TW202012654A - Manufacturing method for hot-dip galvanized steel sheet, and manufacturing method for alloyed hot-dip galvanized steel sheet - Google Patents

Abstract

Provided is a manufacturing method for a hot-dip galvanized steel sheet which can suppress the occurrence of a dross defect, even if bottom dross is formed in a hot-dip galvanizing process. The manufacturing method according to the present invention comprises: a step for forming coarse bottom dross, in which the concentration of free Al CAl in a hot-dip galvanizing bath and a bath temperature T are adjusted so as to fulfill formula (1), and coarse bottom dross is formed in the hot-dip galvanizing bath; and a step for carrying out a hot-dip galvanizing process in which the concentration of the free Al CAl in the hot-dip galvanizing bath, including the coarse bottom dross, and the bath temperature T are adjusted so as to fulfill formula (2), and a hot-dip galvanizing process is carried out to create a hot-dip galvanized layer on a steel sheet. Formula (1): 466.15*CAl+385.14 ≤ T ≤ 577.24*CAl+382.49 Formula (2): 390.91*CAl+414.20 ≤ T ≤ 485.00 The "CAl" in formula (1) and formula (2) is substituted with the concentration of the free Al CAl (mass%) in the hot-dip galvanizing bath.

Description

Manufacturing method of hot-dip galvanized steel sheet and manufacturing method of alloyed hot-dip galvanized steel sheet

The present disclosure relates to a method for manufacturing hot-dip galvanized steel sheet and a method for manufacturing alloyed hot-dip galvanized steel sheet.

The hot-dip galvanized steel sheet (hereinafter also referred to as GI) and the alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA) are manufactured by the following manufacturing method. First, a steel sheet (base material steel sheet) to be subjected to hot-dip galvanizing treatment is prepared. The base material steel plate may be a hot rolled steel plate or a cold rolled steel plate. The prepared base material steel sheet (the above-mentioned hot-rolled steel sheet or cold-rolled steel sheet) is immersed in a hot-dip galvanizing bath, and hot-dip galvanizing treatment is performed to manufacture a hot-dip galvanized steel sheet. When manufacturing an alloyed hot-dip galvanized steel sheet, the hot-dip galvanized steel sheet is further heat-treated in an alloying furnace to manufacture an alloyed hot-dip galvanized steel sheet.

The details of the hot-dip galvanizing treatment performed in the manufacturing steps of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are as follows. The hot-dip galvanizing equipment used in the hot-dip galvanizing process includes a molten zinc pot that houses the hot-dip galvanizing bath, a settling roller disposed in the hot-dip galvanizing bath, and a gas purge device.

In the hot-dip galvanizing process, for example, the steel plate subjected to the passivation treatment is immersed in a hot-dip galvanizing bath. Next, by using a settling roller arranged in the hot-dip galvanizing bath, the traveling direction of the steel plate is turned upward, and the steel plate is pulled up from the hot-dip galvanizing bath. For a steel plate that is pulled up and travels upward, a purge gas is blown onto the surface of the steel plate from the gas purge device. The sprayed gas scrapes off the remaining molten zinc to adjust the amount of plating adhesion on the surface of the steel plate. By the above method, hot galvanizing treatment is performed. In addition, when manufacturing an alloyed hot-dip galvanized steel sheet, the steel sheet in which the amount of plating adhesion is adjusted is further charged into an alloying furnace to perform alloying treatment.

Therefore, in the above hot-dip galvanizing treatment, Fe is eluted from the steel sheet immersed in the hot-dip galvanizing bath to the hot-dip galvanizing bath. When Fe eluted from the steel sheet into the hot-dip galvanizing bath reacts with Al or Zn present in the hot-dip galvanizing bath, an intermetallic compound called slag is formed. There are top dross and bottom dross in the broken slag. Scum is an intermetallic compound with a lighter specific gravity than the hot-dip galvanizing bath. It is a broken dross floating on the liquid surface of the hot-dip galvanizing bath. The bottom slag is the intermetallic compound whose specific gravity is heavier than that of the hot-dip galvanizing bath, and is accumulated in the bottom of the molten zinc pot.

In the hot dip galvanizing process, accompanying flow occurs as the steel sheet in the hot dip galvanizing bath proceeds. The accompanying flow means that as the steel sheet travels, it flows in the hot-dip galvanizing bath. As mentioned above, the scum floats on the liquid surface of the hot-dip galvanizing bath, so it is less affected by the accompanying flow. In contrast, bottom slag accumulates at the bottom of the molten zinc pot. Therefore, there is a case where the bottom of the molten zinc pot that has accumulated due to the flow rises. In this case, the bottom dross floats in the hot-dip galvanizing bath. Such floating bottom slag may adhere to the surface of the steel plate during the hot-dip galvanizing process.

The bottom slag adhering to the surface of the steel sheet may become point defects on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. The surface defects caused by these bottom slags are called "debris defects" in this manual. Debris defects will reduce the appearance of alloyed galvanized steel sheets and galvanized steel sheets, or reduce corrosion resistance. Therefore, it is preferable to suppress the occurrence of chipping defects.

Techniques for suppressing the occurrence of chipping defects have been proposed in Japanese Patent Laid-Open No. 11-350096 (Patent Document 1) and Japanese Patent Laid-Open No. 11-350097 (Patent Document 2).

In Patent Document 1, in the method of manufacturing an alloyed hot-dip galvanized steel sheet, the molten zinc bath temperature is set to T (°C), and the boundary Al concentration defined by Cz=-0.015×T+0.76 is set to Cz (wt%) . In this case, the hot-dip galvanizing temperature T is kept in the range of 435 to 500°C, and the Al concentration in the bath is kept in the range of Cz±0.01wt%. In Patent Document 1, hot dip galvanizing is performed at the boundary between the δ ₁ phase and the ζ phase. In Patent Document 2, in the manufacturing method of the alloyed hot-dip galvanized steel sheet, the Al concentration in the bath is kept within the range of 0.15±0.01wt%. In Patent Document 2, hot-dip galvanizing is performed at the boundary between scum and δ ₁ phase. [Prior Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 11-350096 [Patent Document 2] Japanese Patent Laid-Open No. 11-350097 [Non-patent literature]

[Non-Patent Literature 1] Practical Applications of Phase Diagrams in Continuous Galvanizing, Nai-Yong Tang, Journal of Phase Equilibria and Diffusion Vol. 27, No. 5, 2006

[Problem to be solved by invention]

As described in Patent Documents 1 and 2, it can be seen that when the Al concentration in the hot-dip galvanizing bath is high, most of the broken slag is not bottom slag but scum. The scum floats on the surface of the molten galvanizing bath. Therefore, it is easier to remove the dross from the hot-dip galvanizing bath than to remove the bottom dross from the hot-dip galvanizing bath. Therefore, in the conventional hot-dip galvanizing treatment, the Al content in the hot-dip galvanizing bath is sometimes increased, so that the dross in the hot-dip galvanizing bath is floated on the molten galvanizing bath liquid surface as scum and removed to suppress the dross defects The way it happened. In this way, the operation of generating scum as crushed slag is referred to as a scum operation in this specification.

By the operation of scum, the defects of broken slag can be suppressed. However, when the Al concentration in the hot-dip galvanizing bath is increased, the hot-dip galvanized layer in the alloying process becomes difficult to alloy. Therefore, in order to promote alloying, it is particularly preferable to suppress the Al concentration in the hot-dip galvanizing bath. When the scumming operation is carried out, the Al concentration in the hot-dip galvanizing bath also always becomes high.

The operation of suppressing the Al concentration in the hot-dip galvanizing bath and generating bottom slag as broken slag is called bottom slag operation in this specification. In the case of bottom slag operation, in order to suppress the free Al concentration in the hot-dip galvanizing bath, alloying can be promoted. However, the bottom slag operation requires a method that can suppress chipping defects caused by bottom slag formation.

An object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that can suppress the occurrence of chipping defects even if a bottom dross is generated in the hot-dip galvanizing process. [Means to solve the problem]

The manufacturing method of the hot-dip galvanized steel sheet of the present disclosure includes the following steps: The free Al concentration C _Al (mass %) and the bath temperature T (°C) in the hot-dip galvanizing bath are adjusted in a manner that satisfies the formula (1) to melt A coarse bottom slag generation step for generating a coarse bottom slag having a particle diameter of 300 μm or more in the galvanizing bath, and adjusting the free Al concentration C _Al and the bath temperature T of the molten galvanizing bath after the coarse bottom slag generation step to Satisfying formula (2), using the above-mentioned free Al concentration C _Al and the above bath temperature T satisfying the above-mentioned hot-dip galvanizing bath, performing hot-dip galvanizing treatment, forming a hot-dip galvanizing layer on the steel sheet, a hot-dip galvanizing treatment step,

The manufacturing method of the alloyed hot-dip galvanized steel sheet of the present disclosure includes the following steps: Implementing the above method of manufacturing the hot-dip galvanized steel sheet, the steps of manufacturing the hot-dip galvanized steel sheet, and The alloying step of alloying the aforementioned hot-dip galvanized steel sheet. [Effect of the invention]

The manufacturing method of the hot-dip galvanized steel sheet and the manufacturing method of the alloyed hot-dip galvanized steel sheet of the present disclosure can suppress the occurrence of chipping defects even if a bottom dross is generated in the hot-dip galvanizing process.

[Regarding the causes of slag defects]

The inventors first reviewed the dross that became the cause of dross defects when the bottom dross operation was performed. The slag defects are caused by the slag generated during the hot-dip galvanizing process. It has been reported in previous studies that the following types of slag generated during the hot dip galvanizing process exist. (A) Scum (B) δ ₁ phase crushed slag (C) Γ ₁ phase crushed slag (D) ζ phase crushed slag

The scum is lighter than the hot-dip galvanizing bath as mentioned above. Therefore, scum is likely to float on the surface of the hot-dip galvanizing bath. The chemical composition of the scum, in terms of mass %, is composed of 45% Al, 38% Fe and 17% Zn. Since the scum floats on the liquid surface of the hot-dip galvanizing bath, it is easily removed from the hot-dip galvanizing bath. Therefore, the scum operation can be effectively suppressed by removing scum.

On the other hand, δ ₁ phase slag, Γ ₁ phase slag and ζ phase slag are called bottom slag. The bottom slag is heavier than the hot-dip galvanizing bath. Therefore, the bottom slag is easy to accumulate on the bottom of the hot-dip galvanizing pot that stores the hot-dip galvanizing bath. When carrying out the bottom slag operation, when such bottom slag becomes the cause of defects, it will be considered in previous studies.

