JP3528403B2 - Method for producing hot-dip Zn-Al alloy-plated steel with high corrosion resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high corrosion resistance molten Zn
-In connection with the method for producing an Al alloy plated steel material, more specifically, by increasing the Al concentration in the plating bath on the surface of the steel material regardless of whether it is high Si steel or low Si steel, the corrosion resistance is very high in a relatively low temperature range. The present invention relates to a method for producing a highly corrosion-resistant hot-dip Zn-Al alloy-plated steel material capable of forming a Zn-Al alloy plating film by a one-bath method.

[0002]

2. Description of the Related Art Hot-dip Zn plating has been widely used in building structures, power transmission towers, bridges, and other road facilities because it has excellent corrosion resistance, economy, and workability. Further, in recent years, the social capital is being enhanced, and it is an important subject to improve the durability and prolong the life of steel materials, and the role of hot-dip Zn plating is becoming more and more important.

Pollution control measures by the national government and companies have been successful, and sulfur dioxide and the like emitted from factories have decreased sharply since the 1950s, but the number of automobiles has increased along with economic growth and is included in the exhaust gas. Air pollution caused by nitrogen oxides and sulfur oxides is increasing, and the effect on metal corrosion is also increasing. Specifically, it appears in the form of acid rain, and countermeasures are urgently needed. In addition, since the snow melting agent is sprayed on the road in winter, the corrosion caused by these is also a big problem for the steel materials for road facilities. Furthermore, Japan, which is surrounded by the sea, is rapidly developing the bay area for effective use of the coastline, but the corrosion of steel structures due to sea salt particles is severe, so countermeasures against salt damage are necessary. Therefore, in order to prevent corrosion of various steel structures from air pollutants and salt damage, molten Zn
Various rust preventive measures such as plating and heavy coating are taken.

[0004] However, the plated film formed by hot-dip Zn plating shows excellent corrosion resistance in rural areas and urban suburbs. For example, if the thickness of the plated film is 80 μm (576 g / m ² ), it has a durability of 50 years or more, but sea salt. Particles come in directly, Zn
In an environment where it adheres to the plated structure, the steel base is locally exposed in just 3 years and red rust occurs. Therefore, steel materials for road facilities such as bridges and building structures, which are constructed in the coastal area, are required to form a plating film having higher corrosion resistance.

In recent years, the use of hot-dip Zn plating has expanded, and it has been widely used as a drainage ditch cover for roads and a sand-scattering prevention plate for coastal areas. The groove cover is damaged by the tire as the tire is constantly abraded by the passage of automobiles. In addition, in the case of a sand-scattering prevention plate, the sand particles that are carried by the strong sea breeze are struck, so the surface of the prevention plate becomes as if it had been subjected to sandblasting, and it is further corroded by sea salt particles, creating a severe environment. Will be exposed to. Under the condition that such a physical force acts, the hot-dip Zn plating film is soft, and especially the surface zinc layer is soft, so that there is a weak point that the plating film is rapidly worn.

Therefore, hot-dip Zn-Al alloy plating, which is more excellent in corrosion resistance than hot-dip Zn plating, has come to the fore, and it has been partially put to practical use in special applications. It has been known for a long time that the addition of aluminum to zinc significantly improves the corrosion resistance. For thin steel sheets, in the 1970s, Zn-55% Al-1.6% Si alloy plating galvanium and Zn-5% Al were mixed. A metal-added Gulfan etc. has been developed and plated by the non-oxidizing furnace method. In addition to thin steel plates, wire rods, overhead wire fittings, and furring strip angles have been partially adopted, but molten Zn-Al alloy plating is poor in adhesion with steel materials, and thus, in the past, was first subjected to molten Zn plating, The surface is subjected to a two-step plating process such as hot dip Zn-Al alloy plating, that is, a two-bath method. As described above, the molten Z currently in practical use
N-Al alloy plating is not a method of directly plating in the open to the atmosphere, and since the plating process consists of two processes, the processing cost is high, the cost is high, and its spread is delayed. On the other hand, a so-called one-bath method in which the alloy plating is performed by pre-treatment such as pickling as in the conventional batch type hot dip Zn plating and then immersing the alloy in a Zn-5% Al alloy plating bath is also tried. ing.

