JPH0860319A - Method for removing dross - Google Patents

Abstract

PURPOSE: To make it possible to remove not only the suspended dross but the entire dross as well by setting the temp. of the dross-contg. hot dip metal coating liquid in a removing vessel lower than the hot dip coating liquid temp. in a hot dip coating cell and higher than the m. p. of the hot dip coating metal. CONSTITUTION: The hot dip coating bath contg. the dross in the coating cell 3 is ladled out into the removing vessel 7. The temp. of the dross-contg. hot dip metal coating liquid in the removing vessel 7 is set lower than the coating liquid temp. in the coating cell 3 and higher than the m. p. of the coating metal. The Fe in the coating liquid in the removing vessel 7, then, attains the supersatd. state by as much as a difference in solubility. The Fe combines with the Zn and Al in the coating liquid and crystallizes as an intermetallic compd., i.e., dross. The clean liquid obtd. by separating away this dross is returned to the coating cell 3. This clean liquid is kept at the temp. equal to the temp. of the coating liquid in the coating cell 3 and attains the state that the Fe is not satd. As a result, not only the suspended dross but the dross deposited in the bottom as well are efficiently removed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing dross in a hot-dip galvanizing bath, for example, in the manufacturing process of hot-dip galvanized steel sheet.

[0002]

2. Description of the Related Art As is already well known,
In North America and Canada, road salt is sprayed to prevent roads from freezing to prevent automobile slip accidents in winter.

However, due to the rock salt spraying, the car body is exposed to a severe corrosive environment in terms of corrosion, and surface treated steel sheets having high corrosion resistance are being used in terms of raw materials. Therefore, an alloyed steel sheet that has excellent weldability and workability similar to that of a cold rolled steel sheet and has excellent corrosion resistance is drawing attention. In addition, materials with increasingly stringent specifications are being demanded, such as the need for sharpness for exterior applications.

By the way, in the production of hot-dip galvanized steel sheet, solid particles called dross are present in a molten metal bath to be plated (hereinafter referred to as "plating bath"), and dross floating in the plating bath. Gradually accumulates at the bottom of the plating tank and forms a bottom dross.

The bottom dross deposited on the bottom of the plating tank as described above is wound up in the plating bath by the accompanying flow of the strip generated near the bottom of the sink roll and adheres to the surface of the strip. When such a bottom dross is attached to the strip, a surface non-uniform portion called a press spot is generated during pressing, which not only impairs the image clarity but also forms a non-uniform portion called a dross spot.
A surface defect that forms a local battery and becomes a base for lowering corrosion resistance occurs, causing a quality defect in the plated steel sheet.

In recent years, the production amount of hot-dip Al-plated steel sheets and hot-dip Al-Zn steel sheets, which are more excellent in corrosion resistance than hot-dip galvanized steel sheets, is increasing, and the dross present in the plating baths in the production of these steel sheets are mutually It has a characteristic of being bonded and lumped, and this lumped dross is very strong, and it is difficult to remove the dross if it adheres to the bottom or wall surface of the plating tank.

Therefore, hot-dip Al plated steel sheet, hot-dip Al
-Continuing to manufacture Zn steel plate, the agglomerated dross increases in the plating bath, the agglomerated dross comes into contact with sink rolls or steel strips, and product defects such as scratches occur,
In addition to the defects due to the floating dross of the hot dip galvanizing described above, defects due to agglomerated dross occur.

As a conventional method of removing dross in order to prevent defects due to dross, a method of manually or mechanically using a scoop or bucket,
A method in which Al is added to the plating bath and the dross is floated and removed, JP-A-3-271353 and JP-A-4
In addition to the plating baths proposed in JP-A-221049 and JP-A-4-221050, there is a method of providing a bath outside the system and utilizing the bath outside the system.

[0009]

However, in the method (1), since dross containing a large amount of plating solution is pumped out from the plating bath, it is dangerous that a high-temperature plating solution exceeding 450 ° C. is scattered. Moreover, the dross accumulated on the corners of the bottom of the plating tank cannot be completely removed. On the other hand, the method (1) is effective only for a galvanizing bath having an Al content of 0.2 wt% or less, and the dross formed by a galvanizing bath having other component composition is formed by the galvanizing bath. Unlike dross, the specific gravity of the dross is heavier than that of the plating bath, so the dross settles in the plating bath and cannot be used.

