JPH083706A - Method and apparatus for manufacturing hot-dip galvanized steel sheet - Google Patents

Abstract

(57) [Summary] [Purpose] During hot metal plating on steel sheets, it prevents dross on the surface of the molten metal in the snout from adhering to the steel sheets, and improves the surface quality of the plated steel sheets. [Structure] Molten metal or a non-oxidizing gas is sprayed from the spray nozzle provided below the surface of the molten metal bath in the snout to the surface of the steel sheet in the bath in the snout in the direction opposite to the traveling direction of the steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet capable of preventing the dross from adhering to the hot-dip galvanized steel sheet during the production of the hot-dip galvanized steel sheet and having excellent plating appearance and adhesion. The present invention relates to a method for manufacturing a metal-plated steel sheet and a manufacturing apparatus therefor.

[0002]

2. Description of the Related Art There are various types of continuous hot-dip galvanizing lines. As an example, a continuous hot-dip Zn plating line is shown in FIG. The cold-rolled steel sheet 1 is directly heated in an oxidation-free furnace 2, further annealed and reduced in an annealing furnace 3 in a reducing atmosphere with H ₂ gas, and then introduced into a Zn pot 5 through a snout 4 and a sink. The roll 6 stands up at a right angle, and after the gas wiper 7 on the Zn pot 5 adjusts the weight to a predetermined value, the product is rolled up by a take-up reel 8.

In this case, Zn oxide (ash) is present on the Zn bath surface 5a inside the snout and on the Zn bath surface 5b outside the snout.
Many large dross of Fe-Al system and Fe-Zn system float due to the elution of Fe from the steel sheet. These suspended matters infiltrate into the snout or adhere to the steel plate rising from the bath surface, or are entrained to generate surface irregularity defects and non-plating, which significantly deteriorates the product quality.

Therefore, conventionally, the line is stopped to remove the suspended matter on the Zn bath surface in the snout 4, and the H ₂ in the annealing furnace 3 is stopped.
The gas was purged with N ₂ gas, the snout was released to remove the suspended matter on the Zn bath surface 5a, and the dross on the Zn bath surface 5b outside the snout where the steel plate rises was scraped off periodically.
Since it took a long time to stop the line, purge N ₂ gas in the annealing furnace, remove suspended matters, etc., workability was extremely poor.

Examples of known techniques relating to the removal of these suspended matters and the method for preventing the adhesion of steel sheets include the techniques described in JP-A-60-181259, JP-A-60-230969, and JP-A-3-197657. There is disclosed a method of removing suspended matter on the bath surface with a stirrer or absorbing it with an electromagnetic pump. Further, as a technique similar to the present invention, Japanese Patent Laid-Open No. 63-1346
There is a technique described in Japanese Patent Laid-Open No. 52-52, which is a method of causing a molten metal flow to collide with a steel plate in a plating bath. In the technique described in Japanese Patent Laid-Open No. 2-141563, molten Zn is applied to a strip immediately before entering a molten Zn bath.
A method of spraying a bath is disclosed.

By these methods, the suspended matter on the bath surface is removed to some extent and the adhesion of dross to the steel sheet is suppressed, but the above-mentioned JP-A-60-181259 and JP-A-60-230969,
The inventions described in Japanese Patent Application Laid-Open No. 3-197657 all have a drawback that dross is caught in the bath and easily adheres to the steel sheet because the bath surface is agitated or vibrated. The invention described in JP-A-63-134652 is on the steel plate surface in the bath (between the snout and the sink roll), and the invention described in JP-A-2-141563 is the steel plate on the bath in the snout (before intrusion). The molten metal collides with the surface, but in the former, the molten metal collides with the steel sheet surface in the bath near the sink roll from the snout, so the dross circulates throughout the plating bath.
It easily floats and eventually reattaches to the steel sheet.The latter causes molten metal to collide with the steel sheet from above the bath in the snout, making it difficult to control the temperature of the molten metal to be sprayed, resulting in bath temperature and intrusion plate temperature. However, there are drawbacks such as fluctuations in dross and increased dross, and at present the countermeasures are not sufficient.

[0007]

On the Zn bath surface of the continuous hot-dip galvanizing line, ash generated due to high Zn vapor pressure, Zn oxide oxidized by O _{2 in} the atmosphere, and steel sheet A lot of dross due to Fe elution is floating. In particular, since replenishment of Zn or Al-Zn ingot is often performed from the rear of the snout, it is considered that dross precipitation is likely to occur in the vicinity of the snout portion due to local decrease in bath temperature and increase in Al concentration. In particular, the present inventors have found that these suspended matters often get caught in the snout and adhere to the steel sheet, impairing the surface quality.