The reason why the surface of the steel material cannot be directly subjected to the hot dip Zn-Al alloy plating is that selective oxidation of aluminum occurs on the surface of the hot dip Zn-Al alloy plating bath to prevent contact between the steel material and the plating bath components. This is because zinc chloride or ammonium chloride used as a flux in the hot-dip Zn plating reacts with aluminum to reduce the flux effect. Therefore, the molten Zn-
When Al alloy plating is performed, a plating film defect such as non-plating occurs, and a good plating film is not formed, so that practical application of molten Zn—Al alloy plating for large structures has been hindered. However, as a result of many years of research, the present inventors have developed a new flux as disclosed in Japanese Patent Laid-Open No. 202751/1992, and have developed a one-bath method capable of performing molten Zn-Al alloy plating directly in the atmosphere. Made possible That is, this flux is composed of one or more of zinc chloride, stannous chloride, an alkali metal chloride and an alkaline earth metal chloride, and an alkyl quaternary ammonium salt and an alkyl which are aliphatic nitrogen derivatives. One or two or more of the amines are included.

[0008]

Molten Zn produced by the one-bath method
-5% Al alloy plating, the thickness of the plating film is 50μ
In order to obtain m (about 345 g / m ² ) or more, it is necessary to set the plating temperature to 500 ° C. or more, which causes heat distortion in the steel product to be plated, which is a quality problem. In addition, in hot-rolled steel materials such as angle steel and H-shaped steel, non-uniformity of plating film thickness occurs, which becomes a cause of impairing appearance and corrosion resistance.

The effect of the plating temperature on the growth of the Fe-Al-Zn alloy phase is large, and a dense alloy layer composed of the Fe-Al-Zn alloy phase may be formed at a plating temperature exceeding 500 ° C. Such a plating film has a drawback that it is prone to pitting corrosion, which is a practical problem. Therefore, the development of a method for producing a highly corrosion-resistant hot-dip Zn-Al alloy-plated steel material by the one-bath method, which is economically superior to the two-bath method, can obtain a plated film having a good appearance at a relatively low temperature and a uniform thickness, has been developed. It has been demanded.

[0010]

When the Al concentration in the plating bath is 5% by weight and the plating temperature is 500 ° C. or lower, the coating weight is 140 to 210 g even if the immersion time is long.
It is as small as / m ² (about 20 to 30 μm), and it is difficult to form a thick plated film required for a steel structure used in a severe corrosive environment. However, it has been found that when the Al concentration exceeds 10% by weight, a plating film of 50 μm (about 350 g / m ² ) or more is formed even at a plating temperature near 480 ° C. Al concentration is 20% by weight
Then, although the plating film is formed thicker, the appearance of the plating film is deteriorated and it is not economical, so the upper limit of the Al concentration is set to 20% by weight.

In the present invention, a steel material is fluxed by the flux (Japanese Patent Laid-Open No. 4-202751) already provided by the applicant of the present invention which enables the hot dip Zn-Al alloy plating to be favorably carried out by the one-bath method. After the treatment, an Al concentration of 10 to 20% by weight and a balance of Zn and inevitable impurities in a plating bath with a plating temperature of 450 to 520 ° C. and an immersion time of 0.5 to 10 minutes ( However,
( Excluding the range where the coating weight is less than 450 g / m ² ) , set the plating conditions so that the coating weight becomes 450 g / m ² or more, immerse in an open atmosphere, pull up from the plating bath , and then cool. It is a method of cooling with water . The film thickness (deposition amount) of the plating film can be controlled by changing the plating temperature and the immersion time if the Al concentration is constant,
Further, the alloy structure of the plating film can be controlled to some extent by changing the cooling conditions. In addition, M in the plating bath
It is possible to further improve the corrosion resistance by adding a small amount of g. The addition of Mg does not affect the amount of plating adhered.

