阅读说明：本技术 热浸镀锌钢板的制造方法和热浸镀锌浴的操作方法 (Method for producing hot-dip galvanized steel sheet and method for operating hot-dip galvanizing bath ) 是由 古川直人 小西刚嗣 于 2020-04-16 设计创作，主要内容包括：该热浸镀锌钢板的制造方法通过将钢板连续浸渍在热浸镀锌浴中形成热浸镀锌层来制造热浸镀锌钢板。前述热浸镀锌钢板的制造方法中,在热浸镀锌设备停止时,以顶渣能够产生的方式设定前述热浸镀锌浴的浴温T和游离Al浓度C-(Al),并且除去前述热浸镀锌浴的前述顶渣,在前述热浸镀锌设备运转时,以δ1相能够成核的方式设定前述热浸镀锌浴的前述浴温T和前述游离Al浓度C-(Al)。(The method for manufacturing a hot-dip galvanized steel sheet includes continuously immersing a steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer. In the method for producing a hot-dip galvanized steel sheet, the bath temperature T and the free Al concentration C of the hot-dip galvanizing bath are set so that top dross can be generated when the hot-dip galvanizing facility is stopped Al And removing the top dross of the hot dip galvanizing bath, and setting the bath temperature T and the free Al concentration C of the hot dip galvanizing bath so that a delta 1 phase can be nucleated when the hot dip galvanizing facility is operating Al 。)

1. A method for producing a hot-dip galvanized steel sheet, comprising continuously immersing a steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer, wherein the hot-dip galvanized steel sheet is produced,

setting a bath temperature T and a free Al concentration C of the hot dip galvanizing bath so that top dross can be generated when the hot dip galvanizing facility is stopped_AlAnd removing the top dross of the hot dip galvanizing bath,

setting the bath temperature T and the free Al concentration C of the hot dip galvanizing bath so that a delta 1 phase can nucleate while the hot dip galvanizing facility is operating_Al。

2. The method of manufacturing a hot-dip galvanized steel sheet according to claim 1, characterized in that,

setting the bath temperature T of the hot dip galvanizing bath in a temperature range of 440-460 ℃ and setting the free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is stopped_AlSatisfies the formula (1) in mass%,

setting the bath temperature T of the hot dip galvanizing bath to a temperature range of 480 to 490 ℃ and setting the free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is in operation_AlSatisfies the formula (2) in mass%,

－2.914×10^-5×T+1.524×10^-1＜C_Al＜0.1427 (1)

0.1390＜C_Al＜2.686×10^-4×T+1.383×10^-2 (2)。

3. the method of manufacturing a hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that,

alloying the hot dip galvanized layer to form an alloyed hot dip galvanized layer.

4. A method of operating a hot dip galvanizing bath for forming a hot dip galvanized layer by continuously immersing a steel sheet in the hot dip galvanizing bath,

setting the hot dip galvanizing bath so that top dross can be generated when the hot dip galvanizing facility is stoppedBath temperature T and free Al concentration C_AlAnd removing the top dross of the hot dip galvanizing bath,

setting the bath temperature T and the free Al concentration C of the hot dip galvanizing bath so that a delta 1 phase can nucleate while the hot dip galvanizing facility is operating_Al。

5. The method of operating a hot dip galvanizing bath according to claim 4,

setting the bath temperature T of the hot dip galvanizing bath in a temperature range of 440-460 ℃ and setting the free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is stopped_AlSatisfies the formula (1) in mass%,

setting the bath temperature T of the hot dip galvanizing bath to a temperature range of 480 to 490 ℃ and setting the free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is in operation_AlSatisfies the formula (2) in mass%,

－2.914×10^-5×T+1.524×10^-1＜C_Al＜0.1427 (1)

0.1390＜C_Al＜2.686×10^-4×T+1.383×10^-2 (2)。

6. the method of operating a hot dip galvanizing bath according to claim 4 or 5,

alloying the hot dip galvanized layer to form an alloyed hot dip galvanized layer.

Technical Field

The present invention relates to a method for producing a hot-dip galvanized steel sheet and a method for operating a hot-dip galvanizing bath.

The present application claims priority based on Japanese application laid out in Japanese patent application No. 2019-080277 at 19.4.2019, the contents of which are incorporated herein by reference.

Background

Conventionally, as a method for forming a hot-dip galvanized layer on a steel sheet, a method of continuously dipping the steel sheet in a hot-dip galvanizing bath has been employed. In this method, a steel sheet is annealed, and then the annealed steel sheet is immersed in a hot dip galvanizing bath by passing through the inside of a pipe orifice (snout) having an upper end connected to an annealing furnace and a lower end immersed in the hot dip galvanizing bath. The method of advancing the steel sheet is changed from obliquely downward to upward by the immersion roller in the hot dip galvanizing bath, and the steel sheet is lifted up. Thereafter, the amount of the hot dip galvanized steel sheet adhering to the surface of the steel sheet was controlled by the gas extrusion method.

