JPH11323523A - Hot dip plating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow thick hot dip metallic plating by dipping a metallic material into a hot dip salb flux bath whose m.p. is higher than the plating bath temp. by a specified temp. and thereafter dipping it into a hot dip plating bath of an Al-Zn alloy having a specified compsn. composed of Al, Si and Zn. SOLUTION: A steel material is dipped into a hot dip flux bath, is therearter dipped into a hot dip plating bath of an Al-Zn alloy composed of 45 to 60% Al, 0.005 to <0.5% Si, and the balance substantial Zn and is applied with hot dip plating. At this time, the compsn. of the hot dip flux bath is composed of the combination of crystals and at least one kind of alkali metal chloride and aluminum fluoride as well according to necessary, and its m.p. is made higher than the hot dip plating bath temp. by 5 to 80 deg.C, preferably by 30 to 60 deg.C. In this way, the flux adhered to the material and carried into the plating bath is solidified on the bath surface and is easily removed, and the hot dip plating of the Al-Zn alloy having a uniform thickness and excellent in surface appearance can be formed on the material at the coating weight of >=150 g/m<2> per side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip plating method for a metal material, and more particularly to a hot-dip plating method suitable for applying a flux treatment to a steel material to apply a thick aluminum-zinc (Al-Zn) alloy plating. . Hereinafter, the present invention will be described using a steel material as an example of the metal material.

[0002]

2. Description of the Related Art Iron and steel materials are widely used in various structures as the most general structural materials. However, since they are easily corroded, various rust preventive means have been used. Above all, hot-dip galvanizing has been used as a relatively economical rust prevention method in a wide variety of applications, from small joint parts such as screws and bolts to large structural members such as H-section steel. However, the galvanized film is inferior in corrosion resistance to salt damage corrosion near the shore, so that a corrosion-resistant coating having more excellent corrosion resistance has been required.

[0003] Under such circumstances, it has been found that hot-dip Al-Zn alloy plating has much better corrosion resistance than hot-dip galvanizing. Especially about 55% Al, 1.6% Si,
It has been confirmed that the hot-dip Al-Zn alloy plating consisting of the balance of zinc is the most excellent in terms of achieving both the corrosion resistance of the plating film and the sacrificial corrosion resistance to the steel base, and the corrosion-resistant steel sheet has now reached considerable industrial production. ing.

[0004] Hot-dip coating of a thin steel sheet is generally performed in a continuous hot-dip plating facility in which a hot-dip plating bath is arranged on the outlet side of a continuous annealing facility. In a typical continuous hot-dip plating equipment, first, the surface is cleaned by heating in a weakly oxidizing non-oxidizing furnace, and then the steel sheet is guided to a reducing furnace following the non-oxidizing furnace.
Reduction and annealing are performed in an atmosphere containing hydrogen, and then the molten metal is allowed to enter a hot-dip plating bath without being exposed to the air to perform hot-dip plating. The steel sheet is shielded from the atmosphere from cleaning the surface to entering the plating bath, during which time degreasing,
Oxide reduction is performed and enters the hot-dip plating bath under conditions where the molten metal is easily wetted.

[0005] Such a continuous hot-dip galvanizing apparatus was developed for hot-dip galvanizing, but is also used for hot-dip plating of aluminum and hot-dip plating of Al-Zn alloy. That is, the hot-dip Al-Zn alloy plating can be operated using the hot-dip galvanizing equipment and method only by changing the plating bath composition and operating conditions.

On the other hand, hot-dip plating of steel materials other than thin steel sheets, for example, continuous hot-dip coating of steel materials such as wires, and batch-type hot-dip coating of steel materials such as structural members and various parts are performed by melting steel materials into a molten metal bath in the atmosphere. It has been done by immersion.
In this case, even if the steel material has been degreased and pickled beforehand,
Oxidation before entering the molten metal bath is unavoidable, so the oxide on the surface of the steel material that is unavoidably formed is melted using a flux consisting of a salt generally called a flux, and the molten metal in the hot-dip plating bath is melted. Means for accelerating the wetting of the steel sheet surface by the above have been used. Such a flux treatment method includes a dry method and a wet method.

The dry method is a method in which a flux is attached to a steel material as an aqueous solution, then dried to deposit the flux on the surface of the steel material, and then the steel material is immersed in a molten metal.