Therefore, the present invention prevents the dross from adhering to the steel sheet when the steel sheet passes through the surface of the molten metal in the snout, which has a large amount of such suspended matters and has a large amount of dross attached to the steel sheet. It is an object of the present invention to provide a method for manufacturing a hot-dip plated steel sheet and a manufacturing apparatus therefor capable of obtaining a hot-dip galvanized steel sheet having excellent properties.

[0009]

According to the present invention, when a steel sheet to be plated annealed in a reducing atmosphere is introduced into a molten metal bath through a snout and plated, the steel sheet is provided below the molten metal bath surface in the snout. Molten metal or non-oxidizing gas from the spray nozzle to the surface of the steel sheet in the bath in the snout, a method for producing a hot-dip galvanized steel sheet characterized by spraying in a direction opposite to the traveling direction of the steel sheet. The invention is an apparatus for producing a molten metal-plated steel sheet in which a steel sheet to be plated annealed in a reducing atmosphere is introduced into a molten metal bath through a snout and plated, a molten metal bath surface 100 to 300 mm in the snout, and a steel sheet. An apparatus for producing hot-dip galvanized steel sheet, characterized in that a nozzle for blowing molten metal or non-oxidizing gas to the steel sheet surface in the bath is provided at a position of 10 to 100 mm in the direction opposite to the traveling direction of the steel sheet. Is.

[0010]

The present invention will be described in detail below. When the plating surface defects generated in the hot-dip galvanized steel sheet were investigated, most of the defects were considered to be due to the adhesion of ash, dross, oxides and the like. Therefore, the present inventors investigated the relationship between the oxide and dross morphology in the Zn bath and the adhered dross on the steel sheet, taking the case of continuous hot dip Zn plating as an example. The Zn bath for producing GA and GI on an industrial scale is a coexistence region of Fe-Al-based and Fe-Zn-based dross, and oxide-based dross and Fe are present at the top of the bath.
Large Fe-Zn dross mainly composed of -Al dross
Fe-Al mainly consists of large Fe-Zn dross at the bottom of the bath.
There are many system dross, and the central part of the bath except this part is
The relatively small dross is floating, and most of the dross that adhere to the steel plate are the dross in the snout at the bath top, and the surface defects are large dross (including oxides and ash) that adhere to the surface defects. I found that it was to do. From this, in order to eliminate plating surface defects, it is important to form as many large top dross (including oxides, ash, etc.) present in the snout as possible, or not to adhere to or remove them from the steel plate surface.

The present inventors have conducted various investigations on the latter method of not adhering to the surface of a steel sheet using an actual line and a water model experimental apparatus, and as a result, sprayed molten metal or a non-oxidizing gas onto the steel sheet surface in the bath in the snout. Then, it was found that the dross adhesion inside the snout can be prevented by controlling the bath flow near the steel plate inside the snout. As shown in FIG. 3, the steel sheet 1 is directly heated in the non-oxidizing furnace 2, further annealed and reduced in the annealing furnace 3 in a reducing atmosphere with H ₂ gas, and then reduced and purified, and then passed through a snout 4 to a Zn pot 5. It is guided and adjusted to a predetermined basis weight by the gas wiper 7 on the Zn pot,
In this case, as shown in FIG. 1, spray nozzles 9a and 9b are provided on both sides of the steel plate 1 in the snout (below the bath surface), preferably 100 to 300 mm below the bath surface (h), and a gap (d )Ten
If it is placed at a position of ~ 100 mm and the molten metal or inert gas is sprayed at a flow rate of 5-30 l / min per 1 m of steel plate width,
The bath flow shown in Fig. 2 occurs near the steel plate in the snout (estimated from the water model experiment), and as a result, the adhesion of dross at the steel plate interface is prevented (the velocity boundary layer formed at the advancing steel plate interface is Estimated to suppress or destroy). At this time, the shape of the nozzle to be sprayed and the spray angle are not particularly limited because they may suppress or destroy the velocity boundary layer formed at the steel plate interface, but a slit-like component opposite to the traveling direction of the steel plate is used. It is desirable to spray to have.

