KR100525907B1 - Manufacturing method of galvannealed steel sheets - Google Patents

Abstract

The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet whose surface appearance is as beautiful as an electroplated steel sheet and having excellent paintability. The present invention relates to a method of manufacturing an alloyed hot-dip galvanized steel sheet, injecting nitrogen when the cold rolled strip passes through an air knife, and Install a sealing chamber in the lower part and install a hydrogen-nitrogen mixed gas injector at the lower part, and install the nitrogen curtain at the inlet and outlet of the sealing chamber to maintain the reducing atmosphere in the sealing chamber at a positive pressure (+ pressure) to the upper part of the air knife The main method is a method for producing an alloyed hot-dip galvanized steel sheet, characterized in that passing through the sealing chamber is formed in a reducing atmosphere so as to be blocked from the atmosphere until the molten zinc is solidified.

Description

Manufacturing method of alloyed hot dip galvanized steel sheet {Manufacturing method of galvannealed steel sheets}

The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet using a reducing atmosphere, and more particularly, to a method for manufacturing an alloyed hot-dip galvanized steel sheet having a beautiful surface appearance like an electroplated steel sheet and excellent coating properties.

In general, the alloyed hot-dip galvanized steel sheet is plated in a continuous hot-dip plating process, and as shown in FIG. 1, the cold rolled strip 1 has a snout 2 installed on top of the hot-dip galvanizing bath 3. By passing through the sink roll (4) installed in the plating bath (3), the direction is upward, and hot-dip galvanized, to adjust the amount of plating by the air knife (5) installed on the upper portion of the plating bath (3). do. The strip 1 in which the coating amount of the coating is adjusted in the air knife 5 is heated and alloyed in the alloying furnace 6 installed in the upper portion of the air knife 5, and then cooled in an air cooling zone to be top rolled (deflector). The alloying reaction is completed by passing through the roll 7.

The plated layer of the alloyed hot-dip galvanized steel plated as described above has an iron content of about 10 to 12%, depending on the iron content, zeta phase (ζ, FeZn13), delta phase (δ, FeZn7), and capital gamma phase (Γ, Fe 3 Zn 10), and workability and paintability vary greatly depending on the composition ratio thereof. In particular, the presence of the capital gamma phase is preferably suppressed as much as it causes powdering during molding. In general, the production of hot-dip galvanized steel sheet contains 0.14 ~ 0.2wt% of aluminum in the plating bath to form a ternary alloy of iron-aluminum-zinc to exist at the interface between the base iron and the plating layer to improve the adhesion. However, in the manufacture of alloyed hot-dip galvanized steel sheet, the aluminum in the plating bath is managed to 0.14 wt% or less for the smooth alloying reaction of zinc and iron.

The alloyed hot-dip galvanized steel sheet thus manufactured is widely used in automobiles, home appliances, etc. because the weldability and corrosion resistance after coating is superior to the galvanized steel sheet, and in recent years, the demand for automotive exterior plates is increasing. In particular, strict surface quality is required to be applied for automotive exterior.

The formation of the plating layer of the alloyed hot-dip galvanized steel sheet is first formed of zinc-iron intermetallic compound while zinc adhering to the base iron is redissolved during the alloying treatment, and then the iron particles of the base iron diffuse into the zinc plating layer. A plating layer is formed.

However, since the zinc plating layer passing through the plating bath combines with oxygen in the air to form a thin oxide film on the surface layer, when the zinc plating layer is re-dissolved during the alloying process, the oxide coating layer is not dissolved but remains solid. Dissolved zinc flow is not smooth. That is, the zinc oxide layer of the surface layer hinders the flow of molten zinc and finally solidifies in a rough state when the alloying process is completed, thereby preventing the appearance of the surface.

In order to solve this problem, a method of suppressing an oxide layer of zinc using a plating bath and a method of sealing an air knife is disclosed in US Pat. No. 4,557,952. However, this method has a problem that since the air knife in the sealing box is present, when the zinc scattered during air wiping is attached to the lip of the air knife, it is not easy to remove them. In addition, since the inside of the sealing box maintains a non-oxidizing atmosphere for iron and maintains oxidative properties for zinc steam, there is a problem in that the formation of an oxide film on the surface of the hot dip galvanizing cannot be suppressed during solidification. In addition, the Republic of Korea Patent Application No. 1999-62483 has the advantage of suppressing the formation of the oxide film during solidification by maintaining zinc as a reducing component crisis, but like the US patent, air knife inside the sealing box is attached to the lip portion There is a problem that is difficult to remove the zinc.

