JP3235441B2 - Molten metal bath and molten metal plating method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten metal bath which can reduce surface defects due to dross when a continuously running steel sheet is subjected to a molten metal plating such as a zinc-based hot-dip plating containing zinc as a main component. The present invention relates to a hot metal plating method.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets are widely used as inexpensive surface-treated steel sheets exhibiting excellent high corrosion resistance.

[0003] A typical production method of a hot-dip metal-coated steel sheet is that a steel sheet is continuously subjected to a pretreatment such as degreasing, then the steel sheet is maintained at a high temperature in a predetermined heating pattern in a heating furnace, and after cooling,
Perform hot-dip metal plating by passing through a hot-dip metal plating bath,
Then, after adjusting the amount of molten metal attached to the steel sheet,
This is a method of manufacturing by cooling to normal temperature in a predetermined cooling pattern.

[0004] In the continuous hot-dip metal plating method, when a steel sheet passes through a hot-dip metal plating bath, iron eluted from the steel sheet becomes
It reacts with the component elements in the molten metal plating bath to generate an intermetallic compound, so-called dross. The density of the generated dross is generally higher than the density of the molten metal due to the inclusion of iron, and sediments in the molten metal plating bath. However, since it is an intermetallic compound with the eluted iron, the particle size of this dross is small, and the sedimentation velocity due to the density difference is extremely small.
While floating in the molten metal plating bath, it grows into dross having a large particle size and settles at the bottom of the molten metal plating bath.

If large dross during settling adheres to the steel sheet, it causes surface defects and not only deteriorates the surface quality of the steel sheet after hot-dip coating, but also impairs the smoothness of the hot-dip coated steel sheet surface during forming. In addition, when the amount of dross settled and deposited at the bottom of the bath of molten metal plating increases, the dross that has settled once floats again and adheres to the steel sheet being passed by the flow of the molten metal induced by the passing of the steel sheet. Thus, the surface quality of the steel sheet after plating may be impaired.

In recent years, as a countermeasure against rust on an automobile body, a hot-dip galvanized steel sheet, particularly an alloyed hot-dip galvanized steel sheet in which a galvanized layer is made of an alloy layer of iron and zinc by hot diffusion treatment after hot-dip galvanization, has been developed. Since it has come to be used as a steel sheet for use, reduction of surface defects due to the dross described above has been strongly demanded, and for that purpose, proposals have been made to reduce dross in a hot-dip metal plating bath. .

For example, Japanese Patent Laid-Open Publication No. Hei 5-98406 discloses that, as shown in FIG. 5, an opening 20 for discharging bottom dross is provided on at least one side surface and / or bottom surface.
One set digit molten metal plating bath 5, surrounds the molten metal plating bath 5, using a molten metal plating apparatus provided with the dross deposition chamber 21 for depositing bottom dross 6 discharged from the molten metal plating bath 5, By discharging the dross of the molten metal plating bath 5 to the dross deposition tank 21 by the accompanying flow of the rotating body in the bath such as the sink roll 3 and the steel plate 2, the molten metal plating is performed in the molten metal plating bath 5 without bottom dross accumulation. How to do it is described.

Japanese Patent Application Laid-Open No. Hei 7-268578 discloses that the bottom of the bath has an arc-shaped curve, and the plating in the molten metal plating bath 5 near the upper ends of the side walls 30, 31 as shown in FIG. A plating bath 5 provided with a communication hole 32 through which molten metal enters and / or discharges the precipitation bath 4 and a molten metal bath provided with a precipitation bath 4 for depositing and depositing bottom dross generated in the plating bath are used. Then, the bottom dross generated in the plating bath is discharged from the molten metal plating bath 5 to the sedimentation tank 4 by the flow caused by the rotation of the sink roll 3, and the plating bath from which the bottom dross 6 has been removed in the sedimentation bath 4 is removed from the molten metal plating bath 5. It is described that the plating quality is prevented from deteriorating due to the adhesion of bottom dross to the metal plate by returning to.

[0009]

SUMMARY OF THE INVENTION However, Japanese Patent Laid-Open No. 5-9 / 1993
The technique described in Japanese Patent No. 8406 discloses a technique for circulating the molten metal between the molten metal plating bath and the dross deposition tank because the opening is provided only on the side and / or the bottom of the molten metal plating bath. Since the driving force is weak, circulation of the molten metal between both tanks is not performed smoothly. As a result, dross discharge becomes insufficient, dross deposits locally at the bottom of the bath, and fluctuations in the temperature of the molten metal and the composition of the bath components can increase. There is. In an extreme case, the opening may be closed by the accumulation of dross, which may necessitate stopping the hot-dip metal plating operation.

