CN113950538B - Hot dip galvanization treatment method, method for producing alloyed hot dip galvanized steel sheet using same, and method for producing hot dip galvanized steel sheet - Google Patents

Abstract

Providing: a hot dip galvanization treatment method capable of suppressing occurrence of dross defects and promoting alloying in producing an alloyed hot dip galvanized steel sheet. The hot dip galvanization processing method of the present embodiment is a hot dip galvanization processing method for manufacturing a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet. The hot dip galvanization treatment method comprises the following steps: a sample collection step (S1), a zeta-phase dross amount determination step (S2), and an operating condition adjustment step (S3). In the sample collection step (S1), a sample is collected from a hot dip galvanization bath containing Al. In the zeta-phase dross amount determining step (S2), the zeta-phase dross amount in the collected sample is determined. In the operation condition adjustment step (S3), the operation condition of the hot dip galvanization treatment is adjusted based on the determined zeta-phase slag amount.

Description

Hot-dip galvanizing treatment method and manufacturing method of alloyed hot-dip galvanized steel sheet using the same Method for manufacturing hot-dip galvanized steel sheets

Technical field

The present invention relates to a hot-dip galvanizing process, a manufacturing method of an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing process, and a manufacturing method of a hot-dip galvanized steel sheet using the hot-dip galvanizing process.

Background technique

The hot-dip galvanized steel sheet (hereinafter, also referred to as GI) and the alloyed hot-dip galvanized steel sheet (hereinafter, also referred to as GA) are manufactured by the following manufacturing process. First, a steel plate (base material steel plate) to be hot-dip galvanized is prepared. The base material steel plate can be a hot-rolled steel plate or a cold-rolled steel plate. When the base material steel sheet is a hot-rolled steel sheet, for example, a pickled hot-rolled steel sheet is prepared. If necessary, a hot-rolled steel plate in which a Ni layer is formed on the surface can be prepared by subjecting a pickled hot-rolled steel plate to pre-plating treatment. A hot-rolled steel sheet subjected to other treatments than the above may be prepared. When the base material steel plate is a cold-rolled steel plate, for example, an annealed cold-rolled steel plate is prepared. If necessary, the annealed cold-rolled steel sheet may be pre-plated with Ni to form a cold-rolled steel sheet with a Ni layer on the surface. A cold-rolled steel sheet subjected to other treatments than the above may be prepared. The prepared base material steel sheet (the above-mentioned hot-rolled steel sheet or cold-rolled steel sheet) is immersed in a hot-dip galvanizing bath, and hot-dip galvanizing treatment is performed to produce a hot-dip galvanized steel sheet. When producing an alloyed hot-dip galvanized steel sheet, the hot-dip galvanized steel sheet is further heat-treated in an alloying furnace to produce an alloyed hot-dip galvanized steel sheet.

Details of the hot-dip galvanizing treatment in the manufacturing process of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets are as follows. Hot-dip galvanizing equipment used in hot-dip galvanizing treatment includes a molten zinc pot housing a hot-dip galvanizing bath, a sinking roller disposed in the hot-dip galvanizing bath, and a gas wiping device.

In the hot-dip galvanizing process, the steel plate (base material steel plate) is immersed in a hot-dip galvanizing bath. Then, the traveling direction of the steel plate is switched upward using a sinking roller arranged in the hot-dip galvanizing bath, and the steel plate is pulled up from the hot-dip galvanizing bath. For the steel plate that travels upward after being pulled up, the wiping gas is blown from the gas wiping device to the surface of the steel plate to scrape off the remaining molten zinc and adjust the amount of coating adhesion on the surface of the steel plate. Through the above method, the hot-dip galvanizing process is performed. In addition, when manufacturing an alloyed hot-dip galvanized steel sheet, the steel sheet whose coating adhesion amount was further loaded into an alloying furnace and alloying treatment was performed.

In the above hot-dip galvanizing treatment, Fe is eluted from the steel sheet immersed in the hot-dip galvanizing bath into the hot-dip galvanizing bath. When Fe eluted from the steel sheet into the hot-dip galvanizing bath reacts with Al and Zn present in the hot-dip galvanizing bath, an intermetallic compound called dross is generated. There are top slag and bottom slag in scum. Top slag is scum that has a specific gravity smaller than the intermetallic compounds of the hot-dip galvanizing bath and floats on the liquid surface of the hot-dip galvanizing bath. Bottom slag is the scum deposited on the bottom of the molten zinc pot that has a specific gravity greater than the intermetallic compounds of the hot-dip galvanizing bath. Among these scums, especially in hot-dip galvanizing, the accompanying flow generated as the steel sheet travels in the hot-dip galvanizing bath will roll up the bottom slag from the bottom of the deposited molten zinc pot and float on the hot-dip galvanizing bath. Dip in galvanizing bath. This floating bottom slag sometimes adheres to the surface of the steel plate being hot-dip galvanized. Bottom slag adhering to the surface of the steel plate may become point-like defects on the surface of the alloyed hot-dip galvanized steel plate or hot-dip galvanized steel plate. In this specification, such surface defects caused by bottom slag are called "scum defects". Dross defects reduce the appearance of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets, or reduce corrosion resistance. Therefore, it is preferable to suppress the occurrence of scum defects.

Japanese Patent Application Laid-Open No. 11-350096 (Patent Document 1) and Japanese Patent Application Laid-Open No. 11-350097 (Patent Document 2) propose techniques for suppressing the occurrence of scum defects.

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

In Patent Document 2, in the method of 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 the following. When the Al concentration in the bath is 0.15 wt% or more, the scum becomes the Fe-Al phase. When the Al concentration in the bath is 0.15% or less, the scum becomes the Delta ₁ phase (δ ₁ phase). If the dross repeats the phase transition between the Fe-Al phase and the δ ₁ phase, the dross will become finer. Therefore, Patent Document 2 describes that by maintaining the Al concentration in the bath within the range of 0.15±0.01 wt%, the scum can be made fine, and as a result, the occurrence of scum defects can be suppressed.

existing technical documents

patent documents

Patent Document 1: Japanese Patent Application Publication No. 11-350096

Patent Document 2: Japanese Patent Application Publication No. 11-350097

non-patent literature

Non-patent document 1: Practical Applications of Phase Diagrams in ContinuousGalvanizing, Nai-Yong Tang, Journal of Phase Equilibria and DiffusionVol.27No.5, 2006

Contents of the invention

Invent the problem to be solved

Previous studies have reported that Fe ₂ Al ₅ Zn _x (so-called top slag), δ ₁ phase, Gamma ₁ phase (Γ ₁ phase), and Zeta phase are present in the dross that may be generated during hot-dip galvanizing. (ζ phase) these 4 types. For example, Patent Document 2 proposes operating so that the Al concentration in the bath is near the boundary between the Fe ₂ Al ₅ phase and the δ ₁ phase, thereby minimizing the δ ₁ phase that is a major factor in scum defects.

However, even when the method proposed in Patent Document 1 and Patent Document 2 is performed, dross defects may still occur on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet.

Furthermore, in recent years, there has been an increasing demand for alloying hot-dip galvanizing of steel containing a large amount of alloying elements, such as high-tensile steel. It is known that high-tensile steel containing a large amount of alloying elements is difficult to alloy in the alloying process after hot-dip galvanizing. For this reason, steel sheets composed of high-tensile steel are sometimes referred to as materials that are difficult to alloy. A hot-dip galvanizing treatment method that can easily alloy even materials that are difficult to alloy is sought. In addition, even if the material is not difficult to alloy, when producing an alloyed hot-dip galvanized steel sheet, a hot-dip galvanizing treatment method that can promote the alloying process is preferable.

An object of the present disclosure is to provide a hot-dip galvanizing treatment method that can suppress the generation of dross defects and promote alloying, a manufacturing method of an alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing treatment method, and A method for manufacturing a hot-dip galvanized steel sheet using this hot-dip galvanizing treatment method.

solutions to problems

The hot-dip galvanizing treatment method of the present disclosure is used to manufacture hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets. The hot-dip galvanizing treatment method includes the following steps:

Sample collection process, collect samples from hot-dip galvanizing bath containing Al;

The ζ-phase scum amount determination process uses the collected samples to determine the ζ-phase scum amount in the hot-dip galvanizing bath; and,

The operating condition adjustment step adjusts the operating conditions of the hot-dip galvanizing treatment based on the calculated amount of ζ-phase scum.

The manufacturing method of the alloyed hot-dip galvanized steel sheet of the present disclosure includes the following steps:

The hot-dip galvanizing treatment process is to implement the above-mentioned hot-dip galvanizing treatment method on the steel plate to form a hot-dip galvanized layer on the surface of the steel plate; and,

In the alloying treatment process, the steel plate with the hot-dip galvanized layer formed on the surface is alloyed to produce an alloyed hot-dip galvanized steel plate.

The manufacturing method of the hot-dip galvanized steel sheet of the present disclosure includes the following hot-dip galvanizing processing steps:

The above hot-dip galvanizing treatment method is performed on the steel plate to form a hot-dip galvanized layer on the surface of the steel plate.

Effect of invention

The hot-dip galvanizing treatment method of the present disclosure can suppress the generation of dross defects and can promote alloying even when hot-dip galvanizing and alloying treatments are performed on a steel plate of high-tensile steel. In addition, the manufacturing method of the alloyed hot-dip galvanized steel sheet of the present disclosure can manufacture the alloyed hot-dip galvanized steel sheet in which the generation of dross defects is suppressed. Furthermore, even if the steel sheet of high-strength steel is subjected to hot-dip galvanizing treatment and alloying Alloying can also be promoted in the case of chemical treatment. The manufacturing method of a hot-dip galvanized steel sheet of this disclosure can manufacture a hot-dip galvanized steel sheet in which the generation of dross defects is suppressed.

Description of drawings

FIG. 1 is a functional block diagram showing the overall structure of a hot-dip galvanizing line equipment for manufacturing 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.

FIG. 3 is a side view of a hot-dip galvanizing equipment constructed differently from that of FIG. 2 .

FIG. 4 is a side view of a hot-dip galvanizing equipment constructed differently from that of FIGS. 2 and 3 .

FIG. 5 is a functional block diagram showing the overall structure of a hot-dip galvanizing line equipment different from that of FIG. 1 .

FIG. 6 is a flowchart showing the steps of the hot-dip galvanizing treatment method according to this embodiment.

FIG. 7 is a diagram showing an example of a photographic image of a part of the observation field of view of the sample collected in the sample collection step of the hot-dip galvanizing treatment method of the present embodiment.

Detailed ways

[About the factors causing scum defects]

As mentioned above, previous studies have reported that the following types of dross are generated during hot-dip galvanizing.