The steel materials targeted by the present invention are high Si steels (Si: 0.18% by weight or more), which are often found in type steels such as mountain steels and H-shaped steels, and low Si steels, which are mainly present in steel plates and steel wires. (Si: 0.
However, the effect is remarkable in high Si steel where it is more difficult to form a thick plating film. The alloy structure of the plating film of high Si steel is Z
In the case of the n-5% Al bath, the Zn-Al alloy and the Fe-Al-Zn alloy in the form of particles are dispersed in the Zn matrix. In the case of a Zn-10% Al bath, a Fe-Al-Zn alloy is formed in a portion close to the steel base material, and in the surface layer portion, Zn-rich Fe- is contained in a matrix in which Zn and an alloy having the same bath composition are dispersed. This is a form in which an Al-Zn alloy is formed. In the case of a Zn-15% Al bath, an Fe-Al-Zn alloy is formed in a portion close to the steel material base,
In the surface layer portion, a eutectic of a Zn-rich Al-Zn alloy (β phase) having an Al concentration of about 7% is provided in a gap between crystals of an Al-rich Al-Zn alloy (α phase) having an Al concentration of about 25%. It is a form filled with. Zn In the case of a 20% Al bath, Zn
The form is the same as that of the -15% Al bath. Zn-30
In the case of% Al bath, various Fe-Al with different composition concentration
A form in which the Zn alloy is dispersed throughout.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing a highly corrosion-resistant hot-dip Zn-Al alloy-plated steel material of the present invention having the above contents is as follows.
A method for producing a plated steel material, wherein a steel material is immersed in a molten Zn-Al bath by a one-bath method to form a highly corrosion-resistant Zn-Al alloy plating film on the surface of the steel material; at least a chloride and an aliphatic nitrogen derivative. And one or more of zinc chloride, stannous chloride, an alkali metal chloride and an alkaline earth metal chloride as a chloride, and an alkyl quaternary ammonium salt as an aliphatic nitrogen derivative, and off to the one or more of the alkyl amines as the main constituent
A flux treatment step of immersing in a Lux ; a plating bath having an Al concentration of 10 to 20% by weight and the balance of Zn and inevitable impurities in a plating temperature range of 450 to 520 ° C. and an immersion time of 0.5 to 10 minutes Range (however, plating adheres
( Excluding the range where the amount is less than 450 g / m ² ) and the plating bath immersion step in which the plating conditions are set so that the coating adhesion amount is 450 g / m ² or more, and the plating bath is immersed in the atmosphere.
In order to suppress the development of the Zn-Al alloy layer to the surface layer of the plating film, the cooling step is performed by pulling the Zn-Al alloy layer out of the plating bath and immersing it in cooling water .

Here, the composition of the plating bath is such that the Al concentration is 10 to 20% by weight, the Mg concentration is 0.05 to 2% by weight,
It is more preferable that the balance is Zn and inevitable impurities in terms of corrosion resistance.

Further, as a plating condition in the plating bath dipping step, it is more preferable that the plating temperature is 470 to 500 ° C. and the plating time is 1 to 7 minutes in order to reduce the thermal effect on the steel material.

[0016]

Example First, a test piece (material: Si content 0.2%)
(High Si steel), size: 30 × 75 × 3.2 mm),
Alkali degreasing (15% by weight of caustic soda, 15% by weight of sodium orthosilicate, 70 ° C, 20 minutes), pickling (10% by weight sulfuric acid aqueous solution, 50 ° C, 60 minutes), pretreatment, and flux treatment And then the Al concentration is 10% by weight
And 15% by weight, the balance being Zn and inevitable impurities, and two types of plating baths were used, and the plating temperature was 450 to 540 ° C.
The temperature was changed by 10 ° C. up to 10 ° C., and the immersion Zn-Al alloy plating was performed under each plating condition for the immersion time of 1, 3, and 5 minutes, and the coating adhesion amount (g / m ² ) was measured. The results are shown in Table 1 and FIG.
And shown in FIG.

[0017]

[Table 1]

From this result, the Al concentration is 10% by weight.
It was confirmed that a coating amount of 450 g / m ² or more can be achieved even when the plating temperature is 500 ° C. or lower by controlling the plating temperature and the immersion time by using a Zn-Al alloy plating bath of 15 wt% and.

The flux used for the hot dip Zn-Al alloy plating is that disclosed in JP-A-4-202751, which is composed of at least chloride and an aliphatic nitrogen derivative, and zinc chloride as chloride. (ZnC
l ₂ ), stannous chloride (SnCl ₂ ), one or more of alkali metal chlorides and alkaline earth metal chlorides, and an alkyl quaternary ammonium salt and alkyl as an aliphatic nitrogen derivative. One or two or more of the amines are the main constituent components.

As the alkali metal chloride, chlorides of alkali metals such as lithium, sodium and potassium are used, and as the alkaline earth metal chlorides, there are alkaline earth chlorides such as beryllium, magnesium, calcium, strontium and barium. Metal chlorides are used.

The alkyl quaternary ammonium salt is preferably alkyltrimethylammonium chloride or dialkyldimethylammonium chloride,
Particularly, those having an alkyl group having 7 to 18 carbon atoms are preferable. That is, examples of the alkyl group include octyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, octadecenyl, octadecadienyl and the like.

