Hot-dip galvanizing process: The manufacturing method of hot-dip galvanized steel sheets through alloying using the hot-dip galvanizing process, and the production method of hot-dip galvanized steel sheets using the hot-dip galvanizing process.

Abstract

DEPCT65 What has been provided is a hot-dip galvanizing method that can inhibit the occurrence of... The dross defect and the facilitation of alloying are advantages when producing quenched steel plates. Hot-dip galvanizing is a process of zinc coating through alloying; it is a standard operating procedure for hot-dip galvanizing. This is an example of the hot-dip galvanizing process that has been used for manufacturing. Hot-dip galvanized steel sheet or hot-dip galvanized steel sheet through The alloying process in this hot-dip galvanizing operation includes the aggregation step. Example (S1) procedure for determining the amount of dross phase sine (S2) and adjustment procedure. Working condition (S3) in the sample collection step (S1), samples were collected from the bath. Hot-dip galvanizing with Al in the determination process to obtain dross sine (S2) content. The dross phase saturation in the collected samples was determined during the conditioning procedure. Working (S3) The working conditions of the hot-dip galvanizing operation are adjusted by Based on a defined dross phase quantity. -----------------------------------------------------------

Description

A method for hot-dip galvanizing, a method for manufacturing alloyed hot-dip galvanized steel sheets using the hot-dip galvanizing method, and a method for manufacturing hot-dip galvanized steel sheets using the hot-dip galvanizing method.

[0001] The present invention relates to a hot-dip galvanizing method, a method for manufacturing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and a method for manufacturing a hot-dip galvanized steel sheet using the hot-dip galvanizing method.

[0002] Hot-dip galvanized steel sheets (hereinafter also referred to as GI) and alloyed hot-dip galvanized steel sheets (hereinafter also referred to as GA) are manufactured by the following manufacturing process. First, a steel sheet to be hot-dip galvanized (base steel sheet) is prepared. The base steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet. If the base steel sheet is a hot-rolled steel sheet, for example, an acid-pickled hot-rolled steel sheet is prepared. If necessary, a Ni pre-plating treatment may be performed on the acid-pickled hot-rolled steel sheet to prepare a hot-rolled steel sheet with a Ni layer formed on the surface. A hot-rolled steel sheet with other treatments not mentioned above may also be prepared. If the base steel sheet is a cold-rolled steel sheet, for example, an annealed cold-rolled steel sheet is prepared. If necessary, a Ni pre-plating treatment may be performed on the annealed cold-rolled steel sheet to prepare a cold-rolled steel sheet with a Ni layer formed on the surface. A cold-rolled steel sheet with other treatments not mentioned above may also be prepared. The prepared base steel sheet (the hot-rolled or cold-rolled steel sheet mentioned above) is immersed in a molten zinc plating bath to perform molten zinc plating and produce a molten zinc plated steel sheet. In the case of producing an alloyed molten zinc plated steel sheet, the molten zinc plated steel sheet is further heat-treated in an alloying furnace to produce an alloyed molten zinc plated steel sheet.

[0003] The details of the hot-dip galvanizing process during the manufacturing of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets are as follows: The hot-dip galvanizing equipment used for the hot-dip galvanizing process comprises a molten zinc pot containing a molten zinc galvanizing bath, sink rolls placed in the molten zinc galvanizing bath, and a gas wiping device.

[0004] In the hot-dip galvanizing process, the steel sheet (base steel sheet) is immersed in a hot-dip galvanizing bath. Then, a sink roll placed in the hot-dip galvanizing bath changes the direction of the steel sheet's movement upward, and the steel sheet is lifted out of the hot-dip galvanizing bath. As the steel sheet moves upward after being lifted out, wiping gas is blown onto the surface of the steel sheet from a gas wiping device to scrape off excess molten zinc and adjust the amount of plating adhering to the surface of the steel sheet. The hot-dip galvanizing process is carried out in the manner described above. Furthermore, when manufacturing alloyed hot-dip galvanized steel sheets, the steel sheet with the adjusted amount of plating adhering is charged into an alloying furnace and subjected to an alloying treatment.

[0005] In the hot-dip galvanizing process described above, Fe (Fe) leaches from the steel sheet immersed in the molten zinc plating bath. When the Fe leached from the steel sheet into the molten zinc plating bath reacts with Al (Al) and Zn (Zn) present in the bath, intermetallic compounds called dross are formed. There are two types of dross: top dross and bottom dross. Top dross is an intermetallic compound with a specific gravity lighter than the molten zinc plating bath and floats to the surface of the bath. Bottom dross is an intermetallic compound with a specific gravity heavier than the molten zinc plating bath and accumulates at the bottom of the molten zinc pot. Of these drosses, bottom dross in particular is stirred up from the bottom of the molten zinc pot by the accompanying flow generated by the movement of the steel sheet in the molten zinc plating bath during the hot-dip galvanizing process, and floats in the bath. Such floating bottom dross may adhere to the surface of the steel sheet during the hot-dip galvanizing process. Bottom dross adhering to the surface of a steel sheet may result in point-like defects on the surface of an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. Such surface defects caused by bottom dross are referred to as "dross defects" in this specification. Dross defects reduce the appearance and corrosion resistance of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets. Therefore, it is preferable to suppress the occurrence of dross defects.

[0006] Techniques for suppressing the occurrence of dross defects are proposed in Japanese Patent Publication No. 11-350096 (Patent Document 1) and Japanese Patent Publication No. 11-350097 (Patent Document 2).

[0007] In Patent Document 1, in the method for manufacturing an alloyed hot-dip galvanized steel sheet, when the hot-dip zinc bath temperature is T (°C) and the boundary Al concentration defined by Cz = -0.0015×T + 0.76 is Cz (wt%), the hot-dip zinc bath temperature T is within the range of 435 to 500°C, and the Al concentration in the bath is maintained within the range of Cz ± 0.01 wt%.

[0008] In Patent Document 2, in the method for manufacturing an alloyed hot-dip galvanized steel sheet, the Al concentration in the bath is maintained within the range of 0.15 ± 0.01 wt%. Specifically, Patent Document 2 describes as follows. When the Al concentration in the bath is 0.15 wt% or more, the dross becomes the Fe-Al phase, and when the Al concentration in the bath is 0.15% or less, the dross becomes the delta １ phase (δ １ phase). If the dross repeats phase transformation between the Fe-Al phase and the δ １ phase, the dross becomes finer. Therefore, Patent Document 2 states that by maintaining the Al concentration in the bath within the range of 0.15 ± 0.01 wt%, the dross can be refined, and as a result, the generation of dross defects can be suppressed.

[0009] Japanese Patent Application Laid-Open No. 11-350096 Japanese Patent Application Laid-Open No. 11-350097

[0010] Practical Applications of Phase Diagrams in Continuous Galvanizing, Nai-Yong Tang, Journal of Phase Equilibria and Diffusion Vol. 27 No.5, 2006

[0011] The dross that can occur in the hot-dip galvanizing process includes Fe ２ Al ５ Zn ｘ (so-called top dross), the δ １ phase, the gamma １ phase (Γ １ phase), and the zeta phase (ζ phase). So far, research reports have shown that there are four types of them. For example, in Patent Document 2, when the Al concentration in the bath is Fe ２ Al ５ phase and δ １By operating in a manner that keeps the area near the boundary with the phase, the δ, which is the main cause of dross defects, can be reduced. １ We propose refining the phase.

[0012] However, even when operating using the methods proposed in Patent Documents 1 and 2 above, dross defects may still occur on the alloyed hot-dip galvanized steel sheet or on the surface of the hot-dip galvanized steel sheet.

[0013] Furthermore, in recent years, there has been a growing demand for alloying hot-dip galvanizing treatment on steels containing large amounts of alloying elements, such as high-tensile steel. High-tensile steel containing large amounts of alloying elements is known to be difficult to alloy during the alloying treatment after hot-dip galvanizing. For this reason, steel sheets made from high-tensile steel are sometimes referred to as difficult-to-alloy materials. There is a need for a hot-dip galvanizing method that facilitates alloying even for difficult-to-alloy materials. In addition, even for materials that are not difficult to alloy, when manufacturing alloying hot-dip galvanized steel sheets, a hot-dip galvanizing method that promotes alloying is preferable.

[0014] The object of this disclosure is to provide a hot-dip galvanizing method that can suppress the occurrence of dross defects and promote alloying, a method for manufacturing an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and a method for manufacturing a hot-dip galvanized steel sheet using the hot-dip galvanizing method.

[0015] The hot-dip galvanizing method according to this disclosure is a hot-dip galvanizing method used in the manufacture of a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, comprising: a sample collection step of taking a sample from a hot-dip galvanizing bath containing Al; a ζ phase dross amount determination step of determining the amount of ζ phase dross in the hot-dip galvanizing bath using the collected sample; and an operating condition adjustment step of adjusting the operating conditions of the hot-dip galvanizing treatment based on the determined amount of ζ phase dross.

[0016] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to this disclosure comprises a hot-dip galvanizing step of performing the above-described hot-dip galvanizing treatment method on a steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet, and an alloying step of performing an alloying treatment on the steel sheet on which the hot-dip galvanized layer has been formed to manufacture an alloyed hot-dip galvanized steel sheet.

[0017] The method for manufacturing a hot-dip galvanized steel sheet according to this disclosure comprises a hot-dip galvanizing step of performing the above-described hot-dip galvanizing treatment method on a steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet.

[0018] The hot-dip galvanizing method according to this disclosure can suppress the occurrence of dross defects and can promote alloying even when hot-dip galvanizing and alloying treatments are performed on high-tensile steel sheets. Furthermore, the method for manufacturing alloyed hot-dip galvanized steel sheets according to this disclosure can produce alloyed hot-dip galvanized steel sheets with suppressed dross defects and can promote alloying even when hot-dip galvanizing and alloying treatments are performed on high-tensile steel sheets. The method for manufacturing hot-dip galvanized steel sheets according to this disclosure can produce hot-dip galvanized steel sheets with suppressed dross defects.

[0019] Figure 1 is a functional block diagram showing the overall configuration of a hot-dip galvanizing line equipment used in the manufacture of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets. Figure 2 is a side view of the hot-dip galvanizing equipment in Figure 1. Figure 3 is a side view of hot-dip galvanizing equipment with a different configuration from Figure 2. Figure 4 is a side view of hot-dip galvanizing equipment with a different configuration from Figures 2 and 3. Figure 5 is a functional block diagram showing the overall configuration of a hot-dip galvanizing line equipment with a different configuration from Figure 1. Figure 6 is a flow chart showing the steps of the hot-dip galvanizing process of this embodiment. Figure 7 is a diagram showing an example of a photographic image of a part of the observation field of view of a sample taken in the sample collection step of the hot-dip galvanizing process of this embodiment.