(1)Fe ₂ Al ₅ Zn _x

(2)δ ₁ phase scum

(3)Γ ₁ phase scum

(4)ζ phase scum

Fe ₂ Al ₅ Zn _x is called top slag. The specific gravity of the top slag is smaller than that of the hot dip galvanizing bath. Therefore, the top slag easily floats on the liquid surface of the hot-dip galvanizing bath. The crystal structure of Fe ₂ Al ₅ Zn _x is orthorhombic, and its chemical composition consists of 45% Al, 38% Fe and 17% Zn in mass %. The top slag floats on the surface of the hot-dip galvanizing bath and therefore can be continuously recycled. Therefore, it is known that top dross is less likely to cause dross defects.

δ ₁ phase scum, Γ ₁ phase scum and ζ phase scum are called bottom scum. The specific gravity of the bottom slag is greater than that of the hot dip galvanizing bath. Therefore, bottom slag is easily deposited at the bottom of the molten zinc pot where the hot-dip galvanizing bath is stored.

The crystal structure of δ ₁ phase dross is hexagonal crystal, and its chemical composition is 1% or less Al, 9% or more Fe, and 90% or more Zn in terms of mass %. The crystal structure of Γ ₁ phase dross is face-centered cubic crystal, and its chemical composition is 20% Fe and about 80% Zn in terms of mass %. The crystal structure of ζ phase dross is a monoclinic crystal, and its chemical composition is 1% or less Al, approximately 6% Fe, and approximately 94% Zn in terms of mass %.

In previous studies, many examples were reported in which the main factor of scum defects was δ ₁ phase scum. In the above-mentioned Patent Documents 1 and 2, it is also considered that δ ₁ phase dross is one of the factors causing dross defects. Therefore, the present inventor initially considered δ ₁ phase dross to be the main factor of dross defects and conducted investigation and research. However, even when the generation of δ _1- phase dross is suppressed in the hot-dip galvanizing process, dross defects may still occur on the surfaces of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet.

Therefore, the present inventors believe that the cause of the scum defect is not δ ₁ phase scum but may be other scum. Therefore, the present inventors reanalyzed the chemical composition and crystal structure of the dross defective part using an alloyed hot-dip galvanized steel sheet that produced dross defects. The inventors further reanalyzed the types of dross generated in the hot-dip galvanizing bath. As a result, the present inventors obtained the following insights about dross defects that are different from previous research results.

First, the chemical composition of the dross defect portion on the surface of the alloyed hot-dip galvanized steel sheet was analyzed using EPMA (Electron Probe Micro Analyzer). Furthermore, the crystal structure of the dross defective part was analyzed using TEM (Transmission Electron Microscope). As a result, the chemical composition of the scum defective portion was 2% Al, 8% Fe, and 90% Zn in terms of mass %, and the crystal structure was a face-centered cubic crystal.

The chemical composition of δ ₁ phase dross (which is less than 1% Al, more than 9% Fe, and more than 90% Zn in terms of mass %), which has been considered a major factor in dross defects in the past, and the chemistry of the above-mentioned dross defective parts The composition is similar. However, the crystal structure of _δ1 phase dross is hexagonal crystal, not the face-centered cubic crystal specified in the defective part of dross. Therefore, the present inventors believe that δ ₁ phase dross, which has been considered a major factor in dross defects in the past, is not actually a major factor in dross defects.

Therefore, the present inventors identified dross that causes dross defects. In the above-mentioned dross (1) to (4), the chemical composition of Fe ₂ Al ₅ Zn _x (top dross) is significantly different from the chemical composition of the defective part of the dross. Although the crystal structure of the Γ ₁ phase dross is the same face-centered cubic crystal as that of the defective part of the dross, its chemical composition (20% Fe and 80% Zn in mass %) is the same as the chemical composition of the defective part of the dross. Big difference. The chemical composition of ζ phase dross (less than 1% Al, about 6% Fe, and about 94% Zn in terms of mass %) is different from the chemical composition of the defective part of the dross, and further, the crystal structure (monoclinic) It is also different from the crystal structure (face-centered cubic crystal) of the scum defective part.

Based on the above research results, the inventor believes that the scum defect is not caused by the above-mentioned scum (1) to (4). Furthermore, the present inventors believe that the scum defect may be caused by other types of scum other than the above (1) to (4).

Therefore, the present inventors further conducted an analysis of dross in a hot-dip galvanizing bath. The above-mentioned EPMA and TEM were used for analysis of dross. As a result, the present inventors newly discovered that Gamma ₂ phase (Γ ₂ phase) scum exists as scum generated in a hot-dip galvanizing bath.

The chemical composition of the Γ _2- phase dross is 2% Al, 8% Fe, and 90% Zn in terms of mass %, which is consistent with the chemical composition of the defective portion of the dross analyzed above. Furthermore, the crystal structure of the Γ ₂ phase dross is face-centered cubic crystal, which is consistent with the crystal structure of the defective part of the dross. Therefore, the inventors believe that Γ ₂ phase scum may be the main factor in scum defects. Moreover, the specific gravity of Γ _2- phase dross is greater than that of the hot-dip galvanizing bath. Therefore, Γ ₂ -phase dross is a bottom slag that can be deposited at the bottom of the molten zinc pot.

As mentioned above, the specific gravity of Fe ₂ Al ₅ Zn _x (top slag) is smaller than that of the hot-dip galvanizing bath. Fe ₂ Al ₅ Zn _x (top slag) floats on the liquid surface of the hot-dip galvanizing bath and therefore can be continuously recovered. Therefore, Fe ₂ Al ₅ Zn _x (top dross) is less likely to cause dross defects.

The present inventors further investigated Γ ₂ phase scum and other scums (2) to (4). As a result, it was determined that scum defects were caused by hard scum, while soft scum was less likely to form scum defects.

As a result of further research by the present inventors, it was determined that the Γ 2-phase dross among the above-mentioned dross (2) to (4) and the Γ _{2 -} phase dross is a hard dross. Furthermore, it was determined that δ ₁ phase dross and ζ phase dross are softer than Γ ₂ phase dross and therefore are less likely to cause dross defects. In addition, it was determined that ζ-phase dross is the softest dross among the dross (2) to (4) above, and that ζ-phase dross is least likely to cause dross defects.

Based on the above research results, the inventors reached the following conclusion: The main factor in the scum defects generated on the surface of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets subjected to hot-dip galvanizing treatment is Γ ₂ phase dross Instead of δ ₁ phase scum. Furthermore, the present inventors obtained the following knowledge: The dross classified as bottom dross is any one of Γ ₂ -phase dross, δ _1- phase dross, ζ-phase dross, and Γ _1- phase dross. However, in thermal In the immersion galvanizing bath, there is basically no Γ ₁ phase dross.

The present inventors further obtained the following findings. The ζ phase scum and other phases of scum change from each other. That is, the Γ _2- phase dross and the ζ-phase dross change phase from each other. That is, depending on the conditions of the hot-dip galvanizing treatment, the Γ _2- phase dross changes into the ζ-phase dross or the ζ-phase dross changes into the Γ _2- phase dross. Therefore, if the proportion of ζ phase dross in the bottom slag in the hot dip galvanizing bath increases, the amount of Γ ₂ phase dross in the hot dip galvanizing bath will relatively decrease.

Based on the above insights, the present inventors found that if the operating conditions of hot-dip galvanizing are adjusted to intentionally increase the softest zeta phase scum that has not been paid attention to in the past and is less likely to become a scum defect, hot-dip galvanizing can be reduced. The amount of hard Γ ₂ phase scum in the bath that easily causes scum defects can suppress scum defects. Furthermore, it is considered that in the hot-dip galvanizing treatment method, the above operation can be performed by controlling the amount of zeta phase dross in the hot-dip galvanizing bath.

[About alloying treatment]

The present inventors further studied the case where alloying treatment is performed after hot-dip galvanizing. In the alloying process, Fe contained in the steel plate diffuses into the hot-dip galvanized layer formed on the surface of the steel plate to form an Fe-Zn alloy. It is well known that alloying treatment is easily affected by the Al concentration in the hot-dip galvanizing bath. When the Al concentration in the hot-dip galvanizing bath is high, a large amount of Al is also contained in the hot-dip galvanizing layer. The Al in the hot-dip galvanized layer prevents the Fe in the steel sheet and the Zn in the hot-dip galvanized layer from forming an Fe-Zn alloy. That is, when considering alloying treatment, it is preferable that the Al concentration in the hot-dip galvanizing bath is low.

In addition, high-tensile steel contains a large amount of alloying elements such as Si, P, and Mn. Alloying elements prevent Fe in the steel plate from diffusing into the hot-dip galvanized layer. Therefore, when hot-dip galvanizing and alloying are performed on high-tensile steel, it is preferable that the Al concentration in the hot-dip galvanizing bath be particularly low.

On the other hand, if the Al concentration in the hot-dip galvanizing bath is low, Fe eluted from the steel sheet into the hot-dip galvanizing bath will easily react with Zn in the hot-dip galvanizing bath. Therefore, if the Al concentration in the hot-dip galvanizing bath is low, the amount of bottom slag increases. Conventionally, it has been considered that δ ₁ phase dross contained in bottom dross causes dross defects. Therefore, it is considered that if the Al concentration in the hot-dip galvanizing bath is reduced, dross defects are likely to occur.

However, as a result of research by the present inventors, it was found that if the operating conditions of the hot-dip galvanizing process are adjusted in such a way as to increase ζ-phase scum, even when the Al concentration in the hot-dip galvanizing bath is reduced, the Al concentration in the hot-dip galvanizing bath can be suppressed. Dross defects. As mentioned above, ζ phase scum is a type of bottom slag. However, ζ-phase dross is soft and therefore does not easily cause dross defects. If the Al concentration in the hot-dip galvanizing bath can be reduced, the formation of Fe-Zn alloy can be promoted in the alloying treatment. In this case, even high-tensile steel can be easily alloyed. That is, the present inventors found that by adjusting the operating conditions of the hot-dip galvanizing process in such a manner that zeta phase dross is increased, dross defects can be suppressed and alloying can be promoted.

As described above, the hot-dip galvanizing treatment method of this embodiment is completed based on a discovery different from conventional technical concepts, and is specifically as follows.

The hot-dip galvanizing treatment method of [1] is used to manufacture hot-dip galvanized steel panels or alloyed hot-dip galvanized steel panels,

The hot-dip galvanizing treatment method includes the following steps:

Sample collection process, collect samples from hot-dip galvanizing bath containing Al;

The ζ-phase scum amount determination process uses the aforementioned sample collected to determine the ζ-phase scum amount in the aforementioned hot-dip galvanizing bath; and,

The operating condition adjustment step adjusts the operating conditions of the hot-dip galvanizing treatment based on the determined amount of ζ-phase dross.

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 zeta phase dross in the hot-dip galvanizing bath can be adjusted. In addition, "adjusting the operating conditions of hot-dip galvanizing" refers not only to the act of changing the operating conditions of hot-dip galvanizing, but also to the act of maintaining the operating conditions as they are.