The method (1) is a method of removing the floating dross outside the plating tank or refining it so that there is no obstacle, and it is effective for preventing product defects caused by the floating dross adhering to the steel sheet. Is. However, the floating dross is targeted, and the dross to be removed is only the floating dross having very small particles, and most dross such as bottom dross accumulated on the bottom of the plating tank is not removed. Therefore, there is a problem that most dross must be removed by the method.

The present invention has been made in view of the above problems, and an object thereof is to provide a dross removing method capable of removing not only floating dross but all dross.

[0012]

In order to achieve the above-mentioned object, the inventors of the present invention have added either or both of Zn and Al as a bath component and added Si in an amount of less than 15 wt%. The behavior of dross generation in the plating bath was investigated.
As a result, it was found that the dross was an intermetallic compound in which Fe eluted from the plated steel sheet was combined with the bath components Zn, Al and Si.

As a result of investigating the Fe saturation solubility in the plating bath, it was found that the Fe saturation solubility increases as the plating bath temperature rises, as shown in FIG. Furthermore, as a result of investigating the behavior of the dross when the temperature of the plating bath was changed, the following was found. When the temperature of the plating bath was lowered, it became supersaturated in the plating bath F
e crystallizes as dross and is in an unsaturated state in the plating bath when the temperature of the plating bath is increased. Fe is saturated by dissolving the dross in the plating bath under the condition that dross exists in the plating bath. Try to keep the state.

The present invention has been made based on the above-mentioned findings. The molten metal plating solution containing dross is pumped out from the inside of the molten metal plating tank and guided to the removal tank outside the system, and the dross is removed in the removal tank. In the method for removing the dross which returns the clean solution to the original plating tank after separating, the temperature of the molten metal plating solution containing the dross in the removal tank is lower than the temperature of the plating solution in the molten metal plating tank, and The temperature was set higher than the melting point of the plating metal.

In the present invention, the temperature of the molten metal plating solution containing dross in the removal tank is set higher than the melting point of the plating metal, because the temperature of the melting point or lower causes the entire plating solution to solidify or a solidification phase to form in the plating solution. This is because it will be in a semi-molten state containing it and it is difficult to separate it from the dross to be removed. In addition,
The melting point of the plating solution to which the present invention is applied varies depending on the composition of the molten metal. For example, the melting point of the 0-10 wt% Al-Zn plating solution is about 420 ° C, and the melting point of the 0-15 wt% Si-Al plating solution is about 660 ° C. , 55wt% Al -43.4wt% Zn
The melting point of the -1.6 wt% Si plating solution is about 570 ° C.

[0016]

According to the present invention, the molten metal plating solution containing dross is pumped out from the molten metal plating tank and guided to the removal tank outside the system, and the dross is separated in the removal tank, and then the clean solution is used as the original plating tank. In the method for removing dross, the temperature of the molten metal plating solution containing the dross in the removal tank is set to be lower than the temperature of the plating solution in the molten metal plating tank and higher than the melting point of the plating metal. Fe is crystallized as dross from the molten metal plating solution that has become supersaturated inside.

In FIG. 1, reference numeral 3 denotes a plating tank containing a plating solution. The strip 1 sent into the plating bath through a snout 2 goes around a sink roll 4 and a guide roll 5 to leave the plating bath. Plated during removal,
After the plating is completed, the plating thickness is made uniform with a wiping nozzle (not shown).

Reference numeral 6 denotes a pump for pumping the plating solution containing dross in the plating tank 3 into the removal tank 7, and the plating solution pumped by the pump 6 drops in temperature in the removal tank 7. Supersaturated Fe can be crystallized out. And
The cleaning liquid after the stationary sedimentation treatment in the removal tank 7 is returned to the plating tank 3 by the pump 8. In addition, 9 and 10 are bottom dross deposited on the bottom of each of the plating tank 3 and the removal tank 7.

The method of the present invention is carried out, for example, as described above. However, since the plating solution in the plating tank 3 contains dross, Fe in the plating bath at the temperature T1 has a solubility C1 in a saturated state. . Then, assuming that the solubility of the plating solution in the saturated state at the temperature T2 is C2, the total weight W
When the weight W2 of this plating solution 1 is pumped out of the plating solution in the plating tank 3 into the removal tank 7 and lowered to the temperature T2, Fe in the plating solution in the removal tank 7 has a solubility difference C
Only 1-C2 becomes supersaturated.

In the plating solution in the removing tank 7 in the supersaturated state, Fe is combined with Zn and Al in the plating solution to crystallize as an intermetallic compound, that is, dross. Then, when the cleaning liquid obtained by separating and removing the dross in the removing tank 7 is returned to the plating tank 3, the cleaning liquid becomes equal to the temperature T1 of the plating solution in the plating tank 3 and the difference in solubility C1-C2 is Fe. Is not saturated. Therefore, the dross in the plating solution is dissolved in an attempt to maintain the saturated state, and (C1
The Fe amount of −C2) × W2 is replenished, and the Fe amount of the plating solution in the plating tank 3 becomes saturated again.