Accordingly, the present inventors have proposed a method capable of easily removing zinc adhering to the lip of the air knife and also suppressing the formation of an oxide film during solidification by maintaining a reducing component crisis. The present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet having excellent surface properties as well as beautiful surface appearance by changing the reducing gas from oxidative to reducing the atmosphere gas during the alloying process while maintaining the properties of the existing alloyed hot-dip galvanized steel sheet. Its purpose is to.

The present invention for achieving the above object is to inject nitrogen when the cold rolled strip (1) passes through the air knife (5), the sealing chamber (8) in the upper alloying furnace (6) of the air knife (5) And a hydrogen-nitrogen mixed gas input device 10 at a lower portion thereof, and an inlet and an outlet of the sealing chamber 8 are installed with a nitrogen curtain 9 so that a positive pressure is reduced in the sealing chamber 8. It is maintained at + pressure) to pass through the sealing chamber (8) having a reducing atmosphere is formed so as to block the atmosphere from the upper end of the air knife to the time of the molten zinc solidified provides a method for producing an alloyed hot-dip galvanized steel sheet do.

Hereinafter, the present invention will be described in detail.

The present invention maintains the surrounding atmosphere in a reducing atmosphere of nitrogen-hydrogen during alloying, thereby smoothing the flow of zinc due to re-dissolving of zinc during alloying, and having a beautiful surface appearance. It is characterized by having excellent paintability than conventional alloyed hot dip galvanized steel sheet.

In the manufacturing process of the alloyed hot-dip galvanized steel sheet of the present invention, as described in FIG. 1, the cold rolled strip 1 passes through the snout 2 installed on the hot-dip galvanizing bath 3 to form a plating bath 3. As the direction is advanced upward by the sink roll 4 installed therein, hot dip galvanizing is performed, and the coating amount of the plating is controlled by the air knife 5 installed on the upper portion of the plating bath 3. After that, the strip 1 in which the plating amount is adjusted in the air knife 5 is alloyed by heating the plated layer in the alloying furnace 6 installed at the upper portion of the air knife 5, wherein the strip 1 is air knife. In order to suppress the formation of a thin oxide film by combining with oxygen in the atmosphere on the surface of the zinc plated layer when passing through (5), nitrogen is injected to suppress the formation of an oxide film on the surface as much as possible, and the upper alloying of the air knife 5 is performed. The furnace 6 installs a sealing chamber 8 surrounding the alloying furnace 6 and a hydrogen-nitrogen mixed gas input device 10 below. At this time, the inlet of the sealing chamber 8 is provided with a nitrogen curtain (Nitrogen Curtain) (9) to maintain the reducing atmosphere in the sealing chamber (8). In addition, the reducing atmosphere in the sealing chamber 8 applies a positive pressure (+) to prevent outside air from entering. The reducing atmosphere gas concentration in the sealing chamber 8 introduced by the hydrogen-nitrogen mixed gas input device 10 is limited to 30% or less of hydrogen and 70% to 100% of nitrogen. The upper limit of hydrogen is limited to 30% because there is a risk of explosion above that. In addition, the lower limit of nitrogen to 70% is because there is a risk of explosion if it exceeds by mixing and blowing hydrogen at the same time.

The outlet of the upper part of the sealing chamber 8 also uses a sealing roll or a nitrogen curtain 9 to protect the reducing atmosphere in the sealing chamber 8 from the outside.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

By using the apparatus installed in the alloying furnace as described above to prepare a hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet while maintaining the reducing component atmosphere by adjusting the concentration of hydrogen and nitrogen when alloying the hot-dip galvanized steel sheet. The surface appearance and paintability of the plated specimens as described above were evaluated and the results are shown in Table 1. Plating deposition was 60 ~ 150g / ㎡ in the case of molten zinc, Fe content in the plating layer was 9 ~ 12% in the galvanized steel sheet.

Evaluation method of Table 1 was as follows.