According to the hot-dip galvanizing treatment method having the above constitution, the operation of the hot-dip galvanizing treatment method is adjusted in such a manner that the amount of ζ-phase dross is increased based on the amount of ζ-phase dross in the hot-dip galvanizing bath obtained using the sample. condition. As mentioned above, in the hot-dip galvanizing bath, the amount of ζ phase dross and the amount of Γ ₂ phase dross have a negative correlation. Specifically, it means that if the amount of ζ phase dross in the hot dip galvanizing bath is large, the amount of Γ ₂ phase dross in the hot dip galvanizing bath will be relatively small. Therefore, the amount of ζ-phase dross in the hot-dip galvanizing bath is determined, and the operating conditions are adjusted based on the determined amount of ζ-phase dross to increase the amount of ζ-phase dross, thereby reducing the amount of Γ ₂ in the hot-dip galvanizing bath. The amount of phase scum. As a result, the occurrence of dross defects can be suppressed. In addition, since the amount of Γ ₂ -phase dross is reduced by increasing the ζ-phase dross, dross defects can be suppressed even if the Al concentration in the hot-dip galvanizing bath is reduced. If the Al concentration in the hot-dip galvanizing bath can be reduced, alloying can be promoted.

The hot-dip galvanizing treatment method of this embodiment can be suitably applied to high-tensile steel. The hot-dip galvanizing treatment method of this embodiment can also promote alloying of steels other than high-tensile steel. Therefore, the hot-dip galvanizing treatment method of this embodiment can be suitably applied to steels other than high-tensile steel. High-tensile steel in this specification refers to steel with a tensile strength of 340MPa or more. In this specification, steel other than high-tensile steel refers to steel with a tensile strength lower than 340 MPa.

The hot-dip galvanizing treatment method in [2] is the hot-dip galvanizing treatment method described in [1],

In the aforementioned ζ phase scum amount determination process,

Using the collected sample, the number of ζ-phase scum per predetermined area is determined as the ζ-phase scum amount.

Here, the prescribed area is not particularly limited. The predetermined area may be, for example, the entire area of the observation field of view when observing zeta phase scum in a predetermined observation field of view using a sample, or may be a unit area (cm ² ).

The hot-dip galvanizing treatment method in [3] is the hot-dip galvanizing treatment method described in [1] or [2],

In the aforementioned operating condition adjustment process,

Based on the calculated amount of ζ-phase dross, at least one of (A) or (B) is performed to increase the amount of said ζ-phase dross.

(A) Adjust the bath temperature of the aforementioned hot-dip galvanizing bath.

(B) Adjust the Al concentration of the hot-dip galvanizing bath.

Both of the above (A) and (B) are operating conditions that are effective in converting other phases of scum into ζ-phase scum or increasing the generation of ζ-phase scum. Therefore, by implementing at least one of (A) or (B), the amount of ζ phase scum can be increased and the amount of Γ ₂ phase scum can be reduced.

The hot-dip galvanizing treatment method in [4] is the hot-dip galvanizing treatment method described in any one of [1] to [3],

In the aforementioned operating condition adjustment process,

When the calculated amount of ζ-phase dross is lower than the threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the amount of ζ-phase dross.

In this case, it can be easily determined whether to change the operating conditions based on the ζ-phase scum amount and the threshold value. For example, when the calculated amount of ζ-phase scum is lower than the threshold, the operating conditions may be adjusted so that the amount of ζ-phase scum increases. More preferably, when the calculated amount of ζ-phase dross is lower than the threshold value, the operating conditions of the hot-dip galvanizing treatment are adjusted so that the amount of ζ-phase dross becomes equal to or greater than the threshold value.

The hot-dip galvanizing treatment method in [5] is the hot-dip galvanizing treatment method described in [4],

In the aforementioned ζ phase scum amount determination process,

Using the aforementioned sample collected, the number of ζ-phase scum per specified area is determined as the aforementioned ζ-phase scum amount,

In the aforementioned operating condition adjustment process,

When the calculated amount of zeta phase dross is less than 5.0 pieces/cm ² in terms of unit area (1cm ² ), the operating conditions of the hot dip galvanizing treatment are adjusted to increase the number of zeta phase scum. Amount of scum.

In this case, by maintaining the ζ phase scum amount at a high level, the Γ ₂ phase scum is relatively reduced. As a result, the occurrence of scum defects caused by Γ ₂ phase scum can be further effectively suppressed.

The hot-dip galvanizing method of [6] is the hot-dip galvanizing method described in any one of [1] to [5],

In the aforementioned operating condition adjustment process,

When the Fe concentration in the hot-dip galvanizing bath is defined as Adjust the Fe concentration and Al concentration in the aforementioned hot-dip galvanizing bath.

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1216 (2)

Here, the Fe concentration in the hot-dip galvanizing bath means the concentration of Fe melted in the hot-dip galvanizing bath (so-called Free-Fe concentration). That is, in this specification, the "Fe concentration in the hot-dip galvanizing bath" refers to the Fe content melted in the hot-dip galvanizing bath in addition to the Fe content contained in the dross (top slag and bottom slag). (ie, the Fe concentration in the liquid phase). Similarly, the Al concentration in the hot-dip galvanizing bath refers to the concentration of Al melted in the hot-dip galvanizing bath (so-called Free-Al concentration). That is, in this specification, the "Al concentration in the hot-dip galvanizing bath" refers to the Al content melted in the hot-dip galvanizing bath in addition to the Al content contained in the dross (top slag and bottom slag). (ie, the Al concentration in the liquid phase).

In this case, the amount of ζ phase scum increases, and as a result, the amount of Γ ₂ phase scum relatively decreases. Therefore, the generation of scum defects caused by Γ ₂ phase scum can be further effectively suppressed.

The hot-dip galvanizing treatment method of [7] is the hot-dip galvanizing treatment method described in [6],

In the aforementioned operating condition adjustment process,

When the Fe concentration in the hot-dip galvanizing bath is defined as Adjust the Fe concentration and Al concentration in the aforementioned hot-dip galvanizing bath.

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1066 (3)

In this case, the amount of ζ-phase dross further increases, and as a result, the amount of Γ _- phase dross relatively further decreases. Therefore, the generation of scum defects caused by Γ ₂ phase scum can be further effectively suppressed.

The hot-dip galvanizing method of [8] is the hot-dip galvanizing method described in any one of [1] to [7],

A sinking roller is disposed in the molten zinc pot storing the hot-dip galvanizing bath, and the sinking roller is used to contact the steel plate immersed in the hot-dip galvanizing bath and switch the traveling direction of the steel plate up and down,

In the aforementioned sample collection process,

The sample is collected from the depth range from the upper end to the lower end of the sinking roller in the hot-dip galvanizing bath in the molten zinc pot.

In this case, samples were taken from the same depth range as the sinking roller. Therefore, the correlation between the amount of ζ phase scum and scum defects can be further improved.

The manufacturing method of alloyed hot-dip galvanized steel sheet in [9] includes the following processes:

The hot-dip galvanizing treatment process includes subjecting the steel plate to the hot-dip galvanizing treatment method described in any one of [1] to [8] to form a hot-dip galvanized layer on the surface of the steel plate; and,

In the alloying treatment step, the steel plate having the hot-dip galvanized layer formed on the surface is subjected to alloying treatment to produce the alloyed hot-dip galvanized steel plate.

The hot-dip galvanizing treatment method of the present embodiment described above is applied to the manufacturing method of the alloyed hot-dip galvanized steel sheet of this embodiment. Therefore, it is possible to produce an alloyed hot-dip galvanized steel sheet in which dross defects are suppressed. Furthermore, even when hot-dip galvanizing and alloying are performed on high-tensile steel, alloying can be promoted.

The manufacturing method of hot-dip galvanized steel sheet in [10] includes the following hot-dip galvanizing process:

The hot-dip galvanizing treatment method described in any one of [1] to [8] is performed on a steel plate to form a hot-dip galvanized layer on the surface of the steel plate.

The hot-dip galvanizing treatment method of the present embodiment described above is applied to the manufacturing method of the hot-dip galvanized steel sheet of this embodiment. Therefore, a hot-dip galvanized steel sheet with suppressed dross defects can be produced.

Hereinafter, the hot-dip galvanizing method, the manufacturing method of the alloyed hot-dip galvanized steel sheet, and the manufacturing method of the hot-dip galvanized steel sheet according to this embodiment will be described with reference to the drawings. It should be noted that in this specification and the drawings, components having substantially the same function are designated by the same symbols, and descriptions thereof will not be repeated.

[About the composition of hot dip galvanizing production line equipment]

FIG. 1 is a functional block diagram illustrating an example of the overall configuration of hot-dip galvanizing line equipment for manufacturing alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets. Referring to FIG. 1 , the hot-dip galvanizing production line equipment 1 includes an annealing furnace 20 , a hot-dip galvanizing equipment 10 , and a flat rolling mill (skin pass machine) 30 .

The annealing furnace 20 includes one or more heating zones (not shown) and one or more cooling zones arranged downstream of the heating zone. In the annealing furnace 20, the steel plate is supplied to the heating zone of the annealing furnace 20, and the steel plate is annealed. The annealed steel plate is cooled in the cooling zone and transported to the hot dip galvanizing equipment 10. The hot-dip galvanizing equipment 10 is arranged downstream of the annealing furnace 20 . In the hot-dip galvanizing equipment 10, hot-dip galvanizing is performed on the steel sheet to produce an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. The skin pass rolling mill 30 is arranged downstream of the hot-dip galvanizing equipment 10 . In the skin pass rolling mill 30, the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet manufactured in the hot-dip galvanizing equipment 10 is lightly pressed as needed, and the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet is adjusted. Galvanized steel surface.

[About hot dip galvanizing equipment 10]

FIG. 2 is a side view of the hot-dip galvanizing equipment 10 in FIG. 1 . Referring to FIG. 2 , the hot-dip galvanizing equipment 10 includes a molten zinc pot 101 , a sinking roller 107 , a support roller 113 , a gas wiping device 109 and an alloying furnace 111 .

The interior of the annealing furnace 20 arranged upstream of the hot-dip galvanizing equipment 10 is isolated from the atmospheric atmosphere and maintained in a reducing atmosphere. As described above, the annealing furnace 20 heats the continuously conveyed steel plate S in the heating zone. Thereby, the surface of the steel plate S is activated, and the mechanical properties of the steel plate S are adjusted.

The downstream end of the annealing furnace 20 corresponding to the side away from the annealing furnace 20 has a space where the lower rotating roller 201 is arranged. The downstream end of the annealing furnace 20 is connected to the upstream end of the long mouth 202 . The downstream end of the spout 202 is immersed in the hot-dip galvanizing bath 103 . The inside of the long mouth 202 is isolated from the atmospheric atmosphere and maintained in a reducing atmosphere.

The steel plate S whose conveyance direction is changed downward by the lower rotating roller 201 passes through the long nozzle 202 and is continuously immersed in the hot-dip galvanizing bath 103 stored in the molten zinc pot 101 . A sinking roller 107 is arranged inside the molten zinc pot 101 . The sinking roller 107 has a rotation axis parallel to the width direction of the steel plate S. The width of the sinking roller 107 in the axial direction is larger than the width of the steel plate S. The sinking roller 107 contacts the steel plate S and switches the traveling direction of the steel plate S to above the hot-dip galvanizing equipment 10 .

