JPH0874022A - Continuous hot dip metal coating method for steel sheet and device therefor - Google Patents

Abstract

PURPOSE: To produce a hot dip metal coated steel sheet having a good plating adhesion property by adding an inorg. chloride to the bath surface in a bath and executing plating while further replenishing the bath with Al. CONSTITUTION: Molten metal 12 is housed in a pot 5. The steel sheet 1 is annealed in a reducing atmosphere and is introduced through a snout 4 into the plating bath of the molten metal 12, where its direction is turned perpendicularly upward by a sink roll 6 and the steel sheet is continuously plated. A frame 9 which encloses the steel sheet 1 infiltrating the plating bath, is exposed on the bath surface and is immersed under the bath surface is, thereupon, installed in the snout 4. An inorg. chloride 10 is added to the plating bath surface in this frame 9 enclosing the steel sheet 1. A device 11 for replenishing the bath with the Al is usable for this addition. The bath surface in the frame 9 is thereby replenished with the Al. As result, the hot dip metal coated steel sheet having excellent surface appearance without adhesion of the oxide and dross which are the causes for plated surface defects is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of hot-dip metal-plated steel sheets used for automobile bodies and building materials, and in particular, it has excellent resistance to peeling of plating during processing and has an appearance without surface defects. The present invention relates to an excellent continuous hot-dip galvanizing method for steel sheets and an apparatus therefor.

[0002]

2. Description of the Related Art Conventionally, various types of continuous hot-dip galvanizing lines have been known as a method for continuously performing hot-dip galvanizing on steel sheets. Generally, the continuous hot-dip Zn plating line shown in FIG. The type is widely used.
In this line, the cold rolled steel sheet 1 is
Is directly heated, and then further annealed and reduced in an annealing furnace 3 in a reducing atmosphere with H ₂ gas, then guided to a Zn pot 5 through a snout 4 and rises vertically upward by a sink roll 6, and a gas on the Zn pot 5 is heated. After the weight is adjusted to a predetermined value by the wiper 7, the product is passed through the take-up reel 8.

When the steel sheet is continuously subjected to hot dip Zn plating by the above-mentioned conventional technique, the bath surface 5 in the snout 4 is
a and Zn on the bath surface 5b outside the snout 4 (ash), Fe-Al-based compound, and at the bottom Fe-Zn-based compounds (Fe-Al-based compounds and Fe-Zn-based compounds are collectively referred to as Many dross) are floating or accumulating. It is known in the prior art that these oxides and dross penetrate into the snout, are caught in the steel sheet, adhere to the steel sheet, cause surface irregularity defects and non-plating, and significantly deteriorate product quality.

Further, in order to prevent the product quality from being deteriorated by the oxides and dross adhering to and being entrained in the steel plate, conventionally, the line is stopped to remove the suspended matter on the bath surface 5a in the snout 4. Then, the H ₂ gas in the annealing furnace 3 is purged with N ₂ gas, the snout 4 is opened to remove suspended matter on the bath surface 5a, and the dross on the bath surface 5b on which the steel plate 1 rises is regularly scratched. However, it took a long time to stop the line, purge N ₂ gas in the annealing furnace, remove suspended matters, etc., and the workability was remarkably poor.

As a method for preventing deterioration of product quality such as surface unevenness defects and non-plating of plated steel sheets due to oxides and dross in the above conventional techniques, there have been proposals for prevention and removal of floating substances. There are various. For example, JP-A-56-146869 discloses a method of physically shaking the snout bath surface to prevent dross adhesion.
Japanese Patent Laid-Open No. 60-230969 discloses a method of absorbing dross by an electromagnetic pump, and Japanese Laid-Open Patent Publication No. 3-197657 discloses a method of collecting floating dross in a corner of a bath and stirring and removing Zn attached to the dross.
Further, JP-A-3-277756 proposes a method of controlling the temperature of the inner wall of the snout to prevent the adhesion of oxides and the like.

