JPH05239608A - Manufacture of galvanized steel sheet and device therefor - Google Patents

Abstract

PURPOSE:To manufacture a galvanized steel sheet without generating the defect of dross caused by the adhesion of dross on the surface of a steel sheet in the bath even if it is low Al operation. CONSTITUTION:This method for manufacturing a galvanized steel sheet is a one in which a strip 1 is introduced into a plating bath provided with a sink roll 5 at the inside, and galvanizing is executed while the strip 1 is circulated around the sink roll 5. The lower direction of the plating bath 3 has been provided with a bottom dross deposition tank 8 communicated with the plating bath 3, and bottom dross heaped on the bottom of the plating bath 3 is discharged to the bottom dross deposition tank 8 and is concentrated, by which the bottom dross is discharged to the outside of the plating bath 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for producing a hot-dip galvanized steel sheet which has few surface defects and has an excellent appearance quality.

[0002]

2. Description of the Related Art Today, the specifications required for thin steel sheets (hereinafter simply referred to as "steel sheets") are becoming strict. Particularly in terms of corrosion resistance, the use conditions are becoming severer for building materials, home appliances and automobiles, and further improvement is required. Therefore, plated steel sheets are generally used frequently. The hottest galvanized steel sheet is hot-dip galvanized steel sheet. Normally, this hot-dip galvanized steel sheet is used after being further coated, and not only corrosion resistance but also smoothness of the finished surface after coating and adhesion to the coated surface are required, so the surface quality of the plated surface is extremely important. Is.

In the case of hot-dip galvanized steel sheet, defects caused by dross floating in the plating bath include a so-called press pit which causes a non-uniform surface during press and impairs image clarity, and a non-uniform surface. There is a so-called dross bump that forms a local battery and deteriorates corrosion resistance. On the other hand, examples of the surface defects due to surface oxidation of the plated surface include surface defects caused by the oxides being pushed into the surface of the plated steel sheet and deterioration of appearance due to surface oxidation.

In the present specification, in principle, these are collectively referred to as surface defects, but they are appropriately distinguished from dross defects and surface oxidation defects when necessary. Several attempts have been proposed so far to prevent surface defects caused by dross. For example, in Japanese Examined Patent Publication No. 57-58434, a plating tank and an Al reaction tank are separated from each other by a partition wall so that high Al
A method has been proposed in which bottom dross is converted into top dross by removing the bottom dross by operating and stirring the plating bath.

According to this method, the dross reacts with Al and tends to float during high Al operation, while the dross tends to float during low Al operation.
In other words, at the time of galvannealed steel sheet (alloying treated steel sheet) manufacturing, light dross generated by reaction with Al is rarely generated, and heavy bottom dross generated by reaction with Zn is only accumulated and cannot be removed outside the plating tank. Further, in this method, since it is necessary to separate the plating tank and the Al reaction tank, the entire apparatus has a large capacity, and an increase in equipment cost cannot be avoided. Further, when the reaction tank is stirred to remove dross, oxygen in the air is entrained in the reaction tank, which accelerates the oxidation of the zinc bath, which is a problem in terms of Zn yield and quality control.

On the other hand, in order to prevent surface oxidation defects on the plated surface, it is necessary to prevent surface oxidation of the plated steel sheet.
Since all the conventional hot-dip galvanized steel sheet manufacturing apparatuses are large, for example, even if the galvanized steel sheet is sealed with an inert gas, an extremely large-scale sealing device is required, and an increase in equipment cost cannot be avoided. It is hard to say that the effect of it is sufficient.

[0007]

Particularly in recent years, mainly for automobiles, the demand for alloyed steel sheets has been increasing more and more, and the demand for higher quality is also increasing. Against this background, it is required to establish a manufacturing technique for hot-dip galvanized steel sheets that does not generate surface defects due to dross even in low Al operation. Therefore, an object of the present invention is to provide a manufacturing method and a manufacturing apparatus for a hot-dip galvanized steel sheet that does not have dross defects due to the adhesion of dross on the surface of the steel sheet even in a low Al operation.