Here, the crystal structure of the delta ₁ phase slag is hexagonal. The chemical composition of δ ₁ phase slag, in mass%, is composed of less than 1% Al, more than 9% Fe and more than 90% Zn. The crystal structure of Γ ₁ phase slag is face-centered cubic crystal. The chemical composition of Γ ₁ phase slag, in mass%, is composed of 20% Fe and about 80% Zn. The crystal structure of the ζ phase is monoclinic. The chemical composition of the ζ phase, in mass%, is composed of less than 1% Al, about 6% Fe, and about 94% Zn.

In previous studies, there are many reported cases of bottom slag, the main reason for the slag defect is δ ₁ phase slag. In the above-mentioned Patent Documents 1 and 2, δ ₁ phase slag is also considered to be one of the causes of slag defects. Therefore, the inventors initially considered that the delta ₁ phase slag is the main cause of slag defects, and conducted investigations and studies. However, when suppressing δ ₁ phase dross during the hot-dip galvanizing process, dross defects may still occur on the surfaces of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet.

Therefore, the inventors believe that the cause of the occurrence of the slag defect is not δ ₁ phase slag but other slag. Therefore, the present inventors used the alloyed hot-dip galvanized steel sheet with slag defects to perform the bottom slag operation, and re-analyzed the composition and crystal structure of the slag defects. The present inventors further analyzed the type of slag that occurred in the hot-dip galvanizing bath during the bottom slag operation. As a result, the present inventors have obtained the following insights that are different from the results of previous researches on the slag defects.

First, EPMA (Electron Probe Micro Analyzer: Electron Beam Micro Analyzer) was used to analyze the chemical composition of the slag defect portion on the surface of the alloyed galvanized steel sheet. In addition, the TEM (Transmission Electron Microscope) is used to analyze the crystal structure of the slag portion. As a result, the chemical composition of the defective part of the slag is 2% of Al, 8% of Fe and 90% of Zn, and the crystal structure is a face-centered cubic crystal.

In the past, the chemical composition of δ ₁ phase slag (mainly 1% or less Al, 9% or more Fe, and 90% or more Zn), which is considered to be the main cause of slag defects, and the above slag defect parts The chemical composition is similar. However, the crystal structure of the delta ₁ phase slag is hexagonal crystal, not a face-centered cubic crystal specified in the slag defect part. Therefore, the inventors believe that the δ ₁ phase slag, which was considered to be the main cause of slag defects, is not actually the main cause of the slag defects.

Therefore, the present inventors specified the slag which is the cause of slag defects. Among the above-mentioned bottom slags (B) to (D), the Γ ₁ phase slag has a crystal structure with the same face-centered cubic crystal as the defective part of the slag, but its chemical composition (in mass% is 20 % Fe and 80% Zn) are very different from the chemical composition of the broken slag defects. Regarding the ζ-phase slag, the chemical composition (in terms of mass%, less than 1% of Al, about 6% of Fe, and about 94% of Zn) is different from the chemical composition of the defective part of the slag, and the crystal structure (monoclinic ) Is also different from the crystal structure (face-centered cubic crystal) of the defective part of the slag.

Based on the results of the above review, the inventors believe that the slag defects are not caused by the slag from (B) to (D) above. Therefore, the inventors believe that the slag defects may be caused by other types of bottom slag than (B) to (D).

Therefore, the present inventors further performed bottom slag analysis in the hot-dip galvanizing bath. For the analysis of bottom slag, the aforementioned EPMA and TEM were used. As a result, the present inventors investigated that the bottom slag generated in the hot-dip galvanizing bath exists as Γ ₂ phase crushed slag.

The chemical composition of the Γ ₂ phase slag, in mass%, is composed of 2% Al, 8% Fe, and 90% Zn, which is consistent with the chemical composition of the slag defect part analyzed above. Furthermore, the crystal structure of the Γ ₂ phase slag is a face-centered cubic crystal, which is consistent with the crystal structure of the slag defect. Therefore, the present inventors considered whether the Γ ₂ phase slag is the main cause of slag defects. Moreover, since the proportion of Γ ₂ phase crushed slag is greater than that of the molten galvanizing bath, the Γ ₂ phase crushed slag is equivalent to bottom slag that can accumulate on the bottom of the molten zinc pot.

The inventors further investigated Γ ₂ phase slag and other (B) to (D) slag. As a result, it was found that the slag defects were caused by hard slag, and soft slag was not easy to form slag defects. As a result of further review by the present inventors, it was determined that among the above-mentioned crushed slags (B) to (D) and the Γ ₂ phase crushed slag, the Γ ₂ phase crushed slag is the hardest crushed slag.

Based on the results of the above review, the present inventors believe that the main cause of the slag defects on the surface of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet subjected to hot-dip galvanizing treatment is not δ ₁ phase slag, but Γ ₂ phase slag Slag. Furthermore, the inventors obtained the insight that the broken slag classified into bottom slag is any of Γ ₂ phase slag, δ ₁ phase slag, ζ phase slag, and Γ ₁ phase slag, but In the hot dip galvanizing bath, Γ ₁ phase slag is almost absent.

Therefore, the present inventors further reviewed the slag defects when performing the bottom slag operation to suppress the free Al concentration in the hot-dip galvanizing bath. A part of the bottom slag that accumulates on the bottom of the molten zinc pot due to the accompanying flow that occurs when the steel plate passes through the molten galvanizing bath. Therefore, the raised bottom slag adheres to the steel plate. In this case, slag defects may occur.

In this article, the inventors waited for the review of the bottom slag size. As a result, the inventors obtained the following findings. The bottom slag with a particle size of less than 100 μm is defined as a fine bottom slag. The fine bottom slag may rise from the bottom of the molten zinc pot in the molten zinc plating bath due to the accompanying flow. However, even if the fine bottom slag is raised and attached to the steel plate, the size of the bottom slag is small, so it is not likely to become a slag defect. On the other hand, bottom dross with a particle size exceeding 300 μm is defined as coarse bottom dross. The quality of coarse bottom slag is heavier. Therefore, the coarse bottom slag is less likely to be lifted by the accompanying flow, and is less likely to adhere to the steel plate. As a result of the above investigation, the present inventors have found that the bottom slag that causes the slag defect is a bottom slag having a particle size of 100 to 300 μm (hereinafter also referred to as a medium-sized bottom slag).

Therefore, the present inventors believe that when the bottom slag operation is carried out, during the period during which the hot-dip galvanizing treatment is performed (hereinafter also referred to as during the operation period), if the generation of medium-sized bottom slag can be suppressed, the slag defects can also be effectively suppressed.

The inventors first focused on the growth rate of each bottom slag in order to suppress the formation of medium-sized bottom slag. Among the above bottom slags (B) to (D) and Γ ₂ phase crushed slag, Γ ₂ phase crushed slag grows fastest, and δ ₁ phase crushed slag is the slowest. Thus, Γ ₂ δ ₁ phase ratio of the slag phase grow faster slag, the slag in an earlier phase than the δ ₁ phase, the particle size of the slag phase Γ ₂ exceeds 100μm. In contrast, the growth rate of δ ₁ phase slag is significantly slower than that of Γ ₂ phase slag. Therefore, even if the δ ₁ phase slag core grows, the δ ₁ phase slag does not easily grow earlier than the Γ ₂ phase. Therefore, it is considered that it is better to perform the hot-dip galvanizing treatment on the growth area of the δ ₁ phase than the growth area of the Γ ₂ phase slag during the hot-dip galvanizing treatment step (operation period).

Therefore, the inventors further investigated and reviewed the temperature T (° C.) of the hot-dip galvanizing bath, the free Al concentration C _Al (mass %) of the hot-dip galvanizing bath, and the state of the generated slag. As a result, the present inventors prepared a quasi-stable state diagram of slag in the hot-dip galvanizing bath shown in FIG. 1. The following describes FIG. 1.

The vertical axis of FIG. 1 represents the free Al concentration C _Al (mass %) of the hot-dip galvanizing bath. Herein, the "free Al concentration C _Al "of the hot dip galvanizing bath in this specification means the free Al concentration (mass %) melted in the hot dip galvanizing bath. That is, in this specification, the so-called "free Al concentration C _{Al of the} hot-dip galvanizing bath" means that the Al content contained in the slag (scum and bottom slag) is not included, and is melted in the hot-dip galvanizing bath (that is, liquid Phase) free Al concentration (mass%). The horizontal axis of Fig. 1 represents the bath temperature T (°C) in the hot-dip galvanizing bath.

Referring to FIG. 1, in the range of free Al concentration C _Al shown in FIG. 1 and bath temperature T (° C.), there is a region 1A (hereinafter referred to as a scum generation region 1A) where dross is generated in the hot-dip galvanizing bath, and Γ is generated Region 2 of the _2- phase slag (hereinafter referred to as Γ ₂ phase generation region 2), and region 3 of the δ ₁ phase slag generation (hereinafter referred to as δ ₁ phase generation region 3).

The scum generation region 1A and the Γ ₂ phase generation region 2 are distinguished by the abnormal line F ₁₂ . The scum generation region 1A and the δ ₁ phase generation region 3 are distinguished by the abnormal line F ₁₃ . The Γ ₂ phase generation region 2 and the δ ₁ phase generation region 3 are distinguished by the abnormal line F ₂₃ .

For example, a Γ ₂ phase slag is generated in a hot-dip galvanizing bath with a bath temperature T of 440°C and a free Al concentration of C _Al of 0.135%. Assuming that the free Al concentration C _Al in the hot-dip galvanizing bath is still maintained at 0.135%, and the bath temperature T is increased from 440°C to 470°C. In this case, the state of the hot dip galvanizing bath moves from the Γ ₂ phase generation region 2 to the abnormal line F ₂₃ and moves to the δ ₁ phase generation region 3. Therefore, the Γ ₂ phase slag in the hot-dip galvanizing bath undergoes a phase transformation into δ ₁ phase slag. Further, assume that the bath temperature was 440. T deg.] C, and the free Al concentration C _Al of 0.135% of the molten galvanizing bath of Al concentration of free C 0.140% of _Al to improve the situation. In this case, the state of the hot-dip galvanizing bath moves from the Γ ₂ phase generation region 2 to the abnormal line F ₁₂ and moves to the scum generation region 1A. Therefore, the Γ ₂ phase slag in the hot-dip galvanizing bath undergoes a phase transformation to become scum.