From the above, the Fe amount of (C1 -C2) x W2 is removed from the plating solution in the plating tank 3 per time, and by repeating this step, all the dross existing in the plating solution in the plating tank 3 is removed. Can be removed.
Further, the above steps can be carried out by using either a batch type processing method in which a plating solution having a weight of W2 is pumped out from the plating tank 3 or a method in which the plating solution is continuously pumped out from the plating tank 3. .

When the temperature T1 of the plating solution in the plating tank 3 is higher than the melting point Tm of the plating metal by 200 ° C. or more, when the method of the present invention is used while plating the steel strip, the plating solution and the steel are used. The band may react violently and adversely affect the plating characteristics. In addition, Zn with high vapor pressure
In the case of a plating solution having a composition containing a large amount of such elements as above, when the temperature T1 of the plating solution in the plating tank 3 becomes higher than the melting point Tm of the plating metal by 200 ° C. or more, Zn vapor is vigorously generated and the operation is continued. It will be difficult. However, it is possible to heat the temperature of the plating solution T1 in the plating tank 3 to a temperature higher than the melting point Tm of the plating metal by 200 ° C. or more if measures are taken against the deterioration of the plating characteristics and the environment.

For the above reason, the specific relationship between the temperature T1 of the plating solution in the plating tank 3 and the temperature T2 of the plating solution in the removal tank 7 in the method of the present invention is expressed by the following mathematical formula 1.

[0024]

[Equation 1] Tm + 200 ° C> T1> T2> Tm Tm: melting point of plating metal

Within the above range, the temperature T1 of the plating solution in the plating tank 3 is set to a high temperature, and the temperature T of the plating solution in the removal tank 7 is set to T.
It is desirable to keep 2 at a low temperature. Plating tank 3 and removal tank 7
This is because the larger the temperature difference T1-T2 of the plating solution, the larger the Fe solubility difference C1-C2, and the greater the dross removal efficiency.

The method of separating the dross crystallized in the removal tank 7 from the plating solution is not only a method of allowing the dross to stand still but allowing the dross to settle, but also a method of filtering the dross with a filter or the like, an inert gas in the plating solution. There is a method in which the air is blown in to adsorb dross on the air bubbles to float the air.

[0027]

EXAMPLE An example of the present invention will be described below. Using the apparatus shown in FIG. 1, 20 m ³ of a plating solution having the composition shown in Table 1 was prepared in the plating tank 3, and 5 t of a low carbon steel plate was immersed and dissolved in the plating solution to obtain the temperature of the plating solution in the plating tank 3. T
The dross removal treatment was performed for 1 day by changing the temperature T2 of the plating solution in the removal tank 7 of 1 and 2 m ³ . Tables 2 to 4 show the results of investigating the amount of Fe contained in the dross precipitated in the removal tank 7 after the removal treatment for each plating solution. The pumping amount of the pumps 6 and 8 for pumping the plating solution from the plating tank 3 is 0.2 m.
³ / min.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

From the above results, according to the method of the present invention, dross can be efficiently removed, and it is particularly effective if the temperature difference T1 -T2 between the plating solution in the plating tank and the plating solution in the removal tank is large. it is obvious.

[0033]

As described above, according to the present invention,
Not only the floating dross but also the dross deposited on the bottom can be efficiently removed, and it is possible to manufacture a hot dip plated steel sheet with excellent appearance quality. Further, it is effective to increase the temperature difference between the plating solution in the plating tank and the plating solution in the removal tank.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of equipment for carrying out the method of the present invention.

[Fig. 2] Temperature of plating bath and Fe dissolved in plating bath
It is the figure which showed the relationship with saturated solubility.

[Explanation of symbols]

 1 Strip 3 Plating tank 6 Pump 7 Removal tank 8 Pump 9 Bottom dross 10 Bottom dross

Claims (1)

[Claims] 1. A molten metal plating solution containing dross is pumped out of the molten metal plating tank and led to a removal tank outside the system, the dross is separated in the removal tank, and then the cleaning solution is returned to the original plating tank. In the method for removing dross, the temperature of the molten metal plating solution containing the dross in the removal tank is lower than the temperature of the plating solution in the molten metal plating tank and higher than the melting point of the plated metal. Removal method.