[0020] [Regarding the causes of dross defects] As mentioned above, previous studies have reported that the following types of dross exist in the hot-dip galvanizing process: (1) Fe ２ Al５ Zn ｘ (2) δ １ Phase Dross (3) Γ １ Phase Dross (4) ζ Phase Dross

[0021] Fe ２ Al ５ Zn ｘ This is called top dross. Top dross has a lower specific gravity than the molten zinc plating bath. Therefore, top dross tends to float to the surface of the molten zinc plating bath. Fe ２ Al ５ Zn ｘ Its crystal structure is orthorhombic, and its chemical composition consists of 45% Al, 38% Fe, and 17% Zn by mass. Top dross floats to the surface of the molten zinc plating bath and can therefore be recovered at all times. For this reason, top dross is known to be less likely to cause dross defects.

[0022] δ １ Phase Dross, Γ １ Phase dross and zeta phase dross are called bottom dross. Bottom dross is denser than the molten zinc plating bath. Therefore, bottom dross tends to accumulate at the bottom of the molten zinc pot in which the molten zinc plating bath is stored.

[0023] δ １ The crystal structure of phase dross is hexagonal, and its chemical composition consists of less than 1% Al, more than 9% Fe, and more than 90% Zn, by mass. Γ １ The crystal structure of the dross phase is face-centered cubic, and its chemical composition consists of approximately 20% Fe and 80% Zn by mass. The crystal structure of the zeta phase dross is monoclinic, and its chemical composition consists of less than 1% Al, approximately 6% Fe and approximately 94% Zn by mass.

[0024] Previous studies have identified δ as the main cause of dross defects. １ There were numerous reports that referred to it as phase dross. In the aforementioned Patent Documents 1 and 2, δ １ It seems that phase dross is considered one of the causes of dross defects. Therefore, the inventor also initially considered δ １We considered that phase dross was the main cause of dross defects and conducted investigations and research. However, in the hot-dip galvanizing process, δ １ Even when the generation of phase dross was suppressed, dross defects could still occur on the surface of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets.

[0025] Therefore, the inventors believe that the cause of dross defects is δ １ The inventors suspected that the dross was not phase dross, but rather a different type of dross. Therefore, they re-analyzed the chemical composition and crystal structure of the dross defect areas using alloyed hot-dip galvanized steel sheets in which dross defects had occurred. Furthermore, the inventors re-analyzed the type of dross generated in the hot-dip galvanizing bath. As a result, the inventors obtained the following findings regarding dross defects that differ from previous research results.

[0026] First, the chemical composition of the dross defect areas on the surface of the alloyed hot-dip galvanized steel sheet was analyzed using EPMA (Electron Probe Microanalyzer). Furthermore, the crystal structure of the dross defect areas was analyzed using TEM (Transmission Electron Microscope). As a result, the chemical composition of the dross defect areas consisted of 2% Al, 8% Fe, and 90% Zn by mass%, and the crystal structure was face-centered cubic.

[0027] δ, which was previously thought to be the main cause of dross defects １ The chemical composition of the phase dross (less than 1% Al, more than 9% Fe, and more than 90% Zn by mass) is similar to the chemical composition of the dross defect region described above. However, δ １ The crystal structure of phase dross is hexagonal, not face-centered cubic as identified in the dross defect region. Therefore, the inventors have identified δ, which was previously considered the main cause of dross defects. １ Phase dross was not actually considered the primary cause of the dross defect.

[0028] Therefore, the inventors identified the dross that causes dross defects. Of the dross described in (1) to (4) above, Fe ２ Al５ Zn ｘ Regarding the top dross, its chemical composition differs significantly from that of the dross defect. Γ １ Regarding the phase dross, although its crystal structure is the same face-centered cubic as the dross defect region, its chemical composition (20% Fe and 80% Zn by mass) is significantly different from that of the dross defect region. For the ζ phase dross, its chemical composition (less than 1% Al, about 6% Fe, and about 94% Zn by mass) differs from that of the dross defect region, and furthermore, its crystal structure (monoclinic) also differs from that of the dross defect region (face-centered cubic).

[0029] Based on the above findings, the inventors concluded that the dross defects were not caused by the dross described in (1) to (4) above. Instead, the inventors considered that the dross defects might be caused by other types of dross besides those described in (1) to (4) above.

[0030] Therefore, the inventors further analyzed the dross in the molten zinc plating bath. The EPMA and TEM described above were used for the analysis of the dross. As a result, the inventors determined that gamma was present as the dross generated in the molten zinc plating bath. ２ Phase (Γ) ２ (Partner) We have newly discovered that Dross exists.

[0031] Γ ２ The chemical composition of the phase dross consists of 2% Al, 8% Fe, and 90% Zn by mass, which matches the chemical composition of the dross defect region analyzed above. Furthermore, Γ ２ The crystal structure of the phase dross is face-centered cubic, which matches the crystal structure of the dross defect region. Therefore, the inventors of the present invention, Γ ２ I thought that the phase dross might be the main cause of the dross defect. And then, Γ ２ Since the specific gravity of the phase dross is greater than the specific gravity of the molten zinc plating bath, Γ ２ The phase dross corresponded to bottom dross, which can accumulate at the bottom of a molten zinc pot.

[0032] As mentioned above, Fe ２ Al ５ Zn ｘ (Top dross) has a lower specific gravity than the molten zinc plating bath. Fe２ Al ５ Zn ｘ (Top dross) floats to the surface of the molten zinc plating bath and can therefore be recovered at all times. ２ Al ５ Zn ｘ (Top Dross) is less likely to cause Dross defects.

[0033] The inventors of this invention, Γ ２ Further investigations were conducted regarding the phase dross and the other dross types (2) to (4). As a result, it was found that dross defects are caused by hard dross, while soft dross is less likely to form dross defects.

[0034] As a result of further investigation by the inventors, the dross of (2) to (4) above, and Γ ２ Among the phase dros, Γ ２ The phase dross was found to be a hard dross. Furthermore, δ １ Phase dross and ζ phase dross are Γ ２ It was found that because it is softer than the phase dross, it is less likely to cause dross defects. Furthermore, the zeta phase dross is the softest of the dross types described in (2) to (4) above, and it was found that the zeta phase dross is the least likely to cause dross defects.

[0035] Based on the above findings, the inventors have determined that the main cause of dross defects occurring on the surface of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets subjected to hot-dip galvanizing treatment is δ １ Not phase dross, but Γ ２ It was concluded that it is a phase dross. Furthermore, the inventors determined that dross classified as a bottom dross is Γ ２ Phase dross, δ １ Phase dross, ζ phase dross, and Γ １ Although it is one of the phase dross, in a molten zinc plating bath, Γ １ We have found that there is almost no phase dross present.

[0036] The inventors have further found that the ζ phase dros undergoes phase transformation with other phase dross. That is, Γ ２ Phase dross and ζ phase dross undergo phase transformation with each other. In other words, depending on the conditions of the hot-dip galvanizing process, Γ２ Phase dross undergoes a phase transformation into zeta phase dross, or zeta phase dross becomes Γ ２ It undergoes phase transformation into phase dross. Therefore, if the proportion of ζ phase dross in the bottom dross of the molten zinc plating bath increases, relatively speaking, the Γ phase dross in the molten zinc plating bath will increase. ２ This means that the amount of phase dross decreases.

[0037] Based on the above findings, the inventors have found that by adjusting the operating conditions of the hot-dip galvanizing process to deliberately increase the ζ phase dross, which is the softest and least likely to form dross defects, the hard Γ phase dross, which is more likely to form dross defects, in the hot-dip galvanizing bath, can be increased. ２ We found that the amount of phase dross can be reduced, and as a result, dross defects can be suppressed. Furthermore, we believe that the above operation can be implemented in the hot-dip galvanizing process by controlling the amount of ζ phase dross in the hot-dip galvanizing bath.

[0038] [Regarding Alloying Treatment] The inventors further investigated the case where an alloying treatment is performed after hot-dip galvanizing. In the alloying treatment, Fe contained in the steel sheet diffuses into the hot-dip galvanized layer formed on the surface of the steel sheet, forming an Fe-Zn alloy. It is known that the alloying treatment is affected by the Al concentration in the hot-dip galvanizing bath. If the Al concentration in the hot-dip galvanizing bath is high, a large amount of Al is also contained in the hot-dip galvanized layer. The Al in the hot-dip galvanized layer inhibits the formation of an Fe-Zn alloy between Fe in the steel sheet and Zn in the hot-dip galvanized layer. In other words, when considering the alloying treatment, it is preferable that the Al concentration in the hot-dip galvanizing bath be low.

[0039] Furthermore, high-tensile steel contains large amounts of alloying elements such as Si, P, and Mn. These alloying elements inhibit the diffusion of Fe from the steel sheet into the hot-dip galvanized layer. Therefore, when performing hot-dip galvanizing and alloying treatments on high-tensile steel, it is particularly preferable to have a low Al concentration in the hot-dip galvanizing bath.

[0040] On the other hand, if the Al concentration in the molten zinc plating bath is low, Fe leached from the steel sheet into the molten zinc plating bath is more likely to react with Zn in the molten zinc plating bath. Therefore, if the Al concentration in the molten zinc plating bath is low, the amount of bottom dross increases. Conventionally, the δ contained in the bottom dross... １ Phase dross has been thought to be the cause of dross defects. Therefore, it has been thought that lowering the Al concentration in the molten zinc plating bath would make dross defects more likely to occur.

[0041] However, the inventors' research has shown that by adjusting the operating conditions of the hot-dip galvanizing process to increase ζ-phase dross, dross defects can be suppressed even when the Al concentration in the hot-dip galvanizing bath is reduced. As mentioned above, ζ-phase dross is a type of bottom dross. However, because ζ-phase dross is soft, it is less likely to cause dross defects. If the Al concentration in the hot-dip galvanizing bath can be reduced, the formation of Fe-Zn alloys in the alloying process is promoted. In this case, alloying becomes easier even for high-tensile steel. In other words, the inventors have found that by adjusting the operating conditions of the hot-dip galvanizing process to increase ζ-phase dross, alloying can be promoted while suppressing dross defects.

[0042] As described above, the hot-dip galvanizing method of this embodiment was developed based on an idea different from the conventional technical concept, and specifically, it is as follows.

[0043] The hot-dip galvanizing method of [1] is a hot-dip galvanizing method used in the manufacture of a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, comprising: a sample collection step of taking a sample from a hot-dip galvanizing bath containing Al; a ζ phase dross amount determination step of determining the amount of ζ phase dross in the hot-dip galvanizing bath using the collected sample; and an operating condition adjustment step of adjusting the operating conditions of the hot-dip galvanizing treatment based on the determined amount of ζ phase dross.

[0044] Here, "adjusting the operating conditions of the hot-dip galvanizing process" means adjusting the operating conditions of the hot-dip galvanizing process so that the amount of ζ phase dross in the hot-dip galvanizing bath can be adjusted. Furthermore, "adjusting the operating conditions of the hot-dip galvanizing process" includes not only the act of changing the operating conditions of the hot-dip galvanizing process, but also the act of maintaining the operating conditions as they are.

