JPH11217658A - Method and device for hot dipping - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve quality of a plated metallic strip surface by passing a metallic strip in a snout shifting upward from a snout center and applying hot dipping so as to prevent hot dip bath vapor from condensing/falling down to a upper side wall of the snort. SOLUTION: A steel sheet 1 heated/annealed in an reducing atmosphere is, passing through a snort, is hot dipped by immersing in a hot dip galvanizing bath. At this time a pass line of the steel sheet 1 in the snort is shifted to a snort upper side wall face 3a. As necessary by arranging a temp. holding device or a heating device 9 to the upper side wall face and water cooling or air cooling devices 8 to a snort lower side wall 3b/side wall face 3c, these faces are kept to a lower temp. than the solidification point of a hot dip bath vapor. By this method, a condensate 6 of hot dip bath vapor is prevented from depositing resulting from the gas down flow to the upper side wall face 3a side, gas up flow is caused to the lower side wall face 3b and the side wall face 3c to deposit the condensate 6 thereon, the hot dip bath vapor advancing to an annealing furnace is reduced.

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 continuously immersing a metal strip in molten metal for plating.

[0002]

2. Description of the Related Art A continuous hot-dip plating method is one of the methods for continuously plating a metal such as tin or zinc on the surface of a steel sheet or the like. The following description is made by taking continuous hot-dip galvanizing in which a steel sheet is galvanized as an example.

FIG. 1 is a schematic view of a main part of a conventional hot-dip galvanizing line. The steel sheet 1 is heated and annealed in an annealing furnace 2 in a reducing atmosphere, and immersed in a hot dip galvanizing bath 4 via a snout 3. After immersion, the steel sheet 1 is turned by the sink roll 5, pulled up into the atmosphere, undergoes a step of adjusting the molten coating thickness, and the plating layer solidifies to become a plated steel sheet. Generally, the snout 3 is provided at an angle due to the structure of the equipment.

The inside of the annealing furnace 2 and the snout 3 is kept in a reducing atmosphere, and the surface of the steel sheet 1 is kept in a non-oxidizing state. If there is an oxide film of molten zinc on the surface of the plating bath in the snout 3, the surface of the plating bath in the snout must be clean because it adheres to the surface of the steel sheet and causes surface defects.

However, when there is no oxide film on the surface of the plating bath 4, a large amount of zinc vapor is generated and condensed and accumulated on the inner wall surface of the snout 3 to form a condensate 6. When the condensate 6 accumulated on the snout upper wall surface 3a falls on the surface of the steel sheet by some vibration, the surface of the steel sheet is activated. Therefore, the condensate 6 easily adheres to the surface of the steel sheet 1 or drops on the surface of the plating bath and The substance 7 adheres to the surface and causes the surface flaw.

[0006] In order to prevent the condensation of zinc vapor on the wall surface of the snout, measures are taken to keep the snout warm. However, the zinc vapor also enters the annealing furnace, where zinc solidifies and accumulates in the annealing furnace. Heating the snout alone is not a fundamental solution since it adheres to the surface and causes surface flaws.

The following techniques (a) to (d) have been proposed as methods for suppressing the generation of zinc vapor or removing the generated zinc vapor. (a) Japanese Patent Application Laid-Open No. 7-62512 proposes a technique for preventing the evaporation of zinc from a plating bath by floating a ceramic ball-shaped object on the surface of a plating bath in a snout.

(B) Japanese Patent Application Laid-Open No.
A technique has been proposed for removing zinc vapor (fume) by providing a suction port and a reflux port on a snout wall surface. (c) Japanese Patent Application Laid-Open No. Hei 7-157854 discloses a method in which a zinc vapor suction port and a reflux port are provided on a snout wall surface, and a nitrogen gas is blown downward on a steel sheet in the snout to remove zinc vapor, thereby forming a coagulated steel sheet. There has been proposed a technique for preventing the adhesion to the surface. .

(D) JP-A-7-157855 describes that
A technique has been proposed in which a filter and a fan are provided in an annealing furnace shell above a snout to capture zinc vapor ascending the snout, and a purified atmosphere gas is sprayed on a steel sheet to clean the surface.