However, although the formation of the suspended matter on the bath surface is suppressed or removed to some extent by these disclosed techniques, JP-A-56-146869 and JP-A-60-2309.
In the dynamic methods such as Japanese Patent Laid-Open No. 69 and Japanese Patent Laid-Open No. 3-197657, since the plating bath is vibrated in any case, there is a problem that floating dross in the bath floats and easily adheres to the steel sheet. In addition, even with a static method such as JP-A-3-277756, although it is effective in preventing surface defects, the temperature rise of the steel sheet invading the plating bath alloys the steel sheet and the plating layer, and the plating There is a problem that performance such as adhesion is likely to vary and deterioration is likely to occur.

[0007] Further, an example of deterioration of surface properties and plating adhesion due to adhesion of oxides and dross to the steel sheet in hot dip galvanizing was shown, but oxide formation and deposition in snout, adhesion to plated steel sheet, The formation of insoluble compounds in the plating bath occurs similarly in other molten metal plating such as hot-dip Al plating and hot-dip Zn-Al plating, and due to the adhesion of oxides and dross to the steel sheet, surface properties and plating adhesion It is known that deterioration of sex occurs.

[0008]

As described above, in the case where the steel sheet is continuously subjected to hot metal plating by the prior art, oxides and dross on the bath surface adhere to and are entrained in the steel sheet, resulting in unevenness of the plating surface. There is a problem that the surface quality is deteriorated due to defects and non-plating, and the conventional improvement techniques have not been sufficient to solve the problem.

The present invention provides a steel sheet excellent in appearance and having excellent resistance to delamination during processing by preventing adherence of suspended matters on the bath surface in the snout, for example, oxides and dross in the Zn bath to the steel sheet. It is an object of the present invention to provide a continuous hot-dip metal plating method and its apparatus.

[0010]

DISCLOSURE OF THE INVENTION The present invention is a continuous molten metal in which a steel sheet annealed in a reducing atmosphere is introduced into a molten metal plating bath through a snout and is vertically vertically changed by a sink roll for plating. In the plating method, a frame in which the upper part is exposed on the bath surface and the lower part is immersed under the bath surface is installed in the snout so as to surround the steel plate, and an inorganic chloride is added to the bath surface in the frame, and A continuous hot-dip metal plating method for a steel sheet, characterized by supplementing Al in a frame bath for plating, and the present invention is a hot-dip steel sheet annealed in a reducing atmosphere, in a hot-dip metal bath through a snout. In a continuous hot-dip galvanizing machine that changes the direction vertically upward with a sink roll and performs plating, put a frame in which the upper part is exposed above the bath surface and the lower part is immersed under the bath surface inside the snout so as to surround the steel plate. Installed and A continuous molten metal plating apparatus of a steel sheet, characterized in that the bath of the inner provided with a device for replenishing Al.

[0011]

The present invention will be described in detail below. As an example of a continuous hot-dip metal plating apparatus for steel plates, the present inventors
Using a Zn-Al-based plating line, the flow and adhesion behavior of oxides and dross in the plating bath were investigated and the following findings were obtained. In the plating bath, oxides and dross are divided into those existing above and below the plating bath due to the difference in specific gravity from the plating bath. In particular, the dross at the upper part is present in the snout in a large amount due to Fe that is melted out when the steel sheet enters the plating bath. The oxides and dross existing in the upper part of the bath and the bath surface in the snout are relatively large in many cases, and the surface defects of the plated steel sheet are caused by the adhesion of these oxides and relatively large dross. The flow of molten metal in the plating bath follows the rotation of the steel plate and sink roll, and the steel plate that penetrates into the plating bath with a snout causes oxides and relatively large dross in the upper part of the plating bath inside the snout. Get involved. Oxides and dross in the upper part of the plating bath in the snout are caught in the flow of the molten metal near the steel plate, so that some of them adhere to the steel plate and become surface defects later, but the rest are along the rotation of the sink roll. Reflux into the snout along the flow of molten metal.