In the wet method, a flux is poured into a molten metal bath of a hot dip coating tank, a flux having a low specific gravity is floated on the molten metal, and a flux molten layer having an appropriate thickness is formed on the molten metal. This is a method in which a steel material is immersed in a molten metal via In this case, even when the steel material is pulled up from the molten metal, the steel material passes through the flux molten layer again, so that the adhesion of the flux is inevitable, and the operation of removing the flux from the plating surface after plating is required, and the operation becomes complicated.

In general, a dry method is employed for the flux treatment at the time of hot-dip galvanizing, and an aqueous solution containing zinc chloride and ammonium chloride is generally used as a fluxing agent. When this flux is used for Al-Zn alloy plating, Al in the molten metal is mainly NH ₄ C
As a result, sublimable AlCl ₃ is generated by reacting with l, and the flux is decomposed. As a result, the flux function is significantly impaired, and non-plating occurs frequently.

On the other hand, a wet method using a fluoride has been mainly used for the flux treatment at the time of hot-dip aluminum plating. However, since this flux has a high melting point, an Al—Zn alloy having a lower melting point than aluminum is used. Plating cannot provide a sufficient effect.

From this viewpoint, several fluxes for hot-dip Al-Zn alloy plating have been proposed. For example,
JP-A-58-136759 discloses a flux composition for plating an Al-Zn alloy comprising zinc chloride and at least one of alkali metal or alkaline earth metal chlorides, fluorides or silicofluorides. ing. Since this flux is used in a dry method, it is excellent in operability, but does not have a sufficient flux function, and non-plating tends to occur more frequently as the Al concentration in the molten metal increases. Therefore, this phenomenon becomes severe in the hot-dip plating of a 55% Al-Zn alloy having a high Al concentration, although the corrosion resistance is excellent.

JP-A-3-162557 discloses that the mixing ratio of zinc chloride and ammonium chloride is 10 to 30: 1 by weight.
Which discloses a flux composition for hot-dip plating of an Al-Zn alloy. This flux is also used by the dry method. However, although relatively good plating is possible for plating thin materials, non-plating tends to occur as the temperature of the plating bath rises, and the plating temperature rises to 55%. Al-Zn
In the case of alloy plating, non-plating is likely to occur except for thin materials.

Japanese Patent Application Laid-Open No. 4-293761 discloses a flux composition for hot-dip Al alloy plating comprising four components of zinc, lithium, sodium and potassium chlorides. This flux is used in a wet method in which a flux molten layer is suspended on a molten metal, and expensive lithium is the main component (40 to 60 mol%) of the above four components, which increases the cost.

JP-A-4-323356 discloses a flux composition for hot-dip Al-Zn alloy plating comprising an Al-containing alkali metal fluoride (eg cryolite) and an alkaline earth metal chloride. I have. This flux is also used for the wet method of floating the flux molten layer on the molten metal.
-It has been proposed as suitable for Zn plating, but the flux hanging phenomenon (flux solidifies to form a shelf, creating a cavity between the molten metal and the flux)
Is liable to occur. Further, since this flux contains a fluoride, there is also a problem that the flux attached and solidified when the steel material is pulled up from the molten metal cannot be easily removed by means of washing with water due to the presence of the fluoride. Therefore, the plating appearance becomes poor.

[0015] Other than these methods, for example,
As described in 201767, a two-step plating method in which steel material is previously subjected to hot-dip galvanizing and further to hot-dip Al-Zn alloy plating has also been proposed, but two plating steps are required, and the manufacturing cost is increased. It is clear that it is disadvantageous in this respect.

[0016]

As described above, in the hot-dip plating using the conventional flux composition, in the case of Al-Zn alloy plating in which the Al content is as high as 45% or more, the flux function is not obtained by the dry method. Sufficient and non-plating is easy to occur, and the wet method is problematic in that the flux is expensive, the flux is easily suspended on the shelf, the flux attached after plating is difficult to remove, and the appearance after plating deteriorates Therefore, it was extremely insufficient to industrially perform hot-dip Al-Zn alloy plating.

An object of the present invention is to provide a hot-dip plating method suitable for hot-dip Al-Zn alloy plating in which the problems of the prior art have been solved, and in particular, an Al content of 45 to 60.
% Hot-dip plating method for Al-Zn alloy plating.

[0018]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on the premise that a flux for a wet method having a high flux function is used, and as a result, a novel hot-dip plating method was used. We have successfully solved the above problem.

An outline of the principle of the solution will be described. In the present invention, first, a metal material to be plated, which has been subjected to an appropriate pretreatment, is immersed in a molten salt bath having a melting point higher than a plating bath temperature having a flux function. By the immersion treatment in the molten salt, the metal material is preheated, the surface is activated by the action of the flux, and a flux film is formed on the surface of the metal material when the metal material is pulled out of the molten salt flux bath. You.

