JPH0925548A - Method for removing suspended impurity in molten metal in hot dipping metal coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably remove suspended impurities in molten metal at high removing efficiency by once dropping and holding to a specific temp. range, during introducing molten metal from plating bath to a vessel and returning back to the plating bath. SOLUTION: The molten metal 3 is pumped up from the plating vessel 2 with a pump 7, etc., and introduced into an auxiliary pot 8 through a piping 9 at the auxiliary pot inlet side. The molten metal is held to a temp. range of the solidified point +50 deg.C after once dropping and holding to a temp. range of below the solidified point +50 deg.C during returning back to the plating vessel 2 by shifting to the horizontal direction while rising and dropping in the auxiliary pot 8. By this method, the floated impurities in the bath can sufficiently be precipitated and floated up in a short time and the suspended impurities can efficiently and stably be removed. In this result, the quality and the yield of the hot dip coated band steel can stably be kept at high level in a long time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten metal plating method, and more particularly to a method for removing floating impurities in molten metal.

[0002]

2. Description of the Related Art In molten metal plating baths, there are floating impurities called dross produced by the reaction of iron and the bath components eluted from steel strips and equipment in the bath. The main component of this dross differs depending on the concentration of the bath components and other conditions, such as aluminum in a zinc bath, and it accumulates on the bottom of the bath as bottom dross or floats on the bath surface as top dross due to the difference in specific gravity from the molten metal. However, there are many particles floating in the bath during the process, and these adhere to the plated steel strip pulled from the bath and impair the quality of the product.

Therefore, conventionally, bottom dross and top dross are collected and removed during operation or when operation is stopped, and dross in the bath is melted as disclosed in, for example, Japanese Patent Application Laid-Open No. 4-160141. The metal is filtered by a ceramic filter, or, for example, Japanese Patent Laid-Open No. 4-2210.
As disclosed in Japanese Patent Publication No. 50, the molten metal is removed by statically settling it in a separate tank.

[0004]

However, the removal method using the ceramic filter has a small removal effect when the filter has a large mesh, and if the mesh has a small mesh, clogging occurs and the filtration performance deteriorates. It is difficult to carry out while maintaining a stable state, and the removal method by static sedimentation has a problem that it takes a long time and it takes time to completely recover the precipitated impurities.

In view of the above-mentioned problems of the prior art, the present invention highly removes suspended impurities in the molten metal in order to maintain the quality and yield of the hot-dip galvanized steel strip at a stable and high level for a long period of time. It is an object of the present invention to provide a method that enables efficient and stable removal.

[0006]

In general, the amount of suspended impurities deposited in molten metal is governed by the concentration of the bath component and the temperature of the molten metal. When the concentration of the bath component is high or the temperature of the bath component is low, the amount of impurities The amount of precipitation increases and the amount of floating impurities increases. However, the bath components have the effect of suppressing alloying of the plating film and have an appropriate concentration range. Therefore, if they do not overlap with the preferred range for controlling the amount of impurity precipitation, change the bath components to change the amount of impurity precipitation. It is difficult to control. On the other hand, the temperature of the molten metal has relatively few such restrictions. The present inventors have focused their attention on this point and have completed the present invention as a result of intensive studies.

That is, according to the present invention, the molten metal is taken from the plating bath and guided to a container, and while being moved up and down in the container while being transferred in the horizontal direction and returned to the plating bath, its freezing point is temporarily below + 50 ° C. It is a method for removing floating impurities in molten metal in molten metal plating, which is characterized by holding the temperature in the temperature range above its freezing point + 50 ° C. after holding it in the temperature range. Then, it is preferable that the container is partitioned by a plurality of weirs. The present invention is applicable to hot-dip galvanizing, hot-dip zinc-aluminum alloy plating, and hot-dip aluminum coating.

[0008]

According to the present invention, the molten metal is taken from the plating bath and introduced into a container, and while being raised and lowered in the container while being transferred horizontally to the plating bath, its freezing point is temporarily below + 50 ° C. After falling down and holding in the temperature range of, its freezing point +5
Since the temperature is kept at 0 ° C. or higher, it becomes possible to sufficiently precipitate and float the floating impurities in the bath in a short time.

