JP7136349B2 - Hot-dip galvanizing method, method for producing alloyed hot-dip galvanized steel sheet using the hot-dip galvanizing method, and method for producing hot-dip galvanized steel sheet using the hot-dip galvanizing method - Google Patents

Description

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

Hot-dip galvanized steel sheets (hereinafter also referred to as GI) and alloyed hot-dip galvanized steel sheets (hereinafter also referred to as GA) are manufactured by the following manufacturing processes. First, a steel plate (base steel plate) to be hot-dip galvanized is prepared. The base material steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet. When the base material steel plate is a hot-rolled steel plate, for example, a pickled hot-rolled steel plate is prepared. The pickled hot-rolled steel sheet may be subjected to Ni pre-plating treatment as necessary to prepare a hot-rolled steel sheet having a Ni layer formed on the surface. A hot-rolled steel sheet that has been subjected to other treatments than those described 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. The annealed cold-rolled steel sheet may be optionally subjected to Ni pre-plating to prepare a cold-rolled steel sheet having a Ni layer formed on the surface. A cold-rolled steel sheet that has been subjected to other treatments than those described above may be prepared. The prepared base material steel sheet (the hot-rolled steel sheet or cold-rolled steel sheet described above) is immersed in a hot-dip galvanizing bath to carry out hot-dip galvanizing treatment to produce a hot-dip galvanized steel sheet. When manufacturing a hot-dip galvannealed steel sheet, the hot-dip galvanized steel sheet is further heat-treated in an alloying furnace to manufacture the hot-dip galvannealed steel sheet.

The details of the hot-dip galvanizing treatment in the manufacturing process of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are as follows. Hot-dip galvanizing equipment used for hot-dip galvanizing includes a hot-dip galvanizing pot containing a hot-dip galvanizing bath, a sink roll arranged in the hot-dip galvanizing bath, and a gas wiping device.

In the hot dip galvanizing step, the steel sheet (base material steel sheet) is immersed in a hot dip galvanizing bath. Then, the direction of movement of the steel sheet is changed upward by a sink roll arranged in the hot dip galvanizing bath, and the steel sheet is pulled up from the hot dip galvanizing bath. A wiping gas is blown from a gas wiping device to the surface of the steel sheet that is pulled up and advanced upward to scrape off surplus molten zinc and adjust the coating amount on the surface of the steel sheet. The hot-dip galvanizing process is performed by the above method. In the case of manufacturing a hot-dip galvannealed steel sheet, the steel sheet with the coating weight adjusted is further charged into an alloying furnace and alloyed.

In the hot-dip galvanizing treatment described above, Fe elutes into the hot-dip galvanizing bath from the steel sheet immersed in 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 produced. There are top dross and bottom dross. The top dross is an intermetallic compound having a lighter specific gravity than the hot-dip galvanizing bath, and is dross that floats on the surface of the hot-dip galvanizing bath. Bottom dross is an intermetallic compound having a higher specific gravity than the hot dip galvanizing bath, and is dross deposited on the bottom of the hot dip galvanizing pot. Among these dross, the bottom dross in particular is swirled up from the bottom of the hot-dip galvanizing pot during the hot-dip galvanizing process by the accompanying flow generated by the advancement of the steel sheet in the hot-dip galvanizing bath, and the hot-dip dross is Float in the plating bath. Such floating bottom dross may adhere to the surface of the steel sheet being hot-dip galvanized. The bottom dross adhering to the surface of the steel sheet may become point-like defects on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. Surface defects caused by such bottom dross are referred to as "dross defects" in this specification. Dross defects deteriorate the appearance and corrosion resistance of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets. Therefore, it is preferable to be able to suppress the occurrence of dross defects.

Techniques for suppressing the occurrence of dross defects have been proposed in Japanese Patent Application Laid-Open Nos. 11-350096 (Patent Document 1) and 11-350097 (Patent Document 2).

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

In Patent Document 2, in the method for manufacturing a galvannealed steel sheet, the Al concentration in the bath is kept within the range of 0.15±0.01 wt %. Specifically, Patent Document 2 describes as follows. When the Al concentration in the bath is 0.15 wt % or more, the dross becomes the Fe—Al phase, and when the Al concentration in the bath is 0.15% or less, the dross becomes the delta ₁ phase (δ ₁ phase). If the dross repeats the phase transformation between the Fe—Al phase and the δ1 _phase , the dross becomes finer. Therefore, Patent Document 2 states that by maintaining the Al concentration in the bath within the range of 0.15 ± 0.01 wt%, the dross can be made finer, and as a result, the occurrence of dross defects can be suppressed. there is

JP-A-11-350096 JP-A-11-350097

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

There are four types of dross that can occur in hot-dip galvanizing: _{Fe2Al5Znx (} so-called top dross), δ1 phase, _{gamma1 phase} ( _{Γ1 phase} ), and zeta phase (ζ phase). has been reported in previous studies. For example, in Patent Document 2, by 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, it is proposed to refine the δ ₁ phase, which is the main cause of dross defects. is suggesting.

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

Furthermore, in recent years, there has been an increasing demand for alloying hot-dip galvanizing treatment for steels containing a large amount of alloying elements, such as high-strength steels. It is known that high-strength steel containing a large amount of alloying elements is difficult to be alloyed in an alloying treatment after hot-dip galvanizing treatment. Therefore, a steel plate made of high-strength steel is sometimes referred to as a hard-to-alloy material. There is a demand for a hot-dip galvanizing method that facilitates alloying even for materials that are difficult to alloy. Moreover, even if the material is not hard to be alloyed, a hot-dip galvanizing treatment method that accelerates the alloying treatment is preferable when producing an alloyed hot-dip galvanized steel sheet.

The purpose of the present disclosure is to provide a hot dip galvanizing method capable of suppressing the occurrence of dross defects and promoting alloying, a method of manufacturing an alloyed hot dip galvanized steel sheet using the hot dip galvanizing method, Another object of the present invention is to provide a method for manufacturing a hot-dip galvanized steel sheet using the hot-dip galvanizing treatment method.

A hot-dip galvanizing method according to the present disclosure is a hot-dip galvanizing method used for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet,
A sample collecting step of collecting a sample from a hot dip galvanizing bath containing Al;
A ζ-phase dross amount determination step of determining the ζ-phase dross amount in the hot-dip galvanizing bath using the collected sample;
an operating condition adjusting step of adjusting operating conditions for the hot-dip galvanizing treatment based on the determined ζ-phase dross amount.

A method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present disclosure includes:
A hot-dip galvanizing step of forming a hot-dip galvanized layer on the surface of the steel sheet by performing the above-described hot-dip galvanizing treatment method on the steel sheet;
and an alloying treatment step of performing an alloying treatment on the steel sheet having the hot-dip galvanized layer formed on the surface to manufacture an alloyed hot-dip galvanized steel sheet.

A method for manufacturing a hot-dip galvanized steel sheet according to the present disclosure includes:
A hot-dip galvanizing process is provided for forming a hot-dip galvanized layer on the surface of the steel sheet by performing the above-described hot-dip galvanizing treatment method on the steel sheet.

The hot-dip galvanizing treatment method according to the present disclosure can suppress the occurrence of dross defects, and can promote alloying even when hot-dip galvanizing treatment and alloying treatment are performed on a high-strength steel plate. In addition, the method for manufacturing a hot-dip galvannealed steel sheet according to the present disclosure can manufacture a hot-dip galvannealed steel sheet in which the occurrence of dross defects is suppressed. Alloying can be promoted even when hardening treatment is performed. The method for manufacturing a hot-dip galvanized steel sheet according to the present disclosure can manufacture a hot-dip galvanized steel sheet in which the occurrence of dross defects is suppressed.

FIG. 1 is a functional block diagram showing the overall configuration of a hot-dip galvanized steel sheet and hot-dip galvanized line equipment used to manufacture the hot-dip galvanized steel sheet. 2 is a side view of the hot-dip galvanizing equipment in FIG. 1. FIG. FIG. 3 is a side view of hot-dip galvanizing equipment having a configuration different from that of FIG. FIG. 4 is a side view of a hot-dip galvanizing facility having a configuration different from that of FIGS. 2 and 3. FIG. FIG. 5 is a functional block diagram showing the overall configuration of hot-dip galvanizing line equipment having a configuration different from that of FIG. FIG. 6 is a flowchart showing the steps of the hot-dip galvanizing method of this embodiment. FIG. 7 is a diagram showing an example of a photographic image of a part of the observation field of view of a sample collected in the sample collecting step of the hot-dip galvanizing method of the present embodiment.

[Causes of dross defects]
As described above, conventional research reports that the following types of dross are generated in hot-dip galvanizing.
( _{1 ) Fe2Al5Znx}
(2) δ ₁ -phase dross (3) Γ ₁ -phase dross (4) ζ-phase dross

_{Fe2Al5Znx is} called top _dross . The top dross has a lighter specific gravity than the hot dip galvanizing bath. Therefore, the top dross easily rises to 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 %. Top dross floats on the liquid surface of the hot-dip galvanizing bath, so it can always be collected. Therefore, it is known that the top dross is less likely to cause dross defects.

The δ1-phase dross, Γ1 _{- phase} dross, and ζ-phase dross are called bottom dross. Bottom dross has a higher specific gravity than the hot dip galvanizing bath. Therefore, bottom dross tends to deposit on the bottom of the hot dip galvanizing pot in which the hot dip galvanizing bath is stored.

The crystal structure of δ1 _- phase dross is hexagonal, and its chemical composition consists of 1% or less of Al, 9% or more of Fe, and 90% or more of Zn in mass %. The crystal structure of Γ ₁ -phase dross is a face-centered cubic crystal, and its chemical composition consists of 20% Fe and about 80% Zn in mass %. The crystal structure of the ζ-phase dross is monoclinic, and its chemical composition consists of 1% or less of Al, about 6% of Fe, and about 94% of Zn in mass %.