Next, the metal material having the flux film is immediately immersed in a hot metal plating bath. Until immersion in the plating bath, the flux film has a function of protecting the metal material from oxidation.When the metal material is immersed in the molten metal plating bath, the flux film peels off from the surface of the metal material and melts in the plating bath. Floating on metal. If the melting point of the flux floating on the molten metal is lower than the bath temperature of the molten metal, the flux adheres to the plating surface when forming a liquid film on the molten metal and pulling up the metal material. However, if the melting point of the flux is higher than the bath temperature of the molten metal, the flux floats as a solid on the molten metal, so removal by skimming becomes extremely easy, so that adhesion of the flux at the time of lifting can be prevented, High quality hot-dipped products can be easily obtained.

The basic principle of the present invention is based on the above. The gist of the present invention is that, when applying a molten metal plating to a metal material, the metal material to be plated is at least 5 ° C. higher than the bath temperature of the molten metal plating bath in advance. A hot-dip plating method characterized in that after dipping in a molten salt flux bath having a melting point, the metal material is dipped in a hot-dip metal plating bath to perform hot-dip plating.

The present invention is directed to a product which is required to have high corrosion resistance without post-plating processing. In order to satisfy the corrosion resistance, it is sufficient to simply increase the amount of plating. Considering the viscosity of the molten metal, no matter how much the lifting speed is increased and the lifting amount is increased, 200
The limit is around g / m ² . Further, the upper limit is 150 g / m ^{2 in} consideration of the surface appearance such as plating dripping. Therefore, when the basis weight is 150 g / m ² or more, it is necessary to grow a Fe-Al-based alloy layer at the plating / steel interface at a uniform thickness as the basis weight increases. Was obtained.

In a preferred embodiment of the present invention, the molten metal is
An Al-Zn alloy containing 45-60% by weight of Al and 0.005% by weight or more and less than 0.5% by weight of Si, wherein the flux is composed of cryolite, at least one alkali metal chloride, and aluminum fluoride. Is recommended from the melting point of the molten metal.

[0024]

According to the present invention, hot-dip plating using a flux, particularly hot-dip Al-Zn alloy plating, preferably hot-dip Al-Zn alloy plating containing at least 40% of Al, is performed without any plating. Thus, hot-dip plating can be performed with good operability and excellent surface properties.

In the present invention, it is desirable to provide a flux tank for melting the flux in addition to the molten metal plating tank. A flux whose composition is adjusted to have a melting point higher than the bath temperature of the molten metal plating bath is put into the flux tank and melted. The melting point of the flux needs to be 5 ° C. or more higher than the hot-dip bath temperature, and is preferably 30 ° C.
Increase by at least ℃.

If the temperature difference between the melting point of the flux and the temperature of the hot-dip bath is less than 5 ° C., the solidification of the flux on the molten metal becomes insufficient, and the surface after plating tends to be contaminated with the flux. If the melting point of the flux is too high,
Since the preheating temperature of the metal material becomes too high and adverse effects appear, the difference between the melting point of the flux and the temperature of the hot-dip bath is preferably within 80 ° C, more preferably within 60 ° C.

In the conventional wet flux method, the flux composition is selected so that the melting point of the flux is lower than the bath temperature because the flux molten layer is suspended on the molten metal. In this respect, the present invention using a flux having a melting point higher than the hot-dip bath temperature has a completely different idea from the conventional wet method.

The type of the flux used in the present invention is not particularly limited as long as it has a flux function and has no volatility at the melting temperature of the flux. For example, halides of metals such as alkali metals, alkaline earth metals, aluminum and zinc, especially chlorides and fluorides, alkali metal fluorides and the like can be used. Usually, two or more compounds selected from these are used, and the mixture may be selected so that the melting point of the mixture is 5 ° C. higher than the hot-dip bath temperature.

The molten metal to be plated is 45-60% Al and
In the case of an Al-Zn alloy containing 0.005% or more and less than 0.5% of Si, the hot-dip plating bath temperature is usually 570 to 610 ° C. In this case, a combination of cryolite and at least one alkali metal chloride (eg, lithium chloride, sodium chloride, potassium chloride) or a mixture of cryolite and cryolite (alkali metal and alkali metal chloride) is used as the flux. The combination to which aluminum chloride is added shows a sufficient flux function even in such an Al-Zn alloy plating having a high Al content, and a composition having a melting point higher by 5 ° C. or more than the bath temperature can be easily selected. It is preferable because it is possible.

