[0001]The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanizing bath.
Priority is claimed on Japanese Patent Application No. 2019-080277, filed in Japan on April 19, 2019, the content of which is incorporated herein by reference. [Background Art]
[0002]
Conventionally, as a method for forming a hot-dip galvanized layer on a steel sheet, a method in which a steel sheet is continuously immersed in a hot-dip galvanizing bath has been used. In this method, a steel sheet is annealed, and then the annealed steel sheet is immersed into the hot-dip galvanizing bath through the inside of a snout connected to an annealing furnace at the upper end and immersed in the hot-dip galvanizing bath at the lower end. The travelling direction of the steel sheet is changed from diagonally downward to upward with an immersion roll in the hot-dip galvanizing bath, and the steel sheet is lifted. After that, the amount of the hot-dip galvanized plate attached to the surface of the steel sheet is controlled by a gas wiping method.
[0003]
The steel sheet lifted from the hot-dip galvanizing bath turns into a galvannealed steel sheet by carrying out an alloying treatment in an alloying furnace in
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the latter stage. (Hereinafter, a steel sheet on which an alloying treatment has been carried out (galvannealed steel sheet) and a steel sheet on which an alloying treatment has not been carried out will be collectively referred to as "hot-dip galvanized steel sheet", and, when particularly distinctively mentioned, the steel sheet on which an alloying treatment has not been carried out will be expressed as "non-galvannealed steel sheet".)
[0004]
The inside of the snout is blocked from the atmosphere and is held in a non-oxidative atmosphere such as nitrogen gas, and the oxidative contamination of the surface of a steel sheet to be plated is prevented. Here, when metal eluted from a steel sheet into a bath (for example, Fe eluted from a steel sheet) and Al or Zn present in the bath react with each other, dross that is deposited on the bottom part of the plating bath is generated. Dross generated as described above is referred to as bottom dross. Bottom dross floats in the bath due to an accompanying flow that is generated by the travelling of the steel sheet in the bath, adheres to the surface of the steel sheet that is being immersed in the bath, and acts as a cause for a poor quality (particularly the poor external appearance of the surface of a hot-dip galvanized steel sheet).
[0005]
Conventionally, a variety of techniques have been proposed in order to suppress the poor external appearance of the surfaces of hot-dip galvanized steel sheets. For example, Patent Document 1 proposes a technique in which, at the time of manufacturing a galvannealed steel sheet, when the hot-dip galvanizing bath temperature is represented by T (°C), and the boundary Al concentration that is represented by a formula Cz = -0.0015 x T + 0.76 is represented by Cz (wt%), the hot-dip galvanizing bath temperature T is set within a range of 435°C to 500°C, and the Al
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concentration in the bath is held within a range of Cz ± 0.01 wt%.
[0006]
Patent Document 2 proposes a technique in which, at the time of manufacturing a galvannealed steel sheet, the Al concentration in a bath is held within a range of 0.15 ± 0.01 wt%.
[0007]
It is known that, as dross that can be generated at the time of manufacturing hot-dip galvanized steel sheets, there are four types of dross such as Fe2Als (so-called top dross), a 81 phase, a T2 phase, and a £ phase. The technique proposed in Patent Document 1 proposes that operation is carried out under boundary conditions between conditions under which the £ phase is generated and conditions under which the 61 phase is generated. The technique proposed in Patent Document 2 proposes that operation is carried out under boundary condition between conditions under which the Fe2Ab phase is generated and conditions under which the 81 phase is generated. [Prior Art Document] [Patent Document]
[0008]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. Hll-350096
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. Hll-350097 [Disclosure of the Invention] [Problems to be Solved by the Invention]
[0009]
Conventionally, operation in which the Al concentration of a hot-dip
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galvanizing bath is set to be high to form dross that floats on the hot-dip galvanizing bath surface (so-called "Fe-Al-based top dross") and the Fe-Al-based top dross is appropriately removed (hereinafter, also referred to as top dross operation) has been carried out. As operation that opposes the top dross operation, there is operation called bottom dross operation.
[0010]
In a case where the Al concentration of a hot-dip galvanizing bath is low, dross that settles on the hot-dip galvanizing bath (so-called "Fe-Zn-based bottom dross") is formed. Since it is difficult to remove the Fe-Zn-based bottom dross during the operation of a hot-dip galvanizing facility, the Fe-Zn-based bottom dross is deposited on the bath bottom. The bottom dross deposited on the bath bottom is soon wound up in the bath due to an accompanying flow of the steel sheet, adheres to the steel sheet and a roll in the bath, and acts as a cause for generating a defect (hereinafter, referred to as "dross defect" in some cases) on the surface of the steel sheet.
[0011]
When the bottom dross adheres to the steel sheet, an uneven portion is formed on the plate surface, which causes a poor quality of the external appearance. In addition, as a result of the formation of the uneven portion, a local battery is likely to be formed, a surface defect that acts as a cause for degrading corrosion resistance is generated, and a quality defect of the plated steel sheet is generated. Therefore, in order to maintain the quality of hot-dip galvanized steel sheets during the bottom dross operation, there is a need to periodically bring the line to a standstill and clean the bath in order to remove bottom dross deposited on the bath bottom. As opposed to the top dross operation in which dross can be removed during the operation, the bottom dross operation in which there is a need to bring the line to a standstill and remove dross
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takes man-hours and thus has a problem of a decrease in production volume attributed to the line being brought to a standstill. Therefore, generally, the bottom dross operation is avoided.
[0012]
However, there are cases where an alloying treatment is carried out on a plating layer after a steel sheet is immersed in a hot-dip galvanizing bath. As the Al content in the hot-dip galvanized layer increases, alloying becomes more difficult. Therefore, in the case of carrying out, particularly, an alloying treatment, in order to manufacture high-quality galvannealed steel sheets with highly efficient productivity, the bottom dross operation in which the Al concentration in the hot-dip galvanizing bath is low is advantageous.
[0013]
The present invention has been made in view of the above-described problem. An object of the present invention is to provide a method for manufacturing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanizing bath in which, even in a case where the bottom dross operation is carried out, the poor quality of the hot-dip galvanized steel sheet can be suppressed and the degradation of productivity is suppressed. [Means for Solving the Problem]
[0014]
In order to solve the above-described problem, the present inventors investigated the grain diameter of bottom dross that acts as a cause for generating a dross defect at the time of carrying out the bottom dross operation. As a result, the present inventors found that, when bottom dross having a grain diameter of 100 to 300 um is present in a bath, the number of dross defects increases. In addition, the present
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inventors investigated in detail the conditions of a hot-dip galvanizing bath in which the generation of bottom dross having a grain diameter of 100 to 300 um is suppressed and came up with an idea of the present invention described below in detail.
The gist of the present invention completed based on such a finding is as described below.
[0015]
[1] A method for manufacturing a hot-dip galvanized steel sheet according to an aspect of the present invention is a method for manufacturing a hot-dip galvanized steel sheet, the method including continuously immersing a steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer, thereby manufacturing the hot-dip galvanized steel sheet,
when a hot-dip galvanizing facility is at a standstill, a bath temperature T and a free Al concentration CM of the hot-dip galvanizing bath are set so that top dross is generated, and the top dross in the hot-dip galvanizing bath is removed, and
when the hot-dip galvanizing facility is in operation, the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath are set so that a 81 phase is nucleated.
[2] In the method for manufacturing a hot-dip galvanized steel sheet according to [1], when the hot-dip galvanizing facility is at a standstill, the bath temperature T of the hot-dip galvanizing bath may be set to a temperature range of 440°C to 460°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath may be set to satisfy Formula (1), and,
when the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath may be set to a temperature range of 480°C to 490°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath may be set to
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satisfy Formula (2).
-2.914 x 10"5 x T + 1.524 x 101 < CAI < 0.1427 (1)
0.1390 < CAI < 2.686 x 10"4 x T + 1.383 x 10"2 (2)
[3] The method for manufacturing a hot-dip galvanized steel sheet according to [1] or [2], further including alloying the hot-dip galvanized layer to form a hot-dip galvannealed layer.
[4] A 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, the method including continuously immersing a steel sheet in the hot-dip galvanizing bath to form a hot-dip galvanized layer,
when a hot-dip galvanizing facility is at a standstill, a bath temperature T and a free Al concentration CAI of the hot-dip galvanizing bath are set so that top dross is generated, and the top dross in the hot-dip galvanizing bath is removed, and
when the hot-dip galvanizing facility is in operation, the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath are set so that a 81 phase is nucleated.
[5] In the method for operating a hot-dip galvanizing bath according to [4], when the hot-dip galvanizing facility is at a standstill, the bath temperature T of the hot-dip galvanizing bath may be set to a temperature range of 440°C to 460°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath may be set to satisfy Formula (1), and,
when the hot-dip galvanizing facility is in operation, the bath temperature T of the hot-dip galvanizing bath may be set to a temperature range of 480°C to 490°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath may be set to satisfy Formula (2).
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-2.914 x 10"5 x T + 1.524 x 101 < CAI < 0.1427 (1)
0.1390 < CAI < 2.686 x 10"4 x T + 1.383 x 10"2 (2)
[6] The method for operating a hot-dip galvanizing bath according to [4] or [5], further including alloying the hot-dip galvanized layer to form a hot-dip galvannealed layer. [Effects of the Invention]
[0016]
According to the above-described aspects of the present invention, it becomes possible to provide a method for manufacturing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanizing bath in which, even in a case where the bottom dross operation is carried out, the poor quality of the hot-dip galvanized steel sheet can be suppressed and the degradation of productivity is suppressed. [Brief Description of the Drawings]
[0017]
FIG. 1 is a schematic view showing an example of the configuration of a continuous hot-dip galvanizing facility (hot-dip galvannealing facility) that can be used in the present embodiment.
FIG. 2 is a metastable state diagram in which dross-generating phases in a hot-dip galvanizing bath are arranged with respect to the bath temperature T (°C) and the free Al concentration CAI.
FIG. 3 shows a microphotograph showing the forms of bottom dross generated in a plating bath after 10 days of operation.
FIG. 4 is a graph showing a relationship between the grain diameters and the number of dross grains under manufacturing conditions in each example. [Embodiments of the Invention]
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[0018]
Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to drawings.
[0019]
<1. Configuration of continuous hot-dip galvanizing facility>
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 a method for manufacturing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanizing bath according to the present embodiment will be described in detail. To be exact, this facility is a hot-dip galvannealing facility. In the case of manufacturing a non-galvannealed steel sheet, what is only required is not to bring an alloying furnace into operation. Therefore, in the following description, a hot-dip galvannealing facility will be taken as an example to describe the continuous hot-dip galvanizing facility.
[0020]
FIG. 1 is a schematic view showing an example of the configuration of a hot-dip galvannealing facility. As shown in FIG. 1, a hot-dip galvanizing facility 10 includes, for example, a hot-dip galvanizing bath 103 (hereinafter, also simply referred to as "plating bath"), a hot-dip galvanizing bath tank 101 in which the plating bath 103 is stored, a snout 105, a sink roll 107, a gas wiping device 109, and an alloying furnace 111.
[0021]
An annealing furnace 20 that is provided in the previous stage of the hot-dip galvanizing facility 10 (on the upstream side in the conveyance direction of a steel sheet S) is blocked from the atmospheric atmosphere, and the inside is maintained in a
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reducing atmosphere. In addition, the annealing furnace 20 heats the steel sheet S that is continuously conveyed. With the annealing furnace 20, the surface of the steel sheet S is activated, and the mechanical properties of the steel sheet S are adjusted. The outlet end portion of the annealing furnace 20 is connected to the upstream end portion of the snout 105 through a space in which a turndown roll 30 is provided.
[0022]
The snout 105 is connected to the end portion of the annealing furnace 20 at the upstream end portion and is immersed into the hot-dip galvanizing bath 103 from obliquely above at the downstream end portion. Similar to the annealing furnace 20, the inside of the snout 105 is blocked from the atmospheric atmosphere and is maintained in a reducing atmosphere.
[0023]