The steel sheet taken out of the hot dip galvanizing bath is alloyed in an alloying furnace in the subsequent stage to form an alloyed hot dip galvanized steel sheet. (hereinafter, a steel sheet subjected to alloying treatment (alloyed hot-dip galvanized steel sheet) and a steel sheet not subjected to alloying treatment are collectively referred to as "hot-dip galvanized steel sheet", and particularly, when a steel sheet not subjected to alloying treatment is referred to as "non-alloyed hot-dip galvanized steel sheet")

The inside of the pipe port is isolated from the atmosphere and is kept in a non-oxidizing atmosphere such as nitrogen gas, thereby preventing oxidation contamination of the surface of the steel sheet to be plated. Here, when the metal eluted from the steel sheet into the bath (for example, Fe eluted from the steel sheet) reacts with Al or Zn present in the bath, slag deposited on the bottom of the plating bath is generated. The slag thus produced is called bottom slag. The bottom dross floats in the bath due to the accompanying flow generated by the steel sheet traveling in the bath, and adheres to the surface of the steel sheet immersed in the bath, thereby causing quality defects (particularly appearance defects on the surface of the hot-dip galvanized steel sheet).

In order to suppress the appearance defects of the surface of the hot-dip galvanized steel sheet, various techniques have been proposed. For example, patent document 1 provides a technique of keeping a hot-dip galvanizing bath temperature T within a range of 435 to 500 ℃ and an Al concentration in the bath within a range of ± 0.01 wt% when the hot-dip galvanizing bath temperature is T (° c) and a critical Al concentration expressed by a formula of-0.0015 × T +0.76 Cz (wt%) is used to manufacture an alloyed hot-dip galvanized steel sheet.

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

It is known that Fe is present in slag which may be generated in the production of hot-dip galvanized steel sheet₂Al₅4 types of phases (so-called top dross), δ 1 phase, Γ 2 phase, and ζ phase. The technique proposed in patent document 1 proposes to operate under a boundary condition between conditions for producing the ζ phase and conditions for producing the δ 1 phase. The technique proposed in patent document 2 proposes Fe₂Al₅A scheme operating under boundary conditions of the phase generation conditions and the δ 1 phase generation conditions.

Documents of the prior art

Patent document 1: japanese laid-open patent publication No. 11-350096

Patent document 2: japanese laid-open patent publication No. 11-350097

Disclosure of Invention

Conventionally, slag floating on the surface of a hot-dip galvanizing bath (so-called "Fe — Al-based top slag") is formed by setting the Al concentration of the hot-dip galvanizing bath to a high concentration, and the Fe — Al-based top slag is appropriately removed (hereinafter, also referred to as "top slag operation"). The bottom slag operation is an idea of an operation opposite to the top slag operation.

When the Al concentration of the hot dip galvanizing bath is low, slag (so-called "Fe — Zn-based bottom slag") that has settled in the hot dip galvanizing bath is formed. Fe-Zn-based bottom dross is difficult to remove during the operation of a hot-dip galvanizing facility, and therefore, is deposited on the bottom of the bath. The bottom dross deposited on the bottom of the bath is soon rolled up in the bath by the accompanying flow of the steel sheet, adheres to the steel sheet and the rolls in the bath, and causes defects (hereinafter, sometimes referred to as "dross defects") to occur on the surface of the steel sheet.

If the bottom dross adheres to the steel sheet, uneven portions are formed on the plating surface, resulting in poor appearance quality. In addition, as a result of the formation of the uneven portion, local batteries are easily formed, surface defects which become a factor of reducing corrosion resistance are generated, and quality defects of the plated steel sheet are generated. Therefore, in order to maintain the quality of the hot-dip galvanized steel sheet in the bottom dross removal operation, it is necessary to periodically stop the production line and perform bath cleaning in order to remove the bottom dross deposited on the bottom of the bath. Compared with the top slag operation which can remove slag in the process, the bottom slag operation which requires stopping the production line to remove slag takes time and causes the problem of reducing the production capacity due to stopping the production line. Thus, in general, it is desirable to avoid bottom ash operations.

However, there are cases where the steel sheet is immersed in a hot-dip galvanizing bath and then subjected to alloying treatment of the plating layer. The higher the Al content in the hot-dip galvanized layer, the more difficult it is to alloy. Therefore, particularly when the alloying treatment is performed, in order to produce a high-quality alloyed hot-dip galvanized steel sheet with high productivity, a bottom dross operation in which the Al concentration of the hot-dip galvanizing bath is low is advantageous.

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

In order to solve the above problems, the present inventors investigated the particle size of bottom slag, which is a cause of slag defects when performing bottom slag operation. As a result, the inventors have found that if bottom slag having a particle size of 100 to 300 μm is present in the bath, slag defects increase. Further, the conditions of the hot dip galvanizing bath for suppressing the generation of the bottom dross having the particle size of 100 to 300 μm are studied in detail, and the present invention described in detail below is conceived.

The gist of the present invention completed based on this finding is as follows.