In previous studies, there were many reports that the main factor of dross defects was δ1 _- phase dross. It is believed that the above Patent Documents 1 and 2 also regard the δ1 _- phase dross as one of the factors of dross defects. Therefore, the inventor of the present invention initially thought that the δ1 _- phase dross was the main cause of dross defects, and conducted investigations and studies. However, even when the generation of δ1 _- phase dross is suppressed in the hot-dip galvanizing process, dross defects may still occur on the surfaces of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets.

Therefore, the present inventor considered that the cause of the dross defects was not the δ1 _- phase dross but other dross. Therefore, the present inventor again analyzed the chemical composition and crystal structure of the dross defect portion using a galvannealed steel sheet with dross defects. The present inventor also conducted a new analysis on the types of dross generated in the hot-dip galvanizing bath. As a result, the present inventor obtained the following knowledge regarding dross defects, which is different from the results of conventional research.

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

The chemical composition of δ1-phase dross ( ₁ % or less of Al, 9% or more of Fe, and 90% or more of Zn), which was considered to be the main cause of conventional dross defects, is the above-mentioned dross defect part. similar to the chemical composition of However, the crystal structure of δ1 _- phase dross is hexagonal, not the face-centered cubic crystal structure specified in the dross defect part. Therefore, the present inventor considered that the δ1 _- phase dross, which was conventionally thought to be the main cause of dross defects, is actually not the main cause of dross defects.

Therefore, the inventors identified the dross that causes the dross defect. Among the drosses (1) to (4) described above, the chemical composition of Fe ₂ Al ₅ Zn _x (top dross) is significantly different from that of the dross defect portion. Regarding the Γ ₁ -phase dross, although the crystal structure is the same face-centered cubic crystal structure as the dross defect portion, the chemical composition (20% Fe and 80% Zn in mass%) is significantly different from that of the dross defect portion. different. Regarding the ζ-phase dross, the chemical composition (1% or less of Al, about 6% of Fe, and about 94% of Zn in mass %) is different from the chemical composition of the dross defect part, and the crystal structure (monoclinic ) is also different from the crystal structure of the dross defect portion (face-centered cubic crystal).

Based on the above study results, the present inventor considered that the dross defects were not caused by the dross described in (1) to (4) above. Then, the present inventor considered that the dross defects may be caused by other types of dross other than the above (1) to (4).

Therefore, the inventor further analyzed the dross in the hot-dip galvanizing bath. EPMA and TEM as described above were used for dross analysis. As a result, the present inventor newly found that gamma _two -phase (Γ _two -phase) dross exists as dross generated in the hot-dip galvanizing bath.

The chemical composition of the Γ _two -phase dross consists of 2% Al, 8% Fe, and 90% Zn in mass %, which is consistent with the chemical composition of the analyzed dross defect sites described above. Furthermore, the crystal structure of the Γ _two -phase dross is a face-centered cubic crystal, which matches the crystal structure of the dross defect portion. Therefore, the present inventor considered that the Γ ₂ -phase dross is the main cause of the dross defects. Since the specific gravity of the Γ2 _- phase dross is higher than that of the hot-dip galvanizing bath, the Γ2 _- phase dross corresponds to the bottom dross that can be deposited on the bottom of the hot-dip zinc pot.

As described above, Fe ₂ Al ₅ Zn _x (top dross) has a lighter specific gravity than the hot-dip galvanizing bath. Fe ₂ Al ₅ Zn _x (top dross) floats on the liquid surface of the hot-dip galvanizing bath and can always be recovered. Therefore, Fe ₂ Al ₅ Zn _x (top dross) is less likely to cause dross defects.

The present inventors further investigated the Γ _two -phase dross and the other dross of (2) to (4). As a result, it was found that dross defects are caused by hard dross, and soft dross is less likely to form dross defects.

As a result of further studies by the present inventors, it was found that among the dross (2) to (4) and the Γ _two -phase dross, the Γ _two -phase dross is a hard dross. Furthermore, it was found that the δ1-phase dross and the ζ _- phase dross are softer than the Γ2 _- phase dross, and thus are less prone to dross defects. In addition, it was found that the ζ-phase dross is the softest dross among the dross of (2) to (4) above, and that the ζ-phase dross is the least likely to cause dross defects.

Based on the above study results, the present inventors have concluded that the main factor of dross defects occurring on the surface of alloyed hot-dip galvanized steel sheets and hot-dip galvanized steel sheets that are subjected to hot-dip galvanizing treatment is not δ1 _- phase dross. , to be Γ _two -phase dross. Furthermore, the present inventors believe that the dross classified as bottom dross is any of Γ ₂ -phase dross, δ ₁ -phase dross, ζ-phase dross, and Γ _{1 -} phase dross. It was found that phase dross was almost non-existent.

The present inventor has further obtained the following findings. The ζ-phase dross undergoes phase transformations with other phases of dross. That is, the Γ _two -phase dross and the ζ-phase dross undergo phase transformation with each other. That is, depending on the hot-dip galvanizing conditions, Γ2 _- phase dross transforms into ζ-phase dross, or ζ-phase dross transforms into Γ2 _- phase dross. Therefore, if the proportion of ζ-phase dross in the bottom dross in the hot-dip galvanizing bath increases, it means that the amount of Γ- ₂ -phase dross in the hot-dip galvanizing bath decreases relatively.

Based on the above findings, the present inventors have found that if the operating conditions of the hot-dip galvanizing process are adjusted to daringly increase the ζ-phase dross, which is the softest and least prone to dross defects, which has not been focused on in the past, hot-dip zinc It was found that the amount of Γ ₂ phase dross, which is hard and tends to cause dross defects in the plating bath, can be reduced, and as a result, dross defects can be suppressed. Then, in the hot-dip galvanizing treatment method, it was thought that the above operation could be carried out by controlling the amount of .zeta.-phase dross in the hot-dip galvanizing bath.

[Regarding alloying treatment]
The present inventor further studied the case where the alloying treatment is performed after the hot-dip galvanizing treatment. In the alloying treatment, Fe contained in the steel sheet diffuses into the hot-dip galvanized layer formed on the surface of the steel sheet to form an Fe—Zn alloy. It is known that the alloying treatment is affected by the Al concentration in the hot dip galvanizing bath. When the Al concentration in the hot-dip galvanizing bath is high, the hot-dip galvanized layer also contains a large amount of Al. Al in the hot-dip galvanized layer inhibits Fe in the steel sheet from forming an Fe—Zn alloy with Zn in the hot-dip galvanized layer. That is, considering the alloying treatment, it is preferable that the Al concentration in the hot-dip galvanizing bath is low.

High-strength steels also contain large amounts of alloying elements such as Si, P and Mn. The alloying elements inhibit the diffusion of Fe in the steel sheet into the hot-dip galvanized layer. Therefore, when performing hot-dip galvanizing treatment and alloying treatment on high-strength steel, it is preferable that the Al concentration in the hot-dip galvanizing bath is particularly low.

On the other hand, when the Al concentration in the hot-dip galvanizing bath is low, Fe eluted from the steel sheet into the hot-dip galvanizing bath tends to 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 dross increases. Conventionally, it has been thought that the δ1 _- phase dross contained in the bottom dross causes dross defects. Therefore, it has been thought that dross defects are likely to occur if the Al concentration in the hot-dip galvanizing bath is lowered.

However, as a result of studies by the present inventors, if the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the ζ-phase dross, even if the Al concentration in the hot-dip galvanizing bath is reduced, dross defects will not occur. I have found that it can be suppressed. As described above, the ζ-phase dross is a type of bottom dross. However, since the ζ-phase dross is soft, it is less likely to cause dross defects. If the Al concentration in the hot-dip galvanizing bath can be reduced, the formation of the Fe--Zn alloy is promoted in the alloying treatment. In this case, even high-strength steel can be easily alloyed. That is, the present inventor found that by adjusting the operating conditions of the hot-dip galvanizing treatment so as to increase the ζ-phase dross, it is possible to promote alloying while suppressing dross defects.

As described above, the hot-dip galvanizing method of the present embodiment has been completed based on an idea different from the conventional technical concept, and is specifically as follows.

The hot-dip galvanizing method of [1] is
A hot-dip galvanizing method used for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet,
A sample collecting step of collecting a sample from a hot dip galvanizing bath containing Al;
a ζ-phase dross amount determination step of determining the ζ-phase dross amount in the hot-dip galvanizing bath using the collected sample;
and an operating condition adjustment step of adjusting operating conditions for the hot dip galvanizing treatment based on the obtained ζ-phase dross amount.

Here, "adjusting the operating conditions of the hot-dip galvanizing treatment" means adjusting the operating conditions of the hot-dip galvanizing treatment that can adjust the amount of ζ-phase dross in the hot-dip galvanizing bath. Further, "adjusting the operating conditions of the hot-dip galvanizing treatment" includes not only the act of changing the operating conditions of the hot-dip galvanizing treatment but also the act of maintaining the operating conditions as they are.

According to the hot-dip galvanizing method having the above configuration, the hot-dip galvanizing method increases the ζ-phase dross amount based on the ζ-phase dross amount in the hot-dip galvanizing bath obtained using the sample. adjust operating conditions. As described above, the ζ-phase dross amount and the Γ- ₂ -phase dross amount have a negative correlation in the hot-dip galvanizing bath. Specifically, when the amount of ζ-phase dross in the hot-dip galvanizing bath is large, it means that the amount of Γ- ₂ -phase dross in the hot-dip galvanizing bath is relatively small. Therefore, by obtaining the ζ-phase dross amount in the hot-dip galvanizing bath and adjusting the operating conditions based on the obtained ζ-phase dross amount to increase the ζ-phase dross, the Γ- ₂ phase dross amount in the hot-dip galvanizing bath can be reduced. As a result, the occurrence of dross defects can be suppressed. In addition, since the amount of Γ2 _- 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 lowered. If the Al concentration in the hot-dip galvanizing bath can be reduced, alloying will be promoted.

