip bath temperature.

The bath temperature of the molten flux is the hot-dip bath temperature +
If the temperature is higher than 100 ° C, the temperature of the metal material in the flux bath becomes unnecessarily high, and the penetration temperature of the plating bath becomes too high. Even if the immersion time in the plating bath is shortened, the brittle intermetallic compound layer at the interface will be formed. It becomes difficult to suppress the growth and the workability decreases. On the other hand, if the temperature of the molten flux bath is lower than the molten plating bath temperature −50 ° C., the preheating effect becomes insufficient and it becomes necessary to lengthen the immersion time in the next molten plating bath. Growth occurs and the workability also deteriorates. The molten flux bath temperature is preferably in the range of -20 ° C to + 50 ° C with respect to the molten plating bath temperature.

In the conventional wet flux method, since the flux is melted at the hot dip bath temperature and floated on the molten metal, it is difficult to remove the flux at the time of pulling up and prevent the growth of the brittle intermetallic compound layer. Moreover, the melting point of the flux must be lower than the hot-dip bath temperature, which limits the composition.

In the present invention, a flux bath is separately provided to simultaneously prevent oxidation of steel and preheat, and if SUS304 or 316 is used as the material of the container, a flux composition that can be used semipermanently is selected. Specifically, the flux composition is composed of one kind or two or more kinds of metal fluorides and does not substantially contain chloride. As mentioned above, the bath temperature of the molten flux is -50 ° C to +100 of the hot dip bath temperature.
Since it is in the range of 0 ° C, the melting point of the flux may be higher or lower than the hot dip bath temperature.

As the metal fluoride used for the flux,
Potassium fluoride, aluminum fluoride, cryolite, zinc fluoride,
Examples include sodium fluoride, copper fluoride, titanium fluoride, lithium fluoride and the like.

The molten Al-Zn alloy containing 45 to 60% by mass of Al generally has a plating bath temperature in the range of 570 to 610 ° C. The composition of the flux is selected to obtain it.
The composition is preferably such that the melting temperature of the flux is lower than the temperature of the flux bath to be used by several degrees Celsius to several tens of degrees Celsius. A typical flux composition for obtaining such a melting temperature is 40 to 65% by weight of aluminum fluoride and 60 to 35% by weight of potassium fluoride.
Is. Cryolite and other metal fluorides may be further added to this basic composition.

The immersion time in the molten flux bath may be a very short time, for example, several seconds to several minutes. Since this immersion also serves as preheating, when the thickness of the metal material to be plated is large, the immersion time is set longer so that it is sufficiently preheated. Since it is preheated by this flux treatment, the immersion time in the hot dip bath is significantly shortened,
Even including the immersion time in the flux bath, the total working time required for the hot dipping work can be shortened.

As described above, since the surface of the metal material exiting the flux tank is protected by the flux, the surface of the metal material is not oxidized even when exposed to the atmosphere. Therefore, it is not necessary to shut off the atmosphere during the transfer from the flux bath to the hot dip bath. However, in order to prevent the temperature of the metal material preheated in the flux tank from decreasing, it is preferable to transfer the metal material from the flux tank to the hot dip plating tank promptly.

When the metal material is introduced into the hot dipping bath,
The flux attached to the metal material peels off and floats above the molten metal in the plating bath. The floating peeling flux exists in a solid or molten state depending on the melting point of the flux. Since the amount of flux brought into the hot dip plating tank is small, even if the flux floats on the molten metal, the redeposition of the flux when pulling up the metal material is relatively small. Parts that are unnecessary and that emphasize the appearance can be easily removed by adding water washing. Therefore, a special treatment for removing redeposited flux after plating, which is performed in the conventional wet method flux treatment, is usually unnecessary.

Means can be taken to reduce the redeposition of this flux as much as possible. For example, when the melting temperature of the flux is higher than the plating bath temperature and the floating flux in the plating bath is solid, the floating flux can be removed by a conventional skimming means. The skimming can be performed between the plating operations in the batch type plating, and periodically or constantly in the continuous plating as needed. When the floating flux in the plating bath exists in a molten state, the flux can be driven from the bath surface in the region where the immersed metal material is pulled up by giving a flow to the bath surface. For example, a jet flow may be generated in this region so that the molten metal flows from this region to the periphery on the bath surface.