In the case of the technique described in Japanese Patent Application Laid-Open No. 7-268578, when the running speed of the steel sheet is low, the flow due to the rotation of the sink roll 3 becomes weak, and the plating from the molten metal plating bath 5 to the sedimentation tank 4 is performed. The metal is not discharged well, and the bottom dross formed in the plating bath precipitates and deposits on the bottom of the bath. In addition, at both ends in the sink roll body length direction, the flow is insufficiently formed by the rotation of the sink roll, and grows into a large dross here. There is a problem that the dross adheres to the metal plate during plating and the plating quality is reduced.

The present invention has been made in view of the above-mentioned circumstances, and is intended to provide a molten metal plating bath in which the dross can be increased, and the increased dross floating and accumulation can be prevented by simple equipment and method. It is another object of the present invention to provide a molten metal bath and a molten metal plating method capable of reducing the occurrence of surface defects due to dross by plating with clean molten metal from which dross has been removed.

[0012]

Means for Solving the Problems The present inventors have variously studied the flow of molten zinc in a hot-dip galvanizing bath, the mechanism of dross generation, and the behavior of dross in a hot-dip galvanizing bath used in normal operations. As a result of the survey, the following was confirmed.

The driving force of the flow of the molten zinc in the hot-dip galvanizing bath is that the molten zinc is stirred by the steel sheet running in the hot-dip galvanizing bath, and the hot-dip zinc is kept or heated. Convection due to the non-uniform temperature of molten zinc generated near the inlet of the solid zinc ingot supplied to compensate for the reduced molten zinc adhered to the steel sheet and the electromagnetic force generated by the induction heating device And the flow of molten zinc. Among them, the flow of the molten zinc in the hot-dip galvanizing bath is dominated by the effect of the agitated flow generated by the running of the steel sheet, and the other flows are less affected than the accompanying flow.

Dross is caused by an intermetallic compound formed by reaction of iron powder adhering to the surface of a steel sheet entering a hot dip galvanizing bath and iron eluted from the surface of the steel sheet into the molten zinc bath with the molten zinc. is there. This intermetallic compound is initially fine dross, but due to non-uniformity of the hot-dip galvanizing bath temperature, the temperature rise and fall, and the non-uniform concentration of plating components such as aluminum added to the hot-dip galvanizing bath, Due to a change in the solubility of iron in the hot-dip zinc or a change in the structure of the intermetallic compound, the fine dross becomes a large dross in the hot-dip galvanizing bath.

[0015] The flow in the hot-dip galvanizing bath prevents the dross that floats as it grows from settling out, or the large dross that have settled and deposited re-emerge, causing these dross to adhere to the steel sheet. Quality defects occur on the plated steel sheet.

In order to prevent quality defects due to dross, highly clean zinc is supplied to the hot-dip galvanizing bath, and dross generated during plating is increased in diameter in the hot-dip galvanizing bath so as not to float and deposit. In order to do so, the hot dip galvanizing bath should be a bath in which dross does not accumulate, and a separate sedimentation bath for sedimentation of dross should be provided separately from the hot dip galvanizing bath. It is effective to send molten zinc containing dross from the galvanizing bath to the sedimentation tank, settle and separate the dross in the sedimentation tank, and return the cleaned molten zinc to the galvanizing bath. Discharge of molten zinc to the bathtub, return of molten zinc from the precipitation bath to the hot dip galvanizing bath, steel that substantially controls the flow in the hot dip galvanizing bath Associated flow generated by the running of it was confirmed that available from a number of experiments.

The present invention has been made on the basis of such knowledge, and the characteristic configuration thereof is as follows.

(1) A double-structured molten metal bath having a molten metal plating bath on the inside for performing molten metal plating and a sedimentation bath on the outside for sedimentation and separation of dross generated during molten metal plating. The side wall shape of the inner molten metal plating bath in the running direction of the steel plate is a V-shape having an inclination of 45 degrees or less from the vertical direction, and the upper part of the molten metal immersion part is plated from the precipitation bath to the molten metal plating bath. An upper opening for inflow of the bath, and a lower opening for discharging the plating bath from the molten metal plating bath to the sedimentation bath are provided below the upper opening, and the capacity of the inner molten metal plating bath is set to the outer settling bath. A molten metal bathtub that is 50% or less of the bathtub capacity.