[1] A method for producing a hot-dip galvanized steel sheet according to an aspect of the present invention is a method for producing a hot-dip galvanized steel sheet by continuously immersing a steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer,

setting a bath temperature T and a free Al concentration C of the hot dip galvanizing bath so that top dross can be generated when the hot dip galvanizing facility is stopped_AlAnd removing the top dross of the hot dip galvanizing bath,

setting the bath temperature T and the free Al concentration C of the hot dip galvanizing bath so that a delta 1 phase can nucleate while the hot dip galvanizing facility is operating_Al。

[2]Above-mentioned [1]In the method for producing a hot-dip galvanized steel sheet, the bath temperature T of the hot-dip galvanizing bath may be set to a temperature range of 440 to 460 ℃ and the free Al concentration C of the hot-dip galvanizing bath may be set so that the free Al concentration C of the hot-dip galvanizing bath is lower than a predetermined value when the hot-dip galvanizing facility is stopped_AlSatisfies the formula (1) in mass%,

setting the bath temperature T of the hot dip galvanizing bath to a temperature range of 480 to 490 ℃ and setting the free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is in operation_AlSatisfies the formula (2) in mass%,

－2.914×10^-5×T+1.524×10^-1＜C_Al＜0.1427 (1)

0.1390＜C_Al＜2.686×10^-4×T+1.383×10^-2 (2)。

[3] the method of producing a hot-dip galvanized steel sheet according to [1] or [2], wherein the hot-dip galvanized layer is alloyed to form an alloyed hot-dip galvanized layer.

[4] A method of operating a hot dip galvanizing bath according to another aspect of the present invention is a method of forming a hot dip galvanized layer by continuously immersing a steel sheet in the hot dip galvanizing bath,

setting a bath temperature T and a free Al concentration C of the hot dip galvanizing bath so that top dross can be generated when the hot dip galvanizing facility is stopped_AlAnd removing the top dross of the hot dip galvanizing bath,

when the hot dip galvanizing equipment is in operation, the delta 1 phase is set to be capable of nucleatingThe bath temperature T and the free Al concentration C of the hot dip galvanizing bath_Al。

[5]Above-mentioned [4]In the method for operating a hot dip galvanizing bath, the bath temperature T of the hot dip galvanizing bath may be set to a temperature range of 440 to 460 ℃ and may be set so that the free Al concentration C of the hot dip galvanizing bath is higher than the temperature of the hot dip galvanizing facility_AlSatisfies the formula (1) in mass%,

setting the bath temperature T of the hot dip galvanizing bath to a temperature range of 480 to 490 ℃ and setting the free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is in operation_AlSatisfies the formula (2) in mass%,

－2.914×10^-5×T+1.524×10^-1＜C_Al＜0.1427 (1)

0.1390＜C_Al＜2.686×10^-4×T+1.383×10^-2 (2)。

[6] the method of operating a hot dip galvanizing bath according to any one of [4] and [5], wherein the hot dip galvanized layer is alloyed to form an alloyed hot dip galvanized layer.

According to the above aspect of the present invention, it is possible to provide a method of manufacturing a hot-dip galvanized steel sheet and a method of operating a hot-dip galvanizing bath, which can suppress a quality defect of the hot-dip galvanized steel sheet and suppress a reduction in productivity even when a bottom dross operation is performed.

Drawings

Fig. 1 is a schematic diagram showing an example of a configuration of a continuous hot-dip galvanizing facility (alloying hot-dip galvanizing facility) that can be used in the present embodiment.

FIG. 2 shows the bath temperature T (. degree. C.) and the free Al concentration C_AlA quasi-steady state diagram obtained by conditioning the slag-forming phase of the hot-dip galvanizing bath.

FIG. 3 is a photomicrograph showing the form of the bottom dross formed in the plating bath after 10 days of operation.

FIG. 4 is a graph showing the relationship between the particle size and the number of slag particles under each production condition of examples.

Detailed Description

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

<1. construction of continuous Hot Dip galvanizing facility >

First, before describing the present invention in detail, an example of the configuration of a hot-dip galvanized steel sheet production method and a continuous hot-dip galvanizing facility capable of performing a hot-dip galvanizing bath operation method according to the present embodiment will be described in detail. Again, the plant is precisely an alloying hot dip galvanising plant. In the case of manufacturing a non-alloyed hot-dip galvanized steel sheet, the continuous hot-dip galvanizing facility will be described below by taking an alloyed hot-dip galvanizing facility as an example as long as the alloying furnace is not operated.

FIG. 1 is a schematic view showing an example of the structure of an apparatus for hot dip galvannealing. 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 "plating bath"), a hot-dip galvanizing tank 101 in which the plating bath 103 is stored, a nozzle 105, a guide roller 107, a gas wiping device 109, and an alloying furnace 111.

The annealing furnace 20 provided at the front stage (upstream side in the conveying direction of the steel sheet S) of the hot-dip galvanizing facility 10 is isolated from the atmosphere and the inside is maintained in a reducing atmosphere. In addition, the annealing furnace 20 heats the continuously conveyed steel sheet S. The surface of the steel sheet S is activated by the annealing furnace 20, and the mechanical properties of the steel sheet S are adjusted. The outlet side end of the annealing furnace 20 is connected to the upstream side end of the nozzle 105 via a space in which the lower rolls 30 are provided.