[0010]

SUMMARY OF THE INVENTION The above-mentioned (a)
According to the technology disclosed in Japanese Patent No. 2512, with the vibration of the steel sheet,
The contact between the steel plate and the ceramic ball may cause scratches.

With respect to the technique (b) of Japanese Patent Application Laid-Open No. 7-157853, the inventors simulated the flow of the atmosphere gas. As a result, it was found that an enormous amount of atmospheric gas had to be sucked, and the apparatus for removing zinc vapor also had an enormous operating cost, and was therefore less feasible.

(C) The technique of Japanese Patent Application Laid-Open No. 7-157854 also has the same problem as the technique of Japanese Patent Application Laid-Open No. 7-157854 because it is a method of sucking an enormous amount of atmospheric gas.

[0013] In the technique of (d) JP-A-7-157855, since a filter and a fan are provided in the furnace,
Equipment costs and operating costs become enormous. In order to solve the above problems, an object of the present invention is to prevent the generation of zinc condensate in a snout at low cost equipment and operating costs,
An object of the present invention is to provide a method and an apparatus for producing a plated steel sheet having an excellent surface.

[0014]

Means for Solving the Problems In order to solve the above problems, the present inventors analyzed the behavior of zinc vapor evaporating from the plating bath surface, and performed various simulations and tests. (d) was obtained.

(A) In a snout, a convection of an atmospheric gas (hereinafter, simply referred to as a gas) occurs due to the running of a steel plate. This gas has a downward flow near the steel plate and an upward flow near the snout wall surface. Zinc vapor rides on the ascending flow,
A part is cooled and condensed on the snout wall, and a part enters the annealing furnace.

(B) The solidified material on the upper wall surface of the snout easily falls and adheres to the steel plate, but the aggregate on the lower wall surface of the snout or the side wall surface of the snout does not adversely affect the steel plate.

(C) When the pass line of the steel sheet is brought closer to the upper wall surface of the snout, the upward flow of the gas decreases on the upper wall surface of the snout and increases on the lower wall surface of the snout and the side wall surface of the snout. (d) If zinc vapor is actively agglomerated and trapped on the side wall surface and the lower wall surface of the snout, zinc vapor entering the annealing furnace can be reduced.

Based on the above findings, the gist of the present invention lies in the following (1) to (5). (1) A method for continuously immersing a metal strip in a hot-dip metal plating method, wherein the metal strip is offset above the center of the snout and passed through.

(2) A method for continuously immersing a metal strip in a molten metal plating method, wherein at least one of the lower wall surface and the side wall surface of the snout is maintained at a temperature lower than the condensation point of the metal vapor. Plating method.

(3) A hot dip metal plating apparatus of a continuous immersion type of metal strip, wherein the center of the snout is offset below the pass line of the metal strip.

(4) In a continuous immersion type hot-dip metal plating apparatus for a metal strip, means for maintaining at least one of the lower wall surface and the side wall surface of the snout at a temperature lower than the condensation point of the metal vapor is provided. Metal plating equipment.

In the above (1) and (3), the center of the snout means the center position between the upper wall surface of the snout and the lower wall surface of the snout in the cross section of the snout, and a line connecting the center positions in the direction of the pass line is a straight line. If not, that is, if the snout cross section is locally different or the snout is bent or curved over the substantial snout length, the straight line that minimizes the root mean square of the line connecting the center positions It shall be defined by position.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is a schematic view of a gas flow in a snout portion, wherein FIG. 2 (a) shows a case according to the prior art and FIG. 2 (b) shows a case according to the present invention.

In FIG. 2A, the pass line of the steel plate 1 passes through the center of the snout 3. As the steel sheet 1 travels, the gas near the steel sheet flows downward. This downward flow is reversed on the surface of the plating bath 4 and the upper snout upper wall surface 3a
And snout lower wall surface 3b and snout side wall surface 3
An upward flow along c.

