WO2020213671A1 - Method for manufacturing hot-dip zinc-plated steel sheet, and method for operating molten zinc plating bath - Google Patents

Abstract

A method for manufacturing a hot-dip zinc-plated steel sheet is a method for manufacturing a hot-dip zinc-plated steel sheet by dipping a steel sheet in a molten zinc plating bath continuously to form a hot-dip zinc plating layer. In the method for manufacturing a hot-dip zinc-plated steel sheet, the bath temperature T and the free Al concentration C_Al of the molten zinc plating bath are adjusted so that top dross can be generated and the top dross in the molten zinc plating bath is removed when the hot-dip zinc plating facility is halted, and the bath temperature T and the free Al concentration C_Al of the molten zinc plating bath are adjusted so that a δ1 phase can cause the generation of nuclei when the hot-dip zinc plating facility is operated.

Description

Manufacturing method of hot-dip galvanized steel sheet and operation method of hot-dip galvanized bath

The present invention relates to a method for producing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanized bath.
The present application claims priority based on Japanese Patent Application No. 2019-080277 filed in Japan on April 19, 2019, the contents of which are incorporated herein by reference.

Conventionally, as a method of forming a hot-dip galvanized layer on a steel sheet, a method of continuously immersing the steel sheet in a hot-dip galvanized bath has been used. In this method, after annealing the steel sheet, the upper end is connected to the hot-dip galvanizing furnace, and the lower end is immersed in the hot-dip galvanizing bath through the inside of the snout immersed in the hot-dip galvanizing bath. .. By dipping rolls in the hot-dip galvanizing bath, the method of advancing the steel sheet is changed from diagonally downward to upward, and the steel sheet is pulled up. After that, the amount of hot-dip galvanized adhered to the surface of the steel sheet is controlled by the gas drawing method.

The steel sheet pulled up from the hot-dip galvanized bath becomes an alloyed hot-dip galvanized steel sheet if it is alloyed in the alloying furnace in the subsequent stage. (Hereinafter, those that have been alloyed (alloyed hot-dip galvanized steel sheets) and those that have not been alloyed are collectively referred to as "hot-dip galvanized steel sheets", and in particular, they have been alloyed. When it means something that has not been made, it is expressed as "non-alloyed hot-dip galvanized steel sheet".)

The inside of the snout is shielded from the atmosphere and maintained in a non-oxidizing atmosphere such as nitrogen gas to prevent oxidative contamination of the surface of the steel sheet to be plated. Here, when the metal eluted from the steel plate in the bath (for example, Fe eluted from the steel plate) reacts with Al or Zn existing in the bath, dross deposited on the bottom of the plating bath is generated. The dross generated in this way is called a bottom dross. Bottom dross floats in the bath due to the accompanying flow generated by the progress of the steel sheet in the bath and adheres to the surface of the steel sheet immersed in the bath, resulting in poor quality (particularly poor appearance of the surface of the hot-dip galvanized steel sheet). It causes the occurrence of.

Various techniques have been conventionally proposed in order to suppress the appearance defect of the surface of the hot-dip galvanized steel sheet. For example, in Patent Document 1, when an alloyed hot-dip galvanized steel sheet is manufactured, the hot-dip galvanized bath temperature is T (° C.), and the boundary Al concentration represented by the formula Cz = −0.0015 × T + 0.76 is Cz. When (wt%), a technique has been proposed in which the hot-dip zinc bath temperature T is kept in the range of 435 to 500 ° C. and the Al concentration in the bath is kept in the range of Cz ± 0.01 wt%.

Patent Document 2 proposes a technique for keeping the Al concentration in a bath within the range of 0.15 ± 0.01 wt% when manufacturing an alloyed hot-dip galvanized steel sheet.

It is known that there are four types of dross that can be generated in the production of hot-dip galvanized steel sheets: Fe ₂ Al ₅ (so-called top dross), δ1 phase, Γ2 phase, and ζ phase. The technique proposed in Patent Document 1 proposes to operate under the condition of the boundary between the condition where the ζ phase is generated and the condition where the δ1 phase is generated. The technique proposed in Patent Document 2 proposes to operate under the condition of the boundary between the condition where the Fe ₂ Al ₅ phase is generated and the condition where the δ1 phase is generated.

Japanese Patent Application Laid-Open No. 11-35096 Japanese Patent Application Laid-Open No. 11-350097

Conventionally, by setting the Al concentration of the hot-dip galvanizing bath to a high value, a dross (so-called "Fe-Al-based top dross") floating on the hot-dip galvanizing bath surface is formed, and the Fe-Al-based top dross is appropriately formed. The operation to remove (hereinafter also referred to as the top dross operation) has been performed. There is a bottom dross operation as an operation concept that conflicts with the top dross operation.

When the Al concentration in the hot-dip galvanizing bath is low, dross (so-called "Fe-Zn-based bottom dross") that settles in the hot-dip galvanizing bath is formed. Fe-Zn-based bottom dross is difficult to remove during the operation of the hot-dip galvanizing facility, so it accumulates on the bottom of the bath. The bottom dross accumulated on the bottom of the bath is eventually sprinkled into the bath by the accompanying flow of the steel sheet, adheres to the steel sheet and the rolls in the bath, and causes defects on the surface of the steel sheet (hereinafter sometimes referred to as "dross defects"). It causes to.