The metal material is preheated in the molten flux bath, and the immersion time in the molten plating bath is significantly shortened. As a result, the growth of the brittle intermetallic compound layer at the metal material-plating interface is significantly suppressed and the plating adhesion is improved. Therefore, it is possible to form a high-quality plating film with excellent workability, which is well suited for applications requiring workability, and it is possible to significantly reduce the amount of dross generated per plating amount. In particular,
Unlike the method disclosed in Japanese Patent Publication No. 10-140310, since the flux bath temperature can be lower than the plating bath temperature in the present invention, excessive preheating in the molten flux bath can be avoided,
When the preheating temperature is too high, it is possible to avoid deterioration of plating adhesion and workability due to growth of the intermetallic compound layer.

The corrosion resistance of the plated steel sheet generally depends on the coating weight, but for the 55% Al-Zn alloy plated steel sheet, a thick basis weight material with a particular emphasis on corrosion resistance is demanded. If the amount of adhesion on one side is 100 g / m ² or more, the corrosion resistance in the extremely severe salt damage area can be improved to the level of the conventional material (adhesion amount on one side 75 g / m ² ) in a mild environment. The upper limit of the adhered amount is not particularly limited, but 400 g / m ² is considered to be the economical limit. Also, 40 g / m ^{2 for} indoor use
Substantial and sufficient corrosion resistance can be secured. In this way, the plating adhesion amount may be set according to the corrosiveness of the use environment. When the flux treatment is performed according to the present invention, non-plating in the molten Al—Zn alloy plating can be effectively prevented, so that the thick basis weight material can be stably manufactured.

In order to further improve the corrosion resistance after plating, a chromate treatment may be applied after plating. Chromate treatment is preferably a coating type with good operability, but a reactive type,
Electrolytic type is also possible. The amount of chromate attached may be the same as the conventional one. Instead of chromate, it is also possible to apply a non-chromium type chemical conversion treatment.

The hot-dip Al-Zn alloy-plated steel sheet produced according to the present invention has a beautiful plating appearance and therefore can be used without coating, but even if it is subjected to clear coating to improve scratch resistance. Good. Of course, it is also possible to color paint.

[0052]

[Example] (Example 1) A 200 mm x 300 mm x 2.5 mm thick hot-rolled steel sheet was degreased in an aqueous solution of sodium orthosilicate, washed with water, then pickled with a 10 mass% hydrochloric acid aqueous solution, and plated. Pretreatment was performed. Following this plating pretreatment, flux treatment was performed as described below, and then hot dip Al—Zn alloy plating was performed.

The flux material having the composition and melting point shown in Table 1 was put in a SUS 316 flux tank (500 mm × 500 mm × depth 500 mm) installed in the vicinity of the hot dip plating tank. The flux was melted by heating to the various temperatures shown to form a molten flux bath. In this molten flux bath, the steel sheet that had been subjected to the pre-plating treatment described above was dipped for 1 minute to perform preheating and flux treatment, and then the steel sheet pulled out from the flux tank was dipped into the molten plating tank as quickly as possible.

The hot dip plated metal is 55% Al-1.6% by mass.
% Si-Zn alloy, the temperature of the hot dip bath was 600 ° C, and the immersion time in the bath was 1 minute. The coating weight was adjusted to a unit weight of approximately 120 g / m ^{2 on} one side with the pulling rate from the hot dipping bath being constant. The hot dip coating was continuously operated for 1 week under the condition of hot dipping 42 hot-rolled steel sheets per day (the working time per day was 7 hours). During,
The flux introduced from the flux tank began to float on the molten metal in the plating tank, and the amount gradually increased, but the flux removal treatment in the plating tank was not performed.

The hot-dip Al—Zn alloy-plated steel sheet pulled out from the plating bath was water-cooled, washed with water and brushed, and then the non-plated state was judged by visual observation according to the following criteria. ○ -No non-plating was observed, △ -Pinhole-like non-plating was found within 10 locations, × -Unplating occurred frequently (more than 10 locations).

Table 2 shows the determination results of non-plating of the first and last hot-dipped steel sheets. For hot-dip galvanized material where no plating was observed, test pieces taken from
After T-bending, the appearance of cracks on the outer surface of the bent R portion was visually observed. Further, the tape of the processed portion was peeled off, and the peeling state of the plating film was observed. From these observation results, the workability of the plated steel sheet was judged according to the following criteria, and the results are also shown in Table 2.