In FIG. 2B, the pass line of the steel sheet 1 is shifted upward with respect to the center of the snout. Since the distance between the steel plate 1 and the snout upper wall surface 3a is small, most of the gas near the snout upper wall surface 3a also flows downward, collides with the plating bath 4, and moves downward from the gap between the steel plate edge portion and the snout side wall surface. Wrap around. On the lower surface side, the gas in the vicinity of the steel plate 1 forms a downward flow, but at a distance from the steel plate, the gas flows upward in the vicinity of the snout lower side wall surface 3b, and the gas flowing from the above upper side Both rise. In the vicinity of the side wall surface 3c, the gas has an upward flow because the gas is not affected by the running of the steel plate.

Accordingly, the zinc vapor evaporated from the surface of the plating bath 4 rides on the upward flow near the snout lower wall surface 3b and the side wall surface 3c, and condensate 6 is deposited on the snout lower wall surface 3b and the side wall surface 3c. I do. On the other hand, since no condensate is generated on the snout upper wall surface 3a, it is possible to prevent agglomerates from adhering to the upper surface of the steel plate and reduce plating defects.

FIG. 3 is a schematic diagram showing another example of the pass line according to the present invention. That is, the pass line of the steel sheet 1 does not have to be evenly offset over the entire length of the snout 3, and the zinc vapor only has to sneak to the lower side of the steel sheet as early as possible. The method of shifting the pass line only at the lower part of the snout as shown in the figure has a feature that the existing equipment does not need to be largely modified.

FIG. 4 is a graph showing a simulation result of a gas flow in a snout when the amount of offset of the pass line according to the present invention in the upward direction is changed. FIG. 5A shows the gas flow velocity on the upper wall 3a of the snout (50 mm from the wall,
FIG. 4B shows the gas flow velocity on the snout lower wall surface 3b, and FIG. 4C shows the gas flow velocity on the snout side wall surface.

As shown in FIG. 3A, when the pass line is located at the center, the gas flow velocity on the upper side becomes 0.3 m / s, but when the pass line is displaced 15 mm from the center to the upper side, the upper gas velocity becomes higher. Shows that the gas flow becomes substantially zero, and when the pass line is further brought closer to the upper wall of the snout, the gas flow becomes a downward flow.

FIG. 3B shows that the flow velocity of the upward flow below the snout increases as the pass line is shifted upward. Also, FIG. 3 (c) shows that the flow velocity on the side wall surface is always an upward flow, and becomes smaller as the steel sheet pass line is shifted toward the upper snout wall surface.

The closer the pass line is to the upper wall surface, the more the upward ascending flow can be suppressed. However, in order to prevent the steel plate from coming into contact with the wall surface due to vibration, the offset amount should be within a range of 50 mm or less from the center of the snout. Is preferred. That is, in the example of FIG. 2B, it is preferable that the offset amount of the pass line be in the range of 1/20 to 1/6 of the snout thickness.

FIG. 5 is a schematic view of a snout wall cooling device according to the present invention. FIG. 5 (a) is a side view, and FIG. 5 (b) is a front view taken along the line AA. In the figure, the same parts as those in FIG. 1 are denoted by the same reference numerals. The snout lower wall surface 3b and the side wall surface 3c are cooled by a cooling device 8 using water cooling. Since the zinc vapor evaporated from the surface of the plating bath 4 mainly rises along the lower wall surface 3b and the side wall surface 3c,
Condensed on the cooled lower wall surface 3b or side wall surface 3c. Therefore, zinc vapor entering the annealing furnace can be reduced.

In the case of zinc plating, the condensation temperature of zinc is 42
Since the temperature is 0 ° C., in order to keep the snout lower side wall surface 3b and the side wall surface 3c below the condensing temperature, the cooling device 8 may be a cooling structure using air cooling fins without special water cooling, or more simply a snout structure. The bare steel plate outside the object may be used as it is.

On the other hand, the snout upper wall surface 3a may be provided with a heat retaining device or a heating device 9 as in the prior art in order to prevent the condensation of zinc vapor, but this is not an essential condition of the present invention.

FIG. 6 is a schematic diagram showing a snout portion provided with a space for storing condensate. Figure (a) is a side view,
(b) is a front view as seen from the direction of arrows AA. In the figure, the same parts as those in FIGS. 1 and 5 are denoted by the same reference numerals. In FIGS. 7A and 7B, a cooling device 8 using water cooling is installed on the lower side wall surface 3b and the side wall surface 3c of the snout.