(2) In the molten metal bath of the above (1), the capacity of the inner molten metal plating bath is 40% or less of the capacity of the outer precipitation bath, and the capacity of the inner molten metal plating bath and the outer molten bath are smaller than the capacity of the outer precipitation bath. The total volume of the sedimentation bath is 10 m ³ or more.

(3) Using the molten metal bath of (1) or (2) above, the molten metal plating is performed in the inner molten metal bath, and the molten metal flows out of the lower opening into the precipitation bath. The dross generated by the molten metal plating is settled and separated in the outer precipitation bath to clean it, and the purified molten metal flows into the molten metal plating tank through the upper opening of the molten metal plating tank. I do.

Hereinafter, the operation of the present invention will be described. The shape of the side wall in the traveling direction of the steel plate of the molten metal plating bath is a V-shape having a slope of 45 degrees or less from the vertical direction exceeding the angle of repose of the dross, so that the dross does not settle and deposit in this bath.

In the precipitation bath, dross is settled and separated from the molten metal flowing from the molten metal plating bath to purify the molten metal.

Openings are provided at the upper and lower portions of the hot-dip metal bath, respectively, so that when the steel sheet travels at a high speed, the flow of the molten metal agitated by the steel sheet causes a fine dross from the lower opening. When the molten metal that contains sediment flows out, the molten metal that has settled and separated the dross flows in from the upper opening, and the steel sheet that cannot be expected to be agitated flows at a low speed, the high temperature Natural convection is caused by the inflow of the metal and the outflow of the low-temperature molten metal from the lower opening, thereby circulating the molten metal between the molten metal plating bath and the precipitation bath.

It is desirable that the capacity of the molten metal plating bath is not more than 50%, more preferably not more than 40% of the capacity of the precipitation bath. When this capacity exceeds 50%, the circulation action of the molten metal due to the accompanying flow due to the running of the steel sheet in the molten metal plating bath decreases, the dross increases in the molten metal plating bath, and the sedimentation and separation of the dross in the precipitation bath. There is a possibility that dross defects of the plated steel sheet may increase due to insufficient coating. Also, if this capacity is 40
%, Dross defects can be further reduced.

It is desirable that the total capacity of the precipitation bath and the hot-dip metal bath is not less than 10 m ^{3 and not} more than 40 m ³ . If the total capacity is less than 10 m ³ , sedimentation and separation of dross are not sufficiently performed, so that dross in the molten metal flowing into the molten metal plating bath increases, and dross defects may increase in the plated steel sheet. On the other hand, if the capacity exceeds 40 m ³ , the equipment cost becomes enormous, which is not economically feasible.

With the above-described operation, it is possible to perform clean hot-dip metal plating in the hot-dip metal plating bath, so that quality defects due to dross of the plated steel sheet are reduced.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to FIG.
The same parts as those shown in the already described figures are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 1, 1 is a snout, 4 is a sedimentation bath, 7 is a gas wiping nozzle,
8, 9 are the upper openings of the hot-dip galvanizing bath, 10 is the lower opening of the hot-dip galvanizing bath, 11 is the stabilizing roll, 12 is the collecting roll, 13 is the hot-dip zinc, and 14 is the flow of the hot-dip zinc in the bath. Direction.

The side wall shape of the hot dip galvanizing bath 5 in the running direction of the steel plate 2 has a slope of 45 degrees or less from the vertical direction.
It is shaped like a letter, and upper openings 8 and 9 are respectively provided in the upper portions of the side walls, and lower openings 10 are provided in the lower portions thereof. In addition, hot-dip galvanizing bath 5
A sedimentation bath 4 is provided on the outside.

The molten zinc flows in the direction of arrow 14 due to the accompanying flow of the steel sheet 2 running from the snout 1 on the sink roll 3 and the accompanying flow of the steel sheet 2 turned upward by turning the sink roll 3. Are upper openings 8, 9
Through the precipitation bath 4 into the hot-dip galvanizing bath 5 and out through the lower opening 10 from the hot-dip galvanizing bath 5 to the precipitation bath 4.