When the bottom dross adheres to the steel sheet, non-uniform parts are formed on the plated surface, resulting in poor appearance quality. Further, as a result of the formation of the non-uniform portion, the local battery is likely to be formed, surface defects that cause a decrease in corrosion resistance occur, and quality defects of the plated steel sheet occur. Therefore, in order to maintain the quality of the hot-dip galvanized steel sheet in the bottom dross operation, it is necessary to periodically stop the line and perform bath cleaning in order to remove the bottom dross accumulated on the bath bottom. Compared to the top dross operation, which can remove dross during operation, the bottom dross operation, which requires dross removal by line stop, requires man-hours and causes a problem of a decrease in production due to line stop. For this reason, bottom dross operations are generally avoided.

However, after immersing the steel sheet in the hot-dip galvanizing bath, the plating layer may be alloyed. The higher the Al content in the hot-dip galvanized layer, the more difficult it is to alloy. Therefore, especially when alloying treatment is performed, a bottom dross operation having a low Al concentration in the hot-dip galvanized bath is more advantageous in order to produce a high-quality alloyed hot-dip galvanized steel sheet with high efficiency and productivity.

The present invention has been made in view of the above problems. The present invention provides a method for producing a hot-dip galvanized steel sheet in which quality defects of the hot-dip galvanized steel sheet can be suppressed and a decrease in productivity is suppressed even when a bottom dross operation is performed, and an operation of a hot-dip galvanized steel sheet. The purpose is to provide a method.

In order to solve the above problems, the present inventors investigated the particle size of bottom dross, which causes dross defects during bottom dross operation. As a result, the present inventors have found that the presence of bottom dross having a particle size of 100 to 300 μm in the bath increases dross defects. Then, the conditions of the hot-dip galvanizing bath in which the occurrence of bottom dross having a particle size of 100 to 300 μm is suppressed were examined in detail, and the present invention described in detail below was conceived.
The gist of the present invention completed based on such findings is as follows.

[1] The method for producing a hot-dip galvanized steel sheet according to one aspect of the present invention is to produce a hot-dip galvanized steel sheet by continuously immersing the steel sheet in a hot-dip galvanized bath to form a hot-dip galvanized steel sheet. A method for manufacturing galvanized steel sheets
When the hot-dip galvanizing equipment is stopped, the bath temperature T and the free Al concentration C _Al of the hot-dip galvanizing bath are set so that top dross occurs, and the top dross of the hot-dip galvanizing bath is removed.
When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are set so that the δ1 phase is nucleated.
[2] In the method for producing a hot-dip galvanized steel sheet according to the above [1], when the hot-dip galvanizing facility is stopped, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 440 to 460 ° C. Moreover, the free Al concentration C _{Al in} the mass% of the hot-dip galvanizing bath is set so as to satisfy the formula (1).
When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 480 to 490 ° C., and the free Al concentration C _{Al in} the mass% of the hot-dip galvanizing bath is expressed by the formula. It may be set so as to satisfy (2).
-2.914 x ^10-5 x T + 1.524 x 10 ^-1 <C _Al <0.1427 (1)
0.1390 <C _Al <2.686 × 10 ^-4 × T + 1.383 × 10 ^-2 (2)
[3] In the method for producing a hot-dip galvanized steel sheet according to the above [1] or [2], the hot-dip galvanized layer may be alloyed to form an alloyed hot-dip galvanized layer.
[4] The method for operating a hot-dip galvanizing bath according to another aspect of the present invention is a method for operating a hot-dip galvanizing bath in which a steel plate is continuously immersed in the hot-dip galvanizing bath to form a hot-dip galvanizing layer. hand,
When the hot-dip galvanizing equipment is stopped, the bath temperature T and the free Al concentration C _Al of the hot-dip galvanizing bath are set so that top dross occurs, and the top dross of the hot-dip galvanizing bath is removed.
When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are set so that the δ1 phase is nucleated.
[5] In the operation method of the hot-dip galvanizing bath according to the above [4], when the hot-dip galvanizing facility is stopped, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 440 to 460 ° C. Moreover, the free Al concentration C _{Al in} the mass% of the hot-dip galvanizing bath is set so as to satisfy the formula (1).
When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 480 to 490 ° C., and the free Al concentration C _{Al in} the mass% of the hot-dip galvanizing bath is expressed by the formula. It may be set so as to satisfy (2).
-2.914 x ^10-5 x T + 1.524 x 10 ^-1 <C _Al <0.1427 (1)
0.1390 <C _Al <2.686 × 10 ^-4 × T + 1.383 × 10 ^-2 (2)
[6] In the method of operating the hot-dip galvanizing bath according to the above [4] or [5], the hot-dip galvanizing layer may be alloyed to form an alloyed hot-dip galvanizing layer.

According to the above aspect of the present invention, a method for producing a hot-dip galvanized steel sheet, which can suppress quality defects of the hot-dip galvanized steel sheet and suppress a decrease in productivity even when a bottom dross operation is performed, and It becomes possible to provide a method of operating a hot-dip galvanized bath.