The steel sheet S the conveyance direction of which has been changed downward with the turndown roll 30 is conveyed through the inside of the snout 105 and is continuously immersed into the hot-dip galvanizing bath 103 that is stored in the hot-dip galvanizing bath tank 101. The sink roll 107 is provided inside the hot-dip galvanizing bath tank 101. The sink roll 107 has a rotation axis parallel to the sheet width direction of the steel sheet S, and the width of the outer circumferential surface of the sink roll 107 is equal to or larger than the sheet width of the steel sheet S. The conveyance direction of the steel sheet S is changed upward with sink roll 107.
[0024]
The gas wiping device 109 sprays gas toward both surfaces of the steel sheet S that is brought out from the hot-dip galvanizing bath tank 101, thereby scraping off a part of a hot-dip galvanized plate attached to the surfaces of the steel sheet S. As a result, the amount of the hot-dip galvanized plate attached to the surface of the steel
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sheet S is adjusted.
[0025]
After that, the steel sheet S is further alloyed in the alloying furnace 111 while being vertically lifted. The alloying furnace 111 is made up of three parts of a heating zone, a heat retention zone, a cooling zone in order from the steel sheet S entry side. In the alloying furnace 111, first, the steel sheet S is heated in the heating zone such that the sheet temperature of the steel sheet S becomes substantially uniform. Next, an alloying time in the heat retention zone is secured, whereby the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to turn into an alloyed layer (hot-dip galvannealed layer). After that, the steel sheet S (that is, the galvannealed steel sheet) is cooled in the cooling zone and conveyed to the next step with a top roll 40. In the case of manufacturing a non-galvannealed steel sheet, the alloying treatment using the alloying furnace 111 as described above is not carried out.
[0026]
In the above-described hot-dip galvanizing facility 10, iron eluted from the steel sheet S forms a particulate solid alloy having a high melting point that is called dross in the hot-dip galvanizing bath 103 in the hot-dip galvanizing bath tank 101. When this dross adheres to the steel sheet S, a dross defect is generated on the surface of the steel sheet S.
[0027]
<2. Present Inventors' studies>
What becomes a problem at the time of carrying out the bottom dross operation is that the bottom dross is wound up due to the accompanying flow of the steel sheet S in the plating bath 103 and adheres to the steel sheet S. The generation of the bottom dross is unavoidable in the bottom dross operation, but it is considered
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that, if the grain diameter of the bottom dross is small, a poor quality may not be caused even when the bottom dross adheres to the steel sheet S.
[0028]
The present inventors investigated the grain diameter of the bottom dross, which acts as a cause for the generation of a dross defect. As a result, the present inventors found that, when bottom dross having a grain diameter of 100 to 300 um is present in a bath, a number of dross defects are generated. Since bottom dross having a grain diameter of less than 100 um is sufficiently small, the bottom dross does not act as a cause for the generation of a dross defect even when adhering to the steel sheet S. On the other hand, bottom dross having a grain diameter of more than 300 um is significantly affected by the force of gravity and settles to the bath bottom, which makes the bottom dross unlikely adhere to the steel sheet S. Therefore, in order to suppress the generation of a dross defect, it is important to suppress the amount of bottom dross having a grain diameter of 100 to 300 um as small as possible.
[0029]
Incidentally, the present inventors investigated the growth rate of the grain diameter 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 grain diameter of the bottom dross is fast, and, when the bath temperature of the plating bath 103 is high, the growth rate of the grain diameter of the bottom dross is slow. This is assumed to arise from the fact that the growth rate of the T2 phase, which is stable at a low bath temperature (a temperature range of 455°C to 460°C or lower, that is, 455°C or lower), is fast compared with the growth rate of the 81 phase, which is stable at a high bath temperature (a temperature range of 455°C to 460°C or higher, that is, 460°C or higher).
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[0030]
While the hot-dip galvanizing facility 10 is in operation, since the steel sheet S is continuously passed through the hot-dip galvanizing bath tank 101, local nucleation inevitably occurs. Therefore, during operation, bottom dross is intentionally caused to grow in the nucleation region of the 81 phase, and Fe eluted from the steel sheet S is induced to become fine bottom dross. Specifically, operation is carried out in a high bath temperature range in which the growth rate of the grain diameter of the bottom dross is slow (the nucleation region of the 81 phase), and it is prevented for the grain diameter of fine bottom dross newly nucleated during the operation to become 100 um or more. This makes it possible to suppress the generation of a dross defect.
[0031]
However, in the case of continuing the bottom dross operation for a long period of time, there are cases where bottom dross gradually grows, although at a low rate, but grows up to a grain diameter of 100 to 300 um. A phenomenon in which bottom dross grows as described above is called the Ostwald growth in crystallography. When operation continues for a long period of time in the plating bath 103 in which bottom dross having a variety of grain diameters is present, a substance migrates from bottom dross having a relatively small grain diameter to relatively large bottom dross, the bottom dross having a small grain diameter becomes smaller, and the bottom dross having a large grain diameter becomes larger.
[0032]
Therefore, the bottom dross operation begins from a state where the bottom dross has been removed, and the operation is carried out in a manner that the grain diameters of the bottom dross do not become significantly different even when bottom
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dross is generated. This makes the Ostwald growth unlikely to occur. In addition, even when the grain diameter of the bottom dross becomes large due to the Ostwald growth, the generation of a dross defect can be suppressed as long as the bottom dross is removed before growing to a grain diameter of 100 um or more. Specifically, when the hot-dip galvanizing facility 10 is at a standstill (offline), the bath temperature and the free Al concentration of the plating bath 103 are set so that top dross is generated, the dross in the plating bath 103 is caused to float on the surface of the plating bath, and the dross caused to float is removed as top dross.
[0033]
As described above, even when new fine bottom dross is nucleated during operation, it is possible to remove the bottom dross in the plating bath 103 as top dross before the bottom dross significantly grows and to suppress the generation of a dross defect by changing the conditions of the plating bath 103 between when the hot-dip galvanizing facility is in operation and at a standstill.
[0034]
<3. Method for manufacturing hot-dip galvanized steel sheet and method for operating hot-dip galvanizing bath>
A method for manufacturing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanizing bath according to the present embodiment, which have been completed based on the above-described findings, will be described. In the following description, the method for manufacturing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment will be described to be carried out using a hot-dip galvanizing facility 10 shown in FIG. 1, but the present invention is not limited thereto.
[0035]
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The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment is a method for manufacturing a hot-dip galvanized steel sheet in which the steel sheet S is continuously immersed in the hot-dip galvanizing bath 103 to form a hot-dip galvanized layer, thereby manufacturing the hot-dip galvanized steel sheet. In the present embodiment, a hot-dip galvannealed layer may be formed by, after the formation of a hot-dip galvanized layer, heating the steel sheet S to alloy the hot-dip galvanized layer. In the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment, since the plating bath 103 is operated under bottom dross conditions as described below, the Al content in the hot-dip galvanized layer is suppressed and alloying is easy. As a result, it is possible to manufacture a high-quality galvannealed steel sheet.
[0036]
In addition, the method for operating a hot-dip galvanizing bath according to the present embodiment is a method that is preferably used in the method for manufacturing a hot-dip galvanized steel sheet. In addition, as described above, the method for operating a hot-dip galvanizing bath according to the present embodiment is particularly preferably applied in a case where the hot-dip galvanized layer is alloyed to a galvannealed steel sheet.
[0037]
The steel sheet (base steel sheet) S that is used in the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment is not particularly limited, a well-known steel sheet may be appropriately used depending on a variety of characteristics that are required for a hot-dip galvanized steel sheet to be manufactured (for example, a tensile strength, a variety of strengths, and the like that are required for steel sheets), and a steel sheet that is used for an automobile outer
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sheet can also be used.
[0038]
In the method for manufacturing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment, when the hot-dip galvanizing facility 10 is at a standstill, conditions are set such that the bath temperature T and the free Al concentration CAI of the plating bath 103 are in a top dross region, and top dross is removed. When the hot-dip galvanizing facility 10 is in operation (online), conditions are set such that the bath temperature T and the free Al concentration CAI of the plating bath 103 are in the nucleation region of the 61 phase. That is, when the hot-dip galvanizing facility is at a standstill, the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath are set so that top dross is generated, and the top dross in the hot-dip galvanizing bath is removed. When the hot-dip galvanizing facility is in operation, the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath are set so that the 81 phase is nucleated.
[0039]
This makes it possible to reduce the amount of bottom dross having a grain diameter of 100 to 300 um in the plating bath 103. That is, when the hot-dip galvanizing facility 10 is at a standstill, conditions are set such that the bath temperature T and the free Al concentration CAI of the plating bath 103 are in the top dross region, whereby the top dross that floats on the surface of the plating bath 103 is collected, and coarse dross that can cause a dross defect is removed. On the other hand, when the hot-dip galvanizing facility 10 is in operation, conditions are set such that the bath temperature T and the free Al concentration CAI of the plating bath 103 are in the nucleation region of the 81 phase, operation is intentionally carried out in the
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nucleation region of the 81 phase, and Fe eluted from the steel sheet S is induced to become fine bottom dross.
[0040]
Usually, when the hot-dip galvanizing facility 10 is in operation for a long period of time, due to the Ostwald growth, the grain diameter of generated bottom dross increases. However, the growth rate of bottom dross during operation in the nucleation region of the 81 phase is slow, and the Ostwald growth is unlikely to occur. Therefore, when the hot-dip galvanizing facility 10 is not operated for a somewhat long period of time, the grain diameter of bottom dross does not grow up to 100 um or more. Before the grain diameter of bottom dross grows to 100 um or more, the hot-dip galvanizing facility 10 is brought to a standstill, conditions are set such that the bath temperature T and the free Al concentration CAI of the plating bath 103 are in the top dross region, and dross is removed as top dross, which makes it possible to suppress the generation of a dross defect.
[0041]
Specifically, the condition of the plating bath 103 can be controlled with, 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. 2. FIG. 2 is a metastable state diagram in which dross-generating phases in the hot-dip galvanizing bath are arranged with respect to the bath temperature T (°C) and the free Al concentration CAI in the bath. In FIG. 2, "CAI" indicates the free Al concentration (mass%) in the bath in the plating bath 103. The "free Al concentration in the bath" means the concentration of Al that is contained in a liquid phase in the plating bath 103 and is distinctively used from the total Al concentration of the plating bath 103 that means the average Al concentration in both
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dross and the liquid phase.
[0042]
The free Al concentration CM in the plating bath 103 is measured by the following method. The plating bath liquid is scooped up from the hot-dip galvanizing bath tank 101, and this plating bath liquid is poured into a mold and solidified to produce an ingot. An appropriate amount of chips is scraped from this ingot using a drill, and a part of the chips is dissolved with 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 CAI in the plating bath 103 is obtained.
In addition, the bath temperature T of the plating bath 103 may be measured using a thermometer at a position where the bath temperature is stable.
[0043]
In the present embodiment, the free Al concentration CAI and the bath temperature T of the plating bath 103 are set to, in FIG. 2, the inside of a "81 nucleation" region when the hot-dip galvanizing facility is in operation and set to the inside of a "top dross" region when the hot-dip galvanizing facility is at a standstill. The "51 nucleation" region in FIG. 2 is the above-described nucleation region of the 81 phase. In a case where the free Al concentration CAI and the bath temperature T of the plating bath 103 are included in the "51 nucleation" region, the 51 phase is nucleated in the plating bath 103. In addition, the "top dross" region in FIG. 2 is the above-described top dross region. In a case where the free Al concentration CAI and the bath temperature T of the plating bath 103 are included in the "top dross" region, top dross is generated in the plating bath 103.
Furthermore, in the present embodiment, it is preferable that, in FIG. 2, the