The hot-dip galvanizing method of the present embodiment can be suitably applied to high-strength steel. The hot-dip galvanizing method of the present embodiment can promote alloying of steel other than high-strength steel. Therefore, the hot-dip galvanizing method of the present embodiment can be suitably applied to steel other than high-strength steel. As used herein, high-strength steel refers to steel having a tensile strength of 340 MPa or more. As used herein, steel other than high-strength steel refers to steel having a tensile strength of less than 340 MPa.

The hot dip galvanizing method of [2] is the hot dip galvanizing method of [1],
In the ζ-phase dross amount determination step,
Using the collected samples, the number of .zeta.-phase dross per predetermined area is obtained as the .zeta.-phase dross amount.

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

The hot dip galvanizing method of [3] is the hot dip galvanizing method of [1] or [2],
In the operating condition adjustment step,
At least one of (A) or (B) is executed based on the obtained ζ-phase dross amount to increase the ζ-phase dross amount.
(A) Adjusting the bath temperature of the hot-dip galvanizing bath.
(B) Adjusting the Al concentration of the hot-dip galvanizing bath.

Both of the above (A) and (B) are operating conditions effective for transforming dross of other phases into .zeta.-phase dross and increasing the production of .zeta.-phase dross. Therefore, by performing at least one of (A) or (B), the ζ-phase dross amount can be increased and the Γ- ₂ phase dross amount can be reduced.

The hot-dip galvanizing method of [4] is the hot-dip galvanizing method according to any one of [1] to [3],
In the operating condition adjustment step,
When the obtained .zeta.-phase dross amount is less than the threshold value, the .zeta.-phase dross amount is increased by adjusting operating conditions of the hot-dip galvanizing treatment.

In this case, whether or not to change the operating conditions can be easily determined based on the ζ-phase dross amount and the threshold value. For example, when the determined .zeta.-phase dross amount is less than the threshold value, the operating conditions can be adjusted so that the .zeta.-phase dross amount increases. More preferably, when the obtained .zeta.-phase dross amount is less than the threshold, the operating conditions of the hot-dip galvanizing process are adjusted so that the .zeta.-phase dross amount is equal to or greater than the threshold.

The hot dip galvanizing method of [5] is the hot dip galvanizing method of [4],
In the ζ-phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area is obtained as the ζ-phase dross amount,
In the operating condition adjustment step,
When the obtained ζ-phase dross amount is less than 5.0 pieces/cm ² when converted into a unit area (1 cm ² ), the operating conditions of the hot-dip galvanizing treatment are adjusted to reduce the ζ-phase dross amount. to increase

In this case, the .GAMMA.2 _- phase dross is relatively reduced by maintaining a high .zeta.-phase dross amount. As a result, the occurrence of dross defects caused by Γ _two -phase dross can be suppressed more effectively.

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

Here, the Fe concentration in the hot-dip galvanizing bath means the Fe concentration molten 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" is molten in the hot-dip galvanizing bath (that is, the liquid phase) Fe concentration. Similarly, the Al concentration in the hot-dip galvanizing bath means the Al concentration molten in the hot-dip galvanizing bath (so-called Free-Al concentration). That is, in this specification, "Al concentration in the hot dip galvanizing bath" is molten in the hot dip galvanizing bath (that is, liquid phase) Al concentration.

In this case, the .zeta.-phase dross amount increases, and as a result, the .GAMMA.2 _- phase dross amount decreases relatively. Therefore, the occurrence of dross defects caused by Γ _two -phase dross can be more effectively suppressed.

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

In this case, the .zeta.-phase dross amount further increases, and as a result, the .GAMMA.2 _- phase dross amount relatively decreases further. Therefore, the occurrence of dross defects caused by Γ _two -phase dross can be more effectively suppressed.

The hot-dip galvanizing method of [8] is the hot-dip galvanizing method according to any one of [1] to [7],
A sink roll is arranged in the hot dip galvanizing pot in which the hot dip galvanizing bath is stored to contact the steel plate immersed in the hot dip galvanizing bath and change the traveling direction of the steel plate up and down. ,
In the sample collection step,
The sample is taken from the depth range from the upper end to the lower end of the sink roll in the hot dip galvanizing bath in the hot dip galvanizing pot.

In this case, samples are taken from the same depth range as the sink roll. Therefore, the correlation between the ζ-phase dross amount and the dross defects can be further enhanced.

[9] The method for manufacturing the alloyed hot-dip galvanized steel sheet,
A hot-dip galvanizing step of performing the hot-dip galvanizing method according to any one of [1] to [8] on a steel plate to form a hot-dip galvanized layer on the surface of the steel plate;
and an alloying treatment step of manufacturing the alloyed hot-dip galvanized steel sheet by performing an alloying treatment on the steel sheet having the hot-dip galvanized layer formed on the surface.

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

[10] The method for producing a hot-dip galvanized steel sheet,
A hot-dip galvanizing step of forming a hot-dip galvanized layer on the surface of the steel sheet by performing the hot-dip galvanizing method according to any one of [1] to [8] on the steel sheet.

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

Hereinafter, the hot-dip galvanizing treatment method, the method for manufacturing the alloyed hot-dip galvanized steel sheet, and the method for manufacturing the hot-dip galvanized steel sheet according to the present embodiment will be described with reference to the drawings. In addition, in the present specification and drawings, configurations having substantially the same functions are denoted by the same reference numerals, and the description thereof will not be repeated.

[Regarding the configuration of hot-dip galvanizing line equipment]
FIG. 1 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanized steel sheet and a hot-dip galvanized line facility used to manufacture the hot-dip galvanized steel sheet. Referring to FIG. 1, the hot-dip galvanizing line equipment 1 includes an annealing furnace 20 , a hot-dip galvanizing equipment 10 , and a temper rolling mill (skin pass mill) 30 .

The annealing furnace 20 includes one or more heating zones (not shown) and one or more cooling zones arranged downstream of the heating zones. In the annealing furnace 20, the steel sheet is supplied to the heating zone of the annealing furnace 20, and the steel sheet is annealed. The annealed steel sheet is cooled in a cooling zone and transported to 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, a hot-dip galvanizing treatment is performed on a steel sheet to produce an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. The temper rolling mill 30 is arranged downstream of the hot-dip galvanizing facility 10 . In the temper rolling mill 30, the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet produced in the hot-dip galvanizing equipment 10 is lightly reduced as necessary to produce an alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet. Condition the surface of the galvanized steel sheet.

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

The interior of the annealing furnace 20 arranged upstream of the hot-dip galvanizing equipment 10 is shielded from the atmosphere and maintained in a reducing atmosphere. As described above, the annealing furnace 20 heats the continuously conveyed steel sheet 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.

A downstream end portion of the annealing furnace 20 corresponding to the exit side of the annealing furnace 20 has a space in which a turndown roll 201 is arranged. The downstream end of annealing furnace 20 is connected to the upstream end of snout 202 . The downstream end of snout 202 is immersed in hot dip galvanizing bath 103 . The interior of the snout 202 is shielded from the atmosphere and maintained in a reducing atmosphere.

The steel sheet S whose conveying direction is changed downward by the turndown roll 201 passes through the snout 202 and is continuously immersed in the hot dip galvanizing bath 103 stored in the hot dip galvanizing pot 101 . A sink roll 107 is arranged inside the molten zinc pot 101 . The sink roll 107 has a rotating shaft parallel to the width direction of the steel plate S. The width of the sink roll 107 in the axial direction is larger than the width of the steel plate S. The sink roll 107 comes into contact with the steel sheet S and changes the advancing direction of the steel sheet S to above the hot-dip galvanizing equipment 10 .

The support roll 113 is arranged above the sink roll 107 in the hot dip galvanizing bath 103 . The support roll 113 has a pair of rolls. A pair of rolls of the support rolls 113 has a rotating shaft parallel to the width direction of the steel plate S. As shown in FIG. The support rolls 113 sandwich the steel sheet S whose traveling direction is turned upward by the sink rolls 107 and support the steel sheet S conveyed upward.

The gas wiping device 109 is arranged above the sink roll 107 and the support roll 113 and above the liquid surface of the hot-dip galvanizing bath 103 . The gas wiping device 109 comprises a pair of gas injection devices. A pair of gas injection devices have gas injection nozzles facing each other. During the hot-dip galvanizing treatment, 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 face the surface of the steel sheet S. The gas wiping device 109 blows gas onto both surfaces of the steel sheet S pulled up from the hot-dip galvanizing bath 103 to scrape off a part of the hot-dip galvanization adhering to both surfaces of the steel sheet S. Adjust the amount of hot-dip galvanizing on the surface.

The alloying furnace 111 is arranged above the gas wiping device 109 . The alloying furnace 111 passes the steel sheet S conveyed upward through the gas wiping device 109 , and performs an alloying treatment on the steel sheet S. The alloying furnace 111 includes a heating zone, a holding zone, and a cooling zone in order from the entry side of the steel sheet S toward the delivery side. The heating zone heats the steel plate S so that the temperature (plate temperature) of the steel plate S becomes substantially uniform. The heat-retaining zone keeps the temperature of the steel sheet S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed into an alloyed hot-dip galvanized layer. The cooling zone cools the steel sheet S on which the alloyed hot-dip galvanized layer is formed. As described above, the alloying furnace 111 performs the alloying treatment using the heating zone, the holding zone, and the cooling zone. Note that the alloying furnace 111 performs the above-described alloying treatment when manufacturing a galvannealed steel sheet. On the other hand, when manufacturing a hot-dip galvanized steel sheet, the alloying furnace 111 does not carry out the alloying treatment. In this case, the steel sheet S passes through the non-operating alloying furnace 111 . Here, "not in operation" means, for example, that the alloying furnace 111 is on-line and the power source is stopped (not activated). The steel sheet S that has passed through the alloying furnace 111 is conveyed to the next process by the top roll 115 .