[0008]

[Means for Solving the Problem] Conventionally, the bottom dross accumulated in the plating tank is discharged by scraping the bottom dross that has become a top dross when the Al concentration becomes high during GI operation. It was Here, since this top dross contains as much as 95 wt% or more of Zn and only about 1 wt% of Fe, it seems that Zn is not extracted from the system but extracted from the system. Is something
It was extremely wasteful and a problem.

Further, as the ratio of GA operation increases, bottom dross continues to accumulate without becoming a top dross, and the dross sticking and pushing due to winding increases, which is a problem and some measures have to be taken. We have
As a result of various studies to solve the above problems, the following findings were obtained and the present invention was completed.

In order to discharge the bottom dross accumulated on the bottom of the plating tank to the outside of the plating tank, it is most simple and efficient to discharge the bottom dross from the bottom of the plating tank as it is. Therefore, a discharge port of the plating tank is provided at the bottom of the plating tank, and a bottom dross deposition tank for depositing the discharged bottom dross is installed below the plating tank. Furthermore, between the plating bath and the bottom dross deposition bath
A sliding nozzle provided at the bottom of the tundish used for continuous casting or an equivalent thereof is installed in (or may be the bottom of the plating tank) to control the discharge of bottom dross.

Further, in order to recover the bottom dross accumulated in the bottom dross deposition tank, the bottom dross in a molten state is passed through a distillation apparatus (about 1000 ° C., the boiling point of zinc is 907 ° C.), and Zn is vaporized to a high boiling point (about 1800 ° C.). FeZn compound)
Separated from (dross), the selectively vaporized Zn is cooled and recovered in a liquid state and returned to the plating tank. Also, by supplying consumable Zn and Al, it is possible to manufacture a hot-dip galvanized steel sheet that does not cause dross defects by always operating with a clean plating solution that causes almost no bottom dross.

Here, the gist of the present invention is to immerse the sink roll in a plating bath, preferably, the bottom of the plating tank has a cone shape, and the bottom of the sink roll and the bottom of the plating tank. It is a manufacturing method of hot dip galvanized steel sheet in which the strip is introduced into a plating tank with a distance of 500 mm or more and is galvanized while orbiting around the sink roll. A method for producing a hot-dip galvanized steel sheet, characterized in that a bottom dross deposition tank in communication is provided, and bottom dross accumulated at the bottom of the plating tank is discharged to the bottom dross deposition tank to discharge the bottom dross to the outside of the plating tank. Is.

According to the present invention described above, the concentrated bottom dross in the bottom dross deposition tank is sent to a zinc distillation apparatus connected to the bottom dross deposition tank, vaporized Zn is recovered and circulated / reused as a plating solution, and / Alternatively, vaporized Zn can be used as a heat source for melting the Zn and Al ingots for supplementing the plating solution. Further, by replenishing Al and Zn discharged from the plating tank together with the bottom dross, the plating bath level can be kept constant, the bottom dross can be reduced, and adhesion to the strip surface due to winding of the dross can be prevented.

From another aspect, the present invention is an apparatus for producing a hot-dip galvanized steel sheet having a sink roll inside and preferably a bottom having a cone-shaped plating tank. Hot dip galvanizing, which is arranged below the bath and has a bottom dross deposition bath for depositing and concentrating bottom dross from the plating bath, and (ii) an opening / closing device for opening / closing a communication portion between the plating bath and the bottom dross deposition bath. It is a steel plate manufacturing apparatus.

Further, (iii) a zinc distillation apparatus connected to the bottom dross deposition tank, and (iv) a zinc distillation apparatus,
It may have a zinc replenishment tank for supplying and circulating Zn and Al from the zinc distillation apparatus to the plating tank.

[0016]

The operation of the present invention will be described in detail below. In the present invention, the bottom dross deposition tank is installed below the plating tank so as to communicate with each other, and an opening / closing device such as a sliding gate is installed at the communication portion between the two. Therefore, the bottom dross accumulated on the bottom of the plating tank can be moved to the bottom dross deposition tank, in other words, the bottom dross can be easily and reliably discharged from the plating tank. The flow resistance of the communication part that connects the plating tank and the bottom dross deposition tank also suppresses winding when the bottom dross moves.