[0045] According to the hot-dip galvanizing method with the above configuration, the operating conditions of the hot-dip galvanizing method are adjusted to increase the amount of ζ-phase dross in the hot-dip galvanizing bath, based on the amount of ζ-phase dross in the hot-dip galvanizing bath obtained using a sample. As described above, in the hot-dip galvanizing bath, the amount of ζ-phase dross and Γ ２ It has a negative correlation with the amount of ζ phase dross. Specifically, if the amount of ζ phase dross in the molten zinc plating bath is large, then relatively speaking, the amount of Γ phase dross in the molten zinc plating bath will be large. ２ This means that the amount of ζ phase dross is small. Therefore, by determining the amount of ζ phase dross in the molten zinc plating bath and adjusting the operating conditions based on the determined amount of ζ phase dross to increase the ζ phase dross, the amount of Γ phase dross in the molten zinc plating bath can be increased. ２ The amount of phase dross can be reduced. As a result, the occurrence of dross defects can be suppressed. In addition, by increasing the ζ phase dross, Γ ２ By reducing the amount of phase dross, dross defects can be suppressed even if the Al concentration in the molten zinc plating bath is lowered. Lowering the Al concentration in the molten zinc plating bath promotes alloying.

[0046] The hot-dip galvanizing method of this embodiment is suitably applicable to high-tensile steel. The hot-dip galvanizing method of this embodiment can also promote alloying in steels other than high-tensile steel. Therefore, the hot-dip galvanizing method of this embodiment is suitably applicable to steels other than high-tensile steel. In this specification, high-tensile steel refers to steel with a tensile strength of 340 MPa or more. In this specification, steel other than high-tensile steel refers to steel with a tensile strength of less than 340 MPa.

[0047] The hot-dip galvanizing method of [2] is the hot-dip galvanizing method of [1], wherein in the step of determining the amount of ζ phase dross, the number of ζ phase dross particles per predetermined area is determined as the amount of ζ phase dross using the sample that has been taken.

[0048] Here, the predetermined area is not particularly limited. The predetermined area may be, for example, the total area of ​​the field of view when observing zeta-phase dross in a predetermined field of view using a sample, or a unit area (cm²). ２ ) is also acceptable.

[0049] The hot-dip galvanizing method of [3] is the hot-dip galvanizing method of [1] or [2], wherein in the operating condition adjustment step, based on the determined amount of ζ phase dross, at least one of (A) or (B) is performed to increase the amount of ζ phase dross: (A) Adjust the bath temperature of the hot-dip galvanizing bath. (B) Adjust the Al concentration of the hot-dip galvanizing bath.

[0050] Both (A) and (B) above are effective operating conditions for transforming dross of other phases into zeta phase dross or for increasing the generation of zeta phase dross. Therefore, by implementing at least one of (A) or (B), the amount of zeta phase dross can be increased, and Γ ２ The amount of phase dross can be reduced.

[0051] The hot-dip galvanizing method of [4] is the hot-dip galvanizing method described in any one of [1] to [3], wherein in the operating condition adjustment step, when the determined amount of ζ phase dross is less than a threshold, the operating conditions of the hot-dip galvanizing process are adjusted to increase the amount of ζ phase dross.

[0052] In this case, it is easy to determine whether or not to change the operating conditions based on the amount of zeta-phase dross and a threshold value. For example, if the calculated amount of zeta-phase dross is less than the threshold value, the operating conditions can be adjusted so that the amount of zeta-phase dross increases. More preferably, if the calculated amount of zeta-phase dross is less than the threshold value, the operating conditions of the hot-dip galvanizing process are adjusted so that the amount of zeta-phase dross is equal to or greater than the threshold value.

[0053] The hot-dip galvanizing method of [5] is the hot-dip galvanizing method of [4], wherein in the ζ phase dross amount determination step, the number of ζ phase dross particles per predetermined area is determined as the ζ phase dross amount using the collected sample, and in the operating condition adjustment step, the determined ζ phase dross amount is determined per unit area (1 cm²) ２ When converted using this method, the result is 5.0 pieces / cm². ２ If the number is less than the specified amount, the operating conditions of the hot-dip galvanizing process are adjusted to increase the amount of ζ phase dross.

[0054] In this case, by maintaining a high amount of ζ-phase dross, the relative Γ ２ Reduces phase dross. As a result, Γ ２ This further effectively suppresses the occurrence of dross defects caused by phase dross.

[0055] The hot-dip galvanizing method of [6] is the hot-dip galvanizing method described in any one of [1] to [5], wherein in the operating condition adjustment step, when the Fe concentration in the hot-dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass%), the Fe concentration and Al concentration in the hot-dip galvanizing bath are adjusted to satisfy equations (1) and (2). 0.100 ≤ Y ≤ 0.139 (1) Y ≤ 0.2945X + 0.1216 (2)

[0056] Here, the Fe concentration in the molten zinc plating bath refers to the Fe concentration dissolved in the molten zinc plating bath (the so-called Free-Fe concentration). In other words, in this specification, "Fe concentration in the molten zinc plating bath" refers to the Fe concentration dissolved in the molten zinc plating bath (i.e., in the liquid phase), excluding the Fe content contained in the dross (top dross and bottom dross). Similarly, the Al concentration in the molten zinc plating bath refers to the Al concentration dissolved in the molten zinc plating bath (the so-called Free-Al concentration). In other words, in this specification, "Al concentration in the molten zinc plating bath" refers to the Al concentration dissolved in the molten zinc plating bath (i.e., in the liquid phase), excluding the Al content contained in the dross (top dross and bottom dross).

[0057] In this case, the amount of ζ-phase dross increases, and as a result, relatively, Γ ２ The amount of phase dross decreases. Therefore, Γ ２ This further effectively suppresses the occurrence of dross defects caused by phase dross.

[0058] The hot-dip galvanizing method of [7] is the hot-dip galvanizing method of [6], wherein in the operating condition adjustment step, when the Fe concentration in the hot-dip galvanizing bath is defined as X (mass%) and the Al concentration in the hot-dip galvanizing bath is defined as Y (mass%), the Fe concentration and Al concentration in the hot-dip galvanizing bath are adjusted to satisfy equations (1) and (3). 0.100 ≤ Y ≤ 0.139 (1) Y ≤ 0.2945X + 0.1066 (3)

[0059] In this case, the amount of ζ-phase dross increases further, and as a result, relatively Γ ２ The amount of phase dross decreases further. Therefore, Γ ２ This further effectively suppresses the occurrence of dross defects caused by phase dross.

[0060] The hot-dip galvanizing method of [8] is the hot-dip galvanizing method of any one of [1] to [7], wherein a sink roll is arranged in the molten zinc pot in which the molten zinc galvanizing bath is stored, to contact the steel plate immersed in the molten zinc galvanizing bath and change the direction of movement of the steel plate up and down, and in the sample taking step, the sample is taken from the molten zinc galvanizing bath in the molten zinc pot, from a depth range from the upper end to the lower end of the sink roll.

[0061] In this case, samples are taken from the same depth range as the syncroll. Therefore, the correlation between the amount of zeta-phase dross and dross defects can be further enhanced.

[0062] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to [9] comprises a hot-dip galvanizing step of performing the hot-dip galvanizing treatment method described in any one of [1] to [8] on a steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet, and an alloying step of performing an alloying treatment on the steel sheet on which the hot-dip galvanized layer has been formed to manufacture the alloyed hot-dip galvanized steel sheet.

[0063] The method for manufacturing alloyed hot-dip galvanized steel sheets in this embodiment applies the hot-dip galvanizing treatment method of this embodiment described above. Therefore, alloyed hot-dip galvanized steel sheets with suppressed dross defects can be manufactured. Furthermore, even when hot-dip galvanizing and alloying treatments are performed on high-tensile steel, alloying can be promoted.

[0064] The method for manufacturing a hot-dip galvanized steel sheet according to

[10] comprises a hot-dip galvanizing step of performing the hot-dip galvanizing treatment method described in any one of [1] to [8] on a steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet.

[0065] The method for manufacturing a hot-dip galvanized steel sheet in this embodiment applies the hot-dip galvanizing treatment method of this embodiment described above. Therefore, a hot-dip galvanized steel sheet with suppressed dross defects can be manufactured.

[0066] The hot-dip galvanizing method, the method for manufacturing alloyed hot-dip galvanized steel sheets, and the method for manufacturing hot-dip galvanized steel sheets according to this embodiment will be described below with reference to the drawings. In this specification and the drawings, components having substantially the same function are denoted by the same reference numerals and their descriptions will not be repeated.

[0067] [Configuration of Hot-Dip Galvanizing Line Equipment] Figure 1 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanizing line equipment used in the manufacture of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets. Referring to Figure 1, the hot-dip galvanizing line equipment 1 comprises an annealing furnace 20, a hot-dip galvanizing equipment 10, and a temper rolling mill (skin pass mill) 30.

[0068] The annealing furnace 20 includes one or more heating zones (not shown) and one or more cooling zones located downstream of the heating zones. In the annealing furnace 20, steel sheets are supplied to the heating zones and annealing is performed on the steel sheets. The annealed steel sheets are cooled in the cooling zones and transported to the hot-dip galvanizing equipment 10. The hot-dip galvanizing equipment 10 is located downstream of the annealing furnace 20. In the hot-dip galvanizing equipment 10, hot-dip galvanizing is performed on the steel sheets to produce alloyed hot-dip galvanized steel sheets or hot-dip galvanized steel sheets. The temper rolling mill 30 is located downstream of the hot-dip galvanizing equipment 10. In the temper rolling mill 30, the alloyed hot-dip galvanized steel sheets or hot-dip galvanized steel sheets produced in the hot-dip galvanizing equipment 10 are lightly reduced as needed to adjust the surface of the alloyed hot-dip galvanized steel sheets or hot-dip galvanized steel sheets.

[0069] [About the hot-dip galvanizing equipment 10] Figure 2 is a side view of the hot-dip galvanizing equipment 10 shown in Figure 1. Referring to Figure 2, the hot-dip galvanizing equipment 10 comprises a molten zinc pot 101, a sink roll 107, a support roll 113, a gas wiping device 109, and an alloying furnace 111.

[0070] The annealing furnace 20, located upstream of the hot-dip galvanizing equipment 10, is isolated from the atmosphere and maintained in a reducing atmosphere. As described above, the annealing furnace 20 heats the continuously transported steel sheets S in a heating zone. This activates the surface of the steel sheets S and adjusts the mechanical properties of the steel sheets S.

[0071] The downstream end of the annealing furnace 20, which corresponds to the exit side of the annealing furnace 20, has a space where the turndown roll 201 is located. The downstream end of the annealing furnace 20 is connected to the upstream end of the snout 202. The downstream end of the snout 202 is immersed in the molten zinc plating bath 103. The inside of the snout 202 is isolated from the atmosphere and maintained in a reducing atmosphere.

[0072] The steel plate S, whose conveying direction has been changed downward by the turndown roll 201, passes through the snout 202 and is continuously immersed in the molten zinc plating bath 103 stored in the molten zinc pot 101. A sink roll 107 is positioned inside the molten zinc pot 101. The sink roll 107 has a rotation axis parallel to the width direction of the steel plate S. The axial width of the sink roll 107 is greater than the width of the steel plate S. The sink roll 107 comes into contact with the steel plate S and changes the direction of travel of the steel plate S upward towards the molten zinc plating equipment 10.