When manufacturing a hot-dip galvanized steel sheet, the alloying furnace 111 may be moved off-line as shown in FIG. In this case, the steel sheet S is conveyed to the next step by the top roll 115 without passing through the alloying furnace 111 .

If the hot-dip galvanizing facility 10 is dedicated to hot-dip galvanized steel sheets, the hot-dip galvanizing facility 10 may not include the alloying furnace 111 as shown in FIG.

[Other configuration examples of hot-dip galvanizing line equipment]
The hot-dip galvanizing line equipment 1 is not limited to the configuration shown in FIG. For example, when Ni pre-plating is performed on a steel sheet before hot-dip galvanizing treatment to form a Ni layer on the steel sheet, as shown in FIG. A pre-plating facility 40 may be arranged. The Ni pre-plating equipment 40 includes a Ni-plating cell that stores a Ni-plating bath. Ni plating is performed by an electroplating method. The hot-dip galvanizing line equipment 1 shown in FIGS. 1 and 5 includes an annealing furnace 20 and a temper rolling mill 30 . However, the hot-dip galvanizing line equipment 1 does not have to be equipped with the annealing furnace 20 . Further, the hot-dip galvanizing line equipment 1 does not have to include the temper rolling mill 30 . The hot-dip galvanizing line equipment 1 may include at least the hot-dip galvanizing equipment 10 . The annealing furnace 20 and the temper rolling mill 30 may be arranged as required. Further, the hot-dip galvanizing line equipment 1 may be equipped with a pickling equipment for pickling the steel sheet upstream of the hot-dip galvanizing equipment 10, or other equipment other than the annealing furnace 20 and the pickling equipment. may be provided. The hot-dip galvanizing line equipment 1 may further include equipment other than the temper rolling mill 30 downstream of the hot-dip galvanizing equipment 10 .

[Regarding the generation mechanism of dross defects]
The mechanism by which dross defects occur in the hot dip galvanizing process during the manufacturing process of the alloyed hot dip galvanized steel sheet or the hot dip galvanized steel sheet using the above hot dip galvanizing line equipment 1 is considered as follows.

In the hot-dip galvanizing process, Fe dissolves into the hot-dip galvanizing bath 103 from the steel sheet S immersed in 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. Of the generated dross, top dross floats on the liquid surface in the hot-dip galvanizing bath 103 . On the other hand, among the generated dross, the bottom dross sinks and accumulates on the bottom of the molten zinc pot 101 . As the production of galvannealed steel sheets or hot dip galvanized steel sheets is repeated (that is, as the amount of steel sheet S passed through the hot dip galvanizing bath 103 increases), bottom dross accumulates on the bottom of the hot dip galvanizing pot 101 .

The bottom dross deposited on the bottom of the hot dip galvanizing pot 101 is swept up into the hot dip galvanizing bath 103 by the accompanying flow of the steel sheet S generated near the bottom of the sink roll 107 and floats in the hot dip galvanizing bath 103 . Bottom dross floating in the hot dip galvanizing bath 103 adheres to the surface of the steel sheet S near the sink roll 107 . A portion where the bottom dross adheres to the surface of the steel sheet S may become a dross defect.

If dross defects occur, non-uniform plating occurs on the surface of the coating, and the appearance quality of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is deteriorated. Furthermore, local cells are likely to be formed in dross defect portions on the surface of the steel sheet, and the corrosion resistance of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet is lowered.

As described above, the main cause of dross defects is Γ _two -phase dross, not δ _one -phase dross, which has been reported in many previous studies. Therefore, if the amount of Γ ₂ phase dross in the hot-dip galvanizing bath 103 increases, the possibility of generating dross defects in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet increases.

Furthermore, the ζ-phase dross and the Γ- ₂ -phase dross undergo phase transformation with each other. That is, the ζ-phase dross is transformed into the Γ- ₂ -phase dross, and the Γ- ₂ -phase dross is transformed into the ζ-phase dross. Therefore, in the hot dip galvanizing bath 103, the ζ-phase dross amount and the Γ2 _- phase dross amount have a negative correlation. It means that the amount of Γ _two -phase dross inside is relatively small. Furthermore, the ζ-phase dross is the softest compared to the dross of other phases and is less likely to cause dross defects. Therefore, by obtaining the ζ-phase dross amount in the hot-dip galvanizing bath 103 and adjusting the operating conditions based on the obtained ζ-phase dross amount to increase the ζ-phase dross amount, Γ ₂ in the hot-dip galvanizing bath 103 Phase dross amount can be reduced. As a result, the occurrence of dross defects can be suppressed.

Therefore, in the hot-dip galvanizing method of the present embodiment, the ζ-phase dross amount of the dross in the hot-dip galvanizing bath 103 is obtained. Then, based on the ζ-phase dross amount in the hot-dip galvanizing bath 103, the operating conditions for the hot-dip galvanizing treatment are adjusted. Preferably, based on the amount of .zeta.-phase dross in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the amount of .zeta.-phase dross. As a result, the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be increased, and the amount of Γ- ₂ -phase dross can be kept relatively low. As a result, the occurrence of dross defects in the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet can be suppressed. Preferably, based on the ζ-phase dross amount in the hot-dip galvanizing bath 103, the operating conditions of the hot-dip galvanizing treatment are adjusted so as to increase the ζ-phase dross, and the ζ-phase dross amount in the hot-dip galvanizing bath 103 is increased. Maintain above a certain amount (threshold).

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

[Regarding the hot-dip galvanizing method of the present embodiment]
[About hot-dip galvanizing equipment to be 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, for example, the configuration shown in FIGS. However, as described above, the hot-dip galvanizing line equipment 1 used in the hot-dip galvanizing method of the present embodiment may be the equipment shown in FIGS. Further, other configurations may be added. Also, a known hot-dip galvanizing line facility 1 having a configuration different from that shown in FIGS. 1 and 5 may be used.

[About steel sheets used for hot-dip galvanizing]
The steel type and size (thickness, width, etc.) of the steel sheet (base material steel sheet) used in the hot-dip galvanizing treatment of the present embodiment are not particularly limited. The steel sheet is an alloyed hot-dip galvanized steel sheet to be manufactured, or an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet depending on each mechanical property (for example, tensile strength, workability, etc.) required for the hot-dip galvanized steel sheet. A known steel plate that is applied to is used. You may utilize the steel plate used for a motor vehicle outer plate as a steel plate (base material steel plate) used for a hot-dip galvanizing process.

As described above, in the hot-dip galvanizing treatment of the present 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 for the hot-dip galvanizing treatment of the present embodiment may be a steel sheet made of high-strength steel containing a large amount of alloying elements such as Si and Mn. The steel sheet used for the hot-dip galvanizing treatment of the present embodiment may be a steel sheet made of steel other than high-strength steel.

The steel sheet (base material steel sheet) used in the hot-dip galvanizing treatment of the present 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) Hot-rolled steel sheet subjected to pickling treatment (b) Hot-rolled steel sheet subjected to Ni pre-plating treatment after pickling treatment to form a Ni layer on the surface (c) Cold-rolled steel sheet subjected to annealing treatment Steel sheet (d) Cold-rolled steel sheet subjected to Ni pre-plating treatment after annealing treatment to form a Ni layer on the surface The above (a) to (d) are used for the hot-dip galvanizing treatment of the present embodiment. It is an example of a steel plate to be used. The steel sheet used for the hot-dip galvanizing treatment of this embodiment is not limited to the above (a) to (d). A hot-rolled steel sheet or cold-rolled steel sheet that has been subjected to treatments other than the above (a) to (d) may be used as the steel sheet to be used for 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 further contains Al in addition to Zn. In other words, the hot-dip galvanizing bath 103 used in the hot-dip galvanizing method of the present embodiment is a plating solution containing a specific concentration of Al and the balance being Zn and impurities. If the hot-dip galvanizing bath 103 contains a specific concentration of Al, excessive reaction between Fe and Zn in the bath can be suppressed, and the reaction between the steel sheet immersed in the hot-dip galvanizing bath 103 and Zn can be suppressed. It is possible to suppress the progress of a uniform alloy reaction.

A preferable Al concentration (more specifically, Free-Al concentration) in the hot-dip galvanizing bath 103 is 0.100 to 0.159% by mass. Here, the Al concentration in the hot-dip galvanizing bath 103 means the Al concentration (mass %) dissolved in the hot-dip galvanizing bath 103, ie, the so-called Free-Al concentration. If the Al concentration in the hot-dip galvanizing bath is in the range of 0.100 to 0.159% by mass, it is possible to suppress the occurrence of pattern defects other than dross defects, and furthermore, alloying molten zinc In the alloying treatment during the manufacturing process of the plated steel sheet, it is possible to suppress the occurrence of unalloyed.

Thus, the hot-dip galvanizing bath 103 according to the present embodiment is a plating bath containing Zn as a main component and further containing Al. The hot-dip galvanizing bath 103 may further contain 0.020 to 0.100% by mass of Fe eluted from equipment and steel sheets in the bath. That is, the Fe concentration (% by mass) dissolved in the hot-dip galvanizing bath 103 is, for example, 0.020 to 0.100% by 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 method]
The hot dip galvanizing method of this embodiment uses a hot dip galvanizing bath 103 containing Al. FIG. 6 is a flowchart showing the steps of the hot-dip galvanizing method of this embodiment. Referring to FIG. 6, the hot-dip galvanizing method of the present embodiment includes a sample collection step (S1), a .zeta.-phase dross amount determination step (S2), and an operation condition adjustment step (S3). Each step will be described in detail below.