The inventors further found that in the Γ ₂ phase generation region 2 of the quasi-stabilized state diagram shown in FIG. 1, there is a boundary line F ₂₁₂₂ that distinguishes the Γ ₂ nucleus generation region 21 from the Γ ₂ grain growth region 22. The inventors further found that in the δ ₁ phase generation region 3 of the quasi-stabilized state diagram shown in FIG. 1, there is a boundary line F ₃₁₃₂ that distinguishes the δ ₁ nucleus generation region 31 from the δ ₁ grain growth region 32. Hereinafter, this point will be described.

In the Γ ₂ phase generation region 2, the Γ ₂ nucleus generation region 21 is located on the lower temperature side of the boundary line F ₂₁₂₂ than the Γ ₂ grain growth region 22. The Γ ₂ nucleation region 21 promotes nucleation of Γ ₂ phase slag in the hot dip galvanizing bath compared to the Γ ₂ grain growth region 22. That is, the formation of fine Γ ₂ phase slag is promoted. On the other hand, the Γ ₂ grain growth region 22 promotes the growth (grain growth) of the Γ ₂ phase already existing in the hot dip galvanizing bath compared to the Γ ₂ nucleation region 21.

Similarly, in the δ ₁ phase generation region 3, the δ ₁ nucleus generation region 31 is located on the higher temperature side of the boundary line F ₃₁₃₂ than the δ ₁ grain growth region 32. The δ ₁ nucleation region 31 promotes nucleation of δ ₁ phase slag in the hot dip galvanizing bath compared to the δ ₁ grain growth region 32. That is, the formation of fine δ ₁ phase slag is promoted. On the other hand, the δ ₁ grain growth region 32 promotes the growth (grain growth) of the δ ₁ phase already existing in the hot dip galvanizing bath compared to the δ ₁ nucleation region 31.

再者，邊界線F₂₁₂₂ 可藉下述式(B)定義。

再者，邊界線F₃₁₃₂ 可藉下述式(C)定義。

此處，式(A)~式(C)中之「C_Al 」係代入熔融鍍鋅浴中之游離Al濃度C_Al (質量%)。The abnormal line F ₂₃ in the quasi-stability state diagram can be defined by the following formula (A).

Furthermore, the boundary line F ₂₁₂₂ can be defined by the following formula (B).

Furthermore, the boundary line F ₃₁₃₂ can be defined by the following formula (C).

Here, “C _Al ”in formulas (A) to (C) is the free Al concentration C _Al (mass %) substituted into the hot-dip galvanizing bath.

Based on the quasi-stabilized state diagram of FIG. 1, the inventors of the present invention reviewed the hot-dip galvanizing treatment method that can suppress slag defects. As mentioned above, the slag defects are caused by medium-sized bottom slag with a particle size of 100 to less than 300 μm. During processing in the embodiment of the hot-dip galvanized (during the operation), to suppress the formation of bottom dross of medium, as long as the state of the hot-dip galvanizing bath of FIG. 31 become the molten galvanizing bath adjusted as free [delta] 1 ₁ nucleation region Al concentration C _Al and bath temperature T are sufficient. [delta] ₁ nucleation region 31, although promoting the nucleation of the slag phase [delta] _1, [delta] but inhibited the growth _phase into the slag. Furthermore, as mentioned above, among the bottom slag, the growth rate of δ ₁ phase crushed slag is the slowest. Therefore, if the hot-dip galvanizing bath during the operation is made into the δ ₁ nucleus generation region 31, the growth of the bottom slag (here, the δ ₁ phase crushed slag) can be suppressed to a medium-sized bottom slag.

However, even if the hot-dip galvanizing bath during the operation period is maintained in the δ ₁ nucleus generation region 31, if the execution period (operation period) of the hot-dip galvanizing treatment is a long period, fine δ ₁ phase slag will grow to some extent. Therefore, only the hot-dip galvanizing bath during the operation period is set as the δ ₁ nucleus generation region 31. If the operation period is a long period, there is a possibility that medium-sized bottom dross is generated.

When the medium-sized bottom slag in the hot-dip galvanizing bath is increased, if the bottom slag is removed from the molten zinc pot (hereinafter referred to as bottom slag removal step), the occurrence of chipping defects can be suppressed. However, when the bottom dross removal step is carried out, it is necessary to stop the hot-dip galvanizing process and stop the continuous hot-dip galvanizing production line equipment. The state where the hot-dip galvanizing treatment is stopped in this way is called "stop" in this manual. When the above-mentioned bottom slag removal step is adopted, if the frequency of the bottom slag removal step becomes higher, the production efficiency decreases.

Therefore, the present inventors further reviewed a method for suppressing the frequency of the bottom dross removal step and sufficiently suppressing the dross defects when the hot galvanizing treatment is performed by the bottom slag operation. As a result, the present inventors found that in a hot-dip galvanizing bath, after intentionally containing coarse bottom slag with a particle size of 300 μm or more in advance, hot-dip galvanizing treatment is performed in the δ ₁ nucleus generation region 31, and the medium-sized bottom can be suppressed for a long period of time Slag generation. This point will be described below.

When the bottom slag is at a certain size, Osvald grows and grows. The so-called Osvaldian growth is that when the same type of metal particles with different particle sizes exist in the parent phase (Zn in the liquid phase in this specification), the metal particles with smaller particle sizes shrink or disappear, and the metal particles with larger particle sizes further The phenomenon of growing to coarseness.

In the present embodiment, the hot-dip galvanizing bath before the hot-dip galvanizing treatment contains a coarse bottom slag having a particle diameter of 300 μm or more in advance. Then, a hot-dip galvanizing bath containing coarse bottom dross is used to perform hot-dip galvanizing treatment on the delta ₁ nucleus generation area 31. In this case, during the implementation of the hot-dip galvanizing process, although the nucleus generated δ ₁ phase slag, the nucleated δ ₁ phase slag shrank or disappeared, and it was coarse bottom slag (in this case, the coarse δ ₁ phase slag) absorb. That is, when a hot-dip galvanizing bath containing a coarse bottom slag is used in advance, the fine δ ₁ phase crushed slag grows by Oswald and is absorbed by the coarse bottom slag (coarse δ ₁ phase crushed slag). The growth of the coarse bottom slag also increases the quality of the coarse bottom slag, so it is not easy to rise due to the accompanying flow. In the case of this method, even if the implementation period of the hot-dip galvanizing treatment becomes longer, the generation of fine δ ₁ phase slag can be suppressed. Therefore, the formation of medium-sized bottom slag is further suppressed. As a result, even if the operation period is a long period, the occurrence of chipping defects can be suppressed.

In short, in this embodiment, after the coarse bottom slag is contained in the hot-dip galvanizing bath in advance, the hot-dip galvanizing treatment is performed in the δ ₁ core generation region 31. In this case, the coarse bottom slag is further grown by the growth of Oswald, which not only suppresses the growth of fine bottom slag (fine δ ₁ phase crushed slag) but also suppresses the generation. As a result, it is possible to effectively suppress the formation of medium-sized bottom slag which is the cause of chipping defects.

As described above, the manufacturing method of the hot-dip galvanized steel sheet of the present embodiment means that the hot-dip galvanizing treatment is carried out in a state where the coarse bottom slag contained in the hot-dip galvanizing bath is included in the object that should be removed in the past, and it is based on a completely different from the past The technical thought is completed. Specifically, the manufacturing method of the hot dip galvanized steel sheet of this embodiment is as follows.

[1] A method for manufacturing a hot-dip galvanized steel sheet, comprising the following steps: adjusting the free Al concentration in the hot-dip galvanizing bath C _Al and the bath temperature T in a manner satisfying the formula (1) in the aforementioned hot-dip galvanizing bath A coarse bottom slag generation step that generates a coarse bottom slag having a particle size of 300 μm or more, and the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath after the coarse bottom slag generation step are adjusted to satisfy equation (2) ), using the above-mentioned free Al concentration C _Al and the above bath temperature T satisfying the above-mentioned hot-dip galvanizing bath of formula (2), performing hot-dip galvanizing treatment, forming a hot-dip galvanizing layer on the steel sheet, a hot-dip galvanizing treatment step,

[2] The method of manufacturing the hot-dip galvanized steel sheet is the method of manufacturing the hot-dip galvanized steel sheet described in [1], wherein when the shutdown of the hot-dip galvanizing treatment step is stopped, the melting after the hot-dip galvanizing treatment step is performed. The galvanizing bath performs the aforementioned coarse bottom slag generation step.

[3] The method of manufacturing a hot-dip galvanized steel sheet is the method of manufacturing a hot-dip galvanized steel sheet according to [1] or [2], wherein in the hot-dip galvanizing treatment step, the hot-dip plating after the coarse bottom dross generating step The aforementioned bath temperature T of the zinc bath is increased to prepare the aforementioned hot-dip galvanizing bath satisfying the formula (2).

[4] The method of manufacturing the hot-dip galvanized steel sheet is the method of manufacturing the hot-dip galvanized steel sheet described in [3], wherein the coarse bottom slag generation step and the hot-dip galvanizing treatment step are alternately repeated. After the hot galvanizing treatment step, when performing the coarse bottom slag generation step, in the coarse bottom slag step, the bath temperature T of the hot galvanizing bath after the hot galvanizing treatment step is reduced to satisfy the formula ( 1) The aforementioned hot-dip galvanizing bath.

[5] The manufacturing method of the hot-dip galvanized steel sheet is the manufacturing method of the hot-dip galvanized steel sheet according to any one of [1] to [4], wherein the melting of the coarse bottom slag generation step and the hot-dip galvanizing treatment step The aforementioned free Al concentration C _Al in the galvanizing bath is 0.125% by mass or more.

[6] The method for manufacturing hot-dip galvanized steel sheet is the method for manufacturing hot-dip galvanized steel sheet according to [5], wherein the free Al in the hot-dip galvanizing bath in the coarse bottom slag generation step and the hot-dip galvanizing treatment step The concentration C _Al is 0.138% by mass or less.

[7] The method for manufacturing hot-dip galvanized steel sheet is the method for manufacturing hot-dip galvanized steel sheet as described in any one of [1] to [6], further comprising performing the above-mentioned hot-dip plating The bottom slag removing step of removing at least a part of the aforementioned coarse bottom slag in the zinc bath.