The bottom of the plating tank has a cone shape as shown in FIG. 1 which will be described later, and it is possible to easily collect and discharge bottom dross by arranging a communicating portion with the bottom dross deposition tank at the lowest portion of the shape. Desirable to do. In addition, the distance between the bottom of the sink roll and the bottom of the plating tank is 500 mm or more, preferably 1000 mm or more, from the viewpoint of curbing winding.

The bottom dross is concentrated in the bottom dross deposition tank, but in order to reuse this bottom dross, in the present invention, a zinc distillation apparatus is connected to the bottom dross deposition tank,
Zn is distilled from the bottom dross by a zinc distillation apparatus. That is, conventionally the dross recovered as a top dross was subjected to a melting step from the solid state, and Zn was recovered by separating it from the dross containing Fe by a so-called distillation method.
In the present invention, the bottom dross having a high Fe concentration is extracted from the lower part of the plating tank in a concentrated state and applied to the distillation apparatus in a molten state, so that the thermal loss (heat of fusion + heat of temperature rise) is extremely small,
The efficiency of zinc distillation is also very good. For example, of top dross
Fe content is about 1% by weight, but bottom dross Fe
The content is about 5 to 10% by weight.

Further, in the present invention, vaporized Zn is recovered and circulated / reused as a plating solution, and / or vaporized Zn is used as a heat source for melting Zn and Al ingot for replenishing the plating solution. It is also possible to use.
It is clear that it is effective in saving energy.

As described above, according to the present invention, it is possible to produce a hot-dip galvanized steel sheet without causing a dross defect due to the adhesion of dross on the surface of the steel sheet in the plating bath even in a low Al operation. .. Further, the present invention will be described in detail with reference to examples, but this is an example of the present invention and the present invention is not limited thereto.

[0021]

EXAMPLES FIG. 1 is an explanatory view showing an example of an apparatus for producing a hot-dip galvanized steel sheet according to the present invention. In the figure, the strip 1 is guided to a plating bath 3 through a snout 2, and circulates around a sink roll 5 immersed in a plating bath 4 and is discharged to the outside of the plating bath 3 via support rolls 6 and 7. To be done. The bottom of the plating tank 3 has a V-shaped cone in vertical cross section, and is configured to facilitate accumulation and discharge of bottom dross.

The relationship between the depth D of the plating tank 3 and the plating tank capacity is as shown below, for example. Depth Plating tank capacity 0.2m 200T 0.5m 250T 1.0m 350T Below the plating tank 3, a bottom dross deposition tank 8 connected to the plating tank 3 is connected via a sliding gate 9. The bottom dross deposition tank 8 has a small volume of 100 T, and the bottom has a cone shape like the plating tank 3.

The bottom dross deposition tank 8 is connected to a zinc distillation tank 11 and a zinc replenishment tank 12 through a sliding gate 10 so that the finally recovered zinc can be circulated and recovered in the plating tank 3. Has been done. Using the equipment shown in Fig. 1, GA operation was performed at an effective Al concentration of 0.1% by weight, and the number of dross defects generated under each condition (pieces / 100m ² )
Was evaluated.

When the distance D from the bottom of the sink roll, that is, from the bottom to the bottom of the plating tank 3 is changed to 0.2, 0.5 and 1.0 m, the number of dross defects generated in the apparatus without the dross deposition tank and the bottom dross deposition tank are set. The number of dross defects generated in the provided apparatus (D = 1.0 m) according to the present invention was compared (comparison on the 10th day after removal of dross). The results are shown graphically in FIG. As shown in FIG. 2, it is recognized that the device provided with the bottom dross deposition tank was able to significantly reduce the dross winding and drastically reduce the dross defects.

Further, the Fe concentration in the bottom dross collected in the bottom dross deposition tank with the apparatus stopped was investigated. The results are shown graphically in FIG. In the conventional method (a method of floating bottom dross by adding Al), the Fe concentration can be concentrated only to about 1 wt% (bath composition about 0.03 wt%), but in the present invention in which bottom dross is directly extracted, the Fe concentration is concentrated to about 10 wt%. It was possible to suppress Zn loss to about 1/10 (comparison on the 20th day after removing dross).