The support roller 113 is arranged in the hot-dip galvanizing bath 103 and is arranged above the sinking roller 107 . The support roller 113 includes a pair of rollers. The pair of rollers of the support roller 113 has a rotation axis parallel to the width direction of the steel plate S. The support roller 113 supports the steel plate S conveyed upward by sandwiching the steel plate S whose traveling direction has been switched to the upward direction by the sinking roller 107 .

The gas wiping device 109 is arranged above the sinking roller 107 and the support roller 113 and is arranged above the liquid level of the hot-dip galvanizing bath 103 . The gas wiping device 109 includes a pair of gas injection devices. The pair of gas injection devices have gas injection nozzles facing each other. During the hot-dip galvanizing process, the steel sheet S passes between a pair of gas injection nozzles of the gas wiping device 109 . At this time, the pair of gas injection nozzles faces the surface of the steel plate S. The gas wiping device 109 blows gas onto both surfaces of the steel plate S pulled up from the hot-dip galvanizing bath 103, thereby scraping off part of the hot-dip galvanizing attached to both surfaces of the steel plate S, and adjusting the hot-dip galvanizing on the surface of the steel plate S. The amount of zinc attached.

The alloying furnace 111 is arranged above the gas wiping device 109 . The steel plate S conveyed upward by the gas wiping device 109 is allowed to pass through the inside of the alloying furnace 111, and the steel plate S is alloyed. The alloying furnace 111 includes a heating zone, a heat preservation zone, and a cooling zone in order from the entering side of the steel plate S to the leaving side. The heating zone is heated so that the temperature (plate temperature) of the steel plate S becomes substantially uniform. The heat preservation zone maintains the plate temperature of the steel plate S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed and becomes an alloyed hot-dip galvanized layer. The cooling zone cools the steel plate S formed with the alloyed hot-dip galvanized layer. As described above, the alloying furnace 111 uses the heating zone, the heat preservation zone, and the cooling zone to perform alloying processing. In addition, when manufacturing an alloyed hot-dip galvanized steel sheet, the alloying furnace 111 performs the above-mentioned alloying process. On the other hand, when manufacturing a hot-dip galvanized steel sheet, the alloying furnace 111 does not perform alloying treatment. In this case, the steel plate S passes through the alloying furnace 111 which is not in operation. Here, "not operating" means, for example, a state in which the power supply is stopped (a state in which the alloying furnace 111 is not started) while the alloying furnace 111 is placed online. The steel sheet S that has passed through the alloying furnace 111 is transported to the subsequent process by the upper rotating roller 115 .

In the case of manufacturing hot-dip galvanized steel sheets, the alloying furnace 111 may also be moved offline as shown in FIG. 3 . In this case, the steel sheet S is conveyed to the subsequent process by the top rotating roller 115 without passing through the alloying furnace 111 .

It should be noted that when the hot-dip galvanizing equipment 10 is dedicated to hot-dip galvanizing steel sheets, the hot-dip galvanizing equipment 10 does not need to include the alloying furnace 111 as shown in FIG. 4 .

[About other configuration examples of hot-dip galvanizing production line equipment]

The hot-dip galvanizing line equipment 1 is not limited to the structure of FIG. 1 . For example, when a Ni pre-plating treatment is performed on a steel plate before hot-dip galvanizing and a Ni layer is formed on the steel plate, as shown in FIG. Ni plating equipment 40. The Ni pre-plating equipment 40 includes a Ni plating tank storing a Ni plating bath. Ni plating treatment is performed by electroplating method. In addition, the hot-dip galvanizing line equipment 1 of FIG. 1 and FIG. 5 is equipped with the annealing furnace 20 and the flat rolling mill 30. However, the hot-dip galvanizing line equipment 1 does not need to include the annealing furnace 20 . In addition, the hot-dip galvanizing line equipment 1 does not need to include the skin pass rolling mill 30 . The hot-dip galvanizing production line equipment 1 only needs to have at least the hot-dip galvanizing equipment 10. The annealing furnace 20 and the flat rolling mill 30 can be configured as needed. In addition, the hot-dip galvanizing line equipment 1 may be provided with pickling equipment for pickling steel plates upstream of the hot-dip galvanizing equipment 10, or may be equipped with equipment other than the annealing furnace 20 and the pickling equipment. The hot-dip galvanizing line equipment 1 may further include equipment other than the skin pass rolling mill 30 downstream of the hot-dip galvanizing equipment 10 .

[About the generation mechanism of scum defects]

In the hot-dip galvanizing treatment process in the manufacturing process of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet using the hot-dip galvanizing line equipment 1 described above, the mechanism by which dross defects are generated is considered to be as follows.

In the hot-dip galvanizing treatment process, Fe is eluted 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 generate dross. The top slag among the generated scum floats on the liquid surface in the hot-dip galvanizing bath 103 . On the other hand, the bottom slag in the generated scum settles and is deposited on the bottom of the molten zinc pot 101 . 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), bottom slag is deposited at the bottom of the molten zinc pot 101 .

With the accompanying flow of the steel sheet S generated near the lower part of the sinking roller 107, the bottom slag deposited on the bottom of the molten zinc pot 101 is rolled up into the hot-dip galvanizing bath 103 and floats in the hot-dip galvanizing bath 103. . The bottom slag floating in the hot-dip galvanizing bath 103 adheres to the surface of the steel sheet S near the sinking roller 107 . The portion where the bottom slag adheres to the surface of the steel plate S may become a scum defect.

If a dross defect occurs, uneven parts of the coating will appear on the surface of the coating, and the appearance quality of the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet will deteriorate. Furthermore, local cells are easily formed in the scum defective portion of the steel plate surface, and the corrosion resistance of the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet is reduced.

As mentioned above, the main factor of scum defects is Γ ₂ phase scum rather than δ ₁ phase scum reported extensively in previous studies. Therefore, if the amount of Γ ₂ phase dross in the hot-dip galvanizing bath 103 is large, the possibility of generating dross defects in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet becomes high.

Furthermore, the ζ phase dross and the Γ ₂ phase dross undergo phase change with each other. That is, the ζ phase dross changes into Γ ₂ phase dross, and the Γ ₂ phase dross changes into ζ phase dross. Therefore, in the hot-dip galvanizing bath 103, the amount of ζ-phase dross and the amount of Γ _- phase dross have a negative correlation, which means that if the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is large, then The amount of Γ ₂ phase dross in the hot dip galvanizing bath 103 is relatively small. Furthermore, compared with other phases of dross, ζ phase dross is the softest and is less likely to cause dross defects. Therefore, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is determined, and the operating conditions are adjusted based on the determined amount of ζ-phase dross to increase the amount of ζ-phase dross, thereby reducing the amount of hot-dip galvanizing. The amount of Γ ₂ phase scum in bath 103. As a result, the occurrence of dross defects can be suppressed.

Therefore, in the hot-dip galvanizing treatment method of this embodiment, the amount of zeta phase dross in the dross in the hot-dip galvanizing bath 103 is determined. Then, based on the amount of ζ phase scum in the hot dip galvanizing bath 103, the operating conditions of the hot dip galvanizing treatment are adjusted. It is preferable that the operating conditions of the hot-dip galvanizing treatment are adjusted in a manner to increase the amount of zeta-phase dross based on the amount of zeta-phase dross in the hot-dip galvanizing bath 103 . 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 suppressed to a relatively low level. As a result, the generation 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 scum in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted in a manner to increase the ζ-phase scum, and the ζ-phase scum in the hot-dip galvanizing bath 103 is reduced. The amount remains above a specific amount (threshold).

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

[About the hot-dip galvanizing treatment method of this embodiment]

[About hot dip galvanizing equipment used]

The hot-dip galvanizing line equipment 1 is used in the hot-dip galvanizing treatment method of this embodiment. The hot-dip galvanizing line equipment 1 has the structure shown in FIG. 1 and FIG. 5, for example. However, the hot-dip galvanizing production line equipment 1 used in the hot-dip galvanizing treatment method of this embodiment may be the equipment shown in FIGS. 1 and 5 as mentioned above, or may be the equipment shown in FIGS. 1 and 5 Further equipment consisting of other components can be added. In addition, a known hot-dip galvanizing line equipment 1 having a different configuration from that shown in FIGS. 1 and 5 may be used.

[About steel sheets used in hot-dip galvanizing]

The steel type and dimensions (plate thickness, plate width, etc.) of the steel plate (base material steel plate) used in the hot-dip galvanizing treatment of this embodiment are not particularly limited. The steel plate can be suitable for alloyed hot-dip galvanized steel plate or hot-dip galvanized steel plate according to the various mechanical properties (such as tensile strength, processability, etc.) required by the alloyed hot-dip galvanized steel plate or hot-dip galvanized steel plate to be manufactured. A well-known steel plate. The steel plate used for automobile exterior panels can be used as the steel plate (base material steel plate) used for hot-dip galvanizing.

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 treatment of this embodiment may be a steel sheet composed of high-tensile steel containing a large amount of alloying elements such as Si and Mn. The steel plate used in the hot-dip galvanizing treatment of this embodiment may be a steel plate made of steel other than high-tensile steel.

The steel sheet (base material steel sheet) used in the hot-dip galvanizing treatment of this embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet. As the base material steel plate, for example, the following steel plates are used.

(a) Pickled hot-rolled steel plate

(b) After pickling treatment, Ni pre-plating treatment is performed to form a hot-rolled steel plate with a Ni layer on the surface.

(c) Annealed cold-rolled steel plate

(d) After annealing treatment, Ni pre-plating treatment is performed to form a cold-rolled steel plate with a Ni layer on the surface.

The above (a) to (d) are examples of steel sheets used in the hot-dip galvanizing treatment of this embodiment. The steel sheet used in the hot-dip galvanizing treatment 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 been treated other than the above (a) to (d) may be used as the steel sheet used in the hot-dip galvanizing treatment.

[About hot dip galvanizing bath]

The main component of the hot-dip galvanizing bath 103 is Zn. The hot-dip galvanizing bath 103 contains Al in addition to Zn. That is, the hot-dip galvanizing bath 103 used in the hot-dip galvanizing treatment method of this embodiment is a plating solution containing Al at a specific concentration, and the balance is Zn and impurities. If the hot-dip galvanizing bath 103 contains Al at a specific concentration, excess reaction between Fe and Zn in the bath can be suppressed, and uneven alloying reaction between the steel sheet immersed in the hot-dip galvanizing bath 103 and Zn can be suppressed.

The preferable Al concentration (more specifically, the Free-Al concentration) in the hot-dip galvanizing bath 103 is 0.100 to 0.159% in terms of mass %. Here, the Al concentration in the hot-dip galvanizing bath 103 refers to the Al concentration (mass %) dissolved in the hot-dip galvanizing bath 103, and is a so-called Free-Al concentration. If the Al concentration in the hot-dip galvanizing bath is within the range of 0.100 to 0.159% in terms of mass %, the occurrence of defects other than dross defects can be suppressed, and furthermore, in the manufacturing process of alloyed hot-dip galvanized steel sheets In the alloying process, the occurrence of unalloyed materials can be suppressed.