The dross contained in the molten zinc 13 that has flowed out is settled and separated in the sedimentation bath 4 and is deposited as bottom dross at the bottom of the sedimentation bath 4, and the supernatant of the sedimentation bath 4 becomes clean molten zinc with little bottom dross. .

The side wall shape of the hot dip galvanizing bath 5 in the running direction of the steel sheet is inclined at an angle of 45 degrees or less from the vertical direction and exceeds the angle of repose of the dross, so that dross does not accumulate in this bath.

In the sedimentation bath 4, the dross is settled and separated. In the vicinity of the bath surface of the sedimentation bath 4, top dross mainly composed of zinc oxide or aluminum-zinc floats, so that the cleanliness of the molten zinc decreases. This will be described with reference to FIG. FIG. 2 is a diagram showing the relationship between the distance from the bath surface of the sedimentation tank 4 and the cleanliness of the molten zinc.
The horizontal axis indicates the distance from the bath surface, and the vertical axis indicates the cleanliness of the molten zinc. The cleanliness is evaluated in 0.5 steps from 0 to 4.0, with 0 being the best and 4.0 being the worst, and 1.5 or less being the target level. When the distance from the zinc bath surface is 50 mm or more, no decrease in cleanliness due to top dross is observed. Therefore, the upper opening is desirably provided at a position at least 50 mm away from the bath surface.

The capacity of the hot-dip galvanizing bath 5 is
Is preferably 50% or less, more preferably 40% or less of the capacity of When this capacity exceeds 50%, the circulation action of the molten zinc utilizing the accompanying flow of the steel sheet in the hot-dip galvanizing bath 5 decreases, so that the dross increases in the hot-dip galvanizing bath 5 or the dross in the sedimentation bath 4 increases. Dross defects may increase due to insufficient sedimentation and separation. When the capacity is 40% or less, the effect of reducing dross defects is more excellent. The smaller the capacity of the tank is, the more effective it is. However, it is necessary to select the capacity in consideration of the equipment relation with the in-bath equipment such as a sink roll.

It is desirable that the total capacity of the precipitation bath 4 and the hot-dip galvanizing bath 5 be 10 m ³ or more and 40 m ³ or less. If the total capacity is less than 10 m ³ , sedimentation and separation of dross is not sufficiently performed, so that dross in the molten zinc flowing into the hot dip galvanizing bath increases, and dross defects of the galvanized steel sheet increase. Also, if the capacity exceeds 40 m ³ ,
The equipment cost is enormous, which is not economically feasible.

Further, hot dip galvanizing of a steel sheet is performed as follows using this apparatus. The steel plate 2 passes through the snout 1 whose lower part is immersed in a hot-dip galvanizing bath 5,
After being immersed in the molten zinc 13 of the hot-dip galvanizing bath 5 and turned upward by the sink roll 3, it passes between the stabilizing roll 11 and the collecting roll 12, and is provided on the bath surface and is provided with a gas wiping nozzle. 7, the amount is adjusted to a predetermined amount.

While the steel sheet 2 is immersed in the molten zinc 13 and plated, fine dross is generated. This dross is
Flows out of the lower opening 10 into the sedimentation bath 4 without becoming large or sedimented in the hot-dip galvanizing bath 5 due to the accompanying flow generated by the traveling of. The dross is settled and separated in the sedimentation bath 4, and the molten zinc, which has been purified, returns to the hot dip galvanization bath 5 from the upper openings 8 and 9 by the action of the accompanying flow caused by the running of the steel plate 2. The molten zinc in the hot-dip galvanizing bath 5 becomes clean molten zinc without dross which has become large, and the quality of the steel sheet plated here is reduced by dross. Furthermore, since the molten zinc is stirred and circulated by the accompanying flow of the traveling of the steel sheet 2, the fluctuation of the plating bath components and the fluctuation of the plating bath temperature in the hot-dip galvanizing bath 5 are reduced, and the operation of the hot-dip galvanizing is stabilized. Contributes to

Although the above description has been directed to hot-dip galvanizing, the present invention is not limited to hot-dip galvanizing, but can be applied to hot-dip galvanizing such as zinc-aluminum alloy plating and aluminum plating.