Examples of alkylamines include aliphatic primary amines such as methylamine and ethylamine, aliphatic secondary amines such as dimethylamine and diethylamine, and aliphatic tertiary amines such as trimethylamine and triethylamine. Those having an alkyl group of the number 1 to 18 are preferable.

Specifically, the flux for hot-dip Zn-Al alloy plating contains 10 parts by weight of zinc chloride and / or stannous chloride.
˜50% by weight and 0.1 to 30 of one or more of alkyl quaternary ammonium salts and alkyl amines.
1 to 2 of one or two or more selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides.
Containing 0% by weight and 0.1 to 30% by weight of one or more of alkyl quaternary ammonium salts and alkylamines, zinc chloride and / or stannous chloride
0 to 50% by weight, 1 to 20% by weight of one or more of chlorides of alkali metals and chlorides of alkaline earth metals, and 1 to 20% by weight of alkyl quaternary ammonium salts and alkylamines The thing containing 0.1-30 weight% of 1 type (s) or 2 or more types can be used.

Here, the optimum concentration of zinc chloride and / or stannous chloride is 30 to 40% by weight. For alkali metal chlorides and alkaline earth metal chlorides, for example, the plating temperature is 400 ° C. Within the range of up to 600 ° C., ZnCl ₂ : NaCl = 4: 1 (molar ratio), ZnC
l ₂ : CaCl ₂ = 3: 1 (molar ratio) is preferred,
The optimum concentration is 5 to 10% by weight. Further, the addition amount of the alkyl quaternary ammonium salt and the alkyl amines is optimally 1 to 10% by weight.

Here, the action of each composition used for the flux will be briefly described. First, zinc chloride or stannous chloride is
It is for dissolving the thin oxide layer remaining on the steel surface and the oxide film formed on the plating bath surface. The alkali metal chloride or the alkaline earth metal chloride is for maintaining the molten state of the flux having an appropriate viscosity at the plating temperature. Molten Zn containing one or more of alkyl quaternary ammonium salts and alkyl amines in the flux as an aliphatic nitrogen derivative
-By using a flux for Al alloy plating, when a steel material is immersed in a hot-dip Zn plating bath, Hofmann decomposition is caused on the surface of the steel material to foam, and burnt residue of the flux adhered to the surface of the steel material is promptly removed from the surface. By separating and removing it, it has an effect of improving the wettability between the surface of the steel material and the molten metal, and the adhesiveness and appearance of the plating film are significantly improved.

Next, as shown in Table 2, low-Si steel and high-Si steel were melted under various plating conditions by the one-bath method.
l alloy plating was performed, and the corrosion weight loss (g / m ² ) of each of them was measured by a salt spray test to evaluate the corrosion resistance.
In the plating time in Table 2, A is the time from when the test body starts to be immersed in the plating bath until it is completely immersed, B is the actual immersion time, C is the time from the start of pulling up to the time when it is completely pulled out from the plating bath, and C Is the time to stay in the air before being immersed in cooling water. A part of the result of this salt spray test is shown in FIG. 3 together with other comparative examples. In the figure,
Hi-Si indicates high Si steel and Low-Si indicates low Si steel.

[0027]

[Table 2]

From these results, it was found that the Zn-15% Al alloy plating bath was superior in corrosion resistance to the Zn-10% Al alloy plating bath, and the addition of Mg was superior to the Zn-10% Al alloy plating bath.

FIG. 4 is a graph showing the corrosion weight loss of the high-Si steel specimen plated under the conditions shown in Table 2 after the salt spray test for 1000 hours and 2000 hours, with the Al concentration in the plating bath as the horizontal axis. It is a representation. Before and after the Al concentration is around 15% by weight, the corrosion weight loss becomes very small.
It shows a low peak value in the range of up to 20% by weight.
Although the corrosion weight loss sharply decreases from the Al concentration of 10% by weight to 15% by weight, 10% by weight is sufficiently practical. Further, even if the Al concentration is 30% by weight, the corrosion weight loss is small, but it is not practical in terms of the appearance of the plating film.

FIG. 5 shows the corrosion weight loss of a low Si steel test piece plated under the conditions shown in Table 2 after a salt spray test for 1000 hours, with the Al concentration in the plating bath as the horizontal axis. Is. According to this, the corrosion weight loss is Al concentration 5
Although it tends to decrease from 30% by weight to 30% by weight,
When the Al concentration is 30% by weight, the appearance of the plating film is poor and it is not practical as in the above case.