Thus, the hot-dip galvanizing bath 103 of this embodiment is a plating bath which contains Zn as a main component and also contains Al. The hot-dip galvanizing bath 103 may contain 0.020 to 0.100% by mass of Fe eluted from equipment and steel sheets in the bath. That is, the Fe concentration (mass %) dissolved in the hot-dip galvanizing bath 103 is, for example, 0.020 to 0.100 mass %. However, the concentration of Fe dissolved in the hot-dip galvanizing bath 103 is not limited to the above numerical range.

[Hot-dip galvanizing treatment method]

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

[Sample collection process (S1)]

In the sample collection step (S1), a part of the plating solution is collected from the hot-dip galvanizing bath 103 as a sample. In the sample collection step (S1), samples are collected over time. "Collecting samples over time" means collecting samples every time a specific time passes. The specific time (the period after collecting a sample until the next sample is collected) may be constant or may not be constant. For example, samples can be taken every 1 hour. Alternatively, the next sample may be collected after one hour has elapsed since the sample was collected, and the next sample may have been collected after a further 30 minutes. There is no particular time limit.

The amount of samples collected from the hot-dip galvanizing bath 103 is not particularly limited. In the subsequent ζ-phase dross amount determining step (S2), the amount of sample collection is not particularly limited as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be determined. The sample collection amount is, for example, 100 to 400 g. The collected sample can be brought into contact with a metal at room temperature with high thermal conductivity, and the sample can be quenched to room temperature for solidification. An example of a normal-temperature metal with high thermal conductivity is copper.

The sample collection position in the hot-dip galvanizing bath 103 is not particularly limited. For example, referring to FIGS. 2 to 4 , when the hot dip galvanizing bath 103 is divided into three parts D1 to D3 along the depth direction D, a sample can be collected from the uppermost area D1 in the hot dip galvanizing bath 103 , or Samples can be collected in the middle area D2, and samples can also be collected in the lowermost area D3. The amount of zeta phase scum in the samples collected in each area D1 to D3 is different. However, depending on the collection position, it can be judged to a certain extent whether the amount of zeta phase scum found is large. Therefore, the collection location of the sample is not particularly limited. As shown in FIGS. 2 to 4 , in the hot-dip galvanizing bath 103 , the direction parallel to the width direction of the steel sheet S is defined as the width direction W, and the depth direction of the hot-dip galvanizing bath 103 is defined as the depth direction D. The direction perpendicular to the width direction W and the depth direction D is defined as the length direction L. In this case, it is preferable to collect the sample over time from a specific area divided 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 any case, samples are collected from the same position (within a specific area) within the hot-dip galvanizing bath 103 over time.

It is preferable to collect the sample from the area near the sinking roller 107 as much as possible. Specifically, as shown in FIGS. 2 to 4 , in the hot-dip galvanizing bath 103 , samples are collected in a specific depth range D107 in the depth direction D from the upper end to the lower end of the sinking roller 107 . That is, the specific depth range is set to the depth range D107 from the upper end to the lower end of the sinking roller 107 . There is a high possibility that Γ _2- phase dross adheres to the surface of the steel plate S near the sinking roller 107 . Therefore, the amount of ζ-phase dross in the vicinity of the sinking roller 107 is the most effective index for suppressing dross defects. Therefore, it is preferred to collect samples from depth range D107. In this case, the ζ-phase dross amount is determined based on the sample collected from the range most likely to adhere to the surface of the steel sheet S. Therefore, the correlation between the ζ-phase dross amount and the dross defects can be further improved. Regarding the width direction W and the length direction L, it is also preferable to collect samples from the area near the sinking roller as much as possible. In addition, as mentioned above, the sample was collected from the same area|region in the hot-dip galvanizing bath 103 over time.

[ζ phase scum amount determination process (S2)]

In the ζ-phase dross amount determining step (S2), the collected sample is used to determine the ζ-phase dross amount in the hot-dip galvanizing bath 103. The method for determining the amount of zeta phase scum using a sample is not particularly limited, and various methods are considered.

For example, a test piece for observing zeta phase scum is prepared from the sample collected in the sample collection step (S1). As an example of a test piece for observing zeta phase scum, a rectangular parallelepiped (small plate shape) having a surface (test surface) capable of ensuring an observation field of 15 mm×15 mm and a thickness of 0.5 mm is formed. Using an optical microscope or a scanning electron microscope (SEM) with a predetermined magnification, the entire field of view is observed in the above-mentioned observation field of view (15 mm × 15 mm), and the scum in the entire field of view is identified. The dross can be specified based on the contrast in the field of view, and the top dross and the bottom dross can be distinguished based on the contrast.

FIG. 7 is an example of a photographic image of a part of the observation field of view of the sample collected in the sample collection step (S1). Referring to FIG. 7 , in the photographic image, the matrix phase 200 of hot-dip galvanizing, the top slag 100T, and the bottom slag 100B are observed. The brightness of the top slag 100T is lower (darker) than the brightness of the parent phase 200 and the bottom slag 100B. On the other hand, the brightness of the bottom slag 100B is lower (darker) than the brightness of the parent phase 200 and higher (brighter) than the brightness of the top slag 100T. As mentioned above, top slag and bottom slag can be distinguished based on contrast.

Composition analysis using EPMA was performed on each bottom scum in the specified scum in the above observation field of view (15 mm×15 mm), and the zeta phase scum was identified. It is also possible to specify the zeta phase dross in the observation field by further performing crystal structure analysis using TEM on each bottom dross. It should be noted that, instead of distinguishing the top dross and the bottom dross by contrast, the composition analysis of each dross using EPMA and/or the crystal structure analysis using TEM can be performed to specify the type of each dross in the field of view ( Top slag, Γ ₂ phase scum, δ ₁ phase scum and ζ phase scum).

Based on the specified ζ-phase dross, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is determined. The amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be determined by various indicators. For example, the number of ζ-phase scum per predetermined area can be regarded as the ζ-phase scum amount. Here, the predetermined area is not particularly limited. For example, it may be the area of the entire observation field of view, or it may be a unit area (mm ² ). For example, when the observation field of view is 15 mm × 15 mm, the number of zeta phase dross (pieces/225 mm ² ) in the observation field of view (15 mm × 15 mm = 225 mm ² ) can be regarded as the amount of zeta phase dross. In this case, the number of ζ-phase dross in the observation field of view is determined by the following method. First, the equivalent circle diameter (μm) of the specified ζ phase dross is determined. The diameter when converting the area of each ζ phase dross in the observation field of view into a circle is defined as the circle equivalent diameter (μm). Using the photographic image of the observation field described above, the equivalent circular diameter (μm) of the specified zeta phase dross was determined by known image processing. The number of ζ-phase dross having a circle equivalent diameter of 10 μm or more in the field of view is defined as the number of ζ-phase dross (pieces/225 mm ² ). In this way, the number of ζ-phase dross with a circle equivalent diameter of 10 μm or more in the observation field of view can be defined as the amount of ζ-phase dross. It should be noted that the observation field of view is not limited to the above-mentioned area (15 mm × 15 mm = 225 mm ² ). In addition, the upper limit of the circle equivalent diameter of ζ phase dross is not particularly limited. The upper limit of the circle equivalent diameter of ζ phase dross is, for example, 300 μm.

In addition, other indicators can be used as the amount of zeta phase scum in the hot-dip galvanizing solution. For example, the area of each bottom scum (each Γ ₂ -phase scum, each δ ₁ -phase scum, and each ζ-phase scum) in the observation field of view and the area of each ζ-phase scum are determined. Then, the ratio of the total area of ζ-phase dross to the total area of bottom dross can be used as the amount of ζ-phase dross. In addition, the ratio of the total area of ζ-phase scum to the observation field of view area can be regarded as the ζ-phase scum amount. In addition, the total area (μm ² ) of ζ-phase dross in the above visual field can be regarded as the amount of ζ-phase dross. In addition, X-ray diffraction measurement was performed on the test surface of the above-mentioned sample, and the peak intensity of each bottom scum (Γ ₂ -phase scum, δ _1- phase scum, and ζ-phase scum) was measured. Then, the ζ phase can be calculated with respect to the sum of the peak intensities of each bottom scum (that is, the sum of the peak intensity of the Γ ₂ phase scum, the peak intensity of the δ ₁ phase scum, and the peak intensity of the ζ phase scum). The ratio of the peak intensities of the scum was used as the amount of zeta phase scum. It should be noted that in X-ray diffraction measurement, it is not easy to clearly distinguish between Γ ₂ -phase scum and Γ ₁ -phase scum. However, as mentioned above, it is considered that Γ ₁ phase dross is substantially absent. Therefore, all the peak intensities obtained at the diffraction angle 2θ=43 to 44° are regarded as the peak intensities of the Γ ₂ phase dross. In addition, a Co dry sphere is used as a target in X-ray diffraction measurement, for example. The amount of zeta phase dross can also be determined by methods other than the above.

By the above method, the amount of ζ phase dross in the hot-dip galvanizing bath 103 is determined using the sample collected in the sample collection step (S1). In addition, the ζ phase scum amount determination process (S2) is preferably performed every time a sample is collected in the sample collection process (S1). By collecting samples over time and determining the amount of ζ-phase dross each time a sample is collected, it is possible to grasp changes over time in the amount of ζ-phase dross in the hot-dip galvanizing bath 103 . Therefore, based on the samples collected over time, the amount of zeta phase scum can be determined over time.

[Operating condition adjustment process (S3)]

After the ζ-phase dross amount in the hot-dip galvanizing bath 103 is determined in the ζ-phase dross amount determination process (S2), the operating condition adjustment process (S3) is implemented.

In the operating condition adjustment step (S3), the operating conditions of the hot-dip galvanizing treatment are adjusted based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103. Specifically, when the calculated amount of ζ-phase dross is small, the operating conditions are adjusted (changed) so that the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is increased. If the calculated amount of zeta phase scum is appropriate, the current operating conditions can be maintained. The method for 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 it can be adjusted in such a manner that the amount of zeta phase scum in the hot-dip galvanizing bath 103 can be increased.

As a method of adjusting operating conditions, it is preferable to implement at least one of the following (A) or (B).

(A) Adjust the bath temperature of the hot-dip galvanizing bath 103.

(B) Adjust the Al concentration of the hot-dip galvanizing bath 103.