free Al concentration CAI and the bath temperature T of the plating bath 103 are set to the conditions of a region surrounded by the chain lines in the "81 nucleation" region when the hot-dip galvanizing facility is in operation and set to the condition of a region surrounded by the chain lines in the "top dross" region when the hot-dip galvanizing facility is at a standstill.
[0044]
That is, it is preferable that, when the hot-dip galvanizing facility 10 is at a standstill, the bath temperature T (°C) of the hot-dip galvanizing bath 103 is set to a temperature range of 440°C to 460°C, and the free Al concentration CAI (mass%) in the hot-dip galvanizing bath 103 is set 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 to a temperature range of 480°C to 490°C, and the free Al concentration CAI (mass%) in the hot-dip galvanizing bath 103 is set to satisfy the formula (2).
-2.914 x 10"5 x T + 1.524 x 101 < CAI < 0.1427 (1)
0.1390 < CAI < 2.686 x 10"4 x T + 1.383 x 10"2 (2)
[0045]
When the hot-dip galvanizing facility 10 is at a standstill, if the free Al concentration CAI in the plating bath 103 becomes (-2.914 x 10~5 x T + 1.524 x 10"1) mass% or less in relation to the bath temperature T, there are cases where the conditions deviate from the top dross region and coarse bottom dross remains on the bath bottom. When the hot-dip galvanizing facility 10 is at a standstill, if the free Al concentration CAI in the plating bath 103 is 0.1427 mass% or more, depending on the temperature condition and the like when the hot-dip galvanizing facility 10 is in operation, there is a need to decrease the free Al concentration CAI at the time of
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bringing the hot-dip galvanizing facility 10 at a standstill into operation. Since the free Al concentration CAI in the plating bath 103 is adjusted while passing the steel sheet S, there are cases where the operation becomes complicated. When the hot-dip galvanizing facility 10 is at a standstill, the free Al concentration CAI in the plating bath 103 preferably satisfies the formula (1), but is more preferably set to 0.1400 to 0.1420 mass%.
[0046]
In addition, when the hot-dip galvanizing facility 10 is at a standstill, if the bath temperature of the plating bath 103 is lower than 440°C, depending on the composition of the plating bath 103, the reactiveness becomes low, and transformation from 81 dross to top dross does not sufficiently occur, which makes it impossible to remove the 81 dross. In addition, when the hot-dip galvanizing facility 10 is at a standstill, if the bath temperature of the plating bath 103 is higher than 460°C, while the hot-dip galvanizing facility 10 is at a standstill, the conditions deviate from the top dross region and are likely to enter the bottom dross region. As a result, there are cases where dross in the plating bath 103 are not be sufficiently removed and coarse bottom dross remains on the bath bottom. The bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is at a standstill is preferably set to 440°C to 460°C as described above, but more preferably set to 450°C to 460°C.
[0047]
When the hot-dip galvanizing facility 10 is in operation, if the free Al concentration CAI in the plating bath 103 is 0.1390 mass% or less, there is a need to decrease the free Al concentration CAI when the hot-dip galvanizing facility 10 is in operation. Since the free Al concentration CAI in the plating bath 103 is adjusted while passing the steel sheet S, there are cases where the operation becomes
- 20 -