The upstream end of the nozzle 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 obliquely above. Like the annealing furnace 20, the inside of the nozzle 105 is isolated from the atmospheric atmosphere and maintained in a reducing atmosphere.

The steel sheet S having a downward conveying direction is conveyed inside the nozzle 105 by the lower roll 30, and is continuously immersed in the hot dip galvanizing bath 103 stored in the hot dip galvanizing bath 101. A guide roller 107 is provided inside the hot-dip galvanizing bath 101. The guide roller 107 has a rotation axis parallel to the sheet width direction of the steel sheet S, and the width of the outer peripheral surface of the guide roller 107 is equal to or greater than the sheet width of the steel sheet S. The conveying direction of the steel sheet S is changed to be upward by the guide roller 107.

The gas wiping device 109 blows gas onto both surfaces of the steel sheet S led out from the hot dip galvanizing bath 101 to scrape off a portion of the hot dip galvanizing adhering to the surface of the steel sheet S. Thereby, the hot dip galvanizing adhesion amount on the surface of the steel sheet S is adjusted.

Thereafter, the steel sheet S is further vertically lifted up, and is subjected to alloying treatment in the alloying furnace 111. The alloying furnace 111 is composed of 3 sections of a heating zone, a holding zone, and a cooling zone in this order from the inlet side of the steel sheet S. In the alloying furnace 111, first, heating is performed by a heating belt so that the plate temperature of the steel sheet S becomes substantially uniform. Next, by ensuring an alloying time in the heat-insulating strip, 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). Then, the steel sheet S (i.e., the galvannealed steel sheet) is cooled in a cooling zone and conveyed to the next step by the top roll 40. When manufacturing 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, iron eluted from the steel sheet S in the hot-dip galvanizing bath 101 forms a granular solid alloy having a high melting point called slag in the hot-dip galvanizing bath 103. When the slag adheres to the steel sheet S, slag defects are generated on the surface of the steel sheet S.

<2. investigation of the present inventors >

In the bottom dross operation, it is problematic that the bottom dross is rolled up with the accompanying flow of the steel sheet S in the plating bath 103 and adheres to the steel sheet S. While bottom dross is inevitably generated during the bottom dross manipulation, it is considered that if the particle size of the bottom dross is small, no quality defect occurs even if the bottom dross adheres to the steel sheet S.

The present inventors examined the particle size of the bottom slag, which causes the generation of slag defects. As a result, the present inventors have found that when bottom dross having a particle size of 100 to 300 μm is present in the bath, a large number of dross defects are generated. Since the bottom dross having a particle size of less than 100 μm is sufficiently small, it does not cause a dross defect even if it adheres to the steel sheet S. On the other hand, the bottom dross having a particle size of more than 300 μm is greatly influenced by gravity and is deposited on the bath bottom, so that it is difficult to adhere to the steel sheet S. Therefore, in order to suppress the generation of slag defects, it is important to suppress the amount of the bottom slag 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 grain size of the bottom ash. As a result, it was found that the growth rate of the bottom dross particle size was high when the bath temperature of the plating bath 103 was low, and the growth rate of the bottom dross particle size was low when the bath temperature of the plating bath 103 was high. This is presumably because the growth rate of the Γ 2 phase which is stable at low bath temperatures (455 to 460 ℃ C. or less, that is, 455 ℃ C. or less) is higher than the growth rate of the δ 1 phase which is stable at high bath temperatures (455 to 460 ℃ C. or more, that is, 460 ℃ C. or more).

In the operation of the hot dip galvanizing facility 10, the steel sheet S continuously passes through the hot dip galvanizing bath 101, and thus local nucleation is inevitably generated. Therefore, during the operation, the bottom dross is intentionally grown in the nucleation region of the δ 1 phase, and the iron eluted from the steel sheet S is induced to be fine. Specifically, the operation is performed in a high bath temperature region (a nucleation region of a δ 1 phase) where the growth rate of the bottom slag particle diameter is low, and the particle diameter of the fine bottom slag newly nucleated in the operation is prevented from reaching 100 μm or more. This can suppress the generation of slag defects.

However, when the bottom-dross blowing operation is continued for a long time, the bottom-dross may slowly grow to a particle size of 100 to 300 μm, although the bottom-dross velocity is low. The phenomenon of bottom dross growth is thus known in crystallography as Ostwald (Ostwald) growth. If the operation is continued for a long period of time in the plating bath 103 in which the bottom dross of various particle sizes is present, the transfer of substances from the bottom dross of a relatively small particle size to the bottom dross of a relatively large particle size is caused, and the bottom dross of a small particle size becomes further small and the bottom dross of a large particle size becomes further large.

Therefore, the operation is performed so that the bottom slag operation is started from the state where the bottom slag is removed, and there is no large difference in the particle size of the bottom slag even if the bottom slag is generated. Thus, Ostwald growth is difficult to occur. In addition, even if the bottom slag particle size is increased by Ostwald growth, if the bottom slag is removed before the bottom slag grows to a particle size of 100 μm or more, the generation of slag defects can be suppressed. Specifically, when the hot-dip galvanizing facility 10 is stopped (off-line), the bath temperature and the free Al concentration of the plating bath 103 are set so that top dross can be generated, and the dross in the plating bath 103 is floated on the plating bath surface and removed as top dross.