Regarding the above (A), if the temperature of the hot-dip galvanizing bath 103 is raised, the possibility that the Γ _2- phase dross will transform into the ζ-phase dross becomes high. Therefore, if the temperature of the hot-dip galvanizing bath 103 is raised, the Γ ₂ -phase dross in the hot-dip galvanizing bath 103 decreases, and instead, the ζ-phase dross increases. As mentioned above, ζ phase scum is soft. Therefore, ζ phase scum is less likely to form scum defects. Therefore, when the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is too small, the bath temperature of the hot-dip galvanizing bath 103 can be increased. In this case, the hard Γ ₂ phase dross changes into soft ζ phase dross. As a result, the soft ζ phase scum increases and the hard Γ ₂ phase scum decreases. Therefore, the occurrence of scum defects is suppressed. It should be noted that increasing the bath temperature will increase the energy source unit. Therefore, when the amount of ζ phase scum is sufficient, there is no need to increase the bath temperature excessively. As described above, by adjusting the bath temperature of the hot-dip galvanizing bath 103, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted. Specifically, by raising the bath temperature of the hot-dip galvanizing bath 103, the amount of ζ-phase dross can be increased, and as a result, the amount of Γ _- phase dross in the hot-dip galvanizing bath 103 can be reduced.

Regarding the above (B), if the Al concentration in the hot-dip galvanizing bath 103 is reduced, the possibility that the Γ _2- phase dross will transform into the ζ-phase dross becomes higher. Therefore, when the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is too small, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted by adjusting the Al concentration in the hot-dip galvanizing bath 103 . Specifically, by reducing the Al content in the hot-dip galvanizing bath 103, the amount of ζ-phase dross can be increased. As a result, the amount of Γ _- phase dross in the hot-dip galvanizing bath 103 can be reduced.

Based on the amount of zeta phase scum obtained from the above operating conditions (A) and (B), only one operating condition may be adjusted, or both operating conditions (A) and (B) may be adjusted. For example, when the amount of ζ-phase dross is too small, the bath temperature of the hot-dip galvanizing bath 103 can be raised and the Al concentration of the hot-dip galvanizing bath 103 can be lowered. When the amount of ζ phase scum is appropriate, the current operating conditions of (A) and (B) can be maintained unchanged.

The threshold can be set based on whether the amount of ζ-phase scum obtained in the ζ-phase scum amount determination step (S2) is appropriate as a judgment index. In this case, the operating conditions can be adjusted based on whether the calculated amount of zeta phase scum is lower than the threshold value. Specifically, depending on whether the calculated amount of zeta phase scum is lower than the threshold value, the operating conditions may be changed or may be maintained without changing the operating conditions. For example, when the calculated amount of ζ-phase dross is lower than the threshold value, it is determined that the amount of ζ-phase dross is too small, and the operating conditions are changed so that the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is compared with the current time. Added ways to adjust operating conditions. It is preferable to change the operating conditions so that the amount of ζ-phase dross becomes equal to or more than the threshold value when the calculated amount of ζ-phase dross is lower than the threshold value. On the other hand, when the calculated amount of ζ-phase dross is equal to or greater than the threshold value, it is determined that the amount of ζ-phase dross in the hot-dip galvanizing bath 103 is sufficient, and the current operating conditions are maintained.

When the number of zeta phase scum per predetermined area, for example, the number of zeta phase scum in the above observation field of view is regarded as the amount of zeta phase scum, it is converted into the number per unit area (1cm ² ) The number corresponding to 5.0 pieces/cm ² is used as the threshold. For example, when the number of ζ-phase scum in the observation field of view (15 mm × 15 mm = 225 mm ² ) is regarded as the amount of ζ-phase scum, the threshold value is set to 11.25 pieces (5.0 pieces/cm ² × 225 mm ² ). In this case, the number of ζ-phase scum calculated in the ζ-phase scum amount determination step (S2) is greater than the threshold value (11.25 pieces/225mm ² ), that is, the minimum number when converted in unit area (1cm ² ) When the number is 5.0 pieces/cm ² , it is determined that the amount of ζ-phase dross is too small, and the operating conditions are adjusted so that the amount of ζ-phase dross in the hot-dip galvanizing bath 103 increases. Preferably, when the amount of ζ-phase dross calculated in the ζ-phase dross amount determination step (S2) exceeds the above-mentioned threshold (11.25 pieces/225 mm ² ), it is the calculated amount of ζ-phase dross in terms of unit area. When the number of zeta phase scum is less than 5.0 pieces/cm ² , the amount of zeta phase scum becomes more than the threshold (11.25 pieces/225mm ² ) (that is, when converted in unit area, it becomes 5.0 pieces/cm ² or more number) to adjust the operating conditions. For example, when the amount of ζ-phase scum calculated in the ζ-phase scum amount determination step (S2) is less than 5.0 pieces/cm ² in terms of unit area, perform the operation (A) or (B) above. At least one of the conditions increases the amount of zeta phase scum. For example, the bath temperature of the hot-dip galvanizing bath 103 is raised to increase the amount of zeta phase dross. In addition, for example, the Al content of the hot-dip galvanizing bath 103 is reduced to increase the amount of zeta phase dross. It should be noted that the larger the number of ζ-phase scum per predetermined area, the better, and the upper limit is not particularly limited.

Preferably, in the operating condition adjustment step (S3), the Fe concentration in the hot-dip galvanizing bath 103 is defined as X ( mass %) and the Al concentration in the hot-dip galvanizing bath 103 is defined as Y (mass %), the Fe concentration and Al in the hot-dip galvanizing bath 103 are adjusted to satisfy equations (1) and (2). concentration.

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1216 (2)

Here, the Al concentration refers to the Al concentration in the hot-dip galvanizing bath 103 excluding the Al content contained in the dross, and is a so-called Free-Al concentration (mass %). Similarly, the Fe concentration refers to the Fe concentration in the hot-dip galvanizing bath 103 excluding the Fe content contained in the dross.

Formula (1) represents the range of Al concentration Y (mass %) in the hot-dip galvanizing bath 103. The Al concentration Y in the hot-dip galvanizing bath 103 is related to the production amount of top slag, Γ ₂ -phase dross and ζ-phase dross. If the Al concentration Y is 0.139% or less, the top slag will easily transform into Γ ₂ phase dross and ζ phase dross. In this case, excessive generation of top slag can be suppressed. This can prevent top slag from being sandwiched between the sinking roller 107 and the steel plate to cause surface defects. Therefore, in order to suppress the occurrence of surface defects, the generation of top slag can be suppressed. In order to suppress surface defects, it is sufficient to keep the Al concentration in the hot-dip galvanizing bath 103 to 0.140% or less. However, in actual hot-dip galvanizing operations, there is a possibility of fluctuations of up to ±0.001% in Al concentration management. Therefore, in the formula (1), the upper limit of the Al concentration Y in the hot-dip galvanizing bath 103 is set to 0.139%.

In addition, from the viewpoint of suppressing the occurrence of surface defects, the lower limit of the Al concentration is not particularly limited. However, it is known that over-alloying can be suppressed during the alloying process by setting the Al concentration in the hot-dip galvanizing bath 103 to be constant or higher. In the formula (1), the lower limit value of the Al concentration (the lower limit value of the formula (1)) is set to 0.100%.

It should be noted that the lower limit of the Al concentration Y in the hot-dip galvanizing bath 103 may be 0.100%, 0.105%, or 0.110%. In addition, the upper limit of the Al concentration Y in the hot-dip galvanizing bath 103 may be 0.139%, 0.135%, 0.130%, or 0.125%.

Equation (2) corresponds to the boundary (phase change line) where the Γ ₂ phase dross changes into the ζ phase dross in the hot dip galvanizing bath 103 . If the Al concentration Y in the hot-dip galvanizing bath 103 is higher than the right side of the formula (2), the chemical composition of the hot-dip galvanizing bath 103 becomes a state in which Γ ₂ phase dross can exist more stably than ζ phase dross. In this case, assuming that the Al concentration Y in the hot-dip galvanizing bath 103 satisfies the formula (1), the ζ phase dross easily transforms into the Γ ₂ phase dross. Therefore, in the hot-dip galvanizing bath 103, Γ ₂ phase dross is easily generated.

On the other hand, if the Al concentration Y in the hot-dip galvanizing bath 103 is less than the right side of the equation (2), that is, if the Al concentration Y and the Fe concentration X satisfy the equation (2), then the hot-dip galvanizing bath 103 Assuming that the Al concentration Y in the bath 103 satisfies the formula (1), the chemical composition of the hot-dip galvanizing bath 103 is such that ζ phase dross can exist more stably than Γ ₂ phase dross. Therefore, the Γ _2- phase dross in the hot-dip galvanizing bath 103 is easily transformed into the ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the Γ ₂ phase dross is easily reduced.

Therefore, in the above hot-dip galvanizing treatment, if the Al concentration Y and Fe concentration X in the hot-dip galvanizing bath 103 are adjusted to satisfy equations (1) and (2), then , promote the generation of ζ phase scum, and reduce the amount of Γ ₂ phase scum that is negatively correlated with the amount of ζ phase scum.

More preferably, in the operating condition adjustment step (S3), the Fe concentration in the hot-dip galvanizing bath 103 is defined as (mass %), and when the Al concentration in the hot-dip galvanizing bath 103 is defined as Y (mass %), the Fe concentration and Al concentration.

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1066 (3)

Here, the Al concentration refers to the Al concentration in the hot-dip galvanizing bath 103 excluding the Al content contained in the dross, and refers to the so-called Free-Al concentration (mass %). Similarly, the Fe concentration refers to the Fe concentration in the hot-dip galvanizing bath 103 excluding the Fe content contained in the dross.

Formula (3) is a formula that specifies a region where the Al concentration is lower than the above-mentioned formula (2). The above formula (2) corresponds to the boundary (phase transition line) at which Γ ₂ -phase dross changes into ζ-phase dross in the hot-dip galvanizing bath 103 . Formula (3) is a region in which ζ-phase dross can exist more stably than the specific region in formula (2). Therefore, the Γ ₂ -phase dross in the hot-dip galvanizing bath 103 is further easily transformed into the ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the Γ ₂ phase dross is easily further reduced.

In addition, the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath can be determined by the following method. In the hot-dip galvanizing bath 103 of FIG. 2 , samples are collected from a specific depth range in the depth direction D. More specifically, in the hot-dip galvanizing bath 103 of FIG. 2 , from a specific area divided 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 (Hereinafter, referred to as the sample collection area). When samples are collected sequentially over time, the sample collection positions are set to the same position (within the same sample collection area). Cool the collected samples to room temperature. Using an ICP emission spectrometer, the Fe concentration (mass %) and Al concentration (mass %) in the cooled sample were measured. In addition, the balance other than Fe concentration and Al concentration can be regarded as Zn.

The Fe concentration obtained by the ICP emission spectrometer is a so-called Total-Fe concentration that includes not only the Fe concentration in the hot-dip galvanizing bath (Free-Fe concentration) but also the Fe concentration in the dross. Similarly, the Al concentration obtained by the ICP emission spectrometer is a so-called Total-Al concentration that includes not only the Al concentration in the hot-dip galvanizing bath (Free-Al concentration) but also the Al concentration in the dross. Therefore, the Free-Fe concentration and the Free-Al concentration were calculated using the obtained Total-Fe concentration and Total-Al concentration and the known Zn-Fe-Al ternary system state diagram. Specifically, the state diagram of the Zn-Fe-Al ternary system at the bath temperature when collecting the sample is prepared. As mentioned above, the Zn-Fe-Al ternary system state diagram is well-known and is also disclosed in Figures 2 and 3 of Non-Patent Document 1. In addition, Non-patent Document 1 is a well-known paper among researchers and developers of hot-dip galvanizing baths. On the Zn-Fe-Al ternary system state diagram, points specified by the Total-Fe concentration and the Total-Al concentration obtained by the ICP emission spectrum analyzer are plotted. Then, a connecting line (conjugate line) is drawn from the plotted points to the liquidus line in the Zn-Fe-Al ternary system state diagram. The Fe concentration at the intersection of the liquidus line and the connecting line is equivalent to the Free-Fe concentration, and the Al concentration at the intersection of the liquidus line and the connecting line is equivalent to the Free-Al concentration. By the above method, the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath can be determined.