In order to suppress the dross which flows back into the snout due to the flow of the molten metal along with the rotation of the sink roll, the steel plate 1 penetrating into the plating bath is surrounded in the snout 4 as shown in FIG. The frame 9 which is exposed to the water and immersed under the bath surface is installed. As a result, the dross discharged outside the frame 9 by the action of the molten metal flow along the steel plate 1 is prevented from flowing back into the frame 9 by the flow along the sink roll 6, and the dross inside the frame 9 is prevented. Can be reduced. Inorganic chloride 10 is added to the surface of the plating bath in the frame 9 surrounding the steel plate 1 installed in 1. Oxides and dross on the bath surface are decomposed and reduced by the action of the inorganic chloride 10, and the oxides and dross in the vicinity of the plating bath penetration portion of the steel sheet 1 are reduced.
The method of adding the inorganic chloride 10 is not particularly limited, but Al
It is possible to utilize the device 11 for replenishing The specific gravity of the dross in the lower part of the plating bath is heavy, and the dross accumulates due to the rotation of the sink roll 6 and the molten metal 1 in the plating bath.
It does not cause a plating surface defect because it does not easily float even by the flow of 2.

Therefore, the present inventors say that it is effective to suppress oxides and dross in the vicinity of the plating bath surface in the snout in order to suppress oxides and dross that cause surface defects. With the knowledge obtained, the present invention has been completed. Further, the above-mentioned knowledge of the present inventors was obtained in a molten Zn-Al system plating line, but also in other molten metal plating lines such as a molten Al plating line, generation of oxides and dross, The distribution in the plating bath due to the difference in specific gravity, the flow, the adhesion behavior to the steel sheet, and the like are similar, and the adhesion of oxides and dross to the steel sheet can be prevented by the same method in other molten metal plating lines.

The grounds for the constituent features of the present invention are shown below. (1) Install a frame in which the upper part is exposed above the bath surface and the lower part is immersed below the bath surface in the snout so as to surround the steel plate: Inside the snout along the flow of molten metal as the sink roll rotates. In order to suppress the reflux dross, a cylindrical frame surrounding the steel plate that enters the plating bath is installed inside the snout. In order to prevent the dross outside the frame surrounding the steel plate from entering the frame due to the shaking of the plating bath surface during plating, the frame must be exposed on the bath, and in order to obtain a sufficient effect The exposed height should be 20mm or more. Further, it is immersed in the bath to suppress the reflux of the dross, but the immersion depth is preferably 30 mm or more in order to obtain a sufficient effect. (2) Adding inorganic chloride to the surface of the bath in the frame: By adding inorganic chloride to the surface of the plating bath in the frame surrounding the steel sheet, the action of inorganic chloride causes oxides and dross on the surface of the bath. Decomposed and reduced, oxide near the plating bath penetration part of the steel plate,
Dross is reduced. Suitable inorganic chlorides are zinc chloride, ammonium chloride, calcium chloride, etc.
It is possible to add one kind or a mixture of two or more kinds. The addition amount of the inorganic chloride is determined in consideration of the reducing effect of oxides and dross, but normally it is preferably several kg to several tens kg. (3) Replenishing Al in the bath in the frame: In order to prevent the aluminum chloride in the plating bath from consuming Al as chlorides and reducing the Al concentration in the bath, the inorganic chloride placed in the frame surrounding the steel plate Install a device to replenish Al in the inner bath and to replenish Al for this purpose. It is desirable that the amount of Al replenished in the frame should be adjusted according to the consumption of Al in the bath.

[0015]

EXAMPLE In a continuous hot-dip galvanizing line shown in FIG. 2, a steel plate having a frame 9 in which the upper part is exposed above the bath surface and the lower part is immersed below the bath surface according to the present invention, which is shown in FIG. 1, a device 11 for replenishing Al in the bath in the frame 9 is installed, an inorganic chloride 10 is added to the surface of the bath in the frame 9, and further in the bath in the frame 9. While supplying Al, a Nb-Ti ultra-low carbon steel plate having a plate width of 1200 mm and a plate thickness of 0.7 mm was subjected to hot dip Zn plating with a basis weight of 60 g / m ² . FIG.
(A) is 1 of the continuous hot-dip metal plating apparatus of the steel plate of this invention.
It is a schematic sectional drawing which shows an example, FIG.1 (b) is FIG.1 (a).
AA ^' is a cross-sectional view taken along the arrow.