By changing the conditions of the plating bath 103 during operation and during stoppage in this way, even if nuclei of fine bottom dross are newly formed during operation, the bottom dross in the plating bath 103 can be removed as top dross before the bottom dross greatly grows, and the generation of dross can be suppressed.

<3. method for producing Hot-Dip galvanized Steel sheet and method for operating Hot-Dip galvanizing bath >

The method for producing a hot-dip galvanized steel sheet and the 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 producing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment are described by way of example using the hot-dip galvanizing facility 10 shown in fig. 1, but the present invention is not limited thereto.

The method for producing a hot-dip galvanized steel sheet according to the present embodiment is a method for producing a hot-dip galvanized steel sheet by continuously immersing steel sheet S in hot-dip galvanizing bath 103 to form a hot-dip galvanized layer. In the present embodiment, after the hot-dip galvanized layer is formed, the hot-dip galvanized layer may be alloyed by heating the steel sheet S to form an alloyed hot-dip galvanized layer. In the method for producing a hot-dip galvanized steel sheet according to the present embodiment, as described later, since the plating bath 103 is operated under the bottom dross condition, the Al content in the hot-dip galvanized layer is suppressed, and alloying is facilitated. As a result, a high-quality alloyed hot-dip galvanized steel sheet can be produced.

The method of operating the hot dip galvanizing bath according to the present embodiment is a method suitably used for the method of producing the hot dip galvanized steel sheet. As described above, the method of operating the hot dip galvanizing bath according to the present embodiment is particularly suitable for use in the case where the hot dip galvanized layer is alloyed to produce 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 a known steel sheet may be suitably used according to various characteristics required for the produced hot-dip galvanized steel sheet (for example, tensile strength and various strengths required for the steel sheet), or a steel sheet used for an automobile outer panel may be used.

In the method for producing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment, the bath temperature T and the free Al concentration C of the plating bath 103 are set at the time of stopping the hot-dip galvanizing facility 10_AlThe conditions for forming the top dross region are set, the top dross is removed, and the bath temperature T and the free Al concentration C of the plating bath 103 are set at the time of operation (on-line time) of the hot-dip galvanizing facility 10_AlThe conditions were set to be the nucleation sites for the δ 1 phase. That is, when the hot dip galvanizing facility is stopped, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath are set so that top dross can be generated_AlAnd removing the top slag of the hot dip galvanizing bath, and setting the bath temperature T and the free Al concentration C of the hot dip galvanizing bath so that the delta 1 phase can form nuclei during the operation of the hot dip galvanizing facility_Al。

This can reduce the amount of bottom dross with 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 bath temperature T and the free Al concentration C of the plating bath 103 are adjusted_AlThe conditions for forming the top dross region were set so that the top dross floating on the surface of the plating bath 103 was collected and coarse dross, which may cause a dross defect, was removed. On the other hand, when the hot-dip galvanizing facility 10 is operating, the bath temperature T and the free Al concentration C of the plating bath 103 are adjusted_AlThe conditions for forming the nucleation site of the δ 1 phase are set, and the steel sheet S is intentionally subjected to an operation in the nucleation site of the δ 1 phase to induce Fe eluted from the steel sheet S to form fine bottom dross.

In general, if 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 ostwald growth rate in the operation of the nucleation region of the δ 1 phase is slow, and the ostwald growth is difficult to occur. Therefore, if the hot-dip galvanizing facility 10 is not operated for a certain length of time, the grain size of the bottom dross does not grow to 100 μm or more. Before the grain size of the bottom slag grows to 100 μm or more, the hot dip galvanizing facility 10 is stopped and the plating is performedBath temperature T and free Al concentration C of bath 103_AlIf the slag is removed as the top slag under the condition of the top slag region, the occurrence of slag defects can be suppressed.

Specifically, the conditions of the plating bath 103 can be controlled, for example, according to the composition and temperature of the plating bath 103. Hereinafter, a preferable composition and temperature of the plating bath 103 will be described with reference to fig. 2. FIG. 2 is a graph showing the temperature T (. degree. C.) of the bath and the concentration C of free Al in the bath_AlA quasi-steady state diagram obtained by conditioning the slag-forming phase of the hot-dip galvanizing bath. In FIG. 2, "C_Al"represents the free Al concentration (mass%) in the plating bath 103. The "free Al concentration in the bath" refers to the Al concentration contained in the liquid phase of the plating bath 103, and is used separately from the total Al concentration of the plating bath 103 indicating the average Al concentration of both the slag and the liquid phase.