[0057] ◎ -cracks are fine and there is no peeling, ○ -Cracks are slightly larger, but there is no peeling, △ -Peeling is observed by tape peeling, × -The tape peels off the entire surface of the film.

[0058]

[Table 1]

[0059]

[Table 2] Before the hot dip Al-Zn alloy plating, the hot dip plated steel sheet, which was subjected to the dip treatment in the hot melt flux bath of the composition and temperature according to the present invention, had a thick basis weight of 55% Al-Zn alloy without unplating for one week. The plating could be carried out. Regarding workability, since the coating weight is thick, fine cracks were generated in the plating by the severe processing of 2T bending, but the peeling of the plating did not occur even after 1 week of operation. Flux bath temperature is close to plating bath temperature
+ 50 ° C), the workability was especially good. In every example,
Both the non-plating and the workability showed the same judgment results at the beginning and after 1 week, and the flux effect was not deteriorated during the continuous operation.

On the other hand, the flux composition was the same as that of the example of the present invention, but if the flux bath temperature was too low, the material temperature could not be raised sufficiently in the plating bath, so that the intermetallic compound layer was uniformly formed. However, non-plating occurred, resulting in deterioration of workability. On the other hand, in the comparative example in which the flux bath temperature was too high, the effect of preventing non-plating was sufficient, but the workability was lowered due to the excessive formation of the intermetallic compound layer.

On the other hand, a chloride-containing flux (No. 5 to No. 5) which is a conventional flux for hot dip Al-Zn alloy plating.
In the comparative example using 9), the flux function was high at the start of plating, and the same judgment result as that of the example of the present invention was obtained for both non-plating and workability. But operation 1
After a week, non-plating occurred or frequently occurred in all cases, and the workability could not be evaluated. in this way,
The molten flux containing a substantial amount of chloride cannot be used for a long period of time when stored in a high-grade stainless steel container which is a general corrosion resistant container.

[0062]

According to the present invention, by performing the flux treatment with the molten flux before the hot dip coating, the non-plating is likely to occur in the conventional dry process flux treatment.
It is possible to prevent non-plating of the% Al-Zn alloy plating, and to eliminate the troublesome flux removal treatment after plating, which is required in the conventional wet method, so that the deterioration of the plating appearance can be avoided.

Conventional molten fluxes for Al-Zn alloy plating contained chlorides, but in the present invention, a molten flux substantially free of chlorides is used, and a flux function equivalent to that of chloride-containing fluxes is obtained. Can be obtained.
Since the flux does not contain chloride, even if the molten flux is stored in a high-grade stainless steel container which is a general corrosion resistant container, the molten flux does not corrode the container and the flux function does not deteriorate rapidly, The flux can be used for a long time, and the container can be used semipermanently.

Furthermore, according to the hot dip coating method of the present invention, since the flux treatment also serves as preheating, preheating before plating is not required, and the immersion time in the molten metal plating bath is significantly shortened. Also, the plating work time is reduced. Since the Al-Zn alloy plating bath with an Al content of 45% or more has a high bath temperature, the growth of the brittle alloy bath is significantly suppressed by the drastic shortening of the immersion time in the plating bath, resulting in improved workability of the plating film. The effect of reducing the amount of dross generated is also obtained.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mikio Hashimoto 2-4 Fujinoki, Wakamatsu-ku, Kitakyushu City, Fukuoka Prefecture Shinko Almar Industry Co., Ltd. (56) Reference JP-A-4-323356 (JP, A) Kaihei 10-140310 (JP, A) JP-A-11-323523 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (3)

(57) [Claims] 1. A hot dip plating method in which a metal material is dipped in a molten flux bath and then in a hot metal plating bath to carry out hot dip plating. It is an aluminum-zinc alloy containing Si and has a composition in which the flux consists of one or more metal fluorides and does not substantially contain chloride, and the flux bath temperature is -50 ° C of the plating bath temperature or higher. A hot dip plating method characterized by being within a temperature range of + 100 ° C. 2. A flux aluminum fluoride 40-65%
And 60 to 35 % of potassium fluoride.
The hot dip plating method described. 3. The hot dip plating method according to claim 1, wherein the flux is contained in a stainless steel container.