It is a schematic diagram which shows an example of the structure of the continuous hot-dip galvanizing equipment (alloyed hot-dip galvanizing equipment) which can be used in this embodiment. It is a metastable state diagram which arranged the dross formation phase of a hot-dip galvanizing bath with respect to bath temperature T (° C.) and free Al concentration C _Al . It is a micrograph which shows the morphology of the bottom dross generated in the plating bath 10 days after the operation. It is a graph which shows the relationship between the particle diameter and the number of dross under each manufacturing condition of an Example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

<1. Configuration of continuous hot-dip galvanizing equipment>
First, prior to the detailed description of the present invention, an example of the configuration of a continuous hot-dip galvanizing facility capable of carrying out the method for manufacturing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanized bath according to the present embodiment will be described in detail. .. To be exact, this equipment is an alloyed hot-dip galvanizing equipment. In the case of manufacturing a non-alloyed hot-dip galvanized steel sheet, it is only necessary to operate the alloying furnace. Therefore, the continuous hot-dip galvanized equipment will be described below by taking the alloyed hot-dip galvanized equipment as an example.

FIG. 1 is a schematic view showing an example of the configuration of an alloyed hot-dip galvanizing facility. The hot-dip galvanizing facility 10 includes, for example, as shown in FIG. 1, a hot-dip galvanizing bath 103 (hereinafter, also simply referred to as a “plating bath”), a hot-dip galvanizing bath 101 in which the plating bath 103 is housed, and a snout 105. A sink roll 107, a gas wiping device 109, and an alloying furnace 111 are provided.

The annealing furnace 20 provided in the front stage of the hot-dip galvanizing facility 10 (upstream side in the transport direction of the steel sheet S) is shielded from the atmospheric atmosphere, and the inside is maintained in a reducing atmosphere. Further, the annealing furnace 20 heats the steel plate S which is continuously conveyed. The annealing furnace 20 activates the surface of the steel sheet S and adjusts the mechanical properties of the steel sheet S. The exit side end of the annealing furnace 20 is connected to the upstream end of the snout 105 via a space provided with a turndown roll 30.

The upstream end of the snout 105 is connected to the end of the annealing furnace 20, and the downstream end is immersed in the hot dip galvanizing bath 103 from diagonally above. Like the annealing furnace 20, the inside of the snout 105 is shielded from the atmospheric atmosphere and maintained in a reducing atmosphere.

The steel plate S whose transport direction has been changed downward by the turndown roll 30 is transported inside the snout 105 and is continuously immersed in the hot-dip galvanizing bath 103 housed in the hot-dip galvanizing bath 101. A sink roll 107 is provided inside the hot-dip galvanized bath 101. The sink roll 107 has a rotation axis parallel to the plate width direction of the steel plate S, and the width of the outer peripheral surface of the sink roll 107 is equal to or larger than the plate width of the steel plate S. The sink roll 107 changes the transport direction of the steel sheet S upward.

The gas wiping device 109 scrapes off a part of the hot-dip galvanizing adhering to the surface of the steel sheet S by spraying gas on both sides of the steel sheet S led out from the hot-dip galvanizing bath 101. As a result, the amount of hot dip galvanized on the surface of the steel sheet S is adjusted.

After that, the steel plate S is further alloyed in the alloying furnace 111 while being pulled up vertically. The alloying furnace 111 is composed of three parts, a heating zone, a tropical zone, and a cooling zone, in this order from the entry side of the steel plate S. In the alloying furnace 111, first, heating is performed so that the plate temperature of the steel plate S becomes substantially uniform by the heating zone. Next, by securing the alloying time in the tropical region, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to become an alloyed layer (alloyed hot-dip galvanized layer). After that, the steel sheet S (that is, the alloyed hot-dip galvanized steel sheet) is cooled in the cooling zone and conveyed to the next step by the top roll 40. When producing a non-alloyed hot-dip galvanized steel sheet, the alloying treatment using the alloying furnace 111 as described above is not performed.

In the hot-dip galvanizing facility 10 as described above, in the hot-dip galvanizing bath 101, iron eluted from the steel plate S forms a particulate solid alloy having a high melting point called dross in the hot-dip galvanizing bath 103. When this dross adheres to the steel sheet S, a dross defect is generated on the surface of the steel sheet S.

<2. Examination by the present inventors>
A problem in performing the bottom dross operation is that the bottom dross is rolled up and adheres to the steel plate S due to the accompanying flow of the steel plate S in the plating bath 103. Although the occurrence of bottom dross is unavoidable in the bottom dross operation, if the particle size of the bottom dross is small, it is considered that quality defects do not occur even if the bottom dross adheres to the steel sheet S.

The present inventors investigated the particle size of bottom dross, which causes dross defects. As a result, the present inventors have found that when a bottom dross having a particle size of 100 to 300 μm is present in the bath, many dross defects occur. Since the bottom dross having a particle size of less than 100 μm is sufficiently small, even if it adheres to the steel sheet S, it does not cause dross defects. On the other hand, the bottom dross having a particle size of more than 300 μm is greatly affected by gravity and settles on the bottom of the bath, so that it does not easily adhere to the steel sheet S. Therefore, in order to suppress the occurrence of dross defects, it is important to suppress the amount of bottom dross having a particle size of 100 to 300 μm as small as possible.