[About more preferable bath temperature of hot-dip galvanizing bath]

In addition, the temperature (bath temperature) of the hot-dip galvanizing bath 103 in the said hot-dip galvanizing processing method is preferably 440-500 degreeC. The dross in the hot-dip galvanizing bath 103 mainly transforms into top slag (Fe ₂ Al ₅ Zn _x ) and Γ 2-phase dross depending on the temperature of the hot-dip galvanizing bath 103 and the Al concentration in the hot-dip galvanizing bath ₁₀₃ , δ ₁ phase and ζ phase scum. Γ ₂ phase scum is easily generated in areas with low bath temperatures. ζ phase scum is easily generated in a region where the bath temperature is higher than the region where Γ ₂ phase scum is generated.

In addition, if the bath temperature of the hot-dip galvanizing bath 103 is 500° C. or lower, Zn can be suppressed from evaporating into smoke. When smoke is generated, the smoke adheres to the steel plate and easily causes surface defects (smoke defects). A preferable lower limit of the hot-dip galvanizing bath 103 is 460°C, more preferably 465°C, and still more preferably 469°C. A preferable upper limit of the hot-dip galvanizing bath 103 is 490°C, more preferably 480°C, and still more preferably 475°C. It should be noted that top slag is easily generated in a region where the Al concentration is higher than the region where Γ ₂ phase scum is generated and the region where ζ phase scum is generated.

As described above, in the hot-dip galvanizing treatment method of this embodiment, a sample is collected from the hot-dip galvanizing bath 103 (sample collection step (S1)), and the amount of ζ-phase dross (ζ) in the hot-dip galvanizing bath 103 is determined. Phase scum amount determination step (S2)). Then, the operating conditions of the hot-dip galvanizing treatment are adjusted based on the amount of ζ-phase dross in the hot-dip galvanizing bath 103 (operating condition adjustment step (S3)). By managing the amount of ζ phase scum that is negatively correlated with the amount of Γ ₂ phase scum, operating conditions can be adjusted so as to suppress the occurrence of scum defects.

[Manufacturing method of alloyed hot-dip galvanized steel sheet]

The hot-dip galvanizing treatment method of this embodiment described above can be applied to the manufacturing method of alloyed hot-dip galvanized steel sheet (GA).

The manufacturing method of the alloyed hot-dip galvanized steel sheet according to this embodiment includes a hot-dip galvanizing process and an alloying process. In the hot-dip galvanizing treatment process, the above-mentioned hot-dip galvanizing treatment method is performed on the steel plate to form a hot-dip galvanized layer on the surface of the steel plate. On the other hand, in the alloying treatment process, the steel sheet having the hot-dip galvanized layer formed on the surface by the hot-dip galvanizing process is alloyed using the alloying furnace 111 shown in FIG. 2 . As the alloying treatment method, a known method may be applied.

Through the above manufacturing process, the alloyed hot-dip galvanized steel sheet can be manufactured. In the alloyed hot-dip galvanized steel sheet of this embodiment, the hot-dip galvanizing treatment method of this embodiment described above is used. That is, based on the amount of ζ-phase scum, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the amount of ζ-phase scum. Therefore, the Γ ₂ phase dross in the hot-dip galvanizing bath 103 is relatively reduced, and as a result, the occurrence of dross defects in the manufactured alloyed hot-dip galvanized steel sheet can be suppressed.

In addition, the manufacturing method of the alloyed hot-dip galvanized steel sheet of this embodiment may also include other manufacturing processes other than the hot-dip galvanizing process and the alloying process. For example, the manufacturing method of the alloyed hot-dip galvanized steel sheet according to this embodiment may include a skin-pass rolling process using the skin-pass rolling mill 30 shown in FIG. 1 after the alloying treatment process. In this case, the surface appearance quality of the alloyed hot-dip galvanized steel sheet can be further improved. In addition, other manufacturing processes other than the skin pass rolling process may be included.

[Manufacturing method of hot-dip galvanized steel sheet]

The hot-dip galvanizing treatment method of the present embodiment described above can also be applied to a method of manufacturing a hot-dip galvanized steel sheet (GI).

The method of manufacturing a hot-dip galvanized steel sheet according to this embodiment includes a hot-dip galvanizing process. In the hot-dip galvanizing treatment process, the above-mentioned hot-dip galvanizing treatment method is performed on the steel plate to form a hot-dip galvanized layer on the surface of the steel plate. In the manufacturing method of the hot-dip galvanized steel sheet of this embodiment, the hot-dip galvanizing processing method of this embodiment mentioned above is used. That is, based on the amount of ζ-phase dross, the operating conditions of the hot-dip galvanizing treatment are adjusted to increase the ζ-phase dross. Therefore, the occurrence of dross defects in the manufactured hot-dip galvanized steel sheet can be suppressed.

In addition, the manufacturing method of the hot-dip galvanized steel sheet of this embodiment may include other manufacturing processes other than the hot-dip galvanizing process. For example, the method of manufacturing a hot-dip galvanized steel sheet according to this embodiment may include a skin-pass rolling process using the skin-pass rolling mill 30 shown in FIG. 1 after the hot-dip galvanizing process. In this case, the surface appearance quality of the hot-dip galvanized steel sheet can be further improved. In addition, other manufacturing processes other than the skin pass rolling process may be included.

Example

Hereinafter, the effect of one aspect of the hot-dip galvanizing treatment method of this embodiment will be described in more detail based on examples. The conditions in the examples are an 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 condition example.

In the above-mentioned operating condition adjustment process, the relationship between the Fe concentration X and the Al concentration Y was investigated.

Specifically, the hot-dip galvanizing treatment method was performed using a hot-dip galvanizing equipment having the same configuration as in FIG. 2 . Specifically, the Fe concentration X (mass %) and the Al concentration Y (mass %) of the hot-dip galvanizing bath were adjusted as described in Table 1. As the steel plate, high tensile steel containing C: 0.003%, Si: 0.006%, Mn: 0.6%, P: 0.02%, S: 0.01% and the balance being Fe and impurities was used. This high-tensile steel is a so-called difficult-to-alloy material that is relatively difficult to alloy when manufacturing alloyed hot-dip galvanized steel sheets. The hot-dip galvanized steel sheet is subjected to alloying treatment using an alloying furnace to produce an alloyed hot-dip galvanized steel sheet. The heating temperature in the alloying treatment was constant (510° C.) for each test number.

For each test number, in the hot-dip galvanizing bath 103 of FIG. 2 , a sample was collected in a specific depth range D107 in the depth direction D from the upper end to the lower end of the sinking roller 107 . More specifically, in the hot-dip galvanizing bath 103 of FIG. 2 , from 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, Samples are collected within the area (hereinafter referred to as the sample collection area). In any test number, approximately 400g of sample is collected from the same sample collection area mentioned above. Cool the collected samples to room temperature. Using the cooled samples, the chemical composition of the hot-dip galvanizing bath for each test number was determined using an ICP emission spectrometer. The Fe concentration (mass %) and Al concentration (mass %) obtained by measurement are Total-Fe concentration (mass %) and Total-Al concentration (mass %). Therefore, using the obtained Total-Fe concentration and Total-Al concentration and the known Zn-Fe-Al ternary system state diagram, the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the hot-dip galvanizing bath were calculated. Al concentration in (Free-Al concentration). Specifically, the state diagram of the Zn-Fe-Al ternary system at the bath temperature when collecting the sample is prepared. On the well-known Zn-Fe-Al ternary system state diagram, points specified by the Total-Fe concentration and the Total-Al concentration obtained by an ICP emission spectrometer are plotted. A connecting line (conjugate line) is drawn from the plotted points to the liquidus line in the Zn-Fe-Al ternary system state diagram, and the intersection point of the liquidus line and the connecting line is found. The Fe concentration at the intersection is defined as Free-Fe concentration (mass %), and the Al concentration at the intersection is defined as Free-Al concentration (mass %). By the above method, the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath and the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath were determined. As a result, the Fe concentration in the hot-dip galvanizing bath was in the range of 0.02 to 0.05 mass % in any test number.

[Table 1]

Table 1

In each test number, the Fe concentration X (mass %) of the hot-dip galvanizing bath became constant among the values shown in Table 1, and the Al concentration Y (mass %) of the hot-dip galvanizing bath became the values shown in Table 1. In terms of concentration, Al is appropriately added and adjusted over time. In addition, the conveyance speed of the steel plate during the hot-dip galvanizing process was set to be constant in any test number.

In addition, Table 1 also describes the values on the right side of Formula (2) and Formula (3). In addition, it is described whether the Fe concentration X (mass %) and the Al concentration Y (mass %) in the hot-dip galvanizing bath satisfy equations (1) to (3). For example, when a white circle mark (○) is written in the column of formula (2), it means that the Fe concentration X (mass %) and the Al concentration Y (mass %) in the hot-dip galvanizing bath satisfy formula (2). When a cross mark (×) is written in the column of formula (2), it means that the Fe concentration X (mass %) and the Al concentration Y (mass %) in the hot-dip galvanizing bath do not satisfy formula (2).

For each test number, samples were taken from the hot-dip galvanizing bath under the operating conditions shown in Table 1. Specifically, about 400 g of sample was collected from the above-mentioned sample collection area. A test piece for observing zeta phase scum was prepared from the collected sample. The test surface of the test piece for zeta phase scum observation was 1 cm×1 cm, and the thickness was 0.5 mm. Using a 100-magnification SEM, the entire visual field of the test surface (1 cm × 1 cm) was observed, and dross (top dross, bottom dross) was identified based on contrast. Furthermore, composition analysis using EPMA was performed, and the bottom slag was classified into Γ _2- phase dross, δ _1- phase dross, and ζ-phase dross. Furthermore, the equivalent circle diameter of each specified bottom slag (Γ _2- phase dross, δ ₁ -phase dross, and ζ-phase dross) was determined. Find the number of zeta phase scum having a circular equivalent diameter of 10 μm or more among the zeta phase scum in the above 1 cm×1 cm field of view. The number of ζ-phase dross having a circular equivalent diameter of 10 μm or more (pieces/1cm ² ) in the observation field of view was defined as the amount of zeta-phase dross. The amount of zeta phase scum obtained is shown in Table 1. It should be noted that in this example, no Γ ₁ phase dross was observed in any test number.