complicated. If the free Al concentration CAI in the plating bath 103 becomes (2.686 x 10~4 x T + 1.383 x 10"2) mass% or more in relation to the bath temperature T, depending on the bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is in operation, the conditions approach the top dross region. As a result, the alloying-suppressing effect of Al is excessively exhibited, and there are cases where it becomes difficult to stably alloy the steel sheet S. When the hot-dip galvanizing facility 10 is in operation, the free Al concentration CAI in the plating bath 103 preferably satisfies the formula (2), but is more preferably set to 0.1400 to 0.1420 mass%.
[0048]
When the hot-dip galvanizing facility 10 is in operation, if the bath temperature of the plating bath 103 is lower than 480°C, depending on the composition of the plating bath 103, the conditions approach the top dross region. As a result, the alloying-suppressing effect of Al is excessively exhibited, and there are cases where it becomes difficult to stably alloy the steel sheet S. In addition, when the hot-dip galvanizing facility 10 is in operation, if the bath temperature of the plating bath 103 is higher than 490°C, depending on the composition of the plating bath 103, there are cases where, at the time of alloying a hot-dip galvanized plate that is formed on the surface of the steel sheet S, the alloying proceeds excessively, the adhesion of the alloyed layer (hot-dip galvannealed layer) deteriorates, and the alloyed layer is likely to exfoliate. When the hot-dip galvanizing facility 10 is in operation, the bath temperature of the plating bath 103 is preferably 480°C to 490°C as described above.
[0049]
In conventional methods, in the case of carrying out operation with the hot-dip galvanizing facility 10 in operation under conditions set such that the bath
- 21 -