[0038]

【Example】

 (Example 1) In a molten zinc bath shown in FIG.
Hot-dip galvanizing bath 5 and hot-dip galvanizing bath 5
The lead bath is 5m wide, 2m high, 3m deep and has a capacity of 3
0m ^ThreeAnd the inclination of the side wall of the hot-dip galvanizing bath 5
Both 45 degrees, 1.5 m depth and 6.75 m capacity^ThreeAnd
Also, the upper opening of the side wall is 50-150m deep from the bath surface.
m. The capacity of the precipitation bath 4 is the same as that of the molten zinc bath.
23.2 which is the difference between the capacity and the capacity of the molten metal plating tank.
5m^ThreeAnd the capacity of the hot dip galvanizing bath is
29% of the volume. Using this molten zinc bath,
Deposition of alloyed hot-dip galvanized steel sheet when lead plating is performed
FIG. 3 shows the change over time in the occurrence state of the loss defect. In FIG.
The horizontal axis is the number of days elapsed, and the vertical axis is the degree of dross defect occurrence.
Is shown. Evaluation of the degree of dross defect generation is from 0 to 4
0 is the best, 4 is the worst, and 1.0 or less
High quality alloyed hot-dip galvanized steel sheet
This is the target level for loss defects. Note that, for comparison,
With the same volume of molten zinc bath as above, the inner molten zinc
Occurrence of dross defects without plating bath
Is also shown in FIG.

In the comparative example, the dross defect reaches the limit level by one-day plating operation, whereas in the example of the present invention, the dross defect is below the limit level, and the dross defect can be reduced over a long period of time.

(Example 2) FIG. 4 shows the occurrence of dross defects when the capacity of the hot-dip galvanizing bath 5 is 6.75 m ³ and the capacity of the sedimentation bath 4 is changed. The horizontal axis is the sedimentation tank 4
Ratio of capacity of hot-dip galvanizing bath 5 to capacity of bath (%)
As for the dross defect, the occurrence state of the dross defect of the alloyed hot-dip galvanized steel sheet on the 14th day has been evaluated based on the same criteria as in Example 1.

When the ratio of the tank capacity exceeds 50%, the dross defect increases remarkably. Further, at 40% or less, even on the 14th day, the occurrence of dross defects is 3 or less, which is the target level for a general quality galvannealed steel sheet.

[0042]

According to the present invention, it becomes possible to perform hot-dip metal plating on a steel sheet in a clean hot-dip metal plating bath without dross floating and accumulation, and it is possible to perform hot-dip metal plating with less occurrence of dross defects. it can.

[Brief description of the drawings]

FIG. 1 is a view showing an embodiment of a molten metal tank of the present invention.

FIG. 2 is a graph showing a relationship between a distance from a zinc bath surface and cleanliness of molten zinc.

FIG. 3 is a diagram illustrating a change in a dross defect occurrence state according to the number of elapsed days.

FIG. 4 is a diagram showing a relationship between a tank capacity ratio and a dross defect occurrence state.

FIG. 5 is a view showing a molten metal tank according to the related art.

FIG. 6 is a view showing another molten metal tank in the prior art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Snout 2 Steel plate 3 Sink roll 4 Precipitation bath 5 Hot-dip galvanizing bath 8, 9 Upper opening 10 Lower opening

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (3)

(57) [Claims] 1. A double-structured molten metal bath having a molten metal plating bath on the inside for performing molten metal plating and a sedimentation bath on the outside for sedimentation and separation of dross generated during molten metal plating. The shape of the side wall of the inner molten metal plating bath in the running direction of the steel plate is V-shaped with a slope of 45 degrees or less from the vertical direction, and the plating bath from the precipitation bath to the molten metal plating bath is located above the molten metal immersion part. An upper opening for inflow of the molten metal plating bath, and a lower opening for discharging the plating bath from the molten metal plating bath to the sedimentation bath, and the capacity of the inner molten metal plating bath is reduced to the outer precipitation. A molten metal bathtub having a capacity of 50% or less of the bathtub capacity. 2. The capacity of the inner molten metal plating bath is not more than 40% of the capacity of the outer precipitation bath, and the total capacity of the inner molten metal plating bath and the outer precipitation bath is not less than 10 m ^3. The molten metal bath according to claim 1, wherein: 3. The molten metal bath according to claim 1 or 2, wherein the molten metal plating is performed in an inner molten metal plating bath, and the molten metal is discharged from a lower opening to a precipitation bath. In the sedimentation bath, the dross generated by the molten metal plating is settled and separated and cleaned, and the molten metal plating is performed by flowing the cleaned molten metal into the molten metal plating bath from the upper opening of the molten metal plating bath. A hot-dip metal plating method characterized by performing.