[0073] The support roll 113 is located in the molten zinc plating bath 103 and is positioned above the sink roll 107. The support roll 113 comprises a pair of rolls. The pair of rolls of the support roll 113 have axes of rotation parallel to the width direction of the steel plate S. The support roll 113 supports the steel plate S being conveyed upward by sandwiching the steel plate S, whose direction of travel has been changed upward by the sink roll 107.

[0074] The gas wiping device 109 is positioned above the sink roll 107 and the support roll 113, and above the liquid level of the molten zinc plating bath 103. The gas wiping device 109 comprises a pair of gas injection devices. The pair of gas injection devices have gas injection nozzles that face each other. During the molten zinc plating process, the steel sheet S passes between the pair of gas injection nozzles of the gas wiping device 109. At this time, the pair of gas injection nozzles face the surface of the steel sheet S. The gas wiping device 109 adjusts the amount of molten zinc plating adhering to the surface of the steel sheet S by blowing gas onto both surfaces of the steel sheet S after it has been lifted out of the molten zinc plating bath 103, thereby scraping off a portion of the molten zinc plating adhering to both surfaces of the steel sheet S.

[0075] The alloying furnace 111 is positioned above the gas wiping device 109. The alloying furnace 111 passes the steel sheet S, which has been transported upward after passing through the gas wiping device 109, through its interior and performs an alloying treatment on the steel sheet S. The alloying furnace 111 includes a heating zone, a heat-retaining zone, and a cooling zone, arranged in order from the inlet side to the outlet side of the steel sheet S. The heating zone heats the steel sheet S so that its temperature (plate temperature) becomes approximately uniform. The heat-retaining zone maintains the plate temperature of the steel sheet S. At this time, the hot-dip galvanizing layer formed on the surface of the steel sheet S is alloyed to become an alloyed hot-dip galvanized layer. The cooling zone cools the steel sheet S on which the alloyed hot-dip galvanized layer has been formed. As described above, the alloying furnace 111 performs the alloying treatment using the heating zone, heat-retaining zone, and cooling zone. The alloying furnace 111 performs the above alloying treatment when manufacturing alloyed hot-dip galvanized steel sheets. On the other hand, when manufacturing hot-dip galvanized steel sheets, the alloying furnace 111 does not perform the alloying treatment. In this case, the steel sheet S passes through an inactive alloying furnace 111. Here, "inactive" means, for example, that the alloying furnace 111 is online but the power is turned off (it is not started). After passing through the alloying furnace 111, the steel sheet S is transported to the next process by a top roll 115.

[0076] When manufacturing hot-dip galvanized steel sheets, the alloying furnace 111 may be moved offline, as shown in Figure 3. In this case, the steel sheet S is transported to the next process by the top roll 115 without passing through the alloying furnace 111.

[0077] Furthermore, if the hot-dip galvanizing equipment 10 is equipment specifically for hot-dip galvanized steel sheets, the hot-dip galvanizing equipment 10 does not need to be equipped with an alloying furnace 111, as shown in Figure 4.

[0078] [Other Configuration Examples of Hot-Dip Galvanizing Line Equipment] The hot-dip galvanizing line equipment 1 is not limited to the configuration shown in Figure 1. For example, when a Ni pre-plating treatment is performed on a steel sheet before hot-dip galvanizing to form a Ni layer on the steel sheet, a Ni pre-plating equipment 40 may be arranged between the annealing furnace 20 and the hot-dip galvanizing equipment 10, as shown in Figure 5. The Ni pre-plating equipment 40 includes a Ni plating cell for storing the Ni plating bath. The Ni plating treatment is performed by electroplating. Note that the hot-dip galvanizing line equipment 1 in Figures 1 and 5 includes an annealing furnace 20 and a temper rolling mill 30. However, the hot-dip galvanizing line equipment 1 does not need to include an annealing furnace 20. Also, the hot-dip galvanizing line equipment 1 does not need to include a temper rolling mill 30. The hot-dip galvanizing line equipment 1 only needs to include at least a hot-dip galvanizing equipment 10. The annealing furnace 20 and the temper rolling mill 30 may be arranged as needed. Furthermore, the hot-dip galvanizing line equipment 1 may be equipped with pickling equipment for pickling steel sheets upstream of the hot-dip galvanizing equipment 10, or with other equipment other than the annealing furnace 20 and pickling equipment. The hot-dip galvanizing line equipment 1 may also be equipped with other equipment other than the temper rolling mill 30 downstream of the hot-dip galvanizing equipment 10.

[0079] [Regarding the mechanism of dross defect occurrence] In the hot-dip galvanizing process during the manufacturing process of alloyed hot-dip galvanized steel sheets or hot-dip galvanized steel sheets using the hot-dip galvanizing line equipment 1 described above, the mechanism by which dross defects occur is thought to be as follows.

[0080] In the hot dip galvanizing process, Fe dissolves from the steel sheet S immersed in the hot dip galvanizing bath 103 into the hot dip galvanizing bath 103. The dissolved Fe reacts with Al and / or Zn in the hot dip galvanizing bath 103 to form dross. Among the generated dross, the top dross floats on the liquid surface in the hot dip galvanizing bath 103. On the other hand, among the generated dross, the bottom dross sinks to the bottom of the hot dip galvanizing pot 101 and accumulates. When the production of the alloyed hot dip galvanized steel sheet or the hot dip galvanized steel sheet is repeated (that is, as the amount of the steel sheet S passing through the hot dip galvanizing bath 103 increases), the bottom dross accumulates on the bottom of the hot dip galvanizing pot 101.

[0081] The bottom dross deposited on the bottom of the hot dip galvanizing pot 101 is lifted into the hot dip galvanizing bath 103 by the entrained flow of the steel sheet S occurring near the lower part of the sink roll 107 and floats in the hot dip galvanizing bath 103. The bottom dross floating in the hot dip galvanizing bath 103 adheres to the surface of the steel sheet S near the sink roll 107. The location where the bottom dross adheres to the surface of the steel sheet S may become a dross defect.

[0082] If a dross defect occurs, an uneven part of the plating appears on the plating surface, and the appearance quality of the alloyed hot dip galvanized steel sheet or the hot dip galvanized steel sheet deteriorates. Furthermore, local batteries are likely to be formed at the dross defect part on the steel sheet surface, and the corrosion resistance of the alloyed hot dip galvanized steel sheet or the hot dip galvanized steel sheet decreases.

[0083] As described above, the main cause of the dross defect is not the δ １ phase dross, which has been reported many times in previous studies, but the Γ ２ phase dross. Therefore, if the amount of Γ ２ phase dross in the hot dip galvanizing bath 103 increases, the possibility of dross defects occurring in the alloyed hot dip galvanized steel sheet or the hot dip galvanized steel sheet becomes higher.

[0084] Furthermore, the ζ phase dross and the Γ ２ phase dross mutually undergo a phase transformation. That is, the ζ phase dross transforms into the Γ ２ phase dross, and the Γ ２ phase dross transforms into the ζ phase dross. Therefore, in the hot dip galvanizing bath 103, the amount of ζ phase dross and the Γ２ There is a negative correlation with the amount of ζ-phase dross. If the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is large, the amount of Γ-phase dross in the hot-dip galvanizing bath 103 is relatively small. ２ Furthermore, ζ-phase dross is the softest compared to the dross of other phases and is less likely to cause dross defects. Therefore, by determining the amount of ζ-phase dross in the hot-dip galvanizing bath 103 and adjusting the operating conditions based on the determined amount of ζ-phase dross to increase the amount of ζ-phase dross, the amount of Γ-phase dross in the hot-dip galvanizing bath 103 can be reduced. As a result, the occurrence of dross defects can be suppressed. ２

[0085] Therefore, in the hot-dip galvanizing treatment method of the present embodiment, the amount of ζ-phase dross among the dross in the hot-dip galvanizing bath 103 is determined. Then, based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted. Preferably, based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the amount of ζ-phase dross. Thereby, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be increased, and the amount of Γ-phase dross can be relatively kept low. As a result, the occurrence of dross defects in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet can be suppressed. Preferably, based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the amount of ζ-phase dross, and the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is maintained at a specific amount (threshold value) or more. ２

[0086] The hot-dip galvanizing treatment method of the present embodiment can also be applied to a method for manufacturing an alloyed hot-dip galvanized steel sheet (GA) and can also be applied to a method for manufacturing a hot-dip galvanized steel sheet (GI). Hereinafter, the hot-dip galvanizing treatment method of the present embodiment will be described in detail.

[0087] [About the Hot-Dip Galvanizing Process of This Embodiment] [About the Hot-Dip Galvanizing Equipment Used] In the hot-dip galvanizing process of this embodiment, a hot-dip galvanizing line equipment 1 is used. The hot-dip galvanizing line equipment 1 has, for example, the configuration shown in Figures 1 and 5. However, the hot-dip galvanizing line equipment 1 used in the hot-dip galvanizing process of this embodiment may be the equipment shown in Figures 1 and 5 as described above, or it may be the equipment shown in Figures 1 and 5 with additional configurations. Alternatively, a well-known hot-dip galvanizing line equipment 1 with a configuration different from that shown in Figures 1 and 5 may be used.

[0088] [Regarding the steel sheet used in the hot-dip galvanizing process] The type of steel and size (thickness, width, etc.) of the steel sheet (base steel sheet) used in the hot-dip galvanizing process in this embodiment are not particularly limited. Depending on the mechanical properties required for the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet to be manufactured (for example, tensile strength, workability, etc.), any known steel sheet applicable to the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet may be used. Steel sheets used for automobile body panels may also be used as the steel sheet (base steel sheet) used in the hot-dip galvanizing process.

[0089] As described above, in the hot-dip galvanizing process of this embodiment, dross defects can be suppressed even if the Al concentration in the hot-dip galvanizing bath 103 is reduced. Therefore, alloying can be promoted by reducing the Al concentration in the hot-dip galvanizing bath 103. The steel sheet used in the hot-dip galvanizing process of this embodiment may be a steel sheet made of high-tensile steel containing a large amount of alloying elements such as Si and Mn. The steel sheet used in the hot-dip galvanizing process of this embodiment may also be a steel sheet made of a steel other than high-tensile steel.

[0090] The steel sheet (base steel sheet) used in the hot-dip galvanizing process of this embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet. For example, the following steel sheets can be used as the base steel sheet: (a) a hot-rolled steel sheet that has been pickled (b) a hot-rolled steel sheet that has been pickled and then subjected to Ni pre-plating to form a Ni layer on its surface (c) a cold-rolled steel sheet that has been annealed (d) a cold-rolled steel sheet that has been annealed and then subjected to Ni pre-plating to form a Ni layer on its surface The above (a) to (d) are examples of steel sheets used in the hot-dip galvanizing process of this embodiment. The steel sheet used in the hot-dip galvanizing process of this embodiment is not limited to the above (a) to (d). A hot-rolled steel sheet or a cold-rolled steel sheet that has undergone a process other than the above (a) to (d) may be used as the steel sheet for the hot-dip galvanizing process.