However, even in this case, the composition of the flux is not limited to the above, and a composition not using cryolite is also possible. For example, alkali metal chloride and alkali metal fluoride alone have a flux function,
A mixture having a melting point higher than the plating bath temperature by 5 ° C. or more can be obtained. However, in that case, it is necessary to use a large amount of lithium chloride having a low melting point, so that the cost is increased and the flux function is slightly inferior to that of the mixture using cryolite.

The metal material to which the hot dip coating can be applied by the method of the present invention is not particularly limited, but typical examples are steel materials (eg, shaped steel, deformed steel pipe, steel fittings, etc.). For example, as a building material for roofs and outer walls, not only in areas with strong salt-corrosion such as seaside areas, but also in other areas, hot-dip Al-Zn alloy-plated steel sheets with high corrosion resistance, especially 55% Al-Zn
Alloy plated steel sheets have come to be used, but if the same plating is applied to the small joining members that join these steel sheets, the corrosion resistance of these joining members will be ensured, and at the same time, dissimilar metal materials Also, the dissolution of the plating material due to the local battery action which occurs when the contact is made can be prevented, and the plating durability is also improved. The hot-dip plating method of the present invention can be applied to Al-Zn alloy plating of such a small joint member or to a large member such as a shaped steel.

The metal material to be hot-dip is desirably subjected to a usual plating pretreatment before being immersed in the molten salt flux bath in the flux tank according to the present invention. For example, such a plating pretreatment when the metal material is steel is a degreasing step using a warm aqueous solution of sodium orthosilicate, caustic soda, sodium carbonate, a degreasing step using an organic solvent,
It includes at least one step of pickling with an aqueous solution of an acid such as hydrochloric acid or sulfuric acid.

The temperature of the molten salt flux bath in the flux tank is not particularly limited as long as it is higher than the melting point of the flux, and if a suitable temperature control mechanism similar to that of the plating tank is provided, even if the temperature is several degrees higher than the melting point. Can operate fully. If the temperature of the molten salt flux bath is too high, it is disadvantageous in terms of thermal energy, and may cause thermal degradation of the metal material to be plated. More preferably, the temperature should be within 60 ° C.

The immersion time in the flux bath is very short,
For example, the time may be from one second to several seconds. However, since the immersion also serves as preheating, when the thickness of the metal material to be plated is large, the immersion time may be extended to sufficiently preheat.

As described above, since the surface of the metal material that has exited the flux tank is protected by the flux, the surface does not oxidize even when exposed to the atmosphere. Therefore, there is no need to shut off the atmosphere during the transfer from the flux tank to the hot-dip plating tank. However, in order to prevent the temperature of the metal material preheated in the flux tank from lowering, it is preferable that the transfer from the flux tank to the hot-dip plating tank be performed promptly.

The hot-dip plating tank is preferably provided with a conventional skimming means. In the present invention, since the melting point of the flux is higher than the temperature of the hot-dip bath, the flux removed from the metal material in contact with the hot-dip bath is solidified in the bath and floated as a solid on the hot metal in the bath. Therefore, it can be easily removed by skimming. When the hot-dip plating is of a batch type, skimming may be performed between plating operations. In continuous plating of a wire or the like, skimming can be performed periodically or constantly as necessary. As a result, the flux hardly adheres to the plating surface of the metal material pulled up from the molten metal plating bath, so a special treatment for removing the flux after plating, which is performed by the conventional wet flux method, is not required. Usually unnecessary.

According to the present invention, an aluminum-zinc alloy having a composition of metal plating bath of 45 to 60% Al, 0.005% to less than 0.5% Si, and substantially Zn is obtained.
As mentioned above, to have high corrosion resistance, Al
This is because the Al-Zn alloy composition is excellent. Regarding the amount of Si in the metal plating bath, it is important for the production of a thick material having an excellent surface appearance. When the amount of Si is 0.005% or more and less than 0.05%, Fe-Al
This has the effect of uniformly growing the interface alloy layer. Where Si
If the amount is less than 0.005%, when the metal material is immersed in a molten metal bath, the Fe-Al-based intermetallic compound grows abnormally, so that irregularities are severe and the plating appearance is rough. Since the growth of the Al-based alloy layer is suppressed, it becomes difficult to produce a thick material. Also, trace amounts of Mg, P
Elements such as b, Sn, Fe, Sb, Mn, Ni, Cr, Ti, V, and Sr may be included.