[Sample collection step (S1)]
In the sampling step (S1), a part of the plating solution is sampled from the hot-dip galvanizing bath 103 . In the sample collection step (S1), samples are collected over time. "Taking samples over time" means taking samples after each specified period of time. The specific time (the period from when a sample is taken until when the next sample is taken) may or may not be constant. For example, samples may be taken every hour. Alternatively, the next sample may be collected after 1 hour has passed since the sample was collected, and the next sample may be collected after 30 minutes have passed. The specific time is not particularly limited.

The amount of sample taken from the hot-dip galvanizing bath 103 is not particularly limited. The amount of sample to be collected is not particularly limited as long as the amount of .zeta. The amount of sample taken is, for example, 100-400 g. The collected sample may be brought into contact with a room temperature metal having high thermal conductivity, and the sample may be rapidly cooled to room temperature and solidified. An example of a room temperature metal with a high thermal conductivity is copper.

The sampling 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 equal parts D1 to D3 in the depth direction D, a sample is taken at the uppermost region D1 in the hot dip galvanizing bath 103. Alternatively, the sample may be taken in the middle region D2, or the sample may be taken in the bottom region D3. The amount of ζ-phase dross in the samples collected in each region D1 to D3 is different. However, depending on the sampling position, it is possible to some extent to determine whether the obtained ζ-phase dross amount is large. Therefore, the sampling position 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. A direction perpendicular to the width direction W and the depth direction D is defined as the length direction L. In this case, preferably, 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 Sample over time from within a specific region to collect. In short, samples are taken over time from the same position (within a specific area) within the hot dip galvanizing bath 103 .

Preferably, the sample is taken from an area as close to the sink roll 107 as possible. Specifically, as shown in FIGS. 2 to 4, a sample is taken from within a specific depth range D107 from the upper end to the lower end of the sink roll 107 in the depth direction D in the hot dip galvanizing bath 103. . In other words, the specific depth range is defined as the depth range D107 from the top end to the bottom end of the sink roll 107 . The Γ _two -phase dross is highly likely to adhere to the surface of the steel sheet S in the vicinity of the sink roll 107 . Therefore, the ζ-phase dross amount near the sink roll 107 is the most effective index for suppressing dross defects. Therefore, samples are preferably taken from depth range D107. In this case, since the ζ-phase dross amount is determined based on the samples taken from the range where the ζ-phase dross is most likely to adhere to the surface of the steel sheet S, the correlation between the ζ-phase dross amount and the dross defects can be further enhanced. Also in the width direction W and the length direction L, it is preferable to collect samples from the area near the sink roll as much as possible. In addition, as described above, the samples are taken from the same area in the hot dip galvanizing bath 103 over time.

[ζ-phase dross amount determination step (S2)]
In the .zeta.-phase dross amount determination step (S2), the .zeta.-phase dross amount in the hot-dip galvanizing bath 103 is determined using the collected sample. The method of obtaining the ζ-phase dross amount using a sample is not particularly limited, and various methods are conceivable.

For example, a test piece for observing .zeta.-phase dross is produced from the sample collected in the sample collecting step (S1). An example of the test piece for observing ζ-phase dross is 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. Using an optical microscope or a scanning electron microscope (SEM) with a predetermined magnification, observation of the entire field of view (15 mm×15 mm) is performed to identify the dross in the entire field of view. Contrast in the field of view can identify dross, and contrast can distinguish between top and bottom dross.

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, a hot-dip galvanized mother phase 200, top dross 100T, and bottom dross 100B are observed in the photographic image. The top dross 100T has a lower brightness (darker) than the mother phase 200 and the bottom dross 100B. On the other hand, the bottom dross 100B is lower in brightness (darker) than the mother phase 200 and higher in brightness (brighter) than the top dross 100T. As described above, top dross and bottom dross can be distinguished based on contrast.

Of the dross identified in the observation field of view (15 mm×15 mm), each bottom dross is subjected to composition analysis using EPMA to identify ζ-phase dross. Further, crystal structure analysis using TEM may be performed on each bottom dross to identify the ζ-phase dross in the observation field. In addition, without distinguishing top dross and bottom dross by contrast, each dross was subjected to composition analysis using EPMA and / or crystal structure analysis using TEM to determine the type of each dross in the field of view (top dross, Γ _two -phase dross, δ _one -phase dross, and ζ-phase dross).

Based on the identified .zeta.-phase dross, the .zeta.-phase dross amount 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 indices. For example, the number of .zeta.-phase dross per predetermined area may be used as the .zeta.-phase dross amount. Here, the predetermined area is not particularly limited, and may be, for example, the area of the entire observation field of view or a unit area (mm ² ). For example, when the observation field of view is 15 mm×15 mm, the number of ζ-phase dross (pieces/225 mm ² ) in the observation field of view (15 mm×15 mm=225 mm ² ) may be used as the ζ-phase dross amount. In this case, the number of ζ-phase dross in the observation field is obtained by the following method. First, the equivalent circle diameter (μm) of the identified ζ-phase dross is obtained. The diameter when the area of each ζ-phase dross in the above observation field is converted into a circle is defined as the equivalent circle diameter (μm). Equivalent circle diameters (μm) of the identified ζ-phase dross are obtained by well-known image processing using the photographic image of the observation field of view. The number of ζ-phase dross having an equivalent circle diameter of 10 μm or more in the field of view is defined as the number of ζ-phase dross (number/225 mm ² ). Thus, the number of ζ-phase dross having an equivalent circle diameter of 10 μm or more in the observation field may be defined as the ζ-phase dross amount. Note that the observation field of view is not limited to the above region (15 mm×15 mm=225 mm ² ). Moreover, the upper limit of the equivalent circle diameter of the ζ-phase dross is not particularly limited. The upper limit of the equivalent circle diameter of the ζ-phase dross is, for example, 300 μm.

Another index may be the amount of ζ-phase dross in the hot-dip galvanizing solution. For example, the area of each bottom dross (each Γ ₂ -phase dross, each δ ₁ -phase dross, and each ζ-phase dross) and the area of each ζ-phase dross are obtained in the observation field described above. Then, the ratio of the total area of .zeta.-phase dross to the total area of bottom dross may be used as the .zeta.-phase dross amount. Also, the ratio of the total area of .zeta.-phase dross to the observation field area may be used as the .zeta.-phase dross amount. Also, the total area (μm ² ) of the ζ-phase dross in the field of view described above may be used as the ζ-phase dross amount. Also, X-ray diffraction measurement is performed on the test surface of the above sample to measure the peak intensity of each bottom dross (Γ _two -phase dross, δ1 _- phase dross, and ζ-phase dross). Then, the ratio of the peak intensity of the ζ-phase dross to the total peak intensity of each bottom dross (that is, the sum of the peak intensity of the Γ- ₂ -phase dross, the δ- ₁ -phase dross peak intensity, and the ζ-phase dross peak intensity) is It may be the ζ-phase dross amount. It should be noted that it is not easy to clearly distinguish Γ ₂ -phase dross from Γ ₁ -phase dross by X-ray diffraction measurement. However, as mentioned above, it is believed that the Γ ₁ -phase dross is almost non-existent. Therefore, all peak intensities obtained at diffraction angles 2θ=43 to 44° are regarded as peak intensities of Γ _two -phase dross. A Co dry bulb, for example, is used as a target for X-ray diffraction measurement. The ζ-phase dross amount may be obtained by other methods than those described above.

By the above method, the ζ-phase dross amount in the hot-dip galvanizing bath 103 is determined using the sample collected in the sample collection step (S1). The ζ-phase dross amount determining step (S2) is preferably performed each time a sample is taken in the sample taking step (S1). By taking samples over time and determining the .zeta.-phase dross amount each time a sample is taken, it is possible to grasp the temporal change in the .zeta.-phase dross amount in the hot-dip galvanizing bath 103. Therefore, the ζ-phase dross amount may be determined over time based on samples collected over time.

[Operating condition adjustment step (S3)]
After determining the amount of .zeta.-phase dross in the hot-dip galvanizing bath 103 in the .zeta.-phase dross amount determination step (S2), the operating condition adjustment step (S3) is carried out.

In the operating condition adjustment step ( S<b>3 ), the operating conditions for 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 obtained ζ-phase dross amount is small, the operating conditions are adjusted (changed) so as to increase the ζ-phase dross amount in the hot-dip galvanizing bath 103 . If the obtained ζ-phase dross amount is appropriate, the operating conditions may be maintained as they are. A method for adjusting operating conditions is not particularly limited as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted. Specifically, the method of adjusting the operating conditions is not particularly limited as long as the amount of ζ-phase dross in the hot-dip galvanizing bath 103 can be adjusted to increase.

Preferably, at least one of the following (A) or (B) is carried out as a method for adjusting operating conditions.
(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 (A) above, if the temperature of the hot-dip galvanizing bath 103 is raised, the possibility of phase transformation of Γ2 _- phase dross to ζ-phase dross increases. Therefore, if the temperature of the hot-dip galvanizing bath 103 is increased, the .GAMMA.2 _- phase dross in the hot-dip galvanizing bath 103 is reduced, and instead the .zeta.-phase dross is increased. As mentioned above, ζ-phase dross is soft. Therefore, ζ-phase dross is less likely to form dross defects. Therefore, when the ζ-phase dross amount in the hot-dip galvanizing bath 103 is excessively small, the bath temperature of the hot-dip galvanizing bath 103 may be increased. In this case, the hard Γ _two -phase dross transforms into soft ζ-phase dross. As a result, the soft ζ-phase dross increases and the hard Γ- ₂ -phase dross decreases. Therefore, the occurrence of dross defects is suppressed. It should be noted that increasing the bath temperature increases the energy consumption rate. Therefore, when the ζ-phase dross amount is sufficiently large, it is not necessary to excessively raise the bath temperature. As described above, the ζ-phase dross amount in the hot-dip galvanizing bath 103 can be adjusted by adjusting the bath temperature of the hot-dip galvanizing bath 103 . 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- ₂ -phase dross in the hot-dip galvanizing bath 103 can be reduced.