[8] The manufacturing method of alloyed hot-dip galvanized steel sheet includes the following steps: Implementing the method of manufacturing the aforementioned hot-dip galvanized steel sheet as described in any one of [1] to [7], the steps of manufacturing the aforementioned hot-dip galvanized steel sheet, and The alloying step of alloying the aforementioned hot-dip galvanized steel sheet.

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

[Composition of equipment for hot-dip galvanizing line] FIG. 2 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet used in the manufacture of hot-dip galvanized steel production lines. Referring to FIG. 2, the hot-dip galvanizing production line equipment 1 includes a blunt furnace 20, a hot-dip galvanizing equipment 10, and a tempering and calendering machine (leveler) 35.

The blunt furnace 20 includes one or more heating belts (not shown) and one or more cooling belts arranged downstream of the heating belt. In the sintering furnace 20, the steel sheet is supplied to the heating zone of the sintering furnace 20, and the steel sheet is sintered. The blunt steel plate is cooled by a cooling belt and transported to the hot-dip galvanizing equipment 10. The hot-dip galvanizing equipment 10 is arranged downstream of the sintering furnace 20. In the hot-dip galvanizing facility 10, hot-dip galvanizing treatment is performed on the steel sheet to manufacture an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. The tempering and calendering machine 35 is arranged downstream of the hot-dip galvanizing equipment 10. In the tempering and calendering machine 35, for the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet manufactured in the hot-dip galvanizing facility 10, adjust the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet under light pressure as required .

[About hot-dip galvanizing equipment 10] FIG. 3 is a side view of the hot-dip galvanizing apparatus 10 in FIG. 1. Referring to FIG. 3, the hot-dip galvanizing facility 10 includes, for example, a molten zinc pot 101, a settling roller 107, a support roller 113, a gas purge device 109, and an alloying furnace 111.

The passivation furnace 20 provided in the front stage of the hot-dip galvanizing equipment 10 is maintained in a reducing environment inside. In the sintering furnace 20, the steel sheet S continuously transferred is heated. The steel sheet S is heated by the sintering furnace 20 to activate the surface of the steel sheet S and adjust the mechanical properties of the steel sheet S. The downstream end of the smelting furnace 20 corresponding to the output side of the smelting furnace 20 has a space where the turning roller 30 is arranged. The downstream end of the blunt furnace 20 is connected to the upstream end of the nozzle 105. The downstream end of the nozzle part 105 is immersed in the hot dip galvanizing bath 103. The inside of the nozzle part 105 is blocked from the atmospheric environment and maintained in a reducing environment.

The steel plate S whose conveying direction is changed downward by the reversing roller 30 passes through the nozzle portion 105 and is continuously immersed in the molten galvanizing bath 103 stored in the molten zinc pot 101. A settling roller 107 is arranged inside the molten zinc pot 101. The settling roller 107 has a rotation axis parallel to the width direction of the steel plate S. The axial width of the settling roller 107 is larger than the width of the steel plate S. The settling roller 107 is in contact with the steel plate S to switch the traveling direction of the steel plate S above the hot-dip galvanizing equipment 10.

The support roller 113 is a conventional member. The support roller 113 is tied in the hot dip galvanizing bath 103 and is arranged above the settling roller 107. The support roller 113 includes a pair of rollers. One pair of support rollers 113 has a rotation axis parallel to the width direction of the steel plate S. The support roller 113 holds the steel plate S transferred upward by the settling roller 107 and the traveling direction is changed to the upper direction.

The gas purge device 109 is arranged above the settling roller 107 and the support roller 113 and above the liquid surface of the molten galvanizing bath 103. The gas purge device 109 includes a pair of gas injection devices. A pair of gas injection devices have mutually opposed gas injection nozzles. During the hot dip galvanizing process, the steel sheet S passes between the pair of gas injection nozzles of one of the gas purge devices 109. At this time, the pair of gas injection nozzles face the surface of the steel plate S. The gas purge device 109 blows gas on both surfaces of the steel plate S pulled up from the hot-dip galvanizing bath 103. Thereby, the gas purge device 109 scrapes off a part of the hot-dip galvanizing bath attached to both surfaces of the steel plate S, and adjusts the amount of hot-dip galvanizing adhered to the surface of the steel plate S.

The alloying furnace 111 is arranged above the gas purge device 109. The alloying furnace 111 passes inside the steel plate S conveyed upward by the gas purge device 109, and performs alloying treatment on the steel plate S. The alloying furnace 111 includes a heating zone, a heating zone, and a cooling zone in order from the input side of the steel plate S toward the output side. The heating belt is heated so that the temperature of the steel sheet S (plate temperature) is slightly uniform. The heat preservation zone maintains the temperature of the steel plate S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to become an alloyed hot-dip galvanized layer. The cooling zone cools the steel plate S formed with the alloyed hot-dip galvanized layer. As described above, the alloying furnace 111 uses a heating zone, a heat-preserving zone, and a cooling zone to perform alloying treatment. In addition, the alloying furnace 111 performs the above alloying treatment when manufacturing the alloyed hot-dip galvanized steel sheet. On the other hand, when manufacturing the hot-dip galvanized steel sheet, the alloying furnace 111 is not subjected to alloying treatment. In this case, the steel plate S passes through the alloy furnace 111 which has not been operated. Here, the non-activation means, for example, a state where the power is directly stopped when the alloying furnace 111 is arranged on the production line (a state where it is not started). The steel sheet S passing through the alloying furnace 111 is transported by the top roller 40 to the next step.

When manufacturing a hot-dip galvanized steel sheet, as shown in FIG. 4, the alloying furnace 111 can also be moved offline. In this case, the steel sheet S does not pass through the alloying furnace 111, but is transported to the next step by the top roller 40.

In addition, when the hot-dip galvanizing facility 10 is a dedicated facility for hot-dip galvanized steel sheet, as shown in FIG. 5, the hot-dip galvanizing facility 10 may not include the alloying furnace 111.

[About other configuration examples of hot-dip galvanizing production line equipment] The hot-dip galvanizing line 1 is not limited to the configuration of FIG. 2. For example, when pre-Ni plating is performed on the steel sheet before hot-dip galvanizing to form a Ni layer on the steel sheet, as shown in FIG. 6, between the passivation furnace 20 and the hot-dip galvanizing equipment 10, pre-Ni plating may also be arranged Device 45. The pre-Ni plating equipment 45 is provided with a Ni plating unit that stores a Ni plating bath. The pre-plating Ni treatment is performed by an electroplating method. In addition, the hot-dip galvanizing line 1 of FIGS. 2 and 6 includes a sintering furnace 20 and a tempering and calendering machine 35. However, the hot-dip galvanizing line 1 may not include the sintering furnace 20. In addition, the hot-dip galvanizing line 1 may not include the tempering and calendering machine 35. The hot-dip galvanizing line equipment 1 only needs to include at least the hot-dip galvanizing equipment 10. The blunt furnace 20 and the tempering and calendering machine 35 only need to be arranged as needed. In addition, the hot-dip galvanizing production line equipment 1 may be provided with pickling equipment for pickling steel plates upstream of the hot-dip galvanizing equipment 10, or may be equipped with other equipment other than the sintering furnace 20 and the pickling equipment. The hot-dip galvanizing line equipment 1 may be further equipped with equipment other than the tempering and calendering machine 35 downstream of the hot-dip galvanizing equipment 10.

[About the manufacturing method of the hot dip galvanized steel sheet of this embodiment] [About the equipment used in the hot-dip galvanizing line] The hot dip galvanizing treatment method of this embodiment uses the hot dip galvanizing production line 1. The hot-dip galvanizing line 1 has the structure shown in FIG. 2 or FIG. 6, for example. The hot-dip galvanizing production line equipment 1 used in the hot-dip galvanizing treatment method of this embodiment may be the equipment shown in FIG. 2 or FIG. 6 as described above, or may be added to the equipment shown in FIG. 2 or FIG. 6 Constructor. Furthermore, as described above, the hot-dip galvanizing line 1 may not include the sintering furnace 20. In addition, the hot-dip galvanizing line 1 may not include the tempering and calendering machine 35. The hot-dip galvanizing line equipment 1 only needs to include at least the hot-dip galvanizing equipment 10. The conventional hot-dip galvanizing production line 1 having a configuration different from that in FIG. 2 or FIG. 6 can also be used.

[About steel sheets subject to hot dip galvanizing] The steel type and size (plate thickness, plate width, etc.) of the steel plate (base material steel plate) used in the hot dip galvanizing treatment method of this embodiment are not particularly limited. The steel sheet corresponds to the required mechanical properties (such as tensile strength, workability, etc.) of the alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet, and is suitable for alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet. Knowing the steel plate is enough. The steel plate used for the automobile outer plate can also be used as the steel plate subject to the hot-dip galvanizing treatment. The steel sheet (base metal steel sheet) to be subjected to the hot-dip galvanizing treatment of the present embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet.

[About hot-dip galvanizing bath] The main component of the hot dip galvanizing bath is Zn. The hot dip galvanizing bath contains Al in addition to Zn. That is, the hot-dip galvanizing bath used in the hot-dip galvanizing method of this embodiment contains a specific concentration of Al, and the rest is a plating solution made of Zn and impurities. The impurity is Fe, for example. If the hot-dip galvanizing bath contains Al at a specific concentration, the excessive reaction of Fe and Zn in the bath can be suppressed. As a result, it is possible to suppress the reaction of the non-uniform alloy of the steel sheet immersed in the hot-dip galvanizing bath and Zn.

As shown in Figure 1, the preferred lower limit of the free Al concentration C _Al is 0.125%. If the free Al concentration C _Al is 0.125% by mass or more, the alloying treatment can suppress excessive alloying of the hot-dip galvanized layer. Therefore, the embrittlement of the alloyed hot-dip galvanized layer due to over-alloying can be suppressed. As a result, the adhesion of the alloyed hot-dip galvanized layer to the steel sheet is improved. The further preferred lower limit of the free Al concentration C _Al is 0.127%, more preferably 0.129%, and even more preferably 0.130%.

The preferable upper limit of the free Al concentration C _Al is 0.138% by mass or less. In this case, alloying is further effectively performed to fully form an alloyed hot-dip galvanized layer. The further preferred upper limit of the free Al concentration C _Al is 0.137%, more preferably 0.136%, and even more preferably 0.135%.