On the other hand, the present inventors investigated the growth rate of the particle size of the bottom dross. As a result, it was found that when the bath temperature of the plating bath 103 is low, the growth rate of the particle size of the bottom dross is high, and when the bath temperature of the plating bath 103 is high, the growth rate of the particle size of the bottom dross is slow. This is because the growth rate of the Γ2 phase, which is stable at a low bath temperature (below the temperature range of 455 to 460 ° C, that is, below 455 ° C), is at a high bath temperature (above the temperature range of 455 to 460 ° C, that is, above 460 ° C). It is presumed that this is due to the fact that it is faster than the growth rate of the stable δ1 phase.

During the operation of the hot-dip galvanizing facility 10, the steel plate S is continuously passed through the hot-dip galvanizing bath 101, so that local nucleation inevitably occurs. Therefore, during the operation, the bottom dross is intentionally grown in the nucleation region of the δ1 phase, and Fe eluted from the steel sheet S is induced to become a fine bottom dross. Specifically, the operation is performed in a high bath temperature region (nucleation region of δ1 phase) where the growth rate of the bottom dross particle size is slow, and the particle size of the fine bottom dross newly nucleated during the operation is 100 μm or more. To prevent. As a result, the occurrence of dross defects can be suppressed.

However, if the bottom dross operation is continued for a long period of time, the bottom dross may gradually grow to a particle size of 100 to 300 μm, although it is slow. The phenomenon of bottom dross growing in this way is called Ostwald growth in crystallography. When the plating bath 103 having bottom dross having various particle sizes is continuously operated for a long time, mass transfer occurs from the bottom dross having a relatively small particle size to the bottom dross having a relatively large particle size, and the bottom dross having a small particle size is even smaller. The bottom dross with a large particle size is even larger.

Therefore, the bottom dross operation is started from the state where the bottom dross is removed, and the operation is performed so that there is no large difference in the particle size of the bottom dross even if the bottom dross is generated. This makes Ostwald growth less likely to occur. Further, even if the particle size of the bottom dross is increased due to Ostwald growth, the occurrence of dross defects can be suppressed by removing the bottom dross before the bottom dross grows to a particle size of 100 μm or more. Specifically, when the hot-dip galvanizing facility 10 is stopped (offline), the bath temperature and free Al concentration of the plating bath 103 are set so that top dross occurs, and the dross in the plating bath 103 is set on the plating bath surface. The surfaced dross is removed as a top dross.

By changing the conditions of the plating bath 103 between the operation and the stop in this way, even if new fine bottom dross is nucleated during the operation, the bottom dross in the plating bath 103 is reduced before the bottom dross grows significantly. It can be removed as top dross, and the occurrence of dross defects can be suppressed.

<3. Manufacturing method of hot-dip galvanized steel sheet and operation method of hot-dip galvanized bath>
A method for manufacturing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanized bath according to the present embodiment, which have been completed based on the above findings, will be described. In the following description, the method for manufacturing the hot-dip galvanized steel sheet and the method for operating the hot-dip galvanized bath according to the present embodiment will be described as being carried out using the hot-dip galvanizing facility 10 shown in FIG. , The present invention is not limited to this.

The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment is a hot-dip galvanized steel sheet in which a hot-dip galvanized steel sheet is manufactured by continuously immersing a steel sheet S in a hot-dip galvanized bath 103 to form a hot-dip galvanized steel sheet. It is a manufacturing method of. In the present embodiment, the hot-dip galvanized layer may be formed by heating the steel plate S to alloy the hot-dip galvanized layer after forming the hot-dip galvanized layer. In the method for producing a hot-dip galvanized steel sheet according to the present embodiment, since the plating bath 103 is operated under bottom dross conditions as described later, the Al content in the hot-dip galvanized layer is suppressed and alloying is easy. is there. As a result, it is possible to produce a high-quality alloyed hot-dip galvanized steel sheet.

Further, the method of operating the hot-dip galvanized bath according to the present embodiment is a method preferably used in the method for producing the hot-dip galvanized steel sheet. Then, as described above, the method of operating the hot-dip galvanized bath according to the present embodiment is particularly preferably applied when the hot-dip galvanized layer is alloyed to obtain an alloyed hot-dip galvanized steel sheet.

The steel sheet (base steel sheet) S used in the method for producing a hot-dip galvanized steel sheet according to the present embodiment is not particularly limited, and various characteristics (for example, a steel sheet) required for the hot-dip galvanized steel sheet to be manufactured are not particularly limited. A known steel sheet may be appropriately used depending on the tensile strength and various strengths required for the above, and a steel sheet used for an automobile outer plate can also be used.

In the method for producing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanized bath according to the present embodiment, when the hot-dip galvanized equipment 10 is stopped, the bath temperature T of the plating bath 103 and the free Al concentration C _Al are in the top dross region. The top dross is removed, and when the hot-dip galvanizing equipment 10 is in operation (online), the bath temperature T of the plating bath 103 and the free Al concentration C _Al are the nucleation regions of the δ1 phase. Set to the condition. That is, when the hot-dip galvanizing equipment is stopped, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are set so that top dross occurs, and the top dross of the hot-dip galvanizing bath is removed to perform hot-dip galvanizing. When the equipment is in operation, the bath temperature T and the free Al concentration C _Al of the hot-dip galvanizing bath are set so that the δ1 phase is nucleated.