Regarding (B) above, if the Al concentration in the hot-dip galvanizing bath 103 is lowered, the possibility of phase transformation of Γ _two -phase dross to ζ-phase dross increases. Therefore, when the ζ-phase dross amount in the hot-dip galvanizing bath 103 is excessively small, the ζ-phase dross amount 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 of the hot-dip galvanizing bath 103, the ζ-phase dross amount can be increased, and as a result, the Γ2 _- phase dross in the hot-dip galvanizing bath 103 can be reduced.

Of the operating conditions (A) and (B) described above, only one operating condition may be adjusted based on the obtained ζ-phase dross amount, or both (A) and (B) Operating conditions may be adjusted. For example, when the ζ-phase dross amount is excessively small, the bath temperature of the hot-dip galvanizing bath 103 may be increased and the Al concentration of the hot-dip galvanizing bath 103 may be decreased. If the ζ-phase dross amount is appropriate, the operating conditions (A) and (B) may be maintained as they are.

A threshold value may be provided as an index for determining whether or not the .zeta.-phase dross amount obtained in the .zeta.-phase dross amount determination step (S2) is appropriate. In this case, the operating conditions may be adjusted depending on whether or not the obtained ζ-phase dross amount is less than the threshold value. Specifically, depending on whether or not the obtained ζ-phase dross amount is less than the threshold value, the operating conditions may be changed or maintained without change. For example, if the obtained ζ-phase dross amount is less than the threshold, it is determined that the ζ-phase dross amount is excessively small, and the operating conditions are changed so that the ζ-phase dross amount in the hot-dip galvanizing bath 103 is Adjust the operating conditions so that the Preferably, when the determined ζ-phase dross amount is less than the threshold, the operating conditions are changed so that the ζ-phase dross amount is equal to or greater than the threshold. On the other hand, when the obtained ζ-phase dross amount is equal to or higher than the threshold value, it is determined that the ζ-phase dross amount in the hot-dip galvanizing bath 103 is sufficiently large, and the operating conditions are maintained as they are.

5. When the number of .zeta.-phase dross per predetermined area, for example, the number of .zeta.-phase dross in the observation field as described above, is converted into the number per unit area (1 ^cm.sup.2 ). The number corresponding to 0/cm ² is used as the threshold value. For example, when the number of ζ-phase dross in the above observation field of view (15 mm×15 mm=225 mm ² ) is the amount of ζ-phase dross, the threshold is 11.25 (5.0/cm ² ×225 mm ² ). and In this case, when the ζ-phase dross amount obtained in the ζ-phase dross amount determination step (S2) is greater than the threshold value (11.25 pieces/225 mm ² ), that is, when converted by a unit area (1 cm ² ) If the number is less than 5.0 pieces/cm ² , it is determined that the ζ-phase dross amount is excessively small, and the operating conditions are adjusted so that the ζ-phase dross amount in the hot-dip galvanizing bath 103 increases. Preferably, when the .zeta.-phase dross amount obtained in the .zeta.-phase dross amount determination step (S2) exceeds the threshold value (11.25 pieces/225 ^mm.sup.2 ), that is, when the .zeta.-phase dross amount obtained exceeds the unit area When the number is less ^than 5.0 pieces/cm ² when converted by .0 pieces/cm ² or more), the operating conditions are adjusted. For example, when the ζ-phase dross amount obtained in the ζ-phase dross amount determination step (S2) is less than 5.0 pieces/cm ² in terms of unit area, the above (A) or (B) At least one of the operating conditions is implemented to increase the amount of ζ-phase dross. For example, the bath temperature of the hot-dip galvanizing bath 103 is raised to increase the amount of ζ-phase dross. Also, for example, the Al content of the hot-dip galvanizing bath 103 is reduced to increase the ζ-phase dross amount. The number of ζ-phase dross per predetermined area is preferably as large as possible, and there is no particular upper limit.

Preferably, in the operating condition adjustment step (S3), the Fe concentration in the hot-dip galvanizing bath 103 is defined as X (% by mass) based on the ζ-phase dross amount obtained in the ζ-phase dross amount determination step (S2). , When the Al concentration in the hot-dip galvanizing bath 103 is defined as Y (% by mass), the Fe concentration and Al concentration in the hot-dip galvanizing bath 103 are adjusted to satisfy the formulas (1) and (2) .
0.100≤Y≤0.139 (1)
Y≦0.2945X+0.1216 (2)
Here, the Al concentration means the Al concentration excluding the Al content contained in the dross among the Al in the hot-dip galvanizing bath 103, and means the so-called Free-Al concentration (% by mass). Similarly, the Fe concentration means the Fe concentration of the Fe in the hot-dip galvanizing bath 103, excluding the Fe content contained in the dross.

Formula (1) indicates the range of Al concentration Y (% by mass) in the hot-dip galvanizing bath 103 . The Al concentration Y in the hot-dip galvanizing bath 103 is related to the amount of top dross, Γ _two -phase dross, and ζ-phase dross produced. When the Al concentration Y is 0.139% or less, the top dross is easily transformed into Γ2 _- phase dross and ζ-phase dross. In this case, excessive generation of top dross is suppressed. As a result, it is possible to prevent top dross from being caught between the sink roll 107 and the steel plate, thereby suppressing the generation of surface defects. Therefore, in order to suppress the occurrence of surface flaws, the formation of top dross may be suppressed. In order to suppress surface flaws, the Al concentration in the hot-dip galvanizing bath 103 should be kept at 0.140% or less. However, in the actual operation of hot-dip galvanizing treatment, there is a possibility that a maximum variation of ±0.001% occurs in Al concentration control. 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%.

From the viewpoint of suppressing the occurrence of surface flaws, the lower limit of the Al concentration is not particularly limited. However, it is well known that over-alloying can be suppressed in the alloying treatment by setting the Al concentration in the hot-dip galvanizing bath 103 to a certain level or higher. In formula (1), the lower limit of Al concentration (the lower limit of formula (1)) is set to 0.100%.

The lower limit of the Al concentration Y in the hot-dip galvanizing bath 103 may be 0.100%, 0.105%, or 0.110%. Further, 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 transformation line) where the Γ2 _- phase dross transforms 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 equation (2), the chemical composition of the hot-dip galvanizing bath 103 is a state in which Γ _two -phase dross can exist more stably than ζ-phase dross. It has become. In this case, on the premise that the Al concentration Y in the hot-dip galvanizing bath 103 satisfies the formula (1), the ζ-phase dross is easily transformed into the Γ- ₂ -phase dross. Therefore, in the hot-dip galvanizing bath 103, Γ _two -phase dross is likely to be generated.

On the other hand, if the Al concentration Y in the hot dip galvanizing bath 103 is equal to or less than the right side of the formula (2), that is, if the Al concentration Y and the Fe concentration X satisfy the formula (2), then the Al in the hot dip galvanizing bath 103 Assuming that the concentration Y 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 _two -phase dross in the hot-dip galvanizing bath 103 is likely to transform into the ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the Γ _two -phase dross is likely to decrease.

Therefore, in the hot-dip galvanizing treatment described above, if the Al concentration Y and the Fe concentration X in the hot-dip galvanizing bath 103 are adjusted so as to satisfy the formulas (1) and (2), the hot-dip galvanizing bath 103 , promotes the generation of .zeta.-phase dross, and reduces the .GAMMA.2 _- phase dross amount, which has a negative correlation with the .zeta.-phase dross amount.

More preferably, in the operating condition adjustment step (S3), the Fe concentration in the hot-dip galvanizing bath 103 is defined as X (% by mass) based on the ζ-phase dross amount obtained in the ζ-phase dross amount determination step (S2). Then, when the Al concentration in the hot-dip galvanizing bath 103 is defined as Y (% by mass), the Fe concentration and Al concentration in the hot-dip galvanizing bath 103 are adjusted to satisfy the formulas (1) and (3). do.
0.100≤Y≤0.139 (1)
Y≦0.2945X+0.1066 (3)
Here, the Al concentration means the Al concentration excluding the Al content contained in the dross among the Al in the hot-dip galvanizing bath 103, and means the so-called Free-Al concentration (% by mass). Similarly, the Fe concentration means the Fe concentration of the Fe in the hot-dip galvanizing bath 103, excluding the Fe content contained in the dross.

Formula (3) is a formula for specifying a region with a lower Al concentration than formula (2) above. The above equation (2) corresponds to the boundary (phase transformation line) where the Γ2 _- phase dross transforms into the ζ-phase dross in the hot-dip galvanizing bath 103 . Equation (3) is a region in which ζ-phase dross can exist more stably than the region specified by Equation (2). Therefore, the Γ _two -phase dross in the hot-dip galvanizing bath 103 is more likely to transform into the ζ-phase dross. Therefore, in the hot-dip galvanizing bath 103, the Γ _two -phase dross is more likely to decrease.