Amount of adhesion: In order to improve the corrosion resistance, it is sufficient to simply increase the amount of adhesion. For this reason, a thicker material is demanded. If the adhesion amount is 150 g / m ² or more per side, the corrosion resistance in a severely salted area can be improved by using a conventional steel in a mild environment (weight per area: 75 g per side). / m ² ) Can be improved as much. Although the upper limit of the amount of adhesion is not particularly limited, it is considered that 400 g / m ² is economically the limit. Chromate treatment may be performed to further improve the corrosion resistance after plating.

According to the present invention, the metal material to be plated is preheated in the flux tank to a temperature higher than the plating bath temperature.
It is possible to drastically reduce the immersion time in the hot-dip plating bath, which required a considerable amount of time in the conventional method.

[0040]

EXAMPLES [Example 1] Hereinafter, the present invention will be described more specifically based on examples. In this example, first, 200 mm × 300
Prior to the flux treatment, a hot rolled steel sheet of mm x 2.5 mm was degreased in an aqueous sodium orthosilicate solution, washed with water, and then 10 wt%
It was pickled with an aqueous hydrochloric acid solution to perform a plating pretreatment. Flux treatment was performed following such plating pretreatment,
In this example, flux treatment was performed in the following manner.

That is, a flux tank was installed near the hot-dip plating tank prepared according to the present invention, and each flux having the composition shown in Table 1 was melted therein.
The steel sheet that had been subjected to the above-described plating pretreatment was immersed in the flux tank for 1 minute to perform preheating and flux treatment, and then the steel sheet pulled up from the flux tank was immersed in the hot-dip plating tank as quickly as possible.

[0042]

[Table 1]

The temperature of the molten salt flux in the flux tank was 630 ° C. except for flux Nos. 1 and 2.
And In the case of flux Nos. 1 and 2, the temperature was 5 ° C. higher than the melting point.

The hot-dip metal is 55% Al-0 to 0.08% Si
-Zn alloy, and the temperature of the hot-dip plating bath was changed in the range of 580 to 630 ° C. The immersion time in the plating bath was unified to 1 minute.

The steel sheet pulled up from the hot-dip plating bath was cooled with water, washed with water and brushed, and then visually inspected for non-plating and appearance (adhesion of flux and rough skin). The adhesion amount was determined by immersing the film in a 10% hydrochloric acid solution containing an inhibitor and determining the weight difference before and after dissolution of the film. The results are summarized in Table 2.

[0046]

[Table 2]

[0047]

As described above, according to the hot-dip plating method of the present invention utilizing a wet method in which a flux bath is separately provided, it has been conventionally difficult to obtain a good plating appearance by the flux method. In the Al-Zn alloy plating, it becomes possible to obtain a good plating appearance without contamination, and the function of the flux is sufficiently exhibited, and the occurrence of non-plating is prevented.

Further, according to the hot-dip plating method of the present invention, the flux treatment also serves as preheating, so that preheating before plating is not required, and the plating time is greatly shortened even in the production of a thicker material having high corrosion resistance. The same effect can be obtained. Further, the removal of the flux after plating, which is required in the conventional wet flux method, is not required, and the operability is improved.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroto Masumoto 1850 Minato, Wakayama Sumitomo Metal Industries, Ltd. Inside Wakayama Works (72) Inventor Hiroo Maeda 2-4-8 Fujinoki, Wakamatsu-ku, Kitakyushu-shi Shinko Almar Industries Inside the corporation

Claims (5)

[Claims] 1. A hot-dip plating method for a metal material, comprising immersing a metal material in a molten salt flux bath and immersing it in a molten metal plating bath to perform hot-dip plating, wherein the flux composition of the molten salt flux bath is Has a melting point at least 5 ° C. higher than the temperature of the molten metal plating bath,
And the composition of the hot-dip metal plating bath is Al: 45-60%,
i: A hot-dip plating method comprising an aluminum-zinc alloy of 0.005% or more and less than 0.5%, with the balance being substantially Zn. 2. The hot dip plating method according to claim 1, wherein the molten salt flux bath and the molten metal plating bath are provided in separate tanks. 3. The hot-dip plating method according to claim 1, wherein the molten salt flux bath has a melting point at a temperature not exceeding 80 ° C. from the temperature of the hot-dip metal plating bath. 4. The composition of the molten salt flux bath is a combination of cryolite and at least one alkali metal salt or a combination of cryolite, at least one alkali metal salt and aluminum fluoride. The hot-dip plating method according to any one of claims 1 to 3, wherein 5. The hot-dip plating method according to claim 1, wherein the coating weight is 150 g / m ² or more per one side.