[0091] [About the molten zinc plating bath] The main component of the molten zinc plating bath 103 is Zn. In addition to Zn, the molten zinc plating bath 103 also contains Al. In other words, the molten zinc plating bath 103 used in the molten zinc plating method of this embodiment is a plating solution that contains a specific concentration of Al, with the remainder being Zn and impurities. If the molten zinc plating bath 103 contains a specific concentration of Al, it is possible to suppress the excessive reaction between Fe and Zn in the bath, and to suppress the progress of the non-uniform alloy reaction between the steel plate immersed in the molten zinc plating bath 103 and Zn.

[0092] The preferred Al concentration (more specifically, the Free-Al concentration) in the molten zinc plating bath 103 is 0.100 to 0.159% by mass. Here, the Al concentration in the molten zinc plating bath 103 refers to the Al concentration (by mass) dissolved in the molten zinc plating bath 103, and is what is known as the Free-Al concentration. If the Al concentration in the molten zinc plating bath is within the range of 0.100 to 0.159% by mass, it is possible to suppress the occurrence of pattern defects other than dross defects, and furthermore, it is possible to suppress the occurrence of unalloyed material during the alloying treatment in the manufacturing process of alloyed molten zinc-plated steel sheets.

[0093] As described above, the molten zinc plating bath 103 according to this embodiment is a plating bath mainly composed of Zn and further containing Al. The molten zinc plating bath 103 may also contain 0.020 to 0.100% by mass of Fe leached from the equipment or steel plates in the bath. In other words, the Fe concentration (by mass) dissolved in the molten zinc plating bath 103 is, for example, 0.020 to 0.100% by mass. However, the Fe concentration dissolved in the molten zinc plating bath 103 is not limited to the above numerical range.

[0094] [Hot-dip galvanizing method] The hot-dip galvanizing method of this embodiment uses a hot-dip galvanizing bath 103 containing Al. Figure 6 is a flowchart showing the steps of the hot-dip galvanizing method of this embodiment. Referring to Figure 6, the hot-dip galvanizing method of this embodiment comprises a sample collection step (S1), a ζ phase dross amount determination step (S2), and an operating condition adjustment step (S3). Each step will be described in detail below.

[0095] [Sampling Process (S1)] In the sampling process (S1), a portion of the plating solution is taken from the molten zinc plating bath 103 as a sample. In the sampling process (S1), samples are taken over time. "Taking samples over time" means taking samples at specific intervals. The specific interval (the period between taking one sample and taking the next) may be constant or not. For example, a sample may be taken every hour. Alternatively, one sample may be taken one hour after taking the first sample, and then another 30 minutes later. The specific interval is not particularly limited.

[0096] The amount of sample taken from the molten zinc plating bath 103 is not particularly limited. As long as the amount of sample taken allows for the determination of the amount of ζ-phase dross in the molten zinc plating bath 103 in the next step, the ζ-phase dross amount determination step (S2), the sample amount is not particularly limited. For example, the sample amount is 100 to 400 g. The collected sample may be brought into contact with a metal at room temperature with high thermal conductivity and rapidly cooled to room temperature to solidify. A metal at room temperature with high thermal conductivity is, for example, copper.

[0097] The sampling location in the molten zinc plating bath 103 is not particularly limited. For example, referring to Figures 2 to 4, if the molten zinc plating bath 103 is divided into three equal parts D1 to D3 in the depth direction D, a sample may be taken in the uppermost region D1, in the middle region D2, or in the lowermost region D3. The amount of ζ phase dross in the samples taken in each region D1 to D3 will be different. However, it is possible to some extent to determine whether the amount of ζ phase dross obtained is large or small depending on the sampling location. Therefore, the sampling location is not particularly limited. As shown in Figures 2 to 4, in the molten zinc plating bath 103, the direction parallel to the width direction of the steel plate S is defined as the width direction W, the depth direction of the molten zinc plating bath 103 is defined as the depth direction D, and the direction perpendicular to the width direction W and the depth direction D is defined as the length direction L. In this case, preferably, samples are taken over time from a specific region defined by a specific width range in the width direction W, a specific depth range in the depth direction D, and a specific length range in the length direction L. In short, samples are taken over time from the same location (within the specific region) in the molten zinc plating bath 103.

[0098] Preferably, the sample is taken from an area as close as possible to the sink roll 107. Specifically, as shown in Figures 2 to 4, the sample is taken from within a specific depth range D107 of the molten zinc plating bath 103, from the upper end to the lower end of the sink roll 107 in the depth direction D. In other words, the specific depth range is defined as the depth range D107 from the upper end to the lower end of the sink roll 107. ２The ζ phase dross is highly likely to adhere to the surface of the steel sheet S near the sink roll 107. Therefore, the amount of ζ phase dross near the sink roll 107 is the most effective indicator for suppressing dross defects. Accordingly, it is preferable to take samples from the depth range D107. In this case, since the amount of ζ phase dross is determined based on samples taken from the range most likely to adhere to the surface of the steel sheet S, the correlation between the amount of ζ phase dross and dross defects can be further increased. It is also preferable to take samples from the region as close to the sink roll as possible in the width direction W and the length direction L. As described above, the samples are taken over time from the same region within the molten zinc plating bath 103.

[0099] [Zeta-phase dross amount determination step (S2)] In the zeta-phase dross amount determination step (S2), the amount of zeta-phase dross in the molten zinc plating bath 103 is determined using the collected sample. The method for determining the amount of zeta-phase dross using the sample is not particularly limited, and various methods can be considered.

[0100] For example, a test specimen for observing zeta-phase dross is prepared from the sample collected in the sample collection step (S1). An example of a test specimen for observing zeta-phase dross is a rectangular parallelepiped (small plate shape) with a surface (test surface) that can secure an observation field of view of 15 mm × 15 mm and a thickness of 0.5 mm. The entire field of view (15 mm × 15 mm) is observed using an optical microscope or scanning electron microscope (SEM) at a predetermined magnification, and the dross within the entire field of view is identified. The dross can be identified by the contrast within the field of view, and furthermore, top dross and bottom dross can be distinguished by the contrast.

[0101] Figure 7 is an example of a photographic image of a portion of the observation field of a sample taken in the sample collection process (S1). Referring to Figure 7, the photographic image shows the matrix 200, top dross 100T, and bottom dross 100B of the hot-dip galvanized layer. The top dross 100T is less bright (darker) than the matrix 200 and bottom dross 100B. On the other hand, the bottom dross 100B is less bright (darker) than the matrix 200 and brighter (brighter) than the top dross 100T. As described above, the top dross and bottom dross can be distinguished based on contrast.

[0102] For each bottom dross identified within the above observation field (15 mm x 15 mm), compositional analysis using EPMA is performed to identify the ζ phase dross. Further crystal structure analysis using TEM may be performed on each bottom dross to identify the ζ phase dross within the above observation field. In addition, without distinguishing between top dross and bottom dross by contrast, compositional analysis using EPMA and / or crystal structure analysis using TEM are performed on each dross to identify the type of dross in the field (top dross, Γ ２ Phase dross, δ １ The phase dross and the zeta phase dross may be specified.

[0103] Based on the identified ζ-phase dross, the amount of ζ-phase dross in the molten zinc plating bath 103 is determined. The amount of ζ-phase dross in the molten zinc plating bath 103 can be determined by various indicators. For example, the number of ζ-phase dross particles per predetermined area may be used as the amount of ζ-phase dross. Here, the predetermined area is not particularly limited and may be, for example, the entire area of ​​the observation field, or a unit area (mm²). ２ ) may also be used. For example, if the field of view is 15 mm x 15 mm, the field of view (15 mm x 15 mm = 225 mm) ２ ) Number of zeta-phase dross (number / 225 mm ２ ) may be used as the amount of zeta-phase dross. In this case, the number of zeta-phase dross particles in the observation field is determined by the following method. First, the equivalent circle diameter (μm) of the identified zeta-phase dross particles is determined. The diameter obtained by converting the area of ​​each zeta-phase dross particle in the observation field described above into a circle is defined as the equivalent circle diameter (μm). Using the photographic image of the observation field described above, the equivalent circle diameter (μm) of the identified zeta-phase dross particles is determined by a well-known image processing method. The number of zeta-phase dross particles with an equivalent circle diameter of 10 μm or more in the field of view is defined as the number of zeta-phase dross particles (particles / 225 mm). ２ ) is defined as follows. Thus, the number of ζ-phase dross particles with an equivalent circle diameter of 10 μm or more in the observation field may be defined as the amount of ζ-phase dross. Note that the observation field is the above area (15 mm × 15 mm = 225 mm). ２ ) is not limited to the above. Also, the upper limit of the equivalent circle diameter of the zeta-phase dross is not particularly limited. For example, the upper limit of the equivalent circle diameter of the zeta-phase dross is 300 μm.

[0104] Alternatively, another indicator may be the amount of ζ phase dross in the molten zinc plating solution. For example, in the observation field described above, each bottom dross (each Γ ２ Phase dross, each δ １ The area of ​​the phase dross and each zeta phase dross, and the area of ​​each zeta phase dross are determined. The ratio of the total area of ​​the zeta phase dross to the total area of ​​the bottom dross may be defined as the amount of zeta phase dross. Alternatively, the ratio of the total area of ​​the zeta phase dross to the observation field area may be defined as the amount of zeta phase dross. Furthermore, the total area of ​​the zeta phase dross in the aforementioned field of view (μm) ２ ) may be taken as the amount of ζ phase dross. In addition, X-ray diffraction measurements are performed on the surface of the sample described above to determine each bottom dross (Γ ２ Phase dross, δ １ The peak intensity of the phase dross and the ζ phase dross is measured. Then, the sum of the peak intensities of each bottom dross (i.e., Γ) is calculated. ２ Phase dross peak intensity, δ １ The amount of ζ-phase dross may be defined as the ratio of the peak intensity of ζ-phase dross to the sum of the peak intensities of the phase dross and the ζ-phase dross. Note that in X-ray diffraction measurements, Γ ２ Phase Dross and Gamma １ Phase dross is not easily distinguished clearly. However, as mentioned above, Γ １ It is thought that there is almost no phase dross. Therefore, all peak intensities obtained at diffraction angles 2θ = 43-44° are Γ. ２ This is considered to be the peak intensity of the phase dross. For example, a Co dry bulb is used as the target during X-ray diffraction measurement. The amount of ζ phase dross may also be determined by methods other than those described above.

[0105] By the method described above, the amount of ζ-phase dross in the molten zinc plating bath 103 is determined using the sample taken in the sample collection step (S1). It is preferable that the ζ-phase dross amount determination step (S2) be performed each time a sample is taken in the sample collection step (S1). By taking samples over time and determining the amount of ζ-phase dross each time a sample is taken, the change in the amount of ζ-phase dross in the molten zinc plating bath 103 over time can also be grasped. Therefore, the amount of ζ-phase dross over time may be determined based on samples taken over time.

[0106] [Operating Condition Adjustment Process (S3)] After determining the amount of ζ-phase dross in the molten zinc plating bath 103 in the ζ-phase dross amount determination process (S2), the operating condition adjustment process (S3) is carried out.