The free Fe concentration in the hot dip galvanizing bath 103 is not particularly limited. The free Fe concentration is 0.020 to 0.060% in mass %, for example. Fe in the hot-dip galvanizing bath 103 may be eluted from the steel sheet S, or may be contained in the hot-dip galvanizing bath 103 for other reasons. The hot-dip galvanizing bath 103 may contain impurities other than Fe. The term “impurities” herein means components that are mixed in because of other requirements of the raw materials, and is allowed to fall within a range that does not adversely affect the manufacturing method of this embodiment.

[Measurement method of free Al concentration and free Fe concentration in the hot-dip galvanizing bath 103] The method of determining the free Al concentration and free Fe concentration in the hot-dip galvanizing bath 103 is not particularly limited. For example, the free Al concentration C _Al (mass %) and free Fe concentration (mass %) are obtained based on the Al concentration and Fe concentration obtained by ICP (Inductively Coupled Plasma) emission spectrometry.

Specifically, a sample is taken from the hot-dip galvanizing bath 103. The sample is quenched and solidified. Using the cured sample, the Al concentration and Fe concentration were obtained by ICP emission spectrometry. The Al concentration obtained by ICP emission spectrometry is not only the free Al concentration in the hot-dip galvanizing bath, but also the Al concentration in the slag. That is, the Al concentration obtained by ICP emission spectrometry is the so-called total Al concentration. Similarly, the Fe concentration obtained by the above-mentioned ICP emission spectrometry is not only the free Fe concentration in the hot-dip galvanizing bath, but also the Fe concentration in the slag. That is, the Fe concentration obtained by ICP emission spectrometry is the so-called total Fe concentration. Therefore, the obtained total Al concentration and total Fe concentration were determined using the well-known Zn-Fe-Al ternary system state diagram to determine the free Al concentration C _Al and free Fe concentration.

The method of determining the free Al concentration C _Al and free Fe concentration is as follows. Prepare a state diagram of the Zn-Fe-Al ternary system at the bath temperature T at the time the sample is taken. As described above, the state diagram of the Zn-Fe-Al ternary system is well known and is also disclosed in FIGS. 2 and 3 of Non-Patent Document 1. In addition, Non-Patent Document 1 is a famous paper among researchers and developers of hot-dip galvanizing baths. In the state diagram of the Zn-Fe-Al ternary system, the points specified by the total Al concentration and the total Fe concentration obtained by ICP emission spectrometry are plotted. Next, from the point of the drawing, a connecting line (conjugate line) is drawn in the liquidus of the Zn-Fe-Al ternary system state diagram. The Al concentration at the intersection of the liquidus and the connection line is defined as the free Al concentration C _Al , and the Fe concentration at the intersection of the liquidus and the connection line is defined as the free Fe concentration.

By the above method, the free Al concentration C _{Al in the} hot-dip galvanizing bath and the free Fe concentration in the hot-dip galvanizing bath can be determined. In addition, in the chemical composition of the hot-dip galvanizing bath, the rest other than the free Al concentration C _Al and the free Fe concentration can be regarded as Zn.

[About the manufacturing method of the hot dip galvanized steel sheet of this embodiment] The manufacturing method of the hot dip galvanized steel sheet of this embodiment includes a coarse bottom slag generation step (S1) and a hot dip galvanizing treatment step (S2). The following describes each step.

[Procedure for generating coarse bottom slag (S1)] The coarse bottom slag generation step (S1) is carried out while the hot-dip galvanizing treatment is not performed. That is, the coarse bottom dross generation step (S1) is implemented, for example, while the steel sheet does not pass through the hot-dip galvanizing equipment, while the hot-dip galvanizing production line equipment is stopped (downtime).

其中，式(1)中之「C_Al 」係代入熔融鍍鋅浴103中之游離Al濃度C_Al (質量%)。In the step of generating coarse bottom dross (S1), the free Al concentration C _Al and the bath temperature T in the hot-dip galvanizing bath are adjusted in such a way as to satisfy the formula (1), and a coarse bottom dross with a particle size of 300 μm or more is generated in the hot-dip galvanizing bath .

Among them, “C _Al ”in formula (1) is the free Al concentration C _Al (mass %) substituted into the hot-dip galvanizing bath 103.

"466.15×C _Al +385.14" in formula (1) corresponds to the above formula (B). That is, 466.15×C _Al +385.14 corresponds to the boundary line F _{2122 in} FIG. 1. "577.24×C _Al +382.49" in equation (1) corresponds to equation (A). That is, 577.24×C _Al +382.49 corresponds to the abnormal line F _{23 in} FIG. 1. Therefore, formula (1) means the Γ ₂ grain growth region 22 in FIG. 1.

In the coarse bottom slag generation step (S1), the free Al concentration C _Al in the hot-dip galvanizing bath and the bath temperature T are adjusted to maintain the state of the hot-dip galvanizing bath 103 in the Γ ₂ grain growth region 22. At this time, the bottom slag generated in the hot-dip galvanizing bath 103 is Γ ₂ phase crushed slag. As mentioned above, the Γ ₂ phase slag grows fastest in the bottom slag. Furthermore, the Γ ₂ grain growth region 22 promotes the growth of the Γ ₂ phase debris more than the Γ ₂ nucleation region 21. Therefore, in the coarse bottom slag generation step (S1), coarse bottom slag having a particle diameter of 300 μm or more can be generated in a relatively short period of time.

In the step of growing the coarse bottom dross, the period during which the hot-dip galvanizing bath 103 is maintained in a manner satisfying formula (1) is not particularly limited. As long as a coarse bottom slag having a particle size of 300 μm or more can be generated. In addition, if the hot-dip galvanizing bath 103 without a bottom slag and a newly-built bath is maintained for at least 30 days to satisfy the condition of formula (1), it can be confirmed that Γ ₂ phase slag having a particle size of 300 μm or more is generated. Therefore, in the step of growing the coarse bottom slag, it is preferable to maintain the hot-dip galvanizing bath 103 for at least 30 days in a manner satisfying the formula (1). Furthermore, it is preferable to maintain the hot-dip galvanizing bath 103 for at least 60 days, and more preferably for at least 90 days, in the step of growing the coarse bottom slag.

In this specification, the bottom slag particle size is defined as follows. Referring to FIG. 7, in each bottom slag 100, the largest line segment LS is defined as the “particle size” in the line segment LS connecting any two points of the interface 150 between the bottom slag 100 and the parent phase 200 (that is, the outer periphery of the broken slag) 150. . The particle size can be obtained by using image processing on the photo image of the observation field. In addition, in this specification, slag having a longest diameter of less than 20 μm has little effect on slag defects, so it is excluded from the target.

[Process of hot-dip galvanizing (S2)] The hot-dip galvanizing treatment step (S2) uses the hot-dip galvanizing bath 103 after the coarse bottom dross generating step (S1) to perform hot-dip galvanizing treatment on the steel sheet. Specifically, the steel sheet is passed into the hot-dip galvanizing bath 103 containing coarse bottom dross. At this time, a hot-dip galvanized layer is formed on the surface of the steel sheet.

其中，式(2)中之「C_Al 」係代入熔融鍍鋅浴103中之游離Al濃度C_Al (質量%)。In the hot-dip galvanizing treatment step (S2), during the hot-dip galvanizing process (that is, during the operation period), the free Al concentration C _Al and the bath temperature T in the hot-dip galvanizing bath are adjusted in a manner that satisfies formula (2) .

Among them, “C _Al ”in formula (2) is the free Al concentration C _Al (mass %) substituted into the hot-dip galvanizing bath 103.

"390.91×C _Al +414.20" in the formula (2) corresponds to the above formula (C). That is, 390.91×C _Al +414.20 corresponds to the boundary line F _{3132 in} FIG. 1.

In short, in the hot-dip galvanizing treatment step (S2), the bath temperature T of the hot-dip galvanizing bath 103 after the coarse bottom dross generating step (S1) is raised, and the state of the hot-dip galvanizing bath 103 is increased from the Γ ₂ grain growth region 22 moves to the delta ₁ phase nucleus generation area 31. Next, the state of the hot-dip galvanizing bath 103 is maintained in the δ ₁ phase nucleation region 31. At this time, the coarse bottom slag in the hot-dip galvanizing bath 103 is transformed from the Γ ₂ phase to the δ ₁ phase. During phase transformation, part or all of the coarse bottom slag is not dissolved. That is, when the coarse bottom slag generation step (S1) is transferred to the hot-dip galvanizing treatment step (S2), the size and shape of the coarse bottom slag deposited on the bottom of the molten zinc pot 101 have not changed much since Γ ₂ The phase is perverted to δ ₁ phase.

As described above, in the hot-dip galvanizing treatment step (S2), the bath temperature T of the hot-dip galvanizing bath 103 is raised, and the state of the hot-dip galvanizing bath 103 is shifted from the Γ ₂ grain growth region 22 to the δ ₁ nucleus generation region 31. Next, the state of the hot dip galvanizing bath 103 is maintained in the δ ₁ nucleus generation region 31. At this time, at the bottom of the molten zinc pot 101, there exists a coarse bottom slag that transforms from the Γ ₂ phase to the δ ₁ phase. That is, in the hot-dip galvanizing treatment step (S2), a hot-dip galvanizing bath 103 containing coarse bottom dross is used to perform hot-dip galvanizing treatment.

In the hot-dip galvanizing process step (S2), during the hot-dip galvanizing process (that is, during the operation period), the state of the hot-dip galvanizing bath 103 is the δ ₁ nucleus generation region 31. Therefore, in the hot-dip galvanizing treatment step, fine δ ₁ phase slag is generated in the hot-dip galvanizing bath 103. However, a coarse bottom slag exists at the bottom of the molten zinc pot 101. Therefore, as Oswald grows, the fine δ ₁ phase slag shrinks or disappears, and the coarse bottom slag grows. That is, in the hot-dip galvanizing treatment step (S2), the Oswald growth of the coarse bottom slag is utilized to suppress the generation and growth of fine bottom slag (fine δ ₁ phase crushed slag). In this case, the fine δ ₁ phase slag is further reduced, and the coarse bottom slag is further increased. As a result, the formation of medium-sized bottom slag (δ ₁ phase crushed slag) with a particle size of 100 to 300 μm can be suppressed. In addition, during the implementation of the hot-dip galvanizing step (S2), the coarse bottom slag further grows. However, since the mass of the coarse bottom slag increases, it is less likely to rise in the hot-dip galvanizing bath 103 with the accompanying flow. Therefore, the possibility of coarse bottom slag adhering to the steel plate is extremely low.