This makes it possible to reduce the amount of bottom dross having a particle size of 100 to 300 μm in the plating bath 103. That is, when the hot-dip galvanizing facility 10 is stopped, the top dross floating on the surface of the plating bath 103 is recovered by setting the condition that the bath temperature T of the plating bath 103 and the free Al concentration C _Al are in the top dross region. Then, the coarse dross that can cause dross defects is removed. On the other hand, when the hot-dip galvanizing facility 10 is in operation, the bath temperature T of the plating bath 103 and the free Al concentration C _Al are set to be the nucleation region of the δ1 phase, and the nucleation region of the δ1 phase is intentionally set. Is performed, and Fe eluted from the steel sheet S is induced to have a fine bottom dross.

Normally, when the hot-dip galvanizing facility 10 is operated for a long time, the particle size of the generated bottom dross increases due to Ostwald growth. However, the growth rate of bottom dross in the operation in the δ1 phase nucleation region is slow, and Ostwald growth is unlikely to occur. Therefore, if the hot-dip galvanizing facility 10 is not operated for a certain period of time, the particle size of the bottom dross will not grow to 100 μm or more. Before the particle size of the bottom dross grows to 100 μm or more, the hot-dip galvanizing facility 10 is stopped, and the bath temperature T of the plating bath 103 and the free Al concentration C _Al are set to be the top dross region, and the top dross is set. If the dross is removed, the occurrence of dross defects can be suppressed.

Specifically, the conditions of the plating bath 103 can be controlled by, for example, the composition and temperature of the plating bath 103. Hereinafter, the preferable composition and temperature of the plating bath 103 will be described with reference to FIG. FIG. 2 is a metastable state diagram in which the dross-forming phase of the hot-dip galvanizing bath is arranged for the bath temperature T (° C.) and the free Al concentration C _{Al in the} bath. In FIG. 2, “C _Al ” indicates the free Al concentration (mass%) in the bath in the plating bath 103. The "free Al concentration in the bath" means the Al concentration contained in the liquid phase of the plating bath 103, and means the average Al concentration of both the dross and the liquid phase, and the total Al concentration of the plating bath 103. It is used separately from.

The free Al concentration C _Al in the plating bath 103 is measured by the following method. The plating bath liquid is drawn from the hot-dip galvanizing bath 101, and this plating bath liquid is poured into a mold and solidified to prepare an ingot. Using a drill, an appropriate amount of chips is scraped from this ingot, and a part of the chips is dissolved in hydrochloric acid and nitric acid to prepare a solution. The Al concentration (mass%) is calculated using this solution, an ICP emission spectrophotometer, and a calibration curve calculated in advance. As a result, the free Al concentration C _Al in the plating bath 103 is obtained.
Further, the bath temperature T of the plating bath 103 may be measured using a thermometer at a position where the bath temperature is stable.

In the present embodiment, the free Al concentration C _Al and the bath temperature T of the plating bath 103 are set in the “δ1 nucleation” region during operation and in the “top dross” region when stopped in FIG. The “δ1 nucleation” region in FIG. 2 is the above-mentioned δ1 phase nucleation region. When the free Al concentration C _Al and the bath temperature T of the plating bath 103 are included in the “δ1 nucleation” region, the δ1 phase is nucleated in the plating bath 103. The "top dross" region in FIG. 2 is the above-mentioned top dross region. When the free Al concentration C _Al and the bath temperature T of the plating bath 103 are included in the “top dross” region, top dross occurs in the plating bath 103.
Further, in the present embodiment, in FIG. 2, the free Al concentration C _Al and the bath temperature T of the plating bath 103 are set to the conditions of the region surrounded by the chain line of the “δ1 nucleation” region during operation, and “top” when stopped. It is preferable to set the condition of the area surrounded by the chain line of the "dross" area.

That is, when the hot-dip galvanizing facility 10 is stopped, the bath temperature T (° C.) of the hot-dip galvanizing bath 103 is set to a temperature range of 440 to 460 ° C., and the free Al concentration C _Al (mass) in the hot-dip galvanizing bath 103 is set. %) Is set so as to satisfy the formula (1), and when the hot-dip galvanizing facility 10 is in operation, the bath temperature T (° C.) of the hot-dip galvanizing bath 103 is set in the temperature range of 480 to 490 ° C. It is preferable that the free Al concentration C _Al (mass%) in the plating bath 103 is set so as to satisfy the formula (2).
-2.914 x ^10-5 x T + 1.524 x 10 ^-1 <C _Al <0.1427 (1)
0.1390 <C _Al <2.686 × 10 ^-4 × T + 1.383 × 10 ^-2 (2)

When the hot-dip galvanizing equipment 10 is stopped, the free Al concentration C _Al in the plating bath 103 is (-2.914 × 10 ^-5 × T + 1.524 × 10 ^-1 ) mass% or less in relation to the bath temperature T. In that case, it may deviate from the top dross region and a coarse bottom dross may remain on the bath bottom. If the free Al concentration C _Al in the plating bath 103 is 0.1427% by mass or more when the plating bath 103 is stopped, it is necessary to lower the free Al concentration C _Al when shifting from the stopped state to the operating time depending on the temperature conditions during the operation. There is. Since the adjustment of the free Al concentration C _{Al in} the plating bath 103 is performed while passing the steel plate S, the operation may be complicated. When the hot-dip galvanizing facility 10 is stopped, the free Al concentration C _Al in the plating bath 103 preferably satisfies the above formula (1), but is more preferably 0.1400 to 0.1420% by mass. preferable.