[0009]

Embodiments of the method of the present invention will be described below with reference to the drawings. However, the gist of the present invention is not limited to the figure. (Embodiment 1) FIG. 1 is a sectional view of an apparatus suitable for carrying out the present invention.
Shown in In FIG. 1, 1 is a snout, 2 is a plating tank,
3 is molten metal, 4 is sink roll, 5 is steel strip, 6 is wiping nozzle, 7 is pump, 8 is auxiliary pot (container),
Reference numeral 9 is an auxiliary pot inlet side pipe, and 10 is an auxiliary pot outlet side pipe. In the operation of the apparatus shown in FIG. 1, the steel strip 5 passes through the molten metal 3 in the plating tank 2 by the snout 1 and the sink roll 4, and the basis weight is adjusted by the wiping nozzle 6 as in the prior art. Is. The present invention differs from the prior art in that molten metal 3 (eg, zinc) is used.
Is pumped up from the plating tank 2 by a pump 7 or the like, and is guided to the auxiliary pot 8 through the auxiliary pot inlet side pipe 9, and the auxiliary pot 8
While raising and lowering in the inside, while transferring horizontally and returning to the plating tank 2, once it is lowered and held at a temperature range below its freezing point + 50 ° C. and then maintained at that freezing point + 50 ° C. or higher. is there.

That is, the molten metal 3 is transferred in the horizontal direction while moving up and down in the auxiliary pot 8 and its temperature is
By holding the temperature below the freezing point + 50 ° C. once, a large amount of impurities are deposited in the molten metal 3 in the auxiliary pot 8 and the deposits are formed in a short time.
The remarkable effect of floating above the melt surface appears. This action will be described in more detail.

For example, in the case of hot dip galvanizing or hot dip zinc-aluminum alloy plating, the floating impurities to be floated are Fe--Al alloys containing FeAl ₃ , Fe ₂ Al ₅ or FeAl ₄ Si as the main component, and their specific gravity (3.9 Since ~ 4.2) is smaller than the specific gravity of molten zinc (6.2 to 6.7), it was thought that if it precipitates and becomes a floating impurity, it will easily surface without doing anything. However, in reality, particles having a large particle size easily float in a short time, but particles having a particle size of about 20 μm or less take a long time to float. However, by carrying out the requirement of the present invention that the molten metal 3 is transferred in the horizontal direction while moving up and down in the auxiliary pot 8, an upward flow occurs during the transfer of the molten metal 3, and the force of floating impurities As the upward movement is accelerated, the floating time of even small-sized floating impurities is greatly reduced.

On the other hand, in order to form a precipitate in the molten metal 3, it is necessary to once hold the molten metal 3 in a temperature range below its freezing point + 50 ° C. The reason is as follows. That is, in the plating tank of the actual operation line, since Fe is always leached from the immersed steel strip or the equipment in the bath, it was estimated that the molten metal in the tank was always supersaturated or in a state very close to it. . According to this estimation, molten metal should always be in a state where it is easy to form a precipitate. However, according to the knowledge of the present inventors, the amount of floating impurities to be floated in the auxiliary pot 8 depends on the bath components in the molten metal 3 (Al and Fe in the case of hot dip galvanizing).
When the bath component concentration is low or the temperature is high, the molten metal 3 is in an unsaturated state and the precipitation of impurities in the auxiliary pot 8 does not occur at all. However, by implementing the requirement of the present invention that the molten metal 3 is once lowered and held in the temperature range below its freezing point + 50 ° C., the molten metal 3 in the auxiliary pot 8 is optimal for always precipitating impurities. Since it can be placed in a completely supersaturated state, it becomes possible to stably remove floating impurities.

After the floating impurities are deposited as described above, the temperature of the molten metal 3 is raised to its freezing point + 50 ° C. or higher and then returned to the plating tank 2, whereby the temperature of the plating tank 2 is excessively lowered and It is possible to prevent the precipitation of impurities in the plating tank 2 due to this. Auxiliary pot (container)
FIG. 2 shows a sectional view of a preferred example of No. 8. The same members as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. In FIG. 2, 11 is a primary weir, 12 is a secondary weir, 13 is a tertiary weir, 14a is a heat exchanger for heating, 14b is a heat exchanger for cooling, 15 is a thermocouple, 16 is a flux, and 17 is a first. The first room, 18 is the second room, and 19 is the third room. The first chamber 17 and the second chamber 18 are the primary weir 11, and the second chamber 18 and the third chamber
The chamber 19 is divided into a secondary weir 12 and a third weir 13. The molten metal 3 introduced from the auxiliary pot inlet side pipe 9 into the first chamber 17 at one end of the container 8 is heated to a freezing point of + 50 ° C. by the heating heat exchangers 14a arranged on the side and bottom surfaces of the chamber.
While keeping the temperature as above, it moves over the primary weir 11 and moves to the second chamber 18, where it is once cooled to below the freezing point + 50 ° C by the cooling heat exchangers 14b arranged on the side and bottom surfaces of the same chamber and the secondary When passing through the passage formed by the weir 12 and the tertiary weir 13, it moves to the third chamber 19 while flowing upward. At this time, a large amount of floating impurities are precipitated in the molten metal 3 and, at the same time, float quickly. Floating floating impurities are trapped by the flux 16 (for example, NH ₄ Cl in the case of hot dip galvanizing) and removed to the outside of the system. Then, the molten metal 3 moved to the third chamber 19 is heated again to the freezing point + 50 ° C. or higher by the heat exchanger 14a for heating arranged on the side surface and the bottom surface of the same chamber, and then passes through the auxiliary pot outlet side pipe 10. Then, the process returns to the plating tank 2 shown in FIG. The heat exchanger 14 in each room is controlled based on the measured value of the thermocouple 15 provided in each room.