[Scum defect evaluation test]

After hot-dip galvanizing was performed under the operating conditions of each test number, alloying treatment was performed under the same conditions in each test number to produce an alloyed hot-dip galvanized steel sheet. The surface of the manufactured alloyed hot-dip galvanized steel sheet was visually observed to investigate the presence or absence of dross defects, and the dross defects were evaluated. The criteria for scum defect evaluation are as follows.

A: There is no scum defect (the number of scum defects is 0/m ² )

B: The number of scum defects exceeds 0 and is less than 0.1/ ^m2

C: The number of scum defects exceeds 0.1/ ^m2 and is less than 1/ ^m2

[Alloying evaluation test of difficult-to-alloy materials]

The chemical composition of the alloyed hot-dip galvanized layer on the surface of the alloyed hot-dip galvanized steel sheet produced 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 fluorescence X-ray analyzer (EDX-7000) manufactured by Shimadzu Corporation. The Fe content (mass %) contained in the alloyed hot-dip galvanizing layer was divided by the Zn content (mass %) contained in the alloyed hot-dip galvanizing layer. The alloying was evaluated. The criteria for alloying evaluation are as follows. In addition, when the ratio of Fe content to Zn content is 11% or more, it is judged that it is overalloyed.

A: The ratio of Fe content to Zn content is 10% or more and less than 11%

B: The ratio of Fe content to Zn content exceeds 9% and is less than 10%

C: The ratio of Fe content to Zn content is less than 9%

[Evaluation results]

Referring to Table 1, in the test numbers 1, 2, 5, 6, 8 to 13 in which the amount of ζ-phase scum was controlled to be 5.0 pieces/cm ² or more, the scum defect evaluation was A or B, and the scum can be suppressed more effectively. defect. In test numbers 1, 2, 5, 6, and 8 to 13, the alloying evaluation of the hard-to-alloy material is A or B. Even if the hard-to-alloy material is a hard-to-alloy material, alloying can be promoted more effectively. On the other hand, in test numbers 3, 4, and 7 in which the amount of ζ-phase dross is less than 5.0 pieces/ ^cm2 , the dross defect evaluation and the alloying evaluation of the difficult-to-alloy material were C. Furthermore, referring to test numbers 1 to 13, the larger the amount of zeta phase scum, the better the scum defect evaluation becomes. That is, the amount of ζ phase scum and the number of scum defects show a negative correlation.

From the above results, it is known that scum defects can be suppressed by adjusting operating conditions based on the amount of ζ-phase scum. Furthermore, it was found that it is preferable to set the threshold value of the ζ-phase dross amount to 5.0 pieces/cm ² and to adjust the operating conditions in the hot-dip galvanizing treatment so that the amount of zeta-phase dross becomes 5.0 pieces/cm ² or more. Dross defects can be significantly suppressed.

Among test numbers 1, 2, 5, 6, 8 to 13 that satisfy equations (1) and (2), the scum defect evaluation is A or B, and the scum defect can be suppressed more effectively. In test numbers 1, 2, 5, 6, and 8 to 13, the alloying evaluation of the hard-to-alloy material is A or B. Even if the hard-to-alloy material is a hard-to-alloy material, alloying can be promoted more effectively. Therefore, it can be seen that adjusting the operating conditions in such a manner as to satisfy equations (1) and (2) is effective in suppressing dross defects and promoting alloying of difficult-to-alloy materials.

In the test numbers 1, 5, 8, 9, 11, and 12 that satisfy equations (1) and (3), the scum defect evaluation is A, and the scum defect can be further effectively suppressed. In test numbers 1, 5, 8, 9, 11, and 12, the alloying evaluation of the difficult-to-alloy material was A, and even if it is a difficult-to-alloy material, alloying can be further effectively promoted. Therefore, it can be seen that in the adjustment of operating conditions, adjustment in a manner that satisfies equations (1) and (3) is more effective in suppressing dross defects and promoting alloying of difficult-to-alloy materials.

In addition, in the test numbers 14 and 16 in which the Al concentration Y in the hot-dip galvanizing bath was 0.0990 mass %, the dross defect evaluation was "A". Furthermore, although alloying can also be promoted for materials that are difficult to alloy, However, over-alloying occurs during the manufacturing of alloyed hot-dip galvanized steel sheets. Therefore, it is shown that the Al concentration Y in the hot-dip galvanizing bath more preferably satisfies the formula (1).

In test numbers 15 and 17 in which the Al concentration Y in the hot-dip galvanizing bath was 0.1410% by mass, the alloying evaluation of the difficult-to-alloy material was "C". Therefore, it is shown that the Al concentration Y in the hot-dip galvanizing bath more preferably satisfies the formula (1).

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the gist of the invention.

Explanation of reference signs

10 Hot dip galvanizing equipment

101 Molten zinc pot

103 Hot dip galvanizing bath

107 sinking roller

109 Gas wiping device

111 alloying furnace

202 long mouth

Claims (17)

1. A hot dip galvanization treatment method for manufacturing a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet, the hot dip galvanization treatment method comprising the steps of:

a sample collection step of collecting a sample from a hot dip galvanization bath containing Al;

a zeta-phase dross amount determining step of determining the zeta-phase dross amount in the hot dip galvanization bath using the collected sample; and, a step of, in the first embodiment,

and an operation condition adjustment step of adjusting the operation condition of the hot dip galvanization so as to increase the zeta-phase dross, based on the zeta-phase dross amount obtained.

2. The hot dip galvanization treatment method according to claim 1, wherein,

in the zeta-phase dross amount determining step,

using the collected sample, the number of zeta-phase dross per predetermined area is determined as the zeta-phase dross amount.

3. The hot dip galvanization treatment method according to claim 1, wherein,

in the operation condition adjustment process step,

based on the determined zeta-phase slag amount, at least one of (A) or (B) is performed to increase the zeta-phase slag amount,

(A) Adjusting the bath temperature of the hot dip galvanization bath,

(B) Adjusting the Al concentration of the hot dip galvanization bath.

4. The hot dip galvanization treatment method according to claim 2, wherein,

in the operation condition adjustment process step,

based on the determined zeta-phase slag amount, at least one of (A) or (B) is performed to increase the zeta-phase slag amount,

(A) Adjusting the bath temperature of the hot dip galvanization bath,

(B) Adjusting the Al concentration of the hot dip galvanization bath.

5. The hot dip galvanization treatment method according to claim 1, wherein,

in the operation condition adjustment process step,

and adjusting the operating conditions of the hot dip galvanizing treatment to increase the zeta-phase dross amount when the zeta-phase dross amount is determined to be lower than a threshold value.

6. The hot dip galvanization treatment method according to claim 2, wherein,

in the operation condition adjustment process step,

and adjusting the operating conditions of the hot dip galvanizing treatment to increase the zeta-phase dross amount when the zeta-phase dross amount is determined to be lower than a threshold value.

7. A hot dip galvanizing process as claimed in claim 3, wherein,

in the operation condition adjustment process step,

and adjusting the operating conditions of the hot dip galvanizing treatment to increase the zeta-phase dross amount when the zeta-phase dross amount is determined to be lower than a threshold value.

8. The hot dip galvanizing process according to claim 4, wherein,

in the operation condition adjustment process step,

and adjusting the operating conditions of the hot dip galvanizing treatment to increase the zeta-phase dross amount when the zeta-phase dross amount is determined to be lower than a threshold value.

9. The hot dip galvanizing process according to claim 5, wherein,

in the zeta-phase dross amount determining step,

using the collected sample, obtaining the number of zeta-phase scum per prescribed area as the zeta-phase scum amount,

in the operation condition adjustment process step,

the determined zeta-phase dross amount was calculated as a unit area (1 cm ² ) Is less than 5.0 pieces/cm in conversion ² In the case of the number of (3), the zeta-phase slag amount is increased by adjusting the operating conditions of the hot dip galvanization treatment.

10. The hot dip galvanizing process according to claim 6, wherein,

in the zeta-phase dross amount determining step,

using the collected sample, obtaining the number of zeta-phase scum per prescribed area as the zeta-phase scum amount,

in the operation condition adjustment process step,

the determined zeta-phase dross amount was calculated as a unit area (1 cm ² ) Is less than 5.0 pieces/cm in conversion ² In the case of the number of (3), the zeta-phase slag amount is increased by adjusting the operating conditions of the hot dip galvanization treatment.

11. The hot dip galvanizing process according to claim 7, wherein,

in the zeta-phase dross amount determining step,

using the collected sample, obtaining the number of zeta-phase scum per prescribed area as the zeta-phase scum amount,

in the operation condition adjustment process step,

the determined zeta-phase dross amount was calculated as a unit area (1 cm ² ) Is less than 5.0 pieces/cm in conversion ² In the case of the number of (3), the zeta-phase slag amount is increased by adjusting the operating conditions of the hot dip galvanization treatment.

12. The hot dip galvanizing process according to claim 8, wherein,

in the zeta-phase dross amount determining step,

using the collected sample, obtaining the number of zeta-phase scum per prescribed area as the zeta-phase scum amount,

in the operation condition adjustment process step,

the determined zeta-phase dross amount was calculated as a unit area (1 cm ² ) Is less than 5.0 pieces/cm in conversion ² In the case of the number of (3), the zeta-phase slag amount is increased by adjusting the operating conditions of the hot dip galvanization treatment.

13. The hot dip galvanization treatment method according to any one of claims 1 to 12, wherein,

in the operation condition adjustment process 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 the Al concentration in the hot-dip galvanizing bath are adjusted so as to satisfy the formulas (1) and (2),

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1216 (2)。

14. The hot dip galvanization treatment method according to claim 13, wherein,

in the operation condition adjustment process 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 the Al concentration in the hot-dip galvanizing bath are adjusted so as to satisfy the formulas (1) and (3),

0.100≤Y≤0.139 (1)

Y≤0.2945X+0.1066 (3)。

15. the hot dip galvanization treatment method according to claim 1, wherein,

a sink roll is disposed in the molten zinc pot storing the hot dip galvanizing bath, the sink roll is used for contacting with the steel plate immersed in the hot dip galvanizing bath and switching the travelling direction of the steel plate up and down,

in the step of collecting the sample in the above-mentioned sample collecting step,

the sample is collected from a depth range from an upper end to a lower end of the sink roll in the hot dip galvanization bath within the molten zinc pot.

16. A method for producing an alloyed hot-dip galvanized steel sheet, comprising the steps of:

a hot dip galvanization treatment step of applying the hot dip galvanization treatment method according to any one of claims 1 to 15 to a steel sheet, and forming a hot dip galvanization layer on the surface of the steel sheet; and, a step of, in the first embodiment,

an alloying treatment step of performing an alloying treatment on the steel sheet having the hot-dip galvanized layer formed on the surface to produce the alloyed hot-dip galvanized steel sheet.

17. A method for producing a hot-dip galvanized steel sheet, comprising the following hot-dip galvanization treatment steps: the method of hot dip galvanizing the steel sheet according to any one of claims 1 to 15, wherein a hot dip galvanized layer is formed on the surface of the steel sheet.