temperature T and the free Al concentration CAI of the plating bath 103 are in the nucleation region of 81, the operation was carried out in a manner that the bath temperature of the plating bath 103 was not decreased as much as possible even when the hot-dip galvanizing facility 10 was at a standstill. This is because, if the bath temperature of the plating bath 103 is decreased when the hot-dip galvanizing facility 10 is at a standstill, bottom dross floats and causes the generation of a dross defect. However, as described above, in the present embodiment, it is preferable that, when the hot-dip galvanizing facility 10 is in operation, the bath temperature of the plating bath 103 is set to 480°C to 490°C and, when the hot-dip galvanizing facility 10 is at a standstill, the bath temperature is decreased more than when the hot-dip galvanizing facility 10 is in operation and set to 440°C to 460°C.
[0050]
In the present embodiment, 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 at a standstill is preferably set to 25°C or more. When the difference between the bath temperatures with hot-dip galvanizing facility 10 in operation and at a standstill is set to 25°C or more, it is possible to more stably suppress the poor quality and productivity deterioration of the hot-dip galvanized steel sheet.
[0051]
The plating bath 103 may contain, as liquid-phase components, Zn as a main component and Al, Fe, and impurities. In a case where Fe is contained in the plating bath 103, Fe can be contained in a concentration of, for example, approximately 0.02 to 0.1 mass%. Fe in the plating bath 103 may be derived from the steel sheet S or may be separately added to the plating bath 103. The impurities mean components
- 22 -