[0107] In the operating conditions adjustment process (S3), the operating conditions for the hot-dip galvanizing process are adjusted based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103. Specifically, if the determined amount of ζ-phase dross is small, the operating conditions are adjusted (changed) to increase the amount of ζ-phase dross in the hot-dip galvanizing bath 103. If the determined amount of ζ-phase dross is appropriate, the operating conditions may be maintained as they are. The method of adjusting the operating conditions is not particularly limited as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted. Specifically, the method of adjusting the operating conditions is not particularly limited as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be increased.

[0108] Preferably, as a method for adjusting the operating conditions, at least one of the following (A) or (B) is carried out: (A) Adjust the bath temperature of the molten zinc plating bath 103. (B) Adjust the Al concentration of the molten zinc plating bath 103.

[0109] Regarding (A) above, if the temperature of the molten zinc plating bath 103 is increased, Γ ２ The dross phase is more likely to undergo a phase transformation into Γ phase dross. Therefore, if the temperature of the molten zinc plating bath 103 is increased, the Γ phase in the molten zinc plating bath 103 will be more likely to transform. ２The phase dross decreases, and instead, the ζ phase dross increases. As mentioned above, the ζ phase dross is soft. Therefore, the ζ phase dross is less likely to form dross defects. Consequently, if the amount of ζ phase dross in the molten zinc plating bath 103 is excessively low, the bath temperature of the molten zinc plating bath 103 may be increased. In this case, the hard Γ ２ Phase dross undergoes a phase transformation into soft zeta phase dross. As a result, the amount of soft zeta phase dross increases, and the amount of hard zeta phase dross increases. ２ The ζ phase dross decreases. Therefore, the occurrence of dross defects is suppressed. Note that increasing the bath temperature increases the energy intensity. Therefore, if the amount of ζ phase dross is sufficiently large, it is not necessary to excessively increase the bath temperature. As described above, the amount of ζ phase dross in the molten zinc plating bath 103 can be adjusted by adjusting the bath temperature of the molten zinc plating bath 103. Specifically, by increasing the bath temperature of the molten zinc plating bath 103, the amount of ζ phase dross can be increased, and as a result, the amount of Γ in the molten zinc plating bath 103 ２ Phase dross can be reduced.

[0110] Regarding (B) above, if the Al concentration in the molten zinc plating bath 103 is lowered, Γ ２ The likelihood of the phase dross undergoing phase transformation into ζ phase dross increases. Therefore, if the amount of ζ phase dross in the molten zinc plating bath 103 is excessively low, the amount of ζ phase dross in the molten zinc plating bath 103 can be adjusted by adjusting the Al concentration in the molten zinc plating bath 103. Specifically, by reducing the Al content of the molten zinc plating bath 103, the amount of ζ phase dross can be increased, and as a result, the amount of Γ phase dross in the molten zinc plating bath 103 increases. ２ Phase dross can be reduced.

[0111] Based on the calculated amount of ζ-phase dross, either one of the operating conditions (A) and (B) described above may be adjusted, or both operating conditions (A) and (B) may be adjusted. For example, if the amount of ζ-phase dross is excessively low, the bath temperature of the molten zinc plating bath 103 may be increased and the Al concentration of the molten zinc plating bath 103 may be decreased. If the amount of ζ-phase dross is appropriate, the operating conditions (A) and (B) may be maintained as they are.

[0112] A threshold value may be set as an indicator for determining whether the amount of ζ-phase dross determined in the ζ-phase dross amount determination step (S2) is appropriate. In this case, the operating conditions may be adjusted depending on whether the determined amount of ζ-phase dross is less than the threshold value. Specifically, the operating conditions may be changed or maintained without change depending on whether the determined amount of ζ-phase dross is less than the threshold value. For example, if the determined amount of ζ-phase dross is less than the threshold value, the operating conditions may be changed to increase the amount of ζ-phase dross in the molten zinc plating bath 103 from the current level. Preferably, if the determined amount of ζ-phase dross is less than the threshold value, the operating conditions may be changed so that the amount of ζ-phase dross is equal to or greater than the threshold value. On the other hand, if the determined amount of ζ-phase dross is equal to or greater than the threshold value, the operating conditions may be maintained as they are, based on the judgment that the amount of ζ-phase dross in the molten zinc plating bath 103 is sufficiently high.

[0113] The number of zeta-phase dross particles per unit area, for example, if the number of zeta-phase dross particles in the observation field is defined as the zeta-phase dross amount, then the unit area (1 cm²) is defined as the number of zeta-phase dross particles per unit area. ２ 5.0 pieces / cm when converted to the number of pieces per unit area. ２ The threshold is set to the number corresponding to that. For example, the observation field of view mentioned above (15 mm x 15 mm = 225 mm) ２ If the number of zeta-phase dross particles in ) is defined as the zeta-phase dross amount, the threshold is set to 11.25 particles (5.0 particles / cm²). ２ ×225mm ２ In this case, the amount of ζ-phase dross determined by the ζ-phase dross determination step (S2) is set to the threshold value (11.25 pieces / 225 mm). ２ A number greater than ) that is, a unit area (1 cm²) ２ 5.0 pieces / cm when converted using this method. ２ If the number is less than the threshold (11.25 particles / 225 mm), it is determined that the amount of ζ-phase dross is excessively low, and the operating conditions are adjusted so that the amount of ζ-phase dross in the molten zinc plating bath 103 increases. Preferably, the amount of ζ-phase dross determined in the ζ-phase dross determination step (S2) is equal to the threshold (11.25 particles / 225 mm). ２ When it exceeds 5.0 particles / cm², that is, when the calculated zeta-phase dross amount is converted to a unit area ２When the number is less than the threshold (11.25 particles / 225 mm), the zeta-phase dross quantity is less than the threshold (11.25 particles / 225 mm). ２ ) or more (i.e., 5.0 pieces / cm when converted to a unit area) ２ The operating conditions are adjusted so that the number of particles is greater than or equal to the above. For example, the amount of zeta-phase dross determined by the zeta-phase dross determination process (S2) is 5.0 particles / cm when converted to a unit area. ２ When the number is less than the specified value, the amount of ζ-phase dross is increased by implementing at least one of the operating conditions (A) or (B) described above. For example, the amount of ζ-phase dross is increased by raising the bath temperature of the molten zinc plating bath 103. Alternatively, for example, the amount of ζ-phase dross is increased by reducing the Al content of the molten zinc plating bath 103. Note that the number of ζ-phase dross particles per predetermined area should be as large as possible, and there is no particular upper limit specified.

[0114] Preferably, in the operating conditions adjustment step (S3), based on the amount of ζ-phase dross determined in the ζ-phase dross determination step (S2), the Fe concentration in the molten zinc plating bath 103 is defined as X (mass%), and the Al concentration in the molten zinc plating bath 103 is defined as Y (mass%). Then, the Fe concentration and Al concentration in the molten zinc plating bath 103 are adjusted to satisfy equations (1) and (2). 0.100 ≤ Y ≤ 0.139 (1) Y ≤ 0.2945X + 0.1216 (2) Here, the Al concentration means the Al concentration in the molten zinc plating bath 103 excluding the Al content contained in the dross, and thus represents the so-called Free-Al concentration (mass%). Similarly, the Fe concentration means the Fe concentration in the molten zinc plating bath 103 excluding the Fe content contained in the dross.

[0115] Equation (1) shows the range of Al concentration Y (mass%) in the molten zinc plating bath 103. The Al concentration Y in the molten zinc plating bath 103 is the top dross, Γ ２ This relates to the amount of phase dross and ζ phase dross produced. If the Al concentration Y is 0.139% or less, the top dross will be Γ ２This makes it easier for phase transformation to occur into phase dross and ζ phase dross. In this case, the excessive generation of top dross is suppressed. This prevents top dross from getting trapped between the sink roll 107 and the steel plate, thus preventing the generation of surface defects. Therefore, in order to suppress the occurrence of surface defects, the generation of top dross may be suppressed. To suppress surface defects, it is sufficient to maintain the Al concentration in the molten zinc plating bath 103 at 0.140% or less. However, in the actual operation of the molten zinc plating process, there is a possibility of variation of up to ±0.001% in Al concentration control. For this reason, in equation (1), the upper limit of the Al concentration Y in the molten zinc plating bath 103 is set to 0.139%.

[0116] Furthermore, from the standpoint of suppressing the occurrence of surface defects, the lower limit of the Al concentration is not particularly limited. However, it is well known that over-alloying can be suppressed in the alloying treatment by setting the Al concentration in the molten zinc plating bath 103 to a certain level or higher. In equation (1), the lower limit of the Al concentration (the lower limit of equation (1)) is set to 0.100%.

[0117] The lower limit of the Al concentration Y in the molten zinc plating bath 103 may be 0.100%, 0.105%, or 0.110%. The upper limit of the Al concentration Y in the molten zinc plating bath 103 may be 0.139%, 0.135%, 0.130%, or 0.125%.

[0118] Equation (2) is given in the molten zinc plating bath 103, Γ ２ This corresponds to the boundary (phase transformation line) where the phase dross undergoes a phase transformation to the ζ phase dross. If the Al concentration Y in the molten zinc plating bath 103 is higher than the right-hand side of equation (2), then the chemical composition of the molten zinc plating bath 103 is higher than that of the ζ phase dross. ２ The phase dross is in a state where it can exist more stably. In this case, assuming that the Al concentration Y in the molten zinc plating bath 103 satisfies equation (1), the ζ phase dross is Γ ２ It readily undergoes phase transformation into phase dross. Therefore, in the molten zinc plating bath 103, Γ ２ This creates conditions that facilitate the formation of phase dross.

[0119] On the other hand, if the Al concentration Y in the molten zinc plating bath 103 is less than or equal to the right-hand side of equation (2), that is, if the Al concentration Y and Fe concentration X satisfy equation (2), then assuming that the Al concentration Y in the molten zinc plating bath 103 satisfies equation (1), the chemical composition of the molten zinc plating bath 103 is Γ ２ The ζ phase dross can exist more stably than the Γ phase dross. Therefore, the Γ phase in the molten zinc plating bath 103 ２ Phase dross readily undergoes phase transformation to ζ phase dross. Therefore, in the molten zinc plating bath 103, Γ ２ This creates a condition where phase dross tends to decrease.

[0120] Therefore, in the hot-dip galvanizing process described above, if the Al concentration Y and Fe concentration X in the hot-dip galvanizing bath 103 are adjusted to satisfy equations (1) and (2), the formation of ζ phase dross will be promoted in the hot-dip galvanizing bath 103, and Γ phase dross will have a negative correlation with the amount of ζ phase dross. ２ The amount of phase dross can be reduced.