Furthermore, about the amount extracted from the bottom of the plating tank
100 tons (Zn ≈ 90% by weight, Fe ≈ 10% by weight) of a dross-based slurry liquid (460 ° C) at 1000 ° C (Zn boiling point 907 ° C)
When the temperature was raised to 100 ° C. and the mixture was vaporized and separated in a zinc distillation apparatus, about 80 tons of almost 100% Zn was recovered. At this time, about 20 tons Fe from the bottom of the zinc distillation apparatus
The Zn compound could be separated and concentrated as a residue (Fe ≈ 50% by weight,
Zn ≈ 50% by weight).

It was possible to replenish the recovered Zn with the lost Zn (steel plate adhering portion + zinc distiller lower part escape portion), adjust the Al concentration, and then return to the plating tank 3. In order to concentrate the bottom dross accumulated in the bottom dross deposition tank 8,
The sliding gate 9 shown in FIG. 1 may be always opened and the sliding gate 10 may be closed. The dross generated at this time is concentrated with the passage of time as shown in Fig. 3, but GA
The 100 ton bottom dross deposition tank will be full for about 20 days at 100% operation, and dross will be present in the plating tank after that, which will adversely affect the winding of the dross. Open it, distillate, refine and collect.

[0028]

As described above in detail, according to the present invention,
It has become possible to provide a manufacturing method and a manufacturing apparatus for a hot-dip galvanized steel sheet that has no dross defects due to the adhesion of dross on the surface of the steel sheet even in the Al operation. The significance of the present invention having such effects is extremely remarkable.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an example of a hot-dip galvanized steel sheet manufacturing apparatus according to the present invention.

FIG. 2 is a graph showing the relationship between the distance from the bottom of the sink roll to the bottom of the plating tank and the number of dross defects generated in the example.

FIG. 3 is a graph showing the relationship between the number of days elapsed after bottom dross removal and the Fe concentration in the bottom dross deposition tank.

[Explanation of symbols]

1: Strip 2: Snout 3: Plating bath 4: Plating bath 5: Sink roll 6: Support roll 7: Support roll 8: Bottom dross deposition bath 9: Sliding gate 10: Sliding gate 11: Zinc distillation device 12: Zinc supply tank

Claims (6)

[Claims] 1. A method for producing a hot-dip galvanized steel sheet, which comprises immersing a sink roll in a plating bath, introducing the strip into a plating tank, and performing hot dip galvanizing while orbiting the strip around the sink roll. Then, a bottom dross deposition tank communicating with the lower part of the plating tank is provided, and bottom dross accumulated on the bottom of the plating tank is discharged to the bottom dross deposition tank to discharge the bottom dross to the outside of the plating tank. A method for producing a hot-dip galvanized steel sheet, which is characterized. 2. Further, the discharged bottom dross is sent to a zinc distillation apparatus connected to the bottom dross deposition tank,
The vaporized Zn is recovered and circulated / reused as a plating solution, and / or the vaporized Zn is used as a heat source for melting the Zn and Al ingot for supplementing the plating solution. The method for producing a hot-dip galvanized steel sheet according to. 3. The plating bath level is kept constant by replenishing Al and Zn discharged from the plating tank together with the bottom dross.
A method for producing a hot-dip galvanized steel sheet according to any one of 1. 4. An apparatus for manufacturing a hot-dip galvanized steel sheet having a plating tank having a sink roll inside, which is (i) arranged below the plating tank to deposit and concentrate bottom dross from the plating tank. Bottom dross deposition tank, (ii)
An apparatus for manufacturing a hot-dip galvanized steel sheet, comprising: an opening / closing device that opens and closes a communication part of the plating tank and the bottom dross deposition tank. 5. The hot-dip galvanized steel sheet manufacturing apparatus according to claim 4, wherein at least the bottom of the plating tank has a cone shape. 6. (iii) a zinc distillation apparatus connected to the bottom dross deposition tank, and (iv) a Zn distillation apparatus connected to the zinc distillation apparatus to supplement Zn from the zinc distillation apparatus with new supply.
6. A hot-dip galvanized steel sheet manufacturing apparatus according to claim 4, further comprising: a zinc replenishing tank for supplying and circulating Al and Al in the plating tank.