In this way, in the hot-dip galvanizing treatment step (S2), even in the hot-dip galvanizing process, even if a fine bottom slag (δ ₁ phase crushed slag) is generated, the fine bottom slag is effectively suppressed by using the Oswald growth of the coarse bottom slag Growing into a medium-sized bottom slag. Therefore, it is possible to suppress the formation of slag defects in the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet.

In the above hot-dip galvanizing treatment step (S2), even if hot-dip galvanizing treatment is performed for a long period of time, the formation of medium-sized bottom dross can be effectively suppressed. Therefore, the bottom dross removal step may not be performed, and even if the bottom dross removal step is performed, the frequency of implementation may be suppressed. That is, the frequency (stop frequency) of stopping the hot-dip galvanizing equipment 10 can be suppressed. Therefore, production efficiency can also be improved.

[Implementation time of coarse bottom slag generation step (S1)] In the present embodiment, when a new hot-dip galvanizing bath 103 is built, before the hot-dip galvanizing treatment step (S2), a coarse bottom dross generating step (S1) is performed.

On the other hand, when the hot-dip galvanizing step (S2) is performed for a long period of time, the coarse bottom slag deposited on the bottom of the molten zinc pot 101 grows excessively, and the accumulation amount of the coarse bottom slag becomes excessive. In this case, the molten zinc pot 101 may be taken out from the molten galvanizing equipment 10 and the coarse bottom slag removal step may be performed. In the step of removing bottom dross, at least a part or all of the coarse bottom dross in the hot-dip galvanizing bath 103 is removed. The bottom slag removal method can be implemented by a conventional method. In the step of removing bottom slag, for example, a bucket suspended by a crane is immersed in the molten zinc pot 101, and the bottom slag is extracted with the bucket. Subsequently, the bucket is pulled up from the molten zinc pot 101, and the bottom slag extracted from the bucket is taken out of the molten zinc pot 101. In addition, by pouring Al into the molten zinc pot 101 and scumming the bottom slag, the bottom slag can also be removed.

The molten galvanizing bath 103 in which the bottom slag deposited on the bottom of the molten zinc pot 101 has been removed is subjected to a coarse bottom slag generation step (S1). As a result, coarse bottom slag is generated again in the hot-dip galvanizing bath 103 adjusted by the bath. Next, the hot-dip galvanizing bath 103 containing the coarse bottom slag and adjusted for the bath is set again in the hot-dip galvanizing equipment 10. Subsequently, a hot-dip galvanizing treatment step (S2) is carried out. In short, the coarse bottom slag generation step (S1) and the hot-dip galvanizing treatment step (S2) are repeated a plurality of times.

[Free molten galvanizing bath of 103 C and adjusting the bath temperature T _Al of the Al concentration Method] The coarse bottom dross formation step (S1) and hot dip galvanization step (S2) of the free molten galvanizing bath of Al concentration C _Al 103 The adjustment of the bath temperature T can be carried out by a conventional method.

For example, the free Al concentration C _Al in the hot dip galvanizing bath is adjusted by adding Al to the hot dip galvanizing bath. The addition of Al is performed, for example, by immersing the Al ingot in the hot-dip galvanizing bath. The addition of Al can also be performed by immersing the Al ingot in a method other than the hot-dip galvanizing bath. By immersing the Al ingot in the hot-dip galvanizing bath and adding Al to the hot-dip galvanizing bath, the Al ingot is immersed in the hot-dip galvanizing bath at an immersion rate that can suppress the rapid change in temperature of the hot-dip galvanizing bath . The method of adjusting the free Al concentration C _Al in the hot-dip galvanizing bath is not limited to the above method. The method for adjusting the free Al concentration C _Al in the hot-dip galvanizing bath is preferably a conventional method.

In addition, the bath temperature T of the hot-dip galvanizing bath 103 is adjusted using the heating device provided in the molten zinc pot 101. The heating device is, for example, a high-frequency induction heating device.

When transitioning from the coarse bottom slag generation step (S1) to the hot-dip galvanizing treatment step (S2), only by changing the bath temperature of the hot-dip galvanizing bath 103, the hot-dip galvanizing of formula (1) can be easily satisfied The bath 103 moves to the hot-dip galvanizing bath 103 that satisfies formula (2). Specifically, referring to FIG. 1, in the coarse bottom slag generation step (S1), the state of the hot-dip galvanizing bath 103 (free Al concentration C _Al and bath temperature T) is within the range of the Γ ₂ grain growth region 22. Here, in the case where the coarse bottom slag generation step (S1) is shifted to the hot-dip galvanizing treatment step (S2), if the bath temperature T is increased, the state of the hot-dip galvanizing bath 103 is easily moved from Γ ₂ grain growth region to δ ₁ core generation area. That is, only by changing the bath temperature T, the hot-dip galvanizing bath 103 satisfying formula (1) can be easily changed to the hot-dip galvanizing bath 103 satisfying formula (2).

In addition, in this embodiment, the coarse bottom slag generation step (S1) and the hot-dip galvanizing treatment step (S2) may be alternately repeated. When the coarse bottom dross generation step (S1) is carried out after the hot galvanizing step (S2), if the hot galvanizing bath 103 after the hot galvanizing step (S1) is used in the coarse bottom slag generation step (S1) When the temperature T is lowered, the hot-dip galvanizing bath 103 satisfying the formula (2) can be made from the hot-dip galvanizing bath 103 satisfying the formula (1).

In short, in the method of manufacturing the hot-dip galvanized steel sheet of the present embodiment, only by changing the bath temperature T, the hot-dip galvanizing bath 103 can be easily switched to the state satisfying the formula (1) or the state satisfying the formula (2). Therefore, in this embodiment, by increasing or decreasing the bath temperature T, it is possible to easily switch between the coarse bottom slag generation step (S1) and the hot-dip galvanizing treatment step (S2).

As described above, in the present embodiment, the coarse bottom slag generation step (S1) and the hot-dip galvanizing treatment step (S2) are performed to form a hot-dip galvanized layer on the surface of the steel sheet to manufacture a hot-dip galvanized steel sheet.

In the present embodiment, in the hot-dip galvanizing treatment step (S2), it is difficult to generate a medium-sized bottom dross which is the cause of dross defects. As a result, the generation of slag defects in the hot-dip galvanized steel sheet can be suppressed. Furthermore, the frequency (stop frequency) of stopping the hot-dip galvanizing equipment 10 can be suppressed, and the hot-dip galvanizing process step (S2) can be implemented for a long period of time.

Furthermore, during shutdown, after the hot-dip galvanizing bath 103 performs bath adjustment, before performing the hot-dip galvanizing process step (S2), a coarse bottom dross generating step (S1) is performed. In this way, when the hot-dip galvanizing treatment step (S2) is performed again after shutdown, the hot-dip galvanizing bath 103 containing the coarse bottom slag in advance can be used in the hot-dip galvanizing treatment step (S2).

[Manufacturing method of alloyed galvanized steel sheet] The manufacturing method of the hot-dip galvanized steel sheet of the present embodiment described above can be applied to the manufacturing method of the alloyed hot-dip galvanized steel sheet.

The manufacturing method of the alloyed hot-dip galvanized steel sheet of this embodiment includes the steps of manufacturing the hot-dip galvanized steel sheet and the alloying treatment step. In the step of manufacturing the hot-dip galvanized steel sheet, the above-mentioned manufacturing method of the hot-dip galvanized steel sheet is carried out. In the alloying step, the alloying furnace 111 shown in FIG. 3 is used to perform alloying on the hot-dip galvanized steel sheet produced in the step of manufacturing the hot-dip galvanized steel sheet. The alloying treatment method is sufficient as long as the conventional method is applied. Through the above manufacturing steps, alloyed hot-dip galvanized steel sheets can be manufactured.

In the above, the manufacturing method of the hot dip galvanized steel sheet of this embodiment and the manufacturing method of the alloyed hot dip galvanized steel sheet were demonstrated in detail. In this embodiment, a hot-dip galvanizing bath 103 containing a large bottom dross is used, and a hot-dip galvanizing process step (S2) is performed in the δ ₁ nucleus generation region 31. Therefore, during the hot-dip galvanizing process (that is, during the operation period), the Oswald growth of the coarse bottom dross in the hot-dip galvanizing bath 103 can effectively suppress the generation and growth of fine bottom dross. As a result, during the process of performing the hot-dip galvanizing process, the formation of medium-sized bottom dross which is the cause of dross defects can be suppressed, and the dross defects of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet can be suppressed.

In addition, usually in the case of bottom slag operation, if the amount of bottom slag deposited on the bottom of the molten zinc pot 101 increases, the accumulated medium-sized bottom slag will rise due to the flow, and slag defects are likely to occur. However, in the case of this embodiment, coarse bottom slag is deposited on the bottom of the molten zinc pot 101. The coarse bottom slag has a particle size of 300 μm or more, and is not easily lifted by the accompanying flow due to its mass. Therefore, coarse bottom slag is not likely to be the main cause of slag defects. Therefore, the frequency of the bottom slag removal step in the molten zinc pot 101 can be reduced. As a result, the stop frequency (stop frequency) of the hot-dip galvanizing equipment 10 can be reduced, and the productivity can be improved. [Example]

While showing examples of the present invention and comparative examples, the method for manufacturing the hot-dip galvanized steel sheet of the present embodiment will be specifically described. In addition, the embodiment shown below is only one example of the method of manufacturing the hot-dip galvanized steel sheet of this embodiment. Therefore, the manufacturing method of the hot-dip galvanized steel sheet of this embodiment is not limited to the following examples.

[Example 1] [Experiment considering the step of generating coarse bottom slag] Prepare a hot-dip galvanizing bath that simulates the experiment of a real machine. The free Al concentration C _Al in the hot-dip galvanizing bath was maintained at 0.135% and the bath temperature T was maintained at 455°C for 10 days (240 hours). That is, the hot dip galvanizing bath is maintained in the Γ ₂ grain growth region 22 for 10 days. At this time, the free Fe concentration in the hot-dip galvanizing bath was 0.026%. After holding for 10 days, the shape (phase and particle size) of the bottom slag in the hot-dip galvanizing bath was investigated as follows.