[0121] More preferably, in the operating condition adjustment step (S3), based on the amount of ζ-phase dross determined in the ζ-phase dross determination step (S2), the Fe concentration in the molten zinc plating bath 103 is defined as X (mass%), and the Al concentration in the molten zinc plating bath 103 is defined as Y (mass%). Then, the Fe concentration and Al concentration in the molten zinc plating bath 103 are adjusted to satisfy equations (1) and (3). 0.100 ≤ Y ≤ 0.139 (1) Y ≤ 0.2945X + 0.1066 (3) Here, the Al concentration means the Al concentration in the molten zinc plating bath 103 excluding the Al content contained in the dross, and thus represents the so-called Free-Al concentration (mass%). Similarly, the Fe concentration means the Fe concentration in the molten zinc plating bath 103 excluding the Fe content contained in the dross.

[0122] Equation (3) is an equation that identifies a region with an even lower Al concentration than that of equation (2) described above. Equation (2) described above is an equation that identifies a region with an Al concentration of Γ in the molten zinc plating bath 103. ２This corresponds to the boundary (phase transformation line) where the phase dross undergoes a phase transformation to the zeta phase dross. Equation (3) is a region in which the zeta phase dross can exist even more stably than the region specified by equation (2). Therefore, Γ in the molten zinc plating bath 103 ２ The dross phase is more likely to undergo a further phase transformation into ζ phase dross. Therefore, in the molten zinc plating bath 103, Γ ２ The phase dross becomes even more likely to decrease.

[0123] The Fe concentration (Free-Fe concentration) and Al concentration (Free-Al concentration) in the molten zinc plating bath can be determined by the following method. A sample is taken from a specific depth range in the depth direction D of the molten zinc plating bath 103 shown in Figure 2. More specifically, a sample is taken from a specific region (hereinafter referred to as the sample collection region) in the molten zinc plating bath 103 shown in Figure 2, which is partitioned by a specific depth range in the depth direction D, a specific width range in the width direction W, and a specific length range in the length direction L. When samples are taken sequentially over time, the sample collection position should be the same (within the same sample collection region). The collected sample is cooled to room temperature. The Fe concentration (mass%) and Al concentration (mass%) in the cooled sample are measured using an ICP emission spectrometer. The remainder other than the Fe and Al concentrations can be considered as Zn.

[0124] The Fe concentration obtained by the ICP emission spectrometer described above is the so-called Total Fe concentration, which includes not only the Fe concentration in the molten zinc plating bath (Free Fe concentration) but also the Fe concentration in the dross. Similarly, the Al concentration obtained by the ICP emission spectrometer described above is the so-called Total Al concentration, which includes not only the Al concentration in the molten zinc plating bath (Free Al concentration) but also the Al concentration in the dross. Therefore, the Free Fe concentration and Free Al concentration are calculated using the obtained Total Fe concentration and Total Al concentration and the well-known Zn-Fe-Al ternary phase diagram. Specifically, a Zn-Fe-Al ternary phase diagram is prepared at the bath temperature when the sample was taken. As described above, the Zn-Fe-Al ternary phase diagram is well-known and is disclosed in Figures 2 and 3 of Non-Patent Literature 1. Non-Patent Literature 1 is a well-known paper among researchers and developers of molten zinc plating baths. Points identified from the Total-Fe concentration and Total-Al concentration obtained by ICP emission spectrometer are plotted on the Zn-Fe-Al ternary phase diagram. Then, tie lines (conjugate lines) are drawn from the plotted points to the liquidus line in the Zn-Fe-Al ternary phase diagram. The Fe concentration at the intersection of the liquidus line and the tie line corresponds to the Free-Fe concentration, and the Al concentration at the intersection of the liquidus line and the tie line corresponds to the Free-Al concentration. By this method, the Fe concentration (Free-Fe concentration) and Al concentration (Free-Al concentration) in the molten zinc plating bath can be determined.

[0125] [Regarding a more preferable bath temperature for the molten zinc plating bath] The temperature (bath temperature) of the molten zinc plating bath 103 in the above-described molten zinc plating method is preferably 440 to 500°C. The dross in the molten zinc plating bath 103 is mainly top dross (Fe), depending on the temperature of the molten zinc plating bath 103 and the Al concentration in the molten zinc plating bath 103. ２ Al ５ Zn ｘ ), Γ ２ Phase dross, δ １ It undergoes phase transformations into phases Γ and ζ phase dross. ２ Phase dross is more likely to form in the low bath temperature region. Zeta phase dross is Γ ２Phase dross is more likely to form in regions where the bath temperature is higher than the phase dross formation region.

[0126] Furthermore, if the bath temperature of the molten zinc plating bath 103 is 500°C or lower, the evaporation of Zn and formation of fumes can be suppressed. When fumes are generated, they tend to adhere to the steel plate, causing surface defects (fume defects). The preferred lower limit for the molten zinc plating bath 103 is 460°C, more preferably 465°C, and even more preferably 469°C. The preferred upper limit for the molten zinc plating bath 103 is 490°C, more preferably 480°C, and even more preferably 475°C. Note that the top dross is Γ ２ Formation is more likely to occur in regions where the Al concentration is higher than that of the phase dross formation region and the zeta phase dross formation region.

[0127] As described above, in the hot-dip galvanizing method of this embodiment, a sample is taken from the hot-dip galvanizing bath 103 (sample taking step (S1)), and the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is determined (ζ-phase dross amount determination step (S2)). Then, based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing process are adjusted (operating condition adjustment step (S3)). Γ ２ By controlling the amount of ζ phase dross, which has a negative correlation with the amount of phase dross, operating conditions can be adjusted to suppress the occurrence of dross defects.

[0128] [Method for manufacturing alloyed hot-dip galvanized steel sheet] The hot-dip galvanizing treatment method of this embodiment described above is applicable to the method for manufacturing alloyed hot-dip galvanized steel sheet (GA).

[0129] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to this embodiment comprises a hot-dip galvanizing step and an alloying step. In the hot-dip galvanizing step, the hot-dip galvanizing method described above is performed on the steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet. On the other hand, in the alloying step, an alloying treatment is performed on the steel sheet, on which a hot-dip galvanized layer has been formed in the hot-dip galvanizing step, using the alloying furnace 111 shown in Figure 2. The alloying method can be any well-known method.

[0130] Through the above manufacturing process, alloyed hot-dip galvanized steel sheets can be produced. In the alloyed hot-dip galvanized steel sheets of this embodiment, the hot-dip galvanizing treatment method of this embodiment described above is adopted. That is, the amount of ζ phase dross is increased by adjusting the operating conditions of the hot-dip galvanizing treatment based on the amount of ζ phase dross. Therefore, Γ in the hot-dip galvanizing bath 103 ２ Phase dross is relatively reduced, and as a result, the occurrence of dross defects in the manufactured alloyed hot-dip galvanized steel sheet can be suppressed.

[0131] The manufacturing method for alloyed hot-dip galvanized steel sheets in this embodiment may include other manufacturing steps besides the hot-dip galvanizing process and the alloying process. For example, the manufacturing method for alloyed hot-dip galvanized steel sheets in this embodiment may include a temper rolling process after the alloying process, in which temper rolling is performed using the temper rolling mill 30 shown in Figure 1. In this case, the surface appearance quality of the alloyed hot-dip galvanized steel sheet can be further improved. Furthermore, other manufacturing steps besides the temper rolling process may also be included.

[0132] [Method for manufacturing hot-dip galvanized steel sheet] The hot-dip galvanizing method of this embodiment described above can also be applied to the method for manufacturing hot-dip galvanized steel sheet (GI).

[0133] The method for manufacturing a hot-dip galvanized steel sheet according to this embodiment includes a hot-dip galvanizing step. In the hot-dip galvanizing step, the hot-dip galvanizing method described above is applied to the steel sheet to form a hot-dip galvanized layer on the surface of the steel sheet. The method for manufacturing a hot-dip galvanized steel sheet according to this embodiment employs the hot-dip galvanizing method described above. In other words, the operating conditions of the hot-dip galvanizing process are adjusted based on the amount of ζ-phase dross to increase the ζ-phase dross. Therefore, the occurrence of dross defects in the manufactured hot-dip galvanized steel sheet can be suppressed.

[0134] The manufacturing method for hot-dip galvanized steel sheets in this embodiment may include other manufacturing steps besides the hot-dip galvanizing process. For example, the manufacturing method for hot-dip galvanized steel sheets in this embodiment may include a temper rolling process after the hot-dip galvanizing process, in which temper rolling is performed using the temper rolling mill 30 shown in Figure 1. In this case, the surface appearance quality of the hot-dip galvanized steel sheet can be further improved. Furthermore, other manufacturing steps besides the temper rolling process may also be included.

[0135] The effects of one embodiment of the hot-dip galvanizing treatment method of this embodiment will be further described in detail below with reference to examples. The conditions in the examples are just one example of conditions adopted to confirm the feasibility and effects of the present invention. Therefore, the hot-dip galvanizing treatment method of this embodiment is not limited to this one example of conditions.

[0136] In the aforementioned process of adjusting operating conditions, we investigated the relationship between Fe concentration X and Al concentration Y.

[0137] Specifically, the hot-dip galvanizing treatment method was carried out using a hot-dip galvanizing facility having the same configuration as shown in Figure 2. Specifically, the Fe concentration X (mass%) and Al concentration Y (mass%) of the hot-dip galvanizing bath were adjusted as shown in Table 1. As the steel sheet, high-tensile steel consisting of C: 0.003%, Si: 0.006%, Mn: 0.6%, P: 0.02%, S: 0.01%, and the remainder: Fe and impurities was used. This high-tensile steel is a so-called difficult-to-alloy material, which is relatively difficult to alloy when manufacturing alloyed hot-dip galvanized steel sheets. Alloyed hot-dip galvanized steel sheets were manufactured by performing an alloying treatment using an alloying furnace on the hot-dip galvanized steel sheets. The heating temperature in the alloying treatment was kept constant (510°C) for all test numbers.

[0138] For each test number, a sample was taken from within a specific depth range D107 in the depth direction D of the molten zinc plating bath 103 in Figure 2, from the upper end to the lower end of the sink roll 107. More specifically, a sample was taken from within a specific region (hereinafter referred to as the sample collection region) demarcated by a specific depth range D107 in the depth direction D, a specific width range in the width direction W, and a specific length range in the length direction L in the molten zinc plating bath 103 in Figure 2. For each test number, approximately 400 g of sample was taken from the same sample collection region as described above. The collected samples were cooled to room temperature. Using the cooled samples, the chemical composition of the molten zinc plating bath for each test number was measured using an ICP emission spectrometer. The Fe concentration (mass%) and Al concentration (mass%) obtained from the measurement are the Total-Fe concentration (mass%) and Total-Al concentration (mass%). Therefore, the Fe concentration (Free-Fe concentration) and Al concentration (Free-Al concentration) in the molten zinc plating bath were calculated using the obtained Total-Fe and Total-Al concentrations and a well-known Zn-Fe-Al ternary phase diagram. Specifically, a Zn-Fe-Al ternary phase diagram was prepared at the bath temperature at the time of sample collection. Points identified from the Total-Fe and Total-Al concentrations obtained by ICP emission spectrometer were plotted on the well-known Zn-Fe-Al ternary phase diagram. Tie lines (conjugate lines) were drawn from the plotted points to the liquidus line in the Zn-Fe-Al ternary phase diagram, and the intersection points of the liquidus line and tie lines were found. The Fe concentration at the intersection point was defined as the Free-Fe concentration (mass%), and the Al concentration at the intersection point was defined as the Free-Al concentration (mass%). Using the above method, the Fe concentration (Free-Fe concentration) and Al concentration (Free-Al concentration) in the molten zinc plating bath were determined. As a result, the Fe concentration in the molten zinc plating bath was within the range of 0.02 to 0.05 mass% in all test numbers.