The condensate 6 mainly adheres to the snout lower wall surface 3b and the side wall surface 3c, but the growth rate of the aggregate 6 is higher than when the wall surface is not cooled. If the aggregate 6 grows excessively, the aggregate 6 comes into contact with the steel sheet 1 to cause a flaw or the aggregate 6 may fall into the plating bath 4 and become a deposit on the plating surface. There must be. Further, this is contrary to the aim of keeping the upward flow by moving the pass line according to the present invention away from the lower wall surface 3 of the snout. In the embodiment shown in the figure, in order to solve the above-mentioned problem, a storage space 10 for the aggregates 6 is provided in the snout so that even if the aggregates 6 grow to some extent, they do not come into contact with the steel plate.

[0037]

EXAMPLE As shown in FIG. 5, the pass line was biased to the upper wall of the snout, and the cooling device was installed on the actual machine to conduct a survey in actual operation. A 10-day test run was performed and compared to the 10-day quality data of a normal run before application of the present invention.

Prior to each operation, the condensate on the inner wall of the snout was removed, and after the operation was completed, the condensate was recovered from the inner surface of the snout. Table 1 shows the specifications of the plating line and the manufacturing conditions of the steel sheet. Table 2 shows the operation results.

[0039]

[Table 1]

[0040]

[Table 2]

As can be seen from Table 2, the present invention significantly improved the quality of the plated steel sheet. In addition, the amount of snout condensate recovered increased, and there was an effect of resource recycling. Furthermore, in the comparative example, it is considered that zinc vapor corresponding to the difference in the amount of snout condensate recovered from that of the present invention entered the annealing furnace, which may be a new cause of plating flaws due to falling of condensate in the annealing furnace. is expected. Therefore, it was found that the quality improvement effect of the present invention was further enhanced.

[0042]

According to the present invention, the condensate of zinc vapor on the upper wall surface of the snout can be prevented from adhering, the condensate can be prevented from falling on the steel sheet, and the surface quality of the plated steel sheet can be improved. Furthermore, the penetration of zinc vapor into the annealing furnace can be reduced, and the condensate falling in the annealing furnace can be reduced, so that the surface quality of the coated steel sheet can be further improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a main part of a conventional hot-dip galvanizing line.

FIG. 2 is a schematic view of a gas flow in a snout part, and FIG.
FIG. 3B shows the case according to the prior art, and FIG.

FIG. 3 is a schematic diagram showing another example of the case where the pass line according to the present invention is offset to the upper wall surface of the snout.

4A and 4B are graphs showing simulation results of a gas flow in a snout in the method of the present invention, wherein FIG. 4A shows the vicinity of the upper wall of the snout, FIG. 4B shows the vicinity of the lower wall of the snout, and FIG. ) Is a graph showing the gas flow near the side wall.

FIGS. 5A and 5B are schematic views of a cooling device according to the present invention, wherein FIG. 5A is a side view, and FIG. 5B is a front view taken along the line AA.

FIG. 6 is a schematic diagram of a cooling device having a condensate accumulation space according to the present invention, wherein FIG. 6 (a) is a side view, and FIG.
It is an arrow A front view.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Annealing furnace 3 Snout 3a Snout upper wall surface 3b Snout lower wall surface 3c Side wall surface 4 Plating bath 5 Sink roll 6 Condensate 7 Deposit 8 Cooling device 9 Heating device 10 Storage space

Claims (4)

[Claims] 1. A method for continuously immersing a metal strip in a hot-dip metal plating method, wherein the metal strip is passed to a position higher than the center of the snout. 2. A method for continuously immersing a metal strip in a hot-dip metal plating method, wherein at least one of a lower wall surface and a side wall surface of the snout is maintained at a temperature lower than a condensation point of metal vapor. Method. 3. A hot-dip metal plating apparatus of a continuous immersion type of metal strip, wherein the center of the snout is arranged to be offset below the pass line of the metal strip. 4. A continuous immersion type metal plating apparatus for a metal strip, wherein means for maintaining at least one of a lower wall surface and a side wall surface of the snout at a temperature lower than a condensation point of metal vapor is provided. Metal plating equipment.