that are contained by accident due to a raw material and other causes and are allowed to an extent that the method for manufacturing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment are not adversely affected.
[0052]
The method for removing top dross when the hot-dip galvanizing facility 10 is at a standstill is not particularly limited, and a well-known method can be adopted. Specifically, for example, a method for removing top dross by manually or mechanically scooping up the top dross using a net-like jig is an exemplary example.
[0053]
The grain diameter distribution of dross can be measured as described below.
The plating bath liquid (300 g) is collected from the hot-dip galvanizing bath 103, the collected plating bath liquid is rapidly cooled and solidified, and the solidified substance is polished by a predetermined thickness (for example, approximately 0.5 mm), thereby obtaining a measurement sample. The obtained measurement sample is observed in a plurality of visual fields (for example, approximately five visual fields) using an optical microscope or scanning electron microscope having a predetermined magnification, and the grain diameter and number of dross are measured in each visual field according to a well-known image processing method.
[0054]
Hitherto, the method for manufacturing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment have been described in detail. According to the present embodiment, when the hot-dip galvanizing facility 10 is at a standstill, conditions are set such that the bath temperature T and the free Al concentration CAI of the plating bath 103 are in
- 23 -

the top dross region, and dross is collected, whereby coarse dross can be removed. In addition, when the hot-dip galvanizing facility 10 is in operation, fine bottom dross is generated, but the hot-dip galvanizing facility 10 is operated in a region where the grains of the bottom dross are unlikely to grow (the nucleation region of the 81 phase), and thus there are no cases where the bottom dross adversely affects the quality of the hot-dip galvanized steel sheet. Therefore, in the bottom dross region, the poor quality of the hot-dip galvanized steel sheet is suppressed, and it is possible to manufacture a hot-dip galvanized steel sheet without degrading the productivity. In addition, even in the case of carrying out bottom dross operation that is advantageous for alloying compared with top dross operation, it is possible to improve the quality of a hot-dip galvanized steel sheet to be finally obtained. [Examples]
[0055]
Subsequently, the method for operating a hot-dip galvanizing bath and the method for manufacturing a hot-dip galvanized steel sheet according to the present invention will be specifically described with reference to invention examples and comparative examples. Examples to be described below are merely examples of the method for operating a hot-dip galvanizing bath and the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, and the method for operating a hot-dip galvanizing bath and the method for manufacturing a hot-dip galvanized steel sheet according to the present invention are not limited to the following examples.
[0056]
<1. Preliminary test>
The free Al concentration CM of a plating bath of a continuous hot-dip
- 24 -

galvanizing facility for experiment was set to 0.1400%, the bath temperature of a plating bath with the facility at a standstill was set to 455°C, top dross that had floated was completely removed, then, the bath temperature of the plating bath was set to 455°C and 485°C, and operation was carried out for 10 days.
[0057]
FIG. 3 shows the forms of bottom dross generated on the bath bottom of the plating bath after 10 days from the start of the operation. As shown in FIG. 3, in a case where the bath temperature of the plating bath was 455°C, bottom dross of a coarse T2 phase was generated. This ascertained that, if the bath temperature of the plating bath is 455°C, even when operation is carried out under conditions in the top dross region, bottom dross of the T2 phase is generated on the bath bottom, and the bottom dross coarsen within a relatively short period of time.
[0058]
On the other hand, in a case where the bath temperature of the plating bath was 485°C, as shown in FIG. 3, dross of a fine 51 phase was generated. This ascertained that, even in a case where the bath temperature of the plating bath is 485°C, bottom dross is generated on the bath bottom, but the phase of the bottom dross becomes the 51 phase, and, in the 51 phase, the growth rate of the grain diameter of the bottom dross is slow.
[0059]
The above-described results show a favorable correlation with the phases of dross that is assumed from the Fe-Al liquid phase interface diagram of the hot-dip galvanizing bath shown in FIG. 2. This ascertained that the grain diameter of 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 at a standstill.
- 25 -

[0060]
<2. Test with actual device>
While the free Al concentration CAI of the plating bath of a hot-dip galvanizing facility that is an actual device was fluctuated within a range of 0.1300 to 0.1425 mass%, and the bath temperatures T of the plating bath with the hot-dip galvanizing facility in operation and at a standstill were adjusted within a range of 440°C to 489°C, steel strips were passed through the hot-dip galvanizing facility, thereby manufacturing galvannealed steel sheets. In a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were set to conditions in the top dross region when the hot-dip galvanizing facility was at a standstill, top dross was removed while the hot-dip galvanizing facility was at a standstill. The surfaces of the manufactured galvannealed steel sheets were visually observed to investigate the presence or absence of a dross defect.
[0061]
Table 1 shows the operating conditions of the plating bath at the time of manufacturing the galvannealed steel sheets and the evaluation results of the steel sheet surfaces. Regarding the evaluation results of the steel sheet surfaces, steel sheet surfaces from which no dross defect was observed were evaluated as "A", steel sheet surfaces from which a small number of dross defects were observed were evaluated as "B", and steel sheet surfaces from which a large number of dross defects were observed were evaluated as "C".
[0062]
As is clear from Table 1, in a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the nucleation region of the 81 phase ("61 nucleation" in Table 1) when the hot-dip
- 26 -