Further, if the bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is stopped is less than 440 ° C., the reactivity becomes low depending on the composition of the plating bath 103, and the transformation from δ1 dross to top dross sufficiently occurs. Therefore, the δ1 dross cannot be removed. Further, if the bath temperature of the plating bath 103 at the time of stopping is more than 460 ° C., it is easy to deviate from the top dross region and enter the bottom dross region at the time of stopping. As a result, the dross in the plating bath 103 cannot be sufficiently removed, and coarse bottom dross may remain on the bottom of the bath. The bath temperature of the plating bath 103 at the time of stopping is preferably 440 to 460 ° C. as described above, but more preferably 450 to 460 ° C.

If the free Al concentration C _Al in the plating bath 103 is 0.1390% by mass or less during the operation of the hot-dip galvanizing facility 10, it is necessary to reduce the free Al concentration C _Al during operation. Since the adjustment of the free Al concentration C _{Al in} the plating bath 103 is performed while passing the steel plate S, the operation may be complicated. When the free Al concentration C _Al in the plating bath 103 is (2.686 × 10 ^-4 × T + 1.383 × 10 ^-2 ) by mass or more in relation to the bath temperature T, the plating bath 103 during operation Depending on the bath temperature, it approaches the top dross area. As a result, the alloying suppressing effect of Al works excessively, and it may be difficult to stably alloy the steel sheet S. When the hot-dip galvanizing facility 10 is in operation, the free Al concentration C _Al in the plating bath 103 preferably satisfies the above formula (2), but is preferably 0.1400 to 0.1420% by mass. preferable.

If the bath temperature of the plating bath 103 during operation of the hot-dip galvanizing facility 10 is less than 480 ° C., it approaches the top dross region depending on the composition of the plating bath 103. As a result, the alloying suppressing effect of Al works excessively, and it may be difficult to stably alloy the steel sheet S. Further, when the bath temperature of the plating bath 103 during operation is more than 490 ° C., depending on the composition of the plating bath 103, when the hot dip galvanizing formed on the surface of the steel sheet S is alloyed, the alloying proceeds excessively. However, the adhesion of the alloyed layer (alloyed hot-dip galvanized layer) may decrease, and the alloyed layer may easily peel off. The bath temperature of the plating bath 103 during operation is preferably 480 to 490 ° C. as described above.

In the conventional method, when the hot-dip galvanizing facility 10 is operated by setting the bath temperature T of the plating bath 103 and the free Al concentration C _Al to be the nucleation region of δ1, the operation is performed even when the machine is stopped. The operation was carried out so as not to lower the bath temperature of the plating bath 103 as much as possible. This is because if the bath temperature of the plating bath 103 is lowered when the machine is stopped, the bottom dross floats and causes dross defects. However, as described above, in the present embodiment, the bath temperature of the plating bath 103 is preferably 480 to 490 ° C. during operation, and 440 to 460 ° C., which is lower than that during operation when the machine is stopped.

In the present embodiment, it is preferable that the difference between the bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is in operation and the bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is stopped is 25 ° C. or more. By setting the bath temperature difference between the operating state and the stopped state to 25 ° C. or more, it is possible to more stably suppress quality defects and productivity deterioration of the hot-dip galvanized steel sheet.

The plating bath 103 contains Zn as a main component as a liquid phase component, and may contain Al, Fe and impurities. When Fe is contained in the plating bath 103, it can be contained in a concentration of, for example, about 0.02 to 0.1% by mass. Fe in the plating bath 103 may be derived from the steel plate S, or may be separately added to the plating bath 103. Impurities are components that are mixed due to raw materials and other factors, and are allowed as long as they do not adversely affect the method for producing hot-dip galvanized steel sheets and the method for operating hot-dip galvanized baths according to the present embodiment. ..

The method for removing the top dross when the hot-dip galvanizing facility 10 is stopped is not particularly limited, and a known method can be adopted. Specifically, for example, a method of removing the top dross by scooping the top dross manually or by a machine using a jig in the shape of a net ladle can be mentioned.

The particle size distribution of dross can be measured as follows.
300 g of a plating bath solution is collected from the hot-dip galvanizing bath 103, and the collected plating bath solution is rapidly cooled and solidified and polished to a predetermined thickness (for example, about 0.5 mm) to obtain a measurement sample. The obtained measurement sample is observed in a plurality of fields of view (for example, about 5 fields of view) using an optical microscope or a scanning electron microscope having a predetermined magnification, and the particle size and number of dross are determined for each field of view according to a known image processing method. And measure.

The method for producing the hot-dip galvanized steel sheet and the method for operating the hot-dip galvanized bath according to the present embodiment have been described in detail above. According to the present embodiment, when the hot-dip galvanizing facility 10 is stopped, the bath temperature T of the plating bath 103 and the free Al concentration C _Al are set to the conditions where the top dross region is set, and the dross is recovered to be coarse. Dross can be removed. Then, during the operation of the hot-dip galvanized equipment 10, fine bottom dross is generated, but by operating the bottom dross in a region where grain growth is difficult (nucleation region of δ1 phase), the bottom dross is the quality of the hot-dip galvanized steel sheet. Does not affect. Therefore, in the bottom dross region, the hot-dip galvanized steel sheet can be manufactured without suppressing the quality defect of the hot-dip galvanized steel sheet and lowering the productivity. Then, even when the bottom dross operation, which is advantageous for alloying as compared with the top dross operation, is performed, the quality of the finally obtained hot-dip galvanized steel sheet can be improved.