A sample of 300g was taken from the area at the center of the depth of the hot dip galvanizing bath and at the center of the width and at the center of the length. The sample taken was quenched and solidified. Measurement samples are taken from the cured samples. Grind the measurement sample by 0.5 mm from the quenched surface. Among the measured sample surfaces, the polished surface serves as the observation surface. Any 5 fields in the observation surface were observed with a 200x optical microscope. The area of each field of view is 250 μm×250 μm. In each field of view, the parent phase (Zn) and slag can be easily distinguished by contrast. Based on this, the debris in each field of view was measured.

The slag phase in each field of view is specified as follows. The chemical composition of each slag was analyzed using EPMA. Furthermore, the crystal structure of each crushed slag was analyzed using TEM. As a result, the chemical composition of any of the slags in the five fields of view is composed of 2% of Al, 8% of Fe, and 90% of Zn in mass%, and the crystal structure is a face-centered cubic crystal. Therefore, the dross in the hot dip galvanizing bath is designated as Γ ₂ phase dross. Accordingly, the particle size of the Γ ₂ phase slag in each field of view is specified by the method described above. As a result, the average particle size of all Γ ₂ phase slag in all five fields of view was 100 μm or more. The photographic image of FIG. 8(A) is an example of an image obtained by the scanning electron microscope of Example 1. The particles described as "Γ2" in the figure are Γ ₂ phase slag. Also, the higher the brightness in the image, the higher the Al concentration (refer to Al(%) on the right of FIG. 8).

Through the above test, while maintaining the free Al concentration C _Al in the hot-dip galvanizing bath of the built-up bath at 0.135% and the bath temperature T at 455°C, while maintaining it for 10 days, the particle size in the hot-dip galvanizing bath was 100 μm The above Γ ₂ phase slag. Furthermore, while maintaining the free Al concentration C _Al in the hot-dip galvanizing bath of the built-up bath at 0.135% and the bath temperature T at 455°C, the results were maintained for 90 days while the Γ ₂ phase slag in all five fields of vision became 300 μm the above.

[Example 2] [Test assuming hot-dip galvanizing treatment step] Next, as in Example 1, a hot-dip galvanizing bath for the experiment was prepared. The free Al concentration C _Al in the hot-dip galvanizing bath was maintained at 0.135% and the bath temperature T was maintained at 470°C for 10 days (240 hours). That is, the hot-dip galvanizing bath is maintained in the δ ₁ nucleation zone 31 for 10 days. Furthermore, after holding for 10 days, the form (phase and particle size) of the bottom slag in the hot-dip galvanizing bath was investigated as follows. By the same method as in Example 1, the bottom slag form (phase and particle size) in the hot-dip galvanizing bath was investigated. The photographic image of FIG. 8(B) is an example of an image obtained by the scanning electron microscope of Example 2. The "δ1" indicated by the arrow in the figure is the delta ₁ phase slag.

As also shown in Fig. 8, the chemical composition of any slag in the five fields of view, in terms of mass%, is composed of 1% or less of Al, 9% or more of Fe, and 90% or more of Zn, and the crystal structure is plane Heart cubic crystal. Therefore, the slag in the hot-dip galvanizing bath of Example 2 was regarded as δ ₁ phase slag. The particle size of the δ ₁ phase slag was measured by the above method. As a result, the particle diameters of all δ ₁ phase slags in the five fields of view were much smaller than 100 μm. In five more fields of view, no Γ ₂ phase slag having a particle size of 100 μm or more was confirmed.

The test results of Example 1 and Example 2 above show that they are quite consistent with the slag image predicted from the quasi-stability state diagram of FIG. 1. Therefore, it can be seen that by appropriately adjusting the free Al concentration C _Al (mass %) of the hot-dip galvanizing bath and the bath temperature T (°C), the bottom slag particle size can be adjusted.

[Example 3] Based on the above-mentioned Example 1 and Example 2, the continuous molten galvanizing equipment of a real machine was used and the alloyed molten galvanized steel sheet was manufactured as follows.

In each test number, during the shutdown period, the free Al concentration C _Al (mass %) and the bath temperature T (°C) of the hot-dip galvanizing bath are maintained as shown in the "Al concentration C _Al "column of the "Shutdown" column of Table 1 And "Bath temperature T" column. The holding time is set to 30 days.

After the shutdown period, the hot-dip galvanizing process is implemented. During the process of the hot-dip galvanizing process, the free Al concentration C _Al (mass %) and bath temperature T (°C) of the hot-dip galvanizing bath are maintained as shown in the "Operation" column of Table 1. The hold time is set to 5 days. The steel plate flux during the holding period is the same as each test condition. In addition, the steel sheet after the hot-dip galvanizing treatment is subjected to a well-known alloying treatment. The types of steel plates used for each test number are the same. In addition, the conditions of alloying treatment are the same in each test number. Through the above steps, the alloyed hot-dip galvanized steel sheets of each test number were manufactured.

In addition, the free Al concentration C _Al of each test number was measured over time by the above method, and the free A1 concentration C _{Al of the} hot-dip galvanizing bath was adjusted.

The surface of the alloyed hot-dip galvanized steel sheet subjected to hot-dip galvanizing treatment during the last 2 hours of the holding period was visually observed, and the slag defects were evaluated by the following evaluation indexes.

Specifically, on the surface of the alloyed hot-dip galvanized steel sheet, samples are taken from the center of any width. On the surface of the alloyed hot-dip galvanized layer of the sample taken, a rectangular area of 1 m×1 m was set as 1 field of view, and any 10 fields of view were taken as the measurement object. In each field of view, visually observe debris with a particle size of 100 μm or more. When the slag having a particle diameter of 100 μm or more adheres to the alloyed hot-dip galvanized layer, it is regarded as a slag defect. Specify with 10 fields of view. Calculate the total number of slag defects. Based on the total number of slag defects and the total area of 10 fields of view (10m ² ), the number of slag defects per unit area (pieces/10m ² ) is determined. In addition, in the visual observation, it is judged by using a 100-fold optical microscope to determine whether the particle size is 100 μm or more and it is difficult to distinguish the slag.

The criteria for the evaluation of chipping defects are as follows. Evaluation A: The number of chipping defects per unit area is 0 to 1/10m ² . Evaluation B: The number of broken slag defects per unit area is 1 to 10/10m ² . Evaluation C: The number of chipping defects per unit area is 11 pieces/10m ² or more.

[Evaluation Results] The evaluation results are shown in Table 1.

In addition, the "F ₂₁₂₂ "column in Table 1 shows the F ₂₁₂₂ value of the corresponding test number. The "F ₂₃ "column shows the F ₂₃ value of the corresponding test number. The "F ₃₁₃₂ "column shows the F ₃₁₃₂ value of the corresponding test number. The "Area" column in the "At Shutdown" column in Table 1 indicates the state of the hot-dip galvanizing bath for each test number during shutdown. For example, in the case of Test No. 1, the state of the hot-dip galvanizing bath during shutdown indicates the growth area of Γ ₂ grains. Similarly, the "Area" column in the "Operation" column in Table 1 indicates the state of the hot-dip galvanizing bath for each test number during operation. For example, in the case of Test No. 1, the state of the hot-dip galvanizing bath during operation indicates a growth area of δ ₁ grain.

Refer to Table 1 in the test numbers 3~6, 13~16, 24~26, 33~35. During the shutdown period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath satisfy the formula (1). That is, the state of the hot-dip galvanizing bath is a Γ ₂ grain growth region. In addition, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath satisfy equation (2). That is, the state of the hot-dip galvanizing bath is the δ ₁ nucleation region. Therefore, no slag defects were observed in the produced alloyed hot-dip galvanized steel sheet, and slag defects could be effectively suppressed (Evaluation A).

On the other hand, in the test numbers 1 and 2, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (2) during the operation period. Specifically, during the operation period, the state of the hot-dip galvanizing bath is not the δ ₁ nucleation region, but the δ ₁ grain growth region. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 7, during the shutdown period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy equation (1), and the state of the hot-dip galvanizing bath was δ ₁ grain growth area. In addition, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath do not satisfy the formula (2), and the state of the hot-dip galvanizing bath is δ ₁ grain growth region. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 10, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (2), and the state of the hot-dip galvanizing bath was a growth region of ₂ grains. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 11, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (2), and the state of the hot-dip galvanizing bath was a growth region of ₂ grains. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 12, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (2), and the state of the hot-dip galvanizing bath was δ ₁ grain growth region. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 17, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy equation (2), and the state of the hot-dip galvanizing bath was δ ₁ grain growth region. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test Nos. 18 and 19, during the shutdown period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy Equation (1), and the state of the hot-dip galvanizing bath was δ ₁ nucleus generation area. Therefore, although dross defects were confirmed in the produced alloyed hot-dip galvanized steel sheet (evaluation B).

In the test numbers 20 to 22, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy Equation (2), and the state of the hot-dip galvanizing bath was a growth area of Γ ₂ grains. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 23, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy equation (2), and the state of the hot-dip galvanizing bath was δ ₁ grain growth region. Therefore, although dross defects were confirmed in the produced alloyed hot-dip galvanized steel sheet (evaluation B).

In Test No. 27, during the shutdown period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy equation (1), and the state of the hot-dip galvanizing bath was δ ₁ grain growth area. In addition, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath do not satisfy the formula (2), and the state of the hot-dip galvanizing bath is δ ₁ grain growth region. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 28, during the shutdown period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (1), and the state of the hot-dip galvanizing bath was δ ₁ nucleus generation area. Therefore, although dross defects were confirmed in the produced alloyed hot-dip galvanized steel sheet (evaluation B).

In the test numbers 30 and 31, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy the formula (2), and the state of the hot-dip galvanizing bath was a growth region of Γ ₂ grains. Therefore, dross defects were confirmed in the produced alloyed galvanized steel sheet (evaluation C).

In Test No. 32, during the operation period, the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath did not satisfy Equation (2), and the state of the hot-dip galvanizing bath was δ ₁ grain growth region. Therefore, dross defects were confirmed in the produced alloyed hot-dip galvanized steel sheet (Evaluation B).