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. A sample is taken from within a specific depth range in the depth direction D of the hot-dip galvanizing bath 103 in FIG. More specifically, in the hot-dip galvanizing bath 103 of FIG. A sample is collected from within a specified area (hereinafter referred to as a sample collection area). When samples are collected sequentially over time, the sample collection positions are the same (within the same sample collection area). Cool the collected sample to room temperature. Using an ICP emission spectrometer, the Fe concentration (mass%) and Al concentration (mass%) in the sample after cooling are measured. The balance other than the Fe concentration and Al concentration can be regarded as Zn.

The Fe concentration obtained by the above ICP emission spectrometer is the so-called Total-Fe concentration, which includes not only the Fe concentration (Free-Fe concentration) in the hot-dip galvanizing bath but also the Fe concentration in the dross. Similarly, the Al concentration obtained by the above-mentioned ICP emission spectrometer is the so-called Total-Al concentration, which includes not only the Al concentration (Free-Al concentration) in the hot-dip galvanizing bath but also the Al concentration in the dross. be. Therefore, the Free-Fe concentration and the Free-Al concentration are calculated using the obtained Total-Fe concentration and Total-Al concentration and the well-known Zn-Fe-Al ternary phase diagram. Specifically, a Zn--Fe--Al ternary phase diagram at the bath temperature at which the sample was taken is prepared. As described above, the Zn--Fe--Al ternary system phase diagram is well known and is also disclosed in FIGS. Non-Patent Document 1 is a prominent paper among researchers and developers of hot-dip galvanizing baths. Points identified from the total-Fe concentration and the total-Al concentration obtained by the ICP emission spectrometer are plotted on the Zn--Fe--Al ternary phase diagram. Then, tie lines (conjugate lines) are drawn from the plotted points to the liquidus line in the Zn--Fe--Al ternary phase diagram. The Fe concentration at the intersection of the liquidus line and the tie line corresponds to the Free-Fe concentration, and the Al concentration at the intersection of the liquidus line and the tie line corresponds to the Free-Al concentration. By 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 obtained.

[More preferable bath temperature of hot-dip galvanizing bath]
The temperature (bath temperature) of the hot dip galvanizing bath 103 in the hot dip galvanizing treatment method described above is preferably 440 to 500.degree. The dross in the hot-dip galvanizing bath 103 is mainly top dross (Fe ₂ Al ₅ Zn _x ), Γ _two -phase dross, δ It undergoes a phase transformation to ₁ -phase and ζ-phase dross. Γ _two -phase dross is likely to be generated in a region where the bath temperature is low. ζ-phase dross is more likely to be generated in a region where the bath temperature is higher than in the region where Γ _two -phase dross is generated.

Moreover, if the bath temperature of the hot-dip galvanizing bath 103 is 500° C. or less, Zn can be prevented from evaporating and forming fumes. When fumes are generated, the fumes tend to adhere to the steel sheet and cause surface flaws (fume flaws). A preferable lower limit of the hot-dip galvanizing bath 103 is 460°C, more preferably 465°C, and still more preferably 469°C. The upper limit of the hot-dip galvanizing bath 103 is preferably 490°C, more preferably 480°C, still more preferably 475°C. Note that top dross is more likely to be generated in a region where the Al concentration is higher than in the Γ _two -phase dross generation region and the ζ-phase dross generation region.

As described above, in the hot-dip galvanizing method of the present embodiment, a sample is taken from the hot-dip galvanizing bath 103 (sample taking step (S1)), and the ζ-phase dross amount in the hot-dip galvanizing bath 103 is obtained (ζ Phase dross amount determination step (S2)). Then, based on the ζ-phase dross amount in the hot-dip galvanizing bath 103, the operating conditions for the hot-dip galvanizing treatment are adjusted (operating condition adjusting step (S3)). By controlling the ζ-phase dross amount, which has a negative correlation with the Γ2 _- phase dross amount, the operating conditions can be adjusted so as to suppress the occurrence of dross defects.

[Manufacturing method of galvannealed steel sheet]
The hot-dip galvanizing treatment method of the present embodiment described above can be applied to a method for manufacturing an alloyed hot-dip galvanized steel sheet (GA).

A method for manufacturing a hot-dip galvannealed steel sheet according to the present embodiment includes a hot-dip galvanizing process and an alloying process. In the hot dip galvanizing step, the steel sheet is subjected to the hot dip galvanizing method described above to form a hot dip galvanized layer on the surface of the steel sheet. On the other hand, in the alloying process, the steel sheet having the hot dip galvanized layer formed on the surface thereof by the hot dip galvanizing process is alloyed using the alloying furnace 111 shown in FIG. As for the alloying treatment method, it is sufficient to apply a well-known method.

An alloyed hot-dip galvanized steel sheet can be manufactured by the manufacturing process described above. The galvannealed steel sheet of the present embodiment employs the hot-dip galvanizing method of the present embodiment described above. That is, based on the .zeta.-phase dross amount, the operating conditions of the hot-dip galvanizing process are adjusted to increase the .zeta.-phase dross amount. Therefore, the Γ _two -phase dross in the hot-dip galvanizing bath 103 is relatively reduced, and as a result, it is possible to suppress the occurrence of dross defects in the manufactured alloyed hot-dip galvanized steel sheet.

In addition, the manufacturing method of the alloyed hot-dip galvanized steel sheet of the present embodiment may include manufacturing processes other than the hot-dip galvanizing treatment process and the alloying treatment process. For example, the method for manufacturing the alloyed hot-dip galvanized steel sheet of the present embodiment may include a skin pass rolling step of performing skin pass rolling using a skin pass rolling mill 30 shown in FIG. 1 after the alloying treatment step. . In this case, the appearance quality of the surface of the galvannealed steel sheet can be further improved. Moreover, other manufacturing processes than the temper 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 for manufacturing a hot-dip galvanized steel sheet (GI).

A method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes a hot-dip galvanizing process. In the hot dip galvanizing step, the steel sheet is subjected to the hot dip galvanizing method described above to form a hot dip galvanized layer on the surface of the steel sheet. The hot-dip galvanized steel sheet manufacturing method of the present embodiment employs the above-described hot-dip galvanizing method of the present embodiment. That is, based on the amount of .zeta.-phase dross, the operating conditions of the hot-dip galvanizing process are adjusted to increase the .zeta.-phase dross. Therefore, it is possible to suppress the occurrence of dross defects in the produced hot-dip galvanized steel sheet.

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

Hereinafter, the effect of one aspect of the hot dip galvanizing method of the present embodiment will be described more specifically with reference to examples. The conditions in the examples are examples of conditions adopted for confirming the feasibility and effects of the present invention. Therefore, the hot-dip galvanizing method of the present embodiment is not limited to this example of one condition.

In the operation condition adjustment process described above, the relationship between the Fe concentration X and the Al concentration Y was investigated.

Specifically, the hot-dip galvanizing treatment method was carried out using the hot-dip galvanizing equipment having the same configuration as that shown in FIG. Specifically, the Fe concentration X (% by mass) and the Al concentration Y (% by mass) of the hot-dip galvanizing bath were adjusted as shown in Table 1. As the steel plate, high-strength steel composed of C: 0.003%, Si: 0.006%, Mn: 0.6%, P: 0.02%, S: 0.01% and the balance: Fe and impurities Using. This high-strength steel is a so-called hard-to-alloy material that is relatively difficult to alloy when producing an alloyed hot-dip galvanized steel sheet. An alloying treatment using an alloying furnace was performed on the hot-dip galvanized steel sheet to produce an alloyed hot-dip galvanized steel sheet. The heating temperature in the alloying treatment was constant (510° C.) for all test numbers.

For each test number, a sample was taken from within a specific depth range D107 from the upper end to the lower end of the sink roll 107 in the depth direction D of the hot dip galvanizing bath 103 in FIG. More specifically, in the hot-dip galvanizing bath 103 of FIG. A sample was collected from within a defined specific region (hereinafter referred to as sample collection region). Approximately 400 g of sample was collected from within the same sample collection area as described above for each test number. The collected samples were cooled to room temperature. Using the cooled sample, the chemical composition of the hot-dip galvanizing bath of each test number was measured 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 well-known Zn-Fe-Al ternary system phase diagram, Fe concentration (Free-Fe concentration) and melting in the hot-dip galvanizing bath The Al concentration (Free-Al concentration) in the zinc plating bath was calculated. Specifically, a Zn--Fe--Al ternary phase diagram at the bath temperature at which the sample was taken was prepared. The points specified from the total-Fe concentration and the total-Al concentration obtained by the ICP emission spectrometer were plotted on the well-known Zn--Fe--Al ternary phase diagram. From the plotted points, a tie line (conjugate line) was drawn to the liquidus line in the Zn--Fe--Al ternary system phase diagram to obtain the intersection point between the liquidus line and the tie line. The Fe concentration at the intersection was defined as the Free-Fe concentration (% by mass), and the Al concentration at the intersection was defined as the Free-Al concentration (% by 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 obtained. As a result, the Fe concentration in the hot-dip galvanizing bath was within the range of 0.02 to 0.05% by mass for all test numbers.

In each test number, the Fe concentration X (% by mass) of the hot-dip galvanizing bath was kept constant at the value shown in Table 1, and the Al concentration Y (% by mass) of the hot-dip galvanizing bath was shown in Table 1. The concentration was adjusted by appropriately adding Al over time. The conveying speed of the steel sheet during hot-dip galvanizing treatment was constant for all test numbers.

Note that Table 1 also shows the values of the right-hand sides of Equations (2) and (3). Also, whether or not the Fe concentration X (mass %) and the Al concentration Y (mass %) in the hot-dip galvanizing bath satisfy the formulas (1) to (3) is described. For example, when a white circle (○) is written in the column of formula (2), Fe concentration X (mass%) and Al concentration Y (mass%) in the hot-dip galvanizing bath satisfy formula (2) indicates If a cross (x) is shown in the column of formula (2), it means that Fe concentration X (mass%) and Al concentration Y (mass%) in the hot dip galvanizing bath do not satisfy formula (2). show.