6 and 7 show the results of measuring the polarization resistance R _P of the high-Si steel specimen plated under the conditions shown in Table 2 by the Klostat method and the impedance method, respectively. The larger the value of the polarization resistance R _P, the higher the corrosion resistance, but both measurement methods show the highest value when the Al concentration is 15% by weight, and show that the corrosion resistance is good around 15% by weight. The results also match the results of the salt spray test.

From the above, considering the corrosion resistance, appearance and economy of the plating film, the Al concentration of the plating bath is 1
It can be said that 0 to 20% by weight is suitable. Furthermore, it is more preferable to add 0.05 to 2% by weight of Mg to the plating bath because the corrosion resistance is further improved.

[0033]

The high corrosion resistance molten Z of the present invention as described above
According to the method for producing an n-Al alloy-plated steel material, since it has a low melting point and a low viscosity, it has the advantages of a good surface finish and a low plating temperature, and has a good corrosion resistance. It has become possible to manufacture a plated steel material having more excellent corrosion resistance as compared with Al alloy plating. The present invention, while increasing the Al concentration of the plating bath and setting the plating temperature to a relatively low temperature,
It was possible to improve the corrosion resistance by setting the amount of Zn-Al alloy plating film to be 450 g / m ² or more, which was impossible in the past. In addition, the present invention is based on
By adding, it was possible to improve the physical properties of the plating film itself and improve the corrosion resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of adhesion and the plating time for each plating time using a Zn-10% Al alloy plating bath.

FIG. 2 is a graph showing the relationship between the amount of deposition and the plating time for each plating time using a Zn-15% Al alloy plating bath.

FIG. 3 is a graph showing the results of a salt spray test of a test body that has been subjected to hot dip Zn-Al alloy plating under various conditions.

FIG. 4 is a graph showing corrosion weight loss of high-Si steel test pieces plated under the plating conditions shown in Table 2 after a salt spray test for 1000 hours and 2000 hours, with the Al concentration in the plating bath as the horizontal axis. is there.

FIG. 5 shows the corrosion weight loss of a low Si steel test piece plated under the plating conditions shown in Table 2 after a salt spray test for 1000 hours.
6 is a graph showing the Al concentration in the plating bath on the horizontal axis.

FIG. 6 is a graph showing the results of measuring polarization resistance of a high Si steel test body plated under the plating conditions shown in Table 2 by the Klostat method.

FIG. 7 is a graph showing the results of measuring polarization resistance by the impedance method.

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-6-158257 (JP, A) JP-A-5-195179 (JP, A) JP-A-5-148602 (JP, A) JP-A-4- 202751 (JP, A) JP-A-2-175852 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C23C 2/00 -2/40

Claims (3)

(57) [Claims] 1. A method for producing a plated steel material, which comprises immersing a steel material in a molten Zn-Al bath by a one-bath method to form a highly corrosion-resistant Zn-Al alloy plating film on the surface of the steel material, which comprises at least a chloride. And one or more of zinc chloride, stannous chloride, chlorides of alkali metals and chlorides of alkaline earth metals as chlorides, and alkyl nitrogen as an aliphatic nitrogen derivative. from a quaternary ammonium one of the salts and alkyl amines or more and a flux treatment step of immersing the flux whose main component, Al concentration of 10 to 20 wt%, the balance being Zn and unavoidable impurities Plating temperature of 450-520
C range and immersion time range of 0.5 to 10 minutes (however,
( Excluding the range where the coating weight is less than 450 g / m ² )
Suppress the development of Zn-Al alloy layer up to the surface layer of the plating film and the plating bath dipping process in which the plating conditions are set so that the coating weight is 450 g / m ² or more In order to do so, a cooling step of pulling out from the plating bath and immersing in cooling water , and a method for producing a highly corrosion-resistant molten Zn-Al alloy plated steel material comprising: 2. The composition of the plating bath has an Al concentration of 10
20% by weight, Mg concentration is 0.05 to 2% by weight, and the balance is Zn and inevitable impurities. 3. The high-corrosion-resistant molten Z according to claim 1, wherein the plating conditions in the plating bath immersion step are a plating temperature of 470 to 500 ° C. and a plating time of 1 to 7 minutes.
A method for manufacturing an n-Al alloy plated steel material.