galvanizing facility was in operation, there were no or few dross defects (the evaluations were A or B). On the other hand, in a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the grain growth region of the T2 phase ('T2 grain growth" in Table 1) or in the grain growth region of the 81 phase ("81 grain growth" in Table 1) when the hot-dip galvanizing facility was in operation, a dross defect was generated (the evaluations were C or B).
[0063]
Particularly, when attention is paid to a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the nucleation regions of the 81 phase when the hot-dip galvanizing facility was in operation, in a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the top dross region ("top dross" in Table 1) when the hot-dip galvanizing facility was at a standstill, no dross defect was generated (the evaluations were A). In addition, in a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the nucleation regions of the 81 phase when the hot-dip galvanizing facility was in operation, and the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the grain growth region of the 81 phase or in the grain growth region of the T2 phase when the hot-dip galvanizing facility was at a standstill, a dross defect was generated (The evaluations were B or C). In a case where the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath were conditions that were in the top dross region when the hot-dip galvanizing facility was in operation and at a standstill, no dross defect was generated (the evaluations were A), but poor alloying occurred.
- 27 -

[0064]
[Table 1]

Test
No. Continuous hot-dip galvanizing facility Evaluation result of
steel sheet surface

At standstill In operation

Al concentration Bath
temperature
T(°C) Left-hand
side of
Formula
(1) Region Al concentration
CM0c) Bath
temperature
T(°C) Right-hand
side of Formula (2) Region

1 0.1385 459 0.1390 Top dross 0.1395 485 0.1441 51 nucleation A
2 0.1400 442 0.1395 Top dross 0.1400 482 0.1433 51 nucleation A
3 0.1400 445 0.1394 Top dross 0.1400 485 0.1441 51 nucleation A
4 0.1400 450 0.1393 Top dross 0.1400 485 0.1441 51 nucleation A
5 0.1400 455 0.1391 Top dross 0.1400 489 0.1452 51 nucleation A
6 0.1410 450 0.1393 Top dross 0.1400 485 0.1441 51 nucleation A
7 0.1410 450 0.1393 Top dross 0.1420 485 0.1441 51 nucleation A
8 0.1410 442 0.1395 Top dross 0.1410 482 0.1433 51 nucleation A
9 0.1410 445 0.1394 Top dross 0.1410 485 0.1441 51 nucleation A
10 0.1410 450 0.1393 Top dross 0.1410 485 0.1441 51 nucleation A
11 0.1410 455 0.1391 Top dross 0.1410 489 0.1452 51 nucleation A
12 0.1420 455 0.1391 Top dross 0.1410 485 0.1441 51 nucleation A
13 0.1420 455 0.1391 Top dross 0.1400 485 0.1441 51 nucleation A
14 0.1420 440 0.1396 Top dross 0.1420 485 0.1441 51 nucleation A
15 0.1420 450 0.1393 Top dross 0.1420 485 0.1441 51 nucleation A
16 0.1420 455 0.1391 Top dross 0.1420 485 0.1441 51 nucleation A
17 0.1425 442 0.1395 Top dross 0.1420 485 0.1441 51 nucleation A
18 0.1425 445 0.1394 Top dross 0.1410 485 0.1441 51 nucleation A
19 0.1425 450 0.1393 Top dross 0.1400 482 0.1433 51 nucleation A
20 0.1425 455 0.1391 Top dross 0.1400 485 0.1441 51 nucleation A
21 0.1425 457 0.1391 Top dross 0.1400 488 0.1449 51 nucleation A
22 0.1410 450 0.1393 Top dross 0.1410 450 0.1347 Top dross A * Poor alloying
23 0.1410 450 0.1393 Top dross 0.1410 465 0.1387 Top dross A * Poor alloying
24 0.1350 480 0.1384 51 nucleation 0.1350 480 0.1428 51 nucleation B
25 0.1300 485 0.1383 51 nucleation 0.1420 485 0.1441 51 nucleation B
26 0.1410 450 0.1393 Top dross 0.1330 460 0.1374 51 grain growth B
27 0.1410 450 0.1393 Top dross 0.1350 465 0.1387 51 grain growth B
28 0.1420 455 0.1391 Top dross 0.1360 455 0.1360 T2 grain growth C
29 0.1350 455 0.1391 T2 grain growth 0.1330 460 0.1374 51 grain growth B
30 0.1350 455 0.1391 T2 grain growth 0.1350 465 0.1387 51 grain growth B
31 0.1330 460 0.1390 51 grain growth 0.1330 470 0.1401 51 nucleation B
32 0.1350 445 0.1394 T2 grain growth 0.1410 445 0.1334 Top dross B
33 0.1350 455 0.1391 T2 grain growth 0.1350 455 0.1360 T2 grain growth C
34 0.1350 465 0.1388 51 grain growth 0.1350 455 0.1360 T2 grain growth C
35 0.1350 480 0.1384 51 nucleation 0.1350 455 0.1360 T2 grain growth c
36 0.1350 465 0.1388 51 grain growth 0.1395 485 0.1441 51 nucleation B