Flow pattern: Visual observation, indexes classified by 1 ~ 5

   1: very good: no flow pattern

   2: excellent: some very fine flow pattern mixed

   3: normal: fine flow pattern occurs

   4: Poor (some mixed flow pattern for easy visual identification)

   5: very poor (flow pattern is easily identified by naked eye)

Standard appearance: Visual evaluation

  ◎ very beautiful and fine surface

  △: normal

  X: If you're not beautiful, the surface is rough

Coating Adhesion: Measure the number of coatings dropped out of 100 after tapping through a ball impact test after lattice was made with a knife within 10 × 10 (mm) horizontally and vertically.

division Reducing Ingredient Crisis (Vol%) Surface appearance Flow WCA Paint adhesion Remarks Hydrogen nitrogen Oxygen foot persons ashes One 0 100 0 ◎ One 0.4 0 Reducing Ingredient Crisis 2 5 95 ◎ One 0.41 0 3 10 90 ◎ One 0.38 0 4 15 85 ◎ One 0.42 0 5 20 90 ◎ One 0.39 0 6 25 75 ◎ One 0.41 0 7 30 70 ◎ One 0.40 0 ratio School ashes A 3 95 2 △ 3 1.2 2 Oxidation Component Crisis B 5 92 3 △ 3 1.21 2 C 20 60 10 △ 3 1.24 4 D 25 55 20 X 4 1.42 7 E 30 52 20 X 4 1.45 6 F 35 50 20 X 4 1.48 5 G Waiting X 5 1.50 12

As shown in Table 1, it can be seen that the inventive materials 1 to 7 produced in the reducing atmosphere by the apparatus of the present invention have a beautiful surface appearance and a low wave shape Wa indicating surface roughness. In addition, there was no flow pattern at all, and no peeling of the plating layer appeared at all in the coating adhesion by the ball impact test. Since there is no oxide film on the surface of the hot dip galvanized, the flow of the molten zinc becomes easier when the redissolved during alloying, so that the surface has a smooth surface, and the surface is beautiful. It is judged that this is good and the coating adhesion is improved.

On the other hand, when the comparative material (G) is exposed to the atmosphere in an oxidizing atmosphere, the plating properties are the worst. Also, the surface appearance or coating is also applied to the comparative material (A, B) where some oxygen is present in the reducing atmosphere. It can be seen that the adhesion deteriorates and the wave degree is greatly increased. In addition, the alloyed hot-dip galvanized steel sheet comparative materials (C to F) prepared in an oxidizing atmosphere having 10 to 20 vol% of oxygen were not beautiful in appearance and had high wave shapes and poor flow patterns. .

As described above, when the alloyed hot-dip galvanized steel sheet is manufactured by using the alloying conditions presented in the present invention, the surface appearance is beautiful and the effect of producing an alloyed hot-dip galvanized steel sheet with excellent paintability is obtained.

1 is a schematic view showing a conventional alloyed hot dip galvanizing apparatus.

2 is a schematic view showing an alloyed hot dip galvanizing apparatus according to the present invention.

<Description of the code used in the main part of the drawing>

1: strip 2: snout 3: plating bath

4: sink roll 5: air knife 6: alloying furnace

7: Top roll (deflector roll) 8: Sealing chamber 9: Nitrogen curtain

10: hydrogen-nitrogen mixed gas input device

Claims (5)

In the manufacturing method of alloyed hot-dip galvanized steel sheet excellent in surface appearance and paintability, When the cold rolled strip passes through the air knife, it injects nitrogen, installs a sealing chamber in the upper alloying furnace of the air knife, installs a hydrogen-nitrogen mixed gas inlet device at the bottom thereof, and the inlet and outlet of the sealing chamber By installing a nitrogen curtain to maintain the reducing atmosphere in the sealing chamber of the alloyed hot-dip galvanized steel sheet characterized in that it passes through the sealing chamber formed with a reducing atmosphere so as to be blocked from the atmosphere from the upper end of the air knife to the time of the molten zinc is solidified Manufacturing method. The method of claim 1, Method for producing an alloyed hot dip galvanized steel sheet, characterized in that the alloying furnace is located inside the sealing chamber. The method of claim 1, The composition of the gas blown into the hydrogen-nitrogen mixed gas input device (10) is a vol% hydrogen concentration: 30% or less, nitrogen: 70 ~ 100% method for producing an alloyed hot dip galvanized steel sheet. The method of claim 1, The sealing chamber is a method of manufacturing an alloyed hot-dip galvanized steel sheet, characterized in that the inlet and outlet to the contact with the atmosphere by using a nitrogen curtain. The method according to claim 1 or 2, The pressure inside the sealing chamber for maintaining the reducing atmosphere is a method of producing an alloyed hot-dip galvanized steel sheet, characterized in that the positive pressure (+ pressure) is always made.