Concentration C of free Al in plating bath 103_AlThe following method was used for the measurement. The plating bath solution is drawn from the hot-dip galvanizing bath 101, poured into a mold, and solidified to produce an ingot. Using a drill bit, scraping a proper amount of cuttings from the cast ingot, and dissolving part of the cuttings with hydrochloric acid and nitric acid to prepare a solution. The Al concentration (% by mass) was calculated using the solution, an ICP emission spectrometer, and a pre-calculated calibration line. Thus, the free Al concentration C in the plating bath 103 is obtained_Al。

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 this embodiment, in FIG. 2, the free Al concentration C of the plating bath 103 during operation_AlAnd the bath temperature T is set in the region of "δ 1 nucleation" and in the region of "slag top" at the time of stopping. The "δ 1 nucleation" region of fig. 2 is the nucleation region of the δ 1 phase described above. Concentration C of free Al in plating bath 103_AlAnd the bath temperature T is included in the "δ 1 nucleation" region, nuclei of the δ 1 phase are generated in the plating bath 103. The "slag top" region in fig. 2 is the slag top region described above. Concentration C of free Al in plating bath 103_AlAnd when the bath temperature T is included in the "top dross" region, top dross is generated in the plating bath 103.

Furthermore, in the present embodiment, in fig. 2, it is preferable that the plating bath 103 is movedConcentration of free Al C_AlAnd the bath temperature T is set to a condition in a region surrounded by a chain line of a region of "δ 1 nucleation" during operation, and is set to a condition in a region surrounded by a chain line of a region of "slag top" during stop.

That is, it is preferable that the bath temperature T (C) of the hot dip galvanizing bath 103 is set to a temperature range of 440 to 460 ℃ and the free Al concentration C in the hot dip galvanizing bath 103 is set so that the temperature T (C) is within the range of 440 to 460 ℃ when the hot dip galvanizing facility 10 is stopped_Al(mass%) satisfies the formula (1), and the bath temperature T (DEG C) of the hot dip galvanizing bath 103 is set to a temperature range of 480 to 490 ℃ and the free Al concentration C in the hot dip galvanizing bath 103 is set so that the bath temperature T (DEG C) satisfies the formula (1) when the hot dip galvanizing facility 10 is operated_Al(mass%) satisfies the formula (2).

-2.914×10^-5×T+1.524×10^-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 facility 10 is stopped, if the concentration C of free Al in the plating bath 103_AlThe relationship with the bath temperature T is (-2.914X 10)^-5×T+1.524×10^-1) When the mass% or less is less, the slag may be deviated from the top slag region, and coarse bottom slag may remain in the bath bottom. At the time of stopping, if the concentration C of free Al in the plating bath 103_AlWhen the content is 0.1427 mass% or more, the free Al concentration C needs to be lowered when the operation is shifted from the stop state to the operation state depending on the temperature condition during the operation_Al. Concentration C of free Al in plating bath 103_AlThe adjustment (2) is performed while passing the steel sheet S, and therefore the operation may become complicated. The free Al concentration C in the plating bath 103 at the time of stop of the hot-dip galvanizing facility 10_AlThe amount of the compound (1) is preferably 0.1400 to 0.1420% by mass.

Further, if the bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is stopped is lower than 440 ℃, the reactivity becomes low depending on the composition of the plating bath 103, and the conversion from the δ 1 slag to the top slag does not occur sufficiently, so that the δ 1 slag cannot be removed. Further, if the bath temperature of the plating bath 103 at the time of stopping exceeds 460 ℃, the bath is likely to separate from the top dross region and enter the bottom dross region at the time of stopping. Thus, the slag in the plating bath 103 may not be sufficiently removed, and coarse bottom slag may remain at the bottom of the bath. The bath temperature of the plating bath 103 at the time of stoppage is preferably 440 to 460 ℃ as described above, and more preferably 450 to 460 ℃.

When the hot dip galvanizing facility 10 is in operation, if the concentration C of free Al in the plating bath 103_AlWhen the content is 0.1390 mass% or less, the free Al concentration C needs to be lowered during operation_Al. Concentration C of free Al in plating bath 103_AlSince the adjustment of (2) is performed while passing the steel sheet through S, the operation may become complicated. If the concentration of free Al in the plating bath 103 is C_AlThe relation with the bath temperature T is (2.686 multiplied by 10)^-4×T+1.383×10^-2) At least, the mass% approaches the slag top region depending on the bath temperature of the plating bath 103 during operation. This excessively exerts the effect of suppressing the alloying of Al, and it may be difficult to stably alloy the steel sheet S. The concentration C of free Al in the plating bath 103 when the hot dip galvanizing facility 10 is in operation_AlThe content preferably satisfies the above formula (2), and more preferably 0.1400 to 0.1420 mass%.

When the bath temperature of the plating bath 103 is less than 480 ℃ during the operation of the hot dip galvanizing facility 10, the top dross region approaches according to the composition of the plating bath 103. This excessively exerts the effect of suppressing the alloying of Al, and it may be difficult to stably alloy the steel sheet S. Further, if the bath temperature of the plating bath 103 during operation exceeds 490 ℃, depending on the composition of the plating bath 103, alloying excessively proceeds when the hot-dip galvanizing formed on the surface of the steel sheet S is alloyed, adhesion of the alloyed layer (alloyed hot-dip galvanized layer) decreases, and the alloyed layer is likely to peel off. The bath temperature of the plating bath 103 during operation is preferably 480 to 490 ℃ as described above.