[0065]
In order to investigate the cause for generating a dross defect when the hot-dip galvanizing facility was in operation, the free Al concentration CAI of the plating bath was fixed at 0.1410%, the bath temperature of the plating bath was controlled to be 455°C all the time (Comparative Example 1), 485°C all the time (Comparative Example 2), or 455°C when the hot-dip galvanizing facility was at a standstill and 485 °C when the hot-dip galvanizing facility was in operation (Invention Examples), and the hot-dip galvanizing facility was operated. After the operation of the hot-dip galvanizing facility, the plating bath liquid was scooped out from a position 300 mm deep from the plating bath surface. The plating bath liquid was put into a copper mold, rapidly cooled, and solidified to obtain a sample. Next, the outermost surface of the sample was mirror-polished, and then the grain diameters and the number of dross grains that were included in a 20 mm x 20 mm range were investigated using a laser microscope. Since the sampled plating bath liquid was from the position 300 mm deep from the plating bath surface, the number of top dross grains and the number of coarse bottom dross grains settled to the bottom of the plating bath are not reflected in the investigation results.
FIG. 4 shows the relationship between the grain diameters and the number of dross grains under each manufacturing condition.
[0066]
In a case where the plating bath temperature was 455°C (the top dross region, Comparative Example 1) all the time during the operation, top dross was generated on the plating bath surface, but only an extremely small amount of dross was generated at the position 300 mm deep from the plating bath surface. However, in this case, there was a problem in that it became difficult to alloy the hot-dip galvanized layer as has
- 29 -

been conventionally considered as a problem.
In addition, when the plating bath temperature was set to 485°C (the nucleation region of the 81 phase, Comparative Example 2) all the time, the proportion of fine dross increased. Dross having a grain diameter exceeding 100 um was also observed, which is considered to be a cause for a dross defect.
[0067]
On the other hand, when the plating bath temperature was set to 455°C (the top dross region) when the hot-dip galvanizing facility was at a standstill and to 485°C (the nucleation region of the 81 phase) when the hot-dip galvanizing facility was in operation (invention examples), the number of dross grains having a grain diameter of 100 um or more significantly decreased.
From the above-described facts, it was found that, in a case where, when the hot-dip galvanizing facility is at a standstill, the plating bath temperature is set to the top dross region, and top dross is removed, and, when the hot-dip galvanizing facility is in operation, the plating bath temperature is set to the nucleation region of the 81 phase, and operation is carried out, it is possible to suppress the generation of not only dross having a large dross diameter, which can be a dross defect, but also relatively small dross (dross diameter: 100 to 150 um), and thus it is possible to reliably suppress the generation of a fine dross defect.
[0068]
As a result of continuing the operation of the hot-dip galvanizing bath with the bath temperature T and the free Al concentration CAI of the plating bath set to the top dross region when the hot-dip galvanizing facility was at a standstill and set to the nucleation region of the 81 phase when the hot-dip galvanizing facility was in operation based on the above-described finding, it became possible to manufacture a
- 30 -

high-quality steel sheet in which a dross defect is not a problem while avoiding top dross operation, in which alloying is difficult to carry out and thus the productivity drops.
[0069]
Hitherto, the preferred embodiment of the present invention has been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is evident that a person skilled in the art of the present invention is able to consider a variety of modification examples or correction examples within the scope of the technical concept described in the claims, and it is needless to say that such examples are understood to be in the technical scope of the present invention. [Brief Description of the Reference Symbols]
[0070]
10 Hot-dip galvanizing facility
101 Hot-dip galvanizing bath tank
103 Hot-dip galvanizing bath
105 Snout
107 Sink roll
109 Gas wiping device
111 Alloying furnace

WE CLAIMS

A method for manufacturing a hot-dip galvanized steel sheet, the method
comprising continuously immersing a steel sheet in a hot-dip galvanizing bath to form
a hot-dip galvanized layer, thereby manufacturing the hot-dip galvanized steel sheet,
wherein, when a hot-dip galvanizing facility is at a standstill, a bath temperature T and a free Al concentration CAI of the hot-dip galvanizing bath are set so that top dross is generated, and the top dross in the hot-dip galvanizing bath is removed, and,
when the hot-dip galvanizing facility is in operation, the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath are set so that a 81 phase is nucleated.
2. The method for manufacturing a hot-dip galvanized steel sheet according
to claim 1,
wherein, when the hot-dip galvanizing facility is at a standstill, the bath temperature T of the hot-dip galvanizing bath is set to a temperature range of 440°C to 460°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath is set to satisfy Formula (1), and,
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°C to 490°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath is set to satisfy Formula (2),
-2.914 x 10~5 x T + 1.524x 10'1 < CAI< 0.1427 (1)
0.1390 < CAI< 2.686 x 10"4 x T + 1.383 x 10"2 (2).
- 32 -

3. The method for manufacturing a hot-dip galvanized steel sheet according
to claim 1 or 2, further comprising:
alloying the hot-dip galvanized layer to form a hot-dip galvannealed layer.
4. A method for operating a hot-dip galvanizing bath, the method
comprising continuously immersing a steel sheet in the hot-dip galvanizing bath to
form a hot-dip galvanized layer,
wherein, when a hot-dip galvanizing facility is at a standstill, a bath temperature T and a free Al concentration CAI of the hot-dip galvanizing bath are set so that top dross is generated, and the top dross in the hot-dip galvanizing bath is removed, and,
when the hot-dip galvanizing facility is in operation, the bath temperature T and the free Al concentration CAI of the hot-dip galvanizing bath are set so that a 81 phase is nucleated.
5. The method for operating a hot-dip galvanizing bath according to claim 4,
wherein, when the hot-dip galvanizing facility is at a standstill, the bath
temperature T of the hot-dip galvanizing bath is set to a temperature range of 440°C to 460°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath is set to satisfy Formula (1), and,
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°C to 490°C, and the free Al concentration CAI by mass% of the hot-dip galvanizing bath is set to satisfy Formula (2),
- 33 -

-2.914 x 10"5 x T + 1.524 x 101 < CAI < 0.1427 (1)
0.1390 < CAI < 2.686 x 10"4 x T + 1.383 x 10"2 (2).
6. The method for operating a hot-dip galvanizing bath according to claim 4 or 5, further comprising:
alloying the hot-dip galvanized layer to form a hot-dip galvannealed layer.