For each test number, samples were taken from the hot-dip galvanizing bath under the operating conditions shown in Table 1. Specifically, a sample of about 400 g was collected from the sample collection area described above. A test piece for observing ζ-phase dross was prepared from the collected sample. The test piece for observing ζ-phase dross had a test surface of 1 cm×1 cm and a thickness of 0.5 mm. Using a 100x SEM, the entire field of view (1 cm x 1 cm) of the test surface was observed, and dross (top dross, bottom dross) was identified based on the contrast. Furthermore, composition analysis using EPMA was performed to classify the bottom dross into Γ ₂ -phase dross, δ ₁ -phase dross, and ζ-phase dross. Furthermore, the equivalent circle diameter of each identified bottom dross (Γ ₂ -phase dross, δ ₁ -phase dross, and ζ-phase dross) was determined. Among the ζ-phase dross in the field of view of 1 cm×1 cm, the number of ζ-phase dross having an equivalent circle diameter of 10 μm or more was determined. The number of ζ-phase dross having an equivalent circle diameter of 10 μm or more in the observation field (pieces/1 cm ² ) was defined as the amount of ζ-phase dross. Table 1 shows the obtained ζ-phase dross amount. In this example, no Γ1 _- phase dross was observed in any of the test numbers.

[Dross defect evaluation test]
After the hot-dip galvanizing treatment was performed under the operating conditions of each test number, the alloying treatment was performed under the same conditions for each test number to produce alloyed hot-dip galvanized steel sheets. 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 dross defect evaluation were as follows.
A: No dross defects (the number of dross defects is 0/m ² )
B: The number of dross defects is more than 0 and 0.1/m ² or less C: The number of dross defects is more than 0.1/m ² and 1/m ² or less

[Evaluation test for alloying of hard-to-alloy materials]
The chemical composition of the alloyed hot-dip galvanized layer on the surface of the alloyed hot-dip galvanized steel sheet manufactured under the operating conditions of each test number was investigated, and the alloying of the hard-to-alloy material was evaluated. Specifically, the chemical composition of the galvannealed layer was analyzed using an energy dispersive X-ray fluorescence analyzer (EDX-7000) manufactured by Shimadzu Corporation. The value obtained by dividing the Fe content (% by mass) contained in the alloyed hot-dip galvanized layer by the Zn content (% by mass) contained in the alloyed hot-dip galvanized layer was calculated to evaluate the alloying. The criteria for alloying evaluation were as follows. In addition, when the ratio of the Fe content to the Zn content was 11% or more, it was judged to be a superalloy.
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 is more than 9% and less than 10% C: The ratio of Fe content to Zn content is 9% Less than

[Evaluation results]
With reference to Table 1, in test numbers 1, 2, 5, 6, 8 to 13 in which the ζ-phase dross amount was controlled to 5.0 pieces/cm ² or more, the dross defect evaluation was A or B, and the dross defect was able to be suppressed more effectively. Furthermore, in test numbers 1, 2, 5, 6, 8 to 13, the alloying evaluation of the hard-to-alloy materials was A or B, and even the hard-to-alloy materials were able to promote alloying more effectively. On the other hand, in Test Nos. 3, 4, and 7 in which the amount of ζ-phase dross was less than 5.0 pieces/cm ² , the dross defect evaluation and the alloying evaluation of the hard-to-alloy material were C. Furthermore, referring to Test Nos. 1 to 13, the higher the ζ-phase dross amount, the better the dross defect evaluation. In other words, the ζ-phase dross amount and the number of dross defects showed a negative correlation.

From the above results, it was found that dross defects can be suppressed by adjusting the operating conditions based on the ζ-phase dross amount. Preferably, the threshold value of the ζ-phase dross amount is set to 5.0 pieces/cm ² , and the operating conditions in the hot-dip galvanizing process are adjusted so that the ζ-phase dross amount is 5.0 pieces/cm ² or more. It was found that dross defects can be remarkably suppressed by doing so.

In test numbers 1, 2, 5, 6, 8 to 13, which satisfy the formulas (1) and (2), the dross defect evaluation was A or B, and the dross defects could be suppressed more effectively. Furthermore, in test numbers 1, 2, 5, 6, 8 to 13, the alloying evaluation of the hard-to-alloy materials was A or B, and even the hard-to-alloy materials were able to promote alloying more effectively. Therefore, it was found that adjusting the operating conditions so as to satisfy the formulas (1) and (2) is effective for suppressing dross defects and promoting alloying of hard-to-alloy materials.

In Test Nos. 1, 5, 8, 9, 11 and 12, which satisfy the formulas (1) and (3), the dross defect evaluation was A, and the dross defects could be suppressed more effectively. In Test Nos. 1, 5, 8, 9, 11 and 12, the alloying evaluation of the hard-to-alloy material was A, and even with the hard-to-alloy material, the alloying could be promoted more effectively. Therefore, it was found that adjusting the operating conditions so as to satisfy the formulas (1) and (3) is more effective in suppressing dross defects and promoting alloying of hard-to-alloy materials.

In test numbers 14 and 16, in which the Al concentration Y in the hot-dip galvanizing bath is 0.0990% by mass, the dross defect evaluation is "A", and the promotion of alloying even for hard-to-alloy materials. However, overalloying occurred in the production of alloyed hot-dip galvanized steel sheets. Therefore, it became clear 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 hard-to-alloy material was "C". Therefore, it became clear 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 can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

10 hot-dip galvanizing equipment 101 hot-dip zinc pot 103 hot-dip galvanizing bath 107 sink roll 109 gas wiping device 111 alloying furnace 202 snout

Claims (10)

A hot-dip galvanizing method used for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet,
A sample collecting step of collecting a sample from a hot dip galvanizing bath containing Al;
a ζ-phase dross amount determination step of determining the ζ-phase dross amount in the hot-dip galvanizing bath using the collected sample;
an operating condition adjusting step of adjusting the operating conditions of the hot-dip galvanizing process so as to increase the ζ-phase dross based on the obtained ζ-phase dross amount;
Hot-dip galvanizing treatment method. The hot-dip galvanizing method according to claim 1,
In the ζ-phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area is obtained as the ζ-phase dross amount;
Hot-dip galvanizing treatment method. The hot-dip galvanizing method according to claim 1 or 2,
In the operating condition adjustment step,
increasing the ζ-phase dross amount by performing at least one of (A) or (B) based on the obtained ζ-phase dross amount;
Hot-dip galvanizing treatment method.
(A) Adjusting the bath temperature of the hot-dip galvanizing bath.
(B) Adjusting the Al concentration of the hot-dip galvanizing bath. The hot-dip galvanizing method according to any one of claims 1 to 3,
In the operating condition adjustment step,
when the obtained ζ-phase dross amount is less than a threshold value, adjusting the operating conditions of the hot-dip galvanizing treatment to increase the ζ-phase dross amount;
When the obtained ζ-phase dross amount is equal to or greater than a threshold value, maintaining the operating conditions of the hot-dip galvanizing treatment as they are;
Hot-dip galvanizing treatment method. The hot-dip galvanizing method according to claim 4,
In the ζ-phase dross amount determination step,
Using the collected sample, the number of ζ-phase dross per predetermined area is obtained as the ζ-phase dross amount,
In the operating condition adjustment step,
When the obtained ζ-phase dross amount is less than 5.0 pieces/cm ² in terms of unit area (1 cm ² ), the operating conditions of the hot-dip galvanizing treatment are adjusted to reduce the ζ-phase dross amount. increase ,
When the obtained ζ-phase dross amount is 5.0 pieces/cm ² or more in terms of unit area (1 cm ² ), the operating conditions of the hot-dip galvanizing treatment are maintained as they are.
Hot-dip galvanizing treatment method. The hot-dip galvanizing method according to any one of claims 1 to 5,
In the operating condition adjustment step,
The Fe concentration in the hot dip galvanizing bath is defined as X (mass%), the Al concentration in the hot dip galvanizing bath is defined as Y (mass%), and the Fe concentration and Al in the hot dip galvanizing bath adjusting the concentration to satisfy equations (1) and (2);
Hot-dip galvanizing treatment method.
0.100≤Y≤0.139 (1)
Y≦0.2945X+0.1216 (2) The hot-dip galvanizing method according to claim 6,
In the operating condition adjustment step,
The Fe concentration in the hot dip galvanizing bath is defined as X (mass%), the Al concentration in the hot dip galvanizing bath is defined as Y (mass%), and the Fe concentration and Al in the hot dip galvanizing bath adjusting the concentration to satisfy equations (1) and (3);
Hot-dip galvanizing treatment method.
0.100≤Y≤0.139 (1)
Y≦0.2945X+0.1066 (3) The hot-dip galvanizing method according to any one of claims 1 to 7,
A sink roll is arranged in the hot dip galvanizing pot in which the hot dip galvanizing bath is stored to contact the steel plate immersed in the hot dip galvanizing bath and change the traveling direction of the steel plate up and down. ,
In the sample collection step,
The sample is collected from the depth range from the upper end to the lower end of the sink roll in the hot dip galvanizing bath in the hot dip galvanizing pot.
Hot-dip galvanizing treatment method. a hot-dip galvanizing step of performing the hot-dip galvanizing method according to any one of claims 1 to 8 on a steel plate to form a hot-dip galvanized layer on the surface of the steel plate;
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;
A method for producing an alloyed hot-dip galvanized steel sheet. A hot-dip galvanizing step of forming a hot-dip galvanized layer on the surface of the steel sheet by performing the hot-dip galvanizing method according to any one of claims 1 to 8 on a steel sheet. A method for producing a hot-dip galvanized steel sheet.

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