In this way, the container 8 is divided into chambers by the weirs 11 to 13, and if each chamber is provided with a temperature adjusting means (heat exchanger 14) independently, the molten metal 3 passing therethrough is directed upward. The method of the present invention in which the temperature of the molten metal 3 is once lowered and raised again at the freezing point + 50 ° C. as a boundary can be implemented very easily. Using the apparatus illustrated in FIGS. 1 and 2, the method of the present invention was carried out in the case of hot dip galvanizing. Here, the time taken for 3200 kg of molten metal to pass through the container 8 in the steady state is 1 hr. In addition, the case where the molten metal 3 was allowed to pass through while keeping the temperatures of the first to third chambers of the container 8 constant was set as a comparative example. Table 1 shows the bath component concentration, bath temperature, other conditions, and the floating efficiency represented by the following equation in the examples. Floating efficiency (%) = (number of impurities on the inlet side-number of impurities on the outlet side)
/ (Number of impurities on the inlet side) × 100 Note that the number of impurities on the inlet side and the outlet side is the sample after the molten metal sampled from the inlet side and the outlet side in the container 8 is rapidly cooled and solidified, and the cross-section thereof is obtained using a microscope after polishing. Measured.

[0015]

[Table 1]

As is clear from Table 1, the levitation efficiency of the examples of the present invention (Nos. 1 to 4) is 75% to 97%,
Compared with that (50% to 65%) of the comparative example (No. 5, 6), it was remarkably improved. (Example 2) Using the apparatus illustrated in FIGS. 1 and 2, the method of the present invention was carried out in the case of hot-dip zinc-aluminum alloy plating and hot-dip aluminum plating. In addition, the case where the molten metal 3 was allowed to pass through while keeping the temperatures of the first to third chambers of the container 8 constant was set as a comparative example. Table 2 shows the bath component concentration, the bath temperature, other conditions, and the floating efficiency.

[0017]

[Table 2]

As is clear from Table 2, the levitation efficiency of the examples of the present invention (Nos. 7 and 9) was significantly improved as compared with that of the comparative examples (Nos. 8 and 10).

[0019]

EFFECTS OF THE INVENTION According to the present invention, the molten metal is collected from the plating tank into a container, and while being transferred horizontally in the container, the temperature is lowered to below the freezing point + 50 ° C. and the molten metal is caused to flow upward, thereby floating in the bath. Since impurities can be sufficiently precipitated and floated in a short time, the adherence of floating impurities to the plated steel strip is reduced, the product quality and yield are improved, and the floating floats on the bath surface of the plating tank. Great effects such as reduction of impurities (top dross) are achieved.

[Brief description of drawings]

1 is a cross-sectional view of an apparatus suitable for implementing the present invention.

FIG. 2 is a sectional view of a preferred example of an auxiliary pot (container).

[Explanation of symbols]

 1 Snout 2 Plating tank 3 Molten metal 4 Sink roll 5 Steel strip 6 Wiping nozzle 7 Pump 8 Auxiliary pot (vessel) 9 Auxiliary pot inlet side piping 10 Auxiliary pot outlet side piping 11 Primary weir 12 Secondary weir 13a Heat exchanger for heating 14b Heat exchanger for cooling 15 Thermocouple 16 Flux 17 First chamber 18 Second chamber 19 Third chamber

Claims (3)

[Claims] 1. A molten metal is collected from a plating bath, introduced into a container, and while being raised and lowered in the container while being transferred in the horizontal direction and returned to the plating bath, its freezing point is temporarily +50.
A method for removing floating impurities in molten metal in molten metal plating, which comprises maintaining the temperature in the temperature range of less than ℃ + 50 ° C or more after the temperature is lowered and maintained. 2. The method for removing floating impurities in molten metal in molten metal plating according to claim 1, wherein the container is partitioned by a plurality of weirs. 3. The method for removing floating impurities in molten metal in molten metal plating according to claim 1 or 2, wherein the molten metal is molten zinc, molten aluminum, or a molten zinc-aluminum alloy.