In Test Nos. 8, 9 and 29, during the shutdown period and during the operation period, the bath temperature was constant and above 470°C. In these test numbers, although the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath during the shutdown period did not satisfy the formula (1), the alloyed hot-dip galvanized steel sheet produced did not confirm slag defects (evaluation A). On the other hand, as mentioned above, in test numbers 18, 19, and 28, the bath temperature is constant at 470°C or higher. The free Al concentration C _Al and bath temperature T of the hot-dip galvanizing bath during the shutdown period do not satisfy equation (1) ), so there are few slag defects in the alloyed hot-dip galvanized steel sheet (Evaluation B). Therefore, in order to suppress the slag defects more stably, the free Al concentration C _Al and bath temperature T of the hot-dip galvanizing bath during the shutdown period satisfy the formula (1), and the free Al concentration C _Al of the hot-dip galvanizing bath during the operation period And the bath temperature T satisfies formula (2) is effective.

[Example 4] The test number 4 (Γ ₂ grain growth area during shutdown and δ ₁ nucleus generation area during operation) and test number 11 (both during shutdown and during operation) were investigated by the following method It is the Γ ₂ grain growth area), and the size and number of slag in the hot-dip galvanizing bath of test number 18 (delta ₁ nucleus generation area during shutdown and during operation).

Samples are taken from the area at the center of the length of each hot-dip galvanizing bath after the operation period of test number 4, test number 11, and test number 18, and at the center of the width and at a depth of 300 mm from the liquid surface of the hot-dip galvanizing bath. (Liquid phase). In addition, a position 300 mm deep from the liquid surface of the hot-dip galvanizing bath corresponds to a position near D/10 in the depth direction from the liquid surface when the depth of the hot-dip galvanizing bath is assumed to be D (mm).

The sample taken is quenched with a copper mold and solidified into a rectangular shape. One of the cured sample surfaces is defined as the observation surface. Polish the observation mirror. Among the observation surfaces polished by mirror surface, an arbitrary range of 20 mm×20 mm is designated as the field of view. Using a laser microscope, measure the particle size and number of bottom slag contained in the field of view. Specifically, the field of view of 20 mm×20 mm is divided into 100 fine areas of 2 mm×2 mm. Observe each fine area with an optical microscope to generate a photo image (optical image). In the photo image of the fine area, the contrast between the parent phase (Zn) and the bottom slag is different. Therefore, by performing a binarization process on the photo image of the fine area with an appropriate threshold value, as shown in FIG. 7, the interface 150 between the mother phase 200 and the bottom slag 100 is clarified. The bottom slag 100 in the fine area is specified, and the maximum length LS of each specified bottom slag 100 is obtained by image processing. The maximum length obtained is defined as the particle size (μm) of the corresponding bottom slag 100. The bottom slag in all fine areas is specified, and the specified bottom slag particle size is obtained. Next, the bottom slag in all fine areas is graded according to a specific particle size range. Next, find the number of bottom slags for each class. Columnarize the number of bottom slags in each class.

In addition, the sampling position mentioned above is located above the coarse bottom slag deposited on the bottom of the molten zinc bath 101. Therefore, the sample taken did not include the coarse bottom slag accumulated on the bottom of the molten zinc pot 101.

Based on the measured particle size and the number of bottom slag, a histogram as shown in FIG. 9 is prepared.

[Evaluation Results] Referring to FIG. 9, the test number 4 (Γ ₂ grain growth area during shutdown and δ ₁ nucleus generation area during operation) of the present invention is compared with other test numbers 11 and 18, and the bottom in the field of view The total amount of slag is the least. In Test No. 4, it is considered that the system acts as the following mechanism. The coarse bottom slag is generated by the step of generating the coarse bottom slag during the shutdown period. Next, a hot-dip galvanizing bath containing coarse bottom dross was used, and as a result of hot-dip galvanizing treatment during operation, the formation and growth of fine δ ₁ phase dross was suppressed due to the growth of Oswald. Therefore, the number of bottom slag with a particle size of 100 μm to less than 300 μm is the smallest, and the total number of bottom slag is also the smallest. As a result, it is considered that slag defects were not confirmed.

On the other hand, compared with the test number 4 and the test number 18, the test number 11 (both the Γ ₂ grain growth area during the shutdown period and the operation period) has the most bottom slag having a particle size of 100 μm to less than 300 μm. Since there are many bottom slags with a particle size of 100 μm to less than 300 μm, it is predicted that there are many slag defects in Test No. 11.

Compared with test number 4 in test number 18 (both δ ₁ nucleus generation areas during shutdown and during operation), there are many bottom slags with a particle size of 100 μm to less than 300 μm. In particular, there are many bottom slags with a particle size of 100 μm to less than 150 μm. Therefore, compared with the test number 4, it can be predicted that there are many slag defects. In Test No. 18, since there was no coarse bottom slag, the generation and growth of fine δ ₁ phase slag due to the Oswald growth of the coarse bottom slag could not be sufficiently suppressed during the operation period. The number of bottom slag whose diameter is from 100μm to less than 150μm becomes more.

[Example 5] Based on the above experimental results, the following steps are repeated within 1 year: during the shutdown period of the continuous molten galvanizing equipment, a step of generating a large bottom slag is carried out for 30 to 40 days, and then, the melt containing the large bottom slag is used Galvanizing bath, carry out 30 to 40 days of hot-dip galvanizing process. At this time, the bath temperature T is adjusted to increase or decrease in such a manner that the hot-dip galvanizing bath in the coarse bottom dross generation step satisfies formula (1) and the hot-dip galvanizing bath in the hot galvanizing process step satisfies formula (2). . As a result, the 30-40 day shutdown period and 30-40 day operation period are repeated within 1 year, and during the shutdown period and operation period, the free Al concentration C _Al is constant at 0.130% and the bath temperature is 455°C For comparison of certain operations, the frequency of the bottom slag removal step can be reduced to 1/3.

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

1: Hot-dip galvanizing production line equipment 10: Hot-dip galvanizing equipment 20: Burning furnace 30: flip roller 35: Tempering and Calendering Machine 40: Top roller 45: Pre-plated Ni equipment 100: bottom slag 101: molten zinc pot 103: Hot-dip galvanizing bath 105: nozzle part 107: Settling roller 109: Gas purge device 111: Alloying furnace 113: Support roller 150: Interface 200: mother phase S: Steel plate

Fig. 1 is a quasi-stabilized state diagram of the slag generation phase of the hot-dip galvanizing bath for the bath temperature T (°C) and the free Al concentration C _Al . FIG. 2 is a functional block diagram showing the overall configuration of a hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet used in the manufacture of a hot-dip galvanized steel sheet. Fig. 3 is a side view of the hot-dip galvanizing equipment in Fig. 2. FIG. 4 is a side view of a hot dip galvanizing equipment with a different structure from FIG. 3. Fig. 5 is a side view of a hot-dip galvanizing equipment with a different structure from Fig. 3 and Fig. 4. FIG. 6 is a functional block diagram showing the overall configuration of a hot-dip galvanizing production line with a configuration different from FIG. 2. FIG. 7 is a schematic diagram for explaining the measurement method of the bottom slag particle size. 8 is a microscope photograph showing the shape of the bottom slag formed in the hot-dip galvanizing bath 10 days after the bath was built in Examples 1 and 2. 9 is a graph showing the relationship between the particle size and the number of crushed slag under the manufacturing conditions of Example 5. FIG.

Claims (10)

A method for manufacturing a hot-dip galvanized steel sheet, comprising the following steps: To adjust the free Al concentration in a hot-dip galvanizing bath C _Al and the bath temperature T in a manner that satisfies formula (1), to generate a particle size in the aforementioned hot-dip galvanizing bath A coarse bottom slag generation step of a coarse bottom slag of 300 μm or more, and adjusting the free Al concentration C _Al and the bath temperature T of the hot-dip galvanizing bath after the coarse bottom slag generation step to satisfy equation (2), use The above-mentioned free Al concentration C _Al and the above-mentioned bath temperature T satisfy the above-mentioned hot-dip galvanizing bath of formula (2), and perform hot-dip galvanizing treatment to form a hot-dip galvanizing layer on the steel sheet,

Among them, "C _Al "in formula (1) and formula (2) is the free Al concentration C _Al (mass %) substituted into the hot-dip galvanizing bath. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein when the shutdown of the hot-dip galvanizing treatment step is stopped, the aforementioned coarse dross generating step is performed on the hot-dip galvanizing bath after the hot-dip galvanizing treatment step. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein in the hot-dip galvanizing treatment step, the bath temperature T of the hot-dip galvanizing bath after the coarse bottom dross generating step is increased to satisfy the formula (2) The aforementioned hot-dip galvanizing bath. The method for manufacturing a hot-dip galvanized steel sheet according to claim 2, wherein in the hot-dip galvanizing treatment step, the bath temperature T of the hot-dip galvanizing bath after the coarse bottom dross generation step is raised to satisfy the expression (2) The aforementioned hot-dip galvanizing bath. The method for manufacturing a hot-dip galvanized steel sheet according to claim 3, wherein the aforementioned coarse bottom slag generation step and the aforementioned hot-dip galvanizing treatment step are alternately repeated, After the hot galvanizing treatment step, when performing the coarse bottom slag generation step, in the coarse bottom slag step, the bath temperature T of the hot galvanizing bath after the hot galvanizing treatment step is reduced to satisfy the formula ( 1) The aforementioned hot-dip galvanizing bath. The method for manufacturing a hot-dip galvanized steel sheet as described in claim 4, wherein the aforementioned coarse bottom slag generation step and the aforementioned hot-dip galvanizing treatment step are alternately repeated, After the hot galvanizing treatment step, when performing the coarse bottom slag generation step, in the coarse bottom slag step, the bath temperature T of the hot galvanizing bath after the hot galvanizing treatment step is reduced to satisfy the formula ( 1) The aforementioned hot-dip galvanizing bath. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein the free Al concentration C _Al in the hot-dip galvanizing bath of the coarse bottom slag generation step and the hot-dip galvanizing treatment step is 0.125 mass% or more. The method for manufacturing a hot-dip galvanized steel sheet according to claim 7, wherein the free Al concentration C _Al in the hot-dip galvanizing bath in the coarse bottom dross generation step and the hot-dip galvanizing treatment step is 0.138% by mass or less. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, further comprising a bottom dross removing step of removing at least a portion of the large bottom dross in the hot-dip galvanizing bath before the step of generating the large bottom dross. A method for manufacturing alloyed hot-dip galvanized steel sheet, comprising the following steps: Implementing the method of manufacturing the aforementioned hot-dip galvanized steel sheet according to any one of claims 1 to 9, the steps of manufacturing the aforementioned hot-dip galvanized steel sheet, and The alloying step of alloying the aforementioned hot-dip galvanized steel sheet.

Images