In the present embodiment, the distance between the steel plate 1 and the frame 9 (X and Y in FIG. 1 (b)) was 100 mm, but depending on the sizes of the steel plate 1, the snout 4, and the pot 5, It is preferable to change the distance between the steel plate 1 and the frame 9, and in order to avoid contact between the steel plate 1 and the frame 9, the distance between the steel plate 1 and the frame 9 is 30 mm.
The above is preferable. In FIG. 1A, h is the exposed height above the bath surface of the frame 9, and d is the immersion depth below the bath surface of the frame 9. Further, the frame 9 changes the shape of the upper end and the lower end,
The openings may be fitted, and the frame 9 does not have to be fixed and may be movable back and forth and left and right.

The degree of occurrence of surface defects due to ash and dross on the surface of the obtained plated steel sheet and the plating peeling condition in the 180 ° bending back test were investigated. Table 1 shows the plating conditions, the occurrence of surface defects on the plated steel sheet, and the plating adhesion evaluation results.

[0018]

[Table 1]

The plated steel sheet produced by the conventional technique includes:
Many surface defects due to dross were observed, and plating adhesion was poor due to dross. In accordance with the present invention, a frame 9 having an upper portion exposed above the bath surface and a lower portion immersed under the bath surface is installed in the snout 4 so as to surround the steel plate 1, and the inorganic chloride 10 is formed on the bath surface in the frame 9. In the bath in the frame 9
By performing hot dip Zn plating while adding Al,
It was possible to obtain a hot-dip galvanized steel sheet with excellent plating adhesion by suppressing surface defects due to oxides, dross, and the like.

When only the frame 9 surrounding the steel plate 1 is arranged and the inorganic chloride 10 is not added and Al is not replenished, the effect of the frame 9 causes a decrease in surface defects due to dross. The effect was not sufficient as compared with the invention, and the plating adhesion was poor due to dross. Further, even if the frame 9 was installed and the inorganic chloride 10 was added, if the Al was not replenished, the plating adhesion was significantly deteriorated due to the decrease of Al in the bath.

[0021]

Industrial Applicability According to the present invention, a hot-dip galvanized steel sheet having an excellent surface appearance free from oxides and dross that become plating surface defects and having good plating adhesion can be produced.

[Brief description of drawings]

FIG. 1A is a schematic cross-sectional view showing an example of a continuous hot-dip metal plating apparatus for steel sheets according to the present invention, and FIG. 1B is a cross-sectional view taken along the line AA ^′ of FIG.

FIG. 2 is a schematic cross-sectional view showing a conventional continuous hot-dip Zn plating line.

[Explanation of symbols]

 1 steel plate 2 non-oxidizing furnace 3 annealing furnace 4 snout 5 pot 5a bath surface inside snout 5b bath surface outside snout 6 sink roll 7 gas wiper 8 take-up reel 9 frame 10 device for replenishing inorganic chloride 11 Al 12 molten metal d Immersion depth below bath surface of frame h Exposure height above bath surface of frame X Distance between steel plate and frame Y Distance between steel plate and frame

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Chiaki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. 1 Kawasaki-cho, Ward Kawasaki Steel Corporation Steel Research and Development Division Steel Research Laboratory

Claims (2)

[Claims] 1. A continuous hot-dip metal plating method in which a steel sheet annealed in a reducing atmosphere is introduced into a hot-dip galvanizing bath through a snout, and a vertical direction is changed upward by a sink roll for plating. A frame, which is exposed above and the lower part is immersed under the bath surface, is installed in the snout so as to surround the steel plate, inorganic chloride is added to the bath surface in the frame, and Al is replenished in the bath in the frame. A method for continuous hot-dip metal plating of a steel sheet, which comprises performing a hot dip plating. 2. A continuous hot metal plating apparatus in which a steel sheet annealed in a reducing atmosphere is introduced into a hot metal plating bath through a snout, and is vertically vertically changed by a sink roll to perform plating. A continuous steel plate characterized in that a frame, which is exposed above and the lower part of which is immersed under the bath surface, is installed in the snout so as to surround the steel plate, and a device for replenishing Al in the bath in the frame is provided. Hot metal plating equipment.