In the conventional method, the bath temperature T and the free Al concentration C of the plating bath 103 are set at the time of operating the hot dip galvanizing facility 10_AlWhen the operation is performed under the condition set to the nucleation region of δ 1, the operation is performed without lowering the bath temperature of the plating bath 103 as much as possible even at the time of stop. When the bath temperature of the plating bath 103 is lowered during stoppage, the bottom slag floats and causes slag defects. However, as described above, in the present embodiment, it is preferable that the bath temperature of the plating bath 103 is 480 to 490 ℃ during operation, and that the bath temperature is reduced to 440 to E.C.during stoppage as compared with during operation460℃。

In the present embodiment, the difference between the bath temperature of the plating bath 103 when the hot-dip galvanizing facility 10 is operating and the bath temperature of the plating bath 103 when it is stopped is preferably 25 ℃. By setting the temperature difference between the bath at the time of operation and the bath at the time of stop to 25 ℃ or more, the quality defect and the productivity decrease of the hot-dip galvanized steel sheet can be more stably suppressed.

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 may be contained at a concentration of, for example, about 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 are components mixed by raw materials and other factors, and are allowed within a range that does not adversely affect the method for producing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment.

The method of removing the top dross when the hot-dip galvanizing facility 10 is stopped is not particularly limited, and a known method can be employed. Specifically, for example, a method of removing the top slag by manually or mechanically scooping the top slag using a mesh-spoon-shaped jig is given.

The particle size distribution of the slag can be measured as follows.

300g of the plating bath solution was extracted from the hot dip galvanizing bath 103, and the extracted plating bath solution was rapidly cooled and solidified, and then ground to a predetermined thickness (for example, about 0.5 mm) to be used as a measurement sample. The obtained measurement sample is observed in a plurality of visual fields (for example, about 5 visual fields) using an optical microscope or a scanning electron microscope of a predetermined magnification, and the particle size and the number of the slag are measured for each visual field according to a known image processing method.

The method for producing a hot-dip galvanized steel sheet and the method for operating a hot-dip galvanizing bath according to the present embodiment are described above in detail. According to the present embodiment, the bath temperature T and the free Al concentration C of the plating bath 103 are adjusted when the hot-dip galvanizing facility 10 is stopped_AlCoarse slag can be removed by setting the conditions in the top slag region and recovering the slag. During the operation of the hot dip galvanizing facility 10, although fine bottom dross is generated, the facility operates in a region where the bottom dross is hard to grow into crystal grains (a nucleation region of a δ 1 phase), and the bottom dross is prevented from growing into crystal grainsThe bottom slag does not affect the quality of the hot-dip galvanized steel sheet. Therefore, in the bottom dross region, the quality defect of the hot-dip galvanized steel sheet can be suppressed, and the hot-dip galvanized steel sheet can be produced without reducing productivity. Further, the quality of the finally obtained hot-dip galvanized steel sheet can be improved even in the case of performing the bottom slag operation which is advantageous for alloying as compared with the top slag operation.

Examples

Next, the method for operating the hot-dip galvanizing bath and the method for producing a hot-dip galvanized steel sheet according to the present invention will be specifically described while showing examples of the present invention and comparative examples. The following examples are merely examples of the method for operating a hot dip galvanizing bath and the method for producing 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 producing a hot dip galvanized steel sheet according to the present invention are not limited to the following examples.

<1. preliminary test >

Free Al concentration C of plating bath of continuous hot-dip galvanizing equipment for experiment_AlThe bath temperature of the plating bath at the time of stoppage was 0.1400%, 455 ℃ and the floating top dross was completely removed, and the bath temperature of the plating bath was set at 455 ℃ and 485 ℃ respectively, and the operation was carried out for 10 days.

FIG. 3 shows the form of bottom dross formed in the bottom of the plating bath 10 days after the start of the operation. As shown in FIG. 3, when the bath temperature of the plating bath was 455 ℃, coarse bottom dross of the Γ 2 phase was generated. From this, it was found that when the bath temperature of the plating bath was 455 ℃, bottom dross of the Γ 2 phase was generated in the bath bottom and coarsened in a short time even when the operation was performed under the condition of becoming the top dross region.

On the other hand, when the bath temperature of the plating bath was 485 ℃, as shown in fig. 3, fine δ 1-phase slag was generated. From this, it was found that, even when the bath temperature of the plating bath was 485 ℃, bottom dross was generated at the bottom of the bath, the phase of the bottom dross was δ 1 phase, and the grain size growth rate of the bottom dross was slow in the δ 1 phase.

The above results showed good correlation with the phases of the slag estimated from the Fe-Al liquid phase interface state diagram of the hot dip galvanizing bath shown in FIG. 2. 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 during the operation and stoppage of the hot-dip galvanizing facility.

<2. practical machine test >

The free Al concentration C of the plating bath of the hot dip galvanizing facility of the real machine_AlThe steel strip is passed through a hot-dip galvanizing facility while the bath temperature T at the time of stop and operation is adjusted within a range of 440 to 489 ℃ while varying within a range of 0.1300 to 0.1425 mass%, thereby producing an alloyed hot-dip galvanized steel sheet. When the hot dip galvanizing equipment stops, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath are adjusted_AlWhen the conditions in the top slag region are set, the top slag is removed at the time of stopping. The surface of the produced galvannealed steel sheet was visually observed to examine the presence or absence of slag defects.