[0139]

[0140] In each test number, the Fe concentration X (mass%) of the molten zinc plating bath was kept constant at the value shown in Table 1, and the Al concentration Y (mass%) of the molten zinc plating bath was adjusted by adding Al as appropriate over time to achieve the concentration shown in Table 1. The transport speed of the steel sheet during the molten zinc plating process was kept constant in all test numbers.

[0141] Table 1 also includes the values ​​on the right-hand sides of equations (2) and (3). Furthermore, it indicates whether the Fe concentration X (mass%) and Al concentration Y (mass%) in the molten zinc plating bath satisfy equations (1) to (3). For example, a white circle (○) in the column for equation (2) indicates that the Fe concentration X (mass%) and Al concentration Y (mass%) in the molten zinc plating bath satisfy equation (2). An X (×) in the column for equation (2) indicates that the Fe concentration X (mass%) and Al concentration Y (mass%) in the molten zinc plating bath do not satisfy equation (2).

[0142] For each test number, samples were taken from the molten zinc plating bath under the operating conditions shown in Table 1. Specifically, approximately 400 g of sample was taken from the sample collection area described above. From the collected samples, test specimens for observing ζ-phase dross were prepared. The test surface of the ζ-phase dross observation specimen was 1 cm × 1 cm with a thickness of 0.5 mm. Using a 100x SEM, the entire field of view (1 cm × 1 cm) of the test surface was observed, and dross (top dross, bottom dross) was identified based on contrast. Furthermore, compositional analysis using EPMA was performed to identify the bottom dross as Γ ２ Phase dross, δ １ They were classified into phase dross and zeta phase dross. Furthermore, each identified bottom dross (Γ ２ Phase dross, δ １ The equivalent circular diameters of the phase dross and zeta phase dross were determined. The number of zeta phase dross particles with an equivalent circular diameter of 10 μm or more was determined within the 1 cm × 1 cm field of view described above. The number of zeta phase dross particles with an equivalent circular diameter of 10 μm or more in the observed field of view (particles / 1 cm) ２ ) was defined as the amount of ζ-phase dross. The obtained amounts of ζ-phase dross are shown in Table 1. Note that in this example, in all test numbers, Γ １ No phase dross was observed.

[0143] [Dross Defect Evaluation Test] After performing hot-dip galvanizing treatment under the operating conditions of each test number, alloying treatment was performed under the same conditions for each test number to produce alloyed hot-dip galvanized steel sheets. The surface of the produced alloyed hot-dip galvanized steel sheets was visually inspected to check for the presence or absence of dross defects, and the dross defects were evaluated. The criteria for dross defect evaluation were as follows: A: No dross defects were present (0 dross defects / m²) ２ ) B: Number of dross defects is greater than 0 and is 0.1 per m ２ Below, C: The number of dross defects is 0.1 per meter. ２ Excess 1 piece / m ２ below

[0144] [Alloying Evaluation Test of Difficult-to-Alloy Materials] The chemical composition of the alloyed hot-dip galvanized layer on the surface of alloyed hot-dip galvanized steel sheets manufactured under the operating conditions of each test number was investigated to evaluate the alloying of difficult-to-alloy materials. Specifically, the chemical composition of the alloyed hot-dip galvanized layer was analyzed using an energy-dispersive X-ray fluorescence analyzer (EDX-7000) manufactured by Shimadzu Corporation. The alloying evaluation was performed by calculating the value obtained by dividing the Fe content (mass%) contained in the alloyed hot-dip galvanized layer by the Zn content (mass%) contained in the alloyed hot-dip galvanized layer. The criteria for alloying evaluation were as follows. If the ratio of Fe content to Zn content was 11% or more, it was judged to be an over-alloy. A: Ratio of Fe content to Zn content is 10% or more and less than 11% B: Ratio of Fe content to Zn content is more than 9% and less than 10% C: Ratio of Fe content to Zn content is less than 9%

[0145] [Evaluation Results] Refer to Table 1, and the zeta-phase dross quantity was 5.0 particles / cm². ２ In the controlled tests 1, 2, 5, 6, and 8-13 described above, the dross defect evaluation was A or B, demonstrating more effective suppression of dross defects. Furthermore, in tests 1, 2, 5, 6, and 8-13, the alloying evaluation of difficult-to-alloy materials was A or B, indicating that alloying was more effectively promoted even in difficult-to-alloy materials. On the other hand, the ζ-phase dross amount was 5.0 particles / cm³. ２In tests 3, 4, and 7, the dross defect evaluation and the alloying evaluation of difficult-to-alloy materials were both rated C. Furthermore, referring to tests 1 through 13, the dross defect evaluation improved as the amount of zeta-phase dross increased. In other words, there was a negative correlation between the amount of zeta-phase dross and the number of dross defects.

[0146] From the above results, it was found that dross defects can be suppressed by adjusting the operating conditions based on the amount of zeta-phase dross. Preferably, the threshold for the amount of zeta-phase dross is 5.0 particles / cm. ２ Assuming the zeta-phase dross quantity is 5.0 particles / cm², ２ It was found that dross defects can be significantly suppressed by adjusting the operating conditions in the hot-dip galvanizing process as described above.

[0147] In tests 1, 2, 5, 6, 8-13, which satisfied equations (1) and (2), the dross defect evaluation was A or B, indicating that dross defects were more effectively suppressed. Furthermore, in tests 1, 2, 5, 6, 8-13, the alloying evaluation of difficult-to-alloy materials was A or B, indicating that alloying was more effectively promoted even in difficult-to-alloy materials. Therefore, it was found that adjusting the operating conditions to satisfy equations (1) and (2) is effective in suppressing dross defects and promoting alloying of difficult-to-alloy materials.

[0148] In tests 1, 5, 8, 9, 11, and 12, which satisfied equations (1) and (3), the dross defect evaluation was A, indicating that dross defects could be suppressed even more effectively. Furthermore, in tests 1, 5, 8, 9, 11, and 12, the alloying evaluation for difficult-to-alloy materials was also A, indicating that alloying could be promoted even more effectively, even for difficult-to-alloy materials. Therefore, it was found that adjusting the operating conditions to satisfy equations (1) and (3) is even more effective in suppressing dross defects and promoting alloying of difficult-to-alloy materials.

[0149] In tests 14 and 16, where the Al concentration Y in the molten zinc plating bath was 0.0990 mass%, the dross defect evaluation was "A," and furthermore, although alloying could be promoted even for difficult-to-alloy materials, over-alloying occurred in the production of alloyed molten zinc-plated steel sheets. Therefore, it became clear that it is more preferable for the Al concentration Y in the molten zinc plating bath to satisfy formula (1).

[0150] In tests 15 and 17, where the Al concentration Y in the molten zinc plating bath was 0.1410 mass%, the alloying evaluation of the difficult-to-alloy material was "C". Therefore, it became clear that it is more preferable for the Al concentration Y in the molten zinc plating bath to satisfy formula (1).

[0151] Embodiments of the present invention have been described above. However, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above can be appropriately modified and implemented without departing from the spirit of the invention.

[0152] 10. Hot-dip galvanizing equipment 101. Hot-dip galvanizing pot 103. Hot-dip galvanizing bath 107. Sinkroll 109. Gas wiping device 111. Alloying furnace 202. Snout

Claims

DEPCT651. A hot-dip galvanizing procedure shall be used for the production of hot-dip galvanized steel sheets or hot-dip galvanized steel sheets through alloy formation, in which a sample collection procedure shall be performed from the hot-dip galvanizing bath, in which a dross phasic determination procedure shall be performed, in which the dross phasic content in the hot-dip galvanizing bath shall be determined by the collected samples, and in which the operating conditions of the hot-dip galvanizing procedure shall be adjusted based on the determined dross phasic content.

2. A hot-dip galvanizing procedure per claim 1 shall be performed in which the dross phasic content determination procedure shall be performed, in which the number of dross phasic particles per predetermined area shall be determined as the dross phasic content using the collected samples.3.Hot-dip galvanizing operation methods under Reputation 1 or 2 where at least one of the conditioning steps of (A) and (B) is performed based on the specified dross phase quantity to increase the dross phase quantity (A) adjustment of the hot-dip galvanizing bath temperature and (B) adjustment of the hot-dip galvanizing bath Al concentration.

4. Any one of the hot-dip galvanizing operation methods under Reputation 1 through 3 where, in the conditioning step, if the specified dross phase quantity is less than the operating threshold of the hot-dip galvanizing operation, it is adjusted to increase the dross phase quantity.

5. Hot-dip galvanizing operation methods under Reputation 4 where, in the dross phase quantity determination step, the number of dross phase particles per unit area is determined as the dross phase quantity using collected samples, and in the conditioning step, if the specified dross phase quantity is less than 5.0 / cm² when converted to a unit area (1 cm²), the operating conditions of the hot-dip galvanizing operation are adjusted to increase the amount of dross sulfate.

6. In any one of the hot-dip galvanizing operation methods under claims 1 to 5, in the operating condition adjustment step, when the concentration of Fe in the hot-dip galvanizing bath is determined as X (% by mass) and the concentration of Al in the hot-dip galvanizing bath is determined as Y (% by mass), the concentration of Fe and the concentration of Al in the hot-dip galvanizing bath are adjusted to be true according to formulas (1) and (2).0.100 less than or equal to Y less than or equal to 0.139(1)Y less than or equal to 0.2945X+0.1216(2)7.Hot-dip galvanizing operation method according to claim 6, in which the operating conditions adjustment step when the concentration of Fe in the hot-dip galvanizing bath is determined as X (% by mass) and the concentration of Al in the hot-dip galvanizing bath is determined as Y (% by mass), the concentration of Fe and the concentration of Al in the hot-dip galvanizing bath are adjusted to be true according to formulas (1) and (3) 0.100 less than or equal to Y less than or equal to 0.139 (1) Y less than or equal to 0.2945X + 0.1066 (3) 8. Hot-dip galvanizing operation method In any of the claims 1 through 7, whereby a submersible roller in contact with a steel plate submerged in the hot-dip galvanizing bath and altering the forward and backward movement of the steel plate is placed in a molten zinc bath containing the hot-dip galvanizing bath, and during the collection phase, a sample is collected from the depth from the top end to the bottom end of the submersible roller of the hot-dip galvanizing bath in the molten zinc bath.The manufacturing method of hot-dip galvanized steel sheets through alloyation involves the hot-dip galvanizing operation of any one of the hot-dip galvanizing operations under claims 1 to 8 on the steel sheet to form a hot-dip galvanized coating layer on the surface of the steel sheet, and the alloyation operation of the alloyation operation on the steel sheet surface which has formed a hot-dip galvanized coating layer to produce hot-dip galvanized steel sheets through alloyation.