Subsequently, the operation method of the hot-dip galvanized bath and the manufacturing method of the hot-dip galvanized steel sheet according to the present invention will be specifically described while showing an example of the present invention and a comparative example. The examples shown below are merely examples of the hot-dip galvanized bath operating method and the hot-dip galvanized steel sheet manufacturing method according to the present invention, and the hot-dip galvanized bath operating method and hot-dip galvanized steel according to the present invention. The method for producing a steel sheet is not limited to the following examples.

<1. Preliminary test>
The free Al concentration C _Al of the plating bath of the continuous hot-dip galvanizing facility for experiments was set to 0.1400%, the bath temperature of the plating bath when stopped was set to 455 ° C, and the floating top dross was completely removed before plating. The bath temperature was set to 455 ° C. and 485 ° C., respectively, and the operation was carried out for 10 days.

FIG. 3 shows the morphology of the bottom dross formed on the bottom of the plating bath 10 days after the start of operation. As shown in FIG. 3, when the bath temperature of the plating bath was 455 ° C., a coarse bottom dross of the Γ2 phase occurred. From this, when the bath temperature of the plating bath is 455 ° C, bottom dross of Γ2 phase is generated on the bottom of the bath even if the operation is performed under the condition of the top dross region, and the plating bath becomes coarse in a relatively short period of time. It turned out to be.

On the other hand, when the bath temperature of the plating bath was 485 ° C., fine δ1 phase dross was generated as shown in FIG. From this, even when the bath temperature of the plating bath is 485 ° C., bottom dross is generated at the bottom of the bath, but the phase of the bottom dross becomes the δ1 phase, and the growth rate of the particle size of the bottom dross is slow in the δ1 phase. It has been found.

The above results show a good correlation with the dross phase estimated from the Fe-Al liquid phase interface diagram of the hot-dip galvanizing bath shown in FIG. From this, it was found that the particle size of the bottom dross can be controlled by appropriately controlling the bath temperature of the plating bath when the hot-dip galvanizing facility is in operation and stopped.

<2. Actual machine test>
The free Al concentration C _Al of the plating bath of the hot-dip galvanizing equipment of the actual machine is varied within the range of 0.1300 to 0.1425 mass%, and the bath temperature T of the plating bath during stoppage and operation is 440 to 489 ° C. The alloyed hot-dip galvanized steel sheet was manufactured by passing the steel strip through a hot-dip galvanizing facility. When the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al were set to the top dross region when the hot-dip galvanizing equipment was stopped, the top dross was removed when the hot-dip galvanizing equipment was stopped. The surface of the manufactured alloyed hot-dip galvanized steel sheet was visually observed to investigate the presence or absence of dross defects.

Table 1 shows the operating conditions of the plating bath during the manufacture of alloyed hot-dip galvanized steel sheets and the evaluation results of the steel sheet surface. The evaluation results of the surface of the steel sheet were evaluated as "A" for those without dross flaws, "B" for those with slight dross flaws, and "C" for those with many dross flaws.

As can be seen from Table 1, the conditions under which the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al become the δ1 phase nucleation region (“δ1 nucleation” in Table 1) during the operation of the hot-dip galvanizing facility. If, there was no or few dross defects (evaluation A or B). On the other hand, when the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are in the Γ2 phase grain growth region (“Γ2 grain growth” in Table 1) or the δ1 phase grain growth region (in Table 1). In Table 1, under the condition of “δ1 grain growth”), a dross defect occurred (evaluation C or evaluation B).

In particular, paying attention to the case where the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are the nucleation region of the δ1 phase during the operation of the hot-dip galvanizing facility, when the hot-dip galvanizing facility is stopped. , When the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al were in the top dross region (“top dross” in Table 1), no dross flaws occurred (in evaluation A). there were). Further, when the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are the conditions for forming a δ1 phase nucleation region, and when the hot-dip galvanizing facility is stopped, hot-dip galvanizing is performed. When the bath temperature T and the free Al concentration C _Al were the conditions for the grain growth region of the δ1 phase or the grain growth region of the Γ2 phase, a dross defect occurred (evaluation B or evaluation C). ). No dross flaws occurred when the hot-dip galvanizing bath temperature T and the free Al concentration C _Al were in the top dross region when the hot-dip galvanizing equipment was stopped and operated (evaluation). Although it was A), poor alloying occurred.

In order to investigate the cause of dross flaws during the operation of hot-dip galvanizing equipment, the free Al concentration C _Al of the plating bath was fixed at 0.1410%, and the bath temperature of the plating bath was always 455 ° C (Comparative Example 1). The hot-dip galvanizing equipment was operated by constantly controlling the temperature to 485 ° C. (Comparative Example 2) or 455 ° C. when stopped and 485 ° C. when operating (Example of the present invention). After the hot-dip galvanizing facility was put into operation, the plating bath liquid was scooped out from a position 300 mm deep from the plating bath surface. The plating bath solution was placed in a copper mold and rapidly cooled and solidified to obtain a sample. Next, after mirror polishing the outermost surface of the sample, the particle size and the number of dross contained in the range of 20 mm × 20 mm were investigated using a laser microscope. Since the sampled plating bath liquid is located at a depth of 300 mm from the plating bath surface, the number of top dross and the number of coarse bottom dross settled on the bottom of the plating bath are not reflected in the survey results.
FIG. 4 shows the relationship between the particle size and the number of dross under each manufacturing condition.