Table 1 shows the operating conditions of the plating bath and the evaluation results of the steel sheet surface in the production of the galvannealed steel sheet. As a result of the evaluation of the steel sheet surface, the evaluation was "a" in which no slag defects were found, the evaluation was "B" in which only a few slag defects were observed, and the evaluation was "C" in which a large number of slag defects were found.

As is clear from Table 1, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath during the operation of the hot dip galvanizing facility_AlIn the case of the δ 1 phase nucleation region ("δ 1 nucleation" in table 1), no slag defects or few slag defects were observed (evaluation a or evaluation B). On the other hand, during the operation of the hot dip galvanizing facility, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath are set_AlIn the case of the condition of the Γ 2 phase grain growth region (Γ 2 grain growth in table 1) or the δ 1 phase grain growth region (δ 1 grain growth in table 1), slag defects were generated (evaluation C or evaluation B).

Particularly, when a hot dip galvanizing facility is operating, attention is paid to the bath temperature T and the free Al concentration C of a hot dip galvanizing bath_AlIs a condition of a nucleation region of the delta 1 phase, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath are set at the time of stopping the hot dip galvanizing facility_AlIn the top slag region ("top slag" in table 1), no slag defect was generated (evaluation a). In addition, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath during the operation of the hot dip galvanizing facility_AlIs nucleation of the delta 1 phaseCondition of the zone and at the bath temperature T and free Al concentration C of the hot dip galvanizing bath when the hot dip galvanizing facility is stopped_AlIn the case of the crystal grain growth region of the δ 1 phase or the crystal grain growth region of the Γ 2 phase, slag defects were generated (evaluation B or evaluation C). Further, the bath temperature T and the free Al concentration C of the hot dip galvanizing bath are set at the time of stopping and operating the hot dip galvanizing facility_AlIn the top slag region, no slag defect occurred (evaluation a), but a poor alloying occurred.

TABLE 1

In order to investigate the cause of slag defects during operation of hot dip galvanizing facilities, the free Al concentration C of the plating bath was determined_AlThe temperature of the plating bath was fixed at 0.1410%, and the hot-dip galvanizing facility was operated while controlling the bath temperature at 455 ℃ at all times (comparative example 1), 485 ℃ at all times (comparative example 2), or 455 ℃ at rest and 485 ℃ at operation (inventive example). After the operation of the hot-dip galvanizing facility, the plating bath was scooped up from a position at a depth of 300mm from the surface of the plating bath. The plating bath was put into a copper mold, and rapidly cooled and solidified to obtain a sample. Subsequently, the outermost surface of the sample was mirror-polished, and then the particle size and the number of the slag contained in the range of 20mm × 20mm were examined using a laser microscope. Since the sampled plating bath liquid level was at a position of 300mm in depth from the plating bath surface, the number of coarse bottom dross deposited on the top dross and the bottom of the plating bath was not reflected in the investigation results.

FIG. 4 shows the relationship between the particle size and the number of slag under each production condition.

When the plating bath was always operated at 455 ℃ (slag-top region, comparative example 1), slag was generated on the plating bath surface, but the generation of slag was very small at a position of 300mm in depth from the plating bath surface. However, in this case, as in the conventional case, there arises a problem that the hot dip galvanized layer is difficult to alloy.

In addition, if the plating bath temperature is kept 485 ℃ (the nucleation region of the δ 1 phase, comparative example 2), the proportion of fine slag increases. Slag having a particle size exceeding 100 μm was also found, which is considered to be a cause of slag defects.

On the other hand, when the plating bath temperature was 455 ℃ (top dross region) at the time of shutdown and 485 ℃ (nucleation region of δ 1 phase) at the time of operation (inventive example), the number of dross having a particle size of 100 μm or more was significantly reduced.

From the above, it is found that when the plating bath temperature is set to the top dross region at the time of stop, the top dross is removed, and the operation is performed in the nucleation region of the δ 1 phase at the time of operation, even if the dross having a large dross diameter, which may become a dross defect, is generated, the generation of the dross can be suppressed to be relatively small (the dross diameter is 100 to 150 μm), so that the generation of the fine dross defect can be reliably suppressed.

Based on the above findings, the bath temperature T and the free Al concentration C of the plating bath are adjusted_AlWhen the operation of the hot dip galvanizing bath is continued in the nucleation region which becomes the δ 1 phase at the time of operation while stopping, the slagging operation which is difficult to alloy and therefore the productivity is lowered is avoided, and a high quality steel sheet which does not cause the slag defect problem can be produced.

The preferred embodiments of the present invention have been described in detail with reference to the drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive various modifications and alterations within the scope of the technical idea described in the claims, and it is needless to say that these modifications and alterations also fall within the technical scope of the present invention.

Description of the reference numerals

10 hot-dip galvanizing plant

101 hot-dip galvanizing bath

103 hot dip galvanizing bath

105 nozzle

107 guide roller

109 gas wiping device

111 alloying furnace