The position of the spray nozzle is 100 to 300 mm below the bath surface.
It is desirable that the clearance between the steel plate and the steel plate be 10 to 100 mm. When the nozzle is close to the bath surface and the steel plate, the vibration of the bath surface is remarkable, and conversely, when the nozzle is far, the flow of the bath in the reverse direction near the steel plate is small.
This is because it becomes difficult to obtain the bath flow shown in. The flow rate of the sprayed molten metal and non-oxidizing gas is a steel plate width of 1 m.
5 to 30 l / min is preferable. If it is less than 5 l / min, the desired bath flow cannot be obtained (the bath flow shown in Fig. 2 cannot be obtained), and if it exceeds 30 l / min, the vibration of the bath surface becomes remarkable, and the dross on the bath surface is caught up in the steel sheet. At the same time as the adhesion, other quality defects such as non-plating and deterioration of plating adhesion occur, thereby deteriorating the commercial value. The molten metal sprayed from the inner bath of the snout is the same as the molten metal to be plated, and the non-oxidizing gas is, for example, A
R, N ₂ , and N ₂ + H ₂ mixed gas are suitable.

As described above, in the continuous hot-dip galvanizing line, by installing a spray nozzle on the steel plate surface in the bath in the snout in order to prevent the adhesion of large dross in the snout, a large suspended matter on the bath surface is caused. Unplated,
It is possible to obtain a hot-dip metal-plated steel sheet that is free from uneven defects. The effect of the present invention is particularly great in hot-dip Zn plating, hot-dip Al plating, etc., in which there are many suspended matters on the bath surface.

[0014]

[Example] Ultra-low carbon steel was used as a raw material for Zn plating in the continuous hot-dip Zn plating line shown in FIG. 3 using the spray nozzle shown in FIG. Ashes, dross on the surface of the obtained plated steel sheet,
The degree of surface defects such as oxides was investigated. Table 1 shows the plating conditions and the surface defect investigation results. As a comparative example, the case where the spray nozzle shown in FIG. 1 is not used is also shown. It can be seen that the degree of occurrence of surface defects in the hot-dip galvanized steel sheet obtained by the present invention is significantly reduced.

[0015]

[Table 1]

[0016]

INDUSTRIAL APPLICABILITY According to the present invention, in a continuous hot-dip galvanizing line, in order to prevent the adhesion of large dross inside the snout, the spray nozzle installed in the steel plate surface in the snout at a constant depth from the bath surface and at a constant distance from the steel plate. By spraying molten metal or non-oxidizing gas onto the steel sheet surface in the direction opposite to the steel sheet traveling direction, large ash, dross, oxides, etc. on the bath surface are prevented from adhering. Quality and product quality can be significantly improved.

[Brief description of drawings]

FIG. 1 is a sectional view of a snout portion of a Zn bath having a spray nozzle of the present invention.

FIG. 2 is an explanatory view (estimated from a water model experiment) showing a bath flow in a snout when a molten metal and a non-oxidizing gas are sprayed from a bath in the snout to a steel plate surface.

FIG. 3 is an explanatory diagram showing an example of a continuous hot-dip Zn plating line to which the method of the present invention should be applied.

[Explanation of symbols]

 1 steel plate 2 non-oxidizing furnace 3 annealing furnace 4 snout 5 Zn pot 5a Zn bath surface inside snout 5b Zn bath surface outside snout 6 sink roll 7 gas wiper 8 take-up reel 9a spray nozzle 9b spray nozzle

Claims (2)

[Claims] 1. When a steel sheet to be plated annealed in a reducing atmosphere is introduced into a molten metal bath through a snout and plated, a bath in the snout is provided from a spray nozzle provided below the molten metal bath surface in the snout. A method for producing a hot-dip galvanized steel sheet, characterized in that a molten metal or a non-oxidizing gas is sprayed onto the surface of the intermediate steel sheet in a direction opposite to the traveling direction of the steel sheet. 2. A manufacturing apparatus for a molten metal-plated steel sheet in which a steel sheet to be plated annealed in a reducing atmosphere is introduced into a molten metal bath through a snout and plated, and the molten metal bath surface within the snout is 100 to 300 mm below the surface of the molten metal bath, and Gap between steel plates 10-100 mm
At the position, a nozzle for spraying a molten metal or a non-oxidizing gas onto the surface of the steel sheet in the bath in a direction opposite to the traveling direction of the steel sheet is provided.