When the plating bath temperature was constantly operated at 455 ° C. (top dross region, Comparative Example 1), top dross was generated on the plating bath surface, but the generation of dross at a depth of 300 mm from the plating bath surface was extremely small. However, in this case, there arises a problem that the hot-dip galvanized layer is difficult to alloy, as has been a problem in the past.
Further, when the plating bath temperature was always 485 ° C. (δ1 phase nucleation region, Comparative Example 2), the proportion of fine dross increased. Dross with a particle size exceeding 100 μm is also observed, which is considered to be the cause of dross defects.

On the other hand, when the plating bath temperature is set to 455 ° C. (top dross region) when stopped and 485 ° C (nucleation region of δ1 phase) during operation (example of the present invention), the number of dross having a particle size of 100 μm or more is remarkable. Diminished.
From the above, when the plating bath temperature is set to the top dross region when the machine is stopped to remove the top dross, and when the plating bath temperature is operated as the nucleation region of the δ1 phase during operation, even a dross with a large dross diameter, which can be a dross defect, is relatively small. Since the formation of dross (dross diameter 100 to 150 μm) can be suppressed, it was found that the occurrence of minute dross flaws can be reliably suppressed.

Based on the above findings, when the hot-dip galvanizing bath was continued to operate with the bath temperature T of the plating bath and the free Al concentration C _Al as the top dross region when stopped and the nucleation region of the δ1 phase during operation, alloying was performed. It has become possible to manufacture high-quality steel sheets in which dross defects are not a problem, while avoiding top dross operations, which are difficult to produce and reduce productivity.

Although the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, the present invention is not limited to such examples. It is clear that anyone with ordinary knowledge in the field of technology to which the present invention belongs can come up with various modifications or modifications within the scope of the technical ideas described in the claims. , These are also naturally understood to belong to the technical scope of the present invention.

10 Hot-dip galvanizing equipment 101 Hot-dip galvanizing bathtub 103 Hot-dip galvanizing bath 105 Snout 107 Sink roll 109 Gas wiping equipment 111 Alloying furnace

Claims (6)

1. A method for manufacturing a hot-dip galvanized steel sheet, which manufactures a hot-dip galvanized steel sheet by continuously immersing a steel sheet in a hot-dip galvanized bath to form a hot-dip galvanized layer.
   When the hot-dip galvanizing equipment is stopped, the bath temperature T and the free Al concentration C _Al of the hot-dip galvanizing bath are set so that top dross occurs, and the top dross of the hot-dip galvanizing bath is removed.
   When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are set so that the δ1 phase is nucleated.
   A method for manufacturing a hot-dip galvanized steel sheet.
2. When the hot-dip galvanizing facility is stopped, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 440 to 460 ° C., and the free Al concentration C _{Al in} mass% of the hot-dip galvanizing bath is expressed by the formula. Set to satisfy (1),
   When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 480 to 490 ° C., and the free Al concentration C _{Al in} the mass% of the hot-dip galvanizing bath is expressed by the formula. Set to satisfy (2),
   The method for producing a hot-dip galvanized steel sheet according to claim 1.
   -2.914 x ^10-5 x T + 1.524 x 10 ^-1 <C _Al <0.1427 (1)
   0.1390 <C _Al <2.686 × 10 ^-4 × T + 1.383 × 10 ^-2 (2)
3. The hot-dip galvanized layer is alloyed to form an alloyed hot-dip galvanized layer.
   The method for producing a hot-dip galvanized steel sheet according to claim 1 or 2.
4. It is a method of operating a hot-dip galvanizing bath in which a steel plate is continuously immersed in a hot-dip galvanizing bath to form a hot-dip galvanizing layer.
   When the hot-dip galvanizing equipment is stopped, the bath temperature T and the free Al concentration C _Al of the hot-dip galvanizing bath are set so that top dross occurs, and the top dross of the hot-dip galvanizing bath is removed.
   When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath and the free Al concentration C _Al are set so that the δ1 phase is nucleated.
   A method of operating a hot-dip galvanized bath, which is characterized in that.
5. When the hot-dip galvanizing facility is stopped, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 440 to 460 ° C., and the free Al concentration C _{Al in} mass% of the hot-dip galvanizing bath is expressed by the formula. Set to satisfy (1),
   When the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 480 to 490 ° C., and the free Al concentration C _{Al in} the mass% of the hot-dip galvanizing bath is expressed by the formula. Set to satisfy (2),
   The method for operating a hot-dip galvanized bath according to claim 4, wherein the hot-dip galvanized bath is operated.
   -2.914 x ^10-5 x T + 1.524 x 10 ^-1 <C _Al <0.1427 (1)
   0.1390 <C _Al <2.686 × 10 ^-4 × T + 1.383 × 10 ^-2 (2)
6. The hot-dip galvanized layer is alloyed to form an alloyed hot-dip galvanized layer.
   The method for operating a hot-dip galvanized bath according to claim 4 or 5.

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